The ingredients were procured through different commercial suppliers as follows: Zn(NO₃)₂∙6H₂O (Alfa Aesar, 99%), Ni(NO₃)₂∙6H₂O (Sigma Aldrich, 99.99%, Co(NO₃)₂∙6H₂O (Loba Chemie, 99.8%), In(NO₃)∙xH₂O (Sigma-Aldrich, 99.99%), Mn(NO₃)∙4H₂O (Alfa Aesar, 98%) and NaOH (Sigma-Aldrich, 97%). Ultra-pure water (18. 2 MΩ∙cm) was used in the experiments.

The synthesis of ZnO/M_xO_y nanocomposites were carried out by using 15 ml 0.1M Zn(NO₃)₂∙6H₂O mixed with 15 ml 0.1M along with a desired metal nitrate (Ni(NO₃)₂∙6H₂O, Co(NO₃)₂∙6H₂O, Mn(NO₃)₂∙4H₂O or In(NO₃)∙xH₂O). The mixed solution was ultrasonicated thoroughly and mixed for 30 minutes to get the homogenous mixture. Later, about 24 ml of the suspension was transferred into a 30 ml capacity Teflon liner and closed tightly. The percent fill was kept at 80% in all experiments. The Teflon liner was inserted into SS316 Stainless Steel autoclave and heated up to a particular temperature for a required period of time (Table 1). The autoclave was cooled down to the laboratory temperature after hydrothermal reaction and the resultant material in the Teflon liner was filtered and washed with distilled water and ethanol. Later, the product obtained was dried at around 80˚C in a hot air oven for 12 hours. Table 1 gives the experimental hydrothermal conditions used in the synthesis of ZnO-M_xO_y heterostructure nanocomposites.

The prepared heterostructures were characterized using Rigaku Smart Lab-II X-ray diffractometer (XRD) with CuKα radiation (λ = 1.540598 Å), Carl Zeiss MERLIN Compact field emission scanning electron microscopy (FE-SEM), JASCO 460 plus FTIR spectrometer, a Shimadzu UV-2450 spectrophotometer (UV-Vis DRS), NETZSCH, Germany, Model STA 2500 Regulus, thermo-gravimetric analysis (TGA), and BELSORP MINI 2, BEL, Japan, Brunauer-Emmett-Teller (BET) surface area measurements.

The photocatalytic properties of as synthesized ZnO-M_xO_y nanocomposites were carried out for the study of phenol degradation. The degradation was performed under solar light irradiation to the phenol sample with the catalyst ZnO-M_xO_y nanocomposites. The phenol samples with different concentrations, like 8, 16, 24 and 32 ppm, were prepared and the optimum phenol degradation concentration was investigated for the different catalysts. In typical procedure, the degradation

was carried out by preparing 100 ml of phenol solution with above mentioned different concentration in a 250 cm³ conical flasks. Later, 0.5 g of the catalysts such as ZnO-M_xO_y nanocomposites were added to individual flasks and closed tightly with rubber cork. The degradation reaction was carried out for the period of 3 h (from 11 am to 2 am in a sunny day in April 2018 in Mysuru, India) under solar light irradiation. After the completion of degradation reaction, the samples were filtered and the percentage of phenol degradation was investigated by performing chemical oxygen demand (COD) analysis.

