Ultrafast and Reversible Gas-Sensing Properties of ZnO Nanowire

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Ultra-fast and Reversible Gas Sensing Properties Of ZnO Nanowire Arrays Grown By Hydrothermal Technique Madhumita Sinha, Rajat Mahapatra, Biswanath Mondal, Takahiro Maruyama, and Ranajit Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11012 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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The Journal of Physical Chemistry

Ultra-fast and Reversible Gas Sensing Properties Of ZnO Nanowire Arrays Grown By Hydrothermal Technique Madhumita Sinha,†,‡ Rajat Mahapatra,‡ Biswanath Mondal,† Takahiro Maruyama,§ and Ranajit Ghosh,*† †

CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India

‡

Department of Electronics and Communications Engineering, National Institute of Technology,

Durgapur-713209, India §

Department of Applied Chemistry, Meijo University, Nagoya- 4688502, Japan

ABSTRACT: High-quality single crystal ZnO nanowire arrays with diameter ranging from 30 to 110 nm were synthesized using a two-step process: (first step) synthesis of ZnO thin film by solgel technique which was used as seed layer; and (second step) oriented ZnO nanowire arrays were grown on ZnO seed layer using a hydrothermal reaction process at a low temperature of 90 °C. Experimental results reveal an ultra-high sensitivity about 98% to 100 ppm H2 gas and 93% to 200 ppm CO gas, an ultra-fast recovery of 1–2 ms to CO gas with high repeatability. The current transients demonstrate the reversible type sensing to reducing gas (H2 or CO) detection

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using as-grown ZnO nanowire arrays sensor. The hydrothermally grown ZnO nanowire arrays based gas sensors may have potential application in industry without much modification. INTRODUCTION Recently one-dimensional (1-D) metal oxide nanostructures, such as, nanowires, nanorods, nanotubes, and nanofibers have attracted much attention for their potential application in gas sensors.1–4 1-D nanostructures particularly nanowires are most suited to this application because of their high surface-to-volume ratio, high crystallinity and high electron mobility along with the growth direction. Furthermore, the good chemical and thermal stability under different operating conditions of nanowires could ensure the high-performance sensing behavior. However, semiconducting metal oxides such as SnO2, ZnO, TiO2 and WO3 are very attractive materials for the detection of explosive, hazardous, and toxic gases due to changes in conductance of these materials when oxidizing or reducing species in air chemisorbs onto their surface.5–8 ZnO nanowires are most promising materials due to its sensitivity to many gases (NO2, H2, CO, hydrocarbons etc.), higher response (sensitivity, S= >100), lower detection limit to 10 ppb, fast response (3 s) and recovery (9 s) time.9–14 Sensitivity, response time, recovery time, and detection range are the main performance parameters for gas sensors. For example, the sensitivity (S= ∆R/Ra, where ∆R is the change of resistance and Ra is the resistance in air) of ZnO fibre-mats was reported to be more than 100 towards NO2 at room temperature.9 The relative sensitivity (S=∆R/Ra × 100%) of 57 % of ZnO nanowire was reported towards CO gas at 500 ppm level and at an operating temperature of 320 ºC.10 An array type of sensor containing vertically aligned ZnO nanorods detected NO2 gas with a low detection limit of 10 ppb level at 250 °C.11 The ZnO nanorods of diameter 20–40 nm and length 100 nm showed the very fast response of 3 s and recovery of 9 s towards methanol sensing.12 Tien et al. used single Pt-coated


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ZnO nanowires and achieved a relative response of ~20% to 500 ppm H2.15 Wang et al. employed ZnO nanorods coated with Pd to detect H2, with a relative response of 2.6% at 250ºCfor 10 ppm and a recovery time of less than 10 s.16 Rout et al. employed ZnO nanorods (diameters 30-40 nm), nanowires (diameters 20-30 nm), and nanotubes (diameters 60-100 nm) for sensing H2 gas and obtained a sensitivity (Rair/Rgas ratio) of as-prepared nanowires of about 43 for towards 1000 ppm of at 150 ºC, with the response and recovery times of 54 s and 5 s, respectively.17 A good H2 sensing characteristic (S ≈ 90%) was observed at room temperature with a response time of ∼55 s of a single ZnO nanowire schotttky diode based sensor.18 Generally, the practical usage of a gas sensor is highly dependent on its recovery time; a gas sensor that has a short recovery time (ms) will have greater applications in the commercial market. Moreover, domestic and industrial application of gas sensors requires a reversible-type sensor i.e switched forth and back of conduction between the test gas (during response process) and air (during recovery process).19 So far, there are no reports on reversible type ZnO nanowire sensor towards CO gas with ultra-fast recovery time in millisecond (ms) order. There are many chemical routes reported for synthesis of 1-D ZnO nanostructures. Generally, these include hydrothermal, ultrasonic irradiation in aqueous solution, sol-gel, and solid-state chemical reaction.11–14,20,21A solution approach to synthesize ZnO nanostructures is appealing because of the low growth temperature, simplicity of the process to synthesize controllably novel self-organized architectures and good potential for scale-up. Hydrothermal method is such a lowtemperature, large-scale and versatile synthetic process, which demonstrates the ease of commercial scale-up.22 In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods at 95 °C on conducting glass substrates.23,24 In this paper, we have reported the growth of high-quality transparent ZnO Nanowire arrays using


