## 3 Results and Discussion
### Heterostructures Morphology and Point of Zero Charge (\(pH_{\text{PZC}}\))  
The morphology is an important property that can influence both photocatalytic efficiency and adsorption that take place at the heterostructure's interface with the working environment. The AFM images presented in Figure 3 show that meso- and micro-particles are agglomerated in large aggregates depending on the heterostructure composition.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Sample** & **Line** & **O K** & **Na K** & **Al K** & **Si K** & **S K** & **Ti K** & **Fe K** & **Cu K** & **Total** & **SiAl** & **TiCuSi** \\ \hline \multirow{2}{*}{A10} & Wt. \% & 41.66 & 8.69 & 7.78 & 12.69 & 1.36 & 20.04 & 2.81 & 4.97 & 100 & & \\  & At. \% & 60.39 & 8.77 & 6.69 & 10.48 & 0.99 & 9.70 & 1.17 & 1.81 & 100 & 1.57 & 5.35:15.79 \\ \hline \multirow{2}{*}{B10} & Wt. \% & 40.80 & 6.09 & 4.52 & 7.81 & 0.71 & 34.71 & 2.77 & 2.60 & 100 & & \\  & At. \% & 62.23 & 6.47 & 4.09 & 6.78 & 0.54 & 17.68 & 1.21 & 1.00 & 100 & 1.66 & 17.68:16.78 \\ \hline \multirow{2}{*}{C10} & Wt. \% & 41.95 & 4.30 & 3.67 & 9.94 & 1.19 & 34.64 & 1.29 & 0.56 & 100 & & \\  & At. \% & 63.48 & 4.53 & 3.30 & 8.57 & 0.90 & 17.51 & 3.02 & 1.15 & 100 & 2.60 & 15.22:17.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The weight and atomic percentage of each element in ternary TiO\({}_{2}\)–CuxS–FA materials (from Energy-dispersive X-ray spectroscopy (EDS) spectra).  
Figure 3: The AFM images and distribution curves of ternary heterostructures: (**a**) sample A10, (**b**) sample B10, (**c**) sample C10.  
Sample A10 presented a porous morphology with an average pore size around 1 \(\upmu\)m (Figure 3a), which is considered suitable for photocatalytic application due to the scattering effect [49]. By decreasing the FA quantity and increasing the TiO\({}_{2}\) ratio, the pore size value decreases significantly (\(\sim\)600 nm for sample B10, see Figure 3b, and \(\sim\)250 nm for sample C10, see Figure 3c); even if the size of the pores decreases, the number of pores increases, which is in accordance with other reports [66], suggesting the possibility of tailoring the morphology based on the heterostructure components ratio.  
The Scanning electron microscopy (SEM) images (see Figure 4) indicate that all three samples contain aggregates of various shape and size un-homogenously distributed on the surface. This morphology is characteristic for ternary heterostructures [65] where metal oxides or sulfur compounds (TiO\({}_{2}\), Cu\({}_{2}\)S) adhere to the fly ash particle surfaces. Most of the particles have a spherical shape, with sizes ranging from 200 to 500 nm dispersed over the whole matrix of materials. By maximising the surface area, it is possible to increase the photocatalytic efficiency due to the larger interface between the samples and the aqueous environment.  
Suitable charged surfaces and porous morphologies are considered prerequisites for high efficient photocatalytic and adsorption processes [67]. The photocatalysis and adsorption reaction on the nanostructures materials are influenced by a few factors: (i) the pH of the pollutant, (ii) the ionizing capacity of the solute (acidity, pKa), and (iii) the zero point charge of materials (pH\({}_{\text{PZC}}\)). The values of pH\({}_{\text{PZC}}\) calculated based on Figure 5, i.e., 6.43 for sample A10, 6.32 for Sample B10 and 6.25 for Sample C10, indicated an amphoteric surface of nanostructures favourable for adsorption and photocatalysis.  
Figure 4: SEM images of ternary TiO\({}_{2}\)–CuxS–FA heterostructures: (**a**) Sample A10, (**b**) Sample B10, (**c**) Sample C10.