Template-Free Electroless Plating of Gold Nanowires: Direct Surface

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Template-Free Electroless Plating of Gold Nanowires: Direct Surface Functionalization with Shape-Selective Nanostructures for Electrochemical Applications Falk Muench, Sandra Schaefer, Lorenz Hagelüken, Leopoldo Molina-Luna, Michael Duerrschnabel, Hans-Joachim Kleebe, Joachim Brötz, Alexander Vaskevich, Israel Rubinstein, and Wolfgang Ensinger ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09398 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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ACS Applied Materials & Interfaces

Template-Free Electroless Plating of Gold Nanowires: Direct Surface Functionalization with Shape-Selective Nanostructures for Electrochemical Applications Falk Muench,*,† Sandra Schaefer,‡ Lorenz Hagelüken,‡ Leopoldo Molina-Luna,‡ Michael Duerrschnabel,‡ Hans-Joachim Kleebe,‡ Joachim Brötz,‡ Alexander Vaskevich,† Israel Rubinstein,† and Wolfgang Ensinger‡ †

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 7610001, Israel Department of Materials and Earth Sciences, Technische Universität Darmstadt, Alarich-Weiss-Straße 2, 64287, Germany ‡

ABSTRACT: Metal nanowires (NWs) represent a prominent nanomaterial class, the interest in which is fueled by their tunable properties as well as their excellent performance in e.g. sensing, catalysis and plasmonics. Synthetic approaches to obtaining metal NWs mostly produce colloids or rely on templates. Integrating such nanowires into devices necessitates additional fabrication steps such as template removal, nanostructure purification or attachment. Here, we describe the development of a facile electroless plating protocol for the direct deposition of gold nanowire films, requiring neither templates nor complex instrumentation. The method is general, producing three-dimensional nanowire structures on substrates of varying shape and composition, with different seed types. The aqueous plating bath is prepared by ligand exchange and partial reduction of tetrachloroauric acid in the presence of 4-dimethylaminopyridine and formaldehyde. Gold deposition proceeds by nucleation of new grains on existing nanostructure tips, and thus selectively produces curvy, polycrystalline nanowires of high aspect ratio. The nanofabrication potential of this method is demonstrated by producing a sensor electrode, whose performance is compared with that of known nanostructures and discussed in terms of the catalyst architecture. Due to its flexibility and simplicity, shape-selective electroless plating is a promising new tool for functionalizing surfaces with anisotropic metal nanostructures.

KEYWORDS: electroless plating; pyridine; gold; nanowires; in-situ surface functionalization; electrocatalysis; peroxide sensing 1. INTRODUCTION To facilitate nanomaterial implementation, there is a strong need to develop flexible, simple and robust nanofabrication strategies which do not rely on intricate techniques such as cleanroom processing.1 Wet-chemical surface functionalization techniques, enabling the direct modification of device parts without the need for separate nanostructure synthesis, purification and assembly steps, represent promising candidates for this task. Considering the field of sensor applications, such methods have been used to deposit functional films of one-dimensional (1D) nanostructures, combining large surface areas and efficient diffusive access.2,3 Electroless plating (EP) represents a versatile, solutionbased approach to metal thin film deposition, which can be adapted to nanomaterial production.4-15 EP reactions are based on metastable redox pairs formed by (complexed) metal cations and reducing agents. Due to the reaction energy barrier, metal deposition is confined to catalytic surfaces. The autocatalytic nature of EP results in a contin-

uously growing metal film,16 allowing the adjustment of the metal loading and the realization of nanoscale deposits by controlling the deposition time.8 The conformal growth mode allows to homogeneously coat substrates of almost any shape and composition, including photopolymerized lattices,4,5 track-etched membranes,6 electrospun fiber films,7,8 nanosponges,9 spirulina algae,10 and solid foams.11 Likewise, more conventional materials such as paper12 and carbon powder13 can be employed as substrates for metallization. Due to the wide range of accessible morphologies and the functional versatility of nanostructured metals, the resulting products can be tuned to specific fields, including transparent and flexible electrodes,7 anisotropically conductive composites,10 heterogeneous catalysis,13-15 sensing,6,11 plasmonics,4,8,9 and energy storage.12 Compared to competing deposition techniques such as electroplating16 and chemical vapor deposition,17 EP requires neither special substrate properties (e.g., conductivity or thermal stability) nor costly equipment. It is performed at moderate temperatures by simply immersing substrates in aqueous deposition baths, resulting in easy scalability. While the EP deposition method is simple, the substrates required for nanofabrication are often not. Traditionally, EP has been used for producing smooth coatings, provid-

