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ACS Appl Mater Interfaces ; 11(37): 34010-34019, 2019 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-31453677


Although hybrid photocathodes built by immobilizing molecular catalysts to the surface of semiconductors through chemical linkages have been reported in recent years, systematic and comparative studies remain scarce about the impact of various anchoring groups on the performance, stability, and charge-transfer kinetics of molecular catalyst-decorated hybrid photocathodes for photoelectrochemical (PEC) H2 production. In this study, the molecular cobaloxime catalysts, CoPy-4-X (Py = pyridine, X = PO3H2, COOH, and CONH(OH)), bearing different anchoring groups were synthesized and covalently immobilized to the surface of the porous TiO2 layer coated on a p-Si plate or a fluorine-doped tin oxide glass. The influence of the anchoring groups on the performance of p-Si/TiO2/CoPy-4-X photocathodes was comparatively studied for PEC H2 evolution. Among the tested hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed higher activity and lower charge-transfer resistance than that observed for the electrode with a carboxylate or a phosphonate as the anchoring group. Notably, the catalytic current of p-Si/TiO2/CoPy-4-CONH(OH) was attenuated only by 2.9% in the controlled potential photoelectrolysis tests in borate buffer solution at pH 9 at 0 V versus a reversible hydrogen electrode over 6 h. Moreover, the influence of anchoring groups on the interfacial electron transfer from the TiO2 layer to the immobilized cobaloxime catalyst and electron-hole recombination was studied by transient absorption spectroscopy. These results revealed that the hydroxamate as an anchoring group is superior to the carboxylate and phosphonate groups for speeding up the interfacial electron transfer and firmly immobilizing the molecular catalysts to the metal oxide semiconductors to build efficient and stable hybrid photoelectrodes.

ACS Appl Mater Interfaces ; 11(21): 19132-19140, 2019 May 29.
Artigo em Inglês | MEDLINE | ID: mdl-31062963


Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under conditions compatible with those for photoanodes are desired. Herein, Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO2 protective layer, followed by decorating the electrode surface with Ni and Ni-Mo catalysts. The carbon nanolayer, denoted as CPDA, was formed directly on the Si surface by in situ self-polymerization of dopamine, followed by carbonization of the polydopamine (PDA) coating. The performance of the as-fabricated Si photocathodes with and without the CPDA layer was comparatively studied for PEC hydrogen evolution reaction (HER) in alkaline electrolytes to evaluate the effect of the sandwich CPDA layer in between the Si substrate and the TiO2 layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the CPDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the CPDA layer. Accordingly, the short-circuit photocurrents of the Ni and Ni-Mo-decorated photocathodes with the CPDA layer were enhanced by a factor of 2.8-3.3, and their open-circuit photovoltages were enlarged by 0.14-0.22 V, compared to those of the corresponding electrodes without the CPDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the CPDA layer also exhibited an improved stability for PEC HER in alkaline solutions, with a faradaic efficiency of 90-97% in the initial hour.