Open AccessFeature PaperArticle

Enhanced CO₂ Photoreduction Performance of WO_3−x

Yelan Cheng

Zhaolin Li

and

Xiaofei Yang

College of Science, Nanjing Forestry University, Nanjing 210037, China

Author to whom correspondence should be addressed.

Catalysts 2025, 15(1), 13; https://doi.org/10.3390/catal15010013

Submission received: 6 November 2024 / Revised: 19 December 2024 / Accepted: 21 December 2024 / Published: 27 December 2024

(This article belongs to the Section Photocatalysis)

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Graphical abstract
"> Figure 1
Synthesis diagram of WO3 and WO3−x. "> Figure 2
(a) XRD patterns of WO3 and WO3−x; (b) XPS analysis of W 4f; (c) XPS analysis of O 1s; (d) ESR spectra. "> Figure 3
The structure of the prepared samples: (a) SEM images of WO3 and (b) WO3−x; (c) TEM of WO3 and (d) WO3−x; (e) HRTEM; and (f) Elemental mapping analysis of WO3−x. "> Figure 4
The photocatalytic performance of WO3 and WO3−x: (a) the amount of CO generated, (b) the rate of CO production; ESR spectra of WO3 and WO3−x under dark and light conditions: (c) e−, (d) h+. "> Figure 5
(a) Steady-state fluorescence spectra, (b) transient fluorescence spectra, (c) photocurrent response curves, and (d) EIS Nyquist plots for WO3 and WO3−x. "> Figure 6
(a) UV-Vis-NIR diffuse reflectance spectra, (b) band gaps via Kubelka–Munk (K–M) jump, (c) valence band XPS spectra, and (d) band structures of WO3 and WO3−x. "> Figure 7
In situ FTIR spectra of WO3−x. ">

Versions Notes

Abstract

Converting CO₂ greenhouse gases into high-value-added fuels via semiconductor photocatalysts is an ideal solution to address current environmental and energy issues. Due to its unique physicochemical traits and flexible structure, WO₃ is widely employed in photocatalysis. Nevertheless, it commonly faces problems such as limited light absorption and low reaction selectivity. Here, we effectively tackle the existing issue by introducing an oxygen defect strategy to synthesize two-dimensional WO_3−x nanosheets with rich oxygen vacancies. Due to localized surface plasmon resonance (LSPR), these nanosheets may exhibit broad light absorption and efficient CO₂ adsorption and activation. In the photocatalytic reduction of carbon dioxide (CO₂) to carbon monoxide (CO), WO_3−x nanosheets exhibited 100% selectivity and 16.1 μmol g⁻¹ h⁻¹ yield, 6.2 times higher than WO₃. Oxygen vacancies improve WO₃’s band structure and increase its capacity to absorb visible light. Based on electrochemical tests and fluorescence spectroscopy analysis, the outstanding functionality of WO_3−x nanosheets is related to the improved separation and transport of photocurrents and the restricted radiative recombination of the resulting electron pairs and holes.

Keywords:

photocatalysis; CO₂ reduction; oxygen vacancy; WO_3−x

Graphical Abstract

1. Introduction

The energy crisis and greenhouse effect are the outcomes of fossil fuel overuse and carbon dioxide (CO₂) excess emissions [1,2,3]. Energy and environmental issues may be addressed by employing CO₂ photo-conversion into high-value hydrocarbon fuels [4,5,6]. However, stable catalysts with optimized structures and active sites that overcome the shortages of limited light absorption range and low reactive selectivity are urgently needed for CO₂ photoreduction [7,8,9].