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ing work pieces with electrical conductivity, wear or corrosion protection.18 While EP struggles to produce intensely roughened and nanostructured metal films, these morphologies can be realized using post-synthetic treatments such as thermal wrinkling of underlying polymer substrates.19 To obtain anisotropic nanostructures of special interest (e.g., metal nanotubes,6 NWs,6,14,20 nanorods6,21 or networks composed thereof),6 EP has to be combined with templates, which mold the deposit into such special shapes. To achieve the often desired target morphology of metal NWs,22 EP can be guided either by solid 1D templates (such as DNA assemblies20 or peptide fibers)23 or by templates enclosing 1D pores (such as the nanochannels within track-etched polymers6 or mesoporous silica).14 However, the need for templates complicates the overall fabrication process and interferes with the otherwise favorable characteristics of EP. For instance, for DNA-guided EP of Au NWs, the template first has to be assembled, then covered with pre-synthesized Au NRs, which are finally merged with EP to yield Au NWs of moderate aspect ratios.20 The production of templates can be laborious, and templates often have to be removed prior to application of the deposited nanomaterial. Moreover, templates impair the atom economy of the overall process by introducing auxiliary materials, which are not an essential part of the final product. The need for templates may be overcome by outfitting EP reactions with intrinsic shape control, allowing for the direct deposition of films composed of anisotropic nanostructures. Coordination chemistry is a valuable tool for realizing metal colloids of defined morphology. With the addition of appropriate ligands, the reduction of metal ions in the mostly organic solvents can be tailored to yield dispersed nanoparticles (NPs) of specific shape.24 Ligands affect the reactivity of such colloidal systems both by bonding to the educts (complex formation with the metal ions) as well as to the products (adsorption on the evolving metal nanostructures).24 Here, this strategy is extended to EP as a method for surface-selective metal deposition from aqueous solution. For the first time, metal NWs are produced using EP without replicating the 1D features of either porous or solid templates. The suppression of nanostructure formation in the bulk solution – which is the key to selective material deposition at the solution-substrate interface – distinguishes this approach from colloidal NW syntheses such as polyol reactions.25 Shape control in the EP of Au NWs is achieved by devising a plating bath based on the ligand 4dimethylaminopyridine (DMAP), which fulfills both mechanistic roles described above. First, DMAP serves as a ligand, assisting the formation of metastable Au complexes in the plating bath, which ensures surface-selective, seedmediated deposition. Second, DMAP adsorbed on the evolving Au nanostructures controls the nucleation of new nanocrystals, guiding the NW growth. Metal NW films combine good electrical conductivity with high surface area, reactivity and porosity, furnishing a unique material class for electrochemical applications.26-28 Here, we demonstrate the efficiency of the Au NWs deposited on a common carbon fiber electrode29,30 in amperometric sensing of H2O2.

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2. EXPERIMENTAL SECTION Materials. DMAP (Fluka, ≥99.0%); HAuCl4 30% in diluted hydrochloric acid (Aldrich, 99.99%); HCHO solution 36.5%, stabilized with methanol (Fluka, puriss. p.a.); H2O2 30% in water, stabilized (Sigma-Aldrich); Na2HPO4 × 2 H2O (Sigma-Aldrich; puriss. p.a.); NaH2PO4 × H2O (Sigma-Aldrich; puriss. p.a.); NaAuCl4 × 2 H2O (Alfa Aesar, 99.99%). All procedures were performed with purified water (>18 MΩ). Prior to use, glassware was cleaned with aqua regia. Seeding. Most substrates require the addition of catalytic seeds to initiate metal deposition from the metastable EP solutions. In our case, polycarbonate foil (Makrofol N, Bayer Material Science AG) was coated with Pd NPs by the reaction of absorbed borane dimethylamine complex with [PdCl4]2solution.31 Carbon fiber cloth (SGL Carbon SE) was functionalized with Ag NPs by reaction of Ag(I) with adsorbed Sn(II).32 Gold deposition. The Au plating bath was prepared by dissolving DMAP in a diluted solution of HAuCl4, followed by the addition of formaldehyde at room temperature, and heating to 80 °C. The final bath contained 105 mM DMAP (ligand / adsorbate), 7 mM HAuCl4 (metal source) and 490 mM HCHO (reducing agent). Plating was performed by immersing seeded substrates in the plating bath at 80 °C. The reaction time was determined by the desired amount of Au to be deposited. Typically, it took 20 min to deposit films of highaspect-ratio NWs, such as those used for the electrochemical experiments. After plating, the materials were thoroughly washed with water and dried. Control experiments with 75% and 200% of the DMAP concentration were performed to evaluate the effect of ligand excess on the nanostructure formation. Characterization. Scanning electron microscopy (SEM): Samples were imaged with a JOEL JSM-7401F microscope using an acceleration voltage of 8–10 keV. Scanning transmission electron microscopy (STEM): Samples were prepared as previously described.33 STEM was carried out in a JEOL ARM 200F operating at 120 kV acceleration voltage to reduce beam damage. Further experimental details are similar to those listed in our previous work.33 X-ray diffraction (XRD): The XRD investigations were performed with a Seifert PTS 3003 diffractometer using a Cu anode and an X-ray mirror on the primary side. On the secondary side a long soller slit and a graphite monochromator was used to separate the Cu K-alpha line, operated at 40 mA and 40 kV. Ultravioletvisible (UV-Vis) spectroscopy: A detailed description of the sample preparation can be found in section 1 of the Supporting Information. The measurements were conducted with a Varian Cary 50 Probe spectrophotometer using freshly prepared solutions in a quartz cuvette (1 mm optical path length). Electrochemistry. The electrochemical measurements were conducted with a Gamry Reference 600 potentiostat in a three-


