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Communication

Photocatalysis and Electrocatalysis Properties of a Keggin-Type Inorganic–Organic Hybrid SiW12O40@Ag

by
Xin-Xin Hu
1,†,
Tai-Dan Chen
1,†,
Xiao-Jie Gong
1,*,
Jiu-Yu Ji
1,
Li-Ping Zhao
2,*,
Wen-Xuan Xie
1 and
Kun Zhou
1,*
1
School of Petrochemical Engineering, Liaoning Petrochemical University, Fushun 113001, China
2
Institute of Chemical and Industrial Bioengineering, Jilin Engineering Normal University, Changchun 130052, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Inorganics 2025, 13(5), 132; https://doi.org/10.3390/inorganics13050132
Submission received: 22 March 2025 / Revised: 18 April 2025 / Accepted: 19 April 2025 / Published: 25 April 2025
(This article belongs to the Special Issue Advances in Inorganic–Organic Composite Photocatalysts for Energy and Environmental Applications)
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Abstract

:
An example of an inorganic–organic hybrid compound {[Ag4(SiW12O40)(HBTA)8][Ag4(SiW12O40)(HBTA)8(H2O)]}n·(1) modified by the Keggin-type [SiW12O40]4− polyoxoanion was synthesized hydrothermally, which was determined by single crystal X-ray diffraction. Two 1-dimensional (1D) chains are present in 1: chain a is connected by Ag···Ag interactions and chain b is connected by π···π stacking. Finally, they were extended into 2D and 3D supramolecular structures by hydrogen bonding. The photodegradation of methylene blue (MB) was investigated under visible light irradiation, and the degradation rate reached 99.4% within 200 min. In addition, 1 catalyzes the reduction of sodium nitrite and can be used as a potential electrocatalytic material.

1. Introduction

Methylene blue (MB), a widely used organic dye, is commonly used in textile, paper, and other industries [1,2,3]. However, its high color rendering and difficulty in degradation will affect the photosynthesis of plants when it enters a water body and alters the solubility of oxygen, thus destroying the ecological balance of aquatic environments and seriously harming human health [4,5,6]. Therefore, it is important to explore an effective method to degrade MB pollution. Among available options, photocatalysis is a common degradation technology [7,8,9].
Polyoxometalates (POMs) are important nanoscale metal oxide clusters with structural stability, high-charge, and excellent optical, electrical, and magnetic properties. They are widely used in medical and catalytic applications due to their favorable redox and photo-oxidation properties [10,11,12,13,14]. Among them, Keggin-type clusters are considered to be the most classical POMs, as they have a highly stable symmetric structure, abundant active sites and reversible redox ability on the surface, abundant terminal and bridging oxygen atoms for metal attachment, and strong coordination ability, which is outstanding in the field of photoelectrocatalysis and materials science [15,16,17,18,19]. Therefore, the designed synthesis and potential applications of POM-based inorganic–organic hybrid materials have been of great interest. Among the Keggin-type POMs, [SiW12O40]4− (abbreviated to SiW12), which is easy to synthesize and maintain structural stability, is usually used to construct inorganic modules in inorganic–organic hybrid materials [20,21,22,23].
Organic ligands can be used as template agents in the synthesis of inorganic–organic hybrid materials to guide the structure and regulate the final structure by mixing the size and geometry of the material itself [24,25,26]. Benzotriazole (HBTA) is a common N-heterocyclic organic ligand with three nitrogen atoms, which can be used as an electron donor for connecting metals and POM to form stable coordination bonds. HBTA has different spatial effects, as well as different coordination and bridging modes, and can also be used to build structurally diverse and stable inorganic–organic hybrid materials through π···π stacking and hydrogen bonding [27,28,29,30].
The stable structures contribute to the formation of materials necessary for applications in complex environments (photocatalysis and electrocatalysis). Several studies have shown that the choice of metal ions can have an impact on the structure of inorganic–organic hybrid materials [31,32,33]. Ag, due to its vacant d-orbitals, can efficiently undergo electron transfer and energy leaps upon excitation to high energy states. Ag exhibits a variety of coordination abilities under hydrothermal conditions [34,35,36]; in addition, silver–silver interactions may be formed between silver ions, which can lead to the formation of polynuclear silver complexes, which are excellent linkers for the construction of heterogeneous materials [37,38,39].
This paper provides a description of the properties of an inorganic–organic hybrid material {[Ag4(SiW12O40)(HBTA)8][Ag4(SiW12O40)(HBTA)8(H2O)]}n (1) that has been synthesized by choosing silver atoms as the linker and cleverly connecting the inorganic module SiW12 with the organic module HBTA under hydrothermal conditions. Two one-dimensional (1D) chains are present in compound 1: chain a is connected by Ag ⋯ Ag interactions and chain b is connected by π···π stacking. Finally, they can be expanded into 2D and 3D supramolecular structures by hydrogen bonding. The compound was determined by X-ray powder diffraction, and was also characterized by FT-IR spectroscopy, ultraviolet-visible (UV-Vis) diffuse reflectance spectrum, and cyclic voltammetry characteristics. In the photodegradation experiment of MB, the degradation rate of MB by compound 1 reached 99.4% in 200 min. In the electrochemical performance tests, compound 1 had a catalytic effect on the reduction of sodium nitrite and could be used as a potential electrocatalytic material.

