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Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas
Accepted Manuscript Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas Krishna K. Pawar, Jasmin S. Shaikh, Sawanta S. Mali, Yuvraj H. Navale, Vikas B. Patil, Chang K. Hong, Pramod S. Patil PII:
S0925-8388(19)32763-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.07.248
Reference:
JALCOM 51536
To appear in:
Journal of Alloys and Compounds
Received Date: 23 May 2019 Revised Date:
19 July 2019
Accepted Date: 20 July 2019
Please cite this article as: K.K. Pawar, J.S. Shaikh, S.S. Mali, Y.H. Navale, V.B. Patil, C.K. Hong, P.S. Patil, Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.07.248. 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.
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Graphical abstract
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Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas
1
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Krishna K. Pawar1,2, Jasmin S. Shaikh1, Sawanta S. Mali3, Yuvraj H. Navale4, Vikas B. Patil4, Chang K. Hong3**, Pramod S. Patil1,2*
Thin Film Material Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India 2
Department of Materials Science and Engineering, Chonnam National University, Gwangju
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3
School of Nanoscience and Technology, Shivaji University, Kolhapur 416-004, India
500-757, South Korea
School of Physical Sciences, Solapur University, Solapur 413-002, India
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Corresponding author: Tel.:02312609490
1) *Corresponding author: [email protected] 2) **Co-corresponding author: hongck.chonnam.ac.kr
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E-mail addresses:
ACCEPTED MANUSCRIPT Abstract Herein, we report a facile approach for the synthesis of hollow Indium oxide (In2O3) microcubes (HIMC) by two step process which consist of hydrothermal method and followed by air annealing. Kirkendall effect has been employed to transform Indium hydroxide
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In(OH)3 microcubes (IHMC) to HIMCs. Particularly, the formation of HIMC was studied as function of annealing time. The evolution of hollowness was discussed through Kirkendall effect. Fabricated HIMC films were used to detect NO2 gas up to 100 ppm of concentration. We found that synthesized HIMC had a huge sensing response (S= 1401) towards 100 ppm
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NO2 gas, which demonstrates the effect of HIMC over simple microcubes. Response (Rs=16s) and recovery (Rc=165s) time of sensor were very short, which enables sensor to work rapidly. Also, HIMC sensor found selective for NO2 gas rather ammonia, acetone and
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carbon dioxide gases. NO2 sensing mechanism for the case of HIMC based sensor was described as per well known oxide based chemiresistive theory. From observations, it is found that the fabricated (HIMC)3hr based sensor is highly selective and sensitive towards NO2 gas at ppm level of concentration and could be undertaken for gas sensing application at
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various fields.
Keyword’s: Hollow In2O3microcubes; Kirkendall effect; NO2 gas sensing; Selective;
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Sensitive.
ACCEPTED MANUSCRIPT 1. Introduction Enormous atmospheric degradation has been observed due to rapid growth in industrial and transportation sectors. Due to this, the emission of various toxic gases such as CO, SO2, NO2, CO2 and NO etc have been polluted environment [1]. To avoid environmental
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pollution, it is important to detect the amount of toxic gases and control the atmosphere using an appropriate sensors [2]. Recently, Metal oxide semiconductor (MOS) based solid state gas sensors have been used broadly due to their excellent sensitivity and selectivity with long life. Several attempts have been made by researchers for the improvement of MOS based gas
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sensing properties such as cost effective, sensitive and selective of gas sensors [3][4].
Among various MOS, In2O3 is one of n-type MOS with direct optical band gap
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energy (Eg= 3.5 eV). It has been widely used due to its excellent electric conductivity, electrical mobility (~160 cm2V/s), optical and chemical properties. Different nanostructures of In2O3 such as nanorods, nanowires, nanosheets, nanofibers etc. have been prepared via various physical and chemical methods [5]. Due to its versatility, In2O3 has been utilized for various applications such as solar cells, photocatalysis, gas sensing and transparent
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conducting oxide [6–8]. In recent years, a variety of exciting results have been reported by In2O3 gas sensors for detection of oxidizing and reducing gases such as CO, O3, H2S, NH3 and NO2 etc [9]. Still, It is great challenge to enhance the performance without losing its sensing properties [10–12]. Many attempts have been made by researchers in order to
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enhance the sensing properties of In2O3 nano/microstructures. Mostly, simple and cost effective hydrothermal route has been deployed for engineering various nanoscale
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morphologies of In2O3 such as nanorods, cubes, nanoparticles and nanospheres. Among various nanostructures, hollow nanostructure is one of the unique nanostructures, which are quite beneficial for many important applications such as supercapacitors, gas sensors, solar cells and batteries etc. Hollow structures promote versatile physical and chemical properties such as low material density, large surface area, mesoporous nature and hollow space [13,14]. Also, Controlled and uniform morphology of hollow nanostructure provides immense potential among various nanostructures [15]. So, far a systematic approach of synthesizing hollow In2O3 has not explored. Few reports of hollow In2O3 have been reported but the synthetic strategies lean the understanding. Therefore, it is still great challenge to get hollow micro/nanostructures of In2O3 [16].
ACCEPTED MANUSCRIPT Recently, various synthetic strategies for hollow nanostructures have been developed, which includes Ostwald ripening, etching, Kirkendall effect, hard and soft template methods [17–19]. In 2007, In2O3 hollow microspheres has been prepared by multipore sheets of In2S3 via hydrothermal method [20]. After some primary research and development, Yin et al. have reported single crystalline hollow rhombohedral In2O3 nanocrystals for photocatalysis in 2015
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[21]. Also, In2O3 hollow microspheres have been obtained via spontaneous vesicle template assisted technique [22]. However, hollow structure can be synthesized via template free synthesis, which reduces drawbacks of template contamination and low performance of nanomaterials. In template free synthesis, Ostwald ripening has been generally used for
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synthesis of hollow In2O3 nano/microstructures [23,24]. However, synthesis of hollow nanostructures through Ostwald ripening has limitations of reproducibility. To the best of our knowledge, synthetic route of hollow In2O3 microcubes (HIMCs) through Kirkendall effect
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and its potential application to gas sensors have been not reported so far.
In this work, we have synthesized HIMC by an inexpensive, nontoxic and simple green synthesis via hydrothermal route. Moreover, we discussed growth mechanism of developing HIMC by controlled variation of reaction parameters such as annealing time. The
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morphology and phase change have been systematically investigated. The morphological evolution of HIMC is discussed through Kirkendall effect. Finally, gas sensing performances of HIMC films are analyzed.
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2. Experimental
2.1 synthesis of HIMC based films
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Initially, 45 mM InCl3 was dissolved in double distilled water and 0.15 M of Lalanine was added in solution as a shape controlling agent and mild reducing agent. The solution was transferred to hydrothermal autoclave. The whole system was placed in Muffle furnace at 180oC for 6 h. The white precipitate of In(OH)3 microcubes is washed with ethanol and dried at 70oC. The as-synthesized In(OH)3 microcubes (IHMC) was finally annealed at 450oC for 3h. The In2O3 (HIMC) was carried out at different annealing time such as 1 to 4 h with interval of 1hr and related samples were denoted as (HIMC) 1hr, (HIMC) 2hr, (HIMC) 3hr and (HIMC) 4hr keeping all other parameters fixed. The pastes of synthesized samples were used to deposit material onto glass substrate using screen printing technique.
