Ultrasensitive Room-Temperature Operable Gas Sensors Using p

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Ultra-Sensitive Room-Temperature Operable Gas Sensors using p-Type Na:ZnO Nanoflowers for Diabetes Detection Rawat Jaisutti, Minkyung Lee, Jaeyoung Kim, Seungbeom Choi, TaeJun Ha, Jaekyun Kim, Hyoungsub Kim, Sung Kyu Park, and Yong-Hoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00673 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

Ultra-Sensitive Room-Temperature Operable Gas Sensors using p-type Na:ZnO Nanoflowers for Diabetes Detection Rawat Jaisutti1,2,†, Minkyung Lee3,†, Jaeyoung Kim3, Seungbeom Choi3, Tae-Jun Ha4, Jaekyun Kim5, Hyoungsub Kim2, Sung Kyu Park6,*, and Yong-Hoon Kim2,3,* 1

Department of Physics, Faculty of Science and Technology, Thammasat University, Pathum Thani, Thailand

2

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea

3

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Korea

4

Department of Electronic Materials Engineering, Kwangwoon University, Seoul, Korea

5

Department of Photonics and Nanoelectronics, Hanyang University, Ansan, Korea

6

School of Electrical and Electronic Engineering, Chung-Ang University, Seoul, Korea

ABSTRACT Ultra-sensitive room-temperature operable gas sensors utilizing the photocatalytic activity of Na-doped ptype ZnO (Na:ZnO) nanoflowers (NFs) are demonstrated as a promising candidate for diabetes detection. The flowerlike Na:ZnO nano-particles possessing ultrathin hierarchical nanosheets were synthesized by a facile solution route at a low process temperature of 40 oC. It was found that the Na element acting as a ptype dopant was successfully incorporated in the ZnO lattice. Based on the synthesized p-type Na:ZnO NFs, room-temperature operable chemiresistive-type gas sensors were realized, activated by ultraviolet (UV) illumination. The Na:ZnO NF gas sensors exhibited high gas response (S of 3.35), fast response time (~18 sec) and recovery time (~63 sec) to acetone gas (100 ppm, UV intensity of 5 mW cm-2), and furthermore, sub-ppm level (0.2 ppm) detection was achieved at room-temperature, which enables the diagnosis of various diseases including the diabetes from exhaled breath. Keywords : Na:ZnO, p-type, gas sensor, room temperature, nanoflowers

1

ACS Paragon Plus Environment


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1. INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

With the increasing concern on personal healthcare and environmental safety in a daily life, an effective detection of various volatile biomarkers has become extremely important. In fact, a selective detection of low-concentration volatile organic compound (VOC) gases in exhaled breath can be used as a biomarker for a fast and noninvasive screening diagnosis of various diseases. For example, the

acetone gas

concentrations from the exhalation in patients with lung cancer1 and type-I diabetes2-3 were found to exceed 1.0 ppm and 1.8 ppm, respectively, while those in healthy people are in the range of 0.3-0.9 ppm4. Therefore, sensors with high gas sensitivity and low-concentration-level detectability (< 1 ppm) to various biomarker gases are essential to enable successful diagnosis of diseases using the exhaled breath. Recently, electronic gas sensors based on nanomaterials such as metal oxide nanoparticles5, nanofibers3, graphene6, or carbon nanotube7 have been shown to perform low-concentration-level detectability and fast response owing to their high surface-to-volume ratios. Among these nanomaterials, metal oxide nanoparticle based chemiresistive sensors have been widely studied due to their ease of fabrication, good reproducibility and a wide variety of target gases. However, the main drawback of the metal oxide based gas sensors is the requirement for an external heater, which is used to elevate the temperature of the sensor up to 200~400oC for sufficient gas response activation energy3, 8-10. However, the high temperature operation may lead to high power consumption of the detecting system, and, more importantly, considering the possible applications in wearable and skin-patchable systems, the high temperature operation may bring a severe health risk to the wearers such as thermal skin burns. To avoid these problems, a photo-activated gas sensing approach has been proposed for room temperature operation of metal oxide based gas sensors11-17. Here, sufficient solid-gas reactions is induced by the activation of metal oxide through ultra-violet (UV) illumination and subsequent ionosorption of oxygen species on the metal oxide surface. Among various metal oxide semiconductors, zinc oxide (ZnO) has attracted a huge interest due to its excellent optoelectronic properties of direct wide band gap (~3.37 eV) and a large exciton binding energy at room temperature (~60 meV)14-17. According the previous 2


