Solvothermal Synthesis of Zn2SnO4 Nanocrystals and Their Photocatalytic Properties

Crystalline Zn 2 SnO 4 nanoparticles were successfully synthesized via a simple solvothermal route by using Zn(CH 3 COO) 2 ⋅2H 2 O and SnCl 4 ⋅5H 2 O as source materials, NaOH as mineralizing agent, and water and ethanol as mixed solvents. The used amount of NaOHwas found to have an important influence on the formation of Zn 2 SnO 4 .When themolar ratio of OH : Zn : Sn was set in the range from 4 : 2 : 1 to 8 : 2 : 1, Zn 2 SnO 4 nanoparticles with different shape and size were obtained. However, when the molar ratio ofOH : Zn : Sn was set as 10 : 2 : 1, amixture phase of ZnO andZnSn(OH) 6 instead of Zn 2 SnO 4 was obtained. Photodegradation measurements indicated that the Zn 2 SnO 4 nanoparticles own better photocatalytic property to depredate methyl orange than the Zn 2 SnO 4 nanopolyhedrons. The superior photocatalytic properties of Zn 2 SnO 4 nanoparticles may be contributed to their small crystal size and high surface area.


Introduction
In recent years, metal oxide semiconductors (MOSs) have been paid more and more attentions due to their diverse function and promising application in various fields.Among these MOSs, besides the widely studied binary oxides such as ZnO [1], SnO 2 [2], TiO 2 [3], and Fe 2 O 3 [4], the ternary MOSs, including Zn 2 SnO 4 and ZnSnO 3 , have also been investigated [5,6].Stimulated by the prominent morphology (shape and size) dependent physical and chemical properties of nanostructured MOS, many researchers have devoted their efforts to the design and synthesis of MOS nanostructures.Up to now, a variety of nanostructures, such as zero-dimensional nanoparticles, one-dimensional nanowires, nanorods and nanotubes, twodimensional nanosheets, and three-dimensional hierarchical micro/nanostructures that assembled with low dimensional nanobuilding blocks, have been synthesized and investigated.However, it is still a meaningful and important work to develop facile and feasible techniques for the synthesis of MOS nanomaterials with controlled shape and size in the field of nanoscience and nanotechnology.
Zn 2 SnO 4 , as an important ternary MOS, is a typical ntype semiconductor with the band-gap of 3.6 eV.Because of its high chemical sensitivity, low visible absorption, and excellent optical electronic properties, Zn 2 SnO 4 has many promising applications in gas sensors [5,7,8], solar cells [9,10], photocatalysts [11][12][13][14][15], and negative materials for rechargeable lithium ion batteries [16,17].Driven by these potential applications, an increasing research interest has focused on the synthesis of Zn 2 SnO 4 .In order to achieve Zn 2 SnO 4 , some traditional high-temperature techniques are usually employed, including high-temperature solid-reaction between solid ZnO and SnO 2 [18,19], thermal evaporation method by heating metal or metal oxides at high temperature [20][21][22][23], and the chemical vapor deposition methods [24].However, these reported methods are usually of high energy consumption, and thus not suitable for practical production in industry from the view point of environment protection.Recently, hydrothermal synthesis methods have been successfully developed to prepare Zn 2 SnO 4 [5,11,[25][26][27][28][29].Compared with the high-temperature synthesis methods mentioned above, the hydrothermal synthesis method has the merits of low cost and being friendly to environment and is thus considered as one of the most promising methods that can be applied in practical production.Up to now, various micro/nanostructures of Zn 2 SnO 4 have been synthesized 2 International Journal of Photoenergy by hydrothermal methods, such as nanowires [23,24,29], nanorods [28], irregular particles [17], well-defined polyhedra [22,30], and hierarchical cube-like microstructures assembled with nanoplates [8].As an evolution method of hydrothermal synthesis, the solvothermal synthesis is also an important wet chemical synthesis method that can be used to fabricate MOS nanostructures.However, compared with the widely reported hydrothermal synthesis methods for Zn 2 SnO 4 , few reports are about the synthesis of Zn 2 SnO 4 nanostructure via solvothermal method.
In this paper, a simple and efficient solvothermal route was developed to synthesize crystalline Zn 2 SnO 4 nanoparticles by using water and ethanol as mixed solvents.The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier infrared spectroscopy (FTIR), and nitrogen-sorption techniques.It was found that the crystallinity, particle size, and shape of Zn 2 SnO 4 are strongly dependent on the amount of alkali.The photocatalytic properties of the prepared Zn 2 SnO 4 were investigated by photodegradation of methyl orange dye.Results indicated that the irregular Zn 2 SnO 4 nanoparticles own better photodegradation capacity than Zn 2 SnO 4 nanopolyhedrons perhaps due to their smaller crystal size and higher surface area.were dissolved in 20 mL ethanol under magnetic stirring, followed by dropping 20 mL aqueous solution of NaOH (0.45 M).After stirring for 30 min, the obtained white slurry was transferred to a 50 mL Teflon-lined autoclave and then maintained at 160 ∘ C for 24 h.When the autoclave was cooled down to room temperature naturally, the white precipitates were collected by centrifugation, washed several times with absolute ethanol and distilled water, and finally dried at 80 ∘ C in air for 5 h to get the final products.For comparison, parallel experiments were also carried out by varying the molar ratio of OH − : Zn 2+ : Sn 4+ .The samples prepared at the molar ratio of OH − : Zn 2+ : Sn 4+ of 4 : 2 : 1, 6 : 2 : 1, 8 : 2 : 1 and 10 : 2 : 1 were denoted as 4S, 6S, 8S, and 10S, respectively.

