Recently, researchers at the Institute of Urban Environment, Chinese Academy of Sciences, including Senior Researcher Weng Bo, and their collaborators, have prepared a high-entropy alloy nanoparticle. It is about 3.5 nanometers in size and contains five transition metal elements (Fe, Co, Ni, Cu, and Mn).

When the high-entropy alloy material is used as a co-catalyst and loaded onto a titanium dioxide semiconductor carrier, a composite photocatalyst can be obtained, which can be used for carbon dioxide reduction under simulated sunlight conditions.

The research results show that the introduction of the high-entropy alloy can significantly improve the photocatalytic activity of titanium dioxide in reducing carbon dioxide.

In the photocatalytic carbon dioxide reduction reaction, the titanium dioxide composite material modified with the optimal proportion of high-entropy alloy can achieve a production rate of carbon monoxide and methane of 235.2 µmolg-1h-1 and 19.9 µmolg-1h-1, respectively.

This activity is 23 times higher than that of using titanium dioxide alone and is also the highest value reported to date for non-noble metal nanoparticle-modified titanium dioxide photocatalysts.At the same time, compared to the performance of titanium dioxide photocatalytic materials decorated with some precious metals, the catalytic performance brought by high-entropy alloy materials can basically match the former.

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The reviewer frankly said: "The efficiency of this achievement represents the highest level in this field."

At the same time, titanium dioxide composite materials can operate for up to 36 hours, and still maintain high activity, which fully proves the high stability of the material.

In addition, the research team also used high-energy X-rays for radiation stability testing of the material.

The results show: Even under high-energy radiation of 12keV, the structure of high-entropy alloy materials can remain intact, indicating that it can maintain the stability of structure and performance under extreme conditions, and thus can "serve for a long time".In general, the successful application of high-entropy alloy materials in the field of solar energy conversion has provided important technical support for the development of low-cost and efficient photocatalytic CO2 reduction catalysts, as well as for the reduction of greenhouse gas emissions and the recycling of carbon resources.

Regarding the future practical application prospects of this achievement, Weng Bo believes that the overall efficiency of pure photocatalysis is still relatively low, and the most promising application scenario for this achievement is currently in the field of photo-assisted catalytic technologies, such as photocatalytic hydrogenation of CO2 and photo-thermal removal of VOCs (Volatile Organic Compounds).

Why is the photocatalytic reduction of CO2 significant, and why choose high-entropy alloy materials?

It is understood that in the natural carbon cycle, the reduction of CO2 in photosynthesis can balance the oxidation of carbon in cellular respiration. However, the reduction of carbon in human industrial metabolism is still a missing part.

This imbalance can lead to global warming and trigger extreme weather events. Under the "dual carbon" background, photocatalytic reduction of CO2 using solar energy as the main energy source, as a highly potential negative carbon technology, has attracted much attention.

In summary, the successful application of high-entropy alloy materials in the field of solar energy conversion has provided significant technical support for the development of cost-effective and efficient photocatalytic CO2 reduction catalysts, as well as for the reduction of greenhouse gas emissions and the recycling of carbon resources.

Weng Bo believes that the overall efficiency of pure photocatalysis is still relatively low, and the most promising application scenario for this achievement is currently in the field of photo-assisted catalytic technologies, such as photocatalytic hydrogenation of CO2 and photo-thermal removal of volatile organic compounds (VOCs).

The significance of photocatalytic CO2 reduction lies in the fact that in the natural carbon cycle, the reduction of CO2 in photosynthesis can balance the oxidation of carbon in cellular respiration. However, the reduction of carbon in human industrial metabolism is still a missing part, which can lead to global warming and trigger extreme weather events. Under the "dual carbon" background, photocatalytic reduction of CO2 using solar energy as the main energy source, as a highly potential negative carbon technology, has attracted much attention.

The choice of high-entropy alloy materials is due to their unique properties, such as high entropy, which can enhance the catalytic activity and stability of the materials, making them suitable for photocatalytic CO2 reduction applications.With this technology, carbon dioxide can be catalytically converted into high-value fuels and chemicals, making it one of the important pathways to achieve the "dual carbon" goals and the recycling of carbon resources.

To utilize sunlight to drive the reduction reaction of carbon dioxide, people usually employ semiconductor materials such as titanium dioxide as photocatalysts.

Titanium dioxide can absorb sunlight to generate photogenerated electron-hole pairs. The photogenerated electrons have reducing properties, which can reduce carbon dioxide to carbon monoxide or methane, both of which are excellent chemical fuels and relatively basic chemical products.

However, for the single titanium dioxide photocatalytic material, it can only absorb the ultraviolet part of sunlight, and the utilization rate of sunlight is not high.

Moreover, the electron-hole pairs it generates itself are prone to recombination, leading to low photocatalytic efficiency for the reduction of carbon dioxide, and thus lower yields of products such as carbon monoxide or methane.These factors collectively hinder the efficient conversion of carbon dioxide into fuels and chemicals, limiting the feasibility of solar-driven chemical energy conversion.

Therefore, how to enhance the light absorption performance of semiconductor photocatalytic materials and suppress the recombination of the generated photogenerated electrons and holes, thereby improving the photocatalytic conversion efficiency of carbon dioxide, has always been a research hotspot in the field.

