Enhanced ethanol gas-sensing properties of flower-like p-CuO/n-ZnO heterojunction nanorods

doi:10.1016/j.snb.2014.05.111

Sensors and Actuators B: Chemical

Volume 202, 31 October 2014, Pages 500-507

https://doi.org/10.1016/j.snb.2014.05.111 Get rights and content

Abstract

A facile approach for synthesis of flower-like p-CuO/n-ZnO heterojunction nanorods was reported. The CuO/ZnO nanorods were prepared by co-precipitation of CuO nanoparticles on the hydrothermally grown ZnO nanorods. The obtained samples were characterized by X-ray diffraction and transmission electron microscopy, which confirms that the heterogeneous nanostructure of the CuO/ZnO nanorods was highly crystalline. The ethanol gas-sensing properties of CuO/ZnO nanorods were evaluated with different ethanol vapor concentrations at the working temperature of 300 °C. The response of 0.25:1 CuO/ZnO nanorod sensor to 100 ppm ethanol was 98.8, which is 2.5 times that of ZnO only sample, with a response and recovery time of 7 s and 9 s, respectively. Good selectivity and long-term stability can also be achieved and the response of low concentration as 1 ppm ethanol can reach the value of 9.68 using the flower-like p-CuO/n-ZnO heterojunction nanorods as sensing material. The enhanced ethanol response is mainly attributed to a wider depletion layer on the CuO/ZnO surface resulted from the formation of p–n heterojunctions between p-CuO nanoparticles and n-type ZnO nanorods.

Introduction

Semiconductor gas sensors based on oxides such as ZnO, In₂O₃, SnO₂ and CuO, etc., are widely used in detecting various gases, showing good gas-sensing properties with short response and recovery time [1], [2], [3]. Among these semiconducting materials, wurzite ZnO, with a wide-band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature has been studied intensively. And gas-sensing results of different ZnO nanostructures, such as nanorods, nanowires and nanotubes, to target gases including ethanol, formaldehyde, ammonia, acetone, and nitric oxide, have been reported [4], [5], [6], [7], [8], [9], [10]. Ethanol is a common target gas for comparison of sensing performance with ZnO or other metal oxide sensors. A large number of ethanol sensors based various materials have been reported like ZnO nanorod [11], ZnO nanostructures with MEMS [12], CuO nanosheets [13], nano-coaxial p-Co₃O₄/n-TiO₂ [14].

As the oxide semiconductor gas-sensing mechanism is thought to be a surface controlled process, surface modifications using noble metals (Au, Pd, Pt, etc.) [15], [16], rare earth metals [17], or other types of metals [18] are one solution to improve the gas sensing properties of ZnO-based gas sensors. Combination of n-type ZnO with p-type semiconductor oxide might be another solution. CuO is a p-type semiconductor with an energy band gap of 1.2 eV. ZnO and CuO composite have been reported in photocatalysis [19], degradation [20], humidity sensing [21] and gas sensing [22], [23]. Combination of 1D ZnO and CuO microstructure may also exhibit enhanced gas-sensing property.

In this paper, flower-like p-CuO/n-ZnO heterojunction nanorods were synthesized using a two-step low-temperature template-free hydrothermal method with subsequent calcination. The microstructures with their growth mechanism were discussed, and the gas-sensing properties of the flower-like p-CuO/n-ZnO nanostructures were also discussed, revealing the mechanism for the enhanced gas-sensing.

Section snippets

Preparation of p-CuO/n-ZnO heterojunction hierarchical structures

All the chemical reagents used in the experiments, zinc nitrate (Zn(NO₃)₂·6H₂O), copper nitrate (Cu(NO₃)₂·3H₂O), sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB), ammonia solution (25–28%) and ethanol (C₂H₅OH), were all in analytical grade without further purification. Distilled water was used throughout the experiment.

Morphology of the samples

Fig. 2 shows the low and high magnification FE-SEM images of ZnO and CuO/ZnO samples with different weight ratio. The size of flower-like ZnO structures is estimated to be about 3 μm with well-defined petals of about 1 μm in length, 100–200 nm in width, and 60–80 nm in tip. The morphology of the CuO/ZnO samples is also flowerlike, similar with that of ZnO, but CuO nanoparticles agglomerate on the surface of the 05:1 CuO/ZnO sample with the diameter of 20–40 nm. It can be found clearly that the

Conclusions

In summary, flower-like p-CuO/n-ZnO heterojunction nanostructures composed of nanorods can be fabricated via a co-precipitation method at a low temperature using CTAB as surfactant. The as-prepared flower-like p-CuO/n-ZnO heterojunction nanorods showed ethanol sensing properties, and enhanced responses were observed. Notably, compared to ZnO nanorods, gas sensors based on p-CuO/n-ZnO heterojunction nanorods showed excellent ethanol sensing performance, including high response, fast

Acknowledgment

This research was supported by the National Natural Science Foundation of China under Grant No. 21271139.

Ya-Bin Zhang is a postgraduate working in the area of gas sensors for master degree at Tianjin University of Technology. He obtained his B.Sc. degree in physics from Shanxi Datong University in 2011.

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Jing Yin graduated from Liaocheng University and received her B.Sc. degree in 1989. Her research interests are in the growth of functional crystal materials and the preparation of nano-materials.

Ling Li is a postgraduate working in the area of gas sensors for master degree at Tianjin University of Technology. She obtained his B.Sc. degree in materials science and engineering from University of Jinan in 2012.

Le-Xi Zhang received his Ph.D. degree in materialogy in 2011 from Institute of Coal Chemistry, Chinese Academy of Sciences. His current research is focused on synthesis of semiconductor nanostructures and their gas-sensing applications.

Li-Jian Bie obtained his master's degree in inorganic chemistry from University of Science and Technology of China in 1991, and Ph.D. degree in inorganic chemistry from Peking University in 2002. He is now a professor in Tianjin University of Technology, leading a group in research for the synthesis and property of nano-materials and perovskite-related materials, including their application in sensors.

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Enhanced ethanol gas-sensing properties of flower-like p-CuO/n-ZnO heterojunction nanorods

Abstract

Introduction

Section snippets

Preparation of p-CuO/n-ZnO heterojunction hierarchical structures

Morphology of the samples

Conclusions

Acknowledgment

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Chem. Phys. Lett.

Sol. Energy Mater. Sol. Cells

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

J. Electroanal. Chem.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Mater. Lett.

J. Alloys Compd.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Mater. Sci. Eng.: B

Chem. Eng. J.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

J. Alloys Compd.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Sens. Actuator. B: Chem.

Surface-depletion controlled gas sensing of ZnO nanorods grown at room temperature

Appl. Phys. Lett.