Enhanced room temperature ammonia gas sensing properties of strontium doped ZnO thin films by cost-effective SILAR method

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Abstract

In this present work, pure and Sr doped ZnO films were systematically coated SILAR method is debated. The influence of Sr doping content on microstructure, surface morphology, elemental composition, photoluminescence and ammonia vapor sensing properties of these coated films were analyzed before and after incorporation of Sr. The effect of Sr doping revealed a change in crystal orientation from the (0 0 2) to the (1 0 1). SEM analysis reveals the formation of flower-like morphology formed by a number of nanowires with doping concentration. With the increase in Sr concentration, the bandgap values are increased from 3.02 to 3.20 eV. The addition of Sr also induces a rapid response towards ammonia gas sensing.

Introduction

In recent days, detection of flammable and perilous gases like CO, CO2, NOx (x = 0.5, 1, and 2), SOx (x = 2 and 3), and NH3 has received extensive interest to prevent fire or explosion. Due to the heavy industrialization and rapid economic growth in the world, these hazardous gases are evolved in our environment. The aforementioned gases like NH3 are mostly considered as one of the highly toxic gases widely utilized in manufacturing industries of plastics, fertilizers, explosives, textiles, pesticides, food-based, and refrigerant systems [[1], [2], [3]]. In addition to that, it is naturally a colourless one and mainly produced by the Haber process and it has a threshold limit of 25 ppm, which can easily damage several parts of the human body and leads to death. So it is highly important to develop a gas sensor to detect ammonia to protect our healthy environment in order to continuously track the leakage of this gas [4].

Metal oxides are ideal in this context of gas sensing and these semiconductors have been extensively utilized for proving their excellency in detecting NH3 gas efficiently. Moreover, because of their high sensing response and long-term stability metal oxide, semiconductors-based gas sensors were extensively studied in recent days.Plenty of functional materials like TiO2 [5], WO3 [6,7], SnO2 [[8], [9], [10], [11]], Fe2O3 [12], NiO [13] and ZnO [14] were widely prepared to detect ammonia gas. Among them, ZnO is one of the universally recognized outstanding semiconducting material because of its tunable and attractive properties. ZnO is an n-type semiconducting material and possesses a wide bandgap of 3.37 eV with a wurtzite hexagonal structure. Furthermore, ZnO nanostructure has gained enormous attention in various gas-sensors and suitable to detect gases owing to its chemical sensitivity, thermal stability. Moreover, it shows low electron resistance with more electron mobility by the addition of impurities [15]. SILAR technique is the most useful method to prepare ZnO thin films. It has the following superior advantages than the other methods such as economical, relatively very simple, high purity reproducibility, does not require high-quality substrates and high-cost equipment. Recently, Baktiyar Soltabayev et al. [ 16], C.C. Okorieimoh et al. [ 17] and Mangesh A. Desai et al. [18] prepared ZnO thin films using the SILAR technique.

ZnO shows a noticeable response towards ammonia vapor. To enhance the sensitivity and selectivity of the ZnO films, the addition of impurity was functionalized. Specifically, conventional dopants can tailor various properties such as gas sensing defects, structural, electrical, and energy band structure [19]. In the past, numerous research works have been carried out to detect the leakage of ammonia gas by suitable doping on the ZnO parent atoms. Raghavendra et al. [20] observed increment in visible emissions is attributed to Sr induced oxygen vacancy related recombination in ZnO and also noticed the improved surface morphology at higher concentration of Sr in ZnO: Sr thin film. Moreover, few reports are available with strontium dopant with ZnO. Hence strontium is utilized as the dopant in the ZnO lattice. Handan Aydınk et al. fabricated ZnO:Al thin films for ammonia sensing with different ppm level and shown 5% Al-doped films fast response for 660 ppm [16]. Ganesh Kumar Mani et al. studied selective detection of ammonia by ZnO:Ni films and noticed an enhancement in the recovery time as a 25–1000 ppm level [21]. Ravichandran et al. reported tungsten doped ZnO films to show a fast response towards 100 ppm ammonia levels [22]. Kulandaisamy et al. coated ZnO:Mg films by spray pyrolysis route in which lesser doping shows fast response towards ammonia gas for 100 ppm with reduced response and recovery time [23]. Up to our knowledge, no reports are available for the detection of NH3 for ZnO:Sr. Hence this research work focuses on preparing Sr: ZnO films by using the SILAR technique and study its vapor sensing characteristics towards NH3 gas.

Section snippets

Deposition method

Sr doped ZnO films were grown on soda-lime glass substrates using the SILAR route. Zinc sulfate [ZnSO4], Sodium Hydroxide [NaOH], strontium sulfate [Sr2(SO4)3] materials were used as host and dopant materials. Distilled water was used throughout the deposition process. Before the deposition, the soda-lime glass substrate was cleaned in soapy water, chromic acid, distilled water and acetone to remove the impurities. The SILAR method is similar to chemical bath deposition but it is easier to

XRD studies

Fig. 1(a) presents XRD graphs of ZnO:Sr films and we note three strong reflections such as (100), (002) and (101) which correspond to the hexagonal wurtzite structure and well fit with JCPDS 36–1451 card [24]. In addition to that five small reflections such as (102), (110), (103), (112) and (201) were also noticed. The strong reflection (002) exhibited maximum intensity thereby showing a standard orientation along the c-axis. The addition of impurity content does not lead to the occurrence of

Conclusions

In this study, ammonia vapor sensing characteristics of a ZnO:Sr gas sensor is studied and reported. ZnO:Sr thin films have been successfully developed using the SILAR technique. XRD studies of prepared thin films have confirmed polycrystalline nature formation. The morphological study confirmed the formation of nanowire as structured by the number of nanograins favoring sensing. The vapor sensing results show that among the different Sr-dopant concentrations, film prepared with 5 wt% Sr-doped

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.

Acknowledgments

The authors express their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P.2/84/41.

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