Low-Voltage-Driven Sensors Based on ZnO Nanowires for Room

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24172−24183

Low-Voltage-Driven Sensors Based on ZnO Nanowires for RoomTemperature Detection of NO2 and CO Gases Jae-Hun Kim,† Ali Mirzaei,‡,§ Hyoun Woo Kim,*,§,∥ and Sang Sub Kim*,† †

Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea The Research Institute of Industrial Science and ∥Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea § Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz 71557-13876, Iran Downloaded via UNIV OF SOUTHERN INDIANA on July 19, 2019 at 01:46:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Herein, we report the synthesis of pristine and Aufunctionalized ZnO nanowires (NWs) for low power consumption (self-heated) gas sensors at room temperature. The ZnO NWs were produced via a vapor−liquid−solid growth technique, and Au layers with different thicknesses were sputterdeposited on the ZnO NWs, followed by subsequent annealing. Microscopic characterization methods demonstrated that ZnO NWs were successfully formed. Pristine ZnO NW gas sensors showed the best sensitivity toward either CO or NO2 gases at 300 and 350 °C, respectively. Also, the sensitivities of pristine ZnO NW gas sensors were tested toward NO2 gas under different applied voltages; the sensors revealed a good response and selectivity under an applied voltage of 7 V. Au-functionalized ZnO NW gas sensors exhibited the best response for CO gas at an applied voltage of 7 V and showed a much higher response relative to the pristine ZnO NWs. The sensing mechanisms for pristine and functionalized gas sensors are comprehensively discussed. KEYWORDS: gas sensor, ZnO nanowires, Au functionalization, self-heating, NO2, CO

1. INTRODUCTION Today’s high standard of living has resulted in severe regulations for the control of air pollution. Toxic gases such as NO2 and CO are some of the main air pollutants. In addition to air pollution, some toxic gases such as CO are important biomarkers. Excessive amounts of exhaled CO may indicate diseases such as anemia, respiratory infection, and asthma.1 Accordingly, the type and concentration of toxic gases need to be detected using portable and low-cost electronic devices with fast dynamics, high sensitivity, and good selectivity. Among the different sensing devices, metal resistance gas sensors are the most common types because of their simplicity, small size, fast and reliable response, and high sensitivity.2 Accordingly, they are widely used to sense different gases, including NO2 and CO that are among the most toxic gases for human beings. The morphology of the sensing material significantly affects the sensing performance.3 In this context, one-dimensional materials such as nanowires (NWs),3 nanotubes,4 and nanofibers5 have several advantages over their two-dimensional or three-dimensional counterparts. In fact, they have high stability, a high surface area, and diameters comparable to the Debye length and are easily functionalized.6,7 In particular, NWs can generally be prepared without any special apparatus © 2019 American Chemical Society

and have excellent crystallinity and a limited conduction channel with good electrical transport properties.8 Accordingly, gas sensors with NW morphologies have been studied extensively in recent years worldwide.9,10 Zinc oxide is a multifunctional material.11−14 In particular, it is a well-known metal−oxide−semiconductor for sensing studies because of its availability, ease of synthesis, high electron mobility, good electrical properties, and good intrinsic sensing capacity.15,16 Therefore, different ZnO morphologies have been explored for sensing studies.17,18 Nevertheless, the selectivity of pristine ZnO NWs is poor, and the sensing temperature is high. To overcome these issues, noble metal nanoparticles (NPs), such as Au,19 Pt,20 Pd,21 and Ag,22 can be functionalized on the outer surfaces of NWs. Noble metal NPs have good catalytic activity and work functions that are different from that of ZnO and can improve the chemical and electronic operation of the gas sensor.23 Noble metal NPs can be deposited by a variety of techniques, and each has its own advantages and disadvantages. UV reduction is a low-cost and effective method; Received: April 24, 2019 Accepted: June 18, 2019 Published: June 18, 2019 24172

