Time and Temperature Effects on the Digestive Ripening of Gold

Letter pubs.acs.org/Langmuir

Time and Temperature Effects on the Digestive Ripening of Gold Nanoparticles: Is There a Crossover from Digestive Ripening to Ostwald Ripening? Puspanjali Sahu and Bhagavatula L. V. Prasad* Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune 411008, India S Supporting Information *

ABSTRACT: The effects of time and temperature on the gold nanoparticle sizes obtained by digestive ripening have been investigated. In digestive ripening, a polydisperse colloid, upon refluxing with a surface-active ligand in a solvent, gets converted to a nearly monodisperse one. In this study, a polydisperse gold nanoparticle system was heated in 4-tert-butyltoluene with hexadecanethiol at different temperatures, viz., 60, 90, 120, 150, and 180 °C for different time periods, and the trends in particle size variations were recorded. At lower temperatures such as 60 and 90 °C, after the initial narrowing of the size distribution, the particle sizes remain constant even though the refluxing step is continued for 24 h, substantiating the prevalence of the digestive ripening process. However, at elevated temperatures (120, 150, and 180 °C) particle sizes grow continuously, indicating a deviation from the digestive ripening behavior to an Ostwald ripening-type phenomenon.

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INTRODUCTION Harvesting nanoparticle (NP) properties for applications in electronic and optical systems,1,2 therapeuptics,3,4 catalysis,5,6 and so on entails significant control over their size, shape, and monodispersity. Despite the significant progress achieved in the synthesis of various nanoscale particles, finding conditions that result in the desired size and shape remains a daunting task. This requires a deeper understanding of two crucial phenomena, nucleation and growth. The results of many experimental findings in this direction have been summarized and well explained by the LaMer mechanism and Lifshitz− Slyozov−Wagner model (LSW model)7 proposed in the mid1900s. Among these, the LaMer mechanism concentrated on early-stage nucleation, and the LSW model explained the subsequent growth mechanism. These studies demonstrated how large NPs grow at the expense of smaller ones, commonly known as Ostwald ripening. In the past few years, many experiments and theories have also highlighted the role of aggregation and coalescence of NPs as major contributors to nanoparticle growth.8−10 Although the means of particle growth is different, all of these studies clearly highlight the fact that larger particles are always energetically favored and hence preferably obtained. In spite of the thermodynamic stability associated with larger particles, many synthetic protocols have been developed that could provide very small NPs with a narrow size distribution. In the literature, these are described with different names, such as size focusing, digestive ripening, and inverse Ostwald ripening.11−14 Digestive ripening is one of the most convenient routes to obtaining monodisperse NPs, in which a polydisperse © 2014 American Chemical Society

colloidal dispersion gets converted to a monodisperse one upon refluxing in the presence of excess surface-active agent, commonly referred to as digestive ripening agent (DRA). The final sizes obtained after the digestive ripening process are in between both larger and smaller particles that are present in the initial polydisperse system. Though the breaking of thermodynamically preferred large particles is unconventional, it is known that the presence of defects in larger particles (>10 nm) can induce microstresses. This reduces the lattice stabilization energy, causing these particles to break when the DRA is added to them.15 It has also been claimed that during the reflux step the atoms on the larger particles are etched out by ligand, resulting in a size reduction.16,17 Simultaneously, these atoms or clusters are deposited on smaller particles, which prefer to grow in order to attain a lower surface energy. This combination of the etching of larger particles and the deposition of these etchants on smaller particles leads to particle sizes that are intermediate between the small and large ones. Therefore, the size of the NPs obtained using the digestive ripening protocol has been described as an equilibrium size specific to nanoparticulate systems and the DRA used. The mutual influence of the nature of the ligand and metal used and the detailed steps involved in the digestive ripening process have been thoroughly investigated.18−20 However, the temperature and time dependences of refluxing the polydisperse colloid with a DRA have not been probed Received: March 7, 2014 Revised: August 7, 2014 Published: August 11, 2014 10143

