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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Simultaneous silicon oxide growth and electrophoretic deposition of graphene oxide Pina Atalanta Fritz, Stefanie C. Lange, Marcel Giesbers, Han Zuilhof, Remko M. Boom, and Karin Schroen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03139 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019
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Simultaneous silicon oxide growth and electrophoretic deposition of graphene oxide Pina A. Fritz1,2*, Stefanie C. Lange3, Marcel Giesbers4, Han Zuilhof3,5, Remko M. Boom1, and C.G.P.H. Schroën1 1Wageningen University, Laboratory of Food Process Engineering, Bornse Weilanden 9, 6708 WG Wageningen, the Netherlands. 2Nanyang Technological University, School of Chemical and Biomedical Engineering, 62 Nanyang Drive, 637459, Singapore 3Laboratory of Organic Chemistry, Wageningen University, Stippeneng 4, 6708 WE Wageningen, The Netherlands 4Wageningen Electron Microscopy Centre, Wageningen University, Droevendaalsesteeg 1, 6708PB Wageningen, The Netherlands 5School of Pharmaceutical Science and Technology, Tianjin University, 92 Weijin Road, Tianjin, P.R. China Corresponding author:
[email protected] Abstract During electrophoretic deposition of graphene oxide (GO) sheets on silicon substrates, not only deposition but also simultaneous anodic oxidation of the silicon substrate takes place, leading to a three-layered material. SEM images reveal the presence of GO sheets on the silicon substrate, and this is also confirmed by XPS, albeit that the carbon portion increases with increasing emission angle, hinting at a thin carbon layer. With increasing applied potential and increasing conductivity of the GO solution the carbon signal decreases, while the overall thickness of the added layer formed on top of the silicon substrate increases. Through XPS spectra in which the Si2p peaks shifted under those conditions to 103 - 104 eV we were able to conclude that significant amounts of oxygen are present, indicative of formation of an oxide layer. This leads us to conclude that GO can be deposited using electrophoretic deposition, but that at the same time silicon is oxidized, which may overshadow effects previously assigned to GO deposition.
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Keywords Graphene oxide, electrophoretic deposition, spin coating, anodic oxidation, silicon oxide, MnO2 nano particles.
Introduction Transparent electrodes are used in many electronic devices such as touch screens,1 thin-film photovoltaics,2 and transistors.3 Reduced graphene oxide (GO) thin films are promising cheaper alternatives4,5 to the current standard indium-doped tin oxide (ITO). Graphene oxide is a 2D material synthesized by chemical exfoliation of graphite,6 that in its original state shows low conductivity, but when reduced by either thermal,7 chemical8 or electrochemical methods,9–11 the conductivity can become as high as 100 S/cm.12 Further, their thermal stability, flexibility, and light transmittance of 80-85 % at a wavelength of 550 nm make reduced GO thin films interesting candidates for flexible transparent electrodes.13–16 Many techniques to produce GO thin films have been proposed, amongst others spray-coating,15,17 spin-coating,18,19 and electrophoretic deposition.20–23 When applying the latter technique, GO sheets migrate along an electric field towards the anode due to the negatively charged oxygen groups (typically carboxyl and hydroxyl groups) present on the lateral plane and edges of the GO sheets. Once they reach the surface of the anode, the charged sheets deposit and form a new layer on the electrode, and further reduction can take place.20 This technique can be applied to many conductive substrates such as copper, nickel, aluminum and stainless steel ,and is not limited to planar shapes.20,24–27 A significant advantage of electrophoretic deposition is thus that the GO is directly reduced during the deposition process.22
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Apart from the metallic substrates mentioned above, also silicon has been used as substrate for electrophoretic deposition of GO,21–23 with e.g. the aim to reduce wear and tear in micro- and nano-electromechanical systems (MEMS/NEMS). This use of GO has led to an order of magnitude reduction in both friction and wear volume, as compared to bare silicon, making GO a promising solid lubricant. For this purpose, layer thicknesses of up to 402 nm have been reported,21 based primarily on ellipsometry and cross-sectional SEM images. In these estimates, the deposited GO layer has, however, to the best of our knowledge, never been distinguished from any anodically oxidized silicon layer underneath. To properly evaluate and understand the performance of the layer, a compositional analysis is, of course, required. The anodic oxidation of silicon at the interface of an electrolyte has been described with the following reactions.28 Si + H2O + h SiOH + H+
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2 SiOH Si2O + H2O
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2 Si2O SiO2 + 3 Si
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The majority of the oxygen atoms in an anodically grown oxide layer originate from water that is present at or is generated by redox reactions of salt or solvent molecules at the electrode interface. Water only enters the most outer layer, where proton loss occurs, which leads to hydroxyl or oxide ions traveling further into the silicon structure along the electric field.28 In this paper, we describe the influence of several electrophoretic deposition parameters on the deposition of GO and growth of anodic silicon oxide, using SEM, XPS, ellipsometry, and color analysis. Materials and Methods Electrophoretic deposition of graphene oxide.
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Prior to the coating process, the GO solution (Sigma-Aldrich, 4mg/ml) with a conductivity of 7 S/cm was sonicated and depleted of oxygen using a N2 purge for 30 min. The silicon substrates (1 4 cm) were cleaned with piranha acid (3:1) for 15 min. Later, two silicon substrates were immersed into the deposition cell at a distance of 10 mm, and a constant potential of 5, 20, 30 or 60 V was applied for 1 h (Power supply EPS 1001, Amersham Pharmacia Biotech, USA). To increase the conductivity of the GO solution, 10 or 1000 mM sodium sulfate was added to reach conductivities of 16 and 485 S/cm, respectively. Next to 1 h, silicon substrates were also coated for 0.5 and 2 h at 20 V. As blanks, a silicon substrate was immersed in the GO solution for 1 h without applying a potential, and another silicon substrate was exposed to a sodium sulfate solution with a conductivity of 7 S/cm but without any GO present, while applying a potential of 20 V for 1 h. Since GO solutions can vary in sheet size, oxidation degree and conductivity, as comparison, another GO solution (GO(2)) was synthesized from graphite powder (Sigma-Aldrich,