DOI: 10.1021/acsami.9b07208 ACS Appl. Mater. Interfaces 2019, 11, 24172−24183

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ACS Applied Materials & Interfaces however, there is poor control over the final particle size of deposited NPs.24 Room-temperature, γ-ray irradiation provides a high efficiency. Moreover, the synthesis is contaminationfree. However, it is expensive and requires special apparatuses.1 Sputtering is a well-known physical deposition method that can be used for the deposition of noble metals on materials. It is simple and fast, in which the final size of the NPs depends on the initial thickness of the deposited metal.25 It is well-known that decreasing the particle size, especially when it is in the range of Debye length, significantly enhances the response of gas sensors. 26 Accordingly, in the present work, Au functionalization will produce Au/ZnO heterointerfaces, increasing the depletion region in ZnO, and thus possibly help ZnO conduction volume to reach the size comparable to or smaller than the Debye length. Ordered mesoporous structures significantly enhance the sensor response by increasing the specific surface area.27−29 Generation of Au NPs will certainly increase the specific surface area in comparison to pristine ZnO NWs. Furthermore, Hyodo et al. revealed that the surface engineering of conventional SnO2 powders with mesoporous SnO2 powders enhanced the sensitivity to NO2.29 Accordingly, we expect that the optimized amount of Au functionalization will improve the gas sensing response by a variety of mechanisms which will be explored. Even though noble metal functionalization can effectively reduce the sensing temperature of gas sensors, the sensing temperature is still high, and the sensor consumes a significant amount of energy, making it unsuitable for some applications such as smart phones and other small portable electronic devices. In addition, sensor operation at temperatures much greater than room temperature for extended periods can degrade the sensing ability of a gas sensor.30 Other methods have been explored owing to the difficulties associated with microelectromechanical systems technology, the high cost of production, and high power consumption for micro gas sensors.31 A very cost-effective method for decreasing the energy consumption of the gas sensors is operation in the Joule heating or self-heating modes.32 The self-heating effect refers to a temperature rise of a gas sensor owing to the power dissipated by the voltage.33 Some previous studies developed piezoelectric gas sensors, which can be self-powered.34−37 However, their principle is rather complicated than the metal− oxide resistance sensors because the piezoelectric voltage is influenced not only by compressive strain but also by adsorption of gas molecules. Also, the strain energy should be applied for the operation of piezoelectric gas sensors. Only heating energy is required in metal−oxide resistance sensors; however, the self-heated ones will minimize the power consumption. In previous reports,32,38 we studied the gas sensing properties of ternary structures, namely, noble metal-functionalized core−shell NWs. In the present work, we employed a two-component system, that is, ZnO and Au, which is simpler than ternary composites with regard to the preparation, sensing parameters, and sensing mechanisms. In light of the above ideas, we fabricated ZnO NWs by a vapor−liquid−solid (VLS) technique. Subsequently, Au NPs were functionalized on synthesized ZnO NWs using a sputtering technique, followed by annealing. The sensing capacity of pristine and Aufunctionalized ZnO NW gas sensors was studied for NO2 and CO gases under self-heating conditions. We found that both pristine and Au-functionalized gas sensors can work at very low applied voltages (7 V) with good sensitivity toward

NO2 and CO. The optimized applied voltage in this study may be slightly higher than those already used in some electronic devices. However, it is relatively low compared with values reported in previous reports on self-heated gas sensors (Table S1, Supporting Information), which can be easily provided by rechargeable batteries. Furthermore, the sensors showed a good selectivity to the previously mentioned gases. The sensing mechanism involved in both gas sensors is explained in detail. The present research demonstrates the possibility of integrating ZnO NW-based gas sensors into low power consumption electronic devices, such as smart phones, because of their very low power consumption.

2. EXPERIMENTAL SECTION 2.1. Preparation of ZnO NWs. The synthesis method employed for the synthesis of ZnO NWs was similar to that used in our previous study.39 ZnO NWs were synthesized by heating high-purity Zn powders, whereby Zn powders and Pt-/Ti-coated SiO2/Si substrates were placed in a quartz tube furnace. It has been reported that the morphology and structure of ZnO affected photocatalytic and gas sensing properties.40,41 With change of temperature varying the diameters of ZnO NWs, the growth temperature was set to 950 °C and networked ZnO NWs were grown for 1 h. The growth was performed in the presence of flowing gases (Ar + O2). Figure S1a shows a schematic of the growth conditions of the ZnO NWs. 2.2. Au Functionalization. Au thin layers with different thicknesses (3, 5, 10, 20, and 30 nm) were coated on ZnO NWs by magnetron sputtering under a deposition pressure of 10 m Torr. Au-coated ZnO NWs were subsequently annealed at 500 °C in air for 0.5 h. It is well-known that metal catalysts will enhance the sensitivity in the form of well-dispersed NPs.42,43 Our previous work revealed that the sensor response of Pt-functionalized SnO2 NWs is dependent on the amount of Pt, which can be controlled by varying the thickness of the initial Pt layer.44 As a result, the Au layers were transformed to Au NPs. Au NPs with different thicknesses were obtained because of variations in the thickness of the Au layers. By adjusting the sputtering parameters (current, time, and pressure), as well as the annealing temperature and time, it was possible to reproduce the Aufunctionalized ZnO NWs with almost the same pattern (text S1 in the Supporting Information). Figure S1b illustrates a schematic of the Au deposition steps. 2.3. Materials Characterization. The morphologies of the NWs were studied using a field emission scanning electron microscope (FEI QUANTA FEG250) and a transmission electron microscope (JEM2100F, JEOL). The crystalline nature of the NWs was studied by Xray diffraction (XRD, Philips-X′pert MPD). Ultraviolet photoemission spectroscopy (UPS, Thermo Scientific ESCALAB 250Xi) was used to calculate the work functions of ZnO and Au. He I (21.22 eV) was used as the emission light, and the pressure of the chamber used for the analysis was fixed at ∼2 × 10−8 mbar. 2.4. Gas Sensing Tests. Similar to our previous reports,5,45 for the gas sensing tests, Ti and Pt electrodes with different thicknesses of 50 and 200 nm, respectively, were first sputter-deposited onto the synthesized NWs that were put on SiO2(200 nm thick)-grown Si substrates. As the interface between the sensing layer and electrodes can form either Ohmic or Schottky contact,46,47 we obtained the current−voltage (I−V) characteristics of pristine and Au-functionalized ZnO NW sensors (Figure S2 in the Supporting Information). In both cases, current increases linearly with an increase in the applied voltage, demonstrating Ohmic contacts between ZnO NWs and interdigitated electrodes, indicating that all of the sensing properties are independent of the properties of contacts.46,48,49 Sensing tests were carried out by employing a gas testing system. The desired gas concentrations were injected into a gas chamber (total flow rate of 500 sccm) through the mass flow controllers, while the electrical resistance of the gas sensor was continuously recorded. The resistance of the sensor in air (Ra) and in the presence of the target gas (Rg) was used to calculate the gas response: R = Ra/Rg for the reducing gases 24173


