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Operating Temperature, Repeatability, and Selectivity of TiO2 Nanotube-Based Acetone Sensor: Influence of Pd and Ni Nanoparticle Modifications Partha Bhattacharyya, Senior Member, IEEE, Basanta Bhowmik, Member, IEEE, and Hans-J Fecht Abstract—This paper concerns the tuning of the operating temperatures, selectivity, and repeatability of a resistive acetone sensor based on TiO2 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 TiO2 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. Index Terms—TiO2 nanotubes, Pd modification, Ni modification, optimum temperature, selectivity, repeatability.

I. I NTRODUCTION

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NE dimensional (1D) semiconducting oxide nanostructures are the promising candidates for developing highly efficient and miniaturized sensor devices. In this category, oxide nanotubes, owing to their excessively high surface to volume ratio (both, inner wall and outer wall are available for gas

Manuscript received March 13, 2015; revised June 24, 2015; accepted June 25, 2015. Date of publication July 13, 2015; date of current version September 1, 2015. This work was supported in part by the Indian National Science Academy (INSA) under Grant SP/YSP/81/2013/735 and Grant Intl/DFG/2014/1560 Dt: 16/12/2014, and in part by Deutsche Forschungsgemeinschaft (DFG) under Grant GZ: FE 313/16-1: AOBJ:614734 Dt:11/09/2014. The work of B. Bhowmik was supported by the Ph.D. Fellowship from COE, TEQIP-II, IIEST Shibpur. P. Bhattacharyya and B. Bhowmik are with the Nano-Thin Films and Solid State Gas Sensor Devices Laboratory, Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology Shibpur, Howrah 711 103, India (e-mail: [email protected]; [email protected]). H.-J. Fecht is with the Institute of Micro and Nanomaterials, Ulm University, 89081 Ulm, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2015.2455557

sensing) combined with the unprecedented advantage of 1-D electron transport kinetics, attracted the sensor researches across the globe [1], [2]. Different VOC sensors, based on different device structures employing NT array as the sensing layer, have recently been reported [1]. TiO2 nanotubes, owing to its stability and reproducibility, have recently been investigated for developing efficient and reliable acetone sensors [3], [4]. However, selectivity and operating temperature are the two burning issues which need to be addressed properly. Surface modifications of nanostructured metal oxides, by noble metal catalysts have already been reported by different research groups with the preliminary aim of improving response magnitude [4]–[7]. These noble metal catalysts offered additional adsorption sites on the sensing surface, eventually leading towards higher response magnitude. However, no report has so far been published on the influence of these metal nanoparticle (Ni and Pd) modifications (of TiO2 NT array) on the sensing performance towards acetone vapor. Most of the earlier reported TiO2 NT based gas/vapor sensors suffer either from the high operating temperatures or from poor selectivity and repeatability. Carbon doped TiO2 nanotube array, investigated by Kilinc et al. [8], offered selective H2 sensing at 100 ◦ C, but at the cost of very poor response magnitude (∼31%) and repeatability. Employing Ni and Al doping on p-TiO2 NTs, Li et al. [9], [10] achieved better repeatability towards H2 at 200 ◦ C (for Ni doped sample) and 300 ◦ C (for Al doped sample). High response magnitude (∼97.5%) towards H2 was demonstrated by Varghese et al. [11] using undoped TiO2 NT. In their work, it was found that, though the sensors offered higher response magnitude, other parameters like operating temperature (∼400 ◦ C) and response/recovery characteristics (>150 s) constraint its commercial applications. On the other hand, Sxennik et al. [12], reported on the performance of undoped TiO2 NTs towards H2 sensing at relatively lower operating temperature (150 ◦ C) with very sluggish response time/ recovery time (1800 s/900 s, respectively). Kılınç et al. [4] demonstrated TiO2 NTs based sensor with appreciably fast response time and recovery time (∼18 s and 22 s, respectively) with the maximum response magnitude of only ∼16%. Such poor response magnitude and long with poor repeatability (towards ethanol) were the prime disadvantages of their work. Room temperature formaldehyde sensor by p-TiO2 NTs was investigated by Lin et al. [2]. The sensor showed ∼37% response towards 50 ppm formaldehyde with a sluggish response time of 180 s. Authors did not investigate the repeatability of their device.

