Ultrathin Ti-doped WO₃ nanosheets realizing selective photoreduction of CO₂ to CH₃OH

Abstract

Arduous CO₂ activation and sluggish charge transfer retard the photoreduction of CO₂ to CH₃OH with high efficiency and selectivity. Here, we fabricate ultrathin Ti-doped WO₃ nanosheets possessing approving active sites and optimized carrier dynamics as a promising catalyst. Quasi in situ X-ray photoelectron spectroscopy and synchrotron–radiation X-ray absorption near-edge spectroscopy firmly confirm that the true active sites for CO₂ reduction are the W sites rather the Ti sites, while the Ti dopants can facilitate charge transfer, which accelerates the generation of crucial COOH* intermediates as revealed by in situ Fourier-transform infrared spectroscopy and density functional theory calculations. Besides, the Gibbs free energy calculations also validate that Ti doping can lower the energy barrier of CO₂ activation and CH₃OH desorption by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH₃OH. In consequence, the Ti-doped WO₃ ultrathin nanosheets show a superior CH₃OH selectivity of 88.9% and reach a CH₃OH evolution rate of 16.8 μmol g⁻¹ h⁻¹, about 3.3 times higher than that on WO₃ nanosheets. This work sheds light on promoting CO₂ photoreduction to CH₃OH by rational elemental doping.

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Artificial photosynthesis, which involves the conversion of carbon dioxide (CO₂) into value-added fuels by solar energy, provides a sustainable way to solve the disturbing energy crisis and climate change.^1–4 Various carbon-containing products can be acquired through the CO₂ photoreduction reaction, among which methanol (CH₃OH) is one of the liquid products with the most potential due to its extensive applications.^5–7 For instance, CH₃OH can be directly used in fuel cells⁸ and is the precursor of numerous fundamental chemicals like aromatics, C₂H₄, methyl methacrylate and fatty acid methyl esters.^5,9 It is reported that the global demand for CH₃OH can reach more than 100 million tons per year.¹⁰ Unfortunately, the product selectivity towards CH₃OH in the conversion of CO₂ with H₂O is still insufficient and unsatisfactory. Obstacles arise from the arduous activation of inert CO₂ molecules (C=O dissociation energy of ∼750 kJ·mol⁻¹).^11,12 Most importantly, the 6-electron transfer process during CH₃OH formation causes strict kinetic limitation, making it harder to generate CH₃OH than other products (e.g. CO).^13–15 In addition, many existing photocatalysts suffer from a narrow light adsorption range and the poor separation efficiency of photogenerated carriers, which hinder the charge transfer from the catalysts to the adsorbed CO₂.^16–20 Hence, designing catalysts with outstanding carrier dynamics and approving active sites is essential for realizing highly efficient CO₂ photoreduction into CH₃OH.

To meet the above requirements, constructing ultrathin two-dimensional (2D) nanosheets with appropriate elemental doping is a very promising solution. On the one hand, ultrathin 2D nanosheets can provide abundant active sites for CO₂ adsorption and activation due to their high specific surface area and sufficient uncoordinated atoms.^21–23 Their light absorption ability is also different from that of the traditional bulk catalysts, while the efficiencies of photogenerated carrier separation and charge transfer can be promoted thanks to their reduced migration paths.^24–28 On the other hand, elemental doping can regulate the electron structure of active atoms, further expediting CO₂ activation and reduction.^29,30 It can also change the band structure of catalysts and extend their light absorption range, making the catalysts more suitable for CO₂ photoreduction.^31–33 Furthermore, suitable elemental doping can optimize the photogenerated carrier dynamics, alter the reaction energy barrier and control reaction intermediates, enhancing product selectivity towards the desired species.^29,30,34 Therefore, it is prospective to boost CO₂ photoreduction to CH₃OH through synthesizing ultrathin 2D nanosheets with elemental doping. In this regard, inexpensive and eco-friendly tungsten oxide (WO₃) is one of the promising candidates, thanks to its suitable band gap, good light-harvesting ability and easily tunable electron structure.^35,36 Although WO₃ nanosheets have been applied in many photochemical fields,^36–38 their performance in CO₂ photoreduction to CH₃OH still lacks exploration.

