Micropatternable Double-Faced ZnO Nanoflowers ... - ACS Publications

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Micropatternable Double-Faced ZnO Nanoflowers for Flexible Gas Sensor Jong-Woo Kim,† Yoann Porte,† Kyung Yong Ko,‡ Hyungjun Kim,‡ and Jae-Min Myoung*,† †

Department of Materials Science and Engineering and ‡School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Soedaemun-gu, Seoul 03722, Republic of Korea S Supporting Information *

ABSTRACT: Micropatternable double-faced (DF) zinc oxide (ZnO) nanoflowers (NFs) for flexible gas sensors have been successfully fabricated on a polyimide (PI) substrate with singlewalled carbon nanotubes (SWCNTs) as electrode. The fabricated sensor comprises ZnO nanoshells laid out on a PI substrate at regular intervals, on which ZnO nanorods (NRs) were grown inand outside the shells to maximize the surface area and form a connected network. This three-dimensional network structure possesses multiple gas diffusion channels and the micropatterned island structure allows the stability of the flexible devices to be enhanced by dispersing the strain into the empty spaces of the substrate. Moreover, the micropatterning technique on a flexible substrate enables highly integrated nanodevices to be fabricated. The SWCNTs were chosen as the electrode for their flexibility and the Schottky barrier they form with ZnO, improving the sensing performance. The devices exhibited high selectivity toward NO2 as well as outstanding sensing characteristics with a stable response of 218.1, fast rising and decay times of 25.0 and 14.1 s, respectively, and percent recovery greater than 98% upon NO2 exposure. The superior sensing properties arose from a combination of high surface area, numerous active junction points, donor point defects in the ZnO NRs, and the use of the SWCNT electrode. Furthermore, the DF-ZnO NF gas sensor showed sustainable mechanical stability. Despite the physical degradation observed, the devices still demonstrated outstanding sensing characteristics after 10 000 bending cycles at a curvature radius of 5 mm. KEYWORDS: NO2 gas sensor, ZnO nanoflower, SWCNT, micropatterned array, network-structured flexible device

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INTRODUCTION With the development of technologies such as automation, virtual reality, and augmented reality, the demand for accurate and reliable sensor technology is expanding. Among the various types of sensors, gas sensors have attracted considerable attention due to their role in monitoring of the atmospheric environment and in human health for medical diagnosis, as well as in the detection of pollutants and toxic gases such as nitrogen dioxide (NO2), ammonia (NH3), carbon monoxide (CO), and sulfur dioxide.1−3 Especially, NO2 is the most common air pollutant produced by fossil-fuel combustion and vehicle emissions. Recently, it has been identified as a major component of fine particulate matter, which is a global issue, and efforts are underway to reduce the amount of its emission.4 In addition to environmental problems, NO2 causes skin damage even on very small amounts of exposure, and critical respiratory disorder when exposed to about 50 parts per million (ppm).5 Therefore, it is necessary to develop an improved gas sensor using various techniques and materials for detecting traces of NO2. Metal-oxide semiconductor-based gas sensors have been widely investigated owing to their remarkable sensing proper© 2017 American Chemical Society

ties as well as simple sensing mechanism relying on changes in resistivity.6 Moreover, in the past few years, with the recent advances in nanotechnology, research has been focused on the use of nanomaterials with high surface-to-volume ratios, which highly affected the characteristics of gas sensors.7 Among the metal-oxides, zinc oxide (ZnO) can be easily synthesized as nanomaterials such as nanobelts, nanorods (NRs), and nanowires using a low-temperature solution process.8,9 These ZnO nanomaterials have already been applied to sensing of various gases including hydrogen, NO2, NH3, CO, and ethanol due to their excellent properties, that is, high electrochemical and environmental stability, high crystallinity, low cost, no toxicity, easy synthesis, and large specific surface area.10−12 Despite these advantages, ZnO-based gas sensors generally operate at high temperatures, and their sensing performances still need to be improved in terms of response, saturation behavior, characteristic times, recovery, and selectivity.13 Because most of the conventional ZnO-based gas sensors Received: June 27, 2017 Accepted: September 8, 2017 Published: September 8, 2017 32876

DOI: 10.1021/acsami.7b09251 ACS Appl. Mater. Interfaces 2017, 9, 32876−32886

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the fabrication process for the DF-ZnO NF network structure gas sensor.

