Solar Energy Conversion (SEC) Cluster Research Collection
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- PublicationPhase diagrams of microemulsions containing reducing agents and metal salts as bases for the synthesis of metallic nanoparticlesWe studied the phase diagrams of microemulsions with a view to using these systems for the synthesis of metallic Pt, Pb, and Bi nanoparticles as well as of intermetallic Pt/Pb and Pt/Bi nanoparticles. The microemulsions consisted of H2O/salt – n-decane – SDS – 1-butanol. The salt was either one metal precursor (H2PtCl6·6H2O, Pb(NO3)2, or Bi(NO3)3·5H2O), a mixture of two metal precursors (H2PtCl6·6H2O + Pb(NO3)2 or H2PtCl6·6H2O + Bi(NO3)3·5H2O), or the reducing agent (NaBH4). In addition, other salts needed to be added in order to solubilize the metal precursors, to stabilize the reducing agent, and to adjust the ionic strength. Combining the microemulsion (μe1) that contains the metal precursor(s) with the microemulsion (μe2) that contains the reducing agent leads to metallic nanoparticles. To study systematically how the shape and size of the synthesized metallic nanoparticles depend on the size and shape of the respective microemulsion droplets, first of all one has to find those conditions under which μe1 and μe2 have the same structure. For that purpose we determined the water emulsification failure boundary (wefb) of each microemulsion as it is at the wefb where the water droplets are known to be spherical. We found that the ionic strength (I) of the aqueous phase as well as the hard acid and hard base properties of the ions are the key tuning parameters for the location of the wefb.
Scopus© Citations 23 929 - PublicationGelled Polymerizable Microemulsions. Part 3 Rheology(RSC publications, 2009)
; ; ; ; This is the first report on the rheological properties of oil-gelled polymerizable bicontinuous microemulsions. The polymerizable base system consists of H2O / NIPAm / BisAm – n-dodecane – C13/15E5 (a technical grade n-alkyl polyglycol ether), where NIPAm denotes the monomer N-isopropylacrylamide and BisAm the cross-linker N,N′-methylene bisacrylamide. For the planned polymerization of the aqueous phase a scaffold is needed to preserve the structure of the templating microemulsion during the polymerization. This scaffold is supposed to be a gel, which was formed by adding the gelator 12-hydroxyoctadecanoic acid (12-HOA) to the oil phase of the microemulsion. The influence of the water-to-oil ratio α, of the gelator concentration β, and of the monomer concentration ψ on the rheological behavior of gelled microemulsions has been studied in detail. The most important result of the study at hand is the observation of a transition from a high viscous solution to a gel, i.e. from a transient to a permanent network, with increasing gelator concentration and increasing amount of the oil phase, respectively. In other words, it is only under well-defined conditions that an oil-gelled microemulsion is formed.Scopus© Citations 16 477 - PublicationSpectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes : intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres(RSC publications, 2009)
; ; ; ; ; ; A series of binuclear ruthenium and osmium complexes [(bipy)2Ru(qpy)Ru(bipy)2]4+ (1), [(bipy)2-Os(qpy)Os(bipy)2]4+ (2)[(bipy)2Ru(pytr-bipy)Ru(bipy)2]3+ (3), [(bipy)2Ru(pytr-bipy)Os(bipy)2]3+ (4), [(bipy)2Os(pytr-bipy)Ru(bipy)2]3+(5) and [(bipy)2Os(bpbt)Os(bipy)2]2+ (6) {bipy = 2,2¹-bipyridyl; qpy = 2,2¹:5¹,5¹¹:2¹¹,2¹¹¹-quaterpyridyl; pytr-bipy = 3-(2,2¹-bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato, and bpbt = 5,5¹-bis-(pyrid-2¹¹-yl)-3,3¹-bis-1,2,4-triazolato} are reported. Analysis of the electrochemical data focuses on structural factors and on determining the extent of electronic communication between the metal centres in the mixed valence oxidation state. Intervalence charge transfer (IT) bands could be identified in the spectra of the complexes 4 and 6 only. Analysis of their spectroelectrochemical data leads to the conclusion that the IT is superexchange mediated through the HOMO of the bridging ligand.Scopus© Citations 16 466 - PublicationFirst-principles calculation of nitrogen-tungsten codoping effects on the band structure of anatase-titaniaThe electronic properties and photocatalytic activity of nitrogen (N) and/or tungsten (W)-doped anatase are calculated using density functional theory. For N-doping, isolated N 2p states above the top of the valence band are responsible for experimentally observed redshifts in the optical absorption edge. For W-doping, W 5d states below the conduction band lead to band gap narrowing; the transition energy is reduced by 0.2 eV. Addition of W to the N-doped system yields significant band gap narrowing gap by 0.5 eV. This rationalizes recent experimental data which showed that N/W-doped titania exhibits higher visible-light photocatalytic efficiency than either N- or W-doping alone.
