One-pot synthesis of ZnO/oligoaniline nanocomposites with improved removal of organic dyes in water: Effect of adsorption on photocatalytic degradation
Graphical abstract
Introduction
Organic pollution is a global environmental problem and is harmful to ecosystem and human health. It is very necessary to remove organic pollutants from wastewater before its release into the natural environment [1]. Various methodologies, such as adsorption, biodegradation, chemical and electrochemical reactions, are proposed to eliminate these pollutants from water [2]. Semiconductor heterogeneous photocatalysis is the extensively studied technique for the degradation of many contaminants in the aqueous medium under UV/visible light [3], [4], [5]. Although semiconductor photocatalysts are handily available in the form of powder yet they suffer from several drawbacks [6]. Firstly, the suspended powders exhibit low light utilization efficiency. It has been reported that less than 1% of UV light or about 20% visible light actually passes through a depth of 0.5 m under the water surface [7]. Secondly, the required long setting time and efficient solid-liquid separation lead to both time and money consumption for their recovery [8]. Inconvenient post-treatment also brings loss of catalyst. Continuous efforts have been taken to overcome the above mentioned drawbacks. Immobilization of catalyst is one of the most effective methods. Various supports such as carbon materials [9], [10], [11], fabrics [12], [13], [14], natural porous material [15], [16], and polymers [17] have been applied for immobilizing photocatalysts. In fact, the introduction of a suitable support cannot only simplify the post-treatment, but also increase the specific surface area of catalyst, so as to improve the adsorption capacity and quantum efficiency.
Many studies have shown that pre-adsorption of organic pollutants onto adsorbent results in the enhancement of their photodegradation [18], [19], [20], [21], [22]. It is documented that heterogeneous photocatalysis occurs at the surface rather than in the solution bulk, and the adsorbent support concentrates a target pollutant of low solution-phase concentration on its surface, providing a substrate-rich microenvironment at the interface of catalyst/support. Therefore, the photocatalytic activities were enhanced. However, different views can be also seen in the literatures. H. Yoneyama and his coworkers [23] found that the highest efficiency was obtained at the supports of moderate adsorbability using TiO2-loaded adsorbent supports as photocatalysts. Vigneswaran et al. [24] claimed that pre-adsorption of organic pollutants on activated carbon followed by TiO2 photodegradation led to lower degradation rate compared to simultaneous adsorption and photocatalysis. P. Singh et al. [25] reported that the effect of adsorption on photocatalysis was complicated for the used ZnO-activated carbon and ZnO-brick grain particle catalysts. M. Ye and Z. Chen et al. [26] investigated the removal of ibuprofen using BiOBr microspheres and they observed that approximately 80% of ibuprofen removal rate contributed through BiOBr adsorption in photocatalytic process. From these views, the role of adsorption on photodegradation is still opening.
On the other hand, both zinc oxide (ZnO) and conductive polyaniline (PANI) have received much attention over a long time due to their respective advantageous properties and great potential applications. In recent years, the synthesis of hybrid of polymer/inorganic materials has the goal of obtaining a new composite material presenting synergistic or complementary behaviors between the polymer and inorganic material [17]. ZnO/PANI nanocomposite photocatalysts have been demonstrated having the mutual enhancement of their photocatalytic activity by sensitizing them with conducting polymer chains [27], [28]. The in-situ chemical oxidative polymerization of aniline in the presence of metal oxide nanostructures is a simple and commonly used method to obtain metal oxides anchored on PANI support. However, such an in-situ polymerization method requires the supply of nanoscale metal oxides. Not only that, chemical oxidative polymerization of aniline is usually performed under strong acid conditions, in which most metal oxides were dissolved or their nanostructures were destroyed [29]. As results, less composites or dilapidated nanostructures were received. Recent studies strongly suggested that the initial pH of the reaction system had a meaningful influence on the morphology of aniline polymers [30], [31], [32], [33]. Under alkaline or neutral conditions, aniline oligomers with various hierarchical microstructures can be fabricated. Though these oligoanilines are non-conductive, they have the advantages of low cost, simple preparation and good environmental stability, and can be applied in other fields that do not need conductivity of aniline polymers. For example, aniline oligomers have been demonstrated having excellent adsorptivity for organic dyes and heavy metal ions due to their affinitive structures and powerful coordination [34], [35], [36]. As far as we know, although many available supports were used in immobilized ZnO photocatalyst, aniline oligomers were rarely reported in literatures.
In present work, we reported a novel and facile method for fabrication of ZnO/oligoaniline nanocomposites using zinc salt and aniline monomer as raw materials at ambient condition. ZnO nanoparticles and hierarchical oligoaniline microstructures were simultaneously synthesized in one pot without previous preparation of ZnO. The adsorption and photocatalytic activity of the composite were tested for organic dye removal and the role of adsorption on photocatalysis was investigated. The present work provided an economically responsible strategy to prepare ZnO nanoparticles supported on oligoanilines and the composites exhibited a good performance for removal of organic dyes from water.
Section snippets
Materials and method
Aniline monomers were purchased from Shanghai Aladdin Industrial Corporation and distilled into colorless under reduced pressure prior to use. Methylene blue trihydrate (MB, 98.5%), Rhodamine B (RhB, 98%) and Congo red (CR, 99%) were purchased from Tianjin Fengchuan chemical reagent technologies Co., Ltd. Other chemicals were of analytical grade, purchased locally and used as received. The typical synthesis of ZnO/oligoaniline nanocomposites is described as follows: 2.2 g (10 mmol) Zn(CH3COO)2
Morphology and structure
Fig. 1 shows the morphological features of as-prepared products by SEM and TEM analysis. From the SEM images at different magnifications (Fig. 1a and b) it can be seen that regular microspheres with diameter of 1–3 μm and sheet-like structures with thickness of 50–100 nm were well dispersed. On the surface of these microspheres and nanosheets, irregular particles with sizes from tens to hundreds of nanometers can be observed, indicating hierarchical nanocomposites were successfully obtained by
Conclusion
In this study, a novel and facile method for fabrication of ZnO/oligoaniline nanocomposites using zinc salt and aniline monomer as raw materials at ambient condition was demonstrated. ZnO nanopaticles supported on hierarchical oligoaniline microstructures were synthesized in one pot without previous preparation of ZnO nanostructures. ZnO/oligoaniline exhibited excellent adsorption performance for cationic dyes. The incorporation of oligoaniline support has pronounced effect on ZnO
Acknowledgements
The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 51364040) and the Applied Basic Research Project of Yunnan Province, China (No. 2014FB109).
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