both sides of the shells form a myriad of junctions and develop the double-faced (DF) ZnO nanoflower (NF) network structure. Furthermore, single-walled carbon nanotubes (SWCNTs) were applied as the electrode to improve the flexibility and form a Schottky barrier with ZnO to enhance the gas sensor performance. This highly integrated DF-ZnO NF gas sensor exhibits outstanding sensing characteristics, especially in decay time, recovery, and selectivity, which are a problem in ZnO-based gas sensors. Moreover, the devices were still operating without significant degradation in performance in terms of response, characteristic times, and recovery, even after dynamic bending tests of 10000 cycles at a curvature radius of 5 mm.

generally have cross-sensitivity to various gases, it is necessary to develop highly selective sensors with improved gas response.14 Recently, various studies have been carried out to improve the characteristics of ZnO-based gas sensors such as metal-assisted functionalization, application of porous morphology materials, and structural modulation with multiple junctions by tetrapod structure and nanobrunchs.15−17 These techniques allowed the performances of the sensors to be improved through the effects of large effective area, which induces multiple gas diffusion channels and tunes the concentration of charge carriers.18 However, the high temperature fabrication process and the post-treatments remain critical for their use in industry as they increase the process costs. Moreover, the structures using porous nanomaterials or multiple junctions have limited application in the field of flexible devices due to their fragile geometry and random arrangement of nanomaterials.15−17 Moreover, despite advances in nanotechnology, there still exists a problem of evaluating the properties of nanomaterials and fabricating highly integrated nanodevices.19 With the demand for more compact devices, as well as the need for large area application, accurate position control technology of nanomaterials such as vertical stacking or self-assembly techniques is essential.20 In a recent study, a rubbing technique for precise alignment of fine particles was investigated. This rubbing technique facilitates accurate positioning and assembly of microparticles (MPs) in a short time without additional solvent or post- and surface-treatments.21 The precise alignment of various functional particles can be controlled by using the rubbing technique, thereby realizing a highly integrated device. In this study, we have developed a highly integrated micropatternable ZnO gas sensor with improved sensing properties and stable characteristics under flexibility tests. To maximize the surface area of the nanomaterials, ZnO NRs were grown in- and outside ZnO shells using a low-temperature solution process. The shells were arranged in an island shape using the rubbing technique to relax the strain of the device and ensure stable mechanical characteristics. The NRs grown on

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RESULTS AND DISCUSSION Figure 1 schematically describes the fabrication steps of the micropatternable DF-ZnO NF gas sensor. Firstly, as a template for the DF-ZnO NFs, polystyrene MPs with an average diameter of 5 μm were transferred by a poly(dimethylsiloxane) (PDMS)-transfer technique on a circular well-patterned glass substrate with a diameter of 5 μm and a height of 3 μm prepared by photolithography. Then, the MPs were aligned by being pushed into the circular well patterns by the unidirectional PDMS rubbing technique. This technique is a simple method to align particles in a short time without any solvent or treatment. It facilitates the precise alignment of MPs over spinor spray-coating.21 Next, 100 nm thick ZnO films were deposited on the MP array by using radiofrequency magnetron sputtering. Then, the ZnO-coated MPs were transferred onto a poly(vinyl phenol) (PVP) adhesive-coated polyimide (PI) substrate. This was followed by calcination at 250 °C for 3 h in air on a hot plate to improve the crystallinity of the ZnO films and remove the polystyrene to obtain a high surface area to volume ratio. After removal of the polystyrene MPs, the remaining bowl-shaped ZnO shells possess a large surface area, which can be enhanced further by growing ZnO NRs on their surfaces. Following the growth of ZnO NRs on the in- and outside surfaces of the shells by using the low-temperature aqueous solution process, micropatterned DF-ZnO NF net32877


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Figure 2. (a) Top-view SEM image of MP array in circular well patterns. Inset: Cross-sectional SEM image showing MP array in height-optimized well patterns. (b) Tilted-view SEM image of aligned ZnO shells after transfer and calcination at 250 °C for 3 h in air on a hot plate. Inset: Top-view SEM image showing polystyrene-removed ZnO shells after calcination. (c) SEM image of DF-ZnO NF network structure after growth of ZnO NRs in- and outside the shells, and (d) magnified SEM image of the junctions between NRs.

