1D Nanomaterials 2011View this Special Issue
Research Article | Open Access
Impact of Specifically Adsorbing Anions on the Electroless Growth of Gold Nanotubes
Electroless metal deposition on nanochannel-containing templates is a versatile route towards metal nanotubes and nanowires if the plating reaction can be sufficiently controlled. In this study, disulfitoaurate-formaldehyde-based gold plating baths were modified by the addition of halides, pseudohalides, and EDTA. The introduction of specifically adsorbing anions strongly affected the heterogeneously autocatalyzed plating reaction and allowed the regulation of the reaction rate and the product morphology. The new plating baths showed enhanced stability and allowed the synthesis of homogeneous nanotubes of high aspect ratios (>150) in 30 μm thick ion track-etched polymer templates. Depending on the reaction conditions, solid and porous structures consisting of gold nanoparticles of differing size and shape were accessible. The presented strategy offers adapted gold thin films, nanotubes, and nanowires for applications in catalysis or sensing.
Nanotubes (NTs) are an important class of virtually one-dimensional nanomaterials , which are deployed in various applications of high academical and practical interest. The versatility of NTs is supported by the broad range of materials from which they can be created. Besides carbon , NTs can be prepared from polymers [3, 4], metals [1, 4–8], and compounds such as oxides, sulfides, or nitrides  next to composites [1, 3]. Among the presented materials, metal NTs exhibit a combination of unique physical and chemical properties such as electrical conductivity, plasmon resonance , catalytic activity [6, 7], and a rich surface chemistry involving adsorbates such as thiols [7, 8]. Their superior properties lead to the implementation of Au NTs in a wide range of applications. For instance, they were effectively applied as biosensors , nanoreactors , supportless CO oxidation catalysts , and molecular sieves .
Usually, Au NTs are prepared according to a versatile template-based method introduced by Martin et al. [4, 7–10]. In this protocol, Au is electrolessly plated on ion track-etched polymer membranes. Electroless depositions are solution-based reactions in which metal cations are selectively reduced on a substrate surface by a chemical agent . Electroless Au plating leads to the development of polycrystalline films consisting of interconnected Au nanoparticles (NPs) [9, 10]. If the metal film covers the nanochannel walls inside the template, Au NTs are formed.
While template preparation by ion track etching is well understood and ensures a high degree of control over the NT diameter, length, and shape , less effort has been devoted to the development of new electroless Au baths allowing to adjust the morphology of the plated Au thin films and NTs . Preceding synthetic studies elucidated the effect of different parameters like the Au precursor concentration , the temperature , the pH value [9, 10], and the delayed introduction of the reducing agent  without changing the general bath composition. The results of these studies show relatively large Au NP sizes (typical average diameters lie between 30 and more than 100 nm) and a limitation of the NP shape to globular morphologies [9, 10]. However, control over the size and shape of metal NPs is of major nanotechnological importance since these parameters considerably affect properties such as catalytic selectivity and activity, optical absorption, and local field enhancement [13–15].
As the presence of adsorbates strongly influences the growth of metal NPs , the introduction of adsorbing species is a straightforward measure to control both the deposition kinetics of electroless plating reactions and the morphology of the obtained products . Halides are common reagents that readily adsorb specifically on noble metal surfaces  and show pronounced shape-directing effects in metal NP syntheses [15, 17, 18]. Thus, they are promising additives in the electroless fabrication of Au nanomaterials. In this study, we therefore examine the influence of halides and pseudohalides on the electroless synthesis of Au thin films and NTs and show how they can be used to optimize the obtained nanostructures towards a desired product.
2. Experimental Procedure
2.1. General, Chemicals
All glassware was cleaned with aqua regia prior to use, and all procedures were performed with purified water (Milli Q 18-MΩ water). The sensitization and plating solutions were prepared freshly, and the activation solution was stored dark and cold. The following chemicals were used without further purification: AgNO3 (Grüssing, p.a.), EDTA disodium salt dihydrate (Fluka, puriss. p.a.), ethanol (Brenntag, 99.5%), formaldehyde solution 37% in water, methanol stabilized (Grüssing, p.a.), KBr (Merck, suprapur), KI (Grüssing, 99.5%), KSCN (Merck, cryst. pure), methanol (Sigma-Aldrich, laboratory reagent), NaCl (Merck, suprapur), NaCN (Merck, p.a.), Na2SO3 (Merck, p.a.), NH3 33% in water (Merck, puriss.), SnCl2 dihydrate (Sigma-Aldrich, ACS reagent), sodium hydroxide solution 32% in water (Fluka, puriss. p.a.), and trifluoroacetic acid (Riedel-de Haën, >99%). A commercial electroplating solution (El-Form Galvano Goldbad, Schütz Dental GmbH) was used as the Au source (15 g 99.9% Au per liter, present as (NH4)3[Au(SO3)2]).
