REPORTS the polycrystalline films used in todays crystal and thin films. This has been seen previou LJ. Belenky, D M. Kim, and H P Sun for their help This leads to an upper bound in film thickness ingle crystals, fibers, and powders of BaTiO, and 000 A. The thinness of the ferroelectric lso constrained. First, there is electric phase of the crystal that locally break the a thickness of about 100 A(17, 22-24)and inversion symmetry (30) 30. G.R. Fox, J.K. Yamamoto, D. V Miller, L. E. Cross, S.K supported in part by the Postdoctoral Fellowship banishes for thicknesses in the 10 to 30 rogram of Korea Science and Engineering Foundation ts ( pinholes and nonuniform thickness over the 31. C. B. Eom et al., Science 258, 1766 pporting Online Material amics(Academic Press, London, 1971), p .sciencemag org/cgi/content/full/ 306/5698/1005 26. J w. Matthews, A E Blakeslee, J. Cryst. Growth 27, 33. v Nagarajan et al. /. Appl. Ph 4. R. Ramesh et aL., Appl. Phys. Lett. 61, 1537(1992 29. A small amount of symmetry forbidden SHG signal 37. We thank L E Cross, S.K. Streiffer, and S. Troller is observed even above Te in both the BaTiO, sing McKinstry for useful discussions. We also thank 26 July 2004: accepted 6 October 2004 Cation Exchange Reactions in 000 kJ/mol in the bulk(20, 21). Ag, Se al hibits an interesting temperature-dependent polymorphism: The superionic conducting lonic Nanocrystals phase transition occurs at a relatively low temperature of 133 C in the bulk phase(22) Dong Hee Son,Steven M. Hughes, Yadong Yin, Thus, it may be possible to prepare Ag,Se A. Paul Alivisatos with unusually high cation mobility. Theses two factors favor complete cation exchange Cation exchange has been investigated in a wide range of nanocrystals of nanocrystals and may prove sufficient to varying composition, size, and shape. Complete and fully reversible exchange overcome the fact that the exchange reaction occurs, and the rates of the reactions are much faster than in bulk cation is completely kinetically hindered at ambient exchange processes. A critical size has been identified below which the shapes temperature and pressure in the bulk of complex nanocrystals evolve toward the equilibrium shape with lowest We investigated the reaction by mixing energy during the exchange reaction. Above the critical size, the anion a solution of CdSe nanocrystals(diameter sublattice remains intact and the basic shapes of the initial nanocrystals are 4.2 nm)in toluene with a small amount of retained throughout the cation exchange. The size-dependent shape change ethanolic solution of Ag NO, under ambien can also be used to infer features of the microscopic mechanism. onditions The volume fraction of methanol in the solution mixture is about 1%: the solution contains Agt ion in a slightly larger Chemical transformations from one solid to can be accompanied by a lowering of phase amount than necessary to replace all the another via insertion and exchange of atoms transition temperatures(17, 18). With the Cd2+ ions in the nanocrystals. Methanol can be used to modify the properties of decrease in the volume, statistical averaging more strongly binds to any free binary crystalline materials (1). Recent develop of the kinetics and mechanisms over a cations in solution and thus favors the ments have enabled the production of many distribution of heterogeneous reaction sites forward reaction. A rapid (<l s) change of technologically important crystalline materi- intrinsic to the bulk solid is also reduced, solution color and complete disappearance of als in nanometer sizes, with a wide range of leading to more homogeneous molecule-like fluorescence is observed upon mixing the size-and shape-tunable properties(2-8). Of reaction kinetics and even different reaction solutions. Measurements of the x-ray diffrac particular interest is the creation of nano- mechanisms in nanocrystals(19). The opti- tion (XRD) patterns and optical absorption crystals with nonequilibrium shapes and with mal use of various chemical transformation spectra confirm that the reaction product is higher structural and compositional complex- methods to broaden the range of nanocrystal- Ag, Se(Fig. 1). The reverse reaction is done ity(9-13). In extended solids, reactions in- line materials depends on an understanding under ambient conditions by mixing Ag, Se volving chemical transformation are in general of how chemical transformations in a crys- nanocrystals with an excess amount(typical very slow because of high activation ener- talline solid will be affected by a reduction ly 50 to 100 times the initial Cd+ content )of gies for the diffusion of atoms and ions inin size. We show that cation