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Tuning Magnetic Properties of r-MnO2 Nanotubes by K+ Doping J. Luo,*,† H. T. Zhu,†,‡ J. K. Liang,†,§ G. H. Rao,† J. B. Li,† and Z. M. Du‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China, Department of Materials Science and Engineering, UniVersity of Science and Technology Beijing, Beijing 100083, China, and International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China ReceiVed: January 24, 2010; ReVised Manuscript ReceiVed: March 23, 2010
R-KxMnO2 nanotubes with x ) 0.10, 0.12, 0.15, and 0.17 have been synthesized through acid and alkali treatments of the precursor R-MnO2 nanotubes. For the samples with x ) 0.10 and 0.12, a large divergence of the ZFC and FC susceptibilities and pronounced hysteresis at low temperature are observed. However, the samples with x ) 0.15 and 0.17 are antiferromagnetic with Ne´el temperatures of 25.4 and 25.3 K, respectively. The reciprocal susceptibility and electrical conductivity indicate that the samples with x ) 0.15 and 0.17 undergo a charge separation transition from mixed/averaged valence state manganese ions to Mn3+ and Mn4+ between 250 and 300 K. Below half occupancy of the K+ site (x e 0.125), the strong geometrical frustration due to the triangular lattice configuration and the mixture of Mn3+/Mn4+ result in the spin-glass behavior of the samples with x ) 0.10 and 0.12. Above half occupancy of the K+ site, the samples with x ) 0.15 and 0.17 exhibit an antiferromagnetic feature due to the charge separation of Mn3+ and Mn4+. Our experiments display that the magnetic properties of R-KxMnO2 can be tuned by varying the K+ content. 1. Introduction Owing to its distinctive chemical and physical properties, tetragonal R-MnO2 has been intensively investigated as the molecular/ion sieve,1,2 catalyst,3,4 and electrode material for Li/ MnO2 batteries.5-7 Along its c axis, R-MnO2 has nanoscale 2 × 2 tunnel structures constructed by edge and corner sharing of MnO6 octahedra.8 Because the tunnel cavity is as large as 0.46 nm, it is, therefore, inevitable that some large cations, such as K+, Na+, Ba2+, and others, are introduced to the tunnel from the raw materials during the synthesis process, leading to an uncontrollable chemical composition of the resultant R-MnO2 sample. However, the large tunnel cavity of R-MnO2 also offers a possibility to adjust its chemical and physical properties by cation doping. In our recent work, single-crystal tetragonal R-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in hydrochloric acid solution.9 Considering the change of the local chemical environment due to the insertion of K+ into the tunnel, the magnetic properties of R-K0.09MnO2 have been systematically investigated and a spin-glasslike behavior has been observed at low temperature.10-12 The origin of the spin-glass (SG) behavior has been attributed to the geometrical frustration on the triangular lattices and the mixture of Mn3+ and Mn4+.10 It is well-known that R-MnO2 is antiferromagnetic,13,14 but our above work implies that the physical properties of R-MnO2 could be tuned by varying the content of the cation located at the tunnel cavity. In this work, the K+ content of R-MnO2 is modified by boiling the as-synthesized R-MnO2 nanotubes in HCl and KOH solu* To whom correspondence should be addressed. Tel: +86-10-82648119. Fax: +86-10-82649531. E-mail:
[email protected]. † Institute of Physics, Chinese Academy of Sciences. ‡ University of Science and Technology Beijing. § International Center for Materials Physics, Chinese Academy of Sciences.
tions with different concentrations. The effect of the intercalated K+ on the magnetic properties of the R-MnO2 nanotubes is reported. 2. Experimental Section The R-MnO2 nanotubes used in this study were synthesized as previously reported.9 Typically, the KMnO4 precursor was hydrothermally treated in the HCl solution at 140 °C for 12 h. To obtain the samples with different K+ concentrations, the assynthesized R-MnO2 nanotubes were boiled in the HCl/KOH solution with different concentrations at 120 °C for 4 h. The final product was collected by centrifuging and washed several times by deionized water to remove excess ions. The washed precipitates were then dried in air overnight. The K+ concentration of the sample was cross-checked by energy-dispersive X-ray spectroscopy (EDS) and chemical analysis with inductively coupled plasma-atomic emission spectrometry (ICP-AES). Finally, R-KxMnO2 samples with x ) 0.09 (boiled in HCl solution), 0.10 (boiled in HCl solution), 0.12 (as-synthesized sample), 0.15 (boiled in KOH solution), and 0.17 (boiled in KOH solution) were obtained (in this paper, we focus on the samples with x ) 0.10, 0.12, 0.15, and 0.17 because the magnetic properties of the sample with x ) 0.09 have been reported10). The phase identification, morphology, and structure analysis of the samples were carried out by X-ray powder diffraction (XRD, Rigaku D/max 2500) with Cu KR radiation and transmission electron microscopy (TEM, JEM-2010). Thermogravimetric (TGA) and differential thermal analysis (DTA) was performed on a TA-Q600 analyzer in dry air with a heating rate of 10 °C/min. According to our experimental results, the crystal structure and morphology of the sample were not changed by the boiling treatment (in the text, the XRD, EDS, TEM, and TGA-DTA results of the R-K0.17MnO2 sample have been shown as an example). Magnetic properties were measured by a superconducting quantum interference device (SQUID)
10.1021/jp1006928 2010 American Chemical Society Published on Web 04/23/2010
K+ Doping of R-MnO2 Nanotubes
Figure 1. XRD pattern and EDS spectrum (inset) of the R-K0.17MnO2 sample.
