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Interfacial hydrothermal synthesis of SnO2 nanorods towards photocatalytic degradation of methyl orange
Materials Research Bulletin 60 (2014) 1–4
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Interfacial hydrothermal synthesis of SnO2 nanorods towards photocatalytic degradation of methyl orange L.R. Hou *, L. Lian, L. Zhou, L.H. Zhang, C.Z. Yuan * School of Materials Science & Engineering, Anhui University of Technology, Ma'anshan 243002, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 May 2014 Received in revised form 1 August 2014 Accepted 11 August 2014 Available online 13 August 2014
One-dimensional (1D) SnO2 nanorods (NRs) have been successfully synthesized by means of an efficient interfacial hydrothermal strategy. The resulting product was physically characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscope, etc. The as-fabricated SnO2 NRs exhibited excellent photocatalytic degradation of the methyl orange with high degradation efficiency of 99.3% with only 60 min ultra violet light irradiation. Meanwhile, the 1D SnO2 NRs exhibited intriguing photostability after four recycles. ã 2014 Published by Elsevier Ltd.
Keywords: SnO2 nanorods Semiconductors Photocatalyst Methyl orange
1. Introduction
2. Experimental
Tin dioxide (SnO2), as a wide band gap (3.6 eV) semiconductor, has received increasing attention owing to its potential applications, such as gas-sensor devices, Li-ion batteries, solar cells, etc. [1–3]. Up to now, various synthetic strategies, including sol–gel route [4], microwave-assisted synthesis [3], and hydrothermal method [5], have been well applied to synthesize SnO2 with different morphologies and novel properties. Particularly, the hydrothermal method is widely developed to construct nanostructured materials due to its simplicity, high efficiency, and low cost [5]. Additionally, compared to the conventional homogeneous synthesis, organic/inorganic soft interface has demonstrated unusual merits in the fine synthesis of nanosized materials [6]. Thus, it is highly anticipated that the materials synthesized via the interfacial hydrothermal strategy would be endowed with several collective advantages [7,8]. Herein, we successfully fabricated one-dimensional (1D) SnO2 nanorods (NRs) by the means of a simple yet efficient interfacial hydrothermal strategy, and further utilized it as an advanced photocatalyst for degradation of the dye methyl orange (MO) in water under the ultra violet (UV) light irradiation. The as-prepared SnO2 NRs demonstrated excellent photocatalytic degradation efficiency and striking stability.
All the solvents and chemicals were of analytical purity and used without further purification. SnO2 NRs were typically prepared as follows. In specific, 0.393 g of stannous octanoate (Sn(Oct)2) was dissolved into a mixed solution of 1.787 g of oleic acid (OA) and 20 mL of octadecene (ODE) to obtain solution A. 0.2 mL of HCl (37%) was added into glycerin (20 mL) to form solution B, which was put in a Teflon-lined autoclave. Then, solution A was added upon solution B to obtain the two-phase ODE/glycerin interface. Finally, the Teflon-lined stainless steel autoclave containing the two-phase solution was treated and maintained at 120 C for 6 h. After the autoclave was cooled to room temperature naturally, the octadecene phase was collected, subsequently precipitated by the addition of ethanol, and isolated by centrifugation. The precipitates were finally dried in air. As the above synthetic process, SnO2 nanoparticles (NPs) were prepared in the absence of OA. For comparison, TiO2 was also prepared by the similar method just with the exception that a mixture of 0.3 mL of HCl, 36 mL of H2O and 4 mL of Ti(OC3H7)4 was poured into the autoclave instead. The crystalline phase and morphologies of the samples were determined by X-ray diffractometer (XRD, Max 18XCE, Japan) using a Cu-Ka source, scanning electron microscopy (SEM, LEO 1403VP, Germany) and transmission electron microscope (TEM, JEOL JEM-2100), respectively. UV–vis diffuse reflectance spectrum was performed to determine the band gap energy of the photocatalyst in the wavelength range of 200–600 nm, using a
* Corresponding authors. Tel.: +86 555 2311570; fax: +86 555 2311570. E-mail addresses: [email protected] (L.R. Hou), [email protected] (C.Z. Yuan). http://dx.doi.org/10.1016/j.materresbull.2014.08.006 0025-5408/ ã 2014 Published by Elsevier Ltd.
