Controllable Synthesis and Fabrication of Semiconducting Nanorod

Apr 8, 2008 - electrodes coupled with ingress of Fe2+. (aq) ions, from bulk solution, into the reduced TCNQ crystal lattice via a nucleation-growth pr...
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J. Phys. Chem. C 2008, 112, 6700-6709

Controllable Synthesis and Fabrication of Semiconducting Nanorod/Nanowire Bundles of Fe[TCNQ]2(H2O)2 via Electrochemically Induced Solid-Solid Phase Transformation of TCNQ Microcrystals Ayman Nafady,†,§ Alan M. Bond,*,† and Alexander Bilyk‡ School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, and Materials Science and Engineering, P.O. Box 56 (Graham Road), Highett, Victoria 3190, Australia ReceiVed: NoVember 9, 2007; In Final Form: February 13, 2008

A generic electrochemical approach has been employed to synthesize, fabricate, and characterize 2-D/3-D nanorod/nanowire bundles of semiconducting/magnetic Fe[TCNQ]2(H2O)2 material. The redox-based TCNQ/ Fe[TCNQ]2(H2O)2 solid-solid phase transformation utilizes the one-electron reduction of TCNQ-modified electrodes coupled with ingress of Fe2+(aq) ions, from bulk solution, into the reduced TCNQ crystal lattice via a nucleation-growth process. The reverse oxidation process, involving Fe[TCNQ]2(H2O)2/TCNQ transformation, also proceeds via an analogous nucleation-growth mechanism. The overall chemically reversible solidsolid transformation therefore can be represented by the reaction TCNQ0(S) + 2e- + Fe2+(aq) + 2H2O h {Fe[TCNQ]2(H2O)2}(S). SEM monitoring of the transformation process revealed substantial differences in both the morphology and crystal size of electrochemically produced Fe[TCNQ]2(H2O)2 material and the parent TCNQ crystals. Importantly, this electrochemical approach provides facile access to fabricate and manipulate the morphology of Fe[TCNQ]2(H2O)2 on conducting (Au, Pt, glassy carbon) and semiconducting (indium tin oxide) surfaces. Other aspects of the solid-solid electrochemical conversion have been probed by voltammetric, spectroscopic, and other surface science techniques.

1. Introduction Advances in nanotechnology and molecule-based solid-state chemistry have led to a revived interest in metal-TCNQ materials (TCNQ ) 7,7,8,8-tetracyanoquinodimethane).1-15 In particular, the electrical, optical, and switching properties of CuTCNQ and AgTCNQ have been widely studied,16-22 owing to their potential use in erasable photochromic laser disks and memory storage,23-25 organic field-effect transistors,26-28 biosensors,29-31 and electrochromic devices.32-34 In this respect, a great deal of investigations have been undertaken to tune the intrinsic physical and chemical properties of these MTCNQ materials via controlling of their crystal sizes and morphologies, thereby enhancing their desired reactivity or stability. Thus, different synthetic strategies have been employed for the preparation and fabrication of morphology-tunable MTCNQ micro/nanostructures.35-39 Among these methods are vapor deposition of TCNQ on metal surfaces, reaction of TCNQ- with metal salts, spontaneous electrolysis, and other electrochemical and photochemical approaches.35-39,40-44 In contrast to the MTCNQ case, the semiconducting binary analogue, M[TCNQ]2-based materials (M ) first-row transition metal) only recently have received significant attention.45-47 This has been motivated by the recent discovery by Dunbar and Miller and their co-workers of a diverese range of magnetic properties and the demonstration of relevance to the area of molecular magnets.48-52 These authors have developed many * To whom correspondence should be addressed: Fax 613-9905-4597; e-mail [email protected]. † Monash University. ‡ CSIRO Manufacturing & Materials Technology. § Permanent address: Chemistry Department, Faculty of Science, Sohag University, Sohag, Egypt 82524.

synthetic approaches for the synthesis of different phases of these materials that include the hydrated M[TCNQ]2(H2O)2,50 which is of particular interest to this paper, the alcoholic M[TCNQ]2(MeOH)2,50 and the solvent-free M[TCNQ]2 (M ) Mn, Fe, Co, Ni).51,52 A major synthetic problem that has appeared from chemical synthesis routes is the low solubility of these polymeric materials in most solvents, which makes the recrystallization and purification processes a notoriously difficult task and hence has impeded investigations directed toward the synthesis and fabrication of morphology-tunable M[TCNQ]2based materials. Our recent work on probing the redox-based solid-solid transformation of binary M-TCNQ systems, namely TCNQ/ M[TCNQ]2(H2O)2 (M ) Co, Ni),53,54 has suggested that a facile electrochemical method may be available for other transition metals. Furthermore, this aqueous approach has a significant advantage over chemical methods in controlling the crystal size and morphology of these materials. Importantly, the reaction pathway can be monitored in situ and ex situ by voltammetric, spectroscopic, and surface science tools to provide a unique opportunity to assess the structural and morphological changes that accompany these solid-solid interconversions along with an ability to probe the mechanistic aspects of the underlying redox chemistry.55 In this paper, and on the basis of the knowledge gained from the isostructural M[TCNQ]2(H2O)2 (M ) Co, Ni) analogues,53-55 we report new insights into the formation and fabrication of nanowire/nanorod networks of the semiconducting and magnetic Fe[TCNQ]2(H2O)2 onto conducting (glassy carbon (GC), Pt, Au) and semiconducting indium tin oxide (ITO) surfaces along with electrochemical (voltammetry, chronoamperometry, bulk electrolysis) spectroscopic (IR, Raman) and surface science (XRD,

