Hydrogen Production Through Newly Developed Photocatalytic Nanostructures and Composite Materials

Abstract

Photocatalytic hydrogen (H₂) production offers a promising solution to energy shortages and environmental challenges by converting solar energy into chemical energy. Hydrogen, as a versatile energy carrier, can be generated through photocatalysis under sunlight or via electrolysis powered by solar or wind energy. However, the advancement of photocatalysis is hindered by the limited availability of effective visible light-responsive semiconductors and the challenges of charge separation and transport. To address these issues, researchers are focusing on the development of novel nanostructured semiconductors and composite materials that can enhance photocatalytic performance. In this paper, we provide an overview of the advanced photocatalytic materials prepared so far that can be activated by sunlight, and their efficiency in H₂ production. One of the key strategies in this research area concerns improving the separation and transfer of electron–hole pairs generated by light, which can significantly boost H₂ production. Advanced hybrid materials, such as organic–inorganic hybrid composites consisting of a combination of polymers with metal oxide photocatalysts, and the creation of heterojunctions, are seen as effective methods to improve charge separation and interfacial interactions. The development of Schottky heterojunctions, Z-type heterojunctions, p–n heterojunctions from nanostructures, and the incorporation of nonmetallic atoms have proven to reduce photocorrosion and enhance photocatalytic efficiency. Despite these advancements, designing efficient semiconductor-based heterojunctions at the atomic scale remains a significant challenge for the realization of large-scale photocatalytic H₂ production. In this review, state-of-the-art advancements in photocatalytic hydrogen production are presented and discussed in detail, with a focus on photocatalytic nanostructures, heterojunctions and hybrid composites.

1. Introduction

Photocatalytic hydrogen (H₂) evolution through water splitting represents a promising solution to address the global energy crisis and environmental pollution challenges. Given the distribution of solar energy (UV: 5%, visible: 43%, and infrared: 52%), the efficient harnessing of visible and near-infrared (NIR) light is essential for the development of highly efficient photocatalysts. Over the past few decades, numerous strategies have been used to enhance the light absorption capabilities of photocatalysts. These include modifying the band structure by introducing heteroatoms or defects and integrating semiconductors with other visible light-absorbing materials as a promising approach [1].

Solar energy is considered a highly promising energy source due to its cleanliness, safety, and non-reliance on transportation. Unlike fossil fuels, H₂, as one of the cleanest energy sources, offers distinct advantages such as harmless combustion (without NOx emissions), high energy density, superior conversion efficiency, and renewability. The production of sustainable H₂ energy by utilizing abundant solar energy is widely regarded as the ideal solution to address the global energy crisis and environmental challenges. In this context, photocatalytic technology using semiconductors for solar-driven H₂ production has garnered significant interest [2]. Photocatalytic H₂ production is a promising method to address the energy crisis caused by burning fossil fuels. The development of highly efficient heterojunctions for photocatalytic H₂ production is crucial for the realization of such ambitious goals and commercial application. Photocatalytic H₂ evolution from water under visible light and photocatalytic reduction of CO₂ with H₂O to form hydrocarbons (e.g., methane, methanol, ethanol, etc.) are two critical processes for the use of abundant solar energy that offer significant potential for renewable energy generation and addressing environmental challenges. Various technologies for H₂ production have been developed, mainly including thermal, biological, thermochemical and (photo)electrolytic processes. However, these technologies mostly rely on the utilization of carbon feedstocks (e.g., fossil fuels, biomass, waste) or water as an H₂ source, usually require operation at high pressures and temperatures, and are generally expensive (depending on the cost of feedstocks and related energy costs). For this reason, photoassisted processes that use solar irradiation and water, or potentially renewable feedstocks, are becoming increasingly attractive in the scientific community.

Semiconductor–metal heterostructures, particularly those involving various inorganic semiconductors coupled with platinum (Pt), are widely used for the conversion of solar energy into chemical energy. However, due to the scarcity and high cost of Pt, the development of non-Pt alternatives and efficient assembly strategies is essential. The conversion of sustainable solar energy into clean, cost-effective, and carbon-free H₂ through the use of highly efficient metal-free photocatalysts represents a promising and critical research endeavor [3]. Semiconductor photocatalysts have emerged as promising materials for photocatalytic H₂ evolution due to their ability to efficiently generate electron–hole pairs (e⁻-h⁺) under light irradiation, thereby facilitating reduction reactions. A wide range of semiconductor photocatalysts, including transition-metal oxides, nitrides, and sulfides, have been used for photocatalytic H₂ evolution.

Photocatalytic water splitting, driven by solar energy utilizing the well-known TiO₂ P25 as a benchmark photocatalyst, has gained considerable attention as an environmentally friendly approach and a viable alternative to conventional H₂ production methods. However, TiO₂ P25 exhibits significant limitations, including rapid charge carrier recombination and a narrow light absorption range related to its wide bandgap of approximately 3.2 eV. Recent studies have focused on overcoming these limitations through the development of “new-generation“ TiO₂-based materials, using various strategies of TiO₂ modifications, such as morphology control (nanorods, nanowires, nanosheets), narrowing the bandgap energy via bandgap engineering using doping with elements such as N, C and S, formation of homojunctions (inter-phase junctions, e.g., TiO₂ anatase/rutile homojunctions [4] and heterojunctions by combining TiO₂ with noble metals like Pt and Pd or by other narrow bandgap semiconductors using novel synthesis techniques to improve overall photocatalytic performance [5,6]. Nevertheless, these modifications often encounter challenges, notably in terms of cost-effectiveness and photostability, which have limited their full-scale application across various practical domains, including photocatalytic water splitting, pollutant degradation, and solar energy harvesting via solar cells.

Cobalt monoxide (CoO) is a particularly remarkable metal oxide semiconductor due to its narrow bandgap, which allows it to absorb visible light and facilitate overall water splitting reactions. However, CoO suffers from significant deactivation during photocatalytic processes, primarily due to the aggregation of CoO nanoparticles. To address the deactivation problem associated with CoO, a semiconductor with a larger specific surface area was chosen to minimize the aggregation of nanoparticles [7].

Cadmium sulfide (CdS) has attracted considerable attention as a promising photocatalyst for H₂ evolution due to its excellent response to visible light and its well-positioned conduction band. Despite its potential, the practical application of CdS in photocatalytic water splitting is still limited. This is primarily due to the low separation efficiency of the photogenerated electron–hole pairs and the problem of photocorrosion, which leads to the release of toxic Cd²⁺ ions and subsequent environmental pollution [8]. These challenges can be addressed by optimizing the band energy levels, morphologies, and microcrystal structures of metal sulfide nanoparticles (NPs). In addition, the development of composite materials has proven to be an effective strategy to mitigate the recombination of the photogenerated charge carriers, which is a key factor contributing to the deactivation of photocatalysts.

Polymeric graphitic carbon nitride (g-C₃N₄, CN) has also attracted considerable attention due to its moderate bandgap, unique optical properties, and excellent photochemical stability. Graphitic carbon nitride is an n-type metal-free semiconductor, and presents excellent redox ability to generate H₂ from H₂O under visible light irradiation. However, the practical application of g-C₃N₄ is hindered by its low visible-light utilization efficiency, rapid charge recombination, and limited specific surface area. To address these limitations, several modification strategies have been developed to improve the photocatalytic performance of g-C₃N₄. These strategies include element doping [9], noble metal deposition [10], and the formation of heterojunctions with other semiconductors [11]. Among these strategies, constructing heterojunctions is considered particularly effective in promoting the separation of e⁻-h⁺ pairs and facilitating their transfer across interfaces. In heterojunction-based systems, continuous band bending at the interface enhances visible light absorption and supports efficient charge transfer processes. These features contribute to improved photocatalytic performance by increasing the interaction between charge carriers and light absorption. Metal oxides are commonly employed as semiconductor materials in combination with g-C₃N₄ to enhance photocatalytic activity due to their ability to promote charge separation and reduce recombination [12].

The formation of heterojunctions by coupling CN with semiconductors, such as TiO₂, ZnO, or CdS, minimizes the recombination of photogenerated electron–hole pairs by facilitating the transfer of electrons from a high reduction potential in the conduction band to a lower conduction band potential. This mechanism enhances overall photocatalytic performance, making CN-based systems promising for visible-light-driven H₂ production applications [3]. Besides CN, other carbon nanostructures (CNSs) are also attractive materials for the development of nanocomposites, heterojunctions and nanohybrids applied for H₂ production due to their unique properties such as high electron conductivity, large electron storage capacity, large surface area, suitable morphology and light absorption ability.

The energy stored in H-H bonds can be harnessed through combustion in specially designed engines, but it is more commonly associated with electricity generation in fuel cells. These electrochemical devices typically feature an anode made of platinum on conductive carbon. Fuel cells can achieve relatively high efficiencies, around 30%, in converting chemical energy into electrical energy. Hydrogen can be produced from a variety of primary sources, including natural gas, naphtha, heavy oil, methanol, biomass, waste materials, and coal, typically through a reforming process. The reaction of methane with water, for example, produces CO₂ and H₂ with a reaction enthalpy (ΔH₂₉₈) of 165 kJ mol⁻¹ [13]. Natural gas reforming is the most important industrial method for H₂ production. This process occurs at elevated temperatures (700–1100 °C) and utilizes nickel-based catalysts. The resulting gas mixture typically contains 75–78% hydrogen (H₂), 30% carbon dioxide (CO₂), trace amounts of methane (CH₄), and 10–20 ppm of carbon monoxide (CO) [14]. A brief overview of the various methods of H₂ production and their associated challenges, such as feedstock type, conversion efficiency, and the need for the safe integration of H₂ production systems with H₂ purification and storage technologies, can be found elsewhere [15].

Despite great progress and innovations that have been achieved in recent years, there are still numerous challenges that need to be solved to enable sustainable and applicable photocatalytic hydrogen production. These include several key aspects including the following: (a) the development of advanced composite and hybrid materials using facile synthesis and cost-effective production methods; (b) the further optimization of the composition, structure and morphology of such materials, including the application of advanced characterization techniques in combination with a theoretical modeling approach based on density functional theory (DFT) to predict and explain the electrical structure of advanced materials; and (c) the development of advanced solar reaction systems for continuous operation, the optimization of reaction conditions, and the integration of photocatalytic hydrogen production systems with hydrogen storage and purification technologies. Conflicting information can be found in the literature on specific materials and synthesis methods that provide the highest efficiency, stability and durability of such materials as critical parameters for evaluating their practical viability. Therefore, this article provides a systematic review of the recent literature on photocatalytic nanostructures, heterojunctions and hybrid composites as valuable strategies to improve photocatalytic hydrogen production by enhancing light absorption, charge separation and hydrogen evolution efficiency.

2. Basics of Photocatalytic Water Splitting

Water splitting into hydrogen (H₂) and oxygen (O₂) is a thermodynamically uphill reaction, with a positive Gibbs free energy change (ΔG° = +238 kJ/mol). For efficient photocatalytic H₂ production under visible light, the semiconductor’s bandgap should be between 1.23 and 3 eV [6]. The alignment of the optical bandgap with the valence band (VB) and conduction band (CB) potentials is essential for effective oxidation and reduction reactions. The VB must have a more positive potential than the O₂/H₂O oxidation (1.23 eV), and the CB must be more negative than the H⁺/H₂O reduction (0 V). Common semiconductors, like TiO₂ (3.2 eV), ZnO (3.3 eV), WO₃ (2.6 eV), and Fe₂O₃ (2.1 eV), have different bandgaps [16].

In general, H₂ production by water splitting can be divided into three types: thermochemical, photobiological, and photocatalytic. Photocatalytic water splitting is the most efficient and cost-effective, using light photons without external energy input. In this process, the light excites electrons in the photocatalyst, creating electron–hole pairs. The energy gap between the VB and CB must match the light’s wavelength for effective absorption. The photoelectrons reduce H⁺ ions to H₂, while the holes oxidize water to oxygen. A higher surface area of the photocatalyst improves reaction efficiency by providing more active sites.

The photocatalytic water splitting (sometimes called artificial photosynthesis) can be defined as the reaction that splits H₂O into H₂ and O₂ in a 2:1 ratio using an appropriate semiconductor photocatalyst. It involves two half-reactions, as described by the relevant Equations (1)–(3):

O₂ + 4H⁺ + 4 e⁻ → 2H₂O (water oxidation)

(1)

4H⁺ + 4 e⁻ → 2H₂ (water reduction)

(2)

2H₂O → 2H₂ + O₂ (water splitting)

(3)

Different types of semiconductor materials have been investigated and applied in photoasisted H₂ production with limited efficiency. Research has shown that the main challenges in designing suitable semiconductors include efficient charge carrier separation, fast transfer of photogenerated e⁻-h⁺ pairs and their migration to surface reactive centers, favorable placement of band edges with redox potentials of the desired reactions, and acceptable exposure of active centers as well as simple synthesis. Manzoor et al. [16] proposed a photocatalytic reaction mechanism using copper-doped nanocomposites. In their study [16], the incorporation of copper (Cu) into the ZnO lattice reduced crystal growth, decreasing crystallite size and increasing surface area, which increased photocatalytic activity. Cu doping shifted the light absorption into the visible spectrum and minimized the recombination of photogenerated e⁻-h⁺ pairs. Cu⁺² ions in the ZnO lattice are reduced to metallic Cu⁰ by photogenerated electrons, promoting e⁻-h⁺ separation (Equations (4) and (5)). This reduction occurs because copper has a higher reduction potential than zinc, thereby facilitating the separation of electron–hole pairs and improving photocatalytic performance. To further enhance e⁻-h⁺ pair separation, methanol was introduced as a hole scavenger. Under light illumination, photogenerated h⁺ attacked methanol, leading to the formation of formaldehyde. This formaldehyde reacts with hydroxyl radicals (OH⁻) to produce formic acid (Equations (6)–(8)). The electrons in the ZnO conduction band (CB) react with Cu⁺² ions to reduce them into metallic Cu, resulting in the formation of metallic clusters (Equations (9) and (10)). These Cu nanoparticles act as electron sinks by trapping photogenerated electrons, thereby promoting enhanced electron–hole separation. Furthermore, the presence of Cu nanoparticles shifts the Fermi level toward a more negative potential (Equations (11) and (12)). The photoelectrons migrate toward the Cu nanoparticles until the Fermi level reaches the CB of ZnO. This movement facilitates greater e⁻-h⁺ separation on the photocatalyst’s surface, attributed to the electron sink properties of Cu. As a result, the Cu nanoparticles serve as active sites that trap photogenerated e⁻ or transport them to H⁺ ions, leading to H₂ production (Equation (13)). The results of this study show that the introduction of Cu nanoparticles into ZnO leads to higher photocatalytic H₂ production. This improvement can be attributed to accelerated charge transport, the metallic catalytic properties of Cu, and the observed red shift in optical absorption. In this study, improved photocatalytic H₂ production was observed for the Cu-doped ZnO photocatalyst compared to pure ZnO.

hν + ZnO (e⁻ + h⁺) → ZnO

(4)

H₂O + h⁺ → ·OH + H⁺

(5)

CH₃OH + h⁺ → ·CH₂OH + H⁺

(6)

·CH₂OH + h⁺ → HCHO + H⁺

(7)

·OH + h⁺ + HCHO → HCOOH + H⁺

(8)

2 e⁻ + Cu⁺² → Cu⁰

(9)

ZnO − n (Cu⁰) → ZnO − Cu_n⁰

(10)

hν + ZnO − Cu_n⁰ → ZnO (e⁻ + h⁺) − Cu_n⁰

(11)

ZnO (e⁻ + h⁺) − Cu_n⁰ → ZnO (h⁺) − Cu_n⁰ (e⁻)

(12)

ZnO (h⁺) − Cu_n⁰ (e⁻) + H⁺ → ZnO (h⁺) − Cu_n⁰ + ½ H₂

(13)

The water molecule is very stable, and its thermal dissociation is highly endothermic (∆G° = −237 kJ mol⁻¹). A temperature of 2500 K is required to achieve approximately 5% dissociation at atmospheric pressure. Contrarily, if only the thermodynamics of the process are considered, the photochemical decomposition of water appears feasible by using photons with wavelengths shorter than 1100 nm (around 1.3 eV). However, this photochemical reaction requires overcoming a substantial activation energy. Consequently, dissociation becomes significant only after irradiation with wavelengths shorter than 190 nm, and, fortunately, this process does not occur under sunlight [17]. The application of photocatalysts significantly lowers the energy barrier, making the process feasible with photons from the solar spectrum. It is crucial to note that water splitting, as well as CO₂ photoreduction, is not a spontaneous reaction, but rather an endothermic process (ΔG > 0), meaning it requires the input of energy to drive the formation of chemical bonds in the products. By considering only photons with the necessary energy for water molecule photodissociation, it is theoretically possible to produce approximately 2.8 mmol s⁻¹ m⁻² of H₂ (equivalent to 800 W m⁻²) using solar radiation. However, this value represents an idealized upper limit, as activation losses during the process would inevitably decrease the overall efficiency.

