posted on 2012-11-13, 00:00authored byYuan Ping, Yan Li, Francois Gygi, Giulia Galli
Tungsten oxide (WO3) is a good photoanode
material for
oxidizing water, but it is not an efficient absorber of sunlight because
of its large band gap (2.6 eV). Recently, stable clathrates of WO3 with interstitial N2 molecules (xN2·WO3, x = 0.034–0.039)
were synthesized, with a band gap of 1.8 eV. We studied the structural
and electronic properties of these clathrates using ab initio calculations,
and we analyzed the physical origin of the gap reduction reported
experimentally. We found that both structural changes caused by the
insertion of N2, and a small charge overlap between N2 and WO3, are responsible for the gap decrease.
We compared the effect of N2 intercalation to that of other
closed shell species, in particular CO and rare gas atoms. Our calculations
predicted that CO insertion lowers the band gap by about the same
amount as N2 but it leads to a change of both the oxide
valence and conduction band positions, while the presence of N2 only affects the conduction band minimum. We also predicted
that, in the case of Xe, a strong hybridization between Xe 5p and
O 2p states modifies the valence band edge of WO3, leading
to a reduction of the band gap by approximately 1 eV.