CdPd sulfide heterostructured nanoparticles with metal sulfide seed-dependent morphologies

Abstract

Seed-mediated growth synthesis has provided us with anisotropically phase-segregated CdPd sulfide heterostructured nanoparticles with seed-dependent morphologies and crystal structures.

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Recently, anisotropically phase-segregated heterostructured nanoparticles (NPs) have become accessible and received much attention.¹ The close coupling of different components on the nanoscale may significantly improve the application performance, or even create new properties.² Especially, a combination of distinct metal chalcogenide NPs would have great potential for various photo-related applications.³ They may also serve as building blocks for complex nanostructures in nanodevices.⁴ Recently, we reported anisotropically two and three phase-segregated metal sulfideNPs consisting of Co₉S₈ and PdS_x (CoPd sulfideNPs).^5,6 On the basis of our proposed mechanism for the formation of CoPd sulfideNPs in which the crystalline Co₉S₈ phases anisotropically grow on the amorphous PdS_x seed NPs, the PdS_xNPs are the key material for the formation of anisotropically phase-segregated heterostructures.^5,6 Thus various metal sulfideNPs have potential as seeds for the designed synthesis of heterostructured metal sulfideNPs with seed-dependent morphologies. Herein, in the light of structural control, we report on the synthesis of unique CdPd sulfide heterostructured NPs by means of the PdS_x and CdS seed-mediated growth methods. Flower-shaped NPs were generated through an interesting crystal structural transformation of amorphous PdS_x seeds into crystalline Pd_xCd_yS induced by cation exchange of Cd²⁺ ions with Pd²⁺ ions. Dumbbell-shaped NPs were formed via linking CdS seeds with a small single PdS_x phase, this mechanism being applicable to obtain CdS nanorod chains.

First, the PdS_x seed-mediated growth reaction was carried out to produce PdS_x–CdS heterostructured NPs. Amorphous PdS_x seed NPs of 5.4 ± 0.8 nm in size (Fig. 1a) were synthesized according to our previous work.⁶ The PdS_x seeds were reacted with di-n-octyl ether solution containing Cd(acac)₂·xH₂O and 1-octadecanethiol (C₁₈SH) at 260 °C for 120 min to form flower-shaped heterostructures (nanoflowers, NFs) as shown in Fig. 1b. It was found from the 2D projection of a TEM image that the NFs had faceted dark cores with one to four bright petals of 3 nm (Fig. S1, ESI† ). Local chemical compositional analysis of a single NF measured by nano-spot energy-dispersive X-ray (EDX) spectroscopy showed that the dark cores had Pd-rich Pd_xCd_yS phases and the bright petals were composed of CdS (Fig. S2, ESI† ), suggesting that the Cd²⁺ ions were replaced with Pd²⁺ ions in the PdS_x phases and the nucleation of CdS occurred on the Pd_xCd_yS cores. The size of the Pd_xCd_yS cores is similar to that of the PdS_x seeds. The fractions of the NFs consisting of one, two, three, and four petals were estimated to be 44, 34, 20, and 2%, respectively.

Fig. 1 (a, b) TEM and (c, d) HRTEM images of (a, c) PdS_x seed NPs and (b, d) NFs. (e) XRD patterns of PdS_x seed NPs and NFs. Inverted triangles are assigned to wurtzite-CdS. (f) FFT pattern of rectangular region of (d). Depicted Miller indices of Pd_xCd_yS phase are based on a cubic system.

The crystal structure of CdS/Pd_xCd_yS (CdPd sulfide) NFs was investigated by high-resolution TEM (HRTEM) and X-ray diffraction (XRD). The HRTEM observation revealed that the CdS phases possessed both a zincblende structure with stacking faults and a minor fraction of a wurtzite structure (Figs. 1d and S3, ESI† ). XRD patterns of NFs also demonstrated that the amorphous PdS_x phases completely disappeared and were transformed into crystalline phases and that the CdS phases of NFs were composed of a mixture of zincblende and a small amount of wurtzite (Fig. 1e). The CdS/Pd_xCd_yS interfacial lattice planes of the CdS phases correspond to zincblende(zb)-CdS (002), (111) and wurtzite(w)-CdS (100). Interestingly, the PdS_x seed NPs are amorphous, although the Pd_xCd_yS phases have a crystalline nature (Figs. 1c–e). An ionic crystal containing the elements Cd, Pd and S is not identified in the Powder Diffraction File (JCPDS-ICDD). From the detailed HRTEM and fast Fourier transformation (FFT) investigations (Fig. S3, ESI† ), we assumed that the Pd_xCd_yS phase belongs to a cubic crystal system with a lattice constant of 7.92 Å. Actually, this assumption of Pd_xCd_yS crystal structure can well explain the diffraction peaks at 2θ = 31.9°, 39.2°, 45.8°, 66.2°, and 71.0° (Fig. 1e and Table S1, ESI† ). The detailed crystal structure of the Pd_xCd_yS phase is under investigation using synchrotron-XRD and EXAFS measurements and will be reported in a forthcoming paper.

