On the photo-induced electrical conduction related to gas sensing of the Sb:SnO2/TiO2 heterostructure
Introduction
Nowadays gas sensors based on semiconductors present growing interest [[1], [2], [3], [4]] but have some disadvantages concerning the relatively high operating temperature and high power consumption [5]. To overcome such obstacles, several reports about the use of photoactivation of metal oxide semiconductor sensors have been published, in order to increase the gas sensitivity. Some of these works present the use of UV light to improve the gas sensitivity [4,[6], [7], [8]], and this illumination is an alternative to modulate and activate the sensor signal value [9]. On the other hand, the use of semiconductor heterostructures may improve even further the gas sensing properties, by means of the charge transfer from one material to another. Such heterostructures may also be improved through the UV illumination, by means of the creation of incomplete bonding on the sensor’s surface, enabling the adsorption of more gas molecules [10,11].
Interesting properties may be achieved from different heterostructures, and a very high variety of heterostructure applications have been reported in recent years, such as electrodes for supercapacitors [12], high energy density supercapacitors [13], and gas sensing devices [[14], [15], [16]]. In the case of gas sensor application, the formation of heterostructures enhances the sensing properties, as the use of SnO2:Zn2SnO4 nanowires (NW) [16] that causes an improvement of about 1.5 times in efficiency to sense ethanol, compared with sole SnO2 NW. Such improvement occurs in the principal mechanism for ethanol sensing [17], attributed to the dehydrogenation of ethanol with the addition of Zn2SnO4 to SnO2. The use of n-ZnO/n-In2O3 presents up to 6 times improvement on methanol sensing compared to sole In2O3, ruled by the electron transfer effect at the heterostructure interface [15].
The junction of two materials forming a heterostructure creates a potential barrier at the interface, originated from the valence and conduction bands discontinuities of both semiconductors, after the thermodynamic equilibrium is reached [18]. The electrons tend to migrate from one material to another, according to the electron affinity of the material [18,19]. Considering the SnO2/TiO2 (tin dioxide / titanium dioxide) heterostructure, the equilibrium condition leads to electron transfer from the SnO2 to TiO2, due to the higher electron affinity of the TiO2 [18,20]. The improvement of gas sensing properties in this heterostructure under illumination, compared to sole materials, occurs due to the photo-excitation of electrons in the exposed TiO2 layer, that along with the transferred electrons from the SnO2 layer, at the formation of the junction, increases the concentration of charges exposed on the surface, which act as adsorption centers for gas molecules [11]. The photo-excitation must be done by an UV-light source, with higher energy than TiO2 bandgap, and, after photo-excitation, electron transfer from TiO2 to SnO2 also may occur but due to the potential barrier at their interface this transference may be limited [10].
TiO2 is a semiconductor with wide bandgap of about 3.3 eV [21]. The n-type conductive nature of TiO2 comes from interstitial Ti3+ and oxygen vacancies [22]. The presence of oxygen vacancies in this oxide facilitates the gas adsorption through stronger bonding between the film surface and the gas molecules, with desorption temperature of about 410 K [23,24]. Improved gas sensitivity of TiO2 can be achieved by growing the films in the (101) and (001) (hkl) planes of the anatase structure [23,25,26]. Tin dioxide (SnO2) is also a wide bandgap semiconductor, that presents high transparency in the visible region, and high thermal stability [27,28]. The n-type conductivity nature of SnO2 also originates from oxygen vacancies and interstitial tin atoms [29]. Thin films obtained through sol-gel related techniques present low conductivity related to the small crystallites and consequently high grain boundary scattering that decreases considerably the electron mobility [30,31]. On the other hand, when donor-doped with Sb5+ ions an increase of electron density occurs in the semiconductor [31], which raises the conductivity.
In this work, the investigation of electrical characteristics of the 4at%SnO2/TiO2 heterojunction under photo-excitation is carried out at gas and room atmospheres. Different temperatures and their influences on the photo-induced decay rate are also evaluated and are related to deep level defects in the semiconductor layers. Concerning the improvement of the gas sensing properties, the illumination of the heterojunction, with well-established excitation energy, contributes to increase on the adsorption centers on the surface of the TiO2 exposed layer. The investigation presented in this work contributes to the understanding of the gas sensing mechanisms, and to the improvement in the Sb:SnO2/TiO2 heterostructure under different atmospheres, temperatures and light excitation.
