Among all nanostructured materials, Au nanoparticles with sizes ranging from 1.5 to ≈100 nm are probably the most extensively studied systems as a result of their intriguing properties and fascinating applications.[1] The excellent chemical stability, biocompatibility, surface plasmon resonance effect, and unique catalytic activity associated with Au nanoparticles have enabled a broad range of applications in areas that include biomedical research,[2] electronics,[3] information storage,[4] and photovoltaic devices.[5] Notable examples include cancer diagnosis,[6] molecular ruler,[7] DNA sequence detection,[8] low-temperature catalysis for the conversion of CO to CO₂,[9] as well as thermal[10] and colormetric[11] sensing. Not surprisingly, these and other applications have fueled research related to the preparation of Au nanoparticles with controllable sizes, diverse morphologies, and assorted functionalities.[12]

The first synthesis of Au colloids was reported ≈150 years ago by Michael Faraday who used phosphorous to reduce AuCl₄⁻ ions.[13] Since then, many methods based on the reduction of Au^III salts have been developed by various researchers. For instance, Turkevich employed a mild reducing agent, sodium citrate, to form Au colloids in aqueous solution from AuCl₄⁻.[14] In 1994, another milestone was achieved in the synthesis of Au nanoparticles by the Brust group.[15] In their work, Au nanoparticles with relatively narrow polydispersities and controllable sizes were obtained by reducing AuCl₄⁻ with NaBH₄ in the presence of a capping ligand, dodecanethiol, and a phase transfer agent, tetraoctylammonium bromide. As elemental Au can be easily generated by reducing the salts of Au ions, many different reducing agents, including hydrazine,[16] amines,[17] alcohols,[18] polymers,[19] and even fungus,[20] in addition to the widely used hydrides, have proven to be effective for synthesizing Au nanoparticles. Additionally, various capping ligands, including alkanethiols,[15,21] alkylamines,[17,22] and polymers,[23] together with different preparation techniques, such as wet chemical synthesis,[15] radiolysis,[24] sonochemical approach,[25] and the UV irradiation method,[26] have all been extended to the synthesis of Au nanoparticles.

Although both Au^III and Au^I are common oxidation states of gold, precursors for the synthesis of Au nanoparticles are mainly based on Au^III derivatives, in particular, HAuCl₄. To date, Au^I compounds have rarely been explored as precursors for generating Au nanoparticles. However, it has been proposed that Au^I as an intermediate state may play a critical role in controlling the morphology of Au nanostructures synthesized from Au^III ions.[27] In addition, Liz-Marzán and co-workers have showed that Au^III may oxidize Au⁰ in the presence of cetyltrimethylammonium bromide (CTAB) to alter the morphology of the Au nanoparticles.[28] Therefore, understanding the reduction of Au^I into elemental Au would be helpful to achieve a better control of the size, shape, and crystallinity for the Au nanoparticles. It has been demonstrated that reduction of a Au^I alkylphosphine complex, [AuCl(PR₃)], by B₂H₆ could lead to the formation of Au₅₅ clusters.[29] In an attempt to make Au nanoparticles from Au^I species, Nakamoto and co-workers showed that thermolysis of Au^I complexes, such as [C₁₄H₂₉N(CH₃)₃][Au(SC₁₂H₂₅)₂] or [Au(C₁₃H₂₇COO)(PPh₃)] at 180 °C could yield Au nanoparticles with sizes around 26 nm.[30] Chaudret and co-workers also found that Au^I alkylamine complexes, derived from [AuCl(tetrahydrothiophene)] complex from its reaction with primary amines, could self-assemble on a substrate and form Au nanoparticles.[31] For both cases, however, control over the nanoparticle growth could hardly be achieved as the reaction conditions require either a high temperature or decomposition on a substrate. The special procedures required for preparing these complexes also limit their wide use as Au precursors. Most recently, Stucky and co-workers demonstrated that Au nanoparticles with relatively narrow polydispersity could be obtained from [AuCl(PPh₃)] complex by using tert-butylamine borane as a reductant.[32]

