Sulfur Vacancy Engineering in Photocatalysts for CO₂ Reduction: Mechanistic Insights and Material Design

Abstract

Against the backdrop of increasing global warming, exploring sustainable pathways to mitigate the greenhouse effect has become a central issue for the ecological and energy future. Photocatalytic reduction of CO₂ technology shows a broad application prospect due to its ability to directly convert CO₂ into high-value-added hydrocarbon fuels and to use solar energy, a clean energy source, to drive the reaction. However, traditional semiconductor catalysts generally suffer from insufficient activity and poor product selectivity in the actual reaction, which cannot meet the requirements of practical applications. In recent years, sulfur vacancy, as an effective material modulation strategy, has demonstrated a remarkable role in enhancing photocatalytic performance. This paper reviews a series of research reports on sulfur vacancies in recent years, introduces the methods of preparing sulfur vacancies, and summarizes the commonly used characterization methods of sulfur vacancies. Finally, the mechanism of introducing sulfur vacancies to promote CO₂ reduction is discussed, which improves the photocatalytic activity and selectivity by enhancing light absorption, facilitating carrier separation, improving CO₂ adsorption and activation, and promoting the stability of reaction intermediates. This review aims to provide theoretical support for an in-depth understanding of the role of sulfur vacancies in photocatalytic systems and to provide a view on the future direction and potential challenges of sulfur vacancies.

1. Introduction

1.1. Mechanisms and Challenges of Photocatalytic Reduction of CO₂

Humanity’s long-term reliance on fossil fuels has resulted in excessive CO₂ emissions, disrupting the global carbon cycle and exerting profound impacts on climate change and the energy crisis [1]. In light of the growing threats posed by environmental pollution and energy scarcity, there is increasing recognition of the importance of environmental governance and sustainable development [2,3,4]. Against this backdrop, the development of efficient and clean advanced technologies for the synergistic optimization of carbon emission reduction and renewable energy generation has become imperative [5]. The rapid advancement of new energy technologies offers promising opportunities to rebalance the carbon cycle. Among these, artificial photosynthesis—inspired by natural photosynthesis—stands out as a cutting-edge approach that utilizes solar energy to convert CO₂ and H₂O into high-value hydrocarbon fuels and O₂ [6,7,8]. This process not only mitigates the greenhouse effect but also promotes clean energy regeneration, positioning it as a key strategy toward achieving carbon neutrality [9,10]. Compared with other CO₂ conversion technologies such as electrocatalysis [11], photoelectrocatalysis [12], and photothermal catalysis [13], photocatalysis has garnered significant attention due to its distinct advantages [14,15]. First, it relies solely on renewable solar energy, making the process environmentally benign and cost-effective [16]. Second, the reaction system is inherently safe, is operationally simple, and functions under ambient conditions [17,18]. Despite these merits, the practical application of photocatalysis still faces substantial challenges [19]. A central challenge lies in the development of photocatalysts that are low-cost, highly efficient, and readily available [20,21,22,23]. However, most photocatalysts suffer from poor thermodynamic stability, severe recombination of photogenerated charge carriers, and low quantum efficiency [24,25], all of which significantly limit the performance of photocatalytic CO₂ reduction (CO₂ RR). Therefore, designing and engineering novel photocatalysts with enhanced activity, selectivity, and stability through strategic material modulation and structural modification has become a critical research frontier. Overcoming these limitations is essential to breaking through current technological bottlenecks and advancing photocatalytic CO₂ reduction toward practical implementation [26].

Figure 1 illustrates the specific reaction process of the photocatalytic CO₂ reduction reaction [27]. When a semiconductor photocatalyst is irradiated with light, electrons in the valence band absorb energy and are excited to the conduction band, provided the incident photon energy is equal to or greater than the band gap (Eg) of the material, resulting in the formation of electron-hole pairs [28]. Subsequently, these photogenerated carriers migrate to the catalyst surface, where they participate in reduction and oxidation reactions [29,30]. Electrons reduce adsorbed CO₂ molecules to valuable hydrocarbon fuels (e.g., CO [31], CH₄ [32], CH₃OH [33], and C₂H₄ [34]), while holes oxidize H₂O to produce O₂ [35]. However, electrons and holes are highly prone to recombination during migration, a process that drastically reduces the number of available charge carriers for surface reactions, thereby limiting the overall catalytic efficiency [36]. Understanding the challenges related to the activity and selectivity of photocatalytic CO₂ reduction requires examining its fundamental mechanism, which typically involves three key steps: (1) photon absorption by the semiconductor and excitation to generate electron-hole pairs [37]; (2) efficient separation of charge carriers within the material and their migration to surface reactive sites for oxidation and reduction [38]; and (3) adsorption of CO₂ and H₂O molecules on the catalyst surface, followed by their respective redox reactions through interaction with photogenerated electrons and holes [39]. The simultaneous and efficient operation of all three steps imposes specific requirements on the material properties. First, the energy of the absorbed photons must be equal to or greater than the band gap of the catalyst [40,41,42]. The solar spectrum in the 250–2500 nm range consists of ultraviolet (UV), visible (vis), and near-infrared (NIR) regions, with visible and NIR light accounting for approximately 95% of total solar photons [43]. Therefore, highly active photocatalysts should possess a narrow bandgap to enable strong light absorption across a broad spectral range [44]. Second, regarding charge separation, the material must exhibit efficient carrier separation to minimize electron-hole recombination and ensure sufficient charge carriers reach the surface for catalytic reactions [45,46,47]. Finally, the surface reaction step is particularly crucial, as the chemical inertness of CO₂ necessitates effective activation through strong adsorption capacity and well-distributed active sites on the catalyst surface to facilitate the formation of high-value fuels such as CH4 and C₂H₄ [48].

The photocatalytic reduction of CO₂ is a highly complex multi-electron transfer process. The complete reduction of CO₂ to CH₄ or C2 products requires at least eight consecutive electron injections [49]. This process not only involves complex multi-step reaction pathways but also the formation of multiple intermediates, making selectivity control extremely challenging and one of the most difficult aspects of photocatalytic CO₂ reduction reactions [50]. Moreover, the value of the product is closely tied to selectivity, with high-value products typically requiring more intricate synthetic pathways and higher selectivity. For instance, the direct reduction of CO₂ to C₂ or higher hydrocarbons is considered a highly desirable yet technically challenging goal [51], as it demands that the catalyst not only efficiently activate the CO₂ molecule but also precisely modulate the reaction pathway to promote the formation of specific products [52,53,54]. The fact that the carbon in CO₂ is in the highest oxidation state (+4) results in end products that can range from carbon monoxide (CO), methane (CH₄) to more complex hydrocarbons such as ethylene C₂H₄. If C₂H₄ is the only target product, the specific reaction pathway is as follows (Equations (1)–(5)):

*+CO₂ + e⁻ + H⁺ → *COOH

(1)

*COOH + e⁻ + H⁺ → *CO + H₂O

(2)

*CO → CO+*

(3)

*CO + *CO → *OCCO

(4)

*OCCO + 8e⁻ + 8H⁺ → C₂H₄ + 2H₂O

(5)

Due to the complexity of the CO₂ photoreduction process, numerous factors influence the activity and selectivity of the reaction [55]. Light absorption, charge separation and transport, and surface reactions are the three main factors that determine the selectivity and activity of CO₂ photoreduction [56,57,58]. In practice, it is difficult for pristine catalysts to meet all these requirements simultaneously [59]. The introduction of sulfur vacancies offers an effective strategy to address these challenges [60].

