Materials Chemistry and Physics xxx (2016) 1e7
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Copper doped nickel ferrite nano-crystalline thin films: A potential gas sensor towards reducing gases Pratibha Rao, R.V. Godbole, Sunita Bhagwat* Department of Physics, Abasaheb Garware College, Karve Road, Pune 411 004, India
h i g h l i g h t s Cu:NiFe2O4 thin films are synthesized by low cost spray pyrolysis technique. Addition of Cu content improves magnetic properties. Cu content on the surface of the film enhances the gas response. NiFe2O4 thin films exhibit predominant selectivity towards ethanol. 1 wt% Cu:NiFe2O4 film responses towards ethanol at lower optimum temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2015 Received in revised form 29 October 2015 Accepted 3 January 2016 Available online xxx
NiFe2O4 and (1 wt% and 3 wt%) Cu:NiFe2O4 thin films have been fabricated using spray pyrolysis deposition technique at 350 C and then sintered at 650 C for 3 h. X-ray diffraction, SEM, EDAX, UV-VIS spectroscopy, SQUID VSM were carried out to investigate phase formation, microstructural and influence of Cu doping on magnetic properties of NiFe2O4 thin films. The gas response towards various gases viz. ethanol, Liquid Petroleum Gas (LPG), methanol and hydrogen sulfide (H2S) is investigated. The results of XRD revealed that all samples had shown the principal phase of nickel ferrite and the lattice parameter was found to vary from 8.294 Å to 8.314 Å on an incorporation of Cu, and the crystalline sizes were about 40e45 nm. The effect of Cu concentration on saturation magnetization and coercive force were studied. The maximum value of saturation magnetization calculated from hysteresis loop was 89.16 emu/g at room temperature and 96.88 emu/g at 50 K. Cu content on the film surface was found to be maximum for 1 wt% Cu:NiFe2O4 thin film and this film showed an improved response towards all gases. Response of ethanol for NiFe2O4 thin film was found to be higher as compared to all the other gases. The lowering of the optimum operating temperature is observed in 1 wt% Cu:NiFe2O4 thin film with higher selectivity towards ethanol than other gases. All results indicated that the Cu doping in nickel ferrite thin films has a significant influence on the properties. © 2016 Elsevier B.V. All rights reserved.
Keywords: Thin films SEM EDAX Magnetic properties Surface properties
1. Introduction Semiconductor gas sensors have been expansively investigated for many different applications such as environmental monitoring, automotive applications and air conditioning in airplanes, spacecrafts and house. Moreover, recently semiconducting nanostructures have earned attention due to their huge surface to volume ratios. A gas sensors performance is strongly dependent on the sensor materials surface area. In 1991, Yamazoe demonstrated
* Corresponding author. E-mail address:
[email protected] (S. Bhagwat).
that reduction in crystal size would significantly increase the sensor performance [1]. This is because nano-sized grains of metal oxides are almost depleted of carriers (most carriers are trapped in surface states) and exhibit much poorer conductivity than micro-sized grains in ambient air, hence, when exposed to target gases, they exhibit greater conductance changes as more carriers are activated from their trapped states to the conduction band than with microsized grains. In general, metal oxides are conventionally used to detect most of the reducing gases but their limited selectivity, reproducibility and thermal stability are common problems related to the composition and microstructure of the relevant materials. Semiconductor gas sensors are usually derived from ceramic processing
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P. Rao et al. / Materials Chemistry and Physics xxx (2016) 1e7
and thick film technology. However, they are difficult to miniaturize and are incompatible with integrated circuit fabrication technology. Some of these problems can be overcome using nanometer thin films prepared via physical and chemical routes. Moreover noble metal additives with high effective oxidation catalytic activity can be used to enhance the sensitivity of such sensors. Kapase et al. [2] synthesized NieZn ferrites by citrate sol-gel method and addition of Pd in NieZn ferrite shows improvement in sensitivity and response time towards ethanol. Kadu et al. [3] found that the addition of Pd improves the sensitivity, response and reduces the operating temperature from 300 C to 230 C of MneZn ferrite towards ethanol. Influence of Pd on LPG sensing properties of magnesium ferrite was studied by Darshane et al. [4] and observed highest response and lowering in operating temperature from 350 C to 200 C. In this context, from last few years, researchers have focused on thin films for gas sensors [5e7]. Thin films have good selectivity, enhancement in surface area, low operating temperatures and fast response, though only moderate sensitivity because of various reasons such as crystallite size, high defect density, texture, grain boundaries, substrate used and fabrication methods. Nickel ferrite (NiFe2O4) has been widely studied as a magnetic material [8,9] and considered as gas sensor in the bulk form towards chlorine [10], hydrogen sulfide [11], acetone [12] and LPG [13]. However, the gas sensing properties nickel ferrite in thin film form towards the reducing gases is not explored much. Microstructural, magnetic, electric and dielectric properties of Cu containing nickel ferrite have been investigated earlier [14,15]; though, there are no reports on Cu doped nickel ferrite thin films as a gas sensor. Hence it is appealing to study the gas sensing properties of Cu doped nickel ferrite for reducing gases. Various methods have been used for deposition of ferrite thin films viz. RF sputtering [16], plasma laser deposition (PLD) [17] etc. These methods usually involve elaborate and costly apparatus and complicated process. Furthermore, the high deposition temperature limits the option of the material of the substrates and thus limits the application of the ferrite thin films. Spray pyrolysis technique is versatile and has the unique advantage of producing large surface-area films (about 200 200 ) at low cost. It involves a simple experimental setup and lowers the processing temperatures required to arrive at a stable phase. The response of material as a gas sensor primarily depends upon its pore size, porosity and specific surface area. From our earlier studies [18] it is experienced that these parameters can be controlled easily using spray pyrolysis deposition technique by varying deposition parameters. At the same time doping can be done without many efforts for desired percentage using this technique. In the present work, Cu:NiFe2O4 thin films were deposited on Si (100) and alumina and their structural, magnetic, optical and gas response properties towards ethanol, LPG, methanol, H2S were studied. 2. Experimental 2.1. Synthesis of NiFe2O4 and Cu:NiFe2O4 thin films NiFe2O4 and Cu:NiFe2O4 (NiFe2O4 þ 1 wt% and 3 wt% Cu) thin films were deposited using dual mode automated spray pyrolysis system on Si (100) (5 mm 2 mm) and alumina (5 mm 5 mm) substrates which were cleaned prior to the deposition. Si wafer was dipped for 30s in 1:20 HF:DI water to remove the native oxide layer and any contamination in the oxide from the wafer surface and then strongly rinsed in DI water. The alumina substrates were cleaned using soap solution and distilled water. Then subjected to ultra-sonicator bath and lastly dried under IR lamp. An aqueous
ethanol solution of nickel chloride [NiCl2.6H2O] and iron (III) chloride [FeCl3] (mole ratio 1: 2) were chosen as the precursor solutions for the deposition of NiFe2O4 thin film. The molar concentration of the precursor solution was kept 0.15 M. For the deposition of Cu:NiFe2O4 thin films an aqueous solution of copper chloride [CuCl2.2H2O] (1 and 3 wt%) was added in the precursor solution. All chemicals used were of AR Grade (99.99%). Compressed air was employed as the carrier gas. The solution was sprayed by a spray gun and the resulting mist was deposited on to the Si (100) and alumina by compressed air at a flow rate of 15e17 lpm. The substrate temperature was maintained at 350 C during the deposition. The deposition time depends on the volume of the spraying solution. The nozzle-substrate distance was kept fixed at 30 cm. During decomposition reaction, metal reacts with oxygen, and finally resulted in brown coloured uniform ferrite thin film. After deposition, the coated substrates were allowed to naturally cool down to room temperature before being taken out from the spray chamber. The deposited thin films were then air annealed at 650 C for 3 h to obtain single phase spinel structure. The structural characterization of these nickel ferrite thin films deposited on Si (100) was carried out using Bruker AXS D8 diffractometer, with CuKa radiation. The surface morphology of all the samples was studied using JEOL, JSM 6360A scanning electron microscope (SEM). Reflectance measurements of all the films deposited on Si (100) were carried out using PerkineElmer spectrophotometer in 200e1100 nm range. The thickness of ferrite films was measured using Talystep Profilometer and they were found to be about 6 mm for all the films. MeH curves of ferrite thin films deposited on Si (100) and annealed were recorded using LOTQuantum Design MPMS Superconduting Quantum Interference Device (SQUID) Vibrating Sample Magnetometer (VSM). 2.2. Gas sensitivity measurements The gas-sensing characteristics of NiFe2O4 and Cu:NiFe2O4 thin films deposited on alumina using spray pyrolysis and air annealed at 650 C for 3 h were studied. The sensor material was kept on a heater provided with two probes for electrical measurements in a cylindrically shaped stainless steel chamber (20 cm dia 10 cm height). The gas response was measured after providing the ohmic contacts to the films using silver paste. The known amount of test gas was introduced in the chamber. The gas-sensing characteristics at different temperatures (T ~ 200 Ce400 C) were recorded using a Keithley 2400 source meter. 