ARTICLE pubs.acs.org/JPCC
Evidence of Vacancy-Induced Room Temperature Ferromagnetism in Amorphous and Crystalline Al2O3 Nanoparticles Guijin Yang, Daqiang Gao, Jinlin Zhang, Jing Zhang, Zhenhua Shi, and Desheng Xue* Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, People's Republic of China ABSTRACT: Amorphous and crystalline Al2O3 nanoparticles were synthesized by a solgel method with postannealing at different temperatures. Magnetism measurements have indicated that all Al2O3 nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after vacuum annealing, whereas bulk Al2O3 presents paramagnetism. Electron spin resonance and fitting results of O 1s X-ray photoelectron spectroscopy reveal that the origin of the ferromagnetism in Al2O3 nanoparticles could be attributed to the singly charged oxygen vacancies (F+ centers). The variation of the relative area of oxygen vacancies and the number of free electrons is consistent with the change of saturation magnetization for the samples. Combined with these results, a direct correlation of ferromagnetism with F+ centers exchange mechanism is established.
’ INTRODUCTION Dilute magnetic oxides have received much attention in the past decade due to their theoretically predicted high Curie temperatures (Tc) and potential applications as spintronic materials.13 Up to now, most experimental studies on the origin of ferromagnetism (FM) in transition metal doped oxides, such as ZnO and TiO2, have not yet obtained a consistent conclusion.46 Sometimes contaminants or metallic clusters were found to be responsible for the observed FM in these materials.7,8 Recently, room temperature (RT) FM was even detected in undoped HfO2, giving evidence that defects could be the origin of the observed FM.9,10 Besides HfO2, some other undoped oxides, e.g., ZnO, TiO2, CuO, SnO2, CaO, In2O3, and CoO, also gained more and more focus due to their RT FM.1117 The exchange mechanism for F+ centers (the singly charged oxygen vacancies), as a subcategory of the bound magnetic polaron (BMP) model, was proposed to explain the RT FM in ZnO, CeO2, and TiO2 as oxygen vacancy related.1821 It is worth noting that all of these reported oxides, which showed a RT FM behavior, are crystalline. Compared with the crystalline materials, amorphous oxides with similar chemical compositions should possess many more defects. If magnetism was induced by defects, the RT FM may be improved in the amorphous oxides. Al2O3 with amorphous phase and crystalline phase shows interesting properties with high dielectric constant, wide-gap insulators, low gate leakage, and high thermal stability.2224 It is promising as one of the next-generation insulators to replace SiO2.22 More recently, a RT FM behavior above 390 K has also been reported in Al2O3 nanoparticles, but the origin of the FM was not investigated.1113,25,26 Thus the origin of FM in Al2O3 system calls for in-depth systematic studies. In the present work, amorphous and crystalline phase Al2O3 nanoparticles were r 2011 American Chemical Society
synthesized by a solgel method with postannealing treatment at different temperatures. Interestingly, it was observed that all Al2O3 nanoparticles exhibit intrinsic room temperature ferromagnetism, whereas the bulk ones do not. Series of measurements have indicated that the origin of the ferromagnetism in Al2O3 nanoparticles could be attributed to the singly charged oxygen vacancies (F+ centers). In addition, the relation between ferromagnetism and the F+ center exchange mechanism was discussed.
’ EXPERIMENT Al2O3 nanoparticles were prepared by a solgel method with postannealing at different temperatures. First, 1.88 g of AlCl3 powder was dissolved into 40 mL of ethylene glycol monomethyl ether (C3H8O2) under vigorous magnetic stirring. Then, the obtained solution continued to be stirred for 2 h at 80 °C and sequentially was dried to form the precursor. Finally, the precursor was annealed at 600, 700, 800, 900, and 1000 °C for 2 h in air, respectively. For convenience, the five as-prepared samples were denoted as S600, S700, S800, S900, and S1000, respectively. The crystalline structures of the samples were investigated by X-ray diffraction (XRD; X'Pert PRO PHILIPS, Cu KR radiation, λ = 1.540 56 Å). The morphologies and microstructures of the samples were studied by high-resolution transmission electron microscopy (HRTEM; JEM-2010) equipped with an energy dispersive X-ray spectrometer (EDS) and selected area electron diffraction (SAED). The infrared absorption spectra of the Received: April 28, 2011 Revised: July 20, 2011 Published: July 20, 2011 16814
dx.doi.org/10.1021/jp2039338 | J. Phys. Chem. C 2011, 115, 16814–16818
The Journal of Physical Chemistry C
ARTICLE
Figure 1. XRD patterns of as-prepared samples (S600, S700, S800, S900, and S1000).