1.2. Vacancy Engineering (Sulfur Vacancy)

Vacancy engineering is a materials science strategy that modulates the physical and chemical properties, as well as the functionality, of materials by precisely controlling and utilizing atomic vacancies, i.e., the absence of specific atoms within the crystal structure [61,62,63]. These defects represent deviations from the ideal periodic crystal lattice and can impart new properties to the material or enhance its intrinsic characteristics [64]. Recent studies have demonstrated that this approach, commonly referred to as “defect engineering,” can effectively tune the electronic configuration of catalysts, offering great potential for achieving superior photocatalytic performance [65,66,67]. In particular, the presence of vacancies can profoundly influence key parameters such as the electronic structure, optical properties, and charge transport capability of a material, thereby significantly altering its performance in specific application environments [68]. In photocatalysts, defect sites often possess unique electronic structures capable of forming moderate interactions with reaction intermediates, leading to enhanced catalytic activity and stability [69]. Notably, defect engineering not only increases the number and density of active sites but also optimizes the functionality of these sites [70]. Furthermore, it has been shown that engineering defect sites can effectively promote surface reconstruction of nanocatalysts and induce the formation of highly active species, thereby further accelerating the catalytic process [71].

Among them, sulfur vacancies specifically refer to vacancies formed in compounds or alloys containing elemental sulfur due to the partial removal of sulfur atoms, and they significantly influence the electronic structure and surface chemical activity of the materials [72,73]. In photocatalytic materials, sulfur vacancies can act as electron trapping centers, helping to improve the separation efficiency of photogenerated carriers and thereby enhancing the photocatalytic performance of the materials [74]. Additionally, sulfur vacancies can alter the local charge distribution, enhance the material’s adsorption capacity for reactants, and lower the reaction energy barrier, which is crucial for improving catalytic activity and selectivity [75]. Central to the creation of sulfur vacancies is the modulation of the surface physical and chemical properties of the semiconductor [76,77], including the adsorption-desorption behavior of ions and molecules, the activation state, and the relative stability of intermediates—all of which are key determinants of the reaction pathways in the photocatalytic process [78]. Sulfur vacancies not only perform well in the field of photocatalysis but also show broad application prospects in multiple other fields such as electrocatalysis, battery materials, sensors, photodetectors, and semiconductor devices [72,79]. In electrocatalysis, sulfur vacancies can act as active centers, optimizing the adsorption energy of intermediate products and enhancing the catalytic activity and selectivity of reactions such as hydrogen evolution, oxygen evolution, or oxygen reduction [80]. In battery materials, sulfur vacancies help promote ion diffusion kinetics, improve the conductivity and cycling stability of electrode materials, and are widely used in energy storage systems such as lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries [81]. In the field of sensors, sulfur vacancies can enhance the material’s adsorption and charge response to target gases or biomolecules, significantly improving detection sensitivity and response speed [82]. In photodetectors, sulfur vacancies can regulate the photoconductivity characteristics of materials, increasing the photoresponsivity and detectivity [83]. In semiconductor devices, sulfur vacancies can be used to adjust carrier concentration and band structure, optimizing key performance parameters such as the on-off ratio and mobility of the devices [84]. Therefore, as an important point defect, sulfur vacancies have become one of the key approaches for the design and performance regulation of multifunctional materials. Particularly, sulfur vacancies play a crucial role in photocatalytic reduction of CO₂ to produce hydrocarbon fuels by optimizing the electronic structure and improving the surface reaction activity, further promoting the research progress in this field. Because there are relatively more reports on oxygen vacancies in the current literature, most of the reviews introducing vacancy engineering mainly focus on oxygen vacancies [45]. Although there are some review articles that have introduced sulfur vacancies and their applications in metal sulfides, most of these reviews focus on areas such as electrocatalysis, thermal catalysis, photocatalytic hydrogen production, nitrogen fixation, and water splitting [85]. However, there is still a lack of systematic and in-depth discussions on the research progress of sulfur vacancies in the specific field of photocatalytic reduction of CO₂.

This paper reviews the research progress on sulfur vacancy engineering in the photocatalytic reduction of CO₂, with a focus on the mechanisms by which sulfur vacancies regulate the yield and selectivity in the conversion of CO₂ to hydrocarbons. The key role of sulfur vacancies in enhancing photocatalytic performance is analyzed in depth from three aspects: light absorption by the catalyst, carrier separation, and surface catalytic active sites. The article first introduces the basic properties of metal sulfides as a class of promising photocatalytic materials and summarizes various synthetic strategies for introducing sulfur vacancies into these materials. Meanwhile, the main technical methods currently used for characterizing sulfur vacancies are summarized. By integrating experimental and theoretical studies, the mechanism of sulfur vacancies in the CO₂ photoreduction process is analyzed, including their promoting effects on the adsorption and activation of CO₂ molecules as well as on the stability of reaction intermediates. It is worth noting that, although sulfur vacancies show great potential in enhancing photocatalytic performance, there remains a lack of systematic investigation and in-depth discussion regarding their formation mechanisms, energy level modulation functions, and applications in sulfur-rich catalytic systems. Therefore, the aim of this paper is to systematically review and explore the role of sulfur vacancies in photocatalytic CO₂ reduction and to provide new insights and directions for the future design and development of efficient catalysts.

2. Different Types of Catalysts

2.1. Other Types of Catalysts

In the research of photocatalytic reduction of CO₂, the types of catalysts mainly include oxides, sulfides, nitrides, oxygen-nitrides, and some organic materials. Among them, organic-type photocatalysts have been explored due to their diverse structures and strong designability [86]. However, they are prone to degradation under light conditions, which not only affects the catalytic stability but also may produce by-products, interfering with the accurate analysis of the reaction pathway and mechanism. Therefore, their application in this field is far less widespread than in inorganic systems [87]. Metal oxides such as TiO₂ and CuO, due to their excellent chemical stability and high photoelectric response efficiency, dominated the early research. However, they generally have a large band gap (usually greater than 3 eV), resulting in excitation only in the ultraviolet region. As ultraviolet light only accounts for about 5% of solar energy, this severely restricts the overall utilization efficiency of solar energy [88]. Nitrides such as GaN and oxygen-nitrides such as GaON, due to their tunable energy band structure and good visible light response ability, have attracted attention. However, the synthesis of these materials often faces high technical barriers and requires harsh conditions such as high temperature, high pressure, or an ammonia atmosphere, limiting their scalability and flexibility for performance optimization [89]. Among numerous photocatalyst candidates, sulfides stand out for their outstanding performance and have become one of the most promising materials for photocatalytic CO₂ reduction (

Table 1. Evaluation of photocatalytic CO₂ reduction performance of different catalytic systems.

