Cyanoiron polypyridyl sensitized photoanodes for water oxidation

Abstract

Dye-sensitized photoelectrochemical cells (DSPECs), which convert solar energy to hydrogen fuel via water splitting process, has markedly excelled as a multidisciplinary field in the recent years. In this context, transition metal complexes (TMCs) are employed as efficient photosensitizers because of their unique photochemical and photophysical properties. Ruthenium complexes, have frequently been preferred as both photosensitizers and water oxidation catalysts in DSPECs. However, their toxicity and preciousness have been their main disadvantages. Much research has now devoted to search for highly desirable alternatives. Hexacoordinated Fe-complexes (Fe(II)L6), being earth abundant and chemically stable, have attracted many researchers in this respect. Unfortunately, metal-to-ligand charge transfer (3MLCT) states of Fe-complexes experience ultrafast deactivation process into metal centered (MC) states lying lower in energy with respect to MLCT states, becoming unfavorable for electron injection into TiO2. A fundamental approach is to destabilize these MC states by associating strong field ligands with Fe-center. Given the strong sigma-donating ability of cyanide ligand, the sensitization performance of cyanoiron polypyridyl complexes has also been investigated in earlier studies revealing excited state lifetimes much lower than desired. Herein, my study aims to tackle this problem by assisting donor iron complex not only with electron-donating cyanide groups but also with cobalt ions that are coordinated to nitrogen atoms of cyanide ligands. For this purpose, a series of cyanoiron polypyridyl complexes with different polypyridyl groups and different number of cyanide groups were prepared. These complexes were characterized by multiple techniques including UV-Visible absorption spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and Energy dispersive X-ray analysis (EDX). The foundation of this thesis is mainly built on the development of dyesensitized water oxidation photoanodes. In this study, water oxidation catalytic cobalt sites were connected to iron chromophores through cyanide bridging group affording Prussian blue layer. The effect of cyanide ligands on the rate of charge transfer has also been investigated. Various material characterizations were done to inquire about the effect of cyanide ligands and cobalt catalyst. Photoelectrochemical studies performed on four different dye-sensitized photoanodes reveal that both the type of polypyridyl ligand and the number of cyanide groups play a critical role on the efficiency of the iron photosensitizer. The results of this study suggest that Prussian blue analogues incorporating cyanoiron polypyridyl complexes could be promising assemblies for building efficient DSPECs.