On the Coalescence of Nanoparticulate Gold ... - ACS Publications

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On the Coalescence of Nanoparticulate Gold Sinter Ink Michael B. Cortie,*,† Michael J. Coutts,† Cuong Ton-That,† Annette Dowd,† Vicki J. Keast,‡ and Andrew M. McDonagh† †

Institute for Nanoscale Technology, University of Technology Sydney, P.O. Box 123, Broadway NSW 2007, Australia School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan NSW 2308, Australia

‡

S Supporting Information *

ABSTRACT: We examine the mechanism by which thiol-protected gold nanoparticle inks can sinter at surprisingly low temperatures. At room temperature the sample is comprised of randomly close-packed gold nanoparticles of about 2.3 nm diameter with a ligand shell of about 0.2 nm effective thickness. As the particles are heated through 80 °C they begin to coarsen, reaching about 10 nm diameter at 180 °C. Upon further heating, rapid sintering and grain growth occurs at a temperature that depends on environment and heating rate. Sintering in vacuum requires a higher temperature than in oxidizing environments. Mass spectrometry in the former case is consistent with volatile species such as C4H9, C2SH, and C2H4 being displaced, whereas XPS shows that the exposed surface of the Au is rich in C and S. However, when sintering is performed in the presence of even trace O2, it is the Au−S bond that is cleaved, and the sintering temperature is lowered by up to 50 °C. In this case mass spectrometry shows the generation of alkane and thiol fragments, some S2 and H2S, and oxidized sulfur-containing species, whereas XPS shows that C and S on the Au surface is much reduced.

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INTRODUCTION Aggregations of precious metal nanoparticles such as Au, Pt, or Ag may be readily sintered to provide electrically continuous materials. There are a number of actual or potential commercial applications of this technology as well as considerable recent scientific interest in it.1−8 Recent efforts have been directed toward developing formulations that sinter at the lowest practicable temperatures to enable inks made from these nanoparticles to be applied to polymer or other heat-sensitive substrates.2,9 We have shown previously that sintering can be facilitated by an oxidizing environment, and remarkably, the temperature at which it occurs can even be brought down to 25 °C by these means.9 At the macroscale, the phenomenology of sintering has been well-explored. The process involves the surface diffusion of atoms to points of contact between particles and the formation there of necks. The overall driving force is the reduction in surface area of the system, and the process is thermally activated. The same mechanisms broadly apply to Au nanoparticles that are greater than 10 nm diameter5 although there is the important additional requirement that the capping ligands of the nanoparticles must first melt or desorb before Au-to-Au sintering can begin.8,10 Either phenomenon is achieved by raising the temperature, for example to 200 °C. Thereafter, at higher temperature and longer times, grain growth occurs and defects are annealed out of the lattice.5 In contrast, sufficiently small gold nanoparticles are susceptible to the onset of sudden, exothermic sintering9 (see the Supporting Information). Here we will term this phenomenon “sinter ignition” in order to differentiate it from © XXXX American Chemical Society

the rather more gradual processes normally observed when ordinary powders sinter. The temperature at which sinter ignition occurs has been found to depend markedly on the local environment.9 These attributes indicate that, in detail, the mechanism of the sinter ignition phenomenon differs from that of classic, macroscale sintering. In particular, the marked exothermicity of the process is clearly primarily due to the high specific surface area of very small nanoparticles and the consequent rapid release of this energy as heat when the free surfaces are consumed during sintering.9 The mechanism by which the protective ligand is removed appears to also play a role, and this is not purely a physical process as sometimes assumed (for example refs 2 and 11) because the presence of oxygen or other oxidants can reduce the temperature of sintering (for a particular rate of heating) compared to sintering in a vacuum.9 Here we explore these phenomena using X-ray photoelectron spectroscopy, thermogravimetric analysis, mass spectrometry, and X-ray diffraction techniques to elucidate the processes that occur under high vacuum as well as to examine the nature of the ejected and residual material upon sintering under vacuum and in air.

