Significantly enhanced dye removal performance of hollow tin oxide nanoparticles via carbon coating in dark environment and study of its mechanism
© Yang et al.; licensee Springer. 2014
Received: 24 June 2014
Accepted: 20 August 2014
Published: 28 August 2014
Understanding the correlation between physicochemical properties and morphology of nanostructures is a prerequisite for widespread applications of nanomaterials in environmental application areas. Herein, we illustrated that the uniform-sized SnO2@C hollow nanoparticles were large-scale synthesized by a facile hydrothermal method. The size of the core-shell hollow nanoparticles was about 56 nm, and the shell was composed of a solid carbon layer with a thickness of 2 ~ 3 nm. The resulting products were characterized in terms of morphology, composition, and surface property by various analytical techniques. Moreover, the SnO2@C hollow nanoparticles are shown to be effective adsorbents for removing four different dyes from aqueous solutions, which is superior to the pure hollow SnO2 nanoparticles and commercial SnO2. The enhanced mechanism has also been discussed, which can be attributed to the high specific surface areas after carbon coating.
KeywordsTin oxide Carbon coating Core-shell Dye removal
With the development of society and scientific technology, more attentions have been paid to environmental issues which were caused by the discharge of wastewater. Oil spillage, organic solvents, and synthetic dyes discharged by the textile, paper, and tannery industries are primary pollutants of water sources . It is estimated that more than 100,000 commercially available dyes with over 7 × 105 tonnes of dyestuff are produced annually . Generally, synthetic dyes have complex aromatic structures that make them stable and difficult to biodegrade. As synthetic dyes in wastewater cannot be efficiently decolorized by traditional methods (such as catalytic degradation, cation exchange membranes, and biological processes), the adsorption of synthetic dyes on inexpensive and efficient solid supports was considered as a simple and economical method for their removal from water and wastewater.
The most commonly used absorbent for dye removal is activated carbon, because of its capability for efficiently adsorbing a broad range of different types of dyes . Up to now, there have been many successful methodologies for the fabrication of activated carbon materials, such as pinewood-based activated carbon , coir pith activated carbon , rice husk-based activated carbon , and bamboo-based activated carbon . Although, natural renewable resources have been widely used as raw materials for manufacturing activated carbon, the high production and treatment costs of activated carbon may still hinder its further application.
As a competitive alternative, various nanomaterials have been developed and used to remove the dyes. For example, Zhu and co-workers have prepared hierarchical NiO spheres with a high specific area of 222 m2/g as an adsorbent for removal of Congo red . Mou and co-workers have fabricated γ-Fe2O3 and Fe3O4 chestnut-like hierarchical nanostructures, which can be separated simply and rapidly from treated water by magnetic separation after As(V) adsorption treatment. And the As(V) removal capacity of as-obtained γ-Fe2O3 is maintained at 74% and reaches 101.4 mg/g . And then, they have prepared magnetic Fe2O3 chestnut-like amorphous-core/γ-phase-shell hierarchical nanostructures with a high specific area of 143.12 m2/g and with a maximum adsorption capacity of 137.5 mg/g for As(V) adsorption treatment . Liu and co-workers have prepared various bismuth oxyiodide hierarchical architectures, and their nanomaterials shown enhanced the photocatalytic performance and adsorption capabilities . Recently, the carbon functionalized nanomaterials have recently attracted considerable attention because of their enhanced dye removal performance. For instance, Fan and co-workers have synthesized hybridization of graphene sheets and carbon-coated Fe3O4 nanoparticles as an adsorbent of organic dyes . Li and co-workers have reported Mg(OH)2@reduced graphene oxide composite, which exhibited excellent adsorption behavior for methylene blue (MB) .
Indeed, the adsorption technique is especially attractive because of its simple design, high efficiency, and easy operation, but it requires materials with large specific surface area, well-defined pore size, and shape. Hollow structured materials fit these criteria well, and they have attracted tremendous interest as a special class of materials compared to other solid counterparts, owing to their higher specific surface area, lower density, and better permeation, which have been extensively considered as potential materials applied in adsorption, catalysis, chemical reactors, and various new application fields [14–16]. Therefore, design and fabrication of materials like carbon-coated hollow structure would increase the dye removal abilities.
Herein, a well-defined carbon-coated hollow SnO2 nanoparticles have been designed and fabricated by a facile two-step hydrothermal method without using any surfactants. Carbon coating prepared by hydrothermal treatment of low-cost glucose has aroused much interest. The preparation process belongs to green chemistry as the reaction process is safe and does not incur any contamination of the environment. More importantly, the carbon layer increases the specific area of bare hollow SnO2 nanoparticles, which exhibits an enhanced dye removal performance.
