English

• The rapid advancement of global industrial modernization has led to excessive consumption of fossil fuels, causing a sustained and substantial rise in carbon emissions and subsequent detrimental environmental and climatic consequences [1–4]. To effectively mitigate the intensifying greenhouse effect, there is a global consensus on actively pursuing a green and low-carbon transformation of economy and society to achieve “carbon peaking and carbon neutrality” [5,6]. In recent years, photocatalytic CO₂ conversion technology has emerged as a promising approach, providing robust support for the realization of “dual carbon” goals [7–12]. Among various photocatalytic CO₂ reduction products, liquid-phase products stand out due to their high energy density and convenience in separation, storage, and transportation. Notably, methanol (CH₃OH), as a crucial chemical raw material and alternative energy source, holds a pivotal position in industrial production. Thus, leveraging semiconductor catalysts to convert CO₂ into CH₃OH through photocatalytic method not only addresses environmental challenges but also alleviates the energy crisis, making it a mutually beneficial strategy [13–18]. However, the process of photocatalytic CO₂ reduction to CH₃OH is complex, involving multiple charge transfers and extended reaction pathways, which leads to a higher rate of photogenerated charge recombination compared to their utilization in the reaction. This limits the catalytic activity and selectivity for CO₂ to CH₃OH conversion [19–25].

  In recent years, metal vanadates, such as BiVO₄, InVO₄, and FeVO₄, have garnered considerable attention in photocatalysis due to their exceptional visible light absorption and robust stability [26–29]. Among them, InVO₄, with its suitable energy band structure, stands out as a promising catalyst for photocatalytic CO₂ reduction [30,31]. Moreover, it has been proven to achieve high selectivity for CH₃OH production [32]; BiVO₄ excels in photocatalytic water oxidation, attributed to its relatively high oxidation potential [33–35]. However, both InVO₄ and BiVO₄ suffer from limited photocatalytic activity due to insufficient carrier separation. Integrating InVO₄ and BiVO₄ into a Z-scheme heterojunction through methods, such as hydrothermal [36], dipping-calcination [37] and one-pot microwave [38], can enhance the separation efficiency of photogenerated carriers, leveraging their respective strengths in CO₂ reduction and water oxidation. Nevertheless, traditional methods often result in loose heterojunctions interfaces, limiting carrier separation efficiency. We herein propose a cation exchange strategy, involving the introduction of Bi³⁺ ions onto an InVO₄ substrate for in-situ conversion to BiVO₄, successfully constructing a direct Z-scheme heterojunction of InVO₄/BiVO₄. Compared to traditional self-assembly method, this approach constructs tighter heterojunction interfaces, thus significantly improving charge transport and separation efficiency, and boosting photocatalytic activity.

  The synthesis process for the InVO₄/BiVO₄ heterojunction is illustrated in Scheme S1 (Supporting information), and specific steps are described in experimental section in Supporting information. Initially, an InVO₄ substrate was prepared using a hydrothermal method [27]. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations revealed that the prepared InVO₄ exhibits a nanoflower structure with an average size of approximately 3-4 µm (Fig. 1a and Fig. S1 in Supporting information), which is composed of nanosheets with a thickness of 2–2.5 nm (Figs. S2 and S3 in Supporting information). Subsequently, Bi³⁺ ions were introduced onto the surface of InVO₄, and through cation exchange and in-situ conversion, the InVO₄/BiVO₄ composite material was generated. SEM, TEM and AFM images show that the composite maintains the original nanoflower morphology of the InVO₄ substrate (Fig. 1b, Figs. S4 and S5 in Supporting information), indicating that the cation exchange process did not significantly alter its morphology and size. High-resolution TEM (HRTEM) image of InVO₄/BiVO₄ displays a lattice spacing of 2.7 Å corresponding to the (112) plane of orthorhombic InVO₄ and a lattice spacing of 3.1 Å for the (121) plane of monoclinic BiVO₄ (Fig. 1c), initially verifying the successful conversion of BiVO₄ onto InVO₄. Additionally, energy-dispersive X-ray spectroscopy (EDS) analysis indicated a uniform distribution of In, Bi, O, and V elements in the InVO₄/BiVO₄ composite (Figs. 1d−g), suggesting the uniform growth of BiVO₄ on InVO₄. It is noteworthy that the morphology of InVO₄/BiVO₄ heterojunction differs significantly from that of InVO₄:BiVO₄ composite (prepared via traditional electrostatic attraction method, see details in Supporting information), in which BiVO₄ large particles are positioned on the side of the InVO₄ nanoflower (Fig. S6 in Supporting information). X-ray diffraction (XRD) and Raman spectroscopy measurements further confirmed the presence of both InVO₄ and BiVO₄ in the composite. As illustrated in Fig. S7 (Supporting information), the characteristic diffraction peaks of pure InVO₄ match well with those of orthorhombic InVO₄ (JCPDS No. 48−0898). With the introduction of Bi³⁺ ions, the characteristic diffraction peaks at 28.9° and 30.5° of BiVO₄ (corresponding to JCPDS No. 14−0688) are clearly observed, and as the amount of Bi³⁺ introduced increased, the diffraction peaks of BiVO₄ become more intense. Quantitative analysis of the In and Bi elements within the composite revealed an effective modulation of the conversion ratio of BiVO₄, ranging from 2.45 % to 7.99 % (Table S1 in Supporting information). As presented in Fig. S8 (Supporting information), the InVO₄/BiVO₄ composite exhibits characteristic peaks at 918 and 826 cm⁻¹, attributed to the V−O−In and V−O−Bi stretching vibrations of InVO₄ and BiVO₄, respectively [39,40].

