Synthesis and characterization of Sm2O3 nanorods for application as a novel CO gas sensor

Applied Surface Science 487 (2019) 793–800

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Synthesis and characterization of Sm2O3 nanorods for application as a novel CO gas sensor S. Rasouli Jamnania,b, H. Milani Moghaddamb, S.G. Leonardia, N. Donatoa, G. Neria, a b

T

⁎

Department of Engineering, University of Messina, Messina 98166, Italy Department of Solid State Physics, University of Mazandaran, Babolsar, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Sm2O3 Nanorods Gas sensor Carbon monoxide

Sm2O3 nanorods have been successfully synthesized by a simple hydrothermal process in the presence of cetyl trimethyl ammonium bromide (CTAB) as an organic template. The morphological, microstructural and optical characteristics of the synthesized nanorods were investigated by scanning electron microscopy (SEM-EDX), X-ray diffraction analysis (XRD), micro-Raman spectroscopy and photoluminescence (PL). The complementary investigations substantiated the purity of the material synthesized, constituted of Sm2O3 nanorods in the single cubic crystalline phase having a mean length of about 400 nm and diameter of about 80 nm. A conductometric sensor based on the synthesized Sm2O3 nanorods has been developed. The sensor performances were investigated in the detection of carbon monoxide (CO) in air. At the optimal operating temperature of 250 °C, the sensor showed a response S = Rg/Ra = 1.4 toward 5 ppm of CO, a limit of detection (LOD) of 1 ppm at signal-to-noise ratio S/N = 3, fast response/recovery time (35 s and 110 s, respectively) and selectivity against the most common air pollutant gases. The results demonstrated that the developed sensor based on Sm2O3 nanorods has superior performances in the monitoring of low concentration of CO in air compared to the state of the art rare-earth oxide CO sensors.

1. Introduction Samarium (III) oxide (Sm2O3) is one of most important rare earth oxides from the applicative point of view. It has been extensively studied due to its potential applications in various advanced industrial fields, such as in solar cells, catalysis and optoelectronics [1]. Sm2O3 behave as a p-type metal oxide semiconductor [2] and find remarkable applications in gas sensing in combination with other metal oxides, such as SnO2 and ZnO, to form mixed metal oxides [2–6]. Despite its interesting properties, pure Sm2O3 has been instead less studied for gas sensing. Indeed, while significant work regarding gas sensing has been made for other pure rare earth oxides (e. g. cerium oxide) [7,8], there are only very few reports in the scientific literature on the application of pure Sm2O3 for gas sensing [9,10]. Based on these premises, we focused our efforts to develop a conductometric Sm2O3 gas sensor for CO. To the best of our knowledge, the sensing properties of Sm2O3 for CO detection have been studied only by one research group [10]. Michel et al. applied Sm2O3 microspheres synthesized by the coprecipitation method to the development of an impedimetric CO sensor. However, the sensor shows some important limitations: a) it needs to operate at high temperature (400 °C) and this

⁎

may deteriorate its long-term performance; b) it displays low sensitivity toward CO in air, limiting its applicability only to detection of very high CO concentrations [10]. The monitoring of very low concentrations of CO in ambient air is of outmost importance because of its high toxicity for human life. CO is a gas contributing to air pollution and it is found for example in automobile exhaust, the burning of domestic fuels, and so on. U.S. Environmental Protection Agency (EPA) recommends an ambient air quality of 9 ppm CO, or lower, averaged over 8 h and 35 ppm or lower over 1 h [11]. Therefore, high performance, reliable and low cost CO sensors are demanding for many applications, e. g. detection of smoldering fires, air quality in urban and closed environments [12]. In a previous paper we reported on the preparation of Sm2O3 microspheres, which have been applied for developing a conductometric sensor for volatile organic compounds (VOCs) such as ethanol and acetone [13]. The sensing performances for these VOCs vapors were fairly good, while the response for CO was found to be negligible. We related this behavior to the nanoparticulate structure of the sensing material. It is well known that there is a dependence of sensitivity and selectivity from grain size and shape of sensing materials in MOx semiconductor gas sensors [14–17]. The above cited factors play a

