Synthesis and gas sensing properties of membrane template-grown hollow ZnO nanowires
© The Author(s) 2017
Received: 19 August 2017
Accepted: 4 October 2017
Published: 25 October 2017
One-dimensional, hollow nanostructured materials are among the most promising materials for sensing applications owing to their high surface area that facilitates the adsorption of target gases. Accordingly, for gas sensing studies, hollow ZnO nanowires (NWs) with different surface areas were successfully synthesized herein by using polycarbonate membranes with different pore sizes as templates, and deposition of ZnO via the atomic layer deposition technique. The sensing properties of the synthesized hollow ZnO NWs were examined for CO and NO2, revealing their comparative sensing performances with ZnO nanomaterials-based sensors reported in literature. This study highlights a novel way of synthesizing hollow ZnO NWs by using membrane template and their promising sensing properties as well.
Because of increasing concerns about air pollution, public security, and the high standards of modern life, gas detection has gained increasing importance [1, 2]. Metal-oxide gas sensors are most commonly used for the detection of ambient gases based on their high response, fast and dynamic characteristics, easy fabrication, portability, and cheapness . However, the performance of these sensors must be enhanced to meet the demands of the high standards of living. One promising approach for enhancing the gas-detection capability of metal-oxide-based gas sensors is to increase the surface area of the sensor . In fact, a higher surface area generally results in greater availability of sites for gas adsorption, and accordingly, higher performance. Many researchers have investigated high-surface-area metal-oxides such as nanofibers (NFs) , nanorods , nanowires (NWs) , and hierarchical  and porous materials  for gas sensing applications. In order to further increase the surface areas of such nanomaterials, hollow nanostructured nanomaterials can be employed. Such morphologies offer more adsorption sites as they possess inner and outer surfaces, meaning that the surface-to-volume ratio almost doubles compared with that of the normal solid counterparts; therefore, higher sensing performance is expected .
In our previous work , we fabricated hollow ZnO NFs with different diameters via the electrospinning method. It was found that ZnO NFs with smaller diameters were more sensitive to both reducing and oxidizing gases than those with larger diameters. In another study , we found that the sensing performance of ZnO hollow NFs depended on their wall thickness, where the ZnO hollow NFs with thinner walls showed better sensing performance. More recently , we reported TiO2/ZnO inner/outer double-layer hollow NFs that exhibited sensitive and selective detection of reducing gases. Zhang et al.  compared the CO gas sensing properties of hollow and normal TiO2 NFs; the hollow TiO2 NFs showed better sensing performance because of the effect of the increased surface-to-volume ratio derived from generation of the inner surfaces. Park et al.  reported that a hollow ZnO NFs sensor showed much higher sensitivity to NO2, when compared to normal ZnO NFs, owing to the increased surface area of the former.
In most of these literature studies on hollow nanostructures, the focus was placed on hollow NFs and less attention has been paid to other hollow nanostructures such as hollow NWs. NW gas sensors exhibit many inspiring characteristics such as (i) ultra-sensitivity and fast response time, (ii) higher selectivity and stability, (iii) light weight, (iv) low power consumption, and (v) wireless communication applicability . Therefore, it is of importance to increase the performance of NW gas sensors by increasing the surface area through the fabrication of hollow NWs. Accordingly, in this work, we report the novel synthesis, characterization, and sensing performance of hollow ZnO NWs prepared using cyclopore polycarbonate membranes (with different pore sizes) as templates with subsequent deposition of ZnO via atomic layer deposition (ALD). The membrane templates were removed by combustion at 450 °C over 4 h. Scanning electron microscope (SEM) images demonstrate formation of the hollow ZnO NWs. Gas sensing tests towards CO and NO2 gases reveal the higher performance of the gas sensors with higher surface area. The sensing mechanism is also discussed in detail.
2.1 Synthesis of hollow ZnO NWs
The morphology of the synthesized hollow ZnO NWs was studied by field emission scanning electron microscopy (FE-SEM, S-4300SE, Hitachi). The phase and crystallinity were examined by X-ray diffraction (XRD, X’pert MPD PRO, Philips), and the specific surface areas were measured by Brunauer–Emmett–Teller (BET) analysis.