3. Results and discussion 3.1. Structural studies The NiFe2O4 and Cu:NiFe2O4 thin films were sintered at 650 C for 3 h and characterized using X-ray diffraction (XRD) technique to obtain structural information. The well resolved peaks in the XRD patterns (Fig. 1) clearly indicate the polycrystalline nature of ferrite and match well with the characteristic diffraction peaks of Ni ferrites (JCPDS card # 74-2081). The observed peaks in Fig. 1 for the planes (220), (311), (222), (400), (422) and (511) confirmed the phase formation of NiFe2O4 with cubic spinel ferrite structure. Few peaks of SiO2 are also observed in XRD patterns as SiO2 had grown during the deposition of thin films on Si (100). However, it can be noticed that diffraction lines become broader with Cu incorporation. The crystallite size and lattice structure are known to have their own contributions to the X-ray diffraction peaks, the diffraction peaks in the XRD patterns are strong and sharp, indicating high crystallinity of all the samples. The X-ray diffraction patterns are studied in detail for the
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preferentially occupy B site. With the entering of Cu2þ ions to A site, some of the Fe3þ (r ¼ 0.067 nm) ions transform to Fe2þ (r ¼ 0.083 nm) ions for maintaining the charge neutrality. An increase in the population of Cu2þ cations and a decrease in the Fe3þ cations in the A site contributes to the increase in lattice parameter. 3.2. Morphological studies The morphological characteristics of NiFe2O4 and Cu:NiFe2O4 thin films were studied using SEM. The micrographs show remarkable change in the microstructure and porosity as well. As exhibited in the Fig. 2, petal-like nanoparticles of sizes around 0.7 mm 0.2 mm with small number of voids are observed in NiFe2O4 thin film. However, 1 wt% Cu:NiFe2O4 thin film possesses grain-like structure with sizes ~0.1 mm 0.07 mm with increase in number of voids. SEM image of 3 wt% Cu:NiFe2O4 thin film shows that the nanoparticles are agglomerated intensively with very few voids. Voids are the active sites of the gas sensor.
Fig. 1. XRD spectra of NiFe2O4 and Cu:NiFe2O4 thin films.
3.3. EDAX measurements determination of average crystallite size using Scherrer's formula [19] as given by equation (1),
t¼
0:9l bcosqb
(1)
where b is the angular line width at half maximum intensity and qb is the Bragg angle for the actual peak. The crystallite sizes of all the ferrite thin films are found to be between 40 and 46 nm. The incorporation of Cu shows slight decrease in crystallite size which in turn increases the surface area. It is an established fact that the grain growth depends upon the grain boundary mobility [20]. A possible reason for the decreasing trend of crystallite size is that the increasing concentration of Cu reduces the grain growth probably due to segregation on or near the grain boundaries which hampers its movement. The lattice parameter ‘a’ of these samples was estimated using indexing method given by equation (2) given below,
l2 sin2 q ¼ constant ¼ 2 N 4a
(2)
where N ¼ 2(h2 þ k2 þ l2) and by knowing the sequence of N one can determine the crystal structure of the ferrite thin films [21]. The lattice parameter of nickel ferrite film does not match exactly with the standard bulk value (8.34 nm) which could be attributed to the strains present on the surface of the film during the growth [18,22e24]. The value of calculated crystalline size (t) and lattice parameter (a) are summarized in Table 1. It can be seen that the lattice parameter increases slightly with increasing Cu content. Similar results were also obtained by S. Manjura Hoque et al. [25] and M.A. Gabal et al. [26]. This could be due to the rearrangement of ions which can be explained as follows: Spinel ferrite has tetrahedral (A) and octahedral (B) lattice sites for cations to occupy. According to Neel's sublattice model, Cu2þ (r ¼ 0.070 nm) ions are present at both tetrahedral (A) and octahedral (B) sites of the spinel lattice. Ni2þ (r ¼ 0.078 nm)
Energy Dispersive Analysis by X-ray (EDAX) gives the information of an atomic concentration of different elements present on the surface of the sample. Cu:NiFe2O4 thin films confirm the presence of small amount of Cu along with Ni, Fe and O which are shown in Fig. 3. Various elemental ratios calculated from EDAX are tabulated in Table 2. This table reveals that there is an increase in the ratio of Cu/Ni on the surface of Cu:NiFe2O4 thin films as compared to bulk (in bulk Cu/Ni ¼ 0.011 for 1 wt% Cu and 0.033 for 3 wt% Cu). In particular, it is much higher for 1 wt% Cu:NiFe2O4 thin films (0.11). The ratio of Ni/O on the surface of NiFe2O4 and Cu:NiFe2O4 thin films is found to be less than that of bulk (Ni/O ¼ 0.917). It is found to be minimum for 1wt% Cu:NiFe2O4 thin film. Hence from EDAX it is inferred that both Cu and O ions are mostly present on the surface of 1 wt% Cu:NiFe2O4 which could be attributed to the enhancement in the gas response discussed in the latter section. 3.4. Optical studies To confirm the formation of NiFe2O4 and Cu:NiFe2O4 thin films optical studies were carried out. The reflection spectra of the samples were recorded to determine the optical band gap using Tauc's relation [27] given by equation (3),
ahn ¼ c hn Eg
1=2
(3)
where ‘c’ is a constant, hv is the photon energy and Eg is the band gap and the absorption coefficient a is given by equation (4).
