Nickel nanoparticle-decorated reduced graphene oxide/WO3 nanocomposite – a promising candidate for gas sensing

We report for the first time the combination of WO3 sensing elements with a non-noble metal–carbon composite, namely a nickel metal nanoparticle–carbon composite (Ni@rGO). Previous work with WO3 had used either NiO (as part of the WO3 lattice), solely carbon, Pd-surface decorated WO3 (Pd@WO3), or Pd or Pt@carbon@WO3. We demonstrate the gas response for pure WO3, rGO/WO3 and Ni@rGO/WO3 sensing elements towards NO2 and acetone in air as well as towards CO in N2. The addition of 0.35 wt % Ni@rGO composite to WO3 enables the increase of the sensory response by more than 1.6 times for NO2 vapors. The gas response towards acetone using 0.35 wt % Ni@rGO/WO3 composite was 1.5 times greater for 3500 ppm than for 35,000 ppm acetone. For 0.35 wt % Ni@rGO/WO3 composite and CO gas, a response time (Tres) of 7 min and a recovery time (Trec) of 2 min was determined.


Introduction
Toxic gases as well as volatile organic compounds (VOC) are known air pollutants and their emissions are harmful for humans and ecosystems [1]. Sensor materials that can detect the type and concentration of these gases are therefore needed in various kinds of environments and industries [2]. A gas sensor should be highly sensitive and highly selective with a fast response and recovery rate. Also, it should work at low cost and with low power consumption [3]. In comparison to conven-tional gas sensors, nanostructure-based gas sensors are more sensitive because of their increased detection area [4]. The most common mode used in gas sensing is the resistance mode, where the change in sensor resistance during exposure to the gas is measured directly [5]. Gases can either be oxidizing, such as NO, N 2 O, NO 2 , O 3 , and Cl 2 , reducing, such as H 2 S, NH 3 , CO, H 2 , SO 2 , and CH 4 , or rather inert, such as CO 2 [6,7]. VOCs are organic molecules such as acetone, ethanol, and formaldehyde [8,9].
Metal oxide semiconductors (MOS) are the most commonly used gas sensors [10]. MOS can be divided into n-type and p-type MOS. In n-type MOS electrons are the majority charge carriers, while in p-type MOS holes are the majority charge carriers [6]. The exposure to reducing gases causes a decrease of resistance in n-type MOS and an increase of resistance in p-type MOS and vice versa [8]. MOS have certain advantages such as fast response time and excellent sensitivity towards all kinds of gases [11]. The major disadvantages of MOS are their poor selectivity and high operating temperatures of 200 to 400 °C, which means a high power consumption [4]. WO 3 is a wide-bandgap [12,13] n-type semiconductor [14,15] with good sensitivity towards NO 2 [16] and CO [17].
Known successful routes to improve the MOS gas sensing performance are doping with transition metals, decoration with noble metals, formation of heterojunctions, or size reduction [18,19]. Doping of WO 3 with nickel improves the humidity sensing compared to neat WO 3 . Attributed to a greater number of electrons donated by Ni atoms, higher surface area, and smaller bandgap energy, Ni-doped WO 3 has a faster response, higher sensitivity, and greater stability than pure WO 3 [20]. WO 3 decorated with palladium nanoparticles on the surface can be used as an improved and reusable gas sensor for NH 3 [21].
Metal oxide semiconductor junctions can either be formed between two p-type MOS or two n-type MOS (p-p/n-n homojunctions) or between a p-type MOS and an n-type MOS (p-n heterojunctions) [6,18]. The p-type MOS NiO is not a very popular gas sensing material, because p-type MOS have, in general, a lower gas response than n-type MOS, such as WO 3 , ZnO, or SnO 2 [22,23]. But p-type MOS are ideal doping agents [24]. NiO combined with WO 3 forms a p-n heterojunction, which improves the gas sensing abilities significantly [25].
Carbon-based materials are also promising gas sensors, because of their high surface area and high chemical and thermal stability [26,27]. Pristine graphene is a good conductor but rather inactive for gas sorption, because it has only a few functional groups on its surface, which limits the chemisorption of gas molecules [28]. Graphene oxide (graphite oxide, GO), in contrast, has numerous oxygen functionalities and few remaining π bonds and is therefore electrically insulating [29]. GO can be reduced (reduced graphene oxide, rGO) chemically or thermally. Through the partial removal of oxygen groups, the conductivity can be restored. Additionally, defects and vacancies are created [26]. Because of the ultra-high surface area per atom and the high electron transport along the graphene plane, rGO has a rapid and high response to gas molecules at room temperature [30]. A disadvantage of rGO gas sensors is the long recovery time because of the high binding force between gas molecules and the graphene material [31]. rGO is a p-type semiconductor and can be used for gas sensing of low concentrations of NO 2 at room temperature [32].
The combination of MOS with graphene materials can improve the gas sensing abilities [33,34]. MOS prevent graphene from agglomerating, which leads to a higher specific surface area. Graphene can control the size and morphology of MOS during the synthesis and decreases the resistance of MOS, which leads to a rapid electron transfer from the surface reaction of the target gas with the MOS to the electrodes [35]. Additionally, MOS and graphene can form junctions at their interface. For example, p-p homojunctions can be formed between NiO and rGO to increase the gas sensing responsivity and sensitivity towards NO 2 [36]. In the combination of WO 3 and rGO, p-n heterojunctions are formed. This leads to an increased NO 2 response at room temperature [37]. Overall, MOS@rGO gas sensors are more selective and sensitive with a faster response and recovery rate even at room temperature [8].
The sensing performance of MOS@rGO can further be improved by either chemical doping or by combination with a transition metal as ternary component [38]. Iron oxide-doped WO 3 films showed improved NO 2 sensing at room temperature, when adding a layer of 16 nm p-type rGO on the metal oxide film [39]. Nickel-doped SnO 2 nanoparticles loaded with graphene have an enhanced acetone response at 350 °C with increased graphene loading level (best at 5 wt % graphene) [40]. ZnO nanostructures doped with nickel and rGO were used for hydrogen sensing at 100 °C [34].
The decoration of MOS with a noble metal, such as Pd or Pt, improves the sensitivity, response time and working temperature of MOS/rGO systems [15,41]. TiO 2 /rGO decorated with Pd and Pt nanoparticles was successfully used in the gas sensing of hydrogen gas [19]. The decoration of WO 3 /rGO nanosheets with Pt nanoparticles yielded a faster response for acetone at 200 °C [42]. With the addition of Ag nanoparticles to a dispersion of SnO 2 /rGO, the working temperature was dropped from 55 °C to room temperature in the gas sensing of NO 2 [43]. (For further examples and comparison with other gas sensors see Table S1 in Supporting Information File 1.) The ternary Ni@rGO/WO 3 nanocomposite was synthesized and tested in comparison to pure WO 3 and rGO@WO 3 regarding the response to the oxidizing gas NO 2 (10 ppm in air) and the VOC acetone (35,000 ppm in air). Gas response to CO and recovery times were also determined. An examination of the influence of different gas concentrations on the gas response were measured for 3500 ppm and 35,000 ppm acetone.

