Workshop on Atomic-Scale Challenges in Advanced Materials Defects in Materials ASCAM VI

Hydrogen sensor application of Pd doped anodic TiO

2

film

23. Aug. 2013 Jongyun Moon, Hannu-Pekka Hedman, Risto Punkkinen Department of Information Technology

2 2020-05-01

Introduction

Hydrogen sensor based on semiconductor

   Semiconducting oxides that can be used for hydrogen detection SnO 2 , ZnO, TiO 2 , FeO, Fe 2 O 3 , NiO, Ga 2 O 3 , In 2 O 3 , MoO 3 and WO 3 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 3 2020-05-01

Hydrogen sensor using TiO

2

thin film via anodization

    TiO 2 has a large electric band gap of 3.0 eV.

Crystallized TiO 2 nanostructures prepared by hydrogen sensing performance

anodization

has shown a remarkable (TiO 2 nanotube arrays:to 1000 ppm H 2 a resistance variation of 10 O.K. Varghese, Mater. Res. Soc. Symp. Proc. 835 (2005) 7 ).

Low production cost due to an easy synthesis method Shortcoming: Ti foil which underlines TiO 2 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.

4 2020-05-01 Figure 1. Schematic of a gas sensor using TiO 2 nanotube arrays on Ti metal sheet

Research objective

 Synthesis of TiO 2 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 5 2020-05-01

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Materials and Methods

Anodization of Ti on SiO

2

/Si wafer

      Substrate: SiO 2 (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 : NH 4 F 0.25wt % in Ethylene Glycol Anodization bath temperature : 5 ° C Figure 2. An image of the anodization experiment instrument 7 2020-05-01

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 Heat treatment for crystallization at 500 ° C Formation of Porous TiO 2 film 8 2020-05-01

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 × Ultramic 600, Watlow) 15 mm × 10 mm, 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) 9 2020-05-01

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Results

Current plot during anodization

Voltage: 60V Voltage increase 0-60V 11 2020-05-01

FESEM (Field Emission Scanning Electron Microscope) Thickness : ≈ 20nm Diameter : ≈ 15-20 nm

12 2020-05-01 FESEM image of TiO 2 layer prepared by anodization using 30V

EDS (

Energy-dispersive X-ray spectroscopy) Element O K Si K Ti K Totals Weight% 42.28

22.82

34.90

100.00

Atomic% 63.16

19.42

17.42

TiO 2 area Element O K Ti K Pt M Totals Weight% 29.56

29.38

41.06

100.00

Atomic% 69.16

22.96

7.88

Metal electrode area 13 2020-05-01

Gas sensor measurement



Low concentration of H 2 : 1 – 50 ppm

180 ° C 160 ° C 140 ° C 14 2020-05-01

Sensor response

Operating temperature: 160 ° C Y (Trend line equation) = 1.3219x

0.8914

R² (correlation coefficient) = 0.9652

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Conclusion

 Porous TiO 2 film with Pd thin film was synthesized on SiO 2 /Si substrate with metal electrodes without loss of Ti/TiO 2 layer  Its morphology modification is feasible by the control of the anodization experimental parameters, such as the voltage.

 The formation of TiO 2 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 16 2020-05-01

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 17 2020-05-01

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Thank you for your attention