Enhanced ethanol sensing performance of Fe_ TiO2

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CERAMICS INTERNATIONAL

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Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature K. Vijayalakshmin, S. David Jereil Research Department of Physics, Bishop Heber College, Tiruchirappalli-620 017, Tamilnadu, India Received 23 October 2014; accepted 2 November 2014

Abstract TiO2 and Fe-doped TiO2 nanowires were synthesized by spray pyrolysis technique to study their gas sensing properties towards ethanol. Charge transfer from metal dopant to TiO2, and modification of TiO2 with Fe doping was investigated for their ability to enhance gas sensing activity. The X-ray diffraction results indicate that the Fe dopant was substitutionally incorporated by replacing Ti4 þ cations. Fourier transform infrared spectral analysis confirmed the presence of brookite TiO2. The UV–visible spectra showed the increase in absorption with Fe doping when compared with undoped TiO2 film, and optical band gap decreased slightly with Fe doping. SEM images revealed the presence of one dimensional structure of straight nanowires for undoped TiO2 and curved nanowires for Fe doped TiO2 films. To understand the enhancement of sensing performance of TiO2 film with Fe doping, the gas sensing mechanism of the film towards Ethanol at room temperature was studied and discussed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: TiO2 films; Spray pyrolysis; XRD; UV; Ethanol sensing

1. Introduction Ethanol vapor sensing finds large application in the field of food processing, biomedical, chemical industry, and breath analysis. For these applications, it is essential to provide high sensitivity, high selectivity, high stability, low working temperature, and short response and recovery times. Therefore, a great deal of research has been focused on the development of functional materials for high-performance of ethanol vapor sensing. In the past decades, semiconductors were widely used for gas sensing application, in that metal oxide semiconductors were extensively used because of their significant change in resistance upon exposure of gases to trace concentration of particular gas. Metal oxide semiconductors like ZnO, WO3, SnO2, TiO2 and V2O5 were used as gas sensors. Among them, TiO2 has been investigated extensively for gas sensing due to its higher surface reactivity to gases [1]. In order to enhance the gas n

Corresponding author. Tel.: þ91 9994647287 (mobile). E-mail address: [email protected] (K. Vijayalakshmi).

sensitivity of TiO2, nanostructures such as nanoparticles (0D) [2], nanowires (1D) [3], nanotubes (1D) [4], nanosheets (2D) [5] and hierarchical nanostructures (3D) [6], with high surface area were synthesized [7]. TiO2 nanowires were fabricated to improve gas sensing characteristics on a large scale, as nanowires being one dimensional, nanostructure with uniform morphology and a large surface area with controllable less agglomeration have potential applications. However, there are many disadvantages of TiO2 being used as gas sensors, due to high working temperatures, longer response, recovery time, and lower sensitivity. Recently, many methods were investigated with the focus of improving the gas sensing performance of TiO2 nanowires. Doping with components such as Au, Pt, Pb, and Ag is known to be effective, because active sites can be produced for particular gas species by doping. However, these sensitive materials can be poisoned easily in some gas atmospheres, which can lead to reduction in sensitivity and stability. Iron has been considered an appropriate candidate for doping TiO2, as the radius of Fe3 þ being similar to that of Ti4 þ . Therefore, Fe3 þ ions might easily be incorporated into TiO2 lattice [8]. As the band gap of iron is 2.6 eV, it will

http://dx.doi.org/10.1016/j.ceramint.2014.11.007 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007

