In Situ Monitoring of 2D Nanocrystal Nucleation and Growth Kinetics by Using Time-Resolved Liquid Interface Grazing Incidence X-Ray Scattering Techniques

The design and synthesis of high-quality two-dimensional (2D) materials with desired morphology are essential for property control. One critical challenge that impedes the understanding and control of 2D crystal nucleation and growth is the inability of direct observation of the nanocrystal evolution process with high enough time resolution. Here, we demonstrated an in situ X-ray scattering approach that directly reveals 2D nanocrystal nucleation and growth at the air-water interface.¹ A-few-nm thick wurtzite ZnO nanosheets (NSs) were grown through the ionic layer epitaxy (ILE) method with the guidance of an ionic surfactant monolayer at the air-water interface. Fast monitoring of both in-plane and out-of-plane crystal structures were achieved with a time resolution of only a few minutes by applying grazing incidence X-ray diffraction (GID) and grazing incidence X-ray off-specular scattering (GIXOS) techniques, allowing direct observation of the 2D crystal nucleation and growth processes with a unit-cell resolution. We discovered an obvious lattice deformation in the first two surface layers, which is different from the inner bulk phase. This discovery of surface strain in ILE may open a new route toward strain-induced physical property tuning of 2D nanomaterials by surfactant engineering. Our results also uncovered a lateral to vertical growth kinetics switch phenomenon in the ZnO nanosheet growth. This switch represents the 2D to three-dimensional (3D) crystal structure evolution, which governs the size and thickness of nanosheets, respectively. This phenomenon evidences the significance of the surfactant monolayer in ILE NS growth and can guide 2D nanocrystal synthesis with rationally controlled size and thickness. In addition, we show the impacts of different synthesis parameters on the NS growth behavior. Higher surfactant density enhanced the effects of the electrical double layer extending the lateral growth period, and higher Zn²⁺ concentration accelerated the 2D crystal growth, which is both favorable for large NS morphology formation. These results provide fundamental insights into the surfactant templated 2D growth mechanism in the ILE NS synthesis. The successful in situ diffraction characterization expands the toolbox and opens a new pathway to reveal the fast crystal nucleation and growth kinetics at the liquid interfaces by using time-resolved grazing incidence X-ray scattering techniques.<br/><br/>REFERENCE<br/>1. Zhang, Z. et al. Nucleation Kinetics and Structure Evolution of Quasi-Two-Dimensional ZnO at the Air-Water Interface: An In Situ Time-Resolved Grazing Incidence X-ray Scattering Study. <i>Nano Letters </i><b>22</b>(7), 3040-3046 , doi:10.1021/acs.nanolett.2c00300 (2022).