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Non-Catalytic Benefits of Ni(II) Binding to an Si(111)-PNP Construct for Photoelectrochemical Hydrogen Evolution Reaction: Metal Ion Induced Flat Band Potential Modulation

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journal contribution
posted on 2020-03-12, 18:42 authored by Joseph M. Gurrentz, Michael J. Rose
We report here the remarkable and non-catalytic beneficial effects of a Ni­(II) ion binding to a Si|PNP type surface as a result of significant thermodynamic band bending induced by ligand attachment and Ni­(II) binding. We unambiguously deconvolute the thermodynamic flat band potentials (VFB) from the kinetic onset potentials (Von) by synthesizing a specialized bis-PNP macrochelate that enables one-step Ni­(II) binding to a p-Si(111) substrate. XPS analysis and rigorous control experiments confirm covalent attachment of the designed ligand and its resulting Ni­(II) complex. Illuminated JV measurements under catalytic conditions show that the Si|BisPNP-Ni substrate exhibits the most positive onset potential for the hydrogen evolution reaction (HER) (−0.55 V vs Fc/Fc+) compared to other substrates herein. Thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive VFB value (−0.02 V vs Fc/Fc+) by a wide margin. Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between Von and equivalent circuit element parameters commonly associated with HER. Overall, the resulting paradigm comprises a system wherein the extent of band bending induced by metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni­(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst) in semiconductor|catalyst hybrids for PEC-HER.

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