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. 2026 Sep 27;11(39):59401–59414. doi: 10.1021/acsomega.6c05530

A Hydrothermal Route to Ag–Cu2O/ZnO Heterointerfaced Nanocomposite: Delving into the Synergistic Influence in Visible Light-Driven Synthesis of 1,4-disubstituted 1,2,3-triazoles

Abhinandan Neogi 1, Nilakhi Deuri 1, Geetika Borah 1,*
PMCID: PMC13647948  PMID: 42852317

Abstract

We report a versatile hydrothermal route to synthesize an Ag–Cu2O-decorated ZnO heterointerfaced nanocomposite (Ag–Cu2O/ZnO) for visible-light-driven synthesis of 1,4-disubstituted 1,2,3-triazoles. Extensive characterization with various physicochemical techniques, viz., UV–vis spectroscopy, ICP-OES, FESEM-EDX with elemental mapping, HRTEM with SAED, powder XRD, and XPS, affirmed the formation of Ag–Cu2O nanoparticles on the surface of ZnO. ICP-OES analysis showed an ultralow content of Ag (0.000645 mmol) and Cu (0.00761 mmol) in Ag–Cu2O/ZnO. This heterointerfaced nanocomposite exhibited outstanding photocatalytic efficiency toward the azide–alkyne cycloaddition reaction, affording pharmaceutically important 1,4-disubstituted 1,2,3-triazoles with moderate to excellent yields (up to 98%) under green and sustainable reaction conditions. Surprisingly, the decoration of Ag–Cu2O bimetallic nanoparticles on the surface of ZnO nanoparticles (NPs) reduced the band gap of ZnO NPs and provided active catalytic sites through a synergistic electron-transfer effect. In addition, the nanocomposite exhibited a reduced electron–hole recombination rate, an extended lifespan, and excellent light-harvesting capability. As a result, Ag–Cu2O/ZnO demonstrated excellent photocatalytic efficiency over its monometallic counterparts, substantiating the cooperative influence of the two metals. Notably, the nanocomposite demonstrated magnificent recyclability over six cycles without an apparent loss of activity or structural integrity. Our findings provide experimental validation of a green and sustainable alternative to conventional harsh protocols for the synthesis of 1,4-disubstituted 1,2,3-triazoles.

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Introduction

In the modern era of industrialization, harnessing the power of visible light to synthesize value-added organic compounds has become a significant aspect of contemporary catalysis. The synthesis of 1,4-disubstituted 1,2,3-triazole derivatives has attracted considerable research interest owing to their structural versatility and broad spectrum of biological potential, including anticancer, antimalarial, antidiabetic, antiviral, antifungal, antitubercular, and antibacterial activities. − Triazole derivatives are three-nitrogen-containing five-membered-ring heterocycles that represent an important scaffold in medicinal chemistry due to their synthetic accessibility, high thermal and considerable metabolic stability, and diverse pharmacological activities. ,

Since the pioneering work of the German chemist R. Huisgen on the synthesis of 1,2,3-triazole at the dawn of the twenty-first century, a breakthrough came with the development of the Cu-catalyzed azide–alkyne cycloaddition reaction (CuAAC), i.e., the click reaction, by Sharpless and Meldal in 2002 in this field. Since its discovery, the CuAAC reaction has received a staggering degree of attraction, which revolutionized the field of chemistry and biology by providing a simple, high-yielding method to generate regioselective triazole derivatives with significant applications in material and pharmaceutical sciences. , Since then, several Cu-based catalytic protocols, both homogeneous (e.g., Cu­(I) complex, Cu­(II) complex, etc.) and heterogeneous (e.g., Cu2O nanocrystals, Cu2O nanocubes, etc.), have been reported to synthesize this class of compounds. − However, most of the homogeneous strategies include ligand-stabilized Cu­(I) complexes (Parvin et al. 2020), Cu­(I) salt (Wang et al. 2008; Zheng et al. 2021), surfactant-promoted Cu­(I)/(II) metal precursor (Tanaka et al. 2007), in situ generation of copper­(I) ion from copper­(II) precursors with a reducing agent, − etc. Most of these protocols are usually associated with limitations such as harsh reaction conditions, high Cu loading, high reaction time, and formation of byproducts, bistriazoles, and diacetylenes in some cases. Excessive use of copper can cause genotoxicity in the human body, chemical sensitivities, and oxidative stress. , To cope with these drawbacks, many heterogeneous catalytic methodologies were developed, which eliminate the byproduct generation, high Cu loading, etc., and add advantages, viz., easy handling, easy product separation, and good recyclability. −

Considering the ever-increasing global energy demand, photocatalysis offers a promising solution by enabling the use of light energy for organic transformations. It is a comparatively safer, environmentally friendly, and sustainable approach to replace conventional thermal catalysis. − Semiconductor metal oxides such as TiO2, ZnO, SnO2, CuO, Fe2O3, etc. are promising candidates for photocatalysis. Their suitable band gap between the valence band (VB) and the conduction band (CB) allows them to generate electron–hole pairs under light irradiation, making them especially important in photocatalysis, solar energy conversion, and photochemical cells. Reducing the recombination rate of photogenerated charge carriers leads to an overall improvement of the photocatalytic efficiency. − Effective strategies that can be used to overcome this challenge include metal doping, composite formation, surface modification, and the construction of heterojunctions. −

