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Volume 14, 2005

 

 

Optical properties and applications of lead oxide thin film prepared by chemical Bath deposition Technique

 

D.D. O. Eya,* A.J .Ekpunobi and C.E. Okeke+

Department of Industrial physics, Nnamdi Azikiwe University, Awka, Nigeria.

* Department of physics, federal University of Technology, Owerri, Nigeria

Email: eyadom2003@yahoo.com

+Department of Physics & Astronomy, University of Nigeria, Nsukka, Nigeria

Abstract

Lead oxide (PbO) thin films have been successfully prepared by chemical bath technique and thermally treated under annealing temperatures of 423, 453 and 573 K. The films generally show high transmittance (60-80%) in the visible/near infrared regions of electromagnetic spectrum. The oxide film is therefore a good material for warming applications at home (in temperate regions) and in Agriculture. The absorbance and reflectance are of order of 30% and 20% respectively near the UV region and dwindle with higher wavelength. The band gap of the oxide film is in the range of 2.90 –2.95eV. The values of the refractive are generally greater than 2.0 over the entire spectrum.

Keywords: Lead oxide, optical properties, thin film chemical bath, annealing temperature, band gap.

PACS NOS. : 81.10.DN, 78.66. – w, 78.40.- q, 78.20.Ci

 

 

 

 

  

1.         Introduction

In recent times, several technique have been adopted for thin film deposition such as Sol –gel [1] ionized Cluster Beam Deposition [2-3] dc reactive magnetron, Sputtering [4], pulsed laser deposition [5], chemical bath deposition [6-11] e.t.c. Many of these methods are expensive and require high vacuum and controlled formation conditions [12]. Chemical bath technique has been frequently used for the deposition of metal oxide thin films [6,11,13]. The technique has also been used widely to deposite other material because it is cheap and simple [7,9]. The choice of the technique is largely because it possesses a number of advantages over conventional thin film deposition method such as low cost, low temperature and easy coating of large surfaces. This technology is based on the controlled release of the metal ions. In the case of oxide films, the deposition follows a two-step process; deposition of the hydrous film and its pyrolytic decomposition into the anhydrous film [8].

In this paper, we report on the preparation and optical properties of lead oxide thin film on microscope glass slide substrates using chemical bath deposition technique.

2.         Experimental Details

The      Lead oxide (PbO) thin film were prepared using chemical bath technique. The following constitute the chemical bath system; lead nitrate [Pb(NO3)]2, sodium hydroxide (NaOH) and  ethylenediamine tetra acetate EDTA which served as the complexing agent. Various concentration ratios were used to deposit the oxide films of various thickness such as 0.04M: 0.08M:0.10M, 0.05M:0.10M:0.10M, e.t.c respectively. High bath concentration results in little or no deposition at all. Solutions for the bath were made in 50ml and/or 100ml beakers and 76mmx 26mm x1mm commercial quality glass microscope slides were used as substrates. Before use, the microscope slides were soaked in Aqua Regia (3:1 concentrated HCl and HNO3) solution and then washed thoroughly with detergent and rinsed in distilled water. When the solution is made, the substrate is suspended vertically in it with the aid of a cover to the beaker. 0.10M concentration of the complexing agent, EDTA was found to be the optimum concentration for the deposition. Higher concentration renders the bath unreactive. Initially when solution is made, it appears almost clear but gradually turns white. The optimum period of growth is 6 –18 hours. Below 6 hours, firm deposition may not have taken place while above 18 hours, the resulting film may not be uniform. The colour of the film deposit is white which is lead hydroxide. (Pb(OH)2. The reaction process is of the form (10).

Pb(N03)2 + 2NaOH  ฎ   Pb(OH) 2  +2NaNO3

Pb(NO3)2 +EDTA  ⇌  [Pb(EDTA)]2+ + 2NO3-

[Pb(EDTA)]2+  ⇌   Pb2+  +EDTA 

NaOH  ⇌ Na+ + OH-

Pb2+ + 2OH-  ⇌   Pb(OH)2

The film were annealed at temperatures of 423, 453 and 573K.

