http://hdl.handle.net/10197/1905 Wed, 01 Nov 2017 14:18:24 GMT 2017-11-01T14:18:24Z http://researchrepository.ucd.ie:80/bitstream/id/6339/4460.gif http://hdl.handle.net/10197/1905 http://hdl.handle.net/10197/8720 2D Non-separable Linear Canonical Transform (2D-NS-LCT) based cryptography Zhao, Liang; Muniraj, Inbarasan; Healy, John J.; Malallah, Ra'ed; Cui, Xiao-Guang; Ryle, James P. The 2D non-separable linear canonical transform (2D-NS-LCT) can describe a variety of paraxial optical systems. Digital algorithms to numerically evaluate the 2D-NS-LCTs are not only important in modeling the light field propagations but also of interest in various signal processing based applications, for instance optical encryption. Therefore, in this paper, for the first time, a 2D-NS-LCT based optical Double-random-Phase-Encryption (DRPE) system is proposed which offers encrypting information in multiple degrees of freedom. Compared with the traditional systems, i.e. (i) Fourier transform (FT); (ii) Fresnel transform (FST); (iii) Fractional Fourier transform (FRT); and (iv) Linear Canonical transform (LCT), based DRPE systems, the proposed system is more secure and robust as it encrypts the data with more degrees of freedom with an augmented key-space. Optics and Optoelectronics, Prague, Czech Republic, 24-27 April 2017 Thu, 27 Apr 2017 00:00:00 GMT http://hdl.handle.net/10197/8720 2017-04-27T00:00:00Z http://hdl.handle.net/10197/8716 Constraints to solve parallelogram grid problems in 2D non separable linear canonical transform Zhao, Liang; Healy, John J.; Muniraj, Inbarasan; Cui, Xiao-Guang; Malallah, Ra'ed; Ryle, James P.; Sheridan, John T. The 2D non-separable linear canonical transform (2D-NS-LCT) can model a range of various paraxial optical systems. Digital algorithms to evaluate the 2D-NS-LCTs are important in modeling the light field propagations and also of interest in many digital signal processing applications. In [Zhao 14] we have reported that a given 2D input image with rectangular shape/boundary, in general, results in a parallelogram output sampling grid (generally in an affine coordinates rather than in a Cartesian coordinates) thus limiting the further calculations, e.g. inverse transform. One possible solution is to use the interpolation techniques; however, it reduces the speed and accuracy of the numerical approximations. To alleviate this problem, in this paper, some constraints are derived under which the output samples are located in the Cartesian coordinates. Therefore, no interpolation operation is required and thus the calculation error can be significantly eliminated. Optics and Optoelectronics, Prague, Czech Republic, 24-27 April 2017 Thu, 27 Apr 2017 00:00:00 GMT http://hdl.handle.net/10197/8716 2017-04-27T00:00:00Z http://hdl.handle.net/10197/8712 The choice of optical system is critical for the security of double random phase encryption systems Muniraj, Inbarasan; Guo, Changliang; Malallah, Ra'ed; Cassidy, Derek; Zhao, Liang; Ryle, James P.; Healy, John J.; Sheridan, John T. The linear canonical transform (LCT) is used in modeling a coherent light field propagation through first-order optical systems. Recently, a generic optical system, known as the Quadratic Phase Encoding System (QPES), for encrypting a two-dimensional (2D) image has been reported. In such systems, two random phase keys and the individual LCT parameters (, , ) serve as secret keys of the cryptosystem. It is important that such encryption systems also satisfies some dynamic security properties. In this work, we therefore examine such systems using two cryptographic evaluation methods, the avalanche effect and bit independence criterion, which indicate the degree of security of the cryptographic algorithms using QPES. We compared our simulation results with the conventional Fourier and the Fresnel transform based DRPE systems. The results show that the LCT based DRPE has an excellent avalanche and bit independence characteristics compared to the conventional Fourier and Fresnel based encryption systems.Keywords: Quadratic Phase Encoding system, linear canonical transform, Double Random Phase Encryption, Avalanche effect and bit independence criterion. Wed, 14 Jun 2017 00:00:00 GMT http://hdl.handle.net/10197/8712 2017-06-14T00:00:00Z http://hdl.handle.net/10197/8710 Measuring optical phase digitally in coherent metrology systems Kelly, Damien P.; Ryle, James P.; Zhao, Liang; Sheridan, John T. The accurate measurement of optical phase has many applications in metrology. For biological samples, which appear transparent, the phase data provides information about the refractive index of the sample. In speckle metrology, the phase can be used to estimate stress and strains of a rough surface with high sensitivity. In this theoretical manuscript we compare and contrast the properties of two techniques for estimating the phase distribution of a wave field under the paraxial approximation: (I) A digital holographic system, and (II) An idealized phase retrieval system. Both systems use a CCD or CMOS array to measure the intensities of the wave fields that are reflected from or transmitted through the sample of interest. This introduces a numerical aspect to the problem. For the two systems above we examine how numerical calculations can limit the performance of these systems leading to a near-infinite number of possible solutions. Dimensional Optical Metrology and Inspection for Practical Applications VI, Anaheim, California, United States of America, 9 April 2017 Sun, 09 Apr 2017 00:00:00 GMT http://hdl.handle.net/10197/8710 2017-04-09T00:00:00Z http://hdl.handle.net/10197/6606 Photolytic water oxidation catalyzed by a molecular carbene iridium complex Petronilho, Ana; Rahman, Mahfujur; Woods, James A.; Al-Sayyed, Haris; Müller-Bunz, Helge; MacElroy, J. M. Don; Bernhard, Stefan; Albrecht, Martin The complex IrCl2(Cp*)(trz) (trz = triazolylidene), 2, was prepared from readily available 1,3-dimethyl-4-phenyl-1,2,3-triazolium salt. Under basic conditions, the C-bound phenyl group readily cyclometalates, while under acidic conditions, cyclometalation is reversed. The sensitivity of the Caryl–Ir bond but not the Ctrz–Ir bond towards acidolysis provided a basis for using 2 as a catalyst in CeIV-mediated water oxidation. The catalytic activity is characterized by a robust catalytic cycle, affording excellent turnover