on Indium Tin Oxide and Boron-Doped Diamond - American Chemical

7,7,8,8-Tetracyanoquinodimethane), on Indium Tin Oxide and. Boron-Doped ... conducting phase I CuTCNQ onto indium tin oxide (ITO) and boron-doped ...
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Langmuir 2006, 22, 10499-10505

10499

Electrocrystallization of Phase I, CuTCNQ (TCNQ ) 7,7,8,8-Tetracyanoquinodimethane), on Indium Tin Oxide and Boron-Doped Diamond Electrodes† Anthony P. O’Mullane,‡ Aaron K. Neufeld,*,§ Alexander R. Harris,‡ and Alan M. Bond*,‡ School of Chemistry, Monash UniVersity, P.O. Box 23, Victoria, 3800, Australia, and BlueScope Steel Ltd., Research Laboratories, P.O. Box 202, Port Kembla, NSW 2505, Australia ReceiVed February 12, 2006. In Final Form: May 29, 2006 The electrochemical reduction of TCNQ to TCNQ•- in acetonitrile in the presence of [Cu(MeCN)4]+ has been undertaken at boron-doped diamond (BDD) and indium tin oxide (ITO) electrodes. The nucleation and growth process at BDD is similar to that reported previously at metal electrodes. At an ITO electrode, the electrocrystallization of more strongly adhered, larger, branched, needle-shaped phase I CuTCNQ crystals is detected under potential step conditions and also when the potential is cycled over the potential range of 0.7 to -0.1 V versus Ag/AgCl (3 M KCl). Video imaging can be used at optically transparent ITO electrodes to monitor the growth stage of the very large branched crystals formed during the course of electrochemical experiments. Both in situ video imaging and ex situ X-ray diffraction and scanning electron microscopy (SEM) data are consistent with the nucleation of CuTCNQ taking place at a discrete number of preferred sites on the ITO surface. At BDD electrodes, ex situ optical images show that the preferential growth of CuTCNQ occurs at the more highly conducting boron-rich areas of the electrode, within which there are preferred sites for CuTCNQ formation.

Introduction CuTCNQ (TCNQ ) 7,7,8,8-tetracyanoquinodimethane) is a semiconducting solid that exists in two phases.1,2 Phase I is initially formed in all reported forms of synthesis. Subsequently, this kinetically favored structure may be converted to thermodynamically stable phase II by refluxing phase I in an appropriate solvent1 or by repetitive cycling of the electrode potential when TCNQ is adhered to an electrode in contact with aqueous CuSO4 solution.3 CuTCNQ (phase I) also may be prepared by electrocrystallization onto metallic and glassy carbon electrode surfaces by the reduction of TCNQ in acetonitrile (0.1 M Bu4NPF6) in the presence of Cu+(MeCN) ions according to the reaction scheme4

TCNQ(MeCN) + e- h TCNQ•-(MeCN)

(1)

TCNQ•-(MeCN) + Cu+(MeCN) u CuTCNQ(s)

(2)

In this article, we report the electrocrystallization of semiconducting phase I CuTCNQ onto indium tin oxide (ITO) and boron-doped diamond (BDD) electrodes rather than conventional metallic electrodes. The use of these electrode surfaces allows the nucleation-growth process to be probed under conditions where crystalline deposits are sufficiently large to be monitored by in situ optical methods as well as ex situ by X-ray powder diffraction and other techniques. †

Part of the Electrochemistry special issue. * Corresponding authors. E-mail: [email protected], [email protected]. ‡ Monash University. § BlueScope Steel Ltd. (1) Heintz, R. A.; Zhao, H.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Inorg. Chem. 1999, 38, 144-156. (2) O’Mullane, A. P.; Neufeld, A. K.; Bond, A. M. Anal. Chem. 2005, 77, 5447-5452. (3) Neufeld, A. K.; Madsen, I.; Bond, A. M.; Hogan, C. F. Chem. Mater. 2003, 15, 3573-3585. (4) Harris, A. R.; Neufeld, A. K.; O’Mullane, A. P.; Bond, A. M.; Morrison, R. J. S. J. Electrochem. Soc. 2005, 152, C577-C583.