2.2 Characterizations
ACCEPTED MANUSCRIPT Structural properties of synthesized films were studied using X-ray diffractometer with CuKα radiation of 1.54056 Å (Bruker D2 phaser X-ray diffractometer). The SEM micrographs of the films were recorded using FE-SEM (Model- MIRA3 LMH, TESCAN) operated at 20 KV. The photoluminescence (PL) spectra of synthesized films were carried out by a Flouromax4, Horiba Ltd., Japan. Fourier transform infrared spectroscopy (FTIR)
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spectroscopy was utilized for identification of In-O bonding using Thermo Fisher scientific Nicolet iS10, US. The elemental information about In2O3 film was analyzed using an X-ray photoelectron spectrometer (XPS) (VG Multilab 2000-SSK, USA, Kα) with a multi-channel detector, which can endure high photonic energies from 0.1 to 3 KeV.
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2.3 Gas sensing measurements
Gas sensing measurement of synthesized HIMC films were carried out using variation
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in the electrical resistivity by exposure of toxic gas such as NO2. The variations in the NO2 gas concentrations (from 5ppm to 100 ppm at identical intervals) at various temperatures were examined. All HIMC films sensors were utilized in homemade gas sensing system, made by an airtight stainless steel chamber. A chamber with particular volumetric capacity of 250 cm3 having hot plate with thermocouple and systematic gas inlet-outlet was used. The temperature of system was controlled up to 200oC by use of hot plate with thermocouple
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controller. Probes of sensing chamber were attached with computer and Keithely 6514 electrometer to obtain data. Ends of HIMC film (area 1cm2) were painted with conducting silver paste for contact purpose then film was mounted on hot plate and heated to operating temperature. The known amount of gas was injected using syringe at stable environment. The
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resistance changes were measured under various gases such as CO2, NH3, acetone and NO2. Lastly, all film sensors with invariable film area were used for measurements. The sensor
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response (S) is defined as (Equation 1), =
(1)
Where, Rg is the electrical resistance of the film upon exposure to the target gas and Ra in air.
2.4 Synthesis procedure and Growth mechanism The typical growth mechanism of HIMCs has been discussed using two simple steps Hydrothermal and Kirkendall. Initially, The IHMC sample is synthesized using hydrothermal
ACCEPTED MANUSCRIPT route. As-synthesized IHMCs are found to have prefect cubic crystal structure and morphology too. The typical synthetic approach is represented in following Sch 1. In3+ + 3OH− → In OH
(2)
3
2 In OH 3 → In2O3 + 3 H2O
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(3)
Equation (2) represents reaction for IHMC sample. In second process, IHMC powder is annealed to 4500C for different annealing time intervals (Equation (3)).
Formation of HIMC can be explained using Kirkendall effect with an ionic exchange.
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There are very few case of ionic exchange for synthesis of hollow nanostructure [25]. The case of ionic exchange leads to formation of hollow cubes can be explained via Kirkendall
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effect. The decomposition of IHMC occurred due to sufficient rate of ionic exchange. Anion exchange occurred between OH− and O2−, which leads to hollow cubes rather than only cubes. Generally, weakly bonded atoms can diffuse more rapidly than strongly bonded. Based on thermal analysis, it is observed that In-OH bond is weaker than In-O bond. So, the diffusion rate of OH− ion is greater than O2− Ion [25]. The diffusion flux can be represent
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using Ficks first law of diffusion (Equation (4)). (HIMC)1hr = −
(4)
Where, J is diffusion flux, D is diffusion coefficient and dC/dX is concentration gradient.
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For case of Kirkendall effect, we require gradient in diffusion flux of ions. So, gradient in
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diffusion flux can be balanced using equation as follows
=
−
(5)
Equation (1.4) and (1.5) represent outward diffusion of OH- and inward diffusion of
O2-. The delicate adjustment in the rates of diffusions of anions would results into hollow structures [26]. Initially, vacancies are created inside the cubes, which then grow form voids. Void formation as outcome of the gradient of diffusion flux is indication of the Kirkendall effect. These voids are further grow into the hollow structure [25]. The resultant diffusion flux (Jν) should be large for Kirkendall process. Hence, following condition can be useful For Kirkendall effect (Equation (6)).
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(6)
Transformation from In(OH)3 to In2O3 can be represented using XRD, FE-SEM and FT-IR studies. So, increase in annealing time facilitates increase in diffusional lengths. Based on
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observation, we can judge that In2O3 hollow cubes were formed by controlled annealing time. The phenomena of morphological change over annealing time could be explained using
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Kirkendall effects.
3. Results and Discussion
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XRD pattern of hydrothermally grown as-synthesized IHMC and annealed HIMC films with variations in annealing time are presented in Fig. 1. As-synthesized IHMC exhibits cubic structure (Pn-3m). For IHMC sample, nine diffraction peaks (2θ) were observed at 22.37, 31.79, 39.13, 45.45, 51.32, 56.49,66.62, 70.88 and 75.45 corresponding to (200), (220), (311), (400), (420), (422), (440), (600) and (620) planes as shown in Fig. 1 (a). These diffracted peaks suggest crystalline nature of IHMC by JCPDS card No.00-016-0161.
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Moreover, the calculated lattice constants a=b=c= 7.9580 Å are in good agreement with reported value by JCPDS data. The XRD pattern of prepared In(OH)3 films holds good agreement with other reports. The diffraction pattern consist of a characteristics diffraction
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peak at 2θ = 22.37° along (200) plane confirms formation of IHMC. After annealing, IHMC transforms to HIMC through ionic exchange of OH− and O2-
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ions. The diffraction patterns of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr films were illustrated in Fig. 1 (b). All synthesized crystalline HIMC films have cubic (bixbyite) structure (Ia-3). For all film samples, five diffraction peaks (2θ) were observed at 21.59, 30.6, 35.4, 51.1 and 60.57 corresponding to the (211), (222), (400), (440) and (622) for crystalline In2O3, which suggest polycrystalline nature of In2O3 (JCPDS card No.00-006-0416). The XRD patterns of prepared HIMC films are in good agreement with other In2O3 reports. The diffraction pattern of HIMC films with characteristics diffraction peak at 2θ = 30.6° corresponds to the (222) plane, which confirms formation of crystalline nature [24]. The cubic structure denotes uniform arrangement and growth of atoms at characteristics (222)
ACCEPTED MANUSCRIPT plane. The lattice parameter of HIMC films are determined by using following formula for cubic structure (Eq. 7) =
(7)
!" !#
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Moreover, the calculated lattice constant a=b=c=10.118 Å are in good agreement with reported value by JCPDS data. Observed strong and sharp diffracted peaks indicate high quality synthesis of pure HIMC films, which are free from any type of contamination. The structural investigations of hydrothermally synthesized In2O3 using XRD has been reported in
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the literature [34-35, 37, 38, 42-43].
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Morphological studies of HIMCs were investigated by FE-SEM. Fig. 2 (a) shows FESEM analysis of as-synthesized IHMC sample without annealing. Fig. 2 (a) shows FE-SEM images of HIMC films carried out at different annealing times 1, 2, 3 and 4hr in order to explain the growth mechanism of HIMC. Initially, as-synthesized IHMC is analyzed to check morphology, which is found to be cubic. After 1hr (HIMC)1hr annealing, small cracks on the surface of cubes were found due to outward diffusion of OH- ions. The amount of OH- ions
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diffused outward increases for prolonged time of annealing (2hr to 4hr). Hence, initially developed small cracks grow slowly to form hollow structures with larger voids. The voids become too large to sustain rigidity of cubes after 4hr annealing (HIMC)
4hr
and therefore
cubes collapse (Fig. 2 (b)). Hence, (HIMC)3hr is optimized for further studies based on SEM
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analysis. To confirm hollowness, FESEM images of an optimized (HIMC)3hr film are illustrated with various magnifications in Fig. 3. The FESEM images of (HIMC)3hr clearly
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illustrates the hollowness and opening cracks on the surface.
