Energetic states in SnO₂–TiO₂ structures and their impact on interfacial charge transfer process

In this paper, core(SnO₂)@shell(TiO₂) structures with different SnO₂:TiO₂ molar ratios were obtained by direct hydrolysis of titanium isopropoxide on SnO₂ nanoparticles, forming a composite that provided a direct contact between core and shell. These materials were characterized by TEM, XRD, Raman spectroscopy, XPS, BET, and DRS techniques. The photocatalytic degradation of 4-chlorophenol was evaluated as a probe. The activity was observed to increase until reaching a concentration of 6 mol% of SnO₂, which was followed by subsequent decrease. Recent studies report a maximum of photocatalytic activity, when TiO₂ is coupled with another semiconductor oxide, around 5–7 mol% of the other oxide in the composite, leading to a controversy about why at a higher ratio, the charge carrier separation seems to be accompanied by other processes taking place at the interface of the oxides. To understand this behavior, (photo)electrochemical techniques were employed to characterize semiconducting properties of the composites. The presence of SnO₂ in the composite moved the flat band position toward more negative potentials, and changed the open circuit potential under illumination toward less negative values, as the SnO₂ content increased up to 6 mol%. Additionally, this SnO₂ content generated the highest number of donors, indicating the optimal condition for the formation of SnO₂@TiO₂ heterojunction. These energetic states formed at heterojunction act as traps during the photocatalytic process preventing the recombination of photogenerated charge carriers. However, at higher concentrations of SnO₂, the photocatalytic activity decreases due to changes in the energetic states generated at the heterojunction that directly impact the process of trapping photogenerated electrons.