2. Results and Discussion

2.1. Crystal Structure Description

Compound 1 crystallizes in the triclinic p1 space group, and its asymmetric unit is shown in Figure 1. Compound 1 contains eight silver ions, eight HBTA ligands, two crystallographically independent Keggin-type SiW12 clusters (SiW12a is sky blue and SiW12b is green), and one coordinating water molecule. It is noteworthy that there are two SiW12 polyoxoanions present in 1. SiW12a and SiW12b act as two connected inorganic ligands, coordinating with two silver ions by providing two end-group oxygen atoms.
There are eight crystallographically independent silver ions in compound 1, showing two types of coordination patterns: (i) Ag2, Ag4, and Ag6 form triple coordination sites with the nitrogen atoms in the three HBTA ligands, respectively. The Ag-N bond distance ranges from 2.09(3) to 2.51(3) Å. (ii) Ag1, Ag3, Ag5, Ag7, and Ag8 are tetra-coordinated with N atoms from three BTA ligands and an O atom from the coordinated water molecule. The oxygen atoms coordinated to Ag1 and Ag3 are derived from SiW12a (bond lengths of 2.66(2) Å and 2.54(3) Å for Ag1-O32 and Ag3-O16), the oxygen atoms coordinated to Ag5 and Ag7 are from SiW12b (bond lengths of 2.60(4) Å and 2.57(4) Å for Ag5-O67 and Ag7-O51), and the oxygen atom coordinated to Ag8 is derived from a coordinating water molecule (Ag8-O45 bond length is 2.52(3) Å). As shown in Figure 2a, there is a large number of face-to-face π···π stacking (the distance from center to center is 3.719(2)–3.789(2) Å) between HBTA linked to SiW12a, forming a 1D chain a, [Ag4(SiW12O40)(HBTA)8]n. As shown in Figure 2c, adjacent 1D chains form a 2D layer A via a large number of hydrogen bonds (C2-H2···O37, C17-H17···O19, C20-H20···O17, C35-H35···O23, C38-H38···O35) to build a 2D supramolecular layer A [40]. As illustrated in Figure 2b,d, the distance between Ag5 and Ag7 is 3.192(4) Å. There is an argentophilic interaction, and a 2D layer B is built by linking neighboring SiW12b to form a 1D chain b, [Ag4(SiW12O40)(HBTA)8(H2O)]n, through Ag···Ag interaction. Hydrogen bonding between neighboring 1D chains b can also be used to form a supramolecular 2D layer B (C57-H57···O48 and C74-H74···O52).
As illustrated in Figure 3, adjacent 2D layers are extended through hydrogen bonding to construct a 3D supramolecular structure {[Ag4(SiW12O40)(HBTA)8][Ag4(SiW12O40)(HBTA)8(H2O)]}n (C3-H3···O46, C4-H4···O47, C16-H16···O82, C28-H28···O61, C28-H28···O62, C39-H39···O77, C40-H40···O69, and C62-H62···O58).

