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Article

The Highly Enhanced Efficiency of the Photocatalytic Reduction of CO2 over Bi2WO6 Nanosheets by NaOH Microregulation

by
Chao Song
1,
Yangang Sun
1,*,
Li Zhang
1,
Shuang Liu
1,
Jinguo Wang
1,
Wei An
1,
Yong Men
1,2,* and
Zhenrong Yan
2,3,*
1
School of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
2
Mechanical Industrial Key Laboratory of Boiler Low-Carbon Technology, Shanghai University of Engineering Science, Shanghai 201620, China
3
School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(10), 2827; https://doi.org/10.3390/pr11102827
Submission received: 28 August 2023 / Revised: 10 September 2023 / Accepted: 12 September 2023 / Published: 25 September 2023
(This article belongs to the Section Catalysis Enhanced Processes)
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Abstract

:
The photoreduction of CO2 to other products containing carbon through simulated photosynthesis is a promising area of research. However, given the complexity of the CO2 photocatalytic reduction reaction, it is crucial to adjust the structure of the photocatalysts. The focus of this study was on creating NaOH-modified Bi2WO6 nanosheet photocatalysts via a one-step hydrothermal route and using them to convert CO2 into CO through photocatalytic reduction under the condition of not using an electron sacrifice agent. The results of characterizations and activity data showed that adding an appropriate amount of NaOH significantly improved the photoreduction activity of CO2, as seen in the BWO-2 catalyst. The efficiency of photocatalysts could be improved by tuning the band structure through the addition of an appropriate amount of alkali. This adjustment improves the separation of photogenerated carriers and controls the concentration of oxygen vacancy to reduce recombination. As a result, the photocurrent activity is highly enhanced, leading to better reduction performance compared to unmodified photocatalysts. In experiments, the CO yield of the modified photocatalyst BWO-2 remained above 90 μmol/g after four trials, indicating its effectiveness in reducing CO2. This study offers insights into the regulation of band structure in bismuth-based photocatalysts for efficient CO2 reduction.

1. Introduction

The extensive utilization of fossil fuels and energy sources has resulted in a significant rise in gaseous pollutants in the atmosphere [1,2,3]. This increase is mainly due to the rapid growth of industry and technology in the past century, leading to high levels of carbon dioxide (CO2) in the air, which continues to increase annually. In the atmosphere, the accumulation of CO2 has led to the depletion of the ozone layer, global warming, melting of glaciers, and rising sea levels, resulting in various climate disasters [4]. Catalysts can effectively reduce the activation energy [5,6] of chemical reactions and improve reaction rates [7,8]. Therefore, it is crucial to decrease CO2 emissions or convert it into usable carbon resources.
Scientists have been striving to find better ways to utilize carbon dioxide, but its stable electronic structure makes it difficult to convert into usable carbon-based compounds, especially organic compounds that can be used as fuels [9,10,11,12,13]. Therefore, the catalytic reduction or hydrogenation of CO2 requires specific reaction conditions and appropriate catalysts, such as thermal catalysts [14,15,16,17,18]. Gulzar A. Bhat et al. present recent developments in the catalytic chemistry for coupling of carbon dioxide and epoxides to afford either cyclic or polymeric carbonates and had an in-depth discussion on the development of organic catalysts. The field of electrocatalysts and photocatalysts is also being explored for the conversion of CO2 [19,20]. Afshana Hassan et al. evaluated 27 transition metal-based monatomic catalysts for CO2 reduction to C2 products. It is found that the catalytic activity of all monatomic catalysts is related to the adsorption-free energy of (*COCH2O), providing a novel catalyst design idea for the conversion of CO2 to C2 products on single-atom catalysts. Meanwhile, they propose effective strategies of catalyst design for promoting the electrocatalytic CO2 reduction performance on Zr-based materials.
The use of solar energy for transforming CO2 into chemical energy is a significant process [21], which is essential for plant photosynthesis. To reduce total CO2 emissions [22,23], it is imperative to identify suitable materials for CO2 resource utilization with solar energy. Photocatalysis of CO2 is a promising technology that researchers have utilized for CO2 conversion, among other methods. In 1972, researchers began exploring the use of semiconductor electrodes for photocatalytic decomposition of water. Since then, scientists have been searching for materials that can effectively reduce carbon dioxide. Among the various semiconductor materials [24,25], bismuth-based materials have shown promising photochemical properties [26,27]. However, they are not suitable for photocatalytic carbon dioxide reduction due to their unsuitable band gap [28], poor separation of photogenerated electrons and holes [29], and high recombination rates. Previous studies [30] have found that the catalytic performance of bismuth-based photocatalysts is greatly affected by different preparation and modification methods [31,32,33]. For these reasons, scientists have made various modifications to the physical and chemical properties of this type of catalyst. For example, Xia et al. [34] prepared an ultra-thin Bi2MoO6 catalyst with defects through the template method, which promoted the separation efficiency of the carriers, and the generation rate of carbon monoxide (CO) was 2.55 times higher than that of unmodified material. Zhu et al. [35] prepared a Pd/Al2O3/BiVO4 composite material for the CO2 photocatalytic reduction process, resulting in a CH4 yield of 12.8 μmol·g−1·h−1, and proposed a possible reduction mechanism. Jin et al. [36] designed a bismuth-rich Bi4O5Br2 hollow microsphere, whose hollow structure improves the efficiency of CO2 adsorption and activation. CO and CH4 after CO2 reduction were obtained. Zhao et al. [37] prepared the tremella structure of Bi2WO6 induced by Bi2O3. Compared with Bi2WO6 nanosheets, the photocatalytic reduction rate increased by 5.5 times and showed long-term stability. Wang et al. [38] increased the yield of CO and the selectivity of CH4 in the photocatalytic CO2 process by constructing heterojunctions on Bi2S3. The mechanism of photocatalytic CO2 reduction is generally regarded as the electron–hole theory. When a higher energy is applied to the photocatalysts, the free electrons are excited from the valence band across the band gap width to the conduction band. In this process, holes are created on the valence band due to the transition of electrons, and the conduction band receives free electrons, thus forming an electron–hole, i.e., photogenerated carriers. Photogenerated electrons have a reduction performance, which can reduce CO2 adsorbed on the surface of the photocatalysts and can generate gaseous products such as carbon monoxide, methane, and ethylene and, under appropriate conditions, obtain liquid products such as methanol and ethanol. However, there is still a great prospect for improving the photocatalytic performance of bismuth-based catalysts without the electron sacrificial agents for economic and environmental reasons, even though the sacrificial agent can provide electrons to facilitate the reaction [39]. In the absence of other ions or electron sacrificial agents, this kind of photocatalyst still has excellent photocatalytic potential. CO can be used as a fuel but also as a raw gas for smelting metals. Carbon monoxide is the main component of syngas and various kinds of coal gas. It is an important raw material in the organic chemical industry and the basis of C1 chemistry. It can produce a series of products, such as methanol and acetic acid. At present, the industrialization of C1 chemical production technology mainly includes acetic acid synthesis, acetic anhydride synthesis, Fischer–Tropsch synthesis, etc.
In this study, we developed a synthesis method for Bi2WO6 that involved the addition of sodium hydroxide to adjust the band gap structure. Compared with Bi2WO6 obtained by hydrothermal reaction with the modification of pH [40,41,42], we were able to produce Bi2WO6 nanosheets (BWO-2) with a stable sheet structure and a wider band gap under a moderate condition without impurity ions introduced. As a result, the BWO-2 photocatalysts exhibited better photocatalytic performance, with significantly improved CO yield and stable photocatalytic ability even after multiple activity tests. Our findings could provide valuable insights for the development of bismuth tungstate series photocatalysts.

