ISSN 10231935, Russian Journal of Electrochemistry, 2014, Vol. 50, No. 2, pp. 101–107. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.D. Krotova, Yu.V. Pleskov, A.A. Khomich, V.G. Ralchenko, D.N. Sovyk, V.A. Kazakov, 2014, published in Elektrokhimiya, 2014, Vol. 50, No. 2, pp. 115–121.
Semiconductor Properties of Nanocrystalline Diamond Electrodes M. D. Krotovaa, Yu. V. Pleskova, z, A. A. Khomichb, V. G. Ralchenkob, D. N. Sovykb, and V. A. Kazakovc a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia b Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, 119991 Moscow, Moscow, Russia c Keldysh Research Center, Moscow, Russia Received December 4, 2012
Abstract—The semiconductor properties of nitrogenated nanocrystalline diamond electrodes and their cor rosion transformations caused by electrochemical experiment in indifferent electrolyte (1 M K2SO4) were studied by the electrochemical impedance spectroscopy method. It was shown that after electrochemical measurements a narrow diamond peak at 1335.7 cm–1 appears in the Raman spectrum; formerly the peak was hidden at a background of the intense signal inherent to graphitelike carbon. It was suggested that the cor rosion damage caused by the exposure to electrochemical experiment resulted in a decrease of relative amount of nondiamond (graphitelike) carbon in the subsurface layer in the nanocrystalline diamond. By using Mott–Schottky plots, the nanocrystalline diamond was shown having ntype conductance. Within the bounds of the “effective medium” approach, the nanocrystalline diamond’s flatband potential in aqueous solution and the noncompensated donor apparent concentration were estimated. Keywords: nanocrystalline diamond, semiconductor, corrosion, electrochemical impedance, Mott–Schottky plot, flatband potential, donor concentration DOI: 10.1134/S1023193514020013
INTRODUCTION In early 2000s, great interest arose to search for an alternative to borondoped microcrystalline diamond electrodes; the latter have been thoroughly studied by the time [1]. The search purposed to find an optimal electrode material compromising between the simpli fying of its preparation, acceptable corrosion resis tance, optimal surface properties (i.e., electrocatalyti cal activity and selectivity toward target reactions), and the possibility of the purposefully shaping of its characteristics. A number of diamondlike and dia mondcontaining materials were investigated [2–4], the nanocrystalline diamond among them. Nanocrystalline diamond films comprise nanodi mensional diamond grains (sized a few nanometers or few tens of nanometers) suspended in a continuous phase of amorphous carbon; the latter is formed of grain boundaries whose thickness usually lies within 0.5 to 2 nm [5–7]. The grain boundaries mainly con sist of πbound carbon atoms [2]. The volume fraction of the grain boundaries in nanocrystalline films may be as large as 30 vol %. It is not unexpected therefore that the material of grain boundaries greatly affects the film properties, electrochemical in particular. z
Corresponding author:
[email protected] (Yu.V. Pleskov)
The nanocrystalline diamond films used to be pre pared by Chemical Vapor Deposition, the CH4–H2–Ar reaction mixture being activated by microwave irradi ation. The nanocrystalline diamond as such has poor conductivity. To make the films well conducting, they are nitrogenated during the film growth process, by adding nitrogen to the gas mixture in the reactor. The N content in the film grows, even if not linearly, with N2 content in the reaction mixture [3]. The film con ductance is due to the nitrogenated amorphous carbon phase of the grain boundaries, whereas the diamond nanocrystallites proper remain dielectric. The effec tive resistivity of heavily nitrogenated nanocrystalline diamond films may be as low as tenths or even hun dredths of Ω cm [3, 8]. The nanodiamond electrochemical behavior is studied rather thoroughly [2, 9–11]. The nanocrystal line diamond electrodes are somewhat inferior to tra ditional borondoped microcrystalline diamond elec trodes. With the increasing of nitrogen concentration in the gas mixture in reactor, the electrochemical behavior changes from that characteristic of poor con ductor to a metallike behavior. Moreover, we observed sort of saturation of the material’s electro chemical activity that stops changing at ~30% N2. (And this allowed us using, in our experiment, samples obtained at the nitrogen concentration in the reactor
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Intensity
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Fig. 1. Xray patterns of nanocrystalline diamond film grown at N2 concentration 25% in reaction mixture.