Metal oxide-based photocatalysts which process goodnesses at low cost, that have strong redox capacity, and that are environmentally friendly are vital for photocatalytic CO₂ reduction [10,11,12]. Among them, WO₃ is often used in photocatalysis because of its tunable structure and special physicochemical properties [13]. WO₃, an affordable and low-toxicity n-type semiconductor absorbing the majority of visible light, is a highly promising option for photocatalysis [14]. However, the forbidden bandwidth of WO₃ (E_g = 2.5−2.8 eV) limits the infrared response and is not favorable for CO₂ photocatalytic reduction [15]. In order to improve its light-absorbing ability as well as the selectivity and activity for photocatalytic CO₂ reduction, oxygen vacancies are introduced because of the localized surface plasmon resonance (LSPR) phenomenon, which induces an IR response [16]. In particular, oxygen vacancies (OVs) can affect the disordered production of cations and the structural characteristics of e⁻, which further inhibit e⁻ − h⁺ recombination and increase photocatalytic efficiency [17]. Concurrently, OVs in transition metal semiconductors can change the region surrounding the material’s Fermi surface’s spin polarization, which amplifies the non-uniform DOS and charge distribution in that vicinity. Furthermore, upon photoexcitation, WO₃ exhibits significant charge carrier transfer characteristics. WO₃, in contrast to other metal semiconductors, can be easily adapted to OVs within the lattice, extending its absorption of light into the near-infrared range and enabling the efficient use of sunlight [18]. However, the effect of oxygen vacancies in tungsten oxide on its CO₂ reduction is rarely reported. Moreover, photocatalysts with nanoscale structures typically have high photocatalytic activity [19]. The two-dimensional material (i.e., nanosheets) has a gradient in the potentials of its faces, driving photogenerated electrons and holes to distinct faces. PN junctions form at the nanosheet junctions, significantly facilitating the separation of charge carriers. The photocatalytic activity of WO₃ nanosheets is significantly higher than that of bulk WO₃, indicating that the nanosheet structure holds great potential for improving photocatalytic activity.

In this work, we employed hydrothermal and high-temperature calcination procedures to successfully fabricate two-dimensional WO_3−x nano photocatalysts with numerous oxygen vacancies. The dark blue color of the WO_3−x nanosheets suggested that oxygen vacancies were successfully introduced. Substantial amounts of oxygen vacancies in WO_3−x were further affirmed by ESR and XPS tests. The photocatalytic performance test verified that WO_3−x had robust catalytic activity and excellent selectivity in the photocatalytic conversion of CO₂ to CO. Through electrochemical testing and fluorescence spectroscopy, the beneficial impact of oxygen vacancy on the photogenerated charge kinetics of WO_3−x photocatalysts was further confirmed. In situ Fourier transform infrared spectroscopy (FTIR) was used to investigate WO_3−x’s catalytic mechanism. Important intermediate species and others were found on the catalyst surface to evaluate the photocatalytic CO₂ reduction reaction’s pathway.

2. Results and Discussion

2.1. Preparation and Characterization of Photocatalysts

The WO₃·H₂O precursor in Figure 1 was produced via a straightforward hydrothermal process. The oxygen-deficient WO_3−x nanosheets were synthesized by calcining the precursor at high temperatures in a N₂ atmosphere. For comparison, WO₃ was obtained by treating the precursor in an air atmosphere. The WO₃ and WO_3−x photocatalysts generated were yellow and dark blue, respectively. The observed color shift indicated that the oxygen vacancies in WO_3−x had been successfully introduced. X-ray powder diffraction (XRD) was used to examine the crystal structures of the produced materials. WO₃ shows unique high-intensity diffraction peaks at typical diffraction peaks 2θ of 23.1°, 23.6°, 24.3°, and 34.1°, respectively, as shown in Figure 2a. These peaks correspond to the (002), (020), (200), and (202) crystal planes of monoclinic-phase WO₃ (PDF#83-0951) [20]. The introduction of oxygen vacancies led to a moderate weakening of the diffraction peaks of WO_3−x, possibly due to the increased concentration of oxygen defects. No diffraction peaks related to other compounds were observed, demonstrating that the modified WO₃ nanosheets have high phase purity. In the FTIR spectra (Figure S1), it was found that the chemical bonding on the surface of the samples before and after modification was essentially the same, further confirming the successful preparation of WO_3−x.

