Formation of Hollow Nanocrystals Through the nanoscale kirkendall effect As show, reaction of high quality single"crystal 海。 stals of companying transpo A ed by the injection of sulfur in provide a fast transport path for outward diffusion of cobalt atoms that can then spread on the inner shell surface · 20 nm Evolution of Cose hollow nanocrystals with time by injection of a e ol utom of oe hollow oanmeryatal e r time mD espe hle to a stream aperyista aolution at is k cbo beren t bottom isht oa.10 molar ratio was 1:1 lodal solution at 455 K (A to D) TEM images of the after flow of O /Ar for (A)O min, (B)30 min, (C)80 min, and D)210 min Inset: HRTEM of a Coo hollow nanocrystal Choosing the Optimum Temperature Overcoming the Diffusion Barrier (a)calcined at 500C aks of Li-CO3+ Zroz Need intimate mixture of reactants Can be obtained in several ways overy gmall particle size reactants peaks of LizZ-Oa (monoclinic) ofind molecular precursor that has the needed elements (d) calcined at1400°C in the correct ratio: eg. BalTio(C,)I for BaTiO peaks of LizZrO3+ZrOz out without demixing the components Optimum temperature. carbonates for Brownmillerite (CaFe,Oy XRD Patterns of the Li, ZrO3 crystallize from gels prepared using sol-gel chemistry Calcined at Various Temperatures7 As show, reaction of high quality single-crystal cobalt nanocrystals with oxygen, sulfur, or selenium at relatively low temperatures produces hollow polycrystalline nanocrystals of cobalt oxide, sulfide, or selenide, respectively. As the reaction proceeds in time, more cobalt atoms diffuse out to the shell, and the accompanying transport of vacancies leads to growth and merging of the initial voids. This results in the formation of bridges of material between the core and the shell that persist until the core is completely consumed. These bridges provide a fast transport path for outward diffusion of cobalt atoms that can then spread on the inner shell surface. Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect (A) TEM image of cobalt nanocrystals. (B ) TEM image of the cobalt sulfide phase synthesized by the injection of sulfur in o-dichlorobenzene (5 ml) into cobaltnanocrystal solution with a Co/S molar ratio of 9:12. (C ) HRTEM images of Co3S4 (left) and Co9S8 (right). (D) TEM image of the cobalt sulfide phase synthesized with a Co:S molar ratio of 9:8. Evolution of CoO hollow nanocrystals over time in response to a stream of O2 /Ar mixture (1: 4in volume ratio, 120 ml/min) being blown through a cobalt colloidal solution at 455 K. (A to D) TEM images of the solutions after flow of O2 /Ar for (A) 0 min, (B) 30 min, (C) 80 min, and (D) 210 min. Inset: HRTEM of a CoO hollow nanocrystal. Evolution of CoSe hollow nanocrystals with time by injection of a suspension of selenium in o-dichlorobenzene into a cobalt nanocrystal solution at 455 K, from top-left to bottom right: 0 s, 10 s, 20 s, 1min, 2 min, and 30 min. The Co/Se molar ratio was 1:1. XRD Patterns of the Li2ZrO3 Calcined at Various Temperatures (a)Calcined at 500°C peaks of Li2CO3 + ZrO2 (b) Calcinedat 700°C peaks of Li2ZrO3 (monoclinic) (c) Calcinedat 850°C-1200 °C peaks of Li2ZrO3 (monoclinic) (d) Calcined at 1400°C peaks of Li2ZrO3 + ZrO2 Optimum temperature: 850-1200oC ( a) ( b) (c) ( d) Choosing the Optimum Temperature Overcoming the Diffusion Barrier Need intimate mixture of reactants Can be obtained in several ways: very small particle size reactants find molecular precursor that has the needed elements in the correct ratio：eg. Ba[TiO(C2O4 )2 ] for BaTiO3 Make a solution of needed metals and dry the solution out without demixing the components co-precipitate reactants in a solid solution salt：e.g. carbonates for Brownmillerite (Ca2Fe2O5 ) crystallize from gels prepared using sol-gel chemistry