New opportunities for emerging two-dimensional metastable-phase noble metal oxides in acidic electrocatalytic water splitting

Abstract

Metastable-phase materials possess unique structures, high Gibbs free energy, abundant active sites, and adjustable physicochemical properties, making them ideal candidates for optimizing electrocatalysis. As metal oxides are stable under harsh reaction conditions, by controlling the morphology, defects and phase structure of the material, the surface electronic structure of metal oxides can be adjusted to a great extent, and their catalytic performance can be optimized. As a novel addition to the 2D material family, metastable-phase noble metal oxides exhibit significant promise for catalytic reactions. Here, the latest research progress and advantages of 2D metastable-phase noble metal oxides are reviewed, and their application in acidic electrocatalytic water splitting is presented. Finally, the challenges associated with 2D metastable-phase noble metal oxides and future perspectives are discussed.

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Qun Wang

Qun Wang is currently a graduate student at the Institute of Functional Nano & Soft Materials, Soochow University. He received his bachelor's degree from Yantai University in China in 2022. His current main research interests are the preparation of two-dimensional metastable phase materials and their catalytic applications.

Mingwang Shao

Mingwang Shao is a professor at the Institute of Functional Nano & Soft Materials, Soochow University. He received his PhD degree in chemistry from the University of Science and Technology of China in 2003. His current research interests are inorganic materials and green catalysis.

Qi Shao

Dr. Qi Shao is currently an associate professor at the College of Chemistry, Chemical Engineering and Materials Science, Soochow University. She received her PhD degree in Applied Physics from the City University of Hong Kong in 2016. Her current research interests focus on the metastable-phase catalysts for electrochemical applications.

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1. Introduction

Two-dimensional (2D) nanosheets, consisting of limiting stacked atomic layers with a number less than 10, have attracted extensive attention due to their special physical and chemical properties.^1–3 Since the discovery of graphene, due to its large specific surface area, excellent mechanical properties and quantum Hall effect and outstanding chemical reactivity,^4–7 it has triggered a wave of research on new 2D nanomaterials, such as metal alkenes,⁸ transition metal dichalcogenides (TMDs),^9,10 transition metal carbides,^11,12 nitrides¹³ and group IIIA/IVA monochalcogenides.¹⁴ These atomically thin materials have a wide range of applications in energy storage,^15,16 optoelectronic devices,^17–20 sensors^21–23 and catalysis.^24–26

Hydrogen has emerged as a promising alternative to fossil fuels, offering a clean and renewable energy source. Water splitting electrolysis is an effective method for energy conversion and storage, enabling the environmentally friendly production of hydrogen.²⁴ In the field of renewable energy storage via water electrolysis, proton exchange membrane (PEM) electrolyzers offer several advantages over alkaline electrolyzers. These benefits include lower ohmic losses, improved voltage efficiency, higher gas purity, more compact designs, increased current densities and faster response times. While alkaline electrolyzers can use non-noble metals like Fe, Ni, and Co, the hydrogen evolution reaction at the cathode is significantly limited under alkaline conditions, which negatively impacts the kinetics of the electrolysis process.²⁷ As a result, acidic environments for water electrolysis are seen as more promising. Currently, the use of noble-metal catalysts remains necessary for effective catalysis under acidic conditions, underscoring the urgent need for the development of efficient noble-metal catalysts.

At present, cathodic platinum-based catalysts (Pt/C) and anodic iridium–ruthenium-based catalysts (IrO₂ and RuO₂) are regarded as the most excellent materials for the electrocatalytic production of water.^28–30 Both synthesizing metastable phases and increasing the specific surface area of materials by controlling crystal structures and morphologies to enhance catalytic reaction efficiency are feasible methods. As an emerging class of layered 2D oxides, 2D metal oxides exhibit superior properties and applications compared to traditional bulk oxides due to their ultra-thin nature, which allows most atoms to be in contact with the surface.³¹ Furthermore, in contrast to the inherent thermodynamic instability exhibited by most 2D transition metal chalcogenides in oxidative environments, metal oxides demonstrate greater stability under harsh reaction conditions.³¹

