REPORTS and kinetics can be modeled in a crystal that can be transformed without loss 23. The maximum number of exchange codes in this diffusion kinetics with appropriat of the original shape, this may be ov vercome study is two. with repeated exchange cycles, and mechanical driving-force fie by using an inert and rigid structural support hereas the structural imperfection(such as shown heterogeneous interfaces where the reaction or matrix. Our study also demonstrates that occurs(28). On the nanometer scale, changes inorganic nanocrystals may be far more 4 tat with saturated m ethanolic solution of caf(n o. in the reaction free energy and the height of chemically dynamic than previously realized. the reaction barrier inevitably accompany the sign of compositional or structural chang well-known increase in vibrational amplitude 25. L Dloczik, R. Konenkamp, Nano Lett. 3, 651(2003) and diffusion rate(29, 30). This qualitative Ire in which the root mean square displacement changes the picture for the reaction. the diffusing particle is proportional to the squar Propagation of the reaction zone at th 2. C. B. Murray. Norris, M. C. Bawendi, / Am interface, which typically spans several 3. X Peng et al. Nature 404, 59(2000) the use of the nernst- atomic layers, is central to the description 4. N.R. Jana, L Gerheart, C.J. Murphy /.Phys. Chem. B Planck equation, which does not have such a simple analytical relationsh of the solid-phase cation exchange reaction s 105, 406s 2001 in the bulk(28). In nanocrystals, because of 6. Y Sun, Y Xia, Science 298, 2176(2002) Fischer, J. Phys. Condens. Matter 13, 2425(20 M. Backhaus-Ricoult, Annu. Rev. Mater. Res. 33, 55 he relatively small number of atomic layers 7. L Manna, D ]. Milliron, A. MeiseL, E C. Scher, A.P within a few nanometers(typically two or 29. M. A. Van Hove, J. Phys. Chem. B 108, 14265(2004). three layers per nanometer), the width of the 8. F. X. Redl, K-S. Cho, C B. Murray, S. O'Brian, Nature 30. A. Maradudi reaction zone can become a large part of or 9. Y Yin et al. Science 304, 711(2004) (1964) comparable to the whole width of the crystal. 10. T. Mokari, E Rothenberg L.Popov, R Costi, UBanin, 31. Direct This can have two important consequence arge exothermicity of the reaction in combinatio 11. D ]. Milliron et al., Nature 430, 6996(2004) for reactions in nanometer-sized crystals. 12. L Manna, E CScher, L-S.Li,APAlivisatos./.Am nanometer-sized solid to the liquid environment can First, the slow propagation of the reaction hem.soc.124.7136(2002) A Mews, A. Eychmueller, M. Gier the reaction in solid phase is slower by many orders chemical potential near the reaction zone, 14. ler d, p &. Cansh Korkischk heat will build up in the nanocrystals to melt then may become less important as the rate- Sta limiting process of the reaction. Second, at R E Schak,I. E. Mallouk. Chem. Mater. 14. 1455 32 M02) G.V. Hartland, J. Phys. Chem. B 106, 7029 the early stage of the reaction, the whole 33. The formation of hexagonal CuSe and cubic PbSe 16. 5. Feng, R Xu, Acc. Chem. Res 34, 239(2001). crystal can be in a structurally nonequilibri-17.ANGoldstein,.M.Echer,AP. Alivisatos, Scienc 256,1425(1992 um state where both the cations and anions 18. G. Baldinozzi, D Simeone, D. Gosset, M Dutheil, spectra firmed with XRD patterns and optical absorption are mobile(31, 32). This can result in a 34. Supported by the U.S. Department of Energy change of the morphology to the thermody- 19. C-C Chen, AB. Herhold, C.S. Johnson, A.P. namically more stable shape before all the SooEco nanocrystals from the. cation exchange o旨 ions reach the final equilibrium positions of 20. Pure Substances, Part 1: Elements and Compounds the product phase. As the crystal becomes from AgBr to BaN vol. 19 of Landolt-Boimstein Group Supporting online Materia Physical Chemistry(Springer-Verlag Heidelberg thicker, propagation of the reaction front is observed and the morphology is maintained 21.J Burgess, Metal lons in Solution(Ellis Horwood, Figs. $1 and S2 (Fig. 4B). The change of morphology that 22 M Kobayashi, Solid State ionics 39, 121(1990). 6 August 2004: accepted 21 September 2004 progressively diminishes with the increase in the width of the nanorods is consistent with the idea that the soft reaction zone has a Hysteretic Adsorption and Desorp 503sE9oS finite width, which falls within the size range for nanocrystal synthesis The cation exchange reaction in nano- crystals, investigated mainly with Ag+ ion in tion of Hydrogen by Nanoporous this study, can easily be extended to ex hange with other cations. For example, CaSe Metal-Organic Frameworks nanocrystals can be successfully transformed Xuebo Zhao, Bo Xiao, Ashleigh J. Fletcher, K Mark Thomas into CuSe and PbSe nanocrystals through the cation exchange reaction with Cu2+ and Pb2+ Darren Bradshaw, Matthew