Sensors and Actuators B 224 (2016) 260–265
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Orthorhombic CaFe2 O4 : A promising p-type gas sensor a,b,∗ ˇ Andris Sutka , Margus Kodu b , Rainer Pärna b , Raando Saar b , a Inna Juhnevica , Raivo Jaaniso b , Vambola Kisand b a b
Institute of Silicate Materials, Riga Technical University, Paula Valdena 3/7, LV-1048, Latvia Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
a r t i c l e
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Article history: Received 5 May 2015 Received in revised form 8 October 2015 Accepted 13 October 2015 Available online 21 October 2015 Keywords: CaFe2 O4 Gas sensor Sol–gel
a b s t r a c t The search for p-type metal oxides with high gas response and stability is an important issue in gas sensor technology. In this work the p-type gas sensing properties of orthorhombic CaFe2 O4 are demonstrated for the first time. CaFe2 O4 nanopowders were successfully synthesized by the sol–gel auto-combustion method. The formation of orthorhombic CaFe2 O4 compound was confirmed by X-ray diffraction, energydispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analyses. The synthesized CaFe2 O4 compound showed p-type gas sensing behaviour and high gas response towards ethanol (Rg /Ra = 41.5) at a concentration of 100 ppm. Moreover, the CaFe2 O4 material tested during the experiments showed good stability. © 2015 Elsevier B.V. All rights reserved.
1. Introduction p-Type gas sensors demonstrate several advantages over n-type gas sensors [1]: (1) the ability to chemisorb the higher concentrations of oxygen, since the formation of a hole-accumulation space charge layer in p-type oxide semiconductors is not limited by concentrations of free charge carriers [2]; (2) p-type semiconductor gas sensors promote selective oxidation of various volatile organic compounds (VOCs) [3–6]; and (3) p-type oxide semiconductors show lower humidity dependence [7]. Many authors have demonstrated promising potential for practical applications of p-type oxide semiconductors and, until now, the best results have been presented by Yoon et al. for 3.04 at% Fe-doped NiO electrospun nanofibres, which showed gas responses (Rg /Ra = 245.0) towards 100 ppm C2 H5 OH [8]. Nevertheless, most p-type semiconductor oxides have moderate stability due to the transition of chemisorbed oxygen within the lattice [9], and also a theoretically lower gas response may be predicted for p-type oxide semiconductor gas sensors in comparison with n-type ones at similar morphological configurations [10]. In contrast with the large number of studies related to ntype oxide semiconductor gas sensors, the p-type ones are barely studied and further investigations are required to develop new
∗ Corresponding author at: Institute of Silicate Materials, Riga Technical University, Paula Valdena 3/7, LV-1048, Latvia. ˇ E-mail address:
[email protected] (A. Sutka). http://dx.doi.org/10.1016/j.snb.2015.10.041 0925-4005/© 2015 Elsevier B.V. All rights reserved.
p-type oxide semiconductor gas sensor materials. The five most popular p-type oxide semiconductor gas sensors are CuO, NiO, Co3 O4 , Cr2 O3 , and Mn3 O4 [1]. In this research work we will present the p-type gas sensing properties of the complex metal oxide semiconductor CaFe2 O4 for the first time. Calcium ferrite has already shown high potential for applications to oxide semiconductor ptype photoelectrodes [11], photocatalysts [12], solid catalysts [13], and electrodes of solid oxide fuel cells [14]. The gas sensing properties of CaFe2 O4 , to the best of our knowledge, have not been reported before. The potentially good catalytic properties make CaFe2 O4 particularly interesting for gas sensor research. Moreover, CaFe2 O4 exhibits an orthorhombic structure which is regarded as a highpressure polymorph of spinel, with corner- and edge-sharing distorted FeO6 octahedra forming so-called “double-rutile chains” with pseudo-triangular tunnels occupied by Ca2+ cations [15,16]. The structure of CaFe2 O4 is known to be stable in a wide temperature range [16–18]. Due to its catalytic properties and stability, CaFe2 O4 should be considered as a good candidate for stable p-type gas sensors.
