ZnO Core–Shell Nanorods

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Nanohybrids of Pt-Functionalized Al2O3/ZnO Core−Shell Nanorods for High-Performance MEMS-Based Acetylene Gas Sensor Vijay V. Kondalkar,† Le Thai Duy,‡ Hyungtak Seo,‡,§ and Keekeun Lee*,† †

Department of Electrical and Computer Engineering, ‡Department of Materials Science and Engineering, and §Department of Energy System Research, Ajou University, Suwon 16499, Republic of Korea

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S Supporting Information *

ABSTRACT: Metal oxide nanostructures are the most promising materials for the fabrication of advanced gas sensors. However, the main challenge of these gas sensors is humidity interference and issues related to the selectivity and high operating temperature, which limits their response in real-time applications. In this study, we proposed nanohybrids of Pt-functionalized Al2O3/ZnOcore−shell nanorods (NRs) for a real-time humidity-independent acetylene gas sensor. The core ZnO NRs have been fabricated on microelectromechanical system (MEMS) microheater, followed by a coating of a thin nanoscale moisture-blocking conformal Al2O3 shell by atomic layer deposition (ALD) and decoration of Pt NPs using photochemical deposition and e-beam evaporation. Prior to the fabrication, a COMSOL simulation was performed to optimize the microheater design and moisture-blocking layer thickness. A comparative study of the decoration of Pt NPs on the ZnO surface by photochemical (sPt/ZnO) and e-beam evaporation (e-Pt/ZnO) and a Al2O3 thin moisture-blocking shell layer (Pt/Al2O3/ZnO) in sensor response has been conducted. The fabricated sensors (s-Pt/ZnO) and (e-Pt/ZnO) showed a high response ΔR/R (%) of 96.46% and 68.15% to 200 ppm acetylene at 120 °C and detect trace concentrations of acetylene down to 1 ppm, but the response is influenced by humidity. Moreover, the sensor (Pt/Al2O3/ZnO) exhibited nearly the same sensing characteristics and high acetylene selectivity despite the wide range of humidity variation from 20% RH to 70% RH. The Pt-functionalized Al2O3/ZnOcore−shell NR-based sensor showed better sensing and stable performance than other sensors (s-Pt/ZnO and e-Pt/ ZnO) under humidity conditions. KEYWORDS: acetylene gas sensor; Al2O3/ZnO, Pt nanoparticles, humidity interference, microheater, selectivity

1. INTRODUCTION The emission of toxic and harmful gases due to the rapid development of the economy and industrialization have a significant impact on public safety, air quality and human health.1,2 Among other gas (SOx, NOx, and COX, etc.), hydrocarbons are widely used, and are, therefore, significant contributors to environmental pollution.3 One of the most used hydrocarbons in industry and automobiles is acetylene. Moreover, it is the most effective and versatile fuel for cutting and welding applications.4,5 However, it is a potent and harmful gas. In nature, acetylene is odorless, highly explosive and combustible, with a wide range about its explosive limit in the air: 2.4−83 vol %.6,7 Besides, the dissolved content of acetylene gas in power transformer oil is critical for the safety and reliability of transformer systems.8,9 Therefore, economical and portable acetylene gas sensors are essential to many applications. There are several traditional techniques for acetylene detection, such as gas chromatography (GC)10 or infrared spectral radiometry.11 However, these common measurement methods for acetylene have several drawbacks and require © XXXX American Chemical Society

costly equipment and strict working conditions. Over recent years, the development of effective techniques and sensitive methods for C2H2 gas detection such as photoacoustic spectroscopy12,13 optical fiber14,15 and metal-oxide semiconductors (MOS)16,17 has attracted significant attention. Among them, MOS is potentially applicable to sensors. The most representative materials for sensor and other applications are TiO2,18,19 SnO2,20 ZnO,21,22 NiO,23,24 MoO3,25 WO326 Co3O4,27,28 and Fe2O3.29,30 In particular, ZnO is a widely used material in gas sensors because of its high conductivity and good stability; in addition to its micro and nanostructures, it can be synthesized using versatile low-cost approaches with a wide range of morphologies. Until now, ZnO materials with different sizes and morphologies, including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D), have been successfully prepared.31,32 Moreover, 1D metaloxide (MO) nanostructures are preferable for gas sensors, Received: April 11, 2019 Accepted: July 1, 2019 Published: July 1, 2019 A

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

applications and the feasibility of the proposed methods for the development of other humidity-independent sensors.