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experiment reports, the performance in photocatalytic, photoelectric and gas sensing18-19 properties of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ZnO can be enhanced using hierarchical three-dimensional ZnO nanostructures in which various approaches have been paid considerable attention for ZnO synthesis, such as flame transport approach20, solution-route based hydrothermal21 and microwave assisted22 processes. In addition, it has been reported that by successfully incorporating group-I elements such as sodium (Na) in the ZnO lattice, without causing any significant lattice distortion and potential variation, p-type ZnO could be obtained through the formation of shallow acceptor levels above the valence band23-24. To date, several different methods for fabricating the Na-doped ZnO (Na:ZnO) thin films and nanowires were reported by using pulsed laser deposition25-26, chemical vapor deposition27 or sol-gel process28-30. However, for an enhanced gas sensitivity and low-concentration-level detectability, a facile synthesis method for high surface-to-volume ratio Na:ZnO should be developed. Therefore, in this work, we investigate the synthesis of nanostructured Na:ZnO having a high surface-tovolume ratio and their utilization in photo-activated gas sensors operating at room-temperature. The flowerlike Na:ZnO nanoparticles, or the nanoflowers (NFs), were successfully synthesized by a facile solution route at a low temperature using sodium hydroxide and sodium citrate. The Na:ZnO NFs exhibited hole transport and high sensitivity to acetone and various alcohol gases. Moreover, the Na:ZnO NF-based gas sensors showed a minimum detection range of 0.2 ppm of acetone gas, which may enable the diagnosis of diabetes using the exhaled breath. We systematically investigate the operation of photoactivated Na:ZnO gas sensors as well as their underlying sensing mechanism based on the photocatalytic activity at the Na:ZnO surfaces.

2. EXPERIMENTAL SECTION 2.1 Preparation of hierarchical Na:ZnO NFs. All of the chemical reagents were analytical grade and used as received without further purification. The hierarchical Na:ZnO NFs were synthesized by a facile chemical process according to the method proposed by Ma et al.31. Here, sodium hydroxide (NaOH) was 3



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used as a precipitating and doping agent, and trisodium citrate dihydrate (Na3C6H5O7.2H2O) as a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chelating agent. In a typical process, 0.1 M of zinc acetate dihydrate (Zn(CH3COO)2.2H2O) and 0.24 M of trisodium citrate dihydrate were dissolved in 30 mL of deionized (DI) water under vigorous stirring to form a transparent solution. Then, 0.5 M of NaOH was added into the above solution. The mixed solution was then stirred continuously at a reaction temperature of 40 oC for 30 min and naturally cooled down to room temperature. After that, the white precipitate was collected and washed by centrifugation with DI water and ethanol several times. Finally, the white powder of hierarchical Na:ZnO NFs were obtained after annealing the white precipitate at 60 °C for 24 h in an oven. To investigate the effect of precursor concentrations on the size and morphology of Na:ZnO NFs, various molar ratios of zinc acetate dihydrate and trisodium citrate dihydrate were examined under the same reaction time and temperature. 2.2 Characterization. The crystal structure of the synthesized Na:ZnO samples was characterized though X-ray diffraction (XRD) using a Bruker D8 advance diffractometer with Cu Kα radiation (λ = 1.542 Å) in the 2θ range of 20o to 80o. The surface morphology and microstructure of the samples were studied with a field emission scanning electron microscope (FESEM; JEOL JSM-7600F). The elemental compositions of the Na:ZnO samples were investigated by energy dispersive X-ray spectrometry (EDS). X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of the Na:ZnO samples using a PHI 500 Versa Probe (Ulvac-PHI) spectroscopy at 6.7x10-8 Pa using an Al Kα X-ray source (1486.6 eV). The XPS spectra were charge-calibrated with respect to the adventitious C 1s peak at 284.6 eV. The optical properties of the Na:ZnO samples were observed using UV-VIS-NIR spectrophotometer (Shimadsu UV-3600). The semiconductor-type of Na:ZnO was characterized by Hall effect measurement (Ecopia HMS-300 Hall measurement system) at constant magnetic field of 1.5 T and at room temperature. 2.3 Gas sensor fabrication and measurements. The chemiresistive-type gas sensors based on hierarchical Na:ZnO NFs were fabricated as follows: at first, indium doped tin oxide (ITO) electrodes on a glass substrate were patterned as interdigitated structure with inter-spacing and length of 50 µm and 4


