Valence Regulation of Ultrathin Cerium Vanadate Nanosheets for Enhanced Photocatalytic CO₂ Reduction to CO

Abstract

Atomic valence state regulation is an advantageous approach for improving photocatalytic efficiency and product selectivity. However, it is difficult to precisely control the ratio of the different valence states on the surface and the relationship between the surface valence change and catalytic efficiency in the photocatalytic reaction process is unclear. Herein, CeVO₄ ultrathin nanosheets were fabricated by one-step solvothermal method with ethanolamine (MEA) as the structure-directing agent. The ratio of the concentrations of intrinsic Ce⁴⁺ and Ce³⁺ ions is precisely modulated from 19.82:100 to 13.33:100 changed by the volume of MEA added without morphology modification. The photocatalytic efficiency increases as the concentrations of intrinsic Ce⁴⁺ and Ce³⁺ ions decrease and CV3 (prepared with 3 mL of MEA) shows the highest CO generation rate approximately 6 and 14 times larger than CV (prepared without MEA) and CV1 (prepared with 1 mL of MEA), respectively, in the photocatalytic CO₂ reduction. Interestingly, about 6.8% photo-induced Ce⁴⁺ ions were generated on the surface of the catalysts during the photocatalytic CO₂ reduction without any phase and morphology changes for CV3. The photocatalytic reaction mechanism is proposed considering the intrinsic and photo-induced Ce⁴⁺ ions to obtain efficient photocatalysts.

1. Introduction

Photocatalytic conversion of CO₂ to a specific product is highly important for the use of solar-driven deoxidation and hydrogenation reaction of carbon dioxide to lower CO₂ concentration in an environmentally friendly manner [1,2,3]. Recently, various vanadate compounds have been investigated as photocatalysts for CO₂ reduction, including InVO₄ (E_g: 2.7 eV, CO generation rate: 23.8 µmol g⁻¹ h⁻¹) [4], Co-doped BiVO₄ (E_g: ~2 eV, CO generation rate: 18.28 µmol g⁻¹ h⁻¹) [5], and LaVO₄ (E_g: 2.07 eV, C₂H₅OH generation rate: 12.7 µmol g⁻¹ h⁻¹) [6]. Among these vanadates, CeVO₄ composed of the rare-earth Ce element and ortho-vanadate group is a promising photocatalytic candidate in heterogeneous reaction. In particular, tetragonal CeVO₄ has a zircon structure in which [CeO₉] polyhedra are connected through edge sharing to the alternating [VO₄] tetrahedra along the c axis, and has a low surface energy that facilitates the natural formation of bulk nanostructured materials [7]. As a ternary metal oxide catalyst, CeVO₄ shows superior redox and optical properties due to the 4f electronic structure and abundant electronic transitions of its constituent lanthanide metal. With these promising characteristics, the photocatalytic efficiency of CeVO₄ nanostructures reported is supposed to be improved for application to CO₂ conversion.

Cerium orthovanadate was first reported in the application for photocatalysis in 2007 by Row et al. and it can be applied to photocatalytic reaction because of its advantageous bandgap compared to PrVO₄ and NdVO₄ [8]. The conduction band of CeVO₄ is lowered by excitations from the partially filled 4f orbitals to obtain an effective bandgap of 1.8 eV (band edge: ~680 nm) [9]. The broad light absorption enabled by the low band gap favors the photocatalytic reaction driven by absorption of UV and visible light [10,11]. Ce⁴⁺ and Ce³⁺ ions coexisting on the surface of CeVO₄ nanostructures act as the main catalytic centers, and play the key role in the enhanced photocatalytic activity, as demonstrated by theory and experiments [12,13]. Peng et al. reported that Ce³⁺ ions can be oxidized into Ce⁴⁺ upon the substitution of Ca²⁺ into CeVO₄ and this chemically driven charge transfer is irreversible due to the accompanying lattice contraction [14]. The intrinsic Ce⁴⁺ ions act as recombination centers of photogenerated charge carriers and have a deleterious effect on the catalytic reaction; however, Ce⁴⁺ ions can be changed into Ce³⁺ ions by a post-treatment process. It was found that conversion between Ce³⁺ and Ce⁴⁺ (denoted as Ce_p³⁺/Ce_p⁴⁺) driven by solar light or reactive intermediates is reversible [15] but the charge transfer mechanism is unclear. The relationship between the Ce_p³⁺/Ce_p⁴⁺ redox states and the catalytic activity must be understood for the further improvement of the photocatalytic performance of CeVO₄ nanostructures; however, this relationship has rarely been examined in the studies reported in the literature.

