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Electrical and VOC sensing properties of anatase and rutile TiO2 nanotubes
Journal of Alloys and Compounds 616 (2014) 89–96
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrical and VOC sensing properties of anatase and rutile TiO2 nanotubes Erdem Sennik a, Necmettin Kilinc a,b,⇑, Zafer Ziya Ozturk a,c a
Gebze Institute of Technology, Department of Physics, Kocaeli, Turkey Nigde University, Mechatronics Engineering Department, 51245 Nigde, Turkey c TÜBITAK-Marmara Research Center, Materials Institute, Kocaeli, Turkey b
a r t i c l e
i n f o
Article history: Received 19 April 2014 Received in revised form 23 June 2014 Accepted 13 July 2014 Available online 21 July 2014 Keywords: Nanostructured materials Electrical transport Nanofabrications TiO2 nanotube Chemoresistive sensor
a b s t r a c t The dc electrical and volatile organic compound (VOC) sensing properties of TiO2 nanotubes in both anatase and rutile phases were investigated. TiO2 nanotube arrays were obtained in aqueous HF (0.5 wt.%) electrolytes by anodizing of Ti thin films that deposited on quartz substrates using thermal evaporation. Anodization was performed at 10 V in aqueous HF at 0 °C. Then the fabricated TiO2 nanotubes were annealed at 300 °C and at 700 °C under dry air for 5 h to obtain anatase and rutile phases, respectively. The TiO2 nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) as structural, and UV–vis spectrophotometer as optical. The current voltage characteristics of the nanotubes under dry air flow revealed that the conductivity of the sample with anatase phase was higher than that of the sample with rutile phase. The VOCs sensing properties of the nanotubes were investigated at 200 °C. It was found that the sensor response of anatase was higher than that of rutile for almost all VOC gases. On the other hand, the sensitivities of two sensors are the highest for isopropyl alcohol. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Scientists have been dedicated to the improvement of useful and cheap sensors that can incessantly detect volatile organic compounds (VOCs) [1–8]. VOCs are the basic sources of indoor air pollution and give rise to harmful influences on human health such as breath, skin. Furthermore, human breath includes VOCs such as ethanol and acetone [9–12]. Inasmuch as the detection of VOCs becomes important in air environments, solid-state semiconductor-type gas sensors are designed due to their compact size and low cost. It is clearly understand that the surface shape of sensing layer such as grain size, film thickness, agglomeration, porosity, surface geometry, nano and polycrystalline is an important parameter for gas sensors especially semiconductor gas sensor [13–16]. In order to improve the sensor parameters such as sensitivity, response time, recovery time and reducing optimal working temperatures to low temperatures, researchers have been focused on fabrication of nanostructured metal oxides and functionalization or doping of them [17–22]. High gas sensitivity is related to porous ⇑ Corresponding author. Address: Department of Physics, Science Faculty, Gebze Institute of Technology, P.O. Box 141, 41400 Gebze-Kocaeli, Turkey. Tel.: +90 2626051312; fax: +90 262 6538490. E-mail address: [email protected] (N. Kilinc). http://dx.doi.org/10.1016/j.jallcom.2014.07.097 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
sensing films due to gas diffusion all around the porous film. In general, the gas sensing mechanism of metal oxides at high temperature explained with transfer of electrons between the sensing material and target gases, with the formation or deformation of the depletion layer depending on the material [17–21]. But the gas sensing mechanism of metal oxide at low temperature could be elucidated with a thin layer of condensed humidity on the surface of the metal oxide [23]. This is similar to hydrogenated diamond and/or silicon based sensors. Joshi and Kumar discussed gas sensing mechanism of Si nanowires at room temperature in details [24]. The dimensions of nanostructures could be controlled by using several methods such as anodization, hydrothermal, and chemical vapor deposition (CVD). Hence, large size molecules such as volatile organic compounds (VOCs) can be detected. It is available to utilize TiO2 nanotubes for gas sensing including large sized gas molecules like VOCs. During the past decades, anodized TiO2 nanotubes have been synthesized by controlling the nanotube dimensions with changing the anodization parameters and have been used in many research and technological areas such as catalysis, batteries, solar cells, gas sensors, bio-sensing and biomedical applications [25– 30]. There are more researches on resistive hydrogen gas sensing properties of anodized TiO2 nanotubes, but a few studies on resistive VOC sensor properties of the nanotubes. In generally, VOCs can
