Selective conversion of CO₂ to CH₄ enhanced by WO₃/In₂O₃ S-scheme heterojunction photocatalysts with efficient CO₂ activation

Abstract

Solar-powered CO₂ reduction is a promising approach for mitigating the energy crisis and environmental issues. However, its efficiency is hindered by challenges including difficult CO₂ activation, rapid charge recombination, and uncontrollable selectivity. Here, we develop WO₃/In₂O₃ S-scheme heterojunction photocatalysts by depositing In₂O₃ nanoparticles onto WO₃ nanosheets for CO₂ photoreduction. The Fermi level difference triggers electron transfer upon coupling, generating an internal electric field pointing from In₂O₃ to WO₃ at the interface. This impels oriented charge transfer and effective separation of the powerful photoinduced carriers. With the unique S-scheme mechanism and the efficient activation of CO₂ molecules on the In₂O₃ surface, the resulting WO₃/In₂O₃ heterostructures exhibit enhanced CO₂ photoreduction performance with ∼53.7% selectivity for CH₄, without any molecule cocatalyst or scavenger. In situ irradiation X-ray photoelectron spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, along with density functional theory simulations are conducted to elucidate the photocatalytic and CO₂-reduction mechanism, and the enhanced CH₄-selectivity.

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1. Introduction

Recently, the incessant combustion of fossil fuels has led to a substantial increase in atmospheric carbon dioxide (CO₂) concentration, causing grave environmental pollution and concerns about energy scarcity.^1–9 To tackle these issues, a promising strategy is the photocatalytic reduction of CO₂ to valuable solar fuels driven by abundant, clean, and inexhaustible sunlight.^10,11 Nevertheless, the linear CO₂ molecules exhibit thermodynamic stability with a high dissociation energy of the C=O bond (∼750 kJ mol⁻¹), making their adsorption and activation onto the catalyst surface challenging.^12,13 Additionally, the CO₂ photoreduction process is intricate and involves multiple photoelectron transfer, which leads to diverse reduction products, including CO, CH₄, CH₃OH, etc., with unpredictable selectivity. Although CH₄, the major component of natural gas, has the lowest reduction potential among all the products (−0.24 V vs. NHE), it requires eight photoelectrons, making it more difficult to produce than others from a kinetic perspective. Although a variety of photocatalysts have been reported for CO₂ reduction to CH₄,^14–21 most still suffer from uncontrollable selectivity and low conversion efficiency. Thus, designing highly active photocatalysts with efficient CO₂ activation and high CH₄ selectivity is still a daunting task.

Indium oxide (In₂O₃) is a popular reduction photocatalyst with a high conduction band level, narrow band gap (∼2.8 eV) and visible light absorption.^22,23 It has been reported that stable CO₂ molecules can be highly activated on the In₂O₃ surface to enhance CO₂ photoreduction activity, but the selectivity of CH₄ has not been modulated.²² In addition, the recombination of photogenerated carriers in unitary In₂O₃ occurs more readily than transferring to the catalyst surface to participate in photoreactions, leading to poor photocatalytic performance.^24,25 Therefore, the development of In₂O₃-based heterojunction photocatalysts that enable efficient spatial separation of electron/hole pairs and selective CO₂-to-CH₄ conversion enhancement is desirable but challenging.

Coupling two semiconductors to fabricate an S-scheme heterojunction offers tremendous potential in augmenting photoactivity, owing to the effective separation of photogenerated carriers and their robust redox capability.^8,26–28 Generally, S-scheme heterojunctions are constituted of oxidation and reduction photocatalysts. The variance in their Fermi levels elicits electron transfer upon contact, thereby resulting in the bending of energy bands and the creation of an internal electric field (IEF) at their interface. As a consequence of the bent bands, IEF, and coulombic attraction, the futile photogenerated electrons in the oxidation photocatalyst and the holes in the reduction photocatalyst amalgamate, accomplishing the efficient spatial separation of charge carriers with adequate redox ability to boost the photocatalytic performance.^29–32 Tungsten trioxide (WO₃) is an oxidation photocatalyst that exhibits visible-light response and potent oxidizing ability, attributable to its narrow band gap (2.4–2.8 eV) and positive valence band position.^33,34 Given the interweaved band structures and disparity in work functions, the combination of In₂O₃ with WO₃ might engender a desirable S-scheme heterojunction photocatalyst with anticipated charge separation, strong redox ability, and heightened CO₂ activation, thereby enhancing CO₂ photoreduction performance.^22,35,36

