A photoelectrochemical (PEC) cell for overall water splitting made of a Cu₂O nanowire photocathode modified with a thin film of NiO_x coupled to a WO₃ nanosheet photoanode is presented. The photocathode was prepared by thermal annealing of Cu(OH)₂ nanowires on a Cu foil under N₂, followed by the deposition of a 10 nm NiO_x film on the Cu₂O nanowires (aspect ratio > 40). XPS spectra revealed that the surface species of NiO_x is a mixture of NiO and Ni(OH)₂, which enhances charge separation in photoexcited Cu₂O, as confirmed by electrochemical impedance spectroscopy. The optimized NiO_x modified Cu₂O electrode shows a photocurrent density up to −4.98 mA cm⁻² at −0.33 V and −0.56 mA cm⁻² at 0.1 V vs. the normal hydrogen electrode (NHE) under white-light irradiation (26 mW cm⁻²) in an aqueous electrolyte solution at pH 6 and 25 °C. The formation of H₂ gas was only observed by gas chromatography for NiO_x-modified Cu₂O and was not detectable for unmodified Cu₂O electrodes during prolonged irradiation. The nanocomposite structure also resulted in a three-fold increase in photostability of Cu₂O; 72 ± 3% of the initial photocurrent density remained for the NiO_x modified Cu₂O electrode after 20 min irradiation at 0.1 V vs. NHE. The optimized photocathode was subsequently used in a two-electrode PEC cell with an n-type WO₃ nanosheet photoanode for overall water splitting. The different band gap of Cu₂O (2 eV) and WO₃ (2.6 eV) permits for efficient and complementary light absorption and sunlight-driven water splitting. The p/n heterojunction PEC cell operates with a small output of electricity even in the absence of an external bias. We demonstrate that a Cu₂O-based electrode for H₂ evolution can be prepared free of noble metals and we show its utilization in a PEC water splitting cell made solely from earth abundant elements.

Zinc oxide (ZnO) nanosheets (ZnO-NS) were prepared for the photoanode of a dye-sensitized solar cell (DSSC), first by directly growing layered hydroxide zinc carbonate (LHZC) on an FTO substrate using a chemical bath deposition (CBD) method and then by transforming the LHZC into ZnO through pyrolysis at 300 °C. A light-to-electricity conversion efficiency (η) of 6.06% was achieved for the DSSC with ZnO-NS as its photoanode, under 100 mW cm⁻² illumination, and this (η) was found to be much higher than that of the DSSC with ZnO nanoparticles (ZnO-NP) as the photoanode (2.92%). The far superior performance of the DSSC with ZnO-NS is essentially attributed to (i) higher effective electron diffusion coefficient of ZnO-NS (3.59 × 10⁻³ cm²s⁻¹) than that of ZnO-NP (1.12 × 10⁻³ cm²s⁻¹), and to (ii) higher dye loading on ZnO-NS (2.66 × 10⁻⁷ mol cm⁻²) than that on ZnO-NP (1.99 × 10⁻⁷ mol cm⁻²); this higher electron diffusion coefficient and dye-loading are attributed to the specific morphology of the ZnO-NS. A further improvement in the efficiency of the DSSC with ZnO-NS could be achieved through the electrophoretic deposition (EPD) of a very thin layer (3 μm) of titanium dioxide nanoparticles (TiO₂-NPs of average size 14 nm) onto the ZnO-NS layer (12 μm). Notwithstanding a decrease in the effective electron diffusion coefficient (3.07 × 10⁻³ cm²s⁻¹) in the TiO₂-NP/ZnO-NS film, with reference to that in the ZnO-NS film (3.59 × 10⁻³ cm²s⁻¹), a far higher cell efficiency was obtained in favor of the cell with TiO₂-NP/ZnO-NS (7.07%), compared to that of the cell with bare ZnO-NS (6.06%); this enhancement in the η of the cell with TiO₂-NP/ZnO-NS is ascribed to an increased dye-loading in favor of its cell (3.92 × 10⁻⁷ mol cm⁻²), with reference to that in the case of the cell with bare ZnO-NS (2.66 × 10⁻⁷ mol cm⁻²). As against the common ruthenium dyes, such as N3 and N-719, a metal-free dye, coded as D149, was used in this research. The efficiency achieved for the best DSSC in this work is the highest ever reported for a DSSC with ZnO as the main semiconductor material.

Splitting water into hydrogen and oxygen with molecular catalysts and light has been a long-established challenge. Approaches in homogeneous systems have been met with little success and the integration of molecular catalysts in photoelectrochemical cells is challenging due to inaccessibility and incompatibility of functional hybrid molecule/material electrodes with long-term stability in aqueous solution. Here, we present the first example of light-driven water splitting achieved with precious-metal-free molecular catalysts driving both oxygen and hydrogen evolution reactions. Mesoporous TiO₂ was employed as a low-cost scaffold with long-term stability for anchoring a phosphonic acid-modified nickel(II) bis-diphosphine catalyst (NiP) for electrocatalytic proton reduction. A turnover number of 600 mol H₂ per mol NiP was achieved after 8 h controlled-potential electrolysis at a modest overpotential of 250 mV. X-ray photoelectron, UV-vis and IR spectroscopies confirmed that the molecular structure of the Ni catalyst remains intact after prolonged hydrogen production, thereby reasserting the suitability of molecular catalysts in the development of effective, hydrogen-evolving materials. The relatively mild operating conditions of a pH 3 aqueous solution allowed this molecule-catalysed cathode to be combined with a molecular Fe(II) catalyst-modified WO₃ photoanode in a photoelectrochemical cell. Water splitting into H₂ and O₂ was achieved under solar light illumination with an applied bias of >0.6 V, which is below the thermodynamic potential (1.23 V) for water splitting and therefore allowed the storage of solar energy in the fuel H₂.