Enhanced Photocatalytic CO₂ Reduction with Incorporation of WO₃ Cocatalyst in g-C₃N₄-TiO₂ Heterojunction

Abstract

To enhance the performance of photocatalytic CO₂ reduction, the development of suitable cocatalysts represents an effective strategy. Cocatalysts can interact with photocatalysts to improve light absorption capabilities and facilitate the separation and transfer of photogenerated electrons and holes. Moreover, they provide highly active surface sites that promote the adsorption and activation of CO₂, which leads to acceleration of photocatalytic reduction. Herein, WO₃ is employed as a cocatalyst to promote the CO₂ photoreduction performance of a g-C₃N₄-TiO₂ heterojunction through a facile and scalable calcination method. In pure water, optimal WO₃/g-C₃N₄-TiO₂ (WCT) delivers high selectivity CO and CH₄ formation of 48.31 µmol·g⁻¹ and 77.18 µmol·g⁻¹ in the absence of a sacrificial reagent and extra photosensitizer, roughly 13.9 and 45.7 times higher than that of g-C₃N₄-TiO₂ (CT). WO₃ can strongly interact with g-C₃N₄-TiO₂ electronically, guiding electrons across the interface to the surface. The oxygen vacancies in WO₃, as electron-enriched centers, not only enhance charge separation and form efficient charge transfer channels but also capture photogenerated electrons to suppress charge recombination. This strong interaction and oxygen vacancies in WO₃ jointly improve photocatalytic CO₂ reduction activity and selectivity, offering a feasible way to design efficient cocatalysts.

1. Introduction

Utilizing excess CO₂ as a carbon raw material to generate value-added compounds via catalytic reactions is one of the viable strategies for reducing CO₂ emissions. Thus, exploring photocatalytic CO₂ reduction into C1 solar fuels such as CO and CH₄, which can decrease CO₂ content in the environment and reduce humanity’s reliance on petroleum and other products, holds great promise in curbing global warming [1,2,3,4]. The predominant limitations of reported photocatalysts for CO₂ reduction include low efficiency, high cost, and toxicity [5,6]. In particular, slow charge dynamics including insufficient light absorption and high recombination rates of electron–hole pairs, together with high activation energies, severely restrict photocatalytic CO₂ activity [7,8,9]. Consequently, approaches like band-gap engineering through metal or non-metal element doping, deposition of noble metals, or the incorporation of cocatalysts [10,11] have been proposed to tackle these problems.

The incorporation of cocatalysts into photocatalysts is an effective way to enhance CO₂ photoreduction performance. Cocatalysts can promote surface charge separation, crucial for efficient charge transfer; boost the quantity of active sites by modifying the photocatalyst’s surface structure and electronics; reduce CO₂ conversion activation energy, making the reaction more thermodynamically favorable and accelerating product formation; enhance selectivity toward target molecules like CO, methane, or formic acid, and improve the photocatalytic system’s stability by alleviating excess charge accumulation; optimize the reaction pathway, guiding reactants through beneficial steps to lower the energy barrier and minimize side reactions; and some with unique optical properties can enhance light harvesting by absorbing specific wavelengths of light and transferring energy to photocatalysts, expanding the light-response range. This multifaceted role of cocatalysts shows great potential for developing more efficient and sustainable photocatalytic CO₂ utilization systems [12,13]. Among the reported cocatalysts, tungsten trioxide (WO₃), as an oxide of transition metal elements, exhibits distinctive properties. Characterized by a relatively wide band-gap energy spanning from 2.4 to 2.8 eV, WO₃ is capable of absorbing a broader spectrum of visible light, which endows it with enhanced attractiveness [14]. The synergistic effect with other materials can not only improve the light absorption performance and carrier separation efficiency of the photocatalyst but also adjust the surface properties and active site distribution of the catalyst, thereby improving the overall performance of the photocatalyst in CO₂ adsorption and activation processes, and thereby enhancing catalytic efficiency. Moreover, WO₃ emerges as a highly prospective candidate for photocatalysts on account of its non-toxicity, physicochemical stability, and remarkable resistance to photo-corrosion [15].

