Colloidal Metal Nanocrystals with Metastable Crystal Structures

Janssen, Annemieke; Nguyen, Quynh N.; Xia, Younan

doi:10.1002/anie.202017076

Cited by 21 publications

(40 citation statements)

References 64 publications

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“…[ 7,13,14 ] As a major advantage, template‐directed synthesis can be readily applied to different core and shell combinations. [ 7 ] It has been successfully used to grow Ru shells on a variety of fcc‐ Pd seeds with controlled shapes for the fabrication of metastable fcc ‐Pd@Ru core‐shell nanocrystals and fcc ‐Ru cubic, octahedral, and icosahedral nanocages. [ 6,8,15 ] To our knowledge, this method has not been extended to the synthesis of nanocrystals featuring a metastable hcp structure.…”

Section: Introductionmentioning

confidence: 99%

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

et al. 2021

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enhanced catalytic activities toward various structure-sensitive reactions. [3] In the case of Pt, for example, its activity toward oxygen reduction can be enhanced by five times by switching from cubic nanocrystals enclosed by {100} facets to the octahedral counterparts covered by {111} facets. [4] This and many other examples demonstrate that one can increase the figures of merit of metal nanocrystals in catalytic applications by controlling the arrangement of atoms. [5,6] Most of the prior studies, however, only dealt with metal nanocrystals in their native crystal structures without looking into polymorphism, the ability of a solid to crystallize in metastable phases distinct from the native, thermodynamically stable one. [7] Owing to the corresponding changes in surface and electronic structures, phase-controlled synthesis would offer another viable avenue to maneuver the catalytic properties of metal nanocrystals. [6,[8][9][10] Metastable phases have been achieved for a number of metals, including face-centered cubic (fcc) Co, hexagonally close-packed (hcp) -Ag and Au, 4H-Ag and Au, as well as fcc-Ru and hcp-Rh. [10,11] Recently, hcp-Pd was also reported, but phase-controlled synthesis of Pd nanocrystals remains in its infancy. [12] In general, phase control can be achieved through a variety of methods, including kinetically controlled growth, pressure-induced phase transition, and template-directed synthesis. [7] For kinetically controlled growth, the slow reduction kinetics achieved through control over the reaction temperature and reagents help induce unusual atomic stacking in the resultant nanocrystals, but it is difficult to rationally extend this method to different systems. [7] Pressure-induced phase transition relies on the application of high pressure to nanocrystals for the removal of stacking faults and shifting of the crystal structure, but it is mainly of theoretical interest because the nanocrystals revert to their native phases when the pressure is removed. [7] In template-directed synthesis, nanocrystals with a specific crystal structure can serve as seeds to dictate the nucleation and growth of a metal featuring a different crystal structure. [13] Under proper conditions (such as reduction kinetics, temperature, and use of stabilizing ligands), the crystal structure of the seed can be extended to the shell, resulting in the formation of a metastable core-shell nanocrystal. [7,13,14] As a major advantage, template-directed synthesis can be readily A relatively unexplored aspect of noble-metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core-shell nanocrystals featuring polymorphism, with the core made of hexagonally close-packed (hcp)-Ru while the Pd shell takes either an hcp or face-centered cubic (fcc) phase.The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking the hcp phase whereas the thicker ones revert to fcc. The injection rate pro...

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Section: Introductionmentioning

confidence: 99%

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

et al. 2021

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“…The increased surface energy and decreased lattice energy of nanocrystals lead to reduced activation energies for solid–solid phase transitions in nanocrystals relative to the same transitions in their bulk material analogues. − These differences enable the syntheses of certain nanocrystal phases at temperatures much lower than for the analogous syntheses of bulk crystals. Additionally, differences in free energy between polymorphs change due to, in large part, the role of surface energy in determining thermodynamic stabilities of nanocrystals. − …”

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Crystal Structure of Colloidally Prepared Metastable Ag₂Se Nanocrystals

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Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag2Se nanocrystals has traditionally been referred to as the “tetragonal” phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl2-like structure type (space group P21/n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag2Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl2-like Ag2Se polymorph is a dynamically stable, narrow-band-gap semiconductor. The anti-PbCl2-like structure of Ag2Se is a low-lying metastable polymorph at 5–25 meV/atom above the ground state, depending on the exchange-correlation functional used.

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“…In this regard, several concepts and classes of materials have been discussed and studied. Predominately, this includes small-sized nanoparticles and thin films, nanomaterials with special shapes (e.g., tetrapods or spikecubes), particles with inner surfaces and pores (e.g., hollow nanospheres or nanofoams), and high-porosity bulk materials (e.g., metalorganic frameworks (MOFs) or zeolites) [3][4][5][6]. Furthermore, the interaction between the active sites and the support material has been shown to significantly affect the catalytic activity [7].…”

Section: Introductionmentioning

confidence: 99%

“…In particular, titania and ceria are known to strongly influence the structure of supported noble metal particles or highly dispersed clusters [8]. Besides the catalytic efficiency, chemical and thermal stability are naturally highly relevant, since the catalytic reactions require certain activation energy, and thus elevated temperatures [1][2][3][4][5][6].…”

Section: Introductionmentioning

confidence: 99%

Catalytic CO Oxidation and H2O2 Direct Synthesis over Pd and Pt-Impregnated Titania Nanotubes

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Titania nanotubes (TNTs) impregnated with Pd and Pt nanoparticles are evaluated as heterogeneous catalysts in different conditions in two reactions: catalytic CO oxidation (gas phase, up to 500 °C) and H2O2 direct synthesis (liquid phase, 30 °C). The TNTs are obtained via oxidation of titanium metal and the intermediate layer-type sodium titanate Na2Ti3O7. Thereafter, the titanate layers are exfoliated and show self-rolling to TNTs, which, finally, are impregnated with Pd or Pt nanoparticles at room temperature by using Pd(ac)2 and Pt(ac)2. The resulting crystalline Pd/TNTs and Pt/TNTs are realized with different lengths (long TNTs: 2.0–2.5 µm, short TNTs: 0.23–0.27 µm) and a specific surface area up to 390 m2/g. The deposited Pd and Pt particles are 2–5 nm in diameter. The TNT-derived catalysts show good thermal (up to 500 °C) and chemical stability (in liquid-phase and gas-phase reactions). The catalytic evaluation results in a low CO oxidation light-out temperature of 150 °C for Pt/TNTs (1 wt-%) and promising H2O2 generation with a productivity of 3240 molH2O2 kgPd−1 h−1 (Pd/TNTs, 5 wt-%, 30 °C). Despite their smaller surface area, long TNTs outperform short TNTs with regard to both CO oxidation and H2O2 formation.

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Colloidal Metal Nanocrystals with Metastable Crystal Structures

Cited by 21 publications

References 64 publications

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

Crystal Structure of Colloidally Prepared Metastable Ag₂Se Nanocrystals

Catalytic CO Oxidation and H2O2 Direct Synthesis over Pd and Pt-Impregnated Titania Nanotubes

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Colloidal Metal Nanocrystals with Metastable Crystal Structures

Cited by 21 publications

References 64 publications

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

Facile Synthesis of Palladium‐Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals

Catalytic CO Oxidation and H2O2 Direct Synthesis over Pd and Pt-Impregnated Titania Nanotubes

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Crystal Structure of Colloidally Prepared Metastable Ag₂Se Nanocrystals