Zn₂GeO₄@CeO₂ Core@Shell Nanorods for Efficient Photocatalytic CO₂ Reduction

Abstract

The enduring problem of CO₂ emissions and their consequent influence on the earth’s atmosphere has captured the attention of researchers. Photocatalytic CO₂ reduction holds great significance; however, it is constrained by the effect of carrier recombination. Simultaneously, the structural modification of heterojunction catalysts has emerged as a promising approach to boost the photocatalytic performance. Herein, Zn₂GeO₄@CeO₂ core@shell nanorods were prepared by a simple self-assembly method for photocatalytic CO₂ reduction. The thickness of the CeO₂ shell can be regulated rapidly and conveniently. The photocatalytic results indicate that the structure regulation could affect the photocatalytic performance by controlling the amount of active sites and the shielding effect. X-ray photoelectron spectroscopy (XPS) and Mott–Schottky analyses reveal that Zn₂GeO₄ and CeO₂ formed Type-I heterojunctions, which prolonged the lifetime of the photogenerated carriers. The CO₂ adsorption and activation capacities of CeO₂ also exert a beneficial influence on the progress of CO₂ photoreduction, thus enabling efficient photocatalytic CO₂ reduction. Moreover, the in situ FT-IR spectra show that Zn₂GeO₄@CeO₂ suppresses the formation of byproduct intermediates and shows higher CO selectivity. The best sample of Zn₂GeO₄@0.07CeO₂ can exhibit a CO yield of as high as 1190.9 μmol g⁻¹ h⁻¹.

1. Introduction

In recent decades, the combustion of fossil fuels has led to a substantial increase in the atmospheric concentration of CO₂. As a greenhouse gas, an excessive CO₂ concentration can trigger a series of environmental problems [1,2,3]. Considerable efforts have been devoted to reducing atmospheric CO₂ levels such as photocatalysis, electrocatalysis and thermocatalysis. Among these, photocatalytic CO₂ reduction into high-value-added products is regarded as an effective approach to mitigate energy problems and reduce the greenhouse effect [4,5,6]. A variety of semiconductors, including TiO₂, CdS, and Zn₂GeO₄, have been employed as photocatalysts [7,8,9].

For photocatalytic CO₂ reduction catalysts, the primary limitation lies in the intense photogenerated carrier recombination, which significantly impacts the lifetime of carriers [10,11,12]. In an attempt to surmount this constraint on carrier lifetime, heterojunction photocatalysts have been extensively investigated [13]. The photogenerated carriers in the heterojunctions can transfer between semiconductors, thus generating highly active charge carriers, which is advantageous for reducing the probability of recombination [14,15,16]. Currently, a wide variety of heterojunction photocatalysts with diverse morphologies and structures have been investigated, and their structure and morphology have emerged as a prevalent research focus [7,14,17,18]. It is generally accepted that the carrier transfer process predominantly takes place at the heterostructure interface [19]. In this case, heterojunction catalysts with a special core@shell structure, which possess a large interfacial area, have been widely studied [16]. Structure regulation of the core@shell heterojunction catalysts enables the components to leverage strengths to offset weaknesses, thereby achieving the optimal effect [17]. Moreover, controllable tuning is of great significance in catalyst design.

Zinc germanate (Zn₂GeO₄), owing to its good CO₂ reduction capability, is widely utilized in the field of photocatalysis [20,21]. However, the wide band gap, low charge separation efficiency, and insufficient active sites of Zn₂GeO₄ impose limitations on its photocatalytic efficiency [22]. To remove these obstacles, various methods have been employed by researchers such as adjusting the oxygen vacancies concentration [9]. Cerium dioxide (CeO₂) is an n-type semiconductor, and there are a substantial number of oxygen vacancies on its surface, which can act as reaction sites for the activation of CO₂ [23,24,25]. Inspired by the above analysis, the construction of Zn₂GeO₄@CeO₂ might prove to be an effective way to achieve an ideal photocatalytic effect.

In this work, Zn₂GeO₄@CeO₂ was fabricated by a self-assembly method. The amount of the coated CeO₂ shell was carefully controlled to adjust the amount of active sites. The as-obtained Zn₂GeO₄@CeO₂ catalysts were then comprehensively characterized to identify the optimal amount of the CeO₂ shell that exhibits the best performance on photocatalytic CO₂ reduction. Mott–Schottky analysis and UV–visible spectroscopy (UV-vis) characterizations were conducted to reveal the reaction mechanism. The results suggest that the formation of Zn₂GeO₄@CeO₂ nanorods could extend the carrier lifetime and facilitate the light absorption process, with their catalytic performance being significantly influenced by the coating thickness of the CeO₂ layer. It is believed that this work can provide a novel strategy for the design of heterojunction catalysts for photocatalytic CO₂ reduction.