Ni@rGO synthesis
The synthesis of nickel nanoparticles is well known and different methods such as thermal decomposition [44] or reductive hydrogenation [45] are used. Nickel nanoparticles with sizes below 10 nm can be easily synthesized from the precursor material bis(1,5-cyclooctadiene)nickel(0) (Ni(COD) 2 ) in different ionic liquids without any additional stabilizing or reducing agents [46]. Ionic liquids have the ability to exfoliate graphene oxide into single sheets. Thus, a higher surface area can be achieved [47]. Thermally reduced graphene oxide was tested before with different metals in ionic liquids [48,49]. The decoration of nanoparticles on rGO can be achieved in situ or by mixing previously prepared solutions [50].
Here, we chose the ionic liquid [BMIm][NTf 2 ] for an in situ microwave decomposition approach with rGO synthesized from reduced graphite oxide at 400 °C. It is extremely important that the used rGO is thoroughly dried because of the oxyphilic nature of nickel nanoparticles. Therefore, before the nanoparticle synthesis, the rGO was dried using a turbo molecular pump at 5 × 10 −7 mbar for several days. Then rGO was dispersed with Ni(COD) 2 in [BMIm][NTf 2 ] to gain 0.5 wt % metal nanoparticles and 0.5 wt % rGO. In order to stir the reaction mixture during the microwave decomposition, 0.5 wt % rGO could not be exceeded. The obtained nanomaterial was analyzed using powder X-ray diffraction (P-XRD). The P-XRD pattern shows the reflexes for hexagonal nickel ( Figure 1). TEM images show spherical nickel nanoparticles, which are supported on top of rGO ( Figure 2). The particles have a size of 25 ± 5 nm. All nanoparticles are supported on rGO. The particle size of Ni@rGO increased in comparison to pure nickel nanoparticles from [BMIm][NTf 2 ] (size pure nickel nanoparticles 11 ± 2 nm) [46]. Nickel nanoparticles supported on pristine graphene sheets were synthesized with a size 35 ± 5 nm [51].
The nickel content was measured using atomic absorption spectroscopy (AAS). Ni@rGO contained 8% nickel. A metal loading between 5% and 20% on graphene oxide is common [49].

