Highly Stable Silica-Coated Gold Nanoflowers Supported on Alumina

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Highly stable silica-coated gold nanoflowers supported on alumina Yoshiro Imura, Shiori Koizumi, Ryota Akiyama, Clara Morita-Imura, and Takeshi Kawai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00974 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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Highly stable silica-coated gold nanoflowers supported on alumina Yoshiro Imura a* Shiori Koizumi,a Ryota Akiyama,a Clara Morita-Imurab and Takeshi Kawaia* a

Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

b

Faculty of Core Research, Ochanomizu University,

2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan

[email protected], [email protected]

ACS Paragon Plus Environment

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Abstract Shape-controlled nanocrystals, such as nanowires and nanoflowers, are attractive because of their potential novel optical and catalytic properties. However, the dispersion and morphological stabilities of shape-controlled nanocrystals are easily destroyed by changing the dispersion solvent and temperature. Methods of support and silica coating are known to improve the dispersion and morphological stabilities of metal nanocrystals. The silica-coating method often causes morphological changes to shape-controlled nanocrystals because silica coating is conducted in mixed solutions of water and organic solvents such as ethanol, and this results in aggregation due to changes in dispersion solvent. Furthermore, ligand exchange, designed to improve dispersion stability in the solvent, often causes morphological changes. This paper introduces a method for the preparation of highly stable silica-coated Au nanoflowers (AuNFs) supported on Al2O3. The method of support prevents aggregation and precipitation of AuNFs when the solvent is changed from water to water/ethanol. Through stability improvement, silica coating of AuNFs/Al2O3 was conducted in water/ethanol without ligand exchange that causes morphological changes. Furthermore, silica-coated AuNFs/Al2O3 exhibit high morphological stability under high temperature conditions compared to uncoated AuNFs/Al2O3. These results are very useful when preparing highly morphologically stable, silica-coated, shape-controlled nanocrystals, without ligand exchange.


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Introduction Noble-metal nanocrystals are very important for applications in fields such as electrochemistry, electronics, magnetic storage, catalysis and biotechnology.1-10 The properties of a metal nanocrystal is strongly dependent on its morphology, including size and shape.1-8 Consequently, there have been many reports on the fabrication of various shape-controlled metal nanocrystals such as cubes,11,12 plates,13-15 rods,16-20 wires21-28 and flowers.29-31 Generally, shape-controlled nanocrystals are prepared through the use of capping agents that selectively adsorb onto the surfaces of particular noble metals and/or form assemblies as templates.1-3 However, the morphologies of shape-controlled nanocrystals are easily altered to spherical structures, or to aggregates, by changing temperature, pH or solvent.32-35 Recently, there are many reports to prepare silica nanomaterials with various shapes.36-38 Silica-coating of shape-controlled nanocrystals has become attractive because this method improves the morphological stability of these nanocrystals.35,39 As a result, it is possible to perform catalytic reactions at high temperatures using these nanocrystals, which shows high catalytic activity due to improved morphological stability.39 In addition, the silica-coating method increases dispersion stability of these nanocrystals because surface modifications to silica-coated nanocrystals can be carried out without affecting the morphology of the metal nanocrystal.40 Therefore, it is very important to develop effective silica-coating methods for application to shape-controlled nanocrystals. Generally, silica-coating has been conducted by hydrolysing tetraethoxysilane (TEOS) in NH3/water/ethanol as solvent, and using unsupported metal nanocrystals;35,39-45 hence, metal nanocrystals require high dispersion stability in NH3/water/ethanol.35 For increasing dispersion stability in the solvent, surface modification is often conducted by ligand exchange using poly(vinylpyrrolidone) (PVP), aminopropyltrimethoxysilane (APS) or 3-mercaptopropionic acid