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Figure 1. (a−d) TEM micrographs of nanoparticle systems obtained at 60 °C at 1, 3, 9, and 24 h and (e−h) their respective particle size histograms. The particle size distributions obtained are 5.3 ± 0.6 nm at 1 h, 5.5 ± 0.4 nm at 3 h, 5.5 ± 0.3 nm at 9 h, and 5.7 ± 0.3 nm at 24 h. TEM micrographs of the NP systems obtained at 90 °C at 1, 3, 6, and 9 h are represented by i−l, respectively. m−p are the respective particle size histograms. The particle size distributions obtained are 5.4 ± 0.6 nm at 1 h, 6.1 ± 0.4 nm at 3 h, 6 ± 0.3 nm at 9 h, and 5.8 ± 0.4 nm at 24 h. More images can be found in the Supporting Information (Figure S2a−h,i−p for the 60 and 90 °C cases, respectively). from unbound ligand, DDAB, and other reaction side products by the addition of an excess amount of ethanol. This precipitate was then dried and redispersed in 25 mL of tbt, followed by the addition of another portion of ligand (1:30 metal to ligand ratio). An image of this NP at this stage indicates that they are polydisperse (Figure S1). This polydisperse system was then divided into 5 mL parts, and each part was heated to a different temperature, namely, 60, 90, 120, 150, and 180 °C. At lower temperatures, reaction were carried out for an extended period of time (i.e., 24 h), and the particles remained suspended in solution. However, at higher temperatures, NPs started aggregating with time (about 5 h for 150 °C and 3 h for 180 °C), thus we did not proceed with these reactions for a longer time period. The effect of temperature on particle growth at different time intervals was monitored with the help of transmission electron microscopy (TEM) and UV−visible (UV−vis) spectroscopy. For TEM, NP dispersions after heating, at different temperatures, were drop cast on a TEM grid, and around 300 particles were taken into account for the calculation of the particle size distribution (PSD). For UV−vis spectroscopy, 20 μL NP dispersion are taken and diluted with tbt to make the volume 1 mL for each case. Spectra are recorded using a Jasco V-570 spectrophotometer. For the particle size analysis by UV−vis spectroscopy, we employed the analytical method developed by Haiss et al. According to this, the size of Au NPs can be determined from the ratio of the absorbance at the surface plasmon resonance peak (ASPR) to the absorbance at 450 nm (A450).22 The ratio ASPR/A450

earlier. We envisaged that establishing the effect of time and temperature on the digestive ripening process will provide deeper insights into the mechanistic aspects and offers us a handle to control the particle sizes more intricately. Accordingly, in this letter we describe the effects of time and temperature on the digestive ripening process. For this, we have chosen hexadecanethiol (HDT)-capped Au NPs as the model system. It has been established that Au NPs capped by HDT remain suspended in solution without any aggregation under different experimental conditions,21 thus allowing the investigation of the variation in their sizes and size distribution under different conditions.

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EXPERIMENTAL SECTION

Au NPs were synthesized by employing the digestive ripening protocol.14 To start, 75 mg of AuCl3 was dissolved in 25 mL of a high-boiling-point organic solvent such as 4-tert butyltoluene (referred as tbt later in the text) with the help of long-chain surfactant didodecyl dimethylammonium bromide (DDAB). Subsequently, aqueous NaBH4 (60 μL of 9.4M) was added to the solution, which reduced the Au+3 to Au0, leading to the formation of nanoscale particles of gold. HDT was added to this colloidal dispersion to maintain a metal to ligand molar ratio of 1:30. These ligand-coated NPs were separated 10144

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Figure 2. TEM micrographs and respective particle size histograms of nanoparticle systems obtained at 120 °C for (a−c) 1, (d−f) 3, (g−i) 5, (j−l) 7, and (m−o) 9 h. The particle size distributions obtained are 7 ± 0.7 nm at 1 h, 8.8 ± 1.4 nm at 3 h, 9.4 ± 1.5 nm at 5 h, 9.9 ± 1.3 nm at 7 h, and 10.8 ± 1.5 nm at 9 h. More images can be found in the Supporting Information (Figure S3). shows a linear dependence on ln(d), where d is the particle diameter. However, we would like to add here that the expression derived by Haiss et al. is for the water medium, whereas our NP preparation and digestive ripening are all carried out in a nonpolar organic solvent (i.e., tbt). Hence, the analysis that we carried out here can be considered to be semiquantitative. (For details of the analysis, please see the Supporting Information.)

