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Synthesis and Characterization of ZnO Nano Discs Using Wet Chemical Method for Sensing Applications
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 10763–10770
www.materialstoday.com/proceedings
ILAFM2016
Synthesis and Characterization of ZnO Nano Discs Using Wet Chemical Method for Sensing Applications Pratima Bhata, Uma Ullas Pradhanb, Naveen Kumar S.Ka* a
Research Scholar, Department of Electronics, Mangalore University, Mangalore-574199, India b Department of Electronics, Mount Carmel College, Bangalore - 560052, India
Abstract Fabrication of novel sensing devices using nano structured semiconducting metal oxide (SMO) is gaining prominence in research in recent years due to its notable properties and reduced cost. Among various SMO like ZnO, SnO2, TiO2, In2O3 etc., ZnO is one of the most promising nanomaterial due to its unique properties and when synthesised in various size and morphology can be used for various applications. In this paper, we report the synthesis and characterization of well aligned ZnO (Zinc Oxide) nano discs using wet chemical process. Wet - Chemical synthesis has emerged as one of the most promising technique towards highyield and mass production of nanoparticles of desired particle size, shape and size distributions. The synthesized nanoparticles were characterized for its optical characteristics using UV-Visible spectrophotometer, for structural and compositional studies by XRD, FESEM and EDX. CV (Capacitance–Voltage) characteristics on ZnO based MOM (Metal oxide Metal) and MOS (Metal oxide Semiconductor) structure were studied using Impedance analyser. The XRD showed the peak intensity as 4270.23 at 2θ equal to 36.2095º with FWHM (Full Width Half Maximum) as 0.2453. EDX results showed the sample in pure ZnO phase. The images obtained from FESEM showed a uniform hexagonal disc type structure of approximately 100 nm in diameter and 30-40 nm in thickness. UV absorption peak was observed at 357nm. Sensing property of ZnO highly depends on its size and structure. C-V curves of ZnO based MOS contact showed 3 prominent regions: accumulation, depletion and inversion. C-V curves of MOM contact at 500 Hz behaved like a tunable capacitor. This variation of capacitance with respect to applied voltage exhibited by the MOM sample can be used for biomedical sensing applications to harness bio potential signals generated by the human body. These bio potentials can be sensed using surface electrodes which can show proportional varying sensing parameters like surfaces resistance, capacitance etc. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible
Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016. Keywords:SMO; sensing;morphology; MOM; particle size; wet chemical synthesis. * Dr. Naveen Kumar.S.K. Tel.: +91-9448318252. E-mail address: [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016.
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Pratima Bhat.et.al / Materials Today: Proceedings 5 (2018) 10763–10770
1. Introduction There is a great demand for the development of sensitive, fast, reliable, low power and low cost chemical sensors for health and safety applications [1]. Structure of a material and the impact of its macroscopic properties are very essential parameters towards any practical applications. Semiconducting metal oxide (SMO) materials, having high drifts in their conductivity, are in great demand as gas sensors[2, 3] because of their small size, simple construction, low weight, low cost and low power consumption[4]. SMOs when synthesized in nano scale can be classified into three groups: 0–dimensional, 1-dimensional and 2-dimensional, where 0-dimentional nanostructure are also referred to as quantum dots[5]. 1-dimentional nano structures [4] like nano rods, nano wires, nano tubes, nano discs are highly useful for gas sensing applications as it offers high aspect ratio [6]. There are various semiconducting metal oxides which have applications as gas sensing materials like SnO2 (Tin dioxide), WO3 (Tungsten trioxide), ZnO (Zinc oxide), TiO2 (Titanium dioxide), In2O3 (Indium Oxide) etc. Among these ZnO, II-VI group element and n type of transition semiconductor has been extensively studied as gas sensors as it has excellent chemical sensitivity and stability towards different adsorbed gases [7]. It influences the adsorption of gases by varying the doping level [8] and is non-toxic and low cost. Also ZnO has direct and wide band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature [9]. They can be used in the applications as VOCs (Volatile organic compounds) detection sensor for disease monitoring [2, 10], anti-bacterial activity[11], food processing, for air quality monitoring, functional devices such as energy efficiency devices (solar cells) [12], photo-catalysts, thin film transistors [13], as flexible electrode in optoelectronic and organic luminescent devices (OLED)[14], biosensors, piezoelectric devices etc. ZnO when synthesized in nano dimension (< 100nm), creates large surface to volume ratio, thus offers highly increased sensitivity towards specific gas [16]. The classification of synthesizing techniques as shown in Fig. 1 which yields ZnO in nano-range in the form of colloids, clusters, powder, tubes, rods, wires, thin films, flowers [17] etc. Among the below mentioned methods, shown in Fig. 1, chemical methods have more performance than physical methods in controlling the particle size and morphology at lower temperature (< 350º C).