2.3 Gas sensing test
The process for fabrication of the sensors is described in detail in our previous publications [17, 18]. We applied the interdigitated electrode on the surface of the sensing layer deposited on the substrate. In other words, the interdigitated electrode was made on top of the sensing layer by sputtering with a metal shadow mask. For the interdigitated electrode, Ti (~ 50 nm in thickness) and Pt (~ 150 nm) double layers were sequentially deposited on the sensing layer via sputtering using an interdigital electrode shadow mask.
3 Results and discussion
3.1 Structural and morphological study
The morphologies of the hollow ZnO NWs were observed by FE-SEM. Figure 2a, b show typical FE-SEM images of the hollow ZnO NWs with different surface areas (i.e., 9.33 and 10.17 m2 g−1) prepared from the membranes with pore sizes of 1 and 0.4 µm, respectively. As shown in Fig. 2a, the hollow ZnO NWs synthesized using the membrane with a pore diameter of 1 µm had a relatively smooth surface morphology. The inset in this figure clearly shows the hollow nature of the synthesized ZnO NWs. However, the surfaces of the hollow ZnO NWs prepared using the membrane with a pore diameter of 0.4 µm had bead-like humps, which obviously increased the surface area of this sample. The inset in this figure again shows the hollow nature of the ZnO NWs.
3.2 Gas sensing study
These reactions will result in a decrease in the electron concentration and an increase in the width of the depletion layer, and an increase in the resistance (see Fig. 5b). Accordingly, the width of the conduction channel decreases to D2, which is smaller than D1 (in air), and a high response can be observed.
The released electrons return to the surface of the ZnO sensor, increasing the width of the depletion layers on the inner and outer surfaces of ZnO; the width of the conduction channel will increase to D3, which is larger than D1. Accordingly, the resistance will decrease (see Fig. 5c). For ZnO NWs-based gas sensors, the modulation of depletion layers in the presence of target gas, has been reported in many papers. For instance, Choi et al.  reported modulation of depletion layers in the networked ZnO NWs in the presence of CO gas. Drobek et al.  reported modulation of depletion layers in pristine ZnO and ZnO@ZIF-8 composite NWs in the presence of some reducing gases. Additionally, for other metal oxide NWs such as SnO2 NWs  and In2O3 NWs , the same sensing mechanism has been proposed.
Resistance modulation may also arise from homojunctions formed as a result of intersections between the hollow ZnO NFs. As shown in Fig. 5d–f, when the sensor is exposed to NO2 gas, the initial potential barrier in V1 will increase to V2, and upon exposure to CO gas, it decreases to V3, which is lower than V1. These resistance modulations eventually contribute to observation of a response in the sensors.
The higher response to NO2 relative to CO may be related to the high electron affinity of NO2 (2.28 eV) in comparison with that of adsorbed oxygen (0.43 eV) . NO2 is a strongly oxidizing gas that can extract electrons from the exposed surfaces of the hollow ZnO NFs and significantly decreases the width of the electron depletion layers.
Comparison of the NO2 gas sensing properties of the present sensor (with specific surface area of 10.17 m2 g−1) with those of other ZnO-based gas sensors reported in the literature
NO2 conc. (ppm)
Response (R a /R g )
Hollow ZnO NWs
Hollow ZnO NWs
Branched ZnO NWs
ZnO-reduced graphene oxide
Flower-like ZnO (4.9 m2 g−1)
In summary, a novel approach was applied to the fabrication of hollow ZnO NWs with different specific surface areas. Cyclopore polycarbonate membranes with different pore sizes were used as templates and ZnO was deposited on these templates via the ALD technique. Because of the simplicity of this method, it can be easily applied to other oxide semiconductors. The prepared hollow ZnO NWs had respective surface areas of 9.33 and 10.17 m2 g−1. Gas sensors were fabricated from the hollow ZnO NWs, and the gas sensing properties were investigated in the presence of CO and NO2 gases. The sensor with a surface area of 10.17 m2 g−1 showed excellent sensing of NO2 at 400 °C relative to the lower surface area gas sensor; the responses to 0.1, 1, and 10 ppm NO2 were 15.5, 28.06, and 37.1, respectively.
J-HL, J-YK and J-HK carried out the synthesis of how NWs, device fabrication and measurement. AM, HWK and SSK wrote the manuscript. SSK supervised the research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03935228).
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