Article pubs.acs.org/JPCC
Transformations during Sintering of Small (Dcore < 2 nm) LigandStabilized Gold Nanoparticles: Influence of Ligand Functionality and Core Size Beverly L. Smith and James E. Hutchison* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253, United States S Supporting Information *
ABSTRACT: Ligand-stabilized gold nanoparticles have been investigated as both discrete entities with size-dependent properties and as precursor inks for low-temperature deposition of thin films and patterns. In the first instance it is important to preserve the nanoparticle core, whereas for thin film applications it is desirable for the nanoparticles to sinter at relatively low temperatures. In each case, a detailed understanding of the factors that govern nanoparticle sintering will lead to improved nanomaterial design. An investigation of the sintering behavior of ∼1.4 and ∼0.9 nm gold nanoparticle cores, each passivated with two different ligands, by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) illustrates a clear size and ligand dependency on the sintering process. TGA reveals that free ligand volatilizes at lower temperatures than when bound to the nanoparticle core. Ligands of the same chain length with different terminal functionality show distinctly different volatilities and rates of ligand loss, revealing that volatility is derived from composition rather than merely ligand chain length. Conducting TGA and DSC measurements on nanoparticles of the same ligand passivation but of different core size shows that larger nanoparticles lose ligands and sinter more readily than smaller nanoparticles, suggesting a greater stability of the ligand shell on smaller nanoparticles. TGA, DSC, and X-ray photoelectron spectroscopy (XPS) analyses show that sintering is triggered by a very small amount of ligand loss. Once initiated, the sintering process rapidly excludes ligand from the gold surface, forming a porous film, as shown by scanning electron microscopy (SEM). These studies suggest that both the nanoparticle core size and ligand identity need to be considered together when selecting nanoparticles to either prevent or promote nanoparticle sintering.
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facile synthesis,8 ease of assembly on surfaces and the low processing temperatures (typically below 200 °C) necessary to generate continuous films.4 However, typical syntheses of alkanethiol-stabilized nanoparticles produce cores in the range of 2−6 nm; thus, the sintering behavior, or the retention of core size, of nanoparticles in the size regime below 2 nm remains relatively unexplored. Preserving the size-dependent properties of small nanoparticles is essential for stable catalysts and nanoelectronic materials. On the other hand, these smaller nanoparticle precursors might generate smoother, more continuous thin films upon sintering.9 In addition, little work has been conducted on water-soluble ligand-stabilized nanoparticles, despite the potential advantages of their use in printable inks and thin film fabrication. Here we report insights into the sintering process and investigations into its dependence on nanoparticle size and ligand identity through the use of combined thermal, microscopic, and surface studies of watersoluble gold nanoparticles with diameters less than 2 nm. Recent studies with differential thermal analysis (DTA) and DSC have begun to address the specific role of the passivating ligand during sintering due to heating. It has been shown that
INTRODUCTION In recent years, ligand passivated gold nanoparticles (AuNPs) have garnered significant attention for a wide range of applications including sensing,1 catalysis,2 thin film fabrication,3 printable inks,4 electronic circuits, and transistors.5 For some of these applications, it is desirable to preserve the nanoparticle core due to its size-dependent properties, whereas in others the goal is to use individual nanoparticles as precursors to rapidly convert into continuous structures through sintering. For sensing and catalysis it is crucial to maintain the size-dependent properties of discrete, individual nanoparticles. In contrast, thin film fabrication, nanoparticle inks, circuits, and transistors rely on the significantly lower processing temperatures6,7 of nanoparticles as compared to the bulk to sinter individual nanoparticles together to generate a continuous thin film. Despite these distinctly different applications, in each case it is crucial to understand nanoparticle thermal stability. In particular, understanding how nanoparticles sinter and the factors that influence this process is of paramount importance. An understanding of how ligand identity and nanoparticle size influence the sintering process would provide general guidelines for the selection of appropriate ligands and sizes for preventing, controlling, or inducing sintering. Alkanethiol-stabilized gold nanoparticles have been investigated as thin film precursors as a result of their controllable, © 2013 American Chemical Society
Received: August 13, 2013 Revised: October 17, 2013 Published: October 24, 2013 25127
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from room temperature to 600 °C at a ramp rate of 5 °C min−1. Gold nanoparticle samples were prepared by reconstituting ∼1−7 mg of lyophilized nanoparticles in 40 μL of water and placing in a tared aluminum pan. The sample was then dried under flowing nitrogen until visibly dry and run immediately to avoid adventitious contaminants. Free ligand samples of ∼5−15 mg were placed directly in a tared aluminum pan and run under identical conditions. A minimum of two trials were run for each ligand and nanoparticle sample. The temperature for the onset of mass loss was typically reproducible within 1 °C for identical trials, with a maximum difference of 3.2 °C. The TGA response was independent of sample size for independent runs on identical nanoparticle samples (see Figure S7). The approximate number of ligands passivating the surface of each type of nanoparticle was determined using the average nanoparticle core size by TEM in conjunction with the total mass lost by TGA. The 1.2 nm TMAT AuNPs lost 27% by weight, which results in approximately 17−26 ligands per particle, depending on the specific counterion present (trifluoroacetate or chloride). The 0.9 nm TMAT AuNPs lost 34% by weight, which is equivalent to approximately 10−15 ligands per particle, depending on the specific counterion present. These values are in good agreement with the estimated number of ligands for a particle of equivalent size assuming the literature value17 for the surface area of a thiol (0.214 nm2). Using this value, an estimated 12 and 21 ligands per particle would be expected for a 0.9 and 1.2 nm nanoparticle, respectively. The 1.5 and 1.0 nm MESA AuNPs both lost 19% weight by TGA. However, the approximate number of ligands for each size of the MESA nanoparticles could not be calculated since only a portion of the ligand volatilized. The free MESA ligand only lost 32% mass by 600 °C, as opposed to the 100% mass lost by the free TMAT ligand. The percent weight loss values for each of the nanoparticle size/ligand combinations are reported in Table 1.
