Chapter 6 Formation of Inverted Hexagonal Liquid Crystal in Mixtures
7.2.2 Preparation of silver nanocables
In a typical procedure, the diamminesilver-(І) complex (1), namely Tollen’s reagent, was prepared by the dropwise addition of ammonium hydroxide solution into 1.8g of a 0.1M aqueous AgNO3 solution until the brown precipitate disappears. The thus prepared complex was thoroughly mixed with oleic acid and stirred for 1 hour to obtain a defined molar ratio of oleic acid to Ag+. This led to the formation white, viscous gels of diamminesilver-(І) oleate (2). For the reduction of the silver ions, 0.1g of the complex 2 where dispersed in 5g of ethanol to form an opalescent suspension.
Then, two drops of formaldehyde were added, followed by the slow and dropwise addition of 0.1 ml 1M KOH under continuous shaking at 300 rpm. The color of the suspension changed from pale white to light gray, gray and finally to dark gray with increasing volume of KOH. After the complete addition of KOH, the suspension was kept shaking for 1 hour.
7.2.3 Characterization
The structure of diamminesilver-(І) oleate (2) was identified by Fourier transform infrared (FTIR) spectroscopy using a Nicolet Thermo Nexus. The samples for FTIR measurements were washed by water and ethanol, and then dried under vacuum at room temperature for 8h, after which the samples had the appearance of wax-like solids. The optical microscopy textures were obtained using a Zeiss Axioplan 2 with calibrated image processing software. The products were visualized at room temperature between a glass slide and cover slip. Bright-field transmission electron microscopy (TEM) images were recorded using a Philip CM 10 transmission electron microscope. The Dark-filed TEM image and the elemental profile across a nanocable were obtained by FEI TECNAI F20. The samples before reduction were prepared by dispersing the viscous white gels in ethanol. The samples after reduction were first centrifuged at 4000 rpm for 10 min, and then redispersed in ethanol. Carbon-coated copper grids were used as support.
7.3 Results and discussion
The molar ratio of oleic acid/Ag+ shows a great influence on the product morphology. Below 30, the product agglomerates irreversibly and precipitates in water. Above 30, a homogenous, viscous white gel is obtained. Figure 7.1 shows images of the dark-field optical microscopy for the diamminesilver-(І) oleate gel at a molar ratio of 30. At low magnification, the gel seems to consist of spherical vesicles (Figure 7.1a). Higher magnifications (Figure 7.1b) reveal that these vesicles are composed of nanometer-sized strings, which bundle up to form thicker aggregates.
When this gel is dispersed in ethanol, the agglomerated bundles will separate to individual filaments (Figure 7.1c). In contrast to other objects composed of oleic acid moieties, these filaments are hard and break under the microscope when stressed. This will be covered further on.
a b
c
Figure 7.1 Images of the dark-field optical microscopy for a diamminesilver-(І) oleate gel at a molar ratio of 30 before (a, b) and after (c) dispersion in ethanol.
Figure 7.2 shows unstained micrographs of the oleic acid/Ag+ mixtures. At a molar ratio of 30 (oleic acid/Ag+), large bundles of nanowires are formed (Figure 6.2a). These bundles are composed of solid nanowires and hollow nanotubes as observed in high resolution TEM (data not shown). The length ranges from 5−8 μm and the diameters from 280−500 nm. When the molar ratio of oleic acid/Ag+ reaches 50, long nanowires with high aspect ratios are obtained (Figure 7.2b). High resolution TEM reveals that these nanowires possess hollow cavities (Figure 7.2c and 7.2d). The lengths of these nanotubes are in the order of several ten microns, with outer diameters of 155−200 nm, i.e. an aspect ratio of ca. 250. The wall thickness is 60−70 nm.
c
a b
d
Figure 7.2 TEM micrographs (unstained) of a diamminesilver-(І) oleate gel at a molar ratio oleic acid/Ag+ of 30 (a) and 50 (b-d).
