Cyanide-free preparation of gold nanowires: controlled crystallinity, crystallographic orientation and enhanced field emission

Abstract

The environmentally friendly preparation of nanomaterials with controlled structural features represents a development trend of nanoscience and nanotechnology. In this work, using a cyanide-free bath, gold nanowires with controlled crystallinity and preferred crystallographic orientation have been prepared by electrochemical deposition in home-made polycarbonate ion track-etched templates. Single-crystal and polycrystal gold nanowires with preferred orientations along the [111] and [100] directions have been obtained by selecting fabrication parameters. The influence mechanisms of nanopore diameter, applied voltage, and deposition temperature on structural properties are proposed. In addition, single-crystal nanowires with [100] preferred orientation show enhanced field emission, which may be attributed to their single-crystal structure and the lower work function of loosely packed crystal planes.

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1. Introduction

Metallic nanowires (NWs) have attracted significant interest due to their peculiar mechanical, electrical, optical, thermal and magnetic properties in comparison with their bulk counterparts.^1–4 Based on these, various potential applications have been developed in fields such as plasmonics,⁵ nanobiotechnology,⁶ optics,⁷ nanoelectronics,⁸ photovoltaics,⁹ superconductivity,¹⁰ (photo)catalysis^11,12 and molecular sensing based on Surface Enhanced Raman Scattering.^13,14 Numerous experimental and theoretical studies have confirmed that the physicochemical properties of NWs are considerably dependent upon their structural characteristics.^15–20 For instance, the magnetic properties of Fe NWs are highly dependent upon their crystallinity¹⁸ and the crystallographic orientation of NWs can remarkably influence the thermal expansion properties of Cu NWs.¹⁶ As two dominant structural properties, crystallinity and crystallographic orientation have been a research focus in nanotechnology.

To fabricate NWs, a number of methods have been developed, including template-assisted electrochemical deposition,^21–27 seed-mediated growth,²⁸ chemical vapour deposition,²⁹ lithography,³⁰ hydrothermal reduction³¹ and molecular beam epitaxy.³² Among these methods, the ion-track template approach combined with electrochemical deposition has been extensively applied to fabricate metallic, semiconductive and conductive polymeric NWs in virtue of its simplicity and versatility. Using this method, diameter (>10 nm), length (∼μm), shape (cylindrical, conical, rhombic, etc.), aspect ratio (tens to tens of thousands), and areal density (single to 10¹⁰ cm⁻²) of NWs can be controlled by template structural parameters, heavy ion irradiation, and chemical etching of ion tracks.^21,24,33 In addition, the control over crystallinity and crystallographic orientation of NWs has also been achieved.^{21,24,33–35}

Gold possesses excellent electrical and thermal conductivities, high ductility, and chemical inertness, which has fueled a wave of research on gold NWs.^3,7,31,36 For instance, gold NWs have shown very large yield strength compared to that of bulk nanocrystalline gold (∼100 times).¹ Nanomembranes made of ultrathin gold NWs of 2.5 nm in diameter are mechanically strong, optically transparent and electrically conductive, showing promise for applications in future lightweight foldable optoelectronics.⁹ Highly crystalline gold NWs with atomically smooth surfaces show near-bulk conductivity and can serve as potential interconnects for future nanocircuits.³⁷ The [111]-oriented ultra-twinned gold NWs exhibit near-ideal theoretical strength where crystallographic orientations have a significant influence on their mechanical properties.³⁸ Single-crystal gold NWs have displayed periodic magnetoresistance oscillations induced by superconducting vortices.³⁹ Molecular dynamics simulations have revealed that the structural transition and melting of gold NWs are highly dependent on crystallographic orientations.¹⁷ Clearly, both crystallinity and crystallographic orientation can greatly influence the physicochemical properties of gold NWs.