the heat release due to sintering, previously indicated in calculations,10,11 physically manifests itself as an exothermic transition in DTA and DSC.12−15 Coupled with TGA, several studies have shown12−14 that the identity of the ligand influences the temperature at which sintering occurs. DTA/ TGA studies by Martin et al. and Gupta et al. found that for nanoparticles heated at the same rate the sintering temperature increases with increasing ligand chain length for different alkylthiol passivated gold nanoparticles.13,14 The authors attribute this trend to the volatility of the ligand based on the progressively higher boiling points for longer alkanethiols. What remains unanswered from these studies is whether these trends are based solely on the ligand chain length or the actual ligand composition and strength of binding to the nanoparticle core. Whether sintering is dependent on nanoparticle size is also unknown. It has been suggested that if nanoparticle sintering is solely controlled by ligand passivation, nanoparticle size should have no influence on the sintering temperature.14 Others have been unable to discern differences in sintering temperatures for nanoparticles of different size passivated with the same ligand.13 However, an extensive experimental study to discern the size dependence of nanoparticle sintering has not been reported. Here, we examine the role of nanoparticle size and ligand stabilization upon the sintering process and examine the relationship between the ligand loss and core sintering events using TGA, DSC, SEM, and XPS analyses. By synthesizing nanoparticles of four well-controlled sizes, each stabilized by one of two different ligand shells,16−19 we find that nanoparticle sintering depends on size when comparing nanoparticles of the same ligand passivation. Further, we show that nanoparticle sintering depends on ligand volatility, which depends upon ligand composition. Our results suggest that nanoparticle sintering is triggered by ligand loss. For these smaller sizes, only a very small fraction of ligand loss is necessary to trigger sintering and only partial loss in surface area occurs, forming a porous film. These findings contrast with findings on larger particles that show a larger proportion of ligand loss prior to sintering and the formation of continuous thin films.12,14
Table 1. Onset Temperatures and Total Percent Mass Loss for Free Ligands and Nanoparticles Determined by TGA
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onset temp of mass loss (°C)
total percent mass loss (%)
free 1.2 nm AuNP 0.9 nm AuNP
81 143
100 27
164
34
free 1.5 nm AuNP 1.0 nm AuNP
233 242
32a 19a
245
19a
AuNP or ligand
EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4· 3H2O) was purchased from Strem Chemicals and was used as received. N,N,N-Trimethylammoniumethanethiol trifluoroacetate (TMAT) was prepared according to a known procedure.20 Chloroform was stirred with basic alumina and filtered prior to use to remove any acidic impurities. 18.2 MΩ·cm deionized water was used for all synthetic and purification processes. All other chemicals were purchased from Sigma-Aldrich and used as received. Diafiltration membranes were purchased from Pall Life Sciences. Synthesis and Purification of 1.2 and 0.9 nm TMAT and 1.5 and 1.0 nm Sodium 2-Mercaptoethanesulfonic Acid (MESA) Gold Nanoparticles. Nanoparticles of four well-controlled sizes, each stabilized by one of two different ligand shells, were synthesized according to previously published methods.16−19 A detailed description of the synthesis, purification, and characterization procedures is provided in the Supporting Information. 1H NMR and TEM characterization of the nanoparticles are provided in Figures S1−S6. Thermogravimetric Analysis (TGA) Measurements. TGA measurements were conducted on a TA Instruments Q500 TGA under a nitrogen atmosphere. Samples were run
TMAT
MESA
a
Mass loss for MESA ligand and AuNPs are shown for comparative purposes only because free ligand does not completely volatilize by 600 °C. Therefore, these values do not reflect loss of the total amount of ligand.
