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Synthesis and sensing properties of zirconium-doped hematite nanoparticles
ARTICLE IN PRESS Physica B 404 (2009) 2159–2165
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Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and sensing properties of zirconium-doped hematite nanoparticles Monica Sorescu a,, L. Diamandescu a,b, A. Tomescu b, S. Krupa a a b
Duquesne University, Department of Physics, Pittsburgh, PA, USA National Institute for Materials Physics, Bucharest, Romania
a r t i c l e in fo
abstract
Article history: Received 3 November 2008 Received in revised form 18 March 2009 Accepted 6 April 2009
Zirconium-doped hematite particles of the type xZrO2-(1x)a-Fe2O3 (x ¼ 0.1, 0.5) were synthesized using mechanochemical activation and characterized by X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy. For x ¼ 0.1 all zirconia was dissolved in the hematite lattice after 12 h of ball milling and a particle size of 9 nm was obtained. We obtained the recoilless fraction as function of the ball milling time for each value of the molar concentration x. The appearance of nanoparticles in the system was demonstrated based on these plots. We further correlated the structural properties of the zirconiumdoped hematite system with the sensing properties of the best candidate in the series. These were measured as function of temperature, gas concentration (carbon monoxide and methane) and variable humidity of air. The material system was found to be sensitive over the entire range of CO concentrations and the linearity of the sensor signal was not affected by the relative humidity of air, qualities which make it the ideal system for gas sensing. & 2009 Elsevier B.V. All rights reserved.
PACS: 76.80.+y Keywords: Magnetic materials Milling Mo¨ssbauer spectroscopy Microstructure
1. Introduction Investigations of semiconducting oxides have become increasingly important due to their sensing properties in the detection of toxic gases, such as carbon monoxide or methane [1–3]. Metal oxides are semiconductor materials best suited for gas sensing, since they do not suffer irreversible chemical transformations at the surface and progressive oxidation due to prolonged and repeated heating in the air, as other semiconductors do. If the semiconducting oxides are in addition nanostructured, it is expected that they will exhibit great surface activity due to their enhanced surface areas [4–11]. The mechanism of sensing in xZrO2-(1x)a-Fe2O3 is not well understood, however, due to incomplete understanding of its microstructure characteristics. In this paper we report the mechanochemical synthesis of xZrO2-(1x)a-Fe2O3 nanoparticles for x ¼ 0.1 and 0.5. X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy have been used to correlate the structure and properties in connection with the zirconium concentration and ball milling time. Experimental evidence for compositional transition in this system is given by precise measurements of the recoilless fraction for each value of the molar concentration x. We further correlate the structural properties of the material with the measurements of the sensing properties of the system, in different gases, at Corresponding author. Tel.: +1 412 396 4166; fax: +1 412 396 4829.
E-mail address: [email protected] (M. Sorescu). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.04.006
different temperature and variable air humidity and observe that the zirconium-doped hematite nanoparticle system is an excellent material from the viewpoint of sensor device applications. 2. Experimental Powders of hematite and zirconium dioxide were milled at different molar concentrations (x ¼ 0.1 and 0.5) in a hardened steel vial with twelve stainless steel balls (type 440; eight of 0.25 in diameter and four of 0.5 in diameter) in the SPEX 8000 mixer mill for time periods ranging from 0 to 12 h. The ball/powder mass ratio was 5:1 and all experiments were performed in a glove box under protective argon atmosphere. Prior to their introduction in the ball milling device, the powders were manually ground to obtain ultrafine powders. The structure of the powders was examined using Rigaku D2013 X-ray diffractometer with CuKa radiation (l ¼ 1.540598 A˚). The 57Fe Mo¨ssbauer spectra were recorded at room temperature using a 57Co in Rh source and an MS-1200 constant acceleration spectrometer. For testing the sensing properties, the resulted paste was deposited on supports with electrodes and Pt heater using the screen printing method. Thermal treatment of gas sensitive structures is absolutely necessary for removal of the organic solvent and for stabilization of the crystallite dimensions. This treatment was performed in air flux, in a furnace which was able to monitor the temperature. The temperature of 350 1C was selected, considering the process of evaluation of the gas sensing properties of the material.