Corresponding author. E-mail address: [email protected] (G. Neri).

https://doi.org/10.1016/j.apsusc.2019.05.124 Received 11 December 2018; Received in revised form 5 April 2019; Accepted 12 May 2019 Available online 16 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 487 (2019) 793–800

S. Rasouli Jamnani, et al.

performed using a micro-Raman apparatus with an exciting wavelength of 532 nm from a 10 mW diode laser focused onto the sample surface by means of an Olympus BX 40 microscope equipped with a 100 X objective. The latter was also used to collect the backscattered radiation that was analyzed by an XploRA monochromator and sent to a cooled charge-coupled device detector. Raman spectra were carried out on samples deposited onto glass substrate in order to exclude any contribution of the substrate to the observed sample features. Photoluminescence (PL) measurements were carried out on a NanoLog modular spectrofluorometer Horiba with a Xe lamp as the excitation light source of 340 nm.

crucial role in affecting the reactivity of the semiconductor surface then, as the sensing mechanism of these sensors is linked to surfacecontrolled reactions of the reducing target gas(es) with oxygen adsorbed on the surface, the sensing response is also strongly affected. In this study, we report on the synthesis and characterization of Sm2O3 with a nanorod structure, finalized to the development of a reliable conductometric gas sensor for low CO concentrations in air. Sm2O3 nanorods were first synthesized by a simple hydrothermal process in the presence of cetyl trimethyl ammonium bromide (CTAB) as an organic template. The controllable wet synthesis of different shaped Sm2O3 with the assistance of organic templates has been previously reported [18–20]. However, no data are available so far in literature regarding the influence of particles shape on the sensing performances of Sm2O3-based sensors. Therefore, a comparison of the effect of different particle shapes on the CO sensing performances of the developed Sm2O3-based sensors has been made with the objective to give some guidelines for future studies.

2.3. Gas sensing tests Devices for electrical and sensing tests were fabricated by printing films (about 20 μm thick) of the Sm2O3 nanorods sample dispersed in water on alumina substrates (6 × 3 mm) with Pt interdigitated electrodes and a Pt heater located on the backside. The sensors were then introduced in a stainless-steel test chamber for the electrical and sensing tests. The experimental bench for the electrical characterization of the sensors allows us to carry out measurements in controlled atmosphere. Electrical measurements were carried out in the range from room temperature (RT) to 400 °C, under either dry or humid synthetic air total stream of 100 cm3 min−1. CO coming from a certified bottle can be further diluted at a given concentration by mass flow controllers. An Agilent E3632A power supply was employed to bias the built-in heater of the sensor to perform measurements at high temperatures. The resistance value was obtained by recording the current, which flows through the sensitive layer at an applied potential of 5 V, employing a Keithley 6487 Picoammeter/Voltage Source instrument. This instrument allows to perform direct measurements of high resistance values, and it is able to easily fulfill the measurement range required in this paper (up to 1010 Ω). The gas response, S, is defined as S = Rg/Ra where Rg is the electrical resistance of the sensor at different CO concentrations and Ra is the baseline resistance. The response time, τres, was defined as the time required for the sensor to reach 90% of the saturation signal and the recovery time, τrec, the time needed to bring the signal back to 90% of the baseline signal.