Photocatalyst	Solution	Light Source	Product	Yield (μmol·g−1 h−1)	Ref.
Cu2S	H2O	300 W xenon lamp	CO	1.25	[90]
ZnIn2S4	H2O	300 W xenon lamp	CO	2.97	[91]
BMO/IS-1	H2O	300 W xenon lamp (λ ≥ 420 nm)	CO	7.16	[92]
Vs-AgInS2	H2O	300 W xenon lamp (λ ≥ 780 nm)	CO	8.04	[93]
2Cu-ZISF	Na2SO4 (pH = 7)	300 W xenon lamp with 400 nm cut-off filter	CH4	22.27	[94]
Cu2S1−x	H2O	300 W xenon lamp with an AM 1.5 G filter	CO	13.63	[95]
Sv-ZnS/ZIS	TEOA a aqueous solution (10 vol%, v/v)	300 W xenon lamp (λ ≥ 420 nm)	CO	793.3	[96]
Vs-ZnS/OMNC	TEOA and a mixture of H2O/acetonitrile	300 W xenon lamp (λ ≥ 420 nm)	CO	712.1	[97]
WZ-2	H2O	300 W xenon lamp	CO	11.03	[91]
60ISIO-1h	H2O	300 W xenon lamp with 420 nm cut-off filter	CH4	16.52	[98]
Cu-In2S3	TEOA and a mixture of H2O/acetonitrile	300 W xenon lamp with an AM 1.5 G filter	C2H4 C2H6	6.5 0.4	[99]
Ca-CoS2	H2O	300 W xenon lamp (λ ≥ 420 nm)	CO C2H4	10.01 12.45	[100]

^a TEOA: triethanolamine.

) [85]. Based on the complexity of the composition and structure characteristics of sulfides, they can be classified into single-component sulfides, multi-component sulfides, and composite sulfide catalysts.

2.2. Sulfide Catalysts

2.2.1. Single-Component Sulfide Catalysts

Single-component sulfides usually refer to compounds consisting of a single metallic or non-metallic element and sulfur, which can be divided into metallic sulfides (e.g., CuS and In₂S₃) and non-metallic sulfides (e.g., H₂S) [101]. In the field of photocatalytic CO₂ reduction, metal sulfides have received extensive attention in recent years and become one of the research hotspots due to their unique electronic structures and excellent light response characteristics [79,102]. For example, materials such as CdS [103] and ZnS [104], with band gaps generally between 2 and 3 eV, can effectively respond to visible light and significantly enhance the utilization of solar energy. In addition, these materials have high carrier mobility, and their conduction band position is close to the reduction potential required for the CO₂ reduction reaction, which is beneficial to the efficient migration of photogenerated electrons from the inside of the material to the surface to participate in the reduction reaction [105,106,107]. Meanwhile, the surface of metal sulfides typically features a large number of active sites, which can facilitate the adsorption and activation of carbon dioxide molecules, thereby enhancing the rate of the interfacial reaction. For example, Yan et al. [90]. constructed an S-scheme heterojunction photocatalyst based on ZnIn₂S₄ and Cu₂S using a two-step hydrothermal method. As shown in Figure 2a, the Cu₂S monomer achieves a CO yield of approximately 5 μmol·g⁻¹ after 4 h of visible-light irradiation in the unmodified state, demonstrating its potential for CO₂ photoreduction.

2.2.2. Multicomponent Sulfide Catalysts

Although some progress has been made in the field of photocatalytic CO₂ reduction with single-component metal sulfides, their inherent limitations, such as susceptibility to photocorrosion, mono-composition, and severe carrier complexation, have significantly constrained their stability and efficiency in practical applications [108]. Therefore, the research focus has gradually shifted to multi-component sulfide systems. Multi-component sulfides are compounds consisting of two or more metal elements and sulfur, typically including ternary metal sulfides (e.g., ZnIn₂S₄ [82] and CdIn₂S₄ [109]) and quaternary sulfides (e.g., Cu₂ZnSnS₄ [110]). These compounds not only possess optimized energy band structures [111] but also exhibit strong light absorption and good redox stability, further enhancing their potential in photocatalytic applications. For example, Xu et al. [91] constructed 2D-2D WO₃/ZnIn₂S₄ S-scheme heterojunctions. As shown in Figure 2b, the ZnIn₂S₄ monomer achieved a CO yield of 12 μmol·g⁻¹ after 4 h of light exposure without the addition of a co-catalyst. It is noteworthy that metal sulfides generally possess high crystal symmetry and excellent photogenerated charge transport properties, which provide structural and physical bases for their applications in efficient photocatalytic systems.

2.2.3. Sulfide Composite Catalysts

Despite the theoretically excellent photocatalytic potential of multicomponent sulfides, their performance in practical applications is still limited by a number of factors [84,112,113]. For example, the high carrier complexation rate leads to the difficulty of effective separation of photogenerated electron-hole pairs, and the slow surface reaction kinetics limits the adsorption and activation efficiency of CO₂ molecules [114]. In addition, insufficient product selectivity hinders the efficient generation of targeted reduction products [115]. Together, these problems lead to the fact that the current material systems are still far from practical applications. Therefore, materials engineering strategies are needed to achieve performance modulation and functional optimization [116,117,118]. Among them, sulfur-sulfide composite catalysts refer to composite materials generated by modifying sulfides or combining them with other compounds [119,120,121]. For example, combining with oxides, other sulfides, and carbides to form heterojunctions or composites to enhance photocatalytic reduction of CO₂. As an example, in the study of Yu et al. [92] successfully prepared a Bi₂MoO₆-SOVs@In₂S₃ heterogeneous nanotube photocatalyst with a hierarchical structure using a multi-step controlled synthesis strategy. The structure not only promotes interfacial charge transfer but also enhances CO₂ adsorption and activation through the introduction of sulfur vacancies. As shown in Figure 2c, the catalyst achieved a CO yield of up to 28.54 μmol·g⁻¹·h⁻¹ under visible light irradiation, exhibiting significantly enhanced catalytic activity. It was also reported that Li et al. [94] designed a heterojunction photocatalyst with Cu-doped ZnIn₂S₄ grown in situ on a ZIF-67 substrate. The composite system exhibited excellent CO₂ reduction performance without sacrificial reagents, as shown in Figure 2d, with a CH₄ generation rate of 22.3 μmol·g⁻¹·h⁻¹ and a selectivity of more than 94%, which is 8.3 times higher than the performance of ZnIn₂S₄ alone. These studies indicate that the performance of sulfide composite photocatalysts can be effectively improved by constructing sulfide composite photocatalysts through elemental doping [122], vacancy engineering [123], heterostructure building [124], and co-catalyst loading [125]. Among them, vacancy engineering has been widely considered as an efficient performance optimization strategy. However, current studies on defect modulation mainly focus on the oxygen vacancy system. In fact, sulfur vacancies are easier to form compared to other anionic vacancies (e.g., oxygen vacancies [126]). Therefore, an in-depth study of the mechanism of sulfur vacancies in the photocatalytic process not only helps to reveal the laws of their influence on the carrier kinetic behaviors and surface reaction pathways but also provides new design ideas for the development of high-performance and high-stability photocatalysts [127,128,129]. This not only has an important basic research value but also provides a practical technical path to meet the challenges of energy conversion and environmental governance.

Figure 2. (a) CO production rate of all prepared samples; (b) CO yield of prepared samples; (c) photocatalytic CO₂ reduction activity of different samples; (d) performance of photocatalytic CO₂ reduction reaction [90,91,92,94].