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3. Results and discussion Fig. 1 (a)–(e) represents the XRD patterns of zirconia-doped hematite, xZrO2-(1x)a-Fe2O3 for x ¼ 0.1, corresponding to milling times between 0 and 12 h. The patterns are consistent with the gradual dissolution of zirconia into hematite, as demonstrated by the progressive line broadening of the XRD patterns. From Rietveld refinement, we determined all lattice parameters and particle sizes of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.1) as function of ball milling time and collected the results in Table 1. The refinement was carried out with the hypothesis that the interstitial and substitutional positions of iron are equally populated by Zr4+. In the hematite structure, the Fe3+ ions with coordinates of 0; 0; z occupy 2/3 of the octahedral holes in successive oxygen layers, and 1/3 of the octahedral holes with coordinates 0; 0; 0 are empty. In the case of our samples, the best
Fig. 1. XRD patterns of zirconia-doped hematite for x ¼ 0.1 and milling times (a)–(e) of 0–12 h, respectively.
Table 1 Refinement parameters for xZrO2(1x)a-Fe2O3 for x ¼ 0.1. Mill. Phase time (h)
Content (%)
Lattice parameters a (A˚)
c (A˚)
Particle dim. (nm)
0
Fe2O3 ZrO2 monoclinic
93.9 6.1
5.0187 5.1487
13.6957 5.2931 b ¼ 5.1803
89 38
2
Fe2O3 ZrO2
91 9
5.0366 5.0343
13.7180 5.4057 b ¼ 5.1949
18 14
4
Fe2O3 ZrO2
90.6 9.4
5.0369 5.0758
13.7401 5.4134 b ¼ 5.4134
15 11
8
Fe2O3 ZrO2
90.3 9.7
5.0397 5.0911
13.7624 5.4134 b ¼ 5.1575
15 16
12
Fe2O3 ZrO2
5.0438 –
13.7687 –
9 –
100 0
fit was obtained by allowing the presence of Zr ions in both substitutional 0; 0; z and interstitial 0; 0; 0 sites in the hematite corundum type structure. The lattice parameters were found to increase for longer milling times, i.e. with the number of zirconium ions absorbed in the hematite lattice (the ionic radius of Zr4+ in octahedral coordination is greater than that of iron in the same symmetry). Moreover, the particles dimensions, calculated with the Scherrer formula, decrease down to 9 nm, demonstrating the occurrence of nanoparticles in the zirconia–hematite system. Fig. 2 (a)–(e) represents the XRD patterns of zirconia-doped hematite, xZrO2-(1x)a-Fe2O3 for x ¼ 0.5, corresponding to milling times between 0 and 12 h. The refined parameters extracted from these patterns are tabulated in Table 2. It may be seen that the lattice parameters c and a of the hematite system increase with increasing the ball milling time, showing the expansion of the crystalline lattice due to the incorporation of zirconium atoms in the structure. The particle size of both hematite and monoclinic zirconia phases decreases on increasing the ball milling time down to about 9.5 nm. Moreover, it may be noted that the cubic zirconia phase occurs in the system after 4 h of ball milling and its appearance can be associated with the high local temperatures and pressures inherent to high-energy ball milling. Fig. 3 shows the room-temperature transmission Mo¨ssbauer spectrum of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.1), after 0 h of ball milling. The spectrum was fitted with one sextet, with the hyperfine parameters characteristic to hematite. The spectrum in Fig. 4, recorded after 2 h of milling, was fitted with two sextets, having magnetic hyperfine fields of 48.14 and 44.07 T, respectively. It can be seen that the spectrum exhibits line broadening as the milling time increases, in agreement with the model of local atomic environment. The linewidth increase with the ball milling time means there are possible other resonances to occur, with lower numbers of Fe nearest neighbors. For this reason, the best fit of all subsequent Mo¨ssbauer spectra was obtained by introducing an increased number of sextets in the analysis. For instance, the Mo¨ssbauer spectrum in Fig. 5 (after 4 h of milling) was fitted with three sextets, corresponding to hyperfine magnetic field values of 47.97, 45.7 and 43.19 T, respectively. The addition of a quadrupole-split doublet was necessary in Fig. 6, after 8 h of ball milling and similarly, the spectrum in Fig. 7 was analyzed using four sextets (47.7, 45.88,
