Nitrides, Oxynitrides and Oxysulfides

476 This material evolves H2 and O2 under visible irradiation (X 600 nm) in the presence of sacrificial electron acceptors such as Ag+ and a co catalyst for HER such as Pt.476 Control of solution pH was found to be critical for suppressing the photoanodic corrosion of the photocatalyst which is signaled by N2 evolution. The shrinking of the optical band gap was attributed to a conduction band derived from Ta 5d orbitals and a higher lying valence band derived from N 2p orbitals than the counterpart built from 0 2p orbitals.476 

Partial nitridation affords TaON which is also found to be active for water oxidation or reduction under visible light irradiation (420 nm X 500 nm).477-480 These studies employed aqueous methanol, a Ru co-catalyst (for HER) or a Ag+ electron acceptor for OER.477,478 The band gap of TaON was estimated to be 2.5 eV. 

The final compound in this series is Ge3N4 which was formed by nitridation of GeCh powder under atmosphere NH3 flow at elevated temperatures.484 The band gap of p Ge3N4 is estimated to be - 3.8 3.9 eV the RuCh-loaded material was found to result in overall water splitting under irradiation from a high-pressure Hg lamp.484 

It must be borne in mind that in all the cases above where the band gap of the semiconductor was effectively shrunk to values in the 2.0-2.5 eV range, overall water splitting (that is, evolution of H2 and O2 with no sacrificial reagents) was not observed under visible light irradiation. This contrasts with the P-GesN4 case where, however, UV radiation had to be used. 

The other lanthanide compounds in this series were prepared either by a similar method as above or alternately by a polymerized complex method using Ti(OiPr)4 and Ln(NC)3) 6I I2O to yield the lanthanoid titanate precursor which was subsequently sulfided in a flowing H2S atmosphere.485 The Sm2Ti2S20s compound was 

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