Fabrication of Liquid Crystal Sol Containing Capped Ag−Pd Bimetallic

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J. Phys. Chem. C 2008, 112, 20284–20290

Fabrication of Liquid Crystal Sol Containing Capped Ag-Pd Bimetallic Nanoparticles and Their Electro-Optic Properties Naoto Nishida,† Yukihide Shiraishi,‡,§ Shunsuke Kobayashi,| and Naoki Toshima*,‡,§ Graduate School of Science and Engineering, Department of Materials Science and EnVironmental Engineering, AdVanced Materials Institute, and Liquid Crystal Institute, Tokyo UniVersity of Science, Yamaguchi (TUS-Y), SanyoOnoda-shi, Yamaguchi, 756-0884 Japan ReceiVed: August 30, 2008; ReVised Manuscript ReceiVed: October 24, 2008

Liquid crystal molecule-capped Ag-Pd bimetallic nanoparticles (atomic ratio ) 1/9, 1/4, 1/1, 4/1, and 9/1) were prepared by photoirradiation of the tetrahydrofuran solution of silver perchlorate and palladium(II) acetate in the presence of liquid crystal molecule, 4′-pentylbiphenyl-4-carbonitrile. (5CB is often used for this compound based on conventional nomenclature 4′-pentyl-4-cyanobipenyl. Thus, 5CB is used in this paper.) The prepared bimetallic nanoparticles had an average diameter of 1.8-3.6 nm. Infrared spectra of carbon monoxide adsorbed on the bimetallic nanoparticles suggested that bimetallic nanoparticles had a random alloy structure. The nanoparticles were dispersed in liquid crystal 5CB to construct novel twisted nematic liquid crystal devices (TN-LCDs). The TN-LCDs containing Ag-Pd bimetallic nanoparticles revealed the electro-optic properties depending on the composition of nanoparticles, especially surface composition of nanoparticles, which was shown to be of importance to control not only the properties but also the stability of nanoparticle-doped LCDs by bimetallization. Introduction Recently metal nanoparticles have received much attention from the viewpoint of optical,1-3 electronic,4-6 and magnetic7-9 properties, in addition to catalysis.10-12 These properties of metal nanoparticles depend not only on the size and structure of metal nanoparticles themselves but also on the covering materials that play a role of stabilizer as well.13 The metal nanoparticles covered by organic materials can be well dispersed in organic media. Metal nanoparticle-dispersed organic media may create new properties completely different for those of the original organic media and of the metal nanoparticles themselves. For example, we have successfully dispersed nanoparticles of metal or semiconductor in π-conjugated conducting polymers to produce electroconductive hybrid materials, which may possibly be applied to organic thermoelectric materials.14 For the past decade many researches have been carried out on hybrid materials based on liquid crystals from the viewpoint of supramolecule and soft materials. For example, gelling agents,15-17 surfactant micelles,18,19 colloidal particles,20-22 droplets,23,24 and so forth dispersed in liquid crystal matrixes were reported. Since liquid crystals have many special properties like liquidity, self-assembly, crystallinity, and optical anisotropy, the hybrid materials containing liquid crystals as dispersion media or dispersed phases can be at the same time expected to have many functions arising from such properties. This is also very interesting from the viewpoint of colloid science. On the other hand, liquid crystal molecules themselves have been investigated as raw materials for electronic display devices for more than three decades because of its electro-optic properties and now constructs main parts of information * To whom correspondence should be addressed. E-mail: toshima@ ed.yama.tus.ac.jp. † Graduate School of Science and Engineering. ‡ Department of Materials Science and Environmental Engineering. § Advanced Materials Institute. | Liquid Crystal Institute.

