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Spray pyrolysis deposited CdO: Al films for trimethylamine sensing application
Materials Science in Semiconductor Processing 105 (2020) 104753
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Spray pyrolysis deposited CdO: Al films for trimethylamine sensing application B.K. Balachandar a, T. Logu a, R. Hari Ramprasath a, K. Sankarasubramanian a, P. Soundarrajan a, M. Sridharan b, K. Ramamurthi c, K. Sethuraman a, * a
School of Physics, Madurai Kamaraj University, Madurai, 625021, India Functional Nanomaterials & Devices Lab, Centre for Nanotechnology & Advanced Biomaterials and School of Electrical & Electronics Engineering, SASTRA Deemed to be University, Thanjavur, 613 401, India c Department of Biomedical Engineering, Aarupadai Veedu Institute of Technology, Vinayaga Mission’s Research Foundation, Vinayaga Nagar, Paiyanoor, 603 104, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Al doped CdO CdO Al gas sensors CdO TMA sensors
Cadmium oxide (CdO) thin film was optimized on glass substrate at 250 � C using a chemical spray pyrolysis (CSP) technique followed by Al atoms doped into it with various wt% in order to study its influence on structural, electrical, optical, morphological and gas adsorption properties of the CdO thin films. All the films are deposited in polycrystalline cubic crystal phase is confirmed by XRD. The 90% of the visible as well as near infrared (650–850 nm) region energies is transmitted in pure and Al doped CdO films. Smooth surfaces and spherical shaped nanoparticles morphologies are conspicuous in CdO films after Al doping. The prepared CdO film is of ntype conductivity with having mobility of 64.4 cm2 V-1s-1 and carrier concentrations of 2.2 �1020 cm-3. Impor tantly, the mobility increases and resistivity decreases is observed on CdO film as for Al doping level. After procuring some interesting physicochemical properties in CdO films by Al doping, trimethylamine (TMA) gas sensing analysis has been scrutinized using a simple and cost effective route. Finally, it is found that low resistive and high mobility of 1.5% Al doped CdO film exhibited pronounced sensitivity, 32.12%, is due to its higher specific surface area and as well as existence of more defective sites.
1. Introduction Metallic oxide thin films have been widely used in gas sensing ap plications [1] due to their higher surface tunability which can be easily done by preparation methods that favors an efficient gas molecules adsorption over there [2–6]. It observed that absorption of gases on various metal oxides such as CdO, ZnO, TiO2, WO3 [7–10] is dependent on their physicochemical characteristics. The kinetics of the gas mole cules on the film surface is a key factor that determines the sensing accountability. Surface interaction mechanisms and redox reactions are causing the interactions of different gases that have to be clearly explored in the previous reports [7,8]. The changes in electrical con ductivity/resistivity of the film surface with the interaction of several test gases either transducing or recepting properties have also been taken much attention. Moreover, the interaction of toxic/hazardous gases [11] affects the earth environment and living beings. CdO is a II-VI n-type semiconductor having bandgap energy of
2.2 eV at room temperature and shows good electrical conductivity and optical transparency in the visible and infrared regions [12]. The elec trical conductivity, charge carrier mobility and number of charge car riers in CdO film depend on the generation of interstitial Cd ions and/or O2 vacancies that could be controlled by doping of different metal lic/mineral ions or hindering the physical parameters like electronic work function, electron attractiveness, and film thicknesses. There were some metallic ions such as Ti, Mn and Fe are used as a dopant for CdO [13–15]. There was also a few reports available on Al doped CdO thin films deposited using a CSP technique [16,17]. In the gas sensing aspect, Munde Bhaskar has merely studied ethanol gas sensing on CdO films with respect to Al doping level [17]. CSP is one of the best simplest methods to prepare metal oxide thin films. In this work also we have choosen Al3þ as a dopant due to it can easily reside the position of Cd lattice sites in CdO. On Al doping with CdO, there is one ion excessively available that surely enhance the structural, electrical, conducting, op tical and the sensing properties of resultant films.
* Corresponding author. School of Physics, Madurai Kamaraj University, Madurai, 625 021, Tamil Nadu, India. E-mail address: [email protected] (K. Sethuraman). https://doi.org/10.1016/j.mssp.2019.104753 Received 15 March 2019; Received in revised form 9 September 2019; Accepted 23 September 2019 Available online 1 October 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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There are number of methods used for depositing CdO thin films such as chemical vapour deposition, sputtering, CBD method [18–20]. CSP is a suitable technique for doping into host crystal structures with uniform distributions. This is the main reason why CSP is chosen here to dope Al with CdO semiconductors. Trimethylamine (TMA, (CH3)3N) is a colorless, harmful, poisonous and strong smelling gas found in different fields, is mainly generated from seafood and fish deterioration. The long-term contact is harmful for people’s eyes, nose, throat that can cause dizziness, naupathia, vomit ing, and irritation to both respiratory and nervous systems. TMA is one of the most effective indicators in microbial degradation of TMA N-oxide while assessment of freshness of fish and seafood decay [21]. At above 60 ppm, the freshness is spoiled and denotes that rotten stage of fish. Thus, it is very important to develop a gas sensor for sensing trime thylamine toxic gas. In this work, the chemo-sorption of toxic TMA, ((CH3)3N), as well as environmentally hazardous vapours like ammonia (NH3), benzene (C6H6), dimethylamine ((CH3)2NH) on CSP prepared pure and CdO: Al thin films were studied. When these gases exposed in higher concen trations, cause drowsiness, dizziness, headaches, irregular heartbeat, unconsciousness that leads to death. Indeed, there is still a greater requirement to detect these gases in trace amounts like tens of ppm levels. Already a few researchers have reported TMA gas sensor devices using different nanostructured metal oxides. Cho et al. fabricated WO3 with spheres morphology using an ultrasonic spray pyrolysis. They found sensitivity level of 5 ppm and a response of 57% at 450� C [22]. Tong et al. also reported WO3 thin films by sol–gel technique and showed TMA sensitivity of 50 ppm and a response of only 3% at 70 � C [23]. ZnO thin films prepared by a modified CVD method exhibited sensitivity of 2.8% under 400 ppm at 300 � C [24]. Tong et al. prepared branch-like α-Fe2O3 nanorods/TiO2 nanofibers having a TMA gas con centration of 50 ppm and a response of 13.9% at 250 � C [25]. Finally, in this work, a maximum of 32% TMA gas response is observed on spray pyrolysis deposited 1.5% Al doped CdO thin film at 300 ppm at room temperature.
