Fabrication of Nano-Ag Encapsulated on ZnO/Fe2V4O13 Hybrid-Heterojunction for Photodecomposition of Methyl Orange

Abstract

Novel silver encapsulated nanocomposite zinc oxide/iron tetra-poly-vanadate (Ag-ZnO/Fe₂V₄O₁₃) was synthesized with various wt% of silver (1.0–2.5 wt% of Ag) by cost-effective photo-deposition method under the irradiation of ultraviolet-A (UV-A) light. The nanostructure of the Ag-ZnO/Fe₂V₄O₁₃ was explored by various characterization techniques. The surface functionalities were confirmed by Fourier transform infrared spectra and the crystalline nature of the material was revealed by X-ray diffraction patterns. Furthermore, the surface morphology and the optical properties of the composites were analyzed by scanning electron microscopy, energy dispersive X-ray–elemental color mapping (ECM), high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible diffuse reflectance spectroscopy and photoluminescence. The crystallite size of Ag-ZnO/Fe₂V₄O₁₃ was 28.5 nm which was consistent with HRTEM analysis. The photocatalytic activity was tested against aqueous methyl orange degradation under UV-A light irradiation. In all five runs, the stability of the catalyst was confirmed by reusability measurements and almost 98% of degradation was achieved. A suitable degradation pathway was proposed based on intermediates obtained during the degradation analyzed by gas chromatography–mass spectrometry. Trapping experiments confirmed that the superoxide radical anion (O₂^•−) was considered as the most active species for this degradation process. Complete mineralization was confirmed by the measurements of chemical oxygen demand.

1. Introduction

Textile industries are becoming the chief source of pollution to the environment by releasing dye pollutants [1]. Worldwide, dye pollutants of about 150 tons are released into the aquatic system in a day [2]. The discharge of dye effluent into water bodies is a major cause of non-esthetic degradation and these dyes are also resistant to aerobic degradation and can be converted into carcinogenic aromatic amines under anaerobic conditions [3,4]. Scientists have expressed a great deal of interest in the development of nanotechnology. The removal of contaminants from wastewater has made extensive use of a variety of nanomaterials including nanometals, nanometal oxides, graphene or graphene-based nanomaterials and polymer-based nanomaterials [5]. Advanced oxidation processes (AOPs) producing hydroxyl radical (OH) as one of the most effective oxidants are considered promising techniques. Photocatalytic processes based on the application of semiconductors as a photocatalyst for the degradation of toxic organic contaminants in aqueous phases have been widely studied among various AOPs [6,7,8,9,10]. In the photocatalytic activity of certain dye molecules, zinc oxide (ZnO) is documented to be more effective than titanium dioxide (TiO₂) [11,12]. It has been observed that ZnO is a good choice for photocatalyst applications. In addition, ZnO is found to be an efficient alternative to TiO₂ since ZnO can absorb a more significant portion of ultraviolet (UV) light [13]. However, ZnO has a wide band gap of 3.37 eV, which ultimately limits photodegradation due to rapid electron–hole recombination. Many attempts have been made to enhance the effective charge separation in ZnO to address this limitation, thus strengthening its photocatalytic ability [14,15,16,17,18].

Iron tetra-poly-vanadate (Fe₂V₄O₁₃) has tetrahedrally coordinated VO₄ units and octahedrally coordinated Fe₂O₁₀, where VO₄ units are connected to octahedrally coordinated Fe₂O₁₀ through edge sharing, creating a unique chain structure resembling a horseshoe [19]. There are numerous applications, since it has tetrahedral and hexagonal holes and channels. For example, it has been researched as an effective cathode for lithium secondary batteries [20], a water oxidation [21] photocatalyst to degrade organic pollutants [22] and a photocatalyst to reduce carbon dioxide (CO₂) [23,24].

Encapsulation of noble metal nanocomposite has proven efficacy due to the strong surface plasma resonance of noble metal which extended visible light absorption with charge separation. Recently, due to its high performance photocatalytic ability, nano-silver (AgNPs), when it is combined with semiconductors, has received intense attention. Ag has been identified as the best element to encapsulate with ZnO, due to its high solubility [25]. Recently, photocatalysts based on Ag have been developed as effective photocatalysts for pollutant degradation [26,27,28,29,30].

In this study, a series of different wt% of Ag encapsulated ZnO/Fe₂V₄O₁₃ were prepared via a simple and cost-effective photo-deposition method. The synthesized sample is characterized using techniques such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy combined with energy dispersive X-ray (SEM-EDX), elemental color mapping (ECM), high-resolution transmission electron microscopy (HR-TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV-vis-DRS) and photoluminescence (PL). The photocatalytic properties of nanocomposites Ag-ZnO/Fe₂V₄O₁₃ were assessed through methyl orange (MO) degradation under UV irradiation. The enhancement photodegradation due to Ag can shuttle the photogenerated electron from ZnO in Ag-ZnO/Fe₂V₄O₁₃ and can reduce the recombination of electron–hole pair effectively. Based on the obtained results, a schematic photocatalytic mechanism has been proposed.

2. Experimental

Fabrication of Ag Coated with ZnO/Fe₂V₄O₁₃

The syntheses of Fe₂V₄O₁₃ and Fe₂V₄O₁₃/ZnO were reported in our previous study [22,31,32]. Different concentrations of Ag⁺ ions were added into the suspension (C₂H₅OH/H₂O as solvent) of 3 g of synthesized Fe₂V₄O₁₃/ZnO such that the Ag⁺ concentration was (X wt%, where X = 1, 1.5, 2 and 2.5) in relation to Fe₂V₄O₁₃/ZnO. The mixed suspensions were placed in a photoreactor and then irradiated for 3 h with 8 × 8 W mercury lamps with UV-A light under continuous stirring. During irradiation, Ag⁺ was converted into Ag and deposited on the nanocomposite, ZnO/Fe₂V₄O₁₃. The Ag-ZnO/Fe₂V₄O₁₃ were centrifuged and dried at 100 °C for 2 h and then used as photocatalyst. The chemicals used for this study, instrumental specification for the characterization techniques and the procedures for measuring photocatalytic experimental and chemical oxygen demand (COD) are given in the Supporting Information. The photoreactor used for this study is given in Figure S1 (Supplementary Materials).