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Figure 1. (a) FE-SEM micrograph of the ZnO NWs. (Inset) Low-magnification FE-SEM micrograph of the ZnO NWs on the substrate. (b) Crosssectional FE-SEM images of ZnO NWs on a substrate. (c) Typical TEM micrograph of a single ZnO NW. (d) XRD pattern of the ZnO NWs.

and the final sizes of the Au NPs can be tuned in the range ∼30−175 nm using this procedure. A thick Au layer has a low chance of being broken into fine Au NPs during the heat treatment. However, a thin Au layer can be broken into tiny parts easily, generating smaller Au NPs.51 An XRD pattern of the Au-functionalized ZnO NWs is provided in Figure 2h. In addition to the peaks related to ZnO,52 one additional peak arises from the Au NPs (JCPDS card no. 89-3697),53 proving the existence of Au. 3.2. Gas Sensing Studies. 3.2.1. Temperature Effect. Gas adsorption and related phenomena such as diffusion are thermally activated; therefore, the performance of the gas sensor is related to the sensing temperature.54 Therefore, we first performed a series of preliminary experiments to find the optimal working temperature. The optimal sensing temperature of the pristine ZnO NW gas sensor was explored by exposing the gas sensor to CO gas. Figure 3a displays the dynamic resistance curves of the gas sensor, and Figure 3b shows the corresponding response values versus the working temperature. The sensor exhibits a response to all gas concentrations at all sensing temperatures. However, the response significantly depends on the sensing temperature and the CO concentration. The gas sensor response increases with an increase in the CO concentration at all temperatures owing to the adsorption of more gas molecules on the surface of the gas sensor, causing a greater change in the resistance. For CO gas concentrations of up to 10 ppm, the response increases with an increase in the sensing temperature. However, the highest gas response occurs at 350 °C for 50 ppm CO gas, decreasing at 400 °C. As the adsorption, desorption, reaction, and diffusion processes are strongly temperature-dependent, the sensing temperature plays a vital role for gas sensing. Every gas has an optimal sensing temperature, where the overall sensing process is the most efficient. At a temperature less than the optimal one, the desorption, diffusion, and reaction processes are deactivated, leading to a lower response. Moreover, at a temperature greater than the optimal one, the adsorption will be inefficient, and the desorption is too fast,

and R = Rg/Ra for the NO2 gas. Different voltages were used at room temperature for the self-heating operation mode.

3. RESULTS AND DISCUSSION 3.1. Morphological and Phase Studies. Figure 1a shows a typical field emission scanning electron microscopy (FESEM) image of bare ZnO NWs. Long and continuous ZnO NWs were successfully formed. The inset of Figure 1a provides a low-magnification FE-SEM micrograph showing a substrate covered with ZnO NWs. Figure 1b shows a low-magnification cross-sectional FE-SEM micrograph of a bundle of ZnO NWs that were vertically grown on the substrate surfaces. A typical transmission electron microscopy (TEM) micrograph of a ZnO NW is shown in Figure 1c, where the diameter of the ZnO NW was ∼80−100 nm. XRD spectrum was obtained to study the crystallinity of the ZnO NWs and is shown in Figure 1d. The observed peaks are labeled as a crystalline ZnO with a hexagonal crystal structure (JCPDS card no. 89-1397) in accordance with previous work.50 No impurities or other Zn compounds were observed, demonstrating the good quality and purity of ZnO NWs. Typical FE-SEM micrographs of Au-functionalized ZnO NWs are provided in Figure 2b−f. An FE-SEM micrograph of pristine ZnO NWs is displayed in Figure 2a for comparison. After functionalization with Au NPs, the general morphology of the ZnO NWs did not change. In Figure 2b−f, the sizes of the Au NPs increased with an increase in the initial Au thickness from 3 to 30 nm. Also, the general morphology of the ZnO NWs, which were fairly smooth before the Au functionalization, changed to a relatively rough morphology, which is beneficial for sensing studies. Moreover, Au NPs were almost uniformly distributed over the surface of the ZnO NWs. The insets in Figure 2a−f indicate lower magnification FESEM micrographs, confirming the formation of bundles of ZnO NWs as well as Au-functionalized ZnO NWs on the substrate. Figure 2g shows the relationship between the sizes of the Au NPs and initial Au layer thickness. The final size of the Au NPs increases with an increase in the Au layer thickness, 24174


