EMSL: Chongmin Wang's Publications

Bae IT, W Jiang, CM Wang, WJ Weber, and Y Zhang. 2009. "Thermal evolution of microstructure in ion-irradiated GaN." Journal of Applied Physics 105(8):083514, 1-7. doi:10.1063/1.3106606 Abstract The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. Upon thermal annealing, the thickness of the amorphous layer decreases by ~13.1 %, and nano-beam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to debond and react with the Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an atomic volume change of 18.9 % for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7 % after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.

Chambers SA, T Ohsawa, CM Wang, I Lyubinetsky, and JE Jaffe. 2009. "Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Interface." Surface Science 603(5):771-780. Abstract We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.

Fernandez CA, JG Roberts, EM Hoppes, RJ Wiacek, GE Fryxell, JT Bays, MG Warner, CM Wang, JE Hutchinson, and RS Addleman. 2009. "Advancements Toward the Greener Processing of Engineered Nanomaterials -- Effect of Core Size on the Dispersibility and Transport of Gold Nanocrystals in Near-Critical Solvents." Small 5(8):961-969. doi:10.1002/smll.200801207 Abstract In this work, we explore the dispersibility of octanethiol-stabilized gold nanocrystals of different core sizes in compressed ethane and propane over a wide range of fluid conditions. The dispersibility of the nanocrystals was obtained through the Surface Plasmon Resonance (SPR) absorption spectra of solutions. Three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are briefly discussed as tools to interpret the experimental results. Nanoparticle dispersibility-versus-density plots are strongly dependent on nanoparticle size and solvent conditions, with the dispersion of larger nanocrystals more dependant on changes of pressure or density at a given temperature. These results showed a notable correlation with the calculated sedimentation coefficients of the nanocrystals in both solvents. The Chrastil equation was successfully applied to predict and describe the dispersibility of the gold nanocrystals as a function of density, showing that the high stabilities of the nanocrystals dispersions are a result of the very strong solvent-nanocrystal interactions. For the range of nanoparticle sizes studied, compressed ethane at 25 ºC led to a greater tunability of nanoparticle dispersion when compared with compressed propane at 65 ºC. On the other hand, for equivalent pressures compressed propane was found to provide better solubility than ethane due to its higher density. The results of this study quantitatively demonstrate that compressed fluids can offer pressure tunable, size selective control of nanoparticle solvation and transport. This ability has clear advantages over conventional solvents and direct application to various nanomaterials processes, such as separation, transport and purification of nanocrystals.

Fernandez CA, JG Roberts, EM Hoppes, GE Fryxell, CM Wang, JT Bays, MG Warner, RJ Wiacek, and RS Addleman. 2009. "Effect of the Ligand Shell Composition on the Dispersibility and Transport of Gold Nanocrystals in Near Critical Solvents." Langmuir 25(9):4900-4906. doi:10.1021/la804058x Abstract The development of more efficient and greener methods for the synthesis and manipulation of nanomaterials has been a major focus of research among the scientific community. Supercritical (ScFs) and near-critical fluids (NcFs) offer numerous advantages over conventional solvents for these purposes. Among them, ScFs and NcFs offer dramatic reductions in the volume of organic waste typically generated during advanced material processes with the feasibility of changing a number of physicochemical properties by discrete variations in their pressure or temperature. In this work we study the dispersibility of gold nanocrystals of 3.8 nm core size stabilized with different ligand shells in near-critical (NcF) ethane and propane over a wide range of densities by fine-tuning the pressure of these fluids. Dispersibility-vs-density plots are obtained by following the variation in the Surface Plasmon Resonance (SPR) absorption spectra of the nanoparticles. To better understand the results obtained in this study three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are briefly discussed. The dispersibility of the nanocrystals and its behavior with the variation of the fluid density is strongly dependent on the composition of the ligand shell and solvent employed. A correlation between the measured dispersibility values and the calculated sedimentation coefficients was observed in both compressed solvents. In addition, we successfully applied the Chrastil equation to predict and describe the dispersibility of gold nanocrystals with different shells as a function of density determining that the reason for the high stabilities of some of the nanocrystal dispersions is the strong solvent-nanocrystal interactions. While NcF propane showed larger nanocrystal dispersibilities, NcF ethane led to a greater tunability of nanoparticle dispersion in the pressure range of the study. Therefore, with a judicious selection of the fluid, NcFs seem to offer a remarkable advantage over conventional solvents for manipulation of nanomaterials, including transport, purification, and separation of nanocrystals.