J. Rosseinsky ions, respectively, under ambient conditions (33). On the other hand, attempts to induce Adsorption and desorption of hydrogen from nanoporous materials, such as anion exchange have not been successful activated carbon, is usually fully reversible. We have prepared nanoporous under similar experimental conditions, possi metal-organic framework materials with flexible linkers in which the pore bly because of the much larger size of the openings, as characterized in the static structures, appear to be too small to anions relative to the cations. whic allow H, to pass. We observe hysteresis in their adsorption and desorption diffusion more difficult kinetics above the supercritical temperature of H, that reflects the dynamical Our results show that the cation exchange opening of the"windows"between pores. This behavior would allow H, to be reaction is a versatile route for expanding the adsorbed at high pressures but stored at lower pressures. range of nanoscale materials with diverse produce each individual nanostructure. Al- and cost-effective method of H, storage. kinetics, cost, and safety required for chalis compositions, structures, and shapes without The widespread use of hydrogen as a fuel hydrides, and adsorption orous ma having to develop new synthetic methods is limited by the lack of a convenient, safe, rials) satisfy the criteria of siz though the finite width of the reaction zone None of the current H, storage options transportation (1). An adsorbent material 102 y impose a limit on the size of the nano- (liquefied or high-pressure H2 gas, metal with porosity on the molecular scale could 5NovemBer2004Vol306SciEncewww.sciencemagorgand kinetics can be modeled in a formalism of diffusion kinetics with appropriate chemical and mechanical driving-force fields near the heterogeneous interfaces where the reaction occurs (28). On the nanometer scale, changes in the reaction free energy and the height of the reaction barrier inevitably accompany the well-known increase in vibrational amplitude and diffusion rate (29, 30). This qualitatively changes the picture for the reaction. Propagation of the reaction zone at the interface, which typically spans several atomic layers, is central to the description of the solid-phase cation exchange reaction in the bulk (28). In nanocrystals, because of the relatively small number of atomic layers within a few nanometers (typically two or three layers per nanometer), the width of the reaction zone can become a large part of or comparable to the whole width of the crystal. This can have two important consequences for reactions in nanometer-sized crystals. First, the slow propagation of the reaction front, driven by the gradient of the local chemical potential near the reaction zone, may become less important as the rate￾limiting process of the reaction. Second, at the early stage of the reaction, the whole crystal can be in a structurally nonequilibri￾um state where both the cations and anions are mobile (31, 32). This can result in a change of the morphology to the thermody￾namically more stable shape before all the ions reach the final equilibrium positions of the product phase. As the crystal becomes thicker, propagation of the reaction front is observed and the morphology is maintained (Fig. 4B). The change of morphology that progressively diminishes with the increase in the width of the nanorods is consistent with the idea that the soft reaction zone has a finite width, which falls within the size range for nanocrystal synthesis. The cation exchange reaction in nano￾crystals, investigated mainly with Agþ ion in this study, can easily be extended to ex￾change with other cations. For example, CdSe nanocrystals can be successfully transformed into CuSe and PbSe nanocrystals through the cation exchange reaction with Cu2þ and Pb2þ ions, respectively, under ambient conditions (33). On the other hand, attempts to induce anion exchange have not been successful under similar experimental conditions, possi￾bly because of the much larger size of the anions relative to the cations, which makes diffusion more difficult. Our results show that the cation exchange reaction is a versatile route for expanding the range of nanoscale materials with diverse compositions, structures, and shapes without having to develop new synthetic methods to produce each individual nanostructure. Al￾though the finite width of the reaction zone may impose a limit on the size of the nano￾crystal that can be transformed without loss of the original shape, this may be overcome by using an inert and rigid structural support or matrix. Our study also demonstrates that inorganic nanocrystals may be far more chemically dynamic than previously realized. References and Notes 1. H. Schmalzried, Solid State Reactions (Verlag Chemie, Weinheim, Germany, 1981). 