from 30% to 90%, without emphasizing particular degree of nitrogenation.) At wellnitrogenated nanoc rystalline diamond electrodes outersphere redox reactions (e.g., in the [Fe(CN)6]3–/4– couple) proceed rapidly and practically reversibly. Thus, the nitroge nated nanocrystalline diamond is a promising material for experimental electrochemistry. Despite numerous papers have been devoted to nanocrystalline diamond electrodes, two effects still remain poorly investigated, in particular, (1) gradual changing of the material’s characteristics in the course of its exploitation as electrode and (2) the effect of the material’s semiconductor nature on its electrochemi cal behavior. A restricted study of the electrochemical impedance spectroscopy of nanocrystalline diamond electrodes (grown in electrical arc, rather than in the microwave field) allowed us revealing the ntype of conductance of the material and estimating the donor concentration therein in terms of Mott–Schottky plots [2, 12]. Moreover, the nature of the nitrogenated nanocrystalline diamond conductance, the charge carrier type and concentration were studied by mea suring the Hall effect [7]. On basis of the sign of the effect, it was also concluded that the current in the films is carried by electrons (the ntype conductance). In this work we aimed at an answering the abovefor mulated questions.
varied from 0 to 90%; the argon concentration bal anced the reaction mixture, so the total gas flow was fixed at a value of 0.5 L min–1. The 0.8to3.6 μm thick films were deposited onto polished (class 14) sil icon substrates sized 10 by 10 mm. The substrates were pretreated in a bath with alcohol suspension of deto nation nanodiamond (the particle nominal size was 4–6 nm); the aim was creating of maximal number of the diamond nucleation centers over the substrate sur face. Typical deposition parameters are: pressure 90 Torr, microwave power 2.3 kW, the substrate tem perature 800°С, the process time 2 h. The films’ Raman spectra were recorded at a LabRam HR 800 and a HORIBAJobin Yvon T64000 spectrophotometers (the light scattering was excited at wavelengths of 488 and 244 nm, respectively). The films’ phase composition was studied using a JDX10P (JEOL, Japan) diffractometer with monochromatic CuKα radiation. Nonelectrochemical Characterization of Nanocrystalline Diamond Electrodes The diffraction patterns of nitrogenated samples (Fig. 1) reveal only ground reflections (111), (220), and (311) that relate to the diamond’s cubic form. The grain size was estimated from the width of the (111) reflection by using the Scherrer equation; it is less than 30 nm. No preferred texture along the {111} direction was observed: the (111) texturing degree equals 0.30 for the 15% N2 film; 0.44, for the 25% N2 film. With the UV excitation, the Raman spectra allow better revealing the diamond phase at a background of graphitelike carbon, because of the increase in the diamond Raman scattering cross section when the photon energy approaches the diamond band gap (5.4 eV); whereas in visible light the graphite Raman scattering cross section is two orders higher than that of diamond. In Fig. 2 we give Raman spectra recorded in the UV range for samples synthesized at different nitrogen concentrations in the reaction gas. The “dia mond” peak (1333 cm–1) is observed in all cases, along with broad peak (the Gpeak) near 1583 cm–1, apper taining to the graphitelike phase. The dia mond/graphite quantitative ratio cannot be deter mined from the spectra; however, it is seen decreasing as the reaction gas mixture is enriched with nitrogen. Thus, even at maximal N2 concentration (90%) the films contain the diamond component.
EXPERIMENTAL Deposition of Nanocrystalline Diamond Electrodes and Their Characterization The nanocrystalline diamond films were grown in an UPSA100 plasmachemical microwave reactor (the maximal microwave power 5 kW) at a frequency of 2.45 GHz [7, 13]. In CH4–Ar–H2–N2 mixtures, the methane (2%) and hydrogen (5%) concentrations were maintained constant, whereas that of nitrogen
Electrochemical Measurements A threeelectrode electrochemical cell comprised platinum counter electrode and silver/silver chloride (1 M KCl) reference electrode (all potential values are given against this electrode). The samples (substrate and the film) were mounted at glass holders, electrical contact to substrates being arranged using silver epoxy. The current lead was insulated with highpurity paraf
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Graphite Gpeak 1583 cm–1 Diamond 1333 cm–1 60 90% N2 50% N2 30% N2 40 Intensity
fin. The electrode working surface area was ~0.2 cm2. In what follows, all current and differential capaci tance values are given per 1 cm2 of the electrode’s geo metrical surface. However, as time passed, the elec trode background current density (in the ideal polariz ability region, from 0 to 1.0 V) and the differential capacitance gradually increased (up to ~20%), which is most probably caused by the development of elec trode surface, due to electrochemical attack. Thus, the true values of j and С appear being somewhat lower than those given in figures. However, this phenome non can hardly affect the qualitative conclusions of this paper. The electrochemical impedance spectra over the 1 Hz to 20 kHz frequency range were measured using a SOLARTRON 1280B spectra analyzer; at higher frequencies (up to 100 kHz), an R5021 ac bridge. The measurements were performed in indifferent electro lyte solution (1 M K2SO4). The values of the equiva lent circuit elements were found by the comparing of the experimental impedance spectra (in complex plane plots) with the calculated ones for a chosen equivalent circuit. The latter was the Ershler–Ran dles’ circuit; sometimes, a constantphase element (CPE) was substituted in the circuit for the differential capacitance (see below). The rootmeansquare error used not to exceed 5 × 10–4–10–3.