2. Experimental details Sol–gel auto-combustion was used to produce CaFe2 O4 nanopowders. Iron nitrate nonahydrate (Fe(NO3 )3 ·9H2 O, >98%) and calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O, >99%) with the desired metal ratio of 2:1 were dissolved in Milli-Q water.
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Then citric acid monohydrate (C6 H8 O7 ·H2 O, >99%) was added to the nitrate solution. The molar ratio of nitrates to citric acid was 1:1. Metal nitrates act as an oxidizing agent and citric acid carboxylate groups act as a reducing agent for the combustion reaction. To improve the chelation of metal cations to citrates, a 26% ammonium hydroxide (NH4 OH) solution in water was added to the nitrate–citrate solution. To complete the chelation, a value of pH 7 was chosen, because citric acid is weakly dissociated at low pH. All chemicals were purchased from Sigma–Aldrich and were used as received without additional purification. The obtained solution was poured into a 100-cm3 corundum crucible and evaporated at 80 ◦ C under magnetic stirring. After evaporation of water, a viscous gel was obtained. Additionally the gel was dried for 24 h at 60 ◦ C to remove water residues. Then the gel was heated to 250 ◦ C to initiate a self-sustaining combustion reaction and produce as-prepared ferrite powder. Ferrite samples shaped as 1-mm thick pellets of 10 mm diameter for gas sensing measurements were uniaxially pressed at 5 MPa from the as-prepared powders and sintered at 850 ◦ C for 3 h. To achieve good ohmic contacts on the sample surface, Ag electrodes were applied by using high purity Ag paint (SPI Supplies, USA). Samples were tested by X-ray powder diffraction (XRD) recorded at 2 from 10◦ to 60◦ at a scanning rate of 1◦ min−1 using an Ultima+ X-ray diffractometer (Rigaku, Japan) with Cu K␣ radiation. The morphology and elemental composition of the asprepared powders and sintered pellets were studied using an FEI FIB-SEM Helios Nanolab 600 equipped with Oxford Inca 350 with an X-Max 50 mm SDD-type detector. The electronic structure and chemical state of Fe and Ca in samples were investigated by X-ray photoelectron spectroscopy (XPS). To obtain XPS spectra, powder was pressed into indium. XPS measurements were conducted using a Scienta SES-100 electron energy analyser and a Thermo XR3E2 non-monochromatized twin anode X-ray source (Al K␣ 1486.6 eV). The binding energy scales for the XPS experiments were referenced to the binding energy of the In 3d (444.0 eV) photoemission line. Data analysis was carried out with CasaXPS software (version 2.3.12). Fe 2p, O 1s, and Ca 2p photopeaks were fitted by using symmetric Gaussian–Lorentzian line shapes after subtracting a Shirley-type background. Gas sensing measurements were carried out in a closed test chamber in an ambient atmosphere. Before the gas response measurements were taken, sensor samples were stabilized at the operating temperature in air for 5 h. To evaluate the gas response properties, ethanol (100 ppm) vapour was used as a test gas. The change of the sample electrical resistivity with gas exposure was monitored by an Agilent 34970A digital multimeter. The gas response was determined as Rg /Ra , where Rg is the sample resistance in gas and Ra is the sample resistance in air. For the stability experiment, the sensor structure was prepared by the drop casting method using Al2 O3 substrate with pre-patterned Pt electrodes. CaFe2 O4 nanopowder and ethylene glycol solution were dropped onto the electrode substrate, which was heated to 120 ◦ C. A thin layer of CaFe2 O4 nanopowder was formed on top of the Pt electrodes as a result. After that, the sensor structure was annealed for 12 h in air at 500 ◦ C. Measurements regarding the stability of electrical and gas response characteristics were carried out with a Keithley 2400 sourcemeter, a gas mixing system based on five Brooks mass-flow controllers, and a sample chamber with a small hotplate heater whose temperature was held at 250 ◦ C during the experiment. The voltage applied to the Pt electrodes was 3 V. A mixture of N2 and O2 gases (both 99.999% pure) was used as a carrier gas. The source of NH3 additive was a mixture of 100 ppm NH3 in N2 (99.999% pure). The test was conducted under a dry atmosphere. The oxygen concentration (21%) and the flow rate (200 ml/min) of the gas mixture were kept constant during the measurements.
