- Email: [email protected]
Structured and graphitized boron doped diamond electrodes: Impact on electrochemical detection of Cd2+ and Pb2+ ions
Vacuum 170 (2019) 108953
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structured and graphitized boron doped diamond electrodes: Impact on electrochemical detection of Cd2+ and Pb2+ ions
T
Pavla Štenclováa,∗, Vlastimil Vyskočilb, Ondrej Szabóa, Tibor Ižáka, Štěpán Potockýa, Alexander Kromkaa a
Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00, Prague 6, Czech Republic UNESCO Laboratory of Environmental Electrochemistry, Department of Analytical Chemistry, Faculty of Science, Charles University, Albertov 6, 128 43, Prague 2, Czech Republic
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Boron doped diamond Reactive ion etching Structuring Nanorods Cyclic voltammetry Metal ions Cd2+ Pb2+
Here we introduce the structuring of boron doped diamond (BDD) electrodes using a self-organised mask and reactive ion etching (RIE) in oxygen plasma for electrochemical measurements of selected metal ions. The samples were analysed by SEM, Raman and X-ray photoelectron spectroscopy. The electrochemical performance of structured BDD was compared to as-grown BDD and standard glassy carbon (GC) electrodes employing standard cyclic voltammetry and differential pulse anodic stripping voltammetry. Separate and simultaneous detection of Cd2+ and Pb2+ ions is discussed in relation to the electrode morphology and content of graphitic phase. To determine trace amounts of metal ions we evaluated the repeatability of measurements, which is statistically expressed as the relative standard deviation (RSD) of consecutive determinations. Based on RSD data, the most suitable candidate for the separate detection of Cd2+ and Pb2+ ions was found the structured BDD electrode consisting of nanotips and small nanorods, respectively. Two times better RSD was found for the structured BDD than for the as-grown BDD electrode. Electrochemical measurements provided in the complex mixture consisting both of Cd2+ and Pb2+ ions revealed the small BDD nanorods as the most suitable surface morphology to separate recognition of Cd2+ and Pb2+ ions.
1. Introduction Electroanalytical techniques were established as important methods to detect toxic traces of metals in an environment. It is due to their high sensitivity and easy operational procedures. Boron doped diamond (BDD) have attracted a considerable attention in electroanalytical detection of trace metals [1,2] due to its extremely high chemical stability, a wide potential window in aqueous media with a low background current and stable surface state without the tendency to fouling [3]. For example, McGaw et al. have shown that BDD electrodes exhibit many of the same electrode properties that mercury electrodes do: low detection limits (mid to low ppb range) for Cd2+, Pb2+, Cu2+ and Ag+ with a reproducibility of less than 5% RSD [4]. Tall et al. have noticed 3–5 times higher sensitivity for the BDD electrodes determining Cd2+ and Pb2+ by anodic stripping voltammetry in comparison to glassy carbon electrodes [5]. Extremely low limits of detection at BDD electrodes were measured by Bezerra dos Santos et al. [6], who determined 0.18 μg/L for Cd2+ and 0.08 μg/L for Pb2+ in simultaneous analysis together with very low RSD data (~3.5%). Manivannan et al. reported
∗
that after performing a pre-concentration of the analyte at −1.1 V for 15 min, Pb2+ is detectable at sub-ppb levels in tap water [7]. Recently, Pei et al. replaced mercury electrodes by BDD electrodes for detection of Cd2+ and Pb2+ ions in water sources, achieving detection limits of 3.39 μg/L and 3.62 μg/L, respectively [8]. Thus, it can be concluded that BDD electrodes, particularly used in combination with anodic stripping voltammetry, appear a suitable mercury-free tool for the detection of metal traces. Moreover, very simple miniaturization of BDD electrodes [9,10] makes them ideal working electrodes for the portable miniaturized electrochemical devices – the priceless tools for measurements in small sample volumes. Additionally, BDD can be structured and patterned by different ways to increase electrochemical performance [11,12]. Various patterns, such as nanograss arrays [13], vertical nanowires [14–16], nanorod forest [17] or nanoporous honeycombs [18], have been already reported to increase the response for inorganic as well as organic analytes. In this contribution, we report on fabrication and characterization of structured BDD electrodes featured by nanotips' or nanorods' surface
Corresponding author. E-mail address: [email protected] (P. Štenclová).
https://doi.org/10.1016/j.vacuum.2019.108953 Received 29 November 2018; Received in revised form 19 June 2019; Accepted 18 September 2019 Available online 19 September 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Fig. 1. a) Schematic drawing of the fabrication process for structuring boron doped diamond films including three main steps: evaporation of thin metal layer, the formation of clusters and reactive ion etching of diamond. b) SEM images of surface morphology of BDD films after the formation of gold nanocluster's masks (i.e. after step 2 and before RIE). Two types of the mask were prepared: the so-called “small” and “large” mask depending on the initial thickness of the evaporated Au layer.
2.2. Electrochemical analysis of BDD electrodes
morphology. Their electrochemical performance is compared to the asgrown BDD and standard glassy carbon (GC) electrode. Results from separate and simultaneous detection of Cd2+ and Pb2+ ions are discussed in a relation to the electrode surface morphology and content of the graphitic phase (i.e. sp2 vs. sp3 carbon bonds).
All electrochemical measurements were performed in a three-electrode system using: (i) BDD electrodes as a working electrode, (ii) Ag|AgCl electrode as a reference electrode and (iii) Pt wire as an auxiliary electrode. For a comparison, GC electrode (ED Turnov) based on glassy carbon material was used as a working electrode. Autolab Potentiostat (model PGSTAT302 N, Metrohm) was used to operate the electrochemical processes via Nova 2.0.2 software. Prior to electrochemical measurements, all BDD electrodes were electrochemically activated by applying an electric potential −3 V for 5 min in 2 M HNO3. The electrochemical window was measured in a solution of 0.1 M KCl or in 0.1 M HCl by cyclic voltammetry. The current limit was set to ± 50 μA. Reversibility of the electrode processes was tested by cyclic voltammetry in 1 mM [Fe(CN)6]3−/4− in 0.1 M KCl. The scan rate was 0.1 V/s, with a potential step of 0.001 V. Each measurement was repeated 10 times. The electroactive electrode area was calculated using the RandlesSevcik equation (1):
2. Experimental part 2.1. Fabrication of structured BDD electrodes The boron doped diamond films (~2 μm in thickness) were grown using hot filament chemical vapour deposition on Si substrates by addition of trimethylboron (10 000 ppm) to the H2/CH4 gas mixture [19,20]. The surface of the diamond film was further structured by reactive ion etching (RIE) in pure oxygen plasma either employing gold nanoclusters as the masking material (Fig. 1) [21] or without any mask. The gold nanoparticle's masks were prepared by evaporating a thin Au layer (thickness ~3 or ~12 nm) and further treated by microwave plasma in a hydrogen atmosphere (at 500 °C for 10 min) in order to obtain an array of nano-sized metal droplets on the diamond surface [22]. Two types of the mask were prepared: the so-called “small” and “large” mask pre-defined by the initial thickness of the evaporated Au layer (i.e. 3 or 12 nm, respectively). In the case of the small mask, the surface of the BDD film was covered by circle-shaped nano-sized droplets with a diameter about ~20–30 nm, while for the large mask irregularly-shaped droplets were formed with size ranging from 80 to 120 nm (Fig. 1b). Four types of samples were prepared: (i) as-grown BDD, (ii) BDD nanotips (obtained using RIE in oxygen plasma without any mask), (iii) small and (iv) large BDD nanorods (by applying “small” and “large” mask of gold nanocluster's array) (see Fig. 2). After the RIE process, the remaining Au clusters were etched away by standard wet chemical etching process (HNO3:HCl in 1:3, v/v). The surface morphology of the samples was characterized by scanning electron microscopy (SEM, Maia3, Tescan) under 45° angle view, the chemical composition was studied by Raman spectroscopy (InVia Renishaw Raman spectrometer with excitation wavelength 442 nm) and X-ray Photoelectron Spectroscopy (XPS, AXIS Supra, Kratos) [23].
