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Highly Sensitive, Temperature-Independent Oxygen Gas Sensor based on Anatase TiO2 Nanoparticles-grafted, 2D Mixed Valent VOx Nanoflakelets APPU Vengattoor Raghu, Karthikeyan K Karuppanan, and Biji Pullithadathil ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00544 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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Highly Sensitive, Temperature-Independent Oxygen Gas Sensor based on Anatase TiO2 Nanoparticles-grafted, 2D Mixed Valent VOx Nanoflakelets Appu Vengattoor Raghu†, Karthikeyan K Karuppanan† and Biji Pullithadathil†#* †. #
Nanosensor Laboratory, PSG Institute of Advanced Studies, Coimbatore, 641004, INDIA Department of Chemistry, PSG College of Technology, Coimbatore-641004, INDIA
ABSTRACT: Herein, we report a facile approach for the synthesis of TiO2 nanoparticles tethered on 2D mixed valent vanadium oxide (VOx/TiO2) nanoflakelets using a thermal decomposition assisted hydrothermal method and investigation of its temperature-independent performance enhancement in oxygen-sensing properties. The material was structurally characterized using XRD, TEM, Raman, DSC and XPS analysis. Presence of mixed valent states, such as V2O5 and VO2 in VOx and the metastable properties of VO2 have found to play crucial roles in the temperature-independent electrical conductivity of VOx/TiO2 nanoflakelets. Though pristine VOx exhibited characteristic semiconductorto-metal transition of monoclinic VO2, pure VOx nanoflakelets exhibited poor sensitivity towards sensing oxygen. VOx/TiO2 nanoflakelets showed very low temperature coefficient of resistance above 150°C with improved sensitivity (35 times higher than VOx for 100 ppm ) towards oxygen gas. VOx/TiO2 nanoflakes exhibited much higher response, faster adsorption and desorption towards oxygen as compared to pristine VOx beyond 100°C which endowed the sensor with excellent temperatureindependent sensor properties within 150-500oC. The faster adsorption and desorption after 100oC led to shorter response time (3-5s) and recovery time (7-9 s). The results suggest that 2D VOx/TiO2 can be a promising candidate for temperatureindependent oxygen sensor applications.
Keywords: Temperature-independent sensors, oxygen gas sensors, VOx nanoflakelets, mixed-valent vanadium oxide, VOx/TiO2 nanocomposite.
Oxygen gas sensors are considered to be promising candidates for wide range of potential applications in automobiles, aerospace, soil respiration, ventilator and thermal power generation.1–8 Particularly, semiconducting metal oxide based chemiresistive sensors have gained great attention due to advantages such as, relatively compact device structure, facile fabrication, cost-effective and easy integration into integrated circuits.9–12 Several studies have been reported on sensors working at room temperature as well as at low temperatures.13–16 But, only few reports based on metal oxides have demonstrated stable sensing properties towards oxygen gas at higher temperatures (>200°C).3,17 Among metal oxides, TiO2 has been extensively investigated and recognized as a effective material for oxygen gas detection due to their remarkable catalytic property and superior chemical/mechanical stability.18–22 Recently, lot of efforts have been done to enhance the gas sensing performance of TiO2 by
developing nanostructured materials with ultra-high surface-tovolume ratio, elemental doping and surface modifications to form heterostructures with noble metals and semiconductor nanoparticles.9,21,23 TiO2 is inherently sensitive to oxygen gas from room temperature to high temperature owing to the high oxygen vacancies present in anatase and rutile crystal structures. Recent studies explored that though TiO2 based sensor materials are highly sensitive towards oxygen, such materials suffer from high temperature coefficient of resistance (TCR) (i.e., high activation energy).3,24–26 Hence, one of the major challenges faced by resistive oxygen sensing materials is that though such materials show resistivity changes towards oxygen adsorption, they also respond strongly with temperature which hinders their practical usage under temperature fluctuating conditions. In order to address this issue, many groups used micro heaters in the devices which maintained constant temperature throughout the working conditions.27,28 However, such sensor configuration limited the effective monitoring of oxygen at high temperatures due to high power consumption. The sensor packaging is quite difficult in such devices due to the coupling of output voltage with heater circuit when