Review pubs.acs.org/cm
Hollow Nanocrystals through the Nanoscale Kirkendall Effect Wenshou Wang, Michael Dahl, and Yadong Yin* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: Colloidal hollow nanocrystals with controlled hollow interior and shell thickness represent a class of important nanostructured materials, because of their promising applications for nanoreactors, drug delivery, and catalysis. Since the first report in 2004 on the synthesis of CoS and CoO hollow nanocrystals by sulfidation and oxidation of Co nanocrystals, several different kinds of hollow nanocrystals have been prepared by a similar approach that involves the nanoscale Kirkendall effect. In this review, we introduce the application of this well-known classical phenomenon in metallurgy in the synthesis of hollow nanocrystals. We start with a brief introduction to the synthesis of hollow nanocrystals, then discuss the concepts and applications of nanoscale Kirkendall effect for the synthesis of hollow nanocrystals, and finally touch on the extension of the process to the formation of nanotubes. We conclude with a summary and our personal perspectives on the directions in which future work on this field might be focused. KEYWORDS: hollow nanocrystals, nanoscale, Kirkendall effect, diffusion, vacancy, voids
1. INTRODUCTION Colloidal inorganic nanocrystals have been intensively studied in the past two decades because of their interesting size/shapedependent electrical, optical, magnetic, and chemical properties that cannot be achieved by their bulk counterparts.1,2 Progress in colloidal synthesis, particularly by using the thermolysis method, has shown that various types of high-quality colloidal nanocrystals can now be successfully produced with wellcontrolled size, shape, and surface properties.3−5 Integrating multiple functionalities into individual colloidal nanocrystals through various chemical processes, however, has been a challenging and exciting field in materials science. Colloidal nanocrystals with controlled hollow interior and shell thickness are thus attractive in both fundamental research and practical applications because of their large surface area and low material density, which can serve as ideal building blocks for fabrication of lightweight structural materials and in promising applications for nanoreactors, drug delivery, and catalysis.6,7 The most popular approach for the synthesis of hollow nanoparticles involves coating of the desired materials onto sacrificial templates.7 Hard templates, such as polymers, silica, carbon, and metal oxide nanoparticles, or soft ones, such as emulsion micelles, and even gas bubbles, facilitate the creation of a core− shell composite with subsequent selective removal of the template core via chemical etching or thermal decomposition. However, in some cases, the conventional template-based strategy can be inconvenient especially when the templates themselves are difficult to fabricate in large quantity. Recently, novel self-templated methods, such as those based on nanoscale Kirkendall effect,8 Ostwald ripening,9 galvanic replacement,10 and surface-protected etching,11 have been developed to synthesize various hollow micro/nanospheres. These strategies are particularly attractive because the hollow shells are produced via chemical reaction while the template can be © XXXX American Chemical Society
depleted spontaneously during the hollowing process. It should be mentioned that the hollow nanoparticles obtained via the template-mediated approaches and Ostwald ripening process are often in the submicrometer range, mostly several hundred nanometers, which are larger than the typical size for nanocrystals. Methods based on the nanoscale Kirkendall effect exhibit a number of advantages for synthesizing hollow nanocrystals: high-quality colloidal nanocrystals with uniform sizes and shapes are well-synthesized and their reaction with suitable materials in combination with the nanoscale Kirkendall effect can give rise to highly crystalline hollow nanocrystals with a high yield even in the quantum regime.12,13 In this review, we will focus on the colloidal synthesis of high-quality hollow nanocrystals with sizes smaller than 200 nm through nanoscale Kirkendall effect. This article is organized as follows: we begin with a brief introduction to the nanoscale Kirkendall effect. We then explicitly discuss the recent investigations in this area to introduce basic concepts and methodologies of this new synthetic approach for creating hollow interiors of nanocrystals. Then, we highlight some newly developed nanotubes formed based on nanoscale Kirkendall effect. We conclude with a summary and our personal perspectives on the directions in which future work on this field might be focused. Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: September 24, 2012 Revised: November 28, 2012