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7000 µm, respectively, by using photolithography and wet chemical etching processes. Then, an amount 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of synthesized Na:ZnO NFs was fully dispersed in isopropyl alcohol, spun on the transparent ITO electrodes and dried at 70 oC for 2 h (Figure 1). Finally, the sensor was attached on a print circuit board and bonded to an electrical socket of the sensing chamber. The UV-activated gas sensing properties were measured using a homemade automated gas sensing system attached to a Keithley 2400 source meter. The UV activation source was a single ultraviolet light emitting diode (UV-LED) with maximum intensity located at wavelength of ~377 nm. The irradiated intensity (PL) of UV-LED was controlled by fixing the distance between the sensor and LED light source, and adjusting the bias current (Ibias) supplied to the LED: PL = IbiasV/A, where V and A are applied voltage and active area, respectively. The UV intensity was calibrated by a commercial UV intensity meter (Model-1000, Karl Suss). Also, the temperature and humidity of the chamber were monitored by using a commercial sensor attached inside the chamber (Model sht15, Sensirion AG).

3. RESULTS AND DISCUSSION 3.1 Structural and morphological analyses of Na:ZnO NFs In order to achieve high gas sensitivity using the ZnO NFs as a sensing layer, ZnO NFs having a high surface-to-volume ratio are required. Figure S1 shows a series of FESEM images for ZnO nanostructures synthesized with different trisodium citrate-to-zinc acetate molar ratios (MNaCi/MZnAc). As shown in Figure S1a-b, without trisodium citrate (MNaCi=0), only small clusters of nanosheets were synthesized. However, when the MNaCi/MZnAc was increased to 0.3, a hierarchical flowerlike structure began to appear where multiples of secondary nanosheets are grown from primary nanosheets32 (Figure S1c-d). The typical size of the clusters and the thickness (t) of the nanosheets were 4.3 µm and ~25.4 nm, respectively. Increasing the MNaCi/MZnAc to 1.8 had a little impact on the structure as shown in Figure S1e-f, but, by increasing the MNaCi/MZnAc up to 2.4, ZnO NFs with much thinner (t~16.5 nm) and densely packed nanosheets could be synthesized with an average cluster size of 3.3 µm (Figure S1g-h and Figure 1b). 5