  Figure 1

  

  Figure 1.  SEM images of (a) InVO₄ and (b) InVO₄/BiVO₄. (c) HRTEM image of InVO₄/BiVO₄. (d − g) EDS elemental mapping images (In, V, Bi and O) of InVO₄/BiVO₄.

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  X-ray photoelectron spectroscopy (XPS) measurements further confirmed the successful conversion of BiVO₄ onto the InVO₄ substrate, clearly identifying the characteristic signals of In, Bi, O and V elements (Fig. S9 in Supporting information). Additionally, high-resolution XPS measurements unveiled that the characteristic peaks of In 3d_5/2 and In 3d_3/2 in InVO₄ reside at 444.3 and 451.9 eV, respectively, whereas the characteristic peaks of Bi 4f_7/2 and Bi 4f_5/2 in BiVO₄ are positioned at 159.7 and 165.0 eV (Fig. S10 in Supporting information). Notably, the binding energies of In 3d and Bi 4f in the InVO₄/BiVO₄ composite shifted positively by 0.20 eV and negatively by 0.15 eV, respectively, compared to their individual component values, suggesting strong electronic coupling between InVO₄ and BiVO₄ (Fig. 2a (left) and Fig. S10). This inference can be further confirmed by theoretical simulations. As presented in Figs. 2b and c, there are significant charge density differences at the InVO₄/BiVO₄ heterojunction interface, with charge density decreasing on the InVO₄ side and increasing on the BiVO₄ side. Bader charge analysis indicated a charge transfer of approximately 2.2 electrons from InVO₄ to BiVO₄ at the interface.

  Figure 2

  

  Figure 2.  (a) Binding energy shifts between InVO₄/BiVO₄ and InVO₄ or BiVO₄ in the dark (left), and binding energy shifts of InVO₄/BiVO₄ upon light irradiation (light). (b) Charge density difference of the InVO₄/BiVO₄ heterojunction. (c) Planar-averaged charge density difference along the z-axis. (d) Schematic diagram of the energy band structure and direct Z-scheme charge transfer mechanism under illumination for InVO₄/BiVO₄. The EPR spectra of (e) DMPO-^•O₂⁻ and (f) DMPO-^•OH for InVO₄, BiVO₄ and InVO₄/BiVO₄.

