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

Abstract

Solar energy conversion into chemical form is possible using artificial means. One example of a highly-efficient fuel is solar energy used to split water into oxygen and hydrogen. Efficient photocatalytic water-splitting remains an open challenge for researchers across the globe. Despite significant progress, several aspects of the reaction, including the charge transfer mechanism, are not fully clear. Density functional theory combined with density matrix equations of motion were used to identify and characterize the charge transfer mechanism involved in the dissociation of water. A simulated porous silica substrate, using periodic boundary conditions, with Ti 4+ ions embedded on the inner pore wall was found to contain electron and hole trap states that could facilitate a chemical reaction. A trap state was located within the silica substrate that lengthened relaxation time, which may favor a chemical reaction. A chemical reaction would have to occur within the window of photoexcitation; therefore, the existence of a trapping state may encourage a chemical reaction. This provides evidence that the silica substrate plays an integral part in the electron/hole dynamics of the system, leading to the conclusion that both components (photoactive materials and support) of heterogeneous catalytic systems are important in optimization of catalytic efficiency.

Microscopic mechanisms of cooperative communications within single nanocatalysts

“…These processes are redox reactions that involve the transfer of charged species. Since the charged holes are important for catalytic properties of active sites ( 1 , 4 , 20 – 23 ), we hypothesize that the probability for the redox chemical reaction to fire up at the given site might be a function of the local concentration of charged holes ( 23 , 24 ). In addition, it is assumed that each reaction produces new charged holes with a rate k ( Fig.…”

Photoinduced Charge Transfer at Interfaces of Carbon Nanotube and Lead Selenide Nanowire

“…Next the autocorrelation functions of these couplings were processed with eq . According to Redfield theory, ,, the Fourier transform of M (τ) (eq )­ provides Redfield relaxation tensor elements R ijkl (eq )­ The R ijkl -elements give second order perturbative contributions for the equation of motion for electronic degrees of freedom (eq )­ and being the Liouville and Redfield superoperators acting on the density operator . The initial values of the density matrix at time t = 0, were denoted with symbol ρ ij (A,B) (0) and served as the initial condition for system of differential eqs . Here, indices A and B label specific initial conditions, and Kroneker δ symbol δ ij reflects the fact that off-diagonal elements are set to zero.…”

“…Moreover, for the description of the photoexcited processes taking place at the interface between CNTs and PbSe nanocomposites, methods for time-resolved nonadiabatic dynamics (NAD) have to be applied in order to obtain nonadiabatic electron–phonon couplings responsible for energy relaxation developing simultaneously with and competing with the charge transfer . Recently developed NAD methods based on DFT coupled to either the surface hopping , or the density matrix , techniques have been successful in describing the charge transfer at surfaces of various semiconductor nanostructures, including porous materials, thin films, nanowires, and quantum dots . Although substantial progress has been achieved in modeling photoexcited dynamics in pristine PbSe , nanostructures and various CNTs, − application of NAD for describing charge transfer dynamics at the interface of CNT/PbSe nanocomposites has not been reported yet.…”

Surface Chemistry and Catalysis

“…Kilin D. and co-workers [4] investigated, by means of DFT calculations combined with density matrix equations of motion, the charge transfer mechanism involved in H 2 O dissociation over titanium-doped microporous silica. The results revealed that silica substrates contain electrons and hole trap states, which could facilitate the water splitting.…”

“…It consists of 14 high-quality papers, involving: a comprehensive review article on the surface analysis techniques that can be employed to elucidate the phenomenon of electrochemical promotion in catalysis [3]; two theoretical studies (Density Functional Theory, DFT) on H 2 O dissociation and its implications in catalysis [4,5]; two mechanistic studies by means of temperature-programmed desorption/surface reaction (TPD/TPSR) and/or operando spectroscopy on N 2 O formation over NO x storage-reduction (NSR) catalysts [6] and on methanol reforming over cobalt catalysts [7]; two articles on H 2 production by the steam reforming of ethanol [8] or diesel [9] over transition metal-based catalysts; two articles on the production of commercial fuels by Fisher-Tropsch synthesis [10,11]; two articles on Au-catalyzed CO oxidation [12] and preferential CO oxidation [13]; and three experimental investigations regarding the structure-activity correlation of NO oxidation to NO 2 over Mn-Co binary oxides [14], cyclohexene oxidation on TiZrCo mixed oxides [15] and alkene epoxidation on silica nanoparticles [16].…”