mainly because of their high crystallization and surface-tovolume ratio, which have been proven to enhance the sensitivity and stability of sensor devices. However, pristine ZnO micro- and nanostructures (in addition to other pure metal oxides) are not ideal sensitive materials at low operating temperatures, and they are significantly affected by humidity and issues related to the selectivity. For practical applications, further studies on the improvement of the quality and characteristics of MO sensors are required to overcome the concerns related to the poor selectivity, high operating temperature (about 300−500 °C), and humidity dependence of the gas sensing characteristics. Concerning the chemical viewpoint, doping and surface functionalization are possible routes to achieve the selectivity of acetyleneand reduce the operating temperature of the devices. In general, the state-of-the-art MOS-based acetylene sensor with noble metal doping and/or surface functionalization, Schottky contacts, and hybridization with other MO forming nanoscale heterojunctions effectively lower the operating temperature from 500 to 200 °C.33,34 But still, the present operating temperature of the MOS sensor is relatively high compared to carbon-based materials (e.g., graphene, CNT, Carbon QD) sensor. The carbon-based material sensors work at moderately lower operating temperature, but they show very poor selectivity compared to MOS. For the various applications, the acetylene sensor should function at a lower operating temperature and excellent selectivity (e.g., to detect the C2H2 dissolved in the transformer oil) with a 1−2 ppm detection. Therefore, it is necessary to determine a strategy for the further minimization of the operating temperature of MO using a different structure doping/functionalization, etc. Moreover, humidity interference has been a significant challenge in the MO gas sensors performance and detection accuracy. Humidity induces the formation of low-reactive hydroxyl groups (Zn−OH) and therefore affects the number of reactions on the sensor channel. For the widespread applications of MO gas sensors, it is essential to overcome the humidity dependence due to high ambient moisture and continuously vary upon a change in climate and temperature.35 Considering the issues mentioned above, it is necessary to develop humidity-independent gas sensors with high sensitivity, good selectivity, operate at low temperatures (≤130 °C) is exceptionally critical for practical applications. To the best of our, there are no reports on the combination of various hybridization approaches such as the (i) decoration of metallic nanoparticles (NPs), (ii) doping, and (iii) the formation of a tunneling barrier for the optimization of the overall humidityindependent sensing performance of acetylene MEMS sensors based on Al2O3/ZnOcore−shell. In this study, we present the development of MEMS sensors with a combination of conformally coated Al2O3 ultrathin shell layer on ZnO NRs core, Pt NPs catalyst and chlorine (Cl) doping to lower the operating temperature and enhance the C2H2 selectivity and sensitivity under various humidity conditions (20%−70% RH). The sensors were fabricated with the integration of a microheater. From sensing measurements at different temperatures and humidity levels, the effectiveness of each combination was determined, such as the Pt-decoration with Cl-doping and with/without the Al2O3 on ZnO, to obtain a better performance of the C2H2 gas sensor at low operating temperature and high humidity levels. Moreover, the roles of each component are discussed in detail. Overall, this study confirms the effectiveness of the sensors in practical

2. OPTIMAL PARAMETERS AND ANALYTICAL MODELING 2.1. COMSOL Simulations for Microheater. The MEMS microheater was designed with optimized geometry to minimize the operating power of the gas sensor. COMSOL simulations were performed to predict the performance of the microheater device. The relationship between parameters for the heat generation at the microheater is given by eq 1. ΔT =

V 2t RCpm

(1)

Where ΔT is the temperature variation, V is the voltage, t is the time, R is the resistance, Cp is the specific heat, and m is the mass of substrate. As shown in eq 1, to obtain a high heat with low applied voltage, we need to design a device with low resistance that has a secure enough acceleration distance of the electrons through the relevant adjustment of the microheater configuration. Also, the rate of heat generation and heat dissipation to the surrounding materials (via thermal conductivity, radiation, and convection) should be wellmatched to maintain a constant temperature along the sensor surface for a long time at a fixed applied voltage. Figure 1 presents the COMSOL simulation results of the microheater with the four structures proposed in this study.

Figure 1. Optimization of microheater design with COMSOL simulation for optimal surface heat transfer.

In the case of type 1, two metal lines form the same meandering configuration together. Compared with type 2, a little bit higher temperature was observed at the same applied voltage, but the temperature stability over time was worsened by nonmatching of the heat generation rate and the heat dissipation rate due to the difficulty of the heat sink to the surrounding materials. In the cases of types 3 and 4, several meandering metal resistors are connected in parallel between two metal lines with different size and turns. Compared with type 2, a larger applied voltage was required to reach the same B

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

ultrapure water are the precursors for the formation of Al2O3. We chose Al2O3 because it and its precursor are low-cost and can be conformally deposited on arbitrary surfaces at low temperatures (200 °C or less). After 2-nm-thick Al2O3 layer was formed on ZnO NRs channel at the process temperature of 200 °C, the photochemical decoration of Pt on the samples was conducted, which we named as “s-Pt/Al2O3/ZnO”. The fabrication processes of the samples are illustrated in Figure 2. 3.3. Characterization. X-ray diffraction (XRD) results were obtained using a high-power X-ray diffractometer (Rigaku model Ultima III) with Cu−Kα radiation (λ = 1.54178 Å). Electron microscope images were captured using a field emission scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100F). The chemical composition and valence states of the sensing materials were analyzed using X-ray photoelectron spectroscopy (XPS, VG Multilab 2000Thermo Scientific, USA, K-Alpha). Gas sensing measurements were carried out using Keithley source meters, namely, a Keithley 2400 (for the sensor) and Keithley 2620A (for microheater). Schematic illustration of the sensing measurement system is given in Figure S1.

temperature at the sensor surface because the metal resistances were larger, and the electrons did not have sufficient acceleration distance to generate an efficient heat after collision with constituting atoms in the metal. In addition, thermal stability was not good for a long time because of the structural difficulty of heat dissipation. After all these analyses were considered, the simulation results revealed that the type 2 microheater demonstrated the best performance.

