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J. of Supercritical Fluids 42 (2007) 282–287

The preparation of gold nanoparticle compositesusing supercritical carbon dioxide

Ben Wong a, Satoshi Yoda b, Steven M. Howdle a,∗a School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

b Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology,1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan

Received 15 December 2006; received in revised form 13 March 2007; accepted 14 March 2007

bstract

In this paper we present the synthesis of gold nanoparticles supported on silica using supercritical carbon dioxide (scCO2). Average goldarticle sizes ranged from 3.7 to 6.6 nm. We also demonstrate the flexibility of this supercritical fluid processing technique by successfullyncorporating gold nanoparticles into polyamide, polypropylene and poly(tetrafluoroethylene) (PTFE). Under the conditions employed, it wasound that dimethylacetylacetonato gold(III) (Au(acac)Me2) produced samples with sufficiently high metal loadings which allowed in-depthample analysis. Surprisingly, the fluorinated analogue, dimethylhexafluoroacetylacetonato gold(III) (Au(hfac)Me2) did not yield samples suitable

or characterisation. X-ray diffraction (XRD), transmission electron microscopy (TEM) and solid-state UV–vis were applied to determine averagearticle sizes and to confirm the nature of the metallic particles obtained. To date, scCO2 processing is the only method known to us capable ofoth depositing and impregnating a wide range of substrates with gold nanoparticles.

2007 Elsevier B.V. All rights reserved.

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eywords: Supercritical carbon dioxide; Polymer nanocomposite; Gold nanopa

. Introduction

Gold is widely thought of as being one of the most com-only inert of all the metallic elements. However, when finely

ivided and supported upon certain substrates, preferably on anxide of the first transition series, unique heterogeneous prop-rties are apparent [1,2]. Much of this original research wasstablished by Haruta et al. during the late 1980s. His workemonstrated the catalytic activity of hemispherical gold parti-les with diameters of less than 5 nm [3,4]. In the preparationf low temperature oxidation catalysts, it has been reported thatatalytic activity is strongly related to the preparative methodf the catalyst [5–7]. Conventional deposition-precipitationnd co-precipitation seem better than impregnation methodsn preparing active gold samples [2,8]. It is believed that

eposition-precipitation and co-precipitation methods providehigher level of intimacy of contact between the gold nanopar-

icles and support matrix [7,9,10]. As such our experiments

∗ Corresponding author.E-mail address: steve.howdle@nottingham.ac.uk (S.M. Howdle).

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896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2007.03.005

; Dimethylacetylacetonato gold(III)

ere designed to investigate whether gold nanoparticles could bechieved using supercritical carbon dioxide (scCO2) processing11,12].

The major benefits of scCO2 for fabrication of nanoparticleispersed materials are the ability to make soluble precursorsnd the high diffusivity of the fluid allowing access to narrowores. As the solubility of substances in scCO2 can be controlledith density and the nucleation of the dissolved precursor is

asily achieved by a rapid pressure drop. This fast nucleations quite important for synthesis of fine and monodisperse metalanopatricles [13,14]. The low viscosity, high diffusivity andero surface tension nature of scCO2 has been exploited in aariety of impregnation processes [15,16].

Recently Chatterjee et al. reported the fabrication of goldanoparticles on MCM-48 using scCO2 and an aqueous solutionixture [17]. In this work we chose a non-aqueous system for

he application to polymeric substances. Since gold nanoparticleatalysts work at low temperature, the combination with poly-

eric substrates should be promising. The benefits of scCO2 for

olymer processing are swelling, and plasticization of polymersithout altering physical and thermo-mechanical properties.

cCO2 facilitates the permeation of matrices which are not nor-

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B. Wong et al. / J. of Superc

ally receptive to aqueous or organic solvents. Since scCO2 isaseous at atmospheric pressure, it is rapidly dissipated uponelease, trapping additives inside the polymer substrate.

Most of the previous reports for polymer-metal nanocom-osite via scCO2 route used volatile organometallic precursorsuch as dimethyl(cyclooctadiene)platinum(II) [18], silver(I)omplex containing 1,1,1,5,5,5-hexafluoroacetylacetonate asigands [19–21] and Palladium(II)/Platinum(II) acetylacetonate22]. Our focus in this paper is on gold, for which there is onlyne precursors reported for scCO2 process [23].

Here we demonstrate the first preparation of gold nanopar-icles into both silica gels and conventional polymers usingrganic gold compounds via supercritical impregnation fol-owed by hydrogen reduction of the precursor.