Concurrently, graphite carbon nitride (g-C₃N₄), due to its unique band structure and ease of fabrication, has become a prominent material in the field of photocatalysis [16,17,18]. This particular material exhibits a low band gap of 2.7 eV, minimal particle presence, non-toxicity, cost-effectiveness, favorable controllability of the structure, and superior electronic and optical properties [19]. This material also enhances the photocatalytic activity of the composite by extending the absorption band. Nevertheless, the low electrical conductivity, relatively small surface area of g-C₃N₄, as well as the rapid recombination of electrons and holes, limit its photocatalytic activity [20]. A promising aspect is that the integration of g-C₃N₄ and TiO₂ in a heterojunction configuration has demonstrated remarkable photocatalytic efficiency [21]. The photoreduction of CO₂ based on a g-C₃N₄/TiO₂ heterojunction was studied to produce solar fuel. For instance, Wang et al. [22] fabricated g-C₃N₄–based composite photocatalysts modified with titanium dioxide (TiO₂) through ball-milling and calcination for the photoreduction of CO₂ to methane and carbon monoxide. Zhang et al. [23] constructed 3D/2D TiO₂/p-g-C₃N₄ micro-nano heteroarchitectures through a facile acid hydrothermal route, which improved the performance of photocatalytic CO₂ reduction. Although the g-C₃N₄-TiO₂ heterostructure has shown good stability in CO₂ reduction, the technology based on the g-C₃N₄-TiO₂ heterostructure photocatalyst is not mature enough to obtain the ideal product possessing a high yield along with high selectivity.

In this study, the incorporation of a WO₃ cocatalyst into a g-C₃N₄-TiO₂ heterojunction was attempted to fabricate WO₃/g-C₃N₄-TiO₂ photocatalysts for enhanced photocatalytic CO₂ reduction. A detailed synthesis process was elucidated; a comprehensive characterization of the catalysts was performed; and the mechanisms underlying the enhanced photocatalytic activity were revealed. These materials demonstrated excellent high photocatalytic activity and chemical stability for carbon dioxide reduction due to enhanced charge separation and transfer.

2. Results and Discussion

2.1. Characterizations of WO₃/g-C₃N₄-TiO₂

WO₃/g-C₃N₄-TiO₂ samples were synthesized by the calcining method as depicted in Figure 1a. The Zeta potential of TiO₂ is 1.09 mV and that of WO₃ is −55.41 mV, which favors the formation of the interfaces between TiO₂ and WO₃, and consequently, a strong interaction among these semiconductors, along with more light absorption pathways. The morphology of WO₃/g-C₃N₄-TiO₂ was probed further via SEM, showing a two-dimensional layered structure of g-C₃N₄ and a sphere structure of TiO₂, which are shown in Figure 1b. WO₃ synthesized by the hydrothermal method has a cubic configuration, and uniformly disperses over TiO₂. Some g-C₃N₄ sheets are also observed to be attached to WO₃. In this regard, WO₃/g-C₃N₄-TiO₂ with a good interaction to construct composite is achieved. More interestingly, the HRTEM images in Figure 1c provide further evidence of the strong interaction existing among these three components. The microspheres of TiO₂ with multiple channels were seen as well as WO₃ possessing a cubic structure, which facilitates the formation of oxygen vacancies [24]. Furthermore, the combined structure of these components is characterized by enhanced compactness, facilitating the separation of charge carriers and ensuring efficient electron transport. Thus, incorporation of a WO₃ cocatalyst into the g-C₃N₄-TiO₂ heterojunction resulted in better optical radiation penetration accompanied by an efficient adsorption process to maximize photoactivity. Further characterization of the catalyst’s elemental composition was carried out using energy-dispersive X-ray spectroscopy (EDS). These results presented in Figure 1d reveal that C, N, O, Ti, and W are dispersed throughout the composite photocatalyst. Elemental mapping of WO₃/g-C₃N₄-TiO₂ further indicates the uniform distribution. Moreover, they demonstrate the successful binding of WO₃ to the surface of g-C₃N₄-TiO₂. Based on these findings, the WCT heterojunction has been successfully synthesized, which is conducive to efficient light absorption and charge carrier separation.