2. Experimental Section

2.1. Materials

The chemicals, with the exception of nitric acid, were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China). All reagents require no further purification and can be used directly in this work. The chemicals used include Bi(NO3)3·5H2O (bismuth nitrate pentahydrate, ≥99.0%), Na2WO4·2H2O (sodium tungstate dihydrate, 99.5%), CTAB (hexadecyl trimethyl ammonium bromide, 99%), and NaOH (sodium hydroxide, 97%). HNO3 (Nitric acid, 65%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The ultra-high-purity CO2 (99.999%) was purchased from Shanghai Haoqi Gas Co., Ltd. (Shanghai, China).

2.2. Synthesis

The synthesis process of photocatalysts is very typical (Figure 1). Generally, the addition of 1 mmol of Bi(NO3)3·5H2O to 65 mL of deionized water was followed by thorough mixing. Then, 0.5 mmol of Na2WO4·2H2O and 0.05 g of CTAB are added to the above solution. After ultrasound treatment, the suspension solution is placed on a stirring table for agitation. In this process, a certain concentration of NaOH solution is gradually added to the solution, and the mixed solution is obtained under different pH atmospheres. The different solutions are then transferred to a Teflon-lined autoclave and kept at 120 °C for 24 h. The samples are washed three times, each with anhydrous ethanol and deionized water, and centrifuged at a rotating speed of 3500 r/min. The Bi2WO6 nanosheets needed for the experiment were obtained by vacuum drying at 80 °C for 12 h [43,44,45]. Different Bi2WO6 nanosheets were prepared by adjusting the pH levels with varying amounts of NaOH and were, respectively, labeled as BWO-2, BWO-3, and BWO-5 according to pH = 2, 3, and 5. To confirm that the carbon source came from CO2 and not CTAB, CTAB was used for comparison tests, and BWO-2 was roasted at 450 °C to obtain B-BWO-2. To compare the different catalysts, deionized water, and HNO3 solution were used to adjust the solution pH instead of NaOH, and the resulting samples were labeled as BWO-1.5 (bare Bi2WO6) and BWO-1, respectively. The preparation conditions were kept consistent across all samples.

2.3. Characterization

At a scanning speed of 5°/min, a Cu-Na-α radiation source was used. The diffraction curves of the prepared materials were determined on the X-ray diffractometer (BruNaer D2 Phaser, Karlsruhe, Germany). TEM (Transmission electron microscopy, FEI Talos F200S, Waltham, MA, USA) is used to observe the microscopic structure of catalysts and to perform elemental analysis of catalysts on EDS (energy dispersive spectroscopy). The surface properties and chemical composition and properties were analyzed by Thermo Scientific NA-Alpha (Waltham, MA, USA) (X-ray photoelectron spectroscopy). The optical properties of the catalysts were analyzed by the UV–vis diffuse-reflectance spectra (Shimadzu UV-3600i Plus, Shanghai, China) to reveal the band gap and light absorption range. BET (Brunauere Emmette Teller) surface areas of the materials were measured with N2 as the adsorptive mass at 77.35 K by Micromeritics ASAP 2460, (Atlanta, GA, USA) (automatic surface area and porosity analyzer). The catalyst pore size distribution was obtained by the BJH (Barrett–Joyner–Halenda) method under isothermal adsorption. The separation intensity of photogenerated electrons and holes in the catalysts was observed using the transient photocurrent response test. FTIR (Fourier transform infrared spectra) were acquired by an infrared spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA).

