ARTICLE pubs.acs.org/est
CuO Nanostructures As Quartz Crystal Microbalance Sensing Layers for Detection of Trace Hydrogen Cyanide Gas Mingqing Yang,†,‡ Junhui He,*,† Xiaochun Hu,§ Chunxiao Yan,§ and Zhenxing Cheng§ †
Functional Nanomaterials Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Zhongguancun Beiyitiao 2, Haidianqu, Beijing 100190, China ‡ Graduate University of Chinese Academy of Sciences (CAS), Beijing 100049, China § The No. 3 Department, Institute of Chemical Defence, P.O. Box 1048, Beijing 102205, China
bS Supporting Information ABSTRACT: In this work, quartz crystal microbalance (QCM) sensors for detection of trace hydrogen cyanide (HCN) gas were developed based on nanostructural (flower-like, boat-like, ellipsoid-like, plate-like) CuO. Responses of all the sensors to HCN were found to be in an opposite direction as compared with other common volatile substances, offering excellent selectivity for HCN detection. The sensitivity of these sensors is dependent on the morphology of CuO nanostructures, among which the plate-like CuO has the highest sensitivity (2.26 Hz/μg). Comparison of the specific surface areas of CuO nanostructures shows that CuO of higher surface area (9.3 m2/g) is more sensitive than that of lower surface area (1.5 m2/g), indicating that the specific surface area of these CuO nanostructures plays an important role in the sensitivity of related sensors. On the basis of experimental results, a sensing mechanism was proposed in which a surface redox reaction occurs between CuO and Cu2O on the CuO nanostructures reversibly upon contact with HCN and air, respectively. The CuO-functionalized QCM sensors are considered to be a promising candidate for trace HCN gas detection in practical applications.
’ INTRODUCTION Hydrogen cyanide (HCN) is a colorless gas with a bitter almond smell (although many people do not detect any odor in the presence of HCN). HCN is particularly easy to produce and release into an enclosed space, where it is extremely dangerous to humans and animals as it inhibits consumption of oxygen by body tissues via breath, ingestion, skin absorption, and ocular routes.1,2 Therefore, for human safety, it is extremely important and highly desired to detect any possible presence or leakage of HCN. Spectrophotometric, atomic absorption spectrophotometric, and electrochemical methods have been applied to detect HCN gas, because human beings do not have timely alertness to its presence.36 However, there are intrinsic limitations for these methods. For example, the analysis procedure of the spectrophotometric method is complicated and time consuming, the atomic absorption spectrophotometric method uses large apparatuses that are not available for sensing applications, and electrochemical analysis has poor HCN gas selectivity. Thus, development of novel sensors with fast response, high sensitivity, and high selectivity is attractive for fast in situ and real-time detection of HCN. Since Sauerbrey found experimental verification of the mass frequency shift relation for quartz crystal resonators, quartz crystal microbalance (QCM) has been widely exploited in the field of chemical and biological sensors because of its many r 2011 American Chemical Society
advantages such as intrinsic high sensitivity, low cost, easy installation, and inherent ability to monitor analytes in real time.718 Thus far, how to modify resonator surface properties becomes an essential issue for enhancing the range of application of QCM sensors. A variety of materials, such as ceramics, polymers, dendrimers, oils, and waxes, have been employed as coatings on the surfaces of QCM devices in efforts to attain fast response, high sensitivity, and high selectivity for chemical analytes.12,13 Recently, nanostructured materials have attracted great attention as materials for sensing chemical analytes due to their superior features, such as a very high surface to volume ratio, a lower cost, and the ease with which they can be fabricated as compared to bulk or thin film counterparts.14 CuO, a p-type transition-metal oxide with a narrow band gap (Eg = 1.2 eV), exhibits great potential for broad applications in heterogeneous catalysts, electrode materials for lithium-ion batteries, fieldemission emitters, and gas sensors.1922 Many recent efforts have been directed toward the fabrication of nanostructural CuO to enhance its performance in currently existing applications. To date, well-defined CuO nanostructures with different morphologies, Received: April 6, 2011 Accepted: June 7, 2011 Revised: May 27, 2011 Published: June 24, 2011 6088