. Experimental

.1. Materials

Dimethylacetylacetonato gold(III) (Au(acac)Me2) andhe fluorinated analogue, dimethylhexafluoroacetylacetonatoold(III) (Au(hfac)Me2) (Fig. 1) were obtained from TCLC,ri Chemical Laboratories Inc., Japan. Both complexes weresed without further purification and were refrigerated undern inert nitrogen atmosphere. Both gold precursors are CO2oluble and sufficiently labile to ensure facile decompositiono the metal and CO2 soluble ligands [9,24]. Chromatographyrade CO2 (<20 ppm water) was supplied by Cryoservices UKnd hydrogen was obtained from Air Products UK at 99.995%urity. All gases were used as received. Silica substrate (fineowder) was obtained from Fluorochem Ltd., Derbyshire. Theitrogen adsorption/desoprtion analysis revealed that the BETurface area is 586 cm3/g and that the pore size distributiony BJH method is under 6 nm mostly. The silica substrate wasried in an oven at 573 K for a period of at least 24 h prior to use.

Polyamide (BASF Ultramid C35F, nylon 66/6, FDA-ompliant grade [25]), polypropylene (Moplen, HP462R [26])ere selected since we have significant experiences of fabri-

ation of polymer-silver composites via a scCO2 process [27].oly(tetrafluoroethylene) (PTFE) sheet (1 mm thickness) waslso used as a polymer matrix.

.2. The synthesis of gold nanoparticle composite

One gram of substrate, either silica or polymer was placednto a sintered container and loaded within a 10 cm3 autoclave

ig. 1. Chemical structures of the non-fluorinated and the fluorinated gold pre-ursors used for scCO2 processing.

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Fluids 42 (2007) 282–287 283

Thar Technologies). Approximately 100 mg of gold complexas added into the autoclave outside of the sintered container.here was no direct contact between the substrate and the goldrecursor. The autoclave was filled with CO2 and the pressureas increased to 27.58 MPa (4000 psi) at 313 K. CO2 was sup-lied to the autoclave by means of a refrigerator/compressorump (NWA PM-101) connected to a chromatography gradeO2 cylinder. The temperature was controlled by means ofn externally situated thermocouple connected via a feedbackoop to a set of heating cartridges. The heating cartridges wereocated within an aluminium heating block surrounding the auto-lave. After 24 h of processing, the CO2 was released from theutoclave over a period of approximately 20 s.

To allow for full decomposition and reduction of the infusedomplex to metal atoms and dissociated ligands, the temperaturef the autoclave was set to 353 K and filled with H2 to a pres-ure of 10.34 MPa (1500 psi). The outlet valve of the autoclaveas left slightly opened, thus allowing H2 to continuously flow

hrough the autoclave. After 24 h, the flow of H2 was discon-inued and the outlet valve of the autoclave was fully opened,eleasing H2 over a period of approximately 20 s.

In the final continuous purge stage, to extract the dissociatedigands, the autoclave was purged with CO2 for a further 24 h at7.58 MPa (4000 psi) and 313 K. As in the reduction stage, theutlet valve of the autoclave was left slightly opened, allowinghe scCO2 to continuously flow, extracting the dissociated lig-nds. After 24 h, CO2 was released from the autoclave over aeriod of approximately 20 s. At least three samples were pro-uced per substrate/precursor combination. This experimentalas adapted from a methodology first published by Morley et

l. in 2002 [21].

.3. The analysis of supported and impregnated goldanoparticles

Powder X-ray diffraction (XRD) patterns were acquiredsing a Philips EXPERT system fitted with a PW1710iffractometer control unit operating with Cu K� radiationλ = 0.154 nm). Powdered samples were loaded onto an indentedlass plate and flattened with a microscope cover slip. Diffrac-ion patterns were obtained over angles of 20–80 2θ at 0.02θ steps, with a minimum scan time of at least 6 h. Crystalliteizes were estimated using Philips APD software by applyinghe Scherrer equation to the first four reflections, then averagingy four.

Transmission electron microscope (TEM) images werebtained using a JEOL JEM–2000FXII electron microscopeperating at an accelerating voltage of 200 kV. Samples wererepared by grinding in an agate pestle and mortar prior to soni-ated dispersion in acetone. Two to three drops of the suspensionere transferred onto 3 mm diameter holey carbon film/copperesh #300 TEM grids and allowed to evaporate at room tem-

erature. Bright field TEM images were used to observe particle

orphology.TEM images of gold nanoparticles impregnated into

olyamide, polypropylene and Teflon were obtained using aEOL JEM–2000FXII electron microscope operating at an

2 ritical Fluids 42 (2007) 282–287

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No reflections could be obtained, nor average crystallite sizescalculated from the samples processed using the Au(hfac)Me2precursor. This is assumed to be a consequence of the low goldloadings achieved (<0.5% gold by weight).

