The ZnO microwires were synthesized by the chemical vapor deposition (CVD) method. The well-mixed ZnO powders and graphite powders with a mass ratio of 20 : 3 were placed in a quartz boat and the boat was located in the center of the tube furnace. The cleaned Si substrate was placed horizontally on the downstream side of the source 15 cm far. High purity Ar carrier gas was passed into the furnace at a rate of 200 sccm during the entire synthesis. In the reaction process, oxygen gas was introduced into the reacting chamber at a flow rate of 30 sccm. The temperature was kept at 1000°C for 30 min, and then the furnace was cooled down to room temperature. It could be found that a large-scaled ZnO MW arrays visible to the naked eye were observed on the Si substrate.

The surface morphologies of the samples were characterized by digital camera and scanning electron microscopy (Hitachi TM3030). The elemental analysis of single ZnO microwire was performed by energy dispersive x-ray spectrometer (EDS) equipped in a scanning electron microscope. Photoluminescence (PL) measurements were performed by using a micro-PL system at room temperature. For PL measurements, He–Cd laser with 325 nm in wavelength was used as an excitation source.

In this work, the ethanol gas sensor is based on the principle of non-balance electric bridge by using a single ZnO microwire as one bridge arm R₄ and other three fixed-value resistors R₁ (10 kΩ), R₂ (10 kΩ), and R₃ (50 kΩ) to form the four bridge arms. The schematic diagram is shown in Fig. 1. The sheet glass and the conductive silver paste were used as the substrates and electrodes of single ZnO microwire, respectively. The sheet glass was washed with acetone, ethanol, and deionized water under sonication in sequence. The selected single ZnO microwire was placed on the sheet glass. And the two ends of the microwire were fixed by the conductive silver paste, then connected a single ZnO microwire into the nonbalanced electric bridge as R₄. The as-prepared devices were located in the chamber of our homemade gas sensing system. The Keithley 4200 semiconductor characterization system was used to monitor the sensing property by using the two-point (A and B) measurement as indicated in Fig. 1. Experimental tests were conducted under an applied voltage of DC 5 V at room temperature (25°C) in dark room.

Fig. 1.

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	Fig. 1. Schematic diagram of non-balanced electric bridge ethanol gas sensor based on single ZnO microwire.

Gas-sensing measurements were performed in ambient air at 26% relative humidity. The sensing performance was characterized by the change in current when the sensor was exposed to the ethanol gas, and the sensor response rate was defined as^[14–16] 1 S=|(Ig−Ia)Ia|×100%,

where I_a and I_g are the current value of the sensor in air and in gaseous ethanol, respectively. The response and recovery time of the gas sensor are also crucial parameters.

Figure 2(a) shows the photograph of as-grown ZnO microwires. As shown in Fig. 2(a), a large number of white and transparent microwires are prepared on the entire Si substrate, which have the length ranging from 1 cm–2.5 cm. Furthermore, it can be seen that the ZnO microwires have a large size and high density, which makes it convenient to prepare the gas sensor. Figure 2(b) shows the scanning electron microscope (SEM) image of the individual ZnO microwire. The single ZnO microwire has a perfect smooth surface and a diameter of 30 µm. To determine the composition of ZnO microwires, we perform the EDX analysis, and the result indicates that the sample is composed of O, Zn elements without any impurities. And the molar percentage of O and Zn in microwire are about 48.5% and 51.5%, respectively. That is, the atomic ratio of O/Zn is close to 1:1.

Fig. 2.

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	Fig. 2. (a) Photograph of ZnO microwires arrays, and (b) SEM image of single ZnO microwire.

Room temperature micro-PL measurements of a single ZnO microwire are carried out and shown in Fig. 3. The PL spectrum exhibits a strong near-band-edge (NBE) emission centered at about 377 nm (3.29 eV) and a broad green emission centered around 510 nm (2.43 eV). The NBE is normally ascribed to the recombination of free excitons.^[17–19] The green band emission accounts for the recombination of the excited donor electrons with the deep level donor or acceptor like defects such as zinc interstitials (V_Zn) and oxygen vacancies (V_O).^[20,21]

Fig. 3.

