Sonochemically prepared nanosized BiFeO3 as novel SO2 sensor

Accepted Manuscript Title: Sonochemically prepared nanosized BiFeO3 as novel SO2 sensor Author: S. Das S. Rana S.k.Md. Mursalin P. Rana A. Sen PII: DOI: Reference:

S0925-4005(15)00536-5 http://dx.doi.org/doi:10.1016/j.snb.2015.04.084 SNB 18389

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

20-9-2014 23-4-2015 25-4-2015

Please cite this article as: S. Das, S. Rana, Sk.Md. Mursalin, P. Rana, A. Sen, Sonochemically prepared nanosized BiFeO3 as novel SO2 sensor, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.04.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sonochemically prepared nanosized BiFeO3 as novel SO2 sensor

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CSIR - Central Glass and Ceramic Research Institute 196 Raja S C Mullick Road, Kolkata 700032, India. Presently at Jadavpur University

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b

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S Dasb, S Ranac, Sk Md Mursalina,*, P Ranaa and A Sena

188 Raja S C Mullick Road, Kolkata 700032, India. Presently at Variable Energy Cyclotron Centre

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1/AF Bidhan Nagar, Kolkata 700 064, India.

*Corresponding author. Tel.: +91- 33-24733496; Fax: +91-33- 2473-0957; E-mail: [email protected]

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ABSTRACT Bismuth ferrite (BiFeO3) without any co-formed impurity phase was synthesized by sonochemical method. The average crystallite sizes of synthesized BiFeO3 for two sonication

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times of 40 min and 2 h were found to be 38 and 32 nm and the corresponding band gaps were 2.1 and 2.2 eV, respectively. In contrast to BiFeO3 synthesized by conventional

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precipitation route which gives rise to co-formed impurity phases, the sonochemically

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prepared phase-pure BiFeO3 nanoparticles exhibited an excellent low-ppm SO2 sensing behaviour with fast response and recovery. Also the sonochemically synthesized BiFeO3 was

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selective to SO2 in presence of carbon monoxide and butane and may turn out to be a

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potential functional material for SO2 leak alarms.

Keywords: Sensors ; Nanostructures; Oxides; Semiconductors; Chemical synthesis; Sonochemistry ;

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1. Introduction Sulfur dioxide is one of the toxic gases of great concern because it is hazardous to human beings, animals, plants and even historical structures [1-6]. SO2 is transformed in the

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atmosphere into sulfurous and sulfuric acids which cause acid rain [4]. Also the leakage of sulphur dioxide from wine preservation units and other chemical industries can be hazardous.

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Hence, the development of inexpensive SO2 gas sensors is an essential requirement for

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monitoring the leakage of SO2. So far, different instrumental techniques have been exploited such as chromatography and IR spectroscopy to monitor SO2 [7]. Sulfur

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dioxide can also be measured using electrochemical sensors [8], however such sensors have a short life [9].

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Given the fact that oxide based semiconductor gas sensors are inexpensive, rugged and have a long life, attempts to make such sensors for detection of SO2 so far turned out to

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be of little success. To date, a few semiconducting oxides like SnO2, nickel doped SnO2,

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vanadium doped SnO2, WO3 and noble metal incorporated WO3 had been examined for SO2

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sensing [10-14] and their behaviour towards SO2 is not quite straight forward. Lalauze et al. [10] reported that tin dioxide gas sensor is irreversible modified by the first exposure of SO2 and the modification is due to the adsorption of sulphite and sulphate on the semiconductor surface. Girardin et al. [11] studied the phenomenon at the gas-semiconductor interface during the first and subsequent detections of SO2. According to Berger et al. [12] the increase in the

sensitivity after the first exposure of SO2 is due to the increase in surface acidity

arising from the formation of sulphate groups through the interaction between sulfur dioxide and –OH groups on the surface of tin dioxide. Shimizu et al. [13] showed that SO2 gas on the surface of WO3 behaved as an oxidizing gas at temperatures below 450°C because of the formation of SO2- adsorbates, accompanied by electron transfer from the bulk to the adsorbates leading to an increase in the sensor resistance. However, an opposite trend [13] 3 Page 3 of 26

was observed for 1.0 wt% Ag in WO3, where the resistance decreased after the exposer of SO2 possibly due to formation of SO4-2 (ad) on the Ag loaded SnO2. The reversible decrease in resistance in presence of SO2 may be understood as oxidation of SO2 to SO3 on SnO2