numbers (TON > 20 000). Under cerium-free conditions and in the presence of hematite as a photoelectrode, light-induced activity was observed. The photoelectrochemical reaction is strongly pH-dependent, which requires pH adjustments when running multiple cycle experiments to regenerate the catalytic activity. Analogous chelating complexes display better stability and higher catalytic activity than the monodentate complex 2. Wed, 10 Oct 2012 00:00:00 GMT http://hdl.handle.net/10197/6606 2012-10-10T00:00:00Z http://hdl.handle.net/10197/4231 Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique Awais, Muhammad; Dini, Danilo; MacElroy, J. M. Don; Halpin, Yvonne; Vos, Johannes G.; Dowling, Denis P. In this contribution a novel powder coating processing technique (microblasting) for the fabrication of nickel oxide (NiOx) coatings is reported. ~1.2 μm thick NiOx coatings are deposited at 20 mm2 s−1 by the bombardment of the NiOx powder onto a Ni sheet using an air jet at a speed of more than 180 m s−1. Microblast deposited NiOx coatings can be prepared at a high processing rate, do not need further thermal treatment. Therefore, this scalable method is time and energy efficient. The mechano-chemical bonding between the powder particles and substrate results in the formation of strongly adherent NiOx coatings. Microstructural analyses were carried out using SEM, the chemical composition and coatings orientation were determined by XPS and XRD, respectively. The electroactivity of the microblast deposited NiOx coatings was compared with that of NiOx coatings obtained by sintering NiOx nanoparticles previously sprayed onto Ni sheets. In the absence of a redox mediator in the electrolyte, the reduction current of microblast deposited NiOx coatings, when analyzed in anhydrous environment, was two times larger than that produced by higher porosity NiOx nanoparticles coatings of the same thickness obtained through spray coating followed by sintering. Under analogous experimental conditions thin layers of NiOx obtained by using the sol–gel method, ultrasonic spray- and electro-deposition show generally lower current density with respect to microblast samples of the same thickness. The electrochemical reduction of NiOx coatings is controlled by the bulk characteristics of the oxide and the relatively ordered structure of microblast NiOx coatings with respect to sintered NiOx nanoparticles here considered, is expected to increase the electron mobility and ionic charge diffusion lengths in the microblast samples. Finally, the increased level of adhesion of the microblast film on the metallic substrate affords a good electrical contact at the metal/metal oxide interface, and constitutes another reason in support of the choice of microblast as low-cost and scalable deposition method for oxide layers to be employed in electrochemical applications. Tue, 15 Jan 2013 00:00:00 GMT http://hdl.handle.net/10197/4231 2013-01-15T00:00:00Z http://hdl.handle.net/10197/4200 Dye sensitised solar cells with nickel oxide photocathodes prepared via scalable microwave sintering Gibson, Elizabeth A.; Awais, Muhammad; Dini, Danilo; Dowling, Denis P.; Pryce, Mary T.; Vos, Johannes G.; Boschloo, Gerrit; Hagfeldt, Anders Photoactive NiO electrodes for cathodic dye-sensitised solar cells (p-DSCs) have been prepared with thicknesses ranging between 0.4 and 3.0 mm by spray-depositing pre-formed NiO nanoparticles on fluorine-doped tin oxide (FTO) coated glass substrates. The larger thicknesses were obtained in sequential sintering steps using a conventional furnace (CS) and a newly developed rapid discharge sintering (RDS) method. The latter procedure is employed for the first time for the preparation of p-DSCs. In particular, RDS represents a scalable procedure that is based on microwave-assisted plasma formation that allows the production in series of mesoporous NiO electrodes with large surface areas for p-type cell photocathodes. RDS possesses the unique feature of transmitting heat from the bulk of the system towards its outer interfaces with controlled confinement of the heating zone. The use of RDS results in a drastic reduction of processing times with respect to other deposition methods that involve heating/calcination steps with associated reduced costs in terms of energy. P1-dye sensitized NiO electrodes obtained via the RDS procedure have been tested in DSC devices and their performances have been analysed and compared with those of cathodic DSCs derived from CS-deposited samples. The largest conversion efficiencies (0.12%) and incident photon-to-current conversion efficiencies, IPCEs (50%), were obtained with sintered NiO electrodes having thicknesses of B1.5–2.0 mm. In all the devices, the photogenerated holes in NiO live significantly longer (th B 1 s) than have previously been reported for P1-sensitized NiO photocathodes. In addition, P1-sensitised sintered electrodes give rise to relatively high photovoltages (up to 135 mV) when the triiodide–iodide redox couple is used. Thu, 06 Dec 2012 00:00:00 GMT http://hdl.handle.net/10197/4200 2012-12-06T00:00:00Z http://hdl.handle.net/10197/4197 Spray deposited NiOx films on ITO substrates as photoactive electrodes for p-type dye-sensitized solar cells Awais, Muhammad; Dowling, Denis P.; Rahman, Mahfujur; Vos, Johannes G.; Decker, Franco; Dini, Danilo Spray deposition followed by sintering of nickel oxide (NiO x ) nanoparticles (average diameter: 40 nm) has been chosen as method of deposition of mesoporous NiO x coatings onto indium tin oxide (ITO) substrates. This procedure allows the scalable preparation of NiO x samples with large surface area (~103 times the geometrical area) and its potential for applications such as electrocatalysis or electrochemical solar energy conversion, which require high electroactivity in confined systems. The potential of these NiO x films as semiconducting cathodes for dye-sensitized solar cell (DSC) purposes has been evaluated for 0.3–3-μm-thick films of NiO x sensitized with erythrosine B (ERY). The electrochemical processes involving the NiO x coatings in the pristine and sensitized states were examined and indicated surface confinement as demonstrated by the linear dependence of the current densities with the scan rate of the cyclic voltammetry. Cathodic polarization of NiO x on ITO can also lead to the irreversible reduction