In this work, Ti-doped WO₃ ultrathin nanosheets were fabricated via an acid-assisted method. X-ray photoelectron spectroscopy (XPS), synchrotron–radiation X-ray absorption near-edge spectroscopy (XANES) and element mappings show the successful doping of Ti into WO₃ nanosheets, while photoluminescence (PL) and time-resolved photoluminescence spectra (TRPL) reveal the enhanced separation of photogenerated carriers due to the Ti doping. Besides, quasi in situ XPS spectra and quasi in situ XANES spectra firmly validate that the true active sites are the W sites rather the Ti sites, where the Ti dopants facilitate the charge transfer, which is beneficial for the formation of COOH* species, one of the most important intermediates for carbon products during CO₂ reduction. In situ Fourier-transform infrared (FTIR) spectroscopy demonstrates this conclusion by the stronger intensity of COOH* species on Ti-doped WO₃ nanosheets. In addition, density functional theory (DFT) calculations confirm that Ti doping can strengthen the bonding between COOH* intermediates and catalysts, while the energy barrier of CO₂ activation and CH₃OH desorption is decreased by 0.22 eV and 0.42 eV, respectively, which dramatically facilitates the formation of CH₃OH. As a result, the Ti-doped WO₃ ultrathin nanosheets show a superior CH₃OH selectivity of 88.9% and achieve a CH₃OH evolution rate of 16.8 μmol g⁻¹ h⁻¹, about 3.3 times higher than that on the WO₃ nanosheets. This work offers an effective approach to enhance the conversion of CO₂ and H₂O into CH₃OH through elemental doping.

To obtain a non-toxic and efficient catalyst for CO₂ photoreduction, the ultrathin Ti-doped WO₃ nanosheets (called Ti–WO₃ nanosheets for short) were fabricated via an acid-assisted method. The powder X-ray diffraction (XRD) patterns of the Ti–WO₃ nanosheets could be indexed well to JCPDS no. 89-1287 and no additional peak was detected, indicating the successful synthesis of pure tetragonal WO₃ (Fig. 1A). The transmission electron microscopy (TEM) image showed that Ti-doped WO₃ had a sheet-like morphology (Fig. 1B). Meanwhile, the high-resolution TEM (HRTEM) image showed two interplanar spacings of 0.367 nm and 0.382 nm with a dihedral angle of 90°, corresponding to the (200) and (001) planes of tetragonal WO₃, which demonstrated their [010] orientation (Fig. 1C). The atomic force microscopy (AFM) image demonstrated that the thickness of the Ti–WO₃ nanosheets was about 2.20 nm (Fig. 1D), which was approximately the thickness of the triple-unit-cell slab along the [010] direction, illustrating the successful fabrication of ultrathin Ti–WO₃ nanosheets. For comparison, the undoped WO₃ nanosheets (called WO₃ nanosheets for short) were obtained without the addition of Ti sources during the synthesis. The WO₃ nanosheets had the same crystalline phase, growth orientation and thickness as the Ti–WO₃ nanosheets (Fig. S1†). It is worth mentioning that the crystallinity of the Ti–WO₃ nanosheets was lower than that of the WO₃ nanosheets (Fig. 1A), which could be ascribed to the successful Ti doping into WO₃.^39,40 In addition, the Ti doping also slightly decreased the interplanar spacings in the Ti–WO₃ nanosheets (Fig. 1C and S1†), which was in agreement with a previous study.⁴⁰

Fig. 1 (A) XRD patterns of the Ti–WO₃ nanosheets and the WO₃ nanosheets. (B) TEM image of the Ti–WO₃ nanosheets. (C) HRTEM image of the Ti–WO₃ nanosheets, showing the 0.367 nm and 0.382 nm interplanar distances that matched well with the d₂₀₀ and d₀₀₁ planes, respectively. (D) AFM image of the Ti–WO₃ nanosheets. (E) XPS spectra of W 4f and (F) Ti 2p for the Ti–WO₃ nanosheets and the WO₃ nanosheets.