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electrode setup (Pt mesh counter electrode; Hg|Hg2SO4 reference electrode (in 0.5 M H2SO4); glassy carbon working electrode (3 mm diameter)). To prepare the catalyst electrode, a piece of the Au-coated carbon fiber felt (size: 9 mm²) was glued to the working electrode with carbon paste.6 Phosphate buffer, which was prepared by dissolving sodium hydrogen phosphate and sodium dihydrogen phosphate in water (pH 7, 100 mM total phosphate concentration), served as the base electrolyte. Before the experiments, the electrolyte was purged with nitrogen gas (15 min). During the experiments, the temperature was held constant at 25 °C, and nitrogen gas flow was maintained above the electrolyte. Cyclic voltammetry (CV) measurements employed a scan rate of 20 mV s-1. The working electrode (with / without catalyst) was measured in bare phosphate buffer as well as in buffer containing 10 mM H2O2. During the sensing measurement, 20 µl volumes of H2O2 stock solutions of varying concentration (0.25 mM; 0.5 mM; 5 mM; 25 mM; 100 mM; 500 mM) were injected into 10 mL of stirred electrolyte every 100s under potentiostatic conditions (-0.96 V). 3. RESULTS AND DISCUSSION Bath design and characterization. The starting point of the plating bath development was our observation of short, rod-like NPs in an electroless Au plating reaction which utilized [Au(SO3)2]3- as the metal source and a relatively high concentration of DMAP as a reaction moderator.34 We assume that the formation of anisotropic Au structures can be explained by the ability of DMAP to preferentially adsorb on specific Au facets (depending on pH and potential).35 Mixing DMAP with a second ligand is known to reduce the anisotropy in Au colloids.36 Accordingly, to enhance the anisotropic growth, we aimed to remove sulfite from the EP bath, which shows a strong affinity to Au37 and thus is expected to compete with DMAP. This was realized by switching to [AuCl4]- as the metal precursor, in which the chloride ligands can be exchanged relatively easily. After stabilization of the Au(III) source with excess DMAP, formaldehyde was added. The resulting deep-yellow solution was heated to obtain the final, light-yellow plating bath. Formaldehyde was chosen as the reducing agent to ensure metastability. Homogeneous nucleation of NPs in the bulk solution, common when using strong reducing agents such as NaBH4,36,38 is prevented in the present case. The processes occurring during mixing of the EP bath were analyzed by recording UV-Vis spectra of solutions at the different preparation stages. In addition, the ligand DMAP was characterized in different solutions (Figure 1). While a detailed speciation of the complexes present in the reaction solutions is beyond the scope of this work, several major chemical changes during plating bath preparation could be successfully tracked with this method. The diluted aqueous solution of the deeply orangecolored NaAuCl4 appears colorless (Figure 1 a, orange line). In the UV-Vis spectrum, the lowest energy absorption peak is found at ~ 290 nm, corresponding to a ligand-tometal charge-transfer complex.39 The fact that only a

shoulder-shaped peak is found, which is shifted from the position expected for [AuCl4]- (~ 310 nm), indicates partial hydrolysis and replacement of chloride by hydroxide.39 DMAP solutions in water are alkaline, containing unprotonated molecules as well as pyridinium ions (pKa(DMAP) = 9.7).34 The two species show different absorbance spectra, resulting in an overlapping spectrum (Figure 1 b, straight turquoise line). Excess of either HCl or NaOH was used to differentiate between the species of the conjugate acid-base pair (Figure 1 b, green and blue lines).

Figure 1: (a) UV-Vis spectra of solutions corresponding to the different bath preparation stages, containing 0.175 mM NaAuCl4, and optionally 0.875 mM DMAP / 12.5 mM HCHO. (b) UV-Vis spectra of solutions containing 0.175 mM of DMAP, and optionally an excess of HCl or NaOH to promote / suppress the formation of HDMAP+.

When adding DMAP to the Au(III) solution (Figure 1 a, red line), the lowest energy absorption peak is shifted to ~ 382 nm (Figure 1 a, inset), responsible for the yellow color of the sample. For this experiment, a sufficiently high DMAP:Au ratio of 5:1 was chosen to allow for a practically complete coordination of the Au(III) centers (i.e. > 4 equivalents of DMAP), but at the same time limit the contribution of free DMAP to the absorbance spectra. We ascribe the peak at 382 nm to a DMAP(π)-metal(σ*) transition, which is close to values of the [Au(DMAP)4]3+ complex



