The science describing semiconductor−liquid interfaces is highly interdisciplinary, broad in scope, interesting, and of importance to various emerging technologies. We present a review of the basic physicochemical principles of semiconductor−liquid interfaces, including their historical development, and describe the major technological applications that are based on these scientific principles.

Physical Chemistry of Semiconductor−Liquid Interfaces

Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional crystalline titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water reduction, facilitating electron transport at the cathode–electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradiation, solar-to-hydrogen efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, respectively. The system reaches a value of 0.85 of the theoretical limit for photoelectrochemical water splittin...

Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency

Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional crystalline titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water reduction, facilitating electron transport at the cathode–electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradiation, solar-to-hydrogen efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, respectively. The system reaches a value of 0.85 of the theoretical limit for photoelectrochemical water splitting for the energy gap combination employed in the tandem-junction photoelectrode structure.

Monolithic Photoelectrochemical Device for 19% Direct Water Splitting

Very thin gold films were prepared on glass by ion plating (IP) and by conventional evaporation (CE). Below a certain thickness—∼9 nm for IP and ∼15 nm for CE—the films comprised a metal network; above this thickness we found uniform films. Optical properties were recorded by spectrophotometry. Conspicuous near‐infrared transmittance plateaus were seen in network films. This effect is conducive to high solar transmission. The spectral features were explained from effective medium theories based on the film structure. The uniform films were consistent with the Drude theory, provided that an anomalously large frequency dependence of the relaxation energy was invoked. Significant induced transmission was found in calculations on dielectric/gold/dielectric coatings. Our results lead to improved noble‐metal‐based transparent infrared reflectors for potential use on energy efficient windows.

Noble‐metal‐based transparent infrared reflectors: Experiments and theoretical analyses for very thin gold films

A photoanode protection strategy using a multifunctional NiO_x coating is presented. The ransparency/antireflectivity, low electrochromism, conduction of holes, corrosion protection, and active electrocatalysis for water-oxidation half-reaction are described.

Sputtered NiOx Films for Stabilization of p+n-InP Photoanodes for Solar-Driven Water Oxidation

We report here the optical properties of near-transparent porous platinum films supported on p-type indium phosphide (InP). The deposition of small quantities of catalytically active metals on p-InP increases photoelectrochemical solar-to-hydrogen conversion efficiency by providing a pathway for photogenerated electrons to reduce efficiently protons in solution1. Surprisingly, efficiencies increase slowly even after fairly thick metallic layers have been deposited. These results imply that in certain conditions the metal overlayer is effectively transparent to the incident light, or even promotes the coupling of incident radiation into the bulk of the semiconductor2. We present here both experimental and theoretical evidence to show that these effects can be understood in terms of microstructure: when the metal films are sufficiently porous and built up from particles smaller than the wavelength of the transmitted light, the photon fields are screened out of the metal phase and are forced into the void structure. This increases the effective refractive index of the layer over that of the ambient and provides a better match with the substrate, while incurring negligible absorption loss.

John D. Porter

Papers

Experiment and theory of ‘transparent’ metal films