ZnO is an n-type wide band gap (3.37 eV) semiconductor with several advantages, like unique light harvesting properties, thermal stability, high surface area, low toxicity, biocompatibility, corrosion resistance, economic feasibility, etc., so it gets significant importance in electronics and the chemical industry as a catalyst, flame retardant, smart sensors, etc. − Its versatile nature arises from the easy tunability of its morphology and structure during synthesis, making it a suitable material for diverse applications. ZnO can exist in the form of three crystal structures: hexagonal wurtzite, zinc blende, and rocksalt. Among these, the hexagonal wurtzite structure of ZnO is the most stable phase. Despite these advantages, ZnO as a photocatalyst has a major limitation, i.e., it operates under UV light by generating electron–hole pairs. Extensive research focuses on modifying the surface of ZnO to enable its activation under visible light. − When ZnO, like an n-type semiconductor, is coupled with Cu2O, like a p-type semiconductor, a p–n type heterojunction is formed at their interface. This heterojunction promotes the easy separation of photogenerated electron–hole pairs and effectively narrows the band gap, which enhances the visible-light-driven photocatalytic efficiency. Nonetheless, pristine Cu2O suffers from certain limitations like rapid charge recombination and limited photostability, which restrict its practical application in the field of photocatalysis. , These limitations can be overcome by incorporating noble metal NPs (e.g., Ag NPs), which introduce a localized surface plasmon resonance (LSPR) effect that significantly improves the visible light absorption and charge carrier efficiency. , Thus, the heterojunction formed between ZnO/Cu2O and the metal–semiconductor interface formed between Cu2O and Ag creates an overall heterointerface system that helps toward the effective transfer of electrons necessary for photocatalysis. Moreover, the synergistic interaction between copper and silver enhances the catalytic reactivity beyond that of the individual counterparts. , In addition to boosting overall catalytic efficiency, the partial substitution of noble metals with more abundant non-noble metals reduces material costs, thereby promoting economically viable and scalable nanocatalyst fabrication. ,

Considering these and in line with our ongoing research efforts on the synthesis of nanostructures and catalysis, − here we report the hydrothermal synthesis of the Ag–Cu2O-decorated ZnO heterointerfaced nanocomposite and its catalytic efficiency in visible light-driven synthesis of 1,4-disubstituted 1,2,3-triazoles. As per our literature survey, the visible-light-promoted azide–alkyne cycloaddition reaction catalyzed by heterointerfaced ZnO-supported Ag–Cu2O bimetallic NPs has not been reported earlier. The novelty of the present work is not attributed to the individual components, i.e., Ag, Cu2O, and ZnO, or the CuAAC reaction itself, but to the rational catalyst architecture and its underlying reaction mechanism as a visible-light-responsive photocatalyst for CuAAC under mild conditions. In particular, the synergistic coupling of ultralow amounts of Ag and Cu and the construction of a heterointerface promote effective charge separation, thereby establishing a distinct structure–activity correlation. The identification of the catalytic pathway via a single-electron transfer (SET) process further underscores the mechanistic significance and principal advancement of this study.

Experimental Section

Materials and Reagents

Silver nitrate (AgNO3, 99.9%) and silica gel (mesh 60–120) were purchased from Rankem India. Cupric nitrate trihydrate (Cu­(NO3)2·3H2O, 99%) and sodium citrate tribasic dihydrate (C6H5Na3O7·2H2O, 98% extra pure) were purchased from SRL. Zinc nitrate hexahydrate (Zn­(NO3)2·6H2O, 98% AR), polyethylene glycol-400 (OH­(C2H4O)nH), sodium hydroxide pellets (NaOH, 98% AR), sodium sulfate anhydrous (Na2SO4, 99% extra pure), 2,2,6,6-tetramethylpiperidine-1-oxyl (C9H18NO, 98%), butylated hydroxytoluene (C15H24O, 99% extra pure), 1,4-diazabicyclo­(2,2,2)­octane (C6H12N2, 98%), p-benzoquinone (C6H4O2, 98%), tert-butanol ((CH3)3COH, 99% AR), and N-methyl-2-pyrrolidone (NMP, 99.8% GC) were purchased from Loba Chemie Pvt. Ltd. Poly­(vinylidene fluoride) (PVDF) was purchased from Sigma-Aldrich. The methylene blue dye was purchased from Merck. All azide and alkyne substrates were purchased from Spectrochem, Sigma-Aldrich, and TCI Chemicals and used as received without any prior treatment. Thin-layer chromatography was performed using aluminum sheets coated with silica gel 60 (Loba Chemie) and visualized under UV light.

Synthesis of Ag–Cu2O/ZnO Heterointerfaced Nanocomposite

The Ag–Cu2O/ZnO heterointerfaced nanocomposite was synthesized using several modifications to a reported method. An aqueous solution of Zn­(NO3)2·6H2O (10 mmol in 10 mL DI water) was allowed to stir at room temperature (28 °C) until a clear solution was obtained. To this solution, an aqueous solution of sodium citrate (0.5 mmol in 5 mL DI water) was added, followed by the addition of an aqueous solution of AgNO3 (1 mmol in 10 mL DI water) and an aqueous solution of Cu­(NO3)2·3H2O (1 mmol in 10 mL DI water). The reaction was allowed to stir for 30 min at room temperature (28 °C). To the above solution, an aqueous solution of NaOH (5 M, 20 mL) was added dropwise until pH = 10 was attained, and it was vigorously agitated for 1 h. A brown precipitate was formed. To this mixture, NaBH4 (10 mmol in 10 mL DI water) was added dropwise under constant stirring for 30 min. Upon the addition of NaBH4, the color of the solution changed from brown to black. The black solution was subjected to ultrasonication for 20 min, then transferred to a 100 mL Teflon-lined autoclave. The autoclave was placed in an oven at 180 °C for 12 h. The obtained products were centrifuged, washed with DI water and ethanol (3 times), and then calcined in a muffle furnace at 400 °C for 2 h (Scheme ). The product was named Ag–Cu2O/ZnO.

