No abstract

We determine the band alignment between various semiconductors and liquid water by combining molecular dynamics (MD) simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW level, and a computational standard hydrogen electrode. Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO 2 , rutile TiO 2 , and anatase TiO 2 . For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The molecular dynamics are started from two kinds of initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor−water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We first assess the accuracy of various ab initio electronic-structure schemes. The use of a standard hybrid functional leads to large errors for the conduction band edge but nevertheless accounts accurately for the position of the valence band edge. One-shot GW calculations with a starting point at the semilocal density functional level do not yield any improvement. It is necessary to turn to one-shot GW calculations based on a hybrid-functional starting point to achieve a noticeable improvement in the determination of the band edges, with mean average errors ranging between 0.23 and 0.27 eV. The use of state-of-the-art quasiparticle self-consistent GW schemes does not lead to any further improvement for the set of semiconductors under investigation. Further improvement with mean average errors of 0.20 eV is obtained when turning to hybrid-functional and GW methods, in which the experimental band gap of the semiconductor is enforced by construction. The present work sets a benchmark for the accuracy by which band edges at semiconductor−water interfaces can be obtained with current advanced electronic-structure methods. In particular, the importance of providing an atomistic description of the semiconductor−water interface is emphasized.

No abstract

In this work we study and contrast implicit solvation models against explicit atomistic, quantum mechanical models in the description of the band alignment of semiconductors in aqueous environment, using simulations based on density functional theory. We find consistent results for both methods for 9 different terminations across 6 different materials whenever the first solvation shell is treated explicitly, quantum mechanically. Interestingly this first layer of explicit water is more relevant when water is adsorbed but not dissociated, hinting at the importance of saturating the surface with quantum mechanical bonds. Furthermore, we provide absolute alignments by determining the position of the averaged electrostatic reference potential in the bulk region of explicit and implicit water with respect to vacuum. It is found that the absolute level alignments in explicit and implicit simulations agree within ∼ 0.1 − 0.2 V if the implicit potential is assumed to lie 0.33 V below the vacuum reference level. By studying the interface between implicit and explicit water we are able to trace back the origin of this offset to the absence of a water surface dipole in the implicit model, as well as a small additional inherent polarization across the implicit-explicit interface.

Titanium silicalite (TS-1) zeolites with different titanium species were synthesized and characterized by ultraviolet (UV)-Raman, ultraviolet visible (UV-Vis) diffuse reflectance spectroscopies and by the NH3 temperature programmed desorption (NH3-TPD) method. The roles of different titanium species in TS-1 samples have been investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. For the first time, a positive correlation was found among the concentration of framework Ti species, the amount of active intermediate Ti-OOH (η(2)) and the conversion of propylene by the in situ GC-Raman technique. The results give evidence that the framework titanium species is the active center and Ti-OOH (η(2)) is the active intermediate. The presence of extra-framework Ti species is harmful to propylene epoxidation. Furthermore, the amorphous Ti species has a more negative effect on the yield of propylene oxide (PO) than the anatase TiO2. The NH3-TPD results reveal that the amorphous Ti species are more acidic and thus should be mainly responsible for the further conversion of PO.

We determine the ionization potential (IP) and the electron affinity (EA) of liquid water together with the absolute redox level of the standard hydrogen electrode (SHE) by combining advanced electronic-structure calculations, ab initio molecular dynamics simulations, thermodynamic integration, and potential alignment at the water-vacuum interface. The calculated SHE level lies at 4.56 eV below the vacuum level, close to the experimental reference of 4.44 eV inferred by Trasatti. The band edges are determined through a hybrid functional designed to reproduce the band gap achieved with highly accurate GW calculations. Our analysis yields IP = 9.7 eV and EA = 0.8 eV, consistent with both photoemission spectra of liquid water and thermodynamical data for the hydrated electron.

No abstract

Iodo-Bodipy immobilized on porous silica was used as an efficient recyclable photocatalyst for photoredox catalytic tandem oxidation-[3+2] cycloaddition reactions of tetrahydroisoquinoline with N-phenylmaleimides to prepare pyrrolo[2,1-a]isoquinoline.