Nanostructured Cerium Dioxide Thin Films Biology Essay

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Abstract

Nanostructured Cerium dioxide thin films were deposited on glass substrates at 503K, using spray pyrolysis technique. A 0.05 M solution of Ce(NO3)36H2O diluted in distilled were used as precursors materials. The as-deposited and annealed CeO2 thin films at 573K were characterized by X-ray diffraction, scanning electron microscopy. An optical energy band gap of 3.29 eV for direct transition was estimated by UV-visible spectroscopy. Gas-sensing properties of samples were examined for two different vapours (Ethanol and Trimethylamine) at room temperature as well as for the working temperature of 323K and 373K. In particular, the results obtained clearly show that the transition from n-to p-type conduc­tivity is induced by the variation of the ethanol concentration as a function of working temperature.

Key words: Nanostructured Cerium dioxide thin films, spray pyrolysis technique, scanning electron microscopy, x-ray diffraction spectra, ethanol and trimethylamine

Introduction

Sensing of lower concentrations of toxic/ combustible gases and volatile organic compounds (VOCs) in the environment became more demanding from the point of pollution monitoring and food quality discrimination [1]. Considering this researchers started exploring the surface-effect of metal oxide semiconductor (MOS) for chemical/gas sensing [2, 3] due to its high sensitivity and relatively low cost. The gas sensing detection mechanism in such sensors is as a result of the interaction of a reducing/oxidizing gas like CO, H2,CH4, C2H5OH, NO2 etc. [4] with adsorbed oxygen on the surface of the MOS. The adsorbed oxygen on the surface of the material trap the carriers from the semiconductor, resulting in the formation of a space-charge region at the surface and grain boundaries of the sensor under ambient conditions. These space-charge regions are highly resistive and relative to the bulk of the material. Under such conditions when reducing gas reacts with adsorbed oxygen, number of charge carrier concentration will be increased and hence in turn increases the overall sensitivity of the sensor [5], but at the same time, the selectivity of such sensors is often inadequate. Exposure to any reducing gas results in a decrease in the resistance of the MOS based sensing element. Hence, selective detection of VOCs/gases in an mixed environment is rather difficult. This cross selectivity can be overcome by fixing the operating temperature and by adding suitable dopant to the MOS [5].

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Among various metal oxides like Cr2O3,Mn2O3, Co3O4, NiO, CuO, SrO, In2O3, WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, Nd2O3[6], cerium oxides has been attracting great interest due to its chemical stability and close lattice parameter matching with silicon (0.35 lattice mismatch) [7]. It has potential applicability to silicon-on-insulator structure, stable capacitor devices for large-scale integration and stable buffer layers between high temperature superconducting materials and silicon substrates [8]. Ceria (cerium oxide, CeO2) is an example of a mixed electronic-ionic conductor. Beie and Gnorich [9] and have shown its use as an oxygen gas sensor. CeO2 has considerable interest because of its applications in gas sensors, catalytic supports in automotive exhaust system [10,11] and electrolyte in solid-oxide fuel cell [11,12]. However, no work has been done on CeO2 thin film as Ethanol and Trimethylamine sensor at room temperature as well as at elevated temperatures.

Various deposition techniques like sol-gel [13], pulsed laser evaporation [14, 15], ion beam epitaxy [16, 17], MOCVD [18, 19], thermal relaxation [20] and rf magnetron sputtering [21] have been employed to prepare cerium oxide thin films. However, the former techniques are time consuming and high cost processes. On the other hand spray pyrolysis is found to be simple and economic. This technique can be used to prepare dense and optical quality films [22, 23]. Also this can be adapted for the production of large-area films. Furthermore, it does not require high quality substrates or chemicals because it usually involves atomizing a precursor solution to an aerosol, which is then directed to a heated substrate where the film gets deposited.

Hence in the present work an attempt has been made to deposit CeO2 thin film using spray pyrolysis technique. Also its structural, morphological, optical and electrical properties were studied and reported. The sensing performance of the CeO2 thin film in the detection of ethanol with appreciable sensitivity was observed as a function of the operating temperature. In addition, the cross-selectivity of CeO2 thin film towards trimethylamine is evaluated and reported.

2. Materials and Methodology

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CeO2 thin films was deposited onto the glass substrate using spray pyrolysis technique. Cerium (III) nitrate hexahydrate (Ce(NO3)36H2O) (99% as trace metal Crystals with lumps 1-2%) was used as precursor. During spray pyrolysis process, droplets of the precursor arrive close to the preheated substrates and then they undergo thermal decomposition, which results into the highly adherent cerium oxide thin film formation.

In order to attain nanostructure CeO2 thin film, optimized reaction parameters listed in table 1 were implemented.