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2. KIRKENDALL EFFECT The Kirkendall effect is a classical phenomenon in metallurgy, which normally refers to a nonequilibrium mutual diffusion process through the interface of coupled materials. Prior to 1940s, it was believed that atomic diffusion in metals and alloys took place via a direct exchange or ring mechanism. Both mechanisms assume an interchange of A and B atoms with equal diffusion coefficients (DA = DB). In 1942, Kirkendall for the first time suggested a difference in diffusion rates of two components (DA ≠ DB), based on interdiffusion experiments on copper and brass diffusion couple, which were welded together and subjected to elevated temperature.14 In 1947, Smigelskas and Kirkendall repeated the experiment and demonstrated, for the first time, that diffusive processes operating in a diffusion couple formed by the joining of two compositionally different specimens, such as brass and copper, result in a net directional flow of matter.15 At elevated temperatures, the brass-copper interface was observed to migrate inward and the interface shrinks as a function of time, suggesting that the outward diffusion of zinc into copper is faster than inward diffusion of copper into brass. Experimental observations of unequal matter flows during interdiffusion, now commonly summarized under the term Kirkendall effect, provided the first evidence for vacancymediated hopping of atoms being the predominant mechanism for diffusion in crystalline materials. Figure 1 shows the schematic illustration of the Kirkendall effect in the bulk phase diffusion couple A−B. Because metal A
hollow shells with irregular skeletal remainders inside were produced by subsequent sintering. It was understood that in the core/shell microparticles, excess vacancies annihilate at structural imperfections (such as edge dislocation and grain boundaries) at the core/shell interface, generating a radial stress. As a result of this, stress cracks form and separate the core from the shell.
3. NANOSCALE KIRKENDALL EFFECT From the traditional viewpoint, the formation of Kirkendall voids in alloys and solders is not a desirable process for metallurgical manufacturing because the porosity deteriorates the mechanical properties of the interface or causes wire bond failure in integrated circuits. The main technological motivation for studying the Kirkendall effect in the past was to reduce its negative effect on adhesion or mechanical reliability at the respective interface. For example, engineers try to avoid this effect by introducing diffusion barrier layers between copper and bronze in the multifilamentary superconducting composite. However, after more than 60 years, the Kirkendall effect has received scientific attention in the positive context and presented its “constructive” applications for the generation and design of hollow structures in the field of nanomaterials. Figure 2 shows the schematic illustration of nanoscale Kirkendall effect for the formation of nanocrystals with hollow
Figure 2. Schematic illustration of nanoscale Kirkendall effect for the formation of hollow nanocrystals.
interior. For nanoscale Kirkendall effect in a nanocrystal solution system, the formation of hollow nanocrystals is a onepot, two-step process. The first step involves the synthesis of solid nanocrystals of at least one of the elements of the final shell (A). In the next step, a second element (B) in solution or gas phase is reacted with A to yield an AB compound. During the second step, the solid core nanocrystal’s surface (A) is usually first reacted with reagents (B) to produce a layer of shell materials (AB). The direct conversion of core material to shell material is therefore hindered by the layer and further reaction will continue by the diffusion of atoms or ions through the interface. If the diffusion rate of core material is faster than that of shell material, the preferred outward diffusion of atoms or ions from core material to shell material leads to a net material flux across the nanocrystal interface and simultaneously results in a flow of fast-moving vacancies to the vicinity of the solid− liquid interface. Therefore, the hollow voids are formed through coalescence of the vacancies based on nanoscale Kirkendall effect.