84 B. Wong et al. / J. of Superc

ccelerating voltage of 200 kV fitted with an energy dispersive-ray spectroscopy analyser (EDAX elemental analyser). Poly-er samples were microtomed to a thickness of less than 100 nm.Optical absorption spectra were measured using a Perkin-

lmer Lambda 35 UV–vis spectrophotometer fitted withLabsphere RSA-PE-20 integrating sphere. Spectra were

btained over wavelengths ranging from 200 or 300 to 700 nmt a scan rate of 30 nm/min.

Average particle sizes and distribution analysis were achievedy means of a digital mapping technique. The TEM image wasigitized using image processing software and the average par-icle size was calculated from a minimum of 192 particles.

. Results and discussions

.1. Properties of the silica/gold composite materials

Comparisons of silica during and after the addition ofold precursor revealed significant changes in colour. Postrocessing, all samples took on a uniform pink/red colour-ng indicating the formation of nano-sized particles of gold.he metallic yellow colour commonly associated with bulkold was not observed. Examination during processing (aftercCO2 impregnation, prior to H2 decomposition) yielded nohange in sample colour, with both substrates remaining white.he pink/red colouring was only evident post H2 decompo-ition. The intensity of this colouring was much greater forhe substrates processed with Au(acac)Me2 than it was for theu(hfac)Me2 samples. The disparity in colour would suggest

hat Au(hfac)Me2 did not impregnate or deposit as well as theon-fluorinated Au(acac)Me2. We postulate that the fluorinatedigands in Au(hfac)Me2 preferentially aids the partition of therecursor into the mobile scCO2 phase, thus resulting in pooreposition/processing. The majority of the fluorinated complexikely remains dissolved in the scCO2, therefore venting intotmosphere upon release. Although limited data exist regard-ng the solubility of the two gold precursors in scCO2, it issual for fluorinated complexes to demonstrate a significantlyigher degree of solubility in scCO2 than their non-fluorinatednalogues. This would naturally result in lower process effi-iency and metal loadings. It is also possible that Au(hfac)Me2s not completely stable under the processing conditions usedhus partially decomposing/precipitating onto the surface of theutoclave.

Gravimetric calculations show an increase in mass for allamples processed. According to calculations, for the non-uorinated Au(acac)Me2, loadings of 2 to 3% wt gold metal wasoutinely incorporated using the conditions as outlined above.his is compared to an average uptake of less than 0.5% wt.old for the samples processed with Au(hfac)Me2. After theeduction of the complex, the dissociated, methyl, acetylacet-nate and hexafluoroacetylacetone ligands are gaseous and/or

ighly soluble in scCO2, so it is assumed that all three ligandsre completely removed by scCO2 extraction and that any pos-tive increases in mass are solely due to the incorporation of

etal. We have previously determined this to be the case forimilar Ag complexes [21].

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ndicated are reflections due to gold nanoparticles, whilst the broad amorphouseak at 25◦ 2θ is due to the silica support. An XRD pattern of blank silica (B)s included for comparison.

Powder XRD patterns obtained for the Au(acac)Me2 pro-essed silica samples exhibit a broad reflection correspondingo the amorphous silica matrix (Fig. 2). The remaining foureflections are representative of elemental gold, showing theace-centred cubic structure of metallic gold [28]. No othereflections were observed. Average particle size obtained bypplying the Scherrer equation to the XRD patterns was.1 nm.

ig. 3. Representative TEM images of gold nanoparticles deposited on silicarocessed using the Au(acac)Me2 precursor. Notice the homogeneous distribu-ion of well formed discrete nanoparticles; all are less than 10 nm in diameter.

B. Wong et al. / J. of Supercritical Fluids 42 (2007) 282–287 285

Fig. 4. UV–vis absorption data showing characteristic surface plasmon absorption of supported gold nanoparticles.

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Fig. 5. TEM images showing gold nanoparticles impregnated into P

Representative TEM images demonstrate the homogeneousispersion of gold particles throughout the silica samples.icrographs shown in Fig. 3 confirms the formation of nanopar-

icles all less than 10 nm in diameter.