2at ¼ ln½ðRmax Rmin Þ=ðR Rmin Þ
(4)
where t is the thickness of the films, Rmax and Rmin are maximum and minimum values of reflectance, R the reflectance at a given photon energy hn. A graph of [hn ln{(Rmax e Rmin)/(R e Rmin)}]2 vs hn is plotted and shown in Fig. 4. The extrapolation of the straight line to [hn ln{(Rmax e Rmin)/(R e Rmin)}]2 ¼ 0 axis gives the value of the band gap of the sample.
Table 1 Parameters calculated from XRD and UV-VIS spectroscopy. Sample NiFe2O4 thin film 1 wt% Cu:NiFe2O4 thin film 3 wt% Cu:NiFe2O4 thin film
Crystallite size from XRD t (nm)
Lattice parameter a (Å)
Band gap Eg (eV)
46 41 41
8.294 8.304 8.314
2.03 2.07 2.05
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Fig. 2. SEM of NiFe2O4 and Cu:NiFe2O4 thin films.
It can be seen that the plot varies linearly for all the ferrite thin films in the region of strong absorption near the fundamental absorption edge. The linear variation of absorption coefficient of these thin films at high frequencies indicates that these ferrite films have direct transitions across the energy band gap. The values of energy band gap for NiFe2O4 and Cu:NiFe2O4 thin films are about 2 eV and are tabulated in Table 1. The values indicate that these films are semi-conducting oxides. From Fig. 4 it is observed that optical band gap increases slightly with the 1 wt% of Cu concentration and decreases with further addition of Cu concentration. This result is consistent with SEM results. As experienced from micrographs (Fig. 2), the particle size of 1 wt% Cu:NiFe2O4 thin film is much smaller than NiFe2O4 and 3 wt% Cu:NiFe2O4 thin films and hence the increased band gap is observed.
3.5. SQUID VSM studies
Fig. 3. EDAX measurements of NiFe2O4 and Cu:NiFe2O4 thin films.
Table 2 Parameters calculated from EDAX. Sample
Cu/Ni
Ni/O
Ni/Fe
NiFe2O4 thin film 1 wt% Cu:NiFe2O4 thin film 3 wt% Cu:NiFe2O4 thin film
0.000 0.110 0.067
0.027 0.013 0.071
0.093 0.060 0.240
Fig. 4. Graph of [hn ln{(Rmax e Rmin)/(R e Rmin)}]2 vs. photon energy hn of NiFe2O4 and Cu:NiFe2O4 thin films.
