ic400089s_si_001.cif (5.24 kB)
Design of Medium Band Gap Ag–Bi–Nb–O and Ag–Bi–Ta–O Semiconductors for Driving Direct Water Splitting with Visible Light
dataset
posted on 2013-08-19, 00:00 authored by Limin Wang, Bingfei Cao, Wei Kang, Mark Hybertsen, Kazuhiko Maeda, Kazunari Domen, Peter G. KhalifahTwo new metal oxide semiconductors
belonging to the Ag–Bi–M–O (M = Nb, Ta) chemical systems
have been synthesized as candidate compounds for driving overall water
splitting with visible light on the basis of cosubstitution of Ag
and Bi on the A-site position of known Ca2M2O7 pyrochlores. The low-valence
band edge energies of typical oxide semiconductors prevents direct
water splitting in compounds with band gaps below 3.0 eV, a limitation
which these compounds are designed to overcome through the incorporation
of low-lying Ag 4d10 and Bi 6s2 states into
compounds of nominal composition “AgBiM2O7”. It was found that the “AgBiTa2O7” pyrochlores are in fact a solid solution
with an approximate range of AgxBi5/6Ta2O6.25+x/2 with
0.5 < x < 1. The structure of Ag4/5Bi5/6Ta2O6.65 was determined from
the refinement of time-of-flight neutron diffraction data and was
found to be a cubic pyrochlore with a = 10.52268(2)
Å and a volume of 1165.143(6) Å3. The closely
related compound, AgBiNb2O7, appears to have
an integer stoichiometry and to adopt an orthorhombically distorted
pyrochlore-related structure with a subcell of a =
7.50102(8) Å, b = 7.44739(7) Å, c = 10.5788(1) Å, and V = 590.93(2)
Å3. Density functional theory-based calculations predict
this distortion should result from A-site cation
ordering. Fits to UV–vis diffuse reflectance data suggest that
AgBiNb2O7 and “AgBiTa2O7” are both visible-light-absorbing semiconductors with
the onset of strong direct absorption at 2.72 and 2.96 eV, respectively.
Electronic structure calculations for an ordered AgBiNb2O7 structure show that the band gap reduction and the
elevation of the valence band primarily result from hybridized Ag
d10–O 2p orbitals that lie at higher energy than
the normal O 2p states in typical pyrochlore oxides. While the minimum
energy gap is direct in the band structure, the lowest energy dipole
allowed optical transitions start about 0.2 eV higher in energy than
the minimum energy transition and involve different bands. This suggests
that the minimum electronic band gap in these materials is slightly
smaller than the onset energy for strong absorption in the optical
measurements. The elevated valence band energies of the niobate and
tantalate compounds are experimentally confirmed by the ability of
these compounds to reduce 2 H+ to H2 gas when
illuminated after functionalization with a Pt cocatalyst.