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BHATTACHARYYA et al.: TEMPERATURE, REPEATABILITY, AND SELECTIVITY OF SENSOR

In this endeavor, Pd and Ni modified TiO2 NT array based resistive sensors were tested for detecting acetone. The influence of such modifications on the sensing parameters like operating temperature, response magnitude, response time, recovery time, repeatability and selectivity was investigated. Surface modification was found to be very effective in improving the response characteristics, selectivity and to decrease the operating temperature significantly, though at the cost of poor repeatability. Among Pd and Ni, the first one was found to be more efficient in improving the sensing characteristics including selectivity. II. E XPERIMENTAL The details of the nanotube synthesis technique have already been reported in [13], so here the repetition of the same has been avoided. 99.94% pure Titanium foil (Alfa Aesar) was used as the anode, for room temperature anodization, using two electrode configurations with a cylindrical graphite rod as the cathode. Mixture of 0.5 wt% NH4 F, 199 ml ethylene glycol and 5% (by volume) of water was used as the electrolyte under 20 V bias. The samples were then annealed at 300 ◦ C for 3 hours. The surface modifications of the grown TiO2 nanotubes were achieved using 5 wt% of NiCl2 and 5 wt% of PdCl2 , separately. The samples were dipped into chloride solution for 2 s, 5 s and 10 s using a controlled dip coating unit (Apex, Xdip-SV1). 5 s dipping time was found to be the optimum one for synthesizing NTs with efficient gas sensing capability. After surface modifications, the samples were dried at 110 ◦ C for 10 minutes. Crystallinity of the TiO2 thin films (unmodified and modified) was investigated by X-ray Diffraction (XRD, Rigaku MiniFlex II, Cu Kα radiation, λ = 1.54 Å) analysis. Field Emission Scanning Electron Microscopy (FESEM) (ZEISS SIGMA) was used to examine the surface morphology of the films. Existence of Pd and Ni nanoparticles on TiO2 nanotube surfaces was authenticated by energy dispersive spectroscopy (EDS). Two Pd electrodes having dimensions of 1 mm × 1 mm × 50 nm were deposited at the top of the nanotube surface by e-beam evaporation technique. Sensor study was carried out in a system similar to that reported in [3]. The total flow rate of ∼100 sccm was maintained in the sensing chamber during all the measurements. The change in resistance (in air and in target gas/vapor) was measured using Keithley 6487 picoammeter/programmable voltage source. The sensor response magnitude (RM %) is expressed as [(Ra − Rg )/Ra ] × 100, where Ra is the sensor resistance in air and Rg is the sensor resistance in test vapor (acetone). Response time was calculated as the time required for reaching 90% of the saturation response [3], [14]. III. R ESULTS AND D ISCUSSIONS A. Structural and Electrical Characterizations Fig. 1(a) shows the XRD pattern of the Ni modified unmodified, Pd modified and unmodified TiO2 NTs. All the samples showed anatase (101) crystallinity (JCPDS file no 21-1272)