Polycarbonate foils (Makrofol, Bayer MaterialScience AG, nominal thickness 30 μm) were irradiated with Au ions (energy: 11 MeV per nucleon, fluence: 1·108 ions cm−2) at the Helmholtz centre for heavy ion research (GSI). Subsequently, they were irradiated with UV light in the presence of air (1 h per side, UV source provides 1.5 and 4 Wm–2 in the ranges of 280 nm–320 nm and 320 nm–400 nm, resp.) and etched in stirred sodium hydroxide solution (6 M, 50°C, time depending on desired diameter, ranging from 100 to 200 nm). The as-prepared templates were thoroughly washed with water and dried. Prior to electroless plating, the polycarbonate substrates were activated with Ag NPs according to a previously reported procedure  (for a description of the process, see Section 3.1 and Figure 1). After washing with ethanol and water, the freshly activated substrates were immersed in the electroless plating solutions. The pH of all plating solutions was adjusted to 9.3–9.4 with NaOH since moderately basic pH values favor dense nucleation, low reaction rates and small crystallite sizes, and hence tube formation . Temperature was held constant at 8°C to ensure slow deposition necessary for formation of tubes of homogeneous wall thickness . The Au plating baths contained 7.0 mM Au(I), 125 mM Na2SO3, and 625 mM formaldehyde. All additives have been initially applied in a concentration of 100 mM. In case of strong interference with the plating reaction, the concentrations have been reduced to the values stated in the text. In the cases of mixed addition of EDTA and salts containing specifically adsorbing anions, the concentration of EDTA was held constant at 100 mM while the salt concentrations were varied. After the desired reaction time, the membranes were washed with water and dried.
TEM (FEI CM20 microscope (Netherlands), 200 kV acceleration voltage, LaB6-cathode): the NT-containing templates were embedded in Araldit 502© (polymerization at 60°C for 16 h) and examined as ultrathin sections (70 nm thickness, Reichert-Jung Ultracut E ultramicrotome, DKK diamond knife). EDS (Oxford Model 6767 EDS-system (England)): in combination with TEM, EDS measurements were performed to confirm the composition of the obtained nanostructures. A representative EDS spectrum is shown in Figure 6. SEM (JEOL JSM-7401F, 5–10 kV acceleration voltage): prior to the measurement, the template was removed with dichloromethane. The freed metal structures were collected on silicon wafer pieces sputter-coated with Au.
3. Results and Discussion
3.1. Synthetic Strategy
Generally, electroless plating reactions are not catalyzed by pristine polymer surfaces . Therefore, polymer substrates have to be modified to initiate metal deposition. In case of ion-track-etched polymer templates, catalytic activity is usually established by covering the substrate with Ag NPs [4, 6, 9, 10]. This process involves two steps. First, the template is immersed in an SnCl2 solution, leading to the formation of surface complexes between Sn(II) and polar groups of the polymer (sensitization, see step in Figure 1). After washing off the excess of unbound SnCl2, the template is dipped in an [Ag(NH3)2]NO3 solution. The Ag(I) oxidizes Sn(II) forming Sn(IV), while Ag(I) is reduced to elemental Ag. This leads to the precipitation of Ag NPs on the template surface (activation, see step in Figure 1) [9, 10]. After activation, the template membranes are washed and finally immersed in the plating solution (step in Figure 1). Electroless plating baths applied in the synthesis of Au NTs contain [Au(SO3)2]3- as the stabilized Au(I) source and formaldehyde as the reducing agent [7–10]. During the heterogeneously autocatalyzed plating reaction, formaldehyde is oxidized to formate, while Au(I) is reduced to elemental Au . A side reaction is the cementation of Au on the less noble Ag introduced during activation (1) : Heterogeneous reactions severely respond to interfacial changes. For instance, Pt catalysts are poisoned by strongly bonding species such as CO , whereas the adsorption of chiral molecules such as cinchona alkaloids can lead to enhanced reaction rates and asymmetric induction on them . Adsorbing species such as cationic surfactants, halides, and silver salts play, a major role in the wet-chemical synthesis of anisotropic Au NPs . Analogous to colloidal approaches, electroless plating is based on metal NP formation by the reduction of dissolved cationic precursors. Correspondingly, the presence of adsorbates strongly affects the reaction kinetics and the shape of the evolving nanostructures [6, 21]. Furthermore, adsorbate-forming additives can improve the stability of plating baths towards homogeneous nucleation . This issue is an important quality criterion in electroless plating. If the metal reduction is not limited to the substrate surface, homogeneous nucleation leads to the formation of metal nuclei in the bulk solution. Continued nucleation and the growth of existing particles quickens the depletion of the plating solution and gives rise to undesired metal precipitation. In addition, homogeneous nucleation negatively affects plating rates and often leads to deteriorated metal films . Applying the outlined concept, we investigated the influence of different halides and pseudohalides (chloride, bromide, iodide, cyanide, and thiocyanate) on the electroless growth of Au NTs. Next to adsorbing anions, the universal ligand EDTA was tested as a complementary complexing agent. In contrast to the reactive components Au(I) and formaldehyde, the applied additives are not consumed during the plating reaction (step in Figure 1).