Previously, many studies have reported that introducing metal nanoparticles as co-catalysts on the surface of semiconductor materials such as titanium dioxide can act as collectors of photogenerated electrons, thereby promoting the separation and transfer of photogenerated electron-hole pairs, and effectively overcoming the recombination of electron-hole pairs.

In addition, the introduced large number of metal nanoparticles can also serve as active sites for the carbon dioxide reaction, thereby enhancing the surface reaction rate.

Usually, reducing the size of metal co-catalysts to the nanoscale or even smaller can effectively improve the atomic utilization efficiency of the co-catalysts, increase the number of active reaction sites, and thus enhance photocatalytic activity.Especially ultra-small nanoparticles with a diameter less than 5 nanometers, which usually have higher catalytic activity. The reason is that these ultra-small metal nanoparticles have unique physical and chemical properties, such as quantum size effect, surface geometric effect, and ultra-high specific surface area, etc.

However, these ultra-small metal nanoparticles also have greater surface energy and thermodynamic instability.

Therefore, in the process of material synthesis and photocatalytic reaction, they tend to aggregate into larger nanoparticles, which leads to a decrease in photocatalytic reaction activity or even complete deactivation.

In general, the stability problem of co-catalyst materials has greatly limited the design of efficient photocatalytic carbon dioxide reaction systems.

In recent years, high-entropy alloy materials have attracted widespread attention in the academic community. This kind of material usually contains more than five elements, each element's atomic fraction is between 5% and 35%, and each element's atoms randomly occupy a lattice site.Due to its high mixing entropy and low Gibbs free energy, high-entropy alloy materials exhibit excellent thermal stability and corrosion resistance.

Combining the characteristics of high-entropy alloy materials and the bottleneck issues in the current design of photocatalytic materials, the team used the solvothermal synthesis method to prepare this high-entropy alloy material.

In fact, this work began with a casual conversation in the laboratory.At that time, Weng Bo, who was engaged in postdoctoral research at the University of Leuven in Belgium, and Ph.D. student Guo Hele were conducting experiments in the same laboratory. Through this, Weng Bo learned that the team Guo Hele was part of was preparing high-entropy alloy materials of transition metals.

These materials not only have good structural stability, but the synthesis method is also very simple.

However, at that time, Weng Bo's research focus was still on the morphological structure design of noble metal nanoparticles such as Au and Pd.

But, coincidentally, Weng Bo encountered the instability of the structure of nanoparticle materials. Therefore, in this casual conversation, the two immediately sparked.

Subsequently, Weng Bo and several other colleagues carried out literature research to understand the current state of research on high-entropy alloy materials of transition metals in the field of photocatalysis.The results revealed that this field is still a blue ocean, with very few related studies, especially the research on using high-entropy transition metal alloys for photocatalytic reduction of carbon dioxide has not been reported.

After discussing with his international co-advisor, Weng Bo set the topic of "using high-entropy transition metal alloys for photocatalytic reduction of carbon dioxide."

Subsequently, Weng Bo began to search for suitable transition metal materials. At this time, he and his colleagues made theoretical calculations and simulations for different materials based on the characteristics of the photocatalytic reduction of carbon dioxide reaction, and finally selected the optimal five transition metal materials - Fe, Co, Ni, Cu, and Mn.

They are five consecutive elements in the periodic table, and Cu has a particularly good effect on the reduction reaction of carbon dioxide.

By high-entropy alloying with other metals, the electronic structure of Cu can be optimized, making it easier to drive the reduction reaction of carbon dioxide to obtain the related products.Subsequently, they began to synthesize and prepare, hoping to obtain ultra-small, uniformly sized, and structurally stable high-entropy alloy materials.

Through a series of attempts, they found that the hydrothermal method was not only simple and efficient, but also the size of the obtained high-entropy alloy materials was uniform.

At the same time, they could also control the size below 5 nanometers, which is very much in line with their expectations for high-entropy alloy materials for carbon dioxide reduction.

After the high-entropy alloy materials were prepared, the team immediately modified it onto a titanium dioxide carrier and tested its photocatalytic carbon dioxide reduction activity.

"During the activity test, we were all around the computer staring at the gas chromatogram on the screen, and everyone was very excited when a very high peak of carbon monoxide and methane products appeared," said Weng Bo.During the response period for the peer review of the paper, Weng Bo returned to the country due to work reasons and temporarily lacked the conditions to prepare materials.

"As a result, it was necessary for colleagues abroad to prepare the samples and send them back to the country, going through the customs clearance matters back and forth, and the time for the experimental response was also quite tight, which made me a bit worried at the time," he said.

However, the paper was still smoothly published. Recently, the relevant paper was published in Advanced Materials with the title "Noble-Metal-Free High-Entropy Alloy Nanoparticles for Efficient Solar-Driven Photocatalytic CO2 Reduction" [1].

Dr. Huang Haowei from the University of Leuven, Dr. Zhao Jiwu from Fuzhou University, and doctoral student Guo Hele from the University of Leuven are co-first authors.

Researcher Weng Bo from the Chinese Academy of Sciences' Institute of Urban Environment, Professor Long Jinlin from Fuzhou University, and Professor Maarten B. J. Roeffaers from the University of Leuven serve as co-corresponding authors.