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Self-Assembled Mesoporous Hierarchical-like In2S3 Hollow Microspheres Composed of Nanofibers and Nanosheets and Their Photocatalytic Activity Selvaraj Rengaraj,*,†,‡ Selvaraj Venkataraj,§ Cheuk-wai Tai,|| Younghun Kim,^ Eveliina Repo,† and Mika Sillanp€a€a†,# †
Laboratory of Applied Environmental Chemistry (LAEC), University of Eastern Finland, Patteristonkatu 1, FI-50100 Mikkeli, Finland Department of Chemistry, College of Science, Sultan Qaboos University, Muscat 123, Oman § Solar Energy Research Institute, National University of Singapore, Singapore 117574, Singapore Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden ^ Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Korea # Faculty of Technology, Lappeenranta University of Technology, Pateristonkatu 1, FI 50100 Mikkeli, Finland
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‡
ABSTRACT: Novel template-free hierarchical-like In2S3 hollow microspheres were synthesized using thiosemicarbazide (NH2NHCSNH2) as both a sulfur source and a capping ligand in a ethanol/water system. In this study, we demonstrate that several process parameters, such as the reaction time and precursor ratio, strongly influence the morphology of the final product. The In(NO3)3/thiosemicarbazide ratios were found to effectively play crucial roles in the morphologies of the hierarchical-like In2S3 hollow microsphere nanostructure. With the ratios increasing from two to four, the In2S3 crystals exhibited almost spherical morphologies. The synthesized products have been characterized by a variety of methods, including X-ray powder diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and ultravioletvisible diffused reflectance spectroscopy (UVvis DRS). XRD analysis confirmed the tetragonal structure of the In2S3 hollow microspheres. The products show complex hierarchical structures assembled from nanoscale building blocks. The morphology evolution can be realized on both outside (surface) and inside (hollow cavity) the microsphere. The surface area analysis showed that the porous In2S3 possesses a specific surface area of 108 m2/g and uniform distribution of pore sizes corresponding to the size of pores resulting from the self-assembled structures with flakes. The optical properties of In2S3 were also investigated by UVvis DRS, which indicated that our In2S3 microsphere samples possess a band gap of ∼1.96 eV. Furthermore, the photocatalytic activity studies revealed that the synthesized In2S3 hollow microspheres exhibit an excellent photocatalytic performance in rapidly degrading aqueous methylene blue dye solution under visible light irradiation. These results suggest that In2S3 hollow microspheres will be an interesting candidate for photocatalytic detoxification studies under visible light radiation.
1. INTRODUCTION Controlling the shape, size, and structure of inorganic nanomaterials is an important and interesting field of study to better understand the physical and chemical properties.13 In comparison to the corresponding conventional bulk materials, semiconductor nanomaterials show unique optical, mechanical, electronic, and catalytic properties, which are highly dependent upon size and shape. Among the known semiconductor nanomaterials, the semiconducting metal chalcogenides have been studied most widely. In particular, most studies have focused on IIVI r 2011 American Chemical Society
quantum dots (QDs), such as CdS, ZnS, and CdSe.48 Moreover, IVI QDs, such as Ag2S and Cu2S, have received significant attention.9 In comparison to the semiconductor nanomaterials mentioned above, the optical and electronic properties of metal chalcogenides, which have a 1:1.5 molar ratio of metal/chalcogenide
Received: December 1, 2010 Revised: March 24, 2011 Published: April 05, 2011 5534
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Langmuir in their unit cells, such as In2S3, Bi2S3, and Sb2S3, have not been studied in detail.1012 In2S3, a typical IIIVI group sulfide, is known to crystallize in three polymorphic forms: R-In2S3 (cubic structure), β-In2S3 (spinel structure), and γ-In2S3 (layered structure). Of these, β-In2S3 is a n-type semiconductor with a band gap of 2.0 2.3 eV13 and is a potential candidate for optical,1417 photoconductive,18 and optoelectronic13,14,19 applications because of its defected spinel structure. Recently, it was reported that solar cell devices prepared using β-In2S3 as a buffer layer gave 16.4% conversion efficiency, which is very close to that of the standard CdS buffer layer.1820 On the other hand, much effort has been made to replace highly toxic cadmium with other metals, with regard to environmental reasons.21 Hence, more studies have been performed on the sulfides, especially