Gas detection with sensitive materials based on CeO2.
In this chapter, two important aspects of using CeO2 in the field of gas sensors are presented. Firstly, for CO2 detection in the range of 0–5000 ppm, a binary semiconductor oxides CeO2-Y2O3 was used. Secondly, as a dopants, in oxide semiconductor systems, used to detect the NO2. In this case, CeO2 is used as a dopant in hybride composite, consisting of reduced graphene oxide/ZnO, in order to increase the sensibility in NO2 detection at low concentration in the range of 0–10 ppm. The structural and morphological characterization of sensitive materials by X-ray diffraction, SEM, adsorption desorption isotherms, thermal analysis and RAMAN spectroscopy are presented. Also, the sensing element of the sensor that detects the NO2 is achieved by depositing the nanocomposite material on the interdigital grid. The electronic conditioning signal from the sensing element is achieved by using a Wheatstone bridge together with an instrumentation operational amplifier.
- ceria oxide
- yttrium oxide
- zinc oxide
- reduced graphene oxide
- sensing element
- electronic conditioning
- X-ray diffraction
- scanning electron microscopy
Cerium represents one of the most abundant elements in the Earth’s crust (66.5 ppm) than copper (60 ppm) or tin (2.3 ppm). Ce possesses an unique electronic configuration ([Xe] 4f26s2), and presents two common valence states Ce3+ and Ce4+ [1, 2, 3], which give CeO2 excellent chemical and physical properties: 1/4 O2, at most, can be released from each CeO2 unit cell. It serves as an active oxygen donor in many reactions, such as three-way catalytic reactions to eliminate toxic automobile exhaust [1, 4], the low-temperature water gas shift reaction [1, 5], oxygen sensors, oxygen permeation membrane systems and fuel cells [1, 6]. Cerium oxide CeO2 is a semiconductor oxide with a band gap energy (3.19 eV) [7, 8]. The crystalline structure consists of a cubic fluorite structure (Fm3m) with a cell parameter of 5.41 Å at room temperature and presents a high dielectric constant ɛ = 26, almost of silicon, that it makes use in spintronic devices with silicon microelectronic devices [9, 10]. Synthesis of CeO2 nanoparticles comprise various methods as: solvothermal [2, 11, 12], sol gel [2, 13, 14], sonochemical [2, 15], hydrolysis [2, 16], hydrothermal [2, 17, 18], precipitation [2, 19] and reverse micelles [2, 20]. The dual oxidation state mentioned above means that these nanoparticles have oxygen vacancies or defects . The loss of oxygen and the reduction of Ce4+ to Ce3+ in accord with Eq. 1, is accompanied by creation of an oxygen vacancy. This property is responsible for the interesting redox chemistry exhibited by ceria nanoparticles and makes them attractive for many catalytic applications .
Also all ceria applications are based on its potential redox between Ce3+ and Ce4+, high oxygen affinity and absorption/excitation energy bands associated with the electronic structure . Another important property of CeO2 consists in their ability to release and absorb oxygen during alternating redox conditions and hence to function as oxygen buffer. The addition of dopants leads to increase of concentration of oxygen vacancies and improves the thermal stability of the parent oxide . Also, CeO2 presents a great chemical stability and high diffusion coefficient with values between 10−8 and 10−6 cm2/s in the temperature range of 800–2200 K coming from oxygen vacancies (has been used for gas sensing for oxygen, NOx, acetone and H2S sensors). Besides, CeO2 is also used for improving sensing properties of semiconductor oxides such as ZnO, TiO2 and In2O3 [24, 25]. On the other hand, the ionic conductivity of CeO2 is improved by doping with rare earth oxides such as Sm2O3, Gd2O3 and Y2O3 and the size of conductivity of the doped ceria depends on the ionic radius of the doping ion. The introduction of trivalent ions in ceria leads to production of anion vacancies which may enhance catalytic and gas sensing properties . In Table 1, several sensitive materials based on ceria for gas detection and their gas sensing characteristics is presented.
|Sensitive material||Gas detected||Range concentration, [ppm]/[%]||Operation temperature, [°C]||Detection limit, [ppm]||Response/recovery time, [s]||Ref.|
|CeO2 activated ZnO-TiO2||CO2||286 ppm exposure||290||*||24/72|||
As doping with other ions could lead to enhanced activity for different reasons. Ceria doped with pentavalent ions as Nb, could insert extra oxygen anions that would be more easily removed . In this chapter, the mixed oxides CeO2-Nb2O5, Y2O3-doped CeO2 as sensitive materials for CO2 detection and sensitive materials composed from CeO2-doped rGO (reduced graphene oxide) and CeO2-doped rGO-ZnO for NO2 detection are presented.
To conditioning the signal provided by sensing element, high-performance electronic circuits such as precision operational amplifiers, digital analogue converters and analog multipliers have been used [33, 34].
