Ultrasensitive yttrium modified tin oxide thin film based sub-ppb level NO2 detector

https://doi.org/10.1016/j.snb.2020.129169Get rights and content

Highlights

  • Undoped and Y-doped SnO2 thin films-based gas sensors have been fabricated.

  • Highly reactive in-plane oxygen vacancies were created due to Y-doping.

  • For 3 wt% Y-doped SnO2 sensor, enhanced room-temperature selectivity and high response was obtained towards NO2.

  • The fabricated sensors were able to detect sub-ppb level of NO2.

Abstract

Considering the adverse effect of rising air pollution on environment and human health, highly selective and trace level sensors are needed. Herein, yttrium (Y) incorporated tin oxide (SnO2) thin film gas sensors have been developed for sub-ppb level detection of NO2 at room-temperature. The undoped and Y-doped (1, 3 and 5 wt%) thin films have been prepared via electron beam deposition technique. The XPS studies confirmed the substitution of Sn with Y in SnO2 lattice. The morphological and structural characterizations reveal smooth and crystalline thin films, while Raman and PL spectroscopic studies confirm the presence and growth of oxygen vacancies (OVs), specifically in-plane OVs, in doped samples. Moreover, narrowing of electron absorption spectra in doped samples implied the formation of mid-gap states in energy bandgap of SnO2 leading to enhanced electron transfer. The 3 wt% Y-doped SnO2 sensor exhibited excellent sensing response with high selectivity towards NO2 in 15–240 ppb range. The sensors also showed high stability and repeatability along with humidity insensitivity. Notably, the sensor was observed to be highly sensitive to 0.6 ppb NO2 exhibiting a 26 % response along with response/recovery time of 4/2 min. The improved sensing characteristics have been ascribed to reduced crystallite size, elevated in-plane OVs, enhanced charge transfer and increased specific surface area due to Y integration.

Introduction

Amid the COVID-19 (coronavirus disease of 2019) crisis, the deleterious effects of rising air pollution have been observed in every corner of the world. The satellite images have revealed that with just few months of lockdown, the nitrogen dioxide (NO2) levels were slashed by as much as 40 % over Asia and Europe in comparison to that in 2019 for a brief period. Nevertheless, even with the implementation of strict regulations and stringent measurements, a current report indicates that nearly 98 % of cities in low-income countries fail to meet World Health Organization (WHO) air quality standards, and approximately 3 million people lost their lives each year from pollution-related ailments [1]. Thus, the development of low-cost, easy-to-use and effective gas monitors is critical for averting these life-threatening diseases as well as environmental monitoring.

Until very recently, the prevailing air quality monitoring devices were expensive, bulky, have high working temperatures and low-resolution. Nonetheless, current research trends show that this paradigm is changing at a fast pace with the development of portable, room temperature, low power gas sensors which can detect trace concentration (∼ sub-ppb) of gaseous vapours [[2], [3], [4]]. Towards this end, numerous investigations have been made to find a suitable material possessing these qualities for gas sensing applications. Apparently, tin oxide (SnO2) is a promising material which has attracted wide attention owing to its unique properties such as low toxicity, small size, high charge carrier mobility, wide bandgap, high chemical and thermal stability [5,6]. As it can detect a variety of gases with high sensitivity in a short time, it has been widely explored as a versatile gas sensor. However, traditional SnO2 based sensors have a higher detection limit (DL) normally in the ppm range, higher operating temperatures (150–300 °C) and cross-sensitivity issues, which limits their extensive use.

In view of this, various strategies like doping, nano-structuring, hetero-structuring, self-doping, etc. have been adopted to achieve low working temperatures and enhanced selectivity along with low DL [2,3,7,8]. Among these, the introduction of different valence atoms such as Zn, Pt, Pd, etc. into the SnO2 matrix has been identified as an effective and simple approach to enhance the sensing properties [[9], [10], [11], [12]]. As an example, Pengjian Wang et al. was successful in reducing the DL down to 100 ppb towards H2S with extraordinary stability and low response/recovery time by loading W into ultrafine SnO2 nanoparticles [4]. The observed results have been owed to the reduction in particle size and tuning of the electronic bandgap of SnO2. Lately, rare earth elements (Gd, Er, Y, Ce, etc.) have been used to alter the optical and electrical properties of SnO2 [[13], [14], [15], [16], [17]]. These elements promote gas adsorption by increasing the catalytic activity and creating structural defects like oxygen vacancies (OVs) and tin interstitials (Sni) in host lattice, which in turn modifies the adsorption/desorption properties of the surface. For instance, the loading of Dy in SnO2 nanoparticles reduced the operating temperature by 50 °C and also enhanced the sensing response towards ethanol [18]. A similar result has been obtained with Gd-doped SnO2 thick films where the improved sensing characteristics towards ethanol have been attributed to high surface basicity, small crystallite size and abundant OVs [15]. Reduction in DL to as low as 1 ppm towards formaldehyde has been achieved with Y modified SnO2 flower-like nanostructures; however, the device still performs best at 180 °C [17]. A brief literature review of rare earth doped SnO2 gas sensors has been given in Table 1. Thus, despite these findings, the potential of rare earths in developing low-power SnO2 sensors having low DL has not been fully exploited.

This work is targeted to fabricate 3 wt% Y incorporated SnO2 thin film room temperature (RT) sensor with a sub-ppb experimental detection limit. The sensor showed excellent sensing properties with high selectivity towards NO2 in comparison to undoped SnO2 sensor. The high quality undoped and doped SnO2 thin films (50–150 nm) have been prepared using the electron-beam deposition technique and later characterized by various structural and spectroscopic techniques.

