article ｜China Science Daily reporter Ni Sijie
Li Can (middle), Fan Fengtao (left), and Chen Ruotian, the first author of the paper and associate researcher at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, discussed signal analysis issues in front of the instrument. Photo provided by Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Following scientists taking photos of black holes in the macro universe, Chinese scientists have taken a special "Along the River During the Qingming Festival" from the microscopic world for the first time in the world.
This "photographing" team is the team of researcher Li Can and Fan Fengtao at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (hereinafter referred to as Dalian Institute of Chemical Physics, Chinese Academy of Sciences). They took a full-time and space-time image of the photogenerated charge transfer evolution of photocatalyst . The result was published in Nature on October 12.
reviewers believe that the result "potentially reveals the full picture of different charge transport mechanisms in the photocatalysis process" and solves "a very interesting but unanswered question at present."
"Holy Grail" competition Chinese team
With the help of the catalyst, the sunlight turns water into fuel. This was just an ancient fantasy at first. But scientists have always wanted to turn this fantasy into reality. Until today, photocatalytic decomposition of water is still regarded as the "holy grail" in the field of chemistry.
Since the 1970s, Japanese scientists have begun to organize research teams to find ways to seize the "Holy Grail". At that time, people had already achieved photocatalytic hydrogen production in solar energy, but the efficiency of photocatalysis has not improved. 0.1%, 0.2%, 0.3%... Until the 1990s, the value representing catalytic efficiency only barely rose to about 1%. However, only if the catalytic efficiency reaches more than 5%, can solar photocatalyzed hydrogen production be able to achieve industrialization, and only if it reaches 10%, it can be comparable to fossil energy.
The process of solar photocatalyzed hydrogen production does not seem complicated. Under the action of a photocatalyst, water is exposed to the sun, producing hydrogen and oxygen , or producing oxygen, hydrogen ions and electrons called "photogenerating charges". However, in this not-complex chemical reaction, why can’t the catalytic efficiency improve?
Since 2000, Li Can's team has also joined the "Holy Grail" competition. Unlike other international research teams, they first spent some time looking for the reasons why the photocatalytic efficiency could not be improved until they found three factors that had a decisive impact on the catalytic efficiency - light capture, charge separation, and catalytic conversion.
"Most of the past work has focused on screening photocatalytic materials and optimizing device processes. The research on the microscopic mechanisms of photogenerated charge dynamics and photocatalytic dynamics is relatively weak. In other words, the basic major scientific issues have not been clarified." Li Can said.
They judged that the core scientific challenge of solar photocatalyzed hydrogen production lies in how to achieve efficient separation and transmission of photogenerated charges. Understanding the process of photogenerated charge transfer evolution is what they will do next.
New ideas learned in the ward
The diameter of an electron is only about one billionth of the diameter of the thinnest hair. It is not easy to understand where an electron comes from, where it goes, and what it has experienced in the middle.
In 2008, the experimental team used the independently developed spectral device to discover some experimental phenomena that may be caused by charge transfer evolution in the heterophase junction of titanium oxide. But Li Can still feel that it is almost interesting.
One day in 2010, Li Can was unwell and went to the hospital for examination. He accidentally discovered that doctors often use imaging methods for vascular examinations. After returning to the ward, Li Can, who suddenly realized, excitedly called the team member Fan Fengtao: "Can we use imaging methods to see how the photogenerated charges are transferred and evolved?"
He felt that previous experiments could only infer what the transfer and evolution of the photogenerated charges looked like, but "only by seeing this process with your own eyes can we make people feel more at ease."
Since then, taking photos of photogenerated charges has become a new goal. "Developing advanced photoelectric imaging technology is the prerequisite." Li Can believes.
so they and their partners independently developed various instruments, which gradually improved the resolution and "shunt" speed of the instruments. From nanometers to subnanometers, from microseconds to picoseconds , slowly, instruments that are not found in the world appear one by one in their lab. After
, the researchers integrated a variety of technologies that can be connected at the spatial and temporal scale, combining a variety of advanced characterization techniques and theoretical simulations, allowing the time-resolved light emission electron microscope (femtoseconds to nanoseconds), transient surface photovoltage spectrum (nanoseconds to microseconds), surface photovoltage microscope (microseconds to seconds) and other instruments in the laboratory to take photos of the photogenerated charges on the surface of photocatalyst particles on different time scales and spatial scales, just like participating in a relay race.
"Along the River During the Qingming Festival"
Li Can himself did not expect that he would be surprised when he saw the "photographed" photogenerated charge image at the first glance.
"In theory, we should be able to get this image, but when I really saw the charge separation imaging results at different parts of the photocatalyst surface, I was still surprised." Li Can told " China Science Daily ", "The lifespan of photogenerated charge is very short. It is very challenging to see it spread on the photocatalyst surface and be able to successfully image it."
surprised and amazed image of Li Can, showing the entire time and space process of photogenerated charge on the surface of photocatalyst nanoparticles from appearance to disappearance.
Researchers also found that the effective spatial separation of photogenerated charges and holes on the surface of photocatalyst crystals is jointly determined by the charge transfer mechanism of space-time anisotropic, which can be controlledly adjusted through the anisotropic crystal plane and defect structure.
Li Can likes to use the "Along the River During the Qingming Festival" to compare the use of this achievement. "Through the Qingming River During the River Map, we can see the social culture at that time, and can also be used to verify some historical details; the image of photogenerated charge is like the Qingming River Map, we can see how electrons transfer on the nanoscale and analyze how it affects the photocatalytic efficiency we are concerned about." Li Can said.
Faced with the current situation where photocatalytic efficiency remains at a few percent, Li Can sigh: "Our research is a basic research, which is just the first step to solve the problem of photocatalytic efficiency. We have to do a lot of things after that, and the challenges are also great."
"Holy Grail" competition is still continuing, and the Chinese team is constantly moving forward. "In the future, we will gradually turn our dreams into reality and provide clean and green energy for our production and life." Li Can said.
Related paper information:
https://doi.org/10.1038/s41586-022-05183-1