of the underlying ITO substrate because of the mesoporous nature of the sintered NiO x film that allows the shunting of ITO to the electrolyte. ITO-based reduction processes alter irreversibly the properties of charge transfer through the ITO/NiOx interface and limit the range of potential to NiO x coatings sintered for DSC purposes. Fri, 01 Feb 2013 00:00:00 GMT http://hdl.handle.net/10197/4197 2013-02-01T00:00:00Z http://hdl.handle.net/10197/3750 Molecular dynamics study of water in contact with the TiO2 rutile-110, 100, 101, 001 and anatase-101, 001 surface Kavathekar, Ritwik S.; Dev, Pratibha; English, Niall J.; MacElroy, J. M. Don We have carried out classical molecular dynamics of various surfaces of TiO2 with its interface with water. We report the geometrical features of the first and second monolayers of water using a Matsui Akaogi (MA) force field for the TiO2 surface and a flexible single point charge model for the water molecules. We show that the MA force field can be applied to surfaces other than rutile (110). It was found that water OH bond lengths, H–O–H bond angles and dipole moments do not vary due to the nature of the surface. However, their orientation within the first and second monolayers suggest that planar rutile (001) and anatase (001) surfaces may play an important role in not hindering removal of the products formed on these surfaces. Also, we discuss the effect of surface termination in order to explain the layering of water molecules throughout the simulation box. Thu, 19 May 2011 00:00:00 GMT http://hdl.handle.net/10197/3750 2011-05-19T00:00:00Z http://hdl.handle.net/10197/3744 Evaluation of microwave plasma oxidation treatments for the fabrication of photoactive un-doped and carbon-doped TiO2 coatings Dang, Binh H.Q.; Rahman, Mahfujur; MacElroy, J. M. Don; Dowling, Denis P. The photoactivity of both un-doped and carbon-doped titanium dioxide (TiO2) coatings has been widely reported. In this paper, the use of a microwave plasma as a novel oxidation treatment for the fabrication of these coatings is evaluated. The photoactivity performance of the microwave plasma-formed coatings is benchmarked against those fabricated through air furnace oxidation as well as those deposited using reactive magnetron sputtering. The un-doped and carbon-doped TiO2 coatings were prepared respectively by microwave plasma-oxidizing titanium metal sheets and sputter deposited titanium carbide thin films. The resulting oxides were characterized using XPS, XRD, FEG-SEM, and optical profilometry. The oxide layer thicknesses achieved over the 15 to 45 minute oxidation times were in the range of 0.15 to 3.44 µm. These coatings were considerably thicker than those obtained by air furnace oxidation. The microwave plasma-formed oxides also exhibited significantly higher surface roughness values compared with the magnetron-sputtered coatings. The photoactivity performance of both un-doped and carbon-doped coatings was assessed using photocurrent density measurements. Comparing the un-doped TiO2 coatings, it was observed that those obtained using the microwave plasma oxidation route yielded photocurrent density measurements that were 4.3 times higher than the TiO2 coatings of the same thickness that were deposited by sputtering. The microwave plasma-oxidized titanium carbide coatings did not perform as well as the un-doped TiO2 probably due to the presence of un-oxidized carbide in the coatings, which reduced their photoactivity. Fri, 25 May 2012 00:00:00 GMT http://hdl.handle.net/10197/3744 2012-05-25T00:00:00Z http://hdl.handle.net/10197/3743 Mechanism of Atmospheric CO2 Fixation in the Cavities of a Dinuclear Cryptate El-Hendawy, Morad M.; English, Niall J.; Mooney, Damian A. Using density functional theory (DFT) methods, we have investigated two possible mechanisms for atmospheric CO2 fixation in the cavity of the dinuclear zinc (II) octaazacryptate, and the subsequent reaction with methanol whereby this latter reaction transforms the (essentially) chemically inert CO2 into useful products. The first mechanism (I) was proposed by Chen et al. [Chem. Asian J. 2007, 2, 710], and involves the attachment of one CO2 molecule onto the hydroxyl-cryptate form, resulting in the formation of a bicarbonate-cryptate species and subsequent reaction with one methanol molecule. In addition, we suggest another mechanism that is initiated via the attachment of a methanol molecule onto one of the Zn-centres, yielding a methoxy-cryptate species. The product is used to activate a CO2 molecule and generate a methoxycarbonate-cryptate. The energy profiles of both mechanisms were determined and we conclude that, while both mechanisms are energetically feasible, free energy profiles suggest that the scheme proposed by Chen et al. is most likely. Mon, 07 May 2012 00:00:00 GMT http://hdl.handle.net/10197/3743 2012-05-07T00:00:00Z http://hdl.handle.net/10197/3742 Hydrogen bond dynamical properties of adsorbed liquid water monolayers with various TiO2 interfaces English, Niall J.; Kavathekar, Ritwik S.; MacElroy, J. M. Don Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the hydrogen bonding kinetics of water in contact with rutile-(110), rutile-(101), rutile-(100), and anatase-(101) surfaces at room temperature (300 K). It was observed that anatase-(101) exhibits the longest-lived hydrogen bonds in terms of overall persistence, followed closely by rutile-(110). The relaxation times, defined as the integral of the autocorrelation of the hydrogen bond persistence function, were also larger for these two cases, while decay of autocorrelation function was slower. The increased number and overall persistence of hydrogen bonds in the adsorbed water monolayers at these surfaces, particularly for anatase-(101), may serve to promote possible water photolysis activity thereon. Thu, 12 Apr 2012 00:00:00 GMT http://hdl.handle.net/10197/3742 2012-04-12T00:00:00Z http://hdl.handle.net/10197/3625 Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates Gorman, Paul D.; English, Niall J.; MacElroy, J. M. Don Classical equilibrium molecular dynamics(MD) simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar and up to 250K. For the mixed H2-THF system in which there is single H2 occupation of the small cage (labelled ‘1SC 1LC’), we find that no H2 migration occurs, and this is also the case for pure H2 hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed ‘1SC 4LC’). However, for the more densely-filled H2-THF and pure- H2 systems, in which there is double H2 occupation in the small cage (dubbed ‘2SC 1LC’ and ‘2SC 4LC’, respectively), there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these ‘cage hopping’ events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200 to 250 K range. The ‘breathing modes’, or underlying frequencies governing the variation in the cavities’ radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while a there is some overlap of small cages’ radial breathing modes with lattice acoustic modes. Tue, 24 Jan 2012 00:00:00 GMT http://hdl.handle.net/10197/3625 2012-01-24T00:00:00Z http://hdl.handle.net/10197/3495 Mechanisms for thermal conduction in hydrogen hydrate English, Niall J.; Gorman, Paul D.; MacElroy, J. M. Don Extensive equilibrium molecular dynamics (MD) simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly-filled H2 hydrates (1s4l) and for more densely-filled H2 systems (2s4l), in which four H2 molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity. Mon, 23 Jan 2012 00:00:00 GMT http://hdl.handle.net/10197/3495 2012-01-23T00:00:00Z http://hdl.handle.net/10197/3438 A TD-DFT study of the effects of structural variations on the photochemistry of polyene dyes Agrawal, Saurabh; Dev, Pratibha; English, Niall J.; Thampi, Ravindranathan; MacElroy, J. M. Don We report a TD-DFT study of three polyene dyes namely: NKX-2553, NKX-2554 and NKX-2569 in isolation as well as upon their adsorption on TiO2 nanoparticles. By choosing closely related dyes we wish to focus on the effects of structural variations on the absorption and charge-transfer properties of these systems. These three dyes show a non-intuitive trend in their respective efficiencies and therefore, were chosen to shed light on the structural components that contribute to this behaviour. Although, NKX-2554 has an additional donor group, it is less efficient compared to the simpler NKX-2553 dye that contains only one donor group. When NKX-2554 structure is slightly modified by lengthening the linker-group, one obtains the most efficient dye among this set, namely, NKX-2569. In this work, we show that the changes in the donor moiety has very little or no effect on the efficiency of these dyes as can be seen in the case of NKX-2553 and NKX-2554. On the other hand, the improved performance of NKX-2569-titania complex can be understood to be a result of the longer linker group. A better understanding of these properties within different dye-titania complexes is important for the continual improvement of DSSCs. In this regards, this study will serve to provide guidelines to improve efficiencies of novel organic dyes. Thu, 06 Oct 2011 00:00:00 GMT http://hdl.handle.net/10197/3438 2011-10-06T00:00:00Z http://hdl.handle.net/10197/3416 A facile spin-cast route for cation exchange of multilayer perpendicularly-aligned nanorod assemblies Kelly, Dervla; Singh, Ajay; Barrett, Christopher A.; O'Sullivan, Catriona; Coughlan, Claudia; Laffir, Fathima R.; O'Dwyer, Colm; Ryan, Kevin M. A facile spin cast route was developed to convert perpendicularly aligned nanorod assemblies of cadmium chalcogenides into their silver and copper analogues. The assemblies are rapidly cation exchanged without affecting either the individual rod dimensions or collective superlattice order extending over several multilayers. Fri, 07 Oct 2011 00:00:00 GMT http://hdl.handle.net/10197/3416 2011-10-07T00:00:00Z http://hdl.handle.net/10197/3387 Study of translational, librational and intra-molecular motion of adsorbed liquid water monolayers at various TiO2 interfaces Kavathekar, Ritwik S.; English, Niall J.; MacElroy, J. M. Don Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the vibrational motion of water in contact with rutile-(110), rutile-(100), rutile-(001), anatase-(101) and anatase-(001) surfaces at room temperature (300 K). The vibrational density of states (VDOS) of the first adsorbed monolayer of liquid water has been analysed for each surface. These have been compared with reported experimental INS values involving rutile and anatase polymorph surfaces, along with ab initio MD results. It is observed that good qualitative agreement is obtained for the rutile-(110) and the anatase-(101) surfaces with the corresponding experimental VDOS. A significant contribution from librational dynamics is found for planar rutile surfaces, but no such demarcation is seen in the experimental VDOS. Wed, 19 Oct 2011 00:00:00 GMT http://hdl.handle.net/10197/3387 2011-10-19T00:00:00Z http://hdl.handle.net/10197/3386 Electrophoretic deposition of poly(3-decylthiophene) onto gold-mounted cadmium selenide nanorods Garate, Jose-Antonio; English, Niall J.; Singh, Ajay; Ryan, Kevin M.; Mooney, Damian A.; MacElroy, J. M. Don Molecular mechanisms of electrophoretic deposition (EPD) of poly(3-decylthiophene) (P3DT) molecules onto vertically aligned cadmium selenide arrays have been studied using large-scale, nonequilibrium molecular dynamics (MD), in the absence and presence of static external electric fields. The field application and larger polymer charges accelerated EPD. Placement of multiple polymers at the same lateral displacement from the surface reduced average deposition times due to “crowding”, giving monolayer coverage. These findings were used to develop and validate Brownian dynamics simulations of multilayer polymer EPD in scaled-up systems with larger inter-rod spacings, presenting a generalized picture in qualitative agreement with random sequential adsorption. Wed, 21 Sep 2011 00:00:00 GMT http://hdl.handle.net/10197/3386 2011-09-21T00:00:00Z http://hdl.handle.net/10197/3385 Dynamical and energetic properties of hydrogen and hydrogen–tetrahydrofuran clathrate hydrates Gorman, Paul D.; English, Niall J.; MacElroy, J. M. Don Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the dynamical and energetic properties in hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 30 and 200K and 0.05 kbar, and also at intermediate temperatures, using SPC/E and TIP4P-2005 water models. The potential model is found to have a large impact on overall density, with the TIP4P-2005 systems being on average 1 % more dense than their SPC/E counterparts, due to the greater guest-host interaction energy. For the lightly-filled mixed H2-THF system, in