Compared with extensive studies focusing on the various novel properties, micro-manipulation techniques and applications of gold NWs, less attention has been devoted to controlled fabrication of gold NWs. Using different electrolytes, pore sizes and other fabrication conditions, gold NWs featuring different crystallinity and crystallographic orientation have been reported by different research groups.^{21,24,33–35,40} Namely, the growth of single-crystal gold NWs was favored when gold cyanide electrolytes were used,^1,24,40 while polycrystal structures have usually been achieved by using gold sulfite electrolyte.²³ For crystallographic orientation, using a cyanide electrolyte (Doduco), gold NWs with preferential orientation along the [110] direction were obtained under potentiostatic deposition and the preferential orientation changed to the [100] direction when a reverse pulse deposition was used.^24,33 Furthermore, [111] crystallographically oriented gold NWs were also achieved by using a cyanide-based electrolyte provided by a different company (Orotemp).⁴¹ It is clearly seen that the structural properties of electrochemically deposited gold NWs are highly dependent on electrolyte and other fabrication conditions. The aforementioned findings have been reported by different research groups using different experimental conditions (electrolytes, pore diameters, applied voltages, deposition temperatures, etc.), which makes it difficult to generalize the conditions under which desired structural characteristics can be obtained. Therefore the challenge of achieving systematic control over crystallinity and crystallographic orientation of gold NWs is still unresolved. More importantly, the electrolytes which have been widely adopted to electrochemically fabricate gold NWs are based on cyanide.^{1,24,33,41–43} It is well known that cyanide compounds are extremely toxic and environmentally unfriendly, especially at high temperatures (50–70 °C).⁴⁴ Currently, the environmentally friendly fabrication based on cyanide-free electrolytes represents a development trend of controlled fabrication of gold NWs.

In the present work, an environmentally friendly method for fabrication of gold NWs with controlled crystallinity and crystallographic orientation is reported. The crystallinity and crystallographic orientation are systematically investigated with respect to fabrication conditions such as applied voltage, electrodeposition temperature and nanopore diameter. Furthermore, possible mechanisms of the influences of those parameters on structural properties are discussed. In the end, we show that the preferred crystallographic orientation can dramatically enhance the field emission performance of the gold nanowires.

2. Experimental details

The process of synthesizing gold NWs consists of several steps. First, polycarbonate foils (Makrofol N, Bayer Leverkusen) 30 μm in thickness were irradiated at the UNILAC linear accelerator of GSI (Darmstadt, Germany) with 11.4 MeV per nucleon ²³⁸U²⁸⁺ ions and at the Heavy Ion Research Facility at Lanzhou (HIRFL) with 9.5 MeV per nucleon ²⁰⁹Bi³¹⁺ ions at normal incidence. The irradiation fluence was 1 × 10⁹ ions per cm². After irradiation, each side of the polymer foils was exposed to ultraviolet light for 2 hours to enhance the track etching rate. This process, namely track sensitization, is necessary to produce highly cylindrical nanopores. In the next step, the membranes were chemically etched in a 5 M NaOH solution at 50 °C for between 0.5 to 4 minutes to obtain cylindrical pores with diameters ranging from 15 to 85 nm. During the etching process an ultrasonic field was applied to achieve homogeneous etching. Immediately after etching, the foils were washed with distilled water in an ultrasonic field to remove residual NaOH solution, especially from ion-track pores, avoiding excess etching. Subsequently, a thin gold film was sputtered onto one side of the foils and reinforced electrochemically by a Cu layer with a thickness of several micrometres. This backing layer served as the cathode, and a gold bar with a diameter of 2 mm served as the anode during the following electrochemical deposition of the wires. Gold NWs with different diameters were prepared by direct current electrochemical deposition at various electrodepositing voltages and temperatures. The electrolyte employed was an aqueous solution of Na₃Au(SO₃)₂ (75 g L⁻¹) which is more environmentally friendly compared with cyanide-based counterparts. The deposition processes were monitored by recording curves of deposition current versus time and the processes stopped once wire caps had formed atop the membrane surface, to ensure that the nanopores were entirely filled. The texture of the gold NW arrays was characterized by X-ray diffraction (XRD, RIGAKU D/Max-2400, Cu Kα, λ = 0.15406 nm) while NWs remained embedded in the templates. To make these examinations, both the wire caps and backing layer were removed. After dissolving the polymer membrane in dichloromethane (CH₂Cl₂), the morphology and crystallinity of the gold NWs were examined by scanning electron microscopy (SEM, FEI NanoSEM 450), high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20) and selected area electron diffraction (SAED). The field emission characteristics of the nanowires were measured in a vacuum chamber with a base pressure below 1 × 10⁻⁵ Pa. The measurements were conducted on a standard parallel-plate-electrode configuration where a stainless-steel plate was used as the anode, and each sample was fixed onto a copper stage which served as the cathode. The distance between the nanowire tip and the anode was 300 μm. The I–V (current–voltage) curves were recorded by a computer-controlled measurement system.

