Electrocrystallization of Tetrathiafulvalene Charge-Transfer Salt

Article pubs.acs.org/JPCC

Electrocrystallization of Tetrathiafulvalene Charge-Transfer Salt Nanorods on Gold Nanoparticle Seeds Li Li, Pedram Jahanian, and Guangzhao Mao* Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, United States S Supporting Information *

ABSTRACT: This paper describes electrochemical synthesis of nanorods of tetrathiafulvalene (TTF) charge-transfer salt on gold nanoparticle (GNP) seeds. The seed-mediated process was monitored by cyclic voltammetry, AFM, and field-emission SEM. The electrodeposition of GNPs (nucleation seeds) on highly oriented pyrolytic graphite (HOPG) electrodes was studied as a function of the electrolytic conditions. The GNP size increases with increasing HAuCl4 concentration and decreasing applied overpotential. A morphological transition from quasi-spherical particles to dendritic aggregates occurs when the HAuCl4 concentration increases from 0.5 to 1 mM. The electrocrystallization of (TTF)Br0.76 on the GNP-decorated HOPG was investigated as a function of TTF concentration and GNP morphology. We observed a preferential nucleation of (TTF)Br0.76 on the GNP seed. The seed-mediated (TTF)Br0.76 crystals display a confined crystal morphology in comparison to those nucleated on bare HOPG. (TTF)Br0.76 nanorods as small as 7 nm in height were nucleated on GNPs of 20 nm in height. We also observed preferential nucleation of (TTF)Br0.76 on high-energy facets rather than on the most prominent face of the GNP. The nanoconfinement effect is attributed to the local curvature of the GNP seed that imposes an interfacial strain, thus limiting the cross-sectional dimension of the ensuing (TTF)Br0.76 crystal. This study contributes to the understanding of electrocrystallization at the nanoscale and a solution-based method to incorporate nanorods on nanopatterns and nanodevices.

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INTRODUCTION Seed-mediated nucleation of organic nanorods provides a unique bottom-up approach to placing nanorod interconnects on nanopatterns and nanodevices. 1D nanostructures including nanowires, nanorods, nanobelts, and nanotubes have potential applications as spacers, interconnects, and functional units in electronic, optoelectronic, electrochemical, and electromechanical nanodevices.1−5 A majority of the 1D nanomaterials currently being investigated are inorganic in nature, but organic nanomaterials can be both complementary and even competitive to their inorganic counterparts. Organic nanomaterials are flexible, energy friendly, inexpensive, and compatible with downscaling toward nanodevices.6 In thin film devices, such as single-electron transistors (SETs), electron transport is regulated across multiple phase boundaries among different nanocomponents, for example, nanoparticles connected to nanorods on a patterned circuitry.7 Utilization of individual nanocrystals for thin film devices depends on our ability to precisely place nanocrystals with respect to each other and on patterned circuitry. For example, gold nanoparticle (GNP) patterns have improved stability, sensitivity, and selectivity in ultrasensitive electrochemical sensors.8,9 Nanoparticle electrodes enhance electron transfer between redox agents and bulk electrodes and allow fine-tuning of sensitivity via particle size, morphology, and number.10 GNP © 2014 American Chemical Society

electrodes show better selectivity when organic cross-linking units with π-donors complementary to analyte molecules are incorporated.8,11 GNPs have been shown to facilitate electron transport among redox reaction centers of tetrathiafulvalene (TTF) in TTF-derivatized GNP electrodes.12 Connecting nanoparticles with size-dependent properties and nanorods with unidirectional transport properties can be achieved by placing interconnects, for example, carbon nanotubes (CNTs), on prearranged metal contact nanopatterns, a slow process.13,14 Alternatively, one can design core/shell particles consisting of nanoparticle cores and nanorod branches and devise further strategies to pattern these particles. To that end, inorganic architectures, for example, multipods, nanodendrites, and higher-order geometries, have been synthesized.15−19 In these examples, both seeds and nuclei consist of similar inorganic building blocks through epitaxy mechanisms. On the other hand, when organics are interfaced with inorganic nanocrystals,20−23 they are usually deposited as shells and not as branches with separate geometry. Received: June 9, 2014 Revised: July 28, 2014 Published: July 30, 2014 18771