It is well known that metal deposition initiates at preferred sites, such as step edges, point defects, and domain boundaries of graphite,5,6 metal,7,8 and metal oxide9-11 electrodes. It has also been shown that STM-tip-induced pits on highly oriented pyrolytic graphite (HOPG) surfaces nucleate electrochemical silver deposition.12 Analogous considerations are likely to apply to the electrocrystallization of semiconducting CuTCNQ. In the case of BDD, the boron concentration in the {111} growth sectors is 5 times higher than that in the {100} growth sectors.13 During the doping procedure, boron may also accumulate at defect sites, grain boundaries, and crystal edges of the diamond.14 For BDD, it has been demonstrated that the initial nucleation sites for metal electrodeposition are the more highly conducting dopant-rich areas of the electrode.15-17 Such areas of relatively high and low conductivity are visible as dark and light regions, respectively, under an optical microscope.18 Thus, simple optical14 methods are available to determine if the heterogeneity in the conductivity and location of dopants of a (5) Boxley, C. J.; White, H. S.; Lister, T. E.; Pinhero, P. J. J. Phys. Chem. B 2003, 107, 451-458. (6) Wang, L. L.; Ma, X. C.; Qi, Y.; Jiang, P.; Jia, J. F.; Xue, Q. K.; Jiao, J.; Bao, X. H. Ultramicroscopy 2005, 105 (1-41), 1-5. (7) Kolb, D. M.; Schneeweiss, M. A. Electrochem. Soc. Interface 1999, 8, 26-30. (8) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 2000, 45, 25592574. (9) Min, B. K.; Wallace, W. T.; Santra, A. K.; Goodman, D. W. J. Phys. Chem. B 2004, 108, 16339-16343. (10) Baumer, M.; Frank, M.; Heemeier, M.; Kuhnemuth, R.; Stempel, S.; Freund, H. J. Surf. Sci. 2000, 454-456, 957-962. (11) Wallace, W. T.; Min, B. K.; Goodman, D. W. J. Mol. Catal. A: Chem. 2005, 228, 3-10. (12) Li, W. J.; Virtanen, J. A.; Penner, R. M. Appl. Phys. Lett. 1992, 60, 1181-1183. (13) Kolber, T.; Piplits, K.; Haubner, R.; Hutter, H. Fresenius. J. Anal. Chem. 1999, 365, 0636-0641. (14) Holt, K. B.; Bard, A. J.; Show, Y.; Swain, G. M. J. Phys. Chem. B 2004, 108, 15117-15127. (15) Bennett, J. A.; Show, Y.; Wang, S.; Swain, G. M. J. Electrochem. Soc. 2005, 152, E184-E192. (16) Riedo, B.; Dietler, G.; Enea, O. Thin Solid Films 2005, 488, 82-86. (17) Enea, O.; Riedo, B.; Dietler, G. Nano Lett. 2002, 2, 241-244. (18) Hyde, M. E.; Jacobs, R.; Compton, R. G. J. Phys. Chem. B 2002, 106, 11075-11080.

10.1021/la060408v CCC: $33.50 © 2006 American Chemical Society Published on Web 07/19/2006

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Figure 1. Cyclic voltammograms (a and b) obtained for 10 mM TCNQ (TCNQ0/•- process) in acetonitrile (0.1 M Bu4NPF6) at (a) ITO and (b) BDD electrodes and (c and d) 100 mM Cu(MeCN)4+ in acetonitrile (0.1 M Bu4NPF6) at (c) ITO and (d) BDD electrodes. Scan rates of 50 and 100 mV s-1 were employed for the ITO and BDD electrodes, respectively.

