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] (I; pda = pyridazine), [Cu­(en)2]2­[γ-Mo8O26]
(II; en = ethylenediamine), and [Cu­(o-phen)2]2­[α-Mo8O26] (III; o-phen = o-phenanthroline). The structure of I 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 a axis. The previously reported structures of II and III are constructed from [Cu2­(en)4­Mo8O26] and [Cu2­(o-phen)4­Mo8O26] clusters.
The optical bandgap sizes were measured by UV–vis diffuse reflectance
techniques to be ∼1.8 eV for I, ∼3.1 eV
for II, and ∼3.0 eV for III. Electronic
structure calculations show that the smaller bandgap size of I 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 II and III contain Cu­(II) and exhibit larger
bandgap sizes. Accordingly, aqueous suspensions of I 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, I also exhibits photocatalytic activity
for hydrogen production in 20% aqueous methanol of ∼316 μmol
H2 g–1 h–1. By contrast, II decomposed during the photocatalysis measurements. The
molecular [Cu2­(o-phen)4­(α-Mo8O26)] clusters of III 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 III 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 III 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 III arises from the net contribution of all dissolved
[Cu2­(o-phen)4­(α-Mo8O26)] clusters, compared to only the surface clusters
for the heterogeneous photocatalysis of I.