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(3-MPA).35,40,44,45 Furthermore, surface modification also promotes silica coating on gold surfaces.35,40 However, the morphologies of shape-controlled nanocrystals, such as nanoflowers and nanowires, are easily altered by ligand-exchange surface modifications.34,35 Therefore, improved dispersion stability of shape-controlled nanocrystals, especially morphological stability, is highly desirable. The dispersion stability of metal nanocrystals is improved by the use of a support, such as carbon nanotubes, SiO2 or Al2O3.32,33,46-49 Hence, the use of a support is expected to be an effective method for the preparation of silica-coated nanocrystals that are shape-controlled. Previously, we reported the preparation of Au nanoflowers (AuNFs) supported on alumina with high dispersion stability after removal of the capping agent.32 In this paper, we show that the use of this support is an effective method for preparing silica-coated AuNFs without surface modification and morphological changes, because supported AuNFs have high dispersion stability in aqueous ammonia and ethanol, which are the conditions required for silica-coating (Figure 1). In addition, the silica-coating method improves the morphological stability of the NF structure, due to the lowering the surface energy of Au surface (Figure 1). These results are very useful when preparing shape-controlled nanocrystals with high dispersion and morphological stabilities.


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Figure 1. Schematic illustration of (a) dispersion stability before and after AuNFs are supported on Al2O3, and (b) morphological stability by calcination at 200°C, before and after silica coating. Experimental Materials. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4•4H2O) was obtained from Nacalai Tesque. Aqueous ammonia (28 wt%) was obtained from Wako Pure Chemical Industries. Tetraethoxysilane (3N, TEOS) and -Al2O3 (particle size: 200 nm) were purchased from Kanto Chemicals. Melamine and ascorbic acid were obtained from Tokyo Chemicals. All reagents were used as received, without purification. Unsupported AuNFs. Preparation of AuNFs.32 An aqueous solution of HAuCl4•4H2O (10 mM, 5.0 mL) was added to a 10 mM aqueous solution of melamine (175 mL). Aqueous ascorbic acid (10 mM, 20 mL) was then slowly injected into the above solution. The color of the solution quickly turned blue. Silica coating of unsupported AuNFs. A solution of TEOS (3N, 3 mL) in ethanol (3 mL) was added to the AuNF dispersion (40 mL). Aqueous ammonia (28 wt%, 260 L) was added to the mixture at which time the pH of the mixture was observed to change to about 10.


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Dispersion stability of AuNFs. AuNFs in water (2 mL) were added to 6 mL of water, aqueous ammonia or ethanol. The mixtures were left for 1 h at room temperature. Aqueous ammonia was added to the mixture until the pH value was about 10.

Silica-coated AuNFs supported on Alumina. Preparation of Supported AuNFs. -Al2O3 (1.0 g) was added to the AuNF dispersion (200 mL) and the mixture stirred for 24 h. After heating the dispersion at 80°C, the supported AuNF (AuNFs/-Al2O3) powder was obtained. The nanoflower structure was unaltered after a month (Figure S1a). Stability of AuNFs Supported on Alumina. AuNFs/-Al2O3 powder (0.1 g) was added to 2 mL of water, aqueous ammonia, or ethanol. Aqueous ammonia was added to the mixture until the pH value was about 10. Silica coating of Supported AuNFs. Silica coating method was based on the previous works.35,41 AuNFs/-Al2O3 powder (1.0 g) was added to a solution of TEOS in ethanol (1.5N, 6 or 30 mL). Aqueous ammonia (1.3 mL) was added to the mixture to afford silica-coated AuNFs/-Al2O3. The solvent was removed by evaporation. AuNFs/-Al2O3 with silica layer of 4 nm and 8 nm were obtained by using 6 mL and 30 mL of TEOS in ethanol, respectively. The nanoflower structure was unaltered after a month (Figure S1b and c). Characterization


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Transmission electron microscopy (TEM) was carried out using a JEOL JEM-1011 instrument operating 100 kV. High-resolution TEM (HR-TEM) was performed using a JEOL 2100 instrument equipped with an energy-dispersive X-ray spectrometer operating at 200 kV. UV-vis absorption and reflectance spectra were obtained on a JASCO V-570. -Al2O3 was used as the base sample for UV-vis reflectance spectroscopy (incident and reflectance angle: 30°).