resistance value of films during gas sensing, the copper wire was con nected to a high resistance electrometer (TektronicX DMM 4050 6-1/2) that is interfaced with a computer. The film area of 2 cm � 2 cm was used for TMA gas sensing. The area of the contact electrode is 0.6 cm2 in two edges of the film. The distance between two electrodes is 1.4 cm. Room-temperature sensing studies were conducted using a customized sensing chamber. The schematic of a complete home-made sensing set up is shown in Fig. 1. The sensing chamber was made up of glass with 1 L capacity. To generate TMA vapours inside the gas chamber, an appro priate volume of anhydrous TMA solution (Analytical Grade) was injected through a microlitre syringe. The concentration of the analyte solution was determined using the below mentioned formula [26]. CðppmÞ ¼
δ � Vr � R � T � 106 M � Pb � Vb
(1)
δ – Density of the target gas, Vᴦ – Volume of TMA exposed inside the sensing chamber, R – Universal gas constant, T – Absolute temperature, M Molecular weight, Pb – Chamber pressure, and Vb – Chamber volume in litres. Initially, the sensor was fixed inside the testing chamber and it was allowed to attain a stable baseline resistance. The pre-determined con centration of TMA liquid was placed inside the chamber. It was allowed to become vapour through evaporation. The change in resistance of the sensing element due to TMA vapour inside the chamber was then continuously monitored using an electrometer. After the measurements, the vapour was completely exhausted. The test chamber was thoroughly cleaned with ethanol and deionized water. The same procedure was repeated for all the concentrations and all other vapours. 3. Results and discussion 3.1. Structural characterization
2. Film deposition method and its characterization details
Fig. 2(a) shows the XRD patterns of pure and Al doped CdO thin films on glass substrates with various Al dopant concentrations. CdO film diffraction pattern shows cubic crystal structure with (200) preferential orientation. No any other crystalline phase is found after Al3þdoping. This is well agreed with JCPDS card no. 05–0640. Other crystalline orientations such as (111), (220) and (222) were also observed with comparatively low intensities and well matched with ref [27]. The preferred orientation plane changed from (200) to (111) in Al doped CdO films because incorporated Al3þ ions into the CdO lattice makes considerable changes in the rebuilding of crystallites. In addition, from Fig. 2(b), it can be seen that once Al3þ ion is doped into the CdO lattice, the diffraction peaks slightly shifted towards the higher angle. This may be due to the changes in the ionic radii of Al3þand Cd2þ ions [28]. The Lattice constant ‘a’ value is found decreased at higher Al doping level. In 1.5 wt% CdO: Al thin film, the calculated ‘a’ value is 4.695 Å [29], this agrees with the reported value. The decreased ‘a’ value may be due to shrinkage of lattice by Al doping. The calculated structural parameters are listed in Table 1. Here, the average grain size decreases with increasing Al doping could be due to efficient accumulation and adhesion of the crystallites on the surface of the substrate, which also played an important role in increasing other associated parameters such as strain and dislocation density [30]. This could be ascribed that increases in the packing of crystallites with Al concentration that caused an increase in the thermal and mechanical shock resistance of the film. Texture coefficient TC (200) evaluated from XRD data by using the relation [31].
Pure and Al doped CdO thin films were coated on soda lime glass substrate (75 mm � 25 mm � 1.45 mm) using a CSP method and uti lizing cadmium acetate [Cd(CH3(COO)2)] and aluminum nitrate non ahydrate [Al(NO3)3⋅9H2O] as precursor for Cd and Al. The Al doping concentration is varied from 0.5 to 1.5 wt%. The 30 ml of resultant precursor solution was prepared in all the cases and then sprayed on preheated slides at 250 � 5 � C. The substrate temperature was controlled by thermocouple using PID temperature controller. Nozzle to substrate distance was optimized that is 25 cm. Compressed air having pressure of 2 kg/cm2 was used as carrier and sprayed solution at l ml/ min. The total deposition time was fixed as 30 min for each coating. The X-ray diffraction (XRD) patterns of the pure and CdO: Al films were recorded by using Philips X Pert PRO (Cu Kα1line; λ ¼ 1.540 Å) instrument. Optical transmittance for all the deposited films in the wavelength range of 400–900 nm was recorded by using Shimadzu UV1601 spectrometer. In order to study the surface morphology of the pure and Al doped CdO films, FEI Quanta FEG 200 scanning electron mi croscope (SEM) was used. Further, the surface roughness was measured and calculated using an A100 SGS, APE Research atomic force micro scopy (AFM). Shimadzu ESCA – 3400 X-ray photoelectron spectroscopy instrument was used for investigating elemental composition as well as oxidation states of the deposited films. The electrical properties were analysed using Ecopia HMS-3000 Hall measurement in Vander Pauw configuration. The TMA gas adsorption on the deposited films were recorded using a sensitive Digital Precision Multimeter (TektronicX DMM 4050 6-1/2). For taking contact, copper wire was attached with deposited films using silver paste. Further, in order to determine the 2
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Fig. 1. The schematic diagram of a complete home-made sensing set up.
Fig. 2. (a) XRD patterns of pure and CdO: Al thin films (b) Peak shift of (111) and (200) planes. Table 1 Structural parameters of spray coated CdO: Al thin films. Sample
FWHM
Lattice Parameter (Å)
Crystallite Size (nm)
Micro Strain � 10-3 (lines/m4)
Dislocation Density � 1015 (lines/m2)
Texture Coefficient value of (200)
CdO: Pure CdO: Al (0.5 wt%) CdO: Al (1.0 wt%) CdO: Al (1.5 wt%)
0.3860 0.4224 0.4699 0.8147
4.6993 4.6194 4.6194 4.6244
36.59 32.93 31.63 14.62
1.997 1.984 1.908 1.898
7. 47 9.22 9.95 46.78
3.098 0.970 0.737 0.663
IðhklÞ=IoðhklÞ TCðhklÞ ¼ � � 1 ½IðhklÞ=IoðhklÞ� N
From Table 1, it is seen that TC (200) value is maximum in pure CdO film and then it is reduced with respect to increasing Al doping con centration. The decrement of TC (200) value may be due to the fact that the excess Al difficult to form a critical nucleus for film formation and its surface specific interaction may be reduced.
(2)
where I(hkl) is the measured relative intensity from the (h k l) reflection, I0(h k l) is the reference relative intensity and N is the number of reflections.
Fig. 3. (a) UV–Vis transmittance spectra of pure and CdO: Al films and (b) Tauc Plot for the respective films. 3
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3.2. Optical studies
3.4. Hall characterization
Fig. 3(a) depicts the transmittance of pure and Al doped CdO films measured in the wavelength range from 400 to 900 nm at room tem perature. Pure CdO thin film shows 96% of optical transparency in the visible solar spectrum, which is then slightly reduced to 94%, 93% and 92% with 0.5, 1.0 and 1.5% Al doping level. This clearly indicates op tical transparency decreased with Al doping levels. This could be due to an enhancement in the film thicknesses of CdO after Al doping. This observation is well agreed with the earlier reports [32]. The bandgap values were obtained by the extrapolated linear regression plot of photon energy hν vs. (αhν)2 (Fig. 3(b)) [33]. Pure CdO thin film showed a direct energy gap of 2.11 eV. Further, the energy bandgap changed from 2.15 to 2.23 eV with respect to Al doping. The variation in bandgap values with Al doping can be explained by the Burstein–Moss effect.
