FY 2008 Annual Progress Report DOE Hydrogen Program

Progress Report and Future Directions Our efforts resulted in various new synthetic methodologies that enabled us to tailor the interfacial structures of semiconducting films and crystals. We also observed many intriguing shape-dependent properties, which clearly demonstrate the importance of shape control for maximizing photoelectrochemical properties. Our main research focuses are briefly described below. Electrochemical Construction of Mesoporous Electrodes Incorporating mesoporous structures into inorganic electrodes can generate an enhanced surface area per unit volume and significantly improve the kinetics and mass transport at the interfaces of electrodes. We have developed a new electrochemical method to produce various inorganic films containing uniformly organized mesoporous structures. Our method is based on creating interfacial amphiphilic layers on the working electrode by surface forces and using them as surface templates to electrodeposit inorganic mesoporous films. This method is quite different from and complementary to conventional sol-gel based dip-coating methods where amphiphilic assembly is evaporation-induced and the inorganic wall construction is achieved by the sol-gel process. As a result, this interfacial electrochemical surfactant templating method significantly enhances our ability to assemble various inorganic mesoporous electrodes (e.g. material type, mesostructure type, pore orientation against the substrate) that cannot be produced by previous means. The resulting mesoporous electrodes contain uniform pore sizes and pore connectivities, which allows us to investigate the effect of nanostructural details on photoelectrochemical properties (Figure 1). Regulation of Individual Particle Shapes in Polycrystalline Electrodes The shapes of individual crystals that compose polycrystalline electrodes dictate interfacial atomic arrangements and can significantly affect interfacial organic-inorganic interactions (e.g. dye adsorption), interfacial charge transfer processes, catalytic properties and stabilities of electrodes. Therefore, gaining the ability to uniformly regulate the shape and connectivity of each crystal in polycrystalline electrodes is crucial for identifying optimum interfacial structures that can maximize desired II.K.8 Electrochemical Construction of High Performance, Low Cost Polycrystalline Photoelectrodes for Solar Hydrogen Production Choi – Purdue University II.K Hydrogen Production / Basic Energy Sciences 298 DOE Hydrogen Program FY 2008 Annual Progress Report photoelectrochemical properties. Earlier efforts toward controlling crystal shape and branching have been limited to simply stabilizing a few certain shapes instead of providing a general methodology to systematically evolve shapes. Our research specifically focuses on establishing synthetic strategies/conditions that can methodically control habit formation and branching growth processes of inorganic crystals. Homogeneous habit control enables us to study any dependence of physical and chemical properties on different crystallographic planes (e.g. {100} vs. {111} planes) while controlled branching growth provides means to expose highly reactive surfaces at the interfaces (Figure 2). We also investigate formation of dendritic and fibrous architectures in inorganic electrodes. As crystals in dendrites and fibers form physically continuous networks by nature, regulating the details of dendritic growth (e.g. particle sizes, shapes, and degree of branching) allows us to achieve enhanced surface areas while decreasing the rate of charge recombination due to grain boundary problems (Figure 3). Tuning Band Gaps and Doping Types of Semiconducting Electrodes In addition to gaining higher degrees of freedom in controlling morphological features, our research project simultaneously focuses on tuning doping types, compositions, and band positions of semiconducting electrodes. For example, we can now produce Cu2O as both p-type and n-type electrodes and increase its bandgap energy (Eg) up to 2.6 eV. Bulk Cu2O (Eg = 1.9 eV 2.2 eV) cannot photoelectrolyze water to H2 and O2 without applying an external bias Figure 2. Polycrystalline Cu2O electrodes with only (A) (111) and (B) (100) planes exposed at the interfaces. Photoelectrochemical properties of these electrodes measured with chopped light are also shown. Figure 3. A-B) SEM images of Dendritic Cu2O films with varying nucleation densities and degrees of branching. C-E) SEM images of Sn/SnO2 electrodes with varying growth morphologies. Figure 1. A-B) SEM images show the side and top view of mesoporous SnO2 films; C) TEM shows worm-like mesopores; D) HR-TEM showing the nanocrystalline feature of the mesoporous wall; E) Potential-dependent photocurrent measurement. The inset shows a short-circuit photocurrent with a chopped irradiation. Potential ( V vs. Ag/AgCl) 0.4 8 . 0 6 . 0 4 . 0 2 . 0 0 . 0 2 . 0 C ur re nt ( A ) 50