Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconducto

tpo51托福阅读passage1：originsofthemegaliths原文文本+真题答案TPO51托福阅读passage1：Origins of the Megaliths原文文本+真题答案第一篇：考古类Origins of the MegalithsSince the days of the earliest antiquarians, scholars have been puzzled by the many Neolithic (~4000 .~2200 .) communal tombs known as megaliths alongEurope's Atlantic seaboard. Although considerable variations are found in the architectural form of these impressive monuments, there is a general overriding similarity in design and, particularly, in the use of massive stones.The construction of such large and architecturally complex tombs by European barbarians struck early prehistorians as unlikely. The Bronze Age seafaring civilizations that lived in the region of the Aegean Sea (~ 3000 .~ 1000 .), among whom collective burial and a diversity of stone-built tombs were known, seemed a probable source of inspiration. It was suggested that Aegean people had visited Iberia in southwestern Europe in search of metal ores and had introduced the idea of collective burial in massive tombs, which then spread northward to Brittany, Britain, North Germany, and Scandinavia.Radiocarbon dates for a fortified settlement of megalith builders at Los Millares in Spain appeared to confirm this picture, though dates for megaliths in Brittany seemed too early. When calibrated, however, it became clear that radiocarbon dates were universally too early to support a Bronze Age Aegean origin. It is now clear that the megaliths are a western and northernEuropean invention, not an introduced idea. Even so, they are still a subject of speculation and inquiry. What induced their builders to invest massive efforts in erecting such monumental tombs How was the necessary labor force assembled What underlies their strikingsimilaritiesOne answer to the last question was proposed by Professor Grahame Clark, one ofBritain's greatest prehistorians. Investigating the megaliths of southern Sweden, he noted that one group was concentrated in coastal locations from which deep-sea fish such as cod, haddock, and ling could have been caught in winter. Historically, much of the Atlantic was linked by the travels of people who fished, and this could well have provided a mechanism by which the megalith idea and fashions in the style of tomb architecture spread between coastal Iberia, Brittany, Ireland, western England and Scotland, and Scandinavia. The high concentrations of megaliths on coasts and the surprising number of megaliths found on small islands may support a connection with fishing.Professor Colin Renfrew of the University of Cambridge, England, however, views the similarities as similar responses to similar needs. At the structural level, the passage that forms a major element of many graves could have been devised independently in different areas to meet the need for repeated access to the interior of these communal tombs. Other structural resemblances could be due to similarities in the raw materials available. In answer to the question of why the idea of building monumental tombs should arise independently in a number of areas, he cites the similarities in their backgrounds.Most megaliths occur in areas inhabited in the postglacialperiod by Mesolithic hunter-gatherers (~20000 . ~ 18000 . ). Their adoption of agriculture through contact with Neolithic farmers, Renfrew argues, led to a population explosion in the region and consequent competition for farmland between neighboring groups. Inthe face of potential conflict, the groups may have found it desirable to define their territories and emphasize their boundaries. The construction of megaliths could have arisen in response to this need.Renfrew has studied two circumscribed areas, the Scottish islands of Arran and Rousay, to examine this hypothesis more closely. He found that a division of the arable land into territories, each containing onemegalith, results in units that correspond in size to the individual farming communities of recent times in the same area. 【】Each unit supported between 10 and 50 people. 【】The labor needed to put up a megalith would probably be beyond the capabilities of a community this size. 【】 But Renfrew argues that the cooperation of other communities could be secured by some form of recognized social incentive perhaps a period of feasting at which communal building was one of several activities. 【】Most megaliths contain collective burials. Different tombs used different arrangements, but there seems to have been an underlying theme: people placed in these tombs were representative of their society, but their identity as individuals was not important. The tombs belonged to the ancestors, through whom the living society laid claim to their land. This interpretation reinforces Renfrew’s view of the megaliths a s territorial markers.题目word "collective"in the passage is closest in meaning togroundto paragraph 2, early prehistorians thought the Aegean people of the Bronze Age might have influenced megalith building along the Atlantic seaboard because theyestablished commercial routes along the Atlantic seaboard been in Iberia, where they introduced the idea of burial in very large tombsthought to have found megaliths in Iberia when searching for metals thought to have passed along the concept of burial in monumental tombs as they explored Brittany, Britain, North Germany, and ScandinaviaParagraph 2 is marked withparagraph 3, why does the author discuss the results of radiocarbon datessupport the idea that megaliths spread rapidly during the Bronze Age question the idea that megaliths have a religious originprovide evidence against the theory that Bronze Age Aegean inspired the megalithsargue that the megaliths in Brittany are older than the megaliths in Los MillaresParagraph 3 is marked withword "erecting" in the passage is closest in meaning toto paragraph 4, what did Professor Clark propose as a result of studying the megaliths of southern Swedenmegaliths are nearly identical to megaliths elsewhere.who traveled for fishing may have been responsible for the spread of megaliths in Europe.megaliths were probably built after other European megaliths were built.in Europe were usually located near sites for deep-sea fishing in winter.Paragraph 4 is marked withis the purpose of discussing the passage that forms a major element of many gravesprovide an example of a commonly occurring feature of megaliths that might be related to a similar needargue that similarities in raw materials were responsible for thesimilarity of passagesexplain how repeated access to the interior of the communal tombs was possibleprovide evidence that the builders of the megaliths had similar backgrounds Paragraph 5 is marked withword "consequent" in the passage is closest in meaning to to paragraph 6, Professor Renfrew has argued that one factor leading to the rise of megaliths in Europe wasadoption of farming by Mesolithic hunter-gathererstransition from a glacial to a postglacial climaterelocation of Mesolithic populations from one region to anotherconflict over whether areas inhabited by Mesolithic peoples would be used for farmingParagraph 6 is marked withphrase style "this hypothesis" in the passage refers to the idea that was competition for territory between Mesolithic hunter-gathers and invading Neolithic farmerspopulation explosion brought about a division of theregion's Neolithicfarmers into neighboring groupsneed of neighboring groups to define their territories led to the construction of megalithsconstruction of megaliths was a way of competing for farmlandto paragraph 7, what did Renfrew conclude about the megaliths ofArran and Rousaymegalith was associated with a specific agricultural community.megalith was built by between 10 and 50 people.megaliths were built using stones quarried at other places.megaliths were built gradually over time rather than all at once. Paragraph 7 is marked withword "reinforces" in the passage is closest in meaning tofromonto the passage, all of the following were true of the megaliths along the Atlantic seaboard EXCEPToften had a main passageway.identified the individuals buried within them.were built before the Aegean Bronze Age.differed somewhat in style from region to region. Paragraph 8 is marked withat the four squares 【】that indicate where the following sentence could be added to the passage.So it might seem that megaliths could not have been used by an individual community to mark its land.Where would be the sentence best fit Click on a square toadd the sentence to the passage.14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it.To review the passage, click VIEW TEXT.Answer ChoicesBronze Age Aegeans most probably built some of the tombs in Iberia.evidence supports the idea that the megaliths were a western and northern European invention.megaliths are found in coastal regions, suggesting that people fishing mayhave used them to guide their travels.have found enough indicators to believe that the layout of tombs in the landscape reflects each community's social organization.high proportion of coastal megaliths has given rise to the idea that megalith building was related to fishing.has been suggested that megaliths arose in response to the spread of agriculture and competition for farmland.