exchange re- Cd(NO, mixture of toluene and the solid. For this reason, typical solid-phase actions can occur completely and reversibly in methanol in the presence of tributylphosphine reactions require very high temperatures or ionic nanocrystals at room temperature with (volume fraction <3%) A slower color change pressures(14-16)and therefore would seem unusually fast reaction rates. We also show back to that of CdSe nanocrystals and the to be incompatible with kinetically con- that the crystal structure and morphology of reappearance of fluorescence are observed trolled nonequilibrium nanostructures. the reaction products are strongly dependent over a period of I min. XRD patterns, optical However, in crystals only a few nano- on the size and shape of the nanocrystals. absorption, and fluorescence spectra all indi- meters in size, both the thermodynamics and The prototypical semiconductor nano- cate that CdSe is recovered from the reverse kinetics of reactions can change with size. crystal system of CaSe reacts with Agt ions cation exchange. The XRD linewidths of the For example, a large surface-to-volume ratio to yield Ag, Se nanocrystals by the forward initial and recovered case are nearly identical cation exchange reaction, and vice versa for Moreover, the absorption and fluorescence Materials Sciences Divisio the reverse cation exchange reaction. We ak positions, which show strong size wrence Berkeley chose to work with CdSe nanocrystals dependence due to the quantum confinemer National Laboratory, Berkeley, CA 94720. USA Department of Chemistry, University of California, because of the high degree of control over effect(2), are also nearly identical for the size and shape that has been achieved (2, 3). initial and recovered CaSe nanocrystals. Fi should be addressed. The conversion to Ag, Se is strongly favored nally, transmission electron micrograph E-mail: alvis@berkeley. edu by a thermodynamic driving force of about (TEM) images of the initial and recovered vwwsciencemag org SCIENCE VOL 306 5 NOVEMBER 2004 1009
well as the polycrystalline films used in today’s FeRAM. This leads to an upper bound in film thickness of about 1000 A˚. The thinness of the ferroelectric film is also constrained. First, there is an intrinsic finite-size effect in which the Tc begins to decrease at a thickness of about 100 A˚ (17, 22–24) and eventually vanishes for thicknesses in the 10 to 30 A˚ range (23, 24). The second reason is that extrinsic effects (pinholes and nonuniform thickness over the capacitor area) lead to unacceptably high leakage currents for FeRAM device operation. 26. J. W. Matthews, A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974). 27. J. Schubert et al., Appl. Phys. Lett. 82, 3460 (2003). 28. M. D. Biegalski et al., unpublished data. 29. A small amount of symmetry-forbidden SHG signal is observed even above Tc in both the BaTiO3 single crystal and thin films. This has been seen previously in single crystals, fibers, and powders of BaTiO3 and is suggested to arise from metastable micropolar regions (compositional or physical defects) in the paraelectric phase of the crystal that locally break the inversion symmetry (30). 30. G. R. Fox, J. K. Yamamoto, D. V. Miller, L. E. Cross, S. K. Kurtz, Mater. Lett. 9, 284 (1990). 31. C. B. Eom et al., Science 258, 1766 (1992). 32. B. Jaffe, W. R. Cook Jr., H. Jaffe, Piezoelectric Ceramics (Academic Press, London, 1971), p. 78. 33. V. Nagarajan et al., J. Appl. Phys. 86, 595 (1999). 34. R. Ramesh et al., Appl. Phys. Lett. 61, 1537 (1992). 35. C. B. Eom et al., Appl. Phys. Lett. 63, 2570 (1993). 36. A. Sharan et al., Phys. Rev. B 69, 214109 (2004). 37. We thank L. E. Cross, S. K. Streiffer, and S. TrolierMcKinstry for useful discussions. We also thank L. J. Belenky, D. M. Kim, and H. P. Sun for their help with the experiments. Supported by NSF through grants DMR-0313764, ECS-0210449, DMR-0103354, and DMR-0122638 and a David and Lucile Packard Fellowship (C.B.E.). K.J.C. acknowledges that this work was supported in part by the Postdoctoral Fellowship Program of Korea Science and Engineering Foundation (KOSEF). Supporting Online Material www.sciencemag.org/cgi/content/full/306/5698/1005/ DC1 Materials and Methods Figs. S1 and S2 References 26 July 2004; accepted 6 October 2004 Cation Exchange Reactions in Ionic Nanocrystals Dong Hee Son,1 Steven M. Hughes,2 Yadong Yin,1 A. Paul Alivisatos1,2* Cation exchange has been investigated in a wide range of nanocrystals of varying composition, size, and shape. Complete and fully reversible exchange occurs, and the rates of the reactions are much faster than in bulk cation exchange processes. A critical size has been identified