Differential Scanning Calorimetry (DSC) Measurements. DSC measurements were conducted on a TA Instruments Q2000 DSC under a nitrogen atmosphere. Samples were run from −40 to 550 °C at a ramp rate of 5 °C min−1. Gold nanoparticle samples were prepared by reconstituting ∼0.5−7 mg of lyophilized nanoparticles in 40 25128
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μL of water and placing in a preweighed aluminum pan. The sample was then dried under flowing nitrogen until visibly dry and fitted with an aluminum lid with a pinhole in it and was run immediately. Free ligand samples of ∼0.5−2 mg were placed directly in a preweighed aluminum pan and were also fitted with an aluminum lid with a pinhole and run under the same conditions. Values for the sintering temperature were reproducible within 2.2 °C for independent runs on identical samples. The DSC response was independent of sample size for independent runs on identical nanoparticle samples (see Figure S8). X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS samples for the 1.2 nm TMAT, 1.5 nm MESA, and 1.0 nm MESA AuNPs were prepared by dropcasting aqueous nanoparticle solutions onto mica and drying under nitrogen. Samples of 0.9 nm TMAT AuNPs were prepared in aluminum TGA or DSC pans as outlined above for TGA or DSC measurements. Both heated and unheated samples were run immediately to avoid contamination and prolonged exposure to oxygen. XPS spectra were collected on a ThermoScientific ESCALAB 250 with a monochromatized Al Kα X-ray source operated at 150 W with a nominal spot size of 500 μm. Survey scans were collected at a pass energy of 150 eV. High-resolution Au 4f, C ls, Cl 2p, F ls, I 3d, N 1s, O 1s, and S 2p spectra were collected at a pass energy of 20 eV. The binding energy scale was calibrated by setting the Au 4f7/2 peak to 83.95 eV. This resulted in an actual peak position between 83.90 and 84.00 eV. Three spots per sample were analyzed, and the compositional values and binding energies reported are the average values. Charge neutralization was required on all samples analyzed. An in-lens low-energy electron source was used for charge neutralization, and it was necessary to float the stage. The manufacturer’s Avantage software was used for data reduction and peak fitting. XPS was conducted on each of the nanoparticle size/ligand samples to assess the overall composition and sulfur speciation on the nanoparticle surface. For example, the 0.9 nm TMAT AuNPs had a S:Au ratio of 0.77 ± 0.01, in good agreement with the range of ligands per particle for a nanoparticle of this size.21 Deconvolution of the S 2p peak for the 0.9 nm TMAT AuNPs reveals two pairs of doublets with a spin−orbit splitting of 1.18 eV and an intensity ratio of 2:1 between the 2p3/2 and 2p1/2 S peaks. The S 2p3/2 peak for each pair of peaks appear at 161.5 ± 0.1 and 162.5 ± 0.1 eV (see Figure S9a). The S 2p3/2 peak at 162.5 ± 0.1 eV comprises the majority of the sulfur signal and agrees well with the reported binding energy value for bound thiolate.23,24 With regards to the minor species present at 161.5 ± 0.1 eV, an extensive review of the literature reveals that there is no definitive assignment for the sulfur species at this binding energy. A number of studies have suggested potential sources for this species, including presence of a sulfur-containing impurity,25−27 weakly bound thiolate at low surface coverages,26,28−30 thiol adsorption to different binding or defect sites,26,28,30−34 C−S bond cleavage resulting in gold sulfide or polymeric sulfur species (“atomic” sulfur),24−28,30,32,33,35−40 sample charging or X-ray induced damage,27,33,41−43 or a different bonding arrangement of the ligand.40,44 However, examination of our data and the results of several additional experiments (see Supporting Information for a detailed description of these experiments) suggest that the source of this S species is due to associated disulfide ligand. Spiking of a nanoparticle solution containing a small amount of
this species (which was verified with XPS prior to spiking) with free disulfide results in a marked increase in the amount of this 161 eV species (Figure S10). However, XPS on free disulfide does not show any peaks at this low binding energy, suggesting that the disulfide is not free, but rather associated with the nanoparticle surface (Figure S10). 1H NMR also corroborates this finding, showing no peaks indicative of free disulfide are present in the nanoparticle samples (Figures S1, S2, S5, and S6). Further review of the literature on the use of disulfides to form thiolate self-assembled monolayers on gold films revealed that while thiolate remains the predominant sulfur speciation, there are also numerous reports26,35,44,45 in the literature that show a proportion of the sulfur 2p signal that occurs at binding energies in the region of 161.0−161.5 eV, consistent with our results. One of these examples45 which uses very similar molecules (cysteamine and cystamine) to the TMAT ligand used in our studies reports a higher proportion of the 161.5 eV S species when initially generating self-assembled monolayers from the disulfide form of the molecule versus the thiol, also indicating that the occurrence of this sulfur species is related to the disulfide. The specific nature of the adsorbed disulfide on the surface of the nanoparticle is still under investigation.