Quite gratifyingly, when HDT is added to this polydisperse system and heated to 60 °C (in tbt), a dramatic narrowing of the size distribution to 5.3 ± 0.6 nm was observed within 1 h of heating. (Please see Figures 1a and S2 for TEM images and Figure 1e for the particle size distribution.) Continued heating to 60 °C even for up to 24 h did not lead to any significant change in the particle size except a slight narrowing of the size distribution (5.8 ± 0.4 nm). The same trend, namely, initial narrowing of the size distribution and no significant change in particle size even as the heating was continued for 24 h, was observed when the heating temperature was raised to 90 °C. The representative TEM images at 1, 3, 9, and 24 h and

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RESULTS AND DISCUSSION The representative TEM image (Figure S1a) and the particle size distribution (Figure S1b) clearly reveal that the as-prepared Au NP dispersion is characterized by a broad size distribution. 10145

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Figure 3. TEM micrographs and respective particle size histograms of nanoparticle system obtained at 150 °C for (a−c) 1, (d−f) 3, and (g−i) 5 h. The particle size distributions obtained are 9.5 ± 0.5 nm at 1 h, 10.3 ± 1.6 nm at 3 h, and 9.6 ± 2.1 nm at 5 h. More images can be found in the Supporting Information (Figure S4).

absorbance spectrum was observed even after 24 h of heating. Increasing the temperature to 90 °C does not bring about any significant difference, and here also the spectra obtained after 1 and 24 h of heating almost overlap with those obtained at 60 °C (Figure 5a). This indicates a negligible effect of heating time on particle size with 60 and 90 °C heating. This is in good agreement with the TEM results (vide supra). The UV−vis spectra obtained at different time intervals of heating at 120 °C are very different from those obtained at 60 and 90 °C (Figure 5b). Here, the absorbance intensity increases continuously and significantly up to 7 h of heating. The intensity increase is not so significant at further time intervals. This indicates the growth process to be faster initially, which gets slowed down with time. As the heating temperature is further increased to 150 and 180 °C, the intensity increases significantly within 1 h of heating. However, for longer heating times (5 h at 150 °C and 2 h at 180 °C) a decrease in peak intensity observed with a concomitant broadening of the peak indicates the aggregation of NPs (Figure 5c,d). As mentioned in the Experimental Section, we employed the method developed by Haiss et al. to determine the particle sizes from UV−vis spectra,22 and the

corresponding size distribution histograms at both 60 and at 90 °C are shown in Figures 1 and S2. An increase in temperature to 120 °C changed the scenario completely, where slightly larger particles (7.0 ± 0.7 nm) were obtained compared to those obtained at 60 or 90 °C (∼5 nm) after 1 h of heating. These particles continued to grow with time and reached a size of ∼10.6 ±1.5 nm after 9 h (Figures 2 and S3). At 150 °C, monodisperse particles of ∼9.5 ± 0.5 nm (obtained after 1 h of heating) grew to attain a mean size of ∼10.3 nm at 3 h with a concurrent increase in polydispersity. As the heating time was increased further to 5 h, the mean particle size remained almost the same albeit with an increase in polydispersity (Figures 3 and S4). Here, apart from the large particles, another population of small particles of size ∼7−8 nm could also be seen, which explains the increase in polydispersity. At 180 °C, the initial particle of size of ∼9 nm (obtained after 1 h of heating) grew at a much faster rate but attained a mean size of 9.3 nm (Figures 4 and S5). The UV−vis spectra also revealed interesting changes as a function of time and temperature. For the sample heated to 60 °C, a peak at 520 nm indicative of small Au NPs was observed after the sample was heated for 1 h. No change in the 10146

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Figure 4. TEM micrographs and respective particle size histograms of nanoparticle system obtained at 180 °C for (a−c) 1 and (d−f) 2 h. The particle size distributions obtained are 9 ± 0.6 nm at 1 h and 9.3 ± 2.2 nm at 2 h. More images can be found in the Supporting Information (Figure S5).