参考答案1-5 DBCBB 6-10 ACACA 11-13 DBC 14 BEF文章来源：雷哥托福。

Uncovering the Origin of MetallurgyMetallurgy is the science and technology of extracting metals from their ores, refining them, and preparing them for use. It is a crucial aspect of human civilization and has played a significant role in shaping our world. The origin of metallurgy is a topic of great interest to historians, archaeologists, and scientists alike. In this essay, we will explore the different perspectives on the origin of metallurgy and try to uncover the truth behind this fascinating subject. One perspective on the origin of metallurgy is that it emerged independently in different parts of the world. This theory suggests that humans discovered the properties of metals through trial and error and developed the necessarytechniques to extract and refine them. This is supported by the fact that early metallurgy was found in several regions, including the Middle East, Asia, andSouth America. For example, the ancient Egyptians were known to have used copper and gold for decorative purposes, while the people of the Andes used copper to create intricate jewelry and ornaments. Another perspective on the origin of metallurgy is that it was developed through cultural diffusion. This theory suggests that metallurgy was first developed in one region and then spread toother parts of the world through trade and migration. For example, some historians believe that metallurgy was first developed in the Middle East and then spread to Europe and Asia. This is supported by the fact that the earliest known metal objects were found in the Middle East, such as the copper beads from the 6th millennium BCE found in the area now known as Iraq. A third perspective on the origin of metallurgy is that it was developed as a result of environmental factors. This theory suggests that the availability of certain metals in a particularregion led to the development of metallurgy in that area. For example, the abundance of copper in the Middle East and the Andes may have led to the development of early metallurgy in those regions. Similarly, the availability of iron in certain parts of Africa may have led to the development of ironworking in those areas. Regardless of the origin of metallurgy, it is clear that it played a crucial role in the development of human civilization. The ability to extract and refine metals allowed humans to create tools and weapons that were stronger and more durable than those made from stone or wood. This, in turn, led to thedevelopment of agriculture, trade, and the rise of civilizations. In conclusion, the origin of metallurgy is a fascinating subject that has captured the attention of historians, archaeologists, and scientists for centuries. While there are different perspectives on how metallurgy first emerged, it is clear that it played a crucial role in shaping our world. The ability to extract and refine metals allowed humans to create tools and weapons that were stronger and more durable, leading to the development of agriculture, trade, and the rise of civilizations. As we continue to uncover new evidence about the origin of metallurgy, we will gain a deeper understanding of our shared history and the remarkable achievements of our ancestors.。

TPO51托福阅读passage1：Origins of the Megaliths原文文本+真题答案第一篇：考古类Origins of the MegalithsSince the days of the earliest antiquarians, scholars have been puzzled by the many Neolithic (~4000 B.C.~2200 B.C.) communal tombs known as megaliths alongEurope's Atlantic seaboard. Although considerable variations are found in the architectural form of these impressive monuments, there is a general overriding similarity in design and, particularly, in the use of massive stones.The construction of such large and architecturally complex tombs by European barbarians struck early prehistorians as unlikely. The Bronze Age seafaring civilizations that lived in the region of the Aegean Sea (~ 3000 B.C.~ 1000 B.C.), among whom collective burial and a diversity of stone-built tombs were known, seemed a probable source of inspiration. It was suggested that Aegean people had visited Iberia in southwestern Europe in search of metal ores and had introduced the idea of collective burial in massive tombs, which then spread northward to Brittany, Britain, North Germany, and Scandinavia.Radiocarbon dates for a fortified settlement of megalith builders at Los Millares in Spain appeared to confirm this picture, though dates for megaliths in Brittany seemed too early. When calibrated, however, it became clear that radiocarbon dates were universally too early to support a Bronze Age Aegean origin. It is now clear that the megaliths are a western and northern European invention, not an introduced idea. Even so, they are still a subject of speculation and inquiry. What induced their builders to invest massive efforts in erecting such monumental tombs? How was the necessary labor force assembled? What underliestheir striking similarities?One answer to the last question was proposed by Professor Grahame Clark, one ofBritain's greatest prehistorians. Investigating the megaliths of southern Sweden, he noted that one group was concentrated in coastal locations from which deep-sea fish such as cod, haddock, and ling could have been caught in winter. Historically, much of the Atlantic was linked by the travels of people who fished, and this could well have provided a mechanism by which the megalith idea and fashions in the style of tomb architecture spread between coastal Iberia, Brittany, Ireland, western England and Scotland, and Scandinavia. The high concentrations of megaliths on coasts and the surprising number of megaliths found on small islands may support a connection with fishing.Professor Colin Renfrew of the University of Cambridge, England, however, views the similarities as similar responses to similar needs. At the structural level, the passage that forms a major element of many graves could have been devised independently in different areas to meet the need for repeated access to the interior of these communal tombs. Other structural resemblances could be due to similarities in the raw materials available. In answer to the question of why the idea of building monumental tombs should arise independently in a number of areas, he cites the similarities in their backgrounds.Most megaliths occur in areas inhabited in the postglacial period by Mesolithic hunter-gatherers (~20000 B.C. ~ 18000 B.C. ). Their adoption of agriculture through contact with Neolithic farmers, Renfrew argues, led to a population explosion in the region and consequent competition for farmland between neighboring groups. Inthe face of potential conflict, the groups may have found it desirable to define their territories and emphasize their boundaries. The construction of megaliths could have arisen in response to this need.Renfrew has studied two circumscribed areas, the Scottish islands of Arran and Rousay, to examine this hypothesis more closely. He found that a division of the arable land into territories, each containing onemegalith, results in units that correspond in size to the individual farming communities of recent times in the same area. 