Spinel ferrite nanoparticles for H2S gas sensor

Spinel ferrite nanoparticles for H2S gas sensorAhmad I. Ayesh1  · Mohammad Abu Haija2 · Adel Shaheen3 · Fawzi Banat4 
Received: 18 August 2017 / Accepted: 6 October 2017 © Springer-Verlag GmbH Germany 2017
is a metal oxide that exhibits cubic close-packed structure and tetragonal symmetry [2]. Due to its the magnetic prop- erties, CuFe2O4 utilization for practical applications allows its retrieval and reuses for further applications. Therefore, CuFe2O4 finds a wide range of applications in different fields including gas sensors and catalysis [3].
Nanoparticles are aggregates of atoms with diameter of the order of nanometer [4, 5]. They exhibit novel physical and chemical properties that make them suitable for differ- ent applications [6–8]. The large surface-to-volume ratio of nanoparticles increases their reactive surface; thus, they are, in general, highly reactive [9]. Therefore, they are efficient for applications that require high reactivity including the fabrication of sensitive gas sensors [10–12]. Those sensors have a wide range of practical applications including envi- ronment quality monitoring, safety, and control [13].
CuFe2O4 nanoparticles were synthesized previously using different techniques including: sol–gel [14], sol–gel auto- combustion [15], hydrothermal method [16], auto-combus- tion-assisted sol–gel [17], and microwave-assisted method [18]. Nevertheless, co-precipitation synthesis method is considered an efficient and fast method for CuFe2O4 nano- particle synthesis [19].
Hydrogen sulfide (H2S) is a highly toxic gas that is pro- duced in many fields including petroleum-related industries [20]. Low concentrations of H2S, as low as tens of ppm, may cause sever effects to human and deaths at high concentra- tions. Therefore, highly sensitive and selective H2S sensors are demanded to control its emission.
This work focuses on the fabrication of H2S sensors based on CuFe2O4 spinel ferrite nanoparticles produced by the co- precipitation method. The nanoparticles are pressed in a disc form, and placed between two electrical electrodes forming a conductometric gas sensor. The study includes investigations of nanoparticle composition and morphology as well as their
Abstract We investigate H2S gas sensors based on spinel ferrite CuFe2O4 nanoparticles prepared by the co-precipita- tion method. This technique is an efficient and fast technique to produce nanoparticles. Furthermore, those nanoparticles are metal-oxide nanoparticles with magnetic properties that enable their retrieval and reuse for mutable times and appli- cations. The produced nanoparticles are pressed in a form of disc, and placed between two electrical electrodes, with the top electrode with a grid structure to enable gas exposure. The study includes investigations of crystal structure, com- position, morphology, and charge transport. The results revel that the produced nanoparticles are sensitive and selective to H2S which indicate their potential to be used for practical applications.
1 Introduction
Spinel ferrites are materials of the form MFe2O4 (M = Zn, Cu, Ni, Cd, etc.) and they exhibit magnetic properties that are dependent on their synthesis method, grain size, and annealing temperature [1]. Spinel copper ferrite (CuFe2O4)
* Ahmad I. Ayesh [email protected]
1 Department of Mathematics, Statistics and Physics, Qatar University, P. O. Box 2713, Doha, Qatar
2 Department of Chemistry, Khalifa University of Science and Technology, Petroleum Institute, Abu Dhabi, United Arab Emirates
3 Physics Department, Hashemite University, Zarqa, Jordan 4 Department of Chemical Engineering, Khalifa University
of Science and Technology, Petroleum Institute, Abu Dhabi, United Arab Emirates
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electrical transport properties. The utilization of spinel fer- rite nanoparticles allows their retrieval and reuse for further applications. This study reveals that those nanoparticles can be used for H2S gas sensors for practical applications.
2 Experimental
2.1 Preparation of  CuFe2O4 powder
The co-precipitation method was use to prepare copper fer- rite (CuFe2O4) powders. The procedure started with the preparation of an aqueous solution of Fe(NO3)3·9H2O and Cu(NO3)2·3H2O in a ratio of (2:1) with continuous stirring. Then, the metal nitrate solution was mixed at room tem- perature with NaOH (6M) using (1:1) molar ratio of metal ions to hydroxyl ions. The resulted precipitate was filtered and washed with distilled water. Finally, the precipitate was dried overnight at 120 °C in the air. Hereafter, the obtained powder is designated “as-prepared”. The as-prepared pow- der was then subjected to heat treatments at 500 and 750 °C for 5 h in the air, as presented in Table 1.
