Recent Progress in Metal Oxide-Based Photocatalysts for CO₂ Reduction to Solar Fuels: A Review

Abstract

One of the challenges in developing practical CO₂ photoconversion catalysts is the design of materials with a low cost, high activity and good stability. In this paper, excellent photocatalysts based on TiO₂, WO₃, ZnO, Cu₂O and CeO₂ metal oxide materials, which are cost-effective, long-lasting, and easy to fabricate, are evaluated. The characteristics of the nanohybrid catalysts depend greatly on their architecture and design. Thus, we focus on outstanding materials that offer effective and practical solutions. Strategies to improve CO₂ conversion efficiency are summarized, including heterojunction, ion doping, defects, sensitization and morphology control, which can inspire the future improvement in photochemistry. The capacity of CO₂ adsorption is also pivotal, which varies with the morphological and electronic structures. Forms of 0D, 1D, 2D and 3DOM (zero/one/two-dimensional- and three-dimensional-ordered macroporous, respectively) are involved. Particularly, the several advantages of the 3DOM material make it an excellent candidate material for CO₂ conversion. Hence, we explain its preparation method. Based on the discussion, new insights and prospects for designing high-efficient metallic oxide photocatalysts to reduce CO₂ emissions are presented.

1. Introduction

According to the report from Carbon Emission Accounts and Datasets for emerging economies (CEADs), China has emitted more than 10 billion tons of CO₂ per year since 2018. It is inextricably linked to people’s desire for a more comfortable and convenient life. Nearly 60% of total emissions emanated from the production of cement, steel and coal-fired power generation processes. As a result of the greenhouse effect caused by ever-growing CO₂, the global average temperature is continuously increasing [1] and various types of severe weather will incur more frequently, such as ice melting, land drought, typhoons, hurricanes, earthquakes and tsunamis [2]. It is, therefore, imperative to convert and utilize the CO₂ present in the atmosphere.

The transformation of CO₂ can be driven by illumination, electricity and heat [3]. Solar energy is a safe, clean, renewable and inexhaustible energy source, so it is ingenious to achieve this conversion with sunlight [4]. Additionally, the reaction conditions in photocatalytic processes are mild [5]; thus, it is easy to conduct experimental tests. A new world opened up since H₂ and O₂ were obtained after radiating TiO₂ with light. Extensive research about photocatalyst were conducted until the year of 2000. Since then, a growing number of materials have been designed and studied to absorb solar energy, including oxide semiconductors, sulfides (ZnS, CdS and MoS₂), and polyoxometalates (Bi₂WO₆, Bi₂MoO₆ and BiFeO₃) [6,7]. Organics, organometallic complexes, covalent organic polymers and noncovalent self-assembled supramolecular organic matter are also involved. Among them, inorganic metal oxide materials are widely studied for establishing efficient artificial photosystems due to their low cost, facile synthesis, stable crystal structures and environmental friendliness. TiO₂, Cu₂O, ZnO, WO₃ and CeO₂ show promising research value as the most common materials.

Despite the significant progress that has been achieved, researchers still have difficulties in developing highly active catalysts because of poor light-harvesting capacity, low CO₂ adsorption capacity and rapid recombination of charge carriers. To this end, theoretical foundation and strategies to upgrade photocatalysts are described in detail in this article. Firstly, the advantages of exposed facets adjustment, and the morphology of 0D, 1D, 2D and 3DOM materials are summarized. Then, common co-catalyst materials are introduced. Defects, ion doping and sensitization engineering are also discussed. We provide a basic understanding on these approaches to inspire the future improvement in photocatalytic field. Furthermore, as inorganic metal oxide materials are more suitable for large-scale production, this paper can serve as a model for future industrialization.

2. Theoretical Foundation and Strategies of Photocatalytic CO₂ Reduction

The photocatalytic process in semiconductors can generally be described as shown in Figure 1. Upon being excited by an incident photon with energy equal to or higher than the bandgap (Eg), charge carriers are generated. Electrons (e⁻) at the bottom of conduction band (CB) migrate to the surface of the catalyst to initiate reduction reactions with CO₂. Holes (h⁺) at the top of valence band (VB) conduct oxidative reactions.

The photocatalytic process of CO₂ conversion on semiconductors can be divided into five general steps: (1) Formation of electron–hole pairs under light radiation, (2) Separation and migration of the electrons and positive holes, (3) Adsorption and activation of CO₂, (4) Redox reactions that between surface-adsorbed species and electron–hole pairs, and (5) Desorption of the product. The electrochemical reactions with standard oxidation–reduction potentials (at pH 7 vs. NHE) are as follows:

H₂O + h⁺ → OH• + H⁺

(1)

H⁺ + e⁻ → H•

(2)

H• + H• → H₂

(3)

CO₂ + e⁻ → CO₂⁻

(4)

CO₂ + 2H⁺ + 2e⁻ → HCOOH, E₀ = −0.61 V

(5)

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O, E₀ = −0.53 V

(6)

CO₂ + 4H⁺ + 4e⁻ → HCHO + H₂O, E₀ = −0.48 V

(7)

CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O, E₀ = −0.38 V

(8)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O, E₀ = −0.24 V

(9)

To carry out the conversion, it is necessary to comprehend the structure of CO₂. Due to the great symmetry and high bond energy of 750 kJ/mol, it is particularly difficult to break the C=O double bond. Therefore, the transmutation between CO₂ and bent radical anion of CO₂^•− on the surface of the catalyst is widely recognized as the first step to activate CO₂ for subsequent reaction. Additionally, photoreactions occur favorably only when the CB position of the catalyst presents a more negative potential than the target reduction and the VB position is more positive than the oxidation reaction. Figure 2A displays the CO₂ reduction potentials of some common semiconductors, along with the Eg positions. From a molecular perspective, the adsorbed CO₂ is combined with e⁻, H⁺ or other intermediates on the surface of the catalyst. On the basis of newly-released reports, the photocatalytic mechanism of CO₂ reduction to CH₄/CO is graphically explained in Figure 2B [8,9,10].

Therefore, enough e⁻ and H⁺ are requisites for the entire photoreduction process. A series of fuels can be obtained by means of well-designed catalysts, such as CH₄ [12,13], CH₃OH [14] and HCOOH [15]. It makes sense to create valuable solar fuel from CO₂, which provides a solution to environmental pollution. Products, alcohols, hydrocarbons and even carbon monoxide can be used as feedstock for energy reserves or high-value compounds.

Figure 3 summarizes the current strategies that can boost the photocatalytic CO₂ reduction pace, with detailed information being presented sequentially below.

2.1. Morphology Control

The surface topography of nanocrystals could evidently alter the electronic structure, surface energy and chemical properties of catalysts. Therefore, morphology control is one of the most important issues that concerns researchers in nanoscience, chemistry and physics. Open facets and edges determine the shape of nanocrystals. Thus, the preferential adsorption of additives on certain crystal surfaces provides a good opportunity to tune the surface of nanomaterials.

2.1.1. Exposed Facet Adjustment

Exploring and figuring out the variation that is connected with exposed surfaces is crucial to elucidate shape-related chemical and physical properties. In semiconductor crystals, different facets have distinct electronic band structures that influence the transport of photoexcited carrier charges. It is wise to expose active facets to tune the CO₂ photoreduction efficiency [16].

2.1.2. Quantum Dots (QDs)

Zero-dimensional (0D) semiconductor quantum dots (QDs) have many unique properties, such as quantum confinement effect, high extinction coefficient and multiple exciton generation [17]. Hence, QDs show much better photoactivity in the visible light region. Unlike bulk materials, surface atoms make up the majority of QD semiconductors. The abundant surface sites enhance the interaction between electron donors and acceptors, thus facilitating the photocatalytic charge transfer rate.

2.1.3. One-Dimensional and Two-Dimensional Structures

One-dimensional nanostructured catalysts have high aspect ratios, such as nanowires, nanorods and nanotubes. The morphological tuning of the material makes a difference to their thermal, optical, electrical and magnetic properties [18]. For instance, the TiO₂ nanotube can act as a channel for electron transfer and build up the chemical reactions rate.

Two-dimensional layered materials can protect a tiny particle component from aggregating. The CO₂ adsorption capacity on the surface of 2D photocatalysts can be enhanced due to the large specific surface area and bountiful surface defects [19]. Two-dimensional lamellar nanosheets are widely used in photocatalysts, such as g-C₃N₄, MoS₂ and WO₃.

2.1.4. Macroporous and Three-Dimensional Ordered Macroporous (3DOM) Structures

Macroporous materials are being widely used in massive photocatalytic materials, owing to their excellent properties [20]. Unlike dispersed particles, sunlight can penetrate the pore wall easily and scatter widely inside the hollow structure, thus increasing the efficiency of illumination. Subsequently, the slender walls of pore reduce the transfer length of photo-generated charge carriers. Electrons (e⁻) and holes (h⁺) are separated more efficiently when heterojunctions are loaded on porous materials. The specific surface it provides is so large that more CO₂ molecules have a chance to contact the catalyst for reduction reactions.