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Figure 2. FE-SEM images of (a) ZnO NWs and Au-functionalized ZnO NWs with initial Au film thicknesses of (b) 3, (c) 5, (d) 10, (e) 20, and (f) 30 nm, deposited and subsequently annealed. (g) Diameters of the Au NPs vs Au layer thickness. (h) XRD data of the Au-functionalized ZnO NWs.

deactivating the efficient sensing.55 This is why the response of the gas sensor decreases at 400 °C. Accordingly, the optimal sensing temperature for CO gas was 350 °C. Note that the difference between the responses of the sensor at 350 and 400 °C is not significantly different at other gas concentrations. The sensing behavior of the ZnO NW gas sensor to NO2 as a strong oxidizing gas was also studied. The transient resistance plots of the sensor for various NO2 gas concentrations are shown in Figure 4a. Figure 4b displays the corresponding response curves versus the operating temperatures. In contrast to the case with CO gas, the resistance increases under exposure to the NO2 gas. Such behaviors are resulting from the n-type semiconducting nature of the ZnO NWs. Again, the NO2 gas response depends on the gas concentration as well as the temperature. The maximum gas response occurs at 300 °C for all concentrations. At lower temperatures, there is insufficient energy for the NO2 gas to be effectively adsorbed, and the desorption rate is higher than the adsorption rate at high temperatures. Figure S3 presents the response of the gas

sensor to 50 ppm of CO and NO2 gases. The optimal sensing temperature for NO2 is 300 °C, which increases to 350 °C for the CO gas. Different gases have different energies of adsorption, reaction, and desorption on the surfaces of sensing layers. Accordingly, the optimized gas sensing temperature is different for various gases, and this explains why the optimal sensing temperatures for the NO2 and CO gases in this study were 300 and 350 °C, respectively. 3.2.2. Applied Voltage Effect. To investigate the selfheating effects of the pristine ZnO NW gas sensor, the sensing response to 10 ppm NO2 gas under different applied voltages (1−40 V) was studied, and the dynamic resistance plots are provided in Figure 5. The effect of applied voltage on the electron depletion layer (EDL) is discussed in text S2 in the Supporting Information. As shown, the sensor signal is stable up to 25 V, and a clear response is obtained. However, the stability of the sensor decreases significantly with an increase in the voltage to 30 and 40 V. Figure 6a illustrates the excellent repeatability of the sensor, where the responses curves under two successive cycles are almost the same. The response first 24175


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Figure 3. (a) Dynamic resistance curves of the ZnO NW sensors at different sensing temperatures (250−400 °C) for various CO concentrations. (b) Corresponding responses vs sensing temperatures.

Figure 4. (a) Dynamic resistance plots of ZnO NW sensors at different sensing temperatures (250−400 °C) to different concentrations of NO2 gas. (b) Corresponding responses vs the sensing temperature.

increases with an increase in the applied voltage up to 5 V before decreasing for further increases in the applied voltage. Figure 6b provides the standard deviation of the sensor response under various applied voltages. Figure 6c shows the initial resistance of the sensor at different applied voltages. The resistance of the sensor decreases with an increase in the applied voltage because additional heat is generated with further increase in the voltage because of the Joule heating effect. Therefore, more electrons move to the conduction band of the zinc oxide NWs, improving the electrical conduction of the gas sensor. The self-heating tests were repeated under applied voltages between 1 and 10 V, with 1 V intervals in an atmosphere containing 10 ppm NO2 gas to gain better insights into the sensor behavior and to determine the optimal applied voltage corresponding to a maximum response of the sensor. Figure 7a indicates the dynamic resistance plots of pristine ZnO NW gas sensors to 10 ppm NO2 gas under 1−10 V applied voltage, and Figure 7b presents the response versus applied voltages. As shown, the maximum sensor response occurs at 7 V. As shown in Figure S4, by applying an external voltage to the gas sensor, the temperature of the sensor increases. However, up to 7 V, the rise in the temperature is not significant. Even though a higher applied voltage produces a higher temperature, it is preferable to keep the self-heating sufficiently low with an acceptably high sensor response. The temperatures of the gas sensor under voltages of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 20 V were 23.6, 23.9, 24.0, 24.4, 25.2, 26.9, 29.3, 29.8, 30.3, 34.1, and 57.4 °C, respectively.