Au^I halides, such as AuCl and AuBr, are readily available and their rich chemistry has long been recognized.[33] Even though the low solubility in most solvents limited their use in the synthesis of Au nanoparticles, AuCl and AuBr can form soluble complexes with many ligands, such as alkenes,[34] alkylamines,[35] and alkylphosphines,[36] and these complexes in turn can serve as Au precursors. Both AuCl and AuBr exhibit relatively low stability under ambient conditions and can undergo disproportionation to form Au⁰ and Au^III species even at room temperature. The low stability of AuCl, AuBr, and their derivatives makes it possible to prepare Au nanoparticles under desirable mild reaction conditions without a reducing agent. The low reaction temperatures, the improved solubility of their complexes in various solvents, and the independence of any reducing agent would provide great flexibility for the preparation of Au nanostructures from Au^I halides for various applications. Herein, we report our investigation on the use of AuCl and AuBr as Au precursors to synthesize Au nanoparticles with alkylamine (either oleylamine or octadecylamine) as the coordinating agent and stabilizer. It is found that Au^I halides can be dissolved in many organic solvents including chloroform by forming a complex with oleylamine or octadecylamine. When heated to a relatively low temperature of 60 °C, the Au^I–alkylamine complexes gave rise to Au nanoparticles with a narrow size distribution.

Figure 1 a) TEM image of Au nanoparticles synthesized at 60 °C from AuCl and oleylamine in CHCl3. The concentration of AuCl and oleylamine, respectively, was 20 mm and 0.4 m. The average diameter of the particles was 12.7 nm with a relative narrow size (more ...)

Figure 2 compares the FTIR spectra for oleylamine, the complex formed from oleylamine and AuCl, and the Au nanoparticles. The spectrum of oleylamine is characterized by bands at = 3450 cm⁻¹ for the N–H stretch mode, = 1640 cm⁻¹ for the C=C stretch mode, and = 1380 cm⁻¹ for the CH₂ bending mode, in addition to the typical C–H stretch near = 2850−2960 cm⁻¹. When the oleylamine was mixed with AuCl in chloroform and a [AuCl(oleylamine)] complex had formed, the spectrum was nearly identical to that of the oleylamine, except that the C=C stretch mode was shifted to = 1590 cm⁻¹ from the original = 1640 cm⁻¹ for oleylamine. This shift could be attributed to coordination between the C=C bond of oleylamine and AuCl. Generally, the complexes formed from AuCl and alkenes are not stable and they may rapidly decompose at room temperature. The decomposition of [AuCl(oleylamine)] complex at 60 °C, however, did not complete in 24 h. This additional stability might be provided by the amine group through the formation of a N–H···Cl hydrogen bond.[35] Upon heating, the [AuCl(oleylamine)] complex decomposed to generate Au nanoparticles. The C=C stretch for Au nanoparticles shifted back to 1640 cm⁻¹, at the same wavenumber as the C=C in oleylamine, indicating the disassociation of the C=C bond from the complex.

Figure 2 FTIR spectra of oleylamine, [AuCl(oleylamine)] complex, and Au nanoparticles capped with oleylamine. Note that the C=C stretch mode shifted from = 1640 cm−1 for oleylaimne to a lower frequency of = 1590 cm−1 for AuCl (more ...)

The formation of [AuCl(oleylamine)] complex was also confirmed by using MS analysis. The MS in the positive-ion mode for the mixture of AuCl and oleylamine in chloroform exhibited three peaks at m/z: 268.3 for oleylamine, 464.3 for [Au^I(oleylamine]⁺, and 500.3 for [AuCl(oleylamine)] before the reaction (Figure 3a). The combined peak intensity for [Au^I(oleylamine)]⁺ and [AuCl(oleylamine)] is one fourth of the peak intensity for oleylamine, which is consistent with the starting molar ratio of oleylamine to AuCl at 5:1 for this sample. Once the reaction started at 60 °C, the intensity of [Au^I(oleylamine)]⁺ and [AuCl(oleylamine)] decreased and another peak at m/z: 516.5 started to appear. After 8 h, the relative peak intensity for m/z: 464.3 decreased to about half of its original value, and the intensity for the peak at m/z: 516.5 became stronger than the peak of m/z: 464.3 for oleylamine and AuCl complex (Figure 3b). With the increase of reaction time, the intensity of the m/z: 464.3 peak continued to decrease, while the peak intensity at m/z: 516.5 continued to increase. When the reaction was stopped at 24 h, the peak at m/z: 464.3 nearly disappeared (Figure 3c).

