Voltammetric characterization of boron-doped diamond electrodes for electroanalytical applications

Journal of Electroanalytical Chemistry 862 (2020) 114020

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Voltammetric characterization of boron-doped diamond electrodes for electroanalytical applications ⁎

V. Rehacek a, , I. Hotovy a, M. Marton a, M. Mikolasek a, P. Michniak a, A. Vincze b, A. Kromka c, M. Vojs a a b c

Institute of Electronics and Photonics, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia International Laser Centre, Ilkovicova 3, 841 04 Bratislava 4, Slovakia Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic

A R T I C L E

I N F O

Article history: Received 31 October 2019 Received in revised form 27 February 2020 Accepted 28 February 2020 Available online 29 February 2020 Keywords: Boron-doped diamond Electrode kinetics Electroanalysis Voltammetry Surface treatment Bismuth film

A B S T R A C T

The unique properties of boron-doped diamond make it a promising electrode material for electroanalytical applications, especially for stripping voltammetry. The boron-doped diamond (BDD) electrodes prepared by chemical vapor deposition (CVD) on alumina substrates were electrochemically characterized by cyclic voltammetry. The influence was investigated of the boron dopant concentration, surface termination as well as chemical surface treatment on the kinetics of the electrode reaction. A standard redox couple, ferro/ferricyanide in aqueous solution, was employed to determine important kinetic parameters (anodic transfer coefficient, diffusion coefficient, and electron transfer rate constants) of the reaction. The effect of surface conditions of the boron-doped diamond electrodes on the kinetics of the electrodeposition of bismuth film was investigated for electrochemical applications in which bismuth film electrodes are utilized. A favourable effect of the cleaning procedures was proved, especially for the oxygen plasma-treated BDD electrode with a higher content of boron in alkaline solution. The anodic current response was increased by 109% when compared with as-grown BDD. The effect of the quality of bismuth film on analytical performance of the voltammetric sensor was verified in simultaneous analysis of heavy metals (Pb, Cd and Zn) by anodic stripping voltammetry. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Carbonaceous electrodes based on a diamond structure with sp3 hybridized carbons constitute a prospective electrode material in electroanalysis due to the excellent properties of diamond (chemical inertness, hardness, thermal conductivity) [1,2]. Pure diamond is electrically a highly resistive semiconductor (bandgap of 5.47 eV at 300 K) which electrical conductivity can be significantly increased by doping, most often with boron (p-type dopant) and phosphorus (n-type dopant). At present, boron is a widely used dopant for electrochemical research [2,3]. Here, the conductivity of boron-doped diamond (BDD) is tailored by the boron doping concentration. In literature, the doping level 1–3 × 1020 cm−3 [1–3] was reported for the “metal-like” BDD. A higher boron content is not recommended because of higher capacitance values and a higher content of non-diamond carbon [1]. In general, the BDD electrode has several advantages over the traditional metallic (gold, platinum) and carbonaceous electrodes with sp2 hybridization (pyrolytic graphite, glassy carbon, carbon paste and others). In terms of its use in voltammetry in aqueous media, BDD offers in particular a larger electrochemical potential window (~3–3.5 V) which extends ⁎ Corresponding author. E-mail address: [email protected]. (V. Rehacek).

http://dx.doi.org/10.1016/j.jelechem.2020.114020 1572-6657/© 2020 Elsevier B.V. All rights reserved.

application uses at a lower and stable background current (low capacitance of BDD) which in turn increases the signal-to-noise ratio. Further, the electrochemical properties of BDD can be tuned by the boron concentration in diamond and modified by their surface treatment. Finally, long-term resistance of the surface to fouling by ambient air and analysed environment (weaker adsorption of most pollutants) as well as proper responsiveness for several analytes without pretreatment and no anodic electrode corrosion are further benefits of BDD [1–5]. The most favorite technology of BDD film preparation is synthesis from the gas phase by chemical vapor deposition (CVD) at a higher temperature and low pressure [2]. The reaction of the gas mixture can be activated by plasma (microwave CVD) or thermally (hot-filament CVD). Thermally stable substrates like silicon, alumina, or glass pretreated by seeding with diamond nanoparticles are often used. Growth parameters affect the properties of BDD film such as grain size, texture, the content of non-diamond carbon, and film thickness. Depending on the growth conditions, a polycrystalline BDD film can be prepared with ultra-nanocrystalline (UNC) (< 10 nm), nanocrystalline (NC) (10–1000 nm) or microcrystalline (MC) (> 1000 nm) grain size [2]. During the BDD growth, non-diamond sp2 carbon impurities are also formed that are mainly incorporated at grain boundaries. Since the grain boundary density in ultra- and nano-crystalline diamond is higher than in the micro-crystalline, a significantly stronger