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surface like that of a reducing gas [14]. According to Das et al. [14], the sensitivity of vanadium doped tin dioxide towards SO2 may be understood by considering the oxidation of

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sulfur dioxide to sulfur trioxide on SnO2 surface through redox cycles of vanadium–sulfur–

to be a potential candidate for SO2 gas leak detector.

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oxygen adsorbed species. However, so far, none of the aforementioned materials turned out

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Incidentally, a few studies on multiferroic BiFeO3 have shown its sensitivity towards reducing gases/vapours like alcohols, acetaldehyde, acetone, and ammonia [15-18]. Here we

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first time report that phase-pure BiFeO3 prepared through sonochemical route can detect low concentration of SO2 with excellent sensitivity, response and recovery times. However,

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BiFeO3 prepared through conventional precipitation technique without sonication contains

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impurity phases and does not show such SO2 sensing behaviour.

2. Experimental Procedure

Reagent grade bismuth nitrate,

Bi(NO3)3 and ferric nitrate nonahydrate,

Fe(NO3)3·9H2O were used as starting materials. A 0.2 M Bi(NO3)3 aqueous solution was prepared by dissolving the requisite amount of Bi(NO3)3 in water in presence of HNO3 at a pH value lying between 1 and 2. A 0.2 M ferric nitrate solution was prepared by dissolving the requisite amount of Fe(NO3)3·9H2O in distilled water. The two aqueous solutions of 0.2M concentration were mixed together in equimolar ratio (1:1) in a 500 mL beaker. The solution in the beaker was placed under a high energy sonicator (ultrasonic processor, Sonics, 1500 W, model VCF1500 and the probe (diameter 2 cm) dipping length in the solution was ~ 4 4 Page 4 of 26

cm). During sonication, a 30 wt% NH4OH solution was added drop-wise to achieve the desired pH (~ 9) for complete precipitation. The solution was allowed to cool and at the end of the reaction, a brown orange precipitate was obtained. Two sonication times of 40 min and

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2 h were chosen for the present study. After completion of sonication, the obtained particles were separated out by centrifuging the solution at 10,000 rpm for 15 min. The particles were

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washed several times by centrifuging in water. Finally, the obtained brown orange powder was dried in an oven at 100ºC for 24 h and then calcined for 1 h at 500ºC and 600ºC. The

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present method is a simple version of the earlier reported work of N. Das et al. [19]. For

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comparative studies, BiFeO3 powder was also prepared by conventional precipitation method following the above technique sans the sonication.

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The crystal structure of the BiFeO3 nanoparticles was analyzed using X-ray diffractometer (X’Pert: Pro - MPD, PANalytical with CuKα at 40 kV, 30mA with a step size

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of 0.05º). The particle size and morphology of the powders were studied on a high resolution

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transmission electron microscope (Tecnai G230ST TEM, FEI, Netherlands). The optical

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spectra of the BiFeO3 nanoparticles were measured in the wavelength region (λ) of 200 - 600 nm using a UV–Vis–NIR spectrophotometer (Shimadzu, UV 3101 PC) by dispersing the powders in ethanol.

For fabrication of sensors, the synthesized and calcined BiFeO3 were mixed with a

suitable amount of isopropanol to form pastes. The pastes were then coated on alumina tubes on which two gold electrodes and platinum wires were placed at each end. The gold electrode and platinum lead wires were attached at the two ends of the tubes by curing the system at a high temperature (930ºC) before applying the paste. The BiFeO3 nano-powder coated alumina tubes were cured at 500°C for 1 h. Kanthal heating coils were then placed inside the Al2O3 tubes to generate an optimum operating temperature [Fig 1]. The detailed assembly has