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EXPERIMENTAL METHODS General. Tetrachloroauric acid was prepared using a literature procedure.12 1-Butanethiol (Aldrich), tetraoctylamReceived: February 20, 2013 Revised: May 5, 2013

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dx.doi.org/10.1021/jp401815b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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ment time (∼10 min). The XPS binding energy scale was referenced to the Au 4f7/2 peak (84.0 eV) of a clean gold film. The resulting spectra were processed by deconvolving them into one or two pairs of S 2p doublets using the CASA XPS software package. [CasaXPS is available from http://www. casaxps.com.] Each spectrum was fitted by one or two Voigt curves with two components split by 1.1 eV and with a branching ratio of 1/2 to account for the spin−orbit coupling of the S2p core level. From the peak positions, the possible chemical environments of the sulfur were determined by reference to published values in the literature for different species. The approximate values of the commonly encountered S2p doublet peaks are, in order of increasing energy: atomic S chemisorbed onto an Au surface (160 to 161 eV), gold sulfide or thiolate bound to Au via a sulfur (“bound” or “chemisorbed S”, ∼162 to 163 eV), elemental polymeric S, R−S−S−R or S bound to C only (“unbound or thiol S”, 163 to 165 eV), and finally oxidized S (many peaks in the range 166 to 169 eV).14−20 On this basis it is difficult to differentiate an inorganic gold sulfide from a bound thiol with the signature signal of both being in the vicinity of 162 to 163 eV. On the other hand, a peak in the vicinity of 163 to 164 eV is clearly associated with sulfur that is bonded either to a carbon backbone or to another sulfur, but not to gold. Synchrotron X-ray Diffraction Experiments. Diffraction patterns at the Australian Synchrotron were collected between 2θ of 1.47° and 80° on the Powder Diffraction beamline. The beam stop of the instrument obstructed the beam for values of 2θ less than 1.45°. A wavelength of 0.07741 nm was used, calibrated using LaB6 and diamond standards. The zero error was −0.0018 Å. Samples were held in 0.3 mm silica capillaries and spun. Peak parameters were characterized by fitting Lorentzians to data in an appropriate range of 2θ values, using a cubic polynomial as background. Fitting was achieved using Fityk,21 called via a script written in turn by a Delphi Pascal program which sequentially extracted the X-ray patterns from the synchrotron data set. An estimate on the size of crystalline domains (Deff) in the sample was obtained using the Scherrer equation applied to the Au {111} peak (i.e., strain effects were taken to be negligible).

