The Undoped <scp>CVD</scp> Diamond Electrode: The Effect of Surface Pretreatment on its Electrochemical Properties

Abstract

CVD diamond electrodes have been extensively studied in recent years, from the point of view of fundamental electrochemical properties as well as that of applications. Diamond electrodes are found to be attractive for electrochemical applications due to their physical, chemical, and electronic properties, e.g. high thermal conductivity; high hardness, and chemical inertness; a wide electrochemical potential window in aqueous and non-aqueous media; a very low capacitance and a very high electrochemical stability. The search of new materials for development of chemical and biological sensors of high sensitivity and stability has attracted recently a great attention. The diamond has demonstrated to be an attractive material for electroanalytical applications, in view of its unique properties such as robustness, chemical inertness, and stability, resistance to fouling, wide potential window, and low background currents.1-3 According to large amount of literature the diamond layers used for electrochemical applications were usually heavily boron doped4 in order to make them electrically conducting. Raman spectra of the boron-doped diamond films, except a strong characteristic diamond peak at 1331 cm−1 and broad peak at 1540 cm−1 due to nondiamond sp2 carbon phase, shows additional peak at 1200 cm−1, which is the characteristic for heavily boron doped diamond (BDD) layer.5 It is supposed that Raman band centred at around 1200 cm−1 represents a Fano-type interference resulting from degeneration of the diamond Fermi level within the valence band for high doping levels.6 As the doping level (B/C ratio) increases, the 1332 cm−1 peak widens, its intensity decrease rapidly and the broad band at around 1540 cm−1 increases.7 Such a dependence of Raman spectra on B-doping level indicate on the decrease of diamond layer quality, i.e. on the increase of defects concentration. To defects in the diamond layer one can include grain boundaries or intra-grain dislocations, intergranular amorphous, or graphitic regions and hydrogen which is supposed to be trapped on these intra- or intergranular defects.8-10 It was observed that the concentration of hydrogen concentration is proportional to the amounts of sp2 carbon in grain boundaries.9, 10 Now it is certain that the conductivity can be related to hydrogen11 and several controversial models of electrical transport mechanism have been proposed.12, 13 The electrical properties of hydrogen-terminated diamond surface well account the transfer doping model by Gi et al.12 and later by Maier et al.13 The transport characteristics are governed by a narrow hole accumulation layer at the valence band edge where disorder-induced localized states are also presented.14 Madaleno et al.15, 16 successfully related the macroscopic electrical transport to the microscopic inhomogeneities of the diamond film. They showed that the higher content of amorphous carbon phase admixture leads to electrical transport, which can be well described by SCLC (space charge limited current) mechanism. They showed also that the diodes built on the surface with a relatively high non-diamond defects density present provides better Schottky diodes performance, higher activation energy, suggesting that the defects distribution inside the band gap and near the Fermi level are different. It is commonly known that as-grown CVD diamond film is always hydrogen terminated,8 which generates p-type conductivity restricted to the surface region (10 nm)17 on grain boundaries and gives possibility to use the diamond layer as electrode material in electrochemistry. The amorphous carbon admixture and hydrogen terminated diamond grains are responsible for an electrical contact not only between diamond's grains but also between the diamond layer and the substrate.18 In literature, one can find only few articles on undoped diamond layers used as electrode material having sufficient electrical conductivity18-25 what indicate that the doping of diamond is not a unique way to make diamond conducting. The diamond surface structure and its termination are important factors controlling the electrochemical behavior of the diamond electrode, because all electrochemical reactions are proceed at the interface between electrode surfaces and electrolyte solutions. The surface termination is usually generated on the BDD surface by electrochemical methods (water reduction to produce H-terminations or oxidation for O-terminations) or r.f. plasma treatment (H- and O-terminations) among others.26, 27 The charge transfer rate for some redox couples can change with the surface termination on the BDD electrode. So, many authors have investigated the charge transfer mechanism and the kinetics of several redox couples on BDD electrodes, using for this purpose some physical and electrochemical methods.28-32 It was shown that the change in diamond, the surface chemistry from oxygen to hydrogen terminated, leads