3. Newly Developed Photocatalytic Nanostructures and Composite Materials

The photoactivity of a material suitable for efficient H₂ production is primarily determined by its physicochemical properties, particularly the fundamental electronic properties related to its band-edge energy positions. The following text provides an overview of the different materials that have been developed and investigated for photocatalytic H₂ production.

3.1. Carbon-Based Materials

The development of cost-effective, efficient, photostable and completely metal-free photocatalysts is crucial to advance visible-light-driven photocatalytic production. To address these needs, various carbon-based materials, including graphene and carbon nanotubes (CNTs), have been investigated extensively. However, these materials face limitations related to preparation complexity, resource availability, and scalability. In this regard, g-C₃N₄ or CN has emerged as a more promising alternative to graphene and CNTs.

CN is a nonmetallic π-conjugated polymer with a narrow bandgap of 2.7 eV and a sufficiently negative conduction band edge, rendering it a highly promising candidate for efficient water reduction to produce H₂ [18,19]. CN possesses a range of advantageous properties, including non-toxic nature, excellent photochemical stability, intrinsic basic surface functionalities, and greater natural abundance. Furthermore, CN can be synthesized through a straightforward, one-step polymerization process using inexpensive, readily available nitrogen-rich precursors, such as melamine and urea. CN also demonstrates significant potential when combined with semiconductors like TiO₂ for improved charge transfer and charge carrier separation. Recent research to improve charge separation efficiency has largely focused on the development of composites and heterojunctions. In composite materials, the expected redox potentials can be achieved by using different nanostructured materials or nanocomposites and heterojunctions through the coupling of different semiconductors.

3.2. Composite Photocatalysts and Photocatalysts Multicomponent Heterojunctions

Heterojunction refers to the interface area formed by the contact of two or more semiconductors with unequal band structure [20]. Such an approach enables the expansion of the spectrum absorption range of a single semiconductor, promotes the migration of charges generated by light on different semiconductors through close contact at their interface and improves their spatial separation, thereby enhancing their functional properties and photocatalytic performance [21,22]. Conventional heterojunction composite photocatalysts, which consist of two semiconductors with different properties (semiconductor I (PS I) and semiconductor II (PS II)), can be classified into several types based on their band alignment at the interface and charge transfer mechanism: type I, type II, type III, and S-scheme (Figure 1). Each type of heterojunction has its advantages and limitations, and the proper choice of heterojunction is very important for their practical application. An excellent overview of recent advances in semiconductor heterojunctions, including representative literature on different heterojunctions and their application, is provided by Balapure et al. [23].

In type I heterojunctions (straddling-gap) (Figure 1a), PC II has a narrower bandgap compared to PC I. Because PC I has a more negative CB and a more positive VB, electrons and holes move from PC I to PC II. A type I heterojunction, therefore, does not enhance charge carrier separation since all the charge accumulation occurs in one of the components and, in theory, does not improve photocatalytic activity. However, due to the abundance of wide bandgap semiconductors and the constraints on VB and CB positions, type I heterojunctions are commonly formed when composite materials consist of a narrow bandgap semiconductor and a wide bandgap semiconductor. Despite these limitations, it has been shown that the right choice of materials in type I heterojunction systems exhibits better behavior than the individual components. In type II heterojunctions (staggered-gap) (Figure 1b), the CB position of PC I is more negative than that of PC II, while PC II has a more positive VB position. As a result, electrons and holes move in opposite directions, which enhances charge carrier separation, reduces recombination, increases charge carrier lifetimes, and ultimately improves photocatalytic activity. Therefore, the type II heterojunction is more effective than the type I counterpart. Most of the composite photocatalysts discussed in the literature, such as TiO₂/g-C₃N₄ [24], g-C₃N₄–WO₃ [25], g-C₃N₄–BiPO₄ [26], etc., are examples of type II heterojunctions. In type III heterojunctions (Figure 1d), the charge carrier transfer occurs similarly to type II semiconductors, but the band positions are more widely offset. Due to this significant band alignment, these systems are also referred to as broken-gap configurations, where the charge migration route is not feasible.

Photocatalysts composed of two-dimensional (2D) materials are particularly intriguing because of their unique electrical and optical properties. Type II heterojunctions based on 2D materials have been both theoretically and experimentally explored as efficient photocatalysts. While type II heterojunctions can effectively separate photoinduced charge carriers, p–n heterojunctions, based on the combination of p-type semiconductors (e.g., AgBr, CoO, Cu₂O, NiO) and n-type semiconductors (e.g., ZnO, TiO₂, CdS, g-C₃N₄), can facilitate even faster charge separation by leveraging internal electric fields [27]. There are also Z-scheme semiconductor heterojunctions with the same band configuration as type II heterostructures, which follow a different charge carrier recombination mechanism [28] (Figure 1c). Z-schemes utilize two distinct semiconductors along with a reversible donor/acceptor pair, commonly referred to as a shuttle redox mediator [28]. A Z-scheme heterojunction system is created between two semiconductors due to the small distance between the CB of one semiconductor and the VB of the other. In this arrangement, the electron in the CB of one semiconductor quickly recombines with the holes in the VB of the other semiconductor. As a result, the excited electrons in the CB of one semiconductor drive strong reduction reactions, while the holes in the VB of the other semiconductor facilitate efficient oxidation reactions [29].

Anatase titanium dioxide (TiO₂) nanosheets are intentionally designed as the n-type semiconductor and combined with nickel oxide (NiO) nanoparticles, which act as the p-type semiconductor, leading to the creation of a p–n heterojunction structure. The shortened charge transfer distance and the spontaneous formation of this p–n heterojunction enhance the photocatalyst’s ability to efficiently separate photogenerated electrons and holes in space [30].

This system is inspired by the process of natural photosynthesis in green plants, where photosystems I and II absorb photons with wavelengths of 700 nm and 680 nm, respectively. In contrast to heterojunctions, the semiconductors in this system are typically not in direct contact but are electronically coupled through the redox mediator. This arrangement allows for the combination of semiconductors that are capable of only one of the two reactions—either water reduction or oxidation—due to their band potentials. For instance, WO₃, which cannot reduce H⁺, can produce O₂ from an aqueous solution containing suitable electron acceptors under visible light. This makes WO₃ an effective component for O₂ evolution in Z-scheme water splitting [31]. The redox mediator also plays a crucial role in the Z-scheme water-splitting system by facilitating the transfer of electrons from the O₂ evolution photocatalyst to the H₂ evolution catalyst. As a result, the direction of charge carrier flow in Z-schemes is the opposite of that in the previously described heterojunctions involving two semiconductors [32]. However, conventional Z-scheme photocatalysts have several limitations. Notable issues include light absorption interference caused by the redox mediator, slow charge carrier transfer rates constrained by ion pair diffusion, and, as previously mentioned, pH sensitivity. Additionally, redox mediators are often prone to instability and deactivation, leading to a reduction in reaction rates. Furthermore, the photogenerated electrons in the CB of PS I and the holes in the VB of PS II, which possess strong redox abilities, can also be consumed by the redox mediator. This is referred to as a backward reaction, which negatively affects the photocatalytic process.

The close interaction between the mediator and both semiconductors promotes efficient interfacial charge carrier transfer. Commonly used solid-state electron mediators include noble metal shells/nanoparticles (such as Au, Ag, Cu, Pt) as well as graphene and carbon nanotubes. When the two semiconductors are irradiated with light, the photogenerated electrons in the CB of PS II can recombine with the photogenerated holes in the VB of PS I, facilitated by the electron mediator. This process occurs due to the low contact resistance between the mediator and both semiconductors. Upon light irradiation, the photogenerated electrons in the CB of PS II can recombine with the photogenerated holes in the VB of PS I, a process assisted by the electron mediator. This occurs due to the low contact resistance between the mediator and both semiconductors, facilitating efficient charge transfer. In contrast to the traditional Z-scheme system, backward reactions at direct Z-scheme are significantly suppressed due to the absence of redox mediators. Additionally, the shielding effect caused by redox mediators or charge carrier mediators is minimized. Similar to the all-solid-state Z-scheme, both semiconductors in direct contact are excited by light irradiation. The photogenerated electrons in the CB of PS II can recombine with the holes in the VB of PS I. As a result, the photogenerated electrons in the CB of PS I and the holes in the VB of PS II remain spatially separated, preserving their strong redox potential [33]. A key requirement for the Z-scheme charge transfer mode is a work function difference between the two semiconductors. PS I must have higher CB and VB positions, as well as a smaller work function (higher Fermi level) than PS II. When the two semiconductors come into contact, electron transfer from PS I to PS II occurs due to Fermi level equilibration (similar to the n–n type direct Z-scheme shown in Figure 2).

Thus, the PS I side becomes positively charged, while the PS II side is negatively charged. This results in the formation of an internal electric field (IEF) and band bending. The energy band edges of PS II bend downward due to the accumulation of electrons, while the energy band edges of PS I bend upward due to a decreased electron density [35]. The IEF, along with the additional potential barrier created by band bending and Coulomb repulsion, impedes the transfer of photogenerated electrons from the CB of PS I to the CB of PS II. This also applies to the photogenerated holes in the VB of PS II.

A similar process takes place in the formation of a p–n type direct Z-scheme. However, when the Fermi level of a p-type semiconductor (p-PS) is lower than that of an n-type semiconductor (n-PS), a type II heterojunction is formed. At their interface, the energy band edges bend in an inverted manner (Figure 3). As a result, under light irradiation, photogenerated electrons move from the p-PS to the n-PS, while holes move in the opposite direction, driven by the internal electric field (IEF). While the Fermi level alignment is relatively straightforward for n–n type heterojunctions (Figure 2), it is more challenging to achieve in p–n heterojunctions due to the significant band offset (Figure 3). Consequently, most direct Z-scheme systems are constructed with n–n heterojunctions, and only a few p–n type direct Z-scheme systems, such as CuInS₂/WO₃, CuAl₂O₄/Bi₂WO₆, and Cu₂O/TiO₂ can be found in the literature [34].

As already mentioned, the band alignment in Z-scheme heterojunctions is similar to that of type II heterojunctions, but the mechanisms of charge transfer differ. Direct Z-schemes can also be constructed using a combination of three semiconductors. These photocatalysts are commonly referred to as “dual Z-schemes“ or “ternary Z-schemes“. To better understand the specific charge transfer process, multiple techniques can be employed, including self-confirmation through photocatalytic reaction products and radical species, selective photodeposition of noble metals, in situ irradiated X-ray photoelectron spectroscopy (XPS) analysis, surface photovoltage (SPV) measurements, and time-resolved diffuse reflectance (TDR) spectroscopy [33]. In some cases, the reduced photocorrosion effects can also serve as an indication of a Z-scheme mechanism, as observed in composite materials containing CdS [36]. Despite some limitations, Z-type heterojunctions are included in the list of suitable candidates for industrial applications [22,28,37]. Representative examples of materials belonging to the Z-scheme type photocatalysts active for hydrogen generation are ZnS/ZnO nanosheets, CoTiO₃/Cu/TiO₂, CoTiO₃/Zn_0.5Cd_0.5S, g-C₃N₄/MoO_3-x [23].

Finally, the term S-scheme (or step scheme) was also introduced, which is often used interchangeably with direct Z-schemes (Figure 1e). This type of composite catalyst has been intensively studied for the photocatalytic production of hydrogen and H₂O₂, the photocatalytic reduction of CO₂, bacteria disinfection, and the photocatalytic degradation of organic pollutants [37]. An S-scheme heterojunction consists of a reduction semiconductor and an oxidation semiconductor. For the design of S-scheme heterojunctions, p-type and n-type semiconductors can be used, provided that the CB position and Fermi level of the reduction semiconductor are higher than those of the oxidation semiconductor. This heterostructure provides strong photogenerated charge carriers of holes and electrons in the VB of the oxidation photocatalyst and CB of the reduction photocatalyst, sacrificing those that are useless in recombination. The aforementioned strong charge carriers can thus lead to a strong redox capability, together with efficient charge carrier separation and an extended lifetime of the electron–hole pair. Some of the representative examples of materials belonging to the S-scheme are SnS₂/RGO/g-C₃N₄, TiO₂-X/BiOI, Co₉S₈/In₂O₃, ZnIn₂S₄-NiSe₂/Ti₃C₂ [23]. Despite great progress in the development of S-scheme heterojunctions, there are still major challenges, such as the discovery of new semiconductors with targeted band structures as oxidation semiconductors and reduction semiconductors through machine learning, new strategies for the fabrication of specific materials with unique physicochemical properties, combination of S-scheme heterojunctions with other modification strategies (e.g., heteroatom doping, co-catalyst loading, defect and morphology engineering, etc.).

3.3. Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs) are one (0)-, two (2D)-, or three-dimensional (3D) coordination networks composed of metal ions or clusters connected by organic linker molecules. MOFs have garnered increasing interest in the development of direct Z-scheme photocatalysts. The advantages of MOFs, such as tunable composition and light-harvesting properties, high surface area, and controllable pore size, make them highly attractive for photocatalytic applications [38]. While the visible light photocatalytic performance of pure MOFs is generally limited, significant improvements have been observed when they are used in direct Z-scheme systems. MOFs are a novel class of porous solid materials, renowned for their tunable structure and functionality, as well as their high specific surface area, which facilitates rapid transfer of photogenerated charges to the MOF surface for redox reactions, thus reducing the recombination of photogenerated electrons and holes during photocatalysis. TiATA, a well-known MOF, demonstrates excellent chemical stability and photocatalytic performance under visible light. The coupling of MOFs and carbon nitride is a feasible strategy in the field of H₂ evolution [39].

3.4. Covalent Organic Frameworks (COFs)

Covalent organic frameworks (COFs) are crystalline covalent polymers with a large surface area. These porous materials, which offer tunable topology and functionalities, have attracted significant interest in the field of photocatalysis [40]. The moderate bandgap and the presence of extended π-conjugated electronic networks allow for visible-light responsiveness and provide opportunities for bandgap engineering. As a result, COFs and COF-linked hybrid materials are employed as photocatalysts for H₂ production. Due to the stability of the covalent bonds compared to the coordinate bonds found in MOFs, COFs are known to exhibit high stability in various solvents, even under harsh acidic, basic, oxidative, or reductive conditions. However, most reported photocatalytic COFs are based on imine, hydrazone, or azine linkages, which are considered unstable for prolonged photocatalysis in aqueous environments. COFs are especially excellent platforms for photocatalysis due to their distinctive pore-like architecture and the robust covalent interactions within the frameworks. Nevertheless, obstacles remain in enhancing photocatalytic efficiency. It has been shown that merging the benefits of inorganic semiconductors (different microstructures and manageable bandgap) and organic frameworks to create organic–inorganic hybrid composite photocatalysts (heterojunctions) may enhanced the photocatalytic performance of materials [41].