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Table 2 Refinement parameters for xZrO2(1x)a-Fe2O3 for x ¼ 0.5. Mill. Phase time (h)
a (A˚)
c (A˚)
Particle dim. (nm)
Fe2O3 ZrO2 mono
48 52
5.0311 5.1489
13.7374 5.3160 b ¼ 5.2041
56 40
2
Fe2O3 ZrO2 mono
52.7 47.3
5.0307 5.1282
13.6975 5.1872 b ¼ 5.3130
19 15
4
Fe2O3 ZrO2 mono
55 33.9
5.0372 5.1162
13.7093 5.3162 b ¼ 5.1792
14 12
ZrO2 cubic
11.2
5.0907
Fe2O3 ZrO2 mono
67.8 15
5.0471 5.1483
ZrO2 cubic
17.2
5.0660
Fe2O3 ZrO2 mono
63.8 12.2
5.0578 5.1760
ZrO2 cubic
24.0
5.0510
12
42.9 and 39.19 T) and an additional doublet, having the quadrupole splitting of about 0.89 mm/s and occupying about 3% of the total absorption area. This quadrupole doublet can be assigned to Fe substituting Zr in the zirconia lattice, such that this phase is non-magnetic. For x ¼ 0.1 and ball milling time of 12 h, this phase is below the detection limit of XRD (o1 wt%). This phase can still be seen in the Mo¨ssbauer spectrum under special circumstances, such as long acquisition time, thin absorber and careful spectra fitting. In general, the phase compositions determined by XRD and Mo¨ssbauer spectroscopy are not identical and must be judged on a case-by-case basis. Fig. 8 represents the starting spectrum for the Zr-doped hematite system for the series with x ¼ 0.5 and Figs. 9–12 show the room temperature transmission Mo¨ssbauer spectra of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.5), recorded after 2, 4, 8 and
Lattice parameters
0
8
Fig. 2. XRD patterns of zirconia-doped hematite for x ¼ 0.5 and milling times (a)–(e) of 0–12 h, respectively.
Content (%)
9 13.7429 5.1506 b ¼ 5.3045
14 12 9
13.7653 5.2151 b ¼ 5.1533
12 12 9.5
Fig. 3. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 0 h.
12 h of ball milling time. These spectra were analyzed using two sextets or two sextets and a doublet with the quadrupole splitting of 0.89 mm/s and corresponding to 12.29%, 32.23% and 39.66%, respectively, of the total resonant area of the spectra in Figs. 10–12. This compositional transition between Zr substituting Fe in hematite and Fe substituting Zr in zirconia can be in principle evidenced by the occurrence of an extremum (minimum or maximum) in the recoilless fraction (f) as a function of the ball milling time, provided there is a precise method to do so. We applied the two-lattice method developed by us [12,13] for the precise determination of the recoilless fraction using a second absorber (for instance, a stainless steel foil) and a single room temperature Mo¨ssbauer spectroscopy measurement. The development of such a method is essential considering the low values of the recoilless fraction associated with magnetic nanoparticles.
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Fig. 4. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 2 h.
Fig. 7. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
Fig. 5. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 4 h.
Fig. 8. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 0 h.
Fig. 6. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 8 h.
Fig. 9. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 2 h.
ARTICLE IN PRESS M. Sorescu et al. / Physica B 404 (2009) 2159–2165
Fig. 10. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 4 h.
Fig. 11. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 8 h.
Fig. 12. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 12 h.
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Fig. 13. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 4 h, recorded simultaneously with the stainless steel foil, for the accurate determination of the recoilless fraction.
Fig. 14. The recoilless fraction of the xZrO2(1x)a-Fe2O3 nanoparticles system for x ¼ 0.1 and 0.5 as function of the ball milling time for each value of the molar concentration involved.
