Workshop on
Atomic-Scale Challenges in Advanced Materials
Defects in Materials
ASCAM VI
Hydrogen sensor application of Pd doped anodic TiO2 film
23. Aug. 2013
Jongyun Moon, Hannu-Pekka Hedman, Risto Punkkinen
Department of Information Technology
Introduction
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Hydrogen sensor based on semiconductor
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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.
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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)
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Low production cost due to an easy synthesis method
Shortcoming:
Ti foil which underlines TiO2 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
an electrical short circuit
Ii) vulnerable to mechanical shock or vibrations.
Figure 1. Schematic of a gas sensor using TiO2 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 Methods
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Anodization of Ti on SiO2/Si wafer
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Substrate: SiO2 (1 µm)/Si ( 2 cm × 2.5 cm)
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Anode : Ti film (500 nm) by DC sputtering in argon (Ar) at a pressure of 0.02 mbar
at 150°C
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Cathode : Platinum sheet (99.98%)
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Electric potential : 30 - 60 V
(Voltage ramping rage: 0.5 V/s)
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Electrolyte : NH4F 0.25wt % in Ethylene Glycol
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Anodization bath temperature : 5 °C
Figure 2. An image of the anodization experiment instrument
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Schematic of the sensor preparation
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
Pd thin film depostion
Formation of Porous TiO2 film
Heat treatment for crystallization at 500°C
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Analysis
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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)
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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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Results
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Current plot during anodization
Voltage: 60V
Voltage increase
0-60V
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FESEM (Field Emission Scanning Electron Microscope)
Thickness : ≈ 20nm
Diameter : ≈ 15-20 nm
FESEM image of TiO2 layer prepared by anodization using 30V
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EDS (Energy-dispersive X-ray spectroscopy)
Element
OK
Si K
Ti K
Totals
Weight%
42.28
22.82
34.90
100.00
Atomic%
63.16
19.42
17.42
TiO2 area
Element
OK
Ti K
Pt M
Totals
Metal
electrode
area
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Weight%
29.56
29.38
41.06
100.00
Atomic%
69.16
22.96
7.88
Gas sensor measurement
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Low concentration of H2 : 1 – 50 ppm
180°C
160°C
140°C
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Sensor response
Operating temperature: 160°C
Y (Trend line equation) = 1.3219x0.8914
R² (correlation coefficient) = 0.9652
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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 attention
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