Another approach to enhance photocatalytic activity is the integration of surface plasmon resonance (SPR) into direct Z-schemes. SPR refers to the collective oscillation of electrons that resonate with the oscillating electric field of incident light. Resonances in the visible or near-infrared range can significantly improve the light absorption capacity of the photocatalytic system. In addition, SPR-induced hot plasmon electrons or SPR-enhanced localized electromagnetic fields can facilitate charge separation at the interfaces between the SPR material and the semiconductor [42].

3.5. Hybrid Materials

Photocatalytic H₂ production can effectively tackle energy shortages and environmental challenges. The successful integration of different semiconductor materials can inhibit exciton recombination, rendering it a very efficient approach to boost photocatalytic activity. Although composites share some similarities with hybrid materials, it is important to emphasize that composites cover a broader range of dimensions of mixed components, while hybrid materials usually refer to the constituents at the nanometer or molecular level [43]. Various hybrid materials are used in the context of photocatalytic H₂ production, such as the combination of polymer materials with metal oxide photocatalysts.

3.5.1. Two-Component Composites Based on Graphitic Carbon Nitride (g-C₃N₄) and Organic–Inorganic Hybrid Composites

In

Table 1. Recently developed organic–inorganic hybrid photocatalytic composites for photocatalytic H₂ production.

Photocatalytic Composites, Organic–Inorganic Hybride	Advantage	H2 Production	Ref.
NaYF4:Yb,Er/CdS-380	λ > 400 nm, λ > 600 nm, 300 W Xe lamp Critical role of structural and interfacial engineering in enhancing photocatalytic performance Na2S/Na2SO3 as sacrificial agents NaYF4:Yb,Er enhances photocatalytic activity by converting (NIR) light into (VIS) light, which can then excite the CdS photocatalyst. The role of upconversion processes in promoting photocatalytic performance.	2.539 mmol h−1 g−1 0.019 mmol h−1 g−1	[1]
Polymeric carbon nitride (PCN)/molybdenite nanocomposites (PCN/Mo-x)	Molybdenite replaced the Pt co-catalyst, improve the separation efficiency of photoinduced e− and h+, enhance light absorption, 0.75 M triethanolamine (TEOA) aqueous solution, with TEOA acting as a sacrificial agent, λ > 420 nm, 300 W Xe lamp.	bulk-PCN/Pt-0.067 mmol g−1 h−1 and PCN/Pt-0.08 mmol g−1 h−1 PCN/Mo-1, 2, and 3 0.016, 0.027, and 0.019 mmol g−1 h−1	[2]
Phosphorus-doped graphitic carbon nitride (PCN) 1-Hydroxyethane-1,1-diphosphonic acid (HEDP) as the phosphorus source and carbon nitride (CN)	1000 W xenon lamp, 10% methanol solution	3.0CN–P25 composite–0.92 mmol/g CN alone (0.041 mmol/g) P25 alone (0.099 mmol/g) 2.0PCN–P25 composite–2.531 mmol/g	[3]
CoO quantum dots within hollow-sphere g-C3N4 (SCNO)	λ > 420 nm, 300 W Xe lamp, water and triethanolamine (TEOA)-as the sacrificial agent, in a volume ratio of 9:1	SCNO/CoO (0.2)–11.495 mmol H2 h−1 g−1	[7]
CdS@NC composites; CdS nanoparticles, were simultaneously deposited on nitrogen-doped carbon (NC) CdS@NC-500-4	λ ≥ 420 nm, Na2S/Na2SO3 as sacrificial agents, Assembly of Cd2+ and MO anions into a crystalline phase is essential for optimizing photocatalytic performance.	7.428 mmol h−1 g−1	[8]
Bent g-C3N4 into nanotubes and subsequently merged it with black phosphorus quantum dots (BPQDs)	λ = 420 nm	0.50751 mmol h−1 g−1	[44]
C3N5 conjugated polymer-ceric oxide (CeO2) (10 wt% CeO2)		1.256 mmol g−1 h−1	[45]
Z-scheme two-dimensional (2D)/one-dimensional (1D) g-C3N4/MoO3-x composites MoO3-x nanorods on the g-C3N4 nanosheets	365 nm	0.2092 mmol h−1	[46]

are listed different organic–inorganic hybrid photocatalytic composites for H₂ production.

The PCN/molybdenite nanocomposites (PCN/Mo-x) were constructed from low-cost molybdenite mineral as an effective co-catalyst, and polymeric carbon nitride (PCN) through Na₂SO₄-assisted ball milling [2].

Wadhai et al. [3] investigated the incorporation of phosphorus doping into graphitic carbon nitride (CN) and its subsequent coupling with commercial TiO₂ (P25) to improve photocatalytic H₂ evolution. The phosphorus-doped graphitic carbon nitride (PCN) was synthesized by a simple calcination method using 1-hydroxyethane-1,1-diphosphonic acid (HEDP) as the phosphorus source. It was found that phosphorus doping (anion doping) in CN subsequently coupled with P25 improved its electrical conductivity, optoelectronic and textural properties, leading to a significant improvement in photocatalytic performance. A comparative analysis between CN, PCN and their respective composites with P25 was performed. The optimized PCN-P25 composite showed a significant increase in H₂ production compared to P25, CN, PCN and other CN/PCN-based composites [3].

Ying et al. 2021 prepared CoO quantum dots (QDs) within hollow-sphere g-C₃N₄ (SCNO) [7]. This approach successfully reduced the particle size of CoO QDs and ensured their high dispersion within the SCNO matrix. These effects facilitated the rapid separation and transfer of photogenerated charge carriers, thereby enhancing the photocatalytic H₂ evolution performance. The incorporation of volatile gases during the process effectively inhibited the aggregation of CoO QDs, providing optimal conditions for their uniform distribution [7].

In the case of bare g-C₃N₄ (CN), significant charge recombination and intense fluorescence occur, which have a negative effect on the photocatalytic processes. In their study, Lu et al. (2024) [44] bent g-C₃N₄ into nanotubes and then fused them with black phosphorus quantum dots (BPQDs). The BPQDs functionalized tubular CN (BPTCN) demonstrates a consistent and elevated photocatalytic H₂ production rate of 507.51 μmol h⁻¹ g⁻¹ under visible light, which is four times greater than that of pure TCN. The apparent quantum yield of BPTCN at 420 nm attains 2.58%. The remarkable photocatalytic performance of the BPTCN is due to the reduced recombination of electron–hole pairs and the enhanced carrier transport efficiency, both of which are facilitated by the 1D tubular design and the band alignment of the BPTCN heterojunction.

Augustin et al. (2024) [45] prepared an organic–inorganic hybrid composite from carbon nitride (C₃N₅) conjugated polymer enclosed with a ceric oxide (CeO₂) metal oxide photocatalyst. The encapsulation of the polymer with CeO₂ nanoparticles led to outstanding performance in H₂ production, demonstrating enhanced visible light absorption and a remarkable increase in charge transfer efficiency. This is due to the increased charge transfer and diminished recombination in the composite. Furthermore, the generation of holes by light resulted in a significant reduction in the exciton recombination rate and a simultaneous increase in the photocatalytic H₂ production rate. Remarkably, the measured H₂ evolution for the C₃N₅ composites doped with 10 wt% CeO₂ is 1256 µmol g⁻¹ h⁻¹, while for C₃N₅ alone, it is 125 µmol g⁻¹ h⁻¹. Interestingly, the proposed system represents an innovative approach to H₂ evolution by photocatalysis using CeO₂/C₃N₅ composites [40]. The extended light absorption and inhibited charge recombination are essential for photocatalytic materials based on g-C₃N₄.

Huang et al. (2022) [46] synthesized nonstoichiometric molybdenum oxide (MoO_3-x) nanorods with oxygen vacancies using a small quantity of oleylamine, and they produced the Z-scheme two-dimensional (2D)/one-dimensional (1D) g-C₃N₄/MoO_3-x composites via a simple electrostatic assembly technique. The blue MoO_3-x nanorods with oxygen vacancies are evenly distributed on the g-C₃N₄ nanosheets. The g-C₃N₄/MoO_3-x composite materials show significant absorption in the visible and near-infrared light ranges, along with enhanced charge separation efficiency via the Z-scheme charge transfer mechanism. The g-C₃N₄/MoO_3-x composite exhibits a notably enhanced photocatalytic H₂ production efficiency and excellent cycling stability when compared to sonicated g-C₃N₄ nanosheets. The g-C₃N₄/MoO_3-x composite with a mass percent of 27.5% achieves the highest H₂ generation activity of 209.2 µmol h⁻¹ under solar light and an apparent quantum efficiency of 4.4% when irradiated at 365 nm, which is 2.63 times greater than g-C₃N₄. The weight ratios along with the presence of oxygen vacancies in the smaller MoO_3-x nanorods exhibit a considerable impact on photocatalytic H₂ efficiency. Additionally, successful overall water splitting via photocatalysis can be accomplished with the H₂ and O₂ generation rates of 0.755 and 0.368 µmol h⁻¹ using the g-C₃N₄/MoO_3-x composite. The innovative g-C₃N₄/MoO_3-x composite is expected to have significant potential for photocatalytic applications.

A novel biochar/ZnFe₂O₄ (BZF) composite has been developed to enhance the efficiency of visible-light-driven H₂ evolution through a straightforward microwave hydrothermal synthesis method [47]. The results showed that different biochar/ZnFe₂O₄ (BZF) composite ratios exhibited superior photocatalytic H₂ evolution performance compared to pure ZnFe₂O₄. Notably, the BZF composite with a 5:1 mass ratio (BZF-5) achieved the highest H₂ evolution rate, approximately six times greater than that of pure ZnFe₂O₄. The biochar component serves as an effective electron mediator, promoting the separation of electron–hole pairs and thereby enhancing the photocatalytic H₂ evolution rate. Additionally, the synergistic effects of Eosin Y, the photocatalyst, and triethanolamine (TEOA) contributed to the improved photocatalytic performance. Three consecutive cycles of photocatalytic H₂ evolution using the BZF-5 sample confirmed its excellent stability and reusable performance.

Major Achievements in the Development of Organic–Inorganic Hybrid Photocatalytic Composites for Photocatalytic H₂ Production

The previous studies investigated various strategies for enhancing photocatalytic H₂ evolution through material modifications and composites. Wadhai et al. [3] demonstrated that phosphorus doping in graphitic carbon nitride (PCN) significantly improved its electrical conductivity and photocatalytic performance when coupled with TiO₂ (P25), leading to enhanced H₂ production. Ying et al. (2021) [7] successfully incorporated CoO quantum dots (QDs) within hollow-sphere g-C₃N₄, which improved the dispersion and charge carrier separation, thus enhancing photocatalytic efficiency. Similarly, Lu et al. (2024) [44] achieved four times greater H₂ production by fusing black phosphorus quantum dots with tubular g-C₃N₄, facilitating improved charge transport and reduced electron–hole recombination. Augustin et al. (2024) [45] synthesized an organic–inorganic hybrid composite with CeO₂ and C₃N₅, showing outstanding performance with a remarkable increase in H₂ evolution due to enhanced charge transfer and light absorption. Huang et al. (2022) [46] fabricated a g-C₃N₄/MoO₃-x composite with oxygen vacancies, significantly enhancing photocatalytic efficiency and stability due to better charge separation and visible light absorption. The integration of MoO₃-x into g-C₃N₄ also led to increased overall water splitting, demonstrating the composite’s potential in photocatalytic applications. Additionally, a novel biochar/ZnFe₂O₄ composite was developed with microwave hydrothermal synthesis, achieving six times higher H₂ production than pure ZnFe₂O₄ by promoting electron–hole separation. The biochar acted as an electron mediator, enhancing charge separation, and improving photocatalytic H₂ evolution. These studies underscore the importance of material design, including doping, composite formation, and structural optimization, in achieving efficient photocatalytic H₂ production under visible light. The combined effects of material modifications, such as increased charge separation, improved stability, and extended light absorption, are crucial for advancing photocatalytic technologies for renewable energy generation.

3.5.2. Two-Component and Three-Component Z-Type Heterojunctions

In

Table 2. Recently developed Z-type heterojunction photocatalytic composites for photocatalytic H₂ production.

Photocatalytic Composites Z-Type Heterojunction	Advantage	H2 Production	Ref.
g-C3N4@TiATA composites Ti-MOFs (TiATA) were coupled with metal-free graphitic carbon nitride (g-C3N4)	300 W Xenon lamp, extends the light absorption range of g-C3N4@TiATA to 490 nm, significantly accelerates charge migration	0.2658 mmol h−1	[39]
Organic–inorganic hybrid Covalent organic frameworks (COFs) BDA-THTA-30		9.69184 mmol g−1 h−1	[41]
Z-type heterojunction Pt/Zn0.2Cd0.8S/Cs3PW12O40	Reducing the photocorrosion of sulfides, λ = 420 nm	10.4 mmol g−1 h−1	[48]
CdS/ZnIn2S4, ZnIn2S4 0.3wt%	LED light	5.80 mmol g−1	[49]
Z-scheme 2D-0D MnPS3-Cs4W11O35 composite; Cs4W11O35 (10.9 wt%)		0.0996 mmol g−1 h−1	[50]
Heterojunction B/CoMoO4	Incorporation of nonmetallic atoms like B enhances the specific surface area, increases the bandgap	7.460 mmol g−1 h−1	[51]
CuS/NaNbO3 p–n heterojunction CuS nanoparticles and NaNbO3 nanorods		1.603 mmol g−1 h−1	[52]
2.5% Co1.29Ni1.71O4/TiO2 hybrid heterojunction	365 nm LED	1.685 mmol g−1 h−1	[53]
C@ZnxCd1-xS/Co3O4 (30%)	10 vol% lactic acid aqueous solution, 5 W LED lamp, p–n heterostructure between Co3O4 and ZCS accelerates the separation of carriers, carbon particles accelerated the e− transfer and inhibited the recombination of e− and h+.	(C@ZCS/CO-30)-1.4051 mmol	[54]
5% Co3O4/CdS/g-C3N4 CdS/g-C3N4		30.880 mmol h−1 g−1 4.177 mmol h−1 g−1	[55]
NiS/WO3/g-C3N4 CNWN-4	5 W LED, λ ≥ 420 nm, aqueous solution of 15% TEOA-as a sacrificial hole scavenger, eosin Y (EY) and co-catalyst g-C3N4 Incorporation of WO3 and NiS significantly enhanced photocatalytic activity by increasing the number of active sites and improving electron mobility.	2.9291 mmol g−1 h−1.	[56]
MMT/g-C3N4/15%NiCoP composites	420 nm	12.50 mmol h−1 g−1 at pH 11 under 1.0 mmol L−1 of eosin Y (EY) sensitization	[57]
3D hollow-sphere structure of graphitic carbon nitride (g-C3N4)	300 W Xe lamp, water and triethanolamine (TEOA) at a volume ratio of 9:1, Pd nanoparticles enhancing visible light absorption, acting as e− acceptors, are incorporated into g-C3N4, which increases the material’s ability to trap transition e−, boosting the number of photogenerated carriers.	Pd/SCN (10 h)–0.2679 mmol/h	[58]
CdNCN-CdS heterostructure	Visible light	14.7 mmol g−1 h−1	[59]
Nanostructure composite NdOCl/Fe2O3/NdFeO3	Methanol as the sacrificial agent	0.01279 mmol g−1 h−1	[60]
Nanostructure composite NdOCl/Fe2O3/NdFeO3	Triethanolamine as the sacrificial agent	0.01899 mmol g−1 h−1	[60]

are listed recently developed Z-Type heterojunction photocatalytic composites for photocatalytic H₂ production. Widely used catalysts, such as sulfides, encounter significant issues with photogenerated carrier recombination and with photocorrosion. To address this problem, Xie et al. (2025) [48] recently constructed a Z-type heterojunction Pt/Zn_0.2Cd_0.8S/Cs₃PW₁₂O₄₀ and presented a new approach to reduce photocorrosion of sulfides and utilize polyoxometalates in photocatalytic H₂ generation. Polyoxometalates, such as Cs₃PW₁₂O₄₀, which exhibit a reversible photochromic effect, and Zn_0.2Cd_0.8S, which is photocorrosive, enhance the separation of photogenerated charge carriers while synergistically interacting with the distinct photochromic properties of Cs₃PW₁₂O₄₀ to reduce photocorrosive effects. Furthermore, by incorporating the co-catalyst Pt to create a Schottky barrier that enhances the separation of photogenerated charge carriers, the Pt/Zn_0.2Cd_0.8S/Cs₃PW₁₂O₄₀ Z-type heterojunction ternary composites were effectively designed and used for photocatalytic H₂ generation for the first time. The optimal heterojunction composite exhibited a photocatalytic H₂ precipitation rate of 10.4 mmol g⁻¹ h⁻¹, and its apparent quantum efficiency at a wavelength of 420 nm was able to achieve 8.15%.