Potassium stannate trihydrate (K2SnO3 · 3H2O), commercial SnO2, rhodamine B (RhB), MB, rhodamine 6G (Rh6G), and methyl orange (MO) were purchased from Shanghai Jingchun Chemical Reagent Co., Ltd. (Shanghai, China). Urea (CO(NH2)2), ethylene glycol (EG), ethanol (C2H5OH), and glucose (C6H12O6) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the materials were used without further purification in the whole experimental process. Deionized water was used throughout the experiments.
Synthesis of hollow SnO2 nanoparticles
In a typical process, 0.6 g potassium stannate trihydrate was dissolved in 50 mL ethylene glycol through the ultrasonic method. Urea (0.4 g) was dissolved in 30 mL deionized water and then the solution was mixed together and transferred into a Teflon-lined stainless steel autoclave with a capacity of 100 mL for hydrothermal treatment at 170°C for 32 h. The autoclave solution was removed from the oven was allowed to cool down to room temperature. The product was harvested by centrifugation and washed with deionized water and ethanol and then dried at 80°C under vacuum.
Synthesis of hollow SnO2@C nanoparticles
SnO2@C hollow nanoparticles were prepared by a glucose hydrothermal process and subsequent carbonization approach. In a typical process, 0.4 g of as-prepared hollow SnO2 nanoparticles and 4 g glucose were re-dispersed in ethanol/H2O solution. After stirring, the solution was transferred into a 100-ml Teflon-lined stainless steel autoclave sealed and maintained at 170°C for 8 h. After the reaction was finished, the resulting black solid products were centrifuged and washed with deionized water and ethanol and dried at 80°C in air. Lastly, the black products were kept in a tube furnace at 600°C for 4 h under argon at a ramping rate of 5°C/min.
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed with a JEOL JEM-2100 F transmission electron microscope (Tokyo, Japan) at an accelerating voltage of 200 kV, and all the samples were dissolved in ethanol by ultrasonic treatment and dropped on copper grids. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a D/ruanx2550PC (Tokyo, Japan) using CuKα radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA. The absorption spectra of the samples were carried out on a Shimadzu UV-2550 spectrophotometer (Kyoto, Japan). Raman measurement was performed using a HORIBA Jobin Yvon LabRAM HR spectroscope (Edison, NJ, USA) using an excitation laser wavelength of 488 nm.
Of the samples, 5 mg was re-dispersed in 10 mL of the organic dyes (concentration 10 mg/L) and the mixed solution was stored in the dark for 45 min with gentle stirring. The reaction solution was sampled every 15-min intervals at room temperature; 2 mL solution was sampled and centrifuged to remove the adsorbents, and the corresponding UV-visible spectra were recorded to monitor the progress of the degradation of organic dyes by a Shimadzu 2550 UV-visible spectrophotometer.
Results and discussion
Figure 4e,f further confirms that the removal rate of RhB (10 mg/L) can reach to 94.6%. The results reveal that the as-prepared hollow SnO2@C nanoparticles exhibit excellent removal performance for RhB dyes. Meanwhile, the hollow SnO2@C nanoparticles also showed a good removal performance for MB and Rh6G (5 mg/L); the removal rate can reach to 99.9% and 92.3%, respectively. However, for the MO dyes (5 mg/L), the removal rate can only reach to 41.2%, because the chromophoric structure of MO dye is different from those of RhB and MB, and this will cause a different electrostatic interaction capacity between functional groups of carbon and dye molecules [18–20]. The above results illustrate that the as-obtained hollow SnO2@C nanoparticles exhibit a good dye removal performance.
In summary, hollow SnO2@C nanoparticles have been synthesized on a large scale through a facile hydrothermal method. The as-prepared hollow SnO2@C nanoparticles show excellent adsorption capacity toward RhB, MB, and Rh6G dyes in aqueous solutions. Compared with the naked hollow SnO2 and commercial SnO2 nanoparticles, the adsorption capacity showed about an 89% improvement for RhB organic dye. The porous carbonaceous shells coated on the surface of hollow SnO2 nanoparticles greatly enhanced the specific area, which provides more active sites for dye adsorption. Owing to their unique hollow structures, high surface areas and low cost, the as-obtained hollow SnO2@C nanoparticles are potentially applicable in wastewater treatment. Accordingly, it may be concluded that the developed SnO2@C is an efficient method for the decolorization of RhB, MB, and Rh6G dyes.
We greatly acknowledge financial support by the NSFC (51302325, 51201115, 51471121), New Century Excellent Talents in University (NCET-12-0553), Program for Shenghua Overseas Talent (1681–7607030005) from Central South University, and the partial financial support from the Open-End Fund for the Valuable and Precision Instruments of Central South University (Grant No. CSUZC2013025) and the Fundamental Research Funds for the Central Universities.
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