BDD electrode plays a role in the nucleation and growth of semiconducting CuTCNQ crystals. In the case of ITO electrodes, a much stronger adhesion of CuTCNQ than found on metallic electrode surfaces allows the rapid growth of large crystals that can be monitored by in situ video imaging techniques on this optically transparent surface. Ex situ X-ray diffraction (XRD) and scanning electron microscopy (SEM) and spectroscopic methods (vibrational) can then be used to confirm the identity and phase of the electrocrystallized product. These studies enhance the understanding of the mechanism of CuTCNQ electrocrystallization onto electrode surfaces. CuTCNQ is presently being extensively analyzed for possible applications as a memory storage and field-emission material.19-23 The present study enhances the knowledge concerning the properties of electrochemically synthesized material. Experimental Section Materials. Tetrakis(acetonitrile) copper(I) hexafluorophosphate ([Cu(MeCN)4]PF6), TCNQ, acetonitrile, and tetrabutylammonium hexafluorophosphate (Bu4NPF6) were purchased and purified (when required) as described in ref 4. Analytical-grade copper sulfate was used as received from Aldrich, and all aqueous solutions were prepared from water (resistivity 18.2 MΩ cm) purified by a Milli-Q reagent deionizer (Millipore Corp.). CuTCNQ Phase 1 Synthesis. CuTCNQ crystals were isolated after the reaction of CuI with TCNQ dissolved in acetonitrile for 3 min at 343 K under nitrogen according to Heintz et al.1 The dark(19) Muller, R.; De Jonge, S.; Myny, K.; Wouters, D. J.; Genoe, J.; Heremans, P. Solid-State Electron. 2006, 50, 602-606. (20) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120-1121. (21) Cao, G.; Ye, C.; Fang, F.; Xing, X.; Xu, H.; Sun, D.; Chen, G. Micron 2005, 36, 267-270. (22) Oyamada, T.; Tanaka, H.; Matsushige, K.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2003, 83, 1252-1254. (23) Huang, W. Q.; Wu, Y. Q.; Gu, D. H.; Gan, F. X. Chin. Phys. Lett. 2003, 20, 2178-2181.

blue, needle-shaped crystals (phase I) were recovered by filtration, washed with acetonitrile, and then dried in vacuum at 0.01 Pa. CuTCNQ Phase II Synthesis. The conversion of phase I crystals to phase II was performed by refluxing a suspension of phase I material in acetonitrile at 353 K for 3 h. Recovery of the phase II square-shaped crystals was the same as that for phase I needles.1 Electrochemical Instrumentation and Procedures. Details of voltammetric instrumentation, electrochemical cells, and procedures are contained in ref 4. Potentials are reported versus Ag/AgCl (3 M KCl), which has a potential of 407 mV versus the Fc0/+ (Fc ) ferrocene) reference scale. ITO-coated glass (Prazisions Glas and Optik GmbH) typically having an area of 0.04 cm2 with a sheet resistance of 10 Ω/sq, as quoted by the manufacturer, and BDD disks (Windsor Scientific, boron carrier concentration of 1020 atoms cm-3) having an area of 0.071 cm2 were used as the working electrodes. ITO electrodes were cleaned by sonication in acetone and then in propan-2-ol for 10 min and were then dried with a stream of nitrogen. BDD electrodes were polished with a 0.3 µm alumina slurry (Buehler) on Microcloth polishing cloths, rinsed in deionized water, blown dry with nitrogen, and finally cleaned using the procedure outlined for the ITO electrodes to remove any residual alumina particles. No electrochemical pretreatment of the electrode, such as hydrogenation or oxidation of the surface,24 was undertaken. However, the quality of the electrode performance was monitored by frequently checking the cyclic voltammetric response for the TCNQ0/•-(MeCN) process. Voltammetric experiments were undertaken at 20 ( 2 °C, unless stated otherwise, and were commenced after degassing the acetonitrile (0.1 M Bu4NPF6) electrolyte solutions with solvent-saturated nitrogen for at least 10 min. Other Instrumentation. In situ optical video images of the electrocrystallization of CuTCNQ on ITO surfaces were obtained with an Olympus SZ6045TR-F Zoom stereomicroscope coupled to a DP-11 digital camera. The microscope was positioned by the side of an upright electrochemical cell so that crystal growth on vertically oriented 0.25 cm2 ITO working electrodes (larger-area electrodes (24) Zhang, J.; Guo, S.-X.; Bond, A. M.; Marken, F. Anal. Chem. 2004, 76, 3619-3629.