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  Ultraviolet-visible diffuse reflectance spectra (UV−vis DRS) measurements revealed that both InVO₄ and BiVO₄ possess good visible light response, with corresponding band gaps (E_g) of 2.41 and 2.12 eV, respectively (Figs. S11 and S12 in Supporting information). The valence band maximum potentials (E_VB) of InVO₄ and BiVO₄ were determined by ultraviolet photoelectron spectroscopy (UPS) measurements to be 1.54 and 2.22 V, respectively, versus the standard hydrogen electrode (vs. SHE) (Fig. S13 in Supporting information). Consequently, the conduction band minimum potentials (E_CB) for InVO₄ and BiVO₄ can be calculated as −0.87 and 0.10 V vs. SHE, respectively, which are close to the flat-band potential values obtained from Mott−Schottky plots (Fig. S14 in Supporting information). In addition, the work functions of InVO₄ and BiVO₄ were determined by UPS measurements to be 4.25 and 5.27 eV, respectively. The significant difference in work functions between InVO₄ and BiVO₄ provides a strong driving force for charge transfer at the InVO₄/BiVO₄ heterostructure interface. Specifically, free electrons in InVO₄ tend to transfer to BiVO₄, leading to the formation of a unified Fermi level. This transfer behavior is consistent with aforementioned XPS analysis and theoretical calculation results. In this equilibrium state, negative charges accumulate on the BiVO₄ side and positive charges accumulate on the InVO₄ side, resulting in the formation of a built-in electric field pointing from InVO₄ to BiVO₄. Upon illumination, the electrons in the valence bands (VB) of BiVO₄ and InVO₄ transition to their respective conduction bands (CB). Due to the presence of the built-in electric field, the photogenerated electrons in the CB of BiVO₄ are driven to recombine with photogenerated holes in the VB of InVO₄. This process enables the photogenerated electrons in the CB of InVO₄ and photogenerated holes in the VB of BiVO₄ to remain separated (Fig. 2d).

  The above hypotheses regarding the charge transfer mechanism in the InVO₄/BiVO₄ heterostructure can be confirmed by in-situ illumination XPS (ISI-XPS) and electron spin resonance (ESR) measurements. As shown in Fig. 2a (right) and Fig. S15 (Supporting information), the characteristic peaks of In 3d shifted towards lower binding energy by 0.15 eV, while the binding energy of Bi 4f shifted towards higher value by 0.18 eV in the InVO₄/BiVO₄ under illumination. In contrast, the binding energies of pure InVO₄ and BiVO₄ remained nearly unchanged under the same conditions (Fig. S16 in Supporting information). These photoinduced shifts in binding energies indicate that photogenerated electrons flow from BiVO₄ to InVO₄ within the InVO₄/BiVO₄ heterostructure, supporting the hypothesis that photogenerated carriers follow a Z-scheme transfer path in the heterostructure. Moreover, the DMPO-^•O₂⁻ signal intensity of InVO₄/BiVO₄ is significantly higher than that of InVO₄, while the DMPO-^•O₂⁻ signal of BiVO₄ is nearly undetectable (Fig. 2e). Simultaneously, the DMPO-^•OH signal intensity of InVO₄/BiVO₄ is also significantly higher than that of BiVO₄, while the DMPO-^•OH signal of InVO₄ is barely observable (Fig. 2f). By comparing the redox potentials of O₂/^•O₂⁻ (−0.33 V vs. SHE) and H₂O/^•OH (1.99 V vs. SHE), if the charge transfer mechanism of the InVO₄/BiVO₄ heterostructure is the traditional type-Ⅱ double charge transfer, ^•OH and ^•O₂⁻ radicals should not be produced. However, the experimental results contradicted this assumption, further confirming the formation of a Z-scheme charge transfer pathway, which effectively retained photogenerated electrons in the CB of InVO₄ and photogenerated holes in the VB of BiVO₄, achieving efficient carrier separation. To further investigate the improved charge separation efficiency in the InVO₄/BiVO₄ heterostructure, electrochemical impedance spectroscopy (EIS) and transient photocurrent measurements were conducted. Figs. S17 and S18 (Supporting information) demonstrate that, compared to InVO₄, BiVO₄ and InVO₄:BiVO₄, InVO₄/BiVO₄ exhibits the smallest charge transfer resistance (Table S2 in Supporting information) and the largest photocurrent density. These results indicate that the tightly connected heterostructure formed through the ion-exchange and in-situ partial conversion strategy can significantly accelerate charge carrier migration and separation capabilities.