Next, the formation mechanism of CdPd sulfide NFs was investigated. Although CdS can nucleate independently in the same reaction conditions as for NF synthesis (Fig. S4, ESI† ), the CdS phases preferentially grow on the PdS_x seeds (isolated CdS NPs are rarely observed). This result indicates that the PdS_x seeds behave as the preferential growth substrates for CdS, probably because the CdS phases grow through a sulfur transfer from PdS_x seeds to the Cd precursor, as observed in the PdS_x/Co₉S₈ system.^5,6 Actually, a control experiment of PdS_x seed-mediated growth without an addition of alkanethiol (a sulfur precursor) also demonstrated both the CdS growth on the PdS_x seeds and the formation of Pd_xCd_yS phases (Fig. S5, ESI† ), which indicates that the CdS phases nucleate by consuming S²⁻ ions in the PdS_x seeds. It is quite interesting that the crystal structural transformation from amorphous PdS_x to crystalline Pd_xCd_yS phases is induced by the cation exchange of Cd²⁺ ions with Pd²⁺ ions in PdS_x seeds (cf., the cation exchange of smaller Co²⁺ ions does not occur⁶) and that S²⁻ ions subsequently diffuse from the PdS_x seeds to the outside to form the CdS phases. Change in the XRD patterns of CdPd sulfide NFs with reaction time demonstrates that the CdS phases grow on the initially-formed Pd_xCd_yS phases (Fig. 2a). X-ray photoelectron spectroscopic analysis revealed that the elements Pd and Cd in Pd_xCd_yS phases were in the divalent state (Fig. S6, ESI† ), strongly suggesting that the Pd²⁺ ions dissolve into the solution in the course of both the Cd²⁺ invasion and S²⁻ diffusion to keep the charge balance of Pd_xCd_yS phases (Fig. 2b).

Fig. 2 (a) XRD patterns of CdPd sulfide NFs at various reaction times. (b) Schematic illustration of the formation mechanism of NFs.

On the basis of the fact that the CdS phases preferentially bind to Pd_xCd_yS phases as described above, an effort was made to further develop the seed-mediated growth method, in which the CdS NPs were used as seed NPs. In the CdS seed-mediated growth reaction, nearly spherical w-CdS seed NPs (10.2 ± 2.5 nm, Fig. 3a) were synthesized according to literature protocols.⁷ The reaction of the CdS seeds with Pd(acac)₂ and C₁₈SH at 260 °C for 180 min predominantly gave dumbbell-shaped CdS/PdS_x/CdS NPs (nanodumbbells, NDs), two CdS phases connected by one PdS_x phase, as shown in Fig. 3b. More than 80% of the NPs had a dumbbell-shape (Fig. S7, ESI† ), and the average size was 20 nm (length) × 10 nm (width), the size of the CdS seeds (10.3 ± 1.9 nm) being almost preserved.

Fig. 3 TEM images of (a) CdS seed NPs and (b) NDs. (c) XRD patterns of CdS seed NPs and NDs. (d) HRTEM image of a single ND.

In the XRD pattern of NDs, the diffraction peaks from w-CdS phases were mainly observed even after the PdS_x growth on the CdS seeds (Fig. 3c). Peaks from the PdS_x phases were not detected owing to their volume being smaller than that of w-CdS phases (the CdS/PdS_x volume ratio is estimated to be ∼26.7). According to the HRTEM observation of NDs, an interfacial lattice plane of w-CdS phase was (100) planes of w-CdS, as observed in NFs (Fig. 3d). In contrast to the NFs, the Pd-rich phases of NDs were amorphous-PdS_x, because the Cd²⁺ ions of the CdS seeds could not diffuse into newly-grown PdS_x phases. For comparison, we synthesized similar dumbbell-shaped heterostructures by a one-pot synthesis (see experimental details in ESI† ), in which the CdS phases were grown on the preformed PdS_xNPs to form dumbbell structures. These dumbbell-shaped NPs have zb-CdS/crystalline Pd_xCd_yS/zb-CdS structure (Fig. S8, ESI† ), which supports our claim that the formation of Pd_xCd_yS phases takes place only when the amorphous PdS_xNPs react with Cd²⁺ ions dissolved in solution.