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hydrothermal technique. These ZnO nanowire arrays have shown the high-degree of gas sensitivity and ultra-fast response to CO gas. The models have been proposed to understand the mechanism of the gas sensitivity. To the best of our knowledge and belief, there are no report of high sensitivity of 98% to H2 gas and 93% to CO gas, ultra-fast recovery time of 1–2 ms towards CO gas detection using reversible type ZnO nanowire arrays sensor. EXPERIMENTAL Materials Zinc acetate dihydrate [Zn(CH3COO)2.2H2O] (≥98% extra pure), hexamethylenetetramine [(CH2)6N4] (≥99%), isopropyl alcohol [CH3CHOHCH3], diethanolamine [HN(CH₂CH₂OH)₂], hydrochloric acid [HCl] (35%) were procured from MERCK India Pvt. Ltd. and used as received for the investigation. Ultrapure water (Milli Q, Resistivity >18.2 MΩ·cm) was used for materials synthesis. Synthesis of ZnO nanowire arrays The ZnO Nanowire arrays were grown by hydrothermal technique in two steps. A schematic of the structure is shown in figure 1b. In order to obtain a uniform film, initially ZnO seed layer like thin film was deposited on glass substrates by sol-gel technique. The detailed methodology of thin film deposition was described elsewhere.25 In brief; at first the cornic glass substrates were cleaned and degreased ultrasonically by HCl and soap water followed by hot methanol and acetone successively for 10 minutes each. 0.1 M sol-gel solution of zinc acetatedihydrate [Zn(CH3COO)2.2H2O] was prepared after thoroughly mixing in isopropyl alcohol on a magnetic stirrer until it becomes transparent by using diethanolamine. Drain coating method with a


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drawing rate of 6 cm/min was used to coat all the substrates, then subjected to heat treatments at 120 ºC in hot oven for 20 min followed by 350 ºC in air for 40 min. The process of coating has been repeated for 4 times. In the next step, nanowires (figure 1b) were grown by dipping the thin film in a mixture of equimolar (10 mM) aqueous solution of zinc acetate dehydrate [Zn(CH3COO)2.2H2O] and hexamethylenetetramine [(CH2)6N4] at 90 °C for 1 h. The substrates were then removed from the solution, rinsed in ultrapure water, and dried in purging nitrogen gas. Characterizations A field-emission scanning electron microscope (FESEM) (JSM-6700F, JEOL, Japan) were used to investigate their surface morphology of ZnO thin films and nanowires. Optical transmittance measurements of all samples were carried out using UV-VIS-NIR spectrophotometer (UV 3100, Shimadzu, Kyoto, Japan).The crystallinity and crystal structures of nanowires were observed by a high-resolution transmission electron microscope (HRTEM)((JSM-2010, JEOL, Tokyo, Japan); attached with an energy dispersive x-ray analysis (EDAX) facility, (INCA, Oxford, Oxfordshire, United Kingdom)) with an accelerating voltage of 300 kV. TEM samples were prepared through the dispersion of ZnO nanowires in methanol. Dispersed nanowires were placed onto a Cu TEM grids (Alliance Biosystems, Japan) coated with holey amorphous carbon films. Photoluminescence (PL) measurement of hydrothermally grown ZnO nanowire arrays were carried out at room temperature using a He-Cd laser (Kimmon Koha Co. Ltd., Tokyo, Japan) with 325 nm excitation source and recording the luminescence using a spectrometer (Horiba Jobin Yvon, France) together with a photomultiplier tube.