10.1021/jp7107279 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008

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SEM, EDAX) characterization. Depending on the method of electrode modification and the electrochemical technique employed, the morphology of the generated Fe[TCNQ]2(H2O)2 can be easily manipulated to afford large-scale, differently oriented/ aligned nanostructures including 1-D/2-D nanowires and 3-D nanorod bundles. The present study therefore highlights the importance of our solid-state electrochemical approach in understanding and tuning the intrinsic properties of the moleculebased M[TCNQ]2 family. 2. Experimental Section 2.1. Materials and Synthesis of Fe[TCNQ]2(H2O)2. Analytical grade hydrated FeCl2‚4H2O (99.99%, Aldrich), 7,7,8,8tetracyanoquinodimethane (TCNQ, 99%, Aldrich), and acetonitrile (HPLC grade, Omnisolv) were used as received from commercial sources. Fe[TCNQ]2(H2O)2 was synthesized according to a procedure described by Dunbar et al.50 Thus, an aqueous solution (5.0 mL) of LiTCNQ (0.053 g, 0.25 mmol) was added to an aqueous solution (5.0 mL) of FeCl2·4H2O (0.025 g, 0.125 mmol) with stirring for 30 min at room temperature. The resulting blue precipitate was filtered off and washed with water, ethanol, and diethyl ether followed by drying under vacuum. The solid material was then established to be the same material reported previously by comparison of IR and Raman spectra. These spectroscopic data are presented in the Results and Discussion section (vide infra). 2.2. Procedures. A fresh solution of FeCl2(aq) electrolyte was prepared daily and kept under N2 to minimize air oxidation to ferric (Fe3+(aq)) ions. The working electrodes were modified with solid TCNQ by using either the drop-casting or mechanical attachment methods. Details concerning the immobilization of TCNQ via these two approaches are fully described elsewhere.53 SEM characterization of TCNQ-modified ITO surfaces revealed that the morphology and crystal size of the immobilized TCNQ are highly dependent on the method of electrode modification. The drop-cast method (Figure 1a) normally produces arrays of regularly spaced, rhombus-shaped, large (≈40 × 40 µm) TCNQ microcrystals, upon evaporation of the droplets of acetonitrile containing the dissolved (10 mM) TCNQ. On the other hand, rubbing the electrode surface over a piece of weighing paper containing a small amount of TCNQ microcrystals, as in the mechanical attachment method, produces a compact layer of smaller sized TCNQ microparticles on the electrode surface, as illustrated in Figure 1b. 2.3. Electrochemistry. All electrochemical studies were conducted in aqueous media (water purified by a Millipore System to give a resistivity of 18.2 MΩ cm) with an Autolab PGSTAT100 (ECO-Chemie) workstation and a standard threeelectrode cell configuration. The working electrodes used were glassy carbon (GC) disk (3 mm diameter, Bioanalytical Systems) platinum (Pt) and gold (Au) (1.6 mm diameter, Bioanalytical Systems) as well as semiconducting indium tin oxide (ITO)coated glass (0.05-0.1 cm2 area) having a 10 Ω/sq sheet resistance (as quoted by the manufacturer Prazisions Glas and Optik GmbH). These electrodes were routinely polished according to our standard procedures described elsewhere.53 The auxiliary electrode was made from platinum mesh, and the reference electrode was an aqueous Ag/AgCl (3.0 M KCl; Bioanalytical Systems). All potentials reported in this study are measured against this electrode at ambient temperature of 20 ( 2 °C. 2.4. Other Instrumentation. Infrared spectroscopy (IR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDAX) experiments were

Figure 1. SEM images of solid TCNQ immobilized onto ITO surfaces via (a) drop-casting and (b) mechanical attachment methods before being introduced into electrochemical cell containing FeCl2(aq) electrolyte solution.

carried out as previously described.53 Raman spectra were acquired with a Renishaw RM 2000 Raman spectrograph and microscope using a laser strength of 18 mW at a wavelength 780 nm. 3. Results and Discussion 3.1. Electrochemical Monitoring of the TCNQ/Fe[TCNQ]2(H2O)2 Solid-Solid Transformation. 3.1.1. Voltammetry of Solid TCNQ in the Presence of Fe2+(aq) Electrolyte. Figure 2 contains cyclic voltammograms obtained over the potential range of 0.4 to -0.1 V vs Ag/AgCl at a scan rate of 20 mV s-1 for bare or TCNQ-modified GC electrodes (drop-casting or mechanical attachment methods) in contact with aqueous 0.1 M FeCl2 electrolyte. As can be seen in Figure 2a, Fe2+(aq) ions are electrochemically inactive over the designated potential range when a bare GC electrode is used. Thus, even though Fe(II) is readily oxidized to Fe(III) in aqueous solutions56 and can be reduced to metallic iron, TCNQ redox chemistry involving the one-electron reduction into the corresponding TCNQ- radical anion does not overlap with these processes. As a consequence, GC electrodes modified with solid TCNQ via drop-casting (Figure 2b) or mechanical attachment (Figure 2c) gave rise to cyclic voltammograms exhibiting sharp, symmetrical, and wellseparated reduction and oxidation waves that are similar to those observed in the case of TCNQ/M[TCNQ]2(H2O)2 (M ) Co2+, Ni2+) interconversions.53,54 Presumably, by analogy when FeCl2(aq) is present as the electrolyte, Fe2+(aq) ions are involved in the charge neutralization process to render the TCNQ/Fe[TCNQ]2(H2O)2 solid-solid transformation.

