Ultra-high sensitive ammonia chemical sensor based on ZnO nanopencils
Highlights
► Well-crystalline ZnO nanopencils were synthesized by very simple and facile hydrothermal process using simply available laboratory chemicals. ► Using ZnO nanopencils, a liquid ammonia chemical sensor is fabricated based on convenient and simple current–voltage (I–V) principle. ► The fabricated liquid ammonia chemical sensor exhibits the highest sensitivity ever reported in the literature. ► The obtained detection limit of the fabricated sensor is lower than the previously reported literature values.
Introduction
In recent years, numerous intensive reseacrh efforts in the field of nanotechnology has shown great potential. There has been a significant improvement for the synthesis of desired inorganic nanomaterials as reported by vast literature reported so far. Being at the dimension between approximately 1–100 nm, nanomaterial enabled the various useful applications in diverse fields of the scientific areas.
Nowadays, environmental pollution caused by combustion from vehicles, agricultural sector and industrial leakages of toxic chemical and gases at very alarming rate created havoc in the scientific community [1], [2]. Among various environmental pollutants, the liquid ammonia (i.e. ammonium hydroxide) is one of the highly toxic chemicals for human being and wildlife produced in fertilizer and chemical factories. The dissolution of this chemical into running and drinking water may cause severe health problems such as skin, throat, lung cancer and permanent blindness. Therefore, early detection and monitoring of leakage of liquid ammonia in the environment is highly desirable for public safety. In this regard, there have been reports for the detection of ammonia (gas and liquid both) in various forms such as electrochemical sensor, optical sensor, chemiresistive sensor, polyaniline and metal oxide based sensors, biomaterials (L-glutamic acid·HCl) based sensors, I–V technique based sensor and so on [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Among various detection techniques, the I–V technique is one of the simplest and easy technique for the efficient detection of various hazardous chemicals and environmental pollutants. For this, various nanostructures were used as efficient electron mediators to modify the electrodes for the fabrication of efficient I–V technique based chemical sensor [14], [15], [16], [17], [18]. Among various nanostructures, metal oxide nanostructures possess excellent properties and hence used for wide applications [5], [6], [7], [8], [9], [10], [12], [13], [14], [15], [16], [19], [20], [21]. In metal oxides family, the II–VI wurtzite hexagonal-shaped ZnO possess special place due to its own properties and wide applications. The various exotic properties of ZnO include its direct and wide band gap (∼3.37 eV), high exciton binding energy (60 meV) much larger than other semiconductor materials, biocompatibility, easy and cost effective synthesis, high electron features, optical transparency, non-toxicity and so on [22], [23], [24], [25], [26]. Although, ZnO have excellent properties, there are very few reports of this material as ammonia chemical sensor.
In this paper, we report the fabrication of highly sensitive chemical sensor for the efficient detection of liquid ammonia simply by using ZnO nanopencils. For this, a layer of ZnO nanopencils was coated over the active surface of Glassy carbon electrode (GCE) followed by thermal treatment in an oven in order to obtain a uniform and stable layer. Later, the ZnO modified GCE was used to detect the liquid ammonia by I–V technique. Due to well-crystallinity and higher surface area of the synthesized ZnO nanopencils, the fabricated sensor offers ultra-high sensitivity towards the efficient detection of liquid ammonia. In addition to this, the fabricated sensor also exhibited high regression coefficient and very low detection limit, i.e. in the nanomolar range. Furthermore, it was observed that the proposed ammonia sensor is highly reproducible and stable over a long period of time.
Section snippets
Synthesis of ZnO nanopencils by low-temperature hydrothermal process
Large-quantity synthesis of ZnO nanopencils was done by simple and facile hydrothermal process by using zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and ammonium hydroxide (NH4OH) at low-temperature of 125 °C. All the chemicals utilized for the synthesis of ZnO nanopencils were purchased from Sigma–Aldrich and used as received without further purification. In a typical reaction process, 0.05 M zinc acetate was mixed with 0.05 M ammonium hydroxide solution, both made in 50 mL DI water, under contineous
Morphological, structural and optical properties of as-synthesized ZnO nanopencils
The morphological, structural and optical properties of as-synthesized ZnO nanopencils were examined by various techniques and discussed in this section. To evaluate the general morphologies, the as-synthesized products were characterized by FESEM and the micrographs are reported in Fig. 1. The FESEM images clearly confirm that the synthesized materials are pointed nanorods which are grown in large quantity. Interestingly, it is seen that the pointed nanorods are assembled in bunches and each
Conclusion
In summary, a very simple, reliable, reproducible and facile method has been presented to fabricate ultra-high sensitive liquid ammonia chemical sensor. Low-temperature grown, well-crystalline hexagonal-shaped ZnO nanopencils were used as efficient electron mediators for the fabrication of proposed chemical sensor. The fabricated liquid ammonia chemical sensor exhibits ultra-high sensitivity of ∼26.58 μA cm−2 mM−1 and very detection limit of ∼5 nM. To the best of our knowledge, the fabricated
Acknowledgements
Authors would like to acknowledge the support of the Ministry of Higher Education, Kingdom of Saudi Arabia for this research through a grant for a Collaborative Research Centre on Sensors and Electronic Devices at Najran University, Saudi Arabia, dated 24/3/1432 H, 27/02/2011. A. Umar and S.A. Al-Sayari are thankful to the Deanship of Scientific Research at Najran University, Saudi Arabia for Grant no. 27/11.
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