Ionic liquid-stabilized non-spherical gold nanofluids synthesized using a one-step method
© Zhang et al.; licensee Springer. 2012
Received: 9 September 2012
Accepted: 16 October 2012
Published: 23 October 2012
Ionic liquid (IL)-stabilized non-spherical gold nanofluids have been synthesized by a one-step method in aqueous solution. The whole reaction proceeded in room temperature. In the presence of amino-functionalized ionic liquids, gold nanofluids with long-wave surface plasmon resonance (SPR) absorption (>600 nm) could be obtained by adopting tannic acid as the reductant. The specific SPR absorption was related to the non-spherical gold nanoparticles including gold triangle, decahedra, and icosahedra nanocrystals. All the nanocrystals were observed by transmission electron microscopy. It was deduced that the formation of non-spherical gold nanofluids was related to the hydroxyls in tannic acid while IL acted as the synthesis template.
Heat exchange efficiency is an important property of working media used in heat transfer devices. It affects the economic attractiveness and general performance of the related devices. In the past several years, great efforts were made to improve the efficiency of the working media while nanofluids were explored. In the work of Choi et.al and Eastman et.al, nanofluids showed great enhancements in thermal conductivity when small amounts of metallic or other nanoparticles were dispersed in common heat transfer fluids. When the composition of the nanofluids was settled, the heat transfer efficiency greatly depended on the synthesis procedure.
The current synthesis methods can be generally divided into two types, namely, the two-step method and the one-step method[3, 4]. For the two-step method, nanoparticles are either synthesized or purchased first in the form of dry powders, and the nanofluid formulation process involves the proper separation of the aggregated dried particles into individual particles and keeping them from re-agglomeration under suitable ionic or surfactant conditions. It is a very complicated process, and the performance of the obtained nanofluid will be affected by several factors. In contrast, the one-step method requires less operation. Nanofluids are synthesized through physical or chemical reactions[5, 6]. The advantage of this process lies in the minimized nanoparticle agglomeration. The stability of nanofluids is guaranteed by proper surface functionalization without involving mechanical facilities. Since noble metal shows chemical and physical affinity with many species, the one-step method is usually adopted to fabricate noble metal nanofluids such as gold nanofluids (AuNFs). However, this approach suffers the problem of impurities. For example, residual reactants are generally left in the nanofluids because of incomplete reaction or stabilization. It is difficult to elucidate the nanoparticle effect without eliminating this impurity effect.
Ionic liquid (IL) has been widely studied due to its unique physicochemical properties such as negligible vapor pressure, non-flammability, high ionic conductivity, low toxicity, good solvent for organic and inorganic molecules, high thermal stability, and wide electrochemical window[7–9]. IL containing imidazole ring shows great applicable potential and can be used as the stabilizer of metal nanoparticles[10, 11]. The as-prepared nanofluids can be easily dispersed in aqueous solution, which implies a significant applicable value in thermodynamics. Imidazole IL can also be functionalized with different groups such as carboxyl and amido. Since both carboxyl and amido have a special affinity with noble metal, the functionalized IL-stabilized metal nanofluids have an improved stability[12–14]. For example, imidazole IL-stabilized AuNFs were synthesized in a one-phase and/or two-phase method and had a remarkable thermal conductivity[15–17].
Fundamental research and practical applications of AuNFs are now becoming attractive subjects due to their promising chemical, biological, and physical properties[18, 19]. Frens firstly offered a simple and effective method to synthesize spherical gold nanoparticles. Because the properties of gold nanomaterials were greatly influenced by their size and morphology, the synthesis of AuNFs of controllable shape was a general routine to prepare gold nanomaterials with valuable properties. Non-spherical AuNFs such as nanorods, nanoslices, nanoprisms, nanoflowers, and nanopolyhedrons could be obtained in different ways[27, 28]. However, the reported synthesis processes were very complicated. One or more special conditions such as temperature, pressure, organic solvent, and assisted technique were required.
In our previous work, we synthesized spherical AuNFs in a simple way. NaBH4 or trisodiumcitrate was used as the reductant. In the presence of IL, spherical gold nanoparticles were dispersed in aqueous solution. The average diameter of the spheres was from 3.5 to 100 nm. The surface plasmon resonance (SPR) absorption peaks of the prepared colloids were observed between 512 and 553 nm, which belonged to the typical absorption of spherical good nanoparticles with different sizes[30, 31]. Compared with trisodiumcitrate-stabilized gold nanoparticles, IL-stabilized AuNFs exhibited some advantages such as decrease in aggregation and increase in stability. Moreover, IL belonged to inert ionic compounds, which meant that it would be stable in aqueous solution without decomposition or phase transformation.
In this manuscript, non-spherical AuNFs are synthesized in common conditions. AuNFs consisted of various nanocrystals with different compositions and size can be synthesized by simply changing the amount of reductant during a one-step reacting process. IL is used as the stabilizer, which can improve the stability of AuNFs in aqueous solution. The non-special conditions such as room temperature, atmospheric pressure, and aqueous solution are available everywhere, which would make the nanofluids convenient for further usage.
Chemicals and solutions
Synthesis of AuNFs
All glassware used in the following procedures was cleaned in a bath of freshly prepared HNO3/HCl (1:3, v/v), rinsed thoroughly in purified water, and dried prior to use. A mixture of different volumes of TA solution (1%, w/w) and different volumes of stabilization reagent was added rapidly into the HAuCl4 solution (0.01%, w/w) with vigorous stirring. The total volume of the mixture should be settled as 50 mL. The mixture was stirred continuously during which time a color change from yellow to red was observed. When the color did not change anymore, AuNFs were well prepared and the dispersions were stored at 4°C for future use. Commonly, the whole reaction cost about 0.5 h.
