Kafizas, A.G.; (2011) Combinatorial atmospheric pressure chemical vapour deposition for optimising the functional properties of titania thin-films. Doctoral thesis , UCL (University College London).

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Abstract

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 ≤ Ns: Ti ≤ 11 %, 0 ≤ anatase TiO2 ≤ 100 %, 0 ≤ rutile TiO2 ≤ 41 %) and (iii) pure pseudo-brookite Ti3-δO4N to pure Ni-doped anatase TiO2 phase mixtures. In tailoring high-throughput screening methods to these systems we were able to characterise large numbers of unique states across each combinatorial system and inter-relate their physical and functional properties. It was found that (i) pure Ni-doped anatase TiO2 is a more photocatalytically active material than Ns-doped anatase TiO2 under UVA and visible light (> 420 nm), (ii) un-doped anatase TiO2 is more photocatalytically active than Ns-doped anatase TiO2 under UVA light and (iii) pseudo-brookite Ti3-δO4N is a more active photocatalyst than Ni-doped anatase TiO2 under UVA light. The photocatalytic activity of Nb and W doped TiO2 solid solutions had not previously been investigated; however, their film resistivities for potential applications as more durable transparent conducting oxide materials had been. Using combinatorial APCVD we were able to produce NbxTi1-xO2 (0.0004 ≤ x ≤ 0.0194) and WxTi1-xO2 (0.0038 ≤ x ≤ 0.1380) anatase TiO2 thin-film solid solutions with transitional composition gradients. The Nbdoped system was formed from combining TiCl4, ethyl acetate and NbCl5 precursors. By characterising the film with our screening methods were we able to determine the strong functional inter-relationship between the material’s bandgap, photocatalytic activity and film resistivity in three dimensions; where an increased photocatalytic activity was associated with a lower bandgap energy and electrical resistance. The importance of oxygen vacancies on increasing charge carrier mobility presided over the number of charge carriers in the system (Nb-doping level). The W-doped system was formed from combining TiCl4, ethyl acetate and WCl6 precursors. Using high-throughput screening methods once more, the strong physical inter-relationship between the material’s Raman shift, unit cell volume and W-doping level were determined; where increased W-doping increasingly expanded the unit cell in the a/b axis and caused the prominent Raman active Eg vibrational mode of (144 cm-1) to shift to higher energies. Increased W-doping increasingly disrupted crystallisation, yielding less active photocatalysts. More interestingly however, increased preferred orientation in the (211) plane induced a greater degree of photo-induced surface wetting. Given the mechanism for the photo-induced wetting process in anatase TiO2 is, to our knowledge, yet to be studied, the trends highlighted the importance of the (211) plane in this process. Films synthesized by the combinatorial APCVD route, analysed in conjunction with high-throughput characterisation methods, provide a shortcut to understanding and optimising the functional properties of composition/ phase space.

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