B. Bhowmik

Room temperature alcohol sensing by oxygen vacancy controlled TiO2 nanotube array

Oxygen vacancy (OV) controlled TiO2 nanotubes, having diameters of 50–70 nm and lengths of 200–250 nm, were synthesized by electrochemical anodization in the mixed electrolyte comprising NH4F and ethylene glycol with selective H2O content. The structural evolution of TiO2 nanoforms has been studied by field emission scanning electron microscopy. Variation in the formation of OVs with the variation of the structure of TiO2 nanoforms has been evaluated by photoluminescence and X-ray photoelectron spectroscopy. The sensor characteristics were correlated to the variation of the amount of induced OVs in the nanotubes. The efficient room temperature sensing achieved by the control of OVs of TiO2 nanotube array has paved the way for developing fast responding alcohol sensor with corresponding response magnitude of 60.2%, 45.3%, and 36.5% towards methanol, ethanol, and 2-propanol, respectively.

Repeatability and Stability of Room-Temperature Acetone Sensor Based on

The oxygen vacancy (OV) variation of the electrochemically grown TiO2 nanotube (NT) was achieved through stoichiometry variation by varying the volume of water in the electrolyte (NH4F with ethylene glycol) during anodization. By varying the water content (0%, 2%, and 10% by volume) in the mixed electrolyte, the morphology and stoichiometry of NTs were found to be varied dramatically. After detailed structural and morphological characterization by X-ray diffraction and field-emission scanning electron microscope and photoluminescence spectroscopy, the room-temperature acetone sensing was investigated by employing three distinct nanoforms derived through anodization. The reliability (i.e., repeatability and stability) of the sensors, as well as their response magnitude, was found to be greatly influenced by the variation in stoichiometry. The NTs derived with 2 volume percent H2O was found to offer the most promising response magnitude with excellent repeatability, whereas the best stability was ensured in the case of the NTs derived with 10 volume percent H2O. It was observed that an optimization of stoichiometry (OVs) and the surface-to-volume ratio is important in determining the response magnitude and repeatability. On the contrary, the stability is mainly governed by the stoichiometry only.

\hbox{TiO}_{2}

This paper concerns the tuning of the operating temperatures, selectivity, and repeatability of a resistive acetone sensor based on TiO 2 nanotubes (NTs) through surface modifications (of NTs) using Pd and Ni nanoparticles. Three sets of sensor devices, which employ unmodified, Ni-modified, and Pd-modified TiO 2 NT arrays as the sensing layer, were tested for acetone detection in the temperature range of 50 °C-250 °C, targeting 10-1000 ppm. It was found that both the modified (Pd and Ni) sensors offered a lower optimum operating temperature (100 °C) compared with its unmodified counterpart (150 °C), possibly owing to the requirement of lower activation energy in the case of modified systems. The cross sensitivity toward other interfering species, viz., ethanol, methanol, 2-butanone, and toluene, was investigated. Both the modified sensors were found to offer better selectivity toward acetone than the unmodified sensor. However, the response and selectivity improvement of the modified sensors was achieved at the expense of poor repeatability. Possibly owing to the increased structural defects and the nonidentical oxygen spill over in the repeated cycles, the modified sensors offered relatively poor repeatability. Among the two types of modifications, the Pd-modified sensor offered better response magnitude and transient characteristics (the response time and the recovery time) than the Ni-modified sensor. The underlying mechanism for such improvement has been also highlighted.

