Size Effect in Nanostructured SnO2/TiO2 Gas Sensors
Authors: Zakrzewska Katarzyna Ewa , Radecka Marta Anna 
Published: May 2020

Introduction

Size effect in resistive-type responses to gases has been demonstrated for the first time in the case of SnO2 polycrystalline sensors [1]. Since then it is believed that the smallest the grain size, the higher change in the electrical resistivity of metal oxide nanomaterials can be expected upon interaction with oxidizing or reducing gases. This belief is based on the argument that the surface-to-volume ration increases with a decrease in the grain diameter and as a gas adsorption is a surface process this in consequence leads to an increased density of active sites available for gas-solid interaction [2-3].
Therefore, the size reduction has been considered as a fundamental strategy for nanosensors, justifying an enormous effort towards application of nanotechnology in gas sensing devices.
A closer look at this issue reveals that such simplified reasoning is not always true. Geometrical decrease in the grain size below 10 nm for SnO2 does not necessarily mean to be a good solution for other oxides such as TiO2. In fact, the most important is a comparison between the grain diameter d and the Debye length λD defined as:
λD={(ε
ε0φS)/(eND)}1/2

where ε and ε0
denote relative permittivity of the material and vacuum permittivity, respectively, φ

S
is the surface potential, e represents electron charge and ND stands for donor concentration.
When the condition d << λD is fulfilled, the size effect should play a substantial role in gas sensing. Therefore, the aim of this work is to investigate the size effect in the electrical resistivity of SnO2/TiO2 upon interaction with oxidizing/reducing gases by intentional modification of λD by influencing:

the electrical properties through donor concentration ND

the structural properties through anatase to rutile ratio thus affecting relative permittivity, ε

morphology through its impact on the surface potential, φ
S

Technology
TiO2/SnO2 bi-layer nanoheterostructures have been synthesized in a two-step process: (i) magnetron sputtering, MS, followed by (ii) Langmuir-Blodgett, LB technique [4]. Mixed TiO2/SnO2 composites have been obtained by means of Flame Spray Synthesis, FSS [5], sol-gel [6] and by mechanical mixing of commercial Sigma-Aldrich nanopowders [7].

Experimental Methods

Crystallographic structure of the nanocomposites was studied by X-ray diffraction in Bragg-Brentano geometry while that of thin films was determined by X-ray diffraction in grazing incidence GID geometry using Philips X’pert Pro diffractometer. Scanning electron microscopy SEM investigations were carried out with the use of FEI Nova Nano SEM 200 microscope. Transmission electron microscopy TEM was employed to study the morphology of the nanopowders. FEI Tecnai TF 20 X-TWIN microscope was used. Gas sensing properties were established by measuring resistive-type responses to hydrogen, and NO2 vs. time, within a temperature range of 200-400oC both in dc and ac mode. The experimental setup for sensor characterization was already described in [8]. Sensitivity of the sensors to the reducing gas was defined as R0/R while that to the oxidizing as R/R0, where R0 is the baseline resistance in the reference gas, i.e. in air.

Results and Conclusions

Nanopowders of TiO2 with different crystallite size, grown by FSS, demonstrate a size effect in the electrical conductivity (Fig.1a) which can be attributed to the activation energy decreasing with the particle size. No grain size effect in hydrogen sensing has been observed.
Bi-layered heterostructures synthesized on special gas sensor substrates with precisely defined interdigital electrodes were tested towards NO2 detection over the low concentration range from 200 ppb to 20 ppm.
High response to low concentration of NO2 (Fig. 1b) could be accounted for by two factors:

particular morphology of SnO2 base layer with small, well-dispersed grains indicating possibility of a grain size effect

well-developed, rough surface of TiO2 overlayer grown by the LB technique.

Acknowledgement

National Science Center NCN, Poland project no. 2016/23/B/ST7/00894 is acknowledged.

References

1. C.Xu, Y.Tamaki, N.Miura, N.Yamazoe, Grain size effects on gas sensitivity of porous SnO2-based elements, Sensors and Actuators B. 3 (1991) 147–155. doi:10.1016/0925-4005(91)80207-Z.
2. S.G.Ansari, P.Boroojerdian, S.R.Sainkar, R.N.Karekar, R.C.Aiyer, S.K.Kulkarni, Grain size effect on H2 gas sensitivity of thick film resistor using SnO2 nanoparticles, Thin Solid Films. 295 (1997) 271-276. doi: 10.1016/S0040-6090(96)09152-3.
3. B.Lyson-Sypien, M.Radecka, M.Rekas, K.Swierczek, K.Michalow-Mauke, T.Graule, K.Zakrzewska, Grain-size-dependent gas-sensing properties of TiO2 nanomaterials, Sensor and Actuators B: Chemical. 211 (2015) 67-76. doi: 10.1016/j.snb.2015.01.050.
4. W.Maziarz, TiO2/SnO2 and TiO2/CuO thin film nano-heterostructures as gas sensors, Applied Surface Science. 480 (2019) 361-370. doi:10.1016/j.apsusc.2019.02.139.
5. B.Lyson-Sypien, A.Kusior, M.Rekas, J.Zukrowski, M.Gajewska, K.Michalow-Mauke, T.Graule, M.Radecka, K.Zakrzewska, Beilstein Journal of Nanotechnology. 8 (2017) 108–122. doi:10.3762/bjnano.8.12.
6. A.Kusior, L. Zych, K. Zakrzewska, M. Radecka, Photocatalytic activity of TiO2/SnO2 nanostructures with controlled dimentionallity/complexity, Applied Surface Science. 471 (2019) 973-985. doi:10.1016/j.apsusc.2018.11.226.
7. B.Lyson-Sypien, A.Czapla, M.Lubecka, E.Kusior, K.Zakrzewska, M.Radecka, A.Kusior, A.G.Balogh, S.Lauterbach, H.-J.Kleebe, Gas sensing properties of TiO2-SnO2 nanomaterials, Sensors and Actuators B: Chemical. 187 (2013) 445-454. doi: 10.1016/j.snb.2013.01.047.
8. A.Kusior, M.Radecka, Ł.Zych, K.Zakrzewska, A.Reszka, B.J.Kowalski, Sensitization of TiO2/SnO2 nanocomposites for gas detection, Sensors and Actuators B: Chemical. 189 (2013) 251-259. doi: 10.1016/j.snb.2013.07.029.

Figure 1