Figure 2. (a) CO production rate of all prepared samples; (b) CO yield of prepared samples; (c) photocatalytic CO₂ reduction activity of different samples; (d) performance of photocatalytic CO₂ reduction reaction [90,91,92,94].

3. Methods for Introducing and Controlling Sulfur Vacancies

3.1. Hydrothermal Methods

Compared with other complex synthesis processes, the hydrothermal method has become a common strategy for the preparation of functional materials due to its simplicity of operation, low energy consumption, and ease of regulation of the crystal growth process [130]. Li et al. [131] constructed a recyclable self-supported thin-film-type photocatalyst, ZnS-Vs/Bi₂S₃-PVDF (shown in Figure 3a), using a hydrothermal method, which was unable to form a complete face-centered cubic coordination structure around Zn²⁺ due to the insufficient supply of the sulfur source during the reaction process, which led to the introduction of sulfur vacancy defects in the lattice. Fang et al. [132] utilized hydrothermal conditions to regulate the phase transition process, achieving a controllable transformation from cubic sphalerite to hexagonal wurtzite by adjusting the reaction temperature, and successfully fabricated a new material with a cubic sphalerite structure suitable for use as a PVDF substrate. They successfully constructed a sphalerite-wurtzite heterojunction ZnS system with sulfur vacancies. They explained that the formation mechanism of sulfur vacancies is primarily attributed to the generation of out-of-lattice atoms and unsaturated bonds during the crystal structure reconstruction process and that these local defective states serve as electron trapping centers and active sites within the material.

3.2. Heteroatom Doping

The introduction of low-valence heteroatoms can induce the formation of relevant anionic vacancies on the surface of the material, and this structural modulation strategy can significantly change the electron distribution around the metal atoms, thereby enhancing the local charge density and providing a more favorable electronic environment for the catalytic reaction [135,136,137]. Kumar et al. [138] demonstrated that the introduction of Ni elements on the surface of CdS can effectively induce the generation of a large number of sulfur vacancies, which can improve the photocatalytic performance significantly. Cao et al. [139] proposed a synergistic strategy to construct a composite photocatalyst containing atomically dispersed heterogeneous atoms and controllable anionic vacancies to enhance the CO₂ reduction efficiency. They precisely constructed atomically dispersed Cu⁺ reactive sites as well as sulfur vacancies on the surface of CdS by cation exchange and achieved highly efficient selective reduction of CO₂.

3.3. Organic Treatment

Lai et al. [98] obtained In₂S₃/In₂O₃ heterojunctions enriched with sulfur vacancies by a simple treatment with polyvinylpyrrolidone (PVP). As shown in Figure 3b, in another example, Hu et al. [133] synthesized CuS nanosheets (CS-1, CS-2, and CS-3) with different concentrations of sulfur vacancies by a hydrothermal method under the condition of progressively increasing the dosage of thioacetamide (TAA) and systematically investigated the effect of Vs on their surface-enhanced Raman spectroscopy (SERS) and photocatalysis. Qiang et al. [140] similarly achieved the controllable construction of the sulfur vacancies in SnS₂ crystals using an excessive amount of TAA as a sulfur source. With increased TAA addition, the grain sizes of SnS₂ were all smaller than those of pure SnS₂, which greatly improved the possibility of defect formation.

3.4. Plasma Technology

The strategy of plasma technology treatment is to use high-energy particles (e.g., electrons, ions, and reactive radicals) of plasma to interact with the surface or bulk phase of the sulfide to selectively remove sulfur atoms and form sulfur vacancies [141,142]. As an example, in the study of Wang et al. [143], they used low-pressure argon plasma to surface treat NiCo₂S₄ material and introduced sulfur vacancies under mild conditions. By controlling the reshaping of the surface and the local phase transformation, the formation of sulfur vacancies was regulated simultaneously. In addition, plasma-induced NiCo₂S₄ can also lead to the valence reduction of Ni and Co, producing more sulfur vacancies. In addition, Yang et al. [134] constructed a sulfur vacancy-rich structure on the surface of powdered CdIn₂S₄ by low-temperature Ar plasma bombardment (shown in Figure 3c). Different from the traditional high-temperature heat treatment or chemical reduction methods, this method mainly utilizes the excited states of energetic particles in plasma to bombard CdIn₂S₄ at room temperature within a very short period of time, resulting in the effective fly-away of sulfur atoms.

3.5. Other Methods

In addition to the above strategies, researchers have explored various controllable means for the introduction of sulfur vacancies in recent years [144]. For example, Xu et al. [145] proposed a strategy based on ionic liquid-assisted hydrothermal combined with subsequent annealing treatment (shown in Figure 4a) to synthesize W-doped carbon cloth (Mo_1−xW_xS_y/CC) MoS₂ nanosheets containing sulfur vacancies. The experimental results demonstrated that the replacement of doped W could effectively activate the catalytic activity of the inert substrate surface of MoS₂ due to the creation of sulfur vacancies. In addition, Gao et al. [97] constructed ZnS nanoparticles (Vs-ZnS/OMNC) with abundant sulfur vacancies in ordered mesoporous nitrogen-doped carbon skeletons using an in situ thermal treatment strategy and applied them to an efficient CO₂ photoreduction reaction (shown in Figure 4b). Experimental and theoretical calculations have revealed that sulfur vacancies can induce charge localization on adjacent Zn atoms, thereby enhancing the adsorption and activation capabilities of CO₂ molecules. This study not only reveals the multiscale mechanism of sulfur vacancies in photocatalytic CO₂ conversion but also provides an idea for the development of new efficient photocatalysts by combining defect engineering with ordered mesoporous carbon structures. Although various strategies for introducing sulfur vacancies have been developed, there are still very limited methods that can precisely control their quantity and location. The existing methods often encounter numerous challenges in practical operation, and many of them have complex synthesis processes and are difficult to apply in practice. Moreover, many strategies, while introducing sulfur vacancies, inevitably affect the overall structure or other properties of the catalyst, thereby interfering with the accurate exploration of the intrinsic mechanism of sulfur vacancies. Therefore, in order to gain a deeper and more precise understanding of the role of sulfur vacancies in the catalytic process, more efficient and controllable new construction strategies need to be developed.

4. Characterization of Sulfur Vacancies

With the continuous progress of materials science and characterization technology, especially driven by the development of surface and interface analysis [146], the ability to analyze microstructural features in materials has been significantly improved, providing a solid foundation for an in-depth understanding of the correlation between material properties and structure [80]. In the study of sulfur vacancies, a series of advanced characterization strategies have been developed in recent years, which can be used to probe their presence forms, distribution states, and concentration levels [147,148]. Some of these characterizations can provide direct evidence to confirm the presence of sulfur vacancies. Others assess their impact mainly by analyzing indirect information such as optical response, changes in electronic structure, etc. [149]. The synergistic application of these characterization tools lays the experimental foundation for systematically revealing the mechanism of sulfur vacancies in the photocatalytic process.

