Nanocrystalline Metal oxide gas sensor Materials

Type of sensors: Resistive

• Change in resistance is measured as a response due to interactions between metal oxide surface and gas.

• Typical response looks like this:

Sensor Construction

Gold Wires

Gold Wires

Heating element on the other side.

ElectrodesSilicon Substrate

Materials requirements

Large Grain Size

Bulk Characteristics

Small Grain Size Thin films

Nano Characteristics

Thick films

Critical particle size has to be reached

Important properties of metastable materials

• Many metal oxides in there metastable polymorphic forms are very useful gas sensing materials:

Metal Oxides Polymorphic Form Gas sensed Reference

In2O3 Corundum-type hexagonal structure

Reducing gas, Ethanol 1a

WO3 ε-WO3 (Triclinic) Ferroelectric gas, Acetone 2a

MoO3 Monoclinic Ammonia 3a

SnO2 CaCl2-type orthorhombic

phase

Both reducing gas, CO, H2 and oxidising gas, NO2

4a

ZrO2 Tetragonal phase Reducing gas like, CO, H2 and Oxidising like, Nox.

5a

TiO2 Anatase Reducing gas like CO 6a

Why Nanocrystals?

• In the bulk form these metastable states exist at very high temperatures and are thus inaccessible, due to the equipment constraints and substrate properties, working under such high temperatures is not feasible.

• But in the case when the particle size is small enough these metastable states can exist at ambient conditions of temperature and pressure, making them accessible.

ExamplesMetal Oxide at

ambient conditions

Bulk phase Nano Phase Critical particle size/

Temperature

Ref.

SnO2 (RT and 1 atm)

Rutile-type tetragonal phase

CaCl2-type orthorhombic phase

Below 50nm 5b,6b,7b

In2O3 (RT and 1 atm)

Cubic bixbyite-type structure

Cubic bixbyite-type structure

+

Corundum-type hexagonal structure

Below 80nm 8b,9b,10b

Al2O3 (RT and 1 atm)

Corundum type trigonal α-phase

Cubic γ phase Below 17nm and below 473K

7a,10a

TiO2 (RT and 1 atm) Rutile Anatase Below 11nm 8a,9a

MoO3(RT and 1 atm)

Orthorhombic α phase Orthorhombic α phase

+

Hexagonal MoO3

Below 200nm and 443 to 673K

1b,2b

WO3(RT and 1 atm) Triclinic δ-phase Monoclinic ε phase Nanosize 3b,4b

ZrO2(RT and 1 atm) Monoclinic phase Tetragonal phase Below 18nm 1c,2c

Mechanism for sensing: adsorption

• Gas sensing is mainly due to gas adsorption at the surface. • Gas comes in contact with the crystallographic surface of the

metal oxide and is adsorbed at the surface as can be seen:• Semiconducting nature of metal oxides is important for gas sensing. • Formation of depletion layer is one important reason for this behavior.

Case Study:WO3 thin film sensor

for oxidising gases

WO3 based NO2 sensor[4c]

• Authors prepared thin films of nanocrystalline WO3 calcined at 350°C, 450°C, 550°C, 650°C and observed the NO2 gas response.

• WCl6 precursor was used to prepare the sensor by sol-gel route to get 25nm particle size WO3

Why good response at 550°C?• An monoclinic phase was seen to

bee stable upto 600°C in this case• The response also depends on the

grain size and the authors found that as the grain size decreased the sensitivity was better

• There was a critical point at 550°C, at which optimum temperature and particle size was observed

Response at 500°C

Calcination temperature (°C)

350 450 550 650

Particle size (nm) 18 23 25 70

Crystallite size (nm)

9.3 16.4 17.1 30

Surface area (m2 g−1)

58.4 36.8 32.3 10.7

Other results[5c]

• Morphology results in our lab:

• But it was found that at higher temperatures there was a decreased sensitivity to NO2 and increased sensitivity to ammonia.

• There can be differing results due to grain size differences or different processing route

WO3 Sensitivity to ozone[6c]

• These authors observed a monoclinic phase at similar temperatures as our lab. They used sputtering and alkoxide precursor.

• Between 150 to 400°C, the monoclinic region there is a high sensitivity towards ozone.

Response at 250°C to ozone• Authors found the best response at 250°C

NO2 sensor at 200°C[7c]

• Recent study (2009), demonstrated good sensing behavior of monoclinic WO3 at 200°C similar to ozone sensor.

• They prepared WO3 thin films by acidification of Na2WO4 with particle size 20-50nm

Processing method dependence[8c]

• These authors did a comparison and demonstrated that for different methods used the sensing results don’t change but the operating temperatures and grain size might change

VTE – Vacuum thermal evaporation; SG- Sol-gel ;RFS- rf sputtering.

Conclusion

• Sensing properties remain unchanged with change in processing methods

• Sensing properties depend on grain size and operating temperatures

• Although different processing methods can affect a change in the grain size and operating temperature

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