PL is a powerful and nondestructive tool to study emissions and defects in PL spectra
of HIMC films is shown in Fig. 4. Fig. 4 shows two peaks at 690 and 736 nm, which are attributed to defects in HIMCs. HIMC films have oxygen deficiency due to its nonStoichiometric nature in arrangement. This non-stoichiometric arrangement induces defect levels at longer wavelengths. The formation of enhanced defect levels in material captures more conduction band electron. These captures electron helps to ionize oxygen (O2) into (Oor O2-) ions, which further interacts with toxic gas molecules. So, enhanced defect levels are beneficial for huge surface level interactions. Also, the enhancement in gas response of
ACCEPTED MANUSCRIPT (HIMC) 3hr may indicates the development of large interstitial and antisite oxygen vacancies developed during ionic exchange [29]. In view of numerous reports, the PL emission peaks at 690 and 736 nm match up to defect level emissions in ultraviolet as well as visible region [27][28]. Thus, it is clear that annealing time enhances defect levels in material though ionic
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exchange.
FT-IR spectra of HIMC films were analyzed at different annealing time. FT-IR absorption over 550 to 4000 cm-1 of IHMC sample is shown in Fig. 5. The absorption spectra of as-synthesized IHMC (Fig. 5 (a)) as well as annealed HIMC (Fig. 5 (b)) films were studied
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for the identification and confirmation of transformation. FT-IR absorption spectra of assynthesized IHMC exhibits 5 absorption peaks, which In-OH vibration. The sharp absorption peak at 597 cm-1 is attributed to the In-O stretching vibration. Also, The absorption peak at
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765 cm-1 shows In-O Vibration [30]. For IHMC, the vibrational peaks at 1060, 1147, 3113 and 3230 cm-1 shows O-H bending or deformation vibrations [31]. After annealing, IHMC transforms to HIMC through inter-diffusion of OH- and O- anions Fig. 5 (b). The absorption peaks of hydroxyl group wane with annealing time and for (HIMC) 4hr disappeared, which confirms transformation from IHMC to HIMC. Increase in annealing time decreases in hydroxyl band is observed due to evaporation. However, absence of further peaks in full
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region (4000-400 cm-1) shows perfect HIMC films without any impurity and hydroxyl content. Considering several studies, current study is in good agreement with other reports of
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In2O3 deposited by various techniques [32,33].
In order to determine chemical composition and electronic defect structure of HIMC
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(K3) film, X-ray photoelectron spectroscopy (XPS) analysis is performed. Fig. 6 (a) illustrates the survey spectra of (HIMC) 3hr film show the presence of In and O along with C as surface impurities. The location of the XPS peaks is calibrated to C1s binding energy (BE) at 284.6 eV. Fig. 6 (b) depicts the In 3d core level spectra of (HIMC) 3hr film. The spectra split in doublets namely 3d5/2 and 3d3/2 state. An excellent fit to two peaks located at binding energy 444.3 and 451.8 eV are clearly observed, which can be attributed to the characteristic spin-orbit split 3d5/2 and 3d3/2 respectively. The spectral data reveals that the Indium is in III oxidation state i.e. it has +3 vacancies without any metallic components. Also, Fig. 6 (c) shows O1s core-level spectra of Indium oxide films. It has two peaks at 529.6 and 531.3 eV. The peak at 529.6 eV corresponds to oxygen bonds in In–O–In whereas peak at 531.3 eV is related to oxygen vacancies in the bulk of MOS [34]. The small peak observed at 532.3eV the
ACCEPTED MANUSCRIPT surrounding area of intense oxygen O1s peak indicates the surface contamination or presence of O− present species, such as hydroxyl groups (OH−) or water molecules that are adsorbed on the surface while diffusion of anions [35].
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4. Gas sensing properties: 4.1 Gas sensing measurements:
It is proved that operating temperature of sensor play a vital role for adsorption, accumulation of conduction band electrons, formation of active sites on surface and reaction
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process [36]. Sensing response of HIMC films as function of temperature (50oC to 200oC) were measured at 100 ppm of NO2 gas illustrated in Fig. 7. It is observed that sensing increases at 100oC (Humidity~55%RH) and decreases dramatically with increase of operating
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temperature. So, optimized temperature enhances sensing response due to equilibrium between adsorption and desorption process on and inside of HIMC. Furthermore, compared data of plot suggests highest response is acquired by (HIMC)3hr film with value of 1401 at 100 ppm. The sensor response for (HIMC)1hr, (HIMC)2hr and (HIMC)4hr were 608, 864 and 148 respectively. Comparison of our report with literature shows that the optimizing
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temperature is much lower in case of HIMC [37,38]. The characteristic operating temperature can be considered beneficial to improve the sensing response due to huge adsorption and reaction of NO2 gas on surface of HIMC. Fig. 8 illustrates resistance curves of HIMC films for 100 ppm NO2 gas at 100 0C. It is noticed that all HIMC films sensors displays increment
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in resistance upon the exposure of NO2 as usually observed in n-type semiconductor sensor. The mechanism of n-type MOS sensors is based on the changes in electrical resistance
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observed due to gas adsorption and desorption on surface of films [39]. Initially, the resistance of HIMC films is measured without exposure of NO2 gas for baseline. A drastic increment in resistance of HIMC films is observed. Since, NO2 gas is oxidizing type of gas and has affinity to capture electron from HIMC. A resistance curve reveals that a (HIMC)3hr film acquires huge chemiresistive change than (HIMC)1hr, (HIMC)2hr and (HIMC)4hr films. Therefore, the resistance curve predicts reason behind the huge response of (HIMC)3hr film. The dynamic sensor responses of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples were illustrates in Fig. 9. Gas response of HIMC films sensors examined at various NO2 concentrations (20 to 100 ppm) were investigated with interval of 20 ppm. It is seen that sensing response of all films are increased with increment in concentration of NO2 gas at
ACCEPTED MANUSCRIPT operating temperature. Particularly, the response of (HIMC)3hr film increases linearly with increase in NO2 gas concentration due to more surface level adsorption and reaction. The gas response is observed to be 211, 354, 453, 925 and 1401 at 20, 40, 60, 80 and 100 ppm concentrations of NO2 gas. Development of hollowness inside the cubes is responsible for substantial adsorption and reaction of (HIMC)3hr film in addition to surface level process
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[40]. So, response of (HIMC)3hr film sensor has enhanced sensitivity than other films due to hollowness of microcubes. Reproducibility and stability plots of (HIMC)3hr sensor 100 ppm NO2 gas at 1000C as illustrated in Fig. 10. The identical behavior of (HIMC)3hr sensor for exposure of 100 ppm of NO2 gas represent exact reproducibility. The stability of(HIMC)3hr
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sensor was found 98.23% even after one month of fabrication. The response of sensor at various concentration of NO2 gas found linear. This linearity of sensor can helps to find
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unknown concentration of gases during quantitative analysis as shown in Fig. 11.