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RESULTS AND DISCUSSION Four ligand-stabilized gold nanoparticles (1.2 nm TMAT, 0.9 nm TMAT, 1.5 nm MESA, and 1.0 nm MESA AuNPs) were used to assess the role of nanoparticle size and ligand passivation on the sintering process (Figure 1). These materials
Figure 1. Four synthesized gold nanoparticles of varying core size and of two different ligand passivations (TMAT and MESA). The gold cores are scaled by size relative to one another. The blue circle depicts the approximate size of the ligand shell surrounding each nanoparticle.
were chosen for this study for several specific reasons. First, the small size of these nanoparticles accesses a size regime wherein the sintering behavior had not been explored. Additionally, the controlled size and narrow dispersity of the samples allow for the unambiguous detection of how nanoparticle size influences the sintering process. Further, the versatility of the ligand exchange synthetic method used to prepare these nanoparticles produces nanoparticles with a variety of ligands of different composition, which enables the study of nanoparticles of different ligand passivation while maintaining the same nanoparticle core size. Thus, we can assess how nanoparticles passivated with ligands of the same chain length, but of 25129
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different terminal functionality, impact the sintering process. Further, work in the field to date has focused almost entirely on organic soluble ligand passivated nanoparticles despite the strong desire for water-soluble nanoparticle inks and thin film precursors. Studies of the materials in this work will extend the knowledge of the sintering process to water-soluble systems. All of the nanoparticles were prepared by first synthesizing triphenylphosphine-stabilized gold nanoparticles of the appropriate size, followed by ligand exchange to TMAT and MESA through biphasic reactions. All nanoparticle/ligand combinations were rigorously purified by either column chromatography or diafiltration following synthesis. The purity, size, and composition of each nanoparticle type were evaluated with 1H NMR, TEM, and XPS, respectively. Their purity was confirmed prior to use via 1H NMR (see Figures S1, S2, S5, and S6 for spectra). The absence of sharp signals and the presence of only broadened signals related to the ligand are indicative of a sample of high purity with no remaining free ligand present.46−48 TEM was conducted in order to determine the average nanoparticle size and the polydispersity of each nanoparticle/ligand combination. Size analysis revealed that the TMAT nanoparticles were 1.2 ± 0.4 nm (N = 2741) and 0.9 ± 0.2 nm (N = 2645). MESA passivated particles were 1.5 ± 0.6 nm (N = 3827) and 1.0 ± 0.3 nm (N = 758). Representative TEM images of each sample are found in Figures S3 and S4. Determining the Onset Temperature of Ligand Loss by TGA Based on Ligand Identity and Size. The following two sections describe two chemical processes: the onset of ligand loss and nanoparticle sintering. Both are heating rate dependent and thus are kinetic processes. Therefore, in order to discern differences between nanoparticles of different size and ligand passivation, we chose a slow and common heating rate of 5 °C min−1 for all samples analyzed. As a means for comparison, herein we will refer to the temperature of the onset of ligand loss and the exothermic temperature of maximum heat flow. We began our investigation by measuring the onset temperature of mass loss due to ligand volatilization by TGA. In this study, the onset temperature for ligand loss was defined as the temperature at which the first derivative of the TGA trace signaled the initial change in rate ascribed to mass loss. We conducted TGA on both the free ligand and each of the nanoparticle/ligand combinations synthesized to determine the temperature where free and bound ligand loss occurs. Comparison of free TMAT and MESA to their respective AuNPs (Figure 2a,b) shows the onset of free ligand loss is always at a lower temperature than for the ligand bound to the nanoparticle core for all core sizes. For TMAT, the free ligand volatilization begins at 81 °C. In contrast, 1.2 and 0.9 nm TMAT particles begin losing ligand at 143 and 164 °C, respectively (Figure 2a). For MESA, the free ligand onset is at 233 °C as compared to 242 and 245 °C for the 1.5 and 1.0 nm AuNPs (Figure 2b). These values are summarized in Table 1. The higher temperature of ligand loss from the nanoparticle surface versus unbound ligand suggests that the desorption process of the thiol ligand from the nanoparticle surface differs from that of unbound thiol. Previous reports have suggested that thiol ligands desorb from a gold nanoparticle surface as disulfide,48 which are less volatile than their thiol counterparts and desorb at higher temperatures. Comparison of the onset temperature of mass loss for nanoparticles of the same ligand passivation, but of different
Figure 2. TGA traces of weight (%) versus temperature (°C) for (a) free TMAT ligand, 1.2 nm, and 0.9 nm TMAT AuNPs, (b) free MESA ligand, 1.5 nm, and 1.0 nm MESA AuNPs, and (c) comparison of 1.2 nm TMAT and 1.5 nm MESA AuNPs. Traces have been normalized to 100% after initial residual water loss below 100 °C.