The results of XRD are displayed in Figure 2 to characterize the phase structure of the prepared photocatalysts. The characteristic peaks of pristine WO₃ are detected at 2θ of 23.10°, 23.62°, 24.38°, 26.54°, 28.72°, 33.32°, and 34.22°. These peaks are associated with the monoclinic phase of WO₃ according to the JCPDS, Card No. 83-0951 [25]. For g-C₃N₄, the peaks at 13.4° and 27.3° correspond to (100) and (002) crystal planes of g-C₃N₄ (JCPDS, Card No.87-1526). In g-C₃N₄, the (002) plane is attributed to interplanar stacking of the aromatic system, whereas the (100) plane is associated with the in-plane structural packing motif [26]. The peaks of TiO₂ match those of the standard PDF card (JCPDS, Card No.21-1272), which provides an indication that the experimental specimens are constituted purely of anatase TiO₂ [27]. Regarding the WO₃/g-C₃N₄-TiO₂ composite, characteristic peaks belonging to TiO₂, g-C₃N₄, and WO₃ are clearly visible. Notably, the characteristic (002) diffraction peak of g-C₃N₄ shifted from 27.3° to 26.5° upon composite formation with WO₃/g-C₃N₄-TiO₂, demonstrating lattice expansion and interfacial electronic coupling effects [24]. Moreover, no impurity-related peaks are detected in the XRD pattern, confirming the high purity of the as-synthesized samples. This result further demonstrates successful fabrication of the composite without significant distortion of the original crystal structures of the individual components.

As depicted in Figure 3, to study the structural features of the samples, Raman spectra were obtained. For pure WO₃, characteristic bands are clearly discernible at 131.92, 272.46, 716.92, and 807.41 cm⁻¹. The peak at 272.46 cm⁻¹ is attributable to the bending vibration of the W–O bond, while the peaks at 716.92 and 807.41 cm⁻¹ are indicative of the stretching vibration mode of the O–W–O bands. These results are consistent with the previous ones [28,29]. Analogously, the Raman peaks at 360.1 and 1229.6 cm⁻¹ belong to the polymer-based structure of g-C₃N₄ [30]. The absence of D and G bands was probably accounted for by the relatively low content of g-C₃N₄ revealed by EDS, along with its poor crystallinity indicated by the indistinct XRD peaks [31]. Anatase TiO₂ primarily consisted of peaks at 143.41, 199.35, 396.22, and 519.64 cm⁻¹. Specifically, the peaks at 143.41 and 396.22 cm⁻¹ correspond to vibration of the Ti–O bond, while the peak at 199.35 cm⁻¹ is attributed to vibration of the Ti–O–Ti bond. The peak at 519.64 cm⁻¹ is associated with vibration of the O–Ti–O bond [32,33]. In the distinctive characteristic patterns of the WO₃/g-C₃N₄-TiO₂ (WCT) composite, Raman modes appear at 142.21, 271.21, 716.17, and 808.95 cm⁻¹. It is possible that due to the low content and poor crystallinity of g-C₃N₄, as well as it being encapsulated, the peaks of graphitic carbon nitride were not observed in the WCT composite. A slight shift was observed in the peaks of WO₃ and TiO₂, indicating an interaction between WO₃ and TiO₂ that results in the formation of a composite facilitating efficient charge carrier transfer. Furthermore, the distinct vibrational peak of WO₃ in the Raman spectrum of the composite indicates that WO₃ within the WCT composite possesses remarkable crystallinity.