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were used than for voltammetric studies) could be monitored. Ex situ optical images were captured with an Olympus BX-51M optical microscope (10, 20 and 50× magnification) and a DM-12 digital camera. Ex situ scanning electron microscopy (SEM) measurements employed a Philips XL30 field-emission gun scanning electron microscope with an Oxford Link energy-dispersive X-ray (EDAX) system. Solution remaining on the electrode after removal from the electrochemical cell was carefully removed by use of tissue paper (Kimwipe). Electrodes were also sometimes gently bathed in acetonitrile and then in distilled water in order to reduce the amount of Bu4NPF6 present on the electrode surface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) data were obtained ex situ, with and without the removal of electrolyte, with an inverted Praying Mantis (Harrick Scientific) diffuse reflectance optical accessory. IR spectra from individual islands of crystals were obtained using a Bruker Equinox 55 spectrometer and an IR Scope II infrared microscope. X-ray powder diffraction (XRD) patterns of electrocrystallized CuTCNQ on ITO were obtained with an Inel CPS120 position-sensitive detector and a Cobalt LFF tube source (KR radiation) operated at 35 kV, 35 mA. X-rays were aligned parallel to the CuTCNQ sample (solvent and electrolyte removed) using a multilayer mirror. The X-ray diffraction data were calibrated with Y2O3 powder.

Results and Discussion Voltammetry of TCNQ and Cu(MeCN)4+ in Acetonitrile at ITO and BDD Electrodes. Cyclic voltammograms obtained for 10 mM TCNQ(MeCN) dissolved in acetonitrile (0.1 M Bu4NPF6) exhibit two well-separated, chemically reversible, oneelectron diffusion-controlled reduction processes (eqs 3 and 4) at ITO and BDD electrodes, as is the case at metal electrodes. Figures 1a and b contain voltammograms for the TCNQ0/•process, which are relevant to the electrocrystallization of CuTCNQ. Peak-to-peak separations (∆Ep) of 136 mV (ITO) and 164 mV (BDD) obtained under the conditions of Figure 1 are larger than the theoretically expected value of about 56 mV at 20 °C for a reversible process. This is predominantly attributed to the presence of a significant level of uncompensated IRu drop (I ) current, Ru ) uncompensated resistance) that arises from the combination of the high 10 mM concentration and the largearea ITO and BDD electrodes. The midpoint potentials (Em), calculated as (Epox + Epred)/2, of 200 mV (ITO) and 218 mV (BDD) are slightly more positive than the formal reversible potentials (E0′) of about 180 mV versus Ag/AgCl,4 again attributed mainly to the IRu drop.

TCNQ(MeCN) + e- h TCNQ•-(MeCN)

(3)

TCNQ•-(MeCN) + e- h TCNQ2-(MeCN)

(4)

At potentials where TCNQ is reduced by one electron, a uniform film of green TCNQ•- could be seen to diffuse away from the ITO electrode. Oxidation back to TCNQ led to the observation of the green layer being drawn back toward the electrode, as expected for a diffusion-controlled process. Cyclic voltammograms of Cu+(MeCN) in acetonitrile (0.1 M Bu4NPF6) at ITO and BDD electrodes (Figures 1c and 1d) also exhibit two one-electron processes, but in this case, one involves the reduction (eq 5) and the other involves the oxidation (eq 6) of the univalent copper ion.

Cu+(MeCN) + e- h Cu0(metal)

(5)

Cu+(MeCN) h Cu2+(MeCN) + e-

(6)

Figure 2. Cyclic voltammograms obtained at an ITO electrode for the reduction of TCNQ (9.09 mM) in the presence of an equal concentration of Cu+(MeCN). Five cycles over the potential range of 0.7 to -0.1 V vs Ag/AgCl (TCNQ0/•- region) at scan rates of (a) 50 and (b) 20 mV s-1.