The elemental content of the samples and their related oxidation states were ascertained by XPS analysis. The elements W, O, and C are seen from the complete spectra of WO₃ and WO_3−x (Figure S2). Two energy peaks can be seen in the W 4f XPS energy spectrum (Figure 2b) at 35.7 and 37.9 eV, respectively. W 4f_7/2 and W 4f_5/2 energy peaks of W⁶⁺ in the WO₃ lattice correspond to these [21]. In addition to the binding energy peaks of W⁶⁺, WO_3−x revealed the presence of W⁵⁺’s W 4f_7/2 and W 4f_5/2 energy peaks at 35.1 and 37.0 eV [22], suggesting that W⁶⁺ in WO_3−x was partially reduced to W⁵⁺ to produce OVs. Also, the W⁶⁺ characteristic peak of WO_3−x was pushed toward a higher binding energy in comparison to WO₃, meaning that W 4f_7/2’s binding energy changed from 35.7 eV to 35.9 eV, and W 4f_5/2’s binding energy changed from 37.9 eV to 38.0 eV. Research indicates that the electronic structure of WO_3−x is modulated by the presence of OVs and that the presence of W⁵⁺ raises the electron density of W [23], promoting electron transport and CO₂ reduction. The dominant peak in the O 1s XPS spectrum (Figure 2c) corresponds to the lattice oxygen in the samples, with binding energies for WO₃ and WO_3−x at 530.4 and 530.7 eV, respectively. In contrast, WO_3−x showed two secondary peaks at binding energies located at 533.7 and 532.6 eV, representing adsorbed oxygen in WO_3−x [24] and oxygen in oxygen vacancies [25], respectively. According to the XPS study, oxygen vacancies were successfully created in WO_3−x, which resulted in the buildup of electrons on W and efficient CO₂ photoreduction.

One technique to prove the existence of vacancies in materials is to apply electron spin resonance (ESR) spectroscopy, which focuses on unpaired electrons in the material. In dark settings, WO_3−x produces a substantial ESR signal at g = 2.002, as illustrated in Figure 2d. This suggests that WO_3−x nanosheets contain a substantial amount of unbound electrons [26]. With the addition of light illumination, the intensity of the WO_3−x ESR signal increased significantly. It suggests that more electrons were captured by the oxygen vacancies under light, which in turn implies that the oxygen vacancies can act as electron acceptors [27]. The ESR results suggest that numerous oxygen vacancies form during the synthesis of WO_3−x nanosheets, consistent with the XPS energy spectra analysis. Thus, the presence of oxygen vacancies promotes photogenerated charge separation in addition to increasing light absorption, as they are electron acceptors.

The morphology of WO₃ and WO_3−x was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The WO₃ and WO_3−x samples show regular nanosheets stacked unevenly, as seen in the SEM images in Figure 3a,b. Using TEM, the nanosheet structure of WO₃ and WO_3−x was further verified. Figure 3c shows the structure of the WO₃ sample obtained by calcining the precursor under air conditions. The resulting WO₃ nanosheets have a relatively smooth surface as oxygen in the air fills the oxygen vacancies. When the calcination atmosphere is changed to nitrogen, WO_3−x nanostructures with numerous oxygen defects are obtained. As depicted in Figure 3d, the surface of the WO_3−x sample is relatively rough due to oxygen vacancies. Although WO₃ and WO_3−x contain different oxygen vacancies, both samples have similar monoclinic phases and inherit lamellar structures from their precursors. Figure 3e shows the HRTEM of WO_3−x, revealing slight misalignments and distortions in the lattice of WO_3−x, caused by oxygen defects. At 0.41 nm, the lattice spacing of WO_3−x corresponds to the (002) crystal plane. The elemental mapping of WO_3−x is displayed in Figure 3f, showing evenly dispersed W and O elements. In line with the findings of the XRD study, the results indicate that the oxygen vacancy-modified WO_3−x also exhibits a lamellar structure with distinct borders between the lamellae, indicating a high degree of crystallinity.

2.2. Photocatalytic CO₂ Reduction Performance

A 300 W xenon lamp was applied as the light source to examine the gas–solid phase photocatalytic CO₂ reduction capabilities of WO₃ and WO_3−x. Only the product CO was detected in the entire system. Both WO₃ and WO_3−x demonstrated photocatalytic CO₂ reduction to CO activity, as shown in Figure 4a, and the CO output gradually increased over time. Figure 4b presents the average CO generation rates of WO₃ and WO_3−x after a 5 h exposure to light. The photocatalytic CO₂ reduction efficiency of WO_3−x was higher than that of WO₃, with a CO yield of 16.1 μmol g⁻¹ h⁻¹, which is six times more than that of WO₃ (2.6 μmol g⁻¹ h⁻¹). Additionally, the control and blank experiments are shown in Figure S3. No CO production was observed under either dark or no-photocatalyst conditions. Also, when CO₂ was replaced with Ar gas, no CO production was noticed. Further deduction indicates that CO₂ gas is the source of CO.