3. EXPERIMENTAL SECTION 3.1. Materials. All the chemicals were analytical reagent (AR) grade and purchased from Sigma−Aldrich: Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (98%), Hexamethylenetetramine (HMTA) (C 6H 12N4) (99%), Trimethylaluminum (TMA) (elec. grade, 99.99%), Silane (SiH4), NH3 (25%), Platinum(II) chloride (PtCl2) (99%), Pt metal (99.99%), and deionized water (18 MΩ). 3.2. Device Fabrication. 3.2.1. Integration of a Microheater and a Sensor IDT. The microheaters were fabricated using MEMS technology. First, a photoresist (PR) (AZ-5214E) was spin-coated onto a 4 in. quartz wafer, and then patterned with photolithography using the designed masks. A Ti/Pt electrode layer (thickness of 10 nm/100 nm, respectively) was deposited using an electron-beam evaporator, followed by a lift-off process. To form the insulating layer between the microheater and sensor, we deposited a 1.5-μm-thick layer of silicon nitride (Si3N4) on the entire upper surface of the microheater using plasma-enhanced chemical vapor deposition (PECVD). Finally, an IDT-type sensor using Ti/Pt electrodes was fabricated on the encapsulated microheater area. The structure of the sensor device is illustrated in Figure 2.

4. RESULTS AND DISCUSSION 4.1. Characterization of Sensing Materials. The surface morphologies of the ZnO, e-Pt/ZnO, s-Pt/ZnO, and s-Pt/ Al2O3/ZnO samples were examined using FESEM, as shown in Figure 3. Figure 3a−c reveal the highly networked volume of

Figure 2. Schematic fabrication process of a MEMS-based sensor device using Al2O3/ZnO NRs core−shell channel with catalytic Pt NPs for acetylene detection. 3.2.2. Growth of ZnO and Al2O3/ZnO NRs with the Decoration of Pt NPs. Initially, a seed layer of ZnO nanoparticles was deposited on the IDT sensor by sputtering (100 W, 3 min). The ZnO NR array was fabricated using a simple solution approach. The precursor solution contained Zn(NO3)2·6H2O (0.025 M) and HMTA (0.05 M). The device samples were placed vertically in the precursor solution at a temperature of 95 °C for 6 h. The samples were rinsed using deionized water and dried at 50 °C for 1 h in an oven, prior to the crack prevention in the insulation. After the growth of ZnO NRs, a Pt catalyst is typically decorated using two methods, namely, e-beam evaporation and photochemical deposition. With the use of e-beam evaporation, a 2 nm Pt is deposited on the ZnO NRs channel (rotating) at a rate of 0.1 nm/s. For the wet method, Pt is decorated by mixing PdCl2 (50 μL, 25 mM) with methanol(50 μL, pure) under ultraviolet (UV) light exposure for 5 min at room temperature. In this study, the samples decorated with Pt using the e-beam evaporation and solution approaches are indicated using the letters “e” and “s” (“ePt/ZnO” and “s-Pt/ZnO”), respectively. Regarding Al2O3/ZnO NRs, the samples with bare ZnO NRs were subjected to the atomic layer deposition (ALD) process. TMA and

Figure 3. Top and cross-sectional SEM images of the (a−c) ZnO, (d−f) e-Pt/ZnO, (g−i) s-Pt/ZnO, and (j−l) s-Pt/Al2O3/ZnO at low and high magnification.

the ZnO NRs. The typical diameters of the ZnO NRs were ∼100 nm, and the lengths were up to ∼1.7 μm. The FESEM image in Figure 3c reveals the level of entanglement of the ZnO NRs, which can facilitate the chemo-resistive electrical transport. The NR−NR junction provides the electrical pathway with a resistance that changes as a function of the density of the junctions and the type of the surrounding molecules. Figure 3d−f show ZnO NRs with the decoration of e-Pt NPs. Figure 3g−i present the FESEM results for s-Pt/ ZnO and reveal that the surface morphology of s-Pt/ZnO is very similar to that of e-Pt/ZnO. Here, we carried out FESEM C

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

of functional components, and that the components were only present on the surface of the ZnO NRs. Further TEM and HRTEM measurements are essential to evaluate the changes in the morphology and crystallization of the ZnO NRs with different functional components. To gain further insight into the morphology and crystallographic features of the e-Pt/ZnO, s-Pt/ZnO, and s-Pt/Al2O3/ ZnO NRs, we carried out TEM and HRTEM analyses and shown in Figure 5 and Figures S2 and S3. As seen in these

measurements and comparison at various magnifications to see changes in the morphology of ZnO NRs with different components. Unfortunately, there is no obvious difference between them. It likely proposes that s-Pt and e-Pt NPs on the ZnO NRs network are tiny. Besides, the similarity in ZnO NRs density may hint that there would be no significant difference from the interconnected-ZnO charge transport pathways in our samples. Accordingly, the sensing comparison between ZnO-based samples to understand the contribution of each modifier component would be more dependable. Figure 3j−k present the typical microstructure of the s-Pt/Al2O3/ZnO NRs. The FESEM image reveals that the networked nature of ZnO NRs was maintained after the formation of the Al2O3 shell layer; however, Pt NPs are not clearly visible. The overall FESEM results show a very slight difference in the surface morphology of the ZnO NRs after each functionalization. The synthesized materials were characterized using XRD, TEM, and high-resolution TEM to further investigate the changes in the crystallization and doping and/or loading concentration. XRD investigated the phase purity and crystallographic structure of ZnO, e-Pt/ZnO, s-Pt/ZnO, and s-Pt/Al2O3/ZnO. Typical XRD patterns for all the samples are presented in Figure 4. The XRD patterns, which consist of significant

