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Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application
Journal Pre-proofs Research paper Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application R. Sankar Ganesh, V.L. Patil, E. Durgadevi, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, S. Kawasaki, P.S. Patil, Y. Hayakawa PII: DOI: Reference:
S0009-2614(19)30706-7 https://doi.org/10.1016/j.cplett.2019.136725 CPLETT 136725
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
8 April 2019 25 August 2019 26 August 2019
Please cite this article as: R. Sankar Ganesh, V.L. Patil, E. Durgadevi, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, S. Kawasaki, P.S. Patil, Y. Hayakawa, Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136725
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Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application R. Sankar Ganesh a, b, *, V.L. Patil d, E. Durgadevi c, M. Navaneethan c, S. Ponnusamy b, *, C. Muthamizhchelvan b, S. Kawasaki e, P.S. Patil d, Y. Hayakawa a, c, * a Graduate
School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
b Functional
Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology,
SRM Institute of Science and Technology, Kattankulathur, - 603203, Tamil Nadu, India c Research
Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
d Thin e
Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, India.
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-ku, Showa-ku, Nagoya 466-8555, Japan
*Corresponding authors. E-mail address: [email protected] (S. Ponnusamy), [email protected] (Y. Hayakawa). [email protected] (R. Sankar Ganesh) Abstract In this work, Undoped ZnO and Fe-doped ZnO nanostructures were successfully synthesized by using the simple hydrothermal method and their structural, morphological by various characterization techniques and studied the gas sensing properties. The Field emission electron microscopy (FE-SEM) observed that the nanorods assembled together to form the flower like in undoped ZnO and after Fe doping it turns into nanoellipsoid like morphology. NO2 gas response analysis revealed that Fe-doped ZnO nanoellipsoids displayed a higher selectivity compared to undoped ZnO. Among the different doping concentration 8 wt% exhibited the highest response of 209.8 when exposed to 100 ppm concentration of NO2. Keywords: NO2, Nanoellipsoids, Flower-like nanorods, BET, Fe-doped ZnO.
1. Introduction Due to increase of the population growth, increases the industrialization and adverse effect is that large amount of toxic gases such as NH3, NO2 etc. are evolved from the industries and polluted the atmosphere hence it is directly affected on the human 1
being i.e. scarring to the eye, skin and respiratory tracts, irritation to the throat and respiratory system etc [1]. In addition, another one toxic gas i.e. NO2 which cause severe respiratory diseases, lung function failure, and death [2-6]. Additionally, NO2 and other NOx gases are also precursors for harmful secondary air pollution such as ozone and particulate matter and plays a major role in the formation of acid rain and photochemical smog, if the NO2 level in the environment is high [7-10]. These lots of reasons a great demand of low cost, low temperature and high sensitive NO2 gas sensors. Numerous sensors are commonly used to detect the NO2 gas. However, Nano-structured resistive type metal oxide semiconductors have attained major attraction for NO2 gas detection due to its large surface area, low cost, reliable and simple sensing principle [11-17]. Metal oxide nanostructures are widely used for the detection of both oxidizing gases as well as reducing gases. The working principle of the metal oxide semiconductor is that the conductivity or the resistivity changes during the adsorption of target gas on the material surface [18, 19]. The resistance change in the material exhibits due to the adsorption of the oxidizing gases. The formation of the depletion region between the n-type metal oxide semiconductor grains improves the sensitivity towards the target gas [20-22]. The concentration of adsorbed O- has direct control over the lattice defects. The introduction of oxidizing gas into the n-type semiconducting material decreases the concentration of electron in the surface and increases the resistance of the semiconductor [23]. It is known that electrons are the majority carrier which is responsible for the current flow in the ntype semiconducting material. Due to the presence of oxidizing gas in the semiconductor surface majority carrier flow is decreased. The reduction in current flow corresponds to the high resistivity [24]. Among various metal oxide semiconductor nanostructures (such as SnO2, WO3, TiO2, In2O3 and ZnO) ZnO has drawn major attention due to its wider bandwidth of 3.37 eV, large exciton binding energy of 60meV, high sensitivity, high transparency, high piezo electric constant, large electro-optic coefficient, high ionicity and electrical and thermal stability, which widens the application of ZnO in solar cell, gas sensor, surface acoustic wave devices, piezo electric transducers, optoelectronic devices, light emitting diodes, laser diodes and photodetectors [21-22]. Furthermore, 1-D ZnO nanostructure is the most suitable candidate for gas sensing application. To improve the sensitivity, reliability and reliability of ZnO based gas sensors the properties such as surface to volume ratio, surface defects and active center of the material must be optimized [23-28]. 2
One kind of such optimization is that the addition of impurities into the metal structure which will significantly affect the electrical and optical properties. It has been already proved that the doping of metals or surface modifying oxides can significantly improve the sensitivity since it can effectively modify the parameters of crystal cell and band structure of ZnO [29-32]. Moreover, adding impurities in nanostructure can create defects which provide preferential sites for adsorption of target gas molecules. Doping of transition metals such as Cu, Fe, Mn, Ni and Co into ZnO causes an exchange interaction between S and P electrons of ZnO and d – electron of transition metal ions significantly affects the structural, optical and magnetic property [33, 34]. Fe, Mn and Co has drawn much attention due to their different electronic shell structure and similar ionic size with Zn. Among these Fe ion has multivalence state. It has been reported that the addition of Fe into ZnO appears in the form of both