With the increasing progress in the emerging field of phase engineering of nanomaterials, transition metal dichalcogenides (TMDs) with metastable phases have attracted extensive research interest owing to their unique physicochemical properties and promising potential for various applications. Unlike their semiconducting counterparts, metastable 1T'-phase group VIB TMDs exhibit distinctive metallic behavior, making them highly promising for clean energy electrocatalysis (for example, hydrogen evolution), high-performance electronics and superconducting devices. However, conventional synthetic strategies usually suffer from low yields, poor phase purity, small crystal size and harsh experimental conditions, hindering their fundamental research and practical applications. Here we describe a salt-assisted method for the controlled preparation of various metastable 1T'-phase group VIB TMDs with high crystalline quality and high phase purity. In this context, 'salt-assisted' refers to the co-annealing of commercially available 2H-phase TMDs, chalcogen powders (for example, S or Se) and alkali metal salts (for example, K2C2O4·H2O or K2CO3). When heated in a tube furnace under an H2/Ar atmosphere, these alkali metal salts trigger a phase transformation of TMDs from the thermodynamically stable 2H phase to the metastable 1T' phase. This Protocol describes the details of experimental procedures to prepare a series of 1T'-TMDs, that is, MoS2, MoSe2, WS2, WSe2, MoS2xSe2(1-x) and WS2xSe2(1-x). It takes ~37 h 20 min and yields robust 1T'-TMD crystals with their size of up to hundreds of micrometers at a gram scale (up to ~2 g per batch), overcoming the longstanding synthetic bottlenecks and paving the way for advanced fundamental studies and the exploration of practical applications.

Phase engineering of nanomaterials (PEN) has emerged as a new research field by enabling precise control of crystallographic phases at the nanometer scale. The capability of stabilizing unconventional phases unlocks a vast library of physiochemical properties inaccessible to the conventional, thermodynamically stable crystals. This review provides a comprehensive framework to map the past, present, and future of PEN. We first briefly introduce the concept of PEN. Then, we summarize synthetic methodologies, including direct phase-controlled synthesis and phase transition. The discussion also underscores the pivotal role of advanced characterization, particularly the transition from ex situ to in situ and operando techniques, in probing dynamic phase behaviors under realistic conditions. We present key applications in (electro)catalysis, energy conversion and storage, functional devices, and biomedicine to illustrate the PEN's broad impact. Looking ahead, we identify key challenges and future directions, emphasizing robust phase stabilization, architecture control, and artificial intelligence-integrated phase design. The field is ultimately advancing toward the artificial atomically-assembled structures (AAASs), enabling the deterministic assembly of atoms with precisely controlled phase and architecture to achieve targeted functions and applications.

Semiconducting metal oxide based gas sensors exhibit great promise for convenient detection of acetone, a biomarker gas in the exhaled breath of type-I diabetes patients. However, the detection usually suffers the interference from exhaled moisture. To overcome this challenge, in this work, a novel hierarchical heterojunction structure consisting of SnO2 nanofiber core and Co3O4 nanosheet shell (denoted as SnO2 @Co3O4 core-shell composite) was proposed for fabricating acetone sensor with excellent humidity resistance. Compared with SnO2 nanofibers and Co3O4 nanosheets, the SnO2 @Co3O4 showed the highest sensing response, with a response value (Rg/Ra) of 11.27-50ppm acetone at 110 degrees C. In addition, the hierarchical SnO2 @Co3O4 core-shell structure shows fast response/recovery speed (19/43 s), lower detection limit (125 ppb), excellent selectivity and stability in a humidity environment (relative humidity 30 %-90 %) with a relative change of only 3 %. The enhanced gas sensing performance toward acetone is attributed to the synergistic effect between the two components, the unique core-shell hierarchical structure and the rich oxygen vacancy density. Density functional theory calculations reveal that the SnO2 @Co3O4 has higher acetone adsorption energy than the two components. In addition, a novel SnO2 @Co3O4 gas sensing module and smart portable sensor device enable efficient real-time monitoring of acetone concentrations on a smartphone via Bluetooth communication. (c) 2026 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive detection technique that is used extensively in various analytical applications, including biomedical diagnostics, food safety, and environmental monitoring. Recently, two-dimensional (2D) transition metal dichalcogenides (TMDs) with diverse crystal phases have emerged as promising substrates for SERS detection due to their unique optical and electronic properties. In this perspective, we first introduce the crystal structures and physicochemical properties of TMDs. Then, we briefly summarize the research progress on the preparation of semiconducting and metallic/semi-metallic 2D TMD substrates and their hybrid structures integrated with plasmonic metals for SERS detection. Finally, after we discuss the current challenges and future directions in this exciting field, we highlight the importance of the crystal phases of 2D TMDs in regulating their SERS performance and provide our insights into future research directions in the phase engineering of 2D TMDs for SERS.