To form nanowires, the nanotube dispersions were treated with formaldehyde. As expected, the reaction conditions had a great influence on the morphology of the reduction products (Figure 7.3). Apart from an oleic acid/Ag+ ratio of larger than 30, gradual addition of the reagents under gentle agitation is required to obtain well-defined nanocables. The diameters of these cables match the ones found for the hollow tubes observed before reduction. In the TEM micrograph, a sharp contrast between sheath and core can be clearly observed. The cores are straight and uniform with diameter of 30−45 nm throughout their entire length. The silver profile across a nanocable (Figure 7.3d bottom) further confirms the presence of a solid, inner silver core, while there are some silver particles distribute on the outer sheath. However, the sheath does not seem to be entirely depleted of silver. It remains unclear, whether this is unreacted Ag+-oleate or small silver agglomerations.
b a
c d
Figure 7.3 Bright-field TEM micrographs of nanocables after reduction of a gel with a molar ration oleic acid/Ag+ of 50 (a-c) and a dark-field TEM micrographs (d, top) and the corresponding silver profile across the nanocable (d, bottom).
In contrast, if all the reagents are added at one time under violent stirring, a different morphology is found (Figure 7.4). The product now consists of short rods of 200 nm length (aspect ratio ~5) and even spherical particles of 10−70 nm in diameter.
Clearly, adding all reducing agent at once induces fast nucleation, which results in the formation of spherical particles.[17] It is important to note that the spheres are not randomly distributed, but lined up in string-like aggregates (Figure 7.4b), which bear close reassemblance to the original tubular structure (Figure 7.4b). Higher magnifications (Figure 7.4c) reveal that the rod-like objects are also composed of small spherical nanoparticles of approximately 10 nm diameter arranged linearly in a tubular structure.
b
d a
c
Figure 7.4 TEM micrographs of the reduction products made by fast addition of KOH (a-c) and substituting NaBH4 for formaldehyde (d).
Similar results are obtained when formaldehyde is replaced by NaBH4. Instead of fully developed cables, again particles are found all over the tube (Figure 7.4d). At close examination, several features become evident: (i) most particles are located in
the core of the cable, (ii) the outer surface is densely covered with particles, and (iii) between core and surface, the particles are arranged in stripes parallel to the core. This situation is also seen when formaldehyde is used in low quantities, and appears to be an intermediate state on the way towards the formation of fully the developed cables.
The question now arises as to how the tubes and cables are formed.
~20 layers
Figure 7.5 Schematic structure of the oleic acid/Ag+ nanotube.
It has been reported that metal salts of long chain fatty acids form bilayer aggregates [18] and can assemble to inverted cylindrical structures [19]. The XRD pattern of the nanotube suspensions shows one sharp reflection (100) at 3.18 nm. An individual metal oleate is approximate 2 nm long [20], which suggests the presence of interdigitated bilayers [21].With an observed wall thickness of 60−70nm, the sheath in the presented cables seems to be composed of approximately 20 layers. The layer theory is supported by the formation of particles in a string-like alignment in the sheath. Theoretically, a tubular superstructure composed of a bundle of smaller tubes could also account for the observed string-like arrangement of particles, but such a structure was not seen in high resolution TEM. Furthermore, the inner core diameter seems to be the smallest arrangement of diamminesilver-(І) oleate with a dense enough packing of the alkyl chains to form bilayers. The hollow core before reduction (Figure 7.2d) as well as the silver wire (Figure 7.3c) are, on the other hand, prominent features in the TEM micrographs, and a multi bundle structure should, therefore, be clearly noticeable. Base on these considerations we assume the tubes to have a multi lamellar wall structure as depicted in Figure 7.5.
Upon addition of the reducing agent, the silver ions are reduced to metallic silver.