β-In2S3. The defected spinel structure is obtained in either cubic or tetragonal form. Recently, preparations of hollow micro- and nanostructures by a simple template-free method using some interesting physical phenomena have been demonstrated.2224 For instance, Alivisatos and co-workers have reported on the preparation of hollow CoS spheres through the nanoscale Kirkendall effect.22 Zeng and co-workers have pointed out that hollow nanostructures of Cu2O and TiO2 can be formed through the ripening of solid spheres comprised of numerous small crystallites.23,24 A number of methods have already been developed to fabricate In2S3 with a variety of morphologies, for example, sonochemical method, precipitation, hydrothermal method, solvothermal method, solvent reduction route, microwave-assisted method, and metal organic chemical vapor deposition (MOCVD) method.2530 Hollow spheres with nanometermicrometer dimensions, having modified structural, optical, and surface properties, represent an important class of materials, which are potentially useful for a wide range of applications, such as solar energy conversion, photocatalysis, electrical, optical, and photonic crystals, fillers, and catalysts.3135 However, there are few reports about the In2S3 secondary nanostructure fabrication. Hence, it is desirable to develop the controllable synthesis of In2S3 nanostructures with exceptional properties. Herein, we report the one-step solvothermal synthesis of three-dimensional (3D) hierarchical-like β-In2S3 hollow microspheres, which are composed of two-dimensional (2D) nanosheets. Hierarchical-like In2S3 hollow microspheres can be obtained only using thiosemicarbazide (TSC, NH2NHCSNH2) as both a sulfur source and a capping ligand. To the best of our knowledge, this is the first time that the self-supported simple one-step and template-free growth procedure was reported to prepare In2S3 hollow spheres. Photocatalytic properties of the 3D microspheres will also be discussed in this paper.
2. EXPERIMENTAL SECTION 2.1. Preparation of In2S3 Hollow Microspheres. The hierarchical-like In2S3 hollow microspheres were synthesized using analytical-grade indium nitrate [In(NO3)3] (Sigma-Aldrich, 99%) and TSC (CSN3H5) (Sigma-Aldrich, 99%), without further purification. In a typical synthesis, 3.324 mmol of In(NO3)3 and 6.64813.297 mmol (1:24 molar ratio) of TSC were dissolved in 70 mL of ethanol/water (1:1, v/v) and continuously stirred for 30 min to form a clear solution. Here, TSC serves as both a sulfur source and a capping ligand. The solution was then transferred to an autoclave, maintained at 180 C for 1024 h, and then cooled to room temperature naturally. The orange color precipitate was harvested by centrifugation, washed several times
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using deionized water and ethanol to remove the possible remaining cations and anions, and then dried in an oven at 70 C for 24 h for further characterization purposes. It is noted that the post-treatment of the products after the reaction was carried out in a fume hood to avoid excess H2S (generated in the solvothermal process). 2.2. Characterization of In2S3 Hollow Microspheres. The structural analysis of the samples was performed using a Bruker (D5005) X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ = 1.540 56 Å). An accelerating voltage of 40 kV and emission current of 30 mA were adopted for the measurements. The morphology and microstructure were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). For the highresolution transmission electron microscopy (HR-TEM) study, the samples were dispersed onto a Cu grid with holey carbon supporting films and studied at room tempeature in a JEOL JEM-3010 microscope operated at 200 kV. The ratio between In and S within the sample was analyzed using an energy-dispersive X-ray spectrophotometer installed in the scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was conducted with a Sigma Probe (ThermoVG, U.K.) X-ray photoelectron spectrometer, of which the source is Al KR radiation (1.486 eV). The photoemitted electrons from the sample were analyzed in a hemispherical energy analyzer at a pass energy of Ep = 20 eV. All spectra were obtained with an energy step of 0.1 eV and a dwell time of 50 ms. A software package (Avantage Thermo VG) has been used to analyze the XPS data. The absorption spectrum of the samples in the diffused reflectance spectrum mode was recorded in the wavelength range between 200 and 1000 nm using a spectrophotometer (V-670, JASCO), with BaSO4 as a reference. From the absorption edge, the band gap values were calculated by the extrapolation method. Nitrogen sorption isotherms were measured at 77 K using Quantachrome Instruments (Autosorb 1). The BrunauerEmmettTeller (BET) method and BarrettJoynerHalenda (BJH) model were used for specific surface area calculation and porosity evaluation, respectively. The sample was degassed at 398 K before the sorption measurement. All photoreaction experiments were carried out in a photocatalytic reactor system, which consists of a cylindrical borosilicate glass reactor vessel with an effective volume of 500 mL, a cooling water jacket, and a 150 W sodium vapor lamp (OSRAM Vialox NAV-TS Super 150W) positioned axially at the center as a visible light source. The reaction temperature was kept at 20 C by cooling water. A special glass frit as an air diffuser was fixed at the reactor to uniformly disperse air into the solution.36,37 Visible light photocatalytic activity studies were performed to study the methylene blue degradation in an aqueous solution. For each run, the reaction suspensions were freshly prepared by adding 0.10 g of catalyst into 250 mL of aqueous methylene blue solution, with an initial concentration of 5 mg L1. Prior to the photoreaction, the suspension was magnetically stirred in a dark condition for 30 min to attain the adsorption/desorption equilibrium condition. The aqueous suspension containing methylene blue and photocatalyst was then irradiated with visible light with constant aeration. At the given time intervals, the analytical samples were taken from the suspension, immediately centrifuged at 4000 revolutions per minute (rpm) for 15 min, and then filtered to remove the catalyst. The filtrate was analyzed by an ultravioletvisible (UVvis) absorption spectra instrument (Perkin-Elmer Lambda 45 UVvis spectrometer) to understand the methylene blue degradation. The degree of degradation has been derived from the UVvis absorption intensity, which has been integrated at the wavelength of 664 nm.
3. RESULTS AND DISCUSSION 3.1. Phase and Structure of the Products. The crystal structures of the samples were characterized by X-ray powder 5535
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Figure 2. EDX result of In2S3 hollow microspheres.
Figure 1. XRD patterns of hierarchical-like In2S3 hollow microspheres prepared with different indium nitrate/TSC ratios: (a) In2S3, 1:2; (b) In2S3, 1:3; and (c) In2S3, 1:4.
diffraction (XRD). Figure 1 shows the typical XRD pattern of the samples prepared by a simple template-free solvothermal method. All of the reflection peaks in Figure 1 were carefully compared to the Joint Committee on Powder Diffraction Standards (JCPDS) database and identified as the formation of the tetragonal phase. The observed peak positions: 2θ = 11.80, 27.40, 33.45, 43.80, and 47.90 were indexed as (1 0 1), (1 0 9), (0 0 1 2), (1 0 15), and (2 2 1 2), respectively, according to the tetragonal β-In2S3 phase (a = 7.623 Å, c = 32.36 Å, and JCPDS card no. 731366).38,39 The intensity of the peaks is considerably strong, which indicates that the product is well-crystallized. No characteristic peaks arising from the possible impurities are visible, such as InS, In2O3, and other phases of In2S3. This clearly indicates that pure crystalline tetragonal phase β-In2S3 was formed via a solvothermal process. In Figure 1, it can also be seen that (1 0 9), (0 0 12), and (2 2 12) planes show the three strongest diffraction peaks and the relative diffraction intensity of either (0 0 12)/(1 0 9) or (2 2 12)/(1 0 9) is unusually higher than the corresponding conventional values (JCPDS card no. 73-1366). This observation revealed that the resultant hollow In2S3 microspheres are grown preferentially along the [0 0 12] and [2 2 12] directions. It is interesting to note that similar XRD patterns were observed for all In2S3 hollow microspheres (In2S3, 1:2; In2S3, 1:3; and In2S3, 1:4) prepared with different indium nitrate/TSC ratios, indicating that the samples possess similar crystalline structure and the structure does not change noticeably because of the change in precursor ratios. 3.2. Energy-Dispersive X-ray (EDX) Analysis. To identify the components of the synthesized In2S3 hollow microsphere, the EDX spectrometry microanalysis and elemental mappings have been carried out in SEM. The EDX spectrum and the corresponding elemental mappings recorded for sample In2S3, 1:4 are shown in Figure 2, which illustrate the actual distribution of In and S separately in the sample, with the In/S molar ratio of approximately 2:3. The EDX analysis confirmed that there is no element other than In and S present in the sample. In Figure 2, the EDX spectrum clearly displays three intense peaks between 2.8 and 3.5 keV corresponding to indium L1 (2.92 keV),
