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Preparation and characterization of Photoelectroactive substrates for water splitting and hydrogen production
No full textLa soluzione al problema del fabbisogno energetico mondiale, evitando grandi
danni ambientali, rappresenta la sfida scientifica, tecnologica e sociale dell’immediato
futuro. Le risorse fossili continueranno a giocare ancora un grosso ruolo nei prossimi
decenni, ma il loro costo è destinato a crescere, mentre la loro disponibilità a calare. Oltre
ai combustibili fossili esistono anche molte altre risorse energetiche, come nucleare, eolico,
biomasse e geotermico, tuttavia la più attraente, dal punto di vista dell’inesauribilità e
distribuzione su larga scala, è l’energia solare. Tuttavia per essere utilizzata come risorsa
primaria, l’energia solare deve essere immagazzinata, tramite appropriati processi, sotto
forma di combustibili ad alto valore energetico, per l’utilizzo in centrali elettriche o in
processi chimici secondari. Un esempio di “combustibile solare” è l’idrogeno, ottenuto
dalla “fotosintesi artificiale” tramite scissione fotoindotta dell’acqua.
Rappresentazione di una interfaccia ibrida, basata su elettrodi semiconduttori, per la scissione fotoindotta dell’acqua.
I dispositivi a base di elettrodi semiconduttori rappresentano la via più semplice,
economica e versatile, per la conversione diretta della luce solare in combustibili, ma, per
ottenere efficienze significative, devono essere soddisfatte alcune limitazioni cinetiche e
termodinamiche, le quali rappresentano ancora una sfida nel campo chimico e tecnologico.
Scopo del mio lavoro compiuto all’interno del dottorato di ricerca è stata la preparazione di
substrati fotoelettroattivi, costituiti da elettrodi semiconduttori, funzionalizzati con
sensibilizzatori molecolari e catalizzatori, in modo che entrambe le specie potessero
lavorare in modo sinergico, allo scopo di attivare i processi di trasferimento elettronico
necessari per la fotosintesi artificiale, ed ottenere così l’ossidazione dell’acqua all’interno
di una cella fotoelettrochimica, mentre l’idrogeno viene semplicemente sviluppato al
controelettrodo non illuminato. Diversi approcci sono stati tentati per funzionalizzare i
semiconduttori, e gli elettrodi ottenuti sono stati ampiamente caratterizzati tramite
esperimenti spettroscopici, elettrochimici e fotoelettrochimici.
Dopo una prima discussione generale riguardo i processi fotosintetici artificiali
(Capitolo 1) e sulle tecniche sperimentali applicate in questo lavoro di tesi (Capitolo 2),
sono stati descritti i sistemi fotoelettroattivi studiati.
Nel Capitolo 3 è riportata la sintesi e la caratterizzazione di una nuova diade a base
di rutenio, formata da un cromoforo e un catalizzatore, direttamente legati tra loro, per la
sensibilizzazione di fotoanodi di TiO2. I risultati fotoelettrochimici hanno evidenziato che
il complesso binucleare è in grado di iniettare elettroni nel semiconduttore, ma non si ha un
miglioramento nei confronti dell’ossidazione dell’acqua, rispetto al cromoforo da solo.
Questo fenomeno è stato attribuito ai marcati processi di ricombinazione che sfavoriscono
l’iniezione di carica sul semiconduttore, e all’impossibilità termodinamica di raggiungere
stati alto-valenti del metallo (Ru(V)), responsabile dell’ossidazione dell’acqua.
