Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements

Mildly regulated intrinsic faradaic layer at the oxide/water interface for improved photoelectrochemical performance

Metal oxides are widely used in different fields, including photoelectrocatalysis, photocatalysis, dye-sensitized solar cells, photoinduced superhydrophilicity and so on. It is well-known that there are intrinsic hydrated layers on the surfaces of metal oxides in ambient air or the electrolyte. Generally, interface layers between metal oxides and solutions have significant effects on the performances in these applications. However, the exact roles of the intrinsic hydrated layers are still unclear. In this study, taking TiO2 and Fe2O3 as model materials, we propose a mild heat treatment to increase the hydroxyl concentration in the hydrated surface layers of the oxides, which improves their photoelectrochemical performance remarkably. Moreover, we find that the heat-regulated hydrated layer plays the role of a hole transfer mediator between oxides and the electrolyte, which can accelerate both interface charge collection and oxygen evolution reaction kinetics in acidic solution. The new insights into the intrinsic hydrated interface layer on oxides can offer guidance not only in photoelectrocatalysis, but also in the other applications mentioned above.

In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

Formation of hydroperoxo (-OOH) species on the surface of self-doped Bi2.15WO6: reactivity towards As(iii) oxidation

Bi2+xWO6 is a cost-effective and environmentally friendly photocatalyst that shows high reactivity in the oxidation of various contaminants under visible light. However, under alkaline conditions, the reactive oxidative species in the Bi2+xWO6 system are still not clear yet. In this study, it is observed that the oxidation rates of As(iii) increase with increasing pH values in the Bi2.15WO6 system. Photoluminescence and the Mott-Schottky analyses confirm that OH- promotes the separation and transfer of photogenerated electron-hole pairs over Bi2.15WO6, thus facilitating the oxidation of As(iii). Electron spin resonance spectra analysis and quenching experiments rule out contributions of •OH, O2˙-, 1O2 and superoxo species to As(iii) oxidation and indicate that surface -OOH and/or H2O2 are indeed the predominant species under alkaline conditions. The improved production of H2O2 by H-donors such as glucose and phenol, as well as the UV-vis diffuse reflectance and Raman analyses, further confirms the formation of surface -OOH on Bi2.15WO6 under alkaline conditions. In the dark, the significant higher oxidation rate of As(iii) by H2O2-Bi2.15WO6 than that by H2O2 alone reveals that surface -OOH, instead of H2O2, plays an important role in As(iii) oxidation. This study enriches our understanding of the diversity of reactive oxygen species (ROS) in the Bi2.15WO6 system and gives new insight into the mechanism involved in the oxidation of As(iii) under alkaline conditions.

Direct evidence of the key role of UV-formed peroxide species in photocatalytic gas-solid oxidation in air on anatase TiO2 particles

IR spectroscopy was applied for the in situ investigations of surface intermediates formed on the surface of TiO2 (anatase) and ZnO under UV light illumination in air and their reactivity in the elimination of simple pollutant molecules. UV-irradiation of TiO2 (anatase) in air leads to the generation of peroxo-species with the peaks at 852 and 912 cm-1, but the bands of O2˙-ads were not detected. This is, to our knowledge, the first direct in situ IR spectroscopic detection of O2 photosorption intermediates in moist air. The formation of peroxo-species in these conditions is specific for TiO2 (anatase), whereas on ZnO the predominant species under UV light illumination in air are O2˙-ads and H2O2, desorbing into gas phase. Adsorbed water and surface hydroxyl groups contribute to the formation and stabilization of peroxo-species on TiO2 anatase during UV illumination in an oxygen atmosphere. If UV-irradiation is carried out in the environment of moist argon instead of moist air, the peroxo-species on TiO2 anatase are formed from water in a negligible quantity. Peroxo-species formed after O2 photoadsorption on TiO2 anatase in moist air have band positions similar to peroxo-species formed after photodecomposition of H2O2 (with accompanying color change of this sample from yellow to white). Direct experimental IR-spectroscopic evidence of peroxo-species reactivity as oxidative intermediates on TiO2 (anatase) in CO and ethanol vapor photooxidative processes is firstly obtained. These results confirm our early conclusion that peroxo-species formed under UV-irradiation in O2 on the hydrated surface of TiO2 (anatase) can be responsible for the surprising extreme dependence of the CO photooxidation rate on the adsorbed water coverage with the maximum at ∼0.5 ML. The ZnO sample was not active in the photooxidation of these molecules in air. It is concluded that UV formed peroxo-species are important diamagnetic oxidative intermediates in heterogeneous photochemical gas-solid oxidation processes on TiO2 (anatase).