Figure 1. Schematic illustration of a LCD cell filling by liquid crystal sol containing nanoparticles. A self-assembly property of liquid crystal is speculatively perturbed by containing nanoparticles in liquid crystal medium. Liquid crystal molecules and nanoparticles are presented by bars (a) or ellipses (b) and circles, respectively.

industries. The liquid crystal display, however, has the disadvantage of slow response compared with electroluminescence displays. Thus, it will have a big impact on the design of liquid crystal displays with fast response if liquid crystal sol containing nanoparticles have novel dynamic properties different from the original liquid crystal medium obtained by giving a perturbation to the self-assembly property of liquid crystals (cf. Figure 1). The merging of metal nanoparticles or nanotechnology in a wide sense into self-assembled systems such as liquid crystal devices (LCDs) may attract the attention of researchers who are interested in inaugurating a new kind of combination of different fields.25 In fact, the number of documents, especially patents on a liquid crystal display concerning nanoparticles, has increased rapidly these years. The nanoparticles reported as a dispersed phase in LCDs involves fullerene,26 carbon nanotubes,27,28

10.1021/jp807723j CCC: $40.75  2008 American Chemical Society Published on Web 12/02/2008

Ag-Pd Bimetallic Nanoparticles diamond powders,29 MgO or SiO2 nanoparticles,30 noble metal nanoparticles,31-35 semiconductor nanoparticles,36 Au nanorods,37,38 and so forth. When used as a dopant for LCDs they were expected to improve the contrast, decrease the driving voltage, capture ions, and shorten the response time. In this paper, we would like to describe new liquid crystal sol composed of selfassembled liquid crystal media and metal nanoparticles as a dispersion medium and a dispersed phase, respectively. The miscibility of two materials in completely different categories is the most important criterion to develop such a liquid crystal sol. Thus, we prepared the metal nanoparticles covered by liquid crystal molecules and applied them to the LCDs. In the previous paper, we prepared dispersions of Pd nanoparticles capped by 4′-pentylbiphenyl-4-carbonitrile (abbreviated as 5CB based on the conventional nomenclature 4′pentyl-4-cyanobiphenyl) by ultraviolet light irradiation of the tetrahydrofuran (THF) solution of palladium(II) acetate under nitrogen at room temperature. The twisted nematic (TN) LCDs doped with metal nanoparticles, fabricated by injecting the 5CB sol containing 5CB-capped Pd nanoparticles into a cell, were found to exhibit frequency modulation response, that is, the transmittance of LCDs could be altered not only by the amplitude (applied voltage) but also by the applied frequency.31 Interestingly the electro-optic response could also be enhanced by addition of metal nanoparticles, especially Ag nanoparticles, resulting in fast response,32 which could improve one of disadvantages for the presently available TN-LCDs.39 During a series of investigations for the effect of metal nanoparticles on the electro-optic properties of TN-LCDs, we have found that the effect of nanoparticles on the degree of the response time drastically depends on the kind of metal. In fact, Ag nanoparticles had the largest effect on the response time among Ag, Au, and Pd nanoparticles. However, the effect of Ag nanoparticles decreased rapidly even during the measurement. Thus, we have examined how to prepare liquid crystal moleculecapped Ag-Pd bimetallic nanoparticles and disperse them into a liquid crystal medium to form liquid crystal sol. The liquid crystal sol containing Ag-Pd bimetallic nanoparticles was more stable than those containing Ag nanoparticles. In fact, we have succeeded in fabricating a super twisted nematic (STN)-LCD display by using liquid crystal sol containing Ag-Pd bimetallic nanoparticles.40 Since this display showed a rapid response at -10 °C, the response time at low temperature, which is a disadvantage for the conventional LCD, can be improved by using the new liquid crystal sol. Here, we describe the preparation of 5CB-capped Ag-Pd bimetallic nanoparticles, which are much more stable than Ag nanoparticles, and then the electro-optic properties of TN-LCDs fabricated by liquid crystal sol containing Ag-Pd bimetallic nanoparticles. The relationship between the electro-optic properties of doped LCDs and the composition of doping bimetallic nanoparticles was extensively investigated, as well as the precise structure of Ag-Pd bimetallic nanoparticles. It has been found that the surface composition of bimetallic nanoparticles have an effect on the properties of the LCD. In addition, Ag-Pd bimetallic nanoparticles were superior in long-term stability to Ag nanoparticles as a dopant of LCDs. This fact will be useful to estimate the physical properties of liquid crystal sol containing nanoparticles in general. Experimental Methods Materials. 4′-Pentylbiphenyl-4-carbonitrile (5CB) purchased from Tokyo Kasei Kogyo, Ltd., palladium(II) acetate and silver

