Luísa Andrade

An overview of photocatalysis phenomena applied to NOx abatement.

This review provides a short introduction to photocatalysis technology in terms of the present environmental remediation paradigm and, in particular, NOx photoabatement. The fundamentals of photoelectrochemical devices and the photocatalysis phenomena are reviewed, highlighting the main reaction mechanisms. The critical historical developments on heterogeneous photocatalysis are briefly discussed, giving particular emphasis to the pioneer works in this field. The third part of this work focus mainly on NOx removal technology considering topics such as: TiO2 photochemistry; effect of the operating conditions on the photocatalysis process; Langmuir-Hinshelwood modeling; TiO2 photocatalytic immobilization approaches; and their applications. The last section of the paper presents the main conclusions and perspectives on the opportunities related to this technology.

Influence of sodium cations of N3 dye on the photovoltaic performance and stability of dye-sensitized solar cells.

We report on the effect of substituting the two tetrabutyl ammonium counter ions of the standard N719 dye by sodium ions on the performance and stability of dye-sensitized solar cells (DSCs). The disodium analogue of N719 in conjunction with a non-volatile electrolyte gives a conversion efficiency of 7.6% under standard global AM 1.5 sunlight. Devices maintain 99% of their initial performance after 1000 h under full sunlight aging at 50 degrees C. Electrochemical impedance spectroscopy and photovoltage transient decay studies reveal the evolution of the solar cell parameters during aging. Remarkably, upon aging a decrease in the rate of electron back reaction with the triiodide ions across the TiO(2)/electrolyte interface appears as well as enhanced electronic conduction in the TiO(2) film.

The role of the Ti surface roughness in the self-ordering of TiO2 nanotubes: a detailed study of the growth mechanism

Highly ordered TiO2 nanotubes (NTs) were synthesized by electrochemical anodization of Ti foils. We investigated the effect of the Ti surface roughness (applying different pre-treatments prior to the anodization) on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms related to the TiO2 NT formation and growth were correlated not only with the corresponding anodization curves but also with their appropriate derivatives (1st order) and suitable integrated and/or obtained parameters, to reveal the onset and end of different electrochemical regimes. This enables an in-depth interpretation (and physical–chemical insight), for different levels of surface roughness and topographic features. We found that pre-treatments lead to an extremely small Ti surface roughness, offer an enhanced NT length and also provide a significant improvement in the template organization quality (highly ordered hexagonal NT arrays over larger areas), due to the optimized surface topography. We present a new statistical approach for evaluating highly ordered hexagonal NT array areas. Large domains with ideally arranged nanotube structures represented by a hexagonal closely packed array were obtained (6.61 μm2), close to the smallest grain diameter of the Ti foil and three times larger than those so far reported in the literature. The use of optimized pre-treatments then allowed avoiding a second anodization step, ultimately leading to highly hexagonal self-ordered samples with large organized domains at reduced time and cost.

Modeling, simulation and design of dye sensitized solar cells

It is well known that recombination and transport rule the performance of dye sensitized solar cells (DSCs); although, the influence that these two phenomena have in their performance, particularly on the open circuit-potential (Voc) and on the short circuit current (Jsc), is not fully understood. In this paper a phenomenological model is used to describe the quantitative effect that transport and recombination have on the performance of the solar cell and their influence on its optimal design. The model is used to predict the influence of the recombination reaction rate constant (kr) and diffusion coefficient (Deff) on the Voc and on the Jsc, whether a linear or non-linear recombination kinetic is considered. A methodology is provided for decoupling the conduction band shifts from recombination effects in charge extraction experiments. Results also suggest that the influence of recombination on the Voc and on Jsc is highly dependent on the reaction order considered. This fact highlights the importance of considering the reaction order when modeling data obtained by experimental methods. The combined results are analyzed and discussed in terms of the collection efficiency and the optimization of the photoelectrode thickness. The model provides also a useful framework for exploring new concepts and designs for improving DSCs performance.

Role of temperature in the recombination reaction on dye-sensitized solar cells.

The performance of photovoltaic (PV) devices as a function of temperature is crucial for technical development and for accurate commercial information. Along with solar irradiance, temperature is the most important operating factor of the PV device performance. Normally, it is widely accepted that dye sensitized solar cells (DSC) show minimal energy efficiency dependence with temperature (20-60 °C). The energy efficiency in DSCs depends on the light absorption, charge transport (ohmic resistances) and recombination rates. In this study, the recombination reaction kinetics was studied within a wide temperature range. A unique laser assisted sealing technique that allows studying the temperature effect between -5 °C and 105 °C without electrolyte leakage or external contamination was used. To the best of our knowledge, this is the highest operating temperature ever considered in kinetic studies of liquid state DSCs. The electrochemical reaction between electrons and triiodide/iodide ions was shown to be the most important factor for determining the energy efficiency of DSCs as a function of temperature. It was concluded that the activation energy of the recombination reactions depends on the interface where it happens - TiO2/electrolyte and SnO2-F/electrolyte - and on the temperature. It was found that in addition to temperature having a deep influence on the recombination reaction rate, the energy of the injecting electron is also critical. These conclusions should provide solid ground for further developments in the DSCs and perovskite solar cells, and allow a better comparison of the energy efficiency of different PV technologies over a range of operating temperatures.