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been described elsewhere [20]. The sensors were initially aged at 250°C for 7 days to achieve the desired stability before measurements. Around ten sensors for each composition were made to check the reproducibility. The electrical resistance, response and recovery times of

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the sensors were measured at an operating temperature of 300°C in ambient condition (~ 50% relative humidity) by using a digital multimeter (Agilent U1252A) and a constant

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voltage/current source (Keithley 228A). The sensors to be measured were placed inside a quartz tube (2 cm diameter and 10 cm length and volume ~ 30 cm3) and externally connected

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to the digital multimeter and the constant voltage/current source. The sensors were exposed

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to 5 ppm SO2 (5 ppm SO2 in air, Portagas, Spantech, UK) at a flow rate of 50 sccm and the

S=

Rg Ra

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response, S is measured as

(1)

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where Ra is the sensor resistance in air at the operating temperature, and Rg is the sensor

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resistance in the gas at the same temperature. Sulfur dioxide of 5 ppm concentration was

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selected for this investigation because as per the guidelines of National Institute for Occupational Safety and Health, the STEL (Short Term Exposure Limit) of sulfur dioxide is 5 ppm. In order to compare the cross sensitivity of the sensors, their response towards 1000 ppm butane and 30 ppm carbon monoxide (BOC Scientific, India and flow rate of 50 sccm) were also measured. It may be noted that the aforesaid concentration of butane and carbon monoxide were chosen based on the commercial detection ranges of the two gases. 3. Results and discussion It is known that the preparation of phase pure BiFeO3 using solid-state reaction route is very difficult due to volatilization of Bi2O3, which leads to an incomplete reaction [21]. Coprecipitation [22] and sol-gel [23] methods also generate impurity phases. In addition, sol-gel 6 Page 6 of 26

method may not be cost effective. However, as discussed below, sonochemical route can easily provide phase-pure BiFeO3. The X-ray powder diffraction pattern of the sonochemically synthesized BiFeO3

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powder (with 40 min of sonication time) is shown in Fig. 2(a). The as-prepared powder shows two broad diffraction halos at 2θ ~ 30.56˚ and 2θ ~ 56.58˚, indicating nanocrystalline /

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amorphous nature of the powder. The observed inter-planar spacing (d) values for calcined

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(500°C, 1 h) powder match with those of the rhombohedral structure (JCPDS no. 86-1518) of BiFeO3 (Fig. 2b). Similar powder X-ray diffraction pattern was obtained for 2 h sonication

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time (not shown here). Incidentally, higher calcination temperature (600°C, 1 h, Fig 2c) led to the formation of a small amount of unwanted iron rich phase Bi2Fe4O9, (JCPDS no. 25-0090

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marked by asterisk symbol) and expectedly, a small amount of unreacted Bi2O3 (JCPDS no.74-1375, not distinguishable because of near-overlapping high intensity line with (121) of

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Bi2Fe4O9 ) as impurity phases. From the X-ray diffractograms (Fig 3), it can be inferred that

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the preparation of single phase BiFeO3 powder is not possible through conventional precipitation followed by calcination. The phase formation under high energy ultrasound can

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be appreciated considering the fact that ultrasound gives rise to high temperature and high pressure non-equilibrium reactions in local spots owing to cavitation, vigorous collapse of bubbles and creation of high energy liquid jets [24]. The thermogravimetric analysis (not shown here) of sonochemically synthesized

powders showed an insignificant (~ 0.3 wt%) weight loss up to 600°C, which indicates that the as-prepared powders are pure oxide in nature. The average crystallite sizes (D) of the powders were calculated from the full-width at half maximum (21/2) of the characteristic diffraction peaks from the major reflection using Debye - Scherrer formula [25],

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D

0.89   2  cos  1 2  

(2)

where 2 is the position of the peak in the X-ray diffractogram for a specific crystallographic

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plane (hkl) and λ is the X-ray wavelength. The average crystallite sizes of the sonochemically prepared powders, as found from equation (2), are 38 nm and 32 nm for 40 min and 2 h of

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sonication (followed by calcination at 500°C for 1 h), respectively. The crystallite size of the

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BiFeO3 nanoparticles slightly decreased with the increase in sonication time. Similar observation was reported in sonochemically synthesized MoO3 nanoparticles [26].