Growing attention has been paid to hierarchical composite pores, including photonic crystal catalysis and separation of sub-microns. The slow light effect of photons associated with 3DOM materials have been considered to increase solar radiation absorption and enhance photocatalyst performance [21]. 3DOM products with periodic macrostructures [22], known as inverse opal, have been applied in battery materials, sensors, separation engineering and heterogeneous catalysis. Thanks to the periodic dielectric constants, Bragg diffraction permits certain wavelengths of light to radiate, leading to stop-band reflection. The limited photons reflected back will slow down at the edge of the stop bands, from which it received its name of “slow photons” [23,24]. It accelerates the light absorption rate when the photon energy is consistent with the absorption spectrum of the 3DOM materials [25]. In addition, the light scattering properties are optimized due to the relation between the macrospores and pore walls. For example, a 3D macro mesoporous Mo: BiVO₄ architecture [26] was designed and fabricated, which had a superior photocurrent density.

It has been demonstrated that gas absorption and separation functions benefit from orderly porous architectures [27]. 3DOM chemicals show superior CO₂ absorption/desorption rates than other commercial products, as the special structure can capture CO₂ from the ambient air by utilizing humidity variation. It is a great way to use microporous catalysts because it not only increases visible light absorption but also shortens the transport time of CO₂. They are available at a low cost through simple and rapid methods.

2.1.5. Preparation of 3DOM Materials

Specifically, 3DOM materials are produced by the colloidal crystal template (CCT) method (Figure 4). A uniform and close-connected organic sphere template can be obtained, firstly, through three key processes. Then, seep metallic salt sol into the void of microspheres. After heat treatment, the organic microsphere template fades away and leaves a metal oxide frame. (i) The polymerizable monomer (methyl methacrylate, styrene) and initiator are mixed and heated under the protection of Ar; (ii) the earlier reaction liquid is filtered with microfiltration membrane; (iii) the microsphere mixture is centrifuged at a high speed for a long time, yielding the polymethylmethacrylate (PMMA) or polystyrene (PS) template; (iv) the template is immersed in the precursor solution and (v) is calcinated.

In the absence of ultra-high temperature and expensive equipment, a fantastic 3DOM structure was obtained. Various materials of frame can also be synthesized by altering the precursor formula.

2.2. Heterojunction

Photogenerated charge carriers in a single material tend to recombine rapidly due to coulombic force and they can be separated efficiently in the multi-materials that are in close contact [28]. The heterojunction is an interface between various semiconductors with different energy band structures. The heterojunction structure can reduce the probability of charge recombination and is therefore considered an effective method to enhance the photocatalytic activity [29,30].

2.3. Defects

All semiconductors have surface defects, which are rooted in the absence of host atoms. The amount of oxygen vacancies can be altered by ion doping and nanocrystal modifications. Oxygen defects and other defects originating from the generated vacancy [31] can facilitate the separation of h⁺/e⁻. Over and above, defects can also activate CO₂ molecules, reducing the activation energy of the reaction [32].

2.4. Ion Doping

Elemental doping is a common strategy to modulate the surface electronic structure. Then, the band gap of the semiconductors make a difference, nonmetal doping (such as N, C and O) mainly alters the VB and metal-doping (e.g., Mo, Co and Ni) influences the CB [33,34]. For example, Cu-doped TiO₂ absorbs more visible light due to the Cu 3d-Ti 3d optical transition [35].

2.5. Sensitization

Sensitization means coupling the quantum dots, dyes, etc., with semiconductors to increase the photogenerated carriers and promote the absorption of light by taking advantage of their receptivity in UV, visible or infrared light.

In the following, exuberant measures to optimize TiO₂-, WO₃-, ZnO-, Cu₂O- and CeO₂-based photocatalysts for CO₂ reduction are demonstrated.

3. Photocatalysts with Different Basis Matrices

3.1. TiO₂-Based Photocatalysts

Titanium dioxide (TiO₂) is notable in photochemistry, with advantages such as non-toxic, cheapness, corrosion resistant, good physical and chemical stability. However, owing to the wide Eg of 3.0–3.2 eV, TiO₂ only absorbs energy in the ultraviolet region (3–5% of the solar energy), and photoexcited charge pairs are easy to combine, resulting in quantum inefficiency. Usually, TiO₂ is divided into the rutile phase and anatase phase on the basis of atomic arrangement modes. Rutile TiO₂ is thermodynamically stable and does not distort or decompose at high temperatures. It has a narrower energy gap (3.0 eV) and a wider spectral response than the anatase phase (3.2 eV). Rutile TiO₂ seeds generally grow larger in size and tend to form an agglomerated structure. Smaller anatase TiO₂ particles have a wider lattice gap and abundant surface oxygen defects, which make it favorable for ion doping and photoreactions.

3.1.1. Morphology Control

In hollow nanotube-shaped catalysts, the transport speed of CO₂ and photoproducts can be facilitated. Ru phase is inclined to form methane in CO₂ hydrogenation process [36] and Yang et al. [37] entrapped Ru nanoparticles in TiO₂ nanotubes. Restricted Ru nanoparticles were resistant to sintering and leaching in the Ru-in/TNT catalyst channel (Figure 5). Electrons tend to gather in the tubes because of a confinement effect, which leads to an abundant, accessible metallic phase. It is easier for high-priced Ru species to combine with free electrons and then exhibit a superb CH₄ and CH₃OH yield.

On the contrary, Kar et al. [38] loaded metal nanoparticles onto vertically oriented 1D TiO₂ nanotube arrays (TNAs) platforms via the graft method. In the synthesis process, Au, Ru and ZnPd NPs grow anodically on transparent glass substrates. There is no band bending phenomenon in Au NP-grafted TiO₂, which can be observed from ultraviolet photoelectron spectroscopy (UPS). TPD experiments proved that all NP-grafted samples absorb more CO₂ than TNAs. It is worth noting that, in nanoparticle-grafted TNAs, blue photons close to and below the TiO₂ band edge were excited to drive CO₂ photoreduction process. Ru-TNAs, ZnPd-TNAs and Au-TNAs had the CH₄ formation rates of 26, 27 and 58 µmol·g^–1·h^–1, respectively. Pt and CoO_x growing on the outer and inner layers of a porous TiO₂-SiO₂ frame, respectively, can act as “warehouses” for e⁻ and h⁺, and the selectivity of the new system for reducing CO₂ to CH₄ can reach 94% [39].

Metal–organic frameworks (MOFs), also known as porous coordination polymers, composed of organic linkers and metal nodes (metal ions or clusters), are a kind of porous crystalline inorganic–organic hybrid materials. The unique advantages of MOFs, such as extremely high surface area, uniform adjustable porous structure and high density coordination unsaturated metal sites, have been extensively researched. They have been applied in many fields, such as gas adsorption and separation, sensing, catalysis, capture and conversion of CO₂. The high specific surface area and uniform porous structure of MOFs make it possible to incorporate metal nanoparticles (as electron acceptors) into their frameworks, leading to efficient charge separation. In addition, the eminent CO₂ adsorption capacity of various MOFs resulted in higher concentration of CO₂ in the pores, which helped to accelerate the photoreaction. In [40], a porous material-zirconium-based organic skeleton (UiO-66) was introduced to a TiO₂ photocatalyst as an effective CO₂ adsorbent. The designed two-step strategy endowed the TiO₂/UiO-66 composite with abundant graded pore structure, thus ensuring sufficient catalytic sites and high CO₂ adsorption capacity (78.9 cm³ g⁻¹). The ultrafine TiO₂ nanoparticles were loosely loaded on the UiO-66 surface rather than tightly packed due to the electrostatic repulsion, thus ensuring the exist of microporous of the MOF. Finally, in the weak gas–solid catalytic system with water as the electron donor, the yield of CH₄ was up to 17.9 μmol g⁻¹ h⁻¹, and the selectivity was 90.4%. Additionally, the photocatalytic efficiency was comparable to that of pure CO₂ atmosphere even under low CO₂ concentration conditions (≤2%).

Exposed Facet Adjustment

A Pt-TiO₂ single atomic site catalyst (PtSA/Def-s-TiO₂) was prepared [41] by the “thermal solvent-argon treatment and hydrogen reduction” method. In order to construct Ti–Pt–Ti structures, TiO₂ nanosheets with oxygen deficient sites were used to anchor monatomic Pt particles, which can retain the stability of isolated single atomic Pt and improve photocatalytic performance. The exposed (101) and (001) crystalline of TiO₂ nanosheets (Figure 6A,B) were determined by transmission electron microscopy, and a thickness of 6.9 nm was observed through atomic force microscope. The EPR spectra of the samples confirm that the rich oxygen defect structure can be obtained by heating TiO₂ nanosheets in argon atmosphere. They also indicated that single-atom Pt junctions were formed by occupying the oxygen defect sites, which provides a benchmark for the rational design of highly active and stable single-atom catalysts on metal oxide carriers with defect structures. CH₄ product can be detected when copper oxide nanoparticles are mixed with mesoporous TiO₂ nanorods in close contact [42]. Both the special crystal plane and porous structure contribute to furthering the CH₄ yield.