monium bromide (Aldrich), toluene (Fluka), sodium borohydride (98% Ajax), sodium sulfate (99% Aldrich), cyclohexane (99.5% Lab Scan), methanol (99.9% Aldrich), dichloromethane (99.5% Lab Scan), gold (99.99%, AGR Matthey, Australia), and chromium (99.99% Fluka) were all used as received. Milli-Q water (18 MΩ cm−1) was used in all procedures. Synthesis of Gold Nanoparticles. 1-Butanethiol-stabilized gold nanoparticles were synthesized by a modified Brust procedure13 similar to that of Wu et al.2 Tetraoctylammonium bromide (8.75 g, 16 mmol) was dissolved in toluene (300 mL). A solution of tetrachloroauric acid (HAuCl4·3H2O, 1.57 g, 4 mmol) in water (150 mL) was then added to the toluene with rapid stirring. The organic phase changed from yellow to dark orange in color upon formation of a gold/surfactant complex. After 2 min, a solution of 1-butanethiol (0.36 g, 4 mmol in 50 mL of toluene) was added with vigorous stirring. After stirring for 10 min, the solution mixture was cooled in an ice−water bath. A freshly prepared solution of sodium borohydride (1.51 g, 40 mmol) in 100 mL of water was added over a period of 30 s whereupon the mixture became dark brown/black in color. The mixture was stirred vigorously for 3 h, and then the organic phase was separated, washed three times with water, dried with anhydrous sodium sulfate, and filtered. The filtrate was reduced to a volume of ∼50 mL using a rotary evaporator and 50 °C water bath. The solution was then added dropwise to 200 mL of vigorously stirred methanol. The precipitated particles were isolated by centrifugation. The particles were purified by redissolving in 20 mL of cyclohexane followed by reprecipitation in 200 mL of methanol, centrifuging, and drying, producing 0.755 g of a black powder. Transmission Electron Microscopy. Samples for the TEM were prepared by agitating the black powder in ethanol in an ultrasonic bath for ∼5 min. A drop of the solution was then placed onto a lacey carbon support film and allowed to dry. Imaging was performed in a JEOL JEM-2100 LaB6 TEM operated at 200 kV. Particles that were partially hanging off the edge of the carbon support film were chosen for analysis. As small gold particles are well-known to be modified under the illumination of an electron beam, efforts to capture the images as quickly as possible were made; however, some beam-induced modification remains difficult to avoid. Synchrotron XPS Experiments. Silicon substrates (∼1 cm ×1 cm, p-type, 100) were prepared by sonication in ethanol for 10 min followed by thorough rinsing with water and then drying in a stream of nitrogen. A chromium film (100 nm) was deposited using a Denton bell jar DV-502 vacuum chamber with the film thickness monitored by a calibrated Maxtek TM100 film thickness monitor. A solution of gold nanoparticles (10 mg in 200 μL of dichloromethane) was prepared, and 50 μL drops were deposited onto the substrate to completely cover the surface with particles. Photoelectron spectroscopy was performed using a widerange spherical grating monochromator (WR-SGM) beamline at the National Synchrotron Radiation Research Centre (Taiwan, ROC). XPS data were collected using a VG CLAM2 Triple-Channeltron electron energy analyzer. Specimens were heated in an adjacent chamber equipped with a heating stage and heated for 2 min (either under UHV or air) and then reintroduced to the XPS chamber for data collection. Scans recorded on the same regions to monitor X-ray beam damage revealed no significant changes within the measure-

Deff =

0.94λ (γexp − γinstr) cos(θ )

(1)

Here λ is the X-ray wavelength, 0.94 the Scherrer constant appropriate for spherical particles of cubic crystal structure, γexp the measured fwhm of the peak, and γinstr the instrument broadening (as fwhm of a Lorentzian at the wavelength of the {111} diffraction peak). Instrument broadening was estimated from the line widths measured from a mixed LaB6 plus diamond standard. Mass Spectrometry and Thermal Analysis. Mass spectrometry of samples heated in vacuum was carried out at a facility at the Taiwan Synchrotron. The overall pressure of the chamber was in the 10−8 mbar range and increased to around 10−6 mbar from outgassing during the sintering process. Thermogravimetric analysis (TGA) experiments were performed using SETRAM setsys TG-DSC 15 with simultaneous differential thermal analysis (DTA)−TGA. A heating rate of 2 °C min−1 was used in an argon gas stream of cylinder purity. Simultaneous TGA−mass spectrometry (MS) experiments were conducted using a Quadrupole mass spectrometer (model Thermostar QMS 200 M3) from Balzers Instruments B



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stage, at room temperature, there is a major peak at very low 2θ that we assign to particle-to-particle scattering. Between about 85 and 125 °C, this peak is displaced off the scale to lower 2θ values and replaced by a shoulder of much lower intensity, while simultaneously, the profiles of the Bragg peaks of the gold lattice become more defined. For example, Au{111} (2θ = 18.87°, d = 0.236 nm) is detectable in the starting sample but it only starts to become distinct at about 90 °C, Figure 3a−c. In

in a platinum crucible. Survey scans were performed, and the current intensities of the most prominent masses were examined.