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Environmental Science & Technology such as nanowires, nanorods, nanoribbons, nanorings, nanoellipsoids, nanotubes, nanocages, nanospheres, hollow microspheres, microflowers, and “dandelion”, have been successfully synthesized.2332 Using the novel nanostructural CuO as sensing media on QCM resonators for detection of trace HCN seems to be very promising because of their expected advantages. In our recent work,33 we successfully prepared CuO naonparticles and applied them to the sensing of trace HCN. The CuO nanoparticle (diameter ca. 60 nm) functionalized QCM resonators showed attractive sensing performance. In the current work, we synthesized flower-like, boat-like, ellipsoid-like, and plate-like CuO nanomaterials through simple modulation of hydrothermal treatment time and reagents.34 These CuO nanostructures and commercial CuO powder were used to functionalize both sides of a QCM resonator for detection of HCN gas. The sensor response to HCN is in an opposite direction as compared with other common vapors such as acetone, ethyl ether, water, n-hexane, benzene, acrylic acid, acetic acid, benzyl alcohol, and ethanol, offering excellent selectivity for HCN detection. The sensitivity of these resonators is very high (2.26 Hz/μg) and was found to be dependent on the morphology and specific surface area of CuO nanostructures. A sensing mechanism was also discussed based on experimental findings.
’ EXPERIMENTAL SECTION Reagents and Materials. Copper nitrate, aqueous ammonia
(25 wt %), polyethylene glycol (Mw = 20 000), commercial copper oxide, sodium hydroxide, acetone, ethyl ether, n-hexane, benzene, acrylic acid, acetic acid, benzyl alcohol, and ethanol were purchased from Beijing Chemical Co. All chemicals were analytic grade and used without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ 3 cm was used in all experiments and obtained from a three-stage Millipore Mill-Q Plus 185 purification system. The flower-like, boat-like, ellipsoid-like, and plate-like CuO nanomaterials were synthesized through a hydrothermal treatment method. Detailed synthesis procedures were reported in the literature.34 Characterization of CuO Nanomaterials. X-ray diffraction patterns of the obtained CuO products were recorded on a Bruker D8 Focus X-ray diffractometer using Cu KR radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA. Scanning electron microscopy observations were carried out on a Hitachi S-4300 field emission scanning electron microscope. Transmission electron microscopy observations were performed on a JEOL 2100 transmission electron microscope at an acceleration voltage of 200 kV. Specific surface areas of the CuO products were determined by nitrogen adsorptiondesorption measurements on a QuadraSorb SI automated surface area and pore size analyzer. Mass spectra of effluent gases were recorded on Omnistar with the styles of Scan Bargraph and Multiple Ion Detect. Sensor Fabrication and Characterization. A drop-coating method was used to coat both sides (5 mm in diameter) of a silver-coated QCM resonator with CuO. A dynamic gas-mixing apparatus was used to steadily generate gas containing HCN of low concentration. Vapor generated by liquid HCN was taken away by a mass flow controller manipulated gas flow and further diluted in proportion by another steady gas flow. In order to simulate real environments, all gas flows were air provided by an air compressor. A four-way valve was applied to switch between
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Figure 1. Typical SEM images of CuO products: (a) flower-like CuO, (b) boat-like CuO, (c) ellipsoid-like CuO, (d) plate-like CuO.
air flow and diluted HCN flow. Frequency shifts were recorded by an Agilent 53131A universal counter linked to a computer.
