Potential-Controlled Electrochemical Seed-Mediated Growth of Gold

pubs.acs.org/Langmuir © 2010 American Chemical Society

Potential-Controlled Electrochemical Seed-Mediated Growth of Gold Nanorods Directly on Electrode Surfaces Lina G. Abdelmoti and Francis P. Zamborini* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292 Received April 23, 2010. Revised Manuscript Received June 19, 2010 Here we compare the amount and the morphology of Au nanostructures electrodeposited from a solution containing 2.5 10-4 M AuCl4- and 0.1 M cetyltrimethylammonium bromide (CTAB) onto nonseeded and Au-nanoparticle (NP)-seeded mercaptopropyltrimethoxysilane (MPTMS)-functionalized glass/indium tin oxide (glass/ITO) electrodes as a function of the electrode potential and deposition time. The method is similar to the previously reported seed-mediated chemical synthesis of Au nanorods (NRs) in solution and on surfaces, except that we replace the chemical reducing agent (ascorbic acid) with the electrochemical potential. The deposition can be classified into three different potential ranges on the nonseeded and seeded electrodes on the basis of the amount of Au deposited and the morphology of the deposited nanostructures. On the nonseeded glass/ITO/MPTMS electrode, at potentials ranging from -0.30 to -0.20 V, there are a significant number of Au deposits on the surface with mainly branched morphology. At deposition potentials ranging from -0.10 to 0.27 V, there is very little deposition of Au but the few deposits also have a branched morphology. At 0.27 V and higher, there is no Au deposition on the glass/ITO/MPTMS electrode. Because Au seeds catalyze Au deposition, the three potential ranges, the amount of Au, and the morphologies are quite different on the glass/ITO/MPTMS/Au NP seed electrodes compared to those on the nonseeded glass/ITO/MPTMS electrodes. There is a significant amount of Au (more than on the nonseeded electrode) on the surface over a wider range of potentials from -0.30 to 0.27 V, and they have spherical morphology. From 0.30 to 0.35 V, less Au deposits on the electrode and there are 5-15% Au NRs on the surface in addition to spherical NPs. Above 0.35 V, there is no Au deposition on the glass/ITO/MPTMS/Au seed electrode. For depositions within the potential range of 0.30 to 0.35 V on glass/ITO/MPTMS/Au seed electrodes, the size and shape distributions of the Au nanostructures, including NRs, are similar to those previously synthesized by chemical seed-mediated growth in solution and directly on nonconductive surfaces. The yield, length, and aspect ratio of the Au NRs depend on the deposition time; the average length ranges from about 100 to 400 nm for times of 30 to 120 min. The electrochemical seed-mediated growth of Au is optimal from 0.30 to 0.35 V versus Ag/AgCl under our conditions, which could be useful for enhancing the signal in sensing strategies that employ Au NPs as optical or electrochemical tags.

Introduction Metallic nanostructures have attracted a tremendous amount of attention in recent years. Au nanoparticles (NPs),1 in particular, have been widely studied because they are stable against air oxidation and exhibit fascinating size-2 and shape3,4-dependent *Author to whom correspondence should be addressed. E-mail: f.zamborini@ louisville.edu. Fax: 502-852-8149.

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Published on Web 07/21/2010

DOI: 10.1021/la101639u

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NRs have been previously synthesized by electrochemical,26-29 chemical,30-37 photochemical,38,39 and combined chemical/ photochemical40 methods. The electrochemical methods include the formation of Au NRs in solution29 and their synthesis in hard templates.26-28 Although it was originally used to synthesize sizecontrolled spherical Au NPs,41,42 the seed-mediated growth method has emerged over the past decade as the most common chemical method for synthesizing Au NRs.30-37 The first step of the seed-mediated growth method involves the synthesis of 4-nm-diameter spherical Au seed NPs using sodium borohydride as the reducing agent and citrate as the stabilizer.33 These seeds then act as nucleation sites for the preferential reduction of an Au salt or complex (usually AuCl4-), where ascorbic acid and cetyltrimethylammonium bromide (CTAB) are most commonly used as the reducing agent and surfactant, respectively. The weak reducing agent and surfactant stabilizer prevent the formation of new nucleation sites, ensuring that Au reduction occurs only on seed NPs in solution30-37 or attached to surfaces.43-54 It is thought that CTAB binds preferentially to certain crystal faces of the growing metal seeds, which leads to anisotropic 1D growth for a small number of NPs (4-15%).25,35,37,55 Centrifugation is usually necessary to obtain samples of nearly pure NRs. Variations of the normal procedure have led to the synthesis of other shaped Au nanostructures56 and the high-yield synthesis of short, small-aspect-ratio Au NRs by altering the seed37 or adding Agþ to the solution.57 Improved purity and yield have led to numerous applications for Au NRs, including fluorescence58 and Raman enhancement,59,60 localized surface plasmon (36) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80–82. (37) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957–1962. (38) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316–14317. (39) Sakamoto, M.; Fujistuka, M.; Majima, T. J. Photochem. Photobiol. C 2009, 10, 33–56. (40) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376–2377. (41) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306–313. (42) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313– 2322. (43) Mieszawska, A. J.; Jalilian, R.; Sumanasekera, G. U.; Zamborini, F. P. J. Am. Chem. Soc. 2005, 127, 10822–10823. (44) Mieszawska, A. J.; Slawinski, G. W.; Zamborini, F. P. J. Am. Chem. Soc. 2006, 128, 5622–5623. (45) Mieszawska, A. J.; Zamborini, F. P. Chem. Mater. 2005, 17, 3415–3420. (46) Wei, Z.; Mieszawska, A. J.; Zamborini, F. P. Langmuir 2004, 20, 4322– 4326. (47) Wei, Z.; Zamborini, F. P. Langmuir 2004, 20, 11301–11304. (48) Kambayashi, M.; Zhang, J.; Oyama, M. Cryst. Growth Des. 2005, 5, 81–84. (49) Lin, Z.; Chen, X; Cai, Z.; Oyama, M.; Chen, X.; Wang, X. Cryst. Growth Des. 2008, 8, 863–868. (50) Zhang, J.; Kambayashi, M.; Oyama, M. Electrochem. Commun. 2004, 6, 683–688. (51) Zhang, J.; Kambayashi, M.; Oyama, M. Electroanalysis 2005, 17, 408–416. (52) Lee, K.-H.; Huang, K.-M.; Tseng, W.-L.; Chiu, T.-C.; Lin, Y.-W.; Chang, H.-T. Langmuir 2007, 23, 1435–1442. (53) Liao, H.; Hafner, J. H. J. Phys. Chem. B 2004, 108, 19276–19280. (54) Taub, N.; Krichevski, O.; Markovich, G. J. Phys. Chem. B 2003, 107, 11579–11582. (55) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771–774. (56) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648–8649. (57) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414–6420. (58) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517–523. (59) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372–3378. (60) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17–23. (61) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Stone, J. W.; Sisco, P. N.; Alkilany, A.; Kinard, B. E.; Hankins, P. Chem. Commun. 2008, 544–557. (62) Marinakos, S. M.; Chen, S.; Chilkoti, A. Anal. Chem. 2007, 79, 5278–5283. (63) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 2115–2120. (64) Stone, J. W.; Sisco, P. N.; Goldsmith, E. C.; Baxter, S. C.; Murphy, C. J. NanoLett. 2007, 7, 116–119. (65) Szawinski, G. W.; Zamborini, F. P. Langmuir 2007, 23, 10357–10365. (66) Mieszawska, A. J.; Jalilian, R.; Sumanasekera, G. U.; Zamborini, F. P. Small 2007, 3, 722–756.