below which the shapes of complex nanocrystals evolve toward the equilibrium shape with lowest energy during the exchange reaction. Above the critical size, the anion sublattice remains intact and the basic shapes of the initial nanocrystals are retained throughout the cation exchange. The size-dependent shape change can also be used to infer features of the microscopic mechanism. Chemical transformations from one solid to another via insertion and exchange of atoms can be used to modify the properties of crystalline materials (1). Recent developments have enabled the production of many technologically important crystalline materials in nanometer sizes, with a wide range of size- and shape-tunable properties (2–8). Of particular interest is the creation of nanocrystals with nonequilibrium shapes and with higher structural and compositional complexity (9–13). In extended solids, reactions involving chemical transformation are in general very slow because of high activation energies for the diffusion of atoms and ions in the solid. For this reason, typical solid-phase reactions require very high temperatures or pressures (14–16) and therefore would seem to be incompatible with kinetically controlled nonequilibrium nanostructures. However, in crystals only a few nanometers in size, both the thermodynamics and kinetics of reactions can change with size. For example, a large surface-to-volume ratio can be accompanied by a lowering of phase transition temperatures (17, 18). With the decrease in the volume, statistical averaging of the kinetics and mechanisms over a distribution of heterogeneous reaction sites intrinsic to the bulk solid is also reduced, leading to more homogeneous molecule-like reaction kinetics and even different reaction mechanisms in nanocrystals (19). The optimal use of various chemical transformation methods to broaden the range of nanocrystalline materials depends on an understanding of how chemical transformations in a crystalline solid will be affected by a reduction in size. We show that cation exchange reactions can occur completely and reversibly in ionic nanocrystals at room temperature with unusually fast reaction rates. We also show that the crystal structure and morphology of the reaction products are strongly dependent on the size and shape of the nanocrystals. The prototypical semiconductor nanocrystal system of CdSe reacts with Agþ ions to yield Ag2Se nanocrystals by the forward cation exchange reaction, and vice versa for the reverse cation exchange reaction. We chose to work with CdSe nanocrystals because of the high degree of control over size and shape that has been achieved (2, 3). The conversion to Ag2Se is strongly favored by a thermodynamic driving force of about –1000 kJ/mol in the bulk (20, 21). Ag2Se also exhibits an interesting temperature-dependent polymorphism: The superionic conducting phase transition occurs at a relatively low temperature of 133-C in the bulk phase (22). Thus, it may be possible to prepare Ag2Se with unusually high cation mobility. These two factors favor complete cation exchange in nanocrystals and may prove sufficient to overcome the fact that the exchange reaction is completely kinetically hindered at ambient temperature and pressure in the bulk. We investigated the reaction by mixing a solution of CdSe nanocrystals (diameter 4.2 nm) in toluene with a small amount of methanolic solution of AgNO3 under ambient conditions. The volume fraction of methanol in the solution mixture is about 1%; the solution contains Agþ ion in a slightly larger amount than necessary to replace all the Cd2þ ions in the nanocrystals. Methanol more strongly binds to any free binary cations in solution and thus favors the forward reaction. A rapid (¡1 s) change of solution color and complete disappearance of fluorescence is observed upon mixing the solutions. Measurements of the x-ray diffraction (XRD) patterns and optical absorption spectra confirm that the reaction product is Ag2Se (Fig. 1). The reverse reaction is done under ambient conditions by mixing Ag2Se nanocrystals with an excess amount (typically 50 to 100 times the initial Cd2þ content) of Cd(NO3)2 in a mixture of toluene and methanol in the presence of tributylphosphine (volume fraction G3%). A slower color change back to that of CdSe nanocrystals and the reappearance of fluorescence are observed over a period of 1 min. XRD patterns, optical absorption, and fluorescence spectra all indicate that CdSe is recovered from the reverse cation exchange. The XRD linewidths of the initial and recovered case are nearly identical. Moreover, the absorption and fluorescence peak positions, which show strong size dependence due to the quantum confinement effect (2), are also nearly identical for the initial and recovered CdSe nanocrystals. Finally, transmission electron micrograph (TEM) images of the initial and recovered 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 2 Department of Chemistry, University of California, Berkeley, CA 94720, USA. *To whom correspondence should be addressed. E-mail: alivis@berkeley.edu R EPORTS www.sciencemag.org SCIENCE VOL 306 5 NOVEMBER 2004 1009 on February 4, 2008 www.sciencemag.org Downloaded from
