Gold microsheets having nano/microporous structures fabricated by ultrasonic-assisted cyclic galvanic replacement

Abstract

A novel technique for the fabrication of nano/microporous gold (Au) microsheets using an ultrasonic-assisted cyclic galvanic replacement reaction between a sacrificial silver (Ag) plate and gold ions (AuCl₄⁻) is reported. First of all, AuCl₄⁻ is reduced on the surface of sacrificial Ag via a galvanic replacement reaction. Then, the epitaxial growth of the Au film on the Ag surface is disturbed by the precipitated AgCl by-product of this galvanic replacement reaction. The co-precipitated AgCl and galvanic-generated Au nanostructures induce the formation of an interpenetrated Au/AgCl nanocomposite film on the surface of the sacrificial Ag plate. Finally, ultrasonic radiation enables auto-detachment of the galvanic-generated film along the AgCl/Ag interface. The galvanic replacement and auto-detachment processes continuously occur as a cycle until the scarified Ag is totally consumed. The coral-like Au nanostructures with nano/microporous morphologies are realized after removing AgCl by NH₃ treatment. The hierarchical nano/microporous structures have micropores and nanopores with sizes of 0.15–0.30 μm and 30–60 nm, respectively, separated by chain-like Au structures. The complex porous structures, through which liquids can easily flow, can potentially be used in applications such as high-efficiency free-standing catalysts, super capacitors, electrochemical sensors and surface-enhanced Raman scattering (SERS) substrates. A SERS application of the Au microsheets is also demonstrated.

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Introduction

Nano-porous gold (NPG) films have been excellent candidates for applications such as oxidative catalysts,^1–4 energy storage devices,^5–7 biosensors,^8–10 actuator technologies,^11,12 biomaterials for drug delivery,¹³ and surface-enhanced Raman scattering (SERS).^14,15 Several methods have been used to produce NPG films.^16–20 For example, a NPG film was fabricated on a titanium surface by the chemical reduction of an Au precursor under hydrothermal conditions at 180 °C, and then the structures were annealed at 250 °C under argon protection to increase the porosity of the structures.^9,21 Another method is silver dealloying. In this method, an Au/Ag alloy or blended Au/Ag powders were immersed in HNO₃ to selectively dealloy Ag from Au structures.^2,10,22–25 However, an annealing process at 600 °C is still required as the final step. Direct imprinting techniques can shape the obtained NPG films to obtain the desirable properties.¹⁴ The templating technique is another conventional technique used to fabricate NPG. A template constructed from materials, such as SiO₂ nanoparticles and porous alumina, is built, and then metal is deposited into the cavities of the template using various methods, e.g., electrochemical deposition.^26–28 Finally, the template is etched or dissolved to release a free NPG structure. In this study, a dynamic template consisting of hydrogen bubbles was employed to avoid a template-etching procedure.²⁹

Galvanic replacement is one of the several methods used to prepare isolated NPG particles, e.g., porous gold nanowires, gold nanocages, and cubic Au nanoframes.^30–32 It is a spontaneous electrochemical reaction, which can be used to fabricate Au films on the surfaces of Si and Ge.^33–37 By galvanic replacement, metal ions are reduced to zero-charged metal by receiving electrons from other sacrificial metal templates. In addition, the structures of the metals built would contain the structures of the sacrificial templates. The standard reduction potentials of sacrificial metal templates play an important role in this method. Only metal ions that have higher standard reduction potentials than the sacrificial metal templates can be reduced and deposited on the surface of a sacrificial metal template. For example, the reduction potential of an Au³⁺/Au half-cell is 0.99 V vs. a standard hydrogen electrode (SHE), whereas an Ag⁺/Ag pair has a standard reduction potential of 0.80 V vs. SHE.³¹ Therefore, Au³⁺ can be reduced to Au⁰ by a sacrificial electron from the Ag template as follows:

HAuCl₄(aq.) + 3Ag(s) → Au(s) + 3Ag⁺(aq.) + 4Cl⁻(aq.) (1)

Ag⁺(aq.) + Cl⁻(aq.) → AgCl(s) (2)