Fig. 13 presents the typical composed Mo¨ssbauer spectrum of the ceramic nanoparticles and the stainless steel foil. The recoilless fraction was determined from the relative areas of the two absorbers and was plotted in Fig. 14 as a function of the ball milling time for both x ¼ 0.1 and 0.5. The hematite powder for 0 milling hours already has a low value of the recoilless fraction due to the low particle sizes achieved by manual pre-milling. We further observed the occurrence of a maximum in the values of the recoilless fraction for 4 h of milling for x ¼ 0.5 and 8 h of milling for x ¼ 0.1, at the same time with the appearance of the quadrupole doublets in the respective Mo¨ssbauer spectra. This correlation gives an excellent support for the assignment of doublets in the spectral analysis. The f factor further decreases due to the appearance of nanoparticles in the system. The small increase in the value of the recoilless fraction, observed after 12 h of milling for x ¼ 0.5, can be assigned to agglomerations of the resulted nanoparticles [14,15]. In this paper we obtained additional evidence for this compositional transition while investigating Zr-doped hematite nanoparticles by measuring the
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Fig. 15. The resistance as function of time for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h, for CO and CH4 gases at four different temperatures and RH ¼ 0. Fig. 17. The sensor signal as function of CO concentration at 350 1C and relative humidity RH ¼ 0, 30% and 50%, for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
Fig. 16. The resistance as function of time for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h, at a temperature of 350 1C and RH ¼ 0; the sensor signal as a function of CO concentration for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
recoilless fraction for two molar concentrations separately. In each case, the extremum in the recoilless fraction was related to the occurrence of the doublet in the Mo¨ssbauer spectra, but at
different values of the ball milling times, further reinforcing our assignment of sites. In what follows we present the correlation between structural and sensing properties of this mixed oxide nanoparticle system. Fig. 15 shows the determination of the optimal temperature from the point of view of the sensitivity and stability of the zero resistance for the system xZrO2-(1x)a-Fe2O3 (x ¼ 0.1, ball milling time of 12 h) . In these conditions, the system consists of 9 nm nanoparticles only. The test was performed at four temperatures and high concentrations: 1000 ppm CO and 5000 ppm CH4. It can be seen that the zero resistance, sensitivity (S ¼ Rair/Rgas), response time and recover time are all functions of temperature. From the point of view of the sensitivity to the two gases, it may be inferred that the ceramic nanoparticles are more sensitive to carbon monoxide. Given that all these parameters are of interest, we selected the temperature of 350 1C for further investigations. At this temperature we tested the response of the oxide system to CO over a range of concentrations between 50 and 1000 ppm. It may be observed in Fig. 16 that the material is sensitive to CO over the entire set of concentration values and that the sensor signal tends to saturate at high CO concentrations (250–1000 ppm). We further checked in Fig. 17 the linearity of the signal as function of CO concentration (25–250 ppm) in the presence of air with variable humidity (0, 30% and 50% relative humidity, RH). We could conclude that the material is sensitive over the entire range of CO concentrations and moreover, the sensor signal depends linearly on the CO concentration, a result which demonstrates the outstanding potential of this system for sensing applications. The relative humidity of air influences slightly the reference value of the electrical resistance and the slope of the signal, but does not affect its linearity. Around the detection limit for CO (30–50 ppm) the signal is practically not affected by the variation of the air humidity and this aspect is very important for field applications. We plan to present a detailed investigation of the composition and milling time effects on the sensing properties of the entire series in a study that will be published separately.
4. Conclusions Zirconia-doped hematite nanoparticles, xZrO2-(1x)a-Fe2O3 (x ¼ 0.1 and 0.5), corresponding to milling times between 0 and 12 h, were synthesized by mechanochemical activation.
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The system was further characterized by XRD and Mo¨ssbauer spectroscopy. The occurrence of nanoparticles was demonstrated by the Scherrer formula and by the determination of the recoilless fraction as function of the ball milling time for both values of the molar concentration employed. Their increased surface area of these nanoparticles makes them extremely attractive for the development of gas sensing devices. Indeed, this material system is sensitive over the entire range of CO concentrations and the linearity of the sensor signal is not affected by the relative humidity of air, which makes it the ideal system for sensing devices. References [1] W. Gopel, Sens. Actuators B 18–19 (1994) 1. [2] N. Yamazoe, N. Miura, Sens. Actuators B 20 (1994) 95.
