Hierarchical Ag₃VO₄ Nanorods as an Excellent Visible Light Photocatalyst for CO₂ Conversion to Solar Fuels

Abstract

The potential of photocatalytic CO₂ conversion is significant for the production of fuels and chemicals, while simultaneously mitigating CO₂ emissions and addressing environmental concerns. Despite the current drawbacks of single metal-based photocatalysts, such as lower performance, uncontrollable selectivity, and instability, this study focuses on the synthesis of Ag₃VO₄ nanorods using the sol–gel method. The goal is to create a highly effective catalyst for visible light-responsive CO₂ conversion. The successful synthesis of Ag₃VO₄ nanorods with a nanorod structure, functional under visible light, resulted in the highest yields of CH₄ and dimethyl ether (DME) at 271 and 69 µmole/g-cat, respectively. The optimized Ag₃VO₄ nanorods demonstrated performance improvements, with CH₄ and DME production 6.4 times and 4.5 times higher than when using V₂O₅ samples. This suggests that Ag₃VO₄ nanorods facilitate electron transfer to CO₂, offer short pathways for electron transfer, and create empty spaces within the nanorods as electron reservoirs, enhancing the photoactivity. The prolonged stability of Ag₃VO₄ in the CO₂ conversion system confirms that the nanorod structure provides controllable selectivity and stability. Therefore, the fabrication of nanorod structures holds promise in advancing high-performance photocatalysts in the field of photocatalytic CO₂ conversion to solar fuels.

1. Introduction

The exploration of the photocatalytic reduction of carbon dioxide into valuable chemicals and fuels has become a highly focused area of study in response to concerns about global warming and energy shortages [1,2,3,4,5,6]. In recent years, there has been significant interest in artificial photosynthesis processes aimed at converting CO₂ into beneficial carbon sources like CH₄ and DME [7,8,9,10,11]. These processes are considered attractive alternatives as they contribute to the need for techniques that recycle CO₂ as a natural resource [12,13,14,15,16]. Solar photocatalytic CO₂ reduction into high-value fuels stands out as a particularly appealing strategy [17,18,19]. Consequently, numerous research efforts have been dedicated to developing efficient photocatalysts for CO₂ reduction, including V₂O₅ [20], TiO₂ [21,22,23], Ta₂O₅ [24,25], ZnO [26,27,28], WO₃ [29,30], and CdS [31]. Notably, V₂O₅ has been extensively studied as a semiconductor due to its potent oxidation and reduction capabilities. Furthermore, it is environmentally friendly [32], cost-effective, and can be synthesized in various nanostructures through economical methods. However, its high charge recombination rate has led to subpar photocatalytic performance. To overcome these challenges, various approaches, such as depositing noble metals, surface photosensitization, and forming carbon-based composites, have been explored.

One effective strategy to enhance the activity of vanadium oxide (V₂O₅) in the presence of visible light involves combining it with semiconductors possessing low band gap energy. This combination extends the optical absorption range into the visible portion of the solar spectrum. A promising candidate for this purpose is silver oxide (Ag₂O), a transition metal oxide known for its relatively narrow band gap of approximately 1.89 eV [33,34]. Combining vanadium oxide (V₂O₅) with Ag₂O has the potential to significantly improve the photoactivity when exposed to visible light.

In recent times, there has been significant interest in hierarchical nanostructures such as nanosheets, nanorods, and nanoplates, particularly in the realm of photochemical and catalytic applications [35,36,37]. Ag₃VO₄ nanostructures have shown great promise as efficient photocatalysts due to their unique structural properties, which enhance both the photoactivity and selectivity when compared to other materials like V₂O₅/Ag₂O. The appeal of Ag₃VO₄ lies in its intriguing structural transformations at low temperatures [37], making it a versatile catalyst for a range of reactions. Ag₃VO₄ has been effectively employed in various catalytic applications, including the degradation of organic pollutants and the reduction of CO₂, highlighting its potential in environmental remediation [38,39,40]. While existing studies on Ag₃VO₄-based photocatalysts have explored their use in CO₂ reduction [41], our study specifically focuses on novel structured Ag₃VO₄ photocatalysts, aiming to bridge the gap in understanding their potential for photocatalytic CO₂ conversion under solar energy conditions. Furthermore, the introduction of CH₄ and DME as feedstocks in catalytic processes has gained attention due to their roles in producing valuable chemicals and fuels [42]. Methane (CH₄) is a key component in natural gas and serves as a primary feedstock for hydrogen production, while dimethyl ether (DME) is a promising alternative fuel with applications in combustion engines. By integrating Ag₃VO₄ nanostructures, we aim to enhance the efficiency of these processes, offering a sustainable pathway for the conversion of CH₄ and DME into higher-value products.