dx.doi.org/10.1021/jp505718q | J. Phys. Chem. C 2014, 118, 18771−18782

The Journal of Physical Chemistry C

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scope Probe Station. A three-electrode cell was used with the HOPG or GNP-decorated HOPG as the working electrode (WE), Pt wire (diameter = 0.25 mm) as the counter electrode (CE), and Ag wire (diameter = 0.25 mm) as the quasi-reference electrode (QRE). All the potentials reported in this paper were converted into the values in reference to the SCE. CV was used to characterize the reduction of HAuCl4, total surface area of electrodeposited Au on the HOPG electrode, and the electrochemical activity of the GNP-decorated HOPG. All the CV curves reported in this paper were performed at 100 mV/s scan rate. GNP Electrodeposition. The electrolytic solution for GNP electrodeposition consists of 0.01−10 mM HAuCl4 with 0.1 M KCl as the supporting electrolyte. All solutions have been deoxygenated by purging with N2 for 10 min prior to use. The applied overpotential varies from −0.9 to −0.1 V. The electrodeposition time range is 0.01−10 s. The electrodeposition was conducted using freshly cleaved HOPG at room temperature. After electrodeposition the GNP-decorated HOPG was rinsed with deionized water and dried under N2. The morphology of the electrodeposited GNPs was unchanged for at least 1 month when stored at room temperature. (TTF)Br0.76 Electrocrystallization. Electrocrystallization of (TTF)Br0.76 was conducted in 1−10 mM TTF and 0.1 M TBAB in CH3CN following the literature.31 A 4 s potential pulse of 0.5 V was applied on either HOPG or GNP-decorated HOPG at room temperature. The HOPG substrate was rinsed with ethanol and dried under N2 after electrocrystallization. The morphology of the TTF crystal was unchanged for at least 1 month when stored at room temperature. XRD. The crystalline structure of the GNPs was characterized by a Rigaku SmartLab X-ray diffractometer using Cu Kα1 radiation (λ = 1.540 56 Å) with a Ni filter working at 40 kV and 44 mA. The general medium resolution parallel beam (PB) package was applied with a scan speed of 4°/min. Jade 8 software was used for data analysis. AFM. The nanostructures of electrodeposited GNPs and (TTF)Br0.76 were characterized by a Dimension 3100 AFM (Bruker) in the tapping mode in ambient air. The height, amplitude, and phase images were obtained using silicon tapping tips (nanoScience Instruments, VistaProbes T300) with resonance frequency of 300 kHz and a nominal tip radius less than 10 nm. The scan rate is 1−3 Hz. Integral and proportional gains are approximately 0.4 and 0.8, respectively. Height images were plane-fit in the fast scan direction with no additional filtering operation. The sizes of the GNPs and (TTF)Br0.76 nanocrystals were determined using the sectional height analysis command of Nanoscope 5.30r3sr3 (N = 80− 100 particles for the GNPs and N = 20−70 nanocrystals for the (TTF)Br0.76). The average single particle volume of the GNP was determined using the bearing volume analysis command of the Nanoscope software (N = 80−100 particles). The bearing analysis command was also used to estimate the surface area of the GNPs covering an area of 1 × 1 μm2 (N ∼ 10 spots). Field-Emission SEM. The nanostructures of the GNPs and (TTF)Br0.76 nanocrystals on the HOPG electrode were characterized by field-emission SEM (JEOL JSM 7600F SEM). The SEM including an in-lens thermo electron gun and a γ-filter for detection was operated at an acceleration voltage of 15 kV, working distance of 8 mm, and probe current of 6 μA. The MeasureIT software was used for data analysis. The elemental compositions of the GNPs and (TTF)Br0.76 were obtained by in situ energy-dispersive spectroscopy (EDS)

This study focuses on GNP-mediated nucleation of organic charge-transfer salts to make truly hybrid nanostructures. The method has the potential to deposit small-molecule interconnects using solution chemistry on patterned circuitry. This work represents a significant advance over our original discovery of a seed-mediated mechanism of making ncarboxylic acid nanorods on monolayer-protected nanoparticles of CdSe, CdS, and Au.24−26 The previous work was conducted by coprecipitating the two components from the solution phase on HOPG during spin-coating. This work brings the seedmediated mechanism a step closer to the actual application by demonstrating the synthesis of conductive TTF charge-transfer salt nanorods using highly controlled electrocrystallization. Electrocrystallization is used to synthesize molecular conductors of high purity.27,28 Electrocrystallization offers a much better control of the crystallization conditions than spincoating. In electrocrystallization, the primary driving force is the applied overpotential, η, which can be precisely controlled with a potentiostat. η is defined as the electrochemical potential difference between the applied potential and the standard redox couple potential. The seed-mediated method to connect nanorods and nanoparticles belonging to different material groups promises a room-temperature and solution-based process to generate nanorods and nanowires from diverse crystalline compounds. The TTF system was chosen because their synthesis and crystal structures have been well documented.29−32 TTF (π donor) and 7,7,8,8-tetracyanoquinodimethane (TCNQ, π acceptor) charge-transfer salt, TTF·TCNQ, is often considered the first synthesized organic metal.33,34 Organic metals display metallic electrical conductivity, σ ∼ 10−104 Ω−1 cm−1, which increases with decreasing temperature. (TTF)Br0.7 at room temperature has an electrical conductivity of 1.4 Ω−1 cm−1 (by four-probe method, 0.3 Ω−1 cm−1 by two-probe method).35 The predominant needle crystal habit of TTF salt crystals suggests a high probability of nanorod formation on nanoparticles in maintaining an overall aspect ratio. The electrical conductivities of TTF-based crystals arise from the fact that electrons and holes are delocalized along segregated stacks of donor and acceptor free radicals in the crystal structure. Thus, TTF-based conductors have the highest conductivity along the stacking or needle axis. 1D nanostructures of TTF salts have potential applications as spacers, interconnects, and functional units in nanoscale electronic, optoelectronic, electrochemical, and electromechanical devices.1−5 However, their insolubility and low vapor pressure have delayed their integration into devices.36 This work offers a means for the direct synthesis of TTF-based conductor and semiconductor nanorods and nanowires onto substrates in order to overcome their inherent physical limitations.

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EXPERIMENTAL SECTION Materials. HAuCl4·3H2O (>99.9%, Aldrich), KCl (ACS grade, Fisher Scientific), TTF (97%, Aldrich), tetrabutylammonium bromide (TBAB, ≥99%, Fluka), potassium ferricyanide (K3Fe(CN)6, ACS grade, Fisher Scientific), and acetonitrile (CH3CN, 99.9%, Fisher Scientific) have been used as received. HOPG (ZYB grade, 1 × 1 cm2) has been purchased from MikroMasch and freshly cleaved by adhesive tape just before use. Electrochemical Measurements. The electrochemical measurements were performed using a PAP 263A potentiostat (Princeton Applied Research) coupled to a Signatone Micro18772



