Lightly Fluorinated Graphene as a Protective Layer for n-Type Si(111) Photoanodes in Aqueous Electrolytes
The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F–Gr) was investigated under a range of chemical and electrochemical conditions. The electrochemical behavior of n-Si/F–Gr and np^+-Si/F–Gr photoanodes was compared to hydride-terminated n-Si (n-Si−H) and np+-Si−H electrodes in contact with aqueous Fe(CN)_6^(3-/4-) and Br_2/HBr electrolytes as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. Illuminated n-Si/F–Gr and np^+-Si/F–Gr electrodes in contact with an aqueous K_3(Fe(CN)_6/K4(Fe(CN)_6 solutions exhibited stable short-circuit photocurrent densities of ∼10 mA cm^(–2) for 100,000 s (>24 h), in comparison to bare Si electrodes, which yielded nearly a complete photocurrent decay over ∼100 s. X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F–Gr relative to bare Si electrodes exposed to the same conditions. The variation of the open-circuit potential for n-Si/F–Gr in contact with a series of nonaqueous electrolytes of varying reduction potential indicated that the n-Si/F–Gr did not form a buried junction with respect to the solution contact. Further, illuminated n-Si/F−Gr electrodes in contact with Br_2/HBr(aq) were significantly more electrochemically stable than n-Si−H electrodes, and n-Si/F−Gr electrodes coupled to a Pt catalyst exhibited ideal regenerative cell efficiencies of up to 5% for the oxidation of Br^– to Br_2.
Additional Information© 2016 American Chemical Society. Received: February 22, 2016; Revised: May 19, 2016; Published: June 20, 2016. N.S.L., A.C.N., and A.C.T. acknowledge NSF Grant CHE- 1214152 for support, and the Beckman Institute Molecular Materials Resource Center and Dr. Bruce S. Brunschwig for facilities and valuable advice. A.C.N. and A.C.T. acknowledge Tom Lee and the UCLA Nanoelectronics Research Facility for valuable advice and access to instrumentation. A.C.N. and N.T.P. acknowledge support from NSF graduate research fellowships. C.W.R. thanks the Link Energy Foundation for a graduate research fellowship. J.A.M. thanks the Amgen Scholars program for a fellowship. This graphene material is based upon work supported by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634) and the Nanoelectronics Research Initiative (NRI) through the Institute for Nanoelectronics Discovery and Exploration (INDEX).
Supplemental Material - nl6b00773_si_001.pdf