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Hitachi U-3010 spectrophotometer, where the pure powdered Al2O3 was used as a reference sample. Fourier transform infrared (FT-IR) were recorded on a Nicolet 6700 FT-IR spectrometer. The Brunauer–Emmett–Teller (BET) surface was estimated using the adsorption data by a Micromeritics Tristar 3000 analyzer. Photocatalytic activities of the samples were evaluated by photocatalytic degradation of the MO under UV light irradiation [9]. Typically, 0.1 g of as-synthesized photocatalyst was dispersed in 100 mL of MO aqueous solution with an initial concentration of 1 10 5 M in a quartz reactor. Prior to illumination, the solution was stirred for 30 min in dark to allow the system to reach an adsorption/desorption equilibrium. Afterwards, the solution was irradiated by two 40 W ultraviolet lamps. 5 mL of the solution was drawn at given intervals, and then centrifuged to remove the particles. The optical absorption spectrum for the supernatant solution was recorded by using a double-beam spectrophotometer (UV-2450, Japan and absorption at 464 nm for the MO). 3. Results and discussion The microstructures and morphologies of the SnO2 NRs are investigated, and the results are shown in Fig. 1. Fig. 1a presents the XRD pattern of the as-prepared SnO2 sample. The diffraction features appearing at 2u = 26.60, 33.80, 37.90, 51.90, 54.70 and 64.70 correspond to the (11 0), (1 0 1), (2 0 0), (2 11), (2 2 0) and (11 2) planes of tetragonal SnO2 (space group P42/mnm (1 3 6), a = b = 4.738 Å, c = 3.187 Å; JCPDS card no. 41-1445), respectively. The broadening diffraction peaks indicate the small crystalline size of the as-obtained SnO2 product. Moreover, the OA is observed upon the surface of resultant SnO2 NRs, as shown in FT-IR spectrum (Fig. 1b). In specific, the bands at 2956 and 2861 cm 1 are assigned to the antisymmetric (nasCH2) and symmetric (nas(CH)2) metheylene stretches of the OA molecule. The band at 3167 cm 1 is attributed to the nQC H stretch. For pure ligand, the nCQO stretch appears at 1700 cm 1, while this band is absent in the spectrum (Fig. 1b) of the as-prepared SnO2. The band at
1610 cm 1 is ascribed to the nCQO stretch. Likewise the bands at 1539 and 1405 cm 1 can be assigned to the vibrations of nCOO and nC O, respectively, which strongly confirms the existence of the OA molecule upon the surface of SnO2 NRs. Fig. 1c demonstrates the SEM image of the resultant SnO2 NRs. As evident, the as-synthesized product consists of large-scale NR feature with the diameter of 50–300 nm and length of 1–2 mm. The microstructure of the NRs is further investigated by TEM. Obviously, the 1D NRs with the diameter of 90 nm are observed, as shown in Fig. 1d. Interestingly, just some irregular SnO2 nanoparticles (NPs) were synthesized without the OA during the synthetic procedure (Fig. S1), which indicates that OA plays an important role during the formation process of SnO2 NRs. The electronic structure of a semiconductor affects its optical absorption property greatly, and the optical absorption property is commonly established as the key factor to reflect its photocatalytic property. Fig. 2a demonstrates the UV–vis diffuse reflectance spectrum of the as-prepared SnO2 sample with the steep shape, which reveals that the light absorption is ascribed to the band-gap transition. The sample demonstrates strong light absorption property in the UV region. Generally, the wavelength of the absorption edge is considered as the intersection which is determined by extrapolating the horizontal and sharply rising portion of the curves [9–11]. Thus, the wavelength for the absorption edge of SnO2 is determined as 378 nm on the basis of its UV–vis reflectance spectrum. Given that the wavelength of the absorption edge (lg, nm) of a semiconductor, its band gap (Eg, eV) can be commonly calculated according to the equation of Eg = hc/lg = 1240/lg, in which h,c and lg are the Planck constant, speed of light and wavelength of the absorption edge, respectively. Therefore, the band gap (Eg) of the SnO2 was estimated to be 3.28 eV, suggesting the as-prepared SnO2 has a suitable band gap for photocatalytic degradation of the organic contaminants under UV light irradiation. The photocatalytic activity of the as-prepared sample is evaluated by using the MO as target contaminant under different
Fig. 1. (a) XRD pattern, (b) FT-IR spectrum, (c) SEM and (d) TEM images of the as-prepared SnO2 sample.
