Heating or Cooling: Temperature Effects on the Synthesis of

Nov 25, 2016 - Hence, the good stability of Au25(SR)18 at 40 °C can be explained as the long half-life of the Au25(SR)18 transformation, which is ∼...
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Heating or Cooling: Temperature Effects on the Synthesis of Atomically Precise Gold Nanoclusters Tiankai Chen, Qiaofeng Yao, Xun Yuan, Ricca Rahman Nasaruddin, and Jianping Xie* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 S Supporting Information *

ABSTRACT: Developing an efficient, well-controlled synthesis strategy for gold nanoclusters (Au NCs) is crucial for delivering their expected applications in many fields; and such development requires fundamental understandings on the synthetic chemistry. The synthesis of Au NCs typically consists of a pair of reversible reactions: a fast reduction-growth reaction and a slow size-focusing reaction. Here we demonstrate that the above two reactions can be well-balanced while accelerated in a heated synthesis protocol, thus providing an efficient and scalable synthesis method to obtain thermodynamically favorable Au25(SR)18 NCs (SR denotes thiolate ligand) with high yield (>95% on gold atom basis) and fast kinetics. By investigating the Au NC formation behavior at different temperature, we identified the endothermic nature of the reductive formation of Au25(SR)18 NCs from Au(I)-thiolate complex precursors. More interestingly, if overheated, after the formation of Au25(SR)18, there exists an irreversible first-order reaction, which could transform Au25(SR)18 into Au NCs of mixed sizes. As a result, 40 °C is identified as the optimal temperature to synthesize Au25(SR)18 in aqueous solution, as the half-life of the transformation reaction (67.8 h) is much longer than the time needed to obtain high yield Au25(SR)18. The detailed understandings on the temperature effects of Au NC synthesis would facilitate the development of efficient synthesis strategies for atomically precise Au NCs with predesigned size, composition and structure.

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INTRODUCTION Gold nanoparticles (Au NPs) are of recent interest to heterogeneous researchers from various research communities, as these materials are finding increasing acceptance in many fields such as biomedical science,1−4 catalysis, 5−7 and sensing.8−10 These applications have significantly increased the value of gold as functional materials in addition to their current wealth storage function. More recently, thiolateprotected gold nanoclusters or thiolated Au NCs have emerged as a new type of Au nanomaterials because of their unique physical and chemical properties (from their ultrasmall core size, typically below 2 nm).11 These Au NCs contain several to a hundred gold atoms in the core, with a certain number of thiolate ligands on the surface. Therefore, thiolated Au NCs can be denoted as Aun(SR)m, where n and m are integers representing the number of gold atoms and thiolate ligands in one cluster, respectively.12,13 Due to the strong quantum confinement effects in the sub-2 nm size regime, thiolated Au NCs have discrete electronic states and exhibit some unique molecular-like properties.14,15 For example, Au NCs show quantized charging,16,17 molecular chirality,18,19 and strong photoluminescence,20−22 which are not observed in Au NPs with a particle size above 2 nm. As these properties are highly dependent on the composition of the Au NCs (the value of n and m), it is important to precisely control the Au NC © 2016 American Chemical Society

composition at atomic level. In the past few years, there are many successful attempts in the synthesis of atomically precise Au NCs.23,24 Successful examples include synthesis of Au25(SR)18, Au68(SR)34, Au102(SR)44, and many others.25−34 One of the most popular ways to prepare atomically precise Au NCs is reductive decomposition of Au(I)-thiolate complexes.35−37 In this method, the formation of atomically precise Au NCs typically involves two stages: a fast reductiongrowth stage followed by a slow size-focusing stage.38 These two stages are quite different in the reaction nature. The first stage is the reduction from the Au(I)-thiolate complexes with the help of reducing agent to form mix-sized Au NCs. This process is similar to the rapid formation of small Au NPs in Brust method, in which the Au(I)-thiolate complexes could directly react with the reducing agent, sodium borohydride (NaBH4), to form Au NPs.39,40 It is well demonstrated that cooling the reaction solution or reducing agent (at low temperature, e.g., 0 °C) is a necessary yet efficient step to control the size distribution of the Au NPs.41 For example, in Special Issue: ISSPIC XVIII: International Symposium on Small Particles and Inorganic Clusters 2016 Received: October 28, 2016 Revised: November 24, 2016 Published: November 25, 2016 10743