4.1. Electron Paramagnetic Resonance (EPR)

A variety of material characterization techniques have been used to identify and analyze sulfur vacancies; some of these methods provide direct evidence, while others speculate on their presence mainly through indirect information [140,150,151]. Among them, EPR is the one that can indirectly prove the presence of sulfur vacancies. Sulfur vacancies usually introduce local defect states, which may carry unpaired electrons. EPR technology can detect these magnetic defect centers, thereby directly proving the existence of sulfur vacancies. Yuan et al. [152] successfully verified the existence of sulfur vacancies using EPR spectroscopy (as shown in Figure 5a). The EPR response of the Vs-ZCS/BWO heterojunction constructed in the study was caused by the capture of electrons by the Vs. A stronger resonance peak in the EPR spectrum indicates a higher concentration of sulfur vacancies in the heterostructure. Further research shows that under light conditions, the EPR signal of Vs-ZCS/BWO is significantly enhanced compared to the dark state, but the peak position remains unchanged, indicating that light excitation does not change the spin state of sulfur vacancies but rather strengthens their electron capture effect. This may be attributed to the Vs electron capture effect in Vs-ZCS/BWO, which increases the interfacial potential difference and further enhances the interfacial electric field (IEF).

4.2. Positron Annihilation Lifetime Spectrum (PALS)

PALS is a physical detection technique based on the annihilation of positrons interacting with vacancy-type defects in materials and releasing characteristic radiation signals, and it has been widely used for the identification and quantitative analysis of crystal defects [154]. Among them, positrons tend to be captured and annihilated in low-electron-density regions (e.g., atomic vacancies), and their lifetimes are closely related to the size and concentration of defects, so PALS has become a highly efficient and sensitive means to characterize vacancy defects [155]. For example, Yin et al. [153] took advantage of liquid ammonia-assisted lithiation to synthesize a series of novel porous 1T-phase MoS₂ nanosheets with different structural features and systematically investigated the effects of key factors such as their crystal phase state, the degree of edge exposure, and the concentration of sulfur vacancies on the catalytic performance of the hydrogen-extraction reaction (HER). In order to achieve the exploration of the contribution of each factor, the distribution and concentration of sulfur vacancies were quantitatively analyzed in the study by combining the characterization of EPR and PALS (shown in Figure 5b,c) so as to establish the correlation between the defect structure and the catalytic activity.

4.3. X-Ray Photoelectron Spectroscopy (XPS)

As an analysis technique with surface sensitivity, XPS can provide information on the elemental composition and chemical state of the material surface [156]. In the study of sulfur vacancies, indirect but critical evidence for the existence of defective structures can be provided by analyzing the shifts in the binding energies of S 2p orbitals [157]. Zhang et al. [158] successfully constructed two-dimensional/two-dimensional (2D/2D) WO₃/Vs-ZIS S-scheme heterojunction photocatalysts using a hydrothermal method in combination with a light treatment strategy. The study not only demonstrated the presence of sulfur vacancies by EPR but also utilized XPS analysis in order to determine the elemental composition and chemical state of the catalysts (shown in Figure 6a–d) and to investigate the nature of the vacancies on the catalyst surface. The results show that after the introduction of sulfur vacancies, the S 2p peaks in the Vs-ZIS samples show a significant displacement towards higher binding energies, while the Zn 2p and In 3d orbital peaks are also positively shifted by about 0.1 eV. These displacements indicate that the metal centers are reduced and the sulfur centers are oxidized, which proves the successful construction of sulfur vacancies on the surface of the catalysts. The above changes in the XPS signals provide experimental evidence for understanding the local charge reconstruction induced by sulfur vacancies and reveal their potential impact on interfacial charge transfer and catalytic activity.

4.4. Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS)

The presence of sulfur vacancies can significantly modulate the optical absorption behavior of materials. UV-vis DRS, as an important means to study the light response characteristics of materials, can indirectly evaluate the influence of sulfur vacancies on the material’s light absorption by analyzing the shift of the absorption edge position and the change in absorption intensity [159]. Fang et al. [132] successfully constructed a defective ZnS photocatalytic system with a heterostructured interface between the sphalerite and wurtzite phases by regulating the thermally induced phase transformation process. The study found that sulfur vacancies played a crucial role in the formation of the wurtzite phase, effectively broadening the visible-light response range of ZnS. As shown in Figure 7a, the w-ZS-E sample, which is a nearly pure wurtzite phase rich in sulfur vacancies, exhibits the strongest tail absorption among the samples. The presence of this tail absorption may indicate the existence of defects, leading to the formation of localized states extending into the band gap.

4.5. Synchrotron Radiation X-Ray Absorption Fine Structure (XAFS)

XAFS can also be used to study the local environment around atoms, including how sulfur vacancies affect the coordination of surrounding atoms [162]. For instance, Yan et al. [160] developed a Bi₂S₃@In₂S₃ heterostructure photocatalyst through which unsaturated Bi and In sites were formed, accompanied by abundant sulfur defects. XAFS revealed the defect information of Bi₂S₃@In₂S₃ catalysts. From the Fourier transform (FT) k3-weighted extended XAFS (EXAFS) spectra (shown in Figure 7b,c), it can be observed that the peaks at 2.1 Å for the Bi–S coordination and at 2.0 Å for the In–S coordination have reduced intensities compared with those of the pure Bi₂S₃ nanorods and pure In₂S₃ nanosheets. This indicates the presence of sulfur vacancies in both Bi–S and In–S coordination shells of Bi₂S₃@In₂S₃ catalysts.

4.6. Photoluminescence (PL) Spectrum

The PL spectrum is a technique that obtains the electronic structure information of a material by measuring the photons emitted after the material is excited by light. Specifically, when the material absorbs photons, electrons transition from the valence band to the conduction band, forming electron-hole pairs [163]. These excited-state electrons can return to the valence band through different paths and release photons in the process. Different types of defect states affect the recombination process of these electrons and holes, thereby influencing the characteristics of the PL spectrum [164]. For example, Ganesh et al. [161] successfully prepared CdS nanospheres rich in sulfur vacancies using a one-step chemical synthesis method and achieved effective control of their microstructure by regulating the concentration of TAA. As shown in Figure 7d, at an excitation wavelength of 325 nm, the PL spectrum of CdS (sample S3) shows a clear broad peak at 528 nm, which is caused by sulfur vacancies or surface defects.

4.7. High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM)

HAADF-STEM is a commonly used spherical aberration electron microscopy imaging modality in which the image contrast depends mainly on the atomic number. Heavier elements appear brighter than lighter elements, so different types of atoms and their defects can be distinguished by this imaging mode [165]. As an example, in the study of Sun et al. [166], they constructed a ZnIn₂S_4–x-WO_3−x Z-scheme heterojunction photocatalyst (denoted as ZW) using an in situ hydrothermal synthesis strategy. The presence of oxygen and sulfur vacancies in the ZW-4 sample was clearly confirmed by HAADF-STEM imaging. Figure 8a,b illustrate the typical hexagonal patterns of ZnIn₂S₄ and WO₃. Notably, sulfur defects and oxygen defects can be observed in the middle of the nanosheets (marked with dashed lines). To further verify the defect types and their influence on the optical properties of the materials, the authors also performed PL spectroscopy. As shown in Figure 8c, two characteristic peaks appear at 567 and 486 nm for ZW. Among them, the emission peak at 486 nm is attributed to the localized state jump induced by the oxygen defects in the crystal, while the emission peak at 567 nm is closely related to the energy level jump induced by the sulfur defects.

