Synthesize of ZnO Nano Structure for Toxic Gas Sensing Application

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ScienceDirect Procedia Computer Science 92 (2016) 199 – 206

2nd International Conference on Intelligent Computing, Communication & Convergence (ICCC-2016) Srikanta Patnaik, Editor in Chief Conference Organized by Interscience Institute of Management and Technology Bhubaneswar, Odisha, India

Synthesize of ZnO Nano structure for toxic gas sensing application Argha Sarkara, Santanu Maityb,*, Pinaki Chakrabortyb, Swarnendu Kr. Chakrabortya a

a

Department of Computer Science & Engineering, National Institute of Technology, Arunachal Pradesh-791112 Department of Electronics and Communication Engineering, National Institute of Technology, Arunachal Pradesh-791112

Abstract Gas sensors having zinc oxide (ZnO) nanoparticles as a sensing layer are consumes low power, operates at low temperature, have great level of selectivity as well as inexpensive too. Development of nanostructured metal oxide semiconductor (MOS) for quality sensing is important. So the affect of grain size and thickness is on the sensing materials (MOS) are discussed. A simple and feasible way to synthesis zinc oxide nano particles at 900C temperature is followed. Basic requirements of chemical of this process is zinc powder, hydrogen per oxide (H2O2), and acitic acid (CH3COOH).The synthesized ZnO is deposited on silicon wafer and the characterization is done through Scanning electron microscopy (SEM) and X ray diffraction (XRD). © 2016 2014The TheAuthors. Authors. Published Elsevier © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of scientific committee of Missouri University of Science and Technology. Peer-review under responsibility of the Organizing Committee of ICCC 2016 Keywords: Zinc Oxide, nano particle, gas sensor, metal oxide semiconductor

1. Introduction Nano sized particles of semiconductor materials such as ZnO, SnO2 or WO3 have caught the much more interest in recent years because of their excellent sensing properties like resistivity or conductivity change in presence of target gas molecules, response and recovery time. [1-5] Zinc oxide (ZnO) is a semiconductor which belongs to the

1877-0509 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Organizing Committee of ICCC 2016 doi:10.1016/j.procs.2016.07.346

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group (I) – (VI) family and has extensive applications due to its direct band gap (3.37 eV) and large excite binding energy of 60meV. [6] ZnO is basically n-type in nature. Chemically, it is non toxic. [7] Some key features based on which gas sensing applications are done are, high sensitivity to various oxidizing and reducing gasses, easy to fabricate, and miniaturized as well. [8] Zno is fabricated as nano structured like nanowire, nanorods, nanobelts and 2D structure like nanoplates. High surface to volume ratio , small size of ZnO are the main reason behind detecting with a wide range of concentration by lowering the limit of detection. [9] It is reported that, the adsorption of gas molecules, primarily considered with the chemisorptions of the O2 on the surface causes the change in electrical properties of the metal oxide semiconductor. Here Zno is considered as the metal oxide semiconductor. The toxic gasses like CO,CH4 are made contact with ZnO gas sensor at high temperature, the chemisorbed oxygen stars reaction with the reducing gasses. Electrons are injected into the MOS crystallites of the oxygen as well as the height of the potential barrier at intergranular contacts in figure1.

Fig.1 Intergranular contacts of gas sensing layer

Chemical Reactions: Electrons are extracted through the following reactions,

ZnO  O  e  o ZnO  O 

ZnO  2O  e  o ZnO  O2



As a result resistance is changed increasingly. Electrons are injected when reducing gas molecules (G) react with the chemisorbed oxygen at grain boundary.

G  ZnO  O  o ZnO  GO  e  As a result, resistance changes decreasingly. Similarly for detecting a gas which is oxidizing in nature, the reaction may follow-

ZnO  O  O  e o ZnO  O2



Result is that the resistance is increased even further. Thus by observing the change in resistance or conductivity, the toxic gasses present in atmosphere can be detected. [10]

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Nomenclature

D L ( nt ) ( nb )

Grain diameter Thickness of the depletion layer Density of electrons trapped at the surface Electron density in the bulk material

2. Sensing material: Grain size and thickness For choosing a sensing material the affect of grain size and film thickness is very important. Here in figure 2. Energy band model of MOS powder is shown. The electrons from the depletion layer (Debye Layer) are captured by the chemisorbed oxygen. A few grains are depicted here in figure 2. A formidable barrier is formed at the intergranular contact, which restricts electron to cross and thus the rise in resistance values at those contact points. In most of the polycrystalline materials the grain diameter (D) is usually greater than the thickness of the depletion layer (L). Normally the value of L is 100 nm for MOS films. [11] Sintering of material is very important in the context of sensitivity. Sintering should not be done too much so thatporosity reduces and it becomes compact neck and grain contacts are seen in sintered films.

