Surface modification-induced phase transformation of hexagonal close-packed gold square sheets.

Zhanxi Fan,Hua Zhang,Bing Li,Wenxiong Shi,Xiao Huang,Michel Bosman,Chee Lip Gan,Zhiyuan Zeng,Qingxiao Wang,Shuzhou Li,Yu Han,Yihan Zhu,Jumiati Wu,Qing Liu

doi:10.1038/ncomms7571

Abstract

Conventionally, the phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). Here we report the complete phase transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can also be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs. Depending on the surfactants used during the metal coating process, two transformation pathways are observed, leading to the formation of (100)f-oriented fcc Au@Ag core-shell square sheets and (110)h/(101)f-oriented hcp/fcc mixed Au@Ag nanosheets. Furthermore, monochromated electron energy loss spectroscopy reveals the strong surface plasmon resonance absorption of fcc AuSS and Au@Ag square sheet in the infrared region. Our findings may offer a new route for the crystal-phase and shape-controlled synthesis of inorganic nanocrystals.

Highlights

The phase transformation of inorganic nanocrystals is realized under extreme conditions
The hcp Au square sheets (AuSSs) capped by oleylamine were synthesized by our recently reported method with a slight modification[28]
Transmission electron microscope (TEM) analysis confirmed that the AuSSs have a pure hcp structure, consistent with our previous report (Supplementary Fig. 1)[28]

Summary

Introduction

The phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). We report the complete phase transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs. Depending on the surfactants used during the metal coating process, two transformation pathways are observed, leading to the formation of (100)f-oriented fcc Au@Ag core-shell square sheets and (110)h/(101)f-oriented hcp/fcc mixed Au@Ag nanosheets. Our findings may offer a new route for the crystal-phase and shape-controlled synthesis of inorganic nanocrystals. The study of crystal polymorphs and phase transformations of inorganic nanocrystals is essential to their controlled synthesis and wide applications. Until now it remains a great challenge to realize the complete transformation of a nanocrystal from one crystal phase to another at ambient conditions. The phase transformation of metals and alloys with closepacked structures (that is, hexagonal close-packed (hcp) or fcc) has been well studied both experimentally and theoretically[20,21,22]

Methods

Results

Conclusion