UV-ozone surface pretreatment for high quality ALD-grown ultrathin coatings on bismuth oxyhalide photocatalysts

Abstract

The growth of ultrathin layers of oxides by atomic layer deposition (ALD) is well documented for oxide substrates such as SiO₂, Bi₂O₃, Al₂O₃, in which oxygen is the only negatively charged atom. In contrast, the knowledge regarding ALD growth on substrates containing other negatively charged atoms, such as halogens, is quite limited. The commonly used bismuth oxyhalide (BiOX) family of materials are characterised by a low density of surface hydroxyls, required for the initiation of thermal ALD growth of oxides, thus hampering the ability to grow ultrathin layers of oxides on their surface. This restriction becomes even more severe if the process has to be performed at low temperatures. In this work, we show that high quality Al₂O₃ can be grown on bismuth oxyhalide materials by low temperature ALD, upon performing UV-ozone surface pretreatment. The effect of pretreatment on the BiOX photocatalysts was studied by wettability measurements and FTIR. The coating conformality was monitored by both XPS and via the ability of the ultrathin layers to suppress the photocatalytic activity of the substrates. The capability to form dense, conformal aluminium oxide layers on BiOX substrates opens a door for low-temperature preparation of organic–inorganic hybrid devices on such and similar compounds.