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calculated and found in acetonitrile solvent.40 In addition, a strong peak appears in the UV (maximum at ~ 291 nm), which is assigned to intraligand transition. A similar, but less intense peak is found for the Au-free ligand solution (~ 262 nm for unprotonated DMAP, which is shifted to ~ 282 nm for HDMAP+; see Figure 1 b). Likewise, a red-shift is expected when DMAP bonds to Au cations instead of H+. These observations strongly suggest the replacement of Au(III) precursor ligands (OH-, Cl-) by DMAP. When adding formaldehyde to the solution containing DMAP and NaAuCl4, no marked change is initially observed (compare the dotted violet and the red lines in Figure 1 a). This result is in stark contrast to the immediate reduction of DMAP-Au(III)-complexes by NaBH4,38 and verifies the comparably mild nature of formaldehyde as the reducing agent in our system. After heat treatment (Figure 1 a, dark blue line), a blueshift and weakening of the UV absorbance band is observed (maximum at ~ 287 nm). Similarly, the lowest energy absorption peak is blue-shifted and appears as a shoulder of the main UV peak (Figure 1 a, inset). The overall reduced absorbance in the visible spectral regions explains the faint yellow coloring of the final EP bath. Probably, the color fading during the heat treatment is linked to partial reduction of Au(III) to Au(I).40 Contrary to Au(III), d-d- transitions are forbidden in the Au(I) ion with its d10 electron configuration. The absence of sediments and of any absorbance above ca. 450 nm (see Supporting Information, Figure S1) precludes the presence of plasmonic Au NPs,41 indicating that the Au precursor is not reduced to elemental metal, i.e. homogeneous Au nucleation is substantially suppressed. Electroless nanowire plating. Au deposition can be triggered by immersing a catalytically active substrate into a heated solution containing the Au complex and formaldehyde. As a test substrate, we employed polycarbonate foils, covered with roundish, faceted Pd NPs obtained by the interfacial reaction of absorbed dimethylamine borane with tetrachloropalladate solution.31 The Pd NPs initiate the Au plating, resulting in the formation of a surface film composed of a multitude of particle types, including anisotropic varieties such as bipyramids, bipods and spikes (Figure 2 a). Similar to kinetically controlled nanocrystal syntheses, toning down the EP reaction with the ligand and adsorbate DMAP facilitates selective Au overgrowth and the generation of multiple, distinguishable particle variants such as those as seen in Figure 2 a. While conventional EP leads to the formation of metal islands which at later stages merge to dense deposits,31,42 we find evolution of separate nanostructures. With ongoing Au deposition, 1D growth begins to dominate, and NWs emanate from the particles comprising the initial film (Figure 2 b). The NWs display a curved structure and a modulated diameter. At this stage, wires of several hundred nanometers in length coexist with short stubs, while the

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gaps in the gold surface film continue to close. The formation of elongated NPs and NWs corroborates the (at least partial) reduction of the Au(III) precursor to Au(I) during plating bath preparation, as indicated by the spectroscopic measurements (Figure 1): Direct reduction of Au(III) to the elemental metal in the presence of DMAP was reported to proceed quickly and without directed growth.38 Further Au deposition leads to extension of the NWs, resulting in the formation of a highly porous, web-like coating, which completely covers the initial metal film (Figure 2 c). Scratching off the wire layer reveals that Au deposition almost completely ceases on the surface film, albeit more spiky outgrowths can be found on the remaining particles (see Supporting Information, Figure S2). As determined from the SEM images, the mean wire thickness is 26 ± 8 nm (see Supporting Information, Figure S3). The NW diameter remains approximately constant from base to tip. Lack of thickness increase of existing NW parts despite the ongoing deposition emphasizes the high selectivity of the 1D growth, suggesting passivation of the NW sides. The dynamic formation of high-aspect-ratio nanostructures as a result of differing growth rates points to kinetic control as the underlying reason for the observed shape selectivity.43 The NWs evolve from an immobilized mixture of seed particles. This mechanistically distinguishes the reaction from colloidal approaches, in which metal NWs can be produced by polyol reactions,25 particle agglomeration,44 sacrificial templating45 or directed attachment.46 Seed-mediated approaches commonly rely on careful adjustment of defects present in the seed crystals in order to achieve shape selectivity, and thus are extremely sensitive to the reaction conditions in the seeding stage.25 In contrast, our system favors the growth of formed NWs, resulting in absolute dominance of the NW morphology at later deposition stages, even though the starting film comprises varied deposit shapes. This characteristic greatly simplifies the synthesis: The deposition reaction can be combined with the typically robust, flexible and facile seeding procedures developed for EP.31,32 While Au NW deposition has also been realized by transferring a directed attachment strategy46 to an electrode surface,47 this system still follows the typical characteristics of colloidal NP syntheses.24 It employs organic solvents, and produces NPs in the bulk solution by homogeneous nucleation.46 NWs are then created by aligned attachment of the individual NP building blocks.46 In contrast, EP reactions are characterized by genuinely heterogeneous, continuous deposition on the autocatalytic deposit, which is fed by a steady supply of reducing agents and metal complexes from the plating bath.


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Figure 2: SEM characterization of the evolving Au film. (a) Beginning Au deposition on the seed layer. Various anisotropic particle variants can be found, including spiky outgrowths (triangles), bipyramids (circles) and bipods (squares). Inset: magnifications of marked particles. (b) Beginning NW growth on the Au surface film. The arrows mark wire tips. (c) Fully evolved Au NW film.