2.2 Sensor fabrication
A hydraulic press was used to create a disc of 13 mm in diameter using a presser of 20 MPa. The disc was placed between two electrodes: the back electrode was a copper sheet of diameter of 20 mm, while the top one is a stainless steel (SS) grid with grid dimensions of 250 µm  ×  250 µm, as shown in Fig. 1. The electrodes and disc were fixed with
a Scotch tape, such that the exposed area of the disc was 6 mm × 6 mm. The sensor was connected electrically using silver glue, and fixed on a temperature controlled stage inside a Teflon gas test chamber.
2.3 Characterization
Crystal structure as well as composition of the prepared nanoparticles were tested by X-ray diffraction (XRD) using a PANalytical Powder Diffractometer operated at 40 mA and 40 kV using Cu-K radiation with = 1.5406 . The diffrac- tion pattern was collected in the 2θ range of 10–80° with a step of 2θ = 0.02°.
An FEG-QUANTA-250 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectros- copy (EDS) apparatus was used to image the nanoparticles and to confirm their composition. The SEM was operated at 30 kV, and all samples were coated with gold prior to analy- sis. Furthermore, high-resolution images of nanoparticles were obtained using an FEI-Tecnai-20 transmission electron microscope (TEM) with a 200 kV accelerating voltage. For TEM images, nanoparticles were dispersed in ethanol and sonicated for 5 min, then deposited on carbon coated copper grids. The grids were dried under infrared light.
Current–voltage [I(V)] measurements were conducted using a Keithley Instruments source measurement unit (KI- 236). For sensitivity measurements, each sample was place inside the temperature controlled Teflon chamber. The tem- perature was measured using a K-type thermocouple placed close to the sample. Each target gas was introduced into the test chamber relative to nitrogen, and their flow rates were controlled using Bronkhorst mass flow meters. The gas response for a particular sample was determined by apply- ing a constant voltage across the sample and measurement of the variation of the electrical current, as shown in Fig. 1. Fourier transform infrared (FT-IR) spectra were recorded using an ALPHA spectrometer from Bruker in the range of 300–4000 cm− 1.
3 Results and discussion
Figure 2 represents the XRD patterns of the co-precipitation derived as-prepared and annealed samples. As indicated by the diffraction peaks, the prepared annealed CuFe2O4 can be indexed as spinel cubic phase (JCPDS card 77-0010) [15]. The peaks are labelled to (111), (220), (311), (222), (400), (422), (511), and (440) reflections. The sharp and intense diffraction peaks imply the good crystallization of CuFe2O4 and the growth of crystallite size at high calcina- tion temperatures. However, the diffractograms demonstrate the presence of other phases in the as-prepared and annealed at 500 °C samples. The diffractogram of as-prepared sample
Table 1 Samples of CuFe2O4 used in the present study
As-prepared Sample annealed at 500 °C
Sample annealed at 750 °C
S102 S112 S122
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is mainly due to Cu2(OH)2(NO3)3 (the indices on the as- prepared sample belong to Cu2(OH)2(NO3)3) [21], while the 500 °C annealing sample can be viewed as a mixture of CuFe2O4, CuO, and Fe2O3 [22]. The XRD spectrum at 750 °C represents mainly a cubic phase of CuFe2O4 based on the literature data. However, the XRD pattern at the same temperature (i.e., 750 °C) in reference [15] is related to the tetragonal symmetry of CuFe2O4 which can be assigned to the different preparation method. Furthermore, the XRD fig- ure suggests that a reaction between Fe2O3 and CuO leads to the formation of CuFe2O4. However, the sample at 750 °C still contains CuO as suggested by the XRD spectrum, and does not show any Fe2O3 peaks. Since the reaction between Fe2O3 and CuO is a 1:1 ratio, the absence of Fe2O3 peaks in the XRD spectrum of the sample at 750 °C may suggest that more CuO exist in the 500 °C samples. Herein, the presence of CuO within the sample is likely to support H2S-sensing properties of the sensor, since CuO was reported to exhibit high affinity towards H2S gas [10].