From eqn (1) and (2), AgCl is a solid by-product of this reaction and the epitaxial deposition of Au atoms on an Ag surface might be disturbed by the diffused AgCl in the growing sites.^31,38 This disturbance is a key factor in promoting the formation of porous Au structures.^31,38 To adjust the degree of porosity in the Au structures, the temperature and chloride concentrations were controlled in a previous study.^30,31,39,40 Moreover, excess chloride ion reacts with AgCl and forms a silver chloride complex, which is more soluble than AgCl,⁴¹ as follows:

AgCl(s) + (n − 1)Cl⁻(aq.) → AgCl_n⁽ⁿ⁻¹⁾⁻(aq.) (3)

The ultrasonic sound/wave, which consists of mechanical vibrations with a frequency of at least 20 kHz, can produce expansion and compression cycles that travel through solids, liquids, and gases. When the sound field is applied to a liquid, pressure fluctuations are built up inside the liquid itself. Then, the growth and collapse of microbubbles (acoustic cavitation) create high local turbulence, and heat energy is generated.⁴² Hydrodynamic cavitation is a physical phenomenon of the ultrasonic process that produces mechanical effects such as mixing and shearing in the liquid.^43,44 Ultrasonic waves are employed for several purposes, for example, ultrasonic cleaning⁴⁵ or as a versatile synthetic tool.^46–49

The synthesis of nano/microporous Au microsheets has several limitations, such as the requirement of a block co-polymer to use as the growing template^50,51 and a long growing process.^52,53 In this study, nano/microporous Au microsheets were successfully developed using an ultrasonic-assisted cyclic galvanic replacement reaction between Ag and Au³⁺. The synthesized nano/microporous Au microsheets contain a two-length-scale structure, which has a dual function of facilitating mass transport through micropores and increasing the surface area by the presence of nanopores.^22,26,54 This method combines a chemical technique (i.e. galvanic replacement reaction) and a physical technique (i.e. ultrasonic shearing) and provides the advantages of increasing the porosity of Au structures and reducing the reaction time (within 10 minutes at room temperature). This offers an opportunity for the reaction to be carried out on a large scale for commercialization. The synthesized Au structures are good catalysts at low temperatures,⁵⁵ which do not require any support⁵⁸ and have good thermal stability and resistance to oxidation.^56,57 Moreover, our synthesized Au structures have a potential application as good SERS substrates. By using crystal violet (CV) as a Raman dye and our synthesized Au structures as a substrate, the Raman signal of CV can be strongly enhanced.

Experimental

Chemicals

Nitric acid (HNO₃, 65%), ammonia solution (NH₃, 25%), and hydrochloric acid (HCl, 37%) were purchased from Merck®. All chemicals were of analytical grade and used as received without any additional purification. Deionized water was used as a solvent. Silver plates (≥99.99% purity) and gold beads (≥99.99% purity) with a diameter of 2–3 mm were purchased from a local precious metal retailer (Umicore Precious Metals (Thailand) Co., Ltd). A solution of concentrated gold ions (0.5 M Au³⁺) in the form of tetrachloroauric(III) acid, HAuCl₄, was prepared by dissolving 10 g of gold beads in 40 mL aqua regia under mild agitation and heating (80–100 °C). When all the gold beads were completely dissolved after heating for at least 10 hours, the solution was further heated until almost dry. Deionized water was added to the solution and the volume was adjusted to 100 mL. The concentrated gold ion solution was employed as a source of gold ions for further investigations. Caution: aqua regia, a mixture of HCl and HNO₃ with a volume ratio of 3 : 1, is an acutely toxic chemical and should be handled with care. Prior to use, all glassware and magnetic bars were thoroughly cleaned with detergent, rinsed with deionized water, rinsed with aqua regia, and thoroughly rinsed with copious amounts of deionized water. Silver plates were ultrasonically cleaned before using.