Figure 1. Schematic illustration of a nonequilibrium lattice diffusion at the interface in bulk phase by Kirkendall effect (JA, JB, and Jv are diffuse fluxes of metal A, B, and void, respectively).
diffuses faster into B than that of B into A, the unequal material flow is simultaneously accompanied by vacancy diffusion. Accordingly, condensation of excess vacancies can give rise to void and even gap formation in the zone of the faster diffusing component of the bulk phase diffusion. A typical example is the sulfidation of bulk cobalt by sulfur at temperatures of 773− 1023 K and sulfur partial pressure (1 to 1 × 104 Pa).16 Because outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms, a large gap with a width of ∼40 μm forms between the Co substrate and Co9S8 layer after reaction. Aldinger studied the Kirkendall effect in materials with a reduced dimension and spherical geometry, where the voids tend to collapse in the center to eventually form hollow shells.17 Be microspheres (∼33 μm diameter) were used as the starting material and coated with Ni or Co to form core/shell particles. Because of the dominant outward diffusion of Be, BeNi alloy
4. FORMATION OF HOLLOW NANOCRYSTALS In the past, the nanoscale Kirkendall effect has occurred in many reactions involving nanocrystals but the mechanism was not properly attributed to the interdiffusion of the components B
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cobalt nanocrystals were first covered with a cobalt sulfide shell. Next, the diffusion of cobalt and sulfur atoms in opposite directions took place at the surface of cobalt and cobalt sulfide. As the reaction proceeded, voids formed in the cobalt side of the interface because the outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms. The sulfide shells were composed of either the linnaeite (Co3S4) or the cobalt pentlandite (Co9S8) phase, depending precisely on the sulfur-to-cobalt molar ratio used in the synthesis. Kinematical diffraction simulations of the XRD pattern of cobalt sulfide hollow spheres indicated that observed peak widths were not consistent with a single crystal hollow shell of average dimensions seen in TEM images (Figure 3B). Instead, a satisfactory fit to experimental XRD peak widths was obtained by assuming ∼4 nm cubic crystalline domains. It can be concluded then, that each shell is composed of multiple small crystallites. High resolution transmission electron microscopy (HRTEM) confirmed that the great majority of sulfide hollow nanocrystals were polycrystalline (Figure 3C) although singlecrystalline shells are also occasionally observed. Typical domain sizes seen in HRTEM images were highly consistent with interpretation of the XRD patterns. Close inspection of the HRTEM images shows that each crystal domain extends from the outside surface to the inside of the shell, with a neighboring crystal domain present on each side across a grain boundary. If defects such as grain boundaries and cracks formed during shell growth are indeed predominantly aligned along the radial direction, this would provide fast diffusion paths between the inside and outside of the shell. The reaction between Co nanocrystals and a suspension of fine selenium powder in DCB also yielded CoSe hollow nanocrystals.8 The selenization reaction was slow enough such that it is possible to catch particles at the different stages of the hollowing process (Figure 4). The geometrical evolution of the
during the reactions. For example, Derouane et al. observed that globular copper particles supported on a magnesium oxide substrate undergo a morphological transformation from discrete globular crystallites to toroidal structures when exposed to oxygen at 200 °C.18 Although the formation of toroidal morphology was attributed to the wetting and spreading of the oxide species over the support surface, it may also involve the nanoscale Kirkendall effect. Moreover, the nanoscale Kirkendall effect might have also been the driving force for the formation of some other hollow nanocrystals. Murphy et al. reported a solution-phase synthesis of monodisperse Cu2O nanocubes with hollow interior by reducing copper(II) salts with sodium ascorbate in water.19 The formation mechanism of hollow