of micropatterned MPs in circular well patterns using the PDMS rubbing technique. The optimum height of the well pattern was optimized to stop the MPs from getting out of the wells, and was identified to be about 0.6D (D: particle diameter, 5 μm), as shown in the inset of Figure 2a. Optical microscopy (OM) images before and after placing the MPs in the wells are shown in Figure S3a. After transfer and calcination, the polystyrene was completely removed, leaving only the aligned ZnO shells with large surface area on which the ZnO NRs could grow, as shown in the inset of Figure 2b. Figure 2b shows the aligned ZnO shells partially embedded in the PVP-adhesive layer to form a point contact that minimizes the contact area with the substrate. The island configuration on the flexible substrate relaxes strain by dispersing it into the empty spaces of the substrate from the edge of the pattern. This strain relaxation effect is more effective as the size of the pattern is reduced.23,24 Therefore, the island configuration at regular intervals of the ZnO shells embedded in PVP is favorable for the mechanical flexibility of the device. Figure 2c shows the DF-ZnO NF network structure after growth of the ZnO NRs both in- and outside the shells to maximize the surface area. The alignment of the shells on the PVP adhesive was maintained during the growth of the NRs. The growth time and concentration were optimized to form a network structure through physical contact of the NRs between the shells. The NRs grown in- and outside the shells differ slightly in shape owing to the irregular source supply inside the shells. Therefore, the NRs grown outside the shell are needle-shaped and dense (Figure 2d), whereas those grown inside the shells are much shorter with a lower density, as shown in Figure S3b. Due to the growth of NRs on both sides of the shells and the island configuration, the DF-ZnO NF network structure possesses a large surface area favorable to gas sensing and is highly resistant to strain, which is advantageous for application in flexible devices. The gas sensing performances of the DF-ZnO NF gas sensor were systematically investigated with a constant voltage bias of 1 V applied between each pair of SWCNT electrodes and under

work structures were formed, as shown in Figure S1. Finally, to obtain a high performance gas sensor with gas selectivity and high flexibility, SWCNTs were spray coated as finger-shaped electrode patterns. Due to the maximized surface area, improved crystallinity, and micropatterned network structure, the DF-ZnO NF gas sensor can exhibit optimized performance. Furthermore, the micropatterned network structures are effective for flexible devices because they can reduce the strain.22 The improved crystallinity after calcination and growth of the NRs were confirmed by X-ray diffraction (XRD) and photoluminescence (PL) data analyses. Figure S2a,b show, respectively, the XRD and PL data before and after calcination, and after growth of the NRs. The XRD data confirm the growth of ZnO with a wurtzite structure as the diffraction peaks observed correspond to the Miller indices from the joint committee on powder diffraction standards #36-1541 data sheet. Before calcination, the ZnO film exhibits a (0002) preferred orientation with a secondary (21̅19) orientation. The crystallinity of the ZnO shells was improved after calcination. Finally, after the growth of NRs on the shells, a strong (0002) diffraction peak was measured along with a minor peak for the (211̅ 0) plane, as shown in Figure S2a. This indicates that the ZnO NRs were successfully grown along the c axis, and were highly crystalline. The improvement in crystallinity was also confirmed by PL analyses. Figure S2b shows the evolution of the PL spectra of ZnO before calcination, after calcination, and after growth of the NRs. As the process progresses, the intensity of the near-band edge (NBE) emission at 378.61 nm as well as the broad deep-level emission (DLE) increases considerably due to the improved crystallinity of ZnO. The broad DLE is mainly caused by the various defects induced by the solution process and has significant impact on the sensor response. More detailed PL analyses are discussed later. Scanning electron microscopy (SEM) analyses of the different steps of the fabrication process were carried and are presented in Figure 2. Figure 2a shows a top-view SEM image 32878


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Figure 3. Gas sensing characteristics of DF-ZnO NF gas sensor upon NO2 exposure at 270 °C and relative humidity of 34%. Characteristics under different gas concentrations (50, 100, 200, 300, 400, and 500 ppm): (a) gas response as a function of time, (b) response, and rising and decay times, and (c) percent recovery. Repetitive operating characteristics at 500 ppm: (d) gas response as a function of time, (e) response, and rising and decay times, and (f) percent recovery.

tration of NO2, as shown in Figure 3a. The gas sensor showed perfect rising and decay saturation behavior even at a low NO2 concentration of 50 ppm, and the response of the gas sensor increased linearly from 29 to 218.1 as the NO2 gas concentration increased, as shown in Figure 3b. As the NO2 concentration increased from 50 to 500 ppm, the rising time increased slightly from 25.0 to 30.8 s but the decay time was decreased by half, from 28.1 to 14.1 s, improving the sensing performance. The decay time corresponds to the time it takes for the resistance to return to its original state. It represents an important index in the evaluation of the sensor stability during repetitive operation. Therefore, this short decay time indicates that the DF-ZnO NF gas sensor is capable of stable repetitive measurements. The recovery is an essential factor for gas sensing as it significantly affects the reliability and sustainability of the device. As metal-oxide-based gas sensors often fail to recover, the degree of recovery is an important indicator of the quality of the gas sensor. Therefore, the recovery characteristics of the DF-ZnO NF gas sensor were investigated by calculating the percent recovery, defined as follows.26

an average relative humidity of 34%. The response of the DFZnO NF gas sensor was calculated to conduct a quantitative analysis and is defined as follows.25 response =