2.4. Photocatalytic CO2 Reduction

The results of photocatalytic CO2 were obtained in a custom-made quartz glass container (Education AuLight Technology (CEAuLight) Co., Ltd., Beijing, China). In a typical process (Figure S1), 2.5 mL of deionized water was injected into the reactor, 20 mg of catalysts were placed on a quartz glass platform in the container, and the reactor was sealed with vacuum-sealed silicone grease. After vacuum treatment, the quartz glass reactor was connected to a micro circulating air pump (Chengdu New for Cheng Technology Co., Ltd.). Then, a certain flow rate of high-purity CO2 gas was fed into the reactor for flushing and residual gas replacement. After that, under the irradiation of a 300 W photocatalytic Xenon lamp source (CEL-HXF 300/CEL-HXUV 300) and the power of the micro-circulating air pump, the photocatalytic results were tested. The entire photocatalytic activity test process was conducted in a closed system consisting of a photocatalytic reactor, connecting lines, a mini-gas circulating pump, and a gas chromatograph. The equivalent volume of the closed space was 667 mL. The gas flowed and blended due to the action of the pump. Throughout the photocatalytic activity test, 1 mL of the gas mixture was extracted from the reactor every 0.5 h and analyzed by gas chromatography (GC-9800) containing PQ columns, 13X-5A and SE-30 columns, a flamed ion detector, a thermal conductivity cell detector, and a nickel catalytic reformer. The flame ion detector used N2 as the carrier gas, and the thermal conductivity detector used Ar as the carrier gas, which could analyze methane, carbon monoxide, hydrogen, oxygen, and other products. The products of photocatalytic CO2 reduction were quantitatively detected by gas chromatography with an external standard method.

2.5. Photoelectrochemical Measurement

The transient photocurrent curves of the catalysts were measured using an electrochemical workstation with a three-electrode system (Shanghai Chenhua Instrument Co., Ltd., CHI650E). A certain concentration of Na2SO4 solution was used as the electrolyte during the measurement. Specifically, an Ag/AgCl electrode and a Pt sheet were used as the reference and counter electrode, respectively. The working electrode was prepared by grinding a mixture of catalysts, carbon black, polyvinylidene fluoride (PVDF), and N-methyl pyrrolidone, coating it on a 1 × 1 cm2 carbon paper and drying it at 70 °C for 30 min. The I-t curves of the catalysts were measured using a xenon lamp with a 40 s cycle. All experiments were conducted at room temperature.

