Hierarchical Semiconductor Oxide Photocatalyst: A Case of the SnO 2 Microflower

www.nmletters.org Hierarchical Semiconductor Oxide Photocatalyst: A Case of the SnO2 Microflower Yang Liu, Yang Jiao, Bosi Yin, Siwen Zhang, Fengyu Qu, Xiang Wu∗ (Received 27 July 2013; accepted 13 September 2013; published online 8 October 2013) Abstract: Hierarchically assembled SnO2 microflowers were synthesized by a facile hydrothermal process. Field emission scanning electron microscope results showed these hierarchical nanostructures were built from two dimensional nanosheets with the thicknesses of about 50 nm. Photoluminescence spectrum of the asobtained products demonstrated a strong visual emission peak at 564 nm. The photochemical measurement results indicated that the as-prepared sample exhibits excellent photocatalytic performance. These three dimensional SnO2 hierarchical nanostructures may have potential applications in waste water purification. Keywords: SnO2; Hierarchical structures; Photocatalyst Citation: Yang Liu, Yang Jiao, Bosi Yin, Siwen Zhang, Fengyu Qu and Xiang Wu, “Hierarchical Semiconductor Oxide Photocatalyst: A Case of the SnO2 Microflower”, Nano-Micro Lett. 5(4), 234-241 (2013). http://dx.doi.org/10.5101/nml.v5i4.p234-241 In recent years, the semiconductor photocatalysts with high performances for water contaminant degradation have attracted great interest to solve energy and environmental issues. The purification of waste water by photocatalytic degradation of organic dyes using semiconductor nanocrystals has been proven a very effective method [1-11]. As an important direct wide band gap semiconductor (Eg = 3.6 eV), SnO2 possesses the excellent optical, gas sensing and photocatalytic properties [12-17]. Thus far, the reported SnO2 nanostructures are mostly one dimensional (1D) structures, such as nanorods [18-20], nanotubes [21-23], and nanowires [24-27] and so on. Only a few successful examples of SnO2 nanosheets have been reported [28-30], which may be attributed to the difficulty in controlling the oxidation process of Sn2+ to Sn4+ such that the mixed phases of SnO2 and SnO will coexist in the product [31]. Nevertheless, three dimensional (3D) hierarchical structures by self-assembly of nanosheets building blocks are much more relatively rare [32,33]. Due to the complicated spatial arrangement, the hierarchical architectures can provide both extraordinarily high activated surface area and robustness. It is thus highly desirable to develop a facile and efficient method to fabricate phase-pure nanosheets assembled SnO2 hierarchical structures. In this paper, we reported the synthesis of SnO2 hierarchical architectures by a simple hydrothermal route without the assistant of any templates and surfactants at mild temperature. The effects of growth parameters on morphologies were investigated. A possible growth mechanism of SnO2 hierarchical structures was proposed. The photocatalytic results indicate the assynthesized products may have potential applications in water contaminant treatment. In a typical synthesis, 6 mmol of NH4 F was dissolved in 50 mL of de-ionized water, followed by the addition of 2 mmol of SnSO4 , The solution was then transferred into an 100 mL Teflon-lined stainless steel autoclave, and kept at 180℃ for 16 h. After the hydrothermal procedure, the autoclave was cooled naturally down to room temperature. The yellow-green precipitates were collected by centrifugation, then washed several times with distilled water and absolute ethanol, respectively, Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, P. R. China *Corresponding author. E-mail: Nano-Micro Lett. 5(4), 234-241 (2013)/ http://dx.doi.org/10.5101/nml.v5i4.p234-241 Nano-Micro Lett. 5(4), 234-241 (2013)/ http://dx.doi.org/10.5101/nml.v5i4.p234-241 dard PDF card (No. 41-1445). No obvious diffraction peaks from other impurities are detected, indicating high purity of the as-synthesized product. To further study the crystallization of the obtained product, room temperature photoluminescence (PL) property of the obtained SnO2 hierarchical structures was measured as well (Fig. 1(b)). Only a strong yellow emission band at ∼564 nm was observed. It is known that the energy gap of bulk SnO2 is 3.6 eV. The intrinsic emission peak (∼360 nm) of the SnO2 nanosheets was not found. The strong luminescence emission band from the synthesized products might be related to crystal defects which were produced during the growth [34-40]. During the SnO2 nanosheets growth, a high density of oxygen vacancies, which may mainly locate on the surface of the nanosheets, interact with interfacial tin vacancies, and lead to formation of a considerable amount of trapped states within the bandgap. The results are consistent with previous reports [41]. The morphologies and microstructures of the products were characterized by SEM. Plentiful of flower-like structures assembled by sheet-like subunits with an overall diameter of ∼1 μm could be observed in Fig. 1(c). With a closer examination (Fig. 1(d)), the relatively rigid nanosheets constituents possess a very smooth surface and a thickness of only tens of nanometers. (b) 40 Intensity (a.u.) 310 112 301 002 50 60 2Theta (degree) 321 30 202 20 220 200 Intensity (a.u.) 211 110 (a) 101 and dried in air at 60℃ for 12 h. Finally, the products were annealed in a muffle kiln at 600℃ for 2 h. The crystalline structure of the as-obtained products was characterized using X-ray powder diffraction (XRD, Rigaku Dmax-rB, CuKα radiation, λ = 0.1542 nm, 40 kV, 100 mA). The morphology and microstructure of the samples were characterized by scanning electron microscope (SEM, Hitachi-4800). The photocatalytic experiments of the as-prepared products were conducted as follows: 0.1 g of SnO2 microflowers were suspended in 200 mL methylene blue (MB) aqueous solution (40 mg/L). The solution was continuously stirred for 60 min in the dark to ensure the establishment of an adsorption-desorption equilibrium among the products and MB. After thatthe solution was exposed to UV irradiation from a 500 W Hg lamp at room temperature. The samples were collected at regular time interval to measure the organic dyes degradation by UV-Vis spectra. Subsequently, the experiments of the photocatalytic degradation of eosin red aqueous solution and Congo red (CR) aqueous solution also were conducted in the same conditions. Figure 1(a) shows XRD pattern of the as-synthesized products. All the diffraction peaks can be indexed to the rutile tetragonal SnO2 in accordance with the stan- 70 80 400 (c) 450 500 550 600 Wavelength (nm) 650 700 (d) 500 nm 500 nm Fig. 1 (a) XRD pattern of the as-synthesized SnO2 microflowers; (b) Photoluminescence spectrum of the as-obtained SnO2 product; (c)-(d) SEM images of the as-synthesized SnO2 architectures at different magnification. 235 Nano-Micro Lett. 5(4), 234-241 (2013)/ http://dx.doi.org/10.5101/nml.v5i4.p234 (...truncated)