3.2. Isolated Effect of Additives
Due to its relatively low-binding strength towards low-valent metal cations and its competition with sulfite as a soft sulfur-containing ligand well suited for the coordination of the soft Lewis acid Au(I), the presence of EDTA should not lead to a pronounced additional stabilization of the oxidizing component Au(I). Accordingly, the addition of EDTA does not alter the electroless deposition reaction to a relevant degree.
Similar product morphologies and reaction speeds were obtained for the plating baths containing sulfite and sulfite next to EDTA (Figures 2(a)-2(b)). In both cases, the Au NTs were found to be composed of granular NPs of approximately 15–20 nm size. Also, the Au NTs showed fragmentation when the polycarbonate template was removed. With increasing distance to their openings, the tubes become crumbly and finally break after some micrometers (Figure 2(b), see arrow). Longer fragments were found in the presence of EDTA, revealing slightly improved NT formation. The inhomogeneous deposition of Au alongside the template nanochannels is caused by the relatively high-deposition rate . While the reactive species of the plating bath are consumed on the whole inner template surface, mass transport from the bulk solution is limited by the narrow tube openings. This leads to reagent depletion and reduced plating rates inside the NTs.
The addition of 100 mM chloride or bromide lead to similar changes in Au plating concerning the product morphology and the deposition rate. At the beginning of the plating reaction, a relatively quick color-shift of the activated templates from brownish to pink occurred, which was attributed to Au cementation (1). The electroless deposition of larger amounts of Au, which was indicated by the development of a dark purple or bluish color was decelerated in the presence of chloride and bromide. While chloride leads to a moderate reduction of the deposition rate, bromide caused a drastic deceleration. In both cases, the development of continuous Au films and well-defined Au NTs was hampered. Instead, the growth of separated NPs was observed (Figures 3(a)-3(b), arrow indicates an isolated par
ticle). Compared to chloride, bromide had a more pronounced effect on the product morphology. While in case of chloride tube fragments were found after a reaction of one day (Figure 3(a)), no NTs were observed in the case of the addition of 100 mM bromide even after a deposition time of 10 days (Figure 3(b)). However, the formation of fine NTs of approximately 100 nm diameter was confirmed by TEM (Figure 3(c)). The NTs consist of particles of nm size. As no continuous Au film growth was observed in this experiment, the tubular structures are probably composed of Au NPs stemming from the Ag conversion (1).
The deceleration of electroless plating in the presence of halides can be explained by partial poisoning of the Au surface due to adsorbate formation. Compared to chloride, the heavier halide bromide has a higher polarizability and interacts stronger with Au surfaces . As a result, bromide leads to a higher surface coverage at identical solution concentration . With increasing blockage of the reactive sites on the Au surface, the plating reaction should be increasingly constrained. In accordance with this expectation, bromide has a stronger effect on the electroless Au deposition.
