Control of Crystal Growth in Local Electroless Gold Deposition by

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Control of Crystal Growth in Local Electroless Gold Deposition by Pyridinium Based Surfactants Daniel Mandler, and Roman G. Fedorov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00228 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Crystal Growth & Design

Control of Crystal Growth in Local Electroless Gold Deposition by Pyridinium Based Surfactants Roman G. Fedorov and Daniel Mandler* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel ABSTRACT: The formation and locally deposition of well-shaped Au nanostructures on a non-conducting surface is described. Specifically, the local electroless deposition of Au in aqueous solutions in the presence of various n-alkylpyridinium surfactants is driven by electrochemically generating a flux of AuCl4− at a gold tip close to a 3-mercaptopropyltrimethoxysilane modified Si oxidized wafer. Two reducing agents, NaBH4 and ascorbic acid, were used for the reduction of the gold ions. We studied the effect of the solution temperature, the potential applied to the gold tip and its distance from the surface, the reductant and the nature of the alkylpyridinium on the structure of the gold deposit. The chloride salts of methylpyridinium, butylpyridinium, cetylpyridinium, 4carbamoyl-1-cetylpyridinium and 4-methyl-1-cetylpyridinium were added separately and showed remarkable effect on the shape of the structures that were formed. We find that short chain n-alkylpyridinium salts do not adsorb preferentially on the gold facets whereas the longer chain n-alkylpyridinium ions cause the formation of well-facetted Au structures, such as cubes, hexagons and even multipods. Moreover, comparison between local and bulk deposition revealed significant difference in Au structures that were formed, presumably due to the different concentration profile of the AuCl4−.

Corresponding author address: Email: [email protected], Fax: +972 2 658 5319, Tel: + 972 2 658 5831 INTRODUCTION Metallic nanoparticles (NPs) are commonly prepared in solution.1, 2 Yet, it is possible to form metallic NPs directly on surfaces.3-6 The latter is of special importance in a variety of 1 ACS Paragon Plus Environment

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fields, such as (electro)catalysis, sensing, energy, etc.7, 8. In growing NPs on either conducting or non-conducting surfaces, the particle-surface interaction plays a major role.8, 9 Furthermore, one of the reasons for growing NPs directly on surfaces is to control their location and density for further applications, e.g., catalysis.8, 10 Key factors affecting the growth and properties of the NPs at the solid-liquid interface comprise the nature of the NPs and the surface (including surface defects) and their crystallinity.11 Numerous synthetic approaches for the formation of metallic NP arrays on different surfaces have been reported.12, 13 They can be divided into parallel and serial methods. Parallel processes, such as lithography, (electro)chemical deposition, etching (with or without masking) and stamping, allow the simultaneous formation of patterns on large surface areas. On the other hand, serial processes including printing, local (electro)chemical deposition, laser lithography, etc., are based on the formation of one pattern at a time. Evidently, the parallel processes are characterized by high-throughput, and therefore are attractive for industrial approaches, whereas the serial processes are slower; however, provide high flexibility and in many cases are simpler because they do not require masking and additional processes before and after pattern formation. Scanning probe microscopy (SPM) methods, e.g., AFM, STM, SECM (scanning electrochemical microscopy) offer complex material manipulations with high resolution, sometimes on a molecular or even atomic level.14 It is evident that in order that these methods will be part of an industrial process, their speed, must be significantly increased. An example for a serial process for patterning that reached the market is ink-jet printing. Among the SPM methods, SECM presents one of the most promising approaches for patterning, in particular at the micrometer and sub-micrometer scale, which is becoming highly relevant in the field of printed electronics. Others and we have shown the applicability of SECM as a patterning tool for the local deposition, etching, polymerization and additional chemical and electrochemical reactions.15 Recently, we have shown that SECM enables controlling the crystal growth of metal NPs deposited locally on conducting surfaces.9, 10, 16 We have demonstrated two approaches of controlling the growth of NPs locally on conducting surfaces (Scheme 1). The 2 ACS Paragon Plus Environment

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surfactant-based method 10 (Scheme 1A) involves the addition of shape affecting agents to the solution and generating a flux of metallic ions that are electrochemically reduced on the surface. The second, self-assembled monolayer method (Scheme 1B) comprised the local electrochemical reduction of metallic ions on ω-functionalized self-assembled monolayers.9, 16 The latter controlled significantly the growth of the NPs. Both methods use the “tip generation-surface collection” (TG-SC) mode of SECM,17 by which a flux of metal ions is generated at a metallic microelectrode and reduced electrochemically on the substrate. Yet, and for practical applications, it is much more important to enable patterning and the controlled growth of NPs on non-conducting surfaces, such as polyethylene terephthalate (PET).

