EMSL: Mark Engelhard's Publications

Baer DR, MH Engelhard, AR Felmy, JJ Ford, JZ Hu, AS Lea, P Nachimuthu, LV Saraf, JA Sears, and S Thevuthasan. 2009. "New Approaches for Characterizing Sensor and Other Modern Complex Materials." ECS Transactions 19(6):137-148. doi:10.1149/1.3118546 Abstract Advances in understanding of sensor and other modern complex materials are often enabled by new research tools. This paper highlights three capability development themes used to identify new research tools to be provided to users of the U. S. Department of Energy’s Environmental Molecular Sciences Laboratory. These capability development directions address the importance of dynamic measurements in realistic environments, the need for increased resolution in three dimensional analyses as well as the importance of linking theory and experiment. Capability development involves expanding the range of operation for a number of important techniques, developing and applying new capabilities, and advancing methods of data processing. Examples of current developments are provided including those related to magnetic resonance, x-ray diffraction, application of a focused beam capability to fuel cell aging, and near real time analysis of XPS spectra.

Chang C, S Sankaranarayanan, MH Engelhard, V Shutthanandan, and S Ramanathan. 2009. "On the relationship between non-stoichiometry and passivity breakdown in ultra-thin oxides: combined depth-dependent spectroscopy, Mott-Schottky analysis and molecular dynamics simulation studies." Journal of Physical Chemistry C 113(9):3502-3511. Abstract Understanding the relationship between non-stoichiometry and physical properties of ultra-thin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultra-thin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultra-thin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes; we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott-Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ~3.8nm) in case of photon-assisted oxidation. The denser oxide film results in a ~34% more blockage of chloride ions transport as indicated by XPS analysis. Mott Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces due to atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self limiting thickness of ~ 16 and 22 Å, respectively at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40-70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide-gas interface to 1.30 at metal-oxide interface whereas that formed by natural oxidation was sub-stoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide-gas interface) to 1.05 (metal-oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations.

Gupta S, SVNT Kuchibhatla, MH Engelhard, V Shutthanandan, P Nachimuthu, W Jiang, LV Saraf, S Thevuthasan, and S Prasad. 2009. "Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices." Sensors and Actuators. B, Chemical 139(2):380-386. doi:10.1016/j.snb.2009.03.021 Abstract In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3(0001) exhibit (111) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1mTorr-100Torr) and temperatures (623K to 973K), is significantly lower than that of pure ceria under same conditions. The 6Sm% doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.

Hlaing Oo WM, LV Saraf, MH Engelhard, V Shutthanandan, L Bergman, J Huso, and MD Mccluskey. 2009. "Suppression of conductivity in Mn-Doped ZnO Thin Films." Journal of Applied Physics 105(1):013715. doi:10.1063/1.3063730 Abstract We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition (MOCVD). The ion beam, surface and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was noticed up to ~ 4.5 atom% Mn doping. The presence of Mn2+, confirmed by X-ray photoelectron spectroscopy (XPS), is correlated with the reduction in conductivity. No major change in the activation energy (~40 meV) and a reduction in the Zn/O ratio as a function of Mn concentration in highly sensitive proton induced X-ray emission (PIXE) technique also support this hypothesis. We discuss our results from a view point of homogeneous Mn distribution, elemental XPS ratio offsets and secondary phase formations in ZnO films.

Kim DH, J Szanyi, JH Kwak, X Wang, JC Hanson, MH Engelhard, and CHF Peden. 2009. "Effects of sulfation level on the desulfation behavior of pre-sulfated Pt BaO/Al2O3 lean NOx trap catalysts: a combined H2 Temperature-Programmed Reaction, in-situ sulfur K-edge X-ray Absorption Near-Edge Spectroscopy, X-ray Photoelectron Spectroscopy, and Time-Resolved X-ray Diffraction Study." Journal of Physical Chemistry C 113(17):7336-7341. doi:10.1021/jp900304h Abstract Desulfation by hydrogen of pre-sulfated Pt(2wt%) BaO(20wt%)/Al2O3 with various sulfur loading (S/Ba = 0.12, 0.31 and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), x-ray photoelectron spectroscopy (XPS), in-situ sulfur K-edge x-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved x-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in-situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst, rather than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H2S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H2O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H2S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt BaO/Al2O3 lean NOx trap catalysts is markedly dependent on the sulfation levels.