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[15]. The calculated average crystalline size (from the well known Scherer formula) were ∼13 nm, ∼12.4 nm and ∼12 nm for unmodified, Ni modified and Pd modified samples, respectively. The peak at 52.08◦ corresponds to Ni whereas peaks at 37.25◦ and 62.22◦ refer to Pd. Surface morphology of the unmodified TiO2 NTs is depicted in Fig. 1(b) and inset shows the magnified FESEM images of Ni nanoparticles and Pd nanoparticles, respectively. From the FESEM images, it can be seen that the inner diameters of the nanotubes lie in the range of 50–60 nm; whereas the wall thicknesses are in the range of 15–16 nm. The cross sectional measurement revealed that the lengths of the nanotubes are in the range of 470 ± 10 nm. As the nanoparticles, on TiO2 NT surface, were not clearly evident, Pd and Ni nanoparticles were deposited separately on a smooth surface like FTO glass to estimate the size. Nanoparticles on FTO glass surface were characterized by FESEM. The size of the nanoparticles was found to be in the range of 10–22 nm (for Ni) and 5–14 nm (for Pd), respectively. EDS analysis was carried out to authenticate the existence of Ni and Pd nanoparticles on the NTs surfaces as well as to estimate the percentages of respective chemical compositions. EDS spectra (Fig. 1(c)–(e)), confirmed the successful deposition of Ni and Pd on the TiO2 nanotube surfaces. The weight percentage of Ti, Oxygen and Pd/Ni is shown as the inset of Fig. 1(c)–(e). In case of modified samples, Pd and Ni weight percentage was found to be 1.78% and 0.87%, respectively. Photoluminescence spectroscopy (PL spectra) of the three samples was conducted at room temperature and the results are shown in Fig. 1(f). As evident from Fig. 1(f), all the samples showed three peaks in the wavelength range of 350–550 nm for an excitation wavelength of 300 nm. The emission peaks within 376–382 nm and within 418–423 nm correspond to band to band emission; whereas peaks at relatively higher wavelengths (463–469 nm) are related to oxygen vacancy (OV) induced defect states. Increase in peak intensity related to OVs (at 468 nm and 469.54 nm), in case of modified sensors than that of the unmodified one (at 463 nm) implies the existence of higher oxygen vacancies in modified sensors. De-convolution with Gaussian fittings of the curves (not shown here) further reveals that, amount of oxygen vacancies are in the order of Pd-TiO2 NTs > Ni-TiO2 NTs > TiO2 NTs. Higher oxygen vacancy in case of modified sensors possibly owing to the formation of PdO/NiO by capturing surface oxygen of the TiO2 NTs. Band gap (Eg ) of the samples were found to be 3.3 eV (unmodified), 3.29 eV (Ni modified) and 3.27 eV (Pd modified), respectively. B. Sensor Study Acetone sensing performance of the three devices was tested in the temperature range of 50–250 ◦ C targeting the concentration range of 10–1000 ppm. Fig. 2 shows the response magnitude as a function of operating temperatures for the acetone concentrations of 1000 ppm. Clearly, surface modified sensors (Ni-TiO2 NT and Pd-TiO2 NTs) offered better acetone sensing performance in the entire temperature (50–250 ◦ C) range than that of the unmodified one. The optimum temperature was found to be 150 ◦ C for unmodified sensor and 100 ◦ C for both

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Fig. 1. (a) XRD pattern of unmodified TiO2 NTs, Ni-TiO2 NTs and Pd-TiO2 NTs (b) FESEM image of the top surface of unmodified TiO2 NTs; inset shows the FESEM images of the Ni and Pd nanoparticles grown on FTO glass separately, to measure the particle size (c-e) shows the EDS spectra of unmodified, Ni modified and Pd modified TiO2 NTs (f) Room temperature PL spectra of the unmodified, Ni modified and Pd modified TiO2 NTs for an excitation wavelength of 300 nm.

Ni modified and Pd modified ones. Tuning of the optimum temperature (reducing from 150 ◦ C to 100 ◦ C) in case of both (Ni and Pd) modified sensors is possibly attributed to the lower activation energy (Ea ) requirement of the devices (as the system gains additional activation energy (Ea ) promoted by Pd and Ni nanoparticle clusters) [4]–[7], [16]. The maximum response magnitude (RM%) was found to be ∼52% (for TiO2 NT), ∼82% (for Ni-TiO2 NT) and ∼97% (for Pd-TiO2 NT), respectively, at their respective optimum temperature towards 1000 ppm acetone. Such increase in response magnitude, in case of modified sensors (Ni and Pd), is owing to the effect of enhanced oxygen spill over process around the Pd and Ni nanoparticles [16], [17]. These spilled over oxygen species (around Pd and Ni nanoparticles) offer additional chemisorptions sites leading towards higher response magnitude. Inset of Fig. 2, depicts the response magnitude as a function of acetone concentrations. A linear increase in response magnitude was found, up to 100 ppm for all the sensors and then a saturation

