Yangqiao Liu

Enhanced dye-sensitized solar cell using graphene-TiO2 photoanode prepared by heterogeneous coagulation

In this study, dye-sensitized solar cells (DSSC) were fabricated using graphene-TiO2 composite photoanodes. The graphene-TiO2 nanocomposites were prepared using the heterogeneous coagulation between Nafion-coated graphene and commercial TiO2 (P25) nanoparticles, which ensured a tight interfacial binding between them. The DSSC incorporating 0.5 wt  % graphene in the TiO2 photoanode demonstrates a power conversion efficiency of 4.28%, which is 59% higher than that without graphene. It was found that the incorporated graphene caused both increased dye adsorption and significant longer electron lifetime. Graphene has been proven to be an attractive candidate for improving the performance of DSSC.

Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution

The preparation of polyethersulfone (PES) hollow fiber membranes has been studied using N-methylpyrrolidone (NMP) as solvent, polyethylene glycol 400 (PEG 400) as weak nonsolvent and water as strong nonsolvent. When PEG 400 is used as polymeric additive to the spinning dope the viscosity of the PES solution is strongly enhanced. Furthermore, it was observed that PEG 400 could be added to the solution in large amounts without causing phase separation (NMP/PEG ratio 1:9, PES concentration approximately 11 wt.%). Membranes prepared from a solution containing a NMP/PEG ratio of 1:1 results in higher fluxes than when a ratio of 1:4 is used. Similar fluxes were obtained for PES concentrations of 16 and 20 wt.%. Looking at the fiber cross-section it became clear that macrovoid formation could not be suppressed by the addition of PEG 400 alone, not even at concentrations as high as 38 wt.%. Only when relatively large amounts of water were added to the dope solution macrovoids disappeared and nice spongy structures were obtained. Variation of the bore liquid composition using the components NMP, PEG 400 and water showed to be a powerful method to control the pore size of the bore surface. Pores of 5–28 nm were obtained in combination with high pure water fluxes; e.g. a membrane with pores of 7 nm had a pure water flux of 940 l/(m2 h bar) and showed 100% BSA retention. When an air gap larger than 10 mm was applied the shell surface contained relatively large pores. Spinning directly in water (airgap=0) resulted in shell side pores of 8–10 nm, while an air gap of 10 mm resulted in pore sizes of 40–54 nm.

Investigation of induction period and morphology of CaCO3 fouling on heated surface

The accumulation of unwanted crystalline deposits (fouling) reduces the efficiency of heat exchanger considerably. In order to mitigate fouling, many measures have been taken including the use of low-energy surface and antifoulant. In this investigation, the CaCO3 fouling experiments in both cooling water and pool-boiling systems were performed, the induction period as well as the removal of fouling was studied, and the fouling morphology was also investigated by scanning electron microscopy and atomic force microscopy (AFM). Compared with the copper surface, the self-assembled monolayers low-energy surface can prolong the induction period of fouling in the cooling water system. The induction period increases with decreasing initial surface temperature and fluid velocity. When the heat flux is fixed in different experiments, an increase in the fluid velocity will result in a decrease in the initial surface temperature. Under this condition, owing to the interactional effects between surface temperature and fluid velocity, the induction period increases with increasing fluid velocity. The removal experiments were carried out both in the induction period and in the post-induction period. The results show that only in the induction period can the fouling resistance be reduced owing to the weaker adhesion strength of fouling. In the presence of antifoulant polyacrylic acid (PAA), the crystal forms are changed and the fractal dimensions of CaCO3 morphologies increase for both the cooling water and the pool-boiling systems. AFM images show that the steps are bunched for CaCO3 formed in the pool-boiling system, and in the presence of PAA, the step spacing is widened compared to the case in the absence of PAA.

Stable Nafion-functionalized graphene dispersions for transparent conducting films

Nafion was used for the first time to aid in preparing stable graphene dispersions in mixed water/ethanol (1:1) solvents via the reduction of graphite oxide using hydrazine. The dispersion was characterized by ultraviolet-visible (UV-vis) spectra, transmission electron microscopy, zeta potential analysis, etc. It was found that for Nafion-to-graphene ratios higher than 5:1, graphene solutions with concentrations up to 1 mg ml(-1) and stabilities of over three months were obtained. It was proposed that the Nafion adsorbed onto the graphene by the hydrophobic interaction of its fluoro-backbones with the graphene layer and imparted stability by an electrosteric mechanism. Furthermore, transparent and conductive films were prepared using these highly stable Nafion-stabilized graphene dispersions. The prepared Nafion-graphene films possess smooth and homogeneous surfaces and the sheet resistance was as low as 30 kOmega/sq for a transmittance of 80% at 550 nm, which was much lower than for other graphene films obtained by chemical reduction. X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the p-doping of the graphene by Nafion. It was expected that this p-doping effect, as well as the high dispersing ability of Nafion for graphene and the connection of the sp(2) domains by residual Nafion combined to produce good properties of the Nafion-graphene films.

Assembly of CdSe nanoparticles on graphene for low-temperature fabrication of quantum dot sensitized solar cell

Quantum dot sensitized solar cell (QDSSC) was fabricated in a low-temperature process based on graphene-CdSe composite, which was prepared by the procedures of immobilizing CdSe on graphene oxide (GO) and reduction in GO. It was found that the charge separation between excited CdSe and graphene could be tapped to generate photocurrent from photocurrent measurements, which established the ability of graphene to collect and transport electrons. By optimizing the CdSe-to-GO ratio at 4.5:1, the obtained graphene-CdSe-based cells exhibited significantly higher short-circuit photocurrent and energy conversion efficiency (5.8 mA/cm2 and 0.72%, respectively) than the reported values of the C60 or carbon nanotube related QDSSC, demonstrating that the graphene-CdSe composite is an attractive candidate in energy conversion devices.