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carried out at room temperature and both the reduction-growth reaction and size-focusing reaction can occur simultaneously in aqueous solution, but both reactions might not proceed at the optimal temperature. Increasing the reaction temperature could further improve this process, facilitating both the reductiongrowth reaction and size-focusing reaction. To the best of our knowledge, direct synthesis of atomically precise Au NCs in a one-pot manner and at an elevated temperature is rarely reported. We are particularly interested in investigating the effects of elevated temperature on Au NC synthesis not only because the elevated temperature will not compromise the sizefocusing reaction (instead it will promote this reaction), but also because a relatively higher temperature would promote most of the chemical reactions, shortening the reaction time to achieve the desirable product. Hence, it is important to study how the above two reactions behave and whether they can stay in a comparable time scale for Au NC production when the reaction solution is heated. In addition, some side reactions such as NC degradation, or size transformation might be initiated when the reaction solution is heated,55 and the detailed understandings of these reactions at different reaction temperature can help design an effective synthesis protocol for atomically precise Au NCs. In this paper, we aim to understand the temperature effects on the synthesis of atomically precise Au NCs. We employ the NaOH-mediated reaction environment and report the successful synthesis of Au25(SR)18 at an elevated reaction temperature. Both the reduction-growth and size-focusing reactions are accelerated at the elevated temperature (40 °C), and they were maintained in a comparable time scale, enabling the rapid formation of Au25(SR)18 in a short time period of 0.5 h. The formation of Au 25 (SR) 18 from Au(I)−thiolate complexes is identified as an endothermic reaction according to the temperature-dependent study, and heating could promote this reaction. In addition, when the reaction temperature is above 40 °C, the as-formed Au25(SR)18 will be further transformed into mix-sized Au NCs via a first-order, irreversible reaction in aqueous solution, suggesting 40 °C to be an optimal temperature for achieving high purity Au25(SR)18 at high yield (>95% on gold atom basis). Presented below are the details of this investigation.

the original report by Brust et al., the reaction was proceeded at −18 °C to yield Au NPs in the size range of 1 to 3 nm.39 Similarly, an ice-cold NaBH4 solution was introduced into an aqueous HAuCl4 solution to prepare 1.5−4 nm Au NPs, which are widely used as small seeds for the growth of Au nanorods.42,43 The effectiveness of cooling (or so-called “cooled down” step) is due to the fact that the reducing power of the reducing agent can be tamed down at low temperature, thus creating a mild reducing environment, which could facilitate the formation of small Au NPs. A similar strategy was also used to synthesize ultrasmall thiolated Au NCs, where a series of glutathione-protected Aun NCs were formed by the addition of a cold NaBH4 solution (0 °C).44 The second stage is size-focusing to achieve atomically precise Au NCs in the reaction solution. This size evolution process is similar to the digestive or Ostwald ripening in the synthesis of monodisperse Au NPs. Digestive ripening is one efficient route to obtain monodisperse Au NPs, in which polydisperse Au NPs are converted into monodisperse one(s) owing to their different surface energy.45,46 In particular, the lattice stabilization energy is reduced when large Au NPs are etched into small Au NPs in the presence of digestive ripening agents (e.g., excess thiolate ligands). Simultaneously, the surface energy is reduced when the small Au NPs are growing into larger ones. Hence, this is a thermodynamic-driven process for the formation of the most thermodynamically favorable Au NPs with a certain size. A digestive ripening process typically requires energy input to facilitate the reaction, and heating is the most efficient energy input to promote this process to obtain monodisperse Au NPs.47−49 Similar as digestive ripening, in the synthesis of Au NCs, size focusing is an efficient way to achieve Au NCs at atomic precision (often a thermodynamically favorable NC species). Similarly, heating the reaction solution (or so-called “heated up” step) could also facilitate the size-focusing process, which could efficiently promote the transformation of mix-sized Au NCs into the thermodynamically favorable size under the principle of “survival of the robustness”.50 For example, atomically precise Au25(SR)18 can be obtained after size-focusing of mix-sized Aun NCs (n = 10−39) at 55 °C.51 In this size-focusing process, Au NCs with core size n < 25 were etched into Au(I)-thiolate complexes, while the Au NCs with core size n > 25 were etched into Au25(SR)18, indicating Au25(SR)18 is the thermodynamically stable product. Guided by this thermodynamic selection principle, a number of two-step synthesis methods have been developed to prepare atomically precise Au NCs, which typically involves a “cooled down” stage followed by a “heated up” stage.52,53 The next question we may ask is whether we could integrate the “cooled down” and “heated up” processes into a one-pot synthesis protocol. This is because a simple and rapid one-pot synthesis is desirable for a large-scale production of atomically precise Au NCs (with a quantity sufficient for practical applications), due to its operation simplicity and relatively high yield. There are many successful attempts in developing one-pot synthesis strategies for atomically precise Au NCs. For example, in order to balance the fast reduction reaction and the slow size-focusing reaction in one reaction solution, a NaOHmediated synthesis was recently developed to prepare Au25(SR)18 in aqueous solution.54 In this method, the addition of NaOH could decrease the reducing ability of NaBH4 and increase the etching ability of thiolate ligands, enabling a wellbalanced reaction for Au25(SR)18 formation. This reaction is