Electrical properties of the pure and CdO: Al films are measured by using Hall measurement at room temperature. Generally, the negative sign of the Hall coefficient value shows n-type and positive sign of the Hall coefficient shows p-type conducting nature. The variations of electrical properties in CdO film with different Al doping level is dis played in Fig. 7 and Table 2. The obtained resistivity of pure CdO film was gradually decreased and reached 6.5 � 10-4 Ω cm at 1.5 wt% of Al doping as shown in Fig. 7 (a). This result suggested that Al doping ions donated excess charge carriers when it occupied at Cd2þ sites. The carrier concentration value of 6.89 � 1019 cm-3 is noticed on CdO and then a maximum of 2.20 � 1020 cm-3 is obtained in 1.5 wt% Al doped CdO. This manifest that an order increment of carrier concentration in 1.5 wt% Al doped CdO thin film (Fig. 7 (b)) as compare to pure CdO. The enhanced carrier con centration value suggested that the substitution of A13þ in CdO lattice donates excess of free electrons that leads to high electrical conductivity [34–38]. The mobility of 43.6 cm2 V-1s-1 is obtained on pure CdO film which then reached a maximum of 64.4 cm2 V-1s-1 at 1.5 wt% Al doping as viewed in Fig. 7 (c). When dopant percentage is increased, the number of A13þ ions replaces the Cd2þ ions and provides excess charges. Hence, the number of charge carriers increases in the CdO film at higher Al doping level.
3.3. Surface characterization 3.3.1. AFM studies The 2D and 3D topographs of the pure and CdO: Al thin film is shown in Fig. 4. The 1.5 wt% CdO: Al film clearly shows the uniform surface morphology with large and coarse grains sizes that is also evenly distributed on the entire substrate. Al doping changes the grain size of CdO is noticed. The bigger grains by agglomerations of smaller particles and no cracks or pinholes are observed in the pure CdO to Al doped films up to 1.5 wt%. The roughness values increase from 9.17 to 12.96 nm is observed in CdO films with increasing of Al doping levels.
3.5. XPS studies The available atomic elements and its oxidation states in the Al doped CdO films were investigated using XPS study. Fig. 8(a) presents the XPS survey spectrum of 1.5 wt% Al doped CdO film. Fig. 8(b–d) shows chemical binding energies of Al 2p, Cd 3d3/2, and O1s spectra of CdO: Al thin film. The peak positions of the Cd 3d3/2 and Cd 3d5/2 spectra are centered at 404.2 and 411.2 eV respectively which denotes Cd is in 2 þ oxidation state. Fig. 8(d) show the XPS spectrum of O1s. Here, a sharp peak is positioned at 530 eV. The presence of O1s may be due to the chemi-resistive O2 and the presence of O2 . The occurrence of Al is indicated from the Al 2p spectrum and the intensity is weak because it is a doping ions here. The peak position of the Al 2p spectrum is centered at 74.2 eV which confirmed the presence of Al3þ ion in the Al doped CdO film.
3.3.2. SEM with EDAX studies The SEM studies of pure and CdO: Al films have been carried out with various Al dopant levels and the obtained images are given in Fig. 5. From these images, it is observed that pure and 0.5 wt% Al doped CdO films have highly aggregated nanoparticles (NPs) surface morphology whereas increased Al doping (1.0 and 1.5 wt%) concentration shows conspicuous spherical shaped particles with different sizes. The surfaces were totally covered with nearly uniform sized grains and free from pinholes. When introducing dopant (Al) ions, the pyrolysis of droplet changed under identical deposition conditions. These changes could be played a major role for the changes of CdO morphology. The conclusion is that an irregular aggregated NPs of pure CdO thin film images were changed to uniformed spherical shaped NPs by incorporation of Al dopant ions. The EDAX analysis of CdO: Al (1.5 wt%) thin film is shown in Fig. 6. From the EDAX spectra, the existence of Cd, O and Al atoms is confirmed from the coated CdO: Al (1.5 wt%) thin film.
3.6. Sensor studies The sensing capacity on thin films depend on charge transfer in the metal oxide films and that depends on physical, chemical and structural
Fig. 4. Atomic force microscopic images of CdO: Al thin films. (a) Pure CdO; (b) CdO: Al 0.5 wt%; (c) CdO: Al 1.0 wt% and (d) CdO: Al 1.5 wt%. 4
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Fig. 5. SEM images of pure and CdO: Al thin films. (a) Pure CdO; (b) CdO: Al 0.5 wt%; (c) CdO: Al 1.0 wt% and (d) CdO: Al 1.5 wt%.
Fig. 6. EDX spectrum of 1.5 wt% of Al doped CdO thin film.
parameters like thickness, size, crystallinity, composition, permeability, surface morphology, effective surface area. Generally, negatively charged surface oxygen species in CdO film could be facilitated upward movement of conduction band edge that decreases the conductivity of the film [39]. The change in conductivity of the film was governed by changing of free charge carrier concentration, potential barrier height in the grain boundaries and electro physical parameters of the thin film [40]. Electrical conductivity of semiconducting thin film vapour sensor varies because of two important reactions on the surface of the film. In first reaction, O2 molecules were physisorbed on the surface and gets ionized by pulling charge carrier from conduction band, it became ionosorbed as O- that lowers the conductivity of the sensor [41]. Oxide formulation on the film has been given by
O2 þ 2e →2O
(3)
The vapours sensing phenomena of thin metal oxide semiconductor films have been seen by the interaction between analyte gases and existed reactive oxygen species on the surface of the film. In CdO crystal lattice, the electrons from Cd interstitials migrated to surface oxygen and Cd2þ ions have been ionized to Cdþ level that could act as donors. The conductivity of the film gets increased in the TMA vapour atmo sphere; this may be due to electron transfer between nitrogen that contains lone pair of electrons to chemisorbed oxygen through CH3 active sites present in the TMA molecule. The reaction between functional group (R) of gas with ionosorbed oxygen is
5
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Fig. 7. (a).The variation of resistivity; (b) carrier concentration; (c) mobility as a function of Al concentration.
R þ O →RO þ e
Table 2 Electrical parameters of spray coated CdO: Al thin films.
