Formation of Icosahedral Gold Nanocrystals on the Glass Surface

Song Wei Lu,*,† Veronica Frain, and Mehran ArbabGlass Business and DiscoVery Center, PPG Industries, Inc., 400 Guys Run Road,Cheswick, PennsylVania 15024

ReceiVed: December 30, 2009; ReVised Manuscript ReceiVed: May 24, 2010

Icosahedral gold nanocrystals were formed on the glass surface using a forced aerosol spray process at 600°C. High-resolution transmission electron microscopy analysis reveals the icosahedral structure of variousgold nanocrystals exhibiting the 5-fold symmetry. Scanning electron microscopy images show theself-assembled partial rings of the gold nanocrystals on the glass surface, which is assumed to be formed atthe moment of the aerosol particles hitting the hot glass surface.

1. Introduction

Metal nanocrystals, especially gold nanocrystals, have re-ceived significant research interest due to their unique opticalproperties and potential applications in catalysis, gas sensing,drug delivery, and electronics.1 The shape and size control ofgold nanoclusters has been extensively investigated by varioussynthesis methods and experimental conditions, such as precur-sor concentration, temperature and cooling rate, seeds, andstabilizer concentration.2-9 Kumar et al. observed gold nano-particle arrays on a Si3N4 membrane with different shapes, suchas triangle, diamond, pentagonal, and hexagonal geometries, bya solution-based seed-mediated growth method.10 Rossi andFerrando simulated the freezing of gold nanoclusters intopolydecahedral structures and found that icosahedra are morefavorable at fast cooling rates, whereas lower cooling rates favordecahedral and fcc structures.6 Wang et al. carried out moleculardynamics simulations of melting of icosahedral gold nanoclus-ters and showed that, at low temperatures, the icosahedral goldnanocluster is almost fully faceted with flat facets meeting atedges and vertices.11 At elevated temperatures, that is, about150 °C below the melting temperature, they observed the facetsof the nanoclusters to have shrunken slightly in size, and theedges and vertices have noticeably rounded. At the temperaturejust below the melting temperature, the facets have shrunkento an almost negligible size, and the cluster is almost spherical.Simulations by Cleveland et al. concluded that, when fcctruncated-octahedral and truncated-decahedral gold clusters withhundreds of atoms were heated, they undergo a transformationto the icosahedral structure.12,13 Zhou et al. described that thegold nanocrystalline shape could be controlled by reactiontemperature: at 50 °C, spherical structures dominated, whereasat 100 °C, icosahedral and platelike nanocrystals dominated.14

However, these studies mostly addressed the control of the sizeand shape of gold nanocrystals. Besides the size and shape, thelocation of individual nanocrystals may give very importantinformation on how the nanocrystal was formed, such as in anarray, a circle, or a superlattice, for example, the formation ofgold nanocrystals with a semihexagonal pattern originally fromthe seeds with uniform semihexagonal arrays.10 Zhang et al.formed a 3-D superlattice of monodisperse Ag, Au, and Pd

nanocrystals by self-assembly.7 The control of size, shape, andlocation of nanocrystals would give a better understanding ofthe nanocrystal formation mechanism.

In this paper, we report the formation of icosahedral goldnanocrystals discontinuously along the perimeter of aerosoldroplets at 600 °C on the glass surface by a forced aerosol sprayprocess and the investigation of the gold nanostructures relatedto this unique method and experimental conditions.15

2. Experimental Methods

A 0.1 wt % solution of hydrogen tetrachloroaurate (III)trihydrate (HAuCl4 ·3H2O) was prepared by dissolving it indeionized water. The as-prepared solution is slightly yellow incolor and stable at room temperature. The solution was atomizedat 30-40 psi into an aerosol stream using a TSI single-jetatomizer (model 9302, TSI Incorporated, St. Paul, MN; esti-mated mean aerosol water droplet size of approximately 800nm at an atomizer input pressure of 40 psi using nitrogen gasand around 1.2 µm at a 30 psi atomizer input pressure accordingto the relationship between aerosol water droplet size andatomizer input pressure from the manufacturer). The aerosolstream was carried in a N2 gas stream in an alumina ceramictube of 25 mm in diameter and directed to a heated glasssubstrate (PPG’s Starphire glass, 50 mm ×50 mm in size)horizontally placed inside a vertical furnace. The distancebetween the glass surface and the alumina tube tip wasapproximately 5 mm. The glass substrate was cleaned with a50 wt % isopropanol/50 wt % DI water solution and heated to600 °C prior to deposition, that is, just below its glass softeningpoint of 609 °C. After spraying for 1 or 2 min, the sample wastaken out of the vertical furnace and annealed slowly to roomtemperature overnight in an annealing furnace.