L.R. Hou et al. / Materials Research Bulletin 60 (2014) 1–4
3
Fig. 2. (a) UV–vis diffuse reflectance spectrum of the SnO2 NRs, and (b) photocatalytic degradation efficiencies of the MO under different processes.
Fig. 3. UV–vis spectral changes of MO aqueous in the presence of SnO2 NRs as a function of irradiation time.
experimental conditions (Fig. 2b). The photodegradation efficiency is lower than 4% even after 60 min, which demonstrates that the photolysis of MO is extremely slow without a photocatalyst under UV light irradiation. The degradation efficiency of the MO on SnO2 NRs under the dark condition is slightly higher than that of the direct photolysis test, suggesting the limited adsorption of MO on SnO2 NRs. However, the photocatalytic degradation efficiency of the MO reaches 99.3% only after 60 min UV light irradiation in the presence of SnO2 NRs, and its photocatalytic activity can match
that of the conventional TiO2 (95%). In addition, the Brunauer–Emmett–Teller (BET) surface area of the as-prepared SnO2 NRs and TiO2 were also estimated by using N2 adsorption data. The BET surface area of SnO2 NRs and TiO2 were estimated to be ca. 3.58 and 43 m2/g, respectively. Obviously, the higher degradation efficiency of MO over TiO2 before 15 min should be attributed to its higher BET surface area, which further confirms the higher photocatalytic activity of SnO2 NRs themselves. Simultaneously, SnO2 NRs also exhibit much higher photocatalytic degradation efficiency than that of SnO2 NPs (73%), which should be ascribed to the great role of the OA. The OA not only changes the formation process of SnO2 NRs, but also renders their hydrophobic surface, thus improves their adsorption capacity and even photocatalytic activity toward the dye MO [12]. Additionally, the 1D rod-like structure may be in favor of the photogenerated electrons and holes to be transferred separately to the surface of the semiconductor, which is beneficial to its higher photocatalytic activity [13–15]. To examine the degradation process of MO, the absorption spectral changes of MO aqueous solution in the presence of SnO2 NRs were also investigated as the function of irradiation time. Fig. 3 shows the temporal evolution of the absorption changes of MO. It can be observed that the concentration of MO decreases gradually as the function of irradiation time in the presence of SnO2 NRs under the UV light irradiation, and the peak (l = 464 nm) nearly disappears after 60 min, further confirming that MO was truly photocatalytic degraded [16–18]. The photostability of the SnO2 photocatalyst is furthermore investigated. The circulating runs for the photocatalytic degradation of MO in the presence of SnO2 NRs under the UV light irradiation are checked (Fig. 4). After four recycles for the photocatalytic degradation of the MO, the SnO2 NRs almost exhibit no any obvious loss of photocatalytic activity, which reveals that the resultant SnO2 NRs possess excellent photostability, and do not photocorrode during the photocatalytic degradation of the target MO pollutant, thus the SnO2 NRs demonstrate the significant potential application in waste water treatment as an high-performance photocatalyst. 4. Conclusions
Fig. 4. Cycling runs in the photocatalytic degradation of the MO in the presence of SnO2 under UV light irradiation.
