Surface clean gold nanoflower obtained by complete removal of capping agents: an active catalyst for alcohol oxidation

Abstract

Shape-controlled metal nanocrystals, such as nanowires and nanoflowers (NFs), are attractive due to their potential novel catalytic properties. However, catalytic activity is limited due to capping agents adsorbed on the metal surface, which are necessary for the preparation and dispersion of shape-controlled nanocrystals in solution. Recently, surface clean shape-controlled nanocrystals, which are obtained by removal of almost all of the capping agent, are very interested to bring out the intrinsically novel catalytic property. In this study, to prepare surface clean shape-controlled nanocrystals, firstly, Au NFs obtained by surfactant-free synthesis were supported on γ-Al₂O₃ to increase dispersibility and morphological stability, because unsupported Au NFs formed aggregates by removal of the capping agent. Then, we removed the capping agent from the surface of Au NFs by a calcination procedure and a water extraction procedure, in order to improve their catalytic activity. The water extraction procedure succeeded in removing the capping agent from Au NFs/γ-Al₂O₃ without morphological change and in providing surface clean Au NFs/γ-Al₂O₃, while the calcination procedure changed the morphology from NFs to spherical Au nanoparticles (Au NPs). Surface clean Au NFs/γ-Al₂O₃ was used for the aerobic oxidation of 1-phenylethyl alcohol, and the extraction procedure resulted in an eleven-fold increase in catalytic activity. Furthermore, the formation rate of acetophenone on surface clean Au NFs/γ-Al₂O₃ was ten-fold higher than that on surface clean Au NPs/γ-Al₂O₃ having almost the same diameter, indicating the catalytic superiority of surface clean Au NFs.

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Introduction

Morphological control of noble metal nanocrystals (NCs) is important in the development of modern materials chemistry, because their physical and chemical properties can be easily and widely tuned by tailoring their particle size and shape.^1,2 There have been many reports of the fabrication of shape-controlled NCs, such as rods,^3,4 wires,^5–9 plates,^10,11 and cubes.^12–14 Recently, anisotropically structured NCs have been applied as catalysts in the expectation of novel catalytic properties.^15–24 In general, these shape-controlled NCs are prepared by a colloidal synthesis method in solution using a capping agent that has selective adsorption properties on metal surface, such as cetyltrimethylammonium bromide (CTAB),^3,4 poly(vinylpyrrolidone) (PVP),^25,26 or melamine.²⁰ The capping agent is essential for morphological control and inhibition of aggregates in the preparation of metal NCs.^21,27,28 However, catalytic activity is depressed due to adsorption of the capping agent on the metal surface, because the capping agent acts as a physical barrier, restricting free access by the reactants.^21,27 Therefore, in order to use NCs prepared by colloidal synthesis as catalysts, their preparation has to be carried out using a capping agent which is adsorbed weakly on a metal surface and/or removing the capping agent from the surface after preparation.^{15,20,27–30} The former preparation is called as surfactant-free method.

Recently, there have been some reports on the preparation of shape-controlled NCs, such as nanoflowers (NFs) and dendritic nanowires, under surfactant-free conditions using melamine and ascorbic acid.^15,20,23 Generally, the removal of capping agents from NCs for catalysis has been done by calcination and extraction procedures, with the result that the catalytic activity of the NCs improves upon removal of the capping agent.^30–32 However, morphological stability and dispersibility of metal NCs are easily affected not only by organic/inorganic additives and temperature changes but also by the removal of capping agent.^28,33–37 Therefore, in order to know intrinsically novel catalytic properties of shape-controlled NCs, it is necessary to remove the capping agents without change in their morphology and dispersion. The NCs, which is removing almost the entire capping agent after preparation by surfactant-free synthesis, are called as surface clean NCs, and expected as novel nanocatalysts with high catalytic activity.

The dispersibility and morphological stability of metal NCs is improved by the use of a support, such as carbon nanotubes, SiO₂, or Al₂O₃.^31,38 Hutchings et al. showed that a capping agent, polyvinyl alcohol, was removed from gold spherical nanoparticles (Au NPs) supported on TiO₂ by calcination and water extraction procedures.³⁰ The water extraction procedure was capable of removing the capping agent without changing the morphology of the Au NPs, while the calcination procedure did change the morphology.³⁰ In general, morphology of shape-controlled Au NCs was easily changed compared with Au NPs. For catalytic application, it is a worthy challenge to keep the morphology during the removal of capping agent through the effective approach such as extraction procedure, which could supply the shape-controlled surface clean Au NCs with high catalytic activity.