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20285 perchlorate from Kojima Chemicals Co., Ltd., and THF from Wako Pure Chemical Industries, Ltd. were used without further purification. Preparation of 5CB-Capped Pd and Ag Monometallic Nanoparticles. The THF solution (50 mL) containing palladium(II) acetate (Pd(OAc)2, 0.033 mmol) or silver perchlorate (AgClO4, 0.033 mmol) and 5CB (1.32 mmol, the mole ratio of 5CB against total amount of metal, R ) 40) in a quartz Schlenk tube was degassed by three freeze-thaw-cycles and irradiated by ultraviolet (UV) light with a Ushio 500 W superhigh-pressure Hg lamp for 1.5 h (Pd) or 3 h (Ag) under N2 in a water bath at room temperature. Complete removal of the solvent and volatile byproducts by vacuum evaporation gave 5CB-capped Pd and Ag nanoparticles, respectively in 5CB. Preparation of 5CB-Capped Ag-Pd Bimetallic Nanoparticles. The THF solution (50 mL) containing AgClO4 (0.0166 mmol), Pd(OAc)2 (0.0166 mmol) and 5CB (1.32 mmol, R ) 40) in a quartz Schlenk tube was degassed by three freeze-thawcycles and irradiated by UV light with a Ushio 500 W superhigh-pressure Hg lamp for 2 h under N2 in a water bath at room temperature. Complete removal of the solvent and volatile byproducts by vacuum evaporation gave 5CB-capped Ag-Pd (atomic ratio of Ag/Pd ) 1/1) bimetallic nanoparticles in 5CB. 5CB-Ag-Pd (atomic ratio of Ag/Pd ) 1/9, 1/4, 4/1, and 9/1) bimetallic nanoparticles were prepared by the similar method. Characterization of Nanoparticles. UV-vis (ultraviolet and visible light) absorption spectra were measured with a Shimadzu UV-2500PC recording spectrophotometer using a quartz cell with 10 mm of optical path length. The measurement of Fourier transform infrared (FT-IR) spectra of carbon monoxide (CO) adsorbed on metal surface was carried out with a JEOL JIRWINSPEC 50 spectrometer. Transmission electron microscopy (TEM) images were observed with a JEOL TEM 1230 at accelerated voltage of 80 kV. An average diameter and standard deviation were calculated by counting the diameters of 200 particles on the enlarged TEM photographs. High resolution TEM (HR-TEM) and bright field scanning transmission electron microscopy (BF-STEM) were observed with a JEOL TEM 2010F microscope at accelerated voltage of 200 kV. Energy dispersion X-ray spectroscopy (EDS) measurement was carried out with a NORAN UTW type Si(Li) semiconducting detector with about 1 nm beam diameter attached to the HR-TEM equipment. Elemental analyses of metals of the obtained nanoparticles were conducted by a Hitachi Z-6100 atomic absorption spectrometer for the dissolved solution in aqua regia. Fabrication of TN-LCD Cell. 5CB-capped metal nanoparticles were mixed with 5CB at room temperature resulting in a liquid crystal sol of 5CB containing 0.1 wt % of total metal (sum of Pd and Ag). The sols were injected into an empty cell for a TN mode with a cell gap of 5 µm, supplied by Nippo Denki Co. Ltd. Nematic-Isotropic Phase Transition Temperature of TNLCDs Containing Nanoparticles. The nematic-isotropic phase transition temperature of the TN-LCD containing metal nanoparticles was observed on polarizing microscope images taken with an OLYMPUS BH-2 polarizing microscope equipped with a METTLER Toledo FP82HT hot stage. Electro-Optic Properties of TN-LCDs Containing Nanoparticles. The electro-optic properties, especially applied voltage versus optical transmittance (V-T) curves of TN-LCD cells were measured by applying the 100 Hz square wave alternating current at 25 °C with a LCD evaluation system (Photal Ohtsuka Electronics, Ltd., model LC-5200).

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Figure 3. BF-STEM image and EDS spectra of 5CB-capped Ag-Pd (Ag/Pd ) 1/1) nanoparticles: The magnification is 3,000,000 and EDS spectra are measured in K-edge.

