Communications to the Editor Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 363 http://dx.doi.org/10.5012/bkcs.2013.34.2.363
Nanogold Particles Produced by NaBH
4Reduction of Gold Salt in the Presence of Laponite Sol
Seung-Kyu Yang and Youhyuk Kim*
Department of Chemistry and Institute of Basic Sciences, College of Advanced Sciences, Dankook University, Cheonan, Chungnam 330-714, Korea. *E-mail: [email protected]
Received November 2, 2012, Accepted November 30, 2012 Key Words : Nanogold particles, Laponite, X-Ray diffraction, UV-vis, TEM
In the last few decades, there has been considerable effort in preparing and characterizing nanostructured materials.1 Layered inorganic solids such as polysilicates, double hydr- oxide, perovskites and clay minerals are important constitu- ents of these assemblies because of their ability to provide interlamellar space, and their large active surface area.2 Laponite is a synthetic polycrystalline clay similar in struc- ture and composition to natural hectorite of the smectite group. The sodium ions in the central layer of laponite are exchangeable, and in aqueous dispersions, these ions diffuse into the water, and plate-like particles with negatively charg- ed faces are formed.3,4 Among nanoparticles, colloidal metal or metal oxide particles have been intensively studied because of their unique optical and catalytic properties and their biomedical applications.5 There have been many reports on the synthesis of nanoparticle/clay composites.6-10 Metal hydr- oxides of iron, chromium, cobalt, manganese and cerium in acetic acid solutions were used to exchange the sodium ions in laponite, and their corresponding nanometal oxide- laponite composite products were obtained by calcination of adsorptions of these precursor solutions at 500 oC in air.11
The preparation and characterization of nanogold/laponite composite were examined in this study. For this purpose, gold(III) chloride solution in dilute HCl was reduced with sodium borohydride in the presence of laponite.12 UV-vis spectra of different concentrations of nanogold sols are shown in Figure 1. In Figures 1(a) and (b), very broad absorption peaks are observed, and apparent surface plasmon resonance absorptions of nanogold particles are not observed due to the very low concentration of nanogold particles. Alternatively, characteristic surface plasmon resonance absorption of nanogold particles can be observed at about 520 nm in high concentrations of nanogold (Figures 1(c), (d) and (e)). With the increase of gold content, the absorption peak position is shifted toward higher wavelength, from 518 nm to 521 nm.
This red shift can be related to the growth of nanogold particles with the increase of gold content. It is known that the surface plasmon resonance absorption of gold nano- particles is very sensitive to particle aggregation and vari- ations in surroundings, and strongly influenced by chemical modifications of the particle surfaces.14-16 The growth of nanogold particles has also been observed in TEM micro- graphs, as shown in Figure 2. The particle size is changed
from about 3 nm to about 15-20 nm. With a low concent- ration of nanogold particles, not many particles were observed, and these results agree well with those of UV-vis spectra, Figure 1. UV-vis absorption spectra of gold colloids in the presence of laponite as a function of HAuCl4 concentrations: (a) 0.05, (b) 0.25, (c) 0.50, (d) 2.5, (e) 5.0 mmol.
Figure 2. TEM micrographs of nanogold particles in the presence of laponite prepared in various concentrations of HAuCl4: (a) 0.25, (b) 0.50, (c) 2.5, (d) 5.0 mmol.
364 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 Communications to the Editor
which show very broad peaks.
Figure 3 shows XRD patterns of the clay composite loaded with nanogold particles at different gold content. The XRD patterns of the parent Laponite RD shows broad peaks with a basal d001 spacing of 1.37 nm, corresponding to an interlayer distance of 0.41 nm due to the hydrated sodium ions occupying the interlayer space. The interlayer distance is obtained after subtracting the thickness of the intrinsic silicate layer (~0.96 nm) from the basal spacing.17 A very small increase of the interlayer distance is observed after nanogold generation, as shown in Figures 3(b) and (c). The basal d001 spacing value of Laponite RD after nonogold incorporation in Figures 3(b) and (c) is about 1.42 nm, and corresponds to an interlayer distance of 0.46 nm. This value shows that the hydrated sodium ions are still occupying the interlayer space. The intercalation of nanogold into the interlayer of laponite is excluded, because the particle sizes obtained from the TEM micrographs in Figure 2(a) are too large to be accommodated between the layers of the clay.
The sharpness of the d001 reflection in Figures 3(b) and (c) suggests that the layers of laponite after freeze-drying are stacked face to face in ordered tactoids. With high nanogold concentration, the nanogold particles in laponite showed X- ray patterns characteristic of gold, although the degree of crystallinity differed, as shown in Figures 3(d), (e) and (f).
The broadness of the d001 reflections of the laponite in Figures 3(d), (e) and (f) confirm that laponite containing large-size nanogold forms disordered structure with little face-to-face stacking. Therefore, much of these nanogold particles must be deposited on the external surfaces rather than undergoing intercalation. The size of the nanogold particles from the (311) reflection in the XRD patterns in Figures 3(e) and (f) was calculated using the Scherrer formula in order to obtain a rough estimate of the crystallite size of the nanogold. These values are given as 14.8 and 17.7
nm, respectively. This result is consistent with that of TEM in Figure 2. In conclusion, laponite RD can be used as a protective colloid to give stable nanogold. Although there are many reports about surfactant molecules intercalated in laponite18, the interaction between nanogold and laponite is not reported. This study shows that nanogold particles of cryogel are adsorbed to the external surfaces rather than intercalating into interlayer of laponite.
Acknowledgments. The present research was conducted under the research fund of Dankook University in 2010.
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Figure 3. XRD patterns of cryogels with different concentrations of HAuCl4: (a) the parent Laponite RD, (b) 0.05, (c) 0.25, (d) 0.50, (e) 2.5, (f) 5.0 mmol.