The purpose of this study is to clarify the effect of NF shape on the catalytic properties of Au particles. For achieving this purpose, it is necessary to remove capping agents from the surface of Au NFs, namely, to prepare surface clean NFs. We attached NFs on the surface of γ-Al₂O₃ support to improve the stability against the removal of the capping agent. Furthermore, we applied the mild treatment, water extraction, to remove the capping agent at room temperature. Through these considerations, we finally revealed the higher catalytic activity of surface clean Au NFs than that of surface clean spherical Au NPs in the oxidation of 1-phenylethyl alcohol.

Experimental

Materials

Hydrogen tetrachloroaurate tetrahydrate (HAuCl₄·4H₂O) was obtained from Wako Pure Chemical Industries. Melamine (Kanto Chemicals), γ-Al₂O₃ (JRC-ALO-8, Sumitomo Chemical, BET surface area of 148 m² g⁻¹), SiO₂ (Cariact G-6, Fuji Silysia Co., BET surface area of 640 m² g⁻¹), ascorbic acid (Tokyo Chemical Co.), and 1-phenylethyl alcohol (Tokyo Chemical Co.) were used without further purification.

Preparation of Au NFs/γ-Al₂O₃

Au NFs were prepared by a previously reported surfactant-free method²⁰ using melamine as a capping agent which has a weak adsorption power on Au surface. 10 mM aqueous solution of HAuCl₄·4H₂O (5.0 mL) was added to a 10 mM aqueous solution of melamine (175 mL). Then, 10 mM ascorbic acid aqueous solution (20 mL) was slowly injected into the above solution. The colour of the solution quickly turned to blue. γ-Al₂O₃ (1.0 g) was added to the Au NF dispersion (200 g) and stirred for 24 h. When the dispersion was heated to 80 °C, Au NFs/γ-Al₂O₃ powder was obtained.

Removal procedure for capping agent

We carried out removal of the capping agent by two methods, a calcination procedure and a water extraction procedure. The calcination procedure was conducted by heating the as-prepared Au NFs/γ-Al₂O₃ at 100, 200, 300, 400, or 500 °C for 1 h in air. The water extraction procedure was carried out using the following method. The as-prepared Au NFs/γ-Al₂O₃ (0.4 g) was added to water (40 g) and stirred for 10 min at room temperature. Au NFs/γ-Al₂O₃ was recovered from the solution by centrifugation at 4000 rpm for 10 min. After several repetitions of these treatments, Au NFs/γ-Al₂O₃ powder was obtained by heating at 80 °C for 5 min to remove water.

Catalytic activity before and after extraction procedure

Aerobic oxidation of 1-phenylethyl alcohol (1-PA) was carried out at 60 °C in a batch reactor under an air atmosphere. After addition of K₂CO₃ (0.1 g, 0.72 mmol) to 10 mL of 1.5 mM 1-PA aqueous solution (15 μmol), the mixture was stirred at 60 °C. Au NFs/γ-Al₂O₃ (50 mg) before or after the water extraction procedure were added, and the mixture was stirred at 60 °C under air. After quenching the reaction with 1 M HCl, the products were extracted five times with toluene. The yield of the products was determined by gas chromatography using the internal standard method.

Characterization

Transmission electron microscopy (TEM) observation was carried out using a JEOL JEM-1011 instrument operating at 100 kV. High-resolution TEM (HR-TEM) was performed using a JEOL 2100 instrument equipped with an energy-dispersive X-ray spectrometer (EDX) operated at 200 kV. UV-vis absorption and reflectance spectrum measurements were carried out using a JASCO V-570. γ-Al₂O₃ was used as base sample for UV-vis reflectance spectroscopy (incident and reflectance angle: 30°). FT-IR measurement was carried out using a JASCO FT/IR4200 instrument. The sample (a 20 mm disk) was prepared by pressing Au NCs/γ-Al₂O₃ (3 mg) with KBr (300 mg). TG-DTA measurements were conducted using a SII TG/DTA 7200 instrument. The sample was heated from room temperature to 800 °C at a rate of 10 °C min⁻¹ under a flow of air.

