Scientists subvert the traditional understanding of chemical inertness, aiding i

At present, in the fields of energy, environment, and chemical engineering worldwide, there is a widespread problem of high energy consumption and low efficiency in separation, which is a common issue.

On one hand, the reason lies in the fact that current membrane separation technology mainly uses traditional polymer membranes. These membrane materials do not have a well-defined pore structure, and their thickness is usually in the range of tens of nanometers to several micrometers, resulting in low flux. Moreover, the chemical and thermal stability of the polymer matrix is relatively low.

This leads to a situation where the higher the selectivity of the separation membrane, the worse its permeability.

How to overcome the aforementioned problems is a research direction that is highly concerned in the relevant fields. Utilizing two-dimensional materials to prepare separation membranes may provide an opportunity to break through this issue.

For example, graphene has been proven to be impermeable to any gases and ions in engineering applications, and thus is considered a good matrix material.If one could introduce atom-sized channels of controllable dimensions on the completely impermeable surface of graphene, and fully leverage the interactions between these channels and the transport molecules or ions, it would endow the material with high transport selectivity for different molecules and ions.

Advertisement

Furthermore, due to graphene's atomic-level thickness, the channels introduced on its surface have an extreme atomic-level length, which can maximize the transmembrane flux.

Based on this idea, developing atomic-level porous two-dimensional films can solve the aforementioned issues, achieving maximization in both selectivity and permeability of the separation membrane.

Sun Pengzhan, an assistant professor at the University of Macau, focuses his research on developing new types of two-dimensional membranes and addressing the fundamental scientific issues related to the design of two-dimensional membrane separations.

He utilized a novel device structure that seals graphite single crystals with graphene to measure the gas transmembrane transmission with an accuracy that is 8 to 9 orders of magnitude higher than the previous highest level in the field; and based on this measurement accuracy, he discovered the phenomenon of hydrogen molecules abnormally penetrating the graphene lattice (while helium atoms, which are smaller than hydrogen molecules, cannot penetrate).

In addition, he further precisely fabricated single atomic vacancy channels on the surface of the graphene film, and based on the measurement accuracy of the aforementioned device, revealed the exponential level sieving capability and transport mechanism of the obtained graphene atomic pores for different gas molecules.By committing to the precise construction of atomic-level confined channels and revealing the mechanisms and novel phenomena of material transport processes within them experimentally, as well as using the newly developed confined membrane separation technology to address common problems of high energy consumption and low efficiency in the fields of energy and environment, he has become one of the Chinese inductees for the "35 Innovators Under 35" by MIT Technology Review in 2023.

Developing high-precision gas transmembrane transport detection technology, overturning the conventional understanding of graphene impermeability

Currently, both experimental and theoretical research on graphene has generally proven that although it is only one atom thick, its lattice completely blocks the transmembrane transmission of all gases and liquids.

However, the highest measurement accuracy that can be achieved in experiments is 10^5 to 10^6 molecules per second (with mass spectrometry detection limit at 10^7 molecules per second), which is obtained by the gas transmembrane transmission detector made by sealing the micrometer-sized graphene film on the oxidized silicon microcavity. This accuracy, strictly speaking, does not rule out the possibility of the existence of weaker transport processes.The accuracy of the aforementioned device is limited by the amorphous structure of silicon dioxide and its rough surface. In light of this, Sun Pengzhan chose to use graphene films to seal single-crystal graphite (or hexagonal boron nitride) micrometer-sized cavities. By measuring the positional changes of the suspended film under specific atmospheric and transmembrane pressure difference conditions, he was able to detect the process of gas molecules crossing the film into the microcavity.

With the gas impermeability of the single-crystal cavity walls and the atomic-level seal formed between the smooth surface and graphene, he successfully improved the measurement accuracy of gas transmission by 8 to 9 orders of magnitude compared to the highest level in this field before, being able to detect the extremely weak transport phenomenon of only a few helium atoms passing through the micrometer-sized film per hour [1].

"Because the device has a very high measurement accuracy, we can use it to widely detect different gases, and it may be possible to observe interesting phenomena that were not observed before due to low measurement accuracy," he said.

Based on the measurement accuracy of the device, he found the novel phenomenon that hydrogen molecules can abnormally penetrate the graphene lattice, while no other gases can.

"This discovery was particularly shocking to us at the time and unexpected. Hydrogen molecules can enter the microcavity through the graphene surface, but helium atoms, which are smaller in size, cannot," he said.What is the underlying reason for this?

Through theoretical calculations and further research, Sun Pengzhan found that the above-mentioned abnormal phenomenon can be attributed to a two-step continuous process.

The first step is the catalytic cracking of hydrogen molecules on the corrugated surface of graphene; the second step is the hydrogen atoms overcoming an energy barrier of an electronvolt to mirror-flip to the opposite side of their lattice.