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Figure 4. Cyclic voltammograms (fifth cycle shown) obtained with a TCNQ-modified GC electrode (mechanical attachment method) at scan rate of 20 mV s-1 when placed in contact with aqueous solution of FeCl2 electrolyte at designated concentrations. Figure 2. Cyclic voltammograms (fifth cycle shown) in the presence of aqueous 0.1 M FeCl2 electrolyte obtained at scan rate of 20 mV s-1 over the potential range from 0.4 to -0.1 V at (a) bare GC electrode, (b) a drop-cast TCNQ-modified GC, and (c) a mechanically attached TCNQ-modified GC electrode.

respectively. This attenuation in charge is most likely associated with slow dissolution of solid TCNQ61 or Fe[TCNQ]2(H2O)2 material in aqueous media as found with the congener cobalt and nickel systems.53,54 The maintenance of shape and peak position with prolonged potential-cycling experiments indicates that only one phase of Fe[TCNQ]2-based material, namely hydrated Fe[TCNQ]2(H2O)2, is produced under conditions of cyclic voltammetry. The simplest reaction scheme describing the overall redox chemistry of TCNQ/Fe[TCNQ]2(H2O)2 solid-solid transformation is given in eqs 1 and 2. Red

2TCNQ(s,GC) + Fe2+(aq) + 2e- + 2H2O 98 {Fe[TCNQ]2(H2O)2}(s,GC) (1) Ox

{Fe[TCNQ]2(H2O)2}(s,GC) 98 Fe2+ (aq) + 2TCNQ(s,GC) + Figure 3. Cyclic voltammograms showing the 10-100th cycle of the potential obtained at a scan rate of 20 mV s-1 with a TCNQ-modified GC electrode (mechanical attachment method) immersed in aqueous 0.1 M FeCl2 electrolyte solution.

As seen in Figure 2c, under mechanical attachment conditions a sharp reduction process (peak width at half-height, Wred 1/2 ) 32 mV) with a peak potential (Ered p ) of 16 mV is detected on the forward (negative) potential scan direction (from 0.35 to -0.065 V). On the reverse scan, the oxidation process counterpart οx appears (Eοx p ) 266 mV, W1/2 ) 25 mV). The remarkably large peak separation “inert zone” between the reduction and oxidation red components (∆Ep ) Eοx p - Ep ) 250 mV) is consistent with an electrochemically irreversible solid-solid conversion of TCNQ into Fe[TCNQ]2(H2O)2 taking place by a nucleationgrowth mechanism.53,54,57-60 Interestingly, this ∆Ep value for the iron system is, by far, the largest in TCNQ/ M[TCNQ]2(H2O)2, (M ) Fe, Co, Ni) series, since ∆Ep values of 226 and 153 mV were obtained when M ) Co and Ni, respectively.53,54 Clearly, the identity of the transition metal cation plays a significant role in the kinetics of these redox-based transformations. In contrast to the pronounced electrochemical irreversibility, the magnitude of the charge (Q) associated with the reduction and oxidation components in Figure 2c (Qred ) -1.93 × 10-4 C, Qox ) 1.81 × 10-4 C) implies that the TCNQ/Fe[TCNQ]2(H2O)2 conversion process has a high level of chemical reversibility.53 Upon extensive cycling of the potential (Figure 3) the peak heights progressively decrease; after 100 cycles Qred and Qox diminished to 9.94 × 10-5 and 8.55 × 10-5 C,

2H2O + 2e- (2) The uptake of Fe2+(aq) ions from bulk aqueous solution (eq 1) and its release (eq 2) from the crystal lattice are anticipated to occur more rapidly when the process involves mechanically attached TCNQ nanoparticles than is the case with much larger TCNQ microcrystals produced by the drop-casting method. Thus, the voltammetry associated with the TCNQ/Fe[TCNQ]2(H2O)2 interconversion depends markedly on the method of TCNQ electrode modification (compare parts b and c of Figure 2). Further analysis of the voltammetric behavior reveals that the TCNQ/Fe[TCNQ]2(H2O)2 transformation process is essentially independent of electrode material (Pt, Au, GC, and ITO) and Fe(II) salt counteranions (e.g., ClO4-, Cl-, or SO42-) but is significantly influenced by changes in Fe2+(aq) concentration and scan rate as also found for other TCNQ/Mn+[TCNQ]n systems (M ) Co2+, Ni2+, Cu+, or group I cations).42,53,54,60 The dependence of the midpoint potential (Em ) (Ered p + οx οx 2+ Ep )/2) and peak separation (∆Ep ) Ered E ) on Fe (aq) p p electrolyte concentration (1.0 mM to 1.0 M) was investigated with a TCNQ-modified GC electrode (mechanical attachment) and a scan rate of 20 mV s-1. As illustrated in Figure 4 and shown by analysis of data in Table 1, the highest peak currents and most persistent voltammograms were obtained with Fe2+(aq) concentrations of 0.05 and 0.1 M. At low concentrations (1.05.0 mM) significant broadening of the oxidation and reduction peaks was detected, along with a marked decrease in peak current heights. This is most likely due to exacerbation of iRu drop effects when low electrolyte concentrations are used.

