George Tsekouras

In situ growth of nanoparticles through control of non-stoichiometry

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

Zn−Zn Porphyrin Dimer-Sensitized Solar Cells: Toward 3-D Light Harvesting

Zn-Zn porphyrin dimers have been incorporated into thin dye-sensitized solar cells (DSSCs) to boost their light harvesting efficiency. The photoexcited dimers show efficient and fast electron injection into TiO(2) indicating that both photoexcited chromophores contribute to current generation. The improved light harvesting ability coupled to enhanced DSSC performance demonstrates the potential of 3-D light harvesting arrays as next generation light harvesters for artificial solar energy conversion systems.

Step-change in high temperature steam electrolysis performance of perovskite oxide cathodes with exsolution of B-site dopants

B-site doped, A-site deficient perovskite oxide titanates with formula La0.4Sr0.4Mn+xTi1−xO3−γ−δ (M = Fe3+ or Ni2+; x = 0.06; γ = (4 − n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis at 900 °C. A-site deficiency provided additional driving force for the exsolution of a proportion of B-site dopants at the surface in the form of metallic nanoparticles under reducing SOEC cathode operating conditions. In the case of La0.4Sr0.4Fe0.06Ti0.94O2.97, this represents the first time that Fe0 has been exsolved from a perovskite in such a way. Exsolution was due in part to the inability of the host lattice to accommodate vacancies (introduced (δ) oxygen vacancies () and fixed A-site () and inherent (γ) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Fe0 or Ni0 nanoparticles and higher concentrations dramatically lowered the activation barrier to steam electrolysis compared to the parent material (x = 0). The use of defect chemistry to drive the exsolution of less reducible dopant cations could conceivably be extended to produce new catalytically active perovskites with unique properties.

The role of defect chemistry in strontium titanates utilised for high temperature steam electrolysis

The significant role of perovskite defect chemistry brought about by the A-site doping of strontium titanate with lanthanum on high temperature steam electrolysis properties is demonstrated. Solid oxide electrolysis cells based on oxygen-excess La0.3Sr0.7TiO3+δ, A-site-deficient La0.2Sr0.7TiO3 and undoped SrTiO3 perovskite hydrogen electrodes (cathodes) were considered. Steam electrolysis performance was largely independent of the presence or absence of hydrogen in the cathode inlet, reflecting the redox stability of perovskites and representing a possible advantage over the state-of-the-art Ni/yttria-stabilised zirconia cermet cathode. Electrochemical testing in 47%H2O/53%N2 atmosphere at 900 °C revealed La0.3Sr0.7TiO3+δ to be the best in terms of highest current density and lowest polarisation resistance, followed by La0.2Sr0.7TiO3 and SrTiO3. Surface area effects, electronic conductivity and oxide ion mobility were considered to be among the determining factors. Furthermore a dramatic order-of-magnitude difference between the characteristic relaxation frequencies of oxygen-excess and A-site-deficient titanates was observed. Steam partial pressure (pH2O)-dependency measurements revealed an added benefit of La-doping in that series resistance (Rs) was independent of pH2O for (La,Sr)TiO3 perovskites, while Rs increased with pH2O for undoped SrTiO3. Electrochemical testing was complemented by X-ray diffraction, electronic conductivity, particle size, BET, porosity and scanning electron microscopy measurements.

Enhanced Performance of Dye Sensitized Solar Cells Utilizing Platinum Electrodeposit Counter Electrodes

Enhanced performance was observed for dye-sensitized solar cells (DSSCs) utilizing counter electrodes based on Pt electrodeposits compared to counter electrodes based on sputtered Pt. Scanning electron microscopy of Pt electrodeposits revealed that the use of an initial cathodic overpotential pulse followed by steady electrodeposition at a mild cathodic potential yielded ∼40nm particles, compared to ∼600nm particles when no such pulse was used. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) of electrode materials suggested that Pt electrodeposits would give enhanced performance as DSSC counter electrodes compared to sputtered Pt, and this was confirmed by device testing. EIS characterization of DSSCs under illumination revealed that Pt electrodeposit counter electrodes were found to be more catalytically stable compared to sputtered Pt. The best counter electrode was based on Pt electrodeposited using an initial cathodic overpotential pulse and demonstrated charge-transfer resistance [RCT(Pt)]values of 0.6 and 0.7Ωcm2 within a symmetrical Pt/I−3/I−/Pt cell and within a DSSC, respectively.