At an ITO electrode, for example (Figure 1c), the reduction of 100 mM Cu+(MeCN) to deposited copper metal commences at potentials more negative than about -500 mV versus Ag/AgCl. After reversing the scan direction, Cu0(Metal) can be stripped from the electrode surface at approximately 0 V versus Ag/AgCl. Significant IRu distortion is again evident at these high Cu+(MeCN) concentrations needed to ensure that the solubility of CuTCNQ is exceeded in MeCN. The oxidation of Cu+(MeCN) to Cu2+(MeCN) is not highly favorable in acetonitrile and is not detected voltammetrically until potentials more positive than 1 V versus Ag/AgCl are reached. Furthermore, at such potentials ITO may not be operating in the metallic regime. If this is the case, then the inherent semiconducting properties can influence the Cu+/2+(MeCN) oxidation process. However, importantly, the ideal voltammetric data found for the TCNQ0/•- process reveal that the ITO and BDD electrodes are behaving as metal electrodes in the potential range of interest where CuTCNQ electrocrystallization occurs. That is, the reduction of TCNQ to TCNQ•is facile, and the electrocrystallization of CuTCNQ can be studied within the Cu+(MeCN) reduction and oxidation potential window. Cyclic Voltammetry in Acetonitrile for the Reduction of TCNQ in the Presence of Cu(MeCN)4+. The reduction of 9.09 mM TCNQ to TCNQ•- in the presence of an equal concentration of Cu+(MeCN) produces conditions close to the electrode surface where the CuTCNQ solubility product of 4.9 × 10-7 M2 is substantially exceeded.4 Consequently, electrocrystallization of CuTCNQ can occur under these conditions.4 Cyclic voltammograms for the TCNQ0/•- process at ITO (Figure 2) and BDD (Figure 3) are significantly modified by the presence of high concentrations of Cu+(MeCN), but only under slow scan rate conditions. Then, on switching the scan direction

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Figure 3. Cyclic voltammograms obtained at a BDD electrode for the reduction of TCNQ (9.09 mM) in the presence of an equal concentration of Cu+(MeCN). Five cycles over the potential range of 0.75 to 0.0 V vs Ag/AgCl (TCNQ0/•- region) at scan rates of (a) 100 and (b) 20 mV s-1.

at potentials slightly more negative than the Em value, the diffusion-controlled TCNQ•- f TCNQ + e- oxidation component of the TCNQ0/•- process is no longer detected when the scan rates employed are less than 50 mV s-1 at ITO and 100 mV s-1 at BDD electrodes. This removal of the oxidation process is expected if CuTCNQ has time to electrocrystallize on the voltammetric time scale. A significant difference with data obtained at ITO relative to that at metallic or BDD electrodes is that the removal or stripping of solid is not as readily achieved at the ITO electrode. The oxidative removal of phase I CuTCNQ occurs via a well-defined process having a peak potential in the range of 0.4 to 0.5 V versus Ag/AgCl on metallic4 and BDD surfaces (Figure 3). However, the peak height for this process decreases upon repetitive cycling of the potential, and not all of the CuTCNQ is removed from the surface of the electrode in the stripping process as also observed on metallic electrodes.4 At an ITO electrode, a broad stripping peak at about 0.5 V versus Ag/AgCl is detected in the first cycle (Figure 2). On subsequent cycles of the potential, the magnitude of the TCNQ0/•- reductive current increases, suggesting an increase in the electrode area, and the stripping peak broadens. Additional processes also detected at more negative potentials under these conditions are not discussed in this paper. A visual inspection of the ITO electrode revealed the presence of a blue precipitate on the electrode surface, indicative of CuTCNQ retention, even at potentials where stripping occurs from the electrode surface (which is difficult to detect visually on black BDD surfaces). Superior adhesion of CuTCNQ to ITO is therefore indicated via voltammetry and is also confirmed by optical imaging experiments (see below).

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Figure 4. Ex situ SEM images obtained after electrocrystallization of CuTCNQ from an equimolar 9.09 mM Cu+(MeCN)/TCNQ acetonitrile (0.1 M Bu4NPF6) solution onto an ITO electrode held at a potential of -0.1 V vs Ag/AgCl for 30 s.