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TABLE 1: Voltammetric Parametersa Obtained at a Scan Rate of 20 mV s-1 for TCNQ-Modified GC Electrodes (Mechanical Attachment Method) in the Presence of Variable Concentrations of Aqueous FeCl2(aq) Electrolyte Solutionb [Ni2+(aq)] (M)

Ered p (V)

Wred 1/2 (mV)

Eox p (V)

Wox 1/2 (mV)

Em (V)

∆Ep (V)

0.5 0.1 0.05 0.01 0.005

0.055 0.012 -0.005 -0.056 -0.070

24 35 38 70 128

0.295 0.278 0.264 0.272 0.280

22 37 35 68 85

0.175 0.145 0.130 0.108 0.105

0.240 0.266 0.269 0.328 0.350

ox a Em represents the midpoint potential measured as (Ered p + Ep )/2 in volt vs Ag/AgCl (3 M KCl); ∆Ep is the peak potential separation red red ox calculated as (Eox p - Ep ). Ep and Ep are reduction and oxidation red peak potentials, respectively; W1/2 and Wox 1/2 are the peak widths at half-height for the reduction and oxidation components, respectively. b Voltammetric data were collected after five cycles of the potential.

Unlike the cobalt and nickel cases, voltammetric responses obtained at Fe2+(aq) concentrations greater than 0.5 M are complicated by the proximity to the Fe(II)/Fe(III) redox potential (see Figure S1, Supporting Information). For the above reasons, quantitative analysis of the voltammetric data is limited to the 0.005-0.5 M Fe2+(aq) concentration range. Inspection of the voltammetric data presented in Table 1 and Figure S2 reveals that the midpoint potential, Em, shifts from

0.175 to 0.105 V as the concentration of Fe2+(aq) ions decrease from 0.5 to 0.005 M. The direction and magnitude of this potential change (70 mV total shift or 35 mV per decade change in concentration) are close to theoretical value of 30 mV62 predicted by the Nernst equation assuming an overall twoelectron charge-transfer process and unit activity for the solid materials. Since Em values are measured as an average of red (Eοx p + Ep )/2, the influence of ohmic potential drop (iRu) on Em should be minimal as a result of a nulling effect. However, the inert zone (∆Ep), which increases from 0.240 V at 0.5 M to 0.350 V at 0.005 M, is expected to be influenced by iRu drop, particularly at low electrolyte concentrations. 3.1.2. EVidence for Nucleation-Growth Kinetics in the TCNQ/Fe[TCNQ]2(H2O)2 Transformation Process. (a) Cyclic Voltammetry. A diagnostic feature for rate-determining nucleation-growth kinetics in both the reduction and the oxidation processes of the electrochemically induced TCNQ/Fe[TCNQ]2(H2O)2 interconversion is the observation of current loops at the onset of either the reduction (Figure 5a) or oxidation (Figure 5b) process when the scan directions in cyclic voltammograms are reversed in the foot of each process.63 Further support for the presence of a nucleation and growth mechanism is provided by the scan rate (ν) dependence of the cyclic voltammograms (Figure 5c), obtained for the TCNQ/Feοx [TCNQ]2(H2O)2 conversion process. Thus, Ered p and Ep (Table

Figure 5. Cyclic voltammograms obtained with a TCNQ-modified GC electrode (mechanical attachment method) in contact with 0.1 M FeCl2(aq) solution and scan rate of 20 mV s-1 as a function of switching potential: (a) Third cycle shown followed by the fourth cycle when the potential is switched at the foot of the reduction wave. (b) Cyclic voltammograms obtained under same condition as in (a), but on the fourth cycle the potential is switched at the foot of the oxidation wave. (c) Cyclic voltammograms obtained under same conditions as in (a), but at different scan rates.

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Figure 6. Double-potential step chronoamperograms obtained with a TCNQ-modified GC electrode (mechanical attachment method) in the presence of 0.1 M FeCl2(aq): (a) i-t curve obtained when the potential is initially stepped from Ei ) 350 to Ered ) 35 mV for 30 s to induce reduction and then back to Eox ) 260 mV to induce oxidation. (b) i-t transients obtained under same conditions as in (a) but when the potential is stepped from Ei ) 350 mV to the designated (Ered) potentials to induce reduction. (c) i-t curves obtained when the potential is stepped from Ei ) 350 mV to Ered 35 mV to induce reduction and then back to designated (Eox) potentials to induce oxidation. These data were collected after three cycles of the potential over the range from 350 to -50 mV.

TABLE 2: Voltammetric Parameters Obtained as a Function of Scan Rate for a TCNQ-Modified GC Electrode (Mechanical Attachment Method) Immersed in 0.1 M FeCl2(aq) Solutiona scan rate (mV s-1)

Ered p (V)

Wred 1/2 (mV)

Qred (µC)

Epox (V)

Wox 1/2 (V)

Qox (µC)

Em (mV)

∆Ep (mV)

1 2 5 10 20

0.037 0.031 0.021 0.015 0.007

18 19 22 23 28

109 103 95 90 89

0.250 0.250 0.251 0.258 0.263

11 15 17 20 25

123 105 96 89 88

144 141 136 136 135

213 219 230 243 256

a Voltammetric data collected after three redox cycles over the potential range 0.4 to -0.075 V. Symbols in headings are define in Table 1. Qred and Qox are the charges associated with the reduction and oxidation components.