3. Results and Discussion

The X-ray diffraction patterns of various samples were analyzed, and it was found that all of them had diffraction peaks corresponding to the orthorhombic Bi2WO6 phase (JCPDS, No. 73-2020) (Figure 2). This indicates that the samples were highly pure, which is consistent with the results obtained for pure Bi2WO6 prepared in the past [46]. The intensity of the X-ray diffraction peaks for BWO-1 and other samples prepared by adding HNO3 or NaOH solution did not change significantly as compared to BWO-1.5, which was not adjusted during the preparation process. This suggests that adjusting the pH during the preparation process did not have a notable effect on the crystallinity of Bi2WO6.
The scanning electron microscopy (SEM) analysis of the photocatalysts indicates that BWO-2 has a distinct nanosheet structure compared to BWO-3 and BWO-1.5 (Figure 3b). This is due to the formation of Bi2WO6 into two-dimensional sheets by NaOH during pH adjustment. Meanwhile, the SEM image of BWO-1.5 reveals that the untreated sample shows an irregular stacked structure (Figure 3a), which is believed to be caused by the natural growth of the catalysts during the formation process. This is consistent with the conclusions reported earlier in the literature [47]. Excessive NaOH solution also results in an irregular structure and collapse of the nanosheet structure, indicating a negative impact on catalyst formation.
The formation of nanosheet materials was studied using TEM images to determine the effects of NaOH solution. The images clearly demonstrated a layered two-dimensional nanosheet structure (Figure 3d). Furthermore, the lattice spacing of BWO-2 was measured to be 0.32 nm and 0.24 nm on the (200) and (020) crystal faces of Bi2WO6, respectively, as seen in the TEM image of a single nanosheet (Figure 3e). The BWO-2 material also demonstrated the distribution of O, W, and Bi elements (Figure 3f).
The N2 adsorption–desorption curves were used to study the structural characteristics of three photocatalysts, namely BWO-1.5, BWO-2, and BWO-3. Table 1 compares the BET surface area and average pore size of these three catalysts. BET surface area of BWO-1.5 is slightly larger than BWO-2 and BWO-3, and the average pore size is like that of BWO-2, which is attributed to its non-polar structure. The average pore size of BWO-2 is greater than that of BWO-3, while the BET surface area is slightly smaller, indicating that it has more adsorption sites. Overall, the physical properties of the three catalysts are comparable (Figure 4a). The adsorption isotherms of the three catalysts follow the IUPAC type Ⅳ curves containing H3 hysteresis loops [48], commonly found in microporous and mesoporous materials [49]. The pore sizes of the three catalysts are uniformly distributed within the range of 35 nm (Figure 4b), indicating that the photocatalysts prepared have a mesoporous structure [50,51,52].
XPS pattern was used to analyze the chemical composition and electron states of the elements in the photocatalyst BWO-1.5. All peaks were obtained after correcting for the C 1s peak at 286.4 eV. The Bi 4f spectra of BWO-1.5 without NaOH treatment showed two peaks at 159.2 eV and 164.5 eV (Figure 5b), which were identified as Bi 4f7/2 and Bi 4f5/2, respectively [3]. This indicates the presence of a Bi3+ chemical state in the photocatalysts [53,54]. The W elements in the photocatalysts showed two peaks at 37.6 eV and 35.4 eV, corresponding to the states of W 4f5/2 and W 4f7/2, respectively [55,56,57]. Br 3d peaks were also found in BWO-1.5, BWO-2, and BWO-3, suggesting that Br 3d was bound to the surface of these photocatalysts (Figure 5a).
Figure 6 displays the origin of oxygen vacancy in photocatalysts. The peaks observed at approximately 530.0 eV and 531.3 eV correspond to lattice oxygen and surface-adsorbed oxygen, respectively [58]. The concentration of surface-chemisorbed oxygen can indicate the concentration of oxygen vacancy. It is indicated by comparing the concentrations of BWO-1.5 (9%), BWO-2 (13%), and BWO-3 (18%) that the surface-adsorbed oxygen content of samples increases slightly with the addition of alkali during photocatalyst preparation [59], indicating an increase in oxygen vacancy content. Oxygen vacancy serves as the active center of the reaction, and a higher oxygen vacancy content can enhance the performance of photocatalytic CO2 reduction. Oxygen vacancy can capture photogenerated electrons during the catalytic reaction process, improving the migration efficiency of electrons and holes and preventing their recombination, thus promoting CO2 activation. It should be noted that when BWO-3 is regulated with excessive alkali, it contains more oxygen vacancies, which can act as a center for recombining photogenerated charge carriers. This results in a high efficiency of electron–hole pair recombination. Therefore, during the preparation of photocatalysts, it is beneficial to regulate the pH to a moderate level to accelerate the migration of photogenerated electrons and avoid their recombination process while also promoting the carbon dioxide reduction reaction. Additionally, the band gap structure of Bi2WO6 photocatalysts can be altered through pH adjustment, and the addition of alkali may also change the band structure of Bi2WO6 nanosheets.
The surface characteristics of photocatalysts have been investigated utilizing FTIR (Figure 7). The peak that appears at 3425 cm−1 indicates the stretching and bending of the O-H group in the H2O molecules that are adsorbed onto the photocatalysts’ surface [60,61]. This is consistent with the findings of other researchers [62]. The vibration peaks of CH3-N+ were identified at 2923 cm−1 and 2849 cm−1, which was due to CTAB doping [63]. The peaks observed at 2360 cm−1 and 2340 cm−1 were generated by CO2 in the air that was absorbed by the photocatalysts. From the data presented in Figure 7, it is evident that the intensity of CO2 vibration peaks varies among different photocatalysts. Therefore, we infer that BWO-2 exhibits the strongest CO2 absorption intensity. As per the research conducted, the peaks observed at 1386 cm−1, 734 cm−1, and 579 cm−1 correspond to W-O-W bridging stretch, Bi-O, and W-O stretch vibration, respectively [64,65,66]. This implies that compared to BWO-1.5, the higher alkali or acidic etching of BWO-2 exposes more Bi-O and W-O bonds.
The absorption edges of BWO-2 and BWO-3 in the UV–visible diffuse reflection spectra exhibit a blue shift in comparison to BWO-1.5 (Figure 8a). This phenomenon could be attributed to quantum size effects [67]. This also indicates that BWO-2 has the narrowest range of light absorption compared with the other samples. Furthermore, the absorption edges of photocatalysts are at about 450 nm, illustrating that this type of photocatalyst still has the potential for high catalytic ability in the range of visible light [68]. The steepness of the UV spectra in the visible and ultraviolet ranges demonstrates that the energy absorption is on account of bandgap transitions rather than impurity level transitions [69]. Generally, the band structure of photocatalysts determines the degree of photocatalytic reaction. For direct semiconductors, the equation (αhν)2 = A(hν − Eg) is used, where α, A, ν, h, and Eg, respectively, represent the absorption coefficient, constant, photon incident frequency, Planck constant, and band gap [70,71,72]. According to the formula, the band gaps (Eg) of BWO-1.5, BWO-2, and BWO-3 are 3.15 eV, 3.24 eV, and 3.21 eV, respectively (Figure 8b). Compared to the other two samples, BWO-2 has the widest band gap. A wider band gap weakens the light absorption range of photocatalysts [73]. This also explains why BWO-2 has the narrowest light absorption range. Generally speaking, the lower the band gap energy, the higher the separation ability of photogenerated carriers [74]. However, the recombination of these carriers may also increase, which means that the band gap width should be within a reasonable range. This is necessary not only to ensure a high separation rate of photogenerated carriers but also to effectively inhibit the recombination process of both. Compared to BWO-1.5 and BWO-3, BWO-2 has the widest band gap. This indicates that, in comparison to the BWO-1.5 sample, the generated photogenerated charge carriers have higher energy. The photogenerated electrons have stronger migration ability and can inhibit the photogenerated charge carrier recombination process to a certain extent. Therefore, redox reaction is more likely to occur on the surface, laying a good structural foundation for excellent photocatalytic activity. At the same time, we can observe that the band gap of BWO-3 is narrower compared with that of BWO-2, indicating that excessive alkali has a negative impact on the regulation of the band gap. This is not conducive to inhibiting the photogenerated carrier recombination process, which is in accordance with the reported literature [75,76].
The VB-XPS diagram illustrates that the valence band potential (EVB) of BWO-1.5, BWO-2, and BWO-3 photocatalysts are 2.22 eV, 2.15 eV, and 2.26 eV, respectively (Figure 9). According to the following formula
EVB, NHE = φ + EVB, XPS − 4.44,
where φ is the work function (4.2 eV) of the instrument [77], the valence band positions of the three samples were calculated as EVB,BWO-1.5 = 1.98 eV, EVB,BWO-2 = 1.91 eV, and EVB,BWO-3 = 2.02 eV, respectively. Through the calculated valence band values and the above band gap, the conduction band values of BWO-1.5, BWO-2, and BWO-3 were also obtained [78] as follows: −1.17 eV, −1.33 eV, and −1.19 eV, respectively. The conduction position of BWO-2 is slightly more negative than that of BWO-1.5 and BWO-3, indicating that BWO-2 has a greater reducing capacity [79,80]. Therefore, we can infer that the photocatalytic CO2 reduction rate on BWO-2 is the highest. There is no difference in the conduction values of BWO-1.5 and BWO-3, indicating that the reducing capacity of the two is similar. The band structures of the three catalysts are shown in Figure 10. Due to its better band gap structure, BWO-2 is more suitable for the CO2 photocatalytic reduction process.