To further confirm that the Ti dopants had been doped in WO₃ nanosheets, X-ray photoelectron spectroscopy (XPS) of W 4f and Ti 2p was performed. As shown in Fig. 1E, there were two peaks at around 37.90 eV and 35.75 eV, corresponding to W 4f_5/2 and W 4f_7/2 states, respectively. Notably, both W 4f_5/2 and W 4f_7/2 in the Ti–WO₃ nanosheets exhibited a slightly negative shift, implying that some electrons were transferred to W⁶⁺ after Ti doping, resulting in more W⁵⁺ species.³⁵ This phenomenon was due to the partial replacement of W⁶⁺ by the Ti atoms.⁴¹ In addition, there was no Ti 2p signal detected in the WO₃ nanosheets, while Ti 2p in the Ti–WO₃ nanosheets showed two peaks at 458.7 eV and 464.4 eV, which could be ascribed to the Ti(IV) 2p_3/2 peak and the Ti(IV) 2p_1/2 peak (Fig. 1F). This result was in accordance with the conclusion drawn from synchrotron–radiation X-ray absorption near-edge spectroscopy (XANES), which showed that the valence state of Ti in the Ti–WO₃ nanosheets was +4 (Fig. S2†). The content of Ti in the Ti–WO₃ nanosheets determined using inductively coupled plasma atomic emission spectroscopy (ICP-OES) was 0.87 wt%, which was close to that determined by the XPS results (0.90 wt%). Furthermore, the annular dark-field TEM image and the corresponding element mappings revealed that there was no TiO₂ particle observed and the W, O and Ti elements were distributed uniformly (Fig. S3†). The Raman spectra also demonstrated that there was no additional peak detected in the Ti–WO₃ nanosheets (Fig. S4†), excluding the possibility of TiO₂/WO₃ heterojunction formation. Thus, these results showed that the Ti dopants had been doped in the WO₃ nanosheets and the valence state of Ti in the Ti–WO₃ nanosheets was +4.

To investigate the effect of Ti dopants on the electronic band structures, ultraviolet–visible (UV–vis) absorption spectra were recorded. As shown in Fig. 2A, the absorption spectrum of the Ti–WO₃ nanosheets showed a red-shift compared with that of the WO₃ nanosheets, implying the stronger visible light absorption on the Ti–WO₃ nanosheets.^21,31,35 In addition, the bandgap of the Ti–WO₃ nanosheets determined using the absorption spectra was 2.59 eV (Fig. 2B), which was smaller than that of the WO₃ nanosheets (2.66 eV). To obtain the band edge potentials of the samples, synchrotron–radiation photoemission spectroscopy (SRPES) was applied to measure the work functions and the valence band maxima (VBM).³⁶ As shown in Fig. 2C and D, the work functions of the Ti–WO₃ nanosheets and the WO₃ nanosheets could be calculated to be 4.30 and 4.53 eV, whereas their valence-band edges were located at 2.27 eV and 2.48 eV. Combined with the bandgaps obtained above, the electronic band structures of the Ti–WO₃ nanosheets and the WO₃ nanosheets could be estimated (Table S1†). It was obvious that both samples had suitable electronic band structures for reducing CO₂ to CH₃OH, confirming their potential applications in CO₂ photoreduction.⁵

Fig. 2 (A) UV–vis absorption spectra and (B) the corresponding optical bandgaps for the Ti–WO₃ nanosheets and the WO₃ nanosheets. (C) Secondary electron cutoff and (D) valence band spectra of the Ti–WO₃ nanosheets and the WO₃ nanosheets acquired by synchrotron–radiation photoemission spectra.

