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Distinctions in the Raman Spectroscopy Features of WO3

Materials with Increasing Temperature

Raul F. Garcia-Sanchez, Prabhakar Misra

Department of Physics and AstronomyHoward University

June 15, 2014

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Overview　　Metal Oxide Gas SensorsTungsten Oxide (WO3)

Raman SpectroscopyExperimental SetupSample ComparisonThermal Effects Humidity EffectsSummaryConclusionFuture WorkAcknowledgements

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Metal Oxide Gas Sensors• 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.

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Tungsten Trioxide (WO3)

• An n-type semiconductor material.

• Operational temperatures: • 200-500°C

• WO3 (along with SnO2) 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 (NOx)

• Has been used in research

for sensing: H2S, Cl2, CH4, SO2, CO and others.

Figure 1. Scanning Electron Microscope images of a) WO3:Si, b)

WO3 nanopowder and c) WO3 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:10.1016/j.solidstatesciences.2010.01.019.

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Experimental Setup

Temperature Controlled Environmental Chamber

Objective Lens

WO3

Sample

Collection Lens Notch Filter

Imaging Spectrometer CCD

Detector

CW Laser (780 nm)

780 nm Narrowband

Mirror

Power (mW)

Exposure Time (s)

Laser Spot

Diameter (µm)

Sample Distance (cm) to Objective Lens

Temperature Range (°C)

Temperature Step (°C)

14 120 10

0.8 (Nanowires)

1.0 (Nanopowder)1.3 (WO3:Si)

30-160 10

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Comparison of Raman features of samples at min/max temperatures

Thermal Effects on WO3 on Silicon

Raman Band (cm-1)30°C

Raman Band (cm-1)160°C

Peak Assignments

1537 1553(a) δOH in W-OH34,35

(b) δ(OH-O)23

1361 N/A νOH , δOH36

N/A 1164 δW-OH37

945 948ν(O-W-O)23

ν(W=O terminal)23,35

805 804ν(O-W-O)

(Monoclinic Phase)22,38

715 716 ν (W-O)39

670 N/A γ(O-W-O)34,35

519 516Silicon featureO-lattice37,39

492 N/A O-lattice37,39

366 360 δ(O-W-O)40

326 326 δ (O-W-O)26,39

270 268δ (O-W-O)

in monoclinic structure39

131 131

Low-Frequency Phonon

Temperature Change Marker24,41

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Thermal Effects on WO3 on Silicon

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Thermal Effects on WO3 Nanopowder

Raman Band (cm-1)30°C

Raman Band (cm-1)160°C

Peak Assignments

808 807ν(O-W-O)

(Monoclinic Phase)22,38

717 716 ν(W-O)39

436 438WO2W group bridged

vibrations38

376 372 δ(O-W-O)40

328 328 δ(O-W-O)26,39

273 271δ (O-W-O)

in monoclinic structure39

221 220 W-W42

187 186Low-Frequency

Phonon Temperature Change Marker24,41

136 134Low-Frequency

Phonon Temperature Change Marker24,41

N/A 88Low-Frequency

Phonon Temperature Change Marker24,41

71 N/ALow-Frequency

Phonon Temperature Change Marker24,41

63 68Low-Frequency

Phonon Temperature Change Marker24,41

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Thermal Effects on WO3 Nanopowder

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Thermal Effects on WO3 Nanowires

bRaman Band (cm-1)

30°C

Raman Band (cm-1)160°C

Peak Assignments

N/A 1523(a) δOH in W-OH34,35

(b) δ(OH O)23

N/A 1145 δW-OH37

954 N/Aν(W=O

terminal)23,35

930 N/A νa(WO2)43,44

N/A 924 ν (W-O)35

812 807ν(O-W-O)

(Monoclinic Phase)22, 38

758 749νa(Transition Metal

Oxide bond)45

670 N/A γ (O-W-O)34,35

328 321 δ (O-W-O)26, 39

239 248 ν(O-W-O)46

145 N/A

Low-Frequency Phonon

Temperature Change Marker24,41

108 106

Low-Frequency Phonon

Temperature Change Marker24,41

93 N/A

Low-Frequency Phonon

Temperature Change Marker24,41

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Thermal Effects on WO3 Nanowires

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Humidity Effects on WO3 Nanopowder• 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.

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Humidity Effects on WO3 Nanopowder

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Summary• Raman Spectroscopy of three WO3 samples using 780 nm wavelength laser

at temperatures of 30-160°C.

• Increasing temperature, in most cases, results in red-shift of Raman frequencies.

• The major vibrational modes of WO3 on Silicon substrate and WO3 nanopowder, located at ~807, ~716, and ~271 cm-1, are consistent with the Raman features of a monoclinic WO3 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.

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Conclusions• Understanding the effect of temperature on the

Raman features of WO3 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 WO3 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.

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Future Work• Retake the temperature data of de-hydrolyzed

samples.

• Increase the temperature range to 200oC for nanowires.

• Determine the effect of NOx exposure on the samples.• Different concentrations.• Different temperatures.

• Consider other effects that can affect the Raman Spectroscopy of WO3 samples.

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Acknowledgements • 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 WO3.

• 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.