In layman's terms, hydrogen does not pass through in the form of hydrogen molecules, but will be cracked into smaller protons or adsorbed hydrogen atoms on the surface of graphene, and then cross the graphene lattice into the cavity.

In fact, since graphene was first reported, it has always been considered to have extremely high chemical inertness and does not react with anything. However, the above theoretical research that it can crack hydrogen molecules proves that graphene itself has catalytic activity.Therefore, to further verify this theory, he conducted three independent and complementary experiments, which are the comparison of hydrogen molecule transport characteristics between defect-free graphene and single-layer hexagonal boron nitride lattices, the comparison of Raman spectra of graphene single crystals with nanoscale ripples and atomically flat surfaces in a hydrogen atmosphere, and the catalytic hydrogen isotope exchange reaction of single-layer graphene.

The aforementioned experiments revealed that the nanoscale ripples on the surface of graphene have strong catalytic activity for the dissociation of hydrogen molecules, and this activity is comparable to that of metals and other known catalysts, thus providing a new perspective for controlling the catalytic activity of two-dimensional materials.

That is to say, the surfaces of all two-dimensional crystals are uneven, and the nanoscale ripples brought about by this unevenness are expected to be fully utilized to control their catalytic activity [2].

In addition, by using an electron beam radiation with an energy of only a few thousand electron volts to irradiate micrometer-sized graphene films, he also prepared single graphene atomic vacancy channels, the size of which is only about 2 Å (equivalent to a carbon hexagon) [3].

Furthermore, based on the high precision of the above-mentioned devices, he carried out gas transport tests and found that small molecules such as helium and hydrogen can easily pass through the obtained atomic pores, while larger molecules such as methane and xenon are completely impermeable within the experimental accuracy range."Quantitative analysis results show that the obtained graphene atomic pores exhibit an exponentially large transport selectivity for gas molecules of different sizes," he said.

Obviously, the three research results mentioned above revolve around the same research idea, providing important scientific basis for basic research in the field of physical chemistry, as well as the development of new technologies in the environment, energy, chemical industry, such as high-precision molecular detection, non-noble metal catalysts, and atomic-scale porous two-dimensional separation membranes.

Committed to exploring more unknowns in the material transport process in the atomic-scale confined space to better serve the fields of environment and energy.

It is reported that Sun Pengzhan grew up in an ordinary family in a county town in the mainland.In 2008, he was recommended to Tsinghua University to pursue a Bachelor's degree in Mechanical Engineering and Automation. After graduating in 2012, he continued his studies at his alma mater to pursue a Ph.D. in Materials Science and Engineering.

During his doctoral studies, he mainly engaged in the research of selective mass transfer characteristics and filtration separation performance of two-dimensional layered thin films.

"In the early stages of my research career, my scientific foundation and accumulation were relatively weak. At that time, I mainly hoped to prepare new separation membranes using two-dimensional materials, obtain excellent performance, and put them into practical application in the short term," he said.

However, as his understanding and comprehension of the research subjects gradually deepened, he also realized that although the developed two-dimensional thin films have potential in practical applications such as filtration and separation, he still could not understand some of the fundamental issues closely related to them.

For example, for graphene, where is the limit of molecular impermeability of perfect lattice graphene? And where is the limit of selective molecular sieving performance of atomic-scale porous graphene thin films?In my view, if these fundamental issues do not receive satisfactory responses at the experimental and theoretical levels, it will greatly limit further research and make the so-called 'huge application potential' of two-dimensional thin films empty, he said.

In light of this, to address the aforementioned fundamental scientific issues, Sun Pengzhan went to the University of Manchester in the UK for postdoctoral research after obtaining his doctoral degree in 2016, under the guidance of Professor Andre K. Geim, the 2010 Nobel Prize winner in Physics.

This lasted for a full six years. During this period, he not only achieved a series of breakthrough results. More importantly, this valuable scientific research experience has established a high-quality scientific research value for his future career development and laid a solid foundation.

In November 2022, he came to the University of Macau as an assistant professor, hoping to present more unknowns about the material transport process in the atomic scale confined space through more experimental methods, and deepen people's understanding of this aspect; he also looks forward to transforming the knowledge and wisdom obtained in the basic scientific research exploration process into practical technology to better serve the fields of environment, energy, chemical industry, etc.

According to his introduction, his current research interest focuses on the two-dimensional crystals of clay minerals that are abundant in nature, such as single-layer crystals obtained from exfoliated mica, titanium oxide, and other layered materials, and is committed to using their inherent lattice holes or inherent vacancy defects that are comparable to the size of common gas molecules and ions on the surface to achieve efficient molecular sieving."In other words, we plan to expand the single-atom-level vacancy channels introduced on the surface of graphene to a crystal, and then through self-assembly or other experimental means, transfer the test results in a single porous crystal to the entire macroscopic thin film, thus achieving the original design goal, which is to prepare a large-area porous two-dimensional crystal thin film with atomic-level thickness to achieve higher performance separation," said Sun Pengzhan.