2. Experimental 2.1. Synthesis of Sm2O3 nanorods Sm2O3 nanorods have been synthesized as follows. Samarium nitrate hexahydrate were dissolved in 12.5 ml distilled water under magnetic stirring to obtain 0.06 M solution. Ammonia solution was added dropwise to maintain the pH value to 9–10. This solution was added to 12.5 ml aqueous solution cetyl trimethyl ammonium bromide (CTAB). Mixture was transferred into autoclave of 100 ml capacity sealed and maintained at 120 °C for 3 h natural cooling to room temperature. The obtained precipitate was separated by centrifugation, washed with distilled water and ethanol several times and calcined at 600 °C for 1 h. 2.2. Samples characterization Sm2O3 samples were subjected to a complimentary morphological and microstructural characterization study. SEM analysis was carried out with a field emission scanning electron microscopy (FESEM, Mira3XMU). Powder X-ray diffractometry (XRD) analysis was carried out on a Bruker AXS D8 Advance diffractometer within the 2θ range of 20 to 80° using CuKα1 as X-ray source (λ = 1.5406 Å). The average crystallite size (d) was calculated by applying the Scherrer's equation: d = 0.9 λ/β cosθ, where β is the full width at half maximum (FWHM) in radian of the peak with given (khl) value, λ = 1.5406 Å of the CuKα1 radiation and θ is the diffraction angle. Raman scattering experiments were

3. Results and discussion 3.1. Synthesis and characterization of Sm2O3 nanorods The simple hydrothermal synthesis process followed for synthesizing Sm2O3 nanorods is shown in Scheme 1. CTAB was used as

Scheme 1. Schematic view of the synthesis of Sm2O3 nanorods. 794

Applied Surface Science 487 (2019) 793–800

S. Rasouli Jamnani, et al.

a)

b)

2 µm

Fig. 1. a) SEM image of the synthesized Sm2O3 sample; b) EDX analysis.

200 nm

500 nm Fig. 2. High magnification SEM image of the synthesized Sm2O3 sample.

template (capping agent), with aim to control the final shape and size of the synthesized metal oxide particles. The presence of this organic molecule has the scope to alter the free energy of particle facets [21,22]. The difference of surface energy at each particle face leads to the coalescence of primary particles in specific crystallographic orientation, privileging in this case the one-dimension growth. By following this procedure, we obtained Sm2O3 with an exclusive nanorod structure (see below the attached SEM micrographs). Understanding the mechanism related to morphology control is out of the scope of this paper; more information can be found in previous papers in the literature [18]. The morphological and microstructural features of Sm2O3 sample have been investigated using SEM-EDX, XRD and micro Raman techniques. Low magnification SEM image reported in Fig. 1a reveals clearly the nanorods structure. EDX analysis, shown in Fig. 1b, highlight the good purity of the product obtained, showing only the characteristics peaks corresponding to Sm and O elements. From data of elemental analysis (Sm = 38.95% and O = 61.05%) we draw further information which infer that the synthesized nanorods exhibit the almost stoichiometric Sm2O3 composition. High magnification SEM images of the nanorods obtained are show in Fig. 2. It appears that the nanorods structure is the only one present. As regard the nanorods size, calculations indicate a mean length of about 400 nm and a mean diameter of about 80 nm. The microstructure of the synthesized nanorods was investigated by powder X-ray diffraction and micro Raman. Fig. 3a reports the XRD

pattern registered. The main peaks observed can be indexed as (222), (400), (440) and (622) of the Sm2O3 in the cubic crystal structure. On comparing these peaks with JCPDS data (No. 65-3183), it was confirmed that all the reflection peaks registered belong to this structure. Using the Scherrer equation, the average Sm2O3 crystallites' size is estimated to be 13.9 nm. Raman active phonons spectrum of Sm2O3 nanorods is reported in Fig. 3b. The observed lines at 351 cm−1, 423 cm−1 and 560 cm−1, were in agreement with ones predicted for C-type Sm2O3 structure [23]. The spectrum is also in agreement with that reported by other authors [24], confirming the Sm2O3 composition and the absence of hydroxylated bulk phases, such as Sm(OH)3. Photoluminescence is an important tool allowing to understand the relationship between structure defects and physicochemical properties. The PL spectrum of Sm2O3 sample excited under 340 nm wavelength is shown in Fig. 4. The observed PL spectrum is the results of overlapping of different bands [25]. A broad band of high intensity is centered around 400–450 nm. Two components, with peak maximum at 410 and 435 nm, can be resolved and assigned to near band edge (NBE) and strong emission originated from shallow donor levels of oxygen vacancies present in valence band levels of Sm, respectively [21]. At higher wavelengths, we note two smaller peaks at about 500 and 540 nm, related to oxygen vacancy defects. The intensity of latter emissions coming from oxygen defects is very low with respect to the emission band edge, indicating a good microstructural integrity and 795

