| Second General Session |
Applications In Energy & Photo Catalysis |
Nanostructured Hybrid Assemblies for Catalysis and Solar Cells
Prashant V. Kamat
Radiation Laboratory, Department of Chemistry & Biochemistry and
Department of Chemical & Biomolecular Engineering
University of Notre Dame, Notre Dame, Indiana 46556-0579, USA
(http://www.nd.edu/~pkamat)
Increasing demand for energy in the near future will force us to seek alternate energy resources. Renewable energy such as solar radiation is ideal to meet the demand but require new initiatives to harvest the photons efficiently. Recent efforts to design ordered assemblies of semiconductor and metal nanoparticles and carbon nanostructures provide innovative strategies for designing next generation energy conversion devices. 1,2
Photoactive organic-inorganic composites as two- or three- dimensional assemblies have been designed to obtain nanostructured architectures with tailored optical and catalytic properties (Scheme 1). The ability to convert light energy into electricity or chemical energy makes these materials useful for energy conversion. By controlling the preparative conditions and by chemical functionalization it is possible to tune the properties of semiconductor (e.g., TiO2, ZnO, CdSe) and metal (e.g., Au, Pt) nanostructures. Such strategies have helped us to improve the charge separation efficiency of light-harvesting nanoassemblies and to use them in photochemical solar cells.3 By probing the electron storage property of metal nanoparticle, we have succeeded in demonstrating the size dependent catalytic activity of small (2-5 nm diameter) gold nanoparticles.4,5 The applications of the composite carbon nanostructures for fuel cells will also be discussed.
References
1. Kamat, P. V., Photophysical, photochemical and photocatalytic aspects of metal nanoparticles. J. Phys. Chem. B, 2002, 106, 7729-7744.
2. George Thomas, K. and Kamat, P. V., Chromophore Functionalized Gold Nanoparticles. Acc. Chem. Res., 2003, 36, 888-898.
3. Hasobe, T., Imahori, H., Kamat, P. V. and Fukuzumi, S., Photovoltaic Cells using composite nanoclusters of porphyrins and fullerenes with gold nanoparticles. J. Am. Chem. Soc, 2005, 127, 1216-1228.
4. Subramanian, V., Wolf, E. E. and Kamat, P. V., Catalysis with TiO2/Au Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration. J. Am. Chem. Soc., 2004, 126, 4943-4950.
5. Hirakawa, T. and Kamat, P. V., Charge Separation and Catalytic Activity of Ag@TiO2 Core-Shell Composite Clusters under UV-Irradiation. J. Am. Chem. Soc., 2005, 127, 3928-3934.
Photo-Induced Charge Transfer at Molecular – Semiconductor Interfaces
Gerald J. Meyer
Department of Chemistry, Johns Hopkins University
Baltimore, Maryland 21218, USA
Assemblies of molecular components can be designed to perform relatively elaborate and useful tasks. The concept is currently being applied to the design of systems capable of functioning as molecular devices. In many of these artificial molecular devices, a fundamental role is played by light (either as an energy input or as a signal to be processed), and the function relies on energy or electron transfer processes taking place, with controlled rates and in appropriate sequences, between molecular components within the structure. From this standpoint, supramolecular chemistry (and photochemistry) can be viewed as the basis for a “bottom-up” approach to the challenging fields of molecular electronics and nanotechnology. Interestingly, the opposite, “top-down” tendency is active in the field of materials science, where increasing attention is being shifted from bulk materials to systems of smaller and smaller dimensions. It can be noticed that, working in opposite directions, these two approaches converge to objects of the same, nanometer size. Thus, an appealing idea is that of coupling together, via covalent bonding or other interactions, supramolecular systems and nanoparticles in what may be called “heterosupramolecular” systems. In such systems, discrete particles can play the role of additional molecular components of the supramolecular assembly. Nanoparticles, however, also possess solid-state properties and, as part of macroscopic aggregates can provide an interface between the supramolecular system and the external world. This brings about interesting prospectives for addressing, modulating, and exploiting supramolecular function. The knowledge and control of electronic interactions at the interfaces between molecules and nanometer-sized semiconductors is of fundamental importance for several emerging fields of science and technology.
