A. G. Muñoz

Photoelectrocatalysis: principles, nanoemitter applications and routes to bio-inspired systems

An overview on processes that are relevant in light-induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst/solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio-inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non-oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic (solid state or electrochemical) solar cells and first results with p-Si and p-InP thin films are presented. Solar-to-hydrogen efficiencies reach 12.1% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given.

Epitaxial III–V Films and Surfaces for Photoelectrocatalysis

Efficient photoelectrochemical devices for water splitting benefit from the highest material quality and dedicated surface preparation achieved by epitaxial growth. InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. We propose epitaxial dilute nitride GaPNAs photocathodes on Si(100) substrates to combine close-to-optimum limiting efficiency, lattice-matched growth, and established surface preparation. Prior to a discussion of the challenging III-V/Si(100) heterojunction, we describe the closely related epitaxial preparation of InP(100) surfaces and its beneficial impact on photoelectrochemical water-splitting performance. Analogies and specific differences to GaP(100) surfaces are discussed based on in situ reflectance anisotropy and on two-photon photoemission results. Preliminary experiments regarding GaP/Si(100) photoelectrochemistry and dilute nitride GaPN heteroepitaxy on Si(100) confirm the potential of the GaPNAs/Si tandem absorber structure for future water-splitting devices.

Morphological and chemical optimization of ex situ NH4F (40%) conditioned Si(111)-(1×1):H

Synchrotron radiation photoelectron spectroscopy was employed to investigate the chemical state of Si(111) surfaces upon anisotropic etching in concentrated NH4F solution. Minute amounts of oxidized silicon were detected and attributed to the fast Si–H–OH formation at atomic steps. Combining in situ optical and scanning probe techniques, consecutive chemical treatments were developed to achieve optimized morphological and chemical surface properties. Native oxides and a stressed SiO2/Si layer are removed by a two-step NH4F treatment leading to a terraced surface without triangular etch pits; subsequently, silicon in the Si1+/2+/3+ valence states is dissolved by HF (50%) while the surface topography is preserved.

Advances in Photoelectrocatalysis with Nanotopographical Photoelectrodes

The design of photoelectrodes for high efficiency solar fuel energy conversion devices is based on the search for adequate surface conditioning to achieve efficient light harvesting, stability, minimized surface recombination losses and high electron-transfer rates at the electrolyte interface. An overview on established and novel approaches is given. A recent viable solution is provided by electroplating of nanoscale catalytic metals on passivated semiconductor surfaces, thereby forming reactive centers and avoiding contact between the semiconductor surface and the electrolyte. At these nano-dimensioned Schottky-type junctions, light-induced excess minority carriers are scavenged and transferred to the electrolyte. Various possible device configurations are outlined and envisaged systems for hydrogen or oxygen evolution and carbon dioxide reduction are presented. The role of ultrathin passivating films is emphasized and methods to fabricate open as well as compact conformal films are described.

Fundamental Aspects of Electrodeposition for the Realization of Plasmonic Nanostructures

Electrodeposition is used for the preparation of nanoparticles and nanostructures that allow, in principle, surface plasmon excitation. The (photo)electrodeposition process of Rh and Au nanoparticles as well as of heterodimeric enzymes onto silicon surfaces is investigated and the resulting structures are discussed with regard to applications in photoelectroctalysis and biosensing. Electrodeposition of Rh onto H-terminated p-Si surfaces generates nanostructures of the metal nanoparticles with simultaneous oxidation of the substrate thus forming nano-dimensioned metal-oxide-semiconductor (MOS)-type contacts. The excess minority carrier harvesting in these nanoemitter structures, where semispherical space charge layers underneath the metal exist are discussed based on spectral sensitivity and capacitance measurements The deposition of Au nanoparticles by a combined chemical-electrochemical method on Si is presented as an example for sensing actuators where the resonance frequency is changed by adsorption. Similarly, site-selective deposition of the enzyme reverse transcriptase onto nanostructured (step-bunched) silicon serves as precursor experiment for biosensing in a Kretschmann-type ATR configuration. Future applications based on plasmonically active structures are outlined.