Applied Surface Science 487 (2019) 793–800

S. Rasouli Jamnani, et al. 10

10

250 °C

300 °C

350 °C Resistance (Ω)

Resistance (Ω)

9

10

8

10

400 °C

7

10

7x10

8

6x10

8

5x10

8

4x10

8

10100 10200 10300 10400 10500 Time(s)

5000

10000

15000

Time(s)

Resistance (Ω)

Fig. 5. Response of NRs-Sm2O3 sensor, at different operating temperatures, to pulses of CO at different concentration. In the inset is represented the resistance variation of the sensor, operated at 300 °C, to a pulse of 50 ppm of CO. 10

12

10

11

10

10

a)

10

9

10

8

10

7

200

Series 1 - Dry air Series 2 - Dry air

250

Fig. 3. a) XRD pattern of Sm2O3 nanorods sample; b) Raman spectrum of the sample in the 200–700 cm−1 wave-number region.

300

350

400

Temperature (°C)

Response (R/R0)

1.6

b)

1.5

1.4

1.3

1.2

Fig. 4. Photoluminescence of Sm2O3 nanorods sample.

200

250

300

350

400

Temperature (°C)

crystallinity of the obtained Sm2O3 nanorods, thus confirming the XRD results.

Fig. 6. a) Baseline resistance of two NRs-Sm2O3 sensors at different temperatures; b) Sensor response to 5 ppm of CO computed at different operating temperatures.

3.2. Sensing tests

measurements up to tens of GΩ. Preliminary, it was tested that the bare electrode resistance exceed that of testing apparatus at all temperatures, confirming that the resistance values registered are related to

Sensing tests were conducted with the setup described in detail in the experimental, which allows to perform high resistance 796

Applied Surface Science 487 (2019) 793–800

Ω

Ω

Ω

S. Rasouli Jamnani, et al.

Fig. 7. a–c) Dynamic response of the NRs-Sm2O3 sensor to different concentrations of CO at the working temperatures of 250, 300 and 350 °C. d) Calibration curves of the NRs-Sm2O3 sensor operating at different temperatures.

temperature (Fig. 6a, b). As expected, the baseline resistance decreases with the increase of temperature (Fig. 6a). It is noteworthy that data reported have been collected by two different sensors, fabricated and tested in the same experimental conditions, so highlighting the good reliability in the fabrication process and sensing layer stability. The sensor response to CO shows instead a decreasing trend with increasing of operating temperature (Fig. 6b). This could be explained assuming that, in the range of temperature investigated, the desorption process of the oxygen species on the surface of Sm2O3 nanorods prevails on the adsorption process. Fig. 7 highlight the response to CO pulses of different concentrations registered at different temperatures. As before mentioned, below 250 °C, the sensor has a very high baseline resistance. Further, it has been recognized that at the temperature < 250 °C, the signal recovery (not shown) is very slow. At higher temperatures (Fig. 7a–c), the response and recovery times resulted to be more faster and baseline resistance is much lower. The calibration curves for Sm2O3 sensor to CO (Fig. 7d), demonstrate its very high sensitivity to low concentration of this gas. The response is almost linear for low concentrations (up to 5 ppm) and tends to saturate at higher concentrations. Hence, considering all these factors (the response, baseline resistance and dynamics characteristics), the optimal temperature for NRs-Sm2O3 sensor has been set at 250 °C. At this optimal operating temperature the sensor response, S, at 5 ppm of CO is equal to 1.4. The limit of detection (LOD) at S/N = 3 is down to 1 ppm, and its fast dynamics is demonstrated by the short response/recovery time (35 s and 110 s, respectively).