As seen in Figure 7.4d, this nucleation might take place all over the tube. Slow addition of the reducing agent allows Ostwald ripening to come in to play and provides the mechanism for silver ions to be transported to the centre. As seen in Figure 7.4d, the particle density in the core is higher than anywhere else. This allows for the formation of large particles by agglomeration of smaller ones early in the process. These then grow at the expense of the remaining small particles in the structure. No particles outside the template were found in any of the experiments under the described conditions.
To gain more insight into the structure of the tubes, the FTIR spectra of the nanotubes were compared with oleic acid and the final nanocable (Figure 7.6). The characteristic vibration of oleic acid is a broad OH stretch at ~3000 cm-1 and a sharp C=O stretch at 1710 cm-1. These signals were not found in the spectra of the nanotubes. For the nanotube, the new strong signals at 1563, 1467 and 1419 cm-1 could be assigned to the asymmetric (1563 cm-1) and the symmetric (1467 and 1419 cm-1) stretching vibration of the ionized carboxylate group [20]. The characteristic signals of NH bending and stretching were also found at 1517 and 3410 cm-1, respectively.
The asymmetric and symmetric carboxylate vibrations can be used to determine
the type of coordination by calculating the difference Δ = ν (COO) − νas s(COO). The Δ values indicate the four main coordination types: ionic (Δ = 164 cm-1), monodentate (Δ = 200−300 cm-1), chelating bidentate (Δ = 40−110 cm-1) and bridging bidentate (Δ
= 140−170 cm-1 [21]) . In the present nanotubes, there are two symmetric stretching vibration, thus two Δ values of Δ1 = 96 and Δ2 = 144 cm-1 are obtained. The first one can be assigned to a chelating bidentate type, while the second one indicates a bridging bidentate (Figure 7.7). These results further corroborate the formation of diamminesilver-(І) oleate. After reduction to nanocables, a slight change in the FTIR spectrum is observed as compared to the nanotubes. A new signal at 1577 cm-1 appeared, while the signal at 1419 cm-1 shifted to 1407 cm-1, and the signal at 3410 cm-1 shifted to 3439 cm-1 and became stronger. In addition, a new peak at 1635 cm-1 was found, which might be assigned to C=O vibration of an amide group. It can, therefore, be safely assumed that the sheath consists of oleate molecules, which are partially crosslinked by silver atoms. This would explain the stiffness observed in the optical microscope.
4000 3500 3000 2500 2000 1500 1000 500
1467
Figure 7.6 FT-IR spectra of oleic acid, the nanotubes, and silver nanocables.
Figure 7.7 Coordination types of diamminesilver-(І) oleate.
7.4 Conclusions
A convenient approach to prepare Ag nanocables have been demonstrated by using the self-assembly of tubular diamminesilver-(І) oleate as templates. To obtain the well-defined nanocables, two steps are important. First, in the preparation of the diamminesilver-(І) oleate precursors, the molar ratio oleic acid/Ag+ must reach 50, so that ordered nanotubes with high aspect ratios can be formed. Second, in order to avoid fast nucleation resulting in the formation of spherical particles, the reduction conditions must be carefully controlled by the slow and dropwise addition of the reducing agent under gently shaking. The prepared nanotubes seem to have a multi wall structure. During the reduction, the intermediate state of stripe-arranged particles is formed at early stage, and finally leads to nanocables via Ostwald ripening and particle agglomeration. We believe that this approach could be extended to prepare a variety of metal nanocables by an appropriate choice of the inorganic/organic system and experimental conditions. In addition, metal nanowires also can be prepared by this method, since the organic sheath can be easily removed by dissolving in suitable organic solvents.
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Summary
Chapter 2 presents the study of a series of BAO-based PI-LED devices. These PIs exhibit obvious fluorescent property and the intensities decreases with increasing crystalline degree. It suggests that high crystalline degree would decrease the population of the imide rings-aminophenyl rings face-to-face stacking. Hence, the CT interactions and fluorescence intensity would be reduced. The resultant PIs not only can be a light-emitting layer in a single layer LED, but also act as an electron transport and an electron/hole blocking layer in a double layer PPV-PVA based LED.