Scopus© Citations 122 1538 - PublicationSynergistic effects of Bi/S codoping on visible light-activated anatase TiO2 photocatalysts from first principlesThe electronic properties and photocatalytic activity of S and/or Bi-doped anatase TiO2 are investigated by first-principles density functional theory calculations. For S-doped TiO2, S 3p states locate above the top of the valence band and mix with O 2p states, leading to band gap narrowing. For Bi-doped anatase, the energy levels of the impurity Bi 6s states lie below the bottom of the conduction band while the Fermi level EF lies above the gap states, indicating the gap states are fully occupied. The transition from Bi 6s to Ti 3d states is responsible for a red-shift of the visible light absorption edge. In Bi/S–doped TiO2, both S 3p acceptor states and partially occupied Bi 6s donor states hybridized with S 3p appear simultaneously; this observation suggests that photocatalytic efficiency would be improved significantly due to greater separation of electron-hole pairs. These findings present a reasonable explanation of recent experimental results.
Scopus© Citations 54 851 - PublicationEnergetic and electronic properties of P Doping at the rutile TiO2 (110) Surface from First PrinciplesThe energetic and electronic properties of various P doping configurations at the rutile TiO2 (110) surface are investigated by first-principles density functional theory (DFT) calculations. Several substitution and adsorption configurations for P impurities at the surface and the subsurface are considered. The stability of the P-doped systems is compared on the basis of the calculated formation energy and adsorption energy. Our calculated results indicate that the P impurities replace surface Ti atoms preferentially under O-rich growth conditions,and surface O atoms under Ti-rich conditions. In addition, it was found that the creation of oxygen vacancies favors P incorporation at substitution sites but not at adsorption sites. Doping with a single P atom into an O site may lead to either anionic or cationic states in the dopant. This causes either band-to-band transitions or introduces gap states to band transitions, with the former corresponding to a small band gap narrowing or broadening and the latter resulting in obvious reductions of photon transition energy. Substitutional replacement of Ti atoms by P atoms and adsorption on the surface (P-cation doping) results in either a small band reduction or a slight band gap enlargement, depending on the doping sites. It is speculated that the interaction between P impurities and surface oxygen vacancies will lead to further enhanced photocatalytic activity in the visible light region.
Scopus© Citations 19 834 - PublicationBand gap engineering of (N, Ta)-codoped TiO2 : a first-principles calculationThe electronic properties and photocatalytic activity of X (N, C) / transition metal (TM=Ta, Hf, Fe) – codoped anatase TiO2 have been investigated using density functional theory. It was found that only the (N, Ta)-codoping case narrows the band gap significantly by about 0.48 eV, driven by the continuum-like p-d hybridized states above the top of valence band and d states at the bottom of conduction band. The calculated energy results suggest that codoping of Ta with N can increase the N concentration in N-doped TiO2 based on energy results.