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RESULTS AND DISCUSSION Changes in Electrochemical Characteristics of Nanocrystalline Diamond Electrodes on Service No systematic corrosion tests were performed; we but observed gradual changes in the electrodes’ prop erties during their exploitation for few weeks in a regime of daily measurements (taking of cyclic volta mmograms, measuring of differential capacitance in indifferent electrolyte). The degree of exposure to electrochemical attack can be roughly characterized as follows. We see from the cyclic voltammograms (see [10, 11]) that the ideal polarizability region for the nanocrystalline diamond in 1 M K2SO4 solution (where the background current is 10 μA/cm2, by order of magnitude) extends over the potential range from 0 to 1.0 V. The abrupt current increase is observed at potentials more anodic than 1.5 V; thus, at moderate potential scan rate up to 1.8–1.9 V the electrode receives a charge of ~0.5 C/cm2 (obviously, its over whelming part is consumed in the oxygen evolution). An electrode, during its service life, was used in the recording of ~100 cyclic voltammograms. To reveal the corrosioninduced changes of the electrodes’ properties, we measured their electro chemical impedance. In Fig. 3 we show complex plane plots of impedance spectra (in bilogarithmic coordinates) for two fresh samples deposited at a dif ferent nitrogen concentration in the reactor gas: a higher one (90%) and lower one (25%). The difference RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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Fig. 2. Raman spectra recorded with UVexcitation (λ = 244 nm) for films grown at different N2 concentrations in reaction mixture.
in their appearance is obvious: the curve of the less nitrogenated sample (curve 3) has a kink that allows suggesting that the complexplane plot comprises two semicircles (a highfrequency and a lowfrequency ones). Whereas the curve taken at the heavier nitroge nated sample (curve 1) shows no kink in the studied frequency range and must involve single semicircle. Upon exposure to electrochemical attack, the heavily nitrogenated sample acquires complexplane plot of impedance spectrum (curve 2) similar to the original plot of the lightly nitrogenated sample, which comprises two semicircles. It is not unreasonable to suggest that during the electrochemical exposure of the nitrogenated diamond the relative amount of the conducting (nitrogenated graphitelike) phase therein decreased. One can judge about the processes occurring in the nanocrystalline diamond electrodes, during their exploitation, by the Raman spectra. In Fig. 4 we show the Raman spectra taken before and after the electro No. 2
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chemical measurements for the sample grown from the reaction gas containing 70% N2. The laser spot diam eter (the probe locality) was approximately 2 μm. The broad lines at 1357 cm–1 (the Dpeak) and 1588 cm–1 (the Gpeak) in both spectra are attributed to sp2hybridized C–Cbonds vibrations; they are charac teristic of the disordered (nanocrystalline) graphite [14].
log(–ImZ) [Ω cm2] 3
2 3
The spectrum of the film was measured twice: firstly, before its using as electrode, and secondly, after the removing of the paraffin mask that protected the sample’s periphery (only the central part of its surface contacted the electrolyte). The spectra coincided, thus confirming the reliable paraffin protective action.
2
1 1 0
2 3 logReZ [Ω cm2]
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Fig. 3. Complexplane plots (in bilogariphmic coordi nates) for samples grown at N2 concentration in reaction mixture: (1) 90% (prior to), (2) 90% (after measurements in electrolyte), and (3) 25%.
Diamond 1335 cm–1
Graphite Dpeak 1357 cm–1 Graphite Gpeak 1588 cm–1
We note the following differences between the spectra: In the initial spectrum we observed only the D and Glines caused by the graphitelike (amor phous) carbon with sp2hybridized C–Cbonds. No diamond line at 1332.5 cm–1 is observed, because it is masked by strong signal from sp2component. After the exposure to electrochemical attack, the D and Glines are broadened. Indeed, the width of Dline and Gline in the spectrum 2 is 127 and 112 cm–1, respectively, while in the spectrum 1, 77 and 82 cm–1, respectively. The line broadening points to the disordering of the residual graphitelike phase, due to partial removal of the better crystallized graphene layers surrounding diamond nanorods (or nano plates).