2. Results and Discussion

The schematic synthesis process of a series of Zn₂GeO₄@CeO₂ core@shell nanorods is presented in Figure 1a. Initially, the Zn₂GeO₄ precursor was synthesized via hydrothermal treatment, followed by CeO₂ coating to form the core@shell architecture. A series of Zn₂GeO₄@CeO₂ samples have been prepared, which are named as Zn₂GeO₄@xCeO₂, where x represents the amount of Ce added per 50 mg of Zn₂GeO₄ (Figure 1b–e). Scanning electron microscopy (SEM) was applied to characterize the morphology of the series samples. As shown in Figure 1b and Figure S1, the as-obtained Zn₂GeO₄ shows a rod-shaped morphology. After coating, the surface of these nanorods became rough, implying the formation of the core@shell structure [26,27]. The high-resolution transmission electron microscopy (HRTEM) images of Zn₂GeO₄@xCeO₂ in Figure 1f show that a layer of CeO₂ nanoparticles could be observed clearly on the surface of Zn₂GeO₄. To further investigate the structure of the samples, Zn₂GeO₄@0.07CeO₂ was chosen as the representative sample. As shown in Figure 1g, it can be observed that the lattice spacings of the core and shell of 0.29 and 0.31 nm correspond well to the characteristic (113) plane of Zn₂GeO₄ and the (111) plane of CeO₂, respectively [20,23,28]. Figure 1h and Figure S4 present the X-ray diffraction (XRD) patterns of the samples, where all diffraction peaks of the as-obtained Zn₂GeO₄ and CeO₂ could be well indexed to the standard patterns of Zn₂GeO₄ (JCPDS #11-0687) and CeO₂ (JCPDS #11-0394). The intensities of the signals corresponding to Zn₂GeO₄ decrease when compared to those of Zn₂GeO₄, which should be attributed to the encapsulation of the CeO₂ shell. However, the intensities of the characteristic peaks of CeO₂ gradually increase as the Ce dosage increases [8]. Therefore, as shown in Figure 1i–l and Figure S5, energy dispersive X-ray spectroscopy (EDS) mapping analysis shows that the elements Zn and Ge are present in the core position; however, Ce is only distributed on the nanorod surface. Furthermore, the thickness of the CeO₂ shell increases with the increase in CeO₂ (Figures S5 and S6).

X-ray photoelectron spectroscopy (XPS) was employed to identify the surface chemical environments of the as-obtained samples. As shown in Figure 2a, the peaks centered at 530.7 and 531.6 eV can be assigned to the lattice oxygen and oxygen atoms near oxygen vacancy in Zn₂GeO₄ [29,30,31]. For CeO₂, the O 1s XPS peak at 532.9 eV can be ascribed to the adsorbed (CO₂ or H₂O) oxygen on the surface of CeO₂, indicating the outstanding CO₂-storing ability (Figure 2b) [26,32,33]. The significant peak at 530.8 eV can be attributed to the oxygen atoms near oxygen vacancy defects. It is further substantiated by the Electron Paramagnetic Resonance (EPR) tests that the signal of oxygen vacancy at g = 2.002 can be observed (Figure S7). The abundant O vacancies in CeO₂ could act as active sites for photocatalytic CO₂ reduction [24]. All the peaks displayed in the O 1s signals mentioned above have been included in Zn₂GeO₄@0.07CeO₂ (Figure 2c).