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Further increasing the MNaCi/MZnAc to 4.8, however, resulted in partial loss of the flowerlike structure and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reduction of the cluster size down to ~2.6 µm (Figure S1i-j), which could be detrimental for achieving a high surface-to-volume ratio. Therefore, based on these observations, it can be considered that the ZnO NFs synthesized with MNaCi/MZnAc=2.4 are most suitable for achieving a high surface-to-volume ratio. Interestingly, the synthesized ZnO NFs exhibited a p-type behavior. Table S1 shows the Hall-effect measurement data of ZnO NFs performed under a magnetic field of 1.5 T at room temperature. The results clearly show that the ZnO NFs exhibit hole transport, having a carrier concentration in the range of 1016~1017 cm-3. In addition, to further verify the p-type behavior of the ZnO NFs, a p-n heterojunction device was fabricated which consists of p-type Na:ZnO NF and n-type Si. Figure S2 shows the I-V characteristics of the p-n heterojunction device demonstrating excellent rectification characteristic with a turn-on-voltage of ~2 V. Also, the device exhibited negligible change in the I-V characteristic after 10 days, showing reasonable stability of the ZnO NFs. Since a pristine ZnO film typically exhibits an n-type behavior, these results suggest that p-type dopants may be incorporated in the ZnO lattice during the synthesis, such as Na atoms. To determine the possible incorporation of Na in ZnO lattice, an XPS analysis was carried out. Figure 2a shows a survey XPS spectrum of the ZnO NFs where each peak is assigned to corresponding element. Noticeably, a peak corresponding to Na 1s could be observed at binding energy (BE) of ~1071.8 eV (Figure 2b), suggesting the presence of Na-O bonding states33-34, with an atomic concentration of 0.67 at.% (Table S2). It is reasonable to suppose that these Na atoms are originated from the synthesis precursors, NaOH and trisodium citrate. When they are dissolved in DI water, the NaOH and trisodium citrate can readily dissociate, providing a considerable number of Na+ ions in the solution. Afterwards, during the formation of ZnO lattice, these Na+ cations are likely to be incorporated in the lattice and act as p-type dopants. In addition, the O 1s spectrum was also analyzed to clarify the formation of a ZnO framework (Figure 2c). The O 1s peak can be deconvoluted into three Gaussian peaks centered at BE of 530.4 eV, 531.8 eV and 533.0 eV. The BE peak at 530.4 eV is attributed to the oxygen atoms in metal-oxygen (M-O) bonding states (OM-O), while the BE peak at 531.8 eV is associated with the oxygen atoms in the vicinity of oxygen vacancies (Vo) in ZnO lattice (Ovac). The 6


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BE peak at 533.0 eV is associated with the hydroxyl groups, chemisorbed oxygen species or carbonates35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(OOH). The relative portion of sub-peak area for OM-O (OM-O/(OM-O+Ovac)) was around 0.661 which is similar to other sol-gel-derived ZnO thin films36, suggesting a successful formation of ZnO lattice, along with the Zn 2p spectra (Figure 2d). From the XPS analysis, it can be concluded that a considerable number of Na atoms are incorporated in the ZnO lattice forming p-type Na:ZnO NFs. Also, it was possible to form a well-structured ZnO framework even at a low synthesis temperature. To further determine the crystalline structure of Na:ZnO NFs and to characterize whether the Na doping caused any influence on the crystal structure, an XRD analysis was carried out. As shown in Figure 2e, all the diffraction peaks can be well matched to the hexagonal wurtzite structure of ZnO (JCPDS No. 36-1451), with lattice constants of a = b = 3.253 Å and c = 5.211 Å, indicating that the Na:ZnO NFs are highly crystalline and the Na doping had a negligible impact on the crystalline structure. 3.2 Photo-response characteristics and the gas sensing mechanism Using the Na:ZnO NFs as a sensing layer, photo-activated room temperature gas sensors were fabricated. Initially, in order to select an appropriate light source for the activation of Na:ZnO NFs, the optical bandgap (Eg) of Na:ZnO NFs was extracted from a UV-vis analysis. As shown in Figure 2f, the Na:ZnO NFs exhibited a strong absorption at wavelengths below 400 nm which is associated with the band-toband transition characteristic of ZnO. From the Tauc relation, αhv = (hv-Eg)1/2, where α is the optical absorption coefficient, h is the Planck constant and v is the frequency, an Eg of 3.28 eV was obtained for the Na:ZnO NFs (inset of Figure 2f). Therefore, for an efficient activation of the Na:ZnO NFs, we selected an UV-LED having an emission peak wavelength centered at ~377 nm. To confirm the activation of Na:ZnO NFs by the UV-LED, the photo-response characteristics of Na:ZnO NFs were evaluated. Figure 3a shows the time-dependent photo-response characteristics of a spun-on Na:ZnO NF film under UV illumination. Here, the UV intensity (PL) was varied as 1, 5, 10 and 30 mW cm-2, and the photocurrent was measured from two interdigitated ITO electrodes at an applied bias of 5 V. As shown here, upon UV irradiation, a sharp rise of photocurrent was observed which saturated after 1~2 7



