Rivo Randriatsiferana

Yajian Gan

Fayu Wan

Wenceslas Rahajandraibe

Rémy Vauché

Nour M Murad

Blaise Ravelo

Study and experimentation of a 6-dB attenuation low-pass NGD circuit

Keywords: Circuit theory, low-pass NGD function, negative group delay (NGD), NGD analysis, passive topology, time-domain measurement

Introduction

The delay effects are susceptible to limit certain electronic, electrical and automatic system performances [START_REF] Pal | Delay-Dependent Stability Analysis of Discrete Time Delay Systems with Actuator Saturation[END_REF][START_REF] Hu | Optimal Tracking Performance of NCSs with Time-delay and Encoding-decoding Constraints[END_REF][START_REF] Hwang | Slope Interconnect Effort: Gate-Interconnect Interdependent Delay Modeling for Early CMOS Circuit Simulation[END_REF][START_REF] Kang | A global delay model for domino cmos circuits with application to transistor sizing[END_REF][START_REF] Ravelo | Delay Modelling of High-Speed Distributed Interconnect for the Signal Integrity Prediction[END_REF][START_REF] Groenewold | Noise and Group Delay in Active Filters[END_REF][START_REF] Myoung | Effect of Group Delay in RF BPF on Impulse Radio Systems[END_REF]. The control systems are literally dependent to the delay [START_REF] Pal | Delay-Dependent Stability Analysis of Discrete Time Delay Systems with Actuator Saturation[END_REF] and performances of network control systems with energy constraint can be degraded by delay [START_REF] Hu | Optimal Tracking Performance of NCSs with Time-delay and Encoding-decoding Constraints[END_REF]. The delay is modelled in electronic components such as CMOS circuits [START_REF] Hwang | Slope Interconnect Effort: Gate-Interconnect Interdependent Delay Modeling for Early CMOS Circuit Simulation[END_REF] and Printed Circuit Board (PCB) electrical interconnect since it is one of the main parameters affecting the signal integrity [START_REF] Kang | A global delay model for domino cmos circuits with application to transistor sizing[END_REF][START_REF] Ravelo | Delay Modelling of High-Speed Distributed Interconnect for the Signal Integrity Prediction[END_REF]. In addition to the noise effects, the Group Delay (GD) remains an open problem degrading communication data [START_REF] Groenewold | Noise and Group Delay in Active Filters[END_REF][START_REF] Myoung | Effect of Group Delay in RF BPF on Impulse Radio Systems[END_REF]. To overcome this issue, delay insensitive circuits using time sharing technique have been proposed in [START_REF] Li | Optimised Synthesis of Delay-Insensitive Circuits Using Time-Sharing[END_REF]. However, delay functions have been developed for certain electronic systems [START_REF] Narasimhan | Channel Blind Identification Based on Cyclostationarity and Group Delay[END_REF][START_REF] Heyde | Theoretical Methodology for Describing Active and Passive Recirculating Delay Line Systems[END_REF][START_REF] Vemagiri | Transmission Line Delay-Based Radio Frequency Identification (RFID) Tag[END_REF][START_REF] Wijenayake | RF Analog Beamforming Fan Filters Using CMOS All-Pass Time Delay Approximations[END_REF][START_REF] Alves | A time-delay technique to improve GBW on negative feedback amplifiers[END_REF][START_REF] Gau | Global exponential stability for uncertain bidirectional associative memory neural networks with multiple time-varying delays via LMI approach[END_REF]. For example, by exploiting the GD, a channel blind identification was proposed in [START_REF] Narasimhan | Channel Blind Identification Based on Cyclostationarity and Group Delay[END_REF]. In addition, electronic devices with delay lines have been designed in [START_REF] Heyde | Theoretical Methodology for Describing Active and Passive Recirculating Delay Line Systems[END_REF][START_REF] Vemagiri | Transmission Line Delay-Based Radio Frequency Identification (RFID) Tag[END_REF] and radio frequency filters [START_REF] Wijenayake | RF Analog Beamforming Fan Filters Using CMOS All-Pass Time Delay Approximations[END_REF], and amplifiers [START_REF] Alves | A time-delay technique to improve GBW on negative feedback amplifiers[END_REF] based on the time delay function have been proposed too. To face up the delay effect, a compensation technique using a negative group delay (NGD) function has been initiated in [START_REF] Ravelo | Recovery of Microwave-Digital Signal Integrity with NGD Circuits[END_REF]. The principle of NGD compensation is illustrated in Fig. 1 by considering a delay function which delays of τ the t-time dependent input signal Vin. This delay can be reduced to zero using a NGD function introducing a delay equal to -τ. The global cascaded system also generates an output Vout in phase to Vin. Moreover, NGD circuits have been proposed for bilateral gain compensation and broadband switch-less bi-directional amplifier [START_REF] Kandic | Bilateral Gain-Compensated Negative Group Delay Circuit[END_REF][START_REF] Meng | A Broadband Switch-Less Bi-Directional Amplifier with Negative-Group-Delay Matching Circuits[END_REF]. To understand this NGD function, a brief state of the art is described in the following paragraph. The counterintuitive and unfamiliar NGD effect have been experimented with lumped electronic circuits since 1990s to 2010s [START_REF] Mitchell | Causality and Negative Group-delays in a Simple Bandpass Amplifier[END_REF][START_REF] Mitchell | Negative Group-delay and 'Fronts' in a Causal Systems: An Experiment with Very Low Frequency Bandpass Amplifiers[END_REF][START_REF] Nakanishi | Demonstration of Negative Group-delays in a Simple Electronic Circuit[END_REF][START_REF] Kitano | Negative Group-delay and Superluminal Propagation: An Electronic Circuit Approach[END_REF][START_REF] Munday | Superluminal Time Advance of a Complex Audio Signal[END_REF][START_REF] Jian-Wu | Time-domain nature of group delay[END_REF][START_REF] Wan | Time-Domain Experimentation of NGD Active RC-Network Cell[END_REF][START_REF] Mao | High fidelity of electric pulses in normal and anomalous cascaded electronic circuit systems[END_REF]. Nevertheless, this intriguing effect which induces negative delay [START_REF] Mitchell | Causality and Negative Group-delays in a Simple Bandpass Amplifier[END_REF][START_REF] Mitchell | Negative Group-delay and 'Fronts' in a Causal Systems: An Experiment with Very Low Frequency Bandpass Amplifiers[END_REF][START_REF] Nakanishi | Demonstration of Negative Group-delays in a Simple Electronic Circuit[END_REF][START_REF] Kitano | Negative Group-delay and Superluminal Propagation: An Electronic Circuit Approach[END_REF][START_REF] Munday | Superluminal Time Advance of a Complex Audio Signal[END_REF] remains unfamiliar for most of electronic designers because of its counterintuitive property to operate with output signals in time advance [START_REF] Nakanishi | Demonstration of Negative Group-delays in a Simple Electronic Circuit[END_REF][START_REF] Kitano | Negative Group-delay and Superluminal Propagation: An Electronic Circuit Approach[END_REF] compared to input signals. This time-domain NGD effect is not in contradiction with the causality [START_REF] Mitchell | Causality and Negative Group-delays in a Simple Bandpass Amplifier[END_REF][START_REF] Mitchell | Negative Group-delay and 'Fronts' in a Causal Systems: An Experiment with Very Low Frequency Bandpass Amplifiers[END_REF] and for example the NGD effect has been experimented with audio signals [START_REF] Jian-Wu | Time-domain nature of group delay[END_REF]. Various NGD circuits based on lumped circuits have been designed [START_REF] Bukhman | On the Negative Delay Time of a Narrow-Band Signal as It Passes through the Resonant Filter of Absorption[END_REF][START_REF] Ahn | Synthesis for Negative Group Delay Circuits Using Distributed and Second-Order RC Circuit Configurations[END_REF][START_REF] Abuelma | A new CFOA-based negative group delay cascadable circuit[END_REF]. But so far, despite the work done on this extraordinary function, the NGD function is still not clearly interpreted by non-specialist due to the circuit topology complexity [START_REF] Bukhman | On the Negative Delay Time of a Narrow-Band Signal as It Passes through the Resonant Filter of Absorption[END_REF][START_REF] Ahn | Synthesis for Negative Group Delay Circuits Using Distributed and Second-Order RC Circuit Configurations[END_REF][START_REF] Abuelma | A new CFOA-based negative group delay cascadable circuit[END_REF]. Therefore, further studies are necessary for the understanding of this intriguing function as the common classical electronic functions such as filters, amplifiers, oscillators, phase shifters, etc. For this reason, a more general NGD theory has been initiated by considering the analogy with the linear filter behaviour [START_REF] Ravelo | Similitude between the NGD function and filter gain behaviours[END_REF][START_REF] Ravelo | On the low-pass, high-pass, bandpass and stop-band NGD RF passive circuits[END_REF][START_REF] Ravelo | First-order low-pass negative group delay passive topology[END_REF]. The identification of Low-Pass (LP) and band-pass NGD functions built with lumped circuits have firstly been proposed in [START_REF] Ravelo | Similitude between the NGD function and filter gain behaviours[END_REF][START_REF] Ravelo | On the low-pass, high-pass, bandpass and stop-band NGD RF passive circuits[END_REF]. For further understanding about the low-pass NGD concept, an NGD passive circuit operating with a 6dB attenuation is investigated in this paper which is organized in three main sections as follows. Section 2 theoretically investigates the LP NGD function and especially a LP NGD 6dB topology including its analysis, characterization and synthesis. Section 3 focuses on the results validation. Frequency domain analysis are firstly used to highlight the LP NGD behaviour. Then, time-domain simulations and experimentations are presented to verify the NGD function operation. Then, Section 4 concludes the paper.

Theory on the 6-dB NGD Circuit Under Investigation

The initiated circuit theory is aimed to indicate the basic element allowing to introduce the uncommon LP NGD function. Compared to the previous NGD papers published in literature, this paper focuses on the properties of a low-pass NGD passive circuit operating with a 6dB attenuation.