In conclusion, a simple but efficient interfacial hydrothermal method had been developed first for the synthesis of SnO2 NRs with strong optical absorption property in the UV region. The photocatalytic degradation rate of SnO2 NRs was up to 99.3% with 60 min UV light irradiation. More significantly, the effective, fast, convenient and environmentally friendly interfacial hydrothermal synthetic strategy is further expected to extend to other catalysts with desirable photocatalytic performance.
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Acknowledgement The authors gratefully acknowledge the financial support by the Natural Science Foundation of Anhui Province (KJ2013A051). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.08.006. References [1] W. Zeng, Q.Y. He, K.G. Pan, Y. Wang, Synthesis of multifarious hierarchical flower-like SnO2 and their gas-sensing properties, Phys. E 54 (2013) 313–318. [2] J. Liu, J.M. Huang, L.L. Hao, H.D. Liu, X.L. Li, SnO2 nano-spheres/graphene hybrid for high-performance lithium ion battery anodes, Ceram. Int. 39 (2013) 8623–8627. [3] A. Birkel, Y.G. Lee, D. Koll, X.V. Meerbeek, S. Frank, M.J. Choi, Y.S. Kang, K. Char, W. Tremel, Highly efficient and stable dye-sensitized solar cells based on SnO2 nanocrystals prepared by microwave-assisted synthesis, Energy Environ. Sci. 5 (2012) 5392–5400. [4] F. Gu, S.F. Wang, M.K. Lü, G.J. Zhou, D. Xu, D.R. Yuan, Photoluminescence properties of SnO2 nanoparticles synthesized by sol–gel method, J. Phys. Chem. B 108 (2004) 8119–8123. [5] Y. Liang, B. Fang, Hydrothermal synthesis of SnO2 nanorods: morphology dependence, growth mechanism and surface properties, Mater. Res. Bull. 48 (2013) 4118–4124. [6] L.R. Hou, Q. Zhang, L.T. Ling, C.X. Li, L. Chen, S. Chen, Interfacial fabrication of single-crystalline ZnTe nanorods with high blue fluorescence, J. Am. Chem. Soc. 135 (2013) 10618–10621. [7] Z.H. Ai, L.Z. Zhang, S.C. Lee, W.K. Ho, Interfacial hydrothermal synthesis of Cu@Cu2O core-shell microspheres with enhanced visible-light-driven photocatalytic activity, J. Phys. Chem. C 113 (2009) 20896–20902.
[8] C.Z. Yuan, L.R. Hou, L.F. Shen, X.G. Zhang, Facilewater/ionic liquid/organic triphase interfacial synthesis of coral-like polyaniline toward high-performance electrochemical capacitors, J. Electrochem. Soc. 159 (2012) A1323–A1328. [9] L.R. Hou, C.Z. Yuan, Y. Peng, Synthesis and photocatalytic property of SnO2/TiO2 nanotubes composites, J. Hazard. Mater. 139 (2007) 310–315. [10] M.L. Guan, D.K. Ma, S.W. Hu, Y.J. Chen, S.M. Huang, From hollow olive-shaped BiVO4 to n-p core-shell BiVO4@Bi2O3 microspheres: controlled synthesis and enhanced visible-light-responsive photocatalytic properties, Inorg. Chem. 50 (2011) 800–805. [11] S.F. Chen, W.M. Tang, Y.F. Hu, X.L. Fu, The preparation and characterization of composite bismuth tungsten oxide with enhanced visible light photocatalytic activity, CrstEngComm 15 (2013) 7943–7950. [12] X.J. Ding, T.C. An, G.Y. Li, S.Q. Zhang, J.X. Chen, J.M. Yuan, H.J. Zhao, H. Chen, G.Y. Sheng, J.M. Fu, Preparation and characterization of hydrophobic TiO2 pillared clay: the effect of acid hydrolysis catalyst and doped Pt amount, J. Colloid Interface Sci. 320 (2008) 501–507. [13] J. Ren, W.Z. Wang, S.M. Sun, L. Zhang, J. Chang, Enhanced photocatalytic activity of Bi2WO6 loaded with Ag nanoparticles under visible light irradiation, Appl. Catal. B 92 (2009) 