3DOM Structure Ti-Based Materials

A ternary 3DOM Bi-doped TiO₂ photocatalyst decorated with carbon dots (CDs) was obtained, whose pore engineering of the 3DOM skeleton greatly promoted the response in the whole solar spectrum range [26]. It exhibits enhanced photocatalytic performance because of its excellent exquisite structure and high charge transfer efficiency. Similarly, a BiVO₄/3DOM TiO₂ nanocomposite [43] was synthesized as a highly efficient photocatalytic catalyst for the degradation of dye pollutants. Further studies on its textural, optical and surface properties revealed that connecting pores not only improve the electron transfer rate between coupled materials, but also provide abundant active sites for reactant molecules.

3DOM TiO₂ were prepared [44] by the CCT method (Figure 7A) and CeO₂/3DOM TiO₂ samples (Figure 7B) were obtained by the original bubble-assisted membrane precipitation method. The introduction of CeO₂ nanolayers broadened the photo-absorption range and facilitated the separation of photogenerated electron−hole pairs. The mesoporous structure provided a larger surface area and the catalyst exhibited a higher CO₂ reduction activity.

Pt-particle-decorated 3DOM carbon-coated TiO₂ and g-C₃N₄ were combined [45] to construct an all-solid-state catalyst for CO₂ artificial photoconversion. Slow photon effect and carbon coat optimized the absorption capability of light. The exquisite design drove vectorial electrons from TiO₂@C to Pt particles and then fell to g-C₃N₄, which facilitated the carrier separation. The Z-scheme that consist of two isolated systems has three components and converts CO₂ to CH₄ with H₂O at a yield of 65.6 μmol g⁻¹ h⁻¹. To enhance the surface enrichment of CO₂, Wu et al. [46] fabricated 3DOM perovskite-type Pt_n/SrTiO₃, in which Pt nanoparticles can take in photoelectrons from SrTiO₃ and transfer CO₂ to CO and CH₄ (Figure 8). Pt₂/3DOM SrTiO₃ exhibited the highest CH₄ yield of 26.7 μmol g⁻¹ h⁻¹.

A two-dimensional MoS₂ layers/3DOM TiO₂ photocatalyst was prepared [47] to form heterojunctions, which had a higher performance in the range of 420–900 nm. Cu single atoms were uniformly distributed in 3DOM TiO₂ via the in situ method [48], which provides active sites for CO₂ photoconversion. The main products tested in the gas–solid two-phase system were CH₄ and C₂H₄ with the corresponding generation rates of 43.15 and 6.99 μmol g⁻¹ h⁻¹. Chen’s work provided a new perspective for improving the catalytic efficiency by regulating reaction conditions. Jiao et al. [49] loaded core-shell structure AuPd NPs in 3DOM TiO₂ via one pot of the gas bubbling-assisted membrane reduction method to form a functional photocatalyst. The low Fermi level of AuPd NPs empowered the catalyst system to trap electrons and enhance the separation of charge pairs. Carbon-quantum-dot-decorated 3DOM CaTiO₃ photocatalysts were duly obtained [50], exhibiting an apparent quantum efficiency (QAY). The macro–meso–microporosity structure provided improved charge carrier separation and transport, and it was explored in depth through density functional theory calculations (DFT) and finite difference time-domain simulations.

3.1.2. Heterojunction

p–n heterojunction: A p–n heterojunction is formed by combining p-type and n-type semiconductors. Even without light irradiation, electrons can diffuse from an n-type semiconductor to a nearby p-type one, in the case of the combination of two materials. Correspondingly, the holes on the surface of a p-type semiconductor are transferred to the n-type one, which results in an efficient separation of charge carriers. A ZnFe₂O₄-modified TiO₂ was synthesized by the hydrothermal method [51], and the p-n heterojunction system could reduce CO₂ to methanol at a yield of 75.34 μmol g⁻¹ h⁻¹.

rGO composite: In recent years, graphene materials have been widely used because of their large specific surface area, unique thermal stability and excellent electrical conductivity. Graphite nanomaterials are visible-light-responsive materials with appropriate band gaps, and the energy levels of CB and VB are in optimal positions relative to ordinary hydrogen electrodes. These unique photocatalytic properties have made them prime candidates for photocatalytic CO₂ reduction. Fortunately, tightly contacted ultra-thin graphene layers and TiO₂ compounds and can be prepared with some additives [52]. Seeharaj et al. [53] employed high-intensity ultrasonic waves (ultrasonic horn, 20 kHz, 150 W/cm²) to exfoliate the TiO₂ surface, which led to a highly specific active area and highly reactive nanosheets. The modification of tiny rGO and CeO₂ on the rGO nanosheet surface can improve the CO₂ absorptivity and the charge carriers’ migration efficiency of the catalyst (Figure 9). A kind of d–π electron orbital overlap was formed between TiO₂′s d orbitals and rGO’s π orbitals, which provides a good environment for activated CO₂ and electrons. The complex heterojunction photocatalysts TiO₂/rGO/CeO₂ exhibited high yields of CH₃OH (641 μmol/gcath) and C₂H₅OH (271 μmol/gcath).

BCN composite: The boron carbon nitride (BCN) composite BCN is an adjustable band gap material that has been applied in CO₂ reduction, water splitting and as a detoxification catalyst. Highly negative CB potential of BCN materials makes them suitable for the construction of S-scheme heterojunction, and Kumar et al. [54] designed a novel S-scheme Fe@TiO₂/BCN composite. In situ XPS technology, DFT calculations and finite difference time-domain simulations were adopted to verify the S-scheme transfer mechanism. Under visible light irradiation, the intimately contacted heterojunction reduced the probability of charge recombination. The sample exhibited excellent photocatalytic activity; in addition to converting CO₂ selectively, it also degraded tetracycline antibiotics.

TMS composite: Some transition metal sulfides (TMSs) exhibit the same properties as Pt or Pd in photocatalyst. They form longer-lived charged carriers within the S_3p orbital [55] with a highly negative reduction potential. However, the self-oxidation of metal sulfides confuses researchers and limits their application. It can be alleviated by constructing a Z-scheme, and thus researchers introduced it to Bi-modified TiO₂ [56]. Meanwhile, a spatially coupled heterojunction was enhanced by regularly capsulated CuCo₂S₄ yolk–shell hollow sphere.

3.1.3. Ion Doping

In recent years, elements such as B, N, Co and Bi have been widely applied in TiO₂ doping. A carbon-based hybrid nanocomposite reduced graphene oxide (rGO), belonging to the narrow band gap, with oxygen-containing functional groups on the surface that can be enhanced by π interactions [57]. Laminar graphene carriers not only prevent TiO₂ repolymerization, but also hybridize the function of the catalytic system. Co-doped TiO₂ was loaded on the rGO [58], and the Co peak in EDX spectra and C-O peak in FT-IR spectra confirmed the successful doping and the presence of graphene support, respectively. The size of TiO₂ particles decreased from 48–80 nm to 23–28 nm, which is consistent with earlier reports of changes in titanium doping with transition metal ions.

3.1.4. Sensitization

A growing number of semiconductor materials are being used to modify TiO₂ dioxide, but randomly mixed catalysts are not stable enough to achieve reproducibility. Therefore, Lee [59] grew well dispersed p-type NiS nanoparticles on the surface of a highly aligned n-type TiO₂ film to obtain the NiS-sensitized TiO₂ films. The band gaps of two components were estimated by wavelength relation. Some inferences can be drawn when considering the results of both the ultraviolet and visible spectra. It indicates that more electrons are subpoenaed from the short-Eg NiS and transferred to TiO₂ conduction band. The spectra results reconfirmed the electron contribution of the NiS and the design of a catalyst that produced 3.77-fold CH₄ compared to the TiO₂ film.

3.1.5. Summary

Overall, some of the photocatalytic systems that use TiO₂ is presented in

Table 1. Summary of the recent results of the photoreduction of CO₂ using TiO₂-based materials.

Type of Enhancement	Photocatalyst	Description	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction	Ag-NPs/TiO2	Core/shell	200 W Hg	CO 983 CH4 9.73	[60]
Z-scheme	CuCo2S4@3B-TiO2	CCS yolk–shell hollow spheres, Bi-modified TiO2	300 W Xe	CO 25.5 CH4 42.2	[56]
	Ag-Cu2O/TiO2	P25		CO 13.19 CH4 1.74	[61]
S-scheme	Fe@TiO2/BCN	Fe@TiO2/Boron Carbon nitride	300 W Xe 400 nm	CH4 24.7	[54]
p–n heterojunction	ZnFe2O4/TiO2	Spherical and irregular shapes	500 W Xe	CH3OH 75.34	[51]
Ion doping	Co-doped TiO2/rGO	Equiaxed grain morphologies	500 W Xe	CH3OH 133.7	[58]
3DOM	3DOM Pt@CdS/TiO2		420 nm irradiation light, UV cutoff filter	CH4 36.8	[62]
	AuPd/3DOM-TiO2		300 W Xe	CH4 18.5	[28]
	Pt2/3DOM-SrTiO3		300 W Xe	CH4 26.7	[46]
	g-C3N4/Pt/3DOM TiO2@C		300 W Xe 420 nm	CH4 65.6	[45]
	SnS2/3DOM SrTiO3		300 W Xe 420 nm	CH4 12.5	[63]
	Cu0.01/3DOM TiO2		Xe (320–780 nm)	CH4 43.5	[48]
	MoS2/3DOM TiO2		300 W Xe 420 nm	CH4 11.6	[47]
	3DOM CeO2/TiO2		300 W Xe 420 nm	CO 3.73	[21]

.

