用NMR+研究洋葱碳吸附水后的抗磁性.pdf
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1、文章编号:摇 1007鄄8827(2014)05鄄0392鄄06用 NMR 研究洋葱碳吸附水后的抗磁性A M Panich1,摇 V Yu Osipov2,摇 K Takai3(1. Department of Physics, Ben鄄Gurion University of the Negev, P. O. Box 653, Beer Sheva84105, Israel;2. Ioffe Physical鄄Technical Institute, Polytechnicheskaya 26, St. Petersburg194021, Russia;3. Department of Ch
2、emical Science and Technology, Hosei University, 3鄄7鄄2, Kajino, Koganei, Tokyo184鄄8584, Japan)摘摇 要:摇 对洋葱状碳缺陷位吸附水分子的1H 化学位移进行测试,并比较块状水和石墨烯、氧化石墨烯和纳米金刚石吸附水的化学位移。 纳米金刚石和氧化石墨烯绝缘体的1H 共振位置与块状水接近,然而石墨烯和洋葱碳吸附水呈现明显的化学位偏移,这是由导电的石墨烯片层抗磁屏蔽效应所致。 质子的化学位移和抗磁性之间存在显著的相关性。关键词:摇 洋葱碳;石墨烯;反磁性;核磁共振中图分类号: 摇 TQ 127. 1+1文献标识
3、码: 摇 A收稿日期:2014鄄03鄄21;摇 修回日期:2014鄄10鄄09通讯作者: A M Panich. E鄄mail: pan bgu. ac. ilDiamagnetism of carbon onions probed by NMR of adsorbed waterA M Panich1,摇 V Yu Osipov2,摇 K Takai3(1. Department of Physics, Ben鄄Gurion University of the Negev, P. O. Box 653, Beer Sheva84105, Israel;2. Ioffe Physical鄄Te
4、chnical Institute, Polytechnicheskaya 26, St. Petersburg194021, Russia;3. Department of Chemical Science and Technology, Hosei University, 3鄄7鄄2, Kajino, Koganei, Tokyo184鄄8584, Japan)Abstract: 摇 We measured the1H chemical shift of water molecules adsorbed on defect sites of carbon onions and compar
5、ed it withthe shifts of bulk water and water adsorbed by chemically鄄derived graphene, graphene oxide and nanodiamond. The positions of 1Hresonances for insulator nanodiamond and graphene oxide are close to that of bulk water, while water molecules adsorbed by gra鄄phene, and particularly by carbon on
6、ions, reveal a significant deviation in chemical shift from that of bulk water, which can be at鄄tributed to the diamagnetic screening effect of the conducting graphene layers. This conclusion is supported by a pronounced correla鄄tion established in our experiments between the proton chemical shifts
7、and diamagnetic contribution to the magnetic susceptibility.Keywords:摇 Carbon onions; Graphene; Diamagnetism; Nuclear Magnetic ResonanceFoundation items: Russian Foundation for Basic Research and Japan Society for the Promotion of Science for support within the grantRFBR (12鄄02鄄92107鄄亓囟); Israeli Mi
8、nistry of Science &Technology(3鄄9754).Corresponding author: A M Panich. E鄄mail: pan bgu. ac. ilEnglish edition available online ScienceDirect ( http:蛐蛐www. sciencedirect. com蛐science蛐journal蛐18725805).DOI: 10. 1016/ S1872鄄5805(14)60145鄄11摇 IntroductionNanocarbons with an extended 仔鄄electron systemha
9、ve been a subject of increasing interest for the lasttwo decades after discovery of fullerenes and carbonnanotubes in the beginning of the 1990s. Graphite isknown as one of the strongest diamagnetic materialsamong natural and artificial substances at ambientconditions1,2. This is due to ( i) the abn
10、ormallylarge in鄄plane orbital diamagnetism related to verynarrow (nearly vanishing) energy gap between p鄄bonding and p*鄄 antibonding bands, and (ii) non鄄dissipative p鄄electron diamagnetic ring currents in鄄duced by magnetic field applied perpendicular to thegraphite basal planes. Similar in鄄plane orb
11、ital diamag鄄netism also occurs in large鄄size aromatic molecules,in which its strength depends on the molecule size.The diamagnetic effect becomes greater in graphenemonolayer3鄄5, which is truly a zero鄄gap system.The orbital diamagnetism has been studied for aplenty of graphene鄄related materials, suc
12、h as highlyordered pyrolytic graphite (HOPG), graphite interca鄄lation compounds6鄄8, carbon nanotubes2, carbonnanohorns9, activated carbon fibers and few鄄layergraphenes4,8,10. In the real experiment the grapheneflakes and laminates, consisting of mono鄄and few鄄lay鄄er crystallites of restricted size, a