【】Each unit supported between 10 and 50 people. 【】The labor needed to put up a megalith would probably be beyond the capabilities of a community this size. 【】But Renfrew argues that the cooperation of other communities could be secured by some form of recognized social incentive perhaps a period of feasting at which communal building was one of several activities. 【】Most megaliths contain collective burials. Different tombs used different arrangements, but there seems to have been an underlying theme: people placed in these tombs were representative of their society, but their identity as individuals was not important. The tombs belonged to the ancestors, through whom the living society laid claim to their land. This interpretation reinforces Renfrew’s view of the megaliths as territorial markers.题目1.The word "collective"in the passage is closest in meaning toA.above groundB.publicC.elaborateD.group2.According to paragraph 2, early prehistorians thought the Aegean people of the Bronze Age might have influenced megalith building along the Atlantic seaboard because theyA.had established commercial routes along the Atlantic seaboardB.had been in Iberia, where they introduced the idea of burial in very large tombsC.were thought to have found megaliths in Iberia when searching for metalsD.were thought to have passed along the concept of burial inmonumental tombs as they explored Brittany, Britain, North Germany, and ScandinaviaParagraph 2 is marked with3.In paragraph 3, why does the author discuss the results of radiocarbon dates?A.To support the idea that megaliths spread rapidly during the Bronze AgeB.To question the idea that megaliths have a religious originC.To provide evidence against the theory that Bronze Age Aegean inspired the megalithsD.To argue that the megaliths in Brittany are older than the megaliths in Los MillaresParagraph 3 is marked with4.The word "erecting" in the passage is closest in meaning toA.designingB.constructingC.protectingD.decorating5.According to paragraph 4, what did Professor Clark propose as a result of studying the megaliths of southern Sweden?A.Swedish megaliths are nearly identical to megaliths elsewhere.B.People who traveled for fishing may have been responsible for the spread of megaliths in Europe.C.Swedish megaliths were probably built after other European megaliths were built.D.Megaliths in Europe were usually located near sites for deep-sea fishing in winter.Paragraph 4 is marked with6.What is the purpose of discussing the passage that forms a major element of many graves?A.To provide an example of a commonly occurring feature of megaliths that might be related to a similar needB.To argue that similarities in raw materials were responsible for the similarity of passagesC.To explain how repeated access to the interior of the communal tombs was possibleD.To provide evidence that the builders of the megaliths had similar backgrounds Paragraph 5 is marked with7.The word "consequent" in the passage is closest in meaning toA.intenseB.increasingC.resultingD.continual8.According to paragraph 6, Professor Renfrew has argued that one factor leading to the rise of megaliths in Europe wasA.the adoption of farming by Mesolithic hunter-gatherersB.the transition from a glacial to a postglacial climateC.the relocation of Mesolithic populations from one region to anotherD.the conflict over whether areas inhabited by Mesolithic peoples would be used for farmingParagraph 6 is marked with9.The phrase style "this hypothesis" in the passage refers to the idea thatA.there was competition for territory between Mesolithic hunter-gathers and invading Neolithic farmersB.a population explosion brought about a division of the region's Neolithicfarmers into neighboring groupsC.the need of neighboring groups to define their territories led to the construction of megalithsD.the construction of megaliths was a way of competing for farmland10.According to paragraph 7, what did Renfrew conclude about the megaliths of Arran and Rousay?A.Each megalith was associated with a specific agricultural community.B.Each megalith was built by between 10 and 50 people.C.Some megaliths were built using stones quarried at other places.D.Some megaliths were built gradually over time rather than all at once. Paragraph 7 is marked with11.The word "reinforces" in the passage is closest in meaning toA.representsB.differs fromC.expands onD.supports12.According to the passage, all of the following were true of the megaliths along the Atlantic seaboard EXCEPTA.They often had a main passageway.B.They identified the individuals buried within them.C.They were built before the Aegean Bronze Age.D.They differed somewhat in style from region to region. Paragraph8 is marked with13.Look at the four squares 【】that indicate where the following sentence could be added to the passage.So it might seem that megaliths could not have been used by an individual community to mark its land.Where would be the sentence best fit? Click on a square to add the sentence to the passage.14.D irections: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it.To review the passage, click VIEW TEXT.Answer ChoicesA.The Bronze Age Aegeans most probably built some of the tombs in Iberia.B.Scientific evidence supports the idea that the megaliths were a western and northern European invention.C.Most megaliths are found in coastal regions, suggesting that people fishing mayhave used them to guide their travels.D.Archeologists have found enough indicators to believe that the layout of tombs in the landscape reflects each community's social organization.E.The high proportion of coastal megaliths has given rise to the idea that megalith building was related to fishing.F.It has been suggested that megaliths arose in response to the spread of agriculture and competition for farmland.参考答案1-5 DBCBB 6-10 ACACA 11-13 DBC 14 BEF文章来源：雷哥托福。

Synthesis and catalytic properties of MIL-100(Fe),an iron(III )carboxylate with large pores {Patricia Horcajada,a Suzy Surble ´,a Christian Serre,*a Do-Young Hong,b You-Kyong Seo,b Jong-San Chang,b Jean-Marc Grene `che,c Irene Margiolaki d and Ge ´rard Fe ´rey aReceived (in Cambridge,UK)21st March 2007,Accepted 1st May 2007First published as an Advance Article on the web 15th May 2007DOI:10.1039/b704325bThe large-pore iron(III )carboxylate MIL-100(Fe)with a zeotype architecture has been isolated under hydrothermal conditions,its structure solved from synchrotron X-ray powder diffraction data,while Friedel–Crafts benzylation catalytic tests indicate a high activity and selectivity for MIL-100(Fe).The recent interest in the synthesis of hybrid inorganic–organic solids gives a new dimension to the domain of porous compounds.1–4They offer significant new scientific and technolo-gical opportunities 5by combining attractive features of both inorganic and organic moieties and lead to many potential applications in gas storage,6,7catalysis,8,9insertion,10,11magnet-ism,12,13optical devices,14etc .Most of them are prepared using functionalized organic ligand (phosphonates,carboxylates,sulfo-nates...)and many elements in the Periodic Table have been incorporated in these new framework materials exhibiting novel structures not seen in zeolite chemistry.The introduction of 3d transition metals within the skeleton provides new electronic properties.Among them,iron is an environmentally benign and cheap component with non-toxicity and redox properties.However,to the best of our knowledge,while some MOF materials have been reported to date with iron(II )or iron(III ),15–20only two of them combine a permanent porosity and large pores.21,22We report here the successful synthesis and structure determination of MIL-100(Fe)(MIL:Materials of Institut Lavoisier),a new scarce example of an iron(III )carboxylate with a large accessible and permanent porosity.This solid was isolated as a polycrystalline powder from a reaction mixture of composition 1.0Fe 0:0.661,3,5-BTC :2.0HF :1.2HNO 3:280H 2O (1,3,5-BTC =benzene tricarboxylic or trimesic acid)that was held at 150u C in a Teflon-lined autoclave for 6days with a initial heating ramp of 12h and a final cooling ramp of 24h.The pH remains acidic (,1)throughout the synthesis.The light-orange solid product was recovered by filtration and washed with deionized water.A treatment in hotdeionised water (80u C)for 3h was applied to decrease the amount of residual trimesic acid (typically,1g of MIL-100(Fe)in 350ml of water)followed by drying at room temperature.A laboratory powder X-ray diffraction pattern showed that crystalline phase had been produced,isostructural with the chromium carboxylate MIL-100(Cr).23Elemental analysis indicated that the contents of Fe,F,C and H (obs:13.8%Fe,1.3%F,23.5%C)are in good agreement with the values based on the structure formula,Fe III 3O(H 2O)2F ?