General specifications of LP NGD function

To be familiar with NGD systems, a simple and easy to understand representation is necessary. The present subsection addresses the theoretical approach related to the LP NGD circuit design. (1) By denoting the angular frequency ω, the associated magnitude and phase can respectively be written as follows:

Recall on the circuit and system parameter definitions

( ) ( ) N ω N jω = (2)
 

( ) arg ( ) φ ω N jω =
(3) and the associated group delay (GD) is given by:

() () φω GD ω ω - =  . ( 4
)

Guideline design of LP NGD circuit

Generally, the ideal frequency responses of LP NGD system can be represented as illustrated in Fig. 3. Figure 3(a) shows the ideal group delay (GD) response of a LP NGD function and Fig. 3(b) shows the ideal magnitude response. It is important to underline that the LP NGD should not be confused with the classical LP filter. Compared to usual system analysis, the NGD analysis focuses on the GD response but not the magnitude response. Thus, a LP NGD function is a system having a NGD for low frequencies. However, the NGD effect is obtained for low frequencies with passive circuits at the cost of an attenuation of N0 in the NGD bandwidth BWNGD. The main specifications of LP NGD function are also the GD value and the NGD cut-off frequency ωn defined as follows:

0 ( 0) 0 GD GD ω =   (5)

Recall on the circuit and system parameter definitions

Because of its counterintuitive effect, the NGD function remains so far as an unfamiliar electronic function. Therefore, an academic design method should be initiated to open the NGD engineering to non-specialists. Fig. 4 introduces the basic actions to be fulfilled during the design of a LP NGD circuit.

Passive circuit-based LP NGD analysis

The LP NGD analysis consists in demonstrating the possibility to generate a GD with negative value in function of the circuit parameters.

NGD properties

Fig. 5 presents the simple investigated LP NGD circuit. Its passive topology consists of a RCparallel network followed by a resistive load R. This circuit looks as a basic familiar passive cell but few investigations have been made to illustrate the possibility of generating LP NGD function from such a simple topology.

Figure 5. Passive circuit under study.

Considering the circuit shown in Fig. 5, the VTF introduced in (1) can be expressed as follows:

() 1 R Ns R R RCs = + + . ( 7
)

Analytical investigation on the NGD circuit frequency responses

After analytical calculation, it can easily demonstrate that this VTF can be expressed as the canonical form:

+ = + (8)
with:

τ=RC > 0 (9)
Thus, the magnitude and phase frequency responses of the passive circuit can respectively be written as follows:

( ) ( )

2 2 1 ( ) ( ) 4 τω N ω N jω τω + == + (10)
( ) ( )

( ) atan atan 2 φ ω τω τω =- . ( 11
)
The GD of the circuit shown in Figure 4 is given by:

τ τω GD ω τω τω  -  =     ++     . ( 12
)

NGD characteristics

The NGD characteristics depends on the specifications initially introduced in Fig. 3. The calculation of these characteristics as a function of the considered circuit parameters is proposed in this subsection.

Analysis of the 6-dB NGD circuit

From the VTF expressed in [START_REF] Li | Optimised Synthesis of Delay-Insensitive Circuits Using Time-Sharing[END_REF], the NGD analysis can be elaborated. The associated magnitude and GD at very low frequencies (ω≈0) are given by:

00 1/ 2 6 dB N N dB - =  = - (13) 0 2 τ GD =- (14)
It means that at very low frequencies, the RC-network presents a GD always negative for any parameters R and C. The LP NGD cut-off frequency, which is equal to the NGD bandwidth BWNGD for a LP NGD circuit, can be expressed as follows: (16)

NGD circuit design equations

The synthesis equation of this 6-dB LP NGD circuit aims to calculate the parameter R or C from the desired value of GD0 equal to τn < 0. By fixing for example the resistor value R, the capacitor can be computed from the equation:

2 n τ C R - = . ( 17
)
Table 1 addresses the range of values of RC and considered GD parameters. Then, the abacuses of C=f(R,τn) displayed in Fig. 6(a) and C=f(R,ωn) shown in Figure 6(b) indicate the value of calculated capacitor in function of desired:

• GD value τn supposed between -1 µs to -10 ns,

• and available resistance R supposed between 100 Ω to 1 kΩ.

It can be underlined that the capacitor element varies from Cmin=20 pF to Cmax=20 nF for the considered resistor and GD.

Resistor GD0 Capacitor

Min.

Rmin=100 Ω τn-min=-1 µs Cmin=20 pF Max.

Rmax=1 kΩ τn-max=-10 ns Cmax=20 nF Table 1. Resistor, NGD and capacitor range of values. To verify the validity of the NGD theory, simulations and experimental investigations are discussed in the next section.

Validation Results

The present section introduces the validation of the 6-dB LP NGD function theorized in the previous section. The results are based on simulations run with the commercial tool ADS® electronic circuit design tool from Keysight Technologies® and measurements with discrete components soldered on a Printed Circuit Boards (PCBs).

Description of the 6-dB NGD circuit proof-of-concept

Following the NGD design flow introduced earlier in Fig. 4, a proof of concept circuit has been designed thanks to the theoretical analysis previously established. Accordingly, the LP 6-dB NGD circuit has been simulated in the ADS® schematic environment as displayed in Fig. 7(a) and a photograph of the fabricated prototype is shown in Fig. 7(b). This circuit has been implemented on a classical FR4 dielectric substrate presenting parameters given in Table 2. The PCB conductor traces have been obtained using copper.

Components

Discussion on results in frequency domain of the 6-dB LP NGD prototype

To perform the simulations, an NGD circuit have been designed to operate with a NGD value equal to τn=-0.3 µs. The corresponding LP NGD circuit have been implemented with R=330 Ω and C=1.82 nF. After AC simulations from DC to 2 MHz, frequency domain results have been plotted. Figure 8 presents the magnitude and the GD of the NGD circuit prototype. As expected, a LP NGD behavior is realized with GD=-0.3 µs at very low frequencies. The magnitude and GD at very low frequencies confirms the 6dB LP NGD theoretical analysis. Moreover, the NGD cut-off frequency fn=ωn/(2π)=375 kHz agreed very well with equation (15).

Discussion on time domain results of the 6-dB LP NGD prototype

Simulation and experimental investigations have been realized to validate the previously introduced theoretical analyses.

Simulated results

A typically arbitrary shape smoothed signal is necessary to demonstrate the LP NGD function in the potential applications. To realize the transient analysis, a sinc test input signal have been defined as in [32]:

( ) max ( ) sinc 2 / in v t V π t T =   (17)
with sinc(x)=sin(x)/ x which is equal to 1 when x is equal to 0. This signal was injected as input of the previously designed LP NGD circuit. The sinc pulse width has been set using T=2 µs and the amplitude Vmax has been set to 1 V. Fig. 9(a) plots input signal and output signal, this last being obtained using analytical calculations and ADS® simulations. The calculated and simulated NGD outputs are in very good correlation. For further understanding on such matching, a cross correlation between the input and output versus the input signal bandwidth has been examined. It is very important to underline that the input and output present a very good integrity with almost negligible distortion despite the signal advance aspect. Table 3 indicates the cross correlation between the LP NGD circuit input and output versus the sinc pulse width.

T (µs) 1 2 3 5

x-correlation (%) 94.7 97.25 98.5 99.4

Table 3. Correlation Coefficient Between NGD Circuit Input and Output Versus Sinc Pulse-Width.

Measurement results

The configuration of the experimental setup is illustrated in Fig. 10. The time domain measurement has been settled in order to plot simultaneously the NGD circuit input and output signals, vin and vout, respectively. It has been realized with an arbitrary waveform signal generator and a digital oscilloscope as indicated in

Conclusion

The basic theory allowing to understand and to be familiar to the NGD function has been established. The NGD design method and analyses have been performed on an innovative LP NGD passive function. The investigations demonstrate the possibility to obtain a NGD at the cost of a 6-dB attenuation only. The feasibility of the theory has been illustrated with the help of a commercial tool simulation and experimental validations, these ones being obtained using a prototype built with discrete passive component soldered on a PCB.

Acknowledgement

Figure 1 .

 1 Figure 1. Delay compensation principle with NGD function.

Figure 2

 2 Figure 2 introduces a the LP NGD system having a Laplace transfer function N(s) which links the input signal Vin to the output signal Vout in Laplace domain. As shown in the right part of

Fig. 2 ,

 2 Fig. 2, the output signal can appear in advance in the time domain compared to the input signal thanks to the NGD function.

Figure 2 .

 2 Figure 2. LP NGD system black box and behavior of transient input, and output signals.

Figure 3 .

 3 Figure 3. (a) GD and (b) magnitude ideal response of LP NGD function.

Figure 4 .

 4 Figure 4. LP NGD circuit design flow.

Figure 6 .

 6 Figure 6. Abacuses of capacitor value versus (a) (R,τn) and (b) (R,ωn).

Figure 7 .

 7 Schematic and photograph of LP NGD 6-dB prototype.

Figure 8 .

 8 Figure 8. Frequency responses of the considered LP NGD circuit.

Fig. 9 (

 9 b) shows normalized input and output signals in order to illustrate the time advance effect induced by the LP NGD function. It can be emphasized from this normalized plot that the output is in advance of about 0.3 µs. Moreover, the input and output signals present a very well-correlative behavior with negligible distortion when a sync pulse is considered. It is noteworthy that the apparition of the signal advance depends on the matching between the NGD and input signal bandwidths.

Figure 9 .

 9 Figure 9. LP NGD POC transient simulation results.

•

  Fig. 11(a) for a Gaussian input generated with a Pulse Repetition Frequency (PRF) of 50 kHz, • Fig. 11(b) for a Sinc input generated with a PRF of 20 kHz and, • Fig. 11(c) for a Sinc input generated with a PRF of 10 kHz.

Figure 11 .

 11 Figure 11. Transient measurement results with: (a) Gaussian input pulse having a PRF equal to 50 kHz, (b) sinc input pulse having a PRF equal to 20 kHz and (c) sinc input pulse having a PRF equal to 10 kHz .

Table 2 .