Scopus© Citations 96 1948 - PublicationThe influence of light intensity, active area and excitation wavelength on the temporal response of a dye sensitized solar cell(Society of Photo-Optical Instrumentation Engineers, 2009-08-02)
; ; ; The temporal response of a biomimetic dye-sensitized solar cell (DSSC) is critically linked to the intensity of the incident light. When a DSSC is partially illuminated and the incoming light is of low intensity, the response time of the cell is prolonged dramatically. In this report, the major components of the DSSC are investigated to find the source and to provide a model of the driving mechanisms behind this delay. For low light level conditions,only deep traps states of the TiO2 layer participate in electron transport resulting in a slow temporal response. Increasing the illumination level thus increases the conductivity of the TiO2 electrode by filling these trap states and increases the response time. This study shows a strong correlation between the light intensity, active area and excitation wavelength on the temporal response time of a DSSC.Scopus© Citations 2 1109 - PublicationCharacterising dye-sensitized solar cells(Society of Photo-Optical Instrumentation Engineers, 2009-08-03)
; ; ; ; ; With growing energy and environmental concerns due to fossil fuel depletion and global warming there is an increasing attention being attracted by alternative and/or renewable sources of power such as biomass, hydropower, geothermal, wind and solar energy. In today’s society there is a vast and in many cases not fully appreciated dependence on electrical power for everyday life and therefore devices such as PV cells are of enormous importance. The more widely used and commercially available silicon (semiconductor) based cells currently have the greatest efficiencies, however the manufacturing of these cells is complex and costly due to the cost and difficulty of producing and processing pure silicon. One new direction being explored is the development of dye-sensitised solar cells (DSSC). The SFI Strategic Research Centre for Solar Energy Conversion is a new research cluster based in Ireland, formed with the express intention of bringing together industry and academia to produce renewable energy solutions. Our specific area of research is in biomimetic dye sensitised solar cells and their electrical properties. We are currently working to develop test equipment, and optoelectronic models describing the performance and behaviors of dye-sensitised solar cells (Grätzel Cells). In this paper we describe some of the background to our work and also some of our initial experimental results. Based on these results we intend to characterise the opto-electrical properties and bulk characteristics of simple dye-sensitised solar cells and then to proceed to test new cell compositions.1661 - PublicationSpontaneous room temperature elongation of CdS and Ag2S nanorods via oriented attachment(ACS Publications, 2009-08-06)
; ; ; ; ; ; ; Spontaneous elongation from nanorod to nanowire in the presence of an amine is reported for nanocrystals of cadmium sulfide and silver sulfide (cation exchanged from CdS). Elongation occurs instantaneously where the final aspect ratio is a controllable multiple of the original nanorod length. Transmission electron microscopy (TEM) analysis reveals the influential factors on the attachment processare the concentration of amine, duration and temperature of the reaction. The elongated nanorods are further characterized by X-ray diffraction (XRD), photoluminescence (PL), ultraviolet-visible spectroscopy(UV-vis) and X-ray photoelectron spectroscopy (XPS). A mechanism of oriented attachment is evidenced by the doubling in length of asymmetrically gold tipped CdS nanorods with the corresponding absence of elongation in symmetrically tipped nanorods.826Scopus© Citations 86 - PublicationPhase diagrams of non-ionic microemulsions containing reducing agents and metal salts as bases for the synthesis of bimetallic nanoparticlesPhase diagrams of microemulsions containing metal salt(s) and reducing agent, respectively, were studied in detail. The microemulsions were based on non-ionic surfactants, namely pure tetraethyleneglycol monododecylether, C12E4, and technical grade Brij30. We studied the influence of the metal salts H2PtCl6, Pb(NO3)2, Bi(NO3)3, H2PtCl6 + Pb(NO3)2 (1:1 mixture), and H2PtCl6 + Bi(NO3)3 (1:1 mixture) as well as of the reducing agent NaBH4 on the location of the phase boundaries. The focus was on the water emulsification failure boundary (wefb) where the aqueous phase forms spherical droplets. The temperature shifts of the wefb, which were caused by the presence of the salt(s), are directly related with the shift of the clouding points of the corresponding oil-free systems. The location of the wefb is affected in a complex manner by the pH (the lower the pH the higher the temperature at which the wefb occurred), the ionic strength and by specific salting-in or salting-out effects of the electrolyte ions. The desired overlap of the wefb of the microemulsions containing the metal salt(s) and the reducing agent, respectively, could be achieved by adding NaOH to the C12E4-based microemulsions and by titrating 1-octanol to the Brij30-based microemulsions, respectively.
