LAMMPS Simulation of Tungsten Trioxide By: Sarah Bartley Research Mentor: Dr. P. Misra REU Team members: Dr. D. Casmir, R. Garcia-Sanchez, and C. Craig REU 2015 Howard University 2400 Sixth St NW, Washington, DC 20059

Outline Background Gas Sensors Raman Spectroscopy Molecular Dynamics Methodology Goals for the next 5 weeks

What are gas sensors? It measures the concentration of gases. Each gas causes a particular voltage change that enables the substrate to become electrically conductive. Example: Alcohol breath tests

Qualities of Gas Sensors High Sensitivity Surface-to-volume ratio High Selectivity Stability Mechanical and chemical Low cost to manufacture

Tungsten Trioxide Gas Sensor Able to detect nitric oxide and nitrogen dioxide in temperatures of 200 ˚C – 300 ˚C. Nitrogen Dioxide is a toxic gas is fatal due to inhaling. It is typically associated with respiratory problems.

Raman Spectroscopy with Gas Sensors Explore the sensitivity of gas sensors. Temperature’s impact on the Raman spectra. Gas sensors have a optimal operating temp. Increase in temperature decreases Raman intensity.

Why Model Raman Spectra? Currently we are observing temperature's impact on tungsten trioxide. You can analyze the stretching and bending of the molecule with the Raman spectra. Models in LAMMPS makes this process easier to observe.

What is LAMMPS? LAMMPS is a computer program that models the physical interaction between atoms and/or molecules. LAMMPS can be used to simulate experiments between atoms and molecules. Its output data can be used to compare to the experimental data.

LAMMPS Models

LAMMPS NVE Simulation NVE: Number of particles, volume, and energy are constant (Temperature and pressure are not controlled.)

Using LAMMPS to make WO3 Running file Number of atoms and bonds

Issues with Tungsten Trioxide In the input data file, all the atoms are not connected properly. VMD makes crude approximations about the bonds.

NVE Simulation

Overview of NVE Sim.

NVE Simulation

Goals With the use of fix commands, I can regulate the temperature. Temperature experiments displays the molecule vibrating at different temperature. It gives a closer look into the stability of the molecule.

Acknowledgment NSF for funding Dr. D. Casmir for assistance with LAMMPS Research Mentor: P. Misra REU Teammates: Dr. D. Casmir, R. Garcia-Sanchez, C. Craig

References Garcia-Sanchez, R., Ahmido,T., Casimir, D., Baliga, S., & Misra,P. (2013). Thermal Effect Associated with the Raman Spectroscopy of WO3 Gas Sensor Materials. The Journal of Physical Chemistry, 117 (1),13825-13831. Retrieved from http://pubs.acs.org/doi/ipdf/10.1021/jp408303p Ponzoni,A.,Comini,E.,Sberveglieri,G.,Zhou,J.,Deng,S.,Xu,N.,Ding,Y., & Wang,L.(2006). Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks. AIP Scitation. Advance online publication. Doi : 10.1063/1.2203932 Barsan, N., Koziej, D.,& Weimar,U.(2006). Metal oxide-based gas sensor research: How to?. Science Direct, 121(1), 18-35. Retrieved from http://www.sciencedirect.com/science/article/pii/S0925400506006204 Xie,S.,Iglesia,E.,& Bell,A. (2001). Effects of Temperature on the Raman Spectra and Dispersed Oxides. ACS Publication, 105(22),5144-5152. Retrieved from http://pubs.acs.org/doi/pdf/10.1021/jp004434s