Next, the formation mechanism of NDs was investigated. On the way to forming the NDs, NP dimers consisting of small PdS_x (1–2 nm) and large CdS phases were obtained when the CdS seed-mediated growth reaction was stopped at 10 min (Fig. 4b). After annealing the purified CdS–PdS_x dimers with an excess amount of oleic acid and oleylamine at 260 °C for 60 min, almost all the dimers (>90%) were converted into dumbbell-shaped NPs as shown in Fig. 4c (an excess amount of surfactants was added to prevent the fusion of CdS phases). Note that further Pd and Cd precursors were not added, meaning that no further crystal growth occurred in this annealing process. This result indicates that the dumbbell structures are generated by fusing two PdS_x phases of CdS–PdS_x dimers, as observed in the formation of peanut-shaped PdS_x/Co₉S₈/PdS_xNPs.⁶ However, another mechanism including an attachment of PdS_x phases of dimers to the surface of isolated CdS NPs should also be considered. In order to confirm this possibility, a mixture of the same amounts of CdS–PdS_x dimers and CdS NPs was heated under the same annealing conditions (Fig. S9, ESI† ). As a result, a large amount of free-standing CdS NPs was observed, which indicates that the fusion of PdS_x phases is responsible for the formation of NDs (Fig. 4d).

Fig. 4 TEM images of (a) CdS seed NPs, (b) CdS–PdS_x dimers, and (c) CdPd sulfide NDs obtained by annealing CdS–PdS_x dimers. (d) Schematic illustration of the formation mechanism of NDs.

By utilizing this formation mechanism, the self-assembly of CdS nanorods (NRs) was carried out via a linkage of CdS NRs with PdS_x phases. CdS NRs (6 × 16 nm, Fig. S10, ESI† ) were synthesized using the reported procedure.⁷ The CdS NR seed-mediated growth reaction was carried out in conditions the same as those for spherical CdS seed-mediated growth reaction. As shown in Fig. 5a, PdS_x phases grew preferentially at either end of CdS NRs and fused to link NRs, resulting in the formation of one-dimensional chains. The longest CdPd sulfide nanochains have a length longer than 50 nm aligning more than four CdS NRs. In this case, interfacial planes of CdS seeds were w-CdS (103) or (002) planes, which was different from the case of NDs (w-CdS (100)), probably due to a nonepitaxial growth of PdS_x on the CdS seeds. Because the side walls of NRs may be strongly protected by H⁺, the reactivity of side walls is lower than that of both ends,^7,8 leading to the selective growth of the PdS_x phases on either end of NRs. A minor fragment of T-shaped heterostructures was also observed, which resulted from PdS_x growth on the highly reactive defect site on the side wall of CdS NRs (Fig. 5c).

Fig. 5 (a) TEM image of CdPd sulfide nanochains. (b) HRTEM image of CdPd sulfide nanochain focused on the interface of CdS/PdS_x phases. (c) HRTEM image of T-shaped CdPd sulfide heterostructures.

In conclusion, we have demonstrated that CdPd sulfide heterostructured NPs with metal sulfide seed-dependent morphologies can be selectively synthesized by various seed-mediated growth methods. These results show that various metal sulfides can be candidates as building blocks for creating the desired functional heterostructured NPs. We are currently studying the structure-specific functions of these new NPs such as a directed self-assembly.

This work was supported by a Grant-in-Aid for Exploratory Research (No. 20655027) (T.T.) and a JSPS Research Fellowship for Young Scientists (M.S.). We thank S. Yasuhara and T. Shimotsu for HRTEM–EDX measurements. We also thank Prof. H. Sugimura for XPS measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, characterization techniques, TEM and STEM-HAADF images, HRTEM images, corresponding FFT patterns, and nano-spot EDX results of NFs, TEM image of CdS NPs in the absence of PdS_x seed-NPs, TEM and XRD of CdPd sulfideNPs without an addition of alkanethiol, XPS spectra of Pd_xCd_yS phases, and TEM images of NDs and CdS nanorods. See DOI: 10.1039/b902189b

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