Section snippets

Materials and method

All the precursors including tin chloride pentahydrate (SnCl4·5H2O), yttrium nitrate pentahydrate (Y(NO3)3·5H2O) and ammonium hydroxide (NH4OH) were obtained from Sigma-Aldrich Chem. Co. Ltd. The chemicals and reagents were subsequently used without any further purification throughout the experiment.

Synthesis of pure and Y-doped SnO2 nanoparticles

The facile and low-cost co-precipitation method was used to synthesize pure, 1, 3 and 5 wt% Y-doped SnO2 nanoparticles. In a typical experiment, optimum amount of SnCl4·5H2O was dissolved in 100 mL

Morphological and structural studies

Fig. 1 shows the FESEM micrographs of SnO2 thin films with different doping concentrations. The 1PS thin film is smooth and crystalline with interjoined grains, however with 1% Y addition, distinctive grains start to appear implying the successful incorporation of Y. For 3% Y-doped thin film sample, in addition to the clearly separated grains, reduction in their size was also observed. With excessive doping in 5YS sample, the quality and granular structure of the film was destroyed, and instead

Conclusions

In summary, highly sensitive, selective and reliable Y-doped SnO2 thin film-based RT sensors have been successfully fabricated using physical vapour deposition technique. It has been found that the Y incorporation significantly enhances the sensing performance of the SnO2 sensor towards NO2. The 3 wt% Y-doping improved the sensing response from 19 % to 1445 %, reduced the τres and τrec to few minutes. Notably, the 3YS sensor exhibited outstanding sensitivity towards 0.6 ppb NO2 with 26 %

CRediT authorship contribution statement

Manreet Kaur Sohal: Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Aman Mahajan: Conceptualization, Methodology, Project administration, Supervision, Visualization, Writing - review & editing. Sahil Gasso: Data curation, Writing - review & editing. R.K. Bedi: Conceptualization. Ravi Chand Singh: Project administration, Supervision. A.K. Debnath: Investigation. D.K. Aswal: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are thankful to DST, New Delhi, for providing financial support through Project No. INT/UKP/P-21/2018 in support of the present research work.

Manreet Kaur Sohal received her Bachelor’s and Master’s degree in Physics from Guru Nanak Dev University, Amritsar, India. She is presently working as a research associate in the Department of Physics from the same university. Her research interests lie in the fabrication and characterization of chemiresistive metal oxide thin films based gas sensors.

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  • Cited by (0)

    Manreet Kaur Sohal received her Bachelor’s and Master’s degree in Physics from Guru Nanak Dev University, Amritsar, India. She is presently working as a research associate in the Department of Physics from the same university. Her research interests lie in the fabrication and characterization of chemiresistive metal oxide thin films based gas sensors.

    Dr. Aman Mahajan received his M.Sc. degree (1997) and Ph.D. (2003) in Physics from Guru Nanak Dev University, Amritsar, India. He is presently working as an assistant professor in the same university. His main interests are preparation and characterization of organic and metal oxide thin films for OLEDs, sensors and photovoltaic applications.

    Sahil Gasso received his Bachelor in Non-medical degree from Punjabi University, Patiala, India, in the year 2015 and Master’s degree in Physics from School of Physics and Material Sciences, Thapar University, Patiala, India. Presently, he is working towards his Ph.D. degree in the Department of Physics, Guru Nanak Dev University, Amritsar, India. His research interests are preparation and characterization of metal oxide thin films based gas sensors.

    Dr. R. K. Bedi is a Ph.D. and retired professor from the Department of Physics, Guru Nanak Dev University Amritsar, India. He has held academic positions such as Head of Department, Dean, Science faculty and other administrative positions in the university. His research interests are material Science, thin films, photovoltaics and sensors. He is a fellow of The Institution of Electronics and Telecommunication Engineers (IETE). Presently, he is Director, SIET, Amritsar, India.

    Dr. Ravi Chand Singh received his Ph.D. in Physics from Guru Nanak Dev University, Amritsar, India, in 1989. Since then, he has had an appointment at the same institute for one year and moved to a post-doctoral position at Simon Fraser University, Canada in 1990. He joined Guru Nanak Dev University Amritsar in 1993. He is presently working as a Professor of Physics. His research interests are material research for gas sensing and the development of new experiments for physics education.

    Dr. A. K. Debnath is presently working as Scientific Officer (G) at Technical Physics Division of BARC. He has extensively worked on oxide materials based gas sensor, particularly for H2S detection. His current research interest is to understand the charge transport and gas sensing properties of ultra-thin films of organic semiconductor grown using MBE.

    Dr. D. K. Aswal joined BARC in 1986 and served as Head, Thin Films Devices Section. At, present he is Director, CSIR- National Physical Laboratory, New Delhi, India. His current area of research interests includes physics of organic films and their applications for solar cells, conducting polymer films for flexible electronics, thermoelectric power generators and gas sensors and electronic nose. He is a recipient of several international fellowships including, JSPS fellowship, Japan (1997–1999), IFCPAR fellowship, France (2004–2005), BMBF fellowship, Germany (2006) and CEA fellowship, France (2008). He is recipient of several awards, including “MRSI Medal 2010”, “Homi Bhabha Science and Technology Award-2007”, “DAE-SRC Outstanding Research Investigator Award-2008”, and “Paraj: Excellence in Science Award, 2000”.

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