tendency was prominent at relatively higher concentrations (>100 ppm). At low concentrations (10 ppm), the modified sensors showed relatively higher response magnitude of ∼33% (Ni modified) and ∼41% (Pd modified) than that of the unmodified one (∼25%). Adsorb acetone molecules at the interface of Pd/Ni nanoparticles and TiO2 nanotubes develop a dipole layer with small electrostatic potential (ΔV) [18]. Such electrostatic potential might be the reason of higher effective change of barrier height in case of modified sensors. Details of the effect of such modifications have been discussed in the penultimate section, with the help of a model considering the adsorption desorption kinetics, barrier height (ΦB ) and work function (Φ) with exposure to target species. Transient response characteristics of the sensors (Pd modified, Ni modified and unmodified) at their respective optimum temperature (100 ◦ C for modified and 150 ◦ C for unmodified, respectively) for the acetone concentrations of 10, 50, 100, 400, 700 and 1000 ppm are shown in Fig. 3.

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Fig. 2. Response magnitude as a function of operating temperatures in 1000 ppm acetone; inset shows response magnitude as a function of different acetone concentrations at their respective optimum operating temperature. Fig. 4. (a) Response time (b) Recovery time as a function of operating temperatures at 10 and 1000 ppm acetone.

Fig. 3. Transient response characteristics of the unmodified, Ni modified and Pd modified TiO2 NT based sensors at their respective optimum temperature (for the acetone concentrations of 10, 50, 100, 400, 700, and 1000 ppm).

Fig. 4(a) and (b) demonstrate the response-recovery characteristics of the three sensors as a function of operating temperatures towards 1000 ppm acetone. With increase in temperatures, all the sensors showed appreciably fast response/recovery characteristics than their lower temperature counterparts. Moreover, Pd and Ni modified sensors offered faster response time and recovery time than that of unmodified one for the entire temperature range (50–250 ◦ C). The fastest response time/recovery time of ∼17/26 s (unmodified sensor), ∼11/21 s (Ni modified sensor) and ∼6/10 s (Pd modified sensor) were observed at 250 ◦ C operating temperature (at 1000 ppm acetone). The sensor parameters from earlier reports (for TiO2 nanotube based gas/vapor sensors) have been compared with the developed sensors and are summarized in Table I. However, most of the reports are on H2 sensors with very few on VOC sensing. Most of these sensors do not meet the commercial viability due to relatively high operating temperature, poor response/recovery and/or poor repeatability. In this category, Kilinc and his coworkers [8] reported on carbon doped TiO2 nanotubes, capable of selectively detecting H2 at 100 ◦ C. But,

their senor showed low response magnitude of only ∼31% for 5000 ppm H2 with very poor repeatability. Al and Ni doped p-TiO2 nanotubes for H2 sensing were investigated by Li et al. [9], [10]. It was found that, Ni doped sensor offered lower optimum operating temperature of 200 ◦ C than their Al doped counterpart (300 ◦ C). Further, Ni doped one showed better repeatability, possibly owing to the less oxide formation probability of Ni than that of Al. Using similar approach, Xiang et al. [19] reported H2 sensing performance at relatively lower optimum temperature (150 ◦ C) by doping Pd nanoparticles on the TiO2 NTs surfaces. Poor response magnitude (11.5%) and sluggish response/recovery time (120 s/90 s) even at higher concentrations of H2 , limit the commercial applications of such sensors. High response magnitude towards H2 sensing by undoped TiO2 NT was demonstrated by Sxennik et al. [12] and Varghese et al. [11]. In the work of Varghese et al. [11], it was found that, response magnitude was significantly high (∼97.5%) but at high operating temperature (400 ◦ C). To the contrary, relatively lower operating temperature (150 ◦ C) was found in the work of Sxennik et al. [12]. However, unacceptably high response time and recovery time (of 1800 s and 900 s, respectively) with low response magnitude of 34% were the prime disadvantages of their work [12]. Fast response time/ recovery time of ∼18 s/22 s towards 5000 ppm ethanol by n-TiO2 NTs was reported in another work of Kılınç et al. [4]. Their sensor offered poor repeatability with very low response magnitude of only ∼16%. Room temperature formaldehyde sensing performance by p type TiO2 NTs was investigated by Lin et al. [2]. At room temperature, sensor showed ∼37% response towards 50 ppm formaldehyde with a response time of 180 s. Author did not report on the repeatability of their devices. Significantly better repeatability, response magnitude with relatively fast response time and recovery time are evident in case of present study. Present study attained a maximum response magnitude of ∼97% at relatively lower optimum temperature of 100 ◦ C only towards acetone using Pd nanoparticle modification. Moreover, Pd modified sensor showed fast