2.3. Photocatalytic Mechanism

The photogenerated carrier concentrations of the samples under dark and light conditions were compared by using the electron spin resonance (ESR) technique. Figure 4c and Figure 4d respectively show the ESR spectra of electrons and holes captured by TEMPO before and after the illumination of WO₃ and WO_3−x. In the absence of light, the ESR spectra of WO₃ and WO_3−x exhibit triple signaling peaks with a 1:1:1 intensity ratio. After the introduction of light, the signals of photoelectrons trapped on WO_3−x were much weaker than those on WO₃, suggesting that more photogenerated charges in WO_3−x were involved in the reaction. Moreover, the signal peak intensities of TEMPO-h⁺ for both WO₃ and WO_3−x decreased slightly. The results show that fewer photogenerated electrons reach the surface in WO₃ than in WO_3−x. This illustrates that adding oxygen vacancies improves electron mobility and makes photogenerated electron holes easier to separate [28].

To further investigate the relationship between oxygen vacancies and charge dynamics, photoelectric properties were tested. Figure 5a shows the steady-state fluorescence spectra of WO₃ and WO_3−x. When compared to WO₃, WO_3−x exhibits a decreased fluorescence emission intensity, indicating that photo-generated electron–hole pair recombination is inhibited, which is helpful for charge transfer [29]. Additionally, the lifetime of the photogenerated charges of the samples was explored by transient fluorescence spectroscopy. The fluorescence lifetime of WO_3−x in Figure 5b is longer (1.25 ns) than that of WO₃ (1.20 ns), indicating that WO_3−x has a longer photogenerated charge existence time. Furthermore, Figure 5c shows that WO_3−x has a stronger photocurrent response, indicating that the oxygen defect modification promotes the charge separation rate in WO_3−x. The samples’ photogenerated carrier mobility was evaluated using electrochemical impedance (EIS). The arc radius of WO_3−x in Figure 5d is smaller than that of WO₃, suggesting a lower resistance and a faster charge carrier transfer rate for WO_3−x [30]. To further reveal the light-induced charge separation and transfer process, transient surface photovoltage (SPV) tests were conducted. Figure S4 presents the results, where positive signals were detected for both WO₃ and WO_3−x, indicating that photoelectrons migrated toward the surface of the catalyst under the built-in electric field. Among them, WO_3−x has a strong SPV signal response, indicating that it can effectively capture photogenerated electrons [31]. According to the aforementioned findings, introducing oxygen vacancies can enhance charge separation and transfer, reduce photogenerated carrier recombination, and increase the activity of photocatalytic CO₂ reduction.

The samples’ ability to absorb light was examined using UV-Vis diffuse reflectance spectroscopy. The broad gap of WO₃ is reflected by its absorption band edge, which is approximately 480 nm, as shown in Figure 6a. WO_3−x demonstrates a superior light absorption capacity compared to WO₃, with its light absorption edge extending to cover the entire spectrum above 800 nm. Even in the near-infrared region, WO_3−x exhibits a strong light-absorbing ability, which is caused by defect absorption due to oxygen vacancies. OVs may cause localized surface plasmon resonance (LSPR) effects. To be more precise, if the electrons of the metal and photons oscillate at the same frequency, they will resonate along the metal–dielectric interface, increasing the macroscopic absorption of light, particularly in the near-infrared spectrum. Furthermore, increased light absorption has the potential to generate more hot electrons, which would further enhance photocatalytic activity [32]. The large difference between WO₃ and WO_3−x suggests that the modification of oxygen vacancies has a significant impact on the light absorption capacity of WO₃. Subsequent calculations using the Kubelka–Munk transformation (Text S1) [33] revealed that the introduction of oxygen vacancies changed the energy band structure of WO₃. In the absence of oxygen vacancies, WO₃ has a broad forbidden band (2.61 eV), which limits its effective light absorption in the visible range. When oxygen vacancies are introduced, the forbidden bandwidth of WO_3−x becomes narrower (2.25 eV), which means that the material can absorb visible light more efficiently, thus increasing its photocatalytic activity. Further evidence of the impact of oxygen vacancies on energy band structures comes from valence band XPS measurements, where the valence band energies of WO₃ and WO_3−x are 2.31 and 1.85 eV, respectively (Figure 6c). Lastly, the energy band structure maps of the two materials are displayed in Figure 6d.

