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Magnetic potassium clusters in the nanographite-based nanoporous system
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184 www.elsevier.com/locate/jpcs
Magnetic potassium clusters in the nanographite-based nanoporous system Kazuyuki Takai, Souichiro Eto, Masayasu Inaguma, Toshiaki Enoki Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan Received 29 June 2007; received in revised form 24 October 2007; accepted 29 October 2007
Abstract Novel antiferromagnetic (AF) potassium clusters accommodated in the magnetic nanographene-based porous network are investigated in terms of the host–guest interaction. Raman spectroscopy proves that the potassium clusters whose size is ca. 60 potassium atoms in average are subjected to a slight charge transfer with the host nanographene. This results in a significant reduction in the edge-state spins of the host nanographene. In the high temperature region above 150 K, a large susceptibility contribution of the magnetism of the potassium clusters appears upon the increase in the potassium content. This is explained by antiferromagnetically fluctuating magnetic clusters, where the spins of potassium 4 s electrons having localized nature are suggested to interact antiferromagnetically with each other. The elongated inter-atomic distance and the structural disorder in the cluster are responsible for the localized nature of the 4 s electrons. A simple model with a unique AF spin gap gives the estimate of exchange interaction to be in the temperature region of 500–800 K. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; A. Microporous materials; B. Vapor deposition; C. Raman spectroscopy; D. Magnetic properties
1. Introduction When the crystal size is reduced to nano-dimension, even non-magnetic elements such as carbon, potassium can become magnetic. For example, potassium clusters accommodated in nanopores of zeolite cage are known to be ferromagnet [1]. Interestingly, nano-sized graphite has been known also to enhance magnetism [2,3]. When a singlesheet graphite (graphene) is cut along the zigzag direction, strongly spin-polarized non-bonding p-state (edge state) appears along the created edges; in spite that cutting along the armchair direction produces no such state. Meantime, significant host–guest interaction between graphite and potassium has been well known in graphite intercalation compounds (GIC) [4]. Here, a question arises on what happens when the sizes of the system comprising of graphite host and potassium guest are reduced to nanodimension, since both become magnetic.
Corresponding author. Tel.: +81 3 5734 2610; fax: +81 3 5734 2242.
E-mail address: [email protected] (K. Takai). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.10.056
Activated carbon fibers (ACFs) composed of the network of nanographite are a good candidate for nanoporous host material for creating nanographene host systems with potassium guest. In ACFs, each nanographite domain consists of a stack of 3–4 nanographene sheets with the mean in-plane size of ca. 3 nm [3]. The slit-shaped nanopore produced between nanographite domains accommodates guest species through charge transfer (CT) and van der Waals interaction [5]. In the present study, the magnetic potassium clusters fabricated in the ACFs nanopores are investigated in terms of host–guest interaction by using Raman spectra and magnetic susceptibility.
2. Experimental Potassium was introduced into ACFs (Kuraray Chemicals, FR-20), which was preheat-treated at 1 106 Torr and 200 1C for 12 h, in a vacuum-sealed glass tube through a vapor phase reaction at 250–500 1C. The reaction tube is quenched into room temperature by taking out from the electric furnace after potassium introduction, which is
ARTICLE IN PRESS K. Takai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184
3. Results and discussion
T (emu K g-1)
T (emu K g-1)
×10-5
T (emu K g-1)
important process to form the cluster-shaped potassium in the nanopore. According to the elemental analysis, pristine host ACFs under ambient pressure of air atmosphere (O2, H2O) has a 10% composition of oxygen in atomic ratio, where major amount of oxygen is attributed to the adsorbed air species. Although some oxygen-contained functional groups are also present at the periphery part of nanographite in pristine ACFs, during vacuum heating process at the temperature of 200 1C under 1E 106 Torr, the oxygen-contained functional groups are decomposed and the intrinsic oxygen composition in ACFs become negligible (almost zero), which is supported by the results of the weight reduction and mass spectroscopy during heat treatment. For the synthesized potassium adsorbed ACFs (K-ACFs), atomic ratio K/C was obtained by weight uptake. The Raman spectra were measured using 514.5 nm laser beam. The susceptibility was measured at 2–300 K.