Results and Discussion Preparation and dispersion stability of AuNFs. AuNFs with average diameters of 75 nm were synthesized by reducing HAuCl4 in a solution of melamine as capping agent (Figures 2a and 2b). UV-vis spectroscopy is a very useful tool for examining the shapes of Au nanocrystals, because the surface-plasmon (SP) band of an Au nanocrystal is strongly dependent on shape.20,50,51 The UV-vis spectrum exhibited an SP band at ~600 nm, which confirmed the formation of AuNFs (Figure 2c).29,30 This is in good agreement with the blue color of the as-prepared solution (Figure 2e). The HR-TEM image of AuNFs showed fringes with a periodicity of 0.235 nm, which corresponds to the (111) lattice spacing in gold (Figure 3).22-24 The predominant growth along the [111] direction was due to the weak adsorption of melamine on the Au (111) crystal facet compared with the (100) and (110) crystal facets.29


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Figure 2. (a, b) TEM images of as-prepared AuNFs. UV-vis spectra and photographic images of (c, e) as-prepared AuNF dispersion, and (d, f) Au precipitates after silica coating using TEOS/NH3/ethanol.

Figure 3. HR-TEM image of AuNFs.


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The conventional method for the preparation of silica-coated Au nanocrystals was performed in an ethanol solution of TEOS under alkaline conditions using ammonia. In order to cover the AuNFs with SiO2 shells, an ethanol solution containing TEOS and aqueous ammonia was added to the AuNF dispersion. The pH value of the solution was approximately 10. As a consequence, the color of the solution changed from deep blue to dilute blue, and a black Au precipitate was observed (Figure 2f). UV-vis spectroscopy revealed that the intense peak at 600 nm, corresponding to the SP band, was significantly weaker after silica coating than that before (Figures 2c and 2d). This decrease in peak intensity is due to the formation of the Au precipitate and is in good agreement with the photographic images (Figures 2 d and 2f). Generally, the dispersion stabilities of nanocrystals are altered by external stimuli, such as pH and dispersion solvent.33,35,52 We examined the effect of changes in melamine concentration, pH and the addition of ethanol. Firstly, we added water (6 mL) to the aqueous solution of AuNFs (2 mL) to examine the effect of the melamine concentration. The color remained blue after the addition, and the peak position of the SP band did not change from 600 nm (Figures 4c and 4f). In addition, the TEM image revealed that the NF structure was retained after the addition of water (Figure 4a). These results indicate that AuNFs remain dispersed in a melamine solution that was diluted by a factor of four. Next, we examined the effect of pH on the AuNF dispersion state through the addition of aqueous ammonia (6 mL) to the AuNF solution (2 mL). Following addition, the pH of the mixture was 10 and the color of the solution had changed to dilute blue (Figure 4g). This color change was associated with a change in the SP-band peak, from 600 to ~625 nm, with a concomitant decrease in intensity compared with that of AuNFs after the addition of 6 mL of water (Figures 4c and 4d). AuNF aggregates are observed in the corresponding TEM image (Figure 4b). Clearly, the changes in the SP band and solution color are due to the formation of


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aggregates and precipitates caused by the pH change. Furthermore, we examined the effect of changing the dispersion solvent from water to water/ethanol. Ethanol (6 mL) was added to the AuNF dispersion (2 mL), resulting in a color change from blue to transparent that coincided with the formation of a black Au precipitate (Figure 4h). The SP band at 600 nm disappeared upon addition of ethanol (Figure 4e); in other words AuNFs were not dispersed in ethanol/water. These results indicate that melamine-capped AuNFs are not dispersed in aqueous ammonium or water/ethanol. Therefore, the dispersion stabilities of AuNFs need to be improved in order to obtain silica-coated AuNFs.