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	Fig. 3. Room-temperature micro-PL spectrum of single ZnO microwire.

The large size microwire is chosen for easy manipulation under an optical microscope or naked eye. The single ZnO microwire with a length as long as 2.0 cm and diameter of about 30 µm for gas detector is selected under an optical microscope to prepare the sensor.

Figure 4 showed the I–V characteristics of a single ZnO microwire at room temperature, with the distances between the both electrodes being 1.5 cm. The I–V curves are measured with a bias voltage in a range from −10 V to +10 V. It can be seen that the I–V curve is nearly linear, indicating a fairly good Ohmic contact between silver electrode and ZnO microwire. It ensures that all upcoming behaviors of electrical devices represent the gas-sensing properties but not the contact between the materials and the electrodes. In addition, from the slope of a straight line, the ZnO resistance value and the resistivity can be calculated to be about 87 kΩ and 10 WΩcm, respectively.

Fig. 4.

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	Fig. 4. Room temperature I–V characteristics of a single ZnO microwire.

The time-evolving dynamic response–recovery curves of the non-balanced electric bridge gas sensor based on a single ZnO microwire for ethanol gas concentration sequence of 10, 30, 50, 70, 90 ppm at room temperature are shown in Fig. 5(a). It is clearly seen that the rectangle-shaped sensing behavior and sensor presents sensitive and reversible response to ethanol with different concentrations. The current reveals a drastic decrease with ethanol gas concentration increasing and recovers to its initial value after exposing gas to air. Figure 5(b) shows the corresponding response rate versus ethanol concentration in a range from 10 ppm to 90 ppm. From the curves, it is found that the response rate increases with tested gas concentration increasing, showing its nearly linear correlation with ethanol concentration. When the ethanol concentration is 90 ppm, the response rate of the device can reach 93%.

Fig. 5.

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	Fig. 5. (a) Dynamic response–recovery curves of the non-balanced electric bridge gas sensor based on a single ZnO microwire for different ethanol concentrations at room temperature, (b) response of the sensor to ethanol concentration in a range of 10 ppm–90 ppm, and (c) reproducibility and stability of the sensor exposed to ethanol gas of 90 ppm at room temperature.

Reproducibility and stability are also very important characteristics of gas sensors, and are required by gas sensors for long life service-term and the ability to respond successfully to testing gases without a visible decrease in sensor response rate.^[22,23] Figure 5(c) shows the response rate–recovery curves of gas sensor exposed to four representative reversible cycles of continuous tests for 90 ppm of ethanol gas at room temperature. It can be observed from Fig. 5(c) that when sensors are exposed to gaseous ethanol, the current decreases rapidly and then returns to its initial value when the sensor is re-exposed to air. The gas sensor shows good response and recovery characteristics among the four cycles, which indicates that the gas sensor has a perfect replicability and good stability. Both response and recovery are very fast, taking about 0.3 s and 0.7 s, respectively.

In order to study the difference in gas sensitivity between non-balanced electric bridge structure gas sensor and the conventional resistive gas sensor (the electrodes at both ends of the ZnO microwire are directly connected to the power source), at the same test temperature of 25°C, applied voltage of DC 5 V and ethanol gas concentration of 90 ppm, we test the response curves of the two sensors, and the results are shown in Figs. 6(a) and 6(b). In Figs. 6(a) and 6(b), the conventional resistive sensor has a responsivity of 0.5%, the response and recovery time of 6.5 s and 2.1 s, respectively, and the gas sensor of the non-balanced electric bridge structure has a response rate of 93% and the response and recovery time of 0.3 s and 0.7 s, respectively. It can be clearly seen that device using non-balanced electric bridge structure can significantly increase its sensor response rate, and reduce its response and recovery time.

Fig. 6.

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	Fig. 6. Response and recovery curves of the two kinds of gas sensors at 90-ppm ethanol: (a) non-balanced electric bridge structure, and (b) conventional resistive structure.