2) are strongly affected by slow scan rates (1.0-20 mV s-1). Furthermore, W1/2 and ∆Ep are significantly increased with increasing scan rate. In contrast, Em changes only slightly as ν increased from 1.0 to 5.0 and then remains almost constant (5.0-20) mV s-1. The dependence of these voltammetric parameters on scan rate is highly indicative of nucleationgrowth kinetics.53,60,63 Theoretical models developed for 1-D/2-D “layer-by-layer” nucleation-growth mechanisms64 predict a linear dependence of log(ip, W1/2, and ∆Ep) on log ν at very low scan rates with slopes of x, 1 - x, and 1 - x, respectively, where x is 0.6 or slightly higher if the redox induced transformation involving 2-D nucleation/growth kinetics. In spite of the linear depend-

TABLE 3: Voltammetric Data Derived from Log-Log Plots of ip, W1/2, and ∆Ep vs ν over the Scan Rate Range of (1-20 mV s-1) process/slope

log ip vs log ν

log W1/2 vs log ν

log ∆Ep vs log ν

Red Ox

0.89 ( 0.02 0.88 ( 0.02

0.14 ( 0.02 0.25 ( 0.03

0.063 ( 0.005

encies found for log ip, log W1/2, and log ∆Ep on log ν (not shown), the slopes are either higher (e.g., log ip vs log ν) or smaller (log W1/2 and log ∆Ep) (see Table 3) than the theoretically predicted values for 2-D phase transitions and also for experimentally reported values for both KTCNQ64 and Ni[TCNQ]2(H2O)254 systems, which provided data expected for

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Figure 7. (a) SEM image of Fe[TCNQ]2(H2O)2 formed by reductive electrolysis for 10 min at -0.1 V of a TCNQ single-crystal immobilized onto ITO electrode surface via the drop-casting method as shown in Figure 1a, when placed in contact with 0.1 M FeCl2(aq) electrolyte. Images (b, c) show top view of the crystal in image (a), but at higher magnifications. Image (d) shows Fe[TCNQ]2(H2O)2 nanorods formed from reduction of smaller sized TCNQ crystals present on a different region of the electrode surface.

“layer-by-layer” 2-D nucleation/growth. This discrepancy implies that the TCNQ/Fe[TCNQ]2(H2O)2 conversion process, though it may initially involve 1-D/2-D nucleation processes, may also occur via 3-D nucleation-growth kinetics. SEM images (vide intra) of this solid-solid phase conversion obtained under similar conditions, are consistent with this hypothesis. (b) Chronoamperometry. Additional definitive evidence for the presence of nucleation-growth kinetics in the TCNQ/Fe[TCNQ]2(H2O)2 transformation process is gained by analysis of double potential step chronoamperometric (DPSCA) data53,54 obtained from a TCNQ-modified GC electrode immersed in 0.1 M aqueous FeCl2 electrolyte solution. Figure 6a shows currenttime (i-t) response obtained by stepping the potential from an initial value (Ei ) 350 mV) to a more negative value (Ered ) 35 mV) for 30 s in order to induce the reduction of TCNQ into TCNQ- and concomitant incorporation of Fe2+(aq) ions to form the Fe[TCNQ]2(H2O)2 complex. At least five potential cycles (0.4 to -0.1 V) at a scan rate of 20 mV s-1 were undertaken prior to performing this potential step experiment. On the reverse oxidation step, the potential is stepped back to Eox ) 260 mV for the same period of time. As can be seen in Figure 6a, welldefined peaks follow the rapid onset and decay of the capacitive charging current. Detection of such current maxima under these and related conditions (Figure 6b,c) is generally accepted as a

proof for the existence of a nucleation-growth mechanism.53,54,60,63 3.2. Spectroscopic Characterization of the Electrochemically Produced Fe[TCNQ]2(H2O)2. 3.2.1. IR Spectroscopy. The solid formed as a result of 4.5 potential cycles over the range of 0.35 to -0.1 V or reductive electrolysis for 10 min at Eapp ) -0.1 V using a TCNQ-modified ITO electrodes in contact with 0.1 M Fe2+(aq) electrolyte gave identical IR spectra to that found with chemically synthesized Fe[TCNQ]2(H2O)250 (Table 4 and Figure S3). In all cases, three intense IR bands of the ν(CtN) stretch are located at 2212, 2195, and 2177 cm-1, consistent with the coordination of a TCNQ- radical anion to the divalent Fe(II) ion.50,53,54,65 In addition, two sharp IR bands at 1505 and 825 cm-1, assigned to ν(CdC) stretching and δ(C-H) bending, respectively, are indicative of the TCNQradical rather than the [TCNQ-TCNQ]2- σ-dimer (δ(C-H) ∼ 806 cm-1).50 Analogously to the cobalt- and nickel-TCNQ complexes,53,54 two broad IR bands are observed at much higher energy, 3448 and 3375 cm-1, together with a weak band at ca. 1642 cm-1, consistent with the presence of coordinated water molecules in the iron complex. 3.2.2. Raman Spectroscopy. Raman spectra of solid TCNQ crystals and the electrochemically produced Fe[TCNQ]2(H2O)2 material generated by reductive electrolysis of a TCNQ-modified

