Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO ₂ reduction

Abstract

In 2 O 3-x (OH) y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In 2 O 3-x (OH) y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO 2 reduction.indium oxide | solar fuels | CO 2 hydrogenation | transient absorption | surface defects C oncerns over climate change and the projected rise in global energy demand have motivated researchers to develop alternative, more sustainable ways to generate energy from naturally abundant and renewable sources (1-3). An important challenge associated with using renewable energy sources, such as solar, is their inherent intermittent nature (4-6). The emerging field of solar fuels seeks to address this issue by storing radiant solar energy in chemical bonds, which can then be released on demand and act as a drop-in replacement for traditional fossil fuels (7-11). By using the greenhouse gas, CO 2 , currently regarded as a waste product, as a feedstock and converting it into valuable products such as solar fuels or platform chemicals, we could simultaneously address concerns over climate change and energy security while creating significant economic benefits (12)(13)(14). However, despite recent advances in the development of active materials that can drive the photocatalytic reduction of CO 2 into useful chemical species, much remains unknown about the fundamental physical properties that control the activity of a photocatalyst. To facilitate the rational development and improvement of photocatalytic materials, a detailed understanding of the complex interplay between chemical, optical, and electronic processes is needed.Indium oxide has many favorable optical, electronic, and surface properties that make it a compelling choice as a photocatalyst for CO 2 reduction. It has a relatively high conduction band, and common defects such as oxyge...

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

“…These results have implications for the design of In 2 O 3 based materials optoelectronic applications: Although the ubiquitous hydrogen species and adsorbed OH/oxygen groups do not affect the surface conductivity, their presence may have a strong influence on the optical transparency. In addition, while an n ‐type surface can be unwanted for a particular electronic application, it clearly proves to be beneficial in photocatalysis, enabling the reverse water‐gas‐shift and methanol production reactions . This appears to be particularly relevant for oxygen defect rich In 2 O 3 with nanometer scale particles with high surface to volume ratios .…”

“…In unison with other extrinsic donor‐type defects like interstitial hydrogen, ubiquitously incorporated into the crystal lattice, these states push the E F toward the CNL and produce a 2D gas of itinerant electrons (2DEG) near the surface . Remarkably, theory and experimental studies also reveal the important impact of these surface defects in promoting the reverse water‐gas‐shift reaction . At the (111) surface, in particular, adsorbed OH groups couple with indium sites proximal to an oxygen vacancy, forming a frustrated Lewis pair.…”

Von Sonnenlicht zu Brennstoffen: aktuelle Fortschritte der C1‐Solarchemie

“…[129] Das Ersetzen der ZnO-Trägers durch TiO 2 oder Abbildung 8. [123] Copyright 2016, National Academyo fSciences. a) Wirkung der Vorstufenverbindung auf die photokatalytische Leistungsfähigkeit.…”

“…Abbildung 7. a) Zeitprofil des CO-Verbrauchs, der CO 2 -Entwicklung und der H 2 -Entwicklung über einem Au/TiO 2 -Katalysator (1 Gew.-%) unter Sonnenlichtbestrahlung. Ferner wurde durch transiente Femtosekunden-Absorptionsmessungen nachgewiesen, dass Sauerstofffehlstellen und Oberflächen-Hydroxygruppen eine wesentliche Rolle bei den Ladungsrelaxationswegen des angeregten Zustands spielen, die stark mit der photokatalytischen Aktivitätkorrelieren [123]. GenehmigteW iedergabe [105].…”

Sustainability and Nanomaterials in Concert

“…Such engineering aims at controlling the type and distribution of defects, which can govern the chemical reactivity of the metal oxide toward CO 2 regardless of the type of energy used to activate the process. For example, in photocatalytic CO 2 reduction, oxygen vacancies and hydroxyl radicals can trap the charge carriers (electrons and holes), thus retarding recombination . Alternatively, specific defect types (in specific metal oxides) can drive the formation of low energy midgap states of the metal oxide in its photoinduced excited state, creating surface frustrated Lewis pairs (FLPs) with increased Lewis acidity and basicity that are responsible for enhanced CO 2 hydrogenation…”