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[3] J. Tamaki, C. Naruo, Y. Yamamoto, M. Matsuoka, Sens. Actuators B 83 (2002) 190. [4] J.Z. Jiang, R. Liu, K. Nielsen, F.W. Poulsen, F.J. Berry, R. Clasen, Phys. Rev. B 55 (1997) 11. [5] J.Z. Jiang, R. Liu, K. Nielsen, S. Morup, K. Dam-Johansen, R. Clasen, J. Phys. D Appl. Phys. 30 (1997) 1459. [6] W. Zhu, O.K. Tan, J.Z. Jiang, J. Mater. Electron. 9 (1998) 275. [7] O.K. Tan, W. Zhu, Q. Yan, L.B. Kong, Sens. Actuators B 65 (2000) 361. [8] C.V. Gopal Reddy, W. Cao, O.K. Tan, W. Zhu, Sens. Actuators B 81 (2002) 170. [9] J. Cassedanne, An. Bras. Cienc. 38 (1966) 265. [10] H. Takano, Y. Bando, N. Nakanishi, M. Sakai, H. Okinaka, J. Solid State Chem. 68 (1987) 153. [11] A. Smolira, M. Szymanska, E. Jartych, A. Calka, L. Michalak, J. Alloys Comp. 402 (2005) 256. [12] M. Sorescu, Mater. Lett. 54 (2002) 256. [13] M. Sorescu, L. Diamandescu, V.S. Teodorescu, Physica B 403 (2008) 3838. [14] M. Sorescu, L. Diamandescu, A. Tomescu, D. Tarabasanu-Mihaila, V. Teodorescu, Mater. Chem. Phys. 107 (2008) 127. [15] M. Sorescu, L. Diamandescu, D. Tarabasanu-Mihaila, Mater. Lett. 59 (2005) 22.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and sensing properties of zirconium-doped hematite nanoparticles Monica Sorescu a,, L. Diamandescu a,b, A. Tomescu b, S. Krupa a a b
Duquesne University, Department of Physics, Pittsburgh, PA, USA National Institute for Materials Physics, Bucharest, Romania
a r t i c l e in fo
abstract
Article history: Received 3 November 2008 Received in revised form 18 March 2009 Accepted 6 April 2009
Zirconium-doped hematite particles of the type xZrO2-(1x)a-Fe2O3 (x ¼ 0.1, 0.5) were synthesized using mechanochemical activation and characterized by X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy. For x ¼ 0.1 all zirconia was dissolved in the hematite lattice after 12 h of ball milling and a particle size of 9 nm was obtained. We obtained the recoilless fraction as function of the ball milling time for each value of the molar concentration x. The appearance of nanoparticles in the system was demonstrated based on these plots. We further correlated the structural properties of the zirconiumdoped hematite system with the sensing properties of the best candidate in the series. These were measured as function of temperature, gas concentration (carbon monoxide and methane) and variable humidity of air. The material system was found to be sensitive over the entire range of CO concentrations and the linearity of the sensor signal was not affected by the relative humidity of air, qualities which make it the ideal system for gas sensing. & 2009 Elsevier B.V. All rights reserved.
PACS: 76.80.+y Keywords: Magnetic materials Milling Mo¨ssbauer spectroscopy Microstructure
1. Introduction Investigations of semiconducting oxides have become increasingly important due to their sensing properties in the detection of toxic gases, such as carbon monoxide or methane [1–3]. Metal oxides are semiconductor materials best suited for gas sensing, since they do not suffer irreversible chemical transformations at the surface and progressive oxidation due to prolonged and repeated heating in the air, as other semiconductors do. If the semiconducting oxides are in addition nanostructured, it is expected that they will exhibit great surface activity due to their enhanced surface areas [4–11]. The mechanism of sensing in xZrO2-(1x)a-Fe2O3 is not well understood, however, due to incomplete understanding of its microstructure characteristics. In this paper we report the mechanochemical synthesis of xZrO2-(1x)a-Fe2O3 nanoparticles for x ¼ 0.1 and 0.5. X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy have been used to correlate the structure and properties in connection with the zirconium concentration and ball milling time. Experimental evidence for compositional transition in this system is given by precise measurements of the recoilless fraction for each value of the molar concentration x. We further correlate the structural properties of the material with the measurements of the sensing properties of the system, in different gases, at Corresponding author. Tel.: +1 412 396 4166; fax: +1 412 396 4829.
E-mail address: [email protected] (M. Sorescu). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.04.006
different temperature and variable air humidity and observe that the zirconium-doped hematite nanoparticle system is an excellent material from the viewpoint of sensor device applications. 2. Experimental Powders of hematite and zirconium dioxide were milled at different molar concentrations (x ¼ 0.1 and 0.5) in a hardened steel vial with twelve stainless steel balls (type 440; eight of 0.25 in diameter and four of 0.5 in diameter) in the SPEX 8000 mixer mill for time periods ranging from 0 to 12 h. The ball/powder mass ratio was 5:1 and all experiments were performed in a glove box under protective argon atmosphere. Prior to their introduction in the ball milling device, the powders were manually ground to obtain ultrafine powders. The structure of the powders was examined using Rigaku D2013 X-ray diffractometer with CuKa radiation (l ¼ 1.540598 A˚). The 57Fe Mo¨ssbauer spectra were recorded at room temperature using a 57Co in Rh source and an MS-1200 constant acceleration spectrometer. For testing the sensing properties, the resulted paste was deposited on supports with electrodes and Pt heater using the screen printing method. Thermal treatment of gas sensitive structures is absolutely necessary for removal of the organic solvent and for stabilization of the crystallite dimensions. This treatment was performed in air flux, in a furnace which was able to monitor the temperature. The temperature of 350 1C was selected, considering the process of evaluation of the gas sensing properties of the material.