I risultati insoddisfacenti ottenuti con questo complesso binucleare hanno portato a
considerare un metodo di funzionalizzazione alternativo. Il co-adsorbimento
contemporaneo di indipendenti unità, cromoforica e catalizzatrice, non legate tra loro,
rappresenta un approccio semplice e potenzialmente funzionale. Nel Capitolo 4 è riportata
la sensibilizzazione di fotoanodi di WO3 nanocristallino con un colorante organico
(perilene bis-immide). Sotto irraggiamento di luce visibile, lo stato eccitato del colorante
subisce uno spegnimento ossidativo dovuto all’iniezione elettronica sul WO3, lasciando
una buca fortemente positiva sul perilene (1.7 V vs. SCE) potenzialmente in grado di
condurre reazioni di ossidazione in celle fotoelettrochimiche. Il successivo co
adsorbimento di nanoparticelle di IrO2, come catalizzatore per l’ossidazione dell’acqua, ha
portato ad un incremento di quattro volte della fotocorrente anodica, consistente con il
trasferimento di buca dal colorante ossidato alle nanoparticelle. Risultati interessanti sono
stati raggiunti anche con un catalizzatore molecolare anionico (poliossometallato di
rutenio), ottenendo prestazioni comparabili a quelle riportate in letteratura, in fase
eterogenea, con identici o simili catalizzatori.
Nel Capitolo 5, sono state studiate le dinamiche di trasferimento di carica, su
substrati di TiO2, da parte di un nuovo colorante polipiridinico fosfonato a base di rutenio,
progettato e sintetizzato per foto-generare buche altamente ossidanti per la scissione
dell’acqua. Sotto illuminazione, in presenza di ioduro di litio come agente sacrificale, è stata ottenuta una fotocorrente anodica di 0.2 mA/cm2, ma con bassi valori di fotoazione
(IPCE). Interessante la risposta fotoanodica in presenza di ascorbato di sodio, che risulta
aumentata di quattro/sei volte, a seconda della concentrazione di sacrificale, esplicabile
solo assumendo una variazione nel meccanismo di iniezione di carica. Per la prima volta in
fase eterogenea, è stata dimostrata la presenza dello spegnimento dello stato eccitato del
colorante per via riduttiva (processo anti-biomimetico) ad opera dell’ascorbato di sodio.
Gli esperimenti condotti su fotoanodi di SnO2, dove il colorante subisce il classico
spegnimento ossidativo, hanno permesso di verificare l’interazione con nanoparticelle di
IrO2 co-adsorbite, portando all’ossidazione dell’acqua.
Infine, nel Capitolo 6 è stata valutata la deposizione laser-pulsata (PLD) come
valido metodo di sintesi di nanostrutture amorfe di Fe2O3, da utilizzare come catalizzatore
per la funzionalizzazione di substrati conduttivi. Gli elettrodi così ottenuti, costituiti
principalmente da ferro, metallo molto abbondante ed atossico, sono stati in grado di sostenere elevate correnti (fino a 20 mA/cm2), con un modesto potenziale applicato a pH
11.8, per più di un’ora, come anodi per l’ossidazione elettrochimica dell’acqua. Le ottime
prestazioni ottenute confermano la validità della PLD come metodo di preparazione per
materiali nanostrutturati, con un buon controllo sul grado di ricoprimento superficiale e la
morfologia. Tramite questo approccio è possibile esplorare anche altri ossidi metallici
catalizzatori e funzionalizzare nuove superfici, come per esempio celle fotovoltaiche a
tripla giunzione al silicio, per ottenere quindi l’ossidazione fotoindotta dell’acqua.Meeting the demand of a global energy thirst, while avoiding environmental
damages, poses a major scientific and societal challenge for our future. Hydrocarbons
energy sources will continue to play the major role on the next few decades, but their costs
are doomed to increase and their availability decrease. Besides fossil fuels, there are many
other power sources like nuclear fission, wind, biomass and geothermal, but in the long
term the most appealing and widely distributed is solar, providing ~10000 times the energy
currently consumed worldwide. In order to serve as a primary energy source, solar must be
stored, with appropriate processes, into high-energy chemicals, for its utilization in power
plants or in secondary chemical processes. This leads to the concepts of “artificial
photosynthesis” and “solar fuels”, having, as a main target, the photoinduced water
splitting for hydrogen production.
Schematic semiconductor based hybrid interface for photoinduced water splitting.