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RESULTS Transmission electron micrographs of the as-synthesized nanoparticles are shown in Figure 1. The images indicate that

Figure 1. Transmission electron images of the gold nanoparticles, as synthesized.

the particles are crystalline and have diameters within the range of 2−3 nm, as expected for syntheses of this type. Some of the particles appear to have a faceted surface. However, as this faceting was noted to develop upon illumination of the electron beam, it cannot be concluded that it is a feature of the assynthesized particles. The prior literature suggests that these particles are very likely to be icosohedral or decahedral in structure, with multiple lattice domains separated by stacking fault or twinning type defects.22−26 The process of sintering was examined by a series of in situ XRD experiments. An example of the XRD patterns collected at a heating rate of 2.5 °C min−1 is shown in Figure 2. A number of structural changes occur in the sample during the heating sequence, which can be divided into four stages. In the first

Figure 3. Development of peak due to particle-to-particle scattering compared to the Bragg peaks of Au{111} in sample heated at 2.5 °C min−1. In panel e, we probe the character of the particle-to-particle scattering by logging intensity at 2θ = 1.70° for reasons explained in the text.

an independent study by Bishop et al., similarly sized, hexylamine-capped Au nanoparticles were found to crystallize into an fcc structure at approximately this temperature too.1 The bulk crystal properties of Au become developed in the third stage, between about 125 and 210 °C. During this stage the lattice parameter matches that of bulk gold (literature data27 are shown as a superimposed red line in Figure 3d), and the Bragg diffraction peaks of the Au lattice increase in height

Figure 2. (a) Stacked sequence of X-ray diffraction patterns for sample heated from 47 to 227 °C at a nominal ramp rate of 2.5 °C min−1 and (b) X-ray intensities in the form of a map. C



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interparticle distance would undoubtedly be the increase in thermal energy, which we suggest exceeds the attractive van der Waals potential holding the colloidal glass together at about 80 °C. There is an isothermal component to this meta-melting phenomenon, demonstrated by holding the sample at its critical transition for a few minutes, Figure 4b. Interestingly, the mass

while decreasing in width (plotted here as fwhm). Integrated area under the {111} Bragg peak, which is in principle proportional to the volume fraction of face centered cubic (fcc) gold present, increases somewhat over the whole range of measurements, but this has the most uncertainty of the measured parameters since it depends acutely on the accuracy of the values obtained for peak height, fwhm and background. In the fourth stage of the experiment, which starts at about 190 °C in this example, the remaining signal due to particle-toparticle scattering disappears (Figure 3e) and the Bragg peaks of the gold suddenly and dramatically increase in height while the fwhm declines sharply. In the first stage, the significant peak at 2θ = 1.61° (q = 2.29 nm−1) may be correlated with a physical distance of 2.74 nm. This is evidently a particle-to-particle correlation peak, generated from scattering between ligand-coated particles in physical contact. The sharpness and intensity of this peak indicate that this distance is very prevalent in the sample at the beginning of the experiment. From the particle-to-particle distance a diameter of ∼2.3 nm may be inferred for the gold core (a thickness of ∼0.2 nm is assigned to the 1-butanethiol coating), which is in reasonable agreement with the TEM data. A well-ordered colloidal crystal of such nanoparticles with fcc packing would also have generated additional scattering peaks at 1.99 and 2.29° (corresponding to the colloidal crystal’s {111} and {200} reflections respectively). The absence of such additional peaks here prove that the nanoparticles are not organized in a long-range colloidal crystal structure and are instead somewhat disorganized agglomerates of spheres with short-range order only (cf. Abécassis et al.28 for examples of fcc colloidal crystals of gold nanoparticles that show Bragg peaks due to long-range ordering). In the terminology of Pusey and van Megen, the sample reported here is at this point a colloidal glass.29 In addition, the absence of Bragg peaks for higher order reflections of the actual gold lattice indicate that the nanoparticles themselves are not fully crystalline at this stage either. This concords with observations in the literature that, as mentioned previously, generally find that such tiny particles are icosahedral or decahedral, i.e., not crystalline in the strict sense. As the sample is heated during stage 1, the interparticle spacing expands gradually until at ∼80 °C (at a ramp rate of 2.5 °C min−1) or ∼100 °C (at a ramp rate of 6 °C min−1), after which it enters stage 2, a period of rapid expansion so that by 120 °C the particle-to-particle peak has moved out of the range of the detector (2θ = 1.47° or d = 3.0 nm in our experiments). The breakdown of the particle-to-particle peak at 2.7 nm is quite pronounced and suggests a change in the state of the sample. These starting nanoparticles are agglomerated by van der Waals forces (rather than being irreversibly aggregated) because they may be readily redispersed later in a liquid such as toluene. Therefore, the breakdown of the random close packing with increase in temperature may be due to the phase transition of a colloidal glass to a free-flowing powder. The average distance between nanoparticles rapidly expands during this transition from about 2.75 nm to in excess of 3 nm, and is likely to be associated with a decrease in packing density. If the packing density of the starting sample is taken as being about 60% (the approximate value for a colloidal glass or randomly close packed solid30) then the reduction of packing density of the material caused by the increase in interparticle spacing could reduce the packing density of the material to less than the critical 49.4%, at which point hard-sphere powders become ergodic fluids.30 The factor causing this rapid increase in the

