Now showing 1 - 9 of 9
  • Publication
    Deposition and characterization of NiOx coatings by magnetron sputtering for application in dye-sensitized solar cells
    Nickel oxide (NiOx) due to its p-type nature has considerable potential as a photocathodic material in energy conversion devices such as dye-sensitized solar cells (DSSCs). However,NiOx has not been extensively used for this application mainly because of low light harvesting efficiency due to limited dye loading on the coatings. In this study NiOx coatings were deposited using the dc- magnetron sputtering technique from a nickel target in an argon/oxygen plasma. One of the advantages of magnetron sputtering is the ability to control coating properties such as mechanical performance and chemical stoichiometry. It is anticipated that by enhancing the interconnectivity between NiOx particles and by optimizing surface roughness, it may be possible to enhance dye adsorption and increase its ability to absorb visible light. NiOx coatings were deposited onto both silicon wafer and indium tin oxide (ITO) covered glass substrates. The influence of deposition parameters such as pressure, nickel target current and substrate bias voltage were correlated with the coating properties of surface roughness, thickness, crystallographic structure and surface energy. This evaluation was carried out using optical profilometry, spectroscopic ellipsometry, XRD and contact angle measurements respectively. It was observed that deposited coating morphology and roughness were significantly influenced by the deposition parameters. For example increasing the deposition pressure from 0.20 to 0.40 Pa led to an increase in surface roughness (Ra) from 1.6 to 3 nm. Associated with this increase in roughness the surface energy increased from 36 to 61 mN/mm. The NiOx coatings were spectrally sensitized with Rucomplex dye containing -COOH groups as anchoring moieties. The dye adsorptions on NiOx coatings, deposited on ITO substrates, were investigated in transmission mode using UV-vis spectroscopy in the range of 400 – 800 nm. It was observed that for the coatings with the highest surface energy, there was an increase of up to 60 % in the level of dye adsorption. The electroactivity of the NiOx thin films deposited on Ni substrate at 0.4 Pa has been verified through the occurrence of redox processes of reduction and lithium intercalation within the oxide film.
      7260Scopus© Citations 58
  • Publication
    Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique
    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.
      670Scopus© Citations 29
  • Publication
    Spectroelectrochemical 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
    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.
      347Scopus© Citations 1
  • Publication
    Excited state localization and internuclear interactions in asymmetric Ruthenium (II) and Osmium (II) bpy/tpy based dinuclear compounds
    The synthesis of two asymmetric dinuclear complexes with the formula [M(bpy)2(bpt)Ru(tpy)Cl]2+, where M = Ru (1a), Os(2a); bpy = 2,2’-bipyridyl; Hbpt = 3,5-bis(pyridin-2-yl)1,2,4-triazole and tpy = 2,2’,6’,2”-terpyridine, is reported. The compounds obtained are characterized by mass spectrometry, 1H-NMR, UV/vis/NIR absorption, luminescence and resonance Raman spectroscopy. Deuterium isotope labeling facilitates assignment of the 1H-NMR and resonance Raman spectra. The interaction between the two metal centers, mediated by the bridging 1,2,4-triazolato moiety in the mixed valent state, is assigned as type II based on the observation of metal to metal charge transfer absorption bands at 7090 and 5990 cm-1 for 1a and 2a, respectively. The extent of localization of the emissive excited state was determined by transient resonance Raman and emission spectroscopy. Both 1a and 2a show phosphorescence at the same wavelengths; however, whereas for compound 1a the emission is based on the Ru(tpy)Cl- center, for 2a the emissive state is localized on the Os(bpy)2- unit. This indicates that also in the excited state there is efficient interaction between the two metal centers.
      518Scopus© Citations 23
  • Publication
    Application of a novel microwave plasma treatment for the sintering of nickel oxide coatings for use in dye-sensitized solar cells
    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.
      2416Scopus© Citations 44
  • Publication
    Photoinduced ligand isomerisation in a pyrazine containing ruthenium polypyridyl complex
    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.
      452Scopus© Citations 3
  • Publication
    Spray deposited NiOx films on ITO substrates as photoactive electrodes for p-type dye-sensitized solar cells
    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.
      714Scopus© Citations 35
  • Publication
    Photoinduced rearrangements in transition metal compounds
    (Elsevier, 2010-11) ;
    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.
      819Scopus© Citations 46
  • Publication
    Dye sensitised solar cells with nickel oxide photocathodes prepared via scalable microwave sintering
    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.
      2203Scopus© Citations 66