temperature is maintained above 300°C.29 A comprehensive study shows that the development of micro-oxygen sensors working under fluctuating temperature conditions is still under infancy stage and hence, it is very important to develop alternative materials to overcome these issues. Recently, few research groups have explored the possibility to avoid micro-heater by developing new materials, such as Fedoped SrTiO3 and Fe-doped BaTiO3 which possessed temperature-independent conductivity properties. Although these materials were employed as temperature-dependent oxygen sensing materials, it suffers from low recovery, slow response and high operating temperature ranges30–33 Therefore, metal oxides with partially occupied electron shells have gained significant attention due to their reversible phase transition between metal and insulator states, entirely different from conventional insulators or semiconductors. In this scenario, vanadium dioxide (VOx) is regarded as a unique material due to its reversible semiconductor to metal transition at near room temperature (67°C)34,35 caused by a crystal structure change from monoclinic (M) to tetragonal (A) phase beyond the transition temperature. This transition alters the electronic structure leading to change in resistivity up to five orders of magnitude. Due to this excellent meta-stable property, VOx is useful for several applications, such as optical shutters, thermo-chromic windows, optoelectronic, switching devices and memories.363738 Vanadium, with an electronic configuration of 3d3,4s2 forms multivalent oxides, such as VO, V2O3, V3O7, V4O9, V6O13, VO2 and V2O5, because of the switchable valance of vana-
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dium. Among vanadium oxides, VO2 and V2O5 are the most extensively studied materials because of its unique characteristics and stability.39 Vanadium pentoxide (V2O5) is a band insulator having a more stable 3d0 electronic configuration, is gaining attention for applications in energy storage and sensors due to its unique electrical properties. Similarly, vanadium dioxide (VO2) also has a 3d1 electron configuration which is important in electronic devices, such as resistive random-access memories, switches and thermochromic smart windows.37,40–44 During recent decades, several reports demonstrated the synthesis of vanadium oxides using various methods, such as electrodeposition, electron beam evaporation, RF sputtering, hydrothermal, sol-gel, chemical vapor deposition (CVD) and atomic layer deposition (ALD)38,39,45–52. Thermal decomposition method is considered as a simple, convenient and low-cost method to synthesize vanadium oxides. The oxidation state of vanadium can be prevalently modified by changing the gas pressure and nature of gases.38,53–56 Therefore, it is suggested that the semiconductor to metal transition in VOx can favor the temperature-independent electrical conductivity and integration with TiO2 can enhance oxygen sensing performance. To best of our knowledge, there are no reports available on temperature-independent oxygen sensor based on VOx hybrid nanocomposites. Herein, we report a facile synthesis of mixed valent VOx nanoflakelets grafted with TiO2 nanoparticles using thermal decomposition followed by hydrothermal method which exhibited temperature-independent oxygen sensing characteristics. Atomically stacked layer of mixed-valent VOx nanoflakelets showed transitions from V4O9 to V2O5 and VO2. The presence of oxygen vacancies and surface attached OH groups is found to be responsible for the homogeneous distribution of TiO2 nanoparticles on the surface of VOx nanoflakelets. The synergetic effect of hybrid VOx/TiO2 was found to be the origin of the temperatureindependent properties aroused from VOx nanoflakelets and enhanced gas sensitivity from TiO2. The results emphasized the role of TiO2 nanoparticles on VOx systems towards improvements in sensitivity, selectivity, response and detection limits. EXPERIMENTAL SECTION Ammonium metavanadate (NH4VO3, Aldrich, 99.996%, purity 99.996%), titanium ethoxide (Ti(OC2H5)4, Aldrich), .hydrogen peroxide (Aldrich), glycolic acid (Aldrich) and 25% ammonia (NH4.H2O, Merck) were used as the precursors for the synthesis of nanoflakelets. All reagents were of analytical grade and were directly used without any further purification. Synthesis of Vanadium Oxide Nanoflakelets Vanadium oxide nanoflakelets were synthesized by thermal decomposition method. 