Ternary In₂S₃/In₂O₃ heterostructures and their cathodoluminescence

Abstract

Nanoscale semiconductor heterostructures with superior crystal quality, large surface area and designed interfaces provide more opportunities for building high-performance optoelectronic nanodevices and harvesting clean energy. In this work, we report the rational synthesis of In₂S₃/In₂O₃ heterostructures through a two-channel chemical vapor deposition (CVD) process. The In₂S₃/In₂O₃ heterostructures feature a bundle-like morphology with In₂S₃ nanowires covering randomly distributed In₂O₃ nanoparticles. High-resolution transmission electron microscopy (HRTEM) analysis on the In₂S₃/In₂O₃ heterostructures finds that both In₂S₃ nanowires and In₂O₃ nanoparticles show a crystalline nature, but the lattice matching between the two crystal domains is not observed at the interface, implying that the heterostructure is formed via a simple physical adsorption. Cathodoluminescence (CL) studies on In₂S₃/In₂O₃ heterostructures indicate that the as-synthesized heterostructures have a broad emission in the range of 400–950 nm, covering the whole visible light spectrum from violet to infrared. Finally, the formation process of In₂S₃/In₂O₃ heterostructures and their optical emission behaviors are discussed based on detailed structural analyses.

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Introduction

Low-dimensional semiconductor heterostructures comprised of cations and/or anions from the same elemental group in neighboring crystal domains have received significant research interest due to their intriguing optoelectronic properties, strong interface effect and multi-functions for specific applications in diverse fields.^1–5 Especially, a well-designed interface between different semiconductors or a metal/semiconductor is definitely required to realize an effective separation of photo-generated electrons and holes in a photocatalytic process, with an aim to enhance the internal quantum efficiency (IQE). For instance, Yu et al., Guo et al., and Ni et al. designed ZnS/ZnO heterostructures as a photo-anode for enhanced conversion efficiency for photoelectrochemical applications.^6–8 To design a semiconductor heterostructure, two crystal domains from the same elemental group are preferentially considered due to their advantages in lattice-matching and the same/similar crystallographic symmetries.^9–13 Based on this principle, numerous binary and ternary semiconductor heterostructures made of different group elements have been created in the past years. Typical examples can be found in Si/Ge,¹² InAs/GaAs,¹³ ZnO/ZnS¹⁴ and GaN/AlN¹⁵ systems, and large amount of high-performance optoelectronic nanodevices have been developed using such kind of heterostructure nanostructures. Other alternative ways to achieve a heterostructure can also be realized based on the interface lattice-matching rule even though the two crystal domains have different crystallographic symmetries³ or the phase separation from a solid-solution system due to supersaturation, as reported in GaP/ZnS system.¹⁶ Recently, we also demonstrate that the defects in 3C–SiC semiconductor nanowires can also induce the preferential nucleation of wurtzite-type GaN (WZ-GaN) to form WZ-GaN/3C–SiC heterostructure nanowires with modified morphology.¹⁷

As two important In-containing binary semiconductors, In₂S₃ and In₂O₃ possess a band-gap of 1.8–2.3 eV and 2.6–3.6 eV, and are widely used as transparent conductive electrodes, fast-response photodetectors and highly-sensitive gas sensors for Cl₂, NO₂, NH₃, H₂ and CO detection.^18–21 In the past years, extensive efforts have been made in these two binary compounds from rational synthesis to functional device fabrication.^22–24 Especially, the optical property characterization and tailoring of In₂O₃ and In₂S₃ crystals have attracted tremendous research interests due to their technologically important applications in the field of photovoltaic and optoelectronic devices.^22,25–27 However, the combination of In₂O₃ and In₂S₃ to form a heterostructure and the investigations on their related structural and optical properties are rarely studied. To our best knowledge, Yang et al. and Tien et al. reported the fabrication of type-I In₂O₃–In₂S₃ core–shell nanorods and type-II α-In₂S₃/In₂O₃ nanowire heterostructures through a hydrothermal process;^28,29 Lee et al. fabricated the In₂S₃/In₂O₃ heterojunctions inserted with layered single-walled carbon nanotubes;³⁰ Hara et al. and Sirimanne et al. investigated the nanocrystalline In₂S₃/In₂O₃ electrode through sulfurizing In₂O₃ in H₂S atmosphere;^31,32 and Xu et al. developed the In₂O₃/In₂S₃/Ag nanocubes for photoelectrochemical water splitting.³³ Unfortunately, detailed analysis on the phase interface between In₂S₃ and In₂O₃ crystal domains and their optical behavior are still in demanding, which is essential to understand the formation process behind the heterostructures.