Synthesis of nano/microporous Au microsheets

A solution of 25 mM HAuCl₄ was sonicated in an ultrasonic bath (CREST, TRU-SWEEP model 575D with a sonic power of 135 W) at room temperature. Then, the sacrificial silver plate (5 mm × 5 mm × 0.08 mm, ∼0.02 g) was immersed in the solution. The silver plate immediately turned black. Within 10 seconds, a cloud of black particles spread out over the surface of the silver plate (ESI, Video S1†), and the silver plate completely disappeared within 10 minutes. The black precipitates were obtained after removing the ultrasonic source. The supernatant (the solution of excess gold ions) was removed and the black particles were rinsed with water. The co-developed AgCl was removed by immersing the black precipitates in an ammonia solution (NH₃, 10 mL, 10% w/v) for 15 minutes. The residual ammonia was removed by rinsing the particles with copious amounts of water. The obtained nano/micro Au microsheets were dried under ambient conditions and stored in a desiccator for further investigations.

Characterizations

Scanning electron microscopy (SEM). A galvanized silver plate was attached to a stainless steel stub using a carbon tape. All SEM micrographs were obtained with a JEOL JSM-6510A operated at the acceleration voltages of 10–30 kV in high vacuum mode using secondary electron imaging (SEI). Elemental analysis was carried out using a built-in JEOL energy dispersive X-ray spectrometer (EDS) model JED 2300.

X-ray diffraction (XRD). The samples were investigated using a Rigaku D/MAX-2200 instrument (Cu K_α1 radiation) operated at 50 kV and 250 mA over the range of 30–90 degrees in scanning steps with a step size of 0.02 degree.

Results and discussion

Galvanic replacement reaction without ultrasonic radiation

At first, the galvanic replacement reaction of Au on a sacrificial Ag substrate without ultrasonic radiation was investigated. The reaction was initiated by dipping an Ag plate into gold ion solution for 30 minutes. The generated galvanized layer was detached from the surface of the Ag plate using a corneal forceps. Fig. 1(A1) shows an SEM micrograph of the galvanized layer at the solution-exposed side. This reveals the connected round-edged particles with a particle size of 1–3 μm, which are characteristics of continuous AgCl layers.⁵⁹ Moreover, the grain boundary between the AgCl particles and microchannels with diameters of 0.5–1 μm inside the particles can be observed (ESI, Fig. S1†). The microchannels promote ion transportation and increase the ionic conductivity of the entire layers.⁵⁹ This phenomenon allows the galvanic replacement reaction to take place continuously; even the sacrificial Ag surface is fully covered with galvanized layers.

Fig. 1 (A) Solution-exposed side, (B) side view and (C) Ag-attached side SEM micrographs of the galvanized layer (A1, B1 and C1) before and (A2, B2 and C2) after NH₃ treatment. (B3) Side view vertical panorama SEM of the galvanized layer after NH₃ treatment.

The solid AgCl, which is concurrently developed with the Au nanostructures (eqn (2)), can be dissolved in an ammonia solution:

AgCl(s) + 2NH₃(aq.) → Ag(NH₃)₂⁺(aq.) + Cl⁻(aq.) (4)

After the removal of AgCl using the ammonia solution, the galvanized layer reveals hidden coral-like Au nanostructures (Fig. 1(A2)). This layer consists of connected clusters of Au nanoribbons with a thickness of 25–50 nm (ESI, Fig. S2†). The black spots on the surfaces are vacancies, which were previously occupied by AgCl microparticles. The uniformly-dispersed porous structures in the Au layers indicate that the galvanized layers are formed by the co-deposition of Au and AgCl.³¹ According to eqn (1) and (2), the 3-fold molar ratio of AgCl : Au causes AgCl to be a dominant structure and to act as a hard template for the growth of coral-like Au nanostructures.