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attached to the field-emission SEM at 15 kV in the secondary electron mode. The EDAX Genesis V6.33 software was used for data collection and analysis. Crystal Structure Visualization. The (TTF)Br0.76 crystal structure visualization and modeling were performed using the Materials Studio software from Accelrys based on the (TTF)I5/7 supercell structure. The CCDC THFULI01 contains the supplementary crystallographic data of the (TTF)I5/7 supercell and is available upon request at http://www.ccdc. cam.ac.uk/cgi-bin/catreq.cgi. The (TTF)I5/7 (and (TTF)Br0.76) supercells are monoclinic systems of space group P21/a with the lattice parameter of a = 48.015 Å (46.851 Å), b = 16.041 Å (15.627 Å), c = 24.877 Å (25.004 Å), and β = 91.31° (91.23°).31,37

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RESULTS AND DISCUSSION GNPs were electrodeposited on HOPG in which the GNP morphology was varied by the electrolytic conditions. We varied HAuCl4 concentration, applied overpotential, and electrodeposition time. The electrodeposited GNPs were then used in electrocrystallization of TTF charge-transfer salt, (TTF)Br0.76. The capability of GNPs to nucleate and confine the electrocrystallization of (TTF)Br0.76 was demonstrated. (TTF)Br0.76 nanorods as small as 7 nm in height were nucleated on GNPs of 20 nm in height. The nanostructures were characterized by CV, AFM, and field-emission SEM. Preparation of Nucleation Seeds by Electrodeposition of GNPs on HOPG. Metal nanoparticles supported on bulk electrodes are useful for electroplating, fabrication of microelectronics, electrocatalysis, and understanding of charge transport phenomena including tunneling, single-electron charging, percolation effects, and scattering in granular materials.38−41 Here electrodeposition of GNPs is used as a means of creating nucleation seeds for subsequent electrocrystallization of TTF salts. This study reports correlated AFM and CV measurements in order to relate GNP size, shape, and surface coverage to the electrolytic conditions. The 0.05−10 mM HAuCl4 aqueous solutions with 0.1 M KCl as supporting electrolyte were used for GNP electrodeposition on HOPG electrodes. The electrochemical reduction reaction is

Figure 1. CV curves in 0.05−10 mM HAuCl4 solutions on the HOPG electrode. The potential range is −0.5 to 1.5 V. The scan rate is 100 mV/s. (a) 3D plots of the full potential range. (b) Plots of the cathodic reduction region only.

positive one. The onset potential for HAuCl4 reduction is 0.50−0.55 V for all the HAuCl4 concentrations studied. The crossover potential of the forward and the backward scan (Eco) is 0.66 ± 0.02 V for the investigated concentration range, which is close to the calculated Eeq. The overpotential (η) required for GNP nucleation on HOPG is defined by the difference between the applied potential, E and Eco. The peak current, ip, is defined by the Randles−Sevcik equation for simple redox reactions at 25 °C: i p = (2.69 × 105)n3/2AD1/2Cv1/2

where 2.69 × 10 is a constant, n is the number of electrons transferred in the redox event (n = 3 here), A is the electrode surface area (cm2), D is the diffusion coefficient (cm2/s), C is the concentration of the electroactive species in the bulk solution (mol/cm3), and υ is the scan rate (V/s). Figure 2a displays a linear relationship between the cathodic peak current, ipc, and the HAuCl4 concentration at υ = 100 mV/s. Figure 2b shows a linear relationship between ipc and υ1/2 in the range of 50−500 mV/s in 0.5 mM HAuCl4 solution. Both graphs indicate that the reduction of Au(III) to Au(0) is under the control of reactant diffusion to the electrode surface as opposed to either surface reaction or kinetic control.45 D of the AuCl4− ion in aqueous solution can be calculated from the slope of the graph in Figure 2b. The electrode surface area, A, was estimated by the droplet volume (= 50 μL) and its contact angle on the HOPG substrate (= 50°) to be 0.5 cm2. D of AuCl4− in 0.5 mM HAuCl4/0.1 M KCl was calculated to be 5.6 × 10−6 cm2/s. In comparison, the literature value of D of complex metal ions in aqueous solutions is 10−5 cm2/s.46 AFM was used to study the GNP morphology as a function of the HAuCl4 concentration, η, and deposition time. We define the GNP size as the average volume per particle (by bearing analysis), GNP density as the average number of particles per μm2 on HOPG, and GNP surface area as the total surface area

AuCl4 − + 3e− → Au(0) + 4Cl− 0 (EAu(III)/Au(0) = 0.761 V)

(1)

Considering the HAuCl4 concentration range used, the equilibrium reduction potential (Eeq) was calculated to be 0.77 ± 0.02 V based on the Nernst equation 0 Eeq = EAu(III)/Au(0) = EAu(III)/Au(0) +

(at 25 °C)

(3)

5

[AuCl4 −] 0.059 log n [Cl−]4 (2)

The CV measurements were used to determine the appropriate potential range for the GNP deposition. Figure 1 shows the CV curves on HOPG in 0.05−10 mM HAuCl4 aqueous solutions. All the CV curves display a cathodic peak at 0.2−0.5 V associated with the reduction of Au(III) to Au(0) and an anodic peak at 1.0−1.2 V associated with the oxidation of Au(0) to Au(III).42−44 As the HAuCl4 concentration increases, the absolute values of the cathodic/anodic peak currents increase, the cathodic peak potential shifts to a more negative value, and the anodic peak potential shifts to a more 18773



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Figure 2. Dependence of (a) the reduction peak current (ipc) on the HAuCl4 concentration (scan rate = 100 mV/s) and (b) the reduction peak current (ipc) on (scan rate)1/2 (HAuCl4 concentration = 0.5 mM).

covered by the GNPs on 1 × 1 cm2 HOPG. AFM height images in Figure 3 (low magnification) and Figure 4 (high

Figure 4. AFM images (left) and sectional height profiles measured along the white dashed lines (right) of GNPs electrodeposited on the HOPG using (a) 0.1, (b) 0.5, (c) 1, (d) 5, and (e) 10 mM HAuCl4. The deposition overpotential is −0.5 V, and pulse time is 10 ms. Z range is 20 nm for (a), 30 nm for (b), and 100 nm for (c−e).