The incorporation of BAO-ODPA layer into the PPV-PVA LED provides a significant improvement in the EL efficiency by two order of magnitude.
In Chapter 3, single layer PI-LEDs have been successfully prepared by vapor deposition polymerization from diamine BAO and BAPF with dianhydride 6FDA.
Using VDP process, smoother surface morphology of the LED devices can be obtained than using wet coating process presented in Chapter 2. Effective active PI thin film can be performed as low as 150 Å and shows a low threshold voltage as 4.5V.
Both BAO-6FDA and BAPF-6FDA single layer LED show broad EL spectra. Thicker PI film exhibits higher efficiencies in both types of LEDs. It could be resulted from the more CT sites in the thicker PI film in favor of increasing the intermolecular CT.
BAPF-6FDA LED has a higher EL efficiency than BAO-6FDA LED because of the more balanced charge injection from both electrodes and the stronger intermolecular CT.
In Chapter 4, a series of thermal and mechanical properties of PI/ZnO nanohybrid films were established. The interchain hydrogen bonding being a kind of physical crosslinking exists in PI-ZnO interfacial domain. Owing to this physical crosslinking, the storage modulus, CTE decrement and Tg of the hybrid films can be
effectively improved in comparison with that of pure PI. The improvements on these characteristics of PMDA-ODA/ZnO nanohybrid films are much more significant than that of BTDA-ODA/ZnO nanohybrid film. The physical crosslinking structure also causes BTDA-ODA/ZnO nanohybrid films having two Tgs, one for pure PI domain, the other higher Tg for the PI-ZnO interfacial domain. PMDA-ODA/ZnO nanohybrid films have only one T because the PMDA-ODA having high Tg g cause two possible T s into one. Although the Tg ds of the hybrid films are lower than pure PI, it is good enough for the practical application. From TEM images, the ZnO particles show a uniform dispersion in the BTDA/ODA matrix, but most particles are bigger in size (10−15 nm) and some are elongated in comparison with as-synthetic TPM stabilized ZnO nanoparticle. It can be attributed to the high temperature and long time imidization process. However, the aggregation and irregular shape of ZnO nanoparticles are more notable in the PMDA-ODA/ZnO hybrid film. This means that the matrix structures significantly affect the morphology and characteristic of the PI/ZnO nanohybrid films.
Chapter 5 describes preparation of ZnO nanocrystals by using a thermal coater and an air-circulating. An air-circulating provides a milder and rapid oxidized environment to develop ZnO crystals, so that flexible and thermal stable PI films can also be used as substrates in this process. HRTEM and PL measurements show the produced ZnO nanocrystals are nearly defect-free single crystalline structures.
Besides, deposited ZnO on PI film substrates can obtain individual and well distribution nanocrystals after dispersing by an ultrasonic bath. Because a roll-type of PI film can be used as a substrate to deposit ZnO continually, it can be a simple and efficient method to fabricate ZnO nanocrystals on large scales.
In Chapter 6 shows that the ternary system oleic acid/1-decanol/ammonium hydroxide exhibits an inverse hexagonal (HII) liquid crystalline phase, which is predominate at room temperature and exists in a wide compositional range. Polarized optical microscopy and x-ray diffraction confirms the formation of the hexagonal phase, while conductivity measurements reveal the inverse nature. The system can tolerate up to 45 wt-% of ammonium hydroxide before the hexagonal phase collapses.
Using mixtures of ammonia and oleic acid to prepare in situ the oleate amphiphile is more convenient than using authentic alkali oleates. In particular, this allows for the incorporation of up to 100 mM solutions of metal ions such as Ag+, Cu , Ni , Co , 2+ 2+ 2+
Zn2+, and Cd2+ into the confined aqueous phase due to the formation of ammine complexes. We expect that this inverse hexagonal system will serve as versatile tool for the preparation of nanorods and nanowires.