2. Sensor for CO2 detection with mixed binary oxide CeO2-Nb2O5 sensitive material
Niobium oxide has some properties that make it in principal as promising for catalytic applications. Niobia-based materials are effective catalysts in selective oxidation reactions due to its redox properties. Also, niobia-doped ceria materials have shown a good carbon deposition and excellent properties as solid oxide fuel cell (SOFC) anodes . Nb5+ ions (ionic radius of Nb5+: 78 pm) may initiate the reduction of Ce4+ to Ce3+ by the doping Nb into the CeO2 structure, which results in formation of oxygen vacancies. Using the Kröger-Vink notation, it can mention two mechanisms for the dissolution: one of which occurs by electronic compensation (Eq. 2) and the other by consumption of vacancies (Eq. 3), as shown below [3, 36, 37].
where , represent oxygen and oxygen vacancies on the oxygen sites, , represent cerium (Ce4+) and negatively charged cerium ions (Ce3+) on metal sites M, metal vacancy. Nb2O5 it is known as an n-type oxide semiconductor with a band gap about 3.4 eV. Because of its good physicochemical properties and structural isotropy, it is used in other range of applications such as: in construction of gas sensing, field-emission displays and microelectronics electrochromics display and photoelectrodes .
2.1. Synthesis of mixed oxides CeO2-Nb2O5 sensitive material
In our case, we used the mixed binary oxides CeO2-Nb2O5 for CO2 detection. Sensitive element is composed from mechanical mixing of CeO2 (97%) and Nb2O5 (3%); both reagents purchased from Merck. The powder oxides were treated with a few drops of ethylic alcohol for ink obtaining and then introduced in a ball mill for homogenization followed by calcination at 500, 600 and 800°C for 1 hour. The powder calcined at 600°C was pressed in disc form at 2 tons force/cm2 with the dimensions ∅4 × 1 mm and mounted on the ambasis transistor. The sensor image is showed in Figure 1 .
2.2. Structural characterization
Calcined mixed powder oxides were characterized by X-ray diffraction using a diffractometer-type X Bruker-AXS type D8 ADVANCE in conditions: CuKα radiation (λ =1.54059 Å), 40 kV/40 mA, filter kβ of Ni. pas: 0.04°, measuring time on point: 1 s, measure range 2θ = 10–100°. The mixed oxides powder with composition CeO2–3%Nb2O5 was calcined at 500, 600 and 800°C for 1 hour. It shows a cubic phase for CeO2 and orthorhombic phase for Nb2O5, Figure 2. Also, for this powder that was calcined at 800°C, was identified in addition a hexagonal Ce2O3 phase (Figure 3). It obtain for CeO2 cell parameter a = b = c = 5.407 Å. This is in accord with theoretical value of a = 5.404 Å as well as in according with card number 03–065-5923. The cell parameters for Nb2O5 orthorhombic phase were a = 6.175 Å, b = 29.175 Å and c = 3.930 Å in accord with card number 30-0873 . Corresponding to hkl (Miller indices) 111, 200, 220 and 311, the crystallites size determined with Scherrer formula give values of 160.9, 145.6, 117.4 and 63.5 nm.
2.3. CO2 gas sensor made with mixed oxides CeO2-Nb2O5 sensitive material tested in automated process mode
The gas sensors testing were performed with the apparatus as presented in Figure 4. It is realized by SYSCOM-18 Romania for National Institute for Research and Development in Electrical Engineering ICPE-CA. The voltage measurements were effected by testing module, in automated process mode. A control panel provides a lot of measuring values, at rate 1/10 s. The bench of testing for the gas sensor consists in an enclosure where there are set of testing conditions for the sensor as well as in connected equipment. The whole process of testing is automated, being controlled by a programmable automaton. The gas for testing is introduced in a controlled way in the testing enclosure, through a mass debit meter. In the testing, enclosure is set a constant temperature, controlled by a temperature regulator.
The experimental data shows a good sensor response for CO2 detection with increasing temperature.
2.4. Signal conditioning of the sensing element for CO2 detection with mixed binary oxide CeO2-Nb2O5 sensitive material
The Analog Devices AD620 operational amplifier is used to build the signal conditioning electronic module, provided by the sensing element. A preamp section comprised of Q1 and Q2, Figure 6, provides additional gain up front. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains a constant collector current through the input devices Q1 and Q2, thereby impressing the input voltage across the external gain setting resistor, RG.