Table 1. Particle Size, Density, and Surface Coverage Area per cm2 HOPG of GNPs as a Function of the HAuCl4 Concentration (η = −0.5 V for 10 ms) HAuCl4 concn (mM) 0.1 0.5 1.0 5.0 10.0

Figure 3. AFM height images of electrodeposited GNPs on HOPG. An overpotential of −0.5 V was applied for 10 ms in (a) 0.1, (b) 0.5, (c) 1, (d) 5, and (e) 10 mM HAuCl4. Z range is 20 nm for (a), 50 nm for (b), 100 nm for (c−e), and 50 nm for the insets in (c−e).

particle size (×10−3 μm3/ particle) 0.006 0.024 0.6 7.2 12.2

± ± ± ± ±

0.004 0.008 0.3 5.9 8.0

particle density (particles/μm2)

surface area by AFM (cm2)

± ± ± ± ±

0.005 0.012 0.032 0.115 0.170

13.0 13.3 0.98 0.28 0.22

3.8 1.4 0.15 0.05 0.04

surface area by CV (cm2) 0.007 0.019 0.094 0.206

± ± ± ±

0.003 0.006 0.012 0.019

mM HAuCl4 solution are 11 ± 2 nm in height and 32 ± 6 nm in diameter by AFM sectional analysis. The particle dimensions in 0.5 mM HAuCl4 solution are 20 ± 2 nm in height and 48 ± 7 nm in diameter. The GNP dendrites display a center core with a height of 36−54 nm and dendritic shell whose overall diameter increased from 400 nm in 1 mM HAuCl4 to 1.5 μm in 10 mM HAuCl4 (Figure 4c−e). The HAuCl4 concentration therefore should be kept below 0.5 mM to avoid dendritic growth. The morphological transition is accompanied by an abrupt change in the particle density. The particle density below the transition concentration, ∼13 particles/μm2, is

magnification) illustrate the dependence of the GNP morphology on HAuCl4 concentration. The applied overpotential was fixed at −0.5 V, and the deposition time was fixed at 10 ms. The AFM images show that the GNPs change from smooth hemispheres to irregular dendrites when the HAuCl4 concentration increases above 0.5−1 mM. Similar morphological transitions of electrodeposited GNPs have been reported by others.42,44,47 With increasing HAuCl4 concentration, the GNP size increases while particle number density decreases (Table 1). Typical GNP dimensions made from 0.1 18774



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obtained from the CV curves are in a general agreement with those measured by AFM analysis (Table 1). Electrodeposition of GNPs was also studied as a function of η and pulse time. The HAuCl4 concentration was fixed at 0.1 mM. The data of the deposited GNP height, diameter, particle number density, and surface area are presented in Table 2 based on AFM height images (Figure S1 in Supporting Information). The results show that as the applied overpotential becomes more negative (increasing driving force) the height, diameter, and surface area decrease while the particle density increases. In the applied pulse time range studied particle density decreases with increasing pulse time while no significant changes in the GNP morphology were observed. The electrochemical activities of the GNP-decorated HOPG were further studied by the CV curves of the redox couple of Fe(CN)64−/3−. Figure 6 shows the CVs obtained in 1 mM K3Fe(CN)6 with 0.1 M KCl (potential range = −0.5 to 0.8 V and scan rate = 100 mV/s) on HOPG with and without GNPs. Upon GNP deposition both the anodic peak current (ipa) and the absolute value of cathodic peak current (|ipc|) of Fe(CN)63−/4− redox activities increases while the peak potential separation (ΔEp) decreases (Figure 6b−e) as compared to bare HOPG (Figure 6a). All three attributes point to higher electrochemical activity upon GNP deposition. Figure 6b−e shows an increase in ipa and |ipc| and a decrease in ΔEp with increasing HAuCl4 concentration, further supporting the conclusion that the increase in the HOPG electrode activities is directly related to the GNP deposition. The GNPs electrodeposited on HOPG were examined by XRD (Figure 7). The GNPs were deposited at overpotential of −0.5 V for 10 ms in 0.1 and 10 mM HAuCl4. 0.1 mM HAuCl4 yielded hemispherical GNPs, and 10 mM HAuCl4 yielded dendritic GNPs. The 2θ values of the diffraction peaks at 38.2°, 44.5°, and 82° correspond to the (111), (200), and (222) crystalline planes of the Au FCC crystal structure. The expected peak from the (220) plane at 64.6° was too low to be detected. The expected peak from the (311) plane at 77.5° overlaps with that of the (110) plane of HOPG.47 The peak for (222) from dendritic GNPs splits into two peaks at 81.7° and 82.5° (Figure 7c) possibly due to the reduced symmetry associated with the dendritic shape. The intensity ratio of the (111) to (200) peak is 2.2 and 146 for semispherical and dendritic GNPs, respectively. The intensity ratio for the semispherical GNPs is close to the expected intensity ratio of 1.9.58 The extremely high diffraction intensity of the (111) plane for the dendritic GNPs indicates that they exhibit a higher percentage of the (111) face parallel to the HOPG basal plane. The intrinsic crystalline grain size, τ, was estimated using the Scherrer equation, τ = Kλ/(β cos θ), where K is the shape factor equaling 0.9 for single crystal with a spherical shape, λ is Cu Kα wavelength, and β is the full width at half-maximum peak height. The estimated grain sizes are 28.0 and 26.6 nm for semispherical and dendritic GNPs, respectively, by averaging values from 3 to 4 diffraction peaks. In addition to the hemispherical and dendritic GNPs, we observed other GNP shapes in electrodeposition including truncated octahedron, cubic, and decahedron (Figure 8). The exact conditions for such faceted GNPs have not been determined. The crystal morphology of the FCC Au according to the Wulff’s plot is a truncated octahedron with eight (111) faces of the lowest surface energy and six (100) faces.59−61 Fast growth methods and capping agents have been used to alter the GNP morphology from truncated octahedron to trigonal

