Bin Ding

Electrospinning processed nanofibrous TiO2 membranes for photovoltaic applications

We have recently fabricated dye-sensitized solar cells (DSSCs) comprising nanofibrous TiO(2) membranes as electrode materials. A thin TiO(2) film was pre-deposited on fluorine doped tin oxide (FTO) coated conducting glass substrate by immersion in TiF(4) aqueous solution to reduce the electron back-transfer from FTO to the electrolyte. The composite polyvinyl acetate (PVac)/titania nanofibrous membranes can be deposited on the pre-deposited thin TiO(2) film coated FTO by electrospinning of a mixture of PVac and titanium isopropoxide in N,N-dimethylformamide (DMF). The nanofibrous TiO(2) membranes were obtained by calcining the electrospun composite nanofibres of PVac/titania as the precursor. Spectral sensitization of the nanofibrous TiO(2) membranes was carried out with a ruthenium (II) complex, cis-dithiocyanate-N,N()-bis(2,2()-bipyridyl-4,4()-dicarboxylic acid) ruthenium (II) dihydrate. The results indicated that the photocurrent and conversion efficiency of electrodes can be increased with the addition of the pre-deposited TiO(2) film and the adhesion treatment using DMF. Additionally, the dye loading, photocurrent, and efficiency of the electrodes were gradually increased by increasing the average thickness of the nanofibrous TiO(2) membranes. The efficiency of the fibrous TiO(2) photoelectrode with the average membrane thickness of 3.9xa0µm has a maximum value of 4.14%.

Formation of novel 2D polymer nanowebs via electrospinning

We have found a procedure for generating novel two-dimensional (2D) nanowebs in three-dimensional (3D) fibrous mats by optimization of various processing parameters during electrospinning. The electrospun fibres act as a support for the fishnet-like nanowebs comprising interlinked one-dimensional (1D) nanowires. The average diameter of the nanowires contained in typical nanowebs is about one order of magnitude less than that of conventional electrospun fibres. The formation of the nanowebs of poly(acrylic acid) (PAA) and nylon-6 is considered to be due to the electrically forced fast phase separation of the charged droplets which move at high speed between the capillary tip and the collector. The formation, morphology and area density of the nanowebs in electrospun fibrous mats are strongly affected by the applied voltage, ambient relative humidity, kinds of solvents, solution concentration and distance between the capillary tip and the collector.

Fabrication of a silver-ragwort-leaf-like super-hydrophobic micro/nanoporous fibrous mat surface by electrospinning

Inspired by the self-cleaning silver ragwort leaf, we have recently fabricated a biomimetic super-hydrophobic fibrous mat surface comprising micro/nanoporous polystyrene (PS) microfibres via electrospinning. The rough surface of the silver ragwort leaf fibres, with nanometre-sized grooves along the fibre axis, was imitated by forming micro- and nanostructured pores on the electrospun fibre surface. The solvent composition ratios of tetrahydrofuran (THF) to N,N-dimethylformamide (DMF) in PS solutions were proved to be the key parameter to affect the fibre surface structures due to the various phase separation speeds of the solvents from PS fibres during electrospinning. The combination of the hierarchical surface roughness inherent in electrospun microfibrous PS mats and the low surface free energy of PS yielded a stable super-hydrophobicity with water contact angles as high as 159.5° for a 12 mg water droplet, exceeding that (147°) of the silver ragwort leaf. Moreover, the hydrophobicity of the porous PS mat surface was found to increase on increasing the surface roughness of the microfibres.

Multi-core cable-like TiO2 nanofibrous membranes for dye-sensitized solar cells

Multi-core cable-like TiO2 nanofibres were fabricated by calcination of composite polyvinyl acetate (PVAc)/titania nanofibres with a hot pressing pre-treatment. This resultant novel fibre structure was composed of sheaths of 200?nm in diameter and 25?nm in wall thickness, and cores filled with 24?nm thick TiO2 fibrils. The formation of multi-core cable-like structures of fibres is considered to be due to the enhanced phase separation of PVAc-rich and TiO2-rich phases during the hot pressing process. The BET results showed that the specific surface area of pressed TiO2 membranes was much higher than that of unpressed TiO2 membranes. In this study, the novel multi-core cable-like TiO2 fibrous membranes were used as electrode materials for dye-sensitized solar cells (DSSCs). It was observed that the photocurrent and conversion efficiency of the electrodes increased concurrently with increasing applied pressure and average membrane thickness in the range of 1?9??m. The maximum short circuit photocurrent and energy conversion efficiency were 16.09?mA?cm?2 and 5.77% when the membrane had an average thickness of 9.21??m and 8?MPa applied pressure.

