J. Phys. Chem. C 2009, 113, 2761–2770
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Scanning Electrochemical Microscopy Studies of Redox Processes at Undoped Nanodiamond Surfaces Katherine B. Holt,*,† Christoph Ziegler,† Jianbing Zang,‡ Jingping Hu,§ and John S. Foord§ Department of Chemistry, UniVersity College London, 20, Gordon St., London WC1H 0AJ, United Kingdom, State Key Laboratory of Metastable Materials Science and Technology, College of Material Science and Engineering, Yanshan UniVersity, Qinhuangdao 066004, People’s Republic of China, and Chemistry Research Laboratory, UniVersity of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom ReceiVed: May 1, 2008; ReVised Manuscript ReceiVed: December 1, 2008
The redox behavior of an undoped nanodiamond (ND) film grown by chemical vapor deposition was investigated using cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) and redox mediators Fe(CN)63-, Fe(CN)64-, ferrocenemethanol (FcOH), and Ru(NH3)63+. CV showed extremely sluggish kinetics for all redox couples, but the reduction of Fe(CN)63- was found to be especially slow when compared to the oxidation of Fe(CN)64-. SECM confirmed this trend, with experimental heterogeneous rate constants, obtained by fitting approach curves to theory, being of the magnitude of 10-3 cm s-1. The oxidation of Fe(CN)64- at an overpotential, |η|, of 0.6 V was found to occur 5 times faster than the reduction of Fe(CN)63at the same |η|. The results are explained by assuming conduction takes place through extended sp2 (graphitic and defect sites) through the film. The nondiamond component of the film introduces impurity bands into the band gap that allows limited metallic type conductivity. The slow electron transfer was attributed to the very small percentage of the surface that was electrochemically active and hence relatively narrow impurity bands and limited carrier numbers. About 2% of the surface was calculated to be active in the potential range -0.4 to 0.5 V vs Ag/AgCl. At >0.5 V, the active area was found to increase with applied potential up to about 10% at 0.8 V. This increase in active electrode area explains the faster rate constants obtained for the oxidation of Fe(CN)64- at these potentials. It is postulated that the increase in active area is due to oxidation of defect sites of the film to form electron deficient, hence redox active, centers. This results in the widening of the impurity bands in the band gap and hence an increased density of states. Approach curves to a layer of 5 nm ND powder using the same redox couples exhibited a similar trend, with reduction of Fe(CN)63- taking place much slower than oxidation of Fe(CN)64-. Overall, rate constants were about 10 times faster at the powder interface than the film. It is believed that electron transfer at the ND nanoparticle surface takes place at similar sites as on the ND film but that they are present at higher relative concentrations due to the higher surface to bulk atom ratio of the nanoparticles. 1. Introduction Diamond in its bulk, gemstone form is a well-known insulating material with a band gap of 5.47 eV. At the other extreme, boron-doped diamond (BDD), grown by chemical vapor deposition (CVD), is a well-established and versatile electrode material exhibiting metallic conductivity at high doping levels.1 Although undoped diamond is one of the best known examples of an insulating material, some undoped CVD diamond films exhibit interesting conductivity effects. Hydrogenterminated, diamond thin films have significant p-type surface conductivity2 and have been the subject of much investigation.3 Shin et al4 showed that high-quality, hydrogen-terminated, undoped single crystalline diamond undergoes an insulator-metal transition when in contact with redox solutions having electrochemical potentials below the valence band maximum of H-terminated diamond. The diamond is insulating when in vacuum but acquires surface conductivity due to charge transfer between the diamond valence band and solution redox couple. Such behavior was noted for redox couples Fe(CN)63-/4- and * To whom correspondence should be addressed. E-mail:
[email protected]. † University College London. ‡ Yanshan University. § University of Oxford.