Fe2+ and Fe3+ which has significant effects in band gap and electrical property [34, 35]. Furthermore, Fe-doped ZnO attain ferromagnetism at room temperature. It has been already reported that the defects, vacancies, donor concentration and adsorption-desertion of oxygen are affected by Fe doping in ZnO [35]. Also, that the Fe doped ZnO widens the optical bandgap and shifts the adsorption spectrum to low energy with increasing dopant concentration. Hence it has been established that Fe-doped ZnO has the significant effect in sensing mechanism. Among the various available synthesis process, the hydrothermal method has been considered as an effective method due to its low cost, low temperature, simple process control and environmentally friendly process. In addition to that, better crystallinity of the material can be achieved through this method. Hence, these kinds of nanostructured materials are the suitable candidate for gas sensing mechanism. In this work, we have reported the growth Fe-doped ZnO nanoellipsoids for NO2 gas sensing application at low operating temperature. In synthesis process we have used the simple and one-step hydrothermal method is utilized to control the structure of Fe-doped ZnO and the composition. The prepared materials were characterized by X-ray diffraction, field-emission electron microscopy, energy-dispersive x-ray spectroscopy, Raman spectroscopy, UV-visible spectroscopy. The gas sensing properties of the prepared materials were studied at the low temperature subjected towards NO2 gas. The detailed studies of the material characterization of Fe-doped ZnO in correlation with the gas sensing application has enough application potential and to the best of our knowledge, the results are suitable for to fabricate the gas sensor device for future scope. 3
2. Experimental section 2.1 Materials Zinc nitrate [Zn(NO3)2.H2O], Iron nitrate [Fe(NO3)3], ammonia and solvents were used as received from wako chemicals. 2.2 Preparation of Fe-doped ZnO In the typical synthesis, Fe-doped ZnO nanostructures were prepared by the simple and cost effective hydrothermal method using 1M of zinc nitrate [Zn(NO3)2.H2O] and different weight (2, 4, 6, 8, 10 %) percentages of Fe(NO3)3 were dissolved in 100 mL of deionized water (DI) water and magnetic stirred for 1 h at room temperature. Then, continuously stirring add the ammonia in above solution to get the pH=9. After getting the pH the solution was placed into teflon-lined stainless steel autoclave and maintained at 150 C for 5 h. Furthermore after completion of reaction time period the autoclave was cooled at room temperature, the obtained nanopowder was collected, centrifuged, washed with DI water and ethanol for several times and dried at 100 C in hot air oven. 2.3 Characterization and measurement of Gas sensor The crystalline structure and phase formation of the prepared materials was determined by X-ray diffraction (XRD) on an X'pert PRO (PANalytical) advanced X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) with 2θ scanning range of 20–80° at scanning rate of 0.025°/s. The microstructure of the products was observed by fieldemission scanning electron microscopy (FESEM; QUANTA FEG) and transmission electron microscopy (TEM; JEOL JEM 2100F) operating at accelerating voltages of 15 and 200 kV, respectively. The components of the products were evaluated by energydispersive X-ray spectroscopy (EDX). Raman spectroscopy was performed to evaluate structural disorder, vibrational and defect modes in the prepared samples; the spectra were recorded by a JASCO NR 1800 Raman spectrometer equipped with a Nd:YAG laser. The optical properties of the products were measured using PerkinElmer lambda 5 UV–visible spectrophotometer. To measure the gas sensing characteristics, Fe-doped ZnO (2, 4, 6, 8, 10 %) nanopowders is screen printed on the glass substrate. For screen printing, Fe doped ZnO is mixed with the ethyl cellulose and terpinol in an agate mortar to form a paste. The paste is then coated onto the glass substrates using a screen printing technique and dried at 60 4
°C for 1 h. Finally, the screen printed films are sintered at 400 °C for 5 min to remove the organic elements. These films are then used for gas sensing studies. The silver paste was coated on the sensing element as electrodes to measure the electrical response. The sample was mounted on the test chamber having a volume capacity of 250 cm3 with the provision of gas inlet – outlet and a flat heating plate with temperature controller. A Keithley 6514 electrometer with data acquisition system controlled by computer was connected to the external leads of the thin film sensor. The film area was kept constant for all samples concentrations of gas injected into the system to measure the change in the electrical resistance of the film. The gas sensitivity (%) was calculated by 𝑆=
𝑅𝑎 ― 𝑅g 𝑅a
× 100
where 𝑅a and 𝑅g are the resistance of the sensing element in air and target gas, respectively. 3. Results and discussion 3.1 Structural and morphological analysis of Fe-doped ZnO To study the crystallinity and phase formation of X-ray diffraction pattern of undoped and Fe-doped ZnO nanoellipsoids is shown in fig.1. All the peaks are well indexed with the JCPDS card no. 36-1451 and are more crystalline wurtzite crystal structure. The main peaks of undoped and Fe-doped ZnO situated at the diffraction peaks i.e. 31.64°, 34.44°, 36.11°, 47.45°, 56.56°, 62.92°, 66.33°, 67.83°, 69.06°, 72.53°, and 76.87°, which corresponded to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of ZnO and FZO (2, 4, 6, 8,10 %) respectively [36]. The peak observed at approximately 2θ 43.2 confirmed the presence of Fe in the ZnO nanoellipsoids at higher doping percentage (6, 8, 10 wt%) [37]. Moreover, from the observation fig. 1 shows that the higher intensity of (100), (002), (101) planes leads to the growth of Fe doped ZnO on three direction and also turns the morphology to ellipsoid due to the Fe doping cause Fe incorporation into ZnO lattice and change the morphology of the Fe doped ZnO. Then to calculate the crystalline size of undoped ZnO and Fe-doped ZnO by using the Scherrer’s formula, 𝐾𝜆
𝐷 = 𝛽𝑐𝑜𝑠 𝜃
5
