Synthesis of porous gold nanoparticle/MoS₂ nanocomposites based on redox reactions

Abstract

We describe a facile method for preparing porous gold nanoparticle (Au-NP)/2H-form MoS₂ nanocomposites. The Au-NPs were formed by redox reactions between 2-dimensional MoS₂ nanoflakes with a 2H-form crystal structure and tetrachloroauric acid (HAuCl₄) in a water/ethanol mixture without adding any additional reducing agents. The structures of the Au-NP/MoS₂ nanocomposites were influenced by the molar ratio of the Au ion precursor and MoS₂, as well as the volume ratio of water and ethanol in the reaction mixture. Ethanol allowed the 2H-MoS₂ nanoflakes to maintain their original morphologies after the reaction to serve as the support framework, meanwhile the Au-NPs worked as a junction to construct a 3-dimensional (3D) nanoporous structure. The obtained porous nanocomposites worked as a material for surface enhanced Raman spectroscopy (SERS) due to the formation of many nanoholes with so-called “hot-spots” based on the assembled Au-NPs.

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Introduction

Recently, layered transition metal dichalcogenides (TMDs) have received much attention due to their unique layered 2-dimensional structures and many remarkable properties.^1–4 The interlayer interactions between every layer are based on relatively weak van der Waals forces that are beneficial for exfoliating the structure to ultrathin layers using mechanical or chemical methods.⁵ In TMDs, MoS₂ with a monolayer or few layers has attracted much concern. MoS₂ crystals are classified into the 2H-form and 1T-form based on the differences in their crystal structures. 2H-MoS₂ is semiconducting, while the 1T-form is metallic. Based on the semiconducting or metallic properties, MoS₂ has been used as a material for hydrogen evolution reaction catalysts,^6,7 mechanical resonators,⁸ emerging electronic and optoelectronic devices,³ energy generation and storage devices,⁹ and biotechnological applications.^10–12

MoS₂ exhibits a unique 2D S–Mo–S structure in which the molybdenum atom layer is sandwiched between two close-packed sulfide atom layers through strong covalent bonding.¹³ The outer-layered S atoms can joint different metal or oxide nanoparticles which can expand the use of MoS₂-based materials for many applications.^14–18 Recently, noble-metal decorated MoS₂ has been extensively studied since a possible synergistic effect between the noble metals and MoS₂ might induce a novel function.^19–21 Huang et al. reported a general method to reduce Pt, Au, Ag, and Pd ions to form nanoparticles on the MoS₂ nanosheets using an ascorbic acid aqueous solution and a carboxymethylated cellulose (CMC) aqueous solution as the reducing agent and stabilizer,²² respectively. Huang et al. reported that the Pd, Pt and Ag NPs were prepared by using a reducing agent followed by epitaxial growth on the 2-D MoS₂ nanosheets.²³ The Berry group used both chemical reduction and microwave irradiation to anchor Au-NPs on the MoS₂ ultrathin nanosheets.²⁴ A microwave-assisted hydrothermal method was also reported as an effective approach to decorate Au-NPs on MoS₂ nanosheets.^25,26 Without using such reducing agents or complicated preparation methods, Au-NPs are spontaneously formed and anchored on the MoS₂ nanosheets.²⁷ However, up to now, many studies have focused on growing Au-NPs on 1T-MoS₂ due to their metallic properties and the structure of the Au-NP-decorated MoS₂ nanocomposites shows a sheet-like 2-dimensional structure.

In this paper, we firstly construct porous Au-NP/2H-MoS₂ nanocomposites based on a facile and green in situ growing method, and then apply them as a material for a surface-enhanced Raman spectroscopy (SERS) substrate. We also describe a finding that the morphology of the Au-NP/MoS₂ nanocomposites could be changed by altering the concentration of HAuCl₄ and the volume ratio of ethanol/water. The synthesized Au-NP/MoS₂ substrates exhibited an efficient SERS enhancement based on the formation of porous structures of the compound and assembled Au-NPs.