 2 Proof-of-concept circuit constituting material parameters.

		Description	Parameter	Value
		Material	FR4	-
	Dielectric substrate	Relative permittivity Loss tangent	εr tan(δ)	4.5 0.12
		Thickness	h	1.6 mm
		Material	Copper (Cu)	-
	Metallization	Thickness	t	35 µm
		Conductivity	σ	58 MS/m

Table 4 .

 4 The prototype of LP NGD circuit with 5% tolerances R and C components are designated in this table.Figure 10. Experimental setup of NGD circuit time domain measurement.

	Description	References	Parameters
	Arbitrary function	Textronix AFG3102C DUAL	Sampling rate: 1 GS/s
	generator	CHANNEL	Bandwidth: 100 MHz
	Digital oscilloscope	Agilent infiniium MSO8104A	Sampling rate: 4 GS/s Bandwidth: 1 GHz
	NGD circuit	Resistor Capacitor	R=470 Ω C=0.56 nF

Table 4 .

 4 Correlation coefficient between NGD circuit input and output versus sinc pulsewidth. The measurement results with different types of input test signals presenting amplitude Vmax=1 V are displayed in Fig. 11. It is noteworthy that as predicted in previous simulations, normalized output signals are in advance compared to the input signals as illustrated in:

This research work was supported in part by NSFC under Grant 61971230 and 61601233, and in part by Jiangsu Distinguished Professor program and Six Major Talents Summit of Jiangsu Province (2019-DZXX-022), and in part by the Postgraduate Research & Practice Innovation Program of Jiangsu Province under Grant SJKY19_0974, and in part by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) fund.


K Aguir

Marc Bendahan

S Smiy
email: sabsmiy@gmail.com

M Bejar

E Dhahri

T Fiorido

La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 O 3 (0.00 ≤ 𝑥 ≤ 0.20) thin films-as ozone sensitive layers

Keywords: Perovskites, Drop coating, Co-doped, Thin films, Sensors, Ozone, 𝑠𝑢𝑏 -𝑝𝑝𝑚

La0.8Pb0.1Ca0.1Fe1-xCoxO3 thin films as ozone-sensitive layers

I-INTRODUCTION

Perovskite materials exhibit interesting physicochemical properties which make them promising for gas detection applications. 𝐿𝑎𝐹𝑒𝑂 3 , the mother compound has been found to be a good candidate for NO2 [START_REF] Yoon | The NO2 response of solid electrolyte sensors made using nano-sized LaFeO3 electrodes[END_REF], formaldehyde [START_REF] Zhang | Synthesis and characterization of In-doped LaFeO3 hollow nanofibers with enhanced formaldehyde sensing properties[END_REF], acetone [START_REF] Wang | Gas sensing with yolk-shell LaFeO3 microspheres prepared by facile hydrothermal synthesis[END_REF], ethanol [START_REF] Cao | Enhanced Ethanol Sensing Performance of Au and Cl Comodified LaFeO3 Nanoparticles[END_REF] and 𝑁𝐻 3 detection [START_REF] Chen | Acetone sensing properties and mechanism of nano-LaFeO3 thick-films[END_REF] 𝑒𝑡𝑐. To develop the detection properties of this functional material, it is necessary to perform numerous substitutions in the 𝐿𝑎 and 𝐹𝑒 sites in order to increase the response of these sensors [6 -8]. The need for ozone sensors is increasing in the fields, of textile, food, water treatment, pharmaceutical products and the purification of gases [START_REF] Rocha | Novel ozone gas sensor based on ZnO nanostructures grown by the microwave-assisted hydrothermal route[END_REF]. The exposure to ozone at concentrations greater than 120 𝑝𝑝𝑏 can cause serious health problems (eye irritation, lung damage and respiratory irritation). Moreover, the air quality can be classified referring to the Ozone amount in the atmosphere, as very good (0 -50 𝑝𝑝𝑏) and poor (𝐶 > 50 𝑝𝑝𝑏) [START_REF] Rocha | Novel ozone gas sensor based on ZnO nanostructures grown by the microwave-assisted hydrothermal route[END_REF]. In this respect, the Ozone concentration monitoring in our atmosphere is becoming a serious issue [START_REF] Fan | CO2 gas sensors based on La1-xSrxFeO3 nanocrystalline powders[END_REF][START_REF] Xu | Charge disproportionation in La1-xCaxFeO3-δ (x = 0.4 and 0.5) investigated by mechanical spectroscopy[END_REF]. Recently, several attempts have been made to the development of new materials devoted to ozone gas sensing. In particular, M. Mori et al. have found that introducing Co in place of Fe increases the response of 𝑆𝑚𝐹𝑒𝑂 3 and 𝑆𝑚𝐹𝑒 0.9 𝐶𝑜 0.1 𝑂 3 sensor and the results showed that the substitution of 𝐹𝑒 by 𝐶𝑜 was accompanied by a decrease in the sensor operating temperature [START_REF] Mori | Effect of VOC on ozone detection using semiconducting sensor with SmFe1-xCoxO3 perovskite-type oxides[END_REF].

There are many interests in perovskite structured compounds (with general formula ABO3) due to their colossal magnetoresistance effects [START_REF] Köferstein | Magnetic and optical investigations on LaFeO3 powders with different particle sizes and corresponding ceramics[END_REF], their unique catalytic action [START_REF] Rezlescu | Characterization and catalytic properties of some perovskites[END_REF] and their gas sensing applications [START_REF] Alharbi | Acetylene-and Ethylene-Sensing Mechanism for LaFeO3-Based Gas Sensors: Operando Insights[END_REF]. Ferrite LaFeO3 materials have a great effect on technological development and are widely used in various domains, such as magnetic refrigeration and gas sensing application. The structural characteristics of these compounds can lead to a range of useful physical properties, such as mixed electric, ionic conductivity and ordered magnetism at elevated temperature. While at present, most of the studied perovskites related oxide compounds, such as the rare earth strontium cobaltates, have been investigated for use as cathode materials for solid oxide fuel cells SOFC, magnetic sensors and for high temperature oxygen separation [START_REF] Benali | Magnetic, Raman and Mössbauer properties of double-doping LaFeO3 perovskite oxides[END_REF]. Furthermore, it has been reported that LaFeO 3 (𝐿𝐹𝑂) compound is characterized by higher resistance, which presents a disadvantage for the gas sensing application. In this regard, it has also been reported that the partial substitution of 𝐿𝑎 by lower valent cation elements such as 𝑃𝑏 [START_REF] Song | The structure, electrical and ethanol-sensing properties of La1-xPbxFeO3 perovskite ceramics with x ≤ 0.3[END_REF] and 𝐶𝑎 [START_REF] Kong | Gas-sensing property and mechanism of CaxLa1-xFeO3 ceramics[END_REF] can reduce the resistance of the (𝐿𝐹𝑂) compound. Moreover, and using Kroger-Vink defect notations, its charge carries are holes ℎ . , which are produced by the ionization of 𝐿𝑎 3+ ions vacancy defect [𝑉 𝐿𝑎 𝑥 ] and the ionization reaction is (0.00 ≤ 𝑥 ≤ 0.20) sensors using pellet forms increased the response in the presence of 𝑁𝐻 3 -gas compared to the mother compound (𝑥 = 0.00) [START_REF] Song | The structure, electrical and ethanol-sensing properties of La1-xPbxFeO3 perovskite ceramics with x ≤ 0.3[END_REF].

(𝑉 𝐿𝑎 𝑥 → 𝑉 𝐿𝑎 ,,, + 3ℎ .
Moreover, the responses of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) sensors found in this study were higher than the response of 𝐿𝑎 0.885 𝑃𝑏 0.005 𝐶𝑎 0.11 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 2.95 (0.00 ≤ 𝑥 ≤ 0.15) sensors found in our study [START_REF] Smiy | Correlation between structural, magnetic and gas sensor properties of L a 0.885 P b 0.005 C a 0[END_REF][START_REF] Saoudi | Structural and NH3 gas-sensing properties of La0.8Ca0.1Pb0[END_REF].

In parallel with our study and the study performed by H. compounds as an ozone sensor, for the first time, in the form of thin-film elaborated by the drop-coating method because, For this later application, the perovskite materials have been used in the pellet forms and have showed not enough sensitivity. Therefore, in order to increase their sensitivity, we considered that is necessary to deposit these materials, prepared by the sol gel method, by drop coating [START_REF] Li | Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection[END_REF]. This choice is based on the fact that the sensitivity is directly related to the thickness of the used material.

II-EXPERIMENTAL DETAILS

II-1) Preparation of perovskites nano-powders and thin films deposition

The nanocrystalline 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 have been dissolved in distilled water at 70 °𝐶. Then, the citric acid and the polyethylene glycol (𝑃𝐸𝐺) were added to the mixture to achieve polymerization and well homogenization. Finally, the obtained powder has been sintered at 300, 600 and 900°𝐶

for one day for each temperature.

Nowadays, there are many methods which are used to develop sensors. In this paper we use the drop coating because it is a chemical method and in this context F.L. Pennec et. al [START_REF] Pennec | CO2 detection by barium titanate deposited by drop coating and Screen-Printing Methods[END_REF] carried out a study about methods used in the elaboration of BaTiO3 perovskite materials, authors mentioned that response is more important in case of Drop coating method compared to Screen printing method in presence of 400 ppm of CO2 gas, added to that the recovery time in case of drop coating method is equal to 4min but in case of Screen printing method this time is found to be equal to 5min.

The deposition of the thin film over the 𝑆𝑖/𝑆𝑖𝑂 2 substrate with inter-digitated Ptelectrodes has been performed using the drop-coating method. The coating suspensions have been obtained by mixing 20 𝑚𝑔 of powders from each compound with 20 𝑚ℓ of 2propanol, followed by 10 𝑚𝑖𝑛 ultrasonication at 40 °𝐶 for a good homogenization. The coating sample has been performed drop by drop over the substrate using a micropipette.

The thin films were obtained after the deposition of 8 drops which correspond to 20 µℓ.

The coating process has been followed by heating samples for 1 ℎ at 500 °𝐶.

Film thickness is measured after annealing using a DEKTAK 6M mechanical profilometer.

Ten measurements are made at different locations and the average thickness is found as 300 𝑛𝑚.

