Guangyu Chen

Enhancement of the upconversion radiation in Y2O3:Er3+ nanocrystals by codoping with Li+ ions

We report on an innovative route to increase the upconversion (UC) green radiation by two orders of magnitude in Y2O3:Er3+ nanocrystals through tailoring Er3+ ions’ local environment with Li+ ions under diode laser excitation of 970nm. Theoretical investigations based on the steady-state rate equations indicate that such enhancement arises from a combining effect of the tailored lifetime of the intermediate I11∕24(Er) state, the suppressed cross relaxation H11∕22(Er)+I15∕24(Er)→I9∕24(Er)+I13∕24(Er) process, and the enlarged nanocrystal size induced by the Li+ ions. The proposed route here may constitute a promising step to solve the low efficiency problem in UC materials.

Anomalous power dependence of upconversion emissions in Gd2O3:Er3+ nanocrystals under diode laser excitation of 970 nm

Visible green and red upconversion (UC) emissions with anomalous power dependence were observed in Gd2O3:Er3+ nanocrystals at room temperature under diode laser excitation of 970 nm. The green and red UC radiations both yield an “s”-shape power dependence, in marked contrast to the quadratic ones of the bulk counterparts. A closed positive looping UC mechanism that differs from conventional PA mechanism is proposed to explain the observed five- or six-photon processes in the “s”-shape power dependence. Power dependence analysis of the 1.55 μm emissions from the I413/2 state experimentally demonstrates our proposed model.

Polyelectrolyte Assisted Synthesis and Enhanced Oxygen Reduction Activity of Pt Nanocrystals with Controllable Shape and Size

The shape control of platinum nanocrystals is significant to the enhancement of their catalytic performance in terms of activity and selectivity. However, it still remains a major challenge to prepare Pt nanocrystals with tunable shape and clean surface in an eco-friendly way. This article develops a facile and green strategy to prepare well tuned platinum nanocrystals employing poly(diallyldimethylammonium chloride) (PDDA) as the capping agent, reductant, and stabilizer simultaneously in a facile hydrothermal process. It is identified that the variation of PDDA concentration is crucial to control the growth of crystalline facets, leading to the formation of cubic, truncated cubic, and octahedral Pt nanocrystals with sizes tunable from ca. 17 nm to ca. 50 nm. The resultant Pt nanocrystals exhibit excellent electrocatalytic activity and stability toward the oxygen reduction reaction (ORR) in acidic media compared with those of commercial Pt black and the state-of-the-art Pt/C catalyst. It is proposed that the preferential Pt surface and the decoration of PDDA, which modulates the electronic structures and electrooxidation of Pt nanocrystals, synergistically contribute to the enhanced catalytic performance.

Hydrothermal synthesis, characterization, electronic structure, and thermoelectric properties of (Ca0.85OH)1.16CoO2

A layered cobalt oxide (Ca(0.85)OH)(1.16)CoO(2) was prepared at low temperature by hydrothermal process and characterized by powder x-ray diffraction, Fourier transform infrared, and scanning electron microscopy. The results showed that plate image powders could be obtained at 453 K for 12 h. The electronic calculation of the band structure and density of states revealed that (Ca(0.85)OH)(1.16)CoO(2) is a direct-gap semiconductor material and the conductive model is d-d transition of cobalt. The electrical conductivity, Seebeck coefficient, and thermal conductivity of (Ca(0.85)OH)(1.16)CoO(2) were measured from 290 to 573 K. It was found that the oxide behaves as p-type material in the temperature range measured and there is an M-I transition near 370 K. The ZT increases with the increase in temperature, and the maximum value of 0.02 is obtained at 573 K, indicating (Ca(0.85)OH)(1.16)CoO(2) is a promising thermoelectric oxide candidate at middle temperature usage.

Metal-Organic Coordination Networks: Prussian Blue and Its Synergy with Pt Nanoparticles to Enhance Oxygen Reduction Kinetics.

Oxygen reduction reaction (ORR) is the cornerstone in the electrochemical energy conversion devices such as fuel cells and metal-air batteries. It remains a great challenge to develop the ORR electrocatalysts with fast kinetics and high durability. Herein, we report the synthesis of a novel metal-organic coordination networks material, prussian blue crystalline nanograins mosaicked within amorphous membrane (PB CNG-M-AM). Such unique PB CNG-M-AM is designed to enhance the electrocatalysis of Pt toward the ORR by the electrostatic self-assembly. Thus, obtained Pt-PB/C catalysts form numerous Pt-PB-gas three-phase boundaries and present rather high intrinsic activity, four-electron selectivity and superior stability. Moreover, a completely new synergetic mechanism between PB and Pt is discovered, which delicately alters the ORR route and significantly enhances the ORR kinetics. This work provides not only a new strategy and mechanism for developing highly efficient ORR electrocatalysts, but also an alternative way to utilize metal-organic coordination networks materials.

ZIF-8 with Ferrocene Encapsulated: A Promising Precursor to Single-Atom Fe Embedded Nitrogen-Doped Carbon as Highly Efficient Catalyst for Oxygen Electroreduction

The oxygen reduction reaction (ORR) plays an important role in the fields of energy storage and conversion technologies, including metal-air batteries and fuel cells. The development of nonprecious metal electrocatalysts with both high ORR activity and durability to replace the currently used costly Pt-based catalyst is critical and still a major challenge. Herein, a facile and scalable method is reported to prepare ZIF-8 with single ferrocene molecules trapped within its cavities (Fc@ZIF-8), which is utilized as precursor to porous single-atom Fe embedded nitrogen-doped carbon (Fe-N-C) during high temperature pyrolysis. The catalyst shows a half-wave potential (E1/2 ) of 0.904 V, 67 mV higher than commercial Pt/C catalyst (0.837 V), which is among the best compared with reported results for ORR. Significant electrochemical properties are attributed to the special configuration of Fc@ZIF-8 transforming into a highly dispersed iron-nitrogen coordination moieties embedded carbon matrix.

A review of applications of poly(diallyldimethyl ammonium chloride) in polymer membrane fuel cells: From nanoparticles to support materials

Polymer membrane fuel cells represent important sustainable energy devices because their operation involves zero emissions and low temperatures and their components exhibit low toxicity. Among the various components of such cells, the electrocatalyst plays the vital role of enhancing the output power density and/or working lifetime. Over the past several decades, numerous strategies have been proposed to address the challenges of electrocatalyst activity and/or durability. Herein, we review the applications of polyelectrolytes in electrocatalysts, including the enhancement of both catalytic nanoparticles and support materials. The effects of polyelectrolytes with regard to controlling the size, composition and morphology of catalytic nanoparticles, as well as the modification of support materials were summarized. In addition, the future possibilities for the research and development of polyelectrolytes in the field of catalyst design and synthesis are discussed.