Ru(NH3)62+/3+ but not methyl viologen or Co(sep)2+/3+ whose electrochemical potentials are more negative. This chargetransfer induced surface conductivity requires relatively defectfree diamond and complete surface hydrogen termination and disappears on oxidation of the diamond surface. Recently it was shown that electron transfer also takes place between the hydrogenated diamond surface and oxygen in an aqueous environment.5 This redox process involves the transfer of electrons from the diamond surface to dissolved oxygen, which undergoes electrochemical reduction and results in a surfeit of holes in the near-surface region of the diamond film and hence p-type conductivity. The focus of this paper is nanocrystalline diamond (also known as “ultrananocrystalline diamond” or “nanodiamond”), grown by CVD. Such films are grown using conditions resulting in high nucleation rates and hence consist of diamond crystallites of dimensions of 0.2 V (based on literature values30). The calculated values for percentage of electroactive area are plotted against substrate potential, Esub, in Figure 7. Using this analysis suggests that the asymmetry observed in kexpt for the Fe(CN)63-/4- couple is due to the greater availability of sites for electron transfer on the ND film at Esub > 0.5 V. The conducting portion of the surface at -0.4 V is calculated as ∼2% compared to ∼10% at 0.8 V vs Ag/AgCl. For Ru(NH3)63+/2+ ksite is approximated to 0.25 cm s-1, based on literature values of k0 ) 0.24 cm s-1 at a glassy carbon electrode.32 This corresponds to conducting areas of 0.5 V. Note the widening of the db band as the surface undergoes electrochemical oxidation and the appearance of πCdO and π*CdO states in the band gap leading to increased density of states; EF ) Fermi level. (c)Schematic diagram of the postulated impurity states present in the band gap of ND nanoparticles. πCdO and π*CdO states in the band gap lead to increased density of states; EF ) Fermi level. (d) Schematic diagram showing the relative positions of the Fermi level, EF for the ND nanoparticle and the Fe(CN)64-/3- and Ru(NH3)62+/3+ redox couples in solution (left). On the right the schematic shows how the EF of the ND particle will equilibrate in the Fe(CN)64- solution over time to allow some reduction of Fe(CN)63- to take place. DFe ) energy states for the redox couple in solution.
current distribution (not shown), with any variation attributed to topographical features of the film. This suggests that any active areas had dimensions below the resolution of these images (about 1 µm for the smallest tip), which is consistent with the observed morphology of the surface, consisting of grains of ∼100 nm. This is also consistent with the observation that approach curves for a given mediator, at the same Esub, were almost identical independent of xy position on the ND film. Each approach curve represents the current obtained for approach over an average mixture of conducting and insulating sites, which appear to be evenly distributed over the surface; this is shown schematically in Figure 6c. To sample the activity of the conducting sites only, a tip of dimensions similar to the size of the active sites would be required. Figure 7 shows there is good agreement between the values obtained for percentage of active electrode area for the different
Redox Processes at Undoped Nanodiamond Surfaces redox couples, after taking into account the different heterogeneous rate constants each species experiences at a carbon electrode. In the range Esub ) -0.4 to 0.5 V about 2% of the surface is estimated as active; this is slightly lower than the 5% estimated from CV current peaks but a little higher than the 1% estimated surface sp2 content of the film from XPS studies.8 In the potential region 0.5 V and strongly suggests that similar mechanisms are responsible for the observed redox activity in both materials. However, a notable difference is that the particle surface is already active in the absence of applied potential, whereas the ND film requires applied potentials of >0.5 