Results and discussion

Au NFs with an average size of 75 nm were synthesized by reducing HAuCl₄ in a solution of melamine as capping agent (Fig. 1a and b). UV-vis spectroscopy is very useful for examining the shape of Au NCs, because the surface plasmon (SP) band of Au NCs is strongly dependent on their shape.^2,39–41 For example, the SP band of spherical Au NPs appears at ∼530 nm, while that of Au NFs appears at ∼600 nm.^20,41–45 Therefore, we examined the SP band of the Au NF dispersion by UV-vis spectroscopy. UV-vis measurements showed that peak position of the SP band was at ∼630 nm, indicating the formation of Au NFs (Fig. 2).^20,42 This is in good agreement with the blue colour of the as-prepared solution (Fig. 1a).^20,42 TEM-EDX spectra showed that the Au NFs were composed of pure gold (Fig. S1†). HR-TEM image of Au NFs showed fringes with a periodicity of 0.235 nm, which corresponds to (111) lattice spacing in Au (Fig. S2†).^7,8,36,46 Predominant growth along the [111] direction was due to weak adsorption of melamine on the Au (111) crystal facet compared with (100) and (110) crystals facets.²⁰

Fig. 1 (a) Photographic and (b) TEM images of as-prepared Au NFs before the water extraction. (c) TEM and inserted photographic images of unsupported Au NFs after third extraction.

Fig. 2 UV-vis absorption spectra of unsupported Au NFs (a) before the water extraction (solid line) and (b) after third extraction (dashed line).

When we carried out removal of the capping agent from the Au NF dispersion by the water extraction three times, the Au NFs formed aggregates because Au surface were exposed due to desorb capping agent (Fig. 1c). The black precipitate did not re-disperse into the solution upon shaking and stirring (Fig. 2b). These results indicate that it is very difficult to remove melamine from an unsupported Au NF dispersion without change in morphology and dispersion ability.

Dispersion of NCs can be improved by ligand exchange, coating with silica, or attaching to a support such as silica or γ-Al₂O₃.^31,37,38,47 Au NFs were not supported on silica. Therefore, we supported Au NFs on γ-Al₂O₃, which shows relatively strong interactions with metal particles.^48,49 The Au NFs/γ-Al₂O₃ dispersion was obtained by adding γ-Al₂O₃ to the Au NF dispersion and stirring for 20 h (Fig. 3a and b). After leaving it for 8 h, the colours of the solution and the precipitate were colourless and blue, respectively, indicating that almost all of the Au NFs had been adsorbed onto γ-Al₂O₃ (Fig. 3c). The blue precipitate (Au NFs/γ-Al₂O₃) was easily redispersed by stirring (Fig. 3e). When the Au NFs/γ-Al₂O₃ dispersion was heated to 80 °C to evaporate the solvent, Au NFs/γ-Al₂O₃ was obtained in the form of a purple powder, as shown in Fig. 3d. TEM observation showed that the morphology of Au NFs did not change after the evaporation of water at 80 °C (Fig. 3d). The Au NFs/γ-Al₂O₃ powder was easily redispersed by adding water, and the colour changed back to blue after redispersion (Fig. 3f). Generally, the SP band of Au NCs is affected by the environment, such as the solvent used.^50–54 Therefore, we examined the SP band of Au NFs/γ-Al₂O₃ using UV-vis reflectance spectra before and after the addition of water. The SP band of Au NFs/γ-Al₂O₃ was shifted from 575 nm to 640 nm upon addition of water, and was in good agreement with that of as-prepared Au NFs (Fig. 3, S3 and S4†).

Fig. 3 Photographic and TEM images of (a) Au NFs dispersion, (b) Au NFs/γ-Al₂O₃ dispersion, (c) Au NFs/γ-Al₂O₃ precipitate, (d) Au NFs/γ-Al₂O₃ powder, and (e and f) Au NFs/γ-Al₂O₃ redispersion.

Au NFs/γ-Al₂O₃ contains large amounts of melamine as capping agent. We tried to remove the melamine without significant morphological change by two procedures such as calcination and water extraction.