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EXPERIMENTAL SECTION Chemicals. Hydrogen tetrachloroaurate hydrate (HAuCl4· 3H2O), 6-mercaptohexanoic acid (MHA) and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH) was purchased from Merck. Ultrapure water with a specific resistivity of 18.2 MΩ was used throughout the experiment. All glassware were washed with aqua regia (HCl: HNO3, volume ratio =3:1) and rinsed with ethanol and copious ultrapure water. (Caution: Aqua Regia is a very corrosive oxidizing agent, which should be handled with great care.) All chemicals were used as received. Synthesis of Gold Nanoclusters (Au NCs) at Elevated Temperature. In a typical synthesis, in a 500 mL roundbottom flask, 50 mL of 5 mM MHA aqueous solution and 6.25 mL of 20 mM HAuCl4 solution were mixed in 58.75 mL of water under 500 rpm stirring. After that, 7.5 mL of 1 M NaOH solution was added to the solution to regulate the formation of Au(I)−thiolate complexes. The mixture was then allowed to react in a 40 °C water bath for 30 min. After that, 2.5 mL of NaBH4 solution (prepared by dissolving 0.046 g of NaBH4 in 10 mL of 0.2 M NaOH solution) was added to the solution. 10744

DOI: 10.1021/acs.jpcc.6b10847 J. Phys. Chem. C 2017, 121, 10743−10751

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Figure 1. (a) Time-evolution UV−vis absorption spectra of the Au NCs synthesized at 40 °C. The inset is a digital photo of the Au NC solution after 24 h reaction in a 500 mL round-bottom flask. (b−d) ESI-MS of the Au NCs obtained at 40 °C after 24 h reaction. The zoom-in spectrum in (c) shows peaks of [Au25(MHA)18 − (x + 5) H + x Na]6− (x = 1−8). The red lines in (d) are the simulated isotope patterns with x = 4.

Figure 2. UV−vis absorption spectra of the Au NCs synthesized at (a) 22 °C, (b) 30 °C and (c) 40 °C after 24 h reaction. Insets are the digital photos of the Au NCs synthesized at the corresponding temperature. (d) Plot of absorbance at 670 nm (OD670) against time at different reaction temperature, indicating heating the reaction solution has promoted the formation kinetics, as well as the steady state yield of Au25(SR)18 in the synthesis.

ultrafiltration filter. The synthesis process was repeated in a water bath at different temperature (22, 30, 50, and 60 °C). The Au NCs synthesized at 40 °C (for 24 h) were used to

After the addition of NaBH4, the mixture was reacted in the same water bath for 24 h. The Au NC product was collected and purified using a 5 kDa molecular weight cutoff (MWCO) 10745

