Facet-Dependent Temporal and Spatial Changes in Boron-Doped

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Facet-Dependent Temporal and Spatial Changes in BoronDoped Diamond Film Electrodes due to Anodic Corrosion Francesca Celine Inserto Catalan, Norihiko Hayazawa, Yasuyuki Yokota, Raymond Albert Wong, Takeshi Watanabe, Yasuaki Einaga, and Yousoo Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06085 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry

Facet-Dependent Temporal and Spatial Changes in Boron-Doped Diamond Film Electrodes due to Anodic Corrosion Francesca Celine I. Catalan1, Norihiko Hayazawa1, Yasuyuki Yokota1, Raymond A. Wong1, Takeshi Watanabe2, Yasuaki Einaga3, 4, Yousoo Kim1* 1

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 2

Department of Electrical Engineering and Electronics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 252-5258, Japan 3

4

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan

ACCEL, Japan Science and Technology Agency, 5-3 Yonbancho, Chiyoda-ku, Tokyo 1028666, Japan * Email: [email protected]; Tel: +81-48-467-4073

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ABSTRACT The progression of corrosion in polycrystalline boron-doped diamond (BDD) thin film electrode is explored as the electrode undergoes high-current density anodic treatments with organic compounds. Micro-Raman spectroscopy and spectral mapping indicate that anodic corrosion is initiated by the conversion of sp3 diamond to amorphous sp2 carbon at the surface, which are then removed after longer anodic treatment. Polarized Raman analysis reveals that corrosion-induced changes on the surface are specific to (100)-grain facets and (111)-grain edges. X-ray photoelectron spectroscopic measurements suggest that carbonyl groups consequently form on these specific sites and act as an intermediate towards the etching of the surface. This process exposes and subsequently removes the sub-surface boron atoms, thus reducing the doping density. The observed crystal grain orientation dependence of the corrosion process provides new insights towards a better understanding of degradation in BDD electrodes.


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INTRODUCTION Diamond has been extensively studied recently for its huge potential in various applications in the fields of electrochemistry1, biochemical detection2, 3 and water treatment 4. This renewed interest in diamond stem from its exceptional properties, particularly its extreme hardness and inertness, ease to functionalize its surface to modify its chemical reactivity, and controllability of its conductivity with doping5, 6. While intrinsic diamond is a wide-band gap insulating material, doping it with boron introduces electronic states in the band gap which enhance its conductivity7, 8. In this case, boron acts as an electron acceptor, and the boron concentration [B] in the crystal lattice determines the conductivity of the diamond, i.e., higher [B] makes diamond more conductive. High-quality boron-doped diamond (BDD) thin films can be grown by plasma-assisted chemical vapor deposition (CVD) on various substrates with doping levels of 1019 – 1022 cm-3. At these doping levels, BDD acts as a wide-band gap semiconductor or as a semi-metal, and becomes a promising electrode material with unique properties such as wide electrochemical potential window, low voltammetric background current, and biocompatibility5. Although diamond is known for its mechanical and chemical stability, previous reports have shown that measurable deterioration can occur in BDD electrodes especially in highcurrent density anodic treatments. For example, delamination and textural changes due to surface oxidation were observed in ultra-nanocrystalline BDD at 1 A cm-2 9. It has been reported that in micro-polycrystalline BDD films, anodic corrosion is enhanced with increasing doping levels10, lower solution pH level11, and presence of organic compounds11, 12. Along with morphological changes, electrochemical performance and stability of the BDD electrodes have also been shown to be reduced significantly with anodic corrosion9, 13. In wastewater treatment applications, for example, although an increase in current density results to higher rates of oxidation of contaminant compounds, it also leads to a significant decrease in oxidation 3 ACS Paragon Plus Environment


