Influence of Surface Orientation on Electrochemical Properties of

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Influence of the Surface Orientation on the Electrochemical Properties of Boron-Doped Diamond Tribidasari A. Ivandini, Takeshi Watanabe, Takahiro Matsui, Yusuke Ootani, Shota Iizuka, Ryo Toyoshima, Hideyuki Kodama, Hiroshi Kondoh, Yoshitaka Tateyama, and Yasuaki Einaga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10406 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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The Journal of Physical Chemistry

Influence of the Surface Orientation on the Electrochemical Properties of Boron-Doped Diamond Tribidasari A. Ivandini,1 Takeshi Watanabe,2 Takahiro Matsui,3 Yusuke Ootani,4 Shota Iizuka,5 Ryo Toyoshima,3 Hideyuki Kodama,2 Hiroshi Kondoh,3 Yoshitaka Tateyama5 and Yasuaki Einaga*3,6 1Chemistry

Department, Faculty of Mathematics and Sciences, University of Indonesia, Kampus UI Depok, Jakarta 164424, Indonesia. 2Department of Electrical Engineering and Electronics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa, 252-5258, Japan 3Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan. 4Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan 5Center

for Green Research on Energy and Environmental Materials and International Center for Materials Nanoarchitectonics, National Institute for Material Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 6JST-ACCEL, Hiyoshi 3-14-1, Yokohama 223-8522, Japan.

ABSTRACT: In order to study the influence of crystal orientation on the electrochemical properties of boron-doped diamond (BDD), electrodes comprising (100) and (111) homoepitaxial single crystal layers of BDD were investigated and these were compared with a thin polycrystalline BDD electrode. BDD samples with similar amounts of boron of around 1020 cm-3 and resistivity of around 6 x 10-3 Ω cm were prepared. Evaluation of the electrochemical reactivity of each of the samples with both H- and O-terminated surfaces showed that polycrystalline BDD was the most reactive, while for single crystal BDD, the (111) samples proved to be more reactive than the (100) ones. Moreover, considering the results of firstprinciples molecular dynamics simulations, it is proposed that surface transfer doping is the dominating factor for Hterminated surfaces, whereas the degree of band bending and the thickness of the space-charge layer are the dominating factors for O-terminated surfaces.

Introduction Polycrystalline boron-doped diamond (BDD) electrodes prepared by chemical vapor deposition methods, are well documented as alternatives to conventional solid electrodes due to their exceptional chemical and dimensional stability in harsh conditions, their very low capacitance, and their very wide potential window for water splitting.1–3 These properties have established polycrystalline BDD as the perfect option for applications in electroanalysis,4–13 electrocatalysis13–16 and electrosynthesis. 17–21 The characteristics of polycrystalline BDD electrodes have the intrinsic characteristics of diamond; however, the influence of some other factors needs to be considered, including the boron concentration,22–25 the surface termination,25–31 as well as sp2 impurities at the grain boundaries.22,32–34 Moreover, polycrystalline films contain

various crystal facets, which are considered to affect the electrochemical behaviour.,35–38 While many experimental and theoretical reports regarding the influence of the boron concentration and the surface termination on the electrochemical properties of BDD have been published,22,24,28,29,31 only a limited number of studies have been performed on the effect of different crystal facets.35–37 Diamond crystallites synthesized by CVD methods have well-defined crystal habits of regular polyhedra such as cube, octahedron, or cubo-octahedron with (100) and/or (111) facets.39–42 For the electrochemical application of polycrystalline BDD, therefore, it is important to elucidate the electrochemical properties of crystal facets such as (100) and (111) and to establish the process to control the crystal morphology. Kondo et al. investigated the influence of (100) and (111) crystal facets on the electrochemical behaviour of BDD using homoepitaxial single crystal electrodes.36 The results suggest that (111) facets are more reactive than (100)

