IntroductionA photocatalytic material of Ag₁₀Si₄O₁₃ is widely concerned due to its band structure suitable for visible light response and built-in electric field promoting photogenerated carrier separation. However, in the photocatalysis process, Ag₊ in Ag₁₀Si₄O₁₃ is easily reduced by photoelectrons and accumulates Ag elements, affecting the cycle stability of the photocatalytic system. Graphene oxide can effectively transfer photogenerated electrons, and amorphous SiO₂ can inhibit ion diffusion leaching, which are expected to slow down the photoreduction of Ag₁₀Si₄O₁₃. In this paper, x% SiO₂/Ag₁₀Si₄O₁₃/GO (SAG) composites with different amounts of amorphous SiO₂ were prepared via adding graphene oxide (GO) and silica sol in the process of Ag₁₀Si₄O₁₃ preparation by a sol-gel method. The effect of amorphous SiO₂ addition on the crystal structure, morphology and photoelectrochemical properties was investigated. Methylene blue (MB) was used as a target. The results show that a small amount of amorphous SiO₂ can greatly improve the photoreduction resistance of Ag₁₀Si₄O₁₃ photocatalysts, and broaden the photoresponse range and increase the adsorption capacity of the material for MB. However, the addition of amorphous SiO₂ can simultaneously reduce the migration rate of photogenerated carriers and accelerate the recombination of photogenerated electron-hole pairs. Therefore, the photocatalytic degradation efficiency of the composite system firstly increases and then decreases with the increase of SiO₂ addition. The degradation rate of SiO₂/Ag₁₀Si₄O₁₃/GO (SAG-1) samples can reach 99% under visible light for 40 min, showing a good cyclic stability. After 5 cycles, the crystal structure of the materials becomes stable and the degradation rate can still reach 97%.MethodsIn pre-hydrolysis of ethyl orthosilicate: 0.47 g of citric acid was added to 26 mL of anhydrous ethanol under magnetic stirring until complete dissolution, and then 5 mL of ethyl orthosilicate and 1.65 mL of distilled water were added in dropwise to the solution and the solution was stirred for 1 h and sealed for 1 week aging to obtain the silica sol. In preparation of Ag₁₀Si₄O₁₃/GO, 2.1 mL of silica sol and GO sol containing 1%(in mass) Ag₁₀Si₄O₁₃ were added to 10 mL of distilled water under ultrasound and magnetic stirring. 1.6 mL of 0.38 g/mL AgNO₃ solution was added in dropwise to the mixed solution above under stirring for 1 h and aging for 24 h to obtain the precursor sol. The precursor sol was dried, ground and heat-treated at 400 °C for 5 h to obtain a brick-red Ag₁₀Si₄O₁₃/GO powder=(i.e., AG). In preparation of SiO₂/Ag₁₀Si₄O₁₃/GO, x%(in mass) Ag₁₀Si₄O₁₃ (x=1, 2, 3, 4) of silica sol was added, stirred continuously for 1 h and aged for 24 h to obtain x% SiO₂/Ag₁₀Si₄O₁₃/GO precursor sol. The subsequent steps were the same as for AG to obtain x% SiO₂/Ag₁₀Si₄O₁₃/GO powders (i.e., SAG-1, SAG-2, SAG-3, and SAG-4), respectively.The crystal structure of the materials was characterized by a model D8/axs X-ray diffractometer (XRD, Bruker Co., Germany). The microscopic morphology of the materials was observed by a model JSM-6701F transmission electron microscope (TEM, Electro-Optics Co., Japan). The chemical state was analyzed by a model PHI5702 X-ray photoelectron spectrometer (XPS, Physical Electronics Inc., USA). The photoresponsive properties were determined by a model U-3900H diffuse reflectance spectrometer (UV-Vis, Hitachi High-Technologies Co., Japan). The fluorescence emission spectra of the materials were determined by a model F97 fluorescence spectrophotometer (FSP, Shanghai Prismatic Technology Co., Ltd., China). The impedance, transient photocurrent, and Mott-Schottky curves were determined by a model CHI660D electrochemical workstation (Shanghai Tatsuwa Instrumentation Co., Ltd., China).The sample of 0.1 g was weighed and stirred in 100 mL of 20 