Structure and reactivity of a water-covered anatase TiO2(001) surface

We systematically studied water adsorption and oxidation on the unreconstructed TiO₂(001) surface using first-principles calculations. Water first adsorbs on the surface in a dissociative state and then in a molecular state, as water coverage increases. The geometric properties of all adsorption structures suggest that the dissociative water molecules can induce stress release of the (001) surface at low coverage, reducing reactivity of the surface and thus leading to molecular adsorption of water on the surface at high coverage. The adsorption energy (or the surface energy) monotonously increases (or decreases) with the increase of the coverage, which further confirms that water, irrespective of its dissociative or molecular state, can improve the stability of the (001) surface and reduce its activity. We deeply investigated the mechanism of the oxygen evolution reaction (OER) on the water-covered (001) surface. A new water-assisted OER pathway is identified on the (001) surface, which includes the sequential transfer of protons from molecular water and surface hydroxyls, and O-O coupling processes. During the OER pathway, the O-O coupling step exhibits the largest thermodynamic energy and highest energy barrier, clarifying that it is the rate-determining step in the whole pathway. Our findings provide new insights into the strong dependence of water adsorption modes on coverage for the anatase TiO₂(001) surface and may explain the high oxidation activity of the TiO₂(001) surface in aqueous environments typical of TiO₂ photocatalysis.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Smart, Photocatalytic and Self-Cleaning Asphalt Mixtures: A Literature Review

Nowadays, there is increasing concern in transportation engineering about the use of techniques less harmful to the environment and also about road safety. Heterogeneous photocatalysis based on the application of semiconductor materials onto asphalt mixtures is a promising technology because it can mitigate air pollution and road accidents. The functionalized asphalt mixtures with photocatalytic capability can degrade pollutants, such as damaging gases and oil/grease adsorbed on their surface, from specific reactions triggered by sunlight photons, providing significant environmental and social benefits. In this article, a review of photocatalysis applied in asphalt mixtures is presented. The most important characteristics related to the functionalization of asphalt mixtures for photocatalytic applications and their corresponding characterization are presented, and the achieved main results are also discussed.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

Highly Selective Active Chlorine Generation Electrocatalyzed by Co3O4 Nanoparticles: Mechanistic Investigation through in Situ Electrokinetic and Spectroscopic Analyses

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co₃O₄ nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co₃O₄ nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO₂-based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.

Braiding kinetics and spectroscopy in photo-catalysis: the spectro-kinetic approach

The combination of kinetic and spectroscopic tools has become a key scientific methodology for the understanding of catalytic behavior but its application in photocatalysis has inherent difficulties due to the nature of the energy source of the reaction. This review article provides an overview of its use by, first, presenting mechanistically derived kinetic formulations and spectroscopic data handling methods including intrinsic expressions for light and, second, highlighting representative examples of application. To do it we consider universal catalytic systems, particularly (although not exclusively) titania-based materials, and the most frequent hole and/or electron triggered reaction schemes. This review also provides a general framework to pave the way for the future progress of the spectro-kinetic approach in the photocatalysis area.

Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO2 photoreduction

Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.

Wheel-Like Icosanuclear Peroxotitanate-A Stable Water-Soluble Catalyst for Oxygen Transfer Reactions

Water-soluble wheel-like icosanuclear peroxotitanate K₁₆[Ti₂₀(μ-O)₈(HO₂)₈(O₂)₁₂( R, R-tart)₁₂]·52H₂O (1) chelated by tartrate has been successfully isolated. As the largest peroxotitanate reported, {Ti₂₀} features 20 (hydro)peroxo groups with three kinds of coordination modes in μ-η¹:η², μ-η²:η², and η² fashions. The cluster is stable in solution and solid states. It has been tested for the catalytic oxidations of methyl phenyl sulfide and pyridine with hydrogen peroxide, respectively, which shows reversible elimination and the addition of peroxo groups. This provides a rare example of well-characterized titanium peroxide for homogeneous catalysis and mechanism research.

