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ZnO nanoarchitectures: Ultrahigh sensitive room temperature acetaldehyde sensor
Accepted Manuscript Title: ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor Author: Ganesh Kumar Mani John Bosco Balaguru Rayappan PII: DOI: Reference:
S0925-4005(15)30410-X http://dx.doi.org/doi:10.1016/j.snb.2015.09.103 SNB 19088
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
3-6-2015 2-8-2015 20-9-2015
Please cite this article as: G.K. Mani, ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.09.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ZnO Nanoarchitectures:Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor
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Ganesh Kumar Mani and John Bosco Balaguru Rayappan* Nano Sensors Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
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School of Electrical & Electronics Engineering (SEEE)
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SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
*Corresponding Author
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Prof. John Bosco Balaguru Rayappan, Ph.D.
Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) & School of Electrical & Electronics Engineering SASTRA University
Thanjavur – 613 401 India
Phone: +91 4362 264 101; Ext: 2255 Fax: +91 4362 264120
E-mail: [email protected] (John Bosco Balaguru Rayappan) [email protected] (Ganesh Kumar Mani)
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HIGHLIGHTS ZnO nanoarchitectures were facilely grown on glass substrates
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Effect of doping on the formation of ZnO nanorods was investigated
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Room temperature acetaldehyde sensor using ZnO nanoarchitectures has been reported
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Ultrahigh sensitivity and selectivity observed for Co-doped ZnO
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Abstract In this work, undoped and transition metals (Co, Ni and Cu)doped ZnO nanoarchitectures were deposited on glass substrates by simple chemical spray pyrolysis technique. The obtained
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ZnO nanoarchitectures were characterized using X-ray diffractiometer (XRD), scanning electron microscope (SEM), UV-vis spectrophotometer and electrometer to study their structural,
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morphological, optical and sensing properties respectively. The structural and morphological
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results revealed that transition metal doping effectively changed the microstructures and distorted the preferential orientation of (002) crystal plane with reference to the undoped ZnO.
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Acetaldehyde sensing characteristicsof the undoped and doped ZnO nanoarchitectures were investigated. The sensing response towards 10 ppm wasfound to be 2.85, 800, 2.59 and 21.36 for
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the undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Co-doped ZnO nanostructure showed an excellent sensing response over a wide concentration range of 10 to 500
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ppm of acetaldehyde. Moreover the sensing elements showed high selectivity towards
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acetaldehyde when exposed to 7 different vapours. Ultrahigh sensitivity and selectivity observed forCo-doped ZnO nanoarchitecture revealed its identity asa potential candidate for detectingacetaldehyde in various applications.
Keywords: ZnO; thin film; spray pyrolysis; gas sensor; doping; acetaldehyde.
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1. Introduction Nowadays, development of fast response, cost effective and portable sensors for detecting toxic/hazardous gases are in high demand due to theirnecessity in protecting human
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lives as well as monitoringenvironment[1,2].Acetaldehyde is ubiquitous in daily life and it has been classified as group I carcinogen according to International Agency for Research on Cancer.
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The main routes of acetaldehyde exposure to humans are alcohol consumption, fermented foods,
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cigarette smoking, exhaust from cars and trucks[3,4]. Due to its strong electrophilic nature it was proved that it damages DNA in humans and alter the red blood cell structure[5,6]. In addition,
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acetaldehyde has a strong tendency to combine with Vitamin B1 (nerve vitamin) and inducesa deficiency[7]. Hence it lead to mental illness, poor memory, mental confusion and visual
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disturbances. National Institute for Occupational Safety & Health (NIOSH) and Mine Safety & Health Administration (MSHA) has proposed a permissible exposure limit (PEL) of
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acetaldehyde is 100 ppm as a time weighted average (TWA) of 8 h shift.Occupational Safety and
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Health Administration (OSHA) notified that threshold exposure limit is 25 ppm for 8 h work shift[8]. The above facts necessitates that, it is essential to develop cost effective and high response acetaldehyde sensor to protect human lives. Several methods such as electrochemical, chromatographic, spectrometric, quartz crystal microbalance and chemiresisitve metal oxides have been used to detect acetaldehyde in various applications [3,4,9,10]. Amongst all,nanostructured metal oxidesare found to be suitable candidates for detecting volatile organic compounds (VOCs) / gases of various concentrations due to high surface to volume ratio, tuning selectivity via. crystallographic orientation, enhanced catalytic activity and superior stability[11].Moreover it offerscountless advantageslikehigh
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sensitivity, quick response and recovery times, easy fabrication,portability, requirement of no expensive equipment and mainly applicable for in situ detection. Among many metal oxides, ZnO is one of the popular materials for gas sensing
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applications especially for detecting vapours of ethanol, hydrogen, methanol, trimethylamine, ammonia, acetaldehyde, carbon dioxide, xylene, monoethanolamine, etc[12–14]. Recently,
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various ZnO based acetaldehyde sensors developed using vapour phase, sonochemical, chemical
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precipitation, sol-gel, spray and hydrothermal techniques have been reported, but those sensors were operated at elevated temperatures and demonstrated poor selectivity[15–17]. With respect
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to the available reports, ZnO tetrapods showeda response of 47.5 for 50 ppm of acetaldehyde at 400oC[15]. Rai et al. observed the sensing response of 5.30 towards250 ppm at 400oC[16].
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Flower like ZnO nanostructures exhibited response towards acetaldehyde, carbon monoxide and nitrogen dioxide[18]. Unfortunately, many of the developed sensors have not satisfiedall the
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ideal sensing characteristics such as high stability, low cross selectivity, limit detection limit,
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high response, wide detection range, quick response and recovery times. To overcome these limitations,doping has been widely used as an effective tool to modulate the band structure and hence to obtaindesired sensing characteristics[19–21]. Mainly, noble metals such as Ag, Au, Pd, Pt and transition metals like Fe, Mn, Co, Ni, Cu have been used as additives to promote the gas sensing characteristics [22].In particular, influence of transition metal doping in many instances connected with changes in the grain size and charge carrier concentration often resulted in tunable sensing performances.Chemical vapour deposition (CVD) techniques are best known and widely used in thin film fabrication applications owing to their functional advantages over physical vapour deposition (PVD)techniques, process flexibility, growth rate, large area deposition, etc.Especially, liquid source CVD techniques like spray pyrolysis, sol-gel dip/spin
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coatingarehighly versatile tools for achieving desired functional properties. Under ideal conditions, liquids can be vapourizedcompletely rather than solid sources, hence formation of thin films using liquid source CVD (spray pyrolysis)have been considered as a one of the best
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tools[23]. Complex morphologies have drawn much attention in gas sensor field due to their
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excellent properties like high surface area and enhanced surface catalytic activity[24–26]. As of
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now, seeded growth technologies have been used to prepare various ZnO nanoarchitectures especially complex morphologies like branched nanorods[27–29]. But these kinds of multistep
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methodslimit the applications of such nanostructures due to too many control parameters.Hence, in this work, a facile, novel and template free technique has been developed to synthesize various
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ZnO Nanoarchitectures. In addition the influence of doping on modulating the morphological, transport and sensing properties of ZnO nanoarchitectures has motivated to carry out such
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investigations. To our knowledge,effect of transition metal doping on the room temperature
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acetaldehyde sensing performance of ZnO branched nanorods has not been reported elsewhere. Hence, in this work the development of room temperature acetaldehyde sensor using various undoped and doped ZnO nanoarchitectures has been reported.
2. Materials and methods 2.1 Film Deposition
Undoped and transition metals (Co, Ni and Cu) doped ZnO thin films were deposited on glass substrates by simple chemical spray pyrolysis technique. Spray pyrolysis is a process in which thin film is deposited by spraying a precursor solution on a pre-heated surface, where thermal decomposition of the precursorconstituents lead to the formation of thin film over the
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substrate. Fig. 1 shows the typical spray pyrolysis unit which consists of solution dispenser, pressure regulator, atomizer and heater plate. Atomizer is the heart of spray system where the precursor solution is converted into fine mist with the assistance of compressed air. The atomizer
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was mounted with X and Y axes stepper motor to move throughout the substrates to achieve uniform deposition. Parameters like temperature, flow rate of the solution, spray head movement
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and its speed, spray time, pause time are controlled through a personal computer. The
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temperature of the substrate heater was controlled by K-type thermocouple with PID controller. At first, glass substrates were ultrasonically cleaned with acetone, isopropanol and
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doubly deionized water for 15 min each. The precursor solution wasprepared by dissolving 0.1 M of anhydrous zinc chloridein deionized water. The solution was stirred about 30 min to obtain
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clear solution. Then the solution was sprayed over the pre-heated substrate using spray pyrolysis technique. The optimized deposition parameters were given in Table 1. Effect of transition metal
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doping on the undoped ZnO was executed by mixing 5wt% of precursor salt into the host
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solution. Cupric acetate dihydrate (Cu(CH3COO)2·2H2O), nickel acetate tertahydrate (Ni(CH3COO)2·4H2O), cobalt acetate tertahydrate (Co(CH3COO)2·4H2O) were used as the precursor salts for Cu, Ni and Co doping respectively.