1. Schematic Representation for the Synthesis of Ag–Cu2O/ZnO Heterointerfaced Nanocomposite.

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We have carried out calcination at 350 and 450 °C in addition to 400 °C. But surprisingly, we observed that the nanocomposite resulting from the calcination at 400 °C contains high crystallinity and phase stability with the preservation of active surface sites. Moreover, the 400 °C temperature is suitable for the formation of a highly stable hexagonal wurtzite structure of ZnO. At temperatures lower than 400 °C, the material often consists of amorphous or poorly crystalline phases with small, irregular aggregates, whereas at temperatures higher than 400 °C, highly crystalline aggregates are formed, which reduces the active surface sites for the photocatalytic reaction to take place.

The monometallic counterparts, i.e., Ag/ZnO, Cu2O/ZnO, and pristine ZnO, were synthesized by following the same procedure.

Instrumentation

The powder X-ray diffraction (PXRD) patterns of the as-synthesized materials were obtained by monochromatic Cu Kα radiation (λ = 1.54056 Å) on a Bruker D8 Advance X-ray diffractometer. Field Emission Scanning Electron Microscopy (FE-SEM) images, Energy Dispersive X-ray (EDX) spectra, and elemental mapping of the materials were performed in a Quanta FEG 250. High-Resolution Transmission Electron Microscopy (HRTEM) images and Selected Area Electron Diffraction (SAED) patterns of the materials were obtained from a JEOL/JEM 2100 operating at 200 kV. X-ray Photoelectron Spectroscopy (XPS) analysis of the materials was performed on a Thermo Fisher Scientific K-ALPHA electron spectrometer. The binding energy values were calibrated with reference to the C-1s (284.6 eV) peak. The 1H, 13C, and 19F NMR spectra of the products were recorded in CDCl3 and DMSO-d6, using a Bruker Ascend 500 MHz spectrometer. The electrospray ionization mass spectra (ESI-MS) measurements of the products were collected from a Thermo Scientific TSQ Endura mass spectrometer. UV–vis spectra were recorded on a Shimadzu UV–vis 2600 Spectrophotometer. ICP-OES analysis of the samples was done on a PerkinElmer Avio 220 Max Optical Emission Spectrometer. BET surface area and pore size distribution data were recorded in a Quantachrome Autosorb iQ-C-XR-XR-XR and Quanta Chrome Nova E-2200 instrument. Single-crystal X-ray diffraction of one of the products was performed on a Bruker Smart Apex-II. Electrochemical impedance spectroscopy (EIS) study was carried out using a CHI-760E electrochemical workstation setup with a three-electrode setup consisting of a material-coated working electrode using PVDF as binder and NMP as solvent, a platinum wire as the counter electrode, and an Ag/AgCl electrode as the reference electrode dipped in a 3 M KOH aqueous electrolyte, and all the electrochemical measurements were carried out with respect to it. Electron spin resonance (ESR) spectroscopy was carried out at room temperature on a Bruker A300-9.5/12/S/W spectrometer. The photocatalytic reactions were performed inside a photocatalytic chamber equipped with a 12 W visible light source at room temperature (RT = 26–28 °C). The reactants were charged into an oven-dried round-bottom flask, which was kept at a distance of 6 cm from the light source. The intensity of light was 10,100 lx as measured by a lux meter.

General Procedure for the Synthesis of Triazoles via Azide–Alkyne Cycloaddition Reaction Using Ag–Cu2O/ZnO as a Photocatalyst

A mixture of azide (1 mmol), alkyne (1.1 mmol), and the catalyst, Ag–Cu2O/ZnO (10 mg), was added in a 50 mL round-bottom flask containing 3 mL of water-PEG-400 (1:1) as a solvent, followed by stirring under an optimized visible light (12 W LED) inside a photocatalytic chamber. The round-bottom flask was kept at a distance of 6 cm from the light source, and the reaction temperature was maintained at 26–28 °C by using a water-circulating jacket connected to the chamber. The progress of the reaction was constantly monitored by TLC using ethyl acetate and hexane (1:9) as the mobile phase. After completion of the reaction, the product was extracted from the organic layer using a water and ethyl acetate mixture (1:1), washed several times, and dried over anhydrous Na2SO4. The residual solvent was removed under reduced pressure to get the crude product. The crude product was purified by column chromatography using silica gel mesh (60–120) as the stationary phase and a mixture of ethyl acetate and hexane as the mobile phase to get the pure product. The isolated yields of the products were recorded. The products were characterized by 1H NMR, 13C NMR, 19F NMR (wherever necessary), and ESI-MS analysis.

Procedure for Gram-Scale Synthesis of Triazole

The gram-scale synthesis of triazole was carried out by taking benzyl azide with three different loadings [0.5 g (3.755 mmol), 1 g (7.51 mmol), and 2 g (15.02 mmol)] with phenyl acetylene [0.422 g (4.13 mmol), 0.844 g (8.261 mmol), and 1.688 g (16.522 mmol)] in a round-bottom flask, following the same procedure as mentioned above.

Procedure for Scavenging Experiments

A mixture of azide (1 mmol), alkyne (1.1 mmol), scavenger (2 mmol), and Ag–Cu2O/ZnO (10 mg) was placed in a 50 mL round-bottom flask containing 3 mL of H2O-PEG-400 mixture (1:1), and the round-bottom flask was kept at a distance of 6 cm from the light source. It was allowed to stir inside a photocatalytic chamber equipped with a 12 W visible light source, maintained at a temperature of 26–28 °C with the help of a water circulating jacket connected to the chamber. Various scavengers like TEMPO, BHT, DABCO, tert-butyl alcohol, and p-benzoquinone were used. The progress of the reaction was constantly monitored by TLC using ethyl acetate and hexane (1:9) as the mobile phase. After completion of the reaction, the solid catalyst was separated by filtration from the aqueous layer, while the product was extracted from the organic layer by using a water and ethyl acetate mixture (1:1), washed several times, and dried over anhydrous Na2SO4. The residual solvent was removed under reduced pressure with the help of a rotatory evaporator to get the crude product. The crude product was purified by column chromatography using silica gel mesh (60–120) as the stationary phase and a mixture of ethyl acetate and hexane as the mobile phase to obtain the pure product. The isolated yields of the products were recorded.

