Matter: A laser shot, MOF becomes a metamaterial

Challenges in preparing graphene / metal nanoparticle composites

The composite material of porous graphene and metal nanoparticles has a wide range of application prospects in the fields of energy, environment, chemical industry, medicine, etc., and the preparation of this composite material into continuous cemented blocks can better meet its practical application requirements. However, there is currently no direct method to prepare integrated graphene / metal nanoparticle composite bulk materials. Generally speaking, these methods often synthesize porous graphene and metal nanoparticles separately, and then compound them into one body. Traditional preparation methods are often complicated, time-consuming and energy-consuming, and inefficient. Therefore, the development of a cheap and fast one-step preparation process has important scientific significance for the practical application of porous graphene / metal nanoparticle composites.

Achievements

Recently, Professor Gary Cheng of Purdue University, Professor Deng Hexiang of the School of Chemistry and Molecular Science of Wuhan University, and collaborators with Professor Ye Lei of Huazhong University of Science and Technology and Purdue University based on previously reported laser nano-smelting and patterning work J. Am. Chem. Soc. 2019, 141,5481−5489), further developed laser thermochemical stitching (LTS) technology based on metal-organic frameworks (MOFs) to directly print graphite Graphene-metal nanoparticle monolith (GMM).

Figure 1. Direct printing of graphene / metal nanoparticle superstructures based on laser "thermal stitching" technology. 

Point 1: Laser-scanned MOF transforms into a porous graphene / metal nanoparticle integrated film.

The researchers found that after a fast laser scan of the inexpensive MOF powder crystals between the glass interlayers (Figures 1A, 1B), the MOF can be converted into a monolithic black film (Figures 1C, 1D). Further characterization proved that only the 20 micron-thick black film was cross-linked by porous graphene with uniformly loaded metal nanoparticles. It is worth noting that in the entire preparation process, no MOF crystal powder was added, and no other raw materials were added, which meant that the metal nodes and organic ligands constituting the MOF crystal were converted into metal nanoparticles and porous graphite after laser irradiation Ene. In addition, the process can be performed directly in the air environment, and the laser power consumed is only 5 watts, which is very suitable for large-scale and industrial production.

Figure 2. Structural and light absorption characterization of graphene / metal nanoparticle metamaterials.

Point 2: GMM formed by laser conversion of MOF has super light absorption performance

The scanning electron microscope images of Figures 2A and 2B show that after laser irradiation, the MOF crystals are transformed into a layered porous structure similar to pine cones, and the layered structures are crosslinked to form a complete porous film (Figures 2G, 2H). Test results show that this composite film of graphene and metal nanoparticles has a broad-spectrum absorption performance of more than 99% for sunlight with a wavelength of 250 nm to 2.5 μm (Figure 2F), and exhibits black body-like properties. Further research shows that a large number of cavity structures in porous graphene can form dense optical cavities. After photons are irradiated, they are absorbed by the graphene structure and converted into thermal energy through continuous reflection. In addition, high-density metal nanoparticles in graphene pass through Local plasmon resonance can also convert light energy colleges into thermal energy. The material‘s highly efficient light absorption performance lays the foundation for further utilizing the material to prepare high-performance light-to-heat conversion devices.

Figure 3. Structural characterization of graphene / metal nanoparticle composite film metamaterials.

Point 3: Laser parameters can regulate the defects and number of layers of graphene

PXRD and XPS spectra show that the metals in GMM exist as zero-valent metal nanoparticles. The Raman spectrum analysis in FIG. 3C shows that as the laser power increases, the degree of crystallization of graphene is gradually increasing, and the number of layers is gradually decreasing. The high-resolution electron microscope image also confirms this. This shows that the structure of graphene in GMM can be precisely controlled by adjusting the relevant parameters of laser. In addition, research shows that laser power can also significantly affect the number of hydrophilic groups in graphene, thereby regulating its hydrophilic and hydrophobic properties, and laying a foundation for further expanding its application space.

Figure 4. MOF structural design precisely regulates metal nanoparticle size.

Point 4: Changing the ratio of metal and carbon atoms in MOF can regulate the size of metal nanoparticles

Compared with the traditional pyrolysis method, which requires several hours of pyrolysis time, the laser can heat-treat the material more quickly and accurately. Due to the short interaction time between laser and MOF (irradiation time is on the millisecond scale), the kinetic factors of metal atoms gathering into nanoparticles during the formation of nanoparticles need to be considered. Through the structural design of MOF, the precise control of metal ions and carbon atoms (the main atomic component in MOF) in MOF can be achieved. Based on this guidance, metal nanoparticles with a particle size of less than 2 nanometers can be prepared, which breaks through traditional pyrolysis. The bottleneck of difficult regulation of metal particle size.

Figure 5. High-efficiency GMM-based solar-powered desalination device.

Point 5: GMM for efficient solar-powered desalination

Due to the excellent controllability of laser-made GMM and the extremely high light-to-heat conversion performance of GMM itself, researchers have explored its application prospects in the field of light-to-heat conversion through solar-driven desalination applications. Researchers first prepared a GMM film with good hydrophilic properties and bonded it to a photothermal seawater desalination device as an efficient light-absorbing layer. Studies have shown that photothermal desalination devices with integrated GMM exhibit extremely high photothermal efficiency. Under one solar illuminance, the light-to-heat efficiency can reach 85%, and under higher solar light density, it exhibits more than 90% of light-to-heat conversion efficiency. The fresh water prepared can reach the standard for direct drinking. Because the GMM prepared by this process is thin and has low thermal conductivity, it can quickly reach a stable working state in a short period of time, and its start-up performance exceeds other materials that have been reported so far. This is important for energy saving and emission reduction in practical applications. Practical significance.

summary

In summary, the researchers quickly converted cheap MOF powder materials into porous graphene / metal nanoparticle composites in one step by air in laser. The design of laser parameters and MOF structure can precisely control graphene and metal nanometers. The structure and size of the particles. The establishment of this method laid the foundation for the inexpensive and efficient production of graphene / metal nanoparticle composite materials, and provided ideas for the application of this type of material in other fields such as energy and environment.

references

Jiang, Haoqing, et al. Graphene-Metal-Metastructure Monolith via Laser Shock-InducedThermochemical Stitching of MOF Crystals. Matter (2020).

DOI: 10.1016 / j.matt.2020.03.003

https://www.sciencedirect.com/science/article/pii/S2590238520301168

Source: Nanoman