Engineering g-C₃N₄/Bi₂WO₆ Composite Photocatalyst for Enhanced Photocatalytic CO₂ Reduction

Abstract

As global CO₂ emissions continue to rise, addressing their environmental impact is critical in combating climate change. Photocatalytic CO₂ reduction, which mimics natural photosynthesis by converting CO₂ into valuable fuels and chemicals using solar energy, represents a promising approach for both reducing emissions and storing energy sustainably. However, the development of efficient photocatalysts, particularly those capable of absorbing visible light, remains a challenge. Graphitic carbon nitride (g-C₃N₄) has gained attention for its visible light absorption and chemical stability, though its performance is hindered by rapid electron–hole recombination. Similarly, bismuth tungstate (Bi₂WO₆) is a visible-light-active photocatalyst with promising properties, but also suffers from limited efficiency due to charge recombination. To overcome these limitations, this study focuses on the design and synthesis of a g-C₃N₄/Bi₂WO₆ composite photocatalyst, leveraging the complementary properties of both materials. The composite benefits from enhanced charge separation through the formation of a heterojunction, reducing recombination rates and improving overall photocatalytic performance. The optimized g-C₃N₄/Bi₂WO₆ composite exhibited significant improvements in the production rates of both CH4 and CO, achieving 18.90 and 17.78 μmol/g/h, respectively, which are 2.6 times and 1.6 times higher than those of pure B_i2WO₆. The study explores how optimizing the g-C₃N₄/Bi₂WO₆ interface, increasing surface area, and adjusting material ratios can further enhance the efficiency of CO₂ reduction. Our findings demonstrate the potential of this composite for solar-driven CO₂ conversion, offering new insights into photocatalyst design and paving the way for future advancements in CO₂ mitigation technologies.

1. Introduction

As global carbon dioxide (CO₂) emissions continue to rise, their environmental impact has become a central issue in the fight against climate change [1,2]. CO₂, a significant greenhouse gas, is primarily released from the burning of fossil fuels and industrial processes, contributing to the ongoing rise in global temperatures and leading to a host of negative ecological and socio-economic consequences. Addressing this issue requires not only the reduction of CO₂ emissions, but also the development of technologies that can convert CO₂ into useful products [3,4]. Among the various methods explored for CO₂ mitigation, photocatalytic CO₂ reduction stands out as a promising approach due to its potential to utilize abundant solar energy to convert CO₂ into valuable fuels and chemicals [5,6]. This process not only addresses the need for CO₂ reduction, but also offers a sustainable method of energy storage in the form of chemical bonds.

Photocatalytic CO₂ reduction mimics natural photosynthesis, utilizing sunlight to drive the conversion of CO₂ into carbon-based products such as methane, methanol, or formic acid [7,8,9]. Despite the potential of this process, the development of efficient photocatalysts capable of harnessing sunlight, particularly in the visible spectrum, remains a significant challenge [10,11]. Effective photocatalysts must meet several criteria, including strong light absorption, efficient separation of photogenerated electron–hole pairs, a suitable band structure for CO₂ reduction, and chemical stability [12,13]. While materials such as CO₂ have been widely studied in this field, their limited activity under visible light has motivated the search for alternative photocatalysts [14].

Among the materials investigated, graphitic carbon nitride (g-C₃N₄) has attracted significant attention due to its unique properties. g-C₃N₄ is a metal-free polymeric semiconductor composed of carbon and nitrogen, making it an environmentally friendly option [15,16,17,18,19]. It possesses a moderate band gap of around 2.7 eV, which allows it to absorb visible light, a crucial requirement for solar-driven processes. Additionally, g-C₃N₄ is chemically stable and easy to synthesize from inexpensive, earth-abundant precursors. These features make it an attractive candidate for photocatalytic CO₂ reduction [20,21]. However, g-C₃N₄ suffers from several limitations that restrict its photocatalytic performance. The material has a relatively low surface area, which limits the number of active sites available for catalysis. More critically, it experiences rapid recombination of photogenerated electron–hole pairs, leading to poor charge separation and reduced photocatalytic efficiency. These shortcomings have spurred efforts to modify g-C₃N₄ to enhance its performance, either through structural modification or by combining it with other materials to form composite photocatalysts [22,23]. Bismuth tungstate (Bi₂WO₆) is another promising photocatalyst that has gained attention for its ability to absorb visible light due to its relatively narrow band gap of around 2.8 eV [24,25]. Bi₂WO₆ belongs to the Aurivillius family of layered oxides and exhibits excellent stability and photocatalytic properties [26,27]. Its layered structure is particularly beneficial for photocatalysis as it facilitates the separation of photogenerated charge carriers, which is crucial for maintaining high catalytic activity. Bi₂WO₆ has been studied for various photocatalytic applications, including pollutant degradation and water splitting [28,29]. However, like g-C₃N₄, it faces challenges such as the rapid recombination of electron–hole pairs and a limited surface area, which restricts the number of active sites available for photocatalytic reactions [30,31]. Moreover, its photocatalytic activity under visible light, while notable, still falls short of the performance needed for large-scale applications, particularly for CO₂ reduction.

