RESUMO
Stabilization of different morphologies of iso-material native/non-native heterostructures is important for electron-hole separation in the context of photo-electrochemical and opto-electronic devices. In this regard, we explore the stabilities of different morphologies of rutile ("native", ground state phase) and anatase ("non-native" phase) TiO2 heterostructures through (1) seed-mediated growth and (2) a thermally induced arrested phase transition synthesis protocol. Furthermore, the experimental results are analyzed through a combination of Density Functional Tight Binding (DFTB) and Finite Element Model (FEM) methods. During the seed-mediated growth, anatase is grown over a polydispersed and polycrystalline rutile core through thermal treatment yielding core-shell, Janus and yolk-shell iso-material heterostructures as observed from HRTEM. The arrested phase transition of anatase to rutile at different annealing temperatures yields rutile crystals in the subsurface region of the anatase and rutile/core-thin anatase/shell heterostructures but does not yield a Janus structure. Small particles that can be modeled via DFTB computations suggest that: (1) a heterostructure of the rutile/core-anatase/shell is energetically more stable than the anatase/core-rutile/shell or any other Janus configuration, (2) the off-centered rutile/core-anatase shell is more favorable to the mid-centered rutile/core-anatase shell and (3) Janus heterostructures can be stabilized when the mass ratio of the rutile seed to anatase overgrowth is high. FEM simulations, performed to evaluate the importance of stress relaxation in bicrystalline materials without defects, suggest that Janus structures can be stabilized in larger particles. The present studies add to the heuristics available for synthesizing iso-material heterostructures.
RESUMO
Gas filled Pd nanocontainers can serve as model nanochambers for reaction and phase equilibria studies. In the current study, palladium hollow spheres (PdHS) filled with oxygen are brought in intimate contact with hydrogen filled PdHS at room temperature (with internal pressure in both the spheres at 20 bar). The molecular hydrogen gets chemisorbed in the Pd shell and further diffuses into the oxygen filled sphere. The rapid reaction of hydrogen with oxygen in the inner wall of the oxygen filled sphere leads to a nanoexplosion, with the formation of water. This explosion results in either the complete breakage of the nanoshell or the formation of connected shells via the rupture of the internal wall connecting the shells. Transmission electron microscopy and Raman spectroscopy have been used to establish the sequence of processes. Further, the water in the nanochambers is cooled below sub-zero temperature to crystallize ice. This phenomenon is observed for the first time at room temperature.
RESUMO
The concept of a critical nucleus size (r*) is of pivotal importance in phase transformations involving nucleation and growth. The current investigation pertains to crystallization in nanoscale thin films and study of the same using high resolution lattice fringe imaging (HRLFI) and finite element simulations. Using the CuZrAl bulk metallic glass system as a model system for this study, we demonstrate a liquid like nucleation behaviour in nanoscale free-standing films upon heating. The r* for the formation of the Cu10Zr7 phase in thin films (of decreasing thickness) approaches that of the r* for the formation of the crystal from a liquid (i.e.). Working in the nucleation dominant regime, we introduce the concept of 'depth sensitive lattice fringe imaging'. The thickness of the film is determined by electron energy loss spectroscopy and the strain energy of the system is computed using finite element computations.