The reproducibility, repeatability, and reversibility of the ZnO NW gas sensors under voltages ranging from 6 to 9 V were checked. Figure S5 demonstrates the repeatability and reversibility of the sensor, where the signal comes back to its initial resistance after the gas flow was stopped. Two successive cycles of the injection of gas and air show almost the same trend. Figure S5b demonstrates the good reproducibility of the sensor, in which the response values of the sensor signal were almost the same. As the pristine ZnO NW gas sensor exhibited a maximum response at 7 V, we evaluated the selectivity of the sensor by exposing it to 0.1, 1, and 10 ppm of NO2, CO, C5H5OH, and C6H6 gases under a 7 V applied voltage. The roomtemperature dynamic resistance curves are provided in Figure 8a. The resistance of the sensor increases in a NO2 gas atmosphere (oxidizing gas) and decreases when exposed to CO, C6H6, and C2H5OH gases. The selectivity histograms are presented in Figure 8b. The results given in Figure 8b indicate that the sensor exhibits a good selectivity toward NO2 gas. Indeed, even the response of the gas sensor to 0.1 ppm NO2 gas is greater than those of the large concentrations of interfering gases. For example, the response of the sensor to 0.1 ppm NO2 gas is 1.531, which is greater than that of 10 ppm CO (1.051), C6H6 (1.151), and C2H5OH (1.095). Au-functionalized ZnO NWs with different Au layer thicknesses (3, 5, 10, 20, and 30 nm) were synthesized. Figure S6a shows the dynamic resistance plots of an Au-functionalized gas sensor with different thicknesses of the Au layer to 10 ppm NO2 gas under a 7 V applied voltage. Figure S6b shows a plot 24176


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Figure 5. Dynamic resistance plots of the ZnO NW sensors under 10 ppm NO2 gas for two cycles measured under different applied voltages (1, 3, 5, 10, 20, 25, 30, and 40 V).

Figure 7. (a) Dynamic resistance plots of the ZnO NW sensor under 10 ppm of NO2 gas measured under different applied voltages. (b) Corresponding variation of the sensor responses with the applied voltage.

of the response versus thickness of the Au layer. As shown, the highest response was observed for the pristine gas sensor, namely, the gas sensor without Au functionalization. There are several factors that account for the effects of Au on the NO2 sensing behaviors. First, the Au catalytic effect is expected to enhance the sensing of the CO gas; however, it is not efficient for the adsorption of NO2 gas. Second, the generated ZnO/Au heterointerfaces will widen the EDL on the surface of the ZnO NW sensor. This will affect the sensing behavior based on two different mechanisms, which are explained in text S3 of the Supporting Information. Third, increasing the Au layer eventually decreases the exposed sensing surface of the ZnO NWs, contributing to a reduction in the sensitivity. The response of the Au-functionalized ZnO NW gas sensor to CO gas (10 ppm) under a voltage of 7 V was obtained to find the optimal Au thickness based on the results obtained for the pristine ZnO NW gas sensor. Figure 9a gives the transient resistance plots of the Au-functionalized sensor; all sensors indicated a reproducible response to CO gas. Figure 9b displays a plot of the response as a function of the initial Au layer thickness. The response of the sensor increased with an

Figure 6. (a) Sensor response values of the ZnO NW sensor to 10 ppm of NO2 gas for two cycles measured under various applied voltages (1, 3, 5, 10, 20, 25, 30, and 40 V). (b) Standard deviation of the response of the ZnO NW sensor to 10 ppm of NO2 measured under various applied voltages. (c) Sensor resistance vs applied voltage. 24177


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with increasing Au thickness, and they occupied the available adsorption sites on the surfaces of the ZnO NWs, decreasing the surface area of the ZnO NWs. A comparison between the responses of the functionalized and pristine sensors demonstrates that Au improves the response of the sensor to CO gas. After finding the optimal thickness of the Au layer, we performed self-heating tests for Au-functionalized (10 nm thick) ZnO NW gas sensors. Figure 10a displays the dynamic

Figure 8. (a) Dynamic resistance plots of the ZnO NW sensor for various gas concentrations (0.1−10 ppm) under a 7 V applied voltage for NO2, CO, C2H5OH, C6H6, and C7H8 gas. (b) Comparison between the responses of the ZnO NW sensor to different gases under a 7 V applied voltage.

Figure 10. (a) Transient resistance plots of an Au-functionalized ZnO NW sensor with an initial Au thickness of 10 nm to CO gas (10 ppm) at room temperature under various applied voltages. (b) Corresponding changes of the sensor response with the applied voltage (1−10 V).

resistance plots of an Au-functionalized ZnO NW gas sensor with an initial Au thickness of 10 nm to 10 ppm CO gas at ambient temperature under different applied voltages (1−10 V). Interestingly, the sensor shows a response under all applied voltages. Furthermore, it has good repeatability, as shown over two successive cycles. Figure 10b provides the corresponding response versus the applied voltage. The best sensing performance occurs at 7 V, which is much higher than that of the pristine ZnO NW gas sensor. To evaluate the selectivity of the gas sensor, dynamic resistance curves were obtained for the Au-functionalized ZnO NW sensor at 7 V to 10 ppm of various gases at room temperature (Figure 11). Also, the resistance dynamic plots of the gas sensor to 500, 1000, and 2000 ppm CO2 gas at 7 V are shown in Figure S7a. As expected, there were no responses to the ethanol, benzene, and toluene gases, and the response to the CO2 gas at high concentrations was still lower than that to the lowconcentration CO gas, demonstrating excellent selectivity of Au-functionalized ZnO NWs to CO gas under self-heating conditions. This result contrasts clearly with our previous work, in which optimized Pt-functionalized ZnO NWs selectively sensed toluene gas in the self-heating mode.56 At