50–55. [14] S.S. Yao, J.Y. Wei, B.B. Huang, S.Y. Feng, X.Y. Zhang, X.Y. Qin, P. Wang, Z.Y. Wang, Q. Zhang, X.Y. Jing, J. Zhan, Morphology modulated growth of bismuth tungsten oxide nanocrystals, J. Solid State Chem. 182 (2009) 236–239. [15] Y.Q. Chen, G.R. Li, Y.K. Qu, Y.H. Zhang, K.H. He, Q. Gao, X.H. Bu, Water-insoluble heterometal-oxide-based photocatalysts effective for the photo-decomposition of methyl orange, Cryst. Growth Des. 13 (2013) 901–907. [16] X.B. Ren, M. Chen, D.J. Qian, Pd(II)-mediated triad multilayers with zinc tetrapyridylporphyrin and pydidine-functionalized nano-TiO2 as linkers: assembly, characterization, and photocatalytic properties, Langmuir 28 (2012) 7711–7719. [17] M. Sun, D.Z. Li, W.J. Li, Y.B. Chen, Z.X. Chen, Y.H. He, X.Z. Fu, New photocatalyst, Sb2S3, for degradation of methyl orange under visible-light irradiation, J. Phys. Chem. C 112 (2008) 18076–18081. [18] Y. Yang, H.L. Zhang, S.M. Lee, D.S. Kim, W.B. Hwang, Z.L. Wang, Hybrid energy cell for degradation of methyl orange by self-powered electrocatalytic oxidation, Nano Lett. 13 (2013) 803–808.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Interfacial hydrothermal synthesis of SnO2 nanorods towards photocatalytic degradation of methyl orange L.R. Hou *, L. Lian, L. Zhou, L.H. Zhang, C.Z. Yuan * School of Materials Science & Engineering, Anhui University of Technology, Ma'anshan 243002, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 May 2014 Received in revised form 1 August 2014 Accepted 11 August 2014 Available online 13 August 2014
One-dimensional (1D) SnO2 nanorods (NRs) have been successfully synthesized by means of an efficient interfacial hydrothermal strategy. The resulting product was physically characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscope, etc. The as-fabricated SnO2 NRs exhibited excellent photocatalytic degradation of the methyl orange with high degradation efficiency of 99.3% with only 60 min ultra violet light irradiation. Meanwhile, the 1D SnO2 NRs exhibited intriguing photostability after four recycles. ã 2014 Published by Elsevier Ltd.
Keywords: SnO2 nanorods Semiconductors Photocatalyst Methyl orange
1. Introduction
2. Experimental
Tin dioxide (SnO2), as a wide band gap (3.6 eV) semiconductor, has received increasing attention owing to its potential applications, such as gas-sensor devices, Li-ion batteries, solar cells, etc. [1–3]. Up to now, various synthetic strategies, including sol–gel route [4], microwave-assisted synthesis [3], and hydrothermal method [5], have been well applied to synthesize SnO2 with different morphologies and novel properties. Particularly, the hydrothermal method is widely developed to construct nanostructured materials due to its simplicity, high efficiency, and low cost [5]. Additionally, compared to the conventional homogeneous synthesis, organic/inorganic soft interface has demonstrated unusual merits in the fine synthesis of nanosized materials [6]. Thus, it is highly anticipated that the materials synthesized via the interfacial hydrothermal strategy would be endowed with several collective advantages [7,8]. Herein, we successfully fabricated one-dimensional (1D) SnO2 nanorods (NRs) by the means of a simple yet efficient interfacial hydrothermal strategy, and further utilized it as an advanced photocatalyst for degradation of the dye methyl orange (MO) in water under the ultra violet (UV) light irradiation. The as-prepared SnO2 NRs demonstrated excellent photocatalytic degradation efficiency and striking stability.