4. Photocatalytic Performances for CO2 Reduction

The samples mentioned above have excellent properties and were subjected to a photocatalytic CO2 reduction process with light irradiation and H2O for 4 h. The results showed that all the samples had varying levels of photocatalytic activity, with the production of CO increasing as the duration of light exposure increased (Figure S2). Meanwhile, except for CO, no other C1 products (HCOOH, CH4, and CH3OH) were detected during the photocatalysis process. The high activity of these Bi2WO6 photocatalysts can be attributed to their ability to absorb both UV and visible light, allowing them to effectively utilize photogenerated electrons to activate CO2 and facilitate the photocatalytic reduction reaction. Compared with BWO-1.5 and BWO-3, BWO-2 exhibits better photocatalytic performance at each time interval. With the extension of time, the disparity of activity among BWO-2 and the other samples increases gradually. In addition, except for BWO-2, the activity of all samples decreases with the increase in pH (Figure 11a). Therefore, according to the activity results, it can be considered that the catalyst prepared by adding the appropriate amount of NaOH (BWO-2) is more favorable to the photocatalytic CO2 reduction reaction due to changes in oxygen vacancy and band structure. The activity consequence of BWO-1 prepared by adding nitric acid (Figure S2) also proves that the catalyst prepared under appropriate alkali conditions is favorable to the photocatalytic CO2 reduction process. After light irradiation for 4 h, the yield of CO2 photocatalytic reduction to CO (Figure 11b) on BWO-2 displays 96.64 μmol·g−1, which is a lot more than that of BWO-1.5 (74.44 μmol·g−1) and BWO-3 (78.46 μmol·g−1) (Figure S3). At the same time, the yield of CO produced on BWO-3 is higher than that on BWO-1.5, and the yield of CO produced on BWO-5 is also higher than that on BWO-1. This shows that alkali modification is advantageous to the photocatalytic CO2 reduction reaction. However, compared with the catalytic activity of BWO-2, it indicates that excessive modification and no change are not conducive to the reaction.
To investigate the source of C in CO produced during the CO2 reaction process, a photocatalytic reduction test was conducted on B-BWO-2 and pure CTAB using identical conditions, as shown in Figure S4. The findings revealed that pure CTAB was unable to transform CO2 into CO under light, implying that all the CO generated by BWO-2 was derived from CO2 in the reaction system rather than from CTAB, which also contained C. On the other hand, compared with BWO-2, B-BWO-2 produces a very small amount of CO, indicating that CTAB only acts as a surfactant and has been completely doped in the catalyst BWO-2. In order to verify that the CO produced is from the photocatalytic CO2 reduction process, the control variable method was used to carry out under different conditions with other conditions unchanged (Figure 11c). In the absence of irradiation, no CO can be detected in the reaction system, indicating that light is an indispensable condition for the photocatalytic reaction. No product is detected without catalysts, which confirms that photocatalysts must be involved in the photocatalytic process. When CO2 is replaced by N2 in the reaction system for photocatalytic activity testing, CO and other products can not be found, which proves that CO2 is the only raw material for CO production. Finally, BWO-2 was used to study the stability of this type of photocatalyst. After 4 h, the yield of CO produced by the photocatalyst BWO-2 remains stable at more than 90 μmol·g−1 after being repeated four times (Figure 11d). The cause of the high activity of BWO-2 is that its band gap is relatively suitable, which lays a good structural foundation for the rapid separation of photogenerated carriers to a certain extent. In addition, it has an appropriate amount of oxygen vacancies, which effectively restrains the speed of recombination. Table 2 summarizes the Bi2WO6 photocatalytic system applied in the photocatalytic CO2 reduction field. When compared to other Bi2WO6 photocatalysts that require complex preparation processes, it has been discovered that single-phase Bi2WO6 modified with alkali demonstrates the most superior photocatalytic activity. This manifests that the photocatalytic activity can be effectively improved with alkali modification.
The ability to separate photogenerated carriers can be distinguished through transient photocurrent response tests. BWO-2 exhibits the greatest intensity of photocurrent (as shown in Figure 12), while BWO-1.5 and BWO-3 demonstrate significantly lower photocurrent intensities compared to BWO-2. The I-t test curve reveals that the strong photocatalytic activity of sample BWO-2 with moderate alkali is due to the sensitivity of photogenerated electrons to light radiation [88], which leads to the high efficiency of carrier separation in the sample [89]. Photogenerated electrons are combined with the CO2 on the surface of the photocatalysts, thus improving the activity of the photocatalytic CO2 reduction. This is basically consistent with the influence of the band gap structure of photocatalysts mentioned above in UV–visible and VB-XPS. The results of the photocatalytic reduction reaction of CO2 also correspond well with the consequence of the I-t curves test. The strength of the photocatalytic reduction activity is closely connected with the activity of the photogenerated carriers produced by the photocatalysts. On the other hand, the addition of alkali can not only improve the separation capacity of the photogenerated carrier but also inhibit the recombination process. The results are in line with the observations made in other research on the photoluminescence features of photocatalysts [90].