Transition metal dichalcogenides are a family of quasi-two-dimensional materials that display a high technological potential due to their wide range of electronic ground states, e.g., from superconducting to semiconducting, depending on the chemical composition, crystal structure, or electrostatic doping. Here, we unveil that by tuning a single parameter, the hydrostatic pressure P, a cascade of electronic phase transitions can be induced in the few-layer transition metal dichalcogenide 1T'-WS2, including superconducting, topological, and anomalous Hall effect phases. Specifically, as P increases, we observe a dual phase transition: the suppression of superconductivity with the concomitant emergence of an anomalous Hall effect at P=1.15 GPa. Remarkably, upon further increasing the pressure above 1.6 GPa, we uncover a reentrant superconducting state that emerges out of a state still exhibiting an anomalous Hall effect. This superconducting state shows a marked increase in superconducting anisotropy with respect to the phase observed at ambient pressure, suggesting a different superconducting state with a distinct pairing symmetry. Via first-principles calculations, we demonstrate that the system concomitantly transitions into a strong topological phase with markedly different band orbital characters and Fermi surfaces contributing to the superconductivity. These findings position 1T'-WS2 as a unique, tunable superconductor, wherein superconductivity, anomalous transport, and band features can be tuned through the application of moderate pressures.

Transition metal dichalcogenides display a high technological potential due to their wide range of electronic ground states. Here, we unveil that by tuning hydrostatic pressure P, a cascade of electronic phase transitions can be induced in the few-layer transition metal dichalcogenide 1T’-WS2. As P increases, we observe the suppression of superconductivity with the concomitant emergence of an anomalous Hall effect (AHE) at $$P\approx 1.15$$ GPa. Above 1.6GPa, we uncover a reentrant superconducting state emerging from a state still exhibiting AHE. This superconducting state competes with the AHE state and shows a marked increase in superconducting anisotropy with respect to the ambient pressure phase, suggesting a distinct pairing symmetry. We demonstrate that 1T’-WS2 concomitantly transitions into a strong topological phase with different band orbital characters and Fermi surfaces contributing to the superconductivity. These findings position 1T’-WS2 as a tunable superconductor, wherein superconductivity, AHE, and band features can be tuned reversibly. Transition metal dichalcogenides exhibit diverse and tunable electronic states. Here the authors reveal a cascade of phase transitions upon increasing hydrostatic pressure in the few-layer 1T’-WS2, including a re-entrant superconducting phase emerging from a normal state exhibiting anomalous Hall effect.

Novel core–shell Pd 7 @Pt x heterostructured nanocrystals with Pt-islands shell of unsaturated coordination edges could be directly photodeposited on a reduced graphene oxide support as a highly efficient HER electrocatalyst in a wide pH range.

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an attractive avenue to construct new surface-enhanced Raman scattering (SERS) substrates. Herein, 2D WS2 and MoS2 monolayers with high-purity distorted octahedral phase (1T ') are prepared for highly sensitive SERS detection of analytes (e.g., rhodamine 6G, rhodamine B and crystal violet). 1T '-WS2 and 1T '-MoS2 monolayers show the detection limits of 8.28 x 10-12 and 8.57 x 10-11 M for rhodamine 6G, with the enhancement factors of 4.6 x 108 and 3.9 x 107, respectively, which are comparable to noble-metal substrates, outperforming semiconducting 2H-W(Mo)S2 monolayers and most of the reported non-noble-metal substrates. First-principles density functional theory calculations show that their Raman enhancement effect is mainly ascribed to highly efficient interfacial charge transfer between the 1T '-W(Mo)S2 monolayers and analytes. Our study reveals that 2D TMDs with semimetallic 1T ' phase are promising as next-generation SERS substrates.