However, the development of semiconductor-based heterojunctions at the atomic scale remains a considerable challenge. In their study, Li et al. (2021) [49] introduced innovative atomic CdS/ZnIn₂S₄ heterojunctions created by the in situ epitaxial growth of 2D ZnIn₂S₄ nanosheets on the surface of 1D defective CdS nanorods.

As a potential new photocatalyst for H₂ production, MnPS₃ faced reduced photocatalytic activity due to its increased electron–hole recombination rate and inadequate oxidation capacity of holes. Chen et al. (2021) [50] developed a novel Z-scheme 2D-0D MnPS₃-Cs₄W₁₁O₃₅ composite for photocatalytic H₂ evolution. The MnPS₃-Cs₄W₁₁O₃₅ composites achieve the highest H₂ evolution rate of 99.6 µmol g⁻¹ h⁻¹ when the Cs₄W₁₁O₃₅ content is 10.9% by mass, which is 2.9 times greater than that of MnPS₃ nanosheets alone. Moreover, the photocatalytic performance of MnPS₃-Cs₄W₁₁O₃₅ remains almost stable after four cycles. MnPS₃ is a non-toxic p-type direct bandgap semiconductor known for its strong visible light absorption capabilities. This unique structure makes MnPS₃ a promising candidate for photocatalysts, as it possesses a suitable bandgap and favorable band positions for photocatalytic water splitting. Despite these advantages, the photocatalytic H₂ evolution rate of MnPS₃ nanosheets remains relatively low (21.2 mmol h⁻¹ g⁻¹) due to a high photoelectron–hole recombination rate and insufficient oxygen production. To enhance photocatalytic performance, it is important to exfoliate high-quality MnPS₃ nanosheets to create effective heterojunctions. Cesium tungstate (Cs₄W₁₁O₃₅) is an n-type semiconductor with a bandgap of 3.0 eV [50]. Its valence band potential is more positive than the oxidation potential of oxygen, giving it a strong ability to oxidize holes. Additionally, due to the presence of pentavalent tungsten and oxygen vacancies, cesium tungstate displays significant localized surface plasmon resonance (LSPR) absorption in the visible and near-infrared light regions, which is advantageous for improving photocatalytic reaction activity. Energy level analysis and photocatalytic mechanisms indicate that the MnPS₃-Cs₄W₁₁O₃₅ composite could significantly enhance H₂ production activity. However, MnPS₃ tends to oxidize during the second step of the hydrothermal process, making it difficult to load semiconductors with highly positive valence bands onto its surface using conventional chemical synthesis methods. Therefore, a gentle electrostatic assembly technique is highly desirable to uniformly immobilize Cs₄W₁₁O₃₅ nanoparticles on the MnPS₃ nanosheets, to improve charge separation efficiency and photocatalytic performance.

Li et al. (2025) [51] provided a theoretical backing for improving the photocatalytic H₂ production capability of catalysts through the incorporation of nonmetallic atoms like B or the formation of heterojunctions, presenting a viable concept for the industrial use of photocatalytic H₂ production. Since the B atom is rather electronegative, it can provide abundant electrons for H₂ generation. In this study, a composite catalyst B/CoMoO4 was prepared by the incorporation of B powder during the in situ hydrothermal synthesis of CoMoO4. Doping with nonmetallic B increases the specific surface area of the catalyst and increases the bandgap of the catalyst. Abundant reactive sites and a comparatively low electron shift resistance improve photocatalytic H₂ production. Due to the electron exchange between nonmetal B and CoMoO₄ at the contact interface, the rate of electron shift is enhanced, resulting in a catalyst surface with a robust oxidation-reduction capability. The results of the photocatalytic H₂ evolution test indicate that the H₂ production rate of B/CoMoO₄ attains 7460 μmol g⁻¹ h⁻¹, which is 4.2 times greater than that of the CoMoO₄ catalyst, and exhibits strong stability.

As mentioned, the p–n heterojunction is an important type of heterojunction characterized by an internal electric field due to the different properties of semiconductors. In their study, Chang et al. (2023) [52] described the creation of a new CuS/NaNbO₃ p–n heterojunction through the application of CuS nanoparticles onto the outer surface of NaNbO₃ nanorods to enhance photocatalytic H₂ generation. The best H₂ production activity was achieved at 1603 μmol g⁻¹ h⁻¹, significantly surpassing that of NaNbO₃ (3.6 times greater) and CuS (2.7 times greater). Therefore, the presence of p–n heterojunction interactions between the two materials prevented the recombination of photogenerated charge carriers and enhanced electron transfer efficiency.

Fang et al. (2023) [53] showed that Co_1.29Ni_1.71O₄ serves as a promising heterojunction counterpart material and can enhance photocatalysis via distinct cross-boundary charge transfer. In this study, a new n-type TiO₂ sourced from glycerolate, featuring oxygen vacancies, was hybridized with Co_1.29Ni_1.71O₄ through a straightforward ultrasonication technique to achieve this goal. The optimized 2.5% Co_1.29Ni_1.71O₄/TiO₂ hybrid exhibited a photocatalytic H₂ production rate of 1685 μmol g⁻¹ h⁻¹ when exposed to 365 nm LED light, which is 26.7 times greater than that of pure TiO₂ (63 μmol g⁻¹ h⁻¹). Moreover, the quantity of H₂ produced exhibited a linear rise with the prolongation of irradiation time for the 2.5% Co_1.29Ni_1.71O₄/TiO₂ catalyst throughout a 24-h continuous H₂ release experiment, validating its remarkable catalytic stability. Thorough characterizations showed that the Co_1.29Ni_1.71O₄ nanosheet was in close contact with TiO₂ nanoparticles, enhancing the specific surface area and broadening the light absorption spectrum of TiO₂.

A high-performance catalyst for H₂ production, C@Zn_xCd_1-xS/Co₃O₄, was successfully prepared by Li et al. [54] following a two-step modification process. The light absorption intensity of both C@Zn_xCd_1-xS and C@Zn_xCd_1-xS/Co₃O₄ exhibited an increasing trend compared to that of pure Zn_xCd_1-xS. This improvement was beneficial for visible-light absorption and the utilization of the photocatalyst. Mott–Schottky analysis confirmed that the ZnxCd1-xS catalyst formed a p–n heterojunction with Co₃O₄ nanoparticles, further confirming the successful modification of C@Zn_xCd_1-xS with Co₃O₄. The H₂ production rate of the optimized C@Zn_xCd_1-xS/Co₃O₄ (30%) catalyst reached 1405.1 µmol, which was 6.9 times higher than that of pure Zn_xCd_1-xS.

In their study, Wu et al. [55] synthesized a series of CdS-based ternary composites (Co₃O₄/CdS/g-C₃N₄) by forming a heterojunction through a simple self-assembly approach followed by a solvothermal in situ process to enable efficient photocatalytic H₂ production. The experimental findings revealed that the 5% Co₃O₄/CdS/g-C₃N₄ nanocomposite exhibited a superior photocatalytic H₂ evolution rate of 30 880 μmol h⁻¹ g⁻¹ under visible light illumination—approximately 17.1 times higher than that observed for pure CdS alone. This enhanced photocatalytic performance can be attributed to synergistic interactions among the constituent materials, which promote efficient separation of photoinduced charge carriers. This study highlights the promising potential of CdS-based ternary heterostructures for the further development of photocatalytic H₂ generation.

Pure g-C₃N₄ and CdS exhibit relatively low H₂ evolution rates, with values of 823 and 1806 μmol h⁻¹ g⁻¹, respectively. The apparent quantum efficiency (AQE) of 5% CdS is calculated to be 3.4%. However, when g-C₃N₄ is encapsulated within CdS nanorods to form CdS/g-C₃N₄, the H₂ production rate increases to 4177 μmol h⁻¹ g⁻¹. Notably, the ternary Co₃O₄/CdS/g-C₃N₄ composites showed a significant enhancement in photocatalytic performance, with an optimal loading amount of 5%. The 5% Co₃O₄/CdS/g-C₃N₄ composite achieved the highest photocatalytic H₂ evolution rate of 30,880 μmol h⁻¹ g⁻¹, accompanied by an AQE of 30.7%. These represent approximately 17-fold and 7-fold improvements compared to pure CdS and CdS/g-C₃N₄, respectively. A comparison between 5% Co₃O₄/CdS and 5%Co₃O₄/CdS/g-C₃N₄ also indicated that the superior photocatalytic performance of the latter was due to the synergistic effects of the ternary heterojunction. Recycling experiments and consistent XRD patterns of 5% Co₃O₄/CdS/g-C₃N₄ before and after testing confirmed the composite’s photochemical stability.

Zhang et al. 2020 [56] presents a novel and efficient strategy to significantly enhance the H₂ production performance of g-C₃N₄ by synthesizing a NiS/WO₃/g-C₃N₄ composite. Remarkably, an approximately 8.9-fold improvement in H₂ production was achieved with this approach compared to pristine g-C₃N₄. The enhanced performance is attributed to changes in the electronic structure and charge carrier behavior of g-C₃N₄ facilitated by the in situ growth of a NiS-WO₃ heterojunction system. The NiS-assisted WO₃/g-C₃N₄ heterojunction introduces additional active sites that promote efficient charge separation and transfer processes. When NiS is employed as a co-catalyst in the system, the H₂ evolution reaction rate significantly increases, reaching 2929.1 μmol g⁻¹ h⁻¹ under visible light irradiation.

Xu et al. (2019) [57] enhanced H₂ production by water splitting using NiCoP as a co-catalyst to decorate g-C₃N₄, which was also hybridized with montmorillonite (MMT). The resulting MMT/g-C₃N₄/NiCoP composites achieved a significantly higher H₂ evolution rate of 12.50 mmol h⁻¹ g⁻¹ at pH 11 with 1.0 mmol L⁻¹ eosin Y (EY) sensitization, outperforming MMT/g-C₃N₄ and g-C₃N₄/NiCoP composites. The photocatalytic performance of g-C₃N₄ was improved through hybridization with MMT and NiCoP as the co-catalyst. The highest H₂ evolution rate observed was 12.50 mmol h⁻¹ g⁻¹ for MMT/g-C₃N₄/15%NiCoP under visible light, which was 26 times higher than MMT/g-C₃N₄ (0.48 mmol h⁻¹ g⁻¹) and 1.6 times higher than g-C₃N₄/15%NiCoP (7.69 mmol h⁻¹ g⁻¹). The AQY at 420 nm reached 40.3%. This significant improvement was attributed to the enhanced dispersion of g-C₃N₄ layers, the staggered conduction band potentials between g-C₃N₄ and NiCoP, and the electrostatic repulsion from negatively charged MMT. This study demonstrates that MMT is an excellent support for catalytically active components in photocatalytic H₂ production. Dye sensitization enhances a photocatalyst’s absorption in the visible-light region. However, for effective H₂ generation, the dye must remain stable, and efficient co-catalysts are needed to facilitate electron transfer from excited dye molecules to active sites. One way to stabilize the dye is by immobilizing it on a porous support. Noble metals like platinum (Pt) are common co-catalysts that improve electron transfer, while transition metal phosphides are also promising due to their abundance and ability to capture photoinduced charges in non-dye-sensitized photocatalysts.

Gao et al. (2019) [39] synthetized g-C₃N₄@TiATA composites of Ti-MOFs (TiATA) and metal-free graphitic carbon nitride (g-C₃N₄), forming a heterostructure between g-C₃N₄ and TiATA. The development of heterostructured photocatalysts is an effective approach to improve H₂ production from water splitting due to enhanced charge transport efficiency. This heterojunction not only extends the light absorption range of g-C₃N₄@TiATA to 490 nm, compared to 456 nm for pure g-C₃N₄, but also significantly accelerates charge migration. Photocatalytic experiments reveal that the heterostructure effectively steers charge flow from g-C₃N₄ to TiATA, and subsequently to Pt nanoparticle co-catalysts, achieving a remarkable photocatalytic H₂ production rate of 265.8 µmol h⁻¹ under a 300 W Xenon lamp, which is about 3.4 times higher than that of g-C₃N₄/Pt.

Huang et al. (2019) [58] synthesized a 3D hollow-sphere structure of graphitic carbon nitride (g-C₃N₄) with a large specific surface area and high porosity. This hollow-sphere structure serves as an effective substrate for the uniform dispersion of Pd nanoparticles, improving visible light absorption and exposing more active sites. Pd nanoparticles, acting as electron acceptors, are incorporated into g-C₃N₄, which increases the material’s ability to trap transition electrons, thereby boosting the number of photogenerated charge carriers involved in surface reactions. The Pd/SCN composite showed outstanding photocatalytic H₂ evolution performance under visible light irradiation, with the H₂ production rate of Pd/SCN (10 h) reaching 267.9 µmol/h—nearly 10 times higher than that of Pd/2D g-C₃N₄. The photocatalytic H₂ evolution reaction was carried out in a 250 mL quartz reactor using a 300 W Xe lamp with a 400 nm UV cut-off filter to simulate natural sunlight. For each reaction, 50 mg of catalyst was dispersed in 100 mL of a 9:1 water/triethanolamine (TEOA) mixture. The reactor was evacuated for 30 min to remove air before starting the reaction.

Yi et al. (2024) [41] successfully covalently bonded three COFs (BDA-HTA-COF, BDA-DHTA-COF, and BDA-THTA-COF) with In₂O₃ for the first time to create a heterojunction. The resulting BDA-THTA-30 exhibits the greatest photocatalytic H₂ production activity of 9691.84 μmol g⁻¹ h⁻¹, significantly surpassing BDA-THTA-COF and In₂O₃ by 10.1 and 128.3 times, respectively. Additional research indicates that covalent bonding between In₂O₃ and the elements of BDA-THTA-COF can greatly enhance the separation and transfer rate of light-induced charges in the composites, resulting in improved H₂ evolution activity.

Huang et al. (2024) [59] presented a novel synthesis method for the innovative atomic-level composite semiconductor photocatalyst CdNCN under mild conditions, while also clarifying its formation mechanism. By efficiently utilizing the [NCN]²⁻ group in the thiourea method, they successfully performed the one-pot synthesis of CdNCN-CdS heterostructure photocatalysts. Significantly, the best-performing CdNCN-CdS sample shows an H₂ evolution rate of 14.7 mmol g⁻¹ h⁻¹ when exposed to visible light, making it the most effective catalyst compared to all other reported CdS-based composites lacking any co-catalysts. The remarkable H₂ evolution performance of CdNCN-CdS mainly stems from two crucial factors: (i) the formation of an atomic-scale N-Cd-S heterostructure at the junction of CdNCN and CdS, which enhances efficient electron transfer; (ii) the guided movement of electrons to the (110) crystal plane of CdNCN, which promotes optimal H₂ adsorption and effective involvement in the H₂ evolution reaction.