WO 3 nanopowder synthesis
The tungsten oxide nanopowder was prepared by a sol-gel method according to [52]. The phase composition was analyzed using P-XRD. The P-XRD pattern shows reflexes only of monoclinic tungsten oxide ( Figure 3). Therefore, the thermal decomposition of the WO 3 xerogel leads to the formation of only one crystalline WO 3 phase without crystalline by-products. The average size of WO 3 nanoparticle crystallites, calculated from the powder pattern using the Scherrer equation, is 40 nm. The SEM images show grains of WO 3 nanocrystals in different sizes (hundreds of nanometers to several micrometers). Smaller grains are uniformly distributed on the surface of larger grains ( Figure 3).

Gas sensing measurements on gas permeable pellets
Ni@rGO was then mixed with WO 3 xerogel and pressed into pellets to be tested in gas sensing measurements. Dry air was used as a reference gas. The electrical resistance was measured for the testing gas mixture and air. The response of a semiconductor sensor is the ratio between the electrical resistance in air and that in a gas medium. In the presence of reducing gases (e.g., acetone or CO), the sensor resistance decreases. In the presence of oxidizing gases (e.g., NO 2 ), the electrical resistance increases [7]. Figure 4 shows the sensor characteristics for the Ni@rGO/WO 3 composite in 3000 ppm CO/N 2 at 246 °С. A high sensor response of R air /R gas = 14.8 was detected ( Figure S1 in Supporting Information File 1). It was found that a constant baseline resistance was observed before and after exposure. The response time (T res ) is 7 min. The recovery time (T rec ) is 2 min.
A sensor response of the Ni@rGO/WO 3 composite of R air /R gas = 6.20 in 3500 ppm acetone was detected ( Figure 5 right, Figure S2 in Supporting Information File 1). For a higher acetone concentration of 35,000 ppm, the sensor response was lower with R air /R gas = 4.2 ( Figure 5 left, Figure S2 in Supporting Information File 1).  . This layer is applied to a corundum substrate. In the case of NO 2 , the sensor response (R gas /R air ) is, respectively, 2.9 for 1.00 wt % Ni@rGO + WO 3 , 1.9 for 0.35 wt % Ni@rGO + WO 3 , 1.6 for 0.35 wt % rGO + WO 3 , 1.4 for WO 3 , and 1.2 for Ni@rGO. Thus, the addition of Ni@rGO to WO 3 enables the increase of the sensor response to NO 2 and acetone vapor.  In contrast to sensors in which the sensing element consists only of rGO, sensors based on semiconductor oxide compositions, for example, WO 3 with rGO, have a higher response. In the case of pure rGO, the restoration of the original parameters of the sensors may not be observed at all [8]. In addition, oxidebased composites are mechanically more durable and manufacturing sensors based on them seems to be more economically feasible due to the low content of graphene in the sensing element (up to several percent).
At present, there is no generally accepted mechanism of gas sensitivity of semiconductor oxide compositions with graphene. The reasons for the increase in the response and decrease in the operating temperature of metal oxides combined with non-oxidized graphene are synergetic effects between graphene and metal oxides as a result of chemical bonds between graphene and metal oxide. In the case of reduced graphene oxide (semiconductor), various reasons are considered, such as the appearance of p-n junctions that shift the Fermi level of the oxide. There is evidence of effective charge transfer between graphene and nanospheres through chemical bonds. Emergence of conducting channels from graphene layers is also pointed out, which increase the efficiency of charge carrier transfer in composites [8].
