Selective decoration of nanocrystals on single-crystalline PtTe nanowires based on a solid-state reaction

Abstract

In this study, we demonstrate the fabrication of single-crystalline PtTe nanowires with/without decoration of nanocrystals from different heterostructure nanowires. Sb₂Te₃/Pt and Te/Pt core/shell nanowires were prepared by Pt deposition on single-crystalline Sb₂Te₃ and Te nanowires, respectively, using a sputtering process. When Sb₂Te₃/Pt core/shell nanowires were annealed at 600 °C, Sb nanocrystal-decorated PtTe nanowires were fabricated by a solid-state reaction between the core Sb₂Te₃ nanowires and the Pt shell layers. PtTe nanowires without decoration of nanocrystals were produced by annealing Te/Pt core/shell nanowires at a temperature of 600 °C. Based on the experimental results, the comprehensive mechanism for the formation of PtTe nanowires with/without decoration of nanocrystals is discussed herein. We believe that this facile method, based on solid-state reactions, can be exploited to develop low-dimensional Te-based dichalcogenides to improve functionality in a variety of fields such as energy conversion, energy transition, superconductors and topological insulators.

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Introduction

In recent years, nanocrystal-decorated structures have been studied in efforts to improve the performance of various applications such as thermoelectric devices,^1–3 Li batteries⁴ and solar cells.⁵ For example, Ag nanoparticles that precipitate in a bulk Bi₂Te₃ matrix can increase the scattering of phonons.¹ As a result, the lattice thermal conductivity is decreased and the thermoelectric performance can be greatly improved.^1–3 In one-dimensional nanostructures, in particular, nanocrystal-decorated nanowires (NWs) have been suggested as an alternative to enhance the performance of NW-based applications such as solar cells,⁵ gas sensors,⁶ supercapacitors,⁷ hydrogen production,⁸ surface-enhanced Raman scattering (SERS),^9,10 and photocatalytic-,¹¹ bio-¹² and nano-antenna systems.¹³ To prepare nanocrystal-decorated NWs, solution-based methods such as galvanic displacement,¹⁴ electroless metal deposition (EMD)⁵ and a sol–gel process⁸ mostly have been used.

Te-based dichalcogenide nanostructures such as Sb₂Te₃,^15–17 Bi₂Te₃,^16–18 PbTe,^19,20 Ag₂Te²¹ and CdTe²⁰ have been widely studied for various applications of phase-change random access memory (PRAM), thermoelectric devices, topological insulators and solar cells due to their unique properties. However, the research on PtTe nanostructures has hardly been reported.²² Using a solid-state reaction, therefore, in this paper we introduce an easily applied fabrication method of single-crystalline PtTe NWs by thermal annealing of Sb₂Te₃/Pt and Te/Pt core/shell NWs. Recently, it has been reported that various nanostructures such as NWs and nanotubes (NTs) can be produced by a solid-state reaction between core- and shell-materials in heterostructure NWs or NWs adjoined with a metal electrode.^23–25 The structure and the composition of the final products are dependent on both the reactivity and the diffusion rate among materials. If the core materials diffuse much faster than the shell materials in heterostructure NWs, hollow NTs are obtained instead of NWs.²⁵ If not, NWs are produced. Hence, the selection of core- and shell-materials is very important to fabricate the desired nanostructures using a solid-state reaction. Previously, we demonstrated catalyst-free, spontaneous and selective growth of single-crystalline Sb₂Te₃ and Te NWs via thermal annealing of sputter-deposited thin films at a temperature of 250 °C.¹⁵ As-grown Sb₂Te₃ and Te NWs, prepared by a catalyst-free growth method, are suitable for use as templates because no impurities such as a metal catalyst exist in the NWs and the influence of impurities thus can be ignored to study solid-state reactions with heterostructure NWs.¹⁵

When Sb₂Te₃/Pt core/shell NWs were used as starting materials, PtTe NWs covered with Sb nanocrystals were fabricated by thermal annealing. On the other hand, pure PtTe NWs were obtained by annealing Te/Pt core/shell NWs. This reveals that the morphologies of the PtTe NWs can easily be controlled by changing the core materials (Sb₂Te₃ and Te NWs). We suggested the comprehensive mechanism for selective decoration of Sb nanocrystals of single-crystalline PtTe NWs, which can also explain the fabrication mechanism of pure PtTe NWs.