Intensity
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We compared the Raman spectra with the scatter ing excitation at a wavelength at 488 nm, recorded at the initial (pristine) surface (for this purpose we used the peripheral areas initially buried under the paraffin layer), see Fig. 4, curve 1, and in the central part of the electrode, accessible for electrolyte (curve 2). We shifted the latter spectrum vertically, to keep it off the superposing with the first one. Were this factitious shift disposed of, the intensities of the graphite lines would approach each other.
1400 1500 Raman shift, cm–1
1600
Fig. 4. Raman spectra (excitation wavelength λ = 488 nm) for sample grown at N2 concentration 70% in reaction mixture, recorded (1) prior to and (2) after measurements in electrolyte. The spectra are vertically shifted with respect to each other for the sake of clearness.
Importantly, after the electrochemical measurements have been performed, a narrow peak at 1335 cm–1 appears in the spectrum, which is attributed to dia mond. Its width is 15 cm–1. Because of compression stress, it is shifted toward higher frequencies, as com pared with its normal position (1332.5 cm–1). A simi lar result (appearance of the diamond peak at a back ground of graphite) was observed for the sample deposited from reaction mixture containing 30% N2. Thus, it is not unreasonable suggesting that during the exposure of nanocrystalline diamond electrode to electrochemical attack there occurs selective etching of graphitelike component in the subsurface layer, this component being inferior to diamond in its corro sion resistance. As a result, the electrode surface becomes enriched with the diamond component.
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Semiconductor Properties of the Nanocrystalline Diamond Electrodes
C –2, F–2 cm4 0.010
When discussing semiconductor properties of the nanocrystalline diamond one must keep in mind that it is a twophase system comprising diamond (the nanosized crystallites) and nondiamond (amorphous, graphitelike) carbon (the intercrystalline bound aries). The concepts developed for singlephase semi conductors can be applied to the twophase system within the framework of the “effective medium” approach. The effective medium is an imaginary sin glephase medium whose macroscopic characteristics are the same as those of the studied twophase system (see, e.g., [15]). The applicability criterion of the approach is the infinitesimality of the twophase sys tem’s elements (i.e., the crystallite size and the inter crystalline boundaries’ thickness) as compared with a characteristic length describing the physical phenom enon under study. In the abovementioned Hall mea surements, the characteristic length is the measuring electrodes’s size; in the studies of the semiconduc tor/electrolyte interface differential capacitance C, the width of the spacecharge region at the interface. In the first case (with macroscopic electrodes) the condition is fulfilled a fortiori; below we show that it is also fulfilled in the differential capacitance measure ments. In Fig. 5 we show Mott–Schottky graph plotted for electrode grown at 50% N2 in reactor (curve 1). Qual itatively it resembles similar graphs plotted for typical semiconductors [16, 17]. The extrapolation point at the potential (E) axis is the flatband potential Еfb in the solvent used (here, in water); the slope of the line gives the noncompensated acceptor (NA) or donor (ND) concentration in semiconductor, according to the Schottky formula:
( )
−1 N = ± 2 ⎡⎣d C −2 dE ⎤⎦ , εε 0e
(1)
where the sign “+” and “–” relates, respectively, to donors and acceptors. In our case, the matter is the donors, because the sign of the slope of Mott–Schot tky plots points to the ntype conductance in the material under study. The Mott–Schottky plots measured at various measuring frequencies f show some frequency disper sion of the line slope. The phenomenon occurs for the majority of semiconductor electrodes [17], in particu lar, for the borondoped synthetic CVDdiamond [18, 19]. It goes without saying that the donor concen tration does not depend on the ac frequency; the observed frequency dependence is likely to reflect the fact that the real electrochemical system is more com plicated than that underlying the Schottky theory (from which equation (1) is derived). The problem of the frequency dispersion of the CVDdiamond differ ential capacitance is discussed at length in [1]. Gener ally, the fan of С–2 vs. Е lines can be interpreted as RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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Fig. 5. Mott–Schottky plots (1) for sample grown at N2 concentration 50% in reaction mixture (frequency 16 kHz), (2) for sample grown at N2 concentration 70% in reaction mixture, with hydrogenized surface (frequency 8 kHz).
