Hydrothermal Synthesis of ZnO Structures Formed by High-Aspect-Ratio Nanowires for Acetone Detection
© The Author(s). 2016
Received: 22 May 2016
Accepted: 21 July 2016
Published: 26 July 2016
Snowflake-like ZnO structures originating from self-assembled nanowires were prepared by a low-temperature aqueous solution method. The as-grown hierarchical ZnO structures were investigated by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The results showed that the snowflake-like ZnO structures were composed of high-aspect-ratio nanowires. Furthermore, gas-sensing properties to various testing gases of 10 and 50 ppm were measured, which confirms that the ZnO structures were of good selectivity and response to acetone and could serve for acetone sensor to detect low-concentration acetone.
As wide band gap materials, ZnO has been widely investigated for its potential applications [1–9]. Especially, it can be used in toxic and combustible gas detectors [10–13]. For gas sensor applications, ZnO is one of the promising materials. Moreover, low-dimensional ZnO materials have a large surface-to-volume ratio, which can be used as a potential material on gas-sensing devices [14–20]. ZnO nanostructures with different morphologies have been synthesized through a wide range of preparation methods [21, 22]. Among these methods, the low-temperature route is a suitable choice because of its simplicity, reproducibility, and cost-effectiveness and is attracting considerable attention [23, 24].
Acetone is a commonly used chemical solvent, which has been regarded as an extracting regent in the industry. In addition, acetone is a very important marker for noninvasive diagnosis of diabetes in the human breath aspects . Thus, it is of great significance to develop a new type of acetone gas sensor that can be used as a noninvasive detector. The use of low-dimensional structures is a key technological factor in the creation of new functional and sensing devices .
We proposed a low-temperature method to prepare snowflake-like ZnO structures in this paper. The structures and morphologies have been investigated by X-ray powder diffraction (XRD) and field-emission scanning electron microscopy (FESEM). Micro-Raman and absorption spectrum were also performed to investigate the optical properties of the structures. Meanwhile, a gas sensor was made basing on the snowflake-like ZnO structures, and its gas-sensing properties were investigated. Particularly, the prepared sensor exhibited good selectivity and response to acetone which makes it as a good candidate for detecting low-concentration acetone.
The snowflake-like ZnO structures were grown in aqueous solutions at a low temperature. The typical procedure was to use zinc nitrate (Zn(NO3)2) and hexamethyltetramine (C6H12N4) mixed solutions with the addition of NaF. The typical reaction process was listed as follows: 0.05 M Zn(NO3)2 and 0.02 M NaF were dissolved in distilled water. Then, 0.05 M C6H12N4 was added slowly under stirring condition. Afterward, the mixture solutions were reacted at 90 °C for 5 h. After washing with distilled water and pure ethanol, the sample was dried at 60 °C. Then, the obtained ZnOHF intermediate was baked at 400 °C for 2 h. Finally, the as-grown ZnO samples were cleaned with deionized water and dried in the air.
The gas-sensing properties were performed by using a static state gas-sensing measurement system. As-prepared ZnO nanostructures were dispersed with deionized water to form a paste. Afterwards, it was coated onto a ceramic substrate. In addition, three pairs of gold interdigital electrodes were made to form a ZnO sensing film. The thickness of the ZnO sensing film was about 300 nm.
Results and Discussion
The gas-sensing properties were performed by using a static state gas-sensing measurement system . The as-prepared ZnO nanostructures were coated onto a ceramic substrate (250~300 μm in thickness), dried at 100 °C for 2 h, and annealed at 500 °C for 2 h in air. Finally, a Ni-Cr heating wire was inserted into the ceramic tube to form an inside-heated gas sensor. Voltage division circuit was adopted to measure the export voltage on the sensor. Meanwhile, an external resistor (R L) was connected in series with the obtained sensor under the bias of 10 V. The gas response value (S) was defined as a ratio of the electrical resistance of the sensor in air (R a) to that in a testing gas (R g): S = R a/R g, and R a = R L(10-V air)/V air, R g = R L(10-V gas)/V gas, where V air was the export voltage of R L in air and V gas was the voltage in the testing gas .
The working mechanism of ZnO gas sensors can be attributed to the conductivity changes. The reaction between the testing gas and the oxygen species located on the surface of the ZnO structures could cause resistance change. In our case, when the sensor based on snow-like structures is exposed to acetone gas, the acetone gas is oxidized by the oxygen species to form formaldehyde and then increase conductance. The response to acetone with a low concentration could be ascribed to the snow-like sensor structure, which has a much bigger specific surface area than other conventional sensing structures, which could provide a larger adsorption region then increase the amount of gas molecules adsorbed on the sample surface. Moreover, the formation of nanowire junctions may be another reason for the response enhancement . The junctions are considered as the active sites, which can increase the gas response sensitivity.
In summary, snowflake-like ZnO structures were synthesized by a simple low-temperature way. The structures are constructed of high-aspect-ratio ZnO nanowires. Moreover, the snowflake-like structure sensor exhibits excellent acetone sensing, which are much suitable to detect acetone with a low concentration.
This work was supported by the National Science Foundation of China (Grant No. 51102036, 11474045, 11474036, 61370043), Fundamental Research Funds for the Central Universities DC201502080201, and The Opening Project of State Key Lab of Silicon Materials of Zhejiang University (No. SKL2014-8).
ZC carried out the experiment and drafted the manuscript. YW and ZL commented on the results and revised the manuscript. NY helped in the SEM and gas-sensing characterization. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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