Zinc Germanium Oxynitride as a Photocatalyst for Overall Water Splitting under Visible Light

Abstract

A solid solution of zinc oxide and germanium nitride (Zn 1+x Ge)(N 2 O x ) (x ) 0.44) is demonstrated to be an effective photocatalyst for overall water splitting under ultraviolet and visible light. The catalyst is prepared by reaction of GeO 2 and ZnO under ammonia flow (20 mL‚min -1 ) at 1123 K for 15 h. The crystal structure of the material is investigated by a combination of Rietveld analysis and the maximum-entropy method using neutron powder diffraction data. The (Zn 1.44 Ge)(N 2.08 O 0.38 ) catalyst is confirmed to have a wurtzite-type structure (space group P6 3 mc) and to be the solid solution where the oxygen atoms are substituted for nitrogen atoms. The (Zn 1.44 Ge)(N 2.08 O 0.38 ) catalysts thus prepared exhibit a band gap of ca. 2.7 eV and corresponding activity at visible wavelengths. The decrease in band gap compared to the starting materials is attributed to larger valance band dispersion resulting from the energy difference between O2p and N2p orbitals and from the p-d repulsion between Zn3d and N2p+O2p electrons in the upper valance band, which raises the top of the valance band. (Zn 1.44 Ge)(N 2.08 O 0.38 ) powder modified by surface loading with RuO 2 nanoparticles at 5 wt % achieves overall water splitting under both ultraviolet and visible irradiation.

Nanomaterials for renewable energy production and storage

“…189 A solid solution of zinc oxide and germanium nitride (Zn 1+x Ge)(N 2 O x ) (x = 0.44) was later demonstrated to be an effective photocatalyst for overall water-splitting under ultraviolet and visible light. 193 The catalyst was prepared by reacting GeO 2 and ZnO under ammonia flow at 1123 K for 15 h. 193 Rh 2Àx Cr x O 3 was identified as the most effective co-catalyst for (Zn 1+x Ge)(N 2 O x ), which greatly caused an increase in the activity for hydrogen evolution. Modification of the optimized (Zn 1.44 Ge)(N 2.08 O 0.38 ) sample by loading with Rh 2Àx Cr x O 3 (3.0 wt% Rh, 0.2 wt% Cr) results in an effective photocatalyst for overall water decomposition with a quantum efficiency of ca.…”

Improvement of the Photocatalytic Water Splitting Activity of Silver Tantalate by Photodeposited Platinum and Cobalt-Oxide Nanoclusters

“…1b) and CoO x (Fig. 1c), the peak position of AgTaO 3 did not change, which indicated that Pt and CoO x were just deposited on the surface instead of being inserted in the crystal lattice of AgTaO 3 . No peaks derived from Pt and CoO x were observed.…”

“…To date, a considerable number of photocatalysts have been developed to construct water splitting systems including one-step [2][3][4][5][6][7][8][9][10][11] and two-step 12-17 photoexcitation systems. However, in both systems, the water oxidation process is currently considered to be a key bottleneck in water splitting.…”

Photocatalytic Splitting of Water

“…Among various interesting reactions, the splitting of water into molecular hydrogen and molecular oxygen by visible light is potentially one of the most promising ways for the photochemical conversion and storage of solar energy Lee et al 2007;Bard 1979;Sayama et al 2001). Since the first reported photo splitting of water by Fujishima and Honda in 1972 many authors have published their efforts to split water using semiconductor photocatalysis (Bolton 1996;Esswein and Nocera 2007;Kudo and Miseki 2009;Amouyal 19995).…”

“…As previously discussed, the photocatalyst employed should have a suitable thermodynamic potential for H 2 O splitting, a sufficiently narrow band gap to harvest visible photons, and stability against photocorrosion. These requirements are rather stringent and thereby limit the number of photocatalysts capable of photosplitting H 2 O using the one step mechanism Lee et al 2007). Discussed here is the two-step Z-scheme mechanism for H 2 O splitting, Figure 13.…”