Characterization.
The phase structure and purity of the as-prepared products were characterized by powder X-ray diffraction (XRD) on Bruker D8 diffractometer with Cu K radiation ( = 1.54056 nm).The transmission electron microscopy (TEM) images, selected area electron diffraction (SAED) patterns, and high resolution TEM (HRTEM) images were collected on a JEM-2100 TEM.N 2 adsorptiondesorption isotherms were collected at liquid nitrogen temperature using Quantachrome AsiQM0000-3 sorption analyzer.The specific surface area was calculated using multipoint Brunauer-Emmett-Teller (BET) method.The pore size distribution was determined from the adsorption branch of the isotherms using the DFT method.Before carrying out the measurement, each sample was degassed at 180 ∘ C for more than 6 h.

Measurement of Photocatalytic Activities.
The photocatalytic activities of the as-prepared Zn 2 SnO 4 were measured by photodegradation of methyl orange (MO) at room temperature.In a typical experiment, 50 mg as-prepared catalyst was suspended in 100 mL MO aqueous solutions (20 mg/L) in a water-jacketed reactor with the capacity of 200 mL.The UV lamp (TUV 4W/G4 T5, Philips, wavelength 254 nm) was placed above the reactor, and the distance between the lamp and liquid level was 10 cm.Before UV-light irradiation, the suspensions were stirred in dark for 30 min to ensure the establishment of absorption-desorption equilibrium.During the experiment, 3 mL reaction solution was taken out from the reaction system at a certain time interval and then centrifuged to remove the solid catalyst.After that, the obtained MO solution was analyzed on a UV-Vis spectrophotometer (TU-1810) in the wavelength range of 200-700 nm.

Characterization of the Prepared Samples
3.1.1.XRD Analysis.The phase structure and purity of the prepared samples was analyzed by XRD. Figure 1 shows the typical XRD patterns of the samples prepared at different conditions.It can be seen that when the molar ratio of OH − : Zn 2+ : Sn 4+ was in the range of 4 : 2 : 1-8 : 2 : 1, cubic inverse spinal Zn 2 SnO 4 phases can be successfully obtained.For example, as shown in Figure 1 (c), all the appeared reflection peaks from low angle area to high angle area can be indexed as perfect cubic inverse spinal Zn 2 SnO 4 (JCPDS no.24-1470).In the XRD patterns of 4S, 6S, and 8S (Figure 1 (a) to (c), resp.), no peaks from other crystal phases are detected, such as ZnO and SnO 2 .This result indicates that the samples prepared at present condition are of pure Zn 2 SnO 4 phase.Compared with 8S, the refection peaks of 4S and 6S are seriously broaden, which suggested that the used amount of alkali has important influences on the crystallinity of Zn 2 SnO 4 .In contrast, when the molar ratio of OH − : Zn 2+ : Sn 4+ was increased to 10 : 2 : 1, as shown, the XRD pattern of 10S in Figure 1 (d), two crystalline phases of cubic ZnSn(OH) 6 (JCPDS file no.20-1455), and hexagonal ZnO (JCPDS file no.36-1451) are observed, which indicates that a mixture of ZnSn(OH) 6 and ZnO is obtained instead of the single Zn 2 SnO 4 phase.Such result may be attributed to using excessive amount of alkali.

