Enhanced room temperature gas sensing properties of low temperature solution processed ZnO/CuO heterojunction
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The development of room temperature gas sensors having response towards a specific gas is attracting researchers nowadays in the field. In the present work, room temperature (29 °C) ethanol sensor based on vertically aligned ZnO nanorods decorated with CuO nanoparticles was successfully fabricated by simple cost effective solution processing. The heterojunction sensor exhibits better sensor parameters compared to pristine ZnO. The response of the heterojunction sensor to 50 ppm ethanol is, at least, 2-fold higher than the response of the ZnO bare sensor. Also the response and recovery time of ZnO/CuO sensor to 50 ppm ethanol are of 9 and 420 s whereas the values are 16 and 510 s respectively for ZnO sensor. The vertical alignment of ZnO nanorods as well as its surface modification by CuO nanoparticles increased the effective surface area of the device and the formation of p-CuO/n-ZnO junction at the interface are the reasons for the improved performance at room temperature. In addition to ethanol, the fabricated device has the capability to detect the presence of reducing gases like hydrogen sulfide and ammonia at room temperature.
KeywordsZnO/CuO hierarchical structure Hydrothermal Room temperature gas detection p–n Heterojunction
The effective detection and removal of toxic gases in the atmosphere is important for human as well as any living organisms. The uncontrolled release of toxic gases such as CO, H2S, NH3, CH3CH2OH, etc. from automobiles, industries, laboratories, etc. cause severe health problems and they may even cause death [1, 2, 3]. The use of nanostructured materials for fabricating gas sensors with high sensitivity and selectivity is attracting attention of researchers nowadays because these materials can be easily synthesized and integrated into low cost portable gas detection devices [4, 5]. Among the various nanostructured materials, metal oxide nanostructures belong to the widely accepted category for fabricating gas sensors especially because of their chemical and thermal stability, tunable electrical and optical properties, etc. [6, 7].
Numerous metal oxide nanomaterials such as ZnO, TiO2, SnO2, WO3 etc. [8, 9, 10, 11] are commonly used in the field of gas sensing. Nanomaterials are already established in the field of gas sensing especially because of their high sensitivity originated due to their large surface to volume ratio . One dimensional ZnO nanorods are attractive candidates for gas sensor applications because of their increased surface to volume ratio compared to other morphologies of ZnO and most importantly they provide an easy path way for electron transfer. There are several techniques such as doping, forming hierarchical structures, etc. which can be employed to improve the gas sensing properties especially to lower the operating temperature of metal oxide nanostructure based gas sensors. Among the various methods available, forming hierarchical structures using metals (Au, Ag, Pt, Pd, etc.) or metal oxides (CuO, Cu2O, TiO2, SnO2, etc.) [12, 13, 14] is an effective way to enhance the various properties of metal oxide gas sensors. Researchers have already found the enhanced gas sensing characteristics of metal oxide/metal oxide hierarchical structures [15, 16, 17]. The hierarchical structure can form either p–n, n–n or p–p type semiconductor junctions depending on the nature of the material under consideration. In the present study we have investigated the enhanced gas sensing characteristics of n-ZnO/p-CuO hierarchical structures. Vertically aligned ZnO nanorods were grown by seed mediated hydrothermal method and CuO nanoparticles were loaded on the surface of ZnO nanorods via simple wet chemical method. ZnO is a well known n-type semiconductor having a direct band gap of 3.37 eV . Various nanostructures of ZnO are used in several application such as photovoltaic , gas sensors , spintronics , etc. CuO is a p-type semiconducting material with a band gap of 1.35 eV which is widely being used in the fields of solar energy conversion , gas sensors , batteries , magnetic storage media , transparent electronics etc. p-CuO and n-ZnO can be combined in different ways to utilize the advantages of p-n heterojunction in gas sensor applications. The improvement in sensing performance of these composites have been attributed to many factors, including electronic effects  such as: band bending due to Fermi level equilibration, charge carrier separation, depletion layer manipulation and increased interfacial potential barrier energy. The chemical effects  such as decrease in activation energy, targeted catalytic activity and synergistic surface reactions; and geometrical effects  such as grain refinement, surface area enhancement, and increased gas accessibility also leads to the improvement in sensing. In addition to achieving better sensor characteristics, minimization of operating temperature and power consumption are the current trends in gas sensor technology. Most of the gas sensors based on metal oxides operate at temperatures above 150 °C which increase the power consumption of the gas sensor. Also the high temperature operation inhibits the use of sensors in explosive environments. In this context the development of room temperature gas sensors with enhanced gas sensing performance have significant importance in the gas sensor industry.
Here, we have grown vertically aligned ZnO nanorods on ITO/glass substrates by a seed mediated hydrothermal method. The growth of ZnO nanorods oriented along c-axial direction by seed mediated hydrothermal method have already reported in literature . ZnO/CuO hierarchical structures were synthesized by depositing CuO nanoparticles on ZnO by a wet chemical method followed by annealing at 250 °C in air. The n-ZnO/p-CuO heterojunction device was used to detect ethanol, hydrogen sulfide and ammonia at room temperature (29 °C).
