Zinc acetate dihydrate (≥ 98%, Sigma-Aldrich) was used as the ZnO precursor without further purification. Sodium hydroxide was used as the precipitating agent and de-ionized water as the solvent for all the experiments. Titanium oxysulfate (29%, Sigma-Aldrich) was employed as the titania precursor while ammonium hydroxide (28%, Sigma-Aldrich) was used as the pH adjuster.

The ZnO/TiO₂ nanocomposites were synthesized by incorporating TiO₂ nanopowder into the ZnO sol-matrix. This TiO₂ nanopowder was synthesized by modifying the method proposed by Nankya et al.⁹⁾ The resulting TiO₂ nanopowder was dried at 100°C for 3 h. A weighed sample of zinc acetate dihydrate was dissolved in de-ionized water to form a 0.2 M solution, which was magnetically stirred at 80°C for 30 min. The TiO₂ nanopowder was then added to the ZnO solution. The resulting mixture was stirred for another 30 min before being cooled to room temperature. An aqueous solution of sodium hydroxide was prepared and stirred at 80°C for 20 min. This solution was then cooled to room temperature. The cooled sodium hydroxide solution was added drop-wise to the ZnO/TiO₂ solution at room temperature and the resulting solution was stirred vigorously for 5 h. The ZnO/TiO₂ precipitate so obtained was aged for 24 h, after which it was filtered and washed several times. It was then dried at 100°C for 24 h, calcined at 500°C for 2 h, and ground into powder. Fig. 1 shows the schematic of the procedure used for the synthesis of the ZnO/TiO₂ nanocomposites.

X-ray diffraction (D/Max-2200, Rigaku, Japan) was carried out to examine the crystal structure and calculate the crystallite size of the nanocomposites. Field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6701F, Japan) and energy dispersive X-ray spectroscopy (EDX, line-scan mapping) analyses were carried out to examine the microstructure and elemental composition of the nanocomposites. The morphology of the ZnO/TiO₂ nanocomposites was examined using transmission electron microscopy (TEM, JEOL, JEM-2100F, Japan). UV-vis spectroscopy (Shimadzu, UV-2600) was carried out to investigate the optical properties of the nanocomposites.

The XRD patterns of the calcined samples pure ZnO (Z) and TiO₂ (T) and the ZnO/TiO₂ nanocomposites (Z3T, Z7T)) are shown in Fig. 2. The peaks observed at 2θ = 31.69, 34.36, 36.17, 47.48, 56.52, 62.79, 66.34, 67.88, 69.04, 72.51, and 76.89 correspond to the ZnO hexagonal wurtzite phase (JCPDS card No. 36-1451). On the other hand, the peaks observed at 2θ = 25.24, 37.82, 47.91, 53.92, 62.78, 68.87, and 75.17 correspond to the TiO₂ anatase phase (JCPDS card No. 21-1272). Z3T showed the (101) and (004) diffraction planes, while Z7T showed the (101), (004), and (105) diffraction planes. This indicates that the anatase phase was present in the synthesized nanocomposites. No impurity peaks were observed, indicating that the samples were pure.

The intensity of the XRD peaks decreased with an increase in the weight percentage of the TiO₂ nanopowder added to the ZnO sol-matrix. The Z3T sample with 30 wt% TiO₂ nanopowder showed intense XRD peaks. An increase in the TiO₂ content increased the sharpness and intensity of the anatase phase peaks, as shown in the inset of Fig. 2.

The crystallite sizes of the nanocomposites were calculated using the Scherrer equation:

where t is the crystallite size (nm), k the Scherrer constant (equal to 0.94), λ the wavelength of the incident Cu-K_α radiation (1.54059 A), β the line broadening at full width at half maximum, and θ Bragg’s diffraction angle.¹⁰⁾

The calculated average crystallite sizes of the samples are given in Table 1. The Z3T and Z7T nanocomposites showed ZnO crystallite sizes of 26.8 and 26.3 nm, respectively. The crystallite size of the TiO₂ nanopowder incorporated in the ZnO sol-matrix was about 9 nm.

The morphology of the ZnO/TiO₂ nanocomposites was examined by FE-SEM and TEM. The FE-SEM images of the ZnO/TiO₂ nanocomposites calcined at 500°C are shown in Fig. 3. It was found that the ZnO/TiO₂ nanocomposite powders formed spherical agglomerates and consisted of rod-like ZnO particles (can be observed clearly from the FE-SEM image of Z5T). Fig. 3 also shows the line-scan mapping results of the nanocomposites, which give an insight into the microstructural composition of the nanocomposites. The EDX analysis results revealed that the ZnO/TiO₂ nanocomposites consisted of only Zn, O, and Ti. Following are the atomic ratios in which these elements were present in the nanocomposites: Z3T Zn (77.51), O (21.71), and Ti (0.78); Z7T Zn (63.55), O (23.27), and Ti (13.18).

The TEM images of the samples are shown in Figs. 4 and 5. The TEM results showed that the nanocomposites were made up of rod-like ZnO and spherical TiO₂ particles. The images revealed that the ZnO and TiO₂ particles coexisted in the nanocomposites. It was also observed that some of the TiO₂ particles were embedded in the ZnO particles, while some were attached onto the ZnO surface. The images further confirm the rod-like shape of the ZnO particles in the nanocomposites. The TiO₂ particles on the other hand were spherical and had very small sizes.

Figure 6 shows the UV-vis absorbance spectra of the calcined samples (pure ZnO and TiO₂ and the Z3T and Z7T nanocomposites). Pure TiO₂ (anatase) and ZnO have wide bandgaps of 3.2 and 3.37 eV, respectively, which limits their use of the solar spectrum to only UV light (λ ≤ 385 nm).⁶⁾ The ZnO/TiO₂ nanocomposites exhibited enhanced absorbance in the visible light region compared to pure ZnO and TiO₂. The ZnO/TiO₂ nanocomposites (Z3T, Z7T) showed a strong absorption edge between 380 and 390 nm. However, pure ZnO showed a strong absorption peak between 370 and 380 nm corresponding to ZnO bandgap absorption, while pure TiO₂ showed a broad peak at around 360 nm and a strong absorption peak between 240 and 260 nm ascribed to its 3.2 eV photon energy gap.⁹⁾ The ZnO/TiO₂ nanocomposites also showed enhanced light absorption (> 400 nm), while pure ZnO was more right-shifted compared to TiO₂. Thus, the combination of ZnO and TiO₂ to form ZnO/TiO₂ nanocomposites is an effective approach to improving the physicochemical properties of ZnO and TiO₂. This is because ZnO and TiO₂ have similar photocatalytic mechanisms.⁴⁾ They also have similar bandgap energies with the energy levels located at almost the same positions. The electron-hole recombination in pure ZnO and TiO₂ can also be suppressed by the enhanced charge separation in ZnO/TiO₂ composites.^4,5) Thus, on the basis of the results obtained in this study, it can be stated that the absorption properties of ZnO/TiO₂ nanocomposites can be improved by increasing their TiO₂ contents.