2.2. CO₂ Photoreduction Efficiencies

To assess the effectiveness and catalytic activity of the WO₃/g-C₃N₄-TiO₂ photocatalyst, the CO₂ reduction performance of the prepared materials was assessed under xenon lamp illumination in water without additional organic sacrificial agents (such as triethanolamine) or photosensitizers. And a series of conditional blank control experiments were carried out; the data can be found in Table S1 of the Supplementary File. The main reduction products were identified as CO and CH₄, and nothing was detected in the dark or in the absence of photocatalysts or CO₂. The evolution of CO and CH₄ for all samples was observed in (Figure 4). The WO₃/g-C₃N₄-TiO₂ (WCT) samples exhibited boosted photocatalytic performance in reducing of CO₂ to solar fuel C1 products, among which WCT displays a maximum CO production yield of 48.31 µmol·g⁻¹and a maximum CH₄ production yield of 77.18 µmol·g⁻¹, roughly 13.9 and 45.7 times higher than that of CT (3.53 and 1.70 µmol·g⁻¹, respectively). Furthermore, with increased loading of WO₃, the photocatalytic performance of WCT is markedly enhanced. From Figure 4b,c, the yield of CO and CH₄ for WCT samples are 1.25 and 1.36 times higher than 0.5 WCT samples. After carrying out the reaction for a four-cycle experiment, the activity decrease is negligible, revealing the good photocatalytic stability of WCT (Figure 4d). The apparent quantum efficiency (AQE) of CO₂ photoreduction was measured at different monochromatic wavelengths including 380, 400, 420, 450, 480, and 500 nm over the WCT sample. Clearly, the trend of AQE matches well with the light absorption spectrum of WCT (Figure 4e) and the value of AQE reaches 4.41% at a 400 nm wavelength (Table S4 in Supplementary File), affirming the photocatalytic nature of CO₂ reduction. In order to uncover the underlying reasons for the high production yield of CH₄, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was explored. When CO₂/H₂O is introduced into the system in the dark, the chemisorption of CO₂ on the catalyst surface is detected, evidenced by the appearance of peaks corresponding to HCO₃⁻, m-CO₃²⁻, and b-CO₃²⁻ species [34]. Under light irradiation, adsorption peaks associated with *COOH (at 1669 cm⁻¹), *CHO (at 1012 cm⁻¹), and *CH₃O (at 1042 cm⁻¹) emerge. These species are crucial intermediates in the process of converting CO₂ into CH₄.

Additionally, a comparison of the photocatalytic performance between the WCT samples and other previously reported advanced photocatalysts in the literature is summarized in

Table 1. The photocatalytic activities of other previous advanced photocatalysts reported in the literature are summarized [34,35,36,37].

Catalyst	CO Yield (μmol·g–1·h–1)	CH4 Yield (μmol·g–1·h–1)	Fold Increase in CH4 Production	AQE (%)	Ref.
Our work: WCT	12.08	19.30	>45	4.41	This study
WO3/ZnIn2S4	44.1	0	0	3.63	[34]
Ru@H-MoO3-x	28.0	111.6	8.8	1.05	[35]
strained In2S3	99.25	11.0	2.11	0.45	[36]
(GeH)1–xSix(OH)0.5Hx–0.5	6.91	0	0	5.95	[37]

.

2.3. Insight into Increased Photocatalytic Activity

Figure 5a presents the XPS scan spectrum of WCT. As can be observed from this figure, WCT encompasses signals of the elements of tungsten, carbon, nitrogen, titanium, and oxygen. The signal peaks of these elements can be deconvoluted to conduct a more in-depth analysis of their chemical states. The high resolution W4f spectrum in Figure 5b shows binding energies of 34.8 and 37.0 eV for W4f_7/2 and W4f_5/2, respectively, corresponding to the W⁺⁶ oxidation state of WO₃ [38]. From Figure 5c, the signals of Ti2P_3/2 and Ti2P_1/2 in TiO₂ are located at the peaks of Ti2p at 458.2 and 464.1 eV. Figure 5d depicts the XPS spectrum of O1s with two peaks at 529.5 and 531.1 eV. The strong O1s peak at 529.5 eV is ascribed to the lattice O atom in the WCT composite. Owing to WO₃ lattice O-defects and surface-adsorbed OH⁻/O₂⁻ [39], the intensity of peak at binding energy 531.1 eV decreases. Regarding Figure 5e, it reveals the C1s orbital spectrum of WCT, featuring three characteristic peaks at binding energies of 284.5 eV, 286.1 eV, and 288.4 eV. These peaks are each assigned to the sp² C–C, C–N, and N–C=N bonds in g-C₃N₄. The N1s spectrum in Figure 5f, t can be split into peaks at 398.1 eV and 399.8 eV, which respectively correspond to the C=N–C and N–C bonds in g-C₃N₄ [40]. In conclusion, the successful synthesis of the WO₃/g-C₃N₄-TiO₂ (WCT) composites is further supported by X-ray photoelectron spectroscopy (XPS) analysis of the composite specimens. This analysis also provides additional evidence that aligns with the results obtained from other characterization methods.

