Reports: AC10
46803-AC10 Heterometallic Oxides/Organics For Investigations Of Visible-Light Photocatalysis
The project has focused on two main areas that center around two synthetic approaches for achieving a new molecular-level structural control over the photocatalytic properties of materials: (1) incorporation of organic ligands into heterometallic oxides to enable a finer molecular-level structural control, and (2) molten-salt flux reactions for the controlled growth of metal-oxide particle sizes from nanometers up to micrometers. These investigated materials are those previously proposed in my research plans and contain combinations of transition metals with electron configurations that create an optimal band-energy profile for visible-light absorption and resulting photocatalytic activity. The research has been highly interdisciplinary, and participating graduate and undergraduate students have gained valuable experiences and professional skills relevant to solid-state chemistry, photoelectrochemistry, and physical and inorganic chemistry.
Heterometallic-Oxide/Organic Hybrid Solids: Ligand Effects on the Structures and Properties of Metal-Oxides. Understanding how to use ligand coordination geometries and sizes to modify the internal structures of metal oxides is essential for optimizing their optical and photocatalytic properties at the molecular level. Our hydrothermal synthetic efforts this period have greatly expanded the number of coordinating ligands that can be used to direct the growth of heterometallic-oxide/organic solids (i.e., MM'OL; M/M' = transition metals with d0 and d10 electron configurations; L = coordinating ligand) in the Cu/Re, Ag/Re, Ag/V, Ag/Nb, and Cu/Nb systems. For example, in the MReO4-based hybrids (M = Cu, Ag), about fifteen different ligands have been used to obtain from isolated trimeric and tetrameric cluster units in Ag3(pdc)3(ReO4)3∙1.5H2O and Cu2(pda)3(ReO4)2∙H2O to layered structures in M(bpy)ReO4 (A = Cu, Ag), and network types of structures in Cu(bpy)2ReO4∙ ½H2O, Ag(id)2ReO4, and Ag(dpa)2ReO4, to give just a few examples. These compounds reveal bandgap sizes that range from ~2.1-2.5eV for the CuReO4(L) systems, to ~2.6-3.9eV for the AgReO4(L) systems. Further, we have shown these new hybrids can be structurally modified via subsequent ligand insertion/removal reactions and resulting in lower bandgap sizes of ~0.3eV. XPS measurements have been used to reveal that these bandgap changes are primarily a result of increasing their valence band energies, and thus the hybrids can maintain suitable band energy positions for the water-splitting redox reactions. Solid-state transmittance measurements have also been used to determine their optical absorption coefficients fall within the ranges of ~700-1,000cm-1 and ~1,000-1,500cm-1 for the AgReO4(L) and CuReO4(L) systems, respectively. We are currently investigating the structural features (i.e. Ag-ReO4 connectivity, coordination environments, etc.) that control these bandgap sizes, levels, and absorption coefficients. Further, we have recently had success in extending this work to new M(I)/Nb(V) (M = Ag, Cu) hybrid solids using aqueous solutions of HF as a solvent, resulting in (so far) eight new compounds, including Cu2NbOF5(pyz)2, AgNbOF4(pyz) and MNbOF4(bpy)∙2H2O with bandgap sizes in the range of 2.2 3.2eV. New investigations of their optical and photocatalytic properties are in progress.
Flux Synthesis of Metal-Oxides Particles: Effects of Particle Growth on Optical and Photocatalytic Properties. The molten-salt flux growth of several heterometallic-oxide particles (e.g., MM'O) in the M(I)/Nb(V) (M = Cu, Ag) and M(I)/Ta(V) systems has been investigated. We have confirmed that this approach can be used to achieve a finer control over particle sizes, sample homogeneity, and surface features of these mixed-metal oxides. In ongoing work, we have demonstrated the rapid single-step synthesis of AgNbO3 particles with sizes of ~500 5,000 nm in a Na2SO4 flux at 900 oC in only 1 10 h. Smaller particle-size distributions can be achieved for increasing amounts of flux and shorter reaction times. Surface areas of the AgNbO3 samples from BET measurements range from 0.24 0.59 m2/g. However, we have used FESEM analyses to show that their photocatalytic activity in visible light is correlated with the formation of ~20 50 nm nano-terraced' surface features, and which are absent in the non-active AbNbO3 samples prepared by solid-state methods. The highest observed photocatalytic rates are for flux-prepared AgNbO3 particles of ~60 μmol H2 h-1g-1 in an aqueous methanol solution, with rates in pure water of ~5-30 μmol H2 h-1g-1 for particles with larger amounts of terraced surfaces. The structures of two new copper tantalates have also been determined from a combination of single crystal and powder X-ray diffraction. Ongoing photocatalytic investigations are revealing that flux preparations can activate' the surfaces of these solids for visible-light bandgap absorption, and which are being combined with new surface studies using XPS and FESEM.
