Publications by Year: 2016

Stergiopoulos T, Kontos AG, Jiang N, Milliken D, Desilvestro H, Likodimos V, Falaras P. High boiling point solvent-based dye solar cells pass a harsh thermal ageing test. Solar Energy Materials and Solar Cells [Internet]. 2016;144:457-466. WebsiteAbstract
Dye solar cells (DSCs) have emerged as one of the most efficient third-generation photovoltaic (PV) technologies, whose commercialization is mainly hampered by the lack of sufficient long-term stability compared to conventional PV devices. In this work, it is demonstrated that solvent based DSCs using tetraglyme as a non-nitrile, high boiling point, organic solvent for the iodide/triiodide redox shuttle, can pass a harsh accelerated thermal ageing test of 3000 h light soaking followed by additional 2000 h thermal ageing at 85 °C. Electrochemical and spectroscopic analysis on thermal degradation effects revealed that a conduction band edge shift towards more negative potentials for tetraglyme-DSCs underlies the enhanced photopotential of aged cells, compensating for the thermally induced photocurrent reduction due to slight triiodide loss. The tetraglyme-based solar cells (in contrast to cells based on methoxypropionitrile-MPN) showed exceptional stability, compatible with the established IEC61646 protocol for thin film PVs, keeping ca. 90% of their initial performance under 1 sun illumination. Quite notably, the cells even increased their initial efficiency by 4% when illuminated under 0.1 sun. This is the first time in literature that such a stability record is accomplished for solvent based DSCs utilizing commercially available and cost-effective materials. © 2015 Elsevier B.V. All rights reserved.
Pelaez M, Falaras P, Likodimos V, O'Shea K, de la Cruz AA, Dunlop PSM, Byrne JA, Dionysiou DD. Use of selected scavengers for the determination of NF-TiO2 reactive oxygen species during the degradation of microcystin-LR under visible light irradiation. Journal of Molecular Catalysis A: Chemical [Internet]. 2016;425:183-189. Publisher's VersionAbstract
Although UV-induced TiO2 photocatalysis involves the generation of several reactive oxygen species (ROS), the formation of hydroxyl radicals is generally associated with the degradation of persistent organic contaminants in water. In this study, a variety of radical scavengers were employed to discriminate the roles of different ROS during visible light-activated (VLA) photocatalysis using nitrogen and fluorine doped TiO2 (NF-TiO2) in the degradation of the hepatotoxin, microcystin-LR (MC-LR) in water. The addition of hydroxyl radical scavengers, methanol and tert-butyl alcohol to the reaction mixture resulted in negligible inhibition of NF-TiO2 photocatalytic degradation of MC-LR at pH 3.0 and only partial inhibition at pH 5.7 under visible light. While hydroxyl radicals ([rad]OH) generally play the primary role in UV-TiO2 photocatalysis, the minimal influence of MeOH and t-BuOH on the degradation process under these experimental conditions indicates that [rad]OH are not crucial in VLA NF-TiO2 photocatalysis. However, strong inhibition was observed in VLA NF-TiO2 photocatalytic degradation of MC-LR in the presence of superoxide dismutase, benzoquinone and catalase at pH 3.0 and 5.7 indicating that O2[rad]− and H2O2 play critical roles in the degradation process. Similar degradation rates were observed in the presence of deuterium oxide, which enhances singlet oxygen mediated processes further suggesting singlet oxygen is not a key species in the degradation of MC-LR. Formic acid and cupric nitrate were added to probe the roles of the valence band holes and conduction band electrons, respectively. Under UV–vis light irradiation, almost complete inhibition of MC-LR removal is observed with NF-TiO2 in the presence of [rad]OH scavengers at pH 5.7. These results demonstrate that the solution pH plays a major role in the formation and reactivity of ROS during VLA NF-TiO2 photocatalysis. The adsorption strength of scavengers and MC-LR onto NF-TiO2 as well as the speciation of ROS as a function of pH needs to be carefully considered since they also play a major role in the efficiency of the process. These results indicate that the reduction of molecular oxygen by photo-generated electrons rather than hydroxyl radicals produced by oxidative reactions of photo-generated holes is the key factor in the VLA NF-TiO2 photocatalytic degradation of MC-LR. © 2016 Elsevier B.V.
Likodimos V, Chrysi A, Calamiotou M, Fernández-Rodríguez C, Doña-Rodríguez JM, Dionysiou DD, Falaras P. Microstructure and charge trapping assessment in highly reactive mixed phase TiO2 photocatalysts. Applied Catalysis B: Environmental [Internet]. 2016;192:242-252. Publisher's VersionAbstract
The structural-microstructural characteristics and interfacial charge transfer are key issues to the development of efficient mixed phase TiO2 photocatalysts. In this work, the interplay of lattice deformation and microstrains as well as the identification of charge trapping sites and electron transfer were investigated for a series of nanostructured titania photocatalysts by X-ray powder diffraction analysis, Raman and electron paramagnetic resonance (EPR) spectroscopy. These mixed phase nanomaterials were selected as model sol-gel TiO2 systems based on their exceptional photocatalytic performance over a wide range of hazardous water pollutants (including degradation/mineralization of phenol, 2,4-dichlorophenoxyacetic acid and imazalil) under UV light. Lattice contraction with respect to the bulk anatase together with anisotropic microstrains was identified for the smallest (11 nm) anatase nanoparticles. Both effects gradually relaxed with the increase of calcination temperature and the concomitant particle growth, with microstrains scaling linearly with the relative change of the c-axis lattice constant and the broadening of the main anatase Raman mode. The growth of anatase nanoparticles at 1023 K with minimal lattice deformation and microstrains resulted in the optimal photocatalytic efficiency, outperforming the benchmark Aeroxide® P25 catalyst. This mixed phase catalyst comprised also larger, though more strained, rutile nanocrystals than P25, and presented an additional deeper electron trapping lattice site according to light-induced EPR measurements. More importantly, electron transfer from rutile to anatase lattice traps was identified by EPR under visible light in the mixed phase photocatalyst. The improved crystal quality of the anatase nanocrystals combined with the enhanced charge separation in anatase/rutile interfaces is concluded crucial to the design of competent solar photocatalytic nanomaterials. © 2016.