In principle, semiconductor based devices provide the most straightforward route to
a cheap and tunable process for the direct conversion of sunlight into solar fuels, but, in
order to obtain significant efficiencies, they must satisfy a number of kinetic and
thermodynamic constraints, which pose a great chemical challenge. Scope of this Ph.D.
work was the preparation of photoelectroactive substrates, based on wide band-gap
semiconductor electrodes, functionalized with molecular sensitizers and catalysts, where
both species operate in a concerted way to carry out the electron transfer events at the basis
of the artificial photosynthetic process, thus driving water oxidation while hydrogen is simply evolved at a dark catalytic counter electrode. Several functionalization approaches
of semiconductor electrodes were explored, and the resulting devices were fully
characterized by spectroscopic, electrochemical and photoelectrochemical experiments.
After discussing some general notions about the exploitation of artificial
photosynthetic processes (Chapter 1) and on the main experimental techniques applied
during this Ph.D. thesis (Chapter 2), the study of the photoelectroactive systems is
described.
In Chapter 3 it is reported the modification of TiO2 photoanodes with a new
ruthenium based assembly, constituted by a sensitizer and catalyst units linked together.
The photoelectrochemical results demonstrated that the binuclear complex was able to
inject electrons into the semiconductor, but was not able to drive photoinduced water
oxidation with a significant improvement with respect to sensitizer unit alone. This was
attributed to the unfavourable competition of the charge separation with recombination
processes and to the demanding thermodynamics for generating the high-valent Ru(V),
which was individuated as the active catalytic intermediate for water oxidation.
The unsatisfactory results obtained with the binuclear complex, lead us to consider
the co-adsorption of independent sensitizing and catalytic species which represents a
simpler and potentially more profitable approach. In Chapter 4 it is reported the
modification of nanocrystalline WO3 photoanodes with an organic sensitizer (perylene
bisimide). Under visible irradiation, the dye excited state underwent oxidative quenching
by electron injection into WO3, leaving a strongly positive hole (1.7 V vs. SCE), which
allows to drive demanding photo-oxidation reactions in photoelectrochemical cells. The
co-adsorption of IrO2 nanoparticles, acting as water oxidation catalysts on the sensitized
electrodes, led to a four fold enhancement in photoanodic current, consistent with hole
transfer from oxidized dye to IrO2. Interesting results were also achieved by using an
anionic molecular catalyst (tetra-ruthenium polyoxometalate), producing performances
comparable to some of the best literature results obtained in the heterogeneous phase with
identical or similar catalysts.
In Chapter 5, the charge transfer dynamics on TiO2 substrates, involving a new
phosphonated Ru(II) polypiridine sensitizer, developed to generate highly oxidizing
photoholes for water oxidation, were studied. Under illumination, in presence of LiI as a sacrificial electron donor, an anodic photocurrent of 0.2 mA/cm2 was achieved, with
correspondingly low IPCEs. Interestingly, the photoanodic response were largely increased
in the presence of sodium ascorbate, with photocurrent undergoing a four to six fold
enhancement, depending on the ascorbate concentration, explainable with a modification
of the charge injection mechanism. For the first time in heterogeneous phase, a non
ambiguous evidence for the occurrence of dye excited state reductive quenching (anti
biomimetic pathway) was provided, and the possibility of exploiting such charge transfer
process is discussed. Lastly, experiments performed with sensitized SnO2 photoanodes,
where the Ru(II) dye undergoes the usual oxidative quenching, allowed to verify the
interaction with co-adsorbed IrO2 nanoparticles, capable to drive photoinduced water
oxidation.
Finally, in Chapter 6 the pulsed-laser deposition (PLD) was considered as valuable
tool for the preparation of nanostructured amorphous Fe2O3 catalyst, for the
functionalization of conductive surface. The resulting electrodes, based on earth-abundant
and non-hazardous iron metal, was able to sustain high current densities (up to 20 mA/cm2), at reasonably low applied potential bias, for more than 1 hour when employed as
anodes for electrochemical water oxidation at pH 11.8. The good catalytic performance
achieved proves the validity of PLD as a method to prepare nanostructured materials with
good control over surface coverage and morphology. The methodology lends itself to the
exploration of other metal oxide catalysts and to functionalization of other surfaces, among
which the ohmic contact of multi-junction photovoltaic cells provide the most attractive
example
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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