{C 6H 3(CO 2)3}2?n H 2O (n y 14.5)despite an excess of carbon content (calc.:14%Fe,1.8%F,21.0%C).It has not been possible to prepare sizeable crystals for single-crystal diffraction studies whatever the synthesis conditions.Thermal analysis (TGA2050TA apparatus,O 2flow,heating rate 3K min 21)shows three weight losses between 298and 873K.The first (y 40.1%)at 373K is attributed to the departure of the free water molecules inside the pores.The second at 473K (y 4.5%)comes from the water molecules which interact with the iron trimers.The final weight loss (y 35.3%)at 573K is related to the combustion of the trimesic acid.These latter two losses are,on the whole,in agreement with the theoretical values (calc:4.6and 35.2%).Note that the free water content deduced from the structure determination (y 29.5%)is slightly lower than the experimental value (TGA).This is due to the variable water content in MIL-100solids which vary considerably depending on the atmospheric conditions.The structure of title solid was solved from high-resolution synchrotron X-ray powder diffraction data using coordinates of MIL-100(Cr)as the starting model (ESI {).{The final Rietveld plot is shown in -100(Fe)is an iron(III )carboxylate built up from trimers of iron octahedra sharing a common vertex m 3-O.The trimers are then linked by the benzene-1,3,5-tricarboxylate moieties in such a way that this leads to the formation of hybrid supertetrahedra which further assemble into a zeolitic architecture of the MTN type (Fig.2).This delimits two types of mesoporouscages of free apertures of ca.25and 29A˚,accessible through microporous windows of ca. 5.5and 8.6A˚.Bond valence calculations indicate a trivalent state of iron.As is illustrated in Fig.3,the transmission Mo ¨ssbauer spectra recorded at 300and 77K consist of asymmetrical quadrupolar doublets with broadened and overlapped lines,in agreement with the large number of different environments of Fe atoms.24Whatever the fitting model,the isomer shift values (0.54and 0.42mm s 21at 300K and 77K,respectively)remain consistent with the presence of high spin state of Fe 3+ions located in octahedral units.Taking into account the crystallographic structure,one might describe the spectra by means of sevena Institut Lavoisier,UMR CNRS 8180,Universite ´de Versailles Saint-Quentin-en-Yvelines,45Avenue des Etats-Unis,78035,Versailles Ce ´dex,France.E-mail:serre@chimie.uvsq.fr;Fax:0033139254358;Tel:0033139254305;bCatalysis Centre for Molecular Engineering,Korea Research Institute of Chemical Technology (KRICT),Jang-dong 100,Yuseong-Gu,305-600,Daejon,South Korea cLaboratoire de Physique de L’Etat Condense ´,UMR CNRS 6087,Universite ´du Maine,72085,Le Mans Cedex 9,France dESRF,rue J.Horowitz,38043,Grenoble Cedex,France{Electronic supplementary information (ESI)available:Crystallographic details,TGA,adsorption,Mo ¨ssbauer and catalysis data.See DOI:10.1039/b704325bCOMMUNICATION /chemcomm |ChemComm2820|mun.,2007,2820–2822This journal is ßThe Royal Society of Chemistry 2007D o w n l o a d e d o n 20 S e p t e m b e r 2011P u b l i s h e d o n 15 M a y 2007 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 704325Bdifferent quadrupolar components:different solutions can be found without any clear and sound interpretation.On the contrary,assuming that three main types of Fe octahedral units can be distinguished according to their nearest chemical environ-ments,this gives rise to a reasonable fitting model involving three quadrupolar components,as shown in Fig. 3.This could be explained by the presence of different fluorine environments for the iron atoms within the structure.It has been proved previously with the isostructural solid MIL-100(Cr)that the distribution offluorine atoms bound to chromium atoms from the trimers,was at the origin of the three types of environments for the metal sites as evidenced by CO adsorption experiments.25Thus,as MIL-100(Fe)possesses the same fluorine composition as MIL-100(Cr),i.e.one fluorine atom per trimer,a similar environment is expected.The different values of isomer shift confirm this hypothesis with some Fe ions mainly surrounded by O while the environment of others (with higher isomer shift)also contain F ions.The values of quadrupolar splitting (0.30,0.52and 0.90mm s 21)are consistent with different degrees of distortions and the highest distorted octahedral unit contains F anions.Finally,the significant increase of the absorption spectral area when decreasing the temperature should be noted.This is due to a large increase of the Debye–Waller factor,resulting from the strengthening of the structure,particularly due to its hybrid character which is temperature sensitive.The permanent porosity of the new solid was measured by N 2adsorption experiments performed in liquid -100(Fe)revealed an adsorption isotherm characteristic of micro-porous solids (Fig.4).However,due to the presence of two types of microporous windows and mesoporous cages with different sizes,two secondary uptakes at ca.P /P 0=0.06and 0.12can be distinguished from the isotherm,in agreement with our previous results with MIL-100(Cr).23The corresponding Langmuir surface area is estimated to be .2800(100)m 2g 21.This value is,on the whole,in agreement with that of the isostructural solid MIL-100(Cr)(S =3100m 2g 21).The thermal stability ofMIL-100(Fe)Fig.1Final Rietveld refinement plot for Fe III 3O(H 2O)2F ?{C 6H 3(CO 2)3}2?n H 2O (MIL-100(Fe))in the space group Fd 3¯m .Observed,calculated and difference profiles are plotted on the same scale.Inset is an expanded region of a small part of thedata.Fig.2Structure of MIL-100(Fe).(A)A trimer of iron octahedra and trimesic acid.(B)Schematic view of one unit cell of MIL-100(Fe).(C)the two types of cages in polyhedral mode.(D)Pentagonal and hexagonal windows in balls and sticks (Fe:grey;O:red;C:black).Fig.3Transmission Mo ¨ssbauer spectra of MIL-100(Fe)recorded at 300and 77K.Fig.4N 2adsorption isotherm of MIL-100(Fe)at 77K (P 0=1atm).This journal is ßThe Royal Society of Chemistry 2007mun.,2007,2820–2822|2821D o w n l o a d e d o n 20 S e p t e m b e r 2011P u b l i s h e d o n 15 M a y 2007 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 704325Bhas been studied by X-ray thermodiffractometry and reveals that this solid is stable up to 270u C.The use of heterogeneous catalysts in the liquid phase is highly desirable for Friedel–Crafts type reactions 27because the use of conventional homogeneous catalysts for these reactions leads to several problems,such as difficulty in separation and recovery,disposal of spent catalyst and corrosion.In the light of the importance of heterogenous catalysis,we have performed Friedel–Crafts benzylation to confirm the suitability of iron-containing MIL-100as a new porous catalyst (see ESI {).Fig.5shows the conversion of benzyl chloride in the liquid phase benzylation of benzene by benzyl chloride (BZC)to diphenylmethane (DPM)at 70u C over MIL-100(Fe),MIL-100(Cr)and zeolite catalysts for comparison.We find that MIL-100(Fe)gives high activity and