Interfacial charge-transfer process across ZrO₂-TiO₂ heterojunction and its impact on photocatalytic activity

In this paper, ZrO₂-TiO₂ composites were synthesized by sol-gel method with different ZrO₂:TiO₂ molar ratios (01:99, 05:95 and 10:90). The results identified two trends; at a low ZrO₂ content, the incorporation of Zr⁴⁺ into the TiO₂ lattice was possible provoking generation of oxygen vacancies (1 mol% of ZrO₂); while at higher ZrO₂ contents, ZrO₂-TiO₂ heterojunctions were created (5 and 10 mol% of ZrO₂). The photocatalytic activity was evaluated by measuring photodegradation of phenoxyacetic acid, 2,4-dichlorophenoxyacetic acid or 4-chlorophenol solutions. The ZT-5 composite shows the best performance attributed to surface states at the interface of ZrO₂-TiO₂ heterojunctions. These surface states act as traps for charge carriers favoring the spatial separation of electron-hole pairs until reaching a maximum in the composite with 5 mol% of ZrO₂. The ZT-5 composite showed the most negative flat band potential and the highest donor density indicating that these surface states are in optimal concentration. At higher ZrO₂ contents, charge carrier separation is less effective, which decreases the photocatalytic activity. Nevertheless, the molecule structure has an impact on the direct or indirect charge transfer process as was evidenced by EIS measurements.