Simple Solid-Phase Synthesis of Hollow Graphitic Nanoparticles and their Application to Direct Methanol Fuel Cell Electrodes

Han, S; Yun, Y; Park, K-W; Sung, Y-E; Heon, T. Simple Solid-Phase Synthesis of Hollow Graphitic Nanoparticles and their Application to Direct Methanol Fuel Cell Electrodes. Advanced Materials 15 No. 22 (2003) 1922-1925.

Purpose of the study

To synthesize hollow low graphitic nanoparticles with high crystallinity for the purpose of being integrated into Direct Methanol Fuel Cell (DMFC) electrodes as a catalyst support.

Methods

The source of carbon used for the synthesis of these graphitic nanoparticles was a resorcinol-formaldehyde (RF) gel. This carbon precursor was homogenously mixed in aqueous solution with both cobalt and nickel metal salts. These salts served as the catalytic precursors. The mixture was heated under inert atmospheric conditions in order to generate a metal-carbon composite. This composite consisted of in-situ catalytic metal particles, amorphous carbon, and the desired graphitic nanoparticles. Both acid treatment and KMnO4 oxidation were employed (respectively) in order to remove the metal particle and amorphous carbon impurities and to produce the final product. The crystallinity and basic physical properties of the particles were verified using TEM, HRTEM, XRD, and Raman spectroscopy. To test the practical applications of these particles towards DMFC electrodes, an electrochemical study of a Pt-Ru (1:1) alloy catalyst (synthesized via borohydride reduction) supported on the hollow graphitic nanoparticles was conducted and the results compared to pre-existing catalyst supports.

Key findings

1. TEM images show that the graphitic particles exist in a relatively uniform size range of 30-40nm, with shell thicknesses of 5-8nm, while HRTEM showed that the outer shells of the particles were composed of various graphitic layers.
2. Electrochemical studies showed that the Pt-Ru catalyst supported on the hollow graphitic nanoparticles exhibited higher methanol oxidation, higher electrical conductivity, and a higher maximum power density than both the commercial E-TEK catalyst support and the Vulcan XC-72 catalyst support at 30C (~room temp.) and 60C.
3. In general, hollow graphitic nanoparticle catalyst supports were shown to exhibit superior performance when compared to commercial supports. The synthetic method for these particles uses a relatively cheap polymeric carbon precursor and could be adjusted for large scale production.

Glossary

- TEM: Transmission Electron Microscopy

- HRTEM: High Resolution Transmission Electron Microscopy

- XRD: X-ray diffraction

- KMnO4: potassium permanganate

Controlled Electrophoretic Deposition of Uniquely Nanostructured Star Polymer Films

Suseela Somarajan, Saad A. Hasan, Chinessa T. Adkins, Eva Harth, James H. Dickerson. Controlled Electrophoretic Deposition of Uniquely Nanostructured Star Polymer Films. J. Phys. Chem. B 112 (2008) 23-28.

Purpose of the study
Fabrication and characterization of polystyrene/divinylbenzene (PS/DVB) star polymer nanocrystalline thin films using controlled electrophoretic deposition (EPD) techniques

Methods

The polystyrene/divinylbenzene NMP star polymers were prepared by first degassing a mixture of polystyrene, styrene, and divinylbenzene in chlorobenzene via four freeze-pump-thaw cycles under argon and then heating/stirring the reaction mixture at 124°C for 24 hours. Methanol was used to precipitate the PS/DVB polymers. Nanocrystalline thin films were prepared by applying EDP techniques to a colloidal suspension of star polymer in a stratified hexane:dichloromethane immiscible mixture with a 9:1 volume ratio. This suspension was used in both a mixed and unmixed state. The EPD runs were performed using: silicon dioxide passivated and indium tin oxide coated polished float glass electrodes, 5min deposition time, and an applied voltage of 100V. The resulting thin films were characterized via Atomic Force Microscopy (AFM), Reflectance-Absorption Infrared Spectroscopy (RAIRS), and Scanning Electron Microscopy (SEM).

Key findings

1. EPD of the unmixed PS/DVB solution showed deposition of a translucent film on the (+) electrode while an opaque film deposited onto the (-) electrode. EDP of the mixed PS/DVB solution showed deposition of an opaque film on the (-) electrode [no (+) electrode deposition was observed].
2. Optical microscopy studies showed that micrometer-sized aggregates were dispersed within the tightly packed polymer films. Aggregate diameter lengths ranged from (0.5-5 um) and (> or = to 1 um) for the unmixed negative and positive electrode films (respectively) and (0.5-3um) for the mixed negative electrode film. SEM analysis confirmed these approximations
   
3. AFM studies showed that each of the three collected films possessed a different nanostructure/polymer conformation. The unmixed positive electrode film was found to have the PS/DVB polymers packed in such a way that the individual “arms” of the polymer where absorbed completely to the surface. The unmixed negative electrode film showed intermediate adsorption of the polymer to the surface, resulting in a conformation where only some of the polymer “arms” were absorbed to the surface. The mixed negative electrode film showed weak polymer absorption, resulting in a conformation were only a few of the polymer “arms” were partially absorbed to the surface.
   