roughly 13 times the value obtained in 1 mM HAuCl4 and 40 times the value in 5−10 mM HAuCl4. The total GNP covered surface area on 1 × 1 cm2 HOPG by AFM shows an increase of the GNP coverage with increasing HAuCl4 concentration (Table 1). It suggests that at concentrations higher than 0.5 mM more Au ions are reduced during the secondary nucleation event to form the dendrites while the primary nucleation rate to form the GNPs decreases. The surface area of the GNPs on HOPG was independently determined from the CV curves obtained during electrochemical reduction of GNP-decorated HOPG in 0.05 M H2SO4.48−57 The CV curves were obtained in the potential range of −0.5 to 2.0 V. The GNP-decorated HOPG electrodes were prepared from 0.1−10 mM HAuCl4 solutions at an overpotential of −0.5 V for 10 ms. Figure 5 shows that the

Figure 5. CV curves in 0.05 M H2SO4 from −0.5 to 2.0 V with scan rate 100 mV/s on bare and GNP-decorated HOPG. The GNPdecorated HOPG electrodes were prepared at an overpotential of −0.5 V for 10 ms in 0.5−10 mM HAuCl4 solutions.

cathodic reduction peaks corresponding to the reduction of Au(III) oxide formed during the anodic cycle46,47 to Au(0) occur near 0.4 V on the GNP-decorated HOPG prepared in 0.5−10 mM HAuCl4.50,51 The cathodic reduction peak of the GNP-decorated HOPG made from 0.1 mM HAuCl4 was too small to be measured. The reduction peak current on the GNPdecorated HOPG electrode increases with increasing HAuCl4 concentration. The surface area of electroactive GNPs was thus estimated based on the number of charges consumed during the Au(III) oxide reduction by integrating the catholic peak area in Figure 6.52,53 The number of charges for the reduction of a fully covered monolayer was assumed to be 400 μC/ cm2.54−57 The total surface areas of electroactive GNPs

Figure 6. CV curves in 1 mM K3Fe(CN)6 /0.1 M KCl from 0.8 V to −0.5 V with a scan rate of 100 mV/s on (a) bare HOPG and (b−e) GNP-decorated HOPG. The GNPs are electrodeposited at −0.5 V overpotential for 10 ms in (b) 0.5, (c) 1, (d) 5, and (e) 10 mM HAuCl4 solution. 18775



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Table 2. AFM Analysis of Vertical and Lateral Dimensions, Particle Density, and Surface Area of GNPs Electrodeposited on HOPG with Varying Overpotential and Deposition Time overpotential (V)

deposition time (s)

−0.1 −0.5 −0.5 −0.5 −0.9

0.01 0.01 0.1 1 0.01

vertical height (nm) 18.2 11.3 17.9 20.0 2.3

± ± ± ± ±

lateral diameter (nm)

4.1 2.1 3.4 3.8 0.6

50.3 32.3 43.8 52.5 34.4

± ± ± ± ±

particle density (particles/μm2)

surface area (×10−3 cm2)

± ± ± ± ±

4.9 5.3 4.7 4.2 23.2

11.1 6.2 8.2 7.9 6.4

4.9 13.0 6.2 3.9 50.0

0.8 3.8 1.1 0.9 5.2

Others have shown the dissolution behavior to depend on particle coverage and diffusion profiles.76 The electrochemical stability of metal nanoparticles has also been attributed to oxide formation.77 GNP electrodeposition on HOPG can be described by nucleation and crystal growth steps. In the classical nucleation theory,78−80 the free energy of forming a cluster of radius r with an interfacial tension γ and containing n molecules is ΔG = 4πr 2γ − nΔμ

(4)

This leads to the critical nucleation energy, ΔGc, and critical nucleus size, rc = 2γV/Δμ, for growing into a stable crystal (V = molecular volume of the crystallizing compound). The driving force for electrodeposition is expressed as Figure 7. XRD spectra of (a) bare HOPG and (b, c) GNP-decorated HOPG. The GNPs are electrodeposited on HOPG at −0.5 V overpotential for 10 ms in (b) 0.1 mM and (c) 10 mM HAuCl4.

Δμ = ze|η|

(5)

where z is the valence of the ion, e is the electron charge, and η is the overpotential. When a constant η is applied, reduction of Au ions onto the nucleus is favored when the nucleus is bigger than rc whereas the nucleus will dissolve if its size is smaller than rc. The nucleus size, rc = 2Vγ/ze|η|, is inversely proportional to η. In the case of changing η at the same electrolyte concentration, the higher driving force for the reaction is achieved by applying more negative overpotential for cathodic reduction or more positive value for anodic oxidation.45 Our results show that higher overpotential results in smaller GNP size and higher particle coverage both are consistent with the classical nucleation theory. In the case of changing electrolyte concentration, the overpotential dependence on electrolyte concentration is defined by81 η=

Figure 8. Field-emission SEM images of faceted GNPs with (a, b) truncated octahedron, (c) cubic, and (d) decahedron shapes produced during electrodeposition on HOPG. The bar length = 200 nm.