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min of irradiation. The saturated photocurrent (Iph) exhibited a power-law relationship with UV intensity, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Iph ∝ PL0.66, as shown in Figure S3. Here, the non-integer exponent is due to the complex process of carrier generation, recombination and trapping in the semiconductor film37. Although a higher photocurrent could be obtained by increasing the UV intensity, the photocurrent reproducibility and stability were found to be deteriorated with increased UV intensity as well as a slow current recovery after the UV illumination. Figure 3b shows the gas response characteristics of a Na:ZnO NF sensor when exposed to 100 ppm acetone gas under UV intensity of 5 mW cm-2. Initially, the sensor was stabilized under UV for 180 sec before introducing the acetone gas. Afterwards, when the acetone gas was introduced into the chamber at 240 sec, the sensor exhibited a sharp decrease in current, and after at around 20 sec, the current was stabilized reaching an equilibrium value. Then, when the acetone gas flow was ceased at 300 sec, the current was recovered to the saturated photocurrent level (Iph). Finally, after UV off, the current was decayed to the dark level. Here, it should be noted that the current was decreased when the Na:ZnO NF sensor was exposed to acetone gas. Since the acetone gas is a reducing gas, this result suggests that the Na:ZnO NF is p-type, in a good agreement with previous Hall measurement analysis. Nevertheless, the Na:ZnO NF gas sensors can be operated at room temperature when they are activated from UV illumination (the temperature and relative humidity profiles during the gas measurement are shown in Figure S4). A possible sensing mechanism for the UV-activated gas sensor is illustrated in Figure 3c. It is well-known that chemisorbed oxygen species such as O2- on the surface of an oxide semiconductor play a crucial role as an acceptor, forming a hole accumulation layer (HAL) near the surface38. These chemisorbed oxygen species then result in an upward band bending39 as shown in Figure 3c. Under UV illumination, the Na:ZnO NFs absorb the incoming photons and a substantial number of electron-hole pairs (EHPs) are generated in the NFs (hν → h+ + e-). Then, by a photocatalytic process, the photo-induced electrons interact with the ambient oxygen molecules leading to ionosorption of oxygen molecules on the Na:ZnO NF surface (O2(gas) + e - → O2-(ads))40. This, in turn, induces more upward band 8


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bending near the surface, forming a high concentration HAL region which exhibits increased electrical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

conductivity. It was reported that the ionosorbed oxygen ions are rather weakly bounded to the metal oxide surface41, therefore, they can be easily desorbed from the Na:ZnO NF surface. Upon the introduction of acetone gas to the Na:ZnO NF sensor under continuous UV illumination, the ionosorbed oxygen ions interact with the acetone gas molecules and desorbed from the surface by the following reaction:

CH 3COCH 3 + 4O2−( ads ) → 3CO2 + 3H 2O + 4e −

(1)

By this reaction, the trapped electrons are released near the surface and then readily recombine with the holes in the HAL region. As a result, the upward band bending is relaxed and the hole concentration in the HAL region is lowered, causing a decrease in electrical conductivity (Figure 3c). Furthermore, when the acetone gas is removed from the chamber (still under UV illumination), the number of ionosorbed oxygen molecule is recovered as well as the upward band bending state. As a consequence, the electrical conductivity of the Na:ZnO NF film restores to its original value (Figure 3b). To investigate the conduction contribution in the Na:ZnO NF film, AC impedance spectroscopy was employed. As shown in Figure S5a, both the real and imaginary parts were decreased when the film was illuminated with UV light. Then, when exposed to acetone gas, the real and imaginary parts were increased with the acetone concentration. The data can be fitted nicely with a simple equivalent circuit shown in Figure S5b, consisting of a RC parallel element. According to the RC model39,42, the charge conduction in the Na:ZnO NF film can be ascribed by the parallel competition between the conductive shell and the resistive core, as well as by the serially connected neck component between the clusters. The dominant surface resistor (Rshell) is the potential contribution to the gas-sensitive of the Na:ZnO NF film, while the parallel core resistive component (Rcore) is attributed to the gas-insensitive in sensing layer43. The parallel RC elements result from a complex summation of the inter-granular contact and the electrode contact. For the intergranular contact, the ionosorbed oxygen molecules at the grain surface create a potential Schottky barrier and a deletion layer, which correspond to the resistor of depletion layer (Rneck) and the capacitor (Cneck) at 9