Molecular and “supramolecular” compounds have improved our understanding of electron transfer at nanostructured semiconductor surfaces. Specifically, we have synthesized novel photo- and redox- active molecular compounds that can be positioned on nanocrystalline semiconductor surfaces with high precision and control. By employing both “top-down” and“bottom-up” approaches we have independently tuned both the nanoparticle and the molecular properties to optimize desired behavior. Pulsed light excitation of these molecular materials allows fundamental interfacial electron and energy transfer events to be quantified spectroscopically with high signal-to-noise ratios. The presentation provides examples of how electronic interactions at molecular-semiconductor nanoparticle interfaces can be quantified and ultimately controlled with molecular precision. The applications of these advances in solar energy conversion and in the remediation of environmental pollutants will also be described.
Nanostructured photocatalysts and nanostructured dye-sensitized solar cells: Recent results and future projects
M. S. A. Abdel-Mottaleb
Photoenergy Center and Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt
Tel: + 2012 216 9584 Fax: + 202 634 7683, Email: solar06@photoenergy.org
Photocatalytic degradation of some pollutants that may occur in industrial waste streams is one of the important fields we are concerned with. Supported and non-supported nanoparticles of TiO2 are prepared and used. This lecture will report on our recent results in this field and its impact on the environment; in particular, on water disinfection. The results of a current international project1) will be addressed.
Moreover, we have fabricated and tested simple Graetzel-type solar cells using semiconductor thin films consisting of colloidal TiO2 nanoparticles prepared by sol-gel technique and some electron injecting dyes. The possibility of using xanthenes (rhodamine 101, fluorescein and 5(6)-carboxyfluorescein) and selected azo dyes (alizarin yellow R, alizarin yellow 2G and carboxyaesenazo) as sensitizers has been explored.
Fluorescence and electronic absorption measurements revealed complex formation between the chosen dyes and the surface of the colloidal TiO2. The apparent association constants (Kapp) of the surface complexes have been estimated and are correlated with the dye induced negative shifts of the reduction potential of colloidal TiO2 nanoparticles.
Moreover, due to its utmost importance, photostability of the organic dyes in absence and presence of colloidal TiO2 nanoparticles and the influence of the used electrolyte have been examined. The results point to a remarkable enhancement of photostability in the presence of the electrolyte (I3−/I−), which is attributed to fast regeneration of the neutral dye via the redox couple of the electrolyte. Furthermore, photocurrent action spectrum of the fabricated and tested nanostructured DSC shows the origin of photoelectric output to be optical absorption of the dye used.
Finally, new creative ideas towards more efficient: a- photocatalysts and b- solar cell based on nanowiring alignment of charge generator2) will be also addressed.
1) AQUACAT Project supported by European Union.
2) James T. Mcleskey, Virginia Commonwealth University, Richmond, U.S.A. [Joint U.S. – Egypt project
proposals].
Nanostructured Materials for Photovoltaic and Fuel Cells Applications
A. B. Kashyout
Mubarak City for Scientific Research and Technology Applications, MuCSAT
Borg El-Arab, Alexandria, Egypt
This paper concerns with the nanostructure materials applied for solar cells and fuel cells applications.
Oxide semiconductors are preferential in photoelectrochemistry because of their exceptional stability against photo-corrosion optical excitation in the band gap. Furthermore, the large band gap (> 3 eV) of the oxide semiconductors is needed in dye sensitized solar cells (DSSCs) for the transparency of the semiconductor electrode for a large part of the solar spectrum. In addition to TiO2, semiconductors used in porous nanocrystalline electrodes for dye sensitized solar cells include ZnO, CdSe, CdS, WO3, Fe2O3, SnO2, Nb2O5, and Ta2O5. However, titanium dioxide has been, and still is, the cornerstone semiconductor nanostructures electrodes. Zinc oxide with a band gap similar to that of TiO2, could be an alternative material for solar cell applications. Also, CdS:Sb will be investigated for dye sensitized solar cells.
Fuel cells are considered one of the most promising technologies for clean and reliable transportation systems. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity when suitable catalysts are used. Also, there are many types of fuel cells could be used for transportation systems and could be classified to either direct fuel cell vehicles or processed fuel cell vehicles. This lecture reports the advantages and disadvantages of the two types as well summarizes our activities in the field of fuel cells technology and the preliminary results for the preparation of BaCeO3 films for intermediate temperature fuel cells and preparation of carbon supported nano catalysts and TiO2 films for fuel cells applications.