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be sensed by TiO2 nanostructures up to 250 °C [31–39]. Seo et al. investigated VOC sensing properties at high temperature (450– 550 °C) with TiO2 nanotube by hydrothermal method [31,32]. They observed high sensitivity to ethanol and toluene at high temperature. Rella et al. investigated VOC sensing of a thin layer of TiO2 nanoparticles deposited by matrix assisted pulsed laser evaporation (MAPLE) [33]. In particular, they mentioned that a higher response and a higher sensitivity were found for ethanol at very low concentrations (20–200 ppm in dry air) at high temperatures (350–400 °C) [33]. Galstyan et al. fabricated Nb doped TiO2 nanotubes and investigated gas sensing properties of them towards CO, H2, NO2, ethanol and acetone at the temperature range of 100–300 °C [34]. They found that the doping of Nb enhances gas sensing performance of TiO2 nanotubes [34] Kwon et al. synthesized TiO2 nanotubes with anodization of thin Ti foil and studied ethanol sensing properties of the nanotubes at the temperature of 250 °C [35]. They found a superior response to ethanol. There is a lack of study in literature about effect of TiO2 nanostructure crystallinity on gas sensing properties. On the other hand, another advantage of this study is reproducible TiO2 nanotube sensor devices because Ti thin film on quartz substrate is anodized and gold metal electrodes are evaporated onto the nanotubes and the sensing device is direct integrated. In this study, TiO2 nanotubes were fabricated with anodization of Ti film on quartz substrate in aqueous HF solution and then the nanotubes were annealed at different temperatures to obtain anatase and rutile TiO2 nanotubes. The dc electrical, optical and VOC gas sensing properties of both anatase and rutile TiO2 nanotubes are investigated. 2. Experimental details 2.1. Fabrication of TiO2 nanotubes Ti thin films of 1 lm thickness were evaporated on pre-cleaned quartz substrates in a Leybold Univex 450 coater system with an Inficon Deposition Monitor (XTM/2). Ti film was anodized in an aqueous HF (0.5 wt.%) solution in a thermo-stated bath using a dc power supply, a platinum foil as a cathode, and Ti thin film on the quartz slide as an anode. A constant anodization voltage of 10 V was applied in a two electrodes system at 0 °C. After anodization, the samples were rinsed in deionized water, dried and annealed at temperatures of 300 °C and 700 °C to obtain anatase and rutile phases of TiO2 nanotubes. The scanning electron microscopy (SEM, Philips XL 30S), the X-ray diffraction (XRD, Philips 1820 X-ray diffractometer) and UV–vis spectroscopy (Sinco, UV–vis spectrophotometer) were employed to characterize TiO2 nanotubes. 2.2. Electrical measurements For electrical and gas sensing measurements, we used two TiO2 nanotube samples fabricated with different annealing temperatures: anatase TiO2 nanotube device annealed at 300 °C for 5 h in ambient air, and rutile TiO2 nanotube device annealed at 700 °C for 5 h in ambient air. Gold (Au) interdigital electrodes (IDE) were evaporated onto both anatase and rutile TiO2 nanotubes with a Leybold Univex 450 coater system by using a shadow mask. The IDEs consisted of 10 interdigital pairs of Au fingers on TiO2 nanotubes, with the thickness of 150 nm, the width of 100 lm, and the spacing of 100 lm between adjacent fingers. Fig. 1 shows a schematic illustration about TiO2 nanotube device fabrication steps. The dc electrical measurements were performed under high purity dry air flow (flow rate: 200 ml/ min) in the temperature range of 303–473 K. The current–voltage (I–V) measurement was carried out with a Keithley 6517A Electrometer/High Resistance Meter in the dark, and was recorded with a sweep rate of 50 mV/s between 1 V and 1 V. 2.3. VOC sensing Both anatase and rutile TiO2 nanotube sensor devices were mounted in a homemade test chamber (1 L) which was connected to a conventional gas mixing line. The experimental procedure was performed by heating the devices up to 200 °C and waiting for 10 min at this temperature under the high purity dry air flow in order to obtain a steady state. The VOC gases were generated from cooled bubblers immersed in a thermo-stated bath with dry air as carrier gas. The actual concentration of VOC gases in the mixing gas stream is determined by the saturation vapor pressure at the temperature of the thermo-stated bath. The VOC gases concentrations were calculated by using Antoine’s equation. In order to obtain the desired
VOC concentration the gas stream of VOCs diluted with dry air by using multi gas controller (647C MKS Instruments) and gas flow meters. A constant bias voltage was applied to TiO2 nanotube sensor devices and the dc current of them was measured with a Keithley 6517A Electrometer/High Resistance Meter. The dc current was monitored continuously under dry air flow to establish the baseline of the device and 5000 ppm of VOC gas was exposed into the test chamber and then the chamber was cleaned with dry air. This procedure, exposing to VOC gas and cleaning with dry air, was repeated several times. The measured VOC gases were chloroform, carbon tetrachloride (CCl4), dichloromethane, ethanol, methanol and isopropyl alcohol (IPA). The flow rate kept constant as 200 ml/min and all data recorded using an IEEE 488 data acquisition system incorporated into a personal computer.