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the inter-granular contacts. A similar situation holds for the electrode-Na:ZnO contact, however, in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

contrast to the inter-granular contacts, the Schottky barrier height is determined by the different work function of the semiconductor and metal. 3.3 Optimization of the gas sensing performance Figure 4a presents the UV intensity-dependent gas sensing characteristics of a Na:ZnO NF sensor when exposed to 100 ppm acetone gas. Here, the gas response (S) of the sensor was evaluated: the gas response (S) is defined as S=Rg/R0, where R0 and Rg are the resistance of the Na:ZnO NF film before and after an exposure to acetone gas, respectively. As displayed, the gas response was largely affected by the UV intensity and a maximum S of ~3.35 was obtained at UV intensity of 5 mW cm-2. This dependence of gas response on UV intensity can be related to the kinetics of gas adsorption/desorption processes on the surface of Na:ZnO NFs44. At low UV intensities around 1 mW cm-2, the numbers of photogenerated EHPs and the ionosorbed oxygen species on the surface are low, leading to relatively low gas response (S ~ 2.93). By increasing the UV intensity up to ~5 mW cm-2, the number of photogenerated EHPs is increased. As a result, the ionosorption of oxygen species on the surface is promoted due to the higher number of free electrons. Consequently, these ionosorbed oxygen species react with the acetone gas molecules, contributing to higher gas response of the sensor. Meanwhile, when the UV intensity is increased to ~10 mW cm-2, the gas response of the sensor was again decreased to S ~ 3.02. Further increasing the UV intensity resulted in lower gas response reaching S ~ 2.7. At higher intensities, although the number of electrons that are released can be increased, the relative change of resistance can be smaller since the number of photo-generated carriers has been also increased, leading to lower gas response. Furthermore, the correlations of response time (tres; the time to rise from R0 to R0+0.9·(Rg-R0)) and recovery time (trev; the time to drop from Rg to Rg-0.9·(Rg-R0)) with UV intensity were also evaluated as shown in Figure 4b. It was found that both the tres and trev showed decreasing trends with the increasing UV intensity. For instance, at UV intensity of 5 mW cm-2, the sensor exhibited tres = 18.2 sec and trev = 10


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63.0 sec, while, they decreased to tres = 16.8 sec and trev = 52.0 sec upon increasing the UV intensity to 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mW cm-2. However, considering the sensitivity and also the reproducibility of the gas sensor, we believe that UV intensity of 5 mW cm-2 is more preferable than high intensity operation. Therefore, an optimized UV intensity of 5 mW cm-2 was used for further investigations. 3.4 Low concentration-level detection A low-level detection of target gases below 1 ppm is a prerequisite for applications in ultra-sensitive healthcare monitoring systems. For instance, in order to realize high accuracy declaration of exhaled breath analysis, it is necessary to distinguish between the permissible limit for a driver drinker alcohol (78 ppm)45, lung cancer (~1.0 ppm)1 or type-I diabetes (~1.8 ppm)2-3 patients. Therefore, in this work, we verified the feasibility of the room temperature operated Na:ZnO NF gas sensors for detection of lowconcentration acetone and alcohol gases. Figure 4c shows the dynamic response of a Na:ZnO NF sensor to various acetone concentrations raised from 1 ppm to 500 ppm under a continuous UV intensity (PL = 5 mW cm-2). Here, the acetone gas exposure time was set at 60 sec with a recovery time (reset time) of 120 sec between each exposure (initial photo-stabilization period not shown in the graph). As depicted, the Na:ZnO NF sensor exhibited high gas response, fast response and recovery characteristics even at a concentration of 1 ppm. Particularly, at concentration of 1 ppm, the sensor exhibited S of 1.51, tres and trev of ~15.0 sec and ~32.0 sec, respectively. Also, the Na:ZnO NF sensor showed ultra-fast response and recovery characteristics to various to alcohol gases (inset of Figure 4c), exhibiting gas response of 1.18, 1.15, and 1.11 to 1 ppm-concentration ethanol, methanol and isopropanol gases. Figure 4d summarizes the gas response of a Na:ZnO NF sensor to various concentration (1~500 ppm) of gases including acetone, ethanol, methanol and isopropanol. The relatively high gas response to acetone gas compared to alcohol gases can be attributed to the large dipole moment of acetone gas molecule46 and enhanced interaction with the polar (002) planes of the Na:ZnO NFs47,48. To further discover the lowest limit of detection using the Na:ZnO NF sensors, the gas response characteristics to sub-ppm level acetone gas were evaluated (Figure 4e-f). Particularly, it is possible to 11



