V2O5 Thin Films as Nitrogen Dioxide Sensors

Vanadium pentoxide thin films were deposited onto insulating support by means of rf reactive sputtering from a metallic vanadium target. Argon-oxygen gas mixtures of different compositions controlled by the flow rates were used for sputtering. X-ray diffraction at glancing incidence (GIXD) and Scanning Electronic Microscopy (SEM) were used for structural and phase characterization. Thickness of the films was determined by the profilometry. It has been confirmed by GIXD that the deposited films are composed of V2O5 phase. The gas sensing properties of V2O5 thin films were investigated at temperatures from range 410–617 K upon NO2 gas of 4–20 ppm. The investigated material exhibited good response and reversibility towards nitrogen dioxide. The effect of metal-insulator transition (MIT) on sensor performance has been observed and discussed for the first time. It was found that a considerable increase of the sensor sensitivity occured above 545 K, which is related to postulated metal-insulator transition.


Introduction
Increasing environmental pollution is becoming a vital global concern, particularly in relation to the imperative to reduce emissions of gases causing the greenhouse effect, acid rain, and the depletion of stratospheric ozone. Therefore, there is an urgent need to develop some devices that allow fast, portable, low-cost monitoring of the gases responsible for air pollution and/or pose danger to human health. So far, sophisticated and expensive equipment, such as gas analyzers based on IR and UV spectrophotometry, pulse fluorescence, flame photometry and gas chromatography, to determine air quality were applied. Although this equipment enables very precise gas phase analysis, it has four substantial disadvantages such as high cost, large dimensions (limited portability), slow analysis time and non-continuous monitoring of the gas composition. In this respect, chemical gas sensors may offer advantages in the form of simple construction, low cost and ability to work in situ. One large group of sensors, applied to environmental monitoring, is based on liquid (or wet) electrochemistry. However, these sensors presently suffer from the same four severe disadvantages. Moreover, these sensors cannot be applied in environments warmer than room temperature.
During the past five decades, efforts were made to develop chemical gas sensors based on solid-state technology. The potential advantages of these sensors over the wet technology sensors are (i) miniaturization, (ii) simple calibration and measurement, (iii) low cost, (iv) short response time, (v) resistance to severe conditions, such as high temperature and corrosive environment and (vi) selectivity. Their main advantage is that they can also operate at elevated temperatures, thus meeting the environmental requirements, such as for instance in car and industrial exhaust systems.
There are several types of chemical gas sensors [1]. The largest group consists of electrochemical sensors. According to Wilson et al. [2] and Janata [3] the electrochemical gas sensors have been categorized into three groups: (1) conductometric (measurement of electrical conductivity or related value such as resistivity, impedance); (2) potentiometric (measurement of electromotive force or related value such as voltage of the solid cells); (3) amperometric (measurement of current).
The most widely studied type of the gas sensors are conductometric. Their principle of operation involves a change in the resistance of the sensor sensitive phase (usually metal oxide) upon exposure to a specific component of the gas atmosphere. During adsorption of gas molecules on solid surface, the electronic charges (electrons or electron holes) are created, which change surface electrical conductivity. This process takes place within the thin layer near-surface of the solid of the thickness named the Debye length. Consequently, electrical conductivity may be used as a sensing property for detection of specific gas phase components. Various review works have referred to conductometric gas sensors [4][5][6][7][8][9]. However, there is no simple theory which can predict how the sensor's signal depends on gas concentration.
Generally, semiconductor gas sensors suffer from low selectivity. Several attempts were made to improve their selectivity in respect to the detectable gas. The most frequent approach involves change of the sensor composition by doping, the use of composites with one or more other materials, variation of operating temperature etc. It was found that the successful method improving sensors' selectivity is addition of some heterogeneous catalysts, such as noble metals (Pt, Pd, Au) to the sensor material. In addition, use of the vanadium pentoxide -well-known catalyst -is promising.
Recently, vanadium oxides have attracted considerable interest due to their multi-valence, good chemical stability and excellent catalytic properties [23]. Moreover, unlike the above-mentioned sensing oxides, vanadium oxides show metal-insulator transition (MIT), an interesting electrical property which may have impact on the sensor's performance. V 2 O 5 , the most stable compound among over 15 known vanadium oxides, is a promising NO 2 sensor material [24]. It demonstrated high sensitivity and selectivity for ethanol [25], ammonia [26], hydrogen and hydrocarbons [27]. Table 1   includes examples of conductometric sensors of various gases, except of NO 2 , with V 2 O 5 sensing material or V 2 O 5 as addition to other material such as SnO 2 [33,38], TiO 2 [40,50] and other [29,37,[51][52][53][54]. On the other hand, Table 2 summarizes V 2 O 5 -based NO 2 sensors. For some practical applications, the high operating temperature creates problems with the long-term stability and high costs of the sensor manufacture and maintenance. As can be seen in the Table 1, some authors reported that their sensors based on nanostructured vanadium pentoxide or its composite show sensitivity at room temperature [29,31,36,41,48,49]. Moreover, as per published results [40], the sensor selectivity may be substantially improved for sensors composed of oriented nanoparticles.
In this paper, the application of V 2 O 5 thin films as NO 2 gas sensor is reported. The effect of MIT on sensor performance was studied for the first time.
Nitrogen dioxide, NO 2 is an extremely toxic gas. It is produced by all combustion in air and by industrial processes. NO 2 is responsible for various pollution problems such as smog and acid rain. Therefore, there is an urgent need to develop devices that allow fast, portable, low-cost monitoring of the NO 2 in the interest of environment and human health. Successful development of NO 2 gas sensors for commercialization requires the achieving of four "S", i.e. sensitivity, selectivity, stability and speed (response and recovery rates).

