J Nanopart Res (2011) 13:4987–4995 DOI 10.1007/s11051-011-0480-2
RESEARCH PAPER
Gold nanoparticles enclosed in silica xerogels by high-pressure processing M. T. Laranjo • T. B. L. Kist • E. V. Benvenutti M. R. Gallas • T. M. H. Costa
•
Received: 22 April 2010 / Accepted: 25 June 2011 / Published online: 7 July 2011 Ó Springer Science+Business Media B.V. 2011
Abstract The dispersion of gold nanoparticles (AuNPs) in solid matrices, maintaining their optical properties as in liquid medium, has been explored, enlarging the possibilities of handling and applications of these nanoparticles. In this study, AuNPs were successfully enclosed in silica using the sol–gel method and high-pressure processing at 7.7 GPa, at room temperature, obtaining compacts with densities as high as 85% of silica glass and Vickers microhardness of 50% of quartz. This densification was confirmed by the reduced values of specific surface area and closed pores. UV–Vis spectra showed that the AuNPs maintain their optical properties, in the monoliths and in the compacts, presenting an absorption band in the characteristic region of AuNPs smaller than 20 nm. The slight red-shifts of the absorption peaks cannot be explained in the same way as for AuNPs in
M. T. Laranjo E. V. Benvenutti T. M. H. Costa (&) Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Caixa Postal 15003, Porto Alegre, RS 91501-970, Brazil e-mail:
[email protected] T. B. L. Kist Instituto de Biocieˆncias, Universidade Federal do Rio Grande do Sul, Caixa Postal 15093, Porto Alegre, RS 91501-970, Brazil M. R. Gallas Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, Porto Alegre, RS 91501-970, Brazil
solution, where it is associated to an increase in particle diameter. In solid samples, additional factors as matrix properties should be considered. Transmission electron microscopy (TEM) images showed that the AuNPs have spherical shape both in solution and in the monolith, with an average diameter not exceeding 10 nm. It was also observed that the high-pressure processing did not destroy or misshape the AuNPs. Keywords Gold nanoparticles High pressure Sol–gel Silica nanocomposites Nanoparticle dispersion
Introduction Gold nanoparticles, also called colloidal gold (AuNPs), have received increasing importance in nanoscience (Daniel and Astruc 2004). The AuNPs have fascinating optical (Rechberger et al. 2003), electronic, and magnetic properties that enable applications in several areas like molecular nanotechnology, biological and chemical sensors (Pereira et al. 2007), and catalyses (Kobayashi et al. 2001). The metal nanoparticles are known to exhibit a very intense color, which is absent in the bulk material as well as in individual atoms. These properties are provided mainly by the collective oscillation of the free conduction electrons induced by an interacting electromagnetic field. These resonances are also denoted as surface plasmons and produce a typical absorption
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in the visible spectra (Anker et al. 2008). The optical absorption position of metal nanoparticles can be tailored by controlling several factors like shape, size, composition and concentration of the nanoparticles, and the refractive index of the embedding medium. Also, the large fraction of low coordinated atoms at the surface and the confinement of electrons to a rather small volume, allow electrochemical and electrocatalytic behavior. Among the various methods to synthesize AuNPs the Turkevich method (1951) is distinguished. It is based on the reduction of anion [AuCl4]- by the sodium citrate, the citrate also being the stabilizing agent. There are many applications of AuNPs dispersed in liquid medium (Liz-Marzan 2004) and not so many in solid medium (Cushing et al. 2004). The dispersion of AuNPs in solid matrices has been explored to enlarge the possibilities of handling and applications of these nanoparticles. In this context, the sol–gel process is an attractive route to obtain solid matrices containing AuNPs, because this process starts from a colloidal dispersion of molecules and/or nanostructures. Among the materials produced by this method, silica presents many interesting properties (Daniel and Astruc 2004) as for example, chemical inertness, mechanical resistance, and transparency in the visible optical range, being an efficient matrix for the incorporation of AuNPs (Moraes et al. 2005). This kind of process is very important for several technological applications as catalyst (Anderson et al. 1999), optical devices (Kobayashi et al. 2001), and non linear optical materials (Selvan et al. 1999). The high-pressure technique (up to 8.0 GPa) has been used as an important tool for compaction and densification of nanometric powders, increasing the mechanical resistance and/or producing new materials (Moraes et al. 2005). This is accomplished, using toroidal high-pressure chambers and lead containers as quasi-hydrostatic pressure-transmitting medium, at room temperature. This process promotes the improvement of mechanical properties, allowing the production of hard and dense materials, optically transparent, and crack free (de Andrade et al. 2008; Maus et al. 2004; Mesquita et al. 2009). The mechanism that explains the room temperature densification is called ‘‘cold sintering’’ (Costa et al. 1997a). In this study, we studied the high-pressure processing of silica powders obtained by sol–gel method,
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containing dispersed AuNPs, aiming to produce silica compacts with enclosed AuNPs. The powders and compacts were characterized by UV–Vis spectroscopy, transmission electron microscopy (TEM), and N2 adsorption/desorption isotherms (BET/BJH). The density and hardness were measured by pycnometry and Vickers microhardness, respectively.