size, reveals that the onset of mass loss occurs at a lower temperature for larger nanoparticles as compared to smaller nanoparticles. For example, the 0.9 nm TMAT AuNPs begin losing mass at 164 °C, whereas the 1.2 nm TMAT AuNPs start losing mass at 143 °C (Figure 2a). Similar trends are seen between the 1.5 and 1.0 nm MESA AuNPs (Figure 2b). This trend suggests that the strength of ligand binding to the nanoparticle core varies with nanoparticle size. Hence, the different temperatures for the onset of ligand loss cannot fully be attributed to the nature of the desorbing species (thiol vs disulfide), as samples of the same ligand passivation exhibit different temperatures for the onset of ligand loss. This size dependence presumably arises from the different surface curvatures of each nanoparticle size, which influences the binding energy of the ligand to the nanoparticle core.49 It is also evident from these data that for each ligand, despite the fact that all the ligands in this study are of the same chain length, the onset temperature for loss depends upon the ligand structure. This is consistent with studies of self-assembled monolayers of alkylthiols on planar gold that report slightly different thiol desorption values for alkylthiols of the same length but of different terminal functionality.50 By TGA, it is clear that TMAT is more easily lost and at a faster rate from the nanoparticle surface as compared to MESA (Figure 2c). Assessing the Influence of the Ligand on the Sintering Temperature by Differential Scanning Calorimetry (DSC). DSC analysis was conducted to determine the temperature at which sintering occurs for each nanoparticle core size and ligand type. For each nanoparticle sample, the DSC trace showed a sharp exothermic transition immediately followed by an endothermic transition (Figure 3a−c). The DSC traces for unbound TMAT and MESA did not exhibit this characteristic behavior, only showing endothermic transitions related to ligand volatilization (Figure 3d). The exothermic transition seen in nanoparticle samples has been previously attributed to nanoparticle sintering, as a result of the heat released when the surface area is reduced during coalescence of 25130
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Table 2. Peak Temperatures of Exothermic and Endothermic Transitions for Different Free Ligands and Passivated Gold Nanoparticles AuNP or ligand
peak temp of exotherm (°C)
peak temp of endotherm (°C)
TMAT free 1.2 nm AuNP 0.9 nm AuNP
N/A 147
175 194
174
233
free
N/A
1.5 nm AuNP 1.0 nm AuNP
244, 253
163, 195, 236, 259, 302, 322, 431, 466 259
271
277
MESA
previous predictions14 and experimental results,13 these data definitively show that sintering is in fact size dependent in this size regime. These results suggest that ligand stability is greater on a smaller nanoparticle as opposed to a larger nanoparticle. This likely arises from a greater binding strength to the nanoparticle core due to a higher surface curvature for a smaller nanoparticle.48,49 Determination of Surface Area Loss Due to Sintering by DSC. The DSC data can also be used to determine the amount of surface area that is lost due to sintering by first determining the enthalpy of the exothermic transition in DSC in order to calculate the experimental surface energy released as heat, which is directly correlated to the amount of surface area consumed during sintering.12 The surface energy released as heat during sintering is calculated by integration of the exothermic transition by DSC in conjunction with the percent of the sample composed of gold by TGA. This value reflects the total surface energy released as heat from the reduction in surface area as the nanoparticles coalesce to form a continuous structure. From integration of the sintering peak for the 0.9 nm TMAT AuNPs, an average enthalpy value of −84.5 ± 5.5 J g−1 was shown among integration of multiple samples of the same nanoparticle batch. By TGA, 66% of the sample is composed of gold, which results in a heat release value of 128.2 J g−1 for gold. Similarly, integration of the 1.2 nm TMAT AuNP sintering peak gives an average enthalpy of −83.1 ± 1.9 J g−1. By TGA, the sample is composed of 73% gold, which results in a heat release of 113.7 J g−1. These values along with estimated values for the 1.5 and 1.0 nm MESA AuNPs are depicted in Table S1. Coupled with the experimentally determined surface energy value for gold51 of 0.7−0.8 J m−2, an approximation of the total amount of surface energy that should be released to generate a continuous thin film of limited surface area for a nanoparticle of a specific size can be calculated. This value can be compared with the experimental value to assess whether the actual sintering event reflects a full reduction in surface area. For the 0.9 nm TMAT AuNPs, if one assumes that all surface area is lost due to sintering and a continuous film is formed, one would expect a value for this transition in the range of 276−242 J g−1. This value is well above the experimentally determined value of 128.2 J g−1. For the 1.2 nm TMAT AuNPs, full conversion into a continuous film would produce a heat release in the range of 207−181 J g−1, which is also well above the actual heat released experimentally (113.7 J g−1). This trend is repeated in the 1.5 and 1.0 nm MESA AuNP cases. All
Figure 3. DSC traces for (a) 1.2 nm TMAT and 1.5 nm MESA AuNPs, (b) a comparison of 1.2 and 0.9 nm TMAT AuNPs, (c) a comparison of 1.5 and 1.0 nm MESA AuNPs, and (d) free ligand. DSC traces were normalized by mass. The y-axes were split in graphs a, b, and c in order to provide visual clarity of the exothermic transition of interest.