In spite of this, the search for new 2D materials with higher activity and stability is still ongoing. This is because 2D materials have uniform exposed lattice planes compared to nanoparticles, which makes them an ideal interface for electrocatalysis.³² In addition, the crystal phase has become an important structural parameter to determine the function of materials in addition to their composition, shape and size.^33–39 For example, the metastable-phase 1T MoS₂ showed excellent performance in the electrocatalytic hydrogen evolution reaction compared to the conventional phase MoS₂.⁴⁰ Metastable phases have a unique atomic arrangement, which makes them have significant differences in electronic properties and surface environments compared with stable ones.⁴¹ Therefore, metastable phases often show better electrochemical catalytic behavior than stable ones.⁴² As one of the representatives of new nanomaterials, the magical combination of metastable-phase metal oxides and 2D materials produces incredible synergies. The large specific surface area of 2D materials uniformly exposes the highly active surface lattice of metastable phases, providing numerous surface active sites and unique electronic states.^43–48

This concept review presents the latest progress in 2D metastable-phase noble metal oxides. First, the concept of metastable phases is introduced, followed by a summary of five typical metastable-phase noble metal oxides synthesized by us, including 1T-IrO₂, 3R-IrO₂, IrO₂ nanoribbons (NRs), 1T-PtO₂, trigonal-RhO₂ and 1T-Ru_0.5Ir_0.5O₂. Finally, we discuss the application of acidic electrocatalytic water splitting and present the prospects for the development of 2D metastable-phase noble metal oxides.

2. 2D metastable-phase noble metal oxides

2.1. The development of 2D metastable-phase noble metal oxides

With the in-depth study of 2D materials, researchers have found that in addition to morphology and composition, the atomic arrangement and bonding mode of 2D platforms will have a significant impact on the physical and chemical properties of the material.^48,49 Studies have shown that metastable-phase 2D materials can significantly improve the electrocatalytic performance and will become an ideal substitute for precious metal catalysts in the future.^50,51

The growth of materials is mainly controlled by both dynamic and thermodynamic factors. Therefore, by finely tuning the experimental conditions, various materials can be controlled in a highly controllable manner. When the formation of the final structure of materials is driven by the kinetic control of the synthesis process, a thermodynamically unstable structure, named a metastable phase, can be achieved.⁵²

The representative 2D metastable-phase noble metal oxides are first synthesized using the molten-alkali mechanochemical method. This method successfully synthesizes highly crystalline 1T-IrO₂, 1T-PtO₂ and 1T-Ru_0.5Ir_0.5O₂ by combining mechanochemical treatment with high temperature and strong alkaline conditions (Fig. 1a and b).^44,45,48 Furthermore, the synthesis of 3R-phase IrO₂ was achieved using the microwave-assisted mechano-thermal method (Fig. 1c)⁴⁶ and the synthesis of trigonal RhO₂ using the radio frequency-assisted molten-alkali method (Fig. 1d).⁴⁷

Fig. 1 Schematic diagram of the alkali-assisted mechano-thermal method. (a) Molten alkali force with heat/alkali/mechanochemical synergy. Reproduced from ref. 44 with permission from Springer Nature, copyright 2021. (b) Mechanochemical synthesis unit of molten alkali. Reproduced from ref. 44 with permission from Springer Nature, copyright 2021. (c) Microwave-assisted alkali melt mechanochemical synthesis device. Reproduced from ref. 46 with permission from Elsevier, copyright 2021. (d) Radio-frequency-assisted alkali melt mechanochemical synthesis device. Reproduced from ref. 47 with permission from Springer Nature, copyright 2022.

2.2. Structures and properties

Metastable phases have a unique atomic arrangement, which gives them significantly different electronic properties and surface environments compared to stable ones.

First, 2D materials possess a range of advantages due to their near-atomic thickness, including an exceptionally high surface area, outstanding mechanical strength, and the ease of performance tuning through surface modification and functionalization.⁴⁴ Due to the inherent advantages of 2D materials, 2D metastable-phase noble metal oxides possess a significantly large specific surface area, thereby exposing a majority of atoms and facilitating their participation in catalytic reactions (Fig. 2a).⁵³ Furthermore, the changes in the electrochemical reaction mechanism can be induced by the modification of metastable-phase noble metal oxides. For example, Yu et al. reported the in situ growth of cerium oxide (CeO₂) to modify the in-plane strain of 2D metastable-phase 1T-IrO₂. The strain induced by CeO₂ in 1T-IrO₂ directly leads to the generation of O₂ through an *O–*O radical coupling mechanism, which is fundamentally different from the adsorption-based evolution mechanism observed in traditional pure 1T-IrO₂.⁵⁴ Metastable phases are in a thermodynamically unstable state. In particular, metastable phases have a higher Gibbs free energy compared with stable phase materials. A high energy barrier ensures their metastability and prevents the transformation of metastable phases into stable states (Fig. 2b). Therefore, compared to stable phases, these materials are more likely to exhibit highly active electrochemical catalytic surfaces.⁵⁵

Fig. 2 The advantages of 2D metastable-phase noble metal oxides. (a) Large surface area. (b) High Gibbs free energy. (c) Different coordination environment. (d) Hopping conduction. (e) Magnetic property.