Measurements using in situ Fourier transform infrared spectroscopy (FTIR) are essential to reveal the formation and changes of intermediates during the reaction. Photocatalytic CO₂ reduction over WO_3−x nanosheets containing abundant oxygen defects was taken as an example. As shown in Figure 7, several peaks emerged in the IR spectra and gradually intensified as the time increased from 0 min to 60 min. Characteristic peaks belonging to monodentate carbonate (m-CO₃⁻) were detected at 1539, 1503, 1455, and 1336 cm⁻¹, and characteristic peaks belonging to bidentate carbonate (b-CO₃²⁻) were detected at 1651 and 1556 cm⁻¹. The characteristic peaks of chelate-bridged carbonate (c-CO₃²⁻) at 1731 cm⁻¹ and bicarbonate (HCO₃⁻) detected at 1434 and 1095 cm⁻¹ all originated from CO₂ adsorption, highlighting the excellent CO₂ uptake capacity of WO_3−x [34]. COO⁻ peaks at 1519 and 1360 cm⁻¹ indicate that CO₂ molecules can be well activated during the reaction. In addition, new peaks belonging to the HCOO⁻ group were significantly observed at 1697 and 1574 cm⁻¹, and their peak intensities increased markedly with increasing irradiation time, suggesting that the HCOO⁻ group is a crucial intermediate in the CO₂-to-CO photoreduction process [35]. Based on these analyses, it can be inferred that the photosynthesis of CO₂ on WO_3−x proceeds through the following pathways: First, CO₂ molecules are adsorbed on the surface of the WO_3−x catalyst. This process is one of the key steps in the photocatalytic reduction of the CO₂ reaction; the adsorbed CO₂ reacts with protons (H⁺) produced by hydrolysis to form COOH* intermediates. This step demonstrates the activation of CO₂ on the catalyst surface; subsequently, the COOH intermediate is deprotonated to generate CO. Specifically, COOH reacts with H⁺ and electrons to generate CO while releasing water molecules (COOH* + H⁺ + e⁻ → CO* + H₂O); finally, CO is further decomposed to generate the final product CO (CO → * + CO). This process illustrates the transformation pathway from the intermediate to the final product. Together, these steps constitute the mechanism of carbon dioxide photosynthesis on WO_3−x and enhance the activity and selectivity of the catalyst by introducing defective structures such as oxygen vacancies.

3. Experimental Section

3.1. Chemicals

Sodium tungstate dehydrates (Na₂WO₄·2H₂O, 99.5%), citric acid (≥99.5%), and D-(+)-Glucose (AR) were purchased from Aladdin. Sinopharm Chemical Reagent Co., Ltd. provided anhydrous ethanol (EtOH, AR) and hydrochloric acid (36~38%, AR). 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO, for ESR spectroscopy) was purchased from Sigma-Aldrich (Shanghai, China). The water used in all experiments was ultrapure. Without additional purification, all analytical-grade chemical reagents were utilized exactly as supplied.

3.2. Synthesis of Materials

3.2.1. Preparation of WO₃·H₂O

One milligram of white Na₂WO₄·2H₂O powder was weighed and dissolved in 30 mL of ultrapure water to form a clear solution. Next, 5 mmol of glucose and 1.5 mmol of citric acid were added in that sequence and mixed at room temperature until they dissolved completely. Slowly, 3 mL of HCl solution (6 mol L⁻¹) was added to the mixture and stirred for 30 min until a yellow suspension formed. For the hydrothermal process, the mixture was heated for 24 h at 120 °C in a stainless-steel high-pressure reactor lined with Teflon. After the reaction, the system was allowed to cool naturally to room temperature. The resultant suspension was centrifuged with ultrapure water and anhydrous ethanol, respectively, until the organic matter was entirely removed. The precursor WO₃·H₂O was then extracted by freeze-drying the resultant product.

3.2.2. Preparation of WO_3−x

After grinding the gray-black WO₃·H₂O precursor evenly, it was heated at a rate of 2 °C per minute to 400 °C in a nitrogen atmosphere and held for 2 h. Then, it cooled to room temperature at a rate of 3 °C min⁻¹, resulting in dark blue WO_3−x nanosheets.

3.2.3. Preparation of WO₃

To generate WO₃ samples lacking oxygen vacancies for comparison with WO_3−x, the WO₃·H₂O precursor was calcined in an airtight environment under the same conditions used for preparing WO_3-x.