Kou R, Y Shao, D Wang, MH Engelhard, JH Kwak, J Wang, VV Viswanathan, CM Wang, Y Lin, Y Wang, IA Aksay, and J Liu. 2009. "Enhanced Activity and Stability of Pt catalysts on Functionalized Graphene Sheets for Electrocatalytic Oxygen Reduction ." Electrochemistry Communications 11(5):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 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.

Murugesan V, SN Kerisit, CM Wang, Z Nie, KM Rosso, Z Yang, GL Graff, J Liu, and JZ Hu. 2009. "Effect of Chemical Lithium Intercalation into Rutile TiO2 Nanorods." Journal of Physical Chemistry C 113(32):14567-14574. doi:10.1021/jp904148z Abstract Rutile TiO2 nanorods were synthesized by hydrolysis of TiCl4 followed by a hydrothermal method. Lithium insertion into the rutile nanorods was achieved by a chemical lithium intercalation process. The structural evolution of nano-structured rutile upon lithium intercalation was characterized by several experimental techniques, namely, XRD, TEM and 6Li MAS NMR. The XRD and TEM studies indicate the formation of a new lithium titanate phase (LixTiO2) during lithium intercalation. Additionally, SAED patterns show that the lithium titanate phase has cubic symmetry. Finally, ultra-high magnetic field (21.1T) 6Li MAS NMR reveals that the lithium titanate phase adopts two different structures depending on lithium content. Taken together, the three techniques consistently show that the intercalation of lithium into rutile TiO2 nanorods causes two consecutive structural phase transformations to lithium titanate phases with spinel (Fd m) and rocksalt (Fm m) structures at x=0.46 and 0.88, respectively. In addition, the broad line widths in the 6Li MAS NMR spectrum of the rocksalt phase are indicative of a disordered structure. Density functional theory calculations of the rutile, spinel and rocksalt bulk phases as a function of lithium content corroborate the observed phase transformations. These phase transitions could account for the large irreversible capacity loss of nano-structured rutile anodes observed in electrochemical cycling experiments.

Saraf LV, Z Zhu, CM Wang, and MH Engelhard. 2009. "Microstructure and Secondary Phase Segregation Correlation in Epitaxial/Oriented ZnO Films with Unfavorable Cr Dopant." Journal of Materials Research 24(2):506-515. doi:DOI: 10.1557/JMR.2009.0054 Abstract We discuss the effect of microstructure on the secondary phase segregation region and mobility of carbon impurities in case of poorly soluble Cr as a dopant in ZnO thin films. Thin films of Cr:ZnO ~50 nm in thickness were grown by metal organic chemical vapor deposition (MOCVD) of Zn(TMHD) and Cr(TMHD) precursors in reactive oxygen partial pressure environment. For an accurate comparison among the differences among the grain-boundary density and degree of orientation on the secondary phase segregation and impurity mobility, simultaneous thin film growths were carried out on single crystals of Si (100), c-plane oriented Al2O3 (c-ALO) and r-plane oriented Al2O3 (r-ALO) substrates. High-resolution transmission electron microscopy (HRTEM) measurements across the film substrate interface indicate that growths on Si(100) and c-ALO resulted in highly oriented Cr:ZnO films whereas a good epitaxial growth was observed on r-ALO. The trace carbon impurity detection, secondary phase formation and their mobility properties were studied by sensitive x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). We have observed that secondary phase segregation regions occur near the surface for Cr:ZnO films grown on ALO whereas the region moves near the interface for the growth on Si. Considering the presence of grain boundaries in Cr:ZnO grown on c-ALO and Si, it appears to be a weak relationship between grain boundary density and unfavorable dopant mobility as well as preferred segregation region. A near uniform stress distribution observed at r-ALO/ZnO interface indicates good epitaxial growth by domain matching epitaxy process. We also observe that low carbon impurity distribution in the studied thickness regime remains more or less uniform inside Cr:ZnO. This gives strong evidence that trace amount of carbon is soluble in the Cr:ZnO system as a direct result of oxygen vacancy defects.