Herein, we successfully created and produced innovative Ag₃VO₄ nanorods using the sol–gel method. We examined these materials for their ability to catalyze the reduction of CO₂ by H₂O into fuels when exposed to visible light. The Ag₃VO₄ nanorods demonstrated improved photocatalytic efficiency in converting CO₂ into CH₄ and DME. The notable enhancement in the photocatalytic efficiency was attributed to the exceptional hierarchical nanostructure inherent in Ag₃VO₄ nanorods. This research also systematically investigated the influence of the catalyst content on the photocatalytic activity and stability of the Ag₃VO₄ nanorods.

2. Results and Discussion

2.1. Catalyst Characterization

X-ray diffraction (XRD) analysis was employed to investigate the crystal phase and structure of the photocatalysts. The XRD patterns of the Ag₃VO₄ photocatalyst are depicted in Figure 1. The pure Ag₃VO₄ displayed distinct peaks at 2θ values of 19.6°, 21.6°, 31.7°, 32.6°, 35.3°, 38.9°, 41.5°, 43.3°, 46.1°, 53.7°, and 54.05°, corresponding to the (011), (−111), (−121), (121), (301), (022), (320), (400), (−213), and (132) crystal planes of monoclinic Ag₃VO₄ (JCPDS:43-0542) [43,44], respectively.

The investigation of the Ag₃VO₄ nanorods involved the examination of the morphology and microstructural features using FESEM and HRTEM. It can be seen in Figure 2a that the FESEM image of Ag₃VO₄ contains a large number of nanorods with 1D nanostructures. The elemental composition, determined through EDX analysis and shown in Figure 2b,c, identified silver, vanadium, and oxygen. The TEM images of the pure Ag₃VO₄ photocatalyst, prepared after 24 h of stirring, are presented in Figure 2d–f. Figure 2d displays the microstructure of Ag₃VO₄, composed of numerous compact, smaller nanorods. The interplanar distance was measured at 0.263 nm, corresponding to the (220) plane of Ag₃VO₄, as presented in Figure 2e. The SAED pattern in Figure 2f exhibits a clear polycrystalline ring, indicating the good crystallization of Ag₃VO₄.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the elemental composition and valence states of Ag₃VO₄, as depicted in Figure 3. The overall survey spectra (Figure 3a) reveal the presence of Ag, V, and O in the composite. The high-resolution XPS spectra of Ag 3d, V 2p, and O ls were further examined. In Figure 3b, the high-resolution Ag 3d spectra exhibit distinctive binding energy values of 369.5 and 375.4 eV, corresponding to Ag 3d 5/2 and Ag 3d 3/2 binding energies [44,45], respectively. The peaks observed at 517.9 and 524.1 eV in the high-resolution XPS spectra for V 2p (Figure 3c) are attributed to V 2p 3/2 and V 2p 1/2, aligning with the characteristic features of V⁵⁺ [46,47]. Examining the high-resolution XPS of O1s for Ag₃VO₄ (Figure 3d), two deconvoluted peaks at 531.40 and 529.85 eV are identified, associated with surface-adsorbed oxygen and lattice oxygen, respectively.

Figure 4a illustrates the UV–visible diffuse reflectance absorbance spectra for the Ag₃VO₄ photocatalysts. It is evident that the Ag₃VO₄ nanorods exhibit strong absorption intensities within the visible light range. The energy band gap (E_bg) of the Ag₃VO₄ photocatalyst was determined using the Tauc equation, as depicted in Equation (1).

$${\mathrm{E}}_{\mathrm{g}}(\mathrm{e}\mathrm{V})={\displaystyle \frac{1240}{\lambda }}$$

(1)

The Ag₃VO₄ photocatalyst exhibits a wavelength of 563 nm, corresponding to a calculated E_g value of 2.2 eV. The conduction band position (E_CB) of the Ag₃VO₄ semiconductor was determined using Equation (2).

