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On the work function and the charging of small (r <= 5 nm) nanoparticles in plasmas
Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, Faculty of Science & Engineering.
KTH Royal Institute Technology, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0003-3767-225X
University of Paris 11, France.
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2017 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 1, article id 013702Article in journal (Refereed) Published
Abstract [en]

The growth of nanoparticles (NPs) in plasmas is an attractive technique where improved theoretical understanding is needed for quantitative modeling. The variation of the work function W with size for small NPs, r(NP) amp;lt;= 5 nm, is a key quantity for modeling of three NP charging processes that become increasingly important at a smaller size: electron field emission, thermionic electron emission, and electron impact detachment. Here we report the theoretical values of the work function in this size range. Density functional theory is used to calculate the work functions for a set of NP charge numbers, sizes, and shapes, using copper for a case study. An analytical approximation is shown to give quite accurate work functions provided that r(NP) amp;gt; 0.4 nm, i.e., consisting of about amp;gt; 20 atoms, and provided also that the NPs have relaxed close to spherical shape. For smaller sizes, W deviates from the approximation, and also depends on the charge number. Some consequences of these results for nanoparticle charging are outlined. In particular, a decrease in W for NP radius below about 1 nm has fundamental consequences for their charge in a plasma environment, and thereby on the important processes of NP nucleation, early growth, and agglomeration. Published by AIP Publishing.

Place, publisher, year, edition, pages
AMER INST PHYSICS , 2017. Vol. 24, no 1, article id 013702
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-136218DOI: 10.1063/1.4973443ISI: 000395395100092OAI: oai:DiVA.org:liu-136218DiVA, id: diva2:1086255
Note

Funding Agencies|Knut and Alice Wallenberg foundation (KAW) [2014.0276]; Swedish Research Council via the Linkoping Linneaus Environment LiLi-NFM [2008-6572]

Available from: 2017-03-31 Created: 2017-03-31 Last updated: 2018-06-19
In thesis
1. Quantum chemical studies of deposition and catalytic surface reactions
Open this publication in new window or tab >>Quantum chemical studies of deposition and catalytic surface reactions
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum chemical calculations have been used to model chemical reactions in epitaxial growth of silicon carbide by chemical vapor deposition (CVD) processes and to study heterogeneous catalytic reactions for methanol synthesis. CVD is a common method to produce high-quality materials and e.g. thin films in the semiconductor industry, and one of the many usages of methanol is as a promising future renewable and sustainable energy carrier. To optimize the chemical processes it is essential to understand the reaction mechanisms. A comprehensive theoretical model for the process is therefore desired in order to be able to explore various variables that are difficult to investigate in situ. In this thesis reaction paths and reaction energies are computed using quantum chemical calculations. The quantum-chemical results can subsequently be used as input for thermodynamic, kinetic and computational fluid dynamics modelling in order to obtain data directly comparable with the experimental observations.

For the CVD process, the effect of halogen addition to the gas mixture is studied by modelling the adsorption and diffusion of SiH2, SiCl2 and SiBr2 on the (0001̅) 4H-SiC surface. SiH2 was found to bind strongest to the surface and SiBr2 binds slightly stronger than the SiCl2 molecule. The diffusion barrier is shown to be lower for SiH2 than for SiBr2 and SiCl2 which have similar barriers. SiBr2 and SiCl2 are found to have similar physisorption energies and bind stronger than the SiH2 molecule. Gibbs free-energy calculations also indicate that the SiC surface is not fully hydrogen terminated at CVD conditions since missing-neighboring pair of surface hydrogens is found to be common. Calculations for the (0001) surface show that SiCl, SiCl2, SiHCl, SiH, and SiH2 likely adsorb on a methylene site, but the processes are thermodynamically less favorable than their reverse reactions. However, the adsorbed products may be stabilized by subsequent surface reactions to form a larger structure. The formation of these larger structures is found to be fast enough to compete with the desorption processes. Also the Gibbs free energies for adsorption of Si atoms, SiX, SiX2, and SiHX where X is F or Br are presented. Adsorption of Si atoms is shown to be the most thermodynamically favorable reaction followed by SiX, SiHX, and SiX2, X being a halide. The results in this study suggest that the major Si contributors in the SiC–CVD process are Si atoms, SiX and SiH.

Methanol can be synthesized from gaseous carbon dioxide and hydrogen using solid metal-metal oxide mixtures acting as heterogeneous catalysts. Since a large surface area of the catalyst enhances the speed of the heterogeneous reaction, the use of nanoparticles (NP) is expected to be advantageous due to the NPs’ large area to surface ratio. The plasma-induced creation of copper NPs is investigated. One important element during particle growth is the charging process where the variation of the work function (W) with particle size is a key quantity, and the variation becomes increasingly pronounced at smaller NP sizes. The work functions are computed for a set of NP charge numbers, sizes and shapes, using copper as a case study. A derived analytical expression for W is shown to give quite accurate estimates provided that the diameter of the NP is larger than about a nanometer and that the NP has relaxed to close to a spherical shape. For smaller sizes W deviates from the approximative expression, and also depends on the charge number. Some consequences of these results for NP charging process are outlined.

Key reaction steps in the methanol synthesis reaction mechanism using a Cu/ZrO2 nanoparticle catalyst is investigated. Two different reaction paths for conversion of CO2 to CO is studied. The two paths result in the same complete reaction 2 CO2 → 2 CO + O2 where ZrO2 (s) acts as a catalyst. The highest activation energies are significantly lower compared to that of the gas phase reaction. The presence of oxygen vacancies at the surface appear to be decisive for the catalytic process to be effective. Studies of the reaction kinetics show that when oxygen vacancies are present on the ZrO2 surface, carbon monoxide is produced within a microsecond. The IR spectra of CO2 and H2 interacting with ZrO2 and Cu under conditions that correspond to the catalyzed CH3OH production process is also studied experimentally and compared to results from the theoretical computations. Surface structures and gas-phase molecules are identified through the spectral lines by matching them to specific vibrational modes from the literature and from the new computational results. Several surface structures are verified and can be used to pin point surface structures in the reaction path. This gives important information that help decipher how the reaction mechanism of the CO2 conversion and ultimately may aid to improve the methanol synthesis process.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 65
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1925
National Category
Theoretical Chemistry Nano Technology Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-148757 (URN)10.3384/diss.diva-148757 (DOI)9789176853337 (ISBN)
Public defence
2018-08-30, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-06-19Bibliographically approved

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