Introduction
Boron-doped diamond (BDD) electrodes have been used widely for many applications because of their high performance [1,2]. A wide potential window, low background current, high chemical and mechanical resistance, and controllable surface termination, are the advantages that have made BDD electrodes to be of high interest [3]. Many studies attempted to investigate the effect of surface functionality [4–6], the presence of non-diamond carbon (NDC) [7], boron concentration [8,9], and crystal orientation [10,11]. However, to the best of our knowledge, no studies has engaged in the electrochemical behaviour and surface homogeneity of BDD electrodes with a very high level of boron doping. Therefore, this work aims to conclude the surface homogeneity and the electrochemical performance of several carbon-based materials (BDD, glassy carbon, and highly-ordered pyrolytic graphite electrodes) with the help of the scanning electrochemical microscopy (SECM) and the scanning electrochemical cell microscopy (SECCM) techniques. Hence, this article can assist those who are interested to utilise BDD electrodes with high boron concentrations for any kind of applications especially water and wastewater treatment processes.
In this work, the BDD electrodes with different boron concentrations studied were: 2.94×1020 cm−3 (2000 ppm), 7.09×1020 cm−3 (15000 ppm), and 1.10×1021 cm−3 (30000 ppm). The highest boron concentration that has been previously studied as reported in the literature was 5.5 × 1021 cm−3 [12]. Though, the authors assessed purely the capacitance of the BDD electrodes using a conventional cell. The conclusion they acquired was that the electrochemical behaviour of the BDD electrodes can be affected by the existence of sp2 carbon, which can be eliminated by employing pre-treatments. Therefore, this study attempts to investigate the interface heterogeneity of diamond electrodes with high concentrations of boron dopants which is deemed to be novel to the best of our knowledge.
Other than BDD electrodes, glassy carbon (GC) and highly-ordered pyrolytic graphite (HOPG) electrodes were also investigated by multiple groups. One study concluded that GC electrodes have a high density of states (DOS) resulting in a large double-layer capacitance [12]. Meanwhile, different studies came to the conclusion that HOPG electrodes have a low DOS resulting in low double-layer capacitance and slow kinetic rate for most of the redox mediators [13,14]. In this work, GC and HOPG electrodes were utilised as a standard for comparison to the results obtained from BDD electrodes using the two aforementioned techniques.
Multiple factors exist that affect the electrochemical properties of the BDD electrodes: surface pretreatments, boron doping level, NDC impurities, crystallographic orientations, and grain boundaries [15]. In this work, the different levels of boron doping were studied using the SECM and the SECCM techniques. For the SECM technique, a truncated platinum electrode was brought towards a substrate of interest immersed in the test solutions. As for the SECCM technique, the SECCM cell which consists of the counter electrode, the reference electrode and the test solution was brought towards a substrate. The SECCM cell was prepared using a conventional 3 mL syringe making it cost efficient and easy to make. The results were then supported by other analytical techniques such as Raman spectroscopy, X-ray diffraction spectroscopy, and scanning electron microscopy.
Heavily doped BDD electrodes (>1.1×1021 cm−3 of boron concentration) were also investigated by Watanabe and co-workers [12]. However, only the double-layer capacitance was studied by the authors using Raman spectroscopy and a conventional cell. This study concluded that the amount of the NDC is higher in the heavily doped BDD electrode based on Raman spectroscopy; this also affects the double-layer capacitance as studied by SECCM. Another study reported that NDC can easily be incorporated into the heavily doped diamond during the fabrication process and caution is essential when growing BDD films on a substrate. Even after cleaning the BDD electrodes in a boiling concentrated H2SO4 solution at 300°C for an hour, it was reported that the NDC could not be removed completely [3].
Another factor that can affect the performance of BDD electrodes is the surface termination. There are two methods to modify the surface of the electrodes and they are oxygen and hydrogen terminations. The different forms of oxygen termination that can occur on BDD electrodes are: ketone (C=O), ether (C–O–C), and hydroxyl (C–O–H) [16,17]. The electrode surface with hydroxyl groups could be terminated by an anodic pre-treatment with the transfer of one proton and one electron giving [18,19]:
Hydroxyl termination tends to be hydrophilic and is not stable. The surface of the BDD electrodes could also be terminated by a cathodic pre-treatment which resulted in the presence of hydrogen groups. The proposed reaction is as follows:
The hydrogen-termination has an excellent stability, unlike the anodic pre-treatment, even after exposure to air for months at a time; this roots back to its hydrophobic character [20]. Hydrogen-terminated BDD electrodes with different levels of boron doping: 0.5 ppm, 1 ppm, 5 ppm, and 10 ppm had been studied by Bard and co-workers [8] using SECM. The authors determined that the level of doping for the hydrogen-terminated BDD electrodes is directly proportional to conductor-like behaviour, meaning that the higher level of doping, the more it will behave like a conductor [8]. It was also reported that the resistivity of the BDD electrodes is proportional to the boron-doping level [3].