Fig. 1. Different methods of ZnO Nanoparticle synthesis
The methods that have been established to synthesize different kinds of ZnO nanoparticles with controlled crystalline phases, sizes and shapes, includes co-precipitation [18], thermal decomposition [19], hydro (Solvo) thermalsynthesis [20] and sol-gel process [21]. Other chemical methods for the synthesis of zinc oxide have been reported in various references [17,22]. Among these “Wet chemical Synthesis” is a simple and inexpensive technique which yields good quality nanoparticles [17]. In this technique materials can be obtained in the form of liquid (gel) or can be converted into dry powder or thin film easily in large quantities.This paper aims at preparing ZnO particles using wetchemical process and studying its morphology, structure, optical and electrical (C-V) behavior using FESEM and EDX, XRD diffraction, U-Vis spectrophotometry and impedance analyzer respectively.
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2. Synthesis Procedure Synthesis of ZnO nano structure is done using wet chemical synthesis. 2.1. Material Used Zinc acetate dehydrate (Zn (CH3COO) 22H2O), Potassium hydroxide (KOH) and Ethanol (CH3 3CH2 OH) of analytical grade were used from Merck (Bangalore, India). Double distilled water was used for the preparation of solutions. 2.2. Synthesis Procedure ZnO nano structures were synthesized by wet chemical process in alcohol media with zinc acetate dehydrate and potassium hydroxide as starting precursor taken in 1:20 molar ratio. During the synthesis, 2.195g of zinc acetate dehydrate [Zn (CH3COO)2 2H2O] was mixed in 100 ml ethanol and stirred for 15 minutes using magnetic stirrer in ambient atmosphere. 1.122 g of Potassium hydroxide (KOH) mixed with 10 ml of double distilled water were added drop wise to the above solution under continues stirring for 30 minutes till Ph.was maintained to 7. A milky white solution in a jelly form was obtained which was further heated at 90º C for 3 hour at muffle furnace. The resulting suspension was centrifuged to retrieve the product and was washed 3-5 times with ethanol to remove the impurity. Later it was dried in the furnace at 70ºC for 24 hours to obtain powder [22]. 3. Characterization And Interpretation Characterization of ZnO is done to study various parameters like particle size, lattice parameter, optical energy band gap, peak absorption etc. so as to use it for the specific application. 3.1. XRD The ZnO particles were characterized by X-ray diffraction using Pan analytical Xpert Pro, TESCAN WEGA with Cu Kα radiation of λ = 1.5405 Å for the 2θ range from 10º-80º. The result was plotted using origin61 software into a graph as shown in Fig. 2. The highest peak intensity was observed at 2θ = 36.2095º and corresponding intensity from the figure is observed to be 4270.23.The highest peak correspond to the Miller indices (101) for Bragg’s angle 36.21°.The other indices observed are ( 100), (002), (102), (110), (103), (200), (112), (201) and (202) for diffraction angles 31.75°, 34.38°, 47.49°, 56.52°, 62.82°, 66.305°, 67.86°, 68.97° and 76.89° respectively. All the diffraction peaks were consistent, sharp and match with the standard card Joint Committee on Powder Diffraction Standards (JCPDS36-1451) related to the hexagonal wurtzite structure. The lattice parameters ‘a’ and ‘c’ of hexagonal phase were calculated using equations (1) and (2) given below [15]:
a
c
3sin
sin
(1)
(2)
Where λ is the wavelength of the incident radiation, = 1.5405 Å and θ is the Bragg’s angle. The values of the lattice parameters thus calculated were found to be, a = 0.286 nm, c = 0.495nm and c/a = 1.73. The crystal size of the ZnO particle is obtained using Debye-Scherrer’s equation:
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D
(3)
d cos
Fig. 2. XRD reading for ZnO Nano-Disc
Where D is the crystallite size (nm), λ is the wavelength of incident X-ray (= 1.5405 Å), βd is the FWHM (full width at Half maximum), and θ is the diffraction angle at maximum intensity and k is a constant =0.89. From the obtained XRD reading, 2θ= 36.2093, θ= 18.1046, FWHM =0.2453. The calculated size D = 76 nm was found to be within the observed range. 3.2. FESEM and EDX Structural morphology of the ZnO nano structure were analysed through FESEM (Field Emission Scanning Electron Microscope) as shown in Fig.3, using ZEISS.