selectivity,showing 100%BZC conversion [X (BZC)]with nearly 100%DPM selectivity [S (DPM)]being quickly attained after a short induction period (5min).By contrast,MIL-100(Cr)was poorly active for the reaction,i.e.,42%X (BZC)after 30h.Solid acid catalysts such as HBEA and HY zeolites were not so active under the same reaction conditions:43.4%X (BZC)with 97.6%S (DPM)for HBEA and 54.0%X (BZC)with 95.8%S (DPM)for HY after 5h.These results clearly indicate that iron species in MIL-100(Fe)play a role as catalytically active sites in Friedel–Crafts alkylation.The observed high benzylation activity of MIL-100(Fe)might be attributed to the redox property of trivalent iron species (Fe 3++e 2«Fe 2+)to play a significant role in activating both the reactants,consistent with results observed in iron-containing solid catalysts.26–28The origin of the induction period in the benzylation is generally ascribed to the inhibition effect by moisture present in the catalyst and/or in the reaction mixture 26or the diffusion limitation of reactant molecules into the active site in the pore.In summary,we report the synthesis and characterisation of a new example of a large-pore iron(III )carboxylate under hydro-thermal conditions.First catalytic experiments suggest that iron(III )species metal sites are particularly interesting in catalysis and might lead to new applications.We are currently surveying phases produced from other iron(III )carboxylate systems.This work was supported by CNRS,the EU funding via FP6-Specific Targeted Research Project DeSANNS (SES6-020133),theKorea Ministry of Commerce,Industry and Energy through the Research Center for Nanocatalysis (TS066-26)and the Institutional Research Program (KK-0703-E0).The KRICT s authors thank Dr S.H.Jhung and Dr Y.K.Hwang for helpful discussion.We thank ESRF for provision of synchrotron beam time.Notes and references{Coordinates of MIL-100(Fe)have been deposited with the CCDC data bank,deposition number CCDC 640536.For crystallographic data in CIF or other electronic format see DOI:10.1039/b704325b1G.Fe ´rey,C.Mellot-Draznieks,C.Serre and lange,Acc.Chem.Res ,2005,38,217.2H.Li,M.Eddaoudi,M.O’Keeffe and O.M.Yaghi,Nature ,1999,402,276.3S.Kitagawa and K.Uemura,Chem.Soc.Rev.,2005,34,109.4Y.Liu,V.C.Kravtsov,rsena and M.Eddaoudi,mun.,2006,1488.5U.Mueller,M.Schubert,F.Teich,H.Puetter,K.Schierle-Arndt and J.Pastre,J.Mater.Chem.,2006,16,626.6G.Fe ´rey,troche, C.Serre,T.Loiseau, lange and A.Percheron-Guegan,mun.,2003,2276.7A.G.Wong-Foy,A.J.Matzger and O.M.Yaghi,J.Am.Chem.Soc.,2006,128,3494.8J.S.Seo,D.Whang,H.Lee,S.I.Jun,J.Oh,Y.J.Jeon and K.Kim,Nature ,2000,404,982.9R.-Q.Zou,H.Sakurai and Q.Xu,Angew.Chem.,Int.Ed.,2006,45,2542.10G.Fe ´rey, C.Mellot-Draznieks, C.Serre, lange,J.Dutour,S.Surble ´and I.Margiolaki,Science ,2005,309,2040.11S.Hermes,M.K.Schro ¨ter,R.Schmid,L.Khodeir,M.Muhler,A.Tissler,R.W.Fischer and R.A.Fischer,Angew.Chem.,Int.Ed.,2005,44,6237.12S.Konar,P.S.Mukherjee, E.Zangrando, F.Lloret andN.R.Chaudhuri,Angew.Chem.,Int.Ed.,2002,41,1561.13G.Guillou,C.Livage,M.Drillon and G.Fe ´rey,Angew.Chem.,Int.Ed.,2003,115,5472.14F.Pelle ´,S.Surble ´,C.Serre,lange and G.Fe ´rey,J.Lumin.,2007,122–123,492.15M.Sanselme,J.M.Grene `che,M.Riou-Cavellec and G.Fe ´rey,mun.,2002,2172.16C.Serre,lange,S.Surble´and G.Fe ´rey,Angew.Chem.,Int.Ed.,2004,43,6286.17W.Schmitt,J.P.Hill,M.P.Juanico,A.Caneschi,F.Costantino,C.E.Anson and A.K.Powell,Angew.Chem.,Int.Ed.,2005,44,4187.18T.R.Whitfield,X.Wang,L.Liu and A.J.Jacobson,Solid State Sci.,2005,7,1096.19S.Surble ´,C.Serre,C.Mellot-Draznieks,lange and G.Fe ´rey,mun.,2006,284.20C.Serre,lange,S.Surble ´,J.M.Grene `che and G.Fe ´rey,Chem.Mater.,2004,16,2706.21A.C.Sudik,A.P.Co ˆte ´,A.G.Wong-Foy,M.O’Keefe and O.M.Yaghi,Angew.Chem.,Int.Ed.,2005,118,2590.22J.Jia,X.Lin,C.Wilson,A.J.Blake,N.R.Champness,P.Hubberstey,G.Walker,E.J.Cussena and M.Schro ¨der,mun.,2007,840.23G.Fe ´rey, C.Serre, C.Mellot-Draznieks, lange,S.Surble ´,J.Dutour and I.Margiolaki,Angew.Chem.,Int.Ed.,2004,43,6296.24J.Teillet and F.Varret,in ‘MOSFIT program’.25A.Vimont,J.-M.Goupil,valley,M.Daturi,S.Surble ´,C.Serre,lange,G.Fe ´rey and N.Audebrand,J.Am.Chem.Soc.,2006,128,3218.26V.R.Choudhary and S.K.Jana,Appl.Catal.,A ,2004,224,51.27T.Cseri,S.Bekassy,S.Rizner and F.Figueras,J.Mol.Catal.A:Chem.,1995,98,101.28M.S.Hamdy,G.Mul,J.C.Jansen,A.Ebaid,Z.Shan,A.R.Overweg and T.Maschmeyer,Catal.Today ,2005,100,255.Fig.5Benzyl chloride conversion in the Friedel–Crafts benzylation of benzene over different catalysts.Reaction conditions :70u C,7.8ml benzene,benzene/benzyl chloride =10(molar ratio),0.1g catalyst.2822|mun.,2007,2820–2822This journal is ßThe Royal Society of Chemistry 2007D o w n l o a d e d o n 20 S e p t e m b e r 2011P u b l i s h e d o n 15 M a y 2007 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 704325B。

MXene:A New Family of Promising Hydrogen Storage Medium Qianku Hu,*,†,‡Dandan Sun,†Qinghua Wu,†Haiyan Wang,†Libo Wang,†Baozhong Liu,†Aiguo Zhou,*,†and Julong He§†School of Material Science and Engineering,Henan Polytechnic University,454000Jiaozuo,P.R.China‡Department of Geosciences,Stony Brook University,11794Stony Brook,New York,United States§State Key Laboratory of Metastable Materials Science and Technology,Yanshan University,066004Qinhuangdao,P.R.China *Web-Enhanced Feature*Supporting Informationthe hydrogen molecules(3.4wt%hydrogen storage capacity)reversibly under ambient conditions.Meanwhile,the hydrogenC)were also evaluated,and the results were similar to those ofmembers was expected to be a good candidate for reversibleNowadays hydrogen storage and transport remain a great challenge for its vehicle applications.1,2Storing hydrogen in solid materials is more practical,safe,and economic than that in gaseous or liquid phases.1−4According to the interaction nature between hydrogen and host materials,solid-state storage materials can be classified into two categories:chemisorption of dissociated hydrogen atoms and physisorption of intact hydrogen molecules.Either approach has its own disadvantages. For chemisorption,strong bonding(40−80kJ/mol)5,6between hydrogen atoms and host materials(mainly metal hydrides or complex chemical hydrides)makes it difficult to release hydrogen at moderate temperatures.For physisorption,an ideal sorbent material should possess two fundamental properties:high specific surface area and suitable binding energy(20−30kJ/mol,corresponding to∼0.2−0.3eV)5,7with hydrogen molecules.The sorption-based storage materials studied now mainly comprise carbon-base materials8−12 (including nanotubes,fullerenes,graphene,and nanoporous carbon),metal−organic frameworks(MOFs),13,14and covalent organic frameworks(COFs).15Almost all these materials meet thefirst requirement.For example,both sides of graphene can be utilized to store hydrogen,10,11and the highest Brunauer−Emmett−Teller surface area of MOF tested to date reach about 7000m2/g.16However,the weak binding strength of hydrogen (4−10kJ/mol,physisorption mostly by van der Waals forces)5 is the largest obstacle for the practical application of these materials.17At present these sorption-based hydrogen storage materials can only operate around liquid nitrogen temperature. Metal decorations were employed to increase the bindingenergy of hydrogen on sorption-based materials.A great deal oftheoretical calculation has been conducted to investigate the effects of metal decorations(including alkali,alkaline earth,and transition metals).18−28The transition metal results are more exciting.Via Kubas-type interaction,29the binding energies of hydrogen with the transition metals lies between strong chemisorption and weak physisorption and comes into a desirable range.However,because of their large cohesive energies,transition-metal adatoms have a tendency of aggregating into clusters,20which would significantly reduce the hydrogen storage capacity.30Some calculations show that boron-doping or vacancy defects on carbon adsorbents may prevent the clustering behavior of metal adatoms.27,31,32 However,it is very difficult to fabricate such metal-well-dispersed carbon adsorbents with boron-doping or