which there is single H2 occupation of the small cage (1s1l), we find that the largest contribution to the interaction energy of both types of guest is the van der Waals component with the surrounding water molecules in the constituent cavities. For the more densely-filled mixed H2-THF system, in which there is double H2 occupation in the small cage (2s1l), we find that there is no dominant component (i.e., van der Waals or Coulombic) in the H2 interaction energy with the rest of the system, but for the THF molecules, the dominant contribution is again the van der Waals interaction with the surrounding cage-water molecules; again, the Coulombic component increases in importance with increasing temperature. The lightly-filled pure H2 hydrate (1s4l) system exhibits a similar pattern vis-à-vis the H2 interaction energy as for the lightly-filled mixed H2-THF system, and for the more densely-filled pure H2 system (2s4l), there is no dominant component of interaction energy, due to the multiple occupancy of the cavities. By consideration of Kubic harmonics, there is some evidence of preferential alignment of the THF molecules, particularly at 200 K; this was found to arise at higher temperatures due to transient hydrogen bonding of the oxygen atom in THF molecules with the surrounding cage-water molecules. Mon, 03 Oct 2011 00:00:00 GMT http://hdl.handle.net/10197/3385 2011-10-03T00:00:00Z http://hdl.handle.net/10197/3016 First-principles study of the excited-state properties of coumarin-derived dyes in dye-sensitized solar cells Agrawal, Saurabh; Dev, Pratibha; English, Niall J.; Thampi, Ravindranathan; MacElroy, J. M. Don Using Time-Dependent Density Functional Theory (TD-DFT), we have investigated the optical properties of dye-sensitized solar cells (DSSCs) comprised of TiO2 nanoparticle sensitized with two coumarins, namely, NKX-2311 and NKX-2593. The two sensitizers (dyes) differ only in their linker moieties and are shown to have different absorption spectra when adsorbed on to the TiO2 surface. Knowledge of different light absorption and charge transfer (CT) behavior within these complexes is useful for further improving the photo-dynamics of newer organic dyes presently being designed and investigated worldwide. Moreover, we have also investigated the effect of deprotonation of the sensitizers' carboxylic groups during adsorption on the titania surface and the excited state electronic properties of the resulting species. Thu, 23 Jun 2011 00:00:00 GMT http://hdl.handle.net/10197/3016 2011-06-23T00:00:00Z http://hdl.handle.net/10197/3010 Tunable single-site ruthenium catalysts for efficient water oxidation Bernet, Lucille; Lalrempuia, Ralte; Ghattas, Wadih; Müller-Bunz, Helge; Vigara, Laura; Llobet, Antoni; Albrecht, Martin The catalytic water oxidation activity of mononuclear ruthenium complexes comprising a pyridine-functionalized abnormal triazolylidene ligand can be adjusted by modification of the triazolylidene substituents, which is readily achieved through click-type cycloaddition chemistry, affording some of the most active ruthenium catalysts known thus far for water oxidation(TONs > 400, TOFs close to 7000/h). Thu, 16 Jun 2011 00:00:00 GMT http://hdl.handle.net/10197/3010 2011-06-16T00:00:00Z http://hdl.handle.net/10197/2961 Molecular dynamics study of water in contact with TiO2 rutile-110, 100, 101, 001 and anatase-101, 001 surface Kavathekar, Ritwik S.; Dev, Pratibha; English, Niall J.; MacElroy, J. M. Don We have carried out classical molecular dynamics of various surfaces of TiO2 with its interface with water. We report the geometrical features of the first and second monolayers of water using a Matsui Akaogi (MA) force field for the TiO2 surface and a flexible single point charge model for the water molecules. We show that the MA force field can be applied to surfaces other than Rutile-(110). It was found that water OH bond lengths, H-O-H bond angles and dipole moments do not vary due to the nature of the surface. However, their orientation within the first and second monolayers suggest that planar Rutile-(001) and Anatase-(001) surfaces may play an important role in not hindering removal of the products formed on these surfaces. Also, we discuss the effect of surface termination in order to explain the layering of water molecules throughout the simulation box. Thu, 19 May 2011 00:00:00 GMT http://hdl.handle.net/10197/2961 2011-05-19T00:00:00Z http://hdl.handle.net/10197/2896 Conversion of amorphous TiO2 coatings into their crystalline form using a novel microwave plasma treatment Dang, Binh H.Q.; Rahman, Mahfujur; MacElroy, J. M. Don; Dowling, Denis P. Crystalline titanium dioxide (TiO2) coatings have been widely used in photo-electrochemical solar cell applications. In this study, TiO2 and carbon-doped TiO2 coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO2. This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45 GHz microwave-induced nitrogen plasma where they were heated to approximately 550°C. It was observed that for treatment times as short as 1 minute, the 0.25-µm thick coatings were converted into the anatase crystalline phase of TiO2. The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO2 with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO2 exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating. Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings’ surface morphology and band gap energy, both of which influence the coatings’ photoabsorption efficiency. Mon, 25 Jul 2011 00:00:00 GMT http://hdl.handle.net/10197/2896 2011-07-25T00:00:00Z http://hdl.handle.net/10197/2886 Activation of hematite nanorod arrays for photoelectrochemical water splitting Morrish, Rachel; Rahman, Mahfujur; MacElroy, J. M. Don; Wolden, Colin Andrew Hematite nanorod arrays were activated through proper control of annealing conditions. The 100-fold improvement in photocurrent was correlated with increased absorption and Sn doping from the tin oxide coated glass substrate. The low onset potential is attributed to a reduction in surface defects, while the morphology is credited for promoting tin diffusion and facilitating electron transport. Mon, 18 Apr 2011 00:00:00 GMT http://hdl.handle.net/10197/2886 2011-04-18T00:00:00Z http://hdl.handle.net/10197/2877 A theoretical thermodynamic investigation of cascade reactions in dinuclear octa-azacryptates involving