Size, shape, and compositional effects on the order–disorder phase transitions in Au–Cu and Pt–M (M = Fe, Co, and Ni) nanocluster alloys

Abstract

Au-Cu and Pt-M (M = Fe, Co, and Ni) nanocluster alloys are currently being investigated world-wide by many researchers for their interesting catalytic and nanophase properties. The low temperature behavior of the phase diagrams is not well understood for alloys with nanometer sizes and shapes. We consider two models for low temperature ordering in the phase diagrams of Au-Cu and Pt-M nanocluster alloys. These models are valid for sizes ∼5 nm and approach bulk values for sizes ∼20 nm. We study the phase transitions in nanoclusters with cubic, octahedral, and cuboctahedral shapes, covering the compositions of interest. These models are based on studying the melting temperatures in nanoclusters using the regular solution, mixing model for alloys. From our data, experiments on nanocubes about 5 nm in size, of stoichiometric AuCu and PtM composition, could help differentiate between the models. Dispersion data shows that for the three shapes considered, octahedra have the highest percentage of surface atoms for the same relative diameter. We summarize the effects of structural ordering on the catalytic activity and suggest a method to avoid sintering during annealing of Pt-M alloys.

Atomistic Imaging of Competition between Surface Diffusion and Phase Transition during the Intermetallic Formation of Faceted Particles

“…We exhibited that a spherical structure with an almost round surface could be achieved during the complete ordering transition at 600 °C within 60 min via vigorous surface diffusion from the corner to the terrace. It is slightly lower than the predicted phase transition temperature (∼650 °C) for the 18 nm Pt 3 Co cube and also lower than the temperature in the DHTAP because of the prolonged diffusion at the pretreatment stage at low temperature. We noticed that the surface of the cube after keeping 60 min at 300 °C became rough with a number of dangling atoms (Figure S4A).…”

“…The corresponding fast Fourier transform (FFT) pattern in the inset of Figure A only appears on {200} and {220} planes but does not appear on {100} and {110} planes, which confirms that the Pt 3 Co cube has a disordered face-centered cubic (FCC) alloy structure. ,− The Pt 3 Co NP exhibits a homogeneous random elemental distribution (see Figure S2) at room temperature (RT) with a composition of 75.78:24.22 for Pt:Co (EDS; see Figure S3). In the slow heating process, the Pt 3 Co cubic NP was preannealed at 300 °C, at which the Pt 3 Co NPs could diffuse adequately but not occur at the phase transition . After proceeding for 60 min, the cubic structure was slightly changed with the rounded corner (see Figure S4A) due to the surface diffusion of low-coordination (CN) atoms.…”

“…In addition, the surface segregation, phase transition, and diffusion behaviors of nanoparticles (NPs) are very different from those of bulk materials. , All these changes make the nanometallurgy not easy to understand via utilizing the existing classic theory. Also, the related detailed theoretical and experimental studies are mostly under development, especially those on the eutectic point and phase transition temperature in the equilibrium phase diagrams of NPs. − …”

Hydrogen Adsorption on Ordered and Disordered Pt-Ni Alloys

“…An ordered fcc L1 2 phase for PtM 3 may exist, but the experimental evidence is inconclusive. Computationally determined phase diagrams do predict L1 2 PtM 3 and Pt 3 M phases for Pt-Fe, Pt-Co, and Pt-Ni [14]. In traditional alloy research, "disordered alloys" usually refer to random mixtures of metals that have overall well-defined crystal structures yet whose component elements do not form any well-defined ordered sub-lattices.…”

Моделирование размерных эффектов при фазовых превращениях в субмикронных частицах сплава Au-Pt-Pd

“…Особенности фазовых превращений в малом объеме важны при производстве и эксплуатации порошковых композиционных, ультрамелкозернистых материалов. Для бинарных смесей размерные эффекты при фазовых превращениях описаны во многих работах [1][2][3]5]. Естественно ожидать более сложного поведения от тройных систем, однако, отображение размерных эффектов на фазовых диаграммах для них в литературе практически отсутствуют [6].…”