Figure 4. Evolution of the particle-to-particle scattering peak. (a) Particle-to-particle distance and Scherrer grain size plotted against temperature through to the point at which the particle-to-particle signal leaves the range of the detector. (b) 3D view of stacked X-ray diffraction patterns during an isothermal dwell at 115 °C (sample Au2).

spectrometry data (see below) show no release of molecular species during this transition, so the ligand evidently remains tightly bound to the nanoparticles. Although the particle-to-particle scattering peak moves beyond the detector range at 115 °C, the shoulder of the peak remains within detector range and can be plotted through to ∼180 °C. This permits the evolution of the particle-toparticle scattering peak to be directly compared to the development of the Bragg peaks from the Au atom lattice, Figure 3e. It is clear that crystallization of the individual nanoparticles into the fcc structure, as evidenced by the development of the corresponding Bragg peaks, is coincident with the onset of the meta-melting phenomenon that ended stage 1. Not only does the Au{111} peak increase significantly at this point, but it is only from this temperature onward that the higher order reflections of the Au lattice become visible above the background. We propose that this temperature also corresponds to the change in structure of the nanoparticles from icosohedral to a mixture of cuboctahedral and decahedral (as per Cervellino et al.24 and Barnard et al.25). Stage 3 is characterized by a gradual increase in the intensity of the Bragg peaks (Figure 3) although they remain very broad and of low intensity, as expected from discrete fcc crystallites in the 5 to 10 nm size range. The onset of stage 4 is marked by a sudden and rapid transition in the shape of Bragg peaks. This D



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occurred at 195 °C at a nominal ramp rate of 2.5 °C min−1 and at 185 °C at 1 °C min−1. This phenomenon corresponds to the onset of the exothermic sinter ignition reaction.9 It causes the coalescence of the nanoparticles into a polycrystalline mass and is associated with the onset of macroscopic conductivity in the films.9 Particle-to-particle scattering disappears because the gold nanoparticles are now fused together. Application of the Scherrer equation indicates a rapid increase in grain size to at least 100 nm (see Figure 5). A color change from matt black to

Figure 5. Grain size estimated from Scherrer equation (sample Au0), determined from the Au{111} peak shape. Also shown (red triangles) are the particle-to-particle distances deduced form the scattering peak at very low 2θ.