3.3. Characterization

The Ultima IV Multi-Functional Horizontal X-ray Diffractometer from Rigaku Co. (Tokyo, Japen) was employed to assess the crystal structures of the samples for quality evaluation. A Nicolet NEXUS 470 Fourier Transform Infrared Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was utilized to test the samples. By using the X-ray photoelectron spectrometer PHI500 manufactured by Perkin-Elmer (Waltham, MA, USA), the surface valence states and chemical elements of the material were identified. An electron spin resonance spectrometer (ESR), model JES FA 200 (JEOL, Tokyo, Japan) was adopted to detect the production and changes of e⁻, h⁺, and oxygen vacancies in the samples. The trapping agent was 2,2,6,6-tetramethylpiperidine-nitrogen-oxide (TEMPO). Scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL, Tokyo, Japan) were used to characterize the micro-morphology of the samples. The minority carrier diffusion length was obtained by measuring the surface voltage generated when the light source irradiated the sample surface. The results were tested with a setup composed of a third-harmonic Nd: YAG laser (Polaris II, New Wave Research, Inc., Fremont, CA, USA) and a 500 MHz digital fluorescent screen oscilloscope (TDS 5054, Tektronix Inc., Beaverton, OR, USA). Solid powders were examined by a stable-transient fluorescence spectrometer (FluoroMax-4, HORIBAScientific, Edison, NJ, USA). A UV-Visspectrophotometer (Perkin-Elmer Lambda 950, Waltham, MA, USA) was utilized to investigate the light absorption characteristics of solid samples. The evolution of CO₂ photoreduction reaction intermediates on the surface of the catalyst material was tested in real-time using a Fourier transform infrared spectrometer (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA).

3.4. Electrochemical Measurements

The catalyst was tested on a standard three-electrode electrochemical workstation (CHI 760E) to determine its transient photocurrent density response curve (I-t) and electrochemical impedance spectroscopy (EIS) using a xenon lamp light source with a 420 nm cut-off filter as the incident light source. Ag/AgCl (reference electrode), platinum plate (counter electrode), and FTO conductive glass (working electrode) make up a standard three-electrode testing workstation. The platinum plate electrode’s conducting surface is covered with the substance under test. The area of the mixed solution, approximately 50 μL of sample coated on the conductive surface of FTO, is about 1 cm². After drying, it becomes the electrode to be tested. In total, 5 mg of the sample, 250 μL ethylene glycol, 250 μL anhydrous ethanol, and 40 μL Nafion membrane solution (5%) were combined ultrasonically to create the mixed solution. The test was conducted at room temperature, with the three-electrode system immersed in a quartz bath containing 0.2 mol L⁻¹ Na₂SO₄ electrolyte (pH = 6.8). The photocurrent intensity curve (I-t) test was conducted with an intermittent light cycle of 20 s. The testing for electrochemical impedance was performed in an illuminated environment, using an AC voltage amplitude of 10 mV and a testing frequency range of 10⁻²−10⁻⁶ Hz.

3.5. Photocatalytic Reduction of CO₂

The photocatalytic CO₂ reduction test experiment was carried out using a Labsolar-6A all-glass trace gas automatic online analysis system and a GC Agilent 8890 gas chromatograph (Agilent Technologies, Inc., Santa Clara, CA, USA). Within the reactor, there is a fixed-phase kettle reactor equipped with a gas circulation system and a 300 W Xenon lamp as the light source for the photocatalytic CO₂ reduction test. Firstly, 50 mg of the sample was weighed and dissolved in ethanol. Then, the solution was added dropwise onto a glass fiber filter membrane, dried, and set aside for measurement. After putting the catalyst and 5 mL of ultrapure water into the reactor, the top was sealed, and the system was evacuated for 15 min. Subsequently, high-purity CO₂ gas (purity 99.999%, Nanjing Special Gas Factory Co., Ltd., Nanjing, China) was introduced to the system at a pressure of 60 kPa. The vacuum was continued for 15 min, the gas washing was repeated three times, and the high-purity CO₂ gas was introduced into the system to a pressure of 80 kPa for the final time. The temperature of the reaction system was kept at 5 °C using low-temperature constant temperature circulating water. After adsorption at room temperature for 15 min, a 300 W full-spectrum xenon lamp light source was turned on and fixed vertically 10 cm above the reactor for the CO₂ catalytic reduction experiments. Every 1 h, the gas chromatograph would automatically take the sample for analysis. The GC Agilent 8890 gas chromatography system’s Flame Ionization Detector (FID) and Thermal Conductivity Detector (TCD) were used to identify and analyze reaction products.