V in order for this activity to be “switched on”. Physical characterization methods (XPS, Raman) have shown subtle differences in the concentrations of graphitic sp2 carbon in the surface and subsurface region of these two materials (see Figure 1). Raman spectroscopy reveals greater graphitic content in the film than the powder; however, C1s XPS data shows an sp2 carbon peak present for the powder surface that is not observed for the film. This difference may be a consequence of the surface sensitivity of each technique; XPS samples only the top few atomic layers of the material, in contrast to Raman which can be considered more of a bulk technique. Hence we can interpret that the ND film has a higher concentration of sp2
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2769 carbon within the bulk of the material, but the surface concentration is relatively low. In contrast, the ND powder has a higher surface concentration of sp2 carbon, as detected by XPS. This is not surprising, considering the much higher surface area to bulk ratio expected for powder consisting of 5 nm particles in comparison to the film, consisting of 100 nm crystallites. Although electrochemical processes take place on the surface of the material, the overall carrier properties of a material are usually dominated by bulk properties, as the ratio of bulk to surface atoms is so large. Hence in the case of the ND film the electrochemical behavior is dominated by the availability of conducting sp2 pathways through the film or the density of states due to sp2 defects throughout the film. A schematic of a possible band structure for a ND film below 0.5 V is shown in Figure 8a, based on the theoretical calculations for band structure of ND in ref 14. Impurity bands exist within the band gap of nanodiamond, with π and π* states associated with sp2-bonded carbon atoms placed symmetrically around the EF. A dangling bond (db) state is predicted located around the EF. Also present in the band gap are so-called σ* states due to distortion of sp3 bonds in the grain boundary regions. The conductivity of the film and its electrochemical activity is therefore dominated by the width of these bands and their degree of overlap and that is determined by the concentration and chemical composition of the electrochemically active sites both on the surface and in the bulk of the film. In the case of the powder, the very small size of the individual particles means that that its electrochemical and electronic properties are dominated by the properties of the surface rather than the bulk. Previous studies16 have shown that oxygenated powders with greater surface oxygen termination were more electrochemically active than hydrogenated powders, which suggests that unsaturated surface bonding is responsible for observed redox activity and that the presence of CdO functionalities may be as important as CdC sites. Figure 8c shows a schematic illustration of a possible band structure for the ND particles, taking into account the large degree of π-bonding associated with oxygen termination. We postulate that additional π(CdO) and π*(CdO) states are present, leading to a greater density of states and band overlay in the band gap and hence enhanced redox activity. Figure 8d shows how the EF of the ND particle may relate to the EF for the Fe(CN)63-/4- and Ru(NH3)63+/2+ couples, illustrating how the oxidation of the Fe(II) and Ru(II) species can occur spontaneously at the ND powder surface, as their EF is higher in energy than that of the impurity band of the ND. This explains the large degree of positive feedback observed for the oxidation of these complexes at the powder. A much smaller amount of positive feedback was observed for the reduction of Fe(CN)63-, and this is because for electrons to transfer spontaneously from the ND impurity band to the Fe(CN)63- in solution would require the ND EF to be higher in energy than that of the Fe(CN)63-/4couple, which is probably not the case, as shown in Figure 8d. However, some positive feedback is still observed because the experiment is carried out in the presence of an excess of Fe(CN)64-, which over time can undergo spontaneous oxidation at the ND. This results in the EF of the ND increasing until an equilibrium is reached, and it becomes comparable to the EF of the Fe(CN)63-/4- couple, allowing some degree of reduction of the Fe(III) species at the surface of the powder. We suggest that a change in surface chemistry at the ND film surface at Esub > 0.5 V may be responsible for the increase in electrochemical activity observed above this potential. The background CV for the ND film in Figure 2a shows that this