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facets.36 However, the comparison was performed with samples containing different amounts of boron, as it was reported that the homoepitaxial single-crystal BDD films [i.e. (111) and (100)] had been deposited by means of microwave plasma-assisted chemical vapour deposition (MPACVD) using the same precursor solutions containing acetone, methanol and dissolved B2O3 with a fixed B/C atomic ratio of 1:100.36 Several studies reported that (111) surface typically contain approximately one order of magnitude higher boron concentrations than (100) surface when using precursors with the same B/C ratio.43–46 Pleskov et al. also investigated electrochemical behaviours of semiconductor BDD electrodes using homoepitaxial films oriented as (100), (111), and (110) faces.37 As with the work by Kondo et al.36, their results also showed similar trend for the films grown under constant boron concentration in the feeding gas, due to the different boron concentration in their films.37 In addition, they reported no significant difference in the kinetic behaviour between the equally doped (100)- and (111)-faces of semiconductor BDD homoepitaxial films with acceptor concentration of 6×1018 cm-3.37 The purpose of this work is to clarify the influence of the crystal orientation on the electrochemical behaviour of semi-metallic BDD with different surface terminations. Accordingly, (100) and (111) homoepitaxial single-crystal BDD with similar amounts of boron, as well as polycrystalline BDD films were prepared using resistivity as an indicator. High boron doping at concentrations of around 1020-1021 cm-3 were studied, as these doping levels are commonly employed in many applications.1,47 The investigation of the electrochemical behaviour was performed using inner- and outer-sphere redox markers, i.e. [Fe(CN)6]4-/3- and [Ru(NH3)6]2+/3+, respectively. Furthermore, the effect of surface termination was also investigated as it has previously been reported that a significant change from a reversible to an irreversible CV was observed in the [Fe(CN)6]4-/3- redox couple with the change in surface termination of the electrode.31 The results showed that at both H- and O-terminated BDD the highest reactivity was observed for polycrystalline BDD followed by single crystal BDD with (111) orientation, and finally that with (100) orientation. The main reason for the high reactivity is the heterogeneity of the boron concentration in polycrystalline BDD, which behaves like a micro electrode array with more highly boron-doped grains with (111) facets exposed than (100) facets. On the other hand, the differences between the single crystal BDD samples clearly demonstrate that the surface orientation influences the electrochemical properties rather than the difference in boron concentration. Density functional theory-based molecular dynamics simulations suggest that the different electronic band structures for the two different orientations give rise to the difference in electrochemical reactivity between them. Experimental