In this work, we demonstrated the rational synthesis of In₂S₃/In₂O₃ heterostructure through a well-designed two-channel chemical vapor deposition (CVD) process proposed in our previous work.³⁴ It is found that the In₂S₃/In₂O₃ heterostructures prefer to form bundle-like morphology with In₂S₃ phase as the trunk nanowires and In₂O₃ nanoparticles as decorations. The interface between In₂S₃ and In₂O₃ crystal domains are further investigated and confirmed by spatially resolved energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM) techniques. Optical characterizations using cathodeluminescence (CL) measurement confirmed that separated emission bands originating from In₂S₃/In₂O₃ heterostructures are observed in the wide range from 400–950 nm. It is supposed that the phase interface and structure defects may have a significant influence on the optical emissions.

Experiment section

Growth of In₂S₃/In₂O₃ heterostructures

The In₂S₃/In₂O₃ heterostructure nanomaterials are synthesized on Si(111) substrate through a two-channel CVD approach.³⁴ High purity InP slices (Alfa Aesar A Johnson Matthey Company, 99.9999%) and ZnS powder (Sinopharm Chemical Reagent Co., Ltd, AR) are employed as raw reactants to provide In and S sources, respectively. In a typical experiment, the InP and ZnS powders are placed in two separated quartz tubes and flowing Ar gas with a flowing rate of 150 standard cubic centimeter per minute (sccm) is used as the carrier gas. The schematic diagram of the experimental setup is illustrated in ESI Fig. S1.† The growth reaction is fixed at 1375 K for 80 min based on our experience in previous work.

Structure and optical property characterizations

The morphology of In₂S₃/In₂O₃ heterostructure nanomaterials is characterized by a field emission scanning electron microscopy (FE-SEM INSPECT F50) operating at 20 kV. X-ray powder diffraction (XRD, Bruke D2 PHASER) based on Cu K_α radiation (λ = 1.5418 Å) and high-resolution emission transmission electron microscopy (HRTEM, Tecnai G² F20) with an accelerating voltage of 200 kV are used to analyze the phases, microstructures and crystal domain boundaries of the nanostructures. Chemical composition is measured by an X-ray energy dispersive spectrometer (EDS) attached on the TEM. Room-temperature cathodoluminescence spectrum of In₂S₃/In₂O₃ heterostructure is recorded in an field-emission scanning electron microscope (FE-SEM, Hitachi-S4300) equipped with a CL system (Horiba-MP32S/N) under an accelerating voltage of 5 kV and a beam current of 1000 pA.³⁵

Results and discussion

Fig. 1a–c show the typical morphology of In₂S₃/In₂O₃ heterostructure nanostructures deposited on a Si substrate. An obvious bundle-like morphology can be observed, which consists of trunk nanowires and side nanoparticles. The bundles have an average dimensional size with a length of 12 μm and a diameter of 0.5–1 μm (ESI, Fig. S5†). Magnified SEM image (Fig. 1b) and TEM image (Fig. 2a) reveal that the bundles are consisted of dozens of thin nanowires with a diameter about 100 nm and large amount of nanoparticles with diameters of 40–50 nm. The aggregation of these thin nanowires to form the bundle trunks can be regarded as a self-assembling process and the decoration of nanoparticles on the surface of nanowires is also at random. Series of growth experiments find that the dimensional size of these In₂S₃/In₂O₃ heterostructure nanostructures is strongly dependent on the temperature gradient in the reactor. Slightly reducing the distance between substrate and precursors will lead to a drastic increase of the nanowire and nanoparticle in size, (ESI, Fig. S2†), indicating a faster crystallization rate. Further observation on the nanoparticles shows that these nanoparticles feature a typical octahedron shape (ESI, Fig. S2†), agreeing well with the typical appearance of In₂O₃ nanostructures reported in previous works.^36–38 Preliminary composition analysis on the heterostructure bundles verifies the existence of In, O and S elements (Fig. 2c). However, the back-scattered electron image (BSE, Fig. 1c) shows less difference between the nanowires and nanoparticles due to the approximate relative atomic mass of In₂S₃ and In₂O₃.

Fig. 1 (a) Low-magnification SEM image of In₂S₃/In₂O₃ heterostructure nanomaterials; (b and c) magnified SEM image and BSE image of In₂S₃/In₂O₃ heterostructures, respectively; (d) XRD pattern of as-synthesized sample and standard XRD data of cubic In₂S₃ and In₂O₃; (e and f) crystal unit cells of cubic In₂S₃ and In₂O₃, respectively.