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efficiency14, 15. Thus, in order to aid the current development of BDD thin films as an electrode for various applications, it is important to investigate the mechanisms of anodic corrosion in BDD. Experimental studies on BDD anodic corrosion suggest that degradation is especially initiated at sp2-hybridized carbon sites on the BDD that are either inherent in the electrode from the CVD process or formed by the electrode’s chemical interaction with its surroundings 11, 16. However, the specific mechanisms by which they are formed during the electrochemical process and the consequent role of their formation to corrosion are still unclear. In addition, oxygenated functional groups formed on the surface during anodic treatment has also been shown to affect the electrochemical activity of BDD17, 18. The adsorption and formation of certain surface groups have been reported to be dependent on the crystallographic orientation of the diamond19- 21. Thus, correlating the surface composition with the morphological and electrochemical changes that occur as BDD undergoes anodization can elucidate further the specific mechanisms involved in anodic corrosion. Raman spectroscopy has been regarded as one of the most versatile tools to characterize carbon-based materials. Compared to optical microscopy or scanning electron microscopy, Raman spectroscopy is highly sensitive to chemical species on or near the surface22, providing a comprehensive approach to understanding a material’s composition and interactions with the environment. In diamond films, Raman spectra provide information on the amount of boron dopants23, diamond crystallinity24, presence of various carbon phases and the concentration of diamond defects25. By combining confocal and polarization-sensitive configurations, a spatial resolution of < 1 μm and high sensitivity to grain crystallographic orientation26 can be achieved, allowing the technique to easily analyze grain-dependent surface modifications in polycrystalline BDD compared to other chemical species-sensitive methods.


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The aim of this work is to clarify the corrosion mechanism by evaluating the spatial and temporal changes in highly-doped polycrystalline BDD film under high-current density anodic treatments. The morphological and chemical properties of the BDD were systematically analyzed as the electrolysis time is increased up to 12 hrs. High-resolution spatial mapping of the chemical and morphological corrosion-induced changes on the BDD surface was done by optical imaging and confocal micro-Raman spectroscopy. To our knowledge, our study represents the first application of polarized Raman spectroscopy and Raman spectral imaging to visualize high-resolution, grain-dependent anodic corrosion in polycrystalline BDD films. In addition, X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the surface chemistry of the BDD as it undergoes anodization. The combined temporal study of the morphology of BDD using optical imaging, and investigation of the surface functional groups and chemical species using high-resolution Raman spectroscopy and XPS can give a more complete picture of the mechanisms behind anodic corrosion. METHODS Polycrystalline BDD film was deposited on Si (111) wafer by microwave plasma – assisted CVD facility (CORNES Technologies/ASTeX, AX5400). The deposition was performed at 900˚C using acetone and trimethoxyborane as carbon and boron sources, respectively, with an atomic ratio B/C = 1%. The Si substrate was scratched using abrasion sheets prior to deposition in order to enhance nucleation. The BDD film was marked using a tabletop femtosecond laser micromachining facility prior to any measurements27. This ensures that the same area on the film is processed and analyzed at different stages of the treatment since all measurements are done ex situ. Electrochemical

treatments

were

carried

out

in

a

single-compartment

polytetrafluoroethylene (PTFE) electrochemical cell with Pt wire counter electrode and



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Ag/AgCl (saturated KCl) reference electrode. Anodic pretreatment was performed in 0.1 M H2SO4 aqueous solution at 3.5 V for 10 minutes. Oxidative pretreatment is important as it has been shown to improve the stability and conductivity of the BDD film by removing sp 2 components which may exist on the surface or grain boundaries18,

28

. Constant current

electrolysis were conducted with a potentiostat (HZ-7000, Hokuto Denko Corp.) at a current density of 0.25 A/cm2. The BDD film worked as an anode in 3 M acetic acid (CH 3COOH) aqueous solution with 1 M HClO4 as supporting electrolyte. Acetic acid is a typical intermediate in the oxidation of many organic compounds in BDD electrode applications29, and is thus a suitable model for BDD corrosion studies. The BDD film was electrochemically treated for a total of 12 hrs, and its attributes were analyzed ex situ after 0.5, 2, 6 and 12 hrs using polarized Raman point spectroscopy, large-area Raman imaging, and XPS. Polarized Raman point spectra were measured at room temperature using linearly polarized light from a solid-state laser (λ = 532 nm) focused onto the sample via a 100x, NA 0.9 objective (Nikon LU Plan Fluor EPI). The scattered light was collected using the same objective in a backscattering geometry. A confocal pinhole (Φ = 150 μm) was utilized to spatially filter out the scattered light from out-of-focus parts of the sample. The sampling diameter at the surface of the BDD film is measured to be 0.70 μm. The scattered light is then guided to a spectrometer (Acton SP-2300i, grating = 600g/mm, focal length = 300 mm) and detected by a charge coupled device (CCD) detector, cooled by liquid nitrogen. Polarization measurements were performed by utilizing a fixed analyzer polarization directed into the spectrometer while rotating the scattered light polarization via a half-wave plate positioned prior to the analyzer. This polarization detection scheme is employed in order to avoid the polarization dependence of the grating diffraction efficiency of the spectrometer. The excitation laser power used was 180 μW as measured at the focus. No significant peak shifts were observed at higher excitation power, indicating that the high laser power has no thermal 6 ACS Paragon Plus Environment