Based on the known literature data and the results obtained, it is possible to provide potential reasons for an enhancement of the sensitivity in the case under consideration. When Ni@rGO/ WO 3 sensors are exposed to NO 2 , with NO 2 , which is a strong oxidizing gas accepts electrons from WO 3 . As a result, NO 2 transforms into NO 2 − on the surface of WO 3 . This process leads to accumulation of holes and an increase in sensor sensitivity. In addition, the rGO/WO 3 contact also plays an important role in charge transfer processes and leads to the enhancement in the gas sensing performance due to the synergistic effect between rGO (p-type) WO 3 (n-type) [29]. In [37,53], the formation of C-O-W bonds at the interphase boundaries was observed, when studying a rGO/WO 3 composite using XPS and Raman spectroscopy. Such bonds can play an important role also in our case with regard to charge transfer. It has been established that adsorption of NO 2 molecules will cause upward band bending by capturing free electrons from the conduction band and shift the Fermi level of WO 3 away from the conduction band toward the valence band. Processes on the WO 3 surface make it possible for the work function of WO 3 to lower to a point close to that of rGO [37]. The latter facilitates transfer of electrons at the rGO/WO 3 interface. The continuous capture of electrons by chemisorption of NO 2 gases at the surface of WO 3 facilitates the charge transfer from rGO to WO 3 . At the expense of C-O-W bonds, hole transfer from WO 3 to rGO will occur. Thus, it has been shown that C-O-W bonds are responsible for enhancing the charge carrier transfer rate [54].
In addition, graphene sheets in the composite create a hierarchical nanostructure and facilitate the diffusion of NO 2 molecules, increasing the contacts and enhancing the chemisorption of the gas. rGO has large specific area and more active sites.
Gas sensors of n-type semiconductors based on oxides exhibit resistance changes induced by chemisorption of oxygen adions (O − and O 2 − ) that interact with reducing gases [55,56] such as acetone. A free electron appears in the conduction band of the semiconductor after the interaction of the chemisorbed oxygen adions and the target gas [57]. When acetone adsorbs on the surface of such a sensor material, preadsorbed oxygen adions are released according to the reaction [58]: The same is true for CO [57]: As a result, electrons that were trapped in the oxygen adions return back to the conduction band of WO 3 . Thus, the resistance of WO 3 decreases upon exposure to these gases. An increase in the sensitivity to acetone of the entire composition 0.35 wt % Ni@rGO/WO 3 may be associated with further electronic interaction between WO 3 particles (n-type) and Ni@rGO (p-type). In this case, the transfer of electrons from WO 3 to rGO leads to the formation of spatially separated regions of positive and negative charges (possibly, a p-n junction is formed). It was shown [59] that electrons that were transferred from the n-type semiconductor and stored in the rGO sheets are withdrawn upon exposure to gas, thereby restoring the hole concentration and p-type conductivity of rGO. This is made possible by the energy band alignment between the semiconductor and rGO, the electron acceptor functionalization of the analyzed gas, and the p-type conductivity of the rGO. Thus, the electrondepleted WO 3 surface is more sensitive to the adsorption of acetone molecules and the transition of electrons from the adsorbed gas to the WO 3 conduction band.
When considering the possible mechanism of the influence of nickel on the sensory properties of the Ni/rGO/WO 3 composite, in addition to selective catalytic activity, it is also possible to refer to the existing explanation for the interaction of clusters of metal particles with a semiconductor. In accordance with the literature data, the role of nickel in increasing the sensory sensitivity is most likely associated with the spillover effect [60,61]. When interacting with a gas atom, the barrier height at the Ni/rGO interface decreases due to this effect (the possibility of electron transfer from nickel particles to rGO and WO 3 ), which leads to a greater decrease in the resistance of the composite. In our case, these processes enhance the diffusion of charges at the WO 3 /rGO interface, but the role of nickel particles remains to be further clarified.