vacancy defects.Despite these problems,the Kubas-type hydrogen storage mode is still a promising direction.A material,with lightweight,high specific surface area,no metal-clustering behavior and exhibiting Kubas interactions to store hydrogen,is very hopeful to meet the gravimetric storage capacity targetReceived:September25,2013Revised:November20,2013Published:November21,2013(5.5wt %by 2015)33set by the U.S.DOE.However,the discovery and synthesis of such material is of great challenge.Recently,a new family of graphene-like 2D materials,named as MXene,was prepared by exfoliating the counterpart MAX phases in hydro fluoric acid.34,35MAX phases are a large family (>60members)of layered ternary transition-metal carbides or nitrides with a chemical formula M n +1AX n (n =1,2,or 3),where M is a transition metal,A is an A-group element (mostly IIIA or IVA group),and X is C and/or N.36,37To date,the as-synthesized MXene phases include Ti 2C,Ti 3C 2,Ta 4C 3,(Ti 0.5Nb 0.5)2C,(V 0.5Cr 0.5)3C 2,and Ti 3CN.34,35For a new material,it is very important scienti fically and technically to explore its basic properties and potential applications.Good electrical conductivities of bare MXenes were predicted theoretically and can be tuned by termination/functionalization with di fferent groups.34,35,38−40The electrochemical intercala-tion behaviors of Li ions in MXene structures were also investigated experimentally and theoretically,which prove MXene phases are very promising as anode materials of Li-ion battery.41−44In this paper,we investigated the possibility of employing MXene phases as hydrogen storage media by first-principles calculations.We chose Ti 2C as a representative of MXenes on the basis of the following reasons:(i)Titanium is a commonly used decoration element and has been proved to be e ffective for hydrogen storage in carbon-based materials.(ii)The 2D Ti 2C phase has already been synthesized (though with fluorine (F)and/or hydroxyl (OH)termination).35(iii)Except for Sc 2C,Ti 2C possesses the highest surface area per weight among all possible MXene phases,and thus it is expected to have high gravimetric hydrogen storage capacities.Figure 1gives the crystal structures of bulk Ti 2AlC and 2D Ti 2C.In the layeredTi 2AlC structure,Ti −Al bond is relatively weaker than Ti −C bond.Consequently,the layers of Al atoms can be selectively etched by hydro fluoric acid,which results in the formation of the 2D Ti 2C sheets.35The 2D Ti 2C structure is composed of sharing-edge Ti 6C octahedrons,in which,C atoms occupy the octahedral interstitial sites between near-close-packed Ti atoms.One Ti 2C sheet can be simply considered as one graphene sheet coated with a Ti atoms sheet on each side.These Ti atoms indeed are constituent elements of Ti 2C,and thus the problem of aggregation of Ti atoms can be avoided.With the cleavage of Ti −Al bond,all Ti atoms of Ti 2C lie in an unsaturated coordination state,which is an indispensable condition for metal atoms to exhibit the Kubas interactions.5,45On the basis of aforementioned reasons,we think 2D Ti 2C (even most MXene phases)is very likely to be a reversible and high-gravimetric-capacity hydrogen storage material operated under ambient conditions.The purpose of the calculations in this paper is to con firm this speculation.■THEORETICAL METHODS Our first-principles total energy pseudopotential calculationswere performed using the density functional theory (DFT)asimplemented in CASTEP code.46The exchange and correlation energy is described by the local density approximation (LDA)functional.Vanderbilt ultrasoft pseudopotentials were em-ployed within a plane wave basis set with the cuto ffenergy of 480eV.The numerical integration of the Brillouin zone was performed using 6×6×2(unit cell)and 2×2×1(supercell)Monkhorst −Pack (MP)k -point sampling.Both structural parameters and atomic positions with no constraints were fully relaxed using the BFGS minimization method until the convergence tolerance (energy <5.0×10−6eV,force <0.01eV/Å,stress <0.02GPa and displacement <5.0×10−4Å)was reached.The binding energy (E b )of H 2with the Ti2C host material was calculated according to the followingequation:=+−+E E nE E n()/n b host H host H 22where E host is the total energy of the host structure (bare Ti 2Cstructure or already adsorbed with some H 2or H),E H 2is thetotal energy of an free H 2,E host+n H 2is the total energy of thehost structure adsorbed with new hydrogen molecules,and n is the number of new adsorbed hydrogen molecules.The binding energy of H atom to the surface of Ti 2C was also calculated like this.■RESULTS AND DISCUSSIONTi 2C Model.The 2D Ti 2C structure was constructed by removing the Al element from the parent Ti 2AlC structure.Thereafter,a vacuum space with a thickness of 20Åwas inserted between the neighboring slabs to avoid arti ficial interactions between them.The optimized lattice constant a for the Ti 2C model is 3.002Å,which is in good agreement with the other result of 3.007Å.38A Ti 2C 3×3periodic supercell (containing nine carbon and eighteen titanium atoms,shown in Figure 2a)was used as thehost material.To investigate the hydrogen adsorption,it is important to first find the favorable adsorption sites.On Ti2C surfaces,there exist three types of high-symmetry sites:top site over a Ti atom at the top surface;hollow_1site above the center of three Ti atoms at the top surface,under which there exists one Ti atom at the bottom surface;and hollow_2site above the center of three Ti atoms at the top surface,under which there exists one C atom at the bottom surface.Hydrogen at Ti2C 3×3Supercell.We first studied theadsorption of a single hydrogen molecule on the Ti 2C 3×3supercell.A H 2molecule was put parallel or perpendicularlyabove the three adsorption sites respectively,and thus six models were constructed,which are shown in Figure 2a.After geometry optimization,the H 2initially locating parallel above the three sites and perpendicularly above the hollow_2site is thermally unstable and dissociates into two H atoms.Nearly all dissociated H atoms occupy the hollow_1site.The calculations (Supporting Information)for one hydrogen atom locating above the top,hollow_1and hollow_2sites,respectively,show that the hollow_1site is the most stable one.Thus it canbeFigure 1.(a)Crystal structure of bulk Ti 2AlC.The black solid line labels out the unit cell.(b)Side views and (c)top views of 2D Ti 2C structure.Two Ti 2C octahedrons were labeled out by green color.understood that the dissociated H atoms prefer to occupy the hollow_1site.We turn our eyes onto the stable A and B adsorption sites in Figure 2a where the H 2molecules are still intact and perpendicular to the surface.However,if initially the H 2molecule is put not perpendicular to the A or B sites but is tilted by an angle (>10°),the H 2molecules also dissociate just like the case of parallel to the surface.Therefore,on the basis of the above calculated results,we can presume that the first batch of H 2molecules arriving onto the clean surface of 2D Ti 2C on the hydrogenation process dissociate into H atoms and then the dissociated H atoms occupy the hollow_1site.A model,labeled as Ti 2C(3×3)_18H shown in Figure 2b,was constructed to represent this situation.In this model,the H atoms are adsorbed on the hollow_1sites on both sides of 2D Ti 2C.The calculated binding energy is 5.027eV per H atom with an H −Ti distance of 1.977Å.The large binding energy means the interaction of dissociated hydrogen atoms with the Ti 2C surface is of strong chemisorption.Hydrogen at Ti 2C(3×3)_18H Model.Next,we studied the adsorption of H 2molecules on the Ti 2C(3×3)_18H structure.Similarly,six models were constructed with one H 2molecule parallel or perpendicularly above the top,hollow_1and hollow_2sites,respectively,as shown in Figure 3.After geometry optimization,the H 2molecule parallel above the hollow_2site (labeled as F)relaxes to parallel above the top site (labeled as D).The H 2molecules for the other five con figurations (A,B,C,D,and E;see labels in Figure 3)still keep the initial sites and