Figure 5. TEM micrographs of the s-Pt/Al2O3/ZnO NRs: (a−e) TEM at different magnifications, (f, g) HRTEM, (h) SAED, (i−l) elemental mapping.

supporting figures, the TEM image of e-Pt/ZnO importantly reveals that only the surface of the ZnO NRs exposed to ebeam evaporation was fully covered with Pt NPs. Besides, aggregation of the e-Pt NPs one side edge of ZnO NRs was observed. The TEM and HRTEM images of the s-Pt/ZnO NRs display a high density of Pt NPs with small sizes, which are uniformly anchored on the surface of the ZnO NRs. The uniform decoration of s-Pt NPs increases the Pt/ZnO junction and provides the large active catalytic surface area. Figure 5a−e shows the TEM image of s-Pt/Al2O3/ZnO NRs in which small Pt NPs uniformly anchored on the NR surface. Figure 5e presents the typical core−shell structure of Al2O3/ZnO NRs with the shell thickness of 2 nm, confirming the successful formation of a conformal Al2O3 shell on the ZnO NR cores through ALD process. Figure 5f−g demonstrates well-resolved lattice fringe spacing’s of 2.59 and 2.23 Å, which are attributed to the (002) and (111) planes of ZnO and Pt, respectively. The HRTEM image in Figure 5g reveals that the Pt NPs were spherical in shape, and the average particle size was in the range of 3−5 nm. The corresponding SAED image was shown in Figure 5h. The EDS mapping in Figure 5i−l reveals the presence of Pt, Zn, Al, Pt, and O. Moreover, these TEM and HRTEM results were in good agreement with the FESEM and XRD data regarding (i) the single-crystalline ZnO NR networks preserved after the conformal Al2O3 deposition and (ii) the uniform decoration of Pt NPs on the Al2O3/ZnO NR surface. XPS analysis was performed to determine the chemical composition of the Pt/ZnO and Pt/Al2O3/ZnO samples as shown in Figure 6. In the XPS survey spectra (Figure 6a), the main peaks of Pt, Zn, and O were observed in addition to the peaks of Al, Cl, and C. The Al peak belonged to Pt/Al2O3/

Figure 4. X-ray diffraction patterns of the ZnO, e-Pt/ZnO, s-Pt/ZnO, and s-Pt/Al2O3/ZnO.

diffraction peaks at 2θ = 31.73, 34.45, 36.27, 47.56, and 62.93, can be attributed to the (100), (002), (101), (102), and (103) crystal planes of the hexagonal crystal structure of ZnO, respectively. Moreover, the results are in good agreement with the standard data for ZnO (JCPDS no. 36−145). The significant enhancement of the intensity of the (002) plane, when compared with other planes, suggests that ZnO NR arrays grow along the c-axis in the (002) direction, which indicates that the single crystalline nature of the ZnO NRs. The single-crystalline materials can significantly reduce the charge transfer resistance during targeted gas reactions with chemo-absorbed oxygen and improve the response and recovery. No peaks corresponding to Pt and Al2O3 were detected, which was probably due to the relatively low Pt and Al2O3 contents as compared with those of ZnO. Furthermore, no shifts in the diffraction peaks of ZnO were observed, which confirms that Pt NPs are deposited on the surface of ZnO NRs and Al2O3/ZnO NRs by the formation of Pt/ZnO, Pt/Al2O3/ ZnO nanocomposites, instead of Pt atom substitutes, for Zn2+ or Al2+ as an interstitial atom. The XRD patterns of all the samples were nearly the same, which indicates the low loading D

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. XPS spectra of Pt/ZnO and Pt/Al2O3/ZnO samples. (a) Wide-scan survey spectra. (b−f) narrow-scan spectra of Zn 2p, Al 2p, Pt 4f, O 1s, and Cl 2p core levels, respectively.