The outlined reaction is not restricted to planar geometries, polymer substrates or a specific seed metal. Keeping the polycarbonate foil as substrate while replacing the Pd NPs31 with Ag NPs32 also resulted in the formation of Au NWs. However, narrower NWs, ~ 10 nm in diameter, were obtained (Fig. S4, Supporting Information). This is probably related to the small size of the Ag NPs in conjunction with simultaneous galvanic replacement of Ag by the Au in the plating bath, and potential alloying. As a result, small NPs are found in the Au surface film after initiation of the EP. In contrast, the compact Au shells formed on the Pd seeds result in larger size of the Au NPs in the initial film before wire formation (vide infra). In both systems, the diameter of the NWs is comparable to the size of the Au NPs in the surface film from which they evolve. As the diameter of the NWs is maintained throughout their growth, it can be modified by changing the conditions of their initial nucleation, as shown by variation of the seed type. NW deposition is also viable on curved carbon substrates (fibers), as shown below.Au NW formation can be optically tracked by the color change of the substrate, which turns dark bronze during the deposition reaction (Fig. S5, Supporting Information). While the web-like nanowire coating does not easily delaminate (Fig. S5 c, Supporting Information), it can be displaced by scratching (Fig. S2, Fig. S6, Supporting Information).

line Au. The broad hump at low angles is attributed to the polymer substrate.

The phase composition of the deposited material was analyzed by XRD (Figure 3). All reflections can be assigned to polycrystalline Au. Figure 4 a shows a low magnification STEM bright-field image of the Au surface film. The magnified part of the surface film shown in Figure 4 b reveals that the film consists of single overlapping Au nanoparticles. STEM Zcontrast images of single particles display a core-shell structure (Figure 4 c). The Pd (Z=46) cores are about 7 nm in diameter and are darker than their surrounding Au (Z=79) shells. Figure 4 d shows a high magnification STEM bright-field image of the Au tip of a surface film particle. On top of a twin boundary, a metal outgrowth is found, indicating the starting point of NW growth. Figure 5 illustrates the nanostructure of fully developed Au NWs. Figure 5 a shows a low magnification STEM bright-field image of a representative area of the Au NW film. Figure 5 b-e show magnified parts of several nanowire tips. Figure 5 c and d enable to identify crystal orientations and defective structures, i.e. twin and grain boundaries. In the case of Au NW formation by directed attachment, high angle grain boundaries were not found due to the defined mutual alignment of the constituting NPs.46 Defects strongly affect the reactivity of metal nanocrystals. As the EP reaction yields Au NWs with a high grain boundary density, it represents a promising synthetic tool for designing multicomponent nanostructures48 and active heterogeneous catalysts.49 In the course of the repeated nucleation at the NW tips, the growth direction and the diameter of the NWs continuously change, resulting in the curvy, diameter-modulated appearance of the nanostructures (Figure 2, Figure 5, Figure S8 (Supporting Information)).

Figure 3: X-ray diffractogram of the Au NW covered polycarbonate foil, including the reference positions for polycrystal-



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Figure 4: (a) Low magnification STEM bright-field image of the surface film. (b) Magnified part of (a). (c) HAADF Zcontrast image of single Pd/Au core-shell structures. (d) High magnification STEM bright-field image of a twin boundary, overgrown by Au.

Figure 5: (a) Low magnification STEM bright-field image of a Au NW film. (b-e) magnified parts of several nanowire tips. The nanowire tip shown in (e) is polycrystalline (indicated). Insets in (c) and (e) are fast Fourier transformations of the corresponding images, respectively.

Based on the previous results, we propose the following growth scheme (Figure 6): During bath preparation, both the coordination environment and the oxidation state of the Au source is changed, albeit not necessarily quantitatively (Figure 6 a). The initial chloride ligands are replaced by DMAP, while formaldehyde addition and heating is expected to cause at least partial reduction of Au(III) to Au(I). In the presence of a seeded substrate, the Au precursors are reduced to the elemental state, while formaldehyde is oxidized to formate (Figure 6 b). This reaction results in the formation of Au shells on the Pd seeds, from which NWs start to nucleate. Continued NW growth leads to the evolution of a web-like film composed of intertwined high-aspect-ratio Au nanostructures. While direct observation of the NW formation, using e.g. in situ TEM, would be required to unequivocally identify the dominant growth mechanisms, several key results can


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be pointed out on a phenomenological level. Evidently, Au deposition is heavily restricted to the NW tips. It occurs during repeated nucleation of new grains and change of the growth direction, resulting in the polycrystalline and curvy nature of the Au NWs (Figure 5). The NW sides and the initial NP film appear passive.

Figure 6: Schematic presentation of the shape-selective Au deposition reaction. (a) Reactions occurring during plating bath preparation. (b) Morphology evolution during EP, starting from Pd seeds, which are covered with Au shells, from which NWs start to nucleate. Inset: The redox half-reactions occurring on the autocatalytically active Au deposit.

This growth mode bears an intriguing resemblance to electrocrystallization of elongated multigrain structures in the presence of inhibitors.50 The inhibitors deactivate old grains and direct the metal deposition to the newly produced grains.50 Similarly, in our case, Au is selectively deposited on the fresh surface of active, growing grains at the advancing NW edge. The ligand and NP protecting agent DMAP, which is present in excess, is most likely the inhibitor in our system, deactivating the older parts of the Au deposit, i.e., the NW sides and the surface film. Thus, NW thickness growth and branching are impeded. We speculate that the high NW tip reactivity involves the oxidation half reaction and a delayed formation of a DMAP adsorbate layer. Formaldehyde oxidation involves hydroxide ion consumption, which is likely to result in local pH reduction in the proximity of the NW tip (Fig. 6 b, reaction inset).

