TiO2(B) or “bronze” is a TiO2 polymorph which is difficult to synthesise in a pure form and does not commonly exist in minerals. TiO2(B) potentially plays an important role in applications both as a photocatalytic component alongside anatase for degradation reactions and as an anode material in lithium ion batteries due to its distinctive crystal structure which exhibits large channels and voids. In this research, TiO2(B) has been successfully synthesised by both a hydrothermal route and a Low Pressure Chemical Vapour Deposition (LPCVD) process. Samples were characterised using powder-XRD, Raman spectroscopy, TEM, SEM and UV-Vis spectroscopy. Phase formation mechanisms for both the hydrothermal route and LPCVD process have been proposed.
Initially, in order to investigate the TiO2(B) phase formation mechanism via a sodium titanate phase transformation, hydrothermal synthesis was employed to produce TiO2(B) nanorods including an investigation of the products at each stage of the reaction. The results were used to propose an integrated reaction mechanism which corresponds well with literature. This involved the structural transformation of a sodium titanate intermediate phase which is of interest in relation to the other TiO2(B) fabrication methods where Na+ ions are present in the system such as CVD on glass substrates. As a result, the synthesis of mixed phase TiO2(B) and anatase thin films on a soda lime glass substrate has been achieved, for the first time, by LPCVD synthesis. Titanium isopropoxide (TTIP) and N2 gas were used as the precursor and carrier gas respectively. The optimal LPCVD condition for preparing a mixed phase of TiO2 containing TiO2(B) was 550oC (actual temperature) with a 1 mL/s N2 flow rate. A possible thin-film formation mechanism during the LPCVD process has been proposed.
Subsequently a pre-treatment method involving spraying a Na+-containing solution, such as sodium ethoxide, onto a number of different substrates including silicon wafer, fused quartz, highly ordered pyrolytic graphite (HOPG) and pressed graphite flake (grafoil) was applied in conjunction with the LPCVD method in order to promote the TiO2(B) phase in the thin film products formed on any substrate.
Finally, the effects of different alkali metal ions (Li+, Na+ and K+ from alkali metal hydroxide solutions) during the pre-treatment step were investigated in relation to the phase formation in the thin films produced during the LPCVD process. Only Na+ ions were found to encourage the phase formation of TiO2(B), K+ ions produced only a minority of the TiO2(B) phase, whereas Li+ ions did not produce TiO2(B). Phase formation mechanisms have been proposed based on alkali metal migration from the pre-treatment layer into the deposited nascent titania film and the formation of intermediate titanate phases.