4. Conclusions

In conclusion, we have successfully introduced oxygen vacancies in WO₃ nanosheets using a simple hydrothermal method and high-temperature calcination to improve the light-absorbing ability of WO₃ and increase the photocatalytic CO₂ reduction process’s selectivity and activity. Under simulated light, the CO₂ reduction by photocatalysis to CO selectivity of 2D WO_3−x nanosheets reached 100%. The influencing factors of oxygen vacancies on the photogenerated charge separation and migration within WO_3−x and its photocatalytic mechanism were explored through a combination of energy band structure analysis, free radical trapping, and in situ infrared test. The results showed that oxygen vacancies not only provided abundant CO₂ adsorption sites but also effectively regulated the carrier transport and provided more electron donors to increase the oxygen activation energy, thus improving the photocatalytic redox efficiency. In addition, the COOH* detected by in situ infrared spectroscopy indicates that photocatalytic CO₂ reduction to CO can take place on the WO_3−x surface. This result further confirms the significant role of oxygen vacancies in promoting photogenerated charge separation and migration.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15010013/s1, Figure S1: FTIR spectra of WO₃ and WO_3−x; Figure S2: The survey spectra of WO₃ and WO_3−x; Figure S3: Photocatalytic performance of samples under different reaction conditions; Figure S4: Surface photovoltage of WO₃ and WO_3−x; Text S1: Kubelka–Munk transformation method to obtain band gap energy of photocatalysts.

Author Contributions

Conceptualization, X.Y.; methodology, Y.C. and X.Y.; validation, Y.C. and Z.L.; investigation, Y.C. and Z.L.; resources, X.Y.; data curation, Y.C. and Z.L.; writing—original draft preparation, Y.C., Z.L. and X.Y.; writing—review and editing, X.Y.; supervision, X.Y.; project administration, X.Y.; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Science Fund for Distinguished Young Scholars of Nanjing Forestry University (JC2019002).

Data Availability Statement

All data are contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Synthesis diagram of WO₃ and WO_3−x.

Figure 2. (a) XRD patterns of WO₃ and WO_3−x; (b) XPS analysis of W 4f; (c) XPS analysis of O 1s; (d) ESR spectra.

Figure 3. The structure of the prepared samples: (a) SEM images of WO₃ and (b) WO_3−x; (c) TEM of WO₃ and (d) WO_3−x; (e) HRTEM; and (f) Elemental mapping analysis of WO_3−x.

Figure 4. The photocatalytic performance of WO₃ and WO_3−x: (a) the amount of CO generated, (b) the rate of CO production; ESR spectra of WO₃ and WO_3−x under dark and light conditions: (c) e⁻, (d) h⁺.

Figure 5. (a) Steady-state fluorescence spectra, (b) transient fluorescence spectra, (c) photocurrent response curves, and (d) EIS Nyquist plots for WO₃ and WO_3−x.

Figure 6. (a) UV-Vis-NIR diffuse reflectance spectra, (b) band gaps via Kubelka–Munk (K–M) jump, (c) valence band XPS spectra, and (d) band structures of WO₃ and WO_3−x.

Figure 7. In situ FTIR spectra of WO_3−x.

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Cheng, Y.; Li, Z.; Yang, X. Enhanced CO₂ Photoreduction Performance of WO_3−x. Catalysts 2025, 15, 13. https://doi.org/10.3390/catal15010013

AMA Style

Cheng Y, Li Z, Yang X. Enhanced CO₂ Photoreduction Performance of WO_3−x. Catalysts. 2025; 15(1):13. https://doi.org/10.3390/catal15010013

Chicago/Turabian Style

Cheng, Yelan, Zhaolin Li, and Xiaofei Yang. 2025. "Enhanced CO₂ Photoreduction Performance of WO_3−x" Catalysts 15, no. 1: 13. https://doi.org/10.3390/catal15010013

APA Style

Cheng, Y., Li, Z., & Yang, X. (2025). Enhanced CO₂ Photoreduction Performance of WO_3−x. Catalysts, 15(1), 13. https://doi.org/10.3390/catal15010013

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Enhanced CO₂ Photoreduction Performance of WO_3−x

Abstract

1. Introduction