Rg R0

(1)

Here, Rg and R0 are the resistance of the gas sensor after and before exposure to the target gas, respectively. The response represents the reactivity of the sensor and is a fundamental index for evaluating the selectivity and response rate through comparative analysis. First, the operating temperature for gas sensing was optimized, as shown in Figure S4. Due to the thermal stability of the PI substrate and the degradation of the SWCNTs, the temperature did not exceed 300 °C. The devices exhibited the highest response, as well as the lowest deviation, at an operating temperature of 270 °C. Therefore, all subsequent characteristic analyses of the gas sensor were conducted at this temperature. The comparative analyses of NO2, NH3, and CO gas sensing performances were then carried out to select the optimal target gas and evaluate the operating characteristics of the DF-ZnO NF gas sensor. Figure 3 shows the sensing properties of the DF-ZnO NF gas sensor upon exposure to NO2. The exposure under the target gas and air was maintained for 500 s, while the exposure concentration of NO2 was varied from 50 to 500 ppm. The sensor showed an increase in resistance upon NO2 exposure, and the response of the sensor increased proportionally with increasing concen-

recovery(%) =

Rg − Rr Rg − R0

× 100 (2)

Here, Rg is the resistance value after 500 s of target gas exposure, Rr is the recovered resistance value after 500 s of air exposure, and R0 is the resistance value before exposure to the target gas. As shown in Figure 3c, the DF-ZnO NF gas sensor 32879


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Figure 4. Gas sensing characteristics of DF-ZnO NF gas sensor upon NH3 and CO exposure at 270 °C and relative humidity of 34%. Upon NH3 exposure under different concentrations (50, 100, 200, 300, 400, and 500 ppm): (a) gas response as a function of time, (b) response, and rising and decay times, and (c) percent recovery. Upon CO exposure under different concentrations (50, 100, 200, 300, 400, and 500 ppm): (d) gas response as a function of time, (e) response, and rising and decay times, and (f) percent recovery.

Such a difference occurs due to NO2 acting as an electron acceptor when the gas molecule is adsorbed on the ZnO surface, whereas NH3 and CO act as donors.27 Unlike the NO2 gas response, the gas sensor did not show the saturation behavior of rising and decay upon NH3 exposure regardless of the gas concentration, as shown in Figure 4a. Moreover, the response of the sensor was very low, below 1.4. The response increased from 1.24 to 1.37 in proportion to the NH3 concentration, but was abnormally high in the first cycle. Even though the rising and decay times varied from 87.4 to 130.5 s and from 177 to 204 s, respectively, no significant changes were observed with increasing concentration of NH3, as shown in Figure 4b. The device also exhibited a lower recovery value, below 80%, as shown in Figure 4c. Figure 4d−f shows the sensor characteristics upon CO gas exposure with increasing concentration. The sensor showed a saturation behavior for CO at high concentrations, but little response at low concentrations, as shown in Figure 4d. The sensor characteristics upon CO exposure seem to be similar to NH3 exposure. The response of the sensor was low and increased from 1.19 to 1.46 proportionally to the concentrations, but was abnormally high at 1.33 in the first cycle, as in the case of NH3. The rising time decreased from 121 to 46 s with increasing CO concentration. The decay time was not measureable at concentrations below 200 ppm, and was almost unchanged at about 23 s for concentrations above 300 ppm, as shown in Figure 4e. Although the recovery gradually increased with CO concentration, it remained weak with a peak value of only 56%

showed an outstanding percent recovery value greater than 98% regardless of NO2 concentration. This demonstrates the reliability of the device toward NO2 sensing. To evaluate the sustainability of the sensor, a repeatability test was conducted at a fixed NO2 concentration of 500 ppm, at which the sensing characteristics such as response, and rising and decay times are optimal. The sensor characteristics showed a stable saturation behavior without degradation even after several cycles, as shown in Figure 3d. The response and decay times did not show significant variations, with values around 210 and 17 s, respectively, as shown in Figure 3e. As for the rising time, it gradually stabilized, reaching 27 s. Figure 3f shows a stable recovery above 98%. Additionally, to evaluate the repeatability of the sensor under different concentrations of NO2, the characteristics of the sensor were measured while the exposure concentration of NO2 was decreased from 500 to 50 ppm. As shown in Figure S5, the response characteristics of the DF-ZnO NF sensor remained excellent without showing signs of degradation even after the NO2 concentration was decreased to 50 ppm from a high concentration of 500 ppm. The response of the sensor decreased proportionally with decreasing concentration of NO2. These results confirm the ability of the DF-ZnO NF gas sensor to detect NO2 gas whilst demonstrating outstanding and stable characteristics. The performances of the DF-ZnO NF gas sensor upon NH3 and CO exposure are both shown in Figure 4. In contrast to the NO2 gas response behavior, the DF-ZnO NF gas sensor showed a decrease in resistance under NH3 and CO exposure. 32880