2.2. Spectroscopic and Thermal Analyses

The freshly synthesized PXRD profile of 1 was found to match perfectly with the simulated one (see Figure S1), confirming the high purity of compound 1. The wavelength range of the FT-IR spectrum was 4000–500 cm−1. As shown in Figure S2, in the FT-IR spectrogram, the O-H stretching vibration of compound 1 is located at 3440 cm−1 and the vibrations of the phenyl and triazole ring skeleton are in the range of 1210–1628 cm−1. Bending vibration of the C-H bond on the benzene ring is attributed at 742 cm−1. In addition, there are five distinct characteristic absorption peaks at wavelengths from 1020 cm−1 to 535 cm−1, which are related to the POM structure and can be attributed to the Si-O, W-Oa, W-Ob-W, W-Oc-W, and O-Si-O stretching vibrations, and the specific wavelength designations have been labeled in detail in Figure S2 [41,42]. The UV-Vis diffuse reflectance spectrum was obtained for the crystalline powder of 1 across a wavelength spectrum of 200 to 800 nm. 1 exhibited a wide absorption band from 200–500 nm in the UV-Vis region, as shown in Figure 4. The maximum peaks at 260 nm and 310 nm were attributed to the O-to-W charge transfer involving the edge-sharing and corner-sharing oxygen atoms [18,43]. Additionally, the bandgap of compound 1 was calculated to be 3.33 eV using the Kubelka–Munk approach, implying that they hold potential as viable contenders for semiconductor applications.

2.3. Photodegradation of MB

MB is a common alkaline organic dye, and the wastewater generated during its production as well as its use can cause serious environmental pollution [3,7]. Combined with solid-state UV-Vis diffuse reflectance spectrum analysis, MB was chosen in this experiment to evaluate the photodegradation of 1 on this dye.
First, 5 mg of 1 was added to 12 mg/L of methylene blue solution (100 mL) and stirred for 30 min away from light to ensure that the adsorption equilibrium was reached. The mixed solution was then placed under a xenon lamp and 3 mL of the solution was collected every 40 min to test the degradation process by UV-Vis spectrophotometer [44,45]. As illustrated in Figure 5a, the absorbance of the MB solution gradually decreased with increasing irradiation time, and the degradation rate reached 99.4% at 200 min. This is superior to the reported degradation efficiency of some TiO2 photocatalysts [46,47]. Under ultraviolet irradiation, POM transfers electrons from the highest occupied molecular orbital to the lowest unoccupied molecular orbital, generating an oxygen-to-metal charge transfer. In the excited state, POM has strong oxidizing properties, and can directly oxidize MB, but it can also oxidize with other electron donors in the water to produce OH˙ radicals, and thus achieve the degradation of pollutants [48,49]. After the experiment, 1 was filtered, washed, and collected. The stability of compound 1 was then verified by FT-IR spectroscopy. As illustrated in Figure 5b, the FT-IR spectra of 1 remained unchanged before and after degradation, suggesting that 1 has good stability. Therefore, compound 1 will be further developed and applied for wastewater treatment.

2.4. Electrochemical Characteristics

Redox properties indicate that the material has a wide range of potential and practical applications. The electrochemical behavior of 1 was investigated in neutral condition (0.5 M Na2SO4 + 0.1 M H2SO4) electrolytes in the range of a potential window from 200 to −800 mV by using glassy carbon electrode loaded sample 1. As shown in Figure 6a, 1 exists because of three pairs of redox peaks, I–I’, II–II’, and Ш–Ш’; redox peaks II–II’ and III–III’ corresponded to two consecutive one-electron processes of W, whereas I–I’ was ascribed to a two-electron process [50], with an average peak potential E1/2 = (Epc + Epa)/2 (scanning speed: 100 mV·s−1): −688 mV, −483 mV, and −261 mV, respectively [51]. In addition, an irreversible anodic peak (IV) was also present, which was due to the oxidation process of AgI [28]. As the scanning rate increases from 100 mV·s−1 to 500 mV·s−1, the anodic peak current gradually moves to a high potential and the cathodic peak current gradually moves to a low potential. The peak potential is proportional to the scanning rate, which indicates that the redox process of 1 is controlled by diffusion [52].
Nitrite is widely found in industrial wastewater and is a highly toxic carcinogen. By the electrocatalytic reduction of nitrite, the harmful substances in wastewater can be effectively removed and the pollution to the environment can be reduced [53]. Therefore, the electrocatalytic reduction of sodium nitrite in the same electrolyte was investigated. As shown in Figure 6b, when 2.0 mM NaNO2 was continuously added to the electrolyte solution, the oxidation peak current in the CV curve of compound 1 decreased, and the corresponding reduction peak current increased, which indicated that 1 acted as an electrocatalyst in the reduction process of sodium nitrite, and therefore the compound could be used as a potential electrocatalytic material.