(4)
The sensitivity (S) [42] is denoted as
Samples
Carrier Concentration (cm-3)
Conduction Type
Resistivity (Ω cm)
Mobility (cm2 V-1s-1)
CdO: Pure CdO: Al (0.5 wt %) CdO: Al (1.0 wt %) CdO: Al (1.5 wt %)
6.89Eþ19 1.16Eþ20
n n
1.41E-03 9.62E-04
43.6 49.8
1.53Eþ20
n
8.20E-04
56.0
2.20Eþ20
n
6.52E-04
64.4
% S ¼ [(Ra –Rg) / Ra] � 100
(5)
where Ra and Rg are change in resistances of thin films without TMA vapours and with TMA vapours respectively. The Wolkentein’s model [43] explained TMA sensing mechanism by 4ðCH3 Þ3 N þ 21 O 2 ðadsÞ ¼ 2 N2 þ 12 CO2 þ 18 H2 O þ e
(6)
The enhancement in the TMA sensitivity as shown in Fig. 9 is due to the excess carrier ions present in the CdO crystal system by incorpora tion of Al. Selectivity is a vital aspect for specific analyte gas in a mixed vapour atmosphere, especially having same chemical environment. The interactions between ammonia, benzene and dimethylamine could be ignored because of a slightly larger steric hindrance and low response of DMA, which lacked the higher cloud density of N atoms through methyl
Fig. 8. (a). The survey scan XPS spectrum of the 1.5 wt% Al doped CdO film. Fig. 8(b–d) shows respective elemental binding energy peaks of Al, Cd and O. 6
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Materials Science in Semiconductor Processing 105 (2020) 104753
Fig. 9. (a). The variation of sensitivities for pure and CdO: Al samples for different ppm of TMA vapours. (b) Selectivity of CdO: Al films for TMA vapours over other volatile gases.
and mobility (64.4 cm2 V-1s-1) have been obtained on 1.5 wt% Al doped CdO film. From sensor study, it is found that 1.5 wt% Al doped CdO film shows a maximum sensitivity of 32% for trimethylamine vapour than others such as benzene, ammonia, and dimethylamine. Eventually an important conclusion is that Al doped CdO thin films prepared by CSP technique are a promising candidate for detecting TMA vapours in ambient condition.
bonds in TMA that presented greater response to surface oxygen of CdO: Al film and it is tedious to adsorb ammonia and benzene gases on surface of the CdO: Al film. More number of oxygen ions on film surface leads to higher potential barrier and result in higher resistance. The number of absorbed oxygen highly depends on morphology, grain size, and active surface area of CdO: Al film. The agglomerations in the film have blocked the in-diffusion of analyte vapours to the whole film surface, which makes deteriorated vapour response. Consequently, higher con centrations of vapour accessible nanostructures without sacrificing the smallest optimum particle size were also necessary for enhanced sensor response. The other reason may include the strong reducing nature of TMA than other gases. The reducing vapour helps to emerge out free charge carriers and decrease the resistivity of the film. From the above equation, it is clear that four TMA atoms can liberate 21 electrons in the film [44], an obvious enhanced sensing performance than other vapours were taken. This may be a reason for higher selectivity of TMA towards other candidates. The amount of reducing vapours on the CdO: Al thin film depends on the density of active sites of the film. Vapour response is related to the number of active sites on the surface of sensor and interstitial concentration that present on the CdO: Al thin films. Here, TMA vapours are sensed by CdO: Al film surface at room temperature. Obvious reasons for conduction on CdO: Al film surface include defective sites like missing ions (ie., O2 vacancies), cadmium self-interstitials and as well as through substitutional Al impurity ions in the Cd interstitial positions [45]. Fig. 9(a) demonstrates the TMA gas sensitivity plot for pure and Al doped CdO films and all the films became saturated above 300 ppm of TMA vapour. It gradually increased from 0% till 32% of vapour sensi tivity with increasing Al concentrations whereas benzene sensing attained 5.91%, ammonia got saturated at 5.79% and dimethylamine (DMA) offered only 4.86% sensitivity. Fig. 9(b) show selectivity of pure and CdO: Al thin films. TMA reached a maximum sensitivity of 32.12% on surface reactions with CdO: Al film having 1.5 wt% Al concentration. By this work, it has concluded that 1.5 wt% Al doped CdO film shows effective TMA sensing behavior and reiterates that it could be a useful for ecological and industrial applications in the near future.
Acknowledgment The authors would like to thank DST-PURSE program of School of Physics, Madurai Kamaraj University for providing AFM and SEM facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104753. References [1] K. Wetchakun, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Semiconducting metal oxides as sensors for environmentally hazardous gases, Sens. Actuators B 160 (2011) 580–591. [2] DhiaAldinSleibi Mustafa, RawaaIsam Mohammed Al-Rawi, Structural and optical properties for Zn doped CdO thin films prepared by pulse laser deposition, Iraqi J. Sci. 59 (2B) (2018) 839–846. [3] S.J. Helen, Suganthi Devadason, M. Haris, T. Mahalingam, Transparent conducting Mo-doped CdO thin films by spray pyrolysis method for solar cell applications, J. Electron. Mater. 47 (4) (2018) 2439–2446. [4] G.E. Moreno Morales, M.E. Araiza Garcia, S. Cruz Cruz, B. Rebollo Plata, O. Portillo Moreno, R. Gutierrez Perez, CdCO3 nanocrystalline thin film grown by chemical bath and its transition to porous CdO by thermal annealing treatment, Optik - Int. J. Light and Electron Optics 171 (2018) 347–355. [5] I. Ben Miled, M. Jlassi, I. Sta, M. Dhaouadi, M. Hajji, G. Mousdis, M. Kompitsas, H. Ezzaouia, Influence of In-doping on microstructure, optical and electrical properties of sol–gel derived CdO thin films, J. Mater. Sci. Mater. Electron. 29 (2018) 11286–11295. [6] K. Sankarasubramanian, P. Soundarrajan, K. Sethuraman, R. Ramesh Babu, K. Ramamurthi, Structural, optical and electrical properties of transparent conducting hydrophobic cadmium oxide thin films prepared by spray pyrolysis technique, Superlattice Microstruct. 69 (2014) 29–37. [7] A.S. Kamble, R.C. Pawar, N.L. Tarwal, L.D. More, P.S. Patil, Ethanol sensing properties of chemosynthesized CdO nanowires and nanowalls, Mater. Lett. 65 (2011) 1488–1491. [8] Padmanathan Karthick Kannan, Ramiah Saraswathi, John Bosco Balaguru Rayappan, A highly sensitive humidity sensor based on DC reactive magnetron sputtered zinc oxide thin film, Sens. Actuators 164 (2010) 8–14. [9] Dipak L. Gapale, Sandeep A. Arote, M Palve Balasaheb, Sanjaykumar N. Dalvi, Ratan Y. Borse, Effect of film thickness on humidity sensing of spray deposited TiOthin films, Mater. Res. Express 6 (2019), 026402. [10] Chong Wang, Ruize Sun, Xin Li, Yanfeng Sun, Peng Sun, Fengmin Liu, Geyu Lu, Hierarchical flower-like WO3 nanostructures and their gas sensing properties, Sens. Actuators B 204 (2014) 224–230.