Yang et al. (2023) [60] successfully fabricated neodymium iron composites made up of NdOCl/Fe₂O₃/NdFeO₃, which have a platelet-like nanostructure. They investigated the effects of changes in phase composition due to varying annealing temperatures (750 to 900 °C). The sample obtained at an annealing temperature of 800 °C showed the greatest photocatalytic H₂ evolution rate of 12.79 μmol g⁻¹ h⁻¹ with methanol as the sacrificial agent and 18.99 μmol g⁻¹ h⁻¹ with triethanolamine (TEOA), respectively. They also proposed a potential mechanism involving the separation of photogenerated charge carriers due to different interfacial heterojunctions resulting from the different phase compositions of each sample.

Major Achievements in the Development of the Z-Type Heterojunction Photocatalytic Composites for Photocatalytic H₂ Production

The synthesis of photocatalytic materials for hydrogen production has seen significant advances in recent years, as researchers seek to improve efficiency by addressing common issues such as electron–hole recombination, photocorrosion, and limited light absorption. Xie et al. (2025) [48] pioneered a novel Z-type heterojunction, Pt/Zn_0.2Cd_0.8S/Cs₃PW₁₂O₄₀, which effectively reduces photocorrosion and enhances charge carrier separation, demonstrating a promising photocatalytic H₂ production rate of 10.4 mmolg⁻¹ h⁻¹. However, creating stable, atomic-scale semiconductor heterojunctions remains a significant challenge. In a different approach, Li et al. (2021) [49] created CdS/ZnIn₂S₄ heterojunctions with epitaxial growth, pushing the limits of photocatalyst design at the atomic level. This approach illustrates the ongoing difficulty of synthesizing highly efficient and stable heterojunctions that can effectively manage electron migration.

Meanwhile, MnPS₃, known for its strong visible light absorption, suffers from high electron–hole recombination and insufficient oxidation capacity, though recent developments like the MnPS₃-Cs₄W₁₁O₃₅ composite have significantly enhanced its H₂ production rate by nearly three times [50]. This illustrates the importance of overcoming inherent material limitations, such as recombination rates and oxidation capabilities, which hinder the performance of promising photocatalysts like MnPS₃. The incorporation of nonmetallic elements, such as boron in CoMoO₄, is another example of progress, with Li et al. (2025) [51] demonstrating that B/CoMoO₄ outperforms its non-doped counterpart, achieving a remarkable H₂ production rate of 7460 μmol g⁻¹ h⁻¹. This highlights how modifying the electronic properties of catalysts can lead to substantial improvements in photocatalytic efficiency.

The development of p–n heterojunctions, as seen in the CuS/NaNbO₃ system (Chang et al., 2023) [52], provides an effective strategy to enhance photocatalytic performance by preventing charge carrier recombination. Similarly, Fang et al. (2023) [53] hybridized TiO₂ with Co_1.29Ni_1.71O₄, achieving a 26.7-fold enhancement in H₂ production. These materials benefit from enhanced charge separation and light absorption, showcasing the critical role of heterojunction design in boosting photocatalytic efficiency. Wu et al. (2021) [55] achieved a similar outcome with a Co₃O₄/CdS/g-C₃N₄ ternary composite, which showed a dramatic increase in H₂ production, further confirming the power of synergistic material combinations for improving photocatalytic performance.

However, these advances are not without their drawbacks. For example, while the NiS/WO₃/g-C₃N₄ composite [56] achieved significant improvement in H₂ production, the dye sensitization process requires stable dyes and efficient electron transfer to the active sites for optimum performance, presenting a challenge for long-term stability. Additionally, Gao et al. (2019) [39] developed a TiATA@g-C₃N₄ heterostructure to enhance H₂ production, but the increase in light absorption and charge migration efficiency is still limited by material stability and the scalability of the synthetic process.

Despite these challenges, the continued development of novel heterostructures, the use of co-catalysts, and atomic-level design of composites show promise in overcoming existing limitations, offering a pathway for more efficient and stable photocatalytic systems. The key to future success lies in addressing stability issues, reducing recombination rates, and improving material scalability to enable real-world applications of photocatalytic hydrogen production.

3.5.3. MXene as a New Class of Two-Dimensional (2D) Metal Carbides, Nitrides, or Carbonitrides

A critical step in photocatalytic H₂ evolution is the effective separation of the photogenerated charge carriers. Noble metals, particularly platinum (Pt), are commonly employed to enhance charge carrier separation efficiency, thereby boosting photocatalytic H₂ production. However, due to concerns over the high cost and limited availability of Pt, MoS₂ has emerged as a competitive alternative. Semiconductor–MoS₂ hybrids offer remarkable advantages for photocatalytic H₂ evolution in certain applications, providing potential benefits over semiconductor–Pt hybrids. Furthermore, MXene, a new class of two-dimensional (2D) metal carbides, nitrides, or carbonitrides, has attracted considerable attention in various fields such as batteries, supercapacitors, and sensors. Its unique properties, including excellent metallic conductivity, chemical versatility, and structural stability, make it a highly competitive material for use as a catalyst support in photocatalysis.

The recently developed MXene photocatalytic composites for photocatalytic H₂ production are listed in

Table 3. Recently developed MXene photocatalytic composites for photocatalytic H₂ production.

MXene Photocatalytic Composites	Advantage	H2 Production	Ref.
0D/2D structure of CdxZn1-xS/Ti3C2 ultrathin MXene composites Ti3C2 MXene, CZS, and CZS/Ti3C2 MXene composites	λ > 420 nm, 300 W Xe lamp, 70 mL deionized water and 30 mL of a sacrificial agent mixture (Na2S and Na2SO3) in a 1:1 volume ratio, Na2S/Na2SO3 as sacrificial agents	bare CZS–5.56635 mmol g−1 h−1 CZS/Ti3C2 MXene composites (Ti3C2 content 1 wt%)–15.03581 mmol g−1 h−1	[61]
Binary heterojunction photocatalyst, consisting of Cd0.5Zn0.5S nanorods on 0.5 wt% Ti3C2 MXene nanosheet	λ = 350 nm	15.56 mmol g−1 h−1	[62]
Ti3C2 MXene@TiO2/ZnIn2S4 (MXene@TiO2/ZIS)	300 W Xeon lamp Na2S/Na2SO3 as sacrificial agents	1.1858 mmol g−1 h−1	[63]
Ti3C2Tx MXene-based composites Ti3C2 MXene 6.7 wt% dispersion modified with tetraethyl hydroxyl ammonium (TEHA) alongside rhombic-shaped TiO2 nanoparticles		0.39092 mmol h−1	[64]
g-C3N4 nanotube@polydopamine(pDA)/NiCo-LDH (LPC) composite	λ > 420 nm, triethanolamine (TEOA) as the hole-capturing reagent	1.5551 mmol h−1 g−1	[65]
CdS–BiOCl/PAN composites	lactic acid as a sacrificial agent, 300 W Xenon lamp	CdS powder–0.38948 mmol g−1 h−1 0.09753 mmol g−1 h−1 during the second cycle CdS–BiOCl/PAN composites- C–B–P(3) > C–B–P(2) > C–B–P(1) > C–B–P(4) > C–B–P(5): 0.28865, 0.24371, 0.21504, 0.20763, and 0.15618 mmol g−1 h−1 BiOCl/PAN–0.03601 mmol g−1 h−1	[66]

. Zheng et al. (2021) [61] developed CdxZn_1-xS/Ti₃C₂ ultrathin MXene composites using a simple electrostatic assembly method. The unique 0D/2D structure of these composites exhibited significantly improved photocatalytic H₂ production performance compared to bare CdxZn_1-xS. Spectroscopic analyses confirmed that the electronic interactions and the Schottky barrier formed at the interface between CdxZn_1-xS and Ti₃C₂ MXene play a crucial role in promoting the efficient separation of photoinduced electron–hole pairs. The successful implementation of electrostatic self-assembly to integrate CdxZn_1-xS with ultrathin MXene paves the way for utilizing electrical differences and band theory to design rational semiconductor/MXene Schottky structures for various photocatalytic applications.

Photocatalytic H₂ production was evaluated using an online photocatalytic evaluation system under visible light irradiation from a 300 W Xe lamp equipped with a 420 nm cut-off filter. Typically, 10 mg of each prepared sample was dispersed in 100 mL of solution consisting of 70 mL of deionized water and 30 mL of a sacrificial agent mixture (Na₂S and Na₂SO₃) in a 1:1 volume ratio. Cooling water was circulated through the reactor to maintain the temperature at 5 °C. The amount of H₂ produced was analyzed using gas chromatography with a thermal conductivity detector (TCD).

The photocatalytic H₂ evolution properties of Ti₃C₂ MXene, CZS, and CZS/Ti₃C₂ MXene composites were investigated under visible light irradiation using 0.35 M Na₂S and 0.25 M Na₂SO₃ as sacrificial agents. Ti₃C₂ MXene exhibited metallic properties and supported its role as a co-catalyst rather than a photocatalyst. The H₂ evolution rate of bare CZS was found to be 5566.35 μmol g⁻¹ h⁻¹. In contrast, the H₂ production activity of CZS/Ti₃C₂ MXene composites improved significantly upon the coupling of Ti₃C₂ MXene, showing an initial increase followed by a decrease in activity. When the Ti₃C₂ content reached 1 wt%, the composite achieved the highest H₂ production rate of 15,035.81 μmol g⁻¹ h⁻¹, which is 2.7 times greater than that of bare CZS. However, as the Ti₃C₂ MXene content increased further, the photocatalytic activity gradually declined. This reduction in activity is attributed to several factors, including the coverage of active sites by excess Ti₃C₂ MXene and the formation of recombination centers for photogenerated charge carriers due to the high loading of Ti₃C₂ MXene.

Li et al. (2024) [62] prepared a binary heterojunction photocatalyst, consisting of Cd_0.5Zn_0.5S nanorods on a Ti₃C₂ Mxene nanosheet. In the composite material, the Cd_0.5Zn_0.5S nanorods are evenly coated across the surface of the single-layer Ti₃C₂ nanosheets, creating a Schottky heterojunction at the interface of the materials. This arrangement improves the efficiency of separating photogenerated electrons and holes, thus boosting light utilization. By incorporating 0.5 wt% (mass fraction) of Ti₃C₂ MXene, they noted a maximum photocatalytic H₂ generation rate of 15.56 mmol g⁻¹ h⁻¹, which surpassed the baseline Cd_0.5Zn_0.5S by 2.56 times. Significantly, photocatalytic efficiency stayed mostly the same following five cycles. This composite reached the highest apparent quantum efficiency of 18.4% under 350 nm UV light exposure. This study illustrates the efficacy of the innovative carbon-based material MXene as a co-catalyst to enhance the functionality of photocatalysts and presents a practical method for developing photocatalytic H₂ evolution catalysts containing MXene. The fabrication of Ti₃C₂Tx Mxene-based composites, with precise control over size, shape, and surface properties for high-efficiency photocatalytic applications, is promising and challenging. Two-dimensional MXene materials are promising candidates for co-catalysts due to their hydrophilicity, photostability, extensive specific surface area, and excellent conductivity. Ti₃C₂ MXene is a promising option for a co-catalyst, that could replace noble metals in photocatalytic H₂ generation. Ti₃C₂ acts as a co-catalyst that effectively reduces the recombination of photogenerated electrons and holes, enhancing catalytic efficiency. Thanks to its low cost, non-toxic nature, and plentiful availability of TiO₂ and Ti₃C₂, this stable composite is anticipated to function as a photocatalyst for extensive applications.

Huang et al. (2020) [63] developed a novel hierarchical Ti₃C₂ MXene@TiO₂/ZnIn₂S₄ (MXene@TiO₂/ZIS) photocatalyst with rapid charge transfer channels using a two-step hydrothermal method for efficient H₂ production. The approach involved hydrothermal oxidation for in situ synthesis of Ti₃C₂ MXene embedded with TiO₂ nanosheets (M@TiO₂), which were subsequently used as a support for ZnIn₂S₄ (ZIS). The optimized hybrid photocatalyst exhibited an H₂ production rate of 1185.8 µmol g⁻¹ h⁻¹, which was 9.1 times higher than that of M@TiO₂ and 4.6 times higher than that of pure ZIS. The improved photocatalytic performance was attributed to the enhanced light absorption capabilities of ZIS and Ti₃C₂, the abundant active sites of Ti₃C₂, the strong interfacial interactions, and the efficient separation and transport of photogenerated charge carriers enabled by the heterojunction. Key charge transfer pathways included the type II heterojunction formed between ZIS and TiO₂ nanosheets, the Schottky junction between Ti₃C₂ and the semiconductor, and the excellent conductivity of metallic Ti₃C₂. This study confirmed the formation of Schottky junctions between ZIS and Ti₃C₂ and emphasized that the hierarchical M@TiO₂ structure acts as both a beneficial platform and an effective co-catalyst, facilitating the further development of MXene-based photocatalysts.

Zong et al. (2022) [64] fabricated Ti3C2–TiO2 composites by a two-phase assembly technique using Ti₃C₂ Mxene dispersion modified with tetraethyl hydroxyl ammonium (TEHA) alongside rhombic-shaped TiO₂ nanoparticles that feature exposed {101} facets. The Ti₃C₂ Mxene serves as an effective support for anchoring TiO₂ nanoparticles with a size of 15 nm and excellent facet regulation. The Ti3C2–TiO2 composites achieve significantly improved photocatalytic H₂ evolution activity. The maximum H₂ production rate of 390.92 μmol h⁻¹ is achieved with the Ti3C2–TiO2 composites at a Ti₃C₂ of 6.7 wt%, which is 2.1 times higher than that of pure TiO₂. This remarkable photocatalytic performance can be attributed to the increased light absorption, enhanced charge separation efficiency, and available {101} facets in the TiO₂ nanoparticles. This study presents a novel approach to prepare high-quality MXene Ti3C2–TiO2 hybrids with strong photocatalytic propersties, which could be widely utilized to prepare other MXene-derived composites with outstanding properties.

Meng et al. (2021) [65] develop a novel g-C₃N₄ nanotube@polydopamine(pDA)/NiCo-LDH (LPC) composite photocatalyst by integrating hydrothermal and calcination techniques. In the LPC composite system, the one-dimensional (1D) g-C₃N₄ nanotubes, with their larger specific surface area, provide more active sites and help to reduce the charge migration distance, while the rapid mass transfer in the nanotubes can speed up the reaction process. The g-C₃N₄/NiCo-LDH type II heterojunction can effectively promote the spatial separation of charge generated by light. Furthermore, pDA acts as a metal-free heterojunction interface medium that offers various actions (π-π* electron delocalization effect, adhesive properties, and photosensitization). The optimized LPC nanocomposite shows approximately 3.3 times greater photoactivity for H₂ production compared to the g-C₃N₄ nanotube when exposed to solar light. Moreover, the results from the cyclic experiments indicate that the LPC composite photocatalyst demonstrates excellent stability and recyclability. The resulting g-C₃N₄@pDA/NiCo-LDH composite photocatalyst exhibits potential real-world applications in energy conversion.