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Journal of Electroanalytical Chemistry 862 (2020) 114020

effect of sp2 carbon on the electrochemical properties of UNC and NC is observed in comparison with MC BDD. Except for the surface cleanliness and surface morphology of BDD, three essential factors significantly influence the electroanalytical properties of BDD [1,2,5]. One of the most critical factors is boron doping level. As mentioned previously, a concentration of about 3 × 1020 boron atoms per cm3 was found to be optimum for applications of BDD electrodes in electroanalysis [1]. In CVD technology of BDD films, the content of boron is commonly expressed by the B/C ratio in the gaseous phase in ppm units. In these cases, the values are in the range from 1000 to 20,000 ppm [1]. Some authors report a typical concentration from 500 to 8000 ppm [6,7]. It should be noted that increasing the B/C ratio during the CVD procedure results in an increase not only in electrical conductivity but also in a decrease of the potential window width as well as an increase of sp2 carbon impurities. In all cases, the optimum boron concentration shall be tuned to obtain the highest sensitivity of the analysis [8]. The next factor, the content of non-diamond sp2 carbon in BDD usually expressed by sp3/sp2 ratio, has several effects on its electroanalytical performance. The higher content of sp2-bonded carbon causes a faster electron transfer, reduced potential window, higher background current, and make the surface more sensitive to fouling. The catalytic effect of sp2 carbon in redox reactions is also not negligible in comparison with the catalytically inert sp3 carbon [1,2]. Technological parameters are usually optimized to minimize the concentration of sp2 nondiamond carbon. Surface termination of BDD is the factor that strongly influences the electron transfer kinetics on the electrolyte/electrode interface, and for explanation, it is the most complicated factor. After the CVD procedure, the BDD surface is terminated by hydrogen (H-terminated, HBDD). The native surface of HBDD can be altered to an oxygen-terminated surface (O-terminated, OBDD) using different techniques such as treatment in oxygen plasma, anodic electrochemical pretreatment, photochemical oxidation, reaction in oxygen at high temperature or boiling in acid [1,2]. The surface of as-grown HBDD is positively charged and initially hydrophobic. A weakness of the H-terminated surface is its stability in the air with time, as this surface partially converts to O-terminated [2]. On the contrary, in comparison with HBDD, the OBDD surface is charged negatively, strongly hydrophilic, and possesses long-term stability in air. Moreover, its surface is more stable to fouling and provides a wider potential window for analysis. Also, it allows the surface reactivation to remove adsorbed compounds by anodic pre-treatment before analysis. A question is what termination of the BDD surface is more preferred for electroanalysis. For selectivity and sensitivity of analysis, better results were obtained for some analytes with HBDD, and others with OBDD or no changes in responses of both were observed [1]. It can be concluded that the BDD properties should be tailored particularly for each electroanalytical application. Based on the above advanced features, BDD is a prospective electrode material for electroanalytical applications, especially of organic molecules and biomolecules [1,9,10]. An overview of electrochemical detection of various organic analytes such as drugs (indapamide hydrochloride, leucovorin, imatinib, paracetamol, cysteamine, ibuprofen, warfarin), antibiotics (ciprofloxacin, levofloxacin, metronidazole), pesticides (formetanate), insecticides (imidacloprid) and many other organic compounds (dopamine, melatonin and others) at an unmodified BDD electrode is summarized in [1]. On the other hand, chemical or biochemical modification/functionalization of the BDD surface was applied to improve the response or selectivity of determination [1,11]. The detection was improved of dopamine and proteins by forming an electropolymerized film on the BDD surface. Enhanced response after modification by gold nanoparticles was referred for chloramphenicol, dopamine and neuraminidase. Also, the metals prepared in different ways on BDD successfully influenced the detection of tetracycline, glucose, carbohydrates and aminoacids. Sensors based on chemical modification of BDD by forming a surface with amino groups were developed for glucose and phenol estrogenic derivatives.

Similarly, a sensor was prepared on the BDD surface with aminophenyl functionalities for phenol, p-kresol and 4-CP detection. The BDD electrodes are used for the determination of trace metals concentrations by the electroanalytical method based on anodic stripping voltammetry (ASV). By this fast and cheap method, the metal is accumulated first on the BDD surface at a sufficiently negative potential from the analysed solution, and then it is stripped back to the solution by reoxidation at a positively changing potential. During this step, the oxidation peak is recorded as a response. At the analysis, the metals can be accumulated either directly on the BDD surface [3,12] or simultaneously by codeposition with bismuth. With the so-called Bismuth Film Electrodes (BiFEs) [13–18] which successfully replaced the toxic mercury in voltammetric analysis it is possible to determine various metals such as Pb, Cd, Zn, Tl, In, Sn, Cu, Fe, Mn, Co, Ni, U, Al, V, Mo, As, Sb, Se. The attractive features of BiFEs include in addition to negligible toxicity of Bi, also enhanced sensitivity and selectivity, low limit of detection, insensitivity to dissolved oxygen in the electrolyte, simple preparation, allowing fast and inexpensive analysis [13,18]. This paper presents the results of voltammetric characterization of UNC BDD electrodes prepared by CVD method on alumina substrates for their applications in electroanalysis. The influence was investigated of the boron dopant concentration, their surface termination as well as chemical surface treatment upon the kinetics of the electrode reaction. A standard redox couple, ferrocyanide in aqueous solution, was employed to determine kinetic parameters such as anodic transfer coefficient, diffusion coefficient, and electron transfer rate constants. The electrochemical examination of UNC BDD electrodes is supplemented by structural and morphological characterization, performed by scanning electron microscopy and Raman spectroscopy as well as by electrical measurements for conductivity evaluation. Finally, the influence of the surface conditions of BDD electrodes based on the BiFEs on the kinetics of bismuth electrodeposition is evaluated for electroanalytical applications. 2. Experimental 2.1. Preparation of BDD films, characterization of their morphology and structure The UNC BDD electrodes were deposited on alumina substrates with a relatively rough surface (ra ~ 130 nm). Before deposition, the substrates were purified in NH4OH/H2O2 solution for 15 min at 80 °C and after their rinsing in deionized water they were annealed at 1000 °C for 1 h in air. Then, the substrates were spin-coated with LOL 2000 (lift-off layer) and ultrasonically nucleated for 40 min in a suspension of nanodiamond (5 nm in size) powder in deionized water. For fabrication of BDD electrodes with a defined design, the nuclei from the selected area on the substrate hat to be removed. The substrates were spin-coated with a positive photoresist (S-1813, Shipley) and subsequently the mask of electrodes was patterned using photolithography. During this step, the developer removes not only the exposed resist and lift-off sublayer but also all diamond nuclei from this part of surface. Finally, the resist was removed from the surface by oxygen plasma (PE-200 Plasma Etch). The BDD growth was carried out in a linear antenna microwave chemical vapor deposition (LA MWCVD) reactor (modified SCIA cube 300) with a maximum power of 2 × 6 kW in pulse mode. The 15 hour film growth was performed in a H2/TMB/CH4/CO2 gas mixture where CH4 and CO2 to H2 ratios were 0.5% and 0.2%, respectively. The flow of TMB (trimethylborane) dissolved in hydrogen was set to obtain 40 kppm and 160 kppm of boron to carbon (B/C) concentration. Deposition pressure was 25 Pa and substrate temperature was 650 °C for all experiments. The active area of the electrode was circular in shape with a diameter of 800 μm. A conductive BDD strap between the active area and the BDD contact pad of the electrode was covered by a SiO2 insulating layer with a thickness of 500 nm. The insulation layer was prepared by CVD technique and then patterned by etching through a photolithography mask in buffered hydrofluoric acid. A copper wire was connected to the contact pad of the 2

