Synthesis of Monodisperse Walnut-Like SnO 2 Spheres and Their Photocatalytic Performances

Novel walnut-like SnO 2 spheres have been synthesized using a one-step hydrothermal reaction with SnCl 2 ⋅2H 2 O and KOH as raw materials. The morphology, microstructure, and optical properties of the products were characterized by X-ray powder diffraction (XRD), Raman spectrum, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and ultraviolet-visible (UV-Vis) absorption spectroscopy. The detailed studies revealed that these synthesized spheres are highly monodisperse and have a uniform size of approximately 250 nm. Photocatalytic activity of the prepared SnO 2 spheres was evaluated by the degradation of methylene orange. The synthesized SnO 2 spheres exhibited excellent photocatalytic degradation. In addition, a possible formation mechanism of the walnut-like nanostructures was proposed based on reaction timedependent experiments.


Introduction
Textile industries produce large volume of colored dye effluents which are toxic and nonbiodegradable [1].In the past years, the traditional techniques for the treatment of dye waste effluents are usually inefficient, costly, and nondestructive or just transferring pollution from water to another phase [2].Recently, the purification of wastewater by photocatalytic degradation of dyes using semiconductor particles has proven to be a very effective method, which leads to complete mineralization of the pollutants [3].Many oxide semiconductors, such as TiO 2 , ZnO, and SnO 2 [4][5][6], have been widely studied due to their good potential for photocatalyst.Among the above semiconductors, SnO 2 is a very important n-type oxide and wide band gap (3.6 eV) semiconductor [7].Due to its excellent physicochemical properties, SnO 2 is employed in a wide range of applications, including gas sensors [8] and photocatalysts [9].Some researchers have reported that heterogeneous SnO 2 showed enhanced photocatalytic activity for photodegradation of various organic dyes [10,11].However, use of SnO 2 single-phase as a photocatalyst to decompose organic dyes still remains difficult and necessary [12].It is well known that the size and morphology of nanomaterials can substantially affect their physicochemical properties and their applications [13][14][15].Therefore, novel SnO 2 morphologies may improve its performances in photocatalysis.
The hydrothermal method has been the most widely used method for synthesizing SnO 2 materials due to its advantages of saving energy and convenient manipulation as well as excellent control over size and morphology [16].However, various templates and surfactants, which are typically toxic and expensive, are often introduced into the hydrothermal process to control the morphology, uniformity, and dispersibility of SnO 2 nanomaterials.The removal of these stabilizing molecules requires an additional thermal treatment or chemical etching.Alternatively, if SnO 2 nanomaterials could be synthesized via a hydrothermal reaction without any organic additive or template, the manipulation would be convenient and the extraneous impurities would be avoided.In this paper, we present a one-step hydrothermal method to prepare walnut-like SnO 2 spheres, using only SnCl 2 ⋅2H 2 O and KOH as the raw materials.The products are investigated 2 Journal of Nanomaterials in detail for their structural characteristics and morphologies.A possible growth mechanism of the walnut-like nanostructure is proposed based on the reaction time-dependent experiments.Furthermore, compared with the commercial SnO 2 , the photocatalytic performance of the walnut-like SnO 2 is improved toward methyl orange (MO) solution.

Experimental Section
2.1.Material and Methods.All of the chemical reagents used in this research are of analytical grade without any further purification.In a typical synthesis, 0.002 mol of SnCl 2 ⋅2H 2 O was dissolved in 35 mL of deionized water.Then, KOH (4 M, 5 mL) was added to the above solution under vigorous stirring at room temperature.The resulting solution was transferred to a Teflon-lined stainless steel autoclave (50 mL), sealed, and maintained at 190 ∘ C for 15 h.After cooling to room temperature, the obtained precipitates were washed several times with distilled water and absolute ethanol followed by drying at 60 ∘ C for 12 h.