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.

Kuchibhatla SVNT, P Nachimuthu, F Gao, W Jiang, V Shutthanandan, MH Engelhard, S Seal, and S Thevuthasan. 2009. "Growth-Rate Induced Epitaxial Orientation of CeO2 on Al2O3(0001)." Applied Physics Letters 94(20):204101:1-3. doi:10.1063/1.3139073 Abstract High-quality ceria (CeO2) films were grown on sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy. The epitaxial orientation of the ceria films has been found to be (100) and (111) at low (< 8 Å/min) and higher growth rates (up to ~30 Å/min), respectively. Evidence shows that CeO2 (100) film grows as three-dimensional islands, while CeO2 (111) proceeds with layered growth. Three in-plane domains at 30° to each other are observed in the CeO2 (100), which is attributed to the close match of the oxygen sub-lattices in the film and substrate that has a three-fold symmetry. Molecular dynamic simulations have further confirmed that the CeO2 film retains (100) orientation on the Al2O3 (0001) substrate.

Lim IIS, D Mott, MH Engelhard, Y Pan, S Kamodia, J Luo, P Njoki, S Zhou, L Wang, and CJ Zhong. 2009. "Interparticle Chiral Recognition of Enantiomers: A Nanoparticle-Based Regulation Strategy." Analytical Chemistry 81(2):689-698. doi:10.1021/ac802119p Abstract The ability to regulate how molecular chirality of enantiomeric amino acids operates in biological systems constitutes the basis of drug design for specific targeting of diseases. We report herein nanoparticle-administered chiral recognition of enantiomeric cysteines as a general molecular tuning strategy to regulate the interactions of chiral amino acids. This strategy exploits the pairwise zwitterion interaction of enantiomeric cysteines adsorbed on gold nanoparticles to create a chiral footprint for interparticle enantiomeric recognition. The chiral recognition is evidenced by the sharp kinetic contrast between interparticle homochiral and heterochiral reactivities. The experimental-theoretical correlation of the interparticle reactivity with the enantiomeric ratio reveals that the chiral recognition is tunable both molecularly and by nanoparticles, which has potential applications in drug design for targeting or delivery.

Milbrath BD, JA Caggiano, MH Engelhard, AG Joly, DW Matson, P Nachimuthu, and LC Olsen. 2009. " Using Thin Films to Screen Possible Scintillator Materials ." IEEE Transactions on Nuclear Science 56(3, PT 3): 1650-1654. Abstract The discovery and optimization of new scintillators has traditionally been a rather slow process due to the difficulties of single crystal growth. This paper discusses the production of polycrystalline scintillator thin films (a few microns thick) which were tested in order to determine what characterizations could be made concerning a material’s ultimate potential as a scintillator prior to pursuing crystal growth. Thin films of CaF2(Eu), CeF3, and CeCl3, all known scintillators, were produced by vapor deposition. The hygroscopic CeCl3 was coated with multiple polymer-aluminum oxide bi-layers. Emission spectra peak wavelengths and decay times agreed with single crystal values. The films were too thin to measure gamma photopeaks, but using alpha energy deposition peaks, one could compare the relative photon yield/MeV between materials. The values obtained appear to give a relevant indication of a material’s light yield potential. The technique also appears useful for quickly determining the proper dopant amount for a given material.

Nachimuthu P, YJ Kim, SVNT Kuchibhatla, Z Yu, W Jiang, MH Engelhard, V Shutthanandan, J Szanyi, and S Thevuthasan. 2009. "Growth and characterization of barium oxide nanoclusters on YSZ(111)." Journal of Physical Chemistry C 113(32):14324-14328. doi:10.1021/jp9020068 Abstract Barium oxide (BaO) was grown on YSZ(111) substrate by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as clusters on YSZ(111). During and following the growth under UHV conditions, BaO remains in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation BaCO3 on the surface and partly Ba(OH)2 throughout in the bulk. Based on the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. The variation in the reactivity of BaO between different attempts of the growth is attributed to the cluster size.