In any artificial photosynthetic system, the oxidation of water to molecular oxygen provides the electrons needed for the reduction of protons or carbon dioxide to a fuel. Understanding how this four-electron reaction works in detail is important for the development of improved robust catalysts made of Earth-abundant materials, like first-row transition-metal oxides. Here, using time-resolved Fourier-transform infrared spectroscopy and under reaction conditions, we identify intermediates of water oxidation catalysed by an abundant metal-oxide catalyst, cobalt oxide (Co3O4). One intermediate is a surface superoxide (three-electron oxidation intermediate absorbing at 1,013 cm(-1)), whereas a second observed intermediate is attributed to an oxo Co(IV) site (one-electron oxidation intermediate absorbing at 840 cm(-1)). The temporal behaviour of the intermediates reveals that they belong to different catalytic sites. Knowledge of the structure and kinetics of surface intermediates will enable the design of improved metal-oxide materials for more efficient water oxidation catalysis.

One of the main challenges in developing highly efficient nanostructured photoelectrodes is to achieve good control over the desired morphology and good electrical conductivity. We present an efficient plasma-processing technique to form porous structures in tungsten substrates. After an optimized two-step annealling procedure, the mesoporous tungsten transforms into photoactive monoclinic WO3. The excellent control over the feature size and good contact between the crystallites obtained with the plasma technique offers an exciting new synthesis route for nanostructured materials for use in processes such as solar water splitting.

We demonstrate here for the first time the photoelectrochemical properties of a BiVO 4 photoanode in conjunction with a molecular catalyst. When the Ru-based molecular catalyst (RuCat) is coupled to a BiVO 4 light-absorber the performance of this photoanode improves particularly in the low-bias region (<1.0 V vs RHE). The RuCat-BiVO 4 photoanode shows a higher photocurrent than CoP i -BiVO 4 under front illumination, and a 0.1 V more cathodic onset potential. The former can be partly explained by the low light absorption of the RuCat (<5% light absorption in the UV−vis− NIR range). For the latter, we propose that the linkers in the RuCat reduce the surface recombination in BiVO 4 to a greater extent than CoP i . Finally, we observe that the fill factor of the RuCat-BiVO 4 JV characteristic improves after the stability test. The results presented herein not only show the feasibility and potential of the solid state/molecular heterojunctions but also represent a proof of principle to improve conventional all-solidstate systems such as CoP i -BiVO 4 .