orientation.For the A,B,C,and E con figurations,the calculated binding energies are in the range 30−70meV,and the H −H bond length increases slightly by 0.01−0.04Åfrom the value 0.769Åof an isolated H 2.The weak binding energy and the nearly unchanged H −H bond length illuminate the H 2are physisorbed on the A,B,C,and E sites.An interesting and exciting result is about the con figuration that the H 2is adsorbed parallel on the top site (D in Figure 3).For this con figuration,the binding energy is calculated to be 0.237eV,which falls into the desired range 0.2−0.3eV.From the binding energy and elongated H −H bond length,we speculated this interaction nature should be of Kubas type,which would be con firmed later by the results of density of states (DOS).At room temperatures,the hydrogen molecules by physisorption are very little.Therefore,we can presume that under ambient conditions the second batch of H 2molecules arriving on the surface of 2D Ti 2C are adsorbed only parallel on the top site.A model,labeled as Ti 2C(3×3)_18H_18H 2,was constructed as shown in Figure 4a to represent this situation.Inthis model,onto every top site on both sides was laid one H2molecule whose axis is parallel to the surface.The initial orientation of the axes of all the H 2molecules was set to be the same.After optimization,one-third of the H 2molecules rotated 60°clockwise around the c axis,one-third of the H 2molecules rotated 60°counterclockwise around the c axis,and the remaining one-third kept in the original orientation.Theoptimized structure of Ti 2C(3×3)_18H_18H 2is shown inFigure 4b.The optimized structure is more symmetrical thanthe initial structure.The average binding energy for the H2molecules by Kubas-type interaction is 0.272eV per H 2with an H −H bond length of 0.823Å.Hydrogen at Ti 2C(3×3)_18H_18H 2Model.Now weconsider the hydrogen adsorption by physical forces at liquidnitrogen temperatures.At low temperatures,the hydrogen molecules by Kubas-type interaction and the hydrogen atoms by chemisorption are bound tightly to the Ti 2C surfaces.Therefore,the Ti 2C(3×3)_18H_18H 2model was used as the host material to investigate the physical adsorption of hydrogen molecules.For this model,H 2molecules have occupied the top sites.Only the hollow_1and hollow_2sites could be usedtoFigure 2.(a)Adsorption of a single hydrogen molecule at di fferent sites of Ti 2C 3×3supercell.The blue arrows indicate the moving direction ofthe dissociated H atoms.The black solid line labels out the 3×3supercell.(b)Ti 2C(3×3)_18Hmodel.Figure 3.Adsorption of a single hydrogen molecule at di fferent sites of Ti 2C(3×3)_18H model.The calculated binding energies for H 2atdifferent sites were given.The blue arrow indicates the movingdirection of the unstable H 2molecule.The black solid line labels outthe3×3supercell.The legend of di fferent atoms is the same to that in Figure 2adsorb H 2molecules.Four con figurations,as shown in Figure 5,were constructed in which one H 2molecule is put parallel or perpendicularly above the hollow_1and hollow_2sites respectively.All four adsorption sites are found to be stable.The weak binding energies for the four sites mean the nature of physical interaction.For the hollow_1or hollow_2site,thebinding energies of the parallel and the perpendicularcon figurations are very close.Thus,at liquid nitrogentemperatures,both the parallel and the perpendicularcon figurations are possible.Whereas the perpendicularcon figuration has a little higher binding energy than thecorresponding parallel con figuration,the two perpendicularcon figurations were chosen to represent the physisorption ofH 2molecules above the hollow_1and hollow_2sitesrespectively.A model was constructed to represent this situation,in whichevery hollow_1or hollow_2site on both sides of Ti2C(3×3)_18H_18H 2is occupiedby one H 2molecule with its axisperpendicular to the site.After optimization,the H2moleculesabovethe hollow_1site are stable.However,the H 2moleculesabove the hollow_2site depart from the surfaces along the c -axis direction.These should arise from the short distance and the resulting repulsion between the H 2molecules above thehollow_1site and the neighboring hollow_2site.A Ti2C(3×3)_18H_36H2model with theH 2molecules only occupying thehollow_1site of Ti 2C(3×3)_18H_18H 2was constructed.Partsa andb of Figure 6give the top and side views of thismodel.Figure 4.(a)Initial structure of Ti 2C(3×3)_18H_18H 2model.(b)Optimized structure of Ti 2C(3×3)_18H_18H 2model.The black solid line labels out the 3×3supercell.The legend of di fferent atoms is the same to that in Figure2.Figure 5.Adsorption of a single hydrogen molecule at di fferent sites of Ti 2C(3×3)_18H_18H 2model.The calculated binding energies for H 2at di fferent sites were given.The black solid line labels out the 3×3supercell.Figure 6.(a)Top views and (b)side views of the Ti 2C(3×3)_18H_36H 2model that possesses the maximum hydrogen storage capacity.The black solid line labels out the 3×3supercell.The average binding energy is calculated to be 0.109eV per H 2for these 18hydrogen molecules above the hollow_1site.Maximum Hydrogen Storage Capacity.Now all thepossible sites on the Ti 2C surface have been considered to bind hydrogen.In the Ti 2C(3×3)_18H_36H 2structure,one adsorption site only binds one H 2molecule.We attempted to attach more H 2molecules to di fferent adsorption sites.But unfortunately it failed.The new added H 2molecule directly flies away or pushes the neighboring H 2molecule o ffand then occupies its position.Therefore,the Ti 2C(3×3)_18H_36H 2model possesses the maximum hydrogen storage capacity.In this supercell model,18H atoms (1.7wt %)are bound by strong chemical forces,36H atoms (3.4wt %)are bound in molecule form by weak physical forces,and the remaining 36H atoms (3.4wt %)are bound in molecule form by Kubas-type interaction.Under ambient conditions,desorption of chem-isorbed hydrogen cannot take place,and physisorbed hydro-gens are not easy to bind.Only the hydrogen bound by Kubas-type interaction could be adsorbed and released reversibly under ambient conditions.The reversible capacity of 3.4wt %is still considerable and signi ficant for practical applications.Density of States and Mulliken Populations.The Kubas-type interaction between H 2and transition metals has the following features:19,22,45,47(i)Adsorbed H 2molecules keep intact and the bond length is elongated approximately 10%from the bond length 0.75Åof a free H 2molecule.(ii)The binding energy is between the physisorption and the chemisorption and usually lies in the range 0.2−0.8eV.(iii)The bond axis of the adsorbed H 2molecule is not perpendicular to the transition metal.In this paper,the hydrogen molecules adsorbed upon the top sites possess all these features.Hence,we conclude that the binding nature of these hydrogen molecules with Ti 2C should be of the Kubas-type interaction.This Kubas-type interaction is associated with the electron donation of H 2σorbitals into the empty d orbitals of a transition metal,and simultaneously the electron back-donation from the filled metal d orbitals into the H 2σ*antibonding orbitals.23,45Thus the Kubas-type interaction involves the orbital hybridization between the transition metal d orbitals and the H 2σorbitals (including bonding and antibonding).Figure 7gives the partial density of states (PDOS)for the s orbitals of one H 2molecule above the top site and the 3d orbitals of the underlying Ti atom in the model of Ti 2C(3×3)_18H_36H 2.It can be seen that the Ti 3d orbitals are hybridized with the H 2σorbitals in the range −10to −6eV.And the peaks from −2to 0eV correspond to the hybridization of the Ti 3d with the H 2σ*orbitals.These hybridizations are very similar to the results obtained by otherresearches for the adsorption of H 2on Ti atom.19,47The approximately 10%elongation of H −H bond length arises from the decrease of bonding-orbital electrons and theincrease of antibonding-orbital electrons.If excessive electronsare donated from Ti 3d orbitals to H2σ*antibonding orbitals,the H 2molecule will be unstable and then