Many ways of terminating the surface of the BDD electrodes have been proposed in the literature. The most common method used to terminate the BDD surface is to hold the potential for 30 minutes at either +3.0 V or –3.0 V for an anodic or a cathodic pre-treatment respectively [7]. Images of degraded BDD electrodes surface were reported by Salazar-Banda and co-workers [21], due to this type of cathodic pre-treatment mentioned. It was theorised by the authors that the degraded electrode surface was due to the diffusion of the atomic hydrogen into the cavities on the BDD electrodes surface undergoing the intense hydrogen evolution. However, it was claimed by Hutton et al. that exposing the BDD electrodes in hydrogen plasma is the only way to have a hydrogen-terminated surface [3]. One of the most common oxygen termination treatments is to sweep the potential in acid solutions from the beginning of hydrogen evolution to the foot of oxygen evolution for many times beginning and ending at open circuit potential (OCP) [3]. Oxygen termination can be conducted using other reported methods, such as: polishing with alumina [3], photochemical oxidation [4], annealing in air [6], boiling in acid [5], and exposure to oxygen plasma [7,21]. The work herein reported the oxygen terminated BDD electrodes since it is more plausible than terminating the BDD surface with hydrogen as reported in the literature.
Other than BDD electrodes, HOPG is another carbon-based electrode which is mostly consisted of sp2 carbon bonds that has been widely investigated. Basal and edge planes are the two planes on HOPG electrodes that have been used as the electroactive area. Due to low DOS levels, HOPG electrodes have a low double-layer capacitance, with the density determined to be 2.18 g cm−3 [22]. Due to its uniform surface, HOPG electrodes are usually taken advantage of to calibrate scanning tunnelling microscopy at the atomic level [23]. The intrinsic characteristic of HOPG was studied by McDermott et al. [14], focusing on low defect density on the basal plane. When the contributions of the edge plane to the kinetic are reduced, the defect density becomes important [22]. As concluded from their studies, the HOPG electrode shows a slow kinetic for the ferrocyanide species. It was then claimed by Kneten and McCreery [13] that HOPG electrodes have a slow kinetic process for both ferrocyanide as well as for thirteen other mediators that have been researched by them [13]. In this work, using the SECCM technique in ferrocyanide solution, a bare HOPG electrode was examined on the basal plane. Therefore, a similar experimental data is predicted despite the usage of a different approach in this study.
Another type of carbon-based electrode of interest that has been studied extensively is GC. The advantages of GC electrodes are numerous which include: good electrical conductivity, wide potential window, and high chemical resistance; these criteria further entice interest in their usage [24,25]. The structure of the GC suggested by Jenkins and Kawamura [26] is of twisted ribbon of cross-linked graphite-like (six member ring) sheet with sp2 carbon bond. This structure of the GC includes an intraplanar, La, and an interplanar, Lc, polycrystalline structure. Because it has small and separate void spaces, GC is impermeable to gases and liquids [26]. With a density of 1.5 g cm−3, the density of GC electrode is lower than that of a HOPG electrode [27]. GC electrodes are usually prepared with the help of polymeric resins such as polyacrylonitrile or phenol/formaldehyde polymers by annealing process [26]. A potential window study was undergone by Zittel and Miller [25] for GC electrodes using a wide range of acids. They concluded that GC electrodes is inert to strong acids and oxidizing agents, with HNO3 showing the most constrained potential range in the cathodic region, and HCl showing the most restricted range in the anodic region. In this work, the potential window, the background current, and the redox activity of GC electrodes were researched in dissolved ferrocyanide with potassium chloride (KCl) and in KCl solutions.
When conducting the SECM technique, microelectrodes were employed as the probe tip. Microelectrodes can range from 0.1 to 100 μm in diameter but 5 to 25 μm are typical in electroanalytical studies [28]. The diffusion layer at a microelectrode is different from the diffusion layer at a macroelectrode. Planar diffusion normally occurs at macroelectrodes [28,29]. On the contrary, hemispherical diffusion takes place at microelectrodes [29]. The current measured using this electrode is normally very small, often in nA scale [28]. The hemispherical diffusion would result in steady-state conditions. The limiting current can be determined theoretically using Saito equation [30,31]:
where D is the diffusion coefficient, c∞ is the concentration at the bulk, n is the number of electron transferred, F is the Faraday constant and a is the radius of the electrode. The effect of RG on microelectrodes has been widely studied [32]. RG is the ratio of the insulating sheath, rg, to the radius of the metallic wire, a,
A simulation done by Amphlett and Denuault [33] showed that the limiting current can be at least 40% greater when the RG gets smaller. A small value of RG enables the diffusion from behind the plane of the electrode which increases the flux of the reduced species at the tip of the electrode [33].
One way to study the kinetics parameter is by utilising the SECM technique [34,35]. There are many modes that the SECM can be conducted; the feedback mode has been commonly used to probe the heterogeneous electron transfer reaction of many materials of interest [9]. Meanwhile, the SECCM is a high resolution and adaptable electrochemical imaging technique that can be used for the capacitance measurement at a particular area on substrates of any size [20]. The SECM can be conducted in many modes such as feedback mode, penetration mode, ion transfer feedback mode, substrate generation/tip collection (SG/TC) mode, tip generation/substrate collection (TG/SC) mode, and potentiometric detection mode [36] to study the processes and surface features at the substrates [37]. Feedback mode can be performed on biased or unbiased substrates. For unbiased conducting substrates such as a gold substrate, the reduced species (considering an oxidation at the tip) is being regenerated on the surface of the conducting substrate. The regeneration is driven by the reverse reaction that also occurs on the surface of the substrate [38]. Nernst equation plays a vital role in explaining the phenomenon that occurs at the surface of the conducting substrate. The regeneration of the reduced species increases the flux of the reduced species towards the tip of the electrode [39]. The current increases tremendously as the tip approaches the substrate and this response is called a positive feedback. On the contrary, the diffusion of the reduced species towards the SECM tip is hindered by the surface of an insulating substrate. Consequently, the current drops and this response is called negative feedback or hindered diffusion. A large RG could prevent the electroactive area from moving further towards the substrates. Thus, the RG of the SECM tip needs to be really small to enable the working electrode to be at close proximity with the substrates of interest. With a smaller value of RG, more species can diffuse from behind the plane of the SECM tip as explained earlier [33].
