Seed-mediated synthesis of metal sulfide patchy nanoparticles

Abstract

Anisotropically phase-segregated nanoparticles, so-called patchy nanoparticles, are promising materials, because the close coupling of different components on the nanoscale may significantly improve application performance, or even create new properties. We review the seed-mediated synthesis of various kinds of metal sulfide patchy nanoparticles.

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Toshiharu Teranishi

Toshiharu Teranishi received his PhD from The University of Tokyo under the direction of Prof. N. Toshima in 1994. He spent seven and a half years at Japan Advanced Institute of Science and Technology as an Assistant Professor and an Associate Professor. In 2004, he moved to University of Tsukuba as a Full Professor. Current research interests include precise structural control of inorganic nanomaterials and structure-specific functions for high-performance devices and photo-energy conversion.

Masaki Saruyama

Masaki Saruyama received his undergraduate training at University of Tsukuba. After receiving a BSc degree in chemistry in 2006, he began his graduate work at University of Tsukuba under the supervision of Professor T. Teranishi, and is now carrying out PhD research on the synthesis and novel functions of inorganic patchy nanoparticles.

Masayuki Kanehara

Masayuki Kanehara graduated from Shinshu University in 1999. He completed his PhD at Japan Advanced Institute of Science and Technology in 2004 under the supervision of Professor T. Teranishi. At present he is serving as a Research Associate at University of Tsukuba. His current research interests include functionalization of inorganic materials with various organic molecules.

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Introduction

Chemically synthesized inorganic nanoparticles (NPs) are comprised of inorganic cores surrounded by organic surfactants. When the inorganic cores include more than two chemical species with a combination of metals, metal chalcogenides, and metal oxides, we should control the composition and distribution of each species as novel parameters in addition to size, shape, and crystal structure.¹ Focusing on the distribution of the two chemical species in a single NP, three kinds of structures are available dependent on the distribution manner, that is, the chemically-disordered alloy, layered core–shell, and anisotropic phase-segregation structures. Among these structures, anisotropically phase-segregated NPs, so-called patchy NPs, have recently become accessible and received much attention. The close coupling of different components on the nanoscale may significantly improve the application performance, or even create new properties.^2–29 The representative noble properties peculiar to the structure of patchy NPs are as follows.

(i) Multi-functions, such as a combination of magnetic and photoluminescent NPs;

(ii) directed self-assembly, achieved by modifying different functional ligands on each surface;

(iii) highly efficient charge separation at the hetero-interface in a single NP; and

(iv) creation of innovative materials resulting from atom and ion diffusion at the hetero-interface.

When synthesizing the bi-phasic patchy NPs, we can employ seed-mediated growth methods and heat-induced phase segregation by making use of the large interface energy between the two chemical species (Fig. 1a and 1b). In the case that the two distinct precursors are simultaneously reacted to produce bi-phasic patchy NPs, the reaction mechanism usually follows a seed-mediated growth reaction, that is, the selective nucleation of one chemical species is followed by growth of the second one, which emerges by developing an interface of graded composition.^6,7 To obtain further complex patchy structures, there are two representative formation mechanisms, as shown in Fig. 1c and 1d. Here, we review our recent results to selectively synthesize metal sulfide patchy NPs, based on the seed-mediated growth method.

Fig. 1 Schematic illustrations of possible mechanisms to form patchy NPs; (path a) selective nucleation on a starting seed, (path b) heat-induced phase-segregation, (path c) several nucleations on a starting seed, (path d) fusion of two reactive phases from two distinct bi-phasic patchy NPs.

Bi-phasic metal sulfide patchy NPs

Many efforts have been made toward the synthesis of metal chalcogenide patchy NPs for application to achieve efficient charge separation.^24,25Table 1 summarizes the various patchy NPs containing metal chalcogenides reported so far. The combination of metal sulfides, which are easy to synthesize and exhibit absorption and emission properties in a wide wavelength region, is still rare, and therefore the development of a facile synthesis of metal sulfide patchy NPs is very important.

Table 1 Representative patchy NPs cotaining metal chalcogenides

Material	Shape	Path^a	Ref.
a Formation path shown in Fig. 1.
PdSx/Co9S8	Acorn	a	6,30
Cu2S/In2S3	Acorn, bottle, larva	a	7
CdSe/Au	Dumbbell, bell-tongue	a	8
FePt/CdS	Snowman	b	16
γ-Fe2O3/sufide	Snowman	b	17
CdSe/Au	Rod, tetrapod	c	20
PbS/Au	Tetrapod	c	21
CdS/(Pt, PtNi, PtCo)	Rod	c	22
CdSe/(CdS, CdTe)	Rod, branched rod	c	23
CdSe/CdS	Rod, tetrapod	c	24
CdSe/CdTe	Barbell	c	25
(CdSe, ZnSe, ZnTe)/CdS	Rod, tetrapod	c	26
(CdSe, ZnTe, CdTe)/(CdS, CdTe)	Tetrapod	c	27
PdSx/Co9S8	Peanut	d	29
(PdSx, PdxCdyS)/CdS	Flower, dumbbell	c, d	31

