All-transparent NO2 gas sensor based on free-standing Al doped ZnO

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Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

All-Transparent NO2 Gas Sensors Based on Freestanding Al-Doped ZnO Nanofibers Amit Sanger, Sung Bum Kang, Myeong Hoon Jeong, Chan Ul Kim, Jeong Min Baik, and Kyoung Jin Choi* School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

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S Supporting Information *

ABSTRACT: Transparent optoelectronics can enable a new class of applications such as transparent displays, smart windows, and invisible sensors. Here, we demonstrate all-transparent NO2 gas sensors based on aluminum-doped zinc oxide (AZO) freestanding hollow nanofibers. Freestanding AZO nanofibers are fabricated by sputtering AZO on template polyvinylpyrrolidone (PVP) nanofibers, which are electrospun on a glass frame with indium zinc oxide (IZO) transparent electrodes, followed by a heat treatment to remove the PVP template nanofibers. Not only the gas-sensing active material but also other components such as the substrate and electrodes are all transparent in the visible region. The optical transparency of the nanofibers is controlled by changing the AZO nanofibers density without compromising the sensitivity. The gassensing measurements of the transparent sensor depict n-type response behavior with full recovery even at low NO2 concentrations (0.5 ppm). The high sensitivity of the transparent sensors is attributed to the higher surface area of the hollow nanofibers and the high impact frequency of trapped NO2 gas inside the hollow compared to solid counterpart nanofibers. The unique combination of transparency and high sensitivity can potentially have applications in advanced sensor systems that can be attached to windows integrated with the Internet of Things. KEYWORDS: transparent sensor, freestanding, hollow nanofibers, AZO, NO2

1. INTRODUCTION Transparent electronics is an emerging field focusing on making “invisible” electronics circuit and optoelectronics devices.1,2 It offers a wide range of possibilities for various applications such as sensors, solar cells, transparent displays, smart windows, and many more.3,4 Among various devices, transparent gas sensors are being actively researched to realize advanced sensor systems that can be attached anywhere such as windows and are compatible with the Internet of Things (IoT) technology. Recently, several types of transparent gas sensors have been fabricated by using transitional metal dichalcogenides, graphene, and carbon nanotubes.5−7 Bai et al. prepared the carbon nanotube-coated polyelectrolyte multilayer transparent gas sensors, exhibiting NH3 gas-sensing properties assigned to high carrier mobility and high surfaceto-volume ratio.8 Duy et al. studied the NH3 gas-sensing applications of transparent graphene oxide gas sensors, inspired by the high specific area of the composite film.9 Yoon et al. prepared the graphene-based transparent gas sensor that can detect NO2 gas due to high surface area and excellent carrier mobility.10 Li et al. examined the sensing applications of single and multilayer MoS2 toward NO gas due to high carrier mobility.11 However, the real-field applications of these gas sensors are hindered by a few drawbacks. Yang et al. outlined that transitional metal dichalcogenide-based gas sensors display © XXXX American Chemical Society

some key problems like inadequate gas cross-sensitivity, short lifetimes, slow recovery features, and hard to fabricate largescale devices.12 Mao et al. found out that nanocarbon sensors show high sensitivity except inevitable changes in resistivity, poor cross-sensitivity, extended exposure time, and failure to sense low adsorption energy gases.13 Wang et al. pointed out that graphene sensors display disadvantages with respect to their shallow sensitivity, poor detection limit, and inadequate response repetition.14 In this perspective, the transparency characteristics can be achieved by using transparent conducting oxide (TCO)-based gas sensors, incorporated with transparent electrodes as well as active gas-sensing materials. Resistivities less than 1 × 10−3 Ω cm and transparency more than 85% have been stated for TCOs, such as ZnO, SnO2, and TiO2 doped with other ions (Al, In, and Nb).15−24 TCOs are less expensive than metal electrodes like Pt and Au, and their production processes are adaptable with the semiconductor manufacturing processes.25−28 There are few reports available in the literature in which transparency-controlled gas-sensing properties of TCOs are examined. For example, Miyata et al. examined the Received: April 1, 2019 Accepted: June 19, 2019 Published: June 19, 2019 A

DOI: 10.1021/acsaelm.9b00210 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Electronic Materials

Figure 1. Schematic illustration of the overall fabrication process of the hollow AZO fibers: (a) sputtering deposition of an IZO electrode layer on the PVP fibers, (b) electrospinning of PVP fibers, (c) sputtering deposition of an AZO layer on the PVP fibers, and (d) polymer burnout and calcination of AZO-coated fibers.

