The STH efficiency of these three systems in the first 22 h were 0.17, 0.12 and 0.14%, respectively. However, the activity of Glass-PC rapidly dropped by over 40% after 66 h. This drop was attributed to the loss of Cr species from RhCrOx-Al:SrTiO3 surfaces as evidenced by a significant drop of Cr/Rh ratio from 1.1 to 0.6 after 154 h of irradiation (Supplementary Table ). On the other hand, both untreated SVG-PC and SVG-PC were stable up to 154 h in pure water, with the former retaining 89% of its initial performance and the latter seeing no noticeable change in performance. The performance of SVG-PC already surpassed the performance of Glass-PC after the 3 rd cycle (≥66 h). In artificial seawater, the performance of Glass-PC and untreated SVG-PC dropped significantly compared to their performance in pure water. Both systems only had a STH efficiency of 0.04% in the first 22 h and their H2 evolution rates continued to drop in later cycles. Conversely, SVG-PC performed similarly in both seawater and pure water. Even in artificial seawater, the system retained 80% of its initial performance after 154 h (Supplementary Table ). The slight performance decrease in seawater could be attributed to the fact that a small amount of photocatalyst may be exposed to liquid phase. Contact angle measurements showed that SVG-PC retained its hydrophobicity to a far greater extent than untreated SVG-PC after 154 h of operation (Supplementary Fig. ). Thus, the hydrophobically-treated SVG not only separates the photocatalyst from contaminants in seawater, but also prevents co-catalyst dissolution by keeping the photocatalyst relatively dry. This was verified by post-catalysis ICP-OES analysis of the SVG-PC, which showed that the Rh and Cr content of SVG-PC remained consistent (Supplementary Table ). Post-catalysis SEM images also showed no apparent difference between the as-prepared and used SVG-PC (Supplementary Figs. ). There was no noticeable salt accumulation on the SVG substrate for the sample performed in seawater.