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Figure 5. Schematic energy-band diagrams of the DF-ZnO NF gas sensor (a) before and (b) after exposure of NO2, and (c) room-temperature PL spectra with deconvoluted Gaussian subpeaks of the gas sensor.

at a maximum concentration of 500 ppm, as shown in Figure 4f. Additionally, when comparing Figure 4a,d, a shift in the baseline of the resistance was observed. This baseline shift depends on the degree of recovery of the device. As shown in Figure 4c,f, the percent recoveries of the devices upon NH3 and CO exposure are low. The characteristics of the device were evaluated by continuously exposing NO2, NH3, and CO under the same conditions and a baseline shift was observed when there was a gas exchange without a fully recovery. Thus, the baseline shift occurs due to repetitive exposure to low recovery gases. It is clear from these results that the DF-ZnO NF gas sensor exhibited low sensitivity toward NH3 and CO gases. The responses measured were almost 2 orders of magnitude lower than that with NO2 gas. Furthermore, the recovery for the sensor under NH3 and CO exposure was below 80%, whereas under NO2 exposure, the sensor showed an outstanding recovery of 98%. Therefore, the DF-ZnO NF gas sensor has a high selectivity toward NO2 gas. The DF-ZnO NF gas sensor owes its high selectivity and superior sensing performance for NO2 to several factors, such as the morphology, native point defects, and SWCNT electrode. The first factor is the maximized surface area obtained through the growth of ZnO NRs in- and outside of the ZnO shells. This facilitates the diffusion of gas molecules and the surface reaction on ZnO.28 In addition, the threedimensional (3D) network structures of ZnO shells and NRs improve the gas response. 29,30 Generally, the sensing mechanisms of resistive change-type metal-oxide semiconductor sensors are described using the grain-boundary model and the surface-depletion model.31,32 The grain-boundary model is applicable for polycrystalline grain boundaries in films. The response measured is usually low because only a small fraction near the grain is involved in the reaction. In contrast, the surface-depletion model is applicable for nanomaterials such as nanoparticles, NRs, and nanoneedles. The response is higher due to the large surface area of nanomaterials, which contributes to the formation of the surface-depletion region. This surface-depletion region is caused by the adsorption of

oxygen species acting as electron acceptors at the surface of oxide semiconductors. The adsorbed gases can interact with the electron donor on the surface to form two ionic states below the edge of the conduction band. The electron donor states on the oxide semiconductor surface are usually oxygen vacancies (VO), which possess a large secondary ionization energy.33 Therefore, the donor level becomes an ionic state of +1 valence and the surface-depletion region is formed.34,35 When the sensor is exposed to NO2, NO2 is adsorbed on the surface of ZnO forming a surface-depletion layer and reducing the concentration of electrons by acting as an acceptor. The reaction formulas can be described as follows.17,36 NO2(g) + e− → NO−2(ad)

(3)

NO−2(ad) + O−(ad) + 2e− → NO(g) + 2O2 −(ad)

(4)

Here, g and ad correspond to gas and adsorbate, respectively. After adsorption of NO2, the surface-depletion layer is formed over the entire surface of the DF-ZnO NFs. In this work, the ZnO shells with a film-like structure correspond to the first model, whereas the NRs can be applied to the latter model. The DF-ZnO NF gas sensor combines both models and allows complementary adsorption of the NO2 molecules on the shell grain boundaries and on the large surface area provided by the NRs. An important parameter to consider for performance improvement is the presence of multiple active points in the DF-ZnO NF network structure, corresponding to the junction points between the NRs. These junction points can be assimilated to grain boundaries and induce the formation of potential barriers, as illustrated in Figure 5a. These junctions correspond to active points where electrons from the conduction band of ZnO NRs can be transferred to the adsorbed NO2 acting as an electron acceptor. This leads to the formation of a depletion layer near the junction, and to an increase of the potential barrier, as shown in Figure 5b.37 The electron transport is affected by the height of the potential barriers and can be described by the following equation.17,32 32881


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Figure 6. Gas sensing properties of the DF-ZnO NF flexible gas sensor during the dynamic bending test up to 10 000 cycles with a curvature radius of 5 mm upon NO2 exposure at 500 ppm, 270 °C, and relative humidity of 34%: (a) camera image of the device on the bending machine, (b) gas response as a function of time, (c) response, and rising and decay times, and (d) percent recovery.