4.8. Other Characterizations

In addition, other characterizations include transmission electron microscopy (TEM [167]) and high-resolution transmission electron microscopy (HRTEM [168]); although it is more difficult to observe sulfur vacancies directly, the presence of sulfur vacancies can be indirectly inferred by observing the crystal structure and defect distribution of the material [169]. The presence of sulfur vacancies can also be verified by Raman spectroscopy. Sulfur vacancies cause changes in the vibrational modes of the lattice, resulting in shifts or intensity changes of the characteristic peaks in the Raman spectrum. In addition, the presence of Vs can be demonstrated indirectly using density functional theory (DFT) calculations [170]. Although this is not an experimental method, modeling the electronic structure changes after the formation of sulfur vacancies through theoretical calculations can help to explain the experimental phenomena and predict the effect of sulfur vacancies on material properties. Although there are currently several characterization techniques available for directly or indirectly verifying the existence of sulfur vacancies, due to the immature synthesis methods, many materials are prone to interference by various factors during the characterization process, resulting in significant uncertainty in the identification of sulfur vacancies. Moreover, the existing characterization methods still have significant limitations in accurately determining the concentration of sulfur vacancies, making it difficult to establish a clear correlation between the concentration of sulfur vacancies and catalytic performance. Therefore, more advanced and sensitive characterization techniques need to be developed to achieve precise identification and quantitative analysis of sulfur vacancies.

5. Effect of Sulfur Vacancies on Photocatalytic Reduction of CO₂

The development of efficient catalysts to achieve the conversion of CO₂ into high-value-added fuels still faces many challenges, including the insufficient light responsiveness of the materials, the inefficient separation of photogenerated carriers, and the limited catalytic efficacy of the surface reactive sites [171]. In recent years, the introduction of sulfur vacancies has been widely considered as one of the effective strategies to enhance the photocatalytic performance (Table 1) [83,172]. The role of sulfur vacancies is particularly crucial in the photocatalytic reduction of CO₂. On the one hand, they can improve the overall catalytic activity of the reaction by optimizing the electronic structure and interfacial charge transportation behavior [173]; on the other hand, sulfur vacancies can also modulate the adsorption configuration and reaction pathway of CO₂ molecules on their surfaces, which in turn affects the product selectivity [174]. The effects of sulfur vacancies on the yield and selectivity of CO₂ photocatalytic reduction products are analyzed in the following sections.

5.1. Effect of Sulfur Vacancy on CO₂ Photoreduction Product Yield

The effect of some catalysts on the product yield of CO₂ photocatalytic reduction is not significant due to the low utilization of light [154], poor separation efficiency of photogenerated electrons and holes [175], and low efficiency of surface reactive sites [70]. The introduction of sulfur vacancies effectively improves the problem of low yield due to these reasons.

5.1.1. Enhanced Light Absorption

The light absorption ability of a photocatalyst determines its efficiency in taking advantage of solar energy. It has been shown that the introduction of sulfur vacancies can effectively expand the light responsiveness range of the material and improve light absorption [176]. Sulfur vacancies, as a typical anionic defect, introduce localized defect states in the lattice. These defect states usually occur near the valence band top or within the band gap, forming what are known as “defect energy levels”. These newly generated energy levels can serve as intermediate bridges for electron transitions, enabling electrons that originally require high-energy photons to be excited to the conduction band through multiple-step excitation or sub-bandgap absorption, thereby responding to low-energy photons (such as visible light or even near-infrared light). Additionally, sulfur vacancies may cause lattice distortion and local charge redistribution, further altering the material’s energy band structure, reducing the effective band gap, promoting band-edge movement, and enhancing the excitation efficiency of photogenerated carriers. At the same time, the surface polarization effect and local electron enrichment brought about by sulfur vacancies also help to enhance the material’s photon capture ability and absorption coefficient [177]. The AgInS₂ photocatalyst was enriched with sulfur vacancies (Vs-AgInS₂) synthesized by Wang et al. [93]. It was found by UV-vis-NIR diffuse reflectance spectroscopy and femtosecond transient absorption spectroscopy (fs-TAS) analysis (shown in Figure 9a–c) that the sulfur vacancies not only enhanced the light absorption in the near-infrared region but also significantly contributed to the effective separation of carriers. Shi et al. [95] comparatively investigated the sulfur vacancy-containing Cu₂S_1−x with intact lattice Cu₂S material; the UV-vis DRS is shown in Figure 9d, and the enhanced light absorption intensity of the former is attributed to the defect state modulation effect induced by the surface sulfur vacancies. This enhanced light-responsive behavior directly enhances the photogenerated electron generation efficiency, which in turn optimizes the overall performance of the photocatalytic reduction reaction.

5.1.2. Promoting Photogenerated Electron-Hole Separation

In order to explore the reason why sulfur vacancy enhances CO₂ RR activity, electrochemical and carrier dynamics were employed to analyze the photogenerated charge transport behavior of the catalyst. The introduction of sulfur vacancies is believed to significantly affect the charge dynamics in semiconductor materials. Firstly, sulfur vacancies can introduce defect levels in the band gap, acting as electron traps or migration channels, effectively capturing photogenerated electrons and thereby inhibiting the bulk-phase recombination of electron-hole pairs. Secondly, since sulfur vacancies are usually accompanied by local electron enrichment and lattice distortion, they can form built-in electric fields or band bends, promoting the spatial separation of photogenerated carriers and reducing the recombination probability. Moreover, sulfur vacancies may also enhance the intrinsic conductivity of the material and accelerate the migration rate of carriers to the active sites on the catalyst surface [178]. Liu et al. [96] constructed an S-scheme heterojunction photocatalyst, Sv-ZnS/ZIS, based on the modulation of the sulfur vacancy, as shown in Figure 10a,b, which was confirmed by electrochemical impedance spectroscopy (EIS) and linear scanning voltammetry (LSV) to have significant advantages in photogenerated carrier migration and separation. The separation efficiency of the carriers was further investigated by steady-state photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy to assess the carrier separation efficiency, as shown in Figure 10c,d. The separation efficiency of the photogenerated carriers was further investigated by PL spectroscopy. The presence of sulfur vacancies prolongs the electronic lifetime and effectively enhances the interfacial charge transportation. As an example, the fs-TA results of sulfur vacancy-containing ZnS nanoparticles (Vs-ZnS/OMNC) loaded on an ordered mesoporous nitrogen-doped carbon skeleton (OMNC) designed by Gao et al. [97], as shown in Figure 10e,f, show a significant prolongation of the carrier lifetime of the material. It is shown that the synergistic effect of sulfur vacancy and OMNC structure effectively promotes charge separation and enhances the carrier migration efficiency, which results in an efficient photocatalytic CO₂ reduction activity [179].