Recent reports have demonstrated that the Ce oxidation state is closely correlated with the local structure and chemical environment of the Ce atom. Localization of Ce 4f electrons and the energy levels difference between the V 3d and Ce 4f orbitals generally tends to reduce Ce⁴⁺ to Ce³⁺ [16,17]. The V⁵⁺ species (3d states) remain stable in this redox cycle most likely due to its higher 3d energy levels that are form the bandgap states together with the O 2p states of the VO₄³⁻ ions [18,19]. Based on the Ce_p³⁺/Ce_p⁴⁺ pairs, CeVO₄ is advantageous for combination with other material to improve the efficiency of photocatalysis. For example, Huo et al. reported that the photocatalytic performance of CeVO₄ was greatly improved by the La₂O₃ media combination via electron transfer enhancement [20]. It was also reported that Ag⁺ ions can be directly reduced by Ce³⁺ ions to form CeVO₄/Ag heterojunction nanocrystals (NCs) with a high photothermal conversion efficiency. Moreover, cerium vanadate nanostructures were used to detect clioquinol through the evaluation of the redox reaction on the electrode surface [21]. Since modulation of the surface atomic valence state can strongly alter the catalytic efficiency and product selectivity, it is important to study the mechanism and the effects of the Ce valence change for CeVO₄ photocatalysts.

Herein, CeVO₄ ultrathin nanosheets were synthesized by one-step solvothermal method with ethanolamine (MEA) as the structure-directing agent and then applied to photocatalytic CO₂ reduction. The photocatalytic efficiency of CO₂ conversion for the CeVO₄ nanosheets (referred to CV1–4, corresponding to the different volumes of MEA ranging from 1 to 4 mL) is improved compared to the CV prepared without MEA added. In particular, CV3 shows the maximum CO selectivity (40.97 µmol g⁻¹ h⁻¹) that is 6 and 14 times larger than those of CV and CV1, respectively, which has the lowest concentration of the intrinsic Ce⁴⁺ ions. Moreover, the CO generation rate increases with decreasing intrinsic Ce⁴⁺/Ce³⁺ ions ratio that can be modulated by MEA without affecting phase and morphology of the CeVO₄. Furthermore, it was found that approximately 6.8% of Ce_p⁴⁺/Ce_p³⁺ ion pairs are generated in the photocatalytic reaction process and tend to disappear after thermal treatment. As reactive centers, the Ce_p⁴⁺/Ce_p³⁺ ion pairs are advantageous for the photocatalytic CO₂ conversion to CO by promoting the molecular adsorption of the otherwise inert CO₂. Supported by optical and electrochemical characterization, the possible reactive mechanism is proposed to explain the efficiency and selectivity of the obtained CeVO₄ photocatalyst.

