3d Transition Metal Oxides Research Articles

Orbital entanglement and the double d-shell effect in binary transition metal molecules.

Accurate modeling of transition metal-containing compounds is of great interest due to their wide-ranging and significant applications. These systems present several challenges from an electronic structure perspective, including significant multi-reference characters and many chemically relevant orbitals. A further complication arises from the so-called double d-shell effect, which is known to cause a myriad of issues in the treatment of first-row transition metals with both single- and multi-reference methods. While this effect has been well documented for several decades, a comprehensive understanding of its consequences and underlying causes is still evolving. Here, we characterize the second d-shell effect by analyzing the information entropy of correlated wavefunctions in a periodic series of 3d and 4d transition metal molecular hydrides and oxides. These quantum information techniques provide unique insight into the nuanced electronic structure of these species and are powerful tools for the study of weak and strong correlations in the transition metal d manifold.

Layered 3d Transition Metal‐Based Oxides for Sodium‐Ion and Lithium‐Ion Batteries: Differences, Links and Beyond

AbstractDue to their stable crystal framework, promising energy density, and structural versatility, layered 3d transition metal oxides have emerged as the preferred cathodes for lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs). While extensive research has individually addressed the lithium and sodium 3d transition metal layered oxides, the differences and interconnections between the two types of materials have largely been overlooked. Effectively utilizing these summaries is essential for driving innovative structural designs and inspiring new insights into the structure‐property relationships. This review comprehensively bridges this gap by meticulously examining the disparities and links in the behavior of the layered oxides upon Li+ and Na+ storage and transfer. Key aspects, including atomic and electronic structure, phase transition mechanisms, charge compensation mechanisms and electrochemical kinetics, are carefully summarized. The implications of these aspects on the battery cycle life, energy density, and rate capability are thoroughly discussed. Additionally, by leveraging the unique characteristics of each oxide structure, this review explores the interconnection between lithium and sodium layered oxides in depth. Finally, a concise perspective on future targets and direction of 3d layered oxides is deduced and proposed.

Renewable synthesis of MoO3 nanosheets via low temperature phase transition for supercapacitor application

2D transition metal oxides have created revolution in the field of supercapacitors due to their fabulous electrochemical performance and stability. Molybdenum trioxides (MoO3) are one of the most prominent solid-state materials employed in energy storage applications. In this present work, we report a non-laborious physical vapor deposition (PVD) and ultrasonic extraction (USE) followed by vacuum assisted solvothermal treatment (VST) route (DEST), to produce 2D MoO3 nanosheets, without any complex equipment requirements. Phase transition in MoO3 is often achieved at very high temperatures by other reported works. But our well-thought-out, robust approach led to a phase transition from one phase to another phase, for e.g., hexagonal (h-MoO3) to orthorhombic (α-MoO3) structure at very low temperature (90 °C), using a green solvent (H2O) and renewable energy. This was achieved by implementing the concept of oxygen vacancy defects and solvolysis. The synthesized 2D nanomaterials were investigated for electrochemical performance as supercapacitor electrode materials. The α-MoO3 electrode material has shown supreme capacitance (256 Fg−1) than its counterpart h-MoO3 and mixed phases (h and α) of MoO3 (< 50 Fg−1). Thus, this work opens up a new possibility to synthesize electrocapacitive 2D MoO3 nanosheets in an eco-friendly and energy efficient way; hence can contribute in renewable circular economy.

Oxygen Redox in Alkali-Ion Battery Cathodes

Current high-energy-density Li-ion batteries use stoichiometric Li 3d transition metal oxides as positive electrodes, which are conventionally described purely by transition-metal redox during routine operating windows. Their practical specific capacities (mAh/g) may be increased by widening their operational voltage window, using Li-excess compositions, or a combination of the two, both of which have shown increasing evidence of O participation in the charge-compensation mechanism. Understanding how this influences the electrochemical performance of these cathodes has been of great interest. Therefore, this review summarizes the current understanding of O participation in alkali-ion battery cathode charge compensation. Particular scrutiny is applied to the experimental observations and theoretical models used to explain the consequences of O participation in charge compensation. The charge-compensation mechanism of LiNiO2 is revisited to highlight the role of O hole formation during delithiation and is discussed within the wider context of Li-excess cathodes.