In order to ensure the adsorption of our powder over the 𝑆𝑖/𝑆𝑖𝑂 2 substrate, a view of the deposited thin film has been carried out by an optical microscope with magnifications of 800 µ𝑚 and 150 µ𝑚 (Figure 1).

II-2) Ozone sensing measurements

Ozone-gas is generated using an UV lamp (Stable Ozone Generator UVP/185nm). Moreover, the presence of other peaks characteristic of 𝐹𝑒 3 𝑂 4 secondary phase has been identified in this study using "𝑋0 Pert High Score Plus" program. This phase was found to crystallize in the orthorhombic structure with 𝑃𝑏𝑐𝑚 space group [START_REF] Rocha | Novel ozone gas sensor based on ZnO nanostructures grown by the microwave-assisted hydrothermal route[END_REF][START_REF] Smiy | Correlation between structural, magnetic and gas sensor properties of L a 0.885 P b 0.005 C a 0[END_REF]. The refinement parameters are gathered in Table 1.

The crystallite size of the prepared samples was calculated using the Williamson-Hall's equation (𝑊 -𝐻) [START_REF] Saoudi | Structural and NH3 gas-sensing properties of La0.8Ca0.1Pb0[END_REF] (𝐸𝑞. 1):

𝛽𝑐𝑜𝑠() = 𝑘 𝐷 𝑊-𝐻 ⁄ + 4 sin( 𝜃) (𝐸𝑞. 1)
Where:  is the Bragg diffraction angle, 𝛽 defines the effective strain, 𝑘 is the shape factor and the 𝐷 𝑊-𝐻 presents the average crystallites size. a smaller grain size had a higher activity, which could facilitates the chemisorption of oxygen [START_REF] Benali | Structural, electrical and ethanol sensing properties of double-doping LaFeO3 perovskite oxides[END_REF].

Hence, the decrease of the nanometric crystallite size with increasing the cobalt content on prepared samples has been expected to increase the response.

The SEM micrographs of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) sensors confirm the homogeneous microstructure, with the presence of a porous character of the surface The performances of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) compounds such as response, stability, response time, and recovery time are influenced by the quantity and quality of oxygen adsorbed on the surface of our sensors [START_REF] Liu | Semiconducting gas sensor for ethanol based on LaMgxFe1-xO3 nanocrystals[END_REF]. On the surface, atmospheric oxygen can be adsorbed very easily, and atoms or molecules become chemisorbed ions 𝑂 -or 𝑂 2 -. The increase of the temperature causes the rise of the state of oxygen on the surface to undergo the reactions described in the Eq. 2 to Eq. 5 and this phenomenon was demonstrated theoretically using the 𝑆𝑛𝑂 2 model [START_REF] Pulkkinen | Kinetic Monte Carlo Simulation of Oxygen Exchange of SnO2 Surface[END_REF].

In this context, the thin films present an increase in hole concentration because when oxygen species are adsorbed in 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) materials, which are p-type semiconductors, a depletion region of electrons was formed [START_REF] Xu | Enhanced NH3 gas sensing performance based on electrospun alkaline-earth metals composited SnO2 nanofibers[END_REF].

𝑂 2(𝑔𝑎𝑠) ↔ 𝑂 2(𝑎𝑑𝑠) (𝐸𝑞. 2) 𝑂 2(𝑎𝑑𝑠) + 𝑒 -↔ 𝑂 2(𝑎𝑑𝑠) - (𝐸𝑞. 3) 𝑂 2(𝑎𝑑𝑠) - + 𝑒 -↔ 2𝑂 𝑎𝑑𝑠 - (𝐸𝑞. 4) 𝑂 (𝑎𝑑𝑠) - + 𝑒 -↔ 𝑂 (𝑎𝑑𝑠) 2- (𝐸𝑞. 5)
The chemical reactions process between free adsorption site 𝑆 and the triatomic gases, as Ozone (O3), are illustrated according to the following reactions [START_REF] Smiy | Ozone detection based on nanostructured La0.8Pb0.1Ca0.1Fe0.8Co0.2O3 thin films[END_REF]:

𝑆 + 𝑂 3 → (𝑆 -𝑂) + 𝑂 2 (𝐸𝑞. 6) (𝑆 -𝑂) + 𝑒 -→ (𝑆 + 𝑂) - (𝐸𝑞. 7)
The sensing responses (𝑆 = 𝑅 𝑎 /𝑅 𝑔 ) of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20)

thin-films exposed to various concentrations of ozone were compared to those of several materials such as metal-oxide-semiconductor [START_REF] Guerin | A dynamic response model for the WO3-based ozone sensors[END_REF] and other perovskite materials [START_REF] Mori | Effect of VOC on ozone detection using semiconducting sensor with SmFe1-xCoxO3 perovskite-type oxides[END_REF][START_REF] Mastelaro | Ozone gas sensor based on nanocrystalline SrTi1-xFexO3 thin films[END_REF]. As seen in Table 4, our sensors present a higher response compared to other materials in all ranges of ozone concentrations. Furthermore, 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) sensors are considered as 𝑠𝑢𝑏 -𝑝𝑝𝑚 since the responses in the presence of 50 𝑝𝑝𝑏 were significantly better than those found by the other sensors [START_REF] Mori | Effect of VOC on ozone detection using semiconducting sensor with SmFe1-xCoxO3 perovskite-type oxides[END_REF][START_REF] Mastelaro | Ozone gas sensor based on nanocrystalline SrTi1-xFexO3 thin films[END_REF][32][START_REF] Gurlo | 2O3 and MoO3-In2O3 thin film semiconductor sensors: interaction with NO2 and O3[END_REF][START_REF] Qu | Immunoassay systems based on immunoliposomes consisting of genetically engineered single-chain antibody[END_REF][START_REF] Sathiwitayakul | Ferrite materials produced from self-propagating high-temperature synthesis for gas sensing applications[END_REF][START_REF] Gaddari | A novel way for the synthesis of tin dioxide sol-gel derived thin films: application to 𝑂 3 detection at ambient temperature[END_REF][START_REF] Bejaoui | Theoretical and experimental study of the response of CuO gas sensor under ozone[END_REF][START_REF] Shoa | Discriminative and rapid detection of ozone realized by sensor array of Zn 2+ doping tailored MoS2 ultrathin nanosheets[END_REF].

Commonly, the response time defined is the time needed to reach 90 % of saturation, whereas the recovery time is the time needed to recover 10 % of the original resistance [START_REF] Waser | Nanoelectronics and Information Technology: Advanced Electronic Materials and Novel Devices[END_REF]. The study of the response and recovery times for our 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) thin films indicate significant values compared to other materials considered as potential gas sensors [START_REF] Mori | Ozone detection in air using SmFeO3 gas sensor for air quality classification[END_REF][START_REF] Chou | On the ammonia gas sensing performance of a RF sputtered NiO thin-film sensor[END_REF][START_REF] Chen | Characteristics of a Pt/NiO thin film-based ammonia gas sensor[END_REF]. The values of response(𝜏 𝑟𝑒𝑠𝑝 ) and recovery (𝜏 𝑟𝑒𝑐 ) times of our compounds are gathered in Table 5. It's clear that curves do not return perfectly to the base line which gives an uncertainty on 𝜏 𝑟𝑒𝑐 values but we put the recovery time in order to compare our values with those determined for other materials used in films forms for the detection of harmful gases.

IV-CONCLUSION

In this research paper, the ozone-sensing properties of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) thin films prepared by the drop-coating method have been investigated. The prepared sensors have demonstrated a good sensing performance. The results have shown that introducing cobalt instead of iron increased the response to 𝑂 3gas and decreased the operating temperature from 270 °𝐶 for 𝑥 = 0.00 to 170°𝐶 for 𝑥 = 0.20. Moreover, all the prepared compounds displayed a 𝑠𝑢𝑏 -𝑝𝑝𝑚 behavior. The studied samples, potentially, detected a lower limit of 50 𝑝𝑝𝑏 of ozone with high values of response and were 𝑆 = 1.025, 1.110, 1.586 and 1.753 for 𝑥 = 0.00, 0.05, 0.10 and 0.20, respectively. The finding confirms that the performances of the prepared thin films are a promising material for 𝑂 3 sensing applications. Further works will be extended to test other oxidizing gases such as 𝑁𝑂 2 in order to evaluate the selectivity of these sensors towards ozone gas.

  Firstly, dry air is passed through a quartz tube. The light up of the UV lamp radiation transformed some of the oxygen molecules into 𝑂 3 (ozone). The intensity of the UV radiation has been varied by shifting a shutter around the lamp. The different ozone concentrations were obtained in the range of 50 𝑝𝑝𝑏 to 400 𝑝𝑝𝑏 with a dry air flow rate maintained at 0.5 ℓ / 𝑚𝑖𝑛. A heating source was used to regulate the operating temperature. The applied 𝑑𝑐 voltage for sensors measurement was fixed to 1 𝑉 and the resistance was controlled by a keithley 4250 sourcemeter (Figure2 (a, b)).III-RESULTS AND DISCUSSIONIII-1) 𝑋𝑅𝐷 and EDS Studies:XRD patterns based on the use of 𝐶𝑢 -𝐾  radiation source of the prepared nanopowders 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) are used to identify the structure of our samples. The Rietveld refinement of the XRD patterns using the FullProf program, presented in Figure3. (𝑎), proves that all the samples are indexed in the orthorhombic system with 𝑃𝑛𝑚𝑎 space group. Besides, the substitution of iron by cobalt leads to a decrease in cell-parameters and consequently the decrease of the volume (Figure3. (𝑏)).

Figure. 5 ( 3 .

 53 Figure. 5 (𝑎).Moreover, the EDS-Mapping presented in Figures5 (b, c) Shows the existence of all the elements introduced in our samples, particularly, 𝐿𝑎, 𝑃𝑏, 𝐶𝑎, 𝐹𝑒, 𝐶𝑜, 𝑎𝑛𝑑 𝑂[START_REF] Smiy | Filmthichness influence on ozone detection of La0.8Pb0.1Ca0.1Fe1-xCoxO3 based sensors[END_REF].