Experimental

Al–Sb–Te thin films were deposited by co-sputtering of both Al (99.999%) and Sb₂Te₃ (99.99%) targets. When they were annealed at 250 °C under a N₂ and O₂ gas atmosphere, single-crystalline Sb₂Te₃ and Te NWs, respectively, were spontaneously grown on Al–Sb–Te thin films. The conditions for the selective growth of Sb₂Te₃ and Te NWs were presented in greater detail in our previous report.¹⁵ To prepare Sb₂Te₃/Pt and Te/Pt core/shell NWs, Pt shell layers were deposited on films, which were grown with Sb₂Te₃ or Te NWs, using a radio frequency (RF) sputtering system with a Pt (99.9%) target. The applied RF power on the Pt target was 100 W. The deposition was performed for 200 seconds at room temperature. During the deposition, the flow rate of Ar gas was 20 sccm (standard cubic centimeter per minute) and the working pressure was adjusted to 3.5 × 10⁻² Torr. As-fabricated Sb₂Te₃/Pt and Te/Pt core/shell NWs were annealed in a tube furnace (Lindberg/Blue M) under a N₂ (99.999%) gas atmosphere. The flow rate of N₂ gas and the heating rate were 200 sccm and 5 °C min⁻¹, respectively. Thermal annealing was performed at 400 and 600 °C for 1 hour. The morphological, crystal structural and quantitative analyses of the NWs carried out using transmission electron microscopes (TEM, JEOL/2100F and FEI Tecnai/Tecnai G2 F30 S-Twin) equipped with an energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD, Rigaku D/MAX Rint 2000).

Results and discussion

A high-resolution TEM (HRTEM) image of a Sb₂Te₃ NW clearly shows the single-crystalline nature of Sb₂Te₃ phase (Fig. 1(a)). When Pt shell layers were deposited on Sb₂Te₃ NWs, Sb₂Te₃/Pt core/shell NWs were produced (Fig. 1(b) and (c) and S1†). Fig. 1(b) shows that the Sb₂Te₃/Pt core/shell NWs were linear in shape and smooth. Pt shell layers were densely coated on the Sb₂Te₃ NWs (Fig. 1(c)) and SAED pattern of the Sb₂Te₃/Pt core/shell NWs reveals the poly-crystalline nature of the Pt shell layers (inset in Fig. 1(c)).

Fig. 1 (a) TEM image of as-grown Sb₂Te₃ NWs. (b) and (c) TEM images and SEAD pattern of as-fabricated Sb₂Te₃/Pt core/shell NWs.

When Sb₂Te₃/Pt core/shell NWs were annealed at 600 °C, their surface morphologies were very irregular and rough compared with those of as-fabricated Sb₂Te₃/Pt core/shell NWs (Fig. 2(a) and (b) and S2†). It appears that many nanocrystals covered the NWs and an EDS analysis was performed to verify their composition. The atomic ratio of Pt and Te was about 1 : 1. From the TEM-EDS analyses, interestingly, the Sb content was detected in some regions but not in other regions in a NW (Fig. S3†). We investigated the crystal structures of the non-covered and covered regions of the NW in detail, denoted respectively as #1 and #2 (Fig. 2(b)). The HRTEM image of #1 reveals the single-crystalline nature of monoclinic PtTe (Fig. 2(c)). The d-spacing of the (3–11) crystal plane is 0.196 nm. Fig. 2(d) is a HRTEM image of #2 in Fig. 2(b). The boundary between the core PtTe NW and the nanocrystals was clearly observed (white dotted line in Fig. 2(d)). The crystalline plane (040) of orthorhombic Pt₃Sb₂ was observed at the region close to the core PtTe NW, while the crystalline planes (012) and (015) of Sb phase were detected nearest to the surface of the Sb₂Te₃/Pt core/shell NWs annealed at 600 °C. The TEM analyses reveal two important features: (1) the Sb₂Te₃/Pt core/shell NWs transformed into PtTe NWs covered with Sb nanocrystals (including Pt₃Sb₂ phases). (2) Sb atoms diffused out the surface and crystallized during thermal annealing of the Sb₂Te₃/Pt core/shell NWs.

Fig. 2 (a)–(d) TEM images of Sb₂Te₃/Pt core/shell NWs annealed at 600 °C. Inset in (c) is a SEAD pattern.