2.2.
Characterizations.X-ray powder diffraction (XRD) pattern was collected on a SmartLab X-ray diffractometer with Cu-K radiation ( = 1.5406Å).The Raman spectrum (inVia, Renishaw, Gloucestershire, UK) was excited by a 785 nm diode laser at room temperature.The morphologies of the samples were examined using Zeiss Supra 55 field emission scanning electron microscopy (FE-SEM) operated at 15 kV.Transmission electron microscopy (TEM, JEM-3010, Questar, New Hope, USA) was performed using an acceleration voltage of 300 kV.The optical absorption of the products was carried out using a UV-Vis double-beam spectrophotometer (UV-2450, SHIMADZU).The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method using a surface area analyzer (Micromeritics, Gemini VII 2390).

Photodegradation
Experiments.0.05 g of the SnO 2 photocatalyst was dispersed in 50 mL of a 10 mg/L MO solution followed by stirring in the dark for 30 min to establish absorption equilibrium.Then, the suspension was irradiated by UV light (300 W mercury lamp) under continuous magnetic stirring.After irradiation for a certain time period, the reaction suspension was centrifuged and characterized using a UV-Vis spectrophotometer.

Results and Discussion
The XRD pattern of the walnut-like SnO 2 spheres is shown in Figure 1.All of the diffraction peaks are able to index to the tetragonal rutile structure of SnO 2 based on both the position and relative intensity (JCPDS Card number 71-0652).There is no other peak detected, which indicates that the as-prepared SnO 2 product has high purity.The sharp diffraction peaks imply that the walnut-like SnO 2 sphere in this work is highly crystalline.Based on the Debby-Scherer equation ( = / cos , where  is the mean crystalline size of the powder,  is the wavelength of Cu-K ( = 1.5406Å),  is the full width at half maximum (FWHM) intensity of the peak in radian,  is Bragg's diffraction angle, and  is a constant usually equal to ∼0.9) [17], the average crystallite size of SnO 2 calculated from the main diffraction peak is about 43 nm.The images in Figures 2(a)-2(d) show the morphologies of the products prepared with different concentration of KOH.It is found that the concentration of KOH affects the morphology dramatically.When the KOH concentration was 2 M, the product consists of dispersed nanoplates (Figure 2(a)).The size of some nanoplates is about 400 nm.As the concentration increases to 3 M, some hierarchical structures begin to appear (Figure 2(b)).It can be seen from some broken hierarchical structures that the inner of the products consists of tiny nanoparticles and the surface of the products is covered with numerous nanoplates.Additionally, compared with the concentration of 2 M, the size of nanoplates of the hierarchical structures decreases to ∼200 nm.With an increase of concentration to 4 M, walnut-like SnO 2 spheres are obtained (Figure 2(c)).These SnO 2 spheres are highly monodisperse and have a uniform diameter of approximately 250 nm.From the high magnification SEM image (Figure 2(d)), it can be seen that the surface of walnut-like SnO 2 spheres has many edges, which interlaced with each other to form uniform walnut-like appearance.It denotes that the preferential adsorption of molecules and ions from solution on different crystal faces directs the anisotropic growth of nanoparticles by controlling the growth rate along different crystal axes [18].Yang et al. proposed that OH − ions attached favorably to the specific facets of rutile SnO 2 nanocrystals and accelerated the growth rate of these facets [19].Our results indicate that the KOH influence the anisotropic growth of SnO 2 and play an important role in the synthesis of walnutlike structure.
