Key Takeaways & Executive Findings
- •• • A crystallographic orientation difference >10° between contacting 2H-WS2 grains at 1000 °C reduces the heterogeneous nucleation energy barrier for the 1T phase to 0.005 eV, compared to 2.314 eV for homogeneous nucleation—a 463-fold reduction—enabling spontaneous phase transformation at the contact boundary without catalysts or intercalants. • • The thermodynamic stability of 2H- and 1T-WS2 reverses at 280 K; above this threshold, 1T-WS2 becomes the stable phase, providing a clear temperature window for selective synthesis of the metallic phase in industrial processes operating at elevated temperatures. • • The transformation proceeds via an interface-mediated mechanism: lattice mismatch at large-angle boundaries induces defects and lattice distortion, triggering atomic rearrangement that converts the contact region to 1T-WS2, which then propagates through adjacent 2H nanosheets, ultimately yielding large-area, phase-pure 1T-WS2 films. • • The process uses only 2H-WS2 nanosheets as the raw material and requires no intercalation agents or organic solvents, eliminating contamination risks and simplifying purification, which is critical for catalytic and electronic applications where residual impurities degrade performance.
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Abstract
The synthesis of phase-pure 1T-WS2 remains a persistent challenge due to the thermodynamic metastability of the octahedral phase and the absence of a mechanistic understanding of the 2H-to-1T transformation at the atomic scale. This study demonstrates that when two 2H-WS2 grains with crystallographic orientation differences exceeding 10° are brought into contact at 1000 °C, they coalesce and transform into a single, pure 1T-WS2 grain devoid of orientation mismatch. First-principles calculations reveal a thermodynamic crossover at 280 K: below this temperature, 2H-WS2 is the stable phase, whereas above 280 K, 1T-WS2 becomes energetically favored. Kinetic analysis of nucleation shows that homogeneous nucleation of the 1T phase requires overcoming an energy barrier of 2.314 eV, while heterogeneous nucleation at the contact interface of two nanosheets necessitates only 0.005 eV, a reduction of nearly three orders of magnitude. This dramatic barrier lowering is attributed to the synergistic effect of elevated temperature and lattice mismatch-induced interfacial restructuring, which promotes atomic rearrangement and the formation of 1T-WS2 at the contact boundary. The 1T phase region subsequently expands, consuming the surrounding 2H nanosheets and yielding large-area, phase-pure 1T-WS2 films. This work establishes a straightforward, clean synthesis route for 1T-TMDs and provides a mechanistic framework for interface-driven phase engineering in two-dimensional materials.
1. Introduction
Transition metal dichalcogenides (TMDs) such as WS2 are indispensable in electrocatalysis, energy storage, and optoelectronics, yet their functional performance is phase-dependent. The 1T phase, characterized by octahedral coordination and metallic conductivity, offers a higher density of edge active sites and superior charge transport compared to the semiconducting 2H phase, making it highly desirable for hydrogen evolution and related reactions. However, the thermodynamic metastability of 1T-TMDs means that natural or as-synthesized materials overwhelmingly adopt the 2H structure. Existing preparation methods—intercalation, organic solvent treatment, and chemical vapor deposition—suffer from incomplete phase conversion, residual impurities, and poor scalability, largely because the atomic-scale mechanism of the 2H-to-1T transition has remained unresolved.
This study addresses the mechanistic gap by employing in situ environmental transmission electron microscopy (ETEM) to observe the phase transformation at the atomic scale, combined with first-principles calculations to quantify the thermodynamic and kinetic barriers. The key finding is that large crystallographic orientation differences (>10°) at the contact interface between two 2H-WS2 grains at 1000 °C create a low-energy pathway for heterogeneous nucleation of the 1T phase, with a barrier of only 0.005 eV. This interface-driven mechanism enables the synthesis of phase-pure, large-area 1T-WS2 films from a single 2H-WS2 precursor, providing a clean and straightforward route that bypasses the limitations of conventional methods and offers a general strategy for phase engineering in other TMD systems.
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CUI Hantao, GUO Yunna, JIA Peng, LI Yanxian, YE Zhangran, DENG Lei, MA Chongchong, TAI Chao, ZHANG Liqiang, WEN Bin (2025). Large Crystallographic Orientation Difference Contacts Induce Phase Transformation of WS2 Nanosheets from 2H to 1T. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3505-0
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Frequently Asked Questions
What is the exact kinetic barrier for heterogeneous nucleation of 1T-WS2 at the contact interface, and how does it compare to homogeneous nucleation?
The heterogeneous nucleation barrier at the contact interface of two 2H-WS2 nanosheets is 0.005 eV, whereas homogeneous nucleation requires 2.314 eV. This 463-fold reduction in barrier height explains why phase transformation occurs preferentially at large-angle boundaries and enables rapid, low-energy conversion at 1000 °C.
At what temperature does the thermodynamic stability of 2H-WS2 and 1T-WS2 reverse, and what are the implications for synthesis?
First-principles calculations show that below 280 K, 2H-WS2 is the stable phase, while above 280 K, 1T-WS2 becomes energetically favored. This crossover at 280 K defines a clear processing window: synthesis at 1000 °C ensures that the 1T phase is thermodynamically accessible, and the large-angle contact further lowers the kinetic barrier to realize phase-pure 1T-WS2.
What is the minimum crystallographic orientation difference required to induce the 2H-to-1T phase transformation?
The transformation is triggered when the orientation difference between contacting 2H-WS2 grains exceeds 10°. This threshold ensures sufficient lattice mismatch to generate defects and lattice distortion at the boundary, which promotes atomic rearrangement and the formation of 1T-WS2. Below 10°, the mismatch is insufficient to drive the transformation.
How does the 1T phase propagate from the initial contact boundary to form a large-area film?
Once 1T-WS2 nucleates at the large-angle boundary, the 1T region gradually expands through the host 2H nanosheet. The transformed 1T-WS2 then contacts surrounding 2H-WS2 nanosheets, inducing further transformation at those new interfaces. This self-propagating mechanism continues until the entire assembly converts to a continuous, large-area 1T-WS2 film.
What are the advantages of this synthesis method over conventional intercalation or CVD approaches for 1T-WS2?
The method uses only 2H-WS2 nanosheets as the raw material and requires no intercalation agents, organic solvents, or catalysts, eliminating contamination and simplifying purification. It yields phase-pure 1T-WS2 films with clean surfaces, which is critical for catalytic and electronic applications where residual impurities degrade performance. The process is straightforward and scalable, offering a cost-effective route to high-purity 1T-TMDs.
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