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.


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 [47–49, 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 [50]. 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 [47–49, 52]. Many researchers obtained a band gap narrowing after heat treatment of CeO2 [52] and doping with different metals as Co [52], Gd [53], functionalized by different techniques [47], 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 [54–56]. Table 4 presents UV-Vis spectra parameters of UV-Vis

Blue shift Hyper chromic effect Blue shift Hyper chromic effect band gap narrowing CeO2 -standard 252 1.051 339 0.99 theoretical Commercial type

Figure 17. Diffuse reflectance UV-Vis spectroscopy spectra for CeO2 (red), (1%CeO2/1%rGO) ZnO (blue), 1%rGO/ZnO

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems

3.19, Ref. [47] 3.16

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77

Samples Absmax1 I1 Absmax2 I2 Band gap, [eV]

1%rGO/CeO2 249 1.065 335 1.535 3.05

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

measurements for 1%CeO2/1%rGO/ZnO and 1%rGO/ZnO.

Table 3. UV-Vis spectra parameters of UV-Vis measurements.

(lagun) and 1%rGO/CeO2 (green).

#### 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 ffiffiffiffiffiffiffiffi <sup>α</sup>hv <sup>p</sup> versus photon energy Eg [eV], where <sup>α</sup> is extinction coefficient [cm�<sup>1</sup> ] and h is Planch constant 4.135x10�<sup>15</sup> [eVs], υ is light frequency [s�<sup>1</sup> ] and wavelength [nm] [47–49]. 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.

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

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 77

Figure 17. Diffuse reflectance UV-Vis spectroscopy spectra for CeO2 (red), (1%CeO2/1%rGO) ZnO (blue), 1%rGO/ZnO (lagun) and 1%rGO/CeO2 (green).


Table 3. UV-Vis spectra parameters of UV-Vis measurements.

[33], 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

4. NO2 gas sensor made with rGO-doped CeO2 and CeO2-doped rGO/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.

A. 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 matu-

B. 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

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-

In Table 3, the UV-Vis spectra parameters of UV-Vis measurements, for 1%rGO/CeO2 and

Legend: Aabs represents absorbance plasmon resonance (APR) and I represents intensity of

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

] and h is Planch constant 4.135x10�<sup>15</sup> [eVs], υ is light frequency [s�<sup>1</sup>

<sup>α</sup>hv <sup>p</sup> versus photon energy Eg [eV], where <sup>α</sup> is extinction

] and

concentered sulfuric acid (1 g/150 ml) and graphite [45, 46].

Munk equation with associated plot ffiffiffiffiffiffiffiffi

4.2. Structural and morphological characterizations of the sensitive materials

wavelength [nm] [47–49]. The linearity coefficient was in all case bigger than 0.99.

4.1. Synthesis of sensitive materials rGO-doped CeO2 and CeO2/rGO-doped ZnO

5 pA.

76 Cerium Oxide - Applications and Attributes

ration time.

coefficient [cm�<sup>1</sup>

APR.

CeO2 is also presented.

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 [47–49, 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 [50]. 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 [47–49, 52]. Many researchers obtained a band gap narrowing after heat treatment of CeO2 [52] and doping with different metals as Co [52], Gd [53], functionalized by different techniques [47], 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 [54–56]. 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


Figure 18(a) presents the Raman spectrum of CeO2 powder which reveals a peak situated at 462.5 cm�<sup>1</sup> characteristic for CeO2, corresponding to the Raman active modes F2g for Ce–O symmetric breathing mode of oxygen atoms around the Ce atoms [49]. Figure 18(b) shows the Raman spectrum of 1%rGO/CeO2 with characteristic peak of ceria at 449 cm�<sup>1</sup> corresponding to the Raman active modes of CeO2 and characteristics graphene oxide peaks [69] at 1348.54 cm�<sup>1</sup> (D band), 1593.30 cm�<sup>1</sup> (G band), 2681.42 cm�<sup>1</sup> (2D band), 2938.30 cm�<sup>1</sup> (2D + D' band) and 3180.64 cm�<sup>1</sup> (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) [49], 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�<sup>1</sup> at lower wave

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems

FW cm�<sup>1</sup> <sup>¼</sup> <sup>10</sup> <sup>þ</sup>

(2D band), 2914.68 cm�<sup>1</sup> (2D + D' band) and 3196.75 cm�<sup>1</sup>

teristic peaks of ZnO and active modes F2g, CeO2 (462.79 cm�<sup>1</sup>

ZnO and minor faces of cubic CeO2 and carbon faces.

Figure 19. Raman spectra for rGO (a) and synthetized 1%CeO2/1%rGO/ZnO (b).

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�<sup>1</sup> (D band), 1595.33 cm�<sup>1</sup> (G band), 2681.77 cm�<sup>1</sup>

the Raman spectrum of 1%CeO2/1%rGO/ZnO with characteristic peaks of graphene oxide peaks at 1350.87 cm�<sup>1</sup> (D band), 1605.74 cm�<sup>1</sup> (G band), 2684.22 cm�<sup>1</sup> (2D band) and charac-

evaluate quantitative the crystallinity/disorder degree and are varying between 1.05 and 1.24, lower value indicates the less defects in graphitic structure [69]. 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

124; 7

<sup>d</sup> ½ � nm (9)

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79

(G + D' band). Figure 19(b) shows

), where the ID/IG can be used to

number as a doping effect of 1%rGO.

