Yufeng Liu

Crystallization Behavior of Binary Even-Even n-Alkane Mixtures in Microcapsules: Effect of Composition and Confined Geometry on Solid-Solid phase Separation

The crystallization behaviors of binary even-even normal alkane (n-alkane) mixtures (n-C(18)H(38)/n-C(20)H(42), abbreviated as C(18)/C(20)) with different compositions, both in the bulk state and in nearly monodisperse microcapsules, have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The solid-solid phase separation, usually observed during the cooling process of bulk samples, is greatly suppressed and even eliminated after being microencapsulated, with the orthorhombic-ordered phase dominating in the low-temperature crystal. Such a crystallization transition is attributed to the special interaction between the two even n-alkanes and the confined environment in microcapsules. The triclinic ordered phase, solely formed by the single even n-alkanes (C(18) or C(20)), becomes less stable due to the weakening of the layered structure and the suppression of the terminal methyl-methyl interactions in the confined geometry, which favors the miscibility of the two components. Furthermore, besides the chain-length difference and the composition, the confined environment is proved to be another important factor to exert strong positive influence on suppressing the solid-solid phase separation of C(18)/C(20) binary system.

Controllable synthesis of silver cyanamide as a new semiconductor photocatalyst under visible-light irradiation

Semiconductor photocatalysis has attracted great interest as a promising strategy to achieve sustainable energy generation and realize environmental remediation. High-efficiency and visible-light-driven semiconductor photocatalysts have been pursued during the past decades to overcome the drawbacks of the widely-studied TiO2 photocatalyst. Here, we report a new n-type silver cyanamide (Ag2NCN) semiconductor with a direct band gap of about 2.30 eV, which can utilize visible-light for water photooxidation and organic contaminant decomposition. In order to investigate the optical, photocatalytic and photoelectrochemical properties, various aqueous solution methods are developed to synthesize Ag2NCN nanoparticles (<100 nm), microparticles (up to 20 μm) and nanograined (20 nm) thin films. It is found that the as-prepared nanoparticles exhibit unique absorption and photoluminescence properties. Nano/microparticles show high photocatalytic performance for organic dye degradation, indicating that Ag2NCN is an efficient visible-light-driven photocatalyst. Additionally, nanograined thin films can be assembled into a photoelectrochemical cell for water photooxidation and photocurrent generation. These findings have provided the first evidence that transition metal cyanamides may be promising functional materials for solar energy utilization.

Fe-substituted indium thiospinels: New intermediate band semiconductors with better absorption of solar energy

The indium thiospinels In2S3 and MgIn2S4 are promising host for the intermediated band (IB) photovoltaic materials due to their ideal band gap value. Here, the optical properties and electronic structure of Fe-doped In2S3 and MgIn2S4 have been investigated. All the Fe-substituted semiconductors exhibit two additional absorption bands at about 0.7 and 1.25u2009eV, respectively. The results of first-principles calculations revealed that the Fe substituted at the octahedral In site would introduce a partially filled IB into the band gap. Thanks to the formation of IB, the Fe-substituted semiconductors have the ability to absorb the photons with energies below the band gap. With the wide-spectrum absorption of solar energy, these materials possess potential applications in photovoltaic domain.

Confined Crystallization of n-Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study

Crystallization and phase transition behaviors of n-hexadecane (n-C16H34, abbreviated as C16) confined in microcapsules and n-alkane/SiO2 nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C16 is enhanced in both the microcapsules and SiO2 nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid-solid phase transition is observed only in the microencapsulated C16, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C16/SiO2 composite, however, no novel rotator phase emerges during the cooling process, and C16 crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C16/SiO2 composite, leading to phase transition behaviors similar to those of bulk C16.

Binary n-alkane mixtures from total miscibility to phase separation in microcapsules: enrichment of shorter component in surface freezing and enhanced stability of rotator phases.

The crystallization behaviors of binary normal alkane (n-alkane) mixtures with a series of carbon number difference (denoted as Δn), both in the bulk state and in nearly monodisperse microcapsules, have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As revealed by the DSC data, the surface freezing temperature (denoted as T(s)) of spatially confined binary n-alkane mixtures with large Δn is lower than the calculated value due to the enrichment of shorter component in the surface freezing phase. More alkane molecules with shorter carbon chain are located on the interface between the inner shell of microcapsules and the bulk mixture, thus leading to the decrease of the average chain length of the surface freezing phase and corresponding lower T(s). Furthermore, XRD results have proved that the enhanced surface freezing phenomenon can contribute to the stabilization of the rotator phases in n-alkane mixtures and even induce the crossover of some certain rotator phase (RII) from transient to metastable. However, the decisive reason for such stabilization or crossover is attributed to the suppression of the orienting movement of alkane molecules toward their next-nearest neighbors within the layer of rotator phases.

CuIn(S,Se)2 thin films prepared from a novel thioacetic acid-based solution and their photovoltaic application

Low-cost and high-yield preparation of CuInSe2 films is the bottleneck for promising CuInSe2-based thin film solar cells. Here, we developed a simple, safe and cost-effective method using thioacetic acid to fabricate the absorber films of CuIn(S,Se)2 (CISSe). Dissolution of Cu2O and In(OH)3 in thioacetic acid was attributed to the strong coordination ability of S. The adhesive precursor solution can be prepared without any heating, centrifugation and inert gas protection, superior to the previously reported methods. The precursor CISSe layer was easily deposited in air by spin coating to ensure low cost. Uniform and compact CISSe thin films with well-crystallized and pure-phased CISSe grains were obtained after one step annealing. The as-prepared CISSe thin films were successfully applied to solar cells and a energy conversion efficiency of 6.75% was achieved. This facile preparation provides a low-cost and easy method to fabricate Cu-based thin film solar cells.