Thin Film Preparation
VO x thin films were deposited onto insulating support (either fused silica or alumina) for sample characterization or conductometric sensor substrate type CC1.W (BVT Technologies, Czech Rep.), for electrical measurements, by means of rf sputtering in a reactive atmosphere at working pressure 4.75 Pa (24% O 2 -76% Ar) from a metallic V target. Conductometric supports presented in Figure 1 were provided by BVT Technologies. Details of the film deposition are given elsewhere [27].
Nitrogen dioxide, NO2 is an extremely toxic gas. It is produced by all combustion in air and by industrial processes. NO2 is responsible for various pollution problems such as smog and acid rain. Therefore, there is an urgent need to develop devices that allow fast, portable, low-cost monitoring of the NO2 in the interest of environment and human health. Successful development of NO2 gas sensors for commercialization requires the achieving of four "S", i.e. sensitivity, selectivity, stability and speed (response and recovery rates).

Thin Film Preparation
VOx thin films were deposited onto insulating support (either fused silica or alumina) for sample characterization or conductometric sensor substrate type CC1.W (BVT Technologies, Czech Rep.), for electrical measurements, by means of rf sputtering in a reactive atmosphere at working pressure 4.75 Pa (24% O2 -76% Ar) from a metallic V target. Conductometric supports presented in Figure 1 were provided by BVT Technologies. Details of the film deposition are given elsewhere [27].

Morphology and Structural Characterization
Scanning electron microscopy (SEM) studies were carried out for as-sputtered thin films using NOVA NANOSEM 200 (FEI Europe Company) microscope. Phase composition of as-sputtered thin films was studied by X-ray diffraction at glancing incidence, GIXD. Thickness of the V2O5 thin film was (210  25) nm.

Morphology and Structural Characterization
Scanning electron microscopy (SEM) studies were carried out for as-sputtered thin films using NOVA NANOSEM 200 (FEI Europe Company) microscope. Phase composition of as-sputtered thin films was studied by X-ray diffraction at glancing incidence, GIXD. Thickness of the V 2 O 5 thin film was (210 ± 25) nm.

