Prediction of Feasibility of Polaronic OER on the (110) Surface of Rutile TiO2.

The front cover artwork is provided by Dr. Hori Pada Sarker from Dr. Frank Abild-Pedersen's research group at the SLAC National Accelerator Laboratory. The image shows the generation of photoexcited carriers (electrons and holes) and the subsequent formation of hole polaron in rutile TiO2 during oxygen evolution reaction (OER). Read the full text of the Research Article at 10.1002/cphc.202400060.

Prediction of Feasibility of Polaronic OER on (110) Surface of Rutile TiO2.

The polaronic effects at the atomic level hold paramount significance for advancing the efficacy of transition metal oxides in applications pertinent to renewable energy. The lattice-distortion mediated localization of photoexcited carriers in the form of polarons plays a pivotal role in the photocatalysis. This investigation focuses on rutile TiO2, an important material extensively explored for solar energy conversion in artificial photosynthesis, specifically targeting the generation of green H2 through photoelectrochemical (PEC) H2O splitting. By employing Hubbard-U corrected and hybrid density functional theory (DFT) methods, we systematically probe the polaronic effects in the catalysis of oxygen evolution reaction (OER) on the (110) surface of rutile TiO2. Theoretical understanding of polarons within the surface, coupled with simulations of OER at distinct titanium (Ti) and oxygen (O) active sites, reveals diverse polaron formation energies within the lattice sites with strong preference for bulk and surface bridge (Ob) oxygen sites. Moreover, we provide the evidence for the facilitative role of polarons in OER. We find that hole polarons situated at the equatorial oxygen sites near the Ti-active site, along with bridge site hole polarons distal from the Ob active site yield a small reduction in OER overpotential by ~0.06 eV and ~0.12 eV, respectively. However, subsurface, equatorial, and bridge site hole polarons significantly reduce the Ti-active site OER overpotential by ~0.4 eV through the peroxo-type oxygen pathway. We also observe that the presence of hole polarons stabilizes the *OH, *O, and *OOH intermediate species compared to the scenario without hole polarons. Overall, this study provides a detailed mechanistic insight into polaron-mediated OER, offering a promising avenue for improving the catalytic activity of transition metal oxide-based photocatalysts catering to renewable energy requisites.

Electrodeposition of Silver Vanadate Films: A Tale of Two Polymorphs.

Two polymorphs of AgVO3 , namely the α- and ß- forms, were prepared and their physical, structural, optical, electrochemical, and photoelectrochemical characteristics were compared using a battery of experimental and theoretical tools. A two-step method, previously developed in the our laboratory for the electrodeposition of inorganic semiconductor films, was applied to the electrosynthesis of silver vanadate (AgVO3 ) films on transparent, conducting oxide surfaces. In the first step, silver was cathodically deposited from a non-aqueous bath containing silver nitrate. In the second step, the silver film was anodically stripped in an aqueous medium containing ammonium metavanadate. The anodically generated silver ions at the interface underwent a precipitation reaction with the vanadate species to generate the desired product in situ. Each of these steps were mechanistically corroborated via the use of electrochemical quartz crystal microgravimetry, used in conjunction with voltammetry and coulometry. As-deposited films were crystalline and showed p-type semiconductor behavior. Theoretical insights are provided for the electronic origin of the αâ†’ß phase transformation in AgVO3 and the disparate optical band gaps of the two polymorphs. Finally, implications for the application of this material in solar cells are provided.

Role of f Electrons in the Optical and Photoelectrochemical Behavior of Ca(La1- xCe x)2S4 (0 ≤ x ≤ 1).

This study focuses on a solid solution series, Ca(La1- xCe x)2S4 (0 ≤ x ≤ 1), where the f electron density is absent in CaLa2S4 and is progressively increased until it is maximized in CaCe2S4. Correspondingly, these samples, synthesized by a sealed ampule method, showed progressive variations in color ranging from gray for CaLa2S4 to orange-red for CaCe2S4. The crystal structural nuances of both the end members and three solid solutions with x = 0.25, 0.50, and 0.75 were established with the complementary use of synchrotron X-ray diffraction and neutron scattering. Interestingly, these data were consistent with a two-phase composition centered around each nominal solid solution stoichiometry. Optical characterization via diffuse reflectance spectroscopy and Tauc analyses showed a shrinking of the energy band gap (from the UV to vis range) when Ce was progressively introduced into the host CaLa2S4 structure. These data were in concert with electronic band structure calculations, using density functional theory, which showed the progressive formation of an intermediate f band when Ce was introduced intro the structure. Photoelectrochemical measurements in an aqueous redox electrolyte, as well as surface photovoltage and Kelvin probe measurements, revealed all samples to be n-type semiconductors. The valence and conduction band edge positions of the end members and the three solid solutions could be mapped, on both the redox and vacuum reference energy scales, by combining these measurements with the optical data.

Niobium Doping in BiVO4 : Interplay Between Effective Mass, Stability, and Pressure.

We have applied density functional theory to study the electronic structure changes caused by Nb incorporation in BiVO4 and the application of external pressure. The overall solubility of Nb in BiVO4 is usually high, and the presence of oxygen vacancies affect the dopability of Nb in BiVO4 . Through the analyses of the chemical-potential landscape, we have determined the single-phase stability zone of BiVO4 with the Nb doping. The most favorable Nb doping is simultaneous substitutions at both V- and Bi-sites. Even though Nb substitution at only V-site is next favorable, the band gap change is not very significant which agrees with an earlier experiment. However, it does change the electron effective mass by 20 % owing to the presence of Nb 4d bands in the conduction bands, which explains better catalytic activity by Nb-doped BiVO4 . In addition, application of external pressure the single-phase stability zone in the chemical-potential landscape. We have also focused on the local structural distortions near the Nb doping site, especially on the BiO8 octahedra. We have shown here that pressure-induced symmetrization of BiO8 dodecahedron lowers the electron's effective mass further and therefore can help to improve the photoconduction property of BiVO4 .