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Atomic Force Microscopic and Raman Investigation of Boron-Doped Diamond Nanowires Electrodes and their Activity towards Oxygen Reduction Palaniappan Subramanian, Srikanth Kolagatla, Sabine Szunerits, Yannick Coffinier, Weng Siang Yeap, Ken Haenen, Rabah Boukherroub, and Alex Schechter J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11546 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017
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The Journal of Physical Chemistry
Atomic Force Microscopic and Raman Investigation of Boron-Doped Diamond Nanowires Electrodes and their Activity towards Oxygen Reduction
Palaniappan Subramanian,1 Srikanth Kolagatla,1 Sabine Szunerits,2* Yannick Coffinier,2 Weng Siang Yeap,3,4 Ken Haenen,3,4 Rabah Boukherroub,2 and Alex Schechter1*
1
2
Department of Chemical Sciences, Ariel University, Kriyat Hamada, Ariel 40700, Israel
Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 - IEMN, F-59000 Lille, France 3
Institute for Materials Research (IMO), Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium 4
IMOMEC, IMEC vzw, Wetenschapspark 1, 3590 Diepenbeek, Belgium
*
To whom correspondence should be addressed. Sabine Szunerits (
[email protected]; Tel: +33 3 62 53 17 25; Fax: +33 3 62 53 17 01) and Alex Schlechter (
[email protected]; Tel : 972-39371470 ; Fax : 972-54-7740254
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ABSTRACT: Reactive ion etching of diamond interfaces using oxygen plasma is a widely used approach for the formation of diamond nanowires. In this paper we highlight the influence of the doping level of the etched diamond substrate and density of the resulting nanowires. Heavily boron-doped diamond interfaces result in very dense diamond nanowires, while etching of low boron-doped diamond substrates results in sparsely formed nanostructures, as boron dopant atoms in the diamond act as masks during the etching process. In pursuit of better understanding of doping and plasma etching effect, we demonstrated by performing Raman imaging on single diamond nanowires that the etching process leads to a de-doping of the wire tip and a partial transformation of diamond to sp2 carbon. The etching process does however not alter the initial diamond feature of the rest of the nanowire. Finally, the activity of the different diamond nanowires towards oxygen reduction in alkaline solution was investigated. Interestingly highly boron doped diamond nanowire interfaces reduces oxygen at relatively lower potential of -0.3 V vs. Ag/AgCl despite the boron de-doping at the tip of the wires.
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INTRODUCTION Boron-Doped Diamond (BDD) electrodes have emerged as ideal electrical interfaces for a variety of different electrochemical applications as they are chemically stable and exhibit a large potential window with low background current
1-2
. With the aim of increasing the
surface area of these electrodes while keeping the unique features of a smooth BDD interface, great efforts have been devoted into the formation of nanostructured diamond films3-10. For electroanalytical applications such as the detection of DNA hybridisation events3, or the sensing of small molecules such as glucose4, 9, 11, tryptophan and tyrosine6, 12, boron doped diamond nanowires (herein refered as BDD NWs) exhibited higher sensitivities when compared to planar BDD interfaces. Nebel et al. reported recently that a nickel particle’s coating of diamond nanowires can be sucesssfully applied as three-dimensional electrodes for reduction of hydrogen4. We have recently demonstrated the interest of diamond nanowires decorated with metallic nanoparticles for the immobilization of histidinylated molecules13. While the reports on the fabrication of diamond structured surfaces with diameters as small as 25 µm and hundreds of microns in length date back to the 1960s14, it was only in the last years that further attempts for the synthesis of diamond nanostructures were undertaken. One of the initial attempts for a top-down synthesis of diamond nanostructures was reported by Shiomi using oxygen plasma based Reactive Ion Etching (RIE) of CVD diamond films coated on a 400 nm thick aluminum layer15. The plasma-assisted RIE technology has since then been widely investigated for the top-down fabrication of diamond nanowires and pillars using porous anodic aluminum oxide16 or various nanoparticles17-19 as masks. Mask-less top-down approaches have been recently proposed as alternatives5-8, 20-21. Such methods have the advantage of being simple and straightforward, not requiring complicated processing steps such as mask deposition or template removal. Our group demonstrated that diamond nanowires can be easily prepared from highly boron-doped microcrystalline diamond thin films by RIE using oxygen plasma and be 3 ACS Paragon Plus Environment