13、re rotationally disor鄄dered and show relatively strong diamagnetism, whichis several times smaller than that of HOPG8. Theeffect of suppressed diamagnetism is attributed to the摇第 29 卷摇 第 5 期2014 年 10 月新摇 型摇 炭摇 材摇 料NEW CARBON MATERIALSVol. 29摇 No. 5Oct. 2014摇two main factors. The first factor is 仔鄄ca
14、rrier scatter鄄ing mainly on point鄄like defects and vacancies ran鄄domly distributed in graphene sheets and on grainboundaries11. The second is the shift of the Fermilevel EFaway from the contact point (Dirac point)between p鄄 and p*鄄 bands through the doping causedby intercalated or adsorbed species a
15、nd chemical im鄄purities on defect sites7,12. Such Fermi level shift re鄄sults in a positive Pauli susceptibility contributing tothe total magnetic susceptibility related to appreciablenon鄄vanishing density鄄of鄄states on the Fermi level.Therefore, the distinguishing between the orbital dia鄄magnetism an
16、d temperature鄄independent Pauli para鄄magnetism is not practically possible at | EF| 0. 08eV7,12. Moreover, although the orbital diamagnet鄄ism is noticeably suppressed in powder nanographiticmaterials with crystallite size less than 18 nm13, itstill remains much larger than that in planar aromaticmol
17、ecules consisting of few tens aromatic rings.Graphene鄄based materials usually contain somemoisture adsorbed on the surface, between the exfoli鄄ated layers, and on defect sites of the honeycomb aro鄄matic structure. These water molecules feel the mag鄄netic field induced by the p鄄electron diamagnetic r
18、ingcurrents of graphene layers, whose direction is oppo鄄site to the applied magnetic field, and can serve asmolecular probes in studying diamagnetism. The in鄄teraction between the host basal aromatic layers andguest molecules adsorbed on the interior regions ofgraphene sheets is now under discussion
19、 and may leadto surprising results, such as granular superconductingbehavior with a critical temperature above 300 K inwater鄄treated graphite powder with a micrometer grainsize recently reported by Scheike et al.14.Herein, we used1H nuclear magnetic resonance(NMR) and magnetic susceptibility techniq
20、ues toprobe the interaction between the adsorbed water mol鄄ecules and conducting graphene layers in carbon on鄄ions and chemically derived graphene. These samplesshow somewhat corrugated and defected carbon layersin which water molecules are located. NMR is knownto be an excellent tool in studying ch
21、emical bonding,electron鄄nuclear and host鄄guest interactions in inclu鄄sion, intercalation and porous compounds, inclusiveof carbon nanomaterials15鄄21. For example, chemicalshift of nuclei of the guest molecule is particularlysensitive to its interaction with the host carbon matrix.This is observed in
22、 NMR measurements, which ex鄄hibit a more significant deviation of1H NMR chemi鄄cal shift of the molecules adsorbed by onions and asmaller deviation for graphene compared to the shiftof bulk water. This deviation is attributed to the dia鄄magnetic screening effect of the conducting graphenelayers with
23、well鄄developed p鄄electronic system. Forcomparison, we also measured proton resonance shiftsof water molecules adsorbed by nanodiamond and gra鄄phene oxide, which were found to be nearly the sameas that of bulk H2O. To support our findings, weused magnetic susceptibility technique that providesinforma
24、tion on magnetic state of compounds, disorderof graphene sheets in graphene鄄based materials and isapplicable for studying powder samples. Our magnet鄄ic susceptibility measurements reveal distinct correla鄄tion between the1H NMR chemical shifts and orbitalcontribution to the magnetic susceptibility fo
25、r thecompounds under study, supporting the aforemen鄄tioned conclusions.2摇 ExperimentalCarbon onions were prepared by annealing ofwell purified nanodiamond powder ( grain size 5nm) in a argon atmosphere at 1 650 益 for20min, similar to that described in ref. 22. Theobtained particles with a mean size
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