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effect on the BDD film. The exposure time to measure each spectra was 30 s, making sure that even the low-signal peaks are observed. Raman spectral mapping was performed using a laser-scanning micro-Raman microscope (RAMAN-11, Nanophoton) with excitation at 532 nm. The system utilizes a cylindrical lens and objective lens (100x, NA 0.9 Nikon LU Plan Fluor EPI) to focus the laser beam to a line at the sample which realizes fast Raman spectral mapping (Supplementary Figure S1). The samples were mounted on a computer-controlled XY-stage, and scanned in 2D. During the scan, the full spectra for each point were recorded with an exposure time of 15 s. The excitation laser power used was measured at the sample to be 120 mW. Each Raman spectrum was background-subtracted before it was processed with a fitting program designed in-house using LabVIEW software (National Instruments). The fitting functions used in this work are detailed in the Supplementary Information S1. XPS measurements were performed using Theta Probe angle-resolved XPS system (Thermo Scientific) with monochromated Al Kα X-ray source (1486.6 eV). Binding energies were calibrated with respect to Au 4f7/2 peak at 84.0 eV. XPS spectra were processed using XPST (ver. 1.1) program package for IgorPro software (Wavemetrics). Inelastic background was subtracted from the spectra by applying Shirley’s method. C1s peak was deconvoluted into constituent peaks which were fitted with a combination of Lorentzian and Gaussian line shapes (g/l ratio = 0.3) 30. RESULTS AND DISCUSSIONS Polarized Raman point spectroscopy Figure 1(a) shows an optical image of the pristine BDD film electrode. All grains are discrete with well-distinguishable grain boundaries, which is a clear indication of the polycrystalline nature of the film. The BDD film has an average thickness of 4.2 μm ± 0.29 μm 7 ACS Paragon Plus Environment


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(Supplementary Figure S2), and is composed of randomly distributed crystal grains of various sizes and crystallographic orientation, with an average grain size of 4.0 ± 0.84 μm. The average grain size is determined using the Heyn lineal intercept method where the number of grains intercepted by a test line created across the optical image is counted 31.

Figure 1. Polarized Raman spectroscopy of various grains on the polycrystalline BDD: (a) Optical image of the polycrystalline BDD. Scale bar corresponds to 10 μm. (b) Raman spectra and ((c) – (e)) the corresponding polarization dependence of the diamond ZCP -1

peak (~1332 cm ) measured at 3 different grains. The spectra have been offset vertically from each other for clarity.


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Figure 1(b) reports the polarized Raman spectra measured at the center of three different grains denoted in the optical image. The incident light is polarized along the horizontal direction parallel to the film surface. For this set of spectra, the analyzer polarization is set along the horizontal direction as well, parallel to the incident light polarization. All spectra are accompanied by a broad non-uniform background – a spectral feature caused by intra-band hole scattering typical for highly-doped BDD samples32. No sp2 carbon is detected (G-mode at 1580 cm-1) in all spectra, indicating high-quality BDD film. Other spectral features, however, differ from one location to another, which stems from disparity in the amount of dopants in the crystal grains. The high intensity of the ~1200 cm -1 peak (P2 peak) for Grain-1 denotes more single boron atoms substitutionally incorporated in the diamond lattice8. The broad peak around 500 cm-1 (P1 peak) attributed to the presence of boron pairs in the crystal33, 34 appears in all three spectra, but the P1 peak for Grain-1 is noticeably shifted towards a lower wavenumber compared to that of Grain-2 and Grain-3. We can quantify the amount of boron [B] in the grain by fitting the P1 peak with a combination of Lorentzian and Gaussian functions, and determining the position of the Lorentzian component. The Lorentzian peak position (ω L,P1) is empirically known to be related to [B] by the following equation: [𝐵] = 8.44𝑥1030 𝑒𝑥𝑝(−0.048𝜔𝐿,𝑃1 ) where [B] is in cm-3 and ωL,P1 is in cm-1