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TABLE I A C OMPARATIVE S UMMARIZATION OF THE A NODIZED TiO 2 N ANOTUBES T OWARDS D IFFERENT G AS /VAPOR D ETECTION

Fig. 5. Repeatability study of the three batches of sensor (a) TiO2 NTs at 150 ◦ C, (b) Ni-TiO2 NTs at 100 ◦ C and (c) Pd-TiO2 NTs at 100 ◦ C temperature for the acetone concentration of 1000 ppm (d) Cross sensitivity study of the three devices with respect to other interfering VOCs (Ethanol, Methanol, 2-Butanone and Toluene).

response time/recovery time of ∼19 s/24 s. However, most promising repeatability was achieved in case of unmodified TiO2 NTs. C. Repeatability and Selectivity Study Repeatability of the three devices was tested (for three batches of same sample) at 1000 ppm acetone concentrations considering three consecutive cycles of same concentrations as shown in Fig. 5(a)–(c). Repeatability of the devices in terms of response magnitude, response time and recovery time are summarized in Table II. As evident from Table II, the variations in sensor parameters were more prominent in case of modified sensors. Variations in sensor parameters (among the consecutive cycles) i.e. response magnitude, response time and recovery

time were within ±1.61%, ±2.94%, and ±2.22% for unmodified TiO2 NTs and ±2.43%, ±8.33% and ±8.10% for Ni modified TiO2 NTs, respectively. The maximum response magnitude deviation was found to be ±5.15%, with the corresponding deviations in response time and recovery time of ∼±21% and ∼±16%, respectively in case of Pd modified TiO2 NTs. Such a large deviations in response magnitude, response time and recovery time of the modified sensors is possibly attributed to the increased structural defects and oxygen vacancies after Pd/Ni modifications [7]. Moreover, it can be envisaged that, oxygen spill over around Pd and Ni nanoparticles was not identical for consecutive cycles which is the origin of difference between the same sensor parameter in different cycles (repeated). Variations in the sensor parameters viz. response magnitude, response time and recovery time) among different (three) batches of the same

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TABLE II R EPEATABILITY S TUDY IN T ERMS OF R ESPONSE M AGNITUDE (RM%), R ESPONSE T IME , R ECOVERY T IME OF THE T HREE B ATCHES OF S ENSORS W ITH R EPEATED (C ONSECUTIVE ) C YCLES

type of sensor were tested and the corresponding values were found to vary within ±1.12% (response magnitude), ±2.94% (response time), and ±4.4% (recovery time) for unmodified TiO2 NTs. For Ni modified TiO2 NTs, ±1.21% (response magnitude), ±12.5% (response time), and ±5.8% (recovery time) variations among the different batches were recorded. On the other hand, ±2.06% (response magnitude), ±10.52% (response time), and ±12.5% (recovery time) variations for Pd modified TiO2 NTs were observed. Cross sensitivity of the three sensors towards other interfering species like ethanol, methanol, 2-butanone and toluene were tested at 100 ◦ C for 10 ppm and 1000 ppm concentrations. Fig. 5(d) shows the response magnitude plot in the form of bar diagrams for three sensors. Among the tested VOCs, ethanol was found to offer response magnitude next to acetone with a difference of ∼7%, ∼30% and ∼33% for unmodified, Ni modified and Pd modified sensors, respectively. Both Ni and Pd modified sensors showed a significant improvement in selectivity towards acetone than that of the unmodified ones. Moreover, Pd modified samples offered better selectivity towards acetone than that of Ni modified ones. Such response magnitude difference among the modified sensors can be explained by considering two phenomena; viz., (i) Langmuir adsorption-desorption isotherm and (ii) sticking coefficient (S) of target species considering similar surface coverage f(θ) for all the cases. From the earlier reports [20], it can be concluded that the sticking coefficient of acetone is higher in case of Pd (SPd ) than that of Ni (SNi ). Adsorption of polar gases/vapors on metallic/oxide surfaces follows Langmuir adsorption-desorption isotherm [20]. The rate of adsorption is strongly influenced by; (i) sticking probability (S) of the target species on sensing surface and (ii) flux (F) developed by target molecules on the sensing surface following equation (1).   −Ea p . (1) Rads = S × F = f (θ) exp × kT (2πkmT )0.5 Where f(θ) = function of surface coverage and is same for Pd and Ni, Ea = activation energy, k = boltzmann contant, P =