A

C

B

Scheme 1: Three approaches of controlling the growth of metal NPs locally on different surfaces using SECM: A – Surfactant based method; B – Self-assembled monolayer method; C – Electroless deposition method. This challenge is exactly the focus of the present study. Specifically, we describe an approach, which combines SECM and electroless deposition (Scheme 1C). Electroless deposition has been reported by SECM,18-25 yet, and to the best of our knowledge, the effect of various surfactants on this process has never been reported. The essence of the method involves the TG-SC where a flux of AuCl4− is formed at the tip and is reduced by NaBH4 and/or ascorbic acid (AA) to form Au nanoparticles (Au NPs) that deposit onto a non-conducting surface. We employed an oxidized Si wafer coated with a thin layer of 3-mercaptopropyltrimethoxysilane 3 ACS Paragon Plus Environment


(MPTMS) which is used as a means of attaching the Au NPs to the surface. Cetylpyridinium chloride (CPC) and its derivatives were added to the solution and affected the growth of the Au NPs. CPC preferentially adsorbs on specific crystal facets promoting the formation of well-facetted Au crystallites. Understanding the effect of the surfactant structure on the nucleation and growth of the Au NPs was accomplished by studying first the bulk electroless deposition employing different CPC derivatives. We found that the reductants, their concentration and the ratio between NaBH4 and AA had a major effect on crystal growth. Varying the experimental conditions and/or changing the flux of the metal precursor, resulted in Au NPs with different shapes including cubes, hexagonal prisms and multipods.

EXPERIMENTAL SECTION. Instrumentation The setup of the homemade SECM was previously described.26 The vertical resolution was 0.075 µm. A CHI-750B electrochemical workstation bi-potentiostat (CH Instruments Inc., Austin, TX, USA) was used for the electrochemical measurements as well as for SECM operation. High resolution scanning electron microscopy (HR SEM, Sirion, FEI Company, USA) equipped with an EDX detector was used for imaging the morphology and crystal structures in the micro and nano range and for analyzing the composition of the deposits. For each sample, the average size of Au nanostructures was determined by image processing using "ImageJ". Atomic force microscope (AFM, Nanoscope III, Digital Instruments, USA) was used to measure the depth of the pores in the deposited Au layer.

Chemicals All chemicals were of analytical grade and used as received. 1-Methylpyridinium chloride (>98.0%, MPC), 1-butylpyridinium chloride (>98.0%, BPC), 1-cetylpyridinium chloride monohydrate (CPC, >98.0%), 1-cetyl-4-methylpyridinium chloride hydrate (>98.0%, 4MCP), and 1-cetyl-4-carbamoylpyridinium chloride (>96.0%, 4-CCP) were obtained from TCI 4 ACS Paragon Plus Environment

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Chemicals. Hexaammineruthenium (III) chloride (98%), 3-mercaptopropyltrimethoxysilane (MPTMS, 95%), potassium chloride (98%), sodium borohydride (≥98.0%) and L-ascorbic acid (AA, 99%) were obtained from Sigma Aldrich. Ethanol, hydrochloric acid and nitric acid were purchased from Merck. All solutions were prepared using deionized water (18.3 MΩ cm, EasyPure UV, Barnstead).

Procedures Double-sided polished p-Si(100) wafers (boron-doped, ρ = 1−10 Ω cm) were cleaned with aqua regia (3:1 HCl to HNO3 v/v) for 20 min (caution: aqua regia is a strong oxidizing

agent, which must be handled with extreme care and only in a hood that is empty of other chemicals!). Then, the wafers were heated in the oven for 4 hrs. at 400 °C to form a thick oxide layer. Finally, the wafers were modified by immersing in a deaerated ethanol solution containing 1 v/v % MPTMS for 20 min. The modified wafers were kept in pure water. Local deposition of gold micro- and nanostructures were performed in the tip generation/substrate collection (TG/SC) mode of SECM.17 A 25 µm diameter Au microelectrode was constructed as previously described.10 Control over the flux of gold ions was achieved by varying the potentials of a 25 µm diameter Au microelectrode. A thermostated stainless steel SECM cell was machined that allowed to control the temperature of the SECM solution by flowing water at a fixed temperature. The electrochemical cell consisted of the Au working microelectrode, a silver chloride (1 M KCl) reference electrode, and a Pt wire as counter electrode. Therefore, potentials are quoted vs. this quasi reference electrode. Controlling the distance between the microelectrode and the Si/MPTMS surface was accomplished by the feedback mode of SECM using a solution of 2 mM Ru(NH3)63+ (Etip=−0.21 V) or through oxygen reduction (Etip≤−0.45 V). For both cases, the approach curves were fit to the theoretical negative feedback current of the SECM from which the exact tip-surface distance was determined. Once the Au microelectrode was positioned a few microns above the surface, an anodic potential pulse (1-100 s) was applied to the Au tip in KCl (0.05 M) solution generating a 5 ACS Paragon Plus Environment