4. In general, the above discoveries provide a methodology that permits the synthesis of both positive and negatively charged thin films starting from a neutrally charged polymer.

Glossary

star polymer: macromolecules that have a three-dimensional, compact shape where several polymer chains act as arms linked   to a single core

The characterization of EuO nanocrystals using synchrotoron light

Sukveeradachgul, Pijitrojana. The characterization of EuO nanocrystals using synchrotoron light. Applied Surface Science 254 (2008) 7651-7654.

Purpose of the study

To examine the compositional properties of EuO nanocrystals using X-ray absorption spectroscopy (XAS) techniques: X-ray absorption near-edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS).

Methods

The EuO nanocrystals were prepared  by slowly diffusing oxygen gas (diluted with argon) into a nitrogen purged solution of europium metal and liquid ammonia under “frozen conditions.” The resulting mixture was then warmed to room temperature to yield the EuO nanocrystals. The crystalline structure was verified using X-ray diffraction (XRD), while the physical and surface properties of the nanocrystals were observed using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The electric band character and optical properties were examined using XANES, while EXAFS was used to determine the nanocrystals’ atomic structure.

Key findings

1. XRD patterns showed the nanocrystals to exist in both a crystalline and amorphous phase; with the crystalline phase being either a EuO or Eu(OH)3 structure. The EuO crystal structure possessed a fcc rock-salt packing arrangement.
2. SEM images showed aggregates of EuO nanocrystals forming small grains of less than 1-um in diameter; while the TEM images showed clear lattice fringes. The average EuO nanocrystal had a diameter of 14nm.
3. XANES studies showed that a mixed valence configuration exists between Eu^(2+) and Eu^(3+), and that Eu^(2+) is easier to ionize than Eu^(3+). Using the XANES data it was concluded that the increase in oxidation number depends on the growing of absorption edge energy of Eu-Chalcogenides.
4. EXAFS studies were applied only to EuO nanocrystals where divalent EU was the dominant valence state and showed that the Eu-O and Eu-Eu bond lengths differed from the theoretical lengths by 0.59A and 0.42A (respectively). These differences were due to the size of the crystals decreasing in accordance to quantum confinement properties.
   

Glossary

- FCC: Face Centered Cubic

- XRD: X-ray diffraction

- SEM: Scanning Electron Microscopy

- TEM: Transmission Electron Microscopy

- XAS: X-ray absorption spectroscopy

- XANES: X-ray absorption near-edge structure

- EXAFS: Extended X-ray absorption fine structure

Modelling the size effect on the melting temperature of nanoparticles, nanowires and nanofilms

Safaei, Shandiz, Sanjabi, Barber. Modelling the size effect on the melting temperature of nanoparticles, nanowires and nanofilms. Journal of Physics: Condensed Matter 19 (2007) 1-9.

Purpose of the study

To develop an improved mathematical model for describing the dependence of melting temperature on the size of various nanosolids (such as: nanoparticles, nanowires, and nanofilms.

Methods

Using the proportional relationship between the melting temperature of a nanosolid and its cohesive energy as a starting point, a general mathematical model was derived to explain the effects of size on the nanosolid melting temperatures where the effects of the lattice/surface packing fraction and the coordination numbers for both atoms in the crystal lattice and atoms in the surface crystalline planes are accounted for. The accuracy and practicality of this model were compared to both the liquid drop model (developed previously by Qi) and melting temperature vs. size experimental data for Au and In nanoparticles; Pb and In nanowires; and In nanofilms.

Key findings

1. The general form of the proposed model is described as follows: Tmn/Tmb = 1 – 2(1 – q)[(3 - λ)/3][aX/{(b + (3 - λ)/3)aX}]
2. The use of parameter q allowed the model to fit closely with the experimental data provided. In general, the value q = 1/8 was found to provide the best fit to the experimental data while the value q = 1/4 gave an appropriate fit if experimental errors were accounted for.
3. In regards to the experimental data, the liquid drop model provided a good fit to the Au nanoparticles and Pb nanowires data but was fairly inaccurate when compared to the In nanoparticles, nanowires, and nanofilms data. The proposed method consistently provided good fits to all of the reported experimental data.

Glossary

- Tmn: Nanosolid Melting Temperature

- Tmb: Bulk Melting Temperature

- q: ratio of the coordination number of atoms on the surface to that in the lattice

- λ: type of nanosolid

- a and b: constants dependant on lattice type and atomic diameter

- X: reciprocal of nanosolid size