Experimental The AuNPs solution was prepared starting from the reduction of chloroauric acid 1.5 9 10-3 mol L-1 (Acros 30% w/w solution HCl 99.99%) by a sodium citrate solution 1% (Synth), and using a polyvinylpyrrolidone solution 5% (PVP PM = 40.000 K-30 Vetec) as stabilizing agent (Lee and Meisel 1982) This initial AuNPs solution was diluted in water in the ratio 1:3 and the resulting solution was used to prepare the silica xerogel samples. These samples containing the AuNPs were prepared according to the published procedures, such as hydrolyses and polycondensation of tetraethyl orthosilicate (5.0 ml TEOS), in ethanol solutions (5.0 ml), under acidic conditions (0.1 ml HF 48% wt and 0.02 g citric acid) (Costa et al. 1997b). Two separate solutions, one containing TEOS and ethanol and the other containing the catalysts and diluted AuNPs solution, were mixed under stirring. The mixture is then divided among three small plastic dishes that are put in a large Petri dish and covered without sealing. These sol–gel solutions (AuS1.6 and AuS3.2) are then left to gelify and evaporate slowly for about 7 days to form the xerogel monoliths. For comparison, xerogels were prepared only with water without AuNPs (S1.6 and S3.2), and they were called reference samples. The synthesis conditions of the prepared samples are described in Table 1. The monoliths obtained were crushed in an agate mortar. The resulting powders were pre-compacted in a piston-cylinder apparatus at approximately 0.2 GPa and placed in a lead container with 8 mm of internal diameter that acts as a pressure-transmitting medium. This lead container was put in a ceramic gasket that was assembled in a toroidal-type high-pressure chamber (Khvostantsev and Slesarev 2008). The compaction is then accomplished in a 1,000 ton hydraulic press at 7.7 GPa, at room temperature for 10 min, to promote the cold sintering process (Costa et al.
J Nanopart Res (2011) 13:4987–4995 Table 1 Synthesis conditions for the prepared samples
a
Only water was added, without AuNPs
Sample
4989
AuNPs solution (ml)
AuS1.6
1.6
AuS3.2
3.2
S1.6 S3.2
a
1.6 3.2a
1997a). The samples obtained in this system have volumes of about 300 mm3. The pressure calibration was performed by the fixed point’s technique using Bi and Yb (Shermann and Stadmuller 1987). The UV–Vis absorption spectra of AuNPs solutions were obtained using the transmission technique in the Shimadzu UV-1601PC spectrophotometer. The monoliths and high-pressure compacts were analyzed by the diffuse reflectance technique in the Cary 5000 Varian spectrophotometer, using as accessory an integrating sphere. The samples were analyzed in the wavelength range of 350–800 nm. The samples’ images were obtained in a transmission electron microscope Jeol operating at 200 keV– JEM 2010. It was prepared using suspensions of powdered samples, for both, monoliths and compacts, in acetone (Merck), dispersed in ultrasonic bath for 5 min. The samples were then settled over a copper grid for analysis. The average diameter of AuNPs was made using the Quantikov software (Pinto 1996). The nitrogen adsorption/desorption isotherms of powdered xerogels and compacts, previously degassed at 100 °C for 2 h, were determined at liquid nitrogen boiling point in a homemade volumetric apparatus, connected to a vacuum line system employing a turbo molecular Edward vacuum pump. The pressure measurements were made using a capillary Hg barometer. The specific surface area of the hybrid materials was determined from the BET multipoint method (Brunauer et al. 1938) and the pore size distribution was obtained using the BJH method (Barret et al. 1951). A pycnometer of 5 ml was used to measure the relative density of the compacts and the obtaining results are the average of five measurements for each sample. The Vickers microhardness measurements were performed in a Shimadzu equipment using a load of 100 g and dwell time of 15 s. The obtaining results are the average of six indentations for each sample.