Herein, we synthesized WO₃/In₂O₃ S-scheme heterojunction photocatalysts by depositing In₂O₃ nanoparticles on WO₃ nanosheets through an immersion-annealing technique. Compared to pure WO₃, the resulting WO₃/In₂O₃ heterostructures revealed elevated selectivity in the conversion of CO₂ into CH₄ (53.7%) and enhanced CO₂ reduction efficiency without any molecular cocatalyst or scavenger. Such superior performance was ascribed to the S-scheme charge transfer mechanism and the effective CO₂ activation on the In₂O₃ surface, which were verified through in situ irradiation X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence spectroscopy (TRPL), density functional theory (DFT) calculation and temperature-programmed desorption of CO₂ (CO₂-TPD). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and theoretical simulations were also employed to elucidate the CO₂ reduction mechanism and the potential causes for the high selectivity of CH₄. Our work may present a novel perspective for designing exceptional S-scheme heterojunction photocatalysts that facilitate efficient CO₂ activation and improved selective CO₂-to-CH₄ conversion.

2. Experimental details

2.1 Synthesis of WO₃·H₂O nanosheets

Typically, a total of 3.6 mmol of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and 3.0 mL of tetrafluoroboric acid (HBF₄, 40 wt%) were dissolved in 60 mL of deionized water. The resulting light-yellow solution was transferred to a 100 mL Teflon-lined stainless autoclave and reacted at 100 °C for 12 h. Upon completion of the reaction, yellow precipitates were centrifuged and washed with deionized water and ethanol for three times, followed by drying at 80 °C in an oven to obtain the WO₃·H₂O nanosheets.

2.2 Preparation of WO₃/In₂O₃ heterostructures

A certain amount of indium nitrate (In(NO₃)₃·4H₂O) was added to a 30 mL aqueous solution containing 200 mg of WO₃·H₂O nanosheets. The mixture was magnetically stirred for 2 h, followed by drying in an oven at 90 °C. The resulting solid products were annealed in air at 500 °C for 2 h, with a heating rate of 2 °C min⁻¹. The obtained WO₃/In₂O₃ heterostructures were labelled as WIx (x = 5,10 and 15), where W and I represent WO₃ and In₂O₃ respectively; x is the molar ratio of In₂O₃ in WO₃/In₂O₃ nanohybrids.

3. Results and discussion

3.1 Activation and photoreduction of CO₂ over WO₃/In₂O₃ heterojunctions

The adsorption and activation of CO₂ molecules on catalysts were first investigated by DFT calculations since they are essential steps for CO₂ photoreduction. Fig. 1a and b show that when CO₂ is adsorbed onto the In₂O₃ surface, the O=C=O bond is visibly bent at an angle of 129.33°, and the bond length is stretched in comparison with free CO₂ molecules (1.16 Å). Both the C and O atoms of CO₂ form chemical bonds with the O and In atoms of In₂O₃. Moreover, In₂O₃ donates 0.2 electrons to CO₂ when CO₂ adsorbs on In₂O₃ (Fig. 1c), which is further supported by the planar-averaged charge density difference along the Z direction as shown in Fig. S1.† The investigation of the CO₂ adsorption capability reveals that pure In₂O₃ and WI10 exhibit stronger CO₂ adsorption compared to pristine WO₃, following the same trend as the N₂ adsorption (Fig. S2 and Table S1†). The CO₂-TPD results demonstrate that pure WO₃, In₂O₃ and WI10 all display desorption peaks at 80–200 °C assigned to the physical adsorption of CO₂ (Fig. 1d).^37–39 In the temperature range of 400–800 °C, no significant signals are detected over pristine WO₃. However, both pure In₂O₃ and WI10 reveal three prominent desorption peaks corresponding to the decomposition of b-CO₃²⁻ (∼450 °C) and m-CO₃²⁻ (550–750 °C) species, suggesting strong chemisorption interactions between CO₂ molecules and In₂O₃.^40–42 Furthermore, in situ DRIFTS spectra obtained after adsorption of CO₂ for 15, 30, 45, and 60 min exhibit distinct peaks of formaldehyde (HCHO) (1750 and 1770 cm⁻¹), monodentate carbonate (m-CO₃²⁻) (1590, 1560 and 1540 cm⁻¹), bidentate carbonate (b-CO₃²⁻) (1425, 1410, 1380, and 1340 cm⁻¹), and bicarbonate (HCO₃⁻) (1180, 1110 and 1030 cm⁻¹) over In₂O₃ and WI10 (Fig. S3†). These results indicate that CO₂ can be chemisorbed and activated by In₂O₃, and In₂O₃ is the preferable active site for CO₂ photoreduction in the heterojunctions.