kT a ln ze aeq

(6)

where a and aeq are the activities of the electrolyte concentration being considered and that at equilibrium. Our results show that in the lower concentration (0.5 mM) the primary nucleation event is followed by secondary nucleation that results in the core/shell structure of a spherical core and a dendritic shell. Our data also show that in the higher concentration regime secondary nucleation overtake primary nucleation that results in a reduction in the number of particles. In the case of changing electrodeposition time, longer time results in lower particle coverage that can be attributed to Ostwald ripening. The presence of electrodeposited GNPs has been shown to enhance electrochemical reactivity of the bulk electrodes in the redox reactions of Fe(CN)63−/4− and Ru(NH3)63+ and hydroxymethylferrocene (FcMeOH).58,82 We have shown that GNP deposition on planar HOPG electrodes improve

lamellar plate, icosahedron, octahedron and truncated octahedron, decahedron and truncated decahedron, rhombic dodecahedron, spherical, and dendritic shapes in colloidal solution chemistry.62−71 Our data suggest that it may be possible to create GNPs of various shapes electrochemically as those in colloidal chemistry. This would allow further study on the effect of GNP crystalline faces on seed-mediated electrocrystallization. Electrodeposition of Au on HOPG generally obeys the Volmer−Weber 3D island growth mechanism due to the weak van der Waals interaction.72,73 Despite the weak interaction, the electrodeposited GNPs display high stability on HOPG. One reason is the irreversibility of the electrochemical reaction due to the large band gap energy between the states of the donor Au and the conduction band of the substrate.74 The electrochemical stability of GNPs is a function of particle size. The negative shift in the oxidation potential for smaller GNPs in our study can be predicted by the Oswald ripening.75 18776



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electron transfer kinetics and redox reaction reversibility with characteristics such as narrower peak separation, greater current, and more symmetrical peaks. The increased activity can be attributed to both inherent conductivity difference between Au and HOPG as well as the surface area increase upon 3D GNP deposition on 2D HOPG. Nucleation of (TTF)Br0.76 Nanorods on GNP Seeds by Electrocrystallization. TTF is oxidized at the anode surface in bromide electrolyte solution into a mixed valence salt, (TTF)Br0.76, in the following reaction:31,83−85 TTF + 0.76Br − = (TTF)Br0.76 + 0.76e−

(7) +

E0TTF−TTF+,

The standard reduction potential for TTF/TTF , is 0.3 V (vs SCE). We conducted (TTF)Br0.76 electrocrystallization on HOPG in 5 mM TTF and 0.1 M TBAB in acetonitrile with an anodic potential pulse of 0.5 V for 4 s and obtained needle-shaped crystals (Figure 9). The crystals are 3−13 μm in

Figure 10. Field-emission SEM images of (TTF)Br0.76 on the GNPdecorated HOPG in electrocrystallization. The GNPs were electrodeposited in −0.5 V overpotential for 10 ms. The average diameter of the GNPs is 50−400 nm. The (TTF)Br0.76 rods were deposited in (a) 1, (b) 5, and (c, d) 10 mM TTF and 0.1 M TBAB in acetonitrile at 0.5 V for 4 s. The GNPs in (a−c) were deposited using 10 mM HAuCl4 while the GNPs in (d) were deposited using 0.1 mM HAuCl4.

Table 3. Dimensions of GNP and (TTF)Br0.76 Crystals with Varying TTF Concentration Measured by AFMa TTF concn (mM) 0 1 5 10 10a

Figure 9. Side-by-side comparison between (a) AFM (Z range = 500 nm) and (b) field-emission SEM images of (TTF)Br0.76 crystals electrodeposited on HOPG.

length, 250−800 nm in lateral width, and 100−500 nm in vertical thickness. The crystals have no epitaxial orientation relationship with the HOPG basal plane. The length and width were measured independently by AFM and field-emission SEM. The thickness was measured using the sectional height analysis command on AFM height images. The EDS attachment to the field-emission SEM provides a means to determine the chemical composition of the (TTF)Br0.76 crystal. The atomic ratio of S/Br was determined to be 5.4 ± 1.2 (N = 16). The theoretical S/Br ratio in (TTF)Br0.76 is 5.3. Next we applied the same electrolytic conditions to grow (TTF)Br0.76 crystals on the GNP-decorated HOPG. The GNPs were electrodeposited on HOPG at −0.5 V overpotential for 10 ms in 10 mM HAuCl4. The TTF concentration was varied from 1 to 10 mM. Figure 10a−c displays the field-emission SEM results in different TTF concentrations. The results show that GNPs are capable of nucleating (TTF)Br0.76 crystals and reducing the overall crystal size. Roughly 67% (TTF)Br0.76 crystals are attached to the GNPs when 1 mM TTF is used, and 50% are attached to the GNPs when 5 and 10 mM TTF solutions are used. We also noticed that only one (TTF)Br0.76 is nucleated per GNP consistent with theoretical probability of multiple nucleation on electrode of the GNP size being extremely low.86 The nanorod attachment is strong because attached nanorods cannot be removed by rinsing with ethanol during sample preparation or by AFM probe during normal operation of AFM imaging. It can also be concluded that (TTF)Br0.76 crystals preferentially nucleate on GNPs considering that GNPs occupy less than 20% of the total HOPG surface area. We analyzed the dimensional relationship between GNPs and GNP-induced (TTF)Br0.76 rods. The results are summarized in Table 3. For example, in 5 mM TTF (TTF)Br0.76 crystals are 100 nm in width and 1.3 μm in length in the presence of GNPs, which are significantly smaller than

a

GNP diameter (nm)

(TTF)Br0.76 rod width (nm)

(TTF)Br0.76 rod length (μm)