Figure 5. UV−vis spectra obtained for Au NP dispersions at (a) 60 and 90, (b) 120, (c) 150, and (d) at 180 °C for different time intervals.

temperatures (i.e., 60 and 90 °C) the as-prepared polydisperse system becomes nearly monodisperse within 1 h of heating and there is no size variation (∼5−6 nm) even after 24 h of heating. This supports the argument that at these temperatures a classical digestive ripening process is operative, where the

details of the analysis are provided in the Supporting Information. The particle size variations at different temperature and time intervals from TEM images and UV−vis spectral analysis are presented in Figure 6. It is evident from the figure that at lower 10147

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Figure 6. Particle size vs time graph at different temperatures from (a) TEM and (b) UV−vis spectroscopy.

nm are obtained during the initial period (1 h) of reaction. As the heating at 120 °C continued to 3 h and beyond, the initial monodisperse system (obtained in 1 h) was converted to a polydisperse system with a mean size increment of 1.2 nm and an increase in the standard deviation. We also observed particle coalescence on TEM grid. Simultaneously, a bimodal size distribution of the particles could be seen (Figure 2e). The coalescence and bimodal size distribution could be ascribed to Ostwald ripening or aggregative growth. The former involves the growth of larger particles at the expense of smaller ones.26−28 This growth is predicted to be driven by the sizedependent solubility difference between small and large particles. Another mechanism proposed for the particle size growth involves interparticle aggregative coalescence. The aggregative growth process is normally accompanied by the bimodal size distribution, the presence of polycrystalline particles, and sigmoidal growth kinetics.9 The direct observation of particle coalescence at 3, 5, 7, and 9 h over the TEM grid (Figure S6) strongly suggests the aggregative growth mechanism as one of the reasons for particle size growth with time at these temperatures. Though aggregative growth is evident, it was seen that the particle growth kinetics did not follow a sigmoidal profile but rather a linear profile (Figure S7). This suggests that aggregative growth may not be the sole cause for the increase in nanoparticle size. The linear relationship between particle volume vs time predicts the involvement of Ostwald ripening as asserted by Lifshitz, Slyozov, and Wagner (LSW) model.29 Therefore, we can say that when NPs were heated in tbt at 120 °C, the monodisperse particles (obtained at 1 h) grew via interparticle coalescence. This in turn induced polydispersity in the system, which synergistically drove the NP growth process through both Ostwald ripening and particle coalescence. Defects in NP have been inferred to be the major contributor to the growth process. But the HRTEM images provided in Figure S8 indicate the single-crystalline nature of Au NPs. This rules out the possibility of particle growth induced by the migration of atoms into the defects. In a later stage (i.e., after 3 h), a few particles with defects are also found, but these are fewer in number and may be the result of aggregative growth as explained in the literature.9 Going higher on the temperature scale (i.e 150 °C), we found that the NP growth followed the same trend. Monodisperse particles of ∼9.5 ± 0.5 nm (obtained at 1 h) grew by interparticle coalescence and attained a mean size of ∼10.3 nm with a concurrent increase in polydispersity. With increasing time, the mean particle size exhibited a reduction

etching of surface atoms by the HDT and the redeposition of these atoms on other NP surfaces have been credited to the uniformity of the particle size. We propose that at such low temperatures NPs do not gain sufficient energy to produce the effective collision needed for aggregative growth or for surface atom diffusion (needed for Ostwald ripening). The particle size variation at 120 °C can be explained by invoking arguments similar to those used in classical nucleation theory (CNT).23,24 CNT, stemming from the pioneering work of Gibbs, provided a rationalized description of the formation of a solid phase (nucleation) from a supersaturated solution. It states that the stability of the crystal nuclei on reaching a particular size (critical size) can take the system over a freeenergy barrier. This free-energy barrier results from the competition between favorable changes in the volume excess free energy (reduction in free energy when solute particles segregate) and unfavorable changes in the surface free energy (energy needed to create an additional solid/liquid interface), which is expressed as 4 ΔG = − πr 3kBT ln S + 4πr 2γ V

