Electron Transfer Dynamics in Photocatalytic CO2 Conversion

Electron Transfer Dynamics in Photocatalytic CO2 Conversion

Coal is the workhorse of our power industry, responsible for approximately half of the electricity consumed by Americans. Managing carbon dioxide (CO2) emissions from coal utilization is one of the most challenging issues facing the fossil energy industry today. To cost-effectively capture and manage CO2, new and flexible photocatalytic technologies are being developed that can be used at large storage facilities (geological, oil fields, etc.) to slowly convert stored CO2 into more useful products such as methane and methanol. 

Photocatalysis is the acceleration of a light-induced reaction in the presence of a catalyst. During this process an electron-hole pair is created as a result of exposure to ultraviolet radiation or visible light and the resulting free-radicals are very efficient oxidizers of organic matter. The most common photocatalyst for CO2 conversion is titanium dioxide (TiO2), which suffers from two fundamental inefficiencies preventing industrial scale deployment: 
1) a large UV spectral band gap that only utilizes 1-5% of the sunlight reaching the earth's surface, and 
2) rapid charge carrier recombination that causes electrons and holes to recombine before they can initiate the photoreduction of CO2. 

To overcome these inefficiencies, one approach uses semiconductor nanocrystals such as cadmium selenide (CdSe) or lead sulfide (PbS) to photosensitize TiO2 and improve its optical activity in the visible and near infrared regions of the solar spectrum. The advantage of using these nanocrystals is that they are easily tunable to absorb light at any wavelength, are thermally stable, and are photochemically robust. When combined with TiO2, these semiconductor nanocrystals form a type II band alignment, meaning that photoexcited electrons in the conduction band of the nanocrystal can be injected into the conduction band of TiO2. Photoinjections of electrons are ultra-fast events, causing the electron and hole in the nanocrystal to separate, reducing charge recombination. But the mechanisms by which these events happen are not well understood and they are the key processes in photocatalysis. To simulate electron transfer at interfaces composed of two dissimilar materials such as CdSe and TiO2 is technically challenging.

NETL researchers are investigating the dynamics of these photoinduced electrons at the interface of a CdSe nanoparticle with a TiO2 nanoparticle. The goal is to generate valuable insights into the transfer mechanisms of photoinduced electrons and to provide guidelines for system design and improvement. Simulations are performed using the supercomputers at the National Energy Research Scientific Center (NERSC), where NETL researcher De Nyago Tafen was awarded a total of 600,000 MPP Hours (hours X nodes used X 24 cores per node) for a period of two years starting in January 2012. To tackle this problem, Dr. Tafen uses a variety of techniques and a set of tools in collaboration with a researcher in the Chemistry Department at the University of Rochester. The present work combines nonadiabatic molecular dynamics (NAMD) with time-domain density functional theory (TDDFT) to help understand the mechanisms responsible for the movement of charge through the heterostructure, identify the vibrational motions that promote charge transfer, and provide a better understanding of the electron transfer dynamics. 

In molecular models, approximations assume low-energy density and near equilibrium (adiabatic) situations. NAMD accounts for conditions or subsystems that are quickly driven out of equilibrium by an external perturbation. Time Dependent Density Functional Theory provides a framework to describe electron dynamics out of the electronic ground state. Recent findings at NETL have shown that uniform distribution and close/direct contact of the semi-conductor nanoparticle with the TiO2 nanoparticles are needed for efficient carrier separation (for fast electron transfer to occur) across the semiconductor nanocrystal and TiO2 junction. Additionally the charge injection rate from the semiconductor nanocrystal into the TiO2 nanoparticle depends on the size of the nanocrystal. 

  Figure 1. Schematic of the photoinduced electron injection process in CdSe/TiO2 nanocrystal heterostructure. An absorbed photon promotes an electron from the ground state of the CdSe nanocrystal located inside the TiO2 band gap into an excited state (photoexcited state). Then, the excited electron is transferred into the TiO2 conduction band (CB). VB represents the valence band of TiO2 and ET the electron transfer from the photoexcited state to the TiO2 CB. (Click image for larger view)

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