2.2 Characterization
X-Ray Diffractometer (D8 Focus, Bruker, Germany) was used to investigate the structural parameters such as crystallinity, crystallite size, d-spacing, strain and lattice parameters of the deposited ZnO thin film. Morphology of the thin filmswas investigated using Field Emission Scanning Electron Microscope (FE-SEM) (JEOL, 6701F, Japan). Energy Dispersive X-Ray Spectroscopy was used to along with FE-SEM to confirm the elemental composition in
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the films.Optical properties of the thin films were studied using UV-Vis spectrophotometer (Perkin Elmer, Lambda 25, USA) in the wavelength range of 200 to 800 nm with the scan rate of 100 nm min-1. Electrical and sensing properties were carried out using an electrometer (Keithley,
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6517A, USA).
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2.3 Sensor fabrication and testing
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Based on the deposited undoped and transition metal doped films, four different sensing elements were prepared and investigated their sensing responses at room temperature. Initially,
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Ohmic contacts were established on the surface of the sensing element (10 mm × 10 mm) using zero resistance copper wires and highly conducting silver paste.Sensing studies were carried out
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at room temperature using custom build gas sensing setup and the same described in our previous report[30–32]. The concentration of the target vapour[26] in the testing chamber was
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where,
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fixed using Eq. 1.
is the density of acetaldehyde,
universal gas constant,
is the volume of acetaldehyde injected,
is the absolute temperature,
pressure inside the chamber and calculated using the relation
is the molecular weight,
is the is the
is the volume of the chamber. The sensor response (S) was
where,
and
are the resistance of the sensing elements
is in air and target gas.
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3. Results and Discussion 3.1 Structural studies XRD patterns of the undoped and transition metals doped ZnO thin films are shown in
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Fig. 2 (a) & (b). The observed XRD patterns revealed that all the undoped and transition metals doped ZnO thin films belong to wurtzite crystal structure (JCPDS Card No: 36-1451). No
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anomalouspeaks were observed in doped samples which confirmed the doping levels of
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transition metals in ZnO was within the solubility limit. Various structural parameters like crystallite size, d-spacing, lattice parameter (c) and strain values were estimated and the same is
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presented in Table 2. With reference to undoped ZnO, lattice parameter, d-spacing and strainfor (002) crystal plane was larger/higher for Ni and Cu-doped ZnO, smaller/lower for Co-doped
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ZnO. It emphasized the fact that Ni and Cu ions existed in interstitial sites of ZnO and resulted in increased d-spacing as well as strain. At the same time, substitution of Zn by Co ions confirmed
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through decrease in lattice parameter (c) and d-spacing values. The higher angle peak shift in Co-
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doped ZnO thin films with reference to undoped ZnO might be due to the substitution of high spin state Co2+ ions (ionic radius ~0.58 Å) in tetrahedral coordinated Zn2+ ions (ionic radius ~0.60 Å)[33]. The average crystallite size estimated for (002) crystal plane was found to be 39, 35, 32 and 30 nm for undoped, Co, Ni and Cu doped ZnO thin films respectively. It was clear that when dopant atom introduced into the ZnO lattice, dopants might have distorted the aligned (002) crystal orientation and promotes the crystal growth in multiple directions (001)and(101). Hence, due to the multiple direction growth of crystallites, the dominant crystal growth of ZnO in (002) direction was diminished and resulted in decreased in crystallite size.
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3.2 Morphological and optical studies Fig. 3 (a & a1) – (d & d1) shows the low and high magnification FE-SEM images of the undoped and transition metals doped ZnO thin films. Undoped ZnO thin films showed large
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quantity of well dispersed branched nanorodswith hexagonal tips and uniform diameter along their entire lengths. The typical length of the nanorod was around 300 nm and these branched
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nanorods were heterogeneously grown on both sides from the root (Fig. 3(a&a1)). The
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morphologies of transition metals doped ZnO nanostructures were remarkably different from that of undoped ZnO thin films. For Co-doping (Fig. 3 (b &b1)), evenly distributed branched nanorod
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structure was converted into irregular shaped nanorods along with tiny particles which were well connected to each other. Fig. 3 (c &c1) shows the well dispersed randomly orientated Ni-doped
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ZnO nanorodsand the length was over 200 nm. A greater agglomeration of particles was observed in Cu-doped ZnO thin film (Fig. 3 (d &d1)). The EDS spectra of undoped and doped
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films are shown in Fig. 4 (a-d). The presence of zinc and oxygen elements was confirmed in the
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undoped ZnO thin films. Moreover, the presence of cobalt, nickel and copper elements along with zinc and oxygen was confirmed in the doped samples. The presence of Si and Au peaks may be due to the substrate effect and sputter coating to eliminate the charging effect respectively.
Fig. 5 shows the optical absorbance spectra of undoped and doped ZnO thin films. The increased absorption in doped films might be due to the creation of impurity states below the conduction band as well as strain caused by doping[34]. Also, due to large amount of open space, undoped and Cu-doped ZnO films exhibited lesser absorption as compared with Co and Ni-doped ZnO thin film[35].
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3.3 Sensing studies In the recent past, hierarchical nanostructures have been demonstrated to have great potential for various applications due to their high surface to volume ratio and high catalytic
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activity[36,37]. In particular, it is proven that these complex nanostructures deliver high gas/vapour sensing response with quick response and recovery times. In the same line of thought,
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the sensing response of the observed ZnO nanostructures has been investigated.
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Therefore,electrical contacts were established on the sensing elements to examine their sensing ability. The typical photograph of the sensing set-up is shown in Fig. 6. It consists of testing
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chamber along with vacuum pump, an electrometer and PC interface for storing readings. The sensing responses of undoped and doped sensing elements were studied by exposing them
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towards 7 different vapours at the fixed constant concentration range of 100 ppmin air atmosphere andare shown in Fig. 7. The high sensing response was observed towards
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less than one order of magnitude.
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acetaldehyde for all four nanostructures and the same time responses toward other vapours were
Fig. 8 (a-d) shows the transient response and recovery characteristics of the undoped and doped ZnO nanostructures upon exposure to various concentration levels of vapours ranging from 10 to 500 ppm.All the sensing elements showed rapid response towards acetaldehyde andthe rapidness was observed when the sharp decrease in baseline resistance of n-type ZnO at the introduction of reducing type acetaldehyde vapour. The observed sensing responses of all the four sensing elements are shown in Fig. 9. The sensing response towards 10 ppm were 2.85, 800, 2.59 and 21.36 for undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Moreover 5, 8, 3 and 7 orders of magnitude change was observed with respect to baseline resistance for undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Fig. 10 (a) & (b) shows the
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response and recovery times with respect to various concentrations of acetaldehyde. As the concentration increased, the response timewas decreased due to simultaneous interaction of large number of target molecules with the sensing element. But in the case of recovery, due to slow
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desorption of target molecules at higher concentration lead to longer recovery time athigher concentrations. To the best of our knowledge compared with all other materials, Co-doped ZnO
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nanostructure showed ultrahigh response at room temperature. Comparison between the sensing
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performances of the developed sensors and literature is presented in Table 3.
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3.4 Sensing Mechanism
The sensing mechanism of metal oxides based semiconductors is often explained by
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ionosorption model. Initially, the oxygen molecules wereionosorbed on metal oxide surface by capturing electrons and set the baseline resistance. According to Barsen and Weimer model,
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oxygen adsorption on metal oxide surface is a temperature dependent process. At room
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temperature, molecular oxygen adsorption
is dominant and it can be expressed as follows,
If reducing analyte such as acetaldehyde was injected into the testing chamber, due to oxidation of target analytes, the surface oxygen ions concentration was reduced thereby releasing the trapped electrons to the metal oxide. This process can cause the decrease in resistance from the baseline (Eq. 2),
Thereafter the sensing chamber was flushed with atmospheric air, once steady state change in resistance was observed. All the sensing elements recovered to its original states after many
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cycles of testing. The proposed sensing mechanism (Eq. 2) was validated using simple lime water test and the same was described in detail in our previous work. The high selective nature of the sensing element towards acetaldehyde is possibly due to
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the lower bond dissociation energy[30,35,38] of acetaldehyde (364 kJ mol-1) than other vapours like ammonia (435 kJ mol-1), acetone (393 kJ mol-1), ethanol (436 kJ mol-1), methanol (436.8 kJ
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mol-1), formaldehyde (364 kJ mol-1) and toluene (368 kJ mol-1). Though the acetaldehyde and
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formaldehyde possessed same dissociation energies, the number of electrons released during the
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interaction resulted in high response towards acetaldehyde (Eq. 2) than formaldehyde (Eq. 3).