To address the limitations of both g-C₃N₄ and Bi₂WO₆, researchers have explored the design of composite photocatalysts that combine the advantages of both materials. The g-C₃N₄/Bi₂WO₆ composite photocatalyst has emerged as a promising candidate for enhancing photocatalytic CO₂ reduction [32,33]. By combining g-C₃N₄ and Bi₂WO₆, the composite benefits from the visible-light absorption capabilities of both materials, while the formation of a heterojunction between them enhances charge separation and reduces recombination rates. The heterojunction creates an interface where electrons and holes can migrate more efficiently, thus improving the overall photocatalytic performance. Additionally, the composite structure can increase the surface area and expose more active sites for CO₂ adsorption and conversion, which is critical for improving the efficiency of the reduction process. The g-C₃N₄/Bi₂WO₆ composite takes advantage of the complementary properties of both materials. g-C₃N₄, with its moderate band gap and chemical stability, provides a strong foundation for visible-light absorption, while Bi₂WO₆ contributes to efficient charge separation and additional visible-light activity. The combination of these two materials has been shown to enhance the photocatalytic reduction of CO₂, producing higher yields of valuable products compared to either material alone. Furthermore, the composite offers the potential to tune the electronic structure by optimizing the ratio of g-C₃N₄ to Bi₂WO₆, further improving the material’s photocatalytic properties. However, despite these promising results, there are still several challenges that need to be addressed to fully realize the potential of g-C₃N₄/Bi₂WO₆ composite photocatalysts. One of the primary challenges is optimizing the interface between the two materials to maximize charge separation and minimize recombination. The efficiency of the heterojunction depends heavily on the quality of the interface and the degree of contact between g-C₃N₄ and Bi₂WO₆. Poor contact or mismatched energy levels can hinder charge transfer, reducing the overall photocatalytic performance.

In this study, we aim to engineer a g-C₃N₄/Bi₂WO₆ composite photocatalyst with enhanced efficiency for photocatalytic CO₂ reduction (Figure 1). Our work focuses on optimizing the synthesis of the composite to achieve a well-constructed heterojunction that promotes efficient charge separation and light absorption. We also investigate the structural and electronic properties of the composite to gain insights into the factors that influence its photocatalytic performance. Through a combination of experimental analysis and theoretical modeling, we aim to provide a detailed understanding of how the g-C₃N₄/Bi₂WO₆ composite functions as a photocatalyst and to identify key areas for further improvement.

2. Materials and Methods

2.1. Reagents and Chemicals

In this experiment, high-purity reagents were used to ensure precise synthesis and consistent performance. Bismuth nitrate pentahydrate (Bi(NO₃)₃∙5H₂O) served as the bismuth source, while sodium tungstate dihydrate (Na₂WO₆∙2H₂O) provided the tungsten source for synthesizing Bi₂WO₆. Cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN) was used as a surfactant to control the morphology during synthesis. Melamine (C₃H₆N₆) was a precursor for synthesizing g-C₃N₄, contributing nitrogen and carbon sources during polymerization.

2.2. Synthesis of Bi₂WO₆

Dissolve 1 mmol of Na₂WO₄·2H₂O, 2 mmol of Bi(NO₃)₃·5H₂O, and 0.05 g of CTAB in 80 mL of deionized water. Ultrasonicate the mixture for 15 min, followed by magnetic stirring for 30 min. Afterward, transfer the solution into a 100 mL Teflon-lined autoclave and conduct a hydrothermal reaction at 120 °C for 24 h. Once the reaction is complete, centrifuge the mixture to collect the product, then wash it several times, alternately with deionized water and absolute ethanol. Finally, dry the product at 70 °C for later use.