The extent of modification of the TCNQ0/•- voltammetric process and, by inference, the quantity of electrocrystallized CuTCNQ adhered to the electrode surface depends on the scan rate (less CuTCNQ at faster scan rates) and TCNQ and Cu+ concentrations (less when either concentration is lowered). IR and XRD Characterization of Electrocrystallized CuTCNQ. IR spectra obtained after electrocrystallization when the potential is stepped from 0.6 to -0.1 V and then held at -0.1 V versus Ag/AgCl or the potential is cycled over the range of 0.6 to -0.1 V versus Ag/AgCl (sweep rate 50 to 10 mV s-1) at variable Cu/TCNQ ratios (no removal of Bu4NPF6 electrolyte) exhibited bands associated with CuTCNQ phase I formation (2204, 2172, 1909, and 825 cm-1)1 but provided no evidence for the presence of either phase II formation (2214, 2172, 2141, 1940, 1896, and 825 cm-1)1 or residual TCNQ (2226, 2173, 1996, 1861, and 1807 cm-1)1. Powder XRD data obtained on the electrocrystallized solid obtained after potential step experiments (-0.1 V for 30 s) at ITO electrodes (Figures S1 and S2) and on reference samples of TCNQ and CuTCNQ phase I and II (Figure S3) also confirm that extensive formation of CuTCNQ phase I and not phase II takes place on ITO. Probing the Nature of Electrocrystallized CuTCNQ by Microscopy. Ex situ SEM images of electrocrystallized solid adhered to an ITO electrode (Figure 4) after washing to remove electrolyte show the extensive presence of needle-shaped crystals consistent with CuTCNQ phase I morphology, as found at metallic electrodes.4 The large branched crystals (Figure 4 b) appear to be composed of many small needle-shaped crystals as opposed to one large crystal. Ex situ optical images in Figure 5 also

Electrocrystallization of CuTCNQ

Figure 5. Ex situ optical microscopy images of CuTCNQ electrocrystallized from an equimolar 9.09 mM Cu+(MeCN)/TCNQ acetonitrile (0.1 M Bu4NPF6) solution onto (a) a fresh ITO electrode and (b) a deliberately scratched ITO electrode held at a potential of -0.1 V vs Ag/AgCl for 30 s.

clearly show that crystal growth is not uniform over all parts of the surface. It is noteworthy that preferential crystal growth occurred at the edges of the electrode. Crystal growth also could be deliberately induced by deeply scratching the surface of ITO electrodes (Figure 5b). Such a modification of the ITO surface may provide highly energetic defect sites that are preferential for CuTCNQ nucleation. However, because of the discontinuous nature of the ITO layer after scratching and indeed at the ITO edge-solution interface, high current densities generated at the corresponding microdomains also may contribute to the extent of electrocrystallization being enhanced at these sites. SEM inspection of the surface requires the removal of solvent (electrolyte) from the surface, a process that could contribute to the nature of the crystals detected by this ex situ technique. Thus, in situ optical video measurements (Figures 6 and S4) were also undertaken to monitor the course of the electrocrystallization process. This required the use of vertically rather than horizontally oriented larger-area ITO electrodes (ca. 0.25 cm2) with the glass/ ITO edges exposed to the electrolyte solution. Video images obtained as a function of time are shown in Figure 6. In Figure 6a, the ITO electrode is at open circuit potential, and the characteristic yellow color of TCNQ dissolved in MeCN can be observed. When a potential of -0.1 V versus Ag/AgCl is applied (Figure 6b-d), the green color of TCNQ•- is seen at the electrode surface. After 30 s (Figure 6b), crystals of CuTCNQ can be readily observed that grow rapidly in size over a 90 s period (Figure 6). It can be seen quite clearly that only a limited number of nucleation and growth sites are created over the course of the experiment and that continued growth into large crystals of