Figure 6. Results of mass spectrometry for a sample heated in air/ argon flow at 2 °C min−1 through its sinter ignition temperature (185 °C in this instance). The assigned identities of five significant fragments are indicated. Data for nitrogen and argon are not shown. Data for the remaining fragments (gray lines) are listed in the Supporting Information. The thermogravimetric and heat flow curves are slightly asymmetrical during sinter ignition because the rapid rate of heat release temporarily increases the sample’s temperature above the furnace set point.

metallic gold occurs at the point too, corresponding to the onset of electrical percolation through the sample, as observed previously.9 There is a continuing isothermal aspect to this final crystallization phase too (Supporting Information, Figure S3) with fwhm and height, which are oppositely correlated respectively with the size of grains in the gold, varying in a manner that shows that the crystals in the gold continue to grow but with no significant change in their overall volume fraction. The continuing grain growth at this relatively low temperature is probably driven by the extremely small grain size of these samples, which is well below the several tens of micrometers characteristic of bulk gold. The results of the XRD experiments provide valuable insights into the structural changes that occur upon heating. To gain information about chemical changes that took place upon heating we conducted mass spectrometry and XPS experiments. These experiments also allowed the effects of the local gaseous environment to be examined. When sintering was performed in the presence of oxygen, a range of oxidized sulfur species were released, Figure 6, indicating cleavage of the Au−S bond. In this case, the mass spectrometry data are consistent with the generation of diverse oxidized sulfur-containing species such as SO, SO2, CSO, CH5SO, C2SO, and CH4SO (see the Supporting Information for further details) as well as alkane fragments such as C3H8 (44 amu), C2H5 (29 amu), and C2H2 (26 amu), diverse thiol fragments such as CH4S, C2S, C2H2S, C2H4S,C3H4S, C4H8S, C2HS, and H2S (34 amu). Figure 7 shows mass spectrometry data obtained upon heating the 1-butanethiol-stabilized particles under vacuum (10−4 − 10−6 Pa) up to 275 °C. The concentrations of species detected in the instrument (and thus released by the particles) begins to increase significantly at ∼160 °C. The data are consistent with the evolution of species such as C4H9 and/or

Figure 7. Results of mass spectrometry of a sample heated in vacuum.

C2SH (57 amu), H2 (2 amu), C4H10 and/or C2SH2 (58 amu), and C2H4 and/or N2 and/or CO (28 amu). There is also a constant background of H2O (18 amu). Figure 7 shows that, in vacuum, the evolution of desorbed material reaches a maximum at ∼210−220 °C, a temperature that agrees closely with our previous work showing that these particles sinter in an exothermic manner at ∼220 °C when heated in vacuum at 5 °C min−1.9 Another feature evident in Figure 7 is a second stage E



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of material release at ∼250 °C. After consideration of the recent work reported by Tilley et al.,5 we interpret the current results to show that desorption of stabilizing ligand begins at an appreciable rate at ∼160 °C until sufficient material has been removed to allow the sintering event, in this experiment at ∼220 °C, which results in a large release of organic material. Subsequent defect removal and/or grain coarsening leads to a further release of organic material at ∼250 °C. Although the above data provides useful insights into reaction and/or desorption of the protective ligand, it does not directly provide insight into the material remaining on the gold surface. To examine this, the XPS spectra of samples heated in UHV and air were examined. The intensity ratios, S2p:Au4f and C1s:Au4f, recorded by XPS at room temperature after heating gold nanoparticles to various temperatures are shown in Figure 8. The data follow significantly different trends that depend upon the atmosphere in which the particles were heated. When heated under ultrahigh vacuum conditions, the carbon and sulfur signals increase relative to the Au signal as the temperature is increased