flux of AuCl4− ions. Pulses with different amplitudes and time lengths were repeated at different locations (while maintaining the same distance from the surface). These experiments were repeated with different temperature, tip-surface distance, tip potential and concentration of the reducing agents. Gold bulk deposition on Si/MPTMS was studied in a temperature-controlled vessel under Argon. Specifically, the aqueous solution containing 50 mM of the surfactant (CPC or its derivatives) and 50 mM KCl was constituted by sonication at 50°C during 10 min. Then, the Si/MPTMS was placed in the solution and 1 mM HAuCl4 was added. Finally, 0.2 mM of icecold NaBH4 was quickly added. After 20 sec, the sample was taken out, rinsed with pure water and dried in Argon flow. The effect of ascorbic acid (AA) was investigated by adding 1 mM of AA at various stages of the process: 100 s prior or 10 s after the addition of the surfactant. The samples were examined by HR SEM and in some cases by AFM.

RESULTS AND DISCUSSION Local deposition of metal NPs using SECM can be quite different that bulk deposition. The reason for this lies in the SECM setup, where the flux and profile concentration of the metal precursor is usually different from bulk deposition. The starting conditions for bulk deposition are equal concentration across the solution, whereas the starting concentration profile in SECM, where metal ions are electrochemically generated at the tip, is often time-dependent, highest at the tip and lowest at the substrate. Moreover, the process at the tip may be complicated with diffusion limitation of complexing ligands, adsorption of organic molecules and the presence of competitive processes. In this study, the source of gold ions is the microelectrode, which is anodically dissolved by applying positive potential in the presence of 50 mM KCl to form AuCl4−. As the aim of this research has been to locally deposit metals on insulators, the introduction of a reductant, e.g., AA, or NaBH4, to the solution is necessary. Evidently, this complicates even more the deposition 6 ACS Paragon Plus Environment

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process, vide infra. Furthermore, we added also a surfactant, cetylpyridinium chloride, CPC, as a means of controlling the growth of the Au crystallites. While the addition of the negatively charged reductants is not expected to strongly influence the dissolution of the Au microelectrode, the presence of CPC, which is known to adsorb strongly on Au, has a clear effect as can be seen in Figure 1. Evidently, the presence of CPC suppresses significantly the anodic dissolution of the microelectrode. This can be overcome by increasing the solution temperature, which causes desorption of CPC from the SECM tip at increased temperature (Figure 1A). Figure 1B analyzes the linear sweep voltammetry scans to verify whether the dissolution is reversible, namely, thermodynamically controlled. This method is based on using the Nernst equation, which yields27 a linear dependence (eq. 1) between the applied potential and the natural logarithm of the normalized current with a slope of 8.93 mV (at 40 °C and for a threeelectron process): ோ்

‫ܧ = ܧ‬°ꞌ + ௡ி ln

௜ಮ ସ௔ி஽

[1]

Where ‫ܧ‬°ꞌ is the formal electrode potential, i∞ is the steady-state diffusion-limited current at the disc microelectrode positioned far from the substrate surface, ݊ is the number of electrons per species undergoing the electrochemical reaction, ܽ is the microelectrode radius and ‫ ܦ‬is the diffusion coefficient. Combining the measured tip current of the Au microelectrode dissolution and the Nernst equation yields a slope of ~9 mV in the presence of 50 mM KCl (without CPC surfactant) at 40 °C (Figure 1B (dashed black curve) and Table 1). Addition of 50 mM CPC increases the slope to 24 mV (see Table 1), which is due to obtaining substantially lower currents than the theoretical values. This implies that the electrochemical oxidation of the gold microelectrode is reversible (in the absence of CPC) and is kinetically controlled in the presence of CPC. Furthermore, it is clear that at higher temperatures, the experimental slope gets closer to the theoretical value, which means that the process becomes more reversible, presumably due to the partial desorption of the CPC. Hence, we decided to carry out the experiments at 50 °C. 7 ACS Paragon Plus Environment