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CuTCNQ at these sites is favored over the generation of new nucleation sites. A progressive, rather than instantaneous, nucleation-growth mechanism would appear to be operative. Clearly, only crystals in the micrometer range can be detected with this optical video technique, and nucleation sites at areas of the electrode that appear to be bare with this low-resolution technique may be occurring. However, again it must be emphasized that much higher resolution SEM images taken ex situ, but under otherwise identical conditions, also imply that CuTCNQ electrocrystallization is favored at discrete sites. After electrodeposition times greater than 2 min, some crystals grow so large that they become dislodged from the ITO electrode. Interestingly, after the detachment of a cluster of CuTCNQ crystals from the ITO surface, the growth of new crystals in the region of the former nucleation site was not detected. This may be due to either a chemical or electronic modification of that site by CuTCNQ crystal formation. An analogous process on BDD may account for the gradual decrease in the CuTCNQ oxidation process observed at this electrode surface (Figure 3). When CuTCNQ is removed from specific sites on the surface during the course of the stripping oxidation process, this may eliminate the possibility of these sites participating in subsequent CuTCNQ formation when further cycling of the potential is undertaken. Further evidence for the nucleation and crystal growth process at preferred sites is provided in Figure 6, where the preferential growth of CuTCNQ is again seen to occur at a scratch in the ITO surface (cf. Figures 6a and d) and at the edges of ITO electrodes (Figure S4). These video images confirm that the continuous growth of arrays of needle-shaped crystals occurs at localized regions of the electrode by a nucleation-rapid growth mechanism. That is, nucleation is facilitated at preferred sites, and the growth of needle-shaped crystals in the bulk solution occurs rapidly at regions of the surface where CuTCNQ nuclei have been previously formed. A compacted pellet of CuTCNQ (phase I) has a conductivity1 of 0.025 S cm-1, and it is conceivable that the conductivity of single crystals is even greater. Consequently, the growth of semiconducting CuTCNQ on the ITO surface effectively produces an electrode of increased area that facilitates the formation of more TCNQ•- and hence CuTCNQ formation. Thus, as evident from the micrographs in Figure 4, the rapid growth of large crystal assemblies is achieved via the formation of branched and agglomerated needles of the CuTCNQ (phase I) solid. Optical images of electrocrystallized CuTCNQ adhered to a BDD electrode after the removal of the electrode from the electrochemical cell and washing to remove the electrolyte are shown in Figure 7 and at higher magnification in Figure S5. The dark and light regions corresponding to the higher and lower conducting areas of the electrode, respectively, can be clearly seen. The preferential growth of CuTCNQ is quite evident in the more highly conducting regions. However, it should be noted that CuTCNQ does not electrocrystallize at all of the more highly conducting sites and that again there is preferential growth of CuTCNQ crystals at certain sites within these conducting domains, as seen for ITO. To elucidate whether this type of phenomenon also was present in the case of the electrodeposition of a metal, an ex situ optical image (Figure 7b) was obtained after an experiment that involved the electrodeposition of copper (-0.1 V versus Ag/AgCl for 5 s in 0.1 M CuSO4 solution). The more highly conducting regions again are favored as nucleation-growth sites for the Cu2+(aq) + 2e- f Cu(metal) process. Recent studies on describing the heterogeneous nature of the conductivity of BDD electrodes has been reported on the basis of conducting AFM measurements. The results imply that

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Figure 6. In situ video images of CuTCNQ electrocrystallized from an equimolar 9.09 mM Cu+(MeCN)/TCNQ acetonitrile (0.1 M Bu4NPF6) solution onto an ITO electrode held at a potential of -0.1 V vs Ag/AgCl. Images were taken at (a) 0, (b) 30, (c) 60, and (d) 90 s. Small circles are drawn onto the images to aid the identification of crystal/cluster growth.

individual grains in the size range of 5-20 µm are characterized by conductivities that are higher than for other regions of the electrode surface.25,26 It is noteworthy in our observations that copper deposition seems to occur at almost all of the borondopant-rich sites, unlike the case for CuTCNQ where many such sites do not give rise to the electrocrystallization of CuTCNQ. For the ITO electrode under the same experimental conditions, a uniform coverage of well-spaced copper deposits is observed (Figure S6), again in contrast to the CuTCNQ case where a far more limited number of sites facilitate CuTCNQ formation. The BDD electrode therefore seems to behave as a random array of microelectrodes of various sizes and geometries within which preferred sites are present that facilitate CuTCNQ nucleation and growth.