Water/TEOS molar ratio
Au/silica mg/g
mol/mol
4/1
0.33
9.94 9 10-5
8/1
0.66
1.98 9 10-4
4/1 8/1
0 0
0 0
Results and discussion The AuNPs diluted solution presents a characteristic reddish color and the monoliths prepared presented the same coloration. In Fig. 1 it is showed in the monolith AuS3.2. The powders obtained from AuS3.2 monolith were compacted successfully and a picture is showed in Fig. 2. The UV–Vis absorption spectrum of AuNPs solution obtained by transmission is showed in Fig. 3. The position of the maxima of all the absorption curves was obtained by fitting a Gaussian function to the experimental curve. An absorption peak was observed near 521 ± 1 nm, typical of AuNPs in aqueous suspensions. The monoliths and compacts were analyzed by the diffuse reflectance technique, and the spectra are showed in Figs. 4 and 5. It was an estimated uncertainty of ±3 nm for the peak position of the solid samples. The position of absorption maxima of these samples is in Table 2. The results shown on Table 2 indicate a shift of absorption maximum to higher wavelengths for monoliths and respective compacts comparied with
Fig. 1 Picture of the monolith AuS3.2. The monolith was put over a black line, to show the transparency
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Absorbance (a.u.)
4990
Fig. 2 Picture of the compact AuS3.2 in real size (a) and amplified (b)
400
monolith
compact
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 4 UV–Vis absorption spectra by diffuse reflectance technique of samples AuS1.6 in the form of monolith and compact
0.40 0.38
0.34 0.32 0.30 0.28 0.26 460
480
500
520
540
560
Absorbance (a.u.)
Absorbance
0.36
compact
Wavelength (nm)
Fig. 3 UV–Vis absorption experimental curve (dot line) of AuNPs solution fitted with a Gaussian function (solid line)
AuNPs (521 nm) solution. These results suggest an increase in the nanoparticles size in these samples. In fact, it was reported in the literature (Yguerabide and Yguerabide 1998) that the maximum of the absorption curves of AuNPs in aqueous solutions, shifts to higher wavelengths with the increasing average particles size. However, it should be highlighted that in the present study, the AuNPs are enclosed in solid samples, where additional effects due to the matrix, like density and refraction index, could modify the absorption maxima position. In Table 2, the results also showed that there is a shift of the absorption maxima to lower wavelengths (blue shift) comparing the compacts with the respective monoliths. The same effect is observed comparing the samples with higher content of AuNPs (AuS3.2) to the samples
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monolith 400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 5 UV–Vis absorption spectra by diffuse reflectance technique of samples AuS3.2 in the form of monolith and compact
Table 2 UV–Vis absorption maxima for monoliths and compacts Sample
Absorption maxima (±3 nm)
AuS1.6 Monolith
559
Compact
539
AuS3.2 Monolith
547
Compact
542
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4991
Fig. 6 TEM image of AuNPs solution and a bar graph of particle diameter distribution
250
Frequency
200 150 100 50 0 2
3
4
5
6
7
8
9 10 11 12 13
Particle diameter (nm)
Fig. 7 TEM image of sample AuS1.6 and the bar graph of particle diameter distribution
5
Frequency
4 3 2 1 0 3
4
5
6
7
8
Particle diameter (nm)
AuS1.6, suggesting smaller particles size for the AuS3.2 sample (Yguerabide and Yguerabide 1998). The larger volume of AuNPs solution added in the xerogel synthesis and the presence of dispersant polymers can also influence the physical properties of matrix, like refraction index and porosity, which can change the absorption maximum position. Figures 6, 7, and 8 show images obtained by TEM. It can be observed that the AuNPs have a spherical shape in these three types of samples: suspension, monoliths, and compacts. From these images, the average diameter of AuNPs was determined using the Quantikov software (Pinto 1996). Several TEM images were performed in different sample spots and the average particle size reported are the result for these images. Figure 6 shows a TEM image of the AuNPs solution and a bar graph of diameter distribution. The average diameter is 5.2 nm with a standard deviation of 1.8 nm.