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3. Results and discussion Fig. 1(a) shows the XRD pattern of the as-prepared powders. During the auto-combustion reaction, a mixture of various compounds such as CaCO3 (ICDD 04-007-4989), Fe2 O3 (ICDD 00-002-1047), -Fe2 O3 (ICDD 00-004-0755), and CaFe2 O4 (ICDD 04-007-4989) forms. However, after annealing at 850 ◦ C for 3 h, the admixture of different compounds crystallizes to pure CaFe2 O4 compound (Fig. 1(b)). The peak positions correspond well to the CaFe2 O4 -type orthorhombic unit cell (ICDD 04-007-4989) with lat˚ (b) 10.670 A, ˚ and (c) 3.012 A. ˚ tice constants (Pnam) of (a) 9.160 A, No additional impurity phases were observed. Fig. 2 presents the SEM micrographs of the as-prepared powders and the fractured surface of the gas sensor pellet sample annealed at 850 ◦ C for 3 h. The as-prepared sample powders are composed of amorphous-like anisotropically shaped and closely packed grains (Fig. 2(a)). After annealing at 850 ◦ C for 3 h, the porous structures of interconnected grains were observed (Fig. 2(b)), while the grains keep their anisotropic shape and the size of individual grains of smaller dimensions varies from 70 to 300 nm, while the length of anisotropic nanoparticles is up to 650 nm. Particles are very well interconnected and fused together, at the same time maintaining open structures for gas diffusion. Gas-accessible microstructures are preferred for a high gas response. EDX analysis was employed to investigate the composition and stoichiometry of the CaFe2 O4 compound (Fig. 2(c)). Intense O, Ca, and Fe peaks were observed and a Ca to Fe atomic ratio of 2.01 was observed (see inset in Fig. 2(c)), confirming the successful preparation of the CaFe2 O4 . The Fe 2p photoelectron spectrum of CaFe2 O4 annealed at 850 ◦ C is presented in Fig. 3(a). The photoelectron peak positions are at
Fig. 1. XRD patterns of as-prepared (a) and annealed powders (b).
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Fig. 2. SEM micrographs of as-prepared powder (a) and cross-section of gas sensor pellet annealed at 850 ◦ C for 3 h (b), as well as EDS profile of annealed sample (c).
711.2 eV for Fe 2p3/2 and at 725.1 eV for Fe 2p1/2 , while spin-orbit splitting is at 13.9 eV. The spectrum also exhibits a distinguishable satellite structure at 7.7 eV above the main photoelectron line. Observed spectra demonstrate formation of Fe3+ ions in CaFe2 O4 sample without Fe2+ reduced counterparts. The positions of Fe2p photoelectron lines are at slightly higher binding energies than previously observed in CaFe2 O4 –Ca2 Fe2 O5 -based catalyst [19] but are similar to those of Ca–Mn–Fe3 O4 [20]. The Ca 2p photoelectron spectrum (Fig. 3(b)) demonstrated four components. These belong to Ca2+ photoelectron lines in two different compounds separated by spin orbit splitting. The first two components are positioned at 346 and 349.3 eV. These are related to Ca2+ in sample and confirm formation of CaFe2 O4 in the surface region. In Ca–Mn–Fe3 O4 , Ca 2p is observed at around 347 eV [20]. Other Ca 2p photolines at 347.8 and 351.4 eV are related to CaCO3 residues [21]. Small amount of CaCO3 could be left on the surface after annealing at temperatures around 850 ◦ C. The XPS spectrum of O 1s demonstrated two photoelectron lines (Fig. 3(c)) one at about 530 eV and the other at about 532 eV. The component at 530 eV is attributed to O2− ions bonded with Ca2+ and Fe3+ . The photoelectron peak at higher binding energy can be attributed to surface carbonate species [21], loosely bound oxygen or hydrated oxides. Fig. 4(a) demonstrates the gas response Rg /Ra of CaFe2 O4 gas sensor to 100 ppm ethanol at different temperatures. The gas response to 100 ppm ethanol reached a maximum when operating at 200 ◦ C. When the operating temperature was raised, desorption of inactive surface hydroxyl groups and chemisorption of oxygen increased in the reaction with the test gas [22]. The decrease in the gas response beyond 200 ◦ C is attributed to reduced adsorption ability of the test gas and/or desorption of active oxygen species. At the optimal operating temperature, the sensor gas response Rg /Ra is 41.3, which can be considered very high for a p-type pellet gas sensor element constructed from dense particles with sizes from 70 to 300 nm. For comparison, Table 1 shows the gas responses to ethanol for different p-type oxide semiconductor based gas sensor materials [23–35]. The response–recovery behaviour at the optimal operating temperature (200 ◦ C) of the CaFe2 O4 gas sensor is demonstrated in Fig. 4(b). As expected, the electrical resistance increases when 100 ppm of ethanol is introduced into the chamber, showing ptype conductivity behaviour. During oxygen chemisorption, the
Fig. 3. XPS spectrum of CaFe2 O4 measured from the Fe2p, Ca 2p, and O 1s region with excitation energy of 1486.6 eV.