Figure. 7

 7 Figure. 7 depicts the variation of the operating temperatures as a function of Co-content. The results display that introducing cobalt (Co) instead of iron (Fe) increases the operating temperatures of the sensors. These findings are in agreement with those found by M. Mori and al. where the substitution of Fe by Co in 𝑆𝑚𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 compounds decreased the operating temperature from 290 °𝐶 for 𝑥 = 0.00 to 150 °𝐶 for 𝑥 = 1.00 in the presence of 𝑂 3 -gas [10].

Figure 1 :Figure 2 (

 12 Figure 1: Optical view after drop coating of La 0.8 Pb 0.1 Ca 0.1 Fe 1-x Co x O 3 (0.00 ≤ 𝑥 ≤ 0.20) sensors of a magnifications equal to 800 µ𝑚 and 150 µ𝑚.

Figure. 2 (

 2 Figure. 2(𝑏) : The unit cell volume as a function of cobalt content (𝑥) of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) compounds.

Figure. 3 :

 3 Figure. 3 : Williamson-Hall plots of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) nanopowders prepared by the sol-gel method.

Figure. 4 (Figure. 4 (Figure. 5 (

 445 Figure. 4 (𝑎) : SEM-graphs of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) compounds prepared by the sol-gel method.

Figure. 5 (

 5 Figure. 5 (𝑏) : Ozone gas response of La 0.8 Pb 0.1 Ca 0.1 Fe 0.9 Co 0.1 O 3 (𝑥 = 0.10) sensor as a function of time at several temperatures (200, 220, 250 and 270 °𝐶).

Figure. 6 :

 6 Figure. 6 : Variation of the operating temperature as a function of Co-content of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) sensors.

Figure. 7 :Figure. 8 :

 78 Figure. 7 : 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (𝑥 = 0.00 𝑎𝑛𝑑 𝑥 = 0.05) sensors response (𝑆) to oxidizing gas (𝑂 3 ).

  ). When 𝐿𝑎 3+ is substituted by 𝑃𝑏 2+ , the ionization reaction of this for 𝑥 = 0.00 to 250 °𝐶 for 𝑥 = 0.05 in the presence of 𝑁𝐻 3 -gas[START_REF] Smiy | Correlation between structural, magnetic and gas sensor properties of L a 0.885 P b 0.005 C a 0[END_REF]. In a related study H.Saoudi et al. have reported that introducing Cobalt instead of Iron in 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂

	element is (𝑃𝑏 𝐿𝑎 𝑥 → 𝑃𝑏 𝐿𝑎 , + ℎ

. ) (where, 𝑃𝑏 𝐿𝑎 𝑥 is related to the point defect formed when 𝐿𝑎 3+ is replaced by 𝑃𝑏 2+ in the crystal state and ℎ . are the generated holes). As a result of [𝑃𝑏 𝐿𝑎 𝑥 ] [𝑉 𝐿𝑎 𝑥 ], the mobility of holes increases, and consequently the resistance of the doped compound decreases compared to the (𝐿𝐹𝑂) one

[START_REF] Song | Preparation and ethanol sensitivity of nanocrystalline La0.7Pb0.3FeO3-based gas sensor[END_REF]

.

In our previous published work related to this framework, we have demonstrated that introducing cobalt instead of Iron in 𝐿𝑎 0.885 𝑃𝑏 0.005 𝐶𝑎 0.11 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 2.95 (𝑥 = 0.00, 0.05, 0.10 and 0.15) compounds leads to the decrease of the operating temperature from 270 °𝐶

  [START_REF] Wang | Gas sensing with yolk-shell LaFeO3 microspheres prepared by facile hydrothermal synthesis[END_REF]

  The 𝐷 𝑊-𝐻 of 𝐿𝑎 0.8 𝑃𝑏 0.1 𝐶𝑎 0.1 𝐹𝑒 1-𝑥 𝐶𝑜 𝑥 𝑂 3 (0.00 ≤ 𝑥 ≤ 0.20) samples were obtained by the linear 𝐿𝑎 0.885 𝑃𝑏 0.005 𝐶𝑎 0.11 Fe𝑂 2.95 sensor which is used for 𝑁𝐻 3 sensing application[START_REF] Smiy | New perovskite compound La0.885Pb0.005Ca0.11FeO2.95 for gas sensing application[END_REF].In addition A.Benali et al. mentioned that their samples 𝐿𝑎 0.8 𝐶𝑎 0.2-𝑥 𝑃𝑏 𝑥 Fe𝑂 2.95 present a crystallites size included between 40.19 nm and 65.51 nm and usually nanocrystals with

variation of ( cos() vs. (4 * sin()) (Figure

. 4

). The 𝐷 𝑊-𝐻 values, obtained from the intercept of the linear adjustment values, are regrouped in Table

.

2. The results clearly indicate the decrease of the average crystallites size from 83.023 𝑛𝑚 for 𝑥 = 0.00 to 43.870 𝑛𝑚 for 𝑥 = 0.20. These values are close to those obtained with other compounds considered as nanometric materials for gas-sensing application

[22 -24]

. "As an example we can cited that in our ancient article, crystallite size is found to be equal to 52.11 nm in case of

Table 1 :

 1 The Rietveld refinement results of X-Ray diffraction (𝑋𝑅𝐷) data of La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 O 3 (𝑥 = 0.00, 0.05, 0.10 and 0.20) compounds.

Table 2 :

 2 Values of 𝐷 𝑊-𝐻 determined by Williamson-Hall method for La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 O 3 (𝑥 = 0.00, 0.05, 0.10 and 0.20) samples.

	x	0.00	0.05	0.10	0.20
	Space group	𝑃𝑛𝑚𝑎	𝑃𝑛𝑚𝑎	𝑃𝑛𝑚𝑎	𝑃𝑛𝑚𝑎
	a (Å)	5.5493	5.5431	5.5342	5.5182
	b/√𝟐 (Å)	5.5500	5.5390	5.5277	5.5137
	c (Å)	5.5394	5.5351	5.5283	5.5065
	V (Å 3 )	60.3140	59.3012	59.7922	59.2340
		x		𝑫 𝑾-𝑯 (𝒏𝒎)
		0.00		83.023
		0.05		75.231
		0.10		52.436
		0.20		43.870

Table 3 :

 3 The relationship between response (𝑆 = 𝑅 𝑎 𝑅 𝑔 ⁄ ) and temperature (𝑇) of La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 𝑂 3 (𝑥 = 0.00, 0.05, 0.10 and 0.20) based sensors during ozone exposing time of 1 𝑚𝑖𝑛 for a concentration equal to 100𝑝𝑝𝑏.

		𝒙 = 𝟎. 𝟎𝟎
	𝑻 (°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )	𝑻 (°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )
	200	1.018	250	1.017
	210	1.019	260	1.017
	220	1.016	270	1.022
	230	1.013	280	1.019
	240	1.013
		𝒙 = 𝟎. 𝟎𝟓
	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )
	200	1.042	250	1.064
	210	1.050	260	1 ,051
	220	1.052	270	1.051
	230	1.051	280	1.028
	240	1.056
		𝒙 = 𝟎. 𝟏𝟎
	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )
		1.201	240	1.160
		1.290	250	1.143
		1.254	260	1.140
		1.231	270	1.152
		1.191	280	1.198
		1.168
		𝒙 = 𝟎. 𝟐𝟎
	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )	𝑻(°𝑪)	𝑺(𝑹 𝒂 /𝑹 𝒈 )
		1.330	230	1.160
		1.345	240	1.155
		1.171	250	1.153
		1.237	260	1,139
		1.220	270	1,132
		1.183	280	1,157
		1.211

Table 4 :

 4 Response values (𝑆 = 𝑅 𝑎 𝑅 𝑔 ⁄ ) of La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 O 3 (𝑥 = 0.00, 0.05, 0.10 𝑎𝑛𝑑 0.20) sensors during ozone exposing time of 4 𝑚𝑖𝑛 (Present work : P.W) compared to others materials considered for 𝑂 3 sensing application.

	Sensors	𝑻 (°𝑪)	𝑪 (𝒑𝒑𝒃)	𝑺	References
	𝑺𝒓𝑻𝒊 𝟎.𝟗𝟐𝟓 𝑭𝒆 𝟎.𝟎𝟕𝟓 𝑶 𝟑	250	75	3	[25]
	𝑾𝑶 𝟑	300	50	1.25	[26]
	𝑺𝒏𝑶 𝟑	523	50	3	[27]
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆𝑶 𝟑	270	50	1.025	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟗 𝑪𝒐 𝟎.𝟏 𝑶 𝟑	190	50	1.586	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟖 𝑪𝒐 𝟎.𝟐 𝑶 𝟑	170	50	1.753	𝑷. 𝑾
	𝑾𝑶 𝟑	300	100	1.42	[28]
	𝑾𝑶 𝟑	300	150	1.78	[28]
	𝑰𝒏 𝟐 𝑶 𝟑	473	100	1.5	[25]
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆𝑶 𝟑	270	100	1.068	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟗 𝑪𝒐 𝟎𝟏 𝑶 𝟑	190	100	2.020	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟖 𝑪𝒐 𝟎.𝟐 𝑶 𝟑	170	100	2.111	𝑷. 𝑾
	𝑾𝑶 𝟑 (EG : 50µm)	250	200	1.7	[29]
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆𝑶 𝟑	270	200	1.373	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟗 𝑪𝒐 𝟎.𝟏 𝑶 𝟑	190	200	2.475	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟖 𝑪𝒐 𝟎.𝟐 𝑶 𝟑	170	200	3.264	𝑷. 𝑾
	𝑺𝒎𝑭𝒆 𝟎.𝟖 𝑪𝒐 𝟎.𝟐 𝑶 𝟑	200	400	3	[10]
	𝑾𝑶 𝟑 (EG : 50µm)	250	400	2.3	[29]
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆𝑶 𝟑	270	400	1.716	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟗 𝑪𝒐 𝟎.𝟏 𝑶 𝟑	190	400	3.391	𝑷. 𝑾
	𝑳𝒂 𝟎.𝟖 𝑷𝒃 𝟎.𝟏 𝑪𝒂 𝟎.𝟏 𝑭𝒆 𝟎.𝟖 𝑪𝒐 𝟎.𝟐 𝑶 𝟑	170	400	6.563	𝑷. 𝑾
	𝑾𝑶 𝟑 (EG : 50µm)	250	800	2.8	[29]
	𝑰𝒏 𝟐 𝑶 𝟑	473	1000	2	[25]

Table 5 :

 5 Response (𝜏 𝑟𝑒𝑠𝑝 ) and recovery (𝜏 𝑟𝑒𝑐 ) times of La 0.8 Pb 0.1 Ca 0.1 Fe 1-𝑥 Co 𝑥 O 3 (𝑥 = 0.00, 0.05, 0.10 𝑎𝑛𝑑 0.20) sensors at different concentrations (𝐶) at the operating temperature for each sensor.