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In 2004, we found that the anisotropically phase-segregated acorn-shaped PdS_x/Co₉S₈ (PdCo sulfide nanoacorns) are spontaneously formed, in which one phase is made up of cobalt sulfide and another of palladium sulfide.⁶ The PdCo sulfide nanoacorns were synthesized by the reaction of Co(acac)₂·2H₂O and Pd(acac)₂ in di-n-octyl ether in the presence of 1-octadecanethiol (C₁₈SH). Fig. 2a shows a high-resolution transmission electron microscopy (HRTEM) image of the resulting C₁₈S-protected nanoacorns. The acorn-shaped particles made up of both bright and dark phases having an average size of ∼14 nm (length) × 10 nm (width), and a Co/Pd atomic ratio of 40/60 were predominantly observed. The nano-spot energy-dispersive X-ray (EDX), the detailed HRTEM, and the powder X-ray diffraction (XRD, Fig. 2b) measurements revealed that the PdCo sulfide nanoacorns are made up of crystalline Co₉S₈ and amorphous PdS_x phases with an interfacial lattice plane of a Co₉S₈ phase as the (001) plane.

Fig. 2 (a) HRTEM image, (b) XRD pattern and (c) schematic illustration for the formation mechanism of PdCo sulfide nanoacorns (from ref. 6, T. Teranishi et al., J. Am. Chem. Soc., 2004, 126, 9914, with permission from the American Chemical Society, 2004).

The formation mechanism of the PdCo sulfide nanoacorns was monitored spectroscopically and microscopically and clarified as follows: (i) the reduction of Pd(II) ions with C₁₈SH to yield very small Pd_n(SC₁₈)_mNPs; (ii) the formation of amorphous PdS_xNPs by the spontaneous cleavage of the C–S bond of C₁₈S at the surface of the metallic palladium to liberate the sulfur atoms,^32,33 which transform the Pd NPs into the palladium sulfide NPs; and (iii) the anisotropic growth of the Co₉S₈ phases in the [001̄] direction by the supply of S²⁻ ions from PdS_xNPs to form the PdCo sulfide nanoacorns. The overall formation mechanism of the PdCo sulfide nanoacorns is schematically illustrated in Fig. 2c. The important point is the first nucleation of the PdS_xNPs followed by the anisotropic growth of the Co₉S₈ phases (path a in Fig. 1). This concept is exploited to form bi-phasic type II Cu₃₁S₁₆/CdS patchy NPs from the co-reaction of the respective precursors under suitable conditions.³⁴

Multi-phasic metal sulfide patchy NPs

On the basis of our proposed mechanism for the formation of PdCo sulfide nanoacorns, the PdS_xNPs are the key material for the formation of heterostructures. Accordingly, the PdS_x seed-mediated synthesis of anisotropically phase-segregated NPs was carried out.²⁹ The relatively monodisperse 5.6 ± 1.0 nm PdS_xNPs were synthesized as seeds by making use of the spontaneous cleavage of the C–S bonds of C₁₈S on the surfaces of the Pd NPs. The Co₉S₈ phases were anisotropically grown on the surface of the purified PdS_x seeds by reacting the PdS_x seeds with Co(acac)₂ and C₁₈SH in di-n-octylether at 230 °C for 40 min under nitrogen. Fig. 3a shows an HRTEM image of the resulting NP. The peanut-shaped NPs (nanopeanuts) made up of crystalline Co₉S₈ phases with amorphous PdS_x phases at either end, and having an average size of ∼10 nm (length) × 5 nm (width), were predominantly observed, where the size of the PdS_xNPs was preserved. Further crystal structural investigation of these nanopeanuts using XRD (Fig. 3b) measurements also reveals that the nanopeanuts have an amorphous PdS_x/crystalline Co₉S₈/amorphous PdS_x (PdCoPd sulfide) heterostructure with two interfacial lattice planes of Co₉S₈ phases as the (001) plane, which is observed for the PdCo sulfide nanoacorns.

Fig. 3 (a) HRTEM image of a single nanopeanut when observed in the [110] direction. (b) XRD patterns of PdS_x and PdCoPd sulfide nanopeanuts (from ref. 29, T. Teranishi et al., Angew. Chem., Int. Ed., 2007, 46, 1713, with permission from Wiley-VCH, 2007).