In Fig. 1(B1), a side view SEM micrograph of the galvanized layer shows the thickness of the Au/AgCl composite layer to be 22–23 μm. AgCl in the galvanized layer was dissolved in the NH₃ solution to reveal the deposition pattern of the Au/AgCl composite (Fig. 1(B2)). The detailed structures of coral-like Au micro/nanostructures after AgCl removal are illustrated by a side view vertical panorama SEM micrograph in Fig. 1(B3). The condensed area of Au structures can be observed at <300 nm from the lower boundary of the galvanized layer (Ag-attached side) and structures with micro pores could be noticed at more than 500 nm from the lower boundary. The density of the Au structures decreases when the galvanized layer thickens. This suggests that epitaxial deposition first occurs on a surface of the Ag plate with a high density of Au structures because of a small difference in lattice constants (Au = 0.408 Å, Ag = 0.409 Å).⁴⁴

Moreover, the gradient of the Au structure density can be observed along the thickness of the galvanized layer (Fig. 1(B3)), which is a good evidence of a Cl⁻ concentration effect on the formation of Au/AgCl composites. After immersing the sacrificial Ag plate into HAuCl₄ solution, Ag atoms are oxidized and Ag⁺ ions are produced. At a very high concentration of Cl⁻ and a very low pH (in the solution of HAuCl₄), very soluble silver chloride ion complexes (AgCl_n⁽ⁿ⁻¹⁾⁻) are formed immediately (as expressed in eqn (3)), whereas a small amount of AgCl is generated. As a result, a small amount of AgCl co-deposition occurs during the epitaxial deposition of Au on the Ag surface. This results in high density of Au structures at the lower boundary of the galvanized layer (Ag-attached side).

However, the low density of microporous Au structures can be observed at the top part of the galvanized layer (closed to the solution-exposed side) because later in the reaction, more Ag atoms are oxidized to Ag⁺ ions, which produces more AgCl to co-deposit with the Au atoms. If the Cl⁻ concentration does not affect the formation of Au/AgCl composites, the Au structure density should be similar throughout the layer.

SEM micrographs of the galvanized layer at the Ag-attached side before and after NH₃ treatments are also shown in Fig. 1(C). They indicate that the round-edged particles covering the whole of the Ag-attached surface of the galvanized layer are AgCl, which was removed by soaking in the NH₃ solution. Because of the formation of these AgCl underlayers, the galvanized layer can be easily detached from the surface of the Ag plate by an external mechanical force.

The formation of AgCl underlayers

The formation of AgCl underlayers was investigated by observing time-dependent side view SEM micrographs of the galvanized layer, as shown in Fig. 2. After the first 5 seconds of the galvanic replacement reaction, the color of the Ag plate changes to black with the formation of the galvanized layer. Co-deposited Au/AgCl clusters with a size of 150–300 nm can be observed on the surface of the Ag plate, and the thickness of this co-deposition layer is approximately 700–800 nm (Fig. 2(A1)). At this stage, the galvanized layer cannot be detached from the Ag surface due to a strong epitaxial adhesion between Au and Ag. An insignificant change in the morphology of the layer after NH₃ treatment (Fig. 2(A2)) indicates that this layer is mainly Au. After 60 seconds, the layer can be easily removed from the Ag surface by an external mechanical force because of the immiscibility of AgCl and Ag (Fig. 2(B)). The thickness of the galvanized layer obtained at the 60 seconds of reaction time is approximately 2.6–3.0 μm (Fig. 2(B1)); this is reduced to 1.9–2.3 μm after the NH₃ treatment (Fig. 2(B2)). This indicates that the layer consists of a large proportion of the AgCl generated during this period.

Fig. 2 Side view SEM micrographs of the galvanized layer at (A) 5 and (B) 60 seconds reaction time (A1 and B1) before and (A2 and B2) after NH₃ treatment. The arrow points to the AgCl underlayer.

EDS spectra of the galvanized layers, which were obtained after the 30 minutes of reaction time, before and after NH₃ treatments are shown in Fig. 3. In Fig. 3(A), strong peaks at 2.21, 2.62, and 2.98 are attributed to Au_Mα, Ag_Kα, and Cl_Lα, respectively. Using a conventional ZAF procedure, the Au, Ag, and Cl atomic compositions of the galvanized layer before NH₃ treatment are calculated to be 6.98%, 49.28%, and 43.74%, respectively. After NH₃ treatment, the EDS result shows that the content of Au in the layer increases to 93.94%. This is an evidence of AgCl removal. No Cl atoms can be detected and the content of Ag decreases to 6.06%. The Ag residues possibly occur due to the reduction of Ag⁺, which is generated during the galvanic replacement reaction; they then grow and become widely distributed within the Au structures. EDS mapping images that reveal the distribution of Au, Ag, and Cl in the galvanized layers before and after NH₃ treatment are presented in the ESI, Fig. S3.†

Fig. 3 EDS spectra of galvanized layers (A) before and (B) after NH₃ treatments.