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resonance (LSPR) sensing,61,62 cancer therapeutics,63 and cellular tracking,64 to name a few. Our group has recently used the seed-mediated growth method to synthesize Au43-47 and Ag65 NRs on surfaces that contain attached seed NPs, leading to the formation and assembly of well-spaced NRs directly on surfaces. We demonstrated the synthesis,46,47 patterning,45 alignment,44,65 and use of atomic force microscopy (AFM) to manipulate46 and monitor47 the growth of Au and/or Ag NRs directly on surfaces. We also synthesized 1D Au NR/single-walled carbon nanotubes43 and Au NR/GaAs nanowire66 heterojunctions directly on surfaces using the seed-mediated growth approach. All of these examples involved the direct growth of Au NRs on nonconductive surfaces or on semiconductor nanowires using ascorbic acid as the chemical reducing agent and CTAB as the surfactant. Here we describe the synthesis of Au NRs on indium tin oxidecoated glass (glass/ITO) electrodes by the electrodeposition of Au in the presence of CTAB, where we replace the chemical reducing agent (ascorbic acid) with electrochemical potential. Instead of ascorbic acid, the source of electrons for reduction is the glass/ITO electrode. We note that Oyama and co-workers recently studied the chemical seed-mediated growth of Au on glass/ITO electrodes, where ascorbic acid served as the reducing agent.48-51 Tian et al. recently demonstrated the shape control of Au electrodeposited on an Au film by varying the electrode potential.67 Others studied electrodeposited Au as a function of potential68,69 or time70 on glass/ITO, with Au seed in some cases70 but not in the presence of CTAB. These led to flowerlike or high-surface-area structures with applications in surface-enhanced Raman spectroscopy (SERS)68 and LSPR sensing69 and as superhydrophobic surfaces.70 This report is the first describing the electrochemical seed-mediated growth of Au on glass/ITO in the presence of CTAB as a function of electrode potential and deposition time. This work is important for three main reasons. First, we are able to study the effect of the reduction potential on the growth of the Au nanostructures in the presence of CTAB for the first time. This may lead to improved control over the size and shape of the nanostructures and a better understanding of the seed-mediated growth process. Although the reducing strength of ascorbic acid can be controlled by the pH in solution, other chemical effects may take place by changing the pH; the reducing strength is not as well-controlled compared to using the electrode potential, and in solution the ascorbic acid is used up over time, whereas the source of electrons is constant and unlimited on an electrode surface. Second, if successful, our synthesis method would allow the direct assembly of 1D Au NRs or other shapes on electrode surfaces, which could be useful for size- and shape-dependent electrocatalysis, electrochemical sensing, and nanoelectronics studies. Third, finding the optimal conditions for electrochemical seedmediated growth could lead to improved sensitivity in biosensing schemes that utilize Au NPs as optical71 or electrochemical tags.72

Experimental Section Chemicals. NANOpure water (Barnstead, resistivity=18 MΩ cm) was used for all aqueous solutions and for rinsing substrates. Sodium borohydride (NaBH4, 98.0%) was purchased from (67) Tian, Y.; Liu, H.; Zhao, G.; Tatsuma, T. J. Phys. Chem. B 2006, 110, 23478– 23481. (68) Guo, S.; Wang, L.; Wang, E. Chem. Commun. 2007, 3163–3165. (69) Sakai, N.; Fujiwara, Y.; Arai, M.; Yu, K.; Tatsuma, T. J. Electroanal. Chem. 2009, 628, 7–15. (70) Praig, V. G.; Piret, G.; Manesse, M.; Castel, X.; Boukherroub, R.; Szunerits, S. Electrochim. Acta 2008, 53, 7838–7844. (71) Kim, D.; Daniel, W. L.; Mirkin, C. A. Anal. Chem. 2009, 81, 9183–9187. (72) De la Escosura-Mu~niz, A.; Maltez-da Costa, M.; Merkoc-i, A. Biosens. Bioelectron. 2009, 24, 2475–2482.

