Abstract
Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. We show that metal-organic frameworks (MOFs) are viable surface coatings for enhancing cathodic photovoltages. Under 1-sun illumination, no photovoltage is observed for p-type Si(111) functionalized with a naphthalene diimide derivative until the monolayer is expanded in three dimensions in a MOF. The surface-grown MOF thin film at Si promotes reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The photocurrent is governed by charge diffusion through the film, and the MOF film is sufficiently conductive to power reductive transformations. When grown on GaP(100), the reductions of the MOF linkers are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF films to enhance photocathodic operation.
Highlights
Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer
Electrochemical studies of Zr(NDI) metal-organic frameworks (MOFs) thin films grown on fluorine-doped tin oxide (FTO) showed that charge propagation through the MOF occurs by a diffusion-like linker-to-linker hopping mechanism, with an apparent diffusion coefficient of (5.4 ± 1.1) × 10−11 cm[2] s−1 using KPF6 as the electrolyte
To demonstrate the broader applicability of MOF thin films as surface coatings to increase photovoltage, we extended this work to p-type GaP, a III–V semiconductor with conduction and valence band positions that fulfill the energetic requirements for both hydrogen and oxygen evolution
Summary
Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. In a study of the effects of the linker between a cobaloxime catalyst and TiO2-coated Si(100) wafers, no evidence for the reduction of the molecular species in solution at an illuminated photocathode was found, even though the same species was shown to be reduced when immobilized at the photoelectrode[13] In another example, immobilization of a nickel–phosphine catalyst at a methylated Si(111) surface increased the photovoltage of the functionalized semiconductor by 200 mV compared to the Si photocathode when measured in the presence of the catalyst in solution[14]. Immobilization of a nickel–phosphine catalyst at a methylated Si(111) surface increased the photovoltage of the functionalized semiconductor by 200 mV compared to the Si photocathode when measured in the presence of the catalyst in solution[14] In both cases, the increased performance upon immobilization was tentatively assigned to improved interfacial charge transfer to the immobilized species
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