## 3 Results  
### UV-Visible Spectroscopy  
For observing the formation of NPs, optical absorption measurements are the first step. Figure 1a shows the UV-DRS spectra of ZrO\({}_{2}\), CdWO\({}_{4}\), and ZrO\({}_{2}\)-CdWO\({}_{4}\). Pure ZrO\({}_{2}\) and pure CdWO\({}_{4}\) have an absorption edge at 228 nm nanoparticles and 255 nm nanorods, respectively, while the ZrO\({}_{2}\)-CdWO\({}_{4}\) NC occurs at approximately 240 nm. The absorption band corresponding to these nanorods gets blue-shifted on ZrO\({}_{2}\) doping. The shift of the absorption band towards a shorter wavelength indicates a decrease in particle diameter and the narrowing of the band gap. The narrowing of the band gap could also be due to many interactions on the ZrO\({}_{2}\) deposition and the making of a heterojunction between ZrO\({}_{2}\) and CdWO\({}_{4}\). Here, the optical absorption changes mainly owing to the coupled semiconductor system following the deposition of ZrO\({}_{2}\) on CdWO\({}_{4}\) nanorods. Figure 1: (**a**) UV–Visible diffuse absorbance spectrum of (**a**) ZrO\({}_{2}\), CdWO\({}_{4}\), ZrO\({}_{2}\)-CdWO\({}_{4}\); (**b**) a Tauc plot of ZrO\({}_{2}\), CdWO\({}_{4}\), and ZrO\({}_{2}\)-CdWO\({}_{4}\) as an indirect band gap; and (**c**) a Tauc plot of ZrO\({}_{2}\), CdWO\({}_{4}\), and ZrO\({}_{2}\)-CdWO\({}_{4}\) as a direct band gap. The UV absorption spectrum (Figure 1a) of the sample was recorded for measuring the direct and indirect bandgap of the ZrO\({}_{2}\), CdWO\({}_{4}\), and the ZrO\({}_{2}\)-CdWO\({}_{4}\) NC using the _Tauc_ relation, Equation (1) [16; 17; 18]:  
\[(\alpha\mathrm{h}\
u)^{1/n}=\mathrm{A}\big{(}\mathrm{h}\
u-\mathrm{E}_{g}\big{)} \tag{1}\]  
where A is a proportionality constant, _h\(\
u\)_ is the incident photon energy, \(\alpha\) is the molar extinction coefficient, \(\mathrm{E}_{g}\) is the bandgap of the sample, and n is the exponent to evaluate the type of electronic transition causing the absorption, which has values of \(\frac{1}{2}\) and 2 for direct and indirect band gap transitions, respectively. The bandgap was determined from the Tauc plot [16; 17; 18; 33]. In the Tauc plot, the intercept of the linear portion of the (\(\mathrm{a}\mathrm{h}\
u\))\({}^{2}\) vs. _h\(\
u\)_ on the \(x\)-axis is shown in Figure 1b and (\(\mathrm{a}\mathrm{h}\
u\))\({}^{1/2}\) vs. h\(\
u\) is shown in Figure 1c. The bandgap values for ZrO\({}_{2}\), CdWO\({}_{4}\) and the ZrO\({}_{2}\)-CdWO\({}_{4}\) NC are given in Table 1. The decrease in the band gap suggests the successful coupling of ZrO\({}_{2}\) to CdWO\({}_{4}\). The band gap of the present ZrO\({}_{2}\)-CdWO\({}_{4}\) NC was determined as narrower than that of previously reported CdWO\({}_{4}\)-based NCs such as ZnO-CdWO\({}_{4}\), CuO-CdWO\({}_{4}\), and ZnO-CuO-CdWO\({}_{4}\), as shown in Table 1.  
Apart from the preparation method, the band gap also depends on many other factors, such as plant extract, calcination temperature, and coupled semiconductors. Using estimated band gaps (\(\mathrm{E}_{g}\)) and electronegativity (X) of conjugated semiconductors (ZrO\({}_{2}\) and CdWO\({}_{4}\)), the bands (valence band, VB) and the conduction band, CB) edge position (\(\mathrm{E}_{\mathrm{VB}}\) and \(\mathrm{E}_{\mathrm{CB}}\)) of the current photocatalyst were determined by applying Equation (2) [16], the result of which is given in Table 2.  
\[\mathrm{E}_{\mathrm{VB}}=\mathrm{X}-\mathrm{E}_{\mathrm{e}}+0.5\mathrm{E}_{g}, \mathrm{E}_{\mathrm{CB}}=\mathrm{E}_{\mathrm{VB}}-\mathrm{E}_{g} \tag{2}\]  
Here, \(\mathrm{E}_{\mathrm{e}}\) is the energy of free electrons, which is \(\sim\)4.5 eV. Using the edge positions of these bands (VB and CB), the heterojunction design was proposed as type I [19]. Type I heterojunctions aid charge separation and prevention of electron-hole recombination.