Sm2O3 sensing layer. The electrical and sensing properties of the Sm2O3 nanorods-based sensor toward CO were first tested at different temperatures (Fig. 5). Due to the high electrical resistance of the Sm2O3 sensing layer registered decreasing the operating temperature, the interval range of temperature was in practices restricted between 250 and 400 °C. The response of the Sm2O3 nanorods sensor to pulses of CO at different concentration (2, 5, 10, 20 and 50 ppm) is well evident, too. The resistance of the NRs-Sm2O3 sensor increases in all conditions upon exposure to CO, suggesting that the Sm2O3 sensing layer behaves as a p-type semiconductor in all range of temperature investigated. The inset showed in Fig. 5, highlight the dynamic characteristics of one of these CO pulses. As already pointed out for the previous NPs-Sm2O3 sensor [13], this behavior can be explained as follows: first, in air, occurs the adsorption of negatively charged oxygen on the Sm2O3 nanorods, thus generating holes, h%, which represent the carriers for conduction:

½ O2(g) ↔ O−(ads) + h˙ When CO pulses are admitted into the sensor chamber, carbon monoxide reacts with oxygen ions present on Sm2O3 nanorods surface, releasing electrons into the bulk and, consequently, increasing the resistance, which represents the signal response of the sensor.

CO(g) + 2O−(ads) + 2 h˙ → CO2(g) Data obtained by above test, allowed us to evaluate the performances of Sm2O3 nanorods sensor as a function of the operating 797

Applied Surface Science 487 (2019) 793–800

Ω

Ω

S. Rasouli Jamnani, et al.

Fig. 8. Humidity effect at different temperatures on a) baseline resistance; b) response to CO of the NRs-Sm2O3 sensor.

The effect of humidity on the electrical characteristics and response to the target gas was also evaluated (Fig. 8a, b). Humidity is considered to represent one of the main problems for conductometric sensors. It is well known that water vapor, interacting with the surface of the metal oxide semiconductors can give negative effects, influencing strongly both the baseline resistance and the response to the target gas of these sensors. Anyway, data reported for NRs-Sm2O3 sensor, indicate that the baseline resistance registered at different temperatures in dry and humid air (50% RH) did not changed significantly (Fig. 8a). Further, also the sensor response in the presence of humid air is only slightly decreased (Fig. 8b). These behaviors suggest a low interference of water vapor; further, data obtained can be easily managed in order to correct the sensor response in different humid conditions, providing proper calibration to compensate for effects caused by changes in humidity [26]. The stability and signal reproducibility, as well as the selectivity toward the target gas, need also to be satisfied. Fig. 9a shows five successive response of Sm2O3 sensor toward pulses of CO (10 ppm) at the operating temperature of 250 °C. No obvious variations on sensor response were observed. In addition, the sensor recovered promptly and fully the baseline after removing the CO. The response to some different interferent gases are shown in Fig. 9b. The response is higher when the sensor is exposed to CO, compared to other gases, demonstrating promising characteristics for detecting selectively CO in real conditions.

Fig. 9. a) Reproducibility to five consecutive pulses of 10 ppm CO and b) selectivity tests of NRs-Sm2O3 sensor.

3.3. Sensing performances of Sm2O3 with different particles shape It is widely reported in the scientific literature, that the morphological characteristics (e.g., shape) of gas sensing material can affect largely the sensing properties [27–29]. To evaluate if this can play a role with Sm2O3-based sensors too, we report below a detailed comparison between the microstructural/morphological and sensing performances of different Sm2O3 nanostructures synthesized by us. In Fig. 10 is reported the hierarchical self-assembly nanoparticles of NPsSm2O3 sample previously reported for VOCs sensing [13], compared to nanorods morphology of NRs-Sm2O3 sample. In addition to the obvious different morphology observed, one should also paid attention to the different grains size displayed by two samples. The presence of larger grains on the NPs-Sm2O3 sample, due to large nanoparticles agglomeration, suggests also that this sample has a lower surface area compared to NRs sample one. A comparison regarding their microstructural characteristics is also reported below. XRD analysis, (see Fig. 11a), highlights the larger crystallite size of the NPs-Sm2O3 sample (28.1 nm vs. 13.9 nm). Further, there are some differences in the relative intensity of the diffraction peaks, suggesting that different facets are exposed on the surface of two 798