2.3. Synthesis of g-C₃N₄/Bi₂WO₆ Composite Photocatalyst

First, place 3 g of melamine in a muffle furnace. Heat the furnace at a rate of 10 °C/min until it reaches 550 °C. Maintain this temperature for 4 h, to thermally condense the melamine, resulting in the formation of g-C₃N₄. Next, dissolve 100 mg of Bi₂WO₆ and 5 mg of g-C₃N₄ in 40 mL of deionized water. Ultrasonically mix the solution for 10 min, followed by magnetic stirring for 1 h. After stirring, centrifuge the mixture to collect the product, then wash it several times, alternately with deionized water and absolute ethanol. Finally, dry the product at 70 °C for future use. Following the same method, prepare photocatalysts by mixing 100 mg of Bi₂WO₆ with 10 mg of g-C₃N₄, and 100 mg of Bi₂WO₆ with 15 mg of g-C₃N₄. The photocatalysts with 5 mg, 10 mg, and 15 mg of g-C₃N₄ incorporated into Bi₂WO₆ are referred to as 1g-C₃N₄-Bi₂WO₆, 2g-C₃N₄-Bi₂WO₆, and 3g-C₃N₄-Bi₂WO₆, respectively.

2.4. Characterization

The synthesized samples were characterized using a D8 Advance X-ray diffractometer (XRD, Bruker, Karlsruhe, Germany) with a Cu Kα radiation source (wavelength λ = 1.5406 Å). The diffraction patterns were recorded over a scanning range of 10° to 90° with a step size of 0.02° and a scan speed of 0.2°/min. The resolution of the XRD measurement is approximately 0.01°, and the phase composition and crystalline structure of the samples were identified by comparing the diffraction peaks with standard diffraction cards (JCPDS database). Fourier-transform infrared spectroscopy (FTIR) was performed using an Invenio R model (Bruker) in the range of 4000 to 400 cm⁻¹. The resolution of the FTIR spectra was set to 4 cm⁻¹, and 64 scans were co-added for each measurement to enhance the signal-to-noise ratio. The spectra were recorded using the transmission mode, with samples pressed into KBr pellets. The IR spectra were analyzed to identify characteristic functional groups and molecular vibrations, providing insights into the chemical bonding and structure of the materials. The ultraviolet–visible absorption capacity of the samples was measured using a UV-3900 ultraviolet–visible diffuse reflectance spectrophotometer (Shimadzu, Kyoto, Japan). The resolution of the UV-Vis measurements was 1 nm, and the scanning range extended from 250 to 800 nm. The excitation source used was a 150 W xenon lamp, and the diffuse reflectance was converted into the absorption spectrum using the Kubelka–Munk function. This characterization provides important information regarding the light absorption properties and optical bandgap of the samples. Additionally, the morphology and microstructure of the g-C₃N₄/Bi₂WO₆ composite photocatalyst were examined using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Tokyo, Japan) operating at an accelerating voltage of 5 kV. The samples were coated with a thin layer of gold to prevent charging during imaging. The SEM images allowed for a detailed examination of the surface morphology, particle size, and distribution of the composite materials. These advanced characterization techniques, along with their specific technical parameters, provide comprehensive insights into the structural, optical, and morphological properties of the synthesized materials, which are essential for understanding their photocatalytic behavior.

2.5. Photocatalytic Experiments

In this experiment, the photocatalytic performance was evaluated using a Labsolar-6A all-glass automatic online gas analysis system equipped with a 300 W xenon lamp. The entire reaction system consists of a quartz reaction cell, a vacuum system, and a gas chromatograph. The specific procedure is as follows: 20 mg of the prepared photocatalyst was added to 0.1 mL of deionized water under magnetic stirring, to achieve a uniformly dispersed solution. The solution was then evenly dripped onto a quartz fiber filter membrane, which was placed in the reaction vessel. The quartz fiber filter membrane was positioned at the bottom of the reactor and sealed with a quartz cover. High-purity CO₂ was used to purge the reaction system for 1 h to remove any air or impurities. The system was illuminated using a 300 W xenon lamp as the light source. The reaction was conducted at ambient temperature and pressure, with argon (Ar) as the carrier gas. The gaseous products were analyzed using both a flame ionization detector (FID, Zhejiang Fuli Analytical Instrument Co., LTD, Taizhou, China) and a thermal conductivity detector (TCD, Zhejiang Fuli Analytical Instrument Co., LTD, Taizhou, China). After the reaction, the samples were collected to assess the stability and recyclability of the photocatalyst under the same testing conditions.