3. Materials and Methods

3.1. Material and Instruments

The reagents were commercially purchased and could be used without any purification. X-ray powder diffraction data for 1 were collected using graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å) at a temperature of 296(2) K using a Bruker D8 QUEST purchased from Bruker, Germany [3,42]. Powder X-ray diffraction (PXRD) was performed using the Cu-Kα radiation (λ = 1.54184 Å) by a Bruker D8 VENTURE diffractometer purchased from Bruker, Germany [8,30]. The PerkinElmer Spectrum GX spectrometer was carried out to test the Fourier transform infrared spectroscopy (FT-IR) purchased from Agilent Technologies Inc., America. UV-Vis spectrum was tested on an Agilent Cary5000 spectrometer, purchased from Agilent Technologies Inc., America. A PLS-SXE300/300UV xenon lamp source was used to carry out photodegradation experiments, purchased from Beijing PerfectLight Technology Co., LTD., China. Electrochemical testing was measured by a CHI660E electrochemical workstation, Beijing PerfectLight Technology Co., LTD.

3.2. Synthesis of {[Ag4(SiW12O40)(HBTA)8][Ag4(SiW12O40)(HBTA)8(H2O)]}n (1)

H4SiW12O40·xH2O (0.0810 g, 0.03 mmol), AgNO3 (0.0170 g, 0.10 mmol), and HBTA (0.0200 g, 0.17 mmol) were mixed in H2O (8 mL), and the resulting mixture was stirred for 50 min in air, which was transferred to a 25 mL Teflon-lined autoclave and stored at 160 °C for 3 days. The reaction mixture was cooled to room temperature, and then yellow block crystals of 1 could be obtained (yield of 62.0% based on Ag).

4. Conclusions

An example of a Keggin-type inorganic–organic hybrid compound {[Ag4(SiW12O40)(HBTA)8][Ag4(SiW12O40)(HBTA)8(H2O)]}n (1) was synthesized hydrothermally, which was determined by single crystal X-ray diffraction, FT-IR, and UV-Vis diffuse reflectance spectrum. Two 1D chains are present in 1: chain a is connected by Ag···Ag interactions and chain b is connected by π···π stacking. Finally, the chains were extended into 2D and 3D supramolecular structures by hydrogen bonding. The photodegradation of MB under visible light irradiation was investigated, and the degradation rate reached 99.4% within 200 min. In addition, 1 is catalytic for the reduction of sodium nitrite and will be used as a potential electrocatalytic material.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics13050132/s1: Table S1. Crystallographic data and structure refinement for 1; Table S2. Selected bond lengths [Å] of compound 1; Table S3. Selected bond lengths [Å] of compound 1; Table S4. Hydrogen bond parameters (Å, °) for compound 1; Figure S1. As-synthesized and simulated powder X-ray diffraction patterns of 1; Figure S2. The FT-IR spectrum of 1.

Author Contributions

Conceptualization, X.-X.H.; writing—original draft preparation, T.-D.C.; investigation, X.-J.G.; funding acquisition, J.-Y.J.; supervision, L.-P.Z.; data curation, W.-X.X.; writing—review and editing, K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by National College Students’ Innovation and Entrepreneurship Training Program Project (No. 202410148016), and the Scientific Research Fund of Liaoning Provincial Education Department (Nos. JYTMS20231444, LJ212410148034, and LJ242410148039).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

CCDC 2433224 contains the supplementary crystallographic data for 1 correspondingly. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 10 October 2022), or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Acknowledgments