To investigate light-harvesting capabilities and the band gap of the samples, UV–vis DRS spectra were obtained. Figure 6a depicts the UV–vis DRS spectra of TiO₂, g-C₃N₄, WO₃, CT, and WCT samples. The incorporation of WO₃ significantly enhances the light absorption capacity of g-C₃N₄-TiO₂ (CT), thereby exerting a considerable effect on the photocatalytic process. Moreover, narrower band-gap energies (Egs) can enhance the photoelectric conversion efficiency of composite materials, which is crucial for the photocatalytic process. The band gaps were determined by Tauc plots in Figure 6b. The band-gap energies (Egs) of pure WCT, CT, and WO₃ were calculated to be 2.69, 2.77, and 2.81 eV, respectively [41]. This can be attributable to WO₃–induced mid-gap states (XPS in Figure 5) and interfacial charge transfer. This enhanced light harvesting directly correlates with a 36% improvement in CH₄ production (Figure 4c), confirming the critical role of band engineering.

Photocatalytic activity depends mainly on efficient charge separation and transport. A higher photoluminescence (PL) intensity indicates a lower charge separation efficiency and leads to a higher electron–hole recombination rate. Photoluminescence carrier production and its recombination can be evaluated by photoluminescence (PL) analysis. Figure 7 shows the charge separation efficiency of the TiO₂, g-C₃N₄, WO₃, CT, and WCT composite samples. Using pristine TiO₂, a strong emission peak appears, owning to faster recombination of photogenerated electron–hole pairs on the catalyst surface [42]. When coupling TiO₂ with g-C₃N₄, the CT sample’s PL intensity experiences a gradual yet significant decrease, which is attributed to superior charge carrier separation [43,44]. Moreover, the incorporation of WO₃ in a g-C₃N₄-TiO₂ heterojunction to construct the WCT composite is found to be efficient for the separation and transfer of electrons; analogous discoveries have been presented in the relevant literature [45]. Consequently, the construction of a WO₃/g-C₃N₄-TiO₂ heterojunction can facilitate the efficient generation and separation of charge carriers, which is advantageous for the effective reduction of CO₂ under visible light.

To gain deep insights into the electron excitation processes, charge migration mechanisms, and kinetic characteristics such as charge transfer rates of the as-prepared catalysts, measurements of photocurrent responses and analyses of electrochemical impedance spectroscopy (EIS) were carried out. A Nyquist plot of TiO₂, WO₃, g-C₃N₄, CT, and WCT is depicted in Figure 8a. In electrochemical impedance spectroscopy (EIS), there is generally a positive correlation between the arc diameter in the Nyquist plot and the electron transport resistance exhibited by the catalyst. The arc radius corresponding to the WCT–based electrode is smaller in comparison to TiO₂, g-C₃N₄, and CT, resulting from the incorporation of WO₃ into g-C₃N₄-TiO₂. Therefore, due to the higher photocatalytic activity of WCT compared to CT, this signifies the lowest charge transfer resistance (Rct) among WCT catalysts. This facilitates the separation of charge carriers and enables the rapid separation of photogenerated electron–hole pairs [46,47]. Moreover, the electronic interactions of the prepared photocatalysts were studied through photocurrent measurements using an electrochemical workstation. As depicted in Figure 8b, the chronoamperometric current–time (I–t) curve is presented. The photocurrent measurements were obtained under multiple light-on and light-off conditions. In comparison to CT and WO₃, the photocurrent density of WCT is the highest, manifesting the highest separation efficiency of carriers for WCT. Consistent with the PL results is the fact that the photocurrent response of WCT is higher than that of CT and pure WO₃. The WCT composites exhibit a significantly higher photocurrent, providing compelling evidence of enhanced charge separation efficiency (e⁻-h⁺). This improvement enables the generation of a greater number of photogenerated electrons, thereby substantially augmenting photocatalytic activity. The aforementioned results unequivocally demonstrate that the integration of WO₃ into the g-C₃N₄-TiO₂ heterojunction is conducive to promoting photocatalytic performance by facilitating the separation and transfer of carriers.