where K denotes the Scherrer’s constant (0.89), D is the particle size, λ is the wavelength of the X-ray Cu Kα radiation (1.5406 Å), β is the full-width at half-maximum in radians of respective peaks, and θ is the Bragg’s diffraction angle in degrees. The average crystalline size was observed at peak (002) to all samples and it is found that the 14-18 nm in size. The surface morphology of hydrothermally synthesized undoped ZnO and Fedoped ZnO samples was analyzed by FESEM and TEM techniques. The hexagonal nanorods are assembled together to form the Flower-like morphology was observed for undoped ZnO and the length of nanorods is about 1.2 m as shown in fig. 2a. Similarly Fig. 2b is the lower concentration of Fe doping and observed that the there is no change of morphology i.e. same as undoped ZnO. Moreover, increases the concentration of Fe doping, the flower like morphology turns into nanoellipsoid like morphology can be observed from the fig. 2d-f. So, we strongly confirmed that the incorporation of Fe2+ in Zn2+ lattice and effects is that to change the morphology and structure of the Fe doped ZnO samples. Furthermore, for clear understanding we studied the TEM analysis and it is observed that flower-like morphology consists of hexagonal nanorods of undoped ZnO and which is consistent with the FESEM image (fig. 3a, b). Fig. 3d, e illustrates the transformation of nanorods into nanoellipsoids at lower doping concentration. Fig. 3g, h showed the perfect formation of nanoellipsoids at higher doping concentration (8 wt%) and the size of the nanoellipsoids about 200 nm. The high-resolution TEM (HRTEM) images of the undoped ZnO and FZO samples revealed that the flower nanorods and nanoellipsoids were crystalline (fig. 3c, f, i). 3.2 Chemical bonding and composition of FZO The elemental composition of optimized sample of FZO i.e. 8 wt. % was conducted by EDX and color mapping analysis. The result presented from EDAX analysis was there is no additional impurity except the Zn, O, C and Fe elements. The bird eye view observation obtained that in fig. S1 (a-e) shows that the Fe was evenly distributed on Zinc oxide materials. Fig S1 f confirmed the existence of Zn, O, Fe and C elements in the FZO sample [38]. The C elements are comes from the atmospheric impurities or precursors of the Zn and Fe elements. To study the longitudinal and vibrational modes of undoped-ZnO and Fe doped ZnO was studied using the room temperature raman analysis. The raman spectra of 6
undoped-ZnO and Fe doped ZnO as shown in fig. S2. As well known that ZnO wurtzite structure has eight sets of characteristic optical phonon modes like A1 and E1. These polar branches split into transversal optical (TO) and longitudinal optical components with different frequencies. The low- frequency mode E2L associated to the heavy Zn sub lattice and high frequency mode
E2H
involving only oxygen atom [39]. The peak at 436
cm−1 was ascribed to the non-optical phonon E2H mode, which confirmed the wurtzite structure of ZnO [40]. The weak peaks at 329 and 377 cm−1 were ascribed to multiphonons scattering process E2H - E2L and A1 phonon mode, respectively. The A1 LO and E2L (580 cm−1) phonon mode usually assigned to oxygen vacancies and Zinc interstitials in ZnO materials. 3.3. XPS analysis The undoped and Fe doped ZnO nanoellipsoids samples distribution and oxidations state was found out by using the XPS analysis. In fig. 4 shows the highresolution XPS spectra of Zn 2p, O 1s, and Fe 2p of undoped ZnO and Fe-doped ZnO. In fig. 4a, two strong peaks at 1047.06 and 1024.35 eV correspond to the binding energies of Zn 2p1/2 and Zn 2p3/2, respectively, which confirms that Zn in Zn2+ oxidation state of wurtzite ZnO structure [41]. In addition, the observed spin-orbit splitting between Zn 2p1/2 and Zn 2p3/2 is about 23 eV. Fe-doped ZnO showed a slight shift towards lower binding energy in Zn 2p compared to the undoped ZnO, which clearly indicated the existence of Fe2+ in the Fe-doped ZnO synthesized samples. Fig. 4b shows the XPS spectra of Fe 2p, the peak at 711.8 and 725.1 are attributed to the binding energies of Fe 2p3/2 and Fe 2p1/2, respectively [42]. No additional peaks corresponding to metal or particles are obtained in any spectra. Fig. 4c shows the XPS spectra of O 1s, the peak positioned at 530.9 eV can be attributed to O2− ions that are oxygen deficient regions in the wurtzite structure of ZnO [43]. The Gaussian fitting results indicate that the XPS signals at 532.6 eV indicate that higher amount of surface hydroxyl group and the presence of hydroxyl group facilitates the trapping of electron and holes which enhance the sensitivity in gas sensors [44]. In contrast, Fe-doped ZnO (8 wt%) sample showed the broad peak at 531.4 eV which implies that higher oxygen deficiency in doped sample compared to the undoped ZnO (fig. 4d). XPS analysis clearly reveals that oxygen vacancy increased compared to undoped sample which will enhance the gas sensitivity. 3.4 N2 adsorption and desorption analysis 7
To obtain further information of the surface area and pores nature of the undoped ZnO and Fe-doped ZnO, Brunauer-Emmett- Teller (BET) N2 adsorption-desorption analysis was performed. The adsorption-desorption of undoped ZnO and Fe-doped ZnO nanoellipsoids as observed in the fig. S3 (a, b). The BET specific surface area of the 2% and 8% FZO nanoellipsoids was calculated to be 22 and 51 m2/g from nitrogen adsorption isotherm. And the pure sample of ZnO shows the BET specific surface area is about 18 m2/g from the nitrogen adsorption [45]. So, among these 8 % FZO nanoellipsoid confirmed the high surface area it’s may be due to the effect of the Fe ions in the Zn ions and it is helpful to adsorption of target gas hence to increase the sensitivity of the target gas at low operating temperature. 3.5 NO2 sensing studies In metal oxide gas sensing application operating temperature is the main parameters to detect the target gases. Hence, some of the scientist working on to reduce the operating temperature of the metal oxide and to increase the sensitivity as well as selectivity of metal oxides. Here, to found out the operating temperature of ZnO nanorods authors are very the temperature from 100 C to 250 C at fixed concentration of NO2 gas. The gas response of 8% FZO showed the 209.8 response for 100 ppm NO2 gas concentration which is higher than of the undoped ZnO. In same time authors have studied the cross sensitivity of the 8% FZO nanoellipsoids using different oxidizing and reducing gases. Moreover 8% FZO shows highly sensitive and selective to NO2 gas at low operating temperature which is shown in fig. 5a, b. Also at that operating temperature all doping sample of FZO at fixed amount of NO2 gas concentration was checked then out of these doping samples 8% FZO showed maximum response of NO2 gas because of its extraordinary property in FESEM which adsorb the more NO2 gas and suggested to increase the gas response is shown in fig. 5c. Among these 150 C shows maximum gas response towards 100 ppm NO2 gas concentration as shown in fig. 5d, therefore the 150 C is the operating temperature of ZnO nanorods because of this temperature assists to overcome the energy barriers and activate the surface of ZnO nanorods after adsorption of NO2 gas and hence to increase the gas response Furthermore, we have studied about the linearity of the 8% FZO thin films and it is observed that at gas concentration increases from 10 to 100 ppm of NO2 gas there is increases the gas response hence FZO thin film shows linearity which is shown in fig. 8