Two-site H2O2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley-Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir-Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

Characterization of peroxo reaction intermediates in the water oxidation process on hematite surfaces

We use density functional theory-based calculations to study structural, electronic, and magnetic properties of two key reaction intermediates on a hematite, [Formula: see text]-Fe₂O₃, photoanode during the solar-driven water splitting reaction. Both intermediates contain an oxygen atom bonded to a surface iron atom. In one case, the adsorbed oxygen also forms a peroxo bond with a lattice oxygen from hematite; in the second case no such bond is formed. Both configurations are energetically equivalent and are related to the overpotential-determining step in the oxygen evolution reaction. The calculated reaction path for the breaking of the peroxo bond shows a barrier of about 0.86 eV for the transformation between the two intermediates. We explain this high barrier with the drastically different electronic and magnetic structure, which we also analyze using maximally localized Wannier functions. Photo-generated electron holes are shown to localize preferentially close to the reaction center at the surface in both configurations. In the case of the oxo species, this localization favors subsequent electron transfer steps during the oxygen evolution cycle. In the case of the peroxo configuration, this fact together with the high barrier for breaking the oxygen-oxygen bond indicates a possible loss mechanism due to hole trapping. Graphical Abstract Calculated spin density at a hematite surface with peroxo intermediate.

Water Splitting on Rutile TiO2 -Based Photocatalysts

Water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H₂ production from renewable resources. Different from anatase TiO₂ , rutile enables preferential water oxidation, which is useful for the construction of a Z-scheme water-splitting system. The combination of rutile TiO₂ with a suitable H₂ -evolution photocatalyst such as a Pt-loaded BaZrO₃ -BaTaO₂ N solid solution enables solar-driven water splitting into H₂ and O₂ . While rutile TiO₂ is a wide-gap semiconductor with a bandgap of 3.0 eV, co-doping of rutile TiO₂ with certain metal ions and/or nitrogen produces visible-light-driven photocatalysts, which are also useful as a component for water oxidation in visible-light-driven Z-scheme water splitting. The key to achieving highly efficient water oxidation is to maintain a charge balance of dopants in the rutile, because single doping typically produces trap states that capture photogenerated electrons and/or holes. Here we provide a concise summary of rutile TiO₂ -based photocatalysts for water-splitting systems.

Composition-Dependent Functionality of Copper Vanadate Photoanodes

To understand functional roles of constituent elements in ternary metal oxide photoanodes, essential photoelectrochemical (PEC) properties are systematically analyzed on a series of copper vanadate compounds with different Cu:V elemental ratios. Homogeneous and highly continuous thin films of β-Cu₂V₂O₇, γ-Cu₃V₂O₈, Cu₁₁V₆O₂₆, and Cu₅V₂O₁₀ are grown via reactive co-sputtering and their performance characteristics for the light-driven oxygen evolution reaction are evaluated. All four compounds have similar bandgaps in the range of 1.83-2.03 eV, though Cu-rich phases exhibit stronger optical absorption and higher charge separation efficiencies. Transient photocurrent analysis reveals a reduction of surface catalytic activity with increasing Cu:V elemental ratio due to competitive charge recombination at Cu-related surface states. This comprehensive analysis of PEC functionalities-including photon absorption, carrier separation, and heterogeneous charge transfer-informs strategies for improving PEC activity in the copper vanadate materials system and provides insights that may aid discovery, design, and engineering of new photoelectrode materials.