	Sensors 𝑻 𝑭 (°𝑪)	Detected gas	𝑪 (𝒑𝒑𝒃) 𝝉 𝒓𝒆𝒑 (𝒎𝒊𝒏) 𝝉 𝒓𝒆𝒄 (𝒎𝒊𝒏) References
	𝒙 = 𝟎. 𝟎𝟎	𝐎 𝟑	50	3.4	2.5	𝑷. 𝑾
	𝒙 = 𝟎. 𝟎𝟎	𝐎 𝟑	400	2.6	12.5	𝑷. 𝑾
	𝒙 = 𝟎. 𝟎𝟓	𝐎 𝟑	50	3.38	13.2	𝑷. 𝑾
	𝒙 = 𝟎. 𝟎𝟓	𝐎 𝟑	400	1.98	25.1	𝑷. 𝑾
	𝒙 = 𝟎. 𝟏𝟎	𝐎 𝟑	50	3.30	10.2	𝑷. 𝑾
	𝒙 = 𝟎. 𝟏𝟎	𝐎 𝟑	400	1.95	30.1	𝑷. 𝑾
	𝒙 = 𝟎. 𝟐𝟎	𝐎 𝟑	50	3.51	8.3	𝑷. 𝑾
	𝒙 = 𝟎. 𝟐𝟎	𝐎 𝟑	400	2.1	41.3	𝑷. 𝑾
	𝑺𝒎𝑭𝒆𝑶 𝟑	𝐎 𝟑	50	8.4	-	[31]
	𝑺𝒎𝑭𝒆𝑶 𝟑	𝐎 𝟑	500	4.0	-	[31]
	𝑵𝒊𝑶	𝑵𝑯 𝟑	1000 (𝒑𝒑𝒎)	4.6	13	[32]
	𝑷𝒕/𝑵𝒊𝑶	𝑵𝑯 𝟑	200 (𝒑𝒑𝒎)	6.6	120	[33]
	𝑷𝒕/𝑵𝒊𝑶	𝑵𝑯 𝟑	35 (𝒑𝒑𝒎)	13.3	90	[33]


Luís F Da Silva

Ariadne C Catto

Sandrine Bernardini

Tomas Fiorido

João V N De Palma

Waldir Avansi

Khalifa Aguir

Marc Bendahan

BTEX Gas Sensor Based on Hematite Microrhombuses

Keywords: Fe 2 O 3, hematite, gas sensor, BTEX, benzene, toluene, ethylbenzene, xylenes

Over the past fifty years, gas sensors based on metal semiconducting oxides (MOXs) have drawn attention due to their performance in detecting of various gases. Thus, we report herein on a BTEX gas sensor based on hematite (α-Fe 2 O 3 ) microrhombuses synthesized via the hydrothermal method.

X-ray diffraction and X-ray absorption spectroscopy analyses indicated the presence of a pristine hematite phase after hydrothermal treatment. Electron microscopy analyses revealed that the hematite sample consists of single-crystals with a rhombus-like shape and an average size of 140 nm. Electrical measurements pointed out that hematite microrhombuses were sensitive towards sub-ppm BTEX levels, in which the minimum detected level was 3 ppb and long-term stability was for 1 month. The results presented here demonstrate the potential of hematite microrhombuses as a sensing material to manufacture BTEX gas sensor devices.

INTRODUCTION

The development in industrial and agricultural activities, mainly in developing countries, has resulted in the enhancement of harmful gases dumped into the atmosphere, such as CO, NO, NO 2 , O 3 , and Volatile Organic Compounds (VOCs). [START_REF] Lemieux | Emissions of organic air toxics from open burning: A comprehensive review[END_REF][START_REF] Mirzaei | α-Fe 2 O 3 based nanomaterials as gas sensors[END_REF][START_REF] Catto | Local structure and surface properties of Co x Zn 1-x O thin films for ozone gas sensing[END_REF][START_REF] Avansi | One-dimensional V 2 O 5 /TiO 2 heterostructures for chemiresistive ozone sensors[END_REF][START_REF] Kim | Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview[END_REF][START_REF]World Health Organization -WHO/Health Topics/ Air Pollution[END_REF] The presence of these gases could cause adverse effects on the climate and human life. According to the World Health Organization (WHO), these chemical species may cause serious diseases (e.g. heart and lung diseases, stroke, cancer and acute respiratory infections) or in severe cases, death. [START_REF]World Health Organization -WHO/Health Topics/ Air Pollution[END_REF][START_REF] Abe | Health impact assessment of air pollution in São Paulo, Brazil[END_REF][START_REF] Kampa | Human health effects of air pollution[END_REF][START_REF] Lin | Air pollutants and subsequent risk of chronic kidney disease and end-stage renal disease: A populationbased cohort study[END_REF] It has been estimated that air pollution has been responsible for the death of 7 million people worldwide every year. [START_REF]World Health Organization -WHO/Health Topics/ Air Pollution[END_REF] Furthermore, approximately 80% of the urban population is exposed (indoor and outdoor) towards pollution levels that exceed the guidelines recommended by health organizations. [START_REF] Catto | Local structure and surface properties of Co x Zn 1-x O thin films for ozone gas sensing[END_REF][START_REF]World Health Organization -WHO/Health Topics/ Air Pollution[END_REF][START_REF] Saksena | Exposure of infants to outdoor and indoor air pollution in low-income urban areas -A case study of Delhi[END_REF] Note that adequate levels of air pollution would also provide a lower demand for hospital admissions (or care), since hospitalizations would decrease. [START_REF] Abe | Health impact assessment of air pollution in São Paulo, Brazil[END_REF] Therefore, the continuous and efficient monitoring of atmospheric gas pollutants is essential.

Among the dangerous gases, BTEX (Benzene, Toluene, Ethylbenzene, and the three Xylene isomers) gases belong to the aromatic VOCs group. [START_REF] Lara-Lbeas | Sub-ppb level detection of BTEX gaseous mixtures with a compact prototype GC equipped with a preconcentration unit[END_REF][START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF][START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A Review[END_REF][START_REF] Zhang | Highly sensitive BTEX sensors based on hexagonal WO 3 nanosheets[END_REF][START_REF] Allouch | Portable, miniature, fast and high sensitive real-time analyzers: BTEX detection[END_REF] The main sources for BTEX emissions are indoor and outdoor, such as gas stations, fuel storage, road traffic, and also household products, such as paints, solvents, waxes, pesticides, among others. [START_REF] Lara-Lbeas | Sub-ppb level detection of BTEX gaseous mixtures with a compact prototype GC equipped with a preconcentration unit[END_REF][START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF][START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A Review[END_REF][START_REF] Zhang | Highly sensitive BTEX sensors based on hexagonal WO 3 nanosheets[END_REF] Various investigations focused on BTEX gases due to their toxic, mutagenic, and/or carcinogenic effects. [START_REF] Kampa | Human health effects of air pollution[END_REF][START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF][START_REF] Duarte-Davidson | Benzene in the environment: An assessment of the potential risks to the health of the population[END_REF][START_REF] Rafiee | Lifestyle and occupational factors affecting exposure to BTEX in municipal solid waste composting facility workers[END_REF][START_REF] Correa | The impact of BTEX emissions from gas stations into the atmosphere[END_REF] According to the OSHA (Occupational Safety and Health Administration), permissible employee exposure to BTEX gases is an average concentration of 0.5 ppm (parts-per-million) for benzene, 200 ppm for toluene and 100 ppm for ethylbenzene and xylene. [START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF] Generally, BTEX gases have been identified and quantified using gas chromatography. [START_REF] Lara-Lbeas | Sub-ppb level detection of BTEX gaseous mixtures with a compact prototype GC equipped with a preconcentration unit[END_REF][START_REF] Rafiee | Lifestyle and occupational factors affecting exposure to BTEX in municipal solid waste composting facility workers[END_REF] However, this equipment, besides being expensive, is not portable, making in-situ analyses difficult. [START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A Review[END_REF] Thus, several investigations have focused on the development of portable devices that allow efficient detection and monitoring of BTEX gases in indoor and outdoor environments.

In past decades, MOXs have been widely used in resistive-type gas sensors to detect (non-) toxic gases. Their main advantages are the low cost, small size, high sensitivity and stability, fast response/recovery and portability. [START_REF] Avansi | One-dimensional V 2 O 5 /TiO 2 heterostructures for chemiresistive ozone sensors[END_REF][START_REF] Barsan | Metal oxide-based gas sensor research: How to?[END_REF][START_REF] Da Silva | A novel ozone gas sensor based on one-dimensional (1D) α-Ag 2 WO 4 nanostructures[END_REF][START_REF] Da Silva | UV-enhanced ozone gas sensing response of ZnO-SnO 2 heterojunctions at roomtemperature[END_REF] MOXs have been widely used as sensing layers for detecting BTEX gases, such as WO 3 , ZnO, MoO 3 , [START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF][START_REF] Zhang | Highly sensitive BTEX sensors based on hexagonal WO 3 nanosheets[END_REF][START_REF] Choudhury | BN quantum dots decorated ZnO nanoplates sensor for enhanced detection of BTEX gases[END_REF][START_REF] Sui | Au-loaded hierarchical MoO 3 hollow spheres with enhanced gas-sensing performance for the detection of BTX (benzene, toluene, and xylene) and the sensing mechanism[END_REF] and also some heterojunctions, for example, SnO Motivated by these arguments, we report herein a versatile and efficient approach to obtain hematite microcrystals presenting remarkable properties for application as a BTEX gas-sensing layer. The microcrystals were synthesized via the hydrothermal method and characterized by using X-ray diffraction (XRD), field-emission scanning electron (FE-SEM) and high-resolution transmission electron microscopies (HR-TEM), and X-ray absorption near-edge structure (XANES) spectroscopy. The α-Fe 2 O 3 microcrystals were studied in the detection of BTEX, acetone, and ethanol. Electrical measurements revealed their higher sensitivity to sub-ppb BTEX levels compared to other VOCs. These results have a significant impact on the scientific field due to its practical potential for environmental monitoring applications.