5g of ammonium metavanadate was grinded well for 15 min. The fine powder was then annealed in a tubular furnace for 2 h at various temperature (300, 400 and 500°C) at a heating rate 10°C/min. In order to remove the impurities, the final product was washed several times with de-ionized water and dried in a vacuum oven at 80°C for 8 h. Synthesis of VOx/TiO2 Nanoflakelets TiO2 nanoparticles were anchored on the surface of VOx nanoflakelets by using modified hydrothermal method57,58. The VOx nanoflakelets were uniformly dispersed in 200 mL ethanol and ultrasonicated for 1 h. 20 mL of titanium ethoxide was slowly added into the reaction mixture at constant stirring. Then, 20 mL of 30% ammonia solution, 3 mL glycolic acid and 50 mL of 30% hydrogen peroxide were added to this dispersion and heated at 100°C. The obtained product was transferred into a Teflon lined stainless steel autoclave and kept at 90°C for 6h. After the com-
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pletion of the reaction, the autoclave was cooled to room temperature and the final product was washed several times with ultrapure water (Merck-Millipore, Resistivity: 18.2 MΩ.cm) and dried under vacuum oven at 80 °C for 12 h. The materials prepared under nitrogen and oxygen atmosphere conditions at 500°C are represented as VOx-500N and VOx-500A, respectively. TiO2 nanoparticles were synthesized under similar conditions without VOx nanoflakelets for comparison. Evaluation of Oxygen Gas Sensor Properties Gas sensor testing was carried out using an in-house doublewalled stainless steel gas sensor test station equipped with a temperature-controlled hot stage, sensor holder, mass flow controllers (MFC, Alicat, USA), Keithley source meter (2420, USA) and Agilent Multimeter 34410A connected with a data acquisition system using Labview software. Au interdigitated array (IDA) electrodes (~50 nm) were fabricated on alumina substrates using DC magnetron sputtering (HHV, Bangalore). Thin films of VOx and VOx/TiO2 were drop-casted onto IDA electrodes. During the measurements, O2 gas was mixed with N2 carrier gas to achieve the desired concentrations and the flow rate was maintained at 1 slpm using mass flow controllers. The sensor response (S) was calculated as based on the difference from electrical resistances (S= (Rg-Ra)/Ra) of the sensor in nitrogen (Ra) and oxygen gas (Rg) atmospheres.59 The characterization techniques used are described in supporting information. RESULTS AND DISCUSSION In this investigation, mixed valent VOx nanoflakelets were synthesized via a thermal decomposition method and the mechanism for the formation of VOx can be expressed by the following equations (1-4). 4 ( , 500°) → ( )2 + 2 + … … … … … … … … … … … … … … … … … … … … … … … . . …. (1)
( )2 ( , 500°) → 2( )2 + 2 + … … … … … … … … … … … … … … … … … … … … … … . . … . .. (2) ( )2 ( , 500°) → 2 + 2 + 2 + + () …………………………………………..…….. (3)
( ) − 500°) → ……………………………… (4) The conversion of V4O9 and VO2 from NH4VO3 follows a multistep process. V4O9 was further annealed under oxygen atmosphere to prepare V2O5. (Equation 4). XRD analysis was performed to investigate the crystal structures of synthesized TiO2, VOx and VOx/TiO2 nanoflakelets. The VOx prepared at 300°C clearly indicated the formation of NH4VO3/VO2 mixed phases (VO2 and un-reacted state of vanadium (JCPDS No.09-0411)) and the sample prepared at 400°C and 500°C under N2 atmosphere exhibited two mixed valent states, VO2 (JCPDS 81-2392) and V4O9 (JCPDS-23-0720). (Figure S1, supplementary information). Figure 1 depicted the XRD patterns of the prepared materials, The high intensity peak of VOx-400 located at 2θ, 24.19o corresponds to the (210) plane of V4O9, whereas, VOx-500(N2) located at 2θ, 25.23° corresponds to the (210) plane of tetragonal V4O9 and monoclinic VO2 (110) structures. In addition, other diffraction peaks of V4O9 also showed the peaks located at 13.84o, 28.1o, 43.73o, 46.09o corresponding to the planes, (101), (103), (400), (430) and the peaks located at 24.01o, 33.54 o, 48.43 o and 58.97 o of VO2 shows the planes of (201), (310), (512), (711). VOx-500-A treated under oxygen atmosphere indicate a significant transformation from V4O9 to V2O5 and the confirmation peaks located at 20.28o, 21.73o, 26.15o, 34.31o corresponding to the planes of [(001), (101), (110), (310) and (501) (JCPCDS 41-1426)] along with the pre-existed VO2
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crystals. Moreover, XRD spectrum of TiO2 nanoparticles exhibited all characteristic diffraction peaks of typical anatase structure which confirmed according to the JCPDS database (89-4203). The VOx/TiO2 nanoflakelets shows the peaks of (010, 310), (001, 211 and 312) and (101, 004, 200) indicate the presence of V2O5, VO2 and TiO2 respectively, as shown in Figure1.