The atomic coordination environment greatly affects the performance of catalysts.⁵⁶ The coordination number of the central metal atom in a catalyst can influence its electronic structure and reactivity. For example, metal atoms with higher coordination numbers may exhibit stronger electron-donating capabilities, thereby enhancing catalytic activity.^57,58 Additionally, the geometric configuration of the catalyst (such as planar, tetrahedral, or octahedral) can affect how reactants approach each other and the reaction pathways. Certain geometric configurations may be more favorable for specific reactions to occur.^59,60 2D metastable-phase noble metal oxides have unique bonding patterns due to different atomic fillings compared to their stable phases.⁶¹ For example, the [IrO₆] octahedra in the 1T phase of iridium dioxide (1T-IrO₂) are connected by edge sharing. In contrast, the stable rutile structure of iridium dioxide (rutile-IrO₂) is composed of sharing [IrO₆] octahedra with a mixed edge-sharing and corner-sharing mode (Fig. 2c).⁴⁴ The novel structure allows 2D metastable-phase noble metal oxides to exhibit excellent catalytic performance. Furthermore, the connectivity offered by the shared edges promotes electronic transfer, considering that electron exchange predominantly occurs at the surface of layered materials.⁶² This facilitates the polarization of conducting orbitals along the surface, potentially enhancing catalytic performance (Fig. 2d).

In addition, the application of a magnetic field can enhance the catalytic performance by adjusting the electronic properties of the catalyst. The formation of the O–O bond in O₂ must adhere to the principle of spin conservation, which makes the parallel spin alignment of oxygen intermediates at the active site crucial for achieving high catalytic activity. Theoretically, the effects of such spin alignment can be enhanced by applying a magnetic field to magnetic catalysts.⁶³ Additionally, some studies have also explored the enhancing effect of magnetic fields on the HER electron effects.⁶⁴ The arrangement of crystal structures can influence the magnetic properties of materials. Exploring 2D single crystals of van der Waals noble metal oxide with a long-range magnetic order is an urgent task. We reported a representative large-sized single-crystalline rhodium trioxide, which exhibits robust ferromagnetism at elevated temperatures (Fig. 2e).⁶⁵

2.3. Structural characterization of 2D metastable-phase noble metal oxides

The crystal structure of 2D metastable-phase noble metal oxides markedly differs from that of traditional oxides, highlighting the need to summarize the characterization methods specific to 2D metastable-phase noble metal oxides.

2.3.1. X-ray diffraction. X-ray diffraction (XRD) is the most direct technique for identifying crystal phases and analyzing the crystal structure of materials. The XRD peaks of metastable-phase materials are significantly different from those of stable phases. By analyzing the XRD data, the lattice constant of the 2D metastable-phase noble metal oxides in the c direction can be obtained.⁴⁴

2.3.2. High-resolution transmission electron microscopy. High-resolution transmission electron microscopy (HRTEM) can observe microscopic features such as crystal structure, defects, and phase interfaces within a material, while aberration-corrected electron microscopy offers atomic-level resolution, which is crucial for measuring atomic spacing and determining lattice constants in the a and b directions.⁴⁵

2.3.3. X-ray adsorption spectroscopy. X-ray absorption spectroscopy (XAS) is a powerful technique for analyzing the phase structure and coordination information of solid materials.^46,47 XAS comprises two main components: X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES region provides crucial insights into oxidation states, coordination numbers, and local symmetries of elements within the sample. By examining the characteristics of the absorption edge and the positions of the absorption peaks, one can infer the chemical environment surrounding the elements. In contrast, the EXAFS region offers more detailed information about atomic spacing and coordination arrangements. Through a Fourier transform of the absorption spectrum, it is possible to extract vital parameters such as the distances to coordinating atoms, their coordination numbers, and the types of coordination environments present.⁴⁸

2.3.4. Raman spectroscopy. Raman spectroscopy is a crucial characterization technique for studying metastable phase materials. Raman spectroscopy relies on the phenomenon of Raman scattering, which is closely related to the vibrational modes of materials. This technique offers extensive information about the material structure and chemical environments. In the context of metastable phase materials, Raman spectroscopy can be employed to identify characteristic peaks of different phases, analyze phase transition processes, and investigate interactions between materials.

