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. 2020 Apr 3;20(7):2013.
doi: 10.3390/s20072013.

Observation of CO Detection Using Aluminum-Doped ZnO Nanorods on Microcantilever

Ratno Nuryadi  1 , Lia Aprilia  2 , Makoto Hosoda  2 , Mohamad Abdul Barique  2 , Arief Udhiarto  3 , Djoko Hartanto  3 , Muhammad Budi Setiawan  4 , Yoichiro Neo  2 , Hidenori Mimura  2
Affiliations

Observation of CO Detection Using Aluminum-Doped ZnO Nanorods on Microcantilever

Ratno Nuryadi et al. Sensors (Basel). .

Abstract

An oscillating piezoresistive microcantilever (MC) coated with an aluminum (Al)-doped zinc oxide (ZnO) nanorods was used to detect carbon monoxide (CO) in air at room temperature. Al-doped ZnO nanorods were grown on the MC surface using the hydrothermal method, and a response to CO gas was observed by measuring a resonant frequency shift of vibrated MC. CO gas response showed a significant increase in resonant frequency, where sensitivity in the order of picogram amounts was obtained. An increase in resonant frequency was also observed with increasing gas flow rate, which was simultaneously followed by a decrease in relative humidity, indicating that the molecular interface between ZnO and H2O plays a key role in CO absorption. The detection of other gases of carbon compounds such as CO2 and CH4 was also performed; the sensitivity of CO was found to be higher than those gases. The results demonstrate the reversibility and reproducibility of the proposed technique, opening up future developments of highly sensitive CO-gas detectors with a fast response and room temperature operation.

Keywords: Al doping; CO detection; ZnO nanorods; microcantilever sensor; resonant frequency shift.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Growth process of Al-doped ZnO nanorods on microcantilever consisting of (a) ZnO seeds deposition; (b) the growth of ZnO nanorods; (c) Al doping in ZnO nanorods using RF sputtering.
Figure 2
Figure 2
(a) View of the inside chamber box made of stainless-steel with a size of 8 cm × 5 cm × 2 cm; (b) resonant frequency measurement system based on a microcontroller-controlled direct digital synthesizer (DDS); (c) amplitude of signal B as a function of signal A, which vibrates the microcantilever with a resonant frequency of 36.2 kHz; (d) profile of the vibration amplitude vs frequency for Al-doped ZnO nanorod-coated MC.
Figure 3
Figure 3
(a) Field emission-scanning electron micrograph of the microcantilever and magnified images of ZnO nanorods on the MC surface for (b) upper surface and (c) lower surface taken by JEOL JSM-7600F, Japan at an accelerating voltage of 5 kV. A vertical nanorod structure and hexagonal shape with diameter of around 100 nm were observed.
Figure 4
Figure 4
X-ray diffraction pattern of Al-doped ZnO nanorods on the microcantilever surface using a Rigaku Ultima-III X-ray diffractometer with CuK radiation (λ = 0.15418 nm) operated at 40 kV and 40 mA. The strong (002) peak indicates that the ZnO growth direction tends to be oriented along the vertical axis.
Figure 5
Figure 5
Response characteristics of Al-doped ZnO nanorod-coated MC showing an increase of resonant frequency (blue solid line) and a decrease in RH (black dotted line) due to CO exposure. The increase of resonant frequency and, at same time, the decrease of RH, were also observed with increasing the gas flow rate from 20 mL/min to 100 mL/min. For the uncoated MC (red solid line), there was no response to gas exposure.
Figure 6
Figure 6
Resonant frequency change of the Al-doped ZnO nanorod-coated microcantilever as a function of gas flow rate for various initial relative humidity (RH) inside the chamber, i.e., 46 %, 51 %, 55 %, and 58 % RH.
Figure 7
Figure 7
Comparison of CO response between Al-doped ZnO nanorod-coated MC (blue solid line) and undoped ZnO nanorod-coated MC when CO gas was flow either on, with flow rate of 200 mL/min, or off. The graph also shows no resonant frequency change for the undoped ZnO nanorod-coated MC (red solid line) upon gas exposure.
Figure 8
Figure 8
The possible mechanism of CO adsorption on Al-doped ZnO nanorod-coated microcantilever surface. Here, Al atoms (red circles in Figure 8) replaced Zn atoms on the topmost layer of the slab due to the fact that bonding between Zn and O in ZnO crystal lattices is weaker than that those between Zn–Al [13]. (a) CO exposure causes the desorption of undissociated H2O molecules, and (b) simultaneously, CO molecules are absorbed on the Al-doped ZnO nanorod surface.
Figure 9
Figure 9
Resonant frequency change of Al-doped ZnO nanorod-coated MC upon exposure to CO, CO2, and CH4 with flow rates of 10–100 mL/min.
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