Scalable Production of 2D Non‐Layered Metal Oxides through Metal–Organic Gel Rapid Redox Transformation

Two‐dimensional (2D) nonlayered metal compounds with porous structure show broad application prospects in electrochemistry‐related fields due to their abundant active sites, open ions/electrons diffusion channels, and faradaic reactions. However, scalable and universal synthesis of 2D porous compounds still remains challenging. Here, inspired by blowing gum, a metal‐organic gel (MOG) rapid redox transformation (MRRT) strategy is proposed for the mass production of a wide variety of 2D porous metal oxides. Adequate crosslinking degree of MOG precursor and its rapid redox with NO3‐ are critical for generating gas pressure from interior to exterior, thus blowing the MOG into 2D carbon nanosheets, which further act as self‐sacrifice template for formation of oxides with porous and ultrathin structure. The versatility of this strategy is demonstrated by the fabrication of 39 metal oxides, including 10 transition metal oxides, one II‐main group oxide, two III‐main group oxides, 22 perovskite oxides, four high‐entropy oxides. As an illustrative verification, the 2D transition metal oxides exhibit excellent capacitive deionization (CDI) performance. Moreover, the assembled CDI cell could act as desalting battery to supply electrical energy during electrode regeneration. This MRRT strategy offers opportunities for achieving universal synthesis of 2D porous oxides with nonlayered structures and studying their electrochemistry‐related applications.

Scalable Production of 2D Non-Layered Metal Oxides through Metal-Organic Gel Rapid Redox Transformation.

Two-dimensional (2D) nonlayered metal compounds with porous structure show broad application prospects in electrochemistry-related fields due to their abundant active sites, open ions/electrons diffusion channels, and faradaic reactions. However, scalable and universal synthesis of 2D porous compounds still remains challenging. Here, inspired by blowing gum, a metal-organic gel (MOG) rapid redox transformation (MRRT) strategy is proposed for the mass production of a wide variety of 2D porous metal oxides. Adequate crosslinking degree of MOG precursor and its rapid redox with NO3 - are critical for generating gas pressure from interior to exterior, thus blowing the MOG into 2D carbon nanosheets, which further act as self-sacrifice template for formation of oxides with porous and ultrathin structure. The versatility of this strategy is demonstrated by the fabrication of 39 metal oxides, including 10 transition metal oxides, one II-main group oxide, two III-main group oxides, 22 perovskite oxides, four high-entropy oxides. As an illustrative verification, the 2D transition metal oxides exhibit excellent capacitive deionization (CDI) performance. Moreover, the assembled CDI cell could act as desalting battery to supply electrical energy during electrode regeneration. This MRRT strategy offers opportunities for achieving universal synthesis of 2D porous oxides with nonlayered structures and studying their electrochemistry-related applications.

Spin-orbital Jahn-Teller bipolarons

Polarons and spin-orbit (SO) coupling are distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons originate from strong electron-phonon interaction and are ubiquitous in polarizable materials featuring electron localization, in particular 3d transition metal oxides (TMOs). On the other hand, the relativistic coupling between the spin and orbital angular momentum is notable in lattices with heavy atoms and develops in 5d TMOs, where electrons are spatially delocalized. Here we combine ab initio calculations and magnetic measurements to show that these two seemingly mutually exclusive interactions are entangled in the electron-doped SO-coupled Mott insulator Ba2Na1−xCaxOsO6 (0 < x < 1), unveiling the formation of spin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller lattice activity, converts the Os 5d1 spin-orbital Jeff = 3/2 levels, characteristic of the parent compound Ba2NaOsO6 (BNOO), into a bipolaron 5d2 Jeff = 2 manifold, leading to the coexistence of different J-effective states in a single-phase material. The gradual increase of bipolarons with increasing doping creates robust in-gap states that prevents the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from d1 BNOO to d2 Ba2CaOsO6 (BCOO).