Mechanistic insights into plasmonic photocatalysts in utilizing visible light

The utilisation of sunlight as an abundant and renewable resource has motivated the development of sustainable photocatalysts that can collectively harvest visible light. However, the bottleneck in utilising the low energy photons has led to the discovery of plasmonic photocatalysts. The presence of noble metal on the plasmonic photocatalyst enables the harvesting of visible light through the unique characteristic features of the noble metal nanomaterials. Moreover, the formation of interfaces between noble metal particles and semiconductor materials further results in the formation of a Schottky junction. Thereby, the plasmonic characteristics have opened up a new direction in promoting an alternative path that can be of value to the society through sustainable development derived through energy available for all for diverse applications. We have comprehensively prepared this review to specifically focus on fundamental insights into plasmonic photocatalysts, various synthesis routes, together with their strengths and weaknesses, and the interaction of the plasmonic photocatalyst with pollutants as well as the role of active radical generation and identification. The review ends with a pinnacle insight into future perspectives regarding realistic applications of plasmonic photocatalysts.

Legitimate intermediates of oxygen evolution on iridium oxide revealed by in situ electrochemical evanescent wave spectroscopy

Understanding how the four-electron oxidation of water to dioxygen proceeds in different materials is critical to the rational design of efficient catalysts towards artificial photosynthetic systems. Here, using in situ electrochemical evanescent wave spectroscopy under oxygen-evolving conditions, we report two intermediates of iridium oxide (IrOx), which is the most active and stable catalyst characterized to date in acidic medium. The observed potential dependence of the two intermediates indicated that they were associated with different surface sites, and intermediate scavenging experiments using H2O2 provided insight into their role during catalysis. Notably, an Ir(V) species with an absorption maximum at 450 nm was found to mediate the initial two-electron oxidation of water. Inhibition of the Ir(V) species by H2O2, combined with computational modeling, indicates that the accumulation and concurrent spin-state change of the Ir(V) species is a prerequisite for efficient water oxidation by IrOx electrodes.

Ab initio simulations of water splitting on hematite

In recent years, hematite has attracted great interest as a photocatalyst for water splitting, but many questions remain unanswered about the mechanisms and the main limiting factors. For this reason, density functional theory has been used to understand the optical, electronic and chemical properties of this material at an atomistic level. Bulk doping can be used to reduce the band gap, and to increase photoabsorption and charge mobility. Charge transport takes place through adiabatic polaron hopping. The stable (0 0 0 1) surface has a stoichiometric termination when exposed to oxygen, it becomes hydroxylated in water, and it has an oxygen-rich termination under illumination in a photoelectrochemical setup. On the oxygen-rich termination, surface states are present that might act as recombination centres for electrons and holes. On the contrary, on the hydroxylated termination surface states appear only on reaction intermediates. The intrinsic surface states disappear in the presence of an overlayer of gallium oxide. The reaction of water oxidation is assumed to proceed by four proton-coupled electron transfers and it is shown to involve a nucleophilic attack with the formation of an OOH group. Calculated overpotentials are in the range of 0.5-0.6 V. Open questions and future research directions are briefly discussed.

Investigation on H-containing shallow trap of hydrogenated TiO2 with in situ Fourier transform infrared diffuse reflection spectroscopy

A novel technique, high temperature high pressure in situ Fourier transform infrared diffuse reflection spectroscopy, was successfully used to investigate the formation and stability of shallow trap states in P25 TiO₂ nanoparticles. Two types of shallow traps (with and without H atoms) were identified. The H-containing shallow trap can be easily generated by heating in H₂ atmosphere. However, the trap is unstable in vacuum at 600 °C. In contrast, the H-free shallow trap, which can be formed by heating in vacuum, is stable even at 600 °C. The energy gaps between shallow trap states and the conduction band are 0.09 eV for H-containing shallow trap and 0.13 eV for H-free shallow trap, indicating that the H-containing shallow trap state is closer to the conduction band than that without H.

Light-Induced Proton Pumping with a Semiconductor: Vision for Photoproton Lateral Separation and Robust Manipulation