Fig. 4 shows an XRD pattern of coral-like Au micro/nanostructures after the NH₃ treatment. The peaks at 2θ of 38°, 44°, 64°, and 77° are attributed to the crystal orientation planes of (111), (200), (220), and (311), respectively, which represent a face-centered cubic lattice of Au.^28,60,61 It can be noted that the intensity ratio of the (200) plane to the (111) plane for coral-like Au structures is 0.42, which is lower than that of common Au powder (0.52).^62,63 This suggests that coral-like Au structures contain a greater proportion of (111) planes than a normal Au structure, which implies that the facet of Au nanoplates in coral-like Au structures is mainly a (111) plane.

Fig. 4 XRD pattern of coral-like gold nanostructures after NH₃ treatment.

The mechanism of the co-deposition process in the formation of galvanized layers is depicted in the ESI, Fig. S4.† Since a galvanic replacement reaction always takes place at the interface between a solution of Au³⁺ ions and the sacrificial Ag metal, the oxidation reaction initially arises on an Ag surface (anode) to generate Ag⁺ and free electrons (e⁻). When the electrons are transferred to the aqueous phase, allowing the reduction of Au³⁺, the produced Au atoms are deposited on a sacrificial Ag surface. From eqn (2), Ag⁺ and Cl⁻, which are by-products of the galvanic replacement reaction, combine together and form an AgCl precipitate with an Au : AgCl molar ratio of 1 : 3. Then, the AgCl precipitate is deposited on the Ag surface³¹ and continuously co-deposits with Au atoms to form a film (Fig. S4(B)†). To build up further structures, electrons from the oxidation at the Ag anode are transferred via the connected Au structures, which have been previously produced, to the top surface of the galvanized layers (solution-exposed side). Subsequently, AuCl₄⁻ is reduced to Au atoms as a result of receiving these electrons, and the Au atoms are deposited and grow on the surface of previously grown Au layers. Furthermore, microchannels, which are characteristics of the AgCl structure (ESI, Fig. S1†), are created during this period and play an important role in the transfer of ions (Au³⁺, Ag⁺, and Cl⁻) through the galvanized layers (Fig. S4(C) and (D)†). When galvanic replacement occurs, the Ag surface is gradually eroded, and a gap between the sacrificial Ag surface and the galvanized layer develops, and becomes gradually wider. Therefore, Cl⁻ can diffuse into this gap and react with Ag⁺ from the oxidation of the sacrificial Ag surface to form AgCl underlayers, which are deposited at the bottom of the galvanized layer (Fig. S4(D) and (E)†). This is in good agreement with the SEM micrographs in Fig. 1 and 2 of the galvanized layers before and after NH₃ treatment, which indicate the existence of AgCl underlayers at the bottom of the galvanized layers (Ag-attached side). From the proposed mechanism, only large Au microsheets with porous structures can be synthesized using this chemical reaction. Moreover, the galvanic reaction is terminated after the surface of the Ag plate is fully covered with AgCl underlayers. This coverage leads to the remains of the residual Ag plate.

