Copper-Organic/Octamolybdates: Structures, Bandgap Sizes, and Photocatalytic Activities

The structures, optical bandgap sizes, and photocatalytic activities are described for three copper–octamolybdate hybrid solids prepared using hydrothermal methods, [Cu­(pda)]₄­[β-Mo₈O₂₆] (<b>I</b>; pda = pyridazine), [Cu­(en)₂]₂­[γ-Mo₈O₂₆] (<b>II</b>; en = ethylenediamine), and [Cu­(<i>o</i>-phen)₂]₂­[α-Mo₈O₂₆] (<b>III</b>; <i>o</i>-phen = <i>o</i>-phenanthroline). The structure of <b>I</b> consists of a [Cu­(pda)]₄⁴⁺ tetramer that bridges to neighboring [β-Mo₈O₂₆]^4– octamolybdate clusters to form two-dimensional layers that stack along the <i>a</i> axis. The previously reported structures of <b>II</b> and <b>III</b> are constructed from [Cu₂­(en)₄­Mo₈O₂₆] and [Cu₂­(<i>o</i>-phen)₄­Mo₈O₂₆] clusters. The optical bandgap sizes were measured by UV–vis diffuse reflectance techniques to be ∼1.8 eV for <b>I</b>, ∼3.1 eV for <b>II</b>, and ∼3.0 eV for <b>III</b>. Electronic structure calculations show that the smaller bandgap size of <b>I</b> originates primarily from an electronic transition between the valence and conduction band edges comprised of filled 3d¹⁰ orbitals on Cu­(I) and empty 4d⁰ orbitals on Mo­(VI). Both <b>II</b> and <b>III</b> contain Cu­(II) and exhibit larger bandgap sizes. Accordingly, aqueous suspensions of <b>I</b> exhibit visible-light photocatalytic activity for the production of oxygen at a rate of ∼90 μmol O₂ g^–1 h^–1 (10 mg samples; radiant power density of ∼1 W/cm²) and a turnover frequency per calculated surface [Mo₈O₂₆]^4– cluster of ∼36 h^–1. Under combined ultraviolet and visible-light irradiation, <b>I</b> also exhibits photocatalytic activity for hydrogen production in 20% aqueous methanol of ∼316 μmol H₂ g^–1 h^–1. By contrast, <b>II</b> decomposed during the photocatalysis measurements. The molecular [Cu₂­(<i>o</i>-phen)₄­(α-Mo₈O₂₆)] clusters of <b>III</b> dissolve into the aqueous methanol solution under ultraviolet irradiation and exhibit homogeneous photocatalytic rates for hydrogen production of up to ∼8670 μmol H₂·g^–1 h^–1 and a turnover frequency of 17 h^–1. The clusters of <b>III</b> can be precipitated out by evaporation and redispersed into solution with no apparent decrease in photocatalytic activity. During the photocatalysis measurements, the dissolution of the clusters in <b>III</b> is found to occur with the reduction of Cu­(II) to Cu­(I), followed by subsequent detachment from the octamolybdate cluster. The lower turnover frequency, but higher photocatalytic rate, of <b>III</b> arises from the net contribution of all dissolved [Cu₂­(<i>o</i>-phen)₄­(α-Mo₈O₂₆)] clusters, compared to only the surface clusters for the heterogeneous photocatalysis of <b>I