This creates a differential gain from the inputs to the A1/A2 outputs given by Eq. (4):
The unity-gain subtractor, A3, removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. The value of RG also determines the transconductance of the preamp stage . As RG is reduced for larger gains, the transconductance increases asymptotically to that of the input transistors. The open-loop gain is boosted for increasing programmed gain, thus reducing gain related errors. Also, the gain bandwidth product (determined by C1, C2 and the preamplifier transconductance, Figure 6) increases with programmed gain, thus optimizing the amplifier’s frequency response. In Figure 7, the closed-loop gain of AD620 versus frequency is shown. Finally, the input voltage noise is reduced to 9 nV/√Hz, which is determined mainly by the collector current and base resistance of the input devices. The internal gain resistors, R1 and R2, are laser trimmed to an absolute value of 24.7 kΩ, allowing the gain to be programmed accurately with a single external resistor. The gain equation is Eq. (5).
where the resistor RG in kΩ, according to Eq. (6).
The value of 24.7 kΩ was chosen so that standard 1% resistor values could be used to set the most popular gains. For the input resistors, R1a and R1b were used, capacitor C2p approximately five times to 0.047 μF to provide adequate RF attenuation (Figure 8). With the values shown, the circuit −3 dB bandwidth is approximately 400 Hz and noise levels 12 nV/√Hz. It requires the circuitry preceding the in-amp to drive a lower impedance load and results in somewhat less input overload protection. The output signal VOUT (Figure 8) is a common mode voltage, picked at the output of the operational amplifier. The capacitor groups, 0.01 μF and 0.33 μF make a decoupling of the supply voltage (Figure 8) in the immediate closeness of the operational amplifiers. The supply voltage +Vcc and -Vee, respectively, stabilized is differentiated, ±15Vcc, in comparison with the reference potential bar.
3. Sensor for CO2 detection with Y2O3-doped CeO2 sensitive material
The ion conductivity of CeO2 can be significantly improved upon substitution with some trivalent oxides of lanthanides like Y2O3, Sm2O3 and Gd2O3, because the number of oxygen vacancy will be considerably increased for charge compensation. The electrical conductivity in doped ceria is influenced by factors such as: the dopant ion, the dopant concentration, the oxygen vacancy concentration and the defect association enthalpy. An example is constituted by combination Y2O3-doped CeO2 which has been used usually as the solid electrolyte for moderate temperature solid oxide fuel cells . In our case, we used the Y2O3-doped CeO2 as sensitive material for CO2 detection. For Y2O3-CeO2 synthesis, it utilizes several methods such as hydrothermal , electrospinning , thermolysis  and sol gel .
3.1. Synthesis method
Sol gel method applied for synthesis of Y2O3-doped CeO2 sensitive material, is in accord with ref.  and used as starting reagents Ce(SO4)2 × 4H2O (97% purity, Merck) and Y(NO3)3 × 3H2O (98% purity Karlsrushe GmbH in molar ratio CeO2/Y2O3 = 4:1). The salts were dissolved in deionized water. To 100 ml salt solution, 25 ml solution of 1 M citric acid as chelating agent was added. To obtain gel, the salt solution was heated to 70°C under constant stirring. To this solution, 40 ml ethylene glycol was added to promote citrate polymerization and heated at 90°C. The gel formed was filtered, washed and heat treated in oven at 100°C. The powder obtained was calcined at 800°C for 2 hours. The powder was pressed to disc form using 10 ton force/cm2, with dimensions diameter 4 mm, height 1 mm and then sinterized at 1100°C for 6 hours .
3.2. The construction of the sensor for CO2 detection designed with Y2O3-doped CeO2 sensitive material
On both sides of disc, gold electrodes in circular form with diameter of 1 mm was deposed. The gold was deposed by e-beam evaporation method using Baltzer equipment with conditions: pressure P = 10−5 Torr and current I = 8 mA, for 60 s time deposition. The disc with electrodes deposed was mounted on a 12 pin TO-8 package base. Below the base, the heater element composed of Ni wire with a diameter of 0.1 mm was placed, the winding is composed from 18 turns with a diameter of d = 3 mm. Figure 9 shows how it built the CO2 sensor .
3.3. Structural and morphological characterization of sensitive material Y2O3-doped CeO2
Thermal analysis was performed with NETZSCH STA 409 simultaneous thermogravimetric balance, in the analysis conditions: inert atmosphere of argon, heating rate of 10°C/min in alumina crucible and the mass sample was 15.7 mg. Figure 10 presents the thermal analysis TG, DTA and differential thermogravimetry (DTG) curves for the dried gel. The DTA curve releases two endothermic peaks at 159.2 and 225.1°C. The last one has a correspondent in DTG curve at 219°C, the total mass loss was 86.95% from initial mass. Other thermal transformation appears at 430°C which represents on in a TG curve a loss of 4.91% which correspond to the decomposition of precursors, consisting in cerium sulfate and yttrium nitrate and in the end only Ce-Y-related oxides are obtained .