Proximity effects induced exchange bias and perpendicular net moment in SrIrO3

The magnetic proximity effect enables the incorporation of magnetic properties into non-magnetic materials, thereby achieving high-performance electronic devices. The 5d perovskite-oxide iridates provide a versatile platform for investigating competing electronic states due to their strong spin–orbit coupling. By proximity to conventional 3d transition-metal oxide ferromagnets, originally paramagnetic SrIrO3 is expected to exhibit exotic magnetic phenomena. In this study, we report the emergence of perpendicular net moment and an unprecedented exchange bias effect in SrIrO3 sandwiched between two ferromagnetic LaMnO3 layers. The magnetization measurements clearly demonstrate antiferromagnetic coupling in the heterostructures, resulting in a shift of the magnetic hysteresis loops. Furthermore, magneto-transport measurements provide evidence for the induction of a net moment along the perpendicular direction of the SrIrO3 film plane due to proximity effects. Both the magnetic pinning effect and perpendicular magnetization can be controlled by an external magnetic field. These findings expand the potential applications of iridate-related heterostructures in modern information storage fields such as skyrmions, spin–orbit torques, and antiferromagnet-based spintronics.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Ionomer-Dependent Oxygen Evolution Reaction in a Half-Cell and a Liquid Electrolyzer

Electrocatalytic oxygen evolution reaction (OER) is an important area of research due to its significance in water electrolysis, which contributes to the clean-energy storage and conversion techniques. OER exhibits irreversible and sluggish kinetics, which necessitates a high overpotential to achieve the desired reaction rate. In an alkaline medium, non-precious metals (such as Ni, Fe, Co) or metal-free materials (such as 3d-transition metal oxides and heteroatom-doped carbons) can replace expensive iridium and ruthenium oxides, which is otherwise required in an acidic medium to facilitate the reaction. However, developing new catalysts for alkaline OER is limited by the scaling relations among adsorption energetics of key intermediates (i.e., OOH*, OH*, and O*).The electrochemical reaction kinetics depends critically on the nature of the interface between catalyst and the electrolyte where an electrical double layer (EDL) forms during OER. The current understanding of the EDL structure involves the interfacial interactions between adsorbates and catalyst layer (CL) which can be modified to improve the reaction efficiency. Ionomers are essential components of the CL and are vital in uniformly dispersing catalyst particles in ink solution. The ionomers also act as binder for the catalyst particles in the CL, helping to improve the electrode’s durability and stability over time. In addition, despite the fact that ionomers affect the properties of EDL, the impact of ionomer on the reaction pathway is often overlooked in electrocatalytic research.In this study, we investigate the role of ionomer in the catalyst layer by comparing different polymer dispersions, including PTFE, Nafion, and commercial As-4. The results indicate that the Nafion-contained CL performs better than other polymer CLs in TF-RDE/RRDE, as it facilitates direct O*-O* coupling. In contrast, anionic ionomer demonstrates superior long-term stability in alkaline liquid electrolyzers, despite oxidization degradation.

Unveiling the morphological influence of nitrogen-doped carbonized wood anchored 3D transition metal oxide microarchitectures on catalytic activity: A trifunctional electrocatalyst for sensing and energy conversion

The rational construction and advancement of binder-free, self-standing, and cost-effective electrodes endowed with highly active trifunctional electrocatalysts represent a pivotal nexus between the research domains of energy and healthcare applications. However, the lack of proficient non-precious-metal trifunctional electrocatalysts bridging the aforementioned research domains remains a pinnacle barrier. Herein, we proposed distinct fabrication protocols to construct morphologically diverse three-dimensional (3D) CuO microarchitectures directly docked over the naturally inherited carbon channels of nitrogen-doped carbonized wood (NCW) (CuO@NCW) obtained by the carbonization of waste wood monoliths and explored the efficacy of different catalytic designs towards the trifunctional performances on non-enzymatic glucose sensing and glucose oxidation-assisted water splitting. Among the diverse binder-free engineered electrodes, CuO@NCW obtained via the facile electrodeposition (E) route (dubbed as CuO-E@NCW) demonstrated proficient electrocatalytic trifunctionality benefitting from the allied merits of the robustly anchored 3D porous dendritic architectures equipped with ample porosity and momentous interconnectivity. The as-fabricated CuO-E@NCW electrode as a glucose sensor displays two broad linear ranges between 0.01 and 0.5 mM and 0.5–4.0 mM with corresponding elevated sensitivities of 24.6 and 7.2 mA mM−1 cm−2, with a lower limit of detection of 0.0013 mM (S/N = 3). Besides, this best-performing electrode delivers a high current density of 50 mA cm−2 with a low cell voltage of 1.39 V in the glucose electrolysis system, which is 170 mV lower compared with stereotypical alkaline water splitting.

