Solution-phase synthesis of g-In2Se3 nanoparticles for highly efficient photocatalytic hydrogen generation under simulated sunlight irradiation
Shuang Yang a, b, Cheng-Yan Xu a, b, *, Li Yang a, Sheng-Peng Hu a, b, c and Liang Zhen a, b, * Hexagonal indium selenide (γ-In2Se3) nanoparticles were successfully synthesized by a hot-injection method using triethylene glycol as solvent. The structure and morphology of the obtained nanoparticles were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). Phase-pure γ-In2Se3 nanoparticles could be obtained at relatively low temperature of 220°C. Further increasing of reaction temperature leads to enhanced crystallinity of γ-In2Se3 nanoparticles, which favorites for improving the photocatalytic activity. The γ-In2Se3 nanoparticles synthesized at 250°C exhibit excellent and stable photocatalytic hydrogen generation under simulated sunlight irradiation, which might be attributed to the strong absorbance both in the UV and visible light regions as well as nano-sized morphology.
Photocatalytic water splitting over semiconductor nanomaterials has attracted intense research interest in recent years, because it is a “green” approach to convert solar energy into chemical energy1, 2 . Among the numerous semiconductor nanomaterials, metal selenides (e.g., CdSe and PbSe), which possess appropriate band levels for water splitting, as well as their narrow band gaps allow visible light absorption3, 4, have been extensively studied as promising candidates for the solar energy conversion process. However, lead- or cadmium-based semiconductors are usually viewed as toxic and not environmentally friendly materials. Because of low toxicity and eco-friendly, indium selenide gradually has recently extensive concern to replace CdSe and PbSe in the field of photocatalytic water splitting.
As an important III–VI group inorganic semiconductor, indium selenide (In2Se3 ) with band gap of 1.36−2.0 eV has aFracted much attention due to its highly anisotropic structural, electrical, optical and mechanical properties, which make it attractive for ionic batteries5 , phase change random access memories6 , visible-light photodetectors7-9 and photovoltaic solar cells10. In recent years, various strategies have been utilized to synthesize indium selenide nanostructures, such as thermal evaporation9 , chemical vapor deposition techniques11-13, electrochemical atomic layer epitaxy14 , molecular beam epitaxy15, 16, and solution-phase synthesis17-20 . However, it is difficult to synthesize In2Se3 nanostructures by solution-phase method. On one hand, Indium ions is easy to hydrolyze in aqueous solution due to their deficient electron properties, which will baffle the synthesis of In2Se3 in aqueous solution19. On the other hand, the same stoichiometric In2Se3 exists in several kinds of crystalline phases, such as, room temperature phases of α-In2Se3 , and high temperature phases of β-In2Se3 , γ- In2Se3 , δ-In2Se3 and κ-In2Se3 19, 20, which largely restrict the formation of single phase γ-In2Se3 .
In a previous report, Hsiang et al. synthesized hexagonal shaped γ-In2Se3 particles with sizes of 0.5–2 μm utilized InCl3 ·4H2O and Se as raw materials and oleyamine and octadecene as solvents at 250°C17. Ning et al. found that hexagonal crystal structure In2Se3 nanoplates were formed at high reaction temperature (330°C) while InSe nanocrystals with zinc blende structure were fored at relatively low temperature of 280°C using oleyamine as solvent. 18 Tan et al. demonstrated the synthesis of γ-In2Se3 flower-like hierarchical nanostructures by ascorbic acid (AA)-assisted solvothermal method with InCl3 ·4H2O and Se as starting materials19. Wei et al. developed a two-step strategy for the synthesis of porous γ-In2Se3 tegragonal particles. In-Se based precursor was prepared in a waterethylenediamine mixed solvent under hydrothermal conditions, and was then calcined under N2 atmosphere, forming porous γ-In2Se3 tegragonal particles with improved photocatalyci activity for water splitting compared with In2Se3 nanoparticles20. To obtain the deserved phase and morphology of indium selenides, particular attention should be paid to the reaction temperature as well as the selection of solvents with respect to the starting materials. We are thus motivated to search for alterative solvent for hot-injection synthesis of γ-In2Se3 crystals at mild condtions. Triethylene glycol (TEG) is a cheaper polyols sovlents with merits of less toxicity and eco-friendly, and it can serves as both solvent and reductant for controlled synthesis of indium selenides.
Herein, we report a facile hot-injection synthesis route to prepare γ-In2Se3 nanoparticles through the reaction of InCl3 ·4H2O with Se using triethylene glycol as solvents at 250°C. Because of low price, reduced toxicity, high boiling point and good dissolvability with many inorganic metal salts, triethylene glycol was selected for solvent to prepare the γ-In2Se3 nanoparticles for the first time. Meanwhile, this synthetic approach requires no precursor synthesis or toxic compounds. Furthermore, the obtained γ-In2Se3 nanoparticles exhibit efficient hydrogen evolution capacity under simulated sunlight irradiation.