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Fig. 5. Gas response of CaFe2 O4 to various gases (100 ppm) at 200 ◦ C.
Fig. 4. The gas response Rg /Ra of CaFe2 O4 gas sensor to 100 ppm ethanol at different temperatures (a), the response–recovery behaviour of the CaFe2 O4 gas sensor at the optimal operating temperature (200 ◦ C).
formation of a hole-accumulation layer is observed, because oxygen molecules are absorbed onto the surface of the p-type oxide semiconductor and ionize by attracting electrons, thus increasing the dominating charge carrier (hole) concentration in the p-type material. When a p-type oxide semiconductor gas sensor material is exposed to a reduced gas, the gas molecules are oxidized by pre-adsorbed oxygen on the semiconductor surface and previously attracted electrons are returned to the hole accumulation layer, where they recombine with the free holes and decrease the hole concentration, which in turn increases the electrical resistance of the gas sensor material [36]. The time taken to reach
90% of the maximum change in resistance after 100 ppm ethanol exposure is ∼4 min, but the recovery time to restore the initial resistance is 30 min. The long response and recovery times are attributed to the large volume of the pellet type gas sensor element. For gas sensing experiments, a pellet with a diameter of 10 mm and thickness of 1 mm was used. However, the results shows that orthorhombic CaFe2 O4 is a promising p-type gas sensor and future studies related to other nanostructures of CaFe2 O4 integrated in various architectures of sensor elements are significantly important. Several options and possibilities have been proposed that may be able to increase the gas response–recovery kinetics. For instance, morphology tuning, electronic sensitization by doping, and chemical sensitization by loading noble metals have been proposed [1]. The maximum gas response observed for other target gases at 200 ◦ C is shown in Fig. 5. It was found that gas response falls in the order ethanol > isopropanol > acetone > toluene > hexane > CO. The results indicate that the selective detection of alcohols using a CaFe2 O4 gas sensor may be possible. The tested sensor showed a higher response towards polar VOCs and moderate response to non-polar VOCs and CO. The mechanism of interaction of the target gas with the chemisorbed oxygen is a complex process consisting of many steps, and thus all details regarding the selectivity of the CaFe2 O4 gas sensor are awaiting further investigations. In order to evaluate the repeatability and stability of the gas sensing properties of CaFe2 O4 material over time, the sensor structure was prepared by using the drop coating method. Sensor responses to 100 ppm of NH3 gas were measured during 38 h with a 2.5 h interval between measurements. The results are depicted
Table 1 Gas responses towards ethanol for different p-type metal oxide semiconductor gas sensors. p-Type sensor material
Morphology
NiO
Hollow semispheres Ultralong nanowires Hierarchical microspheres from nanosheets Porous nanosheets Nanoparticles (10 nm)
Ethanol concentration
Gas response Rg /Ra
Sensor architecture
Thickness of sensing layer
Reference
200 ppm 500 ppm 100 ppm
5.0 2.4 3.2
Film on Au finger electrodes Taguchi sensor Taguchi sensor
∼100 nm ∼50 m Not specified
[23] [24] [25]
1000 ppm 100 ppm
3.4 3.0
Taguchi sensor Taguchi sensor
>1 m Not specified
[26] [27]
Taguchi sensor Film on Au finger electrodes Not specified Taguchi sensor
Not specified Not specified Not specified Not specified
[28] [29] [30] [31]
CuO
Nanosheets Featherlike particles Nanoribbons Nanorods
500 ppm 500 ppm 500 ppm 500 ppm
4.5 3.0 2.9 9.5
Co3 O4
Nanosheets Nanocubes Nanorods
100 ppm 100 ppm 500 ppm
57.7 5.2 70.0
Film on Au finger electrodes Taguchi sensor Film on Au–Pd finger electrodes
Not specified Not specified ∼20 m
[32] [33] [34]
Cr2 O3 CaFe2 O4
Nanofibers Nanoparticles (70–300 nm)
1000 ppm 100 ppm
1.7 41.3
Not specified Sensor pellet
Not specified 1 mm
[35] This work
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Fig. 6. Variability of resistance in air Ra , resistance in gas Rg and gas response Rg /Ra to 100 ppm NH3 gas over 35 h at 250 ◦ C.