Optimized Substrate Temperature (K)

Concentration of the precursor solution (M)

Pressure of the precursor gas-Compressed air (Kg/cm2)

Nozzle-Substrate distance (cm)

Spraying time for each cycle (sec)

Time interval between each cycle (sec)

503

0.05

12

40

10

20

Table 1: Optimized Reaction Parameters

The structural characterization of the deposited CeO2 thin films was carried out using X-Ray Diffractometer (XRD) with Cu Kα radiation of wavelength 1.5418 Š(D8 Focus, Bruker, Germany) at the scanning rate of 0.02°/min. The morphology of the film was observed through Scanning Electron Microscopy (FE-SEM, 6701F, JEOL, Japan). The optical characterization was carried out using Perkin-Elmer lamda-35, double beam spectro­photometer. Further sensing behavior of the CeO2 films were observed through electrometer (6517A, Kethiley, Germany).

Gas sensing characterization for CeO2 thin films towards ethanol and trimethylamine has been studied with a homemade testing chamber of water capacity of 5 liters. Desired concentration of ethanol and trimethylamine has been injected through septum provision in the chamber. The working temperature was varied using thermocouple and thermostat and the resistance variations were studied when thin film was exposed to various concentrations of ethanol and TMA using electrometer (Kethiley 6517 A model) interfaced with personal computer. Ohmic contacts to the thin film were made with silver paste [ ].

3. Results and Discussion

3.1 CeO2 film formation:

During spray pyrolysis process, droplets of the precursor arrive close to the preheated substrates and then they undergo thermal decomposition, which results into the highly adherent cerium oxide thin film formation. The reaction mechanism during pyrolysis process is as follows

Ce(NO3)3.6H2O CeO2 + 3NO2 +6H2O +1/2 O2

After the deposition of Ceria thin film, few samples were annealed for 5 hour at 573 K in Muffle furnace.

3.2 Structural and Morphological Characterization

The X-ray diffraction profiles of annealed and as deposited CeO2 thin films are shown in Figure. 1 and 2. The deposited films were found to be polycrystalline in nature with fluorite type face-centered cubic CeO2 type structure with standard JCPDS data cards [JCPDS 81-092, 34-0394]. Observed d-spacing agrees with standard one.

The peak intensity was found to be high in the case of annealed CeO2 for (200) plane because, during annealing process the crystalline particles gets sufficient amount of thermal energy resulting in the increase in radius of the crystalline particle, which in turn decreases the amorphous nature of the thin film and hence the orientation of crystallinity of the annealed CeO2 thin film is improved when compared to the as-deposited CeO2 thin film. However small percentage of orientations of (111),(220),(311) & (400) planes are also observed. The dominance of (200) fluorite type face-centered cubic reflection indicates that the preferential growth of crystallite is in this particular direction. The broad hump that is observed in the background of XRD is due to the amorphous glass substrate and also possibly due to some amorphous phase present in the CeO2 thin film.

The grain size (Dhkl) for the CeO2 thin films are evaluated for the preferred (hkl) planes using Scherrer formula [27]

Dhkl = kλ / β2θCosθ (1)

with k = 0.94, where θ is the Bragg's angle, λ is the wavelength of X-rays used, β2θ is the width of the peak at the half of the maximum peak intensity.

As-deposited CeO2

Annealed CeO2

Fig. 1 XRD for As-deposited CeO2 thin film Fig. 2 XRD for annealed CeO2 thin film

In Figs.1& 2 the XRD pattern of a CeO2 spray pyrolysis film annealed at 300 ËšC is shown. The XRD pattern shows reflections of the cubic fluorite crystal structure type [28]

Position

[°2Th.]

FWHM

[°2Th.]

d-spacing

[Å]

Grain Size

D(nm)

Rel. Int.

[I/I0] [%]

28.5122

1.2026

3.12803

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6.766

57.29

33.0304

0.9228

2.70975

8.879

100.00

47.4004

1.6835

1.91640

5.108

25.44

56.2407

1.5991

1.63433

5.570

19.46

69.3284

2.5562

1.35434

3.736

14.72

Table 2 Grain size calculation for as-deposited CeO2 thin film

Position

[°2Th.]

FWHM

[°2Th.]

d-spacing

[Å]

Grain Size

D(nm)

Rel. Int.

[I/I0] [%]

10.7774

1.1808

8.20917

6.6829

7.34

28.5475

0.7775

3.12424

10.475

54.77

33.0632

0.7741

2.70714

10.59

100.00

47.4707

1.8052

1.91373

4.75

21.63

56.3399

1.6803

1.63168

5.30

18.59

69.2976

1.7829

1.35486

5.35

15.33

Table 3 Grain size calculation for annealed CeO2 thin film

3.3 Scanning Electron Microscope Studies

(a)

Region A

Region B

(b)

(a)

Fig. 3: SEM image for (a) As-deposited (b) Annealed CeO2 thin films Inset: Higher Magnification

The morphological characteristics of the CeO2 thin film have been observed by scanning electron microscopy (SEM). The SEM micrographs are given in Figure. 3 for as-deposited and annealed (573K, 5hr) CeO2 thin film.