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TABLE 4: Frequencies of Infrared Absorption Bands Obtained for Fe[TCNQ]2(H2O)2 Prepared Chemically and Electrochemically method of preparation a

chemical synthesis bulk electrolysisb cyclic voltammetryc

δ(C-H), ν(CdC), cm-1 cm-1 825 m 825 m 825 m

1505 s 1505 s 1505 s

ν(CtN), cm-1 2212 s, 2195 s, 2177 m 2212 s, 2195 s, 2177 s 2211 s, 2193 s, 2178 s

a Reaction of aqueous solution of FeCl2 with LiTCNQ.50 b Bulk electrolysis at -0.1 V for 10 min of a TCNQ-modified ITO electrode in the presence of 0.1 M FeCl2. c Cyclic voltammetry performed at a scan rate of 20 mV s-1 with 4.5 cycles of the potential over the range 0.35 to -0.1 V using either a TCNQ-modified GC or ITO electrodes.

ITO electrode at Eapp ) -0.1 V for 10 min in the presence of 0.1 M FeCl2(aq) electrolyte solution are presented in Figure S4. These spectra clearly confirm the TCNQ/Fe[TCNQ]2(H2O)2 solid-solid interconversion. Upon reduction of TCNQ and formation of Fe[TCNQ]2(H2O)2, the principal vibration modes at 1207 cm-1 (CdCH bending), 1602 cm-1 (CdC ring stretching), and 2224 cm-1 (C-N stretch) only undergo a slight shift. However, the characteristic C-CN wing stretching band at 1454 cm-1 is red-shifted by 63 cm-1 to 1391 cm-1. This shift is similar to that reported for the Mn+[TCNQ]n (M ) Co2+, Ni2+, Cu+) analogues,55 thereby confirming the formation of the hydrated Fe[TCNQ]2(H2O)2 phase. 3.2.3. Powder X-ray Diffraction of Fe[TCNQ]2(H2O)2. XRD patterns were obtained from parent TCNQ crystals immobilized onto an ITO surface and the electrochemically produced Fe[TCNQ]2(H2O)2 material when the TCNQ-modified ITO electrode (drop-cast method) is placed in contact with 0.1 M FeCl2(aq) and subjected to 15 min of reductive electrolysis at Eapp ) -0.1 V. Although the diffraction peaks of Fe[TCNQ]2(H2O)2 are weak and superimposed onto the more intense ITO and unreacted TCNQ diffraction peaks, characteristic peaks at 13.75ο, 15.36ο, 19.45ο, 21.90ο, 25.25ο, 31.39ο, and 40.34ο were detected and are in good agreement with those reported by Dunbar et al.50 for a Fe[TCNQ]2(H2O)2 sample prepared via chemical methods. 3.2.4. Energy DispersiVe X-ray. EDAX elemental analysis of solid generated by the electrochemical reduction of TCNQmodified ITO electrodes in the presence of Fe2+(aq) ions using either reductive electrolysis or cyclic voltammetry showed the presence of iron, carbon, nitrogen, and oxygen consistent with the formation of [Fe(TCNQ)2(H2O)2]. On the basis of the aforementioned electrochemical, spectroscopic, and surface science data, it is established that oneelectron reduction of solid TCNQ attached to a working electrode such as GC, Pt, Au, or ITO in the presence of Fe2+(aq) electrolyte ions yielded only a single-crystalline Fe[TCNQ]2(H2O)2 phase via a nucleation-growth mechanism. 3.3. SEM Monitoring of the Morphological Changes Accompanying the TCNQ/Fe[TCNQ]2(H2O)2 Transformation. Figure 7 contains SEM images obtained for the electrochemically produced Fe[TCNQ]2(H2O)2 material as a result of reductive electrolysis (Eapp ) -0.1 V for 10 min) of large (≈ 40 × 40 µm2) parent TCNQ microcrystals immobilized onto an ITO surface via the drop-casting method (see Figure 1a) when placed in contact with 0.1 M FeCl2 aqueous solution. As can be seen in Figure 7a (low magnification), upon reductive electrolysis, the TCNQ crystal becomes fully covered with nanosized Fe[TCNQ]2(H2O)2 structures. At higher magnifications (Figure 7b,c) these nanostructures are revealed to consist of networks of densely packed, 2-D/3-D nanowires along with

Figure 8. SEM images at different magnifications of Fe[TCNQ]2(H2O)2 nanorod bundles formed by reductive electrolysis for 10 min at -0.1 V of solid TCNQ adhered to an ITO electrode surface via mechanical attachment method as shown in Figure 1b, when placed in contact with 0.1 M FeCl2 (aq) electrolyte.

an interestingly engineered pattern of vertically aligned nanorod bundles. The dimensions of these nanowire/nanorod structures ranged from 2 to 3 µm in length and ∼50-100 nm in diameter. SEM images (Figure 7d) obtained from regions of the ITO electrode surface where smaller parent TCNQ crystals may initially be present gave rise to Fe[TCNQ]2(H2O)2 nanorods having a preferred orientation in which their tips prominently pointed upward. The finding that the characteristic rhombic shape of parent TCNQ crystals remains intact while being completely covered with much smaller sized needle-shaped Fe[TCNQ]2(H2O)2 architectures demonstrates that the latter is preferentially formed by growth from the surfaces (top, edges, and base) of the large TCNQ crystal into the solution phase rather than through the interior part which may be restricted by iRu drop consideration and the more insulating nature of TCNQ crystal.55 This growth, from an energetic perspective, is mostly likely to occur at the conductor|insulator|electrolyte triple-phase junction,66,67 namely ITO electrode|TCNQ(s)/TCNQ-|Fe2+(aq) electrolyte. This threephase junction line presumably generates the region (bottom part of TCNQ crystal directly attached to the electrode surface) for initial nucleation sites.55 Similar to the other M[TCNQ]2(H2O)2 (M ) Co2+, Ni2+) analogues, this type of “network” nucleation-growth is probably facilitated by the semiconducting nature of the Fe[TCNQ]2(H2O)2 material,45,47 which effectively increases the electrode area as crystal growth extends.