mg/L MB solution, which was protected from light. For every 10 min, 3 mL of the sample was removed and the supernatant was centrifuged to determine the concentration of MB by a UV spectrophotometer. The adsorption performance was evaluated by a ratio of the concentration of the solution after adsorption, c_t, to the initial concentration of MB, c₀, at time t (c_t/c₀). After dark adsorption equilibrium, a 300 W xenon lamp was used as a light source, a filter (λ>420 nm) was added to simulate visible light, the distance from the light source to the liquid surface was fixed at 15 cm, the sample of 3 mL was taken for every 5 min, and the supernatant was centrifuged to determine the concentration of MB by a UV spectrophotometer. The photocatalytic performance was evaluated by a ratio of the concentration of the degraded solution to the initial concentration of MB, c₀, at the time of t' (c_t/c₀).Five cycling experiments were performed on AG and SAG-1 to evaluate the stability of the photocatalytic properties of the materials. The stability of the crystal structure of the materials was evaluated based on the XRD patterns after each cycle.Results and discussionThe XRD patterns show that the introduction of SiO₂ has no effect on the crystal structure of Ag₁₀Si₄O₁₃. The TEM images of SAG-1 indicate the presence of graphene oxide, Ag₁₀Si₄O₁₃ and SiO₂. The UV-Vis diffuse reflectance spectra show that the edge of the absorption band of SAG-1 extended to 660 nm, which can be caused by the defects formed during the preparation of the material, but the forbidden bandwidth changes slightly, indicating that the protective layer of SiO₂ does not cause any significant changes in the band gap. The PL spectra show that the fluorescence intensity of the material increases gradually with the increase in the content of amorphous SiO₂, showing that the photogenerated carrier complexation rate increases with the increase of SiO₂ incorporation. The photocatalytic performance of Ag₁₀Si₄O₁₃ decreases significantly from 99% to about 90% after 5 cycles, while the photocatalytic performance of SAG-1 does not change after 5 cycles and still reaches 97%. The results show that the introduction of a small amount of SiO₂ can slow down the reduction production of Ag⁺ during the photocatalytic degradation of Ag₁₀Si₄O₁₃, and the crystal structure remains stable after some cycles, which greatly improves the stability of the material structure.ConclusionsSiO₂/Ag₁₀Si₄O₁₃/GO ternary composites with different SiO₂ composite amounts were synthesized by a sol-gel method with GO and excess silica sol in an one-pot method. The complexing agent citric acid was used as a "bridge" to allow Ag₁₀Si₄O₁₃ nanoparticles to grow uniformly on GO, and the excess silica sol was attached to the precursor surface, forming an amorphous SiO₂ protective layer after heat treatment. The introduction of the amorphous SiO₂ protective layer did not affect the crystal structure of Ag₁₀Si₄O₁₃, which could extend the visible-light response range of the composite photocatalyst and significantly improve the adsorption performance for cationic dyes. However, SiO₂ could reduce the migration rate of photogenerated carriers and accelerate the assembly of photogenerated electron-hole pairs, affecting the photocatalytic performance of the material. An appropriate amount of amorphous SiO₂ protective layer could improve the photoreduction resistance and cycling stability of Ag₁₀Si₄O₁₃ photocatalysts, while maintaining the original photocatalytic activity. The degradation rate of the SAG-1 sample was still 97% after 5 cycles, and the crystal structure remained stable after 5 cycles ($I_{(1\bar{3}2)}/I_{(111)}=1.16$). This work could provide an reference for solving the problem that the materials of Ag-based efficient photocatalysts could be susceptible to photocorrosion during the photocatalytic process.