Nanostructured CeO2 photocatalysts: optimizing surface chemistry, morphology, and visible-light absorption

Emerging photocatalytic applications of cerium dioxide (CeO2) include green hydrogen production, CO2 conversion to fuels, and environmental remediation of various toxic molecules. These applications leverage the oxygen storage capacity and tunable surface chemistry of CeO2 to photocatalyze the chosen reaction, but many open questions remain regarding the fundamental physics of photocatalysis over CeO2. The commonly ascribed 'bandgap' of CeO2 (∼3.1 eV) differs fundamentally from other photocatalytic oxides such as TiO2; UV light excites an electron from the CeO2 valence band into a 4f state, generating a polaron as the lattice distorts around the localized charge. Researchers often disregard the distinction between the 4f state and a traditional, delocalized conduction band, resulting in ambiguity regarding mechanisms of charge transfer and visible-light absorption. This review summarizes modern literature regarding CeO2 photocatalysis and discusses commonly reported photocatalytic reactions and visible light-sensitization strategies. We detail the often misunderstood fundamental physics of CeO2 photocatalysis and supplement previous work with original computational insights. The exceptional progress and remaining challenges of CeO2-based photocatalysts are highlighted, along with suggestions for further research directions based on the observed gaps in current understanding.

Surface- and Structural-Dependent Reactivity of Titanium Oxide Nanostructures with 2-Chloroethyl Ethyl Sulfide under Ambient Conditions

Robust materials capable of heterogeneous reactivity are valuable for addressing toxic chemical clean up. Synthetic manipulations for generating titanium oxide nanomaterials have been utilized to alter both photochemical (1000 nm > λ > 400 nm) and chemical heterogeneous reactivity with 2-chloroethyl ethyl sulfide (2-CEES). Synthesizing TiO₂ nanomaterials in the presence of long-chain alkylphosphonic acids enhanced the visible light-driven oxidation of the thioether sulfur of 2-CEES. Photooxidation reaction rates of 99 and 168 μmol/g/h (quantum yields of 5.07 × 10^-4 and 8.58 × 10^-4 molecules/photon, respectively) were observed for samples made with two different alkylphosphonic acids (C₁₄H₂₉PO₃H₂ and C₉H₁₉PO₃H₂, respectively). These observations are correlated with (i) generation of new surface defects/states (i.e., oxygen vacancies) as a result of TiO₂ grafting by alkylphosphonic acid that may serve as reaction active sites, (ii) better light absorption by assemblies of nanorods and nanowires in comparison to individual nanorods, (iii) surface area differences, and (iv) the exclusion of OH groups due to the surface functionalization with alkylphosphonic acids via Ti-O-P bonds on the TiO₂. Alternatively, nanowire-form H₂Ti₂O₅·H₂O was produced and found to be capable of highly efficient hydrolysis of the carbon-chlorine (C-Cl) bond of 2-CEES in the dark with a reaction rate of 279.2 μmol/g/h due to the high surface area and chemical nature of the titanate structure.

Photoenhanced Degradation of Sarin at Cu/TiO2 Composite Aerogels: Roles of Bandgap Excitation and Surface Plasmon Excitation

Multifunctional composites that couple high-capacity adsorbents with catalytic nanoparticles (NPs) offer a promising route toward the degradation of organophosphorus pollutants or chemical warfare agents (CWAs). We couple mesoporous TiO₂ aerogels with plasmonic Cu nanoparticles (Cu/TiO₂) and characterize the degradation of the organophosphorus CWA sarin under both dark and illuminated conditions. Cu/TiO₂ aerogels combine high dark degradation rates, which are facilitated by hydrolytically active sites at the Cu||TiO₂ interface, with photoenhanced degradation courtesy of semiconducting TiO₂ and the surface plasmon resonance (SPR) of the Cu nanoparticles. The TiO₂ aerogel provides a high surface area for sarin binding (155 m² g^-1), while the addition of Cu NPs increases the abundance of hydrolytically active OH sites. Degradation is accelerated on TiO₂ and Cu/TiO₂ aerogels with O₂. Under broadband illumination, which excites the TiO₂ bandgap and the Cu SPR, sarin degradation accelerates, and the products are more fully mineralized compared to those of the dark reaction. With O₂ and broadband illumination, oxidation products are observed on the Cu/TiO₂ aerogels as the hydrolysis products subsequently oxidize. In contrast, the photodegradation of sarin on TiO₂ is limited by its slow initial hydrolysis, which limits the subsequent photooxidation. Accelerated hydrolysis occurs on Cu/TiO₂ aerogels under visible illumination (>480 nm) that excites the Cu SPR but not the TiO₂ bandgap, confirming that the Cu SPR excitation contributes to the broadband-driven activity. The high hydrolytic activity of the Cu/TiO₂ aerogels combined with the photoactivity upon TiO₂ bandgap excitation and Cu SPR excitation is a potent combination of hydrolysis and oxidation that enables the substantial chemical degradation of organophorphorus compounds.

Degradation of Fatal Toxic Nerve Agents on Dry TiO2

Despite a recent dramatically increased risk of using chemical warfare agents in chemical attacks and assassinations, fundamental interactions of toxic chemicals with other materials are poorly understood, and micromechanisms of their chemical degradation are yet to be established. This represents an outstanding challenge in both fundamental science and practical applications in combat against chemical weapons. One of the most versatile and multifunctional oxides, TiO₂, has been suggested as a promising material to quickly adsorb and effectively destroy toxins. In this paper, we explore how sarin (also known as GB) adsorbs and decomposes on dry nanoparticles of TiO₂ anatase and rutile phases. We found that both anatase and rutile readily adsorb sarin gas molecules because of a strong electrostatic attraction between the phosphoryl oxygen and surface titanium atoms. The sarin decomposition most likely proceeds via a propene elimination; however, the reaction is exothermic on the rutile (110) surface and endothermic on the anatase (101) surface. High energy barriers suggest that sarin would hardly decompose on pristine dry surfaces of TiO₂, and degradation reactions can be triggered by defects or contaminants under realistic operational conditions.