Fig. 3. FESEM images for ZnO Nano Discs.
A uniform hexagonal disc type ZnO structure with a diameter of approximately 100 nm and thickness of 30-40 nm is observed.The obtained ZnO nano structure is in great demand due to its large aspect ratio and thus plays a critical role in sensing low concentration of the gas analyte and VOCs present in exhaled breath. EDX (Energy Dispersive X-Ray Spectroscopy) analysis is done to study the elemental composition of the sample using Oxford Instruments, ZEISS (Fig. 4). The graph shows the sample prepared by the above route has pure ZnO phases. The stoichiometric ratio of the sample is shown in Table 1.
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Table 1. Stoichiometric Ratio of sample (From EDX result). Element
Weight%
Atomic%
Compound %
O(K shell)
0.84
44.23
1.15 1.00
Zn(L shell)
2.60
33.66
C
0.31
22.11
Total
3.75
Fig. 4. EDX data for ZnO Nanoparticle.
3.3. UV-Visible Spectrophotometer Optical parameters were studied with the help of UV – Vis spectrophotometer; UV-1700 Pharma Spec, from SHIMADZU. The obtained result was plotted using origin61 software as shown in Fig.5.
Fig. 5. UV-Vis spectrophotometry images for ZnO Nanoparticle
The sample showed maximum absorbance at 357 nm wavelength. For the optimum sample, band gap energy was calculated using equation (4) [22].
E (eV )
hC * 1.6 * 10 19
(4)
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Where E is band gap energy in eV, h is Plank’s constant = 6.626* 10-34 Joules sec, C is the speed of light = 3*108m/sec and λ is the wavelength with peak absorption and 1.6*10-19 is the conversion factor. The band gap energy was found to be 3.48 eV. 4. Electrical Characteristics 4.1. Thin film synthesis for CV measurement A thin porous layer of synthesized n type of ZnO nano discs were coated on two substrate-(i) Aluminium substrate and (ii) p type of silicon substrate using dip coating method[23] to obtain a MOM (Metal oxide Metal) structure and MOS (Metal oxide Semiconductor) structure respectively. These substrates were dipped in the ZnO nano disc suspension for 30 minutes and dried in the ambient temperature for 24 hours. On the obtained ZnO nano disc coated substrate, silver electrodes were formed to perform CV (Capacitance-Voltage) measurements. CV measurement were done on both the samples using IM 3570 Impedance Analyzer and results were plotted using origin61 software as shown in Fig. 6 (a)and (b), Fig. 7(a) and (b). 4.2. Capacitance- Voltage graphs of MOS Structure: A MOS (Metal oxide semiconductor) structure at 100 Hz frequency is established and studied as seen in Fig. 6 (a).
Fig. 6. (A) C –V variation of –MOS structure at F= 500 Hz. (B) C-F variation –MOS structure
This MOS capacitance is highly dependent on bias voltage. The variation is due to the modulation of the width of the surface charge layer density formed by the field and this is extremely useful in the evaluation of the electrical properties of oxide–silicon interface. Three regions of interest in the characteristics of the MOS capacitor namely accumulation, depletion and inversion is formed as shown in the Fig.6(a). With negative voltage applied at electrode on ZnO layer, the concentration of holes from p-Si substrate will form a second electrode of a parallel plate capacitor with first electrode at the ZnO layer. This increases the charge density and thus capacitance increases and this region is termed as accumulation. Since the accumulation layer is an indirect ohmic contact with the P-type substrate, the capacitance of the structure under accumulation conditions highly depends on the thickness of the (ZnO) oxide layer.[7, 28] As this voltage is made positive, holes are repelled and a region is formed at the surface which is depleted of carriers. This region is called depletion region. Under depletion conditions, the Fermi level near the silicon surface will move to a position closer to the centre of the forbidden region. This voltage is called as flat band voltage. This voltage is nearly 0.5 V. Increasing the positive voltage will tend to increase the width of the surface depletion region and the capacitance from the gate (ZnO layer) to the substrate (p-Si) associated with MOS structure will decrease.
