Regular ArticleFabrication of novel carbon quantum dots modified bismuth oxide (α-Bi2O3/C-dots): Material properties and catalytic applications
Graphical abstract
Introduction
Semiconductor photocatalysis is regarded as a potential and green technology for mitigating environmental pollution problems and eliminating residual water-soluble contaminants by harnessing the entire solar spectrum [1]. Many wide-band gap semiconductors that can be stimulated under UV light only, such as ZnO, TiO2, SnO2 and WO3 etc., have been studied extensively for the degradation of organic pollutants [2], [3], [4]. However, the major drawbacks associated with UV light active photocatalysts such as wide band gap, high recombination rate and inefficient charge separation, limit their applicability. Consequently, the exploration of visible light driven photocatalysts has become a necessity in order to meet promising industrial applications [5], [6]. Many varieties of bismuth-containing semiconductors have been exploited as efficient photocatalyst materials. For example, bismuth oxide (α-Bi2O3) has been used as a promising material for organic contaminants degradation owing to its low energy band gap, non-toxic nature, unique morphology, excellent optical properties and facile route of synthesis [7], [8]. However, limited modifications have been carried out aimed at improving its photocatalytic performance. For example, nanostructures such as nanosheets and hollow mesoporous spheres can enhance the light-harvesting capability and provide more active sites [9]. Moreover, other techniques such as ion doping and heterostructure assembly have proved to be beneficial in suppressing the charge carrier recombination in photocatalytic materials. In addition, up-conversion materials can also be considered to expand the optical spectrum response [10]. However, to the best of our knowledge and based on the information available in the literature, only a limited number of these techniques have been considered in practical applications in pollutant mitigations and novel nanostructures, fast charge separation and broad spectrum photocatalytic activity are highly needed in photocatalytic processes [11]. Consequently, recent research has been focused on the use of hybrid nanostructured semiconductors, that are stable, non-toxic and efficient solar/visible light driven photocatalyst as these materials display efficient charge separation at the interface and expand the spectral response to a wide range of wavelengths [12], [13], [14].
In recent times, carbon quantum dots (C-dots) have been employed in photocatalysis due to their excellent photophysical and chemical properties such as wide absorption spectrum range, low toxicity, electron reservoir, photo-induced electron transfer properties and up-converted photoluminescence [15], [16]. C-dots have the ability to enhance the photocatalytic activity of the conjugated parent photocatalyst to which it is attached to, by efficient charge separation, thereby suppressing the recombination of photogenerated charge carriers [17], [18]. C-dots act as good electron harvesters and converters, thus facilitating their energy band gap to a majority of photocatalysts [19]. A wide range of photocatalysts such as ZnO, TiO2, Fe2O3, BiFeO3, CuInZnS and BiOX (X = Cl, Br and I) have been known to show enhanced photocatalytic activity when attached to C-dots [20], [21], [22], [23], [24], [25]. A number of bismuth-based hybrid systems of C-dots have also been reported for photocatalytic degradation of organic pollutants. Di et al. demonstrated the synergistic effect of C-dots and Bi2MoO6 and reported the photocatalytic degradation of four different pollutants, ciprofloxacin, tetracycline hydrochloride, methylene blue and bisphenol A [26]. Tang et al. fabricated C-dots/m-BiVO4 through hydrothermal and sonochemical technique and employed the nanocomposite for the removal of methylene blue dye [27]. Chen et al. synthesized C-dots/BiOI via hydrothermal approach and studied its photocatalytic activity for the degradation of methylene orange dye. Under optimized conditions, the experiments revealed that C-dots/BiOI showed 2.5 times higher photocatalytic efficiency than pure BiOI [28]. Zhao et al. modified BiOBr with C-dots and reported that BiOBr/C-dots showed 5.3 times higher photocatalytic efficiency than pure BiOBr for the degradation of rhodamine B dye [29]. Additional work in this area is summarized in Table 1 to aid a clearer understanding of work carried out.
In this study, we present a facile sonochemical synthesis of a novel photocatalyst (α-Bi2O3/C-dots), without the use of any surfactant/additive or chelating agent and characterized the prepared photocatalyst in detail using spectroscopic and analytical techniques. To the best of our knowledge, this is the first study that reports the use of visible light responsive photocatalyst, i.e. α-Bi2O3/C-dots for the photocatalytic degradation of indigo carmine dye and levofloxacin (a fluoroquinolone drug).
Section snippets
Materials and methods
All the chemicals used were of analytical grade. The stock solutions of dye and drug were prepared in double distilled water. Levofloxacin was received from Saurav Chemicals, Derrabasi, India. The precursors, bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, >99.0%) and l-Ascorbic acid (C6H8O6, 99.0%) were procured from Sigma-Aldrich, India and Himedia, India, respectively. The model contaminant, IC dye (C.I. 73015), citric acid (C6H8O7, 99.0%), sodium hydroxide (NaOH, 97.0%), sodium chloride
Crystal structure of the as-synthesized photocatalysts
The structural and crystal properties of the synthesized samples were investigated by XRD analysis. The XRD pattern of synthesized C-dots is shown in Fig. 1a. A broad diffraction peak located at 2θ = 20° with a spacing of 0.34 nm was observed, indicating 002 planes of graphitic structure and amorphous nature of C-dots [38]. Fig. 1b represents the XRD pattern of the synthesized α-Bi2O3/C-dots nanocomposite. It was observed that α-Bi2O3 possessed a monoclinic structure along with space group of P
Conclusions
The synthesis of new or novel catalyst materials for use in environmental remediation of toxic pollutants is of high importance, especially in the processing industry. Consequently, on the basis of reported synthetic protocols for photocatalyst materials, we have chosen a surfactant-free sonochemical method to synthesize a novel α-Bi2O3/C-dots. XRD analysis of this novel catalyst material showed high crystallinity and purity, FTIR studies confirmed the successful modification of α-Bi2O3 with
Acknowledgements
Shelja Sharma would like to acknowledge UGC–BSR Government of India for providing financial support through a grant No. F. 25-1/2013(BSR)/5-91/2007 (BSR) and Newton-Bhabha grant by DBT, Govt. of India and British Council, UK. The authors are grateful to TEQIP-III grant of Dr. SSB University Institute of Chemical Engineering and Technology and SAIF, Panjab University, Chandigarh for providing the instrumental characterizations.
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