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Journal of Electroanalytical Chemistry 862 (2020) 114020

electrode conductivity increased after cathodic treatment (Fig. 1, left). With regard to the dependence of peak separation on the scan rate, the separation after additional cathodic treatment of BDD in acid solution rose from 77 mV (at 50 mV s−1) to 104 mV (at 500 mV s−1). This result indicates that the electrode kinetics is controlled by the charge transfer rate on the electrolyte/electrode surface interface. Interestingly, we did not observe any limitation of the peak current increase in dependence on the square root of the scan rate in the range from 50 to 500 mV s−1. In the second case, the surfaces of the as-grown BDD electrodes were oxygen-terminated using either electrochemical treatment at +2 V for 10 min in 0.5 mol L−1 H2SO4 [22,23] or by using oxygen plasma (100 W, 100 sccm, 1 min) (Fig. 1, right). Partial transformation of the surface could also have occurred at the preparation of the SiO2 (500 nm) insulation layer by CVD, which is confirmed by the increased peak separation. The peak separation increased most significantly after the BDD was terminated in oxygen plasma (126 mV at 50 mV s−1 and 168 mV at 500 mV s−1) and less after electrochemical oxidation (103 mV at 50 mV s−1 and 132 mV at 500 mV s−1). Redox reaction is controlled by the charge transfer rate even in this case. From the reversibility point of view this single-electron redox reaction can be considered to be a quasi-reversible reaction because in a given range of the potentials the criterion for a Nernstian behaviour is not fulfilled for any BDD electrodes [24]. The important kinetics parameters such as anodic transfer coefficient, diffusion coefficient, and electron transfer rate constants were determined for both hydrogen- and oxygen-terminated BDD electrodes from cyclic voltammograms of ferro/ferricyanide couple at 50 mV s−1. The anodic transfer coefficient (αa) was calculated from a Tafel plot [25]. The plot was drawn from the ascending part of the anodic peak as the dependence of the logarithm of current vs. potential. The slope of this plot was used for the calculation of αa, according to Eq. (1).

electrode by a silver paste CB 115 (DuPont Electronic Materials). Ohmic contacts were verified using cyclic voltammetry (a linear dependence was measured of current on voltage in the range of ±2 V). The electrical conductivity of BDD layers was evaluated from the slopes of the linear sweep voltammograms recorded by PalmSens2 Instrument in the range of ±0.5 V. The current was measured using two gold tips with a 5 mm distance between them at different temperatures in the chamber PLV 50 (Cascade Microtech) with a temperature controlled table. The morphology of the BDD surface was analysed by scanning electron microscope (SEM) JEOL JSM 7500F. The concentrations of boron dopant in BDD films were determined by secondary ion mass spectrometry (SIMS) (IonTOF SIMS IV) at the ILC in Bratislava (International Laser Center). The diamond layers were investigated by Raman spectroscopy (with wavelength of 514 nm). Using this method it was possible to distinguish the two different levels of boron doping and content of non-diamond sp2 carbon. Raman spectra were acquired with MonoVista CRS 750/BX51 Raman Microscope System. 2.2. Electrochemical measurements Voltammetric measurements were performed with the PalmSens2 sensor interface (Palm Instruments BV). A three-electrode set-up with a BDD working electrode was used with a magnetic stirrer. As a reference, an Ag/AgCl/Cl− (3 mol L−1) electrode and a platinum electrode as an auxiliary electrode were used. Kinetic parameters of electrode reactions of differently treated BDD electrodes were investigated by cyclic voltammetry at different scan rates at room temperature. As the measurements were carried out in relatively concentrated electrolyte (0.1 mol L−1 KCl), with a relatively low concentration of the electroactive substance (1 mmol L−1 ferrocyanide) at low scan rates (5–500 mV s−1), the effect of ohmic losses was not compensated. Square wave voltammetry (SWV) was used for evaluation of the influence of BDD electrodes on the electrodeposition of the bismuth film.