Experimental

Materials

Molybdenum(IV) sulfide nanoflakes in an ethanol/water (45%/55%) solution (Graphene Laboratories, Inc. NY), hydrogen terachloroaurate(III) tetrahydrate (HAuCl₄·3H₂O, Wako), and Rhodamine 6G (R6G) were purchased from commercial sources. All chemicals were used without further purification. All aqueous solutions were prepared using Milli-Q water.

Measurements

Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM, bright field images) were performed using an SU-9000 (Hitachi High-Tech) at 20 kV equipped with an energy dispersive X-ray spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were recorded using an AXIS-ULTRADLD (Shimadzu). UV-vis-near IR (NIR) adsorption spectra were obtained using a V670 spectrophotometer (JASCO). The thickness and surface structures of the MoS₂ and prepared Au-NP/MoS₂ nanocomposites were measured using an atomic force microscope (AFM, SPM-9600, Shimadzu).

Preparation of the Au-NP/MoS₂ nanocomposites

Au-NP decorated MoS₂ nanocomposites were synthesized as follows. The MoS₂ nanoflake solutions were sonicated using a bath-type sonicator for 1 h to obtain a well dispersed solution. A 1 mL portion (18 mg L⁻¹) of the solution was put in a 6 mL glass bottle equipped with a magnetic stirrer bar, then stirred vigorously, to which different aliquots of 1 mM HAuCl₄·3H₂O were immediately added followed by stirring at room temperature for 30 min to provide the Au-NP/MoS₂ nanocomposites, which were collected by centrifugation at 10 000g.

SERS measurements

The SERS analysis was performed on a LabRAM ARAMIS Raman microscope instrument equipped with a standard 632.8 nm HeNe 20 mW laser beam that is directed onto the sample through a 50× objective lens. The SERS of all samples was conducted using similar experimental conditions (acquisition time is 5 s, laser power is 20 mW, while the filter chosen is D1).

Results and discussion

In this study, MoS₂ nanoflakes in water/ethanol were mixed with HAuCl₄ in water without adding any reducing agent. Firstly, the influence of the molar ratio between the Au ions and MoS₂ was investigated by changing the concentration of HAuCl₄ while maintaining the other experimental conditions.

STEM observations were conducted to investigate the morphologies of the prepared composites. As a control sample, commercially available pristine MoS₂ nanoflakes were used. Fig. 1A shows the control sample prepared after 1 h of sonication, in which well-dispersed MoS₂ fragments were observed. On adding HAuCl₄ to the solution with a molar ratio of Au³⁺ : MoS₂ = 0.5 : 1, Au-NPs grew only at the edge of the MoS₂ (Fig. 1B). When the molar ratio of Au³⁺ : MoS₂ was increased to 1 : 1, Au-NPs with diameters of ∼16 nm were decorated on the surfaces of the MoS₂ as shown in Fig. 1C. The composites were well-dispersed in the solution, and the color of the solution did not change until the molar ratio of Au³⁺ and MoS₂ reached 5 : 1 as shown in the insert of Fig. 2. When the molar ratio of Au³⁺ : MoS₂ > 5 : 1, the color of the solution turned to wine red, suggesting that the Au-NPs grew larger. From the SEM images shown in Fig. 1D and E, it was found that both small (2–5 nm) and large (>20 nm) Au-NPs grew on the surfaces of the MoS₂. The average size of the large Au-NPs on the Au-NP/MoS₂ nanocomposites increased from 16.3 nm to 51.1 nm when the molar ratio changed from 1 : 1 to 10 : 1 (Fig. S1†). Compared to a report that describes the destroying of MoS₂ at high HAuCl₄ ratios through a spontaneous redox reaction,²⁵ in our experiments the MoS₂ nanoflakes maintained their flake shape even at the high ratios of Au ions.