3.2. WO₃-Based Photocatalysts

Tungsten-based oxides (WO₃) have been extensively studied in recent decades and various morphologies have been presented. In the WO₃ structure, the crystal in the stoichiometric ratio is connected with a twisted WO₆ octahedra to form a perovskite crystal structure. It has monoclinic, orthorhombic and hexagonal crystal forms. At the same time, the oxygen lattice can be lost easily, resulting in oxygen vacancies and unsaturated, coordinated W atoms. Therefore, tungsten oxide has many non-stoichiometric compounds, such as WO_2.72, WO_2.8, WO_2.83 and WO_2.9. Of these, WO₃ is the most common and has been widely studied as a typical photocatalytic water oxidation semiconductor material. WO₃ is a typical narrow-band gap indirect semiconductor with a forbidden band width of 2.6–2.8 eV, which can absorb part of the visible light [64]. In addition, WO₃ is a research hotspot in the field of photoelectrochemical water splitting because of its high carrier mobility, stability in acidic electrolytes and resistance to photocorrosion.

3.2.1. Morphology Control

Bi₂WO₆ is one of the tungsten-based materials that belongs to Aurivillius crystal oxides. Its crystal has an orthorhombic system, and its narrow band gap (2.7–2.9 eV) structure allows it to meet the response absorption of visible light. Moreover, its stable structure and eco-friendly properties have attracted many scientific researchers to study it. Since the valence band of Bi₂WO₆ is composed of O_2p and Bi_6p, and the conduction band is composed of W_5d-assisted Bi6p orbitals, the VB energy levels can be dispersed broadly. By employing the Kirkendall effect in ion exchange and BiOBr precursor, Huang et al. [65] prepared a bowl-shaped Bi₂WO₆ HMS material. Based on the large specific surface area of the material, its adsorption capacity for CO₂ reaches 12.7 mg g⁻¹ at room temperature and pressure. The material adsorbs a large number of HCO₃⁻ and CO₃²⁻ species on the surface during the reaction, which makes the catalytic reaction easier. The Bi₂WO₆ HMS thus has a high catalytic activity, and the methanol yield is 25 times higher than that of the Bi₂WO₆ SSR.

Iron phthalocyanine FePc is neatly assembled on porous WO₃ under induction and coupled with surface atoms by H-bonding [66]. The optimized FePc/porous WO₃ nanocomposites exhibit enhanced CO₂ photoreduction activity, which is attributed to the synergistic effects of a high specific surface area, a better charge separation and proper central metal cation. A series of mesoporous WO₃ with interconnected networks were synthesized by the silica KIT-6 hard template method [67], which became oxygen-deficient after hydrogenation treatment. Both the ordered porous structure and oxygen vacancies contributed to the increased yield of CH₄ and CH₃OH.

WO₃ with a hollow nest morphology with hierarchical micro/nanostructures (HNWMs) was synthesized [68] by the one-step hydrothermal method (Figure 10), with a particle diameter of about 2.5 μm. The 2D nanosheets, which have an average thickness of 30–40 nm, were assembled to build a distinctive hollow nest structure with a good stability and reusability under visible light. Hao et al. [69] prepared core–shell heterojunctions of two-dimensional lamellar WO₃/CuWO₄ by the in situ method. After the modification of amorphous Co-Pi co-catalyst, the photoanode of ternary homogeneous core–shell structure exhibited a high photocurrent of 1.4 mA/cm² at 1.23 V/RHE, which was 6.67 and 1.75 times higher than that of the pristine WO₃ and 2D homogeneous heterojunction. Ren et al. [70] synthesized unique flower-like Bi₂WO₆/BiOBr catalysts by the simple one-step solvothermal method, and showed that the photocatalytic activity of the composites was significantly enhanced due to the construction of type II heterojunctions. The presence of Br source enhanced the light absorption and improved charge-carrying spatial transfer and separation.

Ti atoms in ultrathin Ti-doped WO₃ nanosheets promoted the charge transfer [71], as they accelerate the generation of key intermediates COOH*, which was revealed by in situ characterization. Furthermore, Gibbs free energy calculations were calculated to verify that ion doping can reduce the CO₂ activation energy barrier and CH₃OH desorption energy barrier by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH₃OH. The ultrathin Ti-WO₃ nanosheets showed an excellent CH₃OH yield of 16.8 μmol g ⁻¹ h ⁻¹. Two-dimensional bilayered WO₃@CoWO₄ were prepared [72] via a facile interface-induced synthesis method. The optical energy conversion efficiency can be improved by both p–n heterojunctions and interfacial oxygen vacancies. The narrow band gap of the WO₃@CoWO₄ heterojunction was proved by DFT calculations and some characterizations, which allows a better visible light absorption. A tree-like WO₃ film was prepared [73] by the hydrothermal process, which has a large specific surface area. The WO₃ product was a unity of hexagonal/monoclinic crystals, which contained W⁵⁺ defects and oxygen vacancies. The products were further subjected to a mild reduction solution at the lower temperature of 333 K to introduce more defects. It turns out that the intermediate state induced by defects diminished the band gap. A reasonable amount of defects benefits the photocatalytic activity of WO₃, while too many defects impair its catalytic capacity. The performance of the treated WO₃ films increased 2.1 times in 48 h compared to that of the annealed WO₃ samples.

Preferentially Exposed Facets

According to studies, infrared (IR) light makes up nearly 50% of solar energy, and it is challenging to make use of the majority of the light. Liang et al. [74] fabricated 2D ultrathin WO₃ with an intermediate band gap. They achieved the first complete decomposition of CO₂ driven by infrared light without the addition of sacrificial agents. Theoretical calculations indicated that the generation of the intermediate energy band resulted from the critical density of the generated oxygen vacancies, which has also been verified by synchrotron valence band spectroscopy, photoluminescence spectroscopy, ultraviolet-visible-near-infrared spectroscopy and synchrotron infrared reflection spectroscopy. The results showed that the WO₃ atomic layer containing oxygen vacancies can achieve the complete decomposition of CO₂ and generate CO and O₂ under infrared light.

Microscopic WO₃ nanocrystals were formed by Chen et al. [75] through solid–liquid phase arc discharge in an aqueous solution. Then, they synthesized ultrathin single-crystal WO₃ nanosheets via a laterally oriented attachment method. The quantization effect of this nanostructure altered the bandgap width of WO₃ nanosheets, enabling the semiconductor to exhibit a high performance at a wide range of nanometer sizes. It is beneficial to control the activity and selectivity of the photoconverted CO₂ products. As a consequence, WO₃ with a strong visible light response has enormous potential in the field of photocatalytic CO₂ reduction.

Bi₂WO₆ has a positive conduction band potential, which is not enough to excite and reduce CO₂ molecules, thus limiting its application in the photocatalytic reduction of CO₂. Therefore, researchers have been devoted to modifying the surface of Bi₂WO₆ to improve its photocatalytic activity in order to obtain a high-efficiency CO₂ reduction ability. Zhou et al. [76] successfully prepared Bi₂WO₆ nanosheets with a monolayered structure by introducing the surfactant CTAB (hexadecyltrimethylammonium bromide) into the precursor solution. A large number of unsaturated Bi atoms were produced, which provided sufficient active sites for photocatalytic reactions. Bi₂WO₆ is composed of a [BiO]⁺-[WO4]²⁻-[BiO]⁺ sandwiched layer structure. Under irradiation, holes and electrons are generated in different [BiO]⁺ layers. This structure promotes the spatial separation of photogenerated electron–holes and greatly reduces the carrier recombination rate of the monolayer Bi₂WO₆ material.

With the assistance of an oil-based primary amine (C₁₈H₃₇N) surfactant, Zhou et al. [77] conducted a hydrothermal reaction at 200 °C for 20 h to prepare an ultra-thin and uniform Bi₂WO₆ nanosheet. The material has a strong response under visible light, and its forbidden band width was about 2.44 eV through theoretical calculations, with a conduction band potential of −0.31 e V. Then, CO₂ could be easily reduced to CH₄, and its yield was 20 times higher than that of SSR Bi₂WO₆. The light-absorbing capacity will decrease if the nanosheets are too thin because of the quantum size effect. Therefore, scholars need to consider roundly when designing fresh material [78].

DOM Structure W-Based Materials

Unexpectedly, it was found that the resistance of 3DOM-WO₃(270) and the Ag₃PO₄ electron absorption band were comparable. By depositing Ag₃PO₄ nanoparticles in the micropores of 3DOM-WO₃, Chang et al. [79] achieved a higher photocatalytic activity and more efficient light harvesting at the wavelengths of 460–550 nm. A Z-scheme g-C₃N₄/3DOM-WO₃ catalyst designed by Tang et al. [80] also has a high CO₂ photoreduction activity. The separation way of the photogenerated electron–hole pairs determined its Z structure, and 3DOM framework heightened the light collecting efficiency. Therefore, an excellent photocatalyst exhibited a high CO evolution rate of 48.7 μmol g⁻¹ h ⁻¹.

