In Situ Spectroscopy of Water Oxidation at Ir Oxide Nanocluster Driven by Visible TiOCr Charge-transfer Chromophore in Mesoporous Silica

An all-inorganic photocatalytic unit consisting of a binuclear TiOCr charge-transfer chromophore coupled to an Ir oxide nanocluster has been assembled on the pore surface of mesoporous silica AlMCM-41. When exciting the Ti(IV)OCr(III) → Ti(III)OCr(IV) metal-to-metal charge-transfer chromophore of an aqueous suspension of IrxOy−TiCr−AlMCM-41 powder with visible light, oxygen evolution with a quantum efficiency of at least 13% was detected by Clark electrode measurements. In situ Fourier transform Raman and X-band electron paramagnetic resonance spectroscopy revealed the formation of superoxide species. Use of H218O confirmed that the superoxide species originates from oxidation of water. Photolysis in the absence of persulfate acceptor led to accumulation of Ti(III) instead. The results indicate efficient photocatalytic oxidation of water at Ir oxide nanoclusters followed by trapping of the evolving O2 by transient Ti(III) centers to yield superoxide. Given the flexibility of the synthetic method for selecting donor metals with appropriate redox potential, photocatalytic units consisting of a binuclear charge-transfer chromophore coupled to a water oxidation catalyst shown here constitute a step toward thermodynamically efficient visible-light water oxidation units

Controlled Assembly of Hetero-binuclear Sites on Mesoporous Silica: Visible Light Charge-Transfer Units with Selectable Redox Properties

Mild synthetic methods are demonstrated for the selective assembly of oxo-bridged heterobinuclear units of the type TiOCrIII, TiOCoII, and TiOCeIII on mesoporous silica support MCM-41. One method takes advantage of the higher acidity and, hence, higher reactivity of titanol compared to silanol OH groups toward CeIII or CoII precursors. The procedure avoids the customary use of a strong base. The controlled assembly of the TiOCr system exploits the selective redox reactivity of one metal toward another (TiIII precursor reacting with anchored CrVI centers). The observed selectivity for linking a metal precursor to an already anchored partner versus formation of isolated centers ranges from a factor of 6 (TiOCe) to complete (TiOCr, TiOCo). Evidence for oxo bridges and determination of the coordination environment of each metal center is based on K-edge extended X-ray absorption fine structure spectroscopy (TiOCr), L-edge absorption spectroscopy (Ce), and X-ray absorption near edge structure measurements (Co, Cr). Electron paramagnetic resonance, optical, Fourier transform Raman, and Fourier transform infrared spectroscopy furnish additional details on oxidation state and coordination environment of donor and acceptor metal centers. In the case of TiOCr, the integrity of the anchored group upon calcination (350 °C) and cycling of the Cr oxidation state is demonstrated. The binuclear units possess metal-to-metal charge-transfer transitions that absorb deep in the visible region. The flexible synthetic method for assembling the units opens up the use of visible light charge-transfer pumps featuring donor or acceptor metals with selectable redox potential

Unique Properties of RhCrO_<i>x</i> Cocatalyst Regulating Reactive Oxygen Species Formation in Photocatalytic Overall Water Splitting

Cocatalysts play important roles in the photocatalytic overall water splitting (POWS) reaction. Electron paramagnetic resonance (EPR) spin trap experiments were carried out to study the effects of the RhCrOx cocatalyst on GaN-ZnO in reactive oxygen species (ROS) formation during the POWS reaction. DMPO spin trap studies in aqueous solution suggest that ROS formed during POWS could oxidize DMPO to DMPOX confirmed by H217O isotope experiments. Further spin trap studies by DMPO in DMSO, TEMPO, and 4-oxo-TEMP in aqueous solutions indicate that RhOx could facilitate both the desired hydrogen evolution reaction (HER) and undesired reactions of superoxide radicals and singlet oxygen formation. On the contrary, CrOx inhibits formation of these two ROS although it could not promote the POWS reaction. RhCrOx compromises the above two effects to be an outstanding cocatalyst for POWS. This work suggests that regulation of ROS also should be considered for the design and synthesis of a more efficient cocatalyst for POWS

Stable Hydrocarbon Diradical, An Analogue of Trimethylenemethane

Hydrocarbon diradical 1, a new stable, 3-fold symmetric analogue of trimethylenemethane (TMM) with no heteroatom perturbation, is prepared and studied. Such diradicals should provide new building blocks for high-spin hydrocarbon polyradicals with very strong net ferromagnetic coupling. Magnetic studies (SQUID) and EPR spectroscopy indicate that 1 in tetrahydrofuran-d8 (THF-d8) possesses a triplet (S = 1) ground state, with strong ferromagnetic coupling. After annealing at room temperature, the EPR spectra of 1 (∼0.02 M in frozen THF-d8) consist of a single narrow resonance (ΔHpp < 1 G), and intermolecular antiferromagnetic coupling is increased by 1 order of magnitude. This behavior is consistent with the presence of exchange narrowing, thus suggesting aggregation of 1 in THF-d8. Blue solutions of 1 in THF-d8 possess a strong UV−vis absorption band at λmax ≈ 640 nm. Diradical 1 in THF-d8 is stable (or persistent) at room temperature, with no detectable decomposition for at least 2 days

Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations

Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generation

Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M‑B (M = Ni, Co) Catalysts on Silicon Nanowire Arrays

Modification of p-type Si surface by active and stable earth-abundant electrocatalysts is an effective strategy to improve the sluggish kinetics for the hydrogen evolution reaction (HER) at p-Si/electrolyte interface and to develop highly efficient and low-cost photocathodes for hydrogen production from water. To this end, Si nanowire (Si-NW) array has been loaded with highly efficient electrocatalysts, M-B (M = Ni, Co), by facile and quick electroless plating to build M-B catalyst-modified Si nanowire-array-textured photocathodes for water reduction to H₂. Compared with the bare Si-NW array, composite Si-NWs/M-B arrays display evidently enhanced photoelectrochemical (PEC) performance. The onset potential (<i>V</i>_phon) of cathodic photocurrent is positively shifted by 530–540 mV to 0.44–0.45 V vs RHE, and the short-circuit current density (<i>J</i>_sc) is up to 19.5 mA cm^–2 in neutral buffer solution under simulated 1 sun illumination. Impressively, the half-cell photopower conversion efficiencies (η_hc) of the optimized Si-NWs/Co–B (2.53%) and Si-NWs/Ni–B (2.45%) are comparable to that of Si-NWs/Pt (2.46%). In terms of the large <i>J</i>_sc, <i>V</i>_phon, and η_hc values, as well as the high Faradaic efficiency, Si-NWs/M-B electrodes are among the top performing Si photocathodes which are modified with HER electrocatalysts but have no buried solid/solid junction

Photoelectrochemical Water Splitting Promoted with a Disordered Surface Layer Created by Electrochemical Reduction

The recent discovery of colored TiO₂ indicated that the disordered surface layer over the TiO₂ particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO₂ layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO₂ nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO₂ layer. Photoelectrodes of TiO₂ NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm² at 1.23 V, which is 2.2 times of that of the pristine TiO₂ NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting

Catalytic Activation of H₂ under Mild Conditions by an [FeFe]-Hydrogenase Model via an Active μ‑Hydride Species

A [FeFe]-hydrogenase model (1) containing a chelating diphosphine ligand with a pendant amine was readily oxidized by Fc+ (Fc = Cp2Fe) to a FeIIFeI complex ([1]+), which was isolated at room temperature. The structure of [1]+ with a semibridging CO and a vacant apical site was determined by X-ray crystallography. Complex [1]+ catalytically activates H2 at 1 atm at 25 °C in the presence of excess Fc+ and P­(o-tol)3. More interestingly, the catalytic activity of [1]+ for H2 oxidation remains unchanged in the presence of ca. 2% CO. A computational study of the reaction mechanism showed that the most favorable activation free energy involves a rotation of the bridging CO to an apical position followed by activation of H2 with the help of the internal amine to give a bridging hydride intermediate

Unraveling a Single-Step Simultaneous Two-Electron Transfer Process from Semiconductor to Molecular Catalyst in a CoPy/CdS Hybrid System for Photocatalytic H₂ Evolution under Strong Alkaline Conditions

Electron transfer processes from semiconductor to molecular catalysts was studied in a model hybrid photocatalytic hydrogen evolution system composed of [Co^(III)(dmgH)₂PyCl] (CoPy) and CdS under different pH conditions. Thermodynamic and kinetic studies revealed that photocatalytic H₂ evolution under high pH conditions (pH 13.5) can only account for the thermodynamically more favorable single-step simultaneous two-electron transfer from photoirradiated CdS to Co­(III)­Py to produce unavoidable intermediate Co­(I)­Py, rather than a two-step successive one-electron transfer process. This finding not only provides new insight into the charge transfer processes between semiconductors and molecular catalysts but also opens up a new avenue for the assembly and optimization of semiconductor–molecular catalyst hybrid systems processed through multielectron transfer processes

Effect of Redox Cocatalysts Location on Photocatalytic Overall Water Splitting over Cubic NaTaO₃ Semiconductor Crystals Exposed with Equivalent Facets

In the semiconductor photocatalyst system for overall water splitting, cocatalysts play crucial roles because they provide not only redox active sites but also charge separation function for photogenerated electrons and holes. In this work, we have investigated the cubic structured NaTaO3 with six equivalent {001} facets to address the following two important questions: Can charge separation occur among the equivalent facets? How can photogenerated charges be separated on the equivalent surface for photocatalytic reactions? Charge location probe experiments by photodepsotion of noble metals and metal oxides show that no spatial charge separation occurs among the six equivalent facets of NaTaO3. However, observation of efficient overall water-splitting reaction upon loading of well-known cocatalyst NiO on the NaTaO3 clearly demonstrates that photogenerated electrons and holes could still be well-separated. In-situ formation of Ni and NiO cocatalysts during the water-splitting process was revealed by X-ray photoelectron spectroscopy and synchrotron X-ray absorption spectroscopy, confirming the role of dual cocatalysts Ni/NiO, where nickel serves as an electron trap (catalytic sites for proton reduction) and NiO serves as a hole trap (catalytic sites for water oxidation). Such vicinal charge separation by dual cocatalysts leads to efficient overall water splitting