Cl2 gas-sensing properties of the (Zn2In2O5)x−(MgIn2O4)1−xbased gas sensor.29 Saeedabad et al. fabricated the SnO2:Sb thin films with different doping and examined the transparency as well as NH3 gas-sensing properties.30 Ganesh et al. and Aydin et al. synthesized the Al-doped ZnO thin films and studied their optical and low-temperature NH3 gas sensitivity.31,32 In these reports the transparency and sensitivity of active material depend on the dopant percentage in the native oxide material. The fabricated devices showed high transparency and sensitivity to a particular dopant percentage and besides that a decrease in both properties noted. Therefore, the dopant percentage in the active material must be optimized to achieve the high transparency as well as sensitivity. To target the above problem, we present the fabrication of transparent freestanding hollow AZO nanofibers on an IZOcoated glass frame. The key objective of this study is to develop a gas sensor with a controlled optical transparency without compromising the sensitivity. The sensors were examined with low NO2 gas concentrations and displayed fully recoverable highly sensitive behavior. The method illustrated here may initiate the realization of other 1-D nanostructure-based transparent gas sensors.

process, the glass was covered with the vinyl sticky hard tape, leaving a square-shaped area (∼1 cm2) open and then dipped in the etchant solution for 10 h. Subsequently, the glass was washed with DI water and dried in N2 gas atmosphere after removing the hard tape. The residue part of glass in uncovered area was removed and successively ground down and polished with SiC abrasive paper. After polishing, two opposite sides of glass frame were sputter-coated with an IZO (2% In, ITASCO, 99.99%) layer (∼200 nm) by applying 100 W at 5 mTorr and RT by covering the other sides with Kapton polymer tape. Synthesis of the Hollow AZO Nanofibers. PVP solution (1.6 g in 25 mL of ethanol) was electrospun on a grounded IZO-coated glass frame distanced at 15 cm by using 26-gauge stainless steel needle under 0.8 mL/h and 9.5 kV conditions as described elsewhere. In addition, a thin layer of 2 wt % Al-doped ZnO (ITASCO, 99.999% pure) was sputtered on nanofibers by using 50 W RF power under 5 mTorr working pressure at RT. Subsequently, postannealing of fibers was done at 400 °C for 1 h after removing the Kapton polymer tape.33,34 Characterizations. A scanning electron microscope (SEM, FEI Quanta 200FEG) and a transmission electron microscope (TEM, JEOL JEM-2100F) were used to examine the morphology of transparent nanofibers. Phase identification of nanofibers was done by a Bruker D8 Advance X-ray diffractometer and TEM selected area diffraction pattern (SAED). X-ray photoelectron spectroscopy (Thermo Fisher) was used to examine the chemical compositions of the nanofibers. A UV−vis spectrometer (Varian Cary) was used to record the optical transmission spectra of nanofibers of different densities. Prior to sensing tests, the sensor devices were steadied in air at a specific measurement temperature in the stainless-steel test chamber (∼125 cm3) equipped with a PID controller, a mass flow controller (MS Tech Korea), and a Keithley 2636A setup. After that, various NO2 concentrations (0.5−10 ppm) blended with air were inserted in the test chamber. The humidity level inside the test chamber was controlled with the ratio of humid carrier gas flow (air flow) and total flow (air + NO2 flow). The humid carried gas “air” was

2. EXPERIMENTAL SECTION Fabrication of the Glass Frame. A glass frame with square hole shape was used as substrate to utilize most of the specific surface area of freestanding electrospun fibers. The freestanding fibers on glass frame allow the better transmittivity and gas−material interaction compared to the plain substrate. The glass frame was prepared by the wet etching process. The etchant solution was made by adding 15 mL of HF (ACS reagent, 48%) and HCl (ACS reagent, 37%) each with 70 mL of DI water in a Teflon container. Before the wet etching B


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Figure 2. (a−e) Optical micrographs of IZO and freestanding hollow AZO nanofibers with different densities. (f−i) SEM images of freestanding hollow AZO nanofibers with different densities. (j) UV−vis spectra of IZO and freestanding hollow AZO nanofibers with different densities.