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reduce the band gap of TiO2, thereby increasing the performance of the sensor at lower temperature with large response and lower recovery time. TiO2 and Fe doped TiO2 thin films were deposited by different methods, such as sol–gel process [9], chemical spray pyrolysis [10,11], sputtering [12], hydrothermal technique [13], reactive pulsed laser deposition [14] and electron beam physical vapour deposition [15]. Among which spray pyrolysis offers a number of advantages over other deposition processes, such as scalability of the process, cost-effectiveness, easiness of doping, operation at moderate temperatures and large uniform surface area. Fefunctionalized Brookite TiO2 nanowires were assessed to detect a range of gases, but their sensing properties toward ethanol gas were not reported as far as we know. Hence, in the present work a novel ethanol sensor based on TiO2 films were fabricated by spray pyrolysis technique. The effect of Fe doping on the structural, optical, and morphological properties of TiO2 were investigated for enhancing sensing performance of TiO2 towards ethanol at room temperature. 2. Experimental procedure Aqueous solution of Titanium isopropoxide (TTIP) and Ferric chloride (FeCl3) were used as precursors for the production of TiO2 and Fe doped TiO2 thin films. The solution was atomized by pneumatic spray system using compressed air as the carrier. TiO2 and Fe doped TiO2 thin films were coated using spray pyrolysis unit as discussed by the author elsewhere [16]. Parameters like solution flow rate, nozzle to substrate distance and deposition time were optimized during deposition to obtain good quality films. TiO2 and Fe doped TiO2 films were deposited using a pulsed solution feed at a flow rate of 5 ml per min. The distance between nozzle and the substrate was 30 cm. The substrate temperature was kept at a constant value of 350 1C. The mixed aqueous solution was sprayed over hot substrate, which undergoes thermal decomposition, TiO2 film and Fe doped TiO2 film was obtained. These films were further used to investigate the structural, vibrational and optical properties. The structural characterization was done using X pert Pro X-ray diffractometer with Cu Kα radiation. Fourier transform infrared spectral analysis was made using FTIR spectrometer. The optical band gap energy of the films was investigated using Lambda 35 UV visible spectrometer. Surface morphology of films was studied using Scanning electron microscope. 3. Results and discussion 3.1. Effect of Fe doping on the structure of TiO2 thin films XRD pattern of the films only show the characteristic peaks of brookite phase at 2θ¼ 31.78 (JCPDS card no. 15-0875) without any characteristic peaks of Fe2O3 as shown in Fig. 1(a). Fe could not be observed in XRD as Fe3 þ and Ti4 þ have similar ionic radii, so Fe can easily substitute Ti4 þ ions in the crystal framework of TiO2 film. Ranjit et al., has also reported that Fe ions can be superseded by TiO2 [17]. The characteristic peaks for Fe-doped TiO2 thin films however, were shifted slightly to lower

Fig. 1. XRD patterns of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

2θ values when iron dopant was incorporated as shown Fig. 1(b). The incorporation of Fe into TiO2 gives rise to the structural expansion of the crystalline lattice, subsequently its structural distortion. The increase in the interplanar distance of the brookite framework causes the XRD peak patterns to shift to lower 2θ direction. Thus, the peak shift can be regarded as indirect evidence of successful iron doping into the TiO2 crystal framework. The crystalline sizes (D) were calculated from peak broadening of principal peaks by Debye Scherer’s formula [18], D¼

0:9 λ β cos θ

ð1Þ

where β is the full-width half maximum (FWHM) of the diffraction peak, λ is the X-ray wavelength (nm) and θ is the diffraction angle. Intensity of brookite TiO2 phase decreased significantly after Fe doping which can be ascribed to substitution of Fe into TiO2 lattice, also the iron oxide content influences the particle size. From the diffraction patterns it is also obvious that materials prepared are in the form of small particles, as the peaks are broad. It can be concluded from Scherer equation that doping iron-ion with proper content decreases the crystal size. For, pure TiO2 film the crystallite size was found to be 215.7 nm, which decreased drastically to 71.87 nm for Fe doped TiO2 film implying that the inclusion of strain due to Fe3 þ has marginal impact on the average crystallite size. Thus, doping modifications may prevent particle agglomeration forming well defined film with high surface area. 3.2. Influence of Fe doping on the vibrational spectra of TiO2 thin film Fig. 2 shows the characteristic peaks of undoped and Fe doped TiO2 samples. The IR bands at 3518 cm  1 revealed the presence

Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007

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Fig. 2. FTIR spectra of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

Fig. 3. Optical absorption of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

of –OH stretching vibrations, while the shoulder at 1782 and 1730 cm  1 can be ascribed to the bending vibrations of adsorbed H2O molecules. The bands at 1666 and 1625 cm  1 were due to organic groups bonded to titanium and iron. The absorptions at 1480, 1436 and 1415 cm  1, recorded for both undoped and Fe-doped films, belong to symmetrical and asymmetrical vibrations of CH2 or CH3 groups, respectively. The metal–OH bands were placed at about 1200 cm  1. The low-frequency regions of the spectrum (below 900 cm  1) illustrate bands attributed to stretching metal–oxygen–organic radical (Ti–O, Ti–O–Ti) [19]. IR bands at 554, 627 and 794 cm  1 were attributed to the vibrations of the pure TiO2 brookite phase. It is also interesting to note the appearance of a new peak at 2200 cm  1, with iron doping. Sharpening of bands with increase in intensity appears in Fe doped TiO2 films, this is due to reduction in particle size. This confirms that Fe doping increases the intensity of brookite peak as observed in XRD peak. For Fe doped TiO2 films, the band corresponding to TiO2 was replaced by the band centered at 540 cm  1 due to metal–oxygen bonds (Fe–O and Ti–O) of the (Ti, Fe)O2 solid solution. Red shift as shown in Fig. 3 also confirmed the incorporation of Fe with Ti molecules.

band and TiO2 [20]. However, new absorption band in longer wavelength region can also be contributed due to the appearance of dopant Fe as impurities, its intensity increasing with amount of doping. The band gap value is obtained by fitting the absorption edge of UV–Visible spectra of TiO2 and Fe doped TiO2 film and it is calculated by Tauc relationship [21].
n αhν ¼ A hν  Eg ð2Þ

3.3. Influence of Fe doping on the optical properties of TiO2 film The optical properties of pure and Fe-doped TiO2 films were studied by measuring the absorption ranging from 250 nm to visible 1100 nm wavelengths and the results are shown in Fig. 3. The abrupt increase in absorption at shorter than 350 nm can be assigned to absorption of pure brookite TiO2. It is clearly seen that Fe doped TiO2 sample reveal a shift in the absorption band edge towards longer wavelength, and increased absorbance in the visible range with that of pure TiO2 films. Comparing TiO2 and Fe doped TiO2 films, Fe doped TiO2 films have remarkable absorption in the visible light regions. Red shift associated with Fe doped TiO2 film can be associated with charge transfer transition between the Fe conduction or valence

where α is the absorption coefficient, A is the constant, h is the Planck’s constant, ν is the photon frequency, Eg is the optical band gap and n is 2 for direct band gap. The Eg values for direct band gap of the TiO2 and Fe doped TiO2 thin films changes according to the doping content. The graphs of (αhν)2 vs E (eV) for direct band gap of TiO2 and Fe doped TiO2 film is shown in Fig. 4. The direct band gap value for pure TiO2 was found to be 3.84 eV. After Fe doping the band gap shifts slightly towards a lower value of 3.77 eV. The decrease in band gap is associated with interaction among d electrons of Fe and the TiO2 conduction or valence band and eventually narrowing the energy gap of the titanium dioxide through the formation of new intermediate energy levels [22]. Yamashita et al. also reported that the overlap of the conduction band due to Ti (d) of TiO2 and the metal (d) orbital of the implanted metal ions on the metal ion-implanted TiO2 can decrease the band gap of TiO2 [23]. Thus, it can be confirmed from the present study that the shift in band gap is due to incorporation of isolated Fe3 þ ions. Optical transmittance spectra of TiO2 and Fe doped TiO2 thin films were shown in Fig. 5. The average transmittance of TiO2 in the visible range is above 92%. The transmittance of Fe doped TiO2 thin films were decreased when compared with pure TiO2 thin films. It is known that the optical transmittance of TiO2 thin films is mainly affected by the scattering on the surface and at the grain boundaries, and also the scattering at the grain boundaries played a great role in the decrease of the optical transmittance. Since Fe doped TiO2 thin film has smaller grains, it will lead to the higher density of grain boundary which will in turn increase the scattering at the grain boundaries.

Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007

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Fig. 4. Energy band gap of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

oxide can decompose into Fe and O2 at high temperature, thus the growth of the nanowire intersections is the culmination of the vapor–liquid solid growth process, in which Fe catalyst particles instigate and are responsible for leading the growth of TiO2 nanowires. The growth of the novel structures proffered here can be separated into two stages. The first stage is fast growth of the TiO2 nanowire. The growth rate is so high that a slow increase in the size of the Fe has marginal impact on the diameter of the nanowire, thus the TiO2 nanowire has a fairly uniform shape along the growth direction as revealed from the micrograph. The blurry diffraction dots in the inset image might hint at the existence of small crystallites segregated around the nanowires. Moreover, Fe doped TiO2 thin films showed a curve nanowire unlike pure TiO2 (Fig. 6(b)), the formation of the curving end on the top of straight nanowires should also be related to inadequate diffusion of Ti atoms at the late growth stage [25]. After a fast growth of the straight trunk, Ti atoms are difficult to diffuse such a long way to arrive at inter-face between Fe particle and TiO2 nanowire at the end. 3.5. Ethanol sensing properties of TiO2 films

Fig. 5. Optical transmittance of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

3.4. Influence of Fe doping on the surface morphology of TiO2 film A mechanistic study of the formation of the TiO2 and Fe doped TiO2 nanowires was performed based on the observation of the surface morphology using FE-SEM as shown in Fig. 6(a) and (b). The growth of the Ti nanowires is facilitated by the Ti oxide, where the TixO vapour promoted by evaporation due to heated substrate temperature plays a significant role. Nucleation of the nanoparticles is assumed to occur on the substrate. The decomposition reaction stimulates the precipitation of Ti nanoparticles, which act as the nuclei of the Titanium nanowires covered by Titanium dioxide. Precipitation, nucleation and growth of the nanowires occur at critical temperature of TiO2, suggesting that the temperature gradient provides the exterior force for the growth of the TiO2 nanowires [24]. Further undoped TiO2 thin film showed a small percentage of straight nanowires as blocks when compared with Fe doped TiO2 and this is due to diffusion of Ti with oxygen. It is known that Iron

The experimental observations may be accounted for by the surface-depletion model, as shown in Fig. 7. Similar to other vapor sensors based on metal–oxide semiconductors, the operation of the ethanol sensors at a concentration of 100 sccm is based on manipulation of the electrical resistance of the TiO2 and Fe doped TiO2 thin films that results from the interactions between the target vapor molecules and the active complexes on the oxide surface as shown in Fig. 8. Resistance of TiO2 thin film at air atmosphere was found to be 250 MΩ and decrease to 188 MΩ for ethanol/air atmosphere. Systematically, as the lower edge of the TiO2 conduction band is higher than the chemical potential of O2, when TiO2 is exposed to air, O2 will be adsorbed dissociatively onto the TiO2 surface, act as effective electron acceptors, capture electrons from the conduction band of TiO2, and form varied surface active complexes. With the formation of the active complexes, a surface depletion region is created within the oxide matrix, leading to an increase of the electrical resistance of the TiO2 layer as a result of the diminishment of charge carrier concentration [26]. In fact, the thickness of the depletion layer may extend throughout the entire nanowire, resulting in minimal electrical conductance. However, upon exposure to reducing vapors such as ethanol, effective electron transfer occurs from the vapor molecules to TiO2 (i.e., ethanol undergoes oxidation reactions), such that the thickness of the surface depletion region decreases accordingly. The increase of charge carrier concentration therefore diminishes the electrical resistance of the TiO2 film, as observed experimentally. On the basis of this sensing mechanism, it can be seen that the detection performance will be strongly contingent upon the charge transfer dynamics between the ethanol molecules and the oxide matrix in TiO2. The following equation explains the reactions between ethanol and oxide matrix to form CO2 and H2O [27]. CH3 CH2 OHðgasÞ -CH3 CH2 OHðadsÞ

