Dissolution Behavior of Isolated and Aggregated Hematite Particles Revealed by in Situ Liquid Cell Transmission Electron Microscopy

Observing the Evolution of Metal Oxides in Liquids

Metal oxides with diverse compositions and structures have garnered considerable interest from researchers in various reactions, which benefits from transmission electron microscopy (TEM) in determining their morphologies, phase, structural and chemical information. Recent breakthroughs have made liquid-phase TEM a promising imaging platform for tracking the dynamic structure, morphology, and composition evolution of metal oxides in solution under work conditions. Herein, this review introduces the recent advances in liquid cells, especially closed liquid cell chips. Subsequently, the recent progress including particle growth, phase transformation, self-assembly, core-shell nanostructure growth, and chemical etching are introduced. With the late technical advances in TEM and liquid cells, liquid-phase TEM is used to characterize many fundamental processes of metal oxides for CO2 reduction and water-splitting reactions. Finally, the outlook and challenges in this research field are discussed. It is believed this compilation inspires and stimulates more efforts in developing and utilizing in situ liquid-phase TEM for metal oxides at the atomic scale for different applications.

Crystal dissolution by particle detachment

Crystal dissolution, which is a fundamental process in both natural and technological settings, has been predominately viewed as a process of ion-by-ion detachment into a surrounding solvent. Here we report a mechanism of dissolution by particle detachment (DPD) that dominates in mesocrystals formed via crystallization by particle attachment (CPA). Using liquid phase electron microscopy to directly observe dissolution of hematite crystals - both compact rhombohedra and mesocrystals of coaligned nanoparticles - we find that the mesocrystals evolve into branched structures, which disintegrate as individual sub-particles detach. The resulting dissolution rates far exceed those for equivalent masses of compact single crystals. Applying a numerical generalization of the Gibbs-Thomson effect, we show that the physical drivers of DPD are curvature and strain inherently tied to the original CPA process. Based on the generality of the model, we anticipate that DPD is widespread for both natural minerals and synthetic crystals formed via CPA.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface

Understanding precipitation formation at lanthanum hydroxide (La(OH)₃) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)₃ nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)₃ nanoparticle surfaces. Low-crystalline La(OH)₃ mostly transformed and high-crystalline ones partly converted to LaPO₄ precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)₃ nanoparticles to LaPO₄ precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Natural nanocolloids regulate the fate and phytotoxicity of hematite particles in water

Hematite (the most abundant iron oxide polymorph) is widely detected in the water environment and has attracted considerable attention. Natural nanocolloids (Ncs) exist ubiquitously in surface waters and play critical roles in biogeochemical processes. However, the influences of Ncs on the fate and phytotoxicity of hematite remain unknown. In this study, the infrared absorption spectra coupled with two-dimensional correlation spectroscopy analysis reveal that the specific binding interactions between Ncs and hematite primarily occur via hydrophilic effects and π-π interactions with an increase in the Ncs contact time. Moreover, binding with Ncs slightly promoted the aggregation rates of hematite particles in the BG-11 medium. Interestingly, Ncs remarkably mitigate the phytotoxicity (e.g., growth inhibition, oxidative stress, and mitochondrial toxicity) of nanosized and submicrosized hematite particles to Chlorella vulgaris after a 96 h exposure. The integrating metabolomic and transcriptomic analysis reveals that the regulated urea cycle, amino acids, and fatty acid-related metabolites (e.g., urea, serine, glutamate, and hexadecenoic acid) and genes (e.g., ACY1, CysC, and GLA) contribute to persistent phytotoxicity. This study provides new insights into the roles and mechanisms of natural Ncs in regulating the environmental risk of iron oxide minerals in aqueous media.

