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Silicon nanowires for solar-to-fuel conversion


Photoelectrochemistry is one of several promising approaches for the realization of efficient solar-to-fuel conversion. Recent work has shown that photoelectrodes made of semiconductor nanowires can have better photoelectrochemical (PEC) performance than their planar counterparts for several reasons including the enhanced light absorption efficiency, release of the high requirement on the minority carrier diffusion length and providing much more catalytic sites for the electrochemical oxidation/reduction reactions to happen. Owing to its earth-abundance, biocompatibility, suitable band structure and stability in aqueous condition, Si nanowire is widely considered as a promising photocathode candidate. Though Si nanowire has been studied for both solar hydrogen evolution and CO2 reduction, our understanding on this photocathode is still not comprehensive, from both fundamental and practical perspectives. Under this context, the subject of my graduate focuses on investigating the properties, understanding the benefits and improving the efficiency of Si nanowire photocathode.

Although much effort has been focused on studying Si nanowire arrays, inhomogeneity in the geometry, doping, defects and catalyst loading present in such arrays can obscure the link between these properties and the nanowires’ PEC performance; correlating the performance with the specific properties of individual wire is difficult because of ensemble averaging. Here, we show that a single-nanowire-based photoelectrode platform can be used to reliably probe the current-voltage (I-V) characteristics of individual nanowires. We found that the photovoltage output of ensemble array samples can be limited by poorly performing individual wires, which highlights the importance of improving the nanowire homogeneity within an array. Furthermore, this platform allows the flux of photo-generated electrons to be quantified as a function of the lengths and diameters of individual nanowires, and the flux over the entire nanowire surface (7-30 electrons/ (nm2∙s)) is found to be significantly reduced as compared to that of a planar analogue (~1,200 electrons/ (nm2∙s)). Such characterization of the photo-generated carrier flux at the semiconductor/electrolyte interface is essential for designing nanowire photoelectrodes that match the activity of their loaded electrocatalysts.

Based on the information obtained from single-nanowire photoelectrode, we moved forward to develop approaches to improving the energy conversion efficiency of Si nanowire photocathode. First, we demonstrate the resonant absorption effect of Si nanowire photoelectrode. Strongly dependent on the nanowire’s diameter, such resonant effect provides guidance to design proper nanowire geometry for maximized light absorption. Second, we try to use Cu to replace Au for the vapor-liquid-solid (VLS) Si nanowire growth. It shows that Cu-catalyzed VLS Si nanowire photocathode outperforms the Au-catalyzed counterpart. Such result highlights the importance of improving the material’s quality, especially avoiding the metal contamination, to realize efficient nanowire-based solar-to-fuel conversion. Third, we use a commercial chemical vapor deposition (CVD) system for wafer-scale Si nanowire growth. Attributed to the capability of precisely-controlled in-situ boron doping, the CVD yields Si nanowire arrays with decent PEC performance. Our approach opens up the opportunities for scale-up production of high-quality Si nanowire photocathode.

Recently the inorganic/microorganism hybrid systems have attracted a lot of interests in the field of microbial electrosynthesis and artificial photosynthesis. However, the electron transfer pathway from electrode to microorganism is still elusive. With Si nanowire/Ni/S. Ovata hybrids as a model system, the last part of my graduate research used sophisticated electrochemical methods to investigate the cathodic electron transfer mechanism in the bacteria-catalyzing CO2-reducing process. The Tafel plot on biotic condition yields fast kinetics (low Tafel slope) at lower over-potential region and slow kinetics (high Tafel slope) at higher over-potential region. Comparison with the abiotic Tafel plot suggests that H2-mediated electron transfer dominates at higher over-potential. The charge transfer resistance extracted from the EIS measurement is consistent with the information obtained from the Tafel plot. The comparison between Ni-based hybrids and Pt-based hybrids system indicate that Ni plays an important role in such kinetics transition. At lower over-potential, the Ni species is oxidized into Ni(OH)2, which is proposed here to bind with the conductive protein complexes on the membrane of S. Ovata bacteria. Such binding would induce the direct electron transfer from Si cathode to the intracellular environment and thus facilitate the kinetics. Our results provide the guidance to design the efficient bio-inorganic interface in the field of microbial electrosynthesis and artificial photosynthesis.

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