Fig. 1 STEM images of (A) the pristine commercial MoS₂ nanoflakes, (B) Au-NP/MoS₂ (with 0.5 : 1 molar ratio), and (C) Au-NP/MoS₂ (with 1 : 1 molar ratio). SEM images of (D) Au-NP/MoS₂ (with 5 : 1 molar ratio), and (E) Au-NP/MoS₂ (with 10 : 1 molar ratio). The scale bar is 100 nm. Insets are enlarged images of the orange areas (scale bar is 20 nm).

Fig. 2 (A) UV/vis spectra of the commercial pristine MoS₂ nanoflakes and specified Au-NP/MoS₂ nanocomposites, and photographs of the samples. (B) UV/vis spectra in the range of 300–800 nm.

The optical properties of the prepared nanocomposites were examined using UV/vis spectroscopy, and the results are shown in Fig. 2, in which two peaks were observed at 606 nm and 665 nm, indicating that the materials contain pristine MoS₂ crystals with the 2H crystal structure.²⁸ No obvious difference was observed between the spectra of the pristine MoS₂ and the nanocomposites with molar ratios of Au³⁺ and MoS₂ lower than 5 : 1. When the amount of the Au³⁺ precursor was increased to 5 : 1, a new peak around 545 nm appeared, indicating the formation of Au-NPs that show surface plasmon resonance (SPR) spectra. Compared to the composite with a 5 : 1 molar ratio, the peak wavelength of the composite with the 10 : 1 molar ratio was red-shifted to 585 nm, indicating the formation of larger Au-NPs. Compared with previous reports describing a SPR peak at 533 nm for ∼50 nm Au-NPs,^29,30 the SPR peak of our Au-NP/MoS₂ was red-shifted to 585 nm even though the particle sizes of the Au-NPs are close (∼50 nm). Such a shift would be due to the effective plasmon interactions of the Au–MoS₂ composites³¹ and/or a partial covering and deformation of the spherical Au-NPs on the MoS₂ surfaces as suggested by Polyakov et al.,³² in which the growth of Au-NPs on the surfaces of MoS₂ might cause distortion of the spherical shape of the Au-NPs leading to a SPR peak red shift. Moreover, the disappearance of the typical absorption peak of the MoS₂ in the composite with the molar ratio of 10 : 1 also derived from the aggregation of the Au-NP/MoS₂ nanocomposites.

The thickness and surface morphologies of the Au-NP/MoS₂ nanocomposites were examined using AFM. As shown in Fig. 3, the thickness of the pristine MoS₂ nanoflakes was larger than 2 nm, indicating that the flakes have a multilayered structure since the thickness of typical single-layered MoS₂ nanosheets is ∼0.7 nm.³³ The AFM image of the Au-NP/MoS₂ (1 : 1 molar ratio) shows that the Au-NPs were aggregated at the edge of the large pieces of MoS₂ nanoflakes, but did not cause large-scale aggregation of the MoS₂, which is in agreement with the STEM results shown in Fig. 1C. Fig. 3C clearly proves that when the Au³⁺ ratio was increased to 10 : 1, many Au-NP/MoS₂ aggregates were generated, and the thickness of the obtained composites was greater than 60 nm. The height profile of the AFM image indicates that the aggregated Au-NP/MoS₂ nanocomposites have a porous structure. The Au-NP deposition on the MoS₂ was also confirmed using EDX and XPS spectral measurements. The intensity of the Au peak in the EDX spectra (Fig. S2†) increased with the increase in the ratio of Au³⁺ to MoS₂. In the XPS spectra (Fig. 4A), typical peaks of Mo 3d_3/2 and 3d_5/2 are observed at around 232.4 eV and 229.3 eV, respectively. There was no significant shift in the doublet peaks of the Mo 3d bands before and after the gold ion reduction, while we observed higher valence forms (Mo⁵⁺, Mo⁶⁺) using XPS (see ESI, Fig. S3 and Table S1†), suggesting partial oxidation of the MoS₂ in the reduction process.^34,35 The peaks in Fig. 4B located at ∼87.2 eV and ∼83.5 eV can be ascribed to Au 4f_5/2 and Au 4f_7/2, respectively, suggesting the reduction of the Au ions.^10,14

Fig. 3 AFM images of (A) the pristine commercial MoS₂ nanoflakes, (B) Au-NP/MoS₂ (with 1 : 1 molar ratio), and (C) Au-NP/MoS₂ (with 10 : 1 molar ratio).