TG-DTA measurement showed that decomposition of melamine occurred at ∼300 °C (Fig. S5†). Thus, to remove the capping agent by the calcination procedure, it is required that NF morphology remains stable even above 300 °C. We examined the morphological stability of Au NFs by calcining Au NFs/γ-Al₂O₃ in air for 1 hour at 100–500 °C. At 100 °C, the Au NFs/γ-Al₂O₃ powder retained its purple colour, but at 200 °C it changed to purple-red (Fig. 4a and b), and above 300 °C it changed to red (Fig. 4c–e). In addition, UV-vis spectra showed that the peak position (∼575 nm) after calcination at 100 °C was almost the same as that obtained for as-prepared Au NFs/γ-Al₂O₃, while the peak positions at 200, 300, 400, and 500 °C were ∼548, 532, 520, and 520 nm, respectively (Fig. 5 and S6†). TEM observation showed that the morphology of Au NFs was retained at 100 °C, while it gradually changed to spherical NPs as the temperature increased from 200 to 400 °C (Fig. 4). Above 400 °C, the morphology was almost spherical (Fig. 4). These results indicate that the morphology of Au NFs is retained after calcination at 100 °C but that deformation takes place above 200 °C. In addition, FT-IR measurement showed that melamine was not removed from Au NCs/γ-Al₂O₃ upon calcination at 100 and 200 °C but was removed after 500 °C (Fig. S7†).

Fig. 4 TEM and photographic images of Au NCs/γ-Al₂O₃ after calcination at (a) 100, (b) 200, (c) 300, (d) 400, and (e) 500 °C.

Fig. 5 Peak position of SP band before and after calcination. Black dotted line is peak position of as-prepared Au NFs/γ-Al₂O₃.

Because the morphology of Au NFs changed upon calcination at 200 °C, as mentioned above, the calcination procedure is not suitable for the removal of capping agents without morphological change. Then, we attempted the removal of the capping agent using the water extraction procedure. In the first extraction, Au NFs/γ-Al₂O₃ was immersed into water and centrifuged. TEM images showed that the morphology, including the shape and diameter, of NFs did not change after the first extraction (Fig. 6a and 7). Upon repeated extraction procedures, the morphology of Au NFs/γ-Al₂O₃ was still retained (Fig. 6). In addition, the UV-vis spectra of them showed that the peak position of the SP band (575 nm) did not change after repeated extraction, indicating a good agreement with the observed colour of purple (Fig. 6, 7, and S8†). This corresponds to the unchanged colour upon the repeated extraction. Thus, the water extraction procedure by using Al₂O₃ support was revealed to retain the morphology of Au NFs.

Fig. 6 TEM and photographic images of Au NFs/γ-Al₂O₃ after water extraction procedure carried out (a) one, (b) two, (c) three, (d) four, and (e) five times.

Fig. 7 Peak positions of surface plasmon band (open circles) and average diameter (filled circles) of Au NFs on Al₂O₃ before and after water extraction procedure.

We subsequently examined the catalytic activity of Au NFs/γ-Al₂O₃ for aerobic oxidation of 1-phenylethyl alcohol before and after the water extraction procedure (Fig. 8). The yield of acetophenone increased with the number of extractions up to four times (Fig. S9 and Table S1†). Similarly, the formation rate of acetophenone was also improved by washing (Fig. 8 and Table 1). As a result, the formation rate after fifth extraction showed an eleven-fold increase over that before the extraction. Because the water extraction procedure did not change the morphology of the Au NFs, the improvement in catalytic activity would be due to remove the melamine, which capping agent decreased catalytic activity, in every extraction. We also carried out elemental analyses (Table 1) and FT-IR measurements (Fig. S10†) to confirm the amount of melamine in Au NFs/γ-Al₂O₃. Elemental analyses showed that almost all of melamine (>98%) was removed after fourth extraction (Table 1). This result was well agreed with the disappearance of the peaks assigned with melamine in FT-IR spectra after extraction procedure (Fig. S10†). Namely, surface clean Au NFs were obtained by this washing method.

Fig. 8 Change in formation rate of acetophenone at 6 h in 1-phenylethyl alcohol oxidation with number of repeated extractions.