± ± ± ±

250−800 173 ± 65 99 ± 48 282 ± 115 36 ± 9

3.00−13.00 2.11 ± 1.03 1.26 ± 0.50 3.31 ± 1.15 0.33 ± 0.14

401 333 415 53

90 85 112 9

The HAuCl4 concentration used to make this sample is 0.1 mM.

those obtained on bare HOPG in the same electrolytic condition. When smaller GNPs were used, the size of the corresponding TTF nanorods was reduced further (Figure 10d). The smaller GNPs were electrodeposited in 0.1 mM HAuCl4. The EDS was used to determine the (TTF)Br0.76 nanorod chemical composition attached to the GNPs (Figure S2, Supporting Information). The S/Br of 5.4 (N = 16) for (TTF)Br0.76 nanorods attached to GNPs is similar to the value measured on (TTF)Br0.76 grown on bare HOPG and the theoretical value. The orientation of the nanorods with respect to the nanoparticle surface was studied. We used the angle between the needle axis of the nanorod and the surface tangent line at the point of contact to define the tangential and radial orientations of the nanorod with respect to the nanoparticle surface. The angle is less than 45° for tangential orientation and between 45° and 90° for radial orientation. The number ratio of nanorods oriented tangentially vs radially to the nanoparticle surface was determined to be 1.8 for 5 mM TTF, 3.7 for 2.5 mM, and 2.6 for 1 mM TTF. It can be concluded that nucleation rates of (TTF)Br0.76 along the tangential direction are higher than along the radial direction of the GNP surface. Next we examined further the GNP size effect on (TTF)Br0.76 electrocrystallization using the electrolytic solution of 5 mM TTF and 0.1 M TBAB in acetonitrile and a 4 s potential pulse of 0.5 V. The GNP size was varied by HAuCl4 concentration. Figure 11 shows the side-by-side comparison between AFM and field-emission SEM results. Table S1 in the Supporting Information summarizes the size measurements of the GNPs and attached (TTF)Br0.76 rods. The AFM and fieldemission results largely agree with each other. In the case of hemispherical GNPs, the smallest (TTF)Br0.76 rods (height = 18777



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Figure 11. AFM and field-emission SEM images of (TTF)Br0.76 electrocrystallization on the GNP-decorated HOPG with various GNP size. (a), (c), and (e) are the AFM height images of (TTF)Br0.76 rods attached to the GNP. The Z range is 40 nm for (a), 500 nm for (c), and 100 nm for (e). (b), (d), and (f) are the corresponding fieldemission SEM images of the AFM images on the left. The inserted images in (b), (d), and (f) show the details of the particle-rod structure by zooming into the feature.

Figure 12. Plots of the average width of the (TTF)Br0.76 rods as a function of (a) the diameter of hemispherical GNPs and (b) local radius of curvature of dendritic GNPs. The lines were drawn as a guide.

6.9 ± 1.1 nm, diameter = 32.1 ± 7.7 nm, and length = 100−500 nm) were induced by the smallest spherical GNPs (height = 19.6 ± 2.1 nm and diameter = 48.1 ± 6.8 nm). We found a general dimensional relationship between the width of the (TTF)Br0.76 nanorods (WTTF) and the GNP seed diameter (DGNP) to be WTTF = 0.33DGNP (Figure 12a). The size of (TTF)Br0.76 grown on dendritic GNPs does not correlate with the overall diameter of the particle but rather with the height and local curvature of the dendrites. The dendritic GNP diameter varies widely from 250 nm to 1.4 μm while the (TTF)Br0.76 nanorod width remains in the 30−100 nm size range. Figure 12b presents the histogram of the width of the (TTF)Br0.76 nanorods as a function of the local radius of curvature of the dendrites. We determined the local curvature by measuring the radius of the dendritic tip to which the GNP is attached. The data point to size confinement of the (TTF)Br0.76 nanorods being imposed by the local radius of curvature at the seed/crystal interface rather than the global seed size. We compare our results with previous related studies. Electrocrystallization of (TTF)Brx was conducted on GNPs deposited on the SiO2 substrate.84 The GNPs with diameter of ∼250 nm aggregated on the substrate, which resulted in (TTF)Br0.7 microcrystals grown on aggregates of GNPs with length of 1−2 μm and width of 200 nm. In another study (TTF)Br0.76 was found to nucleate electrochemically on Pt nanoparticles deposited on HOPG.85 The width of the (TTF)Br0.7 crystals (30−600 nm) was proportional to the diameter of the Pt nanoparticles (70 nm−1.3 μm) with an average ratio of 0.4. The dimensional correlation is very close to the ratio we found in the (TTF)Br0.7 nanorods nucleated on GNPs. The use of electrochemistry to produce the nanorod/ nanoparticle hybrid structure allows a better control of the crystallization process by applied potential, pulse time, and

concentration as well as the production of electroactive nanostructures. The electrodeposited GNPs have higher conductivity than the substrate and act as nanosized electrodes. When an electrical potential pulse is applied to the HOPG surface, GNPs with higher conductivity will have a higher current density than the surrounding areas, which leads to selective electrodeposition and nucleation of (TTF)Br0.7 crystals on the GNPs. The nanorods of (TTF)Br0.7 electrochemically grown on GNPs are remarkably similar to carboxylic acid nanorods grown on monolayer-protected nanoparticles by fast solvent evaporation during spin-coating.24−26 The GNP seed size effect can be understood by the classical nucleation theory. The presence of a 2D seed surface lowers the nucleation energy by the nucleation capability factor, f = ΔGHET /ΔGc = (1/4)(1 − cos c θ)2(2 + cos θ). ΔGHET is the critical nucleation energy on the c seed. θ is the contact angle between the cap-shaped nucleus and the seed surface. cos θ = (γSF − γSC)/γCF. γSF, γSC, and γCF are the substrate/fluid, substrate/crystal, and crystal/fluid interfacial tension, respectively. For 3D seeds f is not only a function of θ but also a function of R′ = RS/rc with RS being the radius of curvature of the seed particle.87−89 When the particle is large, for example, R′ ≥ 10, the particle surface curvature approaches that of a flat surface; the energy barrier is dominated by θ. When the particle is small, for example, R′ ≤ 0.1, f is close to 1; the foreign particle is ineffective in nucleation. When 0.1 ≤ R′ ≤ 10, f decreases drastically with decreasing rc (or increasing Δμ). Therefore, nanoparticles can be effective nucleation seeds only if rc is small and there is a good structural match at the interface. The classical nucleation theory also predicts nucleation on small seed particles at high Δμ to be dominated by nucleation kinetics. Clearly the electrodeposited GNPs are big enough to be effective nucleation seeds. However, 18778