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With further increase in the positive voltage beyond threshold voltage, dynamic carrier generation and recombination take place which causes a region called as Inversion region. The build-up of inversion layer is a threshold phenomenon which is a frequency dependent region. In this inversion region, the capacitance increases as the frequency decreases and vice versa. [26] The variation of the capacitance for different frequencies has been studied for -ZnO based MOS structureand is plotted as shown in the Fig. 6(b). 4.3. Capacitance- Voltage graphs of MOM Structure: A MOM ( Metal oxide Metal) contact at 500 Hz frequency is established and studied as shown in Fig. 7 (a). From the Fig.7 (a) it is clear that, the MOM structure is very sensitive to the applied voltage and is acting like a tunable capacitor. Such capacitor can be considered as varactor diode whose capacitance varies with applied voltage.This MOM structure as capacitor can also be used as sensing element in biomedical sensing applications to harness bio potential signals generated by the human body.
Fig. 7. (A) C –V variation of –MOM structure at F= 500 Hz. (B) C-F variation –MOM structure
Variation of Capacitance for different frequency for MOM structure is studied and it is plotted as shown in the Fig. 7(b). It shows that capacitance exponentially decreases as frequency increases. 5. Conclusions In summary, a uniform hexagonal ZnO nano disc structure of approximately 100 nm were successfully synthesized using wet chemical synthesis at 85º -90ºC. Its optical and electrical characterization was studied. Band gap energy was found to be 3.48 eV. The obtained ZnO nano structure is in great demand due to its large aspect ratio, plays a significant role in sensing low concentration of the gas analyte and VOCs present in exhaled breath. Capacitance-Voltage variation curves of ZnO nanoparticle on p-type Si substrate forming a MOS structure shows 3 prominent regions- accumulation, depletion and inversion. The study of C-V variation of ZnO on Aluminium substrate forming a MOM structure acts as a tunable capacitor. The MOM structure acts as a capacitor that can be used as sensing element in biomedical sensing application to harness bio potential signals generated by the human body. Thus ZnO nanoparticle semiconducting metal oxide can be used as a wearable sensor. References [1] Po-Chiang Chen, Guozhen Shen and Chongwu Zhou, IEEE Transaction on Nanotechnology, 7 (6), (2008) 668-679. [2] Pratima Bhat, Dr. Uma Ullas Pradhan and Dr. Naveen Kumar. S.K, Pariprashna Academic Journal, 2, (2016)330-337. [3] Elena Dilonardo, Michele Penza, Marco Alvisi, Cinzia Di Franco, Francesco Palmisano, Luisa Torsi and Nicola Cioffi, Beilstein Journal of Nanotechnology, 7(2015) 22-31. [4] Jin Huang and Qing Wan, Sensors-Review, 9(2009) 9903-9924. [5] Vladimir A. Fonoberov and Alexander A. Balandin, Journal of Nano electronics and optoelectronics, 1, (2006) 19-38.
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[6] Guozhen Shen, Po-ChiangChen, Koungmin Ryu and Chongwu Zhou, Journal of Material Chemistry, 19, (2009) 828-839. [7] Argha Sarkar, Santanu Maity, Pinaki Chakraborty, Swarnendu Kr. Chakraborty, Procedia Computer Science, 92 (2016), 199-206. [8] Niranjan Ramgir,Niyanta Datta, Manmeet Kaur, S. Kailasaganapathi, A. K. Debnath, D.K Aswal and S. K. Gupta, Article, BARC NEWSLETTER, 325, (2012) 28- 33. [9] Anderson Janotti and Chris G Van de Walle, IOP Publishing, Rep, Prog. Physics, 72, (2009). [10] Pratima Bhat, Uma Ullas Pradhan, IJIRD, 4(12), (2015) 240-246. [11] Amna Sirelkhatim, Shahrom Mahmud, Azman Seeni, N.H.M.Kaus, Ling Chuo Ann, Siti Khadijah, Nano-Micro Letters, 7(3),(2015) 219241. [12] A. Claude, M.Sathya, P.Govindasamy and K.Sudha, Advances in Applied Science Research, 3 (5) (2012) 2591-2598. [13] R.L.Hoffman, B.J. Norris and J.F. Wager, Applied Physics Letters 82 (5), (2003) 733-735. [14] Gaurav Shukla, Journal of Physics D: Applied Physics, 42, (7), (2009). [15] A Dhanalakshmi, B. Natarajan, V. Ramadas, A. Palanimurugan and S. Thanikaikarasan, Pramana- J. Physics, (2016), 