1064Scopus© Citations 18 - PublicationCentimetre scale assembly of vertically aligned and close packed semiconductor nanorods from solutionAssembly of CdS nanorods (8 × 100 nm) into vertically aligned arrays over very large areas on a substrate either as a monolayer or several multilayers is shown by electrophoresis.
Scopus© Citations 52 430 - PublicationFirst-principles study of S doping at the rutile TiO2 (110) surfaceThe structural, energetic and electronic properties of various S doping configurations by substitution and adsorption at the rutile TiO2 (110) surface have been investigated by first-principles density functional theory (DFT) calculations. The stability of these configurations has been compared on the basis of the calculated formation and adsorption energies. Our results indicate that S dopants replace surface O atoms or bind to Ti atoms preferentially. Moreover, implantation of S dopants into the rutile lattice favored the formation of oxygen vacancies, which promotes further S incorporation. Doping of single S atoms into Ti sites (S-cation doping) led to relatively small reductions of the photon transition energy, while S-substitution of O atoms (S-anion doping) and adsorption on the surface (S-cation/anion doping) resulted in significant red-shifts of the optical absorption edge. Our results suggest that the interplay between S impurities and oxygen vacancies does not enhance visible light absorption in an obvious way, and helps to rationalise recent experimental studies.
Scopus© Citations 22 1073 - PublicationMagnetic properties of first-row element-doped ZnS semiconductors : a density functional theory investigationBased on first-principles calculations, we have investigated the magnetic properties of the first-row element-doped ZnS semiconductors. Calculations reveal that Be, B, and C dopants can induce magnetic, while N cannot lead to spin polarization in ZnS. A possible explanation was rationalized from the elements’ electronegativity and interaction between dopant atoms and host atoms. The total magnetic moments are 2.00, 3.16, and 2.38 μB per 2 x 2 x 2 supercell for Be, B, and C doping, respectively, and ferromagnetic coupling is generally observed in these cases. The ferromagnetism of Be-, B-, and C-doped ZnS can be explained by hole-mediated s-p or p-p interactions’ coupling mechanisms. However, the clustering effect was found to be in Be-, B-, and C-doped ZnS but the degree is more obvious in the former two cases than in latter case. Analysis revealed that C-doped ZnS displays better potential ferromagnetic behavior than Be- and B-doped ZnS due to its half-metallic characteristic.
1167Scopus© Citations 78 - PublicationEstimation of zeta potentials of titania nanoparticles by molecular simulationNon-equilibrium molecular dynamics (NEMD) simulations have been performed for static electric fields for a range of positively charged spherical rutile–titania nanoparticles with radii of 1.5 to 2.9 nm for two different salt concentrations in water, in order to simulate electrophoresis directly. Using the observed limiting drag velocities, Helmholtz–Smoluchowski (HS) theory was used to estimate their ζ potentials. These estimates were compared to values from numerical solution of the non-linear Poisson–Boltzmann (PB) equation for representative configurations of the nanoparticles, in addition to idealised analytic and Debye–Hückel (DH) solutions about spherical particles of the same geometry and charge state, for the given salt concentrations. It was found that reasonable agreement was obtained between the various approaches, with the NEMD-HS results some 15%–15% smaller than the numerical PB results for more highly charged nanoparticles.