chemisorbed oxygen, dissociated oxygen, or OH− groups on the surface. A shift in the O 1s peaks of Pt/Al2O3/ZnO (from 530 to 532.3 eV) was also observed. This O 1s peak shift implies that O2− ions on the surface of ZnO were replaced with that of Al2O3. On the other hand, the similarity in the positions of Zn 2p and O 1s peaks of e-Pt/ZnO and s-Pt/ZnO implies that the structure of ZnO was almost preserved after solution decoration of Pt NPs. Thereby, it confirmed that Cl anions of PtCl2 caused no damage to the ZnO NR structure. In Figure 6f, another small difference was detected concerning the trace of Cl in Pt/ZnO and Pt/Al2O3/ZnO. The amount of Cldoping in Al2O3 (1.2 at%) was lower than that in ZnO (4.6 at %), which was probably due to the blocking effect of the Al2O3 layer. Overall, the surface chemical composition of Pt/Al2O3/ ZnO was slightly different from that of Pt/ZnO. 4.2. Microheater Performance. Initially, the temperature sensor is used to determine the surface temperature changes with the change in temperature of microheater. Figure 7a, b shows the top and cross-sectional SEM image of a type 2 microheater integrated with a temperature sensor. Figure 7c showed temperature variations with respect to applied DC voltage (10, 15, 20 V) over time. Moreover, the operating temperature was kept constant on the surface according to the applied voltage, and the temperature increased linearly with the applied voltage. Further, temperature variation on the surface of the microheater according to a surface position was analyzed at three regions is given in Supporting Information (Figure S4) and which confirms the difference in temperature transferred to the surface is within 1−2 °C. The temperature variations were also confirmed by an infrared thermometer as shown in Figure 7d, in which thermal energy is evenly distributed over the surface of the sensor. The measured values were almost the same as the calculated surface temperature. The fabricated microheater decreased the power consumption significantly and lowered the operating temperature which substantially increases the lifetime of the sensor. Moreover, personal and environmental safety is a significant concern; the development of a low power consumable microheater and sensor is essential.

ZnO, whereas the trace of C was assigned to adventitious carbon. Moreover, the Cl peak may be due to the PtCl2 solution during the formation of Pt NPs. From the sensing measurements, a positive influence of the Cl residue (i.e., Cldoping) on the sensor performance was observed, which is discussed in the following section. Figure 6b presents the XPS peak of the Zn 2p core level. It consisted of spin−orbit doublet peaks at 1021.8 and 1044.03 eV, which correspond to Zn 2p3/2 and Zn 2p1/2, respectively. The Zn 2p core level spectra of ePt/ZnO and s-Pt/ZnO were similar. Compared with Pt/ZnO, the Zn 2p peaks of Pt/Al 2 O 3 /ZnO were shifted by approximately 0.61 eV toward higher energy. This peak shift may be due to (i) passivation of Zn interstitials and dangling bonds, in addition to (ii) the difference in the electronegativities of Zn and Al. The interaction between ZnO and Al2O3 was also observed from the Al 2p narrow-scan spectrum of Pt/Al2O3/ZnO. Figure 6c displays the Al 2p core level spectrum, which exhibits two peaks at 71.19 and 75.18 eV, corresponding to Al2P3/2 and Al 2p1/2, respectively. Compared with pure Al2O3, a positive shift in the Al 2p peak of Pt/Al2O3/ ZnO was observed. This shift was probably due to the decreased electron density around Al in the Zn−O−Al. The high-resolution Pt 4f core level spectra of Pt/ZnO and Pt/ Al2O3/ZnO were recorded to obtain more information on the oxidation states and chemical structure of Pt as shown in Figure 6d. The Pt 4f spectrum of Pt/ZnO has two components nearly at 70.62 and 73.92 eV, which correspond to Pt 4f7/2 and Pt 4f5/2 metallic platinum, respectively. There were gradual shifts of Pt 4f peak (toward the higher binding energy at ∼0.68 eV) observed for the Pt/Al2O3/ZnO sample. This may be due to the (i) Pt−Al bond and (ii) the change in the Pt electronic structure toward oxygen (dissociated on Pt).36 This new state has a high electron density at the Fermi level. The O 1s core level spectra for Pt/ZnO and Pt/Al2O3/ZnO (Figure 6e) displays a relatively broad peak, as it is associated with different types of bonds. The main component at 530 eV is attributed to the O2− ions surrounded by Zn in the wurtzite ZnO structure. The second component at 531.70 eV is generally attributed to E

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

as ΔR/R (%) = 100(Rgas − Ro)/Ro; in which Ro and Rg are the device resistances in dry air and during gas exposure. For the ePt/ZnO sensor, its response increased from 10.17% (at 90 °C) to 68.15% (at 180 °C), whereas its recovery reached an almost optimum value at 150 °C. In case of the s-Pt/ZnO sensor, it achieved a good recovery and the maximum response (96.46%) at 120 °C, which is superior to those of the e-Pt/ ZnO NRs at the same temperature. The dynamic sensing response−recovery behavior of the s-Pt/ZnO NRs to acetylene gas (1 to 20 ppm) at optimum operating temperature (120 °C) is given in Figure S5. The sensor could exhibit efficient sensor response (∼11.38%) even in the presence of 1 ppm acetylene concentration. Moreover, the possible reasons for a decrease in the operating temperature and an increase in the response for the s-Pt/ZnO NRs are the uniform decoration of Pt NPs and residual Cl molecules. As shown in the TEM e-Pt NPs, are partially deposited on ZnO NRs and aggregated on the top surface of ZnO NRs. In contrast, the s-Pt NPs are decorated uniformly on the ZnO NRs, which led to an increase in the number of Pt/ZnO junctions. Consequently, there are more reaction sites between the gas molecules with the Pt and ZnO NRs. The second reason is related to the residual chlorine molecules, which act as strong electron acceptors and enable ZnO NRs to receive more electrons donated from the hydrogen molecules of C2H2, resulting in a response at lower operating temperature. This is discussed later in more detail in the sensing mechanism section. In general, both Pt/ZnO sensors exhibited high sensitivity to acetylene at temperatures below the autoignition temperature of acetylene (305 °C). The s-Pt/ZnO device showed a lower operating performance when compared to the e-Pt/ZnO device; its response (∼96.46%) was significantly higher. Therefore, the proposed approach using solution processed s-Pt/ZnO NRs has significant potential for C2H2 sensor applications. 4.3.2. Humidity Effect on Acetylene Sensors. Humidity can affect the sensor response in a similar way as a reducing gas, which interferes with the gas detection process. At low operating temperature, the moisture may have a more significant effect. Therefore, for practical applications, it is essential to test the humidity effect on s-Pt/ZnO sensors. The device response toward 20 ppm of acetylene under various humidity levels (RH) at 120 °C was evaluated. The ratio of the sensor responses under dry and humid conditions was calculated to quantify the effect of humidity on the device sensing signal, i.e., the response ratio, which is defined as Shumid/Sdry or S/S (= ΔR/R(humid)/ΔR/R(dry)). In Figure 9a, the s-Pt/ZnO device shows relatively low S/S values: S/S = 0.64 at an RH of 50% and 0.54 at an RH of 70%. Thus, the s-Pt/ZnO NRs sensor is significantly influenced by the presence of humidity, which results in a reduced gas response and poor recovery. It is probably due to the formation of OH radicals on the surface of the ZnO NRs by the water-poisoning reaction, as expressed by eq 2.