2
1
0 ×10-5 2
0 ×10-5 2
0 0
According to small angle X-ray scattering study, more than a half of the potassium atoms adsorbed in the nanopore form a flat-shaped nano-cluster consisting of ca. 60 atoms (K/C ¼ 0.259) in average [6]. The CT rate per carbon (fc) obtained from the shift of Raman G-band peak (1585 cm1 for pristine ACFs) increases upon the increase in K/C (fc ¼ 0.059 at K/C ¼ 0.282), which is much smaller than that of bulk GIC (fc ¼ 0.075 at K/C ¼ 0.125) in spite that the potassium content is higher [4]. The CT feature is also confirmed by the decrease in the orbital diamagnetic susceptibility w0 of nanographene with the increase in K/C. The findings from these experiments indicate that the potassium atoms adsorbed in the nanopore are less ionized in comparison with those in the bulk K-GIC. The ratio of the ionized potassium to the total potassium content K+/K, steeply increases with increasing K/C and is saturated even in the low K/C region (K/CX 0.05). This suggests that most of the adsorbed potassium atoms in the low K/C range are not subjected to CT but exist merely as neutral clusters in the nanopore of ACFs. Therefore, among the potassium species accommodated in the nanopores, only those directly faced to nanographene surfaces are considered to contribute to the slight CT. The electronic interaction between the host nanographene and the guest potassium clusters modifies the magnetism of the edge states spins of nanographene. The temperature dependence of the susceptibility w is shown in Fig. 1(a), where the wT value is considered to be proportional to the effective localized spin density. Below 150 K, the wT value is roughly independent of the temperature for both of ACFs and K-ACFs except below ca. 50 K, where the weak antiferromagnetic (AF) interaction between edge-state spins reduces the wT value. In the temperature region, where the wT value is roughly temperature-independent, the localized spin density obtained from the wT value decreases as K/C increases. This
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100
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300
T (K) Fig. 1. (a) Susceptibility w vs. temperature plot for ACFs and K-ACFs. The K denotes ACFs, and &, ., +, n, and J denote K-ACFs with K/C ¼ 0.035, 0.058, 0.214, 0.259, and 0.282, respectively. (b) Change in the susceptibility during desorption process of potassium for K-ACFs with K/C ¼ 0. 214. The K, &, n, +, ., and J denote the nonheat-treated sample, the samples heat-treated at 170, 250, 300, 350 and 400 1C, respectively. (c) Fitting with model for K/C ¼ 0.214 (solid line), where K is observed, J and & is the contributions of potassium and carbon, respectively.
can be explained on the basis of the electronic structure of nanographene [5], where the edge state having the localized spins, is located at the degenerate point, at which the linear p- and p*- bands touch to each other. As the Fermi energy EF is located at the degenerate point for the neutral nanographene, the pristine ACFs have the largest localized spin density of edge-state origin. The upshift of EF through CT from potassium to nanographene upon the increase in K/C reduces the contribution of the edge state at EF, resulting in the decrease of the density of the edge-state spins. Interestingly, a strong magnetism of the potassium clusters emerges in the susceptibility behavior in the higher temperature region above 150 K. As shown in Fig. 1(a), an extra contribution of the localized spins appears in the high temperature region upon potassium uptake, where the localized spin density steeply increases with increasing the temperature in K-ACFs. Fig. 1(b) shows the change in the susceptibility during the heat-induced potassium desorption for the sample with the K/C ¼ 0.214. Above the heat-treatment temperature (HTT) of 300 1C, the temperature-dependent contribution at high temperatures suddenly disappears, whereas the temperature-independent wT value in the intermediate temperature range is raised for HTT 4300 1C, showing that the desorption reduces the extent of the CT at the expense of the extra contribution in
ARTICLE IN PRESS 1184
K. Takai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184
the high temperature region. This demonstrates that the extra contribution of the susceptibility originates from the potassium atoms excessively introduced into the nanopores. Taking into account that the spin density of nanographene is independent of temperature in the high temperature range, the contribution of the potassium cluster in the spin magnetism is extracted by subtracting the wT value at 30–80 K from the observed susceptibility. The exponential increase in the wT value upon the increase in the temperature for the potassium cluster is suggestive of the behavior of antiferromagnetically fluctuating spin clusters. The fact that the potassium 4 s electrons is in the half-filled state is favorable for the creation of Mott insulator, which has Heisenberg AF ground state, as the result of the competition between the transfer