S4a. In addition, fig. 9b shows the response time and the recovery time of 8% FZO thin films. Fe-doped ZnO shows fast response towards NO2 gas, the response time decreased from 49 to 6 seconds (s) for 10-100 ppm of NO2 concentration. This trend confirmed the expected trend as decrease in response time as concentration of NO2 increases, due to the availability of higher number of NO2 molecule interacts with sensing surface. But the recovery time of Fe-doped ZnO was found to be 28- 240 s for different concentration of NO2, which might be due to prolonged desorption of NO2 on the surface of the sample. The stability of thin film was recorded for 30 days and reproducible capability of sensing element towards 100 ppm. Fig. S4(c, d) clearly reveals the reproducibility and long-term stability towards NO2 gas. Fig. S4 e represents the gas sensitivity of device at different humidity. The results indicated that increase in humidity reduced the sensitivity of the device. 3.6. Gas sensing mechanism Metal oxide gas sensors are based on the change in resistance of materials due to the chemical and electronic interaction between the gas and sensing matrix. The chemical interaction involves the adsorption of the target gas molecules on the surface, subsequent reaction with oxygen species adsorbed on the surface of ZnO which results in the detection of target gas [38, 46-49]. ZnO films exposed to atmosphere air, the oxygen molecules adsorb on the surface of the ZnO materials to form chemisorbed oxygen anions ― (𝑂2(𝑎𝑑𝑠) ) by capturing electrons from the conduction band, resultant in the formation of
depletion region known as space charge layer. This in turn results in increase in resistance of ZnO [17]. ― 𝑂2 + 𝑒 ― ↔𝑂2𝑎𝑑𝑠
(1)
ZnO films exposed to NO2 oxidizing gas, NO2 gas directly absorbed on the surface of ZnO and subsequently it captures one more electron from the conduction band this in turn leads to chemisorption of NO2 as 𝑁𝑂2― . This results in decrease in majority of charge carriers and increase in the resistance [41]. In addition, adsorbed monomolecule oxygen dissociated when NO2 approaches to the adsorbed site. Monomolecule of oxygen is dissociated and chemisorbed as atomic ionic species due to this the electron concentration decreases in the surface layer of ZnO, which leads to increase in the resistance of ZnO sensor [39, 49]. 9
― 𝑁𝑂2(𝑔𝑎𝑠) + 𝑒 ― →𝑁𝑂2(𝑎𝑑𝑠)
(2)
― ― + 2𝑂(𝑎𝑑𝑠) 𝑁𝑂2(𝑔𝑎𝑠) + 𝑂2― + 2𝑒 ― →𝑁𝑂2(𝑎𝑑𝑠)
(3)
Incorporation of Fe into ZnO showed enhanced sensing response towards NO2 gas. The possibility of getting good response may due to the formation of p-n junction. In formation of p-n junction minority charge carriers in Fe are lesser and they require less energy to move to the conduction band compared ZnO. This in turn increases the sensing response. 4. Conclusion We report a simple hydrothermal method for the controlled growth of flower-like hexagonal nanorods and nanoellipsoids. FE-SEM image clearly indicated the transformation of nanorods to nanoellipsoids on incorporation of Fe. Brunauer-EmmettTeller (BET) N2 adsorption-desorption analysis indicated that Fe-doped ZnO (8 wt%) has larger surface area compared to undoped ZnO. The effect of doping concentration and operating temperature on the gas response of NO2 is analyzed. Higher gas response was observed for Fe-doped ZnO nanoellipsoids 8 wt% compared to other doping concentration. The maximum sensitivity of Fe-doped ZnO sensor was around 209.8 compared to the undoped ZnO. Fe-doped ZnO sensor showed rapid response, reproducibility and stability towards NO2 gas at low temperature of 150 C. Fe-doped ZnO showed rapid response but the recovery time of the sensor is relatively extended. The response time decreased from 49 to 6 seconds (s) for 10-100 ppm of NO2 concentration. Fe-doped ZnO sensor showed good response even at lower detection of 10 ppm. Acknowledgements R. Sankar Ganesh thanks MEXT-Japan for award of a research fellowship. We would like to thank Prof. Murakami, Mr. T. Koyama and Mr. W. Tomoda, Center for Instrumental Analysis, Shizuoka University, Hamamatsu, Japan. 5. Reference [1]
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List of figure caption Fig . 1 XRD patterns of Fe-doped ZnO (2, 4, 6, 8, 12 wt%) nanoellipsoids and undoped ZnO nanoflowers. Fig. 2 FE-SEM micrograph (a). undoped ZnO, (b). 2 % FZO, (c). 4 % FZO, (d). 6 % FZO, (d). 8 % FZO, (e). 10 wt % FZO. Fig. 3 TEM and HRTEM image of (a, b, c). ZnO nanoflowers, (d, e, f). 6 wt %. (g, h, i). 8 wt% Fe-doped ZnO. Fig. 4 XPS analysis of pure and Fe-doped ZnO (a). Zn 2p of undoped and Fe-doped ZnO (8 wt%), (b). Fe 2p core-level spectra of FZO (8 wt%), (c). O 1s of ZnO nanoflower, (d). O 1s of CZO (6 wt%). Fig. 5 (a, b). Selectivity nature of the ZnO and FZO thin films at 100 ppm of various target gases, c). Sensing response of FZO (2, 4, 6, 8, 10 wt%) thin films at 100 ppm of ammonia, d). Response 8 % FZO at different temperature
17
List of figures
Figure 1
18
Figure 2
19
Figure 3
20
Figure 4
21
Figure 5
22
Highlights
Fe-doped ZnO (8 wt%) exhibited the highest response of 209.8
The response time decreased from 49 to 6 seconds(s) for 10-100 ppm of NO2 concentration
BET analysis showed higher surface area compared undoped ZnO.
23
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
1. The research article is original. 2. The article has been written by the stated authors who are ALL aware of its content and approve its submission. 3. The article has not been published previously 4. The article is not under consideration for publication elsewhere 5. No conflict of interest exists, or if such conflict exists, the exact nature must be declared. 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. This research article consists of a main text part, figures, and tables which are uploaded in the web. Thank you very much for your consideration of the manuscript.