To evaluate the effect of Ti dopants on the catalytic activity, photocatalytic CO₂ reduction experiments were carried out. The liquid products were analyzed using nuclear magnetic resonance (NMR) spectroscopy, while the gas products were analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). As displayed in Fig. 3A, CH₃OH was the major product of CO₂ photoreduction on both the samples, since the CO evolution rate was much lower than the CH₃OH evolution rate. The CH₃OH evolution rate on the Ti–WO₃ nanosheets could reach 16.8 μmol g⁻¹ h⁻¹, which was about 3.3 times higher than that on the WO₃ nanosheets, suggesting that Ti doping could efficiently promote the catalytic activity of WO₃ nanosheets. The selectivity towards CH₃OH also increased from 80.0% to 88.9%. In addition, the CH₃OH evolution rate was related to the amount of Ti dopants (Fig. S5†), indicating that Ti doping was a feasible strategy to adjust the performance of WO₃ nanosheets during the CO₂ photoreduction. What's more, the control experiments demonstrated that CO₂, the Ti–WO₃ nanosheets (catalyst) and illumination were prerequisites for the CH₃OH formation, indicating that CH₃OH was derived from photocatalytic CO₂ reduction over the Ti–WO₃ nanosheets (Fig. 3B). This conclusion could be further verified by the ¹³CO₂ labelling experiments performed with synchrotron-based vacuum ultraviolet photoionization mass spectrometry (SVUV-PIM). As shown in Fig. S6,† N₂, CO and CO₂ could not be ionized when the photon energy was set at 11.50 eV, and thus one could eliminate the interference from their fragments.⁴² Hence, the Ti–WO₃ nanosheets produced the product ¹³CH₃OH (m/z = 33), confirming that evolved CH₃OH indeed originated from the photoreduction of CO₂ (Fig. 3C). Furthermore, the CH₃OH evolution rate showed almost no decay after 4 cycles of the photoreduction test (Fig. 3D), indicating the good photocatalytic stability of the Ti–WO₃ nanosheets. The corresponding XRD patterns and TEM images after catalysis could also affirm the good stability of the Ti–WO₃ nanosheets (Fig. S7†).

Fig. 3 (A) Products of the photocatalytic CO₂ reduction over the Ti–WO₃ nanosheets and the WO₃ nanosheets. The error bars represent the standard deviations of three independent measurements. (B) Products of photocatalytic CO₂ reduction over the Ti–WO₃ nanosheets under different conditions. (C) SVUV-PIMS spectrum of the products during ¹³CO₂ labeled photoreduction over the Ti–WO₃ nanosheets at hν = 11.50 eV. Inset: signals of m/z = 33 (¹³CH₃OH). (D) Cycling measurements for CO₂ photoreduction over the Ti–WO₃ nanosheets.

