Frontiers of Materials Science (v.10, #3)

Progress in nanostructured photoanodes for dye-sensitized solar cells by Xueyang Liu; Jian Fang; Yong Liu; Tong Lin (225-237).
Solar cells represent a principal energy technology to convert light into electricity. Commercial solar cells are at present predominately produced by single- or multi-crystalline silicon wafers. The main drawback to silicon-based solar cells, however, is high material and manufacturing costs. Dye-sensitized solar cells (DSSCs) have attracted much attention during recent years because of the low production cost and other advantages. The photoanode (working electrode) plays a key role in determining the performance of DSSCs. In particular, nanostructured photoanodes with a large surface area, high electron transfer efficiency, and low electron recombination facilitate to prepare DSSCs with high energy conversion efficiency. In this review article, we summarize recent progress in the development of novel photoanodes for DSSCs. Effect of semiconductor material (e.g. TiO2, ZnO, SnO2, N2O5, and nano carbon), preparation, morphology and structure (e.g. nanoparticles, nanorods, nanofibers, nanotubes, fiber/particle composites, and hierarchical structure) on photovoltaic performance of DSSCs is described. The possibility of replacing silicon-based solar cells with DSSCs is discussed.
Keywords: dye-sensitized solar cell (DSSC); nanostructure; photoanode

A review on biodegradable materials for cardiovascular stent application by Li-Da Hou; Zhen Li; Yu Pan; MuhammadIqbal Sabir; Yu-Feng Zheng; Li Li (238-259).
A stent is a medical device designed to serve as a temporary or permanent internal scaffold to maintain or increase the lumen of a body conduit. The researchers and engineers diverted to investigate biodegradable materials due to the limitation of metallic materials in stent application such as stent restenosis which requires prolonged anti platelet therapy, often result in smaller lumen after implantation and obstruct re-stenting treatments. Biomedical implants with temporary function for the vascular intervention are extensively studied in recent years. The rationale for biodegradable stent is to provide the support for the vessel in predicted period of time and then degrading into biocompatible constituent. The degradation of stent makes the re-stenting possible after several months and also ameliorates the vessel wall quality. The present article focuses on the biodegradable materials for the cardiovascular stent. The objective of this review is to describe the possible biodegradable materials for stent and their properties such as design criteria, degradation behavior, drawbacks and advantages with their recent clinical and preclinical trials.
Keywords: biodegradable materials; magnesium alloy; polymers; biodegradable stent; cardiovascular stent

Adult central nervous system (CNS) tissue has a limited capacity to recover after trauma or disease. Recent medical cell therapy using polymeric biomaterialloaded stem cells with the capability of differentiation to specific neural population has directed focuses toward the recovery of CNS. Fibers that can provide topographical, biochemical and electrical cues would be attractive for directing the differentiation of stem cells into electro-responsive cells such as neuronal cells. Here we report on the fabrication of an electrospun polypyrrole/polylactide composite nanofiber film that direct or determine the fate of mesenchymal stem cells (MSCs), via combination of aligned surface topography, and electrical stimulation (ES). The surface morphology, mechanical properties and electric properties of the film were characterized. Comparing with that on random surface film, expression of neurofilament-lowest and nestin of human umbilical cord mesenchymal stemcells (huMSCs) cultured on film with aligned surface topography and ES were obviously enhanced. These results suggest that aligned topography combining with ES facilitates the neurogenic differentiation of huMSCs and the aligned conductive film can act as a potential nerve scaffold.
Keywords: human umbilical cord mesenchymal stemcells; neurogenic differentiation; conductive composite film; electrospun nanofibers; electrical stimulation

The combination of graphene oxide (GO) with robust mechanical property, silk fibroin (SF) with fascinating biological effects and hydroxyapatite (HA) with superior osteogenic activity is a competitive approach to make novel coatings for orthopedic applications. Herein, the feasibility of depositing ternary SF/GO/HA nanocomposite coatings on Ti substrate was firstly verified by exploiting electrophoretic nanotechnology, with SF being used as both a charging additive and a dispersion agent. The surface morphology, microstructure and composition, in vitro hemocompatibility and in vitro cytocompatibility of the resulting coatings were investigated by SEM, Raman, FTIR spectra and biocompatibility tests. Results demonstrated that GO, HA and SF could be co-deposited with a uniform, smooth thin-film morphology. The hemolysis rate analysis and the platelet adhesion test indicated good blood compatibility of the coatings. The human osteosarcoma MG63 cells displayed well adhesion and proliferation behaviors on the prepared coatings, with enhanced ALP activities. The present study suggested that SF/GO/HA nanocomposite coatings could be a promising candidate for the surface functionalization of biomaterials, especially as orthopedic implant coating.
Keywords: graphene oxide; silk fibroin; hydroxyapatite; nano-biocomposite coatings; electrodeposition