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A

B

Figure 1: A – Linear sweep voltammetry (scan rate 30 mV/sec) of an Au microelectrode in the presence of 50 mM CPC and 50 mM KCl at different temperatures: 40 °C – red; 45 °C – blue; and 50 °C – green (dash green recorded in the presence of 0.2 mM NaBH4 additive). Dash black curve was recorded in the presence of 50 mM KCl (without CPC) at 40 °C. B – The potential vs. the natural logarithm of the current. Dotted lines correspond to the theoretical reversible current values, calculated using eq. 1. Colors correspond to different temperatures as in A.

Solution Solution Slope calculated from Slope obtained composition temperature Nernst equation from Fig. 1B 50 mM KCl

40 °C

8.93 mM

~9 mM

40 °C

8.93 mM

~24 mV

45 °C

9.07 mM

~22 mV

50 °C

9.21 mM

~17 mV

50 mM KCl 50 mM CPC

Table 1: Experimental and theoretical slopes (eq. 1 and Figure 1B) at different temperatures.

In spite of the previous assumption that BH4− anions do not affect significantly the dissolution process, yet, we find that they increase the oxidation current (Figure 1A), probably due to BH4− oxidation. Specifically, the oxidation of BH4− commences at potentials similar to the 8 ACS Paragon Plus Environment

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oxidation of Au in the presence Cl−. At the same time, we cannot exclude an additional contribution to the oxidation current, due to a catalytic effect by which BH4− reduces AuCl4− and therefore regenerates the chloride. Indeed, current transients recorded on an Au microelectrode (Figure 2A) demonstrate significant intensification of gold dissolution at increased NaBH4 concentration. The kinetic effect discussed above appears at 0.5 mM NaBH4, where the current increases ca. ~2 sec after applying positive potential to the microelectrode (Figure 2A). SEM images of local Au deposits obtained under different conditions (Figure 2B) show the effectiveness of SECM in studying different factors affecting the deposition process. In the frame of a single experiment, conducted using one Si/MPTMS sample, two different parameters, i.e. potential on the tip and concentration of NaBH4, were investigated.

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Figure 2: A – Current transients recorded with an Au microelectrode upon applying potential pulses of 0.9 V for 20 sec at 50 °C in the presence of 50 mM CPC, 50 mM KCl and different concentrations of NaBH4: 0.05 mM – green; 0.20 mM – blue; 0.50 mM – red. The distance between the Au microelectrode and the Si/MPTMS substrate was 4 µm. B – SEM images of the local Au deposits under different NaBH4 concentrations: C – 0.05 mM; D – 0.20 mM (E is higher magnification of D) and F – 0.50 mM NaBH4. 9 ACS Paragon Plus Environment


Interestingly, the concentration of NaBH4 influences not only Au dissolution but also the shape of the deposited Au NPs (Figures 2B-F). At low concentration of NaBH4 (≤0.05 mM), cubes of the order of ca. 35-110 nm are formed (Figure 2C). At moderate concentration, i.e., 0.2 mM NaBH4, hexagonal and pentagonal crystals (35-45 nm in size) are seen (Figures 2D-E) and at higher concentration of NaBH4 (0.5 mM) aggregates of very small roundish NPs (9-13 nm in size) are deposited (Figure 2F). Such remarkable effect of NaBH4 on the shape of Au NPs can be explained in terms of the kinetics of crystal growth. At low concentration of NaBH4 the growth of Au nuclei proceeds through a slow layer-by-layer process, which ends with the formation of well-facetted Au nanostructures. In case that the growth rate is relatively slow, CPC has sufficient time to adsorb on the faces of the growing Au NPs. Higher concentration of NaBH4 promotes rapid growth of Au nuclei, which competes with the adsorption of CPC on Au. Indeed, previous electrochemical and capacity studies, show that the adsorption of pyridine derivatives, such as 3-pyridinecarboxylic acid, on Au(111) proceeds much faster than on the other basal faces (100 and 110)28. Moreover, it was reported29 that the potential of zero charge for vertically adsorbed pyridine on Au(100) is ca. 0.2 V more positive than on Au(111) suggesting that the adsorption is stronger on the latter face. The same study found that the flat adsorption of pyridine on Au(111) was also stronger than on Au(100). In addition, pyridine adsorption is significantly complicated by reconstruction occurring at Au(100) surface and not typical for Au(111)30. This can explain nicely the fact that under low concentration of NaBH4, where CPC adsorbs faster and stronger on Au(111), the formation of Au(100), i.e., cubes, is favored (Figure 2C). At higher concentration of the reductant, where the reduction process is faster, CPC does not adsorb significantly on the Au(111), which results in the most stable form of Au, i.e., Au(111) in a hexagonal form (Figure 2E). Surprisingly is the formation of pentagonal Au NPs (Figure 2E), which is rather rare for such a metal. In fact, these nanocrystals have cyclic penta-twinned structure and can be 10 ACS Paragon Plus Environment