Crystal phase, a critical structural characteristic beyond the morphology, size, dimension, facet, etc., determines the physicochemical properties of nanomaterials. As a group of layered nanomaterials with polymorphs, transition metal dichalcogenides (TMDs) have attracted intensive research attention due to their phase-dependent properties. Therefore, great efforts have been devoted to the phase engineering of TMDs to synthesize TMDs with controlled phases, especially unconventional/metastable phases, for various applications in electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectrics. Considering the significant progress in the synthesis and applications of TMDs, we believe that a comprehensive review on the phase engineering of TMDs is critical to promote their fundamental studies and practical applications. This Review aims to provide a comprehensive introduction and discussion on the crystal structures, synthetic strategies, and phase-dependent properties and applications of TMDs. Finally, our perspectives on the challenges and opportunities in phase engineering of TMDs will also be discussed.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are a typical class of 2D materials with promising potential for future electronics and optoelectronics. Moreover, the rich crystal structures provide polymorphic TMDs with distinct phase-dependent physicochemical properties. Therefore, it would be fascinating to achieve controllable growth and phase transformation of TMDs via phase engineering and utilize them in various application fields. Nowadays, significant efforts have been made to combine the distinct properties of TMD polymorphs to improve device performance and find novel applications. For example, TMDs with metallic phases, such as 1T-MoS 2 and 1T'-MoTe 2 , have been widely utilized as electrodes to achieve ultra-low contact resistance in TMD-based transistors. The resistance changes with the reversible phase transformation also make TMDs attractive for memory devices and emerging neuromorphic computing. This chapter summarizes the recent advances in electronic and optoelectronic applications of TMDs with various polymorphs, including field-effect transistors (FETs), memories, energy harvesting devices, photodetectors, and solar cells. We demonstrate the phase-dependent properties of polymorphic TMDs and discuss the role of different TMD phases in electronic and optoelectronic devices.

Crystal phase is a key factor determining the properties, and hence functions, of two-dimensional transition-metal dichalcogenides (TMDs) 1 , 2 . The TMD materials, explored for diverse applications 3 – 8 , commonly serve as templates for constructing nanomaterials 3 , 9 and supported metal catalysts 4 , 6 – 8 . However, how the TMD crystal phase affects the growth of the secondary material is poorly understood, although relevant, particularly for catalyst development. In the case of Pt nanoparticles on two-dimensional MoS 2 nanosheets used as electrocatalysts for the hydrogen evolution reaction 7 , only about two thirds of Pt nanoparticles were epitaxially grown on the MoS 2 template composed of the metallic/semimetallic 1T/1T′ phase but with thermodynamically stable and poorly conducting 2H phase mixed in. Here we report the production of MoS 2 nanosheets with high phase purity and show that the 2H-phase templates facilitate the epitaxial growth of Pt nanoparticles, whereas the 1T′ phase supports single-atomically dispersed Pt (s-Pt) atoms with Pt loading up to 10 wt

Phasetransition with band gap modulation of materials has gainedintensive research attention due to its various applications, includingmemories, neuromorphic computing, and transistors. As a powerful strategyto tune the crystal phase of transition-metal dichalcogenides (TMDs),the phase transition of TMDs provides opportunities to prepare newphases of TMDs for exploring their phase-dependent property, function,and application. However, the previously reported phase transitionof TMDs is mainly irreversible. Here, we report a reversible phasetransition in the semimetallic 1T '-WS2 driven byproton intercalation and deintercalation, resulting in a newly discoveredsemiconducting WS2 with a novel unconventional phase, denotedas the 1T '(d) phase. Impressively, an on/off ratioof >10(6) has been achieved during the phase transitionofWS(2) from the semimetallic 1T ' phase to the semiconducting1T '(d) phase. Our work not only provides a uniqueinsight into the phase transition of TMDs via proton intercalationbut also opens up possibilities to tune their physicochemical propertiesfor various applications.