3. Results and discussion

The gold in the aqueous solution is in the form of a gold sulphite complex. During the electrochemical deposition of gold nanowires, the discharge of the gold sulphite complex consists of the following two steps:⁴⁵

Au(SO₃)₂³⁻ → AuSO₃⁻ + SO₃²⁻ (1)

AuSO₃⁻ + e⁻ → Au + SO₃²⁻ (2)

The SEM image of a typical polycarbonate template is shown in Fig. 1(a) and indicates that the nanopores are uniform in diameter, which ensures that the fabricated NWs possess a circular cross-section and narrow diameter distribution. Fig. 1(b) shows the optical microscopy image of a polycarbonate template with NWs embedded in it. The uniform color of the template resulted from SPR (Surface Plasmon Resonance), signifying that the NWs are evenly distributed in the template.² By dissolving the template matrix, the morphological characteristics of the Au NWs with a diameter of 75 nm were investigated by SEM. For preparation of this sample, the deposition temperature and the applied voltage were kept at 50 °C and 1.1 V, respectively. The low-magnification SEM image shown in Fig. 1(c) demonstrates that the NWs are homogenously distributed on the backing layer and aggregated after dissolving the template matrix. As the wire length is equivalent to the template thickness (30 μm), the aspect ratio of the NWs is expected to be as high as 400. The aggregation effect can be ascribed to the high aspect ratio, the high ductility of gold and the surface tension effect of solvent droplets when drying samples.⁴⁶ The high-magnification SEM image in Fig. 1(d) displays that each wire has a cylindrical shape and that the wire diameter is uniform along their length.

Fig. 1 (a) SEM image of the polycarbonate template with a nanopore diameter of 75 nm. (b) Optical microscopy image of the polycarbonate template with NWs embedded in it. (c) & (d) show the SEM images of the as-prepared gold NWs at low and high magnifications, respectively.

HRTEM is widely employed to characterise crystallinity of materials. In this work, using HRTEM, we have checked the crystallinity of gold nanowires for several micrometres along the wires’ length. If there is no grain boundary and only lattice fringes can be clearly observed, we conclude that they are single-crystal. By comparison, for polycrystal wires, we have observed some small grains with different crystallographic orientations. For each sample, we have checked several wires to confirm their crystallinity. Fig. 2 shows the HRTEM images of single-crystal and polycrystal NWs deposited at applied voltages of 0.9 V and 2.5 V, respectively, at a deposition temperature of 50 °C. The single and polycrystal structures were clearly illustrated by the images. As shown in Fig. 2(a), the lattice fringes line up along the wire length and the lattice spacing was measured to be 0.239 nm, corresponding to the (111) plane of face-centred cubic Au. We checked several micrometres along the wire length and no grain boundaries were observed. These results demonstrate that the wire possesses a single-crystal structure. By comparison, in Fig. 2(b), the lattice fringe spacing and orientation vary for different crystal grains (detailed within white circles) and they are arranged in an orderly fashion within each grain, which indicates a polycrystalline structure.

Fig. 2 HRTEM images of gold NWs fabricated at 50 °C. (a) Single-crystal (applied voltage of 0.9 V); (b) polycrystal (applied voltage of 2.5 V).