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Figure 9. SEM images obtained by cyclic voltammetry at a scan rate of 20 mV s-1 over the potential range of 0.35 to -0.1 V after completion of 4.5 cycles using ITO electrodes modified with TCNQ via mechanical attachment (a, b) and drop-casting (c, d) when placed in contact with 0.1 M FeCl2 (aq) electrolyte.

SEM monitoring (Figure 8) of the solid-solid conversion at TCNQ-modified ITO surfaces prepared via the mechanical attachment method when also subjected to 10 min of reductive electrolysis at Eapp ) -0.1 V in the presence of 0.1 M FeCl2(aq) revealed that the densely packed “thin layer” of small sized TCNQ particles (Figure 1b) is converted into a compact layer of large-scale, vertically aligned nanorod bundles of Fe[TCNQ]2(H2O)2 material having an average length of about 1-2 µm and a diameter of about 100 nm. If cyclic voltammetry (4.5 cycles over the potential range of 0.35 to -0.1 V) is used instead of constant potential electrolysis, then SEM images obtained via mechanical attachment (Figure 9a,b) or drop-casting (Figure 9c,d) showed the formation of arrays of Fe[TCNQ]2(H2O)2 nanorods. The majority have grown outward when the mechanical attachment method is used. In the case of the drop-casting method, the large parent TCNQ crystals are covered with mostly flat, needle-shaped Fe[TCNQ]2(H2O)2 nanowires as illustrated in Figure 9d. The more intensive generation of Fe[TCNQ]2(H2O)2 crystals with the mechanical attachment method (Figure 9a) compared to the drop-casting method (Figure 9d) correlates with the observation of higher current densities of the cyclic voltammograms. These differences in crystal size, packing density, and growth orientation detected under cyclic voltammetric conditions probably arise from the progressive rather than instantaneous changes in parent TCNQ morphology promoted by repetitive ingress/egress of Fe2+(aq) ions into/from TCNQ-/TCNQ lattice during the chemically reversible TCNQ/Fe[TCNQ]2(H2O)2 transformation process. Ultimately, this multicyclic approach leads to well-defined and more uniform sizes of very small needles of both TCNQ and Fe[TCNQ]2(H2O)2.

The morphological changes associated with the oxidatively induced transformation of Fe[TCNQ]2(H2O)2 back to TCNQ under controlled potential electrolysis conditions and using TCNQ-modified ITO electrodes (mechanical attachment method) were also probed. In these experiments, SEM images of regenerated yellow TCNQ, after back-oxidation (Eapp ) 0.35 V, 5 min) of blue Fe[TCNQ]2(H2O)2 solid formed via reductive electrolysis for 5 min at Eapp ) -0.1 V (morphology as shown in Figure 8b), display the characteristic cubic/rhombus shapes expected for TCNQ crystals.42,53 TCNQ formed in this manner has a significantly different morphology than that of the initially present compact layer of TCNQ microparticles illustrated in Figure 1b. Fragmentation and a marked reduction in size of TCNQ crystals are necessary to accommodate the different preferred morphologies of cubic TCNQ and needle-shaped Fe[TCNQ]2(H2O)2 crystals during the course of exhaustive reduction-oxidation processes. IR spectra of the yellow solid produced by this back-oxidation experiment revealed the presence of the principal TCNQ bands (2228, 1543, and 860 cm-1)65 and indicated full reversion of Fe[TCNQ]2(H2O)2 into TCNQ had occurred under these conditions. These series of SEM observations manifest that crystal size of the electrochemically synthesized Fe[TCNQ]2(H2O)2 material is strongly dependent on the method of electrode modification with solid TCNQ (drop-cast vs mechanical attachment) and on the electrochemical protocol used to induce the chemically reversible TCNQ/Fe[TCNQ]2(H2O)2 solid-solid conversion. Interestingly, Fe[TCNQ]2(H2O)2 exhibits significantly different morphologies and crystals sizes than found with Co[TCNQ]2(H2O)253 (long nanowires 5-8 µm) and Ni[TCNQ]2(H2O)254 (flowerlike architectures) analogues, when prepared under

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Nafady et al. change in TCNQ morphology and crystal size to accommodate the requirements of conversion of rhombus/cubic-shaped TCNQ microcrystals into needle-shaped Fe[TCNQ]2(H2O)2 bundles. 4. Conclusions

Figure 10. SEM mages at low (a) and high (b) magnification showing the morphology of nanocrystals of TCNQ formed after the sequence of reductive electrolysis of a TCNQ-modified ITO electrode (mechanical attachment) at Eapp ) -0.1 V for 5 min when placed in contact with FeCl2(aq) electrolyte to form Fe[TCNQ]2(H2O)2 nanorod bundles as in (Figure 8a) followed by an oxidative back-electrolysis to regenerate TCNQ at Eapp ) 0.35 V for 5 min.