the nanoparticle cores.10,11 It has further been suggested that the endothermic transition following the exothermic transition is the result of ligand loss following sintering.12 Examination of the temperature of the exotherm for nanoparticles of similar size, but different ligand passivation, reveals that sintering is dependent on ligand composition, regardless of chain length. Comparison of 1.2 nm TMAT and 1.5 nm MESA AuNPs (Figure 3a) indicate that the temperature at which sintering occurs depends on the identity of the terminal tail group. The exotherm for the 1.2 nm TMAT nanoparticles is at 147 °C, whereas the peaks for the analogous particle size with MESA are at 244 and 253 °C. These trends are mirrored in the smaller nanoparticle cases. The 0.9 nm TMAT AuNPs (Figure 3b) show an exotherm at 174 °C and the 1.0 nm MESA AuNPs at 271 °C (Figure 3c). Temperatures for the described exothermic peaks due to sintering are listed in Table 2. Effects of Nanoparticle Size on Sintering. DSC clearly shows that sintering is also a size-dependent process for this size regime. When comparing nanoparticles with the same ligand passivation, but of different size, notably the temperature of the exothermic peak due to sintering is at different temperatures when heated at the same rate. For TMATfunctionalized AuNPs, the exotherm is at 147 °C for 1.2 nm nanoparticles versus 174 °C for the 0.9 nm nanoparticles. Nanoparticles passivated with MESA show similar trends. The exotherm for the 1.5 nm particles is at 244 and 253 °C, whereas the 1.0 nm nanoparticles exotherm is at 271 °C. In contrast to 25131
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stabilized nanoparticles are heated, which causes the nanoparticles to sinter. Sintering induces ligand loss that continues with progressively higher temperatures. For model B to be a feasible pathway, either the initial nanoparticles must not be fully passivated to allow for ligand rearrangement or the ligands must desorb from the nanoparticle surface and remain closely associated with the nanoparticle rather than leaving the sample prior to sintering. Previous reports have delineated between these models for sintering by comparison of their TGA and DSC data. This was done by overlaying the mass loss by TGA and the heat flow (or temperature differential between the sample and the reference) by DSC12 (or DTA)13,14 versus temperature to determine whether the temperature of the sintering exotherm precedes or follows the temperature of initial ligand loss. This type of analysis is reasonable when both the TGA and DSC (or DTA) measurements are conducted simultaneously on the same instrumentation, as in previous studies.12−14 In our studies, conducted on two separate instruments, the very narrow temperature range where sintering occurs in relation to the onset of ligand loss is smaller than the error associated with each measurement (≤3.2 °C for TGA and ≤2.2 °C for DSC). However, a rigorous analysis of the sintering models and the predicted signatures in the TGA and DSC data can assist in determining the operative sintering model in our studies. In the case of model A, because a portion of ligand loss triggers the sintering event, one would expect two distinct regions of mass loss by TGA: one ascribed to the initial loss of ligand from the nanoparticle surface and one ascribed to the further loss of ligand following sintering. Since the initial portion of ligand lost is likely from more weakly bound sites52−54 on the nanoparticle surface, one would also predict the rate of mass loss to be different for the two regions of mass loss. Further, since ligand loss is a prerequisite for sintering to occur by model A, one would also expect that the temperature of sintering by DSC to track with ligand volatility for nanoparticles of similar size. In contrast, if sintering proceeds by model B, one would expect only a single region of mass loss by TGA, as the loss of ligand would follow the sintering process. Because of the fact that sintering itself would dictate the initial loss of ligand in model B, the sintering temperature by DSC should be independent of ligand composition for nanoparticles of similar size. Examination of our TGA and DSC data reveals several key features related to the sintering process that allows us to develop a model for sintering. First, by the percent of mass loss in TGA and the experimentally determined nanoparticle size, it is clear that the nanoparticles used in this study are fully passivated. As described earlier, the calculated number of ligands by TGA mass loss reflects a fully passivated nanoparticle. For example, the 1.2 nm TMAT AuNPs lose 27% ligand and have between 17 and 26 ligands per particle, depending on the specific counterion present (trifluoroacetate or chloride). These values are in good agreement with the expected 21 ligands per particle for a 1.2 nm gold nanoparticle, assuming the reported surface area of a thiol of 0.214 nm2.17 Additionally, by using the first derivative of the TGA mass loss to discern the onset of mass loss, it is further evident that at least two distinct regions of mass loss clearly exist in the early stages of ligand loss for all of the nanoparticle sizes and ligand compositions studied (Figures S12−S15). By overlaying the TGA and DSC traces, the data show that the temperature at which sintering occurs is strongly correlated with ligand