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recognized as decahedral nanocrystals, which consist of five neighboring tetrahedral crystallites bounded by (111) planes.31, 32 These five tetrahedra are joined together along (110) edge. It was demonstrated that such decahedral nanocrystals were able to grow in one-dimension along the symmetry axis, forming nanorods with pentagonal cross-sections,31, 33 vide infra. The fact that these Au NPs, which are covered with only (111) facets, were obtained at increased NaBH4 concentration, agrees well with our previous assumption about the effect of NaBH4 concentration. Indeed, it was shown that the yield of such pentagonal Au NPs increases at higher reductant concentration.34 The next step involved studying the effect of the SECM tip potential (Figure 3A). Obviously, applying more positive potentials increases the flux of AuCl4− ions generated at the tip. We found that the applied potential has an effect on the size of the Au NPs that are formed, and to a lesser extent on their shape. A potential of 0.8 V leads to the formation of small (12-14 nm in size) and quite homogeneous Au NPs, possessing hexagonal or pentagonal shape (Figure 3B). Higher potential, i.e., 1.0 V results in the formation of unshaped Au nanostructures of different sizes (Figure 3D). This effect can be explained by electrochemical kinetics. It can be seen that at 50 °C gold dissolution commences at approximately 0.77 V (Figure 1A) and is, initially, kineticallycontrolled. Hence, we worked in the potential window of 0.8-1.0 V, which is kinetically controlled. Under these conditions, higher potentials correspond to higher fluxes of dissolved AuCl4− species from the SECM tip and higher fluxes of this species arriving at the Si/MPTMS surface. In analogy with bulk metal deposition, low concentration of metal ions favors the formation of well-facetted small NPs. Conversely, high concentration of metal ions results in the formation of large structures without clear-cut facets. At more positive potentials (not shown), the process is diffusion-controlled.

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A

B

C

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Figure 3: A – Current transients (shown are only initial 5 sec from a total of 20 sec) recorded with an Au microelectrode at 50 °C at different potentials: 0.8 V – green, 0.9 V – blue, 1.0 V – red. Electrolyte solution was composed of 50 mM CPC, 0.2 mM NaBH4 and 50 mM KCl. The distance between the Au microelectrode and the Si/MPTMS substrate was 4 µm. B-D – SEM images of the local Au deposits corresponding to these current transients: 0.8 V – B; 0.9 V – C; and 1.0 V – D. The magnification is X50000.

Control over the growth of metallic nanostructures can be achieved not only by managing the flux of their precursors, i.e., the metal ions, but also by controlling the kinetics through the reducing and complexing agents. Thus, the kinetics of reduction of metal ions depend on both the concentration of the reducing agent and its reducing power. NaBH4 is a strong reductant, which reduces Au(III) to Au(0) relatively fast. However, the growth of uniform metal NPs requires mild reducing environment allowing slower kinetics. Therefore, we examined the replacement of NaBH4 by AA and the effect of its concentration on the local deposition. Increasing the AA concentration results in increasing the current at the Au microelectrode (Figure 4A). This is not surprising, because AA is easily oxidized on the Au microelectrode at 12 ACS Paragon Plus Environment

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the applied potentials. Moreover, it can be seen that at high AA concentration (≥5 mM) the current at the SECM tip increases with time. This might be contributed to either the regeneration of the chloride ions in the diffusion layer due to the reduction of the gold complex by AA, or to the excess of AA that is prone to oxidation on the Au microelectrode.

A

B

C

D

D'

E

Figure 4: A – Current transients recorded with an Au microelectrode at 50 °C upon applying a constant potential of 0.9 V for 20 sec in the presence of 50 mM CPC, 50 mM KCl and different concentrations of AA: 0.2 mM – black; 1.0 mM – green; 5.0 mM – blue; 20.0 mM – red. The distance between the Au microelectrode and the Si/MPTMS substrate was 4 (continuous lines) and 20 µm (dashed blue). B-E – SEM images of the local Au deposits corresponding to the current transients with different AA concentrations (mM): B – 0.2; C – 1.0; D – 5.0 (D' same as D but d=20 µm); E – 20. Magnification is X50000.