  Gas chromatography analysis revealed that the main photocatalytic CO₂ reduction products for these InVO₄-based materials were CH₃OH, accompanied by a small amount of CO, while pure BiVO₄ exhibits negligible photocatalytic CO₂ reduction activity due to thermodynamic limitation of reduction. As summarized in Fig. 3a, pristine InVO₄ displays a moderate activity with a CH₃OH yield of 58.1 µmol g⁻¹ h⁻¹. The InVO₄:BiVO₄ achieves a slight increase in the performance with a CH₃OH yield of 65.0 µmol g⁻¹ h⁻¹, indicating that the simple electrostatic self-assembly does not effectively enhance carrier separation. In contrast, InVO₄/BiVO₄ demonstrates a significant improvement in photocatalytic activity (Fig. S19 in Supporting information), and the optimized yield of CH₃OH reaches 130.5 µmol g⁻¹ h⁻¹ with a high selectivity of 92 %, which is 2.2 and 2.0 times higher than that of pure InVO₄ and InVO₄:BiVO₄, respectively, and superior to those of semiconductor photocatalysts reported under similar conditions (Table S3 in Supporting information). Additionally, the O₂ yield of InVO₄/BiVO₄ is 221.2 µmol g⁻¹ h⁻¹, indicating a balanced consumption of photogenerated electrons and holes (Fig. S20 in Supporting information).

  Figure 3

  

  Figure 3.  (a) Yields of products with different photocatalysts. (b) Time-on-line amounts of products by InVO₄/BiVO₄.

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  The InVO₄/BiVO₄ heterostructure also demonstrates excellent photoreaction stability. Fig. 3b and Fig. S21 (Supporting information) illustrate that it maintained high catalytic activity throughout a continuous 25 h illumination reaction and displayed no significant decrease in activity after 4 cycles of reaction tests. The crystal structure and surface state of InVO₄/BiVO₄ remained stable during the reaction process, as confirmed by XRD patterns and XPS spectra (Figs. S22 and S23 in Supporting information). Moreover, control experiments revealed that light, catalyst, CO₂, and H₂O are indispensable elements for the generation of CH₃OH, indicating that CH₃OH originates from light-driven CO₂ reduction over photocatalyst with water as electron source (Fig. S24 in Supporting information). Furthermore, when ¹³CO₂ was employed as the reactant, distinctive split peaks attributable to ¹³CH₃OH (δ = 3.04 and 3.40 ppm) were discernible in the ¹H nuclear magnetic resonance spectrum (Fig. 4a). Meanwhile, the replacement of H₂¹⁶O with H₂¹⁸O resulted in unequivocal detection of ¹⁸O₂ (m/z = 36) through mass spectrometry analysis (Fig. 4b). These results further robustly demonstrated that CH₃OH and O₂ originate from CO₂ photoreduction and H₂O oxidation, respectively.

  Figure 4

  

  Figure 4.  (a) ¹H NMR spectrum of the products produced from ¹³CO₂ reduction. (b) MS analyses for solar-driven reaction of H₂¹⁸O to ¹⁸O₂ (m/z = 36) using InVO₄/BiVO₄ as photocatalyst. (c) IS-IR spectra of the CO₂ photoreduction reaction with InVO₄/BiVO₄ as photocatalyst. (d) Calculated free-energy plots for CO₂ photoreduction of InVO₄, and InVO₄/BiVO₄.

