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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.  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 have been published during this time period.

Flux Synthesis of Metal-Oxides Particles:  Effects of Particle Growth on Optical and Photocatalytic Properties.  Completed research during this period included the investigation of two copper tantalates, Cu₅Ta₁₁O₃₀ and Cu₃Ta₇O₁₉, that were synthesized by both solid state and flux synthetic methods, respectively.  A synthetic route yielding Cu₃Ta₇O₁₉ in high purity was found using a CuCl flux at 800^oC and its structure was characterized using powder X-ray Diffraction (XRD) data (Space group: P6₃/m, No. 176).  The solid-state synthesis of Cu₅Ta₁₁O₃₀ was performed using excess Cu₂O that helped to facilitate the growth of single crystals and their characterization by XRD (Space group: P-62c, No. 190).  The atomic structures of both copper tantalates consist of alternating single and double layers of TaO₇ pentagonal bipyramids that are bridged by a single layer of isolated TaO₆ octahedra and linearly-coordinated Cu⁺.  The measured optical bandgap sizes of ~2.59 and ~2.47 eV for Cu₅Ta₁₁O₃₀ and Cu₃Ta₇O₁₉ were located well within visible-light energies and were consistent with their orange-yellow colors.  Each also exhibits optical absorption coefficients at the band edge of ~700 cm^-1 and ~275 cm^-1, respectively, and which were significantly smaller than that for NaTaO₃ of ~1450 cm^-1.  Results of LMTO calculations indicate that their visible-light absorption is attributable mainly to indirect bandgap transitions between the Cu 3d¹⁰ and Ta 5d⁰ orbitals within the TaO₇ pentagonal bipyramids.