Fig. 5 shows variation in magnetization as a function of magnetic field for all the ferrite thin films deposited on Si (100) substrate and air annealed. Hysteresis curves for these films were recorded at 50 K (a) and 300 K (b) between 10 kOe and þ10 kOe. The saturation magnetization (Ms), remanence (MR) and coercivity (Hc) of all the ferrite thin films at 50 K and 300 K are tabulated in Table 3. Cu2þ ions are diamagnetic and will interact with magnetic fields very weakly and occupy both A and B sites; whereas Ni2þ ions preferentially occupy B site. When Cu is added, Cu2þ ions occupy tetrahedral (A) site, the transformation of Fe3þ ions into Fe2þ ions takes place. Due to this the magnetization of A-site decreases, which is consistent with XRD result. Effectively, an appreciable enhancement in magnetization is observed in 1 wt% Cu:NiFe2O4 thin films. From Table 3, it is seen that coercivity increases with the increase in Cu concentration. Coercivity depends on microstructure of the film. The grain size is small, so the number of grain boundaries increase. These boundaries act as pinning sites for domain walls which are considered to be responsible for high value of coercivity [24,28]. Similar observation was reported by Ranjith Kumar et al. [29] and Gabal et al. [30]. Saturation magnetization and coercivity both have larger values at 50 K than at 300 K. Thermally induced randomness at high temperature is more which decreases the magnetization. The higher value of coercivity at 50 K may be due to anisotropic effect [24,31] and/or excitation of spin waves [32]. Fig. 6 shows M-T curves of these films which were recorded up to 300 K at constant magnetic field i.e. the saturation magnetization value. It shows that magnetization reduces continuously which can be attributed to magnetic relaxation [33]. This behavior is typical ferrimagnetic behavior. The similar results were obtained in our earlier work [18]. When the system is cooled down to 50 K under an applied field, the random magnetic moment vectors became parallel to magnetic field. When the temperature is increased magnetization decreases continuously due to misalignment of
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Fig. 5. SQUID VSM of NiFe2O4 and Cu:NiFe2O4 thin films at (a) 50 K and (b) 300 K.
Table 3 Magnetic parameters calculated from SQUID VSM. Sample
NiFe2O4 thin film 1 wt% Cu:NiFe2O4 thin film 3 wt% Cu:NiFe2O4 thin film
Saturation magnetization Ms (emu/cc)
Remanence MR (emu/cc)
Coercivity Hc (Oe)
50 K
300 K
50 K
300 K
50 K
300 K
39.04 78.42 96.88
36.89 72.91 89.16
4.95 20.91 19.18
1.67 12.14 9.81
149.5 255.7 255.3
64.2 128.3 99.1
Fig. 6. M-T curves of NiFe2O4 and Cu:NiFe2O4 thin films.
magnetic moment vectors. 3.6. Gas sensing characteristics When a semiconductor sensor is exposed to a gas, the change in resistance is mainly due to the reaction between the gas and the oxygen species adsorbed on the surface of the semiconductor. The adsorption of gas, which depends on both the type of test gas and the sensor material, may affect the response characteristic. Better response would be expected if a large amount of gas is adsorbed and subsequently the reaction between the adsorbed gas and oxygen species is more favorable. Spray deposited NiFe2O4 and Cu:NiFe2O4 thin films were subjected for studying their gas response and selectivity at the different controlled temperatures towards various reducing gases in the static setup. Thin film has advantages like high surface area, fast recovery, lower energy input, device compatibility,
miniaturization and overall cost effectiveness. NiFe2O4 and Cu:NiFe2O4 thin films were found to be p-type semiconducting oxides as all films responded to all reducing gases by increase in resistance [34,35]. The response of the ferrite thin films are calculated when they are exposed to 5 ppm of these gases. When NiFe2O4 thin film is heated in the presence of air, oxygen ions get adsorbed on the surface. When reducing gas is purged, it combines with oxygen ions and donates electron to the material. Since electrons are accepted by the material, number of majority charge carriers (holes) in the material decreases and hence electrical resistance increases. The response (S) [18,35,36] for a given test gas is calculated using equation (5).