Remarkable synergistic effects in a mixed porphyrin dye-sensitized TiO2 film

A remarkable 300% efficiency enhancement driven by a matching increase in the short circuit current was observed in a mixed porphyrin dye-sensitized solar cell constructed from two dyes in a 3:1 ratio. Absorbed photon-to-current conversion efficiency measurements indicate an improved charge injection yield for both dyes in the mixture. Several possible origins for the observed performance enhancement are discussed.

Gold recovery using inherently conducting polymer coated textiles

The ability of inherently conducting polymer (ICP) coated textiles to recover gold metal from aqueous solutions containing [AuCl4]− was investigated. Nylon-lycra, nylon, acrylic, polyester and cotton were coated with a layer of polypyrrole (PPy) doped with 1,5-naphthalenedisulfonic acid (NDSA), 2-anthraquinonesulfonic acid (AQSA) orp-toluenesulfonic acid (pTS). Textiles coated with polyaniline (PAn) doped with chloride were also used. The highest gold capacity was displayed by PPy/NDSA/nylon-lycra, which exhibited a capacity of 115 mg Au/g coated textile, or 9700 mg Au/g polymer. Varying the underlying textile substrate or the ICP coating had a major effect on the gold capacity of the composites. Several ICP coated textiles recovered more than 90% of the gold initially present in solutions containing 10 ppm [AuCl4]− and 0.1 M HCl in less than 1 min. Both PPy/NDSA/nylon-lycra and PAn/Cl/nylon-lycra recovered approximately 60% of the gold and none of the iron present in a solution containing 1 ppm [AuCl4]−, 1000 ppm Fe3+ and 0.1 M HCl. The spontaneous and sustained recovery of gold metal from aqueous solutions containing [AuCl4]− using ICP coated textiles has good prospects as a potential future technology.

A bistable electrochromic material based on a hysteretic molecular switch immobilised on nanoparticulate metal oxide

Combination of an electrochemically bistable Ru polypyridyl complex (2) with the metal oxide semiconductor Sb:SnO2 formed the basis of an electrochromic hybrid material characterised by a hysteretic response to applied potential. The electrochemical bistability of the molecular component arises from redox-triggered linkage isomerisation where an ambidentate ligand changes reversibly between N- and O-coordination in the Ru(II) and Ru(III) states, respectively. Reversible oxidation and reduction result in a pronounced electrochromic effect (change in UV-Vis absorption at the metal-to-ligand-charge-transfer (MLCT) band) and the potentials for interconversion between the states are separated by ∼0.5 V due to the linkage isomerisation reactions. With a carboxylate anchoring group on the auxiliary ligand, the bistable molecular switch was immobilised on the surface of nanoparticulate Sb:SnO2 films. This resulted in an electrode material featuring a hysteretic electrochemical response where oxidation state and colour of the electrode depended on the electrochemical history of the system.

Gold recovery using fabrics coated with conducting polymers

Conducting polymer coated fabrics were found to recover gold from solutions containing [AuCl 4 ] - in a facile and selective fashion. Although the fabric substrates also demonstrated a significant ability to recover gold, the rate of uptake by the coated fabrics was significantly faster. Conducting polymer coated fabrics also recovered gold from solutions containing [Au(CN) 2 ] - , however, little selectivity was observed when solutions containing both gold and copper cyanide complexes were examined.

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La,Sr)MnO3

The electronic structure of the (La(0.8)Sr(0.2))(0.98)Mn(1-x)Cr(x)O3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented.