In situ synthesis and properties of Ag/Ag₁₀Si₄O₁₃/GO photocatalysts with synergistic effect of plasma Resonance and Schottky junction

Abstract

Ongoing studies on photocatalysts based on the response of the overall band spectrum of visible light to clean water resources have received significant attention. This study focuses on the synergistic effect of plasma resonance and Schottky junction of silver as a promoter in Ag₁₀Si₄O₁₃/GO-based photocatalysts for improving the photocatalytic performance and stability. The nanostructured Ag/Ag₁₀Si₄O₁₃/GO composites were synthesized by an accurate molecular (ion) scale assembly based on the sol–gel method. The response range of visible light was extended to 660 nm, and the combined effect of the Schottky junction and internal polarization electric field of Ag₁₀Si₄O₁₃ showed excellent charge separation efficiency. The highly efficient non-selective degradation for cationic and anionic dyes was demonstrated under visible light, and methylene blue (20 g mL⁻¹) can be completely degraded in 15 min. The excellent cycling stability was obtained due to the inhibition of photoreduction. The novel nano-engineering strategy provided a new reference for highly active photocatalysts.

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1. Introduction

The rapid development and capacity expansion of textile and printing result in the production of a large number of dyes wastewater,^1,2 which has been one of the serious routes for the generation of gray/polluted water.³ Semiconductor photocatalytic oxidation, as a type of advanced oxidation processing with the potential to overcome energy shortage and solve environmental pollution problems, utilizes sunlight for the non-selective and efficient degradation of organic pollutants in the presence of water without secondary pollution.^4–6 Many efforts have been made in the direction of classic photocatalytic materials such as titanium dioxide (TiO₂) to improve the photocatalytic performance and improve the visible light response ability.^7–13 However, the improvement of the intrinsic properties of TiO₂ or other wide band-gap semiconductor materials by ion doping, construction of heterojunction, noble metal-modification, etc., is limited. In recent years, the development of silver-based photocatalysts (Ag₂O,¹⁴ AgBr,¹⁵ AgI2,¹⁶ Ag₃PO₄,^17,18etc.) with small band gaps and unique electronic configurations provides a broad way to solve this problem effectively.

As a new type of silver-based photocatalysts, silver silicate (Ag_xSi_yO_z) photocatalysts could provide empty 5s orbital and potentially the 5p orbital to accept additional electrons due to Ag ions forming FCC cluster-like aggregates.³ Combined with the unique electronic d¹⁰–d¹⁰ structure and the internal polarization electric field induced by the distorted tetrahedral unit [SiO₄], Ag_xSi_yO_z showed a strong charge separation rate.^19,20 As the strength of the internal electric field is determined by the number of the distorted [SiO₄] tetrahedra, Ag₁₀Si₄O₁₃ has attracted considerable attention.²¹ Moreover, the effective unit cell of Ag₁₀Si₄O₁₃ is smaller than the metal, which has a greater propensity to generate photogenerated electrons from the surface to the interface for the photodecomposition of dye molecules.²² Currently, solid-phase synthesis (SPS) has been proposed for the synthesis of Ag₁₀Si₄O₁₃ with excellent photocatalytic activity and has been further developed.^23–25 However, the enhancement of purity and controllable modification of the as-prepared Ag₁₀Si₄O₁₃ are relatively difficult due to ion diffusion in the solid state. In recent years, a high purity nanocrystalline preparation strategy based on the sol–gel process were proposed by our research group, which provided the possibility of atomic scale composition adjustment to further improve the degradation performance and stability of nano-Ag₁₀Si₄O₁₃.^3,26,27

Graphene, as a good conductor of photoelectrons, has been widely used to improve the performance of photocatalysts.^28,29 A high work function (4.42 eV) of graphene makes it a photoelectron acceptor from the conduction band of the photocatalysts.³⁰ Also, a high charge mobility and carrier separation efficiency can be obtained by composite graphene with semiconductors.^31,32 In addition, graphene oxide (GO), a derivative of graphene, possesses a large number of functional groups, which is conducive to the in situ growth of nanoparticles to inhibit agglomeration and fully exploit the utility of photocatalysts.³³ Furthermore, the introduction of silver nanoparticles into semiconductor materials has been shown to effectively improve the photocatalytic efficiency and enhance the stability due to the local surface plasmon resonance (LSPR).^34,35 Meanwhile, the construction of the Schottky junction between silver nanoparticles and semiconductor can synergically improve the photocatalytic performance.^36,37 Therefore, the in situ loading of Ag₁₀Si₄O₁₃ and Ag nanoparticles on the graphene oxide surface is expected to further improve the service stability of Ag₁₀Si₄O₁₃-based photocatalysts by the multiple effects of LSPR, Schottky junction, and internal polarization electric field.

The novelty of our work is in the synthesis of plasmonic nanostructured Ag/Ag₁₀Si₄O₁₃/GO (AAG) composite photocatalytic materials based on the sol–gel method. GO was utilized as a carrier and the in situ growth mechanism of the AAG was revealed. The effects of the Ag nanoparticles dosages on the phase, microstructure, visible light photocatalytic performance, and cycling stability of AAG were studied in detail. In addition, the synergistic coupling mechanism of LSPR, Schottky junction, and internal polarization electric field were discussed. The proposed strategy could provide an important reference for the one-step synthesis of high performance Ag₁₀Si₄O₁₃-based photocatalysts.

2. Experimental details

2.1. Materials and reagents

Tetraethoxysilane (TEOS, ≥98%) and AgNO₃ (≥99.8%) were employed as the raw materials for Ag₁₀Si₄O₁₃. Citric acid (CA, ≥99.5%) was utilized as hydrolysis inhibitor and complexing agent. Methylene blue (MB, ≥98.5%) and methyl orange (MO, ≥98.5%) were the target degradants. AgNO₃, tert-butanol (TB, ≥95%) phthalic acid (PTA, ≥99%), and benzoquinone (P-BQ, ≥99%) served as scavengers for electron (e⁻), hole (h⁺), hydroxyl radical (˙OH), and superoxide radical (˙O₂⁻), respectively. All the chemicals were used as received and without further purification. In all experiments, distilled water was used as a polar solvent. GO hydrosol was prepared from graphite powder using a modified Hummers’ method, as shown in our previous work.³⁸ The XRD pattern and Raman spectrum of GO are given in the ESI.†

2.2. Preparation of AG and AGG composites

Ag₁₀Si₄O₁₃/GO (AG) and AAG were synthesized by a simple one-step sol method to control the homogeneity of the nano-heterojunction from the molecular (ion) level. The specific experimental process is shown in Fig. 1. A certain amount of CA was mixed into absolute ethyl alcohol (25 mL), and 4.8 mL ethyl orthosilicate and 1.5 mL H₂O were added under magnetic agitation for 1 h to get colorless transparent silica sol. Then, an appropriate amount of GO sol was added to the above silica sol; the mass fraction of GO in Ag₁₀Si₄O₁₃ was 1 wt%. On this basis, the AgNO₃ solution was further dropped and the mass fractions of Ag were regulated on the basis of the silver–silicon ratio of Ag₁₀Si₄O₁₃. The mixtures were dried at 70 °C for 24 h and calcined at 400 °C for 5 h to obtain the AG and AGG samples with different mass fractions of Ag simple substance (0 wt%, 0.1 wt%, 0.5 wt%, and 1 wt%, named as 0.1AGG, 0.5AGG, and 1AGG).

Fig. 1 Schematic illustration of the synthesis of AGG composites.

2.3. Characterization and theoretical calculations

Phase analysis was carried out by powder X-ray diffraction (XRD, D8 Advance, Bruker). The morphology of the as-prepared sample was characterized using transmission electron microscopy (TEM, Tecnai F20-G2, FEI Company). Fourier transform infrared (FTIR) spectroscopy was utilized to analyze the evolution of the chemical bond of GO, AG, and AGG (Nexus670, Nicolet). The chemical status of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, PHI-5702, American physical electronics Corp.). The energy band and adsorption edge were evaluated using a UV-vis spectrophotometer (U-3900H, Hitachi). The zeta potential of the photocatalyst was determined by microelectrophoresis (JS94H, PowerEach). The separation and transfer behaviors of photoelectrons was measured by photoluminescence spectroscopy (PL, F97Pro, Lengguang) and an electrochemical work-station (EIS, CHI660E, Chenhua). The work function was calculated based on density functional theory (DFT) with the plane-wave pseudopotential method. The cutoff energy was set to 450 eV; the energy convergence standard was 1 × 10⁻⁶ eV and the force convergence standard for each atom was −0.01 eV Å⁻¹.

2.4. Photocatalytic experiment

The photocatalytic performance of the photocatalyst was evaluated with MB and MO solution (20 mg L⁻¹) as the target degradation materials. The light source used in the photocatalytic test was a 300 W W-XE lamp with a UV cut-off filter (λ > 420 nm) to provide the visible light spectrum. 0.1 g photocatalysts were put into 100 mL pollutant solution for dark adsorption for 60 min, and then the light source was used for degradation. 1 mL irradiated sample was collected every 5 min for photocatalytic evaluation. Also, the absorbance of the supernatant was measured by a UV-vis spectrophotometer (SP-752, Spectrum Instruments) to reflect the degradation efficiency of the photocatalyst.

3. Results and discussion

3.1. Crystallographic and morphological analyses

Fig. 2 shows that the XRD pattern was implemented to determine the phases of the as-prepared samples. Four formulations of composite photocatalysts with a mass fraction of Ag (0 wt%, 0.1 wt%, 0.5 wt%, 1 wt%) in the as-prepared samples were produced to optimize the degradation efficiency of the dye pollutants. The XRD results indicated that the triclinic phase Ag₁₀Si₄O₁₃ (JCPDS No. 71-1365) were obtained in all the samples, and the major peak (111) of the silver simple substance (JCPDS No. 14-0688) at about 38.1° were detected with the introduction of silver nitrate exceeding the silver–silicon molar ratio of Ag₁₀Si₄O₁₃. This indicates that the controlled in situ formation of the silver simple substance by adjusting the composition with molecular (ion) level of raw materials combined with sol–gel method and thermal reduction assistance.

Fig. 2 XRD patterns of AG and AGG composites.

The morphology and microstructure of GO, AG, and 0.5% AAG were characterized by TEM. Fig. 3(a) shows the gauzy GO with wrinkles prepared by improved Hummers’ method. Fig. 3(b) is the TEM image of AG, the nanoparticles grow uniformly on GO, and the selected area electron diffraction (SAED) result confirmed the diffraction spots corresponding to the (21̄3) and (1̄01) crystal planes of Ag₁₀Si₄O₁₃ (Fig. 3(c)). After the introduction of the silver simple substance, the composite material with uniformly dispersed growth of nanoparticles can still be obtained (Fig. 3(d) and (f), 0.5AAG). The direct contact between Ag₁₀Si₄O₁₃ and Ag (Fig. 3(f) and (g)) provides the possibility for the formation of the Schottky junction. The high-resolution (HR) TEM image shows the existence of the (1̄41) plane of Ag₁₀Si₄O₁₃ in alignment with a d-spacing of 0.197 nm and the (111) plane of Ag in alignment with a d-spacing of 0.235 nm (Fig. 3(g)). Meanwhile, the diffraction spots corresponding to the (020) plane of Ag₁₀Si₄O₁₃ and the (111) plane of Ag can be detected in the SAED data (Fig. 3(h)). The average sizes of the 0.5AAG nanoparticles are about 8.7 nm (Fig. 3(e)), which indicates that the utilization of GO as a carrier is helpful for the further grain refinement of Ag₁₀Si₄O₁₃.²⁶

Fig. 3 TEM images of morphology and microstructure of GO and 0.5AGG. TEM image of GO (a). TEM image of AG (b). SAED image of AG (c). TEM image of 0.5AAG (d and f). Size distribution of Ag/Ag₁₀Si₄O₁₃ nanoparticles (e). HRTEM image of 0.5AAG (g). SAED image of AAG (h).

3.2. Chemical state and assembly mechanism of heterojunction analyses

FTIR was employed to analyze the functional group information. Fig. 4 shows the FTIR spectra of GO, AG, and 0.5AGG. In the FTIR spectrum of GO, the peaks of alkoxy/alkoxide C–O stretching (1050 cm⁻¹), carboxy C–O (1390 cm⁻¹), aromatic C=C (1640 cm⁻¹), and C=O (1730 cm⁻¹) were detected.³⁹ Also, the peaks at about 706 cm⁻¹, 1020 cm⁻¹, and 1380 cm⁻¹ in AG and 0.5AGG did not change the position, which were assigned to the Si–O–Si bending vibration, Si–O stretching vibration, and Ag–O stretching vibration, respectively.⁴⁰ This indicates that the addition of Ag has no effect on the crystal structure of Ag₁₀Si₄O₁₃ in AGG. Due to the small dosage of GO, its characteristic peak was not detected in AG and 0.5AGG. However, a weak peak appeared at 1060 cm⁻¹ in both AG and AGG, which may be attributed to the overlap of the peak of the Si–O–C bands with surrounding Si–O–Si bands,⁴¹ which indicates that the complex assembly ultimately ensures the chemical bonding between GO and Ag₁₀Si₄O₁₃.

Fig. 4 FTIR spectra of GO, AG, and 0.5AAG composites.

The surface composition and chemical state of elements of AG and 0.5AAG were studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 5. The main peaks in the survey spectrum correspond to Ag 3d, Si 2p, O 1s, and C 1s orbitals (Fig. 5(a)). Fig. 5(b) shows the fitting spectra of Ag 3d orbitals; the peaks located at 368.0 (367.9) eV and 374.0 (373.9) eV were assigned to Ag⁺ 3d_5/2 and Ag⁺ 3d_3/2, and the peaks of 0.5 AGG located at 369.7 eV and 375.6 eV correspond to metallic Ag⁰, respectively.^42,43 The peaks of Si 2p_3/2 and Si 2p_1/2 partially overlap with Ag 4s in Ag₁₀Si₄O₁₃ and the peaks located at 101.2 eV and 103.0 (103.2) eV correspond to Si 2p_3/2 and Si 2p_1/2, respectively (Fig. 5(c)).⁴⁴ The peaks of 530.0 eV and 532.2 (532.5) eV are the characteristic peaks of O 1s, corresponding to the Ag–O and Si–O bonds, respectively. For the C 1s orbital, the peak of 284.8 eV is the characteristic peak of surface correction carbon, and the characteristic peaks of the C–O bonds contained in GO were detected at 286.5 (286.6) eV.³⁹

Fig. 5 The XPS survey spectra of AG and 0.5AAG (a), and high-resolution spectra of Ag 3d (b), Si 2p (c), O 1s (d), C 1s (e).

The binding energies of the above characteristic peaks of AG and 0.5AGG were all changed. Except for the decrease in the Ag⁺ 3d_5/2 and Ag⁺ 3d_3/2 binding energies of 0.5 AGG, the binding energy of Si 2p_1/2, O 1s of Si–O, and C 1s of C–O were all increased. The increase in the binding energies of O 1s of Si–O and C 1s of C–O may be attributed to the formation of the Si–O-C bond.⁴¹

Based on the above analysis, a possible assembly mechanism of AGG can be proposed, as shown in Fig. 6.^45,46 The adoption of the sol gel method can realize the molecular (ion)-scale dispersion of the Ag-source, which lays a foundation for the controllable assembly of the AAG composites and the controllable introduction of the silver simple substance. The citric acid molecules can coordinate with hydrolyzed products of TEOS. Further complexation will occur between the functional groups of TEOS or citric acid with GO. Then, the negative charges exposed due to the hydrolysis of remanent carboxyl will trap the silver ions with static electricity. Ultimately, the directional grafting of the complexes on the GO surface provides a more potent steric hindrance effect, resulting in a further refinement of the Ag₁₀Si₄O₁₃ grains.

Fig. 6 The assembly mechanism of the AGG precursor.

3.3. Ultraviolet-visible and photoluminescence spectroscopy

The UV-vis diffuse reflectance results of the as-prepared samples with different dosages of silver are shown in Fig. 7(a). The absorption edge of AG is 610 nm. Also, the corresponding values of the AGG samples can reach about 660 nm, which undergo significant red-shifts. Moreover, the light absorption capacity was expanded throughout the 660 nm range. The energy bandgaps (E_g) of the as-prepared samples were calculated using magnified wavelength, corresponding to the Tauc plot.⁴⁷ According to the relationship between (αhv)^1/2 and E_g in the inset of Fig. 7(a), the E_g of AG and 0.5AAG are 1.88 and 1.78 eV, respectively. When the frequency of incident illumination meets the resonance condition of silver nanoparticles, the LSPR is accompanied by absorption and scattering of light, which will induce light absorption occurs in the visible region.^48,49 Therefore, compared to AG, the absorption edge and the light absorption capacity of AGG were expanded.