Sincerely yours R. Sankar Ganesh
24
S0009-2614(19)30706-7 https://doi.org/10.1016/j.cplett.2019.136725 CPLETT 136725
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
8 April 2019 25 August 2019 26 August 2019
Please cite this article as: R. Sankar Ganesh, V.L. Patil, E. Durgadevi, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, S. Kawasaki, P.S. Patil, Y. Hayakawa, Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136725
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Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application R. Sankar Ganesh a, b, *, V.L. Patil d, E. Durgadevi c, M. Navaneethan c, S. Ponnusamy b, *, C. Muthamizhchelvan b, S. Kawasaki e, P.S. Patil d, Y. Hayakawa a, c, * a Graduate
School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
b Functional
Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology,
SRM Institute of Science and Technology, Kattankulathur, - 603203, Tamil Nadu, India c Research
Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
d Thin e
Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, India.
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-ku, Showa-ku, Nagoya 466-8555, Japan
*Corresponding authors. E-mail address: [email protected] (S. Ponnusamy), [email protected] (Y. Hayakawa). [email protected] (R. Sankar Ganesh) Abstract In this work, Undoped ZnO and Fe-doped ZnO nanostructures were successfully synthesized by using the simple hydrothermal method and their structural, morphological by various characterization techniques and studied the gas sensing properties. The Field emission electron microscopy (FE-SEM) observed that the nanorods assembled together to form the flower like in undoped ZnO and after Fe doping it turns into nanoellipsoid like morphology. NO2 gas response analysis revealed that Fe-doped ZnO nanoellipsoids displayed a higher selectivity compared to undoped ZnO. Among the different doping concentration 8 wt% exhibited the highest response of 209.8 when exposed to 100 ppm concentration of NO2. Keywords: NO2, Nanoellipsoids, Flower-like nanorods, BET, Fe-doped ZnO.
1. Introduction Due to increase of the population growth, increases the industrialization and adverse effect is that large amount of toxic gases such as NH3, NO2 etc. are evolved from the industries and polluted the atmosphere hence it is directly affected on the human 1
being i.e. scarring to the eye, skin and respiratory tracts, irritation to the throat and respiratory system etc [1]. In addition, another one toxic gas i.e. NO2 which cause severe respiratory diseases, lung function failure, and death [2-6]. Additionally, NO2 and other NOx gases are also precursors for harmful secondary air pollution such as ozone and particulate matter and plays a major role in the formation of acid rain and photochemical smog, if the NO2 level in the environment is high [7-10]. These lots of reasons a great demand of low cost, low temperature and high sensitive NO2 gas sensors. Numerous sensors are commonly used to detect the NO2 gas. However, Nano-structured resistive type metal oxide semiconductors have attained major attraction for NO2 gas detection due to its large surface area, low cost, reliable and simple sensing principle [11-17]. Metal oxide nanostructures are widely used for the detection of both oxidizing gases as well as reducing gases. The working principle of the metal oxide semiconductor is that the conductivity or the resistivity changes during the adsorption of target gas on the material surface [18, 19]. The resistance change in the material exhibits due to the adsorption of the oxidizing gases. The formation of the depletion region between the n-type metal oxide semiconductor grains improves the sensitivity towards the target gas [20-22]. The concentration of adsorbed O- has direct control over the lattice defects. The introduction of oxidizing gas into the n-type semiconducting material decreases the concentration of electron in the surface and increases the resistance of the semiconductor [23]. It is known that electrons are the majority carrier which is responsible for the current flow in the ntype semiconducting material. Due to the presence of oxidizing gas in the semiconductor surface majority carrier flow is decreased. The reduction in current flow corresponds to the high resistivity [24]. Among various metal oxide semiconductor nanostructures (such as SnO2, WO3, TiO2, In2O3 and ZnO) ZnO has drawn major attention due to its wider bandwidth of 3.37 eV, large exciton binding energy of 60meV, high sensitivity, high transparency, high piezo electric constant, large electro-optic coefficient, high ionicity and electrical and thermal stability, which widens the application of ZnO in solar cell, gas sensor, surface acoustic wave devices, piezo electric transducers, optoelectronic devices, light emitting diodes, laser diodes and photodetectors [21-22]. Furthermore, 1-D ZnO nanostructure is the most suitable candidate for gas sensing application. To improve the sensitivity, reliability and reliability of ZnO based gas sensors the properties such as surface to volume ratio, surface defects and active center of the material must be optimized [23-28]. 2
One kind of such optimization is that the addition of impurities into the metal structure which will significantly affect the electrical and optical properties. It has been already proved that the doping of metals or surface modifying oxides can significantly improve the sensitivity since it can effectively modify the parameters of crystal cell and band structure of ZnO [29-32]. Moreover, adding impurities in nanostructure can create defects which provide preferential sites for adsorption of target gas molecules. Doping of transition metals such as Cu, Fe, Mn, Ni and Co into ZnO causes an exchange interaction between S and P electrons of ZnO and d – electron of transition metal ions significantly affects the structural, optical and magnetic property [33, 34]. Fe, Mn and Co has drawn much attention due to their different electronic shell structure and similar ionic size with Zn. Among these Fe ion has multivalence state. It has been reported that the addition of Fe into ZnO appears in the form of both Fe2+ and Fe3+ which has significant effects in band gap and electrical property [34, 35]. Furthermore, Fe-doped ZnO attain ferromagnetism at room temperature. It has been already reported that the defects, vacancies, donor concentration and adsorption-desertion of oxygen