To verify the formation mechanism of the Sb nanocrystal-decorated PtTe NWs, TEM analyses of Sb₂Te₃/Pt core/shell NWs annealed at a temperature of 400 °C were carried out and XRD patterns of Sb₂Te₃/Pt core/shell NWs as a function of the annealing temperature were investigated (Fig. 3). The different crystalline planes of monoclinic PtTe and hexagonal PtTe₂ phases appeared in the HRTEM image (Fig. 3(a)). The overall morphologies of the NWs were still rough (upper inset of Fig. 3(a)). Sb nanocrystals with very small size (∼10 nm) were also observed at the inner portion of the NW (bottom inset of Fig. 3(a) and S4†). PtTe, PtTe₂, Sb₂Te₃, Pt, Sb and Te peaks were indexed by Joint Committee on Powder Diffraction Standards (JCPDS) no. 89-6166, 88-2277, 15-0874, 04-0802, 35-0732 and 36-1452, respectively, in XRD patterns of Sb₂Te₃/Pt core/shell NWs as a function of the annealing temperature (Fig. 3(b)). When no thermal treatment was performed, Sb₂Te₃, Pt and Te peaks were observed. Sb₂Te₃ NWs are spontaneously grown by stress-induced mechanism, which is related with the volume expansion of oxide in films during a thermal annealing.¹⁵ Films act as sources and can be exhausted during the growth of Sb₂Te₃ NWs. XRD analyses were performed with the films including Sb₂Te₃ NWs and the trace of Te may remain in films. The observation of Sb₂Te₃ and Pt peaks indicates that Sb₂Te₃/Pt core/shell NWs were successfully fabricated. When Sb₂Te₃/Pt core/shell NWs were annealed at 400 °C, however, PtTe and PtTe₂ peaks were newly formed and Sb₂Te₃ peaks disappeared. This reveals that the Sb₂Te₃/Pt core/shell NWs transited into polycrystalline NWs with PtTe and PtTe₂ phases. Although Sb nanocrystals were observed at the TEM analyses (Fig. 3(a) and S4†), no Sb peaks were found in the XRD spectrum. The reason is that very small crystals cannot be detected by XRD measurement.²⁶ When the annealing temperature was increased up to 600 °C, the PtTe₂ peaks disappeared and Sb peaks were observed. This indicates the formation of Sb nanocrystal-decorated PtTe NWs.

Fig. 3 (a) TEM images of Sb₂Te₃/Pt core/shell NWs annealed at 400 °C. (b) XRD patterns of Sb₂Te₃/Pt core/shell NWs with different annealing temperatures.

Although further study is needed about the formation mechanism of Sb nanocrystal-decorated PtTe NWs, considering the results of TEM and XRD analyses, we suggest that the two major reactions that occurred during the transition from Sb₂Te₃/Pt core/shell NWs into Sb nanocrystal-decorated PtTe NWs are as follows:

Sb₂Te_3(s) + 2Pt_(s) → PtTe_(s) + PtTe_2(s) + 2Sb_(s) (at 400 °C)

PtTe_(s) + PtTe_2(s) + 2Sb_(s) → 2PtTe_(s) + 2Sb_(s) + Te_(S) (at 600 °C)

Previously, we directly observed that Sb₂Te₃ NWs were thermally decomposed in Sb₂Te₃/Si core/shell NWs at the annealing temperature of 400 °C. This is two-thirds of the temperature (621 °C) required for bulk-Sb₂Te₃ melting, which is due to the size-effect of nanostructures.²⁷ In addition, this indicates that this temperature (400 °C) is sufficient for decomposing the Sb₂Te₃ phase on a nano-scale and forming new alloy phases (PtTe and PtTe₂) in the Sb₂Te₃/Pt core/shell NWs, as shown in Fig. 3(a) and S4.† The excess Sb was dispersed at the inner region of the NWs and crystallized with very small size, because the Pt–Sb–Te system does not have any ternary phases.²⁸ When Sb₂Te₃/Pt core/shell NWs were annealed at 600 °C, all PtTe₂ transited into PtTe and excess Te was formed. This phase conversion is consistent with a previous report.²⁹ Kjekshus demonstrated that a PtTe single phase was obtained by the reaction between Pt wires and Te powders at a temperature of 900 °C, while both PtTe and PtTe₂ phases were formed at temperature below 900 °C.²⁹ Moreover, it is known that Te is a volatile material due to its low melting point (449.51 °C) and low enthalpy of evaporation (52.55 kJ mol⁻¹).^30,31 Hence, the excess Te may be removed by melt/evaporation above the melting temperature (600 °C). During a heat treatment of polycrystalline solids, solute or unreacted (impurity) atoms in solution tend to be accumulated at defect sites such as grain boundaries and free surfaces for reducing the total energy of system.^32,33 In the case of single-crystalline NWs, the unreacted atoms can be segregated and accumulated at the free surface of NWs. Hence, Sb diffused into the surface of the newly formed PtTe NWs and grew into Sb nanocrystals. In this process, a Pt₃Sb₂ interphase can be produced at the interface between PtTe NWs and Sb nanocrystals (Fig. 2(d)). Based on the experimental results, the possible mechanism for the fabrication of Sb nanocrystal-decorated PtTe NWs is summarized in Fig. 4. We need more study about the formation mechanisms using theoretical and experimental approach with control of the compositions.