Shao Y, R Kou, J Wang, CM Wang, VV Vishwanathan, J Liu, Y Wang, and Y Lin. 2009. "The durability dependence of Pt/CNT electrocatalysts on the nanostructures of carbon nanotubes: hollow- and bamboo-CNTs." Journal of Nanoscience and Nanotechnology 9(10):5811-5815. Abstract The electrochemical durability of Pt/CNT with hollow- and bamboo-structured carbon nanotubes as the support for PEM fuel cells was investigated using cyclic voltammetry (CV, 0.6-1.1V) accelerated degradation test method. Pt/CNT catalysts were characterized with cyclic voltammograms, rotating disk electrodes, and TEM images. The changes in the electrochemical surface area of Pt and the activity toward oxygen reduction reaction (ORR) before and after the degradation indicate that bamboo-structured carbon nanotubes supported Pt (Pt/B-CNT) catalyst exhibited much higher durability. TEM images indicate that the sintering of Pt nanoparticles was much less for Pt/B-CNT. These are attributed to the specific bamboo-like nanostructures which provide more “bamboo-knot” defects and edge plane-like defects. Pt-support interaction was therefore enhanced and the durability was improved.

She X, M Flytzani-Stephanopoulos, CM Wang, Y Wang, and CHF Peden. 2009. "SO2-induced stability of Ag-alumina catalysts in the SCR of NO with methane." Applied Catalysis. B, Environmental 88(1-2):98-105. Abstract We report on a stabilization effect on the structure and activity of Ag/Al2O3 for the selective catalytic reduction (SCR) of NOx with CH4 imparted by the presence of SO2 in the exhaust gasmixture. The reaction is carried out at temperature above 600 8C to keep the surface partially free of sulfates. In SO2-free gases, catalyst deactivation is fast and measurable at these temperatures. Time-resolved TEM analyses of used samples have determined that deactivation is due to sintering of silver from well-dispersed clusters to nanoparticles to micrometer-size particles with time-on-stream at 625 8C. However, sintering of silver was dramatically suppressed by the presence of SO2 in the reaction gas mixture. The structural stabilization by SO2 was accompanied by stable catalyst activity for the NO reduction to N2. The direct oxidation of methane was suppressed, thus the methane selectivity was improved in SO2-laden gas mixtures. In tests with high-content silver alumina with some of the silver present in metallic form, an increase in the SCR activity was found in SO2-containing gas mixtures. This is attributed to redispersion of the silver particles by SO2, an unexpected finding. The catalyst performance was reversible over many cycles of operation at 625 8C with the SO2 switched on and off in the gas mixture.

Shin Y, CM Wang, MH Engelhard, and GE Fryxell. 2009. "A novel low-temperature dendritic cyclotrimerization of 2,6-diacetyl pyridine leading to mesoporous carbon containing pyridine rings." Microporous and Mesoporous Materials 123(1-3):345-348. doi:10.1016/j.micromeso.2009.03.033 Abstract A simple, direct synthesis of a mesoporous carbon containing pyridine rings is described. This synthesis utilizes the SiCl4 induced cyclotrimerization of 2,6-diacetylpyridine to make a dendritic polymer, built of alternating benzene and pyridine rings. The cyclotrimerization allows for a high degree of crosslinking to take place at low temperatures stabilizing the mesostructure and allowing the carbonization to be carried out at only 600°C, the lowest temperature reported to date for an N-doped mesoporous carbon. The functional mesoporous carbon so formed was found to have a surface area of 1275 m2/g, 35Å pores, and contain 6.8% N.