4. Conclusion Pure and Al doped CdO thin films were coated on glass substrates using a facile CSP technique. All the films have built by polycrystalline cubic phase with only preferential plane changing after Al doping result in decreasing crystallite sizes. All the deposited films exhibited above 90% of optical transparency as expected in the visible and IR regions. A slight energy bandgap widening is observed after Al doping. CdO films deposited with uniform, smooth, well-adhered and pinhole free aggre gated NPs and then it modified as clearly visible spherical NPs with increased Al doping. The higher carrier concentration (2.2 � 1020 cm-3) 7
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8
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Spray pyrolysis deposited CdO: Al films for trimethylamine sensing application B.K. Balachandar a, T. Logu a, R. Hari Ramprasath a, K. Sankarasubramanian a, P. Soundarrajan a, M. Sridharan b, K. Ramamurthi c, K. Sethuraman a, * a
School of Physics, Madurai Kamaraj University, Madurai, 625021, India Functional Nanomaterials & Devices Lab, Centre for Nanotechnology & Advanced Biomaterials and School of Electrical & Electronics Engineering, SASTRA Deemed to be University, Thanjavur, 613 401, India c Department of Biomedical Engineering, Aarupadai Veedu Institute of Technology, Vinayaga Mission’s Research Foundation, Vinayaga Nagar, Paiyanoor, 603 104, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Al doped CdO CdO Al gas sensors CdO TMA sensors
Cadmium oxide (CdO) thin film was optimized on glass substrate at 250 � C using a chemical spray pyrolysis (CSP) technique followed by Al atoms doped into it with various wt% in order to study its influence on structural, electrical, optical, morphological and gas adsorption properties of the CdO thin films. All the films are deposited in polycrystalline cubic crystal phase is confirmed by XRD. The 90% of the visible as well as near infrared (650–850 nm) region energies is transmitted in pure and Al doped CdO films. Smooth surfaces and spherical shaped nanoparticles morphologies are conspicuous in CdO films after Al doping. The prepared CdO film is of ntype conductivity with having mobility of 64.4 cm2 V-1s-1 and carrier concentrations of 2.2 �1020 cm-3. Impor tantly, the mobility increases and resistivity decreases is observed on CdO film as for Al doping level. After procuring some interesting physicochemical properties in CdO films by Al doping, trimethylamine (TMA) gas sensing analysis has been scrutinized using a simple and cost effective route. Finally, it is found that low resistive and high mobility of 1.5% Al doped CdO film exhibited pronounced sensitivity, 32.12%, is due to its higher specific surface area and as well as existence of more defective sites.
1. Introduction Metallic oxide thin films have been widely used in gas sensing ap plications [1] due to their higher surface tunability which can be easily done by preparation methods that favors an efficient gas molecules adsorption over there [2–6]. It observed that absorption of gases on various metal oxides such as CdO, ZnO, TiO2, WO3 [7–10] is dependent on their physicochemical characteristics. The kinetics of the gas mole cules on the film surface is a key factor that determines the sensing accountability. Surface interaction mechanisms and redox reactions are causing the interactions of different gases that have to be clearly explored in the previous reports [7,8]. The changes in electrical con ductivity/resistivity of the film surface with the interaction of several test gases either transducing or recepting properties have also been taken much attention. Moreover, the interaction of toxic/hazardous gases [11] affects the earth environment and living beings. CdO is a II-VI n-type semiconductor having bandgap energy of
2.2 eV at room temperature and shows good electrical conductivity and optical transparency in the visible and infrared regions [12]. The elec trical conductivity, charge carrier mobility and number of charge car riers in CdO film depend on the generation of interstitial Cd ions and/or O2 vacancies that could be controlled by doping of different metal lic/mineral ions or hindering the physical parameters like electronic work function, electron attractiveness, and film thicknesses. There were some metallic ions such as Ti, Mn and Fe are used as a dopant for CdO [13–15]. There was also a few reports available on Al doped CdO thin films deposited using a CSP technique [16,17]. In the gas sensing aspect, Munde Bhaskar has merely studied ethanol gas sensing on CdO films with respect to Al doping level [17]. CSP is one of the best simplest methods to prepare metal oxide thin films. In this work also we have choosen Al3þ as a dopant due to it can easily reside the position of Cd lattice sites in CdO. On Al doping with CdO, there is one ion excessively available that surely enhance the structural, electrical, conducting, op tical and the sensing properties of resultant films.
* Corresponding author. School of Physics, Madurai Kamaraj University, Madurai, 625 021, Tamil Nadu, India. E-mail address: [email protected] (K. Sethuraman). https://doi.org/10.1016/j.mssp.2019.104753 Received 15 March 2019; Received in revised form 9 September 2019; Accepted 23 September 2019 Available online 1 October 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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There are number of methods used for depositing CdO thin films such as chemical vapour deposition, sputtering, CBD method [18–20]. CSP is a suitable technique for doping into host crystal structures with uniform distributions. This is the main reason why CSP is chosen here to dope Al with CdO semiconductors. Trimethylamine (TMA, (CH3)3N) is a colorless, harmful, poisonous and strong smelling gas found in different fields, is mainly generated from seafood and fish deterioration. The long-term contact is harmful for people’s eyes, nose, throat that can cause dizziness, naupathia, vomit ing, and irritation to both respiratory and nervous systems. TMA is one of the most effective indicators in microbial degradation of TMA N-oxide while assessment of freshness of fish and seafood decay [21]. At above 60 ppm, the freshness is spoiled and denotes that rotten stage of fish. Thus, it is very important to develop a gas sensor for sensing trime thylamine toxic gas. In this work, the chemo-sorption of toxic TMA, ((CH3)3N), as well as environmentally hazardous vapours like ammonia (NH3), benzene (C6H6), dimethylamine ((CH3)2NH) on CSP prepared pure and CdO: Al thin films were studied. When these gases exposed in higher concen trations, cause drowsiness, dizziness, headaches, irregular heartbeat, unconsciousness that leads to death. Indeed, there is still a greater requirement to detect these gases in trace amounts like tens of ppm levels. Already a few researchers have reported TMA gas sensor devices using different nanostructured metal oxides. Cho et al. fabricated WO3 with spheres morphology using an ultrasonic spray pyrolysis. They found sensitivity level of 5 ppm and a response of 57% at 450� C [22]. Tong et al. also reported WO3 thin films by sol–gel technique and showed TMA sensitivity of 50 ppm and a response of only 3% at 70 � C [23]. ZnO thin films prepared by a modified CVD method exhibited sensitivity of 2.8% under 400 ppm at 300 � C [24]. Tong et al. prepared branch-like α-Fe2O3 nanorods/TiO2 nanofibers having a TMA gas con centration of 50 ppm and a response of 13.9% at 250 � C [25]. Finally, in this work, a maximum of 32% TMA gas response is observed on spray pyrolysis deposited 1.5% Al doped CdO thin film at 300 ppm at room temperature.