Applied Surface Science 487 (2019) 793–800

S. Rasouli Jamnani, et al.

Fig. 10. Typical SEM image of the synthesized a) NPs and b) NRs samples.

Fig. 12. Response of sensors NRs-Sm2O3 sensor and NPs-Sm2O3 sensor, to CO and ethanol at the concentration of 20 ppm.

wavenumber of 1148 cm−1, which can be explained by the larger anisotropy of the rod nanostructure. The effect of the particles shape of Sm2O3 on the sensor response vs. two gases, i. e. CO and ethanol, is then presented in Fig. 12. From this comparison, it appears clear that the nanorod morphology is more effective for CO sensing, respect to nanoparticles one. Indeed, we can observe not only a higher response to the desired CO target gas, but also a decrease of the response to ethanol, which in this way enhance the sensor selectivity. These characteristics highlight that the gas-sensing properties of Sm2O3-based gas sensor are largely dependent on the particles shape. To give a tentative explanation of this behavior, it can be assumed that nanorods provide more active sites for CO adsorption with respect to nanoparticles, so favoring the subsequent reaction with the adsorbed oxygen and consequently enhancing the sensor response to CO. This behavior is likely triggered by the different crystallographic facets exposed on the surface of the different shapes, which in turn also determine the number of atoms located at the edges or corners. These surface atoms have consequently different reactivity and this could explain the different response toward the tested gases [31,32]. On the other hand, gas sensitivity depends not only on particle shape but also on the particle size. Then, we are planning to synthesize Sm2O3 rod and spheres with comparable particle size with aim to investigate the intrinsic role of particle shape in the gas sensing performances. From the practical point of view, this demonstrate that carefully optimizing, among other factors, the shape of the Sm2O3 sensing materials, choosing for example the more appropriate preparation method or synthesis procedure, it is possible to attain the highest sensitivity and

Fig. 11. Comparison of a) micro-Raman and b) XRD of Sm2O3 hierarchical selfassembly nanoparticles and nanorods.

samples. Fig. 11b compare the Raman spectrum of the two samples. In the pattern of nanorods sample, the main vibration modes in the range 300–600 cm−1, display a blue shifts with respect to nanoparticles sample, which could be explained by the phonon confinement effect due to the smaller grain size of the NRs-Sm2O3 sample [30]. In addition, a multiphonon band appears with much higher intensity at a 799

Applied Surface Science 487 (2019) 793–800

S. Rasouli Jamnani, et al.

selectivity to the desired target gas. [13]

4. Conclusions In this study, Sm2O3 nanorods have been synthesized and tested for CO monitoring. Sm2O3 nanorods were found to be promising materials for monitoring very low concentration of CO (5–50 ppm) in air at the operating temperature of 250 °C. To our knowledge, the reported CO sensing properties of Sm2O3 nanorods, are much more better than ones previously reported for Sm2O3 sensors, which are much less sensitive and can detect CO only at higher temperatures. Further, the low LOD value of 1 ppm indicate that this sensor has superior performances in the monitoring of very low concentration of CO in air compared to the state of the art of pure rare-earth oxide CO sensors [33–35]. At last, we demonstrated that the sensitivity and selectivity of NRsSm2O3 conductometric sensor toward CO are higher respect to NPsSm2O3 one, thus indicating that conductometric sensors can be tailored by controlling the particles shape of the Sm2O3 sensing material. In summary, by employing a simple hydrothermal process in the presence of CTAB, Sm2O3 nanorods have been easily obtained ensuring that the most sensitive sites for carbon monoxide are present on their surface, and this has been exploited to maximize both the sensitivity and selectivity of the developed conductometric Sm2O3 CO sensor.