All the reagents were of analytical grade and were used as received without further purification. In a typical procedure, 0.75 mmol Indium chloride tetrahydrate (InCl3•4H2O) was dissolved into 5 mL of triethylene glycol (C6H14O4 , TEG) in a beaker under magnetic stirring at room temperature until a clear indium precursor solution was obtained. Then, 0.01 g selenium (Se) powder was added into 45 mL of TEG in a 100 mL three-necked round-bottom flask. The three necked flask was attached to a reflux condenser and heated to 250°C under magnetic stirring. Then, the indium precursor solution was quickly injected into the three-necked flask with vigorous stirring. The reaction solution was refluxed at 250°C for 2 h. After the completion of reaction, the reaction solution was allowed to cool to room temperature naturally. The products were collected by high speed centrifugation, washed with absolute ethanol for several times, and dried at 60°C in air. In order to investigate the synthetic temperature, injection temperature was set as 210°C, 220°C, 230°C and 250°C, respectively, while the other processing parameters used were unchanged as above conditions.
The phases were identified by means of X-ray diffraction (XRD) with Rigaku D/max 2500 diffractometer using Cu Kα radiation (λ=0.15418 nm). The morphology and microstructures of the products were characterized by fifield-emission scanning electron microscope (FE-SEM, FEI Quanta 200F), transmission electron microscope and high-resolution transmission electron microscope (TEM and HRTEM, JEOL JEM 2100). The elemental compositions of the samples were determined with energy-dispersive X-ray spectroscopy (EDS). The surface chemistry and elemental valence analysis of the products were recorded on the X-ray photoelectron spectra (XPS, Thermofisher Scienticfic Company). The diffusion reflectance spectrum of samples was analyzed by using UV-vis spectrometer (Shimadzu UV-2550). Photocatalytic hydrogen evolutionThe photocatalytic hydrogen evolution was performed with a CEL-SPH2N photocatalytic hydrogen generation system (Beijing Au-light, China). A 300 W xenon lamp (CEL-HXF 300, Beijing Au-light, China, wavelength: 200–1100 nm; I=20 A) was used as simulated sunlight source. The incident light intensity was 364 mW cm−2 (CEL-NP2000 optical power meter). In the photocatalytic process, a cylindrical quartz vessel with diameter of 7 cm and height of 10 cm was employed as the reactor. The temperature of reaction system was kept at about 25°C using recycled chilling water. In a typical photocatalytic experiment, 20 mg γ-In2Se3 powders were suspended in 100 mL of triethanolamine (C6H15NO3 , TEA, 10 vol %) solution. A certain amount of H2PtCl6•6H2O aqueous solution was dripped into the system to load Pt onto the surface of the photocatalyst (as co-catalyst). The concentration of H2 was analyzed by gas chromatography (GC7890 II TECHCOMP, China) equipped with a thermal conductivity detector (N2 carrier), which was connected to the gas circulating line. After catalytic reaction, the used catalyst was centrifuged and washed with absolute ethanol for several times. The recycled catalyst was dried at 60°C in air for the next cycle of catalytic test. The photocatalysis experiments were repeated for three times for each sample.
The apparent quantum yield (AQY) for H2 evolution was determined using a 300 W xenon lamp (CEL-HXF 300, Beijing Au-light, China, I=20 A) with a band-pass filter (λ=500 nm). The irradiation area was about 38.5 cm2 . For the 500 nm monochromatic light, the average intensity was 42.4 mW cm-2 (CEL-NP2000 optical power meter). The AQY was calculated as: AQY= theamount of incident photons theamount of reactionelectrons × 100%= SPtλ 2MNAhc × 100% where S is the irradiation area, P is the intensity of the irradiation, t is the photoreaction time, λ is the wavelength of the monochromatic light, M is the amount of H2 molecules, NA is Avogadro’s constant, h is the Planck constant, and c is the speed of light.
Results and discussion
As previously reported18, γ-In2Se3 nanocrystals could be synthesized by InCl3 ·4H2O and selenourea in the solvent of oleyamine when reaction temperature is high enough (330°C). The reaction temperature of solution-phase synthesis has important influence on the morphology and phase structure of the γ-In2Se3 . Fig. 1 shows XRD patterns of the products synthesized at different injection temperatures. When reacted at 210°C, no significant redox reaction actually occurs between triethylene glycol and Se. The morphologies of selenium have significantly changed at 210°C because following the increase of reaction temperature, the solubility of selenium is increased. Hence, the sphere-like selenium microparticles will decrease or disappear for the growth of wire-like or rod-like selenium crystals21 (Fig. S1 and S2). The EDS spectrums further verify that the phases of both irregular microrods and microparticles were Se (Fig. S3a and b). Selenium has a relatively low melting point (∼216.8°C)21. Hence, Se could occur remelting and recrystallization, when the temperature is above 216.8°C.21 When the temperature was increased to 220°C, all the diffraction peaks of the products can be assigned to that of hexagonal γ-In2Se3 structure (JCPDS no. 40-1407) (Fig. 1). Almost the same characteristic diffraction peaks as injection temperatures of 220°C appear on XRD patterns injection temperatures of 230°C and 250°C, respectively (Fig. 1). As the reaction temperature further increased, the diffraction intensity of γ-In2Se3 was enhanced, suggesting better crystallinty at higher reaction temperature.