Hierarchical Carbon-Silicon Nanowire Heterostructures for Hydrogen Evolution Reaction
Authors: Moon Joonhee , Sim Uk 
Published: February 2020

Silicon nanowires (SiNWs) opened up exciting possibilities in a variety of research fields due to their unique anisotropic morphologies, facile tuning capabilities, and accessible fabrication methods. The SiNW-based photoelectrochemical (PEC) conversion has recently been known to provide an efficiency superior to various photo-responsive semiconductor heterostructures. However, a challenge still remains in designing optimum structures to minimize photo-oxidation and photo-corrosion of Si surface in a liquid electrolyte. Here, we synthesized hierarchically branched carbon nanowires (CNWs) on SiNWs utilizing copper vapors as catalysts in a chemical vapor deposition (CVD) process, which exhibit outperforming photocatalytic activities for hydrogen generation along with excellent chemical stability against oxidation and corrosion.
First, SiNWs were prepared by a metal catalyzed electroless etching method and placed at the center of a quartz reactor in a CVD chamber. Next, a Cu foil was placed above the SiNWs without a physical contact, where Cu vapors from the foil catalyzes the growth of CNWs on SiNWs while flowing 50 sccm H2 and 15 sccm CH­4 for 10~40 min at 1,000 ˚C in an ambient pressure. Scanning Electron Microscopy (SEM), Transmission electron microscopy (TEM), Selected Area Electron Diffraction (SAED), Raman spectroscopy, X-ray Photoemission Spectroscopy (XPS) analysis were employed to characterize the CNWs on the SiNWs, indicating that CNWs with various shapes and diameters are successfully grown on a SiNW.
The PEC performance for the hydrogen evolution reaction shows that the catalytic activity is substantially dependent on the surface morphology of the CNWs-SiNWs. The applied bias photon-to-current efficiency (ABPE) in the optimized CNWs-SiNWs structure (1.86%) exhibits a higher value than in any of the other steps, which is also comparable to other carbon-based catalysts on the Si photocathode reported to date. Enhanced photocatalytic performance is substantially affected by the intrinsic wettability of the surface due to the polarization of electrochemical reaction. The long-term stability of the CNWs-SiNWs photocathode was tested for 300 cycles or for 11 hrs at a pH 0, showing a stable current density. The PEC performance of the Si cell is known to degrade easily due to surface oxidation in aqueous solutions, but our stability testing result implies that the CNWs efficiently suppress the degradation of photoelectrochemical performance by protecting Si surface from oxidation.
In conclusion, various spectroscopic analyses revealed that the CNWs-SiNWs simultaneously played a key role in both as catalysts and electrodes, providing a new strategy to develop a new type of metal-free carbon-based structures with enhanced stability and catalytic reactivity essential for the higher performance of hydrogen evolution reaction. Thus, we believe that the CNWs-SiNWs photoelectrodes would provide a new route to develop high-performing cost-effective catalysts essential for advanced energy conversion and storage technologies.