Moreover, the electron binding energy of the element originates from the Coulomb attraction between the outer electrons of the atom and the atomic nucleus, in which a chemical shift can directly reflect the changes in electron density [17,34,35]. For the XPS spectra of Zn 2p and Ge 3d, the electron transfer process after the formation of the heterojunction was characterized by comparing Zn₂GeO₄ with Zn₂GeO₄@0.07CeO₂ (Figure 2d,e). As shown in Figure 2d, the peaks located at 1021.5 and 1044.5 eV can be fitted into Zn 2p_3/2 and Zn 2p_1/2, respectively. In Figure 2e, the peaks centered at 32.1 eV can be assigned to Ge 3d [20,29]. Compared to Zn₂GeO₄, Zn 2p_3/2 and Zn 2p_1/2 in Zn₂GeO₄@0.07CeO₂ shift to 1022.0 and 1044.9 eV, and Ge 3d shifts to 32.4 eV, which could be attributed to the electron transfer from Zn₂GeO₄ to CeO₂ [23]. In Figure 2f, the Ce 3d XPS spectra can be deconvoluted into eight peaks denoted as V and U, ascribed to the spin−orbit coupling of the Ce 3d_5/2 and Ce 3d_3/2 states, respectively, which also correspond to Ce³⁺ and Ce⁴⁺ [26,36,37]. The presence of Ce³⁺/Ce⁴⁺ redox couples can create oxygen vacancies, which can serve as active sites and enhance the photocatalytic activity [25]. As a result, the Ce³⁺ content in Zn₂GeO₄@0.07CeO₂ is 35.7%, which is significantly higher than the 25.8% in pure CeO₂ (Table S1, Supporting Information). This can be mainly attributed to the electron transfer from Zn₂GeO₄ to CeO₂ [37,38].

Photocatalytic CO₂ reduction was then conducted under full-spectrum irradiation and room temperature to evaluate the catalytic activities of the samples. As shown in Figure 3a, CO, H₂, CH₄, and C₂ are the major gas-phase products. The CO yield of Zn₂GeO₄ is 380.1 μmol g⁻¹ h⁻¹, but the selectivity is only 10.0%. Among the samples, Zn₂GeO₄@0.07CeO₂ has the highest CO yield of 1190.9 μmol g⁻¹ h⁻¹, which is 3.1 times that of Zn₂GeO₄, while its CO selectivity reaches 51.5% (Table S2). For Zn₂GeO₄, Zn₂GeO₄@0.05CeO₂, and Zn₂GeO₄@0.07CeO₂, the CO generation rates increase with the increase in CeO₂, owing to the fact that the active sites on CeO₂ near the interface are crucial for the reaction. The further increase in the thickness of CeO₂ would result in a decline in catalytic performance, which can be attributed to the shielding effect, where the excessively thick CeO₂ restricts the access of CO₂ to the heterojunction interface, thereby impeding its reduction process [39]. ¹H nuclear magnetic resonance (NMR) analysis indicates that negligible amounts of liquid-phase products were identified was detected in the system (Figure S8). As shown in Figure S9, the CO yield of Zn₂GeO₄@0.07CeO₂ drops to 10.9 μmol g⁻¹ h⁻¹ in the absence of a sacrificial agent, comparable to that reported in the literature [29]. It is found that only a negligible amount of CO products are detected when no catalysts or external light source is present, further validating the origin of the product. Zn₂GeO₄@0.07CeO₂ exhibits a similar CO yield over four cycles (Figure 3b). Moreover, there is no significant morphological change in Zn₂GeO₄@0.07CeO₂ after reaction (Figure S10). A comparison of the XRD and XPS spectra of Zn₂GeO₄@0.07CeO₂ before and after reaction indicates that there is no significant change, further demonstrating the structural stability of Zn₂GeO₄ (Figures S11 and S12).

The UV-vis spectra depict the light absorption capacity of the samples. Among them, Zn₂GeO₄@0.07CeO₂ exhibits two absorption edges, which can be respectively attributed to Zn₂GeO₄ and CeO₂ (Figure 4a). The formation of heterojunctions expands the absorption spectrum, thereby enhancing the light absorption performance of Zn₂GeO₄. In addition, the results demonstrate that the absorption edge of Zn₂GeO₄@0.07CeO₂ undergoes a slight red shift as the amount of CeO₂ increases, which can be attributed to the bandgap narrowing caused by the gradual enhancement of the absorption contribution of CeO₂ (Figure S13). As shown in Figure 4b, Zn₂GeO₄@0.07CeO₂ demonstrates a higher photocurrent density compared to Zn₂GeO₄ and CeO₂, further confirming the highly efficient transfer of photogenerated charge and separation of carriers occurring on the Zn₂GeO₄@0.07CeO₂ nanorods. Such efficiency can lead to more effective photogenerated electrons to enhance the ability of photocatalytic CO₂ reduction [40]. Figure 4c demonstrates that Zn₂GeO₄@0.07CeO₂ has a smaller electrochemical impedance spectroscopy (EIS) radius than that of Zn₂GeO₄. This also implies the effective separation and transfer of charge carriers in Zn₂GeO₄@0.07CeO₂. The above results prove that the formation of Zn₂GeO₄@0.07CeO₂ enhances the photocatalytic capability, which is also consistent with the results of the XPS analysis [35,41,42]. The photoluminescence (PL) intensity reveals the defect emission of Zn₂GeO₄, indicating a strong effect of carrier recombination [43]. In comparison, the decrease in the PL of Zn₂GeO₄@0.07CeO₂ demonstrates the extension of the carrier lifetime [15]. Moreover, the CO₂ temperature-programmed desorption (TPD) reflects that the adsorbed HCO₃⁻ peak area increases with the increase in CeO₂ (Figure 4e). This suggests that CeO₂ provides abundant active sites for CO₂ adsorption and activation [24,44,45].