Results and Discussion

The crystallinity and phase purity of the synthesized materials were determined by powder X-ray diffraction analysis. The powder X-ray diffraction pattern of the synthesized Ag–Cu2O/ZnO, depicted in Figure , showed sharp peaks, indicating its crystalline nature. However, compared to the pristine ZnO (Figure S1a) and monometallic counterparts (Figure S1b,c), the PXRD peaks indicate lower crystallinity of Ag–Cu2O/ZnO. The peaks obtained at 31.9°, 34.5°, 36.4°, 47.7°, 56.7°, 62.9°, 66.4°, 68.1°, 69.2°, 72.6°, 76.9°, and 81.5° were indexed with the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202), and (104) crystal planes of ZnO (JCPDS Card No. 96-210-7060) (Figure S1a). From the standard JCPDS pattern, the hexagonal wurtzite crystal structure of ZnO was confirmed, which corresponds to the stable phase of ZnO. In the case of Ag/ZnO, the additional peaks at 38.1°, 44.3°, 64.6°, and 77.1° correspond to (111), (200), (220), and (311) planes of metallic Ag (JCPDS Card No. 01-087-0597) (Figure S1b). As shown in Figure S1c, the Cu2O/ZnO exhibited very low intensity peaks at 42.4°, 61.6°, 73.6°, and 85.3°, attributed to the (200), (220), (311), and (321) planes of Cu2O (JCPDS Card No. 01-074-1230), in addition to peaks for ZnO. The Ag–Cu2O/ZnO nanocomposite displayed peaks for metallic Ag, Cu2O and ZnO as well (Figure ), affirming the formation of Ag–Cu2O/ZnO heterointerfaced nanocomposite.

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Powder XRD pattern of Ag–Cu2O/ZnO.

For an extensive investigation of the surface morphology and composition of the as-synthesized nanocomposite, field emission scanning electron microscopy-energy dispersive X-ray (FESEM-EDX), along with EDX mapping and HRTEM analysis, is carried out. As shown in Figure a–d, upon incorporation of a single metal, the plate-like morphology of pristine ZnO (Figure a) changes slightly (Figure b,c); however, after decoration of both Ag and Cu2O, it changes significantly (Figure d). The EDX mapping and spectra confirm the presence and homogeneous distribution of different elements present in Ag–Cu2O/ZnO, as depicted in Figure e–i. The EDX mapping and EDX elemental spectra of ZnO, Ag/ZnO, and Cu2O/ZnO are given in Figure S2.

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FE-SEM images of (a) ZnO, (b) Ag/ZnO, (c) Cu2O/ZnO, (d) Ag–Cu2O/ZnO, (e–h) EDX mapping of Ag–Cu2O/ZnO, (i) EDX spectra of Ag–Cu2O/ZnO.

The high-resolution transmission electron microscopy (HRTEM) analysis of Ag–Cu2O/ZnO, as demonstrated in Figure a–e, shows that the ZnO NPs mostly appear as irregular, quasi-hexagonal-shaped, as shown in the yellow circles (Figure b,c), resembling the presence of a hexagonal structure of ZnO as confirmed by powder XRD analysis and short-range aggregated domains. The spherical-shaped dark black spots of bimetallic Ag–Cu2O NPs are supported on the ZnO surface with an average particle size of 27.63 nm (Figure d–f). In Figure e, it can be seen that distinct lattice fringes of ZnO and Ag–Cu2O are separated by an interfacial boundary where the dark black portion is occupied by bimetallic Ag–Cu2O NPs, as confirmed by the observed interplanar spacings of 0.24 and 0.23 nm attributed to the (111) planes of Cu2O and Ag. This affirms the formation of a heterointerface between ZnO and Ag–Cu2O. The dark black spots are due to the larger density of Ag and Cu2O compared to ZnO. In Figure g–h, distinct lattice fringes are observed from the HRTEM images with interplanar spacings of 0.29 and 0.26 nm attributed to the (100) and (002) planes of ZnO, while the interplanar spacings of 0.24 and 0.23 nm correspond to the (111) plane of Cu2O and Ag, respectively. The selected area electron diffraction (SAED) pattern exhibits bright spots affirming the polycrystalline nature of the nanocomposite, and it shows clear reflections from the (200), (110), and (100) planes of ZnO, (311) and (222) of Cu2O, and (200) of Ag that match well with the standard XRD pattern (Figure i).

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(a–e) TEM images of Ag–Cu2O/ZnO, (f) particle size distribution curve, (g–h) high-resolution TEM images of Ag–Cu2O/ZnO, (i) SAED pattern.