The X-ray diffraction patterns of the CeO2-Y2O3 oxides powder calcinations at 800°C for 2 hours are shown in Figure 11. For comparison, Figure 12 shows the X-ray diffraction for commercial CeO2 powder. For this oxides system, the XRD pattern reveals the formation of well crystallized phases, CeO2 indexed with the cubic fluorite structure and Y2O3 with cubic structure. Also, a secondary phase with cubic structure and composition Ce0.6Y0.4O1.8 was identified .
Table 2 presents X-ray parameters for Y2O3-doped CeO2, cell parameters and crystallite sizes determined with Scherrer formula.
|Phase||Crystal structure||Unit cell parameter (Å) a = b = c||2Θ||Crystalline face indexes hkl||Crystallites-size (nm)|
|Experimental||Theoretic card no.:|
The morphological structure of Y2O3-doped CeO2 was investigate by SEM measurements using FESEM-FIB type Auriger model Carl Zeiss SMT GmbH at a high voltage acceleration of 2 and 3 kV. The SEM sample morphology was investigated trough SESI (combined detector in SEM chamber–Evernhart Thornley type with Faraday cup). Figure 13 shows the SEM image for disc CeO2-Y2O3 sintered, where it can be seen as a relative homogeneous structure and the crystallite sizes of CeO2 and Y2O3 were in range of 26–54 nm in good accord with X-ray diffraction analysis. Figure 14 shows the SEM images for CeO2-Y2O3 powder calcined at 800°C for 2 hours, where it can be see a nonhomogeneous structure composed by agglomerates .
N2 adsorption desorption isotherms were performed with the AUTOSORB-1, Quantachrome Instruments, United Kingdom in the following conditions: working gas N2, measured temperature: −196°C and relative pressure range P/Po = 0.001–0.99. For binary oxides CeO2-Y2O3, powder calcined at 800°C for 2 hours, BET analysis revealed the results: the specific surface area was 3.13 m2/g, the total volume of the pores was 1.066x10−3 cm3/g and pore sizes of 8.93 Å. There is a specific ratio P/Po = 0.02898 for the pores with diameters smaller than 6.9 Å .
3.4. The CO2 gas sensing mechanism and gas sensors testing
The improved sensing response at CO2 can be attributed to synergistic effects between Y2O3-doped CeO2. In certain conditions such as high temperature, reduced state or pure CeO2, lose some amount of oxygen and generate oxygen vacancies in accord with Eq. (7),
When CO2 comes in contact with CeO2-activated surface, this forms carbonates as a product through the participation of surface oxide ions in accordance with Eq. (8),
The carbonates disappear when they are exposed to oxidizing conditions . The sensor characteristic was performed using test installation presented in Figure 4. The sensor was exposed at CO2 atmospheres in the concentration range of 0–5000 ppm CO2 in the climatic conditions: T = 20°C and two relative humidity testing 40% RH and 80% RH, respectively. The sensor functions at 135°C, temperature provided by the heating resistance (Figure 9, Pos. 7). Figure 15 shows the variation of sensor voltage with the gas concentration. The characteristics show a slow linear decreasing of voltage with CO2 concentration which allows an easy signal conditioning. In the concentration range 0–5000 ppm CO2, the sensor presents a voltage variation as follows: 378.17–377.32 mV for T = 20°C, RH 40% and 377.11–376.61 mV for T = 20°C, RH 80%. The sensor data show a little dependence of voltage with relative humidity that makes usable in environment with high relative humidity. The sensitivity of the sensor was 0.3 V/ppm and the response time was less than 30 s .
3.5. Signal conditioning of the sensing element for CO2 gas detection with Y2O3-doped CeO2 sensitive material
The operational amplifier ADA4627-1, provided by analog devices (Figure 16) is a broadband and high precision amplifier. It is recommended in applications like “sensor conditioning” and electronic conditioning of the signal, due to its exceptional attributes: low noise, very low offset voltage, very high common-mode rejection ratio (CMMR) and very high slew rate. This operational amplifier combines the best “DC” features and very good dynamic characteristics , like: slew rate 60 V/μs; extended range of differential supply voltage: ±5 Vcc …. ± 15 Vcc; opn loop gain 120 dB; low offset voltage maximum 200 μV and the bias current: maximum 5 pA.
4. NO2 gas sensor made with rGO-doped CeO2 and CeO2-doped rGO/ZnO
4.1. Synthesis of sensitive materials rGO-doped CeO2 and CeO2/rGO-doped ZnO
In order to study the CeO2 sensor properties for NO2 detection, two sets of sensitive materials for sensors was synthesized: (a) 1%rGO/CeO2 nanocomposite as sensitive material to study the effect of rGO adding on the sensitivity and (b) 1%(wt. %)CeO2 was added at 1%(wt.%) rGO/ZnO-nanocomposite, in order to study the effect of CeO2 adding on the sensitivity.