nFvD 0.5 ⎞ ic = ia = 0.4463nFAC ⎛ ⎝ RT ⎠
(1)
where ic is an absolute value of cathodic peak current in A, ia is an absolute value of anodic peak current in A, n is a number of electrons transferred in the redox event, F is the Faraday constant in C/mol, A is an active electrode area in cm2, C is a concentration in mol/mL, v is a scan rate in V/s, D is a diffusion coefficient in cm2/s, R is a gas constant in J/(K·mol) and T is temperature in K. The electroactive areas were compared to real (geometric) electrode areas estimated from optical microscope images. The differential pulse anodic stripping voltammetry (DPASV) was used to monitor the signal of metal ions (Cd2+ and Pb2+) in 0.1 mM CdCl2 and/or 0.05 mM Pb(NO3)2 in 0.1 M HCl. The analyte accumulation was performed in a stirred solution at −1.0 V for 60 s. The measurement was realized between −1.0 and 0.1 V with a step potential of 0.003 V. The modulation amplitude was set to 0.05 V, the modulation time to 0.1 s and the interval time to 0.15 s. These parameters resulted in the scan rate of 0.020 V/s. To assure clear electrode surface, the 2
Vacuum 170 (2019) 108953
P. Štenclová, et al.
electrode was cleaned by employing 0.5 V for 60 s after each measurement. Measurements were repeated 14 times. If not otherwise stated, all chemicals were purchased from SigmaAldrich. Deionized water was produced by a Milli-Q Plus system (Millipore, USA). 3. Results and discussion 3.1. Morphological and chemical characterization of BDD electrodes Fig. 2 shows SEM images of the samples with different surface morphology depending on the fabrication process. The surface morphology of as-grown diamond films reveals crystals with a diameter of 600–900 nm (Fig. 2a). If only oxygen RIE plasma is used without any masking material, the formed surface morphology is dominated by needle-like structures or so-called whiskers or nanotips (Fig. 2b). Such nanotips are preferentially formed along grain boundaries because of differences in the chemical composition of grains (which contain mainly sp3 carbon bonds) and grain boundaries (mainly sp2 bonds) in the diamond film due to the different etching rate of sp2 and sp3 carbon. Moreover, nanotips can be formed also within the grains around intrinsic defects in the diamond crystals or in specific grains depending on their orientation [24]. The formation of nanotips can be suppressed by adding CF4 gas to the gas mixture. Even an addition of a small amount of CF4 gas resulted in a flatter surface [25]. Using the gold nanoclusters mask during the RIE process, the final film morphology resulted in nanorods' array. The size/diameter of the nanorods depended on the size of metal clusters used as the masking material (Fig. 1b). The diameter of the nanorods was in the range of 20–30 nm and 80–120 nm for the small and large mask, respectively (Fig. 2c and d). In the case of the sample with large BDD nanorods (Fig. 2d), regions consisting of nanotips are observed as well. Raman spectroscopy measurements showed typical spectra for BDD (Fig. 3a). The observed three characteristic peaks represent: (i) the diamond's zone centre phonon line at ~1315 cm−1, which is attributed to sp3 carbon bonds and it is redshifted relatively to the intrinsic diamond line (1332 cm−1); (ii) the broad G-band (graphite-band) centred at ~1540 cm−1, which is considered as a mixture of sp3/sp2 carbon bonds with sp2 bonds dominance; and (iii) the asymmetric band at ~1207 cm−1, which is characteristic feature of BDD and indirectly expresses the boron concentration in the diamond [26,27]. In addition, there is also the D-band (defect-band) hidden at ~1368 cm−1 (see fitted peaks in supplementary files, Fig. S1). The diamond-peak area and the peak area centred at ~1207 cm−1 were further used to calculate the change of sp3 fraction and relative boron concentration in the BDD films, respectively (Fig. 3b). Fig. 3b shows i) the ratio of the diamond-peak area to sum of all areas (green patterned bars) and ii) the ratio of the peak area at 1207 cm−1 to sum of all areas (red bars) within the spectral range of 900–1700 cm−1 calculated from deconvoluted Raman spectra (Fig. S1). The highest sp3 fraction (i.e. Areadiam/Areaall ratio) was found for asgrown BDD. The RIE process in all cases decreased the relative amount of sp3 carbon bonds, i.e. increased the amount of sp2 bonds. This finding is in an agreement with XPS measurements (see Tables 1 and 2), which also confirmed an increased relative amount of sp2 bonds. The deconvoluted C 1s and B 1s peaks for selected samples are shown in supplementary files (Fig. S2). Raman spectra also confirmed that the lowest relative boron concentration (i.e. Area1207/Areaall ratio) has the sample with nanotips' array. These findings well agree with XPS measurements (Table 1) where a decrease of the boron concentration from 0.5 to 0.3 at. % compared to as-grown BDD sample is observed. The decrease of relative boron concentration should be attributed to the RIE without any masking material. Without metal nanocluster's mask, the oxygen ions reach the diamond surface much easily and etch the BDD at higher etching rate in comparison to other samples when metal mask was
Fig. 2. SEM images of surface morphology of electrodes: a) as-grown BDD, b) BDD nanotips, c) small and d) large BDD nanorods.
3
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Fig. 4. Electrochemical (potential) window of BDD working electrodes measured using cyclic voltammetry in 0.1 M HCl. GC electrode was added for comparison.
applied. Thus, etching without any mask leads to thinner BDD sample. 3.2. Electrochemical characterization of electrodes Cyclic voltammetry of all BDD electrodes measured in 0.1 M KCl showed a relatively broad electrochemical window (from approx. −1.9 V up to ca. 1.7 V). In an acidic medium (0.1 M HCl), the electrochemical window shrinks to ca. −1.3 V up to ca. 1.5 V (Fig. 4). The broadest potential window was observed for as-grown BDD and BDD with nanotips. On the other hand, BDD nanorods' electrodes showed narrower electrochemical window slightly shifted to positive potentials. This behaviour seems to be attributed to the composition and morphology of the electrode surface. As shown in Table 2, the as-grown BDD electrode is considered as a material with a prevalent content of sp3 carbon (acting as diamond), while the structured BDD electrodes represent the material with surface containing both sp3 and sp2 carbon (acting like graphite); GC then represents the material purely attributed to sp2 carbon [3]. Thus, the decreasing ratio of sp3 to sp2 carbon in the electrode material will shift the end of the cathodic potential window to more positive potentials (i.e. decreasing value of hydrogen overvoltage resulting in increasing value of oxygen overvoltage at the BDD-based electrode material, Fig. 4). The morphology of the electrode surface (size of surface nanostructures) also significantly contributes to its overall electrochemical performance [10,28], causing a hydrogen overvoltage at structurally smaller BDD nanotips (Fig. 2b) significantly higher than in the case of structurally larger BDD nanorods (Fig. 2c and d). Cathodic and anodic peak separation of [Fe(CN)6]3−/4− in its 1 mM solution in 0.1 M KCl is found to be very similar for all examined electrodes, with a value of 60 mV, which almost reaches the value for ideally reversible electrochemical systems (Fig. 5).
Fig. 3. a) Normalized Raman spectra of BDD electrodes measured by 442 nm excitation wavelength. b) Ratio of the diamond-peak area centred at 1315 cm−1 (green patterned bars) and the peak area at 1207 cm−1 (red bars) to all areas within spectral range of 900–1700 cm−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Relative concentration of atoms calculated from XPS spectra of BDD electrodes. Sample name
O (at.%)
C (at.%)
B (at.%)
as-grown BDD nanotips small nanorods large nanorods
6.2 11.6 13.4 12.6
93.3 88.1 85.9 86.8
0.5 0.3 0.7 0.6
Table 2 Relative concentration of chemical bonds calculated from deconvoluted C 1s peaks from XPS spectra of BDD electrodes. Sample name
sp2 (%)
sp3 (%)
CHx (%)
C–O–C (%)
C=O (%)
as-grown BDD nanotips small nanorods large nanorods
3 31 31 33
55 49 50 44
30 8 7 10
9 8 9 10
3 3 3 3
Fig. 5. Cyclic voltammograms of 1 mM [Fe(CN)6]3−/4− (displayed 3rd scan from −0.2 to 0.6 V and back to −0.2 V) recorded at structured BDD electrodes and GC electrode in 0.1 M KCl. ∗Current density is defined as current/electroactive area ratio. 4
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Table 3 Electroactive and real (geometric) electrode area of the BDD and GC electrodes. Electrode
Electroactive areaa (mm2)
Real areab (mm2)
Electroactive/real area ratio
as-grown BDD nanotips small nanorods large nanorods GC
2.10 1.13 1.51 1.23 6.15
1.75 1.17 1.62 1.40 7.07
1.19 0.96 0.93 0.88 0.87
*Current density defined as the current divided by the electroactive area. a Electroactive area calculated using Randles-Sevcik equation. b Real (geometric) area estimated from optical microscopy images.