beyond the sintering temperature. The nanoparticles have a calculated starting surface area of >100 m2 g−1 (based on a particle diameter of ∼2.5 nm) but the surface area diminishes considerably upon sintering. Both S and C have negligible solubility in solid gold and so any C and S not lost to the atmosphere should be concentrated on the surface. This is clearly the case for the material sintered in vacuum. However, a different behavior is observed for particles heated in air. In this case, the ratios of C and S to gold are similar to those obtained under UHV up to ∼220 °C after which there is a significant decrease in the C and S to Au ratios. We conclude from these data that a chemical interaction took place between air and sample, the net effect of which was to volatilize the adsorbates. Removal of the stabilizing adsorbates (in this case butanethiolate) may be deemed necessary to sintering2,11 but the acquired data suggest that the fate of the stabilizing butanethiolate ligands differs depending on the environment. The chemical environment of the sulfur atoms in the various samples was probed by deconvolving the binding energy spectra. Figure 9a shows the analysis of the S2p signal for assynthesized particles. The spectrum can be fitted with one doublet accounting for spin−orbit split 2p3/2 and 2p1/2 levels, which are separated by 1.2 eV and have a branching ratio of 1/ 2. After heating in air at 155 and 260 °C, deconvolution of the signals requires two doublets (Figure 9b,c). The doublet at lower binding energies (∼159 eV) may be assigned to individual sulfur atoms bonded to the Au surface, with the slightly reduced binding energy relative to S atoms on a planar Au surface due to the highly curved and strained surface of the nanoparticle. The doublet at higher binding energies (∼162 eV) is assigned to sulfur atoms forming a bridge between Au and some other moiety, presumably the butyl chain (although a polymeric sulfur moiety cannot be excluded on the basis of peak energies alone). Heating in air produced proportionately more signal at ∼159 eV from the individual sulfur atoms. This suggests that while the untreated sample contains its sulfur mainly in the form of covalently bound thiols, a significant portion of it decomposes to S when heated (and sintered) in air. In contrast, removal of chemisorbed thiol by simple volatilization is unlikely to occur at temperatures up to 260 °C, as shown by the spectrum of the sample heated in vacuum. In that case, heating to 271 °C did not result in any marked development of doublets at the lower (∼159 eV) binding energy (Figure 9d), and the resulting spectrum was not dissimilar to that of the pristine sample. This suggests that there was only ‘bound’ thiol present even though this temperature is past that where sintering occurs. There is no peak in any of the samples at 164 eV due to unbound thiols or sulfur as excess thiol from the synthetic procedure is removed during purification and if any had formed during heat treatment, it would have been rapidly volatilized.

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CONCLUSIONS The sintering of gold nanoparticles stabilized with 1butanethiol is a useful and interesting phenomenon in which discrete particles fuse to form electrically continuous materials. During heating, the material undergoes a series of structural transitions, with particle growth and recrystallization commencing at ∼80 °C, which is well before sintering occurs. We propose that the transition at 80 °C corresponds to a change in the material from colloidal glass to loosely packed powder as thermal energy overcomes the van der Waals interactions within the colloidal glass.

Figure 8. Graphs of XPS data showing intensity ratios for (a) S2p/Au4f and (b) C1s/Au4f, measured on 1-butanethiol-stabilized gold nanoparticles as a function of annealing temperature and surrounding environment. F



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Figure 9. Peak shape analysis of the S2p envelope for (a) pristine nanoparticles, (b) nanoparticles heated in air at 155 °C, (c) nanoparticles in air at 260 °C, and (d) nanoparticles heated in an ultrahigh vacuum at 271 °C.

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The sintering of gold nanoparticle ink requires that the protective ligand shell be disrupted and at least in part removed. This occurs at a much lower temperature in oxygencontaining environments than in vacuum, and we have confirmed here that the surface residue differs with environment. Sintering in an oxygen-containing atmosphere gives rise to oxidized sulfur species and a significant portion of S remaining on the sintered surface. In contrast, vacuum sintering yielded no such oxidized species, and the remaining surface species was assigned to predominantly unaffected butanethiol ligand.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +61-2-9514-2208. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Jean-Pierre Guerbois and Ms. Shirin-Rose King (University of Technology Sydney), Dr. Helen Maynard-Casely (Australian Synchrotron) and Dr. Helen Brand (Australian Synchrotron) for technical assistance, and the Australian Research Council, Australian Synchrotron and National Synchrotron Radiation Research Centre (Taiwan) for providing financial support and beam-time.

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ASSOCIATED CONTENT

S Supporting Information *

Data showing heat flow and mass change during an example of “sinter ignition”. Changes in Au {111} X-ray peak of a sample heated at 1 °C min−1. Changes in crystal structure during isothermal annealing at 213 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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