To unravel the true active sites during CO₂ photoreduction over the Ti–WO₃ nanosheets, quasi in situ XPS spectra were used to investigate the change in electron density on the W sites. As shown in Fig. 4A and B, the content of W⁵⁺ increased distinctly upon light irradiation of the reaction system, indicating that the W sites accepted the photoexcited electrons and generated W⁵⁺ active species during the reaction. Once CO₂ was introduced into the system, the ratio of W⁵⁺/W⁶⁺ decreased from 0.38 to 0.25, indicating that the W⁵⁺ ions were reoxidized into the original W⁶⁺ ions by donating electrons to the adsorbed CO₂ molecules.⁴³ In addition, the valence state of the W atoms recovered to the initial state after removing the light irradiation (Fig. 3C) and the same trend was also detected on the WO₃ nanosheets (Fig. S8†). Notably, the W⁵⁺ content in the Ti–WO₃ nanosheets was higher than that in the WO₃ nanosheets in the dark and it increased after the introduction of CO₂, implying that CO₂ was adsorbed on the W sites (Fig. 4D). On the contrary, the quasi in situ X-ray absorption near-edge spectroscopy (XANES) spectra revealed that the L-edge of Ti did not have any distinct changes during the whole process (Fig. S9†), indicating that the doped Ti atoms did not participate in the CO₂ photoreduction directly. This further certified that the true active sites over the Ti–WO₃ nanosheets were the W sites rather the Ti sites. Interestingly, the ratio of W⁵⁺/W⁶⁺ on the Ti–WO₃ nanosheets was lower than that on the WO₃ nanosheets under light irradiation, suggesting that more photoexcited electrons were donated to the adsorbed CO₂ molecules for photoreduction. In other words, the Ti dopants could facilitate the charge transfer on the Ti–WO₃ nanosheets to enhance the CO₂RR performance, where the separation of photogenerated carriers on the Ti–WO₃ nanosheets was also promoted, as revealed by the impedance test, transient photocurrent response, photo-luminescence (PL) and time-resolved photoluminescence (TRPL) spectra (Fig. S10 and S11†).^17,31

Fig. 4 (A) Quasi in situ XPS spectra and (B) the corresponding W⁵⁺/W⁶⁺ ratio of the Ti–WO₃ nanosheets under different atmospheres. (C) Quasi in situ XPS spectra of the Ti–WO₃ nanosheets during the CO₂ photoreduction. (D) The comparison of the W⁵⁺/W⁶⁺ ratio of the Ti–WO₃ nanosheets and the WO₃ nanosheets during the CO₂ photoreduction.

To find out the reaction intermediates of CH₃OH formation on the Ti–WO₃ nanosheets, in situ FTIR measurement was carried out. As shown in Fig. 5A, three peaks appeared at 1158 cm⁻¹, 1365 cm⁻¹ and 1530 cm⁻¹. Similar peaks also appeared in the in situ FTIR spectra of the pristine WO₃ nanosheets, indicating the same CO₂ reduction process (Fig. S12†). The peak at 1530 cm⁻¹ could be assigned to COOH*, which was one of the most important intermediates for carbon products during CO₂ reduction.^36,42,44 Meanwhile, it was found that the peak density of COOH* on the Ti–WO₃ nanosheets was higher than that on the WO₃ nanosheets (Fig. S12†), implying that the Ti dopants might benefit CH₃OH formation through promoting the generation of COOH* intermediates. The peak at 1365 cm⁻¹ was assigned to m-CO₃²⁻, which could be due to the dissolved CO₂ in the water.⁴⁵ In addition, the peak at 1158 cm⁻¹ could be ascribed to CH₃O*, which was one of the intermediates for CH₃OH formation.^42,44 Based on the results of in situ FTIR spectra, the possible reaction pathways could be summarized as follows:

* + CO₂ + e⁻ + H⁺ → COOH* (1)

COOH* + e⁻ + H⁺ → CO* + H₂O (2)

CO* + e⁻ + H⁺ → CHO* (3)

CHO* + e⁻ + H⁺ → CH₂O* (4)

CH₂O* + e⁻ + H⁺ → CH₃O* (5)

CH₃O* + e⁻ + H⁺ → CH₃OH + * (6)

where the asterisks denote catalytically active sites in the reaction.

Fig. 5 (A) In situ FTIR spectra of the Ti–WO₃ nanosheets. (B) Free energy diagrams of CO₂ photoreduction to CH₃OH over the Ti–WO₃ ultrathin layer slab. (C) Free energy diagrams of CO₂ photoreduction to CH₃OH over the WO₃ ultrathin layer slab.