Figure 3. (a) TEM and (b) SAED of S600. The inset in (a) is the corresponding EDS. (c) TEM and (d) HRTEM of S800.
Figure 2. FTIR patterns of samples (S600, S800, and S1000).
samples were conducted with the KBr pellet method on a Fourier transform infrared spectrometer (FTIR; NEXUS 670) in the range from 400 to 4000 cm1. The chemical states of different elements presented in the samples were analyzed by X-ray photoelectron spectroscopy (XPS; VG ESCALAB 210); the standard C 1s peak at 285.0 eV was used as a reference for correcting the shifts. The magnetic properties of Al2O3 nanoparticles were measured by a vibrating sample magnetometer (VSM; Lakeshore 7304) and a quantum design magnetic property measurement system (MPMS) based on a superconducting quantum interference device (SQUID). Electron spin resonance spectra (ESR; JEOL, JES-FA300) were recorded at RT in X-band with magnetic field modulation at 8.985 GHz.
’ RESULTS AND DISCUSSION Figure 1 illustrates the diffraction patterns of the as-prepared Al2O3 nanoparticles. It is observed that no obvious diffraction peaks appear in S600, indicating that S600 may be an amorphous Al2O3 structure. However, the γ-Al2O3 phase (JCPDS card no. 10-0425) begins to be formed in S700 with a rather weak intensity. This indicates that the amorphous Al2O3 begins to transit to γ-Al2O3 phase at about 700 °C. With the increase of annealing temperature, the crystal of γ-Al2O3 completely forms in S800. The characteristic peaks of γ-Al2O3 disappear and totally transform into the phase of R-Al2O3 (JCPDS card no. 46-1212) for S1000.
To further study the chemical compositions and bonds of the samples annealed at different temperatures, FTIR measurement was applied in the range from 400 to 4000 cm1. The FTIR spectra of S600, S800, and S1000 are shown in Figure 2. A broad and smooth absorption band without any fine structure in the wavenumber range from 500 to 900 cm1 is attributed to the disordered distribution of vacancies and the continuous distribution of bond lengths in an amorphous material,27,28 which reveals the formation of amorphous Al2O3 in S600. In S800, the intensive absorption peaks at 598 and 756 cm1 can be ascribed to characteristic AlO stretching vibration modes of γ-Al2O3.29,30 For S1000, significant spectroscopic bands at 446, 590, and 632 cm1 are identified to be the characteristic absorption bands of R-Al2O3.31 These are in good agreement with the above XRD results. The hump at around 2920 cm1 is due to CH stretching.32 The intensive band centered at near 3430 cm1 and the weak band at about 1630 cm1 are assigned to the stretching and bending vibrations of physically adsorbed water, respectively.29 The peak around 2340 cm1 is caused by the absorbed CO2.31 Furthermore, the morphologies and microstructures of the samples were examined by TEM. Figure 3a shows a typical lowmagnification TEM image of S600. The sample is flakelike and made up of small particles. Figure 3b is the corresponding SAED pattern, which exhibits a diffuse ring instead of circular rings or shiny spots, indicating that S600 has an amorphous structure. Moreover, the EDS result shown in the inset of Figure 3a reveals that the sample consists of aluminum and oxygen besides carbon and copper from a C-coated Cu TEM grid used in the TEM measurements. This implies that there are no other impurities in the fabricated samples. Figure 3c and 3d display the low- and high-magnification TEM images of S800, respectively. It is found that the morphology of S800 is also flakelike and assembled by smaller nanoparticles with an average size of 912 nm. The typical lattice fringe spacings shown in the HRTEM image (Figure 3d) are respectively determined to be 0.197, 0.228, and 0.238 nm, corresponding to the (400), (222), and (311) 16815
dx.doi.org/10.1021/jp2039338 |J. Phys. Chem. C 2011, 115, 16814–16818
The Journal of Physical Chemistry C
ARTICLE
Figure 6. Fitted O 1s XPS spectra of (a) S600, (b) S800, and (c) S1000. (d) Variation of relative areas (percentages) of oxygen vacancies of S600, S800, and S1000. Figure 4. (a) RT MH curves of as-prepared samples. The MH curves of bulk Al2O3 shown in the inset. (b) MH curves for S600 at different temperatures. The inset provides a magnified view of the lowfield data. The PM signals of the all samples and the holder have been deducted.
