Selective preparation of zero- and one-dimensional gold nanostructures in a TiO2 nanocrystal-containing photoactive mesoporous template
© Kawamura et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Nanocrystallized SiO2-TiO2 with tubular mesopores was prepared via the sol-gel technique. Gold was deposited in the tubular mesopores of the nanocrystallized SiO2-TiO2. The shape of the gold was varied from one-dimensional [1-D] to zero-dimensional [0-D] nanostructures by an increase in TiO2 content and ultraviolet [UV] irradiation during gold deposition. 1-D gold nanostructures [GNSs] were mainly obtained in the mesopores when a small amount of TiO2-containing mesoporous SiO2-TiO2 was used as a template, whereas the use of a template containing a large amount of TiO2 led to the formation of shorter 1-D or 0-D GNSs. UV irradiation also resulted in the formation of 0-D GNSs.
PACS: 06.60.Jn (sample preparation); 81.07.Gf (nanowires); 81.16.Be (chemical synthesis methods).
Keywordsmesoporous titania template gold nanostructures shape control photocatalysis surface plasmon resonance
Gold nanostructures [GNSs] have been attracting much attention because of the high chemical stability coincident with their unique optoelectronic properties, which are dependent on the morphology of the GNSs [1–4]. Surface plasmon resonance [SPR] is one of the most interesting properties of one-dimensional [1-D] GNSs [2–5]. The wavelength of SPR is affected by the length, diameter, and aspect ratio of the 1-D GNSs [6, 7]. Aligned GNSs perform polarization of light [8–10]. Such multifunctionality of the 1-D GNSs opens up new application fields such as wavelength-sensitive nonlinear optical devices and polarization filters [8, 9, 11]. Several methods for synthesizing GNSs including 1-D GNSs have been reported. These methods include photochemical and electrochemical deposition [12, 13] and seeding growth methods [14, 15]. In these methods, however, the GNSs are suspended in a solvent. Therefore, the GNSs are required to be immobilized in a designed fashion in/on a solid matrix for various kinds of practical applications. The immobilization process for GNSs still requires further development [3, 10, 16].
On the other hand, the use of hard templates such as anodic alumina and mesoporous silica for the synthesis of 1-D GNSs makes the complicated immobilization processes redundant, and several related studies have been reported [17, 18]. Those methods using hard templates are also advantageous to control the diameter and dispersion state of 1-D GNSs because they depend on the pore structure. However, methods that control the morphology of the 1-D GNSs have several problems. For example, the elongation of the 1-D GNSs requires more gold to be deposited in the template. This obstructs, for example, the investigation of the shape-dependent properties of the GNSs. Therefore, a novel method to control the morphology of the 1-D GNSs in hard templates without changing the gold amount is eagerly demanded.
In this work, nanocrystallized SiO2-TiO2 with tubular mesopores was prepared and used as an active template. 0-D and 1-D GNSs were deposited in the tubular mesopores. The shape of the GNSs was observed, and the SPR characteristics were measured. It is known that TiO2 nanocrystals generate electrons through heating and ultraviolet [UV] irradiation. In this study, the generated electrons were found to transfer to the Au3+ ions. As such, the deposition rate of the GNSs can be controlled by controlling the amount of electrons generated. As a result, 0-D and 1-D GNSs are selectively deposited.
Pluronic P123 ((EO)20(PO)70(EO)20, poly(ethylene oxide), and poly(propylene oxide)) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Tetraethoxysilane [TEOS] and 3-aminopropyltriethoxysilane [APTES] were obtained from Shin-Etsu Chemical Co., Ltd. (Tokyo, Japan). Titanium tetra-n-butoxide [TTB] and HAuCl4 were acquired from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and Kishida (Osaka, Japan), respectively.
Synthesis of mesoporous template
The preparation procedure of 20Ti is described as a typical example. A mixture of P123 (1.74 g), NaCl (2.92 g), and 1 mM HCl (100 mM) was added to TEOS (4.18 g) and stirred at 35°C for 24 h. TTB (1.70 g) was then added to the solution and stirred further for 6 h. For the preparation of (100-x)SiO2·x TiO2, only the ratio of TEOS to TTB was varied. The stirred solution was transferred into an autoclave vessel and kept at 100°C for 4 h. The precipitated powder was collected by suction filtration, then washed with ion-exchanged water [IEW] and ethanol, and dried in an ambient environment. The obtained powder was calcined at 550°C for 5 h to remove the surfactant from the mesopore.