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The TEM image of a representative BiFeO3 powder sample (after calcination at 500°C) is shown in Fig. 4a. The average particle (likely to be agglomerated) size turned out

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to be 67 nm (s.d. 19 nm). Fig. 4b shows the selected area electron diffraction (SAD) pattern with sharp diffraction spots indicating the crystalline nature and rhombohedral phase of the

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particles.

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Fig. 5 shows the optical spectra of the BiFeO3 nanoparticles. The overall absorption

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profiles are similar to those reported for the rhombohedral phase of BiFeO3 [27,28]. The intense electronic band at 4.5 eV is assigned to the minority channel dipole-allowed O-p to Fe-d charge transfer excitation [29-31]. The additional electronic band at around 5.6 eV is due to the formation of strong hybridized majority channel O-p and Fe-d to Bi-p state excitation [28,29,31,32]. The long near-infrared tail below 4.0 eV in optical absorption spectra of the nanoparticles primarily arises from the contributions of scattering and reflection [28]. The band gaps of the BiFeO3 nanoparticles were calculated from the optical spectra utilizing Tauc’s equation [33-35]: (αһ) =A(h-Eg)n

(3)

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where α is the absorption coefficient, A is constant and n is equal to 1/2 for a direct band-gap semiconductor. By plotting (αһ)2 vs. h, the optical band gap (Eg) was determined from the

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extrapolation of the straight line portion at (αһ)2 equal to zero on h axis as shown in Fig. 6 . The band gaps of the BiFeO3 powders for sonication times of 40 min and 2 h were found to

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be 2.1 and 2.2 eV, respectively, indicating that the difference in band gap between the two

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sonication times is insignificant. The obtained optical band gap values are consistent with the reported results [27,36,37].

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Fig. 7 shows the response of 2 h sonicated BiFeO3 sensors (40 min sonication showed similar behaviour and not shown here) towards 5 ppm SO2. In contrast, BiFeO3 powder

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prepared through conventional precipitation method (without sonication) is almost insensitive towards 5 ppm SO2 (not shown here), which may be due to the presence of Bi2Fe4O9 as an

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impurity phase in the conventional precipitation method as discussed earlier. With the

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increase in the sonication time from 40 min to 2 h, the average response of the SO2 sensor

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(Rg/Ra) improved from 1.43 to 2.03[standard deviation (s.d.) 0.06] (Fig 7). Incidentally, the response and recovery of the sensors (discussed below) are also quite fast for device applications.

It is known [38] that during calcination, bismuth vacancies are formed due to volatilization of bismuth and effectively, the oxide behaves as a p-type semiconductor (bismuth vacancies act as acceptors). The accepted mechanism of sensing of p-type semiconductor involves the adsorption of oxygen species (O-, O2-) on the surface like that of n-type semiconductors [3941]. Such oxygen adsorption leads to depletion of electrons and in consequence an increase in the concentration of holes (majority carriers) in the valence, (as e- + h. ↔ null), leading to an increase in the conductivity of the sensors. Incidentally, as discussed earlier SO2 can behave as an oxidizing or reducing gas in contact with an oxide semiconductor and the behaviour 9 Page 9 of 26

depends on the nature of the material and operating temperature [10,14,42]. In the present case, in contact with SO2, the resistance of BiFeO3 increases indicating that SO2 is oxidized to SO3 and in the process the bound electrons from the adsorbed oxygen species become free

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leading to lowering of the hole (majority carrier) concentration and in consequence, increase in the resistivity of the sensors.

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Interestingly, the average response time and the recovery time (90% of the final stable

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reading) of the sensors made from 2 h sonicated powder are 20 s (s.d. 1 s) and 50 s (s.d. 2 s), respectively, which are inferior to those of the sensors made from 40 min sonicated powder

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15 s (s.d. 1 s) and 30 s (s.d. 2 s), respectively, not shown here) though, as expected, the sensitivity is higher for 2 h sonicated powder, whose particle size is smaller than that of 40

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min sonicated powder. However, the reason behind the opposite trend of response and recovery time (vis-à-vis the sensitivity) with particle size is not presently clear. Incidentally,

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the sensor is selective to SO2 in presence of butane and carbon monoxide as the response of

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carbon monoxide (Fig. 8).