Figure 9. (a) Transient resistance plots of Au-functionalized ZnO NWs with various initial Au thicknesses toward 10 ppm CO gas at 7 V. (b) Corresponding change in the sensor response to 50 ppm CO gas at room temperature, with varying initial thicknesses of the Au layer (0−30 nm).

increase in the Au layer thickness, and the sensor with a 10 nm thick Au layer showed the highest gas response. Further increases in the Au thickness decreased the gas sensor response. In fact, the continuity of the Au NPs increased 24178


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reproducibility. The calculated limit of detection for the CO gas is 139 ppb (text S6 in the Supporting Information). Figure S9 displays the effect of the relative humidity (RH) on the response of the Au-functionalized ZnO NW sensor to 10 ppm CO gas under an applied voltage of 7 V. For the first cycle, when the RH % increases from 30 to 60%, the response of the gas sensor decreases from 2.90 to 1.97. Also, the initial response time (270 s) and recovery time (158 s) increase to 278 and 210 s, respectively. For the second cycle, the values of the response, response time, and recovery time for 30% RH are 2.94, 264, and 152 s, changing to 1.99, 256, and 195 s after an increase of the RH to 60%, respectively. According to the obtained results, the sensor exhibits good repeatability for two sequential cycles, and the sensor responses are almost invariant. However, the response of the gas sensor decreases at higher RH %. This may be due to the occupation of adsorption sites by water molecules, where the number of available adsorption sites for the CO gas decreases, leading to a lower response.57 Regarding the change of the response time, it can be surmised that the CO molecules require more time to find the vacant sites on the surface of the gas sensor to be adsorbed, and therefore, longer response times are expected. Table S2 compares the sensing performances of the present study with responses of sensor materials reported in the literature. Even though the response values of the present work are not significantly high, in particular the reported CO gas sensor working at room temperature is a rare occurrence, the present sensor is promising for the selective detection of CO gas with a low power consumption. 3.3. Gas Sensing Mechanism. It is known that the resistances of the semiconducting gas sensors change when they are exposed to target gases.58 When pristine ZnO NWs are initially in an air atmosphere, oxygen molecules from the air can be easily adsorbed on the surfaces of NWs and will be converted to oxygen ion species in either atomic (O− and O2−) or molecular form (O2−) because of their high electron affinity.59 The dominant species is different for various metal oxides with different topologies. However, it is generally accepted that the molecular form is dominant at low temperatures and the atomic species are dominant at higher temperatures.60 A very thin EDL will appear on the exposed surfaces of ZnO NWs because of the abstraction of electrons from the ZnO surfaces. The resistance in the EDL is much higher than that in the core regions because of the abstraction of electrons. Accordingly, a conduction channel will form on the ZnO NWs, and the conduction will be confined to this channel.61 For metal oxides with a NW morphology, the overall resistance of the material greatly depends on the diameter of this channel.62 When the ZnO NWs are exposed to CO, which is a reducing gas, CO will react with the already adsorbed oxygen ions, and the resulting electrons come back to the surfaces of the ZnO NWs. Thus, the diameter of the conduction channel will expand significantly, and a noticeable modulation in the resistance contributes to the sensor response. For oxidative gases such as NO2, the diameter of the conduction channel will decrease significantly again (owing to the extraction of more electrons from the surface of the sensor) contributing to a sensor response. As the gas adsorption and diffusion phenomena are strongly temperature-dependent, the sensing temperature plays a vital role for gas sensors. Each gas has its own optimal sensing temperature, where it will be favorably adsorbed on the surface of a particular sensor. It is due to this reason that the pristine ZnO

Figure 11. (a) Dynamic resistance plots of the Au-functionalized ZnO NW sensor under a 7 V applied voltage to 10 ppm of different gases at room temperature. (b) Corresponding sensor response values for NO2, C2H5OH, C6H6, C7H8, and CO gases.

a 20 V applied voltage, the responses to 50 ppm of C6H6, CO, C2H5OH, HCHO, C3H6O, NH3, and C7H8 gases, respectively, were measured to be 1.02, 1.07, 1.03, 1.16, 1.27, 1.60, and 2.743. It has been reported that metal−oxide sensors do not exhibit a noticeable response to CO2 gas (text S4 in the Supporting Information). In addition, to examine the effects of CO2 gas on the sensing to CO gas, we carried out a sensing test with a mixture of 10 ppm CO and 500 ppm CO2 gases (Figure S7b). A sensor response of 2.91 was observed similar to that with 10 ppm of CO gas. The Au-functionalized ZnO NW sensor is not sensitive to C2H5OH and C6H6 gases (Figure 11) (text S5 in the Supporting Information). Further study will be necessary to reveal more detailed mechanisms. Figure S8a provides the dynamic resistance curve of the optimized gas sensor for low concentrations of CO gas at 7 V. As shown by the calibration curves, the responses to 0.04, 0.06, 0.08, 0.1, 1, and 10 ppm CO gas are 1, 1.07, 1.17, 1.33, 2.25, and 2.90, respectively. The long-term stability of the gas sensor is shown in Figure S8b. Even after 12 months, the signal of the gas sensor did not change significantly, demonstrating a high stability of the sensor for long-time use. The response and recovery times of the gas sensor to 10 ppm CO and NO2 gases at 7 V as a function of Au thickness are plotted in Figure S8c, indicating relatively fast dynamics of the gas sensor under the self-heating mode. The reproducibility of the gas sensor is shown in Figure S8d, whereby the sensor exhibits almost the same response under five sequential cycles, confirming a good 24179


