Article pubs.acs.org/Langmuir
Additive-Free Size-Controlled Synthesis of Gold Square Nanoplates Using Photochemical Reaction in Dynamic Phase-Separating Media Shinji Kajimoto,* Daisuke Shirasawa, Noriko Nishizawa Horimoto, and Hiroshi Fukumura* Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan ABSTRACT: Ultrafast phase separation of water and 2butoxyethanol mixture was induced by nanosecond IR laser pulse irradiation. After a certain delay time, a UV laser pulse was introduced to induce photoreduction of aurate ions, which led to the formation of gold nanoparticles in dynamic phaseseparating media. The structure and size of the nanoparticles varied depending on the delay time between the IR and UV pulses. For a delay time of 5 and 6 μs, gold square plates having edge lengths of 150 and 100 nm were selectively obtained, respectively. With a delay time of 3 μs, on the other hand, the size of the square plates varied widely from 100 nm to a few micrometers. The size of the gold square plates was also varied by varying the total irradiation time of the IR and UV pulses. The size distribution of the square plates obtained under different conditions suggests that the growth process of the square plates was affected by the size of the nanophases during phase separation. Electron diffraction patterns of the synthesized square plates showed that the square plates were highly crystalline with a Au(100) surface. These results showed that the nanophases formed during laser-induced phase separation can provide detergent-free reaction fields for size-controlled nanomaterial synthesis.
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a reaction field for photochemical reactions. Esumi et al. have shown that rodlike gold particles are synthesized by a photochemical reaction in a rodlike micellar solution as a template. Soejima and Kimizuka succeeded in obtaining ultrathin single-crystal gold nanosheets using an ionic liquid− water interface as a reaction field for photoreduction of gold ions. As they have proposed, microdroplets of ionic liquid in water provide a unique interface for chemical reactions. However, for increasing the reaction area, vigorous stirring through chemical reactions is necessary. In the case of micelle solutions, such stirring is not required and the size of the droplets or structures in solution can easily be controlled through chemical reactions. Although the soft template methods are recognized as a powerful technique to synthesize anisotropic particles, the shapes of the soft templates are limited and control of the size and shape of nanoparticles still remains challenging. In this work, we introduce a new detergent-free method to synthesize nanostructures using dynamic inhomogeneous solvents. In this method, nanophases formed during laserinduced phase separation in binary liquid mixtures are employed as dynamic reaction fields.27 Some binary liquid mixtures, such as 2-butoxyethanol (2BE)/water and triethylamine/water, have their own lower critical soluble temperatures (LCSTs), and they split into two different phases with an increase in temperature. When such a mixture was irradiated
INTRODUCTION Gold and silver nanoparticles have been extensively studied because of their novel catalytic,1,2 sensing,3 and plasmonic optical properties.4,5 Many wet chemical methods have been proposed for the synthesis of gold nanoparticles of various shapes, such as rods, triangular prisms, hexagonal plates, wires, cubes, and branched particles.6−8 These methods typically use various detergents as capping agents to protect the product surfaces and to control the direction and rate of crystal growth. The kinetics of nanoparticle growth is also considered a key to control the shape and size of the products. When detergents are used as weak reducing agents as well as capping agents, the growth kinetics is relatively slow. The slow growth can cause deviation from the thermodynamically favored shape and anisotropic growth.9,10 As a result of anisotropic growth of crystals, nanoparticles with a variety of shapes were obtained using detergents. Capping agents remaining at nanoparticle surfaces, however, changed the original characteristics of nanoparticles, which required additional treatments, such as chemical or thermal treatment, though such treatments are not always effective.11 Nanoparticle synthesis via photochemical reaction has also been reported for gold12−14 and silver.15−18 Intermediates and the reaction mechanism were investigated using various methods.19−22 One of the advantages of nanoparticle synthesis via photochemical reaction is the versatility of experimental conditions, such as light intensity, irradiation time, excitation wavelength,16 polarization,23 and so on. For control of the size and shape, inhomogeneous systems, such as micelle solutions24,25 and the liquid−liquid interface26 have been applied as © 2013 American Chemical Society
Received: January 28, 2013 Revised: April 2, 2013 Published: April 15, 2013 5889
dx.doi.org/10.1021/la400377k | Langmuir 2013, 29, 5889−5895
Langmuir
Article
photoproducts for analysis, we irradiated solutions with IR and UV pulses for 30 min. After the laser irradiation, a portion of the solution with photoproducts was drop-cast and dried on ITO substrates. These substrates were observed using a scanning electron microscope (Hitachi, FE-SEM-S4300, 10.0 kV). For transmission electron microscope (TEM) measurements, the photoproducts were placed on a copper−carbon grid. TEM images were obtained with a JEOL 2000-EX TEM operated at 200 kV. Shadowgraphic imaging experiments using the fluorescence of a dye solution as a monitor light source34 were also carried out, and the growth of the nanophases during phase separation was observed.