3. Results and discussion 3.1. Structural characterization The formation mechanism of TiO2 nanotubes, which anodized in aqueous HF electrolyte, is well known [25]. The growth of the nanotubes is governed by competition between anodic oxide formation and chemical dissolution of the oxide as soluble fluoride complexes [25]. At first, an oxide layer is grown on Ti surface because of interaction between metal and O2 and then the oxide layer are partially dissolved in the presence of fluoride ions due to usage of HF electrolyte. Previously, TiO2 nanotubes have been fabricated in aqueous HF electrolyte by using anodization of both Ti foil and Ti thin films and the formation mechanism of TiO2 nanotubes is given in details [39,40]. The SEM images of TiO2 nanotubes that anodized Ti thin film in aqueous HF electrolyte with a constant anodization voltage of 10 V at anodization temperature of 0 °C are given in Fig. 2 with different magnifications (a: 50,000, b: 100,000 and c: 250,000). The nanotube structures are clearly seen all over the surface and the diameter of the nanotubes are in the range of 20–50 nm. Fig. 3 shows the XRD patterns of TiO2 nanotubes annealed at 300 °C and 700 °C for 5 h in dry air ambient. The XRD measurements were performed from the nanotubes that synthesized by using anodization of Ti foil in aqueous HF electrolyte at same conditions. The labels A, R, and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium sheet, respectively. The peak of anatase (1 0 1) is clearly seen from Fig. 3 for the TiO2 nanotubes annealed at 300 °C. On the other hand, only rutile (1 1 0) peaks can be seen in XRD patterns of TiO2 nanotubes that heat treated at 700 °C. So, the nanotubes that annealed at 300 °C and 700 °C, are in anatase and rutile phases respectively. Previously, Varghese et al. investigated stability and crystallization of TiO2 nanotubes depending on annealing temperature [41]. They found that as-prepared nanotubes were amorphous and the nanotubes crystallized in the anatase phase at an annealing temperature of about 280 °C and anatase crystallites transformed completely to rutile at about 620 °C in dry environments [41]. Therefore, the heat treated TiO2 nanotubes at 700 °C will be called as rutile TiO2 nanotubes and the annealed the nanotubes at 300 °C as anatase TiO2 nanotubes in the following text. There are a lot of studies that reported the band gap of anatase and rutile TiO2 as about 3.2 eV and 3.0 eV, respectively. However, Alivisatos have reported that the band gap of semiconductor nanomaterials can be controlled by altering the dimensions of them and enhanced absorption coefficient has been observed due to quantum confinement [42]. Bavykin et al. obtained that optical properties of TiO2 nanotubes is a two dimensional behavior and the electronic structure of TiO2 nanotubes is very close to TiO2 nanosheets [43]. Sakai et al. reported that the band gab of TiO2 nanosheet is about 3.8 eV and this large band gab energy explained with quantum size effects [44]. UV–vis analysis of the nanotubes was performed at room temperature. Fig. 4a shows UV–vis absorbance – wavelength graphs of rutile and anatase TiO2 nanotubes. A sharp decrease in the absorbance is observed for both TiO2
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Fig. 1. A schematic illustration for fabrication steps of TiO2 nanotube sensor device.
Fig. 3. The X-ray diffraction patterns of TiO2 nanotubes annealed at 300 °C and 700 °C for 5 h in dry air ambient. The labels A, R, and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium metal, respectively.
Fig. 2. The SEM images of TiO2 nanotubes anodized Ti thin film in aqueous HF electrolyte with a constant anodization voltage of 10 V at anodization temperature of 0 °C (a: 100,000 and b: 250,000).
nanotubes and the absorption edges on to wavelength axis at a wavelength of 375 nm (3.3 eV) and of 345 nm (3.6 eV) was found for anatase and rutile TiO2 nanotubes, respectively, as seen in Fig. 4a. The UV–vis absorption datas of anatase and rutile TiO2 nanotubes were fitted for both indirect and direct band gap transitions to determine the type of band to band transition. Fig. 4b shows (ahv)2 versus (hv) for direct transition and Fig. 4c shows (ahv)1/2 versus hv for indirect transition. The straight line of the curves to the energy axis (hv) in Fig. 4b and Fig. 4c gives the band gaps of anatase and rutile TiO2 nanotubes for direct and indirect transitions. The direct and the indirect band gaps of rutile TiO2 nanotubes were 3.75 eV and 3.36 eV, respectively. Besides, there are two straight lines in anatase graph in Fig. 4b for direct band gap transition, corresponding to 3.9 eV and 3.55 eV. In addition, the indirect band gap of anatase TiO2 nanotubes is about 3.16 eV. We analyzed the absorption edge of Fig. 4b and c in order to establish the transition character (direct or indirect). It is well known, the square of absorption and the square root of absorption coefficients are linear with energy for direct and indirect optical transitions. The plots of (ahv)2 versus hv and (ahv)1/2 versus hv in the
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Fig. 4. UV–vis analysis of TiO2 nanotubes. (a) UV–vis absorbance – wavelength graph, (b) a plots of (ahv)2 versus hv and (c) a plots of (ahv)1/2 versus hv for rutile and anatase TiO2 nanotubes.
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absorption edge region are shown in the inset of Fig. 4b and c, respectively. If we compare the linearity factor of the plots for direct and indirect optical transitions, the linearity factors in indirect transition case for absorption edge of both rutile and anatase TiO2 nanotubes are better than 0.99 as seen in Fig. 4b and c. Therefore, the band to band transition of both rutile and anatase TiO2 nanotubes could be indirect transition and the band gaps of rutile and anatase TiO2 nanotubes should be 3.36 eV and 3.16 eV, respectively.