[14] [15]

References

[24]

[16]

[17]

[18]

[19]

[20]

[21]

[22] [23]

[25] [1] L. Eyring, Progress in the Science and Technology of the Rare Earths, 2 Pergamon Press Ltd, London, 1966. [2] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sensors Actuators B Chem. 192 (2014) 607–627. [3] F. Falsafi, B. Hashemi, A. Mirzaei, E. Fazio, F. Neri, N. Donato, S.G. Leonardi, G. Neri, Sm-doped cobalt ferrite nanoparticles: a novel sensing material for conductometric hydrogen leak sensor, Ceram. Int. 43 (2017) 1029–1037. [4] F.I. Shaikh, L.P. Chikhale, J.Y. Patil, I.S. Mull, S.S. Suryavanshi, Enhanced acetone sensing performance of nanostructured Sm2O3 doped SnO2 thick films, J. Rare Earths 35 (2017) 813–823. [5] C. Peng, J. Guo, M. Liu, Y. Zheng, T. Huang, D. Wu, Enhanced ethanol sensing properties based on Sm2O3-doped ZnO nanocomposites, RSC Adv. 4 (2014) 64093–64098. [6] D. Wang, J. Jin, D. Xia, Q. Ye, J. Long, The effect of oxygen vacancies concentration to the gas-sensing properties of tin dioxide-doped Sm, Sensors Actuators B Chem. 66 (2000) 260–262. [7] S. Chi Tsang, C. Bulpitt, Rare earth oxide sensors for ethanol analysis, Sensors Actuators B Chem. 52 (1998) 226–235. [8] C.R. Michel, A.H. Martínez-Preciado, N.L. López Contreras, Gas sensing properties of Nd2O3 nanostructured microspheres, Sensors Actuators B Chem. 184 (2013) 8–14. [9] B. Renganathan, D. Sastikumar, R. Srinivasan, A.R. Ganesan, Nanocrystalline samarium oxide coated fiber optic gas sensor, Mater. Sci. Eng. B 186 (2014) 122–127. [10] C.R. Michel, A.H. Martínez-Preciado, R. Parra, C.M. Aldao, M.A. Ponce, Novel CO2 and CO gas sensor based on nanostructured Sm2O3 hollow microspheres, Sensors Actuators B Chem. 202 (2014) 1220–1228. [11] www.epa.gov/iaq/co.html. [12] G. Neri, A. Bonavita, G. Micali, G. Rizzo, E. Callone, G. Carturan, Resistive CO gas sensors based on In2O3 and InSnOx nanopowders synthesized via starch-aided

[26]

[27] [28]

[29] [30] [31]

[32]

[33] [34]

[35]