3. Results and Discussion

3.1. FT-IR

Figure 1 shows FT-IR spectra of prepared ZnO, Fe₂V₄O₁₃ and X-Ag-ZnO/Fe₂V₄O₁₃ (X = 1, 1.5, 2 and 2.5 wt%). The stretching vibration of the surface hydroxyl group appeared in the range between 3453 and 3405 cm⁻¹ for all the samples [22,32]. In Figure 1a, the peaks were observed at 420, 447 and 543 cm⁻¹, which could be attributed to the Zn–O stretching modes [31,32,33]. In Figure 1b, the peaks at 1028, 711 and 510 cm⁻¹ are assigned to V–O, V–O–Fe and Fe–O stretching modes, respectively [31]. Bands at 474 and 426 cm⁻¹ were attributed to the characteristic stretching mode of Ag–O and Zn–O bonds [34]. Figure 1c–f shows various functional groups of Ag-ZnO/Fe₂V₄O₁₃ and the metal oxide bond present in the compound, Fe–O, Ag–O and Zn–O vibrations are observed at 1015, 1100, 474, 426 and 413 cm⁻¹, respectively. This indicates that Ag was effectively loaded on the ZnO/Fe₂V₄O₁₃ nanocomposite.

3.2. XRD

Figure 2 shows the typical XRD patterns of the prepared ZnO (Figure 2a), Fe₂V₄O₁₃ (Figure 2b) [31] and ZnO/Fe₂V₄O₁₃ samples with different loading concentrations of Ag (X = 1, 1.5, 2 and 2.5 wt%) (Figure 2c–f). The wurtzite ZnO (JCPDS Card No. 36-1451) planes were observed for the prepared ZnO at 2θ values of 31.75, 34.40, 36.23, 47.53, 56.60 and 62.34° corresponding to (100), (002), (101), (102), (110) and (103) planes, respectively [35,36]. In Figure 2b, Fe₂V₄O₁₃ shows three diffraction peaks at 2θ angles of 12.5, 22.7 and 26.7° that correspond to (002), (022) and (014) planes of Fe₂V₄O₁₃, respectively and the peaks are well matched to the monoclinic phase of Fe₂V₄O₁₃ (JCPDS Card No. 00-039-0893). Good crystallinity is confirmed by high and narrow diffracted peaks [31]. Face-centered cubic (fcc) geometry is indexed to the three additional peaks at 38.06, 44.29 and 64.43° corresponding to (111), (200) and (220) planes of ZnO/Fe₂V₄O₁₃ doped with Ag nanocomposites (JCPDS Card No. 4-0783) [37,38] in Figure 2c–f. The absence of Fe₂V₄O₁₃ peaks in all the Ag-ZnO/Fe₂V₄O₁₃ composites can be attributed to relatively low concentration of Fe₂V₄O₁₃ doped on ZnO and to less intense peaks of Fe₂V₄O₁₃ as compared to ZnO. Moreover, a similar result was observed for ZrS₂/ZnO [39]. The Scherrer equation (eqn. 1) was applied for the calculation of the average crystallite size of 2 wt% of Ag-ZnO/Fe₂V₄O₁₃ and it was found to be 28.5 nm.

D = kλ/βcosθ(1)

where D is the crystalline size, λ is the wavelength of the X-ray used, K is the shape factor, β is the full width at half maximum height of the peak and θ is the Bragg’s angle.

3.3. BET Surface Area

The pore structure and surface area of the prepared Ag-ZnO/Fe₂V₄O₁₃ were analyzed using N₂ adsorption–desorption isotherms are shown in Figure 3. The Ag-ZnO/Fe₂V₄O₁₃ is type II isotherm at IUPAC level [40] and the distribution of pore size is given in the inset of Figure 3. The BET surface area and pore volume of Ag-ZnO/Fe₂V₄O₁₃ are given in Table 1.

3.4. SEM-EDX with Color Mapping

The surface morphology of the Ag-ZnO/Fe₂V₄O₁₃ was described by SEM measurements. Figure 4 shows the 2 wt% Ag-loaded ZnO/Fe₂V₄O₁₃ with different magnifications. The irregular-shaped ZnO/Fe₂V₄O₁₃ agglomerates are shown in all SEM images. Furthermore, EDX can accurately detect a trace amount of metal present on the surface of the base materials. Figure 5 shows the EDX recorded from the selected area, which reveals the presence of Zn, Fe, V, Ag and O in the catalyst. The reason for the unaccomplished atomic ratio between Fe and V may be due to ‘V’ deficiency in the lattice. However, the formation of the composite was clearly confirmed by XRD measurements. A similar result was observed for ZrS₂/ZnO [39]. The presence of these elements in Ag-ZnO/Fe₂V₄O₁₃ was also confirmed by ECM. The different color areas in Figure 6 indicate Ag, Zn, Fe, V and O enriched areas of the Ag-ZnO/Fe₂V₄O₁₃ sample.

3.5. HR-TEM

Figure 7 shows the HR-TEM images of the 2 wt% Ag-ZnO/Fe₂V₄O₁₃. Some of the hexagonal particles are clearly seen in Figure 7a–c, corresponding to ZnO in Ag-ZnO/Fe₂V₄O₁₃, although the identification of Fe₂V₄O₁₃ in Ag-ZnO/Fe₂V₄O₁₃ was impossible. The lattice fringes of the Ag-ZnO/Fe₂V₄O₁₃ catalyst are shown in Figure 7d. The lattice spacing of 0.2 nm is for (101) planes of wurtzite ZnO. The interplanar spacing values for the nanocrystalline Ag-ZnO are shown in Figure 7e. From HR-TEM images, the average diameters (28.545 ± 5.453 nm) of the nanoparticles (Ag-ZnO/Fe₂V₄O₁₃ nanocomposite) were computed by Image J analysis. The histogram shows the average particle diameter of 2wt% Ag-ZnO/Fe₂V₄O₁₃ in Figure 7f.

3.6. UV-Vis-DRS

Figure 8A shows the DRS results of ZnO, Fe₂V₄O₁₃ and samples with various wt% of X-Ag-loaded ZnO/Fe₂V₄O₁₃ (X = 1.0, 1.5, 2 and 2.5 wt%) catalyst. In visible regions, Ag-ZnO/Fe₂V₄O₁₃ composites display higher absorption than ZnO, resulting in higher visible light active catalytic behavior and an increase in the absorption at 350–380 nm (UV-region); this can contribute to enhanced e⁻/h⁺ pair production, which consequently improves the photocatalytic activity under UV light [41]. Figure 8B shows K-M plots for ZnO, Fe₂V₄O₁₃ and ZnO/Fe₂V₄O₁₃ with various wt% of Ag loading. The band gap energies of ZnO, Fe₂V₄O₁₃ and X-Ag-ZnO/Fe₂V₄O₁₃ (X = 1, 1.5, 2 and 2.5 wt%) were 3.25, 2.21, 3.23, 3.23, 3.15 and 3.16 eV, respectively.