2. Results and Discussion

2.1. Morphology and Phase Characterization

Large CeVO₄ crystallites with different sizes (>500 nm) were formed using the hydrothermal method without any structure-directing agents (Figure 1a). Comparatively, the as-prepared samples grew into uniformly nanoscale structures after MEA addition as observed from the scanning electron microscopy (SEM) images (Figure 1b–e). A detailed examination of the CeVO₄ samples showed that most of the as-prepared CeVO₄ (>99.9%) crystallized into translucent nanosheets in the MEA-assisted alkaline environment as shown in the TEM images. These ultrathin nanosheets are monodispersed and the average diameter of the oval-shaped nanosheets is approximately 10 nm (Figure 2). The lattice fringes with d-spacing values of 0.32 nm and 0.27 nm correspond to the (002) and (112) planes of the tetragonal CeVO₄ phase, respectively, and their intersection angle is 63°, in agreement with the theoretical value (Figure 3a,b). Examination of the high-resolution TEM (HRTEM) images of crimpled nanosheets shows that they are approximately 1.7 nm thick (Figure 3c), which is also certified by AFM characterization with non-contact mode (Figure 4d) and their preferentially vertical growth is attributed to the high MEA absorption on the surface of the nanosheets [22,23,24]. The strongly adhesion of MEA blocks the epitaxial growth of CeVO₄ nanosheet toward the longitudinal. The crystal structure of the CeVO₄ nanosheet is schematically illustrated in Figure 3d. The vanadium atom environment is quite symmetric with four equivalent V-O bonds. Each A-site cation (Ce³⁺) has D 2d site symmetry and is surrounded by eight oxygen atoms forming an antiprism with two different sets of A-O bonds of almost equal length [14]. The advantageous crystal symmetry and abundant atom species is listed as a potential candidate for photocatalytic application.

The purity and crystallinity of the prepared samples were characterized by X-ray diffraction (XRD). As shown in Figure 3e, no impurity peaks were detected and all observed diffraction peaks are attributed to the pure tetragonal phase of CeVO₄ (a= b = 7.399 Å, c = 6.496 Å, I4₁/amd). Comparison of the XRD spectra shows that the full width at half maximum (FWHM) values of the stongest peak increase for the CV1–4 samples with more MEA added, indicating that the longitudinal dimensions are effectively reduced relative to CV to obtain ultrathin nanosheets. Moreover, the strongest diffraction peak at 24.03° for (200) facets most likely characterizes the main horizontally exposed planes of the CV1–4 nanosheets [20,25]. UV–vis diffuse reflectance spectra show the light absorbing ability of the as-prepared samples (Figure 1f). Compared to the CV prepared without MEA, the light absorption of the CV1–4 samples is much stronger and extends much further into the visible range (ca. 700 nm) with all samples showing similar bandgaps of approximately 1.76 eV, which is slightly smaller than the previously reported [9,26].

2.2. Photocatalytic CO₂ Reduction Activity

The photocatalytic CO₂ reduction activity of the as-prepared CeVO₄ nanosheets was studied in the gas-phase reaction system (water vapor, CO₂ gas) under simulated solar radiation. During the photocatalytic reaction, carbon monoxide (CO) was detected as the main product and methane (CH₄) was detected as the secondary C1 product (Figure 4a,b). With longer time, the rate of CO generation increased linearly except for the CV2 and CV3 samples for which the products cannot be completely detected due to either light absorption by reactive intermediates or the transformation of multielectron-involved products accompanying the high reaction rate. In the photocatalytic CO₂ reaction, the rate of CO production first increased and then decreased with the increasing amount of the MEA additive in the preparation process, and the maximum CO generation rate reached 40.97 µmol g⁻¹ h⁻¹ for CV3. This value is 6 and 14 times larger than the corresponding values for CV and CV1, respectively (Figure 4c and Figure 5a,b). CV2 showed superior CO generation (28.94 µmol g⁻¹ h⁻¹) compared to CV1 with a much greater increase compared to CV, demonstrating that for these materials, the interfacial effect modulated by MEA plays a more important role in the photocatalytic process than the specific surface area that is largely controlled by the nanomaterial size. Moreover, the catalytic selectivity was calculated as the ratio of the CO and CH₄ production rates and the results are shown in

Table 1. Generation rates of CO and CH₄, and their ratio.

