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Development of highly sensitive and selective ethanol sensor based on lance-shaped CuO nanostructures
Development of highlysensitive and selective ethanol sensor based on lanceshaped CuO nanostructures Ahmad Umar, Jong-Heun Lee, Rajesh Kumar, O. Al-Dossary, Ahmed A. Ibrahim, S. Baskoutas PII: DOI: Reference:
S0264-1275(16)30584-6 doi: 10.1016/j.matdes.2016.05.006 JMADE 1744
To appear in: Received date: Revised date: Accepted date:
22 January 2016 3 May 2016 4 May 2016
Please cite this article as: Ahmad Umar, Jong-Heun Lee, Rajesh Kumar, O. AlDossary, Ahmed A. Ibrahim, S. Baskoutas, Development of highlysensitive and selective ethanol sensor based on lance-shaped CuO nanostructures, (2016), doi: 10.1016/j.matdes.2016.05.006
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ACCEPTED MANUSCRIPT Development of highlysensitive and selective ethanol sensor based on lance-shaped CuO nanostructures
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Ahmad Umar,1,2Jong-Heun Lee3, Rajesh Kumar4, O. Al-Dossary5, Ahmed A. Ibrahim1,2, S. Baskoutas6 1
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Department of Chemistry, College of Science and Arts, Najran University, P.O.Box-1988, Najran11001, Kingdom of Saudi Arabia,2Promising Centre for Sensors and Electronic Devices (PCSED),
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Najran University, P.O.Box-1988, Najran-11001, Kingdom of Saudi Arabia, 3Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea, 4PG Department of Chemistry, JCDAV College, (Punjab University), Dasuya-144205, India, 5 Department of Physics, King Saud University, Riyadh-11442, Saudi Arabia, 6Department of Materials Science, University of Patras, Patras GR-26504, Greece *Corresponding Author: [email protected] (Ahmad Umar)
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Abstract
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Herein, we report the successful synthesis and characterization of lance-shaped CuO nanostructures prepared by a simple andfacile hydrothermal technique. The prepared nanostructures
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were examined by several techniques which revealed that the lance-shaped nanostructures are wellcrystalline, grown in very high density and possessing monoclinic crystal structure. In order to
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develop a nano-device using synthesized nanomaterial, the prepared powder was coated on the alumina substrate with suitable electrodes and the fabricated sensor device was tested for ethanol gas. Interestingly, the gas response (resistance ratio)and response timesfor the fabricated devices were9.1 and 127s, respectively for 100 ppm of C2H5OH at 300C.Further, the selectivity of the developed sensing device was tested against H2 and CO gases and interestingly, it was observed that the fabricated sensor exhibited excellent selectivity towards ethanol gas. Key Words: Lance-Shaped;CuO, Hydrothermal; Ethanol; Gas Sensors
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ACCEPTED MANUSCRIPT 1. Introduction Recently, nanostructured p-type semiconductor metal oxides have been explored for sensing
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reducing as well as oxidizing hazardous, highly flammable and poisonous gases. The morphology,
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surface-to-volume ratio, cationic and anionic surface defects, porosity, particle size,theextent of agglomeration and crystallographic orientations of the metal oxide nanomaterials strongly affect the
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various gas sensing parameters[1-4]. For excellent gas sensing properties, metal oxide nanomaterials
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with high surface area to volume ratio are desired for better adsorption/desorption phenomena of the analyte gas[5]. Among the various nanostructured metal oxides, CuO nanomaterials with a
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monoclinic lattice and band gap range of 1.21–1.51 eV are extensively explored for their gas sensing
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behavior[6, 7]. A number of synthetic methods such as solvothermal, thermal evaporation,
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hydrothermal and microwave-assisted hydrothermal, ultrasonic spray pyrolysis, and electrodepositions have been reported intheliterature for the design of gas sensors using CuO
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nanostructures such as nanowires, nanorods, mesoporous films, nanocubes, nanospikes, nanourchins, nanosheets, hollow spheres etc.[8-14].Aslani et al.[6]reported a synergism between crystallite size and surface area to elaborate the enhanced CO sensing using cloudlike CuO nanoparticles.In a similar study, Yang et al. [8]found that CuO nanorods with high surface to volume ratio having diameter 10–20 nm and length of 45–80 nm synthesized through microwave assisted hydrothermal method using PEG-400 and urea, exhibited a maximum response of 9.8 for 1000 ppm ethanol at the working temperature of 210◦C.Park et al. [12]utilized highly porous CuOnanocubes with average
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ACCEPTED MANUSCRIPT edge size of 90 nm and pore size of 52 nm synthesized through wet solution polyol process as
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HCHO sensors atanoptimum temperature of 300◦C. The author also reported a very low detection
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limit 6 ppb at 250◦C for the same. In contrast, Volanti et al. [15] found that spine-assembled urchin-
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like CuO nanostructures with the lower surface area exhibited a better response to H2 gas as compared to fiber-like CuO nanorods with higher surface area. This was attributed to the multiple
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interconnections between spines of the urchin-like CuO nanostructures which can lead to an
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increased cross-sectional area so as to facilitate the process of charge migration [16 ,17]. In a similar report recently, Yang et al. [18] revealed that 3D hierarchical CuO flowers with BET surface area of
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15.0 m2/g exhibited better gas responses for ethylacetate and ethanol as compared to 2D branching
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CuO nanosheets with BET surface area of 20.8 m2/g. both synthesized through microwave-assisted hydrothermal method. Further Kim et al.[19] reported that sensing behavior of networked CuO
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nanowires was better for reducing gases such as CO, C6H6, C7H8, and H2etc.as compared to oxidizing gases like NO2, SO2 and O3. On the basis of these reports thus, it can be stated that the gas sensing parameters are greatly influenced by not only the morphology, surface to volume ratio and particle size but also on the interconnections present between the secondary structures of the 3D CuO nanomaterials. In is noteworthy to mention that, for p-type semiconductor metal oxides like CuO the conductivity is very little affected by high-temperature ranges as compared to n-type semiconductor nanomaterials[20]. Additionally, p-type metal oxides can easily exchange their lattice oxygen with air in order to maintain their stoichiometry. This is very useful property in order to
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ACCEPTED MANUSCRIPT maintain long-term stability of the sensor. Owing to these properties, CuO nanostructured materials
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can be used for the fabrication of sensors especially for ethanol, which is widely used in biomedical
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applications, food industries, wine quality and breath analysis[21, 22].In this regard, here we report a
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simple hydrothermal method for the synthesis of lance-shaped CuO nanostructures with variable sized branches and explored their sensing properties for the C2H5OH, CO and H2 gases. Experimental Details
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2.