Fig. 4 XPS spectra of the (a) Mo 3d and (b) Au 4f regions of the pristine commercial MoS₂ nanoflakes and the Au-NP/MoS₂ with specified ratios.

It is reported that MoS₂ can adsorb AuCl₄⁻ to form a MoS₂/AuCl₄⁻ redox pair, allowing spontaneous reduction of the Au³⁺ to Au-NPs (metal) and in situ growth on the MoS₂ nanosheets.²⁷ In addition, ethanol would be expected to enhance the reaction due to its reducing ability.^29,36,37 To verify this possibility, a series of experiments was conducted changing the water/ethanol (v/v) ratios. For preparation of the samples, the MoS₂ nanoflakes were collected by centrifugation to remove the solvent, then the precipitate was re-dispersed in water/ethanol solutions, which were sonicated for 1 h using a bath-type sonicator. A SEM image of the sample prepared in only water using the 5 : 1 molar ratio of the Au ion precursor and MoS₂ is shown in Fig. 5A, in which MoS₂ nanoflakes were hardly observed, which implies that, in the absence of ethanol, the formed distorted MoS₂ might cause compact aggregation of the Au-NPs. This idea is supported by the fact that the MoS₂ works as a reducing agent. To further confirm this, XPS measurements were conducted and the obtained results (see ESI, Fig. S4†) demonstrated that when the Au-NPs were reduced by the MoS₂, the Mo atoms were oxidized to higher valence forms (Mo⁵⁺ and Mo⁶⁺). When ethanol was involved in the reaction solution, the molar ratio of the Au ion precursor and MoS₂ was fixed to 5 : 1 and the flake morphology of the MoS₂ remained, as shown in Fig. 5B–E. As exhibited in the XPS results and the quantitative analysis, the amount of oxidized Mo decreased with the increase in the content of ethanol.

Fig. 5 SEM images the Au-NP/MoS₂ samples (with 5 : 1 molar ratio) prepared in solution with different volume ratios of water and ethanol: (A) pure water, (B) 5 : 1, (C) 1 : 1, (D) 1 : 5 and (E) pure ethanol. Scale bars in the figures and in the insets are 100 nm and 20 nm, respectively.

To elucidate the formation process of the Au-NP/MoS₂ nanocomposites, time-dependent investigations of the morphological characterization were carried out. The samples for STEM observation were prepared at specified time intervals under similar reaction conditions. The series of STEM images (Fig. 6) shows the morphology at each reaction stage. After the first 5 min, a few small Au-NPs had formed on the edge of the MoS₂. The number and size of the Au-NPs increased as the time progressed. On prolonging the reaction time to 20 min, the size of the Au-NPs was found to increase and flake-shaped nanocomposites began to assemble. Fig. 6D shows the morphology of the nanocomposites formed after 30 min, which reveals dense aggregation of the Au-NP/MoS₂. As unbound sulfur is enriched at the edge of nanoflakes,³⁸ the Au-NPs incorporated preferentially at the edge of the nanoflakes and large sized Au-NPs were produced. Because the larger Au-NPs have more active surfaces and a greater ability to bind sulfur,³⁹ they would work as a joint to bind the sulfur to other pieces of MoS₂ which would cause aggregation of the Au-NP/MoS₂ nanocomposites. As shown in Fig. 6C, MoS₂ nanoflakes with larger Au-NPs aggregated with each other, while the MoS₂ nanoflakes with smaller Au-NPs did not exhibit such behaviour. Together with the obtained results, the formation process of the Au-NP/MoS₂ nanocomposites can be proposed as schematically shown in Fig. 7.

