T. A. Nirmal Peiris

Insights into mechanical compression and the enhancement in performance by Mg(OH)2 coating in flexible dye sensitized solar cells

The engineering of flexible dye sensitized solar cells (DSCs) by mechanical compression is one of the methods that allow low temperature processing of these devices. However, suppressing the high temperature sintering process also significantly reduces the performance of the cells. In our previous work [J. Phys. Chem. C, 2012, 116, 1211], we have attempted to improve flexible DSC performance by coating the porous TiO2 photoanode with an electrodeposited Mg(OH)2 layer. In that work, we have obtained one of the highest photovoltages reported to date in flexible DSCs (847 mV). In order to gain more insights into the reasons for both poorer performance of compressed cells and the origin of the voltage enhancement achieved by the Mg(OH)2 coating, here we present an in-depth study by means of electrochemical impedance spectroscopy, Mott-Schottky plots analysis and open-circuit voltage decays. The existence of a shunt resistance in the mechanically compressed cells is revealed, causing an additional drawback to the poor inter-particle necking. By introducing the Mg(OH)2 coating the recombination in the cell becomes significantly reduced, being the key reason which is responsible for the higher photovoltage. Additionally, the coating and the compression cause modifications in the surface states and in the nature of the interfaces with the electrolyte. This induces TiO2 conduction band displacements and shifts of the relative position of the modified states that influence the performance.

Electrochemical determination of the density of states of nanostructured NiO films.

Mesoporous p-type NiO films were prepared by aerosol-assisted chemical vapor deposition (AACVD) and characterized by X-ray diffraction (XRD). The nanostructure of the films was investigated by field emission gun scanning electron microscopy (FEG-SEM). The density of states (DOS) in these nanostructured films has been determined by means of electrochemical impedance spectroscopy and cyclic voltammetry. The analysis reveals an exponential distribution of band gap states above the valence band that extends around 1.5 eV. In addition, monoenergetic states were also identified which overlap with the exponential distribution. This distribution of states has an enormous influence in the electronic processes of the devices in which NiO electrodes are employed (electrochromism, water splitting or energy storage). Especially, in p-type dye-sensitized solar cells (p-DSCs), it is thought that intra-band-gap states are responsible for the fast observed recombination processes, whose existence and distribution has not been clearly determined yet and are now confirmed and quantified by our analysis. This provides a better comprehension of the recombination events which represent one of the main losses in p-DSCs.

Preparation and characterization of mesoporous hydroxyapatite with non-cytotoxicity and heavy metal adsorption capacity

Mesoporous hydroxyapatite (MPHA) particles have recently gained a great deal of interest in a broad range of fields including biomedical fields, wastewater treatment and catalysis. In this paper, we describe a novel and convenient route to synthesise MPHA particles using calcium carbonate nanoparticle templates for the formation of mesopores in HA. In this method, both nano-CaCO3 templates and HA are prepared simultaneously using calcium sucrate as a precursor by allowing the nano-CaCO3 to embed in HA. Mesopores in HA are obtained by removing the template. The porosity of HA is confirmed by Brunauer–Emmett–Teller (BET) analysis and the average pore size (below 50 nm) is determined from Transmission Electron Microscopy (TEM) images. The synthesized material is noncytotoxic as confirmed by cytotoxicity studies, which makes it a potential candidate as a biomaterial for biomedical applications. Furthermore, the MPHA shows a superior adsorption ability towards Pb2+ Ni2+ and Cd2+ in aqueous solutions, and could also be used as an environmental-friendly material for wastewater treatment and water purification. Therefore, we believe that this simple and novel synthesis route for the fabrication of porous HA could be useful in fulfilling the current demand for HA in biomedical and water purification applications.