Nanocrystals in compression: unexpected structural phase transition and amorphization due to surface impurities

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

Optimal Sr-Doped Free TiO2@SrTiO3 Heterostructured Nanowire Arrays for High-Efficiency Self-Powered Photoelectrochemical UV Photodetector Applications

Due to their high performance, photoelectrochemical ultraviolet (UV) photodetectors have attracted much attention, but the recombination of photogenerated electrons at the interface of photoanode/electrolyte limited further improvement of photoelectrochemical UV photodetectors (PEC UVPDs). Modification of TiO2 photoanode by SrTiO3 could improve the performance of UVPD, because the energy barrier that is established at the TiO2–SrTiO3 interface could accelerate the separation of the photogenerated electrons-holes pair. However, the recombination center that is caused by the preparation of TiO2@SrTiO3 core-shell heterostructured nanostructure decreases the performance of PEC UVPDs, which is still an important problem that hindered its application in PEC UVPDs. In this paper, we presented a Sr-doped free TiO2@SrTiO3 core-shell heterostructured nanowire arrays as a photoanode for the self-powered PEC UVPD. This will not only accelerate the separation of the photogenerated electrons-holes pair, but it will also reduce the recombination of photogenerated electron-hole pairs in the photoanode. The intrinsic effect of SrTiO3 reaction time on the J variations of UVPDs is investigated in detail. An impressive responsivity of 0.358 A W−1 was achieved at 360 nm for the UVPD based on TiO2@SrTiO3 core-shell heterostructured nanowire arrays, which heretofore is a considerably high photoresponsivity for self-powered photoelectrochemical UVPDs. Additionally, this UVPD also exhibits a high on/off ratio, fast response time, excellent visible-blind characteristic, and linear optical signal response.

Reciprocating Compression of ZnO Probed by X-ray Diffraction: The Size Effect on Structural Properties under High Pressure

Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds.

Enhanced Electrochemical Performance of Fast Ionic Conductor LiTi2(PO4)3-Coated LiNi1/3Co1/3Mn1/3O2 Cathode Material

Layered LiNi_1/3Co_1/3Mn_1/3O₂ (NCM333) is successfully coated by fast ionic conductor LiTi₂(PO₄)₃ (LTP) via a wet chemical method. The effects of LTP on the physicochemical properties and electrochemical performance are studied. The results reveal that a highly layered structure of NCM333 can be well maintained with less cation mixing after LTP coating. LTP of about 5 nm thickness is coated on the surface of NCM333. Such an LTP coating layer can effectively suppress the side reactions between NCM333 and electrolyte but will not hinder the lithium ion transmission. As a result, LTP-coated NCM333 owns an improved capability and cyclic performance, for example, NCM333/LTP delivers an initial capacity as high as 121.0 mA h g^-1 with a capacity retention ratio of 82.3% after 200 cycles at 10 C, whereas NCM333 only has an initial capacity of 120.4 mA h g^-1 with a very low capacity retention ratio of 66.4%. This method of using a fast ionic conductor like LTP as a coating material may provide a simple and effective strategy to modify those electrode materials with poor cyclic performance.

Structural and vibrational properties of corundum-type In2O3 nanocrystals under compression

This work reports the structural and vibrational properties of nanocrystals of corundum-type In₂O₃ (rh-In₂O₃) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In₂O₃. Nanocrystalline rh-In₂O₃ showed stability under compression at least up to 20 GPa, unlike bulk rh-In₂O₃ which gradually transforms to the orthorhombic Pbca (Rh₂O₃-III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In₂O₃ is discussed.