Fig. 6 STEM images the Au-NP/MoS₂ samples (with 5 : 1 ratio) prepared with reaction times of (A) 5 min, (B) 10 min, (C) 20 min and (D) 30 min. Scale bar is 100 nm.

Fig. 7 Schematic illustration for the formation of the Au-NP/MoS₂ nanocomposites.

Due to the unique porous structure of our material, the Au-NP decorated MoS₂ nanocomposites are expected to show a high surface-enhanced Raman scattering (SERS) performance. As shown in Fig. 8A, typical Raman peaks of R6G, for the bare Si substrate and prepared SERS substrates, appeared at ca. 1363 cm⁻¹, 1509 cm⁻¹ and 1650 cm⁻¹ which were ascribed to the ν(C–C) stretching mode, and were in accord with reported R6G molecule SERS spectra.⁴⁰ The intensity of the R6G signal was enhanced when using substrates with a higher content of Au-NPs. Significantly, there was an enhanced intensity for R6G on the Au-NP/MoS₂ with the molar ratio of Au-NPs : MoS₂ = 10 : 1, and this sample exhibited the highest intensity. These results verify that the SERS effect strongly depends on both the increasing size of the Au-NPs and the aggregated nanoporous structure because morphology has an intimate link with so-called “hot spots”.^41–44 To quantitatively study the SERS performance of the prepared nanocomposites, the analytical enhancement factor (EF) was calculated according to the following equation (eqn (1))^25,45,46

EF = [I_SERS]/[I_bulk] × [N_bulk]/[N_SERS] (1)

where I_SERS and I_bulk are the intensities of a vibrational mode in the SERS and Raman spectra, respectively. N_SERS and N_bulk are the number of R6G molecules on the MoS₂-based SERS active substrate and in the bulk solution (0.1 M) effectively illuminated using a laser, respectively.

Fig. 8 (A) SERS spectra of (a) bulk R6G (10⁻¹ M) using a bare Si substrate, and R6G (10⁻⁵ M) collected on different SERS-active substrates: (b) commercial pristine MoS₂ nanoflakes, (c) Au-NP/MoS₂ nanocomposites with a 1 : 5 molar ratio, (d) Au-NP/MoS₂ nanocomposites with a 1 : 1 molar ratio, (e) Au-NP/MoS₂ nanocomposites with a 5 : 1 molar ratio, and (f) Au-NP/MoS₂ nanocomposites with a 10 : 1 molar ratio. (B) SERS spectra of R6G on a 10 : 1 Au-NP/MoS₂ nanocomposite substrate at different concentrations: (a) 10⁻⁷ M, (b) 10⁻⁶ M, (c) 10⁻⁵ M and (d) 10⁻⁴ M. (C) SERS spectra of R6G (10⁻⁵ M) collected on different substrates: (a) pristine MoS₂, (b) 2D-Au-NP/MoS₂ nanocomposites and (c) 3D-Au-NP/MoS₂ nanocomposites.

The SERS intensities for different Au-NP/MoS₂ nanocomposites were monitored for two different Raman peaks (1363 cm⁻¹ and 1650 cm⁻¹) of 10⁻⁵ M R6G to calculate the average enhancement factor of the prepared substrate, and the results are summarized in Table S3.† It was found that the Au-NP/MoS₂ sample with a 10 : 1 ratio exhibited the highest enhancement factor (19.9 × 10⁵). Besides, R6G of different concentrations in the range of 10⁻⁴ to 10⁻⁷ M was applied to monitor the enhancement limit of the substrate which had the highest enhancement ability (Au-NP/MoS₂ sample with 10 : 1 ratio). As shown in Fig. 8B, even at a concentration of R6G of 10⁻⁷ M, the peaks at 1363 cm⁻¹ and 1503 cm⁻¹ were still visible. As shown in Table S4,† the substrate had the highest enhancement factor of 73.4 × 10⁵ when the concentration of R6G was 10⁻⁷ M.