Figure 2. TEM image and size distribution histogram of 5CB-capped monometallic and bimetallic nanoparticles: (a) Pd, (b) Ag-Pd (Ag/Pd ) 1/9), (c) Ag-Pd (Ag/Pd ) 1/4), (d) Ag-Pd (Ag/Pd ) 1/1), (e) Ag-Pd (Ag/Pd ) 4/1), (f) Ag-Pd (Ag/Pd ) 9/1) and (g) Ag.

Stability of TN-LCDs Containing Nanoparticles. The stability of TN-LCD containing metal nanoparticles was examined by an accelerated deterioration test. The TN-LCD cell was kept in an oven at 80 °C for 28 days, and then observed with a polarizing microscope (OLYMPUS BH-2). Results and Discussion Preparation and Characterization of Ag-Pd Bimetallic Nanoparticles. Ag and Pd monometallic, and Ag-Pd (Ag/Pd ) 1/9, 1/4, 1/1, 4/1, and 9/1 in atomic ratio) bimetallic nanoparticles were prepared by photoreduction of AgClO4 and Pd(OAc)2 in THF in the presence of 5CB. Figure 2 shows the transmission electron micrographs (TEMs) and size distribution histograms of the obtained monometallic and bimetallic nanoparticles. The results reveal that Ag-Pd bimetallic nanoparticles examined here have average diameters of about 1.8 ∼ 3.6 nm and relatively small standard deviations. The Ag-Pd bimetallic nanoparticles, in general, have larger size than monometallic Ag (1.4 ( 0.5 nm) and Pd (2.4 ( 1.2 nm) nanoparticles. This result indicates that the obtained dispersions of bimetallic nanoparticles are not composed of the mixture of monometallic Ag and Pd nanoparticles but of alloy nanoparticles, each of which contains both Ag and Pd atoms, probably because the bimetallic alloy nanoparticles have a more stable structure by bimetalization than the mixture of the two. HR-TEM was measured to get more information on the structure of Ag-Pd bimetallic nanoparticles. The HR-TEM

Figure 4. UV-vis absorption spectra of the dispersions of 5CB-capped Pd and Ag monometallic, and Ag-Pd (Ag/Pd ) 1/9, 1/4, 1/1, 4/1, and 9/1) bimetallic nanoparticles in THF.

photographs, shown in Supporting Information, Figure S1, indicate that the nanoparticles are well dispersed and rather homogeneous, suggesting that bimetallic nanoparticles have a random alloy structure. The EDS spectrum taken for each spot of a single nanoparticle in BF-STEM, shown in Figure 3, indicates also that a 5CB-capped Ag-Pd(1/1) nanoparticle contains both Ag and Pd components in a particle. Elemental analysis carried out by an atomic absorption spectroscopy revealed that the atomic ratio of Ag/Pd equaled to 1.00:1.01. This means that the charged Ag and Pd ions were completely converted to 0-valent metal producing Ag-Pd bimetallic nanoparticles. On the basis of these experimental results, it has been confirmed that the obtained particles are Ag-Pd bimetallic nanoparticles capped by 5CB. UV-vis absorption spectra of the THF dispersions of 5CBcapped nanoparticles are shown in Figure 4. After the photoreduction, there appears the tailing reflection at longer wavelength without any surface plasmon resonance (SPR) absorption due to Ag nanoparticles. Even the dispersion of Ag monometallic nanoparticles shows no SPR absorption peak in the visible region probably because the particles are too small in size.41

Ag-Pd Bimetallic Nanoparticles

Figure 5. FT-IR spectra of CO adsorption on 5CB-capped nanoparticles. (a) Pd, (b) Ag-Pd (Ag/Pd ) 1/9), (c) Ag/Pd (1/4), (d) Ag/Pd (1/1), and (e) Ag.