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2.1. Synthesis and characterization of lance-shaped CuO nanostructures Well-crystalline lance-shaped CuO nanostructures were synthesized by a facile hydrothermal
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process using copper nitrate [Cu(NO3)2.3H2O], hexamethylenetetramine [HMTA; C6H12N4] and
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ammonium hydroxide [NH4OH]. To synthesize lance-shapedCuO nanostructures, all the chemicals were purchased from Sigma-Aldrich and used as received without any further purification. De-
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ionized (DI) water was used assolvent for the synthesis. In a typical synthesis process, equimolar aqueous solutions of copper nitrate and hexamethylenetetramine, both made in 50 ml DI water each, were mixed well under vigorous stirring. Further, the pH of the resultant solution was then maintained at pH=11 using NH4OH andtheresultant solution again stirred for 2h at roomtemperature. After stirring, the resultant solution was transferred to Teflon-lines stainless steel autoclave and heated to 160°C for 7h. After desired reaction time, the autoclave was allowed to cool at room temperature and finally black colored precipitate was obtained which was decanted and dried overnight at room-temperature. The obtained powder was further washed with DI water and ethanol
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ACCEPTED MANUSCRIPT and finally dried at 75 ± 10°C for 4 h in an electric oven. The prepared CuO powder was further
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characterized in detail using several analytical techniques.
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The morphologies of the prepared CuO powder was examined by field emission scanning
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electron microscopy (FESEM; JEOL-JSM-7600F), attached with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM; JEOL-JEM-2100F) equipped with high-
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resolution TEM (HRTEM). The crystallinity and crystal phases of the synthesized powder were
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evaluated by the X-ray diffraction (XRD; PANanalyticalXpert Pro.) measured with Cu-Kα Radiation (λ=1.54178Å) in the range of 20-70º. The optical and chemical compositional properties of the
transform
infrared
spectroscopy
(FTIR;
Perkin
Elmer-FTIR
Spectrum-100)
at
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Fourier
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prepared powder were studied by UV-visible spectroscopy (Perkin Elmer-UV/VIS-Lambda 950) and
roomtemperature, respectively. The FTIR spectrum was recorded with KBrdispersion in the range of
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450-4000 cm-1.
2.2. Fabrication of ethanol gas sensor based on as-grown lance-shaped CuO nanostructures The lance-shaped CuO nanostructures were heat-treated at 500 C for 2 h to obtain thermally stable gas sensing characteristics at elevated sensing temperature (300 – 450 C), which weredispersed in DI water to prepare slurry for the functional nanomaterials. The prepared slurry was then coated onanalumina substrate (area: 1.5 x 1.5 mm2, thickness: 0.25 mm) with two Au electrodes on its top surface and micro-heater on its bottom surface. After drying, the sensors were heat-treated again at 450C for 2hto remove residual water solvent completely. The sensor
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ACCEPTED MANUSCRIPT temperatures were controlled using the micro-heater underneath the substrate and were measured
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using an IR temperature sensor (Rayomatic14814-2, EurotonIRtec Co.). The sensor was contained in
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a specially designed, small-volume (1.5 cm3), quartz tube in order to minimize the delays in
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changing the atmosphere. The detailed experimental setup is shown elsewhere [ 2 3 ] . The gas responses (S= Rg/Ra, Rg: resistance in gas, Ra: resistance in air) to 100 ppm C2H5OH, CO, and H2
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were measured at 300 - 450C. Gas concentrations were controlled by changing the mixing ratio of
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the parent gases (100 ppm C2H5OH,100 ppm CO, and 100 ppm H2, all in dry synthetic air balance) and dry synthetic air. The DC 2-probe resistances were measured using an electrometer interfaced
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3. Results and Discussion
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with a computer.
3.1. Structural properties of as-grown lance-shaped CuO Nanostructures
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Fig, 1(a) shows the X-ray diffraction pattern obtained with Bragg angle ranging from 20o to 70o. The observed diffraction peaks are closely matched withastandard data file of CuO (JCPDS #05-0661). The analysis with standard data showed that the synthesized material belongs to crystallographic point group of 2/m or C2h[23, 24].Well-defined diffraction reflections indexed at 2θ = 32.6°, 35.5°, 38.7°, 46.4°, 48.8° , 53.4°, 58.3°, 61.7°, 66.2° and 68.1° were observed corresponding to the monoclinic form of CuO with lattice planes of (110), ( 1 11 002 ), (111-200),
(112) , ( 202) , (020), (202), ( 1 13) , ( 3 11) and (220) respectively. These diffractions match very well
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ACCEPTED MANUSCRIPT with the literature[25]. The results also indicated that lance-shaped shaped CuO nanostructureswere
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formed in asingle phase without impurities corresponding to precursor materials.
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The general morphologies of the as-synthesized lance-shaped CuO nanostructures were
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examined by field emission scanning electron microscopy (FESEM). In Fig. 1(b-d), the low magnification images whereas in Fig. 1(e-f) high magnification images are shown for the lance-
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shaped CuO nanostructures.
Fig. 1. Typical (a) XRD pattern and (b-d)low-magnification and (e-f) high-resolution FESEM images of lance-shaped long leaflets. These FESEM images confirmed that the as-synthesizedCuO nanostructures possess lance-shaped morphologies grown in very high density. Dozens of lance-shaped shaped long leaflets are grown from a common base. These leaflets are wider at their bases with sharper tips. The central part of
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ACCEPTED MANUSCRIPT each leaflet is wider whereas the corners are narrowerforming a lance-shape. The typical length of
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each leaflet is ~2.4 – 2.5 μm with anaverage width of ~50 ± 5 nm (Fig.1(e-f)). Thus, the present
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hydrothermal route is an effective one to synthesize dense lance-shaped CuO nanostructures with
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leaflet of different sizes and morphologies along with very high surface to volume ratio under the applied reaction conditions. These are some of the pre-requisites for the fabrication of highly
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sensitive and selective gas sensors.
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In addition to longer and thinner leaflets, the as-synthesized CuO product also consists of shorter, thicker and broader petals with sharper tips grown on a common base with high density.
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Low and high-resolution FESEM images are shown for these CuO nanostructures with petals like
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morphologies in Fig. 2(a-b) and Fig. 2(c-d) respectively. The typical length of each petal is ~300 ±10
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nm with anaverage width of ~200 ± 10 nm respectively (Fig. 2(a-d)).
Fig.2. (a-b)Low-magnification and (c-d) high-resolution FESEM images of shorter, thicker and broader petals with sharper tips.
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ACCEPTED MANUSCRIPT Detailed microstructural investigations of a single leaflet of lance-shaped CuO nanostructures
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were performed using HRTEM technique. Fig. 3(a) and (b) represent the typical low and high-
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resolution TEM images, respectively. Fig. 3(b) reveals that the leaflet of lance-shaped CuO
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nanostructures is crystalline in nature withaninterplanardistance of 0.27 nm, corresponding to the (110) planes of monoclinic CuO[19, 26]. These result also reveal that the growth of the CuO
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nanostructures is along the (110) plane. As grown leaflets of lance-shaped CuO nanostructures are
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originated from a common base, the CuO leaflets are supposed to have strong binding due to electrostatic attraction. Dey et al. [27] also reported plate-shaped CuO nanostructures arranged to
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form a flower-like topography as well as few bunches of small particles at pH 11. High pH of the reaction solution favors the formation of agglomerated structures probably due to the existence of
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hydrogen bonding between HO- moieties present in the particles.