[Fe(CN)6]3−/4− solution was also used to determine the electrochemically active electrode area. The electroactive area is compared to the real (geometric) electrode area as measured by an optical microscope (Table 3). The highest electroactive to real area ratio (1.19) was calculated for the as-grown BDD electrode. After structuring, this ratio decreased down to 0.88 (see Table 3 for BDD with large nanorods), very similarly to GC (0.87), i.e. it was reduced by 35%. These findings confirm that the electrochemical activity of the as-grown BDD is higher than the activity of the other tested electrode materials. One reason of this behaviour can be attributed to the highest ratio of sp3/sp2 carbon as well as the highest CHx content (Table 2) and the lowest O/C ratio (Table 1). The predominant diamond-like character of the as-grown BDD makes the electrochemical transformation (oxidation and reduction) of [Fe (CN)6]3−/4− faster in comparison with the other tested electrode materials (more graphite-like ones) [3,29], thus causing a significant increase of current density at potentials corresponding to anodic and cathodic peaks (Fig. 5). The decreased electroactive area of the structured electrodes can be attributed to a convoluted effect of the increased resistivity of the formed needles, surface enhancement factor [30], and the increased sp2 content, too. 3.3. Detection of Cd2+ and Pb2+ ions The structured BDD electrodes were also tested for the detection of Cd2+ and Pb2+ions. In our case, Cd2+ and Pb2+ ions in form of 0.1 mM CdCl2 and/or 0.05 mM Pb(NO3)2 in 0.1 M HCl were analysed. First, classical cyclic voltammetry was applied, but this technique was not sensitive enough to provide any electrochemical signal. However, by using differential pulse anodic stripping voltammetry (DPASV), we were able to detect Cd2+ and Pb2+ ions even at low concentrations (Fig. 6). In the presence of Cd2+ ions, the recorded voltammograms exhibit stripping peaks at −0.77 to −0.78 V (Fig. 6a), while in the case of the Pb2+ ions, the stripping peaks are located at −0.49 to −0.51 V (Fig. 6b), and the highest peak current density values for both cases were measured for the as-grown BDD electrode. Thus, the as-grown BDD electrode is very convenient for DPASV determination of the selected analytes. At anodic stripping voltammetric determination of trace amounts of Cd2+ and Pb2+ ions, the repeatability of the measurements represents a crucial factor for the precision of the determination. Cathodic formation of thin metal films, which grow at the electrode surface from initial single crystallization nuclei to final individual layers, very frequently brings inconsistencies and heterogeneities influencing the voltammetric response and its repeatability. Therefore, one of the criteria, which are usually taken into account at seeking and evaluating a suitable electrode material, is its ability to provide the best measurement repeatability (statistically expressed as relative standard deviation (RSD) of consecutive determinations). The measurement repeatability, calculated from 14 independent measurements, is summarized in Table 4.
Fig. 6. DPAS voltammograms of a) 1 mM Cd2+, b) 0.05 mM Pb2+ and c) mixture of 1 mM Cd2+ and 0.05 mM Pb2+ recorded from −1.0 to 0.1 V at fabricated electrodes and GC electrode in 0.1 M HCl. The analyte accumulation was performed in stirred solution at −1.0 V for 60 s. ∗Current density is defined as current/electroactive area ratio.
In the particular case of separate detection, the electrodes consisting of BDD nanotips and small BDD nanorods are the most suitable candidates for DPASV determination of Cd2+ (RSD of 4%) and Pb2+ (RSD
5
Vacuum 170 (2019) 108953
P. Štenclová, et al.
window (−1.9 V to 1.7 V). The broadest potential window was observed for the as-grown BDD and BDD with nanotips' array. The BDD nanorods' electrodes revealed a narrower electrochemical window slightly shifted to positive potentials. This shift was attributed to the increased ratio of sp2 to sp3 carbon due to the RIE process. The BDD morphology also had an impact on the overall electrochemical performance of BDD electrodes. Hydrogen overvoltage was higher for smaller nanotips than for larger nanorods. The highest ratio of electroactive to real area (1.19) was found for the as-grown BDD electrode, while for the structured BDD electrodes this ratio was comparable with the glassy carbon electrode (0.87). It was shown that both an optimized surface morphology and chemical composition of BDD electrodes tune their electrochemical performance with respect to selected separate detection of metal ions. In the case of the separate detection of Cd2+ or Pb2+, the most suitable morphologies were nanotips (RSD of 4%) and small nanorods (RSD of 5%), respectively. Moreover, the morphology also seems to be a crucial requirement for a simultaneous detection of solutions containing various ions. Here, the BDD electrode consisting of small nanorods revealed the highest peak current density together with the lowest RSD value (3%) for Cd2+ ions. Moreover, this electrode also exhibited a very low RSD value of 4% for Pb2+ ions. Thus, for this particular case, this electrode was considered as the most suitable sensors for a simultaneous determination of Cd2+ and Pb2+ ions using DPASV. Finally, the separate and simultaneous detection of metal ions was determined by the content of graphitic carbon phase (sp2 to sp3 ratio).
Table 4 Comparison of relative standard deviation (RSD) for BDD and GC electrodes. Electrode
as-grown BDD nanotips small nanorods large nanorods GC
RSD (%)a Cd2+ separately
Pb2+ separately
Cd2+ in mixture
Pb2+ in mixture
8 4 9 11 5
19 11 5 9 3
9 25 3 11 24
3 1 4 10 2
a
RSD (relative standard deviation) was estimated from 14 measurements in a particular solution.
of 5%) ions, respectively. The RSD improvement by structuring of the as-grown BDD electrode is two times for Cd2+ and Pb2+ ions' determination using BDD nanotips and small BDD nanorods, respectively. The RSD values of the structured BDD electrodes are comparable with values (3.2–4.1%) of as-grown BDD samples published in Ref. [6]. A little different electrochemical behaviour (regarding obtained values of peak current density and measurement repeatability) was observed by simultaneous measurements in a mixture of Cd2+ and Pb2+ ions (Fig. 6c). Here, the electrode based on small BDD nanorods showed the highest peak current density together with the lowest RSD value of 3% for Cd2+ ions (see Table 4). This electrode also exhibited a very low RSD value of 4% for Pb2+ ions. Thus, the electrode consisted of small BDD nanorods represents, for this particular case, the most suitable sensors for simultaneous determination of Cd2+ and Pb2+ ions using DPASV. In the case of the nanotip BDD electrode, a significant decrease of the peak current density was observed for Cd2+ ions in the mixture of Cd2+ + Pb2+ ions (see also Fig. S3 in the supplementary files). In addition, also the RSD for Cd2+ increased to 25% (from 4% calculated for separate detection of Cd2+). On the other hand, the RSD for Pb2+ decreased to 1% (from 11% for separate detection of Pb2+). This behavior is in agreement with the comparative GC electrode, where also an increase of RSD to 24% (for Cd2+) and decrease to 2% (for Pb2+) in the simultaneous detection of ions is observed. We suppose that this effect could be related to the porosity of the electrodes, since both the GC electrode and the BDD electrode with densely distributed nanotips are characterized by a high porosity. It seems that in the case of materials with high porosity, the detection of Cd2+ ions is considerably influenced by the presence of Pb2+ ions. However, to confirm this statement additional studies are required. For evaluation of Cd2+ ions in the mixture of Cd2+ + Pb2+, the electrode with small nanorods reveals the best performance with an exceptionally low RSD (only 3%) in comparison to other electrodes. Except for the BDD electrode with large nanorods, for evaluation of Pb2+ in the mixture of Cd2+ + Pb2+, other electrodes reveal comparable results with RDS lower than 5%. All above mentioned findings assure that the formation of thin films of individual Cd2+ and Pb2+ ions or their composites, especially at various nanostructured electrode surfaces, is a very complex chemical process that deserves a further attention and investigation.