Response and recovery time are two crucial parameters for real time sensing analysis. Fig. 12(a) illustrates response and recovery time of (HIMC)3hr film sensor for 100 ppm NO2. Fig. 12(a) curve shows rapid response and recovery over NO2 gas, which indicates fast adsorption and desorption of gases on the surface of film. To get precise observations of
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response and recovery time, overall behavior of (HIMC)3hr film sensor is measured with various concentrations of NO2 gas as shown in Fig. 12(b). So, the obtain results demonstrates that response time has mere changes but recovery time is increased and comes down after particular concentrations. To check the selectivity of (HIMC)3hr film for 100 ppm of CO2,
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acetone, NH3 and NO2 gas are analyzed at an operating temperature of 100oC is depicted in Fig. 13. The gas response for other gases like CO2, acetone and NH3 are observed to be 32, 13
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and 29 respectively. A (HIMC)3hr film illustrates huge selectivity towards NO2 due to high response i.e. 1401. NO2 is high reactive due its lone pair electron in outer shell. It is observed that the fabricated HIMC based sensor acquires improved sensing properties in comparison with early reported work (Table 1) [41–45].
4.4 Sensing mechanism of (HIMC)3hr film The sensing mechanism of metal oxide like In2O3 is well explained by chemiresistive type sensing process. Sch. 2 displays sensing mechanism of (HIMC)3hr film sensor. It is well known that n-type semiconductor chemisorbs oxygen molecules in air with losing its conduction electrons [47]. As oxygen plays a vital role in electron transportation due to surface depletion model [40]. Model suggests that chemisorbed oxygen gain the electron and
ACCEPTED MANUSCRIPT forms reactive oxygen species such as O2-, O2-, and O- onside and inside of HIMCs. The process of oxygen ionization on the surface of HIMCs are shown in (8-9) ())*
$% + & ' +,,,,,,- $% '
(8)
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())*
$% ' + & ' +,,,,- 2$ '
(9)
This results in the small augmentation of barrier height and consequently depletion on
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the surface of HIMC in air. When sensor is exposed to specific concentration of NO2 gas, it reacts with chemisorbed oxygen species. As an oxidizing gas, NO2 has tendency to acquire electrons from oxygen species and increase the barrier height of HIMC. The reaction of NO2
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with oxygen species is shown in (10-12) [1]
NO2 + e− → NO2− 10 11
NO2 + O− + 2e− → NO2− + O2−
(12)
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NO2 + O2− + 2e− → NO2− +2O−
Obviously, on the basis of mechanism, the detection performance strongly recommends that the hollowness of HIMC enhances the amount of chemisorptions of oxygen onside and inside
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of surface.
5. Conclusions
In Summary, a novel route of synthesis of HIMCs by hydrothermal method using
Kirkendall effect is studied systematically. We have successfully optimized the synthesis of hollow HIMC by optimizing the time required for sufficient air annealing. During this process, OH− and O2− ions get exchanged via diffusion and results in its bulk hollowness. The mechanism of formation of hollow In2O3 from In(OH)3 owing to the well known Kirkendall effect. The gas sensing study reveals that synthesized (HIMC)3hr film exhibits high sensitivity towards NO2 gas at very low (100oC) operating temperature, which is quite low in comparison with the existing literature. The obtained sensing response for (HIMC)3hr film
ACCEPTED MANUSCRIPT was 1401 for 100 ppm of NO2 gas, which is the result of relatively large adsorption and reaction due to hollowness inside the cubes. Also, it is found that fabricated sensor is selective towards NO2 gas as compare to other gases. Additionally, response and recovery time of optimized (HIMC)3hr film sensor were very short. So, optimized (HIMC)3hr film sensor shows appreciable properties of sensor with high response and can be useful for real
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time sensing.
Acknowledgement
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This work was partially supported by University grant commission (UGC) New Delhi, Govt. Of India for Financial assistance through project No. 43-517/2014(SR). This work was partially supported by Korea Research Fellowship Program via the National
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Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) for an outstanding overseas young researcher to Dr. Sawanta S. Mali. Author Dr. Jasmin S. Shaikh, Women Scientist-A is acknowledges to Women Scientist project of Department of Science (DST, grant No. SR/WOS-A/PM1037/2014 (G).), Delhi, India. Also, Prof. Vikas. B. Patil would like to thank DAE-BRNS
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(scheme no. 34/14/21/2015-BRNS) for financial assistance.
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highly sensitive acetaldehyde gas sensors, J. Alloy. Compd. S0925-8388(18)33424-8.
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Table 1.1 represents few reports on In2O3 structures
Operating temperature (oC) 215
100
300
2. 3.
In2O3 nanowires
1
250
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Lotus root slice like In2O3 microspheres Nonporous In2O3 nanosheets Porous rh-In2O3 nanosheets In2O3 nanostructures
50
250
50
250
8.
0.5
hollow In2O3 microcubes
100
Reference
2
[41]
1.8
[42]
2.57
[43]
90
[44]
57
[45]
250
164
[45]
150
74
[46]
100
1401
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In2O3 hollow sphere In2O3 nanofibers
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Figure Caption: Sch 1: Schematic strategy for synthesis of HIMC samples.
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Fig. 1 (a): X-ray diffraction pattern of as-synthesized IHMC sample. Fig. 1 (b): X-ray diffraction patterns of annealed (HIMC)1hr, (HIMC)2hr, (HIMC)3hr (HIMC)4hr samples. Fig. 2 (a) FE-SEM images of synthesized IHMC sample.
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Fig. 2 (b) FE-SEM images of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples.
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Fig. 3: FE-SEM images of (HIMC)3hr film captured at various magnifications.
Fig. 4: PL spectra of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples.
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Fig. 5 (a): FTIR spectra of as-synthesized IHMC sample.
Fig. 5 (b): FTIR spectra of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples. Fig. 6: XPS (a) Survey, (b) Indium and (c) Oxygen spectra in (HIMC)3hr film sample. Fig. 7: The gas response of all HIMC films for 100 ppm NO2 gas at the temperature ranging from 50 to 200oC.
Fig. 8: Resistance change of all HIMC films for 100 ppm NO2 gas.
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Fig. 9: The dynamic response of all HIMC films for various NO2 gas concentrations at 100oC. Fig. 10: The linearity of (HIMC)3hr film sensor for different ppm of NO2 concentrations at 100oC.
Fig. 11 Reproducibility and stability of (HIMC)3hr sensor 100 ppm NO2 gas at 100 0C.
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Fig. 12 (a): Response and recovery time of (HIMC)3hr film for 100 ppm of gas concentrations. Fig.12 (b): Dynamic response and recovery time of (HIMC)3hr film for various gas
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Fig. 13: Selectivity of (HIMC)3hr film towards various test gases for 100 ppm of gas concentration.
Sch. 2 Schematic representation of sensing mechanism of HIMC sensor
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Research highlights
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1. A new route for the synthesis of hollow Indium oxide microcubes (HIMCs) via Kirkendall effect is carried out. 2. Synthesized HIMC samples are characterized and undertaken for NO2 sensing. 3. The obtained results on NO2 sensing predict the high quality of sensor. 4. We found that the sensor shows highly sensitive and selective towards NO2 gas. Also, it is found that the response and recovery time of sensor is very less in comparison with reported sensors. 5. On the basis of results, we may predict that the fabricated sensor can be used for selective NO2 gas detection at very low ppm of concentration at only 100oC of operating temperature.
S0925-8388(19)32763-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.07.248
Reference:
JALCOM 51536
To appear in:
Journal of Alloys and Compounds
Received Date: 23 May 2019 Revised Date:
19 July 2019
Accepted Date: 20 July 2019
Please cite this article as: K.K. Pawar, J.S. Shaikh, S.S. Mali, Y.H. Navale, V.B. Patil, C.K. Hong, P.S. Patil, Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.07.248. 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.