Colloidal dispersion of Ag-Pd bimetallic nanoparticles reveals a curve rather similar to that of Pd monometallic nanoparticles, implicating a considerably strong tailing reflection in the long wavelength. The tailing absorption of dispersion of Ag/Pd bimetallic nanoparticles decreases as the atomic ratio of Ag/Pd in the bimetallic nanoparticles increases in the order of 1/9, 1/4, 1/1, 4/1, and 9/1, suggesting that the electronic state of nanoparticles depends on the composition of particles. In other words, the electronic state of nanoparticle surface can be continuously controlled by the surface composition of bimetallic nanoparticles. There are many reports on Ag-Pd bimetallic nanoparticles.42-44 However, only a few reports have proposed the exact structures except a random alloy. Chu et al. prepared Ag-Pd bimetallic nanoparticles by a wet reduction method and proposed the structure where Pd was rich near the surface.42 Both Ag-core/ Pd-shell43 and Pd-core/Ag-shell structures44 were proposed for the bimetallic nanoparticles prepared by successive reduction, respectively. In the present experiments, we prepared the Ag-Pd bimetallic nanoparticles by a simultaneous photoreduction method. To confirm the structure, FT-IR (Fourier-transform infrared absorption) spectra were measured for carbon monoxide (CO) adsorbed on the surface of metal nanoparticles.45 The results are shown in Figure 5. The CO molecules adsorbed on Ag nanoparticles show a strong bridging peak at 1916 cm-1 and no peak at around 1960 cm-1, while those adsorbed on Pd nanoparticles have two bridging peaks at 1916 and 1960 cm-1. In the case of Ag-Pd (atomic ratio of Ag/Pd ) 1/1, 1/4, and

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20287 1/9) bimetallic nanoparticles prepared in the present experiment, two bridging peaks of CO appeared at 1916 and 1957 cm-1, which means that Pd atoms are located on the surface of bimetallic nanoparticles. Thus, the ratios of the intensity of the absorption peak at 2175 cm-1 to that at 2130 cm-1 were plotted against the component ratios of Pd of nanoparticles to get the information of component ratio on the surface since the CO adsorption occurs only on the surface. The results shown in Supporting Information, Figure S2 reveals that there is a linear relationship between the intensity ratio in IR and the Pd component ratio. This linear relationship is quite consistent with the observed trend of absorption intensities of bimetallic nanoparticles in UV-vis absorption. These results suggest that the Pd content on the surface is the same as that in a whole particle, that is, the bimetallic nanoparticles have the random alloy structure forming a solid solution. Electro-Optic Properties of Liquid Crystal Sol Containing Nanoparticles. Phase transition temperatures between nematic and isotropic (N-I) phases, which are one of the most important properties for a liquid crystal, were measured by using TNLCD cells fabricated by 5CB sol containing nanoparticles. The phase transition temperature of 5CB has a trend to decrease with addition of metal nanoparticles. The decrement became higher with increasing Ag component in Ag-Pd bimetallic nanoparticles. Thus, the N-I phase transition temperatures of the bimetallic nanoparticles with Ag/Pd ) 1/0, 4/1, 1/1, 1/4, and 0/1 were lower than those of pure 5CB by -1.2 °C, -1.0 °C, -0.5 °C, -0.3 °C, and -0.1 °C, respectively. These results suggest the stronger interaction of Ag with the liquid crystal molecule 5CB than Pd. As previously mentioned on the results of UV-vis absorption and CO-FT-IR spectrum, Ag components on the surface of nanoparticles, correlated with the Ag components of the whole particle, have an effect on both spectra. Here again, the Ag components on the surface have a similar effect on the N-I phase transition temperature, suggesting that the surface composition of nanoparticles has a strong effect on physical properties of LCD doped by nanoparticles. The 5CB-capped nanoparticles prepared in the present experiments were easily mixed with the liquid crystal molecule 5CB at room temperature to form liquid crystal sol for twisted nematic liquid crystal devices (TN-LCDs). The TN-LCDs fabricated by injecting the liquid crystal sol containing nanoparticles into empty cells were supplied to measure the electrooptic properties. As we have already reported, TN-LCD fabricated by 5CB sol containing Pd nanoparticle as a dopant showed a frequency modulation response, while that without dopants did not show such a response.43 Curves a-h in Figures 6 show the relationships of applied voltage and transmittance of LCDs fabricated by 5CB sol containing 5CB-capped nanoparticles measured at 25 °C and 100 Hz. The driving voltage of the LCD shifts to higher voltage by doped with nanoparticles. The shift is small in the sol containing Pd nanoparticles and large in that containing Ag nanoparticles. The extent of the shift depends on the composition of bimetallic nanoparticles, increasing with increasing Ag component in the order of Ag/Pd ) 1/9, 1/4, 1/1, 4/1, and 9/1. In Figure 7 the Pd content of bimetallic nanoparticles is plotted against the shift of threshold voltage (Vth) of TNLCDs fabricated by 5CB sol containing Ag-Pd bimetallic nanoparticles from that of pure 5CB, which is defined as the shift in the voltage where the transmittance starts to change from 100%. No linear relationship is observed between the shift in Vth and Pd content. The shift is larger than that expected from the linear dashed line at the Pd content of