23

(1)

. Using equation (1), we calculated the amount of

boron in Grain-1, 2 and 3 as [B]1 = 9.8 x 1020 cm-3, [B]2 = 8.2 x 1020 cm-3, [B]3 = 8.0 x 1020 cm3

, respectively. Moreover, the diamond zone center phonon (ZCP) peak (positioned at 1332 cm-1 for

undoped diamond) was fitted with a Fano function and compared for Grains 1, 2 and 3. The Fano line shape for this peak is derived from the quantum interference between the discrete 9 ACS Paragon Plus Environment


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diamond phonon mode and the continuum of electronic states caused by the presence of dopants35. The fitted values for the peak position (ωZCP), width parameter (Γ), and asymmetry parameter (q) for the diamond ZCP peak obtained for each grain are summarized in the Supporting Information (Table S1). In Table S1, the standard error for each fitted value is placed inside the parentheses. Grain-1 has a notably smaller asymmetry parameter magnitude (|q|1 = 2.4) compared to Grains-2 and 3 (|q| = 4.9, 8.0, respectively), translating to a more pronounced asymmetry of the diamond ZCP peak for Grain-1, as observed in Figure 1(b). The lower relative intensity and more pronounced asymmetry in the diamond ZCP peak indicates degraded crystalline quality due to the incorporation of more dopants. The dynamics involved in the efficient incorporation of dopants is not straightforward in polycrystalline film growth and depends on several factors including the growth parameters and presence of local defects. Studies in homoepitaxial diamond films, however, indicate that crystallographic orientation plays a significant role in doping such that boron is more efficiently incorporated into the lattice in (111)-oriented facets than in (100) facets36. To precisely determine the crystallographic orientation of the grains, we measured the polarization dependence of the diamond ZCP peak (Figure 1 (c) – (e)). The peak is fitted with a Fano function, and the fitted peak intensity is plotted as a function of the rotation angle difference between the incident polarizer and analyzer (θ), i.e. θ = 0˚ when the polarizer and analyzer are parallel. It is observed that the diamond mode in Grain-1 has a very weak polarization dependence, while the same mode in Grain-2 and Grain-3 has a clear polarization dependence with extinction ratio (Imin/Imax) of 0.4 and 0.2, respectively. Based on the Raman tensor26, Grain1 can be assigned as (111)-oriented, Grain-2 as (100)-oriented, and Grain-3 as (110)-oriented facet. Deviations from the expected trend such as asymmetry in the polarization dependence or extinction of the Raman mode can be attributed to dislocation or local defect in the grain, or tilt of the grain facet with respect to the X-Y sample plane – both can affect the propagation of 10 ACS Paragon Plus Environment

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the scattered light and, consequently, the measured intensity. We focus our discussions on (111) and (100) grains because these configurations are most abundant in CVD-grown polycrystalline diamond films37. Figure 2 shows the temporal changes in the surface morphology and polarized Raman spectra of Grain-1 and Grain-2. Both the optical images [Figure 2 (a) – (e)] and Raman spectra [Figure 2(f)] reveal that no significant change occurred at the center of the (111)-oriented grain (Grain-1). Indeed, the fitted values for ωL,P1 (P1 peak) and ωZCP (diamond peak) do not vary considerably even as the electrolysis time is increased to 12 hrs (Table 1). This observation is contrary to previous assertion that the higher boron content in (111)-crystals instigates more degradation due to high electrochemical activity11.