are affected by Fe doping in ZnO [35]. Also, that the Fe doped ZnO widens the optical bandgap and shifts the adsorption spectrum to low energy with increasing dopant concentration. Hence it has been established that Fe-doped ZnO has the significant effect in sensing mechanism. Among the various available synthesis process, the hydrothermal method has been considered as an effective method due to its low cost, low temperature, simple process control and environmentally friendly process. In addition to that, better crystallinity of the material can be achieved through this method. Hence, these kinds of nanostructured materials are the suitable candidate for gas sensing mechanism. In this work, we have reported the growth Fe-doped ZnO nanoellipsoids for NO2 gas sensing application at low operating temperature. In synthesis process we have used the simple and one-step hydrothermal method is utilized to control the structure of Fe-doped ZnO and the composition. The prepared materials were characterized by X-ray diffraction, field-emission electron microscopy, energy-dispersive x-ray spectroscopy, Raman spectroscopy, UV-visible spectroscopy. The gas sensing properties of the prepared materials were studied at the low temperature subjected towards NO2 gas. The detailed studies of the material characterization of Fe-doped ZnO in correlation with the gas sensing application has enough application potential and to the best of our knowledge, the results are suitable for to fabricate the gas sensor device for future scope. 3
2. Experimental section 2.1 Materials Zinc nitrate [Zn(NO3)2.H2O], Iron nitrate [Fe(NO3)3], ammonia and solvents were used as received from wako chemicals. 2.2 Preparation of Fe-doped ZnO In the typical synthesis, Fe-doped ZnO nanostructures were prepared by the simple and cost effective hydrothermal method using 1M of zinc nitrate [Zn(NO3)2.H2O] and different weight (2, 4, 6, 8, 10 %) percentages of Fe(NO3)3 were dissolved in 100 mL of deionized water (DI) water and magnetic stirred for 1 h at room temperature. Then, continuously stirring add the ammonia in above solution to get the pH=9. After getting the pH the solution was placed into teflon-lined stainless steel autoclave and maintained at 150 C for 5 h. Furthermore after completion of reaction time period the autoclave was cooled at room temperature, the obtained nanopowder was collected, centrifuged, washed with DI water and ethanol for several times and dried at 100 C in hot air oven. 2.3 Characterization and measurement of Gas sensor The crystalline structure and phase formation of the prepared materials was determined by X-ray diffraction (XRD) on an X'pert PRO (PANalytical) advanced X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) with 2θ scanning range of 20–80° at scanning rate of 0.025°/s. The microstructure of the products was observed by fieldemission scanning electron microscopy (FESEM; QUANTA FEG) and transmission electron microscopy (TEM; JEOL JEM 2100F) operating at accelerating voltages of 15 and 200 kV, respectively. The components of the products were evaluated by energydispersive X-ray spectroscopy (EDX). Raman spectroscopy was performed to evaluate structural disorder, vibrational and defect modes in the prepared samples; the spectra were recorded by a JASCO NR 1800 Raman spectrometer equipped with a Nd:YAG laser. The optical properties of the products were measured using PerkinElmer lambda 5 UV–visible spectrophotometer. To measure the gas sensing characteristics, Fe-doped ZnO (2, 4, 6, 8, 10 %) nanopowders is screen printed on the glass substrate. For screen printing, Fe doped ZnO is mixed with the ethyl cellulose and terpinol in an agate mortar to form a paste. The paste is then coated onto the glass substrates using a screen printing technique and dried at 60 4
°C for 1 h. Finally, the screen printed films are sintered at 400 °C for 5 min to remove the organic elements. These films are then used for gas sensing studies. The silver paste was coated on the sensing element as electrodes to measure the electrical response. The sample was mounted on the test chamber having a volume capacity of 250 cm3 with the provision of gas inlet – outlet and a flat heating plate with temperature controller. A Keithley 6514 electrometer with data acquisition system controlled by computer was connected to the external leads of the thin film sensor. The film area was kept constant for all samples concentrations of gas injected into the system to measure the change in the electrical resistance of the film. The gas sensitivity (%) was calculated by 𝑆=
𝑅𝑎 ― 𝑅g 𝑅a
× 100
where 𝑅a and 𝑅g are the resistance of the sensing element in air and target gas, respectively. 3. Results and discussion 3.1 Structural and morphological analysis of Fe-doped ZnO To study the crystallinity and phase formation of X-ray diffraction pattern of undoped and Fe-doped ZnO nanoellipsoids is shown in fig.1. All the peaks are well indexed with the JCPDS card no. 36-1451 and are more crystalline wurtzite crystal structure. The main peaks of undoped and Fe-doped ZnO situated at the diffraction peaks i.e. 31.64°, 34.44°, 36.11°, 47.45°, 56.56°, 62.92°, 66.33°, 67.83°, 69.06°, 72.53°, and 76.87°, which corresponded to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of ZnO and FZO (2, 4, 6, 8,10 %) respectively [36]. The peak observed at approximately 2θ 43.2 confirmed the presence of Fe in the ZnO nanoellipsoids at higher doping percentage (6, 8, 10 wt%) [37]. Moreover, from the observation fig. 1 shows that the higher intensity of (100), (002), (101) planes leads to the growth of Fe doped ZnO on three direction and also turns the morphology to ellipsoid due to the Fe doping cause Fe incorporation into ZnO lattice and change the morphology of the Fe doped ZnO. Then to calculate the crystalline size of undoped ZnO and Fe-doped ZnO by using the Scherrer’s formula, 𝐾𝜆
𝐷 = 𝛽𝑐𝑜𝑠 𝜃
5