calculated values are shown in Table S1. For this size regime, all samples gave a consistently lower value for the experimentally determined heat release by gold (between 18.9 and 49.5% lower) as compared to the expected value for full surface area reduction of the appropriate particle size. This suggests that for the nanoparticles in this size regime not all surface area is lost, and a porous film is generated rather than a continuous film possessing significantly lower surface area. The morphology of the nanoparticle film before and after sintering was probed by SEM. Three different samples of 0.9 nm TMAT AuNPs from the same batch were heated in the DSC at the same rate of 5 °C min−1. One sample was heated to 155 °C, before the initial ligand loss/sintering event. Another sample was heated to 185 °C, just after sintering. The final sample was heated to 258 °C, past the temperature range of ligand loss and immediately following the endothermic transition in DSC. The electron micrographs of these samples were compared with an unheated sample. As shown in Figure S11, both low- and high-magnification images of the surface morphology for the unheated and the 155 °C sample appear relatively continuous and smooth with few holes in the film (Figure S11a,b). In contrast, the samples above the temperature of sintering show porous films of greater surface roughness with spherical grains that appear larger with increased temperature (Figure S11c,d). These images support the findings in TGA/ DSC, as the surface area reduction indicated by the porous structure and increased grain size of the 185 and 258 °C samples illustrate that sintering has occurred. As evidenced by the porous structure of the films after sintering (Figure S11c,d), these measurements also confirm that for nanoparticles of this size and ligand passivation only partial loss in surface area occurs, rather than conversion to a continuous film. This is in direct contrast to previous reports12 of gold nanoparticles of larger sizes that resulted in continuous films of reduced surface area. Conversely, the smaller nanoparticles used in this study produce porous films that retain 50−80% of the initial surface area. Evaluating Existing Models for Nanoparticle Sintering for Small, Thiol-Stabilized AuNPs. Previous investigations of larger alkylthiol gold nanoparticles have suggested that sintering occurs by one of two possible models.12−14 In one, sintering is triggered by ligand loss,12,14 whereas in the other model sintering induces ligand loss.13 As depicted in Scheme 1, in the first model (A), gold nanoparticles are heated, which results in an initial loss of ligand. This destabilizes the nanoparticles and results in sintering and further ligand loss with higher temperatures. In the other model (B), ligandScheme 1. Proposed Models of Sintering of LigandStabilized Gold Nanoparticles Due to Heating
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the ligand coordination on the NP surface. Considering the sensitivity of XPS to near surface species and that very little ligand is lost at this temperature (by TGA), the higher S:Au ratio suggests that sintering results in the exclusion of ligand to the surface prior to its loss from the sample. This is the first report of such a process during nanoparticle sintering. With further heating, the S:Au ratio drops back down to similar values as those prior to sintering consistent with the TGA data that show that most of the excess ligand is lost by 258 °C. The fact that ligand persists above this temperature suggests that a small amount of ligand remains entrapped within the gold film. The binding energies of the S 2p peaks provide information about the sulfur speciation throughout the sintering process. The S 2p peaks for the unheated and 155 °C TMAT AuNP samples contain two different sulfur species, whereas there are three pairs of peaks/sulfur species present for the TMAT AuNP samples heated to temperatures beyond the sintering temperature (185 and 258 °C). The binding energies for the S 2p3/2 peak for each pair of peaks are reported in Table 3, and the XPS spectra are shown in Figure S9. Peak fitting of the S 2p3/2 peak for the unheated and heated samples prior to sintering (155 °C) illustrates that the majority of the sulfur signal appears at a binding energy (162.5 eV) previously ascribed to bound thiolate.23,24 Following sintering, the binding energies for the majority of the sulfur signal shift to higher values (∼163−164 eV), in agreement with previous assignments for unbound thiol or disulfide.23,24 The presence of unbound thiol or disulfide after sintering has occurred agrees well with our thermal studies that show ligand desorption, but almost no mass loss, at 185 °C. For the 258 °C sample, a very small amount of bound thiol remains at 162.3 eV. This species is likely not seen in the 185 °C sample because of the large amount of unbound thiol present on the surface of the