Figure 3. (a) XRD spectrum, (b) SAED pattern, and (c) TEM image of fabricated freestanding filled AZO fiber and the corresponding HR-TEM image in the inset. saturated with water vapor in glass bubbler and further mixed with the NO2 gas. The device gas sensitivity was explained as S = Rg/Ra, where Ra and Rg are resistances measured in air and target gas mixed with air, respectively.

that the fabricated nanofibers, electrode, and glass substrate are all transparent. The average diameter of the nanofibers is ∼200 nm. Figure 2c depicts the UV−vis spectra of nanofibers with different densities and IZO contact in the 300−800 nm wavelength range. The transparency of the hollow nanofibers varies from 80% to 50% in the visible range. The decrease in the transmittance is due to change in the density of hollow nanofibers. Because of the Brustein−Moss effect, the nondegeneracy in AZO semiconductor leads to decrease in transmittance at lower wavelength.35 Generally, the bandgap of AZO is very near the cutoff wavelength ∼300 nm; therefore, it significantly adsorbs the UV light near the bandgap range and populates charge carriers, leading to loss in transparency. The transparency of IZO layer was above 80% in the visible range. The average transmittance of the whole device can be controlled by managing the nanofiber density. XRD spectra of

3. RESULTS AND DISCUSSION The schematic diagram of the synthesis procedure is shown in Figure 1. The glass frame was used as a substrate to utilize the high specific surface area of freestanding nanofibers. Before IZO coating, the two opposite sides of wet-etched square glass frame were covered with Kapton tape. Thereafter, PVP fibers were electrospun on a grounded IZO-coated glass frame followed by the sputtering of AZO and subsequent heat treatment in a furnace. Finally, the prototype device was used to examine the gas-sensing properties. Figures 2a and 2b show the optical and SEM images of the sensor prototypes with different transparency, respectively. The optical images show C


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Figure 4. High-resolution core level XPS spectra of AZO hollow fiber: (a) O 1s, (b) Zn 2p, (c) Al 2p, and (d) C 1s peaks.

Figure 5. (a) I−V characteristics curve of the freestanding hollow AZO nanofibers of different densities and PVP fibers. (b) Response and recovery behaviors of the freestanding AZO hollow nanofiber gas sensor of different density to 0.5 ppm of NO2 gas at 250 °C. (c) Gas response curve of freestanding AZO hollow nanofibers as a function of the NO2 concentration (0.5−10 ppm) at 250 °C. (d) Sensing response to 0.5 ppm of NO2 vs operating temperature of freestanding AZO hollow nanofibers.

AZO nanofibers in Figure 3a show the five peaks of (100), (002), (101), (102), and (110) planes corresponding to the wurtzite phase (JCPDS No. 361451). The SAED pattern of transparent nanofibers depicts the polycrystalline nature with

clearly visible rings (Figure 3b). Figure 3c shows TEM and HR-TEM images of typical, uniform, and hollow 1-D nanofiber. The AZO wall thickness was found to be ∼25 nm. The SAED pattern and HR-TEM image agree well with D


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Figure 6. (a) Linear fit of gas response versus concentration of NO2 at 250 °C for hollow AZO nanofibers. (b) Gas selectivity curves of the hollow AZO fibers toward different gases with different concentrations at 250 °C. (c) Cyclic stability curve of hollow AZO fibers toward 0.5 ppm of NO2 gas up to 30 cycles at 250 °C. (d) Gas response curve of hollow AZO fibers toward 0.5 ppm of NO2 gas in different relative humidity conditions (10−100%) at 250 °C.