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Measurement of gas sensing properties The gas sensing measurements were carried out in a stainless steel made chamber equipped with a PID temperature controlled (DCS-PWR-2K5-10AC-024, Dynamic Control System, Mumbai, India) base under flow of gases. The gas dilution system was used for H2 and CO test gases. The flow rates of the gases were controlled using mass flow controllers (AlicatMC-100SCCM-DDS/GAS, Pascal Technologies, Inc., USA). For electrical measurements, several thin gold electrodes of thickness of 50 nm in a circular form of diameter 1 mm with spacing of 3 mm were deposited by a high-vacuum thermal evaporation system through a shadow mask in a lateral geometry as Ohmic contacts on the ZnO nanowire arrays film. Two copper probes (fixed in the gas chamber), one in top and other in bottom, were used to make the electrical contacts for sensing measurements. The current of the sensing elements was recorded at 5 V bias using a source-measure electrometer unit (6517A, Keithley Instruments, USA). The electrometer was interfaced with a PC using KUSB-488B (Keithley Instruments, USA) and LabTracer2.9 (National Instruments, USA) integration software. The transient current was measured with a minimum time interval of 1 ms throughout the experiment. For n-type semiconductor, the relative sensitivity (Sg) of the gas sensor is defined as the ratio of the relative change in the conductivity (∆G) upon exposure to the gas being sensed with respect to its conductivity in an air ambient (Ga)26–28: Sg (%) = ∆G /Ga × 100 = (Gg − Ga)/Ga× 100, where Gg is the conductivity in the presence of the gas being tested, and Ga is the conductivity in air ambient. In term of the electric current measured in the semiconductor-based gas sensor on the application of a constant bias voltage 29,30: Sg(%) = (Ig − Ia) / Ia× 100, where Ig is the current measured in the presence of the gas being sensed and Ia is the current measured in air (i.e., in the absence of the gas being tested).


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RESULTS AND DISCUSSION The top and cross-sectional view of the FE-SEM images of ZnO nanowire arrays grown hydrothermally on ZnO thin film is shown in figure 1a and its inset, respectively. The dense and uniformly grown ZnO nanowire arrays are mostly aligned with an inclination toward the substrate leading to a quasi-aligned growth orientation. The width of the hexagonal ZnO nanowires is in the range of 40-110 nm with a length of about 1.2 µm. In addition, PL measurement was carried out to check the surface defects of as grown ZnO nanowires. Figure S1 (see supporting information) shows the PL spectrum of ZnO nanowires at room temperature. The nanowires exhibit the strong excitonic peak at around 381 nm in the UV region indicating a good optical quality of the nanowires. However, the spectrum in visible region indicates the presence of fewer amounts of defects. Generally, ZnO exhibit two peaks in the PL spectra at around 378 (UV region) and other broad peak centered at 530 nm in visible region.30 The broad peak in visible region originates from oxygen related intrinsic defects.30,31 Figure 1c represents the FE-SEM images of ZnO thin film. ZnO grains are regularly arranged as a uniform surface on glass substrates. The average grain size of ZnO thin films is 60 nm. These ZnO grains were used as seed layer for assisting the growth of ZnO nanowires vertically to the substrate surface. A seeded growth technique was reported to make helical ZnO rods and columns,32 Nanowires.33 It is also verified that ZnO nanowires are randomly distributed on cleaned glass substrate without ZnO thin films (see supporting information, figure S2). Note that vertically aligned nanowires have several important applications and are preferred for integration of devices over a large scale.34 Figure 1d and 1e reveals the corresponding optical image of ZnO nanowire arrays on ZnO thin films and only ZnO thin films. The images distinguish the variation of transparency between