Zhang et al. (2019) [66] developed CdS–BiOCl/PAN composites for H₂ production under UV–vis light, benefiting from the synergy between CdS, BiOCl, and PAN. The CdS content in the composites was the key to their photocatalytic performance. Bismuth oxyhalides (BiOX, X = Cl, Br, I) have gained attention as photocatalysts due to their unique layered structure, which reduces electron–hole recombination and boosts efficiency. Pure BiOCl/PAN absorbed only UV light, with an absorption edge at 370 nm, while CdS absorbed visible light with an edge near 614 nm. The CdS–BiOCl/PAN composites showed a shift in absorption to longer wavelengths, with stronger absorption in the 300–600 nm range, indicating that CdS incorporation improved the photocatalytic performance of BiOCl/PAN. Photocatalytic H₂ production was carried out using a 5 mg sample dispersed in 50 mL of solution containing 5 mL of lactic acid as a sacrificial agent and irradiated by a 300 W Xenon lamp. The system was evacuated for 60 min before the reaction to ensure vacuum conditions. The CdS powder had the highest H₂ evolution rate (389.48 μmol g⁻¹ h⁻¹), but its activity quickly declined in subsequent cycles, dropping to 97.53 μmol g⁻¹ h⁻¹ due to poor recyclability. In contrast, the CdS–BiOCl/PAN composites showed higher H₂ production rates than pure BiOCl/PAN (36.01 μmol g⁻¹ h⁻¹), which can be attributed to CdS incorporation and the formation of a heterojunction structure. As the CdS content increased, the photocatalytic performance of the CdS–BiOCl/PAN composites improved, peaking at C–B–P(3). This indicates that the CdS–BiOCl heterojunction was most effective at this composition. However, beyond a certain CdS threshold, excess CdS nanoparticles agglomerated into larger particles, which acted as recombination centers for photogenerated electron–hole pairs, reducing photocatalytic efficiency. The photocatalytic activity of C–B–P(3) remained stable after six cycles, showcasing its excellent chemical stability. This stability can be attributed to the synergistic effects of CdS, BiOCl, and PAN in the C–B–P(3) composite.

Wang et al. [8] synthesized nitrogen-doped carbon CdS@NC composites through the carbonization of a crystalline complex consisting of cadmium cations and methyl orange anions, [Cd(H₂O)₆(C₁₄H₁₄N₃SO₃)₂]1. These composites were explored as efficient photocatalysts for enhanced H₂ production under visible light irradiation (λ ≥ 420 nm), achieving a remarkable H₂ evolution rate of approximately 7428.0 μmol h⁻¹ g⁻¹. This rate is relatively high compared to previously reported CdS-based nanomaterials. In this study, ref. [8], CdS@NC composites were synthesized using a one-step in situ carbonization strategy involving a pre-assembled crystalline cadmium complex, [Cd(H₂O)₆(C₁₄H₁₄N₃SO₃)₂] (methyl orange = C₁₄H₁₄N₃SO₃Na). This complex consists of cadmium cations and methyl orange (MO) anions arranged into supramolecular scaffolds. It was found that cadmium and sulfur species in the precursors formed CdS nanoparticles (NPs), which were simultaneously deposited on nitrogen-doped carbon (NC) scaffolds during the high-temperature carbonization process and the formation of CdS@NC composites. Among these, the optimal CdS@NC–500–4 composite showed outstanding photocatalytic H₂ evolution, achieving a rate of 7428.0 μmol h⁻¹ g⁻¹ in an aqueous Na₂S/Na₂SO₃ system (as sacrificial reagents). In comparison, the reference material *CdS@NC–500–4, obtained by physically mixing Cd(NO₃)₂·4H₂O and MO and then carbonizing, exhibited a significantly lower H₂ evolution rate of 1300.9 μmol h⁻¹ g⁻¹. These results indicate that combining Cd²⁺ and MO anions into a crystalline phase is essential for optimizing the photocatalytic performance of the resulting composites. Under the given conditions, the pristine crystalline complex 1 produced a negligible H₂ rate of 19.2 μmol h⁻¹ g⁻¹. After carbonization, CdS@NC photocatalysts showed significant improvements, with H₂ production rates ranging from 357.7 μmol h⁻¹ g⁻¹ for CdS@NC–500–1 to 7428.0 μmol h⁻¹ g⁻¹ for CdS@NC–500–4. In contrast, the reference material *CdS@NC–500–4, made by carbonizing a mixture of Cd(NO₃)₂·4H₂O and MO, had a lower rate of 1300.9 μmol h⁻¹ g⁻¹. The enhanced photocatalytic activity was mainly due to the carbonization temperature, which narrowed the bandgap, improved light absorption, and facilitated charge separation. CdS@NC–500–4 showed the best performance, attributed to the effective interfacial contacts between CdS nanoparticles and nitrogen-doped carbon supports. All CdS@NC materials outperformed pure CdS in H₂ production. Stability studies of CdS@NC–500–4 revealed that the photocatalyst maintained its efficiency over a 25-h period (monitored at 5-h intervals) without significant decreases in activity. PXRD analysis confirmed that the phase purity and crystallinity of CdS@NC–500–4 remained stable even after prolonged photocatalytic use.

Major Achievements in the Development of MXene-Based Composites for Photocatalytic H₂ Production

Recent studies have focused on materials like MoS₂, MXenes, and other semiconductor hybrids, which show promising results in improving photocatalytic H₂ production while addressing the cost and sustainability concerns associated with platinum-based systems.

The incorporation of MoS₂ into semiconductor composites has demonstrated notable advantages. MoS₂, with its layered structure, offers excellent charge transfer properties and can effectively enhance the photocatalytic performance of semiconductors like CdxZn_1-xS. The research by Zheng et al. (2021) [61] showed that the integration of CdxZn_1-xS with Ti₃C₂ MXene, a 2D material, resulted in significant improvements in photocatalytic H₂ production, with the Ti₃C₂ hybrid exhibiting 2.7 times greater efficiency than pure CdxZn_1-xS. This is attributed to the Schottky junction formed at the interface, which plays a crucial role in facilitating efficient charge separation. Moreover, the use of MoS₂-based composites, as observed in the study, presents a sustainable alternative to Pt while maintaining high performance.

MXenes, especially Ti₃C₂, have also attracted significant attention as catalyst supports. Ti₃C₂ MXene has excellent metallic conductivity, hydrophilicity, and structural stability, making it a strong candidate for enhancing photocatalytic reactions. In the work of Li et al. (2024) [62], the Cd0.5Zn_0.5S/Ti₃C₂ composite exhibited a remarkable 2.56-fold enhancement in photocatalytic H₂ production compared to bare Cd0.5Zn_0.5S. This was attributed to the efficient separation of charge carriers enabled by the Schottky heterojunction between Cd0.5Zn_0.5S and Ti₃C₂. The stability and recyclability of Ti₃C₂-based composites were further demonstrated by the fact that the composite maintained its efficiency over multiple cycles, emphasizing the potential of MXenes in practical applications. Similarly, other Ti₃C₂-based composites, such as Ti₃C₂@TiO₂/ZnIn₂S₄ [63], also showed substantial improvements in photocatalytic H₂ production by facilitating charge transfer through various heterojunctions, including type II junctions between TiO₂ and ZnIn₂S₄ and Schottky junctions between Ti₃C₂ and the semiconductor.

Despite the impressive performance of Ti₃C₂-based composites, challenges still remain, particularly with the optimal loading of Ti₃C₂. Zheng et al. (2021) [61] found that while a 1 wt% loading of Ti₃C₂ in the composite resulted in the highest photocatalytic activity, further increases in Ti₃C₂ content led to a decline in performance. This reduction was attributed to the excessive coverage of active sites by Ti₃C₂ and the formation of recombination centers for photogenerated charge carriers, highlighting the importance of optimizing Ti₃C₂ loading to achieve the best photocatalytic performance. This is a common issue faced by many photocatalytic systems where excessive loading of the support material can hinder the active sites’ exposure and charge transfer efficiency.

Another key factor in improving photocatalytic performance is the enhancement of light absorption. The study by Zong et al. (2022) [64] demonstrated that Ti3C2–TiO2 composites achieved a 2.1-fold increase in H₂ production due to the improved light absorption and charge separation facilitated by the unique exposed {101} facets of TiO₂ nanoparticles. This facet-specific design improved the photocatalytic efficiency of the composite, showing that the structural modification of semiconductor materials plays an essential role in optimizing photocatalysis.

In addition to the structural and material improvements, the stability of the photocatalysts is a crucial aspect for large-scale applications. The work by Meng et al. (2021) [65] on g-C₃N₄/NiCo-LDH composites showed that the integration of polydopamine (pDA) as a metal-free heterojunction interface material effectively enhanced charge migration and resulted in a photocatalyst with excellent stability and recyclability. The g-C₃N₄@pDA/NiCo-LDH composite demonstrated a 3.3 times greater H₂ production rate than pure g-C₃N₄, while maintaining its activity over multiple cycles. This stability, along with the enhancement in photocatalytic activity, highlights the potential for composites to operate over extended periods without significant degradation.

While the results are promising, there are several challenges that need to be addressed to move these materials from laboratory-scale systems to industrial applications. The stability of Ti₃C₂-based composites in aqueous environments remains a concern, as the high reactivity of Ti₃C₂ in water may lead to material degradation over time. Similarly, the scalability of synthesis methods for MXenes, MoS₂, and other composites is an important consideration. While techniques like electrostatic assembly and hydrothermal processes offer good control over material composition, they may not be easily scalable for large-scale production. The cost of these materials, though lower than platinum, can still be a limiting factor, especially when considering the need for high-quality materials and precise control over their synthesis.

Additionally, while composites like CdS@NC, as reported by Wang et al. (2021) [8], have shown excellent photocatalytic performance, there is still a need for optimization in terms of material design. The carbon-based support material in these composites significantly improved photocatalytic activity, but the challenge lies in ensuring that the carbon content does not adversely affect the material’s performance in the long term. Furthermore, the ability to tailor the surface properties and interfaces of these composites for maximum charge separation efficiency remains a key area for future research.

In conclusion, the integration of MXenes, MoS₂, and other semiconductor materials represents a significant advancement in the development of photocatalysts for hydrogen production. The demonstrated improvements in charge separation, light absorption, and stability show that these materials are highly competitive alternatives to traditional platinum-based photocatalysts. However, challenges, such as optimizing material loading, improving long-term stability, and scaling up synthesis methods, must be addressed for these materials to be viable for industrial applications. Future research should focus on enhancing the scalability of synthesis processes, improving the stability of MXene-based and MoS₂-based composites, and refining the material design to maximize photocatalytic efficiency. With continued advancements, these materials hold the potential to play a critical role in the development of sustainable hydrogen production systems.

4. Interfacial Charge Separation and Transfer in Composite Photocatalysts

The interaction between components in composite photocatalysts plays a critical role in determining their efficiency by affecting the movement of photogenerated charge carriers. While much research has focused on incorporating upconversion materials to extend light absorption into the long-wavelength spectrum, the role of interfacial properties in optimizing performance remains underexplored. One challenge is facilitating efficient charge transfer between non-conductive upconversion materials and semiconductors [1].

The development of composite photocatalysts with suitable material combinations not only broadens the light absorption spectrum but also improves the separation of photogenerated charge carriers. Upconversion materials have the distinctive ability to convert long-wavelength near-infrared (NIR) light into shorter-wavelength visible light so that NIR can be used to activate composite photocatalysts containing these upconversion agents. Among the various upconversion materials, lanthanide-doped hexagonal NaYF4 stands out due to its exceptional stability and low photon energy requirement. When excited by NIR light at around 980 nm [67], it emits mainly in the 540–560 nm range, making it ideal for integration with photocatalysts that have compatible bandgaps and similar hexagonal crystal structures, such as ZnO and CdS [1].

Additionally, molybdenite has emerged as an efficient co-catalyst, replacing platinum to improve electron–hole separation and enhance light absorption, thereby boosting photocatalytic activity. In particular, the PCN/Mo-2 composite demonstrated superior photocatalytic performance due to its excellent light absorption and effective charge carrier transfer [2]. The interface between g-C₃N₄ and molybdenite, formed through ball milling, plays a key role in ensuring efficient electron transfer. This interaction was crucial for the Pt-free photocatalytic activity of PCN/Mo-x composites, highlighting the importance of well-designed interfaces for effective charge separation [3]. Furthermore, the 2.0PCN-P25 composite also demonstrated improved photocatalytic activity due to the reduction in charge carrier recombination, facilitated by the coupling of CN with P25 and phosphorus doping, which lowered the bandgap and enhanced visible light absorption.

The significantly increased photocatalytic activity for H₂ generation under visible light is primarily attributed to the ability of the g-C₃N₄ nanotubes to provide numerous reaction sites and facilitate a reduction in charge carrier transport distances. Additionally, the fast mass transport within the nanotube can accelerate the reaction process, while the g-C₃N₄/NiCo-LDH type II heterojunction improves the spatial separation of the photoinduced charges. Furthermore, pDA serves as a metal-free interface medium for heterojunctions and provides various functions such as electron delocalization effect, adhesive properties and photosensitization [65]. Transient photocurrent response measurements, experiments with reactive oxygen species (·O₂⁻ and ·OH) and photoluminescence (PL) analyses provided evidence of strong electronic interactions between the defective CdS and ZnIn2S4, indicating enhanced charge separation and transport at the interface [49]. These results were further supported by DFT calculations, which confirmed improved charge transfer and electronic coupling within the heterojunctions.

Li et al. [54] observed an improvement in photocatalytic performance, which was attributed to two key factors: (1) the presence of carbon particles that facilitated the storage and transfer of electrons, and (2) the formation of a p–n heterojunction between Co₃O₄ and Zn_xCd_1-xS, which effectively promoted the separation of photogenerated charge carriers. Li et al. [54] showed that the incorporation of amorphous carbon and Co₃O₄ significantly improved electron and hole transfer while suppressing carrier recombination, providing favorable conditions for the development of highly efficient and stable C@Zn_xCd_1-xS/Co₃O₄ photocatalysts.

Manzoor et al. (2020) [16] used ZnO nanocomposites doped with varying concentrations of copper Cu (0.0, 0.01, 0.03, and 0.05 mol%) for photocatalytic H₂ production in a liquid-phase photoreactor with water–methanol splitting. A total of 1 g of photocatalyst particles were introduced into a water–methanol mixture within a 2-L Pyrex reactor. The reaction was performed at a wavelength of 365 nm. Their research showed that the incorporation of Cu into ZnO significantly improved the H₂ production rate from 7.49 μmol g⁻¹ to 41.55 μmol g⁻¹. This enhancement is attributed to the effective reduction of electron–hole recombination during the photocatalytic reaction. Moreover, it was observed that Cu doping reduced the bandgap from 3.19 eV to 3.00 eV. Photoluminescence spectroscopy (PL) analysis showed that the recombination rate of photogenerated electrons and holes decreased with increasing Cu doping levels.

Enhancement in photocatalytic performance arises from two main factors. Firstly, the WO₃/g-C₃N₄ heterojunction effectively suppresses the recombination of photogenerated electron–hole pairs. Secondly, the incorporation of NiS improves electron mobility and introduces additional active sites that facilitate H₂ evolution [56].

Chen et al. (2019) reveal that increasing biochar content in biochar/ZnFe₂O₄ (BZF) composites improves H₂ evolution due to better electron transfer, but excessive biochar reduced the rate by blocking visible light absorption, lowering electron generation [47].

A short summary of experimental (e.g., XPS, PL, TRPL and ESR) evidence typically used to confirm the charge transfer mechanism, particularly for the very promising Z-scheme and S-scheme heterojunctions, is given in

Table 4. A summary of the experimental techniques commonly used to confirm charge transfer mechanisms in Z-scheme and S-scheme heterojunctions in photocatalytic systems.

Experimental Technique	Purpose	Evidence
X-ray Photoelectron Spectroscopy (XPS)	Probes of the electronic structure and chemical states	A directional shift in binding energies can indicate internal electric field formation and interfacial charge migration. Work function measurements via XPS valence band analysis can support band alignment consistent with Z-scheme or S-scheme mechanisms.
Photoluminescence (PL) Spectroscopy	Measurement of the recombination of photogenerated charge carriers	Lower PL recombination in heterojunction compared to single components indicates reduced recombination and supports effective charge separation.
Time-resolved Photoluminescence (TRPL)	Measurement of the carrier lifetime	Longer carrier lifetimes in heterojunctions indicate efficient charge separation and transfer. A complementary technique to the steady-state PL, which enables a temporal resolution of the charge dynamics.
Electron Spin Resonance (ESR)	Detection of the unpaired electrons, radicals, and trapped charge carriers	Information on active electrons/holes, and indication which component is oxidized/reduced (e.g., in an S-scheme confirmation that electrons in the CB of the reductive semiconductor and holes in the VB of the oxidative semiconductor are retained).