partial pressure of target species, m = mass of vapor molecule, T = absolute temperature. Considering the identical flux (F) for all the three types of sensors, only parameter that can influence the adsorption process is the sticking coefficient (S). As already mentioned, sticking coefficient (S) of reducing agent is higher towards Pd than that in Ni (SPd > SNi ); therefore from equation (1) we can assume Rads(Pd) > Rads(Ni) , which is duly supported by the experimental findings also. Fig. 6(a) and 6(b), represent the energy band diagram of Pd modified TiO2 NT and the same for Ni modified one, respectively. The electrodes have almost the similar effect on both Pd and Ni modified sensors. Therefore, we avoid the influence of the electrodes to critically compare the influence of the Pd and Ni nanoparticle modifications only. Interactions of acetone molecules with oxygen species (O− ) finally result in the by-products consisting carbon dioxide, water and (release of) electrons (to the sensing layer) following the equation (2) [3], [14]. CH3 COCH3 + 8O− = 3CO2 + 3H2 O + 8e− .

(2)

As already mentioned, surface modifications using Pd and Ni catalyst increase the chemisorptions sites promoting the oxygen spill over mechanism. As a result effective change in barrier height (ΔΦB ), with respect to its unmodified counterpart, increases. The barrier height (ΦB ) of the three sensors is considered as ΦB1 , ΦB1,Ni and ΦB1,Pd in air. In acetone, its modulated versions are ΦB2 , ΦB2,Ni and ΦB2,Pd for unmodified, Ni modified and Pd modified cases, respectively. Release of electrons to the sensing layer during gas/vapor interactions resulted in barrier height reductions (shown in the band diagram Fig. 6(a) and (b)). This barrier height reduction was more prominent in case of modified sensors than that of unmodified one, due to additional sticking of acetone molecules on the Pd and Ni surfaces. Among Pd and Ni, Pd have higher sticking coefficient towards acetone than that of Ni (SPd > SNi ). Therefore, from equation (2), it is envisaged that the effective

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nanoparticles) is reported in this paper. Such modifications resulted in; (i) lowering of the optimum operating temperature from 150 ◦ C (unmodified) to 100 ◦ C (modified), (ii) higher response magnitude of ∼82% (Ni modified) and ∼97% (Pd modified) (iii) improved response time/recovery time and (iv) improved acetone selectivity. Improvement in sensor response with Pd/Ni modifications possibly attributed to the additional chemisorptions sites and enhanced oxygen spill over around the Pd/Ni nanoparticle islands. This improvement (of modified sensor) was achieved at the expense of relatively poor repeatability. Therefore, for a specific application, judicious optimizations of these parameters are required depending upon the specific requirement of the system. It is worth mentioning that, there is always a trade off among the sensor parameters like working temperature, response magnitude, response time, selectivity and repeatability. One cannot achieve improvement in all parameters at the same time. In the present study, we have successfully achieved improved sensor characteristics with minimal trade off among these mutually conflicting parameters. Deviation in repeatability of the modified sensors took place, due to the increase of surface defects/OVs as authenticated by PL study. Moreover, Pd modified sample offered better response to acetone than that of Ni modified one. Such improvement in response magnitude, response time and recovery time towards acetone has also been correlated with the corresponding sticking coefficient (S) and effective change of barrier height (ΔΦB ) upon exposure to acetone.