The SEM image of as-deposited CeO2 thin film has two major regions as indicated in the Figure 3. The region A shows the agglomeration of the particles due to improper decomposition of the precursor solution and the region B indication void present in the material. So in order to decrease the agglomeration and voids the CeO2 thin film has been annealed at 573 K for about 5 hours and the annealed film was found to be fairly uniform, polycrystalline and free from macroscopic defects like cracks or peeling and the grain size of the particles is about 20-32nm range. Crystallinity of the annealed thin film is good when compared to as-deposited CeO2 thin film.

3.4 Optical Studies

3.4.1 Transmittance and Absorbance

The transmittance spectra of CeO2 thin films are shown in Figure. 4. It is observable that the transmittance decreases to zero in the ultra-violet region and increases with wave­length in the visible region. The high transmittance is due to the existence of an interfacial layer with low refractive index between the deposited CeO2 thin film and the glass substrate [29]. Absorbance spectra of CeO2 thin film is displayed in Figure. 5. It is found that absorbance in the ultra-violet region has a very high value and decreases sharply with increasing wavelength and becomes almost constant towards in the visible region.

Fig. 4.4 Fig. 4.5

In Fig. 4, 5: The Transmission/Absorbance versus wavelengths plots of CeO2 thin film

3.4.2 Optical Band Gap

Eg= 3.29 eV

Fig. 6 Optical band gap of as-deposited/annealed CeO2 thin film

In order to estimate the optical energy band gap for CeO2 thin film, absorption spectra of the sample was determined from optical transmission and reflectivity measurements in the range 200- 800 nm. The absorption coefficient should vary according to the following relation

αhυ = A(Eg − hυ)n (2)

where A is the probability parameter for the transition and that can be taken constant within the observable frequency range, Eg is the optical band gap energy and n is a number which characterizes the transition process. For allowed direct transitions the coefficient n is equal to 1/2 and for indirect allowed transitions n = 2. It is found that n = 1/2 (allowed direct transition) gives the best description of our absorption measurements. Figure. 6 shows the plot of (αhυ)2 versus hυ for a direct transition. The extrapolated value (the straight line) to α = 0 gives a band gap energy Eg = 3.29 eV.

3.5 Hot probe technique

The CeO2 thin films have been confirmed n-type materials by using hot probe technique. In this experiment thermally excited majority free charged carriers are translated within the semiconductor from the hot probe to the cold probe. The mechanism for this motion within the semiconductor is of a diffusion type since the material is uniformly doped due to the constant heating in the hot probe contact. These translated majority carriers define the electrical potential sign of the measured current in the multimeter and it showed positive current reading which conforms the material to be n-type

4.6 Gas Sensing Measurements

The gas sensing properties were evaluated at ambient as well as at various working temperatures, from 323 K to 373 K, by measuring the change in electrical resistance when the flowing aroma was switched between dry air and ethanol/trimethylamine.

Fig 7 Resistance Vs Temperature Characteristics for 5 ppm of Ethanol

Response of the annealed CeO2 thin film was observed towards 5 ppm of ethanol at various operating temperatures was observed to fix the operating temperature of the sensing element. The observed Resistance verses Temperature is plotted and the same is shown in Fig 7. From the observation it is noted that the resistance of the CeO2 thin film is found to increase at 323K and remains almost constant between the temperatures 323 to 373 K. After 373 K, the resistance is found to be constant. This may be due to the relation between the operating temperatures and rate of the chemical reaction on the surface as well as diffusion in the surface [31]. In the case of lower temperatures the sensor response is restricted by the rate of the chemical reaction and at higher temperatures it is restricted by the rate at which the O2- ions getting absorbed on the surface of the material. At intermediate temperatures (323 K and 373 K), the rate of the two processes become equal and at that temperatures the sensitivity attains maximum. Hence, two optimal operating temperature points of 323 K and 373 K were chosen to study the sensing behavior of nanostructured CeO2 thin film towards various concentrations of Ethanol and TMA were studied.

Fig. 8: Sensing characteristics of CeO2 thin film for various concentration of Ethanol and TMA.

Therefore, the gas sensing properties of the annealed CeO2 films were observed at ambient atmospheric condition and at operating temperatures of 323 K and 373 K towards various concentrations of Ethanol and TMA. The observed response is shown in Fig. 8.