Figure 2. Corrosion-induced changes in BDD as a function of electrolysis time: Optical images of BDD surface (a) before anodic treatment, and after (b) 0.5 hrs, (c) 2 hrs, (d) 6 hrs, and (e) 12 hrs of treatment. Scale bar corresponds to 5 μm. Time evolution of the Raman spectra at (f) (111) grain (Grain 1) and (g) (100) grain (Grain 2). (100) and (111) grains are highlighted in the optical images by the yellow square and circle, respectively.



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Table 1. Raman Shift in the P1 Peak and Diamond

ZCP Peak as a Function of Electrolysis time.

To further confirm this observation, we selected another (111)-oriented grain and plotted ωL,P1 as a function of spatial position across the grain before and after anodic treatment (t = 0.5, 2, 6, 12 hrs). Figure 3 shows the results. ωL,P1 is shown to be uniform (average ωL,P1 = 487.0 cm-1) across the (111)-facet, and unchanged even after 12 hrs anodic treatment. Near the edges, however, ωL,P1 increased with anodic treatment, signifying that the boron concentration is reduced due to corrosion.

Figure 3. Spatial line distribution (along the vertical direction) of the Lorentzian peak component of P1, ωL,P1, across a representative (111)-facet at each step of the anodic treatment. Each data point is an average of three adjacent points along the horizontal direction, and the error bars represent the standard deviation for each averaged value. 12 ACS Paragon Plus Environment

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On the other hand, optical images show obvious changes in the relative reflection intensity of the (100)-grain (Grain-2), indicating morphological transformations of the grain as the anodic treatment time is increased. It is important to note that after a total of 12 hrs of anodic treatment, a defect (crack) becomes evident at the center of the grain, indicating severe etching due to long electrolysis time. The Raman spectra measured on Grain-2 after anodic treatment is shown in Figure 2(g). After just 0.5 hrs of treatment, sp 2 carbon peak (G-mode at 1580 cm-1) is distinctly observed, accompanied by a strong increase in the broad background at the high wavenumber region. The broad luminescence originates from continuous distribution of states within the optical band gap of diamond introduced by the disordered forms of amorphous sp2 carbon38, 39. The sp2 carbon spectral features remain even after 2 hrs but disappear at longer times, suggesting that non-sp3 components are removed upon anodic corrosion11, 16. In addition, a broad peak around 2900 cm-1 associated with the stretching mode of CHx species is observed in Grain-2 after 2-hr anodic treatment. This peak is very broad (FWHM ~150 cm-1), but can be sufficiently fitted with two Lorentzian functions at 2928.7 cm1

and 2991.0 cm-1 with FWHM of 64.3 cm-1 and 34.0 cm-1, respectively (Figure S3). Previous

reports have likewise demonstrated that CHx peak with comparable features appeared only after anodic treatment 40, 41. The position of the P1 peak (ωL,P1) is also evidently shifted towards higher wavenumber, signifying depletion of boron with longer electrolysis time (Table 1). For Grain2, [B] decreases by a factor of 4 (from 8.2 x 1020 cm-3 to 1.9 x 1020 cm-3) after 12 hrs of treatment. In addition, the diamond ZCP peak position (ω ZCP) is shifted towards a higher wavenumber and the peak width becomes sharper, indicating a relaxation of the tensile stress on the diamond crystal due to the decline in the amount of dopants in the lattice. We performed polarized Raman spectroscopic measurements and examined the temporal evolution of the spectra on a total of 20 unique grain facets on the BDD surface. Out



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of seven (100)-faceted grains, five grains showed significant changes in the spectra due to corrosion, while only one out of thirteen (111)-faceted grains exhibited any change even after 12 hrs of anodic treatment. Raman spectral mapping We extend our Raman analysis of the effects of anodic treatments on polycrystalline BDD to large-area spectral mapping. While majority of previous studies of BDD films have been performed in small localized spots, spectral mapping can provide qualitative and quantitative assessment of the local distribution of the chemical and crystalline attributes of the diamond film. Here, we monitored the temporal evolution of the spectral maps corresponding to (1) fitted peak intensity of the diamond ZCP line, (2) ωZCP, (3) ωL,P1, and (4) peak intensity of the sp2 carbon peak at 1580 cm-1 (Figure 4). Each spectral map is composed of 75 pixels x 75 pixels corresponding to an area of 16.9 μm x 16.9 μm. The corresponding optical images are shown in the first (top) row of Figure 4.