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distinguish between diabetes patients (acetone gas concentration of 1.8 ppm) and healthy people (acetone 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gas concentration < 0.9 ppm), if sub-ppm level detection of acetone gas is available. From Figure 4e, it can be seen that the Na:ZnO NF sensor can detect sub-ppm level acetone gas down to 0.2 ppm. At 0.2 ppm acetone gas, the Na:ZnO NF sensor exhibited S of 1.31. Also, the S vs. concentration in the sub-ppm range is plotted in Figure 4d showing a linear dependency. Considering the linear fitting equation, S = 0.248C+1.256 (C; acetone gas concentration) and noise level of ~0.02 from the measurement system, the lowest detection limit for acetone gas was ~0.09 ppm for the Na:ZnO NF sensor operated at room temperature. The influence of humidity on gas sensing is important for the exhaled breath monitoring since the high humidity level could affect the VOC detection signal as well as the sensing response45. Figure 5a shows the baseline resistance as a function humidity, where the Rwet denotes the sensor resistance at a given RH. It can be seen that the baseline resistance of the sensor exhibited a linearly increasing trend with the humidity. The Rwet/R0 value changed from ~1 to ~3 when the RH was increased from 25% to 92%. In order to investigate the influence of humidity on acetone gas sensing, the sensing response at different humidity was studied. As shown in Figure 5b, the sensing response (Rg/Rwet) showed a gradual decrease with the increasing RH, indicating that the ambient humidity should be also considered when monitoring the VOCs. In addition, the long-term stability of the gas sensing properties was also investigated (Figure 5c). As shown here, the gas response to 100 ppm acetone gas and the initial resistance (R0) were only slightly changed from 3.35 and ~13 MΩ to 3.33 and ~14 MΩ, respectively, after 3 months, indicating an excellent long-term stability of the gas sensor. Also, in this work, we found that the chelating agent concentration has a significant effect on the size and morphology of the Na:ZnO NFs, as showed in Figure S1. As a result, their sensing responses to acetone gas were also slightly varied when using different Na:ZnO NFs as a sensing layer. Particularly, a slight increase of gas response was observed when using Na:ZnO NFs synthesized with high MNaCi/MZnAc (Figure 5d). However, considering that the flowerlike morphology 12


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can be partially lost at MNaCi/MZnAc=4.8, the Na:ZnO NFs synthesized with MNaCi/MZnAc=2.4 is more 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

promising as a sensing layer, securing a high performance uniformity (Figure 5d). 4. CONCLUSIONS In summary, room-temperature operable ultra-sensitive gas sensors were successfully demonstrated by using p-type Na:ZnO NFs. The synthesized Na:ZnO NFs had self-assembled structure consisting multiples of ultrathin nanosheets. The large surface area of the Na:ZnO NFs lead to high gas sensitivity to acetone and various alcohol gases. Under an optimized UV intensity, the Na:ZnO NF sensor exhibited high gas response, fast response time and rapid recovery time even at room temperature. Furthermore, sub-ppm level (0.2 ppm) acetone gas could be successfully detected by the Na:ZnO NF sensor which will enable the diagnosis of various diseases including the diabetes at room temperature.

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ASSOCIATED CONTENT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information Compositions and semiconductor-type data of Na:ZnO NFs evaluated by XPS and Hall-measurement system, respectively. FESEM images of Na:ZnO NFs synthesized with different molar ratio of sodium citrate and zinc acetate dihydrate, power intensity dependence of photocurrent of Na:ZnO sensor, RC circuit model ZnO NFs gas sensor, the overall percentage relative humidity (%RH) and temperature inside the measurement chamber, and acetone response of ZnO NFs synthesized with different molar ratio of sodium citrate and zinc acetate dihydrate. This information is available free of charge via the internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *Yong-Hoon Kim ([email protected]) *Sung Kyu Park ([email protected]) Notes The authors declare no competing financial interest. †

equal contribution

ACKNOWLEDGMENTS This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource support from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200870), and by the Basic Research Lab. Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A4A1008474).