interiors, although unnoted by the authors, may also relate to the nanoscale Kirkendall effect. Using nanoscale Kirkendall effect to synthesize hollow nanocrystals now has embodiments in compound systems, mostly in binary metal sulfides, selenides, tellurides, oxides, nitrides and phosphines, and ternary metal (mixed) oxides and sulfides. Typically, a metal nanocrystal is exposed to sulfur, selenium, oxygen, phosphorus precursors under elevated temperature resulting in a diffuse couple. When outward diffusion of the metal cations is much faster than the inward diffusion of anions, an inward flux of vacancies accompanies the outward metal cation flux to balance the diffusivity difference. When the vacancies become supersaturated, they coalesce into a void and the reaction products are hollow nanocrystals with binary compositions. The recent progress in hollow nanocrystals synthesized via nanoscale Kirkendall effect can be categorized into two strategies. One is the reaction of metal nanocrystals (Co, Ni, Ag, Fe, et al.) to a binary compound by sulfidation, oxidation, phosphination, or nitridation process. The other is to convert one binary compound into another or into a ternary metal (mixed) oxide or sulfide, which involves diffusions of different rates during a substitution reaction. 4.1. Reaction of Metal Nanocrystals. Metal Chalcogenides. In 2004, we first demonstrated the nanoscale Kirkendall effect for the formation of cobalt sulfide nanocrystals with a hollow interior by reacting colloidal cobalt nanocrystals with elemental sulfur at 180 °C in o-dichlorobenzene (DCB).8 Spherical nanocrystals of ε-Co were first prepared by decomposing Co2(CO)8 in refluxing DCB (Figure 3A), and then reacting with molecular sulfur which had been dissolved in a small amount of DCB and injected into the solution. In aliquots collected immediately following the injection of sulfur, a well-defined hollow morphology was adopted by the cobalt sulfide nanocrystals (Figure 3B). During the transformation,
Figure 4. TEM images showing the evolution of CoSe hollow nanocrystals with time. The reaction was initiated by injection of a suspension of selenium in o-dichlorobenzene into a cobalt nanocrystal solution at 182 °C. TEM images from top-left to bottom-right display the morphology of nanocrystals after reaction for 0 s, 10 s, 20 s, 1 min, 2 min, and 30 min. Adapted with permission from ref 8. Copyright 2004 American Association for the Advancement of Science.
system supports a Kirkendall-type mechanism driving the formation of hollow nanocrystals. The emergence of filaments connecting the core and the growing shell may be attributed to preferential nucleation of voids near the core−shell interface; vacancy concentrations build up first in that region and the interfacial energy lowers the activation energy for void nucleation. As the reaction proceeds further, the core and the shell are connected by thinner and fewer filaments of material, eventually resulting in the complete consumption of the Co core and the filaments.
Figure 3. Hollow nanocrystals of (A) cobalt and (B) cobalt sulfide nanocrystals. An HRTEM image is included in (C) to show the polycrystalline structure of the shell. The hollow cobalt sulfide nanocrystals were produced by reacting Co nanocrystals with elemental sulfur in o-dichlorobenzene at 182 °C. Adapted with permission from ref 8. Copyright 2004 American Association for the Advancement of Science. C
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The nanoscale Kirkendall effect has been a very general procedure for the production of hollow nanocrystals. Gao and co-workers reported a similar experiment of wires constructed by CoSe2 hollow nanocrystals by selenization of Co nanocrystals.20 Co nanocrystals of ∼20 nm were dispersed in a solvent of DCB using trioctylphosphine oxide (TOPO) as a surfactant. Since Co nanocrystals were assembled via magnetic-dipole interaction into necklacelike structures, the injection of a DCB solution of selenium into the dispersion of Co nanocrystals under reflux at 455 K resulted in interconnected hollow nanocrystals of crystalline CoSe2. After being added into the dispersion, selenium reacts with cobalt, and a thin layer of selenium or