As for the other technique, SECCM is a high resolution and versatile electrochemical imaging technique [40,41]. The SECCM technique can be used as an alternative probing technique when fast film formation occurs on the surface of a substrate of interest using other imaging techniques [40] such as the SECM technique. The contact between the meniscus at the mouth of the pipette tip and the working electrode enables the electrochemical experiments to be conducted. The difference between the SECCM technique and the conventional technique (immersing the substrates in a test solution using a conventional cell) is that the cyclic voltammetry can be done at several locations on the substrates. Therefore, this technique can also be used to probe the heterogeneity of the substrates of interest. However, this technique is much easier to use as no cell is required to conduct the experiments. External factors contributing to the measured current such as the titanium substrate of the BDD electrodes or the edge plane of the HOPG electrode can be avoided by employing this technique. Unlike the SECM technique, there is no issue regarding the distance offset as explained above when employing the SECCM technique. In this work, the surface heterogeneity of the carbon-based electrodes was probed using the SECCM technique. The results were then supported with Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. It is hoped that this work can be useful for those who are interested in delving into investigating BDD electrodes with high boron concentrations using two characterisation techniques.
Experimental
2.1 Preparation of electrodes
Regular-shape platinum electrodes: A regular-shape 25 μm diameter platinum microelectrode was employed, to probe the diffusion coefficient and the concentration of the redox mediator used for the SECM experiment. The electrode was polished with 1 μm, 0.3 μm, and 0.05 μm alumina suspension on a polishing cloth (Buehler) for 5 minutes, 10 minutes, and 20 minutes respectively. The electrode was sonicated in acetone for a minute and it was then rinsed with deionized water to remove any remaining alumina powder.
Truncated platinum electrodes: A truncated 25 μm diameter platinum microelectrode was used when conducting the SECM experiments. The side of a regular-shape 25 μm diameter platinum electrode tip was ground using a silicon carbide paper of 1200 grade (3M) until the RG was about 10. RG is the ratio between the radius of the insulating sheaths and the geometric radius of the electroactive materials [42]. The electrode was then sonicated in acetone for a minute and rinsed with deionised water for several times. Then, the tip was further polished using a 0.3 micron alumina lapping film (3M) for a fine finish until the RG was about 5. Next, the tip of the electrode was sonicated in acetone to remove the alumina residue. The RG of the tip was confirmed using the scanning electron microscopy (SEM) under wet mode.
Two reference electrodes were used in this study: a saturated mercury sulphate electrode (SMSE) and a saturated calomel electrode (SCE). The SMSE was used only when the experiments were done in sulphuric acid. Two counter electrodes were used in the experiments, a large platinum gauze (6 cm2) was used during the termination of the BDD electrodes due to their large dimension and a platinum wire was utilised for the SECCM experiments.
2.2 Electrochemical instruments
A thermostated open Perspex® cell was used to conduct the SECM experiments on the BDD electrodes to keep the temperature between 22–25°C. A BDD electrode was placed inside the cell and pushed against small cylindrical blocks that were protruded from the bottom of the cell to define the zero coordinate for the BDD electrode. A gold substrate was also used as a reference which was enclosed in an insulating epoxy and was secured inside a cell with a window. The surface of the gold substrate was exposed to conduct the SECM technique. The substrate was polished with 1 μm alumina suspension for a minute. This cell was put on the platform while conducting the experiments.
2.3 Carbon-based materials preparation
2.3.1 BDD electrodes
Three BDD electrodes with different levels of boron doping were studied: 2.94×1020 cm−3 (2000 ppm), 7.09×1020 cm−3 (15000 ppm), and 1.10×1021 cm−3 (30000 ppm) were labelled as BDD-A, BDD-B, and BDD-C respectively. The rough BDD films were grown on titanium substrates where the dimension of each of them was 25 × 25 mm and the geometric area was 6.25 cm2. The BDD electrodes were washed with ethanol, rinsed with deionized water, and air dried before conducting the SECM and the SECCM experiments. A three-electrode system was employed when functionalising the surface of the BDD electrodes with oxygen by cycling the potential in the test solution.
2.3.2 HOPG electrode
HOPG electrodes (3×10×10 mm), ZYB grade, were employed in this work. A new, clean, and smooth sheet of HOPG was obtained by gently cleaving the top most layers. The HOPG was put on a metal support strip to connect the HOPG with a working electrode cable. The metal support strip along with the HOPG electrode was put on a glass slide and they were secured using Parafilm exposing only a small part of the HOPG electrode for the SECCM experiments.
2.3.3 GC electrode
A 3 mm diameter bare GC electrode was used as a substrate of interest. This electrode was polished using 0.3-micron alumina powder and cleaned using a wet polishing cloth before conducting any experiments. A three-electrode system was used when electrochemical experiments were conducted using this electrode.