Scherrer’s equation allows the calculation of crystal size using
where K is shape factor (dimensionless) and used to be 0.9 in the current calculations [15], is the wavelength of the used XRD, is the full-width at half-maximum, and is the Bragg’s angle. The equation produces nanoparticle size of 24, 21, and 33 nm for the samples: as-prepared, annealed at 500 °C, and annealed at 750 °C, respectively. Here, nano- particle size was calculated using the peak at 2θ = 36.5° for as-prepared sample and the 311 peak for annealed samples.
(1)D = K
cos ,
Those peaks were chosen because of their high intensities. Nanoparticle sizes were calculated using different peaks, and the calculated sizes were consistent with the above results within the range 10–35 nm. It should be noted her that using XRD peaks with low intensities for nanoparticle size calcu- lation is expected produce none accurate results.
Figure 3 shows the FT-IR spectra of as-prepared and annealed samples. FT-IR measurements were performed to verify the formation of the spinel phase. Considering the structure of spinel CuFe2O4, two vibrational modes below 600 cm− 1 are expected in the FT-IR spectra. The high frequency mode is related to the stretching vibration of metal–oxygen ions in tetrahedral sites, whereas the low- frequency mode is associated with stretching vibration of metal–oxygen ions in octahedral sites. Two absorption bands at about 370 and 520 cm− 1 are identified in the FT-IR spectra (Fig. 3), which can be attributed to the spinel ferrite. Accord- ing to the XRD results, the as-prepared powder contains Cu2(OH)2(NO3)3. The FT-IR spectrum of the as-prepared sample shows three peaks at 875, 780, and 670 cm− 1 which can be assigned to the Cu–O–H bond. In addition, the peaks at 1045, 1345, and 1418 cm− 1 can be attributed to the nitrate group. The broad absorption band at about 3400 cm− 1 links to hydrogen bonded of OH groups, while the small peak at 3543 cm− 1 is attributed to structural OH groups. The band at 1630 cm− 1 is assigned to the bending mode of H2O molecules. The bands related to Cu2(OH)2(NO3)3 disappear upon annealing, in agreement with the XRD results (Fig. 2).
The morphology of produced nanoparticles is shown by SEM images, as shown in Fig. 4. The figure reveals large distribution of sizes for nanoparticles of the as-prepared sample. Annealing the sample at 500 °C causes the nano- particles to increase in size and becomes more uniform, as
Fig. 2 XRD patterns of the prepared samples: the as-prepared, annealed at 500 °C, and annealed at 750 °C
Fig. 3 FT-IR spectra of the prepared samples: the as-prepared, annealed at 500 °C, and annealed at 750 °C
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shown in Fig. 4b. Further annealing of the sample at 750 °C causes the nanoparticles to additional growth in size but keep their uniform shape.
Nanoparticle size can be estimated from TEM images, as shown in Fig. 5. The figure reveals nanoparticle size of 10 ± 2, 22 ± 3, and 25 ± 3 for as-prepared, annealed at 500 °C, and annealed at 750 °C, respectively. The lower size of nanoparticles as compared with that calculated from XRD results can be assigned to the agglomeration of nanoparticles during XRD measurements, as shown in Fig. 4.
Nanoparticle agglomeration has a main implication on the gas-sensing properties of the samples as discussed below. Nanoparticle composition is confirmed using EDS meas- urements, as shown in Fig. 6, for the three samples. The figure reveals typical EDS spectra for CuFe2O4 spinel fer- rite nanoparticles with all standard peaks. This confirms the acceptable purity of the produced nanoparticles. It should be noted that the peaks at about 7 and 9 keV in the figure correspond to Fe and Cu of Kβ energy levels (the spectrum shows only alfa levels of the elements).
Figure 7 shows the I(V) measurements of three samples at 260 °C. The figure reveals non-linear characteristics with
low current. The figure also shows that the two samples: as- prepared and annealed at 750 °C have almost similar resist- ance. Nevertheless, the sample annealed at 500 °C reveals higher resistance. The higher resistance here can be assigned to: (1) the existence of different Fe2O3 oxide (see Fig. 2) that has higher resistivity than CuFe2O4 [23, 24] and (2) the minimum agglomeration and uniform nanoparticle size (see Fig. 4) which decrease the contact between nanopar- ticles and causes the lower electrical resistance. The non- linear I(V) characteristics was found common for different systems that include charge transport by quantum tunneling and Schottky barriers within semiconducting materials [10, 25, 26].