Blood compatibility of zinc–calcium phosphate conversion coating on Mg–1.33Li–0.6Ca alloy by Yu-Hong Zou; Rong-Chang Zeng; Qing-Zhao Wang; Li-Jun Liu; Qian-Qian Xu; Chuang Wang; Zhi-Wei Liu (281-289).
Magnesium alloys as a new class of biomaterials possess biodegradability and biocompatibility in comparison with currently used metal implants. However, their rapid corrosion rates are necessary to be manipulated by appropriate coatings. In this paper, a new attempt was used to develop a zinc-calcium phosphate (Zn-Ca-P) conversion coating on Mg-1.33Li-0.6Ca alloys to increase the biocompatibility and improve the corrosion resistance. In vitro blood biocompatibility of the alloy with and without the Zn-Ca-P coating was investigated to determine its suitability as a degradable medical biomaterial. Blood biocompatibility was assessed from the hemolysis test, the dynamic cruor time test, blood cell count and SEM observation of the platelet adhesion to membrane surface. The results showed that the Zn-Ca-P coating on Mg-1.33Li-0.6Ca alloys had good blood compatibility, which is in accordance with the requirements for medical biomaterials.
Keywords: magnesium alloy; lithium; biomaterial; biocompatibility; zinc–calcium phosphate coating

Natural sunlight irradiated flower-like CuS synthesized from DMF solvothermal treatment by Wei Zhao; Zihao Wang; Lei Zhou; Nianqi Liu; Hongxing Wang (290-299).
Three-dimensional CuS hierarchical crystals with high catalytic activity had been successfully fabricated using a facile solvothermal process. The CuS microparticles showed different flower-like morphology and good dispersion by optimizing reaction conditions. It was found that using N,N-dimethylformamide (DMF) as the solvent reagent in the proper temperature conditions was favorable for the growth of hierarchically structured CuS. The hexagonal flower-like CuS synthesized at 170°C for 60 min displayed broad-spectrum photocatalytic properties under ultraviolet (UV) and visible irradiation. The as-prepared CuS crystals exhibited good performance to decolorize methylene blue (MB) solution under visible light irradiation. The total organic carbon (TOC) removal of rhodamine B (RhB) solution was nearly 60% after 5 h of the natural sunlight irradiation, and the property was stable after testing over four recycles, demonstrating a potential application in waster water treatment.
Keywords: photocatalysis; CuS; hierarchical structure; natural sunlight irradiation

In situ anchor of magnetic Fe3O4 nanoparticles onto natural maifanite as efficient heterogeneous Fenton-like catalyst by Hang Zhao; Ling Weng; Wei-Wei Cui; Xiao-Rui Zhang; Huan-Yan Xu; Li-Zhu Liu (300-309).
In situ anchor of magnetic Fe3O4 nanoparticles (NPs) onto the surface of natural maifanite was realized by chemical oxidation coprecipitation in hot alkaline solution. The Fe3O4/maifanite composites were characterized by XRD, FTIR, SEM, and TEM. These results indicated that polycrystalline Fe3O4 NPs with inverse spinel structure were formed and tightly dispersed on maifanite surface. Based on the measurement of surface Zeta potential of maifanite at different medium pHs, the possible combination mechanism between natural maifanite and Fe3O4 NPs was proposed. Then, the asobtained composites were developed as highly efficient heterogeneous Fenton-like catalyst for the discoloration of an azo dye, Methyl Orange (MO). The comparative tests on MO discoloration in different systems revealed that Fe3O4/maifanite composite exhibited much higher Fenton-like catalytic activity than Fe3O4 NPs and the heterogeneous Fentonlike reaction governed the discoloration of MO. Kinetic results clearly showed that MO discoloration process followed the second-order kinetic model. Fe3O4/maifanite composites exhibited the typical ferromagnetic property detected by VSM and could be easily separated from solution by an external magnetic field.
Keywords: maifanite; Fe3O4 NPs; mechanism; Fenton-like catalyst; Methyl Orange (MO)