to an increase in the free carriers concentration, altering the electrochemical behavior from semiconductor to metallic. The present work reports the results on the influence of cathodic and anodic pretreatment of undoped diamond electrode pretreatment on its electrochemical properties. The polycrystalline diamond films were deposited on tungsten wire, 0.5 mm in diameter, as a substrate by hot filament chemical vapor deposition (HF CVD) technique. As a working gas, the mixture of methanol vapor and hydrogen (CH3OH/H2 – 3.0%) was used. The parameters of growth process were as follows: total pressure in reaction chamber was equal to 60 mbar, substrate temperature −1000 K, and working gas flow rate – 100 sccm. In order to enhance the diamond nucleation density, the substrate was scratched with diamond paste before the deposition process was started. The diamond film morphology has been studied by Jeol JSM-820 SEM microscope operating at a voltage of 25 kV. The nature of C–C bonds in diamond films are characterized by means of Raman spectroscopy. The Raman spectra were recorded at room temperature in back scattering geometry using Renishaw inVia Raman spectrometer. The 488 nm line from argon laser was used for excitation. Cyclic voltammetric characteristics were recorded by CS-1087 computer controlled potentiostat (Cypress System Inc., Lawrence, KS) in a three-electrode configuration. A platinum served as the counter and Ag/AgCl/0.5 M H2SO4 served as reference electrodes, respectively. The electrochemical impedance spectra in the frequency range 10 MHz<f<1 MHz with 50 mV ac-amplitude were measured using Hioki type 3522-50/3532-50 LCR HiTESTER equipment. The working electrodes were cleaned in an ultrasonic bath and activated by electrochemical cycling between oxygen and hydrogen evolution before starting the experiments. Figure 1 shows typical morphology of polycrystalline diamond layer deposited on tungsten substrate. The diamond layer is composed from small, randomly oriented microcrystals with well-defined crystalline features. Figure 1a shows diamond electrode at low magnification in order to see overall view of electrode shape while the details of diamond film morphology is presented in Figure 1b. As it is seen in Figure 1b, diamond layer is polycrystalline in nature and consists of well-defined diamond crystallites of the sizes below 1 µm. Diamond films synthesized by CVD with methanol and other carbon sources, in general, contain disordered graphitic carbon phase.8, 33 Raman spectroscopy is one of the best tools to assess the qualities of diamond films because disordered graphitic carbon phase has a distinctly different Raman peak. It is known that cross-section for Raman scattering for graphitic phase is much higher than that for diamond phase, thus allowing even a small amount of graphitic carbon to be recognized in diamond films. Figure 2 presents the Raman spectra of diamond electrode before (Figure 2a) and after (Figure 2b) electrochemical treatment at different places at the electrode surface. In principle, the Raman spectra recorded before and after electrochemical treatment looks similar, only a small difference is observed in intensities of the diamond Raman peaks. Figure 3 presents the two chosen Raman spectra recorded before and after chemical treatment taken from the center of electrode (black curves form Figure 2a and b). There is a single sharp peak at 1332 cm−1 that corresponds to the C–C sp3 bonds. The presence of a broader band around 1580 cm−1 of very low intensity can be attributed to the presence of graphitic carbon impurities (sp2 carbon) contamination. The concentration of non-diamond phase admixture was estimated using the method described elsewhere33 and was in the range from 1.9 to 2.1% what indicates on high-quality diamond films33 with very small amount of impurities. Raman spectra (Figure 2) of diamond electrodes before and after a cathodic/anodic pretreatment (30 min) shows that neither important bulk structural differences nor significant changes in the sp2/sp3 content are introduced into the diamond film by the pretreatment. Thus, the amounts of sp3 carbon (sharp peak at 1332 cm−1) and sp2 carbon carbon impurities (broad peak at 1580 cm−1) are practically the same, indicating that the possible changes in electrochemical response brought on by the pretreatment will be due only to superficial changes, probably in the surface terminations of the diamond film what is in agreement with results observed for BDD electrodes.34 In order to evaluate the quality and the state of the undoped diamond electrode surface after cathodic and anodic pretreatments and the influence of the pretreatment in sulfuric acid electrolyte on the final surface termination, the voltammetric behavior of the redox couple [Fe(CN)6]4−/3− in 0.5 M sulfuric acid supporting electrolyte was investigated by CV. In order to check the influence of the electrode pretreatment on its electrochemical response two similar diamond electrodes were used. After subsequent pretreatments, the