800

sol–gel process for automotive applications, Sensors Actuators B Chem. 132 (2008) 224–233. S.R. Jamnani, H.M. Moghaddam, S.G. Leonardi, G. Neri, A novel conductometric sensor based on hierarchical self-assembly nanoparticles Sm2O3 for VOCs monitoring, Ceram. Int. 44 (2018) 16953–16959. G. Neri, First fifty years of chemoresistive gas sensors, Chemosensors 3 (2015) 1–20. D.N. Oosthuizen, D.E. Motaung, H.C. Swart, Selective detection of CO at room temperature with CuO nanoplatelets sensor for indoor air quality monitoring manifested by crystallinity, Appl. Surf. Sci. 466 (2019) 545–553. H. Liu, H. Sun, R. Xie, X. Zhang, K. Zheng, T. Peng, X. Wu, Y. Zhang, Substratedependent structural and CO sensing properties of LaCoO3 epitaxial films, Appl. Surf. Sci. 442 (2018) 742–749. R. Dhahri, M. Hjiri, L.El. Mir, A. Bonavita, D. Iannazzo, S.G. Leonardi, G. Neri, CO sensing properties under UV radiation of Ga-doped ZnO nanopowders, Appl. Surf. Sci. 355 (2015) 1321–1326. P. Ghosh, S. Kundu, A. Kar, K.V. Ramanujachary, S. Lofland, A. Patra, Synthesis and characterization of different shaped Sm2O3 nanocrystals, J. of Physics D: Applied Physics 43 (2010) 405401. Y. Lixiong, W. Dan, H. Jianfeng, T. Guoqiang, R. Huijun, Controllable synthesis of Sm2O3 crystallites with the assistance of templates by a hydrothermal calcination process, Mater. Sci. Semicond. Process. 30 (2015) 9–13. T.-D. Nguyen, D. Mrabet, T.-O. Do, Controlled self-assembly of Sm2O3 nanoparticles into nanorods: simple and large scale synthesis using bulk Sm2O3 powders, J. Phys. Chem. C 112 (2008) 15226. S. Ahlers, G. Müller, T. Becker, T. Doll, C.A. Grimes, E.C. Dickey, M.V. Pisho (Eds.), Factors Influencing the Gas Sensitivity of Metal Oxide Materials, in Encyclopedia of Sensors, vol. 3, Am. Scientific Publisher, 2006, pp. 413–447. G. Garnweitner, M. Niederberger, Organic chemistry in inorganic nanomaterials synthesis, J. Mater. Chem. 18 (2008) 1171. J.F. Martel, S. Jandal, A.M. Lejuss, Optical crystal field study of Sm2O3 (C- and Btype), J. Alloy and Compouds 275 (1998) 353–355. J.-Gill Kang, B.-Ki Min, Y. Sohn, Synthesis and characterization of Sm(OH)3 and Sm2O3 nanoroll sticks, J. Mater. Sci. 50 (2015) 1958–1964. A. Amali Roselin, N. Anandhan, G. Ravi, M. Mummoorthi, T. Marimuthu, Growth and characterization of Sm2O3 thin films by spin coating technique, Int. J. Chem. Tech. Res. 6 (2014) 5315–5320. J.H. Sohn, M. Atzeni, L. Zeller, G. Pioggia, Characterisation of humidity dependence of a metal oxide semiconductor sensor array using partial least squares, Sensors Actuators B Chem. 131 (2008) 230–235. C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors 10 (2010) 2088–2106. S. Tian, F. Yang, D. Zeng, C. Xie, Solution-processed gas sensors based on ZnO nanorods array with an exposed (0001) facet for enhanced gas-sensing properties, J. Phys. Chem. C 116 (2012) 10586–10591. M. Hjiri, L. El Mir, S.G. Leonardi, N. Donato, G. Neri, CO and NO2 selective monitoring by ZnO-based sensors, Nanomaterials 3 (2013) 357–369. F. Gu, S. Wang, H. Cao, C. Li, Synthesis and optical properties of SnO2 nanorods, Nanotechnology 19 (2008) 095708. A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. B. Geng, C. Fang, F. Zhan, N. Yu, Synthesis of polyhedral ZnSnO3 microcrystals with controlled exposed facets and their selective gas-sensing properties, Small 4 (2008) 1337–1343. D. Majumder, S. Roy, Development of low-ppm CO sensors using pristine CeO2 nanospheres with high surface area, ACS Omega 3 (2018) 4433−4440. N. Izu, S. Nishizaki, T. Itoh, M. Nishibori, W. Shin, I. Matsubara, Gas response, response time and selectivity of a resistive CO sensor based on two connected CeO2 thick films with various particle sizes, Sensors Actuators B Chem. 136 (2009) 364–370. S.M.A. Durrani, M.F. Al-Kuhaili, I.A. Bakhtiari, Carbon monoxide gas-sensing properties of electron beam deposited cerium oxide thin films, Sensors Actuators B Chem. 134 (2008) 934−939.