Gas-Sensing Devices Based on Zn-Doped NiO Two-Dimensional Grainy Films with Fast Response and Recovery for Ammonia Molecule Detection
© Wang et al. 2015
Received: 21 June 2015
Accepted: 23 November 2015
Published: 1 December 2015
Zn-doped NiO two-dimensional grainy films on glass substrates are shown to be an ammonia-sensing material with excellent comprehensive performance, which could real-time detect and monitor ammonia (NH3) in the surrounding environment. The morphology and structure analysis indicated that the as-fabricated semiconductor films were composed of particles with diameters ranging from 80 to 160 nm, and each particle was composed of small crystalline grain with a narrow size about 20 nm, which was the face-centered cubic single crystal structure. X-ray diffraction peaks shifted toward lower angle, and the size of the lattice increased compared with undoped NiO, which demonstrated that zinc ions have been successfully doped into the NiO host structure. Simultaneously, we systematically investigated the gas-sensing properties of the Zn-doped NiO sensors for NH3 detection at room temperature. The sensor based on doped NiO sensing films gave four to nine times faster response and four to six times faster recovery speeds than those of sensor with undoped NiO films, which is important for the NiO sensor practical applications. Moreover, we found that the doped NiO sensors owned outstanding selectivity toward ammonia.
In recent years, air pollution have become one of the most serious problems that every country in the world is facing, which also have attracted considerable attention from a part of the scientific community to investigation of various atmospheric environmental issues [1–5]. Ammonia (NH3) is one of the toxic gases in the earth’s atmosphere and is considered as an artificial source for intensive livestock breeding with the decomposition process of manure and the chemical industry for the production of fertilizers and for refrigeration systems [6, 7]. Generally speaking, upon exposure to around 50 ppm (corresponding to about 40 μg/m3), NH3 is irritating to respiratory system, skin, and eyes and could cause acute poisoning or life-threatening situations [8–11]. Therefore, it became highly desirable to design and fabricate a portable sensing device with low cost and low power, which can real-time detect and monitor NH3 in air . Currently, some miniaturized chemical gas sensors based on nanoscale metal oxide semiconductor, which rely principally on monitoring the direct change in the conductance in adsorption and desorption of gas molecules, have been successfully obtained [13–15]. Gas-sensing performance strongly depends on the dimension, size, and morphology of the sensing materials. Thus, further research on sensing materials is still necessary for enhancing the sensing properties and continuing the expansion of the area of application of the sensor.
During the past several years, various semiconductor metal oxides such as WO3, CuO, TiO2, SnO2, ZnO, and In2O3 have been successfully prepared and their gas-sensing properties have been investigated from a part of the gas-sensing community [16–21]. Among these gas-sensing materials, naturally p-type NiO semiconductors with a wide band gap energy in the range of 3.6–4.0 eV have been recognized as an interesting and prosperous nanomaterials for the detection of gas molecules due to their simplicity of use, high compatibility with microelectronic processing, and environmental friendliness . Recently, one of the most significant breakthroughs in the domain of gas-sensing devices based on metal oxides was the preparation of the doped-nanomaterials [23–25]. Specifically, various catalysts (such as Pt, Ag, and NiO) have been loaded into metal oxide-sensing materials to improve gas-sensing performances. The doping contents, distribution, and size of catalysts are key parameters for enhancing gas molecules response/recovery speeds as well as selectivity . Therefore, doping metal ions in nanoscale NiO two-dimensional grainy films is considered as an effective and simple way to improve the gas-sensing properties (such as decreasing the response and recovering time) via forming micro area p-n junction (e.g., p-type NiO/n-type ZnO), which is very important for the application of gas sensors.
In our previous study, we developed a gas sensor based on pure NiO films for sensing NH3, which owned excellent stability and outstanding sensitivity performance . However, as being a product, the major issue is that the recovery speed is very poor at room temperature because the desorptions of gas molecules adsorbed on sensor need take a long time, and similar problem was proposed via other investigators [28–30]. To solve this problem, we would fabricate the nickel oxide two-dimensional grainy films and doping zinc ions forming in micro region, which enhanced the response and recovery speeds via improving electron transport and gas molecular diffusion in this special nanostructure. In the present work, we present a hydrothermal controllable route combined with high-temperature oxidation approach to prepare Zn-doped NiO films with well-controlled morphologies and systematically investigate Zn-doping effects on the gas-sensing performance of sensors at room temperature. The results showed that the response and recovery speed of NH3 can be remarkably improved via doping zinc ions in nanoscale NiO films as compared with pristine NiO films, which may provide a simple approach to fabricate excellent gas sensors based on metal oxides with desirable gas-sensing performance.