To precisely determine the surface composition and chemical states of Zn, O, Ag, and Cu in the Ag–Cu2O/ZnO nanocomposite, we have performed X-ray photoelectron spectroscopy (XPS). The spectrum for C-1s is calibrated to a value of 284.6 eV, and all other spectra are deconvoluted with respect to this value. , The XPS survey scan showed the presence of C, O, Ag, Cu, and Zn (Figure a). The XPS of Zn 2p showed two peaks at 1021.9 and 1044.9 eV corresponding to Zn 2p3/2 and Zn 2p1/2, respectively. This enumerates the presence of Zn (+2) in the heterointerfaced nanocomposite (Figure c), − whereas for oxygen, the peak at 530.5 eV (O 1s) is assigned to O2– ions, which confirms the Zn–O bond formation, and the other peak at 531.9 eV can be attributed to the interstitial oxygen atoms or surface oxygen in the form of hydroxyl groups (Figure d). − In the case of Ag, the two peaks at 367.7 eV (Ag 3d5/2) and 373.7 eV (Ag 3d3/2), separated by 6 eV, correspond to the metallic form of Ag. The binding energy of Ag is slightly shifted to lower energy compared to pure Ag due to the interaction between ZnO and Ag (Figure e). , For Cu, the peaks at 932.8 eV (Cu 2p3/2) and 952.9 eV (Cu 2p1/2) correspond to the (+1) oxidation state of Cu, i.e., Cu is in Cu2O state, and the satellite peaks at 941.7, 943.5, and 962 eV refer to the (+2) oxidation state of Cu, which indicates that there is a slight amount of Cu present in the CuO state (Figure f). ,, Shifts in Ag and Cu binding energies can indicate electronic interaction and charge redistribution between the components. Figures S3, S4, and S5 show the XPS of bare ZnO, Ag/ZnO, and Cu2O/ZnO, respectively. To further elucidate the interfacial interaction and charge transfer process in Ag–Cu2O/ZnO, we have compared the XPS spectra of Ag–Cu2O/ZnO (Figure ) with pristine ZnO (Figure S3), and it was observed that in the case of Ag–Cu2O/ZnO, there was a slight shift in the binding energy of Zn 2p to a higher value, which indicates the lowering of electron density on ZnO in the nanocomposite. Also, as mentioned earlier, the binding energy of Ag is slightly shifted to a lower value compared to pure Ag, indicating the increase in electron density on Ag in the nanocomposite. This strongly suggests that the electron transfer is from ZnO to Cu2O and finally to metallic Ag, which acts as an electron reservoir.

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Deconvoluted XPS of Ag–Cu2O/ZnO: (a) survey scan, (b) C 1s, (c) Zn 2p, (d) O 1s, (e) Ag 3d, (f) Cu 2p.

The specific surface area of bare ZnO and Ag–Cu2O/ZnO nanocomposite was measured by using the BET method according to their N2 adsorption–desorption isotherms at 77.35 K, displayed in Table . The specific surface areas for ZnO and Ag–Cu2O/ZnO were found to be 50.577 m2/g and 58.756 m2/g, respectively, and the pore volumes were found to be 0.057 cc/g and 1.589 cc/g (Figure ). The enhanced surface area of the Ag–Cu2O/ZnO heterointerfaced nanocomposite compared to pristine ZnO may be attributed to the Ag–Cu2O NP-induced structural defects, affirming the formation of additional active sites, which in turn enhance the photocatalytic efficiency of the nanocomposite. The pore diameters of ZnO and the Ag–Cu2O/ZnO nanocomposite, calculated by using the Barrett–Joyner–Halenda (BJH) model, were found to be 1.909 and 3.416 nm, respectively (insets of Figure a,b). The graphs correspond to the mesoporous nature of the Ag–Cu2O/ZnO nanocomposite, while the pristine ZnO lies at the borderline of mesoporous and microporous. ,

1. N2 Adsorption-Desorption Study of ZnO and Ag–Cu2O/ZnO Heterointerfaced Nanocomposite.

Serial no. Materials Surface area (m2/g) Pore volume (cc/g) Pore diameter (nm)
1 ZnO 50.577 0.057 1.909
2 Ag–Cu2O/ZnO 58.756 1.589 3.416
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N2 adsorption–desorption isotherms of (a) bare ZnO, (b) Ag–Cu2O/ZnO (inset shows the respective pore size distribution graphs between pore volume distribution and pore diameter).

UV–visible analysis was performed to investigate the light-harvesting characteristics of the as-prepared nanostructures (Figure ). All the nanomaterials, i.e., pristine ZnO, Ag/ZnO, Cu2O/ZnO, and Ag–Cu2O/ZnO, showed strong absorption in the UV region. A plasmonic transition appeared at around 461 nm for Ag/ZnO, which could be attributed to the presence of silver (Figure b). In the case of Cu2O/ZnO, there was a small peak in the visible region at 429 nm (Figure c). After anchoring Ag–Cu2O nanoparticles on the surface of bare ZnO, the absorption band experienced a red shift in the visible region (484 nm) with a long absorption tail in the visible region (Figure d). Using Tauc’s plot method, the corresponding band gap energies for bare ZnO, Ag/ZnO, Cu2O/ZnO, and Ag–Cu2O/ZnO were found to be 3.14, 3.1, 3.12, and 2.9 eV, respectively. The synergism between two metal centers (Ag and Cu) is responsible for the reduced band gap energy of Ag–Cu2O/ZnO (Figure d).

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UV–vis spectra and Tauc plot in set: (a) ZnO, (b) Ag/ZnO, (c) Cu2O/ZnO, (d) Ag–Cu2O/ZnO.

Electrochemical impedance spectroscopy (EIS) is a well-established technique employed to study the charge transfer process at the materials’ interface. Figure a shows the electrochemical impedance spectra (Nyquist plot) of the as-synthesized materials. A material exhibiting good synergistic interaction would effectively decrease electron–hole pair recombination and efficiently increase charge separation. As shown in Figure , in the Nyquist plot, Ag–Cu2O/ZnO has the lowest diminished semicircle compared to Ag/ZnO, Cu2O/ZnO, and bare ZnO, which clearly indicates better interfacial charge transfer and efficient charge separation. The insets of Figure b,c show the enlarged portions of the semicircles of Ag/ZnO, Cu2O/ZnO, and Ag–Cu2O/ZnO, respectively.

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(a) EIS spectra (Nyquist plot) of ZnO, Ag/ZnO, Cu2O/ZnO, and Ag–Cu2O/ZnO, (b) and (c) enlarged portions of the semicircles of Ag/ZnO, Cu2O/ZnO, and Ag–Cu2O/ZnO, respectively.

