Preparation of Mesoporous SnO 2 by Electrostatic Self-Assembly

We report a simple and scalable strategy to synthesize mesoporous SnO 2 with tin dioxide nanoparticles of 5-6 nm crystalline walls and 3-4 nmpore diameterwith the assistance ofMo 7 O 24 6− as templating agent at room temperature.The sampleswere characterized by XRD, TEM, UV-DRS, XPS, and BET. The product has a moderately high surface area of 132m g and a narrow mesoporous structure with an average pore diameter of 3.5 nm. The photocatalytic activities of the mesoporous SnO 2 were evaluated by the degradation of methyl orange (MO) in aqueous solution under UV light irradiation.


Introduction
Porous SnO 2 has received great attention because of its wideranging applications, in areas such as in lithium-ion batteries [1], gas sensors [2], catalysis [3][4][5], solar cells [6,7], and supercapacitor electrodes [8] due to its abundance, low cost, and high surface area.Recently, the extensions of soft or/and hard templating procedures to prepare porous SnO 2 with huge specific surface areas have been reported [3].In order to obtain mesoporous SnO 2 , the reported general approach is to use a templating agent, usually a surfactant, which is removed by calcination or chemical extraction after the SnO 2 network is formed.However, the posttreatment, such as high-temperature calcination, may affect the properties of the SnO 2 particles and may reduce the surface areas and pore volume.Therefore, avoiding the use of surfactants is highly desirable and would be beneficial in terms of cost, environmental impact, and scale-up potential.For instance, Mancin and coworkers constructed mesoporous silica through a surfactant-free approach.The templating agent that they used is a hydrolytically unstable inorganic phase and the template could be removed by solvent exchange with water at room temperature [9], but the surface area and pore structure of mesoporous silica had remained unclear.
In this work, we developed a novel, facile, and controllable technique to synthesize mesoporous SnO 2 .The technique of electrostatic self-assembly of charged colloidal particles was exploited to prepare mesoporous SnO 2 .In our previous works, mesoporous NiO has been fabricated with the assistance of MoO 4 2− [10].Our concept is that two kinds of soluble salt in acidic solution do not hydrolyze.The formation of the homogeneous assemblies is initiated with the generation of a large number of nucleuses after the mixing of the acidic precursor with basic solutions.It is well known that the solubility of Sn(OH) 4 in aqueous solution is determined by the pH and the Sn 4+ concentration.When the pH was adjusted to around 4, the Sn 4+ ions are almost transformed to colloidal Sn(OH) 4 However, the isoelectric point of colloidal Sn(OH) 4 solutions is 4.5 [11].The pH value that we used (4) was below the isoelectric point of Sn(OH) 4 ; hence the surfaces of Sn(OH) 4 colloid are positively charged because of the presence of abundant protonated hydroxyl groups.MoO ] (s) ↓ Systems of nanoparticles (NPs) interacting through electrostatic forces have been reported.For example, Grzybowski and coworkers reported that large 3D crystals could be prepared by the electrostatic self-assembly of oppositely charged metallic nanoparticles [13].Crystal growth requires advancing of grain boundaries.Grain boundaries can be "pinned" and hence their motion is restricted by introducing tiny secondary-phase particles along the grain boundaries.The growth of NPs is generally inhibited by surface modification with organic molecules, such as surfactants and ligands [14].In our system, Mo 7 O 24 6− ions play the role of traditional organic passivating ligands that anchor onto the surface of colloidal Sn(OH) 4 particles and form the "pins" at grain boundaries, inhibiting the growth of the particles.Then SnO 2 /MoO 3 is formed following the process of separation and calcination.Mesoporous SnO 2 was prepared by dissolving SnO 2 /MoO 3 with NaOH solution.Figure 1 illustrates the preparation procedure of mesoporous SnO 2 .

Experimental Section
2.1.Synthesis.Chemicals: all of the chemicals (analytic reagent, AR) were used as received without further purification.
In a typical experiment, SnCl 4 ⋅5H 2 O (0.02 M) was dissolved in deionized water (50 mL) and produced a clear precursor solution after stirring for 1 h.(NH 4 ) 6 Mo 7 O 24 ⋅4H 2 O (0.0029 M) was dissolved in deionized water (50 mL) to obtain the solution of (NH 4 ) 6 Mo 7 O 24 , which was added to the solution of SnCl 4 dropwise.After stirring for 30 min, the pH of the solution was adjusted to 4 by dropwise addition of NH 3 ⋅H 2 O solution (2 M).The resulting mixture was stirred for an additional 3 h.Then the precipitate was centrifuged and thoroughly washed with deionized water to remove remaining Cl − and NH 4 + to obtain the precursor.The precursor was calcined at 400 ∘ C in an oven for 3 h with a heating rate of 10 ∘ C min −1 and then cooled to ambient temperature to obtain SnO 2 @MoO 3 .The SnO 2 /MoO 3 was dissolved in NaOH solution (2 M) for 2 days under magnetic stirring, was collected by filtration with a membrane filter, and was dried at 110 ∘ C to obtain mesoporous SnO 2 .