A representative gas sensitivity test for H2S gas of the sample annealed at 500 °C measured at 200 °C is shown in Fig. 8. The figure demonstrates an increase in the electrical current signal upon exposure to gas. The electrical current returns to its original value when the gas is removed from the chamber and pure nitrogen gas is introduced. The rela- tive change in electrical resistance value when the sensor is exposed to gas and nitrogen is defined as the sensor response and given by the equation:
Fig. 4 SEM images for a as-prepared sample, b annealed at 500 °C for 5 h, and c annealed at 750 °C for 5 h. d TEM image of as-prepared sam- ple
Fig. 5 TEM images for a as-prepared sample, b annealed at 500 °C for 5 h, and c annealed at 750 °C for 5 h. d TEM image of as-prepared sam- ple
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Herein, Rgas and R0 are the electrical resistance with and without the target gas, respectively. Figure 9a–c shows gas response measurements for H2S gas of the three prepared samples as a function of temperature. The figure shows that sensor response increases with increasing temperature and H2S concentration. Annealed samples show response to H2S
(2)S =
.
at room temperature with a minimum reasonable response at 25 ppm. Most variation in response occurs at low H2S concentrations that is desirable for safety and environment monitoring applications. The dependence of H2 response for the sensor on its concentration is shown in Fig. 9e–f. The figures reveal response to H2 gas which is sample dependent. The highest response is for the sample annealed at 500 °C. However, sensor response could be measured at a minimum operating temperature of 140 °C. In addition, the sensor is functional for H2 detection at much higher concentrations compared with that of H2S; thus, the fabricated sensors are selective to H2S.
Sensor response time is defined as the time for sensor signal to reach 90% of its maximum; thus, it is essential fac- tor that determines sensor functionality. Negligible depend- ence of response time on temperature and gas concentration was observed. Therefore, the response time was taken as the average response time at different H2S concentrations and temperatures, and the error is one standard deviation. For the
Fig. 6 EDS composition analysis as measured using SEM. a As-pre- pared sample, b annealed at 500 °C for 5 h, and c annealed at 750 °C for 5 h
Fig. 7 Current–voltage measurements for a as-prepared sample, b annealed at 500 °C for 5 h, and c annealed at 750 °C for 5 h
Fig. 8 Representative current measurement for H2S at 200 °C for the samples annealed at 500 °C for 5 h
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present sensors, the response time for H2S gas is 32 ± 10 s. This response time is considered a reasonable fast response time as compared with the published results [15, 27–30]. Table 2 demonstrates a comparison between H2S-sensing properties of CuFe2O4 spinel ferrite nanoparticles for the present sensor as compared with the previously reported sensor that was synthesized by the sol–gel auto-combustion technique [15]. The table reveals that the present sensor is faster compared with that synthesized by the sol–gel auto- combustion technique. The other factors are almost similar. Thus, the present synthesis technique is worth using con- sidering that it is an efficient and fast technique to produce nanoparticles.
The H2S gas-sensing behavior of the produced sensors can be related to number of reactive sites on nanoparticles. Herein, the sample annealed at 500 °C exhibits the minimum agglomeration and uniform nanoparticle size which explains its maximum response for H2S and H2. Furthermore, the low
response of as-prepared sample may be related to the low CuFe2O4 content and the presence of Cu2(OH)2(NO3)3. The gas-sensing mechanism can be assigned to surface intro- duced processes that include adsorption of oxygen ions that includes O− and O2−. Such adsorption causes variation of surface charge and thus electrical conductivity. In general, oxygen adsorption and ionization can be described as [10, 30]
Here, the subscript “ads” refers for adsorbed oxygen ion. The equations reveal that adsorbed oxygen on nanoparticle surface plays a major role in determining gas sensor behav- ior. The adsorbed spices lead to metal reduction that release electrons to the conduction band and increase electrical con- ductivity of the measured sensor [2]. Sensor temperature has a direct impact on its behavior, where it controls the amount of adsorbed gas on nanoparticle surface. Herein, sensor response is dominated by the speed of diffusion of gas mol- ecules at high temperature and by the speed of the reaction at low temperature [15, 31]. Sensor response is minimum at room temperatures, since H2S gas molecules exhibit low thermal energy that is required to react with the adsorbed oxygen ions on nanoparticle surface. At elevated tempera- tures, sensor response increases because of the increase in thermal energy required to overcome the activation energy barrier for surface reaction. Furthermore, increasing tem- perature increases electron concentration and electrical
(3)H2S + 3O−(ads) → H2O + SO2 + 3e−,
(4)H2S + 3O2−(ads) → H2O + SO2 + 6e−.
