Extending Photoinduced Charge Separation. Molecular-semiconductor assemblies for solar energy conversion.
Doktorsavhandling, 2017

The conversion of solar energy into chemical energy by harvesting visible-light with synthetic molecules presents several scientific and technological challenges. This thesis is dedicated to the investigation of approaches to long-lived charge separation, one of the crucial aspects for the photochemical generation of solar fuels. Charge separation was characterized in molecular and molecular-semiconductor hybrid assemblies by using optical spectroscopic techniques. The assemblies studied, were designed with the purpose of either solar fuel generation, or for mere mechanistic understanding and proof of principle studies on relevant aspects for solar-to-chemical energy conversion. The photophysical characterization of a Ru-Mn supramolecular complex for photo-chemical water oxidation, and related Ru(II)-complexes, revealed the reasons behind the difficulty of obtaining a long-lived charge separation state. This prevented the photo-chemical water oxidation, exemplifying the limitations of supramolecular approaches to solar fuels. This work justifies the need to explore other alternatives for the creation of a stable material where all the basic functions of natural photosynthesis can be imitated in a simplified way. A viable option is the construction of nanoarchitectures that incorporate light-harvesting units and catalysts on a semiconductor surface. As demonstrated in this thesis, the modification of the individual components of these assemblies, e.g. macroscopic structure of the dye-sensitized semiconductor and electrolyte, can be used to extend not only the lifetime, but also the distance of charge separation. One of the most remarkable findings of this thesis, is that the lifetime of charge separation in dye-sensitized semicon-ductors can be extended by several orders of magnitude, by implementing a photoanode design consisting of a repetitive patterns of SnO2 and TiO2 µm-thick layers. The main feature of this design is the possibility of trapping electrons at dye-free areas on the film, where they reside for longer times before recombining with dye molecules. In addition, such materials can be used for visible-light generation of catalytically active sites on the surface through electron transfer from the photosensitizer. This process, being facilitated by the conduction band of the semiconductor. The work summarized in this thesis is intended to encourage the development of dye-sensitized semiconductors to expand the possibilities of their application in solar fuel technology.

Artificial photosynthesis

Back-electron transfer

Ruthenium complexes.

Dye-sensitized

Charge recombination

Charge separation

Conduction band mediated

EF-salen, Hörsalsvägen 11, EDIT-huset (Trappa C-Vån 6)
Opponent: Prof. Nikolai Tkachenko, Laboratory of Chemistry and Bioengineering, Tampere University of Technology, Finland

Författare

Valeria Saavedra

Chalmers, Kemi och kemiteknik, Kemi och biokemi, Fysikalisk kemi

Extending charge separation lifetime and distance in patterned dye-sensitized SnO2–TiO2 µm-thin films

Physical Chemistry Chemical Physics,; Vol. 19(2017)p. 22684-22690

Artikel i vetenskaplig tidskrift

Synthesis and Electron-Transfer Processes in a New Family of Ligands for Coupled Ru-Mn-2 Complexes

ChemPlusChem,; Vol. 79(2014)p. 936-950

Artikel i vetenskaplig tidskrift

Ionic Liquid-Induced Local Charge Compensation: Effects on Back Electron-Transfer Rates in Dye-Sensitized TiO2 Thin Films

Journal of Physical Chemistry C,; Vol. 120(2016)p. 20016-20023

Artikel i vetenskaplig tidskrift

Saavedra, B. V, Sundin, E, Abrahamsson, M. Conduction Band Mediated Electron Transfer in Dye/TiO2/Acceptor-Assemblies