similar electrochemical conditions, which underscores the crucial role played by the transition metal cation in controlling the extent and direction of growth in these redox-based solid-solid transformation processes. 3.4. Mechanistic Aspects of the TCNQ/Fe[TCNQ]2(H2O)2 Transformation. A combination of voltammetric, spectroscopic, and surface science data have established that formation of blue nanowire/nanorod bundles of semiconducting Fe[TCNQ]2(H2O)2 takes place via a nucleation-growth process that is induced by one-electron reduction of TCNQ immobilized onto electrode surface to generate the corresponding TCNQ- radical anion and incorporation of Fe2+(aq) ions from bulk solution via an overall two-electron charge-transfer process (eq 3): H2 O

2[TCNQ](s,GC,ITO) + Fe2+(aq) + 2e- y\z {Fe[TCNQ]2)H2O)2}(s,GC,ITO) (3) The location of initial nucleation sites associated with TCNQ/ Fe[TCNQ]2(H2O)2 solid-solid conversion is most likely to be the base of the TCNQ crystal,55 which is directly attached to the electrode surface and in contact with Fe2+(aq) ions at the triple-phase (ITO or GC) electrode|(TCNQ(s)/TCNQ-)|Fe2+(aq) electrolyte junction. Moreover, this solid-solid transformation process, as evident by SEM images, is accompanied by a drastic

Facile electrochemical methodologies used previously to form binary M[TCNQ]2(H2O)2, (M ) Co, Ni) have been applied to synthesize large-scale, oriented 1-D/2-D or 3-D nanorod/ nanowire bundles of the semiconducting/magnetic Fe[TCNQ]2(H2O)2. This is achieved via a redox-based TCNQ/Fe[TCNQ]2(H2O)2 solid-solid phase transformation process that is induced by one-electron electrochemical reduction of TCNQ-modified electrodes into TCNQ- radical anion followed by the ingress of Fe2+(aq) ions from the bulk solution into the crystal lattice. The overall transformation process involves a two-electron charge-transfer process and is controlled by initial nucleation at the triple-phase GC or ITO|(TCNQ(s)/TCNQ-)|Fe2+(aq) junction followed by fast growth kinetics. Similar to the isostructural M[TCNQ]2(H2O)2 (M ) Co2+, Ni2+) compounds, the TCNQ/ Fe[TCNQ]2(H2O)2 voltammetric behavior exhibits a marked dependence on the method of electrode modification, scan rate, and Fe2+(aq) electrolyte concentration. SEM probing of the changes that accompany the TCNQ/Fe[TCNQ]2(H2O)2 conversion revealed vast difference in the morphologies and crystal sizes of the parent TCNQ solid and electrochemically produced Fe[TCNQ]2(H2O)2 nanostructures. More important, the morphology of the latter can be tuned to produced 1-D/2-D nanowires or 3-D nanorod bundles via careful control of the method of electrode modification and the voltammetric technique employed to perform the conversion process. Therefore, this generic electrochemical approach offers an alternative to chemical methods in controlling the synthesis, morphology, and fabrication of the hydrated Fe[TCNQ]2(H2O)2 materials. Acknowledgment. We gratefully acknowledge financial support of this work by the Australian Research Council. We also thank Steven Pentinakis and John Ward from the CSIRO Division of Manufacturing and Materials (CMMT) for technical assistance with SEM and EDAX measurements. Supporting Information Available: Cyclic voltammogram of a TCNQ-modified GC electrode in the presence of 1.0 M FeCl2(aq) (Figure S1), dependence of voltammetric parameters (Em, Epred, Ep°x) on the concentration of Fe2+(aq) ions (Figure S2), and IR (Figure S3) and Raman (Figure S4) spectra of the electrochemically produced Fe[TCNQ]2(H2O)2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xiao, K.; Tao, J.; Pan, Z.; Puretzky, A. A.; Ivanov, I. N.; Pennycook, S. J.; Geohegan, D. B. Angew. Chem., Int. Ed. 2007, 46, 2650. (2) Savy, J.-P.; de Caro, D.; Faulmann, C.; Valade, L.; Almeida, M.; Koike, T.; Fujiwara, H.; Sugimoto, T.; Fraxedas, J.; Ondarcuhu, T.; Pasquier, C. New J. Chem. 2007, 31, 519. (3) Sakai, M.; Nakamura, M.; Kudo, K. Appl. Phys. Lett. 2007, 90, 62101. (4) Xiao, K.; Ivanov, I. N.; Puretzky, A. A.; Liu, Z.; Geohegan, D. B. AdV. Mater. 2006, 18, 2184. (5) Shang, T.; Yang, F.; Zheng, W.; Wang, C. Small 2006, 2, 1007. (6) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.; Kiplinger, J. L. Inorg. Chem. 2007, 46, 5528. (7) Jain, R.; Kabir, K.; Gilroy, J. B.; Mitchell, K. A. R.; Wong, K-C.; Hicks, R. G. Nature (London) 2007, 445, 291. (8) Miyasaka, H.; Izawa, T.; Takahashi, N.; Yamashita, M.; Dunbar, K. R. J. Am. Chem. Soc. 2006, 128, 11358. (9) Taliaferro, M. L.; Palacio, F.; Miller, J. S. J. Mater. Chem. 2006, 16, 2677.

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