CoSe2 grows around the cobalt nanocrystals, without disintegration of the wires. Further reaction between selenium and cobalt through the nanoscale Kirkendall effect process maintains the preassembled nanostructures and affords a 1D assembly of CoSe2 hollow nanocrystals. By replacing selenium by sulfur or tellurium, wires of Co3S4 and CoTe hollow nanocrystals can also be prepared with a similar morphology to CoSe2.20 Cabot et al. reported asymmetric Cd/CdS partial hollow nanospheres through nanoscale Kirkendall effect.21 Partial sulfidation of Cd nanospheres leads to the asymmetric hollow spheres containing a single off-center void region and a single off-center region of unreacted Cd, both with spherical cap geometry. The growth process shows that in the initial stages of Cd sulfidation, vacancies coalesce into voids extending all over the Cd/CdS interface. However, in a slightly advanced stage of sulfidation, vacancies are found to coalesce into a single void with spherical cap shape adhering to one side of the interior hollow CdS shell. This particle arrangement of the core material is explained by the different time scales of Cd diffusion inside the particle and across the shell (The Cd diffusivity through the shell is approximately 4 orders of magnitude slower than the Cd self-diffusivity). Tang et al. reported the formation of Ag2Se hollow nanocrystals through the reaction of selenium with two types of Ag nanoparticles (NP), one single crystalline (SC) and one multiply twinned (MT).22 They found in the reactivity studies that upon exposure of Ag nanocrystals to selenium generated from photodissociation of CSe2 on the surface, the singlecrystal nanocrystals become Ag2Se hollow nanocrystals through the nanoscale Kirkendall effect (Figure 5A), in which the diffusion of silver atoms is faster than that of selenium atoms during reaction, leading to vacancy formation and condensation into one hole. However, the movement of atoms along the defects in the multiply twinned nanocrystals allows interdiffusion of the Se into the Ag, thus creating solid, homogeneous Ag2Se nanocrystals (Figure 5B). This result demonstrates that void formation may be suppressed by the defects contained in the precursor nanoparticles. On the other hand, porosity upon oxidation may become more pronounced for samples with a high surface-to-volume ratio and greater crystalline perfection because of the confinement of vacancies into smaller volumes, and a relative enhancement of vacancy injection rate. Attainment of very pronounced porosity in sulfide nanoshells may thus be attributed to further increase of the surface-tovolume ratio and crystalline perfection when the starting materials are in the form of colloidal nanocrystals. Through UV photodissociation of adsorbed CSe2 on the Ag nanocrystal core surface, Ag@Ag2Se core−shell and Ag2Se hollow nanocrystals are produced, because Ag atoms diffuse faster outward from the core than Se atoms diffuse inward toward the core.23 The net
Figure 5. Chemical transformation of Ag SC- and MT-NPs to Ag2Se nanostructures. A, typical HRTEM image (left) and single-particle electron diffraction (SPED, lower right inset) of SC-NP and schematic diagram of atom diffusion paths (middle); typical HRTEM image (right) and SPED (lower left inset) of SC-Ag2Se NP with hollow core. B, Typical HRTEM image (left) and SPED (lower right inset) of MTNP and schematic diagram of different atom diffusion paths (middle); typical HRTEM image (right) and SPED (lower left inset) of solid SC-Ag2Se NP. Blue and orange spheres represent silver and selenium atoms, respectively. Yellow and red arrows represent outward silver diffusion and inward selenium diffusion through the lattice, respectively. The dark gray spheres (behind some of the orange spheres) highlight silver atoms in the twinning boundaries, providing another fast atom diffusion path for both silver and selenium atoms (white arrows). Scale bar of HRTEM images: 5 nm.22