5.1.3. Providing Reactive Sites and Enhancing CO₂ Adsorption

The photocatalytic reduction of CO₂ is a complex multi-step process, typically divided into four key stages: adsorption, activation, reaction, and desorption. The efficiency of this process is influenced by the properties of the catalyst surface, the charge dynamics behavior, and the interaction between reactants and intermediates. In the adsorption stage, CO₂ molecules need to be physically or chemically adsorbed onto the catalyst surface, which is a prerequisite for the subsequent transformation [180]. However, due to the linear symmetrical structure and high C=O bond energy (approximately 750 kJ/mol) of CO₂ molecules, their chemical inertness is strong, resulting in weak adsorption capacity, which limits the initiation rate of the reaction. Sulfur vacancies, as a typical anionic defect, play multiple theoretical and functional roles in this process. The presence of sulfur vacancies usually leads to local lattice distortion and charge redistribution, forming electron-rich regions. These regions can serve as strong adsorption sites, enhancing the adsorption and enrichment of CO₂ molecules through electrostatic interactions or coordination, which is beneficial for increasing the surface reactant concentration and thereby improving the reaction probability [155]. Additionally, in the activation stage, the adsorbed CO₂ molecules must be effectively activated, that is, breaking their stable linear structure, usually by bending the O=C=O bond angle to form an active intermediate. This process requires the acquisition of electrons and accompanies a large activation energy barrier. Sulfur vacancies play a dual role in this process: on the one hand, as an electron capture center, sulfur vacancies can capture photogenerated electrons, significantly inhibiting the phase and surface recombination of electron-hole pairs and prolonging carrier lifetime; on the other hand, the captured electrons can directly transfer to the adsorbed CO₂ molecules, promoting electron reduction and reducing the activation energy barrier, achieving efficient activation of CO₂ [181]. Cao et al. [139] successfully achieved efficient photocatalytic reduction of CO₂ by constructing atomically dispersed Cu reactive sites and the concomitant generation of sulfur vacancies on the CdS surface. The combination of experimental characterization and theoretical calculations revealed that the presence of sulfur vacancies leads to the formation of local defect energy levels in the forbidden bands, which provide temporary residency states for photogenerated carriers, thus enhancing the light trapping and charge separation efficiency. The dispersion of Cu atoms and the presence of sulfur vacancies promote the adsorption and activation of CO₂, which subsequently leads to the excellent photocatalytic activity. Zhang et al. [158] constructed a 2D/2D WO₃/Vs-ZIS S-scheme heterojunction photocatalyst that exhibited excellent catalytic activity in the reduction of CO₂ to CH₄. The enhanced performance is mainly attributed to the synergistic effect between the sulfur vacancies and the S-scheme heterojunction, which not only improves the spatial separation efficiency of the photogenerated carriers but also enhances the adsorption and decomposition of H₂O. The sulfur vacancies in the heterojunction can act as reactive sites, which markedly improve the adsorption and activation of CO₂ and thus the yield of CO₂ photocatalytic reduction.

5.2. Influence of Sulfur Vacancy on the Selectivity of CO₂ Photoreduction Products

Due to the high thermodynamic stability and kinetic inertness of CO₂ molecules, the photocatalytic reduction reaction pathway of CO₂ is extremely complex, usually involving multi-electron-proton coupling transfer processes, resulting in diverse product types. Besides generating CO, CO₂ can also be further reduced to produce high-value hydrocarbon fuels or chemicals such as CH₄, CH₃OH, and C₂H₄. However, the selectivity of these hydrocarbon products in the actual catalytic process is often low. The main reasons for this are as follows: on the one hand, the key reaction intermediates (such as *COOH, *CO, *CHO, *OCH₂, etc.) have poor stability on the catalyst surface and are prone to dissociation or desorption; on the other hand, CO, as a common primary reduction product, has a relatively weak binding energy with the catalyst surface and tends to preferentially desorb, thereby interrupting the further hydrogenation process and limiting the occurrence of C–C coupling or deep reduction reactions [182]. In the reaction stages of the four steps mentioned above for photocatalytic CO₂ reduction, the adsorbed and activated CO₂ molecules first accept an electron and a proton to form the COOH intermediate and then further reduce to generate CO, which is a key branching point leading to various products. If CO undergoes desorption, the final product is CO; if it remains on the surface and continues to accept electrons and protons, it can gradually be hydrogenated to generate intermediate species such as CHO, CH₂O, OCH₃, etc. and eventually form CH₃OH or CH₄; under specific conditions, the two CO or CHO species may undergo C–C coupling to generate C₂H₄ and other C2+ products. Therefore, the selectivity of hydrocarbons is highly dependent on the adsorption strength, residence time of the *CO intermediate on the catalyst surface, and the subsequent hydrogenation or coupling kinetics rate. The introduction of sulfur vacancies provides an effective strategy for regulating this complex reaction network [183]. From the perspective of electronic structure theory, sulfur vacancies usually introduce localized defect states in the semiconductor energy band, causing a redistribution of electron density near the Fermi level, forming electron-rich regions. These regions can not only serve as electron enrichment centers to enhance the efficiency of charge separation but also significantly change the local chemical environment on the catalyst surface. Specifically, sulfur vacancies can affect the adsorption configuration and binding strength of key intermediates by regulating the local electronic density and coordination desaturation on the surface [184]. For example, moderately enhancing the adsorption energy of CO can effectively inhibit its premature desorption, prolong its residence time on the surface, and thereby promote its further hydrogenation to generate CH₄ or participate in C–C coupling to form C₂H₄. Meanwhile, sulfur vacancies may stabilize *CHO or *COH and other hydrogenation intermediates, lowering the energy barrier of the deep reduction process, thereby improving the selectivity of hydrocarbon products. Additionally, during the desorption stage, the timely desorption of products is crucial for maintaining the catalytic cycle. The presence of sulfur vacancies may regulate the interaction strength between the products and the surface, optimizing the desorption kinetics of the products. For example, adsorption that is too strong can lead to catalyst poisoning or active site blockage, while desorption that is too weak is not conducive to the further conversion of intermediates. Therefore, the rational design of sulfur vacancies is expected to achieve a dynamic balance between “stabilizing intermediates” and “promoting product desorption”, thereby improving the overall catalytic performance [185].

5.2.1. C1 Products

In the C1 product formation pathway, *COOH and *CO are the key intermediates. The sulfur vacancies can modulate the electronic structure on the catalyst surface, thereby altering the adsorption configuration and binding energy of these intermediates. The sulfur vacancies can act as electron traps, capturing photogenerated electrons and effectively inhibiting the recombination of electron-hole pairs, thereby prolonging the carrier lifetime [130]. At the same time, the captured electrons can gradually be released and participate in multi-electron reduction reactions (such as CO₂→CH₄ involving eight electron transfers), providing sufficient reducing equivalents for the deep reduction of C1 products. Additionally, the band bending and built-in electric field caused by sulfur vacancies also facilitate the migration of electrons to the surface, improving the efficiency of interface charge transfer [165]. Moreover, the local charge redistribution around the sulfur vacancies may enhance the adsorption and activation of H₂O molecules, promoting the generation and migration of protons (H⁺), thereby accelerating the proton-coupled electron transfer process, which is crucial for the formation of CH₄, a hydrogen-containing C1 product. Lai et al. [98] prepared an In₂S₃/In₂O₃ S-Scheme heterojunction photocatalyst with a hollow double-shell structure and containing sulfur vacancies. DFT calculations demonstrated that the introduction of sulfur vacancies effectively reduced the work function of In₂S₃, enhanced the built-in electric field strength, and thereby improved the carrier separation efficiency. Meanwhile, DFT calculations showed that the S vacancies led to the accumulation of charges around the In atoms, promoting the adsorption and activation of CO₂ and *CO intermediates. This resulted in a lower formation barrier for *CHO intermediates compared to the CO desorption barrier, leading to high selectivity for CH₄ rather than CO. The Vs changed the electronic orbitals and the highest occupied molecular orbital (HOMO) of the In atoms, enhancing the interaction between the catalyst and *CHO, thereby reducing ∆G *CHO and regulating the selectivity of CH₄. As shown in Figure 11e, the methane yield was 16.52 μmol·g⁻¹·h⁻¹, with a selectivity of up to 95.93%.

