Publications by Year: 2009

Herrera-Alonso, M. ; McAllister, M. J. ; Aksay, I. A. ; Prud'homme, R. K. Bridged graphite oxide materials for hydrogen storage, 2009.Abstract
The present invention is a bridged graphite oxide material, comprising at least two graphite oxide sheets in which a plurality of graphite oxide sheets are bridged to at least one other graphite oxide sheet by at least one diamine bridging group. The bridged graphite oxide is formed by the covalent reaction of one amino group of a diamine with a reactive group on the surface or edge of one graphite oxide sheet and the covalent reaction of another amino group of the same diamine with a reactive group on the surface or edge of another graphite oxide sheet. The bridged graphite oxide material may be incorporated in polymer composites or used in hydrogen adsorption media. [on SciFinder(R)]
Punckt, C. ; Aksay, I. A. Dissolution dynamics of thin films measured by optical reflectance. Journal of Chemical Physics 2009, 131.Abstract
Measuring the dissolution dynamics of thin films in situ both with spatial and temporal resolution can be a challenging task. Available methods such as scanning electrochemical microscopy rely on scanning the specimen and are intrinsically slow. We developed a characterization technique employing only an optical microscope, a digital charge coupled device camera, and a computer for image processing. It is capable of detecting dissolution rates of the order of nm/min and has a spatial and temporal resolution which is limited by the imaging and recording setup. We demonstrate the capabilities of our method by analyzing the electrochemical dissolution of copper thin films on gold substrates in a mild hydrochloric acid solution. Due to its simplicity, our technique can be implemented in any laboratory and can be applied to a variety of systems such as thin film sensors or passive coatings.
Castrillon, S. R. V. ; Giovambattista, N. ; Aksay, I. A. ; Debenedetti, P. G. Effect of Surface Polarity on the Structure and Dynamics of Water in Nanoscale Confinement. Journal of Physical Chemistry B 2009, 113, 1438-1446.Abstract
We present a molecular dynamics simulation study of the structure and dynamics of water confined between silica surfaces using beta-cristobalite as a model template. We scale the surface Coulombic charges by means of a dimensionless number, k, ranging from 0 to 1, and thereby we can model systems ranging frorn hydrophobic apolar to hydrophilic, respectively. Both rotational and translational dynamics exhibit a nonmonotonic dependence on k characterized by a maximum in the in-plane diffusion coefficient, D-parallel to, at values between 0.6 and 0.8, and a minimum in the rotational relaxation time, tau(R), at k = 0.6. The slow dynamics observed in the proximity of the hydrophobic apolar surface are a consequence of beta-cristobalite templating an ice-like water layer. The fully hydrophilic surfaces (k = 1.0), on the other hand, result in slow interfacial dynamics due to the presence of dense but disordered water that forms strong hydrogen bonds with surface silanol groups. Confinement also induces decoupling between translational and rotational dynamics, as evidenced by the fact that TR attains values similar to that of the bulk, while D-parallel to is always lower than in the bulk. The decoupling is characterized by a more drastic reduction in the translational dynamics of water compared to rotational relaxation.
Kou, R. ; Shao, Y. Y. ; Wang, D. H. ; Engelhard, M. H. ; Kwak, J. H. ; Wang, J. ; Viswanathan, V. V. ; Wang, C. M. ; Lin, Y. H. ; Wang, Y. ; et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochemistry Communications 2009, 11, 954-957.Abstract
Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with the commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.
Castrillon, S. R. V. ; Giovambattista, N. ; Aksay, I. A. ; Debenedetti, P. G. Evolution from Surface-Influenced to Bulk-Like Dynamics in Nanoscopically Confined Water. Journal of Physical Chemistry B 2009, 113, 7973-7976.Abstract
We use molecular dynamics simulations to study the influence of confinement on the dynamics of a nanoscopic water film at T = 300 K and rho = 1.0 g cm(-3). We consider two infinite hydrophilic (beta-cristobalite) silica surfaces separated by distances between 0.6 and 5.0 nm. The width of the region characterized by surface-dominated slowing down of water rotational dynamics is similar to 0.5 nm, while the corresponding width for translational dynamics is similar to 1.0 nm. The different extent of perturbation undergone by the in-plane dynamic properties is evidence of rotational-translational decoupling. The local in-plane rotational relaxation time and translational diffusion coefficient collapse onto confinement-independent "master" profiles as long as the separation d >= 1.0 nm. Long-tithe tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.
