Three-dimensional Porous Networks of Ultra-long Electrospun SnO2 Nanotubes with High Photocatalytic Performance
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Recent progress in nanoscience and nanotechnology creates new opportunities in the design of novel SnO2 nanomaterials for photocatalysis and photoelectrochemical. Herein, we firstly highlight a facile method to prepare three-dimensional porous networks of ultra-long SnO2 nanotubes through the single capillary electrospinning technique. Compared with the traditional SnO2 nanofibers, the as-obtained three-dimensional porous networks show enhancement of photocurrent and photocatalytic activity, which could be ascribed to its improved light-harvesting efficiency and high separation efficiency of photogenerated electron–hole pairs. Besides, the synthesis route delivered three-dimensional sheets on the basis of interwoven nanofibrous networks, which can be readily recycled for the desirable circular application of a potent photocatalyst system.
KeywordsElectrospinning SnO2 nanotube Photocurrent Photocatalytic Recyclability
It has been widely demonstrated that semiconducting materials are of great promise for energy and environmental applications such as photocatalytic water splitting for hydrogen production, dye-sensitized solar cells, and photocatalytic remediation of harmful organics from air and water [1, 2, 3, 4]. As a typical n-type semiconductor, SnO2 has received great attention because of its excellent stability, nontoxicity, low-cost, and excellent optical electricity properties . Especially, the exploitations of SnO2 for photocatalytic water splitting and photocatalytic oxidation of organic wastes have been hot topics because of its high reduction potential and low oxidation potential [6, 7, 8].
However, the practical performance of bulk SnO2 is far from the ideal case which has been limited by several factors such as short lifetime of the excited-state carrier (10−12 s), poor oxygen evolution reaction kinetics, and short hole diffusion length (2–4 nm) [9, 10, 11, 12, 13, 14, 15]. As we all know, the separation rate of photoinduced surface and volume charge carriers in a photocatalyst can be significantly increased by reducing its size to the appropriate nanoscale level and, thus, the photocatalytic activity can be enhanced. Therefore, considerable effort has been taken to synthesize photocatalysts with a small size so as to achieve high activities. However, a new disadvantage arises naturally: the recycling of the photocatalysts hinders their applications due to their small size. This has led to an unfavorable balance between the reduced charge recombination and a negative impact on recycling. Therefore, it is highly challenging but desirable to develop a direct effective approach for fabricating a new type of nanostructured photocatalyst with efficient electron–hole utilization, high specific surface areas, and favorable recycling characteristics.
Notably, electrospinning technique provides an effective approach to fabricate the three-dimensional porous supports with large surface area. In particular, it has been demonstrated successfully by different groups including ours that the three-dimensional structure composed of one-dimensional (1D) nanofibers or nanotubes catalysts with large length-to-diameter ratio can allow them to be readily separated from fluid by sedimentation [16, 17, 18, 19, 20, 21]. Moreover, it is known that the structure and morphology also have a strong effect on the physical and chemical properties of photocatalysts, especially on the photocatalytic activities. Among various morphologies, hollow structures have attracted immense attention for their evidently improved performances over particles in photocatalysis and other applications. The hollow structures, especially those with tubular structures, have many useful features: (i) high surface-to-volume ratio enables it to adsorb a large amount of chemicals, (ii) hollow multi-channeled structures makes it convenient for mass transfer, (iii) the unique structure makes better use of light through multiple reflections within its hollow space.
Herein, we reported a successful attempt for the fabrication of the SnO2 nanotubes by the single capillary electrospinning technique. The investigation of photocatalytic ability indicated that the as-prepared composites exhibited high photocatalytic activity in the decomposition of Methyl Orange (MO). Also, free-standing sheets were readily delivered using the current processing routes, which enables circular exploitation of the materials as potent environmental catalysts.