Magnetic measurements
In order to investigate if nickel oxidation had occurred in the Ni@rGO composite during the heat treatment in the study of sensory properties (250 °C), the following model experiment was performed. The original Ni@rGO composite was annealed at a temperature of 250 °C for 2 h. In the following, its magnetic properties, namely the Curie temperature was determined from the temperature dependences of magnetization and magnetic susceptibility.
The results of the magnetic analysis (Figure 7a) indicate that the magnetic phase in the Ni@rGO composite is pure nickel. The Curie temperature of the composite (T c = 630 K, Figure 7, left) and the reference value for pure nickel [62] coincide ( Figure S3, Supporting Information File 1). Moreover, the fraction of the magnetic phase in the Ni@rGO composite, as shown by magnetization measurements, is 7.8 wt %. The same amount of nickel is present in the original Ni@rGO composite without heat treatment.
The temperature dependence of the magnetization was determined during several heating-cooling cycles. It was noted that with an increase in the number of cycles, the Сurie temperature of the sample increased from 345 to 357 °C. Most likely, the increase in the Curie temperature is associated with an increase in the size of the nickel particles as a result of their sintering in agglomerates with an increase in the degree of crystallinity. The maximum temperature during cycling reached 450 °C. It can also be assumed that a sintering of nickel nanoparticles (approximately 25 nm in size), which are located in the partly formed agglomerates in the initial composite, is possible (Figure 2, right).
The increase in the Curie temperature with the size of nanoparticles is known [63,64]. It is explained by a decrease in the fraction of the nickel particle surface layer with a noncollinear spin configuration, which causes the formation of a surface spin canting due to thermal fluctuation of magnetic moments. Figure 7 shows the temperature dependence of the magnetization obtained in the course of the last measurement cycle at which, for nickel particles, ferromagnetic rather than superparamagnetic behavior is manifested. Hence, the results of measurements of the magnetic susceptibility and magnetization indicate the presence of only a metallic nickel phase in the Ni@rGO composite after heat treatment in air.

Сonclusion
Ni@rGO nanocomposites were found to be promising materials that enable the preparation of WO 3 gas and vapor sensor elements with improved sensory response. The addition of a very small amount of Ni@rGO (0.35 wt %) to WO 3 increases the gas response regarding NO 2 traces in air significantly compared to the WO 3 element without the addition of metal and graphene oxide. Low concentrations of acetone (3500 ppm) were better detected by the Ni@rGO/WO 3 composite than the higher con-centration of 35,000 ppm. For CO gas, the response time and the recovery time were T res ≈ 7 min and T rec ≈ 2 min, respectively. The facile preparation of nickel nanoparticles supported on reduced graphene oxide paves the way for their application as dopant in other metal oxide gas sensors.

Experimental
Due to the sensitivity of the precursor substances towards moisture and oxidation, all experiments were carried out in a purified argon (grade 99.998 vol %) or nitrogen (grade 99.996 vol %) atmosphere by using standard Schlenk techniques. Samples were prepared and stored in a MBraun Glovebox. Solvents (acetonitrile, n-hexane, and methylene chloride) were dried by using a MBraun solvent purification system or distilled (1-methylimidazole and 1-chlorobutane) and stored over 4 Å molecular sieves in a nitrogen atmosphere. Final water contents measured by coulometric Karl Fischer titration (ECH/ANALYTIK JENA AQUA 40.00) did not exceed 10 ppm.  [67]. Reduction of the graphite oxide was performed at 400 °C. Before using rGO in the nanoparticle synthesis, it was dried at 100 °C using a turbo molecular pump at 5 × 10 −7 mbar for several days.

Preparation of Ni@rGO in ionic liquid
Nickel nanoparticles on rGO (Ni@rGO) were prepared in septum-sealed 10 mL microwave vessels (CEM GmbH, Germany) in a CEM Discover microwave under argon atmosphere. Ni(COD) 2

Preparation of WO 3 nanopowder
Tungsten oxide nanopowder was prepared according to [52] using the sol-gel method. A 1.23 mol/L aqueous solution of sodium tungstate dihydrate (Na 2 WO 3 ⋅2H 2 O) was added into a 12 mol/L aqueous solution of nitric acid under constant rapid stirring. The prepared sol of tungstic acid was washed in distilled water using multiple centrifugation steps. After drying until the xerogel was formed, the WO 3 nanopowder was calcined at 600 °C for 2 h.

