Global warming and the world energy crisis are urgent challenges for Humanity. Hydrogen (H₂) production from sunlight and water can provide a sustainable means of storing solar energy. Nanoparticle (NP) photocatalysts are projected to be one of the cheapest ways of producing solar H₂. Organic semiconductors, in the form of NPs of blended electron-donating (D) and electron-accepting (A) molecules, are an emerging alternative to conventional photocatalysts, offering better earth abundance, processability and tuneability [1]. They can be easily prepared by miniemulsion in the presence of surfactants, yielding stable aqueous suspensions [2]. Remarkable light-driven H₂ evolution reaction (HER) rates have been recently achieved by these materials in the presence of sacrificial reducing agents and Pt HER co-catalyst, thanks to the optimization of NP morphology through blend composition and surfactant choice [2,3]. However, the effects of surfactant and co-catalyst morphology, and the interplay between the two, on the HER are not well understood. It has been recently shown that a denser and more insulating surfactant layer can hinder in-situ Pt photodeposition on single-component Y6 NPs, drastically reducing their HER rate [4].

Here we show that surfactant and co-catalyst morphology have a direct effect on the light-driven sacrificial HER rates from organic D:A NPs, independently of NP morphology. First, we present a new method to achieve unprecedented control of the loading and morphology of in-situ photodeposited Pt co-catalyst on the organic semiconductor NPs. Using this method, we identify the optimal co-catalyst loading as a tradeoff between catalytic activity and parasitic light absorbance. Moreover, we reach a peak HER rate 8 times higher compared to a typical in-situ photodeposition, normalized by mass of Pt co-catalyst. Finally, we present a method for post-NP synthesis surfactant exchange, enabling the decoupling of surfactant choice for optimal blend morphology and for optimal catalytic activity. Applying this method, we achieve a significant increase in peak HER rate from the same NPs.

We anticipate that the novel photocatalyst preparation methods presented here will lead to a better understanding of the roles of co-catalyst and surfactant on the HER driven by organic semiconductor photocatalysts, and will enable the further improvement of the photocatalytic performance of these materials. These approaches may be applied to other systems were in-situ deposition of co-catalysts is a common practice, or where surfactants are involved.