effect was that all the Ag would move away from the center and get converted into Ag2Se, leaving a hollow core behind. Wang and co-workers prepared PbS hollow nanospheres ∼330 nm in size through sulfidation of Pb nanospheres of ∼200 nm with sulfur vapor under argon at 300 °C.24 The intermediates collected at different reaction times show that a thin layer of polycrystalline PbS can be identified surrounding the Pb core at a reaction time of 5 min. Because Pb atoms diffuse faster than S atoms, outward flow of Pb atoms through the PbS shell resulted in the supersaturation of lattice vacancies in Pb, which condensed to form a void in each core during the reaction. Significant inward sulfur transport could also occur through the grain boundaries of polycrystalline PbS shells. As a result, the increase in shell thickness is believed to originate from both reactions that occurred on the inner and outer surfaces of PbS shell. However, Pb@Ag core−shell particles with cracked shells have been observed instead owing to the faster diffusion of Ag compared to Pb.24 These results demonstrate that voids can be formed either inside the core or in the shell, depending on which component (i.e., the core or shell) has a higher diffusion rate. Conversely, by using selenium or tellurium as core materials to react with metal ions, metal chalcogenide hollow nanocrystals can also be prepared via the nanoscale Kirkendall effect. PbSe and Ni2Se hollow nanospheres with diameters of 150− 180 nm are synthesized by a polyol process from a mixture of Pb(CH3COO)2 or NiCl2, SeO2, hydrazine, and polyvinyl pyrrolidone (PVP) in ethylene glycol (EG).25 Se solid nanospheres with diameters of 120−140 nm are first formed by reducing SeO2 with hydrazine. Then, the generated Pb or Ni atoms obtained from the reduction of Pb2+ or Ni2+ by hydrazine will randomly attach to the surface of Se solid spheres, which leads to the formation of core−shell spheres with few little D
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reaction also transforms the originally smooth Te surface into pleated structures, producing “accordion” shaped Bi2Te3−Te nanostructures. Metal Oxides. We have also demonstrated the formation of CoO hollow nanocrystals based on nanoscale Kirkendall effect via flowing O2/Ar mixture gas into Co colloidal solution.8 The hollowing process is essentially the same as that in cobalt sulfide and selenide hollow nanocrystals. The synthesis of hollow iron oxide nanocrystals based on nanoscale Kirkendall effect has also been explored by several groups in depth because of their interesting superparamagnetic properties. Similar to the sulfidation process of cobalt nanocrystals, the initial oxidation of iron nanoparticles takes place by outward diffusion of cations through the growing oxide shell.27 The net material flow is balanced by an opposite flow of vacancies, which coalesce at the metal/oxide interface. The partial oxidation of colloidal iron nanoparticles leads to the formation of iron core−void−oxide shell nanostructures, whereas the complete oxidation eventually leads to the formation of hollow iron oxide nanoparticles (Figure 7). High-resolution TEM also confirmed the polycrystalline structure of the oxide shells (Figure 7G, H). Interestingly, it was found in this system that the oxide growth slows down significantly for shells thicker than ∼2.0 nm at temperatures lower than 250 °C. In this temperature range, only iron particles smaller than ∼8 nm could be completely converted into hollow oxide particles in solution. The oxidation rate of larger particles becomes imperceptibly small for ∼3 nm thick shells, thus trapping a core inside the oxide shell to form stable core−void−shell nanostructures. Heating the iron particles in solution at temperatures higher than 250 °C leads to the fragmentation of the oxide shells. However, heating the iron particles at 350 °C on a substrate in the absence of a solvent produces hollow oxide particles with smooth shells, regardless of the size of the precursor particles (Figure 7F). Peng et al also observed a core−void−shell intermediate structure during the synthesis of Fe3O4 hollow nanocrystals.28 The Fe3O4 hollow nanocrystals were prepared by controlled oxidation of amorphous Fe−Fe3O4 nanocrystals. The core− shell nanocrystals were synthesized via high-temperature thermal decomposition of Fe(CO)5 in organic solutions and subsequent air oxidation at room temperature. The Fe nanocrystals were not chemically stable, and oxidation when