Interestingly, the trend did not extend to the heaviest homologue iodide. In contrast to the expectation of an efficient reduction of surface activity, electroless plating baths containing 100 mM iodide quickly decomposed to form Au suspensions. Even extremely reduced iodide concentrations below 1 mM accelerated both the homogeneous and heterogeneous gold reduction and were thus detrimental to NT formation. The opposing behavior of iodide compared to its lighter homologues is surprising, but highly reproducible. An increase of the speed of electrochemical reactions on metal surfaces with anion adlayers is a known phenomenon. It can be explained by the lowering of the activation energy of intermediate species by the adsorbed anions . In the case of accelerated Au plating, either formaldehyde oxidation or the integration of Au atoms in the evolving Au film may be supported by iodide. The responsible mechanism is evidently ineffective in the case of chloride or bromide addition. Another example for an unexpected change of surface reactivity in the presence of iodide is the anodic dissolution of Pd . In contrast to the corrosion-enhancing behavior of chloride, which depends on the bulk chloride concentration, even a monolayer of iodide was sufficient to increase the Pd dissolution rate by a factor of nearly 200 .
The formation of halide complexes as an explanation for the altered deposition kinetics is unprobable due to the strong tendency of Au(I) to form sulfito complexes. This is indicated by the significantly more negative standard potential of the disulfitoaurate complex compared to the analogous bromo- and iodoaurate complexes: If the lowered standard potential of a metal complex is ascribed to the reduced concentration of free metal ions (in this case solvated Au(I)), its complex formation constant can be calculated on basis of the Nernst equation. The difference between the standard potentials of two complexes is related to the ratio of their complex formation constants in the following way (: charge, : Faraday constant, : universal gas constant, : temperature): With Au(I) as a monovalent cation () and K, a potential difference of 59 mV corresponds to values deviated by one order of magnitude. The application of (5) to (2)–(4) leads to ratios between the sulfito complex and the bromo- and iodo complexes of 1.9·1014 and 7.0·107, respectively. Due to the markedly stronger complexation of Au(I) by sulfite, it can be stated clearly that even the heavy halides bromide and iodide do not significantly contribute as ligands under the given experimental conditions (similar concentrations of sulfite and halides, no excess of Au(I)).
The presence of 100 mM cyanide completely stopped the electroless deposition reaction. One reason for this behaviour is the formation of the strongly stabilized dicyanoaurate complex. Its standard potential (6) is significantly negative-shifted compared to the disulfitoaurate complex (3): The high thermodynamic driving force towards dicyanoaurate even allows etching of Au by molecular oxygen , a technique which is used in gold extraction as well as in the shape modification of Au nanomaterials . In our plating bath, formaldehyde with a standard potential of −0.927 V  could still reduce [Au(CN)2]−. But in addition to the thermodynamic stabilization of Au(I), cyanide is known to poison Au surfaces in electroless plating . In contrast to cyanide, thiocyanate cannot compete with sulfite as a ligand (7). Application of (5) results in a sulfite complex formation constant, which is 5.1·109 times higher than the corresponding value for thiocyanate. Therefore, the change of the Au deposition induced by thiocyanate can be related to its adsorption behaviour. Its presence has a considerable effect on the Au film growth, leading to a reduced plating rate, improved suppression of homogeneous Au precipitation, and enlarged Au crystallites (Figure 4). During 21 h of deposition, a granular Au film and fragile NTs develop (Figure 4(a)). After extended reaction times, a coarsening of the Au NPs occurs and Au nanowires are formed due to the complete filling of the template nanochannels (Figures 4(b)-4(c)). The rough Au films consist of interconnected, edgy grains, and many small gaps remain between the particles (Figure 4(c)). Therefore, thiocyanate is a suitable additive for the electroless synthesis of roughened Au nanostructures. Surface roughness is an important factor considering implementation in catalysis, superhydrophobicity, and sensing. For instance, rough Au films can be applied as surface enhanced Raman spectroscopy (SERS) substrates  or in adsorbate sensing .
3.3. Combined Effect of EDTA and (Pseudo)halides
The effects of halides on the plating reaction are strongly altered by the addition of EDTA. In all depositions, significantly reduced plating rates and morphology changes were observed. According to the halide adsorption tendency, bromide and iodide have the most intense effect on the electroless plating. All halides allow the synthesis of high aspect ratio Au NTs with homogeneous wall structure (Figure 5). Also the reduced reaction rates correlate with increased bath stabilities. In all mixed EDTA-halide experiments, homogeneous nucleation could be successfully suppressed. The Au NTs obtained with EDTA and thiocyanate were not coherent and completely disintegrated to form NP suspensions during washing of the NT-containing polycarbonate membranes. Some NP loss also occurred in the case of Au films grown in the presence of chloride and bromide.