3.7. PL Emission Spectra

The effective suppression of photogenerated charge carriers and the transfer of the photogenerated e⁻/h⁺ were investigated by PL emission spectra [42]. Figure 9 shows the PL spectra of the prepared ZnO (Figure 9a) and 2 wt% Ag-ZnO/Fe₂V₄O₁₃ (Figure 9b). Three emission bands are observed at 395, 431 and 586 nm. Although all the peaks are almost identical, PL intensities are different. The higher intensity was observed in bare ZnO at 395 nm due to the higher e⁻/h⁺ recombination than Ag-ZnO/Fe₂V₄O₁₃. The Ag particles loaded with the ZnO/Fe₂V₄O₁₃ nanocomposite act as a trapper for photogenerated electrons and quench the PL emission [43]. The maximum intensity shows the higher e⁻/h⁺ recombination and results from low photocatalytic activity [44,45]. The lowest intensity shows the well suppression of e⁻/h⁺ recombination and results from higher photocatalytic activity [46].

3.8. Primary Analysis

The photocatalytic behavior of nanocomposite Ag-ZnO/Fe₂V₄O₁₃ with 1, 1.5, 2 and 2.5 wt% of Ag loading was assessed in terms of MO degradation. Controlled experiments were conducted under different reaction conditions (Figure 10A). The kinetic plot of ln(C₀/C_t) vs treatment time t is given in Figure 10B. The dye/ZnO/UV-A process underwent 68% degradation in 90 min (curve a). The dye/ZnO/Fe₂V₄O₁₃/UV light process showed curve b, which yielded 75% degradation in 90 min. Only 19% adsorption was observed in dye/2 wt% Ag-ZnO/Fe₂V₄O₁₃/dark process (curve e). Curves c, d, f and g demonstrate the degradation of dye on irradiation with the Ag-ZnO/Fe₂V₄O₁₃ catalyst with different wt% of Ag loading. The 2 wt% Ag-ZnO/Fe₂V₄O₁₃ hybrid-heterojunction catalyst showed higher MO degradation (curve f, 93%) in 90 min and almost complete degradation was achieved at 120 min. Hence, 2 wt% of Ag is the optimum loading of Ag-ZnO/Fe₂V₄O₁₃. The inset figure reveals the easy identification of the most active catalysts under variable reaction conditions at the time of 90 min irradiation. Table S1 (Supplementary Materials) shows the corresponding kinetic values of each process and reveals that 2 wt% Ag-ZnO/Fe₂V₄O₁₃ hybrid-heterojunction catalyst showed higher kinetic values (0.0377 min⁻¹) toward MO degradation than other processes.

3.9. Effect of pH

For the application of industrial point of view, pH is an important parameter. By adjusting the pH of the MO solution, the effect of pH on the MO photodegradation was studied. Figure S2 (Supplementary Materials) demonstrates that pH has an important effect on the rate of photodegradation and decolorization. The maximum degradation and decolorization of MO are observed at pH 7. The rate of degradation and decolorization decreases above pH 7. The effect of catalyst loading (Figure S3, Supplementary Materials) and the initial dye concentration (Figure S4, Supplementary Materials) are discussed in the Supporting Information.

3.10. Reusability

The stability of the catalyst for the degradation of MO is shown in Figure 11A. In the first three runs, approximately 99% of MO degradation was achieved. The same catalyst was again reused for further runs. All the remaining two cycles gave almost 98% of degradation in 90 min. Hence, the 2 wt% Ag-ZnO/Fe₂V₄O₁₃ is stable, recoverable and reusable. Figure 11B shows the XRD pattern of (a) first run and (b) fifth run photocatalyst. These are almost like the fresh photocatalyst (Figure 2f) and do not have any impurities. Hence, the 2 wt% Ag-ZnO/Fe₂V₄O₁₃ is stable, recoverable and reusable.

3.11. Mineralization Studies

3.11.1. Gas Chromatography–Mass Spectrometry (GC-MS) Analysis

Sometimes, intermediates are more hazardous than starting materials, so it is necessary to analyze the intermediates for the degradation process. An attempt has been made to find out the degradation intermediates of MO photodegradation with 2 wt% Ag-ZnO/Fe₂V₄O₁₃ hybrid-heterojunction/UV process. The GC-MS studies performed with the solutions obtained after 30 and 60 min of irradiation predicted a degradation pathway for MO by Ag-ZnO/Fe₂V₄O₁₃ based on the m/z ratio, retention time and molecular weight (Scheme 1). For these identified intermediates, molecular ion and fragmentation peak values are stated in Table 2. In photocatalytic degradation of azo dyes, although it was expected that the cleavage of azo bond takes place first, the formation of compounds was observed (D1 and D2) with azo groups at the retention time of 19.796 and 18.533 min, respectively. The hydroxyl radicals were thus considered to be the most reactive species for degradation and the compound D1 undergoes azo link cleavage and replacement of sulfonic acid group by hydroxyl group through the repetitive attack of OH radicals produced intermediates N’-methylbenzene-1,4-diamine (compound I) and 4-aminophenol (D4). The intermediate product D2 further undergoes C–N cleavage produced D3 which further undergoes azo link cleavage produced 4-aminophenol (D4). Finally, it is expected that the compound D4 and compound I would be mineralized to CO₂, water and mineral acids [47,48]. The corresponding total ion chromatogram for D1, D2, D3 and D4 are given in Supporting Information Figures S5–S8.

3.11.2. FT-IR Spectral Analysis

The early adsorption of the dye under dark by 2 wt% Ag-ZnO/Fe₂V₄O₁₃ nanocomposite is 33.4% although complete degradation occurred at 90 min irradiation. The experiments were carried out to determine whether the adsorbed dye molecules had been degraded completely. Comparisons were made with FT-IR spectra of the fresh dye and catalyst (Figure S9a,b; Supplementary Materials) and the dye adsorbed composite before and after irradiations (Figure S9c,d). The characteristic bands of MO (Figure S9a) are observed at 1604, 1366 and 1042 cm⁻¹ due to N=N stretching, C–N bond vibrations and S=O bond of MO, respectively [49]. On comparing Figure S9a,c, the characteristic MO dye peaks are observed in the dye adsorbed catalyst. However, upon irradiation after complete degradation, the FT-IR spectrum of the composite (Figure S9d) resembles the fresh catalyst (Figure S9b) revealing that the adsorbed dye molecules underwent complete degradation. Thus, the 2 wt% Ag-ZnO/Fe₂V₄O₁₃ nanocomposite shows better catalytic performance for MO.