carbon dioxide English, Niall J.; Mooney, Damian A. This paper investigates the thermodynamics of gas-phase CO2 cascade uptake-reactions in the form of carbonate or methoxycarbonate anions in the host cavity of various dinuclear octa-azacryptates of m-CH2C6H4CH2 and 2,5-furano-spaced hosts, L1 and L2 cryptands, using density functional theory (DFT). The cascade process involves two stages, namely the formation of dinuclear cryptate complexes and the subsequent formation of either μ-carbonato cryptate complexes or μ-monomethylcarbonato cryptates. The geometric and electronic structures are also investigated to determine the parameters which affect the stability of the complexes. Natural bond orbital (NBO) analysis has been used to investigate the interactions between the trapped anion and its host. The ion selectivity has been studied in terms of the formation of dinuclear crypate complexes, while the basicity and nucleophilicity of cryptands towards Lewis acids have also been studied, and good agreement was found vis-à-vis available experimental data. Sat, 01 Jan 2011 00:00:00 GMT http://hdl.handle.net/10197/2877 2011-01-01T00:00:00Z http://hdl.handle.net/10197/2839 Tailored adsorption of His6-tagged protein onto nickel(II)–cyclam grafted mesoporous silica Gaffney, Darragh A.; O'Neill, Sarah; O'Loughlin, Mary C.; Hanefeld, Ulf; Cooney, Jakki C.; Magner, Edmond The specific immobilisation of a histidine tagged protein, Spi, onto mono-anchored nickel(II) cyclam functionalised SBA-15 is reported. Sun, 21 Feb 2010 00:00:00 GMT http://hdl.handle.net/10197/2839 2010-02-21T00:00:00Z http://hdl.handle.net/10197/2830 Size controlled gold tip growth onto II–VI nanorods O'Sullivan, Catriona; Gunning, Robert Denis; Barrett, Christopher A.; Singh, Ajay; Ryan, Kevin M. Gold tip size and multiplicity is controlled in hybrid gold-semiconductor nanorods (CdS-Au, CdSe-Au and CdTe-Au) in fast reaction times of less than 2 minutes by optimising precursor type, concentration, and temperature. Controllable gold tips up to as large as 40 nm on a rod diameter of 7 nm are reported with the tip size shown to be directly related to the redox potentials of the dangling atoms on the nanorod and the gold chloride precursors (mono or trivalent). The preference for symmetric (both ends) over asymmetric (single end) is achieved by simply elevating the reaction temperature to 80ºC without changing the reaction time. TEM and XRD analysis were preformed to verify both the gold nanoparticle size and crystallinity of the hybrid nanostructures. Increased quenching of nanorod emission is observed as the size of the gold tip increases suggesting optimisation of charge transfer between the semiconductor and the metal. Tue, 28 Sep 2010 00:00:00 GMT http://hdl.handle.net/10197/2830 2010-09-28T00:00:00Z http://hdl.handle.net/10197/2829 The influence of light intensity, active area and excitation wavelength on the temporal response of a dye sensitized solar cell O'Reilly, Thomas; Andrei, Codrin; O'Reilly, Jennifer; Zerulla, Dominic 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. Poster presentation at SPIE Optics + Photonics: Solar Energy and Technology, San Diego, California, USA, 2nd – 6th August 2009 Sun, 02 Aug 2009 00:00:00 GMT http://hdl.handle.net/10197/2829 2009-08-02T00:00:00Z http://hdl.handle.net/10197/2828 Metal surface nucleated supercritical fluid–solid–solid growth of Si and Ge/SiOx core–shell nanowires Barrett, Christopher A.; Gunning, Robert Denis; Hantschel, Thomas; Arstila, Kai; O'Sullivan, Catriona; Geaney, Hugh; Ryan, Kevin M. High yields of single-crystalline silicon nanowires and germanium/silicon oxide core-shell nanostructures were synthesized using a supercritical fluid-solid-solid growth mechanism. The formation of the one-dimensional nanostructures was investigated on stainless steel and titanium reactor cell walls. The nanowire growth is diffusion controlled with no catalyst particles being observed at the tip of the nanowires, which is contrary to the characteristic feature of a one-dimensional growth based on the vapor-liquid-solid mechanism. The diameter and length of the nanowires were in the ranges of 30-60 nm and 1-10 µm, respectively, as found by high-resolution electron microscopy. The nanowires were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). Fri, 01 Jan 2010 00:00:00 GMT http://hdl.handle.net/10197/2828 2010-01-01T00:00:00Z http://hdl.handle.net/10197/2793 Photoinduced ligand isomerisation in a pyrazine containing ruthenium polypyridyl complex Horn, Sabine; Ahmed, Hamid M. Younis; Hughes, Helen P.; Soman, Suraj; Browne, Wesley R.; Vos, Johannes G. 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. Fri, 01 Jan 2010 00:00:00 GMT http://hdl.handle.net/10197/2793 2010-01-01T00:00:00Z http://hdl.handle.net/10197/2792 Electronic structure and origin of visible-light activity of C-doped cubic In2O3 from first-principles calculations Long, Run; English, Niall J. The origin of the experimentally observed band gap narrowing and red-shift of the adsorption edge of cubic In2O3 induced by C doping has been investigated using density functional theory calculations. We have compared the stability of all these doped systems based on the calculated formation energy as a function of the oxygen chemical potential. The calculated electronic structures show that: (I) at low C concentration, substitutional replacement of O by C could lead to small gap narrowing, owing to C 2p states below the conduction band minimum while interstitial C doping does not induce band gap narrowing; and (II) at high C concentration, C2p states mix well with O 2p states above the valence band, which may account for the experimentally observed red-shift of the absorption edge. Thu, 19 Aug 2010 00:00:00 GMT http://hdl.handle.net/10197/2792 2010-08-19T00:00:00Z http://hdl.handle.net/10197/2791 Photoinduced rearrangements in transition metal compounds Vos, Johannes G.; Pryce, Mary T. In this contribution a range of photo induced ligand rearrangements observed for first and second row transition metal and organometallic compounds are discussed. The processes discussed include photoinduced ligand exchange, linkage isomerisation and changes occurring within the coordination sphere of the compounds such as cis-trans and fac-mer isomerisations. The relevance of these processes for photocatalytic cycles or their application