Surface morphology of the metal oxide semiconducting sensing element plays a vital role
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in determining the sensor response. Gas sensors are of surface-controlled type, where the dimension, surface states and quantities of adsorbed oxygen molecules significantly influences
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the sensing performance. As of now, it is seldom to find correlation between morphology of the
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nanomaterials and sensing response[11]. The incorporation of transition metals is one of the most attractive methods to improve the sensitivity and selectivity of the metal oxides[39,40]. Moreover, it is already proven that undoped ZnO branched nanorods showed high sensing response towards acetaldehyde due to the presence of large amount of open spaces along with tiny grains and networked nanorods [35]. However in the case of transition metal doping, sensing response of Co-doped ZnO was highest among all the sensing elements. According to reference studies[41], doping Co in ZnO creates more zinc vacancies (Eq. 4) lead to decrease in the background charge carrier concentration resulted in increase in baseline resistance of Co-doped ZnO thin film to 9.7 x 1011 Ω from 3.7 x 1010Ω (undoped ZnO). This electronic sensitization
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would have created more active sites which in-turn improved the sensing response of Co-doped ZnO than undoped ZnO.
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Moreover it is well known fact that the p-type dopants (Cr, Co, Ni and Cu) actively promote the oxidation rate due to rich oxygen adsorption and hence show high catalytic activity towards
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volatile organic compounds. Formation of p-n junction could also improve the sensing
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performance[39,40]. But the absence of impurity peaks in the XRD patterns confirmed the absence of the formation of p-n junction. Also the observed SEM images revealed that cobalt
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doping has not completely changed the branched nanorod structure but effectively increased the sensing response towards acetaldehyde.
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However comparing the response of Co and Cu-doped ZnO, Ni-doped ZnO was extremely low. The morphology of Ni-doped ZnO was completely different from undoped and
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Co & Cu-doped ZnO nanostructures. The decreased response in Ni-doped ZnO, might probably
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due to reduced number of active sites available for interaction which was well supported by the observed resistance values of undoped and Ni-doped ZnO thin films. Also Cu has an inherent property of enhanced oxygen adsorption and hence it set the baseline resistance higher (4.3 x 1011 Ω) than that of undoped ZnO. Due to its enhanced oxygen adsorption [42,43] provided more active sites for interaction and lead to higher response than undoped ZnO.Also the baseline resistances of undoped, Co, Ni and Cu doped ZnO thin films are 3.7 x 1010Ω, 9.7 x 1011 Ω, 5 x 107 Ω and 4.3 x 1011 Ω respectively. The resistance values of undoped, Ni and Cu-doped ZnO thin film resistances were lower than Co-doped ZnO thin film. Because of the more number of electrons present on the surface of undoped, Ni and Cu-doped ZnO thin films than the Co-doped
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ZnO thin film, the change in space charge region and hence surface resistance during the reducing gas interaction was less than that of Co-doped film. The morphology and size of the nanostructure determines the degree of electron
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confinement. Thus nanostructure whose configuration is readily accessible to gas molecules is more advantageous to design highly sensitive sensor[44,45]. It has been already proven that
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complex morphologies such as combination of 1D and 2D structures often provided higher
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response compared with traditional thin film structures. In the case of undoped ZnO, due to more open space with loosely packed structure lead to the mass interaction of gas molecules in-turn
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resulted in quicker response and recovery times than others. At the same time, Co, Ni and Cudoped ZnO exhibited compactly packed structures lead to slow response and recovery times.
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The stability nature of Co-doped ZnO thin film was studied in air atmosphere as well as in 100 ppm of acetaldehyde over a period of 10 days and the observed response is shown in Fig. 9.
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There was no serious drift in the baseline resistance and the change in resistance towards 100
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ppm observed. It confirmed the high stability nature of the sensing element and it can be used for continuous monitoring of acetaldehyde in air atmosphere. Further the reproducibility nature of the sensing element was also confirmed by depositing thin film with the same deposition conditions and studied their sensing characteristics.
4. Conclusion
We have demonstrated a simple and cost effective way of synthesizing ZnO nanoarchitectures via spray pyrolysis technique.The undoped and doped ZnO nanostructures showed an excellent response values to acetaldehyde at room temperature. Complete morphology transformation was observed while transition metals were doped into ZnO. All the
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developed sensing elements were highly selective towards acetaldehyde.The transition metal doped ZnO nanostructures was able to detect wide range of acetaldehyde concentration with the detection limit of 10 ppm.It can be concluded that the Co-doped ZnO based acetaldehyde sensor
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is a promising candidate for effective acetaldehyde detection in various applications.
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Acknowledgements The authors wish to express their sincere thanks to the Department of Science & Technology, New Delhi, India for the financial support (Project ID: INT/SWD/VINN/P-04/2011 &
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SR/FST/ETI- 284/2011(C)). They also wish to acknowledge SASTRA University, Thanjavur for
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extending infrastructure support to carry out this work.
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865–871.
and catalyst free grown ZnO nanorods, RSC Adv. 5 (2015) 54952–54962. Z. Lou, F. Li, J. Deng, L. Wang, T. Zhang, Branch-like hierarchical heterostructure (α-
an
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S.S. Kim, H.G. Na, S.-W. Choi, D.S. Kwak, H.W. Kim, Novel growth of CuO-
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functionalized, branched SnO2 nanowires and their application to H2S sensors, J. Phys. D.
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Appl. Phys. 45 (2012) 205301.
O. Lupan, V.M. Guérin, I.M. Tiginyanu, V. V. Ursaki, L. Chow, H. Heinrich, et al., Wellaligned arrays of vertically oriented ZnO nanowires electrodeposited on ITO-coated glass and their integration in dye sensitized solar cells, J. Photochem. Photobiol. A Chem. 211 (2010) 65–73.
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G.K. Mani, J.B.B. Rayappan, Novel and Facile Synthesis of Randomly Interconnected ZnO Nanoplatelets Using Spray Pyrolysis and Their Room Temperature Sensing Characteristics, Sensors Actuators B Chem. 198 (2014) 125–133.
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G.K. Mani, J.B.B. Rayappan, A highly selective and wide range ammonia sensor— Nanostructured ZnO:Co thin film, Mater. Sci. Eng. B. 191 (2015) 41–50.
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G.K. Mani, J.B.B. Rayappan, A highly selective room temperature ammonia sensor using spray deposited zinc oxide thin film, Sensors Actuators B Chem. 183 (2013) 459–466.
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M. Vagadia, A. Ravalia, U. Khachar, P.S. Solanki, R.R. Doshi, S. Rayaprol, et al., Size
ip t
and grain morphology dependent magnetic behaviour of Co-doped ZnO, Mater. Res. Bull. 46 (2011) 1933–1937.
G.K. Mani, J.B.B. Rayappan, Selective Detection of Ammonia Using Spray Pyrolysis
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Deposited Pure and Nickel Doped ZnO Thin Films, Appl. Surf. Sci. 311 (2014) 405–412. G.K. Mani, J.B.B. Rayappan, A simple and template free synthesis of branched ZnO
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nanoarchitectures for sensor applications, RSC Adv. 4 (2014) 64075–64084. Y. Ma, Y. Qu, W. Zhou, Surface engineering of one-dimensional tin oxide nanostructures
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for chemical sensors, Microchim. Acta. 180 (2013) 1181–1200. D. James, S.M. Scott, Z. Ali, W.T. O’Hare, Chemical sensors for electronic nose systems,
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Co-doped ZnO hierarchical microspheres to 1,2-dichloroethane, Sensors Actuators B Chem. 166-167 (2012) 36–43. [40]
H.-S. Woo, C.-H. Kwak, J.-H. Chung, J.-H. Lee, Highly selective and sensitive xylene sensors using Ni-doped branched ZnO nanowire networks, Sensors Actuators B Chem. 216 (2015) 358–366.
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H. Woo, C. Kwak, J. Chung, J. Lee, Co-Doped Branched ZnO Nanowires for Ultraselective and Sensitive Detection of Xylene, ACS Appl. Mater. Interfaces. 6 (2014) 22553–22560.
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ip t
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H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide
us
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cr
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semiconductors: Overview, Sensors Actuators B Chem. 192 (2014) 607–627. P. Shankar, J.B.B. Rayappan, Gas sensing mechanism of metal oxides: The role of
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P. Rai, Y.-T.T. Yu, Citrate-assisted hydrothermal synthesis of single crystalline ZnO
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d
detection of acetaldehyde, Sensors Actuators B Chem. 174 (2012) 402–405.
nanoparticles for gas sensor application, Sensors Actuators B Chem. 173 (2012) 58–65. [48]
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Author Biographies Ganesh
Kumar
Mani
received
his
B.Sc.,
Physics
from
ip t
BharathidasanUniversity, Tiruchirapalli and M.Sc. in Nanoscience and Nanotechnology from BharathiarUniversity, Coimbatore in 2009 and
cr
2011 respectively. He is currently working as Junior Research Fellow in
us
the Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) and School of Electrical & Electronics Engineering, SASTRAUniversity, Thanjavur, India. His
an
current research interests are fabrication of various nanostructured thin film based chemiresistive sensors.He has published more than 30 research articles in peer reviewed international journals
d
M
and filed two patents.
