Photocatalytic water-splitting is a renewable way to store solar energy under chemical energy and, at the same time, produce alternative fuels. The photoconversion of water consists in a reduction half-reaction that produces H₂ and an oxidation half-reaction that produces O₂. O₂ has limited applications (e.g. production of high-purity O₂ for medical purposes), so the performance of the overall process depends only on the H₂ production part. The amount of H₂ produced and the efficiency of the photocatalysts are critical in amortising the environmental and financial costs of the device.

Alternatively, the Marie Skłodowska-Curie OMATSOLFUEL European project explores materials for the photoconversion of industrial effluents rich in sugars (e.g. dairy or brewing effluents). Such an alternative reaction simultaneously produces H₂ and high value-added co-products (e.g. arabinose, erythrose, lactobionic acid), distributing the efforts on both half-reactions. Depending on the quantity, the purity and the cost of the molecules, they can be sold to help achieve a solar H₂ production cost of 1 $/kg [1].

The challenge is therefore not only to design more efficient materials that reduce protons in H₂ and achieve the highest solar to hydrogen value, but also to design materials that are efficient and selective for reduction and oxidation. Proof of concept for photoconversion of sugar-rich reactive mixtures already exists, but more comprehensive studies are needed [2], [3]. Fine structure-activity relationships need to be established between the intrinsic properties of the materials and their photocatalytic activity.

Therefore, we have been working on TiO₂-based model photocatalytic systems; thin films prepared by plasma-based techniques (i.e. magnetron sputtering (MS) and plasma-enhanced chemical vapour deposition (PECVD)). These techniques provide access to a wide range of experimental parameters that can be used to tune the structural, electronic and morphological properties of the photocatalysts.

For example, we studied the impact of the TiO₂ thickness on its catalytic activity for the photoconversion of glucose. The TOC image shows a clear correlation between this thickness and the photocurrent that has been measured for the photoconversion of a Na₂SO₄/glucose mixture. This finding is consistent with a previous work about the degradation of methylene blue with TiO₂ films prepared by PECVD [4].

We are currently investigating two approaches to boost the photocatalytic performance of the thin films. On the one hand, gold nanoparticles have been deposited by MS. The plasmonic effect is expected to increase visible light absorption and the overall photocatalytic activity. By tuning the power of the plasma or the Ar pressure, we can adjust the density and the morphology of the gold nanoparticles. On the other hand, by using a mixture or Ar/O₂/N₂ during the MS procedure we were able to prepare TiO_2-xN_y thin films with different electronic, structural and morphological properties. The experimental parameters were studied in relation to the catalytic activity measured for the photoconversion of sugars.