2.6. Photoelectrochemical Testing

In this experiment, a standard three-electrode system was used, with 0.5 M Na₂SO₄ aqueous solution as the electrolyte. The reference electrode and counter electrode were made of Ag/AgCl and platinum (Pt), respectively, while the sample to be tested served as the working electrode. A 300 W xenon lamp was used as the light source. The preparation of the working electrode was as follows: 5 mg of photocatalyst was added to 1 mL of ethanol and ultrasonically dispersed for 20 min. A mixture of 25 μL of the solution and 25 μL of an adhesive was then dripped onto conductive glass, followed by drying under heat for subsequent testing. Electrochemical impedance spectroscopy was measured under open-circuit voltage conditions, with a frequency range of 0.01 Hz to 100 kHz and an AC amplitude of 0.001 V. This technique is commonly used to study how the capacitance of the photoanode surface affects the charge transfer rate. The size of the charge transfer resistance is related to the radius of the Nyquist plot, which indirectly reflects the separation efficiency of photogenerated electron–hole pairs. The transient photocurrent response was measured under constant voltage conditions, where the working electrode was excited by light energy. Electrons were excited and transitioned, generating a current that varied over time. The resulting curve can be used to explain the separation and transport rates of charge carriers generated in the working electrode after photoexcitation. These photoelectrochemical tests provide critical insights into the charge separation and transport behavior of the photocatalyst, which are key factors influencing its overall photocatalytic efficiency.

3. Results

The X-ray diffraction (XRD) pattern of the g-C₃N₄, Bi₂WO₆ and g-C₃N₄/Bi₂WO₆ composite photocatalyst is shown in Figure 2. The diffraction peaks of Bi₂WO₆ are observed at 28.6°, 33.0°, 47.4°, 56.1°, 59.0°, 69.1° and 75.9°, corresponding to the (113), (200), (220), (313), (226), (040), and (139) crystal planes, respectively. These peaks are consistent with the standard JCPDS card (No. 73-2020) [34], indicating that the prepared Bi₂WO₆ has high crystallinity. The strong intensity and sharpness of the Bi₂WO₆ diffraction peaks, combined with the absence of any impurity peaks, further confirm the high purity and well-ordered crystalline structure of the synthesized Bi₂WO₆. For g-C₃N₄, its characteristic diffraction peaks correspond to the (001) and (110) planes, indicating successful synthesis of g-C₃N₄. However, after the two materials were combined to form the composite, the XRD pattern is primarily dominated by the diffraction peaks of Bi₂WO₆, and no distinct diffraction peaks for g-C₃N₄ are detected. This absence is likely due to the small amount of g-C₃N₄ in the composite, which falls below the detection limit of XRD. Despite not being visible in the XRD pattern, the successful integration of g-C₃N₄ with Bi₂WO₆ is supported by other characterization methods, as its presence can still influence the photocatalytic activity through synergistic interactions between the two materials.

The morphology of the synthesized samples was examined using scanning electron microscopy (SEM), as depicted in Figure 3. The SEM images reveal that Bi₂WO₆ exhibits a flower-like nanostructure comprising nanosheets. Similarly, g-C₃N₄ also exhibits a microstructure composed of numerous irregularly shaped nanosheets. The structural similarity between the two materials, particularly their sheet-like architectures, provides a favorable foundation for forming a heterojunction between them. This structural compatibility is critical in facilitating intimate contact between Bi₂WO₆ and g-C₃N₄, which is essential for efficient charge separation and transfer in the composite photocatalyst. The ultrathin nanosheets of Bi₂WO₆ offer a large surface area, potentially increasing the number of active sites for the photocatalytic reaction. Moreover, the integration of g-C₃N₄, despite its relatively low loading, can enhance light absorption and promote electron–hole separation, further boosting the photocatalytic efficiency. The flower-like arrangement of Bi₂WO₆ nanosheets, combined with the irregular sheet structure of g-C₃N₄, creates a highly porous and interconnected architecture, which could improve the diffusion of reactants and products during the photocatalytic process. This synergistic interaction between the two materials underscores the importance of their morphological characteristics in enhancing the overall performance of the composite photocatalyst.