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Chemicals. Methane (CH4), hydrogen (H2) and hydrogen-diluted trimethoxyborane (TMB) were used as the source gases for growing diamond. These were purchased form Toyoko Kagaku Co., Ltd. All other chemicals were purchased from FUJIFILM Wako Pure Chemical Corp and utilized without further purifications. Synthetic (100) single-crystal diamond substrates were purchased from Element Six Ltd., while synthetic (111) single-crystal diamond substrates were purchased from SYNTEK Co. Ltd. Preparation and characterization of the singlecrystal and polycrystalline BDD electrodes. Single crystal BDD was deposited on the synthetic type Ib (100) and (111) single-crystal diamond substrates. The sizes of the substrates were 3.0×3.0×0.3 mm3 and 2.5×2.5×0.3 mm3, respectively. The polycrystalline BDD was deposited on (100) silicon wafers (2 inch diameter, 0.75 mm thick, p-type, low resistivity < 0.1 Ωcm). Prior to the deposition of the polycrystalline films, the silicon substrates were scratched using nanodiamond particles. The deposition was performed using an MPACVD system (AX6500, Cornes Technologies Corp.). CH4 and TMB were used as the carbon and boron sources, respectively, while H2 was used as the carrier gas. The substrate temperature during growth was monitored by dual wavelength optical pyrometer (Williamson). The surfaces of the single crystal diamond substrates were cleaned according to a procedure given in the literature.48 The quality of the BDD was estimated by Raman spectroscopy (Acton SP2500, Princeton Instruments), with excitation at 532 nm from a green laser diode, while a confocal laser-scanning microscope (VK-X200, Keyence) was used to observe the surface morphology of the polycrystalline BDD. To measure the boron concentration, secondary ion mass spectroscopy (SIMS, IMS-7f CAMECA) was utilized. The chemical bonds at the surface were characterized using X-ray photoelectron spectroscopy (XPS, Shimadzu, Kratos Axis-Ultra) using Al Kα X-rays. The oxygen to carbon (O/C) ratios were calculated from peak areas of O 1s and C 1s in wide-scan XPS spectra using the relative sensitive factors of 0.78 and 0.278, respectively. The resistivity was calculated by the Van der Pauw method using a Keithley source meter 2400 and a Keithley nanovoltmeter 2182A. Before the resistivity measurements of the polycrystalline films, the silicon substrates were removed using a mixed solution of HF and HNO3. The film thicknesses were estimated from the SIMS depth profiles and cross-sectional images using a field emission scanning electron microscope (FE-SEM, SIRION FEI). Low energy electron diffraction (LEED) patterns from the surfaces of the (100) and (111) homoepitaxial single crystal BDD films were measured to ensure the crystallinity and orientation (Figure S1). Electrochemical Measurements. The electrochemical measurements were carried using a three-electrode system in a single compartment cell. The BDDs were employed as working electrodes (WE), while a Pt wire and


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an Ag/AgCl (saturated KCl) were used for the counter electrode (CE) and the reference electrode (RE), respectively. The WE was pressed against the bottom of the electrochemical cell with a Viton O-ring (dia. 2.5 mm) as shown in Figure 1. To provide the electrical contact, a brass plate was placed under the polycrystalline BDD electrode, while gold wires and silver paste were utilized for the single crystal BDD electrodes. To obtain H-terminated surfaces, the BDDs were exposed to a hydrogen plasma after washing in aqua regia, while the conversion from H- to O-termination was carried out by an anodic treatment in 0.5 M sulphuric acid at 3.0 V for 10 min. All electrochemical measurements were performed at room temperature (~ 25oC) using a potentiostat (HZ-5000, Hokuto Denko Corp.) with 1.0 M KCl used as the electrolyte. Theoretical calculation. Density functional theory (DFT)-based first molecular molecular dynamics (FPMD) simulations were performed for the BDD-water interfaces using (111) and (100) surface models containing 254 carbon and 2 boron atoms with the presence of 105 water molecules. The equilibrium trajectories were obtained by the FPMD samplings for about 10 psec. For the 20 selected structures in the trajectories, DFT calculations of the projected density of states (pDOS) were carried out and the averages determined.

Potentiostat CE RE

(111) homoepitaxially BDD compared to that of (100) ones as observed in previous studies.36,37 Therefore, in order to prepare BDD samples with similar amounts of boron, the concentration of boron in the precursor for growing single crystal (100) BDD needs to be increased. After some trials with various conditions, the conditions to deposit BDD with comparable resistivity were fixed as displayed in Table 1. The BDD films produced under these conditions showed the same order of resistivity of around 6 x 10-3 Ω cm (estimated using the van der Pauw method and film thickness from SEM images). This resistivity remained the same before and after oxidation. Therefore, changes in the electrical properties before and after oxidation can be neglected.