where K denotes the Scherrer’s constant (0.89), D is the particle size, λ is the wavelength of the X-ray Cu Kα radiation (1.5406 Å), β is the full-width at half-maximum in radians of respective peaks, and θ is the Bragg’s diffraction angle in degrees. The average crystalline size was observed at peak (002) to all samples and it is found that the 14-18 nm in size. The surface morphology of hydrothermally synthesized undoped ZnO and Fedoped ZnO samples was analyzed by FESEM and TEM techniques. The hexagonal nanorods are assembled together to form the Flower-like morphology was observed for undoped ZnO and the length of nanorods is about 1.2 m as shown in fig. 2a. Similarly Fig. 2b is the lower concentration of Fe doping and observed that the there is no change of morphology i.e. same as undoped ZnO. Moreover, increases the concentration of Fe doping, the flower like morphology turns into nanoellipsoid like morphology can be observed from the fig. 2d-f. So, we strongly confirmed that the incorporation of Fe2+ in Zn2+ lattice and effects is that to change the morphology and structure of the Fe doped ZnO samples. Furthermore, for clear understanding we studied the TEM analysis and it is observed that flower-like morphology consists of hexagonal nanorods of undoped ZnO and which is consistent with the FESEM image (fig. 3a, b). Fig. 3d, e illustrates the transformation of nanorods into nanoellipsoids at lower doping concentration. Fig. 3g, h showed the perfect formation of nanoellipsoids at higher doping concentration (8 wt%) and the size of the nanoellipsoids about 200 nm. The high-resolution TEM (HRTEM) images of the undoped ZnO and FZO samples revealed that the flower nanorods and nanoellipsoids were crystalline (fig. 3c, f, i). 3.2 Chemical bonding and composition of FZO The elemental composition of optimized sample of FZO i.e. 8 wt. % was conducted by EDX and color mapping analysis. The result presented from EDAX analysis was there is no additional impurity except the Zn, O, C and Fe elements. The bird eye view observation obtained that in fig. S1 (a-e) shows that the Fe was evenly distributed on Zinc oxide materials. Fig S1 f confirmed the existence of Zn, O, Fe and C elements in the FZO sample [38]. The C elements are comes from the atmospheric impurities or precursors of the Zn and Fe elements. To study the longitudinal and vibrational modes of undoped-ZnO and Fe doped ZnO was studied using the room temperature raman analysis. The raman spectra of 6
undoped-ZnO and Fe doped ZnO as shown in fig. S2. As well known that ZnO wurtzite structure has eight sets of characteristic optical phonon modes like A1 and E1. These polar branches split into transversal optical (TO) and longitudinal optical components with different frequencies. The low- frequency mode E2L associated to the heavy Zn sub lattice and high frequency mode
E2H
involving only oxygen atom [39]. The peak at 436
cm−1 was ascribed to the non-optical phonon E2H mode, which confirmed the wurtzite structure of ZnO [40]. The weak peaks at 329 and 377 cm−1 were ascribed to multiphonons scattering process E2H - E2L and A1 phonon mode, respectively. The A1 LO and E2L (580 cm−1) phonon mode usually assigned to oxygen vacancies and Zinc interstitials in ZnO materials. 3.3. XPS analysis The undoped and Fe doped ZnO nanoellipsoids samples distribution and oxidations state was found out by using the XPS analysis. In fig. 4 shows the highresolution XPS spectra of Zn 2p, O 1s, and Fe 2p of undoped ZnO and Fe-doped ZnO. In fig. 4a, two strong peaks at 1047.06 and 1024.35 eV correspond to the binding energies of Zn 2p1/2 and Zn 2p3/2, respectively, which confirms that Zn in Zn2+ oxidation state of wurtzite ZnO structure [41]. In addition, the observed spin-orbit splitting between Zn 2p1/2 and Zn 2p3/2 is about 23 eV. Fe-doped ZnO showed a slight shift towards lower binding energy in Zn 2p compared to the undoped ZnO, which clearly indicated the existence of Fe2+ in the Fe-doped ZnO synthesized samples. Fig. 4b shows the XPS spectra of Fe 2p, the peak at 711.8 and 725.1 are attributed to the binding energies of Fe 2p3/2 and Fe 2p1/2, respectively [42]. No additional peaks corresponding to metal or particles are obtained in any spectra. Fig. 4c shows the XPS spectra of O 1s, the peak positioned at 530.9 eV can be attributed to O2− ions that are oxygen deficient regions in the wurtzite structure of ZnO [43]. The Gaussian fitting results indicate that the XPS signals at 532.6 eV indicate that higher amount of surface hydroxyl group and the presence of hydroxyl group facilitates the trapping of electron and holes which enhance the sensitivity in gas sensors [44]. In contrast, Fe-doped ZnO (8 wt%) sample showed the broad peak at 531.4 eV which implies that higher oxygen deficiency in doped sample compared to the undoped ZnO (fig. 4d). XPS analysis clearly reveals that oxygen vacancy increased compared to undoped sample which will enhance the gas sensitivity. 3.4 N2 adsorption and desorption analysis 7
To obtain further information of the surface area and pores nature of the undoped ZnO and Fe-doped ZnO, Brunauer-Emmett- Teller (BET) N2 adsorption-desorption analysis was performed. The adsorption-desorption of undoped ZnO and Fe-doped ZnO nanoellipsoids as observed in the fig. S3 (a, b). The BET specific surface area of the 2% and 8% FZO nanoellipsoids was calculated to be 22 and 51 m2/g from nitrogen adsorption isotherm. And the pure sample of ZnO shows the BET specific surface area is about 18 m2/g from the nitrogen adsorption [45]. So, among these 8 % FZO nanoellipsoid confirmed the high surface area it’s may be due to the effect of the Fe ions in the Zn ions and it is helpful to adsorption of target gas hence to increase the sensitivity of the target gas at low operating temperature. 3.5 NO2 sensing studies In metal oxide gas sensing application operating temperature is the main parameters to detect the target gases. Hence, some of the scientist working on to reduce the operating temperature of the metal oxide and to increase the sensitivity as well as selectivity of metal oxides. Here, to found out the operating temperature of ZnO nanorods authors are very the temperature from 100 C to 250 C at fixed concentration of NO2 gas. The gas response of 8% FZO showed the 209.8 response for 100 ppm NO2 gas concentration which is higher than of the undoped ZnO. In same time authors have studied the cross sensitivity of the 8% FZO nanoellipsoids using different oxidizing and reducing gases. Moreover 8% FZO shows highly sensitive and selective to NO2 gas at low operating temperature which is shown in fig. 5a, b. Also at that operating temperature all doping sample of FZO at fixed amount of NO2 gas concentration was checked then out of these doping samples 8% FZO showed maximum response of NO2 gas because of its extraordinary property in FESEM which adsorb the more NO2 gas and suggested to increase the gas response is shown in fig. 5c. Among these 150 C shows maximum gas response towards 100 ppm NO2 gas concentration as shown in fig. 5d, therefore the 150 C is the operating temperature of ZnO nanorods because of this temperature assists to overcome the energy barriers and activate the surface of ZnO nanorods after adsorption of NO2 gas and hence to increase the gas response Furthermore, we have studied about the linearity of the 8% FZO thin films and it is observed that at gas concentration increases from 10 to 100 ppm of NO2 gas there is increases the gas response hence FZO thin film shows linearity which is shown in fig. 8