Preparation of the Gas-Sensing Materials
Assembling of the Gas-Sensing Devices
The crystallinity and crystalline nature of each synthesized crystal were identified via X-ray powder diffraction (XRD) using a 18-kW advanced X-ray diffractometer (D8 ADVANCE, Bruker, Germany) in a two theta range from 30 to 90° with a Cu Kα (λ = 0.154056 nm) rotating anode point source operating at 40 kV and 40 mA. The specific surface morphology and elemental composition of as-prepared samples were investigated using a field emission scanning electron microscopy (FESEM, Zeiss Ultra 55, Germany) combined with energy-dispersive spectroscopy (EDS) at accelerating voltage of 5 and 20 kV, respectively. In addition, high-resolution transmission electron microscopy (HRTEM) images were recorded in a JEM-2010 transmission electron microscope operating at 200 kV. The electrical signal of the gas-sensing devices was monitored using the Agilent 4156C. The gas-sensing properties of sensor were evaluated via the resistance change at a working voltage of 1 V.
Results and Discussion
Structure and Morphology of Gas-Sensing Materials
Zn-doped NiO films on glass substrate were fabricated via the hydrothermal reaction combined with subsequent high-temperature oxidation method, as shown in Fig. 1. In the first stage of synthesis of gas-sensing materials, a controlled hydrazine reduction approach has been developed to synthesize Ni(1 − x)Znx films on a substrate in a flask. In the second stage, Ni(1 − x)Znx films were transformed Ni(1 − x)ZnxO films via situ chemical oxidation route in open air. The specific chemical reactions for the synthesis can be expressed as follows:
In order to understand the internal microstructure of the doped film in more detail, further HRTEM images recorded from a particle in film are shown in Fig. 4c, d, which shows the crystal lattice structure. The images with no visible or planar defects show the high crystallinity of the Zn-doped NiO film. The inter-planar distance of the fringes in Fig. 4c was measured to be 0.245 (approx. 2.5) nm, which is slightly larger than that for the (111) plane of intrinsic cubilc NiO crystal (0.24120 nm). In the meantime, the interspacing in Fig. 4d was determined to be 0.212 (approx. 2.1) nm, which is also slightly larger than that for the distance of intrinsic NiO (200) plane (0.20890 nm). These results indicate lattice expansion, which implied that zinc atoms might form substitution atoms in NiO film to a great extent.
Gas-Sensing Performances of the Sensors
As well as we know, the stoichiometric NiO crystal is an insulator, however, typical NiO exhibits a reasonable electrical conductivity due to nickel vacancies or interstitial oxygen atoms in NiO crystal (p-type semiconductor) . As-synthesized Zn-doped NiO film exhibited oxygen-rich stoichiometry (see Fig. 5A2–C2), such as the atomic ratio O/Ni of 0 mol% Zn-NiO was about 51.16/48.84. Ammonia-sensing devices based on doped NiO films were fabricated via connecting the gas-sensing materials to the testing sockets using homemade silver paste combined with subsequent sputtering pure Au, as shown in Fig. 2. In our testing experiments, the conductance between the positive and negative electrodes was measured with a precision semiconductor parameter analyzer (Agilent 4156C) to investigate the response of gas-sensing devices. The sensor response is defined via the equation (ΔG/G0 = (G–G0)/G0), where G0 and G are the conductance of sensing films when exposed to the air and target gas with 25 % of relative humidity, respectively. The response time (τresponse) and recovery time (τrecovery) are defined as the time needed to reach 90 % of the final steady state conductance upon exposure to the testing gas and air, respectively .
In summary, semiconductor Zn-doped NiO gas-sensing films on glass substrates have been successfully fabricated via a chemical reaction combined with subsequent high-temperature oxidation method. SEM observations revealed that the doped NiO film composed of nanoscale particles with diameters ranging from 80 to 160 nm, which was larger than the crystalline grain size (about 20 nm) calculated from X-ray diffraction peaks; and Zn-doping concentrations had no influence on the growth of the gas-sensing films. In XRD and HRTEM characterization of doped NiO films, we found that all the XRD peaks shifted toward lower angle and the size of the lattice increased compared with undoped NiO samples, which is assigned to the successful incorporation of zinc ions in the NiO host structure. Significantly, the gas-sensing devices based on as-fabricated Zn-doped NiO films exhibited the short response/recovery time, excellent sensitivity, and selectivity toward ammonia over other organic gases, such as chloroform, dichloromethane, ethylacetate, formaldehyde, heptane, iso-propanol, and toluene. It is suggested that the approach demonstrated here could also be extended to other element-doped sensing films for corresponding gas-sensing devices, which realize gas detection at room temperature.
We thank for the financial support from Key Scientific Research Fund of Xihua University (No. z1320110), Xihua University Young Scholars Training Program (No. 01201418), Chunhui Project of Education Ministry of China (No. Z2011074), and the Open Research Subject of Key Laboratory (Research Base) of Special Materials Preparation and Control (Xihua University, No. szjj2014-059).
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