⎧ −eΔVb ⎫ ⎬ R = R 0 exp⎨ ⎩ kBT ⎭

can act as a resonator, and the WGM phenomenon was observed in the PL spectrum.39 In the case of ZnO, the DLE observed is generally attributed to native point defects such as Zni, VO, oxygen interstitial (Oi), and zinc vacancy (VZn) in ZnO.40 Their presence is caused by the nature of the aqueous solution process, which induces the formation of multiple defects during the growth of the NRs.41 The broad DLE peak was deconvoluted into four Gaussian subpeaks centered at 552.5, 606.7, 702.8, and 749.3 nm, corresponding to the photon energies of 2.24, 2.04, 1.76, and 1.65 eV, respectively. These subpeaks are attributed to the electronic transitions from Zni to VO, Zni to Oi, VO to valence-band maximum, and conductionband minimum to VO, respectively.42 Among the deconvoluted subpeaks, those corresponding to Zni and VO donor defect transitions are predominant, which confirms that they are the main defects in the ZnO structure. The PL analysis confirmed the presence of VO and Zni in the ZnO NFs, which significantly affect the gas sensing performance by improving the reactivity of NO2 on the ZnO surface. In addition to the presence of the native point defects, the use of SWCNTs also plays a positive role in increasing the selectivity to NO2 in the DF-ZnO NF gas sensor. DFT studies on the interaction of various gas molecules with SWCNTs have been carried out, showing that each molecule possesses a different adsorption energy and degree of charge transfer to SWCNTs.43,44 Although the electronic properties of SWCNTs are sensitive to NO2 adsorption, the SWCNTs were not only inactive with the NH3 and CO gases but also demonstrated repulsive forces.45 Additionally, it is also known that the junction barrier height between the electrode and the semiconductor sensing material plays an important role in resistive change-type sensors.46 In the case of a Schottky barrier contact, the barrier height is directly dependent on the electron affinity (χ) of a semiconductor and the work function of a metal

(5)

Here, ΔVb is the change in potential barrier defined as the potential in air minus that in the target gas, R0 is a factor including the intergranular resistance and geometrical effect, e is the charge of an electron, kB is Boltzmann’s constant, and T is the absolute temperature. The electron transport decreases when the potential barrier increases, therefore, the adsorption of NO2 at the junctions of the NRs induces a decrease of the measured current. As a result, the presence of numerous junctions in the DF-ZnO NF network structure is crucial to demonstrate the superior properties of the gas sensor. Another factor affecting the characteristics of the ZnO-based gas sensor is the presence of native point defects as well as surface-depletion layers. In particular, the NO2 gas sensing performance of ZnO is related to donor defects, which give rise to free electrons, such as zinc interstitial (Zni) and VO.38 The high selectivity for NO2 in the ZnO-based gas sensor is influenced by the donor defect state VO. Through density function theory (DFT), it was found that the adsorption energy of NO2 on the VO site was larger than that of NH3 and CO.27 This indicates that NO2 adsorption on the ZnO surface would be improved in the presence of VO. As shown in Figure 5c, the PL spectrum analysis of the DF-ZnO NFs was carried out to identify point defects contributing to the properties of the gas sensor. The room-temperature PL spectrum consists of a sharp peak centered at 378.6 nm and a broad peak centered at about 560 nm corresponding to NBE emission and DLE, respectively. The NBE emission is related to the bandgap of the material, and the DLE is caused by the presence of multiple point defects in the material and whispering gallery mode (WGM) resonances. The WGM resonance is a phenomenon in which light waves circulate around the resonator boundaries due to multiple internal reflections. Hexagonal needle-shaped 1D ZnO 32882


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ACS Applied Materials & Interfaces Table 1. Comparison of the NO2 Gas Sensing Properties of the ZnO-Based Gas Sensor operating temp. (°C)

concentration (ppm)

gas response (R/R0)

rising time (s)

decay time (s)

radius of curvature (mm)

number of bending cycles

solution

270

50−500

29−218.1

25−30.8

28.1−14.1

5

10 000

solution

250

0.01

824

1200

3000

this work 50

solution

200

200

64

41

125

17

solution

200

100

37.2

6.72

52.62

51

solution

290

40

264

solution

25

1

2.61

synthesis method low temp. process low temp. process low temp. process low temp. process low temp. process low temp. process