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Figure 6. V-T curve of TN-LCDs at 100 Hz and 25 °C. (a) Pure 5CB, (b) 5CB-capped Pd, (c) 5CB-capped Ag-Pd (Ag/Pd ) 1/9), (d) 5CBcapped Ag-Pd (1/4), (e) 5CB-capped Ag-Pd (1/1), (f) 5CB-capped Ag-Pd (4/1), (g) 5CB-capped Ag-Pd (9/1), and (h) 5CB-capped Ag.

Figure 8. Schematic illustration of electronic charge transfer in the 5CB-Ag-Pd (atomic ratio of Ag/Pd ) 1/4 and 1/9 (a), 1/1 (b), and 4/1 and 9/1 (c)) bimetallic nanoparticles and Coulomb interaction of a cyano group of liquid crystal molecule 5CB with the charged surface of bimetallic nanoparticles at various Ag/Pd ratios. The green and red arrows mean attractive and repulsive force, respectively.

Figure 7. Relationship between Pd content and shift of threshold voltage from 5CB in the V-T curves of TN-LCDs fabricated by 5CB sol containing Ag-Pd bimetallic nanoparticles. The line is a guide to the eye.

80-90% for bimetallic nanoparticles of Ag/Pd ) 1/4 and 1/9, while smaller at the Pd content of 10-20% for those of Ag/Pd ) 9/1 and 4/1. This nonlinear relationship suggests that the shift in threshold voltage may not depend on the Pd content of a whole particle but on the Pd content of the particle surface. This nonlinear relationship can be explained in the following two ways. The first explanation is based on the difference between the composition of a whole particle and the composition of the particle surface, which can be satisfied by the structure like Pd-core/Ag-shell. The other explanation is based on the variation of the electronic properties of surface Pd depending on the composition of the particles surface. The first explanation based on the core/ shell structure is in conflict with the results observed in UV-vis and CO-FT-IR spectra, described before. Thus, an acceptable explanation should be based on the variation of electronic properties of surface Pd. Here, the bimetallic nanoparticles have a random alloy structure, i.e., the Pd content of particle surface is the same as that of the whole particle. However, atoms of the minor component on the surface of bimetallic nanoparticles change their electronic state owing to charge transfer (electronic) effect by large numbers of adjacent atoms of the main component. This kind

of electronic effect was reported for the catalytic property of core/shell-structured bimetallic nanoparticles.46,47 This charge transfer effect is illustrated as a cartoon in Figure 8. In the case of Ag/Pd ) 1/4 and 1/9 (Figure 8a), the minor component, Ag atom has a rather large positive charge since the Ag atom is surrounded by a lot of Pd atoms. In the case of Ag/Pd ) 1/1 (Figure 8b), electric charges of Ag and Pd atom are opposite but nearly the same in absolute value because atomic numbers of the surface Ag and Pd are the same. In the case of Ag/Pd ) 9/1 and 4/1 (Figure 8c) the minor component Pd atom has a rather large negative charge since the Pd atom is surrounded by a lot of Ag atoms. On the other hand, the liquid crystal molecule 5CB is polarized resulting in a partial negative charge at the cyano end group, which should interact with the surface of bimetallic nanoparticles. Consequently, the Ag/Pd ) 1/4 and 1/9 nanoparticles have a strong Coulomb attraction with the cyano group of 5CB because of large positive charge on the Ag atom, which can result in a large electro-optic effect by giving a stronger perturbation to the liquid crystal matrix. In contrast the Ag/Pd ) 4/1 and 9/1 nanoparticles have a strong Coulomb repulsion to the cyano group of 5CB because of large negative charge of Pd atom, which can result in a small electro-optic effect. Thus, only the small amount of the additive (miner component) can alter the surface properties of nanoparticles. Although LCDs doped with 5CB-capped Ag nanoparticles provided a big change in the electro-optic properties of the LCDs, the change rapidly decreased by deterioration. As mentioned in the previous section, electro-optic properties of nanoparticle-doped LCDs depended on the surface composition of the doping nanoparticles. Thus, we examined the accelerated deterioration by heat treatment of LCD cells at 80 °C for 28 days. After the heat treatment, the cell fabricated by 5CB sol containing 5CB-capped Ag nanoparticles had