In case of chloride, the initially formed Au NPs are only partly connected, leading to porous Au films and NTs in the early stages of the plating reaction. In Figure 5(a), the product obtained after a reaction time of one day is shown, revealing a particle size of approximately 30 nm. In later stages, closed Au films and NTs are obtained. Removing the template from a porous sample leads to the formation of a spongy Au NT network (Figure 5(a)). This structure is interesting because of its hierarchical porosity with macropores defined by the tube interiors and mesoporous tube walls. Similar Ag structures have been implemented as effective substrates for surface-enhanced Raman scattering (SERS) . Also, porous metal NTs are a promising class of unsupported heterogeneous catalysts .
In the EDTA-bromide system, grain size is tunable by adjusting the bromide concentration. A bromide concentration of 2.0 mM leads to the formation of small globular particles of less than 10 nm size, approaching the product morphology obtained in the case of EDTA-free bromide baths (see Figures 3(b)-3(c)). At a concentration of 0.25 mM and a reaction time of 8 days, the Au nanostructures are composed of sharp and interconnected grains with a size of few tens of nm (see right image in Figure 5(b)). Under these conditions, robust Au NTs are formed. Since metal NPs with high curvatures produce high field enhancements [21, 33], the obtained spiky Au films and NTs are interesting for SERS applications. In a seed-mediated approach, similar Au NP morphologies (branched and star-like) were yielded by the reduction of HAuCl4 in the presence of cetyltrimethylammonium bromide (CTAB) and optionally traces of AgNO3 . Another solution-based route towards multibranched Au NPs is the reduction of HAuCl4 with hydrazine in the presence of polyvinylpyrrolidone. These structures have been implemented as effective SERS substrates .
Like in the experiments performed without EDTA, high iodide concentrations increase the deposition rate and the susceptibility to homogeneous nucleation. However, low iodide contents in combination with EDTA delay the plating reaction and suppress bath decomposition. Figure 5(c) shows the product obtained by the addition of 0.25 mM iodide next to 100 mM EDTA after a reaction time of two days. Next to nonfragmenting Au NTs, fibrous deposits are found on top of the solution-sided face of the Au surface film. The nanowires originate from single NPs of the surface films (see arrow in Figure 5(c)) and are even found at a drastically reduced iodide concentration of 25 μM. The EDTA-iodide system yields NT walls composed of densely interconnected Au NPs (Figure 6(a)). By EDS, no significant Ag residues can be detected (Figure 6(b)). The obtained Au NTs are very robust and endure the template removing process without major deformation or fragmentation. A high mechanical strength of metal NTs allows the fabrication of free-standing NT arrays and increases the resistance of the nanostructures under stress. For instance, a quick loss of NPs would decrease the lifetime of metal NT catalysts. In addition, the nanotube structure affects its electrical conductivity. These factors are important for applications in catalysis [35, 36], nanocircuitry , plasmonic biosensing, or the nanoprobing of living cells .
Specifically adsorbing anions strongly affect the deposition kinetics and the product morphology in the disulfitoaurate-formaldehyde plating system. Especially the heavy halides bromide and iodide with their high affinity towards Au surfaces show pronounced effects even in catalytic concentrations. This observation is in accordance with the high sensitivity of colloidal Au crystal growth towards the presence of specifically adsorbing anions [17, 18, 21]. Because of the increased bath stabilities and the improved plating behaviour, the addition of (pseudo)halides and optionally EDTA to disulfitoaurate-formaldehyde-based electroless plating baths is a facile route towards Au thin films and derived nanomaterials of variable morphology. Significantly reduced plating rates allow to cover substrates with complex and badly accessible surfaces with homogeneous Au NP films. In contrast to the standard plating procedure, sophisticated structures such as homogeneous and stable Au NTs of high aspect ratio could be synthesized using the refined plating procedures. With maximum tube entrance diameters between 150 and 200 nm, the presented NTs approach aspect ratios of more than 150. The stable Au NTs could be freed from the template without fragmentation, allowing straightforward processing and application of the nanostructures for example, in heterogeneous catalysis or sensing.
The authors thank Professor Reinhard Neumann and Dr. Christina Trautmann (GSI Helmholtz Centre for Heavy Ion Research) for support with the template preparation and for the possibility to use the SEM of the materials research group. Thanks are given to Ulrike Kunz for sample preparation and TEM imaging. The supply of gold solution by Schütz Dental GmbH is gratefully acknowledged.
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