Synthesis of nano/microporous gold microsheets using ultrasonic radiation

By applying ultrasonic radiation during the galvanic replacement process, the galvanized layers were immediately detached from a sacrificial Ag surface when the layers were formed. Fig. 5 shows time-dependent images of Ag plates dipped in a HAuCl₄ solution under ultrasonic radiation. A video of this synthesis is also shown in the ESI, Video S1.† After dipping the sacrificial Ag plate into the HAuCl₄ solution, the color of the Ag surface rapidly changes to black within a second, as shown in Fig. 5(A)–(C). During the early stages of the reaction, the epitaxial deposition of Au atoms occurs and black galvanized layers are developed at the sacrificial Ag surface. There is no significant difference between the reaction with or without ultrasonic radiation during this period. However, during the later stages of the reaction (after 10 seconds of reaction time, Fig. 5(D) and (E)), the black particles from the surface of the Ag plate were scattered throughout the solution. During this period, AgCl underlayers grow at the bottom of the galvanized layers, as shown in Fig. 1 and 2 and S4.† The growth of these AgCl underlayers causes a decrease in the miscibility between the galvanized layers and the sacrificial Ag surface due to cavitation within the AgCl structures. When ultrasonic waves interact with the plate, the galvanized layers are ejected from the sacrificial Ag surface. This phenomenon is the same as that in a normal ultrasonic cleaning procedure. Finally, the galvanized flakes ceaselessly disperse from the plate into the solution until the sacrificial Ag plate is no longer able to supply electrons for further reaction (Fig. 5(F)).

Fig. 5 Time-dependent images of Ag plates dipped in HAuCl₄ solution under ultrasonic irradiation. (A) A HAuCl₄ solution before galvanic replacement reaction (B–E) galvanic replacement reaction at 0 s, 1 s, 10 s, 20 s, and 7 min of reaction time.

The galvanized flakes that are split from the sacrificial Ag plate were investigated using SEM before and after NH₃ treatments and their SEM micrographs are presented in Fig. 6. The lateral sizes of the galvanized flakes are approximately 5–10 μm and their thickness is in the range of 0.5–1.0 μm, as illustrated in Fig. 6(A). After extracting AgCl from the flakes using the NH₃ treatment, porous Au micro/nanostructures were obtained (Fig. 6(B1)). In Fig. 6(B2) and (B3), micropores and nanopores with the size of 0.15–0.30 μm and 30–60 nm, respectively, in the structures of the Au microsheets can be observed. By measuring 645 pores in the Au microsheets, the ratio of micro (>100 nm) to nano (Fig. S5†). The compositions of galvanized flakes before and after NH₃ treatments were investigated using EDS spectra, as shown in the ESI, Fig. S6.† The results show the same morphologies and compositions as those of galvanized layers produced without ultrasonic radiation. In addition, the existence of AgCl underlayers at the bottom of galvanized flakes is also observed.

Fig. 6 SEM micrographs of (A) galvanized flake ejected from sacrificial Ag plate and (B) porous Au micro/nanostructures after the NH₃ treatment.

Cyclic galvanic replacement mechanism

Fig. 7 depicts a schematic of a cyclic galvanic replacement reaction resulting in the formation of galvanized flakes. First, a clean sacrificial Ag surface is supplied. The sacrificial Ag surface in the presence of AuCl₄⁻ allows the rapid epitaxial growth of Au film on the clean Ag surface. Cl⁻, which is a by-product of this galvanic reaction, reacts with Ag⁺ generated by the oxidation of the Ag surface and then forms an AgCl precipitate on the surface of the previously generated Au film. As a result of the Au/AgCl co-deposition, interpenetrated Au/AgCl nanocomposites are later formed. When a gap between a galvanized layer and an Ag surface is created by oxidative deterioration of the Ag surface, Cl⁻ diffuses into this gap and reacts with Ag⁺ dissolved from the Ag surface to develop AgCl underlayers at the interface between a galvanized layer and the Ag surface. After an AgCl underlayer develops and covers over the interface, the galvanic-generated interpenetrated Au/AgCl nanocomposite film with the AgCl underlayer is detached from the sacrificial Ag surface with the assistance of ultrasonic radiation. The ultrasonic-assisted auto-detachment enables a self-initiated formation of a clean Ag surface on an Ag plate. This self-regeneration enables the cyclic formation of an Au/AgCl nanocomposite film with the AgCl underlayer until the galvanic reaction completely consumes the Ag plate.

Fig. 7 Proposed cyclic galvanic replacement for the formation of nano/microporous Au microsheets. (A) Supply/regeneration of a clean Ag surface. (B) Rapid epitaxial growth of Au film on the clean Ag surface. (C) Precipitation and formation of AgCl precipitates on Au film. (D) Formation of interpenetrated Au/AgCl nanocomposites. (E) Development of an AgCl underlayer. (F and G) Ultrasonic-assisted detachment of the interpenetrated Au/AgCl nanocomposite film with an AgCl underlayer.