5. Possible Photocatalytic Reaction Mechanism

The currently accepted mechanism for the photocatalytic CO2RR [91] can be broken down into two distinct parts: the adsorption–desorption process [77] and the photocatalytic reaction process. This process can then be further divided into five parts [92], namely, light absorption, carrier transfer, CO2 adsorption, surface redox reaction, and product desorption.
In the adsorption–desorption process, physical adsorption of CO2 and H2O molecules occurs on the surface of the photocatalysts, and both diffuse into the internal pore of Bi2WO6. CO2 adsorption and activation are the prerequisites and main obstacles for the photocatalytic reaction. In this process, a high adsorption amount of CO2 is conducive to the formation of ·CO2− anions and ·COOH− [93], and the presence of both can be considered as important signs of the successful activation of CO2. Redox reaction happens at the active site on the pore surface, and the generated CO gradually moves away from the active site and leaves the photocatalyst surface through diffusion. In the course of the photocatalytic CO2 reaction, Bi2WO6 receives enough light energy to generate excited electrons on its energy band, and the photogenerated electrons on the lower energy valence band (VB) transition are transferred to the higher energy vacant orbital conduction band (CB) Meanwhile, this process leaves an equal number of photogenic holes in the valence band, thus generating electron–hole pairs. The photogenerated carrier activation reaction is shown as follows:
BWO + hν (≥Eg) → h+ + e−
where hν represents the energy of the incident light, Eg represents the band gap of the photocatalyst, and h+ and e− represent the photogenerated holes and electrons, respectively.
Photogenerated holes and electrons are represented by h+ and e−, respectively. Electrons produced are highly reducible and can reduce CO2 while oxidizing H2O on the hole. This leads to separate oxidation and reduction semi-reactions on the energy band (Figure 13). The efficiency of photogenerated carriers is mainly reflected in their separation efficiency, but it is important to control the speed of the recombination process to prevent excess light and heat dissipation.
In this work, the average pore size and BET surface area of the prepared BWO-1.5, BWO-2, and BWO-3 photocatalysts are not significantly different. Therefore, we have analyzed their energy band structure and proposed the corresponding reliable mechanism. Generally speaking, for semiconductor photocatalysts, the band structure determines the activity of photogenerated carriers [94], and the lower the band value, the stronger the reduction ability. The reduction reaction can proceed normally if the conduction potential is greater than the required potential for the reduction reaction. Figure 8 shows that BWO-2 has the lowest conduction band position, which ensures that photogenerated electrons generated by light have a stronger reducing ability.
CO2 has a very stable linear structure, and its activation process is very difficult. However, after CO2 adsorption, the structure gradually changes into a curved form, namely the formation of ·CO2− anion and ·HCOO−, which can successfully reduce the reaction barrier [22]. The reaction path of CO2 reduction to CO was mentioned in previous work [95]. It is believed that ·HCOO− is the intermediate of the reaction. The O atom in CO2 is trapped in the vacancy [96] to form the free ·COO− intermediate, which then meets with the proton dissociated from H2O to rapidly form CO. The main reactions [97] involved are as follows:
2H2O + h+ → 4H+ + O2 + 4e−
CO2 + e− → ·CO2−
·CO2− + H+ + e− → ·COOH−
·COOH− + H+ + e− → ·CO−
·CO− → CO
Additionally, scientists have introduced an alternative hypothesis [2], suggesting that ·CO2− is the active intermediate species. They have also provided a corresponding reaction process to support this theory [98].
CO2 + e− → ·CO2−
H2O → H+ + OH−
OH− + h+ → ·OH
In this process, a disproportionation reaction may also occur: ·CO2− + CO2 → CO + CO32−. On the macro level, the following reactions can occur over the photocatalyst: (BWO + V0) + ·CO2− → CO + BWO, where V0 represents oxygen vacancy.
However, according to the infrared spectrogram we obtained, neither the ·COOH− group nor the ·CO2− ion appears at normal temperature and pressure. Therefore, we are more inclined to believe that the first case is valid. This is because the standard electrode potential ENHE for the formation of ·CO2− is −1.85 eV, while it is −0.61 eV for the formation of HCOOH [99]. As the conduction position of BWO-2 is −1.33 eV, it can be inferred that ·COOH− intermediates are easier to form. Therefore, this paper prefers the first mechanism explanation.
As demonstrated by the transient photocurrent response test, the modification method employed enhances the production of photogenerated carriers with greater separation efficiency. BWO-2 boasts the largest band gap within a reasonable range, and there is no significant variance in the light energy range selection. However, it can successfully hinder the recombination process of photogenerated carriers. In essence, the alkali modification approach aims to alter the material band structure and enhance its photogeneration carrier activity, thereby improving its ability to reduce CO2 through photocatalysis.

