Nanoparticle-Assembled Catalysts for Photochemical Water Splitting

Abstract

Over 130 materials are known to catalyze the conversion of water to hydrogen and oxygen (Equation 16.1) [1], but most of them are plagued by either low energy efficiency, inadequate light absorption in the visible range, or material instability under catalytic conditions.DG ¼ þ 237 kJmol À1 ð1:3 eVe À1 ; l min ¼ 1100 nmÞ ð16:1ÞThese issues can potentially be solved with nanostructured catalysts that contain separate components for light absorption, water oxidation and water reduction. By building these structures in modular fashion from separate, preformed nanoparticles, it should be possible to optimize the catalytic function by adjusting the discrete properties of the various components. The working cycle of a hypothetical three-component catalyst is illustrated in Figure 16.1a. As the first step, bandgap absorption of a photon produces an electron-hole pair in the semiconductor (step 1 in Figure 16.1a). The pair becomes separated at the nanointerface, with the electron being guided (step 2) along a path of decreasing energy to the metal particle, where it enters an energy level above the Fermi level and becomes available as a reducing agent for water (step 3). If this step is fast enough (<50 ps), it will prevent recombination of the electron-hole pair -a limiting factor for water-splitting photocatalysts. Subsequently, an electron from the valence band of the metal-oxide particle is injected into

Sequestering High-Energy Electrons to Facilitate Photocatalytic Hydrogen Generation in CdSe/CdS Nanocrystals

“…One approach of reducing the influence of these competing mechanisms is to design new multicomponent systems consisting of two (or more) semiconductor materials. 3 Such systems introduce new possibilities of manipulating electron flow from one material to the other, depending on the energy level differences of the conduction bands (for electrons) and the valence bands (for holes). For example, excitation of a semiconductor material that is in electrical contact with a second semiconductor material with a lower-energy conduction band will result in electron transport into the second material.…”

“…The generation of H 2 via photocatalytic water splitting has received considerable attention in recent years as efforts to optimize solar energy devices have accelerated. − Although visible photons are sufficiently energetic to split H 2 O and generate high-energy H 2 fuel (requiring a 1.23 eV difference of its coupled half-reactions), materials that demonstrate this in any great yield are still not developed. This is due in part to significant overpotentials, rapid recombination of photogenerated charge carriers, poor absorption of the solar spectrum, and undesired photocorrosion reactions. , The recent advent of semiconductor nanoparticle technology has provided researchers with the powerful flexibility to design novel materials to address these limitations.…”

“…This further requires the inhibition of competing mechanisms (i.e., electron/hole recombination, charge trapping, photocorrosion) that decrease catalytic activity. One approach of reducing the influence of these competing mechanisms is to design new multicomponent systems consisting of two (or more) semiconductor materials . Such systems introduce new possibilities of manipulating electron flow from one material to the other, depending on the energy level differences of the conduction bands (for electrons) and the valence bands (for holes).…”