Scientists at EPFL have successfully used X-ray absorption spectroscopy (XAS) to determine the electronic dynamics of photovoltaic materials by examining the trapping dynamics demonstrated by photogenerated electrons in titanium dioxide material.
Transition metal oxides are considered to hold significant promise for converting solar energy into chemical energy (such as in photocatalysis), or electricity through photovoltaics. The structures of transition metal oxides make them preferable not only for generating electrons, electron holes, and other such charge carriers, but also for transporting and trapping them.
Titanium dioxide is a popular transition metal oxide material that is used for various applications, including solar energy conversion. However, researchers have faced significant difficulties in finding out the material’s electron dynamics at room temperature conditions.
Scientists at EPFL have successfully used XAS to address this problem. XAS has helped to discover new information about the movement of electrons on titanium dioxide’s surface region, which opens up new possibilities for photocatalytic and photovoltaic applications.
At EPFL, Majed Chergui’s lab pioneered the usage of XAS for research on femto- and pico-second phenomena in a vast array of of materials science, chemical and molecular systems. In this study, the research team used their knowledge to address a significant obstacle in conversion of solar technology.
A solar material must have the ability to function at room temperature conditions. However, at room temperature conditions, electron dynamics take place very rapidly, and hence, until recently, the materials had to be cooled significantly to very low temperatures. This limits how closely these models can represent reality.
Titanium dioxide is a low cost material that responds to light by trapping and transporting electrical current. It is widely used for solar energy conversion applications, and extensive research has already been carried out on this material.
In this study, Chergui’s team employed femtosecond X-ray absorption spectroscopy to investigate the trapping dynamics of light-generated electrons that were in aqueous colloidal anatase titanium dioxide nanoparticles. They obtained a never-before-seen view of titanium dioxide’s electronic dynamics.
The team used an ultrashort laser pulse to strike the nanoparticles to generate the electrons. Another femtosecond X-ray pulse was used for probing the evolution of electrons. It was observed that the electrons appeared on the titanium atoms within 300 fs, and they acted as a trap with a pentacoordinated geometry. These were mostly on the titanium dioxide nanoparticle’s surface shell region.
The current research provides a new tool for probing a material’s electronic properties under conditions used for the operation of solar devices.
“We have also shown that femtosecond, hard X-ray absorption can be used to gain detailed descriptions of charge-carrier dynamics in transition metal oxides, and our work is being extended to other such systems,” says Majed Chergui.
The EPFL’s Laboratory of Ultrafast Spectroscopy, Paul Scherrer Institute and ETH Zürich had collaborated in this study. The Swiss National Science Foundation funded the research through the NCCR:MUST and contracts. The COST actions, PERSPECT H2O and XLIC, also provided funding for the study.
The study paper has been published in Scientific Reports.