Figure 4. TEM images, UV-vis spectra and photographic images after the addition of 6 mL of (a, c, f) water, (b, d, g) aqueous ammonia, and (e, h) ethanol to 2 mL of the AuNF dispersion.


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Improving AuNF stability through alumina support. We next examined the stability of AuNFs supported on -Al2O3 (AuNFs/-Al2O3, Figure 5) in aqueous ammonia and/or ethanol in order to prepare silica-coated AuNFs/-Al2O3. The AuNFs/-Al2O3 powder retained the NF structure and was purple in color (Figures 5a and 6). The color, more specifically the SP-band peak of the Au nanocrystals, is affected by the environment, such as the solvent used.20,50-52 The AuNFs/-Al2O3 powder changed color, from purple to blue, upon addition of water, due a red shifting of the SP band (Figures 5a and 5b). 32 In addition, the NF structure on -Al2O3 did not change upon addition of water to the AuNFs/Al2O3 powder (Figures 5a and 5b). These results indicate that the purple AuNFs/-Al2O3 retained the NF structure and that the purple color is due to the absence of water. In addition, aqueous ammonia was added to the AuNFs/-Al2O3 powder to examine its stability under the alkaline conditions used during silica coating. Once again, the color changed to blue, and the NF structure was retained when the pH was increased to 10 (Figure 5c). Figure 5d depicts the blue color and NF structure observed when ethanol was added to the AuNFs/Al2O3 powder. Furthermore, addition of aqueous ammonia and ethanol to the AuNFs/-Al2O3 powder resulted in the same color change, and the TEM image revealed that the NF structure had been retained (Figure 5e). These results demonstrate that AuNFs with high stability are formed by the Al2O3-supported method; these NFs do not aggregate under the conditions used for silica coating.


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Figure 5. TEM and photographic (inset) images of (a) the AuNFs/-Al2O3 powder, (b) AuNFs/-Al2O3 in water, (c) AuNFs/-Al2O3 in aqueous ammonia at pH 10.0, (d) AuNFs/Al2O3 in ethanol, (e) AuNFs/-Al2O3 in aqueous ammonia and ethanol, and (b) silica-coated AuNFs/-Al2O3 powder. (g) STEM-EDS spectra of AuNFs/-Al2O3 and silica-coated AuNFs/Al2O3. The Cu peaks arise from the TEM-Cu grid.


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Figure 6. TEM images (left) and STEM-EDS maps (right) of AuNFs/-Al2O3 before (top) and after (bottom) silica coating.

Preparation of highly stable silica-coated AuNFs/Al2O3. AuNFs supported on -Al2O3 were silica coated by the addition of a solution of TEOS in ethanol, under alkaline conditions. Silica layer was efficiently formed on the AuNFs, due to the adsorption of TEOS on Au surface. TEM images reveal that the AuNFs became coated by an 8nm-thick shell (Figures 5f and 6). STEM-EDS measurements indicated that the silica-coated AuNFs/-Al2O3 (SiO2/AuNFs/-Al2O3) were composed of Si, Al and Au, while AuNFs/-Al2O3 were composed of Al and Au (Figure 5g). In addition, STEM-EDS mapping images show that the SiO2/AuNFs/-Al2O3 are coated by silica layers (red dots), while uncoated AuNFs/-Al2O3


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are not coated (Figure 6). Furthermore, the STEM-EDS mapping image reveals that not only AuNFs are coated by silica, but the -Al2O3 is also coated by a silica layer (Figure 6). The color of the SiO2/AuNFs/-Al2O3 is blue, as opposed to purple for the uncoated material (Figures 5a and 5f). The TEM image shows that the NF structure was not changed by the silica-coating method (Figures 5a, 5f, and 6). The SP band peaks, before and after silica coating, were 585 and 625 nm, respectively (Figures 7a and 7c). As mentioned previously, the SP band of an Au nanocrystal is affected by its environment, such as dispersion solvent and silica coating.40,41,45,5052

Generally, silica coating of Au nanocrystals results in a red shift of its SP band.40,41,45

Therefore, the red shift observed for the SP band, and the change in color observed in this study, confirm that Au NFs/-Al2O3 are coated by a silica layer.