Samples were analyzed with transmission electron microscopy(TEM) using a JEOL 2000FX system with an accelerationvoltage of 200 kV. The samples were thinned down by Ar ionmilling with a Bal-tec system and then placed on a copper grid.SEM surface images of samples were obtained by coating thesurface with a conductive carbon film. Samples were imagedusing a LEO 1530 FESEM, and EDX spectra were collectedusing a Noran Vantage EDS detector at acceleration voltagesof 15-20 kV. UV-vis-IR optical spectra were measured usinga Lamda 9 optical spectrophotometer from 300 to 800 nm. X-rayphotoelectron spectroscopy (XPS) data were collected on a VGScientific ESCALAB MkII system. A nonmonochromated Al

* To whom correspondence should be addressed. E-mail: slu@ppg.com.Tel: (724) 325-5831. Fax: (724) 325-5325.

† Current address: PPG Industries, Inc., Monroeville Chemicals Center,440 College Park Drive, Monroeville, PA 15146.

J. Phys. Chem. C 2010, 114, 12850–1285412850

10.1021/jp912254f 2010 American Chemical SocietyPublished on Web 07/12/2010

KR X-ray source was used at 240 W and a slit size allowing aroughly 1 × 3 mm region to be analyzed. Narrow scan datawere collected at pass energy of 20.0 eV.

3. Results and Discussion

Figure 1 is the optical absorbance spectra of gold nanocrystalson Starphire glass samples after spraying for 1 and 2 min, alongwith an uncoated glass sample in the 300-800 nm range. Thesurface plasmon resonance (SPR) of gold nanocrystals isobserved for both pinkish-gold-decorated samples. A slight colordifference was observed with the 2 min spray sample deep incolor and more toward to purplish-pink. The SPR band centersat 570 nm and at 620 nm for the 1 and 2 min sprayed samples,respectively. In addition, the SPR band broadens and decreasesin intensity for the 2 min sprayed sample. The band shiftsuggests the presence of larger gold nanocrystals with theextended spray time. Doremus found that, for gold colloidsprecipitated in glass with a nanocrystalline size smaller than10 nm, the absorption peak position is almost independent ofthe particle size, while the absorption intensity increasessignificantly from 3.5 nm particles to 5.2 nm particles.16 The80 nm gold nanocrystals in glass has a peak at around 605 nm.Link and El-Sayed also reported the red shift of the plasmonpeak of gold nanoparticles in aqueous solution from 517 to 575nm for 8.9 and 99.3 nm particles, respectively.17 The discrepancyof the absorption peak position of different gold nanocrystalssizes reported in the literature may arise from different synthesismethods, different media or matrices, different measurementmethods for sizes, and the shapes of the particles.16,17 Theoreticalcalculation shows that the absorption intensity of the plasmonresonances increases with the increasing gold nanocrystal sizefor spherical particles.18 In this study, the intensity of the 2 minsprayed sample is lower than that of the 1 min sprayed sample.This is possibly due to the decrease of the uniformity of thegold nanocrystals and/or surface defects of the gold nanocrystalson the glass after extended spray times. This also reflects onthe broadening of the absorption peak and slight change of thecolor from pinkish to purplish-pink.

The presence of larger gold nanocrystals with increasing spraytime is also verified by the XRD results (Figure 2). After 1 minof spraying, there is a very small XRD peak at 38° correspond-ing to the (111) face of an fcc gold crystal. After 2 min ofspraying, it is evident that larger nanocrystals form withprominent (111), (200), and (220) diffraction peaks representa-tive of a gold fcc structure (JCPDS file No. 4-0784).19,20 It isalso likely that the increased XRD peak intensity is due to a

higher density of the gold nanoclusters resulting from extendedspraying time. Kim et al. reported that the XRD orientation maychange with the shape of gold nanocrystals, with the (200) faceas the preferred orientation for a cubic shape and the (111) faceas the preferred orientation for icosahedron and tetrahedronshapes.21

Figure 3 shows the narrow XPS spectrum of the goldnanocrystals on the glass surface exhibiting the characteristicdouble peaks with binding energies of 83.8 and 87.4 eVcorresponding to Au 4f7/2 and Au 4f5/2, respectively.22,23 This istypical for the presence of the Au0 state. The presence of goldon the glass surface was also confirmed by the gold peak in theenergy-dispersive spectroscopy (EDS) spectrum.