Sabourin, J. L. ; Dabbs, D. M. ; Yetter, R. A. ; Dryer, F. L. ; Aksay, I. A. Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion. ACS Nano 2009, 3 3945-3954.Abstract
We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.
Aksay, I. A. ; Milius, D. L. ; Korkut, S. ; Prud'homme, R. K. Functionalized graphene sheets having high carbon to oxygen ratios for polymer composite, 2009.Abstract
Functionalized graphene sheets having a carbon to oxygen molar ratio of at least about 23:1. A polymer composite comprises the functionalized graphene sheet and ≥1 polymer. [on SciFinder(R)]
Wu, H. ; Wang, J. ; Kang, X. H. ; Wang, C. M. ; Wang, D. H. ; Liu, J. ; Aksay, I. A. ; Lin, Y. H. Glucose biosensor based on immobilization of glucose oxidase in platinum nanoparticles/graphene/chitosan nanocomposite film. Talanta 2009, 80, 403-406.Abstract
The bionanocomposite film consisting of glucose oxidase/Pt/functional graphene sheets/chitosan (GOD/Pt/FGS/chitosan) for glucose sensing is described. With the electrocatalytic synergy of FGS and Pt nanoparticles to hydrogen peroxide, a sensitive biosensor with a detection limit of 0.6 mu M glucose was achieved. The biosensor also has good reproducibility, long-term stability and negligible interfering signals from ascorbic acid and uric acid comparing with the response to glucose. The large surface area and good electrical conductivity of graphene suggests that graphene is a potential candidate as a sensor material. The hybrid nanocomposite glucose sensor provides new opportunity for clinical diagnosis and point-of-care applications.
Kang, X. H. ; Wang, J. ; Wu, H. ; Aksay, I. A. ; Liu, J. ; Lin, Y. H. Glucose Oxidase-graphene-chitosan modified electrode for direct electrochemistry and glucose sensing. Biosensors & Bioelectronics 2009, 25, 901-905.Abstract
Direct electrochemistry of a glucose oxidase (GOD)-graphene-chitosan nanocomposite was studied. The immobilized enzyme retains its bioactivity, exhibits a surface confined, reversible two-proton and two-electron transfer reaction, and has good stability, activity and a fast heterogeneous electron transfer rate with the rate constant (k(s)) of 2.83 s(-1). A much higher enzyme loading (1.12 x 10(-9) mol/cm(2)) is obtained as compared to the bare glass carbon surface. This GOD-graphene-chitosan nanocomposite film can be used for sensitive detection of glucose. The biosensor exhibits a wider linearity range from 0.08 mM to 12 mM glucose with a detection limit of 0.02 mM and much higher sensitivity (37.93 mu A mM(-1) cm(-2)) as compared with other nanostructured supports. The excellent performance of the biosensor is attributed to large surface-to-volume ratio and high conductivity of graphene, and good biocompatibility of chitosan, which enhances the enzyme absorption and promotes direct electron transfer between redox enzymes and the surface of electrodes.
Wang, D. H. ; Choi, D. W. ; Li, J. ; Yang, Z. G. ; Nie, Z. M. ; Kou, R. ; Hu, D. H. ; Wang, C. M. ; Saraf, L. V. ; Zhang, J. G. ; et al. Self-Assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano 2009, 3 907-914.Abstract
We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in the presence of a percolated graphene network embedded into the metal oxide electrodes.
Hirata, Y. ; Hara, A. ; Aksay, I. A. Thermodynamics of densification of powder compact. Ceramics International 2009, 35, 2667-2674.Abstract
This paper established a necessary condition for the sintering of powder compacts by examining the total free energy balance in terms of the particle size, neck size and contact number. The thermodynamic analysis of the proposed model clarifies the relation of shrinkage (q) of powder compact-contact angle (phi)-relative density at a given dihedral angle (phi(e)) of a grain boundary. Faster densification proceeds in the region with a larger coordination number (n) of particles at a small q value. A large shrinkage is needed to eliminate the large pores formed in the structure of small n value. Full density can be achieved in the range of 117 degrees