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Figure 4. Optical (1st row) and Raman (2nd – 5th rows) images of the same location in the BDD film before and after successive anodic treatments. Green and red arrows in the optical images point to the center of representative (100) and (111)

grains, respectively. Square boxes and dashed circles on the Raman images highlight the (100) and (111) grains, respectively. (scale bar = 5 μm)

As expected, the intensity map of the diamond ZCP peak is non-homogeneous, showing the polycrystallinity of the film and heterogeneity in the doping of each crystal grain. Representative (100)-grains and (111)-grains are highlighted in the optical images by green arrows and red arrows, respectively. In the sample’s pristine state, (100)-oriented grains are observed to have higher crystallinity compared to (111)-oriented grains, as evidenced by the



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high diamond ZCP fitted peak intensity and the small deviation of its peak position from that of the undoped diamond (1332 cm-1). In addition, ωL,P1 is relatively higher at these grains, indicating low amount of dopants based on equation (1). Similar trend is observed at this large area: (1) a significant amount of sp2 carbon is detected exclusively on (100) grains (black arrows in Figure 4) and disappears after longer anodization, and (2) a shift towards higher wavenumber for both diamond ZCP and P1 peaks can be observed particularly in (100)-oriented grains and near the grain boundaries of most grains as the anodic treatment is performed longer. We measured the Raman spectra in four different areas in the sample (Area A, B, C and D as shown in Supplementary Figure S4) and monitored the temporal evolution of the spectral attributes in each area. Each measured area consists of 400 x 80 pixels2 (90 μm x 18 μm) comprising of 32000 spectra. Supplementary Figures S5 – S8 show the temporal evolution of the Raman spectral maps of P1 peak position in the four areas. We calculated the mean ωL,P1 and the corresponding average doping concentration [B] for each one. Figure 5 shows the trend of the average [B] as a function of the electrolysis time. The large standard deviation at the 0.5hr data points, especially for Area C, is due to the abundance of grains with broad and strong luminescence due to amorphous sp2 carbon (Figure S11). As the treatment progressed, the doping concentration is reduced in all areas. In Area A, which included the analyzed area in Figure 4, the average [B] continuously decreased from 4.7 x 1020 cm-3 in its pristine state to 1.5 x 1020 cm-3 after 12 hrs. The reduced average doping may explain the diminished electrochemical activity due to anodization as reported previously9,14,15. Variation in the


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amount of degradation for the various areas may be attributed to the inhomogeneity of the film attributes.

Figure 5. Temporal evolution of the average boron concentration [B] calculated from the mean Raman shift of the P1 peak measured at four different locations on the BDD film. Bars represent the standard deviation.

We note, however, that not all (100)-oriented grains in the BDD film are observed to be affected, and several grains are modified only after longer anodization. In addition to the P1 spectral maps, we show the temporal evolution of the Raman spectral maps of the sp 2 carbon G-mode intensity in Supplementary Figures S9 – S12. The temporal evolution of the spectra taken from two other representative (100)-grains in sample area C are likewise shown in Supplementary Figure S13. As shown in the spectral maps, a few (100)-grains in Area C, for example, are graphitized as early as 0.5 hrs of treatment, but [B] in these grains are not significantly reduced. XPS measurements In order to complement and extend the chemical information provided by Raman spectroscopic measurements, high-resolution core-level C1s XPS spectra were measured after 2 hrs, 6 hrs and 12 hrs of anodization and were compared to that of pristine BDD film. The X-



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ray beam spot diameter is 1 mm. While the spatial resolution of XPS is poor compared to Raman spectroscopy, it benefits from its extreme sensitivity to elemental composition and chemical states present on the surface. Figure 6 shows the C1s spectra and the corresponding deconvoluted constituent peaks. The evolution of the relative percentages of the chemical components on the surface as a function of the electrolysis time is plotted in Figure 7.