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ACS Applied Materials & Interfaces NW sensor showed different sensitivities to the NO2 (300 °C) and CO (350 °C) gases at two different temperatures. We did not study the role of the ZnO NW diameter on the gas response; however, for SnO2 NWs, the gas response increased with a decrease in the diameter of NWs.63 When the average diameter of ZnO NWs decreases, the response to the target gas will be increased. The ZnO NW diameter can be decreased by decreasing the size of the metal catalyst droplets used for the VLS growth of ZnO NWs.64 Another factor contributing to the variation of the resistance is the presence of ZnO−ZnO homojunctions. In fact, many ZnO−ZnO homojunctions will form because of the random alignment of ZnO NWs, where a potential barrier for the transfer of electrons will appear. The SEM and TEM images in Figure S10 indicate the interconnections among the NWs. The NWs are seemingly in intimate contact with each other, forming well-defined homojunctions. Although the contact pressure may exist, which generates plastic deformation by increasing the dislocation density, the dislocations are not severely bent or distorted. Upon exposure to gases, the height of this barrier decreases (for the CO gas) or increases (for the NO2 gas), leading to changes in the resistance.65 These sensing mechanisms are schematically shown in Figure 12a. Further sensing mechanisms occur owing to the presence of Au in Au-functionalized ZnO NW gas sensors. Schottky barriers can form on the interfaces between Au and ZnO owing to the difference between the different work functions (φ) of Au and ZnO. It is possible that the wide EDL on the ZnO NWs will affect the sensing behaviors (text S3, Figure S11 in

the Supporting Information).56,66 This sensing mechanism is schematically shown in Figure 12b. Furthermore, Au has a good catalytic ability toward gases and can effectively adsorb the target gases on its surfaces, transferring them to the surfaces of the sensing material in a socalled spill-over mechanism. Accordingly, more gas is adsorbed on the surface of ZnO NWs for Au-functionalized ZnO NW gas sensors, resulting in a high resistance modulation. Au NPs are widely used as CO oxidation catalysts.67 The high sensitivity and selectivity to CO gas can also be attributed to a lower oxidation barrier for the CO gas in the presence of Au, which is only 1.20 eV. The interactions between CO and Au depend on the overlap of the 5σ and 2π* orbitals of CO with the “d” orbitals of Au. Upon interaction, CO donates electrons from its π orbitals to the dσ orbitals of Au, and the filled dπ orbitals of Au donate electrons back into the empty π orbitals of CO.68 Figure 12c schematically shows a sensing mechanism in the self-heating operation. In the self-heating operation mode, which is performed at room temperature, heat is generated via a Joule heating effect when applying a voltage. The Joule heating effect is due to the heat dissipation at a small scale.69 Any obstacle that leads to the deviation of electrons from their pathway can result in heat generation. Electron−electron collisions, grain boundaries, heterojunctions, and homojunctions can all be different sources of heat generation. For our networked ZnO NWs, the heat dissipation concentrates in some regions, which are named as hot spots. Indeed, the most resistive parts, such as the ZnO−ZnO NW homojunctions and grain boundaries, are the most likely regions for the dissipation of heat.69 In this regard, the NWs are very efficient materials.69 In the case of NWs with a cylindrical cross section, the increase in the temperature is inversely related to the square of the diameter of the NWs69 ΔT =

1 rNW 2

(1)

Here, rNW is the diameter of the NWs. As the diameters of synthesized ZnO NWs are on the nanoscale, the temperature generated as a result of an applied voltage is significant. Target gas molecules require energy to reach the surface of the sensor and be adsorbed. For low applied voltages (7 V) will result in a decrease in the sensor’s sensitivity. This can be due to thermal losses to the substrate. In fact, both the transport of energy through the sensing materials in the Joule heating mode and the losses of generated heat affect the final temperature of the gas sensor. Heat dissipation in the substrate or contacts can decrease the effectiveness of self-heating.69,70 Therefore, this suggests that higher applied voltages (>7 V) are associated with higher power dissipation to the substrate and loss of the sensor response. ZnO is a semiconductor with a relatively high conductivity, so low applied voltages, such as 7 V, can induce sufficient heat and electrical currents. This explains why the sensors can easily work at 7 V, which is a rather low applied voltage. The calculated activation energy values for the response and recovery phenomena of 10 ppm CO gas for the surface of Aufunctionalized ZnO NWs were 1.26 and 1.06 eV, respectively (see text S7 in the Supporting Information), and those for 10 ppm NO2 gas on the surface of pristine ZnO NWs were 0.799