Fig.3. (a)low-magnification and (b) high-resolution TEM images of lance-shaped long leaflets. 9
ACCEPTED MANUSCRIPT To examine the elemental composition of the lance-shaped CuO nanostructures energy
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dispersive spectroscopy (EDS) attached with FESEM was applied. Fig 4(a) clearly revealed that the
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lance-shaped CuO nanostructures are made of copper and oxygen. As no other peak related with any
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other element is present in the spectrum, as prepared lance-shaped CuO nanostructures are pure. The results are in good correlation with XRD analysis (Fig. 1(a)).
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The chemical composition of the as-preparedlance-shaped CuO nanostructures was examined
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by Fourier transform infrared (FTIR) spectroscopy at roomtemperature(Fig.4(b)). The observed FTIR spectrum exhibited various well-defined absorption peaks at 509, 605, 1389, 1628, 2931 and
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vibrations of M-O (Cu–O).
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3421 cm-1. The presence of strong peaks at ~509 and 605 cm-1 are attributed to the stretching
Fig. 4. (a) EDS and (b) FTIR spectra oflance-shaped CuO nanostructures.
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ACCEPTED MANUSCRIPT The results confirmed that the lance-shaped CuO nanostructures possess monoclinic CuO phase[28-
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30].The absorption peak at 1389 cm-1 may be dueto CO 32 , which usually appears in the spectrum
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when the FTIR sample are prepared and measured in the air[30]. Further, a small and wide band at
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3421 cm−1 and additional sharp band at2931 cm−1 may be ascribed to the asymmetric and symmetric vibrational modes oftheO-H bond of the H2O molecules physisorbed on the surface of the CuO[31].
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The weak band at 1628 cm−1 may be attributed to the bending vibration modes of O–H groups of
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these adsorbed H2O molecules[30].
3.2. Optical properties of as-grown lance-shapedCuO Nanostructures
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The optical properties of CuO nanostructures were examined by UV-visible spectrum
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measured at room-temperature and demonstrated in Fig.5 (a). In order to have a precise determination of the energy position of the absorption edge, one can take the edge position to be
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determined by the maximum of the first derivative of the optical absorption with respect to the energy and it corresponds to the optical band gap[32, 33].The first derivative of the spectrum as is shown in Fig. 5 (b) indicates a peak at 1.24 eV which is similar to the bulk value. As is also well known by utilizing the Tauc’s formula, the relationship between absorption coefficient and the incident photon energy of semiconductors can be obtained. Hence, the optical band gap,Egcan be experimentally obtained from absorption coefficient according to the Tauc’s equation as follows (Eq. 1): (αhν) = A(hν – Eg)n …………(1)
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ACCEPTED MANUSCRIPT where α is the absorption coefficient, A is constant, and n is equal 2 for indirect transition
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semiconductor. The corresponding graph for indirect transition is shown in Fig. 5 (c). As is seen
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from Fig. 5 (c), the optical band gap obtained through extrapolation, is 1.23 eV corresponds well
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with the energy value obtained in Fig. 5 (b)andalso matches well with the reported value[34]. The peak which appears at 1.44 eV in Fig. 5b is an artifact signal from the instrument and, therefore,
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does not correspond to any transition.
Fig. 5.(a)UV-visible spectrum (b) first order derivative of UV –visible absorbance and (c) band gap calculations from Tauc’s formula for CuO nanostructures. Here, we are using the potential morphing method within the effective mass HartreeFockapproximationto obtain the size dependent exciton energy[35-37].The observed results are depicted in Fig. 6, where we have used the following materials parameters: me* 0.35 , mh* 1.47 , 0 12 , 6.45 and LO 74 meV andEg(bulk) = 1.2 eV[38-41].
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ACCEPTED MANUSCRIPT As is seen from Fig. 6 the size of synthesized CuOlance-shaped nanostructure are in the
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weak confinement regime, e.g. their radius is greater than the exciton Bohr radius of CuO, which
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varies from 6.6–28.7 nm[38-42].
Fig.6. Variations of optical band gap as a function of atomic radius
Nanostructures
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3.3. Plausible growth mechanism for the formation of as-grown lance-shaped CuO
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As reported earlier, the pH of the growth solution plays an important role in the formation of Cu(OH)2 growth nuclei from theprecursor. At pH ˃ 10, initially formed copper (II) hydroxynitrate [Cu2(OH)3NO3] is easily converted to Cu(OH)2 by the replacement of NO3 ions (Eq.-2,3)[27].
2Cu(NO3 ) 2 3NH 4 OH [Cu 2 (OH)3 NO3 ] 3NH 4 3NO3 ……..(2) [Cu 2 (OH)3 NO3 ] HO 2Cu(OH)2 NO3 …….(3) The presence of the HMTA in the reaction solution also accelerates the formation of Cu(OH)2 growth nuclei through the generation of HO- ions. HMTA undergoes hydrolysis producing HO- ions according to the chemical reactions: (Eq-4,5)
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ACCEPTED MANUSCRIPT N 6 HCHO + 4 NH3
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N + 6 H2O
N N
NH4+ + OH-
……………………..….(5)
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NH3 + H2O
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…………(4)
Higher the concentration of these OH- ions greater is the amount of Cu(OH)2 growth nuclei.
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In addition to the formation of these Cu(OH)2 growth nuclei (Eq.-6), the concentration of OH- ions
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also significantly affects the nucleation and crystal growth process which in turn controls the morphology of the CuO nanostructures[43].According to Wen et al.[44] at sufficiently higher pH,
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OH- ions prefer a square planar confirmation around Cu2+ ions along dz 2 orbitals resulting in the
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formation one-dimensional Cu(OH)2 nanostructures which are the critical requirement for the formation of ID lance-shaped CuO nanostructures in the present study. As indicated in Fig.
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3(b),aninter-planar distance of 0.27 nm corresponds to the (110) planes of monoclinic CuO suggesting that the preferential epitaxial growth of as-synthesizedlance-shaped CuO nanostructures are along (110) plane. Additionally, at higher concentration of OH- diffusion layers are supposed to be formed on the surfaces of the CuO nanostructures which favor anisotropic growth along crystallographic (110) planes[38, 45]. During hydrothermal growth at 160°C for 7h, these Cu(OH)2 growth nuclei are converted into CuO nanocrystals which grow to form lance-shaped CuOnanostructures with longer and thinner leaflets along with some shorter, thicker and broader petals like morphologies with sharper tips (Fig. 7).
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ACCEPTED MANUSCRIPT Δ Cu(OH)2 2 CuO H 2 O …………………….….(6)
3.4. Ethanol gas sensing applications
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The lance-shaped CuOnanostructures based sensorwas characterized in a static system wherein a
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particular amount of gas was inject in text chamber with air as background and the device temperature was raised. The resistance of the sensor was measured as a function of temperature with
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and without test gas to determine the sensitivity factor which was calculated as the ratio of resistance
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in two ambient (S= Rg/Ra, Rg: resistance in gas, Ra: resistance in air). The responses to 100 ppm
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C2H5OH, 100 ppm CO and 100 ppm H2were measured at 300 – 450C (Fig. 8a).