87:57. [16] Eric R. Waclawik Jin Chang, Andrea Ponzoni, Isabella Concina, Dario Zappa, Elisabetta Comini, Nunzio Motta, Guido Faglia and Giorgio Sberveglieri, Beilstein Journal of Nanotechnology, 3(2012), 368-377. [17] Z.L. Wang, Materials Today, 7(6), (2004)26-33. [18] Mergoramadhayenty Mukhtar, Lusitra Munisa, Rosari Saleh, Materials Sciences and Applications, 3, (2012)543-551. [19] B.Hutera, A. Kmita, E. Olejnik, T.Tokarski, Archives of Metallurgy and Materials, 58(2), (2013) 489-491. [20] Ankica Saric, Goran Stefanic, Goran Drazic, Marijan Gotic, Journal of Alloys and Compounds, 652, (2015)91-99. [21] T.V.Kolekar, H.M.Yadav, S.S.Bandgar, A.C.Raskar, S.G.Rawal, G.M.Mishra, Indian Streams Research Journal, 1 (1), 2011. [22] Chittaranjan Bhakat and Prasoon Pal Singh, IJMER, 2(4), (2012) 2452-2454. [23] N. V. Kaneva, C. D. Dushkin, Bulgarian Chemical Communications, 43, (2) (2011) 259-263. [24] Khalid Omar, M. D. Johan Ooi and M. M. Hassin, Modern Applied Science, CCSE journal, Vol (3) 2, 2009. [25] Priyadarshini Jena, Rajesh Ku Mishra, K. Hari Krishna, IJCET, l2, (4) (2012). [26]Liu Chen, Zhang Yu- Ming, Zhang Yi-Men and Lii Hong-Liang, Chin. Phys. B, Vol 22(7), (2013) 076701. [27] Rajesh Kumar. O, Al-Dossary, Girish Kumar, Ahmad Umar, Nano-Micro Letter. (2015) 7(2), 97–120. [28] E.H. Nicollian and J.R. Brews, MOS Physics and Tech (NY: Wiley, 1982), 224.
ScienceDirect Materials Today: Proceedings 5 (2018) 10763–10770
www.materialstoday.com/proceedings
ILAFM2016
Synthesis and Characterization of ZnO Nano Discs Using Wet Chemical Method for Sensing Applications Pratima Bhata, Uma Ullas Pradhanb, Naveen Kumar S.Ka* a
Research Scholar, Department of Electronics, Mangalore University, Mangalore-574199, India b Department of Electronics, Mount Carmel College, Bangalore - 560052, India
Abstract Fabrication of novel sensing devices using nano structured semiconducting metal oxide (SMO) is gaining prominence in research in recent years due to its notable properties and reduced cost. Among various SMO like ZnO, SnO2, TiO2, In2O3 etc., ZnO is one of the most promising nanomaterial due to its unique properties and when synthesised in various size and morphology can be used for various applications. In this paper, we report the synthesis and characterization of well aligned ZnO (Zinc Oxide) nano discs using wet chemical process. Wet - Chemical synthesis has emerged as one of the most promising technique towards highyield and mass production of nanoparticles of desired particle size, shape and size distributions. The synthesized nanoparticles were characterized for its optical characteristics using UV-Visible spectrophotometer, for structural and compositional studies by XRD, FESEM and EDX. CV (Capacitance–Voltage) characteristics on ZnO based MOM (Metal oxide Metal) and MOS (Metal oxide Semiconductor) structure were studied using Impedance analyser. The XRD showed the peak intensity as 4270.23 at 2θ equal to 36.2095º with FWHM (Full Width Half Maximum) as 0.2453. EDX results showed the sample in pure ZnO phase. The images obtained from FESEM showed a uniform hexagonal disc type structure of approximately 100 nm in diameter and 30-40 nm in thickness. UV absorption peak was observed at 357nm. Sensing property of ZnO highly depends on its size and structure. C-V curves of ZnO based MOS contact showed 3 prominent regions: accumulation, depletion and inversion. C-V curves of MOM contact at 500 Hz behaved like a tunable capacitor. This variation of capacitance with respect to applied voltage exhibited by the MOM sample can be used for biomedical sensing applications to harness bio potential signals generated by the human body. These bio potentials can be sensed using surface electrodes which can show proportional varying sensing parameters like surfaces resistance, capacitance etc. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible
Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016. Keywords:SMO; sensing;morphology; MOM; particle size; wet chemical synthesis. * Dr. Naveen Kumar.S.K. Tel.: +91-9448318252. E-mail address: [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016.
