Controllable high-throughput fabrication of porous gold nanorods driven by Rayleigh instability

Abstract

Nanoporous gold (NPG) nanorods have recently attracted tremendous interest and research effort due to their vast applications in biomedical engineering, catalysis and photonics areas. However, the rational fabrication of large volumes of low-aspect-ratio NPG nanoparticles with well-defined geometries remains a difficult challenge. Here we demonstrate a controllable fabrication of porous gold nanorods by a novel template-assisted electrodeposition and in situ fragmentation of Au–Ag alloy nanowires, followed by a dealloying process to convert the resulting Au–Ag nanorods into desired NPG nanorods. The thermal-induced fragmentation process was believed to be associated with Rayleigh instability of the alloy nanowires confined within anodic aluminum oxide (AAO) templates, which has been further confirmed by in situ TEM experiments for geometrically confined gold nanowires upon Joule heating. More importantly, the status of the nanowire-breakdown process can be monitored in situ by macroscopic current–voltage (I–V) measurements of the over-grown nanowires–AAO sandwich structure. Together with a one-step dealloying finishing process, our method could facilitate the mass production of high quality NPG nanorods with well-controlled diameters, which could open up many opportunities for low-cost, high-throughput fabrication of low-aspect-ratio porous metallic nanorods for biomedical (such as drug delivery) and other applications.

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Introduction

By varying their geometries (e.g. aspect ratio) and dimensions (e.g. diameters), one-dimensional (1-D) noble metal nanostructures, such as gold nanowires with a low aspect ratio (i.e. nanorods), will have various properties suitable for desired potential applications.^1–9 For instance, Mitamura et al. proved that rod shaped nanoparticles display more particular and valuable optical properties than spherical or cylindrical ones.¹⁰ While spherical gold nanoparticles have only one surface plasmon band (at 510–575 nm) that depends on the size and dielectric environment, anisotropic shaped nanorods provide two intrinsic (transverse and longitudinal) bands. On the other hand, introducing a nanoporous structure into the normal 1-D nanostructures (such as nanoporous gold (NPG) nanoparticles or nanoporous silicon particles) and tuning their porosity can further bring additional features with tremendous advantages in sensing, catalysis, and photonic applications, and in particular as drug/gene delivery vehicles.^12–16

Therefore, combining porous nanostructures with one-dimensional geometry, nanoporous metallic nanostructures could further broaden their applications towards catalytic activity,^17,18 chemical sensing,^14,19 plasmonics^10,11 or drug delivery.²⁰ Among them, 1-D nanoporous gold (NPG) nanostructures have recently attracted particular attention for its intrinsic chemical and electrical characteristics, favourable optical properties, strong affinity to biomolecules and biocompatibility: e.g. nanoporous gold nanowires fabricated by Ji and Searson show high surface area and well-defined morphology, which are dependent on the precursor solution composition and etching conditions. Their single-NPG devices also have been reported as an efficient chemical sensor due to the resistance change dominated by surface scattering of the NPG structure.^14,21 Compared with long-aspect-ratio nanowires, the NPG nanorods, on the other hand, generate nearly 20-fold increase in the active surface area, so as to encapsulate large quantities of drug payloads.²² Meanwhile, the optical properties depend not only on the length but also the surface roughness. Compared with common gold nanorods with smooth surface, the longitudinal modes of nanoporous gold nanorods are red-shifted, which provide an alternative method for various functionalities. Tailoring aspect ratio and porosity can be used to tune near-infrared absorbance of in vivo nanorods, aiding in medical molecular imaging and drug release.

However, despite their great value in drug delivery and molecular imaging, rational fabrication of nanoporous gold nanorods with low aspect ratio remains quite challenging. For example, the conventional template-assist method¹³ can be used to fabricate shorter nanowires via short-time deposition but the yield will be rather low. Recently, researchers reported that annealing treatment can be induced to fragment long nanowires into shorter nanorods or nanoparticles,^23–25 which inspired us to combine this technique with the template-assist growth method for high throughput fabrication of nanoporous gold nanorods. This fragmentation process was thought to be associated with Rayleigh instability,²³ which was a phenomenon caused by cylindrical liquids or solids having the tendency to minimize their free surface energies, thus decay into a row of spherical droplets. This phenomenon was pioneered by Plateau theoretically on instability of cylindrical geometry and Lord Rayleigh²⁶ on the linear stability of non-viscous liquid jets against periodic perturbations. Then this Rayleigh's perturbation was further extended to solids even below their melting point and studied systematically by Nichols and Mullins.²⁷ More importantly, by over-growth of the Au–Ag alloy nanowires, a conductive sandwich structure was obtained. This technique could allow macroscopic I–V measurement of the whole sample, providing real-time monitoring of the internal nanorod fragmentation status.