integral and the Coulomb energy. In bulk potassium, the 4s electrons are completely delocalized due to the large transfer integral, which is generated by the strongly extended feature of their wavefunction. In the potassium clusters in the nanopores of ACFs, the surface potassium atoms faced to nanographene are seriously affected by the host–guest interaction, as well evidenced by the above discussion. The random potential coming from the surrounding nanographite domains makes the arrangement of potassium atoms disordered in the marginal region faced to nanographene. In addition, the slit-shaped nanopores between nanographite domains confine the clusters to two dimension. The random potential, the elongated interatomic distance and the reduced dimensionality bring about the localization of 4s electrons of the surface potassium atoms of the clusters. Here, we analyze the AF behavior of potassium clusters based on a simple AF cluster model with a unique spin gap
D [7], where the susceptibility is described as wK ¼ (CK/T) exp[D/kBT]. CK is the Curie constant representing the number of localized spins in the clusters. By fitting the data with this formula, D is calculated as 800 K for K/C ¼ 0.035, and it decreases upon the increase in K/C (500 K at K/C ¼ 0.282). This means that the growth of the potassium cluster size tends to reduce the spin-gap energy. This finding that smaller potassium clusters have larger spin gap D strongly supports the fluctuating AF spin cluster model for the potassium magnetism. Acknowledgments The authors are grateful to Profs. K. Iio and S. Saito for their valuable discussion. The present work was supported by the Grant-in-aid for Scientific Research No. 15105005 from Japan Society for Promotion of Science. References [1] Y. Nozue, T. Kodaira, T. Goto, Phys. Rev. Lett. 68 (1992) 3789. [2] M. Fujita, K. Wakabayashi, K. Nakada, K. Kusakabe, J. Phys. Soc. Jpn. 65 (1996) 1920. [3] T. Enoki, K. Takai, Unconventional magnetic properties of nanographite, in: F. Palacio, T. Makarova (Eds.), Carbon-based Magnetism, Elsevier, Amsterdam, 2006, p. 397. [4] T. Enoki, M. Suzuki, M. Endo, Graphite Intercalation Compounds and Applications, Oxford University Press, New York, 2003. [5] K. Takai, H. Kumagai, H. Sato, T. Enoki, Phys. Rev. B 73 (2006) 035435. [6] K. Takai, S. Eto, M. Inaguma, T. Enoki, H. Ogata, M. Tokita, M. Watanabe, Phys. Rev. Lett. 98 (2007) 017203. [7] D. Arcon, R. Blinc, D. Mihailovic, A. Omerzu, P. Cevc, Europhys. Lett. 46 (1999) 667.
Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184 www.elsevier.com/locate/jpcs
Magnetic potassium clusters in the nanographite-based nanoporous system Kazuyuki Takai, Souichiro Eto, Masayasu Inaguma, Toshiaki Enoki Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan Received 29 June 2007; received in revised form 24 October 2007; accepted 29 October 2007
Abstract Novel antiferromagnetic (AF) potassium clusters accommodated in the magnetic nanographene-based porous network are investigated in terms of the host–guest interaction. Raman spectroscopy proves that the potassium clusters whose size is ca. 60 potassium atoms in average are subjected to a slight charge transfer with the host nanographene. This results in a significant reduction in the edge-state spins of the host nanographene. In the high temperature region above 150 K, a large susceptibility contribution of the magnetism of the potassium clusters appears upon the increase in the potassium content. This is explained by antiferromagnetically fluctuating magnetic clusters, where the spins of potassium 4 s electrons having localized nature are suggested to interact antiferromagnetically with each other. The elongated inter-atomic distance and the structural disorder in the cluster are responsible for the localized nature of the 4 s electrons. A simple model with a unique AF spin gap gives the estimate of exchange interaction to be in the temperature region of 500–800 K. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; A. Microporous materials; B. Vapor deposition; C. Raman spectroscopy; D. Magnetic properties
1. Introduction When the crystal size is reduced to nano-dimension, even non-magnetic elements such as carbon, potassium can become magnetic. For example, potassium clusters accommodated in nanopores of zeolite cage are known to be ferromagnet [1]. Interestingly, nano-sized graphite has been known also to enhance magnetism [2,3]. When a singlesheet graphite (graphene) is cut along the zigzag direction, strongly spin-polarized non-bonding p-state (edge state) appears along the created edges; in spite that cutting along the armchair direction produces no such state. Meantime, significant host–guest interaction between graphite and potassium has been well known in graphite intercalation compounds (GIC) [4]. Here, a question arises on what happens when the sizes of the system comprising of graphite host and potassium guest are reduced to nanodimension, since both become magnetic.
Corresponding author. Tel.: +81 3 5734 2610; fax: +81 3 5734 2242.
