Claudia Longo

Dye-sensitized solar cells: a successful combination of materials

Dye-sensitized TiO2 solar cells, DSSC, are a promising alternative for the development of a new generation of photovoltaic devices. DSSC are a successful combination of materials, consisting of a transparent electrode coated with a dye-sensitized mesoporous film of nanocrystalline particles of TiO2, an electrolyte containing a suitable redox-couple and a Pt coated counter-electrode. In general, Ru bipyridyl complexes are used as the dye sensitizers. The light-to-energy conversion performance of the cell depends on the relative energy levels of the semiconductor and dye and on the kinetics of the electron-transfer processes at the sensitized semiconductor | electrolyte interface. The rate of these processes depends on the properties of its components. This contribution presents a discussion on the influence of each of the materials which constitute the DSSC of the overall process for energy conversion. An overview of the results obtained for solid-state dye-sensitized TiO2 solar cells assembled with polymer electrolytes is also presented.

Reduction of Carbon Dioxide to Formate at Low Overpotential Using a Superbase Ionic Liquid

A new low-energy pathway is reported for the electrochemical reduction of CO2 to formate and syngas at low overpotentials, utilizing a reactive ionic liquid as the solvent. The superbasic tetraalkyl phosphonium ionic liquid [P66614][124Triz] is able to chemisorb CO2 through equimolar binding of CO2 with the 1,2,4-triazole anion. This chemisorbed CO2 can be reduced at silver electrodes at overpotentials as low as 0.17 V, forming formate. In contrast, physically absorbed CO2 within the same ionic liquid or in ionic liquids where chemisorption is impossible (such as [P66614][NTf2]) undergoes reduction at significantly increased overpotentials, producing only CO as the product.

Iron Insertion and Hematite Segregation on Fe-Doped TiO2 Nanoparticles Obtained from Sol–Gel and Hydrothermal Methods

Iron-doped TiO(2) (Fe:TiO(2)) nanoparticles were synthesized by the sol-gel method (with Fe/Ti molar ratio corresponding to 1, 3, and 5%), followed by hydrothermal treatment, drying, and annealing. A similar methodology was used to synthesize TiO(2) and α-Fe(2)O(3) nanoparticles. For comparison, a mixture hematite/titania, with Fe/Ti = 4% was also investigated. Characterization of the samples using Rietveld refinement of X-ray diffraction data revealed that TiO(2) consisted of 82% anatase and 18% brookite; for Fe:TiO(2), brookite increased to 30% and hematite was also identified (0.5, 1.0, and 1.2 wt % for samples prepared with 1, 3, and 5% of Fe/Ti). For hematite/titania mixture, Fe/Ti was estimated as 4.4%, indicating the Rietveld method reliability for estimation of phase composition. Because the band gap energy, estimated as 3.2 eV for TiO(2), gradually ranged from 3.0 to 2.7 eV with increasing Fe content at Fe:TiO(2), it can be assumed that a Fe fraction was also inserted as dopant in the TiO(2) lattice. Extended X-ray absorption fine structure spectra obtained for the Ti K-edge and Fe K-edge indicated that absorbing Fe occupied a Ti site in the TiO(2) lattice, but hematite features were not observed. Hematite particles also could not be identified in the images obtained by transmission electron microscopy, in spite of iron identification by elemental mapping, suggesting that hematite can be segregated at the grain boundaries of Fe:TiO(2).

Nanostructured homogenous CdSe–TiO2 composite visible light photoanodes fabricated by oblique angle codeposition

A unique fabrication method, oblique angle codeposition, is used to deposit well-aligned nanorod arrays and thick films of homogenously mixed CdSe–TiO2 composites. The composite films are characterized structurally, optically, and photoelectrochemically using a variety of experimental techniques. The CdSe–TiO2 composites are compared with pure CdSe and TiO2 films in order to determine their utility for photoelectrochemical (PEC) applications and to understand the mechanisms underlying the observed behaviors. The evaporation process of CdSe creates three different cluster types within the TiO2 film structures: isolated Se, Se-deficient CdSe, and Se-rich CdSe. The prevalence of each cluster type is dependent on predicted film composition, and each is affected differently by open-air annealing. Isolated Se can be incorporated into the TiO2 lattice, resulting in low energy rutile phase. Se-deficient CdSe clusters crystallize preferentially into cubic CdSe and are easily oxidized into CdO, while Se-rich CdSe clusters crystallize into hexagonal CdSe and are more stable. Furthermore, each of these cluster types interacts differently with the surrounding TiO2 matrix, resulting in diverse optical and PEC behaviors. Importantly, the composite nanorod structure is a more efficient photoanode under visible light illumination than both the pure CdSe and TiO2 nanorod array films. The stoichiometry of the CdSe domains is more important than overall CdSe content within the film in determining the structural, optical, and PEC properties of the films.

Dye-sensitized solar cells and solar module using polymer electrolytes: Stability and performance investigations

We present recent results on solid-state dye-sensitized solar cell research using a polymer electrolyte based on a poly(ethylene oxide) derivative. The stability and performance of the devices have been improved by a modification in the method of assembly of the cells and by the addition of plasticizers in the electrolyte. After 30 days of solar irradiation (100 mW cm-2) no changes in the cells efficiency were observed using this new method. The effect of the active area size on cell performance and the first results obtained for the first solar module composed of 4.5 cm2 solid-state solar cells are also presented.