Ac ce pt e
John BoscoBalaguruRayappanreceived his M.Sc. and Ph.D. degrees in Physics from St. Joseph’s College, Bharathidasan University, Tiruchirapalli, India in 1996 and 2003, respectively. He is currently
working asAssociate Dean (Research) in the School of Electrical &
Electronics Engineering and Centre for Nanotechnology &Advanced Biomaterials (CeNTAB) of SASTRA University. His current research interests include development of gas/chemical sensors, biosensors and functional nanomaterials. He is also working in the field of embedded systems and steganography. He has published more than 160 research articles in peer reviewed international journals and filed six patents.
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Table Captions Table 1 Optimized deposition parameters.
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Table 2 Structural parameters of undoped and doped thin films obtained from XRD spectrum. Table 3 Acetaldehyde sensing performance comparison of various ZnO nanostructures with the
us
cr
present work.
Figure Captions
an
Figure 1 Spray Pyrolysis set-up.
Figure 2 XRD patterns of a) undoped and b) doped ZnO thin films.
M
Figure 3 FE-SEM micrographs of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. (a1) – (d1) shows the high magnification FESEM images respectively.
Ac ce pt e
films.
d
Figure 4 EDS Spectrum of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin
Figure 5 Optical absorbance spectra of undoped and doped ZnO thin films. Figure 6 A photograph of sensor testing setup. Figure 7 Selectivity nature of the sensing elements. Inset shows the highlighted view of response towards interfering vapours.
Figure 8 Transient resistance response of a) undoped, b) Co-doped, c) Ni-doped and d) Cudoped ZnO thin films.
Figure 9 Response of undoped and doped ZnO thin films towards various concentrations of acetaldehyde at room temperature. Figure 10 Typical a) response and b) recovery times of undoped and doped ZnO nanostructures.
25
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us
cr
ip t
Figure 11 Stability nature of Co-doped ZnO thin film towards 100 ppm over a period of 5 days.
Table 1 Deposition parameters
Values
an
Parameters Precursor
Anhydrous zinc chloride (0.1 M)
Doping element precursors
M
Cupric acetate dihydrate
Nickel acetate tertahydrate Cobalt acetate tertahydrate 0.2 mm
Solution flow rate
2 ml. min-1
Ac ce pt e
d
Spray nozzle diameter
Compressed air pressure
1 bar
Substrate temperature
523 ± 2 K
Annealing temperature
623 K
Annealing duration
6h
Substrate to nozzle distance
15 cm
Spray angle
90º
Deposition area per min.
15 cm2
26
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ip t cr us
Table 2 Structural parameters of undoped and doped thin films obtained from XRD spectrum.
ZnO Co-doped
Size (nm)
(Å)
35.1253
39
35.6282
35
Ni-doped ZnO
Cu-doped ZnO
34.9764
34.858
32
30
Lattice
an
(deg.)
Ac ce pt e
ZnO
d-spacing
M
Undoped
Crystallite
parameter
Strain
5.1060
0.002985
2.5181
5.0362
0.003207
2.5635
2.5720
Resistance (Ω)
“c” (Å)
2.5530
d
Sample
2 (θ)
5.1271
5.1439
0.003677
0.003829
3.7 x 1010 9.7 x 1011 5.5 x 107 4.3 x 1011
27
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Table 3 Acetaldehyde sensing performance comparison of various ZnO nanostructures with the
Concentration
Response
Operating
structures
(ppm)
(S)
temperature (oC)
Powders
2
5.73
Platelets
50
50
Particles
250
~ 45
400
[47]
Tetrapods
50
47.5
400
[15]
Sheets
1
77
220
[48]
Rods
250
5.30
400
[16]
Flowers
250
8
400
[18]
Petals
100
14
Room Temperature
[49]
10
2.85
Room Temperature
Nanorod
cr us
450
an
Room Temperature
M
Ac ce pt e
Branched
ip t
ZnO Nano-
d
present work
Branched
10
[46]
[30]
Present
Co-doped ZnO
Ref.
Work 800
Room Temperature
Nanorods
28
Page 28 of 42
ip t cr us an M d Ac ce pt e
Figure 0 Spray Pyrolysis set-up.
29
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(002)
0
35
40
45
M
30
an
ip t
us
cr
Intensity (a.u.)
(a)
50
55
60
(110)
(102)
Cu-doped ZnO
Intensity (a.u.)
(b)
d
(101)
(002)
Ac ce pt e
(001)
2θ (deg.)
Ni-doped ZnO
Co-doped ZnO
30
35
40
45
50
55
60
2θ (deg.)
Figure 0XRD patterns of a) undoped and b) doped ZnO thin films.
30
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31
Page 31 of 42
d
Ac ce pt e us
an
M
cr
ip t
ip t cr us an M d Ac ce pt e Figure 0FE-SEM micrographs of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. (a1) – (d1) shows the high magnification FESEM images respectively.
32
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33
Page 33 of 42
d
Ac ce pt e us
an
M
cr
ip t
34
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d
Ac ce pt e us
an
M
cr
ip t
ip t cr us an
3.0
M d
2.0
1.5
Ac ce pt e
Absorbance (a.u.)
2.5
Undoped ZnO Co-doped ZnO Ni-doped ZnO Cu-doped ZnO
1.0
0.5
0.0 350
400
450
500
550
600
Wavelength (nm)
Figure 1 Optical absorbance spectra of undoped and doped ZnO thin films. 0
35
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ip t cr us an M d Ac ce pt e
Figure 1A photograph of sensor testing setup.
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Undoped ZnO Co-doped ZnO Ni-doped ZnO Cu-doped ZnO
Response for 100 ppm
cr
800 700
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600
400 300
an
6
Response (S)
Response (S)
8
500
4
M
2
200
0
0 ia tone mon Ace Am
l l yde ano ethano deh Eth M mal For
d
100
ip t
900
ene Tolu
Ac ce pt e
e e e ia ne hanol hanol hyd Toluen ldehyd mon Aceto Et a Met rmalde Am t Ace Fo
Figure 1 Selectivity nature of the sensing elements. Inset shows the highlighted view of response towards interfering vapours.
37
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(b)
(a)
12
10
12
10 11
10
11
10
0
1000
2000
3000
4000
0
Time (s)
2000
ip t
4000
6000
500 ppm
Acetaldehyde OFF
250 ppm
10
100 ppm
Acetaldehyde ON
5
cr
6000
50 ppm
5000
10
75 ppm
7
10
us
6
10
500 ppm
Acetaldehdye OFF
8
10
6
250 ppm
Acetaldehdye ON
7
10
100 ppm
75 ppm
50 ppm
10
30 ppm
20 ppm
8
9
10
20 ppm
Resistance (Ω )
Resistance (Ω )
9
10
10
10
10 ppm
10
10
8000
10000
12000
Time (s)
(d)
an
(c)
13
10
8
10
12
10
11
0
2000
4000
6000
8000
10000
7
10
75 ppm
Acetaldehyde ON Acetaldehyde OFF
6
10
5
12000
10
0
2000
4000
6000
500 ppm
8
10
250 ppm
50 ppm
10
20 ppm
9
10 ppm
M Resistance (Ω )
d
Acetaldehyde OFF
10
10
100 ppm
5
250 ppm
Ac ce pt e Acetaldehyde ON
10
100 ppm
75 ppm
50 ppm
6
10
500 ppm
7
10
20 ppm
10 ppm
Resistance (Ω )
10
8000 10000 12000 14000 16000
Time (s)
Time (s)
Figure 1Transient resistance response of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. 251694080
38
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40000
ip t
30000 20000
cr
10000
Cu-doped ZnO
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0
an
200 0
Ni-doped ZnO
M
Response (S)
400
4000000
Ac ce pt e
d
2000000 0
Co-doped ZnO
8000 6000 4000 2000
Undoped ZnO
0
0
50 100 150 200 250 300 350 400 450 500 550
Concentration (ppm) Figure 1 Response of undoped and doped ZnO thin films towards various concentrations of acetaldehyde at room temperature.