The SEM image of the g-C₃N₄/Bi₂WO₆ composite, shown in Figure 4, reveals that the composite retains the microstructural features of Bi₂WO₆. The ultrathin nanosheets not only maintain their characteristic morphology, but also play a crucial role in enhancing the photocatalytic performance. These nanosheets provide an abundance of surface-active sites, which are essential for catalyzing reactions, and also shorten the migration path of charge carriers, thereby improving their mobility. The high surface area and well-dispersed sheet-like structure of Bi₂WO₆ facilitate the effective separation and migration of photo-induced electron–hole pairs. This characteristic is crucial for the composite’s performance in photocatalytic CO₂ reduction, where efficient charge separation is necessary to drive the reduction reaction. Additionally, the integration of g-C₃N₄, although not directly visible in the SEM image, likely contributes to light harvesting and enhances the composite’s photocatalytic efficiency through its role in further improving charge separation. The combination of these two materials, with their complementary properties, creates a microstructure that is highly conducive to photocatalysis. The interconnected nanosheets offer pathways for rapid charge transport, while the heterojunction between g-C₃N₄ and Bi₂WO₆ promotes effective charge transfer between the two phases. This synergistic effect not only boosts the catalytic activity, but also makes the g-C₃N₄/Bi₂WO₆ composite a promising candidate for photocatalytic CO₂ reduction, enabling the conversion of CO₂ into valuable chemicals under light irradiation [35,36]. The overall microstructural characteristics of the composite provide a solid foundation for achieving high photocatalytic efficiency.

The infrared absorption spectra (FTIR) of the g-C₃N₄/Bi₂WO₆ composite photocatalyst, as shown in Figure 5, provide further insights into the structural composition of the materials. Bi₂WO₆ exhibits seven distinct infrared absorption peaks, two of which can be attributed to background absorption resulting from atmospheric interference and instrumental factors. The peaks at 669 and 720 cm⁻¹ correspond to the infrared vibrational absorption of Bi-O and Bi-W bonds, respectively, indicating the presence of the metal–oxygen framework. Additionally, the peak at 1385 cm⁻¹ is associated with the absorption of hydroxyl groups (OH⁻) on the surface of Bi₂WO₆, highlighting the material’s surface chemistry. The peaks observed at 2850 and 2926 cm⁻¹ are attributed to the infrared vibrational modes of cetyltrimethylammonium bromide (CTAB), which was used during the synthesis of ultrathin Bi₂WO₆ nanosheets to prevent excessive stacking of the layers. On the other hand, g-C₃N₄ primarily shows three infrared absorption peaks, with two corresponding to background absorption. The peak at 806 cm⁻¹ is characteristic of the vibrational mode of the C-N bonds within the g-C₃N₄ structure, confirming the presence of g-C₃N₄ in the material. After the combination of g-C₃N₄ and Bi₂WO₆, the FTIR spectrum is dominated by the absorption peaks of Bi₂WO₆. However, the peak at 806 cm⁻¹, corresponding to the C-N vibrational mode, is still clearly visible, demonstrating the successful incorporation of g-C₃N₄ into the composite. This preservation of the C-N vibrational peak, alongside the dominant Bi₂WO₆ absorption features, confirms the successful formation of the g-C₃N₄/Bi₂WO₆ heterojunction. The FTIR analysis, in conjunction with the previously discussed XRD and SEM results, provides strong evidence that the composite retains key structural elements from both g-C₃N₄ and Bi₂WO₆. The preservation of characteristic vibrational modes indicates that the synthesis process has not altered the core structural integrity of the individual components, while the presence of CTAB during synthesis further enhances the structural stability of the Bi₂WO₆ nanosheets by preventing agglomeration. The successful formation of the heterojunction, evidenced by both FTIR and SEM, suggests that this composite is well-positioned to take advantage of the complementary properties of g-C₃N₄ and Bi₂WO₆ for enhanced photocatalytic activity, particularly in applications such as CO₂ reduction. Note that the peaks at 2341 and 2361 cm⁻¹ are attributed to atmospheric CO₂ interference, which may bond to the surface of the material from the air.