The band structure of Zn₂GeO₄ and CeO₂ was conducted by Mott–Schottky spectrum measurements. The plots of Zn₂GeO₄ and CeO₂ exhibit positive slopes, suggesting that they are n-type semiconductors (Figure 4f) [7,17,44]. The flat-band potentials (E_fb) of Zn₂GeO₄ and CeO₂ are −0.82 and −0.65 V (vs. normal hydrogen electrode (NHE)), respectively [9,23,46,47]. The E_fb could roughly be equal to the conduction band (CB) potential of an n-type semiconductor [7,17,18]. Moreover, the band gaps (Eg) of Zn₂GeO₄ and CeO₂ are determined by the UV-vis spectra and Tauc plots (Figure 4a), which are 4.58 eV and 2.85 eV, respectively [47,48]. Therefore, the valence band maximum (VBM) potentials of Zn₂GeO₄ and CeO₂ are 2.20 and 3.76 V (vs. NHE), respectively. Due to the standard reduction potential of CO₂/CO being −0.52 V (vs. NHE at pH = 7), the photocatalytic CO₂ reduction can thermodynamically proceed under the photocatalysis of Zn₂GeO₄ and CeO₂, and this is consistent with the experimental results [1,4]. The band structures in Figure 4i show that Zn₂GeO₄ and CeO₂ should be ascribed to Type-I heterojunction. Under light excitation, the electrons of Zn₂GeO₄ and CeO₂ will be excited from the valence band to the conduction band, respectively [42]. Due to the different conduction band energies, the photogenerated electrons with higher energy on the conduction band of Zn₂GeO₄ will migrate to the conduction band of CeO₂, thus extending the carrier lifetime. The carriers will accumulate at the heterojunction interface of CeO₂. Furthermore, the migration rate of holes is generally about two orders of magnitude slower than that of electrons. This effectively reduces the probability of carrier recombination [11,49]. The formation of heterojunction endows Zn2GeO4@0.07CeO2 with superior catalytic performance (Tables S3 and S4) [50,51,52,53,54,55,56,57,58,59,60].

To further clarify the reaction mechanism, in situ FT-IR measurements were performed on Zn₂GeO₄@0.07CeO₂ and Zn₂GeO₄ during the CO₂ photoreduction process. As shown in Figure 4g, the peak attributed to COOH^* was detected at 1550 cm⁻¹ [61]. The COOH^* species are considered to be a key intermediate during CO₂ reduction to CO. The bands located at 1478 cm⁻¹ and 1637 cm⁻¹ can be assigned to HCO₃^* species [9]. A negative peak corresponding to CO₂^* can be observed at 1683 cm⁻¹, which can be attributed to the CO₂ consumption by Zn₂GeO₄@0.07CeO₂, demonstrating the high activity and fast reaction rates [62]. Furthermore, the distinct peaks observed at 1302 cm⁻¹ and 1350 cm⁻¹ can be assigned to the bidentate carbonate species (b-CO₃²⁻), which indicates the abundant oxygen vacancies on the surface of Zn₂GeO₄@0.07CeO₂ [62,63]. There will be a large amount of CO₂ adsorbed near the oxygen vacancy of CeO₂, and this adsorption is further enhanced due to the electron accumulation near the interface [24,62]. In contrast, Zn₂GeO₄ demonstrates more pronounced characteristic peaks indicative of intermediate species diversity, which correlates well with the poor selectivity. Moreover, Zn₂GeO₄ exhibits a certain capability to catalyze water splitting, thereby providing an abundant source of hydrogen species for the hydrogenation of adsorbed CO₂ molecules, leading to the formation of CH₄ [9,47]. Furthermore, due to the presence of asymmetric Zn-O-Ge sites, the CO* intermediates adsorbed on the Zn and Ge atoms exhibit different charge distributions, which facilitates C-C coupling [9]. As a result, Zn₂GeO₄ is more inclined to produce C₂ products compared to CeO₂. After coating with CeO₂, the surface CeO₂ serves as the adsorption and activation site for CO₂ reduction, with CeO₂ favoring the production of CO, thus altering the reaction pathway selectivity and reducing byproduct intermediates. Based on the in situ FT-IR spectra and the above analysis, the specific reaction pathway of Zn₂GeO₄@0.07CeO₂ and Zn₂GeO₄ is depicted in Figure 4j [61,62,63,64,65].

