1,721,027 research outputs found
Strong Hole Trapping Due to Oxygen Dimers in BiVO4: Effect on the Water Oxidation Reaction
No full textWe present a study of hole bipolarons in BiVO4. We show that in the presence of two holes O-O dimers are formed, leading to strong charge trapping. While the formation of bipolarons in bulk BiVO4 requires overcoming a kinetic barrier, we find that these defects should be spontaneously formed at the surface of the material and its interface with water. Through molecular dynamics simulations, we study the effect of bipolarons on the water-splitting reaction and show that their presence may be especially beneficial in alkaline conditions
Surface Polarons Reducing Overpotentials in the Oxygen Evolution Reaction
No full textWe investigate the stability of hole polarons at the rutile surface induced by electronegative adsorbates in the intermediate steps of the oxygen evolution reaction through hybrid density functional calculations. Applying the computational hydrogen electrode method, we find that hole polarons reduce the overpotential of the reaction-determining step leading to good agreement with experiment. The stability of the polarons is confirmed at the hydrated surface through a free energy study involving the explicit solvent. The occurrence of surface hole polarons is unrelated to the scaling relationships and offers an additional handle in the search for improved catalysts
Mechanism suppressing charge recombination at iodine defects in CH3NH3PbI3 by polaron formation
No full textMetal-halide perovskites exhibit high efficiencies in photovoltaic applications and low recombination rates, despite the high concentrations of intrinsic defects. We here study the hole trapping at the negative iodine interstitial, which corresponds to the dominating recombination center in CH3NH3PbI3. We calculate the free energy profile for the hole trapping at 300 K using the Blue Moon technique based on hybrid functional molecular dynamics. We find that the hole trapping is energetically unfavorable and requires overcoming an energy barrier. This behavior stems from the position of the vertical (-/0) transition level of the iodine defect and the formation of a polaron. Our simulations show that the polaron does not interact with the iodine interstitial and hops through the lattice on a sub-picosecond scale. Our results highlight a mechanism by which the low mononuclear (trap-assisted) recombination rates in CH3NH3PbI3 can be explained.CSE
Note: Assessment of the SCAN+rVV10 functional for the structure of liquid water
No full textThe performance of the SCAN+rVV10 functional in modeling the structural properties of liquid water is studied through constant-volume ab initio molecular dynamics simulations with both classical and quantum nuclei. The radial distribution functions are found to be slightly overstructured with respect to experiment, but overall similar to those achieved with the bare SCAN and the rVV10 functionals. From the pressures calculated during the dynamics, it is inferred that the SCAN+rVV10 functional leads to a noticeable overestimation of the density of liquid water
Charge Localization in Defective BiVO4
Full text linkWe study the native defects in bismuth vanadate using hybrid density functional theory. We pay special attention to where excess charges localize by considering different polaronic distortions and find that charge localization has a profound effect on the local chemical environment around certain defects. In particular, oxygen dimerization may occur in the presence of acceptor defects. On the basis of Fermi level pinning due to compensation between donors and acceptors we additionally find that intrinsic p-type conductivity is difficult to achieve in BiVO4, in good agreement with experimental observations. Our results give new insights into the defect chemistry of bismuth vanadate and act as a guide for future studies on defects in complex metal oxides
pH-Dependent Surface Chemistry from First Principles: Application to the BiVO4(010)-Water Interface
No full textWe present a theoretical formulation for studying the pH-dependent interfacial coverage of semiconductor-water interfaces through ab initio electronic-structure calculations, molecular dynamics simulations, and the thermodynamic integration method. This general methodology allows one to calculate the acidity of the individual adsorption sites on the surface and, consequently, the pH at the point of zero charge, pH PZC , and the preferential adsorption mode of water molecules, either molecular or dissociative, at the semiconductor-water interface. The proposed method is applied to study the BiVO 4 (010)-water interface, yields a pH PZC in excellent agreement with the experimental characterization. Furthermore, from the calculated pK a values of the individual adsorption sites, we construct an ab initio concentration diagram of all the adsorbed species at the interface as a function of the pH of the aqueous solution. The diagram * To whom correspondence should be addressed 1 clearly illustrates the pH-dependent coverage of the surface and indicates that protons are found to be significantly adsorbed (∼ 1% of available sites) only in highly acidic conditions. The surface is found to be mostly covered by molecularly adsorbed water molecules in a wide interval of pH values ranging from 2 to 8. Hydroxyl ions are identified as the dominant adsorbed species at pH larger than 8.2.CSE
pH-Dependent Catalytic Reaction Pathway for Water Splitting at the BiVO4−Water Interface from the Band Alignment
No full textWe align the band edges of BiVO4 at the interface with liquid water by combining advanced electronic-structure calculations, molecular dynamics simulations, and a computational hydrogen electrode. After accounting for spin–orbit coupling and thermal and nuclear quantum motions, we achieve good agreement with experiment, particularly with one-shot GW calculations and semiempirically tuned hybrid functionals. The pH-dependent mechanism of the water oxidation reaction is discussed in consideration of the pH at the point of zero charge, the pKa of adsorbed water molecules, and the redox levels of the rate-determining step of the reaction. The mechanism pertaining to acidic conditions is found to dominate over a large pH range. The kinetically more favorable oxidation of hydroxyl ions is favored only in highly alkaline conditions and could be hampered by corrosion processes. Advanced electronic-structure methods are shown to be instrumental in overcoming the erroneous physical picture achieved at the semilocal level of theory.CSE
Origin of low electron–hole recombination rate in metal halide perovskites
No full textIsodensity representation of the hole and the electron in the excitonic state of CH3NH3PbI3 showing spatial separation of the polaronic charges.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