E-mail address: [email protected] (K. Takai). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.10.056
Activated carbon fibers (ACFs) composed of the network of nanographite are a good candidate for nanoporous host material for creating nanographene host systems with potassium guest. In ACFs, each nanographite domain consists of a stack of 3–4 nanographene sheets with the mean in-plane size of ca. 3 nm [3]. The slit-shaped nanopore produced between nanographite domains accommodates guest species through charge transfer (CT) and van der Waals interaction [5]. In the present study, the magnetic potassium clusters fabricated in the ACFs nanopores are investigated in terms of host–guest interaction by using Raman spectra and magnetic susceptibility.
2. Experimental Potassium was introduced into ACFs (Kuraray Chemicals, FR-20), which was preheat-treated at 1 106 Torr and 200 1C for 12 h, in a vacuum-sealed glass tube through a vapor phase reaction at 250–500 1C. The reaction tube is quenched into room temperature by taking out from the electric furnace after potassium introduction, which is
ARTICLE IN PRESS K. Takai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184
3. Results and discussion
T (emu K g-1)
T (emu K g-1)
×10-5
T (emu K g-1)
important process to form the cluster-shaped potassium in the nanopore. According to the elemental analysis, pristine host ACFs under ambient pressure of air atmosphere (O2, H2O) has a 10% composition of oxygen in atomic ratio, where major amount of oxygen is attributed to the adsorbed air species. Although some oxygen-contained functional groups are also present at the periphery part of nanographite in pristine ACFs, during vacuum heating process at the temperature of 200 1C under 1E 106 Torr, the oxygen-contained functional groups are decomposed and the intrinsic oxygen composition in ACFs become negligible (almost zero), which is supported by the results of the weight reduction and mass spectroscopy during heat treatment. For the synthesized potassium adsorbed ACFs (K-ACFs), atomic ratio K/C was obtained by weight uptake. The Raman spectra were measured using 514.5 nm laser beam. The susceptibility was measured at 2–300 K.
2
1
0 ×10-5 2
0 ×10-5 2
0 0
According to small angle X-ray scattering study, more than a half of the potassium atoms adsorbed in the nanopore form a flat-shaped nano-cluster consisting of ca. 60 atoms (K/C ¼ 0.259) in average [6]. The CT rate per carbon (fc) obtained from the shift of Raman G-band peak (1585 cm1 for pristine ACFs) increases upon the increase in K/C (fc ¼ 0.059 at K/C ¼ 0.282), which is much smaller than that of bulk GIC (fc ¼ 0.075 at K/C ¼ 0.125) in spite that the potassium content is higher [4]. The CT feature is also confirmed by the decrease in the orbital diamagnetic susceptibility w0 of nanographene with the increase in K/C. The findings from these experiments indicate that the potassium atoms adsorbed in the nanopore are less ionized in comparison with those in the bulk K-GIC. The ratio of the ionized potassium to the total potassium content K+/K, steeply increases with increasing K/C and is saturated even in the low K/C region (K/CX 0.05). This suggests that most of the adsorbed potassium atoms in the low K/C range are not subjected to CT but exist merely as neutral clusters in the nanopore of ACFs. Therefore, among the potassium species accommodated in the nanopores, only those directly faced to nanographene surfaces are considered to contribute to the slight CT. The electronic interaction between the host nanographene and the guest potassium clusters modifies the magnetism of the edge states spins of nanographene. The temperature dependence of the susceptibility w is shown in Fig. 1(a), where the wT value is considered to be proportional to the effective localized spin density. Below 150 K, the wT value is roughly independent of the temperature for both of ACFs and K-ACFs except below ca. 50 K, where the weak antiferromagnetic (AF) interaction between edge-state spins reduces the wT value. In the temperature region, where the wT value is roughly temperature-independent, the localized spin density obtained from the wT value decreases as K/C increases. This
1183
100
200
300
T (K) Fig. 1. (a) Susceptibility w vs. temperature plot for ACFs and K-ACFs. The K denotes ACFs, and &, ., +, n, and J denote K-ACFs with K/C ¼ 0.035, 0.058, 0.214, 0.259, and 0.282, respectively. (b) Change in the susceptibility during desorption process of potassium for K-ACFs with K/C ¼ 0. 214. The K, &, n, +, ., and J denote the nonheat-treated sample, the samples heat-treated at 170, 250, 300, 350 and 400 1C, respectively. (c) Fitting with model for K/C ¼ 0.214 (solid line), where K is observed, J and & is the contributions of potassium and carbon, respectively.