Synthesis of 1%rGO/CeO2: 1%(wt.%) rGO/CeO2 nanocomposite was synthesized in situ by precipitation method using Ce(NO3)3 and NH3 (25% conc) at 90°C and 30 min maturation time.
Synthesis of 1%CeO2/1%rGO/ZnO: The 1%(wt.%)GO and 1%CeO2 was mixed with ZnO in ethanol. The resulted powder after ethanol evaporation was heat treated at 150°C. The GO was synthesized by Hummers’ modified method using as strong oxidant potassium permanganate (mass ratio C:oxidant = 1:3) in a solution of sodium nitrate and concentered sulfuric acid (1 g/150 ml) and graphite [45, 46].
4.2. Structural and morphological characterizations of the sensitive materials
UV-Vis diffuse reflectance spectroscopy measurements were performed a Jasco V-570 Spectrophotometer, Japan, equipped with integrating sphere for diffuse reflectance measurement mode and SPECTRALON reference as etalon, and bang gap software in order to evaluate the optical properties and band gap values of the CeO2, doped CeO2 with 1%rGO and doped 1%GO-ZnO nanocomposite with 1%CeO2. The diffuse reflectance spectrum was converted in absorbance spectrum and presented in Figure 17. The band gap was calculated using Kubelka-Munk equation with associated plot versus photon energy Eg [eV], where α is extinction coefficient [cm−1] and h is Planch constant 4.135x10−15 [eVs], υ is light frequency [s−1] and wavelength [nm] [47, 48, 49]. The linearity coefficient was in all case bigger than 0.99.
In Table 3, the UV-Vis spectra parameters of UV-Vis measurements, for 1%rGO/CeO2 and CeO2 is also presented.
|Samples||Absmax1||I1||Absmax2||I2||Band gap, [eV]|
|Blue shift||Hyper chromic effect||Blue shift||Hyper chromic effect||band gap narrowing|
|CeO2 -standard||252||1.051||339||0.99||theoretical||Commercial type|
|3.19, Ref. ||3.16|
Legend: Aabs represents absorbance plasmon resonance (APR) and I represents intensity of APR.
The effect of doping of CeO2 with 1%rGO leads to blue shift of APR presented in Table 3 and Figure 17 accompanied by the hyperchromic effect for both peaks, while the band gap are narrows, keeping the same characteristic shape of the ceria spectrum. The same effect—a blue shift has been described in literature for both TiO2 aditived with GO and for ZnO aditived with GO [50, 51]. The introduction of 1%GO(wt.%) in CeO2 leads to a decrease in the effective optical band gap value from 3.16 eV to 3.05 eV, with a variation of 0.11 eV. This shows that the 1%GO(wt.%) acted as a band gap modifier [47, 48, 49, 52]. The same effect has been described in literature for the introduction of GO and related materials rGO in TiO2 leads to a decrease in band gap . Earlier, a lot of researchers attempt to tailor the properties of oxide semiconductors by using band gap modifiers and in this way to improve the catalytic, photovoltaic and sensing properties; this new trend is named bend gap engineering [47, 48, 49, 52]. Many researchers obtained a band gap narrowing after heat treatment of CeO2  and doping with different metals as Co , Gd , functionalized by different techniques , etc. Rare earth oxides present a high basicity related to ordinary oxide semiconductors such as TiO2, WO3, SnO2 and ZnO, fast oxygen ion mobility and interesting catalytic properties which are important in gas sensing application [54, 55, 56]. Table 4 presents UV-Vis spectra parameters of UV-Vis measurements for 1%CeO2/1%rGO/ZnO and 1%rGO/ZnO.
In the case of doping with 1%(wt.%)CeO2 of the 1%(wt.%)rGO/ZnO nanocomposite, the effect is the same, an increasing of APR accompanied by the hyperchromic effect with the preservation of the characteristic spectra shape. But in opposite with the first case of CeO2 doped with 1%GO(wt.%), there is a decrease in effective optical band gap value from 3.24 to 3.19 eV, with a variation of 0.05 eV. This shows that the 1%CeO2(wt.%) acted as a band gap modifier. The UV-Vis spectra present a strong absorption bands below 400 nm in UV region for the nanocomposites with the main component ZnO which are attributed to ZnO NP. The APR of ZnO nanocomposites are lower, in generally, than the absorption band of bulk ZnO (373 nm) that had a wide direct band gap at room temperature of 3.37 eV [48, 57, 58, 59, 60]. CeO2 adding on ZnO nanocomposite surface leads to a significant increase of the absorption in the UV light spectrum and decrease in the visible light spectrum. Based on the above results, UV-Vis and transformed Kubelka?Munk function plots suggested that are necessary energy for generation of electrons in conduction bands and holes in valence bands is smaller for the doped 1%rGO/CeO2 than the CeO2, this makes the doped CeO2 more reactive and sensitive. Other researchers tried to improve the sensing properties of ZnO sensor, for ethanol detection, by adding noble metals such as Pd , Pt  and Au , other metals such as Al, In, Cu, Fe and Sn , oxides as TiO2 , CuO , CoO , RuO2  and SnO2 and not in the end Ce and CeO2 . There is a current practice to use band gap modifiers. Many researchers use a band gap modifier in order to improve the functional properties of nanocomposite based on semiconductors oxides. The functional properties are ranging from the photocatalytic properties, sensitivity and selectivity for different sensor types, catalysts and others . Raman spectroscopy measurements was performed with Raman dispersive spectrometry–LabRam HR Evolution, Horiba Jobin Yvone, France, equipped with Laser wave 532 nm, acquisition time 5 s, 10 accumulation, 0.1% laser power, used in order characterized the order-disorder degree in the synthetized nanocomposite. Figure 18 shows the RAMAN spectra for CeO2 and rGO/CeO2.