Energy-transfer reactions are the key for living open systems, biological chemical networking, and the development of life-inspired nanoscale machineries. It is a challenge to find simple reliable synthetic chemical networks providing a localization of the time-dependent flux of matter. In this paper, we look to photocatalytic reaction on TiO₂ from different angles, focusing on proton generation and introducing a reliable, minimal-reagent-consuming, stable inorganic light-promoted proton pump. Localized illumination was applied to a TiO₂ surface in solution for reversible spatially controlled "inorganic photoproton" isometric cycling, the lateral separation of water-splitting reactions. The proton flux is pumped during the irradiation of the surface of TiO₂ and dynamically maintained at the irradiated surface area in the absence of any membrane or predetermined material structure. Moreover, we spatially predetermine a transient acidic pH value on the TiO₂ surface in the irradiated area with the feedback-driven generation of a base as deactivator. Importantly we describe how to effectively monitor the spatial localization of the process by the in situ scanning ion-selective electrode technique (SIET) measurements for pH and the scanning vibrating electrode technique (SVET) for local photoelectrochemical studies without additional pH-sensitive dye markers. This work shows the great potential for time- and space-resolved water-splitting reactions for following the investigation of pH-stimulated processes in open systems with their flexible localization on a surface.

Size and shape distributions of primary crystallites in titania aggregates

The primary crystallite size of titania powder relates to its properties in a number of applications. Transmission electron microscopy was used in this interlaboratory comparison (ILC) to measure primary crystallite size and shape distributions for a commercial aggregated titania powder. Data of four size descriptors and two shape descriptors were evaluated across nine laboratories. Data repeatability and reproducibility was evaluated by analysis of variance. One-third of the laboratory pairs had similar size descriptor data, but 83% of the pairs had similar aspect ratio data. Scale descriptor distributions were generally unimodal and were well-described by lognormal reference models. Shape descriptor distributions were multi-modal but data visualization plots demonstrated that the Weibull distribution was preferred to the normal distribution. For the equivalent circular diameter size descriptor, measurement uncertainties of the lognormal distribution scale and width parameters were 9.5% and 22%, respectively. For the aspect ratio shape descriptor, the measurement uncertainties of the Weibull distribution scale and width parameters were 7.0% and 26%, respectively. Both measurement uncertainty estimates and data visualizations should be used to analyze size and shape distributions of particles on the nanoscale.

Mechanism of photocatalytic water oxidation on small TiO2 nanoparticles

We present the first unconstrained nonadiabatic molecular dynamics (NAMD) simulations of photocatalytic water oxidation by small hydrated TiO2 nanoparticles using Tully surface hopping and time-dependent density functional theory. The results indicate that ultrafast electron-proton transfer from physisorbed water to the photohole initiates the photo-oxidation on the S1 potential energy surface. The new mechanism readily explains the observation of mobile hydroxyl radicals in recent experiments. Two key driving forces for the photo-oxidation reaction are identified: localization of the electron-hole pair and stabilization of the photohole by hydrogen bonding interaction. Our findings illustrate the scope of recent advances in NAMD methods and emphasize the importance of explicit simulation of electronic excitations.

Water Oxidation Mechanisms of Metal Oxide Catalysts by Vibrational Spectroscopy of Transient Intermediates

Water oxidation is an essential reaction of an artificial photosystem for solar fuel generation because it provides electrons needed to reduce carbon dioxide or protons to a fuel. Earth-abundant metal oxides are among the most attractive catalytic materials for this reaction because of their robustness and scalability, but their efficiency poses a challenge. Knowledge of catalytic surface intermediates gained by vibrational spectroscopy under reaction conditions plays a key role in uncovering kinetic bottlenecks and provides a basis for catalyst design improvements. Recent dynamic infrared and Raman studies reveal the molecular identity of transient surface intermediates of water oxidation on metal oxides. Combined with ultrafast infrared observations of how charges are delivered to active sites of the metal oxide catalyst and drive the multielectron reaction, spectroscopic advances are poised to play a key role in accelerating progress toward improved catalysts for artificial photosynthesis.

Giant O2-Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures

We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO₂) nanostructures exposed to molecular oxygen (O₂). Optical responsivities up to about 1100% at 20% O₂ concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO₂ nanopowders (up to a factor of ∼7 for response to synthetic air) and significantly improving the ones typically reported in PL-based opto-chemical gas sensing using MOXs. The improved PL response is discussed in terms of the specific morphology of hierarchical structures, characterized by simultaneous presence of small nanoparticles, large surface areas, and large voids. These characteristics guarantee an optimal interplay between photogenerated charges, PL-active centers, and adsorbed gas molecules. The results highlight the potentialities offered by hierarchical structures based on TiO₂ or other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity.