Photocathodes Modified By CdSe Quantum Dots And A Molecular Cobalt Catalyst For Photoinduced Hydrogen Evolution Reaction

The environmental pollution and energy shortage have become serious problems due to the fast consumption of fossil fuels. The hydrogen which is the clean, efficient, pollution-free renewable energy has been considered as an ideal alternative to fossil fuels. Scientists have been trying hard to realize the conversion of the solar energy into hydrogen by water splitting. The photocatalytic hydrogen production systems containing three components have achieved some progresses in recent years, but the practical application of these systems will brought some problems, such as the consumption of large amount of sacrificial electron donors and unrecyclable catalysts.Firstly, a NiO thin film electrode was prepared by screen printing method, and then the water soluble CdSe quantum dots (QDs) were grafted onto the NiO electrode surface by in situ growth method. The sensitized NiO film was characterized by SEM, EDX, XRD and ultraviolet spectroscopy. The cobaloxime complexes bearing a phosphonate or a carboxylate anchoring group, namely [Co(dmgH)2(4-PO3H2-py)Cl] (1) (dmgH2= dimethylglyoxime) and [Co(dmgH)2(4-CO3H2-py)Cl] (2), were respectively linked to the surface of a CdSe quantum dot-sensitized p-type NiO film to fabricate two NiO-based photocathodes PEC1 and PEC2. The photoelectrochemical tests show that both PEC1 and PEC2 electrodes are catalytic active for water reduction to hydrogen under illumination of visible light at low bias potential. The PEC1 electrode with a phosphonate linkage between the cobalt catalyst 1 and the electrode surface displayed higher activity and better stability for hydrogen generation from neutral water compared to the PEC2 electrode with the cobalt catalyst 2 tethered onto the electrode surface through a carboxylate linkage.Secondly, this paper introduced carbon nanotubes (CNTs) to photoinduced hydrogen production systems in order to recycle photosensitizer CdSe quantum dots and the cobaloxime catalysts. Under the optimal conditions, the hydrogen evolved from the water-soluble CdSe QDs and CNTs systems twice that of the system without CNTs after a 9-h irradiation. The CNTs were wrapped with the cobaloxime catalyst Co-C12 bearing a long chain and combined with water-soluble CdSe QDs. The hydrogen evolution from the CdSe QDs-CNTs/Co-C12 system reached 108.42 μmol during 9-h illumination. The CNTs/Co-C12 assembly was centrifuged and reused for photocatalytic reaction in the presence of newly added CdSe QDs. The catalytic efficiency did not show considerable change, which proves that the cobalt catalysts relatively stable on the surface of CNTs and can be recycled. Subsequently, the CNTs were first adsorbed with the photosensitizer, CdSe QDs, by in situ growth procedure and then wrapped with the cobalt catalyst Co-C12 by a long chain. The hydrogen evolution for the system of the CdSe QDs/CNTs/Co-C12 assembly reached 3.19 μmol after a 12-h irradiation. This assembly was also centrifuged and reused for photocatalytic reaction. The hydrogen evolution efficiency did not show apparent change, indicating that CdSe QDs and Co-C12 can be recycled at the same time.