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Scheme 1. Setup for the Electrochemical Seed-Mediated Growth of Au

Aldrich, citric acid trisodium salt was purchased from BioRad, cetyltrimethylammonium bromide (CTAB, g 99.0%) and mercaptopropyltrimethoxysilane (MPTMS, g97.0%) were purchased from Fluka, isopropyl alcohol (99.8%) was purchased from Pharmco-AAPER, acetone (99.9þ%) was purchased from Burdic & Jackson, and HAuCl4 3 3H2O was synthesized from metallic Au. Synthesis of Au Seed Nanoparticles (NPs). A solution of 2.5 10-4 M citric acid trisodium salt and 2.5 10-4 M HAuCl4 was prepared by combining 2.5 mL of 0.01 M citric acid trisodium salt, 2.5 mL of 0.01 M HAuCl4, and 95 mL of NANOpure water, which gave a yellow solution. Then, 3.0 mL of 0.1 M ice-cold NaBH4 was added to the solution, turning the color from yellow to reddish brown, indicating the formation of Au seed NPs. The solution was allowed to stir for 2-5 h prior to use. This seed solution contained 4 nm average-diameter citrate-capped Au NPs.33 Electrode Modification. Single-sided 7 50 0.9 mm3 indium tin oxide (ITO)-coated glass electrodes with an 8-12 Ω resistance (Delta Technologies, Stillwater, MN) were cleaned by sonication for 10 min each in NANOpure water, acetone, and isopropyl alcohol sequentially. The cleaned glass/ITO electrode was functionalized by immersion for 20 min in a solution containing 10 mL of isopropyl alcohol, 1 to 2 drops of water, and 100 μL of mercaptopropyltrimethoxysilane (MPTMS) that was heated to just below boiling (50-60 °C). This led to a surface terminated with thiol groups. The glass/ITO/MPTMS was then rinsed and sonicated for 10 min in isopropyl alcohol and dried under N2 before being immersed in the Au NP seed solution for 20 min, which led to the attachment of the Au NPs to the glass/ITO/ MPTMS surface through strong Au-thiolate interactions. The resulting glass/ITO/MPTMS/Au seed electrode was rinsed with nanopure water and dried under N2 before electrochemical studies. Cyclic Voltammetry. Cyclic voltammograms of glass/ITO/ MPTMS and glass/ITO/MPTMS/Au seed were obtained in 0.1 M CTAB, 0.1 M CTAB plus 2.5 10-4 M HAuCl4, 0.1 M KBr, and 0.1 M KBr plus 2.5 10-4 M HAuCl4 as noted in the main text using a CH Instruments electrochemical workstation (Austin, TX) with an Ag/AgCl reference electrode and Pt wire counter electrode at a scan rate of 100 mV/s. Au Electrodeposition. The nonseeded glass/ITO/MPTMS and seeded glass/ITO/MPTMS/Au seed electrodes were used as two working electrodes along with an Ag/AgCl reference electrode and a Pt wire counter electrode in an electrochemical cell with a deposition solution containing 2.5 10-4 M HAuCl4 and 0.1 M CTAB electrolyte. Scheme 1 shows the setup. The Au Langmuir 2010, 26(16), 13511–13521

Figure 1. Cyclic voltammograms (CVs) of a glass/ITO/MPTMS electrode in (A) 0.1 M KBr (---) and 0.1 M KBr plus 2.5 10-4 M HAuCl4 (-) from 1.2 to -1.2 V and (B) 0.1 M CTAB (---) and 0.1 M CTAB plus 2.5 10-4 M HAuCl4 (-) from 1.0 to -1.0 V. deposition was performed using a CH Instruments electrochemical workstation (Austin, TX) operating in cyclic voltammetry mode as follows. First, both the nonseeded and seeded electrodes were attached to the working electrode lead and held out of the solution while the reference and counter electrodes were placed into the deposition solution. Next, with the two working electrodes still out of the solution, the potential was scanned from þ1.00 V to a final potential x, where x was varied from þ0.50 to -0.30 V. Once potential x was reached, it was held constant (paused) and the two working electrodes were lowered into the solution under potential control for an amount of time ranging from 30 to 120 min as indicated in the main text. After the desired time, both working electrodes were then removed from the solution while the potential was held constant at x, rinsed thoroughly with nanopure water, and dried under N2 prior to spectroscopic and microscopic characterization. With this procedure, the two working electrodes were inserted into and removed from the deposition solution under potential control for a desired period of time, ensuring that the samples never experienced another potential other than the desired potential x for any amount of time. Nanoparticle Characterization. The Au nanostructures deposited on glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed were characterized optically using a Varian Cary 50 Bio UVvisible spectrometer by scanning from 300 to 1000 nm in fast scan mode using the same glass/ITO/MPTMS slide prior to Au electrodeposition as the background because using a different glass/ITO slide led to artifacts in the spectra such as negative absorbance values. We believe that these are due to variations in the ITO film from slide to slide. Scanning electron microscopy (SEM) images were obtained using either a Carl Zeiss SMT AG Supra 35 VP field-emission scanning electron microscope (FESEM) or an FEI NOVA nanoSEM 600. The accelerating voltages were 20.0 and 7.00 keV, respectively.