Figure 2. TEM image, high-resolution TEM image (bottom-left), and selected-area FFT (top-right) of the R-K0.17MnO2 sample.
magnetometer. To detect their electrical conductivities, the samples were pressed into tablets and then sintered at 200 °C for 12 h. A standard four-probe method was used to measure the resistance of the samples. 3. Results and Discussion Figure 1 shows the XRD pattern and EDS spectrum of the R-K0.17MnO2 sample. All the diffraction peaks can be indexed as the tetragonal R-MnO2 (JCPDS 44-0141), and no other impurities are observed. The EDS spectrum confirms the existence of K element with an average K/Mn atomic ratio around 17%. A representative R-K0.17MnO2 nanotube with a diameter around 100 nm is shown in Figure 2. The tubular structure is normally not uniform throughout the nanotube; instead, a diameter reduction is observed along the nanotube, which is consistent with the etching formation mechanism of the nanotubes.9 The high-resolution TEM image and its fast Fourier transform (FFT), shown in Figure 2, clearly indicate that the R-K0.17MnO2 nanotube is of single-crystal. Figure 3 shows the TGA and DTA curves of the R-K0.17MnO2 sample. Only a small amount of absorbed water, approximately 1% mass percent of the sample, is detected based on the weight loss below 200 °C. The weight losses at 670 and 890 °C should correspond to the phase transformations from MnO2 to Mn2O3 and from Mn2O3 to Mn3O4.15 The final product after the TGA and DTA measurement has been examined by XRD, which is composed of K2Mn4O8 and Mn3O4 (see the Supporting Informa-
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Figure 3. TGA and DTA curves of the R-K0.17MnO2 sample.
tion, Figure S1). The calculated weight loss based on the chemical composition of R-K0.17MnO2 is 7.5%, which agrees with the weight loss of 8.3% according to the TGA measurement. Our above experiments indicate that R-MnO2 nanotubes are very stable. On the one hand, the R-MnO2 sample does not decompose into other phases below 670 °C. On the other hand, the morphology and size of the nanotube are not altered by the acid and alkali treatment. According to the previous report, acid treatment is an effective way to remove the intercalated K+ from the R-MnO2 sample.16 However, the K+ cannot be completely eliminated based on our experiment. The proposed reason is that the large K+ cation may be essential for the formation and stabilization of R-MnO2 nanostructures.17,18 In addition, we find that the K+ concentration of the sample can also be increased by increasing the K+ content of the aqueous alkali. Thus, the intercalated K+ content of the as-synthesized R-MnO2 nanotubes can be easily modified by the effective follow-up treatment. Figure 4 shows the magnetic properties of the R-MnO2 nanotubes with different K+ contents. The temperature dependence of the magnetization was recorded on heating the sample in a field of 100 Oe using zero-field-cooled (ZFC) and fieldcooled (FC) modes. Similar to the R-K0.09MnO2 sample,10 pronounced ZFC-FC irreversibility is observed at low temperature for the samples with x ) 0.10 and 0.12. However, as shown in Figure 4e,g, both ZFC and FC susceptibilities show sharp cusps at the same temperature for the samples with x ) 0.15 and 0.17 (25.4 K for x ) 0.15 and 25.3 K for x ) 0.17). The magnetic transition temperatures of the samples with x ) 0.15 and 0.17 are nearly identical to the Ne´el temperature of the bulk R-MnO2 (24.5 K).13 Despite that the ZFC and FC curves show small differences at low temperature, the samples with x ) 0.15 and 0.17 are dominated by the antiferromagnetic coupling at low temperature with the Ne´el temperatures of 25.4 and 25.3 K, respectively. The field dependence of the magnetization of the R-KxMnO2 sample was measured at both 5 and 300 K. As shown in Figure 4b,d,h, the magnetization of these samples does not show any hysteresis at 300 K, indicating that all the samples are paramagnetic at high temperature. At 5 K, considerable hysteresis with coercivities as large as 6.65 and 8.65 kOe is observed for the samples with x ) 0.10 and 0.12, respectively. However, the hysteresis effect at 5 K is much weaker for the samples with x ) 0.15 and 0.17. Figure 5 shows the central portion of the hysteresis loop at 5 K for the samples with x ) 0.15 and 0.17. Coercivities around 610 and 550 Oe are detected for the samples with x ) 0.15 and 0.17, respectively. These small coercivities at 5 K together with the difference between ZFC and FC susceptibilities at low temperature can be attributed
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Figure 4. Magnetic susceptibilities in an applied field of 100 Oe (left column) and hysteresis loops at 5 and 300 K (right column) of the R-KxMnO2 samples.