Experimental

In Fig. 1, we show schematically the template-assist fabrication and in situ fragmentation process of gold nanoporous nanorods, as well as the associated I–V measuring setup for in situ status monitoring. Typical template-assist electrodeposition method can be found in literatures.^22,28,30 The electrolyte was prepared by mixing commercial electroplating solutions of Au (Technic™ Orotemp®, major component K(Au(CN)₂) 0.1 M) and Ag (Technic™, major component K(Ag(CN)₂) 0.1 M) with a molar ratio of Au⁺/Ag⁺ = 1 : 2. Then Au–Ag alloy nanowires were formed inside AAO templates (Whatman® Anodisc™, channel size ∼100–200 nm in diameter) by direct current (DC)-electrochemical deposition method.²⁹ Two-electrode system was used to complete this process, with a graphite counter electrode at room temperature under a dc power supply (Extech™, voltage ∼ 1 to 3 V, current ∼ 0.02 A).

Fig. 1 Schematic illustration of the nanorods fabrication process and in situ monitoring of the fragmentation process: (a) Au–Ag alloy nanowires were electrodeposited into the AAO (anodic aluminum oxide) channels with one side sputtered one layer of silver film serving as electrode; (b) over growth of the Au–Ag alloy nanowires can form a top metal layer, serving as another electrode for real time I–V measurements. SEM image of alloy nanowires with uniform diameter in AAO template before in situ fragmentation at T₀ was shown in (c). Then by (d) heating and (e) rapid cooling thermal processes, alloy nanowires will breakdown into short nanorods, with SEM images shown in (f). EDS spectrum of the alloy nanowire in AAO template was shown in (g). The whole process can be monitored by (h) real-time I–V characterization of the nanowire–AAO sandwich structure.

Here, the alloy nanowires formed inside the AAO template have almost the same lengths as that of the AAO channel. To get nanorods with lower aspect ratio, we established the following fragmentation steps: in order to achieve in situ heating-induced fragmentation and status monitoring, we intentionally over-grew the Au–Ag alloy nanowires to form an over-coated metallic alloy layer on the AAO surface (Fig. 1a and b) as another electrode. Together with the bottom layer of silver coating (as growth electrode in electrochemical deposition process) and fully-filled nanowires in-between, we obtained a sandwich-like electric resistance structure, with top alloy layer and bottom silver layer serving as two electrodes. The status information of the inside alloy nanowires fragmentation process (as illustrated in Fig. 1d) can thus be monitored, by sweeping the voltage through the sandwich structure and read the current (instead of the conventional ex situ SEM/TEM visual examinations with multi-step processes). For the heating-induced fragmentation process, the template supported alloy nanowires were treated by isothermally heating up to 940 °C in vacuum condition, followed by rapid cooling process (Fig. 1d and e). Throughout the multiple thermal treatment processes, real time I–V characterizations of the sandwich structure were carried out ex situ, to provide information about the formation (fragmentation) status of shorter nanorods (Fig. 1e) based on the principle of Ohm's law (the more nanorods formed, the higher the electrical resistances measured).

After we confirmed that fragmentation process completed by just reading the I–V output, we dissolved the AAO template (by NaOH/KOH) along with the front and back coating layers (by acid), and carried out standard dealloying process^30,31 to completely etch away the silver component in the samples using concentrated nitric acid, resulted in the formation of final nanoporous gold nanorods.