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Fig.7. Schematic Growth Mechanism for the formation of CuO nanostructures.
The sensing characteristics at < 300C were not measuredbecause of slow response and recovery speed. The response to C2H5OH was decreased from 9.10 to 2.30 as the sensor temperature increased from 300 to 450C while those to CO and H2 were negligibly low (1.30 – 1.83) at the entire sensor temperatures. This means that C2H5OH can be measured in a selective manner. As a measure of selective detection of C2H5OH, the Sethanol/Sgas (Sethanol: response to C2H5OH; Sgas: response to interference gas) values were calculated (Fig. 8b). The Sethanol/SCOandSethanol/SH2values
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ACCEPTED MANUSCRIPT showed the highest values (4.97 and 5.87) at 300 C and monotonously decreased to ~1.7 as
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increasing sensor temperature to 450 C. This shows that the selective detection of C2H5OH can be
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best accomplished at 300 C.
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The sensing transients to 100 ppm C2H5OH at 300 – 450C were shown in Fig. 9a-d. The sensor showed the reproducible increase and recovery of resistance upon exposure to C2H5OH and air,
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respectively. The sensor resistances in air at 300, 350, 400, and 450 C were 37.57 0.10, 23.32
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0.03, 17.370.02,and 15.590.02 k, respectively. The decrease of sensor resistance with the increase of temperature indicates the semiconducting nature of CuO. In order to estimate the sensing
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and recovering speed, the 90% response time (res) and 90% recovery times (recov),the times to reach 90% variation in the resistance upon exposure to C2H5OHand air, respectively, were calculated from
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the sensing transients (Fig. 9e). The res value at 300C was 127 s, which decreased to 40 s at 450C. (b)
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(a)
Fig.8. (a) Variations of responses to C2H5OH, CO and H2(100 ppm) with temperatureranging from 300–450C (b) Selective detection of C2H5OH at different operating temperatures.
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ACCEPTED MANUSCRIPT The recov value showed also the highest value at 300C and tended to decrease as the temperature
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increases. In the sole viewpoint of sensitive and selective detection of C2H5OH, the sensor
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temperature of 300C can be regarded as optimum. However, if fast detection becomes more
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important, the sensor operation at 350 and 400C at the expenses of response decrease will be more
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advantageous.
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Fig.9. (a-d)Sensing transients for100 ppm of C2H5OH at 300–450C (e) Variations of recovery and response times as a function of operating temperature for 100 ppm of C2H5OH.
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The adsorption of O2 gas molecules, dissociation into oxygen atom, and ionization of oxygen by taking the electrons fromthe surface of the CuO nanostructures result in the formation of negatively charged surfaceoxygen ions (Eq.-7,8)[46]. ……….(7) …………..(8) The adsorption of thisoxygen withanegative charge on the p-type oxide semiconductors such as CuOleads to the formation of hole(h+) accumulation layer near the surface (Fig. 10). When reducing gases such as C2H5OH, CO and H2 are present in the surrounding atmosphere, the reaction between
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ACCEPTED MANUSCRIPT reducing gas and negatively charged surface oxygen releases the electron to the holes (h+), which in turn increases the resistance of p-type semiconductor by electron-hole recombination(Eq.-9,10).
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C 2 H 5 OH(Chemisorbed) 6O (Chemisorb ed) 2CO 2 (g) 3H 2 O (g) 6e ……..(9)
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e h Null ……….(10)
In general, the gas diffusion does not increase to a significant degree as the sensor temperature
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increases from 300 to 450C. Accordingly, the faster response and recovery at the higher sensor
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temperature indicate that both of the reaction between reducing gas and negatively charged surface oxygen during sensing and the adsorption of oxygen with negatively charge during recovery are
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thermally activated ones[47].
Fig. 10. Sensing mechanism of the CuO nanostructures for ethanol. The C2H5OH responses of various CuO nanostructures in the literature were summarized in Table 1. The response of Rg/Ra = 9.10 for100 ppm of C2H5OH at 300C in the present study was among the highest values reported recently in the literature for CuO sensors. The high gas response can be attributed to the formation of the abundant hole accumulation layer near the surface of thin
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ACCEPTED MANUSCRIPT leaflets and enhanced gas accessibility to the sensor surface through lance-shaped morphology. The
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C2H5OH sensor should be able to detect [C2H5OH] > 200 ppm (0.5 g of C2H5OH per liter of blood)
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in order to screen an intoxicated automobile[48].Accordingly, highly sensitive and selective
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C2H5OH sensors using lance-shaped CuO nanostructures in the present study can be used for breath alcohol analyzer.
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Nanoleaves Lance-shaped
Rg/Ra
Ref.
2.35 3.5 ~3
[44] [50] [51]
300
100
1.14
[52]
220 400 200 240 300 180 320 300 150
200 500 100 200 1000 12.5 2.0 300 500
~6 7.31 4.11 1.7 1.5 2.2 3.1 1.51 ~7
[53] [54] [55] [56] [57] [58] [59] [60] [61]
260 300
1500 100
8.22 9.1
[62] This study
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Wormlike structures CuO/MWNT films Nanoplates Microspheres Nanowires Thin films Hollow microspheres Nanocubes Nanoplates
[C2H5OH] (ppm) 2000 1000 10
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Nanorods Nanoribbons Entangled quasi-1D nanoarchitectures Nanowires
Operating Temp.(oC) 300 200 200
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Sensing materials
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Table 1:Reported C2H5OH responses of various CuO nanostructures
4. Conclusion Lance-shaped CuO nanostructures with longer and thinner leaflets along with some shorter, thicker and broader petals like morphologies with sharper tips were synthesized through low cost, facile and simple hydrothermal synthesis. These self-organizedlance-shaped CuOnano-morphologies
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ACCEPTED MANUSCRIPT exhibited excellent gas response for 100 ppm of C2H5OH at 300C as compared to H2 and CO
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gases.Thegasresponse and response time of9.1 and 127s were observed, respectively for 100 ppm of
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C2H5OH at 300C for lance-shaped CuOnanostructures.Theresponse as well as recovery timeswere
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found to decrease at high operating temperatures. In the sole viewpoint of sensitive and selective detection of C2H5OH, the sensor temperature of 300 C can be regarded as optimum. The results
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confirm thatlance-shaped CuO nanostructures can be utilized as excellent gas sensors more
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efficiently for reducing gases rather than oxidizing gases. Acknowledgements:
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This Project was funded by the National Plan for Science, Technology and Innovation
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(MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number. 12-NAN2551-02. References:
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights: Facile synthesis of CuO nanostructures
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Fabrication of highly sensitive and selective ethanol gas sensor
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High selectivity of the developed sensing device against H2 and CO gases
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S0264-1275(16)30584-6 doi: 10.1016/j.matdes.2016.05.006 JMADE 1744
To appear in: Received date: Revised date: Accepted date:
22 January 2016 3 May 2016 4 May 2016
Please cite this article as: Ahmad Umar, Jong-Heun Lee, Rajesh Kumar, O. AlDossary, Ahmed A. Ibrahim, S. Baskoutas, Development of highlysensitive and selective ethanol sensor based on lance-shaped CuO nanostructures, (2016), doi: 10.1016/j.matdes.2016.05.006
