Nitrides Oxynitrides and Oxysulfides
We have seen that introduction of nitrogen into the TiO2 lattice has a favorable effect in terms of sensitizing it to the visible range of the electromagnetic spectrum (Table 6). The line between doping and new phase formation is one of degree and the studies on nitridation of a given parent oxide exemplify this point. Thus the band gap of Ta2O5 shrinks from ~ 4.0 eV to ~ 2.1 eV by nitriding it in a NH3 atmosphere to yield
Ta3N5.476 This material evolves H2 and O2 under visible irradiation (k < 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 O 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 < k < 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.
Two other oxynitrides, namely LaTiO2N and Y2Ta2O5N2,483 have been reported to be effective for evolving H2 or O2 from H2O under visible light irradiation. The former is derived from La2Ti2O7 (see Entry 3, Table 11 for the Nb or Ta analog) by nitriding in NH3 atmosphere.481,482 The two structures La2Ti2O7 and LaTiO2N have been compared;482 the oxynitride has the same structure as a perovskite of the ABO3 genre and is composed of a TiOxNy octahedral skeletal structure (x + y = 6). The band gap of LaTiO2N is estimated to be ~ 2.1 eV. Y2Ta2O5N2 is obtained by nitriding YTaO4 power, which in turn is synthesized via a solid-state reaction between Y2O3 and Ta2O5.483 While the YTaO4 has a band gap of 3.8 eV, it shrinks to ~ 2.2 eV on nitridation to Y2Ta2O5N2.483 Water reduction and oxidation were observed under visible light irradiation for Y2Ta2O5N2 modified with a Pt-Ru co-catalyst.483
The final compound in this series is Ge3N4 which was formed by nitridation of GeO2 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 RuO2-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-Ge3N4 case where, however, UV radiation had to be used.
Other than N, sulfur is another non-metallic element that has worked for sensitizing TiO2 to visible light (Table 6). Thus oxysulfides have been examined as potential photocatalysts for water splitting. Two sets of studies on Sm2Ti2S2O5 and Ln2Ti2S2O5 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) have been reported from a photocatalytic water splitting perspective.484,485 The samarium compound was prepared by heating a mixture of Sm2S3, Sm2O3 and TiO2 in sealed tubes at elevated temperatures.484 This compound has the same structure as the Ruddlesden-Popper type layered perovskites (see Section 6.7 this Chapter). This material, with a band gap of ~ 2 eV, evolves H2 in the presence of Ag+ and O2 in the presence of Na2S, Na2SO3 or methanol under visible light irradiation (k < 650 nm).485
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(NO3)^6H2O to yield the lanthanoid titanate precursor which was subsequently sulfided in a flowing H2S atmosphere.485 The Sm2Ti2S2O5 compound was found to have the highest activity amongst all the homologues tested. The band gaps vary from a low value of i.94 eV (Er compound) to 2.i3 eV (for the Sm compound).
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