The electrochemical deposition is an intricate process that can be affected by many factors. Accordingly, the crystal structure of metal NWs synthesized by template-assisted electrochemical deposition is significantly influenced by deposition conditions and growth dynamics. The formation of single-crystalline and polycrystalline structures of Au NWs can be ascribed to a 2D layer-by-layer epitaxial mechanism and 3D nucleation–coalescence growth mechanism, respectively.^34,41,47,48 According to these two mechanisms, new grains will appear when the size of an initial cluster exceeds the critical dimension N_c during the deposition process. A larger N_c favours the single-crystal growth of NWs from an initial seed grain.^41,48 For 2D growth, N_c is expressed as

where s, ε, z, e, η and b are the area occupied by one atom on the surface of nucleus, the edge energy, the effective electron number, elementary charge, the overpotential and a constant, respectively. The overpotential η is defined as the difference between the external current induced potential E_i and the equilibrium potential of the electrode E₀ (η = E_i − E₀). For 3D growth, N_c is expressed aswhere V_m, σ and B are the volume occupied by one metallic atom on the surface of the nucleus, the surface energy and a constant, respectively. A low applied voltage corresponds to a low overpotential η. In accordance with formula (3) and (4), the lower the η, the larger the critical dimension N_c. Namely, a low applied voltage favours single-crystal growth of the NWs. With increasing applied voltage, N_c decreases and 3D clusters tend to grow independently of each other, coalesce and ultimately form polycrystalline NWs. These analyses are in good agreement with our experimental results shown in Fig. 2.

It should be mentioned that a higher deposition temperature not only promotes the surface diffusion of atoms and thus favours a 2D-like nucleus, but also accelerates the growth rate of the wires, which hinders 2D growth.⁴¹ Previous studies showed that a high overpotential may give rise to the surface adsorption of H ions or micelles in the electrolyte and thus considerably enhance the electrochemical reaction driving force, which increases the deposition rate of metallic ions during electrodeposition.³⁴ A fast growth rate of the NWs makes it difficult for atoms to diffuse freely and thoroughly, therefore they form polycrystalline structures. At a lower overpotential, it is easy to reach a steady diffusing process which favors single crystal growth. However, due to the constraint of driving force, metallic ions cannot be deposited in pores if the applied voltage is too low. In template-based electrochemical deposition of NWs, the growth progress can be monitored by a current–time curve.⁴⁹ The current rises sharply as wires reach the top surface of the template and the average growth rate is calculated as the ratio of template thickness and the time of current rise. In our experiments, as shown in the inset of Fig. 3, the time of current rise decreases dramatically and the average growth rate increases by a factor of four when the temperature increases from 30 °C to 80 °C. In addition, NWs can continue to grow at a lower overpotential in the presence of higher temperatures. That is, high temperatures can also enhance the electrochemical reaction driving force. For single-crystal growth, the presence of a high temperature increases the growth rate of NWs, but promotes the surface diffusion of atoms and reduces the minimum applied voltage required to grow wires, resulting in a larger N_c. For polycrystal growth, the enhancement of the growth rate caused by high overpotentials and high temperatures promotes 3D nucleation, although high temperatures can improve the ion diffusion thereby impeding 3D nucleation.

Fig. 3 Current–time curves for NWs deposited at different temperatures at 1.3 V. The inset shows the average growth rate and time of current rise versus temperature for these wires.

In addition to crystallinity, the preferential growth orientation of deposited NWs is found to be dependent on deposition conditions. Fig. 4(a) shows that the XRD spectra for the NWs of 25 nm diameter deposited at room temperature under applied voltages from 1.5 to 1.7 V exhibit a single peak corresponding to the (200) crystalline plane, indicating that the NWs possess a preferred growth orientation along the [100] direction. Upon decreasing the applied voltage to 1.3 V and lower, the NWs have a strong [111] orientation that dominates over the [100] orientation. At an elevated temperature of 50 °C, the orientation has a similar transition and the turning point occurs at 1.4–1.2 V as shown in Fig. 4(b).

