Raul F. Garcia-Sanchez, Prabhakar Misra Department of Physics and Astronomy Howard University June 15,

Metal Oxide Gas Sensors Tungsten Oxide (WO 3 ) Raman Spectroscopy Experimental Setup Sample Comparison Thermal Effects Humidity Effects Summary Conclusion Future Work Acknowledgements 2

Metal Oxide Gas Sensors (MOGS) are solid-state gas detecting devices for commercial and industrial applications. Metal Oxide Gas Sensors can be used in the detection of various compounds: Carbon and Nitrogen Oxides Hydrogen Ammonia Other gases Metal Oxides are one of the most researched materials in gas sensing applications. Metal oxides selected for gas sensors can be determined from their electronic structure. 3

An n-type semiconductor material. Operational temperatures: °C WO 3 (along with SnO 2 ) is one of the most used metal oxides for gas sensing applications. It is the most used metal oxide for the detection of nitrogen oxides (NO x ) Has been used in research for sensing: H 2 S, Cl 2, CH 4, SO 2, CO and others. 4 Figure 1. Scanning Electron Microscope images of a) WO 3 :Si, b) WO 3 nanopowder and c) WO 3 nanowires, at 600, 100 and 200 nm scales, respectively. [1] "Gas sensing selectivity of hexagonal and monoclinic WO3 to H2S," I.M. Szilagyi, S. Saukko, J. Mizsei, A.L. Toth, J. Madarasz and Gyorgy Pokol, Solid State Sciences 12 (2010) 1857; doi: /j.solidstatesciences

5 Power (mW) Exposure Time (s) Laser Spot Diameter (µm) Sample Distance (cm) to Objective Lens Temperature Range (°C) Temperature Step (°C) (Nanowires) 1.0 (Nanopowder) 1.3 (WO 3 :Si)

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Raman Band (cm -1 ) 30 ° C Raman Band (cm -1 ) 160 ° C Peak Assignments (a) δ OH in W-OH 34,35 (b) δ (OH-O) N/A ν OH, δ OH 36 N/A1164 δ W-OH ν (O-W-O) 23 ν (W=O terminal) 23, ν (O-W-O) (Monoclinic Phase) 22, ν (W-O) N/A γ (O-W-O) 34, Silicon feature O-lattice 37,39 492N/AO-lattice 37, δ (O-W-O) δ (O-W-O) 26, δ (O-W-O) in monoclinic structure Low-Frequency Phonon Temperature Change Marker 24,41 7

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Raman Band (cm -1 ) 30 ° C Raman Band (cm -1 ) 160 ° C Peak Assignments ν (O-W-O) (Monoclinic Phase) 22, ν (W-O) WO 2 W group bridged vibrations δ (O-W-O) δ (O-W-O) 26, δ (O-W-O) in monoclinic structure W-W Low-Frequency Phonon Temperature Change Marker 24, Low-Frequency Phonon Temperature Change Marker 24,41 N/A88 Low-Frequency Phonon Temperature Change Marker 24,41 71N/A Low-Frequency Phonon Temperature Change Marker 24, Low-Frequency Phonon Temperature Change Marker 24,41 9

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bRaman Band (cm -1 ) 30 ° C Raman Band (cm -1 ) 160 ° C Peak Assignments N/A1523 (a) δ OH in W-OH 34,35 (b) δ (OH O) 23 N/A1145 δ W-OH N/A ν (W=O terminal) 23,35 930N/A ν a (WO 2 ) 43,44 N/A924 ν (W-O) ν (O-W-O) (Monoclinic Phase) 22, ν a (Transition Metal Oxide bond) N/A γ (O-W-O) 34, δ (O-W-O) 26, ν (O-W-O) N/A Low-Frequency Phonon Temperature Change Marker 24, Low-Frequency Phonon Temperature Change Marker 24,41 93N/A Low-Frequency Phonon Temperature Change Marker 24,41 11

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Sample 1 (Moist) was made by leaving 0.9 g of nanopowder in a ~60% humidity environment for 5 days. Sample 2 (Damp) was made by leaving 0.9 g of nanopowder in a ~75% humidity environment for 5 days. Sample 3 (Wet) was prepared by mixing 0.5 mL of water with 0.9 g of nanopowder. 13

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Raman Spectroscopy of three WO 3 samples using 780 nm wavelength laser at temperatures of °C. Increasing temperature, in most cases, results in red-shift of Raman frequencies. The major vibrational modes of WO 3 on Silicon substrate and WO 3 nanopowder, located at ~807, ~716, and ~271 cm -1, are consistent with the Raman features of a monoclinic WO 3 structure. Alternatively, this suggests the nanowires sample is not strictly monoclinic. Some features begin fading with increasing temperature and low-frequency phonon temperature change markers also vary. Humidity effects become clearer with increasing temperature, as OH-related bonds vibrate due to the increased thermal energy. De-hydrolyzing the sample reduces these humidity-dependent peaks. 15

Understanding the effect of temperature on the Raman features of WO 3 has helped extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. Features such as 750 cm -1 for nanowires and 492 and 670 cm -1 for WO 3 on Silicon substrate, appear to slowly fade as temperature increases. Interestingly enough, these are related to bonds involving metal oxides rather than O-H bonds. This suggests that, as temperature increases, O-H bonds are dampening the vibrations of WO-like bonds. This is further reinforced by the appearance of intense O-H bonds at the ~1500 cm-1 range with increasing temperature. 16

Retake the temperature data of de-hydrolyzed samples. Increase the temperature range to 200 o C for nanowires. Determine the effect of NO x exposure on the samples. Different concentrations. Different temperatures. Consider other effects that can affect the Raman Spectroscopy of WO 3 samples. 17

Would like to thank: My research advisor: Dr. Prabhakar Misra. My committee chairperson, Dr. Silvina Gatica and committee members, Dr. Kimani Stancil and Dr. Belay Demoz. Mr. Daniel Casimir for initial data acquisition on WO 3. The Physics Department staff, Ms. Anne Cooke, Dr. Julius Grant and Mr. Ronald Crutchfield. AGEP staff, for their support and funding, Dr. Kamla Deonauth. BCCSO staff, Ms. Katherine Cooke Mundle and Ms. Teria Powell. 18