Table 1. Parameters used to deposit single crystal and polycrystalline BDD films using MPACVD. Single Crystalline (100) B/C Ratio in gas precursor (%)

1

0.1

0.3

Microwave Power (W)

6000

6000

6000

Substrate Temperature (℃)

950

850

1000

Chamber Pressure (Torr)

85

80

85

H2 flow (sccm)

511.4

530

530

CH4 flow (sccm)

20

10

20

TMB flow (sccm)

20.6

1

6

O2 flow (sccm)

3

0.5

1.5

Time (h)

4

4

4

Resistivity (Ω cm)

5.8 x 10-3

6.8 x 10-3

6.2 x 10-3

Thickness (µm)

12.6

4.8

6.2

O-ring Electrode 2.5 mm

(111)

Polycrystalline

WE

Figure 1. Schematic illustration of the electrochemical cell. Results and discussion Preparation and Characterization of Single Crystal and Polycrystalline BDD Electrodes It has been reported that when precursors with a high boron concentration are used in MPACVD, the dominant facets are (111), due to the much slower growth rate in the [111] direction compared to the [100] and [110] directions.49 Such a difference in the growth rate is the probable reason for dependence of the incorporation efficiency of boron on the surface orientations as it has been previously reported that employing a precursor with the same B/C ratio generally provides (111) faces with a boron concentration approximately 10 times higher than that of (100) faces.43–45 Consequently, CVD growth with constant condition leads to the higher conductivity of



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indicate that the deposited BDD layers were of comparable quality. Furthermore, Raman spectra of the polycrystalline film are shown in Figures 2(c) and (d). The (100) and (111) crystal facets marked by a green square and a yellow triangle, respectively, in the image in Figure 2(e) were selected to be measured. The relative intensity of the peak for (100) BDD at 1330 cm-1 is clearly higher than that for (111), whereas the peak for disordered diamond in the (100) spectrum is lower than that in the (111) spectrum. The spectra suggest that there is a much higher boron concentration in grains with the (111) facet exposed than in grains with the (100) facet exposed.35,36 The spectrum of the (111) facet is quite similar to typical spectrum of heavily boron-doped diamond with boron concentration higher than 1021 cm-3. These results are typical of those found for (100) and (111) facets all over the surface of the polycrystalline BDD. Therefore, since the surface contains mostly (100) and (111) facets (Figure 2(e)), the heterogeneity of the boron doping concentration at the polycrystalline BDD surface is very high. 　

Figure 2. Raman spectra of (a) (100) and (b) (111) single crystal BDD as well as the (c) (100) and (d) (111) crystal facets of different grains in the polycrystalline film. Figure (e) shows a confocal laser scanning microscopy image of the polycrystalline BDD, with marks showing the areas where the measurements were made for the spectra shown in (c) and (d). SIMS was utilized to measure the amounts of various elements in the deposited BDD layers (Figure S2). SIMS was performed at the end of all characterizations since the samples are destroyed during the analysis. The amounts of carbon and boron in each of the samples were of comparable order. Boron concentrations of 4.2 × 1020 and 3.8 × 1020 cm-3, respectively, were observed for the (100) and (111) single-crystal BDD layers, while the concentration in the polycrystalline BDD film was 7.6 × 1020 cm-3. Although the polycrystalline film had a higher boron concentration, the resistivity was similar to the single crystal films. This is due to the resistivity on the grain boundaries and heterogeneity of boron concentration. Raman spectroscopy measurements were performed for the single crystal and polycrystalline BDD samples (Figure 2). For all the BDD samples, the typical sharp peak of diamond can be seen at around 1330 cm-1 together with a couple of peaks at around 500 and 1200 cm-1. This couple of peaks are generally observed in medium- to high-doped BDD,27 and indicates the formation of a disordered diamond film due to the presence of boron. 1,18,25 No peak for amorphous or the sp2 configuration at around 1500 cm-1 was observed for any of the samples indicating that the quality of all the BDD samples was high.18,25 Relatively similar peaks for the sp3 configuration and disordered diamond, which appear in the (100) and (111) single crystal BDD spectra (Figure 2(a) and (b)),