Samples	CO (μmol g−1h−1)	CH4 (μmol g−1h−1)	CO/CH4
CV	6.68	0.0325	205.61
CV1	2.75	0.0655	41.91
CV2	28.94	0.477	60.64
CV3	40.97	0.136	301.92
CV4	17.68	0.0905	195.38

and Figure 4d. The catalytic selectivity results show the same trend as that for the CO generation rate and the improved selectivity may be correlated with surface properties controlled by the MEA agent, with CV3 showing the highest CO selectivity with a CO/CH₄ ratio of 301.92. In addition, tiny amounts of C2 products were also detected in the reaction system (Figure 4e and Figure 5c). Comparison of the XRD spectra of the as-prepared sample and the sample after the reaction reveals no changes (Figure 5d), indicating that the phase of these nanosheets remain stable during the photocatalytic CO₂ reaction process. No products were detected in the absence of either simulated solar radiation or catalysts. Moreover, isotopic substitution experiments showed that for CV3, the obtained ¹³CO product definitely originates from the reduction in gaseous ¹³CO₂ rather than from other C-containing compounds (Figure 4f). The CO generation rate of the CV3 sample is higher than those for the other reported vanadate photocatalysts [2,3,4]. which shows adverse interfacially electronic structure [27].

2.3. Spectroscopy Analysis

To investigate the electronic state of the surfaces of the as-prepared samples, X-ray photoelectron spectroscopy (XPS) characterizations were carried out using the C1s peak (294.8 eV) for calibration and Ce 3d, V 2p, O 1s peaks for analysis (Figure 6a). The ratio of Ce and V element is in accordance with the stoichiometric ratio of CeVO₄. The Ce 3d XPS spectra of the as-prepared samples display four peaks at the binding energies of 881.6, 885.3, 900.1 and 903.5 eV labeled as ʋ₀, ʋ, u₀ and u, respectively, corresponding to the Ce³⁺ state (Figure 6b). The u’ and u’’ signature peaks or Ce⁴⁺ at the binding energies of 905.7 and 914.3 eV [13,28] are also observed in the XPS spectra. Generally, the integrated areas of the XPS spectra and the Ce⁴⁺/Ce³⁺ molar concentration ratio are positively correlated. Therefore, the XPS spectra were deconvoluted into the contributions of the two different Ce 3d valence states in order to obtain the Ce⁴⁺/Ce³⁺ molar concentration ratio according to the following formula [29]:

$$\frac{{\mathrm{Ce}}^{4+}}{{\mathrm{Ce}}^{3+}}=\frac{{\mathrm{A}}_{{\mathrm{u}}^{\prime }}+{\mathrm{A}}_{{\mathrm{u}}^{″}}}{{\mathrm{A}}_{\mathrm{v}0}+{\mathrm{A}}_{\mathrm{v}}+{\mathrm{A}}_{\mathrm{u}0}+{\mathrm{A}}_{\mathrm{u}}}$$

(1)

where A_i is the total integrated area of fitting peak i. The results of the calculations are shown in Figure 6c. It is observed that as the amount of the added MEA increases, the Ce⁴⁺/Ce³⁺ molar concentration ratio gradually decreases for the CV1–3 samples, and CV3 shows the lowest Ce⁴⁺/Ce³⁺ proportion (13.33:100). This change can be understood based on the molecular formula of MEA. MEA contains reducing amino groups and oxidizing hydroxyl groups (Figure 6d); when MEA is used as an addictive in small amounts, the amino groups absorbing on the surface of Ce³⁺ act as stabilizers and protect the Ce³⁺ ions from oxidation in the solvent. However, when excessive MEA is introduced in the reactor, hydroxyl groups oxidize Ce³⁺ to the Ce⁴⁺ state [30,31,32], increasing the concentration of the Ce⁴⁺ ions in the CV4 sample.