2770 J. Phys. Chem. C, Vol. 113, No. 7, 2009 increased activity corresponds with the onset of the oxidation processes on the film itself. We postulate that at these potentials some defect sites on the ND film surface (e.g., isolated sp2 inclusions, sp/sp2/sp3 amorphous carbon clusters, intercrystallite regions, and edge planes and steps) begin to be oxidized, losing electrons to the solvent and resulting in electron deficient sites with unsaturated bonding or “dangling bonds”. This results in the introduction of “surface” π, π* and dangling bond impurity bands in the band gap, increased band overlap, and hence an increased density of states in the band gap, as shown in Figure 8b. CdO termination may begin to be formed on the ND surface at these potentials leading to the formation of π(CdO) and π*(CdO) states, as suggested for the powder, again leading to an increased density of states. Oxidation to higher potentials (>2 V) will result in the oxidation and permanent loss of these surface groups (as CO2) and hence explains the passivating effect of oxidative etching on this material. At a microscopic level, in the presence of a oxidizable redox probe, such as Fe(CN)64-, electron transfer between the probe and the film can take place at an increased rate as more of these active sites become activated or “switched on” and the percentage of conducting area increases. Note that in Figure 7, the y axis representing the conducting area of the surface could readily be interchanged for one corresponding to the concentration of carriers in the impurity band, or the density of π states in the band gap and would be describing the same phenomenon. 5. Conclusion Undoped ND film and ND particles are nonconducting materials; however both undergo electron transfer with redox probes Ru(NH3)63+, Fe(CN)63-/4-, and FcOH. The redox behavior of the ND film is potential dependent, with greater activity noted as the film becomes mildly oxidized. For both forms of ND heterogeneous rate constants for oxidation of Fe(CN)64- were faster than those for reduction of Fe(CN)63- at the diamond interface. If the redox active sites on the diamond surface are due to sp2 bonding and CdO termination, resulting surface states may be thermodynamically more inclined to gain electrons than lose them, i.e., they can be thought of as occupying a higher oxidation state than the redox probes. Alternatively, conduction can be thought of as via impurity π bands in the band gap, and surface CdO bonding results in additional bands being introduced into the band gap, increasing the density of states and resulting in greater conductivity. The different activity toward the Fe(CN)64- and Fe(CN)63- species may be explained by the Fermi level of the ND being at a higher potential (lower energy) than that of the redox couple. The observed behavior of the film did not provide any evidence of n-type conductivity, although some results were consistent with either intrinsic or induced surface p-type conductivity. However, as such a mechanism would contradict previous studies of this material, at this stage a metallic conduction mechanism via extended sp2 pathways through the film is preferred. However, due to the heterogeneity of the surfaces under investigation, it is quite likely that a mixture of sites of metallic and semiconducting nature may be present and the observed results may be attributed to complex and mixed mechanisms. Acknowledgment. K.B.H. acknowledges EPSRC for an Advanced Research Fellowship. J.Z. acknowledges the support of Fok Ying Tung Education Foundation (No. 91049) and the