The UV–visible absorption spectra and bandgap diagrams of the g-C₃N₄/Bi₂WO₆ composites are shown in Figure 6. For pure Bi₂WO₆, the absorption edge in the UV region is observed at around 450 nm, while g-C₃N₄ shows a broader absorption edge extending to approximately 550 nm. In the g-C₃N₄/Bi₂WO₆ composites, the absorption edge of the 1g-C₃N₄/Bi₂WO₆ and 2g-C₃N₄/Bi₂WO₆ samples remains similar to that of pure Bi₂WO₆, with a maximum absorption edge around 450 nm. However, the 3g-C₃N₄/Bi₂WO₆ composite shows a slight redshift in the absorption edge, with its maximum absorption edge extending to approximately 470 nm. This redshift in the 3g-C₃N₄/Bi₂WO₆ composite suggests enhanced light absorption in the visible region, which is beneficial for photocatalytic applications. The shift indicates that with an increasing g-C₃N₄ content, the composite can harness a broader range of visible light, thus potentially improving photocatalytic efficiency. The incorporation of g-C₃N₄ into the Bi₂WO₆ matrix modifies the optical properties of the composite, allowing for more effective utilization of solar energy. The bandgaps of g-C₃N₄ and Bi₂WO₆, as shown in the inset of the figure, further confirm their respective electronic properties. Bi₂WO₆ has a bandgap of 3.21 eV, which corresponds to its ability to absorb UV light, while g-C₃N₄ has a smaller bandgap of 2.72 eV, allowing it to absorb more visible light. This complementary relationship between the two materials in terms of their bandgaps enables the composite to cover a wider range of the solar spectrum, thus improving the potential for photocatalysis. The adjustment of the bandgap in the composite materials, particularly in the 3g-C₃N₄/Bi₂WO₆ sample, suggests that the combination of g-C₃N₄ and Bi₂WO₆ can optimize light absorption and increase the generation of photoinduced charge carriers. This is crucial for enhancing photocatalytic performance, as the ability to absorb more visible light directly correlates to the photocatalyst’s ability to drive chemical reactions, such as CO₂ reduction, under solar irradiation. Therefore, the UV–visible absorption and bandgap analysis further supports the suitability of g-C₃N₄/Bi₂WO₆ as an efficient photocatalyst, with the potential for tunable optical properties through compositional adjustments.

The photocatalytic performance of the g-C₃N₄/Bi₂WO₆ composite for CO₂ reduction is illustrated in Figure 7. During the photocatalytic reaction, the main products observed are CO and CH₄. After 4 h of photocatalysis, the CH₄/CO production rates for Bi₂WO₆, g-C₃N₄, 1g-C₃N₄/Bi₂WO₆, 2g-C₃N₄/Bi₂WO₆, and 3g-C₃N₄/Bi₂WO₆ were measured as 7.21/10.98, 2.63/19.63, 18.90/17.78, 4.80/29.82, and 18.80/9.91 μmol/g/h, respectively. It is evident that the g-C₃N₄ content plays a crucial role in product selectivity and the overall performance of photocatalytic CO₂ reduction. The 1g-C₃N₄/Bi₂WO₆ composite exhibited significant improvements in the production rates of both CH₄ and CO, which were 18.90 and 17.78 μmol/g/h, respectively. These rates are much higher compared to pure Bi₂WO₆, which only produced CH₄ and CO at rates of 7.21 and 10.98 μmol/g/h. The significant enhancement in photocatalytic performance can be attributed to synergistic effects observed specifically in two composites (1g- and 3g- C₃N₄/Bi₂WO₆) and specifically for CH₄ production. These synergistic effects enhance light absorption, charge separation, and reaction kinetics. For the 2 g-C₃N₄/Bi₂WO₆ composite, the CH4 production rate was lower at 4.80 μmol/g/h, while the CO production rate increased to 29.82 μmol/g/h. This indicates a high selectivity for CO over CH₄ in this composition, suggesting that adjusting the g-C₃N₄ content in the composite can influence the product distribution between CO and CH₄. Such tunable selectivity is a valuable trait for photocatalysts, allowing for targeted production of desired products, depending on the application. In the case of the 3g-C₃N₄/Bi₂WO₆ composite, the CH₄ and CO production rates were measured at 18.80 and 9.91 μmol/g/h, respectively. This composition showed a notable preference for CH₄ production, similar to the 1g-C₃N₄/Bi₂WO₆ composite, but with a slightly reduced CO production rate. Overall, the results clearly demonstrate that the g-C₃N₄ content has a significant impact on both the product selectivity and the overall photocatalytic efficiency of the g-C₃N₄/Bi₂WO₆ composites for CO₂ reduction. The 1g-C₃N₄/Bi₂WO₆ composite, in particular, showed the highest total production of CH₄ and CO, indicating that an optimal balance between g-C₃N₄ and Bi₂WO₆ can maximize the photocatalytic activity. Comparative analysis with some previous studies (see

Table 1. Comparison of CO and CH₄ production rates in CO₂ photoreduction for g-C₃N₄/Bi₂WO₆ composites and some previous studies.

