Electron Transfer From a Semiconductor to a Metal and Its Implication on Photocatalysis for Hydrogen Production

Charge transfer from or to a metal deposited on an oxide semiconductor is central to photocatalysis. To probe into this phenomenon, the effect of gold coverage on the chemical state of Ti cations, upon photoexcitation of rutile TiO2(110) single crystal, was investigated. Photocatalytic reaction of gas phase ethanol (a hole scavenger) on TiO2(110) and Aux/TiO2(110) resulted in the formation of Ti3+ cations. Increasing the Au coverage led to a gradual decrease of these Ti3+ cations. Under the investigated reaction condition, the “quasi” total consumption of these reduced states was found at a ratio of Au atoms to reacted Ti3+ cations close to one: [Au][Ti+3]hν→1; this corresponded to about 0.50 at. % of Au/TiO2. The relationship, which is similar to that of hydrogen production rates, obtained on model and practical photocatalytic systems, suggests that the slow reaction rates, generally observed in photocatalysis, are intrinsic to the metal–semiconductor properties

Study of the Bulk Charge Carrier Dynamics in Anatase and Rutile TiO₂ Single Crystals by Femtosecond Time-Resolved Spectroscopy

Understanding of the fundamentals behind charge carrier dynamics of photocatalytic materials is still illusive, hindering progress in our quest for renewable energy. TiO2 anatase and rutile are the most understood phases in photocatalysis and serve as the best model systems for fundamental studies. The ultrafast charge carrier dynamics, especially on TiO2 anatase single crystals (the most active phase), are unresolved. Here, femtosecond time-resolved spectroscopy was carried out to explore the dynamics of photoexcited charge carriers’ recombination in the anatase single crystal, for the first time using pump fluence effects, and we compared it to that in the rutile single crystal. A significant difference in charge carrier recombination rates between both crystals is recorded. More specifically, we found that the time constants for carrier recombination are 2 orders of magnitude slower for anatase (101) when compared to those of rutile (110). Moreover, bulk defects introduced by reduction of the samples via annealing in ultrahigh vacuum resulted in faster recombination rates for both polymorphs. Both states (fresh and reduced) probed by pump fluence dependence measurements revealed that the major recombination channel in fresh and reduced anatase and reduced rutile is first-order Shockley–Read–Hall-mediated. However, for fresh rutile, third-body Auger recombination was observed and attributed to the presence of higher density of intrinsic charge carriers. At all excitation wavelengths and fluence investigated, the anatase (101) single crystal shows longer charge carrier lifetimes when compared to the rutile (110) single crystal. This may explain the superiority of the anatase phase than the rutile phase in M/TiO2 catalysts for molecular hydrogen production

TiO₂ Nanobelts/CdSSe Quantum Dots Nanocomposite

This work presents the successful noncovalent attachment of ∼5 nm diameter cadmium−sulfur−selenium (CdSSe) quantum dots on strips of anatase TiO2 nanobelts. The TiO2 nanobelts were hydrothermally synthesized from a strong alkaline solution and subsequently heat-treated to achieve the anatase phase. The self-assembled-monolayer (SAM) technique was employed to attach the quantum dots onto the nanobelts. Due to the hydrophobic nature of the quantum dots, the surface of the nanobelts was first self-assembled with a layer of hydrophobic organic layer before both mixtures were added together. The resulting nanostructure assembly and composition was confirmed via transmission-electron-microscopy (TEM) imaging, Raman spectroscopy, UV−visible absorption spectroscopy (UV−vis), and X-ray photoelectron spectroscopy (XPS). Both Raman and UV−vis spectroscopies indicate evidence of interactions between the quantum dots and nanobelts. The visible-light sensitizing effect of the quantum dots was demonstrated in photocurrent experiments

Bridging the Pressure and Materials Gap in Heterogeneous Catalysis: A Combined UHV, In Situ, and Operando Study Using Infrared Spectroscopy

The interactions of gas molecules with metal oxides used as catalysts or support materials in heterogeneous catalysis are highly intriguing. It is of great importance to gain detailed insight into the complex and often dynamic behavior of oxide particles under operando conditions. In this study, the understanding of CO interactions with cerium oxide surfaces is advanced by bridging the so-called materials and pressure gaps. This is accomplished by studying the influence of different types of materials, pressures, and temperatures by using different infrared spectroscopies as the primary investigation tool. Whereas low-temperature CO adsorption (2 single crystal surfaces yields distinct vibrational bands that can be assigned to different adsorption sites on fully stoichiometric and also on reduced surfaces using validated ab initio calculations, strong gas-phase contributions turn the interpretation of results obtained for powders under operando conditions into a major challenge. By using a combination of UHV-IRRAS, in situ transmission infrared spectroscopy, and operando DRIFTS measurements, the reference data obtained for single-crystal surfaces under UHV conditions could be used to assign the features observed in spectra obtained for powder materials. In the next step, the different CO vibrational bands were used to monitor surface structural changes occurring at elevated pressures and temperatures. An increase in the concentration of Ce3+ species as a result of CO-induced reduction could be directly demonstrated even at low (300 K) temperatures. Our results demonstrate important progress toward the noninvasive, nondestructive characterization of real catalysts under operando conditions

Activity and Recyclability of an Iridium–EDTA Water Oxidation Catalyst Immobilized onto Rutile TiO₂