Results and Discussion Cyclic Voltammetry Studies. Figure 1A shows cyclic voltammograms (CVs) of a glass/ITO/MPTMS electrode in 0.1 M KBr only and 0.1 M KBr plus 2.5 10-4 M HAuCl4, and Figure 1B shows a glass/ITO/MPTMS electrode in 0.1 M CTAB only and 0.1 M CTAB plus 2.5 10-4 M HAuCl4. These CVs DOI: 10.1021/la101639u

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show the effect of the CTAþ cation in comparison to Kþ on the electrodeposition of Au. The CVs of the electrode in KBr and CTAB solutions are similar only in that there is very little electrochemical activity over the scan range until about -1.0 V (dashed CVs in frames A and B). The reduction peak below -1.0 V is likely due to the reduction of water. In the presence of HAuCl4, the CVs are very different (solid CVs in frames A and B). In the presence of KBr electrolyte, the first reduction peak is at about 0.4 V, followed by two other reduction peaks at around -0.4 and -0.8 V. We attribute the first peak to the reduction of AuBr4- according to one or both of the following electrochemical reactions: AuIII Br4 - þ 3e - a Au0 þ 4Br - E0 ¼ 0:858 V vs NHE ð1Þ AuIII Br4 - þ 2e - a AuI Br2 - þ 2Br - E0 ¼ 0:805 V vs NHE ð2Þ

The standard reduction potentials of both of these reactions are similar and consistent with the first peak observed in the CV considering that our reference electrode is Ag/AgCl (∼0.222 V vs NHE) and there is a large concentration of Br- in solution (0.1 M). Note that we discuss the reactions of AuBr4- instead of those of AuCl4-. This is because the excess Br- from KBr (or CTAB) will displace the Cl- of AuCl4- according to the following reaction: AuCl4 - þ 4Br - a AuBr4 - þ 4Cl -

ð3Þ

We observe a noticeable color change from AuCl4- to AuBr4and have direct spectroscopic evidence of this reaction. There is another possible electrochemical and chemical reaction that can occur upon reduction of AuBr4- to AuBr2- as follows: ðelectrochemicalÞ AuI Br2 - þ e - a Au0 þ 2Br E0 ¼ 0:963 V vs NHE ðchemicalÞ 3AuI Br2 - a 2Au0 þ AuIII Br4 - þ 2Br -

ð4Þ ð5Þ

Reaction 4 occurs at a more positive potential than does reaction 2; therefore, Au0 should form upon the formation of AuBr2-. Reaction 5 is a chemical disproportionation reaction that can result in the deposition of Au0 on the electrode after the formation of AuBr2-, but we do not have direct evidence that this reaction occurs. We are not certain about the reactions leading to the reduction peaks at -0.4 and -0.8 V and did not probe these further because we mainly focused on Au reduction in the presence of CTAB as described next. The peaks at 0.8 and 1.1 V on the reverse scan correspond to the oxidation of Au to form AuBr2-, AuBr4-, or surface oxides. The CV of glass/ITO/MPTMS in the presence of CTAB plus AuCl4- is drastically different. Because of the excess Br- in solution, the Au complex should again be in the form of AuBr4because of reaction 3. The AuBr4- reduction peak shifts drastically negative by ∼1.1 V in the presence of CTAþ compared to the reduction in the presence of Kþ. There is no significant reduction peak until about -0.7 V as compared to þ0.4 V in KBr. Also, the magnitude of the current is smaller than that in KBr electrolyte. The formation of CTAþ-AuBr4- complexes within micelles leads to the drastic negative shift in the reduction potential. The crossover of the CV upon the reverse scan in frame B occurs because the reduction potential of AuBr4- shifts positively after 13514 DOI: 10.1021/la101639u

Figure 2. CVs of (A) glass/ITO/MPTMS in 0.1 M CTAB (---) and 0.1 M CTAB plus 2.5 10-4 M HAuCl4 (-) and (B) glass/ITO/ MPTMS/Au seed in 0.1 M CTAB (---) and 0.1 M CTAB plus 2.5 10-4 M HAuCl4 (-) from 0.1 to -0.9 V. Linear sweep voltammograms (LSVs) of glass/ITO/MPTMS (---) and glass/ITO/MPTMS/ Au seed (-) in 0.1 M CTAB plus 2.5 10-4 M HAuCl4 from (C) 0.1 to -0.9 V and from (D) 0.0 to 1.0 V following four cycles from 0.1 to -0.9 V.