2. C. B. Murray, D. J. Norris, M. C. Bawendi, J. Am. Chem. Soc. 115, 8706 (1993). 3. X. Peng et al., Nature 404, 59 (2000). 4. N. R. Jana, L. Gerheart, C. J. Murphy, J. Phys. Chem. B 105, 4065 (2001). 5. R. Jin et al., Science 294, 1901 (2001). 6. Y. Sun, Y. Xia, Science 298, 2176 (2002). 7. L. Manna, D. J. Milliron, A. Meisel, E. C. Scher, A. P. Alivisatos, Nature Mater. 2, 382 (2003). 8. F. X. Redl, K.-S. Cho, C. B. Murray, S. O’Brian, Nature 423, 968 (2003). 9. Y. Yin et al., Science 304, 711 (2004). 10. T. Mokari, E. Rothenberg, I. Popov, R. Costi, U. Banin, Science 304, 1787 (2004). 11. D. J. Milliron et al., Nature 430, 6996 (2004). 12. L. Manna, E. C. Scher, L.-S. Li, A. P. Alivisatos, J. Am. Chem. Soc. 124, 7136 (2002). 13. A. Mews, A. Eychmueller, M. Giersig, D. Schooss, H. Weller, J. Phys. Chem. 98, 934 (1994). 14. V. A. Pedrov, V. A. Ganshin, Y. N. Korkishko, Phys. Stat. Sol. 139, 9 (1993). 15. R. E. Schaak, T. E. Mallouk, Chem. Mater. 14, 1455 (2002). 16. S. Feng, R. Xu, Acc. Chem. Res. 34, 239 (2001). 17. A. N. Goldstein, C. M. Echer, A. P. Alivisatos, Science 256, 1425 (1992). 18. G. Baldinozzi, D. Simeone, D. Gosset, M. Dutheil, Phys. Rev. Lett. 90, 216103 (2003). 19. C.-C. Chen, A. B. Herhold, C. S. Johnson, A. P. Alivisatos, Science 276, 398 (1997). 20. Pure Substances, Part 1: Elements and Compounds from AgBr to Ba3N2, vol. 19 of Landolt-Bo¨rnstein Group IV Physical Chemistry (Springer-Verlag, Heidelberg, Germany, 1999). 21. J. Burgess, Metal Ions in Solution (Ellis Horwood, Sussex, UK, 1978). 22. M. Kobayashi, Solid State Ionics 39, 121 (1990). 23. The maximum number of exchange cycles in this study is two. With repeated exchange cycles, the average volume of the nanocrystals is invariant, whereas the structural imperfection (such as shown in Fig. 1H) may accumulate. 24. The XRD pattern of powdered CdSe crystal in con￾tact with saturated methanolic solution of Cd(NO3)2 containing excess Cd(NO3)2 for 2 weeks indicates no sign of compositional or structural change. 25. L. Dloczik, R. Konenkamp, Nano Lett. 3, 651 (2003). 26. This conclusion is based on a simplified diffusion picture in which the root mean square displacement of the diffusing particle is proportional to the square of the diffusion time. A general description of the ion exchange reaction requires the use of the Nernst￾Planck equation, which does not have such a simple analytical relationship. 27. M. A. Hamilton, A. C. Barnes, W. S. Howells, H. E. Fischer, J. Phys. Condens. Matter 13, 2425 (2001). 28. M. Backhaus-Ricoult, Annu. Rev. Mater. Res. 33, 55 (2003). 29. M. A. Van Hove, J. Phys. Chem. B 108, 14265 (2004). 30. A. Maradudin, J. Melngailis, Phys. Rev. 133, A1188 (1964). 31. Direct thermal melting of the nanocrystals, from the large exothermicity of the reaction in combination with the size-dependent melting temperature, is unlikely to take place. The dissipation of heat in the nanometer-sized solid to the liquid environment can occur at time scales of 10–11 to 10–10 s (32), whereas the reaction in solid phase is slower by many orders of magnitude; hence, not enough heat will build up in the nanocrystals to melt them. 32. M. Hu, G. V. Hartland, J. Phys. Chem. B 106, 7029 (2002). 33. The formation of hexagonal CuSe and cubic PbSe nanocrystals from the cation exchange of CdSe nanocrystals with Cu2þ and Pb2þ has been con￾firmed with XRD patterns and optical absorption spectra. 34. Supported by the U.S. Department of Energy under contract DE-AC03-76SF00098. Supporting Online Material www.sciencemag.org/cgi/content/full/306/5698/1009/ DC1 Figs. S1 and S2 6 August 2004; accepted 21 September 2004 Hysteretic Adsorption and Desorp￾tion of Hydrogen by Nanoporous Metal-Organic Frameworks Xuebo Zhao,1 Bo Xiao,1 Ashleigh J. Fletcher,1 K. Mark Thomas,1 * Darren Bradshaw,2 Matthew J. Rosseinsky2 Adsorption and desorption of hydrogen from nanoporous materials, such as activated carbon, is usually fully reversible. We have prepared nanoporous metal-organic framework materials with flexible linkers in which the pore openings, as characterized in the static structures, appear to be too small to allow H2 to pass. We observe hysteresis in their adsorption and desorption kinetics above the supercritical temperature of H2 that reflects the dynamical opening of the ‘‘windows’’ between pores. This behavior would allow H2 to be adsorbed at high pressures but stored at lower pressures. The widespread use of hydrogen as a fuel is limited by the lack of a convenient, safe, and cost-effective method of H2 storage. None of the current H2 storage options (liquefied or high-pressure H2 gas, metal hydrides, and adsorption on porous mate￾rials) satisfy the criteria of size, recharge kinetics, cost, and safety required for use in transportation (1). 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