The photochemical reactivity of the SrTiO3 surface is affected by the pH of the surrounding aqueous solution. Scanning electron microscopy and atomic force microscopy have been used to quantify the amount of silver that is photochemically reduced on the surfaces of (100), (110), and (111) oriented crystals as a function of pH. For all orientations, the reactivity increases from pH 3, reaches a maximum, and then decreases at higher pH. The pH associated with the maximum reactivity depends on the crystallographic orientation of the surface. The results indicate that the solution pH influences the charge on the SrTiO3 surface. The amount of surface charge influences band bending within SrTiO3, and the maximum reactivity is achieved at a surface charge where neither the photocathodic nor the photoanodic reaction limit the overall reaction rate.

Photocatalytic hydrogen production rates have been measured from Al‐doped SrTiO3 with a range of controlled shapes and sizes using a high‐throughput parallelized and automated photochemical reactor. It is found that the photocatalytic reactivity is influenced by crystal shape and that crystals with a {1 1 0} to {1 0 0} surface area ratio between 1.3 and 1.8 yield more H2 than crystals with other ratios. Crystals with a {1 1 0}/{1 0 0} surface area ratio of 1.8 generate hydrogen at 550 μmol h−1 g−1 at pH 7, whereas crystals with only {1 0 0} facets exposed generate hydrogen at 300 μmol h−1 g−1 under the same condition. It is likely that the surface area ratio provides the appropriate balance between the photoanodic reaction on the {1 1 0} surface and the photocathodic reaction on the {1 0 0} surface. In the size range of 250–450 nm, larger crystals produce hydrogen at a rate of 400 μmol h−1 g−1 at pH 7, whereas smaller crystals only produce 200 μmol h−1 g−1, suggesting that the larger crystals reduce the rate of electron–hole recombination or back reaction and that the widths of the space charges within the crystal are comparable to the particle radius.

This has motivated attempts to develop photocatalysts with enhanced charge-separation properties using defect engineering, [5,6] facet engineering, [7][8][9] and the construction of heterojunctions. [10][11][12] Recently, molten salt treatments have been used for synthesis and to optimize photocatalyst properties. [13,14] In a molten salt, the mass transfer between precursors is accelerated and, compared with conventional solid-state synthesis, lower reaction temperatures can be used. [15,16]

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