S4a. In addition, fig. 9b shows the response time and the recovery time of 8% FZO thin films. Fe-doped ZnO shows fast response towards NO2 gas, the response time decreased from 49 to 6 seconds (s) for 10-100 ppm of NO2 concentration. This trend confirmed the expected trend as decrease in response time as concentration of NO2 increases, due to the availability of higher number of NO2 molecule interacts with sensing surface. But the recovery time of Fe-doped ZnO was found to be 28- 240 s for different concentration of NO2, which might be due to prolonged desorption of NO2 on the surface of the sample. The stability of thin film was recorded for 30 days and reproducible capability of sensing element towards 100 ppm. Fig. S4(c, d) clearly reveals the reproducibility and long-term stability towards NO2 gas. Fig. S4 e represents the gas sensitivity of device at different humidity. The results indicated that increase in humidity reduced the sensitivity of the device. 3.6. Gas sensing mechanism Metal oxide gas sensors are based on the change in resistance of materials due to the chemical and electronic interaction between the gas and sensing matrix. The chemical interaction involves the adsorption of the target gas molecules on the surface, subsequent reaction with oxygen species adsorbed on the surface of ZnO which results in the detection of target gas [38, 46-49]. ZnO films exposed to atmosphere air, the oxygen molecules adsorb on the surface of the ZnO materials to form chemisorbed oxygen anions ― (𝑂2(𝑎𝑑𝑠) ) by capturing electrons from the conduction band, resultant in the formation of
depletion region known as space charge layer. This in turn results in increase in resistance of ZnO [17]. ― 𝑂2 + 𝑒 ― ↔𝑂2𝑎𝑑𝑠
(1)
ZnO films exposed to NO2 oxidizing gas, NO2 gas directly absorbed on the surface of ZnO and subsequently it captures one more electron from the conduction band this in turn leads to chemisorption of NO2 as 𝑁𝑂2― . This results in decrease in majority of charge carriers and increase in the resistance [41]. In addition, adsorbed monomolecule oxygen dissociated when NO2 approaches to the adsorbed site. Monomolecule of oxygen is dissociated and chemisorbed as atomic ionic species due to this the electron concentration decreases in the surface layer of ZnO, which leads to increase in the resistance of ZnO sensor [39, 49]. 9
― 𝑁𝑂2(𝑔𝑎𝑠) + 𝑒 ― →𝑁𝑂2(𝑎𝑑𝑠)
(2)
― ― + 2𝑂(𝑎𝑑𝑠) 𝑁𝑂2(𝑔𝑎𝑠) + 𝑂2― + 2𝑒 ― →𝑁𝑂2(𝑎𝑑𝑠)
(3)
Incorporation of Fe into ZnO showed enhanced sensing response towards NO2 gas. The possibility of getting good response may due to the formation of p-n junction. In formation of p-n junction minority charge carriers in Fe are lesser and they require less energy to move to the conduction band compared ZnO. This in turn increases the sensing response. 4. Conclusion We report a simple hydrothermal method for the controlled growth of flower-like hexagonal nanorods and nanoellipsoids. FE-SEM image clearly indicated the transformation of nanorods to nanoellipsoids on incorporation of Fe. Brunauer-EmmettTeller (BET) N2 adsorption-desorption analysis indicated that Fe-doped ZnO (8 wt%) has larger surface area compared to undoped ZnO. The effect of doping concentration and operating temperature on the gas response of NO2 is analyzed. Higher gas response was observed for Fe-doped ZnO nanoellipsoids 8 wt% compared to other doping concentration. The maximum sensitivity of Fe-doped ZnO sensor was around 209.8 compared to the undoped ZnO. Fe-doped ZnO sensor showed rapid response, reproducibility and stability towards NO2 gas at low temperature of 150 C. Fe-doped ZnO showed rapid response but the recovery time of the sensor is relatively extended. The response time decreased from 49 to 6 seconds (s) for 10-100 ppm of NO2 concentration. Fe-doped ZnO sensor showed good response even at lower detection of 10 ppm. Acknowledgements R. Sankar Ganesh thanks MEXT-Japan for award of a research fellowship. We would like to thank Prof. Murakami, Mr. T. Koyama and Mr. W. Tomoda, Center for Instrumental Analysis, Shizuoka University, Hamamatsu, Japan. 5. Reference [1]
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List of figure caption Fig . 1 XRD patterns of Fe-doped ZnO (2, 4, 6, 8, 12 wt%) nanoellipsoids and undoped ZnO nanoflowers. Fig. 2 FE-SEM micrograph (a). undoped ZnO, (b). 2 % FZO, (c). 4 % FZO, (d). 6 % FZO, (d). 8 % FZO, (e). 10 wt % FZO. Fig. 3 TEM and HRTEM image of (a, b, c). ZnO nanoflowers, (d, e, f). 6 wt %. (g, h, i). 8 wt% Fe-doped ZnO. Fig. 4 XPS analysis of pure and Fe-doped ZnO (a). Zn 2p of undoped and Fe-doped ZnO (8 wt%), (b). Fe 2p core-level spectra of FZO (8 wt%), (c). O 1s of ZnO nanoflower, (d). O 1s of CZO (6 wt%). Fig. 5 (a, b). Selectivity nature of the ZnO and FZO thin films at 100 ppm of various target gases, c). Sensing response of FZO (2, 4, 6, 8, 10 wt%) thin films at 100 ppm of ammonia, d). Response 8 % FZO at different temperature
17
List of figures
Figure 1
18
Figure 2
19
Figure 3
20
Figure 4
21
Figure 5
22
Highlights
Fe-doped ZnO (8 wt%) exhibited the highest response of 209.8
The response time decreased from 49 to 6 seconds(s) for 10-100 ppm of NO2 concentration
BET analysis showed higher surface area compared undoped ZnO.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
1. The research article is original. 2. The article has been written by the stated authors who are ALL aware of its content and approve its submission. 3. The article has not been published previously 4. The article is not under consideration for publication elsewhere 5. No conflict of interest exists, or if such conflict exists, the exact nature must be declared. 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. This research article consists of a main text part, figures, and tables which are uploaded in the web. Thank you very much for your consideration of the manuscript.
Sincerely yours R. Sankar Ganesh
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