We found that the size and shape of deposited Au NPs changes considerably by increasing the AA concentration. At low AA concentrations, the formation of small (11-17 nm in size) 13 ACS Paragon Plus Environment


roundish Au seeds (Figure 4B) are seen. Increasing the AA concentration results in the deposition of large (100-400 nm in size) well-defined Au structures, of which part possess (111) structure, i.e. triangular or hexagonal shape (Figure 4C-D). Further increase in AA concentration promotes unshaped Au NPs (80-125 nm in size) without preferential crystal symmetry (Figure 4E). Remarkably, varying the distance between the Au microelectrode and the substrate increases significantly the yield of (111) deposited Au structures. Increasing the distance up to 20 µm leads to the formation of large (450-670 nm in size) hexagonal Au crystals with flat upper side. This very interesting phenomenon might be attributed to lowering the flux of AuCl4− that reaches the Si/MPTMS surface. We exclude the effect of the positive potential on the positively charged CPC and the diffusion into the tip to surface gap because of the relatively high ionic strength and the CPC concentration (50 mM), and the large distance between the tip and the surface. The geometrical characteristics of such Au hexagonal crystals, especially their thickness, remain unclear. Unfortunately, in this study we were unable to use the SECM for estimating the amount of deposited metal and to compare this value with the theoretical one.9 Such measurements become possible only in the case of conductive surface, which serves as a second working electrode (both the microelectrode and substrate are biased). To better understand the effect of the CPC surfactant and reveal the role played by the pyridine ring, functional groups and the alkyl chain, we performed bulk Au deposition in the presence of different n-alkylpyridinium derivatives (Figure 5). Specifically, we added 50 mM of methylpyridinum chloride (MPC), butylpyridinium chloride (BPC), 4-carbamoyl-1cetylpyridinium chloride (4-CCP) and 4-methyl-1-cetylpyridinium chloride (4-MCP) each separately and applied under the same conditions as described above (0.2 mM NaBH4, 50 mM KCl at 50 °C) with the addition of 1 mM HAuCl4. The SEM images obtained after exposing the Si/MPTMS samples for 20 s in the solutions containing the various pyridinium salts are shown in Figure 5. First, it is clear that the length of 14 ACS Paragon Plus Environment

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the hydrocarbon chain linked to the pyridinium, has a strong effect on the deposit morphology (Figure 5A-C). In the presence of methyl- and butyl- derivatives, we observe the growth of a thin (5-10 nm) Au layer with hexagonal pores. The AFM image and the profile of the individual pore (Figure 5B-B') confirms that it is indeed a pore. Most surprisingly is the long range ordering of the pores along the same axes (Figure 5A-B, for clarification, we drew the axes in Figure 5A). This long range ordering is of great interest by itself and will be further studied. Additional spectroscopic and AFM measurements are required in order to elucidate the growth mechanism of this porous Au layer on Si/MPTMS substrate in the presence of short n-alkylpyridinium surfactants. A longer chain, i.e., CPC, promotes the formation of Au rods (Figure 5C). We attribute this effect to preferential adsorption of CPC on basal faces of Au nuclei, which favors the formation of decahedral Au NPs, vide supra. As noticed above, the decahedral Au nanostructures can grow in one-dimension along the symmetry axis, forming nanorods with pentagonal cross-sections.31, 33

We believe that such Au nanorods are not observed in local deposition because of the

relatively short time window of the SECM process. The presence of functional groups in the para-position of the pyridine ring disturbs the selective adsorption of the surfactant on the Au crystal faces. In this case, the surfactant serves as a capping agent slowing the kinetics of gold reduction. This results in the formation of large and typical Au(111) structures, i.e., hexagons and triangles (Figure 5D-E). This effect is stronger in the case of the carbamoyl functional group, as compared with the methyl group. It is conceivable that the positively charged nitrogen atom of the carbamoyl-group can interact with the AuCl4− ions leading to an additional kinetic effect, slowing down Au reduction35. Moreover, the binding energy of the –NH2 (in carbamoyl-group) to Au surface is compatible with that of the pyridine,36 which means that the adsorption of 4-CCP on Au can be realized by both ways. Therefore, the presence of a carbamoyl functional group in the CPC surfactant has a strong effect on the growth of Au structures. 15 ACS Paragon Plus Environment