Figure 5. (a) ZFC and FC curves of S600 at the dc field of 100 Oe. (b) XPS survey spectrum of S600; the inset shows the high-resolution XPS spectrum of Al 2p at the binding energy range from 66 to 82 eV.
lattice planes of γ-Al2O3, respectively. All of these results are consistent with the above XRD and FTIR results. Figure 4a shows the magnetization versus magnetic field (MH) curves of as-prepared samples. These samples were measured at RT with a maximum applied magnetic field of 8 kOe, and the paramagnetism (PM) signals of the samples and the holders were deducted. It can be seen that all as-prepared samples exhibit hysteresis curves with different saturation magnetization (Ms), which indicates that all samples show clear RT FM and Ms decreased monotonically as the annealing temperature increased.
The Ms of 0.012 emu/g observed from the amorphous Al2O3 nanoparticles is much higher than those of crystal Al2O3 (0.0048 emu/g for γ-Al2O3 and 0.0011 emu/g for R-Al2O3). Bulk Al2O3 was obtained by annealing the precursor at 1000 °C for 36 h, and the MH curve is shown in the inset of Figure 4a. It can be seen that bulk Al2O3 shows PM at RT. Moreover, the MH curves of S600 measured at different temperatures from 10 to 200 K are shown in Figure 4b. It can be seen that coercivity (Hc) and Ms for S600 decreases monotonically with increase of the measuring temperature. The low-field data are shown in the inset of Figure 4b for better clarity of the hysteresis loop. The Hc values (211.5, 228.7, 305.2, and 439.1 Oe at 200, 100, 50, and 10 K, respectively) of the sample in low temperature are larger than that measured at RT. In order to understand the physical origin of RT FM existing in Al2O3 nanoparticles and to clarify that all RT FM behavior comes from pure Al2O3, the evidence for the purity of samples were obtained from SQUID and XPS. Figure 5a exhibits zerofield-cooled (ZFC) and field-cooled (FC) magnetization curves of S600 in the temperature range 2300 K under the field of 100 Oe. It can be seen that no blocking temperature occurred in this temperature range, indicating that there are no ferromagnetic clusters formed in the sample. The FC curve exhibits an obvious deviation from the ZFC curve until the temperature above 300 K. This observation indicates that the sample is ferromagnetic at RT, which is consistent with the VSM results. A representative XPS spectrum of S600 is shown in Figure 5b. The result indicates that S600 was composed of Al and O elements. The absence of the feature peaks at 778.1 and 706.6 eV reveals that there are no impurities of Co and Fe metals, consistent with the above XRD and SAED results. From these, the incorporation of any additional magnetic impurities can be ruled out. The inset of Figure 5b shows the XPS spectrum of Al 2p core level which exists as Al3+ with a binding energy of 74.3 eV.33 Therefore, it can be concluded that the observed RT FM of Al2O3 nanoparticles should be intrinsic. The O 1s XPS spectra of S600, S800, and S1000 are shown in Figure 6a, 6b, and 6c, respectively. The broad and slightly asymmetric nature of the peak is suggested to be due to the various coordinations of oxygen in Al2O3 nanoparticles. The O 16816
dx.doi.org/10.1021/jp2039338 |J. Phys. Chem. C 2011, 115, 16814–16818
The Journal of Physical Chemistry C
ARTICLE
Figure 7. (a) ESR spectra of as-prepared samples. (b) Variation of Ms and Ns of as-prepared samples.