Figure 6. C1s XPS spectra of the BDD film measured on the same spot before (pristine) and after successive treatments.


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Figure 7. Temporal evolution of the surface functional groups.

In the pristine state, the main component (centered at 285.2 eV) is attributed to bulk sp3 carbon species, corresponding to the C-C single bond in the diamond surface and, possibly, a contribution from C-H from the hydrogen termination of the film20. Small contribution from C-O groups can also be observed which may be ascribed to the natural oxidation of the film surface from exposure to air. No sp2 C=C peak is observed. After 2 hrs of anodic treatment, a significant amount of sp2 C=C (284.5 eV) is detected, replacing some sp3 carbon on the surface as shown by the decrease in the sp3 C-C concentration. At longer electrolysis time, the sp2 C=C concentration lessens but does not completely disappear – a result of the non-uniform degradation rate within the measured area. The slight increase in the sp3 C-C after 12 hrs suggests that the surface have been etched, revealing the sub-layer sp3 carbon atoms. Interestingly, carbonyl (C=O) peak at 287.2 eV is detected and it is observed to significantly progress with anodization. This peak only appears after electrochemical treatment, and increases in concentration at longer treatment times. It has been previously shown that carbonyl cannot reasonably exist in unreconstructed (111) diamond surface 25, 42, 43. Thus, the evolution of the carbonyl species on the surface must be correlated to the preferential degradation of (100)-grains as observed in optical imaging and Raman studies. Other oxidized 19 ACS Paragon Plus Environment


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species O-C=O was also observed to increase after electrochemical treatment, albeit in small amount. The O-C=O group may arise from carboxyl (COOH) which is a likely intermediate in the corrosion of the BDD surface13. Corrosion mechanism in BDD The electrochemical oxidation of acetic acid has been previously explained by Kapalka et al using differential electrochemical mass spectrometry (DEMS)44. Surface-adsorbed CH3COOH oxidizes to CO2 through its decomposition to methyl radicals (CH3·) via successive H-abstraction by hydroxyl radicals (·OH) formed from the oxidation of the aqueous electrolyte. It has been previously shown that the CH3· intermediate is an essential initial step in the corrosion process11. In the present results, the small manifestation of the CH x Raman peak measured on the BDD surface after anodic treatment along with other spectral changes confirm its importance in the process. In parallel, ·OH also reacts with the BDD to create an OH-terminated surface13. Methyl radicals from the surrounding electrolyte then abstract -OH from this surface, leaving carbon dangling bonds11. Dangling carbon can reconstruct to form double bonds with adjacent carbon atoms and rehybridize from sp3 to sp2. Because of the differences in the surface structures of the (100) and (111) diamond facets [Figure 8 (a)], different activation energies and possibly different pathways are needed for graphitization to occur 45, 46. The anodization parameters particularly employed in this work seem to favor graphitization in (100)-facets, possibly due to the two dangling bonds for a single surface carbon atom in these facets compared to one available bond in (111)-facet surface atom. Surface defects and application of even higher current density or other anodization parameters may activate the graphitization process in (111)-facets.


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Figure 8. (a) Schematic diagram of the idealized (100) and (111) diamond facets showing carbon dangling bonds. (b) Proposed mechanism for anodic corrosion of diamond.

While glassy carbon electrodes have been previously shown to be easily electrochemically etched due to weaker bonding of the top layer carbon atoms to the subsurface atoms47, our results indicate that carbonyl formation may play an even more important role in the corrosion process [Figure 8 (b)]. Likewise due to the available bonds on the surface, carbonyl can only occur in (100) diamond surface or at the edges of the (111) grains. Carbonyl can form from the interaction of the dangling bonds of surface sp 3 carbon atoms with ·OH or via the graphitized surface. Previous reports based on DFT calculations suggest that carbonyl can be further oxidized to form carbonic acid (H2CO3) via carboxyl (COOH), depending on the functionality of neighboring carbon atoms13. H2CO3 then dissociates to the surrounding electrolyte solution, etching the surface and exposing the sub-layer sp3 carbon. Raman measurements showed that boron atoms are removed from the lattice later in the process (in this case, starting at 6 hrs), and are continuously removed as the anodic treatment is prolonged. Similar reduction in boron content after 12 hrs anodic treatment was observed near the surface region (depth < 0.40 μm) using glow discharge optical emission spectroscopy11. Because boron is more energetically stable at the subsurface layers48, 49, we suppose that it is exposed and removed only after the upper carbon atoms are etched from the surface. Due to 21 ACS Paragon Plus Environment