Anomalous negative resistance in SrTaO3 thin film: The nonuniform electronic state with possible superconductivity around 40 K

Transition metal oxides provide opportunities to induce and modulate the superconducting state, due to the delicate interplay between charge, spin, orbital, and lattice order. In this work, 5d transition metal oxide SrTaO3 films were deposited by pulsed laser deposition, and an anomalous negative resistance was observed. The negative resistance can be ascribed to the nonuniform conductivity, associated with nonuniform electronic states in the SrTaO3 thin films, as confirmed by conductive atomic force microscopy and x-ray photoelectron spectroscopy. Moreover, a sharp drop in the resistance together with a decrease in magnetization was observed at around 40 K. This might imply a possible superconducting state. The negative resistance can be well explained based on an equivalent circuit comprising normal and superconductive states, also supporting the possible superconducting state in SrTaO3 thin films. This work promotes the exploring and revealing of the mechanism in tantalates superconductivity.

First-principles investigations on the electronic structure, optical properties, and giant magnetic-optical Kerr effect in double-perovskite Ba2NiBO6 (B = Os, Ir)

The hybrid 3d–5d transition-metal oxides have been frequently studied for their novel electronic and magnetic properties, in which the strong spin-orbit coupling (SOC) effect is added to the interplay among charge, spin, and orbital degrees of freedom. Recently, perfectly ordered 3d–5d double-perovskite oxides of Ba2NiOsO6 and Ba2NiIrO6 were synthesized: Ba2NiOsO6 was measured to be a rare ferromagnetic semiconductor and predicted to have a giant magneto-optical Kerr effect (MOKE); and Ba2NiIrO6 showed a competition between antiferromagnetic and ferromagnetic interactions. In this work, we systematically investigated the magnetic, electronic, and optical properties, as well as the MOKE of Ba2NiBO6 (B = Os, Ir) using density functional theory. The long-ranged B–B interaction was found to play a key role in determining the magnetism of the compounds. For Ba2NiOsO6, the experimentally observed semiconductive nature was reproduced only by taking into account both the electron correlation and SOC effect. In Ba2NiIrO6, however, the SOC effect on 5d Ir was weak. Owing to the d→d or p→d interband transitions, both materials had a strong absorption in the ultraviolet–visible spectral region. In particular, the absorption spectrum of Ba2NiIrO6 covered almost the entire visible region. They also exhibited considerable MOKE in the visible region. The maximum of Kerr rotation for Ba2NiOsO6 and Ba2NiIrO6 could reach 5.3° and 4.1°, respectively. The characteristics of the Kerr spectra were primarily dominated by the off-diagonal optical conductivity, which reflects the contributions of band exchange splitting and SOC to MOKE. The Ba2NiBO6 (B = Os, Ir) are very promising optoelectronic and magneto-optical materials.

Promising cathode material MnO2/CoO2 heterostructure for the Li and Na ion battery: A computational study

Although two-dimensional (2D) transition metal oxide monolayers have shown potential for applications in metal-ion batteries, the heterostructures of this family are yet to be studied in details for energy storage applications. In this work, we have made the heterostructure by taking half-metallic ferromagnetic 2D transition metal oxide CoO2 and semiconducting MnO2 monolayers and demonstrated its potential application as a cathode material in lithium and sodium-ion batteries by performing first-principles calculations using density functional theory approach. We have systematically studied the electronic structure and stability of the MnO2/CoO2 heterostructure. We have carefully examined the adsorption and diffusion behavior of metal ions (lithium and sodium). Our structure has offered a maximum adsorption energy of −3.84 eV, which is greater than the adsorption energy of individual monolayers. We found that the lowest diffusion barrier is 0.4 eV for lithium ion and 0.32 eV for sodium ion. Also, our system has shown a maximum open circuit voltage of 2.18 V for lithium ion battery and 0.32 V for Na-ion battery. The specific capacity is found to be 584 mAh g−1 for lithium ion and 529 mAh g−1 for sodium ion battery. These findings can serve as a proof that the MnO2/CoO2 heterostructure should be considered as a potential cathode for lithium- and sodium-ion batteries.