Fig. 2 (a) TEM image of as-prepared In₂S₃/In₂O₃ heterostructures; inset shows a single In₂S₃ nanowire with In₂O₃ nanoparticle; (b and c) EDS spectra collected from the local areas labeled in (a); (d) STEM image of In₂S₃/In₂O₃ heterostructure; (e and f) line-scan composition profiles taken along radial and axial directions of In₂S₃/In₂O₃ heterostructure in (d), respectively; (g) STEM image and its corresponding EDS elemental mapping of In, S, O elements in a single nanowire.

The designed formation of In₂S₃/In₂O₃ heterostructures starts from the original precursors InP and ZnS powders, which comprise of In, P, Zn and S elements and can provide numerous of possibilities for the formation of diverse binary, ternary or quaternary compounds like solid-solutions or heterostructures (In₂S₃, In₂O₃, ZnS, ZnO, ZnP, InP, (Zn, In)S, (Zn, In)O, Zn(S, O), In(S, O), (Zn, In)(S, P)).^34,39 Therefore, it is definitely essential to examine their composition and phase purity of the as-synthesized nanostructures. Fig. 1d shows the XRD pattern of the nanostructures. The peaks centered at 2θ = 14.2°, 23.4°, 27.5°, 28.7°, 33.3°, 43.7°, 47.8°, and 55.8° can be indexed to the (111), (220), (311), (222), (400), (511), (440), and (533) planes of cubic In₂S₃ (JCPDS no. 65-0459), while these peaks with 2θ = 21.1°, 30.5°, 35.4°, 37.6°, 50.9°, and 55.8° correspond well to the (211), (222), (400), (411), (440), and (611) planes of cubic In₂O₃ (JCPDS no. 65-3170). The peak centered at 2θ = 41.6° arises from the Si substrate and 2θ = 20.5° is assigned to SiO₂ because of the oxidation of silicon substrate at high temperature. The appearance of In₂S₃ and In₂O₃ peaks implies the co-existence of both phases in the form of separated crystal grains or heterostructures. The bright field (BF) and dark field (DF) TEM images further demonstrate the heterostructure feature of In₂S₃/In₂O₃ with obvious boundary (ESI, Fig. S3†).

From above analyses, the In₂S₃/In₂O₃ heterostructure can be confirmed. However, it is still required to define the exact composition of different crystal domains (the bundles and the octahedral nanoparticles) in local regions. The composition measurement performed on the heterostructure concludes that it is made of In, S and O elements which originate from the In₂S₃/In₂O₃ samples, while the detected Cu signal is ascribed to the Cu TEM grid. Further composition analysis using EDS in point mode confirms that both the central thin nanowires and edge nanoparticles share the same In element. In addition, the nanowire portion shows a strong peak intensity of S element rather than O element and the In/S atomic ratio approaches to the stoichiometric value of In₂S₃, suggesting that the thin nanowires are actually In₂S₃ phase. Meanwhile, the edge octahedral nanoparticles with regular geometrical shapes exhibit obvious signal intensity in O element and the In/O atomic ratio also matches well with In₂O₃.^36,37 The line-scan composition profiles taken along the radial and axial directions, as well as spatial resolved elemental mapping (shown in the Fig. 2e–g), further verify the spatial distributions of each elements and prove the heterostructures nature of In₂O₃/In₂S₃. O signal collected in nanowires suggests that some adsorbed oxygen or doped oxygen exist in In₂S₃ nanowires. Some impurities related to Zn and P with weak peak intensity can also be detected due to the inevitable contamination from the starting precursors (ESI, Fig. S4†). These results are also in good accordance with the structure and phase data measured by XRD and BSE shown in Fig. 1.