Fig. 4 XRD patterns of Au nanowire arrays synthesized under different conditions (nanopore diameter, applied voltage and deposition temperature): (a) 25 nm, 1.0–1.7 V, room temperature; (b) 25 nm, 0.8–1.5 V, 50 °C; (c) 17–75 nm, 1.1 V, room temperature; (d) 17 nm, 0.9–1.4 V, room temperature.

In the present work, the NWs are deposited on the sputtered gold films which are polycrystalline. As reported previously, the polycrystal gold substrate has no effect on the preferential growth orientation of NWs.^27,35,48 In this case, in the initial stage of the wire growth, the crystallographic orientation of individual nuclei is random.⁵⁰ From an energy point of view, grains with lower surface energy grow faster than those with higher surface energy. The competition between adjacent crystal grains is inevitable because of the spatial restriction effect of the nanopores. Once a grain with preferential growth orientation survives, the consequent growth will be along this direction and form a single-crystal wire, according to the 2D growth mode. The transition length from 3D growth to 2D growth greatly depends on the deposition conditions.⁵⁰ If the conditions disfavour 2D nucleation, the NWs will grow under the 3D nucleation mode all the time and possess a polycrystal structure.

The surface energy of the Au crystals decreases in the sequence of (220), (200), and (111) planes, and thus the (111) plane is more likely to appear during deposition. However, as reported previously, the surface energy of crystalline planes can be highly influenced by adsorption of H ions in electrolytes and a high applied voltage may make H ions appear on the cathode and thereby promote their adsorption.^34,51 Compared with intrinsic low-surface-energy planes, high-surface-energy planes are more preferable for the adsorption of H ions to meet the principle of minimum free energy. This adsorption could make the surface energy of the crystal planes decrease and even lower than that of previous low surface energy planes.^21,47,48 The surface energy of the (220) plane is high, such that the adsorption of H ions is not sufficient to make it possess the lowest surface energy, but the (200) plane has a relatively low surface energy which can. Therefore, the NWs prefer to grow along the [100] direction (perpendicular to the (200) plane) at higher voltage. Temperature could lower the voltage exhausted in electrolyte by accelerating the mobility of ions. Hence, the effective voltage at 50 °C is higher than that at room temperature when applying a constant voltage. That is why the voltage at which the growth direction of wires changes from [111] to [100] is lower at 50 °C than at room temperature. In Fig. 4(b) the relatively weak peak of the (200) plane appears at low voltage at 50 °C because the temperature causes a change of the H adsorption and overpotential and thus disturbs the competition between the (111) and (200) planes, but at higher voltage the effect of adsorption is dominant and refrains that of temperature. Fig. 4(c) displays the XRD patterns of the NWs with different diameters deposited at voltage of 1.1 V at room temperature. The NWs with diameters of 17 and 25 nm have a preferential orientation along the [111] direction. With increasing NW diameter, the relative intensity of the diffraction peak corresponding to the (111) plane gradually abates and that of the (200) plane heightens. When the diameter reaches 75 nm, a strongly preferred orientation along the [100] direction is observed. It is interesting that, as shown in Fig. 4(d), the NWs of 17 nm diameter deposited at 0.9–1.4 V at room temperature all possess a strongly preferred [111] orientation and this preferential orientation is also observed for the NWs of the same diameter but deposited at 50 °C under a similar voltage range (results not shown here). These results can be ascribed to the effect of interface energy. In the electrochemical deposition, the growth of metallic nanowires is a complex process, which is influenced by factors such as charge transfer, ion diffusion, and H ion or micelles absorption. However, from an energy point of view, crystal planes with low interface energy have greater chances of being deposited to meet the principle of minimum free energy. The total interface energy includes two parts: the top surface of the wire/electrolyte and the side surface of the wire/polycarbonate nanopore wall. At small diameters, the interface energy of the top surface of the wire/electrolyte gives less contribution to the total energy and in this case the total energy is mostly influenced by the side interface. Therefore, the most densely packed (111) plane has the lowest energy and thus has a greater possibility of appearing. At large diameters, the interface energy of the top surface of the wire/electrolyte has more contribution to the total energy and it is influenced by parameters such as voltage and temperature. Accordingly the crystallographic orientation can be controlled by these parameters.