Figure 1. XRD patterns of VOX nanoflakelets annealed at 500°C in nitrogen atmosphere and air atmosphere, TiO2 nanoparticles and VOx/TiO2 nanocomposite. Further insights into structural and morphological features of the VOx and hybrid VOx/TiO2 nanoflakelets were analyzed by TEM studies, as depicted in Figure 2. As can be seen from Figure 2a, the VOx synthesized under nitrogen atmosphere exhibited sheet-like structures assembled together forming thicker sheets. On the other hand, further treatment of VOx under air atmosphere, transformed the structure into aligned flakelet-like arrangements (Figure 2b). Corresponding SAED patterns indicated the internal microstructural details of the materials. The SAED analysis revealed the presence of mixed phases such as, VO2 and V2O5.
Figure 2. TEM images of (a) VOx 500-N, (b) VOx 500-A. (c and d) pristine hybrid VOx/TiO2 Composite (insets show the corresponding SAED patterns).
The morphology of TiO2 was determined as shown in Figure S2. The inter-planer spacing of TiO2 was estimated to be 0.35 nm and corresponding SAED pattern verified the anatase crystal structure of TiO2 nanoparticles. TEM image analysis indicated the uniform dispersion of anatase TiO2 nanoparticles with an average particle size of ~15 nm anchored on the surface of VOx nanoflakelets (Figure 2c and d). Further analysis using energy dispersive Xray spectrum shown in Figure S3 (supplementary information) confirmed the hybridization of TiO2 on VOx. The AFM images of VOx–500N and VOx-500A nanoflakelets shown in Figure S5, also indicated thin and layered structure of VOx. The thickness of VOx–500N and VOx-500A were calculated from the height profiles of AFM images, which was found to be 3.85 and 3.75 nm, respectively corroborating with the TEM analysis.
Figure 3. DSC curve of VOx-500A (The curve with exothermic peak records the heating process (black solid line) and endothermic peak records the cooling process (red solid line). Raman spectroscopy is an important tool to examine the structural characteristics of VOx nanoflakelets and hybrid VOx/TiO2 composites (Figure S4). The peaks centered at 140 and 990 cm-1 are the characteristic scattering bands of orthorhombic V2O5.60 Particularly, the peak at 990 cm-1 band is related to vanadyl V=O bonds of distorted VO6 sites in the VOx valance of +5 oxidation state. The peaks appeared at 278, 405, 694 cm-1 could be attributed to the vibration modes of VO2 (M).48,49 As expected, the Raman spectra of composite (VOx/TiO2) displayed anatase active vibration peaks at 150cm-1 (Eg), 523 cm-1 (B1g + A1g), 636cm-1 (Eg) as well as a weak wide contribution around 805-840 cm-1 (B1g overtone scattering) corresponding to the successful integration of nano-sized titania on VOX nanoflakelets.61,62 DSC analysis of VOX-500A (Figure 3) manifested the transition of VO2 (M) to VO2 (A) occurred at near room temperature. The transition of V4O9 to V2O5 occurred at 390°C in the presence of oxygen is shown in Figure S5 (supplementary information). Typically, vanadium dioxide (VO2) showed the first order transition of semiconductor to metal (Tc) at 67°C. But, this transition temperature of VO2 (M) to VO2 (A) was found to be shifted to 69.1 °C for VOx-500A, which is due to the presence of mixed phases (VO2 and V2O5) present in the material.35 Figure 3a shows the DSC heating and cooling curves of VO2 (M) to VO2 (R) which clearly suggested the first order phase transition from a highly resistive semiconducting monoclinic (M1) phase to metallic rutile phase.
