Novel Cu2O quantum dots coupled flower-like BiOBr for enhanced photocatalytic degradation of organic contaminant
Graphical abstract
Introduction
The poisonous wastewater containing phenol and organic dyes were most hazardous to human's health, and the treatment for hazardous industrial chemical effluents was the growing needs of the present time [1]. Semiconductor photocatalysis has been considered to be a potentially promising approach for the abatement of environmental pollutants especially for the wastewater [2]. However, the high rate of recombination of photogenerated electron–hole pairs still limits its practical applications. Two main methods can be used to accelerate the separation of photo-induced electron–hole pairs. The first approach is through using noble metals to decorate semiconductor photocatalysts [3], [4]. In this configuration, the photo-induced electrons can easily transfer from the semiconductor to the noble metal through the Schottky barrier formed between them, since the Fermi level of noble metal is more positive [5]. Meanwhile, the electron capture trap formed in the surface of the semiconductor can prolong the lifetime of the excitation electrons, and promote their interaction with dissolved O2, thus inhibiting the recombination of the charge carriers. The second approach is combining two or more semiconductors with appropriate band positions, since the coupled energy levels can provide good internal potential driving forces to improve the separation of the photogenerated charge carriers [6]. In particular, p–n heterojunction semiconductor composites can effectively facilitate the photogenerated electron migration, due to the well-matched overlapping band-structures and intimately contacted interfaces [7], [8]. In addition, the conjugative π structure material exhibit special conductivity which can accelerate the transfer of the photogenerated charges when assembled with semiconductors [9], [10], [11]. A limitation of coupled semiconductors lies in their large sizes, and resulting decrease in the surface/volume ratio and active sites [12], which decreases their photocatalytic performance.
Recently, quantum dots-modified semiconductor composites have attracted considerable attention [13]. The semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon due to their unique quantum effect [14]. Furthermore, the high specific surface area provides more active sites which favors for the reactants adsorption and then improving the photocatalytic activity observed [15]. A major advantage of these quantum dots decorated semiconductors is that a larger charge transfer rate can be obtained due to the short charge-carrier transport paths and the intimately contacted interface [16]. Ultra-small Cu2O nanoparticles, possessing uniform sizes of approximately 10 nm, were recently loaded onto TiO2 nano-sheets or multi-walled carbon nanotubes to facilitate the separation rate of the photogenerated chargers and to enhance the photocatalytic activity [17], [18].
Three-dimensional structures are also of interest in photocatalysis, and the well-aligned pore structures present provide high specific surface areas and enhance the photoenergy conversion efficiencies obtainable [19]. Three-dimensional flower-like BiOBr, possessing internal electric fields in sandwich-like crystals, are able to effectively separate photogenerated charges and exhibit excellent photocatalytic performance [20]. In addition, the flower-like BiOBr structure was found to increase the active sites present in porous microspheres and improve the photoenergy conversion efficiency, due to its quantum confinement effects in the ultrathin thickness [21]. Despite these advantages, the flower-like BiOBr still need to further improve the photocatalytic performance to meet the practical application. Many attempts have been attempted, including noble metal modification [22], [23] and fabricate hetero-structure materials with C3N4 [24], ZnFe2O4 [25], AgBr [26], etc. In addition, methods used to enhance electronic separation have also been attempted using semiconductor quantum dots to decorate BiOBr catalysts. For example, Liu [27] et al. reported that cadmium sulphide (CdS) QDs sensitized BiOBr prepared via a solvothermal approach exhibited enhanced photocatalytic activity. Alternatively, Cu2O QDs exhibit considerable visible light absorption, and the well-aligned overlapping band-structures of BiOBr and Cu2O are expected to effectively separate the photogenerated carriers. To the best of our knowledge, the synthesis of Cu2O QDs decorated flower-like BiOBr has not been reported to date.
In this work, we successfully loaded Cu2O QDs onto the surface of BiOBr with a uniformly dispersion. The Cu2O QDs extended the visible light absorption and effectively promoted the migration of photogenerated charge carriers, and improved photocatalytic degradation of organic contaminant. This work suggests that the Cu2O QDs tolerated the three-dimensional microspheres BiOBr has a grant potential application in degradation of organic contaminant.
Section snippets
Experimental
All chemicals were analytical purity and used as received. Detailed synthesis procedures, characterization methods, and methodology related to photocatalytic reaction are given in the Supporting Information (SI).
Results and discussion
Fig. 1(a) shows the XRD patterns for the as-prepared samples. For the spectra of pure BiOBr, characteristic diffraction peaks were detected at 2θ angles of 8.11°, 25.2°, 31.7°, 32.2°, 46.2°, and 57.1°, attributed to the (0 0 1), (1 0 1), (1 0 2), (1 1 0), (2 0 0), and (2 1 2) crystal planes, respectively, which can be ascribed to the tetragonal BiOBr (JCPDS 09-0393). The diffraction peaks for the QDs-Cu2O/BiOBr composites prepared were similar to the BiOBr, indicated that the introduction of the Cu2O QDs
Conclusions
In summary, novel Cu2O QDs decorated flower-like BiOBr composites were successfully prepared via a simple method. The Cu2O QDs used to decorate BiOBr imparted an enhanced photocatalytic activity in degrading of organic contaminant due to the quantum effects, which were beneficial for the improvement of visible light absorption and increasing photon conversion efficiencies. In addition, the coupled energy levels could accelerate the separation of photogenerated charge carriers at the intimately
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant nos. 51172063, 51202056, 51372068), Hebei Natural Science Funds for Distinguished Young Scholar (Grant no. B2014209304), Hebei Natural Science Funds for the Joint Research of Iron and Steel (Grant no. B2014209314), Hebei Provincial Foundation for Returned Scholars.
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