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ESI-MS results together, we can confirm the Au NCs synthesized at 40 °C were [Au25(MHA)18]− with high purity. The synthesis of Au NCs was then repeated in a water bath at different temperature (22, 30, 40, 50, and 60 °C). As shown in Figure 2a−c, the UV−vis absorption spectra of Au NCs synthesized at a lower temperature (below 40 °C) were similar to that of the Au NCs synthesized at 40 °C (Figure 1a), although the intensities of the absorption peaks were different, suggesting the formation of the same Au NC species − Au25(SR)18. Figures S1 and S2 present the formation processes of Au25(SR)18 at 22 and 30 °C, respectively, which were monitored by UV−vis absorption spectroscopy. Compared with the formation of Au25(SR)18 at 40 °C, where, within half an hour, well-defined Au25(SR)18 absorption spectrum could be observed, the appearance of the characteristic absorption peaks of Au25(SR)18 at 22 or 30 °C was much slower. It took at least 3 h to observe the well-defined absorption spectrum of Au25(SR)18. To trace the growth of Au25(SR)18, we monitored the absorbance of the NCs at 670 nm (i.e., the optical density at 670 nm or OD670) with the reaction progressed (Figure 2d). The absorbance at 670 nm is a strong indicator of the formation of Au25(SR)18, and its intensity is proportional to the concentration of Au25(SR)18.44 Within the first 3 h, the conversion of Au25(SR)18 increased ∼20% if the reaction solution was heated to 40 °C, compared with the conversion at 22 or 30 °C (e.g., OD670 at t = 3 h was 0.222 and 0.267 for 20 and 40 °C, respectively), suggesting a faster formation kinetics of Au25(SR)18 at a higher temperature. Besides the formation kinetics, the yield of Au25(SR)18 at steady state also changed with the increase of reaction temperature. In particular, the yield of Au25(SR)18 increased with temperature increasing, as the plateau absorbance at 670 nm increased with temperature. When the reaction solution was heated to 30 °C, OD670 was increased ∼5% compared with that at 22 °C (from 0.333 to 0.349). The increase in OD670 could reach as high as 15% if the reaction solution was further heated up to 40 °C (from 0.333 to 0.384). Moreover, the digital photos in Figure 2a−c (insets) showed the Au NC solution getting darker with the temperature increasing, also suggesting that a higher yield of Au NCs was achieved at a higher temperature. These observations are consistent with the ICP-OES measurement of the Au25(SR)18 concentration in the product, where the yields of Au25(SR)18 at 22, 30, and 40 °C were 82.2%, 86.6% and 95.1%, respectively. Taken together, we can conclude that the formation reaction of Au25(SR)18 (Reaction I) has shifted to the right with temperature increasing, indicating the endothermic nature of Reaction I. At the reaction temperature of 40 °C, the rapid formation of Au25(SR)18 within 0.5 h suggests an accelerated reductiongrowth reaction and size-focusing reaction, which are also wellbalanced in a comparable time scale. Although the Au NC mixture initially formed might be more widely distributed in size due to the stronger reducing power of the reducing agent at a higher temperature, Au25(SR)18 is still the only Au NC species after the size-focusing process. The higher likelihood and faster rate of size-focusing when increasing reaction temperature makes possible the conversion of mix-sized Au NCs into thermodynamically stable Au25(SR)18 in a shorter time period. As a result, the initial formation time of Au25(SR)18 can be shortened to 0.5 h. With the reaction proceeds, more of the Au(I)-thiolate complexes can be reduced and almost immediately transforms into Au25(SR)18. By comparison, at 22 and 30 °C, before the third hour of reaction, mix-sized Au NCs

study the transformation process of Au NCs. In particular, 5 mL of the Au NC solution was taken and heated in a water bath at different temperature (40, 60, 65, and 70 °C) for another 24 h. The temperature variance was within ±1 °C throughout the reaction process. Material Characterization. UV−vis absorption spectra were recorded on a Shimadzu UV-1800 UV−vis spectrometer. Electrospray ionization mass spectra (ESI-MS) were taken on a Bruker MicroTOF-Q ESI time-of-flight system operated at the negative ion mode. The samples were directly injected into the chamber at a flow rate of 180 μL·h−1. The instrument parameters were set as capillary voltage, 4 kV; nebulizer, 0.4 bar; dry gas, 4 L·min−1 at 120 °C; and m/z range 50−6000.