Electrochemical properties of BDD electrodes with (100) and (111) orientations The electrochemical behaviour of BDD electrodes was studied using solutions of K4[Fe(CN)6] and [Ru(NH3)6]Cl3 as inner- and outer-sphere redox markers, respectively. All the different types of BDD electrode with both H- and Otermination were examined. As-deposited BDD given a hydrogen plasma treatment was used for the H-terminated BDD, as it is generally known that the surface of asdeposited BDD contains some oxygen termination after exposure to the ambient.38 The as-deposited polycrystalline BDD contains oxygen with an O/C ratio of around 0.02 from wide-scan XPS spectra. The Oterminated BDD was prepared by anodic treatment of the H-terminated BDD at +3.0 V in 0.1 M H2SO4 using a potentiostat to get the saturated oxygen on the surface. This treatment increased the O/C ratio up to around 0.09 in the polycrystalline BDD film. Both of single crystalline (100) and (111) BDD showed O/C ratio of around 0.06 after the anodic oxidation. These values contain contribution of carbon atoms in subsurface regions but not only at surface. Lower values of single crystalline films compared to polycrystalline films is possibly attributed to difference in surface roughness. Figure 3 shows XPS spectra of the O-terminated surfaces for (100) and (111) single-crystal BDD after deconvolution of the carbon 1s peak (C 1s) at binding energies around 280-290 eV. The horizontal axis was calibrated with Au 4f7/2 using gold films evaporated on the surface of the BDD. For both (100) and (111) BDD electrodes (Figure 4(a) and (b), respectively), the sharp, high peak due to sp3 can be seen, at 284.2 eV and 284.6 eV, respectively. The smaller peak at lower binding energy in each of the spectra is attributed to sp2–bonded carbon50 and peaks for C-O-C or C-OH can also be seen at higher binding energies (+1.0-1.2 eV greater than the sp3 peak).51 However, in comparison with the sp3 peak, the latter peak


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at 285.4 eV for the (100) surface (Figure 3(a)) is relatively high, whereas for the (111) surface this peak is relatively small compared to the sp3 peak (Figure 3(b)), indicating that (100) BDD is easier to be oxidized than (111) BDD. Otherwise, it may be attributed to a difference in escape depth of photoelectron between (100) and (111) surfaces. It should be noted that a C=O peak at a binding energy of 286.9 eV can be seen in the spectra for the (100) BDD electrode, while it was not observed for (111) BDD. This difference can be due to the different maximum number of oxygen atoms that can be presented to each orientation. The absence of this peak for the (111) surface is reasonable as the (100) surface has two-dangling C bonds, while the (111) surface has only one, so is unable to form C=O. The comparative amounts of the different bonding types for C 1s spectra of the (100) and (111) surfaces are summarized in Table 2.

sp3

sp3

C-OH/ C-O-C

C-OH/ C-O-C

sp2

C=O

response of the (111) BDD electrode is comparable to the response of the polycrystalline one. Consistent with these peak separations, the current observed with the (100) electrode is also lower, while they are similar for the (111) and polycrystalline electrodes.

sp2 (b)

(a)

Figure 3. Deconvoluted XPS C1s spectra of (a) (100) and (b) (111) single crystal BDD after anodic treatment at +3.0 V (vs. Ag/AgCl) for 10 min. Table 2. Percent peak area of the XPS C1s spectra for (100) and (111) single crystal BDD after anodic treatment. (100)

(111)

sp2

5.1

4.7

sp3

70.5

84.7

C-OH/C-O-C

19.9

10.6

C=O

4.5

N.A.

Cyclic voltammograms (CVs) of [Fe(CN)6]4-/3- in 1.0 M KCl with single crystal and polycrystalline BDDs are shown in Figure 4. The CVs for both H- and O-terminated surfaces are shown. For the H-terminated surfaces, the behaviour of the CVs is similar although the peak separation (ΔEp) of 0.220 V observed with the (100) electrode is larger than the those with the (111) and polycrystalline electrodes (0.070 V and 0.063 V, respectively). Since all the BDD electrodes were prepared with similar conductivities, the differences in ΔEp between both single crystalline BDD films can be due to the lower reactivity of the (100) surface compared to the (111) surface. On the other hand, the

