Piyush Dua

Photocatalytic and Photoelectrochemical Water Oxidation over Metal‐Doped Monoclinic BiVO4 Photoanodes

The visible-light-induced water oxidation ability of metal-ion-doped BiVO(4) was investigated and of 12 metal ion dopants tested, only W and Mo dramatically enhanced the water photo-oxidation activity of bare BiVO(4); Mo had the highest improvement by a factor of about six. Thus, BiVO(4) and W- or Mo-doped (2 atom %) BiVO(4) photoanodes about 1 μm thick were fabricated onto transparent conducting substrate by a metal-organic decomposition/spin-coating method. Under simulated one sun (air mass 1.5G, 100 mW cm(-2)) and at 1.23 V versus a reversible hydrogen electrode, the highest photocurrent density (J(PH)) of about 2.38 mA cm(-2) was achieved for Mo doping followed by W doping (J(PH) ≈ 1.98 mA cm(-2)), whereas undoped BiVO(4) gave a J(PH) value of about 0.42 mA cm(-2). The photoelectrochemical water oxidation activity of W- and Mo-doped BiVO(4) photoanodes corresponded to the incident photon to current conversion efficiency of about 35 and 40 % respectively. Electrochemical impedance spectroscopy and Mott-Schottky analysis indicated a positive flat band shift of about 30 mV, a carrier concentration 1.6-2 times higher, and a charge-transfer resistance reduced by 3-4-fold for W- or Mo-doped BiVO(4) relative to undoped BiVO(4). Electronic structure calculations revealed that both W and Mo were shallow donors and Mo doping generated superior conductivity to W doping. The photo-oxidation activity of water on BiVO(4) photoanodes (undoped<W doped<Mo doped) was in accordance with the results from electrochemical impedance spectroscopy, Mott-Schottky analysis, and theoretical electronic structural calculations. Thus, Mo or W doping enhanced the photocatalytic and photoelectrochemical water oxidation activity of monoclinic BiVO(4) by drastically reducing its charge-transfer resistance and thereby minimizing photoexcited electron-hole pair recombination.

Effect of Coulomb correlations on one-dimensional monatomic nanowire

The experimental realisation of one–dimensional (1D) monatomic chains of transition elements has opened a new route to study the magnetism in nanostructures in the field of condensed matter physics. We studied a so–called extended Hubbard model which includes all the off–diagonal matrix elements of Coulomb interactions, to explain the phenomenon of ferromagnetism in 1D monatomic chain of itinerant ferromagnets like Fe, Co and Ni. We have used the Greens function equation of motion approach to get energy eigenvalue of quasi–particle. Within the mean field approximation, among the off–diagonal matrix elements, the effect of correlated hopping is very crucial, because the magnitude is larger than exchange interaction. Therefore, in 1D itinerant ferromagnets, the ferromagnetism is driven by combined effect of band splitting and band narrowing/broadening. As a result of it, for the systems with less than half filled band, the up spin band gets broadened and down spin band narrowed down and vice versa for more than half filled band systems. From available Density Functional Theory (DFT) results it can be seen that up spin channel is present for conduction near Fermi level in the systems with less than half filled band and vice versa for more than half filled band systems. This effect is more pronounced in the presence of external magnetic field. Presence of one spin channel is one of the requirements for spintronic devices. The obtained results are in good agreement with existing DFT results.

Role of orbital degeneracy in itinerant ferromagnetism

In this work we have considered a doubly degenerate extended Hubbard model to study the magnetic (and optical) properties of itinerant ferromagnets such as Fe, Co, Ni, and their compounds. Using Green’s function equation of motion technique, we have studied ground state phase diagram as a function of band filling n. Our results for magnetization M as a function of n reproduce the observed behavior of Fe1−xCoxS2, Co1−xNixS2. We have studied magnetic susceptibility, magnetoresistance, spectral weight, and effective mass as a function of temperature. We have found that in the presence of magnetic field, the effective mass for up-spin electrons decreases and their spectral weight increases at all temperatures. The reduction of the effective mass of up-spin electrons, on applying magnetic field, has been observed in EuB6.