Photocatalytic Activity of Ag–Cu2O/ZnO Heterointerfaced Nanocomposite toward Azide–Alkyne Cycloaddition Reaction

To examine the photocatalytic activity of the Ag–Cu2O/ZnO heterointerfaced nanocomposite toward the azide–alkyne cycloaddition reaction, a series of reactions were carried out using benzyl azide and phenyl acetylene as model substrates to ascertain the optimal reaction conditions. Initially, we carried out the reactions in different solvents like water, ethanol, ethylene glycol, PEG-200, PEG-400, acetonitrile, DMF, DMSO, toluene, etc., using 10 mg of the catalyst (Table , entries 1–17). Among these, the water-PEG-400 (1:1) mixture was found to be the best one, affording a 98% isolated yield of the product in 20 min. The increase in product yield in a short duration of time may be attributed to the better dispersion of the catalyst in the water:PEG-400 solvent medium. With 3 mL of solvent, we varied the amount of catalyst and observed that with a decrease in the amount of catalyst loading to 5 mg, the yield decreased, while the yield remained the same (98%) even with enhanced catalyst loading (Table , entries 18–20). Figure shows the change in the product yield with the change in catalyst loading. Further, the reaction was optimized using Ag/ZnO, Cu2O/ZnO, Cu2O, and bare ZnO, which afforded either a trace amount or a low isolated yield of the product, except Cu2O/ZnO and Cu2O, which afforded yields of 66% and 60%, respectively (Table , entries 24–27). In the absence of the catalyst, the reaction did not proceed despite the presence of visible light (Table , entry 21). Again, while we performed the reaction with 10 mg of the catalyst in the dark, it ended with no product formation (Table , entry 22). This implies that the presence of both a catalyst and visible light is essential for the progress of the said reaction. Applying direct sunlight instead of an artificial LED light afforded 74% product yield in 5 h (Table , entry 23). The reaction was carried out with various salts of zinc, silver, and copper as the catalyst in the presence of visible light in water-PEG-400 (1:1) solvent, which gave either a very poor yield or a trace amount of the product, except for CuI, which afforded a yield of 62% (Table , entries 28–33). So, 1.0 mmol of benzyl azide with 1.1 mmol of phenyl acetylene in 3 mL of water-PEG-400 (1:1) as a solvent under visible light in the presence of 10 mg of Ag–Cu2O/ZnO as a catalyst gave the highest yield of the desired product in 20 min at room temperature.

2. Optimization of Reaction Conditions with Benzyl Azide and Phenyl Acetylene for the Synthesis of 1,4-Disubstituted 1,2,3-Triazoles .

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Entry Catalyst Amount (mg) Solvent (mL) Time (h/min) Isolated yield %
1 Ag–Cu2O/ZnO 10 Water 4 h 83%
2 Ag–Cu2O/ZnO 10 Ethanol 5 h 74%
3 Ag–Cu2O/ZnO 10 Isopropanol 2 h 79%
4 Ag–Cu2O/ZnO 10 Ethylene glycol 3 h 82%
5 Ag–Cu2O/ZnO 10 PEG-200 2.5 h 82%
6 Ag–Cu2O/ZnO 10 PEG-400 2 h 83%
7 Ag–Cu2O/ZnO 10 Chloroform 24 h Trace
8 Ag–Cu2O/ZnO 10 DMSO 4 h 60%
9 Ag–Cu2O/ZnO 10 Acetonitrile 10 h 35%
10 Ag–Cu2O/ZnO 10 Water:Ethanol (1:1) 3 h 88%
11 Ag–Cu2O/ZnO 10 Water:PEG-200 (1:1) 35 min 95%
12 Ag–Cu 2 O/ZnO 10 Water:PEG-400 (1:1) 20 min 98%
13 Ag–Cu2O/ZnO 10 Water:Ethylene glycol (1:1) 50 min 94%
14 Ag–Cu2O/ZnO 10 Water:Isopropanol (1:1) 1 h 92%
15 Ag–Cu2O/ZnO 10 DMF 7 h 42%
16 Ag–Cu2O/ZnO 10 DCM 24 h Trace
17 Ag–Cu2O/ZnO 10 Toluene 24 h Trace
18 Ag–Cu2O/ZnO 5 Water:PEG-400 (1:1) 1 h 90%
19 Ag–Cu2O/ZnO 15 Water:PEG-400 (1:1) 20 min 98%
20 Ag–Cu2O/ZnO 20 Water:PEG-400 (1:1) 20 min 98%
21 Water:PEG-400 (1:1) 24 h NR
22 Ag–Cu2O/ZnO (dark) 10 Water:PEG-400 (1:1) 24 h NR
23 Ag–Cu2O/ZnO (direct sunlight) 10 Water:PEG-400 (1:1) 5 h 74%
24 Ag/ZnO 10 Water:PEG-400 (1:1) 10 h 27%
25 Cu2O/ZnO 10 Water:PEG-400 (1:1) 3 h 66%
26 Cu2O 10 Water:PEG-400 (1:1) 5 h 60%
27 ZnO 10 Water:PEG-400 (1:1) 10 h Trace
28 Zn(NO3)2 ·6H2O 10 Water:PEG-400 (1:1) 12 h Trace
29 ZnCl2 10 Water:PEG-400 (1:1) 16 h Trace
30 AgNO3 10 Water:PEG-400 (1:1) 8 h 22%
31 Cu(SO4)2 10 Water:PEG-400 (1:1) 10 h Trace
32 Cu(NO3)2 ·3H2O 10 Water:PEG-400 (1:1) 11 h Trace
33 CuI 10 Water:PEG-400 (1:1) 4 h 62%
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a

Reaction conditions: benzyl azide1 (1 mmol), phenyl acetylene2 (1.1 mmol), solvent (3 mL), visible light irradiation at RT (26–28 °C), 10 mg of catalyst (0.11430 mmol Zn, 0.000645 mmol Ag, and 0.00761 mmol Cu).