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Figure 1. (a) Schematic illustrations of patterning of ITO interdigitated electrodes; solution processing of Na:ZnO NF film on the ITO electrodes; and gas sensing operation of acetone molecules under continuous UV-LED irradiation (emission peak wavelength centered at ~377 nm); (b) A series of FESEM images for the Na:ZnO NFs synthesized with a trisodium citrate-to-zinc acetate molar ratio (MNaCi/MZnAc) of 2.4. The inset graph shows the distribution of Na:ZnO NF cluster size.

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Figure 2. (a) A survey XPS spectrum of Na:ZnO NFs with each peak assigned to corresponding element, and (b-d) XPS spectra for Na 1s (centered at BE ~ 1071.8 eV), O 1s (centered at BE ~ 530.4, 531.8 and 533.0 eV), and Zn 2p (centered at BE ~ 1021.8 and 1044.9 eV). The O 1s peak is deconvoluted to show the relative portion of sub-peak area for OM-O, Ovac and OOH. (e) An XRD spectra, and (f) UV-vis analysis data for the Na:ZnO NF film. The inset shows the Tauc plot, αhv = (hv-Eg)1/2, to extract the optical bandgap (Eg) of Na:ZnO NFs.

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Figure 3. (a) Time-dependent photo-response characteristics of a Na:ZnO NF film under UV illumination. The UV intensity (PL) was varied from 1 to 30 mW cm-2. (b) The gas response characteristics of a p-type Na:ZnO NF sensor exposed to 100 ppm acetone gas under UV intensity of 5 mW cm-2 (bias = 5 V). (c) Schematics showing the gas sensing mechanism for a p-type Na:ZnO NF sensor under UV illumination. The band bending at the surface of Na:ZnO nanosheets are displayed under UV illumination and with the presence of acetone gas molecules.

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Figure 4. (a) UV intensity-dependent gas response (S), and (b) response time of a Na:ZnO NF sensor exposed to acetone gas (concentration of 100 ppm). Inset in (a) shows a photograph of a Na:ZnO NF gas sensor. (c) The dynamic response of a Na:ZnO NF sensor to various concentrations of acetone gas from 1 to 500 ppm operated at room temperature (PL = 5 mW cm-2). Inset shows the gas response to ethanol, methanol and isopropanol (1~10 ppm). (d) Gas response vs. concentration plots for various concentrations of acetone, ethanol, methanol and isopropanol gases (1~500 ppm). (e) The dynamic response of a Na:ZnO NF sensor to sub-ppm level acetone gas from 0.2 to 1 ppm (PL = 5 mW cm-2). (f) A gas response vs. concentration plot for sub-ppm level acetone gas.

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b

4 5

mW.cm-2

16 acetone gas (100 ppm)

3

3

Rg/Rwet

Rwet/R0

4

UV intensity

2 1

12

2

R2 = 0.989

1

8 4

R2 = 0.984

0 20

40

60

80

0 20

100

40

Relative Humidity (% RH)

d

4

25

3

acetone gas (100 ppm)

3.3 20

3.2

15

3.1 3.0

R0 (MΩ)

3.4

0 100

80

Relative Humidity (% RH) 30

Response (Rg/R0)

c

60

Response (Rg/R0)

a

Response (Rg/R0)

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0

20

40

60

80

2 1 0

10

0

1

2

3

4

5

MNaCi / MZnAc

Time (day)

Figure 5. Humidity-dependent sensing properties of a Na:ZnO NF sensor under UV illumination (intensity of 5 mW cm-2); (a) Rwet/R0 response and (b) gas response under 100 ppm acetone gas. (c) Longterm stability test data for a Na:ZnO NF sensor exposed to 100 ppm acetone gas over a period of 3 months. (d) Acetone gas sensing characteristics using different Na:ZnO sensing layers. The Na:ZnO NFs were synthesized with MNaCi/MZnAc of 0.3 to 4.8.

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Ultrasensitive Room-Temperature Operable Gas Sensors Using p

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