39
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251694080
300 Undoped ZnO Co-doped ZnO Ni-Doped ZnO Cu-doped ZnO
ip t
200
cr
150
100
us
Response Time (s)
250
0 0
100
200
an
50
300
400
(a)
500
M
Concentration (ppm)
250
d
Undoped ZnO Co-doped ZnO Ni-Doped ZnO Cu-doped ZnO
Recovery Time (s)
Ac ce pt e
200
150
100
50
(b)
0
0
100
200
300
400
500
Concentration (ppm)
Figure 1Typical a) response and b) recovery times of undoped and doped ZnO nanostructures.
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0
ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature 12
5x10
Acetaldehyde Sensor
5
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8.2x10
Ganesh Kumar Mani and John Bosco Balaguru Rayappan*
5
cr
8x10
5x10
In Air
us
School of Electrical & Electronics Engineering (SEEE)
5
SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
7.8x10
an
In 100 ppm
10
5
7.6x10
M
5x10
9
5x10
2
3
4
5
6
5
7.4x10
7
8
9
10
d
1
Resistance (Ω)
Resistance (Ω )
Nano Sensors 11 Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
Ac ce pt e
Time (Days)
Figure 1Stability nature of Co-doped ZnO thin film towards 100 ppm over a period of 5 days.
41
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ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor
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Ganesh Kumar Mani and John Bosco Balaguru Rayappan* Nano Sensors Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
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School of Electrical & Electronics Engineering (SEEE)
Ac ce pt e
d
M
an
us
SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
Page 42 of 42
S0925-4005(15)30410-X http://dx.doi.org/doi:10.1016/j.snb.2015.09.103 SNB 19088
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
3-6-2015 2-8-2015 20-9-2015
Please cite this article as: G.K. Mani, ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.09.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ZnO Nanoarchitectures:Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor
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Ganesh Kumar Mani and John Bosco Balaguru Rayappan* Nano Sensors Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
cr
School of Electrical & Electronics Engineering (SEEE)
M
an
us
SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
*Corresponding Author
d
----------------------------------------------------------------------------------------------------------------
Ac ce pt e
Prof. John Bosco Balaguru Rayappan, Ph.D.
Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) & School of Electrical & Electronics Engineering SASTRA University
Thanjavur – 613 401 India
Phone: +91 4362 264 101; Ext: 2255 Fax: +91 4362 264120
E-mail: [email protected] (John Bosco Balaguru Rayappan) [email protected] (Ganesh Kumar Mani)
1
Page 1 of 42
HIGHLIGHTS ZnO nanoarchitectures were facilely grown on glass substrates
•
Effect of doping on the formation of ZnO nanorods was investigated
•
Room temperature acetaldehyde sensor using ZnO nanoarchitectures has been reported
•
Ultrahigh sensitivity and selectivity observed for Co-doped ZnO
Ac ce pt e
d
M
an
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cr
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•
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Page 2 of 42
Abstract In this work, undoped and transition metals (Co, Ni and Cu)doped ZnO nanoarchitectures were deposited on glass substrates by simple chemical spray pyrolysis technique. The obtained
ip t
ZnO nanoarchitectures were characterized using X-ray diffractiometer (XRD), scanning electron microscope (SEM), UV-vis spectrophotometer and electrometer to study their structural,
cr
morphological, optical and sensing properties respectively. The structural and morphological
us
results revealed that transition metal doping effectively changed the microstructures and distorted the preferential orientation of (002) crystal plane with reference to the undoped ZnO.
an
Acetaldehyde sensing characteristicsof the undoped and doped ZnO nanoarchitectures were investigated. The sensing response towards 10 ppm wasfound to be 2.85, 800, 2.59 and 21.36 for
M
the undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Co-doped ZnO nanostructure showed an excellent sensing response over a wide concentration range of 10 to 500
d
ppm of acetaldehyde. Moreover the sensing elements showed high selectivity towards
Ac ce pt e
acetaldehyde when exposed to 7 different vapours. Ultrahigh sensitivity and selectivity observed forCo-doped ZnO nanoarchitecture revealed its identity asa potential candidate for detectingacetaldehyde in various applications.
Keywords: ZnO; thin film; spray pyrolysis; gas sensor; doping; acetaldehyde.
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1. Introduction Nowadays, development of fast response, cost effective and portable sensors for detecting toxic/hazardous gases are in high demand due to theirnecessity in protecting human
ip t
lives as well as monitoringenvironment[1,2].Acetaldehyde is ubiquitous in daily life and it has been classified as group I carcinogen according to International Agency for Research on Cancer.
cr
The main routes of acetaldehyde exposure to humans are alcohol consumption, fermented foods,
us
cigarette smoking, exhaust from cars and trucks[3,4]. Due to its strong electrophilic nature it was proved that it damages DNA in humans and alter the red blood cell structure[5,6]. In addition,
an
acetaldehyde has a strong tendency to combine with Vitamin B1 (nerve vitamin) and inducesa deficiency[7]. Hence it lead to mental illness, poor memory, mental confusion and visual
M
disturbances. National Institute for Occupational Safety & Health (NIOSH) and Mine Safety & Health Administration (MSHA) has proposed a permissible exposure limit (PEL) of
d
acetaldehyde is 100 ppm as a time weighted average (TWA) of 8 h shift.Occupational Safety and
Ac ce pt e
Health Administration (OSHA) notified that threshold exposure limit is 25 ppm for 8 h work shift[8]. The above facts necessitates that, it is essential to develop cost effective and high response acetaldehyde sensor to protect human lives. Several methods such as electrochemical, chromatographic, spectrometric, quartz crystal microbalance and chemiresisitve metal oxides have been used to detect acetaldehyde in various applications [3,4,9,10]. Amongst all,nanostructured metal oxidesare found to be suitable candidates for detecting volatile organic compounds (VOCs) / gases of various concentrations due to high surface to volume ratio, tuning selectivity via. crystallographic orientation, enhanced catalytic activity and superior stability[11].Moreover it offerscountless advantageslikehigh
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sensitivity, quick response and recovery times, easy fabrication,portability, requirement of no expensive equipment and mainly applicable for in situ detection. Among many metal oxides, ZnO is one of the popular materials for gas sensing
ip t
applications especially for detecting vapours of ethanol, hydrogen, methanol, trimethylamine, ammonia, acetaldehyde, carbon dioxide, xylene, monoethanolamine, etc[12–14]. Recently,
cr
various ZnO based acetaldehyde sensors developed using vapour phase, sonochemical, chemical
us
precipitation, sol-gel, spray and hydrothermal techniques have been reported, but those sensors were operated at elevated temperatures and demonstrated poor selectivity[15–17]. With respect
an
to the available reports, ZnO tetrapods showeda response of 47.5 for 50 ppm of acetaldehyde at 400oC[15]. Rai et al. observed the sensing response of 5.30 towards250 ppm at 400oC[16].
M
Flower like ZnO nanostructures exhibited response towards acetaldehyde, carbon monoxide and nitrogen dioxide[18]. Unfortunately, many of the developed sensors have not satisfiedall the
d
ideal sensing characteristics such as high stability, low cross selectivity, limit detection limit,
Ac ce pt e
high response, wide detection range, quick response and recovery times. To overcome these limitations,doping has been widely used as an effective tool to modulate the band structure and hence to obtaindesired sensing characteristics[19–21]. Mainly, noble metals such as Ag, Au, Pd, Pt and transition metals like Fe, Mn, Co, Ni, Cu have been used as additives to promote the gas sensing characteristics [22].In particular, influence of transition metal doping in many instances connected with changes in the grain size and charge carrier concentration often resulted in tunable sensing performances.Chemical vapour deposition (CVD) techniques are best known and widely used in thin film fabrication applications owing to their functional advantages over physical vapour deposition (PVD)techniques, process flexibility, growth rate, large area deposition, etc.Especially, liquid source CVD techniques like spray pyrolysis, sol-gel dip/spin
5
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coatingarehighly versatile tools for achieving desired functional properties. Under ideal conditions, liquids can be vapourizedcompletely rather than solid sources, hence formation of thin films using liquid source CVD (spray pyrolysis)have been considered as a one of the best
ip t
tools[23]. Complex morphologies have drawn much attention in gas sensor field due to their
cr
excellent properties like high surface area and enhanced surface catalytic activity[24–26]. As of
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now, seeded growth technologies have been used to prepare various ZnO nanoarchitectures especially complex morphologies like branched nanorods[27–29]. But these kinds of multistep
an
methodslimit the applications of such nanostructures due to too many control parameters.Hence, in this work, a facile, novel and template free technique has been developed to synthesize various
M
ZnO Nanoarchitectures. In addition the influence of doping on modulating the morphological, transport and sensing properties of ZnO nanoarchitectures has motivated to carry out such
d
investigations. To our knowledge,effect of transition metal doping on the room temperature
Ac ce pt e
acetaldehyde sensing performance of ZnO branched nanorods has not been reported elsewhere. Hence, in this work the development of room temperature acetaldehyde sensor using various undoped and doped ZnO nanoarchitectures has been reported.