In Figs. 7 and 8 are shown the TEM images of AuNPs for sample AuS1.6, monolith and compact, respectively. In the monolith, the AuNPs have an average diameter of 4.7 nm with a standard deviation of 1.2 nm, and in the compact the average diameter is 8.8 nm. In this case, it was not possible to obtain a satisfactory bar graph because of the small number of AuNPs observed in this image. Figures 9 and 10 show the TEM images of AuNPs in the monolith AuS3.2 and in the compact, respectively. In Fig. 9, the particles present an average diameter of 6.6 nm with a standard deviation of 2.1 nm and in Fig. 10, they present an average diameter of 10 nm. For this image, it was also not possible to make a bar graph of diameter distribution. TEM results show that the AuNPs observed in the xerogel AuS1.6 have an average diameter near that of the AuNPs in solution (5.5 nm). The AuNPs observed in the xerogel AuS3.2 have an average diameter of 6.6 nm, which is a little larger than that in the
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Fig. 8 TEM image of AuS1.6 compact
solution. For the compacted samples, it was observed that the AuNPs have an average diameter larger than that observed for the AuNPs in the respective powders, possibly due to agglomeration or image superposition. According to the behavior of AuNPs in aqueous solution reported by Yguerabide and Yguerabide 1998, the maximum of the absorption curves of AuNPs shifts to higher wavelengths with an increase in the average particles size. In the present study, a red-shift of absorption maximum for both monoliths and respective compacts was observed, compared to solution. However, the average diameter obtained for the AuNPs by TEM in the silica matrix did not show an increase in the diameter compared with AuNPs in solution. For this reason, we explain the red-shift observed by effects of density, opacity, and refraction index of the solid medium where the AuNPs are
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dispersed, and not due to an increase in particle size (Tuzovskaya et al. 2007; Morris et al. 2002; LizMarzan et al. 1996). We would like to highlight that the TEM images of the compacted samples demonstrate that the AuNPs are intact and maintain their spherical shape. This indicates that the high-pressure compaction did not damage the AuNPs, which is in accordance to the UV–Vis results, where these samples present absorption properties similar to the xerogels or solutions. The specific surface area and the specific pore volume of all samples are presented in Table 3. An increase in surface area was observed while comparing the AuNPs monoliths (AuS1.6 and AuS3.2) to the reference material (S1.6 and S3.2). This can be explained by considering the AuNPs solution containing stabilizing agents, sodium citrate, and PVP, which will influence the sol–gel morphology, and consequently the surface area. Comparing the monoliths to the compacts, there is an expressive reduction in surface area. This reduction in surface area values for the compacts was already observed in previous studies of similar sol–gel systems of pure silica and silica with organic dyes. It was ascribed to a cold sintering process between the silica particles, which promotes the closing of pores. (Costa et al. 1997a, b, 2004). A surface area reduction comparing the samples with lower Au content (AuS1.6) and the samples with higher Au content (AuS3.2) was observed. This result can be explained by an increase in the molar ratio of water/TEOS for samples AuS3.2, which influences the textural properties of xerogels. A water/TEOS molar ratio, up to 4 (AuS1.6) produces an increase in the porosity and surface area, possibly due to a fast
Fig. 9 TEM image of AuNPs in sample AuS3.2 and the respective particle diameter distribution bar graph
5
Frequency
4 3 2 1 0 3
4
5
6
7
8
9
10
Particle diameter (nm)
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11
12
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4993
0.14
-1
0.10
3
dV/dr (cm nm )