Slope ¼ αa F=2:3 RT

ð1Þ

where αa is the anodic transfer coefficient, F is the Faraday constant (C mol−1), R is the gas constant (J K−1 mol−1), and T the temperature (K). The values of 0.59 and 0.48 for hydrogen- and oxygen-terminated BDDs were determined, respectively. They are closely to 0.5 which is a typical value for most reactions. The transfer coefficient is used to describe the symmetry between the forward and reverse electron transfer steps, and its value is from the range from 0 to 1. In general, the higher its value, the more significant the impact of potential on the rate constant of the reaction [19,26,27]. For both electrodes, the diffusion coefficients from the dependence of peak current on the square root of the scan rate in 1 mmol L−1 ferrocyanide in 0.1 mol L−1 KCl were determined through the Randles-Sevcik equation (Eq. (2)) [24,25].

2.3. Chemicals All solutions of ferrocyanide, KCl, dopamine, ascorbic acid, H2SO4, NH4OH, H2O2, HCl, HNO3, NaNO3, BiNO3 and acetate buffer (pH 4.5) were prepared in 18.2 MΩ cm deionized water (Millipore) from analytical grade chemicals. 3. Results and discussion 3.1. Surface termination of BDD films For quality assessment of differently terminated BDD films, cyclic voltammetric measurements with the redox couple of ferro/ferricyanide in electrolytes were used. Although this pair belongs to outer-sphere redox systems, reactions of whom are generally insensitive to surface properties, it is known from the literature [4,19–21] that the kinetic parameters of this redox reaction can be strongly influenced by the state of the electrode surface. Four samples of BDD, one with a native surface (as-grown) and others with surfaces treated either by electrochemical method (cathodically or anodically) or in oxygen plasma, were used for investigation of their kinetic parameters. Semiconductive BDD electrodes with a boron concentration of 6.4 × 1019 atoms cm−3 were used for these investigations. Since the electrode is hydrogen-terminated after preparation by CVD, its surface was strongly hydrophilic. However, it is also known [2,11] that with time the hydrogen-terminated surface of BDD in air is changed to a surface terminated with oxygen due to the presence of oxygen. Therefore this native BDD electrode was compared with the electrode that was polarized additionally at −2 V in 0.5 mol L−1 H2SO4 for 10 min immediately before measuring. A significant change was found for the surface termination since the peak separation was reduced from 148 to 77 mV at 50 mV s−1, and the

I p ¼ 2:69  105 n3=2 ACD1=2 υ1=2

ð2Þ

where Ip is the peak current (A), n is the number of electrons, A is the surface area (cm2), C is the concentration (mol L−1), D is the diffusion coefficient (cm2 s−1), and υ the scan rate (V s−1). A higher coefficient (1.76 × 10−5 cm2 s−1) was found for oxygenterminated BDD in comparison with hydrogen–terminated (7.2 × 10−6 cm2 s−1). Probably, the negatively charged surface of oxygen-terminated BDD promotes the anodic oxidation of ferrocyanide. We used the Nicholson method [27] to determine the heterogeneous electron transfer rate constant k0, which is based on the peak separation according to Eq. (3). k 0 ¼ Ψ ½πD0 nFυ=RT 1=2

ð3Þ

where the k0 is the electron transfer rate constant (cm s−1), Ψ is a dimensionless parameter and the other parameters were mentioned previously. 3

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as-grown -2V in H2SO4for 10 min

1.2

oxygen plasma (100W) for 1min +2V in H2SO4 for 10min after SiO2 mask formation

0.8

Current [ A]

Current [ A]

0.8

0.4

0.0

-0.4

0.0

-0.4

-0.8

-0.6

0.4

19

-0.8

-3

6.4 x10 at.cm of boron -0.4

-0.2

0.0

0.2

0.4

0.6

19

-3

6.4 x10 at.cm of boron

0.8

1.0

-0.6

-0.4

-0.2

0.0

Potential [V]

0.2

0.4

0.6

0.8

1.0

Potential [V]

Fig. 1. Cyclic voltammograms of 1 mmol L−1 ferrocyanide in 0.1 mol L−1 KCl for H-terminated (left) and O-terminated (right) BDD electrodes. Scan rate 50 mV s−1.

where Λ is the electrochemical reversibility, f is the ratio of F to RT, and the other parameters were mentioned previously. According to this equation, Λ values 2.70 for hydrogen-terminated and 0.61 for oxygen-terminated BDD at a scan rate of 50 mV s−1 were calculated. Whereas Λ values are in the range from 15 to 10–2(1+α) [20] for both electrode materials, the redox reaction of ferrocyanide taking place on them is evaluated as a quasi-reversible reaction. This conclusion has confirmed our initial results mentioned above. The impact of BDD termination on the response of the analyte is illustrated by a well-known example of the simultaneous determination of dopamine in presence of L-ascorbic acid [29,30]. Using a hydrogenterminated BDD electrode, both substances could not be detected simultaneously (Fig. 2, left) due to the peaks overlapping (the anodic peak separation was only 75 mV when the substances were measured individually). On the contrary, with oxygen-terminated BDD, the distance between the two anodic peaks was even 580 mV at single measurements, which makes it possible to distinguish the two substances (Fig. 2, right). Supposed mechanism of these interactions is described in details in [31].