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It is worth mentioning that the pyridinium-based surfactants are not the only component in the system, which can potentially adsorb onto the Au nuclei and affect their crystal growth. The MPTMS thin layer attaches the Au nanostructures to the oxidized Si substrate through a thiol group that is pointed towards the solution. However, the possible interaction between the surfactant and the MPTMS, affecting the deposition of the Au, should not be neglected. Therefore, we examined the MPTMS layer before and after exposure to a solution of CPC by XPS. No change in the XPS spectrum of the MPTMS was observed even at 50 °C, clearly indicating that such interaction does not play a major role.

A

B

B'

C

D

E

Figure 5: SEM images of Au bulk deposits obtained after 20 s at 50 °C in the presence of 1 mM HAuCl4, 0.2 mM NaBH4, 50 mM KCl and 50 mM of different n-alkylpyridinium derivatives (the structures are shown): A – MPC; B – BPC; C – CPC; D – 4-CCP; and E – 4-MCP. The magnification is X10000. In the lower-right corner of B is presented the AFM image of an individual pore in the Au thin layer. B' – is a profile along the red line on the AFM image.


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The role of the reductant has also been studied on bulk deposition. Clearly, AA is a milder reducing agent than NaBH4, yet, it has its own role. AA is incapable of reducing Au(III) to Au(0) in solution as no Au is deposited as a result of adding AA into an AuCl4− solution.

Yet, we find that the interaction of CPC with gold ions depends on the redox state of the gold. This is clearly shown in Figure 6, which shows the SEM images of gold deposits that are formed by adding to the AuCl4− solution first the AA before CPC is added (Figure 6A) and vice versa by adding first the CPC to the AuCl4− solution followed by the AA (Figure 6B). In the first case, an incomplete Au layer is formed that has many well-shaped facetted holes, whereas if CPC is present in the solution prior to adding AA, gold nanorods are formed. Since AA is known to reduce Au(III) to Au(I) it suggests that the interaction of Au(I) with CPC results in the formation of the layer with the faceted holes, similar to that obtained in the presence of MPC and BPC surfactants (Figure 5A-B). Therefore, the formation of Au nanorods requires the preliminary interaction of Au(III) ions (as AuCl4−) with long chain n-alkylpyridinium surfactants. On the other hand, short n-alkylpyridinium molecules, which do not form wellorganized adlayers preferentially on certain facets, will not result in the formation of anisotropic structures such as nanorods.

A

B

Figure 6: SEM images of Si/MPTMS samples after exposure to a solution of 1 mM HAuCl4, 50 mM KCl (at 50 °C) and: A – 1 mM AA was added first, 100 s later 50 mM of CPC was added; B – 50 mM CPC was added first followed by 1 mM of AA 10 s later. 0.2 mM of NaBH4 was added to both solutions and the samples were removed after 20 s. The magnification is X10000. 17 ACS Paragon Plus Environment


To summarize the bulk deposition, we can conclude that in the presence of a strong reducing agent, i.e., BH4−, short chain n-alkylpyridinium lead to the formation of a thin continuous and long range ordered layer of Au having mostly hexagonal pores. Increasing the chain length causes the formation of individual nanorods. Introducing functionalities on the pyridinium ring avoids the selective adsorption and, therefore, cause the formation of Au(111) deposits. If a milder reductant is added to form Au(I) then a continuous layer of Au is formed even in the presence of long chain alkylpyridinium. On the other hand, if the long chain alkylpyridinium is added prior to the mild reductant, then again the selective adsorption leads to anisotropic structures.

Now, we returned to local deposition by the SECM. The effect of the same n-alkylpyridinium derivatives in the presence of 5 mM AA was examined as a result of generating the flux of AuCl4− by a microelectrode (Figure 7). The distance between the latter and the Si/MPTMS substrate was ca. 20 µm. It can be seen that the presence of a short nalkylpyridinium surfactant, i.e., MPC, results in the deposition of unshaped Au structures without clear facets (Figure 7A). We attribute this effect to the inability of the short chain MPC surfactant, to form well-organized adlayers on specific crystal facets of Au (see above). Interestingly and as opposed to bulk deposition, the introduction of 4-CCP leads to the formation of different multipods, i.e., tripods and tetrapods (110-140 nm in size, Figure 7B-C). This observation is very surprising, as we did not observe such structures in bulk deposition. The difference in AuCl4− diffusion in local deposition, as compared with bulk, might be the main reason for this effect. To the best of our knowledge, the formation of such Au multipods requires either foreign metal seeds37 or Pt(II)/Pt redox couple as a catalyst38 and is unusual in direct chemical synthesis. Changing carbamoyl group to methyl group (4-MCP) promotes the formation of Au(111) structures, i.e., hexagons and triangles (130-510 nm in size, Figure 7D). 18 ACS Paragon Plus Environment

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This confirms the crucial role of the carbamoyl functional group in the formation of Au multipods.