dissociate.It is the reason that in this study the first H 2arriving at Ti atomdissociates into H atoms.This phenomenon is also observed in other literatures.19,20,48,49The Mulliken charge population calculations show every dissociated H atom gains 0.33e totally from the three nearest Ti atoms.This means that every Ti atom donates 0.33e totally to the three nearest H atoms.Meanwhile,due to the smaller electronegativity of Ti than C,every Ti atom donates about 0.35e to the three nearest C atoms.Thus when the second H 2arrives,the Ti atom has no enough charges to destabilize the dihydrogen state.As a result,the second arriving H 2does not dissociate and locates above the top site in a molecular form.And even,no more charges in the Ti atom can be transferred to bind extra H 2molecules.It is the reason thatone Ti atom can bind 4−6H2molecules in other literatures,19,22,47,48but one Ti atom in the 2D Ti 2C can onlybind one H 2molecule by Kubas-type interaction.Ab Initio Molecular Dynamic Simulations.From thebinding energies results,we speculated that the hydrogen molecules bound by Kubas-type interaction could adsorb and desorb reversibly under ambient conditions.To verify this and ascertain the exact desorption temperature,desorption behaviors of hydrogen on Ti 2C were investigated by ab initio molecular dynamic (MD)simulations using the Nosealgorithm.The simulation temperature was set to be 300and400K.The total simulation time was set to be 1.5ps with a time step of 1.0fs.Figure 8gives the snapshots of the Ti 2C(3×3)_18H_36H 2model after 1.5ps molecular dynamic simulations at 300and 400K.(Movies 1,2,3,and 4in mpg format for top and side views of desorption processes of hydrogen are available as Web Enhanced Objects.)At both 300and 400K,all the H 2molecules by chemisorption still stay at the initial sites and all the H 2molecules by physisorption depart from the surfaces.These phenomena can be easily understood from the binding energy results.For the eighteen H 2molecules by Kubas-type interaction,the results at 300K are di fferent with those at 400K.At 400K,nearly half of the H 2molecules by Kubas-type interaction depart from the surface.It should be reminded that the simulation system does not reach the balancebecause the 1.5ps simulation time is not long enough (however,already quite costly in computation time).Thus the temperature 400K provides enough energy for the release of the H 2molecules bound by the Kubas-type interaction.At 300K,only three H 2molecules fly away.And an important phenomenon was observed on the simulation process of 300K.When a released H 2molecule flies over the region of a vacanttop site,the vacant top site can catch and adsorb this H 2molecule again.Thus at 300K mostly the top sites should be saturated with H 2molecules.From the above discussions,the adsorption and desorption of hydrogen by Kubas-typeinteraction can be accomplished in the narrow temperature range 300−400K.Therefore,the MD results give clear evidence that 2D Ti2C is a reversible hydrogen storage material under ambientconditions.Figure 7.Partial density of states (PDOS)for thes orbitalsof one H 2molecule above the top site and the 3d orbitals of the underlying Ti atom in Ti 2C(3×3)_18H_36H 2model.The black dash line represents the Fermi level.Perspectives of MXene Phases as Hydrogen Storage Materials.An important experimental result should be noted that because MXene phases were made in aqueous hydro fluoric acid,the as-fabricated 2D Ti 2C are chemically terminated with fluorine (F)and/or hydroxyl (OH)groups.34,35The binding interactions are so strong.Thus,although e ffort has been made,42bare MXene phases with no surface termination have not been prepared.From our point of view,two routes are possible to obtain bare MXene structures:(i)removing the F and OH groups from the synthesized MXenes by chemical or physical methods;(ii)finding a new way to exfoliate parent MAX phases.This is a big challenge and task for materials scientists and chemists.When the writing of this paper came to a close,we realized that the F-or OH-terminated MXene phases may be also a good hydrogen storage material due to the electrostatic interactions between the F (or OH)anions and the adsorbed H 2.The corresponding calculations are now in progress.When the A elements are removed from the corresponding MAX family,which includes more than 60members,theoretically there exist more than 20MXene phases.These MXene phases have structures and compositions similar to Ti 2C and thus are also expected to be good hydrogen storage materials.To test it,the hydrogen storage properties of 2D Sc 2C and V 2C were calculated in a simple way.We replaced Ti atoms in Ti 2C(3×3)_18H,Ti 2C(3×3)_18H_18H 2,and Ti 2C-(3×3)_18H _36H 2models by Sc and V atoms,respectively.The optimized geometry structures (Supporting Information)for hydrogen adsorbed on Sc 2C and V 2C surfaces are similar to those on the Ti 2C surface.The hydrogens are also bound by three modes:physisorption,chemisorption,or Kubas-type interaction.The binding energies for the H 2on Sc 2C and V 2C by Kubas-type interaction were calculated to be 0.164and 0.242eV per H 2respectively,which are also suitable.Therefore,the studies in this paper opened the door of a house that contains a series of reversible and high-gravimetric-capacity hydrogen storage materials operated under ambient conditions.And the hydrogen adsorption and desorption behaviors could be adjusted by using di fferent MXene phases as hydrogen storage materials.■CONCLUSIONSIn summary,using first-principles total energy pseudopotential calculations,we systematically investigated the possibility of 2D Ti 2C structure (a representative MXene)as hydrogen storage materials.The calculations show that hydrogen can be adsorbed on di fferent sites on both sides of Ti 2C layered structure.Considering all adsorbed hydrogen molecules and atoms,the maximum hydrogen storage capacity was calculated to be 8.6wt %,which meets the gravimetric storage capacity target (5.5wt %by 2015)set by the U.S.DOE.These hydrogen are bound by three modes:chemisorption of the H atom (1.7wt %),physisorption of the H 2molecule (3.4wt %),and Kubas-type binding of the H 2molecule (3.4wt %)with calculated binding energies of 5.027,0.109,and 0.272eV,respectively.The binding energy of 0.272eV for the H 2molecule by Kubas-type interaction just falls into the desired range for a reversible hydrogen storage material under ambient conditions.Ab-initioMD simulations con firmed that the hydrogen molecules bound by Kubas-type interaction can be adsorbed and released reversibly in the temperature range 300−400K.The di fferent binding energy values for the three modes imply that 2D Ti 2C can store hydrogenat low,room,and high temperatures.The hydrogen storage properties of Sc 2C and V 2C MXene phases were also evaluated in a simple way.The results are similar to that for Ti 2C and are also fascinating.Therefore,MXene phases including more than 20members should be a new family of hydrogen storage materials.Experiments areexpected to con firm the results of this work.And furtherexperimental and computational investigations should be conducted on other MXene phases.Discovery of MXene phases with better hydrogen storage performances is anticipated in the near future.■ASSOCIATED CONTENT *Supporting Information Adsorption geometries and total energies and structural parameters of all the optimized con figurations involved in this paper.This material is available free of charge via the Internet at .*Web-Enhanced Features Four animations in mpg format are available in the HTML version of the paper.■AUTHOR INFORMATION Corresponding Authors*Q.Hu:e-mail,hqk@.*A.Zhou:e-mail,zhouag@.NotesThe authors declare no competing financial interest.■ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos.51202058,50802024,Figure 8.Top and side snapshots of Ti 2C(3×3)_18H_36H2modelafter 1.5ps moleculardynamic simulations at 300and400K.Severalhydrogen molecules departing too far from the surfaces were not given.The legend of di fferent atoms is the same to that in Figure 2.。