FT-IR Analysis.
The chemical-bond types of the prepared Zn 2 SnO 4 were investigated by FT-IR. Figure 2 shows the typical FT-IR spectra of the prepared 4S, 6S, and 8S.From the FT-IR spectra, we can see that the three samples exhibit similar characteristics of infrared adsorption bands, which are quite similar to the spinal Zn 2 SnO 4 reported in previous International Journal of Photoenergy literatures [11,31].Therein, the broad absorption peaks at 3426 and 1602 cm −1 can be ascribed to the vibration of absorptive water, and the absorption peaks at 546, 1038, and 1410 cm −1 are due to the vibration of M-O or M-O-M groups in Zn 2 SnO 4 .The results given by FT-IR analysis further confirm the formation of Zn 2 SnO 4 and thus corroborate the results obtained in XRD analysis.

TEM Analysis.
In order to obtain the detailed structural information of the as-prepared Zn 2 SnO 4 , further measurements by TEM and HR-TEM were performed.Figures 3(a)-3(c) show the typical TEM images of the prepared Zn 2 SnO 4 samples.From Figures 3(a) and 3(b), it can be seen that a large scale of nanosized particles with irregular shapes are obtained in 4S and 6S, respectively.The size of these nanoparticles is measured to be about 5-8 nm for 4s and 12-15 nm for 6S.In order to minimize their surface energy, most of these formed nanoparticles aggregated together loosely.As for 8S, besides the irregular nanoparticles, many polyhedron-like particles with the size about 30 nm are also formed (Figure 3(c)), being obviously different from that observed in 6S and 8S.The different crystallite size and shape in the obtained Zn 2 SnO 4 samples indicated that the used amount of alkali may have important influence on the formation of Zn 2 SnO 4 .Relatively high concentration of NaOH is favorable for achieving Zn 2 SnO 4 with larger size.Moreover, it is worthy to be mentioned that in the three samples numerous nanopores with different size were formed in the interspaces among the nanoparticles due to their loosely aggregated characteristics, as shown in Figures 1 (a), (b), and (c).The existence of porous structure in the prepared Zn 2 SnO 4 samples can be further convinced by following N 2 -sorption analysis.The insets in Figures 3(a)-3(c) show the corresponding selected area electron diffraction (SAED) images of 4S, 6S, and 8S, respectively.The appeared diffraction rings in the SAED patterns demonstrate the polycrystalline nature of the prepared Zn 2 SnO 4 samples.Figure 3(d) displays a representative HR-TEM image taken from one of the observed Zn 2 SnO 4 nanoparticles in Figure 3(b).The distance between the adjacent lattice fringes is measured to be 0.263 nm, which can be attributed to the (311) crystal plane of cubic inverse spinal Zn 2 SnO 4 (JCPDS no.24-1470).This result is consistent with the XRD analysis, which further proved the preparation of crystalline Zn 2 SnO 4 by the present solvothermal method.
3.1.4.N 2 -Sorption Analysis.N 2 -sorption measurements were further performed on the prepared Zn 2 SnO 4 samples to obtain the information of specific surface area and pore structure.Figure 4 depicts the nitrogen adsorption-desorption isotherms and the corresponding pore size distributions of 4S, 6S, and 8S.From Figure 4(a) it can be seen that the adsorption-desorption isotherms of all three samples exhibit a type of IV-like behavior including a type H3 hysteresis loop according to the IUPAC classification.Such result demonstrated the existence of mesoporous structure in the prepared samples, which is in agreement with the TEM observation.The pore size distribution curves (Figure 4(b)) of the prepared Zn 2 SnO 4 samples, being estimated based on the DFT method from the adsorption branch of the isotherm, indicated that the pore size distribution range for 4S and 6S was mainly centered at 1.7-10 nm, which was smaller than that of 8S (3-14.5 nm).As for the three Zn 2 SnO 4 samples, the relative wide pore-size distribution may be mainly contributed to the random and disordered aggregation characteristics of Zn 2 SnO 4 nanocrystals in the products.The calculated BET surface areas for 4S, 6S, and 8S were found to be 226, 166, and 91 m 2 ⋅g −1 , respectively.Obviously, the surface area of 4S and 6S was higher than that of 8S.Combined with the TEM analysis, it can be concluded that the surface area of the prepared samples seriously decreased with the increasing of crystal size of Zn 2 SnO 4 .