Figure 4. Cyclic voltammograms of 1.0 mM K4[Fe(CN)6] in 1.0 M KCl with (a) 100 and (b) 111 single crystal BDD electrodes, as well as (c) polycrystalline BDD. The scan rate was 100 mV/s. The [Fe(CN)6]4-/3- redox couple is well known as producing an inner-sphere redox system with carbonbased electrodes, in which the electron transfer occurs through interactions with the reactive sites on the electrodes. The impurities, surface defects, and surface functional groups on the electrodes are generally proposed to be the reactive sites.38 The minor differences in the electrochemical behaviour of the [Fe(CN)6]4-/3- couple for all BDD electrodes with H-termination in Figure 4, suggest



that the BDD electrodes employed in this work are relatively clean, defect-free, and contain similar functional groups. On the other hand, for O-terminated BDD, which contains some C-O bonds as well as C=O bonds, the reactivity is generally lower, a reflection of the sluggish kinetics of the redox reaction. As can be seen in Figure 4, greater ΔEp with lower peak current is typically observed for O-terminated rather than H-terminated BDD electrodes. However, the magnitudes are in the same order as those for H-terminated BDD, with the largest value of ΔEp being 2.70 V for the (100) electrode, followed by 1.47 V for the (111) electrode, and 0.15 V for the polycrystalline electrode. Although the values of ΔEp for O-terminated BDD are greater than those for H-terminated BDD and the corresponding peak currents are smaller, the difference for the polycrystalline electrode is very small in comparison to the differences for the single crystal electrodes. It is possible that grain boundaries and minor plane orientations such as (110) and (311) facets play a role as reaction sites in the O-terminated polycrystalline BDD films. However, it is more likely that the heterogeneity of the boron concentration in the polycrystalline BDD is the reason for higher electrochemical activity compared to the single crystalline BDD. Raman spectra indicated that the (111) facets in the polycrystalline BDD have a higher boron concentration than the (100) ones and both of single crystalline BDD. Since the electrochemical properties are generally dominated by the more highly conducting component, the polycrystalline BDD electrode can act as a micro-array or partially blocked electrodes,52 resulting in higher electrochemical activity than the (111) and (100) single crystal electrodes. As opposed to H-termination with similar functional groups, the XPS results for O-termination in Table 2 show that (100) surfaces have more oxidized sites than (111) surfaces. Therefore, slightly higher electrostatic repulsion of the negatively charged [Fe(CN)6]4-/3- was expected. However, the effect of this is considered to be small because all these oxidized surfaces have similar negative charge. Nevertheless, the decrease in the ability of the redox couple to interact with the electrode due to the presence of the oxidized surface is considered to be one of the reasons for the difference in reactivity between the (100) and (111) surfaces. Furthermore, the electronic density of states near the Fermi energy level affects the electrochemical behaviour. Similar behaviour for the H-terminated BDD electrodes was found for an outer-sphere redox system, i.e. with [Ru(NH3)6]2+/3+. The values of ΔEp are 0.205 V with the (100) electrode, 0.114 V with the (111) electrode, and 0.063 V with the polycrystalline electrode (Figure 5). It is generally assumed that outer-sphere electrode interactions can be neglected. Therefore, the electrochemical behaviour of [Ru(NH3)6]2+/3+ is sensitive to the density of states. Particularly for semimetal electrodes such as BDD, the electrochemical properties depend greatly on the density of states. The differences between the peak separations observed for the H-terminated

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surfaces are probably due to differences between the electronic densities of states near the Fermi energy. After anodic oxidation, both single crystal BDD electrodes have wider peak separations for [Ru(NH3)6]2+/3+ than H-terminated surfaces, similar to the results for [Fe(CN)6]4-/3-. Because single crystal BDD with a boron concentration of 4×1020 cm-3 has insufficiently high carrier density, a space charge layer can be formed depending on the relative energy position with respect to the redox potential of the electrolyte. As will be described in detail later, the formation of a space charge layer is considered to be the main reason for the sluggish kinetics at the Oterminated surfaces of both types of single crystal BDD.