The composition of the thin films was determined using energy dispersive X-ray fluorescence (EDXRF). The analysis was done with the aid of an ANNULAR 25mCi109Cd excitation source that emits Ag-K X-rays (22.IkeV).

The optical absorbance/Transmittance of the film were studied in the spectral range of 340 – 1000 nm using Unicam Helios Gamma UV-Visible Spectrophotometer.

3.         RESULTS AND DISCUSSION

The thin film as grown (at 300k) is white in colour which is Pb(OH)2.

In the beginning, the ionic product of the solution (IP) is greater than the solubility product of the solution (SP), which leads to film deposition. As the ions (cations and anions) react, the IP decreases with time up to the moment it becomes equal to the SP [7]. At this point, the white precipitates have either been deposited on the substrate or remained in the solution (partly sedimented) and maintaining its colour. As stated earlier, the optimum period of deposition is 6 – 18 hours. These films were annealed at temperatures 423, 453 and 573K. When the temperature is about 573K, the films gradually assume the colour of lead oxide (PbO), yellow [14]

Figures 1 and 2 show the spectral dependence of the transmittance/reflectance of the films under various annealing temperatures. The properties tend to assume more definite trend at higher annealing temperatures. A manifestation of this is shown in the distribution of points about the curves for various annealing temperatures. One could attribute the behaviour to the transition from Pb(OH)2 to PbO. The transmittance rises with higher wavelength, which indicates high transmittance in the infra-red region. The variation is between  40% and 20% transmittance. This indicates that PbO thin film is a good material for warming applications in temperate regions; it could be used as window glazing to create warmth in the house. It could also be very useful in Agriculture especially creating warmth for chicken in a poultry farm. On the other hand, reflectance decreases rapidly with increase in wavelength falling from about 20% near UV region to about 13% near the infrared region.

Figures 3 and 4 show the spectral dependence of the absorbance of the films under the various annealing temperatures. The figure shows that absorbance decays exponentially with increase in wavelength but infinitely large near the UV region.

The absorption coefficient of the film is related to the photon energy for direct transition by

 (ahn) =(hn – Eg)1/2

(ahn)2 =hn – Eg

where a is the absorption coefficient, h, plank’s constant, n, the frequency and Eg, the band of the film.

 

 


 

 


 

 


 

 


Eg of the film is obtained when the straight portion of the (ahn)2 versus hn plot is extrapolated to (ahn)2  = 0 [10,15] as shown in figure 5. Band gap of 2.95 – 3.00 eV were obtained for PbO thin film. However, for the film as grown [Pb(OH)2], band gap of 2.40 – 2.80eV were obtained.


Figures 6 and 7 show the spectral dependence of refractive index real. It shows exponential decay with higher wavelength but the values tend to become infinitely large towards the UV region. Apart from sample annealed at 423K, which so far had been showing anomalous behaviour, when compared with others, the refractive indices of other samples decay exponentially with increasing wavelength. This is an indication that


the rate at which light slows down in the oxide film is very high in the UV region and relatively low in the NIR region. Figures 8 and 9 show plots of the real dielectric constant as a function of wavelength. The figure shows the dielectric constant decaying exponentially with increasing wavelength. In other words values of the constant are infinitely large in the UV region but decay to very low values in the NIR region.

 


 

 

 

 

 


 

 

 

 


4.         Conclusion

Lead oxide thin films have been successfully prepared by chemical bath deposition technique using lead nitrate [Pb(NO3)2] and a sodium hydroxide (NaOH) with EDTA as the complexing agent. The band gap of the film is 2.90eV. The films as grown have to be annealed at temperatures equal or greater than 573K to obtain the oxide (PbO) from the hydroxide [Pb(OH)2]. Lead oxide thin film shows high transmittance in the visible/near-infra-red regions of the electromagnetic spectrum but either absorbs or reflects most of the incident radiations in the UV region.

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