Results and discussion

Fig. 1c and f show the corresponding status of the Au–Ag alloy nanowires (which was confirmed by EDS spectrum in Fig. 1g) inside AAO channels after annealed isothermally at 940 °C (for reference, the melting point of bulk silver is ∼961 °C and gold ∼1064 °C). By monitoring real time current–voltage measurement of the sandwich structure, the effect of thermal treatment on the nanowire breakdown process can be monitored macroscopically. As shown in Fig. 1h, before thermal annealing at time T₀, the resistance of the “alloy–AAO with nanowires inside-silver” sandwich network is rather low (∼0.3 Ω) because of the high metallic conductivity. Due to Rayleigh instability and geometrical confinement from the cylindrical template channels,^23,32–37 at the first stage of thermal treatment after 45 min (T₁), the nanowire would experience shrinking in diameter and become non-uniform due to “Rayleigh instability”, resulted in the increasing of the overall resistance of the sandwich structure (but not that prominent, due to the high conductivity nature of gold and silver). However, as the annealing time increased, the resistance of the “alloy–nanowires–silver” sandwich structure will significantly increase due to the fact that more and more nanowires broke into disconnected pieces (short nanorods). Eventually, after 135 min at time T₂, the nanowires in the template are mostly broken, resulting in an open circuit. Fig. 1e and f represented the fragmentation status at T₀ and T₂. Further annealing of 45 min showed a slight increase in the resistance, indicating that almost all the inside nanowires were broken into short disconnected nanorods, i.e. the successful fragmentation of the samples. The template-confined nanowire fragmented into short nanorods rather than spherical particles because of the geometrical confinement of the AAO channels,²³ as shown in Fig. 1e and f. The nanorods within one single channel have uniform diameter. Most of them have two ends with nearly spherical surface and are distributed with regular spacing.

After the completion of fragmentation, Au–Ag alloy nanorods were released form the AAO template. Then standard dealloying process was applied for turning the Au–Ag nanorods into the desired nanoporous Au nanorods. Fig. 2 shows the free-standing porous Au nanorods well dispersed on Si substrate. The length of the NPG nanorods (or nanoparticles when having low aspect ratio) ranges from 200 nm towards 1600 nm, while the diameter lies in around 100–200 nm, which makes a low aspect ratio from about 2 : 1 to 8 : 1 (Fig. 2a and b). According to some earlier reports,^32,37 the template-confined fragmentation of alloy nanowires would form short nanorods with an aspect ratio of about 9 : 1. In our case, most of our final porous nanorods have aspect ratio around 9 : 1, or a little bit smaller which could be due to further anisotropic dealloying process, as well as the possible fracture during releasing. However, the diameter of resulted nanorods can be well controlled via tuning the template channel size, while the porosity and surface morphology of the NPG nanorods can be controlled by tailoring gold/silver plating solutions of different composition ratios and modifying etching conditions. In the case shown in samples from Fig. 2c, when the mole fraction in the Au–Ag binary alloy nanowire is assumed 1 : 2, the final porosity is around 67% in our sample, almost equal to the fraction of Au⁺ in precursor solution.²¹ The high magnification TEM image (Fig. 2d) confirmed that typical NPG nanorods exhibit a sponge-like nanoporous structure with a ligament size of about 10–30 nm and a pore size of about 20–40 nm.

Fig. 2 As-fabricated nanoporous gold (NPG) nanorods: SEM images of (a) dispersed NPG nanostructures on the Si substrate and (b) one slightly longer NPG nanorods; TEM image of (c) one NPG nanorods, and magnified TEM image of (d) the NPG nanorod, showing the uniform nanoporous and ligament microstructure.

3.1 Rayleigh instability-induced nanorod formation mechanism

The most important step in this fabrication process is the thermal-induced fragmentation of nanowires into short nanorods. The nanorod formation mechanism was believed to be associated with Rayleigh instability, as suggested by several researchers.^23,32–37 Karim et al.³² claimed that morphological evolution in nanowires is mainly the result of atomic movement by surface diffusion, when the nanowires have high surface-to-volume ratio and the annealing temperature is significantly lower than the melting point. The driving force for the instability of cylindrical rod is the chemical potential gradient induced by a perturbation in the rod radius.^32,33 And in our study, thermal treatment provided this perturbation. Then the mass transport mechanism controlled the nanowire decay process. However, the effect of the template on the Rayleigh instability cannot be neglected. In our case, the tendency of interface energy reduction between the alloy and the Al₂O₃ channel could be the driving force for this Rayleigh instability behaviour. According to Nichols and Mullins,²⁷ free-standing nanowires will break up into nanospheres with larger diameter, while cylindrical template supported nanowires will fragment into shorter nanorods as the diameter has been confined.²³ In Fig. 3d, cylindrical template confined geometry evolution process was illustrated: in step (i), Au–Ag alloy was formed inside AAO channels. If no AAO template, step (ii) will achieved, where alloy nanowires will fragment into nanospheres with a radius R = 3.78r and an inter-sphere spacing λ = 8.89r, r denotes the radius of the alloy nanowire²⁶ and we assumes it equals to the inner radius of AAO pore. If AAO was induced, the over-growth layers are less firm than the ceramic AAO channels, leaving the room for nanowire expanded in longitudinal direction as shown in step (iii). So the aspect ratio of the resulted nanorods will be in some degree higher than normal nanoparticles or nanospheres, which in our study ranging from 2 : 1 to 8 : 1. The whole process due to Rayleigh-instability in a confined volume has been schematically shown in Fig. 3c.

