Temperature dependent selective and sensitive terbium doped ZnO nanostructures

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Abstract

In the present work, influence of terbium doping on structural, morphological, optical and gas sensing properties of zinc oxide has been studied. A chemical route was adopted for synthesis of pure and terbium (Tb) doped zinc oxide. X-ray diffraction study confirmed the formation of hexagonal wurtzite structure for synthesized materials. Raman analysis revealed the shifting and broadening of peaks with increase in Tb concentration. The presence of terbium in ZnO and its oxidation states was confirmed using X-ray photoelectron spectroscopy. Photoluminescence emissions indicated increase in concentration of oxygen vacancies with introduction of dopant. Gas sensors were fabricated out of synthesized samples and it was observed that doped samples have significantly high sensing response, temperature dependent selectivity toward ethanol and acetone, and sensors were able to detect even low concentration (∼10 ppm) of these vapors. The temperature dependent selectivity of terbium doped ZnO depends on target gas which may be ascribed to interaction of target gas molecules and doped metal oxide surface at optimum operating temperature. It was found that 4% Tb doped ZnO sensor exhibited maximum sensor response toward ethanol and acetone. The enhanced sensing response has been attributed to increase in oxygen vacancies, reduction in particle size, large structural disorders and high surface basicity.

Introduction

A variety of volatile organic compounds such as alcohol or acetone are invariably used in research laboratories and chemical industries for various purposes. Their usage under precautionary conditions may not cause serious health issues but inhalation or ingestion beyond permissible limit may cause irritation in throat, nose, eye; giddiness, nausea, headache and their chronic exposure may affect liver, kidney and heart [1], [2], [3]. Therefore for conducive working environment it becomes imperative to install highly sensitive acetone and ethanol sensors at work places to monitor volatile vapor in the working ambiance. Zinc oxide, an n-type semiconductor, is an ecofriendly metal oxide and possesses interesting properties such as wide band gap, good thermal and chemical stability, high electron mobility, large exciton binding energy, and its anomalous surface properties that allows the interaction of various gases on its surface making it a suitable candidate as a gas sensor [4], [5]. Various research groups have reported improvement in gas sensing properties of ZnO by employing a variety of physical and chemical procedures for the synthesis of oxides, addition of dopants, modification of surface properties, etc. [6], [7], [8], [9], [10], [11]. For obvious reasons, doped nanostructures have always attracted significant attention for gas sensing applications because doping could alter the electrical, optical and structural properties of host lattice by introducing lattice distortions, grain size refinements, oxygen and cation vacancies etc. Dopant such as transition metals (Cr, Sn, Mn, Ni, etc.) and noble metals (Pd, Pt, Ag and Au) have been effectively employed for improving gas sensing behavior of semiconductor metal oxides [12], [13]. In addition to this, rare earth metals have been found to be suitable candidates for improving the sensing properties because of their extraordinary catalytic nature, high surface basicity and rapid oxygen ion mobility [14]. Bagheri et al. have reported ethanol sensor based on Sm2O3 loaded ZnO [15]. Dar and coworkers have prepared Ce-doped ZnO nanorods and they found it as a better chemical sensor for hydroquinone detection [16]. Zhang et al. synthesized rare earth doped (La, Er, Yb) In2O3 hollow spheres and observed their sensitivity toward alcohol as a consequence of high surface activity and chemisorbed oxygen [17]. Xu et al. reported La doped ZnO bead like structure as an effective acetone sensor due to increase in surface adsorption and catalytic nature of La [18]. However, to the best of our knowledge the sensing properties of Tb doped ZnO for VOCs (ethanol and acetone) have not been reported yet. Terbium is one of the rare earth elements that can exist in +3 and +4 oxidation states relatively easily as compared to other rare earths [19]. It will be interesting to find how the sensing properties of host metal oxide are affected with hopping of terbium ions between +3 and +4 oxidation state in Tb doped ZnO. A number of research groups have investigated the luminescence properties of Tb doped ZnO and have found that terbium induces defects (green emission) which are associated with oxygen vacancies [20]. The increase in oxygen vacancies in n-type semiconductors is favorable for enhancement in gas sensing response. Furthermore, since terbium does not tarnish easily in air therefore it is suitable as a dopant in various applications. The properties of terbium make it a promising element to be used as a dopant in gas sensing applications.