5.2.2. C2+ Products

The formation of C2+ products usually relies on the C–C coupling reaction between two or more CO intermediates. Sulfur vacancies can enhance the adsorption strength of CO intermediates by regulating the surface electronic structure, preventing their premature detachment, and providing sufficient time windows for C–C coupling [155]. Moreover, sulfur vacancies may change the geometric structure and electronic environment of the catalyst surface, causing the *CO intermediates to arrange in a more favorable way for coupling, shortening the C–C distance, and reducing the energy barrier of the coupling reaction, thereby promoting the formation of C2+ products [186]. Xing et al. [99] developed a Cu-doped In₂S₃ photocatalyst and calculated experimentally and theoretically (shown in Figure 12a) that the asymmetric sulfur vacancy (Cu-Vs-In) could provide electrons as active centers, which enhanced the adsorption of CO₂ on the In₂S₃ surface and also significantly reduced its adsorption energy barrier. The presence of sulfur vacancies provided additional reactive sites that also promoted the weakening of C=O bonds in CO₂. The increased C₂H₄ selectivity (shown in Figure 12b) was due to the formation of asymmetric sites by the sulfur vacancies, which lowered the overall activation energy barrier and stabilized the surface CO* species (shown in Figure 12c), thus facilitating the intermediate *OCOO produced by further C–C coupling, one of the key intermediates for the generation of C₂H₄ in the CO₂ reduction process. In another example, An et al. [100] synthesized an efficient photocatalyst for converting CO₂ to ethylene (C₂H₄) by implanting Ga atoms into CoS₂, which promoted the formation of adjacent sulfur vacancies on the original CoS₂. DFT theoretical calculations (as shown in Figure 12e) demonstrated that this asymmetric electron distribution enhanced the attraction between adjacent atoms, resulting in a shortened distance between adjacent Co^δ+ (δ = 2, 3) atoms. The coexistence of adjacent Co^δ+ atoms brought the d-band center closer to the Fermi level, facilitating the strong adsorption of *CO and subsequent *COCO dimerization. The asymmetric electronic structure between adjacent Co^δ+ atoms on Ga-CoS₂ provided a lower activation energy barrier for C–C coupling, significantly promoting the generation of C₂H₄. As shown in Figure 12d, the strategy proposed in this study can regulate the selectivity of photocatalytic reduction of CO₂ to C₂H₄. Ga-CoS₂ exhibited outstanding C₂H₄ selectivity (94.5%) and high C₂H₄ yield (12.45 μmol·g⁻¹·h⁻¹), which was 20 times that of CoS₂. This work not only deepened the understanding of the structure-performance relationship of catalysts but also further revealed the key role of sulfur vacancies in regulating the electronic structure, promoting key reaction steps, and enhancing catalytic performance.

From the literature mentioned above, it can be seen that sulfur vacancies can effectively enhance the performance of photocatalytic reduction of CO₂ by optimizing the electronic structure of the catalyst, enhancing light absorption capacity, improving charge separation efficiency, and improving the adsorption and activation process of reactants. However, when sulfur vacancies are introduced, other changes also occur in the catalyst, which cannot well prove the specific influence of sulfur vacancies on the catalyst. In conclusion, although the strategy of using sulfur vacancies provides a new idea for the development of efficient and stable photocatalysts and opens up new ways for green energy conversion and environmental remediation, further research is still needed to fully understand its specific mechanism of action.

6. Conclusions and Outlook

The conversion of CO₂ to high-value-added hydrocarbons using photocatalytic reduction has been shown to be feasible. The modification of catalysts is particularly critical in this process. In this paper, we reviewed the effect of sulfur vacancies on the generation of hydrocarbon products by photocatalytic reduction of CO₂ and summarized a variety of methods for introducing sulfur vacancies, as well as their characterization, which not only verified the existence of sulfur vacancies but also revealed their effects on the material properties. In addition, this paper explored how sulfur vacancies can enhance the activity and selectivity of catalysts by enhancing light absorption, improving carrier separation efficiency, providing adsorption and activation sites, and stabilizing reaction intermediates. It was demonstrated that sulfur vacancies not only markedly improve the yield of CO₂ reduction products but also modulate product selectivity, leading to more efficient energy conversion. In conclusion, the engineering of sulfur vacancies provides a new idea for the development of highly efficient and stable photocatalysts and opens up a new way for green energy conversion and environmental remediation.

Although enhancing the performance of photocatalysts for CO₂ reduction through the introduction of sulfur vacancies shows significant potential for application, it also faces formidable challenges. These include how to control the location and concentration of the introduced sulfur vacancies, how to more accurately characterize the concentration of sulfur vacancies, and how to more accurately analyze the mechanism of action of sulfur vacancies without the influence of other factors. Therefore, solving these problems is an urgent task in the study of sulfur vacancies.

6.1. Precise Control of Sulfur Vacancy Concentration and Position Is Still the Core Problem of Material Design

Currently, sulfur vacancies are introduced in various ways, including plasma treatment, organic treatment, heteroatom doping, etc., but it is often difficult for these methods to achieve precise regulation of their concentration and distribution. The ideal distribution of sulfur vacancies should maximize the catalytic activity without causing an increase in non-radiative compounding or other negative effects due to excessive defects. In addition, the difference in the distribution of sulfur vacancies in different crystalline or interfacial regions of the material will also affect its catalytic efficacy. Therefore, it is necessary to develop more elaborate synthesis strategies to achieve the controlled construction of sulfur vacancies at specific locations (e.g., surfaces, interfaces, and grain boundaries) and to establish a clear correlation between defects and structural properties.

6.2. Means of Characterization of Sulfur Vacancies Need to Be Further Improved

The characterization techniques for sulfur vacancies require advancement because most of the literature only proves the existence of vacancies through surface characterization, without revealing their actual quantity. Although various techniques have been applied to detect sulfur vacancies, quantitative analysis remains challenging, especially in complex heterogeneous structures or multi-component systems. This makes it difficult to determine the correlation between the photocatalytic performance and selectivity of catalysts and the concentration of sulfur vacancies. Therefore, there is an urgent need for more advanced and sensitive techniques to accurately analyze and quantitatively characterize the vacancy density in nanomaterials.

6.3. The Role of Sulfur Vacancies Under Multifactorial Coupling Is Still Unclear

Although there are various methods to create sulfur vacancies, they often lead to changes in other properties of the catalyst, making it difficult to clearly determine their actual role in photocatalytic CO₂ reduction. Therefore, new strategies are still needed to precisely introduce sulfur vacancies while maintaining the stability of the main structure in order to clarify the intrinsic relationship between their concentration and activity. Regulating sulfur vacancies is the key to improving the photocatalytic performance of sulfides, but it still faces challenges. In the future, attention should be focused on their roles in electronic structure and reaction kinetics, combined with high-precision characterization and theoretical simulation, to deeply understand the microscopic correlation between vacancy concentration and performance.