Loading of Au
The obtained powder was immersed in the 1 wt.% APTES solution (in ethanol) and stirred at 25°C for 3 h. The powder was then filtered with suction, washed with ethanol, and dried at 60°C in air. The amino-functionalized powder was mixed into a 1-mM HAuCl4 aqueous solution and stirred at 25°C for 2 h. After the suction filtration, the product was washed with IEW and dried in an ambient environment. The product was then calcined at 350°C for 3 h (at a heating rate of 1°C/min) with or without UV irradiation (USHIO SP-9, 230-440 nm, 2.5 mW/cm2 at 365 nm).
X-ray diffraction [XRD] measurements were performed using a Rigaku RINT 2000 diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKα radiation (λ = 1.5406 Å). Transmission electron microscopy [TEM] images and energy dispersive spectroscopy [EDS] were taken using a Hitachi H-800 transmission electron microscope (High-Technologies Corporation, Chiyoda, Tokyo, Japan) and a JEOL JEM-2100F (JEOL, Ltd., Akishima, Tokyo, Japan) transmission electron microscope operating at 200 kV. UV/visible-near infrared diffuse reflectance [Vis-NIR DR] spectra were measured using a JASCO V-670 UV-Vis-NIR spectroscope (JASCO Corporation, Tokyo, Japan).
Results and discussion
1-D GNSs deposited in 0Ti showed two extinction peaks in the diffuse reflectance [DR] spectrum: a sharp extinction peak at 500 nm and a broad extinction peak spreading over the whole region of the NIR region (Figure 3D). The shorter- and longer-wavelength extinctions are attributed to the transverse mode of SPR and the light scattering by fairly long 1-D GNSs , respectively. The length of the 1-D GNSs was shortened when 20Ti was used. An extinction peak appeared at around 600 nm, and the extinction intensity at wavelengths longer than 1,200 nm increased. This is presumably due to the shortening of the 1-D GNSs, which leads to a decrease in the light scattering intensity of the long 1-D GNSs (appearing over the whole NIR region) and an increase in the longitudinal SPR [LSPR] mode caused by the short 1-D GNSs (appearing at the NIR region toward the shorter wavelength side, e.g. 600 nm and approximately 2,000 nm in this case). With 30Ti, the LSPR peaks blue-shifted and appeared at 580 and approximately 900 nm. When 50Ti was used, only 0-D GNSs were deposited accompanied by a 520-nm peak, which is attributed to the SPR of the 0-D GNSs. By the use of a mesoporous SiO2 template containing less than 30 mol% TiO2, 1-D GNSs exhibiting LSPR, which is excited by NIR light, are deposited regardless of the presence of TiO2 nanocrystallites in the template.
The results obtained suggest the probable mechanism of Au deposition by the simultaneous heat treatment and UV irradiation, where the predominant formation of 0-D GNSs was observed (Figure 4E). The heat treatment causes the decomposition of organic matter adsorbed on the wall of the template. This results in the partial reduction of Au3+ ions, followed by the formation of scattered Au nanoclusters. The partially reduced Au ions are released from their electrostatic adsorption to the amino groups and associated with the oxygen atoms on the wall surface of the matrix, enabling mobility of the Au ions . The Au ions, therefore, can reach the neighboring Au nanoclusters and are reduced on the surface of the nanoclusters by autocatalysis of Au [21, 22], resulting in the formation of 1-D GNSs because the growth of Au occurs in the tubular mesopores. Thermally excited electrons of TiO2 accelerate the reduction rate of the Au ions; thus, a large content of TiO2 in the template leads to the preferential formation of 0-D GNSs. Furthermore, UV irradiation also accelerates the reduction rate of the Au ions. Therefore, the combination of heat treatment and UV irradiation leads to a fast rate of Au deposition. The time taken for the movement of the Au ions is shortened, and the formation of 0-D GNSs instead of 1-D GNSs becomes dominant. By optimizing the heating and UV irradiation condition of our method, 1-D and 0-D GNSs are selectively deposited regardless of the composition of the template, where the amount of deposited Au atoms is constant but the shape of the GNSs is different.
We have demonstrated the preparation of TiO2 nanocrystal-containing mesoporous templates and deposited 1-D or 0-D GNSs in the as-formed tubular mesopores. Since the provision of thermally generated electrons from TiO2 increased when a template contained a large amount of TiO2, Au ions were rapidly reduced and deposited as shorter 1-D or 0-D GNSs. Similarly, UV irradiation during Au deposition in the TiO2-containing template produced electrons photocatalytically and accelerated the Au deposition rate, leading to the dominant formation of 0-D GNSs.
energy dispersive spectroscopy
longitudinal surface plasmon resonance
surface plasmon resonance
transmission electron microscopy
This work was supported by Grants-in-Aid for Young Scientists (Start-up) 21860045 and Young Scientists (B) 22760539 from the Japan Society for the Promotion of Science (JSPS).
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