INTRODUCTION

Existing sensor for detecting trace amount of volatile vapours in the environment was based on measuring the change in electrical conductivity of semiconducting materials. Also these sensors are operated at elevated temperature for better sensitivity and selectivity (Suryawanshi et al., 2008; Lee and Reedy, 1999). Micro-heaters were used which consume minimum of 35 to 50 mW power (Lim et al., 2001) to operate sensor at elevated temperature. Hence, there is a need of room temperature vapour sensing to minimize the power consumption.

FOS is a good alternative one. Existing FOS are designed either modifying cladding (Vijayan et al., 2008; Grant et al., 2006; El-Sherif et al., 2007) or core of the fiber (Akita et al., 2010; Iga et al., 2003; Liu et al., 2003; Topliss et al., 2010) which makes the design part too complex. Instead, in the present work, the sensing material (cladding) was deposited as thin film over a glass rod (core) of 1 mm diameter. The deposited film will serve as a transducer part which makes the fabrication work simple.

MATERIALS AND METHODS

Fabrication of FOS: Chemical solution methods such as SILAR (Shinde et al., 2007) and Chemical Bath Deposition (CBD) (Khallaf et al., 2009) were used to obtain ZnO films. To prepare core of optical fiber, commercially available borosilicate glass (1.51-1.54 refractive index across the visible range) of 4 mm diameter is dropped into 1 mm. Zinc-ammoniate complex solution ([Zn(NH₃)₄]²⁺) serves as cationic precursor was prepared by adding drop by drop (interval of 2/3 sec) of ammonium hydroxide (40%) in 0.1 M of zinc nitrate solution under constant stirring kept at room temperature. Deionised water kept at 95°C serves as anionic precursor. ZnO thin films were grown heterogeneously by dipping the glass rod on cationic and anionic precursor solution for 3 and 5 sec, respectively. The number of dipping cycles was varied from 25 to 100 insteps of 25 to obtain different film thickness. The as-grown films were annealed at 250°C for 1 h to improve the crystallinity. Figure 1 shows the photograph image of prepared ZnO film on glass rod for different dipping cycle.

Experimental analysis: The grain size and crystalline nature of the prepared ZnO film was investigated using PANalytical X-ray diffractometer (Model D/MAX ULTIMA III). Surface morphology of the ZnO films was investigated by using HITACHI made Scanning Electron Microscope (SEM) of Model S-3000H with 18 kV accelerating potential. The film thickness was measured using Stylus Profilometer (Mitutoyo Stylus Profilometer SJ-301).

Fig. 1:	Photograph image of prepared ZnO film on glass rod for different dipping cycle

Fig. 2:	Photograph image of experimental setup

The optical studies were carried out using ELICO SL 159 UV-Vis Spectrophotometer. The ethanol vapour response measurements of the prepared FOS were carried out in the home made test chamber made. The light source used in the present work is of 850 nm obtained from MOS 850-ST operated at 9V dc. The detector was INFOS-M100 optical power meter. The accuracy of the detector is 0.01 dBm. During testing, the whole chamber is kept in vibration free table and optically made dark. The prepared FOS was connected to source and detector with the help of plastic fiber. Figure 2 shows the photograph image of experimental setup.

RESULTS AND DISCUSSION

Structural studies: Figure 3 shows the X-ray diffraction pattern of SILAR grown ZnO film deposited at different dipping cycles. All the pattern shows a preferential diffraction peaks at 2θ of 34.6 which corresponds to (002) orientation of hexagonal structure of ZnO (JCPDS card [05-0640]). Peaks such as (100) (101) were also observed and indicating polycrystalline nature of film.

Film thickness: Figure 4 shows that the film thickness increases as dipping cycle increased and attains saturation above 75 dipping cycle. Hence, films were deposited up to 100 dipping cycle.

Fig. 3(a-d):	XRD pattern of ZnO films prepared by SILAR technique different dipping cycles; (a) 25, (b) 50, (c) 75 and (d) 100

Fig. 4:	Variation of film thickness with respect to dipping cycles

Fig. 5:	Scanning Electron micrograph of ZnO thin film

Fig. 6:	Optical absorbance and transmittance of ZnO thin film

SEM analysis: Figure 5 shows the Scanning electron micrograph of ZnO film annealed at 250°C. The presence of discrete grain confirms the polycrystalline nature of prepared ZnO film. Also each grain can be indexed to hexagonal structure.

Optical studies: Figure 6 shows the optical absorbance and transmittance of ZnO film. It shows a smooth increase in transmission above 400 nm. The maximum transmission found to be 43% at 1100 nm and less than 25% at 850 nm which is much preferable for sensing application since, the working of the sensor is based on the transmittance changes with respect to the ethanol concentration.

Sensing properties: The amount of light gets attenuated depends on the ZnO cladding length, thickness and the ethanol vapour concentration. Therefore, sensitivity studies have been carried out for four FOS prepared from different dipping cycle (labeled as S₂₅, S₅₀, S₇₅ and S₁₀₀) with four different lengths (10, 20, 30, 40 mm). Ethanol vapour concentration was fixed to 100 ppm. Appreciable response was observed for S₅₀, S₇₅. Figure 7 shows the ethanol response of S₅₀ and S₇₅ as a function of sensing length. The lesser response of S₂₅ and S₁₀₀ may be due to the presence of voids and inhomogeneity. Therefore, S₅₀ and S₇₅ were tested for dynamic studies to find response and recovery time.

Fig. 7:	Sensitivity of S50 and S75 as a function of sensing length

Fig. 8(a-b):	Dynamic response of (a) S50 and (b) S75 FOS towards ethanol vapour

Response and recovery time studies: The response and recovery trend of the fabricated FOS for ethanol vapour was shown in Fig. 8. The stair-case like changes indicates a slow adsorption and desorption of ethanol vapour. The response and recovery time were found to be 200 and 400 sec, respectively. Also it shows S₅₀ is better than S₇₅ due to lesser ZnO film thickness.

CONCLUSION

Preliminary results of ZnO thin film coated glass rod as optical based ethanol sensor at room temperature were studied. Experiment shows that the transmitted light intensity decreases when ethanol of 100 ppm is injected. For the same ZnO thin film length, it was observed that as thickness increases, sensitivity found to decreases. In conclusion, the reported technique for ethanol sensing at room temperature is encouraging and can be tuned to reduce response and recovery time.