A Simple Surfactant-Free Solution Phase Synthesis of Flower-like In 2 S 3 Hierarchitectures and their Photocatalytic Activities

Flower-like In2S3 hierarchical nanostructures were successfully prepared via a facile solution-phase route, using thiacetamide as both sulfur source and capping agent. Our experimental results demonstrated that the morphology of these In2S3 nanostructures can be easily modified by changing the ratio of In(NO3)3/thiacetamide. With the ratio increasing from 1:1.5 to 1:6, the In2S3 crystals exhibited flower-like morphology of varying size. XRD and HRTEM of the flowers revealed the cubic structure of In2S3; morphological studies examined by SEM and TEM showed that the synthesized In2S3 nanostructure was a flower-like hierarchitecture assembled from nanoscale flakes. XPS and EDX analysis confirmed the stoichiometry of In2S3 nanoflowers. The optical properties were investigated by UV-vis DRS, which indicated that the In2S3 nanoflower samples possess a band gap from 1.90 to 1.97 eV. Furthermore, photocatalytic activity studies revealed that the prepared In2S3 nanoflowers exhibit an excellent photocatalytic performance, degrading rapidly the aqueous methylene blue dye solution under visible light irradiation. These results suggest that In2S3 nanoflowers will be a promising candidate for a photocatalyst working under the visible light range.


Introduction
he rational design and fabrication of nanomaterials with special morphologies of nanostructures have attracted a great deal of attention because the properties of a material depend not only on its phase, shape and size, but also on its organization [1,2].In particular, controlled hierarchical semiconductor nanostructures have attracted much attention because of their potential in new applications in catalysis [3], solar cells [4], chemical sensors [5], and supercapacitors [6].Thus, controllable synthesis of hierarchical nanostructures is of great interest and urgently called T for.At present, scientists have been devoting considerable efforts to three dimensional (3D) hierarchical architectures assembled by the low dimensional nanoscale building blocks, such as nanoplates, nanoparticles, nanorods, and so forth [7,8].However, it is still important, from the perspective of both science and industry, to develop a simple method to fabricate 3D hierarchical nanostructures with desired structures and properties and to explore their potential applications.
Indium sulfide (In 2 S 3 ), as an important semiconductor material, has stimulated great interest because of its wide applications in optical [9], photoconductive [10], optoelectronic [11] and catalytic fields [12].These applications result from its unique physicochemical properties, which depend not only on its phase but also on its shape and organization.For instance, Liu et al. [13] have reported that β-In 2 S 3 nanotubes showed high photocatalytic performance on the decomposition of Rhodamine B under solar irradiation.Du et al. [14] have synthesized ultrathin β-In 2 S 3 nanobelts via an improved hydrothermal route, and found that these showed a good photocatalytic performance on the degradation of methylene blue (MB).Moreover, β-In 2 S 3 hollow microspheres also exhibited an excellent photocatalytic performance in degrading MB under visible light irradiation [15].These results suggest that In 2 S 3 with various morphologies is a promising candidate for photocatalysis under visible light.So far, considerable effort has been devoted to fabricating various morphologies, including particles [16], flakes [17], wires [18], tubes [19], belts [20], spheres [21], sheets [22], dendrites [23] and so on.Despite these advances having been made, the ability to synthesize 3D In 2 S 3 hierarchical architectures with fine morphology control still remains a significant challenge.
In this paper, we present a facile solution-phase synthesis of 3D flower-like β-In 2 S 3 hierarchical nanostructures, which are composed of two-dimensional (2D) nanosheets.Flower-like In 2 S 3 hierarchitectures can be obtained only using thiacetamide as both a sulfur source and a capping agent.The effect of molar ratios of precursors on the morphologies of flower-like In 2 S 3 hierarchitectures and their formation mechanism are also discussed.Furthermore, optical and photocatalytic properties of the flower-like nanostuctures are also reported.