as synthetic tools is discussed where appropriate. Mon, 01 Nov 2010 00:00:00 GMT http://hdl.handle.net/10197/2791 2010-11-01T00:00:00Z http://hdl.handle.net/10197/2790 First-principles calculation of electronic structure of V-doped anatase TiO2 Long, Run; English, Niall J. The energetic and electronic structures of V-doped anatase TiO2 have been investigated systematically by the GGA+U approach, including replacement of Ti by V in the absence and presence of oxygen vacancies and the presence of an interstitial site. It was found that V should exist as a V4+ ion in the replacement of Ti in the anatase lattice, the electron transitions of which to the conduction band from V 3d states are responsible for the experimentally observed visible-light absorption. The influence of V dopant concentration on the electronic and magnetic properties is also discussed, such as the influence of the U value in systems containing oxygen vacancies and spin flip phenomena for interstitial V-doping. Mon, 23 Aug 2010 00:00:00 GMT http://hdl.handle.net/10197/2790 2010-08-23T00:00:00Z http://hdl.handle.net/10197/2789 Electronic properties of F/Zr co-doped anatase TiO2 photocatalysts from GGA + U calculations Long, Run; English, Niall J. The energetic and electronic properties of F and/or Zr-doped anatase TiO2 are investigated by first-principles calculations. For F-doping, reduced Ti3+ ions are formed and Ti orbitals lie slightly below the conduction band, leading to band gap narrowing. For Zr-doping, Zr 4d orbitals reside well into the conduction band, with essentially no band gap change. For F/Zr–codoping, the electronic structure is similar to that for F–monodoping, where Ti3+ gap states are induced by both the oxygen vacancy and F dopant. The influence of oxygen vacancies indicates that interplay between dopants and oxygen vacancies is key for improvement of photocatalytic activity. The theoretical findings present a reasonable explanation of recent experimental results. Fri, 08 Oct 2010 00:00:00 GMT http://hdl.handle.net/10197/2789 2010-10-08T00:00:00Z http://hdl.handle.net/10197/2770 Characterising dye-sensitized solar cells Tobin, Laura L.; O'Reilly, Thomas; Zerulla, Dominic; Sheridan, John T. Kafafi, Zakya H.; Lane, Paul A. 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. SPIE: SPIE Optics + Photonics: Solar Energy and Technology, 2nd – 6th August 2009, San Diego, California, USA Mon, 03 Aug 2009 00:00:00 GMT http://hdl.handle.net/10197/2770 2009-08-03T00:00:00Z http://hdl.handle.net/10197/2741 Investigating dye-sensitised solar cells Tobin, Laura L.; O'Reilly, Thomas; Zerulla, Dominic; Sheridan, John T. At present there is a global concern of environmental issues and future energy supplies, for instance global warming and the rapid depletion of fossil fuels. This trepidation has initiated a more critical investigation into alternative and renewable sources of power such as geothermal, biomass, hydropower, wind and solar energy. The immense dependence on electrical power in today’s society has prompted the manufacturing of devices such as photovoltaic (PV) cells to help alleviate and replace current electrical demands of the power grid. The most popular and commercially available PV cells are silicon solar cells which have to date the greatest efficiencies for PV cells. The drawback however is that the manufacturing of these cells is complex and costly due to the expense and difficulty of producing and processing pure silicon. One relatively inexpensive alternative to silicon PV cells that we are currently studying are dye-sensitised solar cells (DSSCs or Grätzel Cells). DSSCs are biomimetic solar cells which are based on the process of photosynthesis. The SFI Strategic Research Centre for Solar Energy Conversion is a research cluster based in Ireland formed with the express intention of bringing together industry and academia to produce renewable energy solutions. Our specific research area is in DSSC and their opto-electrical properties. We are currently developing testing equipment for multiple DSSC and optoelectronic models which will describe the performance and behaviour of DSSCs. SPIE Photonics Europe: Photonics for Solar Energy Systems III, 12th- 16th April, Brussels, 2010. Tue, 13 Apr 2010 00:00:00 GMT http://hdl.handle.net/10197/2741 2010-04-13T00:00:00Z http://hdl.handle.net/10197/2730 Application of a novel microwave plasma treatment for the sintering of nickel oxide coatings for use in dye-sensitized solar cells Awais, Muhammad; Rahman, Mahfujur; MacElroy, J. M. Don; Dini, Danilo; Vos, Johannes G.; Dowling, Denis P. In this study the use of microwave plasma sintering of nickel oxide (NiOx) particles for use as p-type photoelectrode coatings in dye-sensitized solar cells (DSSCs) is investigated. NiOx was chosen as the photocathode for this application due to its stability, wide band gap and p-type nature. For high light conversion efficiency DSSCs require a mesoporous structure exhibiting a high surface area. This can be achieved by sintering particles of NiOx onto a conductive substrate. In this study the use of both 2.45 GHz microwave plasma and conventional furnace sintering were compared for the sintering of the NiOx particles. Coatings 1 to 2.5 μm thick were obtained from the sintered particles (mean particle size of 50 nm) on 3 mm thick fluorine-doped tin oxide (FTO) coated glass substrates. Both the furnace and microwave plasma sintering treatments were carried out at ~ 450 °C over a 5 minute period. Dye sensitization was carried out using Erythrosin B and the UV-vis absorption spectra of the NiOx coatings were compared. A 44% increase in the level of dye adsorption was obtained for the microwave plasma sintered samples as compared to that obtained through furnace treatments. While the photovoltaic performance of the DSSC fabricated using the microwave plasma treated NiOx coatings exhibited a tenfold increase in the conversion efficiency in comparison to the furnace treated samples. This enhanced performance was associated with the difference in the mesoporous structure of the sintered NiOx coatings. Mon, 25 Jul 2011 00:00:00 GMT http://hdl.handle.net/10197/2730 2011-07-25T00:00:00Z http://hdl.handle.net/10197/2724 Energetic and electronic properties of P Doping at the rutile TiO2 (110) Surface from First Principles Long, Run; English, Niall J. The 