52 180

300

53

NF gas sensor showed saturation behavior and was still operating as a NO2 detector, even after 10 000 cycles of bending tests. These results demonstrate the stability and high performance of the DF-ZnO NF gas sensor. To relate the performance of the DF-ZnO NF gas sensor presented in this work, its characteristics are compared to those of reported ZnO nanomaterial-based NO2 gas sensors from the literature in Table 1. Other comparisons with NO2 sensors using ZnO films or other metal-oxide nanostructures are shown in Table S1. Numerous reported sensor devices showed sensitive properties at very low concentrations, but had long characteristic times and did not exhibit saturation behavior. Some reports showed considerable failure in recovery. Moreover, although sensors fabricated on flexible substrates have been reported, the stability of the sensors after repetitive deformation has not been evaluated. The comparative assessment reveals that the DF-ZnO NF gas sensor possesses superior sensing characteristics toward NO2 and sustainable mechanical stability by maintaining sensing properties even after 10 000 cycles of bending tests with a curvature radius of 5 mm. Therefore, the combination of the NF network structure with a maximized surface area, donor point defects, numerous active points, and the use of SWCNTs as electrode in the DFZnO NF sensor provides excellent sensing characteristics, stable mechanical properties, and enables high integration of nanodevices.

(⌀m). The Schottky barrier height can be described as follows.47,48 ⌀B = ⌀m − χ

ref

(6)

Here, ⌀B is the Schottky barrier under zero bias. The work function (⌀m) of the SWCNTs was measured to be 5.02 eV using a surface-analysis photoelectron spectrometer, as shown in Figure S6. The electron affinity (χ) of ZnO is considered to be 4.35 eV from the reported literature.49 Considering ZnO as the sensing material, the Schottky barrier height with the SWCNT electrode is 0.67 eV, as is schematically illustrated in Figure 5a. Therefore, it is believed that SWCNTs are a suitable electrode material for ZnO NRs to form a Schottky barrier, thus improving the characteristics of the DF-ZnO NF gas sensor. Lastly, to evaluate the performance of the DF-ZnO NF gas sensor as a flexible device, a mechanical bending test was conducted under NO2 exposure. Figure 6a shows a camera image of the flexible DF-ZnO NF gas sensor on a bending machine. The bending test was performed at a curvature radius of 5 mm. As shown in the response−time characteristic curve from Figure 6b, even though the electrical fluctuation of the device gradually increases with increasing number of bending cycles, the response characteristic appears to improve. In quantitative analyses, the response of the sensor increased from 202.2 to 297.6 after 10 000 cycles, as shown in Figure 6c. Moreover, the characteristic times were also improved. The rising and decay times were decreased from 36.0 to 19.2 and 20.3 to 11.5 s, respectively. Furthermore, even the recovery increased from 89.4 to 94.8%, as shown in Figure 6d. Overall, the characteristics of the gas sensor appear to improve as the bending cycle increases from 0 to 10 000. However, despite an improvement of the numerical values during the evaluation of the sensor characteristics, the resistance gradually increased while under the same conditions, along with significant fluctuations. This behavior implies a deterioration of the sensor. As shown in Figure S7, the degradation aspects of the sensor with increasing bending cycle were confirmed by SEM analyses. Overall, the aligned network structure embedded in the PVP-adhesive layer was maintained but partial fractures occurred. After flexibility tests of 10000 cycles, the NRs appeared as bent, indicating that a constant strain was applied to the NRs during the repetitive flexibility test. As a result of repetitive strain, breakaway of the NRs from the shells occurred and was the first sign of device failure. On the other hand, the shells appeared to maintain their structure, and cracked shells were very rarely observed. Despite the deterioration of the sensor after bending cycles observed under SEM, the DF-ZnO

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CONCLUSIONS A micropatterned DF-ZnO NF flexible gas sensor with maximized surface area was successfully fabricated on a PI substrate by growth of ZnO NRs on ZnO shells. As a sacrificial layer to form the ZnO shells, polystyrene MPs were arranged at regular intervals by a PDMS rubbing technique and the arrangement was maintained after transfer and calcination. This island configuration at regular intervals allows the mechanical flexibility to be improved by dispersing the strain into the empty spaces of the substrate. The maximized surface area of the DF-ZnO NF network structure was obtained by growth of the NRs on the ZnO shells. The operating temperature of the sensor was optimized at 270 °C considering the stability of the SWCNT electrode and the PI substrate. The DF-ZnO NF gas sensor under NO2 exposure showed perfect saturation behavior and outstanding sensing performances. The sensor showed a response of 218.1, rising time of 25.0 s, decay time of 14.1 s, and percent recovery greater than 98% upon NO2 exposure. Interestingly, the sensor showed higher selectivity toward NO2 than NH3 and CO. The comparative analyses revealed that the high response and selectivity of the sensor were due to the 32883