exposed to air gave core−shell Fe−Fe3O4 nanocrystals with both Fe and Fe3O4 in the amorphous state. The controlled oxidation of Fe−Fe3O4 nanocrystals in the presence of an oxygen-transfer reagent, trimethylamine N-oxide, led to the formation of intermediate core−void−shell Fe−Fe3O4 nanocrystals, and further to Fe3O4 hollow nanocrystals (∼16 nm in size) with controlled sizes via the nanoscale Kirkendall effect. The HRTEM image of a single hollow nanocrystal shows that the shell also contains multiple crystal domains. It was observed that when the reaction was quenched by lowering the temperature in the middle of the reaction process, multiple voids were formed in the particle. By adjusting the reaction temperature and time, it was possible to obtain a series of intermediate structures from the solid particle to the core− void−shell, the yolk−shell, and, finally, the hollow structures, which are good evidence for the Kirkendall effect. Railsback et al. discussed the size-dependent nanoscale Kirkendall effect during the oxidation of Ni nanocrystals in air to form hollow (single void) or porous (multiple voids) NiO at different temperatures and times.29 For oxidation of Ni nanocrystals with small sizes (∼9 and 26 nm), fast self-
voids between them. The mutual diffusion process could be initiated and progressed through the bridge structure. The void becomes larger with increasing reaction time and the core material ultimately disappears, leaving behind PbSe and Ni2Se hollow nanocrystals. We have recently developed a one-pot solution process to synthesize colloidal Bi2Te3−Te heterogeneous nanocrystals with hollow interiors from a mixture of Bi(NO3)3, Na2TeO3, hydrazine, poly(acryl acid), PVP and a trace amount of FeCl3 in EG.26 The reaction of TeO32‑ and Bi3+ in the presence of hydrazine first produces uniform Te nanorods, and then grows Bi2Te3 nanoplates on the tips and surfaces of these Te nanorods, forming various shapes including “nails,” “barbells, “syringes”, “accordions” with hollow interiors, and hollow nanocrystals. The reaction involves three sequential processes, including nucleation and growth of Te nanorods, heterogeneous nucleation of Bi2Te3 on the tips and/or surface of Te nanorods followed by its growth into disks, and the direct reaction of Bi precursor and Te nanorods to form hollow structures via the nanoscale Kirkendall Effect. Figure 6 shows
Figure 6. TEM images of Bi2Te3−Te hollow heteronanocrystals prepared by using (A, B) 170 uL, and (C, D) 300 uL of diluted hydrazine added (2 M). Adapted with permission from ref 26. Copyright 2010 American Chemical Society.
the Bi2Te3−Te hollow heterogeneous nanocrystals. The first step is the reduction of TeO32− by hydrazine, producing Te nanorods. The Te formation is relatively faster so that it consumes most of the TeO32‑ precursor and only allows limited growth of Bi2Te3 disks on the tips of the nanorods through reaction between Bi3+ and remaining TeO32−. As a result, the extra Bi3+ ions can only react with preformed Te nanorods to form Bi2Te3 through the reaction 6Te + 3N2H4 + 4 Bi3+ + 12OH− = 2 Bi2Te3 + 3N2 + 12H2O. This reaction proceeds by outward diffusion of Te atoms and produces voids inside the Te nanorods through the nanoscale Kirkendall effect. Basically, during this solid−liquid reaction, the inward diffusion of Bi3+ is hindered so that Te atoms diffuse out to the interface to form Bi2Te3, thus creating vacancies inside the nanorods. As the reaction proceeds, more vacancies form and eventually condense into large voids, which can be clearly observed in the enlarged TEM images B and D in Figure 6. The interfacial E
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to 423 K, with a summary of the results shown in Figure 8A.31,32 When Cu, Zn, and Al nanoparticles were oxidized,
Figure 8. (A) Summary of the critical diameters for obtaining hollow oxide nanoparticles from oxidation of metal nanoparticles at 423 K. (B) Self-diffusion coefficients of both metal and oxygen ions in Cu2O, ZnO, and α-Al2O3 as a function of the reciprocal temperature. Adapted with permission from ref 31. Copyright 2007 American Institute of Physics.
Figure 7. TEM images of iron/iron oxide nanocrystals exposed to dry 20% oxygen: (A)