For this method, a tabulated dimensionless parameter Ψ is assigned to various peak separations [25,27]. In our case, for the hydrogenterminated BDD electrode parameter Ψ had a value equal to 1.539 at 50 mV s−1. After calculating according to Eq. (3), this value corresponds to the k0 constant with the value of 1.01 × 10−2 cm s−1. Similar values ranging between 0.01 and 0.1 cm s−1 for hydrogen-terminated BDD electrodes without any treatment were reported in [21,28]. For the OBDD made by oxygen plasma, parameter Ψ was lower (0.320), and the k0 constant was 3.6 × 10−3 cm s−1. Lower k0 value, i.e. the slower electron transfer through the interface, in the case of the oxygen-terminated BDD is caused by the oxygen functionalities on the BDD surface. These functionalities are known to inhibit the electrode kinetics for ferro/ferricyanide at BDD, and the constant can be reduced by over two orders of magnitude for oxygen-terminated BDD [28]. Based on the known values of the electron transfer rate constant k0 and the diffusion coefficient D, the electrochemical reversibility could be quantified for the redox reaction of the ferrocyanide. The electrochemical reversibility Λ should be defined correctly according to [20] as a ratio of the charge transfer rate to mass transfer rate, given by Eq. (4):

3.2. Concentration of boron dopant in BDD film Λ ¼ k 0 =ðDfυÞ0:5

ð4Þ

The effect of two boron concentrations 1.4 and 6.4 × 1019 atoms cm−3 (this corresponds to 40 and 160 kppm of boron in the gas phase,

2.0

1.5

3.0

Ascorbic acid Dopamine Dopamine + Ascorbic acid

2.5

Ascorbic acid Dopamine Dopamine + Ascorbic acid

Current [ A]

Current [ A]

2.0 1.0

0.5

Eox separation = 75 mV

1.5 1.0 0.5

0.0

Eox separation = 580 mV 0.0

-0.5 19

-0.5

-3

19

6.4 x10 at.cm of boron

-3

6.4 x10 at.cm of boron

-1.0

-1.0 -0.5

0.0

0.5

1.0

1.5

-0.5

Potential [V]

0.0

0.5

1.0

1.5

Potential [V]

Fig. 2. Cyclic voltammograms of aqueous solution of dopamine (1 mmol L−1), L-ascorbic acid (1 mmol L−1) and mixture of dopamine and L-ascorbic acid (0.5 mmol L−1 of each) for H-terminated (left) and O-terminated (right) BDD electrodes. Scan rate 50 mV s−1. 4

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respectively) in the BDD electrodes on their electrochemical properties was investigated. It is known [1] that increasing the content of boron in BDD leads to a decrease in the peak separation of ferrocyanide reaction, which can be caused not only by increasing BDD conductivity but also by the number of active sites for electron transfer (Fig. 3, left). It was shown that the properties of oxygen-terminated BDD containing less boron were significantly influenced, in particular when the surface was treated in oxygen plasma. Besides to lower electrical conductivity, peak separation increased considerably up to 484 mV at 50 mV s−1 (Fig. 3, right). The anodic transfer coefficient was found to be 0.22 for this sample, which means that the voltammetric curve is less symmetrical, and the influence of varying potential on the rate constant of the reaction should be decreased [26,27]. The diffusion coefficient of the ferrocyanide redox reaction was lower (7.9 × 10−6 cm2 s−1) when compared with a highly doped sample (1.76 × 10−5 cm2 s−1). The electron transfer rate constant for oxygenterminated BDD with lower content of boron could not be calculated by the Nicholson method because the peak separation is out of the tabulated range of parameter Ψ. The kinetic parameters of electrode reaction for variously treated BDD electrodes are summarized in Table 1. Both BDD electrodes were investigated for their electrical conductivity. Electrical characterization of BDD layers with Ag contacts revealed an increase of the conductivity with temperature in the range from 30 to 210 °C for both electrodes. The Arrhenius equation was used to determine the conductivity activation energy Ea. The values of 16.7 and 27 meV for samples with a higher and a lower boron concentration respectively, were calculated from the slope of the semilogarithmic dependence of σ on T−1 (Fig. S1). The obtained activation energies are similar to published results of the activation energy vs. boron concentration [32]. Based on these results, we can deduce that both BDD electrodes with lower and higher boron content possess semiconductive properties. However, a different dominant electrical charge transport mechanism is expected for each of these electrodes. For BDD with boron concentration of 6.4 × 1019 atoms cm−3 the hopping mechanism should dominate, while for BDD with lower content (1.4 × 1019 atoms cm−3) the band conduction should be the typical mechanism according to [32].