Experimental section 2.1 Material preparation
The flower-like In 2 S 3 nanocrystals were synthesized using analytical grade indium nitrate In(NO 3 ) 3 .5H 2 O (Sigma-Aldrich 99%) and thioacetamide (TAA) (C 2 H 5 NS) (Sigma-Aldrich 99%).As a typical synthesis, 10 mmol of In(NO 3 ) 3 .5H 2 O and 15-60 mmol (1:1.5 to 1:6 molar ratio of In(NO 3 ) 3 to C 2 H 5 NS) of TAA were dissolved in 100 mL of deionized water and continuously stirred for 30 min to form a homogenous solution.The above mixture was then transferred to a round bottomed flask and refluxed at 105 o C for 30 min, which yielded an orange precipitate.The product was collected by centrifugation, washed with deionized water and ethanol several times, and dried at 70 o C for 24 h.The final product was calcined at 350 o C for 2h for further characterization.

Material characterization
The products were characterized by X-ray diffraction (XRD) using a Bruker (D5005) X-ray diffractometer equipped with graphite monochromatized CuK  radiation ( = 1.54056Å).Morphology observations were performed on a Hitachi S-4800 field-emission scanning electron microscope.The morphologies and microstructures of the prepared products were analyzed by the transmission electron microscope (TEM) (JEOL JEM-3010).The chemical states of the samples were studied by X-ray photoelectron spectroscopy (XPS).The X-ray source was AlK  radiation (1.4867 eV).The photo-emitted electrons which escaped from the sample were analyzed in a hemispherical energy analyzer at a pass energy of E p = 20 eV.All spectra were obtained with an energy step of 0.1 eV and a dwell time of 50 ms.The absorption spectrum of the samples in the diffused reflectance spectrum (DRS) mode was recorded in the wavelength range of 200 and 1000 nm using a spectrophotometer (Jasco-V670), with BaSO 4 as a reference.
All photoreaction experiments were carried out in a photocatalytic reactor system, which consisted 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 150 W) positioned axially at the center as a visible light source.The reaction temperature was kept at 20 o C by cooling water.A special glass frit was fixed at the reactor as an air diffuser to disperse air uniformly into the solution.
Photocatalytic activities of the prepared samples were examined by the degradation of methylene blue under visible light irradiation.For each run the reaction suspension was freshly prepared by adding 0.10 g of catalyst into 250 mL of aqueous methylene blue solution, with an initial concentration of 5 mg L -1 .Prior to the photoreaction, the suspension was magnetically stirred under dark conditions for 30 min to attain the adsorption/desorption equilibrium condition.The aqueous suspension containing methylene blue and photocatalyst was then irradiated under visible light with constant aeration.At the 10-min intervals, the analytical sample was 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 using an ultraviolet-visible (UV-vis) spectrophotometer (Perkin-Elmer Lambda 45), and the concentration of MB was monitored at 664 nm.

The morphology and crystal structure of the products
Figure 1a shows the XRD pattern of the prepared product from the In(NO 3 ) 3 /C 2 H 5 NS molar ratio of 1:6.The peaks were indexed to a cubic phase of β-In 2 S 3 (JCPDS card No. 65-0459).The morphologies and structures of the product were examined with TEM and high-resolution TEM (HRTEM).Figure 1b shows the flower-like β-In 2 S 3 hierarchitecture in a large area, while an individual flower-like hierarchitecture shown in Figure 1c is about 300-350 nm in diameter.It can be seen that the hierarchitecture is constructed from the aggregation of many thin petal-like building units.The nanopetals are of layered structure and consist of the nanoflakes growing on both sides uniformly.The HRTEM image (Figure 1d) of individual nanopetals indicates a high degree of crystallinity of In 2 S 3 nanoflowers.The distance between adjacent lattice planes is about 0.27 nm, corresponding to the (400) plane.Furthermore, the EDX spectrum recorded from the highlighted region in Figure 1c confirms that the components of the products are only In and S with a ratio of 2.02:3 (Figure 3g), which is consistent with the XRD result above.The Cu signal is attributed to the TEM copper supporting grid.The elemental mapping of In and S did not show a strong composition variation (Figures 1e and 1f).The contrast difference was contributed by the morphology of the selected petals only.
To confirm the composition of the In 2 S 3 nanoflowers, XPS analysis was carried out (Figure 2).The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.6 eV.The survey XPS spectrum is shown in Figure 2a.No peak of any element other than C, O, In, and S was observed.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 sample surface and the carbon supporting film.The XPS spectra of the In 2 S 3 sample is consistent with the typical In 2 S 3 spectrum reported in the literature [24].Figures 2b and 2c show the high-resolution XPS spectra of In 3d and S 2p, respectively.The two strong peaks at 445.3 and 452.6 eV are assigned to the binding energies of In 3d 5/2 and In 3d 3/2 .The observed peaks at 160.9 and 162.4 eV are attributed to the binding energy of S 2p 3/2 and S 2p.These values are in good agreement with the reported data [25].According to the measurements of the In 3d and S 2p peak areas, the atomic ratio of In/S is 2:3, very close to the nominal composition of In 2 S 3 , which further confirms that the final products were stoichiometric In 2 S 3 with no impurity.This result is consistent with the measurements of energy-dispersive X-ray (EDX) and with the element map.(see Figures 1e-g).