Oxygen vacancies (Ovs), as intrinsic defects, can modulate the local geometric structure of metal oxides, thereby tailoring their physical and chemical properties [48]. This modulation influences the band alignment of metal oxides and introduces defect levels, which significantly enhances light absorption. Additionally, the existence of Ovs alters the charge separation and transfer processes within photocatalysts [49]. To investigate the unpaired electrons in WO₃ with oxygen vacancies, electron paramagnetic resonance (EPR) measurements were conducted. As can be seen from Figure 9a, the signals of different samples (WCT, WO₃, TiO₂, CT, and g-C₃N₄) exhibit varying intensity around g = 2.003, which corresponds to unpaired electrons located at the Ovs sites, indicating differences in their unpaired-electron environments, likely related to distinct electronic structures or surface properties. It is evident that after the calcination process, the signal intensity at g = 2.003 of WCT markedly increases, confirming the substantial introduction of Ovs into WO₃ through calcination. The trap states induced by Ovs effectively promote charge separation and facilitate rapid charge transport channels at the heterointerface. Moreover, Ovs enhance light absorption, leading to an increase in photogenerated carriers.

Furthermore, time-resolved photoluminescence (TRPL) spectra can be utilized to explore the dynamic photogenerated charge transfer process. This approach provides more in-depth information, which is beneficial for further validating the underlying mechanism. The TRPL spectra clearly demonstrate that WO₃/g-C₃N₄-TiO₂ enables ultrafast electron extraction (τ₁ = 0.19 ns) and promotes long-lived charge separation (with the contribution of τ₃ reduced to 18.7%). The presence of oxygen vacancies can change the charge distribution on a material’s surface, which in turn affects the separation of electron–hole pairs. On one hand, these oxygen vacancies act as electron-trapping centers, capturing photogenerated electrons from the area near the holes. This process increases the distance between the electrons and holes, reducing their recombination and promoting long-lived charge separation. On the other hand, oxygen vacancies can create a specific electric field distribution on the material’s surface. This electric field helps drive the electrons and holes to move in opposite directions, further enhancing the efficiency of charge separation. Consequently, this reduces the contribution of τ₃ to 18.7% and supports the achievement of sustained charge separation. These features are crucial for CO₂ reduction processes, where multi-electron transfer often limits the reaction rate. The 58% reduction in τ compared to TiO₂ implies that WO₃ not only effectively suppresses electron–hole recombination but also efficiently channels electrons to the active sites for CO₂ adsorption and protonation, which are key steps in the formation of CH₄ or CO [49].

Based on the above research, we propose the following possible mechanisms in Figure 10: under visible light irradiation in a WO₃/g-C₃N₄-TiO₂ (WCT) composite catalyst, photogenerated electrons from WO₃ are effectively separated from holes in the TiO₂ valence band (VB), while electrons from the TiO₂ conduction band (CB) are separated from holes in g-C₃N₄ VB, resulting in increased charge carrier separation efficiency. The incorporation of a WO₃ cocatalyst into g-C₃N₄-TiO₂ reduces recombination rates, leading to enhanced production of hydroxyl radicals, thereby improving the overall photocatalytic activity of the prepared ternary composite catalysts. The optimized photocatalyst demonstrates efficient performance under visible light illumination with a prolonged lifetime suitable for industrial applications as well.

3. Materials and Methods

3.1. Materials

Sodium tungstate hydrate, hydrochloric acid, anhydrous ethanol, urea, anatase titanium dioxide, and other chemicals were purchased from Shanghai Maklin Biochemical (Shanghai, China). The experimental water used in this study was deionized water sourced from the laboratory facilities.