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  In-situ infrared spectroscopy (IS-IR) measurements were carried out to monitor the evolution of CO₂ on the InVO₄/BiVO₄ surface during photocatalytic reaction. As shown in Fig. 4c, the characteristic absorption peaks emerged for various intermediates like m-CO₃²⁻ (1321, 1508 and 1556 cm⁻¹), b-CO₃²⁻ (1476 cm⁻¹), HCO₃⁻ (1454 cm⁻¹) and *COOH (1542 cm⁻¹) [41–43], with increasing illumination time. Additionally, the characteristic signal peaks of *CH₂O (1163 cm⁻¹) and *CH₃O (1362 cm⁻¹) [44,45], which are crucial intermediates in the reduction of CO₂ to CH₃OH, were also identified. Based on these observations, the reaction pathway for photocatalytic CO₂ reduction on the InVO₄/BiVO₄ heterojunction surface can be speculated as follows. Photoexcited InVO₄/BiVO₄ generates electron-hole pairs separated via a Z-scheme mechanism, retaining electrons on InVO₄ and holes on BiVO₄. Electrons on InVO₄ drive CO₂ reduction through proton-coupled electron transfer, forming *CO via *COOH protonation and dehydration, followed by consecutive reduction and hydrogenation to CH₃OH. Furthermore, density functional theory (DFT) calculations determined the Gibbs free energies of the reaction intermediates. As illustrated in Fig. 4d, CO₂ adsorbed at the reduction site was first activated to form *CO₂, and the adsorption effect of InVO₄/BiVO₄ on CO₂ is more significant than that of InVO₄, mainly attributed to the transfer of free electrons in InVO₄/BiVO₄, rendering InVO₄ in an electron-deficient state. The rate-determining step for both InVO₄ and InVO₄/BiVO₄ is the transition from *CO₂ to *COOH, but with a significantly lower energy barrier for InVO₄/BiVO₄ than that of InVO₄. Subsequently, *COOH undergoes protonation and dehydration to form a stable *CO intermediate. Notably, hydrogenation of *CO to *CHO has a lower energy barrier than *CO desorption, contributing to high selectivity of InVO₄-based materials in reducing CO₂ to CH₃OH.

  In summary, a Z-scheme heterojunction of InVO₄/BiVO₄ was successfully synthesized through an innovative cation exchange strategy and utilized for photocatalytic CO₂ reduction with H₂O as the electron source. Microstructural analysis unveiled that the InVO₄/BiVO₄ features a tightly connected interface, enabling efficient separation of photogenerated carrier through a Z-scheme mechanism, as validated by diverse measurements encompassing ESR, ISI-XPS and photoelectrochemical techniques. Furthermore, DFT calculations revealed that the formation of heterojunction can promote the adsorption and activation of CO₂, attributed to the deficient-electron nature of the InVO₄ component. Consequently, the InVO₄/BiVO₄ heterojunction demonstrates remarkable efficiency and selectivity in photocatalytic CO₂ reduction to CH₃OH, achieving a yield and selectivity of 130.5 µmol g⁻¹ h⁻¹ and 92 % respectively, outperforming other materials reported thus far.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Cheng-Cheng Jiao: Writing – original draft, Methodology, Investigation. Guang-Xing Dong: Writing – original draft, Methodology, Investigation. Ke Su: Writing – review & editing, Supervision, Project administration, Methodology. You-Xiang Feng: Investigation. Min Zhang: Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. Tong-Bu Lu: Methodology, Funding acquisition.

  Acknowledgments

  This work was financially supported the National Key R&D Program of China (No. 2022YFA1502902), the National Natural Science Foundation of China (NSFC, Nos. 22475152 and U21A20286), and the 111 Project of China (No. D17003).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110752 .

  
  
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  Figure 1  SEM images of (a) InVO₄ and (b) InVO₄/BiVO₄. (c) HRTEM image of InVO₄/BiVO₄. (d − g) EDS elemental mapping images (In, V, Bi and O) of InVO₄/BiVO₄.

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  Figure 2  (a) Binding energy shifts between InVO₄/BiVO₄ and InVO₄ or BiVO₄ in the dark (left), and binding energy shifts of InVO₄/BiVO₄ upon light irradiation (light). (b) Charge density difference of the InVO₄/BiVO₄ heterojunction. (c) Planar-averaged charge density difference along the z-axis. (d) Schematic diagram of the energy band structure and direct Z-scheme charge transfer mechanism under illumination for InVO₄/BiVO₄. The EPR spectra of (e) DMPO-^•O₂⁻ and (f) DMPO-^•OH for InVO₄, BiVO₄ and InVO₄/BiVO₄.

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  Figure 3  (a) Yields of products with different photocatalysts. (b) Time-on-line amounts of products by InVO₄/BiVO₄.

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  Figure 4  (a) ¹H NMR spectrum of the products produced from ¹³CO₂ reduction. (b) MS analyses for solar-driven reaction of H₂¹⁸O to ¹⁸O₂ (m/z = 36) using InVO₄/BiVO₄ as photocatalyst. (c) IS-IR spectra of the CO₂ photoreduction reaction with InVO₄/BiVO₄ as photocatalyst. (d) Calculated free-energy plots for CO₂ photoreduction of InVO₄, and InVO₄/BiVO₄.

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