s¼
Ra Rg Ra
(5)
where Ra and Rg are the electrical resistances of the sensor in air
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and in test gas respectively. The gas response (S) of NiFe2O4, 1 wt% and 3wt% Cu:NiFe2O4 thin films deposited on alumina against various reducing gases viz. ethanol, LPG, methanol, H2S are shown in the Fig. 7. All these films have started responding to various gases above 200 C. The bar graph shows that these films are highly selective towards ethanol as compared with the other test gases. In the bar graph, the optimum operating temperature, the temperature at which the gas response is maximum, for sensing of each gas with 5 ppm concentration is mentioned. All the gases such as LPG, methanol and hydrogen sulphide vapors show response (S) less than 1.5 while ethanol shows above 2 for NiFe2O4 thin film whereas above 3 for Cu:NiFe2O4 thin films at remarkably less optimum operating temperature for sensing as compared with other reducing gases. As mentioned earlier, the response of a sensor depends on removal of adsorbed oxygen molecules by reaction with a target gas and generation of electrons. As the ethanol gas is introduced into the test chamber, the conductance of nickel ferrite thin films increases due to the exchange of electrons between ionosorbed species and nickel ferrite. The overall reactions of ethanol gas with the chemisorbed oxygen may take place by two ways as follows [3],
C2 H5 OHðgasÞ þ O 4 CH3 CHO þ H2 O þ e C2 H5 OHðgasÞ þ O 4 Hþ þ C2 H5 O ðsurfaceÞ C2 H5 O 4Hþ þ CH3 CHO CH3 CHO þ OðbulkÞ / CH3 COOH þ oxygen vacancies Fig. 8 shows response (S) towards ethanol at various operating temperatures which indicates 375 C and 325 C as the optimum temperature for NiFe2O4 and Cu:NiFe2O4 thin films respectively. The ethanol response decreases slightly for 3 wt% Cu:NiFe2O4 thin film as compared to 1 wt% Cu:NiFe2O4 thin film whereas optimum operating temperature retains. The similar behavior was observed for almost all the gases. The structural features like crystallite size, shape, phase composition, surface morphology also decides the sensing activities for different gases. The selectivity shown by NiFe2O4 and Cu:NiFe2O4 thin films towards ethanol in comparison with LPG can be attributed to the higher electron affinity of ethanol towards the acidic ferrite surface. Researchers have shown that the ferrite (bulk) surfaces are strongly acidic in nature [37e40]. This strong Lewis acidity of ferrites
Fig. 8. The variation of gas response parameter (S) with temperature for NiFe2O4 and Cu:NiFe2O4 thin films towards 5 ppm for C2H5OH gas.
originates from the presence of tri-positive ferric ions. Fe3þ is highly acidic as compared with Ni2þ due to its higher charge to ionic radius ratio. Here, acidic nature of ferrite is due to active sites present on the surface. An enhanced sensitivity of 1 w/o Cu:NiFe2O4 thin film is due to the proper dispersion of Cu atoms within nickel ferrite structure. This leads to substantial increase in sensing properties at higher temperature. At the same time Cu content in Cu:NiFe2O4 thin films on the surface enhances the acidity (due to increase in active sites) therefore may hamper affinity towards LPG and thereby reduces the response towards LPG. To summarize it is observed that maximum response of ethanol was obtained in 1 wt% Cu:NiFe2O4 thin films with lower operating temperature (325 C). This shift in response towards the lower operating temperatures on Cu incorporation may be originating due to the catalytic activity of copper that enhances the rate of dissociation and diffusion of oxygen species on the surface of oxide. Doping with copper accelerates chemical reactions that give rise to various oxygen adsorbates and surface states causing enhancement in the gas sensing even at low operating temperatures. Similar observations were reported by Darshane et al. [4] for Pd-doped MgFe2O4 nanoparticles. Further incorporation of Cu (3 wt%) decreases the response which could be due to the agglomeration of particles that leads to increase in grain size and decrease in porosity as clearly seen from SEM images. Apparently there seems to be plausible correlation between magnetic behavior (saturation magnetism) and gas sensing property of thin films. It is observed that 1 wt% Cu:NiFe2O4 thin film has lower magnetization and better gas response than 3 wt% Cu:NiFe2O4 thin film. Such a behavior may be explained due to the presence of optimum amount of Cu2þ ions on the surface of the spinel lattice with proper distribution at A-site and B-site, enhancement in the magnetization than NiFe2O4 thin film is observed. In 3 wt % Cu:NiFe2O4 thin film, Cu content on the surface of spinel lattice decreases (Pl. refer Table 2) and also more transfer of Cu2þ ions from B-site to A-site accountable for higher magnetization that effectively lowers the adsorption of the gas and therefore may hinder gas response.
4. Conclusions
Fig. 7. Response of NiFe2O4 and Cu:NiFe2O4 thin films towards 5 ppm for various gases.
NiFe2O4 and Cu:NiFe2O4 thin films were synthesized using less expensive spray pyrolysis technique. All the films possess spinel structure. Crystallite size of Cu:NiFe2O4 thin films decreases on
Please cite this article in press as: P. Rao, et al., Copper doped nickel ferrite nano-crystalline thin films: A potential gas sensor towards reducing gases, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.016
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Please cite this article in press as: P. Rao, et al., Copper doped nickel ferrite nano-crystalline thin films: A potential gas sensor towards reducing gases, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.016