Mass spectrometry (MS) is a precise analytical technique that involves ionizing, identifying, and detecting ions. The process begins with sample ionization and proceeds to ion characterization based on mass-to-charge (m/z) ratios using analyzers, followed by ion detection using sensitive particle detectors.
Beyond its analytical capabilities, MS also serves as a preparative method, involving the manipulation of materials in ionic form. Preparative MS extends to organic synthesis, allowing for the collection of substantial amounts of products. In the context of metamaterials, mass spectrometry is valuable for both characterization and synthesis purposes.
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A Brief History of Mass Spectrometry
A century ago, Sir J.J. Thomson created the initial mass spectrometer, originally known as a parabola spectrograph. This invention marked the beginning of mass spectrometry (MS), assessing the mass-to-charge ratio (m/z) values of ionized gaseous molecules.
Initially focused on analyzing organic compounds, MS gradually expanded to include more intricate substances like sugars, DNAs, and peptides, as per the article published in Materials Advances. However, progress was hindered by the absence of efficient, non-destructive ionization methods. In the late 1980s, the introduction of electrospray ionization (ESI) and laser desorption methods overcame these limitations, particularly benefiting protein analysis.
Simultaneously, mass spectrometers evolved to become more sensitive and accurate, featuring high resolving power. This significantly increased the significance of this method for the discovery and characterization of new types of materials.
Mass Spectroscopy of Plasmonic Metamaterials for Efficient Light-Driven CO2 Hydrogenation
Solar-driven conversion of CO2 into valuable fuels and chemicals, as an eco-friendly method for recycling released CO2, holds promise for achieving carbon neutrality. Essentially, a typical light-driven catalytic process involves light absorption to generate charges or heat, supplying localized energy to activate CO2 molecules on the catalyst surface.
Plasmonic metamaterial absorbers, artificially engineered structures, show potential for light-driven catalytic applications. These materials have found applications in optical sensing, solar steam generation, and photo-detection due to their strong interactions with electromagnetic waves. However, the interactions of plasmonic metamaterials with electromagnetic waves, crucial for their design and working mechanism, have not been fully explored in the context of light-driven catalysis.
In addressing these challenges, an article published in Advanced Materials is focused on uncovering the potential of plasmonic metamaterial absorbers in light-driven CO2 hydrogenation, using Au as a model.
The plasmonic metamaterials are synthesized in three layers, with the top layer of patterned Au used to suppress the reflection of electromagnetic waves. To incorporate catalytic sites, a 3 nm-thick film of Cu single-atom alloy (specifically, Ag8Cu1 as confirmed by mass spectrometry and X-ray absorption spectroscopy (XAS)) was added to the plasmonic metamaterial absorber obtained. The molar fraction of Ag/Cu in the AgCu alloy was determined to be 8:1 based on energy-dispersive X-ray spectroscopy analysis via high-resolution transmission electron microscopy (HRTEM).
CO2 hydrogenation involves a solid-gas reaction that facilitates catalyst separation and recycling, making catalyst durability crucial for practical applications. To confirm the carbon sources of CO and CH4 produced by CO2 hydrogenation, isotopic CO2 was used as the reactant for light-driven catalytic CO2 hydrogenation. Gas chromatography–mass spectrometry (GC-MS) identified emerging peaks corresponding to CO and CH4, confirming that the CO and CH4 indeed originated from CO2.
Laser-Based Mass Spectrometry for Metamaterials
Metamaterials with unique 3D morphologies at the micro- or nanoscale, known as mechanical metamaterials, exhibit exceptional properties influenced by their 3D building blocks. For over a decade, researchers have focused on studying static mechanical properties such as stiffness, strength, mechanical resilience, energy absorption, and negative Poisson’s ratios.
However, the exploration of the dynamic properties of these materials has been significantly limited. Design principles can be directed towards frequency-dependent properties and resilience during high-strain-rate deformation, making them versatile for applications in lightweight impact resistance, acoustic wave-guiding, and vibration damping. Nevertheless, accessing dynamic properties at small scales remains challenging due to low throughput, destructive characterization, or the absence of established testing protocols.
Researchers from MIT have published an article in Nature developing a novel laser-based mass spectroscopy framework that can be used to characterize the dynamic response of 3D metamaterials. This will aid in developing metamaterials with the required properties optimized for specific functions.
This technique, termed Laser-induced resonant acoustic spectroscopy (LIRAS) comprises two distinct modules for pump and probe. In the pump module, a picosecond pulsed laser is directed at a three-dimensional sample with a thin chromium coating, absorbing the laser pulse and generating broadband acoustic waves. The resulting surface displacements on the sample are measured by the probe module, employing a phase-mask interferometer.
Mass Spectrometry for Situ Pyrolysis of 3D Printed Building Blocks for Nanoscale Materials
Novel printing techniques have made it feasible to create highly intricate micro- and nano-architectures with ease. Researchers have recently employed in situ scanning electron microscopy (SEM) and mass spectrometry in their research published in Advanced Functional Materials to examine the size-defining shrinkage step of 3D-printed microstruts, serving as building blocks for mechanical metamaterials. This allowed them to observe and quantify structural changes under actual isothermal conditions and in various surrounding atmospheres.
The researchers selected a range of micro strut structures with a length of 10 µm, incorporating springs and disc-shaped pedestals as decoupling supports for sample morphology and characterization. Protruding "ears" at both ends of the micro strut were utilized as tracking markers to more accurately determine the actual (projected) lengths during shrinkage when exposed to heating.
Mass spectrometry confirmed the formation of volatile byproducts, including CH4, H2O, CO, HCOH, CO2, etc., which degas from the printed structures during pyrolysis. For further In-situ pyrolysis study, mass spectroscopy along with SEM was used.
The findings indicated that the effective activation energy needed for pyrolysis-induced morphological shrinkage is approximately four times higher under vacuum conditions compared to a nitrogen atmosphere (2.6 eV vs. 0.5–0.9 eV, respectively). Additionally, a subtle enrichment of oxygen on the surfaces of the structures for pyrolysis in nitrogen was observed through a postmortem electron energy loss spectroscopy study.
In short, mass spectrometry is a fundamental tool for material characterization and synthesis.
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References and Further Reading
Chu, J. (2023). New laser setup probes metamaterial structures with ultrafast pulses. (Online). Available at:
https://news.mit.edu/2023/new-laser-setup-probes-metamaterial-structures-ultrafast-pulses-1115
Sun, Q. et. al. (2023). In Situ Pyrolysis of 3D Printed Building Blocks for Functional Nanoscale Metamaterials. Advanced Functional Materials, 2302358. Available at: https://doi.org/10.1002/adfm.202302358
Kai, Y. et al. (2023). Dynamic diagnosis of metamaterials through laser-induced vibrational signatures. Nature 623, 514–521. Available at: https://doi.org/10.1038/s41586-023-06652-x
Shao, T. et. al. (2022). A Stacked Plasmonic Metamaterial with Strong Localized Electric Field Enables Highly Efficient Broadband Light‐Driven CO2 Hydrogenation. Advanced Materials, 34(28), 2202367. Available at: https://doi.org/10.1002/adma.202202367
Comby-Zerbino, C. et. al. (2021). The emergence of mass spectrometry for characterizing nanomaterials. Atomically precise nanoclusters and beyond. Materials Advances, 2(15), 4896-4913. Available at: https://doi.org/10.1039/D1MA00261A
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