sensing behavior of hollow fibers electrospun on plain glass substrate was also examined under the same experimental conditions (Figure S1). The fibers on the plain substrate exhibited almost 2 times lower sensitivity and comparatively slower response−recovery time than the hollow fibers. The enhancement in gas sensitivity of freestanding fibers compared to their plain glass counterparts is due to the geometric enhancement in the specific surface to volume ratio of freestanding fibers. As shown in Figure 5c, the response behaviors of transparent sensors to 10−0.5 ppm of NO2 concentrations at 250 °C displayed the enhancement in sensitivity with respect to higher concentration. Figure 5d shows the response curve of the transparent sensor as a function of temperature at 0.5 ppm of NO2. The hollow nanofibers response displayed a noticeable rise with high temperature and exhibited remarkable RT sensitivity. The high temperature gives rise to the activation energy of metal oxide leading to faster chemical reactions with the analyte gas, resulting in high gas sensitivity. The gas-sensing behavior fully recovered at RT with a response time ∼11 min and recovery time ∼8 min for 0.5 ppm of NO2 at RT. To elaborate the slow response−recovery behavior, the sensing behavior of freestanding filled fibers was also examined under similar experimental conditions (Figure S2). Here, the inner core of fibers filled with the insulated Al2O3 material. The fabrication process of filled fibers is discussed in the Supporting Information. The filled fiber sensor depicted the fast response−recovery behavior with comparative lower sensitivity with respect to hollow fibers at 250 °C. The sensor showed the full recovery behavior with a response time ∼23 s and recovery time ∼40 s for 0.5 ppm of NO2 at 250 °C. This observed gassensing behavior is easy to describe with gas−material diffusion

the phase identification by the XRD measurements. In addition, XPS measurements were performed to examine the constituents of the nanofibers (Figure 4). The peaks at 531 and 533 eV correspond to Zn−O and Zn−O−C bonds in O 1s XPS spectra, respectively (Figure 4a).36 The Zn 2p (Figure 4b) spectra shows the typical peaks of Zn 2p1/2 and 2p3/2 at 1021 and 1044 eV, respectively.37 As shown in Al spectra (Figure 4c), the peak at ∼74 eV advocates that Al donor atoms replaced the Zn atoms in the ZnO lattice.38 The C 1s spectra of AZO nanofibers show three peaks at 284, 287, and 289 eV, corresponding to residual carbon, Zn−O−C bond, and structural carbonate CO bond, respectively (Figure 4d). Ansari et al. and Zhang et al. suggested that presence of the Zn−C bond in AZO nanostructures may enhance the photocatalytic activities by introducing the new energy band.36,39 In addition, the gas-sensing properties of transparent sensor were examined. The Ohmic behavior in current−voltage (I− V) characteristics confirmed that transparent nanofibers are properly joined with IZO electrodes (Figure 5a). Notably, the AZO nanofiber samples revealed a monotonic decrease in resistance with increasing nanofiber density. The high nanofiber density provides more electron per unit length to carry the current flow, resulting in decrease in resistance. For comparison, the gas-sensing results of AZO nanofibers with different density and transparency toward 0.5 ppm of NO2 gas at 250 °C are displayed in Figure 5b. Irrespective of transparency variation from 80 to 50% in visible region, AZO nanofibers exhibit almost identical and reproducible sensitivity (∼1% change) along with response−recovery times of 9 and 10 min, respectively. This relationship indicates a dominant ensemble effect of nanofiber density.40 The gasE


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ACS Applied Electronic Materials kinetics. Initially, the adsorption of NO2 molecules takes place on the nanofibers surface; after that, gas molecules penetrate through the nanofiber layer and are eventually trapped inside the hollow core. The total resistance change comes from the reactions inside the core as well as the surface reactions. Because of trapping of gas molecules, the probability of the gas−material interaction would be higher inside the hollow core, leading to a high sensitivity of nanofibers. However, in the case of filled fibers the gas−material interaction mainly takes place on the surface of active material as inner side of active material covered with the insulated Al2O3 core and unreactive to the gas molecules. The total change in the resistance comes from the change in potential barrier from outer side of active material. This behavior leads to the faster response−recovery time with lower sensitivity compared to hollow fibers. The power law dependence behavior of transparent nanofibers with respect to NO2 concentration at 250 °C is shown in Figure 6a. The lowest detection limit (LDL) was found to be ∼0.2 ppm. LDL can be calculated from linear fit data of sensor response versus gas concentration curve. LDL =

3.3 × standard deviation in intercept slope

(1)

The standard deviation in intercept is b=a n

(2)

Figure 7. Schematic illustration of the interaction of the NO2 gas molecules with the inner and outer surfaces of a hollow AZO fiber with energy band diagram.