Figure 1 shows the powder XRD patterns of pure ZnO and heterogeneous semiconductor ZnO-M_xO_y nanocomposites. It is clear indicates that, all the diffraction peaks of ZnO can be indexed to the hexagonal wurtzite structure according to JCPDS card N (JCPDS: 36-1451) [19] and peaks corresponds to heterostructured composites consist of different phases. The XRD pattern of heterostructured ZnO-Co₃O₄ composite indicates that it consists of two kinds of phases, hexagonal ZnO and spinel structured cubic Co₃O₄ (JCPDS card # 43-1003) confirming the coexistence of Zn and Co oxide structure together to form a composite however, the peaks of Co₃O₄ are weak compared with ZnO and it is matched with the data reported by Zhang et al. [20] . The XRD pattern of heterostructured ZnO-NiO composite indicates that it consists of two phases, hexagonal ZnO and cubic structured rocksalt type NiO phase (JCPDS-78-0643) showing the coexistence of Zn and Ni oxide structures together to form heterogeneous composite. The XRD pattern of heterostructured ZnO-MnO₂ composite specifies that it consists of two kind of phases, hexagonal ZnO and birnessite δ-MnO₂ crystal phase (JCPDS 80-1098) [21] which indicates that the coexistence of Zn and Mn oxide structures together to form heterogeneous heterostructure

composite [22] . The powder XRD pattern of heterostructured ZnO-In₂O₃ composite postulates that it consists of two kind of phases, hexagonal ZnO and bixbyite type In₂O₃ of cubic system crystal phase indicates that the coexistence of Zn and In oxide structures together to form heterogeneous composite. The XRD pattern of ZnO-In₂O₃ is in agreement with the earlier reported pattern [23] . The intensity of standard ZnO decreased in all heterogeneous composites, due to the major reflections of other metal oxides which were entrapped in intense ZnO reflections. This mechanism leads to the growth of heterogeneous composites with different crystal phases. The powder patterns show well crystallized metal oxides with a higher degree of crystallinity forming M_xO_y nanocomposites.

The FTIR analysis of heterostructured ZnO-M_xO_y nanocomposites was performed. Figure 2 shows the FTIR spectra and the bands are observed near 3445 cm⁻¹ and 1630 cm⁻¹ correspond to the O-H stretching and bending vibration of water molecules adsorbed on the surface of nanocomposites due to the porosity as noticed in the images of electron microscopic study.

The carbonate peaks are prominent in all the composites and it is again attributed to the absorption of CO₂ from the atmosphere. The finger print region of metal oxygen bond shows the characteristic absorption bands in the wavenumber region 400 to 600 cm⁻¹ [11] . This finger print range shows the metal oxygen bond presence of different metal oxygen of ZnO-M_xO_y nanocomposites. Usually ZnO shows flower like, or interlocked rod like structure, and other metal oxides like CO₃O₄ , In₂O₃, NiO, and MnO₂, show a different morphology (either tabular, or massive rounded to sub-rounded). The bonding of composites have metal and oxygen bond between Zn-O, Ni-O, Mn-O, In-O and Co-O along with the absorption peak at wavenumber range 1402 - 1506 cm⁻¹ indicates the presence of CO₂, which was due to the atmospheric CO₂ interference during analysis [11] [19] . Apparently, the FTIR results further support the formation

of the heterostructured nanocomposites and agree well with XRD results.

Figure 3 shows FE-SEM images of ZnO-M_xO_y nanocomposites and it is observed that ZnO-Co₃O₄ and ZnO-In₂O₃ show fine interlocked nanorods showing porous structure, whereas ZnO-NiO and ZnO-MnO₂ show still finer structures with smaller rods and also showing the interlocked porous structure. This is a very typical characteristic of ZnO-M_xO_y nanocomposites. It is further observed that the particles have narrow-size distribution. The average particle size distribution of ZnO-M_xO_y nanocomposites is shown in Figure 4. Among the nanocomposites synthesized, ZnO-In₂O₃ have highest particle size (140 nm) and ZnO-NiO have the least particle size (40 nm). This kind of interlocked structure shows higher porosity and in turn higher BET surface area. It is also evident from FTIR spectra that, ZnO-M_xO_y nanocomposites have the surface adsorption of CO₂ and O-H.