Table 1 Summary of the kinetics parameters for various BDD electrodes and analytes. BDD [at.cm −3 ] 1.4 6.4 6.4 1.4 6.4

1019 1019 1019 1019 1019

Electroactive substance

αa

αc

Diffusion coefficient [cm2 s−1]

k0 [cm s−1]

OBDD HBDD OBDD OBDD OBDD

Ferrocyanide Ferrocyanide Ferrocyanide Bi3+ Bi3+

0.22 0.59 0.48 – –

– – – 0.66 0.95

7.9 × 10−6 7.2 × 10−6 1.76 × 10−5 0.12 0.37

– 0.0101 0.0036 0.45 1.55

predominantly. This can have an undesirable influence on the electroanalytical properties of the BDD electrodes (a faster electron transfer rate, narrowed potential window, higher current background, or unwanted catalytic activity) [1,2], especially in the case of ultra-nanocrystalline boron doped diamond possessing an increased amount of grain boundaries. There is a possibility to remove/reduce the sp2 phase from/on the surface of completed BDD by wet chemical etching [20,33]. The treatment is carried out in various acidic or alkaline solutions at higher temperature. Two different cleaning procedures were evaluated in this experiment. In the first case, a hot alkaline mixture (70 °C) of NH4OH/H2O2/H2O in the volume ratio of 1/1/5 was applied for 15 min. In the second case, the acidic treatment took place in two steps, first in the solution of HCl/HNO3 (3/1 in vol. ratio) for 30 min at 70 °C, and after rinsing in deionized water in a solution of concentrated H2SO4/HNO3/NaNO3 (1/1/1 in vol. ratio) for 30 min at 70 °C [21]. In Fig. 4, the influence of BDD treatment is shown on its electrochemical properties. A favourable effect of the cleaning procedures was proved, especially for the BDD with a higher content of boron. The current response was increased when compared with an untreated electrode. It is known that the treatment of BDD in strong acids or alkalis at higher temperatures [2,11] is one of the methods how to form an oxygenterminated surface. Therefore, in addition to removing the sp2 carbon phase during the purification process, the oxygen-terminated surface is probably also formed. We found notable changes in voltammograms for as-grown highly doped BDD regarding the current responses before and after treatment. Although the electrical resistance higher by 50 Ω was found for the electrode which was cleaned in alkaline solution, its electrochemical activity for ferrocyanide redox couple was higher. This result is inconsistent with the results presented in [20], where a decrease in electrode activity was observed after the sp2 quantity was reduced. The electrical resistance of both electrodes (as-grown HBDD and post-treated OBDD in alkaline solution) was measured by electrochemical impedance spectroscopy (EIS) (at 1 V

3.3. Non-diamond sp2 carbon phase in BDD film Polycrystalline BDD is essentially a composite material consisting of majority diamond sp3 and minority non-diamond sp2 carbon phases. Since the formation of sp2 phase cannot be entirely prevented during the CVD preparation, the parameters of the technology should be optimized to minimize the content of sp2. Non-diamond carbon is considered as impurity incorporated in BDD on the grain boundaries and intergranular regions 1.4

1.4 19

-3

6.4 x 10 at.cm of boron 19 -3 1.4 x 10 at.cm of boron

1.2 1.0

19

1.2

-3

6.4 x 10 at.cm of boron 19 -3 1.4 x 10 at.cm of boron

1.0

0.8

0.8

Current [ A]

Current [ A]

× × × × ×

Termination

0.6 0.4 0.2 0.0 -0.2

0.6 0.4 0.2 0.0 -0.2

-0.4

-0.4

Anodic treatment +2V in H2SO4 for 10min

-0.6

Treatment in oxygen plasma 100sccm O2, 100W for 1min

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Fig. 3. Cyclic voltammograms of 1 mmol L−1 ferrocyanide in 0.1 mol L−1 KCl for BDD electrodes with lower and higher content of boron after their termination by anodic treatment in 0.5 mol L−1 H2SO4 (left) and oxygen plasma (right). Scan rate 50 mV s−1. 5

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1.4 1.2 1.0

1.4

Alkaline treatment Acidic treatment as-grown

1.2 1.0 0.8

Current [ A]

Current [ A]

0.8 0.6 0.4 0.2 0.0 -0.2

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Fig. 4. Cyclic voltammograms of 1 mmol L−1 ferrocyanide in 0.1 mol L−1 KCl for as-grown BDD electrodes and electrodes after their treatment in alkaline and acidic solutions. Scan rate 50 mV s−1.

in 0.1 mol L−1 KCl). The higher electrical resistance of BDD after purification is related to a lower content of sp2 carbon, since the non-diamond carbon species can be considered as the primary pathway of charge transfer with a significantly higher of charge transfer rate constant [20]. The morphology of the BDD surface was analysed by SEM. The images showed that the films are polycrystalline with the ultra-nanocrystalline grain size with a thickness of 780 nm (Fig. S2) without any significant defects such as cracks or pinholes. Raman spectroscopy, in general, is a method appropriate for the quality characterization of the BDD films. In Fig. S3, Raman spectra are shown of BDD films with a higher and lower boron content for treated surfaces. The measured Raman spectra contain typical maxima of heavily boron doped diamond at about 470 cm−1 (B1), 1200 cm−1 (B2), 1290 cm−1 (sp3) and 1530 cm−1 (sp2). The sp3 maximum belongs to the remains of zone center phonon maximum of diamond and is shifted due to the interference of electronic and optical phonons transitions from its original position for single crystal diamond (1333 cm−1). The B2 band relates to the optical phonon confinement at lattice imperfections caused mainly by boron incorporation. The B1 band also relates to the boron content and was recently connected to the acoustical phonon confinement [33]. Small maxima of transpolyacetylene (tPA) at 1140 cm−1 and 1470 cm−1 and amorphous carbon at 1350 cm−1 are also visible in the recorded spectra. Because the laser source with the wavelength of 514 nm was used, the responses come from the entire volume of BDD film, not just from its surface. Then the effect of cleaning cannot be evaluated quantitatively from these spectra since the sp2 carbon is removed only from the surface. Nevertheless, we observed slight changes in Raman spectra. While the ratio of sp 3 to sp 2 intensity was calculated to be 2.7 for as-grown BDD, in the case of the oxygen-terminated BDD after consecutive treatment in the alkaline bath this ratio was increased to 3.9. These results are consistent with the results of electrochemical measurements (Fig. 4, right) which showed that increased electrode activity after chemical treatment was accompanied by a decrease in the sp 2 carbon content on the BDD surface [34]. A typical nanocrystalline diamond spectrum was recorded in the case of sample with lower boron doping level showing the sp3 peak at 1335 cm−1, amorphous carbon band at 1350 cm−1 as well as the peaks at 1140 and 1470 cm−1, originating from polyacetylene type of molecule [2].

