Noble-metal-free metal hydroxide Co-catalyst coupled Mn(ii)-doped CdS nanorods with bridged charge transport for enhanced photocatalytic hydrogen generation

Abstract

Clean and renewable energy from sunlight utilization offers significant opportunities for addressing on-going energy and environmental crises. In this work, Mn(II) doped 1-dimensional (1D) CdS nanorods (NRs) modified with metal hydroxides (M(OH)_x, M = Ni²⁺, Co²⁺, and Fe³⁺) were synthesized as noble metal free photocatalysts for hydrogen generation from water splitting. The incorporation of Mn(II) dopants inside the CdS NRs with a longer lifetime (∼ms) than that of the host CdS (∼ns) leads to efficient charge separation and subsequent electron transfer, which is more efficient due to the longer lifetime Mn(II) dopants, to M(OH)_x results in higher photocatalytic activity towards water splitting. Interestingly, Ni(OH)₂ has the highest efficiency towards photocatalysis under neutral conditions (2.1 fold increase), while the other M(OH)_x (M = Co²⁺ and Fe³⁺) decorated NRs utilized higher pH values (>13, 2.9 and 2.2 fold increase, respectively) for increased photocatalysis. This can be understood through the changing stable phase in a potential/pH diagram (Pourbaix diagram); at higher pH values, the stable phase of Ni(OH)₂ is Ni(OH)₃⁻ which has a Coulombic repulsive force lowering the ability of photoexcited electrons to transfer to the co-catalytic surface for photo–redox reactions. Enhanced charge transport from a small number of Mn(II) dopants (∼0.9% Mn dopants in a CdS NR) efficiently results in increased photocatalysis for the resulting Mn:CdS-M(OH)_x NRs.