’ RESULTS AND DISCUSSION Structures and Morphologies of As-Prepared CuO. Figure S1, Supporting Information, shows typical XRD patterns of as-prepared CuO products and commercial CuO. Diffraction peaks were observed at 2θ of 32.5°, 35.5°, 38.7°, 48.8°, 53.4°, 58.3°, 61.6°, 66.3°, and 68.1° and could be assigned to the (110), (002), (200), (202), (020), (202), (113), (311), and (220) planes of monoclinic CuO. Except for these CuO peaks, no other peaks were observed, indicating the high purity of CuO products. As shown by Figure 1a, flower-like CuO nanostructures formed when the time of hydrothermal treatment was 0.5 h. An individual flower-like nanostructure was assembled by CuO nanoplates of ca. 80 nm in thickness. When the time of hydrothermal treatment was 2 h, well-defined boat-like structures formed in high yield (98%) (Figure 1b). They have nearly uniform shape and size. The average length and width of boatlike structures were estimated to be 10 and 5 μm, respectively. From the enlarged image in Figure 1b, inset, each boat-like structure is composed of nanoplates. The thicknesses of nanoplates are in the range of 100150 nm. As shown in Figure 1c, without addition of NH3 3 H2O, ellipsoids of uniform shape and size were obtained on a large scale. The average length and width were estimated to be ca. 3 and ca. 2 μm, respectively. A magnified image of the rectangled ellipsoid (Figure 1c) shows that the ellipsoids consist of stacked CuO layers. When Cu(NO3)2, NaOH, and NH3 3 H2O were hydrothermally treated at 150 °C without addition of PEG, 2D nanoplates of polygonal shapes were obtained in high yield (98%) (Figure 1d). The thicknesses of nanoplates are ca. 100 nm. The structure of the plate-like CuO was further investigated by TEM (Figure S2a, Supporting Information), which again indicates that it is composed of 2D nanoplates. HRTEM image was taken at the head part of 6089
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Figure 2. Profiles of the frequency shift of (a) flower-like CuO, (b) boat-like CuO, (c) ellipsoid-like CuO, (d) plate-like CuO, and (e) commercial CuO-functionalized QCM resonators upon exposure to air containing 50 ppm HCN at 25 °C.
nanoplate (Figure S2b, Supporting Information), which reveals the single-crystal nature of nanoplate. The d spacings of [200] planes were measured to be 0.23 nm, which correspond to those of monoclinic CuO. The corresponding fast Fourier transform (FFT) pattern (Figure S2b, inset, Supporting Information) of a nanoplate exhibits a monoclinic single-crystal pattern. Nitrogen physisorption represents the most widely used technique to measure the specific surface areas of materials. By nitrogen adsorptiondesorption measurements, the specific surface areas of flower-like, boat-like, ellipsoid-like, and platelike CuO and commercial CuO powder were measured to be 5.2, 5.1, 1.9, 9.3, and 1.5 m2/g, respectively, following the order of plate-like CuO > flower-like CuO ≈ boat-like CuO > ellipsoidlike CuO > commercial CuO. Gas Sensing Properties. Sensing properties of CuO-functionalized QCM resonators were examined by placing them in a
glass chamber inside of an incubator where temperature (25 °C) and relative humidity (10%) were accurately controlled. The concentration of HCN in the carrier gas (air) was controlled at 550 ppm. A naked silver-coated QCM resonator was also tested as control for comparison (Figure S3, Supporting Information). While the solid arrow indicates the time at which air containing 50 ppm HCN was introduced into the testing chamber, the dashed arrow indicates the time at which air started to flush the testing chamber. Clearly, the naked silver-coated QCM resonator had no significant response to HCN of 50 ppm. Figure 2 shows response curves of as-prepared CuO and commercial CuO-functionalized QCM resonators under otherwise identical conditions. Compared with the naked silver-coated QCM resonator, all the CuO-functionalized QCM resonators have fast, significant response to HCN of 50 ppm. Analysis of the curves in Figure 2 (especially Figure 2a and 2d) shows that they 6090
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Table 1. BET Specific Surface Areas of CuO Products, Frequency Shifts of CuO-Functionalized QCM Resonators 10, 30, and 300 s after Introduction of 50 ppm HCN, and the Corresponding Sensitivitiesa
a
CuO
BET specific surface
frequency shift
frequency shift
frequency shift
sensitivity
nanostructures
area (m2/g)
at 10 s (Hz)
at 30 s (Hz)
at 300 s (Hz)
(Hz/μg)
commercial CuO ellipsoid-like CuO
1.5 1.9
11 ( 2 17 ( 2
21 ( 2 28 ( 2
41 ( 2 65 ( 2
0.31 0.49
boat-like CuO
5.1
28 ( 3
42 ( 3
97 ( 3
0.80
flower-like CuO
5.2
29 ( 3
45 ( 3
99 ( 3
0.83
plate-like CuO
9.3
79 ( 5
105 ( 5
148 ( 5
2.26
The coating amounts were all 35 μg.