Section snippets
Experimental details
Sol–gel production of TiO2 was accomplished by hydrolysis and condensation of titanium (IV) isopropoxide using a high molar ratio of water:alkoxide (200:1), isopropanol as co-solvent, HNO3 as catalyst, and Triton X-100 as surfactant [32,33]. Initially 2.6 ml of Nitric acid was added to 185 ml of deionized water and 57 ml of isopropanol, followed by dropwise addition of 15 ml of titanium (IV) isopropoxide alkoxide, under stirring and heating at 85 °C during 4 hours. The concentration of solution
Results
Results on structural and optical properties of the Sb:SnO2/TiO2 film were obtained by XRD and UV–vis measurements, shown in Fig. 1, to obtain information about the optical properties and bandgap, as well as the crystalline phase and crystallite size of the sample. These properties are important factors that affect directly the electrical character in oxide semiconductors. The XRD presented in Fig. 1(a) shows that the most intense diffraction peak is attributed to plan (101) of anatase TiO2
Conclusion
The investigation presented in this work showed that the formation of the heterostructure of TiO2 with 4at%Sb:SnO2, is promising to gas sensors devices. The architecture with the TiO2 semiconducting layer exposed to atmosphere gases shows higher gas sensitivity when operating under UV light.These results are attributed to the electron transfer from the SnO2 to the TiO2 layer at junction formation, and under light irradiation, due to the further increase of electrons in the titanium oxide film.
Acknowledgements
The authors acknowledge Professor José Humberto D. da Silva for support with transmittance measurements, Professor Dayse Iara dos Santos for support with XRD measurements, which were performed at Multiuser Lab. at UNESP/DF Campus Bauru, and to PNPD/CAPES and CNPq for financial support.
R. A. Ramos Jr. graduated in Physics (2015) and has Masters in Materials Science and Technology (2018) by the São Paulo State University Professor Júlio de Mesquita Filho. During his Master’s, Ramos Jr. studied thin films of titanium and tin dioxides for gas sensing applications. Currently Ramos Jr. is PhD student in Materials Science and Technology at the São Paulo State University and works with semiconductor thin films aiming non-volatile memories.
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R. A. Ramos Jr. graduated in Physics (2015) and has Masters in Materials Science and Technology (2018) by the São Paulo State University Professor Júlio de Mesquita Filho. During his Master’s, Ramos Jr. studied thin films of titanium and tin dioxides for gas sensing applications. Currently Ramos Jr. is PhD student in Materials Science and Technology at the São Paulo State University and works with semiconductor thin films aiming non-volatile memories.
M. H. Boratto graduated in Physics and has Master’s and Ph.D. in Materials Science and Technology by the São Paulo State University (UNESP). During his Ph.D. Boratto participated as researcher at the University of Western Ontario – Canada. Miguel is currently a PostDoc at the Physics Department of the Federal University of Santa Catarina (UFSC). His main works involve the development and study of semiconductors (TiO2 and SnO2) and dielectric (Al2O3, ZrO2) materials for the obtainment of electronic devices such as transistors, capacitors, memristors, and gas sensors.
L. V. A. Scalvi: Chemical Engineer: Federal University of São Carlos (Brazil-1983), Ph.D. Applied Physics: University of São Paulo (Brazil-1991). Research Associate: Oregon State University, U.S.A. (06/1988-06/1990); Visiting Researcher: Laboratory Synchrotron SOLEIL, France (1-4/2009). Director of Meteorological Research Institute, Brazil (11/2011-5/2013). Coordinator of Materials Science and Technology graduate Program (6/2013- 10/2017). Professor at State University of São Paulo for 25 years. Experience: Solid State Physics and Materials Science, with about 90 publications. Presently working in: oxide semiconductors, heterostructures, thin films, graphene, electrical transport. Recently participated as UNESP delegate in the First Meeting of BRICS Working Group on Materials Science and Nanotechnologies (Ekaterinburg, Russia, 2017).