Figure 8. (a) MH curves and (b) ESR spectrum before and after vacuum annealing for S600.
1s XPS peak can be fitted into three peaks by Gaussian simulation.17 The lowest binding energy peak located at 530.2 eV (Oa) can be assigned to O2 ion in the Al2O3 structure.34 The highest binding energy peak at 532.6 eV (Oc) is attributed to the near-surface oxygen, such as oxygen atoms in carbonate ions, surface hydroxylation, adsorbed H2O, or O2.35 The medium binding energy located at 531.4 eV (Ob) is related to the loss of oxygen in the sample and associated with the O2 ions in the oxygen-deficient regions (O vacancies) within the matrix of Al2O3.36,37 Thus, the intensity of the Ob relative area may be related to the concentration of the oxygen vacancies. From Figure 6d, it is seen that the Ob relative area of S600 is larger than those of S800 and S1000, indicating a higher concentration of oxygen vacancies in S600. The variation of the Ob relative area in S600, S800, and S1000 is consistent with that of the FM behavior of the samples. Thus, it might be deduced that oxygen vacancies may be responsible for the RT FM in Al2O3 nanoparticles, which seems to be similar to those previously reported for other pure metal oxide nanoparticles, such as HfO2, ZnO, TiO2, and CuO.8,1113 To further verify this deduction, ESR which is often used to directly characterize singly charged oxygen vacancies is introduced. Figure 7a shows the results of ESR measurements at RT for as-prepared samples. All samples were performed under the same instrument parameters and almost the same sample masses during the ESR measurement. It can be seen that there is a single sharp peak of all as-prepared samples with the response field of 320.7 mT which may be due to the presence of singly charged oxygen vacancies (unpaired electrons). These can be confirmed by the g value. The g value was calculated using the relation
unpaired electrons participating in the resonance has been calculated by using the formula38
hν ¼ gμB H0
ð1Þ
where h is Planck’s constant, ν is the microwave frequency (8.985 GHz), μB is the Bohr magnetron, and H0 is the resonance magnetic field. In Figure 7a, H0 is 320.7 mT. The value of g is 2.0018, which nearly equals the free electron value of 2.0023, indicating the existence of unpaired electrons in the samples which are due to oxygen vacancies, with the agreement of the fitting O 1s XPS results. The relative number of spins (Ns)
Ns µ IðΔHÞ2
ð2Þ
where I is the peak-to-peak height and ΔH is the line width. Figure 7b displays the variation of the relative Ns vs Ms of asprepared samples. It can be seen that the Ms decreases with the decrease of Ns for the samples. Thus, based on the consistency of the variation of the oxygen vacancies, Ns, and Ms, it is suggested that oxygen vacancies with single charge boost the RT FM. Generally, vacuum annealing will enhance the number of oxygen vacancies in the sample due to deficiency of oxygen. The effect of vacuum annealing on the magnetic properties of S600 was investigated, and the annealing treatment was conducted under 103 Pa at 500 °C for 2 h. Figure 8a shows the MH curves of S600 before and after vacuum annealing. After annealing the Ms and Hc increase from 0.012 emu/g and 185 Oe to 0.021 emu/g and 209 Oe, respectively. Figure 8b depicts the ESR spectrum at RT for S600 before and after vacuum annealing. Calculating by eq 2, the relative Ns increases after vacuum annealing, showing that the number of oxygen vacancies with single charge is obviously enhanced by vacuum annealing. Therefore, the enhancement of Ms for Al2O3 nanoparticles via vacuum annealing could be attributed to the increased density of oxygen vacancies. In other words, this demonstrates that oxygen vacancies with single charge are the origin of the observed RT FM. Singly charged oxygen vacancies induce the RT FM which can use the F+ center exchange mechanism with one trapped electron to explain, such as in ZnO, CeO2, and TiO2.1921 As previously reported that electrons in these singly charged oxygen vacancies (F+) are strongly localized,18,39 the localization radius is ε(m/ m*)a0, where ε is the dielectric constant, m and m* are the mass and effective mass of an electron, respectively, and a0 is the Bohr radius. Once the F+ center density reaches the critical value for magnetic percolation, these F+ centers will overlap each other, and this will result in a long-range ferromagnetic ordering. For the amorphous sample, they possess many more defects than the crystalline sample. Thus, the F+ center density in amorphous 16817
dx.doi.org/10.1021/jp2039338 |J. Phys. Chem. C 2011, 115, 16814–16818
The Journal of Physical Chemistry C Al2O3 is higher than that of crystalline Al2O3 nanoparticles, accordingly leading to stronger FM. The increase of F+ centers can also account for the increase of FM from the vacuumannealing samples. Therefore, the strength of FM should be directly related to the concentration of F+ centers in the Al2O3 nanoparticles.