Sensing Characterization
The responses S of films to the target gas (NO 2 ), defined as changes in electrical resistance (S = R NO2 /R air ), were measured by custom-made setup at different NO 2 concentrations (0-20 ppm). The sample was placed in a gas chamber on a workholder, where the temperature and gas atmosphere (gas composition and humidity) were stabilized. The relative humidity was set to 50 ± 0.1%. The requested NO 2 concentration was obtained by controlling the ratio of gas to air flow rate, and total flow rate was set to 500 cm 3 /min. The total flow rate was maintained at the same level of 500 cm 3 /min. The film resistance changes upon NO 2 were measured with Keithley 6517 electrometer working in constant voltage mode (U = 0.1 V). At the beginning, the sensor response has been stabilized in pre-set conditions (at the lowest temperature, constant gas flow, pure air of 50% humidity). Then two types of measurement procedures were performed. During the first stage, the measurement of sensor response under varying temperature (range 483-623 K), and at constant NO 2 concentration (20 ppm) fed alternately with air for purging purposes, was realized. After that, the second stage was performed: The measurement at constant temperature under increasing gas concentrations (0-20 ppm with 4 ppm step), also while purging the chamber with air. The sensor resistance was sampled every 2 s. The sensor measurements were performed within the temperature range extending from RT to 700 K. Equipment applied for measurements of the sensor characteristics was described in detail elsewhere [55].    The determined lattice parameters (a = 1.145 ± 0.003 nm; b = 0.436 ± 0.004 nm; c = 0.355 ± 0.003 nm) well agree with those from literature reports [56]. Presented XRD patterns were used for determination of the crystallite size. Crystallite size, d XRD , was calculated according to Scherrer's method: d XRD = 20.0 ± 1.8 nm. No effect of sintering temperature on obtained XRD results was observed.

Structural and Microstructural Characteristics
As can be seen, the as-sputtered thin films (Figure 3a) are poly-dispersed, and the grains are mostly columnar in shape with a length of 565 ± 100 nm and diameter of 220 ± 40 nm. On the other hand, after sintering (Figure 3b) they are rather spherical (mean diameter = 500 ± 75 nm). Chemical analysis performed by EDS technique revealed presence of high peaks coming from the silicon support and short peaks corresponding to oxygen and vanadium elements. As can be seen, the as-sputtered thin films (Figure 3a) are poly-dispersed, and the grains are mostly columnar in shape with a length of 565 ± 100 nm and diameter of 220 ± 40 nm. On the other hand, after sintering (Figure 3b) they are rather spherical (mean diameter = 500 ± 75 nm). Chemical analysis performed by EDS technique revealed presence of high peaks coming from the silicon support and short peaks corresponding to oxygen and vanadium elements.

Sensing Characteristics
An example of V2O5 sensor responses to 20 ppm NO2 are shown in Figure 4a

Sensing Characteristics
An example of V 2 O 5 sensor responses to 20 ppm NO 2 are shown in Figure 4a,b.  As can be seen, the as-sputtered thin films (Figure 3a) are poly-dispersed, and the grains are mostly columnar in shape with a length of 565 ± 100 nm and diameter of 220 ± 40 nm. On the other hand, after sintering (Figure 3b) they are rather spherical (mean diameter = 500 ± 75 nm). Chemical analysis performed by EDS technique revealed presence of high peaks coming from the silicon support and short peaks corresponding to oxygen and vanadium elements.