Acknowledgement The work was supported by the CSF 17-19968S project and Operational Programme Research, Development and Education financed by the European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21CZ.02.1.01/0.0/0.0/16_019/0000760). Authors would like to thank Dr. A. Artemenko for X-ray photoelectron spectroscopy measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.108953. References [1] T.S. Le, P. Da Costa, P. Huguet, P. Sistat, F. Pichot, F. Silva, L. Renaud, M. Cretin, Upstream microelectrodialysis for heavy metals detection on boron doped diamond, J. Electroanal. Chem. 670 (2012) 50–55, https://doi.org/10.1016/j.jelechem.2012. 02.015. [2] C. Prado, S.J. Wilkins, F. Marken, R.G. Compton, Simultaneous electrochemical detection and determination of lead and copper at boron-doped diamond film electrodes, Electroanalysis 14 (2002) 262–272, https://doi.org/10.1002/15214109(200202)14:4<262::AID-ELAN262>3.0.CO;2-D. [3] J.H.T. Luong, K.B. Male, J.D. Glennon, Boron-doped diamond electrode: synthesis, characterization, functionalization and analytical applications, Analyst 134 (2009) 1965–1979, https://doi.org/10.1039/b910206j. [4] E.A. McGaw, G.M. Swain, A comparison of boron-doped diamond thin-film and Hgcoated glassy carbon electrodes for anodic stripping voltammetric determination of heavy metal ions in aqueous media, Anal. Chim. Acta 575 (2006) 180–189, https:// doi.org/10.1016/j.aca.2006.05.094. [5] O. El Tall, N. Jaffrezic-Renault, M. Sigaud, O. Vittori, Anodic stripping voltammetry of heavy metals at nanocrystalline boron-doped diamond electrode, Electroanalysis 19 (2007) 1152–1159, https://doi.org/10.1002/elan.200603834. [6] V. Bezerra dos Santos, E.L. Fava, N. Sá de Miranda Curi, R.C. Faria, T.B. Guerreiro, O. Fatibello-Filho, An electrochemical analyzer for in situ flow determination of Pb (ii) and Cd(ii) in lake water with on-line data transmission and a global positioning system, Anal. Methods 7 (2015) 3105–3112, https://doi.org/10.1039/ C5AY00012B. [7] D. Dragoe, N. Spătaru, R. Kawasaki, A. Manivannan, T. Spătaru, D.A. Tryk, A. Fujishima, Detection of trace levels of Pb2+ in tap water at boron-doped diamond electrodes with anodic stripping voltammetry, Electrochim. Acta 51 (2006) 2437–2441, https://doi.org/10.1016/j.electacta.2005.07.022. [8] J. Pei, X. Yu, C. Zhang, X. Liu, Development of a boron-doped diamond electrode for the simultaneous detection of Cd2+ and Pb2+ in water, Int. J. Electrochem. Sci.
4. Conclusion In this article, we reported on the RIE structuring and characterization of boron doped diamond film and its utilization as electrode for targeted electrochemical detection of selected metal ions (Cd2+ or Pb2+). SEM measurements confirmed morphological changes from the flat diamond surface to surface with nanotips' or nanorods' array. The structured BDD samples showed higher content of sp2 carbon phases as observed by XPS and Raman measurements. All BDD electrodes featured a relatively broad electrochemical 6
Vacuum 170 (2019) 108953
P. Štenclová, et al.
(2019) 3393–3407, https://doi.org/10.20964/2019.04.19. [9] K. Peckova, J. Barek, Boron doped diamond microelectrodes and microelectrode arrays in organic electrochemistry, Curr. Org. Chem. 15 (2011) 3014–3028, https:// doi.org/10.2174/138527211798357164. [10] K. Pecková, J. Musilová, J. Barek, Boron-doped diamond film electrodes—new tool for voltammetric determination of organic substances, Crit. Rev. Anal. Chem. 39 (2009) 148–172, https://doi.org/10.1080/10408340903011812. [11] Y. Yu, L. Wu, J. Zhi, Diamond nanowires: fabrication, structure, properties, and applications, Angew. Chem. Int. Ed. 53 (2014) 14326–14351, https://doi.org/10. 1002/anie.201310803. [12] S. Szunerits, Y. Coffinier, R. Boukherroub, Diamond nanowires: a novel platform for electrochemistry and matrix-free mass spectrometry, Sensors 15 (2015) 12573–12593, https://doi.org/10.3390/s150612573. [13] M. Wei, C. Terashima, M. Lv, A. Fujishima, Z.-Z. Gu, Boron-doped diamond nanograss array for electrochemical sensors, Chem. Commun. (2009) 3624–3626, https://doi.org/10.1039/b903284c. [14] N. Yang, H. Uetsuka, E. Osawa, C.E. Nebel, Vertically aligned nanowires from boron-doped diamond, Nano Lett. 8 (2008) 3572–3576, https://doi.org/10.1021/ nl801136h. [15] J. Shalini, K.J. Sankaran, C.-L. Dong, C.-Y. Lee, N.-H. Tai, I.-N. Lin, In situ detection of dopamine using nitrogen incorporated diamond nanowire electrode, Nanoscale 5 (2013) 1159–1167, https://doi.org/10.1039/c2nr32939e. [16] P. Subramanian, I. Mazurenko, V. Zaitsev, Y. Coffinier, R. Boukherroub, S. Szunerits, Diamond nanowires modified with poly[3-(pyrrolyl)carboxylic acid] for the immobilization of histidine-tagged peptides, Analyst 139 (2014) 4343–4349, https://doi.org/10.1039/C4AN00146J. [17] D. Luo, L. Wu, J. Zhi, Fabrication of boron-doped diamond nanorod forest electrodes and their application in nonenzymatic amperometric glucose biosensing, ACS Nano 3 (2009) 2121–2128, https://doi.org/10.1021/nn9003154. [18] M. Yoshimura, K. Honda, R. Uchikado, T. Kondo, T.N. Rao, D.A. Tryk, A. Fujishima, Y. Sakamoto, K. Yasui, H. Masuda, Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes, Diam. Relat. Mater. 10 (2001) 620–626, https://doi.org/10.1016/S0925-9635(00)00381-2. [19] V. Malcher, A. Mrška, A. Kromka, A. Šatka, J. Janı́k, Diamond film coated on WC/ Co tools by double bias-assisted hot filament CVD, Curr. Appl. Phys. 2 (2002) 201–204, https://doi.org/10.1016/S1567-1739(02)00064-0. [20] R. Šelešovská, M. Štěpánková, L. Janíková, K. Nováková, M. Vojs, M. Marton, M. Behúl, Surface and electrochemical characterization of boron-doped diamond
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
7
electrodes prepared under different conditions, Monatsh. Chem. 147 (2016) 1353–1364, https://doi.org/10.1007/s00706-015-1640-3. T. Ižák, M. Domonkos, O. Babchenko, M. Varga, B. Rezek, V. Jurka, K. Hruška, A. Kromka, Technological aspects in fabrication of micro- and nano-sized carbon based features: nanorods, periodical arrays and self-standing membranes, J. Electr. Eng. 66 (2015) 282–286, https://doi.org/10.2478/jee-2015-0046. O. Babchenko, A. Kromka, K. Hruska, M. Michalka, J. Potmesil, M. Vanecek, Nanostructuring of diamond films using self-assembled nanoparticles, Cent. Eur. J. Phys. 7 (2009) 310–314, https://doi.org/10.2478/s11534-009-0026-8. M. Varga, T. Izak, V. Vretenar, H. Kozak, J. Holovsky, A. Artemenko, M. Hulman, V. Skakalova, D.S. Lee, A. Kromka, Diamond/carbon nanotube composites: Raman, FTIR and XPS spectroscopic studies, Carbon 111 (2017) 54–61, https://doi.org/10. 1016/j.carbon.2016.09.064. H. Uetsuka, T. Yamada, S. Shikata, ICP etching of polycrystalline diamonds: fabrication of diamond nano-tips for AFM cantilevers, Diam. Relat. Mater. 17 (2008) 728–731, https://doi.org/10.1016/j.diamond.2007.12.071. T. Izak, A. Kromka, O. Babchenko, M. Ledinsky, K. Hruska, E. Verveniotis, Comparative study on dry etching of polycrystalline diamond thin films, Vacuum 86 (2012) 799–802, https://doi.org/10.1016/j.vacuum.2011.07.023. D. Kumar, M. Chandran, M.S. Ramachandra Rao, Effect of boron doping on firstorder Raman scattering in superconducting boron doped diamond films, Appl. Phys. Lett. 110 (2017) 191602, https://doi.org/10.1063/1.4982591. V. Mortet, Z. Vlčková Živcová, A. Taylor, O. Frank, P. Hubík, D. Trémouilles, F. Jomard, J. Barjon, L. Kavan, Insight into boron-doped diamond Raman spectra characteristic features, Carbon 115 (2017) 279–284, https://doi.org/10.1016/j. carbon.2017.01.022. G. Piret, C. Hébert, J.-P. Mazellier, L. Rousseau, E. Scorsone, M. Cottance, G. Lissorgues, M.O. Heuschkel, S. Picaud, P. Bergonzo, B. Yvert, 3D-nanostructured boron-doped diamond for microelectrode array neural interfacing, Biomaterials 53 (2015) 173–183, https://doi.org/10.1016/j.biomaterials.2015.02.021. P. Chen, R.L. McCreery, Control of electron transfer kinetics at glassy carbon electrodes by specific surface modification, Anal. Chem. 68 (1996) 3958–3965, https://doi.org/10.1021/ac960492r. S. Szunerits, Y. Coffinier, E. Galopin, J. Brenner, R. Boukherroub, Preparation of boron-doped diamond nanowires and their application for sensitive electrochemical detection of tryptophan, Electrochem. Commun. 12 (2010) 438–441, https://doi. org/10.1016/j.elecom.2010.01.014.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structured and graphitized boron doped diamond electrodes: Impact on electrochemical detection of Cd2+ and Pb2+ ions
T
Pavla Štenclováa,∗, Vlastimil Vyskočilb, Ondrej Szabóa, Tibor Ižáka, Štěpán Potockýa, Alexander Kromkaa a
Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00, Prague 6, Czech Republic UNESCO Laboratory of Environmental Electrochemistry, Department of Analytical Chemistry, Faculty of Science, Charles University, Albertov 6, 128 43, Prague 2, Czech Republic
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Boron doped diamond Reactive ion etching Structuring Nanorods Cyclic voltammetry Metal ions Cd2+ Pb2+
Here we introduce the structuring of boron doped diamond (BDD) electrodes using a self-organised mask and reactive ion etching (RIE) in oxygen plasma for electrochemical measurements of selected metal ions. The samples were analysed by SEM, Raman and X-ray photoelectron spectroscopy. The electrochemical performance of structured BDD was compared to as-grown BDD and standard glassy carbon (GC) electrodes employing standard cyclic voltammetry and differential pulse anodic stripping voltammetry. Separate and simultaneous detection of Cd2+ and Pb2+ ions is discussed in relation to the electrode morphology and content of graphitic phase. To determine trace amounts of metal ions we evaluated the repeatability of measurements, which is statistically expressed as the relative standard deviation (RSD) of consecutive determinations. Based on RSD data, the most suitable candidate for the separate detection of Cd2+ and Pb2+ ions was found the structured BDD electrode consisting of nanotips and small nanorods, respectively. Two times better RSD was found for the structured BDD than for the as-grown BDD electrode. Electrochemical measurements provided in the complex mixture consisting both of Cd2+ and Pb2+ ions revealed the small BDD nanorods as the most suitable surface morphology to separate recognition of Cd2+ and Pb2+ ions.