(glassy carbon, carbon paste, carbon fiber, carbon tubes, pencil-lead, pyrolyzed photoresist film, pyrolytic graphite, screen-printed carbon ink, diamond-like carbon, boron-doped diamond) but also metals (platinum, gold and others) or silver amalgam were reported [13,15,17,18,35,36]. Among them, the boron-doped diamond is a perspective electrode material due to its superior mechanical, electrical, and electrochemical properties. The bismuth film serves as a matrix in which the analysed substances accumulate. Therefore, for the best analytical results, optimum conditions have to be provided for electroplating of bismuth on the BDD supporting electrode. In this respect, electrodeposition on variously treated surfaces of BDD was investigated. In Fig. S4 the voltammograms are compared for the oxygen-terminated BDD electrodes (treated by oxygen plasma) with a different concentration of boron dopant. The kinetic parameters of the cathodic reduction of Bi3+ (300 μg L−1) in 0.1 mol L−1 acetate buffer solution (pH 4.5) calculated from cyclic voltammograms (Fig. S4) are shown in Table 1. It can be concluded that the mass transport rate as well as the electron transfer rate on the interface were significantly higher for reaction at the more conductive BDD electrode, of course. The influence of treatment of the BDD surface on the bismuth deposition/dissolution kinetics for BDD with lower and higher boron contents is shown in Fig. 5. A considerably slower reaction kinetics was observed for the as-grown BDD electrode. In this case, not only a lower electrode activity of as-grown BDD but probably also repulsive forces between bismuth cation and the surface positively charged contribute to this [21]. The effect of the treatment procedure of BDD on bismuth deposition was verified using a square wave voltammetric method that is the most frequently method used for analysis with BiFEs. Evaluation of the SWVs confirmed that after surface treatment by oxygen plasma and consecutive alkaline treatment the BDD with a higher boron content had the most suitable properties for the formation of the bismuth film electrodes. Using an oxygen plasma-treated BDD electrode, the SWV response of bismuth increased by 64% and after its subsequent cleaning in an alkaline solution by 109% in comparison with as-grown BDD (Fig. 6). The weak peak at ~ 0 V originates from reoxidation of bismuth, probably localized at the grain boundaries. The effect of the quality of bismuth film on analytical performance of the voltammetric sensor was verified in simultaneous analysis of heavy metals (Pb, Cd and Zn) by anodic stripping voltammetry. For comparison, the square wave voltammograms which were performed on higher doped OBDD (treated by oxygen plasma and modified in alkaline bath) and HBDD (as-grown) as well as on less doped OBDD (treated by oxygen plasma and modified in alkaline bath) for simultaneous determination of Pb(II), Cd

3.4. BDD as support for bismuth film electrodes As an electrode support for bismuth film formation from electrolyte by cathodic reduction of Bi3+, a variety of the carbonaceous materials 6

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Journal of Electroanalytical Chemistry 862 (2020) 114020

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Fig. 5. Cyclic voltammograms recorded for 300 μg L−1 of Bi3+ in 0.1 mol L−1 acetate buffer with pH 4.5 for as-grown BDD electrodes and electrodes after their treatment in oxygen plasma (100 sccm, 100 W for 10 min) and alkaline solution (NH4OH/H2O2/H2O in the volume ratio of 1/1/5 at 70 °C for 15 min). Scan rate 50 mV s−1.

4. Conclusions

(II) and Zn(II) in an aqueous solution are given in Fig. 7. The quality of the bismuth film, which depends on the surface properties of BDD, obviously significantly affects the accumulation of the target metals. It should be noted that no response was recorded if bismuth was not present in the analyte. Excellent performance of BiFE was observed only in the case of the pre-treated OBDD electrode with a higher content of dopant. Fig. 8 displays representative stripping voltammograms for this electrode. All of the peaks are without any interference, well resolved and increase linearly with the metal concentration in the range from 1 to 10 ppb. A preconcentration time of 180 s was selected for the calibration. The analytical sensitivities for Pb(II), Cd(II) and Zn(II) were 0.083 (R = 0.999), 0.066 (R = 0.999) and 0.092 (R = 0.997) μA ppb−1, respectively. The resulting calibration plots (Fig. S5) were used for estimating the limit of detection (LOD). On the basis of the criteria for calculation of LOD (Y intercept +3 × SD), the detection limits were estimated as 0.31, 0.37 and 0.81 ppb for Pb(II), Cd(II) and Zn(II), respectively. For comparison, in the case of as-grown HBDD electrode with the same dopant concentration the sensitivities were considerably lower (0.004, 0.004 and 0.010 μA ppb−1 for Pb, Cd and Zn, respectively). The values of LOD were 1.49, 1.89 and 1.40 ppb for Pb(II), Cd(II) and Zn(II), respectively.