This would in turn increase the degree of DMAP protonation, which is known to reduce the DMAP-Au interaction strength.35,51 The interpretation based on adsorbed DMAP as the underlying cause of surface deactivation is supported by control experiments, in which the amount of DMAP in the Au plating bath was varied. Using a reduced excess of DMAP prevented the evolution of Au NWs, while an increased DMAP concentration resulted in NW formation, which however possess a reduced aspect-ratio and less defined structure. Both observations are in line with the outlined mechanistic interpretation. To promote NW growth, a sufficient DMAP concentration is required to achieve efficient surface deactivation. Further increase of the DMAP concentration slows down the deposition reaction by affecting the autocatalytically active Au deposit.34 Amperometric peroxide sensing. The applicability of the Au NW film was demonstrated using carbon felt as an open-porous, electrically conductive support material (Figure S7, Supporting Information). Applying the Au NW plating reaction to this substrate resulted in the formation of a hierarchical wire-on-wire architecture (Figure 7), with a NW film thickness of ~ 600– 700 nm (Fig. S6 b, Supporting Information). The as-obtained material can be directly employed in electrochemical experiments. Given the direct growth of the NWs on the freestanding fiber support, no binder is required to attach the catalyst nanostructures to the electrode. The carbon fibers provide stability and continuous paths for electron conduction. Based on the mass gain after EP, the metal loading was determined as 0.69 mg cm-2 (geometric area of the carbon felt). Hydrogen peroxide sensing was chosen as a model application, allowing to compare our catalysts’ performance with that of a variety of structurally or compositionally related metal nanostructures. The electrochemical response of the catalyst was first evaluated using CV (Figure 8 a). Without the catalyst, only a negligible current response was measured in the tested potential range, in the presence and absence of the analyte. With the Au NW catalyst, a redox pair related to Au surface oxidation / reduction is observed.52 Using the Au oxide reduction charge to calculate the corresponding Au surface area,29 the specific surface area of the NW film was estimated as 8.5 m2Au g-1Au.



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Figure 7: SEM characterization of the electrocatalyst, showing (a) a NW-decorated carbon fiber imaged at low magnification and (b) the Au NW film surrounding the fiber.

Upon addition of H2O2, faradaic currents are observed which correspond to the catalytic reduction and oxidation of the analyte to H2O and O2, respectively. The peroxide oxidation regime overlaps the oxidation of the Au substrate. The reduction current is attributed to reduction of H2O2 and possibly oxygen (resulting from H2O2 disproportionation).53 The disappearance of the Au oxide reduction peak in the presence of H2O2 indicates surface oxide reduction by the analyte.54

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Figure 8: (a) CV of electrodes with and without Au NW catalyst (indicated) in phosphate buffer, in the absence / presence of 10 mM H2O2 (indicated). Scan rate: 20 mV s-1. (b) Amperometric peroxide sensing with the Au NW catalyst at a constant potential of -0.96 V. Inset: Two early additions of H2O2, magnified. (c) Peroxide sensitivity results, extracted from the measurements in (b).

Amperometric peroxide detection was performed by injecting defined volumes of H2O2 stock solutions into the stirred electrolyte under potentiostatic conditions, resulting in a stepwise increase of the reduction current (Figure 8 b). The constant potential chosen for the measurements (-0.96 V vs. Hg|Hg2SO4, i.e. ~-0.5 V vs. Ag|AgCl), is well within the H2O2 reduction regime of our catalyst (Figure 8 a). Similar values have been frequently employed in previous sensing studies.36,55-58 In the present study peroxide reduction rather than oxidation was chosen for the sensing experiments, for several reasons. In agreement with previous studies,36,55,56 higher currents (and thus sensitivities) can be realized with the reduction reaction (see Figure 8 a). Furthermore, with oxygen gas as the product under oxidizing conditions, bubbles form easily on the electrode, blocking its surface.