Holt et al. Service Center of Expert and Scholars of Hebei Province (No. 200410). Dr. Daren Caruana (UCL, UK) and Dr. Enrique Millan-Barrios (Universidad de Los Andes, Merida, Venezuela) are both gratefully acknowledged for helpful discussions. The helpful comments of two anonymous referees are also acknowledged. References and Notes (1) Compton, R. G.; Foord, J. S.; Marken, F. Electroanal. Chem. 2003, 15, 1349. (2) Landstrass, M. I.; Ravi, K. V. Appl. Phys. Lett. 1989, 55, 975. (3) (a) Ristein, J. J. Phys. D: Appl. Phys. 2006, 39, R71. Nebel, C. E.; Rezek, B.; Shin, D.; Watanabe, H. Phys. Status Solidi A 2006, 203, 3273. (4) (a) Shin, D.; Watanabe, H.; Nebel, C. E. J. Am. Chem. Soc. 2005, 127, 11236. Nebel, C. E.; Kato, H.; Rezek, B.; Shin, D.; Takeuchi, D.; Watanabe, H.; Yamamoto, T. Diamond Relat. Mater. 2006, 15, 264. (5) Chakrapani, V.; Angus, J. C.; Anderson, A. B.; Wolter, S. D.; Stoner, B. R.; Sumanasekera, G. U. Science 2007, 318, 1424. (6) Fausett, B.; Granger, M. C.; Hupert, M. L.; Wang, J.; Swain, G. M.; Gruen, D. M. Electroanal. Chem. 2000, 12, 7. (7) Ferreira, N. G.; Azevedo, A. F.; Beloto, A. F.; Amaral, M.; Almeida, F. A.; Oliveira, F. J.; Silva, R. F. Diamond Relat. Mater. 2005, 14, 441 (. (8) Foord, J. S.; Hu, J. P. Phys. Status Solidi A 2006, 203, 3121. (9) Williams, O. A. Semicond. Sci. Technol. 2006, 21, R49. (10) Pleskov, Yu, V.; Krotova, M. D.; Elkin, V. V.; Ralchenko, V. G.; Saveliev, A. V.; Pimenov, S. M.; Lim, P.-Y. Electrochim. Acta 2007, 52, 5470. (11) (a) Hian, L. C.; Grehan, K. J.; Compton, R. G.; Foord, J. S.; Marken, F. Diamond Relat. Mater. 2003, 12, 590,Hian, L. C.; Grehan, K. J.; Goeting, C. H.; Compton, R. G.; Foord, J. S.; Marken, F. Electroanal. Chem. 2002, 15, 169. (12) Raina, S.; Kang, W. P.; Davidson, J. L. Diamond Relat. Mater. 2008, 17, 896. (13) Pleskov, Yur, V.; Krotova, M. D.; Ralchenko, V. G.; Khomich, A. V.; Khmelnitskiy, R. A. Electrochim. Acta 2003, 49, 41. (14) Zapol, P.; Sternberg, M.; Curtiss, L. A.; Frauenheim, T.; Gruen, D. M. Phys. ReV. B 2001, 65, 045403 (. (15) Achatz, P.; Garrido, G. A.; Stutzmann, M.; Williams, O. A.; Gruen, D. M.; Kromka, A.; Steinmuller, D. Appl. Phys. Lett. 2006, 88, 101908 (. (16) Holt, K. B.; Ziegler, C.; Caruana, D. J.; Zang, J. B.; Millan-Barrios, E.; Foord, J. S.; Hu, J. P. Phys. Chem. Chem. Phys. 2008, 10, 303,. (17) Chen, L. H.; Zang, J. B.; Wang, Y. H.; Bian, L. Y. Electrochim. Acta 2008, 53, 3442. (18) Zhu, J. T.; Shi, C. G.; Xu, J. J.; Chen, H. Y. Bioelectrochem. 2007, 71, 243. (19) Holt, K. B.; Bard, A. J.; Show, Y. J. Phys. Chem. B 2004, 108, 15117. (20) Wilson, N. R.; Clewes, S. L.; Newton, M. E.; Unwin, P. R.; Macpherson, J. V. J. Phys. Chem. B 2006, 110, 5639. (21) Szunerits, S.; Mermoux, M.; Crisci, A.; Marcus, B.; Bouvier, P.; Delabouglise, D.; Petit, J. P.; Janel, S.; Boukherroub, R.; Tay, L. J. Phys. Chem. B. 2006, 110, 23888. (22) Wang, S. H.; Swain, G. M. J. Phys. Chem C. 2007, 111, 3986. (23) Neufeld, A. K.; O’Mullane, A. P. J. Solid State Electrochem. 2006, 10, 808. (24) Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. J. Am. Chem. Soc. 2006, 128, 11635. (25) Goeting, C. H.; Marken, F.; Gutierrez-Sosa, A.; Compton, R. G.; Foord, J. S. New Diamond Front. Carbon Technol. 1999, 9, 207. (26) Prawer, S.; Nugent, K. W.; Jamieson, D. N.; Orwa, J. O.; Bursill, L. Al.; Peng, J. L. Chem. Phys. Lett. 2000, 332, 93. (27) Ferrari, A. C. J. Phys. ReV. B 2001, 63, 121405. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons, 2000. (29) Bard, A. J.; Mirkin, M. V. Scanning Electrochemical Microscopy; Marcel Dekker: New York, 2001; p 158. (30) McDermott, M. T.; McDermott, C. A.; McCreery, R. L. Anal. Chem. 1993, 65, 937. (31) Fischer, A. E.; Show, Y.; Swain, G. M. Anal. Chem. 2004, 76, 2553. (32) Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 3115. (33) C. Bourdillon, C.; Demaille, C.; Moiroux, J.; Saveant, J.-M. J. Am. Chem. Soc. 1995, 117, 11503.
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