Optical Aliphatic Hydrocarbon Gas Sensor based on

2015 XVIII AISEM Annual Conference

Optical Aliphatic Hydrocarbon Gas Sensor based on Titanium Dioxide thin film A. Zotti, S. Zuppolini, M. Giordano, M. Zarrelli, A. Borriello

G. De Luca Dipartimento SCIFAR University of Messina Messina, Italy

Institute for Polymers, Composites and Biomaterials (IPCB) National Research Council of Italy (CNR) Portici, Naples, Italy Abstract— This work analyses the performances of an optoelectronic sensor based on a sensitive nanoscale film of Titanium Dioxide (TiO2), prepared via sol-gel method. The sensor was developed for fast and high resolution detection of aliphatic hydrocarbon gases. In particular, the change in reflectivity in presence of butane (C4H10) has been used as transduction property. Reflectance measurements have been performed by using a fiber optic refractometer, coated with either one or three TiO2 films, which shows a high sensitivity and an excellent reversibility performance. The final sensor was characterized by a layered configuration, with a thin layer of Silicon Dioxide (SiO2) deposited directly onto the fiber optics tip to ensure the homogeneity of the overlying Titanium Dioxide film. Optical microscopy and Atomic Force Microscopy (AFM) have been used to study the morphology of the sensible layer. The chemical structure of the deposited oxides (TiO2 and SiO2) has been analyzed using Fourier Transform Infrared (FTIR) Spectroscopy. Refractometer results have shown a better sensing behaviour for the sample obtained with a single dip-coating, in term of sensibility to butane, compared to the sample coated with three layer of TiO2. Keywords—Optical gas sensor, titanium dioxide, sol-gel method, hydrocarbon

I.

INTRODUCTION

In the last decades, due to the increasing demand the transportation frequency of fossil fuels, both on road and on rail, are enormously increased. Hydrocarbons are very hazardous substances, even at low concentrations, due to their high flammability and volatility. Thus, for safety reasons, it has become essential a real-time monitoring system of possible losses during the transportation operation of these substances (liquids and/or gaseous). Actually, the great part of hydrocarbon gas sensors is built on planar architecture, exploiting the variation of electrical conductivity of certain material in presence of these gases. However, these sensor devices generally require high operative temperature [1] and may not be suitable for use in small spaces. At contrary, fiber optic sensors are widely used for detecting very small amounts of chemical, gaseous and biological species at room temperature. Over the past two decades optical fibers have undergone a wide development in sensors field, also thanks to their versatility and easy processing [2].

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In this work, a fiber optic sensor for the detection of aliphatic hydrocarbon gas, with particular attention to butane, was developed and tested. The presented optical sensor is based on Titanium Dioxide (TiO2), prepared via sol-gel route and deposited by dip-coating technique on the tip of the optical fiber. TiO2 can be deposited on the optical fiber tip using different methods, such as spray pyrolysis deposition [3], however sol-gel method ensures an uniform layer deposition at low costs. A layered configuration was realised and tested revealing interesting results as butane detector. II.

EXPERIMENTAL SECTION

A. Fiber Tip Fabrication Silica (SiO2) and Titanium Dioxide (TiO2) were synthesized using a Sol-Gel process. Silica was prepared starting from TEOS (tetraethoxysilane), by acid catalyzed reaction. Even Titanium Dioxide was synthesized with an acid catalyzed reaction, using the titanium isopropoxide as precursor. Depositions were performed using the dip coating technique. The fiber tip was firstly cut by using a price manual cleaver and then washed in ethanol and acetone to remove possible coating traces and residuals. The so cleaned fiber was dipped in the silica sol, dried in air for 5 minutes, and then immersed in the titanium dioxide sol. Two different samples were prepared according to the number of dipping in the TiO2 sol, respectively. An aging of the coated tip was performed in oven for 24 hours @100°C, before testing the device as gas sensor. B. Detection System The sensor response was studied using a fiber optic refractometer set-up [4], which monitors the reflectivity as function of time in presence of different butane concentrations. A schematic view of the basic sensor design is reported in Fig. 1. The light source was a superluminescent diode (40 nm bandwidth) operating at 1310 nm. As shown in (1), the reflectance itself depends on the refractive index of the optical fiber, nf, of the sensible film, n and of the external medium, next, so as on the film thickness d and the wavelength of the probe light λ:


( r + r ) 4r r sin 2 δ I=α R=α 12 23 2 12 23 (1 + r12 r23 ) − 4r12 r23 sin 2 δ 2

r12 =

nf − n nf + n

; r23 =

n − next 2π nd ;δ = ; λ n + next

Eq. 1

Eq. 2

where r12 , r23 are the reflectivity coefficients at the fiber– layer and layer–external medium interfaces, respectively, while δ is the phase shift that the λ wavelength light undergoes when it passes through the sensible layer of thickness d , and α an optical coupling factor. The setup, described above, allows to perform an on-line monitoring of the active nano-architecture controlling the various fabrication stages. III.

RESULTS AND DISCUSSION

Fig. 2 shows the FTIR spectra relative to TiO2 and SiO2 powders, obtained drying the prepared sols. It can be noticed that the silica spectra can be separated in two region: WR (wave number) > 2500cm-1, which corresponds to the hydroxyl group stretching (present in the absorbed water and in the hydrolysed silica), and WR < 1300cm-1, associated to the Si-O-Si bonds. This behaviour find support in the work of Fidalgo et al.[5].

Fig. 1. Optoelectronic set-up.

Also in the TiO2 spectra, there are evidences of the hydroxyl groups signals, typical of the sol-gel prepared materials. In particular are very intense the bands at 3403cm-1 and 1631cm-1 which are related to the stretching mode of the OH group and the bending mode of molecular water. The broadband over the not fully resolved range of 400-1000cm-1 is related to bending and stretching mode of Ti–O–Ti bonds, and is characteristic of well-ordered TiO6 octahedrons. Combination between optical microscopy and AFM has allowed to study the morphology of the titanium dioxide surface.

Fig. 3. Optical micrographs and relative zooming with AFM performed on: a) 3 dip in TiO2 sol without SiO2 wet substrate, b) 1 dip in SiO2 sol followed by 3 dip in TiO2 sol, c) 1 dip in SiO2 sol followed by 1 dip in TiO2 sol.

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Fig. 5. Testing of the 1 TiO2 layer sample behaviour as butane detector, by monitoring the reflectance signal going from air to butane vapours Fig. 2. FTIR spectra relative to TiO2 and SiO2 powders.

It is clear from Fig. 3, that performing the deposition of TiO2 layers without a silica wet substrate, the formation of wide fractures on the titanium dioxide surface will occur. This feature could be related to the affinity between optical fiber silica and silica sol, as well as the elasticity of the wet silica substrate, which reduces the shrinkage of the titanium dioxide layers, preventing unwanted surfaces of fracture. Instead, there are not remarkable differences, in term of roughness, between samples prepared with three dip (Fig 3.b) and one dip (Fig 3.c) in TiO2 sol. The realised optical device was tested to verify its performance as label-free sensor in detecting aliphatic hydrocarbons vapours, such as butane. It is interesting to note that fibers coated with one titanium dioxide layer (Fig. 5) show a higher response to butane compared to those coated by three identical layers (Fig. 4). IV.

CONCLUSIONS

This work has shown interesting experimental results of an aliphatic hydrocarbon optical-based sensor device operating at room temperature by using titanium dioxide thin layered configuration directly deposited onto a simple optical fiber.

FTIR spectroscopy has revealed the chemical nature of the deposited material, showing that is possible to obtain an oxide system of good purity using a sol-gel technique avoiding high temperature annealing procedure. The surface morphology of the deposited sensible layers has been studied using an Optical and an AFM Microscope. It is noteworthy that the presence of a silica wet substrate under the TiO2 layer remarkably reduces the surface fractures enhancing the sensitivity of the device. Qualitative results have shown the good potentiality of this system and the quantitative aspects of the sensor characteristics are currently under investigation. V.

REFERENCES

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. Fig. 4. Testing of the 3 TiO2 layers sample behaviour as butane detector, by monitoring the reflectance signal going from air to butane vapours

978-1-4799-8591-3/15/$31.00 ©2015 IEEE