Figure 12. Sensing mechanism in the (a) ZnO NWs and (b) Aufunctionalized ZnO NWs. (c) Resistance of the electrical currents crossing the ZnO−ZnO homojunctions and resistance of the electrical currents by additional EDL because of the Au−ZnO heterojunctions. 24180


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low power consumption, suitable for integration into portable devices.

and 1.02 eV, respectively (see text S8 in the Supporting Information). Although the response process is comprised by many factors, it can be assumed that it corresponds to the adsorption energy (i.e., the activation energy for the adsorption of gas molecules). Similarly, it can be assumed that the activation energy for the recovery process corresponds to the desorption energy (i.e., the activation energy for the desorption of gas molecules). As the activation energy of the recovery process is less than that of the response process, we surmised that the desorption is more efficient than the adsorption in the present case. Yamazoe et al. for the first time reported a systematic study on the chemical and electronic effects of noble metals on resistive-based gas sensors.71,72 Previously, we reported the effects of noble metals on the sensing properties. We studied the effects of Au on the sensing performance of SnO2−ZnO core−shell NWs in terms of CO sensing.30 In addition, we employed the Pt functionalization of SnO2−ZnO core−shell (C−S) NWs to enhance the selective sensing of C7H8 gas.32 While we studied SnO2−ZnO C−S NWs as sensing materials in previous works, in this study we employed pristine ZnO NWs. Generally, the synthesis of ternary composites is not only difficult and requires different steps, but also their sensing mechanism is complex. Accordingly, control over the sensing parameters can be very difficult. However, this article considers a two-component system, that is, ZnO and Au. Therefore, its synthesis and sensing mechanism are much simpler than ternary systems, and this is very important for practical applications. In terms of chemical sensitization, Pt NPs enhanced the adsorption of C7H8 gas,32 whereas Au NPs promoted the adsorption of CO gas.38 In the present work, we employed Au NPs for CO sensing, additionally revealing that pristine NWs without Au NPs exhibit selective sensing to NO2 gas. Regarding electronic sensitization, we know that different sensing materials can generate different heterojunctions with metal NPs and changes the sensing performances. Even though both Au-functionalized SnO2−ZnO C−S NWs and Aufunctionalized ZnO NWs have Au−ZnO heterojunctions; the presence of SnO2−ZnO heterojunctions certainly will affect the sensing features. Moreover, in this work, we studied the gas sensing in the self-heating mode. Both Au-functionalized SnO2−ZnO C−S NWs and Pt-functionalized of SnO2−ZnO C−S NWs exhibited higher responses at an applied voltage of 20 V, without further optimization.32,38 At 20 V and 10 ppm, the Au- and Pt-functionalized SnO2−ZnO core−shell NWs exhibited responses of 1.40 and 1.85, respectively. However, in the present work, we optimized the applied voltage with regard to the CO sensing, obtaining a low voltage of 7 V for a high response of 2.90.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07208. Schematic illustration of the preparation of ZnO NWs and Au functionalization; I−V characteristics of pristine and Au-functionalized ZnO NW sensors between two adjacent electrodes; response of ZnO NW gas sensors at different temperatures; temperature of Au-functionalized gas sensors as a function of applied voltage; dynamic resistance plots of NW sensors; effect of RH on the response of NW sensors; SEM and TEM images exhibiting the interconnections between ZnO NWs; optimized voltage of self-heated gas sensors in previous literature studies; CO and NO2 gas sensing properties of different metal−oxide−semiconductors; preparation of Au NPs; effects of applied voltage on the EDL; explanation of sensing mechanisms based on UPS data; sensing behaviors of CO2 gas; possible mechanisms of Au-functionalized ZnO NW sensing for not responding to C2H5OH and C6H6 gases; calculation of limit of detection; and activation energies for adsorption and desorption (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected] (H.W.K.). *E-mail: [email protected] (S.S.K.). ORCID

Hyoun Woo Kim: 0000-0002-6118-9914 Author Contributions

S.S.K. and H.W.K. conceived the study and designed the experiments. S.S.K., H.W.K., and A.M. wrote the manuscript. J.-H.K. carried out the experiments. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1A2C1006193).

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4. CONCLUSIONS We synthesized pristine and Au-functionalized ZnO NWs. The ZnO NWs were prepared using a VLS growth method, and the Au functionalization was performed using sputtering, followed by a thermal treatment. Gas sensing tests under self-heating conditions were performed using different applied voltages. The pristine and Au-functionalized ZnO NW gas sensors exhibited their best performances under an applied voltage of 7 V, where they showed good responses to the NO2 and CO gases, respectively. In addition, they showed good selectivity toward either NO2 or CO gases. The present work demonstrates the possibility of realizing gas sensors with very

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