Scopus© Citations 14 1105 - PublicationBlock copolymer mediated stabilization of sub-5 nm superparamagnetic nickel nanoparticles in an aqueous medium(IOP Science, 2009-10-14)
; ; ; ; ; The paper presents a facile method for decreasing the size of water dispersible Ni nanoparticles from 30 nm to 3 nm by the incorporation of a passivating surfactant combination of pluronic triblock copolymer and oleic acid into a wet chemical reduction synthesis. A detailed study revealed that the size of the Ni nanoparticles is critically governed not only by the concentration of the tri block copolymers but also dependent on the hydrophobic nature of the micelle-core formed. The synthesized Ni nanoparticles were thoroughly characterized by TEM, XRD, XPS and temperature and field dependent magnetic measurements along with a comprehensive FTIR analysis to predict a possible mechanism of formation.Scopus© Citations 34 778 - PublicationDensity functional theory description of the mechanism of ferromagnetism in nitrogen-doped SnO2Based on first-principles calculations, we have studied the occurrence of spin polarization in the magnetic metal oxide SnO2 doped with nonmagnetic nitrogen (N) impurities. It was found that the local magnetic moments are localized mainly on the N dopant, causing a total moment of 0.95μB per cell. The long-range magnetic coupling of N-doped SnO2 may be attributed to a p-p coupling interaction between the N impurity and host valence states.
1036Scopus© Citations 35 - PublicationPhotoinduced ligand isomerisation in a pyrazine containing ruthenium polypyridyl complex(RSC publications, 2010)
; ; ; ; ; Photochemically induced ligand rearrangements for the N2 and N4 coordination isomers of the complex [Ru(bpy)2(Hpztr)]2+ and its deprotonated analogue [Ru(bpy)2(pztr)]+, where bpy is 2,2’-bipyridyl and Hpztr is pyrazine-1,2,4-triazole ligand, are reported. 1H NMR spectroscopic and HPLC studies indicate that in acetone and acetonitrile the complexes are photostable when the triazole ring is deprotonated. Irradiation of the protonated N2 isomer in acetone results in formation of the N4 isomer, with the N4 isomer being photostable. In acetonitrile both isomers show photolability of the triazole based ligand and full dissociation to form [Ru(bpy)2(CH3CN)2]2+ is observed. The activation parameters for the population of the 3MC state from the lowest 3MLCT manifold, as obtained from temperature dependent emission lifetime studies, are reported and their relevance to the observed photochemical behaviour is considered. The results obtained are discussed in relation the analogous pyridine-triazole complexes.Scopus© Citations 3 545 - PublicationDirecting semiconductor nanorod assembly into 1D or 2D supercrystals by altering the surface chargeCoulomb repulsion due to the surface charge on semi-conductor nanorods works against the dipole–dipole attraction that tends to direct the nanorods to self-assemble; the nature of this self-assembly for CdSe nanorods can be thus altered by pyridine washing, which charges the rods surface—thereby allowing the Coulomb repulsion to tailor the alignment.
Scopus© Citations 49 663 - PublicationA spatially resolved study on the Sn diffusion during the sintering process in the active layer of dye sensitised solar cellsDye sensitised solar cells (DSSCs) use a mesoporous TiO2 scaffold, typically assisted by an adsorbed dye, as the main active element, responsible for the photon absorption, exciton generation and charge separation functionality. The sintering process employed in the TiO2 active layer fabrication plays a crucial role in the formation of the nanoparticle scaffold and hence the performance of a dye sensitised solar cell, as it allows the particles to form efficient inter-crystalline electric contacts to provide high electron conductivity. The sintering temperature, with typical values in the range of 450–600 °C, is of particular importance for the formation as it reduces the amount of unwanted organics between the individual crystallites and determines the formation of interfaces between the nanoparticles. Furthermore, the cell design requires a conductive transparent top electrode which is typically made of fluorinated tin oxide or indium tin oxide. Here we report on a highly spatially resolved scanning electron microscopy study including focussed ion beam (FIB) milling and energy dispersive X-ray (EDX) mapping of the distribution of all relevant elements within a DSSC subsequent to a classical sintering process. We find that the above quoted temperatures cause the Sn of the transparent conductive oxide (TCO) to migrate into the TiO2 scaffold, resulting in unwanted alterations in the composition of the complex scaffold which has a direct effect on the DSSC performance. One potential solution to this problem is the invention of novel concepts in the manufacturing of DSSCs using lower sintering temperatures.
Scopus© Citations 12 701