Ongoing advances in superconductors continue to revolutionize technology thanks to the increasingly versatile and robust availability of lossless supercurrent. In particular high supercurrent density can lead to more efficient and compact power transmission lines, high-field magnets, as well as high-performance nanoscale radiation detectors and superconducting spintronics. Here, we report the discovery of an unprecedentedly high superconducting critical current density (17 MA/cm2 at 0 T and 7 MA/cm2 at 8 T) in 1T'-WS2, exceeding those of all reported two-dimensional superconductors to date. 1T'-WS2 features a strongly anisotropic (both in- and out-of-plane) superconducting state that violates the Pauli paramagnetic limit signaling the presence of unconventional superconductivity. Spectroscopic imaging of the vortices further substantiates the anisotropic nature of the superconducting state. More intriguingly, the normal state of 1T'-WS2 carries topological properties. The band structure obtained via angle-resolved photoemission spectroscopy and first-principles calculations points to a Z2 topological invariant. The concomitance of topology and superconductivity in 1T'-WS2 establishes it as a topological superconductor candidate, which is promising for the development of quantum computing technology.

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Tunable physicochemical properties of bimetallic core-shell heterostructured nanocrystals (HNCs) have shown enormous potential in electrocatalytic reactions. In many cases, HNCs are required to load on supports to inhibit catalyst aggregation. However, the introduction of supports during the process of growing core-shell HNCs makes the synthesis much more complicated and difficult to control precisely. Herein, we reported a universal photochemical synthetic strategy for the controlled synthesis of well-defined surfactant-free core-shell metal HNCs on a reduced graphene oxide (rGO) support, which was assisted by the fine control of photogenerated electrons directly transferring to the targeted metal seeds via rGO and the precisely tuned adsorption capacity of the added second metal precursors. The surface photovoltage microscopy (SPVM) platform proved that photogenerated electrons flowed through rGO to Pd particles under illumination. We have successfully synthesized 24 different core-shell metal HNCs, i.,e., MA@MB (MA = Pd, Au, and Pt; MB = Au, Ag, Pt, Pd, Ir, Ru, Rh, Ni and Cu), on the rGO supports. The as-prepared Pd@Cu core-shell HNCs showed outstanding performance in the electrocatalytic reduction of CO2 to CH4. This work could shed light on the controlled synthesis of more functional bimetallic nanostructured materials on diverse supports for various applications.

The freshness of seafood can be judged by detecting the concentration of triethylamine (TEA). In this work, 2D CuO porous nanosheets (CuO PNs) were prepared by a graphene oxide template method and their particle sizes were regulated by changing the calcination temperature. Their structure, morphology and gas sensing performances were investigated by various characterization methods. The response (Rg/Ra) of the gas sensor based on CuO PNs calcined at 700 oC was as high as 440-100 ppm TEA at the operating temperature of 40 °C. The detection limit was as low as 0.25 ppm. In addition, the gas sensor has good selectivity and stability. The excellent TEA sensitivity is mainly resulted from the appropriate particle size and loose porous framework. This work not only paves the way to explore the novel low temperature TEA gas sensors, but also provides deep insight on improving the structure and properties of gas sensitive materials by controlling the calcination temperature.

Phase engineering of nanomaterials (PEN) has demonstrated great potential in the fields of catalysis, electronics, energy storage and conversion, and condensed matter physics. Recently, transition metal dichalcogenides (TMDs) with unconventional metastable phases (e.g., 1T and 1T') have attracted increasing research interest due to their unique and appealing physicochemical properties. However, there is still a lack of a simple, universal, and controlled method for the preparation of large-scale and high-purity unconventional-phase TMD crystals, restricting their further fundamental study and practical applications. Here, a facile, one-step salt-assisted general strategy is reported for the controlled phase transformation of commercially available TMDs with conventional 2H phase, yielding a large amount of metastable 1T'-phase TMDs, including WS2 , WSe2 , MoS2 , and MoSe2 . It is found that the easily accessible metal salts, such as K2 C2 O4 ·H2 O, K2 CO3 , Na2 CO3 , Rb2 CO3 , Cs2 CO3 , KHCO3 , NaHCO3 , and NaC2 O4 , can be used to assist the 2H-to-1T' phase transformation, greatly simplifying the synthetic process for producing metastable 1T'-TMDs. Importantly, this method can also be used to prepare 1T'-TMD alloys, such as 1T'-WS2x Se2(1-x) . This newly developed strategy is robust and highly effective, which can also be used for the phase engineering of other materials with various polymorphs.