REPORTS CaSe(Fig. 1, A and C)also indicate that the This indicates that the effective tropic nonequilibrium shapes, such size and shape are preserved. This remarkable barrier is much lower in nanomo eres preservation of volume over multiple com- crystals than in larger systems, theret Figure 2 shows TEM plete exchange cycles demonstrates a funda- facilitating molecule-like dynamic CdSe nanorods of different sizes and their mental feature of cation exchange reactions in um between the reactant and product phases. transformed Ag, Se nanocrystals. It is readily crystals: The number of anions per nano- High-resolution TEM images of the apparent that thinner nanorods(Fig. 2A) at room temperature in the nanocrystals is crystal is not necessarily preserved in the that the anion sublattice is completely surprising. In a separate experiment with recovered sample. Figure IG shows an ex- disrupted during the reaction(Fig. 2B). micrometer-sized powders of Case, we ample of the recovered CaSe nanocrystals Thicker nanorods maintain their nonequilib found the cation exchange to be virtually without noticeable structural defects; Fig 1H rium shapes( Fig. 2, E, F, L, and D). The prohibited under similar experimental con- shows one with stacking faults. Moreover, degree of size control in this system is ditions over a period of weeks(24). In the although it is observed much less frequently, sufficiently high that it is possible to captur bulk phase, these materials were typically a coagulated crystal formed from the merging intermediate cases(Fig. 2, C, D, G, and H subjected to molten salts at very high of two smaller initial nanocrystals(Fig. In) where the shape partially anneals, yielding temperatures to effect the exchange of cat- can be found, which shows a distinct boundary Ag, Se of rather irregular shape. Thus, there ions(14). The reaction time for the forward between two different crystal domains. This exists a certain size limit below which the cation exchange reaction ( 1 s) is much raises an important question about whether the structural rigidity of the anion sublattice is shorter than can be deduced from the cation exchange reaction is topotaxial, where not maintained during the cation exchang reaction time obtained in related systems of the structural rigidity of the anion subframe is reaction. The width and length dependence N larger size [e. g, -10 hours for 100-nm maintained, or whether substantial morpholog- of the morphology changes we observed t Cds wire(25))on the basis of simple scaling ical reorganization accompanies the reaction. (e.g, compare Fig. 2, E and F, with Fig. 2 of the size in diffusion -controlled reaction schemes, where the reaction time is roughly this question, we performed cation exchange proportional to the square of the size(26). reactions on nanocrystals with highly aniso Cdse Ag2Se : CaSe D REcovered Cd 59o品E853sE9品°5 40 nm X-ray diffraction Fluoresence 203040506070450550650750850400500600700800 3 nm Diffraction angle(20) Fig. 1. TEM images of(A)initial CdSe(diameter 4.2 nm), (B)Ag, Se transformed from the forward cation exchange reaction, and()recovered CdSe nanocrystals from ation exchange Fig. 2. TEM images of CdSe nanorods of differ- reaction.(D to F)XRD patterns, fluorescence emission, and optical ectra of initi rger than 6 nm in diameter. An additional nanorods are(A)3. 2 nm x 15 nm, (C)3.4 nr fluorescence emission feature near 700 nm seen in the recovered CdSe(e)is due to the increased 17 nm, (E)5.3nm x 29 nm, (G 3.6nm x 37 nm. emission. Vertical lines in(E)and()are a guide for the comparison of peak positions. and(0)5.6 58 of two 6-nm nanocrystals o. The distinct boundary of the different structures is clea 1010 5NovemBer2004Vol306SciEncewww.sciencemagorg