Conclusions The reduction of TCNQ in acetonitrile at ITO and BDD electrode surfaces in the presence of Cu+(MeCN) facilitates the electrocrystallization of sparingly soluble semiconducting phase I CuTCNQ. This process is detected at potentials more negative than for the commencement of the reduction of TCNQ to TCNQ•-. The nucleation of CuTCNQ is followed by a rapid growth stage that generates aligned and hyperbranched needles that are strongly adhered, particularly to the ITO electrode surface. Stripping of CuTCNQ from the BDD electrode surface occurs at potentials similar to that formed on metallic surfaces but over a much broader potential range from the ITO electrode surface. CuTCNQ adheres and grows so large at an ITO surface even when potential cycling is undertaken that they eventually fall off vertically oriented electrodes during the course of electrocrystallization. This inability to rapidly strip the newly formed CuTCNQ from ITO may be related to an energy barrier arising from the interaction (25) Wilson, N. R.; Clewes, S. L.; Newton, M. E.; Unwin, P. R.; Macpherson, J. V. J. Phys. Chem. B 2006, 110, 5639-5646. (26) Colley, A. L.; Williams, C. G.; Johansson, U. D.; Newton, M. E.; Unwin, P. R.; Wilson, N. R.; Macpherson, J. V. Anal. Chem. 2006, 78, 2539-2548.

Figure 7. Ex situ optical microscopy images of (a) CuTCNQ electrocrystallized from an equimolar Cu+(MeCN)/TCNQ acetonitrile (0.1 M Bu4NPF6) solution onto a BDD electrode held at a potential of 0 V vs Ag/AgCl for 2 s and (b) Cu metal electrocrystallized from an aqueous 0.1 M CuSO4 solution onto a BDD electrode held at a potential of -0.1 V vs Ag/AgCl for 5 s.

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between CuTCNQ and ITO acting as a semiconductor at positivepotentials. Deliberate scratching of the electrode surface and exposure of the edge of the glass/ITO surface to solution enhances the abundance of prime nucleation sites available for CuTCNQ electrocrystallization and also allows higher current densities to be obtained at these sites. The growth of CuTCNQ is also highly favored at the dopant-rich and hence more highly conducting domains of BDD electrodes, but within which there are also preferential growth regions, as seen for ITO electrodes. Thus, the one-electron reduction of TCNQ(MeCN) at ITO or BDD electrodes in the presence of Cu+(MeCN) may be summarized by the following reaction scheme over the potential range of 0.7 to -0.1 V versus Ag/AgCl, which is also the case at metal surfaces:4 ITO,BDD

TCNQ(MeCN) + e- y\z TCNQ•-(MeCN) TCNQ

•(MeCN)

+ Cu

(7)

preferred site

+ (MeCN)

98 CuTCNQ(nuc) (8)

Rapid crystal growth is then facilitated by the process

where CuTCNQ(nuc) represents the nucleation of CuTCNQ at a preferred site. The difference with ITO electrodes is that the very strong adherence of solid facilitates the growth of very large crystal assemblies of CuTCNQ. Thus, the in situ formation of branched needle-shaped crystals may be monitored at ITO electrodes by video imaging. Acknowledgment. Technical support from Ian Madsen and Nicki Scarlett in obtaining X-ray diffraction data is gratefully acknowledged, as is financial support from the CSIRO division of Manufacturing and Infrastructure Technology, the Australian Research Council and the American Electroplaters and Surface Finishers Society. Supporting Information Available: Diffraction patterns obtained after electrocrystallization experiments on ITO. XRD patterns for TCNQ and chemically synthesized CuTCNQ phases I and II. GIF movie of the electrocrystallization process on an optically transparent ITO electrode. Additional optical microscopy images of electrocrystallized CuTCNQ phase I and Cu on ITO and BDD electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

CuTCNQ(nuc)

TCNQ(MeCN) + Cu+(MeCN) + e- y\z CuTCNQ(s) (9)

LA060408V