In order to prove that the current changes of the device in the ethanol gas are from the ZnO microwire, we make four non-balance electric bridge devices by replacing the bridge arm R₄ with the fixed resistances of 10 kΩ, 20 kΩ, 50 kΩ, and 100 kΩ, respectively. Figure 7 shows the responses of the five sensors to ethanol gas of 90 ppm at room temperature with applying biase voltage of 5 V. It can be seen that the device is immersed into ethanol gas for 20 s and then taken out and placed in the air for 60 s. It is found that when R₄ value is any of the four fixed resistance values, the device does not respond to the ethanol gas. In addition, it is also found that with resistance R₄ decreasing, the current value between points A and B in Fig. 1 decreases gradually.

Fig. 7.

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	Fig. 7. Response curve of fixed value resistance acting as bridge arm R4 at ethanol gas concentration of 90 ppm.

The ZnO gas sensing mechanism, which has been widely accepted, is the depletion theory, including the model of charge depletion layers induced by the adsorption/desorption of oxygen molecules on the surface of sensing material.^[24–26] When the gas sensing material is exposed to air, oxygen molecules are adsorbed on the ZnO microwire surface. The adsorbed oxygen molecules can extract electrons from the sensing ZnO material due to their large electronegativity and form oxygen species such as molecules (O2−) and (O⁻, O²⁻) by capturing electrons from the ZnO conduction band at temperatures of 25°C–500°C, as described in the following reaction equations:^[27] 2 O2(gas)→O2(ads), 3 O2(ads)+e−→O2(ads)−(O2−dominatesat25°C−150°C), 4 O2(ads)−+e−→2O(ads)−(O−dominatesat150°C−350°C), 5 O(ads)−+e−→O(ads)2−(O2−dominatesabove350°C).

Of these oxygen species, O2− is dominant at room temperature.^[28]

Thus, the electron depletion layers are formed in the surface regions of ZnO microwire as shown in Fig. 8. Here, the concentration of electrons, as majority carriers, is very low and eventually increases the resistance of the gas sensor. Secondly, when ethanol is added into the atmosphere, the O2− ions on the surface of ZnO microwire react with the ethanol gas molecules at room temperature, releasing the trapped electrons back to the conduction band of ZnO. This process leads the resistance of the sensor to increase, which can be described with the following reaction equation: 6 C2H5OH+3O2(ads)−→2CO2+3H2O+3e−.

Fig. 8.

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	Fig. 8. Schematic illustration forsensing mechanism of ZnO microwire sensor to ethanol: (a) exposed to air, and (b) exposed to ethanol.

According to the analysis of the possible gas sensing mechanism, when the ZnO microwires are placed in the ethanol gas, the resistance of the microwire sensor decreases, that is, the resistance R₄ decreases.

The expression of the output voltage V_g between the two points A and B is given. The output voltage V_g between the two points A and B can be formulated as follows:^[29] 7 Vg=(R2R2+R3−R1R1+R4)V=R2(R1+R4)−R1(R2+R3)(R1+R4)(R2+R3)V,

where V is the bias voltage.

The equivalent resistance R between points A axs 8 R=R1R4R1+R4+R2R3R2+R3=R1R4(R2+R3)+R2R3(R1+R4)(R1+R4)(R2+R3).

Therefore, the current I_g can be obtained as 9 Ig=VgR=R2(R1+R4)−R1(R2+R3)R1R4(R2+R3)+R2R3(R1+R4)V=R2R4−R1R3R1R4(R2+R3)+R2R3(R1+R4)V.

In this experiment, V = 5 V, R₁ = R₂ = 10 kΩ, and R₃ = 50 kΩ. For easy calculation, the unit of R₄ is kΩ and the unit of I_g is mA. The above equation (9) can be simplified into 10 Ig=R4−5022R4+100.

According to the above analysis, the ZnO microwires serving as resistance R₄ is strongly dependent on ethanol concentration. When the ZnO microwires are placed in the ethanol gas, the resistance value R₄ decreases, and the current also decreases as indicated by formula (10). This result is consistent with the decrease in current value in Fig. 5(a). Furthermore, we can see from the relevant literature that the non-balanced electric bridge structure can amplify a small amount of resistance change,^[30] so the gas sensor can significantly enhance the response rate, response time, and recovery time of the device.