some Au0 is deposited on the surface. This leads to a significant cathodic current at -0.4 V upon the reverse scan because the Au deposited on the forward scan catalyzes the reduction of AuBr4in solution at more positive potentials. In other words, reactions 1 and 2 occur at more positive potentials on a glass/ITO/MPTMS/ Au electrode than on a glass/ITO/MPTMS electrode. This is the basis for seed-mediated growth, where metal ion reduction is more favorable on existing metal seed particles in comparison to reduction on a nonactive surface. Figure 2 (frames A and B) shows CVs of glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed electrodes in 0.1 M CTAB and 0.1 M CTAB plus 2.5 10-4 M HAuCl4, focusing on the region from þ0.1 to -0.9 V. Frame A shows the glass/ITO/MPTMS electrode, which is very similar to Figure 1. There is very little electrochemical activity in CTAB only and significant Au reduction around -0.7 V in the presence of HAuCl4 with the characteristic crossover in the CV discussed earlier. Frame B shows the glass/ITO/MPTMS/Au seed electrode, which has two main differences compared to glass/ITO/MPTMS. First, the CV in CTAB alone shows a higher current from -0.6 to -0.9 V. This is likely due to faster electron-transfer kinetics for oxygen or solvent reduction on Au seed NPs compared to that on glass/ITO, leading to a larger current in the electrolyte only. In the presence of HAuCl4, the potential for significant Au reduction occurs at about -0.5 V, which is ∼200 mV more positive than for glass/ ITO/MPTMS, and there is less crossover in the CV on the reverse scan. The linear sweep voltammograms (LSVs) from 0.1 to -0.9 V in Figure 2C show the difference in the reduction of AuBr4- on glass/ITO/MPTMS versus that on glass/ITO/MPTMS/Au seed more clearly because there was no crossover in the plots after we removed the reverse scan. There is clearly a significant cathodic current at -0.5 V for glass/ITO/MPTMS/Au seed in comparison to -0.7 V for glass/ITO/MPTMS, showing that the Au seed NPs attached to the surface catalyze the reduction of AuBr4-. Figure 2D shows the subsequent oxidation of the Au metal on the surface at ∼0.6 V after four reduction cycles on glass/ITO/ MPTMS and glass/ITO/MPTMS/Au seed from 0.1 to -0.9 V. The Langmuir 2010, 26(16), 13511–13521


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Figure 3. UV-visible spectroscopy of glass/ITO/MPTMS (blue)

and glass/ITO/MPTMS/Au seed (red) electrodes held at (A) -0.30, (B) 0.0, (C) 0.30, and (D) 0.50 V for 30 min in 0.1 M CTAB plus 2.5 10-4 M HAuCl4 solution.

Figure 5. UV-vis spectra of a glass/ITO/MPTMS/Au seed electrode held at (A) 0.30, (B) 0.32, and (C) 0.35 V in 0.1 M CTAB plus 2.5 10-4 M HAuCl4 for 30 (blue), 60 (red), and 120 min (black). The absorbance increased with increasing Au deposition time in all cases.

Figure 4. Potential versus (A) the average maximum absorbance of glass/ITO/MPTMS (blue) and glass/ITO/MPTMS/Au seed (red) electrodes after being held at the indicated potential for 30 min in a solution of 0.1 M CTAB plus 2.5 10-4 M HAuCl4 and (B) the difference in the maximum absorbance values from frame A as a function of electrode potential. Averages and standard deviations are from at least three electrodes. The dashed lines were drawn manually to show the general trend. The potential ranges defined as regions Ia, IIa, and IIIa refer to deposition potentials on nonseeded glass/ITO/MPTMS electrodes, and those defined as regions Ib, IIb, and IIIb refer to deposition potentials on seeded glass/ITO/MPTMS/Au seed electrodes as described in the text.

larger peak current for the glass/ITO/MPTMS/Au seed shows that more Au deposited on the Au-seed-coated surface, again due to the ability of Au NPs to catalyze the reduction of AuBr4-. Spectroscopy Studies. We obtained UV-vis spectra for all of the glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed working Langmuir 2010, 26(16), 13511–13521

Figure 6. Plot of the average maximum absorbance as a function of Au deposition time for three slides of (A) glass/ITO/MPTMS and (B) glass/ITO/MPTMS/Au seed electrodes held at -0.30 (black dashed line), 0.30 (blue diamonds), 0.32 (red triangles), and 0.35 V (green diamonds) in 0.1 M CTAB plus 2.5 10-4 M HAuCl4. (C) Difference in the average maximum absorbance between the two electrodes (B - A) as a function of time. DOI: 10.1021/la101639u

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Figure 7. SEM images of glass/ITO/MPTMS (A, C, E) and glass/ITO/MPTMS/Au seed electrodes (B, D, F) held at -0.2, 0.0, and 0.2 V, respectively, in 0.1 M CTAB plus 2.5 10-4 M HAuCl4 for 30 min.

electrodes after the Au deposition procedure to qualitatively determine the amount of Au deposited on the surface as a function of electrode potential and time. Metallic Au has a characteristic localized surface plasmon resonance (LSPR) band in the visible/ near-infrared range (520 - 2000 nm), depending on the exact size and shape of the structures,5,6 whose magnitude can be used to qualitatively determine the amount of Au on the surface. Figure 3 shows an example of the spectra of glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed obtained after Au deposition for 30 min at -0.3, 0.0, 0.3, and 0.5 V (frames A-D, respectively). In all cases, the maximum absorbance decreased as the potential increased, as expected because a more negative reduction potential will facilitate the reduction of AuBr4- to form Au0 deposits. At potentials of -0.3, 0.0, and 0.3 V, the absorbance for the glass/ ITO/MPTMS/Au seed exceeded that of the glass/ITO/MPTMS, showing more Au deposition on the Au-seeded surfaces. This is consistent with the voltammetry and the fact that the Au seed catalyzes Au deposition. Also, the Au deposited on glass/ITO/ MPTMS/Au seed surfaces has a well-pronounced peak at about 530-550 nm, which is attributed to the LSPR band for spherical 13516 DOI: 10.1021/la101639u