EXPERIMENTAL SECTION

Preparation of microcrystals

The α-Fe 2 O 3 microcrystals were obtained by the hydrolysis of iron (III) chloride hexahydrate (FeCl 3 .6H 2 O, 99.9%, Sigma-Aldrich) in ethyl alcohol anhydrous at room temperature. The concentration of iron chloride diluted in alcohol was 0.06 M. Afterwards, deionized water was slowly added to the reactional solution to obtain a molar ratio of Fe(III) : H 2 O = 1:500. The chloride anions were removed by the dialysis process, and the precipitate was then dried in an electric oven during 12 h at 80 o C. To obtain the hematite phase, the precipitate powder was diluted in deionized water (1 g. L -1 ) and then submitted to hydrothermal treatment for 4 hours at 200 o C with a heating rate of 2 o C min -1 . The precipitated powders were washed with deionized water and isopropyl alcohol and dried at 80 o C overnight under the air atmosphere.

Characterization Techniques

X-ray diffraction (XRD) measurements were carried out using CuKα radiation (Rigaku diffractometer, model D/Max-2500PC) in a 2θ range from 20 o to 80 o with a step of 0.02 o at a scanning speed of 2 o min -1 . The morphological properties were investigated using a field emission scanning electron microscopy (FE-SEM, Zeiss Supra35) and a transmission electron microscopy (TEM, FEI Tecnai G2 F20) operated at 200 KeV, respectively. The mean particle size was estimated by analyzing FE-SEM micrographs through the measure of approximately 100 particles.

X-ray absorption spectroscopy (XAS) experiments were carried out at the D08B-XAFS2 beamline on the Brazilian Synchrotron Light Laboratory (LNLS). The experiments were carried out at the Fe K-edge (7112 eV) in a transmission mode at room temperature using a flat Si(111) double crystal monochromator. X-ray Absorption Near-Edge Structure (XANES) spectra were collected for each sample between 7090 and 7190 eV using energy steps of 0.5 eV. For the sake of comparison, all spectra were background removed and normalized using as unity the first EXAFS (Extended X-ray Absorption Fine Structure) oscillation using MAX software. [START_REF] Michalowicz | MAX: Multiplatform Applications for XAFS[END_REF] 2.3.

Gas-sensing Experiments

Hematite microcrystals were dispersed in isopropyl alcohol (20 mg mL -1 ) using an ultrasonic cleaner for 30 min and then the suspension was dropped three times onto a Si/SiO 2 substrate containing 100 nm thick Pt interdigitated electrodes separated by a distance of 50 µm. After deposition, the sensor was then treated at 500 o C in an electric oven under the air atmosphere for 1 h at a heating rate of 10 o C min -1 . The photography of the sensing platform and FE-SEM images of the hematite microcrystals used in the experiments are presented in Fig. 1.

Insert Figure 1 Gas-sensing experiments were carried out in a dynamic chamber that allows us to control the operating temperature and to obtain various gas concentrations by using Mass Flow Controllers (MFC). [START_REF] Favard | Detection and measuring of BTEX traces at the ppb level using metal oxide gas sensor[END_REF] This workbench, Fig. 2, can produce low BTEX concentrations from 1 to 400 ppb S1. Further details regarding the gas-sensing workbench may be found in previous work in Ref. 12. The applied DC voltage was kept constant, while the electrical resistance was monitored using a sourcemeter (Keithley, model 2540). The sensor response (S) was defined as S(%) = ((R air -R BTEX )/R air ).100, where R air and RBTEX are the electrical resistances of the sensor upon exposure to dry air and BTEX gases, respectively. The response time was defined as the time required for the electrical resistance to reach 90% of the initial value when exposed to the BTEX gases.

Similarly, the recovery time was defined as the time required for the electrical resistance to recover 90% of the initial value after switching off the BTEX gas flow. To obtain short recovery times of the sensing material, we chose 280 o C as the optimum working temperature for further experiments.

Insert Figure 4

The hematite microrhombuses were kept at 280 o C and then exposed to several different BTEX levels, from 3 till 400 ppb. It should be mentioned that for gas levels lower than 3 ppb, the microcrystals did not exhibit any sensitivity; considering here the abovementioned temperature.

Fig. 5 shows that the electrical resistance of the microrhombuses decreased during exposition to BTEX gases. When an n-type MOX, such as hematite, is exposed to a reducing gas (e.g. BTEX)

the ionized oxygen anions adsorbed at the surface sample oxidize the analyte. During this adsorption process, the release of the electrons occurs from the MOX surface, consequently decreasing the electrical resistance of the sensing material. [START_REF] Kim | Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview[END_REF][START_REF] Peeters | Au/ε-Fe 2 O 3 nanocomposites as selective NO 2 gas sensors[END_REF][START_REF] Flak | Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO 2 , ZnO and SnO 2 nanoparticles[END_REF][START_REF] Malagù | Electrical and spectroscopic analysis in nanostructured SnO 2 : 'Long-term' resistance drift is due to in-diffusion[END_REF] Regarding the sensor responses, the hematite microcrystals showed good sensitivity even for very low BTEX levels, in addition to complete recovery after each exposure cycle. Furthermore, the sample also exhibited high repeatability of its response, reinforcing its reliability, as demonstrated by the seven exposure cycles displayed in Fig. 5(d).

Insert Figure 5 Fig. 6 displays the sensor response of α-Fe 2 O 3 microrhombuses, kept at 280 o C, exposed towards different BTEX levels, from 3 to 400 ppb. The response enhancement as a function of the BTEX level is clear, with no evidence of saturation in the evaluated range. Despite the fact that the microcrystals had detected sub-ppm gas levels, the inset of Fig. 6 shows that the response has a low variation between 3 and 10 ppb, meaning a low accuracy in this range. Nevertheless, this finding does not debunk the hematite for practical applications, as long as just BTEX levels above 500 ppb become harmful to human health, as recommended by the WHO.

Regarding the sensor response behaviors with BTEX levels, an exponential trend in the sensor response of hematite microrhombuses was obtained. It is well established that the sensor response (S) of MOXs complies with the following power-law equation: S(C) = A(C) n , where A is a constant, n the exponent, and C is the concentration of the investigated analyte. [START_REF] Lu | UV-enhanced room temperature NO 2 sensor using ZnO nanorods modified with SnO 2 nanoparticles[END_REF] Herein, the best fitting law acquired from the experimental data was found to be the equation: S(C)= 7,617.(C) 0,461 , respectively.

Insert Figure 6

The influence of relative humidity on the BTEX gas sensing performance was also Insert Figure 7 Gas selectivity and long-term stability are desirable characteristics in the development of gas sensor devices. Fig. 8(a) illustrates the response (S) of the microrhombuses, at 280 o C, exposed to BTEX gases, ethanol, and acetone gases. Notably, the hematite microrhombuses exhibited a higher response to BTEX gases compared to the investigated VOCs gases. Regarding the long-term stability, it was evaluated by exposing the microrhombuses towards 100 ppb of BTEX gases for 30 days, and the results are presented in Fig. 8(b). It can be noted after the mentioned period that the sample still detected a very low BTEX level. It means that the lifetime of the hematite microrhombuses can be superior to 1 month since the sample surface was not poisoned/damaged due to the several exposures to BTEX gases.

Insert Figure 8

Table 1 shows the performance of some MOXs synthesized by different methodologies, applied as a BTEX gas sensing material. As seen, the range of working temperatures of such MOXs is similar to those found here for the microrhombuses. Nevertheless, we must draw attention to the sensitivity of hematite microrhombus that detected a very low BTEX level, i.e., 3 ppb. Based on the findings presented here, it is reasonable to consider the potential of hematite microcrystals as a sensing material for BTEX sensors.

Table 1. Gas-sensing performance of the MOXs used as a BTEX sensing material.

*Minimum BTEX level, experimentally, detected.

Gas-Sensing Mechanism

We propose herein a possible sensing mechanism of the hematite microrhombuses to detect gases. According to the literature, for resistive-type gas sensors, the sensing mechanism is based on the adsorption process of the oxygens and the target gas molecules which occurs on the MOX surface, depending on the sensing material characteristics (e.g. particles size, and morphology), working temperature, analyte concentration, environment conditions (e.g. pressure, and humidity), etc. [START_REF] Malagù | Electrical and spectroscopic analysis in nanostructured SnO 2 : 'Long-term' resistance drift is due to in-diffusion[END_REF][START_REF] Suematsu | Effect of ambient oxygen partial pressure on the hydrogen response of SnO 2 semiconductor gas sensors[END_REF][START_REF] Hübner | Influence of humidity on CO sensing with p-type CuO thick film gas sensors[END_REF][START_REF] Bârsan | Understanding the fundamental principles of metal oxide based gas sensors; The example of CO sensing with SnO 2 sensors in the presence of humidity[END_REF][START_REF] Korotcenkov | Spray pyrolysis deposition of undoped SnO 2 and In 2 O 3 films and their structural properties[END_REF][START_REF] Miller | Nanoscale metal oxide-based heterojunctions for gas sensing: A Review[END_REF] Regarding the hematite microrhombuses, when the oxygen molecules are adsorbed on the sample surface, they trap free electrons from the hematite conduction band. This ionosorption process increases the band bending (qV s ), forming a depletion region "d" on the hematite microrhombuses surface, as illustrated in Fig. 9(a)-(b). At operating temperatures higher than 280 o C, the ionic O -species are dominant. [START_REF] Kim | Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview[END_REF][START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A Review[END_REF][START_REF] Bârsan | Understanding the fundamental principles of metal oxide based gas sensors; The example of CO sensing with SnO 2 sensors in the presence of humidity[END_REF] This process can be expressed by:

O 2(ads) + 2e -® 2O - (ads) (1)
Upon exposure to a reducing gas, such as BTEX gases, the electrical resistance of the hematite decreases due to the reaction between the molecules of the reducing gas and the ionosorberd oxygens (O -) at the rhombus surface, releasing the electrons to the conduction band.