Preparação de eletrodos opticamente transparentes

A simple experiment for the preparation of transparent conducting glass electrodes by deposition of pure and fluorine doped SnO2 films is described. This procedure was tested in the undergraduate inorganic course at IQ-UNICAMP. The success in achieving a conducting layer was easily checked using the standard probes of a volt-ohm meter. The optical transmittance and thickness were studied by UV-vis spectrophotometry. To discuss the experimental results we place significant emphasis on molecular orbital and energy band model theories. The undergraduate students can also discuss the concepts related to the electronic properties of solids and to interesting new materials, such as transparent conducting films, which are the subject of significant current research and technological applications.

Characterization of Fe-TiO2 films synthesized by sol-gel method for application in energy conversion devices

Fe-TiO2 particles were synthesized by sol-gel process from hydrolysis of titanium tetra-isopropoxide with nitric acid and ferric nitrate aqueous solutions (relative Fe:Ti molar ratio ranging from 1 to 6 at %) followed by hydrothermal treatment. Thin films were deposited onto conducting glass electrodes from a suspension with polyethylene glycol and heating at 450 °C for 30 min, which resulted in 1.5 μm thick transparent porous films. Crystalline samples, 93 % anatase and 7 % brookite, were obtained. Increasing the iron amount, the crystallite size estimated from XRD patterns ranged from 18 to 11 nm and the color varied from slightly yellow to brown. The optical properties have also changed; the absorption edge shifted towards longer wavelengths, with band gap energy decreasing from 3.0 to 2.7 eV. The films exhibited photocatalytic activity for phenol degradation that indicates promising applications in solar energy conversion.

Photoelectrochemical and photocatalytic properties of nanocrystalline TiO2 electrodes

The electrochemical and photocatalytic properties of a TiO2 film deposited on transparent electrodes were investigated. Its electrochemical behavior was typical of an n-type semiconductor electrode. Its photocatalytic activity, investigated for phenol degradation on an optical bench (area of 1 cm2, 5 mL of solution), revealed small currents (3 μA) and poor total organic carbon (TOC) removal (5 %) when the electrode was biased at + 1.1 V in the dark for 3 h. Under polychromatic irradiation, the electrode presented 25 μA of current and 12 % of phenol degradation. A better performance was achieved for photoelectrocatalytic configuration, when the electrode was irradiated and biased with + 0.6 V. Experiments done under irradiation with a metallic vapor lamp using 9 cm2 electrodes and 10 mL of solution revealed that heterogeneous photocatalysis configuration (HPC) resulted in 50 % of TOC removal, while 85 % was achieved by the electro-assisted process (EHPC). Both the configurations exhibited pseudo-first order kinetics for phenol degradation, but the rate constant was two times that of EHPC. The application of a potential bias to the TiO2 porous electrode must enhance the photogenerated electron/hole separation, which minimize the charge recombination and increases its photocatalytic activity towards organic pollutant degradation.

Electrochemical and photocatalytic properties of TiO2/WO3 photoelectrodes

Porous films of TiO2 and TiO2/WO3 were deposited onto transparent electrodes from aqueous suspensions with polyethylene glycol, TiO2 particles and different amounts of tungistic acid. After annealing, crystalline samples were obtained. The band gap energy, approximately 3.1 eV for TiO2, decreased from 2.9 to 2.7 eV for varying W/Ti molar ratios from 3 to 12 %. The electrochemical properties were investigated in Na2SO4 aqueous solution; for the TiO2 electrode, the open circuit potential changed from 0.18 V in the dark to -0.25 V under irradiation from a solar simulator. For hybrid TiO2/WO3 electrodes, the VOC values were almost independent of the WO3 content and corresponded to 0.3 V in the dark and -0.1 V under irradiation; however, photocurrent and interfacial capacitance increased with a higher WO3 concentration. The electrodes were then used as photocatalysts for 17-α-etinylestradiol removal from water, and the mixed TiO2/WO3 exhibited better performance for photocatalytic oxidation of estradiol than TiO2. Adding WO3 enhances the visible light harvesting and minimizes the charge recombination resulting in higher efficiency for solar energy conversion.

TiO2 and TiO2/WO3 porous film electrodes for application in solar energy conversion

TiO2 and TiO2/WO3 porous films were deposited onto transparent conducting glass electrodes, resulting in uniform films consisted of agglomerated particles with average diameters ranging from 50 to 200 nm; Ti, O and W atoms were homogeneously distributed at the surface of hybrid film. Comparable electrochemical properties were observed in the dark, with small capacitive currents and similar potentials for O2 and H2 evolution reactions in aqueous solution. Under polychromatic irradiation, the hybrid film electrode, molar ratio WO3/TiO2 = 12 %, reveled higher photocurrent and photocatalytic activity for oxidation of phenol and 17-α-ethinylestradiol. The visible light harvesting ability of hybrid film, with band gap energy estimated as 2.3 eV, and the relative position of conduction and valence band edges that inhibits charge recombination, should improve its photocatalytic activity for organic pollutant removal.