Au NPs. The absence of this peak on the glass/ITO/MPTMS surface (especially at -0.3 V) suggests that the deposited Au has a very different morphology. At 0.5 V (frame D), there is no Au deposition, showing that the potential is not negative enough to reduce AuBr4- on either surface. It is interesting that Au deposition occurs on both surfaces at a potential far more positive than the onset of current observed in the LSVs of Figure 2C, which is about -0.6 and -0.4 V for the glass/ITO/MPTMS and glass/ITO/ MPTMS/Au seed electrodes, respectively. The presence of CTAB slows the reaction rate so that there is no observable current in the LSV until these potentials are reached, but the reaction (reduction of AuBr4-) is thermodynamically favorable at more positive potentials than that observed in the LSV. Over the 30 min deposition, the presence of Au became noticeable on the surface up to potentials of about 0.2 and 0.35 V for glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed, respectively, in the UV-vis data and/or scanning electron microscopy (SEM) images as described later. The Au deposition process is considered to be seed-mediated electrochemical deposition only if Au deposition occurs selectively Langmuir 2010, 26(16), 13511–13521


Article

Figure 8. SEM images of glass/ITO/MPTMS (A, C, E) and glass/ITO/MPTMS/Au seed (B, D, F) electrodes held at 0.30 V for 30 (A, B), 60 (C, D), and 120 min (E, F) in a solution of 0.1 M CTAB plus 2.5 10-4 M HAuCl4.

on the glass/ITO/MPTMS/Au seed and not the glass/ITO/MPTMS surface. Figure 4A very roughly quantifies the amount of Au deposited on both electrodes by plotting the maximum absorbance from the UV-vis spectrum of each electrode as a function of the electrode potential using 30 min deposition times. The average and standard deviations shown are from at least three measurements at each potential. The average absorbance as a function of potential was very different for the nonseeded and Au-seeded glass/ITO electrodes. For both electrodes, the average absorbance generally decreased with increasing potential. As shown in Figure 4A, we define three different potential regions on the basis of the amount of Au deposited on the nonseeded (regions Ia, IIa, and IIIa) and Au-seeded (regions Ib, IIb, and IIIb) glass/ITO electrodes. For the nonseeded glass/ITO/MPTMS electrode, there was significant Au deposition at potentials defined by region Ia (-0.3 Langmuir 2010, 26(16), 13511–13521

to -0.2 V) leading to an average absorbance of between 0.02 and 0.03. Au deposition in region IIa (-0.1 to 0.2 V) led to a lower average absorbance of about 0.005, indicating a much smaller amount of Au deposition. At potentials defined by region IIIa (g0.27 V), the absorbance was zero, indicating no Au deposition at or above this potential. For the glass/ITO/MPTMS/Au seed electrode, three regions also existed, but the potential ranges and amount of Au deposited were very different compared to those of the nonseeded electrode because Au seed catalyzes Au deposition. For example, significant deposition occurred over a wider range of potentials defined as region Ib (-0.3 to 0.27 V), where the average absorbance was between 0.04 and 0.06, or 2-10 times larger than for the nonseeded electrode. For depositions at potentials defined by region IIb (0.30 to 0.35 V), the absorbance dropped to between 0.01 and 0.03, showing a smaller but still significant amount of Au deposition on the surface. At these DOI: 10.1021/la101639u

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potentials, the absorbance was zero on the nonseeded electrode. At potentials defined by region IIIb (g0.40 V), the absorbance was insignificant, showing no Au deposition. In comparison, no Au deposition on the nonseeded surface occurred at g0.27 V. Figure 4B shows the difference in the average maximum absorbance between the glass/ITO/MPTMS/Au seed and glass/ ITO/MPTMS as a function of potential for deposition times of 30 min. The difference is small at potentials e-0.2 V because a significant amount of Au deposits on both electrodes. The difference is zero at potentials g0.4 V because no Au deposition occurs on either electrode. The maximum difference between Au deposition on the two electrodes occurs between -0.1 and 0.27 V, however, there is still some deposition up to 0.2 V on the nonseeded surface. For seed-mediated growth, there should ideally be no Au deposition in the absence of the Au seed. On the basis of the UV-vis data and SEM images shown later, the optimum region for seed-mediated growth is in the range of 0.30 to 0.35 V because there is a fairly large difference in Au deposition between the 13518 DOI: 10.1021/la101639u

seeded and nonseeded electrodes and the absorbance is essentially zero on the nonseeded electrode. We note that there are large standard deviations at some of the potentials but we do not know the exact reason for this. The lines in the plots of Figure 4 were manually drawn to serve as a guide to the eye showing the general trend. We focused on the potentials from 0.30 to 0.35 V for timedependent studies because these potentials appeared to have the least amount of Au deposited on the glass/ITO/MPTMS surface, indicating that primarily seed-mediated growth occurred. Figure 5 shows the UV-vis spectra of glass/ITO/MPTMS/Au seed electrodes held at 0.30 (frame A), 0.32 (frame B), and 0.35 V (frame C) for 30, 60, and 120 min in a solution of 0.1 M CTAB plus 2.5 10-4 M HAuCl4. At these three potentials, the amount of Au deposited did not change significantly as a function of potential, but the data clearly shows that the amount of Au deposited increased with increasing deposition time. Figure 6 shows plots of the maximum absorbance as a function of deposition time (30, 60, 120 min) of glass/ITO/MPTMS Langmuir 2010, 26(16), 13511–13521


