Fx1 Highlights: • Thin alumina barrier coating can enhance photo-activity of bismuth vanadate photoanode materials • There are two distinct potential domains with different enhancement characteristics and mechanisms • The thickness of alumina films can be controlled with a sol-gel coating to provide optimized performance Abstract Nanostructured semiconductor photoanodes play an important role in solar fuel generation, and the design of the semiconductor -aqueous electrolyte interface can be crucial in enhancing the energy conversion efficiency. We have investigated the effects on photoelectrochemical oxygen evolution for monoclinic nanostructured BiVO4 films uncoated and coated with microporous sol-gel Al2O3 "over-layers". Variation of the thickness of the Al2O3 coating (formed by surface sol-gel deposition and annealing at 435 o C) led to a reduction of pseudo-capacitance and allowed optimization of the quantum efficiency. Exploration of the photocurrent enhancement as a function of applied potential reveals two distinct potential domains/mechanisms: (i) a low bias region enhancement effect (assigned to a lowering of the rate of external recombination of electrons with oxygen) and (ii) a high bias region of enhancement (assigned to higher charge carrier mobility due to less trapping in surface states).

Polymers of intrinsic microporosity (PIM or here PIM-EA-TB) offer a highly rigid host environment into which hexachloroplatinate(IV) anions are readily adsorbed and vacuum carbonised (at 500°C) to form active embedded platinum nanoparticles. This process is characterised by electron and optical microscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical methods, which reveal that the PIM microporosity facilitates the assembly of nanoparticles of typically 1.0 to 2.5-nm diameter. It is demonstrated that the resulting carbonised BPt@cPIM^from drop-cast films of ca. 550-nm average thickness, when prepared on tin-doped indium oxide (ITO), contain not only fully encapsulated but also fully active platinum nanoparticles in an electrically conducting heterocarbon host. Alternatively, for thinner films (50-250 nm) prepared by spin coating, the particles become more exposed due to additional loss of the carbon host. In contrast to catalyst m a t e r i a l s p r e p a r e d b y v a c u u m-t h e r m o l y s e d hexachloroplatinate(IV) precursor, the platinum nanoparticles within Pt@cPIM retain high surface area, electrochemical activity and high catalyst efficiency due to the molecular rigidity of the host. Data are presented for oxygen reduction, methanol oxidation and glucose oxidation, and in all cases, the high catalyst surface area is linked to excellent catalyst utilisation. Robust transparent platinum-coated electrodes are obtained with reactivity equivalent to bare platinum but with only 1 μg Pt cm −2 (i.e.~100% active Pt nanoparticle surface is maintained in the carbonised microporous host).

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