Metal Oxide Nanomaterials for Solar Energy to Hydrogen Fuel Conversion.

摘要

Photoactive metal oxide nanomaterials enable full or partial water splitting by reducing water to hydrogen and oxidizing water into oxygen through transfer of photogenerated electrons and holes, respectively, upon absorption of light of certain frequencies. Scanning Transmission Electron Microscopy (STEM) is one of the useful instruments to study these materials through observation of their atomic structures using high resolution imaging and through chemical analyses using complementary analytical techniques. Combinations of z-contrast imaging, selected area electron diffraction (SAED), electron dispersive x-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS) were used to elucidate the structures of IrO2, H2Ti4O 9, H2K2Nb6O17 and WO 3 photocatalysts. STEM techniques were also employed to observe the reduction of V2O5 nanoribbons into photoactive VO 2 and to monitor the effect of sonication on the size and crystallinity of TBACa2Nb3O10 (TBA = tetrabutylammonium) nano sheets.;Aberration-corrected STEM equipped with a fluid stage was utilized to examine water catalysis by TBACa2Nb3O10 in situ under the electron beam. Phenomena associated with calcium niobate catalysis such as photodeposition of Pt and IrO2 co-catalysts and the surface poisoning with Ag particles during water oxidation were observed in real time. Formation of gas bubbles during water reduction was also detected as it occurs using dark field imaging and EELS.;Electron microscopy was also employed to probe charge separation and distribution of redox-active sites on photolabeled TBACa2Nb 3O10. The sizes, shapes, and particle densities vary with the precursor concentration and the presence of sacrificial agents. Photogenerated electrons and holes were shown to be accessible throughout the nanosheets, without evidence for spatial charge separation across the sheet.;To measure the relative catalytic activities of multiple photocatalysts, a comparative quantum efficiency (QE) study was carried out on the H 2Ti4O9 nanobelts, H2K2Nb 6O17 nanoscrolls, PA2K2Nb6O 17 (PA = propylammonium) and TBACa2Nb3O10 nanosheets, and their platinated counterparts. Hydrogen and oxygen evolved upon irradiation with a Xe lamp were measured using gas chromatography (GC). The QEs of these catalysts were found to be dependent on the quasi-Fermi levels (QFLs) and the mobility of the charge carriers as measured by surface photovoltage spectroscopy (SPV).;A similar photocatalytic study was employed to measure the effects of exfoliation, sacrificial charge donors, presence of co-catalysts, and co-catalyst deposition conditions on the TBACa2Nb3O10 nanosheets. Factorial analysis on the hydrogen and oxygen evolution results showed the degree of dependence of catalytic activity on these factors. High resolution STEM and cyclic voltammetry showed the structural and electronic features of the nanosheets that give rise to the observed effects of the factors studied.