CdSe (Fig. 1, A and C) also indicate that the size and shape are preserved. This remarkable preservation of volume over multiple complete exchange cycles demonstrates a fundamental feature of cation exchange reactions in nanocrystals: The number of anions per nanocrystal is invariant over multiple cycles (23). The speed and reversibility of the reaction at room temperature in the nanocrystals is surprising. In a separate experiment with micrometer-sized powders of CdSe, we found the cation exchange to be virtually prohibited under similar experimental conditions over a period of weeks (24). In the bulk phase, these materials were typically subjected to molten salts at very high temperatures to effect the exchange of cations (14). The reaction time for the forward cation exchange reaction (¡ 1 s) is much shorter than can be deduced from the reaction time obtained in related systems of larger size Ee.g., È10 hours for È100-nm CdS wire (25)^ on the basis of simple scaling of the size in diffusion-controlled reaction schemes, where the reaction time is roughly proportional to the square of the size (26). This indicates that the effective reaction barrier is much lower in nanometer-sized crystals than in larger systems, therefore also facilitating molecule-like dynamic equilibrium between the reactant and product phases. High-resolution TEM images of the recovered CdSe spheres indicate that the wurtzite structure of the initial CdSe nanocrystal is not necessarily preserved in the recovered sample. Figure 1G shows an example of the recovered CdSe nanocrystals without noticeable structural defects; Fig. 1H shows one with stacking faults. Moreover, although it is observed much less frequently, a coagulated crystal formed from the merging of two smaller initial nanocrystals (Fig. 1I) can be found, which shows a distinct boundary between two different crystal domains. This raises an important question about whether the cation exchange reaction is topotaxial, where the structural rigidity of the anion subframe is maintained, or whether substantial morphological reorganization accompanies the reaction. To obtain a more conclusive answer to this question, we performed cation exchange reactions on nanocrystals with highly anisotropic nonequilibrium shapes, such as rods, tetrapods, and hollow spheres (3, 7, 9). Figure 2 shows TEM images of the initial CdSe nanorods of different sizes and their transformed Ag2Se nanocrystals. It is readily apparent that thinner nanorods (Fig. 2A) reorganize to the equilibrium spherical shape during the forward reaction, which indicates that the anion sublattice is completely disrupted during the reaction (Fig. 2B). Thicker nanorods maintain their nonequilibrium shapes (Fig. 2, E, F, I, and J). The degree of size control in this system is sufficiently high that it is possible to capture intermediate cases (Fig. 2, C, D, G, and H) where the shape partially anneals, yielding Ag2Se of rather irregular shape. Thus, there exists a certain size limit below which the structural rigidity of the anion sublattice is not maintained during the cation exchange reaction. The width and length dependence of the morphology changes we observed (e.g., compare Fig. 2, E and F, with Fig. 2, G and H) also suggest that nanorod thickness Fig. 1. TEM images of (A) initial CdSe (diameter 4.2 nm), (B) Ag2Se transformed from the forward cation exchange reaction, and (C) recovered CdSe nanocrystals from the reverse cation exchange reaction. (D to F) XRD patterns, fluorescence emission, and optical absorption spectra of initial CdSe (red), Ag2Se (blue), and recovered CdSe (green) nanocrystals, respectively. In the recovered CdSe, the peak positions of the emission and absorption show a slight redshift from those of the initial CdSe, which becomes negligible for nanospheres larger than 6 nm in diameter. An additional fluorescence emission feature near 700 nm seen in the recovered CdSe (E) is due to the increased surface trap emission. Vertical lines in (E) and (F) are a guide for the comparison of peak positions. (G to I) High-resolution TEM images of the recovered CdSe nanocrystals without structural defects (G), nanocrystals with stacking faults (H), and a large coagulated nanocrystal formed by merging of two 6-nm nanocrystals (I). The distinct boundary of the different structures is clearly visible. Fig. 2. TEM images of CdSe nanorods of different sizes (A, C, E, G, and I) and their transformed Ag2Se crystals (B, D, F, H, and J). The average dimensions (width length) of CdSe nanorods are (A) 3.2 nm 15 nm, (C) 3.4 nm 17 nm, (E) 5.3 nm 29 nm, (G) 3.6 nm 37 nm, and (I) 5.6 nm 58 nm. As the nanorods become thicker from (A) to (I), the shape change during the cation exchange reaction is suppressed. R EPORTS 1010 5 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org on February 4, 2008 www.sciencemag.org Downloaded from