Examination of the XPS results shows that there is no variation in the integrated areas of the V 2p and O 1s XPS spectra of the as-prepared samples (Figure 7a,b). Two V⁵⁺ peaks located at 517.2 and 524.8 eV are due to V 2p_3/2 and V 2p_1/2, respectively, and are accompanied by a small peak at 523.1 eV corresponding to the V 2p_1/2 state of V³⁺. Only two types of surface oxygen (O_lattice and O_adsorbate) are observed in the XPS spectra for these samples, corresponding to the two peaks at 529.8 and 531.2 eV, respectively. Compared to CV, the peak position of CV1–4 gradually moves to the lower binding energies, which is likely due to the increasing electron enrichment of the O atoms in these samples. The positive slope of the Mott-Schottky (MS) curve further shows that the as-prepared CeVO₄ is a characteristic n-type semiconductor (Figure 7c). The tangent of the MS curve has an x-axis intercept of −0.757 V corresponding to the flat charged potential (vs. Ag/AgCl, pH = 7). Generally, the CB potential of an n-type semiconductor is approximately equal to the flat charging potential. According to the conversion formula [33]:

$${\mathrm{E}}_{\left(\mathrm{NHE}\right)}={\mathrm{E}}_{\left(\frac{\mathrm{Ag}}{\mathrm{AgCl}}\right)}-{\mathrm{E}}^{\mathsf{\theta }}+0.059\mathrm{pH}$$

(2)

where E (Ag/AgCl) = 0.197 V at pH = 7. The conduction band (CB) potential of CV3 is determined to be −0.54 V (vs. NHE, pH = 7) and the corresponding valence band (VB) is 1.2 V (vs. NHE, pH = 7), effectively driving the photocatalytic conversion from CO₂ to the CO, CH₄ and C2 products kinetically. Moreover, the reactive barrier for catalytic reduction can be lowered with the intrinsic Ce⁴⁺ defective states, as confirmed by the LSV curves of the as-prepared samples (Figure 8a). The Tafel slope value for the CV3 nanosheets is smaller than that for the CV2 and other samples (363, 388, 311, and 371 mV decade⁻¹ for CV1, CV2, CV3, and CV4, respectively) due to its lower energy input in the catalytic reaction. In addition, the Ce_p⁴⁺/Ce_p³⁺ ratio increased by 6.78:100 from 13.33:100 to 20.11:100 for CV3 after the photocatalytic reaction (Figure 7d), as is also confirmed by the results of Raman spectroscopy analysis (Figure 8b,c) carried out to explore the interactions between various groups (

Table 2. Raman peaks and various groups.

Raman Peaks	Group	Raman Vibrations Modes
831 cm−1	-VO43−	symmetric stretching (A1g)
760 cm−1	-VO43−	anti-symmetric stretching (B1g)
446 cm−1	-VO43−	bending mode (B1g)
354 cm−1	-VO43−	bending mode (A1g)
below 250 cm−1	Ce-VO4	external mode (Eg)

).

The characteristic Raman peaks of the symmetrical stretching (A_1g) and the antisymmetric stretching (B_1g) of the VO₄³⁻ tetrahedron are observed at 831 and 760 cm⁻¹, respectively. The Raman peaks at 446 and 354 cm⁻¹ are assigned to the B_1g and A_1g bending modes of VO₄³⁻. The external mode of Ce-VO₄ vibration appearing below 250 cm⁻¹ confirms the formation of vanadate (Figure 8b) [10,34]. There is no change in the positions of the characteristic peaks of these groups after the photocatalytic reaction (Figure 8c). By contrast, the Raman peaks of the B_1g bending modes of VO₄³⁻ (448 cm⁻¹) are clearly red-shifted, particularly for the CV3 and CV2 samples that show much higher photocatalytic activities. The red-shift corresponding to stronger Raman scattering originates from the greater energy capture that is likely to be related to the emergence of Ce⁴⁺ that lowers the geometric symmetry of the CeVO₄ crystal structure. Moreover, the lower photoluminescence (PL) intensity of CV3 compared to CV2 demonstrates its superb photogenerated charge carrier transport behavior that is advantageous for the photocatalytic CO₂ reduction (Figure 8d). The PL intensity change after the photocatalytic reaction further confirm presence of photo-induced Ce⁴⁺ ions that then reduced into Ce³⁺ after the post-treatment at 400 °C (Figure 8e and Figure 7d). Additionally, the CV1–4 samples display similar specific surface areas of 22.5332, 23.8444, 21.2346 and 22.9847 m² g⁻¹ (Figure 8f), respectively, as evaluated by the Brunauer-Emmet-Teller (BET) measurements, showing that the photocatalytic activity difference is not attributed to the surface area.