The high-resolution images of gold nanocrystals on the glasssurface sprayed for 1 min are shown in Figures 4 and 5. InFigure 4, a Fourier transformed TEM image is also presented,clearly showing the lattice structure of the nanocrystal. The5-fold axes are clearly seen from all gold nanocrystal images.The icosahedral structure can be considered as an assembledset of 20 tetrahedra that share an apex. This configurationindicates strain induced within the particle to compensate forspatial gaps between the tetrahedral.2,24-26 The varying gray scalein the HRTEM image, particularly the dark color of the rightside tetrahedron, is possibly caused by the strain in this particulargold nanocrystal (Figure 4). We estimate the average size ofthe nanocrystals at about 29 nm.

The particular nanocrystal in Figure 4 is nearly spherical withsoft edge lines. The nanocrystals in Figure 5 are all spherical.This agrees well with Wang’s molecular dynamics simulationsof the melting of icosahedral gold nanoclusters that, at hightemperatures slightly below the melting point of gold nano-

Figure 1. Optical absorption spectra of gold nanocrystals depositedon the Starphire glass surface: (1) spray for 2 min at 600 °C, (2) sprayfor 1 min at 600 °C, and (3) uncoated glass.

Figure 2. X-ray diffraction patterns of gold nanocrystals depositedon the glass surface: (1) sprayed for 2 min at 600 °C, (2) sprayed for1 min at 600 °C, and (3) unsprayed glass. The vertical lines are fromJCPDS file No. 4-0784 of fcc gold crystals.

Figure 3. Characteristic XPS spectrum of the gold nanocrystals onthe glass surface exhibiting double peaks with the binding energies of83.8 and 87.4 eV.

Icosahedral Gold Nanocrystals on the Glass Surface J. Phys. Chem. C, Vol. 114, No. 30, 2010 12851

crystals, the facets have shrunken to an almost negligible size,and the cluster is almost spherical.11 Because these goldnanocrystals were deposited at 600 °C on the glass surface andthe samples were immediately taken out to anneal in a 500 °Cfurnace, the high-temperature structure of the gold nanocrystalswere preserved by quenching. In addition, according to Rossiand Ferrando’s simulation that the icosahedra are more favorableat fast cooling rates,6 we determine that these gold nanocrystalsare icosahedral not decahedral.

Figure 6 shows the SEM images with different magnificationsof gold nanocrystals on Starphire glass sprayed for 1 min. Theaverage size of the gold nanocrystals from these images is 29nm. It is shown that the gold nanoparticles are not denselypacked on the glass surface; instead, they appear to have alignedthemselves in rings (Figure 7). The typical ring diameter isbetween 500 and 900 nm (average around 800 nm), which isclose to the mean aerosol droplet size of 800 nm at 40 psiaccording to the relationship between aerosol droplet size andatomizer input pressure from the manufacturer of the aerosolgenerator (Figure 7a). However, when the atomizer inputpressure was reduced to 30 psi, the typical ring diameterincreased to around 1.2 µm, which is in agreement with thelarge aerosol droplet size from the reduced atomizer inputpressure (Figure 7b). In addition, when the atomizer inputpressure was reduced, the aerosol output has also reduced. Thiswill probably lead to formation of few particles. It is unclearwhy the gold nanoparticles have a poor size uniformity whenusing an atomizer input pressure of 30 psi. The phenomenonof gold nanocrystals aligned around the perimeter of a ring maybe explained as follows: The aerosol droplet is initially generated

with a mean diameter of 800 nm using an atomizer inputpressure of 40 psi. The subsequent flow of the aerosol streamforced rapidly by the nitrogen gas through a heated aluminaceramic tube presumably allows only a small portion of eachliquid droplet to evaporate before impinging on the hot glasssurface. At the moment that the aerosol droplet hits the hot glasssurface, water evaporates immediately and the liquid dropletcollapses on the glass surface. During the short, but finite,evaporation period, the remaining materials originally containedwithin a droplet phase separate and segregate to the perimeterof the droplet to lower its interfacial energy with the glasssurface. At the same time, HAuCl4 decomposes above 254 °Cto liberate gold atoms, which then form gold nanocrystals. Thehigh concentration of Fe2+ in the high redox glass may havefacilitated the reduction of the Au3+ cations.