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Graphical abstract
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Hollow In2O3 microcubes for sensitive and selective detection of NO2 gas
1
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Krishna K. Pawar1,2, Jasmin S. Shaikh1, Sawanta S. Mali3, Yuvraj H. Navale4, Vikas B. Patil4, Chang K. Hong3**, Pramod S. Patil1,2*
Thin Film Material Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India 2
Department of Materials Science and Engineering, Chonnam National University, Gwangju
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School of Nanoscience and Technology, Shivaji University, Kolhapur 416-004, India
500-757, South Korea
School of Physical Sciences, Solapur University, Solapur 413-002, India
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Corresponding author: Tel.:02312609490
1) *Corresponding author: [email protected] 2) **Co-corresponding author: hongck.chonnam.ac.kr
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E-mail addresses:
ACCEPTED MANUSCRIPT Abstract Herein, we report a facile approach for the synthesis of hollow Indium oxide (In2O3) microcubes (HIMC) by two step process which consist of hydrothermal method and followed by air annealing. Kirkendall effect has been employed to transform Indium hydroxide
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In(OH)3 microcubes (IHMC) to HIMCs. Particularly, the formation of HIMC was studied as function of annealing time. The evolution of hollowness was discussed through Kirkendall effect. Fabricated HIMC films were used to detect NO2 gas up to 100 ppm of concentration. We found that synthesized HIMC had a huge sensing response (S= 1401) towards 100 ppm
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NO2 gas, which demonstrates the effect of HIMC over simple microcubes. Response (Rs=16s) and recovery (Rc=165s) time of sensor were very short, which enables sensor to work rapidly. Also, HIMC sensor found selective for NO2 gas rather ammonia, acetone and
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carbon dioxide gases. NO2 sensing mechanism for the case of HIMC based sensor was described as per well known oxide based chemiresistive theory. From observations, it is found that the fabricated (HIMC)3hr based sensor is highly selective and sensitive towards NO2 gas at ppm level of concentration and could be undertaken for gas sensing application at
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Keyword’s: Hollow In2O3microcubes; Kirkendall effect; NO2 gas sensing; Selective;
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Sensitive.
ACCEPTED MANUSCRIPT 1. Introduction Enormous atmospheric degradation has been observed due to rapid growth in industrial and transportation sectors. Due to this, the emission of various toxic gases such as CO, SO2, NO2, CO2 and NO etc have been polluted environment [1]. To avoid environmental
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pollution, it is important to detect the amount of toxic gases and control the atmosphere using an appropriate sensors [2]. Recently, Metal oxide semiconductor (MOS) based solid state gas sensors have been used broadly due to their excellent sensitivity and selectivity with long life. Several attempts have been made by researchers for the improvement of MOS based gas
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sensing properties such as cost effective, sensitive and selective of gas sensors [3][4].
Among various MOS, In2O3 is one of n-type MOS with direct optical band gap
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energy (Eg= 3.5 eV). It has been widely used due to its excellent electric conductivity, electrical mobility (~160 cm2V/s), optical and chemical properties. Different nanostructures of In2O3 such as nanorods, nanowires, nanosheets, nanofibers etc. have been prepared via various physical and chemical methods [5]. Due to its versatility, In2O3 has been utilized for various applications such as solar cells, photocatalysis, gas sensing and transparent
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conducting oxide [6–8]. In recent years, a variety of exciting results have been reported by In2O3 gas sensors for detection of oxidizing and reducing gases such as CO, O3, H2S, NH3 and NO2 etc [9]. Still, It is great challenge to enhance the performance without losing its sensing properties [10–12]. Many attempts have been made by researchers in order to
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enhance the sensing properties of In2O3 nano/microstructures. Mostly, simple and cost effective hydrothermal route has been deployed for engineering various nanoscale
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morphologies of In2O3 such as nanorods, cubes, nanoparticles and nanospheres. Among various nanostructures, hollow nanostructure is one of the unique nanostructures, which are quite beneficial for many important applications such as supercapacitors, gas sensors, solar cells and batteries etc. Hollow structures promote versatile physical and chemical properties such as low material density, large surface area, mesoporous nature and hollow space [13,14]. Also, Controlled and uniform morphology of hollow nanostructure provides immense potential among various nanostructures [15]. So, far a systematic approach of synthesizing hollow In2O3 has not explored. Few reports of hollow In2O3 have been reported but the synthetic strategies lean the understanding. Therefore, it is still great challenge to get hollow micro/nanostructures of In2O3 [16].
ACCEPTED MANUSCRIPT Recently, various synthetic strategies for hollow nanostructures have been developed, which includes Ostwald ripening, etching, Kirkendall effect, hard and soft template methods [17–19]. In 2007, In2O3 hollow microspheres has been prepared by multipore sheets of In2S3 via hydrothermal method [20]. After some primary research and development, Yin et al. have reported single crystalline hollow rhombohedral In2O3 nanocrystals for photocatalysis in 2015
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[21]. Also, In2O3 hollow microspheres have been obtained via spontaneous vesicle template assisted technique [22]. However, hollow structure can be synthesized via template free synthesis, which reduces drawbacks of template contamination and low performance of nanomaterials. In template free synthesis, Ostwald ripening has been generally used for
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synthesis of hollow In2O3 nano/microstructures [23,24]. However, synthesis of hollow nanostructures through Ostwald ripening has limitations of reproducibility. To the best of our knowledge, synthetic route of hollow In2O3 microcubes (HIMCs) through Kirkendall effect
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and its potential application to gas sensors have been not reported so far.
In this work, we have synthesized HIMC by an inexpensive, nontoxic and simple green synthesis via hydrothermal route. Moreover, we discussed growth mechanism of developing HIMC by controlled variation of reaction parameters such as annealing time. The
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morphology and phase change have been systematically investigated. The morphological evolution of HIMC is discussed through Kirkendall effect. Finally, gas sensing performances of HIMC films are analyzed.
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2.1 synthesis of HIMC based films
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Initially, 45 mM InCl3 was dissolved in double distilled water and 0.15 M of Lalanine was added in solution as a shape controlling agent and mild reducing agent. The solution was transferred to hydrothermal autoclave. The whole system was placed in Muffle furnace at 180oC for 6 h. The white precipitate of In(OH)3 microcubes is washed with ethanol and dried at 70oC. The as-synthesized In(OH)3 microcubes (IHMC) was finally annealed at 450oC for 3h. The In2O3 (HIMC) was carried out at different annealing time such as 1 to 4 h with interval of 1hr and related samples were denoted as (HIMC) 1hr, (HIMC) 2hr, (HIMC) 3hr and (HIMC) 4hr keeping all other parameters fixed. The pastes of synthesized samples were used to deposit material onto glass substrate using screen printing technique.
2.2 Characterizations
ACCEPTED MANUSCRIPT Structural properties of synthesized films were studied using X-ray diffractometer with CuKα radiation of 1.54056 Å (Bruker D2 phaser X-ray diffractometer). The SEM micrographs of the films were recorded using FE-SEM (Model- MIRA3 LMH, TESCAN) operated at 20 KV. The photoluminescence (PL) spectra of synthesized films were carried out by a Flouromax4, Horiba Ltd., Japan. Fourier transform infrared spectroscopy (FTIR)
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spectroscopy was utilized for identification of In-O bonding using Thermo Fisher scientific Nicolet iS10, US. The elemental information about In2O3 film was analyzed using an X-ray photoelectron spectrometer (XPS) (VG Multilab 2000-SSK, USA, Kα) with a multi-channel detector, which can endure high photonic energies from 0.1 to 3 KeV.