2.4. Photocatalytic Mechanism

These two Ce⁴⁺ ion species play important roles in the photocatalytic reaction process. The intrinsic Ce⁴⁺ ions act as recombination centers and are unfavorable for photocatalytic CO₂ reduction, in which the CO product selectivity is inversely proportional to the intrinsic Ce⁴⁺/Ce³⁺ ratio. By contrast, the Ce_p⁴⁺ are active and inferred to improve the selective photocatalytic CO₂ reduction to CO with the valence change. The pathway of photocatalytic CO₂ reduction with H₂O vapor is illustrated in Figure 9. First, the photogenerated carriers (holes and electrons, denoted as h⁺ and e⁻, respectively) are generated on the surface of CeVO₄ through the absorption of photon energy (hʋ). Simultaneously, a large number of Ce_p⁴⁺ ions appear on the surface of the photocatalysts under solar light irradiation. Next, a Ce_p⁴⁺ ion can be reduced to a Ce_p³⁺ ion by capturing the photogenerated electrons and generating an oxygen vacancy (Vo), providing the active site for CO₂ adsorption as described by

$${\mathrm{Ce}}_{\mathrm{p}}^{4+}+{\mathrm{e}}^{-}\to {\mathrm{Ce}}_{\mathrm{p}}^{3+}+{\mathrm{V}}_{\mathrm{O}},$$

(3)

$${\mathrm{CO}}_2+{\mathrm{V}}_{\mathrm{O}}\to {\mathrm{CO}}_2\left(\mathrm{s}\right).$$

(4)

As a reactive intermediate, Vo does not contributed to the XPS O 1s spectrum (Figure 7b). Meanwhile, H⁺ is generated by

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}\to 2{\mathrm{H}}^{+}+\frac{1}{2}{\mathrm{O}}_2$$

(5)

Then, the Ce_p³⁺ ions capture adsorbed CO₂ with Ce_p⁴⁺ and the CO product is generated with assistance of H⁺ [27,35] as described by

$${\mathrm{Ce}}_{\mathrm{p}}^{3+}+{\mathrm{CO}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{Ce}}_{\mathrm{p}}^{4+}+\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$$

(6)

The conduction band position (−0.54 eV vs. NHE for CV3) is more negative than the standard redox potentials of CO and CH₄. The existence of the Ce_p³⁺/Ce_p⁴⁺ pairs not only improves the effective separation of the photogenerated electrons and holes, but also promotes the absorption and extensive activation of CO₂.

3. Materials and Methods

CeVO₄ were synthesized by one-step solvothermal method. Firstly, Ce(NO₃)₃.6H₂O 1.0 mmol (≥99.0%, Sinopharm, Shanghai, China) and NH₄VO₃ 1.0 mmol (≥99.0%, Sinopharm, Shanghai, China) were added into 20 mL distilled water, respectively. Secondly, MEA (≥99.0%, Lingfeng, Shanghai, China) was dropped into the prepared solutions as structure-directing agent. The volume of MEA was varied from 1 to 4 mL with the sample of MEA scarcity for comparison. Then, the mixture was transferred to a Teflon-lined stainless-steel autoclave with a 30 mL inner volume after stirring for 2 h and put in an oven heated at 200 °C for 36 h. After the reaction, these samples cooled down to room temperature naturally. The bottom precipitates were centrifuged and washed for 3 times with ethanol and deionized water, respectively. The dark brown CeVO₄ samples were successfully obtained after dry in freeze dryer.