In Chapter 7, a convenient approach to prepare Ag nanocables have been demonstrated by using the self-assembly of tubular diamminesilver-(І) oleate as templates. To obtain the well-defined nanocables, two steps are important. First, in the preparation of the diamminesilver-(І) oleate precursors, the molar ratio oleic acid/Ag+ must reach 50, so that ordered nanotubes with high aspect ratios can be formed.
Second, in order to avoid fast nucleation resulting in the formation of spherical particles, the reduction conditions must be carefully controlled by the slow and dropwise addition of the reducing agent under gently shaking. The prepared nanotubes seem to have a multi wall structure. During the reduction, the intermediate state of stripe-arranged particles is formed at early stage, and finally leads to nanocables via Ostwald ripening and particle agglomeration. We believe that this approach could be extended to prepare a variety of metal nanocables by an appropriate choice of the inorganic/organic system and experimental conditions. In addition, metal nanowires also can be prepared by this method, since the organic sheath can be easily removed by dissolving in suitable organic solvents.
Publication
S. C. Hsu
[1] , W. T. Whang, S. C. Chen, ″ Electroluminescence characteristics of aromatic polyimides by vapor deposition polymerization″, Journal of Polymer Research 2003, 10, 7.
S. C. Hsu
[2] , W. T. Whang, C. H. Hung, P. C. Chiang, Y. N. Hsiao, ″ Effect of the polyimide structure and ZnO concentration on the morphology and characteristics of polyimide/ZnO nanohybrid films″, Macromolecular Chemistry and Physics 2005, 206, 291.
S. C. Hsu
[3] , W. T. Whang, C. H. Hung, ″ Large-scale fabrication of ZnO nanocrystals by a simple two-step evaporation oxidation approach″, Materials Characterization (Accepted).
Shou-Chian Hsu
National Chiao Tung University, Department of Materials Science and Engineering 1001 TA HSUEH ROAD, HSINCHU, TAIWAN 30050, ROC
Tel : +886-3-5712121 # 55347 E-mail : u8818552.mse88g@nctu.edu.tw
EDUCATION ¾ Doctor of Science
Sep 2000 ~ Jul 2006 National Chiao Tung University - Hsinchu, Taiwan
Major in materials science and engineering
(Exchange student to DWI an der Aachen, Germany, from Sep 2004 to Mar 2006)
¾ Master of Science Sep 1999 ~ Jun 2000 National Chiao Tung University - Hsinchu, Taiwan
Major in materials science and engineering
¾ Bachelor of Science Sep 1997 ~ Jun 1999 National Taiwan University of Science and
Technology - Taipei, Taiwan Major in chemical engineering
¾ Associate Degree Sep 1992 ~ Jun 1997 National I-Lan Institute of Technology - I-Lan, Taiwan
Major in chemical engineering
CAPABILITIES Synthesis of inorganic nanoparticles, nanorods and nanowires
¾
¾
¾
¾
Design and preparation of organic/inorganic nanocomposites Fabrication of organic light-emitting devices
Operation and maintenance of thermal coater and sputtering Work individually or as a team member in a laboratory setting
¾
¾ Report research results in both Chinese and English written and oral forms
CONFERENCES ¾ Electronic Display Forum 2003 - Tokyo, Japan Co-sponsored by Japan Electronics and Information Technology Industries Association and Semiconductor Equipment and Materials International
¾ The Development and Tread of OLED 2003 - Taipei, Taiwan
Sponsored by Photonics Industry and Technology Development Association
PROJECTS ¾ Preparation of polymer/inorganic hybrid quantum dot laser 2003
¾ Thin Film Polymer Light Emitting Laser 2002 2000
¾ Preparation and characteristic study of light-emitting aromatic polyimides
PhD THESIS Synthesis and characterization of polyimide light-emitting diodes and nanocomposites
PUBLICATIONS ¾ Large-scale fabrication of ZnO nanocrystals by a simple two-step evaporation
PUBLICATIONS ¾ Large-scale fabrication of ZnO nanocrystals by a simple two-step evaporation