Fig. 1 (a and b) Optimized structure and (c) the corresponding charge density difference images of the CO₂ molecule adsorbed on the In₂O₃ (111) surface. (d) CO₂-TPD spectra of In₂O₃, WI10 and WO₃. Photocatalytic CO₂ reduction activities over WO₃, WIx, and In₂O₃ during the 12 h experiment performed under visible light irradiation: time course of (e) CO and (f) CH₄ production yields. (g) SVUV-PIMS spectra of the products collected after ¹³CO₂ photoreduction for WI10 at hν = 14.2 eV. (h) The selectivity of CO and CH₄ over WO₃, WIx, and In₂O₃ samples.

The photocatalytic performance toward CO₂ reduction was evaluated in an online closed gas-circulation system (OLPCRS-2, Shanghai Boyi Scientific Instrument Co., Ltd) with a quartz reactor under visible light irradiation (Fig. S4†). CO and CH₄ were identified as the reduction products, and O₂ was determined as the oxidation product. Control experiments revealed that no product can be detected in the dark or in the absence of CO₂. As shown in Fig. 1e and f, pristine WO₃ and In₂O₃ both reveal lower production yields of CO and CH₄ due to the rapid recombination of photogenerated charge carriers within the unitary photocatalyst. After coupling WO₃ with In₂O₃, the CO₂ reduction efficiency is enhanced, reaching the highest evolution yield of 6.6 and 5.4 μmol g⁻¹ for CO and CH₄ over WI10. Excessive loading of In₂O₃ nanoparticles on WO₃ nanosheets, i.e., WI15, deteriorates the photocatalytic performance owing to the light shielding effect. During CO₂ photoreduction, the photogenerated holes oxidize H₂O to produce O₂ in the absence of sacrificial agents, and the evolution rate of O₂ (Fig. S5†) follows the same tendency as that of CO and CH₄. The apparent quantum efficiency (AQE) of CO₂ photoreduction was measured at different monochromatic wavelengths including 365, 380, 400, 420, 450, and 500 nm over WO₃/In₂O₃ nanohybrids. Clearly, the trend of AQE matches well with the light absorption spectrum of WI10 (Table S2, Fig. S6†), affirming the photocatalytic nature of CO₂ reduction.^43–45 The synchrotron-radiation vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was employed to disclose the origin of the products by using isotope-labeled carbon dioxide (¹³CO₂) as the substitute source gas. At a photon energy of 14.2 eV, the WO₃/In₂O₃ heterojunctions yield a mixture of ¹³CH₄ (m/z = 17) and ¹³CO (m/z = 29) as shown in Fig. 1g, which confirms that the evolved reduction products indeed originate from the photoreduction of CO₂ rather than any carbonaceous impurities.⁴⁶

Notably, the selectivity of CH₄ over WO₃/In₂O₃ nanohybrids is higher than that of CO and reaches the maximum of 53.7% over WI10 as revealed in Fig. 1h. In order to uncover the underlying reasons for the high selectivity of CH₄, as well as the reaction mechanism, in situ DRIFTS experiments along with DFT calculations were carried out. As shown in Fig. 2a, when CO₂/H₂O is introduced into the system in the dark, peaks of HCHO, m-CO₃²⁻, b-CO₃²⁻ and HCO₃⁻ are detected, strongly remanifesting the chemisorption of CO₂ on the catalyst surface.^47,48 Under light irradiation, new adsorption peaks assigned to *COOH (1669 cm⁻¹), *CO (2079 cm⁻¹), *CHO (1012 cm⁻¹), *CH₂O (2855 cm⁻¹) and *CH₃O (1042 and 1181 cm⁻¹) are observed, which are significant intermediates in the conversion of CO₂ to CH₄.^49–51 Accordingly, a probable CO₂ photoreduction mechanism over WO₃/In₂O₃ composites is proposed as follows, where * stands for the active sites on WO₃/In₂O₃ heterojunctions.