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Figure 4. Full survey scan XPS spectra (a & d), high resolution spectrum of vanadium (b & e), high resolution spectra of oxygen (c & f) for VOx 500-N and VOx 500-A samples respectively. date nanoflakelets also was enhanced by the formation of oxygen vacancies.19,61,63,64. This phase transition in a narrow temperature range strongly implied a drastic change in electrical resistivity with a magnitude of 3-4 orders. In rutile phase, the vanadium atoms form straight and equally spaced c-axis chains. In contrast, the insulating monoclinic (M1) phase, vanadium atoms dimerize to form unequal spacing zigzag chains.51 In order to further investigate structure and the oxidation states of materials at different conditions, XPS analysis was performed. As shown in Figure 4, the XPS survey scan of VOx nanoflakelets which confirmed the presence of oxygen, vanadium, OH groups and chemisorbed CO2 (Figure 4a and 4d for VOx 500N and VOx 500-A samples respectively). The deconvoluted peaks of vanadium located at 515.8, 517.2 and 530.3 eV were ascribed to the V4+ (2p3/2), V5+ (2p3/2) and O1s, respectively. As evident from Figure 4c and 4f, O1s spectra appearing at 531.5 eV clearly indicated less quantity of oxygen vacancies present in VOx-500A to the transformation of V4O9 to V2O5. Figure 4b and 4e shows the high resolution spectrum of vanadium in VOx500-N (Figure 4b), which indicated that the +4 oxidation state of vanadium in VOx 500-N (~24%) is higher than VOx 500-A (~15%), which is mainly due to the mixed valance states (+4 and +5) of V4O9 and VO2 (+4). During oxygen treatment process, V4O9 was completeFigure 5. Full survey scan XPS spectra (a), high resolution specly converted into V2O5 and this can be understood from the reductrum of vanadium (b), high resolution spectra of titanium (c) and tion in +4 valance state. But still, the presence of +4 oxidation high resolution spectra of oxygen (d) of VOx/TiO2 nanoflakelets. state could be due to the pre-existed VO2 species. The highresolution O1s spectra of VOx 500-N (Figure 4c) and VOx 500-A As displayed in Figure 5a, the survey scan spectrum of hy(Figure 4f) exhibit the peaks at 529.9, 531.7 and 533.9 eV correbrid VOx/TiO2 confirmed the presence of elements such as, V, Ti, sponding to lattice oxygen (V-O), oxygen vacancies (Vo) and O and C. The high resolution XPS spectrum for V 2p3/2 could be adsorbed oxygen species. The higher binding energy value cendeconvoluted into two characteristic peaks (Figure 5b) at the bindtered at 531.4 eV is mainly due to evolution of V-OH species. The ing energy value of 515.8 eV and 517.2 eV corresponding to V4+ formation of hydroxyl groups act as nucleation sites for TiO2 and V5+ oxidation states, respectively. The presence of +4 and +5 nanoparticles to grow on the surface of vanadate nanoflakelets. oxidation states of vanadium manifested the retention of mixedMoreover, the anchoring between TiO2 nanoparticles and vanavalence species even after the TiO2/VOx composite formation. As
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per literature, it is understood that V4+ species originate from VO2 phase present in VOx. The peaks appeared at 458.70 eV and 464.42 eV are attributed to Ti2p3/2 and Ti2p1/2, which are the characteristics of Ti4+ in TiO2 (Figure 5c). The peaks observed at 457.45 eV could be assigned to Ti3+ (2p3/2) due to the oxygen vacancies associated with the edge shared atoms present in the TiO2 nanoparticles which can contribute to the active adsorption of oxygen. The O1s spectrum was deconvoluted into three peaks centered at 529.8, 530.3 eV which are ascribed to the Ti-O linkage of TiO2 and V-O linkage of VO2 and V2O5, respectively (Figure 5d). The presence of oxygen vacancies in both vanadium oxides and titanium dioxide were confirmed from the peak appeared at 532.6 eV, as shown in Figure 6d. The lattice oxygen (V-O and Ti-O) and oxygen vacancies (Ov) were revealed from two deconvoluted O 1s peaks appeared at (529.8 & 530.2 eV) and (531.5 & 532 eV), respectively. Strong in-plane covalent bonds and relatively weak van der Waals out-of-plane interactions provided the structural stability for this atomic-scale