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Pratima Bhat.et.al / Materials Today: Proceedings 5 (2018) 10763–10770
1. Introduction There is a great demand for the development of sensitive, fast, reliable, low power and low cost chemical sensors for health and safety applications [1]. Structure of a material and the impact of its macroscopic properties are very essential parameters towards any practical applications. Semiconducting metal oxide (SMO) materials, having high drifts in their conductivity, are in great demand as gas sensors[2, 3] because of their small size, simple construction, low weight, low cost and low power consumption[4]. SMOs when synthesized in nano scale can be classified into three groups: 0–dimensional, 1-dimensional and 2-dimensional, where 0-dimentional nanostructure are also referred to as quantum dots[5]. 1-dimentional nano structures [4] like nano rods, nano wires, nano tubes, nano discs are highly useful for gas sensing applications as it offers high aspect ratio [6]. There are various semiconducting metal oxides which have applications as gas sensing materials like SnO2 (Tin dioxide), WO3 (Tungsten trioxide), ZnO (Zinc oxide), TiO2 (Titanium dioxide), In2O3 (Indium Oxide) etc. Among these ZnO, II-VI group element and n type of transition semiconductor has been extensively studied as gas sensors as it has excellent chemical sensitivity and stability towards different adsorbed gases [7]. It influences the adsorption of gases by varying the doping level [8] and is non-toxic and low cost. Also ZnO has direct and wide band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature [9]. They can be used in the applications as VOCs (Volatile organic compounds) detection sensor for disease monitoring [2, 10], anti-bacterial activity[11], food processing, for air quality monitoring, functional devices such as energy efficiency devices (solar cells) [12], photo-catalysts, thin film transistors [13], as flexible electrode in optoelectronic and organic luminescent devices (OLED)[14], biosensors, piezoelectric devices etc. ZnO when synthesized in nano dimension (< 100nm), creates large surface to volume ratio, thus offers highly increased sensitivity towards specific gas [16]. The classification of synthesizing techniques as shown in Fig. 1 which yields ZnO in nano-range in the form of colloids, clusters, powder, tubes, rods, wires, thin films, flowers [17] etc. Among the below mentioned methods, shown in Fig. 1, chemical methods have more performance than physical methods in controlling the particle size and morphology at lower temperature (< 350º C).
Fig. 1. Different methods of ZnO Nanoparticle synthesis
The methods that have been established to synthesize different kinds of ZnO nanoparticles with controlled crystalline phases, sizes and shapes, includes co-precipitation [18], thermal decomposition [19], hydro (Solvo) thermalsynthesis [20] and sol-gel process [21]. Other chemical methods for the synthesis of zinc oxide have been reported in various references [17,22]. Among these “Wet chemical Synthesis” is a simple and inexpensive technique which yields good quality nanoparticles [17]. In this technique materials can be obtained in the form of liquid (gel) or can be converted into dry powder or thin film easily in large quantities.This paper aims at preparing ZnO particles using wetchemical process and studying its morphology, structure, optical and electrical (C-V) behavior using FESEM and EDX, XRD diffraction, U-Vis spectrophotometry and impedance analyzer respectively.
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2. Synthesis Procedure Synthesis of ZnO nano structure is done using wet chemical synthesis. 2.1. Material Used Zinc acetate dehydrate (Zn (CH3COO) 22H2O), Potassium hydroxide (KOH) and Ethanol (CH3 3CH2 OH) of analytical grade were used from Merck (Bangalore, India). Double distilled water was used for the preparation of solutions. 2.2. Synthesis Procedure ZnO nano structures were synthesized by wet chemical process in alcohol media with zinc acetate dehydrate and potassium hydroxide as starting precursor taken in 1:20 molar ratio. During the synthesis, 2.195g of zinc acetate dehydrate [Zn (CH3COO)2 2H2O] was mixed in 100 ml ethanol and stirred for 15 minutes using magnetic stirrer in ambient atmosphere. 1.122 g of Potassium hydroxide (KOH) mixed with 10 ml of double distilled water were added drop wise to the above solution under continues stirring for 30 minutes till Ph.was maintained to 7. A milky white solution in a jelly form was obtained which was further heated at 90º C for 3 hour at muffle furnace. The resulting suspension was centrifuged to retrieve the product and was washed 3-5 times with ethanol to remove the impurity. Later it was dried in the furnace at 70ºC for 24 hours to obtain powder [22]. 