Sharmila M. Mukhopadhyay

Surface chemical states of barium titanate: Influence of sample processing

The composition and chemistry of the near-surface region of BaTiO 3 have been studied using x-ray photoelectron spectroscopy (XPS). It is found that the Ba3 d photoclectron peak shows two chemical states, one of which is attributed to the bulk perovskite and the other to a special surface state unrelated to contamination. The bulk component is reduced and the surface component increases when the material is annealed at high temperatures (either in reducing or oxidizing atmosphere). Both the components are unaltered if the sample is exposed to air, solvents, or water: processes that lead to adsorption of impurities. The surface peak, therefore, attributed to a relaxation related and not contamination-related state, has been compared with those in other Ba-containing oxides. The oxygen photoelectron peak consists of a normal perovskite peak typical of most titanates and a higher energy component clearly related to surface contamination. Annealing in reducing atmosphere results in drastically different optical and electrical properties, and in chemical reduction of some Ti 4+ ions to Ti 3+ . The overall stoichiometry, however, does not change with annealing atmosphere. These results have been discussed in light of our current understanding of this and other related oxides.

Plasma assisted hydrophobic coatings on porous materials: influence of plasma parameters

Cellulose-based filter paper was plasma treated for hydrophobicity and the permeation of the coating investigated. A five-layer stack of filter paper was the model porous medium and perfluoromethylcyclohexane (PFMCH) was the model monomer used in this study. Water-drop-absorption time (time needed for certain size droplet of water to be completely absorbed) was recorded as a measure of coating effectiveness. Video contact angle measurements were used to evaluate surface hydrophobicity and x-ray photoelectron spectroscopy was used to analyse the surface chemistry of each surface in the stack. It is clear that the outer surface of normally hydrophilic filter paper can easily be made hydrophobic by treatment with PFMCH plasma. By adjusting plasma power, pressure and time, a filter paper can be treated to have one side hydrophobic (water repellent) and the other side hydrophilic (water absorbent). The control of coating penetration into inner layers needs more careful study: it is seen that plasma permeation depends to a small extent on plasma power, negligible extent on background pressure, but to a very large extent on treatment time. During initial deposition, both, the water-drop-absorption rate and the contact angle, have exponential relationships with plasma time. It is shown that CF2 and CF3 functional groups are related to hydrophobic behaviour and that the contact angle and water-absorption time can be correlated to the total fluorine concentration on the surface. A very small amount of surface F is needed for hydrophobic behaviour.

Hierarchical nanostructures by nanotube grafting on porous cellular surfaces

Natural biological systems make use of capillary-type hierarchical structures in order to enhance surface functionality within limited size. This paper discusses fabrication of similar synthetic structures by grafting carbon nanotubes (CNTs) on microcellular substrates such as graphitic foam. A major hurdle so far had been deposition of dense CNT layers inside uneven pores. This has been overcome in this study by pre-coating the porous surface with plasma-derived silica molecules. It is seen that the pre-coating not only increases the density of nanocatalyst attachment on the surface but also makes each nanocatalyst more effective in nucleation and growth of nanotubes. The CNT layers formed are strongly attached to the substrate, which makes them particularly suitable for use in robust hierarchical devices in the future.

Structural investigation of graphitic foam

Structural and chemical characteristics of pitch-based graphitic foam have been studied using scanning electron microscopy, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy. Chemically, the carbon atoms in these materials are found to have identical bonding states as those in pure graphite single crystals. Microstructural studies indicate that they have a cellular morphology with the cell walls made up of graphitic layers. The walls can be smooth or stepped depending upon the orientation of graphitic layers with respect to the cells. Ligaments between neighboring cells and junctions of ligaments (corners of three or more cells) distinctly show layers of graphitic planes, irregular flakes, and beam-like protruding structures made up of folded layers of graphite. The network of interconnecting pores have openings on the cell walls that have ruptured edges and sharp corners. This indicates that they were formed after hardening of the precursor, resulting in brittle fracture of the ...