2.4 SECM
After the pre-treatment of a BDD electrode was done, the positioning arm connected to the three-dimensional translation stages was positioned at a specific location on the BDD electrode. The micro-positioning arm was brought to zero coordinate in Z-axis. A truncated 25 μm diameter platinum electrode was mounted on the micro-positioning arm. Then, the micro-positioning arm was brought up in Z-axis approximately 1000 μm above the substrate from the initial position. A 1 mM ferrocenemethanol (FcMeOH) with aerated 0.5 M potassium chloride (KCl) and 2% ethanol was poured into the cell. The SECM tip was moved at 300 s after the potential was stepped once the transient current recorded by the platinum electrode had stabilized. The relative position of the SECM tip was known from the PIMikro-Move software. The position of the SECM tip was observed using a telescope-microscope. The SECM approach curves were done at five different points on the substrate; A–E. Each point was located 5 mm apart from each other. An approach curve was also done on a gold substrate to compare the experimental data with the approach curves done on the BDD electrode.
2.5 SECCM
A SECCM cell was made using a conventional 3 mL syringe. Two small holes were made at the sides of the syringe to fit a platinum wire for the counter electrode and the tip of the agar-salt bridge SCE for the reference electrode. The SECCM cell was filled with the test solution of interest by using the plunger. The micro-positioning arm was brought to zero coordinate at Z-axis. The SECCM shaft was secured at the micro-positioning arm and brought to the surface of the substrate as close as possible. The SECCM tip was then brought to approximately 500 μm above the substrate. A minute amount of the test solution was allowed to hang at the tip of the SECCM cell to form a meniscus. The SECCM tip was brought towards the substrate at 1 μm s−1. The meniscus at the tip deformed as it made contact with the substrates and the movement of the SECCM tip was stopped. Conventional cyclic voltammograms were recorded on the samples in deoxygenated 0.5 M KCl at 0.1 V s−1 for the background current study. Cyclic voltammetry was also done in 5 mM ferrocyanide (K4Fe(CN)6) with deoxygenated 0.5 M KCl at several random locations on the substrates of interest.
Results and Discussion
3.1 SECM
The SECM approach curves were done at five different points on the substrate; A–E. An approach curve was also done on a gold substrate to compare the experimental data with the approach curves done on the BDD electrode. The SECM tip was electrochemically cleaned in between every approach curve by sweeping the potential from the start of hydrogen evolution to the foot of oxygen evolution for 40 cycles at 0.5 V s−1. At point A, the SECM technique was done on the BDD-A (2000 ppm) in 1 mM FcMeOH with aerated 0.5 M KCl and 2% ethanol [43]. A cyclic voltammogram was recorded by sweeping the potential from 0 to 0.4 V vs. SCE for 1 cycle at 0.002 V s−1. Fig. 1(a) shows the steady steady-state current obtained using a 25 μm diameter platinum electrode with the RG of approximately 4 in the bulk of the test solution. A transient current can be seen in Fig. 1(b) as the potential was stepped from zero current to the diffusion-controlled regime. The SECM tip was moved towards the substrate at 300 s after the transient current had stabilised as indicated by an arrow.
Fig. 1(a) shows that the limiting current is ca. 3.22 nA. The hysteresis between the forward and backward scans is also small which implies that the working electrode was thoroughly polished. The sigmoidal shape cyclic voltammogram is a typical electrochemical response for microelectrodes which is due to the hemispherical diffusion. Meanwhile, a stable current can be seen for a period of time as shown in Fig. 1(b). The current has remained unchanged even when the working electrode was brought towards the substrate. However, the current begins to increase as the working electrode was approaching the substrate.
The graph of current as a function of time was then converted to a graph of current as a function of distance with the offset as shown in Fig. 2(a). The distance was then normalized to the radius of the electrode, a, and the current was normalised to the limiting current at the bulk as shown in Fig. 2(b). The approach curve was then compared to a theoretical approach curve developed by Lefrou and Cornut [44]. The three adjustable parameters (RG, kfeedback, and Loffset) were obtained from the fitting. Loffset is the distance between the substrate and the electroactive material which may exist when the insulating sheath is hindering the electroactive area from getting closer to the substrate.
Fig. 2(b) shows that the theoretical data fits perfectly to the experimental data. This fitting enables the determination of certain parameters including the feedback kinetic rate constant, kfeedback. The specific rate constant is a proportionality constant relating the rate of a reaction to the concentrations of the reactants. A large rate constant value indicates a faster reaction, while a small value indicates a slower reaction.
The SECM technique was also done at the other four locations: B, C, D, and E. All the approach curves were treated using the same data treatment done for the experimental data obtained at point A. The limiting currents at the bulk were 3.21, 3.12, 3.12, 3.04, and 3.36 nA for Point B, C, D, E and the gold substrate respectively. The SECM tip was stopped at 31, 340, 230, 34, and 217 μm above the BDD-A electrode for Point A, B, C, D, and E respectively. The SECM tip was stopped at 414 μm above the gold substrate. These values are not the real tip-substrate distances but the distances relative to the position in the bulk.
Fig. 3 shows the approach curves done on the BDD-A at all locations: Point A, B, C, D, and E after subtracting the experimental data with the Loffset obtained from the fittings. An approach curve was also done on a gold substrate as a standard for comparison. The same feedback can be seen at all locations except for point E. The feedbacks obtained at point E might be because the SECM tip had crashed into the substrate. The feedback obtained from the SECM technique on the BDD electrode seems to be quite similar with the normalized approached curve obtained from the gold substrate. The three adjustable parameters acquired from the fitting of the approach curves at all locations including at the gold substrate are shown in Table 1.