resistance value of films during gas sensing, the copper wire was con nected to a high resistance electrometer (TektronicX DMM 4050 6-1/2) that is interfaced with a computer. The film area of 2 cm � 2 cm was used for TMA gas sensing. The area of the contact electrode is 0.6 cm2 in two edges of the film. The distance between two electrodes is 1.4 cm. Room-temperature sensing studies were conducted using a customized sensing chamber. The schematic of a complete home-made sensing set up is shown in Fig. 1. The sensing chamber was made up of glass with 1 L capacity. To generate TMA vapours inside the gas chamber, an appro priate volume of anhydrous TMA solution (Analytical Grade) was injected through a microlitre syringe. The concentration of the analyte solution was determined using the below mentioned formula [26]. CðppmÞ ¼
δ � Vr � R � T � 106 M � Pb � Vb
(1)
δ – Density of the target gas, Vᴦ – Volume of TMA exposed inside the sensing chamber, R – Universal gas constant, T – Absolute temperature, M Molecular weight, Pb – Chamber pressure, and Vb – Chamber volume in litres. Initially, the sensor was fixed inside the testing chamber and it was allowed to attain a stable baseline resistance. The pre-determined con centration of TMA liquid was placed inside the chamber. It was allowed to become vapour through evaporation. The change in resistance of the sensing element due to TMA vapour inside the chamber was then continuously monitored using an electrometer. After the measurements, the vapour was completely exhausted. The test chamber was thoroughly cleaned with ethanol and deionized water. The same procedure was repeated for all the concentrations and all other vapours. 3. Results and discussion 3.1. Structural characterization
2. Film deposition method and its characterization details
Fig. 2(a) shows the XRD patterns of pure and Al doped CdO thin films on glass substrates with various Al dopant concentrations. CdO film diffraction pattern shows cubic crystal structure with (200) preferential orientation. No any other crystalline phase is found after Al3þdoping. This is well agreed with JCPDS card no. 05–0640. Other crystalline orientations such as (111), (220) and (222) were also observed with comparatively low intensities and well matched with ref [27]. The preferred orientation plane changed from (200) to (111) in Al doped CdO films because incorporated Al3þ ions into the CdO lattice makes considerable changes in the rebuilding of crystallites. In addition, from Fig. 2(b), it can be seen that once Al3þ ion is doped into the CdO lattice, the diffraction peaks slightly shifted towards the higher angle. This may be due to the changes in the ionic radii of Al3þand Cd2þ ions [28]. The Lattice constant ‘a’ value is found decreased at higher Al doping level. In 1.5 wt% CdO: Al thin film, the calculated ‘a’ value is 4.695 Å [29], this agrees with the reported value. The decreased ‘a’ value may be due to shrinkage of lattice by Al doping. The calculated structural parameters are listed in Table 1. Here, the average grain size decreases with increasing Al doping could be due to efficient accumulation and adhesion of the crystallites on the surface of the substrate, which also played an important role in increasing other associated parameters such as strain and dislocation density [30]. This could be ascribed that increases in the packing of crystallites with Al concentration that caused an increase in the thermal and mechanical shock resistance of the film. Texture coefficient TC (200) evaluated from XRD data by using the relation [31].
Pure and Al doped CdO thin films were coated on soda lime glass substrate (75 mm � 25 mm � 1.45 mm) using a CSP method and uti lizing cadmium acetate [Cd(CH3(COO)2)] and aluminum nitrate non ahydrate [Al(NO3)3⋅9H2O] as precursor for Cd and Al. The Al doping concentration is varied from 0.5 to 1.5 wt%. The 30 ml of resultant precursor solution was prepared in all the cases and then sprayed on preheated slides at 250 � 5 � C. The substrate temperature was controlled by thermocouple using PID temperature controller. Nozzle to substrate distance was optimized that is 25 cm. Compressed air having pressure of 2 kg/cm2 was used as carrier and sprayed solution at l ml/ min. The total deposition time was fixed as 30 min for each coating. The X-ray diffraction (XRD) patterns of the pure and CdO: Al films were recorded by using Philips X Pert PRO (Cu Kα1line; λ ¼ 1.540 Å) instrument. Optical transmittance for all the deposited films in the wavelength range of 400–900 nm was recorded by using Shimadzu UV1601 spectrometer. In order to study the surface morphology of the pure and Al doped CdO films, FEI Quanta FEG 200 scanning electron mi croscope (SEM) was used. Further, the surface roughness was measured and calculated using an A100 SGS, APE Research atomic force micro scopy (AFM). Shimadzu ESCA – 3400 X-ray photoelectron spectroscopy instrument was used for investigating elemental composition as well as oxidation states of the deposited films. The electrical properties were analysed using Ecopia HMS-3000 Hall measurement in Vander Pauw configuration. The TMA gas adsorption on the deposited films were recorded using a sensitive Digital Precision Multimeter (TektronicX DMM 4050 6-1/2). For taking contact, copper wire was attached with deposited films using silver paste. Further, in order to determine the 2
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Fig. 1. The schematic diagram of a complete home-made sensing set up.
Fig. 2. (a) XRD patterns of pure and CdO: Al thin films (b) Peak shift of (111) and (200) planes. Table 1 Structural parameters of spray coated CdO: Al thin films. Sample
FWHM
Lattice Parameter (Å)
Crystallite Size (nm)
Micro Strain � 10-3 (lines/m4)
Dislocation Density � 1015 (lines/m2)
Texture Coefficient value of (200)
CdO: Pure CdO: Al (0.5 wt%) CdO: Al (1.0 wt%) CdO: Al (1.5 wt%)
0.3860 0.4224 0.4699 0.8147
4.6993 4.6194 4.6194 4.6244
36.59 32.93 31.63 14.62
1.997 1.984 1.908 1.898
7. 47 9.22 9.95 46.78
3.098 0.970 0.737 0.663
IðhklÞ=IoðhklÞ TCðhklÞ ¼ � � 1 ½IðhklÞ=IoðhklÞ� N
From Table 1, it is seen that TC (200) value is maximum in pure CdO film and then it is reduced with respect to increasing Al doping con centration. The decrement of TC (200) value may be due to the fact that the excess Al difficult to form a critical nucleus for film formation and its surface specific interaction may be reduced.
(2)
where I(hkl) is the measured relative intensity from the (h k l) reflection, I0(h k l) is the reference relative intensity and N is the number of reflections.
Fig. 3. (a) UV–Vis transmittance spectra of pure and CdO: Al films and (b) Tauc Plot for the respective films. 3
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3.2. Optical studies
3.4. Hall characterization
Fig. 3(a) depicts the transmittance of pure and Al doped CdO films measured in the wavelength range from 400 to 900 nm at room tem perature. Pure CdO thin film shows 96% of optical transparency in the visible solar spectrum, which is then slightly reduced to 94%, 93% and 92% with 0.5, 1.0 and 1.5% Al doping level. This clearly indicates op tical transparency decreased with Al doping levels. This could be due to an enhancement in the film thicknesses of CdO after Al doping. This observation is well agreed with the earlier reports [32]. The bandgap values were obtained by the extrapolated linear regression plot of photon energy hν vs. (αhν)2 (Fig. 3(b)) [33]. Pure CdO thin film showed a direct energy gap of 2.11 eV. Further, the energy bandgap changed from 2.15 to 2.23 eV with respect to Al doping. The variation in bandgap values with Al doping can be explained by the Burstein–Moss effect.