Novel Platform Development Using an Assembly of Carbon Nanotube, Nanogold and Immobilized RNA Capture Element towards Rapid, Selective Sensing of Bacteria

This study examines the creation of a nano-featured biosensor platform designed for the rapid and selective detection of the bacterium Escherichia coli. The foundation of this sensor is carbon nanotubes decorated with gold nanoparticles that are modified with a specific, surface adherent ribonucleiuc acid (RNA) sequence element. The multi-step sensor assembly was accomplished by growing carbon nanotubes on a graphite substrate, the direct synthesis of gold nanoparticles on the nanotube surface, and the attachment of thiolated RNA to the bound nanoparticles. The application of the compounded nano-materials for sensor development has the distinct advantage of retaining the electrical behavior property of carbon nanotubes and, through the gold nanoparticles, incorporating an increased surface area for additional analyte attachment sites, thus increasing sensitivity. We successfully demonstrated that the coating of gold nanoparticles with a selective RNA sequence increased the capture of E. coli by 189% when compared to uncoated particles. The approach to sensor formation detailed in this study illustrates the great potential of unique composite structures in the development of a multi-array, electrochemical sensor for the fast and sensitive detection of pathogens.

METALLIZATION OF ELECTRONIC POLYMERS: A COMPARATIVE STUDY OF POLYVINYLIDENE FLUORIDE, POLYTETRAFLUOROETHYLENE, AND POLYETHYLENE

The initial stages of bond formation between a piezoelectric fluropolymer, polyvinylidene fluoride (PVDF) (‐CF2‐CH2−)n and a moderately reactive metal (Ni) has been studied using x‐ray photoelectron spectroscopy. In order to understand the two different types of C bond present (‐CF2− and ‐CH2−), these studies have been extended to include the polymers poly‐tetrafluoroethylene (PTFE) (‐CF2−)n and polyethylene or PE (‐CH2−)n. It is found that Ni attacks the C and F in the ‐CF2− component, forming nickel fluoride and at least two new C containing products (perhaps a combination of ‐CFNi‐ and graphite‐like C). On the other hand, the ‐CH2− species is unaffected by Ni. Therefore, among the polymers studied, the PTFE surface is most severely altered by Ni followed by PVDF. PE is totally inert to this metal. A semiquantitative analyses of these interactions has been presented in light of the information currently available in the literature.The initial stages of bond formation between a piezoelectric fluropolymer, polyvinylidene fluoride (PVDF) (‐CF2‐CH2−)n and a moderately reactive metal (Ni) has been studied using x‐ray photoelectron spectroscopy. In order to understand the two different types of C bond present (‐CF2− and ‐CH2−), these studies have been extended to include the polymers poly‐tetrafluoroethylene (PTFE) (‐CF2−)n and polyethylene or PE (‐CH2−)n. It is found that Ni attacks the C and F in the ‐CF2− component, forming nickel fluoride and at least two new C containing products (perhaps a combination of ‐CFNi‐ and graphite‐like C). On the other hand, the ‐CH2− species is unaffected by Ni. Therefore, among the polymers studied, the PTFE surface is most severely altered by Ni followed by PVDF. PE is totally inert to this metal. A semiquantitative analyses of these interactions has been presented in light of the information currently available in the literature.

Surface properties of perovskites and their response to ion bombardment

X‐ray photoelectron spectroscopy has been used to study the surface composition and chemistry of two perovskites: SrTiO3 and Pb(Zr,Ti)O3 (commonly known as PZT). It is seen that ion bombardment, which is a common surface modification technique, can cause substantial changes in these oxides. The PZT surface undergoes surface depletion of lead along with chemical reduction of the Pb2+ ion to its metallic state. The Zr/(Ti+Zr) ratio also changes with sputtering, but the total oxygen to cation ratio is unchanged. On the other hand, the surface stoichiometry of SrTiO3 is almost unaffected by ion bombardment. In all the perovskites, irrespective of whether the composition changes or not, a substantial amount of surface Ti is reduced to a lower valency state on sputtering. Most of this component is restored back to the original Ti4+ state when Ni is evaporated on these surfaces, indicating that the reduced state is associated with a damaged outermost surface that can be repaired with an adsorbate. The implicatio...