Nanotubes: Influence of Stoichiometry Variation

In this paper, we report on the development of a highly sensitive, relatively low-temperature ethanol sensor based on sol-gel derived p-TiO2 thin film. The p-type anatase TiO2 thin film was deposited by sol-gel technique on a thermally oxidized <;100> p-Si (resistivity 5 Ω cm) substrate. Anatase TiO2 phase with <;101> nanocrystallinity was confirmed with an average particle size of ~11 nm from X-ray diffraction and field emission scanning electron microscopic study. Ethanol sensor study, in the resistive mode, was carried out at a relatively low operating temperature range (75 °C-175 °C) for sensing low concentrations of ethanol in air (5-100 ppm). Response magnitude of ~146% was observed at 150 °C toward 100-ppm ethanol (in air) with corresponding response time and recovery time of 39 and 15 s, respectively. The sensor showed appreciably high-response magnitude (129%) even at low ethanol concentration (5 ppm) with acceptable response and recovery time (54 and 22 s, respectively) at the same operating temperature (150 °C). At a particular temperature, for all the ethanol concentrations, sensor showed minimal base line resistance drift, thereby offering highly repeatable and stable sensing performance. Ethanol selectivity study against other volatile organic compounds, such as methanol, acetone, and 2-butanone, was also investigated and was found to be quite promising. Ethanol sensing mechanism for such p-type TiO2 has also been discussed in the light of corresponding oxygen vacancy model.

Operating Temperature, Repeatability, and Selectivity of TiO 2 Nanotube-Based Acetone Sensor: Influence of Pd and Ni Nanoparticle Modifications

The dimensionality of the nanostructures plays a pivotal role in determining the sensing performance of metal oxide semiconductors. In this paper, 2-D TiO2 nanosheets were assembled to form hierarchical 3-D nanoflowers by a low-temperature hydrothermal process and tested for volatile organic compound (VOC) sensing. Structural characterization, such as X-ray diffraction, revealed the anatase crystallinity of the TiO2 nanoflowers. The chemical composition of the nanoflowers was confirmed through energy dispersive spectroscopy and X-ray mapping techniques. Field emission scanning electron microscopy results demonstrated the 3-D nanoflower like structure, consisting of 2-D nanosheets of length 120 nm and a thickness of 12-23 nm. A suitable growth mechanism for hierarchical nanoflowers has been proposed. The nanoflowers were found to offer promising sensing performance toward VOCs, such as acetone, methanol, 2-butanone, toluene, and 2-propanol at relatively low optimum operating temperature of 60 °C, 90 °C, 60 °C, 120 °C, and 60 °C, respectively. The sensor appreciably showed fast response (~6-15 s) and recovery (~15-39 s) characteristics. Furthermore, the sensor offered high selectivity toward acetone. The sensing performance of the nanoflowers has been correlated with a space charge model at the grain boundary.

Highly Repeatable Low-ppm Ethanol Sensing Characteristics of p-TiO 2 -Based Resistive Devices

1-D oxide nanostructures-based metal-insulator-metal structures represent potential gas sensor devices, owing to their vertical electron transport feature. In this paper, we demonstrate that for achieving optimum gas sensing by a TiO2 nanotube (NT) array in vertical mode, tuning of NTs (1-D) surface area as well as carrier transport path length by tailoring the NT length can be a valuable approach. For anodization times of 1, 4, 8, 12, and 16 h, the corresponding NT length was found to be 280-320, 506-514, 1730-1790, 2000-2200, and 2380-2420 nm, respectively, with almost no variation in tube diameters or separations. The carrier concentrations of the NTs were found to be decreasing with increasing tube length. The vertical device structure, employing NT arrays of different lengths as the sensing layer, was investigated in the temperature range of 50 °C-250 °C for sensing acetone, as a test gas/vapor, in the concentration range of 10-1000 ppm. The response magnitude of the sensor was increased with increased NT length, possibly owing to the availability of larger amounts of gas interaction sites due to higher surface area at increased length. The response time and recovery time of the developed sensor also increased with increasing tube length and became excessively sluggish after the critical tube length exceeded 2250 nm owing to slower adsorption/desorption and diffusion kinetics.