6. Conclusions

This study synthesized Bi2WO6 nanosheets by a one-step typical hydrothermal route and researched the impact of alkali preparation conditions on photocatalytic activity through experimental design. Various factors affect photocatalytic activity. Characterization of the photocatalysts revealed that the BWO-2 sample had high purity, excellent photoelectric effect, and more exposed Bi-O and W-O sites. BWO-2 nanosheets improved the energy band structure, promoting the separation capacity of photogenerated carriers and inhibiting recombination rate, resulting in stronger reducing performance under light sources. The photocatalytic reduction of CO2 on BWO-2 samples showed increased CO accumulation, with considerable reaction rate and reduction stability even after prolonged reaction time and multiple cycles. This study proposes a mechanism for CO2 photocatalytic reduction and investigates the factors contributing to the high activity of BWO-2 synthesized with an appropriate amount of alkali. The addition of NaOH improved the band structure of bare Bi2WO6, adjusted the conduction band position of the photocatalyst, and provided an appropriate amount of oxygen vacancy after modification, resulting in strong activity of photogenerated carriers, and thus improving the photocatalytic CO2 reduction competence. These findings provide valuable insights for further research on the band gap structure of photocatalytic CO2 reduction systems.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/pr11102827/s1. Figure S1. Schematic diagram of CO2 photocatalytic reduction process; Figure S2. CO production over four hours of BWO-1, BWO-2, and BWO-3; Figure S3. CO production after four hours of BWO-1, BWO-1.5, BWO-2, BWO-3, and BWO-5; Figure S4. CO production over four hours of CTAB, B-BWO-2, and BWO-2.

Author Contributions

Methodology, Y.S. and J.W.; formal analysis, S.L.; resources, Y.M.; data curation, L.Z.; writing—original draft preparation, C.S. and Y.S.; writing—review and editing, C.S. and Y.M.; investigation, W.A. and Z.Y.; funding acquisition, Y.M. and J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (22179081, 22076117), Class III Peak Discipline of Shanghai—Materials Science and Engineering (High-Energy Beam Intelligent Processing and Green Manufacturing), and Science and Technology Commission of Shanghai Municipality (20ZR1422500).