Fig. 7 The UV-vis DRS (a) and PL spectra (b) of AG and AAG composites.

Some photogenerated electrons and holes of the excited semiconductors will be combined and released in fluorescence if the spin-state is conserved. Therefore, innate fluorescence intensity reflects the recombination rate of the photogenerated electrons and holes. Fig. 7(b) shows the PL spectra of the as-prepared samples with different dosages of silver. The AGG samples exhibited lower fluorescence intensity compared to AG, and 0.5AGG shows the lowest fluorescence intensity. Due to the difference of WF of Ag, GO, and Ag₁₀Si₄O₁₃, the photoelectrons produced in the conduction band of Ag₁₀Si₄O₁₃ will inject into Ag or GO under the double action of the interface electric field of the heterojunction and the internal polarized electric field of Ag₁₀Si₄O₁₃. The Schottky junction will prevent photoelectron backflow to the barrier. As a result, a higher separation efficiency of photogenerated electrons and holes will be obtained by the AGGs. However, when the Ag dosage is much higher, the overlapping Ag nanoparticles can provide a good conductor of electrons and become their recombination center, which results in a higher recombination rate.

3.4. Electrochemical properties

Electrochemical properties were tested to further explore the generation, separation, and transfer mechanisms of the photoelectron. The flat band potentials obtained from the Mott–Schottky curve infer the energy level structure, and then determine the positions of the valence band or conduction band of the semiconductors. Fig. 8(a) and (b) shows the Mott–Schottky curves of AG and 0.5AGG at 1000 Hz. The slopes of these curves are positive values, indicating that AG and 0.5AGG are all n-type semiconductors. Compared with the saturated calomel electrode (SCE), the flat band potentials of AG and 0.5AGG are 0.37 eV and 0.31 eV, respectively, which possess flat band potentials of 0.61 eV and 0.55 eV relative to the standard hydrogen electrode (NHE). For the n-type semiconductor, its flat band potential is approximate to the conduction band.⁵⁰ Therefore, the valence band position of AG and 0.5AGG can be calculated as 2.49 eV and 2.33 eV according to the formula E_CB = E_VB − E_g, which was close to the valence band spectra of 2.56 eV and 2.41 eV obtained by XPS (Fig. 8(c)). Compared with AG, not only the narrower E_g of 0.5AGG contributes to the charge separation, its valence band is still in a position with strong oxidation ability (Fig. 8(d)).

Fig. 8 Mott–Schottky plot of AG (a) and 0.5AAG (b) composites. The valence band spectra of AG and 0.5AAG composites (c). Bandgap position of AG and 0.5AAG composites (d). Nyquist plot of AG and AAG composites (e). Photocurrent response plot of AG and AAG composites (f).

The EIS measurement was employed to determine the charge transfer capability of the photocatalysts, as shown in Fig. 8(e). The Nyquist arc radius determines the resistance value of the photocatalyst.⁵¹ The radii of AGGs are all lower than AG and decrease with the increase in silver dosages. This indicates that the introduction of Ag effectively lowered the interface charge transfer resistance. The transient photocurrent response reflects the rapid generation, separation, and transfer of charge carriers under on and off conditions of illumination. The photocurrent intensities of the as-prepared samples exhibit consistent evolution law as PL (Fig. 8(f)). Under the synergistic action of LSPR and Schottky junction, the photoelectron separation ability of AGGs is greatly improved. Similarly, excess silver provides photoelectron recombination centers that reduces the strength of the photocurrent intensity.

3.5. Zeta potential and adsorption property

The capture of pollutants by adsorption contributes to the improvement of the degradation performance. Fig. 9(a) shows the zeta (ζ) potential of the as-prepared samples in suspended distilled water. Overall, the potentials of these photocatalysts are all negative, and the absolute potential of AGGs are all higher than that of AG. 0.5AGG has the relatively highest absolute value of potential, indicating that its particle surface can provide the most adsorption active sites. Confirmed by dark adsorption measurement, the adsorption performance of AGGs is better than that of AG. The difference in the adsorption performance between AGGs was small, and 0.5AGG showed the best performance, which was consistent with the potential test (Fig. 9(b)). The silver oxide layer produced by the oxidation of silver can react with H₂O molecules.⁵² Although most of the Ag⁺ ions ionize into water and contribute to the formation of the double-layer surrounding the particles, Ag(OH)²⁻ can provide a highly hydrophilic surface of the photocatalyst. This provides more active sites for the adsorption of MB. However, excessive silver may lead to a relative reduction of the exposed interfaces due to the overlapping effect, resulting in a slight decrease in the adsorption performance.