A

B

C

D

Figure 7: SEM images of Au local deposition obtained at 50 °C using SECM as a result of applying a constant potential pulse of 0.9 V for 20 sec to the Au tip in the presence of 5 mM AA, 50 mM KCl and 50 mM of different n-alkylpyridinium derivatives: A – MPC; B, C – 4-CCP; and D – 4-MCP. The distance between the Au microelectrode and the Si/MPTMS substrate was ca. 20 µm. The magnification is X50000 for A, B, D and X200000 for C.

CONCLUSIONS

In this work, we examined the effect of various surfactants on the local electroless deposition of Au nanocrystals driven by scanning electrochemical microscopy (SECM). A flux of gold ions in the form of AuCl4− is generated at the SECM tip by its controlled dissolution. Two reducing agents, i.e., ascorbic acid (AA) and NaBH4, were added to the solution, and are responsible for the reduction of the AuCl4−. In addition, we added derivatives of n-alkylpyridinium, which are positively charged and known to bind to both the AuCl4− as well as



specific gold metal facets. The gold that was formed adsorbed onto an oxidized Si wafer that was modified by 3-mercaptoproplytrimethoxysilane, MPTMS.

By controlling carefully the concentration of the reactants, temperature, potential applied to the SECM tip and its distance from the surface, as well as the type of the surfactant and the sequence of its addition, we were able to manipulate the structure of the locally deposited Au nanocrystals. For example, Au multipods with high yield were formed in the presence of 4-carbamoyl-1-cetylpyridinium, and a weak reducing agent (ascorbic acid, AA). On the other hand, in the presence of cetylpyridinium added before adding AA (that reduced the Au(III) to Au(I)) nanorods were formed. Clearly, understanding the role played by the reducing agent and the surfactant are crucial to controlling the crystal growth and the formation of homogeneous nanostructures.

The unique advantage of SECM, i.e., its capability to control the flux of metal precursors close to a surface, enables the formation of Au nanostructures, which are not obtained in bulk deposition. To better understand the exact mechanism of crystal growth, additional spectroscopic and microscopic tools are required to be combined with the SECM, which are currently undertaken in our laboratory. Clearly, much more information on the chemical and physical processes, such as phase formation, nucleation and dissolution on a local scale, could be, in principle, extracted from the current transients. This has been previously demonstrated by Unwin et al.39, 40 Yet, the fact that the oxidation current is a result of not only the dissolution of the gold microelectrode but also the oxidation of the BH4− and ascorbic acid, makes this quantitative approach quite impossible. Simpler systems would be required to get some insight into these local and important processes as well as employing nanoelectrodes for studying them on a nanometer scale. In this respect, our study demonstrates also how SECM could be used also for local imaging of active sites for metal nucleation, i.e. structural defects of the surface or oxygen containing functional groups, such as oxygen containing groups on graphene oxide.41 20 ACS Paragon Plus Environment

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ACKNOWLEDGMENT

This work is supported by the Israeli Ministry of Science and Technology (project 3-12610). The Harvey M. Krueger FamilyCentre for Nanoscience and Nanotechnology of the Hebrew University is acknowledged. We would like to express our sincere gratitude to Dr. A. Vaskevich and Prof. N. Eliaz for their helpful discussion.

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For Table of Contents Use Only Control of Crystal Growth in Local Electroless Gold Deposition by Pyridinium Based Surfactants Roman G. Fedorov and Daniel Mandler TOC graphic

Synopsis Local deposition of well-shaped Au nanostructures on a Si/(3-mercaptopropyl)trimethoxysilane (MPTMS) substrate in the presence of NaBH4 as reductant and pyridinium-based surfactant using scanning electrochemical microscopy (SECM) is demonstrated. Using cetylpyridinium chloride (CPC) and its derivatives enabled manipulation of the structures that were formed and resulted in cubes, hexagons and multipods.


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Control of Crystal Growth in Local Electroless Gold Deposition by

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