All the solvents and chemicals were of analytical purity and used without further purification. SnO2 NRs were typically prepared as follows. In specific, 0.393 g of stannous octanoate (Sn(Oct)2) was dissolved into a mixed solution of 1.787 g of oleic acid (OA) and 20 mL of octadecene (ODE) to obtain solution A. 0.2 mL of HCl (37%) was added into glycerin (20 mL) to form solution B, which was put in a Teflon-lined autoclave. Then, solution A was added upon solution B to obtain the two-phase ODE/glycerin interface. Finally, the Teflon-lined stainless steel autoclave containing the two-phase solution was treated and maintained at 120 C for 6 h. After the autoclave was cooled to room temperature naturally, the octadecene phase was collected, subsequently precipitated by the addition of ethanol, and isolated by centrifugation. The precipitates were finally dried in air. As the above synthetic process, SnO2 nanoparticles (NPs) were prepared in the absence of OA. For comparison, TiO2 was also prepared by the similar method just with the exception that a mixture of 0.3 mL of HCl, 36 mL of H2O and 4 mL of Ti(OC3H7)4 was poured into the autoclave instead. The crystalline phase and morphologies of the samples were determined by X-ray diffractometer (XRD, Max 18XCE, Japan) using a Cu-Ka source, scanning electron microscopy (SEM, LEO 1403VP, Germany) and transmission electron microscope (TEM, JEOL JEM-2100), respectively. UV–vis diffuse reflectance spectrum was performed to determine the band gap energy of the photocatalyst in the wavelength range of 200–600 nm, using a
* Corresponding authors. Tel.: +86 555 2311570; fax: +86 555 2311570. E-mail addresses: [email protected] (L.R. Hou), [email protected] (C.Z. Yuan). http://dx.doi.org/10.1016/j.materresbull.2014.08.006 0025-5408/ ã 2014 Published by Elsevier Ltd.
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Hitachi U-3010 spectrophotometer, where the pure powdered Al2O3 was used as a reference sample. Fourier transform infrared (FT-IR) were recorded on a Nicolet 6700 FT-IR spectrometer. The Brunauer–Emmett–Teller (BET) surface was estimated using the adsorption data by a Micromeritics Tristar 3000 analyzer. Photocatalytic activities of the samples were evaluated by photocatalytic degradation of the MO under UV light irradiation [9]. Typically, 0.1 g of as-synthesized photocatalyst was dispersed in 100 mL of MO aqueous solution with an initial concentration of 1 10 5 M in a quartz reactor. Prior to illumination, the solution was stirred for 30 min in dark to allow the system to reach an adsorption/desorption equilibrium. Afterwards, the solution was irradiated by two 40 W ultraviolet lamps. 5 mL of the solution was drawn at given intervals, and then centrifuged to remove the particles. The optical absorption spectrum for the supernatant solution was recorded by using a double-beam spectrophotometer (UV-2450, Japan and absorption at 464 nm for the MO). 3. Results and discussion The microstructures and morphologies of the SnO2 NRs are investigated, and the results are shown in Fig. 1. Fig. 1a presents the XRD pattern of the as-prepared SnO2 sample. The diffraction features appearing at 2u = 26.60, 33.80, 37.90, 51.90, 54.70 and 64.70 correspond to the (11 0), (1 0 1), (2 0 0), (2 11), (2 2 0) and (11 2) planes of tetragonal SnO2 (space group P42/mnm (1 3 6), a = b = 4.738 Å, c = 3.187 Å; JCPDS card no. 41-1445), respectively. The broadening diffraction peaks indicate the small crystalline size of the as-obtained SnO2 product. Moreover, the OA is observed upon the surface of resultant SnO2 NRs, as shown in FT-IR spectrum (Fig. 1b). In specific, the bands at 2956 and 2861 cm 1 are assigned to the antisymmetric (nasCH2) and symmetric (nas(CH)2) metheylene stretches of the OA molecule. The band at 3167 cm 1 is attributed to the nQC H stretch. For pure ligand, the nCQO stretch appears at 1700 cm 1, while this band is absent in the spectrum (Fig. 1b) of the as-prepared SnO2. The band at