The formation of nanocrystal rings has been reported for goldand other nanocrystals. Govor et al. reported the CoPt3 nano-particle ring formation in evaporating micrometer-sized dropletswith a ring diameter ranging from 0.6 to 1.5 µm and a particlediameter of 6 nm self-assembled along the contact line.27,28

Formations of similar rings with nanoparticles on the dropletperimeter were observed for silver,29 copper, cobalt, silver sulfideand CdS,30 barium hexaferrite,31 and gold on heterogeneous softsurfaces.32 Ohara and Gelbart explained the nanoparticle ringformation in terms of wetting properties.33 Maillard et al.interpreted the phenomenon as a Marangoni effect that the ringpattern is driven by surface tension gradients that inducesBenard-Marangoni instability in the liquid film containingnanoparticles.30 Although most in the literature reported com-plete rings with nanoparticles self-assembled around the pe-

Figure 4. Icosahedral structure of gold nanoparticles: (a) original HRTEM image and (b) Fourier transformed image. Atomizer input pressure )40 psi.

Figure 5. Five-fold orientation of gold nanocrystals (bar ) 5 nm). Atomizer input pressure ) 40 psi.

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rimeter of the ring, we observe the formation of discontinuousrings with few isolated gold nanocrystals on the perimeter ofthe ring with a relatively large spacing between the individualparticles. We believe that this is due to a low concentration ofHAuCl4 in the aqueous solution used in this work.

This work differs from other studies on gold nanocrystals,especially gold nanocrystals on a glass surface or embeddedinside glass, in that the gold nanocrystals form an icosahedralshape and discontinuous rings of nanocrystals. The forcedaerosol spray process is a unique way to deposit gold nano-crystals on the hot glass surface in that the precursor solutionis a stable uniform aqueous solution without preformed nano-particles. The gold nanocrystals formed on the glass surfaceinstantly as the aerosol stream reaches the glass surface. Incontrast, Gerber et al. first prepared gold colloids with differentsizes in solutions and then deposited the gold nanocrystal thinfilm on the glass surface.34 Under experimental conditions ofthis work, the gold nanocrystals have no agglomeration withan icosahedral shape. The distance of the individual goldnanocrystals is large enough that the effect of the gap betweengold nanocrystals could be negligent. In addition, because ofthe large distance between nanocrystals, the surface is still notconductive, unlike the conductive thin film multilayer of goldon glass as reported by Gerber et al.34 The discontinuous ringsof gold nanocrystals appear as an ordered structure in near order,however, it appears as homogeneity in a long distance. Thisgives the uniformity of such a nanostructure on the glass surface.It differs significantly from the complete rings of nanocrystals,such as reported by Shafi et al.31 and Govor et al.27,28 In Shafi’swork, the barium hexaferrite nanocrystals are located on thecontinuous Olympic ring, and the nanocrystals nonuniformly

appeared throughout the surface.31 In addition, their ringdiameter differs greatly from area to area. The possibleapplication of this work will be the architectural decorationsusing the uniform and unique color of gold nanoparticles onthe glass surface by an aerosol spray process that can be operatedduring the glass production, while still maintaining the non-conductivity of the surface. Other possible applications includegas sensors using gold nanocrystals on the glass surface.1 Interms of processing, Dubiel et al. used an expensive andcompleted ion implantation setup to form gold and silvernanoparticles under the glass surface.35 Their sample was limitedto a very small size due to the limitation of the ion implantationprocess. By using the forced aerosol spray process, the goldnanoparticle coatings can be deposited through a continuousprocess on a large area of glass sheet during the glass production.

4. Conclusions

We experimentally demonstrated the formation of ∼29 nmicosahedral gold nanocrystals on the glass surface by a forcedaerosol spray process at 600 °C. The 5-fold axes were clearlyobserved from high-resolution TEM images. The near sphericalshape of icosahedral gold nanocrystals was discussed to haveresulted from their high-temperature formation at slightly belowthe melting point of the nanocrystals. These nanocrystals formeddiscontinuous rings by rapid evaporation of the solvent on the

Figure 6. SEM images with different magnifications of gold nano-crystals on the glass surface. Atomizer input pressure ) 40 psi.

Figure 7. SEM images of the formation of gold nanoparticles aroundthe perimeter of rings on the glass surface. Rings were drawn to suggestthe arrangement of the gold nanocrystals along the perimeter of thering. More rings could be drawn with the rest of the nanocrystals.Sample (a): average ring diameter around 800 nm with a range between500 and 900 nm and an atomizer input pressure of 40 psi. Sample (b):average ring diameter around 1.2 µm and an atomizer input pressureof 30 psi.

Icosahedral Gold Nanocrystals on the Glass Surface J. Phys. Chem. C, Vol. 114, No. 30, 2010 12853

hot glass surface, with the nanocrystals located on the perimeterof the discontinuous rings.

Acknowledgment. The authors would like to acknowledgeL. Rukavina for XRD, E. G. Goralski for XPS, and M. Valarikand C. Pociernicki-Stahl for optical measurements. All are fromthe Glass Business and Discovery Center, PPG Industries, Inc.

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