Figure 7. SEM image of microheater and temperature sensor (a) top view; (b) cross-sectional view; (c) surface temperature at the microheater when 10, 15, and 20 V DC voltage applied; and (d) infrared thermometer view when the voltage was applied.

4.3. Gas-Sensing Characteristics. 4.3.1. Working Temperature of Sensors. It is well-known that at room temperature, semiconductor-based hydrocarbon sensors have prolonged responses, sluggish recovery to the baseline, and they cannot produce reliable sensing outputs upon exposure to low concentrations of target gas molecules. In the view of the attractive and repulsive forces between atoms, physical interactions (e.g., doping and charge induction) usually occur earlier and thus contribute mainly to the rapid response region. Chemical interactions, which occur later and more slowly due to relating to decomposing and recomposing of molecules, are the main contribution to the slow response region. As an incomplete reversion of chemical interactions in the desired period, the devices exhibited baseline drifts.37,38 Therefore, it is necessary to optimize the operating temperature of the gas sensor. The both e-Pt/ZnO and s-Pt/ZnO sensors were exposed to 200 ppm of acetylene at various temperatures (ranging from room temperature to 180 °C) to evaluate the temperature dependency as shown in Figure 8a, b. Here, we

Figure 8. Dynamic response transitions toward 200 ppm acetylene at different temperature, (a) e-Pt/ZnO NRs sensor, (b) s-Pt/ZnO NRs sensor.

fixed exposure and purging times (10/10 min) for comparison the gas detectability of our materials, especially the exposure time to reach the response saturation and the magnitude of the response. They are the key factors indicating the response speed or sensitivity of the sensors. At room temperature, the response of the e-Pt/ZnO and sPt/ZnO sensors to acetylene was negligible. When the operating temperature was increased (≥90 °C), the responses gradually increased. In this study, the gas response was defined

2− 2Znlat + O−ad or Oad + H 2O

→ 2(Znlat−OH) + e− (or 2e−)

(2)

Consequently, the number of adsorbed oxygen species (O2−, O−, or O2−) decreases, which deteriorates the gas response. Thus, humidity can hamper the implementation of the s-Pt/ ZnO sensor applications. The use of the conformal coating of an Al2O3 shell on the core ZnO NRs was considered to obtain F

DOI: 10.1021/acsami.9b06338 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

protection layer for the ZnO NRs-based sensors, their dynamic responses and selectivity’s were measured at 120 °C. The dynamic responses of the s-Pt/Al2O3/ZnO sensor toward acetylene concentrations ranging from 1−20 ppm was measured. As can be seen in Figure 10a, the response

Figure 9. (a) e-Pt/ZnO NRs sensor performance toward 20 ppm in different humidity level, (b) simulation prediction of humidity diffusion in the presence of different thickness of Al2O3 layer on ZnO. s-Pt/Al2O3/ZnO NRs sensors performance toward acetylene at in different humidity level of (c) 20 ppm, (d) 1 ppm. Figure 10. s-Pt/Al2O3/ZnO NRs sensor performance toward acetylene: (a) dynamic response-recovery behavior at 120 °C toward 20 to 1 ppm, (b) calibration curve of sensors in 1−20 ppm, and (c) selectivity of the sensor toward different gas.