3.2.2. Heterojunction

Quantum dot composite: CuO quantum dots (QDs) were combined with WO₃ nanosheets by a self-assembly method and the diameter of 6%CuO QDs/WO₃ NSs was mainly located at 1.6 nm [81]. The bandgap energy of CuO/WO₃ fell in 2.28 eV and the complex catalyst possessed a lower resistance for charge carrier transfer that showed in UV-vis DRS and EIS analysis. Due to the low CB position, CO cannot be obtained when using pure WO₃. However, the photogenerated electrons gathered in the WO₃ CB position was able to reach the CuO VB position when the Z-scheme (Figure 11) was formed by intimate heterojunctions. At the same time, the reduction reaction that transformed CO₂ into CO occurred at the CuO CB position. The high yield rate of about 1.58 mmol g⁻¹ h⁻¹ also benefited from a longer fluorescence lifetime, and reduced the overlap of electron pore pairs.

Perovskite composite: For WO₃, the negative potential energy of the sheet shape (−31.5 mV) was lower than that of the rod (−21.0 mV). In addition to its potential advantages, S–WO₃ offers a higher specific surface area. Defects occurred because of the exposed interior atoms in nanosheets surface, which promoted the CO₂ adsorption. Positively charged perovskite cesium lead tribromide (CsPbBr₃, CPB) with a long electrically diffused length was selected to be combined with S–WO₃, affording a high field rate of CO and CH_4. Zhang and others [82] employed a three-dimensional hydrophobic porous melamine foam to support the mixture, not only to protect the CPB from dissolution but also to reduce toxic Pb²⁺.

3.2.3. Ion Doping

Molybdenum with a similar ionic size was chosen to dope the WO₃ as a low-valence metal species. The W⁵⁺/W⁶⁺ ratio of the catalyst was increased, making it easier to exchange electrons with reactants. The conductivity of protons was enhanced by the presence of hydrogen bronze, which originated from a chemical reaction between WO₃ and Brønsted protons and excess electrons in their lattices. Wang et al. [83] prepared molybdenum-doped WO₃·0.33 H₂O by the hydrothermal method. The E_cb and E_vb energy were both higher at 3%Mo-WO than WO₃. After 20 min of the FTIR spectra, rare intermediate CO²⁻ was observed, verifying the activation of CO₂, which was more common in low-valent meta species. The content of potassium hydroxide in an aqueous solution obtained by photocatalytic water oxidation was higher and the CH₄ yield was 4.2 times higher than WO₃.

3.2.4. Summary

To date, Bi₂WO₆ semiconductor photocatalytic materials have made great progress in the field of environmental management. Some photocatalytic systems using WO₃-based materials in CO₂ conversion are listed in

Table 2. Summary of the recent results on the photoreduction of CO₂ by WO₃-based materials.

Type of Enhancement	Photocatalyst	Description	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction	Pd-Au/TiO2–WO3	0.5%Pd−0.1 wt%Au	400W Hg	CO 225.5 CH4 15.1	[84]
	Bi2WO6/Si	Bi2WO6: Si = 1:1	300 W Xe 420 nm	C2H5OH 201	[85]
S-scheme	CdS/WO3	CdS nanoparticles at the WO3 surface	300 W Xe 420 nm	CO 35.625 CH4 3.75	[86]
	WO3/CsPbBr3	3D porous melamine foam-supported	300 W Xe	CO+CH4 600	[82]
Z-scheme	WO3-TiO2/Cu2ZnSnS4	Mesoporous ternary heterostructure	400 W Xe420 nm	CO 15.37 CH4 1.69	[87]
	WO3/g-C3N4	3D/2D hollow microspheres	35 W HID car lamp and 300 W Xe (H2 reduction)	CO 145 CH4 133	[88]
	TiO2/WO3/Pt	1D continuous fibrous structure	300W Xe	H2 128.66	[89]
	CuO Dots/WO3	CuO quantum Dots/WO3 nanosheets	300 W Xe400 nm	CO 1.58	[81]
Ion doping	3%Mo-WO	Mo-doped WO3·0.33H2O nanorods	500 W Xe	CH4 5.3	[83]
3DOM	g-C3N4/3DOM-WO3	300 W Xe 420 nm	visible light (≥420 nm)	CO 48.7 CH4 7.5	[80]

.

3.3. ZnO-Based Photocatalysts

ZnO, a common metal oxide, is a n-type semiconductor with an Eg value of 3.37 eV. It is a kind of amphoteric oxide that has the advantages of nontoxic harmlessness, low cost, abundant reserves, convenient preparation, low dielectric constant and low optical coupling rate. ZnO has three main lattice structures: wurtzite structure, zinc-blended structure and tetragonal rock salt structure. The wurtzite structure is considered the most stable and common structure in nature. It is a kind of hexagonal crystal, in which the O and Zn atoms are aligned with the hexagonal density stacking. The photodegradation process of ZnO is similar to TiO₂ and it has been widely used in photocatalysts, solar cells and conducting materials.

3.3.1. Morphology Control

The 3nm Pt particles were uniformly dispersed over ZnS@ZnO with a mesoporous heterostructure [90] and more CH₃OH was obtained. Reactant charge carriers entered the pore channels of the porous heterozygous layer, thus reducing the likelihood of flow resistance and electron–hole recombination. The S-scheme photocatalyst delivered a high CH₃OH formation rate of 81.1 μmol g⁻¹ h⁻¹, which is roughly 40 and 20 times larger than that of bare ZnO (3.72 μmol g⁻¹ h⁻¹) and ZnO–ZnS (4.15 μmol g⁻¹ h⁻¹). On the other hand, a porous ZnO@ZnSe core/shell nanosheet array material (Figure 12A) was prepared in a controlled manner [91]. The final n-type semiconductor composites had a proper negative CB band edge. In comparison to ZnO or ZnSe, more pairs of electron–holes were formed under visible light. Electrons tend to land on ZnO, which is aimed at methanol production. Mei et al. prepared a ZnO microsphere with different numbers of shells [92] and the photoelectric performance of ZnO was optimal when the number of shells reached three.

It is challenging to coat uniform 2D g-C₃N₄ nanofilm on the surface of 3D materials because of the difficulty in exfoliation process. Thus, Wang et al. [93] proposed an electrostatic method and incorporated g-C₃N₄ nanofilm with porous ZnO nanospheres that has a strong interaction. The heterojunction was then anchored on 3D graphene aerogels (GAs). The compound g-C₃N₄/ZnO/GA has extraordinary stability, maintaining a high CO₂ conversion rate, which is 92% of its original activity after 100 h.

ZnO/ZnS nanoflowers (Figure 12B) were combined with g-C₃N₄ nanosheets (Figure 12C) to construct a double Z-scheme structure [94]. ZnO/ZnS nanoflowers provide a large specific surface area and g-C₃N₄ helps to absorb more photons under solar light irradiation. Optimized interfacial charge transfer dynamics in ternary heterostructure can be characterized by photocurrent measurements. As a result, the formation rate of H₂ product over the novel double Z-scheme mixture increases to 301 μmol g⁻¹ h⁻¹ on water splitting.

Hierarchical CuO/ZnO nanocomposites with p-n heterojunction were prepared [95] by the modified hydrothermal method. The photocatalysts are found to converse CO₂ to methanol in aqueous solution containing dimethylformamide (DMF) and triethylamine (TEA) as electron donor under visible light irradiation. ZnO/NiO porous hollow spheres with sheet-like subunits were obtained [96] by calcination of Ni-Zn MOFs. Numerous p-n heterojunctions with n-type ZnO and p-type nickel monoxide were formulated in mixed ZnO/NiO. The porous hollow structure with the large specific surface area can increase the absorption capacity of CO₂ and light.

Based on the vapor to solid mechanism, a novel ternary Ag/CeO₂/ZnO nanocomposite [97] was synthesized by the facile thermal decomposition method. Oxygen vacancies introduced by line structure contributied to a narrow band gap of 2.66 eV, and this was further confirmed by DRS characterization. After the preparation of the ZnO/TiO₂ nano-tree arrays, Ag₂S and ZnS were synthesized to modify the nano-tree arrays by cation exchange methods [98]. The core-shell structure of ZnO@ZnS prevented the decomposition of ZnO, and the modification of Ag₂S reduced the Eg value of the composite and promoted the red-shifted of light absorption.

3.3.2. 3DOM Structure Zn-Based Materials

Wang et al. [99] published metal-organic-framework-derived 3DOM N-C doped ZnO (Figure 13) for efficient CO₂ reduction. The ultra-tiny CoO_x clusters were anchored on the surface of catalyst and no Co-Co peak was found in CoO_x/N-C-ZnO. The charge transfer rate was jacked up by ion doping and the recombination of electron-hole pairs was tamed because of the CoO_x clusters. Furthermore, CoO_x on the orderly connected channels can act as an electron trap to capture electrons, which makes a contribution to photoreaction efficiency. The density theory calculations (DFT) was also used to detect the CO₂ adsorption ability, and CoO_x/N-C-ZnO exhibited the most negative CO₂ binding energy due to improved electron structure of adsorption site.