3. Characterization And Interpretation Characterization of ZnO is done to study various parameters like particle size, lattice parameter, optical energy band gap, peak absorption etc. so as to use it for the specific application. 3.1. XRD The ZnO particles were characterized by X-ray diffraction using Pan analytical Xpert Pro, TESCAN WEGA with Cu Kα radiation of λ = 1.5405 Å for the 2θ range from 10º-80º. The result was plotted using origin61 software into a graph as shown in Fig. 2. The highest peak intensity was observed at 2θ = 36.2095º and corresponding intensity from the figure is observed to be 4270.23.The highest peak correspond to the Miller indices (101) for Bragg’s angle 36.21°.The other indices observed are ( 100), (002), (102), (110), (103), (200), (112), (201) and (202) for diffraction angles 31.75°, 34.38°, 47.49°, 56.52°, 62.82°, 66.305°, 67.86°, 68.97° and 76.89° respectively. All the diffraction peaks were consistent, sharp and match with the standard card Joint Committee on Powder Diffraction Standards (JCPDS36-1451) related to the hexagonal wurtzite structure. The lattice parameters ‘a’ and ‘c’ of hexagonal phase were calculated using equations (1) and (2) given below [15]:
a
c
3sin
sin
(1)
(2)
Where λ is the wavelength of the incident radiation, = 1.5405 Å and θ is the Bragg’s angle. The values of the lattice parameters thus calculated were found to be, a = 0.286 nm, c = 0.495nm and c/a = 1.73. The crystal size of the ZnO particle is obtained using Debye-Scherrer’s equation:
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D
(3)
d cos
Fig. 2. XRD reading for ZnO Nano-Disc
Where D is the crystallite size (nm), λ is the wavelength of incident X-ray (= 1.5405 Å), βd is the FWHM (full width at Half maximum), and θ is the diffraction angle at maximum intensity and k is a constant =0.89. From the obtained XRD reading, 2θ= 36.2093, θ= 18.1046, FWHM =0.2453. The calculated size D = 76 nm was found to be within the observed range. 3.2. FESEM and EDX Structural morphology of the ZnO nano structure were analysed through FESEM (Field Emission Scanning Electron Microscope) as shown in Fig.3, using ZEISS.
Fig. 3. FESEM images for ZnO Nano Discs.
A uniform hexagonal disc type ZnO structure with a diameter of approximately 100 nm and thickness of 30-40 nm is observed.The obtained ZnO nano structure is in great demand due to its large aspect ratio and thus plays a critical role in sensing low concentration of the gas analyte and VOCs present in exhaled breath. EDX (Energy Dispersive X-Ray Spectroscopy) analysis is done to study the elemental composition of the sample using Oxford Instruments, ZEISS (Fig. 4). The graph shows the sample prepared by the above route has pure ZnO phases. The stoichiometric ratio of the sample is shown in Table 1.
Pratima Bhat..et.al / Materials Today: Proceedings 5 (2018) 10763–10770
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Table 1. Stoichiometric Ratio of sample (From EDX result). Element
Weight%
Atomic%
Compound %
O(K shell)
0.84
44.23
1.15 1.00
Zn(L shell)
2.60
33.66
C
0.31
22.11
Total
3.75
Fig. 4. EDX data for ZnO Nanoparticle.
3.3. UV-Visible Spectrophotometer Optical parameters were studied with the help of UV – Vis spectrophotometer; UV-1700 Pharma Spec, from SHIMADZU. The obtained result was plotted using origin61 software as shown in Fig.5.
Fig. 5. UV-Vis spectrophotometry images for ZnO Nanoparticle
The sample showed maximum absorbance at 357 nm wavelength. For the optimum sample, band gap energy was calculated using equation (4) [22].
E (eV )
hC * 1.6 * 10 19
(4)
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Where E is band gap energy in eV, h is Plank’s constant = 6.626* 10-34 Joules sec, C is the speed of light = 3*108m/sec and λ is the wavelength with peak absorption and 1.6*10-19 is the conversion factor. The band gap energy was found to be 3.48 eV. 4. Electrical Characteristics 4.1. Thin film synthesis for CV measurement A thin porous layer of synthesized n type of ZnO nano discs were coated on two substrate-(i) Aluminium substrate and (ii) p type of silicon substrate using dip coating method[23] to obtain a MOM (Metal oxide Metal) structure and MOS (Metal oxide Semiconductor) structure respectively. These substrates were dipped in the ZnO nano disc suspension for 30 minutes and dried in the ambient temperature for 24 hours. On the obtained ZnO nano disc coated substrate, silver electrodes were formed to perform CV (Capacitance-Voltage) measurements. CV measurement were done on both the samples using IM 3570 Impedance Analyzer and results were plotted using origin61 software as shown in Fig. 6 (a)and (b), Fig. 7(a) and (b). 4.2. Capacitance- Voltage graphs of MOS Structure: A MOS (Metal oxide semiconductor) structure at 100 Hz frequency is established and studied as seen in Fig. 6 (a).