1610 cm 1 is ascribed to the nCQO stretch. Likewise the bands at 1539 and 1405 cm 1 can be assigned to the vibrations of nCOO and nC O, respectively, which strongly confirms the existence of the OA molecule upon the surface of SnO2 NRs. Fig. 1c demonstrates the SEM image of the resultant SnO2 NRs. As evident, the as-synthesized product consists of large-scale NR feature with the diameter of 50–300 nm and length of 1–2 mm. The microstructure of the NRs is further investigated by TEM. Obviously, the 1D NRs with the diameter of 90 nm are observed, as shown in Fig. 1d. Interestingly, just some irregular SnO2 nanoparticles (NPs) were synthesized without the OA during the synthetic procedure (Fig. S1), which indicates that OA plays an important role during the formation process of SnO2 NRs. The electronic structure of a semiconductor affects its optical absorption property greatly, and the optical absorption property is commonly established as the key factor to reflect its photocatalytic property. Fig. 2a demonstrates the UV–vis diffuse reflectance spectrum of the as-prepared SnO2 sample with the steep shape, which reveals that the light absorption is ascribed to the band-gap transition. The sample demonstrates strong light absorption property in the UV region. Generally, the wavelength of the absorption edge is considered as the intersection which is determined by extrapolating the horizontal and sharply rising portion of the curves [9–11]. Thus, the wavelength for the absorption edge of SnO2 is determined as 378 nm on the basis of its UV–vis reflectance spectrum. Given that the wavelength of the absorption edge (lg, nm) of a semiconductor, its band gap (Eg, eV) can be commonly calculated according to the equation of Eg = hc/lg = 1240/lg, in which h,c and lg are the Planck constant, speed of light and wavelength of the absorption edge, respectively. Therefore, the band gap (Eg) of the SnO2 was estimated to be 3.28 eV, suggesting the as-prepared SnO2 has a suitable band gap for photocatalytic degradation of the organic contaminants under UV light irradiation. The photocatalytic activity of the as-prepared sample is evaluated by using the MO as target contaminant under different
Fig. 1. (a) XRD pattern, (b) FT-IR spectrum, (c) SEM and (d) TEM images of the as-prepared SnO2 sample.
L.R. Hou et al. / Materials Research Bulletin 60 (2014) 1–4
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Fig. 2. (a) UV–vis diffuse reflectance spectrum of the SnO2 NRs, and (b) photocatalytic degradation efficiencies of the MO under different processes.
Fig. 3. UV–vis spectral changes of MO aqueous in the presence of SnO2 NRs as a function of irradiation time.
experimental conditions (Fig. 2b). The photodegradation efficiency is lower than 4% even after 60 min, which demonstrates that the photolysis of MO is extremely slow without a photocatalyst under UV light irradiation. The degradation efficiency of the MO on SnO2 NRs under the dark condition is slightly higher than that of the direct photolysis test, suggesting the limited adsorption of MO on SnO2 NRs. However, the photocatalytic degradation efficiency of the MO reaches 99.3% only after 60 min UV light irradiation in the presence of SnO2 NRs, and its photocatalytic activity can match
that of the conventional TiO2 (95%). In addition, the Brunauer–Emmett–Teller (BET) surface area of the as-prepared SnO2 NRs and TiO2 were also estimated by using N2 adsorption data. The BET surface area of SnO2 NRs and TiO2 were estimated to be ca. 3.58 and 43 m2/g, respectively. Obviously, the higher degradation efficiency of MO over TiO2 before 15 min should be attributed to its higher BET surface area, which further confirms the higher photocatalytic activity of SnO2 NRs themselves. Simultaneously, SnO2 NRs also exhibit much higher photocatalytic degradation efficiency than that of SnO2 NPs (73%), which should be ascribed to