3.2. Electrical characterization The dc electrical characterizations of anatase and rutile TiO2 nanotubes were performed under dry air flow at the temperature range from 303 to 473 K. Generally, Au metal makes Schottky contact to TiO2 material both anatase and rutile phases [45,46]. Fig. 5 shows the I–V characteristics of rutile and anatase TiO2 nanotubes at the indicated temperatures. The I–V characteristic of rutile TiO2 nanotubes has a hysteresis at low temperature up to 393 K and the hysteresis disappears with increasing temperature as seen in Fig. 5a. There is a linear relationship between the current and the voltage in the measured voltage interval (1 V to 1 V) for rutile TiO2 nanotubes and the slope of the curve increases with enhancing temperature as seen in Fig. 5a. But, the I–V graph of anatase TiO2 nanotubes has not linear for all temperatures as given in
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Fig. 5b. The symmetric I–V relation from the Au–anatase TiO2–Au confirms Schottky junction formation between Au and anatase TiO2 for all temperatures. The I–V characteristics of a Schottky diode are represented by [47]
qV 1 I ¼ I0 exp nkT
ð1Þ
where I0 is the saturation current based on thermionic emission theory, n is the ideality factor, k is the Boltzmann’s constant, T is the absolute temperature. The magnitude of the current of anatase TiO2 nanotubes enhances with rising temperature as seen in Fig. 5b. Fig. 6 shows the logarithm of the current at 1.0 V as a function of the reciprocal of temperature for both rutile and anatase TiO2 nanotubes. A linear relationship between ln I and 1000/T is obtained for rutile TiO2 nanotubes (Fig. 6). This may be depending on extrinsic behavior of rutile TiO2 nanotubes. Two distinct regions which may be related to extrinsic and intrinsic behavior are observed for anatase TiO2 nanotubes approximately before and after 350 K (Fig. 6). The relationship between ln I and 1000/T for both rutile and anatase TiO2 nanotubes is linear and the linearity factor is better than 0.99 as seen in Fig. 6. Therefore, conduction mechanism could be explained with the thermally activated type of conduction. For thermally activated conduction, the dependence of dc conductivities on the reciprocal absolute temperature (1/T) is defined using the relation:
Fig. 5. The I–V characteristics of rutile (a) and anatase (b) TiO2 nanotubes under dry air flow at indicated temperatures.
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Fig. 6. The dependence of logarithmic current, ln I, on the inverse of the temperature, 1000/T, for rutile and anatase TiO2 nanotubes.
rdc ¼ r0 : exp
EA kT
ð2Þ
where EA is the activation energy, T the temperature, k the Boltzmann’s constant and r0 is the constant of proportionality. The activation energies of both rutile and anatase TiO2 nanotubes are calculated from ln I to 1000/T graphs. The activation energy of rutile TiO2 nanotubes is 0.8 eV at indicated temperature range. Two different activation energies (EA1 and EA2) are obtained depending on extrinsic and intrinsic behavior of anatase TiO2 nanotubes as 0.19 eV and 0.41 eV, respectively. The activation energies which obtained from electrical measurements for both rutile and anatase TiO2 nanotubes are smaller than the optical band gap energies of them. These activation energies for both rutile and anatase TiO2 nanotubes could be corresponding with trap level located below the conduction band. 3.3. VOC sensing Two different TiO2 nanotubes devices that annealed at 300 °C (anatase) and at 700 °C (rutile) are used for VOC sensing measurements. For sensor measurements, 5000 ppm of VOCs was exposed to the devices, and after 20 min waiting time the chamber was cleaned with 200 sccm high purity dry air flow for 30 min. The current versus time graphs for rutile and anatase TiO2 nanotube sensor devices exposed to the 5000 ppm isopropyl alcohol (IPA) at 200 °C are shown in Fig. 7. After the first 5000 ppm IPA exposure, the current of both rutile and anatase TiO2 nanotube sensor devices increased rapidly, and then the rate of increase slow down and the current came to saturation as seen in Fig. 7. During purging time, the current decreased rapidly, and then the rate of decrease slows
Fig. 7. The current versus time for rutile (a) and anatase (b) TiO2 nanotubes sensor exposure to 5000 ppm isopropyl alcohol (IPA) at 200 °C.
down and the current recovers. Similar behavior was observed for the second and third 5000 ppm IPA exposure to the sensor as seen in Fig. 7. The increase in the current could be explained as follow: the VOC gas molecules diffuse into the sensing layer through pores and react with the adsorbed oxygen on TiO2 [48]. This reduces the thickness of the surface depletion layer in TiO2, thereby increasing the electric current. Similar behavior was observed for other VOCs by exposing them to the sensor devices. The sensing properties of rutile and anatase TiO2 nanotube devices were characterized by sensor response. The sensor response (%) is defined as
Sensor Responseð%Þ ¼ ðDI=I0 Þ 100
ð3Þ
where DI is the change in the current when exposed to VOCs, and I0 is the reference value of the devices when exposed to high purity dry air. Fig. 8 shows the sensor responses of rutile and anatase TiO2 nanotube devices exposed to the 5000 ppm VOCs at 200 °C. The maximum sensor response of both sensor devices is observed for IPA and the sensor response of anatase TiO2 nanotubes is higher than that of rutile TiO2 nanotubes.
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Fig. 8. The sensor responses of rutile and anatase TiO2 nanotubes sensor devices exposure to the 5000 ppm VOCs at the temperature of 200 °C.
4. Conclusions TiO2 nanotubes are fabricated by anodization of Ti thin films in aqueous HF electrolyte, at 0 °C with anodization voltage of 10 V and then annealed at 300 °C and 700 °C to observe anatase and rutile phases of TiO2 nanotubes respectively. The temperature dependence dc conduction mechanism of both anatase and rutile TiO2 nanotubes could be explained with the thermally activated conduction. The both anatase and rutile TiO2 nanotube sensor devices showed good response to alcohols and the maximum sensor response is observed for IPA. In addition, anatase TiO2 nanotube sensor showed better sensor response than the rutile one.