film. Since all bound thiol should desorb by this temperature, the small fraction of the sample attributed to this binding energy is most likely occluded ligand within the porous gold film. As described earlier, the minor species present at ∼161.5 eV is persistent across all of the samples studied and appears to be due to adsorbed disulfide ligand. Kinetic Analysis of the Sintering Process Suggests Ligand Loss Is a Key Initial Step. Kinetic analysis of the sintering temperature for different heating rates by DSC of the 1.2 nm TMAT AuNPs yields a calculated activation energy that closely matches literature values for thiol desorption from a gold surface, further supporting the idea that sintering proceeds by model A. By conducting DSC at rates of 2, 5, and 10 °C min−1 for the 1.2 nm TMAT AuNPs (Figure 4a), an activation energy value was obtained through the use of nonisothermal Arrhenius kinetics.55 Kissinger analysis61−63 was performed to determine the activation energy of the sintering process according to eq 1:
composition for each of the nanoparticle size and ligand compositions studied (Figures S12−15). What is further evident from these data, in contrast to the previous cases presented in the literature for larger nanoparticles,12−14 is that very little mass loss occurs in the temperature range between sintering and the onset of ligand loss, even when accounting for error. In previous studies of larger nanoparticles, a much larger proportion of ligand was lost prior to sintering.12−14 Additional analysis of the DSC data shows that the reduction in surface area due to sintering greatly outpaces ligand loss when comparing the values for heat released in the process of sintering. As previously discussed, the heat released due to sintering reflects a reduction in surface area between 19% and 50% for all of the nanoparticles studied. Ligand loss is necessary to achieve such a significant loss in surface area. Hence, examination of our TGA and DSC results in conjunction with the expected signatures for each of the models indicate that model A in Scheme 1 most closely describes the sintering process for nanoparticles of this size and ligand passivation. XPS Investigations Provide Evidence for the Role of the Ligand during Sintering. Our thermal studies suggest that sintering greatly outpaces ligand loss. XPS spectra of heated samples can complement these studies by providing information about the fate of the ligand during these transformations. XPS spectra were collected for samples used in TGA and DSC experiments on 0.9 nm TMAT AuNPs before sintering (155 °C), immediately after sintering (185 °C), and following the endothermic transition (258 °C). The S:Au ratios and chemical shifts of the sulfur species (Table 3) were evaluated at each temperature and compared to an unheated sample. Table 3. Sulfur to Gold Ratios from XPS for Unheated 0.9 nm TMAT AuNPs As Compared to TGA Samples before and after Sintering sample temp (°C)
S:Au ratio
unheated
0.77 ± 0.01
155 (just before sintering)
0.76 ± 0.01
185 (just after sintering)
6.11 ± 0.51
258 (after endothermic transition)
0.85 ± 0.04
S 2p3/2 binding energies (eV) 161.5 162.5 161.4 162.5 161.7 163.3 164.2 161.4
± ± ± ± ± ± ± ±
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
162.3 ± 0.1 164.1 ± 0.1
The S:Au ratios determined for each heated nanoparticle sample allowed us to assess the loss and/or changes in spatial distribution of the ligands during sintering. The S:Au ratio remains constant prior to sintering and increases 8-fold immediately following sintering. Further heating after sintering results in a decrease in the S:Au ratio to a value similar to that observed prior to heating. The constant value for the S:Au ratio prior to sintering suggests that the ligand shell is unchanged during initial heating. This correlates well with our TGA and DSC data, which show that no ligand loss or thermal events occur below 155 °C. The 8-fold increase in the S:Au ratio immediately following sintering suggests that there is a significant change in
⎡ ⎤ E ⎡ AR ⎤ β − a ln⎢ 2 ⎥ = ln⎢ ⎥ ⎦ ⎣ E RTp ⎢⎣ Tp ⎥⎦
(1) −1
where β is the heating rate (K min ), Tp is the peak temperature (K), A is the pre-exponential factor (min−1) and is assumed to be independent of temperature, R is the universal gas constant (8.314 J mol−1 K−1), and E is the activation energy (J mol−1). Using the slope of the line generated from the plot of ln(β/Tp2) vs 1/T (K) (Figure 4b) for the exothermic peak in DSC at each heating rate, an activation energy of 20.3 kcal mol−1 (84.9 kJ mol−1) was obtained. 25133
dx.doi.org/10.1021/jp408111v | J. Phys. Chem. C 2013, 117, 25127−25137
The Journal of Physical Chemistry C
Article
functionality, but of the same chain length, are lost at different temperatures and rates as well, suggesting that ligand volatility is dictated by composition, rather than merely ligand chain length. By comparing TGA with DSC results, we found that nanoparticles of the same ligand passivation, but of different size, showed that the larger nanoparticles lost ligand and sintered at lower temperatures than smaller nanoparticles. For this class of small nanoparticles, only a small amount (