3.3.3. Heterojunction

Recently, zeolitic framework (ZF) composite fabricated by the microwave-hydrothermal synthesis method (MWH) has attracted attention, which can provide a fast heating-speed and produce morphologically uniform samples. With biodegradable template, the zeolitic framework (ZF) was synthesized via MWH method from volcanic ashes. The NaAlSiO₄ (NAS) framework was composed of 50 nm circular channels and has a large surface area. Hip’olito et al. [18] embedded ZnO/CuO hybrid structure in the NAS channels, resulting in a ternary composite. The synergistic effect among ZnO, CuO and ZF support accelerated the photocatalytic process of water splitting and CO₂ reduction, which offers higher H₂ and HCOOH evolution rate.

The imidazole framework-8 (ZIF-8) molecular as a widely used zeolite (ZIF) composite, has an excellent competence of absorbing CO₂, which is apt for CO₂ converting. Selective-breathing effect was wielded to boost CO₂ conversion efficiency through monolithic NF@ZnO/Au@ZIF-8 (Figure 14A) catalyst [100]. Au particles were loaded on ZnO nanorods that grown on Ni foam (NF), the mixture was immersed in methylimidazole solution (Figure 14B). Based on the results of electrochemical impedance spectroscopy (EIS) Nyquist plots, the alternating magnetic field was introduced to create magnetic heat, which leading to increased carrier density and improved photocatalytic performance. It is found that the selectivity of CH₄ cheeringly achieved 89% from 61% under photo-thermal-magnetic coupling effect.

3.3.4. Ion Doping

Many metal elements have been doped in ZnO to minish its band gap, such as Sb, Cu and Mn, among them Co distinguished itself because of the similar ion radius. In addition, the conversion of Co³⁺ and Co²⁺ results in oxygen vacancies, which greatly enhances photocatalytic efficiency. Xie et al. [101] introduced Co³⁺ with different mole ratio to ZnO microspheres precursor (including Zn²⁺, urea, and PVP), and the introduction of Co³⁺ did not disrupt lattice structure as seen in the XRD model. It was revealed that the presence of both Co²⁺ and Co³⁺ in Co-ZnO from high-resolution spectrum, and Zn_2p exhibited higher binding energy due to Zn²⁺ charge transfer in 7% Co-ZnO. As the ratio of Co/Zn increased, the conversion from Co²⁺ to Co³⁺ decreased, and the light response range gradually expanded. The results showed that the electrochemical impedance of 7% Co-ZnO sample was the lowest band gap of 2.56 eV.

3.3.5. Summary

Great efforts have been made to design catalysts for CO₂ reduction on ZnO and the relevant research data are summarized in

Table 3. Summary of the recent results on the photoreduction of CO₂ by WO₃-based materials.

Type of Enhancement	Photocatalyst	Description	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction	Cu-ZnO Cel-T	Cellulose template designed porous ZnO	300 W Xe	CO 30.17	[102]
	OD-ZnO/C	Carbon dots, spherical morphology	400 W Xe	CO 118.8(600 °C)	[103]
	Pt/ZnO–ZnS	Porous ZnS–ZnO	300 W Xe, 420 nm	CH3OH 81.8	[90]
Z-scheme	Ag-Cu2O/ZnO	0.6Ag−0.4Cu2O ZnO nanorods	300 W Xe	CO 3.36	[104]
	g-C3N4/ZnO/graphene aerogel	Porous ZnO nanosphere	300 W Xe, 420 nm	CO 33.87	[93]
	ZnO/ZnS/g-C3N4	Spherical ZnS, ZnO nanoflowers	300 W	H2 301	[94]
p–n heterojunction	ZnO/CuO/Zeolite	ZnO/CuO in NaAlSiO4 channels	two 20 W halogen lamps	H2 62.3 HCOOH 907	[18]
Ternary catalyst	R-ZnO@ LDH	Core–shell Structure, belt-like ZnO hierarchical LDH	300 W Xe	CH4 11.4	[105]
3DOM	CoOx/N-C-ZnO	N, C doped	420 nm	CO 26.4	[99]

.

3.4. Cu₂O-Based Photocatalysts

Cuprous oxide (Cu₂O) is a potential p-type semiconductor with a wide visible-light response range and high photo-electric conversion efficiency (18%) [106], and it displays attractive prospects in solar energy conversion and heterogeneous photocatalysis. Although Cu₂O possesses many excellent properties, photocorrosion and the rapid recombination of e⁻/h⁺ pairs affect its activity and limit its application. The photocorrosion is believed to occur in two ways: (1) self-reduction caused by generated electrons and (2) self-oxidation caused by the generated holes.

Self-reducing photocorrosion: Cu₂O + H₂O + 2e⁻ → 2Cu + 2OH⁻

(10)

Self-oxidative photocorrosion: Cu₂O + 2OH⁻ + 2h⁺ → 2CuO + H₂O

(11)

Therefore, developing Cu-based catalysts with excellent activity, selectivity and stability has become the research hotspot in the area of the photocatalytic reduction of CO₂. Many successful attempts have been made to improve the photostability and photocatalytic performance of Cu₂O. In general, most studies focus on enhancing the charge transfer from Cu₂O to reactants or cocatalysts to prevent charges from accumulating within the particles. A series of methods for improving the performance of Cu₂O are discussed in detail in what follows.

3.4.1. Morphology Control

A branch-like Cd_xZn_1-xSe nanostructure was obtained [107] by the cation-exchange method, which was then mixed with Cu₂O@Cu to form heterojunctions. Selenium (Se) vacancies were created during the ion exchange process and the crystal growth was limited due to the additive diethylenetriamine (DETA), leading to insufficient coordination of the surface atoms, which then become active adsorption sites. Highly hierarchical branching-like structures assembled by one-dimensional structural materials not only facilitate electron accumulation at their tips but also increase the light-accepting area, and characterization results show that branching structures can effectively absorb visible light. Cd_0.7Zn_0.3Se/Cu₂O@Cu step-scheme heterojunction exhibited a CO release yield of 50.5 μmol g⁻¹ h⁻¹.

Ultrafine cuprous oxide U-Cu₂O (<3 nm) was grown on the polymeric carbon nitride (PCN) (Figure 15) by the in situ method [108]. PCN has a narrow band gap of 2.7 eV that can capture visible light. Both ultrafine nanoclusters and Z-scheme heterojunction can protect U-Cu₂O from degradation. The photocatalyst U-Cu₂O-LTH@PCN has high stability, maintaining more than 95% activity after five cycles of testing, while bare Cu₂O grades completely within three cycles. A large number of heterojunctions were formed by U-Cu₂O particles and lamellar PCN, expediting the electron transfer efficiency. The product can convert CO₂ to methanol with water vapor under light irradiation at the high yield of 73.46 μmol g⁻¹ h⁻¹. Ultrathin Ti₃C₂MXene with a high fraction of coordinated unsaturated surface sites was fabricated by Zhang et al. [109]. Via the HF etching method, different amounts of Cu₂O were combined with Ti₃C₂ nanosheets under the hydrothermal condition. The unique hexagram morphology of Cu₂O, the 2D layer structure and excellent conductivity of Ti₃C₂T_x nanosheets and the synergistic effect between the two composites promote the improvement of photoactivity. Zhang et al. reported the bifunctional catalyst of Cu₂O@Fe₂O₃. Cu₂O nanoparticles coated with an Fe₂O₃@carbon cloth electrode were used for both overall water splitting and CO₂ photoreduction [110].

Preferentially Exposed Facets

Cu₂O is an ideal compound to study the influence of electron-related effects. The rare occurrence of the O-Cu-O 180^o linear coordination of Cu₂O makes its (111), (100) and (110) facets chemically active. Zhang et al. [111] successfully achieved the morphology control of Cu₂O nanocrystals by utilizing the selective surface stabilization of PVP on the (111) plane of Cu₂O. With different amounts of PVP, the surface area ratio of (111) to (100) was subtly tuned, which resulted in the shape evolution of the system and various Cu₂O structures (Figure 16). The detailed modification mechanism was elucidated from the structural and kinetic perspectives.

Octahedral copper oxide that exposes the (111) crystal faces was decorated with low Fermi energy Ag nanoparticles. After coating with rGO, the ternary heterojunction catalyst [112] exhibits selective photocatalytic superiority towards CH₄. The CO* radicals can be characterized via DRIFT spectra and DFT calculations, which is the key intermediate for the conversion of CO₂ to CH₄. Two types of MoS₂ (p-type and n-type) and two shapes of Cu₂O (cubic and octahedron) were synthesized and combined with each other [113], and the compositions possessed different electronic and structural properties. The heterostructures formed by the p-type MoS₂ with intrinsic conductivity had a higher photocatalytic activity, and the methanol production yield was as high as 76 μmol g⁻¹ h⁻¹. Both Z-type and ii-type charge transfer mechanisms were built using an n-type MoS₂ mixture. For Cu₂O, the cubes of the exposed (100) crystal plane with a higher binding affinity with MoS₂ transferred electrons more efficiently and produced methanol at a higher rate.