This behavior reduces the height of potential barrier qV s , consequently decreasing the depletion region "d" and enhancing the conductivity of the hematite microrhombuses. [START_REF] Suematsu | Effect of ambient oxygen partial pressure on the hydrogen response of SnO 2 semiconductor gas sensors[END_REF][START_REF] Hübner | Influence of humidity on CO sensing with p-type CuO thick film gas sensors[END_REF][START_REF] Gurlo | An n-to p-type conductivity transition induced by oxygen adsorption on α-Fe 2 O 3[END_REF] Insert Figure 9 CONCLUSION In summary, we report here the synthesis of hematite microcrystals via hydrothermal route for use as BTEX gases-sensing material. XRD pattern and Fe K-edge XANES spectra revealed the presence of the pristine hematite phase. FE-SEM and HR-TEM analyses showed that hematite microcrystals exhibit a well-defined rhombus-like shape with a single-crystal nature. Electrical measurements showed a good sensitivity towards BTEX gases at an operating temperature of 280 o C. The microrhombuses detected the lowest gas levels that ranged from 3 ppb to 400 ppb, exhibiting excellent repeatability and stability, and good selectivity towards acetone and ethanol.

Finally, the findings presented here highlighted the promising properties of hematite microrhombuses as BTEX gas sensors for practical applications.

Corresponding Author

*E-mail: lfsilva83@gmail.com.

Title of Figures

(

  part-per-billion) from the dilution with dry air. The total gas flow rate was kept constant and equal to 500 SCCM. The workbench also allows the control of relative humidity (% RH) from 0 to 90%. The cylinder with the BTEX gases mixture was purchased from Air Liquide company (France). The concentration of each component in the BTEX mixture is displayed in Table

  diffraction pattern of the as-synthesized sample prepared via the hydrothermal route is shown in Fig.S1(a). All reflections were indexed to a pristine rhombohedral hematite (α-Fe 2 O 3 ) phase with an R3c space group (JCPDS file 87-1164). Fe K-edge spectra of iron-oxide based compounds and the as-prepared hematite sample are displayed in Fig.3(a)-(b). The comparison of reference compound spectra with the sample confirms the presence of the pristine α-Fe 2 O 3 phase, with iron atoms in octahedral coordination, FeO 6 .[START_REF] Wu | Synthesis of hematite (α-Fe 2 O 3 ) nanorods: Diameter-size and shape effects on their applications in magnetism, lithium ion battery, and gas sensors[END_REF] FE-SEM image in Fig.3(c) reveals a homogeneity of the morphology of the as-studied microcrystals that exhibit a well-defined rhombus-like shape with an average size of approximately 140 nm. The HRTEM image of a selected region of an individual crystal and their respective Fourier Transform (FT), Fig.3(d), shows the monocrystalline nature of the as-studied sample. performances of α-Fe 2 O 3 microrhombuses were evaluated towards BTEX gases. Initially, the optimum operating temperature of the sample was evaluated by exposing it to 100 ppb of the analyte and varying only the operating temperature, as seen in Fig.4. The electrical measurements carried out at different working temperatures are presented in Fig.S2. For temperatures below 200 o C, no sensitivity towards BTEX gases was observed. The highest sample responses were achieved in the temperature range of 240 to 280 o C, as shown in Fig.4(a). In addition, the behavior of response and recovery times as a function of operating temperature is displayed in Fig.4(b). We can observe that despite the sample required a long time for the complete desorption of BTEX molecules, its response time was relatively fast at around 50 s. To assert the high sensitivity of the microrhombuses, they were then exposed to 100 ppb of BTEX during different time intervals (from 5 s to 360 s), as shown in Fig.4(c). The sample was sensitive to various exposure times to BTEX gases, mainly, for the shorter time, i.e., 5 s. Moreover, the sample saturation level was reached at exposure times longer than 180 s, as illustrated in the inset of Fig.4(c). It is interesting to note that, even after a long sequence of exposure cycles (> 8 h) the sample exhibited a total recovery, indicating that its surface was not damaged.

7 .

 7 investigated. For that, the sample was kept at 280 o C and then exposed towards 100, 200, 300, and 400 ppb of BTEX gases under different relative humidity (0, 20, and 40 % RH) values. It can be seen that the BTEX sensor response was affected by the presence of water molecules, as shown in Fig.This behavior is due to the competition of water and BTEX molecules by actives sites on the sample's surface, this reducing its sensing performance towards BTEX gases. Zhang and co-workers reported similar effect of the humidity for WO 3 nanosheets used as sensing material to BTEX (Benzene, Toluene, Ethylbenzene, and p-Xylol)vapors.[START_REF] Zhang | Highly sensitive BTEX sensors based on hexagonal WO 3 nanosheets[END_REF]

Fig. 9

 9 (b)illustrates the possible sensing mechanism of hematite microrhombuses to detect BTEX gases. In this figure, it is suggested that the possible sub-products formed by the interaction between the BTEX gases and sample surface are H 2 O and CO 2 .

  Figure 1. (a) Photography of sensing platform based on hematite microcrystals compared to a U.S quarter-dollar coin. (b)-(c) FE-SEM images show the homogeneity of the thick film.

Figure 2 .

 2 Figure 2.Experimental workbench used in the gas-sensing experiments of BTEX gases.

Figure 3 .

 3 Figure 3. The as-prepared α-Fe 2 O 3 microcrystals prepared via hydrothermal treatment. (a) Fe

Figure 4 .

 4 Figure 4. (a) Gas-sensing performance of α-Fe 2 O 3 microrhombuses, exposed towards

Figure 5 .

 5 Figure 5. Gas-sensing response of the α-Fe 2 O 3 microrhombuses kept at 280 o C and exposed to

Figure 6 .

 6 Figure 6. Sensor response of the α-Fe 2 O 3 microrhombuses vs. BTEX level. The inset shows the

Figure 7 .

 7 Figure 7. Sensor response of the α-Fe 2 O 3 microrhombuses vs. BTEX level under different

Figure 8 .

 8 Figure 8. Sensor performance of α-Fe 2 O 3 microrhombuses at 280 o C. (a) Comparison of the

Figure 9 .

 9 Figure 9. Schematic illustration of the sensing mechanism of hematite rhombus-like crystals to

Fig. 2

 2 Fig.2

  Fig3

Fig. 6

 6 Fig.6

Fig. 7 28Fig. 8 29Fig. 9

 789 Fig.7

Fe 2 O 3 compound has been studied as sensing layer for detection of a wide range of analytes.[33-38] For example, Tian and co-workers prepared hematite nanoboxes via metal- organic frameworks method. The authors observed such nanoboxes were highly sensitive to H 2 S gas for an optimum temperature of 200 o C, besides exhibiting a good selectivity towards NO, CO, and NH 3 gas.[38] Despite the remarkable gas-sensing properties of hematite, few researchers have paid attention to its performance for detecting BTEX gases. Kim co- workers prepared vertically hematite nanotubes applied as a gas-sensing layer. They observed a higher response towards acetone compared to other analytes (benzene, toluene, acetone, ethanol, and CO), for an operating temperature relatively high equal to 350 o C.[27] Also, Thu and co-workers reported good selectivity and sensitivity to ethanol of a hematite nanoporous sample working at 400 o C. In a similar work, Park and co-workers observed for hematite microcubes a high sensitivity to VOCs, especially formaldehyde, at an operating temperature of 300 o C.[31]

2 /V 2 O 5 , and CuO/SnO 2 .

[START_REF] Ren | Enhanced BTEX gas-sensing performance of CuO/SnO 2 composite[END_REF][START_REF] Zhang | Selective BTEX sensor based on a SnO 2 /V 2 O 5 composite[END_REF]

Hematite (α-Fe 2 O 3 ) is an n-type wide band-gap semiconductor (E g = 2.1 eV, at 300 K).

[START_REF] Bailly | Microwave gas sensing with hematite: Shape effect on ammonia detection using pseudocubic, rhombohedral, and spindlelike particles[END_REF]

It is the most abundant and the cheapest MOX obtained in the Earth'crust,

[START_REF] Kim | Vertically ordered hematite nanotube array as an ultrasensitive and rapid response Acetone Sensor[END_REF]

exhibiting potential applications in photocatalysis, energy generation, lithium batteries and the gas sensor area.

[2,26- 32]

The

  BTEX, i.e., Benzene (C 6 H 6 ), Toluene (C 6 H 5 CH 3 ), Ethylbenzene (C 8 H 10 ) and Xylene (C 6 H 4 (CH 3 ) 2 )

Synchrotron Radiation (LNLS, Project D04B-XAFS2-20180311), Campinas, SP, Brazil. We also thank the Laboratory of Structural Characterization (LCE/DEMa/UFSCar) for the TEM facilities. The authors acknowledge the financial support of the Brazilian research funding institution FAPESP (under grants No. 2017/12437-5, 2019/18082-2, 2018/18208-0 and 2013/07296-2), CNPq (under grants No. 405140/2018-5, 426511/2018-2, 308706/2018-8, and 311463/2017-7) and CAPES.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

The authors thank Dr. Julio C. Sczancoski for the XRD data and Mr. Rorivaldo Camargo who operated the FE-SEM microscope. This investigation was initiated in the Institute of Material and Microelectronics of Provence in France during a period of visiting professor at the Aix-Marseille University. This research was also partially performed at the Brazilian Nanotechnology National Laboratory (Microfabrication team: Project LMF-18580), and the Brazilian Laboratory of