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. Mon, 20 Apr 2009 00:00:00 GMT http://hdl.handle.net/10197/2724 2009-04-20T00:00:00Z http://hdl.handle.net/10197/2723 First-principles study of S doping at the rutile TiO2 (110) surface Long, Run; English, Niall J.; Dai, Ying The 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. Tue, 15 Sep 2009 00:00:00 GMT http://hdl.handle.net/10197/2723 2009-09-15T00:00:00Z http://hdl.handle.net/10197/2722 First-principles calculation of nitrogen-tungsten codoping effects on the band structure of anatase-titania Long, Run; English, Niall J. The 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. Thu, 02 Apr 2009 00:00:00 GMT http://hdl.handle.net/10197/2722 2009-04-02T00:00:00Z http://hdl.handle.net/10197/2721 Spontaneous room temperature elongation of CdS and Ag2S nanorods via oriented attachment O'Sullivan, Catriona; Gunning, Robert Denis; Sanyal, Ambarish; Barrett, Christopher A.; Geaney, Hugh; Laffir, Fathima R.; Ahmed, Shafaat; Ryan, Kevin M. 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. Thu, 06 Aug 2009 00:00:00 GMT http://hdl.handle.net/10197/2721 2009-08-06T00:00:00Z http://hdl.handle.net/10197/2720 Synergistic effects on band gap-narrowing in titania by codoping from first-principles calculations Long, Run; English, Niall J. The large intrinsic band gap in TiO2 has hindered severely its potential application for visible-light irradiation. In this study, we have used a passivated approach to modify the band edges of anatase-TiO2 by codoping of X (N, C) with transition metals (TM=W, Re, Os) to extend the absorption edge to longer visible-light wavelengths. It was found that all the codoped systems can narrow the band gap significantly; in particular, (N+W)-codoped systems could serve as remarkably better photocatalysts with both narrowing of the band gap and relatively smaller formation energies and larger binding energies than those of (C+TM) and (N+TM)-codoped systems. Our theoretical calculations provide meaningful guides for experiments to develop more powerful visible-light photocatalysts. Fri, 12 Feb 2010 00:00:00 GMT http://hdl.handle.net/10197/2720 2010-02-12T00:00:00Z http://hdl.handle.net/10197/2719 Band gap engineering of (N, Ta)-codoped TiO2 : a first-principles calculation Long, Run; English, Niall J. The 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. Wed, 29 Jul 2009 00:00:00 GMT http://hdl.handle.net/10197/2719 2009-07-29T00:00:00Z http://hdl.handle.net/10197/2718 Magnetic properties of first-row element-doped ZnS semiconductors : a density functional theory investigation Long, Run; English, Niall J. Based 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. Tue, 22 Sep 2009 00:00:00 GMT http://hdl.handle.net/10197/2718 2009-09-22T00:00:00Z http://hdl.handle.net/10197/2717 Electronic structures of N- and C-doped NiO from first-principles calculations Long, Run; English, Niall J.; Mooney, Damian A. The large intrinsic band gap of NiO has hindered severely its potential application under visible-light irradiation. In this study, we have performed first-principles calculations on the electronic properties of N- and C-doped NiO to ascertain if its band gap may be narrowed theoretically. It was found that impurity bands driven by N 2p or C 2p states appear in the band gap of NiO and that some of these locate at the conduction band minimum, which leads to a significant band gap narrowing. Our results show that N-doped NiO may serve as a potential photocatalyst relative to C-doped NiO, due to the presence of some recombination centres in C-doped NiO. Mon, 15 Feb 2010 00:00:00 GMT http://hdl.handle.net/10197/2717 2010-02-15T00:00:00Z http://hdl.handle.net/10197/2716 Phase diagrams of non-ionic microemulsions containing reducing agents and metal salts as bases for the synthesis of bimetallic nanoparticles Magno, Miguel; Angelescu, Daniel G.; Stubenrauch, Cosima Phase 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. Tue, 01 Sep 2009 00:00:00 GMT http://hdl.handle.net/10197/2716 2009-09-01T00:00:00Z http://hdl.handle.net/10197/2715 Block copolymer mediated stabilization of sub-5 nm superparamagnetic nickel nanoparticles in an aqueous medium Bala, Tanushree; Gunning, Robert Denis; Venkatesan, Munuswamy; Godsell, Jeffrey F.; Roy, Saibal; Ryan, Kevin M. 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. Wed, 14 Oct 2009 00:00:00 GMT http://hdl.handle.net/10197/2715 2009-10-14T00:00:00Z http://hdl.handle.net/10197/2714 Centimetre scale assembly of vertically aligned and close packed semiconductor nanorods from solution Ahmed, S.; Ryan, Kevin M. Assembly 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. Tue, 08 Sep 2009 00:00:00 GMT http://hdl.handle.net/10197/2714 2009-09-08T00:00:00Z http://hdl.handle.net/10197/2713 Synergistic effects of Bi/S codoping on visible light-activated anatase TiO2 photocatalysts from first principles Long, Run; English, Niall J. The 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. Mon, 20 Apr 2009 00:00:00 GMT http://hdl.handle.net/10197/2713 2009-04-20T00:00:00Z http://hdl.handle.net/10197/2712 Density functional theory studies of doping in Titania Long, Run; English, Niall J. The structural and electronic properties of rutile and anatase, and the influence of both mono- and co-doping, have been studied using Density Functional Theory. Ge-doped anatase and rutile exhibit different band gap-narrowing mechanisms; in particular, host Ti 3d states move to lower energy regions in anatase and Ge 4s impurities states locate below the conduction band of rutile. For S-doping, S 3p states locate above the top of the valence band and mix with O 2p states, leading to band gap narrowing. For Bi-doping, the energy levels of the 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. For Bi/S–codoping, both S 3p acceptor states and partially occupied Bi 6s donor states hybridised with S 3p appear simultaneously. For N- and W-monodoping, isolated N 2p states above the top of the valence band and W 5d states below the conduction band lead to band gap narrowing. N/W codoping yields significant band gap narrowing. Both studies for Bi/S and N/W codoping rationalise recent experimental data which show that these doped anatase systems exhibit higher visible-light photocatalytic efficiency than respective monodoping. Tue, 01 Jun 2010 00:00:00 GMT http://hdl.handle.net/10197/2712 2010-06-01T00:00:00Z