* + CO₂ → *CO₂ (1)

*CO₂ + H⁺ + e⁻ → *COOH (2)

*COOH + H⁺ + e⁻ → *CO (3)

*CO + H⁺ + e⁻ → *CHO or *CO → CO + * (4)

*CHO + H⁺ + e⁻ → *CH₂O (5)

*CH₂O + H⁺ + e⁻ → *CH₃O (6)

*CH₃O + H⁺ + e⁻ → CH₄ + *O (7)

*O + H⁺ + e⁻ → *OH (8)

*OH + H⁺ + e⁻ → * + H₂O (9)

Fig. 2 (a) In situ DRIFT spectra of WI10 after adsorption of CO₂/H₂O for 15, 30, 45, and 60 min in the dark, and then under light irradiation for 75, 90, 105, and 120 min. (b) Gibbs free energy diagram of CO₂ photoreduction over the In₂O₃ (111) slab.

Gibbs free energy (ΔG) calculation of CO₂ photoreduction over the In₂O₃ surface was further conducted to confirm the reaction pathway described above, and the results are presented in Fig. 2b. It is worth noting that the change in ΔG for the protonation of *CO to *CHO (pathway II, −1.61 eV) is much smaller than that for CO desorption form the catalyst surface (pathway I, −0.28 eV), providing a reasonable explanation for the high selectivity of CH₄ during CO₂ photoreduction.⁵² Overall, the strong adsorption and activation of CO₂ molecules on the In₂O₃ surface are critical factors contributing to the enhanced CH₄ selectivity and CO₂ reduction performance.

3.2 Characterization of WO₃/In₂O₃ heterojunctions

Structural characterization studies provide additional insights into understanding the boosted photocatalytic activity. The X-ray diffraction (XRD) pattern of pure WO₃ is consistent with the standard monoclinic phase (JCPDS No. 20-1324), and pristine In₂O₃ is indexed to the cubic phase (JCPDS No. 71-2195), respectively (Fig. S7†). In the WIx nanohybrids, the intensity of diffraction peaks attributed to In₂O₃ increases with its content (x), indicating the synthesis of WO₃/In₂O₃ heterojunctions. The precursor WO₃·H₂O exhibits uniform square nanosheets with a smooth surface as revealed in the field emission scanning electron microscopy (FESEM) image (Fig. S8†). After dehydration, pristine WO₃ maintains its nanosheet microscopy with some tiny surface wrinkles (Fig. 3a).⁵³ In FESEM and transmission electron microscopy (TEM) images of WO₃/In₂O₃ nanohybrids (WI10) (Fig. 3b and c), In₂O₃ nanoparticles are clearly observed to be deposited on the surface of WO₃ nanosheets. The high-resolution TEM image of WI10 (Fig. 3d) reveals distinct lattice fringes with spacings of 0.292 and 0.375 nm, corresponding to In₂O₃ (222) and WO₃ (020) planes, respectively. The energy-dispersive X-ray spectroscopy (EDX) elemental mappings of WI10 confirm the existence of W, O and In elements, further affirming the formation of WO₃/In₂O₃ nanohybrids. The phase and morphology of spent WI10 were also evaluated, and there are no detectable changes compared with the fresh sample, indicating substantial photostability of the catalysts (Fig. S9†).

Fig. 3 (a) FESEM image of pure WO₃ nanosheets. (b) FESEM, (c) TEM and (d) HRTEM images of WO₃/In₂O₃ nanostructures (WI10). (e) EDX elemental mappings of W, O, and In elements in WO₃/In₂O₃ nanohybrids.

