Article Cite This: Langmuir 2017, 33, 10765-10771
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Immobilization of Reduced Graphene Oxide on HydrogenTerminated Silicon Substrate as a Transparent Conductive Protector Yudi Tu, Toru Utsunomiya, Sho Kokufu, Masahiro Soga, Takashi Ichii, and Hiroyuki Sugimura* Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan S Supporting Information *
ABSTRACT: Silicon is a promising electrode material for photoelectrochemical and photocatalytic reactions. However, the chemically active surface of silicon will be easily oxidized when exposed to the oxidation environment. We immobilized graphene oxide (GO) onto hydrogen-terminated silicon (HSi) and reduced it through ultraviolet (UV) and vacuumultraviolet (VUV) irradiation. This acted as an ultrathin conductive layer to protect H-Si from oxidation. The elemental evolution of GO was studied by X-ray photoelectron spectroscopy, and it was found that GO was partially reduced soon after the deposition onto H-Si and further reduced after UV or VUV light irradiation. The VUV photoreduction demonstrated ca. 100 times higher efficiency compared to the UV reduction based on the irradiation dose. The saturated oxygen-to-carbon ratio (RO/C) of the reduced graphene oxide (rGO) was 0.21 ± 0.01, which is lower than the photoreduction of GO on SiO2 substrate. This indicated the H-Si played an important role in assisting the photoreduction of GO. No obvious exfoliation of rGO was observed after sonicating the rGO-covered H-Si sample in water, which indicated rGO was immobilized on H-Si. The electrical conductivity of H-Si surface was maintained in the rGO-covered region while the exposed H-Si region became insulating, which was observed by conductive atomic force microscopy. The rGO was verified capable to protect the active H-Si against the oxidation under an ambient environment.
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INTRODUCTION Silicon, which is a semiconductor with a band gap of 1.12 eV, is an attractive material for use in visible-light photoelectrochemical and photocatalytic reactions.1−4 The hydrogen-terminated Si (H-Si) surface is electrically ideal when initially formed, but this surface readily oxidizes in the ambient environment and aqueous solutions. An insulating oxide overlayer will form on the Si surface, which quickly prevents current flow through it.5,6 To stabilize the Si surface, a transparent conductive inhibitor for oxidation is required. Several strategies have been developed such as coating Si substrates with Ni,7 epitaxial SrTiO3,8 and alkyl chains.9 As an impermeable and transparent protector, graphene can be effective barriers for preventing the oxidation of Si surfaces. Lewis et al. reported that durable photoelectrochemical performances were achieved in graphenecoated n-type Si(111) photoanodes.10,11 However, Schriver et al. reported that graphene could not prevent the oxidation of Si surface in the long term.12 One reason can be that graphene contains few functional groups which can act as anchors for tight bonding onto the substrates; this will result in the invalidation of passivation. The high cost of the chemical vapor deposition (CVD) process for preparing graphene also limits the application of the graphene-coated Si surface. To overcome these problems, another precursor of graphene, graphene oxide (GO), can fulfill the demands of anchoring the protection layer onto the Si substrates and reduce the fabrication cost. Similar to graphene, GO has a good optical property on visible light transmission, which is important for © 2017 American Chemical Society
the further photoelectrochemical application of Si substrate. Because of the oxidative preparation, GO is heterogeneously decorated by oxygen-containing functional groups (OFGs), which can work as sufficient anchors. Previous researches have reported hydroxy groups, carbonyl groups, and epoxy groups can be covalently bonded onto H-Si via photoassisted reactions (as shown in Scheme 1).13−18 On the basis of these findings, it can be predicted that GO can be possibly immobilized onto HSi via the reaction between OFGs and H-Si. Meanwhile, ultraviolet (UV) and vacuum-ultraviolet (VUV) irradiation can reduce the GO sheets, which transform GO into the conductive reduced GO (rGO) to maintain the conductivity of Si surface.19−23 In this article, we report a simple process to immobilize GO onto H-Si. The GO sheets were then photochemically reduced by using UV and VUV irradiation. The elemental evolutions of GO were studied by X-ray photoelectron spectroscopy (XPS). The difference in photoreduction efficiency between VUV and UV light was demonstrated, and the mechanism was discussed. The topographic features of the H-Si surface and the GO sheets were observed by atomic force microscopy (AFM). The electrical properties of the rGO-coated H-Si surface were studied by conductive AFM (CAFM). It demonstrated that rGO prevented the H-Si surface from the oxidation. Received: May 20, 2017 Revised: September 10, 2017 Published: September 20, 2017 10765
DOI: 10.1021/acs.langmuir.7b01688 Langmuir 2017, 33, 10765−10771
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surface. After that, the substrate was spun at 500 rpm for 15 s and then 2000 rpm for 100 s. The samples were submitted to sonication for 20 min in the ultrasonic cleaner (same as mentioned above) to confirm the immobilization of GO on H-Si after spin-coating. Photoreduction. Photoreduction was performed by UV and VUV light, which was generated by high-pressure Hg lamp (100 mW cm−2, REX-250, Asahi spectra Co., Ltd.) and Xe2 excimer lamp (172 nm, 13.8 mW cm−2, UEM-20-172, Ushio Inc.), respectively. The light intensity of UV lamp was measured by a laser power meter (PM-335, Neoark Co.) while the light intensity of VUV lamp was measured in N2 environment by an accumulated UV meter (UIT-150, Ushio Inc.). The as-coated H-nSi and H-pSi substrates were set into a high-vacuum chamber (