a reliable and humidity-independent acetylene sensor. The COMSOL simulations were performed to predict the humidity effect on the Al2O3-coated ZnO NRs as shown in Figure 9b. The background material in this simulation system was air, whereas the moisture was placed on the top surface of the Pt/ ZnO and Pt/Al2O3/ZnO. The diffusion rate was higher for the Pt/ZnO NRs than for the Pt/Al2O3/ZnO NRs. Moreover, Pt was in an outermost surface of Al2O3/ZnOcore−shell NRs because the water poisoning reaction has an insignificant effect on Pt and, we wanted to ensure its catalytic role. From the simulation results, Pt/Al2O3/ZnO exhibited better stability under high humidity conditions. The conformal Al2 O 3 protection layer lowered the penetration of moisture and thus accelerated its desorption process. The sensing results of s-Pt/Al2O3/ZnO at different humidity were shown in Figure 9c, d. As expected, the Pt/Al2O3/ZnO device responded to 20 ppm of acetylene, and it was a little influenced by humidity (Figure 9c). Moreover, its S/S values were approximately 0.94 at an RH of 50% and 0.86 at an RH of 70%, which is significantly better than that of the s-Pt/ZnO sensor, as shown in Figure 9a. However, its acetylene response was slightly slower than that of the s-Pt/ZnO sensor under the dry condition. The optimum thickness of the Al2O3 film was 2 nm, which increased the sensor stability and slightly degraded the acetylene sensitivity and response. Although the s-Pt/ Al2O3/ZnO devices responded well to 1 ppm of acetylene, as shown in Figure 9d, the responses were affected by humidity, as the gas concentration was minimal in comparison with the RH of 50 and 70%. Furthermore, the results confirmed that the conformal Al2O3 coating layer significantly minimized the water-poisoning reaction on the surface of the ZnO NRs. For further improvement of the sensor performance concerning the humidity effect, optimization of the Pt NPs density and Al2O3 thickness, or deposition of ultrathin protective oxides should be carried out. 4.3.3. Acetylene Sensing Performance of s-Pt/Al2O3/ZnO Sensor. To better understand the device performance, in addition to the advantages and disadvantages of using an Al2O3

amplitude of s-Pt/Al2O3/ZnO increased with an increase in the gas concentration. However, there was a sudden increase in the gas response toward 20 ppm of acetylene. The results confirm the effective gas blocking characteristics of the 2 nm thick Al2O3 layer. Besides, the s-Pt/Al2O3/ZnO device demonstrated response of approximately ∼6.70% to 1 ppm of acetylene, and its recovery was quite good in comparison with that of the s-Pt/ZnO device. Figure 10b presents the fitted sensing data of s-Pt/ZnO and s-Pt/Al2O3/ZnO. As shown, the sensitivity of s-Pt/ZnO was better than that of s-Pt/ Al2O3/ZnO. However, both the sensors exhibited an excellent linear response to low acetylene concentrations ranging from 1−20 ppm with high sensitivities. For s-Pt/Al2O3/ZnO, its sensitivity was approximately 2.87%/ppm and 0.986 linearity. Further, the comparison of the sensor performances with other sensors reported in the literature is given in Table 1. With respect to selectivity, the response magnitudes of the s-Pt/ZnO and s-Pt/Al2O3/ZnO devices to C2H2 (20 ppm), NH3 (20 ppm), NO (20 ppm), CH4(0.1%), and C2H2OH (20%) at 120 °C were compared as shown in Figure 10c. In general, they responded similarly to C2H2, and their acetylene responses (∼67% at 20 ppm) were the highest among the tested gases, which indicates good selectivity to acetylene at 120 °C. Their selectivity can be attributed to (i) the highly selective absorption capability of the Pt catalyst to C2H2 molecules, and (ii) the smaller bond energy of H−CC−H (490 kJ mol−1). From a comparison of the two sensors, although they had similar response magnitudes and selectivity’s to acetylene, the responses of s-Pt/Al2O3/ZnO to other tested gases were slightly lower. The 2 nm Al2O3 layer retained the gas molecules on the surface of the NRs, and therefore prolonged their diffusion rates through Al2O3 to ZnO. This confirms that the selective gas blocking behavior of Al2O3 is dependent on the G

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ACS Applied Materials & Interfaces Table 1. Comparison of the Sensing Performances of Various Acetylene Sensors materials NiO/SnO2 Ag-ZnO NWs Au-ZnO ZnO/rGO ZnO Mesoporous Sm2O3/SnO2 NP WO3 microstructures Ni-ZnO Ag-ZnO-Gr hybrid Pt functionalized Al2O3/ZnO core− shell

response @gas exposure (Rg/Ra or Ra/Rg) (%)

OT (°C)

LOD (ppm)

13.8 @ 100 ppm 30.8 @ 1000 ppm 311.3 @ 100p pm 18.2 @ 100 ppm 101.1 @ 100 ppm 63.8 @ 1000 ppm 58.53 @ 200 ppm 6 @ 5 ppm 21.19 @ 100 ppm 17.6 @ 100 ppm (43%RH) 97.46 @ 200 ppm 67.67 @ 20 ppm 65.02 @ 20 ppm (50%RH) 59.87 @ 20 ppm (70% RH)

206 220 183.5 250 400 180 300 250 150

1 1 1 10 1

120

3 1

humidity effect studied (RH%)

ref

no no no yes (humidity-dependent) no yes (humidity-dependent) no no yes (humidity-dependent above 31% RH)

42

yes (humidity-independent up to 70% RH)

This work

43 44 22 6 45 46 47 34

Figure 11. Schematic illustration of the sensing mechanism of s-Pt/ZnO and s-Pt/Al2O3/ZnO upon exposure to ambient air and C2H2 gas at temperature 120 °C.

