Yuebin Cao

Highly Electrocatalytic Cu2ZnSn(S1–xSex)4 Counter Electrodes for Quantum-Dot-Sensitized Solar Cells

Traditional Pt counter electrode in quantum-dot-sensitized solar cells suffers from a low electrocatalytic activity and instability due to irreversible surface adsorption of sulfur species incurred while regenerating polysulfide (S(n)(2-)/S(2-)) electrolytes. To overcome such constraints, chemically synthesized Cu(2)ZnSn(S(1-x)Se(x))(4) nanocrystals were evaluated as an alternative to Pt. The resulting chalcogenides exhibited remarkable electrocatalytic activities for reduction of polysulfide (S(n)(2-)) to sulfide (S(2-)), which were dictated by the ratios of S/Se. In this study, a quantum dot sensitized solar cell constructed with Cu(2)ZnSn(S(0.5)Se(0.5))(4) as a counter electrode showed the highest energy conversion efficiency of 3.01%, which was even higher than that using Pt (1.24%). The compositional variations in between Cu(2)ZnSnS(4) (x = 0) and Cu(2)ZnSnSe(4) (x = 1) revealed that the solar cell performances were closely related to a difference in electrocatalytic activities for polysulfide reduction governed by the S/Se ratios.

A new synthetic route to hollow Co3O4 octahedra for supercapacitor applications

Co3O4 hollow octahedra were successfully synthesized via a facile one-step solvothermal route. Time-resolved electron microscopy and X-ray diffraction analyses revealed that the Co3O4 hollow octahedra were formed through the self-assembly of primary nanocrystals followed by subsequent Ostwald ripening. Subtle control over the reaction conditions led to different morphologies (hexagonal plates and nanocubes) and crystal structures (β-Co(OH)2–Co3O4 composite). The unique hollow nanostructure rendered our Co3O4 potentially useful for charge-storage applications. To prove its usefulness, the pseudocapacitive performance of the Co3O4 hollow octahedra as a supercapacitor electrode was evaluated and exhibited a charge storage capacity of 192 F g−1 with good long-term cyclability.

Fabrication of ZnO nanorod-assembled multishelled hollow spheres and enhanced performance in gas sensor

In this work, ZnO multishelled hollow spheres with an average diameter of 5 μm were prepared by a facile solvothermal process in a ternary solvent system including water, ethanol and ethylene, and as-synthesized products were constructed by highly directional interactions of anisotropic single-crystalline ZnO nanorods. A two-step assembly process followed symmetric Ostwald ripening process is proposed to explain the formation mechanism of obtained products, which highlights the driving force of the solvents in promoting the nanorod aggregation and the solid evacuation of final products. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy were used to characterize the structure of synthesized products. The investigation of the gas-sensing properties indicated that control of the shape-defined building units and their assembled structure provides ZnO with high performance in gas sensing, and the double-wall hollow structures exhibit the highest sensitivity to formaldehyde gas than the nanorods and hollow spheres, which is contributed to their high donor-related and the low acceptor-related intrinsic defects in ZnO crystals.

1,3-Diphenyl-5-(9-phenanthryl)-4,5-dihydro-1H-pyrazole (DPPhP): structure, properties, and application in organic light-emitting diodes

A novel hole-transport material, 1,3-diphenyl-5-(9-phenanthryl)-4,5-dihydro-1H-pyrazole (DPPhP), was synthesized and fully characterized. The crystal structure of DPPhP was determined by X-ray diffraction analyses. DSC and AFM analysis demonstrate that DPPhP has a high Tg of 96 °C and good film forming ability. The hole-transport performance of DPPhP was examined by fabricating a multilayer device with structure of ITO/DPPhP (60 nm)/AlQ (60 nm)/LiF (0.8 nm)/Al, using DPPhP as the hole-transport layer along with an emitting-material, tris-(8-hydroxyquinolino)aluminium (AlQ). Both the brightness and efficiency of the device are about 30% higher than those of the device using N,N′-di-1-naphthenyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) as the hole-transport layer.