.

5. Photocatalytic H₂ Production with Composite Photocatalysts

Zhu et al. (2022) [1] synthesized NaYF₄:Yb,Er/CdS composites using NaYF₄:Yb,Er nanorods and CdS nanoparticles through an ion adsorption/precipitation method and subsequent annealing under an argon atmosphere at varying temperatures to tune their microstructures. The annealing process facilitated the structural transformation from low-crystallinity cubic CdS to high-crystallinity hexagonal CdS, while also promoting improved interface contact between NaYF₄:Yb,Er and CdS. The study [1] showed that a robust interface between NaYF₄:Yb,Er and CdS greatly enhanced photocatalytic H₂ production. This improvement is likely attributed to the efficient charge transfer enabled by the optimized interface. Specifically, under light irradiation of λ > 400 nm the H₂ evolution activity increased from 171 to 2539 μmol h⁻¹ g⁻¹, while under light irradiation of λ > 600 nm it improved from 0 to 19 μmol h⁻¹ g⁻¹.

The PCN/molybdenite nanocomposites (PCN/Mo-x) exhibited a remarkable noble-metal-free photocatalytic H₂ evolution reaction rate under visible light [2]. The photocatalytic H₂ evolution reaction was conducted on a quartz glass under simulated light irradiation in a nitrogen-sealed environment. Typically, 100 mg of the photocatalyst was dispersed in 100 mL of a 0.75 M triethanolamine (TEOA) aqueous solution, with TEOA acting as the sacrificial agent. For bulk-PCN and PCN, 1 mL of H₂PtCl₆ aqueous solution was added to achieve a mass ratio of 3 wt% of Pt co-catalyst to photocatalyst, while H₂PtCl₆ was not added to the PCN/Mo-x and PCN/Mo-2 mixtures [2]. The setup was then degassed with high-purity nitrogen for 15 min to remove air completely and sealed with a quartz cover. The reactor was vertically irradiated with a 300 W Xe lamp, fitted with a cut-off filter (λ > 420 nm), to provide visible light and reduce H₂PtCl₆ into the Pt co-catalyst. After purging with high-purity nitrogen again, the sealed reactor was irradiated with the same Xe lamp. The reaction was maintained at a constant temperature with circulating condensed water and conducted under magnetic stirring. The resulting H₂ production was measured using gas chromatography equipped with a 5A-zeolite column and a thermal conductivity detector. The photocatalytic activities of the samples were evaluated based on H₂ production from water splitting under simulated visible light irradiation. Stable and linear H₂ evolution rates were observed within 4 h for bulk-PCN and PCN in the presence of Pt (bulk-PCN/Pt and PCN/Pt, respectively), as well as for PCN/Mo-x without Pt [2]. Both bulk-PCN and PCN generated H₂ only after the incorporation of Pt nanoparticles, exhibiting H₂ evolution rates of 67 and 80 μmol g⁻¹ h⁻¹, respectively. The improved performance of PCN was attributed to its enhanced specific surface area resulting from ball milling. Notably, even without the Pt co-catalyst, the PCN/Mo composites (PCN/Mo-1, 2, and 3) showed favorable H₂ evolution rates of 16, 27, and 19 μmol g⁻¹ h⁻¹, corresponding to 23.9%, 40.3%, and 28.4% of the H₂ production rate of bulk-PCN/Pt, respectively [2].

All photocatalytic H₂ evolution experiments with CN, PCN, PCN-P25 and other CN/PCN-based composites were conducted at room temperature under simulated solar light conditions (irradiation provided by a 1000 W xenon lamp) using a 200 mL Pyrex quartz reactor [3]. In each typical experiment, 40 mg of the catalyst and 100 mL of a 10% methanol solution (as a sacrificial agent) were used. Prior to the reaction, the quartz reactor containing the methanol solution and catalyst was sonicated for 10 min and sealed with a silicone rubber septum to ensure airtight conditions. The system was then purged with N₂ gas for 10 min to establish an inert atmosphere. Following this preparation, the quartz reactor was exposed to simulated solar light irradiation, and the amount of H₂ gas evolved was quantitatively monitored on an hourly basis using a gas chromatograph (GC) instrument [3].

The photocatalytic performances of P25, CN, and PCN were individually assessed for their ability to produce H₂. Additionally, various weight percentages of CN and PCN composites with P25 were tested. The trend observed in photocatalytic H₂ production was as follows: CN < PCN < P25 < CN-P25 < PCN-P25. Among the different CN-P25 weight ratios, the 3.0CN-P25 composite exhibited the highest performance, with an optimal balance between photocatalytic activity and structural properties. Notably, increasing the CN content beyond 3% reduced performance due to the light-shielding effect of CN over the P25 surface. The 2.0PCN-P25 composite was identified as the optimal composition. The 3.0CN-P25 composite showed a nearly 22-fold increase in H₂ production (920 μmol/g) compared to CN alone (41 μmol/g) and a 10-fold increase compared to P25 alone (99 μmol/g), highlighting the significant enhancement achieved by optimizing the composite composition [3]. The 2.0PCN-P25 composite exhibited a nearly 40-fold increase in H₂ production (2531 μmol/g) compared to PCN alone and a 25-fold increase compared to P25 (99 μmol/g). A comparative analysis of CN, PCN, CN-P25, and PCN-P25 revealed that the 2.0PCN-P25 composite not only demonstrated the highest performance among all tested photocatalysts but also showed nearly three times greater H₂ production than the 3.0CN-P25 composite. This highlights the pivotal role of phosphorus doping in improving the performance of the composite photocatalyst [3].

Photocatalytic H₂ evolution experiments revealed that the optimal sample, hollow-sphere g-C₃N₄ (SCNO) SCNO/CoO (0.2), exhibited an impressive catalytic activity of 11,495 μmol H₂ h⁻¹ g⁻¹, which was approximately four times higher than that of SCNN and two times higher than SCNO, and is attributed to the significantly enhanced rate of charge carrier transfer [7]. However, a reduction in the H₂ production rate was observed for SCNO/CoO (0.3), probably due to the agglomeration of CoO QDs. The photocatalytic H₂ evolution reaction was performed in a 250 mL quartz reactor. A 300 W xenon (Xe) lamp equipped with a 420 nm ultraviolet cut-off filter was used as the visible light excitation source to simulate natural sunlight. Triethanolamine (TEOA) was used as the sacrificial agent to consume the photogenerated holes in the valence band. For each reaction, 50 mg of the synthesized photocatalyst was dispersed in 100 mL of a solvent mixture composed of water and triethanolamine (TEOA) in a volume ratio of 9:1. Prior to initiating the reaction, the reactor was evacuated for 30 min to ensure the complete removal of air. Hydrogen gas generation was analyzed using a gas chromatograph with argon as the carrier gas. To determine the (AQY), the measurements were carried out with a 420 nm band-pass filter and an optical power meter (NP2000-2) positioned 10 cm from the light exit port.

The photocatalytic H₂ evolution rate of SCNO was significantly higher than that of SCNN. This indicates that the wider pore structure and larger specific surface area of SCNO enhance its catalytic performance. Furthermore, the SCNO/CoO composite exhibited superior photocatalytic H₂ activity, highlighting the effectiveness of the novel one-step synthesis method to achieve highly dispersed CoO quantum dots (QDs) within the carbon nitride matrix. The large specific surface area of SCNO provided optimal conditions for the uniform distribution of CoO QDs. The results also showed that SCNO/CoO (0.2) outperformed CoO/g-C₃N₄ composites synthesized through a step-by-step method. The average particle size of CoO QDs on g-C₃N₄ was 5 nm, which was much smaller than the 30–50 nm CoO nanoparticles deposited on g-C₃N₄ using the step-by-step method. These smaller particles were uniformly dispersed over the g-C₃N₄ matrix, resulting in a higher number of active sites per gram of catalyst. This suggests that the reaction rate is proportional to the number of CoO QDs present on the support. The (AQY) of SCNO/CoO (0.2) was measured at 5.63% under the same experimental conditions using a 420 nm band-pass filter. Additionally, the AQY measurements were extended to other band-pass filters to assess the photocatalytic performance of SCNO/CoO (0.2) across a range of wavelengths. The results demonstrate that the SCNO/CoO (0.2) sample exhibits exceptional photocatalytic activity, outperforming most reported g-C₃N₄-based catalysts.

The photocatalytic H₂ evolution experiments for all samples are performed under visible light excitation (λ > 420 nm), using triethanolamine (TEOA) as the hole-capturing reagent. The photocatalytic performance of pure g-C₃N₄ is also lacking, with H₂ production merely reaching 58.1 μmol over 5 h, primarily due to the inadequate visible light absorption ability of the single-component photocatalytic material, leading to a rapid recombination of the photogenerated charge carriers. To address the aforementioned drawbacks, pDA has been deposited onto the surface of g-C₃N₄ nanotubes, which significantly increased the H₂ evolution activity of g-C₃N₄, with an H₂ evolution quantity reaching 121.3 μmol [65]. The enhanced rate of H₂ production results from two primary factors: firstly, the delocalization effect between pDA and g-C₃N₄ provides a driving force for the separation and movement of charge carriers. Secondly, the pDA exhibits enhanced electron transport ability, as it is closely wrapped around the surface of the g-C₃N₄ nanotubes, creating a larger contact area between them. This configuration provides numerous pathways for charge carrier transfer and effectively manages charge carrier recombination [65]. The photocatalytic performance of LDH/CN (292.6 μmol) has been evaluated to confirm that NiCo-LDH is a widely recognized photocatalytic substance in the photocatalytic arena, with its energy band alignment being suitable for g-C₃N₄ to create a type II heterojunction, thereby enhancing charge carrier separation and significantly boosting photocatalytic H₂ evolution activity [65]. The resulting LPC sample shows the maximum H₂ evolution activity of 398.7 μmol, approximately 1.4 times greater than that of LDH/CN. The amount of H₂ evolution for g-C₃N₄, CN@pDA, LDH/CN, and LPC increases linearly with the duration of light exposure, confirming that H₂ generation can be sustained for 5 h [65]. The H₂ evolution reaction (HER) rate of pure NiCo-LDH is negligible under visible light exposure, whereas the rates for pure g-C₃N₄, CN@pDA, LDH/CN, and LPC are 232.5, 485.4, 1170.5, and 1555.1 μmol h⁻¹ g⁻¹, respectively [65]. In comparison, it is evident that LPC exhibits the highest HER (1555.1 μmol h⁻¹ g⁻¹). Consequently, LPC can enhance the effective use of solar energy and photocatalytic efficiency [65]. The optimized LPC composite has the greatest photocatalytic efficiency, with its H₂ evolution reaction (HER) of LPC (1555.1 μmol h⁻¹ g⁻¹) being about 6.7 and 3.2 times greater than that of g-C₃N₄ (232.5 μmol h⁻¹ g⁻¹) and CN@pDA (485.4 μmol h⁻¹ g⁻¹), respectively [65].

The CdS/ZnIn₂S₄ hybrids show superior photocatalytic H₂ production activity compared to pristine CdS [49]. After 4.5 h of light irradiation, H₂ production was 1.28 mmol/g for the pure CdS sample, while the pristine ZIS sample achieved an H₂ yield of 5.14 mmol/g. Under identical photocatalytic conditions, all CdS/ZIS-X heterojunctions exhibited enhanced H₂ production following the incorporation of ZIS. Notably, when the ZIS loading amount was increased to 0.3, the ZIS/CdS-0.3 heterojunction achieved the highest activity, producing 5.80 mmol/g—surpassing both pure CdS (1.28 mmol/g) and the pristine ZIS sample (5.14 mmol/g). However, further increases in the ZIS loading amount resulted in reduced H₂ production. This decline is attributed to the excessive ZIS coverage blocking active sites on the CdS surface, thereby hindering redox reactions and lowering overall efficiency. Additionally, introducing excessive ZIS reduced the light absorption capacity of CdS, which negatively impacted photocatalytic performance [49].

The photocatalytic H₂ generation activities of the Ti₃C₂ MXene@TiO₂/ZnIn₂S₄ (MXene@TiO₂/ZIS) photocatalysts [63] were assessed through water splitting under simulated sunlight illumination (300 W Xeon lamp). A sample of 15 mg of the synthesized photocatalyst was suspended in 80 mL of an aqueous solution containing 0.35 M Na₂S·9H₂O and 0.25 M Na₂SO₃. Before starting light irradiation, the solution was degassed by vacuum treatment. The pristine ZIS and M@TiO₂ materials exhibited low H₂ evolution rates, primarily due to inefficient charge separation and limited light absorption. In contrast, when ZIS was combined with M@TiO₂, the resulting M@T/ZIS composites showed a marked improvement in photocatalytic H₂ production, suggesting a synergistic effect between the Ti₃C₂ metallic co-catalyst, TiO₂ nanosheets, and ZIS nanosheets. For the M@T/ZIS composites with varying ZIS content, H₂ production increased proportionally with longer light irradiation, indicating the stable catalytic activity of the photocatalysts. Among these, the M@T/ZIS-50 composite exhibited the highest H₂ production rate. However, a further increase in ZIS content led to a decrease in H₂ evolution efficiency, which was attributed to a “light-shielding effect“.

Li et al. (2020) [54] dispersed 10 mg of the catalyst in a 10 vol% lactic acid aqueous solution within a 62 mL quartz glass bottle and deaerated with N₂ for 10 min. The quartz bottle was then placed into a multi-channel reaction system and exposed to a 5 W LED lamp for 5 h. The H₂ production of the C@ZCS composite catalyst increased with higher glucose content. At 30% glucose by mass, the H₂ yield was 583.34 μmol, 2.8 times higher than pure ZCS. The carbon particles acted as electron acceptors, boosting H₂ production. However, as the glucose content increased further, H₂ production declined due to excessive carbon particles blocking ZCS active sites. Additionally, H₂ production rose with increased Co₃O₄ content, reaching a maximum of 1405.1 μmol for C@ZCS/CO30. The p–n heterostructure between Co₃O₄ and ZCS improved carrier separation, while carbon particles facilitated electron transfer. However, excessive Co₃O₄ reduced H₂ production by blocking ZCS active sites, limiting contact with reagents and water molecules. Hydrogen production also decreased due to lactic acid consumption.

Chen et al. (2019) [47] have studied the photocatalytic H₂ evolution by the biochar/ZnFe₂O₄ (BZF) composite, which was tested using a 300 W xenon lamp with a UV cut-off filter (λ > 420 nm). A mixture of 0.02 g photocatalyst, 0.02 g Eosin Y photosensitizer, and 1 mL of triethanolamine (TEOA) in 20 mL aqueous solution was sonicated for 5 min. The reactor was purged with N₂ gas and irradiated for 30 min. The photocatalytic H₂ evolution of BZF composites, pure ZnFe₂O₄, and biochar (BC) was investigated under visible light over 3 h. A control with biochar alone showed no H₂ production, confirming that biochar is non-photosensitive and cannot generate photogenerated electrons. In contrast, BZF composites outperformed pure ZnFe₂O₄, which had a low H₂ evolution rate of 173 µmol g⁻¹ h⁻¹ due to the rapid recombination of electron–hole pairs. The BZF-5 composite, with the highest biochar content, achieved a rate of 1219 µmol g⁻¹ h⁻¹, about six times that of pure ZnFe₂O₄ [47].