Electrical properties of the pure and CdO: Al films are measured by using Hall measurement at room temperature. Generally, the negative sign of the Hall coefficient value shows n-type and positive sign of the Hall coefficient shows p-type conducting nature. The variations of electrical properties in CdO film with different Al doping level is dis played in Fig. 7 and Table 2. The obtained resistivity of pure CdO film was gradually decreased and reached 6.5 � 10-4 Ω cm at 1.5 wt% of Al doping as shown in Fig. 7 (a). This result suggested that Al doping ions donated excess charge carriers when it occupied at Cd2þ sites. The carrier concentration value of 6.89 � 1019 cm-3 is noticed on CdO and then a maximum of 2.20 � 1020 cm-3 is obtained in 1.5 wt% Al doped CdO. This manifest that an order increment of carrier concentration in 1.5 wt% Al doped CdO thin film (Fig. 7 (b)) as compare to pure CdO. The enhanced carrier con centration value suggested that the substitution of A13þ in CdO lattice donates excess of free electrons that leads to high electrical conductivity [34–38]. The mobility of 43.6 cm2 V-1s-1 is obtained on pure CdO film which then reached a maximum of 64.4 cm2 V-1s-1 at 1.5 wt% Al doping as viewed in Fig. 7 (c). When dopant percentage is increased, the number of A13þ ions replaces the Cd2þ ions and provides excess charges. Hence, the number of charge carriers increases in the CdO film at higher Al doping level.
3.3. Surface characterization 3.3.1. AFM studies The 2D and 3D topographs of the pure and CdO: Al thin film is shown in Fig. 4. The 1.5 wt% CdO: Al film clearly shows the uniform surface morphology with large and coarse grains sizes that is also evenly distributed on the entire substrate. Al doping changes the grain size of CdO is noticed. The bigger grains by agglomerations of smaller particles and no cracks or pinholes are observed in the pure CdO to Al doped films up to 1.5 wt%. The roughness values increase from 9.17 to 12.96 nm is observed in CdO films with increasing of Al doping levels.
3.5. XPS studies The available atomic elements and its oxidation states in the Al doped CdO films were investigated using XPS study. Fig. 8(a) presents the XPS survey spectrum of 1.5 wt% Al doped CdO film. Fig. 8(b–d) shows chemical binding energies of Al 2p, Cd 3d3/2, and O1s spectra of CdO: Al thin film. The peak positions of the Cd 3d3/2 and Cd 3d5/2 spectra are centered at 404.2 and 411.2 eV respectively which denotes Cd is in 2 þ oxidation state. Fig. 8(d) show the XPS spectrum of O1s. Here, a sharp peak is positioned at 530 eV. The presence of O1s may be due to the chemi-resistive O2 and the presence of O2 . The occurrence of Al is indicated from the Al 2p spectrum and the intensity is weak because it is a doping ions here. The peak position of the Al 2p spectrum is centered at 74.2 eV which confirmed the presence of Al3þ ion in the Al doped CdO film.
3.3.2. SEM with EDAX studies The SEM studies of pure and CdO: Al films have been carried out with various Al dopant levels and the obtained images are given in Fig. 5. From these images, it is observed that pure and 0.5 wt% Al doped CdO films have highly aggregated nanoparticles (NPs) surface morphology whereas increased Al doping (1.0 and 1.5 wt%) concentration shows conspicuous spherical shaped particles with different sizes. The surfaces were totally covered with nearly uniform sized grains and free from pinholes. When introducing dopant (Al) ions, the pyrolysis of droplet changed under identical deposition conditions. These changes could be played a major role for the changes of CdO morphology. The conclusion is that an irregular aggregated NPs of pure CdO thin film images were changed to uniformed spherical shaped NPs by incorporation of Al dopant ions. The EDAX analysis of CdO: Al (1.5 wt%) thin film is shown in Fig. 6. From the EDAX spectra, the existence of Cd, O and Al atoms is confirmed from the coated CdO: Al (1.5 wt%) thin film.
3.6. Sensor studies The sensing capacity on thin films depend on charge transfer in the metal oxide films and that depends on physical, chemical and structural
Fig. 4. Atomic force microscopic images of CdO: Al thin films. (a) Pure CdO; (b) CdO: Al 0.5 wt%; (c) CdO: Al 1.0 wt% and (d) CdO: Al 1.5 wt%. 4
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Fig. 5. SEM images of pure and CdO: Al thin films. (a) Pure CdO; (b) CdO: Al 0.5 wt%; (c) CdO: Al 1.0 wt% and (d) CdO: Al 1.5 wt%.
Fig. 6. EDX spectrum of 1.5 wt% of Al doped CdO thin film.
parameters like thickness, size, crystallinity, composition, permeability, surface morphology, effective surface area. Generally, negatively charged surface oxygen species in CdO film could be facilitated upward movement of conduction band edge that decreases the conductivity of the film [39]. The change in conductivity of the film was governed by changing of free charge carrier concentration, potential barrier height in the grain boundaries and electro physical parameters of the thin film [40]. Electrical conductivity of semiconducting thin film vapour sensor varies because of two important reactions on the surface of the film. In first reaction, O2 molecules were physisorbed on the surface and gets ionized by pulling charge carrier from conduction band, it became ionosorbed as O- that lowers the conductivity of the sensor [41]. Oxide formulation on the film has been given by
O2 þ 2e →2O
(3)
The vapours sensing phenomena of thin metal oxide semiconductor films have been seen by the interaction between analyte gases and existed reactive oxygen species on the surface of the film. In CdO crystal lattice, the electrons from Cd interstitials migrated to surface oxygen and Cd2þ ions have been ionized to Cdþ level that could act as donors. The conductivity of the film gets increased in the TMA vapour atmo sphere; this may be due to electron transfer between nitrogen that contains lone pair of electrons to chemisorbed oxygen through CH3 active sites present in the TMA molecule. The reaction between functional group (R) of gas with ionosorbed oxygen is
5
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Fig. 7. (a).The variation of resistivity; (b) carrier concentration; (c) mobility as a function of Al concentration.
R þ O →RO þ e
Table 2 Electrical parameters of spray coated CdO: Al thin films.
