Photoinduced superhydrophilic and catalytic properties of ZnO vs. TiO2 based thin films

Heterogeneous photocatalysis by semiconductor oxides is a fast growing and promising area of applied research, especially regarding the degradation of various air and water pollutants. In this context, titanium dioxide, a semiconductor harvesting radiation in the near-UV region, has become the principal material of choice for photocatalytic environmental remediation and hydrophilic self-cleaning surfaces. A highly appealing candidate as an alternative to TiO2 is zinc(II) oxide, which, though being known as a photocorrosive material, possesses similar physico-chemical characteristic and photodegradation mechanism [1]. In particular, both photoinduced superhydrophilicity (PSH) and photocatalytic oxidation (PCO) are initiated by photogeneration of electron (e-) and hole (h+) pairs in the semiconducting material. In both TiO2 and ZnO, the energy position of valence and conduction bands enable an effective separation of the photogenerated e- and h+ resulting, in the case of PCO, in strong oxidizing ability of the latter. Nevertheless, there are other important factors influencing the photocatalytic activity of thin semiconducting films, such as surface morphology and crystallinity, which, in turn, can be tailored by the use of versatile synthetic routes. Among the various adopted preparative strategies, Chemical Vapor Deposition (CVD) and Sol-Gel (SG) processes offer the possibility of exerting a direct control on the system morphogenesis even under relatively mild conditions, providing several degrees of freedom to properly modify the system functional performances. In the present contribution, the PSH and PCO properties of ZnO and TiO2 thin films, obtained by CVD and SG routes, are compared and discussed.