2. Materials and methods 2.1 Film Deposition
Undoped and transition metals (Co, Ni and Cu) doped ZnO thin films were deposited on glass substrates by simple chemical spray pyrolysis technique. Spray pyrolysis is a process in which thin film is deposited by spraying a precursor solution on a pre-heated surface, where thermal decomposition of the precursorconstituents lead to the formation of thin film over the
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Page 6 of 42
substrate. Fig. 1 shows the typical spray pyrolysis unit which consists of solution dispenser, pressure regulator, atomizer and heater plate. Atomizer is the heart of spray system where the precursor solution is converted into fine mist with the assistance of compressed air. The atomizer
ip t
was mounted with X and Y axes stepper motor to move throughout the substrates to achieve uniform deposition. Parameters like temperature, flow rate of the solution, spray head movement
cr
and its speed, spray time, pause time are controlled through a personal computer. The
us
temperature of the substrate heater was controlled by K-type thermocouple with PID controller. At first, glass substrates were ultrasonically cleaned with acetone, isopropanol and
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doubly deionized water for 15 min each. The precursor solution wasprepared by dissolving 0.1 M of anhydrous zinc chloridein deionized water. The solution was stirred about 30 min to obtain
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clear solution. Then the solution was sprayed over the pre-heated substrate using spray pyrolysis technique. The optimized deposition parameters were given in Table 1. Effect of transition metal
d
doping on the undoped ZnO was executed by mixing 5wt% of precursor salt into the host
Ac ce pt e
solution. Cupric acetate dihydrate (Cu(CH3COO)2·2H2O), nickel acetate tertahydrate (Ni(CH3COO)2·4H2O), cobalt acetate tertahydrate (Co(CH3COO)2·4H2O) were used as the precursor salts for Cu, Ni and Co doping respectively.
2.2 Characterization
X-Ray Diffractometer (D8 Focus, Bruker, Germany) was used to investigate the structural parameters such as crystallinity, crystallite size, d-spacing, strain and lattice parameters of the deposited ZnO thin film. Morphology of the thin filmswas investigated using Field Emission Scanning Electron Microscope (FE-SEM) (JEOL, 6701F, Japan). Energy Dispersive X-Ray Spectroscopy was used to along with FE-SEM to confirm the elemental composition in
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the films.Optical properties of the thin films were studied using UV-Vis spectrophotometer (Perkin Elmer, Lambda 25, USA) in the wavelength range of 200 to 800 nm with the scan rate of 100 nm min-1. Electrical and sensing properties were carried out using an electrometer (Keithley,
ip t
6517A, USA).
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2.3 Sensor fabrication and testing
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Based on the deposited undoped and transition metal doped films, four different sensing elements were prepared and investigated their sensing responses at room temperature. Initially,
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Ohmic contacts were established on the surface of the sensing element (10 mm × 10 mm) using zero resistance copper wires and highly conducting silver paste.Sensing studies were carried out
M
at room temperature using custom build gas sensing setup and the same described in our previous report[30–32]. The concentration of the target vapour[26] in the testing chamber was
Ac ce pt e
where,
d
fixed using Eq. 1.
is the density of acetaldehyde,
universal gas constant,
is the volume of acetaldehyde injected,
is the absolute temperature,
pressure inside the chamber and calculated using the relation
is the molecular weight,
is the is the
is the volume of the chamber. The sensor response (S) was
where,
and
are the resistance of the sensing elements
is in air and target gas.
8
Page 8 of 42
3. Results and Discussion 3.1 Structural studies XRD patterns of the undoped and transition metals doped ZnO thin films are shown in
ip t
Fig. 2 (a) & (b). The observed XRD patterns revealed that all the undoped and transition metals doped ZnO thin films belong to wurtzite crystal structure (JCPDS Card No: 36-1451). No
cr
anomalouspeaks were observed in doped samples which confirmed the doping levels of
us
transition metals in ZnO was within the solubility limit. Various structural parameters like crystallite size, d-spacing, lattice parameter (c) and strain values were estimated and the same is
an
presented in Table 2. With reference to undoped ZnO, lattice parameter, d-spacing and strainfor (002) crystal plane was larger/higher for Ni and Cu-doped ZnO, smaller/lower for Co-doped
M
ZnO. It emphasized the fact that Ni and Cu ions existed in interstitial sites of ZnO and resulted in increased d-spacing as well as strain. At the same time, substitution of Zn by Co ions confirmed
d
through decrease in lattice parameter (c) and d-spacing values. The higher angle peak shift in Co-
Ac ce pt e
doped ZnO thin films with reference to undoped ZnO might be due to the substitution of high spin state Co2+ ions (ionic radius ~0.58 Å) in tetrahedral coordinated Zn2+ ions (ionic radius ~0.60 Å)[33]. The average crystallite size estimated for (002) crystal plane was found to be 39, 35, 32 and 30 nm for undoped, Co, Ni and Cu doped ZnO thin films respectively. It was clear that when dopant atom introduced into the ZnO lattice, dopants might have distorted the aligned (002) crystal orientation and promotes the crystal growth in multiple directions (001)and(101). Hence, due to the multiple direction growth of crystallites, the dominant crystal growth of ZnO in (002) direction was diminished and resulted in decreased in crystallite size.
9
Page 9 of 42
3.2 Morphological and optical studies Fig. 3 (a & a1) – (d & d1) shows the low and high magnification FE-SEM images of the undoped and transition metals doped ZnO thin films. Undoped ZnO thin films showed large
ip t
quantity of well dispersed branched nanorodswith hexagonal tips and uniform diameter along their entire lengths. The typical length of the nanorod was around 300 nm and these branched
cr
nanorods were heterogeneously grown on both sides from the root (Fig. 3(a&a1)). The
us
morphologies of transition metals doped ZnO nanostructures were remarkably different from that of undoped ZnO thin films. For Co-doping (Fig. 3 (b &b1)), evenly distributed branched nanorod
an
structure was converted into irregular shaped nanorods along with tiny particles which were well connected to each other. Fig. 3 (c &c1) shows the well dispersed randomly orientated Ni-doped
M
ZnO nanorodsand the length was over 200 nm. A greater agglomeration of particles was observed in Cu-doped ZnO thin film (Fig. 3 (d &d1)). The EDS spectra of undoped and doped
d
films are shown in Fig. 4 (a-d). The presence of zinc and oxygen elements was confirmed in the
Ac ce pt e
undoped ZnO thin films. Moreover, the presence of cobalt, nickel and copper elements along with zinc and oxygen was confirmed in the doped samples. The presence of Si and Au peaks may be due to the substrate effect and sputter coating to eliminate the charging effect respectively.
Fig. 5 shows the optical absorbance spectra of undoped and doped ZnO thin films. The increased absorption in doped films might be due to the creation of impurity states below the conduction band as well as strain caused by doping[34]. Also, due to large amount of open space, undoped and Cu-doped ZnO films exhibited lesser absorption as compared with Co and Ni-doped ZnO thin film[35].
10
Page 10 of 42
3.3 Sensing studies In the recent past, hierarchical nanostructures have been demonstrated to have great potential for various applications due to their high surface to volume ratio and high catalytic
ip t
activity[36,37]. In particular, it is proven that these complex nanostructures deliver high gas/vapour sensing response with quick response and recovery times. In the same line of thought,
cr
the sensing response of the observed ZnO nanostructures has been investigated.
us
Therefore,electrical contacts were established on the sensing elements to examine their sensing ability. The typical photograph of the sensing set-up is shown in Fig. 6. It consists of testing
an
chamber along with vacuum pump, an electrometer and PC interface for storing readings. The sensing responses of undoped and doped sensing elements were studied by exposing them
M
towards 7 different vapours at the fixed constant concentration range of 100 ppmin air atmosphere andare shown in Fig. 7. The high sensing response was observed towards
Ac ce pt e
less than one order of magnitude.
d
acetaldehyde for all four nanostructures and the same time responses toward other vapours were
Fig. 8 (a-d) shows the transient response and recovery characteristics of the undoped and doped ZnO nanostructures upon exposure to various concentration levels of vapours ranging from 10 to 500 ppm.All the sensing elements showed rapid response towards acetaldehyde andthe rapidness was observed when the sharp decrease in baseline resistance of n-type ZnO at the introduction of reducing type acetaldehyde vapour. The observed sensing responses of all the four sensing elements are shown in Fig. 9. The sensing response towards 10 ppm were 2.85, 800, 2.59 and 21.36 for undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Moreover 5, 8, 3 and 7 orders of magnitude change was observed with respect to baseline resistance for undoped, Co, Ni and Cu-doped ZnO nanostructures respectively. Fig. 10 (a) & (b) shows the
11
Page 11 of 42
response and recovery times with respect to various concentrations of acetaldehyde. As the concentration increased, the response timewas decreased due to simultaneous interaction of large number of target molecules with the sensing element. But in the case of recovery, due to slow
ip t
desorption of target molecules at higher concentration lead to longer recovery time athigher concentrations. To the best of our knowledge compared with all other materials, Co-doped ZnO
cr
nanostructure showed ultrahigh response at room temperature. Comparison between the sensing
us
performances of the developed sensors and literature is presented in Table 3.