The emerging nonlayered 2D materials (NL2DMs) are sparking immense interest due to their fascinating physicochemical properties and enhanced performance in many applications. NL2DMs are particularly favored in catalytic applications owing to the extremely large surface area and low-coordinated surface atoms. However, the synthesis of NL2DMs is complex because their crystals are held together by strong isotropic covalent bonds. Here, nonlayered molybdenum phosphide (MoP) with well-defined 2D morphology is synthesized from layered molybdenum dichalcogenides via surface-confined atomic substitution. During the synthesis, the molybdenum dichalcogenide nanosheet functions as the host matrix where each layer of Mo maintains their hexagonal arrangement and forms isotropic covalent bonds with P that substitutes S, resulting in the conversion from layered van der Waals material to a covalently bonded NL2DM. The MoP nanosheets converted from few-layer MoS2 are single crystalline, while those converted from monolayers are amorphous. The converted MoP demonstrates metallic charge transport and desirable performance in the electrocatalytic hydrogen evolution reaction (HER). More importantly, in contrast to MoS2 , which shows edge-dominated HER performance, the edge and basal plane of MoP deliver similar HER performance, which is correlated with theoretical calculations. This work provides a new synthetic strategy for high-quality nonlayered materials with well-defined 2D morphology for future exploration.

Two-dimensional (2D) covalent organic frameworks (COFs) possess designable pore architectures but limited framework topologies. Until now, 2D COFs adopting the kgd topology with ordered and rhombic pore geometry have rarely been reported. Here, an isoreticular series of 2D COFs with the kgd topology and controllable pore size is synthesized by employing a C6-symmetric aldehyde, i.e., hexa(4-formylphenyl)benzene (HFPB), and C3-symmetric amines i.e., tris(4-aminophenyl)amine (TAPA), tris(4-aminophenyl)trazine (TAPT), and 1,3,5-tris[4-amino(1,1-biphenyl-4-yl)]benzene (TABPB), as building units, referred to as HFPB-TAPA, HFPB-TAPT, and HFPB-TABPB, respectively. The micropore dimension down to 6.7 Å is achieved in HFPB-TAPA, which is among the smallest pore size of reported 2D COFs. Impressively, both the in-plane network and stacking sequence of the 2D COFs can be clearly observed by low-dose electron microscopy. Integrating the unique kgd topology with small rhombic micropores, these 2D COFs are endowed with both short molecular diffusion length and favorable host-guest interaction, exhibiting potential for drug delivery with high loading and good release control of ibuprofen.

Crystal phase engineering of noble-metal-based alloy nanomaterials paves a new way to the rational synthesis of high-performance catalysts for various applications. However, the controlled preparation of noble-metal-based alloy nanomaterials with unconventional crystal phases still remains a great challenge due to their thermodynamically unstable nature. Herein, we develop a robust and general seeded method to synthesize PdCu alloy nanomaterials with unconventional hexagonal close-packed (hcp, 2H type) phase and also tunable Cu contents. Moreover, galvanic replacement of Cu by Pt can be further conducted to prepare unconventional trimetallic 2H-PdCuPt nanomaterials. Impressively, 2H-Pd67Cu33 nanoparticles possess a high mass activity of 0.87 A mg-1Pd at 0.9 V (vs reversible hydrogen electrode (RHE)) in electrochemical oxygen reduction reaction (ORR) under alkaline condition, which is 2.5 times that of the conventional face-centered cubic (fcc) Pd69Cu31 counterpart, revealing the important role of crystal phase on determining the ORR performance. After the incorporation of Pt, the obtained 2H-Pd71Cu22Pt7 catalyst shows a significantly enhanced mass activity of 1.92 A mg-1Pd+Pt at 0.9 V (vs RHE), which is 19.2 and 8.7 times those of commercial Pt/C and Pd/C, placing it among the best reported Pd-based ORR electrocatalysts under alkaline conditions.