Synthesis, structure and properties of photofunctional oxide materials

Abstract

Photofunctional materials convert light into other forms of energy and are key to a number of emerging technologies such as solar cells, light sensors and photocatalysts. Crystalline metal oxides are cheap, robust and tuneable options for such applications. In this thesis, five oxide materials are thus identified and systematically studied as viable photofunctional candidates: Cr modified perovskite BiFeO3 (BFCO); Ti4+ and M2+ modified BiFeO3 perovskites (M = Fe, Ni, Mg, Zn and Cu, labelled as BFFTO, BFNTO, BFMTO, BFZnTO and BFCuTO respectively); Aurivillius Bi(m+1)Fe(m-3)Ti(3)O(3m+3) where m = 4-6; (V,N) modified anatase TiO2 (VTON) and V modified rutile TiO2 (VTO). BFCO was synthesized in bulk for the first time with a quartz crucible at near ambient pressure. Systematic X-ray, neutron and electron diffraction studies reveal that it has a chemically disordered R3c structure. This leads to exhibition of antiferromagnetism (AFM) above room temperature, measured with magnetometry and muon spin resonance spectroscopy. The material is also ferroelectric as observed with piezoresponse force microscopy, making it multiferroic, in addition to also having a narrow band gap (~ 1.5 eV) and ability to generate photocurrent. BFNTO and BFMTO were synthesized at ambient pressure with solid-state reaction and metal organic decomposition (MOD) methods respectively. Their light absorption, photo-excited electronic and ferroelectric properties were investigated for the first time. The magnetic properties of each are complex, with BFMTO paramagnetic to 3 K contrary to previous reports, and BFNTO possibly ordered below ~ 200 K (and thus potentially multiferroic). The dilution of paramagnetic centres and likely random chemical ordering (from Moessbauer spectroscopy) explain this result. Aurivillius phases were also synthesized using an MOD method at ambient pressure, avoiding magnetic impurities that have plagued other studies. Their structures were refined from X-ray diffraction and Moessbauer data, appearing to be chemically disordered. Like the perovskite systems, random ordering and dilution lead to paramagnetism at room temperature, but an ordered state with AFM-like interactions may exist below 100 K in m = 5, 6. Striped ferroelectric domains in the bulk samples were imaged for the first time in m = 4, 6. Combined with visible light absorption, this leads to a clear photocurrent generation, with m = 6 being very promising for photofunctional application. VTON anatase nanoparticles were produced using a solvothermal method new to the literature. Up to 3.5 at. % V could be introduced to make a phase pure product. However, the V exists as V4+ and V3+ and the V3+ increases with increasing N incorporation. In conjunction with electron spin resonance studies, it appears V4+ is incorporated substitutionally and a V3+ - N association exists in a likely non-substitutional environment. The reduced V appears to form a gap state within the band structure and is likely the origin of low catalytic performance in degrading Rhodamine B under visible light. VTO rutile was synthesized in the solid-state under an N2 atmosphere to increase V levels to ~ 50 at. % (VTO50). This method incorporates V in multiple valance states (V5+ and V4+ from X-ray photoelectron spectroscopy). The optical band gap is narrowed from the 3 eV of TiO2 to below visible in VTO50. Semiconductor behaviour with an unusual two-mechanism charge transport was observed with impedance spectroscopy for the first time. This dual valence state also seems to preclude the metal-insulator behaviour noted in other works. Across each study, there are links between the synthesis methods, structure type, element choice, chemical ordering, valence states, and the physical properties of these photofunctional materials. This holistic understanding of linked factors contributes to gauging crystalline oxide materials for future energy transformation technologies.