For semiconductor heterostructures, the interface plays a key role in governing the optoelectronic properties such as photocatalytic and photoelectrochemical performance. The sharpness and atomic arrangement at the grain boundary will strongly affect the electrical transport and the migration of electrons, photo-generated electrons and holes, and phonons.^30,40,41 Therefore, it is of great significant to understand how the In₂S₃ nanowires combine with In₂O₃ nanoparticles. Fig. 3a shows a typical TEM image of In₂S₃ nanowire bundles decorated with octahedral In₂O₃ nanoparticles. Unlike most nanowire/nanoparticle heterostructures, it is found that the In₂O₃ nanoparticles randomly cover on the surface of trunk nanowires and no preferential/regular distribution of the particles is observed. To examine the crystallinity of both In₂S₃ nanowires and In₂O₃ nanoparticles, HRTEM analyses performed on individual nanoparticle and nanowire edge (Fig. 3a) are carried out, and their corresponding atomic lattice images are presented in Fig. 3b and c. Apparently, both In₂S₃ nanowires and In₂O₃ nanoparticles are well-crystallized with obvious single crystal nature. The HRTEM image shown in Fig. 3b and its corresponding fast Fourier transformation (FFT) with succinct diffraction spots verify that the bundled nanowires are In₂S₃ with a cubic structure and a space group of Fd3̄m. The distance of 0.38 nm between adjacent lattice planes matches well with the d-spacing of (220) plane in cubic In₂S₃ (ESI, Table S1†). Similarly, the nanoparticles can be confirmed to be cubic In₂O₃ with a space group of Ia3̄ (shown in Fig. 3c) and the inter-planar distance of 0.34 nm can be attributed to the d-spacing of (220) plane in cubic In₂O₃ (ESI, Table S1†). As is known to all, the two materials have the same structure (cubic structure) and very close lattice constants (10.774 Å for In₂S₃ and 10.140 Å for In₂O₃), it is plausible to assume that In₂S₃ and In₂O₃ may have an epitaxial relationship in lattice-matching. However, carefully observing the In₂S₃/In₂O₃ interface using HRTEM does not find obvious epitaxial relationship even though In₂S₃ and In₂O₃ have the similar crystallographic symmetry and close lattice parameters (Fig. 3b–e). The difference of space group and atomic configuration of In₂O₃ and In₂S₃ crystals (Fig. 1e and f) makes them quite difficult to form epitaxial growth at their phase boundaries. ​One can see that the protruding In₂O₃ nanoparticles with octahedral tip have a good crystalline lattice image, whereas the atomic arrangement in the In₂S₃ area shows a poor crystallization (Fig. 3e). Therefore, it can be found that the decoration of In₂O₃ nanoparticles on the surface of In₂S₃ nanowires to form In₂S₃/In₂O₃ heterostructures is mainly realized through a self-assemble physical contact rather than epitaxial nucleation. This assertion can be further supported by the random distribution of numerous In₂O₃ nanoparticles. Interestingly, the thin In₂S₃ nanowires prefer to assemble together to present bundle-like morphology. It is believed that such tight aggregation is beneficial to the stabilization of In₂S₃/In₂O₃ heterostructures based on the principle of surface energy minimization, as observed in WO₃/Ag heterostructures.⁴²

Fig. 3 (a) TEM images of In₂S₃/In₂O₃ heterostructure bundles; (b and c) HRTEM images of nanowire and nanoparticle indicated in (a), respectively; (d) TEM image of a single heterostructure product; (e) HRTEM image corresponding to the region marked in (d).

To disclose the formation of In₂S₃/In₂O₃ heterostructures, a tentative growth model is schematically illustrated to describe the possible nucleation and crystallization processes, as shown in Fig. 4. First, InP and ZnS reactants will be decomposed into the elemental precursors in the form of sublimated In, P, Zn and S atoms under high temperature (stage I). Second, these active atoms will be reorganized to nucleate on the Si surface and evolve into one dimensional In₂S₃ nanowires and In₂O₃ nanoparticles (stage II). It should be mentioned that the O source in In₂O₃ nanoparticles comes from the residual O₂ in the reaction chamber. Finally, the In₂S₃ nanowires decorated with In₂O₃ nanoparticles will be assembled into heterostructure bundles (stage III). Here the wire-like appearance of In₂S₃ crystal domain and the octahedral shape of In₂O₃ nanoparticles can be attributed to their own growth nature. Generally, the In₂S₃ nanowires are commonly found in the CVD process under a high temperature, as reported in previous works.^43,44 For In₂O₃ crystal domain, it has been reported that the In precursor type has a significant influence on its morphology. For example, octahedral nano/micro-particles are usually observed when pure In is used as the raw reactant, while In₂O₃ nanowires are often obtained with In₂O₃ powder as precursor and the higher reaction temperature from 1225 K to 1575 K will lead to the size reduction of In₂O₃ octahedrons from 1 μm to 100 nm.³⁶ The shape of In₂O₃ nanoparticles is greatly influenced by the surface energy of different planes and the growth rates of each crystallographic facet. It has been reported that the geometrical shape of In₂O₃ nanoparticles is strongly dependent on the ratio of v{100}/v{111}.⁴⁵ Hence, the appearance of In₂S₃ nanowires and In₂O₃ octahedral nanoparticles are the integration function of thermodynamics and kinetics, while more than just only kinetics or thermodynamics effects. It should be mentioned that the absence of Zn and P elements in the In₂S₃/In₂O₃ heterostructures is possibly due to the faster diffusion rate of Zn and P precursors at high temperature, as observed in GaP–ZnSe and (GaN)_1−x(ZnO)_x pseudobinary solid-solutions.^34,46

Fig. 4 The schematic diagram describing the formation process of In₂S₃/In₂O₃ heterostructures.