5.1. Apparent Quantum Efficiency (AQE) for H₂ Evolution

Li et al. (2021) [49] and Huang et al. (2019) [58] have used the following equation to calculate AQY for H₂ evolution:

$$A\mathrm{Q}\mathrm{Y} = {\displaystyle \frac{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{e}\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{d} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n} \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s} \times 2}{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{i}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}}} \times 100\%$$

Li et al. (2021) [49] determined AQE using monochromatic LED light sources at wavelengths of 395, 420, 500, and 590 nm. The CdS/ZIS-0.3 heterojunctions exhibited a strong wavelength dependence, indicating that the H₂ production arises from the reduction of H₂O, driven by photoexcited electrons under light irradiation.

Huang et al. (2019) [58] performed measurements with a 400 nm band-pass filter and an optical power meter to determine the (AQY). The light source was positioned 10 cm from the light exit port. The AQY of Pd/SCN (10 h) was 3.52%, attributed to Pd nanoparticles embedded in g-C₃N₄ which provide abundant active sites for the water reduction reaction and enhance the separation of photogenerated electrons [58]. As the oil bath time increased to 10 h, the H₂ evolution rate correlated positively with the duration. However, for Pd/SCN (12 h), the rate decreased, likely due to Pd nanoparticle agglomeration. In contrast, Pd/SCN (10 h) outperformed Pd/BCN in H₂ production, likely due to SCN’s larger surface area and porous hollow structure. The hollow-sphere structure of g-C₃N₄ offers more active sites and improves visible light absorption, while reduced surface area can negatively impact photocatalytic performance, underscoring the importance of surface area in efficiency.

Transient absorption (TA) spectroscopy is a powerful and reliable method for analyzing real-time photogenerated charge carriers and interfacial charge transfer dynamics in nanosystems. Suitable pump pulses can induce electron transitions from the valence band to the conduction band, allowing for the investigation of charge migration pathways. A 365 nm central wavelength was selected for the pump pulses [7].

Ying et al. [7] proposed the mechanism of photocatalytic H₂ production on SCNO/CoO composites based on experimental data carried out by transient absorption (TA) spectroscopy. During photocatalysis, electrons in the valence band (VB) of SCNO and CoO QDs are excited and migrate to their respective conduction bands (CB). The resulting electron holes are effectively scavenged by triethanolamine (TEOA). Because the CB of CoO QDs lies at a lower energy level than that of SCNO, photogenerated electrons preferentially transfer from SCNO to the CB of CoO QDs. The large specific surface area of CoO QDs provides a significant number of active sites, which accelerates the reaction rate and suppresses the recombination of photogenerated electrons and holes. Additionally, the defect energy levels within the CoO QDs play a crucial role. Some photogenerated electrons are trapped in these defect states, creating additional space for further photogenerated electrons and indirectly enhancing the number of electrons available for the reaction. These mechanisms collectively suggest that the high photogenerated electron–hole pair separation rate in SCNO/CoO composites is a key factor leading to their superior photocatalytic H₂ evolution activity.

Chen et al. (2019) [47] proposed the mechanism for photocatalytic H₂ production using biochar/ZnFe₂O₄ (BZF) composite. In the photocatalytic H₂ evolution system, Eosin Y (EY) serves as a photosensitizer, being excited under light to form the singlet excited state (EY¹*), which then undergoes intersystem crossing (ISC) to the triplet excited state (EY³*), a strong oxidant. The triplet state (EY³*) is quenched by triethanolamine (TEOA), forming EY⁻ anions. These photogenerated electrons from the conduction band (CB) of ZnFe₂O₄ reduce protons from water to produce H₂. EY⁻ anions donate electrons to the conduction band of ZnFe₂O₄, enhancing photocatalytic H₂ evolution. Additionally, the photogenerated holes (h⁺) in the valence band (VB) of ZnFe₂O₄ react with TEOA, promoting efficient electron–hole pair separation, as confirmed by previous photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) studies. The presence of biochar in the BZF composites further boosts photocatalytic performance by facilitating rapid electron transfer from ZnFe₂O₄ to biochar, which aids in electron–hole separation and accelerates H₂ production. Additionally, the photogenerated holes (h⁺) in the valence band (VB) of ZnFe₂O₄ can react with TEOA, facilitating the efficient separation of photogenerated electron–hole pairs. The presence of biochar in the BZF composites further enhances photocatalytic activity by effectively promoting the rapid transfer of photogenerated electrons from ZnFe₂O₄ to biochar. The electrons on the biochar’s surface also participate in water reduction. Together, EY as a photosensitizer, TEOA as a sacrificial agent, and biochar as an electron mediator synergistically enhance the photocatalytic H₂ evolution efficiency of the BZF composite system.

5.2. Role of Sacrificial Agent

Although some photocatalysts are capable of evolving H₂ or O₂ in the presence of electron donors or acceptors (sacrificial reagents), the number of visible-light-driven photocatalysts that can split water into H₂ and O₂ without sacrificial reagents is still limited. Different compounds have been investigated as potential sacrificial agents, including methanol, ethanol and bio-available compounds such as ethanol, glycerol and sugars originating from biological substrates or wastes. The optimization of methanol concentration was performed using the optimized 2.0PCN–P25 composite under consistent experimental conditions, including fixed catalyst concentration and total solution volume, under simulated solar light irradiation. The methanol concentration was varied from 0% to 20% (V/V) [3]. It was observed that in the absence of methanol (0% methanol solution, i.e., pure water), the amount of H₂ production was significantly lower compared to solutions containing methanol. A 10% methanol solution exhibited the highest H₂ evolution capacity, with a production of 2531 μmol/g, confirming methanol’s critical role as a sacrificial agent. Methanol acts by trapping the photogenerated holes from the semiconductor’s valence band, thereby reducing the recombination of photogenerated electron–hole pairs and enhancing the efficiency of H₂ evolution [3]. However, increasing the methanol concentration beyond 10% led to a slight decrease in H₂ evolution efficiency. This decline is likely due to the formation of intermediate byproducts, such as CO₂ and methane, which consume photogenerated electrons and protons (H⁺), thereby reducing the overall rate of H₂ production [3].

6. Additional Challenges in the Development of Photocatalytic Nanostructures and Composite Materials for H₂ Production

6.1. Effect of Annealing Temperature on Crystallinity

Zhu et al. [1] discovered that dispersion of CdS nanoparticles on NaYF4:Yb,Er nanorods and the strong interfacial interactions between the nanoparticles and nanorods enhanced photoluminescence (PL) emission. Compared to the unmodified NaYF4:Yb,Er, the photoluminescence (PL) emission peak of NaYF4:Yb,Er-380 remains centered around 550 nm, but its intensity is notably reduced by 80%. This reduction is attributed to the enhanced crystallinity achieved through annealing, which helps minimize the quenching of photogenerated charge carriers. The CdS and CdS-380 samples exhibit weak emission spectra with peaks at 640 nm and 780 nm, respectively. The 640 nm emission in CdS is due to its intrinsic bandgap emission, while the 780 nm emission in CdS-380 is linked to defect-related emissions. In the composite systems, NaYF4:Yb,Er/CdS and NaYF4:Yb,Er/CdS-380 show spectral characteristics similar to those of their individual components (CdS and CdS-380), but the PL intensities of the composites are higher, especially for NaYF4:Yb,Er/CdS-380. The annealing temperature-dependent PL spectra of NaYF4:Yb,Er/CdS reveal that all annealed samples exhibit significantly higher emission intensities compared to the pristine NaYF4:Yb,Er/CdS. Among these, NaYF4:Yb,Er/CdS-380 shows the weakest emission peak, which correlates with its lowest absorbance in the defect-related absorption tail. The annealing process in an argon atmosphere results in the loss of sulfur atoms from CdS, leading to the creation of sulfur vacancies. These vacancies introduce electronic states at the lower edge of the CdS conduction band. As the annealing temperature increases, the number of sulfur vacancies increases, which in turn enhances the PL emission due to the electronic states associated with these vacancies.

Annealing at 380 °C resulted in a significant boost in photocatalytic activity, with CdS and NaYF4:Yb,Er/CdS showing increases of 5.1 and 59.7 times, respectively, under λ > 400 nm light irradiation. The large difference in these enhancements—nearly an order of magnitude—emphasizes the importance of the strong interfacial interaction between NaYF4:Yb,Er and CdS in enhancing the photocatalytic performance of the NaYF4:Yb,Er/CdS-380 composite. The annealing process promotes phase transformation and improves interfacial bonding between NaYF4:Yb,Er and CdS. It also facilitates the formation of sulfur vacancies, as indicated by the changes observed in the light absorption and PL spectra. However, when the concentration of sulfur vacancies becomes too high, it leads to an increase in the recombination of photogenerated charge carriers, which ultimately reduces the photocatalytic H₂ evolution activity. This trend is particularly evident in the NaYF4:Yb,Er/CdS-450 sample, where a higher density of sulfur vacancies corresponds to a marked decrease in activity [1].

6.2. Photocatalyst Characterization

XPS, FTIR, and DFT calculations confirmed the presence of a strong electronic coupling between CdS and ZnIn₂S₄. These synthesized CdS/ZnIn₂S₄ heterojunctions showed enhanced photocatalytic water-splitting performance compared to pristine CdS [49]. In the study [3], CN and PCN were successfully synthesized by a facile calcination method and their composites were prepared with P25. All the synthesized photocatalysts were thoroughly characterized by XRD, FTIR, Raman spectroscopy, UV–visible DRS, FESEM and LRTEM-HRTEM techniques. These results are additionally confirmed by comprehensive characterizations, such as XRD, FT-IR, TEM, SEM, BET, UV–vis DRS, XPS, photoelectric performance measurements, and steady-state/transient fluorescence analyses [56].

In general, composite materials are extensively characterized using various techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis/DRS), photoluminescence spectroscopy (PL) and electrochemical impedance spectroscopy (EIS) [47].

6.3. Durability and Recyclability of Selected Composite Materials

The photostability and recyclability of composite materials are key parameters for assessing their practical viability and are of crucial importance for large-scale hydrogen production [68]. Photostability is usually evaluated by longevity under irradiation, expressed in hours of sustained activity, by structural integrity, determined by e.g., XRD, TEM and/or XPS before and after testing, or by the loss of activity over time. On the other hand, recyclability is usually assessed by the number of cycles before a significant drop in photoactivity, recovery efficiency or hydrogen yield after regeneration, depending on the regeneration method. However, it is important to point out that it is very difficult to compare the results published in the literature, as there are no standardized procedures. Therefore, the published results often refer to the comparison of composite materials that differ in their chemical composition or were produced using different synthesis methods, and their testing is often carried out under different operating conditions. Considering the above-mentioned, MOF- and COF-based composites probably exhibit the best photostability and recyclability, while the CdS- and perovskite-based photocatalysts are sensitive due to photocorrosion and exhibit phase instability.

The stability of PCN/Mo-2 was investigated by repeated photocatalytic H₂ evolution cycles without Pt co-catalyst. The H₂ evolution rate of PCN/Mo-2 remained linear and constant over four cycles, demonstrating its satisfactory stability in visible-light-driven photocatalytic H₂ evolution [2]. Four recycling experiments confirmed the stability of SCNO/CoO (0.2) and there no significant deactivation during the H₂ evolution, indicating its excellent potential for long-term sustainable use [7].

The H₂ evolution performance of BZF-5 remained stable over three cycles and showed only minimal changes. In a 7-h long-term photocatalytic test under visible light, BZF-5 showed good stability. These results confirm that the BZF-5 composite photocatalyst exhibits excellent photocatalytic performance and remarkable stability during extended H₂ evolution under visible light [42]. The CdS/ZIS-0.3 heterojunctions, revealed high photocatalytic activity even after six test cycles, demonstrating excellent long-term stability [49]. The Pd/SCN sample (10 h) was subjected to cyclic tests under the same conditions. After four cycles, the photocatalytic H₂ production rate remained high at 259 µmol/h, demonstrating excellent stability and sustained performance [58]. After four cycles, a slight decrease in photocatalytic H₂ production by M@T/ZIS-50 was observed, probably due to mass loss during the photocatalyst separation and recovery process [63].

There are several strategies, which are mostly based on coating technologies, that can be used to reduce the toxicity of the CdS- and perovskite-based composites while simultaneously maintaining or enhancing their photocatalytic performances for H₂ production. For this purpose, the following approaches are most commonly used: (i) encapsulation with protective coatings (e.g., carbon or polymer coating, SiO₂ or Al₂O₃ shell, etc.) to isolate toxic constituents, such as Cd²⁺ and similar components, from leaching into the environment; (ii) doping/alloying to replace toxic elements or passivate their release (e.g., Zn alloying in CdS, production of lead-free perovskites, etc.); and (iii) immobilization on stable substrates to fix the material on non-leachable supports (e.g., encapsulation of CdS in a MOF/COF matrix), etc.

7. Conclusions and Challenges

Photocatalytic H₂ production is a promising approach to tackle the global energy crisis and reduce dependence on fossil fuels. However, achieving high efficiency in this process remains a challenge due to persistent issues such as charge carrier recombination, narrow light absorption ranges, and material instability. The development of non-Pt photocatalysts and material modifications, including doping and heterojunction formation in materials like TiO₂, CoO and g-C₃N₄, is critical to improving performance. TiO₂ (P25) has demonstrated potential as a benchmark photocatalyst, but its limitations in charge separation and stability necessitate the development of next-generation materials that perform more efficiently. Materials such as g-C₃N₄ and CdS show promise but their low efficiency and environmental concerns still limit real application. To maximize the potential of photocatalytic H₂ production, overcoming these barriers is critical for scalability and economic viability. Optimizing the physicochemical properties, especially the electronic structures, of materials is key to developing efficient and cost-effective photocatalysts. Carbon-based materials, especially g-C₃N₄, are characterized by their non-toxic nature, stability, and ease of synthesis. Combining these materials with semiconductors to form heterojunctions or composites can improve charge separation and, thus, photocatalytic performance. In addition, the integration of advanced materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), can further enhance photocatalytic efficiency due to their tunable properties and stability.

Recent advances in photocatalytic H₂ production, such as the integration of materials like phosphorus-doped graphitic carbon nitride (PCN), CoO quantum dots, and MoO_3-x, have led to significant improvements in efficiency. These improvements are largely due to better charge carrier separation, increased stability, and expanded light absorption. For example, the coupling of PCN with TiO₂ (P25) has improved its electrical conductivity and photocatalytic performance, leading to higher H₂ production. Similarly, the incorporation of CoO quantum dots into g-C₃N₄ hollow spheres has enabled improved charge separation, which has increased photocatalytic efficiency. Despite these advancements, challenges remain in optimizing charge carrier dynamics and material stability. Nonmetallic elements, such as boron in CoMoO₄, have improved photocatalytic performance, but high recombination rates and insufficient oxidation capacity are still limiting factors. The use of MoS₂ and MXenes in semiconductor composites has shown improved charge transfer and photocatalytic efficiency and is a promising alternative to platinum-based systems. However, there are still stability and scalability issues with these materials. For example, excessive loading of Ti₃C₂ can block the active sites and reduce photocatalytic performance, while the reactivity of the material in an aqueous environment can lead to decomposition. The scalability of synthesis methods for materials such as MXenes and MoS₂ composites remains a major hurdle. Large-scale production is not yet cost-effective, and the stability of these materials over time needs further investigation. The integration of sustainable materials, including MoS₂ and MXenes, plays a critical role in reducing dependence on precious metals such as platinum, thereby reducing costs and increasing the economic viability of photocatalytic H₂ production.

Further research should focus on improving the scalability and long-term stability of photocatalytic systems. Optimization of material loading, heterojunction design and composite interfaces will be crucial for maximizing photocatalytic efficiency. In addition, structural modifications of semiconductor materials, such as those observed in Ti3C2–TiO2 composites, are crucial for improving light absorption and charge separation. Ultimately, the development of photocatalysts that maintain high stability, recyclability, and efficiency over long periods of time is essential for practical large-scale H₂ production. With continued innovation and refinement, these materials have the potential to revolutionize sustainable H₂ production and contribute to the transition to renewable energy sources.