Figure 5. Central part of the hysteresis loop measured at 5 K: (a) R-K0.15MnO2, (b) R-K0.17MnO2.
to either the magnetic anisotropy or the charge separation of Mn4+ and Mn3+.16,19 To unravel the origin of these magnetic behaviors, the ZFC and FC hysteresis loops of the samples with x ) 0.12 and 0.17 have been recorded at 5 K. The FC hysteresis loop has been measured after cooling the sample from 300 to
Luo et al. 5 K in an applied field of 50 kOe. As shown in Figure 6a, the ZFC and FC loops are virtually identical and both loops are symmetric about the origin for the R-K0.12MnO2 sample. However, for the R-K0.17MnO2 sample, the FC loop exhibits the typical features of an exchange bias system, which is evidenced by a shift of the hysteresis loop toward negative applied field and enhanced coercivity (Figure 6b). This may imply that the R-K0.12MnO2 sample is homogeneous with the Mn ions in the averaged valence state, whereas the R-K0.17MnO2 sample is composed of antiferromagnetic and ferromagnetic regions due to the separation of Mn4+ and Mn3+. Thus, both the divergence of ZFC and FC susceptibilities at low temperature and the small coercivity at 5 K of the samples with x ) 0.15 and x ) 0.17 are originated from weak ferromagnetism due to the charge separation of Mn4+ and Mn3+. Because all the investigated R-KxMnO2 samples have the same crystal structure and morphology, it is to be expected that the difference in their magnetic properties results from their different K+ concentrations. To clarify the magnetic properties of the R-KxMnO2 samples, it is essential to have a clear understanding of the crystal structure of the K+-containing R-MnO2 compound. First, the K+ is located at the tunnel cavity and occupies the 4e site. [The K cation has two crystal positions at (0,0,z) and (0,0,-z), but these two positions cannot be occupied by K+ simultaneously due to space restriction. Therefore, the maximum K+ occupancy corresponds to x ) 0.25.]20,21 Second, the intercalated K+ leads to the mixed/ averaged valence state of Mn4+ and Mn3+ (here, the mixed/ averaged valence state means that the Mn4+ and Mn3+ are randomly distributed on the 4e sites).10,20 Third, the magnetic Mn ions are triangularly arranged.10 Finally, the maximum occupancy of the K+ site corresponding to x ) 0.25 can only be achieved by high-pressure synthesis.20 Below half occupancy of the K+ site (x e 0.125), the K+ is randomly distributed on the 4e site, but an ordering of K+ is expected by taking into account the strong electrostatic repulsion when more than half of the K+ site is occupied (x g 0.125).16,20 The ordering of K cations has been reported in R-K1.33Mn8O16,20 R-K1.5(H3O)xMn8O16,16 and other hollandites.22,23 To relax the Coulomb repulsion, the Mn3+ and Mn4+ prefer to occupy the Mn sites close to the K cation and vacancy, respectively. Thus, the ordering of K+ results in a charge separation of Mn3+ and Mn4+, which are in the mixed/averaged valence state when the K cation is randomly distributed.16 Figure 7 shows the reciprocal ZFC susceptibilities of the R-KxMnO2 samples at the high-temperature region (150-300 K). The reciprocal susceptibilities are linearly dependent on the temperature for both x ) 0.10 and x ) 0.12, but a deviation from the straight line is observed between 250 and 300 K for the sample with x ) 0.15. For the sample with x ) 0.17, this offset at high temperature becomes more evident. Figure 8 shows the temperature-dependent resistance of the R-KxMnO2 samples, which displays the typical feature of a semiconductor. The electrical conductivity at high temperature is shown in Figure 9. For the samples with x ) 0.10 and 0.12, ln(σT) decreases in approximate linearity with the increase of 1000/T. A deviation from the linearity is observed at about 250 K for the samples with x ) 0.15 and 0.17. The reciprocal susceptibilities and electrical conductivities of the samples with x ) 0.15 and 0.17 indicate that the Mn ions in the mixed/averaged valence state at high temperature may undergo a charge separation at about 250 K. Note that our in situ XRD analysis from 120 to 300 K (the XRD patterns are not shown in the text) excludes the possibility of the existence of structure transitions for the
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Figure 6. ZFC and FC hysteresis loops measured at 5 K: (a) R-K0.12MnO2, (b) R-K0.17MnO2 (only the central portion is shown).