To confirm the advantages of the 3D-nanocomposites, the SERS ability of pristine MoS₂ nanoflakes, 2D-Au-NP/1T-MoS₂ nanosheets and 3D-nanoporous Au-NP/MoS₂ nanocomposites were compared and the results are shown in Fig. 8C. Although the charge transfer and dipole–dipole coupling of the semiconductor MoS₂ sheets have been reported to enhance the Raman signal of molecules,⁴⁷ the signal detected on the pristine MoS₂ nanoflakes was extremely weak. In addition, in order to discuss the structural influence on the SERS, Au-NP/1T-MoS₂ nanosheets with a 2D structure were also prepared as a control according to a previous report,²⁷ in which, the Au-NPs were decorated on the surfaces of the MoS₂ without aggregation, leading to the formation of 2D-structured Au-NP/MoS₂ nanocomposites (denoted as 2D-Au-NP/MoS₂). The Au-derived SPR peak of the 2D-Au-NP/MoS₂ appeared at 580 nm, which is close to that of the 3D porous Au-NP/MoS₂. As can be seen in the STEM, the average size of the Au-NPs in the 2D-Au-NP/MoS₂ was ∼50.0 nm, which is similar to that (∼51.1 nm) of the Au-NPs of the 3D-Au-NP/MoS₂. However, as shown in Table S3,† the EF of the 2D-Au NP/MoS₂ was calculated to be 3.48 × 10⁵, which is smaller than that (19.9 × 10⁵) of the 3D porous Au-NP/MoS₂ nanocomposites (19.9 × 10⁵), even though the Au-NP sizes and the amount of Au-NP/MoS₂ nanocomposites dropped on the Si substrate were the same, for which the deposited quantities were adjusted based on the absorbance in the UV/vis absorption spectra. It is suggested that the 3D porous architecture provided by the MoS₂ frameworks would increase the specific surface area and increase the Z-distribution of the Au-NPs which would build up the amount of “hot spots” longitudinally as well as horizontally and enhance the SERS signal as observed in other studies.^45,47

Conclusions

In summary, we described a facile method to prepare Au-NP/MoS₂ nanocomposites with porous structures in water/ethanol without using any additional reducing agent, in which both MoS₂ and ethanol reduced the Au ions to form Au-NPs on the MoS₂. The size of the Au-NPs depended on the molar ratio of the Au ions and MoS₂. The large sized Au-NPs (>30 nm) on the MoS₂ flakes worked as a joint to connect each MoS₂ flake, leading to the formation of a 3D-porous structure. The morphology of the MoS₂ flakes worked as a support framework since ethanol replaced the Mo⁴⁺ ions to reduce Au ions efficiently. The 3D-porous Au-NP/MoS₂ nanocomposites had a 19.9 × 10⁵ EF value for R6G enhancement in Raman spectroscopy, which corresponds to ∼20 times larger than the EF (0.95 × 10⁵) of the pristine MoS₂ nanoflakes and ∼5.7 times larger than the EF (3.48 × 10⁵) of the 2D-Au-NP/1T-MoS₂ nanosheets, indicating that the 3D-porous-structured nanocomposites are a suitable material for SERS. The present work could be of importance in the preparation of many different nanometal/MoS₂ materials with a 3D porous structure through a process based on the reducing ability of MoS₂ and ethanol, and such materials would be useful for developing future applications such as new sensor materials.^48–52

Acknowledgements

Xiaojing Yu acknowledges the China Scholarship Council (CSC) for their support. This work was supported in part by the Nanotechnology Platform Project (Molecules and Materials Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan and by the Japan Science and Technology Agency (JST) through its Center of Innovation Science and Technology-based Radical Innovation and Entrepreneurship Program (COI Program).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15421a

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