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used for electrochemical sensing, formation of superhydrophobic interfaces as well as for mass spectrometric applications6-8, 11-13. Using RIE without masks, concerns were raised regarding the possibility of de-doping of the highly doped diamond wires. To shed more light on this issue, in this paper we report on AFM and Raman investigations of smooth diamond interfaces of low, moderate and high boron doping levels and compared them to those of diamond nanowires formed by using an oxygen plasma etching in a mask-less approach. Raman images obtained from the tip, the middle and the base of a single diamond nanowire indicated that indeed, dedoping occurs during the etching of highly doped diamond films resulting in diamond nanowire. While the very tips of highly doped BDD NWs became insulating and showed the presence of sp2 contributions, the rest of the BDD NWs remained unaltered. We furthermore investigated the possibility to use the BDD NWs electrodes for the reduction of oxygen. Oxygen reduction is a widely studied reaction due to its importance in fuel cells and sensor applications. Next to gold and platinum electrodes, diamond has been suggested as stable cathode for the reduction of oxygen. Several groups have looked into the use of oxygenated and hydrogenated BDD for electrocatalytic oxygen reduction22-25 in light of the high chemical stability of these materials. Yano et al. investigated the behavior of BDD thinfilm electrodes towards oxygen reduction in alkaline and acidic solutions25-26. It was found that the reaction is highly inhibited on BDD electrodes compared to platinum and glassy carbon and that oxygen reduction reaction (ORR) was a two-electron process based on rotating disk electrode measurements. The electrocatalytic activity of BDD could be enhanced through the modification of BDD with gold nanoparticles, which was higher than that of massive gold electrodes27. We were interested if nanostructured BDD interfaces such as the differently doped BDD NWs developed would be efficient interfaces for oxygen reduction. Electrochemical tests conducted on these BDD NW interfaces indicated an improvement in the oxygen reduction activity with increase in doping levels.
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METHODS AND MATERIALS Preparation of Boron-Doped Diamond Nanowires (BDD NWs). BDD films were grown by microwave plasma-enhanced chemical vapor deposition from methane/hydrogen mixtures (1% CH4) in an ASTeX 6500 reactor, at 30 Torr and 950 °C substrate temperature. The substrates were p-type doped (100) silicon wafers (thickness 500 ߤm, resistivity from 1 to 20 Ωcm). Trimethyl-borane gas was added during the growth at 1,000, 5,000 or 10,000 ppm with respect to the used methane concentration. The final film thickness was 15 µm. The boron concentrations in the films determined by SIMS were NA= 8×1017 B cm-3 (1,000 ppm, BDD-low), 1.5×1019 B cm-3 (5,000 ppm, B-moderate) and 2×1020 B cm-3 (10,000 ppm, BDDhigh). Sheet resistances (Rs) as measured by a four point probe measurement, were determined being 1000, 500 and 207 Ω sq-1 for BDD-low, BDD-moderate and BDD-high, respectively which agrees well with the typical values for diamond films grown under these conditions28-30. The BDD NWs were prepared using RIE of BDD using an oxygen plasma (Plasmalab 80plus) with a radio frequency generator (13.56 MHz) for 30 min. Operating oxygen pressure, flow speed, and plasma power were: 150 mT, 20 sccm, and 350 W. The resulting BDD-NWs were immersed for 15 min in an aqueous solution of HF (5% v/v) to dissolve the SiO2 deposited on the wires during the etching process. Hydrogenation of as-grown BDD NW electrodes was performed in an Ultra High Vacuum CVD chamber using a hot-filament chemical vapor deposition mode (HF CVD) as described elsewhere31. To release the BDD NWs from the silicon substrate, back etching of silicon was performed by dipping the interface into NH4OH (50%) at 70 °C during 24 h. The interfaces were then rinsed in water and dispersed in ethanol. Instrumentation.
Scanning electron microscopic images (SEM) of the films were
obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field 5 ACS Paragon Plus Environment
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emission emitter, three different detectors (ESB detector with filter grid, high efficiency Inlens SE detector, Everhart-Thornley Secondary Electron Detector) and an X-ray energy dispersive analysis device (EDX analysis) (Bruket AXS). An accelerating voltage between 5 kV (nitrogen detection) and 15 kV was used. High-resolution transmission electron images and Selected Area Electron Diffraction (SAED) pattern of BDD-high were acquired from FEI F20 Philips-Tecnai microscope. Micro-Raman spectroscopy measurements were performed on an XploRA ONE™ system (Horiba Scientific, France) using 532 nm (power
