Nanoparticles: Synthesis, Stabilization ... - American Chemical Society

Chapter 7

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch007

Functional Nanostructures by Wet Chemistry: A Tool to Ordered 1D and 3D Nanostructures 1

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Lidiany Gonzalez , Eunice Mercado , Priscila Santiago , Madeline Leon , Marissa Morales , Roberto Irizarry , and Miguel E. Castro 1

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Chemical Imaging Center, University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680 DuPont Electronic Technologies, 14 T. W. Alexander Drive, Research Triangle Park, NC 27709 2

Hollow nanostructures are functional structures suitable for high energy material storage, controlled drug release, light weight structural materials, catalysts and filters. We present evidence, based on scanning and transmission electron microscopy measurements, for the formation of one and three dimensional hollow nanostructures from chemically modified silver particles. One dimensional structures form in acid media while an increase in the solution ionic strength result in the formation of hollow nanostructures that exhibit three dimensional order. Such structures can find applications in high energy material storage, catalysis, electrochemistry and biomedicine.

© 2008 American Chemical Society

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In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction There is a tremendous interest in the synthesis of nanostructures suitable for the storage of high energy materials. This interest drives from the need to replace our dependence on fossil fuels with environmentally friendly alternatives (1,2). Hydrogen is the most abundant element in the universe and does not create environmental pollution when used as a fuel. Overcoming hydrogen storage is among the important steps toward widespread commercialization and implementation of fuel cells in vehicles. Organic and inorganic molecules with high hydrogen content and which can release the hydrogen in their decomposition reactions are as important as hydrogen itself. Depending of the application, storage of hydrogen and hydrogen containing molecules in adequate structures is central to find green solutions to our energy needs. The use of nanotechnology for this task promises to result in a reduction of space required for high energy materials storage and transport, which can trickle down to the miniaturization of fuel cells. The broader impact of cost effective and efficient functional nanostructures for hydrogen and hydrogen materials storage resides in (i) facilitation of fuel transportation and (ii) the miniaturization of fuel cells and the (iii) acceleration in the implementation of environmentally friendly energy alternatives in vehicles. Nanotubes and hollow nanospheres are excellent candidates for hydrogen storage. Short diffusion distances and surface sites to dissociate hydrogen are among the characteristics of ideal nanostructures for hydrogen storage materials. The low density and high strength of nanotubes has made them ideal for their exploration as hydrogen storage material. Research in this area has been very active since the end of the last decade and the beginning of the new millennium. The availability of new methods for the synthesis of carbon nanotubes (3-6), as well as our energy needs, accelerated work in the area. Recent work suggests that titanium decorated SWCN may be able to store about 8 % H per weight, slightly higher than the 6 % per weigh minimum required by the Freedom Car Research Partnership Act set forth by the Department of Energy and major American car manufacturers (7). The synthesis of hollow nanostructures has received considerable less attention than pure metal or semiconductor counterparts (8-12). Ideal hollow nanostructure synthesis platforms should take advantage of methods to functionalize nanoparticles into structures suitable for high energy storage. As illustrated in Figure 1 below, such structures may include nanotubes and nanospheres. In the case of nanotubes with a very thin diameter and decorated with metal nanoparticles, new effects may be brought about such as electron conduction across their wall, resulting in nanoscaled electrochemical cells suitable for hydrogen generation from electrolysis of water or other chemicals. In the last few years, we have worked in the ensemble of silver into hollow nanostructures like those displayed in Figures la, b and lc. From the perspective 2


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of a starting material that consists of silver nanoparticles, we can think about the hollow nanostructures in la and lb as one and three dimensional constructions from nanoparticle building blocks. These are already functional nanostructures, suitable for high energy material storage. The nanostructure displayed on Figure lc, on the other hand, is clearly a step ahead in complexity and is suitable for energy conversion chemistry thru electrochemical processes.

Figure 1. The scheme shows possible hollow nanostructure suitable for hydrogen storage in (a) and (b). A more specialized hollow nanostructure is displayed in (c), where metal nanoparticles added to the outside wall of a hollow nanotube can be used as electrodes for the electrochemistry offluids moving inside the tube. The inset at the right illustrates the electron flow from the - to a + charged nanoparticle.

Results The approach to the synthesis of one dimensional nanostructures, like the one displayed on Figure lb, is based on wet chemistry. Wet chemical approaches methods are a promising tool to the mass production of functional nanostructures. The thiol itself must have several structural restrictions, besides just simply a sulfur atom to bond to the silver surface.



Hydrogen bonding: particle assembly 1 d order

Figure 2. General approach employed to the synthesis of one dimensional nanostructures. A thiol capped silver particle is assembled into one dimensional nanostructure.

Chain Length: Control particle-particle spacing Control removal

Sulfur end: Anchor to particle surface


81 The terminal group is also important. Carboxylic acids are ideal for this purpose: hydrogen bonding interactions can be used for inter-particle links while pH control may be used to turn such links "on" and "off. We have demonstrated that mercapto acetic acid (HOOCCH SH) capped silver nanoparticles is capable of driving such order. As displayed on Figure 3, hydrogen bonding interactions among the carboxylic acid terminal group in HOOCCH S- Ag can be used as a linker. 2


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Bifunctional thiol + A g

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Assembly wet chemical approach

Linear arrangement of particles

Figure 3. Cartoon illustrating the concept of hydrogen bonding interactions among carboxylic acid end of alkyl thiols bonded to silver surfaces.