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2.3 Gas sensing measurements
Gas sensing measurement of synthesized HIMC films were carried out using variation
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in the electrical resistivity by exposure of toxic gas such as NO2. The variations in the NO2 gas concentrations (from 5ppm to 100 ppm at identical intervals) at various temperatures were examined. All HIMC films sensors were utilized in homemade gas sensing system, made by an airtight stainless steel chamber. A chamber with particular volumetric capacity of 250 cm3 having hot plate with thermocouple and systematic gas inlet-outlet was used. The temperature of system was controlled up to 200oC by use of hot plate with thermocouple
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controller. Probes of sensing chamber were attached with computer and Keithely 6514 electrometer to obtain data. Ends of HIMC film (area 1cm2) were painted with conducting silver paste for contact purpose then film was mounted on hot plate and heated to operating temperature. The known amount of gas was injected using syringe at stable environment. The
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resistance changes were measured under various gases such as CO2, NH3, acetone and NO2. Lastly, all film sensors with invariable film area were used for measurements. The sensor
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response (S) is defined as (Equation 1), =
(1)
Where, Rg is the electrical resistance of the film upon exposure to the target gas and Ra in air.
2.4 Synthesis procedure and Growth mechanism The typical growth mechanism of HIMCs has been discussed using two simple steps Hydrothermal and Kirkendall. Initially, The IHMC sample is synthesized using hydrothermal
ACCEPTED MANUSCRIPT route. As-synthesized IHMCs are found to have prefect cubic crystal structure and morphology too. The typical synthetic approach is represented in following Sch 1. In3+ + 3OH− → In OH
(2)
3
2 In OH 3 → In2O3 + 3 H2O
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(3)
Equation (2) represents reaction for IHMC sample. In second process, IHMC powder is annealed to 4500C for different annealing time intervals (Equation (3)).
Formation of HIMC can be explained using Kirkendall effect with an ionic exchange.
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There are very few case of ionic exchange for synthesis of hollow nanostructure [25]. The case of ionic exchange leads to formation of hollow cubes can be explained via Kirkendall
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effect. The decomposition of IHMC occurred due to sufficient rate of ionic exchange. Anion exchange occurred between OH− and O2−, which leads to hollow cubes rather than only cubes. Generally, weakly bonded atoms can diffuse more rapidly than strongly bonded. Based on thermal analysis, it is observed that In-OH bond is weaker than In-O bond. So, the diffusion rate of OH− ion is greater than O2− Ion [25]. The diffusion flux can be represent
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using Ficks first law of diffusion (Equation (4)). (HIMC)1hr = −
(4)
Where, J is diffusion flux, D is diffusion coefficient and dC/dX is concentration gradient.
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For case of Kirkendall effect, we require gradient in diffusion flux of ions. So, gradient in
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diffusion flux can be balanced using equation as follows
=
−
(5)
Equation (1.4) and (1.5) represent outward diffusion of OH- and inward diffusion of
O2-. The delicate adjustment in the rates of diffusions of anions would results into hollow structures [26]. Initially, vacancies are created inside the cubes, which then grow form voids. Void formation as outcome of the gradient of diffusion flux is indication of the Kirkendall effect. These voids are further grow into the hollow structure [25]. The resultant diffusion flux (Jν) should be large for Kirkendall process. Hence, following condition can be useful For Kirkendall effect (Equation (6)).
ACCEPTED MANUSCRIPT ≫
(6)
Transformation from In(OH)3 to In2O3 can be represented using XRD, FE-SEM and FT-IR studies. So, increase in annealing time facilitates increase in diffusional lengths. Based on
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observation, we can judge that In2O3 hollow cubes were formed by controlled annealing time. The phenomena of morphological change over annealing time could be explained using
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Kirkendall effects.
3. Results and Discussion
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XRD pattern of hydrothermally grown as-synthesized IHMC and annealed HIMC films with variations in annealing time are presented in Fig. 1. As-synthesized IHMC exhibits cubic structure (Pn-3m). For IHMC sample, nine diffraction peaks (2θ) were observed at 22.37, 31.79, 39.13, 45.45, 51.32, 56.49,66.62, 70.88 and 75.45 corresponding to (200), (220), (311), (400), (420), (422), (440), (600) and (620) planes as shown in Fig. 1 (a). These diffracted peaks suggest crystalline nature of IHMC by JCPDS card No.00-016-0161.
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Moreover, the calculated lattice constants a=b=c= 7.9580 Å are in good agreement with reported value by JCPDS data. The XRD pattern of prepared In(OH)3 films holds good agreement with other reports. The diffraction pattern consist of a characteristics diffraction
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peak at 2θ = 22.37° along (200) plane confirms formation of IHMC. After annealing, IHMC transforms to HIMC through ionic exchange of OH− and O2-
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ions. The diffraction patterns of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr films were illustrated in Fig. 1 (b). All synthesized crystalline HIMC films have cubic (bixbyite) structure (Ia-3). For all film samples, five diffraction peaks (2θ) were observed at 21.59, 30.6, 35.4, 51.1 and 60.57 corresponding to the (211), (222), (400), (440) and (622) for crystalline In2O3, which suggest polycrystalline nature of In2O3 (JCPDS card No.00-006-0416). The XRD patterns of prepared HIMC films are in good agreement with other In2O3 reports. The diffraction pattern of HIMC films with characteristics diffraction peak at 2θ = 30.6° corresponds to the (222) plane, which confirms formation of crystalline nature [24]. The cubic structure denotes uniform arrangement and growth of atoms at characteristics (222)
ACCEPTED MANUSCRIPT plane. The lattice parameter of HIMC films are determined by using following formula for cubic structure (Eq. 7) =
(7)
!" !#
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Moreover, the calculated lattice constant a=b=c=10.118 Å are in good agreement with reported value by JCPDS data. Observed strong and sharp diffracted peaks indicate high quality synthesis of pure HIMC films, which are free from any type of contamination. The structural investigations of hydrothermally synthesized In2O3 using XRD has been reported in
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the literature [34-35, 37, 38, 42-43].
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Morphological studies of HIMCs were investigated by FE-SEM. Fig. 2 (a) shows FESEM analysis of as-synthesized IHMC sample without annealing. Fig. 2 (a) shows FE-SEM images of HIMC films carried out at different annealing times 1, 2, 3 and 4hr in order to explain the growth mechanism of HIMC. Initially, as-synthesized IHMC is analyzed to check morphology, which is found to be cubic. After 1hr (HIMC)1hr annealing, small cracks on the surface of cubes were found due to outward diffusion of OH- ions. The amount of OH- ions
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diffused outward increases for prolonged time of annealing (2hr to 4hr). Hence, initially developed small cracks grow slowly to form hollow structures with larger voids. The voids become too large to sustain rigidity of cubes after 4hr annealing (HIMC)
4hr
and therefore
cubes collapse (Fig. 2 (b)). Hence, (HIMC)3hr is optimized for further studies based on SEM
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analysis. To confirm hollowness, FESEM images of an optimized (HIMC)3hr film are illustrated with various magnifications in Fig. 3. The FESEM images of (HIMC)3hr clearly
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illustrates the hollowness and opening cracks on the surface.
