Harnessing low energy photons (635 nm) for the production of H₂O₂ using upconversion nanohybrid photocatalysts

This study demonstrates, for the first time in literature, in situ photocatalytic synthesis of hydrogen peroxide (H₂O₂) through sensitized triplet-triplet annihilation (TTA) upconversion (UC) of low-energy, sub-bandgap photons. The aqueous phase TTA-UC and subsequent photocatalytic oxygen reduction were achieved by a newly developed ternary nanohybrid that consists of three components: (1) a nano-scale silica core-shell structure that encapsulates TTA-UC chromophore-containing media; (2) a low-bandgap CdS photocatalyst on the surface of the silica nanocapsule; and (3) a graphene oxide nanodisk (GOND) as a co-catalyst. In this study, we employed a benchmark TTA-UC chromophore pair, palladium(ii) tetraphenyltetrabenzo-porphyrin sensitizer and 9,10-bis(phenylethynyl)anthracene acceptor, to upconvert red photons (λ_Ex = 635 nm and 1.95 eV) to green photons (λ_Em = 505 nm and 2.45 eV). CdS is sensitized by upconverted green light to produce charge carriers, but not by incident red light without TTA-UC. The photogenerated electrons are efficiently transferred to a GOND to retard rapid charge recombination in CdS, which subsequently reduce dioxygen to produce H₂O₂ up to a 100 micromolar level per hour (or 3 mg L^-1 h^-1 with 0.5 g L^-1 of GOND/CdS component only). Wrapping of CdS by a GOND was also found to markedly enhance the stability of CdS against photocorrosion without light shielding owing to its small size (ca. ∼80 nm) and transparency (α_635nm = 1.85 g^-1 dm³ cm^-1).