the great role of the OA. The OA not only changes the formation process of SnO2 NRs, but also renders their hydrophobic surface, thus improves their adsorption capacity and even photocatalytic activity toward the dye MO [12]. Additionally, the 1D rod-like structure may be in favor of the photogenerated electrons and holes to be transferred separately to the surface of the semiconductor, which is beneficial to its higher photocatalytic activity [13–15]. To examine the degradation process of MO, the absorption spectral changes of MO aqueous solution in the presence of SnO2 NRs were also investigated as the function of irradiation time. Fig. 3 shows the temporal evolution of the absorption changes of MO. It can be observed that the concentration of MO decreases gradually as the function of irradiation time in the presence of SnO2 NRs under the UV light irradiation, and the peak (l = 464 nm) nearly disappears after 60 min, further confirming that MO was truly photocatalytic degraded [16–18]. The photostability of the SnO2 photocatalyst is furthermore investigated. The circulating runs for the photocatalytic degradation of MO in the presence of SnO2 NRs under the UV light irradiation are checked (Fig. 4). After four recycles for the photocatalytic degradation of the MO, the SnO2 NRs almost exhibit no any obvious loss of photocatalytic activity, which reveals that the resultant SnO2 NRs possess excellent photostability, and do not photocorrode during the photocatalytic degradation of the target MO pollutant, thus the SnO2 NRs demonstrate the significant potential application in waste water treatment as an high-performance photocatalyst. 4. Conclusions
Fig. 4. Cycling runs in the photocatalytic degradation of the MO in the presence of SnO2 under UV light irradiation.
In conclusion, a simple but efficient interfacial hydrothermal method had been developed first for the synthesis of SnO2 NRs with strong optical absorption property in the UV region. The photocatalytic degradation rate of SnO2 NRs was up to 99.3% with 60 min UV light irradiation. More significantly, the effective, fast, convenient and environmentally friendly interfacial hydrothermal synthetic strategy is further expected to extend to other catalysts with desirable photocatalytic performance.
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Acknowledgement The authors gratefully acknowledge the financial support by the Natural Science Foundation of Anhui Province (KJ2013A051). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.08.006. References [1] W. Zeng, Q.Y. He, K.G. Pan, Y. Wang, Synthesis of multifarious hierarchical flower-like SnO2 and their gas-sensing properties, Phys. E 54 (2013) 313–318. [2] J. Liu, J.M. Huang, L.L. Hao, H.D. Liu, X.L. Li, SnO2 nano-spheres/graphene hybrid for high-performance lithium ion battery anodes, Ceram. Int. 39 (2013) 8623–8627. [3] A. Birkel, Y.G. Lee, D. Koll, X.V. Meerbeek, S. Frank, M.J. Choi, Y.S. Kang, K. Char, W. Tremel, Highly efficient and stable dye-sensitized solar cells based on SnO2 nanocrystals prepared by microwave-assisted synthesis, Energy Environ. Sci. 5 (2012) 5392–5400. [4] F. Gu, S.F. Wang, M.K. Lü, G.J. Zhou, D. Xu, D.R. Yuan, Photoluminescence properties of SnO2 nanoparticles synthesized by sol–gel method, J. Phys. Chem. B 108 (2004) 8119–8123. [5] Y. Liang, B. Fang, Hydrothermal synthesis of SnO2 nanorods: morphology dependence, growth mechanism and surface properties, Mater. Res. Bull. 48 (2013) 4118–4124. [6] L.R. Hou, Q. Zhang, L.T. Ling, C.X. Li, L. Chen, S. Chen, Interfacial fabrication of single-crystalline ZnTe nanorods with high blue fluorescence, J. Am. Chem. Soc. 135 (2013) 10618–10621. [7] Z.H. Ai, L.Z. Zhang, S.C. Lee, W.K. Ho, Interfacial hydrothermal synthesis of Cu@Cu2O core-shell microspheres with enhanced visible-light-driven photocatalytic activity, J. Phys. Chem. C 113 (2009) 20896–20902.
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