3.2. Material Preparation

The g-C₃N₄-TiO₂ sample was fabricated by a well-documented one-step vapor deposition method in the presence of urea and anatase titanium dioxide [50]; WO₃ was obtained through a hydrothermal process [51]. The WO₃/g-C₃N₄-TiO₂ samples were prepared via a facile calcination method, in particular, as-synthesized WO₃ and g-C₃N₄-TiO₂ were put into an alumina crucible at a mole ratio of (0.1–1):1 to modulate the quantity of WO₃, which was subsequently placed in a vacuum tube furnace, calcined for 4 h, cooled to room temperature, and ground into fine powders. The resultant products were denoted as (0.1–1) WCT composite photocatalysts.

3.3. Characterization

Characterization methods including powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman measurements, scanning electron microscopy (SEM) images, a zeta sizer, high-resolution transmission electron microscopy (HRTEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectra (EIS), transient photocurrent responses, electron paramagnetic resonance (EPR), time-resolved photoluminescence (TRPL) spectra, and in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) were employed. Information about the instruments used for these tests is provided in the Supplementary File.

3.4. Photoreduction of CO₂

Photocatalytic CO₂ reduction was investigated using an automatic online trace gas analysis apparatus (Labsolar-6A, Beijing Perfect Light, Beijing, China). Irradiation was carried out using a 300 W xenon lamp equipped with an AM 1.5 filter and operating at a current of 20 A, positioned 2 cm away from the reactor. The gaseous products were then introduced into a gas chromatograph (GC, GC-9790II, Fuli Instruments, Wenling, China; with argon as the carrier gas and a methanizer attached) for detection. Carbon-containing products, including methane, carbon monoxide, methanol, and multi-carbon compounds, were analyzed by a flame ionization detector (FID), while hydrogen, oxygen, and nitrogen were quantified using thermal conductivity detection (TCD). Error bars indicated the standard deviation derived from three independent measurements conducted with 10 mg of fresh sample for each trial.

4. Conclusions

In summary, through a facile and scalable calcination method, we have successfully synthesized a WO₃/g-C₃N₄-TiO₂ nanohybrid characterized by a unique 3D/2D-3D hierarchical heterostructure, accompanied by the presence of oxygen vacancies. A comprehensive suite of advanced characterization techniques, including UV-DRS, PL, EIS, and EPR, was employed to elucidate the underlying mechanisms. The band-gap energy (Eg) of pure WCT was calculated to be 2.69 eV; compared to TiO₂, g-C₃N₄, and CT, the arc radius corresponding to the WCT–based electrode is smaller; the photocurrent density of WCT is the highest; and the signal intensity at g = 2.003 of WCT corresponds to unpaired electrons located at the oxygen vacancy (Ovs) sites. The photocatalytic performance of the WO₃/g-C₃N₄-TiO₂ (WCT) composite for CO₂ reduction was evaluated under simulated solar light irradiation without any external photosensitizer or sacrificial agent. Impressively, the WCT composite exhibited outstanding CO₂ photoreduction activity, yielding CO and CH₄ with formation rates reaching up to 48.31 µmol·g⁻¹ and 77.18 µmol·g⁻¹, respectively. These values represent a remarkable improvement over previously reported g-C₃N₄-TiO₂–based photocatalysts for CO₂ reduction photocatalysts. The presence of oxygen vacancies plays a pivotal role in this process. Oxygen vacancies in WCT act as active sites that enhance the adsorption and activation of CO₂ molecules, which facilitate the electron transfer processes to enable more efficient conversion of CO₂ to intermediates. Specifically, the unpaired electrons around oxygen vacancies interact with CO₂ molecules, weakening C–O bonds and thus promoting the formation of *COOH. Subsequently, the reaction pathways leading to *CHO and *CH₃O are also accelerated due to the modified electronic environment created by oxygen vacancies. Thus, the reaction rate is significantly increased, while selectivity for CH₄ is simultaneously improved, attributed to the essential role of oxygen vacancies in the CO₂ photoreduction process. This research opens up novel prospects for the rational design of other high-efficiency cocatalysts aiming to enhance CO₂ photoreduction performance.