Finally, the field emission performance of an array of vertically standing gold nanowires was measured. Fig. 5(a) shows the measured I–V curve, i.e. emission current density J versus applied electric field E. The turn-on field of the array is 6.3 V μm⁻¹ which is comparable to the reported value of 6.3 V μm⁻¹ for gold nano-urchin and significantly lower than that for gold nanothorn (13.3 V μm⁻¹).⁵² The field emission property is further analyzed by the Fowler–Nordheim (F–N) theory which is expressed as:^53,54

where A_E = 1.54 × 10⁻⁶ A eV V⁻², B_E = 6.83 × 10³ eV^−3/2 μm⁻¹, J is the current density, β is the field enhancement factor, E is the applied field, and ϕ is the work function of the emitter material (5.00 eV for Au⁵²). As shown in the inset of Fig. 5(a), the linear relationship of ln(J/E²) versus 1/E indicates that the electron emission from the gold nanowires follows F–N behaviour, i.e. a process of tunnelling of electrons through a potential barrier. According to formula (5), the field enhancement factor is deduced to be 281. It is well known that the enhancement of field emission generally results from local electric field enhancement at the structure apex of protrusion. For a protrusion on a flat plane, the enhancement factor β may be expressed as β = h/r, where h is the height of the tip and r is the radius of curvature of the tip apex.⁵⁴ In the present work, the SEM image of the gold nanowire array for the field emission measurement is displayed in Fig. 5(b). The height and diameter of the nanowires are approximately 2 μm and 120 nm, respectively. Therefore, the enhancement factor is calculated to be 33, which is significantly lower than the value of 281 extracted from the field emission measurement. The enhancement may be attributed to the crystallographic nature of the gold nanowires, i.e. single-crystal and [100] preferred orientation. On the one hand, it is notable in Fig. 5(b) that the tips possess single-crystal features consisting of crystal facets rather than a single flat plane. Consequently, some sharp ridges are formed at the junctions of crystal facets and these ridges are essentially small protrusions where the local electric field is dramatically enhanced to facilitate the field emission process. On the other hand, it has been reported that different crystal planes have different work functions.^55,56 For materials including gold⁵⁵ and aluminium,⁵⁶ loosely packed planes have lower work functions than closely packed planes and polycrystal gold. In this work, the gold wires from the field emission measurement have preferred orientation along the [100] direction. Therefore, the relatively lower work function could be the other contribution to the enhanced field emission.

Fig. 5 Field emission J–E curve (a) and SEM image (b) of vertically standing gold nanowires. The inset in (a) is the corresponding F–N plots.

4. Conclusions

In summary, by using a cyanide-free electrolyte, we have synthesized gold nanowires whose crystallinity and preferred orientation can be controlled by tuning deposition conditions. While the applied voltage determines the crystal structure in a straightforward manner, the deposition temperature can have either a restriction or promotion effect on the final crystal structure, depending on the nanopore diameter of the ion track-etched template. We have also found that the growth orientation of the nanowires has a transition from [111] to [100] and the turning point depends on the applied voltage and nanopore diameter. These observations can be well explained by the adsorption of H ions modulated by effective voltage and the nanopore diameter-determined interface energy. Single-crystal wires with a [100] preferred orientation show enhanced field emission which may be attributed to their single-crystal structure and the lower work function of loosely packed crystal planes.

Acknowledgements

We thank the members of the Materials Research Department at the GSI Helmholtz-Zentrum (Darmstadt, Germany) for preparation and irradiation of polycarbonate foils. The financial supports from the National Natural Science Foundation of China (Grant no. 11175221, 11304261, 11375241, 11179003, 11275237, and 11205215) and the Hong Kong Research Grants Council (ECS Grant no. 509513) are acknowledged.

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