can be broken into two parts: the “rapid” (steep slope) and “slow” (shallow slope) response. Figure 2a shows a response curve of flower-like CuO-functionalized QCM resonator (coating amount 35 μg). When air containing 50 ppm HCN was introduced into the testing chamber, the QCM frequency began to rise. The frequency shift initially increased rapidly, reaching 29 Hz in 10 s. After 30 s, the frequency shift reached 45 Hz. Then the frequency shift increased less rapidly after 30 s. After 320 s, when the testing chamber was flushed by air, the frequency shift decreased rapidly. In addition, 300 s later, the frequency shift recovered basically. Figure 2b, 2c, 2d, and 2e show response curves of boat-like, ellipsoid-like, plate-like, and commercial CuO-functionalized QCM resonators, which display that the frequency shifts reached 28, 17, 79, and 11 Hz, respectively, after 10 s HCN exposure. The sensitivities of CuO-functionalized QCM resonators were assessed by the (frequency shift)/(coating amount) ratio, in order to compare the performance of different CuO nanostructures. The results show that the sensitivity of the plate-like CuO-functionalized QCM resonator is significantly higher (2.26 Hz/μg) than those of flower-like (0.83 Hz/μg), boat-like (0.80 Hz/μg), ellipsoid-like (0.49 Hz/μg), and commercial (0.31 Hz/μg) CuO-functionalized QCM resonators. A detailed comparison of the sensitivities of these QCM resonators was made in Table 1. As shown in column 2 of Table 1, the specific surface areas of as-prepared CuO and commercial CuO follow the order of plate-like CuO > flower-like CuO ≈ boat-like CuO > ellipsoid-like CuO > commercial CuO. Column 3 of Table 1 shows the frequency shifts recorded 10 s after HCN exposure. Clearly, the sensitivities of as-prepared CuO and commercial CuO-functionalized QCM resonators follow the order of plate-like CuO > flower-like CuO ≈ boat-like CuO > ellipsoid-like CuO > commercial CuO, in striking agreement with that of their specific surface areas. Column 4 of Table 1 shows the frequency shifts recorded 30 s after HCN exposure. Column 5 of Table 1 shows the frequency shifts recorded 300 s after HCN exposure. They are higher than those recorded at 10 s with the frequency shift of plate-like CuO-functionalized QCM resonator reached to 148 Hz. Column 6 of Table 1 shows the sensitivities of CuO-functionalized QCM resonators. Similarly, the sensitivities of as-prepared CuO and commercial CuO-functionalized QCM resonators also follow the order of their specific surface areas. From Figure 2 and Table 1, the as-prepared CuO-functionalized QCM resonators have higher sensitivities (0.492.26 Hz/ μg) than the commercial CuO-functionalized QCM resonator (0.31 Hz/μg). Among them, the plate-like CuO-functionalized QCM resonator has the highest sensitivity (2.26 Hz/μg). Thus, we took the plate-like CuO-functionalized QCM resonator as a typical resonator and studied its reproducibility, stability, and
Figure 3. Profiles of the frequency shift of a plate-like CuO-functionalized QCM resonator upon exposure to air containing 50 ppm HCN at 25 °C in three continuous cycles.