A 3D porous Cu was produced by electrodeposition method [114], being transformed into CuO₂ after following high-temperature annealing. Three-dimensional Cu₂O delivers a 24-fold production of CO compared with the unremarkable and non-porous Cu. Additionally, more CO₂ accumulated and took reactions in the hollow space of 3D Cu₂O to form C₂ products.

3DOM Structure Cu-Based Materials

The 3DOM Cu₂O structure was luckily obtained [115] via polystyrene crystal templates. Under the contrived “sunlight” irradiation, incident light was reflected and absorbed around and around again. In the ultra-visible absorption spectra (350 to 800 nm), Cu₂O with large orifices absorbs more photons than bulk samples, making it more advisable for solar applications. 3DOM Cu₂O was prepared [116] by the electrochemical method to reduce CO₂, and its Faraday efficiency was five times higher than that of Cu film. The CO₂·⁻ intermediate in 3DOM channels is more stable and leads to the possibility of forming CO and HCOOH products (Figure 17). We look forward to the applications of inverse opal Cu₂O in photocatalysis.

3.4.2. Heterojunction

Liu et al. [117] reported a facile solution and chemistry route to synthesize rGO-incorporated crystal Cu₂O with various facets as visible-light-active photocatalysts for CO₂ reduction. The enhanced activity was attributed to the formation of the heterojunction and the existence of rGO as the electron transport mediator. M. Flores et al. [118] adopted the microwave-hydrothermal method to couple the powders of Mg(OH)₂, CuO and Cu₂O. The synthesis method allowed a sufficient interaction between Mg(OH)₂/CuO and Cu₂O without inhibiting the gas adsorption capacity of Mg(OH)₂. They found that the presence of Cu₂O favored the selectivity towards CH₃OH production because a higher Cu⁺ concentration led to better selectivity. Niwesh et al. [119] reported the formation of a p–n heterojunction between Cu₂O and the SnS₂/SnO₂ nanocomposite that offered favorable reductive potentials and high stability, mainly owing to their intimate interfacial contact. In the absence of a sacrificial agent, the generation rate of NH₄⁺ was 66.35 μmol g⁻¹ h⁻¹ for Cu₂O/SnS₂/SnO₂, which is 1.9-fold higher than that of SnS₂/SnO₂. However, the work of Trang et al. also demonstrated the instability and photo-oxidation of Cu₂O heterojunctions. Generally, most p–n heterojunctions are found to reduce the redox capacity of photogenerated charges. This is especially evident for Cu₂O p–n type heterojunctions, as CuO is excessively formed on the surface of Cu₂O under continued illumination, so there are recombination problems at the heterojunction interface. The construction of Z-scheme heterojunctions overcomes the limitations of p–n heterojunctions, namely the reduction in redox potential and charged carrier recombination at the p–n heterojunction interface. In a Z-type heterojunction, the redox potential can be maintained under the premise of high photo-induced electron transport rate.

Zhang et al. [120] synthesized coal-based CNPs with an sp² carbon and multilayer graphene lattice structure, and loaded them onto the surface of Cu₂O nanoparticles prepared by the in situ reduction of copper chloride. The rapid recombination of electron–hole pairs was suppressed by the introduction of CNPs. The energy gradient formed on the surface of Cu₂O/CNPs facilitates the effective separation of electron–hole pairs for CO₂ reduction, improving the photocatalytic activity. Atomically dispersed In-Cu bimetallic catalysts were prepared [121] by the in situ pyrolysis method, in which carbon nitride acted as a carrier. The light-harvesting and charge separation efficiency were enhanced by regulating the loading amount of Cu and In, and the supreme generation rate of the photoreduction of CO₂ to ethanol reached 28.5 μmol g⁻¹ h⁻¹ with 92% selectivity. The DFT calculations showed that the introduction of an In atom in copper can accelerate electron transfer from carbon nitride to metal, improve the charge separation efficiency and increase the electron density of copper active sites. The presence of In–Cu sites exerted a synergistic effect, which could promote C–C coupling, lower the energy barrier of *COCO generation and increase ethanol yield. Zhao et al. [122] reported the indirect Z-scheme heterojunction of UiO-66-NH₂/Cu₂O/Cu, which achieved a high CO₂ photocatalytic conversion to CO. The SEM results of Cu₂O, UiO-66-NH₂ and U/C/Cu-0.39 are shown in Figure 18A–C. In this catalytic system, UiO-66-NH₂ slowed down the photo-corrosion rate of Cu₂O and increased the CO₂ adsorption capacity (Figure 18D).

3.4.3. Summary

In conclusion, photochemical methods offer the opportunity to modulate the persistence and selectivity of Cu-based catalysts photoreduction to value-added compounds. The most recent photocatalytic CO₂ reduction outcomes of Cu-based materials are listed in

Table 4. Summary of the recent results on the photoreduction of CO₂ by Cu₂O-based materials.

Type of Enhancement	Photocatalyst	Description	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction S-scheme	Cu2O/CNPs	Coal-based carbon nanoparticles, multilayer graphene lattice structure	300 W Xe	CH3OH 24.86	[120]
	g-C3N4 foam/Cu2O QDs	3D g-C3N4 foam, 0D Cu2O QDs	300 W high-pressure Hg	CO 8.182	[123]
	Cu2O/rGO	Rhombic dodecahedra	300 W Xe 420 nm	CH3OH 17.77	[117]
	Ag/Cu2O@rGO	Octahedral Cu2O	300 W Xe 380 nm	CH4 82.6	[112]
Z-scheme	UiO-66-NH2/Cu2O/Cu	Octahedron UiO-66-NH2	300 W Xe	CO 4.54	[122]
p–n heterojunction	Mg(OH)2/CuO/Cu2O	Microwave-hydrothermal method	Liquid phase: two halogen lamps of 20W	CH3OH 6 HCHO 9	[118]
	Cu2O/SnS2/SnO2	Hierarchical flower-like SnS2/SnO2	300 W Xe	CO 3.18 CH4 2.27	[119]
3DOM	3D porous Cu2O		300 W Xe, 420nm	CO 13.4 nmol cm−2h−1	[114]

.

3.5. CeO₂-Based Photocatalysts

Cerium oxide (CeO₂) has an octahedral face-centered cubic fluorite structure, in which the coordination numbers of Ce and O are 8 and 4. When reduced at a high temperature, it can be converted to nonstoichiometric CeO_2−x (0 < x < 0.5). Notably, CeO_2−x maintains a fluorite crystal structure and forms oxygen vacancies after losing a certain amount of oxygen. CeO_2−X materials with different Ce/O ratios were also obtained in different conditions and it could be reconverted to CeO₂ again if it returned to an oxidizing environment. Because of the unique electrical structure, cerium oxide (CeO₂) is famous for the conversion sates between Ce⁴⁺ and Ce³⁺, which have been studied as oxygen storage catalytic materials and solid oxide full cells by many scholars [124,125]. In summary, CeO₂ is a rare-earth metal oxide with a good photochemical stability, low cost and environment friendly characteristics. It is an important n-type semiconductor with a wide bandgap, and credible photocatalysts have been designed to reduce CO₂ and degrade pollutants [126].

3.5.1. Morphology Control

Yb-, Er-doped CeO₂ hollow nanotubes were synthesized [127] using silver nanowires coated with silica, and the products had a narrower band gap of 2.8 eV. The core–shell structured CeO₂ was converted into mesoporous hollow spheres by the Ostwald ripening method in the presence of urea and hydrogen peroxide [128]. CeO₂ nanocages can be fabricated by mixing (NH₄)₂Ce(NO₃)₂ with templates of Cu₂O nanocubes [129], in which Cu₂O is finally sacrificed. The photocatalytic results [130] indicated that CeO₂ nanocages exhibit higher activity than hollow spheres.

Preferentially Exposed Facets

It was found that molecular CO₂ can be distorted and participate in reactions at a low energy on the CeO₂ surface [131]. A p-type NiO material was designed to modify the rod-like CeO₂ nanostructure [132], allowing electrons and holes to migrate to opposite directions. They then operated the Mott–Schottky test, which showed a typical p–n junction. The presence of hexagon-shaped NiO plates broadened the range of light responses, which can be verified in the UV-Vis absorption spectra. Graphene oxide (rGO) was introduced as a “network” of for photoreduction electron transportation (Figure 19A–C). The impedance can be seen in the EIS Nyquist plot, which shows that the NiO/CeO₂/rGO achieved the minimum value. The HCHO production rate of the ultimate catalyst was 421 μmol g⁻¹h⁻¹ with the synergy of several favorable factors. It is worth mentioning that a range of in situ techniques have been used to detect oxygen vacancies, structural changes, free radicals and formate on the surface of CeO₂.

Macroporous

Mesoporous N-doped CeO₂(NMCe), a relatively ordered intermediate structure with enhanced CO₂-capturing capability, was prepared without any convoluted procedures or expensive equipment [124]. In the Roman spectrum, the bands from 550 to 650 cm⁻¹, which are closely related to oxygen vacancy, were more salient than the MCe band. In addition, N-doped porous CeO₂ has a higher CO₂ absorption capacity than porous CeO₂. All the above results conformed to the photoluminescence spectrum (PL) analysis, and the reduction gross yield of CO and CH₄ was 3.5 times higher than that of OMCe.