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3. Results and discussion Fig. 1 (a)–(e) represents the XRD patterns of zirconia-doped hematite, xZrO2-(1x)a-Fe2O3 for x ¼ 0.1, corresponding to milling times between 0 and 12 h. The patterns are consistent with the gradual dissolution of zirconia into hematite, as demonstrated by the progressive line broadening of the XRD patterns. From Rietveld refinement, we determined all lattice parameters and particle sizes of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.1) as function of ball milling time and collected the results in Table 1. The refinement was carried out with the hypothesis that the interstitial and substitutional positions of iron are equally populated by Zr4+. In the hematite structure, the Fe3+ ions with coordinates of 0; 0; z occupy 2/3 of the octahedral holes in successive oxygen layers, and 1/3 of the octahedral holes with coordinates 0; 0; 0 are empty. In the case of our samples, the best
Fig. 1. XRD patterns of zirconia-doped hematite for x ¼ 0.1 and milling times (a)–(e) of 0–12 h, respectively.
Table 1 Refinement parameters for xZrO2(1x)a-Fe2O3 for x ¼ 0.1. Mill. Phase time (h)
Content (%)
Lattice parameters a (A˚)
c (A˚)
Particle dim. (nm)
0
Fe2O3 ZrO2 monoclinic
93.9 6.1
5.0187 5.1487
13.6957 5.2931 b ¼ 5.1803
89 38
2
Fe2O3 ZrO2
91 9
5.0366 5.0343
13.7180 5.4057 b ¼ 5.1949
18 14
4
Fe2O3 ZrO2
90.6 9.4
5.0369 5.0758
13.7401 5.4134 b ¼ 5.4134
15 11
8
Fe2O3 ZrO2
90.3 9.7
5.0397 5.0911
13.7624 5.4134 b ¼ 5.1575
15 16
12
Fe2O3 ZrO2
5.0438 –
13.7687 –
9 –
100 0
fit was obtained by allowing the presence of Zr ions in both substitutional 0; 0; z and interstitial 0; 0; 0 sites in the hematite corundum type structure. The lattice parameters were found to increase for longer milling times, i.e. with the number of zirconium ions absorbed in the hematite lattice (the ionic radius of Zr4+ in octahedral coordination is greater than that of iron in the same symmetry). Moreover, the particles dimensions, calculated with the Scherrer formula, decrease down to 9 nm, demonstrating the occurrence of nanoparticles in the zirconia–hematite system. Fig. 2 (a)–(e) represents the XRD patterns of zirconia-doped hematite, xZrO2-(1x)a-Fe2O3 for x ¼ 0.5, corresponding to milling times between 0 and 12 h. The refined parameters extracted from these patterns are tabulated in Table 2. It may be seen that the lattice parameters c and a of the hematite system increase with increasing the ball milling time, showing the expansion of the crystalline lattice due to the incorporation of zirconium atoms in the structure. The particle size of both hematite and monoclinic zirconia phases decreases on increasing the ball milling time down to about 9.5 nm. Moreover, it may be noted that the cubic zirconia phase occurs in the system after 4 h of ball milling and its appearance can be associated with the high local temperatures and pressures inherent to high-energy ball milling. Fig. 3 shows the room-temperature transmission Mo¨ssbauer spectrum of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.1), after 0 h of ball milling. The spectrum was fitted with one sextet, with the hyperfine parameters characteristic to hematite. The spectrum in Fig. 4, recorded after 2 h of milling, was fitted with two sextets, having magnetic hyperfine fields of 48.14 and 44.07 T, respectively. It can be seen that the spectrum exhibits line broadening as the milling time increases, in agreement with the model of local atomic environment. The linewidth increase with the ball milling time means there are possible other resonances to occur, with lower numbers of Fe nearest neighbors. For this reason, the best fit of all subsequent Mo¨ssbauer spectra was obtained by introducing an increased number of sextets in the analysis. For instance, the Mo¨ssbauer spectrum in Fig. 5 (after 4 h of milling) was fitted with three sextets, corresponding to hyperfine magnetic field values of 47.97, 45.7 and 43.19 T, respectively. The addition of a quadrupole-split doublet was necessary in Fig. 6, after 8 h of ball milling and similarly, the spectrum in Fig. 7 was analyzed using four sextets (47.7, 45.88,