’ CONCLUSIONS In conclusion, amorphous and crystalline Al2O3 nanoparticles were synthesized by a solgel method with postannealing at different temperatures. Magnetic measurements show that all Al2O3 nanoparticles exhibit intrinsic RT FM. Simulation of O 1s XPS spectra and measurement of ESR of Al2O3 nanoparticles reveal that the origin of the FM could be attributed to the singly charged oxygen vacancies. According to the consistency of the variation of the oxygen vacancies, Ns, and Ms, a direct correlation of ferromagnetism with the F+ center exchange mechanism is established in the Al2O3 nanoparticles. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Science Fund for Distinguished Young Scholars (Grants 50925103 and 11034004), the Keygrant Project of Chinese Ministry of Education (Grant 309027), NSFC (Grant 50902065), and the Fund for Academic Newcomer of Ph.D. of Lanzhou University. ’ REFERENCES (1) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (2) Venkatesan, M.; Fitzgerald, C. B.; Lunney, J. G.; Coey, J. M. D. Phys. Rev. Lett. 2004, 93, 177206. (3) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharma, R.; Ahuja, R.; Guillen, J. M. O.; Johansson, B.; Gehring, G. A. Nat. Mater. 2003, 2, 673. (4) Jin, Z. W.; Fukumara, T.; Kawasaki, M.; Ando, K.; Saito, H.; Sekiguchi, T.; Yoo, Y. Z.; Murakami, M.; Matsumoto, Y.; Hasegawa, T.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 3824. (5) Fukumura, T.; Toyosaki, H.; Yamada, Y. Semicond. Sci. Technol. 2005, 20, S103. (6) Ungureanu, M.; Schmidt, H.; Wenckstern, H. V.; Hochmuth, H.; Lorenz, M.; Grundmann, M.; Fecioru, M. M.; G€untherodt, G. Thin Solid Films 2007, 515, 8761. (7) Shinde, S. R.; Ogale, S. B.; Higgins, J. S.; Zheng, H.; Millis, A. J.; Kulkarni, V. N.; Ramesh, R.; Greene, R. L.; Venkatesan, T. Phys. Rev. Lett. 2004, 92, 166601. (8) Zhou, S. Q.; Talut, G.; Potzger, K.; Shalimov, A.; Grenzer, J.; Skorupa, W.; Helm, M.; Fassbender, J.; Cizmar, E.; Zvyagin, S. A.; Wosnitza, J. J. Appl. Phys. 2008, 103, 083907. (9) Venkatesan, M.; Fitzgerald, C. B.; Coey, J. M. D. Nature 2004, 430, 630. (10) Coey, J. M. D.; Venkatesan, M.; Stamenov, P.; Fitzgerald, C. B.; Dorneles, L. S. Phys. Rev. B 2005, 72, 024450. (11) Gao, D. Q.; Zhang, Z. H.; Fu, J. L.; Xu, Y.; Qi, J.; Xue, D. S. J. Appl. Phys. 2009, 105, 113928. (12) Kim, D.; Hong, J.; Park, R. Y.; Kim, K. J. J. Phys.: Condens. Matter 2009, 21, 195405. (13) Gao, D. Q.; Yang, G. J.; Li, J. Y.; Zhang, J.; Zhang, J. L.; Xue, D. S. J. Phys. Chem. C. 2010, 114, 18347.