2 Experimental Section
2.1 Fabrication of SnO2 Nanotubes and SnO2 Nanofibers
The morphologies of the as-prepared nanofibers were observed by scanning electron microscopy (FESEM, JSM-7500F) at an accelerating voltage of 20 kV, transmission electron microscopy (TEM; FEI Tecnai G2 F20) at an accelerating voltage of 300 kV. X-ray diffraction (XRD) was carried out with the 2θ range from 20 to 80° at a scan rate of 1° min−1 using a D/max 2500 XRD diffractometer (Rigaku) with Cu Kα radiation (0.1541 nm). The Brunauer-Emmett-Teller (BET)-specific surface area determination was performed by N2 gas adsorption using an America Micromeritics ASAP 2010 surface analytical instrument.
2.3 Photoelectrochemical Experiment
Photoelectrochemical measurements were performed using the conventional three electrode setup connected to an electrochemical station (CH Instrument 660C, Shanghai Chenhua, China). The setup had SnO2 NFs/FTO, and SnO2 NTs/FTO (effective area was 1 cm2, effective amount was 0.01 g) as working electrodes, and a Pt wire and an Ag/AgCl (saturated KCl) electrode were used as the counter electrode and reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution. A 50 W high-pressure mercury lamp with main emission wavelength of 313 nm was used as the visible light source. The photocurrent response spectroscopy was carried out at a constant potential of +0.6 V to the working photoanode.
2.4 Photocatalytic Test
3 Results and Discussion
3.1 SEM of the As-Prepared Composite Nanofibers
3.2 TEM of the As-Prepared Composite Nanofibers
3.3 XRD Patterns
3.4 Photocatalytic Activity
3.5 Postulated Photocatalytic Mechanism of the SnO2 NTs
It was well known that the photocatalytic activity was mainly governed by phase structure, adsorption ability, and separation efficiency of photogenerated electrons and holes. As could be seen from the XRD analysis, the crystal phase structure of the SnO2 NTs was nearly similar to that of SnO2 NFs. An adsorption experiment was performed to evaluate the adsorption ability of the SnO2 NFs and SnO2 NTs photocatalysts in the dark. As could be seen from Fig. 6, after equilibration in the dark for 30 min, 84 and 80 % of MO remained in the solution with SnO2 NFs and SnO2 NTs, respectively. Obviously, there were significant changes in the BET surface area (19.231 and 32.832 m2 g−1 for SnO2 NFs and SnO2 NTs). The enhancement of adsorption could be contributed to the increased surface area of the nanotubes. MO molecules could be adsorbed on the surface of SnO2 NTs until an adsorption–desorption equilibrium was reached. Compared with SnO2 NFs, the enhanced adsorptivity was a good supplement for the high photocatalytic activity of the SnO2 NTs photocatalyst.
As we know, the structure and morphology also have a strong effect on the efficiency of charge separation. Compared with SnO2 NFs, the unique nanotube structure makes better use of light through multiple reflections within its hollow space. Through enhancing the efficiency of light absorbance, the number of photoexcited charge carriers will be increased. What’s more, the underlying but probably more important advantages are the shorter bulk diffusion length produced by nanotubes with ultrathin thickness, and the hollow multi-channeled structure makes it convenient for mass transfers, which plays an important role in prolonging the lifetime of charge carriers and improve the quantum yield.
In summary, we describe herein an effective route to synthesize three-dimensional porous networks of ultra-long SnO2 nanotubes through the single capillary electrospinning technique. Compared with the traditional SnO2 nanofibers, the as-obtained three-dimensional porous networks show enhancement of photocurrent and photocatalytic activity, which could be ascribed to its improved light-harvesting efficiency and electron transport ability along the in-plane direction, and increased lifetime of photoexcited charge carriers. Besides, the synthesis route delivered three-dimensional sheets on the basis of interwoven nanofibrous networks, which can be readily recycled for the desirable circular application of a potent photocatalyst system. Notably, the free-standing 3D nanotubes network structure could improve photocatalyst’s performance of separation and reuse. Also, it is expected that the SnO2 NTs network will promote their industrial application as clean energy materials.
The present work is supported financially by the National Natural Science Foundation of China (Nos. 51001091, 111174256, 91233101) and the Fundamental Research Program from the Ministry of Science and Technology of China (No. 2014CB931704), and Project funded by China Postdoctoral Science Foundation(No. 2014M560602).
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