Computational Search for a Descriptor of Electrocatalysis in Transition Metal-Oxides

Currently, the most effective electrocatalysts for oxygen reduction (ORR) and oxygen evolution reactions (OER) are comprised of noble metal oxides, which bring issues of high cost and scarcity. Discovering earth-abundant alternatives is challenging due to the costly and time-consuming experiments. High-throughput atomistic simulation of catalysts can be an efficient route for enabling the better design and discovery of new electrocatalysts. The efficient use of this approach relies on an activity descriptor that can capture the qualitative and quantitative catalytic trends. In the past, multiple descriptors based on electronic properties have been proposed either reaction-specific or system specific. Therefore, we need a descriptor, which can capture the bonding strength accurately for any sort of reaction or system.In this work, we study the trends of O*/OH* adsorption energies for a wide range of pure metal oxide polymorphs[1] namely rocksalt (MO), corundum (M2O3), rutile (MO2), pentoxide (M2O5) and MO3 in all conceivable oxidation states i.e., +2, +3, +4, +5 and +6 respectively. The main drivers of the adsorption energy are the extent of filling of the bonding and anti-bonding orbitals, along with the spin-dependent coupling strength between the metal-d and oxygen-2p atomic orbitals. This property can be easily captured with the help of crystal orbital Hamiltonian population (COHP). We have observed a strong correlation between the integrated COHP I(COHP) of the bulk system and the adsorption energy, which indicates expensive surface-level electronic calculations can be replaced by bulk-level computations. However, this correlation does not hold on two classes of systems: (i) d0 systems and (ii) systems with significant surface distortion during the reaction. We have considered a different approach to address these systems, where instead of a single bond ICOHP, we account for the summation of the ICOHP around the active site.In search of promising catalysts, we also report tetrahedral systems for 3d, 4d and 5d transition metal oxides, i.e., zinc blende, wurtzite and square planar systems. This will broaden the horizon of possible catalysts along the octahedral systems. We correlate the bulk ICOHP with the surface reactivity of these systems, which highlights the applicability of ICOHP as a universal descriptor across a series of metal oxide systems and guide the design of novel catalysts.This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.[1] "Unravelling Electronic Trends in O* and OH* Surface Adsorption in the MO2 Transition-Meta Oxide Series"; Benjamin M. Comer, Jiang Li, Frank Abild-Pedersen, Michal Bajdich, Kirsten Winther, 2022, (ACS J. Phys. Chem C.,)[2] “Predicting O* and OH* Surface Adsorption on Transition Metal Oxides from Bulk Structure”; Benjamin M. Comer, Neha Bothra, Jaclyn Lunger, Frank Abild-Pedersen, Michal Bajdich, Kirsten Winther, Manuscript under preparation.

(Invited) Electrocatalysis and Surface Chemistry in Lithium Sulfur Batteries

This talk will focus on the application of electrocatalysis and surface/interface chemistry to solve the cycling stability problem of lithium sulfur batteries, which are a type of future high energy rechargeable battery that can deliver a few times more energy than the prevalent lithium ion batteries of the same mass. One critical problem is the adsorption of lithium polysulfides, the key reaction intermediates that control the electrochemical performance of the sulfur cathode. We find that 3d transition metal oxides effectively bind polysulfides via oxygen-lithium or metal-sulfur bonding, whereas metal phosphides and chalcogenides only do so with their oxidized surfaces. When the electrolyte/sulfur ratio is lowered for even higher energy density (lean electrolyte condition), we discover that catalysis is needed to overcome the potential limiting reaction barrier associated with the conversion of reaction intermediates. With this understanding, we have developed high capacity sulfur cathode materials achieving hundreds of stable charging-discharging cycles. This surface/interface chemistry approach also enables us to analyze the cycling behavior of the lithium/sodium metal anode and to develop electrode/electrolyte interphases for improving the charging-discharging efficiency and cycling stability, including lithium metal electrodes working with solid-state electrolyte.