(4)
The sensitivity (S) [42] is denoted as
Samples
Carrier Concentration (cm-3)
Conduction Type
Resistivity (Ω cm)
Mobility (cm2 V-1s-1)
CdO: Pure CdO: Al (0.5 wt %) CdO: Al (1.0 wt %) CdO: Al (1.5 wt %)
6.89Eþ19 1.16Eþ20
n n
1.41E-03 9.62E-04
43.6 49.8
1.53Eþ20
n
8.20E-04
56.0
2.20Eþ20
n
6.52E-04
64.4
% S ¼ [(Ra –Rg) / Ra] � 100
(5)
where Ra and Rg are change in resistances of thin films without TMA vapours and with TMA vapours respectively. The Wolkentein’s model [43] explained TMA sensing mechanism by 4ðCH3 Þ3 N þ 21 O 2 ðadsÞ ¼ 2 N2 þ 12 CO2 þ 18 H2 O þ e
(6)
The enhancement in the TMA sensitivity as shown in Fig. 9 is due to the excess carrier ions present in the CdO crystal system by incorpora tion of Al. Selectivity is a vital aspect for specific analyte gas in a mixed vapour atmosphere, especially having same chemical environment. The interactions between ammonia, benzene and dimethylamine could be ignored because of a slightly larger steric hindrance and low response of DMA, which lacked the higher cloud density of N atoms through methyl
Fig. 8. (a). The survey scan XPS spectrum of the 1.5 wt% Al doped CdO film. Fig. 8(b–d) shows respective elemental binding energy peaks of Al, Cd and O. 6
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Fig. 9. (a). The variation of sensitivities for pure and CdO: Al samples for different ppm of TMA vapours. (b) Selectivity of CdO: Al films for TMA vapours over other volatile gases.
and mobility (64.4 cm2 V-1s-1) have been obtained on 1.5 wt% Al doped CdO film. From sensor study, it is found that 1.5 wt% Al doped CdO film shows a maximum sensitivity of 32% for trimethylamine vapour than others such as benzene, ammonia, and dimethylamine. Eventually an important conclusion is that Al doped CdO thin films prepared by CSP technique are a promising candidate for detecting TMA vapours in ambient condition.
bonds in TMA that presented greater response to surface oxygen of CdO: Al film and it is tedious to adsorb ammonia and benzene gases on surface of the CdO: Al film. More number of oxygen ions on film surface leads to higher potential barrier and result in higher resistance. The number of absorbed oxygen highly depends on morphology, grain size, and active surface area of CdO: Al film. The agglomerations in the film have blocked the in-diffusion of analyte vapours to the whole film surface, which makes deteriorated vapour response. Consequently, higher con centrations of vapour accessible nanostructures without sacrificing the smallest optimum particle size were also necessary for enhanced sensor response. The other reason may include the strong reducing nature of TMA than other gases. The reducing vapour helps to emerge out free charge carriers and decrease the resistivity of the film. From the above equation, it is clear that four TMA atoms can liberate 21 electrons in the film [44], an obvious enhanced sensing performance than other vapours were taken. This may be a reason for higher selectivity of TMA towards other candidates. The amount of reducing vapours on the CdO: Al thin film depends on the density of active sites of the film. Vapour response is related to the number of active sites on the surface of sensor and interstitial concentration that present on the CdO: Al thin films. Here, TMA vapours are sensed by CdO: Al film surface at room temperature. Obvious reasons for conduction on CdO: Al film surface include defective sites like missing ions (ie., O2 vacancies), cadmium self-interstitials and as well as through substitutional Al impurity ions in the Cd interstitial positions [45]. Fig. 9(a) demonstrates the TMA gas sensitivity plot for pure and Al doped CdO films and all the films became saturated above 300 ppm of TMA vapour. It gradually increased from 0% till 32% of vapour sensi tivity with increasing Al concentrations whereas benzene sensing attained 5.91%, ammonia got saturated at 5.79% and dimethylamine (DMA) offered only 4.86% sensitivity. Fig. 9(b) show selectivity of pure and CdO: Al thin films. TMA reached a maximum sensitivity of 32.12% on surface reactions with CdO: Al film having 1.5 wt% Al concentration. By this work, it has concluded that 1.5 wt% Al doped CdO film shows effective TMA sensing behavior and reiterates that it could be a useful for ecological and industrial applications in the near future.
Acknowledgment The authors would like to thank DST-PURSE program of School of Physics, Madurai Kamaraj University for providing AFM and SEM facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104753. References [1] K. Wetchakun, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Semiconducting metal oxides as sensors for environmentally hazardous gases, Sens. Actuators B 160 (2011) 580–591. [2] DhiaAldinSleibi Mustafa, RawaaIsam Mohammed Al-Rawi, Structural and optical properties for Zn doped CdO thin films prepared by pulse laser deposition, Iraqi J. Sci. 59 (2B) (2018) 839–846. [3] S.J. Helen, Suganthi Devadason, M. Haris, T. Mahalingam, Transparent conducting Mo-doped CdO thin films by spray pyrolysis method for solar cell applications, J. Electron. Mater. 47 (4) (2018) 2439–2446. [4] G.E. Moreno Morales, M.E. Araiza Garcia, S. Cruz Cruz, B. Rebollo Plata, O. Portillo Moreno, R. Gutierrez Perez, CdCO3 nanocrystalline thin film grown by chemical bath and its transition to porous CdO by thermal annealing treatment, Optik - Int. J. Light and Electron Optics 171 (2018) 347–355. [5] I. Ben Miled, M. Jlassi, I. Sta, M. Dhaouadi, M. Hajji, G. Mousdis, M. Kompitsas, H. Ezzaouia, Influence of In-doping on microstructure, optical and electrical properties of sol–gel derived CdO thin films, J. Mater. Sci. Mater. Electron. 29 (2018) 11286–11295. [6] K. Sankarasubramanian, P. Soundarrajan, K. Sethuraman, R. Ramesh Babu, K. Ramamurthi, Structural, optical and electrical properties of transparent conducting hydrophobic cadmium oxide thin films prepared by spray pyrolysis technique, Superlattice Microstruct. 69 (2014) 29–37. [7] A.S. Kamble, R.C. Pawar, N.L. Tarwal, L.D. More, P.S. Patil, Ethanol sensing properties of chemosynthesized CdO nanowires and nanowalls, Mater. Lett. 65 (2011) 1488–1491. [8] Padmanathan Karthick Kannan, Ramiah Saraswathi, John Bosco Balaguru Rayappan, A highly sensitive humidity sensor based on DC reactive magnetron sputtered zinc oxide thin film, Sens. Actuators 164 (2010) 8–14. [9] Dipak L. Gapale, Sandeep A. Arote, M Palve Balasaheb, Sanjaykumar N. Dalvi, Ratan Y. Borse, Effect of film thickness on humidity sensing of spray deposited TiOthin films, Mater. Res. Express 6 (2019), 026402. [10] Chong Wang, Ruize Sun, Xin Li, Yanfeng Sun, Peng Sun, Fengmin Liu, Geyu Lu, Hierarchical flower-like WO3 nanostructures and their gas sensing properties, Sens. Actuators B 204 (2014) 224–230.
4. Conclusion Pure and Al doped CdO thin films were coated on glass substrates using a facile CSP technique. All the films have built by polycrystalline cubic phase with only preferential plane changing after Al doping result in decreasing crystallite sizes. All the deposited films exhibited above 90% of optical transparency as expected in the visible and IR regions. A slight energy bandgap widening is observed after Al doping. CdO films deposited with uniform, smooth, well-adhered and pinhole free aggre gated NPs and then it modified as clearly visible spherical NPs with increased Al doping. The higher carrier concentration (2.2 � 1020 cm-3) 7
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