The correlation coefficients, R2, for all the fittings are close to 1 which shows that the experimental data has a good fit [44] with the theoretical approach curves, except for Point E. The value of RG obtained from the fittings is approximately 2, close to the value of RG assessed using the SEM which is ca. 4. The kfeedback obtained at all locations are near to the kfeedback obtained on the gold substrate. This implies that BDD-A behaves as a metal and the surface of the BDD electrode is homogeneous. The values of kfeedback obtained from the fitting are in the range of the apparent heterogeneous electron transfer reported by Neufeld and O’Mullane [45]. The authors used a 10 μm diameter platinum electrode for the SECM study. However, they did not mention the value of RG and the boron concentration of the BDD electrode.
The peaks that can be seen in the Raman spectra of BDD-A (see supporting information) suggest metallic conductivity behaviour which will further be discussed in Section 3.3.1. This could be the reason why BDD-A behaves like a metal at all locations. Surface termination might also play an important role in affecting the kfeedback [3,5,16,46]. The electrode conditioning also affects the kfeedback of the electrode. The typical features such as the hydrogen adsorption and desorption for a platinum electrode should be seen when cycling the potential in H2SO4.
3.2 SECCM
A SECCM cell consists of a cell made of a conventional 3 mL syringe, a reference electrode, a counter electrode, a test solution, and pipette tip. Conventional cyclic voltammetry was employed to probe the substrate in this work. The difference between the conventional method and this technique is that the conventional cell was replaced with the SECCM cell. Another difference is that the SECCM cell was brought towards the substrate which acted as a working electrode.
In this work, a background current study was conducted for all the carbon-based electrodes of interest in deoxygenated 0.5 M KCl. The double layer capacitance and the potential window of the carbon-based electrodes can be determined by conducting this study. The solvent window study provides the information regarding the potential limit of an electrode to electrolyse the solvent [47]. The potential window can be seen by sweeping the potential where the oxygen and hydrogen evolution take place [3].
The SECCM technique allows the determination of the background current study without having any external factors contributing to the measured current such as the metallic substrates that films of interest are grown. The BDD films were grown on titanium substrates with the sides of the titanium exposed. Therefore, this technique is a much reliable technique to probe the BDD electrodes so as to reduce any external factors that might contribute to the measured currents.
Fig. 4 shows the cyclic voltammograms recorded for the carbon-based electrodes of interest done in deoxygenated 0.5 M KCl. In terms of the potential window, the BDD electrodes have wider potential windows than the GC and the HOPG electrodes as shown in Fig. 4(a). The GC and the HOPG electrodes have fairly similar potential windows as shown in the figure. BDD-A has the widest potential window among the carbon-based electrodes of interest. This might be due to the lack of charge carriers on the lowest doped BDD electrode [3]. The efficiency of water hydrolysis increases with the number of catalytic sites on the surface of the electrode [48]. The water decomposition is hindered as the boron concentration increases. This might be due to the limited binding sites on the oxygen-terminated BDD electrodes that facilitate the heterogeneous electron transfer. Hutton et al. [3] concluded that the presence of the NDC could affect the potential window to be narrower.
As for the GC electrode, the water electrolysis becomes more efficient with the presence of quinine-like groups. A narrower potential window was obtained because the water electrolysis was more effective [3]. The narrow potential window might also be because of the supporting electrolyte used. Studies had been conducted on a bare GC electrode in different type of supporting electrolytes, such as sodium acetate, sodium hydroxide, potassium perchlorate, and potassium sulfate which show a slight different in the potential window [47,49,50]. Yu et al. [51] reported that the aqueous electrolyte could contribute to corrosion and low stability-window issues that affect the electrochemical performance and stability.
It has been reported that a low-doped BDD electrode has a lower background current and low amount of NDC incorporated in the BDD films [3,12]. Hutton et al. [3] suggested that the NDC can be easily incorporated into the BDD films as the concentration increases. However, in this study, BDD-B has the smallest background current among the BDD electrodes. For heavily-doped BDD electrodes, Watanabe and co-workers [12] suggested that the electronic interactions between the σ-valence bond (heavily doped matrix) and the π-bond (sp2-bonded), contribute to the increase in the carrier density and electronic DOS near the Fermi level. Therefore, the presence of the sp2 carbon increases the capacitance of the BDD electrode. This is consistent with the result obtained for the BDD-C electrode. BDD-A has a wide background current may be due to the presence of the NDC. Macpherson [47] reported that the NDC cannot be removed completely even by cleaning the BDD electrodes in boiling concentrated H2SO4, concentrated HNO3, and saturated KNO3 for one hour. The amount of the NDC can be reduced by controlling the BDD films growth [3]. Nevertheless, the area of contact at the SECCM tip between the test solution and the substrate of interest could have been different for each time because it was quite difficult to retain the same size droplet every time which may also have affected the current magnitude.
Features, such as a sp2 oxidation peak, that can be seen in the anodic region of the cyclic voltammograms at around 0.7 V vs. SCE for the BDD-A and BDD-C can be attributed to the presence of the NDC. This is because the binding sites provided by the NDC enable the sp2 oxidation and oxygen reduction reaction to occur [47]. The sp2 oxidation features should not be seen in the anodic region for the BDD electrodes with the absence of sp2 carbon [47].
The Raman spectra for the BDD-B electrode, discussed in Section 3.3.1, show that the peak intensity for the G-band is similar to the highest doped BDD films. Thus, the BDD-B electrode should also exhibit the features associated to the NDC surface driven electrochemical features. Interestingly, the cyclic voltammogram for BDD-B shows no feature that can be ascribed to the presence of the sp2 carbon incorporated in the films. The acid treatment might have removed much of the NDC on BDD-B, although the same cleaning procedure was adopted for the other BDD electrodes. It might also be that the preparation of BDD-B was done slightly differently from the other BDD electrodes which reduced the amount of the NDC incorporated in the BDD films [3].
