ReviewInvestigation of CdO nanostructures synthesized by microwave assisted irradiation technique for NO2 gas detection
Introduction
It is well recognized that quantum mechanical effects become dominant when the nanometer size range is reached, typically at distances of 100 nm or less. Additionally, a number of physical (thermal, electrical, optical, etc.) and mechanical properties change when compared to macroscopic systems. One example is the increase in surface area to volume ratio altering thermal and catalytic properties of materials. Furthermore, it is known that as the particle diameter, a, of semiconductor metal oxides decreases down to critical size ∼80 – 100 nm, the redistribution of electrons from bulk donors to the surface vacancies (“surface electron traps”) takes place. This leads to a significant (by three orders of magnitude) drop in the bulk conduction electron density. As a result, conductance of the material becomes very sensitive to the concentration of gas molecules adsorbed on the surface [1].
CdO is considered one of the most promising metal oxide materials. It is an n-type semiconductor with band gap of ∼2.5 eV, showing low resistivity due to defects of oxygen vacancies and cadmium interstitials. Then, due to its high electrical conductivity and optical transmittance in the visible region of solar spectrum, it has great potential for advanced applications (flat panel display, organic light emitting diodes, gas sensors, etc.).
Different techniques such as sol–gel [2], DC magnetron sputtering [3], radio-frequency sputtering [4], spray pyrolysis [5], pulsed laser deposition [6], chemical vapor deposition [7], chemical bath deposition [8], hydrothermal [9] and solvothermal [10] have been used to prepare CdO nanostructure. Microwave-assisted synthesis has more advantages with respect to above conventional methods such as very short reaction times, production of small particles with a narrow particle size distribution, and high purity [11], [12], [13], [14]. Further, it allows the controlled synthesis of nanostructures, a theme of outmost importance due to recent interests toward the search of nanomaterials with peculiar size and shape dependent properties having superior performances [2], [15].
In the present work, we report the preparation of CdO nanostructures with a simple and cost-effective chemical route assisted by microwave irradiation without any post-synthesis annealing treatment. Only few articles reported the preparation of crystalline CdO nanostructures without high temperature treatments [2]. Compared to previous reports, we have successfully prepared crystalline CdO nanostructures within 5–15 min.
Despite its interesting electrical properties, very little literature is concerning devices which employ CdO for gas sensors. So far, CdO has been used for detecting liquefied petroleum gas [16], carbon dioxide [17] and other reducing gases. To our knowledge, there are instead only limited reports on gas sensing studies towards oxidizing gases, such as NO2 [18]. The present work aims therefore investigating the NO2 sensing properties at mild temperature of CdO samples prepared by the chemical route assisted by microwave irradiation without any post-synthesis annealing treatment.
Section snippets
Synthesis
A cadmium hydroxide solution (0.1 M) was prepared by dissolving cadmium acetate with double distilled water. The ammonia solution was added to above precursor solution under constant stirring until the pH of the solution is 8. Resulting precipitate was washed using double distilled water. The obtained precipitate was subdivided and treated in two ways: the first one, the precipitate was directly dried at 120 °C in a conventional oven without implementation of microwave irradiation (sample-A)
Materials characterization
The XRD pattern of the dried precipitate (sample-A), i.e. before microwave irradiation, confirms the existence of cadmium hydroxide phase, Cd(OH)2, as clearly indicated by diffraction reflections shown in Fig. 1(A). The low intensity and large broadening of FWHM in the XRD spectrum of sample not irradiated suggest the presence of small Cd(OH)2 particles that are not completely crystallized.
The XRD patterns of microwave irradiated samples for different time intervals, i.e. 5 (sample-B), 10
Conclusion
A simple and low cost microwave assisted technique has been proposed for the synthesis of cadmium oxide nanostructures without any post-synthesis annealing procedure. The characterization techniques revealed that cadmium hydroxide gets converted into cadmium oxide due to microwave irradiation. The formation of spherical and rod-like CdO nanostructures was found to be dependent on the microwave time irradiation. Furthermore, the synthesized cadmium oxide nanostructures were found to be highly
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2022, Materials Science and Engineering: BCitation Excerpt :With a direct bandgap of 2.5 eV and an indirect bandgap of 1.98 eV, CdO is an n-type semiconductor that belongs to the II-VI group [20]. These NPs are attractive candidates for various technical applications, such as liquid crystal displays [21], solar cells [22], infrared detectors, gas sensors [9], photovoltaic devices and photodiodes [23,24]. As in one works, the aqueous extract of Convolvulus arvensis was used as the capping agent, while hydrazine acted as the reducing agent in the plant-mediated production of CdO NPs [25].
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2021, Journal of Materials Research and TechnologyCitation Excerpt :The band at 476 cm−1 was suggested by the vibration of Cd(OH)2 [27]. Moreover, the continuous large broad band from 624 to 1400 cm−1 could be related to the vibration of the Cd–O bond [28,29]. Therefore, CdO modified the structural units of phosphate groups (P–O bonds in Q2 and Q1 and P–O–P linkage), and a new linkage such as P–O–Cd could be formed, which is stronger than the P–O–P linkage.