can be explained on the basis of the electronic structure of nanographene [5], where the edge state having the localized spins, is located at the degenerate point, at which the linear p- and p*- bands touch to each other. As the Fermi energy EF is located at the degenerate point for the neutral nanographene, the pristine ACFs have the largest localized spin density of edge-state origin. The upshift of EF through CT from potassium to nanographene upon the increase in K/C reduces the contribution of the edge state at EF, resulting in the decrease of the density of the edge-state spins. Interestingly, a strong magnetism of the potassium clusters emerges in the susceptibility behavior in the higher temperature region above 150 K. As shown in Fig. 1(a), an extra contribution of the localized spins appears in the high temperature region upon potassium uptake, where the localized spin density steeply increases with increasing the temperature in K-ACFs. Fig. 1(b) shows the change in the susceptibility during the heat-induced potassium desorption for the sample with the K/C ¼ 0.214. Above the heat-treatment temperature (HTT) of 300 1C, the temperature-dependent contribution at high temperatures suddenly disappears, whereas the temperature-independent wT value in the intermediate temperature range is raised for HTT 4300 1C, showing that the desorption reduces the extent of the CT at the expense of the extra contribution in
ARTICLE IN PRESS 1184
K. Takai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1182–1184
the high temperature region. This demonstrates that the extra contribution of the susceptibility originates from the potassium atoms excessively introduced into the nanopores. Taking into account that the spin density of nanographene is independent of temperature in the high temperature range, the contribution of the potassium cluster in the spin magnetism is extracted by subtracting the wT value at 30–80 K from the observed susceptibility. The exponential increase in the wT value upon the increase in the temperature for the potassium cluster is suggestive of the behavior of antiferromagnetically fluctuating spin clusters. The fact that the potassium 4 s electrons is in the half-filled state is favorable for the creation of Mott insulator, which has Heisenberg AF ground state, as the result of the competition between the transfer integral and the Coulomb energy. In bulk potassium, the 4s electrons are completely delocalized due to the large transfer integral, which is generated by the strongly extended feature of their wavefunction. In the potassium clusters in the nanopores of ACFs, the surface potassium atoms faced to nanographene are seriously affected by the host–guest interaction, as well evidenced by the above discussion. The random potential coming from the surrounding nanographite domains makes the arrangement of potassium atoms disordered in the marginal region faced to nanographene. In addition, the slit-shaped nanopores between nanographite domains confine the clusters to two dimension. The random potential, the elongated interatomic distance and the reduced dimensionality bring about the localization of 4s electrons of the surface potassium atoms of the clusters. Here, we analyze the AF behavior of potassium clusters based on a simple AF cluster model with a unique spin gap
D [7], where the susceptibility is described as wK ¼ (CK/T) exp[D/kBT]. CK is the Curie constant representing the number of localized spins in the clusters. By fitting the data with this formula, D is calculated as 800 K for K/C ¼ 0.035, and it decreases upon the increase in K/C (500 K at K/C ¼ 0.282). This means that the growth of the potassium cluster size tends to reduce the spin-gap energy. This finding that smaller potassium clusters have larger spin gap D strongly supports the fluctuating AF spin cluster model for the potassium magnetism. Acknowledgments The authors are grateful to Profs. K. Iio and S. Saito for their valuable discussion. The present work was supported by the Grant-in-aid for Scientific Research No. 15105005 from Japan Society for Promotion of Science. References [1] Y. Nozue, T. Kodaira, T. Goto, Phys. Rev. Lett. 68 (1992) 3789. [2] M. Fujita, K. Wakabayashi, K. Nakada, K. Kusakabe, J. Phys. Soc. Jpn. 65 (1996) 1920. [3] T. Enoki, K. Takai, Unconventional magnetic properties of nanographite, in: F. Palacio, T. Makarova (Eds.), Carbon-based Magnetism, Elsevier, Amsterdam, 2006, p. 397. [4] T. Enoki, M. Suzuki, M. Endo, Graphite Intercalation Compounds and Applications, Oxford University Press, New York, 2003. [5] K. Takai, H. Kumagai, H. Sato, T. Enoki, Phys. Rev. B 73 (2006) 035435. [6] K. Takai, S. Eto, M. Inaguma, T. Enoki, H. Ogata, M. Tokita, M. Watanabe, Phys. Rev. Lett. 98 (2007) 017203. [7] D. Arcon, R. Blinc, D. Mihailovic, A. Omerzu, P. Cevc, Europhys. Lett. 46 (1999) 667.