calibration curves of frequency shift versus coating amount and frequency shift versus HCN concentration. Reproducibility and stability are regarded as important aspects for sensors. As shown in Figure 3, the response curves are similar for three continuous cycles with nearly no changes in response, response time, and recovery time, indicating good reproducibility and stability. The high reproducibility and stability also indicate the robustness of the sensing layers, though they were applied to QCM electrodes by a simple drop-coating method. Doubtlessly, such high reproducibility and stability are critical for applicable sensing devices. The effluent air flow passing the testing chamber was analyzed in real time by a mass spectrometer for clarification of possible sensing mechanisms. Interestingly, after the plate-like CuOfunctionalized QCM resonator was exposed to HCN of 50 ppm, a new species of m/z = 52 was detected by the mass spectrometer, which is attributed to (CN)2.35 We also monitored the variation of (CN)2 ion intensity with time. As shown in Figure S4, Supporting Information, the variation profile of (CN)2 ion intensity is similar to that of QCM frequency shift (Figure 3). When air containing 50 ppm HCN was introduced into the testing chamber, the (CN)2 ion was detected. When the testing chamber was flushed by air, the (CN)2 ion intensity began to decrease, and reached the initial baseline eventually. The (CN)2 ion intensity variations are also similar for the three continuous cycles. Such striking agreements between the QCM frequency shift and the (CN)2 ion intensity doubtlessly point to production of (CN)2 as a result of the contact of HCN with the plate-like CuO. 6091
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Figure 4. Profiles of the frequency shift with increasing coating amount of plate-like CuO-functionalized QCM resonator upon exposure to air containing 50 ppm HCN at 25 °C.
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Figure 6. Profiles of the frequency shift of a plate-like CuO-functionalized QCM resonator upon exposure to saturated acetone, ethyl ether, water, n-hexane, benzene, acrylic acid, acetic acid, benzyl alcohol, and ethanol.
resonator decreases proportionally with the decrease of HCN concentration. Figure S6, Supporting Information, shows the calibration curve obtained using the frequency shifts recorded 10 s after HCN exposure.13 Clearly, the frequency shift shows a linear relationship with HCN concentration in the HCN concentration range of 050 ppm. A linear equation was derived as follows ΔF ¼ 1:52c + 0:43, r ¼ 0:9935
Figure 5. Profiles of the frequency shift of a plate-like CuO-functionalized QCM resonator upon exposure to HCN gases of varied concentrations at 25 °C (coating amount 35 μg).
A series of sensors with varied amounts of plate-like CuO were prepared to investigate the relationship between response and coating amount. In Figure 4, profiles of frequency shift are shown with increasing the coating amount of plate-like CuO under a constant HCN concentration (50 ppm). Figure S5, Supporting Information, shows the calibration curve obtained using the QCM frequency shifts recorded 10 s after exposure.13 Clearly, the QCM frequency shift increases linearly with the coating amount of plate-like CuO in the range of 872 μg, from which a linear equation was derived as follows ΔF ¼ 2:51m 5:15, r ¼ 0:9967ð25 °C; HCN, 50 ppmÞ where ΔF is frequency shift (Hz), m is coating amount (μg), and r is the regression coefficient. From this linear equation, the more coating amount of plate-like CuO on QCM resonator, the higher the QCM frequency shift. Figure 5 displays the response curves of a plate-like CuOfunctionalized QCM resonator (coating amount 35 μg) to HCN of varied concentrations. When a HCN gas of given concentration was introduced into the testing chamber, a frequency shift was recorded and all responses were fast within the initial few seconds toward HCN of varied concentrations. The response profiles are similar in shape, and the response of the