Title 1	Light Source	Production	Ref.
g-C3N4/Bi2WO6	300 W Xe lamp with λ > 420 nm filter	CO: 5.19 μmol/g/h	[37]
g-C3N4@UIO66	400–800 nm	CO: 9.9 μmol/g/h	[38]
Amine-g-C3N4	300 W Xe lamp	CH4: 0.34 μmol/g/h	[39]
BiOI/g-C3N4	300 W Xe lamp	CO: 3.44 μmol/g/h	[40]
g-C3N4/Bi4O5I2	300 W Xe lamp with λ > 400 nm filter	CO: 45.6 μmol/g/h	[41]
Bi2WO6/RGO/ g-C3N4	300 W Xe lamp with λ > 420 nm filter	CH4: 2.51 μmol/g/h CO: 15.93 μmol/g/h	[42]
AgBr/g-C3N4	UV light, 365 nm	CH4: 30.76 μmol/g/h	[43]
WO3/g-C3N4	8 W Hg Lamp	CO: 14.6	[44]
g-C3N4/Bi2WO6	300 W Xe lamp	CH4: 18.90 μmol/g/h CO: 29.82 μmol/g/h	This work

) indicates that the CO and CH₄ production rates achieved in this study are competitive with existing research findings. The enhanced performance is likely due to the improved light absorption and better charge carrier separation in the heterojunction, leading to more efficient photoreduction of CO₂. Additionally, the varying selectivity toward CO or CH₄ observed in different composites highlights the potential for tuning the reaction pathways by adjusting the material composition, providing flexibility for specific catalytic applications.

To elucidate the underlying mechanism, photoelectrochemical tests were conducted, and the photocurrent responses of the g-C₃N₄/Bi₂WO₆ composites are presented in Figure 8. In photocatalysts, the efficiency of electron–hole pair separation under illumination is reflected in the photocurrent density; a higher photocurrent density indicates faster separation of photogenerated charge carriers [45]. It can be observed that both pure g-C₃N₄ and Bi₂WO₆ exhibit the lowest photocurrent responses, indicating poor charge carrier separation efficiency (Figure 8a). However, after the formation of a heterojunction between g-C₃N₄ and Bi₂WO₆, the photocurrent responses of the composites surpass those of the individual materials. Among the composites, 3g-C₃N₄/Bi₂WO₆ shows the highest photocurrent response, suggesting that it has the most efficient charge carrier separation. This enhanced photocurrent in the composites can be attributed to the formation of a heterojunction, which facilitates the separation of photogenerated electrons and holes. In pure g-C₃N₄ and Bi₂WO₆, rapid recombination of these carriers limits their photocatalytic efficiency. However, in the composite materials, the heterojunction creates a favorable pathway for charge carriers to be separated and transferred, thereby reducing recombination and enhancing overall photocatalytic activity. The superior performance of the 3g-C₃N₄/Bi₂WO₆ composite, in particular, implies that the optimal balance between g-C₃N₄ and Bi₂WO₆ allows for maximum charge separation and transfer, leading to the highest photocatalytic efficiency. Electrochemical impedance spectroscopy (EIS) results, shown in Figure 8b, provide further insights into the interfacial charge transfer properties of the photocatalysts. The semicircles observed in the high-frequency region of the Nyquist plots correspond to the charge transfer resistance at the photocatalyst–electrolyte interface. Generally, a smaller semicircle radius indicates lower interfacial resistance and higher charge carrier mobility. Bi₂WO₆ exhibits the largest semicircle, indicating the highest charge transfer resistance and, thus, the lowest charge carrier mobility [46,47]. Upon the introduction of g-C₃N₄ into the composite, the impedance decreases significantly, confirming improved charge carrier mobility in the g-C₃N₄/Bi₂WO₆ heterostructure. Among the composites, 3g-C₃N₄/Bi₂WO₆ displays the smallest semicircle, representing the lowest interfacial resistance and the highest charge carrier mobility. The improved charge transfer and reduced recombination rates observed in the g-C₃N₄/Bi₂WO₆ composites, particularly in the 3g-C₃N₄/Bi₂WO₆ sample, can be explained by the effective formation of the heterojunction. This junction facilitates the separation of photogenerated electrons and holes, allowing them to migrate efficiently to the catalyst surface, where they can participate in the photocatalytic reaction. Additionally, the reduced impedance in the composites highlights the improved electrical conductivity and charge transfer properties, which are crucial for enhancing the overall photocatalytic performance. In summary, the photoelectrochemical analysis confirms that the formation of a g-C₃N₄/Bi₂WO₆ heterojunction plays a vital role in boosting charge carrier separation and mobility, ultimately leading to enhanced photocatalytic activity. The 3g-C₃N₄/Bi₂WO₆ composite, in particular, demonstrates the best performance, due to its superior photocurrent response and lowest interfacial resistance, making it an ideal candidate for high-efficiency photocatalysis.