(1)

where V is the molecular volume, r is the radius of the nuclei formed, kB is the Boltzmann constant, T is the temperature, and γ is the surface free energy per unit area. As the two terms on the right-hand side of eq 1 are of opposite signs and have different dependences on r, G passes though a maximum. This maximum corresponds to a critical nucleus r* and is given by r* =

2Vγ 3kBT ln S

(2)

And the free energy corresponding to critical radius r* is ΔG* =

16πγ 3V 2 3(kBT ln S)2

(3)

These equations show the inverse relationship of supersaturation with both r* and G*. With increasing temperature, supersaturation decreases, leading to increases in both r* and G*.25 Thus, the size of the critical nucleus increases with temperature. We propose that the ∼5 nm particles, which are present in the as-prepared system, are probably smaller than a critical size specific to 120 °C, the temperature to which the dispersion is being heated. Therefore, when they are heated to 120 °C, they get dissociated and these dissociated atoms and clusters get attached to the stable nuclei, leading to an increase in particle size. Thus, at this temperature larger particles of ∼7 10148

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Langmuir with a simultaneous enhancement in polydispersity unlike the trend observed at 120 °C (Figure 3). This possibly happens because of the continued growth of some particles, when others start to disintegrate. This phenomenon happened at a much faster rate at 150 °C than at 120 °C. A considerable number of particles having a size of less than 9.5 nm (starting size) were observed on the TEM grid, which confirmed the persistence of the Ostwald ripening process. The coalescence of these smaller particles with each other and with larger particles was also observed simultaneously (Figure S9). At a much higher temperature (i.e., at 180 °C), the rate of particle coalescence and diffusion seems to be much faster than that observed at 150 or 120 °C. Here, the initial monodisperse system of particle size of ∼9 nm was converted to a bimodal system within a very short time period (2 h). The resultant bimodal system had one set of particles with a size of 6−8 nm, smaller than 9 nm. This type of bimodal distribution of particles, where one set of particles is smaller than the starting size, proved Ostwald ripening as one of the growth mechanisms (Figures 4d,e and S5e−h), and the coalescence of particles (Figure S10) confirms that the aggregative process is present as well. If we look into the particle size variation graph with time at different temperatures, it seems that in all cases the growth process became sluggish or almost stopped when particles attained a size of ∼9−10 nm (Figure 6). So, this size can be considered to be the most stable size for the Au-HDT system. In conclusion, this present study furnishes new insights into the Au NP growth mechanism. The digestive ripening process leads to the conversion of a polydisperse colloid to a nearly monodisperse colloid at low temperature (60 and 90 °C) and higher temperature (120, 150 and 180 °C) within a short time period. The findings indicate that the initial particle size is controlled by a free-energy barrier resulting from the competing surface and volume free energies, which increase with temperature in a manner similar to the arguments invoked in classical nucleation theory. At low temperatures, the particle size remains unaffected by time, whereas at higher temperature upon prolonged heating particle growth is initiated by the process of interparticle coalescence. This induces polydispersity, causing both interparticle aggregation and Ostwald ripening, which operate in a combined manner, leading to the growth of NPs. It was observed that with increasing temperature, Ostwald ripening becomes more pronounced within a short time. Finally, this study provides an easy means to tune the Au NP size by simply controlling the temperature and time.

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ACKNOWLEDGMENTS

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REFERENCES

P.S. thanks the Council of Scientific and Industrial Research for a fellowship. We gratefully acknowledge CSIR, New Delhi for financial support through XII five year plan network project BSC0112 (Nano-SHE). We also acknowledge the Indo-US Science and Technology Forum (IUSSTF) for support through the Joint Virtual Centre “From Fundamentals To Applications of Nanoparticle Assemblies”.

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ASSOCIATED CONTENT

S Supporting Information *

NP size determination from UV−vis spectra. TEM images of nanoparticle systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-20-25902013. Fax: +91-20-25902636. Notes

The authors declare no competing financial interest. 10149

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