2.3.5. Differential scanning calorimetry. Differential scanning calorimetry (DSC) is a crucial thermal analysis technique that provides valuable insights into the thermal properties and phase transition behavior of materials by measuring changes in heat flow during heating or cooling. A metastable phase is one that is relatively stable under certain conditions but does not represent the most stable phase from a thermodynamic perspective. Upon heating, a metastable phase may gradually transform into a more stable equilibrium phase, and this transition typically involves significant changes in heat flow. By measuring the temperature difference between the sample and a reference material, as well as the heat flow over time, DSC can elucidate important thermodynamic parameters such as phase transition temperatures, enthalpy changes, and melting points of the material.⁶⁶

2.4. Typical 2D metastable-phase noble metal oxides

Recently, 2D metastable-phase noble metal oxides have shown remarkable performance in electrocatalysis. We summarize and review the latest research results of our research group in the electrocatalysis of metastable-phase 2D materials.

2.4.1. 1T-IrO₂. The stable phase of iridium oxide is the rutile structure with the space group of No. 136 (P4₂mnm). The [IrO₆] octahedra of rutile IrO₂ are connected through a mixture of two vertex-sharing and two edge-sharing modes (Fig. 3a).

Fig. 3 Summary of the crystal structures and characterization of stable phase and 2D metastable-phase noble metal oxides. (a) Crystal structures and TEM image of rutile-IrO₂. Reproduced from ref. 44 with permission from Springer Nature, copyright 2021. (b) Crystal structures and TEM image of 1T-IrO₂. Reproduced from ref. 44 with permission from Springer Nature, copyright 2021. (c) Crystal structures and TEM image of 3R-IrO₂. Reproduced from ref. 46 with permission from Elsevier, copyright 2021. (d) Crystal structures and TEM image of 1T-PtO₂. Reproduced from ref. 45 with permission from Royal Society of Chemistry, copyright 2023. (e) Crystal structures and TEM image of trigonal RhO₂. Reproduced from ref. 47 with permission from Springer Nature, copyright 2022. (f) Crystal structures and TEM image of 1T-Ru_0.5Ir_0.5O₂. Reproduced from ref. 48 with permission from Springer Nature, copyright 2023. (g) Crystal structures and TEM image of IrO₂ NR. Reproduced from ref. 67 with permission from Springer Nature, copyright 2023.

1T-IrO₂ is a layered structure with AA layered superposition, and Ir–O₆ octahedrons are connected in an edge-sharing form. The coordination numbers and bond lengths of the two coordination shells of Ir–O₆ and Ir–Ir₆ shells are 6.0/2.00 Å and 6.0/3.12 Å, respectively. The thickness of 1T-IrO₂ nanosheets is about 3–5 nm and the structure of the material undergoes significant changes. It has been reported that 1T-IrO₂ belongs to the space group P3̄m1 (164), with the corresponding unit cell parameters a = b = 3.11 Å, c = 6.91 Å (Fig. 3b), which differ obviously from the iridium oxide nanomaterial in the rutile phase.⁴⁴

2.4.2. 3R-IrO₂. As a new ultra-thin 2D metastable phase, the space group of the newly prepared 3R-IrO₂ phase is R3̄m (a = b = 3.158 Å, c = 13.617 Å). In 3R-IrO₂, three layers of IrO₆ octahedra form a unit and are stacked in an order of ABC, and each IrO₆ octahedron is surrounded by six other IrO₆ octahedra, sharing six edges (Fig. 3c).⁴⁶

2.4.3. 1T-PtO₂. The crystal structure and thermodynamic stability of platinum oxides have been studied and five platinum oxide phases have been identified, namely PtO (space group: P4₂/mmc), Pt₃O₄ (Na_xPt₃O₄ structure), α-PtO₂ (space group: P3̄m1), β-PtO₂ (space group: Pnnm), and β′-PtO₂ (space group: P4₂/mnm).^68,69

Metastable-phase monolayer 2D platinum oxide, 1T-PtO₂, was also synthesized using a melt-alkali mechanical chemical method.⁴⁵ This fabrication yields an oxygen–platinum–oxygen (O–Pt–O) monolayer consisting of edge-shared [PtO₆] octahedra, with space group P3̄m1 (164) (a = b = 3.07 Å and c = 6.95 Å). The thickness of 1T-PtO₂ is about 1.0 nm and the average diameter of 1T-PtO₂ is about 200 nm (Fig. 3d).⁴⁵