Ag-Pd Bimetallic Nanoparticles black spots and disordered parts in the liquid crystal orientation on a photograph taken with a polarization microscope, while that containing 5CB-capped Ag-Pd (1/ 1) bimetallic nanoparticles did not (shown in Supporting Information, Figures S3a and S3b.). Thus, 5CB-capped Ag nanoparticles look like they easily aggregate, which may decrease the practical concentration of dispersed nanoparticles in the liquid crystal medium and the effect of nanoparticles upon the behavior of liquid crystal as well. In the case of Ag-Pd bimetallic nanoparticles, by contrast, the particles can disperse well in a liquid crystal medium because the surface of bimetallic nanoparticles can be partially covered by Pd, which can suppress the aggregation of nanoparticles. This explanation was confirmed by TEM observation of nanoparticles in the 5CB sol containing Ag nanoparticles and Ag-Pd (Ag/Pd ) 1/1) bimetallic nanoparticles after heat treatment at 80 °C for 120 h not in the cells but in separate test tubes. Thus, the TEM photograph of 5CB-capped Ag nanoparticles after heat treatment (Supporting Information, Figure S4b) shows actually the presence of large aggregates with about 30 nm in diameter, while that of 5CB-capped Ag-Pd bimetallic nanoparticles (Supporting Information, Figure S4d) reveals the presence of large single nanoparticles with about 8 nm in diameter along with many small metal nanoparticles with about 2 nm in diameter. In the latter case, although a kind of so-called Ostwald ripening may have occurred, the resulting dispersions contain a small number of the nanoparticles which are large in size but still are dispersed well. The presence of aggregates in the heat-treated 5CB sol containing Ag nanoparticles will give a explanation for the fact that the cell fabricated by 5CB sol containing Ag nanoparticles has black spots in a polarization microscope. In contrast, heat-treated 5CB sol containing Ag-Pd bimetallic nanoparticles has no aggregates, which may result in a rather clear surface in fabricated cells. Thus, the control of surface composition and structure of nanoparticles could be of importance to stabilize the liquid crystal sol containing nanoparticles. This kind of stable liquid crystal sol may provide novel functions as hybrid materials containing nanoparticles. Conclusion 5CB-capped Ag-Pd bimetallic nanoparticles were successfully synthesized by simultaneous photoreduction of the corresponding metal ions in THF at the atomic ratios of Ag/ Pd ) 1/9, 1/4, 1/1, 4/1, and 9/1. The produced bimetallic nanoparticles had an average diameter of 1.8-3.6 nm and were relatively monodispersed. They had a random alloy structure and the surface composition was nearly the same as that of whole particle, which was supported by the results of UV-vis and CO-FT-IR spectra as well as N-I phase transition temperature. 5CB-capped nanoparticles can be well dispersed in a liquid crystal medium of 5CB. The liquid crystal sol containing nanoparticles can be injected into a cell to fabricate a LCD. The electro-optic properties such as driving voltage shift and deterioration behavior of LCDs varied by doping with nanoparticles. However, the change was not linear to the composition of nanoparticles. Only the small amount of additive had a rather large effect. This kind of enhancement effect of a small amount of additive could be explained by a charge-transfer effect. In other words, it has been revealed that the composition of the dispersed phase is very important to the design of the liquid crystal sol containing nanoparticles. Bimetalization is an available

