1,721,024 research outputs found
Photoelectrochemical hydrogen production from aqueous solution employing nanostructured semiconductors
No full textThe focus of this thesis is the exploitation of various types of photoelectrodes based on
nanostructured materials to be employed in photoassisted electrolysis schemes, aimed to
convert solar energy into chemical energy.
Since the world demand of energy is projected to increase in the next years,
environmental issues like global warming and climate change impose a change in our
way of producing and storing energy, currently and mainly based on the combustion of
fossil fuels. Both a stable improvement of the quality of life for an increasing world
population and the preservation the environment will require the use of progressively
larger amounts of new energy sources which must be carbon neutral, renewable and
low cost. This defines a grand scientific and technological challenge for the XXI
century.
Sunlight provides by far the largest of all carbon-neutral energy sources. More energy
from sunlight strikes the Earth in one hour (4.3 x 1020 ) than all the energy consumed on
the planet in a year (4.1 x 1020 J). Yet solar electricity provides only approximately 1
millionth of the total electricity supply, and renewable biomass provides less than 0.1%
of the total energy consumed. There is a huge gap between our present use of solar
energy and its enormous undeveloped potential.
Hydrogen production from solar water splitting is regarded by many as the “holy grail”
of photochemistry, providing a practically inexhaustible fuel source and, since the
experiment of Fujishima and Honda, the use of semiconductor photoelectrodes capable
of operating with a virtually unitary quantum yield has appeared as a convenient mean
to realize water splitting in a photoelectrochemical cell.
In particular, a large part of this thesis work has been devoted to the development of
efficient WO3 photoelectrodes which is a promising material for water splitting,
coupling favourable energetics for oxygen evolution, high photochemical stability in
aqueous media and visible photon absorption up to 500 nm. .
At first, different synthetic approaches for obtaining nanostructured WO3 on transparent
conductive oxides have been explored by keeping the Santato-Augustynski recipe as a
main guideline. In general the preparation of nanostructured photoelectrodes involves
the formation of stabilized H2WO4 colloidal precursors in the presence of different
organic dispersing agents which control both nanoparticle size and film porosity. The
relationships between crystallite size, film porosity, electrochemically active surface
and photoelectrochemical behavior under simulated solar illumination have been
studied.
The work has then focused on the preparation of WO3 films by potentiostatic
anodization of metallic tungsten in different solvent/electrolyte compositions with the
aim of improving the charge transfer and charge transport kinetics by creating tightly
interconnected porous oxide structures directly bound to a metal collector. In the
NMF/H2O/NH4F solvent mixture the anodization leads to highly efficient WO3
photoanodes, which, combining spectral sensitivity, high electrochemically active
surface and improved charge transfer kinetics, outperform, under simulated solar
illumination, most of the reported nanocrystalline substrates produced by anodization
in aqueous electrolytes and by sol gel methods. The use of such electrodes results in
high water electrolysis yield, of the order of 70% in 1 M H2SO4 under a potential bias of
1 V vs SCE and close to 100% in the presence of methanol acting as a hole scavenger.
The requirement to harvest large part of the solar spectrum and photooxidize small
molecules like H2S (largely present as a contaminant of hydrocarbon sources) has led
us to the study of heterointerfaces between TiO2 and group VI semiconductors like
CdS, CdSe, CdTe and Bi2S3, which are characterized by a relatively narrow band gap
(from 2.4 eV for CdS to 1.4 eV for Bi2S3).
The use of colloidal and nanotubular TiO2 as a substrate for growing sensitizing
semiconductors by chemical and electrochemical means was instrumental in reducing
significantly degradative self photoxidation and recombination leading to good photon
to electron quantum yields, ranging from 20% (Bi2S3) to 60% (CdS)
The photoelectrochemical behavior of TiO2 photoanodes sensitized by water stable
molecular Ru(II) dyes was finally explored in the context of photoinduced hydrogen
generation. The photoanodes sustained 240 h of irradiation without undergoing
appreciable hydrolysis and decomposition in an aqueous environment at pH 3. Despite a
favourable driving force for direct water oxidation, the performances in pure water were
far from an applicative interest. However, the use of organic sacrificial donors, like
isoprapanol and ascorbic acid considerably enhanced their photoanodic response and,
interestingly, the exploitation of iodide, a well known electron donor in
photoelectrchemical cells, was more problematic because the adsorption of
photogenerated I3
- from aqueous media favored charge recombination with conduction
band electrons, thus limiting the efficiency of the photoelectrosynthetic device.
However, experiments performed in a three-compartment cell, where the photolectrode
was in contact with an organic solvent preventing triiodide adsorption, revealed a
remarkable photocurrent, with an electrolysis yield close to 87%.
Although the problem of finding the ideal system for photoassisted electrolysis has not
been entirely solved, this thesis work suggests some feasible approaches for obtaining
high photon to electron conversion efficiencies, pointing out that the basis of an
efficient photoelectrochemical system reside in the combination of an high
electroactive area with fast interfacial charge transfer kinetics, necessary to overcome
losses arising from recombination.
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Conductive PEDOT Covalently Bound to Transparent FTO Electrodes
No full textA new 3,4-ethylenedioxythiophene (EDOT) monomer derivatized with aminopropyl-triethoxysilane (APTES-EDOT) was prepared via a simple two step reaction in high yield. The new monomer can be firmly grafted to the fluorine−tin-oxide (FTO) conductive glass, where the irreversible electro-oxidation of surface bound APTES-EDOT, in the presence of unsubstituted EDOT monomers in solution, triggers the cationic polymerization of EDOT, resulting in the incorporation of PEDOT chains into APTES-EDOT. As a result, the modified PEDOT film (Si-PEDOT) is covalently bound to the FTO surface and easily withstands mechanical stresses that are critical for the adhesion of regular PEDOT. When tested with Co(III)/(II) redox shuttles, electrodeposited Si-PEDOT films showed decreased charge transfer and mass transport resistances with respect to both platinum and conventional PEDOT films, leading to enhanced relative efficiencies (≈10%) when employed as counter electrode in transparent dye sensitized solar cells
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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