All the reagents used were analytically pure and used without further purification. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), sodium hydroxide (NaOH) and copper acetate hydrate (Cu(CO2CH3)2H2O) were purchased from fisher scientific. Ammonia solution, isopropyl alcohol and ethanol were purchased from Merck Millipore. De ionized water was obtained from an ultra filter system. ITO/glass substrates were purchased from Sigma Aldrich (surface resistivity 15–25 Ω/sq). The substrates were cleaned by standard cleaning procedure.
Synthesis and characterization
A thin layer of ZnO seed layer was deposited by immersing the cleaned ITO/glass substrate in a solution containing zinc acetate (0.025 M) and sodium hydroxide (0.05 M) in 100 ml ethanol. The substrate was immersed in the solution for 5 min and the dipping process repeated for 8 times to obtain a uniform ZnO layer over a considerable area of the substrate. In between each dipping process the sample was kept at 80 °C on a hot plate. The annealing of the substrates at the optimized temperature 250 °C in air results in the formation of ZnO nanoparticles. The ITO/glass substrate with ZnO nanoparticle seed layer will act as a lattice matched substrate for the hydrothermal growth of aligned ZnO nanorods. The precursor solution for hydrothermal experiment was prepared by dissolving zinc acetate (0.1 M) and ammonia (25%) in 100 ml de-ionized water. The solution was transferred into a Teflon lined autoclave with the seed layer coated substrate immersed horizontally facing up and kept at 180 °C for 1 h in a laboratory oven. After hydrothermal experiment the samples were taken out and sonicated in iso propyl alcohol for few seconds to remove the unaligned nanorods lying over the vertically aligned nanorods. CuO nanoparticles were deposited by a wet chemical method. 0.05 M copper acetate solution was prepared in ethanol at room temperature and ZnO sample was immersed in the solution for 1 h. After deposition the sample was annealed at 250 °C for 2 h in air to form ZnO/CuO heterostructure.
The crystal phase and crystallinity of ZnO/CuO hierarchical structure was investigated by glancing angle X-ray diffraction taken using PANalytical X’pert PRO high resolution X-ray diffractometer (HRXRD) with CuKα (λ = 1.5418 Å). The detailed microstructure of the samples was analyzed using JEM2100 transmission electron microscopy (TEM) measurements. Raman spectra were recorded using Horiba Jobin–Yvon LABRAM HR Raman spectrometer excited with the 514 nm line of an Ar+ laser. The surface morphology of the samples was analyzed using Carl Zeiss field emission scanning electron microscopy (FESEM). The absorption spectra of the samples were recorded using JASCO V-570 spectrophotometer. Room temperature photoluminescence (PL) of the samples were measured using Horiba Jobin–Yvon Fluoromax-3 spectrofluorimeter using Xe lamp as the excitation source. The p–n junction characteristics of the device were studied using Keithley 4200 Semiconductor analyzer.
Results and discussion
Evaluation of the development of gas sensors based on ZnO/CuO structures
Method of synthesis
Sensor working temperature (°C)
Target gas concentration (ppm)
Response time (s)
Recovery time (s)
Solid state reaction
Pulsed laser deposition
Hydrogen sulphide 15
CuO hierarchical structure exhibit good response to various reducing gases and the fabricated devices are more selective to ethanol at room temperature (29 °C). The basic gas sensing mechanism of metal oxide semiconductors relies on the interaction between the adsorbed oxygen molecules on the surface of the sensor material and the target gas [5, 7, 48, 49, 50, 51]. Generally O 2 − at temperature < 100 °C and O− and O2− at temperature > 100 °C are the dominant oxygen species adsorbed on the semiconductor. The adsorption of oxygen ions on the surface of oxide semiconductor forms an electron depletion region by withdrawing electrons from the conduction band. The interaction between the adsorbed oxygen ions and ethanol gas release electrons back to the semiconductor consequently the depletion layer width and resistance of the semiconductor decreases.
These reactions release free electrons resulting in the enhanced room temperature gas sensing performance of p-CuO/n-ZnO heterojunction device.
ZnO/CuO heterojunction gas sensor has been successfully fabricated by low temperature solution processing and its room temperature (29 °C) response to various reducing gases has been investigated. Working at room temperature, the response to ethanol gas of the fabricated device is higher than to hydrogen sulfide or ammonia gases. All the gas sensor parameters have been improved by the incorporation of CuO nanoparticles on ZnO nanorods. The easy preparation technique and room temperature gas sensing of the samples will make the practical use of these devices with reduced power consumption a reality.
PPS has made significant contribution in the preparation and characterizations of samples, collected data, analyzed and wrote the manuscript. MKJ has revised the manuscript for intellectual content and corrected accordingly. Both authors read and approved the final manuscript.
The work was supported by Nanomission council (DST NO. SR/NM/NS-22/2008), Department of Science and Technology, India. Author PPS thanks the University Grant Commission (UGC) for research fellowship.
The authors declare that they have no competing interests.
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