Thin Films for Coating Nanomaterials

Abstract For nano-structured solids (those with one or more dimensions in the 1-100 nm range), attempts of surface modification can pose significant and new challenges. In traditional materials, the surface coating could be several hundreds nanometers in thickness, or even microns and millimeters. In a nano-structured material, such as particle or nanofibers, the coating thickness has to be substantially smaller than the bulk dimensions (100 nm or less), yet be durable and effective. In this paper, some aspects of effective nanometer scale coatings have been discussed. These films have been deposited by a non-line of sight (plasma) techniques; and therefore, they are capable of modifying nanofibers, near net shape cellular foams, and other high porosity materials. Two types of coatings will be focused upon: (a) those that make the surface inert and (b) those designed to enhance surface reactivity and bonding. The former has been achieved by forming 1-2 nm layer of —C—CF 2 — (and/or CF 3 ) groups on the surface, and the latter by creating a nano-layer of SiO 2 -type compound. Nucleation and growth studies of the plasma-generated film indicate that they start forming as 2-3 nm high islands that grow laterally, and eventually completely cover the surface with 2-3 nm film. Contact angle measurements indicate that these nano-coatings are fully functional even before they have achieved complete coverage of 2-3 nm. They should therefore be applicable to nano-structural solids. This is corroborated by application of these films on vapor grown nanofibers of carbon, and on graphitic foams. Coated and uncoated materials are infiltrated with epoxy matrix to form composites and their microstructure, as well as mechanical behaviors are compared. The results show that the nano-oxide coating can significantly enhance bond formation between carbon and organic phases, thereby enhancing wettability, dispersion, and composite behavior. The fluorocarbon coating, as expected, reduces bond formation, and therefore, effective as an inert layer to passivate nanomaterials.

Nanoscale Multifunctional Materials: Science and Applications

Preface. Section I. Overview. 1. Key attributes of nano-scale materials and functionalities emerging from them (S. M. Mukhopadhyay). 2. Societal Impact and Future Trends in Nanomaterials (S. M. Mukhopadhyay). Section II. Processing and Analysis. 3. Fabrication Techniques for Growing Carbon Nanotubes (I. T. Barney). 4. Nanoparticles and Polymer Nanocomposites (G. A. Jimenez, B. J. Lee, and S. C. Jana). 5. Laser-Assisted Fabrication Techniques (T. Murray). 6. Experimental Characterization of Nanomaterials (A. Jackson). 7. Modeling and Simulation of Nanoscale Materials (S. Patnaik and M. Tsige). Section III. Applications. 8. Nanomaterials for Alternate Energy (H. Huang and B. Z. Jang). 9. Enhancement of Through-Thickness Thermal conductivity in Adhesively Bonded Joints Using Aligned Carbon Nanotubes (S. Sihn, S. Ganguli, A. K. Roy, L. Qu, and L. Dai). 10. Applications of Metal Nanoparticles in Environmental Cleanup (S. R. Kanel, C. Su, U. Patel, and A. Agrawal). 11. Application of Carbon Nanomaterials in Water Treatment: Removal of Common Chemical and Biological Contaminants by Adsorption (V. K. K. Updahyayula, J. R. Ruparelia, and A. Agrawal). 12. Peptide Nanotubes for Biomedical and Environmental Applications (B. W. Park and D. S. Kim). Index.

Surface Modification of Graphitic Foam

This article discusses surface-related issues and possible modification approaches in high-porosity graphitic foam. The microcellular solid is made of graphitic carbon walls, ligaments, and beams supporting a network of interconnected pores. This makes the exposed surface area very high and its understanding a necessity for most applications. Graphitic planes are seen to be stacked at various orientations with respect to exposed surfaces. Therefore, a simplified analytical model that assumes “random” graphitic planes forming a three-dimensional array of tetrahedral cells may be an appropriate approximation. The influence of oxidizing chemicals such as nitric acid and hydrogen peroxide on surface properties has been studied using electron microscopy, photoelectron spectroscopy, and water absorption tests. Bulk properties such as density measurements and mechanical tests have been performed in parallel. It is seen that exposure to nitric acid results in an increase in oxygen-containing functional groups on ...