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electrodes (frame A) that we poised at -0.30, 0.30, 0.32, and 0.35 V in the Au deposition solution as compared to that of glass/ITO/ MPTMS/Au seed electrodes (frame B) that we poised at the same potentials and times. On the nonseeded electrodes, the absorbance remained at about zero for all times at 0.30-0.35 V, indicating no Au deposition even at longer times. At -0.3 V, the absorbance reached an average of about 0.2 after 120 min, indicating significant Au deposition that increased with time. The absorbance increased exponentially, likely because of a nucleation and growth process on the electrode surface. On the seeded electrodes, the absorbance increased approximately linearly at deposition potentials of 0.30-0.35 V, with a final average absorbance of about 0.1. This indicates a kinetically controlled deposition process on the Au NP seed nucleation sites. If the deposition was diffusion-controlled, then the amount of Au deposited would increase rapidly at short times and level off at longer times. At -0.3 V, the deposition of Au increased exponentially up to an average absorbance of about 0.4. The overall absorbance was likely due to the combination of Langmuir 2010, 26(16), 13511–13521

nucleation and growth directly on the electrode and seeded growth on the Au NP seeds. Frame C of Figure 6 shows a plot of the difference in the absorbance between the Au seeded and nonseeded electrodes held at the various potentials for different times. This data is important because it shows that the absorbance signal for Au NPs on an electrode can be enhanced electrochemically. This could be useful for chemical or biochemical sensing if the Au NPs served as an optical or electrochemical tag.71,72 Seed-mediated electrochemical deposition could enhance the signal and lower the detection limits. The plot shows that the differences in absorbance between a glass/ITO/MPTMS/Au seed electrode and a glass/ITO/MPTMS electrode are similar at all potentials for 30 and 60 min deposition times. After 120 min, the electrode held at -0.3 V begins to separate from the electrodes held at 0.30-0.35 V. Considering the fact that Au deposition occurs on the electrode at -0.3 V even in the absence of Au NPs, electrode potentials of 0.30-0.35 V would be better for the signal amplification of Au NPs as optical tags. DOI: 10.1021/la101639u

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Table 1. Statistical Data on the Nanorods and Nanoparticles Grown by Electrochemical Seed-Mediated Growth As a Function of Potential and Time potential (V) growth time (min)

0.30 30

60

0.32 120

rod length (nm) 163 ((31) 292 ((110) 185 ((68) aspect ratio 5.4 ((2.0) 7.8 ((5.1) 5.1 ((2.3) particle diameter (nm) 77 ((19) 94 ((34) 76 ((24) rod % yield 8.1 ((3.6) 10.0 ((2.9) 5.1 ((2.8) 9 ((3) 16 ((16) 32((21)a coverage (particles/μm2) a Performed on a different batch of glass/ITO electrodes.

60

120

30

60

120

181 ((57) 6.1 ((2.2) 64 ((20) 7.4 ((1.2) 10 ((6)

204 ((72) 7.3 ((3.7) 69 ((24) 8.3 ((3.3) 36 ((29)

272 ((95) 8.4 ((4.6) 67 ((17) 9.9 ((1.1) 42((15)a

176 ((100) 8.3 ((5.7) 68 ((20) 9.8 ((2.8) 11 ((3)

418 ((150) 6.7 ((4.7) 140 ((56) 9.6 ((1.8) 6 ((1)

389 ((190) 15.7 ((9.3) 99 ((24) 13.0 ((3.1) 13 ((3)

At these potentials, there is relatively little or no signal in the absence of Au NPs even up to 120 min of deposition time (Figure 6A). Microscopy Studies. We obtained SEM images of the various glass/ITO/MPTMS and glass/ITO/MPTMS/Au seed electrodes at the different potentials shown in Figure 4 and at different times (30, 60, and 120 min). Figure 7 shows SEM images of glass/ITO/ MPTMS and glass/ITO/MPTMS/Au seed electrodes that were held at -0.2 (frames A and B), 0.0 (frames C and D), and 0.2 V (frames E and F) for 30 min in the Au deposition solution. In all cases, there was some Au deposition on the glass/ITO/MPTMS electrodes as expected on the basis of the UV-vis data. The images show regions that contained Au, but there were also regions largely absent of any deposits. Note that the larger amount of Au shown in frame E does not necessarily indicate more Au deposition on this electrode because many other areas showed little or no Au deposited at all on the surface. Interestingly, the Au deposited in the form of branched structures on the nonseeded electrodes. As suggested by the UV-vis data (Figure 3), this morphology was quite different from the spherical morphology observed on all of the glass/ITO/MPTMS/Au seed electrodes. Previous studies on the seed-mediated growth of Au NPs in solution and on surfaces in the presence of 0.1 M CTAB with ascorbic acid as the reducing agent showed the presence of 5-15% Au NRs.34,46,54 In contrast, we did not observe any obvious NR structures at these potentials during electrochemical reduction, except for possibly a few very short rods at 0.0 V. Figure 8 shows glass/ITO/MPTMS and glass/ITO/MPTMS/ Au seed electrodes after the deposition of Au at 0.30 V for 30, 60, and 120 min. At this potential, there were fewer Au deposits on the glass/ITO/MPTMS electrodes, consistent with the UV-vis data, and there were a significant number of Au NRs on the glass/ ITO/MPTMS/Au seed electrodes along with several spherical particles. The average lengths of the Au NRs were 163 ( 31, 292 ( 112, and 185 ( 68 nm, and the average aspect ratios were 5.4 ( 2.0, 7.8 ( 5.1, and 5.1 ( 2.3 for the 30, 60, and 120 min deposition times, respectively. The NR yield ranged from 5 to 10%. When comparing the morphology of the Au deposited at 0.30 V with that deposited at 0.20 V, which contained only spherical NPs, it is clear that the electrode potential has a drastic effect on the shape of the Au nanostructures. Figure 9 shows glass/ITO/MPTMS and glass/ITO/MPTMS/ Au seed electrodes after the deposition of Au in 0.1 M CTAB plus 2.5 10-4 M HAuCl4 at 0.32 V for 30, 60, and 120 min. Again, no visible Au was deposited on the glass/ITO/MPTMS at this potential, and a significant percentage of NRs formed on the glass/ITO/MPTMS/Au seed electrodes. The average lengths of the NRs were 181 ( 57, 204 ( 72, and 272 ( 95 nm, and the aspect ratios were 6.1 ( 2.2, 7.3 ( 3.7, and 8.4 ( 4.6 for depositions times of 30, 60, and 120 min, respectively. The yield of the NRs ranged from 7 to 10%. Figure 10 shows glass/ITO/MPTMS and glass/ ITO/MPTMS/Au electrodes after the deposition of Au at 0.35 V for 30, 60, and 120 min. No deposition occurred on the electrode 13520 DOI: 10.1021/la101639u