$${\mathrm{E}}_{\mathrm{V}\mathrm{B}}={\mathrm{E}}_{\mathrm{C}\mathrm{B}}+{\mathrm{E}}_{\mathrm{b}\mathrm{g}}$$

(2)

Valence band X-ray photoelectron spectroscopy (VB-XPS) was utilized to examine the valence band of the Ag₃VO₄ photocatalyst. As illustrated in Figure 4b, the valence band edges of Ag₃VO₄ were observed to be situated at around 1.86 eV. The energy band gap (Ebg) for Ag₃VO₄ was determined to be 2.2 eV. Therefore, the conduction band of the Ag₃VO₄ photocatalyst was calculated to be at −0.34 eV.

Figure 5 illustrates the photoluminescence (PL) analysis of V₂O₅ and Ag₃VO₄ samples excited at 325 nm. The pure V₂O₅ exhibits broad and intense PL signals in the same spectral regions. In contrast, the Ag₃VO₄ nanorods exhibit a lower PL emission intensity compared to V₂O₅. The diminished intensity of the peak signifies a lower likelihood of free charges recombining, suggesting that the sol–gel method, involving the coupling of vanadium with Ag₂O to form the ternary Ag₃VO₄ structure, effectively mitigates the recombination of photogenerated electron–hole pairs.

2.2. Photocatalytic Activity and Stability

The experiments examining the conversion of CO₂ with H₂O were conducted in the presence of visible light. During these tests, it was observed that when the lamp was turned off, there were no carbon-containing compounds present in the reaction system. Conversely, under light exposure, the consistent generation of CH₄ and dimethyl ether (DME) was identified. This observation indicates that both the samples and the photoreactors were effectively cleaned, and any carbon-containing compounds were exclusively formed through the reduction of CO₂.

Figure 6 illustrates the impact of varying V₂O₅ and Ag₃VO₄ samples on the photoactivity of photocatalytic CO₂ conversion under visible light. The assessment of the photocatalysts focused on the yields of CH₄ and DME, the two resulting products. The pure V₂O₅ photocatalyst, synthesized through the sol–gel process, demonstrated minimal CO₂ reduction and exhibited poor efficiency in generating CH₄ and DME. In contrast, the utilization of the innovative Ag₃VO₄ nanorods significantly increased the production of CH₄ and DME. This improvement can be ascribed to the nanorods’ effective absorption of visible light, proficient charge transfer characteristics, and heightened electron mobility achieved through the coupling of vanadium with silver using the sol–gel method.

Figure 7 depicts the impact of varying photocatalyst preparation durations (6 h, 12 h, and 24 h) under room conditions and atmospheric pressure on the Ag₃VO₄ photocatalyst intended for CO₂ conversion with H₂O under visible light. The efficiency analysis of different stirring durations focused on the yields of CH₄ and DME, the two products resulting from the process. The findings reveal that the Ag₃VO₄ sample stirred for 24 h exhibits significantly enhanced photoactivity in the evolution of CH₄ and DME compared to samples stirred for 6 h and 12 h. Consequently, the Ag₃VO₄ sample prepared with a stirring duration of 24 h was chosen for further investigation into the effects of the irradiation time and stability analysis.

Figure 8 demonstrates the impact of the irradiation time on the visible-light-driven photocatalytic conversion of CO₂ with H₂O to CH₄ and DME over Ag₃VO₄ nanorods. The gradual increase in CH₄ and DME generation becomes evident as the irradiation time extends. When employing Ag₃VO₄ nanorods, CH₄ emerges as the main product in the process of CO₂ photoreduction. Significantly, the catalyst exhibits sustained activity even after 4 h of continuous irradiation, ensuring the ongoing production of CH₄ and DME. Consequently, these innovative Ag₃VO₄ nanorods offer heightened photoactivity and stability, thereby enhancing the conversion of CO₂ into solar fuels.