Fig. 4 Schematic diagrams showing the entire fabrication process of PtTe NWs with/without decoration of Sb nanocrystals.

If this mechanism is indeed valid, we wondered what would transpire if Te NWs were used as a templates instead of Sb₂Te₃ NWs? It is evident that pure PtTe NWs with no decoration of Sb nanocrystals can be obtained through a solid-state reaction between core Te NWs and Pt shell layers, as illustrated in Fig. 4. Although excess Te was present during the reaction, it can be easily removed by melt/vaporization. In previous reports, moreover, Te NWs have been used as templates for synthesizing Te-based dichalcogenide NWs such as PbTe,^19,20 Ag₂Te²¹ and CdTe²⁰ using vapor transport, solid-state reaction and solution-phase methods. Hence, we prepared Te/Pt core/shell NWs and examined them with the same experimental conditions as employed for Sb₂Te₃/Pt core/shell NWs. Fig. 5 shows TEM images, SAED pattern and EDS mapping images of Te/Pt core/shell NWs annealed at 600 °C. The morphologies of the NWs are smooth and linear (Fig. 5, S5 and S6†). The atomic percent was 46.25 at% for Pt and 53.75 at% for Te, showing that the ratio was approximately 1 : 1 (Fig. S5†). Fig. 5(b) reveals the single-crystalline nature of monoclinic PtTe, while Te/Pt core/shell NWs annealed at 400 °C are not single-crystalline (Fig. S7†). This is similar with the case of Sb₂Te₃/Pt core/shell NWs. TEM-EDS mapping images show that Pt and Te atoms were well dispersed through the NW and no other atoms such as Sb existed as shown in Fig. 5(c), which strongly supports our hypothesis. From these experimental results, it is confirmed that pure PtTe NWs were successfully fabricated and the selective decoration of nanocrystals on PtTe NWs can be controlled by changing the core NWs.

Fig. 5 (a) and (b) TEM images and SAED pattern and (c) EDS mapping images of Te/Pt core/shell NWs annealed at 600 °C.

Notably, the characteristics of PtTe are not well known thus far, except that it is a superconductor (T_c = 0.59 K).³⁴ Considering that Te-based dichalcogenide nanostructures have been widely used in various applications such as thermoelectric devices,¹⁹ PRAM^15,35 and topological insulators,^16–18 research for finding the unknown properties of PtTe nanostructures is worthwhile, and hence is undergoing. Moreover, we believe that our facile method can be used to fabricate nanostructures of chalcogenide alloys and the selective decoration of nanocrystals on NWs can be utilized to enhance the performance of thermoelectric nano-devices,^36,37 solar cells³⁸ and gas sensors.^39,40

Conclusions

In summary, we successfully fabricated single-crystalline PtTe NWs with/without decoration of Sb nanocrystals via thermal annealing of Sb₂Te₃/Pt and Te/Pt core/shell NWs. Poly-crystalline Pt shell layers were coated on single-crystalline Sb₂Te₃ NWs using a sputtering process at room temperature. When as-fabricated Sb₂Te₃/Pt core/shell NWs were annealed at 400 °C, PtTe and PtTe₂ phases were formed through solid-state reactions between core Sb₂Te₃ NWs and Pt shell layers. When the annealing temperature was increased up to 600 °C, PtTe₂ phases transited into PtTe phases. The excess Sb atoms diffused out and crystallized at the surface of the PtTe NWs. As a result, Sb nanocrystal-decorated PtTe NWs were obtained. Single-crystalline PtTe NWs without decoration of nanocrystals were also produced by thermal annealing of Te/Pt core/shell NWs at a temperature of 600 °C.

Acknowledgements

This work was supported by a Fundamental Research and Development Program for Core Technology of Materials grant funded by the Ministry of Trade, Industry and Energy, Republic of Korea (10048035). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057767).

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Footnote

† Electronic supplementary information (ESI) available: TEM images, TEM-EDS line profile, TEM-EDS spectra and SAED pattern of Sb₂Te₃/Pt and Te/Pt core/shell NWs as a function of annealing temperature. See DOI: 10.1039/c5ra13933c

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