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RESULTS AND DISCUSSION The synthesis of Au NCs was conducted at an elevated temperature (e.g., 40 °C) according to a reported protocol with some modifications, as shown in Scheme 1.54 As a proof-ofconcept, 6-mercaptohexazonic acid (MHA) was chosen as the model thiolate ligand. UV−vis absorption spectroscopy was used to monitor the progress of the reaction, where the time for NaBH4 addition was set as 0 h. As shown in Figure 1a, within 0.5 h reaction at 40 °C, three distinct absorption peaks at 440, 670, and 760 nm could be observed, matching well with the characteristic absorption peaks of Au25(SR)18.56,57 The welldefined absorption peaks suggested the high-purity of the Au25(SR)18 in the product formed in 0.5 h. As the reaction proceeded, the intensity of these absorption peaks increased while their positions remained unchanged until t = 24 h, suggesting the accumulation of Au25(SR)18 in the reaction solution during this time period (Reaction I in Scheme 1 shifts to right). Further elongating the reaction time (t > 24 h) would not cause obvious changes in the UV−vis absorption spectra of the reaction solution, indicating the reaction reached the steady state at 24 h. At steady state, a typical yield for Au25(SR)18 is ∼95.1% on the Au atom basis, as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). In addition, the synthesis of Au25(SR)18 can be easily scaled up. For example, > 30 mg of Au25(SR)18 can be produced in one batch, as shown in a digital photo (inset of Figure 1a) of Au25(SR)18 synthesized in a 500 mL round-bottom flask. The composition of the Au NC product obtained at 40 °C heating was further determined by electrospray ionization mass spectroscopy (ESI-MS) in negative ion mode, as shown in Figure 1b−d. Only two sets of peaks at ∼1276 and ∼1549 were observed in the m/z range between 1200 and 3000. The zoomin spectrum (Figure 2c) of the peaks at ∼1276 reveals that they were comprised of several bunches of peaks spaced 3.67 apart. Further zooming in the spectrum (Figure 2d), we found the isotope peak spacing was 0.17 (= 1/6), indicating the ionized Au NCs carried six negative charges. Hence, the molecular weight of the Au NCs was determined to be 7656 Da (1276 × 6). The formula can then be assigned as [Au25(MHA)18 − 9 H + 4 Na]6−, which was also supported by the perfect match between the experimental (black line) and simulated (red line) isotope patterns. The peak bunches spaced 3.67 apart in Figure 2c had a mass difference of ∼22 (3.67 × 6), which was attributed to the addition of a Na+ and the removal of a H+. Hence, these bunches of peaks can be assigned to a general formula of [Au25(MHA)18 − (x + 5) H + x Na]6−, where x is an integer from 1 to 8. Similarly, the peaks at m/z ∼ 1549 can be assigned to [Au25(MHA)18 − (y + 4) H + y Na]5−, where y is an integer from 4 to 14. Taking the UV−vis absorption and 10746

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Figure 3. Time-evolution UV−vis absorption spectra of the synthesis of Au NCs at 60 °C. The reaction process can be divided into two stages: (a) the growth of Au25(SR)18 within the first 3 h of reaction; and (b) the as-formed Au25(SR)18 transforming into mix-sized NCs after 3 h of reaction. Inset in (b) is the digital photo of the Au NCs after 24 h of reaction at 60 °C.

Figure 4. UV−vis absorption spectra of Au NC mixture synthesized at 60 °C for 24 h, followed by incubation at (a) 22 °C and (b) 40 °C for another 24 h.

solution. The featureless spectrum and the brownish color of the product after 24 h reaction (Figure 3b) suggest the degradation products were most probably Au NCs of mixed sizes.58 Therefore, at 60 °C, Reaction II will take the place of Reaction I to be the dominant reaction in the system at t > 3 h. A similar trend for the first formation of Au25(SR)18 followed by a consumption was also observed at 50 °C, with the plateau OD670 occurring at t = 8 h and the spectrum gradually becoming featureless after that (Figure S3 and S4). Au25(SR)18 is considered as one of the most stable NC species due to its 8 e− close-shell electron configuration and highly symmetric atomic packing structure.59−61 Therefore, Reaction II is of research interest not only because it helps one understand and optimize the synthesis of Au NCs at elevated temperature, but also because the thermal degradation of Au25(SR)18 at high temperature may provide information on the thermal stability of Au NCs. In order to study the reversibility of the reaction, we first prepared the mix-sized Au NCs at 60 °C (t = 24 h), where Reaction II was considered to proceed to completion. After that, the solution was cooled down to either 22 or 40 °C, which is the temperature favoring the formation of Au25(SR)18 in the forward reaction. However, as shown in Figure 4, after 24 h of reaction at 22 or 40 °C, both UV−vis absorption spectra from the Au NCs are featureless. It is also worth noting that there was little change in the UV−vis absorption spectra compared with that of the Au NCs before cooling down (pink line in Figure 3b), indicating that the Au NC mixture were likely remaining unchanged in the cooling