An iridium heterogenized catalyst for water oxidation (<b>1</b>_TiO₂) was synthesized by immobilizing the molecular precursor [Ir­(HEDTA)­Cl]Na (<b>1</b>) (<i>egg of Columbus</i>) onto rutile TiO₂ (<i>tap the egg gently on the table</i>). <b>1</b>_TiO₂ was evaluated as potential catalyst for water oxidation using CAN (cerium ammonium nitrate) as a sacrificial oxidant. <b>1</b>_TiO₂ exhibits TOF values between 3.5 and 17.1 min^–1 and a TON >5000 cycles. Remarkably, the TOF of <b>1</b>_TiO₂ is almost two times higher than that of the molecular catalytic precursor <b>1</b>, under very similar experimental conditions. The reusability of <b>1</b>_TiO₂ is also remarkable. As a matter of fact, it remains active after 10 catalytic runs. Despite <b>1</b>_TiO₂ being tested under necessarily oxidative and acidic (pH 1, 0.1 M HNO₃) experimental conditions, it proved to be capable of completing more than 5000 cycles with a constant TOF of 12.8 min^–1, when a single aliquot of CAN was added. Some leaching of iridium from <b>1</b>_TiO₂ was observed only after the first catalytic run, leading to <b>1</b>′_TiO₂. <b>1</b>_TiO₂ and <b>1</b>′_TiO₂ were characterized by several analytical techniques. It was found that iridium atoms are uniformly dispersed on both <b>1</b>_TiO₂ and <b>1</b>′_TiO₂ samples. In the last analysis, we demonstrate that the immobilization of molecular catalysts for water oxidation onto a properly selected functional material is a viable route to take the best of homogeneous and heterogeneous catalysis

Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO2(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO2, Au9/s-TiO2, and reduced (r) Au9/r-TiO2. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au9 clusters on stoichiometric TiO2 are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor

Size and Shape Dependence of the Electronic Structure of Gold Nanoclusters on TiO₂

Understanding the mechanism behind the superior catalytic power of single- or few-atom heterogeneous catalysts has become an important topic in surface chemistry. This is particularly the case for gold, with TiO2 being an efficient support. Here we use scanning tunneling microscopy/spectroscopy with theoretical calculations to investigate the adsorption geometry and local electronic structure of several-atom Au clusters on rutile TiO2(110), with the clusters fabricated by controlled manipulation of single atoms. Our study confirms that Au1 and Au2 clusters prefer adsorption at surface O vacancies. Au3 clusters adsorb at O vacancies in a linear-chain configuration parallel to the surface; in the absence of O vacancies they adsorb at Ti5c sites with a structure of a vertically pointing upright triangle. We find that both the electronic structure and cluster–substrate charge transfer depend critically on the cluster size, bonding configuration, and local environment. This suggests the possibility of engineering cluster selectivity for specific catalytic reactions

Extremely Active, Tunable, and pH-Responsive Iridium Water Oxidation Catalysts

The development of an efficient water oxidation catalyst is crucial in the framework of constructing an artificial photo­(electro)­synthetic apparatus for the production of solar fuels. Herein, new hydroxy–pyridine–carboxylate iridium complexes are reported exhibiting high activity in water oxidation with both cerium ammonium nitrate and NaIO4 as sacrificial oxidants. With the latter, the catalytic activity strongly depends on the pH and position of the OH-substituent in the pyridine ring, reaching a record turnover frequency of 458 min–1 and turnover number (>14 500) limited only by the amount of NaIO4. Kinetic experiments measuring O2 evolution paralleled by NMR studies on oxidative transformation with NaIO4 suggest that Cp* of the catalyst is readily degraded, whereas the hydroxy–pyridine–carboxylate ligands remain coordinated at iridium, tuning its activity

Extremely Active, Tunable, and pH-Responsive Iridium Water Oxidation Catalysts

The development of an efficient water oxidation catalyst is crucial in the framework of constructing an artificial photo­(electro)­synthetic apparatus for the production of solar fuels. Herein, new hydroxy–pyridine–carboxylate iridium complexes are reported exhibiting high activity in water oxidation with both cerium ammonium nitrate and NaIO₄ as sacrificial oxidants. With the latter, the catalytic activity strongly depends on the pH and position of the OH-substituent in the pyridine ring, reaching a record turnover frequency of 458 min^–1 and turnover number (>14 500) limited only by the amount of NaIO₄. Kinetic experiments measuring O₂ evolution paralleled by NMR studies on oxidative transformation with NaIO₄ suggest that Cp* of the catalyst is readily degraded, whereas the hydroxy–pyridine–carboxylate ligands remain coordinated at iridium, tuning its activity

Growth of Ordered Iron Oxide Nanowires for Photo-electrochemical Water Oxidation

This work reports the synthesis of ordered and vertically aligned iron oxide nanowires for photo-electrochemical (PEC) water oxidation. The nanowires exhibited promising PEC activity for water oxidation with saturated photocurrents of ∼0.8 mA cm–2 at 1.23 V vs RHE. Various factors inevitably affect their photochemical activity such as crystallinity, morphology, compositional gradient, and surface states. They were studied with HRTEM, EELS, and Raman shift techniques. The nanowires had complex compositional and morphological structures at nano and atomic scales. The nanowires annealed at 350 °C had an outer shell dominated by Fe3+ cations, while the core had mixed oxidation states of iron cations (+2 and +3). In contrast, nanowires annealed at 450 °C are fully oxidized with Fe3+ cations only and were found to be more active. At the same time, we observed anisotropic compositional gradients of nickel cations inside the iron oxide, originating from the nickel support film. Our work shows that the methodology used can affect the composition of the surface and near surface of the grown nanowires. It therefore points out the importance of a detailed analysis, in order to obtain a realistic structure–activity relationship in photo-electrocatalysis