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Effect of catalyst amount on isolated yield.

After confirming the optimal reaction conditions, we next examined the versatility of the catalyst for diverse azides and alkynes. A plethora of azides and alkynes having functional groups such as halogens, methoxy, esters, etc., were employed as substrates. It was observed that both electron-donating and electron-withdrawing groups have nearly the same effect toward the product formation, affording yields in the range of 66–98% (Table ). However, aliphatic terminal alkynes were less effective compared with the aromatic alkynes. Figure demonstrated the light on/off experiment, and it was observed that when the light was turned off, the reaction did not proceed; when the light was turned on, it proceeded.

3. Substrate Scope of the Ag–Cu2O/ZnO Catalyzed Photocatalytic AAC Reaction with Different Azides and Alkynes .

graphic file with name ao6c05530_0016.webp

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a

Reaction conditions: azide (1 mmol), alkyne (1.1 mmol), Ag–Cu2O/ZnO (10 mg), water: PEG-400 (3 mL), visible light irradiation at RT (26–28 °C), time (20 min). The isolated yield refers to the pure and isolated products. TON was calculated with respect to the number of moles of Ag.

9.

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Light on–off experiment.

Plausible Mechanism

The reaction proceeds through a single-electron transfer (SET) pathway by absorbing light at the ZnO’s surface. Upon irradiation with light, the electrons from the valence band (VB) are excited to the conduction band (CB) of ZnO, leading to an electron–hole (e–/h+) pair generation (Figure ). The excited electrons were subsequently transferred to the conduction band of Cu2O and finally migrated to the surface of metallic Ag, which acts as an electron repository. Simultaneously, the holes (h+) generated in the VB of ZnO promote single-electron oxidation of the alkyne, which results in an alkyne radical cation A. This reactive species A rapidly combines with the azide to form azide–alkyne radical intermediate B. Afterward, intermediate B produces the triazole precursor C by cyclization. In the last and final step, electrons stored in the metallic Ag surface are transferred back to intermediate C, resulting in its reduction and producing the required 1,4-disubstituted 1,2,3-triazole product. , This continuous process is a result of the cooperative roles of metallic Ag, Cu2O, and ZnO, which provide efficient and continuous electron transport, while the alkyne oxidation refills the continuous flow of electrons. The coexistence of traces of CuO as observed from XPS analysis may facilitate the Cu2+/Cu+ redox couple, promote interfacial charge transfer, and contribute to the photocatalytic process.

10.

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Plausible reaction mechanism of photocatalytic azide–alkyne cycloaddition reaction.

Now, to get a better insight into the mechanism, some quenching experiments were performed (Table S1). First, we introduced 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO; 2.0 mmol) into the reaction mixture, which resulted in the formation of a TEMPO adduct and a trace amount of the desired triazole, thus suggesting the formation of free radicals (Table S1, entry 1). Introducing butylated hydroxytoluene (BHT; 2.0 mmol) afforded a trace amount of triazole formation (Table S1, entry 2). The other radical scavengers, namely, tert-butyl alcohol (TBA, scavenger for ·OH), p-benzoquinone (BQ, scavenger for ·O2 –), and 1,4-diazabicyclo­[2,2,2]­octane (DABCO, scavenger for 1O2), exhibited minimal impact on the yields of the triazole product (Table S1, entries 3–5).

To confirm the formation of radical intermediates during the progress of the reaction, an ESR spectrum was recorded at room temperature for the species obtained from entry 2, Table S1, after product and catalyst separation. In the presence of BHT, a distinct 4-peak ESR signal was observed, with the strongest peak having a g value of 2.006 (Figure ), which confirms the formation of the BHT-phenoxy radical and strongly indicates that the radical species A (Figure ) captured the labile hydrogen of BHT and thereby suppressed the free radical mechanism during the photocatalytic reaction with a trace amount of triazole product formation. Therefore, this confirms that during the course of the photocatalytic reaction, radical intermediates were formed. The ESI-MS spectrum of the BHT-phenoxy radical is shown in Figure S87.

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ESR spectrum of BHT-phenoxy radical.

Photocatalytic Degradation of Methylene Blue

To further assess the photocatalytic efficacy of the prepared catalyst in an additional photocatalytic application, methylene blue (MB) degradation experiments were conducted under visible-light irradiation. A 300 mL aqueous MB solution with a concentration of 5 mg/L was prepared and equally distributed into three round-bottom flasks, each containing 100 mL of the solution. The first flask was used as a control without any photocatalyst, whereas 0.5 and 1.0 g of the Ag–Cu2O/ZnO photocatalyst were added to the second and third flasks, respectively. Before visible-light irradiation, the two catalyst-containing suspensions were stirred in the dark for 1 h to establish adsorption–desorption equilibrium between MB molecules and the photocatalyst surface. The photocatalytic degradation of MB with time was determined by measuring the optical absorption of the MB solution at different time intervals (Figure ). The performance of the photocatalytic degradation was compared with other reported methods under identical conditions. Encouragingly, the Ag–Cu2O/ZnO photocatalyst exhibited excellent photocatalytic performance, with both 0.5 and 1.0 g catalyst loadings achieving complete degradation of the MB solution within 20 min under visible-light irradiation. In contrast, the MB solution without photocatalyst showed negligible degradation throughout the irradiation period.

12.

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UV–vis absorption spectra of MB: (a) without catalyst, (b) with 0.5 g of Ag–Cu2O/ZnO, and (c) with 1 g of Ag–Cu2O/ZnO.