3.11.3. UV-Vis Spectral Analysis

Figure 12 shows UV-vis spectra for the degradation and decolorization of MO by 2 wt% Ag-ZnO/Fe₂V₄O₁₃ nanocomposite under UV-A light irradiation for 0–90 min. UV-vis spectrum of MO shows strong absorption in the range of 200–600 nm with λ_max at 464 nm, which is due to the presence of the azo group (N=N) and another band is observed at 272 nm in the UV region due to aromatic part of the dye. On continuous irradiation with 2 wt% Ag-ZnO/Fe₂V₄O₁₃ nanocomposite, both peaks were diminished with respect to time and finally almost completely disappeared. From these observations, we conclude that the degradation of the dye occurs with respect to time [50]. Moreover, no new peaks were observed during irradiation, indicating that MO was degraded gradually and intermediates do not absorb at analytical wavelengths. The color of the suspension changed from orange to colorless (inset of Figure 12).

3.11.4. COD Measurements

The mineralization of the dye was further confirmed with the reduction of COD values. Under optimum conditions, COD measurements were made and the percentage of COD reduction of the dye at different times of UV-A light irradiation is given in Table 3. Percentages of COD reduction increases with respect to irradiation time reveals the mineralization of the dye.

3.12. Mechanism

Under light irradiation, the photoexcited electrons in the CB of Fe₂V₄O₁₃ can jump to the CB of ZnO. The CB potential of ZnO was found to be more electronegative than the reduction potential of O₂/O₂^•− (–0.34 eV vs. NHE) and therefore, the electrons concentrated in the CB of ZnO can generate O₂^•− for dye degradation (Figure 13). Further, Ag can shuttle the photogenerated electron from ZnO in Ag-ZnO/Fe₂V₄O₁₃ and reduces the recombination of electron–hole pair effectively. However, owing to the less positive VB potential of Fe₂V₄O₁₃, the holes do not generate ^•OH. Meanwhile, Fe₂V₄O_13′s holes created in the VB would be transferred to its surface and then directly involved in the MO degradation. Through these two processes, the lifetime of electrons and holes increase, leading to enhanced photocatalytic activity.

The increased transfer of charge between the interface of Fe₂V₄O₁₃ and ZnO strongly suppressed the rate of recombination, which is beneficial to improving the photocatalytic ability. The reduction of PL intensity in Ag-ZnO/Fe₂V₄O₁₃ as compared to that of pure undoped ZnO reveals the effective prohibition of recombination of e⁻/h⁺ pairs, indicating that loading of Ag and Fe₂V₄O₁₃ could substantially suppress the charge transfer rate in ZnO. The band gap energy of Ag-ZnO/Fe₂V₄O₁₃ is lower than that of pure ZnO.

In general, for a higher formation rate of ^•OH radicals, the separation efficiency of electron–hole pairs would be greater. The photocatalytic activity, therefore, shows a positive association with the rate of radical formation of ^•OH, that is, a faster rate of radical formation of •OH contributes to a higher photocatalytic activity of the nanocomposite. Moreover, the formation of superoxide radical anions increased due to electrons transferred to the adsorbed oxygen molecules by Ag via the ZnO conduction band. Hydroxyl radicals and superoxide radical anions are potent oxidants so that organic molecules and intermediate species can completely oxidize to their respective end-products.

Trapping experiments were performed to identify the active species involved in the photocatalytic degradation process [51,52] and the results are shown in Figure 14. With no scavenger, the degradation efficiency was 99% at a time of 90 min. Under the same condition, with 0.1 mmol of TBA (^•OH scavenger), KI (h⁺ scavenger), BQ (O₂^•− scavenger) and AgNO₃ (e⁻ scavenger) gave 93.9, 86.8, 84.6 and 89.6% of degradation, respectively. From these values, more or less all the species are contributing equally to the degradation process. However, the addition of BQ suppresses the photocatalytic activity of the composite in some extent, hence the superoxide radical anion (O₂^•−) has been considered as the most active species for this degradation process.

4. Conclusions

Ag-loaded ZnO/Fe₂V₄O₁₃ nanocomposites have been successfully fabricated by a cost-effective photo-deposition method. The prepared nanocomposite was characterized by various surface analytical techniques. In XRD, the crystallite size of Ag-ZnO/Fe₂V₄O₁₃ is 28.5 nm which is consistent with HR-TEM analysis. The photocatalytic activities of Ag-ZnO/Fe₂V₄O₁₃ nanocomposite were evaluated using degradation of MO under UV-A light irradiation. In pH 7, almost 99% degradation was observed at the time of 90 min with 3 g L⁻¹ catalyst concentration. The stability of the catalyst was observed by multiple runs of the catalyst. This nanocomposite is stable and reusable for multiple runs. All five runs gave nearly 98% degradation. The GC-MS studies reveal the formation of three azo compounds (D1, D2 and D3) and 4-aminophenol (D4) as intermediates during the degradation process. Trapping experiments confirm that the superoxide radical anion (O₂^•−) can be considered as the most active species for this degradation process. The complete mineralization was confirmed by COD measurement. A suitable degradation mechanism is also proposed.