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Already the addition of the smallest amount of H2O2, corresponding to a concentration of 0.5 µM, could be clearly resolved with the sensor (Figure 8 b, red magnified curve). After analyte injection, new steady state currents were quickly reached (response time < 4 s). By plotting the reduction current density increase vs. the analyte concentration (Figure 8 c), the sensitivity within the linear range (up to 6 mM H2O2) was calculated as 6.43 mA mM-1 cm-2. Comparison of the sensor performance with literature data is complicated by the variability in the measurement conditions (varying electrolyte, pH, potential, electrode setup, sample geometry etc.). Nevertheless, the sensitivity achieved with the Au NW-coated carbon fiber electrode is quite outstanding: It exceeds by a wide margin values for related catalyst designs, most of whom were investigated under identical or similar experimental conditions (see Supporting Information, Table S2).6,29,36,55-60 Au has several favorable properties for electrochemical sensing (such as extraordinary chemical stability or convenient biomolecule attachment),55 and in nanostructured form, it can be unexpectedly active in the reduction of H2O2 and O2.61 However, it should be noted that H2O2 reduction proceeds on other metals (most prominently, Pt) with lower overpotentials. In the case of the Au NW catalyst, less negative reduction potentials can be employed without losing much of the H2O2 current response: At -0.2 V vs. Ag|AgCl, a sensitivity of 4.7 mA mM-1 cm-2 was found (Fig. S9, Supporting Information). Several factors may contribute to the very high catalyst sensitivity. Open-porous networks composed of 1D nanostructures combine continuous pathways for electron conduction and diffusion with a large active surface area, thus displaying especially efficient electrocatalyst architectures. Typical metal (Ag,6,58 Au,56 Pt)28 nanonetwork catalysts for H2O2 detection exhibit sensitivities between 1.34 and 1.70 mA mM-1 cm-2 (Supporting Information, Table S2). Arrays of separated 1D nanostructures usually exhibit lower sensitivities (e.g. 0.56 mA mM-1 cm-2 for Au nanotubes,55 or 0.365 mA mM-1 cm-2 for Ag NWs).57 Lower sensitivity is found for 0D catalysts (e.g. 0.1923 mA mM-1 cm-2 for Au NPs, 0.0337 mA mM-1 cm-2 for Ag NPs,59 and 0.0127 mA mM-1 cm-2 for graphene-supported Au NPs).60 Similarly, the pronounced electrochemical response of the web-like Au NW film can be attributed to its specific morphology. First, it consists of a high density of highaspect-ratio nanostructures, which provide a large electrochemically active surface area. Second, the NWs form an open-porous catalyst layer, which is not as compact as in the case of 0D particles and thus retains improved accessibility.61 Third, the NWs are directly grown on the support, in the form of a hierarchical wire-on-wire structure. No polymer binders are required to attain an adhering film. These additives, necessary for immobilization of catalyst powders,59,60 result in occlusion of active sites, and impose diffusion barriers.62 The high density of grain boundaries found in the NWs (Figure 5, Figure S8) could also contribute to their catalytic activity. Defects and low-coordinated atoms play a critical role in Au catalysis,63-65 a previous indicated a promoting effect of active surface sites on the reduction of hydrogen peroxide.65

Finally, the choice of DMAP for Au nanomaterial fabrication is advantageous when envisioning catalytic applications. The interaction of the DMAP molecule with Au surfaces is strong enough to induce anisotropic nanostructure growth34,36,38 or to stabilize Au NPs,51 but the binding remains reversible. Accordingly, the DMAP can be easily removed by simple washing procedures to expose unprotected and reactive Au surfaces.34,51

Figure 9: Selectivity test using injections of H2O2 and four interferents (concentration increase: 50 µM) under potentiostatic conditions (-0.96 V vs. Hg2SO4).

The addition of urea, ethanol, ascorbic acid and NaCl in concentrations equal to the analyte did neither cause noticeable current responses nor disturb the detection of a second H2O2 aliquot (Fig. 9). However, the relatively large current response found for the first two H2O2 addition series indicates that the system is not selective toward oxygen, which is present in the stock solutions and can compete with the analyte at such low concentrations (Fig. S10, Supporting Information). The fast response, the maximum of the linear range and the low detection limit of the NW-decorated carbon fibers are rather noteworthy; however, similar values could be achieved with other nanostructured catalysts (see Supporting Information, Table S2). 4. CONCLUSION We devised a solution containing the ligand and shapedirecting agent DMAP, the reducing agent formaldehyde and an Au source (tetrachloroauric acid), which can be used for direct deposition of Au NW films. It is important to note that we adhere to conventional activation procedures employed in EP, which introduce seed mixtures. Thus, the shape selectivity reported here does not originate from the specific nanostructure of uniform seeds, but is effected during metal deposition. The system follows the classical reactivity of EP: The bath is metastable, metal deposition occurs on catalytic surfaces which initiate the reaction, and then continues by autocatalysis. At the same time, the deposition reaction differs from conventional plating (overcoating substrates with compact films), as it produces nanostructures of a defined elongated shape. In contrast to traditional EP approaches, no hard, soft or sacrificial templates are needed to achieve the anisotropic product morphology. The underlying reason for shape



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selectivity is kinetically controlled nanostructure growth, promoted by the adsorbate-forming pyridine ligand. Following NW nucleation, competitive deposition with strong preference to the nanostructure tips results in the formation of high-aspect-ratio Au NWs, displaying a polycrystalline morphology due to frequent nucleation of new grains. The reaction combines traditional advantages of EP with the shape control achieved in the synthesis of metal nanocrystal colloids. It is a simple, fast, water-based approach which does not require sophisticated equipment and has a high NW yield. Intensely nanostructured Au films are directly deposited due to the specific bath chemistry, not by an extrinsic, post-synthetic modification. The reaction is generally applicable: Selective NW deposition can be performed using random seed mixtures and different seed metals, and is not restricted to certain substrate materials and shapes. Given its favorable characteristics, shape-selective EP is a promising novel route toward functional coatings. We are currently attempting to extend this approach to other metals and product shapes. Anticipated uses include fields already established for anisotropic metal nanostructures, such as heterogeneous catalysis, sensing, plasmonics, surface-enhanced Raman spectroscopy, and nanoelectrodes. In the field of micro- and nanofabrication, the use of defined seeding to create patterned NW films and arrays deserves further investigation.