heterostructures. Therefore, the presence of oxygen vacancies in the VOx/TiO2 nanoflakelets could make more number of active sites for oxygen adsorption Electrical characteristics In order to study the electrical behavior and stability, temperature-dependent electrical properties of the synthesized materials were performed by analyzing their current-voltage (I-V) characteristics (see supporting information Figure S7). As evident from Figure 6(a), TiO2 nanoparticles were exhibiting high temperature-dependent electrical conductivity. A perfect oxygen sensor is likely to respond only during change in the oxygen partial pressure rather than temperature variations. Figure 5b indicates the presence of stable form of mixed valance (4+/5+) in vanadium oxides which were found to be more beneficial for temperatureindependent conductivity after 150°C. In VOx500-A nanoflakelets, VO2 (V4+) exhibited temperature-independent electrical conducting property from 150°C (Figure 6b), because of the semiconductor to metal transition phenomenon. In the case of VOx/TiO2 nanoflakelets, the initial change in resistivity is mainly due to the removal of surface adsorbed species and the typical temperature dependant electron transport (thermal motion of lattice atoms) factor of metal oxides nature (figure 6c). The repeated cycles of experiments demonstrating the good stability through in resistance variation during the temperature range 20-500°C with an error bar of ±0.5%). The temperature-independent electrical conductivity was observed from 150°C, which is due to the SMT behavior of VO2. Similarly, the Arrhenius plots of all materials (Figure 6 (d, e, and f)) also demonstrated a clear picture of temperature-independent electrical properties of TiO2 nanoparticles, VOx and VOx/TiO2 nanoflakelets from 150 to 500°C. The thermal activation energies were calculated from temperature dependent electrical resistances of VOx, TiO2 and VOx/TiO2 nanoflakelets using the equation 5,66 R(T) = $% &'( )
*+
,- .
3
/ + 024 $′ %,2 &'( )
∆*2
,- .
/ '67 %,2 ……… (5)
Where, R0 and R0,i are constants, Eg is the thermal band gap energy, Ei is the impurity level ionization energies, KB is the Boltzmann constant, and T is the temperature. The conductivity of the materials at lower temperature was initiated from the free carriers created by ionization of impurity levels. The intrinsic conductivity of the materials at relatively higher temperatures was further triggered because of their band-to-band transitions. Two types of linear trends were observed in Arrhenius plots which imply the existence of forbidden band gap and band gap impurity levels. The overall R(T) considerably depends upon the generation of
intrinsic charge carriers, which can be expressed using equation (6),65,66 ∆E = 9: &'( ;
? (@(.)) B C
A) /
D ……………….. (6)
According to the equation (6), the calculated activation energy (∆E) of TiO2 nanoparticles from Arrhenius plot was found to be 0.28 eV at lower temperature (50-200°C) and 0.13 eV high temperature (200-500°C) regions. The superior activation energy value of TiO2 at high temperature region revealed the temperature-dependent electrical conductivity. VOx displayed activation energy of 0.28 eV at low temperature (20-80°C), 0.64 eV at near SMT temperature region and 0.0074 eV at high temperature (130500°C) region. The results were found to be corroborating with the theoretical predictions from molecular oxygen isotopic exchange over mixed valance in VOx/TiO2.65 This reduction in activation energy at higher temperature region is mainly due to the SMT property of VO2, which could drastically reduce the temperature-dependent electrical conductivity of the sensor system. More significantly, the VOx/TiO2 nanoflakelets also manifested the activation energy of 0.23 eV, 0.60 eV and 0.00078 eV at lower, medium and high temperature regions respectively. The negligible change in temperature-dependent conductivities of the composite materials is mainly due to presence of VOx. The overall electrical conductivity (σ) changes primarily depend on the oxygen partial pressure and the variation in temperature of chemiresistive materials, as expressed in equation 67 /J
E(FG , H) = FG7 &'( )−
*K
L.
