Lead Bismuth Oxyhalides PbBiO₂X (X = Cl, Br) as Visible-Light-Responsive Photocatalysts for Water Oxidation: Role of Lone-Pair Electrons in Valence Band Engineering

Abstract

We show that layered oxyhalides PbBiO2X (X = Cl, Br, I) with a Sillén-type structure possess band levels appropriate for visible-light-induced water splitting. Under visible light, PbBiO2Cl and PbBiO2Br with band gap (BG) of 2.51 and 2.48 eV, respectively, stably oxidized water to O2 in the presence of an Fe3+ electron acceptor. A comparison with structurally related SrBiO2Cl and BaBiO2Cl (BG = 3.55 and 3.54 eV) combined with DFT calculations revealed a significant interaction between O 2p and Pb 6s orbitals leading to the upward shift of the valence band maximum in PbBiO2X as compared with (Sr,Ba)­BiO2Cl. Z-scheme water splitting into H2 and O2 has been demonstrated using PbBiO2Cl as an O2-evolving photocatalyst, coupled with an appropriate H2-evolving photocatalyst in the presence of an Fe3+/Fe2+ redox mediator.

Light Absorption Properties and Electronic Band Structures of Lead‐Vanadium Oxyhalide Apatites Pb5(VO4)3X (X=F, Cl, Br, I)

“…The band gaps were estimated to be approximately 3.0 eV for the X=F, Cl and Br materials and 2.7 eV for the X=I sample. It has been reported that the band gaps of SrBiO 2 X and PbBiO 2 X (X=Cl, Br, I) are reduced as the atomic number of X is increased. This effect is attributed to the ability of less electronegative X atoms to form valence bands at more negative potentials, as typically found in mixed anion compounds such as oxynitrides and oxysulfides .…”

“…Oxyhalides have been studied as heterogeneous photocatalysts for pollutant decomposition and water splitting . Of particular interest is the development of visible‐light‐responsive photocatalysts that can utilize a wide range of the solar spectrum.…”

“…Oxyhalides have been studied as heterogeneous photocatalysts for pollutant decomposition [2][3][4][5][6][7][8] and water splitting. [9][10][11][12][13] Of particulari nteresti st he development of visible-light-respon-sive photocatalysts that can utilize aw ide range of the solar spectrum.I nt his regard,t he use of anions less electronegative than oxygen (such as in the case of oxyhalides that contain less electronegativeh alogens) could allow the development of photocatalysts activeu nder visible light. This may be possible because less electronegative halogen atoms, in principle, will produce av alence band at an energy level more negative than the O2 po rbitals in oxides [1,2,6,7,9,14] (although there are some exceptions [10][11][12][13][15][16][17] ).…”

“…For this reason, the light absorption characteristics and band structures of oxyhalides have been studied so as to understand correlationso ft hese properties with their crystal structures. [2,8,[10][11][12][13][14][15][16][17][18] Apatites, which can be represented as M 10 (AO 4 ) 6 X 2 or M 5 (AO 4 ) 6 X (M, A:c ations, X:a nion), are one of the most commonm ineralsf ound in nature,w ith aw ide variety of compositions available. [19,20] The crystal structure of apatite is shown Figure 1.…”

“…In this regard, the use of anions less electronegative than oxygen (such as in the case of oxyhalides that contain less electronegative halogens) could allow the development of photocatalysts active under visible light. This may be possible because less electronegative halogen atoms, in principle, will produce a valence band at an energy level more negative than the O 2p orbitals in oxides (although there are some exceptions). For this reason, the light absorption characteristics and band structures of oxyhalides have been studied so as to understand correlations of these properties with their crystal structures …”