Author Contributions

Conceptualization, Methodology, G.K.; Software; Visualization, S.C.; Validation, U.D.P.; Resources, V.R.; Investigation; T.G.; Formal analysis, S.T.; Conceptualization, Supervision, Project administration, Funding aquation, Investigation, Writing—reviewing and editing, M.I. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors (I.M.) thanks University Grants Commission (UGC), New Delhi, India for the financial support [F.No. 43-222/2014(SR)]. The authors thank International research center, Kalasalingam Academy of Research and Education, Kalasalingam University, Krishnankoil–626 126, Tamil Nadu, India for taking SEM-EDS and ECM analyses and DST-SAIF Cochin, Cochin University of Science & Technology Campus, Kochi–682 022, Kerala, India for taking HR-TEM analysis.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

ZnO	Zinc Oxide
Fe2V4O13	Iron tetrapolyvanadate
FT-IR	Fourier transform infrared spectroscopy
XRD	X-ray diffraction
BET	Brunauer–Emmett–Teller analysis
EDX	Energy dispersive X-ray
ECM	Electron color mapping
HR-TEM	High-resolution transmission electron microscopy
UV-vis-DRS	Ultraviolet–visible diffuse reflectance spectroscopy
PL	Photoluminescence spectroscopy
MO	Methyl orange
AOPs	Advanced oxidation processes

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Figures, Scheme and Tables

Figure 1. FT-IR spectra of (a) ZnO, (b) Fe2V4O13, (c) 1 wt% Ag-ZnO/Fe2V4O13, (d) 1.5 wt% Ag-ZnO/Fe2V4O13, (e) 2 wt% Ag-ZnO/Fe2V4O13 and (f) 2.5 wt% Ag-ZnO/Fe2V4O13.

Figure 2. XRD patterns of (a) ZnO; (b) Fe2V4O13; (c) 1 wt% Ag-ZnO/Fe2V4O13; (d) 1.5 wt% Ag-ZnO/Fe2V4O13; (e) 2 wt% Ag-ZnO/Fe2V4O13; (f) 2.5 wt% Ag-ZnO/Fe2V4O13.

Figure 3. N2 Adsorption–desorption isotherm of 2 wt% Ag-ZnO/Fe2V4O13. The inset shows pore size distribution.

Figure 4. SEM images of 2 wt% Ag-ZnO/Fe2V4O13: (a) and (b) 1 μm; (c) and (d) 200 nm.

Figure 5. EDX of 2 wt% Ag-ZnO/Fe2V4O13.

Figure 6. ECM images of 2 wt% Ag-ZnO/Fe2V4O13. (a) 2 wt% Ag-ZnO/Fe2V4O13 composition; (b) O; (c) Ag; (d) V; (e) Fe; (f) Zn; (g) Analyzed SEM image.

Figure 7. HR-TEM images of 2 wt% Ag-ZnO/Fe2V4O13: (a) 100 nm; (b) 50 nm; (c) 20 nm; (d) 2 nm; (e) SAED pattern; (f) particle size distribution.

Figure 8. (A) UV-DRS and (B) K-M plot of (a) ZnO; (b) Fe2V4O13; (c) 1 wt% Ag-ZnO/Fe2V4O13 (d) 1.5 wt% Ag-ZnO/Fe2V4O13 (e) 2 wt% Ag-ZnO/Fe2V4O13; (f) 2.5 wt% Ag-ZnO/Fe2V4O13.

Figure 9. PL spectra of (a) ZnO and (b) 2 wt% Ag-ZnO/Fe2V4O13.

Figure 10. (A) The primary analysis of Ag-ZnO/Fe2V4O13 catalyst with MO under UV-A light. [MO] = 4 × 10−4 M; catalyst suspended = 2 g L−1; airflow rate = 8.1 mL s−1; pH = 7.0; IUV = 1.381 × 10−6 Einstein L−1 s−1The inset shows variable reaction conditions at 90 min. (B) Corresponding kinetic plot for the photodegradation of MO.

Figure 11. (A) Reusability. [MO] = 4 × 10−4 M; 2 wt% Ag-ZnO/Fe2V4O13 = 3 g L−1; airflow rate = 8.1 mL s−1; pH = 7.0; irradiation time = 90 min; IUV = 1.381 × 10−6 Einstein L−1 s−1. (B) (a) First run and (b) fifth run of XRD pattern of 2 wt% Ag-ZnO/Fe2V4O13.

Scheme 1. Degradation pathway of MO with Ag-ZnO/Fe2V4O13.

Figure 12. Overlay spectrum of MO. [MO] = 4 × 10−4 M; 2 wt% Ag-ZnO/Fe2V4O13 = 3 g L−1, pH = 7.0. (a) 0 min; (b) 15 min; (c) 30 min; (d) 45 min; (e) 60 min; and (f) 90 min.

Figure 13. Schematic representation of Ag-ZnO/Fe2V4O13 nanocomposite mechanism of dye degradation.

Figure 14. Effect of different scavengers on MO degradation with Ag-ZnO/Fe2V4O13.

Table 1

Texture parameters of 2 wt% Ag-ZnO/Fe₂V₄O₁₃.

SBET (m2 g−1)	Vp (cm3 g−1)	Dp (nm)	Smicro (m2 g−1)	Vmicro (cm3 g−1)
0.9043	0.1044	461.86	16.53	0.0089

S_BET = BET surface area, V_p = total pore volume, D_p = average uniform pore size distribution, S_micro = surface area of micropores, V_micro = pore volume of micropores.

Table 2

Mass spectral data of intermediates (MO).

Compounds	Retention Time (min)	m/z Values
C13H13N3O3S (D1)	19.796	292.0 (M + 1), 258.0, 229.0, 181.0, 152.0, 126.0, 111.0, 75.0
C12H9N2NaO3S (D2)	18.533	284.0 (M+), 282.0, 211.0, 111.0, 85.0, 71.0, 57.0, 54.0
C6H5N2NaO3S (D3)	16.665	209.0 (M+), 111.0, 97.0, 85.0, 71.0, 57.0
C6H7NO (D4)	15.512	109.0 (M+), 83.0, 71.0, 67.0, 57.0

Table 3

Percentages of COD reduction at different radiation times for the degradation of MO by 2 wt% Ag-ZnO/Fe₂V₄O₁₃ catalyst.

Time (min)	0	30	60	90	120
COD reduction (%)	0	31.6	57.4	79.3	88.0

[MO] = 4 × 10⁻⁴ M; 2 wt% Ag-ZnO/Fe₂V₄O₁₃ = 3 g L⁻¹; air flowrate = 8.1 mL s⁻¹; pH = 7; I_UV = 1.381 × 10⁻⁶ Einstein L⁻¹ s⁻¹.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/su142316276/s1, Figures S1–S9, Table S1, the chemicals used for this study, instrumental specification for the characterization techniques and the photocatalytic experimental and COD measurements procedures are provided in Supporting Information.

References

1. Weber, E.J.; Adams, R.L. Chemical- and Sediment-Mediated Reduction of the Azo Dye Disperse Blue 79. Environ. Sci. Technol.; 1995; 29, pp. 1163-1170. [DOI: https://dx.doi.org/10.1021/es00005a005] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/22192007]

2. Zollinger, H. Colour Chemistry—Synthesis, Properties of Organic Dyes and Pigments; VCH Publishers: New York, NY, USA, 1987; pp. 92-100.