Modeling the Growth Kinetics of Anodic TiO2 Nanotubes.

The fundamental understanding of the barrier layer (δ(b)) growth in TiO2 nanotubes (NTs) is here established and compared with the classical metal oxidation theory from Mott and Cabrera. The role of δ(b) in the anodization of TiO2 NTs under different applied potentials and times was analyzed using scanning transmission electron microscopy (STEM). Contrary to the well-known case of anodic aluminum oxide, we found that δ(b) of TiO2 NTs progressively grows over time due to the nonsteady anodization regime. We then establish a relation between the phenomenological growth of the barrier layer with time and applied voltage, δ(b)(V,t) using the high-field Mott and Cabrera conduction theory. The developed model was found to be in excellent agreement with the experimental data from both STEM and anodization curves. On the basis of these results, the relationship between δ(b) and the anodization time and potential can now be quantitatively understood.

Influence of Different Cations of N3 Dyes on Their Photovoltaic Performance and Stability

The N3 dye was modified by substituting two of its protons by potassium or sodium cations. The performance and stability of dye-sensitized solar cells incorporating the new dyes were evaluated under light soaking at . Photocurrent measurements demonstrated that proton substitution by potassium cations rends the system more stable. Further characterization of the potassium-based devices was performed by electrochemical impedance spectroscopy to investigate the charge-transfer phenomena occurring at the different interfaces of the cells.

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Optimization of the NO photooxidation and the role of relative humidity

Photocatalysis was recognised as a suitable process for the photoabatement of atmospheric pollutants. The photooxidation mechanism on TiO2 has been widely studied. However, recent studies demonstrated that the very often-assumed photooxidation intermediated by the hydroxyl radical cannot explain all the experimental observations. Indeed, this study contributes for a new understanding of NO photooxidation. First, the adsorption equilibrium isotherms of NO, NO2 and H2O on the photocatalyst, Aeroxide® P25 from Evonik Industries, were obtained. Also, the concentration of hydroxyl radicals was determined by photoluminescence. A comprehensive design of experiments was then followed; NO conversion and selectivity were obtained as a function of the relative humidity, irradiance, NO inlet concentration and residence time, following a response surface methodology (RSM). These results were then used to discuss the photooxidation mechanism of NO.

Highly Ordered Hexagonal Arrays of TiO2 Nanotubes

Highly-ordered TiO2 nanotubes (NTs) have gained much importance for application in hydrogen production by water splitting (photoelectrochemical cells) and the dye-sensitized solar cells (DSCs) [1,2]. The TiO2 NTs can be synthesized using a titanium foil in fluoride containing electrolytes via electrochemical anodization method. The NTs geometry depends on different anodizing parameters (electrolyte type and concentration, pH, time, applied potential) that determine the tube features (length, pore diameter, wall thickness, etc.). TiO2 NTs arrays were synthesized by an electrochemical anodization of a Ti foil (two-electrode cell) with an anodization potential of 60 V for 17 h, in an ethylene glycol solution containing NH4F (0.3 wt%) and H2O (2 wt%) at room temperature [3]. It was implemented, prior to the anodization, three different pre-treatments on the Ti foil: a chemical etching with 4% HF solution, a mechanical polishing and an electropolishing in a H2SO4/HF solution, with an applied potential of 10 V during 4 min. In this work, it is described the impact that a simple pre-treatment has to the template growth and final thickness after a single anodization step, as well as on the template quality (NTs organization and domain size). For this purpose, the topography of the Ti surface (prior to the anodization) with these 3 pre-treatments and an as-rolled Ti sample was investigated by Atomic Force Microscopy. Roughness studies were compared with the NTs template thickness and organization quality. Highly self-ordered arrays of TiO2 NTs were obtained, and found that pre-treatments that decrease the Ti surface roughness are a crucial step in the TiO2 NTs electrochemical anodization syntheses for obtaining: a fast NTs growth attaining a highest final template thickness; an enhancement in the NTs organization quality reaching highly ordered hexagonal NTs arrays in larger areas [3].