HOPG electrode has the smallest background current among the substrates studied. This is because the basal plane of the HOPG electrode has a low DOS at the Fermi level resulting in low background current and slow kinetic reaction [22]. On the contrary, the GC electrode has high DOS at the Fermi level which contributes to the large double layer capacitance [12].
The SECCM technique was also done in 5 mM ferrocyanide in deoxygenated 0.5 M KCl at 0.01 V s−1. Cyclic voltammograms were recorded on the samples at several different locations on the substrates of interest using the SECCM technique as exhibited in Fig. 5. This study was done to determine the reactivity of the surface of the substrates.
The reference and counter electrodes were fixed at the same position for all experiments when using this technique. Thus, the issue of the position of the counter and the reference electrodes can be excluded. Although the working and the reference electrodes were quite far from one another (approximately 5.5 cm), the ohmic drop effect was not observed in the cyclic voltammograms of the GC electrode, which acted as a standard.
As the pipette tip has the external diameter of 795 μm, the working electrodes still behave similar to a macroelectrode [28] as seen in the results. The cyclic voltammograms for the GC electrode are very reproducible at the three locations as can be seen in Fig. 5(a). This shows that the GC electrode has a homogeneous surface. The cyclic voltammograms for the GC electrode are characteristic of a reversible electrochemical reaction with rapid electron transfer kinetics. The average peak to peak separation, ΔEp, for the GC electrode is 0.07 ± 0.01 V vs. SCE, indicating a fast electron transfer process. It has been reported that the GC electrode shows a fast electron transfer when using the K4Fe(CN)6 regardless of the amount of oxide groups on the surface of the electrode [45,52].
The cyclic voltammograms for the HOPG electrode have higher ΔEp than the GC electrode. The HOPG electrode has been reported to exhibit a slow kinetic transfer for most of the mediators [13,14]. The peaks for the oxidation of ferrocyanide are stable and reproducible. However, the cathodic peaks show no similarity as can be seen in Fig. 5(b). There are several factors that could affect the kinetic process of the HOPG electrode as explained by McDermott and colleagues [14]. The writers suggested that this is probably due to some defects on the HOPG electrode that are not visible to the naked eyes [14]. There are a few types of defects that can be found on a HOPG electrode such as step edges, missing atoms, atomic scale ridges and fissures [53–55]. The defects will act in the same way as random microelectrode arrays at slow scan rate (0.1–10 mV s−2) [14]. Cline et al. [56] reported that the K4Fe(CN)6 species is sensitive to the chemical, physical, and mechanical pretreatments that include morphological defects introduced on the HOPG electrodes. Another plausible reason is that the basal plane of the HOPG electrode had been exposed to the air for too long after being cleaved [14]. Other than that, McDermott et al. [14] also suggested that the distortion might be caused by a potential-dependent transfer coefficient, α, which is important as the ΔEp increases. This is because at larger ΔEp the α varies, resulting in the oxidation and reduction reactions to occur at different potentials [14]. Kneten and McCreery [13] added that the hydrophobicity and low DOS reduced the kinetic reaction on the HOPG electrode.
As the SECCM technique was used to probe the homogeneity of the HOPG electrode, the edge plane effect can be ruled out. The SECCM technique was used to investigate the surface of the HOPG electrode on the basal plane. However, the defects on the HOPG electrode especially at the atomic levels cannot be easily avoided even though a plastic pipette was used in this experiment. The HOPG electrode has a lower mechanical resistance than the BDD electrodes [57].
As for the BDD electrodes, the cyclic voltammograms show that all the BDD electrodes have an inhomogeneous surface. The average ΔEp seems to be affected by the boron concentration in the BDD films. From the results, the BDD electrode with the lowest boron doping has the largest average ΔEp (0.54 ± 0.09 V) and the lowest average jpa among the BDD electrodes. On the contrary, BDD-C has the smallest average ΔEp (0.11 ± 0.06 V). This might be because BDD-C was doped sufficiently to have enough charge carriers to provide an efficient heterogeneous electron transfer [3]. Watanabe et al. [12] reported that the electrode kinetic rate increases as the boron concentration increases. Hutton et al. [3] stated that the average ΔEp is larger as the boron concentration decreases because of lower amount of electronic DOS. Interestingly, the cyclic voltammograms for the BDD-C do not show the typical diffusion-controlled shape akin to the GC electrode. The anodic peak current densities, jpa, for the BDD-C at certain locations are much higher than at other locations. On a BDD electrode, the heterogeneous electron transfer differs at all locations but at a small order of magnitude. Table 2 summarises the average ΔEp and the average jpa for all the carbon-based electrodes of interest.
Hutton et al. [3] reported that the method used to terminate the electrodes could influence the heterogeneous electron transfer of the BDD electrodes. They reported that alumina polishing produces a reversible redox reaction and higher jp due to more oxidized groups available on the surface. Thus, the amount of oxygen functional groups on the surface of the electrode affects the reversibility of the redox reaction and the peak current.
The diffusion coefficient of the test solution was also assessed by conducting the cyclic voltammetry at different scan rates using the SECCM technique in 5 mM K4Fe(CN)6 with deoxygenated 0.5 M KCl on a GC electrode as shown in Fig. 6(b). Cyclic voltammograms were recorded at different scan rates using freshly polished GC electrodes using the conventional electrode configuration and the SECCM technique. This experiment was conducted to determine the effect of using the SECCM technique on the diffusion coefficient of the redox mediator. The cyclic voltammetry with different scan rates was also done using a conventional cell done at the bulk of the same redox mediator as shown in Fig. 6(a).