3. Rajasri, S.; Krishnakumar, B.; Sobral, A.J.F.N.; Balachandran, S.; Swaminathan, M.; Muthuvel, I. Development of Cd₃(PO₄)₂/rGO coupled semiconductor system for effective mineralization of Basic Violet 10 (BV 10) under UV-A light. Mater. Today Proc.; 2019; 15, pp. 471-480. [DOI: https://dx.doi.org/10.1016/j.matpr.2019.04.109]

4. Tanaka, K.; Padermpole, K.; Hisanaga, T. Photocatalytic degradation of commercial azo dyes. Water Res.; 2000; 34, pp. 327-333. [DOI: https://dx.doi.org/10.1016/S0043-1354(99)00093-7]

5. Ruan, W.; Hu, J.; Qi, J.; Hou, Y.; Zhou, C.; Wei, X. Removal of dyes from wastewater by nanomaterials: A review. Adv. Mater. Lett.; 2019; 10, pp. 9-20. [DOI: https://dx.doi.org/10.5185/amlett.2019.2148]

6. Muthuvel, I.; Swaminathan, M. Photoassisted Fenton mineralisation of Acid Violet 7 by heterogeneous Fe(III)–Al₂O₃ catalyst. Catal. Commun.; 2007; 8, pp. 981-986. [DOI: https://dx.doi.org/10.1016/j.catcom.2006.10.015]

7. Muthuvel, I.; Swaminathan, M. Highly solar active Fe(III) immobilized–Alumina for the degradation of Acid Violet 7. Sol. Energy Mater. Sol. Cells; 2008; 92, pp. 857-863. [DOI: https://dx.doi.org/10.1016/j.solmat.2008.02.007]

8. Shirzad-Siboni, M.; Samadi, M.T.; Yang, J.; Lee, S. Photocatalytic reduction of Cr(VI) and Ni(II) in aqueous solution by synthesized nanoparticle ZnO under ultraviolet light irradiation: A kinetic study. Environ. Technol.; 2011; 32, pp. 1573-1579. [DOI: https://dx.doi.org/10.1080/09593330.2010.543933]

9. Rezaee, A.; Masoumbeigi, H.; Soltani, R.D.C.; Khataee, A.R.; Hashemiyan, S. Photocatalytic decolorization of methylene blue using immobilized ZnO nanoparticles prepared by solution combustion method. Desalination Water Treat.; 2012; 44, pp. 174-179. [DOI: https://dx.doi.org/10.1080/19443994.2012.691700]

10. Zhang, N.; Zhang, Y.; Xu, Y.-J. Recent progress on graphene-based photocatalysts: Current status and future perspectives. Nanoscale; 2012; 4, pp. 5792-5813. [DOI: https://dx.doi.org/10.1039/c2nr31480k]

11. Lee, H.J.; Kim, H.; Park, S.S.; Hong, S.S.; Lee, G.D. Degradation kinetics for photocatalytic reaction of methyl orangver-allAl-doped ZnO nanoparticles. J. Ind. Eng. Chem.; 2015; 25, pp. 199-206.

12. Ansari, S.A.; Khan, M.M.; Ansari, M.O.; Lee, J.; Cho, M.H. Biogenic synthesis, photocatalytic, and photoelectrochemical performance of Ag–ZnO nanocomposite. J. Phys. Chem.; 2013; 117, pp. 27023-27030. [DOI: https://dx.doi.org/10.1021/jp410063p]

13. Muñoz-Fernandez, L.; Gomez-Villalba, L.; Milošević, O.; Rabanal, M. Influence of nanoscale defects on the improvement of photocatalytic activity of Ag/ZnO. Mater. Charact.; 2022; 185, 111718. [DOI: https://dx.doi.org/10.1016/j.matchar.2021.111718]

14. Gurugubelli, T.R.; Ravikumar, R.V.S.S.N.; Koutavarapu, R. Enhanced Photocatalytic Activity of ZnO–CdS Composite Nanostructures towards the Degradation of Rhodamine B under Solar Light. Catalysts; 2022; 12, 84. [DOI: https://dx.doi.org/10.3390/catal12010084]

15. Dharmana, G.; Gurugubelli, T.R.; Masabattula, P.S.R.; Babu, B.; Yoo, K. Facile synthesis, characterization, and photocatalytic activity of hydrothermally crown Cu²⁺-doped ZnO-SnS nanocomposites for MB dye degradation. Catalysts; 2022; 12, 328. [DOI: https://dx.doi.org/10.3390/catal12030328]

16. Alam, M.W.; Azam, H.; Khalid, N.R.; Naeem, S.; Hussain, M.K.; Baqais, A.; Farhan, M.; Souayeh, B.; Zaidi, B.; Kahan, K. Enhanced photocatalytic performance of Ag₃PO₄/Mn-ZnO nanocomposite for the degradation of Tetracycline Hydrochloride. Crystals; 2022; 12, 1156. [DOI: https://dx.doi.org/10.3390/cryst12081156]

17. El-Sayed, F.; Hussien, M.S.A.; Alabdulaal, T.H.; Ismail, A.; Zahran, H.Y.; Yahia, I.S.; Abdel-Wahab, M.S.; Khairy, Y.; Ali, T.E.; Ibrahim, M.A. Comparative degradation studies of Carmine Dye by photocatalysis and photoelectrochemical oxidation pro-cesses in the presence of Graphene/N-Doped ZnO nanostructures. Crystals; 2022; 12, 535. [DOI: https://dx.doi.org/10.3390/cryst12040535]

18. Landge, V.K.; Huang, C.M.; Hakke, V.S.; Sonawane, S.H.; Manickam, S.; Hsieh, M.C. Solar-energy-driven Cu-ZnO/TiO₂ nanocomposite photocatalyst for the rapid degradation of Congo Red azo dye. Catalysts; 2022; 12, 605. [DOI: https://dx.doi.org/10.3390/catal12060605]

19. Lakkepally, S.; Kalegowda, Y.; Ganganagappa, N.; Siddaramanna, A. A new and effective approach for Fe₂V₄O₁₃ nanoparticles synthesis: Evaluation of electrochemical performance as cathode for lithium secondary batteries. J. Alloys Compd.; 2018; 737, pp. 665-671. [DOI: https://dx.doi.org/10.1016/j.jallcom.2017.12.096]