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Table 2 Refinement parameters for xZrO2(1x)a-Fe2O3 for x ¼ 0.5. Mill. Phase time (h)
a (A˚)
c (A˚)
Particle dim. (nm)
Fe2O3 ZrO2 mono
48 52
5.0311 5.1489
13.7374 5.3160 b ¼ 5.2041
56 40
2
Fe2O3 ZrO2 mono
52.7 47.3
5.0307 5.1282
13.6975 5.1872 b ¼ 5.3130
19 15
4
Fe2O3 ZrO2 mono
55 33.9
5.0372 5.1162
13.7093 5.3162 b ¼ 5.1792
14 12
ZrO2 cubic
11.2
5.0907
Fe2O3 ZrO2 mono
67.8 15
5.0471 5.1483
ZrO2 cubic
17.2
5.0660
Fe2O3 ZrO2 mono
63.8 12.2
5.0578 5.1760
ZrO2 cubic
24.0
5.0510
12
42.9 and 39.19 T) and an additional doublet, having the quadrupole splitting of about 0.89 mm/s and occupying about 3% of the total absorption area. This quadrupole doublet can be assigned to Fe substituting Zr in the zirconia lattice, such that this phase is non-magnetic. For x ¼ 0.1 and ball milling time of 12 h, this phase is below the detection limit of XRD (o1 wt%). This phase can still be seen in the Mo¨ssbauer spectrum under special circumstances, such as long acquisition time, thin absorber and careful spectra fitting. In general, the phase compositions determined by XRD and Mo¨ssbauer spectroscopy are not identical and must be judged on a case-by-case basis. Fig. 8 represents the starting spectrum for the Zr-doped hematite system for the series with x ¼ 0.5 and Figs. 9–12 show the room temperature transmission Mo¨ssbauer spectra of the xZrO2-(1x)a-Fe2O3 system (x ¼ 0.5), recorded after 2, 4, 8 and
Lattice parameters
0
8
Fig. 2. XRD patterns of zirconia-doped hematite for x ¼ 0.5 and milling times (a)–(e) of 0–12 h, respectively.
Content (%)
9 13.7429 5.1506 b ¼ 5.3045
14 12 9
13.7653 5.2151 b ¼ 5.1533
12 12 9.5
Fig. 3. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 0 h.
12 h of ball milling time. These spectra were analyzed using two sextets or two sextets and a doublet with the quadrupole splitting of 0.89 mm/s and corresponding to 12.29%, 32.23% and 39.66%, respectively, of the total resonant area of the spectra in Figs. 10–12. This compositional transition between Zr substituting Fe in hematite and Fe substituting Zr in zirconia can be in principle evidenced by the occurrence of an extremum (minimum or maximum) in the recoilless fraction (f) as a function of the ball milling time, provided there is a precise method to do so. We applied the two-lattice method developed by us [12,13] for the precise determination of the recoilless fraction using a second absorber (for instance, a stainless steel foil) and a single room temperature Mo¨ssbauer spectroscopy measurement. The development of such a method is essential considering the low values of the recoilless fraction associated with magnetic nanoparticles.
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Fig. 4. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 2 h.
Fig. 7. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
Fig. 5. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 4 h.
Fig. 8. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 0 h.
Fig. 6. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.1 and ball milling time of 8 h.
Fig. 9. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 2 h.
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Fig. 10. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 4 h.
Fig. 11. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 8 h.
Fig. 12. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)aFe2O3 system for x ¼ 0.5 and ball milling time of 12 h.
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Fig. 13. Room-temperature transmission Mo¨ssbauer spectrum of the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 4 h, recorded simultaneously with the stainless steel foil, for the accurate determination of the recoilless fraction.
Fig. 14. The recoilless fraction of the xZrO2(1x)a-Fe2O3 nanoparticles system for x ¼ 0.1 and 0.5 as function of the ball milling time for each value of the molar concentration involved.