Effects of precursor ratio on the morphology of In 2 S 3 nanocrystals
In order to investigate the influence of molar ratio of reactants on the morphology of In 2 S 3 nanocrystals, the morphologic variety at different molar ratios of indium nitrate to thioacetamide was examined by SEM (Figure 3).When molar ratio of indium nitrate to thioacetamide is 1:1.5, the product is composed of the smallest flower-like nanostructures illustrated in Figure 3a, with a diameter of about 400-650 nm.When the molar ratio is further decreased to 1:3.5, the diameters are 550-650 nm (Figure 3b).When the molar ratio is 1:4.5, the diameters are 400-450 nm (Figure 3c).When the molar ratio is 1:6, the diameters are 300-350 nm (Figure 3d).These SEM results indicate that the general flower-like morphology of these In 2 S 3 nanostructures is almost the same.However, it is apparent that the diameter of the individual flowers can be controlled by changing the concentration of thioacetamide.All the above samples of In 2 S 3 have the same crystal structure, as confirmed by XRD (Figure 4).

Growth mechanism of In 2 S 3 nanoflowers
As is well known, the morphology of nanocrystals can be determined by their intrinsic crystal structures [26].Figures 1a and 1d demonstrate that the crystal structure of the prepared In 2 S 3 is cubic close packing.Thus, In 2 S 3 crystals usually present a morphology of 2D nanoplates.High concentration of In 3+ and S 2-could lead to fast nucleation and congregation of In 2 S 3 in the early stages, resulting in sphere-like cores, which could then serve as heterogeneous nucleation sites, providing high-energy sites for crystalline growth.Then a secondary nucleation would be favored, and cubic angular protuberances would grow into nanopetals combining with the remaining primary particles, finally forming the 3D flower-like hierarchitectures, a process which is similar to that described as "the terrace-step-kink model" [27].This is similar to the formation process of those flower-like nanocrystals reported previously, such as CoS [28], CuO [29], TiS 2 [30], In 2 O 3 [31], AlO(OH) [32], ZnO [33], Bi 2 MoO 6 [34], Ni(OH) 2 [35] and so on.
We observed that the sizes of In 2 S 3 nanoflowers decreased with increasing the concentration of C 2 H 5 NS (Figure 4).This can be explained as follows.The thioacetamide ions can be easily absorbed on the In 2 S 3 surface, which slows down the rate of growth units moving to the growing crystal surface.This contributes to the growth units having enough time to move towards the lowest or favorite energy position.Thermodynamics plays a dominant role in this process.The adsorbed thioacetamide ions can stabilize and protect the In 2 S 3 exposed facet; this will be helpful to form a bigger nanoplate (Figure 4a).However, as the concentration of thioacetamide increases, the higher concentration of S 2-leads to easier nucleation and a faster reaction rate, resulting in a lack of time to age and thus producing small sized nanoflowers (Figure 4d).In this process kinetic growth dominates the strongest effect.This indicates that there is a synergistic effect between thioacetamide ions protecting the crystal facets and the nucleation rate, causing thioacetamide to tune the growth of In 2 S 3 nanoplates.Therefore, the growth rate and morphology of flower-like In 2 S 3 crystals can be controlled by adjusting the thioacetamide concentration in the mixed solvent.