Acknowledgements This work has been funded by The Scientific and Technological Research Council of Turkey (TUBITAK), Project Number: 111M261 and 113F403. The authors thank KUYTAM and Dr. Barıs Yagci for SEM measurements.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrical and VOC sensing properties of anatase and rutile TiO2 nanotubes Erdem Sennik a, Necmettin Kilinc a,b,⇑, Zafer Ziya Ozturk a,c a
Gebze Institute of Technology, Department of Physics, Kocaeli, Turkey Nigde University, Mechatronics Engineering Department, 51245 Nigde, Turkey c TÜBITAK-Marmara Research Center, Materials Institute, Kocaeli, Turkey b
a r t i c l e
i n f o
Article history: Received 19 April 2014 Received in revised form 23 June 2014 Accepted 13 July 2014 Available online 21 July 2014 Keywords: Nanostructured materials Electrical transport Nanofabrications TiO2 nanotube Chemoresistive sensor
a b s t r a c t The dc electrical and volatile organic compound (VOC) sensing properties of TiO2 nanotubes in both anatase and rutile phases were investigated. TiO2 nanotube arrays were obtained in aqueous HF (0.5 wt.%) electrolytes by anodizing of Ti thin films that deposited on quartz substrates using thermal evaporation. Anodization was performed at 10 V in aqueous HF at 0 °C. Then the fabricated TiO2 nanotubes were annealed at 300 °C and at 700 °C under dry air for 5 h to obtain anatase and rutile phases, respectively. The TiO2 nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) as structural, and UV–vis spectrophotometer as optical. The current voltage characteristics of the nanotubes under dry air flow revealed that the conductivity of the sample with anatase phase was higher than that of the sample with rutile phase. The VOCs sensing properties of the nanotubes were investigated at 200 °C. It was found that the sensor response of anatase was higher than that of rutile for almost all VOC gases. On the other hand, the sensitivities of two sensors are the highest for isopropyl alcohol. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Scientists have been dedicated to the improvement of useful and cheap sensors that can incessantly detect volatile organic compounds (VOCs) [1–8]. VOCs are the basic sources of indoor air pollution and give rise to harmful influences on human health such as breath, skin. Furthermore, human breath includes VOCs such as ethanol and acetone [9–12]. Inasmuch as the detection of VOCs becomes important in air environments, solid-state semiconductor-type gas sensors are designed due to their compact size and low cost. It is clearly understand that the surface shape of sensing layer such as grain size, film thickness, agglomeration, porosity, surface geometry, nano and polycrystalline is an important parameter for gas sensors especially semiconductor gas sensor [13–16]. In order to improve the sensor parameters such as sensitivity, response time, recovery time and reducing optimal working temperatures to low temperatures, researchers have been focused on fabrication of nanostructured metal oxides and functionalization or doping of them [17–22]. High gas sensitivity is related to porous ⇑ Corresponding author. Address: Department of Physics, Science Faculty, Gebze Institute of Technology, P.O. Box 141, 41400 Gebze-Kocaeli, Turkey. Tel.: +90 2626051312; fax: +90 262 6538490. E-mail address: [email protected] (N. Kilinc). http://dx.doi.org/10.1016/j.jallcom.2014.07.097 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
sensing films due to gas diffusion all around the porous film. In general, the gas sensing mechanism of metal oxides at high temperature explained with transfer of electrons between the sensing material and target gases, with the formation or deformation of the depletion layer depending on the material [17–21]. But the gas sensing mechanism of metal oxide at low temperature could be elucidated with a thin layer of condensed humidity on the surface of the metal oxide [23]. This is similar to hydrogenated diamond and/or silicon based sensors. Joshi and Kumar discussed gas sensing mechanism of Si nanowires at room temperature in details [24]. The dimensions of nanostructures could be controlled by using several methods such as anodization, hydrothermal, and chemical vapor deposition (CVD). Hence, large size molecules such as volatile organic compounds (VOCs) can be detected. It is available to utilize TiO2 nanotubes for gas sensing including large sized gas molecules like VOCs. During the past decades, anodized TiO2 nanotubes have been synthesized by controlling the nanotube dimensions with changing the anodization parameters and have been used in many research and technological areas such as catalysis, batteries, solar cells, gas sensors, bio-sensing and biomedical applications [25– 30]. There are more researches on resistive hydrogen gas sensing properties of anodized TiO2 nanotubes, but a few studies on resistive VOC sensor properties of the nanotubes. In generally, VOCs can