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ACCEPTED MANUSCRIPT Development of highlysensitive and selective ethanol sensor based on lance-shaped CuO nanostructures
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Ahmad Umar,1,2Jong-Heun Lee3, Rajesh Kumar4, O. Al-Dossary5, Ahmed A. Ibrahim1,2, S. Baskoutas6 1
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Department of Chemistry, College of Science and Arts, Najran University, P.O.Box-1988, Najran11001, Kingdom of Saudi Arabia,2Promising Centre for Sensors and Electronic Devices (PCSED),
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Najran University, P.O.Box-1988, Najran-11001, Kingdom of Saudi Arabia, 3Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea, 4PG Department of Chemistry, JCDAV College, (Punjab University), Dasuya-144205, India, 5 Department of Physics, King Saud University, Riyadh-11442, Saudi Arabia, 6Department of Materials Science, University of Patras, Patras GR-26504, Greece *Corresponding Author: [email protected] (Ahmad Umar)
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Abstract
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Herein, we report the successful synthesis and characterization of lance-shaped CuO nanostructures prepared by a simple andfacile hydrothermal technique. The prepared nanostructures
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were examined by several techniques which revealed that the lance-shaped nanostructures are wellcrystalline, grown in very high density and possessing monoclinic crystal structure. In order to
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develop a nano-device using synthesized nanomaterial, the prepared powder was coated on the alumina substrate with suitable electrodes and the fabricated sensor device was tested for ethanol gas. Interestingly, the gas response (resistance ratio)and response timesfor the fabricated devices were9.1 and 127s, respectively for 100 ppm of C2H5OH at 300C.Further, the selectivity of the developed sensing device was tested against H2 and CO gases and interestingly, it was observed that the fabricated sensor exhibited excellent selectivity towards ethanol gas. Key Words: Lance-Shaped;CuO, Hydrothermal; Ethanol; Gas Sensors
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ACCEPTED MANUSCRIPT 1. Introduction Recently, nanostructured p-type semiconductor metal oxides have been explored for sensing
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reducing as well as oxidizing hazardous, highly flammable and poisonous gases. The morphology,
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surface-to-volume ratio, cationic and anionic surface defects, porosity, particle size,theextent of agglomeration and crystallographic orientations of the metal oxide nanomaterials strongly affect the
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various gas sensing parameters[1-4]. For excellent gas sensing properties, metal oxide nanomaterials
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with high surface area to volume ratio are desired for better adsorption/desorption phenomena of the analyte gas[5]. Among the various nanostructured metal oxides, CuO nanomaterials with a
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monoclinic lattice and band gap range of 1.21–1.51 eV are extensively explored for their gas sensing
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behavior[6, 7]. A number of synthetic methods such as solvothermal, thermal evaporation,
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hydrothermal and microwave-assisted hydrothermal, ultrasonic spray pyrolysis, and electrodepositions have been reported intheliterature for the design of gas sensors using CuO
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nanostructures such as nanowires, nanorods, mesoporous films, nanocubes, nanospikes, nanourchins, nanosheets, hollow spheres etc.[8-14].Aslani et al.[6]reported a synergism between crystallite size and surface area to elaborate the enhanced CO sensing using cloudlike CuO nanoparticles.In a similar study, Yang et al. [8]found that CuO nanorods with high surface to volume ratio having diameter 10–20 nm and length of 45–80 nm synthesized through microwave assisted hydrothermal method using PEG-400 and urea, exhibited a maximum response of 9.8 for 1000 ppm ethanol at the working temperature of 210◦C.Park et al. [12]utilized highly porous CuOnanocubes with average
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ACCEPTED MANUSCRIPT edge size of 90 nm and pore size of 52 nm synthesized through wet solution polyol process as
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HCHO sensors atanoptimum temperature of 300◦C. The author also reported a very low detection
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limit 6 ppb at 250◦C for the same. In contrast, Volanti et al. [15] found that spine-assembled urchin-
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like CuO nanostructures with the lower surface area exhibited a better response to H2 gas as compared to fiber-like CuO nanorods with higher surface area. This was attributed to the multiple
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interconnections between spines of the urchin-like CuO nanostructures which can lead to an
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increased cross-sectional area so as to facilitate the process of charge migration [16 ,17]. In a similar report recently, Yang et al. [18] revealed that 3D hierarchical CuO flowers with BET surface area of
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15.0 m2/g exhibited better gas responses for ethylacetate and ethanol as compared to 2D branching
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CuO nanosheets with BET surface area of 20.8 m2/g. both synthesized through microwave-assisted hydrothermal method. Further Kim et al.[19] reported that sensing behavior of networked CuO
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nanowires was better for reducing gases such as CO, C6H6, C7H8, and H2etc.as compared to oxidizing gases like NO2, SO2 and O3. On the basis of these reports thus, it can be stated that the gas sensing parameters are greatly influenced by not only the morphology, surface to volume ratio and particle size but also on the interconnections present between the secondary structures of the 3D CuO nanomaterials. In is noteworthy to mention that, for p-type semiconductor metal oxides like CuO the conductivity is very little affected by high-temperature ranges as compared to n-type semiconductor nanomaterials[20]. Additionally, p-type metal oxides can easily exchange their lattice oxygen with air in order to maintain their stoichiometry. This is very useful property in order to
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ACCEPTED MANUSCRIPT maintain long-term stability of the sensor. Owing to these properties, CuO nanostructured materials
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can be used for the fabrication of sensors especially for ethanol, which is widely used in biomedical
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applications, food industries, wine quality and breath analysis[21, 22].In this regard, here we report a
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simple hydrothermal method for the synthesis of lance-shaped CuO nanostructures with variable sized branches and explored their sensing properties for the C2H5OH, CO and H2 gases. Experimental Details
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2.
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2.1. Synthesis and characterization of lance-shaped CuO nanostructures Well-crystalline lance-shaped CuO nanostructures were synthesized by a facile hydrothermal
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process using copper nitrate [Cu(NO3)2.3H2O], hexamethylenetetramine [HMTA; C6H12N4] and
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ammonium hydroxide [NH4OH]. To synthesize lance-shapedCuO nanostructures, all the chemicals were purchased from Sigma-Aldrich and used as received without any further purification. De-
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ionized (DI) water was used assolvent for the synthesis. In a typical synthesis process, equimolar aqueous solutions of copper nitrate and hexamethylenetetramine, both made in 50 ml DI water each, were mixed well under vigorous stirring. Further, the pH of the resultant solution was then maintained at pH=11 using NH4OH andtheresultant solution again stirred for 2h at roomtemperature. After stirring, the resultant solution was transferred to Teflon-lines stainless steel autoclave and heated to 160°C for 7h. After desired reaction time, the autoclave was allowed to cool at room temperature and finally black colored precipitate was obtained which was decanted and dried overnight at room-temperature. The obtained powder was further washed with DI water and ethanol
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ACCEPTED MANUSCRIPT and finally dried at 75 ± 10°C for 4 h in an electric oven. The prepared CuO powder was further
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characterized in detail using several analytical techniques.