20. Li, S.-R.; Yesibolati, N.; Qiao, Y.; Ge, S.-Y.; Feng, X.-Y.; Zhu, J.-F.; Chen, C.-H. Electrostatic spray deposition of porous Fe₂V₄O₁₃ films as electrodes for Li-ion batteries. J. Alloys Compd.; 2012; 520, pp. 77-82. [DOI: https://dx.doi.org/10.1016/j.jallcom.2011.12.092]

21. Tang, D.; Rettie, A.J.E.; Mabayoje, O.; Wygant, B.R.; Lai, Y.; Liu, Y.; Mullins, C.B. Facile growth of porous Fe₂V₄O₁₃ films for photoelectrochemical water oxidation. J. Mater. Chem. A; 2016; 4, pp. 3034-3042. [DOI: https://dx.doi.org/10.1039/C5TA07877F]

22. Muthuvel, I.; Gowthami, K.; Thirunarayanan, G.; Suppuraj, P.; Krishnakumar, B.; Sobral, A.J.F.N.; Swaminathan, M. Gra-phene oxide–Fe₂V₄O₁₃ hybrid material as highly efficient Hetero-Fenton catalyst for degradation of Methyl Orange. Int. J. Ind. Chem.; 2019; 10, pp. 77-87. [DOI: https://dx.doi.org/10.1007/s40090-019-0173-8]

23. Li, P.; Zhou, Y.; Li, H.; Xu, Q.; Meng, X.; Wang, X.; Xiao, M.; Zou, Z. Correction: All-Solid-State Z-Scheme System arrays of Fe₂V₄O₁₃/RGO/CdS for visible light-driving photocatalytic CO₂ reduction into renewable hydrocarbon fuel. Chem. Commun.; 2015; 51, pp. 800-813. [DOI: https://dx.doi.org/10.1039/C4CC08744E] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25424013]

24. Li, P.; Zhou, Y.; Tu, W.; Liu, Q.; Yan, S.; Zou, Z. Direct growth of Fe₂V₄O₁₃ nanoribbons on a stainless-steel mesh for visible-light photoreduction of CO₂ into renewable hydrocarbon fuel and degradation of gaseous isopropyl alcohol. ChemPlusChem; 2013; 78, pp. 274-278. [DOI: https://dx.doi.org/10.1002/cplu.201200289]

25. Yu, J.; Sun, D.; Wang, T.; Li, F. Fabrication of Ag@AgCl/ZnO submicron wire film catalyst on glass substrate with excellent. visible light photocatalytic activity and reusability. Chem. Eng. J.; 2018; 334, pp. 225-236. [DOI: https://dx.doi.org/10.1016/j.cej.2017.10.003]

26. Hashim, F.S.; Alkaim, A.F.; Salim, S.J.; Alkhayatt, A.H.O. Effect of (Ag, Pd) doping on structural, and optical properties of ZnO nanoparticles: As a model of photocatalytic activity for water pollution treatment. Chem. Phys. Lett.; 2019; 737, 136828. [DOI: https://dx.doi.org/10.1016/j.cplett.2019.136828]

27. Sampaio, M.J.; Lima, M.J.; Baptista, D.L.; Silva, A.M.; Silva, C.G.; Faria, J.L. Ag-loaded ZnO materials for photocatalytic water treatment. Chem. Eng. J.; 2017; 318, pp. 95-102. [DOI: https://dx.doi.org/10.1016/j.cej.2016.05.105]

28. Zhang, F.; Li, Y.H.; Qi, M.Y.; Tang, Z.R.; Xu, Y.J. Boosting the activity and stability of Ag-Cu₂O/ZnO nanorods for photo-catalytic CO₂ reduction. Appl. Catal. B; 2020; 268, 118380. [DOI: https://dx.doi.org/10.1016/j.apcatb.2019.118380]

29. Ahmad, M.; Ahmad, I.; Ahmed, E.; Akhtar, M.S.; Khalid, N. Facile and inexpensive synthesis of Ag doped ZnO/CNTs composite: Study on the efficient photocatalytic activity and photocatalytic mechanism. J. Mol. Liq.; 2020; 311, 113326. [DOI: https://dx.doi.org/10.1016/j.molliq.2020.113326]

30. Moradi, M.; Haghighi, M.; Allahyari, S. Precipitation dispersion of Ag–ZnO nanocatalyst over functionalized multiwall carbon nanotube used in degradation of Acid Orange from wastewater. Process. Saf. Environ. Prot.; 2017; 107, pp. 414-427. [DOI: https://dx.doi.org/10.1016/j.psep.2017.03.010]

31. Gowthami, K.; Krishnakumar, B.; Sobral, A.J.F.N.; Thirunarayanan, G.; Swaminathan, M.; Siranjeevi, R.; Rajachandrasekar, T.; Muthuvel, I. Fabrication of Hybrid Fe₂V₄O₁₃/ZnO Heterostructure for Effective Mineralization of Aqueous Methyl Orange Solution. J. Cluster Sci.; 2020; 31, pp. 839-849. [DOI: https://dx.doi.org/10.1007/s10876-019-01691-9]

32. Gowthami, K.; Suppuraj, P.; Thirunarayanan, G.; Krishnakumar, B.; Sobral, A.J.F.N.; Swaminathan, M.; Muthuvel, I. Fe₂V₄O₁₃ assisted hetero-Fenton mineralization of methyl orange under UV-A light irradiation. Iran. Chem. Commun.; 2018; 6, pp. 97-108.

33. Babu, B.; Aswani, T.; Thirumala Rao, G.; Joyce Stella, R.; Jayaraja, B.; Ravikumar, R.V.S.S.N. Room temperature ferromagnetism and op-tical properties of Cu²⁺ doped ZnO nanopowder by ultrasound assisted solid state reaction technique. J. Magn. Magn. Mater.; 2014; 355, pp. 76-80. [DOI: https://dx.doi.org/10.1016/j.jmmm.2013.11.038]

34. Shah, A.H.; Manikandan, E.; Basheer Ahmed, M.; Ganesan, V. Enhanced bioactivity of Ag/ZnO nanorods-A comparative antibacterial Study. J. Nanomed. Nanotechnol.; 2013; 4, 1000168.

