Mechanical Exfoliation of Plate-stratiform Structured Bi12O17Cl2 for Enhanced Photocatalytic Performance
Authors: Zhao Mei , Guo Zhipeng , Fu Congqi , Wang Siwei , Zhao Linlin , Dong Lifeng 
Published: February 2020

Bismuth oxychlorides including BiOX (X=Cl, Br, or I) have been the subject of novel photocatalyst series due to their unique layered structure and high chemical stabilities. Internal static electric field perpendicular to layers can effectively separate electron-hole pairs and decrease the distance to reactive sites on surface, which thus make possible higher photocatalytic performance. In this series, oxygen-rich bismuth oxychlorides BixOyXz (X=Cl, Br, or I) with modest band gaps and visible-light-response ability have become fast-growing candidates, such as Bi12O17Cl2, Bi3O4Cl, Bi4O5Br2, Bi24O31Br10 and Bi5O7I. However, their photodegradation efficiency is not acceptable and their practical application is limited due to high recombination rate of photogenerated charge carriers. Therefore, current work has been put to couple oxygen-rich bismuth oxychlorides with other semiconductors or metal dots to improve their photodegradation efficiency. More effectively unfold their distinct layer structure to expose larger surface area by some physical technique has been investigated as a “clean” and targeted strategy, such as exfoliation.

In our previous work, plate-stratiform nanostructured Bi12O17Cl2 with visible-light-response was prepared and characterized. The plate-stratiform Bi12O17Cl2 is stacked by a parallel array of ultrathin nanosheets, which parallelly aligned and closely superimposed. Since the exfoliation of graphene and C3N4 nanosheets from their bulk counterparts can bring unfolding exceptional properties, a facile and reliable method for high-yield production of Bi12O17Cl2 ultrathin nanosheets to make full exploration of Bi12O17Cl2 photocatalytic potentials is urgently needed. Besides chemical exfoliation and thermal decomposition, mechanical exfoliation using high-energy ball milling has been accepted as a simple and efficient way to produce large quantities of two dimensional nanomaterials.

In this study, a simple one-step high-energy ball milling method is firstly presented for mechanical exfoliation of plate-stratiform structured Bi12O17Cl2 into nanosheets with thickness only in several nanometers. The produced nanosheets in desired nanometer scale possess much larger surface area and shorter transferring distance, which benefit photocatalytic activity. TEM observation proves gradual size reduction of nanosheet thickness with increasing grinding time of Bi12O17Cl2 plate-stratiform nanomaterials. Photoluminescence spectra prove that Bi12O17Cl2 untrathin nanosheet has higher emission intensity than original plate-stratiform Bi12O17Cl2, representing improved separation efficiency of photogenerated carriers. Adsorption ability and photocatalytic efficiency of Bi12O17Cl2 untrathin nanosheet were greatly enhanced after exfoliation. The crucial role of its exfoliation refinement on enhanced photocatalytic performance under visible light is discussed. This mechanical exfoliation method is not only easy, low cost and high-yielding, but also provides a more “clean” way for layer-structured bismuth-based materials into two-dimensional nanosheets with broad spectrum of properties.