manifestation of some slow process occurring on semiconductor diamond surface (e.g., adsorption, charging of surface states, etc.) or in the spacecharge region (e.g., ionization of a deep donor), which is not taken into consideration in the Schottky theory. One more manifestation of the slow process is the appear ance of a constant phase element (CPE), rather than a frequencyindependent capacitance, in the equivalent circuit of the electrodes under study. The CPE imped ance is ZCPE = σ–1(iω)–a,
(2)
where the frequencyindependent factor σ is mea sured in Fa Ωa – 1 cm–2; the parameter a determines the character of frequency dependence; ω = 2πf is the angular frequency; and i is the imaginary unity. The frequency dispersion of the differential capac itance results in the apparent frequency dependence of the flatband potential Еfb determined from the Mott– Schottky plots, as well, as of the donor concentration ND (table). The limited information does not allow revealing the dependence of ND and Еfb on the nanocrystalline diamond film’s nitrogenation degree; the difference lies within the limits of experimental error. The values of ND found in this work allowed esti mating, even if formally, the thickness of spacecharge region in the semiconductor material, Lsc. The latter equals [16, 17] Lsc = LD × 21/2(|Y| – 1)1/2,
(3)
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The donor apparent concentration (ND) and flatband po tential (Efb), determined partly from Fig. 4, for the sample grown at N2 concentration 70% in reaction mixture Frequency, kHz
ND × 10–21, cm–3
17 14 13 12 11 10 8 6 4
–Efb, V
3.7 5.3 5.5 5.3 5.5 6.1 6.9 8.1 11
1.8 2.2 1.95 1.9 1.6 1.6 1.4 1.37 1.3
is the Debye length in the semiconductor; εsc is the semiconductor permittivity; ε0 is the permittivity of vacuum; k is the Boltzmann constant; T is the absolute temperature; e is the electron charge; and n is the cur rent carrier concentration. In equation (3), Y is the dimensionless potential: Y = Esc/kT, where Esc is the potential drop in the spacecharge region (the value of Esc is counted off from the flatband potential). Keep ing in mind that n < ND (incomplete donor ionization in the diamond bulk), we obtain LD > 10–7 cm for ND = 1018 cm–3; for Esc ~ 1 V we have Lsc > 10–6 cm. Thus, the thickness of the spacecharge region exceeds the crystallite size in the nanocrystalline dia mond (~10–7 cm) by order of magnitude. And this allows, at least, formally, applying the concept of the C –2, cm4 μF–2 1
2
1 2
0
0.5
1.0
1.5 E, V
Fig. 6. Mott–Schottky plots for sample grown at N2 con centration 30% in reaction mixture (1) prior to and (2) after the annealing in air at 400°C for 10 min. Frequency 15 kHz.
effective medium to the interpreting of the results obtained. Despite the formal conformity of the parameters of the nanocrystalline diamond film to the condition of applicability of the effective medium theory, the phys ical meaning of the quantities ND and Еfb still remains obscure. The flatband potential Еfb, undoubtedly, is kind of characteristic of the film surface state, in par ticular, the degree of its oxidation. In Fig. 5 we also show Mott–Schottky plot for a film whose surface was hydrogenised by exposure to hydrogen plasma after the depositing (curve 2); hence, initially it was in the reduced state (hydrogenterminated). By comparing the two curves in Fig. 5 (and ignoring, in the qualita tive comparison, the difference in nitrogenation degree, see Introduction), we see that the value of Еfb is much more negative for the reduced electrode. The shift of the semiconductor flatband potential [17] (as well, as of the metal zerocharge potential) upon the electrode surface oxidation (that is, putting adsorbed oxygen thereon) toward positive values by ca. 1 V is well known, in particular, for the borondoped dia mond [20–22]. To additionally confirm the abovegiven explana tion, we oxidized electrodes by their annealing in air and compared the Mott–Schottkyplot extrapolation potentials for the oxidized and nonoxidized electrodes (Fig. 6). We see that the oxidation results in the posi tive shift of Еfb in this case. As to the quantity ND, it can be considered as a measure of the material’s conductivity. CONCLUSIONS The measurements of electrochemical impedance and the Raman spectra of nanocrystalline diamond revealed a decrease in the relative amount of nondia mond (graphitelike) carbon and clearer manifesta tion of the diamond component at a background of the nanografite one in the subsurface layer of the electrodes upon their exposure to electrochemical attack during measurements in indifferent electrolyte (1 M K2SO4) solution. The Mott–Schottky plot measurements revealed ntype conductance in the nanocrystalline diamond deposited with microwave activation of reaction gas. By using the effectivemedium approach, the flat band potential and noncompensated donor apparent concentration in nanocrystalline diamond were esti mated. ACKNOWLEDGMENTS Authors are grateful to A.V. Saveliev for the films preparation. This work was supported by the Russian Founda tion for Basic Research, projects nos. 100300011 and 130300660.
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Translated by Yu. Pleskov
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