where ΔF is the frequency shift (Hz), c is the HCN concentration (ppm), and r is the regression coefficient. Selectivity is one of the most important factors that determine the eventual applicability of any chemical sensors. The sensitivity of the plate-like CuO-functionalized QCM resonator was investigated by exposing the resonator to saturated vapors of a variety of common volatile chemicals (25 °C), including acetone, ethyl ether, water, n-hexane, benzene, acrylic acid, acetic acid, benzyl alcohol, and ethanol. As shown in Figure 6, the plate-like CuO-functionalized QCM resonator not only showed large and rapid response to all the examined chemicals but also demonstrated fast recovery when the chemical vapors were removed. However, surprisingly and interestingly, reversed signals were observed for the examined chemicals as compared with HCN. These characteristics would allow unambiguous report on HCN among these volatile substances. On the basis of comprehensive analyses of experimental results, a plausible mechanism was proposed. A surface redox reaction might have occurred reversibly between CuO and Cu2O.36,37 When CuO nanostructures were exposed to HCN, the surface CuO was reduced to Cu2O by HCN, forming (CN)2 species. In this process (from CuO to Cu2O), one oxygen atom was lost. Thus, the total mass on the QCM resonator decreased, and a positive frequency shift was recorded according to the Sauerbrey equation. Initially, the speed of the redox reaction was high because of the high HCN concentration, which resulted in the “rapid” response in the detection. Then, production of Cu2O on the surface of CuO reduced the speed of the redox reaction, which resulted in the “slow” response. When the testing chamber was flushed by air, the surface Cu2O was reoxidized to CuO by O2 and the original frequency shift recovered. These processes are reversible, which brings the excellent reproducibility and stability of the CuO-functionalized QCM resonator. Verification of Cu2O presence is doubtlessly very important. However, it is 6092
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’ ASSOCIATED CONTENT
bS
Supporting Information. Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +86 10 82543535; e-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 20871118), the National Basic Research Program of China (Grant No. 2010CB934103), the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS) (Grant Nos. KGCX2-YW-111-5 and KSCX2YW-G-059), and “Hundred Talents Program” of CAS. This work was also partially supported by the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS). ’ REFERENCES (1) Patnaik, P. Cyanides, Inorganic; John Wiley & Sons, Inc.: New York, 2006; pp 317335 (2) Ballantyne, B.; Bismuth, C.; Hall, A. H. Cyanides: Chemical Warfare Agents and Potential Terrorist Threats; John Wiley & Sons, Ltd.: New York, 2007; pp 495542 (3) Toth, K.; Pungor, E. Determination of cyanides with ion-selective membrane electrodes. Anal. Chim. Acta 1970, 51, 221–230. (4) Meyer, R. E.; Lantz, P. M. Reactions of the cyanide ion with the packed-bed silver electrode: Analysis for the cyanide ion. J. Electroanal. Chem. 1975, 61, 155–163. (5) Sweileh, J. A. Study of equilibria in cyanide systems by gasdiffusion measurement of hydrogen cyanide. Anal. Chim. Acta 1996, 336, 131–140. (6) Langmaier, J.; Janata, J. Sensitive layer for electrochemical detection of hydrogen cyanide. Anal. Chem. 1992, 64, 523–527. (7) O’Sullivan, C. K.; Guilbault, G. G. Commercial quartz crystal microbalances - theory and applications. Biosens. Bioelectron. 1999, 14, 663–670. (8) Kimura, M.; Sugawara, M.; Sato, S.; Fukawa, T.; Mihara, T. Volatile Organic Compound Sensing by Quartz Crystal Microbalances Coated with Nanostructured Macromolecular Metal Complexes. Chem. Asian J. 2010, 5, 869–876. (9) Gomes, M. T. S. R.; Nogueira, P. S. T.; Oliveira, J. A. B. P. Quantification of CO2, SO2, NH3, and H2S with a single coated piezoelectric quartz crystal. Sens. Actuators B: Chem. 2000, 68, 218–222. (10) Shafiqul Islam, A. K. M.; Ismail, Z.; Ahmad, M. N.; Saad, B.; Othman, A. R.; Shakaff, A. Y. M.; Daud, A.; Ishak, Z. Transient parameters of a coated quartz crystal microbalance sensor for the detection of volatile organic compounds (VOCs). Sens. Actuators B: Chem. 2005, 109, 238–243. (11) Matsuguchi, M.; Kadowaki, Y. Poly(acrylamide) derivatives for QCM-based HCl gas sensor applications. Sens. Actuators B: Chem. 2008, 130, 842–847. (12) Liang, C.; Yuan, C.-Y.; Warmack, R. J.; Barnes, C. E.; Dai, S. Ionic Liquids: A New Class of Sensing Materials for Detection of Organic Vapors Based on the Use of a Quartz Crystal Microbalance. Anal. Chem. 2002, 74, 2172–2176. (13) Lee, S.-W.; Takahara, N.; Korposh, S.; Yang, D.-H.; Toko, K.; Kunitake, T. Nanoassembled Thin Film Gas Sensors. III. Sensitive
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