As expected, the samples prepared using Au(acac)Me2 shows

greater number of nanoparticles per unit area than those pre-ared using Au(hfac)Me2. No electron diffraction data could bebtained from any of the substrate/precursor combinations, as

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ig. 6. TEM images showing gold nanoparticles impregnated into polyamide, note th

note the highly dispersed and round nature of each discrete particle.

here was insufficient particle density in the field of view foruccessful diffraction spotting. Digital analysis on representa-ive TEM images (597 particles measured) yielded an averageold particle size of 6.6 nm. The value is in good agreement with

hat from XRD analysis.

To prove these particles were in fact metallic gold in nature,he solid-state UV–vis spectra were obtained. The surfacelasmon absorption of gold occurs at a wavelength range of

e gradient effect indicating the possibility of multiple particle distributions.

286 B. Wong et al. / J. of Supercritical Fluids 42 (2007) 282–287

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pproximately 520–560 nm, whilst Silica absorption occurs atavelengths less than 300 nm. As both regions are widely sep-

rated, the absorption of silica is far enough removed from therea of interest. All UV–vis spectra were recorded at room tem-erature against the same reference sample.

Fig. 4 shows representative UV–vis absorption spectra char-cteristic of gold nanoparticles. By the fine control of particleizes, their colour and consequently absorption can be systemat-cally varied from pink to red to violet. This directly correspondso a shift of the UV–vis absorption peak, where, large gold par-icle sizes will cause a red-shift to longer wavelengths, lowerrequency and lower energies [29–31]. Conversely smaller par-icle sizes will be blue-shifted to shorter wavelengths, higherrequency and higher energies. The control of gold particle sizesnd their corresponding shift in the λmax value will be addressedn a future paper.

.2. Properties of the gold/polymer composites

Using identical processing conditions as employed above,old nanoparticles were successfully incorporated into threeypes of polymer; PTFE, polyamide and polypropylene. Basedn the TEM images below (Figs. 5–7), it would seem thathe nature of the polymer substrate plays an important partn the size, shape and the dispersion of the gold particleschieved. All three impregnated polymer types showed theharacteristic pink/red colouring indicative of gold nanoparti-les.

.3. Particle size analysis of the gold/polymer composites

The particle sizes and shapes of Au were different on eachample and much less homogeneous than those on inorganicubstances. Average particle sizes were calculated from repre-

entative TEM images. Digital analysis on the Teflon samples,ig. 5 (192 particles measured) yielded an average gold par-

icle size of 4.4 nm. In Fig. 6, representative TEM imagesf the polyamide samples show a gradient effect, with larger

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ene. Note the variation of particle shapes as well as particle elongation.

nconsistent particles evident on the left hand side within theame field of view of smaller more homogeneous particles onhe right. The left hand side of each TEM image correspondsirectly to the surface of the polymer sample, whereas the rightand side corresponds to polymer located towards the cen-re of each sample. Analysis of the smaller particles yieldedn average gold particle size of 3.2 nm (518 particles mea-ured). The larger particles towards the surface of the polyamideielded an average particle size of 18.7 nm (273 particles mea-ured). For polypropylene (Fig. 7) large erratic non-circulararticles were observed throughout all TEM images. Particlenalysis yielded an average particle size of 23.1 nm (231 par-icles measured). The variety of sizes and shapes in polymersan be affected by interaction between polymers and precur-ors/nanoparticles, and by diffusion of scCO2/precursors insidef the polymers. Polyamide and polypropylene could have lowerffinity to scCO2 than PTFE and hence the precursor con-ent in polymer, the rate of depressurizing, the gold particlesucleation and the growth of the particles could all appear sig-ificantly different from those obtained in PTFE and inorganicubstances.

. Conclusions

Novel gold nanocomposites have been prepared using scCO2.wo gold precursors were initially investigated, with the non-uorinated Au(acac)Me2 producing gold samples showingreater promise. Au(hfac)Me2 did not yield samples suitable forharacterisation. Analysis of the composite materials by TEMnd XRD has allowed the measurement of average crystalliteizes, whilst solid state UV–vis and indexed Miller indices haveonfirmed these nano-crystallites to be gold in nature. Silicaupported samples were prepared with average gold particleizes ranging from 3.7 to 6.6 nm. There are wide range of tech-

iques for obtaining free gold nanoparticles with controlledize and shape [32], but this SCF route represents a facile wayf preparing supported gold nanoparticles in a wide range ofubstrates.

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cknowledgements

We thank the EPSRC for studentship support (B.W.) and Mr.. Fields and Mr. R. Wilson for technical support. S.M.H. is aoyal Society Wolfson Research Merit Award Holder.

S.Y. thanks Prof. M. Haruta, Tokyo Metropolitan Univerityor his kind advice and help on this cooperative research.

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