Figure 18(a) presents the Raman spectrum of CeO2 powder which reveals a peak situated at 462.5 cm−1 characteristic for CeO2, corresponding to the Raman active modes F2g for Ce–O symmetric breathing mode of oxygen atoms around the Ce atoms . Figure 18(b) shows the Raman spectrum of 1%rGO/CeO2 with characteristic peak of ceria at 449 cm−1 corresponding to the Raman active modes of CeO2 and characteristics graphene oxide peaks  at 1348.54 cm−1 (D band), 1593.30 cm−1 (G band), 2681.42 cm−1 (2D band), 2938.30 cm−1 (2D + D’ band) and 3180.64 cm−1 (G + D’ band). According to the Raman line, broadening is equivalent with lattice constant cell crystallographic parameter ao of CeO2 can be estimated by Eq. (9) , with 0.9 nm for CeO2 powder and 0.43 nm for the CeO2 from the 1% rGO-CeO2 nanocomposite. The characteristic peak of CeO2 was shifted with 13.05 cm−1 at lower wave number as a doping effect of 1%rGO.
where the FW is full wide at half-maximum of the Raman active mode F2g and d is the diameter particle in nm. Figure 19(a) shows the Raman spectrum of GO with characteristic peaks of graphene oxide peaks at 1347.96 cm−1 (D band), 1595.33 cm−1 (G band), 2681.77 cm−1 (2D band), 2914.68 cm−1 (2D + D’ band) and 3196.75 cm−1(G + D’ band). Figure 19(b) shows the Raman spectrum of 1%CeO2/1%rGO/ZnO with characteristic peaks of graphene oxide peaks at 1350.87 cm−1 (D band), 1605.74 cm−1 (G band), 2684.22 cm−1 (2D band) and characteristic peaks of ZnO and active modes F2g, CeO2 (462.79 cm−1), where the ID/IG can be used to evaluate quantitative the crystallinity/disorder degree and are varying between 1.05 and 1.24, lower value indicates the less defects in graphitic structure . Figure 20 shows the morphologies for the three sensitive materials reveals for (a) CeO2 was evidentied a polycrystalline structure, for (b) CeO2/rGO - the micrographic image presents a 3-D layered structured of GO mixed with small polycrystalline particles of ceria and for (c) CeO2/rGO/ZnO was evidentied a mixed polycrystalline structure of preponderant small particles of wurtzite hexagonal types ZnO and minor faces of cubic CeO2 and carbon faces.
4.3. The construction of the sensing element for NO2 gas detection designed with rGO-doped CeO2 and CeO2-doped rGO/ZnO sensitive material
The sensor module is constituted from printed circuit board (PCB), substrate with interdigitated Ag array electrode deposed by photolitografic technology and the sensitive material in amounts 15–20 mg was deposited on surface electrode. The active area for sensitive material was 10 mm × 0.5 mm, Figure 21(a) and (b).
4.4. The NO2 gas sensing mechanism
In metal oxide semiconductor gas sensors, the resistance is measured as a function of the gas concentration. Generally, this devices function at elevated temperature between 200 and 600°C in air. The grain of metal oxide is covered by adsorbed oxygen molecules. Oxygen molecules present the character of electronegativity, they extract electrons from the conduction band of metal oxide causing the formation of oxygen ions , , , adsorbed at the surface of metal oxide. Since electrons are removed from the metal oxide, the concentration of free charge carriers is reduced forming a depletion layer at grain boundaries. The surface reactions can be written according with Eqs. (10–12):
As is it known, nitrogen oxides specify as NOx have the character of oxidizing gases with very high electron affinity 2.28 eV as compared with oxygen 0.43 eV. The NOx molecules interact with the surface of metal oxide through surface adsorbed oxygen ions, thus increasing the potential barrier at grain boundaries. The redox reactions taking place on the surface of a metal oxide can be written according with Eqs. (13–14) .