with a near-IR laser pulse, which vibrationally excited water molecules,28,29 the temperature of the mixture rose instantaneously and the phase separation process, including scission of hydrogen bonding between unlike molecules and the formation and growth of macroscopic phase domains, was observed.30,31 In this study, we initiated photoreduction of aurate ions during laser-induced phase separation, which resulted in the formation of gold nanoparticles. We found that the size and shape of the formed nanoparticles depended upon the timing of the initiation of photoreduction. Additionally, by changing the total irradiation time of the IR and UV pulses, the size of nanoparticles was also changed, which indicated that the nanoparticles grew in size by repetitive irradiation of these pulses. On the basis of these results, we concluded that the size of nanophases would affect the process of nanoparticle growth.
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RESULTS AND DISCUSSION Microscopic shadowgraphs of phase separating media of 2BE and water with aurate ions are shown in Figure 2. Similar to previous reports,30−32 the growth of bicontinuous phases from several hundreds of nanometers to micrometers after IR laser irradiation was observed during phase separation for solutions containing aurate ions. The Fourier transform images in the insets indicate that phase separation occurred isotropically and the phases had characteristic lengths, which grew with time after IR laser irradiation. We confirmed that the addition of aurate ions (4 × 10−5 mol/dm3) did not affect the phaseseparation dynamics. As discussed in previous reports,32 we consider that bicontinuous phases start to grow in size from several tens of nanometers by maintaining their shape and obeying a simple power law. From the power law, the sizes of the nanophases at 3 and 5 μs were estimated to be 160 and 200 nm, respectively. Thus, we expected to control the size of the reaction fields by changing the delay time between an IR pulse and the UV laser pulse. The formation of gold nanostructures, that was a function of the delay time, was confirmed with images obtained by scanning electron microscopy (SEM) as shown in Figure 3. In these experiments, the irradiation time was fixed at 30 min as explained in Experimental Section. The formation of gold nanostructures results from the direct photoexcitation of aurate ions, Au(III), by UV irradiation. The excited ions are readily reduced to Au(II) by 2BE. Subsequently, the reduction of Au(II) via disproportionation takes place and nanoparticles form.19,21 It was clear that the size and shape of the produced nanostructures varied, depending on the delay time. For a delay time of 3 μs, square gold plates were obtained and the edge length varied from several tens of nanometers to a few micrometers. Square plates were also obtained with a delay time of 5 μs; however, the sizes of the plates were much smaller than those obtained with a delay time of 3 μs. On the other hand, square plates were not found and aggregates of nanoparticles were obtained for a delay time of 10 μs. Without IR pulse irradiation, phase separation was not initiated and only a small amount of gold nanoparticles was obtained after UV laser irradiation. These results clearly indicated that the nanophases in the phase separating media influence the size and shape of gold nanostructures through photochemical reaction. For detailed characterization of the obtained square gold nanoplates, TEM was employed. One example of a TEM image is shown in Figure 4. The inset shows a typical selected area electron diffraction pattern of a square nanoplate. For reference, a TEM image and an electron diffraction pattern of a gold nanoprism obtained from reduction of gold ions by citric acid33 are also shown (Figure 4b). From the comparison of the photographic contrast of these TEM images, the thickness of
EXPERIMENTAL SECTION
As a sample solution, 4 × 10−5 mol/dm3 of potassium tetrachloroaurate (III) (Wako Pure Chemical Industry) was dissolved in a mixture of 2BE (Wako) and water (Millipore, Simplysity UV) having a critical composition under atmospheric pressure (mole fraction of 2BE: 0.052). The solution (150 mL) was circulated through a quartz flow cell having a thickness of 100 μm with a flow rate of 25 mL/min. For the duration of laser irradiation, the solution temperature was kept at 313 K. Figure 1 shows a schematic of the experimental setup used. The experimental setup is similar to the setup used in our previous
Figure 1. Schematic of the experimental setup for laser fabrication of gold nanostructures during laser-induced phase separation. The phase separation was induced in mixtures of 2-butoxyethanol and water in a flow cell by the irradiation of an IR pulse. After a certain delay time, photoreduction of Au ions was initiated with a UV pulse (M: mirror for 1.9 μm, DM: dichroic mirror for 355 nm, FC: flow cell). works.29−32 Briefly, a nanosecond near-IR pulse was generated by Raman shifting from the fundamental beam of a Nd:YAG laser (Spectra Physics, Quanta Ray GCR200, 1064 nm, 8 ns, 10 Hz,