1. Introduction Electroanalytical techniques were established as important methods to detect toxic traces of metals in an environment. It is due to their high sensitivity and easy operational procedures. Boron doped diamond (BDD) have attracted a considerable attention in electroanalytical detection of trace metals [1,2] due to its extremely high chemical stability, a wide potential window in aqueous media with a low background current and stable surface state without the tendency to fouling [3]. For example, McGaw et al. have shown that BDD electrodes exhibit many of the same electrode properties that mercury electrodes do: low detection limits (mid to low ppb range) for Cd2+, Pb2+, Cu2+ and Ag+ with a reproducibility of less than 5% RSD [4]. Tall et al. have noticed 3–5 times higher sensitivity for the BDD electrodes determining Cd2+ and Pb2+ by anodic stripping voltammetry in comparison to glassy carbon electrodes [5]. Extremely low limits of detection at BDD electrodes were measured by Bezerra dos Santos et al. [6], who determined 0.18 μg/L for Cd2+ and 0.08 μg/L for Pb2+ in simultaneous analysis together with very low RSD data (~3.5%). Manivannan et al. reported
∗
that after performing a pre-concentration of the analyte at −1.1 V for 15 min, Pb2+ is detectable at sub-ppb levels in tap water [7]. Recently, Pei et al. replaced mercury electrodes by BDD electrodes for detection of Cd2+ and Pb2+ ions in water sources, achieving detection limits of 3.39 μg/L and 3.62 μg/L, respectively [8]. Thus, it can be concluded that BDD electrodes, particularly used in combination with anodic stripping voltammetry, appear a suitable mercury-free tool for the detection of metal traces. Moreover, very simple miniaturization of BDD electrodes [9,10] makes them ideal working electrodes for the portable miniaturized electrochemical devices – the priceless tools for measurements in small sample volumes. Additionally, BDD can be structured and patterned by different ways to increase electrochemical performance [11,12]. Various patterns, such as nanograss arrays [13], vertical nanowires [14–16], nanorod forest [17] or nanoporous honeycombs [18], have been already reported to increase the response for inorganic as well as organic analytes. In this contribution, we report on fabrication and characterization of structured BDD electrodes featured by nanotips' or nanorods' surface
Corresponding author. E-mail address: [email protected] (P. Štenclová).
https://doi.org/10.1016/j.vacuum.2019.108953 Received 29 November 2018; Received in revised form 19 June 2019; Accepted 18 September 2019 Available online 19 September 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Fig. 1. a) Schematic drawing of the fabrication process for structuring boron doped diamond films including three main steps: evaporation of thin metal layer, the formation of clusters and reactive ion etching of diamond. b) SEM images of surface morphology of BDD films after the formation of gold nanocluster's masks (i.e. after step 2 and before RIE). Two types of the mask were prepared: the so-called “small” and “large” mask depending on the initial thickness of the evaporated Au layer.
2.2. Electrochemical analysis of BDD electrodes
morphology. Their electrochemical performance is compared to the asgrown BDD and standard glassy carbon (GC) electrode. Results from separate and simultaneous detection of Cd2+ and Pb2+ ions are discussed in a relation to the electrode surface morphology and content of the graphitic phase (i.e. sp2 vs. sp3 carbon bonds).
All electrochemical measurements were performed in a three-electrode system using: (i) BDD electrodes as a working electrode, (ii) Ag|AgCl electrode as a reference electrode and (iii) Pt wire as an auxiliary electrode. For a comparison, GC electrode (ED Turnov) based on glassy carbon material was used as a working electrode. Autolab Potentiostat (model PGSTAT302 N, Metrohm) was used to operate the electrochemical processes via Nova 2.0.2 software. Prior to electrochemical measurements, all BDD electrodes were electrochemically activated by applying an electric potential −3 V for 5 min in 2 M HNO3. The electrochemical window was measured in a solution of 0.1 M KCl or in 0.1 M HCl by cyclic voltammetry. The current limit was set to ± 50 μA. Reversibility of the electrode processes was tested by cyclic voltammetry in 1 mM [Fe(CN)6]3−/4− in 0.1 M KCl. The scan rate was 0.1 V/s, with a potential step of 0.001 V. Each measurement was repeated 10 times. The electroactive electrode area was calculated using the RandlesSevcik equation (1):
2. Experimental part 2.1. Fabrication of structured BDD electrodes The boron doped diamond films (~2 μm in thickness) were grown using hot filament chemical vapour deposition on Si substrates by addition of trimethylboron (10 000 ppm) to the H2/CH4 gas mixture [19,20]. The surface of the diamond film was further structured by reactive ion etching (RIE) in pure oxygen plasma either employing gold nanoclusters as the masking material (Fig. 1) [21] or without any mask. The gold nanoparticle's masks were prepared by evaporating a thin Au layer (thickness ~3 or ~12 nm) and further treated by microwave plasma in a hydrogen atmosphere (at 500 °C for 10 min) in order to obtain an array of nano-sized metal droplets on the diamond surface [22]. Two types of the mask were prepared: the so-called “small” and “large” mask pre-defined by the initial thickness of the evaporated Au layer (i.e. 3 or 12 nm, respectively). In the case of the small mask, the surface of the BDD film was covered by circle-shaped nano-sized droplets with a diameter about ~20–30 nm, while for the large mask irregularly-shaped droplets were formed with size ranging from 80 to 120 nm (Fig. 1b). Four types of samples were prepared: (i) as-grown BDD, (ii) BDD nanotips (obtained using RIE in oxygen plasma without any mask), (iii) small and (iv) large BDD nanorods (by applying “small” and “large” mask of gold nanocluster's array) (see Fig. 2). After the RIE process, the remaining Au clusters were etched away by standard wet chemical etching process (HNO3:HCl in 1:3, v/v). The surface morphology of the samples was characterized by scanning electron microscopy (SEM, Maia3, Tescan) under 45° angle view, the chemical composition was studied by Raman spectroscopy (InVia Renishaw Raman spectrometer with excitation wavelength 442 nm) and X-ray Photoelectron Spectroscopy (XPS, AXIS Supra, Kratos) [23].