As result, the thickness and resistance of the depletion layer increase and resistance change is reversible at operating temperature . The oxygen vacancies can significantly enhance the adsorption of oxygen molecules and electrons will transfer from the oxygen vacancies from CeO2 to the oxygen molecules, resulting in more oxygen species (especially O2−). These oxygen species will react with NO2, resulting in an abrupt change in the conductivity of the sensor . The graphene sheets by their good properties as: high surface area 2630 m2/g, thermal conductivity in the range of 3000–5000 W/mK at room temperature carrier mobility up to 200,000 cm2/Vs , electrical conductivity of 7200 S/m , coming from their structure two-dimensional (2D) single atom layer is used in gas sensing and in the composite leads to increase of the electrical conductivity of CeO2 and thus improve the performance to gas sensing room temperature .
4.5. The NO2 gas sensors testing and sensing characteristics
The sensors with sensitive materials 1%rGO-doped CeO2, and 1% CeO2/1%rGO-doped ZnO were tested in NO2 atmosphere in concentrations 5 and 10 ppm. The gas testing was effected with testing installation presented in Figure 4. The gas testing was performed in order to establishment of the sensitivity sensors and response time. The sensor sensitivity was expressed in accord with Eq. (15), as the ratio of resistance in air to that in target gas, in this case NO2,
where is the resistance of sensor in air and is the resistance of sensor in gas.
The response time is expressed by formula:
Notations are the same with Eq. (15) . Having the resistance values, from the graph, the response time can be determined. Figure 22 shows the resistance variation with time exposure gas and Figure 23 shows the sensitivity (response) for sensing element with time exposure gas for two sensitive material: 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO. All the characteristics are considered for the 1 hour time exposure. Since the resistance of sensors decreases sharply, for a good view we opted for a semilogarithmic scale representation of resistance and sensors response with exposure time. The decreases of resistance denotes a character of type p semiconductors for both sensitive materials in oxidant gas like NO2, character given by reduced graphene oxide which is a semiconductor type p.
The sensors performances can be resumed in Table 5.
|Sensitive material||Concentration NO2, [ppm]||Resistance in air, [kΩ]||Resistance in gas, [kΩ] (after 3600 s exposure)||Sensitivity,||Response time, [s]|
Analyzing the obtained results, it can be concluded that the both sensitive materials show the good performance at NO2 exposure at room temperature. However, the sensitive material composed by 1%rGO/CeO2 presents very good sensitivity at NO2 exposure for 5 and 10 ppm concentrations of 2000 and 1818 and very short response time of 2.5and 3.5 s. Thus, sensitive materials with CeO2 in majority concentration in matrix with reduced oxide graphene presents the best performance at NO2 detection, face to sensitive materials 1%CeO2/1%rGO/ZnO where ZnO is majority and are a promising sensitive materials for NO2 detection.
4.6. Signal conditioning of the sensing element for NO2 gas detection designed with rGO-doped CeO2 and CeO2-doped rGO/ZnO sensitive material
Resistance of sensor sensing element ES, R + ΔR, Figure 24 may vary from less than 10 kΩ to several hundred kΩ, depending on the design of the sensor and the physical environment to be measured. The sensing element ES of the NO2 gas sensor is disposed in one of the Wheatstone bridge arms and shows the resistance R for a NO2 concentration of zero ppm. The resistances of resistors disposed in all of other branches of the bridge show the same value, namely R. A DC voltage excitation source U1 is connected to one of the bridge diagonals .
If the gas concentration of NO2 is zero ppm, the sensing element ES shows the resistance R. The Wheatstone bridge is in this case at equilibrium so that the voltage measured on the other diagonal of the bridge is 0 V. Variation of NO2 gas concentration in the range from zero ppm to 10 ppm causes a voltage variation with ΔU0, which can be measured on the other diagonal of the bridge. The voltage variation up to ΔU0 is given by the relation (17):
The operational amplifier that can be used with the best performance is instrumentation type amplifier (in-amp), “resistor programmable” (Figure 24). Considering the transfer function of the electronic amplifier module and taking into account the relation (17), we obtain :
where A is the amplification factor, depending on the Rg resistance value and , is a constant. In-amps such as the AD620 family, the AD623 and AD627, Analog Devices type can be used in single (or dual) supply bridge applications.
4.6.1. Realization of the continuous U1 excitation voltage source
The continuous U1 excitation voltage source is made using a D/A digital/analog converter, a Uref reference voltage and an operational amplifier (OA) (Figure 25). Thus, depending on the values set for the least significant bit (LSB) up to the most significant bit (MSB), the resulting word can establish a desired U1 continuous excitation voltage. Figure 25 shows the schematic of the electronic block for the U1 excitation voltage source.