PL is a powerful and nondestructive tool to study emissions and defects in PL spectra
of HIMC films is shown in Fig. 4. Fig. 4 shows two peaks at 690 and 736 nm, which are attributed to defects in HIMCs. HIMC films have oxygen deficiency due to its nonStoichiometric nature in arrangement. This non-stoichiometric arrangement induces defect levels at longer wavelengths. The formation of enhanced defect levels in material captures more conduction band electron. These captures electron helps to ionize oxygen (O2) into (Oor O2-) ions, which further interacts with toxic gas molecules. So, enhanced defect levels are beneficial for huge surface level interactions. Also, the enhancement in gas response of
ACCEPTED MANUSCRIPT (HIMC) 3hr may indicates the development of large interstitial and antisite oxygen vacancies developed during ionic exchange [29]. In view of numerous reports, the PL emission peaks at 690 and 736 nm match up to defect level emissions in ultraviolet as well as visible region [27][28]. Thus, it is clear that annealing time enhances defect levels in material though ionic
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exchange.
FT-IR spectra of HIMC films were analyzed at different annealing time. FT-IR absorption over 550 to 4000 cm-1 of IHMC sample is shown in Fig. 5. The absorption spectra of as-synthesized IHMC (Fig. 5 (a)) as well as annealed HIMC (Fig. 5 (b)) films were studied
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for the identification and confirmation of transformation. FT-IR absorption spectra of assynthesized IHMC exhibits 5 absorption peaks, which In-OH vibration. The sharp absorption peak at 597 cm-1 is attributed to the In-O stretching vibration. Also, The absorption peak at
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765 cm-1 shows In-O Vibration [30]. For IHMC, the vibrational peaks at 1060, 1147, 3113 and 3230 cm-1 shows O-H bending or deformation vibrations [31]. After annealing, IHMC transforms to HIMC through inter-diffusion of OH- and O- anions Fig. 5 (b). The absorption peaks of hydroxyl group wane with annealing time and for (HIMC) 4hr disappeared, which confirms transformation from IHMC to HIMC. Increase in annealing time decreases in hydroxyl band is observed due to evaporation. However, absence of further peaks in full
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region (4000-400 cm-1) shows perfect HIMC films without any impurity and hydroxyl content. Considering several studies, current study is in good agreement with other reports of
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In2O3 deposited by various techniques [32,33].
In order to determine chemical composition and electronic defect structure of HIMC
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(K3) film, X-ray photoelectron spectroscopy (XPS) analysis is performed. Fig. 6 (a) illustrates the survey spectra of (HIMC) 3hr film show the presence of In and O along with C as surface impurities. The location of the XPS peaks is calibrated to C1s binding energy (BE) at 284.6 eV. Fig. 6 (b) depicts the In 3d core level spectra of (HIMC) 3hr film. The spectra split in doublets namely 3d5/2 and 3d3/2 state. An excellent fit to two peaks located at binding energy 444.3 and 451.8 eV are clearly observed, which can be attributed to the characteristic spin-orbit split 3d5/2 and 3d3/2 respectively. The spectral data reveals that the Indium is in III oxidation state i.e. it has +3 vacancies without any metallic components. Also, Fig. 6 (c) shows O1s core-level spectra of Indium oxide films. It has two peaks at 529.6 and 531.3 eV. The peak at 529.6 eV corresponds to oxygen bonds in In–O–In whereas peak at 531.3 eV is related to oxygen vacancies in the bulk of MOS [34]. The small peak observed at 532.3eV the
ACCEPTED MANUSCRIPT surrounding area of intense oxygen O1s peak indicates the surface contamination or presence of O− present species, such as hydroxyl groups (OH−) or water molecules that are adsorbed on the surface while diffusion of anions [35].
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4. Gas sensing properties: 4.1 Gas sensing measurements:
It is proved that operating temperature of sensor play a vital role for adsorption, accumulation of conduction band electrons, formation of active sites on surface and reaction
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process [36]. Sensing response of HIMC films as function of temperature (50oC to 200oC) were measured at 100 ppm of NO2 gas illustrated in Fig. 7. It is observed that sensing increases at 100oC (Humidity~55%RH) and decreases dramatically with increase of operating
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temperature. So, optimized temperature enhances sensing response due to equilibrium between adsorption and desorption process on and inside of HIMC. Furthermore, compared data of plot suggests highest response is acquired by (HIMC)3hr film with value of 1401 at 100 ppm. The sensor response for (HIMC)1hr, (HIMC)2hr and (HIMC)4hr were 608, 864 and 148 respectively. Comparison of our report with literature shows that the optimizing
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temperature is much lower in case of HIMC [37,38]. The characteristic operating temperature can be considered beneficial to improve the sensing response due to huge adsorption and reaction of NO2 gas on surface of HIMC. Fig. 8 illustrates resistance curves of HIMC films for 100 ppm NO2 gas at 100 0C. It is noticed that all HIMC films sensors displays increment
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in resistance upon the exposure of NO2 as usually observed in n-type semiconductor sensor. The mechanism of n-type MOS sensors is based on the changes in electrical resistance
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observed due to gas adsorption and desorption on surface of films [39]. Initially, the resistance of HIMC films is measured without exposure of NO2 gas for baseline. A drastic increment in resistance of HIMC films is observed. Since, NO2 gas is oxidizing type of gas and has affinity to capture electron from HIMC. A resistance curve reveals that a (HIMC)3hr film acquires huge chemiresistive change than (HIMC)1hr, (HIMC)2hr and (HIMC)4hr films. Therefore, the resistance curve predicts reason behind the huge response of (HIMC)3hr film. The dynamic sensor responses of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples were illustrates in Fig. 9. Gas response of HIMC films sensors examined at various NO2 concentrations (20 to 100 ppm) were investigated with interval of 20 ppm. It is seen that sensing response of all films are increased with increment in concentration of NO2 gas at
ACCEPTED MANUSCRIPT operating temperature. Particularly, the response of (HIMC)3hr film increases linearly with increase in NO2 gas concentration due to more surface level adsorption and reaction. The gas response is observed to be 211, 354, 453, 925 and 1401 at 20, 40, 60, 80 and 100 ppm concentrations of NO2 gas. Development of hollowness inside the cubes is responsible for substantial adsorption and reaction of (HIMC)3hr film in addition to surface level process
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[40]. So, response of (HIMC)3hr film sensor has enhanced sensitivity than other films due to hollowness of microcubes. Reproducibility and stability plots of (HIMC)3hr sensor 100 ppm NO2 gas at 1000C as illustrated in Fig. 10. The identical behavior of (HIMC)3hr sensor for exposure of 100 ppm of NO2 gas represent exact reproducibility. The stability of(HIMC)3hr
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sensor was found 98.23% even after one month of fabrication. The response of sensor at various concentration of NO2 gas found linear. This linearity of sensor can helps to find
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unknown concentration of gases during quantitative analysis as shown in Fig. 11.
Response and recovery time are two crucial parameters for real time sensing analysis. Fig. 12(a) illustrates response and recovery time of (HIMC)3hr film sensor for 100 ppm NO2. Fig. 12(a) curve shows rapid response and recovery over NO2 gas, which indicates fast adsorption and desorption of gases on the surface of film. To get precise observations of
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response and recovery time, overall behavior of (HIMC)3hr film sensor is measured with various concentrations of NO2 gas as shown in Fig. 12(b). So, the obtain results demonstrates that response time has mere changes but recovery time is increased and comes down after particular concentrations. To check the selectivity of (HIMC)3hr film for 100 ppm of CO2,
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acetone, NH3 and NO2 gas are analyzed at an operating temperature of 100oC is depicted in Fig. 13. The gas response for other gases like CO2, acetone and NH3 are observed to be 32, 13
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and 29 respectively. A (HIMC)3hr film illustrates huge selectivity towards NO2 due to high response i.e. 1401. NO2 is high reactive due its lone pair electron in outer shell. It is observed that the fabricated HIMC based sensor acquires improved sensing properties in comparison with early reported work (Table 1) [41–45].
