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Abstract
Au clusters, embedded in a dendrimer nanoparticle, have been successfully fabricated by photoreduction of gold ion-dendrimer complex nanoparticles that were fabricated using the reprecipitation method. The electron microscope observations suggested that the Au cluster was formed in an individual gold ion-dendrimer complex after 10 min of UV irradiation. As a result, Au clusters with 1.3 ± 0.6 nm were obtained as a minimum size. In addition, the final size of Au clusters can be controlled by changing UV irradiation time through the aggregation (or fusion) of Au clusters and/or Au ions in several dendrimer molecules.
© 2017 Optical Society of America
1. Introduction
Noble metal nanoparticles (NPs) have attracted much attention in recent years because of remarkably higher catalytic activities than the corresponding bulk metals [1,2]. The limited availability of metal resources has led to a high requirement for developing high-performance catalysts. Especially, it is important to avoid the aggregation of noble metal NPs in order to maximize the specific surface area. For example, the metal NPs have been supported on a matrix such as a metal oxide [3] and surface-modified with ligand molecules such as thiol derivatives [4]. On the other hand, since dendrimers, perfectly monodisperse macromolecules with precisely controlled structure, have nano-size void around their core, they can act as a host for various organic compounds and/or ions [5]. Tomalia-type poly(amidoamine) dendrimer (PAMAM dendrimer) has been utilized to fabricate gold [6], copper [7], platinum, and silver [8] NPs. The catalytic activities and the relationship between the generation number of PAMAM and the size of Pt and Pd NPs have been also investigated in the detail [9]. In addition, Au-PAMAM composites have been reported to form Au NPs with 2–5 nm [10]. Au NPs are one of the most promising catalysts, in spite of bulk Au as an inactive material. Their remarkable catalytic activities, however, can be observed within sub-nanometer in size.
Dendritic phenylazomethine (DPA), shown in Fig. 1, has rigid architecture and shows stepwise radial complexation from the inner imines to the outer imines with various metal ions [11]. They exhibit chemical properties depending on numbers of metal ions accumulated in DPA molecule [12–14]. The greatest feature of DPA is that the accumulation number of metal ions in one DPA molecule is controllable as the coordination number of metal atoms, where precise size-control of metal clusters has also been carried out [15]. Furthermore, metal clusters show high dispersion stability due to encapsulation in DPA molecule. However, it is a still challenging theme to mass-produce precisely size-controlled metal clusters because reduction of metal ion-dendrimer complex must perform under dilute condition.
In this article, we report the fabrication of a large number of Au clusters embedded in a DPA NP by photoreduction of Au ion-DPA complex NPs fabricated using reprecipitation method. The size of Au clusters has been investigated on photoreduction condition such as UV irradiation time.
2. Experimental
2.1 Materials
DPA was synthesized according to the literature [16,17]. Dehydrated chloroform, dehydrated cyclohexane, dehydrated acetonitrile, and gold(III) chloride were purchased from Wako Pure Chemical Industries, Ltd., and used without further purification.
2.2 Fabrication of Au-DPA ((14Au3+)@DPA) NPs
(14Au3+)@DPA NPs were fabricated by the reprecipitation method as follows. First, a solution of gold chloride in dehydrated acetonitrile was added to a solution of DPA (fourth-generation dendritic phenylazomethine with a phenylene core) in dehydrated chloroform. The mixture was stirred for a few minutes, and changed from yellow to orange, which indicates the formation of DAP complex with fourteen Au3+ ions, (14Au3+)@DPA complexes. The concentration of (14Au3+)@DPA was adjusted in the range from 0.07 mM to 0.27 mM. Then, a 53 µL of the solution was injected into vigorously stirred dehydrated cyclohexane (10 mL) at room temperature using a microsyringe.
2.3 Photoreduction of (14Au3+)@DPA NPs
Photoreduction of (14Au3+)@DPA NPs was induced by UV irradiation with a super-high-pressure mercury lamp (USH500SC, USHIO). The irradiation time was in the range from 10 min to 5 h.
2.4 Characterization
SEM and TEM images were collected using a field-emission scanning electron microscope (JSM6700F, JEOL) and a sub-Angstrom-resolution analytical electron microscope (TITAN80-300, FEI), respectively. The extinction spectra of (14Au3+)@DPA NPs dispersion liquid were measured using UV-visible spectrometer (V-570DS, JASCO) before and after UV irradiation. The XPS measurement was performed using a SHIMADZU ESCA-3400 spectrometer with Kα radiation. The charging effect of the sample was corrected using C(1s) signal with a binding energy of 284.2 eV as an internal standard.