Table 4. UV-Vis spectra parameters of UV-Vis measurements for 1% CeO2/1%rGO/ZnO and 1%rGO/ZnO.

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, 57–60]. 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 [61], Pt [62] and Au [63], other metals such as Al, In, Cu, Fe and Sn [64], oxides as TiO2 [65], CuO [66], CoO [67], RuO2 [68] and SnO2 and not in the end Ce and CeO2 [55]. 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 [52]. 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. Raman spectra for CeO2 powder (a) and synthetized 1%rGO/CeO2 (b).

Figure 18(a) presents the Raman spectrum of CeO2 powder which reveals a peak situated at 462.5 cm�<sup>1</sup> characteristic for CeO2, corresponding to the Raman active modes F2g for Ce–O symmetric breathing mode of oxygen atoms around the Ce atoms [49]. Figure 18(b) shows the Raman spectrum of 1%rGO/CeO2 with characteristic peak of ceria at 449 cm�<sup>1</sup> corresponding to the Raman active modes of CeO2 and characteristics graphene oxide peaks [69] at 1348.54 cm�<sup>1</sup> (D band), 1593.30 cm�<sup>1</sup> (G band), 2681.42 cm�<sup>1</sup> (2D band), 2938.30 cm�<sup>1</sup> (2D + D' band) and 3180.64 cm�<sup>1</sup> (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) [49], 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�<sup>1</sup> at lower wave number as a doping effect of 1%rGO.

$$FW[\text{cm}^{-1}] = 10 + \frac{124,7}{d} \text{ [nm]} \tag{9}$$

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�<sup>1</sup> (D band), 1595.33 cm�<sup>1</sup> (G band), 2681.77 cm�<sup>1</sup> (2D band), 2914.68 cm�<sup>1</sup> (2D + D' band) and 3196.75 cm�<sup>1</sup> (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�<sup>1</sup> (D band), 1605.74 cm�<sup>1</sup> (G band), 2684.22 cm�<sup>1</sup> (2D band) and characteristic peaks of ZnO and active modes F2g, CeO2 (462.79 cm�<sup>1</sup> ), 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 [69]. 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.

Figure 19. Raman spectra for rGO (a) and synthetized 1%CeO2/1%rGO/ZnO (b).

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, 57–60]. 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 [61], Pt [62] and Au [63], other metals such as Al, In, Cu, Fe and Sn [64], oxides as TiO2 [65], CuO [66], CoO [67], RuO2 [68] and SnO2 and not in the end Ce and CeO2 [55]. 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 [52]. 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

Samples Absmax1 I1 Absmax2 I2 Band gap, [eV]

Blue shift Hyper chromic effect Blue shift Hyper chromic effect Band gap narrowing

— — — Theoretic

3.37

Ref.: [48], [57–60]

1%CeO2/1%rGO-ZnO 260 1.104 293 1.103 3.19

1%rGO-ZnO-etalon 265 1.059 294 1.056 3.25

Table 4. UV-Vis spectra parameters of UV-Vis measurements for 1% CeO2/1%rGO/ZnO and 1%rGO/ZnO.

Legend: Aabs represents absorbance plasmon resonance (APR) and I represents intensity of APR.

RAMAN spectra for CeO2 and rGO/CeO2.

Figure 18. Raman spectra for CeO2 powder (a) and synthetized 1%rGO/CeO2 (b).

ZnO 376

78 Cerium Oxide - Applications and Attributes

Ref. [60]

metal oxide causing the formation of oxygen ions O�

oxide can be written according with Eqs. (13–14) [70].

NO=NO2ð Þþ gas O�

4.5. The NO2 gas sensors testing and sensing characteristics

written according with Eqs. (10–12):

to 200,000 cm<sup>2</sup>

sensing room temperature [71].

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

Figure 21. The NO2 gas sensing element structure (a) PCB substrate with interdigitated Ag array electrode; (b) the

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems

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

� ! NO�=NO�

<sup>2</sup> ð Þ! ads NO�=NO�

As result, the thickness and resistance of the depletion layer increase and resistance change is reversible at operating temperature [70]. 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 [71]. The graphene sheets by their good properties as: high surface area 2630 m2

thermal conductivity in the range of 3000–5000 W/mK at room temperature carrier mobility up

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

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

/Vs [72], electrical conductivity of 7200 S/m [73], coming from their structure

� ! O�

O<sup>2</sup>ð Þ gas þ e

sensing element made with PCB substrate and sensitive material deposed on surface electrode.

<sup>1</sup>=2O2ð Þþ gas e

O� þ e

NO=NO<sup>2</sup>ð Þ gas þ e

<sup>2</sup> , <sup>O</sup>�, <sup>O</sup><sup>2</sup>�, adsorbed at the surface of

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81

<sup>2</sup> ð Þ ads (10)

<sup>2</sup> ð Þ ads (13)

<sup>2</sup> ð Þþ ads O2ð Þ gas (14)

/g,

� ! O�ð Þ ads (11)

� ! <sup>O</sup><sup>2</sup>�ð Þ ads (12)

Figure 20. SEM images for: (a) CeO2; (b) CeO2/rGO; (c) CeO2/rGO/ZnO.