Layer-by-layer structured films of TiO2 nanoparticles and poly(acrylic acid) on electrospun nanofibres

We report a new approach for fabricating layer-by-layer (LBL) structured ultrathin hybrid films on electrospun nanofibres. Oppositely charged anatase TiO2 nanoparticles and poly(acrylic acid) (PAA) were alternately deposited on the surface of negatively charged cellulose acetate (CA) nanofibres using the electrostatic LBL self-assembly technique. The fibrous mats were characterized by wide-angle x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and Brunauer–Emmett–Teller (BET) surface area techniques. The crystalline phase of anatase TiO2 remained unchanged in the resultant TiO2/PAA films coated on CA fibrous mats. Moreover, the TiO2/PAA film coated fibres showed rough surfaces with grains due to the deposition of aggregated TiO2 particles. The average diameter of the fibres increased from 344 to 584xa0nm and the BET surface area of the fibrous mats increased from 2.5 to 6.0xa0m2xa0g−1 after coating with five bilayers of TiO2/PAA films.

Conversion of an electrospun nanofibrous cellulose acetate mat from a super-hydrophilic to super-hydrophobic surface

We report a new approach to convert an electrospun nanofibrous cellulose acetate mat surface from super-hydrophilic to super-hydrophobic. Super-hydrophilic cellulose acetate nanofibrous mats can be obtained by electrospinning hydrophilic cellulose acetate. The surface properties of the fibrous mats were modified from super-hydrophilic to super-hydrophobic with a simple sol–gel coating of decyltrimethoxysilane (DTMS) and tetraethyl orthosilicate (TEOS). The resultant samples were characterized by field emission scanning electron microscopy (FE-SEM), x-ray photoelectron spectroscopy (XPS), water contact angle, Brunauer–Emmett–Teller (BET) surface area, atomic force microscopy (AFM), and UV–visible measurements. The results of FE-SEM and XPS showed that the sol–gel (I) films were formed on the rough fibrous mats only after immersion in sol–gel. After the sol–gel (I) coating, the cellulose acetate fibrous mats formed in both 8 and 10 wt% cellulose acetate solutions showed the super-hydrophobic surface property. Additionally, the average sol–gel film thickness coated on 10 wt% cellulose acetate fibrous mats was calculated to be 80 nm. The super-hydrophobicity of fibrous mats was attributed to the combined effects of the high surface roughness of the electrospun nanofibrous mats and the hydrophobic DTMS sol–gel coating. Additionally, hydrophobic sol–gel nanofilms were found to be transparent according to UV–visible measurements.

Polyoxometalate nanotubes from layer-by-layer coating and thermal removal of electrospun nanofibres

We have recently fabricated Keggin-type polyoxometalate (H4SiW12O40) nanotubes by calcining layer-by-layer (LBL) structured ultrathin hybrid film coated electrospun fibrous mats. The hybrid film coated electrospun fibrous mats were obtained from the alternate deposition of positively charged poly(allylamine hydrochloride) (PAH) and negatively charged H4SiW12O40 on the surface of negatively charged cellulose acetate (CA) nanofibres using the electrostatic LBL self-assembly technique. The fibrous mats were characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared (FT-IR) spectroscopy, and wide-angle x-ray diffraction (WAXD). It was found that the morphology of hybrid PAH/H4SiW12O40 film coated fibrous mats was strongly influenced by the number of deposition bilayers and the pH of dipping solutions. The fibrous mats were contracted and H4SiW12O40 nanotubes with a wall thickness of about 50 nm can be fabricated after calcination at high temperature. Additionally, the FT-IR and WAXD results indicated that the pure H4SiW12O40 nanotubes were obtained with a Keggin structure.

Ammonia sensors based on electrospun poly(acrylic acid) fibrous membranes

A novel gas sensor composed of electrospun nanofibrous membranes (FM) and quartz crystal microbalance (QCM) was successfully fabricated. The electrospun nanofibers can be deposited on the QCM electrode by electrospinning the homogenous blend solutions of cross-linkable poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). Moreover, the PAA fibrous membranes with different morphology can be deposited on the QCM electrode by electrospinning the PAA solutions with various solvent compositions of H/sub 2/O and ethanol. Sensing experiments were examined by measuring the resonance frequency shifts of QCM due to the additional mass loading. The results showed that the sensing properties were mainly affected by the content of the PAA component in nanofibrous membranes, the morphology of the fibrous membranes, the concentration of NH/sub 3/, and the relative humidity. Additionally, the sensitivity of the FM coated QCM (FM-QCM) sensor was much higher than that of a continuous film coated QCM (CF-QCM) sensor. Furthermore, the PAA FM-QCM sensors exhibited high sensitivity towards low concentrations of ammonia, as low as 130 ppb at the relative humidity of 40 %. The pre-sorbed water in the fibrous membranes was proved to be the key factor affecting the sensitivity of FM-QCM sensors for ammonia.