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.