Self-assembled synthesis and surface photovoltage properties of polyhedron-constructed micrometer solid sphere and hollow-sphere In₂S₃

Abstract

β-In₂S₃ solid micro-spheres composed of wedge-like octahedra, and hollow micro-spheres composed of nanorods have been fabricated by a simple one-step hydrothermal treatment. The unique surface photovoltage properties of the as-prepared In₂S₃ samples were investigated through surface photovoltage (SPV) spectroscopy.

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Nowadays, the controllable synthesis of micro- and nano-scale materials with unique morphology has attracted intensive interest due to their significance in basic scientific research and technology applications.¹ Several researchers have synthesized and characterized varied morphology of metal oxides or chalcogenides. Studies have focused on functional metal chalcogenides/oxides materials with special micro- or nano-scale morphology, such as two-dimensional monolayer MoS₂,² hollow/solid Ag₂S/Ag heterodimers as antibacterial,³ floral Ga₂O₃ nanorods as pholocatalyst⁴ and hollow spherical TiO₂ for dye-sensitized solar cells.⁵

As a III–VI chalcogenide and n-type semiconductor material, In₂S₃, has potential special optical, photo-conductive and electronic properties because of its defected spinel structure.⁶ In₂S₃, a band gap of 2.0–2.3 eV (ref. 7) responding to visible light, is a potential photoelectrical substitute for the highly toxic CdS as a buffer layer in CuInSe₂ and CuInS₂ based solar cells to reduce toxicity.⁸ In₂S₃ nano- or micro-material has a promising future in many fields. So far, many physical and chemical methods such as vapor–liquid–solid growth,⁹ chemical vapor deposition,¹⁰ solvothermal¹¹ and wet chemical synthesis with various templates or without¹² have been devoted to the synthesis of varied In₂S₃, most of them are single monolithic structure such as nanoflakes,¹³ nanowires,¹⁴ nanotubes,¹⁵ nanobelts,¹¹ hollow microspheres¹⁶ and nanorods.¹⁰ Nano/micro-sized In₂S₃ with composite structure can be rarely found in literatures. Recently, preparations of hollow nano/micro-particles structures by a simple template method with some interesting physical phenomena have been demonstrated.¹⁷ This communication presents a simple one-step hydrothermal synthesis of solid micro-spheres composed of wedge-like octahedra. And hollow micro-spheres β-In₂S₃ composed of one-dimensional (1D) nanorods are synthesized with sodium dodecyl sulfate (SDS) as the template. This is the first time studying surface photovoltage property about In₂S₃ powders.

In a typical procedure, 1.0 mmol indium chloride (the reaction product of elemental indium and hydrochloric acid) and 1.5 mmol thiourea were added to 30.0 mL deionized water in a 50 mL Teflon-lined stainless steel autoclave and stirred for 30 min. Then the autoclave was sealed and maintained at 165 °C for 24 h in a preheated oven and cooled to room temperature naturally. Repeatedly centrifugation and washed with absolute ethanol and distilled water, then volatilized naturally in the ventilation at room temperature. Red powder was obtained as sample 1. The procedure of coordination sample 2 was similar to the sample 1 except 0.35 mmol SDS was added in the raw material. The transparent solution was obtained, and then heated at 165 °C for 24 hours. Red products were collected after treatment.

X-ray diffraction (XRD) was used to investigate the crystalline structure of the as-prepared In₂S₃ (Fig. 1). All the reflections could be indexed to β-In₂S₃ (JCPDS no. 65-0459). No characteristic peaks were observed for the other impurities such as In₂O₃, InS and In(OH)₃. It shows that in all the samples main characteristic peaks are observed at 27.4°(311), 33.2°(400), 43.6°(511) and 47.7°(440). According to the Scherrer equation,¹⁸ the average crystallite sizes of sample 1 and 2 are ca. 25 nm and 22 nm.

Fig. 1 XRD patterns of In₂S₃ samples.