UV-vis diffuse reflectance spectroscopy (DRS) was employed to investigate the optical absorption properties of WO₃, In₂O₃ and WI10. The absorption edges of pure WO₃ and In₂O₃ are located at 470 and 465 nm, respectively, corresponding to the bandgap energies of 2.81 and 2.97 eV (Fig. S10†). The light harvesting of WI10 is slightly improved compared to pristine WO₃, indicating the successful hybridization of WO₃ and In₂O₃. XPS was carried out to analyze the chemical states and surface compositions of the resultant samples. The survey spectrum of WI10 demonstrates the presence of the In element, as well as W and O elements derived from the WO₃ matrix (Fig. S11a†). The W 4f XPS spectra (Fig. 4a) exhibit two distinct peaks at 35.6 and 37.7 eV, corresponding to hexavalent W⁶⁺.^54,55 The binding energies (BEs) of In 3d_5/2 and In 3d_3/2 are observed to be 444.2 and 451.7 eV, respectively, confirming the existence of trivalent In³⁺ (Fig. 4b).^56,57 After the photoreaction, the chemical states of WO₃/In₂O₃ heterojunctions exhibit imperceptible changes as manifested by the W 4f and In 3d XPS spectra (Fig. S12†), further confirming the stability of catalysts. The O 1s XPS spectra (Fig. S11b–d†) reveal the presence of lattice oxygen (530.5 eV), adsorbed oxygen (533.2 eV), and surface hydroxyls (531.9 eV) in the samples.⁵⁸ Notably, the BEs of W 4f in WI10 display negative shifts in comparison with those in pure WO₃, while the peaks of In 3d in WI10 shift positively relative to pure In₂O₃. Such an intriguing phenomenon suggests the migration of electrons from In₂O₃ to WO₃ upon hybridization, creating an IEF with the direction from In₂O₃ to WO₃ at their interface to facilitate the efficient separation of photogenerated charge carriers (as discussed below).

Fig. 4 High-resolution XPS spectra of (a) W 4f, (b) In 3d of WO₃, In₂O₃, and WI10. In situ irradiation XPS measurement was conducted under UV light irradiation. (c) Schematic illustration of the formation and charge separation of the WO₃/In₂O₃ S-scheme heterojunction. (d) The charge density difference of the WO₃/In₂O₃ heterojunctions; cyan and yellow regions represent electron depletion and accumulation, respectively; grey, green, and red spheres represent W, In and O atoms, respectively. (e) TRPL spectra of WO₃ and WI10 in different atmospheres.

3.3 Insights into the S-scheme charge separation mechanism

The band structure of WO₃ and In₂O₃ was first investigated to unravel the charge transfer and separation mechanism over WO₃/In₂O₃ heterojunctions. The potentials of the VB maximum and CB minimum of WO₃ and In₂O₃ are calculated to be 2.88 and 0.07 V, as well as 2.28 and −0.69 V (vs. RHE), respectively, based on Mott–Schottky plots, valence band XPS spectra (Fig. S13†) and the band gap (Fig. S10b†). According to the electrostatic potentials along the z-axis of WO₃ (001) and In₂O₃ (111) (Fig. S14†), the work function (Φ) of WO₃ and In₂O₃ is estimated to be 6.54 and 5.87 eV, respectively, indicating that In₂O₃ has a higher Fermi level (E_F) than WO₃. Additionally, the contact potential difference (CPD) between the samples and the standard gold tip of the Kelvin probe instrument reveals that the calculated Φ level of WO₃ is larger than that of In₂O₃ (Fig. S15†), aligning with the DFT results. As a consequence, electrons in In₂O₃ will spontaneously diffuse to WO₃ upon contact until reaching the same E_F at their interface, which will lead to the bending of their energy bands and the creation of an IEF pointing from In₂O₃ to WO₃ (Fig. 4c). The charge density difference of DFT simulation (Fig. 4d) shows that In₂O₃ donates electrons (cyan area) while WO₃ gains electrons (yellow area) at their interface, further confirming the above electron transfer pathway between WO₃ and In₂O₃.

Under light irradiation, the electrons in WO₃ and In₂O₃ VBs are initially photoexcited to their CBs. Due to the bent bands and IEF, the photogenerated electrons in the WO₃ CB tend to transfer to the In₂O₃ VB and recombine with its holes, indicating the formation of S-scheme heterojunctions between WO₃ and In₂O₃. Ultimately, the powerful photoelectrons in the In₂O₃ CB and photoholes in the WO₃ VB are reserved to participate in photocatalytic reactions (Fig. 4c). Such an S-scheme charge transfer route not only efficiently separates photoinduced carriers, but also preserves the strong redox ability of surviving electrons and holes, thereby definitely enhancing CO₂ photoreduction performance and selectively producing CH₄.