It is observed from the response of CeO2 thin film is that the resistance is found to be decreasing at room temperature as the concentration of Ethanol and TMA concentrations is increased. This decreasing trend of resistance at room temperature is mainly attributed to the following reaction mechanism

O2 + e-CeO2 O2- (3)

2C2H5OH + O2- (surf) 2CH3CHO + 2H2O + e- (4)

(CH3)N + O2- (surf) NO2 +CO2 + H2O + e- (5)

Eqn. 3 indicates the oxidation reaction between CeO2 surface and with ambient oxygen. When Ethanol and TMA aroma exposed to CeO2 thin film, its resistance decreased, as both VOCs undergoes reduction process and donate electrons to the surface of the material as given in Eqn. 4 and 5, this results in an increase in the charge carrier concentration and in turn decreases the resistance.

Further, the sensing characteristics of annealed CeO2 thin films at operating temperatures of 323 K and 373 K has been studied and same is shown in Fig 8. The response of the annealed film at temperatures 323 K and 373 K, the resistance of CeO2 films was found to show an increasing trend instead of a decreasing trend for various concentration of Ethanol. This may be attributed to the influence of concentration as well as the operating temperature [32,33] towards the decomposition of ethanol. This is because as the concentration of ethanol is increased and also due to the temperature effect the number of O2- ions getting adsorbed on the material increases resulting in the formation of inversion layer which increases the material resistance and is responsible in the change in the type of the material that is from n to p type. Hence the number of O2- ions adsorbed on the surface of the material significantly increases at 373 K as the concentration of ethanol is increased till 50ppm and hence the maximum increase in the resistance when compared to the increase in resistance at 323 K.

A. Gurlo et al. [32] observed the transition from p to n-type conductivity for polycrystalline α-Fe2O3 thick film and also a similar change in the type in the material type that is from n to p-type was also observed by T. Siciliano et al. [33], these workers showed the reason for the switching in the type of the material mainly due to the adsorption of O2- ions and formation of inversion layer on the surface of the material which depends on the concentration of ethanol and the working temperature.

But the response of CeO2 thin films towards various concentrations of TMA even at elevated operating temperatures was found to follow the same trend of the response as that of ambient atmospheric condition. This may be due the presence of strong reducing components evolved in the decomposition process of trimethylamine, which suppress the effect of adsorption of O2- ions on the surface of the material.

Hence the influence of operating temperature towards the sensing behavior of CeO2 thin film in the presence of Ethanol compared to TMA can be utilized to improve the selectivity of the sensor element (CeO2 thin film).

Fig 9 Response and Recovery Characteristics of annealed CeO2 thin film for Ethanol

Fig 9 showning the response and recovery characteristics of CeO2 thin film for Ethanol at ambient as well as at elevated temperatures (323 K and 373 K). From this characteristics it was found that at 373 K for 50 ppm of ethanol the response time is 35 sec and recovery time is about 65 sec which is very good when compared to the response and recovery characteristics at 323 K for the same comcentration of ethanol which was about 79 sec for response time and 66 sec for recovery time. The sensitivity of the CeO2 thin film for ethanol at 373 K is found to high ~140 as shown in Fig 10. and at room temperature the senstivity of CeO2 thin film is found to be high for TMA ~16 when compared to Ethanol which is about ~8. By this it is clear that at room temperature the CeO2 thin film is more sensitive to TMA and at elevated temperatures (323 K and 373 K) for ethanol and hence this characteristics can be used for cross selectivity purpose between two reducing VOCs such as ethanol and TMA.

Fig 10 Sensitivity measurement of annealed CeO2 thin film for Ethanol and TMA

Further, the long term stability and reproducibility of the CeO2 thin film, was ascertained by repeatedly observing the resistance of CeO2 thin film towards 50 ppm ethanol and dry air over a period of 25 days is shown in Fig 11. The experiments were carried out after 3 days of stabilisation with the films stored for a period of six months. The observed response were in well aggrement with the freshly prepared CeO2 thin film. Hence the excellent stability of the CeO2 thin film and its reproducibility were confirmed.

Fig 11 The long-term stability of the Vapour Sensor at 373K in (a) dry air (b) 50ppm of Ethanol Vapour.

Conclusions

In summary, CeO2 thin film were prepared by spray pyrolysis technique under optimized conditions. The as-deposited and annealed CeO2 thin films were characterized by X-ray diffraction, scanning electron microscopy, and UV-vis spectroscopy. The XRD result confirms fluorite type face-centered cubic CeO2 type structure with preferential orientation along (2 0 0) The fundamental optical absorption edge corresponds to a direct allowed transition with an energy gap located at about 3.29 eV.

The sensing properties of CeO2 thin film were examined at various operating temperatures (309, 323 and 373 K). And it is concluded that the transition from n to p type conductance was induced by variation of ethanol concentration at elevated temperatures.

At room temperature the sensitivity of CeO2 thin film was found to be better for TMA and at elevated temperature mainly at 373 K it showed high sensitivity for ethanol.