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20289 method to get metal nanoparticles having both useful characteristics and lifetime, as a practical example in this report. It has been clarified in the present experiments that not only the structure and composition of metal nanoparticles but also those of protecting compounds (5CB in the present experiments) that form organic corona surrounding nanoparticles will be important to develop novel hybrid systems containing metal nanoparticles. Acknowledgment. This work was supported by a Consortium R&D project for regional revitalization (No. H16 S6001) from Ministry of Economy, Trade and Industry, Japan and Cooperation for innovative technology and advanced research in evolutional area (City area program: Nano LCD) from Ministry of Education, Culture, Sports, Science and Technology, Japan. Supporting Information Available: Further details are given in Figures S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M. K.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007. (2) Tunc, I.; Guvenc, H. O.; Sezen, H.; Suzer, S.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. J. Nanosci. Nanotechnol. 2008, 8, 3003. (3) Myroshnychenko, V.; Rodrı´guez-Ferna´ndez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; L. Liz-Marza´n, M.; Garcı´ade Abajo, F. J. Chem. Soc. ReV. 2008, 37, 1792. (4) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209. (5) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (6) Zhang, H.; Yasutake, Y.; Shichibu, Y.; Teranishi, T.; Majima, Y. Phys. ReV. B 2005, 72, 205441. (7) Matsushita, T.; Masuda, J.; Iwamoto, T.; Toshima, N. Chem. Lett. 2007, 36, 1264. (8) Nakaya, M.; Kanehara, M.; Teranishi, T. Langmuir 2006, 22, 3485. (9) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (10) Metal Nanoparticles in Catalysis and Materials Science: The Issue of Size Control; Corain, B., Schmid, G., Toshima, N., Eds.; Elsevier: New York, 2008. (11) Matsushita, T.; Shiraishi, Y.; Horiuchi, S.; Toshima, N. Bull. Chem. Soc. Jpn. 2007, 80, 1217. (12) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (13) Toshima, N. In Nanoscale Materials; Liz-Marzan, L. M., Kamat, P. V., Eds.; Kluwer Acad. Pub.: New York, 2003; Chap. 3, pp 79-96. (14) Toshima, N.; Marutani, H., to be published. (15) Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. ReV. 2007, 36, 1857. (16) Mizoshita, N.; Hanabusa, K.; Kato, T. AdV. Mater. 1999, 11, 392. (17) Mizoshita, N.; Hanabusa, K.; Kato, T. AdV. Funct. Mater. 2003, 13, 313. (18) Yamamoto, J.; Tanaka, H. Nature 2001, 409, 321. (19) Caggioni, M.; Giacometti, A.; Bellini, T.; Clark, N. A.; Mantegazza, F.; Maritan, A. J. Chem. Phys. 2005, 122, 214721. (20) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (21) Loudet, J. C.; Barois, P.; Poulin, P. Nature 2000, 407, 611. (22) Fukuda, J.; Yokoyama, H. Phys. ReV. Lett. 2005, 94, 128301. (23) Fukuda, J.; Yokoyama, H. Eur. Phys. J. E 2006, 21, 341. (24) Nazarenko, V. G.; Nych, A. B.; Lev, B. I. Phys. ReV. Lett. 2001, 87, 075504. (25) Toshima, N. Macromol. Symp. 2003, 204, 219. (26) Suzuki, M.; Furue, H.; Kobayashi, S. Mol. Cryst. Liq. Cryst. 2001, 368, 191. (27) Lee, W.; Wang, Y.-C.; Shin, Y.-C. Appl. Phys. Lett. 2004, 85, 513. (28) Chen, Y.-H.; Lee, W. Appl. Phys. Lett. 2006, 88, 222105. (29) Chen, S.-P.; Huang, C.-C.; Liu, W.-Y.; Chao, Y.-C. Appl. Phys. Lett. 2007, 90, 211111. (30) Haraguchi, F.; Inoue, K.; Toshima, N.; Kobayashi, S.; Takatoh, K. Jpn. J. Appl. Phys. 2007, 46, 796. (31) Shiraishi, Y.; Toshima, N.; Maeda, K.; Yoshikawa, H.; Xu, J.; Kobayashi, S. Appl. Phys. Lett. 2002, 81, 2845. (32) Miyama, T.; Thisayukta, J.; Shiraki, H.; Sakai, Y.; Shiraishi, Y.; Toshima, N.; Kobayashi, S. Jpn. J. Appl. Phys 2004, 43, 2580.

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