Fig. 3 Rayleigh instability-induced nanowire fragmentation inside AAO template: (a) SEM image showing on one side of the fully grown AAO membrane after heat treatment (inset showing the alloy nanowires embedded under the overgrown top metal layer), with (b) magnified SEM image showing the nanowire bumping (with corresponding sizes same as the channel diameters, indicating the nanowire under thermal expansion and Rayleigh instability). (c) Schematic illustration showing the thermal-induced fragmentation and bumping process. (d) Schematic illustration of evolution steps from alloy nanowire to nanorods confined in AAO template driven by Rayleigh instability.

To check the above hypothesis, we took one of the specimens right after heating up to ∼900 °C (but below the melting points of bulk Au and Ag), as shown in Fig. 3a, and found those surface bumps on the over-growth side of the AAO template (formed by over-grown Au–Ag thin layer). The bumping hemispheres have almost the same diameters as the pore size (Fig. 3b), indicating that the Au–Ag alloy nanowires indeed expanded longitudinally inside the confined AAO channels upon the heating/annealing, due to the thermal expansion and template-confined Rayleigh instability. After stripping part of the top over-grown alloy layer (inset in Fig. 3a), we can indeed see the nanowires under those bumping areas, with rather uniform diameters, indicating that during heating process Au–Ag nanowires indeed underwent thermal expansion and Rayleigh instability until broken down into short nanorods.

3.2 In situ TEM fragmentation of gold nanowires by Joule heating

In order to further confirm Rayleigh instability as the underlying mechanism in this thermal-induced fragmentation of metallic nanowires, in situ fragmentation of a confined gold nanowire was performed (Fig. 4b and c) inside a transmission electron microscope (TEM). In this experiment, in situ nanowire manipulation and heating was performed by a Nanofactory™ TEM-scanning tunnelling microscopy (STM) sample holder platform,³⁸ in which an Au nanowire was encapsulated within a thick layer of amorphous carbon (analog to AAO template channel, but it is optically transparent, allowing the monitoring of the internal sample structure/morphology evolution). To simulate the heating process described above, we chose to apply Joule heating on the single nanowire. The individual nanowires were first attached on Au supporting substrate by conductive silver paste as positive electrode (Fig. 4a right side), while an Au STM probe on the other side of the holder acted as the counter electrode which will be manipulated and attached onto the nanowire sample through precise movement. Upon a moderate bias (∼1.5 mV) was applied to the Au nanowire sample by the STM probe between these two electrodes, we observed that the confined ultrathin Au nanowire instantly broke down into several shorter Au nanorods pieces, which was attributed to the Joule heating induced Rayleigh instability. The fragmented nanorods confined in the carbon channel show uniform diameters in accordance with the channel size, as illustrated in the inset of Fig. 4c, as a direct evidence for the Rayleigh-instability fragmentation process in our previous nanorod formation process. It is worth noting that, despite this analogy of heating process on this thinner gold nanowires, regular 100–200 nm diameter Au–Ag alloy nanowires in our high-throughput fabrication requires reasonably longer heating time and higher heating temperature than that required for ultrathin gold nanowires under Joule heating.^38,39

Fig. 4 In situ TEM fragmentation of an encapsulated gold nanowire: (a) setup for the in situ Joule heating experiment, and TEM image of (b) one Au nanowire encapsulated by a thick layer of amorphous carbon. TEM images showing (c) the fragmentation of the Au nanowire into shorter Au nanorods when passing current through the sample via a gold probe (bottom); inset shows the fragmentation process indeed occurred within the carbon layer confinement.

Conclusions

The present work demonstrates a multi-step fabrication of nanoporous gold nanorods by template-assisted electrodeposition combined with Rayleigh-instability induced fragmentation and dealloying techniques. The critical fragmentation process can be monitored in situ macroscopically by measuring the I–V status for the whole piece of the nanowire–AAO sandwich structure, without the need of costly and destructive ex situ electron microscopy examinations. The simple and low-cost method allows the controllable, high throughput fabrication for large quantity of porous metallic nanorods with well-defined diameter and porosity.

Acknowledgements

The work described in this paper was partially supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project # CityU 11209914) and the National Natural Science Foundation of China (Project # 51301147). J. Lou acknowledges the funding support from the Welch Foundation (Grant C-1716). Part of this work was performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000). Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11050a

‡ These authors contributed equally to this work.

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