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its instability at the surface, boron can easily detach from the lattice by reacting with hydroxyl or other functional groups that are present in the solution. Further studies, however, are needed to support this hypothesis. Nevertheless, it is also important to note that while boron atoms may not be directly involved in the etching of the diamond surface, previous reports have suggested that substitutional boron may help in enhancing the electrochemical activity of carbon by weakening the C-C bonds in the lattice50 . CONCLUSIONS Anodic corrosion was investigated in highly-doped polycrystalline BDD film under high-current density anodic treatments. Systematic assessment of the morphological and chemical attributes of the BDD described the chronological progression of corrosive degradation. Raman and XPS measurements confirmed that the degradation starts via the graphitization of sp3 carbon on the surface by methyl radicals. Further reactions with ·OH creates carbonyl groups specifically on (100)-facets and edges of (111)-facets. Raman imaging showed that as the sp3 carbon atoms are etched from the surface, sub-surface boron dopants are detached from the lattice as the BDD is treated longer. The observed grain-orientation dependence of the corrosion process in BDD offers new and essential insights into the chemical interactions happening on the surface of a BDD electrode, allowing for future works to be geared towards minimizing degradation without sacrificing its efficiency. Moreover, the combined sensitivity of high-resolution Raman spectroscopy and XPS to the corrosion-induced modifications in BDD allows for a potentially important modality for analyzing diamond surfaces as well as other electrodes. SUPPORTING INFORMATION Details about the fitting functions used in this work and other supplementary figures are included in the Supporting Information. 22 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS This work is supported by the Japan Science and Technology Agency (JST) under the ACCEL project entitled, “Fundamentals and Applications of Diamond Electrodes.” Raman spectral mapping was done using the RAMAN-11 facility at the Synthetic Organic Chemistry Laboratory in RIKEN with the help of Dr. Keisuke Dodo. We also gratefully acknowledge Dr. Koji Sugioka, Dr. Jian Xu and Dr. Daniela Serien of the RIKEN-SIOM Joint Research Unit for their help using the femtosecond laser micromachining facility. REFERENCES (1)Kraft, A. Doped Diamond: A Compact Review on a New, Versatile Electrode Material. Int. J. Electrochem. Sci.2007, 2, 355 – 385. (2) Suzuki, A.; Ivandini, T. A.; Yoshimi, K.; Fujishima, A.; Oyama, G.; Nakazato, T.; Hattori, N.; Kitazawa, S.; Einaga, Y. Fabrication, Characterization, and Application of Boron-Doped Diamond Microelectrodes for In Vivo Dopamine Detection. Anal. Chem. 2007, 79, 8608 – 8615. (3) Preechaworapun, A.; Ivandini, T. A.; Suzuki, A.; Fujishima, A.; Chailapakul, O.; Einaga, Y. Development of Amperometric Immunosensor Using Boron-Doped Diamond with Poly(o-Aminobenzoic Acid). Anal. Chem. 2008, 80, 2077 – 2083. (4) Comninellis, C.; Kapalka, A.; Malato, S.; Parsons, S. A.; Poulios, I.; Mantzavinos, D. Advanced Oxidation Processes for Water Treatment: Advances and Trends for R&D. J. Chem. Technol. Biotechnol. 2008, 83, 769 – 776. (5) MacPherson, J. V. A Practical Guide to Using Boron Doped Diamond in Electrochemical Research. Phys. Chem. Chem. Phys. 2015, 17, 2935 – 2949. (6) Kalish, R. Doping of Diamond. Carbon 1999, 37, 781 – 785.



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Facet-Dependent Temporal and Spatial Changes in Boron-Doped

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