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The morphologies of the prepared CuO powder was examined by field emission scanning
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electron microscopy (FESEM; JEOL-JSM-7600F), attached with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM; JEOL-JEM-2100F) equipped with high-
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resolution TEM (HRTEM). The crystallinity and crystal phases of the synthesized powder were
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evaluated by the X-ray diffraction (XRD; PANanalyticalXpert Pro.) measured with Cu-Kα Radiation (λ=1.54178Å) in the range of 20-70º. The optical and chemical compositional properties of the
transform
infrared
spectroscopy
(FTIR;
Perkin
Elmer-FTIR
Spectrum-100)
at
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Fourier
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prepared powder were studied by UV-visible spectroscopy (Perkin Elmer-UV/VIS-Lambda 950) and
roomtemperature, respectively. The FTIR spectrum was recorded with KBrdispersion in the range of
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450-4000 cm-1.
2.2. Fabrication of ethanol gas sensor based on as-grown lance-shaped CuO nanostructures The lance-shaped CuO nanostructures were heat-treated at 500 C for 2 h to obtain thermally stable gas sensing characteristics at elevated sensing temperature (300 – 450 C), which weredispersed in DI water to prepare slurry for the functional nanomaterials. The prepared slurry was then coated onanalumina substrate (area: 1.5 x 1.5 mm2, thickness: 0.25 mm) with two Au electrodes on its top surface and micro-heater on its bottom surface. After drying, the sensors were heat-treated again at 450C for 2hto remove residual water solvent completely. The sensor
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ACCEPTED MANUSCRIPT temperatures were controlled using the micro-heater underneath the substrate and were measured
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using an IR temperature sensor (Rayomatic14814-2, EurotonIRtec Co.). The sensor was contained in
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a specially designed, small-volume (1.5 cm3), quartz tube in order to minimize the delays in
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changing the atmosphere. The detailed experimental setup is shown elsewhere [ 2 3 ] . The gas responses (S= Rg/Ra, Rg: resistance in gas, Ra: resistance in air) to 100 ppm C2H5OH, CO, and H2
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were measured at 300 - 450C. Gas concentrations were controlled by changing the mixing ratio of
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the parent gases (100 ppm C2H5OH,100 ppm CO, and 100 ppm H2, all in dry synthetic air balance) and dry synthetic air. The DC 2-probe resistances were measured using an electrometer interfaced
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3. Results and Discussion
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with a computer.
3.1. Structural properties of as-grown lance-shaped CuO Nanostructures
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Fig, 1(a) shows the X-ray diffraction pattern obtained with Bragg angle ranging from 20o to 70o. The observed diffraction peaks are closely matched withastandard data file of CuO (JCPDS #05-0661). The analysis with standard data showed that the synthesized material belongs to crystallographic point group of 2/m or C2h[23, 24].Well-defined diffraction reflections indexed at 2θ = 32.6°, 35.5°, 38.7°, 46.4°, 48.8° , 53.4°, 58.3°, 61.7°, 66.2° and 68.1° were observed corresponding to the monoclinic form of CuO with lattice planes of (110), ( 1 11 002 ), (111-200),
(112) , ( 202) , (020), (202), ( 1 13) , ( 3 11) and (220) respectively. These diffractions match very well
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ACCEPTED MANUSCRIPT with the literature[25]. The results also indicated that lance-shaped shaped CuO nanostructureswere
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formed in asingle phase without impurities corresponding to precursor materials.
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The general morphologies of the as-synthesized lance-shaped CuO nanostructures were
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examined by field emission scanning electron microscopy (FESEM). In Fig. 1(b-d), the low magnification images whereas in Fig. 1(e-f) high magnification images are shown for the lance-
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shaped CuO nanostructures.
Fig. 1. Typical (a) XRD pattern and (b-d)low-magnification and (e-f) high-resolution FESEM images of lance-shaped long leaflets. These FESEM images confirmed that the as-synthesizedCuO nanostructures possess lance-shaped morphologies grown in very high density. Dozens of lance-shaped shaped long leaflets are grown from a common base. These leaflets are wider at their bases with sharper tips. The central part of
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ACCEPTED MANUSCRIPT each leaflet is wider whereas the corners are narrowerforming a lance-shape. The typical length of
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each leaflet is ~2.4 – 2.5 μm with anaverage width of ~50 ± 5 nm (Fig.1(e-f)). Thus, the present
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hydrothermal route is an effective one to synthesize dense lance-shaped CuO nanostructures with
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leaflet of different sizes and morphologies along with very high surface to volume ratio under the applied reaction conditions. These are some of the pre-requisites for the fabrication of highly
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sensitive and selective gas sensors.
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In addition to longer and thinner leaflets, the as-synthesized CuO product also consists of shorter, thicker and broader petals with sharper tips grown on a common base with high density.
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Low and high-resolution FESEM images are shown for these CuO nanostructures with petals like
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morphologies in Fig. 2(a-b) and Fig. 2(c-d) respectively. The typical length of each petal is ~300 ±10
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nm with anaverage width of ~200 ± 10 nm respectively (Fig. 2(a-d)).
Fig.2. (a-b)Low-magnification and (c-d) high-resolution FESEM images of shorter, thicker and broader petals with sharper tips.
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ACCEPTED MANUSCRIPT Detailed microstructural investigations of a single leaflet of lance-shaped CuO nanostructures
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were performed using HRTEM technique. Fig. 3(a) and (b) represent the typical low and high-
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resolution TEM images, respectively. Fig. 3(b) reveals that the leaflet of lance-shaped CuO
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nanostructures is crystalline in nature withaninterplanardistance of 0.27 nm, corresponding to the (110) planes of monoclinic CuO[19, 26]. These result also reveal that the growth of the CuO
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nanostructures is along the (110) plane. As grown leaflets of lance-shaped CuO nanostructures are
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originated from a common base, the CuO leaflets are supposed to have strong binding due to electrostatic attraction. Dey et al. [27] also reported plate-shaped CuO nanostructures arranged to
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form a flower-like topography as well as few bunches of small particles at pH 11. High pH of the reaction solution favors the formation of agglomerated structures probably due to the existence of
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hydrogen bonding between HO- moieties present in the particles.
Fig.3. (a)low-magnification and (b) high-resolution TEM images of lance-shaped long leaflets. 9
ACCEPTED MANUSCRIPT To examine the elemental composition of the lance-shaped CuO nanostructures energy
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dispersive spectroscopy (EDS) attached with FESEM was applied. Fig 4(a) clearly revealed that the
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lance-shaped CuO nanostructures are made of copper and oxygen. As no other peak related with any
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other element is present in the spectrum, as prepared lance-shaped CuO nanostructures are pure. The results are in good correlation with XRD analysis (Fig. 1(a)).