Gram-Level Scalability

graphic file with name ao6c05530_0018.webp

To explicate the synthetic productiveness of this protocol, three gram-scale reactions have been performed taking the substrate benzyl azide with three different loadings 1a [0.5 g (3.755 mmol), 1g (7.51 mmol), and 2g (15.02 mmol)] with phenyl acetylene, 2a [0.422g (4.13 mmol), 0.844g (8.261 mmol), and 1.688g (16.522 mmol)] under the optimized reaction conditions. As per our observation, reactions afforded the consequent product, 1-benzyl-4-phenyl-1,2,3-triazole (3aa), in excellent isolated yields (85%, 83%, and 82%, respectively). Thus, it affirmed the feasibility of the reported catalyst toward large-scale production. The graphical representations of the outcomes are shown in Figure .

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Gram-level scalability.

Heterogeneity Test

A hot filtration test was performed to examine the catalyst’s heterogeneity. A round-bottom flask was charged with 10 mg of the catalyst and 4 mL of a distilled water-PEG-400 (1:1) mixture as the solvent and heated at 80 °C for 1 h. The catalyst was then carefully filtered from the mixture and cooled to room temperature (28 °C). Two mL of the filtrate was used as a solvent with benzyl azide (1 mmol) and phenyl acetylene (1.1 mmol) as the model substrate for 20 min under the illumination of a 12 W LED at room temperature, and no product formation was observed, whereas a parallel reaction under optimized reaction conditions by using the remaining 2 mL of the above filtrate with the catalyst showed 98% product formation. The ICP-OES analysis of the filtrate revealed a residual amount of Ag and Cu (<0.01 ppm). This suggests the catalyst’s genuine heterogeneous nature throughout the reaction (Figure S6).

Reusability Test

Reusability is an important aspect in catalysis for the sustainable use of catalysts. The recyclability experiment was conducted by using benzyl azide (1 mmol) and phenyl acetylene (1.1 mmol) under optimized conditions. Upon completion of the reaction, the catalyst was recovered by filtration, repeatedly washed with distilled water and ethyl acetate, dried in an oven, and used in a fresh reaction. To our delight, the catalyst Ag–Cu2O/ZnO demonstrated good recyclability (6 cycles) with no apparent loss in yield of the product and no noticeable deactivation (Figure S7). The FE-SEM image of the recycled catalyst after the sixth cycle exhibited no significant agglomeration or morphological changes, which indicates good structural stability and reusability of the catalyst. The TEM image of the recycled catalyst after the sixth cycle showed minimal loss of structural integrity compared with the fresh catalyst, whereas a PXRD pattern of the catalyst recorded after the sixth cycle retained the pattern of the fresh catalyst, except for two additional peaks at 2θ = 35.6° and 38.8°, which could be attributed to the presence of a small amount of CuO and Ag2O (Figure S8).

Conclusions

The present study reports a facile hydrothermal route to integrate ZnO, Cu2O, and Ag in a heterointerfaced nanocomposite, Ag–Cu2O/ZnO. The visible-light-promoted photocatalytic efficiency of the as-synthesized nanocomposite was examined for the synthesis of 1,4-disubstituted 1,2,3-triazole via the azide–alkyne cycloaddition reaction. The room-temperature catalytic reaction under visible light, excellent product yield (up to 98%), use of water and PEG-400 mixture (1:1) as a solvent, very short reaction time (20 min), multiple-time retrievability, and gram-level scalability of the product affirmed the reported protocol as a green and sustainable one. The broad-spectrum applicability of the catalyst toward diverse substituted azides and alkynes is an added advantage of it.

Supplementary Material

ao6c05530_si_001.pdf (5.7MB, pdf)

Acknowledgments

The authors express their gratitude to the Department of Science and Technology, Govt. of India, for the financial support under the scheme DST-FIST from 2021 to 2026 (no. SR/FST/CS-I/2020/152) and DST-PURSE (no. SR/PURSE/2022/143(C)). N.D. remains grateful to the Ministry of Tribal Affairs for the NFST (National Fellowship and Scholarship for Higher Studies of ST Students, Award number: 202425-NFST-ASS-00085) fellowship. The authors are thankful to STIC, Kochi University, Kochi, for HRTEM and powder XRD analysis; BITS Pilani, Goa Campus, for FESEM-EDX with elemental mapping; ARF-JNCASR for XPS analysis; SIF-BITS Pilani, Rajasthan, for BET analysis; Materials Analysis & Research Centre, Bengaluru, for BET and ESR analysis; and SAIC Tezpur University for single crystal XRD analysis. The CSIC Dibrugarh University and the Department of Physics, Dibrugarh University, are gratefully acknowledged for NMR and UV–vis analysis, respectively. The authors are also thankful to the Department of Chemistry, Dibrugarh University, for ICP-OES, ESI-MS, and EIS facilities.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05530.

  • General information; powder XRD pattern of ZnO, Ag/ZnO, Cu2O/ZnO (Figure S1); EDX mapping and EDX elemental spectra of ZnO, Ag/ZnO, Cu2O/ZnO (Figure S2); XPS of bare ZnO (Figure S3); XPS of Ag/ZnO (Figure S4); XPS of Cu2O/ZnO (Figure S5); heterogeneity test (Figure S6); reusability test (Figure S7); FE-SEM image of recycled catalyst after 6th cycle, TEM image of recycled catalyst after 6th cycle, and PXRD pattern of fresh and recycled catalyst after 6th cycle (Figure S8); table for scavenging experiments (Table S1); comparison table for azide alkyne cycloaddition reaction (Table S2); 1H NMR, 13C NMR, 19F NMR, and ESI-MS spectra of the products (Figures S9–S85); ESI-MS spectrum of tempo adduct (Figure S86); ESI-MS spectrum of BHT-phenoxy radical (Figure S87); single crystal XRD study of 3aa (Figure S88); crystallographic parameters of product 3aa (Table S3) (PDF)

All authors have made equal contributions to the preparation of the manuscript. All the authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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