PacSurf2014 Session NM-MoE: Nanomaterials Characterization & Reactivity I

A detailed understanding of molecular interactions at the interface of two-dimensional GaAs systems under ultra-high vacuum (UHV) conditions has been achieved over the decades. While research on the understanding of such interactions with lower-dimensional GaAs-based structures, such as one-dimensional nanowires (NWs), has not been performed despite the potential importance of these structures in developing nano-electronic devices. Moreover, surface characterization of GaAs under more realistic conditions rather than the UHV studies, are critical in any attempt to correlate surface chemistry with device properties.

Due to recent advances in the surface characterization techniques, and especially the development of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP XPS) [1], we are able to monitor in-situ surface chemistry under elevated pressures and temperatures.

In our present work, we performed NAP XPS studies for different GaAs morphologies:

the simple planar GaAs(100) crystal and a radically non-planar GaAs surface comprised of an ensemble of GaAs NWs under elevated pressures of O₂ or H₂O molecules. The evolution of O₂ and H₂O molecule dissociation on GaAs NWs was tracked under in-situ conditions as a function of temperature and gas pressure to establish whether the processes of dissociation leading to oxidation an hydroxylation depend on surface morphology. In contrast to ideally flat GaAs single crystal surfaces [2], gas molecules experienced the enhanced dissociation on GaAs NW ensembles due to an increase in the surface area ratio and the presence of stepped edges, atom vacancies, and other defects on non-flat semiconductor surfaces [3].

The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533.

[1] D. E. Starr, Z. Liu, M. Havecker, A. Knop-Gericke, and H. Bluhm, "Investigation of solid/vapor interfaces using ambient pressure X-ray photoelectron spectroscopy. ," Chemical Society Reviews, vol. 42, 5833-5857, 2013

[2] X. Zhang, S. Ptasinska, Dissociative Adsorption of Water on an H₂o/GaAs(100) Interface-in-Situ near Ambient Pressure XPS Studies, J. Phys. Chem. C 2014, 118, 4259-4266.

[3] X. Zhang, E. Lamere, X. Liu, J. K. Furdyna, S. Ptasinska, Morphology Dependence of Interfacial Oxidation States of Gallium Arsenide under near Ambient Conditions, Appl. Phys. Lett. 2014, 10.1063/1.4874983.

Pd is a typical material that absorbs hydrogen in its bulk, and hydrogen absorbed in Pd clusters was shown to play an essential role in olefin hydrogenation reactions [1,2]. We have recently studied absorption of hydrogen in Pd(110) [3] and Pd₇₀Au₃₀(110) [4], and shown that hydrogen can be efficiently absorbed in Pd₇₀Au₃₀(110) [4]. In the present work, we have studied reactivity of Pd(110) and Pd₇₀Au₃₀(110) alloy surfaces towards olefin hydrogenation reactions with thermal desorption spectroscopy (TDS) and nuclear reaction analysis (NRA) that allows for high-resolution depth profiling of hydrogen [5].

When a Pd(110) surface was exposed to H₂ at a low temperature, TDS revealed desorption features at ~150 K and ~300 K, which are attributed to hydrogen absorbed in the bulk and adsorbed on the surface, respectively [3]. Whereas coadsorption of C₄H₈ with surface H on Pd(110) revealed no hydrogenation reaction, hydrogenated products of C₄H₁₀ were clearly observed in presence of H in the absorbed state. When the Pd₇₀Au₃₀(110) surface was exposed to H₂, on the other hand, a single desorption feature was recognized at ~250 K, which is different from both the pure Pd(110) and Au(110) surfaces [4]. Examination of the Pd₇₀Au₃₀(110) surface with low-energy electron diffraction and Auger electron spectroscopy revealed that Au segregates at the surface of the alloy. Hydrogen is dissociated at minor Pd sites on the surface and absorbed into bulk through the Pd site without spillover onto the Au site [4]. When C₄H₈ was adsorbed on the D-absorbed Pd₇₀Au₃₀(110) surface, TDS showed no hydrogenated products of C₄H₁₀, which is in remarkable contrast with the Pd(110) surface. Instead of the hydrogenation reaction, H-D exchange reactions were clearly observed. We discuss the reaction mechanisms on these two surfaces.

References

1) A. M. Doyle et al., Ang. Chem. Int. Ed. 42 (2003) 5240.

2) M. Wilde et al., Ang. Chem. Int. Ed. 47 (2008) 9289.

3) S. Ohno et al., J Chem. Phys. 140 (2014) 134705.

4) S. Ogura et al., J. Phys. Chem. C 117 (2013) 9366.

5) K. Fukutani, Curr. Opin. Sol. Stat. Mater. Sci. 6 (2002) 153.

In this work we have utilized a novel Chemical-physical method for synthesis of SnO₂ nanoparticles. In two previous works [1,2], we have reported about the homogeneous precipitation synthesis of SnO₂ powders by two different precipitation agents, Urea and Ammonia; those powders were further ball milled to manufacture SnO₂ pellets and then tested for CO gas sensing. Compared to other methods [3-4] our synthesis route offers SnO₂ particles with very less agglomeration, particle size in the order of 15-20 nm, and homogeneous size distribution of the particles. An research group reported a maximum sensitivity for SnO₂ pellets around 8, for 1000 ppm of CO at operating temperatures of 300⁰C [5], whereas in our pellets, sensitivities were around 300 and 550 for CO when were measured at 300 ppm for 200and 300⁰C, respectively. Later the ball milled powders were mixed with Al₂O₃ powders (particle size around 1µm) with different ratios like 1:1, 2:1 and 4:1 in order to save tin oxide powder and also for increasing the oxygen trapping by increasing the porosity of the pellets. The effect of alumina mixing ratio on the pellets sensitivity was also studied. Maximum sensitivity obtained in pellets manufactured from ball milled SnO₂ powders, at 300 ⁰C for 300 ppm, by two different powder preparation routes were 548 and 262, whereas for mixing pellets with alumina at a 2:1 ratio (SnO₂:Al₂O₃), were 483 and 340, for the same two preparation methods. T he tendency of increasing the sensitivity with the operation temperature and the gas concentration was achieved successfully. The sensitivities obtained for pure ball milled SnO₂ and alumina 2:1 mixed pellets were almost in the same range. Therefore, high sensitivities can be achieved with less sensing material.

