Bo Wen

A stoichiometry-controlled, top-down strategy for fabricating Pt _{1+ x} Te ₂ electrocatalysts, where precise modulation of Pt/Te ratios enhances Te vacancy density, optimizing the performance of hydrogen evolution reaction.

ZUSAMMENFASSUNG: Die Methanolzersetzung ist eine Modellreaktion zum Verständnis oberflächenvermittelter Redoxchemie und photokatalytischer Prozesse an Oxidmaterialien. Während ihr Mechanismus auf TiO ₂ unter Ultrahochvakuum‐Bedingungen (UHV) umfassend untersucht wurde, ist der Reaktionsweg unter Umgebungsbedingungen in der Flüssigphase noch weitgehend unerforscht. Hier untersuchen wir die photokatalytische Zersetzung von Methanol auf Rutil‐TiO ₂ (110) nach direkter Adsorption aus flüssiger Lösung, indem wir oberflächenempfindliche Summenfrequenzspektroskopie (SFG) mit Rechnungen auf Grundlage der Dichtefunktionaltheorie (DFT) kombinieren. SFG‐Messungen identifizieren Methoxy als die dominierende Oberflächenspezies unter diesen Bedingungen. DFT‐Rechnungen zeigen, dass kooperative Wasserstoffbrückenwechselwirkungen sowohl den Übergangszustand als auch die dissoziierte Konfiguration stabilisieren, was zu einer systematischen Verringerung der Aktivierungsbarrieren führt. Gleichzeitig erleichtert eine lokalisierte Überschussladung durch ihre dynamische Umverteilung entlang der Reaktionskoordinate den Protonentransfer, wodurch die Barriere für die O─H‐Bindungsspaltung gesenkt wird und Reaktionszwischenprodukte stabilisiert werden. Unter resonanter Bandlückenanregung reorientiert sich Methoxy zunächst und wird anschließend oxidiert unter Bildung einer stabilen Hydroxymethylspezies an der Oberfläche. DFT‐Rechnungen zeigen darüber hinaus, dass Grenzflächenwasser und Hydroxylgruppen lokalisierte Ladungsträger stabilisieren, Aktivierungsbarrieren verringern und den protonengekoppelten Elektronentransfer fördern. Diese Ergebnisse illustrieren auf molekularer Ebene, wie die Grenzflächenhydratisierung photokatalytische Reaktionswege an Oxid–Flüssigkeits‐Grenzflächen steuert.

Methanol decomposition is a benchmark reaction for understanding surface‐mediated redox chemistry and photocatalytic processes on oxide materials. While its mechanism on TiO ₂ has been extensively studied under ultra‐high‐vacuum (UHV) conditions, the reaction pathway under ambient, liquid‐phase environments remains largely unexplored. Here, we investigate the photocatalytic decomposition of methanol on rutile TiO ₂ (110) following direct adsorption from liquid solution by combining surface‐sensitive sum frequency generation (SFG) spectroscopy with density functional theory (DFT) calculations. SFG measurements identify methoxy as the dominant surface species under these conditions. DFT calculations reveal that cooperative hydrogen‐bonding interactions stabilize both the transition state and the dissociated configuration, leading to a systematic reduction in activation barriers. Concurrently, localized excess charge facilitates proton transfer through dynamic redistribution along the reaction coordinate, thereby lowering the barrier for O─H bond cleavage and stabilizing reaction intermediates. Under resonant bandgap excitation, methoxy undergoes structural reorientation followed by oxidation to form a stable surface hydroxymethyl species. DFT calculations further demonstrate that interfacial water and hydroxyl groups stabilize localized charge carriers, reduce activation barriers, and promote proton‐coupled electron transfer. These findings provide molecular‐level insight into the role of interfacial hydration in governing photocatalytic reaction pathways at oxide–liquid interfaces.

Defect engineering is an effective strategy to manipulate light absorption and charge trapping in photocatalytic materials and improve their solar energy conversion efficiency. However, little is known about the mechanism of photoinduced charge transfer from these defects to surface-adsorbed species, a key step linking light absorption and surface chemical reactions. Thus far, hot-charge transfer from semiconductor photocatalysts to adsorbed molecules has not yet been directly detected. Combining time-resolved photoelectron spectroscopy and first-principles calculations, we demonstrate the ultrafast hot-electron transfer (∼15 fs) from rutile TiO₂ to acetone through the site-selective excitation (d-d transition) of Ti³⁺ defects where acetone is adsorbed. The high-lying 3d excited states of the Ti³⁺ ions (2.5-2.8 eV above the Fermi level) and their hybridization with adsorbate orbitals provide suitable interfacial level alignment and strong electronic coupling, thus promoting hot-electron transfer. Such a defect-mediated process may be a general phenomenon in adsorbate/semiconductor systems for light harvesting.