REPORTS is a more relevant variable than nanorod Initial Cds hollow sphere Ag2 S hollow sphere Recovered Cds hollow sphere length in determining the shape change Other even more complex and high-energy nonequilibrium shapes of nanocrystals, such as hollow spheres and branched tetrapods(9, 12) maintain their overall shapes throughout complete cation exchange cycles, provided they have a dimension thicker than 5 nm XsEE9iRes Fig. 3). Cds hollow spheres maintain overall Ag?Te tetrapods morphology during the cation exchange, although a smoothing of the rough surface d a small increase in volume are observed In the case of CdTe tetrapods, slight expan- sion(5%)of the width of each branch is observed after the transformation to Ag te The observed changes in size can be counted for by changes in the crystal ur cell symmetry and lattice parameters during the ig. 3. (A to C) TEM images of (A)initial Cds hollo transformation. In Fig. 4A, the structures of cation exchange of CdS, and(C)recovered CdS from the reverse change reaction. (D to Se2- sublattices in wurtzite CdSe and various TEM images of(D) initial CdTe tetrapods, (E) Ag, Te tetrapods from cation exchange phases of Ag, Se are presented to show the CdTe, and (F)recovered CdTe from the reverse cation exchange topotaxial relationship between the reactant and product phases and associated changes in dimension. Small increases in the width observed in the transformation of thicker cdse rods to tetragonal Ag, Se rods(Fig. 2, F and D) CdSe reflect the changes Ag2Se of the crystal structure as shown in Fig. 4A. Wurtzite Tetragonal Cubic These observations reveal a second fundamen. tal feature of cation exchange in nanocrystal The anion sublattice connectivity is preserved ···中中·:·····中平 during exchange in large nanocrystals 5o品Eg品 There are two possible explanations for the crossover of morphology change at widths of 4 to 5 nm observed in CdSe nanorods. First, the 4“…:# structure of the reaction product is progres- sively changing from a cubic to a tetragonal 4444 phase and eventually adopts an orthorhombic 444 44444,444 phase in the bulk material in crystals of larger 4444·,,· size. For small spheres and thin, short rods of 4444 44 CdSe, like those in Figs. I and 2A, the Ag, 4444a4a product is cubic. For thicker rods such as E and J, the Ag tetragonal (fig. S1). The cubic phase of Ag, Se is a superionic conductor, with a diffusion Wx1.0 15 Wx1.15 coefficient for Ag+ ions similar to that in x1.0 x1.0 liquid solvent(10 cm/s), unlike in other phases of Ag Se(22, 27). Because the smaller CdSe nanocrystals are those that form the reaction zone width a crystal size reaction zone width crystal size cubic phase of Ag, Se and lose structural rigidity, it is conceivable that the high mobility of the Agt ions influences the orphology of the crystal during the reaction. However, we consider this unlikely, because it H●。【圆 is the anion sublattice that forms the structural framework of the crystal in the cation final nitial final exchange reaction, and this should occur Fig. 4.(A) Comparison of the projection of the selenium anion sublat Am山以如 i we anions in different atomic layers inthe direorods. Dark and light colors are used to distinguish the regardless of the degree of cation mobility consider that 4 to 5 nm is comparable to the best of a unit cell is also shown superimposed on the anion sublattice structure to facilitate the available estimates of the width of the reaction arison. Anion sublattices show simple topotaxial relationships, where the transformation phase transformation and microscopic morphol- perpendicular to the long axis(c axis)of CdSe. (B)Illustration of the size-dependent morphology ogy changes in the bulk have been considered product phase, respectively The green region indicates the reaction zone where the structural extensively. The evolution of the reaction front equilibrium is not yet established. www.sciencemag.orgSciEnceVol3065NovemBer2004 1011
is a more relevant variable than nanorod length in determining the shape change. Other even more complex and high-energy nonequilibrium shapes of nanocrystals, such as hollow spheres and branched tetrapods (9, 12), maintain their overall shapes throughout complete cation exchange cycles, provided they have a dimension thicker than È5 nm (Fig. 3). CdS hollow spheres maintain overall morphology during the cation exchange, although a smoothing of the rough surface and a small increase in volume are observed. In the case of CdTe tetrapods, slight expansion (È5%) of the width of each branch is observed after the transformation to Ag2Te. The observed changes in size can be accounted for by changes in the crystal unit cell symmetry and lattice parameters during the transformation. In Fig. 4A, the structures of Se2– sublattices in wurtzite CdSe and various phases of Ag2Se are presented to show the topotaxial relationship between the reactant and