Ni@rGO mixing of WO 3 xerogel
The Ni@rGO admixing of WO 3 samples was done by preparing a physical mixture of the WO 3 xerogel and Ni@rGO. At a pressure of 150 kPa, tablets were pressed from the powder (diameter 10 mm, thickness 2.5 mm, weight 0.75 g), which were sintered in air at 450 °C (4 h).
Characterization Powder X-ray diffraction, P-XRD data were measured at ambient temperature on a Bruker D2 Phaser using a flat sample holder and Cu Kα radiation (λ = 1.54182 Å, 35 kV). Samples had been precipitated with acetonitrile from the nanoparticle/ ionic liquid dispersion and washed several times with acetonitrile. P-XRD patterns were recorded for 1 h (2θ = 5-100°).
Atomic absorption spectroscopy, AAS for metal analysis was performed on a PerkinElmer PinAAcle 900T, equipped with a flame furnace. Flame-AAS with an air/acetylene flame was used for the determination of the nickel content. Samples were digested in hot aqua regia two times (30 mL). The residues were re-dissolved in aqua regia, filtered and brought with water to a total volume of 10 mL. For the nickel measurements the samples were diluted 1:100.
Transmission electron microscopy, TEM was performed with a FEI Tecnai G2 F20 electron microscope [68] operated at 200 kV accelerating voltage or FEI Titan 80-300 TEM operated at 300 kV accelerating voltage [69]. Conventional TEM images were recorded with a Gatan UltraScan 1000P detector. TEM samples were prepared by drop casting the with acetonitrile diluted material on 200 µm carbon-coated copper grids, followed by washing the grid several times with acetonitrile to remove the excess ionic liquid. The size distribution was determined manually or with the aid of the Gatan Digital Micrograph software from at least 50 individual particles.
Gas sensing properties of the sensor elements were characterized using a custom-designed flow-type sensing measurement system inside a corundum chamber with precisely controlled temperature and atmosphere. Calibrated according to STB ISO 9001-2009, a CO/N 2 mixture was received from Joint Stock Company "Kryon". Liquid acetone was classified as "chemically pure" (purissimum). The NO 2 gas was obtained by dissolving Cu in nitric acid ("chemically pure" grade). The CO/air mixture was measured in flow mode. NO 2 and acetone were measured in a static mode. Here, a given amount of gas was introduced into a sealed chamber with a volume of 120 cm 3 . Electrical resistance of samples WO 3 , rGO/WO 3 , 0.35 wt % Ni@rGO/WO 3 in the range of 20-240 °С was measured by the two-probe method in a corundum cell using an Agilent 34401 digital multimeter. The cell was placed in a tube furnace with a temperature regulator. To enhance electrical conductivity and to improve contact Ag electrodes were deposited on parallel sides of the pellets. The measurement procedure was carried out for NO 2 and acetone in a stationary regime in a precisely controlled atmosphere (10 ppm NO 2 and 3500 ppm acetone in air) according to the method proposed in [70]. The sensing element was placed into a preheated and thermostabilized chamber. Then, calibrated testing gas mixtures were injected into the chamber and measurements were carried out at the indicated temperature. Sensitivity to CO was measured in a dynamic regime using the CO/N 2 gas mixture in flow mode. The CO/N 2 mixture was fed at a rate of 2 L/h for aperiod of 10 min.
We note that it was not possible to set a repeated exposure to analyte gas and T rec in a stationary mode for technical reasons.
In the stationary regime, the flanges of the measuring cell have rubber plugs through which the required amount of analyte is injected from a microdispenser with a needle. To determine parameters such as T rec and T res in this stationary regime the operation of purging the cell with air would be necessary. For this, the cell flanges need to be replaced with other flanges. The temperature at which the measurements were carried out in the stationary mode excludes quick manipulations of these flanges. Even at room temperature, it takes 1 to 2 min to partially disas-semble the chamber and replace the flanges with plugs with flanges with air purge pipes. Therefore, it was practically impossible to measure the relaxation time in a heated cell. Even at room temperature, at best, the measurement procedure will be incorrect, since the possible recovery time is comparable to the time of manipulations with the cell components.
Magnetic measurements were carried out by using the ponderomotive method with automatized installation for measuring magnetic characteristics and for the determination of magnetic impurities in substances by nondestructive testing with a precision to 0.01%. The measurement error for the specific magnetization of the measured samples is equal to ±0.005 A·m 2 /kg, for the magnetic susceptibility of samples with known mass it is equal to ±1 × 10 -11 m 3 /kg.

Supporting Information
Supporting Information File 1 Comparison with other gas sensors as well as the sensor response figures for CO and acetone.