Figure 5. Cyclic voltammograms of 1.0 mM [Ru(NH3)6]Cl3 in 1.0 M KCl with (a) (100) and (b) (111) single-crystal BDD electrodes and (c) a polycrystalline BDD electrode. The scan rate was 100 mV/s.


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With regard to the polycrystalline surface (Figure 5(c)), the peak separations as well as the peak currents for the [Ru(NH3)6]2+/3+ system are not significantly different between H- and O-terminated BDD, suggesting high reactivity with both terminations. This can also be related to the heterogeneity of the boron concentration for the (100) and (111) crystal facets, similar to [Fe(CN)6]4-/3-. In polycrystalline BDD, the B content of the exposed (111) facets is sufficiently high for it to exhibit metallic behaviour (>1021 cm-3).25 Therefore, it is considered that a space charge layer is not formed beneath the (111) surfaces even after anodic treatment, opposed to the (100) facets, resulting in heterogeneous distribution of electrochemical activity. Such heterogeneities in polycrystalline BDD derived from grain-dependent boron uptake53 and crystallographic structure,54,55 have been reported. From the microscopical image (Fig. 2e), the surface of the polycrystalline BDD used in this work comprises of predominant grains with relatively large (100) or (111) facets (> 2μm of the side length) and more irregular shaped grains. When the (111) facets behave as electro-active zones, this polycrystalline BDD can be roughly regarded as a microelectrode array of more than 2-µm-diameter discs with center-to-center nearest separations of less than 10 µm. According to study by Compton et al.,52 such a geometry is expected to sufficiently overlap the diffusion layers of the discs and behave like an electrochemically active macroelectrode, which is consistent with the experimental results.　 A summary of the peak separations for the [Fe(CN)6]4/3- and [Ru(NH ) ]2+/3+ systems with the various types of 3 6 BDD electrodes and various terminations is displayed in Table 3. Comparing the (100) and (111) single crystalline BDDs, the (100) BDD clearly shows more sluggish response also for [Ru(NH3)6]2+/3+ system. Accordingly, it is suggested that the electronic energy structure at the surface is rather influenced by the orientation and bonding structure of the surface.

A first theoretical study of BDD interfaces based on FPMD recently reported by our group56 concluded that the water interface with H-termination has a reductive character, which conforms with the surface transfer doping model proposed by Maier et al.57 On the other hand, the interface covered by carbonyl oxygen is oxidative; nevertheless, the behaviour of hydroxyl termination is less oxidative due to the influence of the hydrogen-bonding network provided by the termination groups. The FPMD simulations of H-terminated (100) and (111) surfaces indicate similar pDOSs where the energies of the valence band maxima (VBM) of the BDD surfaces are much higher than those of water (Figure 6), confirming the reductive character with surface transfer doping. This similarity can account for the similar peak separations shown in Table 3, while slightly higher VBM for the (111) surface reflects the higher reactivity of this surface. On the other hand, the pDOSs of O-terminated (100) and (111) surfaces, shown in Figure 7, indicate that the VBM positions are much lower than the H-terminated cases, probably due to the interfacial dipole, and the tendency of surface transfer doping is significantly suppressed at the O-terminated surfaces. These explain the less reactivity of the O-terminated surfaces, demonstrated in Table 3. Comparing between the (100) and (111) surfaces, we found in Figure 7 that the (100) VBMs are lower in energy than the (111), which can account for the larger peak separation (less reactivity) of (100) surface, as shown in Table 3.