product phases and associated changes in dimension. Small increases in the width observed in the transformation of thicker CdSe rods to tetragonal Ag2Se rods (Fig. 2, F and J) reflect the changes in dimension upon change of the crystal structure as shown in Fig. 4A. These observations reveal a second fundamental feature of cation exchange in nanocrystals: The anion sublattice connectivity is preserved during exchange in large nanocrystals. There are two possible explanations for the crossover of morphology change at widths of 4 to 5 nm observed in CdSe nanorods. First, the structure of the reaction product is progressively changing from a cubic to a tetragonal phase and eventually adopts an orthorhombic phase in the bulk material in crystals of larger size. For small spheres and thin, short rods of CdSe, like those in Figs. 1 and 2A, the Ag2Se product is cubic. For thicker rods such as those in Fig. 2, E and J, the Ag2Se is tetragonal (fig. S1). The cubic phase of Ag2Se is a superionic conductor, with a diffusion coefficient for Agþ ions similar to that in liquid solvent (È10–5 cm2/s), unlike in other phases of Ag2Se (22, 27). Because the smaller CdSe nanocrystals are those that form the cubic phase of Ag2Se and lose structural rigidity, it is conceivable that the high mobility of the Agþ ions influences the morphology of the crystal during the reaction. However, we consider this unlikely, because it is the anion sublattice that forms the structural framework of the crystal in the cation exchange reaction, and this should occur regardless of the degree of cation mobility. A more likely explanation arises when we consider that 4 to 5 nm is comparable to the best available estimates of the width of the reaction zone. Solid-state reactions and the associated phase transformation and microscopic morphology changes in the bulk have been considered extensively. The evolution of the reaction front Fig. 3. (A to C) TEM images of (A) initial CdS hollow spheres, (B) Ag2S hollow spheres produced from cation exchange of CdS, and (C) recovered CdS from the reverse cation exchange reaction. (D to F) TEM images of (D) initial CdTe tetrapods, (E) Ag2Te tetrapods produced from cation exchange of CdTe, and (F) recovered CdTe from the reverse cation exchange reaction. Fig. 4. (A) Comparison of the projection of the selenium anion sublattice in the wurtzite CdSe nanorod and different phases of Ag2Se nanorods. Dark and light colors are used to distinguish the anions in different atomic layers in the direction corresponding to the long axis of CdSe. Projection of a unit cell is also shown superimposed on the anion sublattice structure to facilitate the comparison. Anion sublattices show simple topotaxial relationships, where the transformation between different structures can be accomplished by movement of ions mostly in the planes perpendicular to the long axis (c axis) of CdSe. (B) Illustration of the size-dependent morphology change during the reaction. Orange and blue colors indicate the regions of initial reactant and final product phase, respectively. The green region indicates the reaction zone where the structural equilibrium is not yet established. R EPORTS www.sciencemag.org SCIENCE VOL 306 5 NOVEMBER 2004 1011 on February 4, 2008 www.sciencemag.org Downloaded from
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.sciencemagorg
and 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 ratelimiting process of the reaction. Second, at the early stage of the reaction, the whole crystal can be in a structurally nonequilibrium state where both the cations and anions are mobile (31, 32). This can result in a change of the morphology to the thermodynamically 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 nanocrystals, investigated mainly with Agþ ion in this study, can easily be extended to exchange 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, possibly 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. Although the finite width of the reaction zone may impose a limit on the size of the nanocrystal 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 contact 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 NernstPlanck 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 confirmed 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 Desorption 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 materials) satisfy the criteria of size, recharge kinetics, cost, and safety required for use in transportation (1). An adsorbent material with porosity on the molecular scale could R EPORTS 1012 5 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org on February 4, 2008 www.sciencemag.org Downloaded from