It is expected from the shape of the nanopeanuts that they are formed by fusing together two PdCo sulfide nanoacorns. Actually, once the spherical PdCo sulfide nanoacorns are formed, the less-passivated Co₉S₈ phases of the nanoacorns are easily fused at their further growth stage in the [001̄] direction to yield PdCoPd sulfide nanopeanuts. In the fusion process, two PdCo sulfide nanoacorns appear to fuse together by facing each other with their Co₉S₈ phases aligned in the same crystallographic orientation, following path d in Fig. 1. Here, we consider that the key factor determining whether the fusion of the two Co₉S₈ phases proceeds or not is the amount of passivating agent, C₁₈SH, added to grow the Co₉S₈ phases. It was reasonably concluded from additional experiments that the less-passivated PdCo sulfide nanoacorns (C₁₈SH/Co(acac)₂ ≤ 2) easily form nanopeanuts via the fusion of two Co₉S₈ phases to reduce interface free energies of the less-passivated Co₉S₈ phases, whereas the fusion of the nanoacorns synthesized at C₁₈SH/Co(acac)₂ ≥ 3 is suppressed due to the sufficient passivation of the Co₉S₈ surfaces by thiols, as shown in Fig. 3c.

Extension of the kinds of metal sulfide seeds is requisite for the designed synthesis of various metal sulfide patchy NPs. Recently, we have demonstrated that CdPd sulfide patchy NPs can be selectively synthesized from various seed-mediated growth methods.³¹ In the PdS_x seed-mediated growth, the crystal structural transformation from amorphous PdS_x to crystalline Pd_xCd_yS phases by the cation exchange of Pd²⁺ in PdS_x with Cd²⁺ in solution (cf., the cation exchange of Pd²⁺ with smaller Co²⁺ ions does not occur during the formation of PdCo sulfide nanoacorns and PdCoPd sulfide nanopeanuts) and the subsequent diffusion of S²⁻ takes place to form the flower-shaped Pd_xCd_yS/CdS patchy NPs, as shown in Fig. 4a,b, following path c in Fig. 1. The Pd_xCd_yS/CdS interfacial lattice planes of the CdS phases correspond to zinc blende (zb)-CdS (002), (111) and wurtzite (w)-CdS (100). An ionic crystal containing the elements Cd, Pd and S was not identified in the powder diffraction file (JCPDS-ICDD). From the detailed HRTEM and fast Fourier transformation (FFT) investigations, we assume that the Pd_xCd_yS phase belongs to a cubic crystal system with a lattice constant of 7.92 Å. Sophisticated dumbbell-shaped w-CdS/PdS_x/w-CdS patchy NPs with the interfacial lattice planes of the w-CdS phase as (100) planes, were formed by bridging CdS seeds with PdS_x phases (Fig. 4c,d) using the CdS seed-mediated growth reaction, following path d in Fig. 1. Contrary to the flower-shaped Pd_xCd_yS/CdS patchy NPs, the Pd-rich phases were amorphous PdS_x, because the Cd²⁺ ions of the CdS seeds could not diffuse into the newly-grown PdS_x phases. The dumbbell-shaped w-CdS/PdS_x/w-CdS patchy NPs were found to be generated by fusing together two PdS_x phases of w-CdS-PdS_x dimers, as observed in the formation of peanut-shaped PdS_x/Co₉S₈/PdS_xNPs. By utilizing this formation mechanism, the self-assembly of CdS nanorods (6 × 16 nm) successfully proceeded via the linkage of CdS nanorods with PdS_x phases (Fig. 4e,f). Recently, a similar concept was demonstrated to assemble semiconductor nanorods by Manna and co-workers.³⁵ These results confirm that various metal sulfides can be considered as candidate building blocks for creating desirable functional patchy NPs. The partial cation exchange of metal sulfidenanostructures was revealed to be useful to obtain the complex metal sulfide patchy structures like the CdS/Ag₂S nanorods reported by Alivisatos and co-workers.^36,37

Fig. 4 TEM images of (a,b) flower-shaped Pd_xCd_yS/CdS, (c,d) dumbbell-shaped CdS/PdS_x/CdS patchy NPs and (e,f) CdS/PdS_x/CdS nanochains, where (a), (c) and (e) are each starting seed, respectively. Insets show the schematic illustrations of the formation mechanism. (from ref. 29, T. Teranishi et al., Chem. Commun., 2009, 2724, with permission from Royal Society of Chemistry, 2009).

Conclusions and prospects

We demonstrated that the seed-mediated growth synthesis effectively works for creating desirable metal sulfide patchy NPs. This would lead to the following prospects for the patchy NPs;

(i) enrichment of the patchy NP library;

(ii) programmable assembly of NPs induced by the selective modification of the NP surface by the different ligands;

(iii) creation of various innovative materials by the hetero-interfacial atom or ion diffusion; and

(iv) realization of hetero-interfacial charge separation.

Patchy NPs have a great potential to provide novel functional materials by making use of their structure specificity.

Acknowledgements

This work was supported by a Grant-in-Aid for Exploratory Research (No. 20655027) and Scientific Research on Priority Area “Strong Photon-Molecule Coupling Fields” (No. 19049007) (T.T.) and a Research Fellowship of JSPS for Young Scientists (No. 20429) (M.S.).

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