In situ irradiation XPS was carried out to verify the proposed S-scheme charge transfer pathway within the WO₃/In₂O₃ heterostructure. As presented in Fig. 4a and b, the BEs of W 4f shift positively under illumination, while the In 3d BEs show negative shifts with reference to their respective values measured in the dark. These results support the transfer of photogenerated electrons from WO₃ to In₂O₃ and validate the suggested S-scheme photocatalytic mechanism. To gain a better understanding of how charge carriers are transferred in WO₃/In₂O₃ heterostructures, TRPL spectra of pure WO₃ and WI10 were collected under different atmospheres. As shown in Fig. 4e and Table S3,† the shorter lifetime (τ₁) of WI10 is only 0.76 ns in an Ar atmosphere, which is shorter than that of pure WO₃ (0.96 ns). This suggests that the photoexcited electrons in WO₃ transfer to In₂O₃ and the radiative recombination is retarded. Consequently, only a small fraction of photoelectrons of WI10 engage in the photocarrier recombination to emit fluorescence, resulting in shorter lifetimes of τ₂ and τ₃. These results provide further evidence for the S-scheme charge separation mechanism in WO₃/In₂O₃ heterojunctions. In addition, the average lifetime (τ_a) of WI10 in a CO₂ atmosphere is shorter compared to that in an Ar atmosphere (3.38 vs. 3.56 ns), which indicates that a great deal of photoinduced electrons in the In₂O₃ CB are delivered to CO₂ molecules for surface photoreduction and fewer photocarriers are involved in detectable recombination.^59,60 Photoelectrochemical measurements were further conducted to analyze the charge separation efficiency. As shown in Fig. S16,† WI10 displays higher photocurrent density and smaller charge transfer resistance (R_ct) as compared with pristine WO₃, indicating superior electron/hole separation efficiency and lower resistance for charge transfer in the WO₃/In₂O₃ S-scheme heterojunctions.^33,61

Based on the aforementioned analyses, it is suggested that the combination of WO₃ and In₂O₃ to construct S-scheme heterojunctions can effectively promote charge transfer and reduce electron–hole recombination. In concert with the efficient adsorption and activation of CO₂ molecules on the In₂O₃ surface, the resulting WO₃/In₂O₃ S-scheme heterojunctions possess more photoelectrons with high reduction capacity to participate in the CO₂ photoreduction reaction and produce multi-electron products such as CH₄.

4. Conclusion

To summarize, an S-scheme WO₃/In₂O₃ heterojunction photocatalyst was constructed by depositing In₂O₃ nanoparticles on WO₃ nanosheets via a facile immersion-annealing method. DFT calculations, CO₂-TPD and in situ DRIFTS revealed that the WO₃/In₂O₃ heterostructures displayed efficient chemisorption and activation of CO₂ molecules. The difference in Fermi levels caused electrons to transfer from In₂O₃ to WO₃, thus forming an IEF and bending the energy bands at the interface, as evidenced by XPS and DFT results. Under light irradiation, photoinduced electrons in the WO₃ CB recombined with the holes in the In₂O₃ VB, driven by the bent bands and IEF, following an S-scheme charge transfer pathway and achieving efficient separation efficiency of powerful photogenerated charge carriers. As a result, the optimized WO₃/In₂O₃ heterojunctions exhibited enhanced CO₂ photoreduction performance with ∼53.7% selectivity for CH₄, in the absence of any molecule cocatalyst or scavenger. The CO₂ photoreduction mechanism and underlying reasons for high CH₄-selectivity were uncovered via in situ DRIFTS, along with Gibbs free-energy calculations. This work offers a perspective on designing novel S-scheme heterojunction photocatalysts for efficient CO₂ photoreduction and selective conversion of CO₂ to CH₄.

Author contributions

Y. H., J. Y., and F. X. conceived and designed the experiments. Y. H. and Z. Y. conducted material synthesis, the characterizations of the materials, and the photocatalytic test. C. L. and Y. P. performed the SVUV-PIMS test. Y. H., J. Y., D. X., W. M., and F. X. contributed to data analysis. Y. H. wrote the manuscript. F. X. supervised the project, performed DFT calculations, and revised the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2022YFE0115900), National Natural Science Foundation of China (52003213, 22238009, 51932007, and 22261142666), China Postdoctoral Science Foundation (2022M712958) and the Natural Science Foundation of Hubei Province of China (2022CFA001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta02951d

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