Azoxybenzene, azobenzene and their derivatives have attracted significant attention in synthetic chemistry due to their widespread applications in dyes, pharmaceuticals, agrochemicals. The existing protocols for the synthesis of these important compounds suffer from several drawbacks including high reaction temperatures, the use of additional reagents as reducing agent and/or pressured hydrogen gas. Consequently, developing a sustainable and efficient protocol for synthesizing azo derivatives under mild conditions is a significant objective in modern synthetic chemistry. On the other hand, hydrogen energy has become a beacon of hope in addressing escalating environmental concerns due to its clean nature. Among the various hydrogen production methods, photocatalytic water splitting has gained prominence, utilizing semiconductor-based photocatalysts to convert sunlight into fuel efficiently. In this study, we report a novel one-pot solvothermal synthesis of Red-Black Phosphorus/Pt-PtP2 heterostructures (RP-BP/Pt-PtP2) and their use as photocatalysts in the tandem photocatalytic hydrogen evolution and hydrogenation of nitroarenes using water as hydrogen donor. The presented protocol is unique as it enables i) the generation of BP nanosheets from RP under mild conditions and ii) the in-situ generation of Pt and PtP2 NPs deposited onto RP-BP heterojunctions in one-pot reaction. RP-BP/Pt-PtP2 photocatalysts first tested in hydrogen evolution reaction (HER) under visible light irradiation, providing a high HER activity of 543.27 µmol H2 cm−2 (illuminated area) under one sun AM1.5G, which is 39 and 16 times higher than pristine RP and BP, respectively. The HER results prompted us to design a RP-BP/Pt-PtP2 catalyzed tandem photocatalytic HER and hydrogenation of nitroarenes using water as sole hydrogen donor. The developed photocatalytic tandem HER and hydrogenation protocol provided high selectivity for converting nitrobenzene derivatives to the corresponding azoxy compounds with yields reaching up to 99 %. Deuterium labelling experiments confirmed that water serves as the hydrogen source.