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We reported for the first time a facile method for the preparation of the spinel CuCo2O4 at 140 °C by using sulfates of copper and cobalt in KOH. The compound crystallizes in a cubic system (SG: F d \( \bar{3} \) m) with a lattice constant of 8.0590 A. The material was characterized by physical and photo-electrochemical techniques. The X-ray diffraction (XRD) pattern confirms the single phase with a high purity. The UV–Visible spectroscopy of the black product revealed a direct optical transition of 1.38 eV. The capacitance-potential (C−2-E) displays a negative slope and a flat band potential (+0.28 VSCE) characteristic of p-type semiconductor behavior. Further, the Nyquist plot shows a semicircle followed by straight line at low frequencies, indicating a diffusion process. To show effectively the photocatalytic performance, we tested our product in the hydrogen production upon visible light irradiation. H2 evolution rate of 7 µmol h−1 mg−1 and a quantum efficiency of 1.1% were obtained upon visible light illumination.

The hetero-junction 5% CuO/ZnO elaborated by co-precipitation is studied by thermal analysis, X-ray diffraction, ATR spectroscopy, and optical reflectance to assess its performance for the H2 photoproduction. The specific surface area averages ~7 m2 g−1 while the crystallite size varies from 20 to 50 nm. The diffuse reflectance spectra shows an indirect transition at 3.13 eV for ZnO and a direct transition at 1.60 eV for CuO. The capacitance-potential plot of the sensitizer CuO exhibits p-type conduction, due copper deficiency, with a flat band potential of 0.075 VSCE. ZnO works as an electron bridge; its conduction band of ZnO (−0.68 VSCE) is cathodically localized with respect to that of CuO and CuOH2O/H2 level (~−0.5 VSCE), yielding H2 evolution under visible light illumination. H2 liberation rate of 340 µmol h−1 (g catalyst)−1 and a quantum yield of 0.38% were obtained under full light (27 mW cm−2) for a catalyst dose of 0.25 mg/mL and pH ~ 7 in presence of \({{\text{SO}}_{3}}^{2 - }\) as reducing agent; H2 liberation rate of 340 µmol h−1 (g catalyst)−1 and a quantum yield of 0.38% are determined.

We investigated the technical feasibility of photochemical hydrogen release based on CuFe2O4 powder dispersion in an aqueous electrolyte containing a reducing agent (\({\text{S}}_{2} {{\text{O}}_{3}}^{ - 2}\)). The oxide combines an average resistance to corrosion and an optimum inter-referred band Eg of 1.67 eV. The intercalation of a small amount of oxygen should be accompanied by partial oxidation of Cu+ to Cu2+ involving a p-type semi-conductivity. The oxidation S2− inhibits the photo-corrosion, and the evolution of H2 increases in parallel with the formation of polysulfides. Most of H2 is produced when p-CuFe2O4 is connected to n-ZnO formed in situ. The release of H2 is mainly on CuFe2O4, whereas, the oxidation of S2− takes place on the surface of ZnO and the hetero-system CuFe2O4/ZnO is optimized with respect to certain physical parameters. The photoelectrochemical production of H2 is a multi-step process whose decisive step is the arrival of electrons at the interface due to their low mobility. Remarkable performance with a rate of 6.74 (μmol g−1 min−1) cm3 h−1 hydrogen evolution in 0.1 M \({\text{S}}_{2} {{\text{O}}_{3}}^{ - 2}\) (pH 13) is recorded.

Ternary oxides are practically employed in most technological fields and continue to attract much interest because of their low cost and simple preparation. In this work, we have synthesized La2NiO4 by sol-gel method and characterized by X-ray diffraction, BET surface area, SEM analysis, laser granulometry and electrical conductivity. The XRD pattern shows that the pure La2NiO4 is obtained beyond 900 °C. The BET measurements give relatively a small surface area (10 m2 g−1). The elemental chemical analysis by laser granulometry, confirmed the agglomerated nature of the synthesized powder observed by SEM analysis and attributed to the fine powder obtained by sol-gel method. The diffuse reflectance gives an optical gap of 1.3 eV, in agreement with the dark color. The transport properties show a semi-conductor behavior with p-type conductivity and activation energy of 44 meV. The results of the absorption analysis show that La2NiO4 exhibits an excellent chemical stability in the pH range (6–14). The capacitance measurement (C−2-E) in basic electrolyte reveals a linear behavior from which a flat band potential of 0.1 VSCE is obtained. La2NiO4 is tested successfully as photocatalyst for the hydrogen production upon visible light and the best performance is obtained in alkaline solution (NaOH 0.1 M, Na2S2O3 10−3 M) with an average rate of 23.6 μmol mn−1 (g catalyst)−1. The system shows a tendency toward saturation whose deceleration is the result of the competitive reduction of the end product S4O62−.

10% NiO/γ–Al2O3 was synthesized by wet impregnating γ–Al2O3 with Ni(NO3)2, 6H2O solution. After evaporation, the sample was calcined in air at 450 °C. The insulator γ–Al2O3 was used as support to offer a large distribution of NiO, leading to a higher specific surface area, an enhanced photosensibility. The sensitizer NiO was characterized by physical and photoelectrochemical techniques. The XRD pattern exhibits the peaks of both γ–Al2O3 and NiO. The particle size of NiO (18 nm) was calculated from the full width at half maximum. The optical gap was found at 1.51 eV and the transition is directly permitted. The capacitance measurement indicates n-type semiconductor and the potential of conduction band (−0.35 VSCE), is more cathodic than the H2O/H2 level (∼−0.3 VSCE) at pH ∼13. Such condition generates a direct water reduction under visible light illumination. The hydrogen yield of 10% NiO/γ–Al2O3 was compared with those produced with NiO alone.