Photocatalytic Properties Studies.
In general, the photocatalytic properties of MOS can be influenced by many factors, such as the crystal size, exposed crystal plane, morphology, and band structure.In terms of our experiment, the crystal size and the surface area of Zn 2 SnO 4 can be controlled by adjusting the used amount of alkali in the reaction system.Therefore, it was expected that the Zn 2 SnO 4 samples prepared under different conditions can bring different photocatalytic activities.So, in order to evaluate the photocatalytic activity of the prepared Zn 2 SnO 4 , the experiments of photodegradation of methyl orange (MO) were performed under UV-light irradiation.It can be seen that when there was no catalyst in the reaction system, the decrease of  0 / is very slow and almost negligible.After irradiating for 100 min, only about 0.5% of total MO molecules were depredated.However, once the catalyst of Zn 2 SnO 4 was added, a rapid decrease of / 0 was observed, indicating that the prepared Zn 2 SnO 4 samples are effective to depredate MO molecules.Under the same irradiation period, the values of  0 / for 4S and 6S were larger than thos for 8S, indicating their better photocatalytic activity than 8S.After 100 min irradiation, the  0 / values for 4S, 6S, and 8S are about 0, 0.06, and 0.6, corresponding to the degradation about 100%, 96%, and 40% of total MO molecules, respectively.The superior photocatalytic activity of 4S and 6S can be explained by their smaller crystal size and larger surface area.It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of organic dye molecules on the surface of the catalyst.The higher specific surface area of 4S (226 m 2 ⋅g −1 ) and 6S (166 m 2 ⋅g −1 ) can provide more unsaturated surface coordination sites exposed to the solution and more opportunities for dye molecules to absorb on the surface of catalysts, resulting in the production of more active reaction sites.Moreover, the mesoporous structures in the catalysts enable storage of more dye molecules, which can also promote the photocatalytic properties.Furthermore, the bandgap of the catalyst material may be another important factor that can influence the photocatalytic properties.In our experiment, the crystallite size of Zn 2 SnO 4 in 4S and 6S was observed to be less than 20 nm.Therefore, the bandgap of Zn 2 SnO 4 catalyst may be broadened by the quantum size effect.The broadened bandgap can not only bring higher redox potentials but also promote electrons transferring form the conductive band of Zn 2 SnO 4 with high electric potential to those with low electric potential.Thus, the recombination of the photogenerated electron-hole pair can be hampered, which in turn results in the enhancement of the chargetransfer rates in the catalyst.

Conclusions
In summary, a simple solvothermal route was successfully developed for controlled synthesis of Zn 2 SnO 4 nanocrystals with different shape and size.Irregular Zn 2 SnO 4 nanoparticles about 5-8 nm (4S) and 12-15 nm in size (6S) and polyhedron-like Zn 2 SnO 4 nanoparticles about 30 nm in size (8S) can be easily obtained by adjusting the used amount of alkali.The BET surface areas of the prepared samples of 4S, 6S, and 8S were measured to be 226, 166, and 91 m 2 ⋅g −1 , respectively.Due to the higher specific surface area and quantum size effects, the 4S and 6S exhibited higher photocatalytic activity to degrade MO than 8S.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: TEM images of (a) 4S, (b) 6S, and (c) 8S and (d) the HR-TEM image corresponding to (c).The SAED patterns are shown as an inset in corresponding TEM images.
Figures 5(a)-5(c) show the time-dependent absorption spectra of MO solution containing different Zn 2 SnO 4 catalysts during the UV-light irradiation.It can be seen that the characteristic absorption International Journal of Photoenergy peak of MO centered around 465 nm decreased rapidly in intensity with the irradiation time and almost disappeared after undergoing a certain time of irradiation.The decreased speed of MO concentration in Figures 5(a) and 5(b) was found to be faster than that in Figure 5(c), indicating that the photodegradation capacity of 4S and 6S is stronger than that of 8S.The photodegradation plots of MO under UV-light by using different Zn 2 SnO 4 samples as catalysts are shown in Figure 5(d), in which  0 / was substituted by  0 /, where  0 and  are the initial and actual concentration of MO, respectively, because the normalized concentration of the solution equals the normalized maximum absorbance.For comparison, the degradation plot of MO without any catalyst is also displayed in Figure 5(d).
2.1.Synthesis of the Zn 2 SnO 4 .All of the chemical reagents were analytical grade and used as received without further purification.In this paper, Zn 2 SnO 4 was prepared via a simple solvothermal method by using Zn(CH 3 COO) 2 ⋅2H 2 O and SnCl 4 ⋅5H 2 O as starting materials, NaOH as mineralizing agent, and distilled water and absolute ethanol as mixed solvents.In a typical experiment for synthesizing Zn 2 SnO 4 , the molar ratio of OH − : Zn 2+ : Sn 4+ was set as 6 : 2 : 1.In detail, 0.525 g SnCl 4 ⋅5H 2 O and 0.659 g Zn(CH 3 COO) 2 ⋅2H 2 O