Fig. 9 Sensor response as a function of gas concentration and temperature for a–c H2S gas, and d–f H2 gas. a, d as-prepared sample, b, e annealed at 500 °C for 5 h, and c, f annealed at 750 °C for 5 h
Table 2 Comparison between H2S-sensing properties of CuFe2O4 spinel ferrite nanoparticles in the current study and nanoparticles pre- pared by sol–gel auto-combustion technique [15]
Present work: co- precipitation
Sol–gel auto-com- bustion
Particle size 10–25 nm 25–36 nm Average response time 32 ± 10 s 51.5 ± 3.4 s Maximum response to H2S ~ 0.3 ~ 0.4 Minimum sensitivity 25 ppm 25 ppm
Spinel ferrite nanoparticles for  H2S gas sensor
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conductivity due to the high conversion of adsorbed oxygen from O2
− to O− [31]. Optimum sensor response is achieved at a temperature, where the speeds of diffusion and reaction are maximum. Such a temperature is different depending on gas type and nanoparticle composition. Accordingly, sen- sor response increases with gas concentration and tempera- ture until the optimum temperature is reached, and then, it remains constant. It should be noted her that the present sensor is operational at low temperatures, as low as 20 °C which indicates its low power consumption.
The electrical resistance of each gas sensor was meas- ured after 2 min exposure to 300 ppm of H2S gas at differ- ent temperatures, and the results are shown in Fig. 10. The behavior of the resistance with temperature is similar quali- tatively, where the resistance increases with temperature to a maximum then decreases. The nanostructure of CuFe2O4 is considered semiconducting material, where the resistance is exhibit negative temperature coefficient with temperature [15]. Therefore, the increase in resistance at low temperature can be assigned to the release and accumulation of molecules such as H2O as a result of the reaction between H2O and O2 on the surface of nanoparticle. At the optimum operation temperature, H2O can be operated efficiently which results in the normal behavior of CuFe2O4. This is supported by the results in Fig. 9, where the maximum response is observed the optimum operation temperature, for example, for the sen- sor based on nanoparticles annealed at 750 °C exhibits the maximum response at 140 °C. Beyond this temperature, the response is maximum.
4 Conclusion
CuFe2O4 nanoparticles were prepared by co-precipitation method and used for H2S gas sensor applications. The results
reveal an average nanoparticle size of 10 ± 2, 22 ± 3, and 25 ± 3 nm for as-prepared, annealed at 500 °C, and annealed at 750 °C, respectively. The composition of nanoparticles was confirmed by dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) measurements. XRD meas- urements reveal a spinel cubic structure of the produced nanoparticles. The fabricated sensors were produced by pressing nanoparticles into discs using a hydraulic. Each disc was placed between two metallic electrical electrodes of capacitor structure, where the top electrode exhibits a grid structure to allow nanoparticle exposure to H2S gas. Gas response measurements were established by applying a constant voltage across the electrodes and observation of electrical current signal. The results reveal that nanoparti- cles annealed at 500 °C exhibit the best response to H2S gas at an operation temperature of 140 °C. Nevertheless, the sensors are functional at low temperatures, as low 20 °C, which confirms its low power consumption. The presented results are promising and can be built on to further enhance the sensitivity of the H2S sensor. Herein, controlled doping of CuFe2O4 nanoparticles can increase their affinity (and sensitivity) to H2S.
Acknowledgements This work was supported by the Petroleum Insti- tute under Grant No. RIFP-14312 and Qatar University under Grant No. QUUG-CAS-DMSP-15\16–20.
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Abstract
2.2 Sensor fabrication