3. Results and discussion
Figure 2 shows the SEM images of (14Au3+)@DPA NPs fabricated under different concentrations of the injected solution in the reprecipitation method. (14Au3+)@DPA NPs were roughly spherical. Figure 3 indicates the relationship between concentration of the injected solution and size of (14Au3+)@DPA NPs. The NPs slightly became larger with increasing concentration of the injected solution.
Fig. 2 SEM images of (14Au3+)@DPA NPs under different concentration of injected solution in the reprecipitation method: 0.07 mM (left) and 0.27 mM (right).
Photoreduction of Au3+ ions has been carried out by UV irradiation to (14Au3+)@DPA NPs dispersion liquid. Figure 4 shows XPS spectra of (14Au3+)@DPA NPs before and after UV irradiation. Two peaks for Au(4f7/2) (87.6 eV) and Au(4f5/2) (91.4 eV) of Au3+ ion were clearly observed before UV irradiation. On the other hand, they were shifted to 84.5 eV for Au(4f7/2) and 88.1 eV for Au(4f5/2) corresponding to bulk gold after UV irradiation. These peak shifts indicate that Au3+ ions were reduced even in NPs [18].
The UV irradiation to (14Au3+)@DPA NPs also resulted in a change in the extinction spectrum as shown in Fig. 5. The extinction peak at λ = 400 nm, assigned to the π-π* transition of (14Au3+)@DPA, decreased after UV irradiation, which indicates decomplexation of (14Au3+)@DPA. In addition, no significant change was observed in long wavelength region as shown in the inset of Fig. 5. These facts suggest that very small Au clusters without localized surface plasmon resonance (LSPR) peak would be formed in DPA NPs.
Fig. 5 Extinction spectral change of (14Au3+)@DPA NPs dispersion liquid before (blue) and after (red) UV irradiation. The inset is an enlarged spectra in a long-wavelength region.
Then, (14Au3+)@DPA NPs were observed using SEM before and after the photoreduction (Fig. 6). These SEM samples were prepared by filtrating (14Au3+)@DPA NPs with a previously platinum-sputtered filter (VMTP04700, Millipore). Namely, no platinum was coated on the surface of (14Au3+)@DPA NPs. We can distinguish bright spots of Au NPs from dark area of DPA. Although only dark NPs corresponding to (14Au3+)@DPA NPs were observed before and after 10 min of UV irradiation, large bright spots corresponding to Au NPs were observed on the surface of dark NPs after 5 h of UV irradiation. The Au NPs was 48 ± 26 nm in size and irregular in shape, so it seems that Au clusters have grown up during long irradiation time.
Fig. 6 SEM images of (14Au3+)@DPA NPs before UV irradiation (left), and after 10 min (center) and 5 h (right) of UV irradiation.
TEM image before UV irradiation shows black spots with 1.9 ± 0.6 nm in size (Fig. 7). Judging from molecular size of DPA (2–3 nm) [11], these spots would be attributed to (14Au3+)@DPA molecules as schematically depicted in the inset of Fig. 7(a). The size of black spots decreased to 1.3 ± 0.6 nm after 10 min of UV irradiation, which indicates that Au3+ ions accumulated in an individual DPA molecule aggregated to form a Au cluster by photoreduction as shown in the inset of Fig. 7(b). In addition, it is also found that each Au cluster was dispersed in DPA NPs without aggregation or fusion after 10 min of UV irradiation.
Fig. 7 TEM images (a and b) and size histograms (c and d) of (14Au3+)@DPA NPs before (left) and after 10 min (right) UV irradiation. The insets in (a) and (b) are schematic depiction of (14Au3+)@DPA and Au@DPA molecules, respectively.
Finally, the size of the resulting Au clusters was investigated on the UV irradiation time (Fig. 8). The size of Au clusters was evaluated using TEM images. As the UV irradiation time increased, the size and the standard deviation of Au clusters increased and their shape became irregular. Although Au cluster seems to be formed in an individual (14Au3+)@DPA molecule as a template, excess UV irradiation would induce aggregation or fusion of Au clusters or Au3+ ions in several DPA molecules. These results suggest that the size of Au clusters can be controlled by optimizing UV irradiation time.
4. Summary and conclusion
We have successfully fabricated Au clusters embedded in a DPA NP by photoreduction of (14Au3+)@DPA NPs fabricated using reprecipitation method. The SEM and TEM observations suggested that Au clusters were formed in an individual (14Au3+)@DPA molecule after 10 min of UV irradiation. As a result, Au clusters with 1.3 ± 0.6 nm were obtained. In addition, the final size of Au clusters can be controlled by changing UV irradiation time through the aggregation (or fusion) of Au clusters and/or Au3+ ions in several DPA molecules.
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