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be sensed by TiO2 nanostructures up to 250 °C [31–39]. Seo et al. investigated VOC sensing properties at high temperature (450– 550 °C) with TiO2 nanotube by hydrothermal method [31,32]. They observed high sensitivity to ethanol and toluene at high temperature. Rella et al. investigated VOC sensing of a thin layer of TiO2 nanoparticles deposited by matrix assisted pulsed laser evaporation (MAPLE) [33]. In particular, they mentioned that a higher response and a higher sensitivity were found for ethanol at very low concentrations (20–200 ppm in dry air) at high temperatures (350–400 °C) [33]. Galstyan et al. fabricated Nb doped TiO2 nanotubes and investigated gas sensing properties of them towards CO, H2, NO2, ethanol and acetone at the temperature range of 100–300 °C [34]. They found that the doping of Nb enhances gas sensing performance of TiO2 nanotubes [34] Kwon et al. synthesized TiO2 nanotubes with anodization of thin Ti foil and studied ethanol sensing properties of the nanotubes at the temperature of 250 °C [35]. They found a superior response to ethanol. There is a lack of study in literature about effect of TiO2 nanostructure crystallinity on gas sensing properties. On the other hand, another advantage of this study is reproducible TiO2 nanotube sensor devices because Ti thin film on quartz substrate is anodized and gold metal electrodes are evaporated onto the nanotubes and the sensing device is direct integrated. In this study, TiO2 nanotubes were fabricated with anodization of Ti film on quartz substrate in aqueous HF solution and then the nanotubes were annealed at different temperatures to obtain anatase and rutile TiO2 nanotubes. The dc electrical, optical and VOC gas sensing properties of both anatase and rutile TiO2 nanotubes are investigated. 2. Experimental details 2.1. Fabrication of TiO2 nanotubes Ti thin films of 1 lm thickness were evaporated on pre-cleaned quartz substrates in a Leybold Univex 450 coater system with an Inficon Deposition Monitor (XTM/2). Ti film was anodized in an aqueous HF (0.5 wt.%) solution in a thermo-stated bath using a dc power supply, a platinum foil as a cathode, and Ti thin film on the quartz slide as an anode. A constant anodization voltage of 10 V was applied in a two electrodes system at 0 °C. After anodization, the samples were rinsed in deionized water, dried and annealed at temperatures of 300 °C and 700 °C to obtain anatase and rutile phases of TiO2 nanotubes. The scanning electron microscopy (SEM, Philips XL 30S), the X-ray diffraction (XRD, Philips 1820 X-ray diffractometer) and UV–vis spectroscopy (Sinco, UV–vis spectrophotometer) were employed to characterize TiO2 nanotubes. 2.2. Electrical measurements For electrical and gas sensing measurements, we used two TiO2 nanotube samples fabricated with different annealing temperatures: anatase TiO2 nanotube device annealed at 300 °C for 5 h in ambient air, and rutile TiO2 nanotube device annealed at 700 °C for 5 h in ambient air. Gold (Au) interdigital electrodes (IDE) were evaporated onto both anatase and rutile TiO2 nanotubes with a Leybold Univex 450 coater system by using a shadow mask. The IDEs consisted of 10 interdigital pairs of Au fingers on TiO2 nanotubes, with the thickness of 150 nm, the width of 100 lm, and the spacing of 100 lm between adjacent fingers. Fig. 1 shows a schematic illustration about TiO2 nanotube device fabrication steps. The dc electrical measurements were performed under high purity dry air flow (flow rate: 200 ml/ min) in the temperature range of 303–473 K. The current–voltage (I–V) measurement was carried out with a Keithley 6517A Electrometer/High Resistance Meter in the dark, and was recorded with a sweep rate of 50 mV/s between 1 V and 1 V. 2.3. VOC sensing Both anatase and rutile TiO2 nanotube sensor devices were mounted in a homemade test chamber (1 L) which was connected to a conventional gas mixing line. The experimental procedure was performed by heating the devices up to 200 °C and waiting for 10 min at this temperature under the high purity dry air flow in order to obtain a steady state. The VOC gases were generated from cooled bubblers immersed in a thermo-stated bath with dry air as carrier gas. The actual concentration of VOC gases in the mixing gas stream is determined by the saturation vapor pressure at the temperature of the thermo-stated bath. The VOC gases concentrations were calculated by using Antoine’s equation. In order to obtain the desired
VOC concentration the gas stream of VOCs diluted with dry air by using multi gas controller (647C MKS Instruments) and gas flow meters. A constant bias voltage was applied to TiO2 nanotube sensor devices and the dc current of them was measured with a Keithley 6517A Electrometer/High Resistance Meter. The dc current was monitored continuously under dry air flow to establish the baseline of the device and 5000 ppm of VOC gas was exposed into the test chamber and then the chamber was cleaned with dry air. This procedure, exposing to VOC gas and cleaning with dry air, was repeated several times. The measured VOC gases were chloroform, carbon tetrachloride (CCl4), dichloromethane, ethanol, methanol and isopropyl alcohol (IPA). The flow rate kept constant as 200 ml/min and all data recorded using an IEEE 488 data acquisition system incorporated into a personal computer.