Table 1 Amount of melamine in 0.05 g of Au NFs/γ-Al₂O₃, and formation rate of acetophenone at 6 h in 1-phenylethyl alcohol oxidation

Number of extraction	Melamine/μmol	Formation rate/μmol h^−1
0	57.38	0.12
1	14.40	0.30
2	1.90	0.57
3	0.18	1.16
4	0.12	1.38
5	0.12	1.40

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The effect of melamine on the catalytic activity was further investigated by titration of melamine to the washed catalyst; aerobic oxidation of 1-phenylethyl alcohol was carried out after the addition of melamine to Au NFs/γ-Al₂O₃ that had undergone the extraction procedure five times. The formation rate of acetophenone decreased as the amount of melamine increased (Fig. 9). Interestingly, the dependence of the formation rate of acetophenone on the amount of melamine well agreed with that for the extraction procedure. These results strongly indicate that (1) the removal and addition procedures are reversible; (2) the change in catalytic activity depends exclusively on the amount of melamine. Therefore, we conclude that the extraction procedure using γ-Al₂O₃ as support can remove the capping agent and improve catalytic activity, without changing the morphology of Au NFs.

Fig. 9 Change in the formation rate of acetophenone in 1-phenylethyl alcohol oxidation over Au NFs/γ-Al₂O₃ with amount of added (open squares) or residual (filled circles) melamine. Reaction conditions: 1-phenylethyl alcohol (15 μmol), catalyst (50 mg), K₂CO₃ (0.1 g), water (10 g), air (1 atm), 60 °C, 6 h.

Finally, we compared the catalytic activity of surface clean non-spherical Au NFs/γ-Al₂O₃ with that of surface clean spherical NPs (Au NPs/γ-Al₂O₃) for 1-phenylethyl alcohol oxidation. The former catalyst was prepared by repeating the extraction procedure for five times, and the latter one was obtained by heating the former catalyst at 200 °C in air for 1 h. FT-IR showed no peak from melamine (Fig. S11†). Fig. 10 shows the TEM images of typical NF and NP. The average diameter of spherical NPs was about 70 nm, which was close to that of NFs, indicating that no aggregation of Au particles occurred during the deformation of NFs into NPs at 200 °C. Fig. 10 also shows the change in acetophenone yield with reaction time on surface clean Au NFs/γ-Al₂O₃ and surface clean Au NPs/γ-Al₂O₃. The former exhibited much higher acetophenone yield. The formation rates of acetophenone were 1.40 (NFs) and 0.135 μmol h⁻¹ (NPs), respectively. The shape of Au particles indicates a significant decrease in Au surface area during the thermal treatment. However, the difference between the Au surface areas would be much smaller than that between formation rates. The Au atoms on NFs must have higher intrinsic activity than those on NPs. Au atoms at corner and/or edge sites may be such highly active sites, because the NF particle looks like an aggregate of smaller crystallites having higher proportion of these sites. These results clarify the advantage of NF structure as the catalyst for aerobic oxidation. In addition, after washing the recovered Au NFs catalyst by water extraction, the catalyst was reused under same catalytic reaction condition. The formation rate of acetophenone was 1.33 μmol h⁻¹ without significant loss of activity.

Fig. 10 Acetophenone yield of surface clean Au NFs/γ-Al₂O₃ and Au NPs/γ-Al₂O₃. Reaction conditions: 1-phenylethyl alcohol (15 μmol), catalyst (50 mg), K₂CO₃ (0.1 g), water (10 g), air (1 atm), 60 °C.

Conclusions

This article reveals the advantage of surface clean Au nanoflowers over surface clean spherical particles as metal catalysts. Au NFs supported on the surface of γ-alumina have higher morphological stability than unsupported ones. With the help of γ-alumina support and by using capping agent weakly bound to NFs, the capping agent is removed completely by moderate water extraction treatments keeping the morphology of NFs intact. A bare nanoflower particle thus obtained consists of small crystallites, which have higher proportion of edge and corner sites as well as higher surface area than a spherical nanoparticle with the same particle size. This must be the reason for the much higher catalytic activity of Au NFs for the alcohol oxidation. The findings in this article will open new possibilities of shape-controlled nanocrystals as novel metal catalysts.

Acknowledgements

This work was partially supported by a Grant-in-Aid for JSPS Fellows from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM-EDX, HR-TEM image, photographic images, UV-vis spectra, TG-DTA, FT-IR, catalytic performance, elemental analyses. See DOI: 10.1039/c5ra27146k

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