LR1 (3.29 keV), and Lβ (3.5 keV) and an equally strong sulfur KR1 peak at 2.33 keV, which are similar to the major constituents of the In2S3 hollow microsphere. The peak observed at 1.5 keV originated from the Al substrate holder, upon which the In2S3 hollow microspheres were dispersed. The quantitative analysis indicated that the atomic ratio of In/S in the sample is approximately 2:3, which is close to the stoichiometry of bulk In2S3, suggesting that the synthesized hollow microspheres possess nearly stoichiometric composition. 3.3. XPS Analysis. To further confirm the chemistry of the samples, XPS analysis was carried out. The XPS spectra show the quality and composition of the samples. All of the spectra were calibrated using C (1s) (284.6 eV) as the reference. Figure 3 depict the typical XPS spectrum recorded for In2S3, 1:3. The typical survey spectrum of In2S3 is shown in Figure 3a. It reveals the absence of other elements, except In, S, O, and C. The observed C peak is due to the carbon-supporting film on the copper TEM grid, and the O peaks can be attributed to the absorption of oxygen on the surface of the hollow microspheres because of their exposure to the atmosphere. The XPS spectra of In2S3 samples were similar and consistent with the typical In2S3 spectrum reported in the literature.38 High-resolution core spectra of In 3d and S 2p of In2S3 are shown in panels b and c of Figure 3, respectively. The observed two strong peaks at 445 and 452.5 eV are attributed to binding energies of In 3d5/2 and In 3d3/2, respectively. The peaks at 161.3 and 162.50 eV are attributed to the binding energy of S 2p3/2 and S 2p transition, respectively. These values are in good agreement with the reported data.3941 In panels b and c of Figure 3, high-resolution spectra of In 3d and S 2p peaks show the presence of symmetric peaks, indicating their single oxidation states (In3þ and S2).42,43 The atomic ratio of In/S is about 2:3 according to the peak areas of In 3d5/2 and S 2p, which further confirms that the final products are stoichiometric In2S3 with no impurity. 3.4. Morphology and Formation Mechanism. 3.4.1. SEM. The morphology of the samples was examined by SEM. Panels ad of Figure 4 are the typical SEM images of In2S3 prepared at 180 C for 24 h with a molar ratio of 1:4. The SEM results at low magnification show that the as-prepared In2S3 samples have spherical-like morphology with sizes from 3 to 7 μm in diameter. The high-magnification SEM images reveal that In2S3 hollow microspheres are built from small nanosheets with a thickness of 1020 nm and nanofibers with a length of 80100 nm. The wall thickness of the hollow microsphere is around 300500 nm. These nanosheets and nanorods construct the surface of the 5536
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Figure 3. XPS results of the hierarchical-like In2S3 hollow microspheres: (a) survey spectrum, (b) In 3d binding energy spectrum, and (c) S 2p binding energy spectrum.
Figure 4. SEM images of hierarchical-like In2S3 hollow microspheres prepared at 180 C for 24 h with a molar ratio of 1:4 taken at different magnifications. The images taken at high magnification reveal that a microsphere is constructed by several nanosheets and nanofibers.
hollow In2S3 microspheres. The surfaces of the hollow microspheres are analogous to the hierarchical structure. The relationship between the observed morphology and the synthesis time has also been investigated. The SEM images of the samples prepared at 10 and 15 h are shown in Figure 5. Initially, after 10 h of synthesis, it has been observed that the spherical morphology is not completely achieved (see Figure 5a). When the synthesis time increased, after 15 h of synthesis, clear and ordered spherical morphology has been observed (Figure 5b).
Figure 5. SEM images of hierarchical-like In2S3 hollow microspheres prepared at 180 C for (a and b) 10 h and (c and d) 15 h. The overview and single microspheres are shown.
These spheres were constructed by nanosheets and nanofibers. When the reaction time further increased (up to 24 h), the microsphere structure is retained and the increasing synthesis time did not change the morphology. These results suggest that the wall thickness of these hollow microspheres increased with the increase of both the molar ratio and reaction time and, also, that the latter is the main parameter of the formation of In2S3 hollow microspheres constructed by 2D nanosheets and 1D nanofibers. It is evident that the process undergoes Ostwald ripening.4447 5537
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Figure 6. (a and b) Low magnification of bright-field TEM image of the nanostructured In2S3, 1:3 and In2S3, 1:4, respectively. (c) SAED pattern of In2S3. (d) TEM image of the region at the edge of nanostructured In2S3. (e) TEM image of the thin areas showing nanofibers and nanosheets. (f) Æ100æ HR-TEM images showing lattice fringes of (103) and (109) planes.