Characterizations.
The phase structures of as-prepared products were characterized by X-ray diffraction (XRD) (Bruker D8 advance) with Cu-K radiation ( = 1.5418Å) over a 2 range between 5 ∘ and 70 ∘ and the scanning speed of 4 ∘ min −1 with a step of 0.02 ∘ .The sample morphology was examined by transmission electron microscope (TEM) with the accelerating voltage of the electron beam of 200 kV (JEOL JEM-3930EX).Diffuse reflectance spectra (DRS) were obtained for the dry-pressed disk samples using a scan UV-vis spectrophotometer (Hitachi U-3010) equipped with an integrating sphere assembly, using BaSO 4 as reflectance sample.The spectra were recorded at room temperature in air, in the range of 200-800 nm.Specific surface area and pore diameter of samples were measured by the BET method using N 2 adsorption measurement (ASAP 2020 V3.01H) on nitrogen adsorption at 77 K after the pretreatment at 393 K for 2 h.The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and pore size distribution was obtained from desorption plots by a Barrett-Joyner-Halenda (BJH) analysis.X-ray photoelectron spectroscopy (XPS) spectra were acquired with a Kratos Axis Ultra DLD spectrometer (Kratos Analytical-A Shimadzu group company) using a monochromatic Al K source (1486.6 eV).The analyzer uses hybrid magnification mode (both electrostatic and magnetic) and take-off angle is 90 ∘ .Under slot mode, the analysis area is 700 × 300 m.Analysis chamber pressure is less than 5 × 10 −9 Torr.Pass energy of 160 eV and 40 eV is normally used for survey spectra and narrow scan spectra, respectively.The BE scale was calibrated according to the C 1s peak (284.8 eV) of adventitious carbon on the analyzed sample surface.

