Interfacial Engineering of a Carbon Nitride-Graphene Oxide-Molecular Ni Catalyst Hybrid for Enhanced Photocatalytic Activity

Carbon nitrides (CNx) are a promising class of photocatalyst for fuel and chemical synthesis as they are nontoxic and readily synthesized at a low cost. This study reports the enhanced photocatalytic activity for simultaneous alcohol oxidation and proton reduction when graphene oxide (GO) or reduced graphene oxide (RGO) is employed as an interlayer between a cyanamide-functionalized melon-type carbon nitride ((CNx)-C-NCN) and a phosphonated Ni-bis(diphosphine) H-2-evolution catalyst (NiP). Introduction of the GO/RGO enhanced the activity three times, reaching a specific activity of 4655 +/- 448 mu mol H-2 (g (CNx)-C-NCN)-1 h(-1) with a NiP-based turnover frequency of 116 +/- 3 h(-1). Mechanistic studies into this closed photoredox system revealed that the rate of electron extraction from (CNx)-C-NCN is rate limiting. GO/RGO is commonly employed to improve the electron transfer dynamics on nanosecond time scales, but time-resolved photoluminescence and transient absorption spectroscopy reveal that these properties are not significantly affected in our (CNx)-C-NCN-GO hybrid on fast time scales (<0.1 s). However, long-lived trapped-electrons generated upon photoexcitation of (CNx)-C-NCN in the presence of organic substrates are shown by photoinduced absorption spectroscopy to be quenched faster with GO/RGO, supporting that GO/RGO improves electron transfer from (CNx)-C-NCN to NiP on time scales >0.1 s. The absorption profile of NiP in the presence of different GO loadings reveals that GO acts as a conductive interfacial binder between NiP and (CNx)-C-NCN. The enhancement in activity therefore does not primarily arise from changes in the photophysics of the (CNx)-C-NCN, but rather from GO/RGO enabling better electronic communication between (CNx)-C-NCN and NiP.