4. Discussion

Figure 9 illustrates the mechanistic process of the Bi₂WO₆/g-C₃N₄ hybrid based on a type-II heterojunction. According to literature reports, the conduction band (CB) position of g-C₃N₄ is higher (more negative) than that of Bi₂WO₆ [48,49]. As a result, photogenerated electrons (e⁻) in the conduction band of g-C₃N₄ are rapidly transferred to the conduction band of Bi₂WO₆. Simultaneously, the photogenerated holes (h⁺) in the valence band of Bi₂WO₆ migrate to the valence band of g-C₃N₄. This charge transfer process effectively prevents the fast recombination of charge carriers, a common issue in photocatalytic systems. The prevention of charge recombination is crucial, because it increases the lifetime of the photogenerated electrons and holes, allowing them to participate in redox reactions. In this heterojunction system, the holes accumulated in the valence band of g-C₃N₄ are strong enough to oxidize H₂O molecules, producing O₂ and H⁺ ions. The production of oxygen is a clear indicator of the system’s oxidative capabilities, and supports the photocatalytic water-splitting process. On the reduction side, CO₂ molecules are adsorbed onto the surface, where the electrons transferred to the conduction band of Bi₂WO₆ play a key role. These electrons are capable of reducing CO₂ into valuable products like CO and CH₄. The photocatalytic reduction of CO₂ is a critical step in solar fuel production, and the ability of the composite material to facilitate this reaction is a major advantage. The type-II heterojunction between g-C₃N₄ and Bi₂WO₆ is particularly effective for this photocatalytic system, because it optimally aligns the energy bands of the two materials, allowing for efficient charge separation and transfer. The higher conduction band of g-C₃N₄ promotes electron transfer to Bi₂WO₆, while the lower valence band of Bi₂WO₆ allows for hole transfer to g-C₃N₄. This dual pathway minimizes electron–hole recombination, thus enhancing the photocatalytic efficiency of both oxidation and reduction processes. In summary, the g-C₃N₄/Bi₂WO₆ heterojunction system not only facilitates effective charge separation, but also supports both oxidative and reductive photocatalytic reactions. The oxidation of water at the g-C₃N₄ valence band produces oxygen, while CO₂ is reduced at the Bi₂WO₆ conduction band to produce useful products like CO and CH₄.

5. Conclusions

In this study, Bi₂WO₆ was successfully synthesized using a hydrothermal method, while g-C₃N₄ was prepared by the calcination of melamine. Subsequently, g-C₃N₄ was combined with Bi₂WO₆ to form the g-C₃N₄/Bi₂WO₆ composite photocatalyst, which was thoroughly characterized, and its photocatalytic performance in CO₂ conversion was evaluated. The experimental results demonstrate that when the loading of g-C₃N₄ is below 15% of the total Bi₂WO₆ weight, the introduction of g-C₃N₄ does not significantly alter the microstructure of Bi₂WO₆. However, it substantially enhances its photocatalytic and photoelectrochemical performance. The addition of g-C₃N₄ significantly reduces the recombination rate of photogenerated electron–hole pairs in Bi₂WO₆, thereby improving its photocatalytic efficiency for CO₂ conversion. These findings reveal the promising potential of the g-C₃N₄/Bi₂WO₆ composite for solar-driven photocatalytic CO₂ conversion, offering a new pathway to address climate change and energy challenges. Future research should focus on optimizing the structural design of these composite materials and exploring more efficient mechanisms for photocatalytic CO₂ conversion, aiming for broader applications and greater environmental benefits.