Fig. 6. (a) Band diagram of Pd modified TiO2 NT (b) band diagram of Ni modified TiO2 NT.

change in barrier height (ΔΦB ) upon exposure to acetone is in the order; ΔΦB,Pd−TiO2 > ΔΦB,Ni−TiO2 > ΔΦB,TiO2 . The higher effective change of barrier height (in addition with oxygen spill over in case of Pd modified sample) is the reason for better sensing performance towards acetone. Oxygen spill over around Pd and Ni nanoparticles generates weakly bonded PdO− and NiO− which provide additional depletion layer to the system. Such weakly bonded PdO− and NiO− easily interact with target species. Accumulation of acetone on the Pd/Ni/TiO2 interfaces produce a dipole layer as shown in Fig. 6(a) and (b). Each dipole develops an electrostatic potential (ΔV) at the Pd/Ni/TiO2 interfaces following equation (3) [18]. ΔV =

μNi θi . ε

(3)

Where μ is the effective dipole moment, Ni is the chemisorptions site per unit area at the interface, θi is the acetone surface coverage at the TiO2 NTs interface, ε is the dielectric permittivity of TiO2 . IV. C ONCLUSION Tuning of optimum operating temperature, repeatability and acetone selectivity of the TiO2 nanotube array based sensor employing the surface modification approach (with Pd or Ni

R EFERENCES [1] A. Hazra and P. Bhattacharyya, “Tailoring of the gas sensing performance of TiO2 nanotubes by 1-D vertical electron transport technique,” IEEE Trans. Electron Devices, vol. 61, no. 10, pp. 3483–3489, Oct. 2014. [2] S. Lin, D. Lin, J. Wu, and X. Li, “A selective room temperature formaldehyde gas sensor using TiO2 nanotube array,” Sens. Actuators B, Chem., vol. 156, pp. 505–509, 2011. [3] B. Bhowmik, A. Hazra, K. Dutta, and P. Bhattacharyya, “Repeatability and stability of room-temperature acetone sensor based on TiO2 nanotubes: Influence of stoichiometry variation,” IEEE Trans. Device Mater. Rel., vol. 14, no. 4, pp. 961–967, Dec. 2014. [4] N. Kılınç, E. Sennik, ¸ and Z. Z. Öztürk, “Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing,” Thin Solid Films, vol. 520, pp. 953–998, 2011. [5] J. Guo et al., “High-performance gas sensor based on ZnO nanowires functionalized by Au nanoparticles,” Sens. Actuators B, Chem., vol. 199, pp. 339–345, Aug. 2014. [6] G. Neri et al., “O2 sensing properties of Zn- and Au-doped Fe2 O3 thin films,” IEEE Sensors J., vol. 3, no. 2, pp. 195–198, Apr. 2003. [7] M. Hari, S. A. Joseph, S. Mathew, P. Radhakrishnan, and V. P. N. Nampoori, “Band-gap tuning and nonlinear optical characterization of Ag: TiO2 nanocomposites,” J. Appl. Phys., vol. 112, pp. 074307–074314, 2012. [8] N. Kılınç et al., “Fabrication and gas sensing properties of C-doped and un-doped TiO2 nanotubes,” Ceramics Int., vol. 40, no. 1, pp. 109–115, Jan. 2014. [9] Z. Li, D. Ding, Q. Liu, C. Ning, and X. Wang, “Ni-doped TiO2 nanotubes for wide-range hydrogen sensing,” Nanoscale Res. Lett., vol. 9, no. 1, pp. 118–126, Mar. 2014. [10] Z. Li, D. Ding, and C. Ning, “p-Type hydrogen sensing with Al- and V-doped TiO2 nanostructures,” Nanoscale Res. Lett., vol. 8, pp. 25–33, 2013. [11] O. K. Varghese, D. Gong, M. Paulose, K. G. Ong, and C. A. Grimes, “Hydrogen sensing using titania nanotubes,” Sens. Actuators B, Chem., vol. 93, no. 1–3, pp. 338–344, Aug. 2003. [12] E. Sxennik, Z. Colak, N. Kılınc, and Z. Z. Ozturk, “Synthesis of highlyordered TiO2 nanotubes for a hydrogen sensor,” Int. J. Hydrogen Energy, vol. 35, pp. 4420–4427, 2010.