To investigate the growth process of the walnut-like SnO 2 spheres and the possible growth mechanism, reaction timedependent experiments were performed.The corresponding results are shown in Figures 3(a)-3(d).When the hydrothermal time is 4 h, the product consists of some aggregates with sharp corners (Figure 3(a)).When the hydrothermal reaction time is prolonged to 6 h, the nanostructure evolves To obtain more details of the walnut-like SnO 2 spheres, the structure is characterized by TEM and SAED.As shown in Figure 4(a), the products have a spherical appearance with an average size of ∼250 nm, which agrees with the results from SEM.Furthermore, the as-prepared SnO 2 spheres are solid and composed of some nanoparticles.Based on the experimental results, a formation mechanism is proposed, as illustrated in Figure 5.The reaction at basic conditions can be shown as follows [19][20][21][22]: (5) When SnCl 2 ⋅2H 2 O is added into the water, the Sn(OH) 2 is obtained efficiently by the rapid hydrolysis of Sn 2+ cations (see ( 1)), and some of them further undergo dehydration and oxidation to form SnO 2 (see ( 2)-( 3)), while some others are subsequently oxidized to produce Sn(OH) 4 precipitation (see ( 4)).When KOH is added into the solution, the amphoteric Sn(OH) 4 precipitations immediately react with the excessive OH − to produce dissoluble Sn(OH) 6 2− complex ions (see (5)).During hydrothermal treatment, the complex Sn(OH) 6 2− ions decompose and massive SnO 2 nuclei are formed (see ( 6)) [23].In the present study, the formation of walnut-like structure is a typical Ostwald ripening process.At first, the supersaturation of reaction system is high; thus the nucleation is dominant.Large quantities of SnO 2 nuclei or particles aggregate to reduce the overall Gibbs free energy [24].Subsequently, SnO 2 nuclei or particles grow preferentially with growth inhibition in the [001] direction to form platy structure [19].In the latter stage, the nucleation is gradually reduced as the supersaturation falls with time in the surrounding solution.The new nucleated particles spontaneously "land" on the as-formed larger plates because of the surface energy minimization and further grow into another plate, leading to the irregular assembly [25].When the reaction is further progressed, the nucleation stops because the concentration falls below the critical nucleation concentration [26].However, the dissolution and recrystallization process through the competition growth under Ostwald ripening process still continue [27,28].Therefore, the larger SnO 2 plates grow at the cost of the smaller ones due to the different solubility between relatively larger and smaller particles.Then the thickness of these larger SnO 2 plates increases gradually.Also, their edge becomes smoother.Finally, walnut-like SnO 2 spheres are formed as the reaction time is further extended to 15 h.As is well known, Raman spectroscopy is a powerful method for the investigation of the structural properties of nanoparticles, because the variations in Raman spectra with decreases in particle size can be easily detected [29].The tetragonal rutile SnO 2 belongs to the space group D 4 h , of which the normal lattice vibration at the Γ point of the Brillouin Zone is given by the following equation [30,31]: Among these modes, A 1g , E g , B 1g , and B 2g modes are Raman active; A 2u and 3E u modes are IR active; and 2B 1u and A 2g modes are inactive.Figure 6(a) shows the Raman spectrum of the walnut-like SnO 2 spheres at room temperature.The Raman peaks located at 467, 622, and 755 cm −1 were assigned to the E g , A 1g , and B 2g vibrational modes of tetragonal rutile SnO 2 , respectively.These peaks show that the walnut-like SnO 2 spheres actually possess the main optical characteristics of a tetragonal rutile structure.In addition to these classical vibration modes, some additional Raman scattering peaks at 209, 334, 541, and 679 cm −1 , which are not found in Raman spectrum of bulk SnO 2 crystals, can be observed in the present SnO 2 spheres.According to some previous studies [32][33][34], the peak at 334 cm −1 may be related to the surface defects or the SnO 2 nanocluster formation, which constituted a new kind of vibration mode.The 209 cm −1 peak position is similar to the Eu(TO) mode observed in the IR spectra of crystalline SnO 2 [35,36].The 679 cm −1 peaks and the 541 cm −1 peak correspond to the A 2u mode [37,38] and inactive B 1u mode [39], respectively.These additional Raman peaks can be explained as follows.As particle size decreases due to the increase in the surface-to-volume ratio, the surface modes become dominant and some new bands may be observed.In the present SnO 2 spheres, the presence of IR modes and other forbidden Raman modes can be attributed to the relaxation of the Raman selection rules purportedly due to the reduced dimension of the nanostructure, as well as the lattice disorder and a high concentration of defects such as oxygen vacancies [40].Another feature observed on Raman spectrum is that the peaks shifts can be found by comparing with the data from bulk SnO 2 [41].This is consistent with the findings reported by others, which can be explained using phonon dispersion [42][43][44].