Fig.2 Schematic presentation of grain size effect

It was reported that when D ! 2 L , it is under grain boundary control where higher resistance is seen at grain boundary contact. Grained boundary controlled is desirable for relatively large particle MOS sensors. D # 2 L , it is under neck control and moderate resistance. But for D  2 L the grain resistance dominates other resistance and grains control the sensitivity. [12] These three conditions are depicted in Figure2. The trapped charge density of MOS depends on the grain diameter D .

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Charge density

f

1 D

The grain size can be made smaller, even below of the thickness of depletion layer which will result extraction of electrons. [13] Such nanostructure should be fabricated for gas sensor materials with uniform changing resistance upon different concentration of gasses. The density of electrons trapped at the surface (nt) and electron density in the bulk material (nb),

eVs

e 2 nt

2¦o

¦ ¦ o

r

permittivity, then the grain boundary potential barrier, [14]

2

(1)

¦n r

b

The equation 1 helps to find out the mathematical analysis of finding resistance of MOS layer like ZnO. The grain size, thickness are the main factors regarding the performance of gas sensor. We need a thick flim of lightly sintered nanocrystalline material. [15]

3. Working principle The important function is the change in electrical conductivity of heated ZnO layer in presence of toxic gasses. ZnO is n-type semiconductor which operates which operates at temperature 2000C or more than that. So fabrication of heater and interdigited electrode is necessary for the activation of sensing layer. [16] That implies the formation of electron depletion by atmospheric oxygen. It adsorbs on surface and collects electrons from Zno films to form reactive O - when the toxic gasses make contact with that oxygen species at the sensing layer surface,the thickness of depletion layer changes. It causes measurable changes in resistance of the material.For reducing gasses reduction of depletion layer and for oxidizing gasses the reverse affect is seen in depletion layer. [17] The changes in depletion layer thickness create a noticeable change in overall resistance and thus it becomes easy to sense gasses through conductivity or resistivity.

Fig.3 Schematic representation for gas analyzing set up

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4. Experimental setup All chemicals with high level of purity are used. The chemicals required for this process is Zn powder, Hydrogen per oxide (H2O2), and acitic acid. In a typical reaction, 1.3 g of Zn powder is first added to a biker. Water (60ml) and 30% (60ml) H2O2 solution is made and the solution is added to this biker. The solution is stirred continuously. Here magnetic bit is used for siring purpose. This solution is kept on heater at 90 0C for 5 hrs. Water to H2O2 ratio is 1:1 (v/v) for all the reactions. Initially the colour of the solution seems to grey. After 40 minutes acitic acid is added to the reaction mixture. The continuous stirring is done all through the process. A colour change is seen after 35 minutes of adding catalyst. Initially a white layer is formed at top. It is found that at 90 0 C during the initial stage of reaction, primarily ZnO2 is formed a long with ZnO, and the ZnO2 later converts to ZnO for longer duration reaction. After few hours a white coloured precipitates are formed in the reaction solution it is the indication of forming ZnO.

Chemical reaction: Catalyst 3Zn  2H 2O2 ' o ZnO2  2ZnO  2H 2  H 2O Catalyst ZnO2  H 2  H 2 O ' o ZnO  2H 2 O

Fig4 Schematic experimental set up for ZnO nanopaletes

5. Result and discussion The synthesized ZnO is then deposited in the silicon wafer for SEM and XRD purposes. Silicon wafer is first cleaned in sanicator and the solution of ZnO and Ethanol or Isopropyl alcohol (IPA) is uniformly deposited in the wafer by spin coater. Finally, wafer is made for XRD and SEM analysis.