2.4.4. Trigonal-RhO₂. The stable phase of Rh oxide is Rh₂O₃, which exists in two polycrystalline forms: α-Rh₂O₃ and a low-temperature form with a corundum structure. The space group of rutile RhO₂ is P4₂/mnm (a = b = 4.49 Å, c = 3.09 Å).⁷⁰

Fan et al.⁴⁷ reported a new 2D metastable phase of RhO₂, known as the trigonal phase RhO₂ (Tri-RhO₂). Tri-RhO₂ is prepared using the radio frequency-assisted molten-alkali method with a thickness of 1.39 nm, and its crystal structure belongs to space group P3̄m1 (164) with lattice parameters a = b = 3.091 Å and c = 4.407 Å (Fig. 3e).⁴⁷ Furthermore, in 2023, Fan et al. reported a large-size trigonal single-crystal rhodium oxide (SC-Tri-RhO₂), with crystal parameters of a = b = 3.074 Å, c = 6.116 Å, and a space group of P3̄m1 (164), exhibiting strong ferromagnetism (FM) at a rather high temperature.⁷¹ In 2024, Chen et al. reported a remarkable Hall effect in the trigonal phase RhO₂ single crystals, where the Hall coefficient transitions from negative values at temperatures below 100 K to positive values above 150 K. This shift indicates that the trigonal phase of RhO₂ is n-type at low temperatures and transitions to p-type at higher temperatures, suggesting that a phase transition occurs in the range of 130–140 K.⁶⁵

2.4.5. 1T-Ru_0.5Ir_0.5O₂. Zhu et al.⁴⁸ reported a solid solution 1T-Ru_0.5Ir_0.5O₂ nanosheet with a thickness of 1.9 nm. The atomic ratio of Ru : Ir : O is approximately 0.46–0.50 : 0.54–0.50 : 2. Based on the XRD result, the lattice parameters of 1T-Ru_0.5Ir_0.5O₂ can be determined as a = b = 3.00 Å, c = 6.95 Å (Fig. 3f).

2.4.6. IrO₂ NR. Liao et al.⁶⁷ reported a monoclinic phase IrO₂ nanoribbon with a C2/m space group, which is significantly different from the stable tetragonal phase (P42/mnm). Through a molten-alkali mechanochemical method, researchers developed a unique strategy to convert the monoclinic phase K_0.25IrO₂ (I2/m (12)) precursor into a layered nanobelt structure. The parameters of the unit cell are as follows: a = 4.43 ± 0.01 Å, b = 3.14 ± 0.02 Å, c = 6.95 ± 0.03 Å and γ = 90° (Fig. 3g).

3. Electrochemical performance of 2D metastable-phase noble metal oxides

3.1. Oxygen evolution reaction (OER)

IrO₂ is the most promising commercial catalyst in acidic media. However, traditional rutile phase IrO₂ has a high overpotential and poor stability. Therefore, it is feasible to search for excellent metastable-phase iridium-based catalysts.

In 2021, Dang et al. reported new metastable-phase 1T-IrO₂,⁴⁴ which exhibited exceptional performance in the 0.1 M HClO₄ electrolyte, with an ultralow overpotential of 197 mV to reach the current density of 10 mA cm⁻². At the same time, 1T-IrO₂ has superior catalytic activity and electrochemical stability compared with commercial catalysts. The mass activity is 296.8 mA mg_Ir⁻¹ and the TOF is 4.2 s_UPD⁻¹ (Fig. 4a). The working stability is a key issue due to the significant activity loss of most metal oxides under acidic OER conditions in practical applications. At a high current density of 50 mA cm⁻², 1T-IrO₂ retained 98% of its initial activity after a 45 h stability test (Table 1). The catalytic origin of 1T-IrO₂ in the OER can be attributed to the optimized free energy increase of binding hydroxyl groups on Ir atoms (Fig. 4b).

Fig. 4 The OER performance of 2D metastable-phase noble metal oxides. (a) The comparison of OER performance among 2D metastable-phase noble metal oxides and commercial catalysts. (b) OER mechanism of 1T-IrO₂. Reproduced from ref. 44 with permission from Springer Nature, copyright 2021. (c) OER mechanism of 1T-Ru_0.5Ir_0.5O₂. Reproduced from ref. 48 with permission from Springer Nature, copyright 2023. (d) OER mechanism of 3R-IrO₂. Reproduced from ref. 46 with permission from Elsevier, copyright 2021. (e) OER mechanism of IrO₂ NR. Reproduced from ref. 67 with permission from Springer Nature, copyright 2023.

