HDR Seminar: Mr Junwei Li; The University of Sydney
Monday, 21 March 4:00pm – 5:00pm
This seminar will be delivered Online Zoom. Please email chemistry.researchsupport@sydney.edu.au for zoom link and password.
Speaker: Mr Junwei Li; The University of Sydney
Host: Prof. Brendan Kennedy
Title: Investigation of Structural-Catalytic Relationship of Mixed-Metal Layered Oxide Materials for Photocatalytic Overall Water Splitting
Abstract: Alternate fuel sources are needed to replace fossil fuels to reduce the emission of greenhouse gases that are driving global warming. Hydrogen gas is one popular choice to replace fossil fuels as an energy storage medium, due to its high energy density per unit weight. Hydrogen can be generated renewably by sunlight driven, photocatalytic water-splitting. Metal oxides, including those with a Ruddlesden-Popper layered perovskite structures were studied as potential photocatalysts. The structure contains multiple cationic sites, which allows for different combinations of metal cations, which in turn allows tuning of the bandgap.
KLaTiO4 is a n=1 Ruddlesden-Popper oxide that can be used as a Hydrogen Evolution Catalyst (HEC). The main disadvantage of KLaTiO4 is its high bandgap (4.09 eV) that is above the visible light region, which makes it a poor choice for a solar activated HEC. It was observed that synthesis of KLaTiO4 above 900 °C invariable resulted in the presence of K2La2Ti3O10 as an impurity phase. K2La2Ti3O10 is structurally similar to KLaTiO4, both being Ruddlesden-Popper type oxides built on layers of TiO6.
The synthesis of ALaTiO4 and A2La2Ti3O10 (A = Na+, K+) was studied in detail to understand the co-existence of these two phases. The first factor to consider is the volatility of alkaline metal ions at elevated temperatures. Due to this volatility, excess Na or K needs to be included in the initial reagent mixture. Successful synthesis of NaLaTiO4 or Na2La2Ti3O10 required 50 % (minimum tested) excess alkaline metal reagent. The second factor to consider is the heating temperature. Multiple samples of KLaTiO4 and K2La2Ti3O10 were made using traditional solid-state synthesis methods at temperature between 750 °C and 950 °C. KLaTiO4 was made at 800 °C and K2La2Ti3O10 was made at 850 °C, which is lower than the literature reports. The low synthesis temperature of K2La2Ti3O10 suggests that care must be taken when synthesising and analysing KLaTiO4 to ensure no impurities are present in the sample for successful characterisation of the photocatalyst.
Reduction of the bandgap of KLaTiO4 for sunlight driven hydrogen evolution was attempted by cationic and anionic doping. The crystal structures, and sample purity, was determined using synchrotron X-ray powder diffraction (PXRD) and Rietveld refinement. Cationic doping of KLaTiO4 was achieved by partially replacing lanthanum with praseodymium or ytterbium, yielding two solid solution series: KLaxPr1-xTiO4 and KLaxYb1-xTiO4 (x = 0.005, 0.01 and 0.03). While none of the Pr doped samples produced were active HEC, all the KLaxYb1-xTiO4 were. In comparison to KLaTiO4, ytterbium-doped samples have reduced catalytic activity compared to KLaTiO4.
Anionic doping of KLaTiO4 was attempted with partial replacement of oxygen with nitrogen. Attempts to synthesise KLaTiO4-xNx were done by using TiN as a reagent in place of TiO2 with annealing the sample under N2 flow at 800 °C. PXRD patterns of initial samples show good crystallinity, but no observable structural difference to KLaTiO4. When tested as HEC in identical testing condition, KLaTiO4-xNx had one third rate of hydrogen evolution compared to KLaTiO4.