The typical scanning electronic microscopy (SEM) images of β-In₂S₃ samples are presented in Fig. 2. Inside, Fig. 2a–c showed SEM pictures of the sample 1. From Fig. 2a, the morphology of sample 1 is uniform solid-spheres. The particles’ diameter is about 9 μm, which belongs to solid micro-spheres, and the solid spheres are composed of many tiny particles of polyhedron packing together densely (Fig. 2b). Furthermore, these tiny polyhedral particles on the spheres are similar to octahedron, except the vertex of octahedron grows into the arris, as four faces of octahedron have four edges, and other faces have three edges (Fig. 2c). Compared with other researches,¹⁹ not only the diameter of sample 1 is larger, but also the mutation of octahedron structure unit is different from typical octahedron, and this octahedron structure unit tends to be wedge-like.²⁰ And the size of this construction unit is 750 nm × 600 nm × 800 nm (length × width × height). SEM pictures of the sample 2 showed in Fig. 2d–f. The average size of these micro-spheres with many holes on the surface is about 6 μm, smaller than sample 1 (solid-spheres) (Fig. 2d). The enlarged drawing of hollow-spheres displays that these spheres with many holes are hollow-spheres (Fig. 2e). Apparently, the particles form hollow-spheres are parallelepiped with the length about 350 nm, the height about 100 nm and the thickness about 50 nm (Fig. 2f). Therefore, tiny nanocrystals are 1D nanorods. In some sense, In₂S₃ hollow-sphere samples are composed of nanocrystals. On the basis of Fig. 2, sample 2 has smaller size than sample 1.

Fig. 2 SEM images of (a–c) sample 1 (solid-spheres) from low to high magnification; (d–f) porous hollow-spheres In₂S₃ particles (sample 2) with from different magnifications.

In the aqueous solution, ionization of InCl₃ generates In³⁺ ions. It is generally believed that metal ions can coordinate with thiourea in aqueous solution to form metal-thiourea compound.²¹ A possible reaction mechanism is shown in Scheme 1. Free In³⁺ ions combined with thiourea to form [In(SC(NH₂)₂)_n]³⁺ complexes, which would decompose into In₂S₃ nuclei under the hydrothermal condition. To decrease the total surface free energy under the constraint of fixed volume, tiny In₂S₃ nuclei grow up to form polyhedral shape. Once the reactant ions were depleted, crystal dissolution might occur at the high-energy surfaces, and driven by the minimization of the total energy of the system. Tiny In₂S₃ particles can be self-assembled into microspheres, which can be due to aggregation-based mechanism under hydrothermal conditions.¹⁹ The formation of In₂S₃ hollow micro-spheres can be attributed to the SDS generated a core–shell type that the negative charge R–SO₄⁻ coming from SDS molecule was outside during the hydrothermal process. SDS, an anion surfactant, often acts as a template during synthesis of various sulfides/oxides.²² Those negative charges attracted [In(Thiourea)_n]³⁺ complex that were carried from water to the interface of SDS–water. Attributed to the interreaction between R–SO₄⁻ anion and [In(Thiourea)_n]³⁺ complex, reaction condition of reflux may have led to the formation of sheet-like micelles of SDS as anionic template. And SDS makes nanoparticles grow along these templates which serves as the nuclei for the nanoparticles growth as nanorod-like with a hexahedral shape. With the hydrothermal carrying out, the common result of both self-assembly tiny In₂S₃ particles and micelles of SDS might be the formation of hollow microspheres. With SDS removed, hollow-spheres In₂S₃ are formed with many holes.

Scheme 1 Schematic drawing of the possible mechanism for the formation of In₂S₃ samples.

In order to examine the quantum confinement effect of as-prepared β-In₂S₃ samples, room temperature powder ultraviolet and visible (UV-Vis) absorption were recorded with BaSO₄ used as a reference. Fig. 3 showed the UV-Vis absorption of as-prepared In₂S₃ samples. The energy gap of bulk In₂S₃ is 2.2 eV with the corresponding absorption edge ca. 560 nm.⁷ From UV characterization, λ₁ = 655 nm and λ₂ = 642 nm, which are red-shifted by almost 80–90 nm compared to the absorption edge in bulk In₂S₃. Energy gap by solid ultraviolet test could be calculated according to formula E_g = 1240/λ, so E_g(1) = 1.89 eV, E_g(2) = 1.93 eV. The step-like characteristic of the absorption spectrum correlates well with that of the In₂S₃ nanocrystals prepared by many other researchers using organic capping media and have been explained to be due to valence-conduction band transition in In₂S₃.²³ A blue-shift by almost 100 nm of the optical absorption edge of hollow-spheres sample was observed from the UV-Vis absorption spectra. This can be attributed to the size of octahedron-like polyhedrons from solid-spheres In₂S₃ is much larger than nanorods with hexahedral shape which is hollow-sphere, because particles with large size could increase scattering of incident light.