The authors thank Liaoning Provincial Department of Education for funding support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00132 g001
Figure 1. The asymmetric unit of 1. Color code: Ag, pink; O, red; N, blue; C, gray; SiO4 polyhedron, yellow; WO6 polyhedron in SiW12a, sky blue; WO6 polyhedron in SiW12b, green. All hydrogen atoms are omitted.
Figure 1. The asymmetric unit of 1. Color code: Ag, pink; O, red; N, blue; C, gray; SiO4 polyhedron, yellow; WO6 polyhedron in SiW12a, sky blue; WO6 polyhedron in SiW12b, green. All hydrogen atoms are omitted.
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Figure 2. (a) 1D chain a of 1 constructed from π···π stacking (black dashed line). (b) 1D chain b constructed from Ag···Ag interactions. (c) 2D supramolecular layer A connected by hydrogen bonds (black dashed line). (d) 2D supramolecular layer B connected by hydrogen bonds (black dashed line). Hydrogen atoms were omitted.
Figure 2. (a) 1D chain a of 1 constructed from π···π stacking (black dashed line). (b) 1D chain b constructed from Ag···Ag interactions. (c) 2D supramolecular layer A connected by hydrogen bonds (black dashed line). (d) 2D supramolecular layer B connected by hydrogen bonds (black dashed line). Hydrogen atoms were omitted.
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Figure 3. Compound 1 forms a 3D supramolecular structure through hydrogen bonding (black dashed line).
Figure 3. Compound 1 forms a 3D supramolecular structure through hydrogen bonding (black dashed line).
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Figure 4. The UV-Vis diffuse reflectance spectrum of 1; inset: crystal powder photograph (left) and (Ahυ)2 versus hυ (eV) of 1 (right).
Figure 4. The UV-Vis diffuse reflectance spectrum of 1; inset: crystal powder photograph (left) and (Ahυ)2 versus hυ (eV) of 1 (right).
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Figure 5. (a) The UV-Vis absorption spectra of degraded MB solution; insert: the plot of blank and the MB concentrations containing 1 versus stirring time. (b) The FT-IR spectra of 1 before and after degradation.
Figure 5. (a) The UV-Vis absorption spectra of degraded MB solution; insert: the plot of blank and the MB concentrations containing 1 versus stirring time. (b) The FT-IR spectra of 1 before and after degradation.
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Figure 6. (a) The CVs of 1 in the neutral condition at different scan rates ranging from 100, 150, 200, 250, 300, 350, 400, 450, to 500 mV s−1. (b) CVs of 1 in different concentrations of NaNO2. Scan rate: 100 mV s−1.
Figure 6. (a) The CVs of 1 in the neutral condition at different scan rates ranging from 100, 150, 200, 250, 300, 350, 400, 450, to 500 mV s−1. (b) CVs of 1 in different concentrations of NaNO2. Scan rate: 100 mV s−1.
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MDPI and ACS Style

Hu, X.-X.; Chen, T.-D.; Gong, X.-J.; Ji, J.-Y.; Zhao, L.-P.; Xie, W.-X.; Zhou, K. Photocatalysis and Electrocatalysis Properties of a Keggin-Type Inorganic–Organic Hybrid SiW12O40@Ag. Inorganics 2025, 13, 132. https://doi.org/10.3390/inorganics13050132

AMA Style

Hu X-X, Chen T-D, Gong X-J, Ji J-Y, Zhao L-P, Xie W-X, Zhou K. Photocatalysis and Electrocatalysis Properties of a Keggin-Type Inorganic–Organic Hybrid SiW12O40@Ag. Inorganics. 2025; 13(5):132. https://doi.org/10.3390/inorganics13050132

Chicago/Turabian Style

Hu, Xin-Xin, Tai-Dan Chen, Xiao-Jie Gong, Jiu-Yu Ji, Li-Ping Zhao, Wen-Xuan Xie, and Kun Zhou. 2025. "Photocatalysis and Electrocatalysis Properties of a Keggin-Type Inorganic–Organic Hybrid SiW12O40@Ag" Inorganics 13, no. 5: 132. https://doi.org/10.3390/inorganics13050132

APA Style

Hu, X.-X., Chen, T.-D., Gong, X.-J., Ji, J.-Y., Zhao, L.-P., Xie, W.-X., & Zhou, K. (2025). Photocatalysis and Electrocatalysis Properties of a Keggin-Type Inorganic–Organic Hybrid SiW12O40@Ag. Inorganics, 13(5), 132. https://doi.org/10.3390/inorganics13050132

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