Fig. 6. (A) C –V variation of –MOS structure at F= 500 Hz. (B) C-F variation –MOS structure
This MOS capacitance is highly dependent on bias voltage. The variation is due to the modulation of the width of the surface charge layer density formed by the field and this is extremely useful in the evaluation of the electrical properties of oxide–silicon interface. Three regions of interest in the characteristics of the MOS capacitor namely accumulation, depletion and inversion is formed as shown in the Fig.6(a). With negative voltage applied at electrode on ZnO layer, the concentration of holes from p-Si substrate will form a second electrode of a parallel plate capacitor with first electrode at the ZnO layer. This increases the charge density and thus capacitance increases and this region is termed as accumulation. Since the accumulation layer is an indirect ohmic contact with the P-type substrate, the capacitance of the structure under accumulation conditions highly depends on the thickness of the (ZnO) oxide layer.[7, 28] As this voltage is made positive, holes are repelled and a region is formed at the surface which is depleted of carriers. This region is called depletion region. Under depletion conditions, the Fermi level near the silicon surface will move to a position closer to the centre of the forbidden region. This voltage is called as flat band voltage. This voltage is nearly 0.5 V. Increasing the positive voltage will tend to increase the width of the surface depletion region and the capacitance from the gate (ZnO layer) to the substrate (p-Si) associated with MOS structure will decrease.
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With further increase in the positive voltage beyond threshold voltage, dynamic carrier generation and recombination take place which causes a region called as Inversion region. The build-up of inversion layer is a threshold phenomenon which is a frequency dependent region. In this inversion region, the capacitance increases as the frequency decreases and vice versa. [26] The variation of the capacitance for different frequencies has been studied for -ZnO based MOS structureand is plotted as shown in the Fig. 6(b). 4.3. Capacitance- Voltage graphs of MOM Structure: A MOM ( Metal oxide Metal) contact at 500 Hz frequency is established and studied as shown in Fig. 7 (a). From the Fig.7 (a) it is clear that, the MOM structure is very sensitive to the applied voltage and is acting like a tunable capacitor. Such capacitor can be considered as varactor diode whose capacitance varies with applied voltage.This MOM structure as capacitor can also be used as sensing element in biomedical sensing applications to harness bio potential signals generated by the human body.
Fig. 7. (A) C –V variation of –MOM structure at F= 500 Hz. (B) C-F variation –MOM structure
Variation of Capacitance for different frequency for MOM structure is studied and it is plotted as shown in the Fig. 7(b). It shows that capacitance exponentially decreases as frequency increases. 5. Conclusions In summary, a uniform hexagonal ZnO nano disc structure of approximately 100 nm were successfully synthesized using wet chemical synthesis at 85º -90ºC. Its optical and electrical characterization was studied. Band gap energy was found to be 3.48 eV. The obtained ZnO nano structure is in great demand due to its large aspect ratio, plays a significant role in sensing low concentration of the gas analyte and VOCs present in exhaled breath. Capacitance-Voltage variation curves of ZnO nanoparticle on p-type Si substrate forming a MOS structure shows 3 prominent regions- accumulation, depletion and inversion. The study of C-V variation of ZnO on Aluminium substrate forming a MOM structure acts as a tunable capacitor. The MOM structure acts as a capacitor that can be used as sensing element in biomedical sensing application to harness bio potential signals generated by the human body. Thus ZnO nanoparticle semiconducting metal oxide can be used as a wearable sensor. References [1] Po-Chiang Chen, Guozhen Shen and Chongwu Zhou, IEEE Transaction on Nanotechnology, 7 (6), (2008) 668-679. [2] Pratima Bhat, Dr. Uma Ullas Pradhan and Dr. Naveen Kumar. S.K, Pariprashna Academic Journal, 2, (2016)330-337. [3] Elena Dilonardo, Michele Penza, Marco Alvisi, Cinzia Di Franco, Francesco Palmisano, Luisa Torsi and Nicola Cioffi, Beilstein Journal of Nanotechnology, 7(2015) 22-31. [4] Jin Huang and Qing Wan, Sensors-Review, 9(2009) 9903-9924. [5] Vladimir A. Fonoberov and Alexander A. Balandin, Journal of Nano electronics and optoelectronics, 1, (2006) 19-38.
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