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BiFeO3 towards SO2 is much higher than its response towards 1000 ppm butane and 30 ppm

4. Conclusions

Phase-pure bismuth ferrite (BiFeO3) nanoparticles can be synthesized by

sonochemical method. In contrast, the conventional precipitation route gives rise to impurity phases along with rhombohedral BiFeO3. The average crystallite sizes of BiFeO3 nanoparticles were found to be 38 and 32 nm for 40 min and 2 h sonication (followed by calcination at 500ºC for 1 h), and the corresponding band gaps were 2.1 and 2.2 eV, respectively. The sensors fabricated from sonochemically synthesized BiFeO3 nanoparticles show very good sensitivity towards 5 ppm SO2 and the response and recovery are also quite 10 Page 10 of 26

fast. The sensors are also selective to low concentration of SO2 in presence of relatively high concentrations of CO and butane indicating that sonochemically prepared BiFeO3 can be a

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potential material for SO2 sensing.

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Acknowledgements

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The authors are grateful to the Director of CSIR-Central Glass and Ceramic Research Institute, Kolkata for his permission to publish the paper. One of the authors (SR)

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acknowledges financial support from Council of Scientific and Industrial Research,

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Government of India.

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Figure captions: Fig. 1: Sensor in TO package

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Fig. 2: X-ray diffractograms of sonochemically synthesized BiFeO3 nanoparticles (sonication

at 600ºC for 1 h (asterisks represent the Bi2Fe4O9 impurity phase).

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time 40 min) (a) as synthesized, (b) after calcination at 500ºC for 1 h and (c) after calcination

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Fig. 3: X-ray diffractograms of BiFeO3 powder obtained by conventional precipitation method (a) as synthesized and (b) after calcination at 500ºC for 1 h. (asterisks represent the

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Bi2Fe4O9 impurity phase).

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Fig. 4: (a) TEM image and (b) corresponding selected area electron diffraction pattern of BiFeO3 nanoparticles.

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sonication.

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Fig. 5: Absorption spectra of BiFeO3 nanoparticles (a) 40 min sonication and (b) 2 h

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Fig. 6: Band gap analysis of BiFeO3 nanoparticles (a) 40 min sonication and (b) 2 h sonication.

Fig. 7: Response of BiFeO3 sensors (2 h sonicated) towards 5 ppm SO2 at the operating temperature of 300ºC.

Fig. 8: Response of BiFeO3 sensors (for two different sonication times) towards SO2, butane and CO at the operating temperature of 300ºC.

17 Page 17 of 26

S. Das : After completing her M.Tech in Nanoscience and Technolgy, Ms Das has been persuing her Ph.D at Jadavpur University, Kolkata. Her research interest includes nanostructured gas sensors, CNT & graphene based materials.

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S. Rana : Dr. Rana has been presently active at Variable Energy Cyclotron Centre, Kolkata. His research interest focuses on nanostructured materials for biosensing and gas sensing applications.

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Sk Md Mursalin : Mr Mursalin has received his M.Sc degree in chemistry. He mainly focuses on the synthesis and characterization of metal oxide nanostructured material for gas sensing applications.

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P. Rana: Mr Rana has recently received his M.Tech degree in Nanoscience and Technology. His research interests include synthesis and characterization of nanostructured materials and chemical gas sensors.

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M

A.Sen : Dr A Sen is currently the Head of Sensor & Actuator division, CSIR-CGCRI, Kolkata. His research interest includes gas sensors, bio-medical sensors and piezoelectric actuators.

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Fig. 1

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Fig. 2

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Fig. 3

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(b)

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(a)

(024)

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Fig. 4

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(104)

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Fig. 5

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Fig. 6

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Fig. 7

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40 min sonication 2 h sonication

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2.0

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Response

(a)

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1000 ppm butane

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5 ppm SO2 30 ppm CO

Fig. 8

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