The cyclic voltammograms recorded using both techniques demonstrate similar features. The peak current increases as the scan rate increases for both configurations. The diffusion coefficient of the test solution can be calculated from the plot of cathodic peak currents, jpc as a function of the square root of the scan rate, v1/2, as described by Eq. 5:
where n is the number of electrons transferred, D is the diffusion coefficient, c∞ is the concentration of the test solutions, R is the gas constant, T is the temperature, F is the Faraday constant, and v is the scan rate. Therefore, graphs of current as a function of square root as scan rate were plotted from the previous data (Fig. 6) as exhibited in Fig. 7.
Fig. 7 shows that the jpc is linearly proportional to the v1/2 [28]. The value of the diffusion coefficient was calculated using the slope from the linear fittings given in Fig. 7. Both techniques have the same slope gradient. The diffusion coefficient of the 5 mM ferrocyanide in aqueous solution found using a conventional cell is ca. 1.45×10−6 cm2 s−1. This value is smaller than the value of the diffusion coefficient for the ferrocyanide reported in the literature; 6.6×10−6 cm2 s−1 [28]. This might be because the experiment was performed at 18°C. The temperature was not controlled for both techniques because the SECCM cell was not designed to allow the temperature to be fixed. However, the calculated diffusion coefficient of the mediator using the SECCM technique is similar to the calculated diffusion coefficient of the mediator using the conventional technique. Thus, it can be concluded that the diffusion coefficient of the redox mediator is not affected when employing the SECCM technique.
3.3 Supporting analyses
3.3.1 Raman spectroscopy
Typical Raman spectra for the BDD electrodes showed a diamond zone centre optical phonon peak at 1332 cm−1 which corresponds to the sp3 carbon bond [3]. Fig. S1 shows the Raman spectra for the BDD electrodes before and after cleaning the electrodes in a mixture of concentrated acid. The diamond zone centre optical phonon peak can be seen in the figure. This peak can be observed clearly in the Raman spectra for the BDD-A. However, this peak is shifted to a lower wavenumber for the BDD electrodes with a higher level of boron doping (BDD-B and BDD-C) at around 1317 cm−1. This shift is reported to be dependent with the concentration of boron [1]. Asymmetric distortion and decrease in the sp3 peak as seen in the figure, for all BDD electrodes [58]. This distortion is normal for high concentration of boron (>1020 boron atoms cm−3) [3]. The shift in the D-band [59,60] and the asymmetric distortion can be attributed to the quantum interference between the phonon and a continuum of electronic transitions which is also called as the Fano effect [12]. Two broad peaks centred at ca. 470 cm−1 and 1250 cm−1 for all the BDD electrodes suggest metallic conductivity where these two bands have been thought to be due to the maxima of the phonon DOS of diamond [1]. Any sharp peaks that can be seen in the Raman spectra can be attributed to noise.
Bernard and co-workers [61] concluded that the peaks that can be seen at 500 and 1220 cm−1 are related to the boron concentration and should not be attributed to the metallic conductivity. They reported that the peaks at 500 and 1220 cm−1 cannot be seen at a BDD electrode with 14000 ppm of boron concentration in the Raman spectra. On the contrary, the Raman spectra in Fig. S1 do show the two peaks at 500 and 1220 cm−1 even for BDD-C which has the highest concentration. Therefore, their conclusion is not plausible and further investigation needs to be done.
In a Raman spectrum, sp2 carbon bonds can be analysed from 1430 to 1650 cm−1 (G-band) and sp3 carbon bond can be examined from 1300 to 1350 cm−1 (D-band) [3,59,60]. Pure diamond should only contain sp3 carbon. The higher the boron concentrations, the harder it gets to prevent the growth of sp2 carbons [47]. The presence of the NDC impurity, e.g. graphite and amorphous carbon can be observed between 1400 and 1600 cm−1 for all the three BDD electrodes before and after cleaning with concentrated acids for 30 minutes. The peak intensity for the G-band of BDD-B decreased remarkably, after the acid cleaning, which is almost the same as BDD-C. The G-band for the BDD-A also decreased slightly after the acid treatment. The G-band for the highest doped BDD electrode remains the same even after cleaning the electrodes in the concentrated acids. Hutton et al. [3] concluded that the NDC cannot be thoroughly removed even after immersing the BDD electrodes in boiling acid for an hour at 300°C. From the Raman spectra, a G-band can be seen at about 1550 cm−1 for all the electrodes, ascribed to C=C bond (sp2) stretching mode [62]. Nonetheless, Fig. S1 also shows that the intensity of the G-band decreases as the boron concentrations increases. This is contrary to the common understanding of the relationship between the intensity of the G-band and the concentration of boron [47]; this is because the G-band which correlates to the amount of sp2 carbon should increase as more boron atoms are incorporated into films. It is important to take note that the excitation wavelength that was used for this work was fairly high which was 785 nm. The higher the excitation wavelength, the scattering becomes more sensitive towards sp2 carbon than sp3 carbon. Nevertheless, it has been reported that growth techniques have been improved that it is now possible to grow BDD with high boron concentrations but with negligible amount of sp2 carbon or non-diamond carbon [47]. Therefore, it is believed that the growth technique used to prepare the BDD films for this work was able to suppress the formation of the NDC as suggested by the Raman spectra.