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XRD pattern of synthesized ZnO is shown in Fig.6.The pattern shows distinct peaks related to the newly formed nano structure of the ZnO, and a very low intensity peak corresponding to ZnO2. Sharp peaks mean good crystal growth. Here sharp peak at (001) is seen and it seems to be a good sensing properties. It is found that line widths of the ZnO related peaks in bothcases are almost identical. Narrow line width with high intensity of the spectra indicates the formation of good crystalline NPs. The crystallinity of the product varies with growth span and reaction temperature. It is found that crystallite size grows with increasing reaction time at an appropriate temperature. Initially ZnO2 peak is prominent but with time it decreases and the peak corresponding to ZnO starts increasing. The line width of XRD pattern decreases with increasing reaction time as a result the size of NPs stars growing. It was reported that ignoring the effect of anisotropic strain on the XRD line shape broadening and using Scherrer formula the average size of ZnO crystallites can be obtained at different instant of the reaction time. [18] In Fig.6 shows SEM images of nanoparticles where the EHT 7.00kV, working distance 8.1 mm and magnification 9.37KX.

Intensity

(101)

Intensity (Au)

100

(002)

(102)

(110)

50

0

20

40

60

2*Theta (Degree) Fig5 XRD pattern of synthesized ZnO for (002), (101), (102) and (110) orientation

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Fig.6 Scanning electron microscopic image of synthesized ZnO nano structures

6. Conclusion Zinc oxide nanoparticles are synthesized using the easy and simple method at comperatively low temperature and normal atmospheric pressure. Tha sharp peak in XRD analysis indicates the formation of ZnO in the above method. SEM image has characterized the synthesized zinc oxide nano particle. References 1. Wan, Q.; Li, Q.H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L., Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, , Appl. Phys. Lett. 2004, 84, 3654−3656. 2. Li, C. C.; Du, Z. F.; Li, L. M.; Yu, H. C.; Wan, Q.; Wang, T. H., Surface-depletion controlled gas sensing of ZnO nanorods grown at room temperature, Appl. Phys. Lett. 2007, 91, No. 032101. 3. Kim, Y. S.; Ha, S. C.; Kim, K. W.; Yang, H. S.; Choi, S. Y.; Kim, Y. T.; Park, J. T.; Lee, C. H.; Choi, J. Y.; Park, J. S.; Lee, K. Y., Synthesis and room-temperature hydrogen sensing properties of polycrystalline sno2 coating on multi-walled carbon nanotubes, Appl. Phys. Lett. 2005, 86, No. 213105. 4. Yu, H. Y.; Kang, B. H.; Pi, U. H.; Park, C. W.; Choi, S. Y.; Kim, G. T., Zinc Oxide Nanostructure Thick Films as H2S Gas Sensors at Room Temperature, Appl. Phys. Lett. 2007, 86, No. 253102. 5. Feng, P.; Wan, Q.; Wang, T. H., Contact-controlled sensing properties of flowerlike ZnO, Appl. Phys. Lett. 2007, 87, No. 213111. 6. R.N.Viswanath, S Ramasamy, R . Ramamoorthy, P. Jayavel, t. Nagarajan, preparation and characterization of nanocrystalline ZnO based materials for varistor applications, Nanostruct.Mater.6 (1995) 993-996) 7. Ismail, B.; Abaab, M.; Rezig, B., Structural and electrical properties of ZnO films prepared by screen printing technique, Thin Solid Films 2001, 383, 92− 94. 8. Yi Zeng; Tong Zhang; Mingxia Yuan; Minghui Kang; Geyu lu; Rui Wang; Huitao Fan; yuan He; Haibin Yang;, Growth and selective acetone detectio based on ZnO nanorod arrays,Sensors and Actuators B:Chemical,143 (2009),93-98 9. Sugato Ghosh; Chirasree RoyChaudhuri; Raghunath Bhattacharya; Hiranmay Saha; Nillohit Mukherjee;, Palladium−Silver-Activated ZnO Surface: Highly Selective Methane Sensor at Reasonably Low Operating Temperature, Applied materials & Interfaces,2014,3879-3887 10. Swati Sharma; Marc Madou;, A new approach to gas sensing with nanotechnology, Phil. Trans. R. Soc. A 370, 2448-2473 11. Madou, M. & Morrison, S. R. Chemical sensing with solid state devices. New York, NY: Academic Press, 1989 12. Xu, C., Jun Tamaki, J., Miura, N. & Yamazoe, N. , Grain size effects on gas sensitivity of porous SnO2-based elements. Sens. Actuat. B 3, 1991 , 147–155. (doi:10.1002/anie.200903801)

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