Fig. 9 The zeta potential in distilled water (a) and adsorption rate for MB of AG and AGG composites (b).

3.6. Photocatalytic reactivity of dye degradation

Cationic dyes methylene blue (MB) and anionic dyes methyl orange (MO) were used to evaluate the photocatalytic performance of the as-prepared samples. For the adsorbed equilibrium MB solution, the AG sample can reach a degradation rate of 99% at 25 min under visible light and a significant performance improvement occurred in the AAG samples, as shown in Fig. 10(a). MB was completely degraded in 15 min by AGGs, which takes 40 percent less time than AG. According to the Langmuir-Hinshelwood dynamics model, the degradation process of MB by nano-Ag₁₀Si₄O₁₃ conforms to the pseudo-first-order kinetic model (Fig. 10(b)). The reaction constants of AGGs are 0.2330 min⁻¹, 0.2436 min⁻¹, and 0.2352 min⁻¹, respectively, which is much higher than the value of AG (0.0995 min⁻¹). Fig. 10(c) shows the UV-vis absorption spectrum of the MB degradation process, which inferred that the degradation occurs within 15 min exposure to visible light, confirming carbon bond fission of the heterocycle of MB. The degradation of MO by as-prepared samples was relatively slower (Fig. 10(d) and (e)). The degradation rate of AG to MO at 70 min is 75%. The degradation rates of AGGs were improved, and 0.5AGG showed the most outstanding performance, with a degradation rate of 87% at 70 min.

Fig. 10 The visible light degradation rate of AG and AGG composites for MB (a). The pseudo-first-order kinetic plots of AG and AGG composites for MB (b). The UV-vis absorption spectrum of the degradation process of 0.5AGG (c). The visible light degradation rate of AG and AGG composites for MO (d). The pseudo-first-order kinetic plots of AG and AGG composites for MO (e). A performance comparison of 0.5AAG with other Ag-based photocatalysts under visible light (f).

For the positive MB molecules, the electrostatic adsorption by the as-prepared samples can capture its molecules to efficiently in situ degrade. As for MO molecules, the dark adsorption measurements confirmed that the photocatalysts showed no adsorption effect due to electrostatic repulsion. The degradation of MO mainly depends on the dynamic contact between its molecules with active substances on the photocatalyst interface; thus, the degradation efficiency is relatively lower. This indicates that AGG for the degradation of dyes is non-selective. Fig. 10(f) shows a performance comparison with other Ag-based photocatalysts under visible light. Compared to Ag₂MoO₄,⁵³ Ag₃VO₄,⁵⁴ Ag₃PO₃,⁵⁵ and AgIO₄,⁵⁶ the degradation rates based on 0.5AAG in this work have obvious advantages for MB. The performance of silver silicate (Ag₆Si₂O₇,⁵⁷ SPS-Ag₁₀Si₄O₁₃,^22,23 and nano-Ag₁₀Si₄O₁₃^3,26,27)-based photocatalysts is outstanding, but there is still a certain gap compared with that of AGG.

3.7. Degradation mechanism

The active radical capture experiments were carried out to evaluate the photocatalytic degradation mechanism of the photocatalyst (0.5AGG). The effects of electron (e⁻), hole (h⁺), superoxide radical (˙O₂), and hydroxyl radical (˙OH) on the photocatalytic performance are reported here. As shown in Fig. 11(a), the introduction of AgNO₃ as electron scavenger has no effect on the photocatalytic process, demonstrating that the electrons are hardly involved in the photocatalytic decomposition of dyes. The photocatalytic efficiency is significantly reduced by adopting TEOA and PTA chemical inhibitors, and the effect of TEOA is the most drastic. In addition, although the conduction band potential of 0.5AGG was far from reaching the formation position of ˙O₂⁻ (−0.33 vs. NHE), the ˙O₂⁻ exhibited a slight effect on the photocatalytic performance. This may be the result of interaction of photoelectrons transferred to GO and Ag nanoparticles with dissolved O₂ molecules on the surface.^58,59 This indicates that holes play a major role in the degradation of dyes by AGGs, and hydroxyl groups and superoxide free radicals also promote the degradation.