ð3Þ

Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007

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Fig. 6. Typical SEM image of (a) pure TiO2 and (b) Fe doped TiO2 thin films.

Fig. 7. Surface depletion sensing model for (a) air and (b) ethanol gas.

CH3 CH2 OHðadsÞ þ 6O  ðadsÞ -2CO2ðgasÞ þ 3H2 OðgasÞ þ 6e  ðadsÞ ð4Þ Resistance of Fe doped TiO2 composite nanowires was found to be as low as 160 MΩ, this is due to Fe metal nanoparticles supported on reducible Titanium oxide surfaces, partial charge transfer may occur from the FeTiO2 centers to the metal nanoparticles, leading to the accumulation of negative charges on the metal nanoparticles surface. This may then facilitate the dissociative adsorption of oxygen onto the particle surface and consequently enhance the formation of electron deficient depletion layer. In fact, such a unique interfacial charge transfer

phenomenon has been exploited in the electro catalytic reduction of oxygen by using metal nanoparticles supported on oxide surfaces. Response and Recovery time was faster for Fe doped TiO2 thin films when compared with TiO2 films since Fe doped TiO2 intimate interfacial contacts may lead to the formation of defects and trap states within the oxide band gap [28]. These may serve as the surface active sites for the adsorption of oxygen and alcohol vapor molecules, which is beneficial to the improvement of the ethanol sensing sensitivity. Both structural defects and the resulting increased effective surface area are anticipated to improve the ethanol sensing performance of Fe doped TiO2 nanowires as compared with TiO2.

Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007

K. Vijayalakshmi, S. David Jereil / Ceramics International ] (]]]]) ]]]–]]]

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Fig. 8. Dynamic responses of (a) pure TiO2 and (b) Fe doped TiO2 nanowires for 100 sccm concentration of ethanol, recorded at room temperature.

[9]

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4. Conclusion TiO2 and Fe doped TiO2 thin films were synthesized by spray pyrolysis method. The effects of Fe doping on the structural, optical, and morphological properties of TiO2 thin films were discussed. XRD patterns of pure and Fe doped TiO2 thin films reveal that Fe3 þ ions were incorporated into the structure of Ti4 þ ions of Brookite TiO2 thin films without the formation of Fe2O3, thereby enhancing the quality of crystallites. The crystal grain size of TiO2 and Fe doped TiO2 thin films was about 215.7 nm and 71.87 nm, respectively. Fourier transform infrared spectra confirm the presence of Brookite phase for TiO2. The increase in the intensity of absorption after Fe doping on TiO2 is due to scattering effect of metal nanoparticles in TiO2 lattice. Decrease in band gap with Fe doping on TiO2 can be attributed to the formation of new intermediate energy levels. SEM micrographs of the synthesized TiO2 thin films produced a bunch of straight nanowires, and Fe doping has minimal influence over the growth of TiO2 nanowires with a bend on the edge of nanowires. The films were found to be very sensitive to ethanol gas at room temperature, and the Fe doped TiO2 nanowires revealed an enhanced sensing performance towards ethanol at room temperature.

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Please cite this article as: K. Vijayalakshmi, S. David Jereil, Enhanced ethanol sensing performance of Fe: TiO2 nanowires and their mechanism of sensing at room temperature, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.11.007