were formed, but the majority of Au NCs were not transformed into Au25(SR)18. A good balance between the two reactions did not occur, until some Au(I)−thiolate complexes were consumed and some initially formed Au NC mixture were accumulated with reaction. Although elevating the reaction temperature will favor the reaction of Au25(SR)18 formation from precursors both thermodynamically and kinetically, overheating (e.g., at 60 °C) would evoke degradation reaction of Au25(SR)18. As shown in Figure 3a, when the reaction solution was heated to 60 °C, the well-defined absorption peaks at 440, 670, and 760 nm began to appear in a similar but even faster manner like that observed in the system of 40 °C (OD670 = 0.390 at 60 °C vs OD670 = 0.267 at 40 °C) within the first 3 h of reaction. This is expected as a higher temperature could accelerate the reaction kinetics. However, unexpectedly, after 3 h of reaction, the absorbance at 670 nm began to drop and the well-defined absorption spectrum of Au25(SR)18 gradually turned into a featureless spectrum as of 24 h. It should be noted that the maximum of the absorbance at 670 nm, which occurred at 3 h after NaBH4 addition, was of similar value to the plateau absorbance when the reaction was heated to 40 °C (0.390). Since Reaction I will shift to the product side with temperature increasing, Au25(SR)18 should have a higher theoretical conversion at 60 °C than 40 °C. Hence, the identical maximum yields of Au25(SR)18 at 40 and 60 °C suggest the existence of an additional reaction pathway, which could consume the asformed Au25(SR)18 (Reaction II in Scheme 1) in the reaction 10747

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Figure 5. (a) Time-evolution UV−vis absorption spectra of the transformation process from Au25(SR)18 (synthesized at 40 °C) to mix-sized Au NCs at 60 °C. (b) Plot of absorbance difference at 670 nm compared with the final absorbance (denoted as [A]) with the reaction time at 60 °C. (c) Linear fit of ln(−d[A]/dt) to ln([A]) to determine the order of the reaction. (d) Regression analysis from the integrated first-order reaction rate law: linear fit of ln([A]) to reaction time. (e) Arrhenius fit of 1000/T to ln(k), where T and k denote reaction temperature and the rate constant at the respective reaction temperature, respectively.

roughly the same as the consumption rate of Au25(SR)18 (Reaction II and reverse reaction of Reaction I). After 1 h, we could observe the intensity of the Au25(SR)18 peaks at 440 and 670 nm began to decrease and the absorption spectrum gradually turned into featureless, similar to that observed in the direct synthesis of Au NCs at 60 °C. In this stage, on one hand, Reaction II was the dominant reaction in the system as Au25(SR)18 exists in a large quantity. After 24 h heating at 60 °C, the majority of Au25(SR)18 were consumed and transformed into the Au NCs with mixed sizes. On the other hand, the formation of Au25(SR)18 from Reaction I became slower with the reaction proceeding as Au(I)-thiolate complexes were gradually consumed. As a net result, after the solution was kept at 60 °C for some time, the contribution from Reaction I was negligible in comparison to that of Reaction II, and it was reasonable to assume that the concentration change of

process. The cooling study suggests that Reaction II is an irreversible reaction, allowing us to further study the reaction kinetics. However, the origin of the irreversibility is still unclear at this moment. In order to study the kinetics of Reaction II, we tried to separate Reaction I and Reaction II. We first prepared Au25(SR)18 at 40 °C, allowing Reaction I to reach its steady state at t = 24 h. After that we immediately heated up the solution to 60 °C to initiate Reaction II. The reaction process was also monitored by UV−vis absorption spectrometry, as shown in Figure 5a. We found that the UV−vis absorption spectra remained almost unchanged in the first hour after the solution was heated to 60 °C, suggesting the total amount of Au25(SR)18 in the solution was constant in the first hour. This is because the formation rate of Au25(SR)18 (from Reaction I that favors the formation of Au25(SR)18 at a higher temperature) is 10748

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kinetics. The higher yield of Au25(SR)18 achieved at a higher temperature suggests the formation reaction of Au25(SR)18 from Au(I)-thiolate complexes is an endothermic reaction. In addition, we observed an irreversible degradation reaction from Au25(SR)18 to Au NCs of mixed sizes at a relatively high temperature (>40 °C). By investigating the kinetics of the degradation reaction at different temperature (60, 65, and 70 °C), we found the degradation reaction is most probably a firstorder reaction. The rate constants derived from the rate equations also fit well with the Arrhenius equation, with an activation energy of ∼72.67 kJ·mol−1 for this reaction. Hence, the good stability of Au25(SR)18 at 40 °C can be explained as the long half-life of the Au25(SR)18 transformation, which is ∼67.8 h as predicted from the Arrhenius equation. This study not only presents a fast and high-yield Au NC synthesis strategy, but also provides some physical chemistry insights into the Au NC synthesis and transformation reactions at elevated temperature, which will contribute to the understandings of the synthesis process of atomically precise metal NCs.