Particle size distributions of as prepared nanocomposites are shown in Figure 4. The analysis was carried out by DLS instrument, in which the size measurement was based on the Brownian motion of particle in the solvent media. The results as evident from Figure 4, indicate that the average particle size (nm) of the nanocomposites prepared by hydrothermal reaction conditions. The average particle size of pure ZnO, composites ZnO-Co₃O₄, ZnO-In₂O₃, ZnO-MnO₂ and ZnO-NiO was 38, 47, 143, 46 and 44 respectively. The pure ZnO shows the lowest and composite ZnO-In₂O₃ shows highest average particle size. The results obtained from DLS corresponding with the results of FESEM images. It was described that the highest particle size from the composite ZnO-In₂O₃ due to the high atomic/ionic radius (167 pm) of Indium atom, when compared to the other metals from the composite. Further the composites, prepared from the hydrothermal reaction show the size within 100 nm range except ZnO-In₂O₃ and indicates that the confirmation of nanocomposite formation.

The DRS UV-Vis spectra of as-synthesized ZnO-M_xO_y nanocomposites were recorded to determine their light absorption characteristics and Figure 5 shows the DRS UV-Vis spectra of heterostructured nanocomposites. ZnO-MnO₂ shows the highest visible light absorption while ZnO-NiO shows the lowest visible light absorption. The band gaps of the various heterostructures were obtained from Tauc plot displayed in Figure 5(b) [24] [25] . Obviously, the band gap of the four heterostructures followed the trend of ZnO-NiO (3.1 eV) > ZnO-In₂O₃ (2.71 eV) > ZnO-Co₃O₄ (2.64 eV) > ZnO-MnO₂ (2.19 eV).

The thermal analysis of as synthesized ZnO-M_xO_y nanocomposites was carried out to get information about change in composites properties as function of temperature. Figure 6 shows the TGA curves obtained under inert nitrogen environment for the nanocomposites. TGA curve indicate that the change in weight loss (%). The TGA curve of ZnO-Co₃O₄ shown that, the gradual decrease

in the weight at 105˚C, attributed to the evaporation of any residual water. Further decrease in the weight loss up to 290˚C was due to the evaporation of CO₂. The presence of CO₂ was evident from the FTIR analysis as well as the sudden loss in the weight% at temperature 290˚C to 350˚C was due to the decomposition of Co₃O₄ which in turn form Co₃O₄ and O₂. The total weight loss of ZnO-Co₃O₄ was ~15% and it shows high thermal stability. TGA curve at the temperature range between 40˚C - 180˚C and 190˚C - 375˚C of ZnO-NiO composite shows the weight loss occurred due to evaporation of water and CO₂ respectively. Again the sudden increase in the weight at temperature range between 380˚C - 390˚C occurred due to the re-adsorption of CO₂ with the influence of fluid N₂. Further the sudden decrease in the weight loss at temperature between 440˚C - 520˚C was due to the decomposition of composites. The TGA curve of ZnO-MnO₂ composite shows the curve with gradual decrease in the weight loss at temperature 40˚C - 150˚C due to evaporation of water molecules. Further the weight loss at temperature 420˚C - 550˚C was due to the thermal decomposition of ZnO-MnO₂ to MnO₂, and also the total weight loss observed was only ~8%, which depicts the higher thermal stability. The TGA of ZnO-In₂O₃ composite shows that the gradual decrease of weight loss at 40˚C - 120˚C and 150˚C - 240˚C indicate the evaporation of water and CO₂ respectively. Further the sudden decrease in the weight loss at 280˚C - 330˚C was due to the decomposition of composite, with a larger weight loss (35%).

The N₂ sorption isotherms of all samples can be classified as type IV with H₃ hysteresis loops (Figure 7), which means the existence of slit-like mesopores (2 - 50 nm) formed between aggregated nanoparticles [26] . The hysteresis loops of all samples spanned a large P/P0 range (0.3 - 1.0), corresponding to the presence of mesopores. The surface area of the various samples follows the order: ZnO-MnO₂ (58.4 m²/g) > ZnO-NiO (41.6 m²/g) > ZnO-Co₃O₄ (39.6 m²/g) > ZnO-In₂O₃ (22.7 m²/g). In comparison with DLS results, the lowest surface area is shown by ZnO-In₂O₃ with the larger particle size. The pore size distribution of the samples (Figure 7) further confirms the formation of mesopores. As can be seen, the pore size distribution curve is quite broad (from 2 to 100 nm) with small and large mesopores (in the range of 2 - 50 nm). The small mesopores may reflect porosity within the nanoparticles and the large mesopores are associated to the pores formed between the nanoparticles [27] .