ASSOCIATED CONTENT Experimental details of the UV-Vis measurements, sample photographs, additional SEM, STEM and amperometric characterization, wire thickness determination, synthesis of wires of reduced diameter, comparison of the sensor performance with the literature. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The synthesis was devised by F.M., the study was designed by F.M. and S.S., the depositions were performed by S.S., L.H. and F.M., the electrochemical experiments were conducted by F.M., S.S. and L.H., and the solutions / structures were characterized by F.M. (SEM, UV-Vis), J.B. (XRD), L.M.-L., J.K. and M.D. (STEM). Experiments and conclusions were discussed with A.V. and I.R. The manuscript was written by F.M., and refined through contributions of S.S., L.M.-L., M.D., A.V., I.R. and W.E.; all authors have given approval to the final version of the manuscript.

Funding Sources F.M. acknowledges a DFG Research Fellowship (MU 4125/1-1) by the German Research Foundation. Additional funding was provided by the Minerva Foundation with funding from the German Ministry of Education and Research (A.V. and I.R.) and the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE (S.S., M.D., L.M.-L.). The transmission electron microscope used in this work was partially

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funded by the German Research Foundation (DFG/ 682 INST163/2951). ACKNOWLEDGMENT We thank Ulrike Kunz for TEM sample preparation and SGL Carbon SE (Wiesbaden) for supplying a free sample of the carbon fiber support.

ABBREVIATIONS 0D, zero-dimensional; 1D, one-dimensional; CV, cyclic voltammetry; DMAP, 4-dimethylaminopyridine; NW, nanowire; NP, nanoparticle; SEM, scanning electron microscopy; STEM, scanning transmission electron microscopy; UV-Vis, ultraviolet-visible; XRD, X-ray diffraction.

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Table of contents / abstract graphic


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Figure 1. (a) UV-Vis spectra of solutions corresponding to the different bath preparation stages, containing 0.175 mM NaAuCl4, and optionally 0.875 mM DMAP / 12.5 mM HCHO. (b) UV-Vis spectra of solutions containing 0.175 mM of DMAP, and optionally an excess of HCl or NaOH to promote / suppress the formation of HDMAP+. 124x183mm (300 x 300 DPI)


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Figure 2. SEM characterization of the evolving Au film. (a) Beginning Au deposition on the seed layer. Various anisotropic particle variants can be found, including spiky outgrowths (triangles), bipyramids (circles) and bipods (squares). Inset: magnifications of marked particles. (b) Beginning NW growth on the Au surface film. The arrows mark wire tips. (c) Fully evolved Au NW film. 49x14mm (300 x 300 DPI)



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Figure 3. X-ray diffractogram of the Au NW covered polycarbonate foil, including the reference positions for polycrystalline Au. The broad hump at low angles is attributed to the polymer substrate. 44x23mm (300 x 300 DPI)


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Figure 4. (a) Low magnification STEM bright-field image of the surface film. (b) Magnified part of (a). (c) HAADF Z-contrast image of single Pd/Au core-shell structures. (d) High magnification STEM bright-field image of a twin boundary and a gold overgrowth. 84x84mm (300 x 300 DPI)



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Figure 5. (a) Low magnification STEM bright-field image of a Au NW film. (b-e) magnified parts of several nanowire tips. The nanowire tip shown in (e) is polycrystalline (indicated). Insets in (c) and (e) are fast Fourier transformations of the corresponding images, respectively. 84x147mm (300 x 300 DPI)


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Figure 6. Schematic presentation of the shape-selective Au deposition reaction. (a) Reactions occurring during plating bath prepara-tion. (b) Morphology evolution during EP, starting from Pd seeds, which are covered with Au shells, from which NWs start to nucleate. Inset: The redox half-reactions occurring on the autocatalytically active Au deposit. 117x164mm (300 x 300 DPI)



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Figure 7. SEM characterization of the electrocatalyst, showing (a) a NW-decorated carbon fiber imaged at low magnification and (b) the Au NW film surrounding the fiber. 128x195mm (300 x 300 DPI)


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Figure 8. (a) CV of electrodes with and without Au NW catalyst (indicat-ed) in phosphate buffer, in the absence / presence of 10 mM H2O2 (indicated). Scan rate: 20 mV s-1. (b) Amperometric peroxide sensing with the Au NW catalyst at a constant potential of -0.96 V. Inset: Two early additions of H2O2, magnified. (c) Peroxide sensitivity results, extracted from the measurements in (b). 138x224mm (300 x 300 DPI)



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Figure 9: Selectivity test using injections of H2O2 and four interferents (concentration increase: 50 µM) under potenti-ostatic conditions (-0.96 V vs. Hg2SO4). 49x29mm (300 x 300 DPI)


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TOC graphic 83x44mm (300 x 300 DPI)


Template-Free Electroless Plating of Gold Nanowires: Direct Surface

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