cyclic voltammograms were recorded in 1 × 10−3 M [Fe(CN)6]4−/3− + 0.5 M H2SO4. The results shown in Figure 4 indicate that short time cathodic treatment does not change significantly the electrochemical response of undoped diamond electrode. ΔEp slightly increased from the value of 122.44 mV to value of 166.53 mV. When even short time anodic treatment is performed the electrode lost its electrochemical sensitivity (Figure 4 curve 3). The anodic pretreatment converts the hydrogen terminations of diamond surface to oxygen terminations. This is due to the decomposition of water molecules to form hydroxyl radicals and subsequent adsorption of its reaction products on the diamond surface.35 Similar phenomenon was also observed for non-doped diamond which is conducting when its surface is H-terminated and insulating when O-terminated.36, 37 According to Duo et al.38 the decrease in the diamond electrode activity toward the ferro/ferricyanide redox couple after mild anodic polarization is due to the elimination of “active species” (sp2 carbon) leading to a “non-active” polycrystalline diamond surface. It is well known that ref.8 the hydrogen concentration in the CVD diamond appears to be proportional to the sp2 bonds ratio therefore the elimination of sp2 carbon phase can lead to the dehydrogenation and oxidation of diamond surface. The conductivity of the oxidized diamond surface can be partially reinstated when the diamond surface is hydrogenated again by a cathodic treatment (see Figure 4 curve 4) but the electrode did not receive their initial sensitivity but it is still comparable with that of standard Pt electrode (Figure 5). The partial loss of the electrochemical sensitivity can be associated with the partial etching of sp2 sites, at which hydrogen is mostly adsorbed, during earlier anodic pretreatment.8 The results shown in Figure 6 confirm the possibility of using cathodic pretreatments to insure highly conductive diamond electrode surfaces that facilitate the interaction and adsorption of the electrochemical species with the electrode surface in contrary to the other pretreatments are commonly reported in the literature, for instance as anodic and thermal.39 The short time cathodic pretreatment causes only small change in ΔEp value, which decreases from the value of 336.98 mV to the value of 254.91 mV (Figure 6. CV's 1 and 2). If the pretreatment is longer ΔEp decreases to the value of 87.80 mV (Figure 6. CV 3), what indicate on totally reversible character of [Fe(CN)6]4−/3− redox couple. These changes must be related to increase of superficial hydrogen due to hydrogenation of diamond surface via cathodic pretreatment. The above results are confirmed by EIS spectroscopy measurements presented in Figure 7. In the theory, the impedance data of materials that have capacitive and resistive components, when represented in the Nyquist diagram [namely, the negative of the imaginary part-Im Z in the y-axis and the real part, Re Z in the x-axis] each point corresponding to a different frequency, leads to a succession of semicircles in the high frequency region.40 In this case, the radius of the semicircle corresponds to the bulk resistance R of the sample. As one can see from Figure 7, the bulk resistance of the electrode after cathodic pretreatment decreases what is in good agreement with CV measurements are presented in Figure 5. According to literature,34, 41, 42 the cathodic pretreatment only increases hydrogen termination, but does not change the original diamond surface physical and chemical characteristics, e.g. a hydrophobic surface, with negative electron affinity and high conductivity. Hayashi et al.43 reported that the surface p-type conductivity (should not be mistaken for the bulk conductivity of the material) is observed on hydrogenated BDD surfaces and disappears after dehydrogenation or oxidation of the surface what is in accordance with our observation. They suggested from different experimental results43, 44 that the hole accumulation layer (about 20 nm thick) is directly linked with the presence of hydrogen on surface and not in the bulk. As it is seen from Figure 2, the Raman spectrum of diamond electrode after cathodic/anodic pretreatment remained unchanged indicating that bulk properties of diamond layer are not changed. The obtained results show that the hydrogenated surface activates the ferri/ferrocyanide charge transfer reaction; on the other hand the oxygen surface termination inhibits its kinetics for diamond surface specific active sites. Hydrogenation of diamond surface trough cathodic pretreatment can be considered as a effective way of improvement in its electrochemical activity. Diamond electrode itself can be described as a composite electrode consisting of a non-active matrix of polycrystalline diamond (sp3 carbon) in which species of sp2 carbon phase are presented. The sp2 carbon species can be considered as the primary pathway for the charge transfer process, assuming that electron transfer is much more significant at non-diamond than at diamond sites.