The photocatalytic activities of the as prepared nanocomposites were performed by using phenol as a model organic pollutant in an aqueous solution for the degradation process. Figure 8 shows Chemical Oxygen Demand level of phenol

degradation in different concentrations of various composites under sunlight irradiation for the period of 3 hours. The order of phenol degradation follows the trends: ZnO-Co₃O₄ > ZnO-In₂O₃ > ZnO-MnO₂ > ZnO-NiO. Apparently, the order of phenol degradation does not follow the order of surface area, but it may occur because of the chemical composition of composites. The photocatalytic degradation was maximum by using ZnO-Co₃O₄, where 97% of 8 ppm of phenol have been degraded followed with 95% degradation by ZnO-In₂O₃, 84% degradation by ZnO-MnO₂ and 29% degradation by ZnO-NiO respectively. The degradation efficiency decreased when the concentration of phenol increased from 8 ppm to 32 ppm, which may be due to phenol molecules around the photocatalytic active sites resulted in inhibiting the penetration of light to the surface of the catalyst. Hence, the generation of relative amount of ?OH and ˙ O 2 − on the surface of the catalyst decreased [28] . The heterostructure catalysts were further tested for repetitive processes of photocatalytic degradation. It was found that the photocatalytic degradation efficiency did not decrease even after 3 cycles of photodegradation.

The band gaps and band positions were calculated and the band structure diagrams of the different nanocomposites were presented in Figure 9, which is

helpful in understanding the mechanism of phenol degradation. The enhancement in the photocatalytic activity of ZnO-Co₃O₄ and ZnO-In₂O₃ can be attributed to the small band gap of ZnO-In₂O₃ (2.71 eV) and ZnO-Co₃O₄ (~2.64 eV) which results in improving the visible light absorption, and the enhancement in the charge separation occurs because of coupling ZnO with Co₃O₄ and In₂O₃ separately [29] [30] [31] . In case of Co₃O₄-ZnO nanocomposite, the electrons are accumulated in the conduction band (CB) of ZnO and the holes in the valance band (VB) of Co₃O₄ because the difference in the band edges positions. Similarly, in In₂O₃-ZnO nanocomposite, the electrons migrate to the CB of ZnO and the holes to the VB of In₂O₃ leading to inhibiting the electron hole recombination and enhancing the charge carrier separation efficiency and thus enhanced the photocatalytic activity. The electronic structure and band gap movement and also defect states have been studied in detail using XPS and photoluminescence spectroscopy. These results will be published elsewhere as a part of a detailed mechanism for heterostructure metal oxides nanocomposites.

Although the band gap of ZnO-MnO₂ (2.19 eV) is smaller than ZnO-In₂O₃, ZnO-Co₃O₄ and ZnO-NiO shows lower photocatalytic activity and the reason is charge separation efficiency of ZnO-MnO₂ is very low when compared with ZnO-Co₃O₄ and ZnO-In₂O₃. This process is indicated clearly in Figure 9, where the electrons and holes are accumulated in the CB and VB of MnO₂ which leads to high electron-hole recombination rate and thereby low photocatalytic activity. Moreover, the CB edge of MnO₂ is lower than the potential required to produce superoxide radicals ( ˙ O 2 − ), hence only hydroxyl radicals (?OH) take part in the phenol degradation over ZnO-MnO₂ leading to further decrease in the photocatalytic activity [32] . The low photocatalytic activity of ZnO-NiO under sunlight irradiation due to the large band gap of ZnO-NiO (3.1 eV) leads to low light absorption and hence low photocatalytic activity occurred.