For example, in the case of HGM/CNx|Pt, doubling the composite concentration from 3 to 6 mg mL -1 increases the H2 yield by a factor of 2.5, while doubling again from 6 to 12 mg mL -1 only increases yield by a factor of 1.5. Based on these findings, a composite loading of 12 mg mL -1 was selected for further trials to allow for efficient use of material and balance specific and areal activities. The mechanism of PR over HGM/CNx|Pt and HGM/CNx|Ni2P is expected to proceed according to PR over CNx|Pt or CNx|Ni2P in the absence of the inert silica support material as described in previous work. Briefly, photo-generated electrons and holes drive proton reduction and substrate oxidation, respectively, with the Pt or Ni2P co-catalyst serving as an electron collector and hydrogen evolution co-catalyst. Substrate oxidation proceeds non-selectively through direct electron transfer to CNx rather than an OH• mediated pathway and produces a wide variety of intermediate products with a final mineralised product of CO3 2-in alkaline media. Though the mechanism should remain the same, CNx or HGM/CNx light absorbers may be affected by altered exposure to light and decreased kinetics, likely from reduced mass transfer between the substrate and the photocatalyst in the absence of stirring, particularly if the catalyst is not uniformly distributed across the solution surface in a floating system. In this study, a lower specific activity was therefore observed under such conditions (Figure ). Evaluating the effect of light orientation on photocatalytic activity is challenging as gravity remains a factor that affects heterogenous catalyst distribution, particularly for floating systems, and liquid geometry. Nevertheless, it is anticipated that the overall effect of light orientation would be minimal.