nFvD 0.5 ⎞ ic = ia = 0.4463nFAC ⎛ ⎝ RT ⎠
(1)
where ic is an absolute value of cathodic peak current in A, ia is an absolute value of anodic peak current in A, n is a number of electrons transferred in the redox event, F is the Faraday constant in C/mol, A is an active electrode area in cm2, C is a concentration in mol/mL, v is a scan rate in V/s, D is a diffusion coefficient in cm2/s, R is a gas constant in J/(K·mol) and T is temperature in K. The electroactive areas were compared to real (geometric) electrode areas estimated from optical microscope images. The differential pulse anodic stripping voltammetry (DPASV) was used to monitor the signal of metal ions (Cd2+ and Pb2+) in 0.1 mM CdCl2 and/or 0.05 mM Pb(NO3)2 in 0.1 M HCl. The analyte accumulation was performed in a stirred solution at −1.0 V for 60 s. The measurement was realized between −1.0 and 0.1 V with a step potential of 0.003 V. The modulation amplitude was set to 0.05 V, the modulation time to 0.1 s and the interval time to 0.15 s. These parameters resulted in the scan rate of 0.020 V/s. To assure clear electrode surface, the 2
Vacuum 170 (2019) 108953
P. Štenclová, et al.
electrode was cleaned by employing 0.5 V for 60 s after each measurement. Measurements were repeated 14 times. If not otherwise stated, all chemicals were purchased from SigmaAldrich. Deionized water was produced by a Milli-Q Plus system (Millipore, USA). 3. Results and discussion 3.1. Morphological and chemical characterization of BDD electrodes Fig. 2 shows SEM images of the samples with different surface morphology depending on the fabrication process. The surface morphology of as-grown diamond films reveals crystals with a diameter of 600–900 nm (Fig. 2a). If only oxygen RIE plasma is used without any masking material, the formed surface morphology is dominated by needle-like structures or so-called whiskers or nanotips (Fig. 2b). Such nanotips are preferentially formed along grain boundaries because of differences in the chemical composition of grains (which contain mainly sp3 carbon bonds) and grain boundaries (mainly sp2 bonds) in the diamond film due to the different etching rate of sp2 and sp3 carbon. Moreover, nanotips can be formed also within the grains around intrinsic defects in the diamond crystals or in specific grains depending on their orientation [24]. The formation of nanotips can be suppressed by adding CF4 gas to the gas mixture. Even an addition of a small amount of CF4 gas resulted in a flatter surface [25]. Using the gold nanoclusters mask during the RIE process, the final film morphology resulted in nanorods' array. The size/diameter of the nanorods depended on the size of metal clusters used as the masking material (Fig. 1b). The diameter of the nanorods was in the range of 20–30 nm and 80–120 nm for the small and large mask, respectively (Fig. 2c and d). In the case of the sample with large BDD nanorods (Fig. 2d), regions consisting of nanotips are observed as well. Raman spectroscopy measurements showed typical spectra for BDD (Fig. 3a). The observed three characteristic peaks represent: (i) the diamond's zone centre phonon line at ~1315 cm−1, which is attributed to sp3 carbon bonds and it is redshifted relatively to the intrinsic diamond line (1332 cm−1); (ii) the broad G-band (graphite-band) centred at ~1540 cm−1, which is considered as a mixture of sp3/sp2 carbon bonds with sp2 bonds dominance; and (iii) the asymmetric band at ~1207 cm−1, which is characteristic feature of BDD and indirectly expresses the boron concentration in the diamond [26,27]. In addition, there is also the D-band (defect-band) hidden at ~1368 cm−1 (see fitted peaks in supplementary files, Fig. S1). The diamond-peak area and the peak area centred at ~1207 cm−1 were further used to calculate the change of sp3 fraction and relative boron concentration in the BDD films, respectively (Fig. 3b). Fig. 3b shows i) the ratio of the diamond-peak area to sum of all areas (green patterned bars) and ii) the ratio of the peak area at 1207 cm−1 to sum of all areas (red bars) within the spectral range of 900–1700 cm−1 calculated from deconvoluted Raman spectra (Fig. S1). The highest sp3 fraction (i.e. Areadiam/Areaall ratio) was found for asgrown BDD. The RIE process in all cases decreased the relative amount of sp3 carbon bonds, i.e. increased the amount of sp2 bonds. This finding is in an agreement with XPS measurements (see Tables 1 and 2), which also confirmed an increased relative amount of sp2 bonds. The deconvoluted C 1s and B 1s peaks for selected samples are shown in supplementary files (Fig. S2). Raman spectra also confirmed that the lowest relative boron concentration (i.e. Area1207/Areaall ratio) has the sample with nanotips' array. These findings well agree with XPS measurements (Table 1) where a decrease of the boron concentration from 0.5 to 0.3 at. % compared to as-grown BDD sample is observed. The decrease of relative boron concentration should be attributed to the RIE without any masking material. Without metal nanocluster's mask, the oxygen ions reach the diamond surface much easily and etch the BDD at higher etching rate in comparison to other samples when metal mask was
Fig. 2. SEM images of surface morphology of electrodes: a) as-grown BDD, b) BDD nanotips, c) small and d) large BDD nanorods.
3
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Fig. 4. Electrochemical (potential) window of BDD working electrodes measured using cyclic voltammetry in 0.1 M HCl. GC electrode was added for comparison.
applied. Thus, etching without any mask leads to thinner BDD sample. 3.2. Electrochemical characterization of electrodes Cyclic voltammetry of all BDD electrodes measured in 0.1 M KCl showed a relatively broad electrochemical window (from approx. −1.9 V up to ca. 1.7 V). In an acidic medium (0.1 M HCl), the electrochemical window shrinks to ca. −1.3 V up to ca. 1.5 V (Fig. 4). The broadest potential window was observed for as-grown BDD and BDD with nanotips. On the other hand, BDD nanorods' electrodes showed narrower electrochemical window slightly shifted to positive potentials. This behaviour seems to be attributed to the composition and morphology of the electrode surface. As shown in Table 2, the as-grown BDD electrode is considered as a material with a prevalent content of sp3 carbon (acting as diamond), while the structured BDD electrodes represent the material with surface containing both sp3 and sp2 carbon (acting like graphite); GC then represents the material purely attributed to sp2 carbon [3]. Thus, the decreasing ratio of sp3 to sp2 carbon in the electrode material will shift the end of the cathodic potential window to more positive potentials (i.e. decreasing value of hydrogen overvoltage resulting in increasing value of oxygen overvoltage at the BDD-based electrode material, Fig. 4). The morphology of the electrode surface (size of surface nanostructures) also significantly contributes to its overall electrochemical performance [10,28], causing a hydrogen overvoltage at structurally smaller BDD nanotips (Fig. 2b) significantly higher than in the case of structurally larger BDD nanorods (Fig. 2c and d). Cathodic and anodic peak separation of [Fe(CN)6]3−/4− in its 1 mM solution in 0.1 M KCl is found to be very similar for all examined electrodes, with a value of 60 mV, which almost reaches the value for ideally reversible electrochemical systems (Fig. 5).
Fig. 3. a) Normalized Raman spectra of BDD electrodes measured by 442 nm excitation wavelength. b) Ratio of the diamond-peak area centred at 1315 cm−1 (green patterned bars) and the peak area at 1207 cm−1 (red bars) to all areas within spectral range of 900–1700 cm−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Relative concentration of atoms calculated from XPS spectra of BDD electrodes. Sample name
O (at.%)
C (at.%)
B (at.%)
as-grown BDD nanotips small nanorods large nanorods
6.2 11.6 13.4 12.6
93.3 88.1 85.9 86.8
0.5 0.3 0.7 0.6
Table 2 Relative concentration of chemical bonds calculated from deconvoluted C 1s peaks from XPS spectra of BDD electrodes. Sample name
sp2 (%)
sp3 (%)
CHx (%)
C–O–C (%)
C=O (%)
as-grown BDD nanotips small nanorods large nanorods
3 31 31 33
55 49 50 44
30 8 7 10
9 8 9 10
3 3 3 3
Fig. 5. Cyclic voltammograms of 1 mM [Fe(CN)6]3−/4− (displayed 3rd scan from −0.2 to 0.6 V and back to −0.2 V) recorded at structured BDD electrodes and GC electrode in 0.1 M KCl. ∗Current density is defined as current/electroactive area ratio. 4
Vacuum 170 (2019) 108953
P. Štenclová, et al.
Table 3 Electroactive and real (geometric) electrode area of the BDD and GC electrodes. Electrode
Electroactive areaa (mm2)
Real areab (mm2)
Electroactive/real area ratio
as-grown BDD nanotips small nanorods large nanorods GC
2.10 1.13 1.51 1.23 6.15
1.75 1.17 1.62 1.40 7.07
1.19 0.96 0.93 0.88 0.87
*Current density defined as the current divided by the electroactive area. a Electroactive area calculated using Randles-Sevcik equation. b Real (geometric) area estimated from optical microscopy images.