As two important semiconductors, cubic In₂S₃ and In₂O₃ crystals have a band-gap of 1.8–2.3 eV and 2.6–3.6 eV, respectively, corresponding to an emission wavelength in the range of 539–689 nm and 344–476 nm.^{25,28,47–49} The combination of In₂S₃ and In₂O₃ phases will undoubtedly broaden the emission wavelength from violet to visible light, which enables a wider optical emission for efficient photocatalytic applications. To study the optical properties of semiconductor nanostructures, cathodoluminescence technology with high-resolution of wavelength in nano-scale area has been demonstrated to be quite useful and reliable.^46,50–53 Fig. 5 shows the CL spectrum collected from the In₂S₃/In₂O₃ heterostructure samples under an accelerated voltage of 5 kV and a beam current of 1000 pA. A broad emission band covering the whole visible range can be observed. The CL spectrum can be further divided into three separated bands with typical peaks centered at 482 nm, 645 nm and 800 nm by the Gaussians deconvolution function. Obviously, the emission band with a peak wavelength centered at 645 nm shows the strongest intensity and matches well with the luminescence data of In₂S₃ crystal. Therefore, it can be concluded that the 645 nm emission should originate from the In₂S₃ nanowires. The strong luminescence intensity of 645 nm emission is also in good agreement with the predominant In₂S₃ phase in the heterostructures. The left shoulder with a peak wavelength located at 482 nm can be assigned to the cubic In₂O₃ phase, even though the 482 nm green emission has been ascribed to the deep trap states or defects in In₂S₃ nanowires.⁴⁸ The broad emission band with a peak wavelength centered at 800 nm can be regarded as defect-related luminescence since some structural defects like phase boundary and vacancies are involved in the heterostructure crystals. It has been reported that nitrogen doping in In₂O₃ nanostructures can also induce a red-light emission at 740 nm.^54,55 Previous studies also pointed out that high-density oxygen vacancies in In₂O₃ nanostructures can also lead to a strong visible light emission due to the recombination of donor band (oxygen vacancy, V_O) and acceptor band (indium vacancy, V_In).^26,55–57

Fig. 5 Room-temperature CL spectrum of In₂S₃/In₂O₃ heterostructures collected under an accelerated voltage of 5 kV and a beam current of 1000 pA.

From above results, it can be seen that the In₂S₃/In₂O₃ heterostructures synthesized from a CVD process have predominant merits in crystal quality, large surface areas and phase interfaces, as well as a broad emission band covering from violet to the infrared region, which paves a solid way for efficient visible light absorption and carrier separation towards photocatalytic applications in hydrogen clean energy harvesting and organic solution photodegradation.^28,29 Based on current work and the crystallographic characteristics of In₂O₃ and In₂S₃ crystals, the rational growth of In₂S₃/In₂O₃ heterostructures with sharp interface, tunable emission wavelength and controllable heterostructure configuration (core–shell or hierarchical structure) can be further achieved through careful design and precise control of the nucleation and crystallization process. In this way, the In₂S₃/In₂O₃ heterostructure with excellent crystal quality and optical properties will open up more opportunities in the field of optoelectronic nanodevices, clean energy and environment processing.

Conclusions

In summary, ternary In₂S₃/In₂O₃ heterostructure nanostructures comprised of In₂S₃ nanowire bundles and In₂O₃ octahedral nanoparticles have been fabricated through a two-channel CVD process. Both In₂O₃ and In₂S₃ domains exhibit superior crystal quality without obvious structural defects such as stacking faults and twins. However, the lattice matching and the sharp interface in In₂O₃/In₂S₃ boundary are not observed due to the self-assembled decoration of In₂O₃ nanoparticles on the surface of In₂S₃ nanowires. Room-temperature cathodoluminescence measurement indicates that the In₂S₃/In₂O₃ heterostructures show a broad emission band covering from the violet to infrared region, which holds great promise for efficient visible light absorption toward enhanced photocatalytic applications in the field of clean energy harvesting and the photodegradation of organic pollutants.

Acknowledgements

This work was partially supported by the Knowledge Innovation Program of Institute of Metal Research with grants No. Y2NCA111A1 and Y3NCA111A1, and the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No. Y4NC711171).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09520h

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