Fig. 13 presents the typical composed Mo¨ssbauer spectrum of the ceramic nanoparticles and the stainless steel foil. The recoilless fraction was determined from the relative areas of the two absorbers and was plotted in Fig. 14 as a function of the ball milling time for both x ¼ 0.1 and 0.5. The hematite powder for 0 milling hours already has a low value of the recoilless fraction due to the low particle sizes achieved by manual pre-milling. We further observed the occurrence of a maximum in the values of the recoilless fraction for 4 h of milling for x ¼ 0.5 and 8 h of milling for x ¼ 0.1, at the same time with the appearance of the quadrupole doublets in the respective Mo¨ssbauer spectra. This correlation gives an excellent support for the assignment of doublets in the spectral analysis. The f factor further decreases due to the appearance of nanoparticles in the system. The small increase in the value of the recoilless fraction, observed after 12 h of milling for x ¼ 0.5, can be assigned to agglomerations of the resulted nanoparticles [14,15]. In this paper we obtained additional evidence for this compositional transition while investigating Zr-doped hematite nanoparticles by measuring the
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Fig. 15. The resistance as function of time for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h, for CO and CH4 gases at four different temperatures and RH ¼ 0. Fig. 17. The sensor signal as function of CO concentration at 350 1C and relative humidity RH ¼ 0, 30% and 50%, for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
Fig. 16. The resistance as function of time for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h, at a temperature of 350 1C and RH ¼ 0; the sensor signal as a function of CO concentration for the xZrO2(1x)a-Fe2O3 system for x ¼ 0.1 and ball milling time of 12 h.
recoilless fraction for two molar concentrations separately. In each case, the extremum in the recoilless fraction was related to the occurrence of the doublet in the Mo¨ssbauer spectra, but at
different values of the ball milling times, further reinforcing our assignment of sites. In what follows we present the correlation between structural and sensing properties of this mixed oxide nanoparticle system. Fig. 15 shows the determination of the optimal temperature from the point of view of the sensitivity and stability of the zero resistance for the system xZrO2-(1x)a-Fe2O3 (x ¼ 0.1, ball milling time of 12 h) . In these conditions, the system consists of 9 nm nanoparticles only. The test was performed at four temperatures and high concentrations: 1000 ppm CO and 5000 ppm CH4. It can be seen that the zero resistance, sensitivity (S ¼ Rair/Rgas), response time and recover time are all functions of temperature. From the point of view of the sensitivity to the two gases, it may be inferred that the ceramic nanoparticles are more sensitive to carbon monoxide. Given that all these parameters are of interest, we selected the temperature of 350 1C for further investigations. At this temperature we tested the response of the oxide system to CO over a range of concentrations between 50 and 1000 ppm. It may be observed in Fig. 16 that the material is sensitive to CO over the entire set of concentration values and that the sensor signal tends to saturate at high CO concentrations (250–1000 ppm). We further checked in Fig. 17 the linearity of the signal as function of CO concentration (25–250 ppm) in the presence of air with variable humidity (0, 30% and 50% relative humidity, RH). We could conclude that the material is sensitive over the entire range of CO concentrations and moreover, the sensor signal depends linearly on the CO concentration, a result which demonstrates the outstanding potential of this system for sensing applications. The relative humidity of air influences slightly the reference value of the electrical resistance and the slope of the signal, but does not affect its linearity. Around the detection limit for CO (30–50 ppm) the signal is practically not affected by the variation of the air humidity and this aspect is very important for field applications. We plan to present a detailed investigation of the composition and milling time effects on the sensing properties of the entire series in a study that will be published separately.
4. Conclusions Zirconia-doped hematite nanoparticles, xZrO2-(1x)a-Fe2O3 (x ¼ 0.1 and 0.5), corresponding to milling times between 0 and 12 h, were synthesized by mechanochemical activation.
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The system was further characterized by XRD and Mo¨ssbauer spectroscopy. The occurrence of nanoparticles was demonstrated by the Scherrer formula and by the determination of the recoilless fraction as function of the ball milling time for both values of the molar concentration employed. Their increased surface area of these nanoparticles makes them extremely attractive for the development of gas sensing devices. Indeed, this material system is sensitive over the entire range of CO concentrations and the linearity of the sensor signal is not affected by the relative humidity of air, which makes it the ideal system for sensing devices. References [1] W. Gopel, Sens. Actuators B 18–19 (1994) 1. [2] N. Yamazoe, N. Miura, Sens. Actuators B 20 (1994) 95.
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