Synthesis of nickel sulfide-reduced graphene oxide for application in gas sensors

Abstract

The detection of toxic substances are crucial for the environment and human health. Recently, composite structures have shown great capabilities in detecting these toxic substances due to their unique chemical and physical properties. In this study, simple microwave-assisted and other wet chemical methods were used to produce nickel sulfide-reduced graphene oxide composites and applied in the detection of acetone, ethanol, methanol and toluene vapours. Firstly, the influence of different solvents on microwave produced Ni3S2 structures was investigated. Water, ethanol, ethylene glycol, a mixture of water and ethanol and a mixture of water and ethylene glycol were used separately as solvents during a microwave reaction that was operated at 600 W for 6 minutes. The as-prepared materials were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Laser Raman spectroscopy, Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) surface area analysis. The X-ray diffraction planes obtained for the materials synthesized when using water, ethylene glycol and a mixture of water and ethylene glycol solvents were in alignments with the reported Ni3S2 diffraction planes, while a mixture of nickel sulfide phases were obtained when ethanol and the mixture of water and ethanol solvents were used. TEM micrographs revealed that the structures synthesized in water, ethylene glycol and a mixture of water and ethylene glycol produced flowerlike nanostructures. The nanoflowers produced in only water consisted of smaller nanosheets. These nanoflowers also had the highest BET surface area. The materials synthesized in water were also exposed to annealing in a hydrogen gas atmosphere at temperatures between 300 and 500°C. Annealing at 300°C caused an increase in the crystallinity of the material and a further increase in the annealing temperatures resulted in a decrease in the crystallinity of the material, which was also accompanied by a phase transformation of the materials from Ni3S2 to a mixture of NiS, Ni3S2 and Ni7S6. This phase transition was also accompanied by a morphological transition from flowerlike structures to a mixture of rod-like flowers or quasi-spherical nanoparticles. Graphene oxide (GO) was synthesized using the Improved Hummers’ method and further reduced with or without the use of a reducing agent by employing a facile microwave technique in order to form reduced graphene oxide (rGO). XRD results showed an increase of the interlayer spacing upon oxidation of graphite due to the incorporated oxygen groups between the layers. After reduction this interlayer spacing decreased. Raman results demonstrated that rGO synthesized without any reducing agent had a comparable defect density with the rGO synthesized in hydrazine hydrate. However, TEM showed that the sheets in the control sample were smaller in diameter and were more agglomerated. XPS analysis showed an increased C/O ratio from 2:1 in GO to 7.8:1 in hydrazine reduced GO. Thereafter, Ni3S2-rGO composites were prepared in situ and ex situ and characterized. XRD and XPS analysis confirmed the formation of the composites. TEM showed that the ex situ prepared composites were made up of 2D-2D junctions whereas the sample prepared in situ consisted of quasi-spherical nickel sulfide nanoparticles (1D) on 2D rGO sheets. Finally, different sensors were prepared by dispersing the sensing material (Ni3S2, rGO, (5%) Ni3S2-rGO, (10%) Ni3S2-rGO and in situ NS-rGO) in a N,N-dimethylformadide/ethylene glycol solution and drop casted on gold electrodes. Gas sensing results showed that ex-situ prepared (5%) Ni3S2-rGO could potentially be used as a room temperature ethanol sensor. This sensor demonstrated a good response, the highest sensitivity and most linear sensing performance when compared with the other fabricated sensors