in Fig. 6. The sensor response was stable during the investigated time interval and Rg /Ra value varied slightly between 4.3 and 4.5, which is fairly high response to inorganic gas for a p-type gas sensor material. For previously investigated p-type sensor materials, the responses Rr /Ra to inorganic gaseous air pollutants are typically up to 1–4 [1]. Generally, n-type oxides are considered to be thermally stable and, in contrast, many p-type oxides are unstable due to oxidation of metal ions in the crystal lattice [9]. The insufficient stability of p-type gas sensors is one of the most critical drawbacks for their use in practical applications. Our sensor based on CaFe2 O4 material showed a stable and reproducible response to 100 ppm NH3 gas and is therefore promising as a candidate material for sensor applications. 4. Conclusions In summary, a p-type orthorhombic CaFe2 O4 nanopowder was successfully synthesized by the sol–gel auto-combustion method. The formation of orthorhombic CaFe2 O4 compound was confirmed by XRD, EDX, and XPS analyses. The results showed that CaFe2 O4 is a promising p-type gas sensor which could be used for selective detection of alcohols. Moreover, the CaFe2 O4 material tested during the experiments showed good sensing stability. The lack of stability is the most critical drawback to the use of p-type gas sensors in practical applications. Orthorhombic CaFe2 O4 could be a promising p-type gas sensor in this sense. Acknowledgements Support for this work was provided by the Riga Technical University through the Scientific Research Project Competition for Young Researchers No. 34-14100-ZP-2014/24. This work was also supported by institutional research funding IUT (34-27) of the Estonian Ministry of Education and Research. References [1] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B: Chem. 192 (2014) 607–627. [2] S.-W. Choi, A. Katoch, J.-H. Kim, S.S. Kim, Remarkable improvement of gas-sensing abilities in p-type oxide nanowires by local modification of the hole-accumulation layer, ACS Appl. Mater. Interfaces 7 (2015) 647–652. [3] N.G. Cho, H.-S. Woo, J.-H. Lee, I.-D. Kim, Thin-walled NiO tubes functionalized with catalytic Pt for highly selective C2 H5 OH sensors using electrospun fibers as a sacrificial template, Chem. Commun. 47 (2011) 11300–11302. [4] F. Zhang, A. Zhu, Y. Luo, Y. Tian, J. Yang, Y. Qin, CuO nanosheets for sensitive and selective determination of H2 S with high recovery ability, J. Phys. Chem. C 114 (2010) 19214–19219.
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Biography Andris Sˇ utka received his Ph.D. in engineering sciences in 2012 after performing research on transition metal oxide physical and chemical properties. His main scope of interests during the Ph.D. studies covered defect chemistry of metal oxides, chemical oxide semiconductor gas sensors ˇ and photocatalysts. A. Sutka is the first author of more than 30 ISI scientific publications and 3 patent applications. In 2014 he received the Riga Technical University award “Young Scientist of the Year” and the Latvian Academy of Science award in physics “Prize of Ludvigs and Maris Jansons”.