3. Results and discussion 3.1. Structural characterization The formation mechanism of TiO2 nanotubes, which anodized in aqueous HF electrolyte, is well known [25]. The growth of the nanotubes is governed by competition between anodic oxide formation and chemical dissolution of the oxide as soluble fluoride complexes [25]. At first, an oxide layer is grown on Ti surface because of interaction between metal and O2 and then the oxide layer are partially dissolved in the presence of fluoride ions due to usage of HF electrolyte. Previously, TiO2 nanotubes have been fabricated in aqueous HF electrolyte by using anodization of both Ti foil and Ti thin films and the formation mechanism of TiO2 nanotubes is given in details [39,40]. The SEM images of TiO2 nanotubes that anodized Ti thin film in aqueous HF electrolyte with a constant anodization voltage of 10 V at anodization temperature of 0 °C are given in Fig. 2 with different magnifications (a: 50,000, b: 100,000 and c: 250,000). The nanotube structures are clearly seen all over the surface and the diameter of the nanotubes are in the range of 20–50 nm. Fig. 3 shows the XRD patterns of TiO2 nanotubes annealed at 300 °C and 700 °C for 5 h in dry air ambient. The XRD measurements were performed from the nanotubes that synthesized by using anodization of Ti foil in aqueous HF electrolyte at same conditions. The labels A, R, and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium sheet, respectively. The peak of anatase (1 0 1) is clearly seen from Fig. 3 for the TiO2 nanotubes annealed at 300 °C. On the other hand, only rutile (1 1 0) peaks can be seen in XRD patterns of TiO2 nanotubes that heat treated at 700 °C. So, the nanotubes that annealed at 300 °C and 700 °C, are in anatase and rutile phases respectively. Previously, Varghese et al. investigated stability and crystallization of TiO2 nanotubes depending on annealing temperature [41]. They found that as-prepared nanotubes were amorphous and the nanotubes crystallized in the anatase phase at an annealing temperature of about 280 °C and anatase crystallites transformed completely to rutile at about 620 °C in dry environments [41]. Therefore, the heat treated TiO2 nanotubes at 700 °C will be called as rutile TiO2 nanotubes and the annealed the nanotubes at 300 °C as anatase TiO2 nanotubes in the following text. There are a lot of studies that reported the band gap of anatase and rutile TiO2 as about 3.2 eV and 3.0 eV, respectively. However, Alivisatos have reported that the band gap of semiconductor nanomaterials can be controlled by altering the dimensions of them and enhanced absorption coefficient has been observed due to quantum confinement [42]. Bavykin et al. obtained that optical properties of TiO2 nanotubes is a two dimensional behavior and the electronic structure of TiO2 nanotubes is very close to TiO2 nanosheets [43]. Sakai et al. reported that the band gab of TiO2 nanosheet is about 3.8 eV and this large band gab energy explained with quantum size effects [44]. UV–vis analysis of the nanotubes was performed at room temperature. Fig. 4a shows UV–vis absorbance – wavelength graphs of rutile and anatase TiO2 nanotubes. A sharp decrease in the absorbance is observed for both TiO2
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Fig. 1. A schematic illustration for fabrication steps of TiO2 nanotube sensor device.
Fig. 3. The X-ray diffraction patterns of TiO2 nanotubes annealed at 300 °C and 700 °C for 5 h in dry air ambient. The labels A, R, and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium metal, respectively.
Fig. 2. The SEM images of TiO2 nanotubes anodized Ti thin film in aqueous HF electrolyte with a constant anodization voltage of 10 V at anodization temperature of 0 °C (a: 100,000 and b: 250,000).
nanotubes and the absorption edges on to wavelength axis at a wavelength of 375 nm (3.3 eV) and of 345 nm (3.6 eV) was found for anatase and rutile TiO2 nanotubes, respectively, as seen in Fig. 4a. The UV–vis absorption datas of anatase and rutile TiO2 nanotubes were fitted for both indirect and direct band gap transitions to determine the type of band to band transition. Fig. 4b shows (ahv)2 versus (hv) for direct transition and Fig. 4c shows (ahv)1/2 versus hv for indirect transition. The straight line of the curves to the energy axis (hv) in Fig. 4b and Fig. 4c gives the band gaps of anatase and rutile TiO2 nanotubes for direct and indirect transitions. The direct and the indirect band gaps of rutile TiO2 nanotubes were 3.75 eV and 3.36 eV, respectively. Besides, there are two straight lines in anatase graph in Fig. 4b for direct band gap transition, corresponding to 3.9 eV and 3.55 eV. In addition, the indirect band gap of anatase TiO2 nanotubes is about 3.16 eV. We analyzed the absorption edge of Fig. 4b and c in order to establish the transition character (direct or indirect). It is well known, the square of absorption and the square root of absorption coefficients are linear with energy for direct and indirect optical transitions. The plots of (ahv)2 versus hv and (ahv)1/2 versus hv in the
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Fig. 4. UV–vis analysis of TiO2 nanotubes. (a) UV–vis absorbance – wavelength graph, (b) a plots of (ahv)2 versus hv and (c) a plots of (ahv)1/2 versus hv for rutile and anatase TiO2 nanotubes.
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absorption edge region are shown in the inset of Fig. 4b and c, respectively. If we compare the linearity factor of the plots for direct and indirect optical transitions, the linearity factors in indirect transition case for absorption edge of both rutile and anatase TiO2 nanotubes are better than 0.99 as seen in Fig. 4b and c. Therefore, the band to band transition of both rutile and anatase TiO2 nanotubes could be indirect transition and the band gaps of rutile and anatase TiO2 nanotubes should be 3.36 eV and 3.16 eV, respectively.