Radiolysis and Radiation-Driven Dynamics of Boehmite Dissolution Observed by In Situ Liquid-Phase TEM

Over the last several decades, there have been several studies examining the radiation stability of boehmite and other aluminum oxyhydroxides, yet less is known about the impact of radiation on boehmite dissolution. Here, we investigate radiation effects on the dissolution behavior of boehmite by employing liquid-phase transmission electron microscopy (LPTEM) and varying the electron flux on the samples consisting of either single nanoplatelets or aggregated stacks. We show that boehmite nanoplatelets projected along the [010] direction exhibit uniform dissolution with a strong dependence on the electron dose rate. For nanoplatelets that have undergone oriented aggregation, we show that the dissolution occurs preferentially at the particles at the ends of the stacks that are more accessible to bulk solution than at the others inside the aggregate. In addition, at higher dose rates, electrostatic repulsion and knock-on damage from the electron beam causes delamination of the stacks and dissolution at the interfaces between particles in the aggregate, indicating that there is a threshold dose rate for electron-beam enhancement of dissolution of boehmite aggregates.

In Situ Visualization on Surface Oxidative Corrosion with Free Radicals: Black Phosphorus Nanoflake as an Example

Free radicals exert a significant impact on the fate of redox-active substances and play a crucial role in the surface corrosion of solid in environment. Dynamic visualization on the response of the surface to the free radicals at nanoscale is essential to explore the mechanism. Environmental transmission electron microscopy will be a powerful tool for dynamic changes of the interface redox process of solid surface with electron beams induced free radicals, to simulate the redox process of a solid in the environment. Black phosphorus (BP), an environment-sensitive material, is selected as an example to visualize the degradation pathways with environmental transmission electron microscopy. The distribution of the corrosion initiation points, formation and growth of corrosion areas, and the eventual splintering and disappearance of BP nanoflakes are recorded vividly. In situ results are substantiated by the ex situ experiments and density functional theory (DFT) calculations. Results show that degradation originates at the edges and defect structures when the humidity reaches high enough. The microscopic structural oxidative etching of solid surface with radicals in natural light is simulated with radicals produced by electron beam irradiation on suspending medium O₂ and H₂O for the first time. This method will offer unprecedented details and valuable insights into the mechanism involved in the oxidative etching with natural light.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

The studies on wet chemical etching via in situ liquid cell TEM

Wet chemical etching is a widely used process to fabricate fascinating nanomaterials, such as nanoparticles with precisely controlled size and shape. Understanding the etching mechanism and kinetic evolution process is crucial for controlling wet chemical etching. The development of in situ liquid cell transmission electron microscopy (LCTEM) enables the study on wet chemical etching with high temporal and spatial resolutions. However, there still lack a detailed literature review on the wet chemical etching studies by in situ LCTEM. In this review, we summarize the studies on wet etching nanoparticles, one-dimensional nanomaterials and nanoribbons by in situ LCTEM, including etching rate, anisotropic etching, morphology evolution process, and etching mechanism. The challenges and opportunities of in situ LCTEM are also discussed.

Developments and advances in in situ transmission electron microscopy for catalysis research

Recent developments and advances in in situ TEM have raised the possibility to study every step during the catalysts' lifecycle. This review discusses the current state, opportunities and challenges of in situ TEM in the realm of catalysis.

Further insights into the Fe(ii) reduction of 2-line ferrihydrite: a semi in situ and in situ TEM study