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The chemical composition of the as-preparedlance-shaped CuO nanostructures was examined
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by Fourier transform infrared (FTIR) spectroscopy at roomtemperature(Fig.4(b)). The observed FTIR spectrum exhibited various well-defined absorption peaks at 509, 605, 1389, 1628, 2931 and
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vibrations of M-O (Cu–O).
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3421 cm-1. The presence of strong peaks at ~509 and 605 cm-1 are attributed to the stretching
Fig. 4. (a) EDS and (b) FTIR spectra oflance-shaped CuO nanostructures.
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ACCEPTED MANUSCRIPT The results confirmed that the lance-shaped CuO nanostructures possess monoclinic CuO phase[28-
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30].The absorption peak at 1389 cm-1 may be dueto CO 32 , which usually appears in the spectrum
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when the FTIR sample are prepared and measured in the air[30]. Further, a small and wide band at
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3421 cm−1 and additional sharp band at2931 cm−1 may be ascribed to the asymmetric and symmetric vibrational modes oftheO-H bond of the H2O molecules physisorbed on the surface of the CuO[31].
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The weak band at 1628 cm−1 may be attributed to the bending vibration modes of O–H groups of
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these adsorbed H2O molecules[30].
3.2. Optical properties of as-grown lance-shapedCuO Nanostructures
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The optical properties of CuO nanostructures were examined by UV-visible spectrum
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measured at room-temperature and demonstrated in Fig.5 (a). In order to have a precise determination of the energy position of the absorption edge, one can take the edge position to be
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determined by the maximum of the first derivative of the optical absorption with respect to the energy and it corresponds to the optical band gap[32, 33].The first derivative of the spectrum as is shown in Fig. 5 (b) indicates a peak at 1.24 eV which is similar to the bulk value. As is also well known by utilizing the Tauc’s formula, the relationship between absorption coefficient and the incident photon energy of semiconductors can be obtained. Hence, the optical band gap,Egcan be experimentally obtained from absorption coefficient according to the Tauc’s equation as follows (Eq. 1): (αhν) = A(hν – Eg)n …………(1)
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ACCEPTED MANUSCRIPT where α is the absorption coefficient, A is constant, and n is equal 2 for indirect transition
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semiconductor. The corresponding graph for indirect transition is shown in Fig. 5 (c). As is seen
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from Fig. 5 (c), the optical band gap obtained through extrapolation, is 1.23 eV corresponds well
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with the energy value obtained in Fig. 5 (b)andalso matches well with the reported value[34]. The peak which appears at 1.44 eV in Fig. 5b is an artifact signal from the instrument and, therefore,
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does not correspond to any transition.
Fig. 5.(a)UV-visible spectrum (b) first order derivative of UV –visible absorbance and (c) band gap calculations from Tauc’s formula for CuO nanostructures. Here, we are using the potential morphing method within the effective mass HartreeFockapproximationto obtain the size dependent exciton energy[35-37].The observed results are depicted in Fig. 6, where we have used the following materials parameters: me* 0.35 , mh* 1.47 , 0 12 , 6.45 and LO 74 meV andEg(bulk) = 1.2 eV[38-41].
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ACCEPTED MANUSCRIPT As is seen from Fig. 6 the size of synthesized CuOlance-shaped nanostructure are in the
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weak confinement regime, e.g. their radius is greater than the exciton Bohr radius of CuO, which
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varies from 6.6–28.7 nm[38-42].
Fig.6. Variations of optical band gap as a function of atomic radius
Nanostructures
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3.3. Plausible growth mechanism for the formation of as-grown lance-shaped CuO
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As reported earlier, the pH of the growth solution plays an important role in the formation of Cu(OH)2 growth nuclei from theprecursor. At pH ˃ 10, initially formed copper (II) hydroxynitrate [Cu2(OH)3NO3] is easily converted to Cu(OH)2 by the replacement of NO3 ions (Eq.-2,3)[27].
2Cu(NO3 ) 2 3NH 4 OH [Cu 2 (OH)3 NO3 ] 3NH 4 3NO3 ……..(2) [Cu 2 (OH)3 NO3 ] HO 2Cu(OH)2 NO3 …….(3) The presence of the HMTA in the reaction solution also accelerates the formation of Cu(OH)2 growth nuclei through the generation of HO- ions. HMTA undergoes hydrolysis producing HO- ions according to the chemical reactions: (Eq-4,5)
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ACCEPTED MANUSCRIPT N 6 HCHO + 4 NH3
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N + 6 H2O
N N
NH4+ + OH-
……………………..….(5)
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NH3 + H2O
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…………(4)
Higher the concentration of these OH- ions greater is the amount of Cu(OH)2 growth nuclei.
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In addition to the formation of these Cu(OH)2 growth nuclei (Eq.-6), the concentration of OH- ions
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also significantly affects the nucleation and crystal growth process which in turn controls the morphology of the CuO nanostructures[43].According to Wen et al.[44] at sufficiently higher pH,
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OH- ions prefer a square planar confirmation around Cu2+ ions along dz 2 orbitals resulting in the
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formation one-dimensional Cu(OH)2 nanostructures which are the critical requirement for the formation of ID lance-shaped CuO nanostructures in the present study. As indicated in Fig.
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3(b),aninter-planar distance of 0.27 nm corresponds to the (110) planes of monoclinic CuO suggesting that the preferential epitaxial growth of as-synthesizedlance-shaped CuO nanostructures are along (110) plane. Additionally, at higher concentration of OH- diffusion layers are supposed to be formed on the surfaces of the CuO nanostructures which favor anisotropic growth along crystallographic (110) planes[38, 45]. During hydrothermal growth at 160°C for 7h, these Cu(OH)2 growth nuclei are converted into CuO nanocrystals which grow to form lance-shaped CuOnanostructures with longer and thinner leaflets along with some shorter, thicker and broader petals like morphologies with sharper tips (Fig. 7).
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ACCEPTED MANUSCRIPT Δ Cu(OH)2 2 CuO H 2 O …………………….….(6)
3.4. Ethanol gas sensing applications
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The lance-shaped CuOnanostructures based sensorwas characterized in a static system wherein a
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particular amount of gas was inject in text chamber with air as background and the device temperature was raised. The resistance of the sensor was measured as a function of temperature with
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and without test gas to determine the sensitivity factor which was calculated as the ratio of resistance
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in two ambient (S= Rg/Ra, Rg: resistance in gas, Ra: resistance in air). The responses to 100 ppm
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C2H5OH, 100 ppm CO and 100 ppm H2were measured at 300 – 450C (Fig. 8a).
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Fig.7. Schematic Growth Mechanism for the formation of CuO nanostructures.
