Nanostructure Approaches for Photoelectrocatalysis

Abstract

Processes, relevant for light-induced fuel generation (water photoelectrolysis, CO 2 reduction) are shortly reviewed. Stabilization strategies for development of efficient tandem cell structures that are based on non-oxidic semiconductors are presented. The recently introduced nanoemitter concept that involves surface passivation by a nanoporous film is described and first results on lightinduced hydrogen generation with p-Si(100) and p-InP(100):2x4 thin films are given. Solar-to-hydrogen efficiencies reach 14% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, successful enzyme immobilization at an electrochemically nanostructured Si surface that shows step bunching has been achieved as evidenced by tapping mode atomic force microscopy. Background and introductory remarksCatalysis is known since long and processes and definitions reach back into the 19 th and early 20 th century [1][2][3][4]. In the current situation, three major routes are followed. Among them is still the use of noble metals with established catalytic properties such as Pt, Rh, Ir or Ru [5][6][7]. Although considered scarce and expensive, architectures and recycling processes (compare traditional photography [8]) can be envisaged and have partly been realized that make such systems cost efficient [9]. In a second approach, the accumulating information on the composition, reaction centers, reaction intermediates and on excitation energy transfer, learned from photosynthesis [10] is used as guiding principle for development of low cost, abundant material photelectrocatalysis [11]. It should be kept in mind, however, that the efficiency of, for example, CO 2 reduction by plants corresponds to an electrochemical current of ~2µA, much too small for any serious application [12] and that the water oxidase in photosystem II has to be regenerated in about every 30 minutes [13]. Also, conversion efficiency is lost in an entropic way in photosynthesis related to the creation and maintainance of highly ordered structures. Also, an artificial photosynthetically active system will have to possess a considerably increased robustness and an architecture that is scalable for terrestrial applications. Finally, the field is also characterized by a considerable effort is made in what could be called a guided combinatorial approach where, based on experience and incidental findings, classes of materials are investigated that are considered promising. This includes the extensive work on transition metal oxides and their modification towards increased photon absorption [14].