where a is the standard error of the intercept and n is the total calculated points.41 Figure 6b shows the gas selectivity of transparent AZO nanofiber sensor toward 100 ppm of H2 and 50 ppm of CO gases at 250 °C. The sensor shows high selectivity for NO2 with a response (Rg/Ra ∼ 11) compared to other gases (Rg/Ra < 3.6). The cyclic stability of transparent AZO nanofiber is verified under 0.5 ppm of NO2 at 250 °C, displaying the stable response over 30 cycles (Figure 6c). Figure 6d shows the sensitivity curve under varying relative humidity (RH) (10− 100%). The sensitivity decreased about 2% at 100% RH which is because humidity reacts as the reducing gas for AZO nanofibers, mitigating the oxidation characteristics of NO2. The minute change in the sensitivity with respect to humidity is due to the low accumulation of moisture on sensor surface at high operating temperature of 250 °C, warranting the real field application of the sensor.42 The typical gas-sensing behavior of the transparent nanofibers is explained as follows: Initially, oxygen molecules (from air) adsorb on the fiber surface by taking out electrons which results in changing the carrier concentration Nd and potential barrier (qVs1) formation.43−45 The type of adsorbed oxygen species depends on the temperature. As shown in Figure 7, an increase in the height of qVS1 to qVS2 under exposure to NO2 leads to further decrease in the current. O2(air) → O2(adsorbed on AZO)

(3)

− − O2(adsorbed) + e(from AZO) → O2(adsorbed)

(4)

− − O−2(adsorbed) + e(from AZO) → 2O(adsorbed)

(5)

− NO2 + 2O(adsorbed) → NO−2 + O−2(adsorbed)

(6)

Vs =

2πQ s 2 εNd

response =

=

2π (qNs)2 εNd

i e(V − Vs1) yz zz ≈ expjjjj s2 z Ra kT k {

(7)

Rg

(8)

where e is the elementary charge, Qs is the surface charge density, k is the Boltzmann constant, T is the operating temperature, and ε is the dielectric constant of AZO.41 Here, the high sensitivity may be associated with the high permeability of the freestanding nanofiber networks to the gas molecules, which facilitates the diffusion of gaseous molecules. The freestanding nanofiber network provides a high surface area of the active material so that the effect of the potential barrier height and interaction with the target gas molecules leads to higher sensitivity. Thus, the higher NO2sensing properties can be justified by the transfer of electrons from the AZO nanofibers to the NO2 molecules following their adsorption on the surface. Furthermore, the hollow characteristics of nanofibers play an important role in high sensitivity of the gas sensor. During gas sensing, the gas molecule−active material interactions are determined by mainly three factors: energy of gas molecules, number of active sites, and collision frequency.46,47 The highly energetic gas molecules can easily adsorb on available active sites of material surface. However, the gas molecules with low energy may have difficulty to adsorb. The availability of active sites depends on surface area of the target material. At high concentration, the competition between gas molecules is very high to be adsorbed on active sites. Here, the collision frequency is defined as the average rate of collision between the gas molecules and active gassensing material in a definite system and can be controlled by

The gas response based on the potential barrier height can be represented as follows: F


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temperature as well as confinement effect of material’s morphology.48,49 The mean free path of gas molecules is comparatively higher on outer layer of the hollow fiber due to larger available free space than that of inner core of nanofibers. The collision frequency of gas molecules increases due to trapped motion and short mean free path inside the hollow core. The change in collision frequency of inner/outer sides leads to variation in the corresponding potential barrier heights.50 This difference in collision frequency contributes to the high gas sensitivity of transparent nanofibers.

4. CONCLUSION The transparent AZO hollow nanofibers were fabricated on the IZO-coated glass frame. In the fabricated sensors, all components including the glass substrate, active sensing nanofibers, and electrodes are transparent in the visible spectrum. The optical transparency of the sensing AZO nanofibers was controlled by the density of nanofibers without compromising the sensitivity. The transparent nanofibers displayed the high sensitivity toward NO2 gas with full recovery characteristics. The high sensitivity was attributed to the high collision frequency of trapped gas molecules inside the nanofiber core. We expect that the combination of transparency and high sensitive gas-sensing operation may have potential applications in the Internet of Things for the realization of advanced sensor systems which can be even attached anywhere like a window pane.

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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/acsaelm.9b00210.

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Figures S1−S2 and Table S1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kyoung Jin Choi: 0000-0003-2884-8006 Author Contributions

A.S. and S.B.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Institute for Information & Communications Technology Program (IITP) through the Korea government (MSIT No. 2017-0-00910) and the Midcareer Researcher Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2018R1A2B2003720).

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REFERENCES

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All-transparent NO2 gas sensor based on free-standing Al doped ZnO

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