On the basis of the above results, a growth process of the hollow microspheres by means of In2S3 2D nanosheets can be proposed. It is well-known that, the higher the density of surface atoms, the stronger the peak from XRD. When the surfactant is used, the surfactant molecules can cap the crystal surface selectivity. Because TSC can act as both a sulfur source and capping agent, the TSC molecule can cap the crystal surface selectivity.48 3.4.2. HR-TEM. Bright-field TEM images of the hollow microspheres of In2S3, 1:3 and In2S3, 1:4 are shown in panels a and b of Figure 6, respectively. The surfaces of the microspheres are not even and contain 2D nanosheet structures. It is clearly shown in Figure 6b. TEM not only characterizes the morphology but also gives crystallographic information of a material. Figure 6c is a selected-area electron diffraction (SAED) pattern taken from several dozen microspheres. The 1 0 9, 0 0 12, 3 0 9, and 2 2 12 reflections have been observed and are indexed according to space group I41a/amd. A general view of the regions at the microsphere surface is shown in Figure 6d. It is looks like a structure that consists of fibers and sheets, of which contrasts are dark and light, respectively. Both of them are, in fact, the nanosheets but oriented along different directions with respect to the electron beam. When the surface of the nanosheet is parallel to the electron beam, therefore, observing the edge, it appears as dark contrast. On the contrary, the light contrast shows the surface of nanosheets. The observed various contrasts can be explained as a thickness effect, therefore, given by the combination of the different oriented and curved nanosheets. Figure 6e is a HR-TEM image showing nanofiber-like regions embedded in the continuous nanosheet. It can be seen that the lattice fringes are continuous but slightly distored from the dark region to the nanosheet, indicating that both regions have the same lattice plane. We can confirm that the observed contrast in Figure 6d is given by curved nanosheets. A HR-TEM image taken along close to a Æ100æ zone axis is shown in Figure 6f. The lattice spacings of 0.62 and 0.32 nm are identified as (103) and (109), respectively. We should emphasize that the image is a projection of a
Figure 7. Possible formation mechanism of In2S3 hollow microspheres.
curved or even bended region, giving various thicknesses and, hence, a range of contrasts. Some regions having the same lattice spacing but lighter contrasts suggest the change of density, i.e., the presence of small voids embedded in the nanosheet. The discontinuity of lattice fringes suggests that the region is composed of various crystallites with similar orientations. The areas without lattice fringes could be amorphous materials or at different heights. 3.4.3. Formation Mechanism. A series of experiments was carried out to investigate the formation mechanism of tetragonal β-In2S3 hollow microspheres. Hence, we deduce the formation mechanism of hierarchical-like tetragonal β-In2S3, pictorially shown in Figure 7. The growth mechanism of the hierarchicallike In2S3 hollow microsphere nanostructures is proposed. At first, TSC and In3þ can form indiumTSC complexes. Upon exposure to the high temperature and pressure in the solvothermal process, the SC bond is broken. It results in the formation of In2S3 nuclei. Nanoparticles were further obtained with the 5538
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Figure 8. UVvis DRS spectrum of In2S3 microspheres.