Table 1 Comparison of OER performance of representative 2D metastable-phase noble metal oxides with that of representative non-noble catalysts reported in the literature

Catalyst	Electrolyte	Overpotential@10 mA cm^−2 (mV)	Stability	Ref.
1T-IrO2	0.1 M HClO4	197	45 h@50 mA cm^−2	44
3R-IrO2	0.1 M HClO4	188	511 h@10 mA cm^−2	46
1T-Ru0.5Ir0.5O2	0.5 M H2SO4	151	618.3 h@10 mA cm^−2	48
IrO2 NR	0.5 M H2SO4	205	138 h@10 mA cm^−2	67
1T-MoS2	0.5 M H2SO4	420	2 h@10 mA cm^−2	72
Ba–Co3O4	0.1 M HClO4	278	110 h@10 mA cm^−2	73
NiFeP	0.5 M H2SO4	540	30 h@10 mA cm^−2	74
Ni2Ta	0.5 M H2SO4	570	66 h@10 mA cm^−2	75
Mn-doped FeP/Co3(PO4)2	0.5 M H2SO4	390	8.3 h@10 mA cm^−2	76
Co3O4	1 M H2SO4	360	40 h@10 mA cm^−2	77
Co3O4@C	0.5 M H2SO4	370	82 h@10 mA cm^−2	78
C–Fe3O4	0.5 M H2SO4	650	24 h@10 mA cm^−2	79
CuMnO:F	0.5 M H2SO4	325	24 h@16 mA cm^−2	80
HNC-Co	0.5 M H2SO4	265	20 h@10 mA cm^−2	81
CoCl2@Th-BPYDC	0.1 M HClO4	388	25 h@10 mA cm^−2	82

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Furthermore, Zhu et al. reported that stable and oxidative charged Ru in 2D ruthenium–iridium oxide enhances the activity.⁴⁸ The 1T-Ru_0.5Ir_0.5O₂ exhibited high activity in acid with a low overpotential of 151 mV at 10 mA cm⁻² and a high turnover frequency of 6.84 s⁻¹ at 1.44 V (Fig. 4a). It also demonstrated good stability, with an operational time of 618.3 h (Table 1). The local structure of Ru–O–Ir in the 2D Ru_0.5Ir_0.5O₂ solid solution enhances the stability of these Ru centers (Fig. 4c).

In the following study, Fan et al. reported 3R-IrO₂ and evaluated the OER performance in 0.1 M HClO₄.⁴⁶ 3R-IrO₂ delivers a record overpotential as low as 188 mV at 10 mA cm⁻², further suggesting an important role for phase engineering on achieving high activity (Fig. 4a). Furthermore, the overpotential of 3R-IrO₂ shifts only about 30 mV after a 511 h test at a current density of 10 mA cm⁻² (Table 1). At the same time, 3R-IrO₂ largely retains its layered structure after the stability test, which proved the phase stability. The unique active sites in the edge-shared octahedra of 3R-IrO₂ contribute to enhanced activity. Proton enhancement transport along the 2D closed structure in the plane and vertical pathway through iridium vacancies provide a novel working mechanism for the acidic OER (Fig. 4d).

Liao et al. evaluated the OER activity of IrO₂ NR in O₂-saturated 0.5 M H₂SO₄. IrO₂ NR exhibits excellent OER activity with a low overpotential of 205 mV at a current density of 10 mA cm⁻² (Fig. 4a). The Tafel slope of the IrO₂ NR is 46.2 mV dec⁻¹, which is significantly lower than that of C-IrO₂ (61.2 mV dec⁻¹) and C-Ir/C (97.3 mV dec⁻¹), indicating faster OER kinetics. At 1.5 V, the mass activity of the IrO₂ NR reaches 2354.5 mA mg_Ir⁻¹. The overpotential for the IrO₂ NR exhibits an increase of approximately 1.6% at 10 mA cm⁻² after 500 000 s in 0.5 M H₂SO₄ (Table 1). Furthermore, the turnover frequency (TOF) of the IrO₂ NR at 1.5 V is 14.01 s⁻¹, demonstrating the high quality of its active sites. In the OER cycle, the exposed Ir atoms in the IrO₂ NR exhibit a lower d band center compared to rutile phase IrO₂. This phenomenon results in weaker adsorption of OER intermediates, thereby self-regulating the four-electron OER process to achieve a balanced free energy distribution at a low overpotential (Fig. 4e).