Table 3. Summary of the peak separations for the [Fe(CN)6]4-/3- and [Ru(NH3)6]2+/3+ systems with various types of BDD electrodes. Peak Separation / V System

Surface

Single Crystalline

Polycrystalline

(100)

(111)

H-termination

0.22

0.07

0.06

O-termination

2.70

1.47

0.15

H-termination

0.20

0.11

0.06

O-termination

1.83

0.87

0.07

Figure 6. The calculated projected densities of electronic states for H-terminated BDD/water interfaces at the (a) (100) and (b) (111) surfaces. The energy origin is set to the valence band maxim (VBM) of water, and the arrow designates VBM of the BDD.

[Fe(CN)6]4-/3-

[Ru(NH3)6]2+/3+



Figure 7. The calculated projected densities of electronic states for the O-terminated BDD/water interfaces at (100) surfaces with OH termination and Obridge ((a) and (b) respectively), and (c) at (111) surface with OH termination. The energy origin is set to the VBM of water, while the arrow designates VBM of the BDD. Schematic electronic band diagrams for O-terminated BDD (100) and (111) interfaces are proposed in Figure 8, where BDD and water are indicated in the left- and righthand sides, respectively. The VBM, conduction band minimum and Fermi energy are shown with Ev, Ec and EF, respectively, while the target redox potentials of [Ru(NH3)6]2+/3+ and [Fe(CN)6]4-/3- are represented by Eredox symbolically, which are usually located in the band gap of the O-terminated diamond.2 In the BDD surface region, the VBM position is lower in the (100) surface than the (111), as shown above, the subsurface band bending, shown with xd in Figure 8, is longer in the (100) case for the target Eredox. This seems to induce larger thicknesses of the space charge layers and more concentration of holes around the surface region. Note that the dopant concentrations for the samples for the two surfaces are similar. As a result, the (100) oriented single crystal BDD has a more sluggish response due to having a wider space charge layer than the (111) oriented one.

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Figure 8. Schematic electronic band diagrams of Oterminated BDD-water (100) and (111) interfaces for the redox reaction. When the redox reaction of the solute occurs, the Fermi energy (EF) of BDD coincides with the target redox potential (Eredox). As the surface VBM position is lower at the (100) surfaces than the (111), longer band bending (xd region) is induced, leading to less reactivity at the (100) surfaces. Conclusions Samples of (100)- and (111)-homoepitaxial single crystal BDD with comparable boron concentrations of around 4×1020 cm-3 were prepared by MPACVD method in order to elucidate the influence of surface orientation on the electrochemical properties of BDD. Polycrystalline BDD with comparable resistivity to the homoepitaxial samples was also prepared. Investigation of these BDD samples with inner- and outer-sphere complexes, i.e. [Fe(CN)6]4-/3and [Ru(NH3)6]2+/3+ showed that at both H- and Oterminated surfaces, the (111) plane was more reactive than the (100) plane, while the highest reactivity was achieved with the polycrystalline surface due to the heterogeneity of the boron distribution. The difference in reactivity was enhanced when the termination was changed to O-termination. The difference in reactivity between (111) and (100) single crystal BDD can be explained by the difference in magnitude in band bending and the thickness of the space charge layer. We expect that further characterization of surface orientation dependences on electrochemical properties such as durability towards oxidation55 and corrosion58 will yield critical insights that will help facilitate the practical applications of polycrystalline BDD electrodes.

ASSOCIATED CONTENT Supporting Information LEED patterns from the surfaces and depth profiles of the elements for the samples by SIMS [Figure S1 and S2]. The


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Asai, K.; Ivandini, T. A.; Falah, M. M.; Einaga, Y. Surface Termination Effect of Boron-Doped Diamond on the

AUTHOR INFORMATION Corresponding Author * Y.E. [email protected].

Author Contributions All authors have given approval to the final version of the manuscript.

ABBREVIATIONS BDD Boron Doped Diamond.

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Boron-doped diamond electrodes

Single crystal

(100)

Polycrystalline

(111)

TOC Graphic


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Influence of Surface Orientation on Electrochemical Properties of

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