Ruthenium Complex-Sensitized Hybrid Systems for Solar Chemical Conversion

Abstract

The photosensitization of TiO2 by visible light active dye-sensitizer is a very efficient and cost effective way of solar energy conversion. In addition, the hybridization of nanofunctional species is suggested as an innovative technology to overcome the limitation of a single structure photocatalytic system such as low photoconversion efficiency. Therefore, the hybridized dye-sensitized photocatalytic system was investigated for developing the highly efficient and easily fabricable photoconversion process. Mainly employed sensitizer to develrop hybrid systems was tris(2,2？-bipyridine)ruthenium(II), Ru(bpy)32+ (abbreviated as RuL), which has been the most successful and widely used as a visible light sensitizer. On the Rul or RuL/TiO2 base system, the functional materials such as polymer, carbon nanotubes, and chemical additives are hybridized to develop more efficient solar chemical converision system. The hybrid dye-sensitized systems approached to energy and environmental applications including the environment-friendly hydrogen energy production and the detoxification process such as desulfurization of aromatic sulfur compounds. Firstly, we investigated a photoelectrochemical system employing a hybrid electrode which consists of TiO2 nanoparticles, nafion resin, dye sensitizers, and carbon nanotubes (CNTs). By dispersing TiO2 nanoparticles in nafion matrix in the hybrid electrode, dye sensitizers (ruthenium bipyridyl complexes or cationic organic dyes) that do not adsorb on the surface of TiO2 could be easily attached to the electrode through a simple ion exchange process. The CNTs incorporated in the nafion matrix interconnect TiO2 nanoparticles embedded in the insulating polymer matrix and serve successfully as an electron collector and conduit that transports electrons from the excited sensitizers to the underlying conducting electrode (FTO) base. The photoelectrochemical activity of the hybrid electrode was assessed by measuring the hydrogen production and the photocurrent generation under visible light irradiation. Both the hydrogen production and the photocurrent generation were drastically enhanced with CNTs present in the hybrid electrode. Next, we studied the role of guanidinium cations (abbreviated as G) hybridized in the dye-senstized photocatalytic system. G enhanced the visible light-induced hydrogen production by three times in the RuL-sensitized system using TiO2 nanoparticles coated with nafion polymer. The enhanced photoactivity in the presence of G adsorbed on the nafion coating was related to the retardation of the charge recombination between the electrons injected into TiO2 conduction band (CB) and the oxidized dye molecules, which was confirmed by time-resolved diffuse reflectance spectroscopic measurements. It is proposed that G cations located near the TiO2 surface repel the oxidized dyes to retard the recombination with CB electrons. In addition, the dye luminescence in nafion was enhanced in the presence of G, which was ascribed to the retardation of self-quenching of the excited dyes. The dual roles of G in nafion layer increase the photoelectron density in TiO2 CB and subsequently enhance the production of hydrogen.Lastly, an oxidative desulfurization was investigated by the hybridization of inorganic oxidant with dye-sensitizer to remove aromatic sulfur compounds which are present in transporting fuels and induce the exhaust gas containing SOx as one of the major sources of air pollution. The addition of inorganic oxidant in photocatalytic oxidation process has been demonstrated to enhance the rate of degradation of organic contaminants remarkably because they trap the photogenerated electrons more efficiently than O2. The oxidative radicals for the desulfurization were generated by the electron transfer from the dye to the oxidant under visible light irradiation, which effectively allow the oxidation of dibenzothiphene to the corresponding sulfoxide and sulfone, which are highly polarized compounds, so can be removed from the non-polar fuel oil by conventional separation process. In the present study, we tried the combinative desulfurization method of peroxydisulfate anion (S2O82-) as an inorganic oxidation agent and RuL as a photocatalyst to activate the peroxydisulfate anion to produce a powerful oxidant known as the sulfate free radical (SO4？-). Appendix included the mask hybridiziation on dye-sensitized solar cells (DSSCs). Since a mask was physically attached on a DSSC for more accurate measurement of its photovoltaic performances, it should be mentioned that the mask hybridization is not pursuing a chemical process for solar conversion. The mask aperture size with respect to dye-adsorbed TiO2 area affects on the response of photocurrent, voltage, fill factor and efficiency of DSSCs and the overall efficiency could be overestimated when measuring a DSSC without mask having adequate aperture size. In this Appendix, beside the aperture size, the effects of glass substrate thickness and geometry, thickness and layer structure of TiO2 film on photovoltaic parameters were included. Photovoltaic parameters, mostly photocurrent density, were found to be significantly influenced by the glass substrate thickness and the TiO2 layer structure. Data analysis suggests that photovoltaic characteristics before and after mask are dependent not only on measuring condition such as mask aperture size but also on substrate thickness and TiO2 layer structure.