Fabrication of Porous TiO2 Hollow Spheres and Their Application in Gas Sensing

In this work, porous TiO2 hollow spheres with an average diameter of 100 nm and shell thickness of 20 nm were synthesized by a facile hydrothermal method with NH4HCO3 as the structure-directing agent, and the formation mechanism for this porous hollow structure was proved to be the Ostwald ripening process by tracking the morphology of the products at different reaction stages. The product was characterized by SEM, TEM, XRD and BET analyses, and the results show that the as-synthesized products are anatase phase with a high surface area up to 132.5 m2/g. Gas-sensing investigation reveals that the product possesses sensitive response to methanal gas at 200°C due to its high surface area.

Formation of cubic Cu mesocrystals by a solvothermal reaction

Cubic Cu mesocrystals were prepared via a facile solvothermal reaction in a mixed solvent of glycerol–water. The microstructure of synthesized mesocrystals can be successfully controlled by adjusting the volume ratio of glycerol/water. The evolution of these cubic Cu mesocrystals was investigated and the mechanism was discussed. The oriented attachment process controls the initial stage in the formation of scabbled cubic Cu mesocrystals and then Ostwald ripening process contributes to the formation of the final Cu mesocrystals by shearing the scabbled mesocrystals. The influences of reaction temperature and NaOH concentration on the morphology of the synthesized mesocrystals were also investigated. Isotropic conductive adhesives (ICA), filled with cubic Cu mesocrystals, exhibit a low bulk resistivity, of about 5.21 × 10−4 Ω cm.

Dark current characteristics of GaAs-based 2.6 µm InGaAs photodetectors on different types of InAlAs buffer layers

GaAs-based In0.83Ga0.17As photodetectors (PDs) with cut-off wavelengths up to 2.6 µm are demonstrated. The effects of continuously-graded or fixed-composition InAlAs buffers on the device performances are investigated. The dark current characteristics of the PDs at various temperatures are analysed in detail. The photocurrents are also measured at 300 K; the detectivity of the PDs is extracted. The two GaAs-based PDs with different buffer schemes show different temperature-dependent dark current behaviours. The around room temperature performances of the GaAs-based device on the fixed-composition buffer are not as good, but comparable to those of InP-based devices, revealing a promising candidate for the GaAs-based PDs and focal plane arrays for many low-end applications.

2.7 μm InAs quantum well lasers on InP-based InAlAs metamorphic buffer layers

This work reports 2.7 μm InAs/In0.6Ga0.4As quantum well lasers on InP-based metamorphic InxAl1−xAs graded buffers. X-ray diffraction measurement shows favorable strain compensation effect in the quantum wells. Type-I photoluminescence emission is observed around 2.7 μm at 77 K and red-shifts to 3 μm at 300 K. The continuous-wave lasing wavelength of the laser reaches 2.7 μm at 77 K, which is the longest wavelength from the interband lasing of InP-based antimony-free structures. The threshold current density is as low as 145 A/cm2 and the continuous-wave output power at injection current of 400 mA is over 5 mW.

Shape-controlled synthesis and luminescence properties of yttria phosphors

Y(OH)3 hexagonal tubes with embedded polyhedrons (HTEPs) were successfully prepared by a facile hydrothermal method using sodium citrate as the crystal modifier, and as-synthesized products exhibit uniform diameter and length of about 2 μm and 2–3 μm, respectively. An oriented aggregation and subsequent dissolution-recrystallization process was proposed to explain the growth mechanism of obtained products. Y2O3 : Eu3+ microstructures with similar morphologies were obtained after thermal treatment of the as-prepared Y(OH)3 : Eu3+ microstructures at 1000 °C for 2 h. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and fluorescence spectrometer were used to characterize the samples. The photoluminescence properties investigation reveals that Y2O3 : Eu3+ HTEPs show a strong red emission with the strongest peak centered at 614 nm under the excitation of 259 nm ultraviolet light.

InAs/In0.83Al0.17As quantum wells on GaAs substrate with type-I emission at 2.9 μm

This work reports on InAs quantum wells (QWs) grown on GaAs-based metamorphic In0.83Al0.17As buffers for type-I mid-infrared (MIR) emission. X-ray diffraction and Raman measurements show that the GaAs-based quantum wells have similar lattice and strain conditions with the InP-based structure. Atomic force microscope shows the smoother surface of the structure on GaAs substrate. For the GaAs-based quantum wells, favorable photoluminescence emission at 2.9 μm at 300 K has been achieved, and the optical quality is comparable to the structure on InP substrate. It is promising to employ this metamorphic quantum well structure for the demonstration of GaAs-based antimony-free mid-infrared lasers.