Conformal Filling of TiO2 Nanotubes with Dense MxSy Films for 3D Heterojunctions: The Anion Effect

It is appealing to fill up TiO2 nanotube arrays with other semiconductors to form three-dimensional heterojunctions of interdigitated feature. This benefits from the large interfacial area for rapid charge separation and the ordered vertical channels for oriented charge transport. However, it remains a great challenge to fill the nanotubes with dense films and more importantly in a conformal manner. In this study, the TiO2 nanotubes are conformally filled with ZnS by successive ionic layer adsorption and reaction. The ZnS exists in the form of either discrete nanoparticles or dense films when using nitrate or acetate precursor solutions, respectively. This is attributed to the hydrolysis process of the acetates in aqueous solutions, which releases extra hydroxyl ions beneficial to film formation. The CuS/TiO2 heterojunctions, resulted from ZnS dense films, show better rectifying behaviors in view of the interdigitated structure. Such a conformal filling process with dense films can also be used to form other MxSy/TiO2 bulk heterojunctions toward applications in optoelectronics, photovoltaics, photo-catalysis, and so on.