Fig. 11 Radical scavenging experiment (a) schematic diagram (b) of 0.5AAG.

Ag₁₀Si₄O₁₃ is endowed with high visible light photocatalytic potential due to its inherent narrow bandgap and internal polarized electric field. On this basis, Ag₁₀Si₄O₁₃ with finer grains were obtained by use of GO as the carrier, the shorter pathway of electrons and holes transfer to the surface, and the more abundant reaction sites can ensure the carriers to participate in the photocatalytic reaction efficiently. With the introduction of silver nanoparticles, the synergistic effect of LSPR and Schottky junction can effectively prevent the backflow of hot electrons and photoelectron to silver nanoparticles and improve light utilization. The presence of GO can provide a pathway for electron transfer and contribute to more efficient separation of charge carriers, as shown in Fig. 11(b). In this way, the participation of holes in Ag₁₀Si₄O₁₃ and silver nanoparticles for the photodecomposition of dyes improved the photocatalytic performance significantly.

3.8. Cyclic service characteristic

Multiple cycle performance tests were used to evaluate the serviceability of photocatalysts. The results of cycle test performed five times are shown in Fig. 12(a) and (b). The photocatalytic performance of AG shows a decrease after the third cycle and decreased by 10% after the fifth cycle. The performance of 0.5AGG maintained an excellent stability; the degradation performance of the fifth cycle can still reach 98%. To further explore the evolution of performance stability, photocatalysts from each cycle test were collected and tested for XRD, as shown in Fig. 12(c) and (d). The peak intensities of the (111) plane of Ag all increased due to the photoreduction of Ag⁺. According to the peak strength ratio of the (111) plane of Ag₁₀Si₄O₁₃ and the (13̄2) plane of Ag, the contents of silver in AG and 0.5AGG increases with the number of cycles (Fig. 12(e)). For the 0.5AAG sample, Ag contents increase slowly after 3 cycles, indicating the effective protection of the Schottky junction on the stability of material composition and photocatalytic performance. As a whole, AGG has a considerable efficiency and stable service characteristic, which provides a new strategy to design and develop visible-light-driven photocatalysts.

Fig. 12 The photocatalytic stability of AG (a) and 0.5AAG (b) composites. The XRD patterns of the AG (c) and 0.5AAG (d) composites after the degradation of MB. The peak intensity ratio of the (13̄2) plane of Ag₁₀Si₄O₁₃ with the (111) plane of Ag (d). The contents of silver in AG and 0.5AGG increases with the number of cycles (e).

4. Conclusion

In summary, a novel synthetic strategy for fabricating AAG nano-heterojunction was proposed by accurate molecular (ion) scale assembly based on the sol–gel method. The coordination between Ag-source, Si-source, and GO supported by CA molecules acted as chelated pendant, resulting in the in situ synthesis of nanostructured Ag/Ag₁₀Si₄O₁₃ Schottky junction uniformly loaded on the GO surface. The red-shifted edge of the absorption band of the AAG photocatalyst confirmed a prominent visible light response capacity. The outstanding charge separation efficiency was obtained due to the multiple function of LSPR, Schottky junction, and the internal polarization electric field of Ag₁₀Si₄O₁₃. The AAG photocatalyst was found to be highly efficient in the non-selective degradation of anionic and cationic dyes under visible light; the degradation efficiency of MB and MO (20 g mL⁻¹) can reach 100% (15 min) and 87% (70 min), respectively. Photodecomposition reactions were driven by photogenerated holes (h⁺) assisted by ˙OH and ˙O₂⁻ radical species. An excellent cycle performance and compositional stability were confirmed by five cycles (reached 98% after the fifth cycle). The synthetic strategy provides a simple route for the design and fabrication of highly active visible light-responsive photocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 522020096), the Natural Science Foundation of Gansu Province, China (No. 22JR5RA284), Gansu Province Science and Technology Program Grant (No. 22JR5RA281).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ma00743f

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