35. Bandekar, G.; Rajurkar, N.S.; Mulla, I.S.; Mulik, U.P.; Amalnerkar, D.P.; Adhyapak, P.V. Synthesis, characterization, and photocatalytic activity of PVP stabilized ZnO and modified ZnO nanostructures. Appl. Nanosci.; 2014; 4, pp. 199-208. [DOI: https://dx.doi.org/10.1007/s13204-012-0189-2]

36. Rekha, K.; Nirmala, M.; Nair, M.G.; Anukaliani, A. Structural, optical, photocatalytic and antibacterial activity of zinc oxide and manganese doped zinc oxide nanoparticles. Phys. B: Condens. Matter; 2010; 405, pp. 3180-3185. [DOI: https://dx.doi.org/10.1016/j.physb.2010.04.042]

37. Mohammadzadeh, S.; Olya, M.E.; Arabi, A.M.; Shariati, A.; Nikou, M.R.K. Synthesis, characterization and application of ZnO-Ag as a nanophotocatalyst for organic compounds degradation, mechanism and economic study. J. Environ. Sci.; 2015; 35, pp. 194-207. [DOI: https://dx.doi.org/10.1016/j.jes.2015.03.030]

38. Yin, X.; Que, W.; Fei, D.; Shen, F.; Guo, Q. Ag nanoparticle/ZnO nanorods nanocomposites derived by a seed-mediated method and their photocatalytic properties. J. Alloys Compd.; 2012; 524, pp. 13-21. [DOI: https://dx.doi.org/10.1016/j.jallcom.2012.02.052]

39. Krishnakumar, B.; Imae, T.; Miras, J.; Esquena, J. Synthesis and azo dye photodegradation activity of ZrS₂–ZnO nano-composites. Sep. Purif. Technol.; 2014; 132, pp. 281-288. [DOI: https://dx.doi.org/10.1016/j.seppur.2014.05.018]

40. Qiu, Z.; He, D.; Wang, Y.; Zhao, X.; Zhao, W.; Wu, H. High performance asymmetric supercapacitors with ultra-high energy density based on hierarchical carbon nanotubes and NiO core–shell nanosheets and defect-introduced graphene sheets with hole structure. RSC Adv.; 2017; 7, pp. 7843-7856. [DOI: https://dx.doi.org/10.1039/C6RA27369F]

41. Zheng, M.; Wu, J. One-step synthesis of nitrogen-doped ZnO nanocrystallites and their properties. Appl. Surf. Sci.; 2009; 255, pp. 5656-5661. [DOI: https://dx.doi.org/10.1016/j.apsusc.2008.10.091]

42. Baeissa, E. Photocatalytic degradation of methylene blue dye under visible light irradiation using In/ZnO nanocomposite. Front. Nanosci. Nanotechnol.; 2016; 2, pp. 1-5. [DOI: https://dx.doi.org/10.15761/FNN.1000134]

43. Suppuraj, P.; Thirumalai, K.; Parthiban, S.; Swaminathan, M.; Muthuvel, I. Novel Ag-TiO₂/ZnFe₂O₄ nanocomposites for effective photocatalytic, electrocatalytic and cytotoxicityapplications. J. Nanosci. Nanotechnol.; 2020; 20, pp. 709-718. [DOI: https://dx.doi.org/10.1166/jnn.2020.16893] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31383066]

44. Wang, Q.; Hui, J.; Li, J.; Cai, Y.; Yin, S.; Wang, F.; Su, B. Photodegradation of methyl orange with PANI-modified BiOCl photocatalyst under visible light irradiation. Appl. Surf. Sci.; 2013; 283, pp. 577-583. [DOI: https://dx.doi.org/10.1016/j.apsusc.2013.06.149]

45. Ge, L.; Han, C.; Liu, J. In situ synthesis and enhanced visible light photocatalytic activities of novel PANI–g-C₃N₄ composite photocatalysts. J. Mater. Chem.; 2012; 22, pp. 11843-11850. [DOI: https://dx.doi.org/10.1039/c2jm16241e]

46. Ghaly, H.A.; El-Kalliny, A.S.; Gad-Allah, T.A.; El-Sattar, N.E.A.A.; Souaya, E.R. Stable plasmonic Ag/AgCl–polyaniline photoactive composite for degradation of organic contaminants under solar light. RSC Adv.; 2017; 7, pp. 12726-12736. [DOI: https://dx.doi.org/10.1039/C6RA27957K]

47. Krishnakumar, B.; Subash, B.; Swaminathan, M. AgBr–ZnO–An efficient nano-photocatalyst for the mineralization of Acid Black 1 with UV light. Sep. Purif. Technol.; 2012; 85, pp. 35-44. [DOI: https://dx.doi.org/10.1016/j.seppur.2011.09.037]

48. Senthilraja, A.; Krishnakumar, B.; Hariharan, R.; Sobral, A.J.; Surya, C.; John, N.A.A.; Shanthi, M. Synthesis and characterization of bimetallic nanocomposite and its photocatalytic, antifungal and antibacterial activity. Sep. Purif. Technol.; 2018; 202, pp. 373-384. [DOI: https://dx.doi.org/10.1016/j.seppur.2018.04.015]

49. Kalyani, D.C.; Telke, A.A.; Govindwar, S.P.; Jadhav, J.P. Biodegradation and detoxification of reactive textile dye by isolated Pseudomonas sp. SUK1. Water Environ. Res.; 2009; 81, pp. 298-307. [DOI: https://dx.doi.org/10.2175/106143008X357147]

50. Deng, J.; Jiang, J.; Zhang, Y.; Lin, X.; Du, C.; Xiong, Y. Hybrid copper doped Titania/polythiophene nanorods as efficient visible light-driven photocatalyst for degradation of organic pollutants. Appl. Catal. B; 2009; 84, pp. 468-473. [DOI: https://dx.doi.org/10.1016/j.apcatb.2008.04.029]

51. Liu, H.; Hu, Y.; Zhang, Z.; Liu, X.; Jia, H.; Xu, B. Synthesis of spherical Ag/ZnO heterostructural composites with excellent photocatalytic activity under visible light and UV irradiation. Appl. Surf. Sci.; 2015; 355, pp. 644-652. [DOI: https://dx.doi.org/10.1016/j.apsusc.2015.07.012]

52. Liu, H.; Zhai, H.; Hu, C.; Yang, J.; Liu, Z. Hydrothermal synthesis of In₂O₃ nanoparticles hybrid twins hexagonal disk ZnO heterostructures for enhanced photocatalytic activities and stability. Nanoscale Res. Lett.; 2017; 12, 466. [DOI: https://dx.doi.org/10.1186/s11671-017-2233-3]