Spin Unrestricted Excited State Relaxation Study of Vanadium(IV)-Doped Anatase

Abstract

Atomistic modeling of light driven electron dynamics are important in studies of photoactive materials. Spin-resolved electronic structure calculations become necessary when dealing with transition metal, magnetic, and even some carbon materials, intermediates, and radicals. An approximate treatment can be pursued in the basis of spincollinear density functional theory. Most transition-metal compounds exhibit open shell nonsinglet configurations, necessitating special treatment of electrons with α/β spin projections. By separate treatment of electronic states with the α/β spin components one is able to describe a broader range of materials, identify new channels of relaxation and charge transfer, and provide knowledge for rational design of new materials in solar energy harvesting and information storage. For this methodology, named spin-resolved electron dynamics, spin-polarized DFT is used as the basis to implement nonadiabatic molecular dynamics. At ambient temperatures, the thermal lattice vibrations results in orbital and energy fluctuations with time. Nonadiabatic couplings are then calculated, which control the dissipative dynamics of the spin resolved density matrix. Different initial excitations are then analyzed and used to calculate relaxation dynamics. Spin-resolved electronic dynamics approach (SREDA) is applied to study vanadium(IV) substitutionally doped bulk anatase in a doublet ground state. The results show that a difference in the electronic structure for α and β spin components determines consequences in optical excitations and electronic dynamics pathways experienced by electrons with α and β spin projections. Specifically, the lone occupied V 3d α-orbital increases the range of absorption and defines the rates and pathways of relaxation for both holes and electrons with α-spin projection. Optical excitations involving occupied V 3d α-orbital are responsible for IR-range absorption, followed by nonradiative relaxation. Certain transitions involving orbitals of α-spin component occur in the visible range and induce localization of a negative charge on the V ion for an extended time period. The slower nonradiative relaxation rate of α-excitations is rationally explained as a consequence of difference of electronic structure for α and β spin projections and specific pattern of energy levels contributed by doping. Specifically, excitations involving orbitals with α-projection of spin experience transitions through larger subgaps in the conduction band compared to the ones experienced by similar excitations involving orbitals with β-projection of spin. It is anticipated that this methodology can be broadly implemented on multiple applications of transition metal based materials, including optoelectronics, information storage, laser crystals, dyes, photovoltaic materials, and metal oxides for photoelectrochemical water splitting.

Dynamics of charge at water-to-semiconductor interface: Case study of wet [0 0 1] anatase TiO2 nanowire

“…45,46,47,48,49,50,51,52,53 There were recent approaches to combine Redfield theory of electron relaxation with on-the-fly coupling of electrons-to-lattice through a molecular dynamics (MD) trajectory in the basis of density functional theory (DFT). 54,55,56,57,58,59,60,61,62 The photo-induced electron transfer at the interfaces of a molecule or a water layer, a graphenelayer, a semiconductor quantum dor or perovskite layer with rutile TiO 2 surfaces has been investigated by Prezhdo et al 63,64,65,66,67,68 using real-time nonadiabatic MD. 69,70,71,72 These works have provided an understanding of the mechanisms of electron transfer, relaxation, and recombination dynamics, and the effect of these processes on the solar cell efficiencies.…”

Charge Transfer Mechanism in Titanium-Doped Microporous Silica for Photocatalytic Water-Splitting Applications

“…were used to compose the density matrix for each instant of time in such a way that it matches the initial conditions Equation (14) [6][7][8]22]. The electron in the conduction band and the hole in the valence band are indicated by e and h, respectively.…”

“…In addition, index e was used with the appreciation that equations for holes are the same as for electrons. Utilizing the above information, the charge density distribution, rate of energy dissipation, and rate of charge transfer were calculated as follows [6][7][8]22,33].…”

“…The addition of titanium atoms to the inner surface of the silica pore walls provides a photoactive material [5] and introduces numerous charge transfer states, which can be analyzed by atomistic computational modeling [6][7][8]. It has been known since the 1970s that titanium dioxide (TiO 2 ) can be used in water-splitting applications [9].…”

“…This information helps to identify orbital transitions due to excitations, which are of interest. Molecular dynamics [19] are simulated, which leads to couplings [20] and eventually to electron dynamics of photoexcitation [21] that then provides excitation lifetimes [22] and details of the charge transfer meachanism [6][7][8]. These lifetimes are an integral part of making conclusions about a material and its photocatalytic properties.…”