Figure 9 illustrates the cycling performance of hierarchical Ag₃VO₄ nanorods in the continuous generation of CH₄ and DME under visible light. To assess the stability, the reactor and catalyst were purged with nitrogen gas for 30 min after each photocatalytic reaction before initiating the next cycle. It is important to note that there was no significant decrease in catalytic activity observed during the stability test over four consecutive runs. While the CH₄ production remained consistent throughout all cyclic runs, the DME production showed a slight decline after the third cycle. This decline may be attributed to the reduced adsorption capacity of Ag₃VO₄ towards CO₂, possibly resulting from the inefficient desorption of intermediate products and hydrocarbons within the vacant spaces of the nanorods. Overall, the prolonged stability of the hierarchical Ag₃VO₄ nanorods in this study can be attributed to the gas–solid heterogeneous system and the use of heterostructure materials. Consequently, the enhanced photoactivity and selectivity of Ag₃VO₄ nanorods are evidently linked to their hierarchical structure, which facilitates the dynamic movement of electron–hole pairs and hinders their recombination under visible light.

2.3. Reaction Mechanism

Ag₃VO₄ nanorods serve as photocatalysts for the assessment of photocatalytic activity through CO₂ conversion to CH₄ and DME. In the course of the reduction process, key reaction steps are observed, which are succinctly outlined in Equations (3)–(7).

$${\mathrm{A}\mathrm{g}}_3{\mathrm{V}\mathrm{O}}_4+\mathrm{h}\mathrm{v}\to {\mathrm{h}}^{+}{+\mathrm{e}}^{-}$$

(3)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to {}^{•}\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}$$

(4)

$${\mathrm{CO}}_2{+\mathrm{e}}^{-}\to {{\mathrm{CO}}_2^{•}}^{-}$$

(5)

$${\mathrm{CO}}_2{+8\mathrm{H}}^{+}{+8\mathrm{e}}^{-}\to {\mathrm{CH}}_4{+\mathrm{H}}_2\mathrm{O}$$

(6)

$${\mathrm{CO}}_2{+12\mathrm{H}}^{+}{+12\mathrm{e}}^{-}\to {\mathrm{C}}_2{\mathrm{H}}_6{\mathrm{O}+\mathrm{H}}_2\mathrm{O}$$

(7)

Equation (3) illustrates the generation of electron–hole pairs upon photoexcitation. The conversion of CO₂ takes place in the conduction band through electron participation, whereas holes in the valence band facilitate the oxidation of H₂O, as elucidated in Equations (4) and (5). The mechanisms for the production of CH₄ and DME via the reduction of CO₂ involving 6, 8, and 12 electrons are detailed in Equations (6) and (7). The investigation of the photoactivity and reaction pathways provides valuable insights into the reaction mechanism.

The diagram in Figure 10 illustrates the reaction mechanism. When exposed to visible light, electrons excited from the valence band (VB) of the Ag₃VO₄ nanorods migrate to the conduction band (CB). Holes in the VB of the Ag₃VO₄ nanorods interact with H₂O, leading to the generation of O₂ and H⁺. Concurrently, absorbed CO₂ molecules undergo reduction to form CH₄ and DME, facilitated by the enriched electrons on the surface of Ag₃VO₄. In the context of CO₂ reduction with H₂O, the Ag₃VO₄ nanorods predominantly yield CH₄ as the main product, likely due to the suitable reduction potential of CO₂/CH₄ (−0.24 V). The reaction is more favorable for CH₄ production as the reduction potential of CO₂/CH₄ (−0.24 V) is lower than the conduction band of Ag₃VO₄. While C₂H₆O requires more electrons and has a conduction band closer to that of CH₄ compared to Ag₃VO₄ nanorods, the proper band alignment of Ag₃VO₄ contributes to the selective production of CH₄ during CO₂ conversion under visible light. Consequently, the novel Ag₃VO₄ nanorods exhibit significantly enhanced CH₄ production due to effective visible light absorption, a suitable band structure, and higher electron mobility with inhibited recombination.

3. Experimental Section

3.1. Chemicals

The chemicals used for the preparation of the Ag₃VO₄ photocatalyst were silver nitrate (AgNO₃—purity 99%) and ammonium vanadate (NH₄VO₃—purity 99%) from Sigma Aldrich (Saint Louis, MO, USA).

3.2. Synthesis of Ag₃VO₄ Nanorods

Ag₃VO₄ nanorods were prepared through the sol–gel method. Typically, 10 g AgNO₃ was added to 40 mL of distilled water and stirred for 45 min to form solution A. Simultaneously, 2.3 g NH₄VO₃ was dissolved in 40 mL of distilled water and stirred for 45 min to form solution B. After both solutions were prepared, solution B was carefully transferred to solution A in a dropwise manner while continuously stirring to ensure uniform mixing. The combined solution was then stirred continuously for 24 h. Finally, the obtained products were washed with distilled water and dried overnight at 80 °C for 12 h. Pristine Ag₃VO₄ nanorods were obtained. The schematic for the fabrication of the Ag₃VO₄ nanorods is illustrated in Figure 11.

3.3. Material Characterization

The crystalline structures of the photocatalysts were investigated with X-ray diffraction (XRD) using the Rigaku Mini Flex II (Woodlands, TX, USA) at 30 kV and 15 mA, employing Cu Kα radiation (λ = 1.5406 Å). The structure and morphology of the samples were assessed through field emission scanning electron microscopy (FESEM) using the Tescan (Brno, Czech Republic) Mira 3 LMU and high-resolution transmission electron microscopy (HRTEM) with the JEM-2100 F model (JEOL Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical states of the samples, utilizing the JEOL (Peabody, MA, USA) JPS-9200 Photoelectron Spectrometer. UV–vis diffuse reflectance (DR) spectra were obtained using an Agilent (Santa Clara, CA, USA) UV–vis spectrophotometer (Cary 100 model) equipped with an integrating sphere. Photoluminescence (PL) spectra were taken using a HORIBA (Irvine, CA, USA) spectrometer operating at a wavelength of 325 nm.

3.4. Photocatalytic Activity Test

The as-prepared photocatalysts were tested for the photocatalytic conversion of CO₂ using a gaseous-phase fixed-bed photoreactor. To activate the photocatalytic reactions, a 50 W LED flood light was used as the light source and the focused light intensity at the surface of the photocatalyst was 32 mW/cm². A quartz window was employed to optimize the light exposure. A total of 150 mg of powder photocatalyst was evenly distributed at the bottom of the photoreactor to ensure uniform exposure. Prior to introducing the reactant gases, the reactor was purged with nitrogen to remove residual gases. High-purity compressed CO₂, regulated by a mass flow controller, was passed through a water saturator to introduce moisture. For continuous CO₂ reduction, the gas flowed through the reactor at a rate of 10 mL/min. Product analysis was conducted using an online gas chromatograph with FID and TCD detectors (GC/FID/TCD). No carbon-containing products were detected in the absence of light irradiation or catalysts.

The as-prepared photocatalysts were tested for the photocatalytic conversion of CO₂ using a gaseous-phase fixed-bed photoreactor. A 50 W LED flood light, coupled with a concentrator, served as the illumination source to activate the photocatalytic reactions, with a quartz window optimizing light involvement. First, 150 mg of powder photocatalyst was dispersed at the bottom of the photoreactor to ensure proper distribution. Prior to introducing the reactant gases, the reactor was purged with nitrogen to remove other gases. High-purity compressed CO₂, regulated by a mass flow controller, was passed through a water saturator to introduce moisture. For continuous CO₂ reduction, the gas flowed through the reactor at a rate of 10 mL/min. Product analysis was performed using an online gas chromatograph with FID and TCD detectors (GC/FID/TCD). In the absence of light irradiation or catalysts, carbon-containing products were not detected.

4. Conclusions

In this study, we successfully employed the sol–gel method to create Ag₃VO₄ hierarchical nanorods for the purpose of the photocatalytic reduction of CO₂ with H₂O in a fixed-bed photoreactor. The Ag₃VO₄ nanorods exhibited improved photocatalytic performance and suppressed charge recombination rates during the process of converting CO₂ with H₂O into CH₄ and DME. The hierarchical structure of the Ag₃VO₄ nanorods specifically favored efficient and stable CH₄ production. Under visible light, the highest efficiency for CH₄ production with the Ag₃VO₄ nanorods reached a yield rate of 271 μmol/g-cat. The elevated photocatalytic performance and stability of the Ag₃VO₄ nanostructure can be ascribed to its hierarchical composition, which enhances charge separation. This research suggests that hierarchical ternary materials hold promise as effective photocatalysts for the solar-driven reduction of CO₂ into valuable solar fuels.