Article pubs.acs.org/cm
Reversible Crystal Phase Interconversion between Covellite CuS and High Chalcocite Cu2S Nanocrystals Yang Liu,† Maixian Liu,‡ and Mark T. Swihart*,† †
Department of Chemical and Biological Engineering, ‡Department of Pharmaceutical Science, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States S Supporting Information *
ABSTRACT: Copper deficient copper sulfides (Cu2−xS, 0 ≤ x ≤ 1) are earth abundant, nontoxic materials with size-, phase-, and composition-dependent localized surface plasmon resonance (LSPR). Although synthesis of Cu2−xS nanocrystals (NCs) has attracted substantial research attention, understanding of the transformations of copper sulfides between their many possible stoichiometries and crystal phases is still lacking. Here, we develop a reversible transformation between CuS, which has a high density of free charge carriers and strong LSPR, and high chalcocite Cu2S with no LSPR. Initial CuS nanoplatelets (NPls) with a diameter of 55 nm and thickness of 4 nm were transformed into round high chalcocite Cu2S NPls with a diameter of 29.2 ± 2.0 nm and a thickness of 10.8 ± 0.7 nm by treatment with 1-dodecanethiol (DDT), which can reduce disulfide bonds in covellite. Treatment with an oleic acid− sulfur complex (OA-S), which serves as a sulfur source, can restore the hexagonal shape of the original CuS NPls during the reverse transformation from Cu2S to CuS, producing hexagonal NPls with a diameter of 43.1 ± 2.0 nm and thickness of 11.2 ± 0.9 nm. We also treated monodisperse, spherical roxbyite (Cu1.78S) NCs with OA-S and obtained hexagonal CuS NPls, showing that the tendency of CuS to form hexagonal NPls is an intrinsic result of its crystal structure. For comparison, we used different sulfur precursors to drive the transformation from Cu2S to CuS, illustrating the different reactivities of S-sources with Cu2S. This interconversion not only provides a better understanding of possible transformations in copper sulfide nanostructures but also provides new possibilities for the well-controlled colloidal synthesis of these nanomaterials with combinations of phase, size, shape, and LSPR energy not previously obtainable.
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INTRODUCTION Over the past decade, copper sulfide nanocrystals (NCs) have captured the attention of many researchers because of their potential for use in thermoelectric,1 electrocatalytic,2 and photovoltaic applications.3 The composition-, crystal structure-, size-, and shape-dependent properties of copper sulfides have been well explored, especially for materials prepared by colloidal synthesis.4−15 Copper sulfide NCs exhibit a wide range of stable or metastable stoichiometries and crystal structures, including Cu2S,4−8 Cu31S16 (Cu1.96S),9 Cu9S5 (Cu 1.8 S), 10 Cu 7 S 4 (Cu 1.75 S), 11 Cu 9 S 8 (Cu 1.12 S), 12 and CuS.13−15 Among all these copper sulfides, covellite (CuS) has recently attracted considerable attention because it has strong p-type metallic character, as well as the highest concentration of free carriers in the copper sulfide class of materials.16 Solid−solid phase transformations in NCs are of great importance not only for providing an understanding of nanostructures themselves but also as novel pathways to produce new nanostructures. Pressure-induced phase transformation of NCs can occur by single nucleation events.17,18 Other parameters, including temperature and magnetic field, © 2017 American Chemical Society
have been shown to play key roles in phase transformations of NCs.4,19 Reports of reversible transformations of NCs between crystal phases containing the same elements at different stoichiometries are relatively rare. The copper-sulfide system, by virtue of the many possible binary phases of different stoichiometries, provides unique opportunities to investigate such transitions. While transformations that require adding or removing atoms from a solid are likely to be kinetically limited by solid state diffusion in bulk samples, they are more easily observed in discrete NCs where the length scale for diffusion is very small. Methods of obtaining NCs of Cu2−xS (0 ≤ x ≤ 1) with a specific targeted stoichiometry and crystal phase are wellknown. However, understanding of the transformation between these copper sulfides is still lacking. Understanding the mechanism of transformation starting from arbitrary Cu2−xS has been a challenge because most reported Cu2−xS NCs contain a mixture of crystal phases of x value from 0 to 1. Received: February 10, 2017 Revised: May 19, 2017 Published: May 19, 2017 4783
DOI: 10.1021/acs.chemmater.7b00579 Chem. Mater. 2017, 29, 4783−4791
Article
Chemistry of Materials
carriers and strong localized surface plasmon resonance (LSPR), the NCs are converted to Cu2S NCs with no LSPR and then transformed back to covellite CuS. The forward transformation produces high chalcocite Cu2S NCs in the form of round NPls with smaller diameter compared to the hexagonal NPl templates. This intrinsic shape evolution is induced by 1-dodecanethiol (DDT), which acts as a reducing agent to break disulfide bonds in covellite. Subsequent treatment with oleic acid-S (OA-S), a sulfur source, can restore the hexagonal shape during the reverse transformation from Cu2S to CuS. The overall process overcomes the limited growth of CuS along the c-axis, producing NPls with a thickness of ∼11 nm. In addition, we treated Cu2S NPls with other organo-sulfur complexes to provide insight into their ability to induce the formation of covellite or other copper-deficient Cu2−xS phases and their effect on the NPl morphology.
Moreover, the positions of copper and sulfur ions in these crystal phases are not well-defined. Therefore, reports of obtaining a controlled pure phase via the transformation of another copper sulfide phase are limited. In consideration of these challenges, the two stoichiometric limits (CuS and Cu2S) are the ideal initial templates to obtain other compositions.20 The transformation from CuS to Cu2S by adding Cu precursors into a system containing CuS NCs has been reported,16 leading to an increased Cu/S atomic ratio, crystal phase transformation, LSPR damping, and changes in the oxidation states of Cu and S. However, this approach may introduce foreign ions like F− and Cl−. Although many studies have demonstrated the ability to preserve the morphology of NCs during changes in composition,16,21−23 compositioninduced morphology evolution is also of great interest, especially if it can be controlled.24 There are few reports demonstrating the production of thicker (>10 nm) Cu2S NPls,25,26 and these all relied on processes carried out at high temperature (>180 °C). Therefore, the idea of intrinsic shape evolution emerges to describe the transformations between nanostructures without introducing extrinsic elements into the system. On the other hand, the transformation from Cu2S to CuS has rarely been considered. On the basis of our previous work, covellite CuS can be prepared by mixing CuCl2 with ammonium sulfide (AS) solution at ambient conditions.13 The sulfur precursor and ligand should be carefully chosen for the synthesis of covellite CuS; other sulfur donors and ligands would lead to Cu2−xS with nonuniform NC size or shape.27 In addition, most covellite nanoplatelets (NPls) have been limited to a fixed thickness (