Figure 7. UV-vis reflectance spectra of (a) AuNFs/-Al2O3 powder, (b) SiO2/AuNFs/-Al2O3 with 4 nm silica layer, and (c) SiO2/AuNFs/-Al2O3 with 8 nm silica layer. UV-vis reflectance spectra after calcination of (d) AuNFs/-Al2O3 powder, (e) SiO2/AuNFs/-Al2O3 with 4 nm silica layer, and (f) SiO2/AuNFs/-Al2O3 with 8 nm silica layer at 200°C for 1 h.


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Finally, we confirmed the high morphological stability of SiO2/AuNFs/-Al2O3. The uncoated AuNFs/-Al2O3 were observed to transform from the NF structure to a spherical structure by calcination at 200°C (Figures 5a and 8a); the color also changed from purple to red (Figures 5a and 8a). It is known that spherical Au nanocrystals are red in color.53,54 The SP band was observed at 530 nm (Figure 7d), and in good agreement with the red color. On the other hand, the TEM image of the silica-coated AuNFs/-Al2O3 shows that its NF structure was unaltered by calcination at 200°C (Figures 5f and 8b), and the color and the SP band remained blue and 625 nm indicating that the morphological stability of the NF structure is improved by silica-coating, respectively (Figures 7c and 7f). We also prepared the ultrathin silica-coated AuNFs/-Al2O3 with average thickness of 4 nm, by the reduction of TEOS addition (Figure 9a). The color and the SP band were blue and 625 nm as well as SiO2/AuNFs/-Al2O3 (Figures 7b and 9a). It found that AuNFs with 4 nm silica layer changed by calcination at 200°C (Figure 9b), and the color and SP band were changed to purple and 570 nm, respectively (Figure 7e and 9b). We concluded that the thickness of silica shell greatly affected the stability of AuNFs. These results show silica coating of shape-controlled nanocrystals is easily achieved when supported on Al2O3, without ligand exchange that often causes morphological changes. The silica-coated AuNFs/-Al2O3 have high stability compared to uncoated AuNFs/-Al2O3.


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Figure 8. TEM and photographic (inset) images of (a) AuNFs/-Al2O3 and (b) SiO2/AuNFs/Al2O3 with 8 nm silica layer after calcination at 200°C.

Figure 9. TEM and photographic (inset) images of SiO2/AuNFs/-Al2O3 with 4 nm silica layer (a) before and (b) after calcination at 200°C for 1 h.

Conclusions This paper introduces a method for the preparation of highly stable silica-coated Au nanoflowers (AuNFs) supported on -Al2O3 (SiO2/AuNFs/-Al2O3). The Al2O3 support prevents aggregation of AuNFs and precipitation when the solvent is changed. By improving stability, the silica coating of AuNFs/-Al2O3 was conducted in water/ethanol under alkaline conditions, without


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the need for ligand exchange that often causes morphological changes. Furthermore, the SiO2/AuNFs/-Al2O3 exhibit high morphological stabilities at high temperatures compared to uncoated AuNFs/-Al2O3. These results are very useful when preparing highly morphologically stable, silica-coated, shape-controlled nanocrystals, without ligand exchange.

ASSOCIATED CONTENT Supporting Information. TEM images of AuNFs/-Al2O3 and SiO2/AuNFs/-Al2O3 after a month. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Y. Imura), [email protected] (T. Kawai) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 16K7490).


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Highly Stable Silica-Coated Gold Nanoflowers Supported on Alumina

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