Nonlinear optical properties of 3d transition metal oxides and applications in fiber lasers: Review and prospect

Transition metal oxides (TMOs) are distinguished by their thermal and chemical stability as well as excellent optical, electrical, and magnetic properties. Moreover, the 3[Formula: see text]-orbital electrons of the transition metal ions in TMOs have a strong [Formula: see text] Coulomb interaction, enabling them to exhibit a variety of unusual physical phenomena, dramatically different from conventional metals and semiconductors. Additionally, TMOs possess excellent nonlinear optical properties, such as third-order nonlinear response and saturation absorption, which make them broadly applicable for ultrafast photonics devices. Herein, a review is presented of the nonlinear optical properties of typical TMOs, as well as their employment as saturable absorbers in near-infrared pulsed fiber lasers in recent years. Finally, some challenges and promising prospects for nonlinear ultrafast photonic devices based on TMOs are discussed.

Efficient charge to spin conversion in iridium oxide thin films

Many 5d transition metal oxides have a unique electronic structure, where the density of states near the Fermi level is dominated by only 5d electrons with strong spin–orbit coupling. IrO2, a Dirac nodal line semi-metal, is the simplest of these oxides. The presence of 5d electrons and gap opening of Dirac nodal lines via strong spin–orbit coupling allows for the hybridization of the 5d electrons of the oxide with the itinerant d electrons of a ferromagnet, while simultaneously increasing the intrinsic spin Hall effect. We report large charge-to-spin conversion in thin films of this material using spin-torque ferromagnetic resonance experiments. By independently performing line shape analysis and linewidth modulation experiments, we conclusively determine the spin Hall angle of optimized IrO2 films to be ∼8 times larger than that of Pt.

In-situ Raman unveiled Nb-O-bond-dependency selectivity for methanol electro-oxidation at high current density

Selective methanol oxidation reaction (MOR) to value-added formate can maximize the energy utilization by replacing the high onset-potential and sluggish oxygen evolution reaction. However, to date, only first-row 3d transition metal oxides exhibit the capacity of selective MOR, motivating the design of new kinds of electrocatalysts to further enhance the performance. Here we develop amorphous Nb2O5 as an efficient electrocatalyst, for the first time, for selective MOR to formate, with a low operating potential of 1.47 V vs. RHE to achieve the industry-relevant current densities (>100 mA cm−2) with a formate Faradaic efficiency of ∼100 %. We demonstrate, via operando Raman measurements, that this superior electrochemical MOR capacity originates from the in-situ induced short Nb-O bonds from the amorphous structure. Besides, the interface between amorphous Nb2O5 and nickel foam could also faciltate the MOR. This work may provide insights for designing novel transition metal oxides as efficient catalysts for selective MOR.

Angstrom-confined Electrochemical Synthesis of Sub-unit Cell non van der Waals 2D Metal Oxides.

Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-van der Waals (vdW) materials. Thicknesses below few nm have not been reported yet, posing the question how thin can non-vdW materials be electrochemically synthesized? This is important as materials with (sub-) unit cell thickness often show remarkably different properties compared to their bulk form or thin films of several nm thickness. Here, we introduce a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels to obtain a centimeter-scale network of atomically thin (< 4.3 Å) 2D-transition metal oxides (2D-TMO). The angstrom-confinement provides a thickness limitation, forcing sub-unit cell growth of 2D-TMO with oxygen and metal vacancies. We showcase that Cr2 O3 , a material without significant catalytic activity for OER in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit cell form. Our method displays the high activity of sub-unit cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. We show that while retaining the advantages of bottom-up electrochemical synthesis like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub-unit cell dimensions. This article is protected by copyright. All rights reserved.