/ ……………… (7)
Where, Po2 is the oxygen partial pressure, EA is the activation energy and k is the Boltzmann constant and T is the temperature. As the temperature rises, more electrons jump to the conduction band and therefore, the resistance of the material decreases on account of the large number of charge carriers due to thermally excited electrons and also the thermal scattering of conduction band electrons hinders the flow of electrons at high temperatures. The second part of the equation 7 shows the relative change in resistance with respect to temperature changes in semiconductor metal oxides. Due to the SMT nature of VO2 existed on VOx nanoflakelets the temperature-dependent conductivity was found to be reduced. A schematic representation of monoclinic phase to rutile phase transformation of VO2 is shown in Figure 8b. However, VOx have the tendency of behaving as a semiconductor due to the other phase (V2O5) existing in VOx flakelets. A slight variation in the resistance occurred in VOx flakelets as a function of temperature as depicted in Figure 6b (inset). In addition, complete transition of metallic state in VO2 in VOx/TiO2 could not be an alternate option for temperature-independent materials as it can lead to an inefficient monitoring of resistance changes caused by surface adsorptions. As shown in Figure S9a, the oxygen gas response of VOx nanoflakelets was studied under various concentration levels (100-300 ppm) at different working temperatures such as, 100, 200, 300, 400 and 500°C. The presence of oxygen gas increased the resistance of the VOx nanoflakelets owing to their n-type characteristics. The relative sensor response (Figure S9b) as a function of concentration (100 ppm to 300 ppm) clearly indicated the linear response of VOx nanoflakelets due to the presence of active sites (oxygen vacancy) in vanadium oxides. But, the poor sensitivity of VOx nanoflakelets towards oxygen gas impeded the investigation on temperature-independent sensing properties. The VOx/TiO2 showed much narrower conduction channel and high active sites as compared to VOx nanoflakelets.
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Figure 6. (a) Temperature-dependent electrical characteristics (Inset shows the corresponding selected area) and Arrhenius plots of (a and b) pure TiO2, (c and d) VOx500-A and (e and f) VOx/TiO2 nanocomposite respectively. Therefore, it is significantly behaved as a high-performance gas sensing element. Figure 7a demonstrates the temperaturedependent gas sensing responses of VOx/TiO2 towards various concentration levels of oxygen, (100-500 ppm). The VOx/TiO2 nanoflakelets also was found to exhibit the typical n-type behaviour of semiconductors similar to VOx system and their sensitivity was enhanced 35 times higher than VOx nanoflakelets for 100 ppm concentration at 200°C as shown in Figure 8b. The electrical conductivity measurements (Figure 6b and e for VOx flakelets and Figure 6c and f for VOx/TiO2 nanocomposite) indicating that the temperature independent behavior initiating from 150°C, and hence the sensing characteristics of VOx-500A and VOx/TiO2 nanocomposites for the temperatures of 150, 200, 300, 400 and 500°C were analyzed as shown in Figures S9a and 7a respectively. The oxygen sensing studies of both VOx-500A nanoflakelets and VOxTiO2 nanoflakelets at 100°C as shown in Figure S10. Along with the significant enhancement in the sensitivity of VOx/TiO2 nanoflakelets towards O2 gas, the recovery also was found to be faster compared to the individual components. This effect could be mainly due to adsorption of oxygen gas from am bient air on VOx/TiO2 nanoflakelets and further ionization of O2 into superoxo (O-), peroxo (O2-/O2-) species depending on the temperature. The capturing of electrons from the conduction band of TiO2 due to ionosorption of oxygen resulted in the formation of a depletion layer in the oxide surface and simultaneous increase in the resistivity.68,69 Further adsorption of O2 leading to the formation of Schottky barriers on the grain boundaries. During the adsorption of O2 molecules on the surface of TiO2, the electrons dissociated from oxygen vacancies and Ti interstitials on anatase (101) will transfer to the lowest unoccupied molecular orbital (LUMO) of the molecule forming a negative charge layer at the semiconductor surface. This phenomenon is mainly because of the lower energy level of LUMO rather than the Fermi level(saturation level shown in Figure S8).69 Meanwhile, the upward band bending during oxygen