Figure 3 Mass spectra for [AuCl(oleylamine)] complex before and after heating up to 60 °C in CHCl3. a) Once mixed, the AuCl and oleylamine formed a complex of [AuCl(oleylamine)], as confirmed by the peaks at m/z: 464 for [AuI(oleylamine)]+ and m/z: 500 (more ...)

The fragmentation spectrum for the peak at m/z: 516.5 was obtained in the positive mode (Figure 3d). The resultant spectrum shows two major peaks at m/z: 266.3 and 280.4, which can be attributed to [C₈H₁₇–CH=CH–(CH₂)₈–NH]⁺ and [C₈H₁₇–CH=CH–(CH₂)₈–NH-CH₂]⁺, respectively. Therefore, the peak at m/z: 516.5 can be assigned to molecular dioleylamine [C₈H₁₇–CH=CH–(CH₂)₈]₂–NH. Based on the experimental observation and spectroscopic analysis, we propose the following reaction mechanism for the formation of Au nanoparticles:

When the [AuCl(oleylamine)]complex was heated up to 60 °C in chloroform, a slow thermolysis occurred. The [AuCl(oleylamine)]complex decomposed to give elemental Au⁰ and the oleylamine in the complex condensates to form dioleylamine. The Au⁰ atoms nucleated and grew into nanoparticles stabilized by oleylamine and/or dioleylamine. The mechanism for the formation of dioleylamine remains unclear but it may be attributed to the strong noncovalent Au^I–Au^I interaction between two [AuCl(oleylamine)]complexes due to the hybridization and relativistic effect.[37,38] This so-called “aurophilic” interaction has been recognized as being responsible for the aggregation of Au^I–thiolate complexes,[39] and it has also been observed in Au^I–amine complexes.[35,38]

The reaction was closely monitored as the [AuCl(oleylamine)] complex decomposed and the formation of Au nanoparticles proceeded. Figure 4 shows the TEM images of the Au nanoparticles obtained at different reaction times. When the reaction was stopped at 3 h, very few Au nanoparticles were found. The size of the nanoparticles was ≈4.1 nm, much smaller than those obtained after 24 h. With the progress of the reaction, the size of the Au nanoparticles increased. At 6, 12, and 18 h, the average diameter of the Au nanoparticles became 5.8, 9.2, and 10.1 nm, respectively. The diameter reached 12.7 nm when the reaction was stopped at 24 h. When the reaction time was further prolonged, the size distribution decreased because of the involvement of Oswald ripening.

Figure 4 Morphological evolution of the Au nanoparticles obtained from 20 mm AuCl with 0.4 m oleylamine at different reaction times: a) t = 3 h, d = 4.1 nm; b) t = 6 h, d = 5.8 nm; c) t = 12 h, d = 9.2 nm; and d) t = 18 h, d = 10.1 nm. The size distribution decreased (more ...)

During the reaction process, the color of the solution changed gradually from clear to pink, indicating the slow decomposition of [AuCl(oleylamine)] and formation of Au nanoparticles. The UV/Vis spectra of the Au nanoparticles obtained at different reaction times are presented in Figure 5. As mentioned above, Au^I halide compounds are generally unstable and they tend to disproportionate to form Au⁰ and Au^III. However, disproportionation of [Au-Cl(oleylamine)] complex was not observed when it was heated up to 60 °C in chloroform, as no absorption band for an Au^III species, typically at λ = 330 nm, was observed during the progress of the reaction. The absorption maximum at λ = 530 nm, a peculiar characteristic of the localized surface plasmon resonance peak of Au colloids, started to appear after three hours of reaction and the intensity increased steadily with the progress of the reaction, consistent with the observation by TEM, showing both the size and the number of particles increased with the reaction time.