Highly Selective Low-Temperature Acetone Sensor Based on Hierarchical 3-D TiO 2 Nanoflowers

In this paper, sol-gel derived undoped p-type nanotitania thin films particularly aimed to detect low ppm (5-100 ppm) methanol at low temperature (75-150°C) is reported. Anantase TiO2 thin film was deposited by sol-gel technique and p-type conductivity of the film was confirmed by Hall measurement. Nanocrystalline structure (12-15 nm) of the TiO2 film was confirmed by X-ray diffraction study and field emission scanning electron microscopy, respectively. Photoluminance spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the defect structure. Methanol sensor study was carried for detecting the concentration range of 5 to 100 ppm in the temperature range of 75 to 150°C. Highly promising sensing performance was observed at an optimum operating temperature of only 100°C with 102% response magnitude (in 100 ppm methanol) and appreciably fast response and recovery time (~18 and ~21 s, respectively). The possible mechanism behind the origin of p-type conductivity has been correlated with the defect distribution model. Detailed sensing mechanism of the p-nano-titania has been explained using Schottky barrier model.

Vertical Mode Gas Sensing Performance of TiO 2 Nanotube Array by Tuning of Surface Area and Carrier Transport Length

Selective detection of vapor species, especially belonging to the same group, is a crucial bottleneck for resistive sensor devices. In this paper, methanol, ethanol, and 2-propanol detection by Pd/TiO2 nanorod array/Ti device in capacitive mode was achieved, with improved selectivity, employing a resonant frequency tuning technique. The optimal capacitive response of the device was obtained by identifying the resonant frequency over the frequency range of 0.01–200 kHz. The imaginary part of the impedance (−|Z| sinθ as a function of frequency) in air and in alcohols revealed a shift in resonant frequency towards lower frequency regime upon exposure to alcohols. Moreover, the difference in the resonant frequencies (Δf ) from one alcohol to another were found to be almost constant which facilitated the frequency-selective sensing even for varying concentrations of target species (1, 50, and 100 ppm in present case). Such frequency selective nature of the device possibly originated from different degree of polarization of different alcohol molecules at different frequencies which was duly correlated to the corresponding equivalent circuit model. However, for real selective detection, when the target species is unknown, the proposed device does not work properly and an array of devices is necessary.

Low Temperature Methanol Sensing by p-Type Nano-titania: Correlation with Defects States and Schottky Barrier Model

Thin film Sol-gel grown nano TiO2 based resistive sensor was developed to detect acetone at low temperature. Structural characterization were carried out using XRD, FESEM and AFM to ensure the crystalline, grain size and surface roughness of the TiO2 thin film and anatase phase TiO2 having particle size in order of 10 to 20 nm with average roughness 100nm. The thin film with catalytic Pd contact were investigated for acetone vapor sensing in the range of 150-250°C for acetone concentration 500-1500 ppm with N2 as carrier gas. The maximum response 178 % approximately was obtained at the optimum temperature of 150oC for 500 ppm acetone. The dynamic range of the sensor was also found to be quite appreciable (500-1500 ppm). The response and recovery time 42s and 44s respectively was recorded at 500 ppm at optimum temperature. The short term stability study indicated that the sensor is appreciably stable with a nominal baseline drift of ±6 %.

Resonant frequency tuning technique for selective detection of alcohols by TiO 2 nanorod based capacitive device

Selective detection of vapor species, especially belonging to the same group, is a crucial bottleneck for resistive sensor devices. In this paper, methanol, ethanol, and 2-propanol detection by Pd/TiO2 nanorod array/Ti device in capacitive mode was achieved, with improved selectivity, employing a resonant frequency tuning technique. The optimal capacitive response of the device was obtained by identifying the resonant frequency over the frequency range of 0.01–200 kHz. The imaginary part of the impedance (−| Z | sinθ as a function of frequency) in air and in alcohols revealed a shift in resonant frequency towards lower frequency regime upon exposure to alcohols. Moreover, the difference in the resonant frequencies (Δ f ) from one alcohol to another were found to be almost constant which facilitated the frequency-selective sensing even for varying concentrations of target species (1, 50, and 100 ppm in present case). Such frequency selective nature of the device possibly originated from different degree of polarization of different alcohol molecules at different frequencies which was duly correlated to the corresponding equivalent circuit model. However, for real selective detection, when the target species is unknown, the proposed device does not work properly and an array of devices is necessary.