an
3.4 Sensing Mechanism
The sensing mechanism of metal oxides based semiconductors is often explained by
M
ionosorption model. Initially, the oxygen molecules wereionosorbed on metal oxide surface by capturing electrons and set the baseline resistance. According to Barsen and Weimer model,
d
oxygen adsorption on metal oxide surface is a temperature dependent process. At room
Ac ce pt e
temperature, molecular oxygen adsorption
is dominant and it can be expressed as follows,
If reducing analyte such as acetaldehyde was injected into the testing chamber, due to oxidation of target analytes, the surface oxygen ions concentration was reduced thereby releasing the trapped electrons to the metal oxide. This process can cause the decrease in resistance from the baseline (Eq. 2),
Thereafter the sensing chamber was flushed with atmospheric air, once steady state change in resistance was observed. All the sensing elements recovered to its original states after many
12
Page 12 of 42
cycles of testing. The proposed sensing mechanism (Eq. 2) was validated using simple lime water test and the same was described in detail in our previous work. The high selective nature of the sensing element towards acetaldehyde is possibly due to
ip t
the lower bond dissociation energy[30,35,38] of acetaldehyde (364 kJ mol-1) than other vapours like ammonia (435 kJ mol-1), acetone (393 kJ mol-1), ethanol (436 kJ mol-1), methanol (436.8 kJ
cr
mol-1), formaldehyde (364 kJ mol-1) and toluene (368 kJ mol-1). Though the acetaldehyde and
us
formaldehyde possessed same dissociation energies, the number of electrons released during the
an
interaction resulted in high response towards acetaldehyde (Eq. 2) than formaldehyde (Eq. 3).
Surface morphology of the metal oxide semiconducting sensing element plays a vital role
M
in determining the sensor response. Gas sensors are of surface-controlled type, where the dimension, surface states and quantities of adsorbed oxygen molecules significantly influences
d
the sensing performance. As of now, it is seldom to find correlation between morphology of the
Ac ce pt e
nanomaterials and sensing response[11]. The incorporation of transition metals is one of the most attractive methods to improve the sensitivity and selectivity of the metal oxides[39,40]. Moreover, it is already proven that undoped ZnO branched nanorods showed high sensing response towards acetaldehyde due to the presence of large amount of open spaces along with tiny grains and networked nanorods [35]. However in the case of transition metal doping, sensing response of Co-doped ZnO was highest among all the sensing elements. According to reference studies[41], doping Co in ZnO creates more zinc vacancies (Eq. 4) lead to decrease in the background charge carrier concentration resulted in increase in baseline resistance of Co-doped ZnO thin film to 9.7 x 1011 Ω from 3.7 x 1010Ω (undoped ZnO). This electronic sensitization
13
Page 13 of 42
would have created more active sites which in-turn improved the sensing response of Co-doped ZnO than undoped ZnO.
ip t
Moreover it is well known fact that the p-type dopants (Cr, Co, Ni and Cu) actively promote the oxidation rate due to rich oxygen adsorption and hence show high catalytic activity towards
cr
volatile organic compounds. Formation of p-n junction could also improve the sensing
us
performance[39,40]. But the absence of impurity peaks in the XRD patterns confirmed the absence of the formation of p-n junction. Also the observed SEM images revealed that cobalt
an
doping has not completely changed the branched nanorod structure but effectively increased the sensing response towards acetaldehyde.
M
However comparing the response of Co and Cu-doped ZnO, Ni-doped ZnO was extremely low. The morphology of Ni-doped ZnO was completely different from undoped and
d
Co & Cu-doped ZnO nanostructures. The decreased response in Ni-doped ZnO, might probably
Ac ce pt e
due to reduced number of active sites available for interaction which was well supported by the observed resistance values of undoped and Ni-doped ZnO thin films. Also Cu has an inherent property of enhanced oxygen adsorption and hence it set the baseline resistance higher (4.3 x 1011 Ω) than that of undoped ZnO. Due to its enhanced oxygen adsorption [42,43] provided more active sites for interaction and lead to higher response than undoped ZnO.Also the baseline resistances of undoped, Co, Ni and Cu doped ZnO thin films are 3.7 x 1010Ω, 9.7 x 1011 Ω, 5 x 107 Ω and 4.3 x 1011 Ω respectively. The resistance values of undoped, Ni and Cu-doped ZnO thin film resistances were lower than Co-doped ZnO thin film. Because of the more number of electrons present on the surface of undoped, Ni and Cu-doped ZnO thin films than the Co-doped
14
Page 14 of 42
ZnO thin film, the change in space charge region and hence surface resistance during the reducing gas interaction was less than that of Co-doped film. The morphology and size of the nanostructure determines the degree of electron
ip t
confinement. Thus nanostructure whose configuration is readily accessible to gas molecules is more advantageous to design highly sensitive sensor[44,45]. It has been already proven that
cr
complex morphologies such as combination of 1D and 2D structures often provided higher
us
response compared with traditional thin film structures. In the case of undoped ZnO, due to more open space with loosely packed structure lead to the mass interaction of gas molecules in-turn
an
resulted in quicker response and recovery times than others. At the same time, Co, Ni and Cudoped ZnO exhibited compactly packed structures lead to slow response and recovery times.
M
The stability nature of Co-doped ZnO thin film was studied in air atmosphere as well as in 100 ppm of acetaldehyde over a period of 10 days and the observed response is shown in Fig. 9.
d
There was no serious drift in the baseline resistance and the change in resistance towards 100
Ac ce pt e
ppm observed. It confirmed the high stability nature of the sensing element and it can be used for continuous monitoring of acetaldehyde in air atmosphere. Further the reproducibility nature of the sensing element was also confirmed by depositing thin film with the same deposition conditions and studied their sensing characteristics.
4. Conclusion
We have demonstrated a simple and cost effective way of synthesizing ZnO nanoarchitectures via spray pyrolysis technique.The undoped and doped ZnO nanostructures showed an excellent response values to acetaldehyde at room temperature. Complete morphology transformation was observed while transition metals were doped into ZnO. All the
15
Page 15 of 42
developed sensing elements were highly selective towards acetaldehyde.The transition metal doped ZnO nanostructures was able to detect wide range of acetaldehyde concentration with the detection limit of 10 ppm.It can be concluded that the Co-doped ZnO based acetaldehyde sensor
Ac ce pt e
d
M
an
us
cr
ip t
is a promising candidate for effective acetaldehyde detection in various applications.
16
Page 16 of 42
Acknowledgements The authors wish to express their sincere thanks to the Department of Science & Technology, New Delhi, India for the financial support (Project ID: INT/SWD/VINN/P-04/2011 &
ip t
SR/FST/ETI- 284/2011(C)). They also wish to acknowledge SASTRA University, Thanjavur for
Ac ce pt e
d
M
an
us
cr
extending infrastructure support to carry out this work.
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Author Biographies Ganesh
Kumar
Mani
received
his
B.Sc.,
Physics
from
ip t
BharathidasanUniversity, Tiruchirapalli and M.Sc. in Nanoscience and Nanotechnology from BharathiarUniversity, Coimbatore in 2009 and
cr
2011 respectively. He is currently working as Junior Research Fellow in
us
the Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) and School of Electrical & Electronics Engineering, SASTRAUniversity, Thanjavur, India. His
an
current research interests are fabrication of various nanostructured thin film based chemiresistive sensors.He has published more than 30 research articles in peer reviewed international journals
d
M
and filed two patents.
Ac ce pt e
John BoscoBalaguruRayappanreceived his M.Sc. and Ph.D. degrees in Physics from St. Joseph’s College, Bharathidasan University, Tiruchirapalli, India in 1996 and 2003, respectively. He is currently
working asAssociate Dean (Research) in the School of Electrical &
Electronics Engineering and Centre for Nanotechnology &Advanced Biomaterials (CeNTAB) of SASTRA University. His current research interests include development of gas/chemical sensors, biosensors and functional nanomaterials. He is also working in the field of embedded systems and steganography. He has published more than 160 research articles in peer reviewed international journals and filed six patents.
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Table Captions Table 1 Optimized deposition parameters.
ip t
Table 2 Structural parameters of undoped and doped thin films obtained from XRD spectrum. Table 3 Acetaldehyde sensing performance comparison of various ZnO nanostructures with the
us
cr
present work.
Figure Captions
an
Figure 1 Spray Pyrolysis set-up.
Figure 2 XRD patterns of a) undoped and b) doped ZnO thin films.
M
Figure 3 FE-SEM micrographs of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. (a1) – (d1) shows the high magnification FESEM images respectively.
Ac ce pt e
films.
d
Figure 4 EDS Spectrum of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin
Figure 5 Optical absorbance spectra of undoped and doped ZnO thin films. Figure 6 A photograph of sensor testing setup. Figure 7 Selectivity nature of the sensing elements. Inset shows the highlighted view of response towards interfering vapours.
Figure 8 Transient resistance response of a) undoped, b) Co-doped, c) Ni-doped and d) Cudoped ZnO thin films.
Figure 9 Response of undoped and doped ZnO thin films towards various concentrations of acetaldehyde at room temperature. Figure 10 Typical a) response and b) recovery times of undoped and doped ZnO nanostructures.
25
Page 25 of 42
us
cr
ip t
Figure 11 Stability nature of Co-doped ZnO thin film towards 100 ppm over a period of 5 days.
Table 1 Deposition parameters
Values
an
Parameters Precursor
Anhydrous zinc chloride (0.1 M)
Doping element precursors
M
Cupric acetate dihydrate
Nickel acetate tertahydrate Cobalt acetate tertahydrate 0.2 mm
Solution flow rate
2 ml. min-1
Ac ce pt e
d
Spray nozzle diameter
Compressed air pressure
1 bar
Substrate temperature
523 ± 2 K
Annealing temperature
623 K
Annealing duration
6h
Substrate to nozzle distance
15 cm
Spray angle
90º
Deposition area per min.
15 cm2
26
Page 26 of 42
ip t cr us
Table 2 Structural parameters of undoped and doped thin films obtained from XRD spectrum.
ZnO Co-doped
Size (nm)
(Å)
35.1253
39
35.6282
35
Ni-doped ZnO
Cu-doped ZnO
34.9764
34.858
32
30
Lattice
an
(deg.)
Ac ce pt e
ZnO
d-spacing
M
Undoped
Crystallite
parameter
Strain
5.1060
0.002985
2.5181
5.0362
0.003207
2.5635
2.5720
Resistance (Ω)
“c” (Å)
2.5530
d
Sample
2 (θ)
5.1271
5.1439
0.003677
0.003829
3.7 x 1010 9.7 x 1011 5.5 x 107 4.3 x 1011
27
Page 27 of 42
Table 3 Acetaldehyde sensing performance comparison of various ZnO nanostructures with the
Concentration
Response
Operating
structures
(ppm)
(S)
temperature (oC)
Powders
2
5.73
Platelets
50
50
Particles
250
~ 45
400
[47]
Tetrapods
50
47.5
400
[15]
Sheets
1
77
220
[48]
Rods
250
5.30
400
[16]
Flowers
250
8
400
[18]
Petals
100
14
Room Temperature
[49]
10
2.85
Room Temperature
Nanorod
cr us
450
an
Room Temperature
M
Ac ce pt e
Branched
ip t
ZnO Nano-
d
present work
Branched
10
[46]
[30]
Present
Co-doped ZnO
Ref.
Work 800
Room Temperature
Nanorods
28
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ip t cr us an M d Ac ce pt e
Figure 0 Spray Pyrolysis set-up.
29
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(002)
0
35
40
45
M
30
an
ip t
us
cr
Intensity (a.u.)
(a)
50
55
60
(110)
(102)
Cu-doped ZnO
Intensity (a.u.)
(b)
d
(101)
(002)
Ac ce pt e
(001)
2θ (deg.)
Ni-doped ZnO
Co-doped ZnO
30
35
40
45
50
55
60
2θ (deg.)
Figure 0XRD patterns of a) undoped and b) doped ZnO thin films.
30
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31
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d
Ac ce pt e us
an
M
cr
ip t
ip t cr us an M d Ac ce pt e Figure 0FE-SEM micrographs of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. (a1) – (d1) shows the high magnification FESEM images respectively.
32
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33
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d
Ac ce pt e us
an
M
cr
ip t
34
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d
Ac ce pt e us
an
M
cr
ip t
ip t cr us an
3.0
M d
2.0
1.5
Ac ce pt e
Absorbance (a.u.)
2.5
Undoped ZnO Co-doped ZnO Ni-doped ZnO Cu-doped ZnO
1.0
0.5
0.0 350
400
450
500
550
600
Wavelength (nm)
Figure 1 Optical absorbance spectra of undoped and doped ZnO thin films. 0
35
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ip t cr us an M d Ac ce pt e
Figure 1A photograph of sensor testing setup.
36
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Undoped ZnO Co-doped ZnO Ni-doped ZnO Cu-doped ZnO
Response for 100 ppm
cr
800 700
us
600
400 300
an
6
Response (S)
Response (S)
8
500
4
M
2
200
0
0 ia tone mon Ace Am
l l yde ano ethano deh Eth M mal For
d
100
ip t
900
ene Tolu
Ac ce pt e
e e e ia ne hanol hanol hyd Toluen ldehyd mon Aceto Et a Met rmalde Am t Ace Fo
Figure 1 Selectivity nature of the sensing elements. Inset shows the highlighted view of response towards interfering vapours.
37
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(b)
(a)
12
10
12
10 11
10
11
10
0
1000
2000
3000
4000
0
Time (s)
2000
ip t
4000
6000
500 ppm
Acetaldehyde OFF
250 ppm
10
100 ppm
Acetaldehyde ON
5
cr
6000
50 ppm
5000
10
75 ppm
7
10
us
6
10
500 ppm
Acetaldehdye OFF
8
10
6
250 ppm
Acetaldehdye ON
7
10
100 ppm
75 ppm
50 ppm
10
30 ppm
20 ppm
8
9
10
20 ppm
Resistance (Ω )
Resistance (Ω )
9
10
10
10
10 ppm
10
10
8000
10000
12000
Time (s)
(d)
an
(c)
13
10
8
10
12
10
11
0
2000
4000
6000
8000
10000
7
10
75 ppm
Acetaldehyde ON Acetaldehyde OFF
6
10
5
12000
10
0
2000
4000
6000
500 ppm
8
10
250 ppm
50 ppm
10
20 ppm
9
10 ppm
M Resistance (Ω )
d
Acetaldehyde OFF
10
10
100 ppm
5
250 ppm
Ac ce pt e Acetaldehyde ON
10
100 ppm
75 ppm
50 ppm
6
10
500 ppm
7
10
20 ppm
10 ppm
Resistance (Ω )
10
8000 10000 12000 14000 16000
Time (s)
Time (s)
Figure 1Transient resistance response of a) undoped, b) Co-doped, c) Ni-doped and d) Cu-doped ZnO thin films. 251694080
38
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40000
ip t
30000 20000
cr
10000
Cu-doped ZnO
us
0
an
200 0
Ni-doped ZnO
M
Response (S)
400
4000000
Ac ce pt e
d
2000000 0
Co-doped ZnO
8000 6000 4000 2000
Undoped ZnO
0
0
50 100 150 200 250 300 350 400 450 500 550
Concentration (ppm) Figure 1 Response of undoped and doped ZnO thin films towards various concentrations of acetaldehyde at room temperature.
39
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251694080
300 Undoped ZnO Co-doped ZnO Ni-Doped ZnO Cu-doped ZnO
ip t
200
cr
150
100
us
Response Time (s)
250
0 0
100
200
an
50
300
400
(a)
500
M
Concentration (ppm)
250
d
Undoped ZnO Co-doped ZnO Ni-Doped ZnO Cu-doped ZnO
Recovery Time (s)
Ac ce pt e
200
150
100
50
(b)
0
0
100
200
300
400
500
Concentration (ppm)
Figure 1Typical a) response and b) recovery times of undoped and doped ZnO nanostructures.
40
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0
ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature 12
5x10
Acetaldehyde Sensor
5
ip t
8.2x10
Ganesh Kumar Mani and John Bosco Balaguru Rayappan*
5
cr
8x10
5x10
In Air
us
School of Electrical & Electronics Engineering (SEEE)
5
SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
7.8x10
an
In 100 ppm
10
5
7.6x10
M
5x10
9
5x10
2
3
4
5
6
5
7.4x10
7
8
9
10
d
1
Resistance (Ω)
Resistance (Ω )
Nano Sensors 11 Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
Ac ce pt e
Time (Days)
Figure 1Stability nature of Co-doped ZnO thin film towards 100 ppm over a period of 5 days.
41
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ZnO Nanoarchitectures: Ultrahigh Sensitive Room Temperature Acetaldehyde Sensor
ip t
Ganesh Kumar Mani and John Bosco Balaguru Rayappan* Nano Sensors Lab @ Centre for Nano Technology & Advanced Biomaterials (CeNTAB) and
cr
School of Electrical & Electronics Engineering (SEEE)
Ac ce pt e
d
M
an
us
SASTRA University, Thanjavur 613 401, Tamil Nadu, India.
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