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ZnO nanowires and thin films. “Z’ letter is brightly visible by ZnO thin film as it is highly transparent to visible light; obviously “O” letter is little bit blur due to low transparency of ZnO nanowires. The transparency of the ZnO nanowires and ZnO thin film was confirmed by the measurement of transmittance using UV-VIS-NIR spectrophotometer. The nanowire arrays (figure 1f) show lower transmittance of about 78% compared to ZnO thin film (figure 1g) of about 94%. Therefore, ZnO nanowires grown on ZnO thin films should be useful independently as transparent medium for solar cell applications as well as gas sensing applications. The structures and crystallinity of ZnO nanowires was confirmed by TEM measurement. Figure 2 shows the TEM image of an oriented nanowire assemble which also confirms the diameter of the nanowires in the range of 30-60 nm and length about 1.2 µm. HRTEM image and the corresponding selected-area electron diffraction (SAED) pattern shown in inset in figure 1b indicate that each ZnO nanowire is hexagonal single crystal growing along (002) orientation. The interplanar spacing, as calculated from the fringe pattern of the HRTEM image is 0.26 nm, which corresponds to the distance between two (002) planes, confirming that growth occurs in the caxis [001] direction.35 Gas sensing performance Gas sensors are very much useful in industrial applications due to their high sensitivity, low cost and simplicity. In accordance to the apparent nature of the current/resistance transients (partially or fully recovered), the gas sensing processes can be categorized into irreversible and reversible types, respectively. The gas sensor is termed as irreversible-type, when the base current/resistance recovers partially. While, for reversible gas sensor the base current/resistance is fully recovered.19 The transient characteristics of reversible-type sensor are switched forth and


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back to the base in presence (during the response process) and in absence (during the recovery process) of gas, respectively. The reversible-type gas sensors are most commercially viable due to their characteristics of quick response, and complete recovery (fully recoverable baseline). It is thus very interesting to confirm the reversible-type sensor. Figure 3a illustrates the typical current transients of ZnO nanowire arrays sensing element during the 100 ppm detection of H2 gas at an operating temperature of 250 °C, and at a constant biased voltage of 5 V. In due course of time, the current is switched forth to ~2.76 × 10-6A in presence of 100 ppm H2 gas and switched back to base current of 5 × 10-9A when H2 gas was switched off, i.e. only in presence of air. The response amplitude [Sg(%) = (Ig – Ia)/Ia × 100] estimated from typical response plot (figure 3a) is very high of 98%. As other important parameters of gas sensors, the response and recovery times was derived from the typical response plots for ZnO nanowire arrays gas sensor. The response time is defined as the time it takes for conductance/resistance of the gas sensor to increase to 90% of the maximum conductance/resistance when a specific amount (e.g. 100 ppm) H2 gas was introduced into the sensor test chamber. The recovery time is the time required for a 90% reduction in conductance/resistance when the H2 gas was turned off and air was reintroduced into the chamber (i.e the time to reach 10% conductance/resistance in H2). Figure 3b shows the representative response curve for the first cycle of figure 3a. The response time is 60 s to reach 90% of its current in presence of 100 ppm H2 gas and the recovery time is 14s due to 90% reduction of current when H2 gas was turned off of ZnO nanowire arrays sensor. Recovery time is very fast in comparison to the response time. It is also to be interesting that the fast response (60 s) and recovery time (14 s) were constant for successive three cycles for detection to 100 ppm H2 gas, which implies that hydrothermally grown ZnO nanowire arrays based H2 gas sensors are very much stable. Table 1 presents a comparison between the gas sensing


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performances of the ZnO nanowire arrays in the present work and reported ZnO based gas sensors. It is seen from Table 1 that the as-grown ZnO nanowires sensor is very much sensitive (~98%) to H2 gas as compared to previous reported results. The performance of 1-D nanostructured ZnO sensors depends greatly on the processing techniques, crystallinty, surface morphology, sensor fabrication arrangements and operating temperature. It has also been seen that single ZnO nanorod and single ZnO nanowire sensor assemblies can detect H2 gas at room temperature in presence of dry air.36,37 However, at room temperature, the sensitivity [Sg(%) = │(Ra – Rg)│/Rg × 100] of ZnO nanowires is only 4% for 200 ppm H2 gas.37 The addition of catalysts was found to increase the sensitivity of ZnO nanorods. Wang et al.16 reported H2 sensitivity of ZnO nanorods with Pd coated and found the response increased by approximately a factor of 5 relative to an uncoated nanostructure. Although many reports in the literature state that ZnO nanostructures have poor sensitivity towards H2 gas,20,38 it is important to note that the present hydrothermally grown vertically aligned ZnO nanowire arrays on ZnO thin film have shown the very high sensitivity of 98% at high temperature of 250 °C. However, Sadek et al.39 found that ZnO nanobelts showed a considerable sensitivity (Sg = Ra/Rg) of 14.3 for 1% H2 concentration at the optimum working temperature of 385 °C. It may be the case that the low response of ZnO nanostructures found in the previous literature is due to the low working temperature. It was found that the recovery time of Pd coated ZnO nanorods were

Ultrafast and Reversible Gas-Sensing Properties of ZnO Nanowire

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