The synthesis of the precursors is made by adding HSCH COOH to silver nitrate in water. Scanning electron microscopy (SEM) measurements of the wet solution are summarized in Figure 4. Large micelles, like the one displayed on Figure 4a, are the dominant structure observed. Once in a while we will observe 2


82 some of these micelles broken and some one dimensional structure inside the area as displayed on Figures 4b to 4f. In favorable cases, we can observe the formation of tubes in some of the micelles, as displayed in the inset of Figure 4f. The large amount of water present solvates the carboxylic acid ends of the thiol molecules and prevents the formation of links and one dimensional structures, although there are enough fluctuations to allow their occasional detection. Typical scanning and transmission electron microscopy images of dry deposits of a mixture of AgN0 and thiol diluted with 10.0 mL of water are displayed on Figures 5 and 6, respectively. A bundle of nanofibers is identified in SEM. These one dimensional structures are readily identified in the TEM image. The primary beam energy of the TEM employed for the measurements reported here allowed us to obtain clear images of the edge of the deposits. The TEM does allow imaging these nanostructures in greater detail than in SEM measurements. The nanostructures are between 15 and 30 nm in diameter while their length variedfromabout 230 nm up to several micrometers. It is interesting that, not a single "free" particle is observed in TEM images of dry deposits of aqueous dispersions of thiol-silver mixtures, despite the fact that their existence is suggested in independent UV-visible absorption measurements (X 3240-380 nm). The particles, if present, must be inside the nanofibers. Inspection of the regions indicated by the arrows at higher magnifications confirms the presence of small particles inside the fibers. We were able to observe these formations in all the nanofibers examined in the TEM. The size of the particles is very difficult to determine from the images. We estimate an upper limit of 1 nm for the diameter of the particles that can be distinguished in the image. This particle size is considerable smaller than those reported in the literature for silver nanoparticle synthesis in basic media. Smaller structures are observed on the edges of the walls of the nanofibers, but the images are not resolved well enough to interpret them in terms of the silver dimer, Ag . In passing, we note that the pH of the solution is below 3.0 . The density of one dimensional nanostructures decreases with solution pH and disappear when a pH>7 is attained, indicating that hydrogen bonding interactions are responsible for the formations displayed in Figures 5 and 6. If particles are inside the one dimensional structures, we hypothesized that the one dimensional structures must be hollow and a strong binding molecule should be able to remove the particles from the inner volume of the formation. Figure 7 shows SEM images of dry deposits of the thiol-silver mixture treated with a cysteine solution. The nanoparticles are displaced to the outside walls of the one dimensional formation, a result that confirms the hollow nature of the formation, consistent with the images displayed in Figure 4f, which shows the formation of a hollow nanotube. These particles have probably undergone several chemical processes and their sizes are likely to be different from those of the particles contained in the original nanostructure.


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m a x

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Figure 4. From (a) to (f) illustrate wet SEM measurements of aqueous colloidal solutions containing of mercapto acetic acid and silver. Continued on next page.



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Figure 4. Continued.



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Figure 5. Scanning electron microscopy image of a dry deposits of a silver-thiol mixture diluted with 10.0 mL of water.

Figure 6. Transmission electron microscopy images ofdry deposits of a silver-thiol mixture diluted with 10.0 mL of water. The insets corresponds to the regions indicated by the arrows magnified by a factor of 500 to indicate the formation of the particles. The region of a wall magnified 1000 times is also displayed in the inset.



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Figure 7. SEM image of a dry deposit of a thiol-silver material treated with cysteine. The particles are observed on the walls of the tube and the inner volume of the tube is now empty.

There are several approaches to bring three dimensional order to colloidal solutions. For instance, electrostatic or magnetic interactions among particles may result in specific long range order among particles in solution. To test this possibility, a few micro liters of an aqueous sodium nitrate (NaN0 ) solution were added to the colloidal solution of the thiol and silver. Wet SEM measurements performed on the colloid are summarized in Figure 9. The first thing to note is the order that is exhibited by the micelles. Nearly linear formations are clearly distinguished in the image. When the electron beam is 3

oo£ oooo oorV—-oooo og oooo Q

disordered

ordered

Figure 8. Bringing order to a disordered array in solution.


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focused on one of the micelles it is easy to see that they are hollow spheres. The size of each sphere is comparable to the size of the dominant nanostructures found in solution (see Figure 4), which are several micrometers in diameter. The formation of spheres is a surprising result, since we expect the electrostatic interactions to affect only the long range order and have little or no effect on internal structure of the micelle Vibrational spectroscopy work is needed to fully characterize the chemical composition of these micelles and learn on the tools that can be used to bring one and two dimensional order at the nanoscale.

Figure 9. Three dimensional ordered silver-thiol colloids. The SEM image at the right hand side shows the electron beam burned hole in one of the hollow nanospheres.

Conclusions Hydrogen bonding interactions among the carboxylic acid end of the thiol on Ag drive 1 dimensional order of nanoparticles while longer range electrostatic interactions may be effective in driving the 3D order of suspension. The hollow nanostructures presented here may be useful for applications in materials storage, for fuel cell applications, dug delivery and nanostructured batteries.

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Nanoparticles: Synthesis, Stabilization ... - American Chemical Society

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