continuous supply of the building blocks. It is well-known that the surface area of a sphere is the smallest under the same volume. The particles in the solution can aggregate to form the spherelike crystals, driven by minimizing the surface energy and hydrogen-bond interaction, which is similar to the results reported by Zhang et al.49 It is emphasized that 3D In2S3 hollow microspheres were obtained when no surfactant was added. In fact, the formation process is different from that of 3D hierarchical-like In2S3 hollow microspheres. TSC decomposition provides S2 at 180 C, and S2 reacts with In3þ to form the In2S3 nucleus. To decrease the surface energy, microcrystals forms by either the further growth of the crystalline nucleus or aggregation of small crystals. XRD shows that both peak intensity of (0 0 12) and (2 2 12) increase abnormally, suggesting the formation of 1D nanofibers and 2D nanosheets because of anti-isotropy. A lot of microbubbles of H2S produced in the TSC decomposition provide aggregation centers. Driven by the minimization of interfacial energy, the 1D or 2D In2S3 nanocrystals may aggregate around the gasliquid interface between H2S and water and, finally, hollow In2S3 microspheres form.50 By means of the SEM images taken at high magnification (Figure 4), the surfaces of the hollow microspheres are assuredly composed of 1D nanofibers and 2D nanosheets. The prepared 3D hierarchical-like hollow microspheres of In2S3 could not be destroyed and broken into individual nanosheets even by subjecting its aqueous and non-aqueous solution mixture to centrifugation at the rate of 4000 rpm for 30 min, which shows that they are stable. Because weak van der Waals interactions are not strong enough to stabilize the 3D structures, it is believed that a strong chemical bond exists between the contacting surfaces at the inner end of the sheets formed during growth. 3.5. Optical Properties. In addition to the structural properties, the optical properties of the In2S3 samples were also studied by UVvis diffused reflectance spectroscopy (DRS). The DRS spectra of In2S3 samples were recorded in the wavelength range between 200 and 1000 nm and are shown in Figure 8. The reflectance data were converted to F(R) values according to the KubelkaMunk theory.51 The function F(R) = (1 R)2/2R is used as the equivalent of absorbance (plotted in Figure 7). The band gap (Eg) of bulk-like In2S3 is reported to vary between 2.0 and 2.2 eV, which corresponds to 620550 nm.14 In contrast, we observed that the absorption edges of all of our samples vary between 610 and 620 nm. The values correspond to the absorption edge of a semiconductor material. It is also interesting to note that the absorption edge values of all samples were very
Figure 9. Nitrogen sorption isotherms of In2S3 at 77 K. (Inset) Corresponding BJH pore size distribution curve calculated from the desorption branch.
close, indicating that they have similar optical properties. Furthermore, the steep absorption edge is evidence of a narrow size distribution and uniform crystallites of In2S3 microspheres;52 the particle size and uniform distribution is confirmed by SEM (Figure 4). The band gap values of these samples were obtained by extrapolating the absorption edge by a linear fit method. The band gap values of all samples are very close, around 1.9 eV, indicating that the band gap energy value is not sensitive to the molar ratio of the precursors. The steep shape of the visible region reveals that the absorption band of In2S3 is due to the transition from the valence band to the conduction band,53 instead of the transition from the metal impurity level to the conduction band, as observed for the metal-ion-doped semiconductors.54,55 The band structure indicates that charge transfer upon photoexcitation occurs from the S 3p orbital to the In 5p empty orbital. Thus, the steep absorption edge implies singlephase In2S3, which is in good agreement with our XRD studies. Because In2S3 absorbs a significant amount of visible light, it can be used as a visible-light-active photocatalyst.56 Hence, the photocatalytic properties of these In2S3 microspheres have been systematically studied and are discussed in the following sections. 3.6. Surface Area Analysis. The surface area and porosity are important properties of a catalyst to determine its activity to apply for photocatalytic degradation applications. The pore architectures of the heterogeneous catalysts control the transport phenomena and govern the selectivity in various catalyzed reactions. In addition to the SEM and TEM studies, the pore nature of these In2S3 hollow microspheres was confirmed by pore size distribution measurements, which was obtained by the nitrogen adsorptiondesorption isotherm and BJH methods on a Quantachrome Instruments Autosorb 1 accelerated surface area and porosimetry system. Figure 9 shows the typical sorption isotherms and the corresponding pore size distribution (inset of Figure 9) of the In2S3 3D hollow microspheres. It is seen that the isotherm is type IV, indicating the presence of mesoporous materials according to the International Union of Pure and Applied Chemistry (IUPAC) classification.56 The type-IV isotherm with a hysteresis loop (H3) in the range of 0.40.9P/P0 was obtained in our samples. Accordingly, the pore size distribution curve displays a narrow size distribution, with maxima at 4.5 nm. These measured data indicate that the In2S3 3D hollow microspheres possess a spherical pore shape with mesoporous structures. The 5539
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and oxidative decomposition of the electron-deficient methylene blue. To the best of our knowledge, the photocatalytic performances of tetragonal β-In2S3 hollow microspheres have not been reported and the catalytic mechanism of β-In2S3 has not been fully understood until now. In our future work, we will investigate the catalytic mechanism of tetragonal β-In2S3 hollow microspheres in detail.
Figure 10. Time-dependent UVvis absorption spectra of methylene blue (5 mg/L) degradation in In2S3 hollow microspheres under visible light irradiation. (Inset) Photocatalytic degradation curve of methylene blue with In2S3 hollow microspheres under visible light irradiation.
BJH pore size distribution was found to be less than 10 nm. These observed