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air was used as the purging gas. A more detailed description of the gas sensor measurement can be found in Figure S8.

combination of high surface area and numerous active points from the NRs, as well as the ZnO point defects of the donor state, and the SWCNT electrode. Furthermore, the DF-ZnO NF gas sensor demonstrated sustainable mechanical stability. Although some physical degradation of the device was observed under SEM after the repeated flexible test, the sensor functioned even after 10 000 cycles with a curvature radius of 5 mm. These results are important for the development of nanomaterial-based sensor devices as we successfully improved the performance in NO2 sensing by using a 3D micropatterned network structure. This structure with maximized surface area not only greatly improves the response and characteristic times, but also enhances the selectivity and recovery in a metal-oxidebased gas sensor. Moreover, the micropatterning technique on a flexible substrate enables fabrication of highly integrated nanodevices and application to flexible devices. Finally, it provides stable mechanical sustainability in flexible electronic devices based on nanomaterial semiconductors.

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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.7b09251. SEM images of DF-ZnO NFs formed by the growth of ZnO NRs on both sides of the ZnO shells; XRD and PL peaks of ZnO film (before calcination), shells (after calcination), and NRs; OM image after placing the MPs in circular well patterns; response at different operating temperatures; work function measurement result of SWCNTs; SEM images after dynamic bending test of 10 000 cycles with a curvature radius of 5 mm; schematic illustration of gas sensing system; and comparison of the ZnO film to other metal-oxide nanostructure sensors for detecting NO2 gases (Table S1) (PDF)

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EXPERIMENTAL SECTION

Micropatterned Arrays of Polystyrene MPs. First, the glass substrates were cleaned by sonication in acetone, methanol, and deionized (DI) water consecutively for 15 min each. Then, circular patterns with a diameter of 5 μm and a height of 3 μm were defined on the substrates by photolithography. Next, the MPs with a diameter of 5 μm purchased by Sigma-Aldrich were placed on the patterned substrates by using PDMS-transfer. After transfer, the MPs were aligned by being pushed into circular patterns by a PDMS rubbing technique. The PDMS rubbing technique was performed unidirectionally. Construction of Micropatterned ZnO Shells on a Flexible Substrate. The ZnO film with a thickness of 100 nm was deposited on the micropatterned MPs by using radiofrequency magnetron sputtering with a plasma power of 150 W and a pressure of 15 mTorr in an Ar environment. As a flexible receiver substrate, a PVP-coated PI substrate was prepared by spin coating of a 10% PVP solution with a cross-linking agent, poly(melamine-co-formaldehyde) in propylene glycol monomethyl ether acetate, and was softly baked at 80 °C for 1 min. Then, the ZnO-coated MPs were transferred onto the PVPcoated PI substrate by applying a pressure of 7.5 g/cm2 and then heating at 120 °C for 1 min. After that, to finalize the micropatterned ZnO shells, the transferred MPs were calcined at 250 °C for 3 h on a hot plate. Fabrication of Micropatterned ZnO NF Flexible Gas Sensor. The ZnO NRs were grown on the micropatterned ZnO shells by using an aqueous solution process. The ZnO shell arrays on the flexible substrate were immersed in a chemical bath containing an equimolar solution of 10 mM zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ACS reagent 96459) and hexamethylenetetramine (C6H12N4, ACS reagent 398160). The reaction was carried out at 95 °C for 3 h. After the reaction was complete, SWCNTs were deposited using a spray coater and maintained at 120 °C in air to remove the solvent. To get rid of residual surfactant and dispersant, the SWCNT electrodes were rinsed with DI water for 5 min. Characterization and Measurement. The surface morphologies of all materials used in this work were characterized by using SEM (Hitachi S-5000). The structural properties of the materials were investigated by using XRD (Rigaku SmartLab). The work function of the SWCNT electrode was analyzed by a surface-analysis photoelectron spectrometer (AC-2, Riken Keiki Co. Ltd). To investigate the optical properties of the ZnO, room-temperature PL spectroscopy measurements were performed using an IK3252R-E He-Cd laser (325 nm) source coupled with MonoRa 320i monochromator (Dongwoo Optron) and Andor SOLIS simulation package. The gas sensing performances were monitored in a sealed sensing chamber coupled with an electrical feed-through and gas inlet and outlet. The target gas was diluted with nitrogen gas (concentration: 50−500 ppm) and dry

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kyung Yong Ko: 0000-0002-7934-5620 Jae-Min Myoung: 0000-0002-9895-4915 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFCMA1301-07.

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REFERENCES

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