3.2. Hydrogen evolution reaction (HER)

Platinum oxide is an ideal catalyst for redox and selective hydrogenation reactions.^83,84 In 2023, Yang et al. reported a new metastable-phase single-layer 2D platinum oxide, 1T-PtO₂.⁴⁵ The HER performance of 1T-PtO₂ was evaluated in 0.5 M H₂SO₄. 5% 1T-PtO₂/C showed the best HER activity with the lowest overpotential of 12 mV at a current density of 10 mA cm⁻² (Fig. 5a). The 5% 1T-PtO₂/C exhibited good stability at 10 mA cm⁻² in a 100 h stability test (Table 2). Based on the DFT simulation, the author found that when using the new metastable-phase 1T-PtO₂ as the model catalyst, the reaction process exhibits a novel and distinct Pt–O active site, where the formation of Pt–O active sites dominates the high HER activity of 1T-PtO₂ (Fig. 5b).

Fig. 5 The HER performance of 2D metastable-phase noble metal oxides. (a) The comparison of HER performance among 2D metastable-phase noble metal oxides and commercial catalysts. (b) HER mechanism of 1T-PtO₂. Reproduced from ref. 45 with permission from Royal Society of Chemistry, copyright 2023. (c) HER mechanism of Rh-NA/RhO₂. Reproduced from ref. 47 with permission from Springer Nature, copyright 2022.

Table 2 Comparison of HER performance of representative 2D metastable-phase noble metal oxides with that of representative non-noble catalysts reported in the literature

Catalyst	Electrolyte	Overpotential@10 mA cm^−2 (mV)	Stability	Ref.
1T-PtO2	0.5 M H2SO4	12	100 h@10 mA cm^−2	45
Rh-NA/RhO2	0.5 M H2SO4	9.8	100 h@750 mA cm^−2	47
Cu@MoS2	0.5 M H2SO4	131	6.94 h@10 mA cm^−2	85
Co0.6Fe0.4P/CNTs	0.5 M H2SO4	67	24 h@10 mA cm^−2	86
Fe–Mo2C@NCF	0.5 M H2SO4	129	12 h@10 mA cm^−2	87
S–Co0.85Se-1	0.5 M H2SO4	108	48 h@10 mA cm^−2	88
N–MoS2/CN	0.5 M H2SO4	114	10 h@40 mA cm^−2	89
Mo–Fe(1/1)-Se-CP	0.5 M H2SO4	86.9	24 h@10 mA cm^−2	90
Mo0.5W0.5S2	0.5 M H2SO4	138	10 h@10 mA cm^−2	91
P–MoP/Mo2N	0.5 M H2SO4	89	48 h@20 mA cm^−2	92
Fe0.9Co0.1S2/CNTs	0.5 M H2SO4	155	40 h@25 mA cm^−2	93
NiCo2Px/CF	0.5 M H2SO4	104	30 h@20 mA cm^−2	94
Ni2P/Ni3Se4-5.0/NF	0.5 M H2SO4	76	50 h@15 mA cm^−2	95
CoP/NCNHP	0.5 M H2SO4	140	24 h@20 mA cm^−2	96
CoS2@WS2/CC	0.5 M H2SO4	97.2	7 h@50 mA cm^−2	97

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Rhodium-based catalysts are considered to be the second most promising HER catalysts after platinum. However, poor durability and high hydrogen adsorption energy (ΔG_H) on the surface of Rh hinder its application in the HER process.⁹⁸ In 2022, Shao et al.⁴⁷ proposed a triangular phase 2D rhodium oxide, which provided a platform for constructing ordered fcc Rh nanocrystal arrays. The coupling of ordered Rh arrays with metastable-phase substrates improved the hydrogen spillover effect. The overpotential of the catalyst at −10 mA cm⁻² was only 9.8 mV (Fig. 5a). Rh-NA/RhO₂ demonstrated excellent stability during a long-term test lasting 100 h, with an average current density of −750 mA cm⁻² (Table 2). This work provides an important method for manufacturing ordered nanocrystal arrays with sub-nanometer spacing, which can be used for future advanced applications (Fig. 5c).⁴⁷

4. Conclusions and outlooks

This concept paper aims at describing the characteristics and advantages of 2D metastable-phase noble metal oxides in electrocatalysis. Several typical 2D metastable-phase noble metal oxides, such as 1T-IrO₂, 1T-PtO₂, 1T-Ru_0.5Ir_0.5O₂, trigonal RhO₂ and IrO₂ NRs, are introduced (Table 3). Their applications in acidic electrocatalytic water splitting are briefly analyzed. The 2D metastable-phase noble metal oxides show significant improvement in electrochemical activity and stability compared with traditional noble metal oxides and non-noble metal materials (Tables 1 and 2).