The UV-Vis absorption spectrum of the walnut-like SnO 2 spheres is shown in Figure 6(b).The band gap can be determined by (ℎ]) 2 = (ℎ] − E g ), where , , and ℎ] are the absorption coefficient, a constant, and the photon energy, respectively [45].According to the intercept of the tangent to the plot of (ℎ]) 2 as a function of ], the band gap is approximately equal to 3.46 eV (inset of Figure 6(b)), which is lower than that for bulk SnO 2 .
The experiments related to photocatalytic degradation of MO were carried out to demonstrate the photocatalytic properties of walnut-like SnO 2 nanospheres.Figure 7(a) shows the photocatalytic rate of walnut-like SnO 2 spheres and commercial SnO 2 powders under UV light at different periods of time.After 180 min of illumination, in the case of walnut-like SnO 2 spheres, the degradation of the MO concentration is about 88%, whereas in the case of commercial one, the degradation is lower than 9%.In order to evaluate the photostability, the cyclic photodegradation of MO over the walnut-like SnO 2 spheres is also examined.As shown in Figure 7(b), after three cycles of photocatalytic degradation, no significant loss of activity is represented.It indicates that the photocatalysts with high photocatalytic activity could be easily recovered by sedimentation and would greatly promote their industrial application in eliminating organic pollutants from wastewater.
The basic photocatalytic principle relies on the photogenerated electrons (e − ) and holes (h + ) (when absorbed energy is equal to or higher than its band gap) [1].The electrons and holes react with the adsorbed surface substances, like O 2 and OH − , to form reactive species • O 2− and OH • , which are the major oxidative species for the decomposition of organic pollutants.Generally, the larger exposed surface can provide more surface sites, where OH • radicals can be generated by the reaction between the photogenerated charge carriers and react with MO molecule.The as-prepared SnO 2 spheres possess the small size and rough surface, which benefit from improving their specific surface area.The BET surface area of the walnut-like SnO 2 spheres is estimated by nitrogen sorption and is about 47.4 m 2 g −1 , which is much higher than that of the commercial SnO 2 (3.5 m 2 g −1 ).Moreover, the walnut-like SnO 2 spheres are highly monodisperse without agglomeration in the solution.This good dispersion stability is of considerable importance in the practical water treatment applications.

Conclusions
Novel walnut-like SnO 2 spheres have been synthesized successfully via a facile one-step hydrothermal method.The asprepared SnO 2 spheres are highly monodisperse and have a uniform size of approximately 250 nm.The formation mechanism of the SnO 2 walnut-like structure has been proposed.In addition, these walnut-like SnO 2 spheres exhibit excellent photoactivity for the degradation of MO.Our synthesis method is environmental-friendly and inexpensive.It provides SnO 2 materials with a novel morphology that can be exploited for various applications.

Figure 4 (
b) shows high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) pattern (inset of Figure 4(b)), respectively.Based on the HRTEM image, the measured spacing is 0.26 nm, which corresponds to the (101) planes of the tetragonal rutile SnO 2 .The sharp diffraction spots observed in the SAED pattern indicate that the individual nanoparticles are highly crystalline.Furthermore, the spots can be easily divided into two sets, which are caused by the (101) and (110) crystal planes, and correspond to the [011] zone axis.These crystallographic results agree well with the XRD analysis.

Figure 7 :
Figure 7: (a) Photocatalytic degradation of MO under UV irradiation and (b) stability for photocatalytic activity of walnut-like SnO 2 spheres.