3. Materials and Methods

3.1. Synthesis of Zn₂GeO₄

A total of 1100 mg of Zn(CH₃COO)₂·2H₂O was added to 25 mL of deionized water and stirred magnetically for 10 min, and then 450 mg of GeO₂ was added to the solution and stirred for 20 min. Next, 300 mg of sodium oleate (NaOA) was added slowly and stirred for another 20 min. Then, 5 mL of 3 mol L⁻¹ NaOH aqueous solution was added dropwise to maintain the pH value at 7.5. After stirring for 20 min, the mixture was transferred to a 50 mL hydrothermal high-pressure reactor, sealed and heated at 140 °C for 24 h, and cooled naturally to room temperature. Then, the mixture was centrifuged and washed twice with 5 mol L⁻¹ NaOH solution and dispersed into ethanol and water for washing. The sample was centrifuged five times and dried in a vacuum. Finally, the product was calcined in a N₂ atmosphere at 400 °C for 3 h.

3.2. Synthesis of CeO₂

A total of 1 mmol of Ce(NO₃)₃·6H₂O was dissolved in a mixed solution of 20 mL of water and 20 mL of ethanol. Next, 25 mL of 0.02 g L⁻¹ hexamethylenetetramine (HMT) solution was added. The mixture was heated to 70 °C, held for 2 h, and then cooled to room temperature. The product was purified by centrifugation, washed three times with water, and dried at 60 °C. Finally, the product was calcined in a N₂ atmosphere at 400 °C for 3 h.

3.3. Synthesis of Zn₂GeO₄@xCeO₂

First, 50 mg of uncalcined Zn₂GeO₄ crystals was dispersed by ultrasound in a mixed solution of 40 mL of water and 40 mL of ethanol. Next, 0.05 mmol Ce(NO₃)₃·6H₂O and 0.1 mmol HMT were added sequentially. Then, the mixture was heated at 60 °C for 2 h and cooled to room temperature. The product was purified by centrifugation, washed three times with water, and dried at 60 °C. Finally, the product was calcined in a N₂ atmosphere at 400 °C for 3 h. The prepared samples are named Zn₂GeO₄@0.05CeO₂ based on the amount of added Ce(NO₃)₃·6H₂O. Using the same method, Zn₂GeO₄@0.07CeO₂ and Zn₂GeO₄@0.1CeO₂ were prepared by controlling the amount of added Ce(NO₃)₃·6H₂O and HMT, where x represents the amount of Ce added per 50 mg of Zn₂GeO₄. The corresponding amount of HMT is added proportionally to the amount of Ce(NO₃)₃·6H₂O, maintaining a molar ratio of 2:1 between HMT and Ce(NO₃)₃·6H₂O.

3.4. Characterization

The XRD patterns were obtained on an XRD-6000 (Shimadzu Corporation, Kyoto, Japan) X-ray diffractometer. The morphologies of the samples were measured by a field-emission scanning electron microscope on the Quanta 250FEG scanning electron microscope (FEI, Hillsboro, OR, USA). The XPS measurements were performed by a K-Alpha electron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The quantification of liquid products was performed using NMR spectroscopy (Bruker Corporation, Billerica, MA, USA) with dimethyl sulfoxide as the internal standard. The UV-vis analysis was carried out on a UV-300 UV Visible Spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The CO₂-TPD experiments were conducted in a quartz tube reactor under a flow of 20 vol % of CO₂/Ar (20 mL/min) over 100 mg of catalysts, and the desorption was conducted under a flow of N₂ (20 mL/min). The photoluminescence (PL) emission spectra of samples were collected using an FLS-1000 luminescence spectrometer (Edinburgh Instruments, Edinburgh, UK). The in situ FT-IR spectra were measured through Fourier transform infrared spectroscopy (Bruker Corporation, MA, USA).