This paper presents the results of voltammetric characterization of polycrystalline BDD electrodes prepared by CVD on alumina substrates. The influence was investigated of the boron dopant concentration, surface termination and content of sp2 impurities in BDD upon the kinetics of the electrode reaction. A standard outer-sphere redox couple, ferro/ferricyanide in aqueous solution of 0.1 mol L−1 KCl, was employed for determining the transfer and diffusion coefficients and electron transfer rate constants by cyclic voltammetry. The results can be summarized as follows: - Ultra-nanocrystalline BDD electrodes with different boron dopant concentrations (1.4 and 6.4 × 1019 atoms cm−3) were grown on alumina substrates by chemical vapor deposition. - The conductivity activation energy 16.7 and 27 meV were determined for BDD with the lower and higher boron concentration, respectively. Both BDD electrodes possess semiconductive properties. - Four samples of BDD with boron concentration 6.4 × 1019 atoms cm−3, one with a native surface (as-grown) and others with surfaces treated either by electrochemical method (cathodically or anodically) or in

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Fig. 6. Square wave voltammograms of 300 μg L−1 of Bi3+ in 0.1 mol L−1 acetate buffer with pH 4.5 for as-grown BDD electrodes and electrodes after their treatment in oxygen plasma (100 sccm, 100 W for 10 min) and alkaline solution (NH4OH/H2O2/H2O in the volume ratio of 1/1/5 at 70 °C for 15 min). Edep = −1.3 V, tdep = 60 s, Estep = 10 mV, amplitude = 50 mV, and frequency = 50 Hz. 7

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Journal of Electroanalytical Chemistry 862 (2020) 114020

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chemical treatment of BDD, the redox reaction of ferrocyanide taking place on them was at scan rate of 50 mV s−1 evaluated as a quasireversible reaction controlled by the charge transfer rate. - The influence of the surface conditions of BDD electrodes on the kinetics of bismuth electrodeposition (300 μg L−1 of Bi3+) in 0.1 mol L−1 acetate buffer solution was investigated. High values of the diffusion coefficient (0.37 cm2 s−1) and rate constant (1.55 cm s−1) were found for OBDD electrode (6.4 × 1019 atoms cm−3) treated in oxygen plasma. The effect of the treatment procedure of BDD on bismuth deposition was verified by square wave voltammetry. Using an oxygen plasmatreated BDD electrode, the SWV response of bismuth increased after its subsequent cleaning in an alkaline solution by 109% in comparison with the as-grown BDD. - The effect of the quality of bismuth film on analytical performance of the voltammetric sensor was verified in simultaneous analysis of heavy metals (Pb, Cd and Zn) by anodic stripping voltammetry. With a highly doped chemically treated OBDD electrode the detection limits were estimated as 0.31, 0.37 and 0.81 ppb for Pb(II), Cd(II) and Zn(II), respectively.

Bi

OBDD 6.4x10 at.cm (O2 plasma + alkaline treatment) HBDD 6.4x10 at.cm (as-grown) 19 -3 OBDD 1.4x10 at.cm (O2 plasma + alkaline treatment)

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Potential [V] Fig. 7. Square wave stripping voltammograms of target metals with concentration 10 ppb in 0.1 mol L−1 acetate buffer solution with pH 4.5 for as-grown BDD electrodes and electrodes after their treatment in oxygen plasma (100 sccm, 100 W for 10 min) and alkaline solution (NH4OH/H2O2/H2O in the volume ratio of 1/1/5 at 70 °C for 15 min). Edep = −1.3 V, tdep = 180 s, Estep = 10 mV, amplitude = 50 mV, and frequency = 50 Hz. Concentration of Bi3+: 100 μg L−1.

CRediT authorship contribution statement V. Rehacek: Supervision, Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. I. Hotovy: Funding acquisition, Project administration. M. Marton: Investigation, Resources, Writing - review & editing. M. Mikolasek: Funding acquisition, Investigation, Formal analysis. P. Michniak: Investigation. A. Vincze: Investigation. A. Kromka: Writing - review & editing. M. Vojs: Funding acquisition, Resources, Project administration.

oxygen plasma were used for investigation of their kinetic parameters. Depending on termination, evident changes were observed in voltammograms from which the kinetics parameters were calculated. The highest diffusion coefficient (1.76 × 10−5 cm2 s−1) was found for OBDD electrode treated in oxygen plasma. The calculated rate constant for this electrode was 3.6 × 10−3 cm s−1. - Attempts were made to remove the unwanted sp2 impurities from the surface of completed BDD by wet chemical etching. Two different cleaning procedures (in alkaline or acidic bath solution) were evaluated in this experiment. A favourable effect of the cleaning procedures was proved, especially for the BDD with a higher content of boron in alkaline solution. The anodic current response was increased by 36% when compared with an untreated electrode. - Independently of the concentration of boron, surface termination and

Zn

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and of the Slovak Academy of Sciences VEGA 1/0563/20 and 1/0558/17 and Slovak Research and Development Agency under the contract APVV-16-0266, APVV-17-0169 and APVV-16-0124.

Pb

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2020.114020.

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References

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