Optical properties
The optical property of the obtained In 2 S 3 nanoflowers was investigated by an UV-vis absorption spectrophotometer.As shown in Figure 5, the absorption edges of the In 2 S 3 nanoflowers were found between 652 and 630 nm, corresponding to the absorption edge of a semiconductor material.It is interesting that the absorption edges for all samples were very close to each other.However, the absorption edge for the samples from In 2 S 3 -1:1.5 to In 2 S 3 -1:6 showed a small blue shift, which can be attributed to the quantization size effect of the In 2 S 3 nanoflowers [36].For a crystalline semiconductor, the optical absorption near the band edge follows the equation αhv=A(hν-E g ) n/2 , where α, ν, E g and A are the absorption coefficient, the light frequency, the band gap and a constant, respectively.Among them, n decides the characteristics of the transition in a semiconductor [37].The band gaps are calculated to be about 1.90, 1.92, 1.97, and 1.95 eV for In 2 S 3 -1:1.5,In 2 S 3 -1:3, In 2 S 3 -1:4.5 and In 2 S 3 -1:6, respectively, from the onset of the absorption edge (inset of Figure 5).It should be noted that those values are smaller than the one reported in our previous studies (2.00 eV) [15].The main reason may attribute to the large size of In 2 S 3 nanoflowers.This indicates that the In 2 S 3 nanoflowers have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation.

Photocatalytic performance of In 2 S 3 nanoflowers
In order to study the photocatalytic degradation properties of the prepared In 2 S 3 samples, methylene blue was used as a probe pollutant to investigate the photocatalytic performance of In 2 S 3 nanoflowers (In 2 S 3 -1:6 sample, for example).The maximum absorptive energy of methylene blue was at 664 nm. Figure 6 shows that the absorptive intensity of methylene blue at 664 nm gradually decreased with prolonging the irradiation time, when the mixed solution of methylene blue and In 2 S 3 nanoflowers was exposed to visible light irradiation by a sodium vapor lamp at room temperature.This result indicates that methylene blue underwent degradation behavior under the catalysis of In 2 S 3 nanoflowers.If the degradation ratio is defined as the ratio between the decreased absorptive intensity and that of the initial methylene blue solution, the degradation ratio was about 85% when the mixed solution was irradiated for 3 h.This implies that the prepared In 2 S 3 nanoflowers have good photocatalytic activity for methylene blue and are likely to be an efficient photocatalyst.As to the photocatalytic mechanism of In 2 S 3 nanoflowers on methylene blue, we assume that the electron injection from photoexcited methylene blue molecules to In 2 S 3 nanoflowers leads to oxidative decomposition of the consequently electron-deficient methylene blue.To the best of our knowledge, the photocatalytic performance of β-In 2 S 3 nanoflowers has not been reported and the catalytic mechanism of β-In 2 S 3 has not been well understood until now.

Conclusion
In summary, we have reported a simple method to prepare flower-like β-In 2 S 3 hierarchitectures by a solutionphase process, using (In(NO 3 ) 2 and thioacetamide as the precursors.The XRD analysis confirmed that the In 2 S 3 nanoflowers are in cubic phase.The SEM and TEM analysis revealed that the products are flower-like in shape, with a diameter from 300 nm to 650 nm.On the basis of the structural and morphological studies, a possible growth mechanism has been proposed to explain the formation of the In 2 S 3 nanoflowers.The compositional analysis made by EDX and XPS measurements confirmed that the stoichiometric In/S ratio is approximately 2:3, which indicates highpurity In 2 S 3 .The UV-vis DRS measurements demonstrated that the samples have the band gap from 1.90 to 1.97 eV and slightly change with varying crystal sizes.Thus, the narrow band gap of our samples combined with the pronounced catalytic activity for methylene blue degradation suggests that it is a highly efficient photocatalyst.

Acknowledgment
One of the authors (R.S.) thanks Sultan Qaboos University for financial support for this work under a SQU Internal Research Grant (IG/SCI/CHEM/14/02).

Figure 1 .
Figure 1.Characterizations of prepared In 2 S 3 nanoflowers from the In(NO 3 ) 3 /C 2 H 5 NS molar ratio of 1:6.(a) XRD patterns; (b) low-, and (c) high-magnification TEM images; (d) HRTEM image of the selected area marked with a yellow rectangle in (c); (e and f) the corresponding elemental mapping of In and S in the same area; (g) EDX spectrum.