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(TTF)Br0.76 crystals nucleated on GNPs are much smaller than the ones nucleated HOPG. A possible explaination for the size confinement is the strain energy of a crystal nucleated at the curved seed surface.90 The high curvature of a nanosized seed imposes a high strain energy of the nucleated crystal. The strain energy will limit the cross-sectional area of the crystal eminating from the seed surface. The dimensional analysis results of (TTF)Br0.76 nanorods attached to hemispherical and dendritic GNPs are both consistent with the curvature confinement theory. In addition, we also observed (TTF)Br0.76 nanorods unattached to but in the vicinity of the nanoparticles. Similar observations have been made in other cases in which crystals nucleated on a particle seed detach due to the high interfacial strain energy.26,90 The curvature effect does not apply in the cases when (TTF)Br0.76 nanorods nucleate from faceted GNPs. When (TTF)Br0.76 nanorods are nucleated by faceted GNPs, they prefer to grow on the vertices or sharply truncated corners instead of the dominant (111) face of the GNPs (Figure 13). This may be attributed to the different electronic states of the crystalline faces and should warrant further study.

Figure 14. (a) Crystal structure of (TTF)Br0.76 exposing the (010) face generated by the Materials Studio program. The structure is based on a single-crystal X-ray structure in ref 37. (b) Hypothesized crystal structure of the smallest (TTF)Br0.76 nanorod attached to the GNP. The cross-sectional area contains 4 unit cells in the [100] and [010] directions.

be either in the tangential or radial direction of the GNP surface. The height and width of the smallest (TTF)Br0.76 nanorods observed by AFM are slightly larger than 4× the interplanar spacing of (010) and (100) planes, respectively. We note that the actual terminal surfaces in the width direction of the nanorod are probably high-index planes such as the (210) and (310) planes. Figure 14b shows the crystallographic orientation of such a nanorod with respect to the GNP seed. The GNP seed is therefore capable of nucleating (TTF)Br0.76 whose cross-sectional size is approaching the unit cell size of the molecular crystal.

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CONCLUSIONS Our work demonstrates a methodology of using inorganic nanoparticles as nucleation seeds for the creation of functional organic nanorods. By using electrochemical method, the electrodeposition of GNPs on HOPG shows controllable morphology by changing electrolyte concentration and overpotential. HOPG shows an increased electrochemical activity upon GNP deposition. The electrocrystallization of TTF bromide salt displays preferential nucleation on the GNPs rather than on planar HOPG. The resulting (TTF)Br0.76 crystal size is confined and controlled by the TTF concentration, GNP size, and GNP morphology. The confinement effect is directly related to the GNP size and local curvature. The study contributes to the understanding of electrocrystallization at the nanoscale and a solution-based method to incorporate nanorods on nanopatterns and nanodevices. It provides direct experimental evidence of the capability of nanosized electrodes in reducing crystal size of organic conductors down to the nanometer size. It further provides a one-step synthesis method of charge-transfer nanorods by electrocrystallization that may overcome the inherent physical limitations of the materials and enable their integration into patterned circuitry.

Figure 13. Field-emission SEM images of (TTF)Br0.76 electrocrystallization on the GNP-decorated HOPG with various GNP shapes: (a, b) truncated octahedron, (c) cubic, and (d) decahedron. The bar length = 200 nm.

The ratio of the number of tangentially oriented to the number of radially oriented (TTF)Br0.76 nanorods indicates that the nucleation rate along the tangential direction is higher than along the radial direction. The difference stems from a higher probability of molecules attached parallel to the GNP surface that leads to the radial orientation of the nanorod. This preference is supported by parallel attachment of TTF to the surface of bulk gold electrodes in its electrocrystallization.91 This is different from our previous study of n-carboxylic acid crystallization on GNPs that show equal probability of molecular attachment due to high supersaturation. Figure 14 represents the likely crystal structure of (TTF)Br0.76 on the GNP. Figure 14a shows the crystal structure of (TTF)Br0.76 crystal using the data from Cambridge Crystallographic Database. The crystal structure of (TTF)Br0.76 is monoclinic, P21/a, Z = 12, similar to that of (TTF)7I5.37,92−94 The lattice parameters for (TTF)Br0.76 is a = 4.6851 nm, b = 1.5627 nm, c = 2.4943 nm, and β = 91.23° according to singlecrystal X-ray diffraction (Supporting Information).31,93 When the crystal is electrochemically nucleated on the GNPdecorated HOPG, we expect the most prominent (010) face to parallel the HOPG basal plane and its needle axis [001] to

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ASSOCIATED CONTENT

S Supporting Information *

AFM and field-emission images, data analysis of the GNP, and TTF salt crystal structures and chemical compositions. This 18779



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material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Science Foundation (CHE-1404285, CBET-0619528, CBET0755654, and DMR-0922912).

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Electrocrystallization of Tetrathiafulvalene Charge-Transfer Salt

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