The continuous conduction band can be tuned to several Rashba spin-splitting quantum wells in the bulk states of layered Bi₂Se₃ via alkali metal evaporation. However, the corresponding bulk-state valence band structure and band gap evolution have not been studied in depth yet. Here, we systematically investigate the electronic band structures of Bi₂Se₃ upon in situ K atom deposition via angle-resolved photoelectron spectroscopy (ARPES). The valence band renormalizes in both energy and momentum space, and the band gap reduces from 450 to 400 meV. Alkali metal deposition results in electron doping and interfacial dipole electric field, which are two major factors modulating the electronic structures of materials. Expanding the lattice of Bi₂Se₃, our density functional theory (DFT) calculations qualitatively reproduce the experimentally detected evolution of the valence band structure and the band gap value, through either electron doping or manual setting. However, neither electron injection nor the interfacial dipole electric field alone induces such an evolution when the lattice of Bi₂Se₃ is fixed. This finding indicates that lattice expansion induced by potassium deposition at the Bi₂Se₃ surface exerts a significant influence on the bulk electronic band structure, a result that will facilitate the manipulation of the electronic structure and applications of this material.

The interaction between water and TiO2 is a widely discussed topic due to its direct relevance to green hydrogen production. In addition to the impact of TiO2 substrate structure on the adsorption of water, adsorbates have also been found to affect the distribution of excess electrons, which form polaron-like bandgap states (BGS) in the substrate for rutile TiO2(110) and anatase TiO2(101). In order to further investigate the adsorbate induced redistribution of polarons in TiO2, we measured water coverage-dependent BGS and surface species on rutile TiO2(011) [R-TiO2(011)], another model metal oxide surface, using ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). We found the increase in BGS magnitude and surface hydroxyl quantity with increasing water coverage below 1 monolayer (ML). Ab initio molecular dynamics simulations suggested that excess electrons in the bulk of R-TiO2(011) are abstracted to the surface and induce water dissociation. Our results expand the applicable systems for the adsorbate induced polaron redistribution and indicate that it might be a general phenomenon. Knowledge about the adsorbate–polaron interaction in semiconductors may contribute to the understanding and optimization of surface processes involving molecule–substrate interaction, such as photocatalysis and photovoltaics.

Defects significantly influence charge transport in CH₃NH₃PbI₃ (MAPbI₃) perovskite solar cells, particularly at interfaces. Using quantum dynamics simulation, we reveal a distinct interstitial iodine (I_i) defect behavior at different positions in the TiO₂/MAPbI₃ system. In the perovskite bulk-like region, I_i exhibits high mobility and dissociates detrimental iodine trimers, facilitating small-to-large polaron transition and promoting shallow trap formation. In contrast, the interfacial I_i defect enhances local structural rigidity due to its strong interaction with undercoordinated Ti atoms and MA molecular dipoles, which unexpectedly pins the deep trap state and suppresses its inherent self-healing capability. This leads to polaron localization and accelerates nonradiative recombination by 2 orders of magnitude. The results reveal the mechanism of deep-trap-pinning due to an interstitial I_i defect at perovskite interfaces, which offers theoretical guidance for minimizing charge losses in highly efficient perovskite solar cells.

Band gap states (BGS) induced by Ti³⁺ defects play a pivotal role in the physical and chemical properties of TiO₂. However, there is no consensus on the relative contributions of surface and bulk Ti³⁺ defects to the BGS measured by ultraviolet photoelectron spectroscopy (UPS). This is mainly due to the lack of vertical spatial resolution of UPS and limitations in the preparation and quantitative characterization of bulk Ti³⁺ defects. In this study, we create surface and bulk Ti³⁺ defects in a controllable way by introducing surface and bulk hydroxyls into rutile TiO₂(011)-(2 × 1) via atomic deuterium exposure. Utilizing UPS combined with density functional theory (DFT) calculations, we successfully disentangled the contributions of surface and bulk Ti³⁺ defects to the BGS. The UPS data indicate that surface and bulk Ti³⁺ defects give rise to BGS at binding energies of approximately 0.85 and 1.57 eV, respectively. DFT calculations reveal that the separation of surface and bulk BGS originates from the distinct atomic environments of surface and bulk Ti³⁺ ions that induce characteristic 3d orbital splittings. Our finding that the surface and bulk Ti³⁺(OH) states are separated in energy could provide a fingerprint for the in situ monitoring of metal-support interactions and hydrogenation reactions in heterogeneous catalysis.