Au25(SR)18 after 3 h was predominantly caused by Reaction II. The change in OD670 came from the consumption of Au25(SR)18 together with the formation of final Au NC species (which is from the same reaction as Au25(SR)18 consumption), hence the concentration of Au25(SR)18 at a specific time was proportional to the difference between OD670 at that time and OD670 of the final product (the OD670 difference denoted as [A] hereafter). To determine the order of the reaction, we first used the differential method, in which we connected the data points in [A]-reaction time plot, as shown in Figure 5b. For an nth order reaction, the rate law is −d[A]/dt = k[A]n, where k is the rate constant. The logarithm form of this equation will be ln(−d[A]/dt) = ln k + n ln([A]). With the help of OriginPro 8.5 software, we were able to obtain (d[A]/dt) from the slope of the tangent line of the connected line in Figure 5b. We then plotted ln(−d[A]/dt) against ln([A]) and fitted the plots with linear regression (Figure 5c). It can be seen that the data points were mostly linearly distributed in Figure 5c, and R2 of the fitting was 0.997. The slope of the linear fit was 0.954 from Figure 5c, and it is equivalent to the order of the reaction n. It is close to a first-order reaction or a pseudo-first-order reaction if we simplify this reaction into an integer order. This was further proved from a linear fit with high R2 (0.999) using the integrated form of the first-order reaction rate law (ln([A]) against reaction time, as shown in Figure 5d. The slope derived from the linear fit, which was identical to the addictive inverse of the reaction rate constant, was −0.0549. Hence, the rate constant of this first-order (or pseudo-first-order) reaction is k60 = 0.0549 h−1, and the half-life (t1/2 = ln2/k) is 12.6 h. Using a similar strategy, we also studied the transformation process of Au25(SR)18 at 65 and 70 °C, as presented in Figures S5 and S6, respectively. From the linear fit of ln(-d[A]/dt) to ln([A]) in Figure S5c and S6c, the orders of the reaction at 65 and 70 °C were 0.833 and 1.110, respectively. The order of both reactions was also close to 1, with a high R2 in the linear fit of their ln([A]) against reaction time (Figure S5d and S6d). The firstorder (or pseudo-first-order) rate constants for 65 and 70 °C reactions were k65 = 0.0808 h−1 and k70 = 0.118 h−1, respectively, and the half-life was shortened to 8.58 and 5.87 h, respectively. With the experimental k60, k65 and k70 values, we were able to plot the Arrhenius curve ln(k) = −EA/(RT) + ln(A), where EA, R, and A are activation energy, universal gas constant and pre-exponential factor, respectively. As shown in Figure 5e, a good linear relationship was observed between ln(k) and 1/T: ln(k) = −8.740 × 103/T + 23.34 (the units were omitted for clarity). Hence, the activation energy of the reaction (EA = 8.740 × 103 × R) was 72.67 kJ·mol−1. Using the Arrhenius equation, the first-order (or pseudo-first-order) rate constant and the half-life at 40 °C was determined to be 0.0102 h−1 and 67.8 h, respectively. This data also well explained why Au25(SR)18 remained almost intact at 40 °C in our previous experiments, as the half-life (67.8 h) of the product is much longer than the time window of our experiment (24 h).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10847. Time-evolution UV−vis absorption spectra of Au NC synthesis at 22, 30, and 50 °C, and kinetics analysis of Reaction II at 65 and 70 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianping Xie: 0000-0002-3254-5799 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Ministry of Education, Singapore, under the grant R279-000-481-112. T.C. acknowledges the National University of Singapore for his research scholarship.

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REFERENCES

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CONCLUSION In conclusion, we successfully demonstrated that a proper (and rational) heating of the reaction solution (e.g., to 40 °C) could simultaneously accelerate and well-balance the formation and the size-focusing of Au NC intermediates, making possible a fast and scalable one-pot synthesis of thermodynamically stable Au25(SR)18. The synthesis of Au NCs at an elevated temperature features a higher conversion and faster reaction 10749

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