Adsorption of salicylic acid, 5-sulfosalicylic acid and Tiron at the alumina–water interface

Abstract The influences of three multivalent anionic electrolytes: salicylic acid, 5-sulfosalicylic acid and 1,2-dihydroxy-3,5-benzenedisulfonic acid disodium salt (Tiron) on the properties of alumina aqueous suspensions have been investigated in this paper. Zeta potential measurements show that the addition of Tiron results in a more dramatic increase in the absolute zeta potential in the alkaline region, as well as a shift of isoelectric point to the more acidic region than salicylic acid and 5-sulfosalicylic acid. Adsorption of a dispersant is promoted strongly by forming a complex between dispersant and Al atom. The adsorption ability is also related to molecular volume of the dispersant and its number of functional group which creates surface charge. The surface properties of alumina suspensions are examined by using FTIR analysis. The efficiency of the dispersant is characterized by observing the morphology of the sediments using scanning electron microscopy and measuring the sediment volume. The results show that Tiron is more effective than the other two dispersants.

A facile method to observe graphene growth on copper foil.

A novel scanning electron microscope (SEM) method is presented for high contrast identification of each layer of pyramidal graphene domains grown on copper. We obtained SEM images by combining the advantages of the high resolution property of the secondary electron signal and the elemental sensitivity of the backscattering electron signal. Through this method, we investigated the difference in the growth mechanisms of mono-layer and few-layer graphene. Due to different lattice mismatches, both the surface adsorption process and the epitaxial growth process existed under the atmospheric growth conditions. Moreover, the copper oxidation process can be easily discovered. It is obvious from the SEM images that the graphene greatly delayed the oxidation process of the copper surface. Finally, the nucleation and growth speed of graphene domains was found to depend on the linear array distribution of surface ledges and terraces of annealed rolled copper foil. This result explains the linear rows of graphene during the growth process and accords with theoretical results.

A bilayer structure of a titania nanoparticle/highly-ordered nanotube array for low-temperature dye-sensitized solar cells

A chemical-free, environmentally friendly and harmless water immersion method is developed to delaminate anode oxidationtitania nanotube (TNT) arrays from Ti foils to obtain a free-standing TNT membrane. The as-synthesized TNT membrane with tunable thickness which is controlled by anodization time are used in low-temperature dye-sensitized solar cells (DSSCs) as a second layer to improve the DSSC efficiency. Compared with the common titania nanoparticle electrode, a water-delaminated TNT membrane electrode of 16 μm in thickness can significantly enhance the DSSCs photovoltaic performance with a short-circuit current density from 3.87 to 7.63 mA cm−2 and a fill factor from 0.637 to 0.708, which combinatorially lead to a remarkable increment of photo conversion efficiency from 1.70% to 3.68%. The results show that, owing to its vertically aligned ordered nanotube structure, the TNT arrays exhibit trifunctional behavior when used as an electrode, which are charge generation, light scattering and I3− diffusion improvement.

Effect of acrylic copolymer adsorption on the colloidal stability of a 3Y-TZP suspension

This article investigates the adsorption of an acrylic acid/acrylate copolymer on 3Y-TZP and its influence on the stability of colloidal 3Y-TZP aqueous suspensions. Potentiometric titration shows that ionization degree of the copolymer increases from 0 at pH 2.5 to 1 at pH 9. The adsorption is characterized using the chemical oxygen demand (COD) method. The influences of pH, copolymer concentration and electrolyte concentration are investigated. The adsorption density is found to increase pronouncedly with increasing KNO3 concentration at higher pH, while is almost independent upon KNO3 concentration at lower pH. The zeta potential of Y-TZP decreases as a result of copolymer adsorption. The optimum copolymer concentration at pH 8.8 is around 2 dry mass base (dmb)%, determined by both effective diameter measurements and rheological characterization. The stabilization of the copolymer for the suspension is also evidenced by scanning electron microscopy (SEM) analysis of the sediments. The fractal dimensions of sediments obtained with and without dispersant were determined and compared using the limited fractal dimension method. # 2002 Elsevier Science Ltd. All rights reserved.

A facile way to prepare nanoporous PbI2 films and their application in fast conversion to CH3NH3PbI3

In this report, we demonstrate a facile way to prepare PbI2 films with interpenetrating nanopores. The nanoporous PbI2 (n-PbI2) films were prepared by the solvent–solvent extraction (SSE) method, in which the DMF solvent was effectively extracted by isopropanol (IPA) within seconds, resulting in well-crystallized n-PbI2 films without annealing. The mechanism involved in the preparation of n-PbI2 films using the SSE method was studied further, and some universal rules for fabricating n-PbI2 films with the SSE method were proposed. The interpenetrating nanoporous morphology enabled fast penetration of the CH3NH3I (MAI) solution, so most of the PbI2 converted into CH3NH3PbI3 within 10 s even with a perovskite overlayer of 300 nm. Moreover, the perovskite layer was pinhole-free and smoother than that based on a conventional PbI2 film. Consequently, perovskite solar cells based on n-PbI2, with the setup as FTO/compact TiO2/bilayer CH3NH3PbI3/P3HT/Ag, delivered an impressive power conversion efficiency of 10.1%, compared with 5.9% for its counterpart based on a conventional compact PbI2 film. This work unveils the PbI2-morphology-related reaction kinetics in the two-step method, and will contribute to understanding the role PbI2 films play in the preparation of perovskites.