Characterization of AuNFs
As-synthesized AuNFs were subsequently characterized by high-resolution transmission electron microscopy (HRTEM) (JEOL-2010, Jeol Ltd., Akishima, Tokyo, Japan) and UV-visible (vis) spectroscopy (8453 UV–vis spectrophotometer, Agilent Technologies, Inc., Santa Clara, CA, USA). When a nanocolloid sample was characterized by HRTEM, the liquid sample was dipped onto a copper grid. After the solvent of the sample evaporates from the grid, colloids remained and could be easily observed by microscopy. The obtained images showed a partial area of the copper grid. The HRTEM photomicrographs were further edited by Adobe Photoshop CS (Adobe Systems Inc., San Jose, CA, USA).
Results and discussion
Typical functionalized IL-stabilized AuNFs
Effect of different stabilizers
As described, the AuNFs were synthesized by a relatively rapid and simple reaction. In order to investigate the formation mechanism of multiple morphologies, reagents involved in the synthesis process were changed slightly. The SPR absorption was used to characterize the AuNFs macroscopically. The long-wave absorbance indicated the relative abundance of the non-spherical particles.
In conclusion, no non-spherical particles were formed when the synthesis process underwent without IL. When IL was used as the stabilizer, long-wave absorbance appeared. The absorbance was not related to bromine anion. The imidazole cation played an important role in the formation of non-spherical particles, and the amino IL displayed a better performance than the carboxyl IL. It was reported[29, 34] that the amino group could form the covalent bond (Au-N) with gold atom. The carboxyl functional group also had a conjugated interaction with gold atom[35, 36]. The covalent bond was a more powerful interaction. Therefore, [AEMIM]·Br could be stabilized in an efficient way and has made the non-spherical particles grow much easier.
Effect of the concentration of amino ILs
Effect of the concentration of reductant
It is clear that the SPR property of gold nanomaterial is related to its morphology and size of particles. For the regular gold nanomaterial, the general relationship between SPR property and geometric parameter was established theoretically and experimentally. Millstone and co-workers pointed out that the gold nanotriangles had SPR absorbance in the NIR area. Based on the side length of the triangle, the peak would be distributed from 800 nm (side length 60 nm) to 1,300 nm (side length 150 nm). Shankar and co-workers also reported that the SPR absorbance of gold nanotriangles started from 650 nm and increased with the wavelength. A maximum appeared near 1,300 nm. The absorbance in the long-wave area was very similar with the curves in Figure6. For the other crystals in AuNFs, there were also some research about its SPR property. It was reported that the SPR absorption of gold nanodecahedrons was greatly concerned with its size. The absorbance appeared from 700 to 800 nm, which was confirmed by the calculating results of discrete dipole approximation. The icosahedrons had a complicated tridimensional structure. Various factors would affect the growth of a perfect icosahedron crystal, which made the icosahedrons grow into a sphere easily. Therefore, its SPR absorption appeared very similar to that of the spherical gold nanoparticles.
According to the references and TEM images in Figure7, it could be deduced that the long-wave absorbance peak might exist in the area from 1,100 to 1,300 nm when the volume of TA was less than 0.19 mL. Our spectrophotometer could not collect the data from 1,100 to 1,300 nm. When the peak shifted to 1,080 nm (0.19 mL TA was added), the maximum datum was collected very well as shown in Figure6. The peak continued shifting with the amount of TA increased. The observed shifts were from 1,080 to 697 nm. Meanwhile, gold nanotriangles and nanodecahedrons were clearly identified in TEM images. With the cited literatures, we concluded that the shifts corresponded to the change of size and abundance of gold nanotriangles and nanodecahedrons. In other words, AuNFs consisted of various nanocrystals with different compositions, and size could be synthesized by simply changing the amount of reductant during the one-step reacting process.
A testified experiment on the effect of amino IL
The formation of non-spherical AuNFs
IL could stabilize gold nanoparticles by static interaction. The imidazole ring was a positive ion and had a special static property. It preferred to interact with a metal atom from a special angle[42, 43]. Besides, IL molecules could form a specific structure by π-π stack interaction. The specific structure acted as a perfect mode for the synthesis of non-spherical AuNFs[44–46]. Therefore, we deduced that the special angle and structure caused by IL would result in the asymmetry of electric field around gold nanoparticles. The properties decreased the growing energy of some special crystal angle and prevented the nanoparticles from aggregating together. Non-spherical AuNFs were formed subsequently.
Both TA and amino ILs were necessary in the formation of non-spherical AuNFs.
The amount of amino ILs could increase the abundance of non-spherical nanoparticles. However, a limitation existed. When the amount of amino ILs reached a fixed value, the abundance would no longer increase.
Amino ILs had a better performance than carboxyl ILs in the synthesis process of non-spherical AuNFs.
The alteration of TA volume would change the composition and size of non-spherical nanoparticles.
The results enrich the research of surface functionalization of gold nanoparticles. Different shapes of IL-stabilized AuNFs can easily be prepared in room temperature. Moreover, the as-prepared AuNFs are dispersed in aqueous solution, which makes it more valuable in industrial applications. Besides the heat transfer efficiency, more research should also be focused in the stability of AuNFs in the near future.
This work was supported by the National Natural Science Foundation of the People's Republic of China (grant nos. 20625517 and 20573101) and the Overseas Outstanding Young Scientist Program of the Chinese Academy of Sciences.
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