adsorption decreases the energy level between the Fermi level and LUMO, as illustrated in Figure 8c. Along with the significant enhancement in the sensitivity of VOx/TiO2 nanoflakelets towards O2 gas, the recovery also was found to be faster compared to the individual components. This effect could be mainly due to adsorption of oxygen gas from ambient air on VOx/TiO2 nanoflakelets and further ionization of O2 into superoxo (O-), peroxo (O2-/O2-) species depending on the temperature. The capturing of electrons from the conduction band of TiO2 due to ionosorption of oxygen resulted in the formation of a depletion layer in the oxide surface and simultaneous increase in the resistivity.68,69 Anatase TiO2 (101) crystal facets have the tendency to adsorb oxygen molecules as O- and O2- ions at lower and higher temperature regions respectively. Moreover, the ionosorbed O2- species are comparatively unstable and energetic than O2, O2- and O-. The reactive O2- species act as an electron acceptor which create negative charge layer on the semiconductor surface which can enhance the contact resistance between the VOx/TiO2 nanoflakelets at the grain–grain interface regions. The fast recovery of oxygen gas sensing demonstrates the physisorption of oxygen molecules on VOx/TiO2 surface. The oxygen adsorption reaction kinematics can be described as follows, ( M) ↔ ( OMP Q&O) ( OMP Q&O) + & R → R R + & R ↔ 2
R
R 2
R + & R ↔ + R
The gas sensor testing setup is schematically represented in Figure 8a. The VOx nanoflakelets exhibited n-type semiconducting behaviour towards oxygen gas and the sensing performance at 100°C for both VOx and VOx/TiO2 flakelets as shown in Figure S10. For further understanding of the sensing properties of the material, response time, recovery time and sensitivity were esti-
6 ACS Paragon Plus Environment
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ACS Sensors
mated. At high-temperature region, the response time was found to be lower
Figure 7. (a) Temperature-dependent (150-500°C) oxygen sensing performance of VOx/TiO2 nanocomposite at various concentration levels (100-500 ppm) and (b) Plot of sensitivity versus concentration of oxygen gas at various temperatures. (ranging from 4.5 to 2.9s) because of the faster kinetics associated with desorption of adsorbed oxygen from oxide surface and oxygen vacancies. Similarly, the recovery time (7.4 to 8.5 s) of the system was also found faster at higher temperature, as shown in Figure S11d. The initial response of VOx/TiO2 nanoflakelets based sensor started from 0.2 s. Figure 8b reveals that the sensitivity of the VOx/TiO2 reaches a maximum at higher temperature (400oC) and minimum at lower temperature (100 °C). In order to study the adsorption kinetics, Elovich model has been used for detail understanding of sensor performances as shown in figure S11a. The general model is described given below,63,66 q=1+
U>? (VW U)
+
U>? (X)
…………………. (8)
Where, q is the quantity of gas adsorbed during time t, a' is the initial adsorption rate and α is a measure of potential barrier for successive adsorption. These values (Figure S11b and c) were derived from Figure S11a and C0 and Ct are the conductance of VOx/TiO2 system at times t=0 and t=t on O2 gas, respectively. The linearity in the plot shown in Figure S11 a confirmed the validity of Elovich model (equation 6). The value of α is high (1.26x104 ohm) at 500 °C, where the response was high and initial adsorption rate was maximum (9.54x10-5±0.05) shown in figure S11c. The observed adsorption rate at low temperature (100°C) and high temperature (500°C) were found to be 7.35x10-5 ± 0.05 and 9.54x10-5 ± 0.05, respectively. A detailed analysis of initial adsorption rate and barrier potential (inset) for successive adsorption from 100°C to 500°C are shown in Figure S11 (c). At high temperature, VOx/TiO2 based sensor exhibited faster response towards oxygen gas which is corroborating with Elovich plots. The Elovich plots at different temperatures are shown in Figure S12.
The repeatability of the VOx/TiO2 nanoflakelets based sensor was investigated to establish its stability after several oxygen sensing cycles. No significant decrease in the sensing response was observed while the temperature was around 500°C. Multiple exposures have been conducted and presented in Figure S14a. The long term stability was tested by extending exposure time upto 5 and 9 minutes in order to understand the stability of gas sensor at longer time exposure time as demonstrated in Figure S14b. The results showed that the sensor have better stability towards oxygen gas even after prolonged exposure time. The reproducibility of VOx/TiO2 nanoflakelets towards oxygen was studied by repeated adsorption-desorption cycles which showed the stability of sensor (shown in Figure S14). The gas sensing performances towards percentage levels of oxygen (1-4%) and stability of the sensor under prolonged exposure time are shown in Figure S15. The present work rationally designs an ideal temperature-independent hybrid sensor configuration for oxygen sensing applications at high temperature regions (>500°C). The sensor can affect its performance during the lower temperature region (