Omvandling av solenergi till kemisk energi genom att skörda synligt ljus med syntetiska molekyler medför flera vetenskapliga och tekniska utmaningar. Denna avhandling är inriktad på utveckling av material och metoder för långlivad laddnindseparation, en av de avgörande aspekterna inom fotokemisk produktion av solbränslen.
Laddningsseparation karakteriserades i molekylära och hybridmaterial med användning av spektroskopiska tekniker. Dessa konstruerades med syftet att antingen generera sol-bränsle eller enbart för mekanisk förståelse (och bevis på principstudier) om relevanta aspekter för sol-till-kemisk energiomvandling.
Den fotofysiska karaktäriseringen av ett Ru-Mn-komplex för fotokemisk vattenoxi-dation underströk orsakerna till svårigheten att erhålla ett laddningsseparat tillstånd i komplexet, vilket förhindrade den önskade vattensplittringsreaktionen. Detta exemplifie-rar begränsningarna av supramolekylära komplex för solbränslegenerering. Detta arbete motiverar behovet av att utforska andra alternativ för skapandet av ett stabilt material där alla grundläggande funktioner för naturlig fotosyntes kan imiteras på ett förenklat sätt.
Ett genomförbart alternativ är att bygga nanoarkitekturer som innehåller ljus-infångande enheter och katalysatorer på en halvledaryta. I denna avhandling presenteras strategier för modifiering av de individuella komponenterna i dessa enheter, t.ex. makroskopisk struktur hos färgamnessensibiliserad halvledare och elektrolytens sammansättning. Dessa modifieringar kan användas för att förlänga båda livslängden och avståndet för laddnings-separation. Dessutom har forskningen som presenteras här visat att sådana material kan användas för att generera, med hjälp av synligt ljus, katalytiskt aktiva tillstånd på ytan genom elektronöverföring från fotosensibiliseraren. Detta är en process som medieras av halvledarens ledningsband.
Arbetet som sammanfattas i denna avhandling syftar till att uppmuntra utvecklingen av färgämnessensibiliserade halvledare för att utöka möjligheterna för deras tillämpning inom solbränsle produktion teknik.

The conversion of solar energy into chemical energy by harvesting visible-light with synthetic molecules presents several scientific and technological challenges. This thesis is dedicated to the investigation of approaches to long-lived charge separation, one of the crucial aspects for the photochemical generation of solar fuels. Charge separation was characterized in molecular and molecular-semiconductor hybrid assemblies by using optical spectroscopic techniques. The assemblies studied were designed with the purpose of either solar fuel generation or for mere mechanistic understanding and proof of principle studies on relevant aspects for solar-to-chemical energy conversion.

The photophysical characterization of a Ru-Mn supramolecular complex for photochemical water oxidation, and related Ru(II)-complexes, revealed the reasons behind the difficulty of obtaining a long-lived charge separation state. This prevented the photochemical water oxidation, exemplifying the limitations of supramolecular approaches to solar fuels. This work justifies the need to explore other alternatives for the creation of a stable material where all the basic functions of natural photosynthesis can be imitated in a simplified way.

A viable option is the construction of nanoarchitectures that incorporate light-harvesting units and catalysts on a semiconductor surface. As demonstrated in this thesis, the modification of the individual components of these assemblies, e.g. macroscopic structure of the dye-sensitized semiconductor and electrolyte, can be used to extend not only the lifetime, but also the distance of charge separation. One of the most remarkable findings of this thesis, is that the lifetime of charge separation in dye-sensitized semiconductors can be extended by several orders of magnitude, by implementing a photoanode design consisting of a repetitive patterns of \ce{SnO2} and \ce{TiO2} µm-thick layers. The main feature of this design is the possibility of trapping electrons at dye-free areas on the film, where they reside for longer times before recombining with dye molecules. In addition, such materials can be used for visible-light generation of catalytically active sites on the surface through electron transfer from the photosensitizer. This process, being facilitated by the conduction band of the semiconductor.

The work summarized in this thesis is intended to encourage the development of dye-sensitized semiconductors to expand the possibilities of their application in solar fuel technology.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Fysikalisk kemi

ISBN

978-91-7597-621-1

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 0346-718X

Utgivare

Chalmers tekniska högskola

EF-salen, Hörsalsvägen 11, EDIT-huset (Trappa C-Vån 6)

Opponent: Prof. Nikolai Tkachenko, Laboratory of Chemistry and Bioengineering, Tampere University of Technology, Finland

Mer information

Senast uppdaterat

2018-10-19