4.4 Sensing mechanism of (HIMC)3hr film The sensing mechanism of metal oxide like In2O3 is well explained by chemiresistive type sensing process. Sch. 2 displays sensing mechanism of (HIMC)3hr film sensor. It is well known that n-type semiconductor chemisorbs oxygen molecules in air with losing its conduction electrons [47]. As oxygen plays a vital role in electron transportation due to surface depletion model [40]. Model suggests that chemisorbed oxygen gain the electron and
ACCEPTED MANUSCRIPT forms reactive oxygen species such as O2-, O2-, and O- onside and inside of HIMCs. The process of oxygen ionization on the surface of HIMCs are shown in (8-9) ())*
$% + & ' +,,,,,,- $% '
(8)
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())*
$% ' + & ' +,,,,- 2$ '
(9)
This results in the small augmentation of barrier height and consequently depletion on
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the surface of HIMC in air. When sensor is exposed to specific concentration of NO2 gas, it reacts with chemisorbed oxygen species. As an oxidizing gas, NO2 has tendency to acquire electrons from oxygen species and increase the barrier height of HIMC. The reaction of NO2
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with oxygen species is shown in (10-12) [1]
NO2 + e− → NO2− 10 11
NO2 + O− + 2e− → NO2− + O2−
(12)
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NO2 + O2− + 2e− → NO2− +2O−
Obviously, on the basis of mechanism, the detection performance strongly recommends that the hollowness of HIMC enhances the amount of chemisorptions of oxygen onside and inside
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of surface.
5. Conclusions
In Summary, a novel route of synthesis of HIMCs by hydrothermal method using
Kirkendall effect is studied systematically. We have successfully optimized the synthesis of hollow HIMC by optimizing the time required for sufficient air annealing. During this process, OH− and O2− ions get exchanged via diffusion and results in its bulk hollowness. The mechanism of formation of hollow In2O3 from In(OH)3 owing to the well known Kirkendall effect. The gas sensing study reveals that synthesized (HIMC)3hr film exhibits high sensitivity towards NO2 gas at very low (100oC) operating temperature, which is quite low in comparison with the existing literature. The obtained sensing response for (HIMC)3hr film
ACCEPTED MANUSCRIPT was 1401 for 100 ppm of NO2 gas, which is the result of relatively large adsorption and reaction due to hollowness inside the cubes. Also, it is found that fabricated sensor is selective towards NO2 gas as compare to other gases. Additionally, response and recovery time of optimized (HIMC)3hr film sensor were very short. So, optimized (HIMC)3hr film sensor shows appreciable properties of sensor with high response and can be useful for real
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time sensing.
Acknowledgement
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This work was partially supported by University grant commission (UGC) New Delhi, Govt. Of India for Financial assistance through project No. 43-517/2014(SR). This work was partially supported by Korea Research Fellowship Program via the National
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Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) for an outstanding overseas young researcher to Dr. Sawanta S. Mali. Author Dr. Jasmin S. Shaikh, Women Scientist-A is acknowledges to Women Scientist project of Department of Science (DST, grant No. SR/WOS-A/PM1037/2014 (G).), Delhi, India. Also, Prof. Vikas. B. Patil would like to thank DAE-BRNS
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EP
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(scheme no. 34/14/21/2015-BRNS) for financial assistance.
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[38] C. Cantalini, W. Wlodarski, H.T. Sun, M.Z. Atashbar, M. Passacantando, S. Santucci, NO2 response of In2O3 thin film gas sensors prepared by sol-gel and vacuum thermal evaporation techniques, Sensors Actuators, B Chem. 65 (2000) 101–104.
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highly sensitive acetaldehyde gas sensors, J. Alloy. Compd. S0925-8388(18)33424-8.
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Table 1.1 represents few reports on In2O3 structures
Operating temperature (oC) 215
100
300
2. 3.
In2O3 nanowires
1
250
4.
Lotus root slice like In2O3 microspheres Nonporous In2O3 nanosheets Porous rh-In2O3 nanosheets In2O3 nanostructures
50
250
50
250
8.
0.5
hollow In2O3 microcubes
100
Reference
2
[41]
1.8
[42]
2.57
[43]
90
[44]
57
[45]
250
164
[45]
150
74
[46]
100
1401
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In2O3 hollow sphere In2O3 nanofibers
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Figure Caption: Sch 1: Schematic strategy for synthesis of HIMC samples.
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Fig. 1 (a): X-ray diffraction pattern of as-synthesized IHMC sample. Fig. 1 (b): X-ray diffraction patterns of annealed (HIMC)1hr, (HIMC)2hr, (HIMC)3hr (HIMC)4hr samples. Fig. 2 (a) FE-SEM images of synthesized IHMC sample.
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Fig. 2 (b) FE-SEM images of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples.
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Fig. 3: FE-SEM images of (HIMC)3hr film captured at various magnifications.
Fig. 4: PL spectra of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples.
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Fig. 5 (a): FTIR spectra of as-synthesized IHMC sample.
Fig. 5 (b): FTIR spectra of (HIMC)1hr, (HIMC)2hr, (HIMC)3hr and (HIMC)4hr samples. Fig. 6: XPS (a) Survey, (b) Indium and (c) Oxygen spectra in (HIMC)3hr film sample. Fig. 7: The gas response of all HIMC films for 100 ppm NO2 gas at the temperature ranging from 50 to 200oC.
Fig. 8: Resistance change of all HIMC films for 100 ppm NO2 gas.
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Fig. 9: The dynamic response of all HIMC films for various NO2 gas concentrations at 100oC. Fig. 10: The linearity of (HIMC)3hr film sensor for different ppm of NO2 concentrations at 100oC.
Fig. 11 Reproducibility and stability of (HIMC)3hr sensor 100 ppm NO2 gas at 100 0C.
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Fig. 12 (a): Response and recovery time of (HIMC)3hr film for 100 ppm of gas concentrations. Fig.12 (b): Dynamic response and recovery time of (HIMC)3hr film for various gas
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Fig. 13: Selectivity of (HIMC)3hr film towards various test gases for 100 ppm of gas concentration.
Sch. 2 Schematic representation of sensing mechanism of HIMC sensor
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Sch 1:
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Fig. 1 (a):
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Fig. 1 (b):
(HIMC)1hr
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In2O3JCPDS No.00-006-0416
(HIMC)2hr
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Fig. 5 (a):
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Fig. 6: 5
In 3d3/2 In 3d5/2
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O1s
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In 3p1/2
In 3p1/2
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Fig. 7:
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(HIMC)1hr (HIMC)2hr (HIMC)3hr
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Fig. 8: gas out
gas out
(HIMC)1hr
(HIMC)2hr
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4x10
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9
3x10
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gas in
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Fig. 9: 100 ppm
80 ppm
60 ppm
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0 1000
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0 1000
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40 ppm
20 ppm
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Research highlights
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1. A new route for the synthesis of hollow Indium oxide microcubes (HIMCs) via Kirkendall effect is carried out. 2. Synthesized HIMC samples are characterized and undertaken for NO2 sensing. 3. The obtained results on NO2 sensing predict the high quality of sensor. 4. We found that the sensor shows highly sensitive and selective towards NO2 gas. Also, it is found that the response and recovery time of sensor is very less in comparison with reported sensors. 5. On the basis of results, we may predict that the fabricated sensor can be used for selective NO2 gas detection at very low ppm of concentration at only 100oC of operating temperature.