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Jang, Jeonghwan,Lee, Seung-Yong,Park, Hwanyeol,Yoon, Sangmoon,Park, Gyeong-Su,Lee, Gun-Do,Park, Yongjo,Kim, Miyoung,Yoon, Euijoon American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.48
<P>Solid-phase epitaxy (SPE), a solid-state phase transition of materials from an amorphous to a crystalline phase, is a convenient crystal growing technique. In particular, SPE can be used to grow α-Al<SUB>2</SUB>O<SUB>3</SUB> epitaxially with a novel structure that provides an effective substrate for improved performance of light-emitting diodes (LEDs). However, the inevitable two-step phase transformation through the γ-Al<SUB>2</SUB>O<SUB>3</SUB> phase hinders the expected improved crystallinity of α-Al<SUB>2</SUB>O<SUB>3</SUB>, and thereby further enhancement of LED performance. Herein, we provide a fundamental understanding of the SPE growth mechanism from amorphous to metastable γ-Al<SUB>2</SUB>O<SUB>3</SUB> using transmission electron microscopy (TEM) and density functional theory (DFT) calculations. The nanobeam precession electron diffraction technique enabled clear visualization of the double-positioning domain distribution in the SPE γ-Al<SUB>2</SUB>O<SUB>3</SUB> film and emphasized the need for careful selection of the viewing directions for any investigation of double-positioning domains. Void and stacking fault defects further investigated by high-resolution scanning TEM (STEM) analyses revealed how double-positioning domains and other SPE growth behaviors directly influence the crystallinity of SPE films. Additionally, DFT calculations revealed the origins of SPE growth behavior. The double-positioning γ-Al<SUB>2</SUB>O<SUB>3</SUB> domains randomly nucleate from the α-Al<SUB>2</SUB>O<SUB>3</SUB> substrate regardless of the α-Al<SUB>2</SUB>O<SUB>3</SUB> termination layer, but the large energy requirement for reversal of the γ-Al<SUB>2</SUB>O<SUB>3</SUB> stacking sequence prevents it from switching the domain type during the crystal growth. We expect that this study will be useful to improve the crystallinity of SPE γ- and α-Al<SUB>2</SUB>O<SUB>3</SUB> films.</P> [FIG OMISSION]</BR>
Kim, Ji-Yong,Park, Hwanyeol,Joo, Wonhyo,Nam, Dae-Hyun,Lee, Sungwoo,Kim, Hyoung Gyun,Ahn, In-Kyoung,Kang, Ho-Young,Lee, Gi-Baek,Jung, In-ho,Kim, Mi-Young,Lee, Gun-Do,Joo, Young-Chang The Royal Society of Chemistry 2019 Journal of materials chemistry. A, Materials for e Vol.7 No.13
<P>Although numerous first-principle studies have suggested candidate materials that may be used as enhanced electrocatalysts, the synthesis of such materials is quite another challenge. Furthermore, it has been still necessary to combine the active material with a supporting matrix in an optimized structure for stability and surface accessibility. Herein, we report a predictive fabrication method for the system where NiP2 nanoparticles has beenevenly embedded in carbon nanofibers (CNFs), which is expected to be superior by theoretical calculation but not successfully made so far. Through thermodynamic considerations on the inter-component reactions against processing variables, such as temperature, partial pressure of oxygen and phosphorus (pO2, pP4), a suitable two-step synthesis route consisting of successive pO2-controlled carbonization and pP4-controlled phosphidation was deduced. The obtained NiP2/CNFs had the desired electrocatalytic properties as well as physical features, including maximized surface area and well-combined structure. Consequently, the catalysts had overpotentials of only 71 mV at 10 mA cm<SUP>−2</SUP> and long-term stability of over 100 hours with less than 10% degradation under the operating conditions.</P>