[Fe(CN)6]3−/4− solution was also used to determine the electrochemically active electrode area. The electroactive area is compared to the real (geometric) electrode area as measured by an optical microscope (Table 3). The highest electroactive to real area ratio (1.19) was calculated for the as-grown BDD electrode. After structuring, this ratio decreased down to 0.88 (see Table 3 for BDD with large nanorods), very similarly to GC (0.87), i.e. it was reduced by 35%. These findings confirm that the electrochemical activity of the as-grown BDD is higher than the activity of the other tested electrode materials. One reason of this behaviour can be attributed to the highest ratio of sp3/sp2 carbon as well as the highest CHx content (Table 2) and the lowest O/C ratio (Table 1). The predominant diamond-like character of the as-grown BDD makes the electrochemical transformation (oxidation and reduction) of [Fe (CN)6]3−/4− faster in comparison with the other tested electrode materials (more graphite-like ones) [3,29], thus causing a significant increase of current density at potentials corresponding to anodic and cathodic peaks (Fig. 5). The decreased electroactive area of the structured electrodes can be attributed to a convoluted effect of the increased resistivity of the formed needles, surface enhancement factor [30], and the increased sp2 content, too. 3.3. Detection of Cd2+ and Pb2+ ions The structured BDD electrodes were also tested for the detection of Cd2+ and Pb2+ions. In our case, Cd2+ and Pb2+ ions in form of 0.1 mM CdCl2 and/or 0.05 mM Pb(NO3)2 in 0.1 M HCl were analysed. First, classical cyclic voltammetry was applied, but this technique was not sensitive enough to provide any electrochemical signal. However, by using differential pulse anodic stripping voltammetry (DPASV), we were able to detect Cd2+ and Pb2+ ions even at low concentrations (Fig. 6). In the presence of Cd2+ ions, the recorded voltammograms exhibit stripping peaks at −0.77 to −0.78 V (Fig. 6a), while in the case of the Pb2+ ions, the stripping peaks are located at −0.49 to −0.51 V (Fig. 6b), and the highest peak current density values for both cases were measured for the as-grown BDD electrode. Thus, the as-grown BDD electrode is very convenient for DPASV determination of the selected analytes. At anodic stripping voltammetric determination of trace amounts of Cd2+ and Pb2+ ions, the repeatability of the measurements represents a crucial factor for the precision of the determination. Cathodic formation of thin metal films, which grow at the electrode surface from initial single crystallization nuclei to final individual layers, very frequently brings inconsistencies and heterogeneities influencing the voltammetric response and its repeatability. Therefore, one of the criteria, which are usually taken into account at seeking and evaluating a suitable electrode material, is its ability to provide the best measurement repeatability (statistically expressed as relative standard deviation (RSD) of consecutive determinations). The measurement repeatability, calculated from 14 independent measurements, is summarized in Table 4.
Fig. 6. DPAS voltammograms of a) 1 mM Cd2+, b) 0.05 mM Pb2+ and c) mixture of 1 mM Cd2+ and 0.05 mM Pb2+ recorded from −1.0 to 0.1 V at fabricated electrodes and GC electrode in 0.1 M HCl. The analyte accumulation was performed in stirred solution at −1.0 V for 60 s. ∗Current density is defined as current/electroactive area ratio.
In the particular case of separate detection, the electrodes consisting of BDD nanotips and small BDD nanorods are the most suitable candidates for DPASV determination of Cd2+ (RSD of 4%) and Pb2+ (RSD
5
Vacuum 170 (2019) 108953
P. Štenclová, et al.
window (−1.9 V to 1.7 V). The broadest potential window was observed for the as-grown BDD and BDD with nanotips' array. The BDD nanorods' electrodes revealed a narrower electrochemical window slightly shifted to positive potentials. This shift was attributed to the increased ratio of sp2 to sp3 carbon due to the RIE process. The BDD morphology also had an impact on the overall electrochemical performance of BDD electrodes. Hydrogen overvoltage was higher for smaller nanotips than for larger nanorods. The highest ratio of electroactive to real area (1.19) was found for the as-grown BDD electrode, while for the structured BDD electrodes this ratio was comparable with the glassy carbon electrode (0.87). It was shown that both an optimized surface morphology and chemical composition of BDD electrodes tune their electrochemical performance with respect to selected separate detection of metal ions. In the case of the separate detection of Cd2+ or Pb2+, the most suitable morphologies were nanotips (RSD of 4%) and small nanorods (RSD of 5%), respectively. Moreover, the morphology also seems to be a crucial requirement for a simultaneous detection of solutions containing various ions. Here, the BDD electrode consisting of small nanorods revealed the highest peak current density together with the lowest RSD value (3%) for Cd2+ ions. Moreover, this electrode also exhibited a very low RSD value of 4% for Pb2+ ions. Thus, for this particular case, this electrode was considered as the most suitable sensors for a simultaneous determination of Cd2+ and Pb2+ ions using DPASV. Finally, the separate and simultaneous detection of metal ions was determined by the content of graphitic carbon phase (sp2 to sp3 ratio).
Table 4 Comparison of relative standard deviation (RSD) for BDD and GC electrodes. Electrode
as-grown BDD nanotips small nanorods large nanorods GC
RSD (%)a Cd2+ separately
Pb2+ separately
Cd2+ in mixture
Pb2+ in mixture
8 4 9 11 5
19 11 5 9 3
9 25 3 11 24
3 1 4 10 2
a
RSD (relative standard deviation) was estimated from 14 measurements in a particular solution.
of 5%) ions, respectively. The RSD improvement by structuring of the as-grown BDD electrode is two times for Cd2+ and Pb2+ ions' determination using BDD nanotips and small BDD nanorods, respectively. The RSD values of the structured BDD electrodes are comparable with values (3.2–4.1%) of as-grown BDD samples published in Ref. [6]. A little different electrochemical behaviour (regarding obtained values of peak current density and measurement repeatability) was observed by simultaneous measurements in a mixture of Cd2+ and Pb2+ ions (Fig. 6c). Here, the electrode based on small BDD nanorods showed the highest peak current density together with the lowest RSD value of 3% for Cd2+ ions (see Table 4). This electrode also exhibited a very low RSD value of 4% for Pb2+ ions. Thus, the electrode consisted of small BDD nanorods represents, for this particular case, the most suitable sensors for simultaneous determination of Cd2+ and Pb2+ ions using DPASV. In the case of the nanotip BDD electrode, a significant decrease of the peak current density was observed for Cd2+ ions in the mixture of Cd2+ + Pb2+ ions (see also Fig. S3 in the supplementary files). In addition, also the RSD for Cd2+ increased to 25% (from 4% calculated for separate detection of Cd2+). On the other hand, the RSD for Pb2+ decreased to 1% (from 11% for separate detection of Pb2+). This behavior is in agreement with the comparative GC electrode, where also an increase of RSD to 24% (for Cd2+) and decrease to 2% (for Pb2+) in the simultaneous detection of ions is observed. We suppose that this effect could be related to the porosity of the electrodes, since both the GC electrode and the BDD electrode with densely distributed nanotips are characterized by a high porosity. It seems that in the case of materials with high porosity, the detection of Cd2+ ions is considerably influenced by the presence of Pb2+ ions. However, to confirm this statement additional studies are required. For evaluation of Cd2+ ions in the mixture of Cd2+ + Pb2+, the electrode with small nanorods reveals the best performance with an exceptionally low RSD (only 3%) in comparison to other electrodes. Except for the BDD electrode with large nanorods, for evaluation of Pb2+ in the mixture of Cd2+ + Pb2+, other electrodes reveal comparable results with RDS lower than 5%. All above mentioned findings assure that the formation of thin films of individual Cd2+ and Pb2+ ions or their composites, especially at various nanostructured electrode surfaces, is a very complex chemical process that deserves a further attention and investigation.