BHATTACHARYYA et al.: TEMPERATURE, REPEATABILITY, AND SELECTIVITY OF SENSOR

[13] A. Hazra et al., “Formation mechanism of anodically grown free-standing TiO2 nanotube array under the influence of mixed electrolytes,” Sci. Adv. Mater., vol. 6, no. 4, pp. 714–719, 2014. [14] B. Bhowmik, K. Dutta, A. Hazra, and P. Bhattacharyya, “Low temperature acetone detection by p-type nano-titania thin film: Equivalent circuit model and sensing mechanism,” Solid State Electron., vol. 99, pp. 84–92, Sep. 2014. [15] Natl. Bur. Stand. (U.S.), Monogr. 25 7, 82 1969. [16] P. Bhattacharyya, P. K. Basu, H. Saha, and S. Basu, “Fast response methane sensor using nanocrystalline zinc oxide thin films derived by sol-gel method,” Sens. Actuators B, Chem., vol. 124 no. 1, pp. 62–67, Jun. 2007. [17] T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree, and M. Salmeron, “Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies,” Nature, vol. 422, no. 17, pp. 705–707, Apr. 2003. [18] C. S. Hsu et al., “The impact of TiO2 interface layer on a Pd/n-LTPS Schottky diode hydrogen detecting performances,” IEEE Trans. Electron Devices, vol. 57, no. 8, Aug. 2010. [19] C. Xiang et al., “A room-temperature hydrogen sensor based on Pd nanoparticles doped TiO2 nanotubes,” Ceramics Int., vol. 40, no. 10, pp. 16343–16348, Dec. 2014. [20] M. Johansson, O. Lytken, and I. Chorkendorff, “The sticking probability of hydrogen on Ni, Pd and Pt at a hydrogen pressure of 1 bar,” Topics Catalysis, vol. 46, no. 1/2, Sep. 2007.

Partha Bhattacharyya (SM’14) received the B.E. degree in electronics and telecommunication engineering, the M.E. degree in electron devices, and the Ph.D. degree in MEMS-based gas sensor and its integration with CMOS circuits from Jadavpur University, Kolkata, India, in 2002, 2004, and 2008, respectively. Currently, he is an Assistant Professor with the Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology Shibpur, Howrah, India. In 2015, he was a Visiting Scientist with the Institute of Micro and Nanomaterials, Ulm University, Ulm, Germany, for his postdoctoral research. He is the author or coauthor of about 110 research articles in reputed journals and conferences. His current research interests include nanomaterial-based sensors, MEMS-based chemical sensors, and its CMOS integration. Dr. Bhattacharyya was the recipient of the Young Engineer’s Award from the Institution of Engineers (India) in 2010, the Career Award for Young Teachers from the All India Council for Technical Education in 2011–2012, the “Young Engineer Award” from the Indian National Academy of Engineering in 2012, and the “Young Scientist Award” from the Indian National Science Academy in 2012 for his teaching and research contributions.

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Basanta Bhowmik (M’14) received the M.Tech. degree, with specialization in VLSI Design, from the Indian Institute of Information Technology and Management, Gwalior, India, in 2012. He is currently working toward the Ph.D. degree in electronics and telecommunication engineering with the Nano-Thin Films and Solid State Gas Sensor Devices Laboratory, Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology Shibpur, Howrah, India, focusing on thin-film solid-state gas sensors. His research interests include the fabrication of thin-film solid-state chemical gas sensor devices to detect different volatile compound gases (VOC).

Hans-J Fecht received the Ph.D. degree in materials science from the University of Saarbrücken, Saarbrücken, Germany, in 1984. He is a Chair Professor with the Faculty of Engineering, and Computer Science, Ulm University, Ulm, Germany, where he is also the Director of the Institute of Micro and Nanomaterials. He served as a Professor with the University of Augsburg, Augsburg, Germany, after postdoctoral fellowships with the University of Wisconsin–Madison, Madison, WI, USA, and with the California Institute of Technology, Pasadena, CA, USA. In 1993, he was appointed a Full Professor with the Technical University of Berlin, Berlin, Germany, and in 1997, he became a Full Professor with Ulm University. Dr. Fecht is a member of the European Academy of Sciences and Arts. He was a recipient of the Leibniz Prize of the German Research Foundation (DFG), the “Pioneer of Nanotechnology” Award from Deutsche Bank, and the Innovation Award of the Association of German Engineers (VDI).