Sensing Characteristics
An example of V2O5 sensor responses to 20 ppm NO2 are shown in Figure 4a  As one can see from Figure 4, the resistance of the investigated V2O5 thin films increases upon exposure to NO2, which is characteristic of n-type semiconductors.
NO2 sensing mechanism can be explained in terms of defect structure of vanadium pentoxide As one can see from Figure 4, the resistance of the investigated V 2 O 5 thin films increases upon exposure to NO 2 , which is characteristic of n-type semiconductors. NO 2 sensing mechanism can be explained in terms of defect structure of vanadium pentoxide [57]. V 2 O 5 equilibrated in air shows deficit, x, of oxygen (V 2 O 5−x ). Doubly positive ionized oxygen vacancies are compensated by V 4+ ions. Electrical conductivity is achieved by small polaron mechanism, via electron hopping from neighbouring two ions V 4+ and V 5+ (marked as 1 and 2): Electrical conductivity is proportional to concentration of V 4+ ions. When NO 2 is added to gas atmosphere, adsorption of NO 2 molecules takes place according to the reaction: Equation (2) indicates that the increase of the resistance upon exposure of NO 2 , as observed in Figure 4a,b, results from decrease of the concentration of V 4+ ions. Figure 5a illustrates sensor response, S = R NO2 /R air , versus temperature. The abrupt increase in the sensor response is observed at 545-547 K. This behaviour may be explained by the occurrence of the metal-insulator transition, MIT.
One of the most spectacular phenomena occurring in most vanadium oxides is the abrupt change of the electrical resistivity from the values typical for semiconductor to those typical of metal phase. This phenomenon, called semiconductor-metal phase transition, SMPT [58] or more frequently metal-insulator transition, MIT, offers immense prospects for various practical applications, in particular oxide electronics, photoelectronics [59,60] and gas sensors. Strelcov et al. [61] studied gas sensor properties of nanowire VO 2 close to temperature of MIT (T MIT = 338 K). They observed that varying the temperature of the nanowire close to the T MIT , the conductance of the nanowire becomes extremely responsive to the tiny changes in the ambient gas environment. According to our knowledge, the effect of MIT on gas sensor properties of V 2 O 5 have not been studied yet.
The single valence vanadium oxides V 2 O 3 , VO 2 , and most of double valence vanadium oxides (i.e., Magneli or Wedsley series) show metal-insulator transition. During this transition, the polymorphic phase transition occurs from lower to higher symmetrical crystallographic structure. However, vanadium pentoxide, V 2 O 5 thin film is an exception. At temperatures close to 540 K (T MIT = 530 ± 5 K, Blum [59]; T MIT = 553 K, Kang et al. [60]), MIT takes place in nanostructured and thin films of V 2 O 5 without any phase transition. This fact can be explained that MIT in V 2 O 5 is limited to the surface layer [59]. Our recent studies revealed that MIT is observed in V 2 O 5 thin films annealed in air, while in V 2 O 5 thin films annealed in gas atmosphere containing NO 2 MIT effect was not observed. From these facts and Equation (2), we postulate that sufficient V 4+ ion concentration on the surface is a prerequisite to MIT in vanadium pentoxide. The abrupt change (Figure 5a) of sensor response S, which is observed close to T MIT , results from MIT in sample in air (i.e. abrupt decrease of R air , and much smaller increase in R NO2 . As Figure 5b indicates, below T MIT (T < T MIT = 546 K), the sensor response S increases slightly with temperature. On the other hand, for T > T MIT the sensor response exhibits much larger increase.

Conclusions
In the present work, we fabricated V2O5 thin films by rf reactive sputtering. The film structure and morphology were studied by X-ray diffraction at glancing incidence and scanning electronic microscopy. Gas sensing studies showed that the V2O5 thin films were sensitive to NO2 at relatively low operating temperatures. The considerable increase of the sensor sensitivity was observed above 545 K, which is related to postulated metal-insulator transition. Presented studies on vanadium pentoxide thin film NO2 sensor will be continued regarding selectivity (especially in respect to water vapor).

Conclusions
In the present work, we fabricated V 2 O 5 thin films by rf reactive sputtering. The film structure and morphology were studied by X-ray diffraction at glancing incidence and scanning electronic microscopy. Gas sensing studies showed that the V 2 O 5 thin films were sensitive to NO 2 at relatively low operating temperatures. The considerable increase of the sensor sensitivity was observed above 545 K, which is related to postulated metal-insulator transition. Presented studies on vanadium pentoxide thin film NO 2 sensor will be continued regarding selectivity (especially in respect to water vapor).