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of synthesis process of Bi2WO6 series photocatalysts.
Figure 1. Schematic diagram of synthesis process of Bi2WO6 series photocatalysts.
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Figure 2. XRD patterns of BWO-1, BWO-1.5, BWO-2, BWO-3, and BWO-5.
Figure 2. XRD patterns of BWO-1, BWO-1.5, BWO-2, BWO-3, and BWO-5.
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Figure 3. SEM images of (a) BWO-1.5, (b) BWO-2, and (c) BWO-3; (d,e) TEM images of BWO-2; (f) STEM; and the corresponding elemental mapping images of BWO-2.
Figure 3. SEM images of (a) BWO-1.5, (b) BWO-2, and (c) BWO-3; (d,e) TEM images of BWO-2; (f) STEM; and the corresponding elemental mapping images of BWO-2.
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Figure 4. (a) N2 adsorption isotherms at 298 K; (b) pore diameter distribution curves of BWO-1.5, BWO-2, and BWO-3.
Figure 4. (a) N2 adsorption isotherms at 298 K; (b) pore diameter distribution curves of BWO-1.5, BWO-2, and BWO-3.
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Figure 5. (a) XPS spectra, (b) Bi 4f spectra, and (c) W 4f spectra of BWO-1.5, BWO-2, and BWO-3.
Figure 5. (a) XPS spectra, (b) Bi 4f spectra, and (c) W 4f spectra of BWO-1.5, BWO-2, and BWO-3.
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Figure 6. O 1s spectra of (a) BWO-1.5, (b) BWO-2, and (c) BWO-3.
Figure 6. O 1s spectra of (a) BWO-1.5, (b) BWO-2, and (c) BWO-3.
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Figure 7. FTIR spectra of different photocatalysts: BWO-1, BWO-1.5, BWO-2, BWO-3, and BWO-5.
Figure 7. FTIR spectra of different photocatalysts: BWO-1, BWO-1.5, BWO-2, BWO-3, and BWO-5.
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Figure 8. (a) UV–visible spectra and (b) plots of band gap energy of photocatalysts: BWO-1.5, BWO-2, and BWO-3.
Figure 8. (a) UV–visible spectra and (b) plots of band gap energy of photocatalysts: BWO-1.5, BWO-2, and BWO-3.
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Figure 9. VB-XPS diagram of photocatalysts: (a) BWO-1.5, (b) BWO-2, and (c) BWO-3.
Figure 9. VB-XPS diagram of photocatalysts: (a) BWO-1.5, (b) BWO-2, and (c) BWO-3.
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Figure 10. Band gap structure diagram of photocatalysts: BWO-1.5, BWO-2, and BWO-3.
Figure 10. Band gap structure diagram of photocatalysts: BWO-1.5, BWO-2, and BWO-3.
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Figure 11. (a,b) CO production after CO2 photocatalytic reduction, (c) comparison of CO production by photocatalytic reduction of CO2 with different conditions, and (d) repetition of the photocatalytic CO2 reduction four times.
Figure 11. (a,b) CO production after CO2 photocatalytic reduction, (c) comparison of CO production by photocatalytic reduction of CO2 with different conditions, and (d) repetition of the photocatalytic CO2 reduction four times.
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Figure 12. Photocurrent response curves of BWO-1.5, BWO-2, and BWO-3.
Figure 12. Photocurrent response curves of BWO-1.5, BWO-2, and BWO-3.
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Figure 13. Model of photocatalytic CO2 reduction process mechanism.
Figure 13. Model of photocatalytic CO2 reduction process mechanism.
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Table 1. BET test for three photocatalysts: BWO-1.5, BWO-2, and BWO-3.
Table 1. BET test for three photocatalysts: BWO-1.5, BWO-2, and BWO-3.
CatalystsBET Surface Area (m2/g)Average Pore
Volume (cm3/g)		Average Pore
Diameter (nm)
BWO-1.540.00.213.5
BWO-233.50.213.3
BWO-333.90.112.1
Table 2. Bi2WO6 photocatalysts for photocatalytic CO2 reduction.
Table 2. Bi2WO6 photocatalysts for photocatalytic CO2 reduction.
PhotocatalystSynthesis MethodProductCO Yield (4 h)Light SourceReferences
Bi2WO6
(BWO-2)		Hydrothermal routeCO96.6 μmol·g−1300 W Xe lampOur work
Bi2WO6/rGO/3D-GCNMicrowave-assisted routeCO8.9 μmol·g−1300 W Xe lamp (λ > 420 nm)[81]
Bi2WO6/RGO/g-C3N4Hydrothermal routeCO, CH463.8 μmol·g−1visible light[82]
Bi2O3/Bi2WO6Hydrothermal routeCO, CH469.6 μmol·g−1visible light (λ > 400 nm)[3]
Bi2WO6/InVO4Hydrothermal routeCO, CH471.9 μmol·g−1Xe lamp (300 W, λ > 420 nm)[83]
g-C3N4/Bi2WO6Hydrothermal routeCO2.6 μmol·g−1Xe lamp (300 W, λ > 420 nm)[84]
Cs3Bi2I9/Bi2WO6Solvothermal routeCO30.0 μmol·g−1300 W Xe lamp[85]
CdS/Bi2WO6−SColloidal two-phase routeCO, CH454.9 μmol·g−1 (8 h)300 W Xe lamp (λ > 420 nm)[86]
Bi2WO6Colloidal two-phase routeCO, CH47.0 μmol·g−1 (24 h)300 W Xe lamp (visible light)[62]
Bi2WO6Mild aluminothermic reductionCO, CH423.8 μmol·g−1visible light[87]
Cs2AgBiBr6/Bi2WO6Ultrasonic-assisted
process		CO, CH415.4 μmol·g−1 (8 h)300 W Xe lamp (visible light)[14]
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Song, C.; Sun, Y.; Zhang, L.; Liu, S.; Wang, J.; An, W.; Men, Y.; Yan, Z. The Highly Enhanced Efficiency of the Photocatalytic Reduction of CO2 over Bi2WO6 Nanosheets by NaOH Microregulation. Processes 2023, 11, 2827. https://doi.org/10.3390/pr11102827

AMA Style

Song C, Sun Y, Zhang L, Liu S, Wang J, An W, Men Y, Yan Z. The Highly Enhanced Efficiency of the Photocatalytic Reduction of CO2 over Bi2WO6 Nanosheets by NaOH Microregulation. Processes. 2023; 11(10):2827. https://doi.org/10.3390/pr11102827

Chicago/Turabian Style

Song, Chao, Yangang Sun, Li Zhang, Shuang Liu, Jinguo Wang, Wei An, Yong Men, and Zhenrong Yan. 2023. "The Highly Enhanced Efficiency of the Photocatalytic Reduction of CO2 over Bi2WO6 Nanosheets by NaOH Microregulation" Processes 11, no. 10: 2827. https://doi.org/10.3390/pr11102827

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