The sensing characteristics at < 300C were not measuredbecause of slow response and recovery speed. The response to C2H5OH was decreased from 9.10 to 2.30 as the sensor temperature increased from 300 to 450C while those to CO and H2 were negligibly low (1.30 – 1.83) at the entire sensor temperatures. This means that C2H5OH can be measured in a selective manner. As a measure of selective detection of C2H5OH, the Sethanol/Sgas (Sethanol: response to C2H5OH; Sgas: response to interference gas) values were calculated (Fig. 8b). The Sethanol/SCOandSethanol/SH2values
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increasing sensor temperature to 450 C. This shows that the selective detection of C2H5OH can be
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best accomplished at 300 C.
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The sensing transients to 100 ppm C2H5OH at 300 – 450C were shown in Fig. 9a-d. The sensor showed the reproducible increase and recovery of resistance upon exposure to C2H5OH and air,
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respectively. The sensor resistances in air at 300, 350, 400, and 450 C were 37.57 0.10, 23.32
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0.03, 17.370.02,and 15.590.02 k, respectively. The decrease of sensor resistance with the increase of temperature indicates the semiconducting nature of CuO. In order to estimate the sensing
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and recovering speed, the 90% response time (res) and 90% recovery times (recov),the times to reach 90% variation in the resistance upon exposure to C2H5OHand air, respectively, were calculated from
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the sensing transients (Fig. 9e). The res value at 300C was 127 s, which decreased to 40 s at 450C. (b)
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(a)
Fig.8. (a) Variations of responses to C2H5OH, CO and H2(100 ppm) with temperatureranging from 300–450C (b) Selective detection of C2H5OH at different operating temperatures.
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ACCEPTED MANUSCRIPT The recov value showed also the highest value at 300C and tended to decrease as the temperature
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increases. In the sole viewpoint of sensitive and selective detection of C2H5OH, the sensor
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temperature of 300C can be regarded as optimum. However, if fast detection becomes more
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important, the sensor operation at 350 and 400C at the expenses of response decrease will be more
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advantageous.
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Fig.9. (a-d)Sensing transients for100 ppm of C2H5OH at 300–450C (e) Variations of recovery and response times as a function of operating temperature for 100 ppm of C2H5OH.
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The adsorption of O2 gas molecules, dissociation into oxygen atom, and ionization of oxygen by taking the electrons fromthe surface of the CuO nanostructures result in the formation of negatively charged surfaceoxygen ions (Eq.-7,8)[46]. ……….(7) …………..(8) The adsorption of thisoxygen withanegative charge on the p-type oxide semiconductors such as CuOleads to the formation of hole(h+) accumulation layer near the surface (Fig. 10). When reducing gases such as C2H5OH, CO and H2 are present in the surrounding atmosphere, the reaction between
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ACCEPTED MANUSCRIPT reducing gas and negatively charged surface oxygen releases the electron to the holes (h+), which in turn increases the resistance of p-type semiconductor by electron-hole recombination(Eq.-9,10).
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C 2 H 5 OH(Chemisorbed) 6O (Chemisorb ed) 2CO 2 (g) 3H 2 O (g) 6e ……..(9)
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e h Null ……….(10)
In general, the gas diffusion does not increase to a significant degree as the sensor temperature
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increases from 300 to 450C. Accordingly, the faster response and recovery at the higher sensor
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temperature indicate that both of the reaction between reducing gas and negatively charged surface oxygen during sensing and the adsorption of oxygen with negatively charge during recovery are
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thermally activated ones[47].
Fig. 10. Sensing mechanism of the CuO nanostructures for ethanol. The C2H5OH responses of various CuO nanostructures in the literature were summarized in Table 1. The response of Rg/Ra = 9.10 for100 ppm of C2H5OH at 300C in the present study was among the highest values reported recently in the literature for CuO sensors. The high gas response can be attributed to the formation of the abundant hole accumulation layer near the surface of thin
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ACCEPTED MANUSCRIPT leaflets and enhanced gas accessibility to the sensor surface through lance-shaped morphology. The
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C2H5OH sensor should be able to detect [C2H5OH] > 200 ppm (0.5 g of C2H5OH per liter of blood)
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in order to screen an intoxicated automobile[48].Accordingly, highly sensitive and selective
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C2H5OH sensors using lance-shaped CuO nanostructures in the present study can be used for breath alcohol analyzer.
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Nanoleaves Lance-shaped
Rg/Ra
Ref.
2.35 3.5 ~3
[44] [50] [51]
300
100
1.14
[52]
220 400 200 240 300 180 320 300 150
200 500 100 200 1000 12.5 2.0 300 500
~6 7.31 4.11 1.7 1.5 2.2 3.1 1.51 ~7
[53] [54] [55] [56] [57] [58] [59] [60] [61]
260 300
1500 100
8.22 9.1
[62] This study
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Wormlike structures CuO/MWNT films Nanoplates Microspheres Nanowires Thin films Hollow microspheres Nanocubes Nanoplates
[C2H5OH] (ppm) 2000 1000 10
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Nanorods Nanoribbons Entangled quasi-1D nanoarchitectures Nanowires
Operating Temp.(oC) 300 200 200
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Sensing materials
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Table 1:Reported C2H5OH responses of various CuO nanostructures
4. Conclusion Lance-shaped CuO nanostructures with longer and thinner leaflets along with some shorter, thicker and broader petals like morphologies with sharper tips were synthesized through low cost, facile and simple hydrothermal synthesis. These self-organizedlance-shaped CuOnano-morphologies
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ACCEPTED MANUSCRIPT exhibited excellent gas response for 100 ppm of C2H5OH at 300C as compared to H2 and CO
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gases.Thegasresponse and response time of9.1 and 127s were observed, respectively for 100 ppm of
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C2H5OH at 300C for lance-shaped CuOnanostructures.Theresponse as well as recovery timeswere
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found to decrease at high operating temperatures. In the sole viewpoint of sensitive and selective detection of C2H5OH, the sensor temperature of 300 C can be regarded as optimum. The results
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confirm thatlance-shaped CuO nanostructures can be utilized as excellent gas sensors more
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efficiently for reducing gases rather than oxidizing gases. Acknowledgements:
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This Project was funded by the National Plan for Science, Technology and Innovation
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(MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number. 12-NAN2551-02. References:
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[60] H.J. Park, N.J Choi, H. Kang, H.K. Lee, S.E. Moon, M.Y. Jung, K.H. Park, D.S. Lee, CuOnanocubegassensor for ethanol detection, Sensor Letters, 12 (2014)1156–1159. [61] D. Su, X. Xie, S. Dou, G. Wang, CuO single crystal with exposed {001} facets - A highly efficient material for gas sensing and Li-ion battery applications, Scientific Reports 4 (2014)5753 [62] Y. Cao, S. Liu, X. Jian, G. Zhu, L. Yin, L. Zhang, B. Wu, Y. Wei, T. Chen, Y. Gao, H. Tang, C. Wang, W. He and W.Zhang, Synthesis of high-purity CuOnanoleaves and analysis of their ethanol gas sensing properties,RSC Advances, 5(44) (2015) 34788-34794.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights: Facile synthesis of CuO nanostructures
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Fabrication of highly sensitive and selective ethanol gas sensor
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High selectivity of the developed sensing device against H2 and CO gases
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