Figure 5 UV/Vis absorption spectra for the decomposition of AuCl in CHCl3 with the presence of oleylamine showing the intensity of the surface plasmon resonance peak gradually increases with reaction time.

To optimize the synthesis, reactions with various concentrations of AuCl and molar ratios of oleylamine to AuCl were investigated. Figure 6 shows the TEM images of the Au nanoparticles formed by varying the reaction conditions. It was found that a narrow size distribution (< 10 %) could only be achieved for the nanoparticles when the molar ratio of oleylamine to AuCl was above 20. Further increases of the molar ratio of oleylamine to AuCl to 30 did not change the size distribution. When the molar ratio is below 20, the nanoparticles exhibited polydispersity. For example, the sizes of the nanoparticles formed at an oleylamine/AuCl molar ratio of 5 and 10 showed the relative standard deviation of 28 and 18 %, respectively, much higher than 8 % for the nanoparticles obtained at an oleylamine to AuCl molar ratio of 20. Once formed, the Au nanoparticles are well protected by the oleylamine and they can be readily dispersed in many organic solvents including chloroform, hexane, and toluene, even after multiple washings.

Figure 6 TEM images of Au nanoparticles obtained from the reaction of AuCl with different concentrations in CHCl3 at 60 °C with oleylamine as the capping ligand. a) [AuCl] = 20 mm, oleylamine/AuCl 5:1; b) [AuCl] = 20 mm, oleylamine/AuCl 10:1; c) [AuCl] (more ...)

Figure 7 TEM images of Au nanoparticles obtained from the reaction of a) AuCl with octadecylamine and b) AuBr with oleylamine in CHCl3 at 60 °C. While Au nanoparticles made from 20 mm AuCl and 0.4 m oleylamine were 12 nm in size, the particles made from (more ...)

We have successfully developed a facile wet chemical method for synthesizing Au nanoparticles with narrow size distribution by using Au^I halide compounds. Both AuCl and AuBr have proven to be effective precursors for generating Au nanoparticles simply by thermolysis under mild reaction conditions without the need for any reducing agent. When coordinated with oleylamine, AuCl could decompose slowly at 60 °C in chloroform and controlled growth of Au nanoparticles was achieved. The average size of the Au nanoparticles was ≈12 nm in diameter with a low polydispersity of 8 % when the molar ratio of oleylamine to AuCl was 20. The UV/Vis and mass spectrometry measurements during the reaction process revealed a slow thermolysis of the [AuCl(oleylamine)] complex to form Au nanoparticles and dioleylamine. When octadecylamine instead of oleylamine was used as the capping ligand, much larger particles with sizes around 100 nm were obtained. Although both capping ligands are similar in terms of the length of the hydrocarbon chains and the basicity, the oleylamine could form a more stable complex with AuCl than octadecylamine because the coordination bond between AuCl and C=C is believed to be stronger than that between AuCl and RNH₂. The higher stability and thus slower decomposition of [AuCl(oleylamine)] than [AuCl(octadecylamine)] led to a better control over the size of Au nanoparticles in the presence of oleylamine. When AuBr was used as the Au precursor to decompose in chloroform in the presence of oleylamine, a mixture of nanospheres and nanobars with average aspect ratio of 1.28 was obtained, indicating that one-dimensional growth was promoted for the case of AuBr, possibly due to the effect of local oxidative etching caused by Br⁻. This one-step Au^I-based preparation method is independent of reducing agent and is expected to find applications that require mild reaction conditions.

This work was supported in part by a DirectorRs Pioneer Award from the NIH (1DPOD000798) and a research grant from the NSF (DMR-0451788). Y.X. is a Camille Dreyfus Teacher Scholar (2002−2007). X.L. is an INEST Postdoctoral Fellow supported by Philip Morris USA.