Combinatorial Atmospheric Pressure Chemical Vapour Deposition for Optimising the Functional Properties of Titania Thin-Films

Titanium dioxide (TiO2) is the leading material for self-cleaning applications due to its chemical inertness, mechanical robustness, durability to extended photocatalytic cycling, low cost and high photocatalytic activity. There has been a concerted effort to try and improve the material’s functional properties through impurity doping; altering the band structure and electronic transport properties. However, any improvements are difficult to optimise using traditional methods. Thin-film combinatorial methods have heralded the discovery of more than 20 new families of materials since their resurgence in the mid-90’s. Such methods enable a high diversity of states to be produced in a single deposition and are now being used more prominently to optimise the functional properties of existing materials. Atmospheric pressure chemical vapour deposition (APCVD) has been applied in a combinatorial fashion to deposit thin-films containing compositional gradients and is also the native method in which thin-films of TiO2 are mass-produced. Utilising combinatorial APCVD, we investigated N, Nb and W doped TiO2 thin-film systems. The Ndoped TiO2 system has been studied most prominently for improved visible light photocatalysis. Nitrogen can either substitute oxygen sites (substitutional doping Ns) or enter within the TiO2 framework (interstitial doping Ni), yet there is little consensus on which type of doping or dopant concentration yields the more active photocatalyst. Using the combinatorial APCVD approach, TiCl4 and ethyl acetate precursors were used to form the host TiO2 matrix with either NH3 or t-butylamine used as the N-sources. From three separate investigations we were able to produce combinatorial films with transitional composition/ phase gradients of (i) Ns/ Ni-doped to pure Ni-doped anatase TiO2 (0 ≤ Ns: Ti ≤ 8.4 %, 0.57 ≤ Ni: Ti ≤ 3.3 %), (ii) Ns-doped anatase TiO2 and rutile TiO2 phase mixtures (0 ≤