In addition to this, we found that the gas (ethanol and acetone) selectivity of Tb-doped ZnO is highly dependent on operating temperature. Sensor selectivity is very preferable in gas sensing applications and not many sensors show this property. The temperature dependent selectivity of Tb doped ZnO has not been studied yet.

In the present work, ZnO and Tb doped ZnO nanoparticles have been synthesized using a simple and cost effective co-precipitation method. With optimized Tb doping in ZnO, the high performance and temperature selective ethanol and acetone gas sensor has been fabricated. An attempt has been made to correlate sensing response to dopant induced defects.

Section snippets

Chemicals used

All chemical reagents, zinc acetate dihydrate [(CH3COO) 2 Zn·2H2O, Sigma–Aldrich], terbium(III) nitrate pentahydrate (Tb (NO3)2·5H2O, Sigma–Aldrich) and ammonia used in the present work were of analytical grade and used without further purification.

Synthesis process

Pure and terbium doped zinc oxide powder samples were prepared by co-precipitation technique. To start with, 0.2 M aqueous solution of zinc acetate dihydrate was prepared. Ammonia solution was added drop wise to the above aqueous solution with

X-ray diffraction

XRD measurements were performed to investigate the crystalline structure of synthesized ZnO and Tb doped ZnO. The XRD patterns shown in Fig. 1(a) confirm formation of hexagonal wurtzite structure for pure and Tb doped ZnO (JCPDS 80–0074). Log of intensity verses 2θ is plotted in order to visualize the low intensity peak corresponding to dopant. It is clear from Fig. 1(a) that no peak for terbium or terbium oxide (Tb2O3) appears up to 4% doping. However for 6% Tb doped ZnO, a low intensity peak

Conclusion

Pure and Tb3+ doped ZnO nanoparticles were synthesized by co-precipitation route and were found to have hexagonal wurtzite structure. A secondary phase corresponding to Tb2O3 was detected for 6% Tb doped ZnO. XRD analysis provided information regarding the reduction in crystallite size, expansion of unit cell leading to disorders in lattice due to increase in Tb concentration. The increase in defect concentration predominantly oxygen vacancies in doped samples is evidenced by PL and XPS

Acknowledgements

Anita Hastir would like to acknowledge financial support from the INSPIRE Fellowship, Department of Science & Technology, India. Thanks to UGC-UPE, India and DST-FIST, India for providing instrumental facilities. We would also like to thank Prof. Robert L. Opila at the University of Delaware for his help in providing XPS facilities.

Anita Hastir received her M.Sc. Physics (Hon. School) degree from Guru Nanak Dev University, Amritsar, India in 2010. Presently she is pursuing Ph.D. in the field of synthesis and characterization of zinc oxide based materials and their applications as gas sensors from the same institute.

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    This may be expected as the C-H BDE of acetone is higher than that of formaldehyde. Hastir et al. [129] reported the temperature dependent selectivity of a Tb-doped ZnO sensor. Pristine ZnO sensor shows maximum response to acetone and ethanol at the same optimized operating temperature of 400 °C.

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Anita Hastir received her M.Sc. Physics (Hon. School) degree from Guru Nanak Dev University, Amritsar, India in 2010. Presently she is pursuing Ph.D. in the field of synthesis and characterization of zinc oxide based materials and their applications as gas sensors from the same institute.

Nipin Kohli received her Ph. D. in Physics in 2013 and M.Sc. Physics degree in 2005 from Guru Nanak Dev University, Amritsar, India. Her work interest includes thin films coating and fabrication of gas sensors based on metal oxides, composite oxides and polymer materials.

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 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 development of new experiments for Physics Education.

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