However, the cyclic galvanic replacement would be inhibited by excessive Cl⁻. With the addition of NaCl, the formation of galvanized flakes can be observed during the early stages. Then, the surface of the Ag plate is covered with AgCl, which is generated from dissolved Ag⁺ and added Cl⁻. The generated AgCl on the surface of the Ag plate protects the Ag surface from oxidation. Therefore, the Au³⁺/Ag⁺ galvanic reaction does not proceed and the cyclic galvanic replacement is terminated, which causes the remains of the Ag plates to be covered by AgCl. The galvanized flakes produced with additional NaCl and the residual Ag plates are shown in the ESI, Tables S1 and S2.†

SERS application

In addition, SERS application of the constructed nano/microporous Au microsheets is demonstrated in the ESI, Fig. S7.† Crystal violet (CV), which is a common Raman dye, was employed as a molecular probe to investigate the enhancement of the Raman signal. The results show that the SERS signal of CV is enormously stronger than the normal Raman signal of CV. The rise of the Raman signal enhancement is due to the porous structures in the Au microsheets, which provide a large number of “hot-spots” with a high electric field. This high electric field couples with the incident and scattering light on the surface of Au microsheets and then produces a strong Raman scattering signal of adsorbed molecules. Moreover, the pores in the Au microsheets also increase the surface area for the adsorption of molecules, which slightly improves the Raman intensity. The enhancement factor (EF) of the constructed nano/microporous Au microsheets was calculated to be 6.1 × 10⁷ (the details of the calculation are described in the ESI†). This suggests that synthesized nano/microporous Au microsheets are a good candidate for application as a sensitive SERS substrate.

By observing the SEM images in Fig. 6, it can be seen that the porous structure is quite uniform throughout the Au microsheets. The uniformity of the Au microsheets is good enough for them to be used as a SERS substrate because the smallest size of the laser spot in a conventional Raman spectrometer is approximately 1 μm, which covers the entire Au microsheets.

The morphology and porosity of the Au microsheets mainly depend on the nature of the co-precipitation of Au/AgCl and the galvanic replacement of Ag/Au³⁺. Especially, the co-precipitation process plays an important role in building the porous structure in the Au microsheets. Using the proposed technique, we found that the morphology and porosity is not affected by the concentration of any of the reagents. However, it is possible to control the co-precipitation and galvanic replacement processes by adjusting the temperature of the synthesis. Moreover, applying different powers of ultrasonic radiation may alter the auto-detachment of the galvanized flakes, which may also cause differences in the morphology and porosity.

Conclusions

Nano/microporous Au microsheets are successfully fabricated using an ultrasonic-assisted cyclic galvanic replacement technique. This synthesis method does not require any capping agents, templates or stabilizers. The pores in free standing micrometer-sized Au film are created by an AgCl by-product, which co-deposits with Au. The AgCl underlayer is a key factor in the detachment of the galvanized layer from the Ag surface under ultrasonic irradiation. The advantages of this fabrication method are a simple procedure with a low synthetic cost, no requirement for expensive and complex instruments, a high increase of porosity in the Au structures, a short reaction time (within 10 minutes at room temperature), and suitability for large-scale production. Moreover, the fabricated structures could potentially be used as SERS substrates and catalysts.

Acknowledgements

This research has been supported by the National Research University Project, Office of Higher Education Commission (WCU-018-FW-57).

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Footnote

† Electronic supplementary information (ESI) available: Solution-exposed side SEM micrograph of galvanized layer, SEM micrographs of galvanized layer after NH₃ treatment, EDS mapping images of galvanized layers, the mechanism of co-deposition process in the formation of galvanized layers, pore size distribution of the synthesized nano/microporous Au microsheets, EDS spectra of galvanized flakes, video of nano/microporous gold microsheets synthesis using ultrasonic radiation, SEM micrographs of galvanized flakes produced with additional NaCl and the residual Ag plates, SERS application of the constructed nano/microporous Au microsheets. See DOI: 10.1039/c5ra11193e

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