molecule size. Overall, the obtained results validated the effectiveness of the sensors for the selective detection of acetylene with a detection limit of 1 ppm at 120 °C in the air. 4.4. Sensing Mechanism. To clary the detailed mechanism of the sensors in this study, we fabricated ZnObased thin film transistors (TFTs) to check the charge transfer characteristics of ZnO with other components.39,40 The details of device fabrication and analysis are described in Supporting Information (Figure.S6). Here, we briefly discussed the sensing mechanism by using the conductivity-type diagrams as presented in Figure 11. For s-Pt/ZnO, there are three sites for the gas reactions to take place, namely, the ZnO surface, Cl-doped ZnO surface, and Pt/ZnO junction, as shown in the

panel in the left-hand side of Figure 11. It is well-known that ZnO is generally an n-type semiconductor. The Cl anions occupied oxygen vacancies of ZnO surface and thus acted as an electron acceptor which decrease the electron concentration in ZnO. Owing to the work function difference between Pt NPs (5.3 eV) and ZnO (4.45 eV), a Schottky contact is formed at their interfaces due to the transfer of electrons from the ZnO conduction band to Pt. It leads to an upward bending of the ZnO energy band that locally contributes to the n-type nature of ZnO, as shown in the panel in the left-hand side of Figure 11. Furthermore, Pt is known as a chemical sensitizer by its “spillover effects” and catalytic effect. It can catalytically dissociate oxygen molecules into oxygen species (O2−, O−, or H

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ACS Applied Materials & Interfaces O2−). However, the oxygen ions O2−, O−, O2− are stable in the following range below 130 °C, 130 to 300 °C, and above 300 °C, respectively.41 Therefore, most of the gas reactions in this study (at 120 °C) were with O2−. The results agreed with the O 1s XPS spectra (Figure 6e). When the sensor surface was exposed to the air, due to the presence of oxygen species, a low-conductivity depletion region was formed near the ZnO surface, which could narrow the conduction channels at the NR−NR junctions. Upon exposure to a reducing gas (acetylene in this study), its molecules reacted and desorbed the active oxygen ions. Moreover, Al2O3 is an insulator; however, it exhibits p-type behavior. Thus, it can cause a decrease in the electron concentration of ZnO at the interface of Al2O3/ZnO. Although the work function of Al2O3 is higher. However, its thickness is negligible in comparison with that of ZnO. Hence, the charge transport (tunneling) from ZnO through Al2O3 to Pt is almost the same as in the case of Pt/ ZnO. Upon exposure to acetylene, the ZnO Fermi level in Pt/ Al2O3/ZnO changed similarly to that in Pt/ZnO, as presented in Sections (i) Humidity effect and (ii) Acetylene sensing performance. However, the most significant occurrence is the charge redistribution due to Cl doping. In the case of e-Pt/ ZnO, when the increased Fermi level of ZnO in air approached its conduction band, the response entered the saturation state. With Cl-doping, the charge redistribution between n-doped ZnO (Pt) and Cl-doped ZnO areas limited the response saturation, which led to more significant changes in the resistance of s-Pt/ZnO than that of e-Pt/ZnO. Concerning the selectivity, it is closely related to the energy level of the oxide surfaces, which has two possible causes. First, the dissociation or decomposition of gases at 120 °C is different. Among them, acetylene has many π bonds, which are significantly more reactive than those of the other tested gases. The second reason is based on the property of oxidized materials to react with reducing gases.

environmental monitoring with high accuracy and convenience.

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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.9b06338. Additional characterization and figures including sensing measurement setup, TEM and HRTEM micrograph of e-Pt/ZnO and s-Pt/ZnO, microheater performance, cyclic gas response of s-Pt/ZnO, and fabrication and characterization of thin film transistors (TFTs) based on ZnO NRs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Hyungtak Seo: 0000-0001-9485-6405 Keekeun Lee: 0000-0001-9472-0209 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant number: 20172220200110).

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REFERENCES

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5. CONCLUSIONS In summary, we have designed and fabricated nanohybrids of Pt-functionalized Al2O3/ZnO core−shell NRs for MEMScompatible high-performance humidity-independent acetylene gas sensor. COMSOL simulations were performed to predict the microheater design and optimize the conformal coating of Al2O3 thickness on the surface of ZnO to minimize the humidity interference. The improved gas sensing performance is attributed to the photochemical sensitization of Pt NPs, due to the uniform anchoring of Pt NPs on the ZnO and Al2O3/ ZnO core−shell NRs when compared with e-beam Pt functionalized. Although the Pt/ZnO sensor showed high response and selectivity to acetylene, it is the gas response was significantly worsening by humidity because of the waterpoisoning reaction. In contrast, in the Pt/Al2O3/ZnO core− shell, the NR sensor gas response remained nearly the same despite the wide variation in humidity from 20% RH to 70% RH. These humidity-independent gas-sensing characteristics were achieved by minimizing water-poisoning reaction and maintaining the concentration of the adsorbed oxygen species on the ZnO by a conformal coating of 2 nm Al2O3 shell. The sensor comprising Pt/Al2O3/ZnO core−shell NRs exhibited a high response to acetylene in a humid environment with high sensitivity of 2.87%/ppm and 0.986 linearity. The results confirm that Pt-functionalized Al2O3/ZnO core−shell NRbased sensor can provide a fully air-stable real-time acetylene sensor with high sensitivity, selectivity, and repeatability for I

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