TiO2/g-C3N4 heterojunctions: In situ fabrication mechanism and enhanced photocatalytic activity by Li-Cheng Wu; Hang Zhao; Li-Guo Jin; Huan-Yan Xu (310-319).
In situ fabrication of TiO2/g-C3N4 (TCN) heterojunctions was achieved by a modified sol-gel method. TG analysis was employed to determine the content of TiO2 in TCN composites. XRD, FTIR, TEM and HRTEM were used to analyze the phase composition, functional groups, morphology and microstructure of as-obtained products, respectively. Based on the measurement of surface Zeta potential of g-C3N4, a possible mechanism on in situ fabrication of TCN heterojunctions was concluded. The control experiments indicated that TCN heterojunctions exhibited better photocatalytic performance than either TiO2 or g-C3N4, suggesting that the enchanced photocatalytic activity could be realized by TCN heterojunctions. Then, the evaluation of parameters affecting the photocatalytic performance of TCN heterojunctions was investigated. Even after five cycles, TCN heterojunctions still maintained high photocatalytic activity, exhibiting the good photocatalytic stability. UV-vis absorption spectra showed that almost all MB molecules were decomposed in the photocatalytic process. Finally, the possible mechanism on enhanced photocatalytic performance of TCN heterojunctions was discussed.
Keywords: TiO2 ; g-C3N4 ; heterojunctions; photocatalytic mechanism

Preparation of microstructure-controllable superhydrophobic polytetrafluoroethylene porous thin film by vacuum thermal-evaporation by Na Yi; Shanhu Bao; Huaijuan Zhou; Yunchuan Xin; Aibin Huang; Yining Ma; Rong Li; Ping Jin (320-327).
The three-dimensional porous network polytetrafluoroethylene (PTFE) thin films were achieved by a vacuum technique through evaporating the pure PTFE powders. The surfaces of PTFE thin films showed various morphologies by adjusting the evaporation temperature and the corresponding contact angle ranging from 133° to 155°. Further analyses of surface chemical composition and morphology by FTIR and FE-SEM revealed that the origin of hydrophobicity for the PTFE thin films could be ascribed to the fluorine-containing groups and the surface morphologies, indicating that abundant -CF2 groups and network structures with appropriate pore sizes played a vital role in superhydrophobicity. By characterization of UV-Vis, the films also showed high transmittance and antireflection effect. The films prepared by this simple method have potential applications such as waterproof membrane and self-cleaning coating.
Keywords: polytetrafluoroethylene (PTFE); vacuum; superhydrophobicity

Up-converted ultraviolet luminescence of Er3+:BaGd2ZnO5 phosphors for healthy illumination by Ya Zhang; Qingzhi Cui; Zhanyong Wang; Gan Liu; Tian Tian; Jiayue Xu (328-333).
Moderate level of exposure to the solar irradiation containing UV component is essential for health care. To incorporate the UV-emitting phosphors into the commercial YAG-based white light-emitting diode introduces the possibilities of healthy illumination to individuals’ daily lives. 1 mol.% Er3+-doped BaGd2ZnO5 (BGZ) particles were synthesized via sol-gel method and efficient up-converted luminescence peaked at 380 nm was detected under 480 nm excitation. The mixed phosphors with varied mass ratio of Er3+:BGZ and Ce3+:YAG particles were encapsulated to form LEDs. The study of the LEDs indicated that the introduction of BGZ component favored the enhancement of color-rendering index and the neutralization of the white light emitting. The WLED with the BGZ/YAG ratio of 8:2 was recommendable for its excellent overall white light luminous performances and UV intensity of 84.55 mW/cm2. The UV illumination dose of the WLEDs with mixed YAG and BGZ was controllable by adjusting the ratio, the illumination distance and the illumination time. Er3+:BGZ phosphors are promising UVemitting phosphors for healthy indoor illumination.
Keywords: Er3+:BaGd2ZnO5 ; up-converted ultraviolet; LED; healthy illumination