The biotic or abiotic reduction of nano-crystalline 2-line ferrihydrite (2-line FH) into more thermodynamically stable phases such as lepidocrocite-LP, goethite-GT, magnetite-MG, and hematite-HT plays an important role in the geochemical cycling of elements and nutrients in aqueous systems. In our study, we employed the use of in situ liquid cell (LC) and semi in situ analysis in an environmental TEM to gain further insights at the micro/nano-scale into the reaction mechanisms by which Fe(ii)_(aq) catalyzes 2-line FH. We visually observed for the first time the following intermediate steps: (1) formation of round and wire-shaped precursor nano-particles arising only from Fe(ii)_(aq), (2) two distinct dissolution mechanisms for 2 line-FH (i.e. reduction of size and density as well as breakage through smaller nano-particles), (3) lack of complete dissolution of 2-line FH (i.e. "induction-period"), (4) an amorphous phase growth ("reactive-FH/labile Fe(iii) phase") on 2 line-FH, (5) deposition of amorphous nano-particles on the surface of 2 line-FH and (6) assemblage of elongated crystalline lamellae to form tabular LP crystals. Furthermore, we observed phenomena consistent with the movement of adsorbate ions from solution onto the surface of a Fe(iii)-oxy/hydroxide crystal. Thus our work here reveals that the catalytic transformation of 2-line FH by Fe(ii)_(aq) at the micro/nano scale doesn't simply occur via dissolution-reprecipitation or surface nucleation-solid state conversion mechanisms. Rather, as we demonstrate here, it is an intricate chemical process that goes through a series of intermediate steps not visible through conventional lab or synchrotron bulk techniques. However, such intermediate steps may affect the environmental fate, bioavailability, and transport of elements of such nano-particles in aqueous environments.

Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

Uranium (U) is a ubiquitous element in the Earth's crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 as nanocrystals (~1-2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO2 nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.

Benefits at the nanoscale: a review of nanoparticle-enabled processes favouring microbial growth and functionality

Nanoparticles are ubiquitous and co-occur with microbial life in every environment on Earth. Interactions between microbes and nanoparticles impact the biogeochemical cycles via accelerating various reaction rates and enabling biological processes at the smallest scales. Distinct from microbe-mineral interactions at large, microbe-nanoparticle interactions may involve higher levels of active recognition and utilization of the reactive, changeable, and thereby 'moldable' nano-sized inorganic phases by microbes, which has been given minimal attention in previous reviews. Here we have compiled the various cases of microbe-nanoparticle interactions with clear and potential benefits to the microbial cells and communities. Specifically, we discussed (i) the high bioavailabilities of nanoparticles due to increased specific surface areas and size-dependent solubility, with a focus on environmentally-relevant iron(III) (oxyhydr)oxides and pyrite, (ii) microbial utilization of nanoparticles as 'nano-tools' for electron transfer, chemotaxis, and storage units, and (iii) speculated benefits of precipitating 'moldable' nanoparticles in extracellular biomineralization. We further discussed emergent questions concerning cellular level responses to nanoparticle-associated cues, and the factors that affect nanoparticles' bioavailabilities beyond size-dependent effects. We end the review by proposing a framework towards more quantitative approaches and by highlighting promising techniques to guide future research in this exciting field.

Kinetics and Mechanisms of Protein Adsorption and Conformational Change on Hematite Particles

Adsorption kinetics and conformational changes of a model protein, bovine serum albumin (BSA, 0.1, 0.5, or 1.0 g/L), on the surface of hematite (α-Fe₂O₃) particles in 39 ± 9, 68 ± 9, and 103 ± 8 nm, respectively, were measured using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. As the particle size increases, the amount of adsorbed BSA decreases, but the loss in the helical structure of adsorbed BSA increases due to the stronger interaction forces between adsorbed BSA and the larger particles. On 39 or 68 nm hematite particles, refolding of adsorbed BSA can be induced by protein-protein interactions, when the protein surface coverage exceeds certain critical values. Two-dimensional correlation spectroscopy (2D-COS) analysis of time-dependent ATR-FTIR spectra indicate that the increase in the amount of adsorbed BSA occurs prior to the loss in the BSA helical structure in the initial stage of adsorption processes, whereas an opposite sequence of the changes to BSA conformation and surface coverage is observed during the subsequent refolding processes. Desorption experiments show that replacing the protein solution with water can quench the refolding, but not the unfolding, of adsorbed BSA. A kinetic model was proposed to quantitatively describe the interplay of adsorption kinetics and conformational change, as well as the effects of particle size and initial protein concentration on the rate constants of elementary steps in protein adsorption onto a mineral surface.