Table 3 Phase, morphology and performance of some representative 2D metastable-phase noble metal oxides

Oxides	Phase	Morphology	Application	Performance	Ref.
IrO2	1T	Nanosheet	OER	η 10 = 197 mV	44
IrO2	3R	Nanosheet	OER	η 10 = 188 mV	46
PtO2	1T	Nanosheet	HER	η 10 = 12 mV	45
RhO2	Trigonal	Nanosheet	HER	η 10 = 9.8 mV	47
Ru0.5Ir0.5O2	1T	Nanosheet	OER	η 10 = 151 mV	48
IrO2 NR	Monoclinic	Nanoribbon	OER	η 10 = 205 mV	67

---

Although the development of 2D metastable-phase noble metal oxides has been significant, there are still challenges. First, only a few precious metal materials such as Ir, Pt, and Rh have been reported. So, it is necessary to expand the candidates of 2D metastable-phase oxide materials in the periodic table. Comprehensive theoretical studies are also needed to determine their properties. Based on crystal symmetry and crystal chemistry, we theoretically propose 192 metal oxides dispersed in 10 space groups and 32 metallic elements, which can provide beneficial theoretical guidance for the development of 2D materials.⁹⁹ Yet there is still a lot of work to be done in this regard.

Secondly, there are still limited methods for synthesizing 2D metastable-phase noble metal oxides. According to our experience and previous reports, the synthesis of metastable noble metal oxides usually requires three key factors: high temperature, strong alkali and mechanical force. The high temperature provides enough heat energy to overcome the energy barrier required for the reaction. Mechanical forces and alkali environments may induce phase transition of the material, thus contributing to the formation of a new phase. It is necessary to develop more efficient and feasible synthetic methods.

Thirdly, the research on the regulation strategy of the 2D metastable-phase noble metal oxides still needs to be further studied. Recently, there have been some relevant reports, involving nanocrystal array coupling, doping, applying strain, and so on. For example, Fan et al. induced hydrogen spillover by coupling an ordered rhodium array with metastable trigonal phase RhO₂, thus enhancing the acidic hydrogen evolution reaction.⁴⁷ Zhu et al. synthesized Ru–Ir solid solution 1T-Ru_0.5Ir_0.5O₂ through a two-step molten-alkali process, and the results showed that stable and oxidative charged Ru in 2D ruthenium–iridium oxide greatly enhanced the activity of the OER.⁴⁸ Yu et al. adjusted the in-plane strain of 1T-IrO₂ using the in situ growth method and found that the stress generated by loading 5% CeO₂ triggered a completely different OER path compared to that of 1T-IrO₂.⁵⁴ However, more research studies are needed to explore the regulatory strategies of 2D metastable-phase noble metal oxides. For instance, by employing functionalization strategies to modify carriers, one can construct multi-phase catalysts such as core–shell structures and heterogeneous architectures.

In addition, in situ characterization is crucial for understanding the intrinsic electrocatalytic behavior of metastable-phase materials at the nanoscale. For instance, Huang et al. monitored the electrochemical phase transition of 1T-MoS₂ using in situ Raman mapping and atomic force microscopy, discovering that the continuous HER transforms 1T-MoS₂ into a stable 2H-1T mixed phase, which exhibits optimal hydrogen adsorption energy.¹⁰⁰ However, there is a lack of effective and real time experimental observations on the catalytic mechanisms of 2D metastable-phase noble metal oxides, and it is believed that the integration of advanced characterization techniques such as in situ X-ray absorption and in situ XRD can further reveal the atomic environment of metastable-phase oxides, thereby deepening the understanding of reaction mechanisms.

Finally, expanding the applications of 2D metastable phase materials is of great importance. Oxides offer new possibilities for tuning the activity and selectivity of catalysts. Therefore, it is recommended to explore the use of 2D metastable phase oxides in thermally assisted propane dehydrogenation and photochemically assisted carbon dioxide hydrogenation reactions. Beyond the field of catalysis, future research studies in flexible electronics, data storage, mechanical engineering, and biological applications based on 2D metastable phase oxides are equally significant.

Author contributions

Qun Wang: investigation and writing original draft. Mingwang Shao: supervision and writing – review & editing. Qi Shao: supervision and writing – review & editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Ministry of Science and Technology of China (2024YFA1509500), the National Natural Science Foundation of China (22475143), the Young Elite Scientists Sponsorship Program by CAST (grant no. 2023QNRC001), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

Notes and references

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