3.3.2 XRD analysis
The G-band as seen in the Raman spectra was attributed to the presence of sp2 carbon as discussed in Section 3.3.1. However, the Raman spectroscopy was incapable of determining the type of the NDC. The presence of the NDC was also confirmed after conducting the XRD analysis on the BDD samples which was done under grazing mode at 1° angle as shown in Fig. S2. The peaks that can be seen at approximately 44° for all BDD electrodes can be attributed to the diamond carbon. Sp2 carbon peaks can be seen in the figure at 22 and 40° angle for the BDD-A. More sp2 carbon peaks can be seen for BDD-B and BDD-C that are ascribed to the buckminsterfullerene and the graphite. As stated in Section 3.3.1, it is impossible to remove the NDC thoroughly even after treating the BDD electrodes in boiling acid as reported by Hutton et al. [3].
Interestingly, the existence of titanium, titanium carbide and rutile was also confirmed by the XRD spectra. Titanium substrates were used to grow the BDD films for different levels of boron concentration. The titanium carbide was formed as the intermediate layer between the titanium substrate and the BDD films during the growth process [63]. This natural event was crucial as it ensures a good electrical conductivity between the titanium substrate and the diamond carbon [63]. Peng and Clyne [64] reported that the growth rate and the thickness of the titanium carbide interlayer are low and small respectively. It has been reported that the titanium alloy is a better substrate than Si due to its mechanical strength and electrical conductivity [63].
The XRD analysis was also performed after cleaning the BDD electrodes in the mixture of concentrated acids as shown in Fig. S3. Traces of the NDC can still be seen in the XRD spectra even after cleaning the BDD electrodes using the concentrated acids. The intensities of some of the peaks, including the peaks that are assigned to the diamond carbon, are lower than the XRD spectra for the BDD electrodes before cleaning the electrodes with the concentrated acid mixture. This might have shown that the concentration of the NDC in the BDD films had decreased slightly after the acid treatment.
The peaks after 70° for the BDD-A and the BDD-B are not observed in the spectra. This might be because the beam was obstructed by the wires of the BDD electrodes which were not removed. However, after the 70°, most peaks can be attributed to the titanium substrate. From the XRD spectra after 70°, for the BDD-C, it can be seen that the intensity for the peak that can be attributed to the carbon had also decreased slightly.
For BDD-C, the peak that can be seen at ca. 28° which was ascribed to rutile increased drastically. A new peak at approximately 37° can also be seen for BDD-C which can be attributed to rutile. This might be because the acid treatment terminated the surface of the BDD electrode with more oxygen functional groups [3].
3.3.3 BDD morphologies and thickness
The morphologies and the thickness of the BDD films were confirmed using the SEM. The BDD films have a rough electrode surface, as shown in Fig. S4. This implies that the deposition is not homogenous across the surface of the BDD electrodes. The size and shape of the granules on the BDD electrodes are not similar as can be seen in the SEM images at lower magnifications; 500× and 1000×. The diamond crystal structure can also be seen in the figure at higher magnification: 5000× and 10000×.
The SEM images were also taken at the cross sections of the BDD electrodes in order to determine the thickness of the BDD films. The thickness of the BDD films is between 1–5 μm for all the BDD electrodes as seen in Fig. S5. The images show the rough surface of the BDD electrodes from the side. However, BDD-A is not as rough and as thick compared to the other BDD electrodes as seen in Fig. S5(a,b). The thinner and smoother BDD films for BDD-A might be because of the low boron concentration.
Conclusions
It was shown by the SECM experiment in the FcMeOH solution that the BDD-A behaves as gold substrate at all locations. This result was backed up by Raman spectra where two peaks can be seen at both 500 and 1220 cm−1 that can be attributed to metal conductivity. Both the Raman spectra and the XRD analysis confirmed the existence of the NDC incorporated in the BDD electrodes. Though, the features that can be ascribed to the NDC are not able to be seen in the background study of BDD-B. This behaviour may be due to the acid treatment or the preparation of the BDD electrodes. The SECCM technique employed in this work shows encouraging results which can be perfected by utilising a smaller pipette tip. Both the GC and the HOPG electrodes have similar potential windows and the narrowest between the carbon-based electrodes. It can be seen that the potential window decreases as the boron concentration increases for the BDD electrodes. Meanwhile, the background current increases as the boron concentration increases. The GC electrode has relatively large background current, while oppositely the HOPG electrode displays the smallest background current. The fastest heterogeneous electron transfer among the carbon-based electrodes of interest is the GC electrodes. The HOPG electrode has a slow electron transfer kinetic, unlike the GC electrode. As the boron concentration increases, the heterogeneous electron transfer of the BDD electrodes become faster. This may be because of the amount of DOS at the Fermi level.
) with an offset compared to the theoretical current (thick line) developed by Cornut and Lefrou [
) electrodes. (b) Cyclic voltammograms for the BDD electrodes: BDD-A (
), BDD-B (
), and BDD-C (
) done in deoxygenated 0.5 M KCl at 0.1 V s−1 for 5th cycle using the SECCM technique. The inset shown in (a) is the cyclic voltammogram for the HOPG electrode. The inset shown in (b) is the cyclic voltammograms for BDD-A (
) 0.005, (
) 0.01, (
) 0.025, (
) 0.05, (
) 0.1, (
) 0.15, (
) 0.2, (
) 0.25, and (
) 0.3 V s−1; temperature: 18°C.
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