Phase Control in Inorganic Nanocrystals through Finely Tuned Growth at an Ultrathin Scale

Crystalline polymorphs have been considered a prevailing phenomenon in inorganic nanocrystals and provide approaches to modulate fundamental properties and innovative advanced applications. As a basic demand for phase engineering, accessible and controllable synthetic methodologies are indispensable for acquisition of high-quality products in expected phases. Phase stability is also a non-negligible issue that determines continuous gains of functionality and long-term sustainability of characteristic features. Maintaining structural stability of metastable phases provides challenges and opportunities for investigations on fascinating properties and intriguing applications of inorganic nanocrystals. Phase engineering is of great significance to acquire metallic (1T) and semiconducting (2H) Mo- and W-based dichalcogenides for hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR), respectively. The catalysts in 1T phase have superior electron transfer kinetics and abundant active sites on both basal planes and edges for HER, while ones in 2H phase are preferentially deployed for CO₂RR to utilize edge sites for catalysis and restrain competitive HER activity. In addition, the photocatalytic performance for HER has been enhanced by combining anatase and rutile phases because electron transfer between the two phases during photocatalysis facilitates the separation of charge carriers and thus impedes the recombination of electron-hole pairs. Although ample effort has been devoted to developing phase engineering, principle understanding at an ultrathin scale remains obscure. In this Account, we provide comprehensive insight into work from our group regarding controllable synthesis of inorganic nanocrystals with phase engineering, critical effects on phase stability, and noteworthy studies on phase-related properties and applications. For bulk materials, phase control and transition have a large energy barrier, so they can only be achieved under rigorous conditions. However, at the initial stage of synthesis, especially for nucleation, there are a small quantity of chemical bonds that contribute to regulate phase and structure with ease. In our work, we mainly modulate nucleation and growth at an ultrathin scale to demonstrate facile approaches for phase engineering. This unique perspective makes for a distinct guidance of controllable synthesis and deliberate stabilization of inorganic nanomaterials with phase engineering. We have developed a series of synthetic strategies for phase engineering to fabricate inorganic nanocrystals in a specific phase with controlled size and composition and adjustable morphologies and surface features. Four sorts of models (MoS₂, ZrO₂, In₂O₃, and TiO₂) are used for demonstrating finely tuned growth at an ultrathin scale. However, phase engineering has been regarded as immature because only one phase in polymorphs is thermodynamically stable generally. Phase stability of metastable nanocrystals has attracted much interest. Our substantial investigations illustrate several crucial factors on phase stability, leading to inspiration for facilitating persistent emergence of characteristics and functionalities. By full use of the features of a specific phase, we spotlight ligand-induced surface interactions on coverage-dependent electronic structures and chemisorption effects at one-unit thickness of TiO₂(B) nanomaterials with phase engineering. Meanwhile, an energy conversion system for overall water splitting (OWS) drives forward steps in function-oriented synthesis of MoS₂-based nanomaterials with phase engineering. In the last section, we summarize this theme and highlight several promising directions for future development.