Chinese scientists "shot" full-space-time images of photogenerated charge transfer evolution of photocatalysts

Solar photocatalytic reaction can decompose water to produce hydrogen and reduce carbon dioxide to produce solar fuel, which is a "holy grail" topic in the field of science and has attracted worldwide attention
.
Although great efforts have been made in photocatalyst preparation and photocatalytic reaction research in the past half century, the basic mechanism of this process has been unclear
due to the separation, transfer and spatiotemporal complexity of participating in chemical reactions in photocatalytic reactions.

The Dalian Institute of Chemical Physics of the Chinese Academy of Sciences released news that recently, Li Can, academician of the Chinese Academy of Sciences and researcher of the institute, and Fan Fengtao, a researcher, have uncovered this mystery
.
The researchers integrated a variety of technologies that can be connected at the spatiotemporal scale, and "took" full-space-time images
of photogenerated charge transfer evolution for the first time in the world.

The researchers clarified the essential relationship between the charge separation mechanism and the efficiency of photocatalytic water splitting, which provided a new understanding and research strategy
for breaking through the "bottleneck" of solar photocatalytic reactions.
The findings were published Oct.
12 in Nature
.

The core scientific challenge of photocatalytic water splitting is how to achieve efficient separation and transport
of photogenerated charges.
Because this process spans huge space-time scales from femtoseconds to seconds, from atoms to microns, unraveling the microscopic mechanisms of this whole process is extremely challenging
.
The researchers integrated a variety of advanced technologies and theories to track the whole process
of the separation and transfer evolution of photogenerated charges in nanoparticles in the whole space-time domain.

Full-space-time dynamic "image" of the photogenerated charge separation process of a single photocatalytic particle from femtoseconds to seconds

In the photocatalytic process, photogenerated electrons and holes need to be separated from inside the micro- and nanoparticles and transferred to the surface of the catalyst to initiate a chemical reaction
.
But at such tiny physical scales, photocatalysts often lack the driving force needed to separate charges, so an efficient electric field
is required to achieve efficient charge separation.
In order to form a directional rearranged electric field in the photocatalyst particles, researchers selectively synthesize a specific trapping state (e.
g.
, defective structure) to a specific crystal plane of the particles, effectively promoting the separation
of charges.

To better understand the efficient charge separation mechanism in the nanosecond range, the researchers used time-resolved photoemission electron microscopy and found that photogenerated electrons can be selectively transferred to specific crystal plane regions on subpicosecond time scales, and electrons can move from one surface to another on ultrafast time
scales.
It attributes the ultrafast charge transfer to a new quasi-ballistic transport mechanism, in which carriers travel at extremely high speeds and are accelerated by an electric field built into the crystal face, spanning the entire particle
before interacting with the lattice.

Subsequently, in order to directly observe the charge transfer process, the researchers performed instantaneous voltage analysis and found that as the timescale progressed from nanoseconds to microseconds, holes gradually appeared in crystal planes
containing defective structures.

In summary, this study shows that the effective spatial separation of photogenerated electrons and holes on the crystal plane is determined by the charge transfer mechanism of space-time anisotropy, which can be adjusted controllably by the anisotropic crystal plane and defect structure
.
The ability to track charge transfer in time and space will greatly promote the understanding of the complex mechanism in the energy conversion process, and provide new ideas and research methods
for rationally designing photocatalysts with better performance.

Professor Ulrich Aschauer of the University of Bern, Switzerland, believes that "this method of tracking electron and hole transfer and distribution at the single-particle scale with high spatial and temporal resolution has established an unprecedented research method, which can greatly expand our in-depth understanding of the basic microscopic mechanism behind the photocatalytic function, thus bringing great hope for diagnosing the bottleneck problem of the photocatalytic process and how to successfully develop the regulation strategy of high-efficiency photocatalytic particles.
" Through the photocatalyst control strategies developed based on these understandings, such as the precise design of defect structures, rational assembly of other materials and cocatalysts, etc.
, it is expected to achieve the maximum efficiency
of photocatalytic water splitting water to produce hydrogen.
”