Measurements of Photocatalytic Activity.
The photocatalytic activity of samples was evaluated in terms of the degradation of methyl orange (MO, 10 mg/L).The photocatalyst (0.05 g) was added into a 100 ml quartz photoreactor containing 50 ml of a MO solution.The mixture was stirred for 0.5 h in the dark in order to reach the adsorptiondesorption equilibrium.Ultraviolet light was obtained by a 200 W high-pressure mercury lamp with main emission wavelength 313 nm as light source and the distance between the light and the reaction tube is 20 cm.After a given irradiation time, the samples were withdrawn for subsequent analysis with a UV-vis spectrophotometer (Shanghai 760).
No diffraction peaks of other impurities were detected, indicating that MoO 3 exists in the product in amorphous form.The crystallite size calculated from the widened (110) diffraction peak at 2 = 26.6 ∘ according to the Scherrer equation is 5.1 nm, which is in good accord with the thickness observed on HRTEM (Figure 3).The morphology and microstructure of the prepared SnO 2 @MoO 3 and mesoporous SnO 2 were examined by TEM measurements.Figures 3(a) and 3(b) show representative SnO 2 @MoO 3 samples and Figures 3(c) and 3(d) show representative mesoporous SnO 2 , which aggregated from ultrafine nanoparticles with a typical diameter of 6 nm.It can be clearly seen that amorphous MoO 3 layer was coated onto SnO 2 nanoparticles in SnO 2 @MoO 3 in Figure 3(b), confirming that the formation mechanism of SnO 2 /MoO 3 is exactly as described in Figure 1.The SnO 2 nanoparticles with a size of about 5-6 nm can be identified.The SAED (inset of Figure 3(b)) image presents the polycrystalline behavior of the materials due to the spot and ring patterns.The spacing between two adjacent lattice fringes are 0.24 nm and 0.17 nm, corresponding well to the spacing of the (200) and (220) plane of SnO 2 (Figure 3(d)), respectively.This is perfectly in agreement with the XRD analytical results.Since SnO 2 is insoluble and MoO 3 is highly soluble in NaOH solution, the content of Mo atoms in SnO 2 @MoO 3 is reduced from 16.6% before dissolution to 6.9% after dissolution in NaOH solution (Figure 4), which suggests that Mo 7 O 24 6− plays the role of the protective and stabilizing agent coated onto SnO 2 to inhibit the growth of the particles during calcination.
The nitrogen adsorption-desorption isotherm (a) and BJH-derived pore size distributions (b) of the sample are shown in Figure 5.Its micro/mesoporous properties are indicated by the hybrid isotherm (I + IV) type in Figure 5(a).The sample has a BET surface area of 132 m 2 g −1 , a narrow mesoporous structure with an average pore diameter (Figure 3(b)) of 3.5 nm, and a pore volume of 0.12 cm 3 g −1 .
The BET surface area of samples increased to 132 m 2 g −1 from 21 m 2 g −1 , and the average pore diameter reduced from 4.2 nm to 3.5 nm after dissolution with NaOH, which suggests that MoO 3 plays the role of the protective and stabilizing agent coated onto SnO 2 to inhibit the growth of the particles during calcinations.
The diffuse reflectance spectra of the samples show photoresponse in visible light region (Figure 6).The absorption edge of mesoporous SnO 2 is estimated to be 420 nm with a band gap of 3.0 eV, which is less than the bulk band gap (3.6 eV).Meanwhile, it can be clearly observed that SnO 2 @MoO 3 exhibits a higher absorbance in the visible light range than mesoporous SnO 2 .This may be caused by the fact that MoO 3 shell can serve as a component in the composites and the interfacial coupling effect between MoO 3 and SnO 2 particles is possibly for the red-shift of the band gap.Such an enhancement in the visible light harvesting, including absorbance and scattering, can increase the number of photogenerated electrons and holes to participate in the photocatalytic reaction and enhance the photocatalytic performance [15].
The surface chemical composition and chemical states of mesoporous SnO 2 were further confirmed by X-ray photoelectron spectroscopy (XPS).Only Sn, Mo, C, and O species were detected (Figure 7(a)).In Figure 7(b), the Sn3d5 binding energy peaks are observed at ∼486.7 and ∼495.1 eV, corresponding to the spin orbit of Sn3d5/2 and Sn3d3/2, which suggests that Sn in the samples exists in a tetravalentoxidation state [16].The Mo 3d core level spectrum of the MoO 3 consists of a spin orbit doublet with peaks at 232.6 and 235.8 eV, associated with Mo cations in the higher oxidation state (Mo 6+ ) (Figure 7(c)) [17].Additionally, the peak with an O1s binding energy around 530.8 eV is attributed to oxygen in the SnO 2 crystal lattice, which corresponds to the Sn-O-Sn bonds [18], while the other peak positioned at 530.4 eV should be ascribed to the typical Mo−O bond [19].
The photocatalytic activities of the investigated samples were evaluated by measuring the time-dependent degradation of MO in an aqueous catalyst suspension.Figure 8 illustrates the relative concentration C/C 0 of MO as a function of time under ultraviolet light irradiation in the presence of the catalysts.Prior to irradiation, the suspension of the catalyst and dye solution was stirred in the dark for 0.5 hours to reach the equilibrium adsorption.It was noteworthy that the adsorption of MO was found to gradually increase with the increase of Mo content and MO could be completely decolorized after 2.5 hours.
In order to evaluate the photocatalytic activity of the synthesized mesoporous SnO 2 , photocatalytic degradation of methylene blue (MB) was also studied under ultraviolet light irradiation.Mesoporous SnO 2 also performed on photocatalytic activity of degradation of MO.The effect of the pH value of the MO solution on the photocatalytic activity of mesoporous SnO 2 was tested.It can be observed that the pH value can significantly affect the photocatalytic activity.The degradation rates are 60% and 100% under the conditions of pH = 4 and pH = 7, respectively, while it is only 10% when the pH value is further increased to 9.

Conclusion
In summary, we have synthesized SnO 2 nanoparticles 5-6 nm in size with the assistance of Mo 7 O 24 6− in aqueous solution at room temperature and assembled them into mesoporous materials.Mesoporous SnO 2 shows photocatalytic activities by the degradation of MO in aqueous solution under UV light irradiation.

Figure 4 :Figure 5 :
Figure 4: The EDX patterns of the content of Mo atoms in SnO 2 @MoO 3 (a) and mesoporous SnO 2 (b).
4 2− ions could be condensed in acid solution, and the extent of condensation depended on the pH level.The dominant species are MoO 4 2− ions in pH range of 7-12, and the protonated species were those of Mo 7 O 24 6− ions and of Mo 8 O 26 4− ions in pH ranges 3-5 and at pH below 2, respectively [12].Therefore, the positively charged Sn(OH) 4 and the negatively charged Mo 7 O 24 Schematic illustration of the preparation of mesoporous SnO 2 .ions began to coprecipitate (2) when Mo 7 O 24 6− ions were added to colloidal solutions of Sn(OH) 4 at a pH value of 4