Acknowledgement The work was supported by the CSF 17-19968S project and Operational Programme Research, Development and Education financed by the European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21CZ.02.1.01/0.0/0.0/16_019/0000760). Authors would like to thank Dr. A. Artemenko for X-ray photoelectron spectroscopy measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.108953. References [1] T.S. Le, P. Da Costa, P. Huguet, P. Sistat, F. Pichot, F. Silva, L. Renaud, M. Cretin, Upstream microelectrodialysis for heavy metals detection on boron doped diamond, J. Electroanal. Chem. 670 (2012) 50–55, https://doi.org/10.1016/j.jelechem.2012. 02.015. [2] C. Prado, S.J. Wilkins, F. Marken, R.G. Compton, Simultaneous electrochemical detection and determination of lead and copper at boron-doped diamond film electrodes, Electroanalysis 14 (2002) 262–272, https://doi.org/10.1002/15214109(200202)14:4<262::AID-ELAN262>3.0.CO;2-D. [3] J.H.T. Luong, K.B. Male, J.D. Glennon, Boron-doped diamond electrode: synthesis, characterization, functionalization and analytical applications, Analyst 134 (2009) 1965–1979, https://doi.org/10.1039/b910206j. [4] E.A. McGaw, G.M. Swain, A comparison of boron-doped diamond thin-film and Hgcoated glassy carbon electrodes for anodic stripping voltammetric determination of heavy metal ions in aqueous media, Anal. Chim. Acta 575 (2006) 180–189, https:// doi.org/10.1016/j.aca.2006.05.094. [5] O. El Tall, N. Jaffrezic-Renault, M. Sigaud, O. Vittori, Anodic stripping voltammetry of heavy metals at nanocrystalline boron-doped diamond electrode, Electroanalysis 19 (2007) 1152–1159, https://doi.org/10.1002/elan.200603834. [6] V. Bezerra dos Santos, E.L. Fava, N. Sá de Miranda Curi, R.C. Faria, T.B. Guerreiro, O. Fatibello-Filho, An electrochemical analyzer for in situ flow determination of Pb (ii) and Cd(ii) in lake water with on-line data transmission and a global positioning system, Anal. Methods 7 (2015) 3105–3112, https://doi.org/10.1039/ C5AY00012B. [7] D. Dragoe, N. Spătaru, R. Kawasaki, A. Manivannan, T. Spătaru, D.A. Tryk, A. Fujishima, Detection of trace levels of Pb2+ in tap water at boron-doped diamond electrodes with anodic stripping voltammetry, Electrochim. Acta 51 (2006) 2437–2441, https://doi.org/10.1016/j.electacta.2005.07.022. [8] J. Pei, X. Yu, C. Zhang, X. Liu, Development of a boron-doped diamond electrode for the simultaneous detection of Cd2+ and Pb2+ in water, Int. J. Electrochem. Sci.
4. Conclusion In this article, we reported on the RIE structuring and characterization of boron doped diamond film and its utilization as electrode for targeted electrochemical detection of selected metal ions (Cd2+ or Pb2+). SEM measurements confirmed morphological changes from the flat diamond surface to surface with nanotips' or nanorods' array. The structured BDD samples showed higher content of sp2 carbon phases as observed by XPS and Raman measurements. All BDD electrodes featured a relatively broad electrochemical 6
Vacuum 170 (2019) 108953
P. Štenclová, et al.
(2019) 3393–3407, https://doi.org/10.20964/2019.04.19. [9] K. Peckova, J. Barek, Boron doped diamond microelectrodes and microelectrode arrays in organic electrochemistry, Curr. Org. Chem. 15 (2011) 3014–3028, https:// doi.org/10.2174/138527211798357164. [10] K. Pecková, J. Musilová, J. Barek, Boron-doped diamond film electrodes—new tool for voltammetric determination of organic substances, Crit. Rev. Anal. Chem. 39 (2009) 148–172, https://doi.org/10.1080/10408340903011812. [11] Y. Yu, L. Wu, J. Zhi, Diamond nanowires: fabrication, structure, properties, and applications, Angew. Chem. Int. Ed. 53 (2014) 14326–14351, https://doi.org/10. 1002/anie.201310803. [12] S. Szunerits, Y. Coffinier, R. Boukherroub, Diamond nanowires: a novel platform for electrochemistry and matrix-free mass spectrometry, Sensors 15 (2015) 12573–12593, https://doi.org/10.3390/s150612573. [13] M. Wei, C. Terashima, M. Lv, A. Fujishima, Z.-Z. Gu, Boron-doped diamond nanograss array for electrochemical sensors, Chem. Commun. (2009) 3624–3626, https://doi.org/10.1039/b903284c. [14] N. Yang, H. Uetsuka, E. Osawa, C.E. Nebel, Vertically aligned nanowires from boron-doped diamond, Nano Lett. 8 (2008) 3572–3576, https://doi.org/10.1021/ nl801136h. [15] J. Shalini, K.J. Sankaran, C.-L. Dong, C.-Y. Lee, N.-H. Tai, I.-N. Lin, In situ detection of dopamine using nitrogen incorporated diamond nanowire electrode, Nanoscale 5 (2013) 1159–1167, https://doi.org/10.1039/c2nr32939e. [16] P. Subramanian, I. Mazurenko, V. Zaitsev, Y. Coffinier, R. Boukherroub, S. Szunerits, Diamond nanowires modified with poly[3-(pyrrolyl)carboxylic acid] for the immobilization of histidine-tagged peptides, Analyst 139 (2014) 4343–4349, https://doi.org/10.1039/C4AN00146J. [17] D. Luo, L. Wu, J. Zhi, Fabrication of boron-doped diamond nanorod forest electrodes and their application in nonenzymatic amperometric glucose biosensing, ACS Nano 3 (2009) 2121–2128, https://doi.org/10.1021/nn9003154. [18] M. Yoshimura, K. Honda, R. Uchikado, T. Kondo, T.N. Rao, D.A. Tryk, A. Fujishima, Y. Sakamoto, K. Yasui, H. Masuda, Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes, Diam. Relat. Mater. 10 (2001) 620–626, https://doi.org/10.1016/S0925-9635(00)00381-2. [19] V. Malcher, A. Mrška, A. Kromka, A. Šatka, J. Janı́k, Diamond film coated on WC/ Co tools by double bias-assisted hot filament CVD, Curr. Appl. Phys. 2 (2002) 201–204, https://doi.org/10.1016/S1567-1739(02)00064-0. [20] R. Šelešovská, M. Štěpánková, L. Janíková, K. Nováková, M. Vojs, M. Marton, M. Behúl, Surface and electrochemical characterization of boron-doped diamond
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
7
electrodes prepared under different conditions, Monatsh. Chem. 147 (2016) 1353–1364, https://doi.org/10.1007/s00706-015-1640-3. T. Ižák, M. Domonkos, O. Babchenko, M. Varga, B. Rezek, V. Jurka, K. Hruška, A. Kromka, Technological aspects in fabrication of micro- and nano-sized carbon based features: nanorods, periodical arrays and self-standing membranes, J. Electr. Eng. 66 (2015) 282–286, https://doi.org/10.2478/jee-2015-0046. O. Babchenko, A. Kromka, K. Hruska, M. Michalka, J. Potmesil, M. Vanecek, Nanostructuring of diamond films using self-assembled nanoparticles, Cent. Eur. J. Phys. 7 (2009) 310–314, https://doi.org/10.2478/s11534-009-0026-8. M. Varga, T. Izak, V. Vretenar, H. Kozak, J. Holovsky, A. Artemenko, M. Hulman, V. Skakalova, D.S. Lee, A. Kromka, Diamond/carbon nanotube composites: Raman, FTIR and XPS spectroscopic studies, Carbon 111 (2017) 54–61, https://doi.org/10. 1016/j.carbon.2016.09.064. H. Uetsuka, T. Yamada, S. Shikata, ICP etching of polycrystalline diamonds: fabrication of diamond nano-tips for AFM cantilevers, Diam. Relat. Mater. 17 (2008) 728–731, https://doi.org/10.1016/j.diamond.2007.12.071. T. Izak, A. Kromka, O. Babchenko, M. Ledinsky, K. Hruska, E. Verveniotis, Comparative study on dry etching of polycrystalline diamond thin films, Vacuum 86 (2012) 799–802, https://doi.org/10.1016/j.vacuum.2011.07.023. D. Kumar, M. Chandran, M.S. Ramachandra Rao, Effect of boron doping on firstorder Raman scattering in superconducting boron doped diamond films, Appl. Phys. Lett. 110 (2017) 191602, https://doi.org/10.1063/1.4982591. V. Mortet, Z. Vlčková Živcová, A. Taylor, O. Frank, P. Hubík, D. Trémouilles, F. Jomard, J. Barjon, L. Kavan, Insight into boron-doped diamond Raman spectra characteristic features, Carbon 115 (2017) 279–284, https://doi.org/10.1016/j. carbon.2017.01.022. G. Piret, C. Hébert, J.-P. Mazellier, L. Rousseau, E. Scorsone, M. Cottance, G. Lissorgues, M.O. Heuschkel, S. Picaud, P. Bergonzo, B. Yvert, 3D-nanostructured boron-doped diamond for microelectrode array neural interfacing, Biomaterials 53 (2015) 173–183, https://doi.org/10.1016/j.biomaterials.2015.02.021. P. Chen, R.L. McCreery, Control of electron transfer kinetics at glassy carbon electrodes by specific surface modification, Anal. Chem. 68 (1996) 3958–3965, https://doi.org/10.1021/ac960492r. S. Szunerits, Y. Coffinier, E. Galopin, J. Brenner, R. Boukherroub, Preparation of boron-doped diamond nanowires and their application for sensitive electrochemical detection of tryptophan, Electrochem. Commun. 12 (2010) 438–441, https://doi. org/10.1016/j.elecom.2010.01.014.