With the help of a suitable crosslinked and pyrogenic solvent, doped CeO₂ was uniformly fixed on transparent polymers by the in situ polymerization pathway [133]. By increasing the Ca/Ce molar ratio (wt.% < 20%), no peaks relating to CaO were observed in the p-XRD of samples and the original diffraction peak intensity became more strident due to the foreign ion’s minuscule radius (Ca²⁺ 0.1 nm, Ce⁴⁺ 0.184 nm). The specific surface areas of three catalysts (CeO₂, (20: 80) CaO/CeO₂ and CaO/CeO₂ NC-dispersed polymers as a whole) were 28.4, 58.3 and 224.7 m² g⁻¹, respectively, and heterojunction nanocomposites had the highest photocatalytic efficiency.

3DOM Structure Ce-Based Materials

Under the protection of poly alcohol, Zhang et al. [134] synthesized 3DOM CeO₂ that was loaded with Au–Pd alloys. 3DOM CeO₂ photocatalytic materials are expected to emerge in the field of CO₂ emission reduction, which could open up more possibilities for the development of super-catalysts.

3.5.2. Heterojunction

Researchers tried to combine CeO₂ with g-C₃N₄, which is popular for its energy bands and chemical stability. Through hard work, a three-dimensional porous g-C₃N₄(3DCN) was achieved, with the advantages of multi-channel structure. To accommodate more electrons and heighten the density of photoelectric currents, Zhao et al. [125] loaded Pt nanoparticles (5–6 nm) on CeO₂/3DCN using photodeposition techniques that require UV lamp radiation. The photoreduction rate gradually increased as the CeO₂ amount rose in the range of 15~45% and the yield rates of 4.69 and 3.03 μmol·h⁻¹·g⁻¹ for CO and CH₄ were achieved, respectively, after decorating with Pt crystalline grains.

Under mild reaction conditions, carbon-doped hexagonal boron nitride (h-BN), known as boron carbon nitride (BCN), can reduce more carbon dioxide after dispersing cerium oxides on it. In the BCN/CeO₂ heterostructure [135], the N-O-Ce bond was formed by thermal precipitation method. It is of interest that the proportion of Ce³⁺/(Ce³⁺ + Ce⁴⁺) that influences electron transfer rate fluctuates with CeO₂-loading amount, and the yield of CO peaked on 30%CeO₂ (selected from 10% to 70%). When exposed to UV light, CeO₂ crystals could absorb more photons than visible light, whereas h-BN does not exhibit absorption in the UV range. The establishment of heterojunction expanded the effective wavelengths and improved the absorption capability in ultraviolet and visible light. The •OH species was detected by ESR analysis to identify the type of the heterostructure and the result of no newly generated •OH species was in accord with the II-scheme system.

3.5.3. Summary

As a whole, the wider light harvesting range and longer separation time of charge pairs improved the photoreduction upshot. Overall, recent results on CO₂ photoreduction by CeO₂-based materials are presented in

Table 5. Summary of the recent results on the photoreduction of CO₂ by CeO₂-based materials.

Type of Enhancement	Photocatalyst	Description	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction	modified TiO2/rGO/CeO2	Sono-assisted, 0.75 wt% rGO and 1 wt% CeO2	UV light (a 15 W UV-C mercury)	CH3OH 106.83 C2H5OH 45.17	[53]
	S-C/In2O3-CeO2	S-doped, hollow hexagonal prisms with carbon coating	300 W Xe	CH4 60.6	[136]
	CeO2/Fe3O4	G-C3N4 QDs (CN QDs) CeO2/Fe3O4 micro-Flowers (MFs)	UV-Vis light	CO 28.0 CH4 9.5	[137]
	Ni/CeO2	Pure cubic fluorite structure CeO2	Xe 4.7 kW m-2 >500 °C	H2 6.53 CO 6.27	[138]
	CeO2@Ti3C2TX	Layered Ti3C2TX nanosheets, high-density CeO2	300 W Xe	CH3OH 76.2 C2H5OH 33.7	[139]
Z-scheme	CoAl-LDH/CeO2	CeO2 and RGO on the flower-like CoAl-LDHs	ultraviolet (UV) light (200 W)	CO 5.5	[140]
p-n heterojunction	NiO/CeO2/rGO	CeO2 nanorods, hexagon-shaped NiO plates	300 W Xe	HCHO 421.09	[132]
Ternary composites	N-TiO2/CeO2/CuO	N-doped TiO2	2 Xe lamps (20 W each)	HCOOH 33	[141]
Ion doping	Fe-Ni@CeO2	Spherical Fe and hexagonal Ni-doped CeO2 nanorods	20 W white LED	CH3OH 293.29	[142]

.

4. Other 3DOM Materials

In order to introduce advanced porous structures to slow the self-aggregation of quantum dots, Wang et al. [143] devised 3DOM N-doped carbon (NC) to support CdS and ZnO QDs (Figure 20). They filled the interspace in an ordered PS microsphere template and then employed a pyrolytic treatment and in situ growth methods. Compared to bulky CdS, the 3DOM compounds have a larger cathodic current density and enhanced light harvesting, bearing a satisfactory carbon monoxide yield of 5210 μmol g⁻¹ h⁻¹. A three-dimensional SnO₂ inverse opal structure was synthesized as gas sensors, soot oxidation catalyst and photoanode. The 3DOM BiVO₄/SnO₂ heterostructure was obtained by adding a BiVO₄ precursor to fill the space between SnO₂ skeleton and periodic PS template. The compatibility of energy states with SnO₂ significantly reduced their photoluminescence intensity. Meanwhile, Au nanoparticles enhanced the slow photon effect, which in turn increased the incident light utilization efficiency.

Moreover, some photoreduction results on 3DOM materials are listed in

Table 6. Summary of the recent results on the photoreduction of CO₂ and H₂O by 3DOM materials.

Type of Enhancement	Photocatalyst	Conditions (Lamp, Cutoff Filter)	Product Yield/ μmol·g−1·h−1	Refs.
Heterojunction	3DOM Au-CsPbBr3	300 W Xe 420 nm	CO 12.6 CH4 2.1	[82]
	3DOM CdS QDs/N-doped carbon	visible light irradiation, acetonitrile solution	CO 5210	[143]
Water splitting to produce H2	CdS/Au/3DOM-SrTiO3	300 W Xe 420 nm	5.39 × 103	[144]
	3DOM Pt/ZnS@ZnO	300 W Xe	87.6	[145]
	TiO2-Au-CdS	34 mW/cm2 UV light, 158 mW/cm2 visible light	1.81 × 103	[146]

. From the above analysis, we can infer the great potential of macroporous materials for further development in the field of photocatalysis.

5. Conclusions

The photoconversion of CO₂ into solar fuels seems to curb greenhouse effect and resolve the energy crisis. In this review, the major research progresses of different metal oxide materials on solar-light-driven CO₂ conversion to fuels were carefully summarized. TiO₂, WO₃, ZnO, Cu₂O and CeO₂ are the most common materials for photocatalysts among the numerous semiconductors. Even though considerable progress has been achieved, creating superb catalysts still presents several challenges. Researchers have presented treatment options to boost catalytic activity after apprehending the fundamental principles of objective reactions.

The main formulas for designing photocatalysts are as follows.

• Absorb more photons to produce excitons.
• Improve the migration efficiency of charge carriers.
• Reduce the recombination rate of electron–hole pairs.
• Uplift the absorption capacity of CO_2.

In terms of material selection, it is wise to choose metal oxides as one of the active components because of their relatively suitable Eg and CB bands. From the perspective of morphology, compared with the solid and regular morphology, the structure of hollow ordered pores can promote the absorption of reactants and the desorption of products. A larger contact area between the catalyst and reactants and more active sites for the reaction can be provided. From the point of view of light absorption, the CB or VB positions can be adjusted after ion doping, which affects Eg and influences the position of spectral absorption; the slow photon effect in the 3DOM structure can also improve the light utilization. As for charge separation efficiency, it can be promoted at the interface of the heterojunction, and photonic crystals can also improve their separation efficiency by shortening the distance of charge movement to the interface. Numerous attempts have been made in the synthesis of metal oxide photocatalysts, which can yield CO, CH₄, HCHO, HCOOH, CH₃OH and C₂H₅OH. However, it remains a great challenge for current photocatalysts to satisfy actual industrial production demands. The efficiency and selectivity for target products cannot meet the requirements for industrial and commercial implementation.

To sum up, the migration–separation efficiency of photoinduced pairs is important to improve the catalytic activity. Scholars should concentrate on the multiple advantages of photonic crystals to design catalysts with better performance based on them. This review provided a wealth of experience and ideas for the exploitation of photocatalyst material selection, morphology control and active site design. There is still a considerable work to conduct in converting CO₂ from solar energy to fuel, and we believe more significant breakthroughs can be achieved regarding the efficiency, mechanism and durability of the photocatalyst.