0.35

30

without Au seeds, and Au NRs formed on the Au NP-seeded electrode. The average lengths of NRs were 176 ( 100, 418 ( 150, and 389 ( 190 nm, and the average aspect ratios were 8.3 ( 5.7, 6.7 ( 4.7, and 15.7 ( 9.3 for deposition times of 30, 60, and 120 min, respectively. The yield ranged from 9 to 13%. Clearly, reduction potentials of 0.30-0.35 V favor the growth of Au NRs compared to more negative reduction potentials, where spheres primarily formed. The dimensions of the NRs varied within the 0.30-0.35 V range as a function of potential and growth time. Table 1 compares the average length, aspect ratio, and yield of Au NRs deposited on glass/ITO/MPTMS/Au seed electrodes at 0.30, 0.32, and 0.35 V for 30, 60, and 120 min. In general, the average length of the NRs increased from 30 to 60 min at all potentials. It continued to increase after 120 min of deposition time at 0.32 V but decreased or remained similar at potentials of 0.30 and 0.35 V. These results may be due to some irreproducibility in the deposition process. The length of the NRs also generally appears to increase with increasing deposition potential. The density of nanostructures (particles, rods, and other shapes) on the surface ranged from 6 to 16 per μm2 for 30 and 60 min depositions at 0.30 and 0.32 V and for all deposition times at 0.35 V. The density was larger (32-42 per μm2) for 120 min depositions at 0.30 and 0.32 V. For seed-mediated growth, the density of the nanostructures should be approximately the same for all samples (equal to the initial Au seed density on the surface). We note that we conducted the 120 min depositions at 0.30 and 0.32 V at a later date than for the other samples and on a different batch of glass/ITO electrodes and attribute this to the larger density observed in those two samples. This shows that there may be some irreproducibility in density associated with the batch of electrodes but that the general observations of seed-mediated growth as a function of potential and time are consistent.

Conclusions The morphology of Au deposited on glass/ITO/MPTMS electrodes in CTAB depends on the presence or absence of Au NP seeds on the surface, the electrode potential, and the deposition time. For nonseeded glass/ITO/MPTMS electrodes, Au is deposited in the form of branched or snow flake structures at potentials ranging from -0.30 to 0.20 V. Above 0.0 V, there is very little Au deposition on the nonseeded surface but sporadic regions with some Au on the electrode exist. On the glass/ITO/MPTMS/Au seed electrodes, Au deposits from -0.30 to 0.35 V and the amount of Au deposited is greater than that on the nonseeded surfaces at all potentials. The morphology is primarily spherical from -0.30 to 0.27 V. From 0.30 to 0.35 V, the seed-mediated growth is optimal because there is little or no Au deposition on nonseeded surfaces. Also, from 0.30 to 0.35 V, there are a significant number of Au NRs on the electrode surfaces. The length, aspect ratio, and yield of the Au NRs generally increase with increasing deposition time up to 120 min and increasing potential. By controlling the Langmuir 2010, 26(16), 13511–13521


deposition time and potential, the average rod length ranged from 163 to 418 nm and the yield ranged from 5 to 13%. It is interesting that the optimal potential for the electrochemical seed-mediated growth of Au NRs is 0.30-0.35 V, which is slightly larger than the open circuit potential for the chemical (ascorbic acid) seedmediated growth of Au NRs, which we measured to be about 0.27 V. Optimized electrochemical seed-mediated growth and the controlled formation of Au NRs and other shaped metal nanostructures on electrode supports have potential applications in chemical sensing, plasmonics, and nanoelectronics.

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Acknowledgment. We gratefully acknowledge the National Science Foundation (CHE-0848883) for the financial support of this research. The facilities at the Electrooptics and Research Institute and Nanotechnology Center (ERINC) and the Conn Center for Renewable Energy Research at the University of Louisville made it possible to obtain the scanning electron microscopy images presented in this article. Finally, we acknowledge Corey Knox and Elizabeth M. Heck for their preliminary work on the electrochemical set-up.

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Potential-Controlled Electrochemical Seed-Mediated Growth of Gold

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