0.12
0.08 0.06 0.04 0.02 0.00 3
4
5
6
7
8
9
10
11
12
Pore diameter (nm)
Fig. 11 Pore diameter distribution for samples AuS1.6, reference (circle), monolith (triangle), and compact (square). Fig. 10 TEM image of AuS3.2 compact Table 3 Specific surface area and specific pore volume of the prepared samples
AuS1.6 Reference
320 ± 15
0.58 ± 0.06
Monolith
424 ± 18
0.57 ± 0.06
36 ± 6
0.03 ± 0.02
Reference
255 ± 13
0.55 ± 0.06
Monolith
361 ± 15
0.66 ± 0.07
Compact
35 ± 6
0.03 ± 0.02
Compact AuS3.2
0.04 -1
Specific pore volume (cm3 g-1)
3
Specific area (m2 g-1)
dV/dr (cm nm )
Sample
0.05
0.03 0.02 0.01 0.00 2
3
4
5
6
7
8
9
10 11 12 13 14
Pore diameter (nm)
Fig. 12 Pore diameter distribution for samples AuS3.2, reference (circle), monolith (triangle), and compact (square).
hydrolysis reaction and decrease in the aggregation rate. When the water/TEOS molar ratio is higher than 6 (AuS3.2), it is observed as an inverse effect due to an inhibition of polycondensation reaction (Xi et al. 1995; Azolin et al. 2004). The pore size distribution of these samples is shown in Figs. 11 and 12 and they are typical of mesoporous materials. The reference sample S1.6 and the sample AuS1.6 showed in Fig. 11 have a pore diameter region between 4 nm and 8 nm with a maximum near 6 nm. For the compact, it was observed that the pores are closed, as already discussed. In Fig. 12, a similar behavior was observed for samples S3.2 and AuS3.2, which have pore region between 4 nm and 9 nm. These pores were also closed by high-pressure compaction. The density of the compacted samples AuS1.6 and AuS3.2 was 2.01 ± 0.01 and 2.02 ± 0.01 g cm-3, respectively. The density value reported in the
literature for high- pressure compacts prepared with pure silica obtained by sol–gel method is 2.02 ± 0.03 g cm-3 (Costa et al. 2004). Comparing these density values, we can assert that the presence of AuNPs did not cause significant change on density of compacted samples. The high densification degree (85% of silica glass), as well as the reduction of surface area values obtained with high-pressure compaction at room temperature are comparable to values obtained for silica xerogels treated at temperatures higher than 1,000 °C. The Vickers microhardness measurements for AuS1.6 and AuS3.2 compacts were 350 ± 11 and 336 ± 7 HV, respectively. The values found in the literature for compacts obtained from pure silica and silica with organic dyes powders, prepared by sol–gel
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method are in the same range between 300 and 400 HV (Costa et al. 1997a, b). This hardness corresponds to nearly 50% of the quartz hardness, confirming the impressive effect of high-pressure compaction at room temperature.
Conclusions The optical properties of AuNPs, in the monoliths and in the compacts, were maintained, and confirmed by the UV–Vis spectra. An absorption band in the characteristic region of the AuNPs was observed. The slight shifts of the absorption maxima cannot be explained in the same way as for AuNPs in solution, where a red-shift is associated to an increase in particle diameter. In solid samples, additional factors as matrix properties should be considered. The TEM images show that the AuNPs have spherical shape in solution and in the monoliths, and the high-pressure processing did not destroy or misshape them. For the silica AuNPs compacts, the measurements of N2 adsorption/desorption isotherms showed a marked reduction in specific surface area and closed pores. According to the density measurements (85% of silica glass) and Vickers microhardness (50% of quartz), a high degree of densification was achieved. Acknowledgments The authors would like to thank CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico) and CEME-UFRGS (Centro de Microscopia Eletroˆnica da Universidade Federal do Rio Grande do Sul), Brazil. MRG and TMHC also thank AFOSR (Air Force Office for Scientific Research) for grant FA9550-08-1-0426, USA.
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