Figure 9. Temperature dependence of the electrical conductivity of R-KxMnO2 samples. The solid line is a guide for the eyes.
Figure 7. Reciprocal ZFC susceptibilities of the R-KxMnO2 samples. The solid line is a guide for the eyes.
Figure 8. Temperature-dependent resistances of the R-KxMnO2 samples.
sample with x ) 0.17. A similar charge separation of Mn ions has been detected in R-K1.5(H3O)xMn8O16 by measuring its electrical conductivity and magnetic properties.16 Very recently, Umek et al. reported the phase segregation of R-K0.02Fe0.18Mn1.16O2 into
mixed Mn3+/Mn4+ and nearly pure Mn4+ regions through the measurement of magnetic susceptibility and electron paramagnetic resonance.19 Finally, the effect of K+ doping on the magnetic properties of the R-KxMnO2 samples can be proposed on the basis of the distribution of K+, Mn3+, and Mn4+. Below half occupancy of the K+ site, the Mn ions in the mixed/averaged valence state are randomly distributed on the triangular lattice. The strong geometrical frustration from the triangular lattice configuration together with the mixture of Mn3+/Mn4+ leads to the SG behavior of the samples with x ) 0.10 and 0.12,10,24,25 which is manifested as the large divergence of the ZFC and FC susceptibilities and the considerable hysteresis effect at 5 K. The randomly distributed Mn3+ increases with the stuffing of K+ when the K+ content x is less than 0.125, leading to a stronger competition between the ferromagnetic component and antiferromagnetic base. Therefore, both the freezing temperature (39.2 K for x ) 0.10 and 43.3 K for x ) 0.12) and the coercivity (6.65 kOe for x ) 0.10 and 8.65 kOe for x ) 0.12) increase with the K+ concentration. Above the half occupancy of the K+ site, the R-KxMnO2 sample with x g 0.125 returns to the antiferromagnetic state due to the charge separation of Mn3+ and Mn4+.16
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4. Conclusions
References and Notes
In summary, the effect of K+ doping on the magnetic properties of the R-KxMnO2 samples has been investigated by magnetic and electric measurements. The K+ content of the precursor R-MnO2 nanotube can be modified without changing its structure and morphology by acid and alkali treatments. The magnetic properties of the R-KxMnO2 samples are dramatically influenced by the K+ content. When less than half of the K+ sites are occupied, the mixture and random distribution of the magnetic Mn3+ and Mn4+ ions on the triangular lattice lead to the spin-glass behavior of the sample with x e 0.125. Thus, the samples with x ) 0.10 and 0.12 show a large ZFC-FC irreversibility and pronounced hysteresis at low temperature. When more than half of the K+ sites are occupied, a charge separation of Mn3+ and Mn4+ is induced due to the ordering of K+. Both reciprocal susceptibilities and electrical conductivities display that the samples with x ) 0.15 and 0.17 undergo a charge separation transition between 250 and 300 K. Thus, the samples with x ) 0.15 and 0.17 are antiferromagnetic with the Ne´el temperatures of 25.4 and 25.3 K, respectively. Our experimental results indicate that tunable magnetic and electric properties of the R-KxMnO2 compound can be available through the adjustment of the K+ located at the tunnel cavity, such as modifying the K+ content, replacing the K+ with other cations, etc.
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Acknowledgment. The work was financially supported by the National Natural Science Foundation of China (Grant No. 20851001), the National Basic Research Program of China (Grant No. 2007CB925003), and the Chinese Academy of Sciences. Supporting Information Available: Figure showing the XRD pattern of the R-K0.17MnO2 sample after being heated to 1100 °C and a table listing the magnetic transition temperature, Curie-Weiss temperature, and coercivity of the R-KxMnO2 samples with x ) 0.10, 0.12, 0.15, and 0.17. This material is available free of charge via the Internet at http://pubs.acs.org.
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