Hydrogen sensor application of Pd doped anodic TiO2 film 

23. Aug. 2013

Jongyun Moon, Hannu-Pekka Hedman, Risto Punkkinen

Department of Information Technology

Workshop onWorkshop onAtomic-Scale Challenges in Advanced MaterialsAtomic-Scale Challenges in Advanced Materials

Defects in MaterialsDefects in MaterialsASCAM VIASCAM VI

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IntroductionIntroduction

Hydrogen sensor based on semiconductor

Semiconducting oxides that can be used for hydrogen detection SnO2, ZnO, TiO2, FeO, Fe2O3, NiO, Ga2O3, In2O3, MoO3 and WO3

Hydrogen is detected by the change of the electrical properties when the metal oxide are exposed to target gases.

Advantages:

High sensitivity, feasibility of miniaturization, low production cost Shortcoming:

low selectivity toward carbon monoxide, methane, alcohols, humidity etc. Decoration with catalytic materials can achieve improvements in selectivity

and sensitivity

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Hydrogen sensor using TiO2 thin film via anodization

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TiO2 has a large electric band gap of 3.0 eV.

Crystallized TiO2 nanostructures prepared by anodization has shown a remarkable hydrogen sensing performance (TiO2 nanotube arrays:to 1000 ppm H2 a resistance variation of 107).

O.K. Varghese, Mater. Res. Soc. Symp. Proc. 835 (2005)O.K. Varghese, Mater. Res. Soc. Symp. Proc. 835 (2005)

Low production cost due to an easy synthesis method Shortcoming:

Ti foil which underlines TiOTi foil which underlines TiO22 film, limit the usage of the material in various applications. film, limit the usage of the material in various applications.

i) metal electrode atop the oxide layer may diffuse into the Ti metal layer and cause i) metal electrode atop the oxide layer may diffuse into the Ti metal layer and cause an electrical short circuitan electrical short circuit

Ii) vulnerable to mechanical shock or vibrations.Ii) vulnerable to mechanical shock or vibrations.

Figure 1. Schematic of a gas sensor using TiOFigure 1. Schematic of a gas sensor using TiO22 nanotube arrays on Ti metal sheet nanotube arrays on Ti metal sheet

Research objective

Synthesis of TiO2 thin film on foreign substrate with metal electrodes

by using anodization → Reliable sensor structure.

Decoration of the sensor material with catalyric material (ex. Pd)

Improvement of gas sensor performance

→ Sensitivity, response/recovery time and selectivity to other gases

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Materials and MethodsMaterials and Methods

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Anodization of Ti on SiO2/Si wafer

Substrate: SiO2 (1 µm)/Si ( 2 cm × 2.5 cm)

Anode : Ti film (500 nm) by DC sputtering in argon (Ar) at a pressure of 0.02 mbar at

150°C

Cathode : Platinum sheet (99.98%)

Electric potential : 30 - 60 V

(Voltage ramping rage: 0.5 V/s)

Electrolyte : NH4F 0.25wt % in Ethylene Glycol

Anodization bath temperature : 5 °C

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Figure 2. An image of the anodization experiment instrumentFigure 2. An image of the anodization experiment instrument

Schematic of the sensor preparation

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Pt

Al

Au/Al metal electrode deposition by DC sputtering Heat treatment at 300 °C for 10 min

Ti film (500 nm) deposition by DC sputtering at 150°C

Anodization

Formation of Porous TiO2 film Pd thin film depostion

Heat treatment for crystallization at 500°C

Analysis

Material characteristics

i) Observation of current behavior during the andization

ii) FESEM (Field Emission Scanning Electron Microscope) analysis

iii) EDS (Energy-dispersive X-ray spectroscopy)

Gas sensor measurement

i) Sensor Temperature control: Heater plate (15 mm × 15 mm × 10 mm, Ultramic 600, Watlow)

ii) Measurement chamber: 56 l glass test chamber with continuous air circulation

iii) Desired volume of hydrogen was inserted to chamber.

* Concentration was verified by a commercial sensor (SX-917, Sensorex, Finland)

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ResultsResults

Current plot during anodization

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Voltage increaseVoltage increase0-60V0-60V

Voltage: 60VVoltage: 60V

FESEM (Field Emission Scanning Electron Microscope)

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Thickness : ≈ 20nmThickness : ≈ 20nm

Diameter : ≈ 15-20 nmDiameter : ≈ 15-20 nm

FESEM image of TiOFESEM image of TiO22 layer prepared by anodization using 30V layer prepared by anodization using 30V

EDS (Energy-dispersive X-ray spectroscopy)

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Element Weight% Atomic%O K 29.56 69.16Ti K 29.38 22.96Pt M 41.06 7.88Totals 100.00

Element Weight% Atomic%O K 42.28 63.16Si K 22.82 19.42Ti K 34.90 17.42Totals 100.00

TiOTiO 22 area area

Metal Metal electrode electrode

areaarea

Gas sensor measurement

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Low concentration of H2 : 1 – 50 ppm

180°C180°C

160°C160°C

140°C140°C

Sensor response

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Y (Trend line equation) = 1.3219xY (Trend line equation) = 1.3219x0.89140.8914

R² (correlation coefficient) = 0.9652

Operating temperature: 160Operating temperature: 160°C°C

Conclusion

Porous TiO2 film with Pd thin film was synthesized on SiO2/Si

substrate with metal electrodes without loss of Ti/TiO2 layer

Its morphology modification is feasible by the control of the

anodization experimental parameters, such as the voltage.

The formation of TiO2 nanostructure can be interpreted by

monitoring the anodic current variation

The sensor exhibited a three order magnitude drop in resistance on

exposing to 10,000 ppm hydrogen gas at 160°C

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Future work

Since the study is still ongoing, more material characteristics are

required.

Selectivity measurement to various gases

Modification of the nanostructure to improve sensor’s performance

Material decoration using various doping methods

Miniaturization for the mass production

Integration of the sensor into a practical electric device

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Thank you for your attentionThank you for your attention