3.5. Photocatalytic Performance Measurements

A total of 25 mg of the photocatalyst samples and 20 mL of deionized water with 10 mL of trolamine were added into a 230 mL reactor (CEL-APR100H, China Education Au-light Technology Co. Ltd, Beijing, China) under ultrasonic dispersion. After that, high-purity CO₂ gas was injected for 30 min to achieve a partial pressure of 1.0 atm. The reactor was irradiated by a 300 W Xe lamp (CEL-PUV300-T8, China Education Au-light Technology Co., Ltd., Beijing, China) with circulating cooling water to keep it at room temperature. After the irradiation reaction had occurred for 3 h, 1 mL of the evolved gas was pumped from the reactor and examined by a gas chromatograph.

For the cycle tests, the photocatalyst after reaction is centrifugally separated, washed with deionized water and ethanol sequentially, and then vacuum-dried prior to subsequent testing under the same conditions.

The gas product (CO, CH₄, H₂, C₂) yield (μmol g⁻¹ h⁻¹) and product selectivity of gas product $i$ were calculated using the following equations:

$${Yield}_i={\displaystyle \frac{c_i\times \left(\frac{pV}{RT}\right)}{Wt}}\times {10}^6$$

(1)

$${Selectivity}_i={\displaystyle \frac{ c_i\times \left(\frac{pV}{RT}\right)\times number of electrons}{n_{CO}\times 2+n_{{CH}_4}\times 8+n_{H_2}\times 2+n_{C_2{H}_6}\times 14+n_{C_2{H}_4}\times 12}}$$

(2)

Here, $c_i$(ppm) represents the molar concentration of $i$ obtained by gas chromatography (GC), $p$ (Pa) is the pressure inside the reaction system, $V$ (m³) is the volume of the reactor, $R$ is the universal gas constant, $T$ (K) is the temperature under light irradiation, $W$ (g) is the mass of the catalyst, $t$ (h) is the reaction time, and $n_i$ is the mole number of $i$.

3.6. Photoelectrochemical Measurements

For the photoelectrochemical testing, 5 mg of the sample was added to a solution of 1000 μL of isopropanol and 50 μL of naphthol, sonicated for 40 min, and stirred for 5 min. After uniform dispersion, 80 μL of the dispersion solution was applied to an ITO glass slide to form a square area of 1 cm × 1 cm and dried for 15 min. Photoelectrochemical testing was conducted using a CH650E electrochemical workstation in a three-electrode system, with the electrolyte being 0.5 M sodium sulfate solution. A Ag/AgCl electrode and Pt electrode were used as the reference electrode and counter electrode, respectively. The ITO glass sheet coated with the sample was used as the working electrode. The electrochemical impedance test was conducted under a forward bias voltage of the open circuit voltage, amplitude of 0.005 V, and frequency range of 0.001–100,000 Hz. The photocurrent test was conducted under a bias voltage of 0 V, using a 300 W xenon lamp (CEL-PUV300-T8, China Education Au-light Technology Co. Ltd., Beijing, China) to illuminate the working electrode, and turning on/off the light source every 30 s for testing. Mott–Schottky measurements were also tested at frequencies of 1000/2000/3000 Hz.

4. Conclusions

In this work, Zn₂GeO₄@CeO₂ nanorods with a special core@shell structure have been successfully fabricated through a simple self-assembly method. By precisely controlling the amount of Ce(NO₃)₃·6H₂O, a series of rod-shaped Zn₂GeO₄@CeO₂ composites with varying amounts of CeO₂ can be obtained. The photocatalytic CO₂ reduction experiments demonstrated that an appropriate CeO₂ thickness is beneficial for catalytic performance, in which Zn₂GeO₄@0.07CeO₂ exhibited the highest activity with a CO yield of 1190.9 μmol g⁻¹ h⁻¹. Further characterization indicates that Zn₂GeO₄ and CeO₂ formed a Type-I heterojunction. The excited electrons will transfer from Zn₂GeO₄ to CeO₂, which could extend the carrier lifetime. Furthermore, the CO₂ TPD results demonstrated the enhanced CO₂ adsorption capacity of the CeO₂ shell. Finally, the in situ FT-IR spectra confirmed the high CO selectivity of Zn₂GeO₄@0.07CeO₂ and demonstrated that the CeO₂ coating modifies the pathway selectivity of CO₂ adsorption and reduction, suppresses the formation of byproduct intermediates, and thereby achieves enhanced CO selectivity.