The phases of these prepared powder samples were characterized by X-ray diffractometry (XRD; Rigaku Ultima III, Rigaku, Tokyo, Japan) at room temperature. Cu-Ka radiation (λ = 0.154178 nm) was used at 40 kV and 40 mA in a scan range of 10°–90° at a scan rate of 20° min⁻¹. The morphology of the prepared samples was characterized by scanning electron microscopy (SEM, JSM-7800F, JEOL, Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken by TEM (JEM 200CX, JEOL, Tokyo, Japan). UV–visible diffuse reflectance spectra were obtained at room temperature using an ultraviolet-visible spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) and directly converted to absorption spectra by using the Kubelka-Munk equation in software. AFM images were taken using a Park XE7 (Park SYSTEMS, South Korean) atomic force microscope in non-contact mode. The PL spectra were obtained by using (F-7000, Tianmei, China) FL Spectrophotometer (excitation wavelength 280 nm). Raman spectroscopy is the use of the model Raman spectrometer (WITec alpha 300 M⁺, WITec, German) under 532 nm laser. The chemical state was examined by X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, American), and charge correction was performed based on the binding energy of the indefinite C1s peak at 284.8 eV. Using a electrochemical workstation (ZaHNER PP211, Zahner, German), a 0.2 M Na₂SO₄ solution was used as the electrolyte, Ag/AgCl was used as the reference electrode, and Pt was the counter electrode as the standard of three-electrode system. The Mott-Schotty test was carried out at a frequency of 5 kV, Scan rate is 5 mV/min, and voltage rage at −0.9–0.1 V. The Tafel test by using the other electrochemical workstation (CHI660E, Huachen, Shanghai, China). Scan rate is 5 mV/min, and voltage rage at 0–0.6 V. Working electrode was prepared by deposited catalyst on the F-doped SnO₂ conductive glass (FTO).

The photocatalytic reaction of carbon dioxide reduction was carried out at room temperature, and the 0.05 g sample of the prepared powder was evenly weighed on the top of a glass reactor with an area of 4.91 cm². The bottom of the glass reactor was tied with a magnetic stirrer, it was guaranteed that during the reaction pure CO₂ gas is slowly and evenly injected until it fills the reactor. The excess gas was collected with NaHCO₃ solution, and finally, the glass reactor was put into a gas-tight reaction system with a volume of about 440 mL. 0.4 mL of pure water was injected from the silicone rubber diaphragm into the reactor through a micro syringe as a reducing agent and injected into the reaction system. After stirring the stirrer for 1 h, the adsorption of CO₂-H₂O atmosphere was balanced to ensure complete adsorption. A solar simulator (Micro-solar 300 W xenon lamp) was used to illuminate the vertical sample. From the start of the xenon lamp, the lighting time is 6 h. Draw about 1 mL of gas from the reaction cell. After injection, a gas chromatograph (GC-2014, Shimadzu Corporation, Kyoto, Japan) was used to analyze the subsequent reaction products. The isotope-labelled experiments were implemented using ¹³CO₂ as the original reactant, and the products were analyzed using gas chromatograph-mass spectrometry (QP2010SE, Shimadzu Corporation, Kyoto, Japan).

4. Conclusions

In summary, ultrathin CeVO₄ nanosheets with a thickness of approximately 1.7 nm were synthesized by one-step hydrothermal method with MEA as the structure-directing agent. This photocatalyst has a narrow bandgap of approximately 1.74 eV, extending the optical absorbance edge to 700 nm, and exhibits superior light capture. The proportion of Ce⁴⁺ ions can be decreased by MEA without either morphology or phase changes. The optimal photocatalytic activity is obtained when the Ce⁴⁺/Ce³⁺ molar ratio decreases to 13.33:100 for the CV3 sample. The maximum yield rate of CO is 40.97 µmol g⁻¹ h⁻¹ for CV3, showing an enhancement by 6 times compared to CV. Moreover, the Ce⁴⁺/Ce³⁺ ratio is negatively correlated with and modulates the C1 product selectivity. Finally, the likely reaction mechanism is proposed based on the Ce_p⁴⁺/Ce_p³⁺ pairs that give rise to the photocatalytic reaction process and drive the selective photocatalytic CO₂ reduction to CO. The atomic valence regulation demonstrated in this work provides an effective alternative approach for the improvement of the product selectivity and efficiency enhancement in photocatalytic reactions.