Keywords: Gas Sensors; Homogenous Precipitation; Sensitivity; CO, Pellets; Tin Oxide Powders.

References:

[1] Karthik, T.V.K. Maldonado, A., de la L Olvera, M., “Synthesis of tin oxide powders by homogeneous precipitation. Structural and morphological characterization”, IEEE Proceedings, Sept. 2012.

[2] Karthik, T.V.K. Maldonado, A., de la L Olvera, M., “ Manufacturing of Tin Oxide Pellets and their application for CO and C3H8 Gas Sensors ”, IEEE Proceedings, Sept. 2013.

[3] T. Seiyama, A. Kato, K. Fujiishi, and M. Nagatani, Anal. Chem. 34,1502 (1962).

[4] K. Ihokura and J. Watson, “The Stannic Oxide Gas Sensor—Principles and Applications.” CRC Press, Boca Raton, FL, 1994.

Presenting author’s email: krishnakarthik.tv@gmail.com

In order to comprehend the properties affecting the catalytic performance of metal nanoparticles, their dynamic nature and response to the environment must be taken into consideration. The working state of a nanoparticle catalyst might not be the state in which the catalyst was prepared, but a structural and/or chemical isomer that adapted to the particular reaction conditions. This work provides examples of recent advances in the preparation and characterization of nanoparticle catalysts with well-defined sizes and shapes. It discusses how to resolve the shape of nm-sized Pt, Au, Pd, Cu, and PtNi catalysts via a combination of in situ microscopy (AFM, STM, TEM), operando spectroscopy (XAFS, GISAXS) and modeling, and how to follow its evolution under different gaseous or liquid chemical environments and in the course of a reaction. It will be highlighted that for structure-sensitive reactions, catalytic properties such as the reaction rates, onset reaction temperature, activity, selectivity and stability against sintering can be tuned through controlled synthesis. Examples of catalytic processes which will be discussed include the gas-phase oxidation of alcohols (methanol and butanol), the oxidation of NO, and the electrochemical reduction of CO₂. Emphasis will be given to elucidating the role of the nanoparticle size, shape, and chemical state in the activity and selectivity of the former reactions.

The adsorption of environmentally and biologically relevant molecules on the surface of metal oxide nanoparticles can impact the properties of these small particles and thus their behavior. In particular, the impact of surface adsorption of environmentally and biologically relevant molecules from the gas and liquid phase on the properties of nanoparticles in aqueous suspensions (dissolution, aggregation and reaction chemistry) will be presented. Additionally, the role of size, particularly for nanoparticles below 10 nm in diameter, will be discussed. The approach in these studies is to combine nanomaterial characterization using a wide range of techniques including microscopy, spectroscopy, light scattering measurements, along with molecular probes of surface adsorption and surface chemistry to better understand the behavior of oxide nanomaterials in the presence of environmentally and biologically relevant ligands.

Titanium dioxide nanotubes (TiO₂ NTs) have attracted significant attention in biomedical applications due to their biocompatibility and photocatalytic properties. Such a nanomaterial can be coupled with near-infrared irradiation to heat bio-molecules such as cancer cells causing them to death so-called cancer photothermal therapy. In this work, we proposed a novel method to fabricate isolated TiO₂ NTs and utilized them as a therapeutic agent in cancer photothermal therapy. TiO₂ NTs were synthesized by anodization of titanium electrode using diethylene glycol (DEG) +2 vol% HF as electrolyte. TiO₂ NTs were then isolated from Ti substrate by sonication in isopropanol (IPA) for 20 mins. The hepatic cancerous cells (HepG2) were treated with different concentrations of isolated TiO₂ NTs; 0, 6.25, 12.5 and 25 mg/ml under three exposure conditions; dark (no irradiation), UV and Near-Infrared (NIR) irradiation. After the treatment, in vitro cell experiment was performed to measure the viability of the cells. According to the results, the viability of cells under NIR irradiation dropped with the increment of the concentration of TiO₂ NTs. At the concentration of 25 mg/ml the viability was reduced by 31%. Thus, isolated TiO₂ NTs shows promising results for cancer photothermal therapy.

binder was investigated to produce quality pellets by properly controlling the surface properties of

minerals and to beneficiate low grade/fine iron ores. The results showed that zeta potential of

hematite-bentonite mixture did not change with the bentonite dosage. On the other hand, the type and

amount of gangue minerals greatly affected the zeta potential of the mineral mixture with bentonite.

Specifically, the amount of silica presented in the system governed the changes in zeta potential due to

the bentonite adsorption on silica surface and its charge while alumina had no effect. This is due to

their nature of the charges and interaction with bentonite. This study indicated that the mineral

composition of iron ores significantly affected the surface charge of the ore, and can noticeably

influence the quality of pellets formed by iron ores with the binder.