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. 2019 Dec 21;20(1):74.
doi: 10.3390/s20010074.

TiO2-x/TiO2-Structure Based 'Self-Heated' Sensor for the Determination of Some Reducing Gases

Simonas Ramanavicius  1 , Alla Tereshchenko  2   3 , Renata Karpicz  1 , Vilma Ratautaite  1   3 , Urte Bubniene  1   3 , Audrius Maneikis  1 , Arunas Jagminas  1 , Arunas Ramanavicius  1   3
Affiliations

TiO2-x/TiO2-Structure Based 'Self-Heated' Sensor for the Determination of Some Reducing Gases

Simonas Ramanavicius et al. Sensors (Basel). .

Abstract

In this research we report the gas-sensing properties of TiO2-x/TiO2-based hetero-structure, which was 'self-heated' by current that at constant potential passed through the structure. Amperometric measurements were applied for the evaluation of sensor response towards ethanol, methanol, n-propanol and acetone gases/vapours. The sensitivity towards these gases was based on electrical resistance changes, which were determined by amperometric measurements of current at fixed voltage applied between Pt-based contacts/electrodes deposited on the TiO2-x/TiO2-based layer. X-ray diffraction (XRD) analysis revealed the formation of TiO2-x/TiO2-based hetero-structure, which is mainly based on Ti3O5/TiO2 formed during the hydro-thermal oxidation-based sensing-layer preparation process. Additionally, photoluminescence and time-resolved photoluminescence decay kinetics-based signals of this sensing structure revealed the presence of TiO2 mainly in the anatase phase in the TiO2-x/TiO2-based hetero-structure, which was formed at 400 °C annealing temperature. The evaluation of TiO2-x/TiO2-based gas-sensing layer was performed at several different temperatures (25 °C, 72 °C, 150 °C, 180 °C) and at these temperatures different sensitivity to the aforementioned gaseous materials was determined.

Keywords: Ti3O5/TiO2; TiO2-x/TiO2; anatase; nonstoichiometric titanium oxides; photoluminescence; rutile; sensor for reducing gases.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
(A) Schematic view (layer-by-layer) of initial structure, which was used for the design of sensor: 1—Si-based wafer; 2—thin layer (100 nm) of SiO2; 3—thin layer (100 nm) of Ti deposited by magnetron sputtering. (B) Schematic view (layer-by-layer) of TiO2-x/TiO2-based sensor structure: 1—Si-based wafer; 2—thin layer (100 nm) of SiO2; 3—nonstoichiometric TiO2-x/TiO2 layer formed by hydrothermal oxidation; 4—Pt electrodes/(contact zones) deposited by magnetron sputtering.
Figure 2
Figure 2
Representation of typical analytical signal registered by TiO2-x/TiO2 (400 °C)-based structure. It should be noted that ΔI, the duration of signal development and the regeneration of sensor after measurements were different for different gases and different concentrations of those gases.
Figure 3
Figure 3
Scanning electron microscope (SEM) images of TiO2-x/TiO2 (400 °C)-based hetero-structure at different magnification: (A)—at ×25000; (B)—at ×50000.
Figure 4
Figure 4
X-ray diffraction (XRD) patterns of: A(metallic Ti)—metallic Ti layer of 100 nm thickness, which was formed by magnetron sputtering; B(TiO2-x/TiO2-structure)— TiO2-x/TiO2 (400 °C)-based hetero-structure, which was formed from above mentioned metallic 100 nm thick Ti layer; C(TiO2 powder)—TiO2-powder, which was used as ‘control sample’ and by supplier (Sigma-Aldrich) was declared as 99.3% TiO2 in the anatase phase.
Figure 5
Figure 5
(A) Photoluminescence spectra of TiO2-x/TiO2 (400 °C) sample (1), and silicon substrate (2); (B) Photoluminescence spectra of TiO2-x/TiO2 (600 °C) sample (1), and silicon substrate (2); (C) Photoluminescence spectra of TiO2-x/TiO2 (800 °C) sample; (D) Photoluminescence decays of TiO2-x/TiO2 (400 °C) sample at different photoluminescence emissions bands under 375 nm excitation.
Figure 6
Figure 6
(A) Photoluminescence spectra of TiO2-x/TiO2 (400 °C)-based sample at different applied voltage; (B) The changes in spectral position of photoluminescence maximum vs. voltage of TiO2-x/TiO2 (400 °C)-based sample; (C) The changes in photoluminescence maximum intensity vs. voltage of TiO2-x/TiO2 (400 °C)-based sample; (D) The plot of temperature vs. voltage of TiO2-x/TiO2 (400 °C)-based sample.
Figure 7
Figure 7
Temperature dependence of electrical resistance (R(T)) for the TiO2-x/TiO2 (400 °C)-based hetero-structure. Temperature was changed in two ways (indicated by black and red arrows): (i) black cycles shows points measured by cooling down, (ii) red squares shows points by increasing temperature. Measured was performed in vacuum using helium cryostat.
Figure 8
Figure 8
The response of TiO2-x/TiO2 (400 °C)-based hetero-structure towards humidity (H2O), methanol, ethanol, n-propanol, and acetone at different temperatures (25–180 °C).
Figure 9
Figure 9
The response of TiO2-x/TiO2 (400 °C) hetero-structure towards ethanol, at different temperatures: 25 °C, 72 °C, 150 °C and 180 °C.
See this image and copyright information in PMC

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