In situ imaging of amorphous intermediates during brucite carbonation in supercritical CO2

Armoring of MgO by a Passivation Layer Impedes Direct Air Capture of CO2

It has been proposed to use magnesium oxide (MgO) to separate carbon dioxide directly from the atmosphere at the gigaton level. We show experimental results on MgO single crystals reacting with the atmosphere for longer (decades) and shorter (days to months) periods with the goal of gauging reaction rates. Here, we find a substantial slowdown of an initially fast reaction as a result of mineral armoring by reaction products (surface passivation). In short-term experiments, we observe fast hydroxylation, carbonation, and formation of amorphous hydrated magnesium carbonate at early stages, leading to the formation of crystalline hydrated Mg carbonates. The preferential location of Mg carbonates along the atomic steps on the crystal surface of MgO indicates the importance of the reactive site density for carbonation kinetics. The analysis of 27-year-old single-crystal MgO samples demonstrates that the thickness of the reacted layer is limited to ∼1.5 μm on average, which is thinner than expected and indicates surface passivation. Thus, if MgO is to be employed for direct air capture of CO2, surface passivation must be circumvented.

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Water Film-Driven Brucite Nanosheet Growth and Stacking

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH)2] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH)2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH)2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH)2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

Supercritical CO2 Directional-Assisted Synthesis of Low-Dimensional Materials for Functional Applications

Supercritical CO2 (SC CO2 ), as one of the unique fluids that possess fascinating properties of gas and liquid, holds great promise in chemical reactions and fabrication of materials. Building special nanostructures via SC CO2 for functional applications has been the focus of intense research for the past two decades, with facile regulated reaction conditions and a particular reaction field to operate compared to the more widely used solvent systems. In this review, the significance of SC CO2 on fabricating various functional materials including modification of 1D carbon nanotubes, 2D materials, and 2D heterostructures is stated. The fundamental aspects involving building special nanostructures via SC CO2 are explored: how their structure, morphology, and chemical composition be affected by the SC CO2 . Various optimization strategies are outlined to improve their performances, and recent advances are combined to present a coherent understanding of the mechanism of SC CO2 acting on these functional nanostructures. The wide applications of these special nanostructures in catalysis, biosensing, optoelectronics, microelectronics, and energy transformation are discussed. Moreover, the current status of SC CO2 research, the existing scientific issues, and application challenges, as well as the possible future directions to advance this fertile field are proposed in this review.

Moisture-Induced Degradation of Quantum-Sized Semiconductor Nanocrystals through Amorphous Intermediates

Elucidating the water-induced degradation mechanism of quantum-sized semiconductor nanocrystals is an important prerequisite for their practical application because they are vulnerable to moisture compared to their bulk counterparts. In-situ liquid-phase transmission electron microscopy is a desired method for studying nanocrystal degradation, and it has recently gained technical advancement. Herein, the moisture-induced degradation of semiconductor nanocrystals is investigated using graphene double-liquid-layer cells that can control the initiation of reactions. Crystalline and noncrystalline domains of quantum-sized CdS nanorods are clearly distinguished during their decomposition with atomic-scale imaging capability of the developed liquid cells. The results reveal that the decomposition process is mediated by the involvement of the amorphous-phase formation, which is different from conventional nanocrystal etching. The reaction can proceed without the electron beam, suggesting that the amorphous-phase-mediated decomposition is induced by water. Our study discloses unexplored aspects of moisture-induced deformation pathways of semiconductor nanocrystals, involving amorphous intermediates.

4D nanoimaging of early age cement hydration

Despite a century of research, our understanding of cement dissolution and precipitation processes at early ages is very limited. This is due to the lack of methods that can image these processes with enough spatial resolution, contrast and field of view. Here, we adapt near-field ptychographic nanotomography to in situ visualise the hydration of commercial Portland cement in a record-thick capillary. At 19 h, porous C-S-H gel shell, thickness of 500 nm, covers every alite grain enclosing a water gap. The spatial dissolution rate of small alite grains in the acceleration period, ∼100 nm/h, is approximately four times faster than that of large alite grains in the deceleration stage, ∼25 nm/h. Etch-pit development has also been mapped out. This work is complemented by laboratory and synchrotron microtomographies, allowing to measure the particle size distributions with time. 4D nanoimaging will allow mechanistically study dissolution-precipitation processes including the roles of accelerators and superplasticizers.

Molecular-scale mechanisms of CO2 mineralization in nanoscale interfacial water films

The calamitous impacts of unabated carbon emission from fossil-fuel-burning energy infrastructure call for accelerated development of large-scale CO₂ capture, utilization and storage technologies that are underpinned by a fundamental understanding of the chemical processes at a molecular level. In the subsurface, rocks rich in divalent metals can react with CO₂, permanently sequestering it in the form of stable metal carbonate minerals, with the CO₂-H₂O composition of the post-injection pore fluid acting as a primary control variable. In this Review, we discuss mechanistic reaction pathways for aqueous-mediated carbonation with carbon mineralization occurring in nanoscale adsorbed water films. In the extreme of pores filled with a CO₂-dominant fluid, carbonation reactions are confined to angstrom to nanometre-thick water films coating mineral surfaces, which enable metal cation release, transport, nucleation and crystallization of metal carbonate minerals. Although seemingly counterintuitive, laboratory studies have demonstrated facile carbonation rates in these low-water environments, for which a better mechanistic understanding has come to light in recent years. The overarching objective of this Review is to delineate the unique underlying molecular-scale reaction mechanisms that govern CO₂ mineralization in these reactive and dynamic quasi-2D interfaces. We highlight the importance of understanding unique properties in thin water films, such as how water dielectric properties, and consequently ion solvation and hydration behaviour, can change under nanoconfinement. We conclude by identifying important frontiers for future work and opportunities to exploit these fundamental chemical insights for decarbonization technologies in the twenty-first century.