4.6.2. Bridge-linearization electronic circuit
Schematic of the electronic linearization block of the signal generated by the Wheatstone bridge, via an operational amplifier, in-amp uses an analog multiplier , AD 534 or AD 734, produced by analog devices (Figure 26). The transfer function associated with the AD 534 or AD 734 analog multiplier is written [76, 77]:
where A0 is the open loop gain, X1, X2, Y1, Y2, Z1 and Z2 represent the inputs of the analog multiplier, SF a scale factor, typically SF = 10 V and W = OUT, according to Figure 26.
Since A0 → 72 dB can be considered as W/A0 → 0 and the relation (19) becomes:
Since Z1 = W it is obtained:
Since Z2 = U2, Y1-Y2 = βU2, 0 ≤ β < 1, X2 = 0 and X1 = Z1 = W=U3, according to Figure 26. Finally,
The relation (6) together with the relation (2) represents the calculation method regarding the linearization of the signal generated by the Wheatstone bridge, via an operational in-amp instrumentation amplifier.
4.6.3. Resulting structures for the electronic block for signal conditioning generated by the sensing element
By considering the three previously analyzed electronic blocks, the electronic block for signal conditioning generated by the sensing element is obtained. Figure 27 shows the schematic of the electronic block for signal conditioning generated by the sensing element, single supply bridge applications.
It is possible to reconfigure circuits so as to improve the performance in terms of reduces the dc common-mode voltage to zero. Figure 28 shows how the use of split U1 tension in order to reduce the dc common-mode voltage to zero.
An isolation amplifier can be useful for this application, with respect to the signal-conditioning, so that it does not exist galvanic connections between the bridge and grounded instrumentation circuitry.
Cerium, by its unique electronic configuration ([Xe] 4f26s2) and by the two common valence states Ce3+ and Ce4+ allowing a redox reaction between them which gives CeO2 excellent chemical and physical properties, is used in many applications, like as: three-way catalytic reactions to eliminate toxic automobile exhaust, the low-temperature water gas shift reaction, oxygen permeation membrane systems for fuel cells as well as gas sensors. For gas sensing applications, several sensitive elements based on CeO2 were tested to determine both this detection function as well as this performances:
By doping the CeO2 with oxides semiconductor, for example, Nb2O5 introduced in CeO2 structure, the following mechanism is triggered: Nb5+ ions initiate the reduction of Ce4+ to Ce3+ resulting in the formation of oxygen vacancies with consequences in increasing the sensitivity.
The ionic conductivity of CeO2 is improved by doping with rare earth oxides such as Sm2O3, Gd2O3 and Y2O3. The size of conductivity for doped ceria depends on the ionic radius of the doping ion. Therefore, the introduction of trivalent ions in ceria leads to the production of anion vacancies which may enhance catalytic and gas sensing.
CO2 detection using sensitive material based on mixed binary oxide CeO2-Nb2O5 in ratio 97%/3%, for 10,000 ppm CO2 at the 25, 50 and 70°C chamber test temperature, the sensor was developed voltage values of 48, 50 and 770 mV.
CO2 detection with Y2O3-doped CeO2 molar ratio CeO2/Y2O3 = 4:1 with characteristics: the CO2 concentration in the range of 0–5000 ppm, function temperature 135°C, climatic conditions T = 20°C, 40% RH and 80% RH, voltage values 378.17–377.32 mV for T = 20°C, 40% RH and 377.11–376.61 for T = 20°C, 80% RH. Sensitivity is 0.3 V/ppm and response time 30 seconds;
Sensitive materials based on 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO was analyzed with UV-Vis spectroscopy showing that a decreasing of band gap of CeO2 in matrix with rGO from 3.19 eV at 3.05 eV what allows for sensor to function at room temperature. The sensors were tested for 5 and 10 ppm NO2 obtaining the sensitivities of 2000 and 1818, response times of 2.5 and 3.5 s for sensitive material 1%rGO/CeO2 and sensitivities of 41.29 and 321.2, response times of 2.8 and 2.2 s for sensitive material 1%CeO/rGO/ZnO. The sensitive materials made so that the matrix in which CeO2 is in majority presents the best performance.
Also, the sensing mechanism in CO2 and NO2 detection was discussed.
Based on these results, it can be stated that CeO2 is a good candidate in gas sensors applications.
The research was performed with the support of Ministry of Research and Innovation, NUCLEU Programme Research Projects - Romania, Contract PN18240301/2018, “Sensors and electronic actuators based on new active materials”, and UEFISCDI, Program 2 - Romania, Contract 4SOL/2017, “Development and implementation of modern solutions for gas turbine propulsion systems and their associated systems”.
Conflict of interest
No conflict of interest exists with regard to this chapter.