Fig. 3 UV-Vis spectra of In₂S₃ samples.

The SPV spectroscopies of the prepared In₂S₃ samples with solid-spheres and hollow-spheres structure were taken with zero bias. As characterizing In₂S₃ samples, it is assembled to be a sandwich-like device consisting of indium tin oxide (ITO) glass/powder/conductor substrate and light permeate ITO electrode to effect on sample (Fig. S1†). To the best of our knowledge, there are many researches studying the optical property and electrical property of In₂S₃ crystalline material,⁶ but no optoelectronic property investigation of indium sulfide by the SPV spectroscopy method has been reported yet. Fig. 4 presents the result of surface photovoltage characterization. A surface photovoltaic spectrum is a sensitive method to investigate the structural properties of the surface, surface state, the separation and recombination of the photo-carriers. Since the nanorod thickness of the hollow-spheres particle is less than 1 μm, the built-in field for these particles is not as significant as that in a bulk semiconductor, in which the built-in field within the space charge region is well formed and guarantees an intense SPV response. The special hollow-spheres morphology of these microparticles is responsible for a higher contact electric resistance compared with the adjacent particles for their loose aggregation and point contact between their curved surface. This point contact blocks the charges transferring along the particles.

Fig. 4 SPV spectra of In₂S₃ samples.

Therefore, a weak SPV response is obtained for the aggregated hollow particles. It is noticed that a strong SPV response to the light from 760 nm to 300 nm. At same photonic energy, the rod-shape may be easy to drive more electron carriers by a larger proportion in the conduction band of micro-hollow spheres to hop across the interfaces between them toward the probing electrode. Therefore, the edge of the SPV response band of sample 1 which is 688 nm, red-shifts compared with the sample 2 (676 nm), which is in well agreement with the result of UV characterization. On the other hand, maximum photovoltage of solid micro-spheres and hollow micro-spheres In₂S₃ samples come up to 23.9 mV and 18.6 mV at 465 nm and 464 nm in visible region respectively. Sample 1 shows a much higher SPV signal than sample 2, which benefits from their larger average particle size. Therefore, a perfect energy band can be formed, and the photogenerated charges can be distinctly separated by the built-in field.²⁴ The SPV signal intensity of the as-prepared β-In₂S₃ samples can achieve millivolt level, which is stronger than other metal chalcogenides/oxides materials.²⁵ In fact, the pattern of SPV response maximum in the UV region is a collaborative and complicated process of energy spectral distribution of excitation light source and optical pre-absorption by the probing blank ITO electrode and the intrinsic SPV response produced in the sample.

In summary, the pure solid-spheres composed of wedge-like octahedra and hollow-spheres composed of 1D nanorods In₂S₃ microparticles which index (JCPDS no. 65-0459) were successfully synthesized by one-step hydrothermal method. Their E_g are 1.89 eV and 1.93 eV respectively. The edge of the SPV and UV response band of solid-spheres is all red-shifts compared to hollow-spheres. Maximum photovoltage of solid micro-spheres and hollow micro-spheres In₂S₃ samples come up to 23.9 mV and 18.6 mV at 465 nm and 464 nm respectively.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21371040 and 21171044), National key Basic Research Program of China (973 Program, no. 2013CB632900), and Supported by Program for Innovation Research of Science in Harbin Institute of Technology.

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Footnote

† Electronic supplementary information (ESI) available: Details of characterization, and the sandwich-like structure of SPV measurement. See DOI: 10.1039/c3ra42021c

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