To further reveal the reason for the promotion of CH₃OH formation on the Ti–WO₃ nanosheets, the Gibbs free energy of these key reaction pathways was calculated using DFT calculation (Fig. 5B and C and Table S4†). The calculation results suggested that the rate-limiting step was the formation of COOH* intermediates for both the samples,^46,47 while the reaction energy from CO₂ to COOH* was decreased from 1.37 to 1.15 eV over the Ti–WO₃ ultrathin layer slab, indicating a lower activation barrier of CO₂ on the WO₃ nanosheets after Ti doping. This could be attributed to the strengthened bonding between COOH* intermediates and the catalyst (Fig. S13†),⁴⁴ which could stabilize and generate more COOH* intermediates during the CO₂ photoreduction, well consistent with the in situ FTIR results. Furthermore, the energy barrier of CH₃OH desorption over the Ti–WO₃ ultrathin layer slab was −0.06 eV, obviously lower than that of 0.36 eV over the WO₃ ultrathin layer slab, implying that the Ti dopants could also facilitate the CH₃OH desorption, which was an important process for CH₃OH formation. Thus, it was rational to conclude that doping Ti into WO₃ nanosheets could lower the reaction energy barrier, stabilize the COOH* intermediates and make CH₃OH easier to desorb, thus improving the CH₃OH yield during the CO₂ photoreduction.

Conclusions

In summary, the ultrathin Ti-doped WO₃ nanosheets were successfully fabricated as an efficient catalyst for selectively photoreducing CO₂ and H₂O into CH₃OH. XPS, XANES and element mappings showed the successful doping of Ti into the WO₃ nanosheets, while the PL and TRPL spectra demonstrated that the separation of photogenerated carriers on the Ti-doped WO₃ nanosheets were significantly promoted. Quasi in situ XPS spectra and quasi in situ XANES spectra firmly confirmed that the true active sites were the W sites rather the Ti sites, where the Ti dopants could facilitate the charge transfer, which was beneficial for the formation of COOH* species. In situ FTIR spectroscopy demonstrated this conclusion through the stronger intensity of COOH* species on the Ti-doped WO₃ nanosheets, while DFT calculations showed that Ti doping could strengthen the bonding between COOH* intermediates and the catalyst. In addition, the Gibbs free energy calculations also indicated that the activation barrier of CO₂ was decreased from 1.37 to 1.15 eV, while the energy barrier of CH₃OH desorption was decreased from 0.36 to −0.06 eV on the Ti–WO₃ nanosheets, thus promoting the formation of CH₃OH. In consequence, the Ti-doped WO₃ ultrathin nanosheets exhibited a superior CH₃OH selectivity of 88.9% and achieved a methanol evolution rate of 16.8 μmol g⁻¹ h⁻¹, about 3.3 times higher than that on the WO₃ nanosheets. This work offers an effective approach to photoreduce CO₂ into CH₃OH and renders a deep insight to promote the performance of catalysts during the photoreduction reaction.

Author contributions

Y. Xie, Y. F. Sun, P. Q. Ling and J. C. Zhu conceived the idea and co-wrote the paper. P. Q. Ling and Z. Q. Wang carried out the sample synthesis, characterization and CO₂ photoreduction measurement. J. C. Zhu conducted DFT calculations. J. Hu and J. F. Zhu conducted the quasi in situ XPS and quasi in situ XANES measurement. W. S. Yan conducted the synchrotron–radiation XANES measurement. All the authors contributed to the overall scientific interpretation and edited the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2019YFA0210004), the National Natural Science Foundation of China (22125503, 21975242, U2032212, 21890754), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB36000000), the Youth Innovation Promotion Association of CAS (CX2340007003), the Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2020HSC-CIP003), the Users with Excellence Program of Hefei Science Center CAS (2020HSC-UE001) and the University Synergy Innovation Program of Anhui Province (GXXT-2020-001). Supercomputing USTC and National Supercomputing Center in Shenzhen are acknowledged for computational support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr02364d

‡ These authors contributed equally to this work.

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