3.2. Electrical characterization The dc electrical characterizations of anatase and rutile TiO2 nanotubes were performed under dry air flow at the temperature range from 303 to 473 K. Generally, Au metal makes Schottky contact to TiO2 material both anatase and rutile phases [45,46]. Fig. 5 shows the I–V characteristics of rutile and anatase TiO2 nanotubes at the indicated temperatures. The I–V characteristic of rutile TiO2 nanotubes has a hysteresis at low temperature up to 393 K and the hysteresis disappears with increasing temperature as seen in Fig. 5a. There is a linear relationship between the current and the voltage in the measured voltage interval (1 V to 1 V) for rutile TiO2 nanotubes and the slope of the curve increases with enhancing temperature as seen in Fig. 5a. But, the I–V graph of anatase TiO2 nanotubes has not linear for all temperatures as given in
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Fig. 5b. The symmetric I–V relation from the Au–anatase TiO2–Au confirms Schottky junction formation between Au and anatase TiO2 for all temperatures. The I–V characteristics of a Schottky diode are represented by [47]
qV 1 I ¼ I0 exp nkT
ð1Þ
where I0 is the saturation current based on thermionic emission theory, n is the ideality factor, k is the Boltzmann’s constant, T is the absolute temperature. The magnitude of the current of anatase TiO2 nanotubes enhances with rising temperature as seen in Fig. 5b. Fig. 6 shows the logarithm of the current at 1.0 V as a function of the reciprocal of temperature for both rutile and anatase TiO2 nanotubes. A linear relationship between ln I and 1000/T is obtained for rutile TiO2 nanotubes (Fig. 6). This may be depending on extrinsic behavior of rutile TiO2 nanotubes. Two distinct regions which may be related to extrinsic and intrinsic behavior are observed for anatase TiO2 nanotubes approximately before and after 350 K (Fig. 6). The relationship between ln I and 1000/T for both rutile and anatase TiO2 nanotubes is linear and the linearity factor is better than 0.99 as seen in Fig. 6. Therefore, conduction mechanism could be explained with the thermally activated type of conduction. For thermally activated conduction, the dependence of dc conductivities on the reciprocal absolute temperature (1/T) is defined using the relation:
Fig. 5. The I–V characteristics of rutile (a) and anatase (b) TiO2 nanotubes under dry air flow at indicated temperatures.
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Fig. 6. The dependence of logarithmic current, ln I, on the inverse of the temperature, 1000/T, for rutile and anatase TiO2 nanotubes.
rdc ¼ r0 : exp
EA kT
ð2Þ
where EA is the activation energy, T the temperature, k the Boltzmann’s constant and r0 is the constant of proportionality. The activation energies of both rutile and anatase TiO2 nanotubes are calculated from ln I to 1000/T graphs. The activation energy of rutile TiO2 nanotubes is 0.8 eV at indicated temperature range. Two different activation energies (EA1 and EA2) are obtained depending on extrinsic and intrinsic behavior of anatase TiO2 nanotubes as 0.19 eV and 0.41 eV, respectively. The activation energies which obtained from electrical measurements for both rutile and anatase TiO2 nanotubes are smaller than the optical band gap energies of them. These activation energies for both rutile and anatase TiO2 nanotubes could be corresponding with trap level located below the conduction band. 3.3. VOC sensing Two different TiO2 nanotubes devices that annealed at 300 °C (anatase) and at 700 °C (rutile) are used for VOC sensing measurements. For sensor measurements, 5000 ppm of VOCs was exposed to the devices, and after 20 min waiting time the chamber was cleaned with 200 sccm high purity dry air flow for 30 min. The current versus time graphs for rutile and anatase TiO2 nanotube sensor devices exposed to the 5000 ppm isopropyl alcohol (IPA) at 200 °C are shown in Fig. 7. After the first 5000 ppm IPA exposure, the current of both rutile and anatase TiO2 nanotube sensor devices increased rapidly, and then the rate of increase slow down and the current came to saturation as seen in Fig. 7. During purging time, the current decreased rapidly, and then the rate of decrease slows
Fig. 7. The current versus time for rutile (a) and anatase (b) TiO2 nanotubes sensor exposure to 5000 ppm isopropyl alcohol (IPA) at 200 °C.
down and the current recovers. Similar behavior was observed for the second and third 5000 ppm IPA exposure to the sensor as seen in Fig. 7. The increase in the current could be explained as follow: the VOC gas molecules diffuse into the sensing layer through pores and react with the adsorbed oxygen on TiO2 [48]. This reduces the thickness of the surface depletion layer in TiO2, thereby increasing the electric current. Similar behavior was observed for other VOCs by exposing them to the sensor devices. The sensing properties of rutile and anatase TiO2 nanotube devices were characterized by sensor response. The sensor response (%) is defined as
Sensor Responseð%Þ ¼ ðDI=I0 Þ 100
ð3Þ
where DI is the change in the current when exposed to VOCs, and I0 is the reference value of the devices when exposed to high purity dry air. Fig. 8 shows the sensor responses of rutile and anatase TiO2 nanotube devices exposed to the 5000 ppm VOCs at 200 °C. The maximum sensor response of both sensor devices is observed for IPA and the sensor response of anatase TiO2 nanotubes is higher than that of rutile TiO2 nanotubes.
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Fig. 8. The sensor responses of rutile and anatase TiO2 nanotubes sensor devices exposure to the 5000 ppm VOCs at the temperature of 200 °C.
4. Conclusions TiO2 nanotubes are fabricated by anodization of Ti thin films in aqueous HF electrolyte, at 0 °C with anodization voltage of 10 V and then annealed at 300 °C and 700 °C to observe anatase and rutile phases of TiO2 nanotubes respectively. The temperature dependence dc conduction mechanism of both anatase and rutile TiO2 nanotubes could be explained with the thermally activated conduction. The both anatase and rutile TiO2 nanotube sensor devices showed good response to alcohols and the maximum sensor response is observed for IPA. In addition, anatase TiO2 nanotube sensor showed better sensor response than the rutile one.
Acknowledgements This work has been funded by The Scientific and Technological Research Council of Turkey (TUBITAK), Project Number: 111M261 and 113F403. The authors thank KUYTAM and Dr. Barıs Yagci for SEM measurements.
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