ARTICLE
(14) Hong, N. H.; Poirot, N.; Sakai, J. Phys. Rev. B 2008, 77, 033205. (15) Gao, D. Q.; Li, J. Y.; Li, Z. X.; Zhang, Z. H.; Zhang, J.; Shi, H. G.; Xue, D. S. J. Phys. Chem. C. 2010, 114, 11703. (16) Panguluri, R. P.; Kharel, P.; Sudakar, C.; Naik, R.; Suryanarayanan, R.; Naik, V. M.; Petukhov, A. G.; Nadgorny, B.; Lawes, G. Phys. Rev. B 2009, 79, 165208. (17) Yang, G. J.; Gao, D. Q.; Shi, Z. H.; Zhang, Z. H.; Zhang, J.; Zhang, J. L.; Xue, D. S. J. Phys. Chem. C. 2010, 114, 21989. (18) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173. (19) Panigrahy, B.; Aslam, M.; Misra, D. S.; Ghosh, M.; Bahadur, D. Adv. Funct. Mater. 2010, 20, 1161. (20) Singhal, R. K.; Kumari, P.; Samariya, A.; Kumar, S.; Sharma, S. C.; Xing, Y. T.; Saitovitch, E. B. Appl. Phys. Lett. 2010, 97, 172503. (21) Singhal, R. K.; Kumar, S.; Kumari, P.; Xing, Y. T.; Saitovitch, E. Appl. Phys. Lett. 2011, 98, 092510. (22) Robertson, J. Rep. Prog. Phys. 2006, 69, 327. (23) Zhang, X. H.; Domercq, B.; Wang, X. D.; Yoo, S.; Kondo, T.; Wang, Z. L.; Kippelen, B. Org. Electron. 2007, 8, 718. (24) Novikov, Y. N.; Vishnyakov, A. V.; Gritsenko, V. A.; Nasyrov, K. A.; Wong, H. Microelectron. Reliab. 2010, 50, 207. (25) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. N. R. Phys. Rev. B 2006, 74, 161306. (26) Sundaresan, A.; Rao, C. N. R. Nano Today 2009, 4, 96. (27) Shek, C. H.; Lai, J. K. L.; Gu, T. S.; Lin, G. M. Nanostruct. Mater. 1997, 8, 605. (28) Giri, V. S.; Sarathi, R.; Chakravarthy, S. R.; Venkataseshaiah, C. Mater. Lett. 2004, 58, 1047. (29) Costa, T. M. H.; Gallas, M. R.; Benvenutti, E. V.; Jornada, A. H. J. Phys. Chem. B 1999, 103, 4278. (30) Lian, J. B.; Ma, J. M.; Duan, X. C.; Kim, T.; Li, H. B.; Zheng, W. J. Chem. Commun. 2010, 46, 2650. (31) Li, J.; Pan, Y. B.; Xiang, C. S.; Ge, Q. M.; Guo, J. K. Ceram. Int. 2006, 32, 587. (32) Jayaraman, V.; Gnanasearan, T.; Periaswami, G. Mater. Lett. 1997, 30, 157. (33) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (34) Chen, M.; Wang, X.; Yu, Y. H.; Pei, Z. L.; Bai, X. D.; Sun, C.; Huang, R. F.; Wen, L. S. Appl. Surf. Sci. 2000, 158, 134. (35) Yang, H. M.; Ouyang, J.; Tang, A. D. J. Phys. Chem. B 2007, 111, 8006. (36) Fan, J. C. C.; Goodenough, J. B. J. Appl. Phys. 1977, 48, 3524. (37) Samariya, A.; Singhal, R. K.; Kumar, S.; Xing, Y. T.; Sharma, S. C.; Kumari, P.; Jain, D. C.; Dolia, S. N.; Deshpande, U. P.; Shripathi, T.; Saitovitch, E. Appl. Surf. Sci. 2010, 257, 585. (38) Sagar, R. V.; Buddhudu, S. Adv. Sci. Lett. 2010, 3, 461. (39) Shah, L. R.; Ali, B.; Zhu, H.; Wang, W. G.; Song, Y. Q.; Zhang, H. W.; Shah, S. I.; Xiao, J. Q. J. Phys.: Condens. Matter 2009, 21, 486004.
16818
dx.doi.org/10.1021/jp2039338 |J. Phys. Chem. C 2011, 115, 16814–16818
