Synthesis of PANi nanoarrays anchored on 2D BiOCl nanoplates for photodegradation of Congo Red in visible light region

https://doi.org/10.1016/j.jiec.2019.09.012Get rights and content

Abstract

Photocatalytic processes have attained considerable attention of late years, especially for environmental remediation. Despite extensive research in this area, the need for safer, more efficient, and cost-effective processes has encouraged researchers to develop novel photocatalysis. However, the low active surface area and narrow bandgap limit their photocatalytic performances. In the present research, the 2D BiOCl sheets were successfully synthesized by a new hydrothermal method and decorated by PANi nanoarrays through in-situ oxidative polymerization of aniline. The UV–vis diffuse-reflectance and photoluminescence spectroscopy revealed the synergistic effects between PANi nanoarrays and 2D BiOCl by enhancing the absorption in the visible light region and reduction of bandgap down to 2.9 eV. Furthermore, the morphology analysis showed the proper decoration of PANi nanoarrays on 2D BiOCl nanoplates. The synthesized nanocomposite with different weight loadings of PANi was taken to evaluate the decolorization efficiency of it. The result exhibited an optimum value of 88.35% at 60 min irradiation under visible light in the photodegradation of Congo Red (CR). Moreover, the probable photocatalytic mechanism for degradation of CR by PANi/BiCOl photocatalyst was proposed based on the scavenger experiments. The outcomes indicated that the PANi promoted the absorption intensity of the pure BiOCl in the visible region. To that, the well-arranged array and considerably high specific surface area of PANi could encourage the transfer of electrons witch generated by the photo to 2D BiOCl substrate and repel the recombination of electron-hole pairs.

Introduction

Environmental pollution with natural contaminants such as antibiotics, organic dyes, and pesticides has become a challenging task due to their carcinogenic and toxic effects on human and organisms [1], [2], [3]. Because of increasing daily usage of Congo Red (CR) in different industries including textile, printing, leather, paper, pulp, and cosmetic, remediation of the colored effluents is regarded as a solution to these environmental problems [4]. The mutagenic nature of these effluents impedes their treatment by physical and biological approaches [5]. Degradation of naphthalene and benzene rings of CR cannot apply with ordinary methods. Moreover, because of its aromatic structure, CR possesses high physiochemical, thermal, and optical stability [6].

Various techniques like biodegradation [7], adsorption [8], [9], ozonation, photocatalytic degradation, physiochemical treatment, catalytic reduction, and coagulation/flocculation are reported for treatment of dyes [10]. Advanced oxidation processes (AOPs) have drawn considerable attention in recent decades and be more useful for the oxidation of organic pollutants [11]. Despite the economic effectiveness of several adsorbents over CR removal, CR is not degraded by adsorbents and just transferred from liquid phase to solid phase. Therefore, the implementation of AOPs and among them light-driven photocatalytic processes for its high energy and environmental efficiency [12].

Nowadays, tremendous researches have focused on the design and development of innovative, efficient, and cost-effective approaches for remediation of wastewaters [13]. Because of the light-driven photocatalytic system is activated at the visible region (can be sunlight at large scales) and no additional reagent or chemical added to the CR solution, make it a great choice for degradation of this pollutant. Recently, bismuth-based compounds, specially BiOX (X = Cl, Br, I) have been received great attention in photocatalytic degradation of water pollutants [14], [15]. As a key V–VI–VII ternary semiconductor compounds, all BiOX have a tetragonal PbFCl-type structure, a layered structure described by [Bi2O2] sheets inserted by double sheets of halogen atoms [16]. The high photocatalytic activities of BiOX were mostly related to the indirect optical transition and their open crystalline structures. The first one means that the excited electron has to trip a certain distance emitted to the valence band (VB), which decreases the recombination likelihood of the excited electron and hole [17]. Several researchers have reported that nanostructured BiOCl compared to TiO2 displays the higher photocatalytic degradation performance under irradiation of visible light [18], [19], [20]. Furthermore, some dye molecules, including Rhodamine B can easily adsorb on the BiOCl surface, and under irradiation of visible light can be further degraded due to the dye sensitization phenomenon [21]. On the other hand, due to the determining role of morphology and the size of BiOCl particles on its photocatalytic performance, many approaches have been investigated for the synthesis of BiOCl with various morphologies such as solvothermal [22], precipitation [23], the reverse microemulsion route [24], the molecular precursor route [25], calcination [26], microwave irradiation [27], and hydrolysis [21]. Besides its significant photocatalytic activity, the overall photocatalytic efficiency of BiOCl still needs to be improved. In this regard, some modification approaches have been used, including co-catalyst use [28], [29], doping [30], [31], coupling [32], sensitization [33], graphene use [34], and making defects [35], [36]. Also, the conductive polymers including π-conjugated electron systems including polyaniline (PANi), polythiophene, polypyrrole, and their derivatives indicated the great encouraging in photocatalytic processes thanks to their attractive features including broad absorption in the visible light region and excellent stability [37], [38]. Besides, PANi is relatively cheap with simple synthesis approach in comparison with doped noble metals. Some reports depict the mixture of conductive polymers and some semiconductors to advance their photocatalytic performance as sensitization agents. Wang et al. [39] have reported the modification of BiOCl by PANi via chemisorption approach. The obtained PANi/BiOCl photocatalyst declared many plates of BiOCl with anchored PANi particles on their surface. The main morphological difference between the nanocomposites synthesized here with the earlier one reported by Wang by al. one is the creation of PANi nanoarrays by in-situ polymerization approach compared to nanoparticles of PANi attached on the surface of BiOCl sheets via chemisorption method. Therefore, we can consider the improvement in synthesis approach and morphological features of PANi/BiOCl to its enhanced performance. In another study, a multifunctional Ni0.5Zn0.5Fe2O4@PANi modified BiOCl was developed by Tanwar et al. [40] as highly effective and visible light-driven photocatalyst in the degradation of four toxic dyes. In that composite, the ferrite and PANi components absorb photons to create electron-hole pairs. This group in another work [41] prepared PANi/Fe0 doped BiOCl by chemisorption of PANi/Fe0 fibers on BiOCl plates. The nanocomposite showed 1.62 eV decrease in the estimated bandgap value compared to pure BiOCl with improved photocatalytic performance in the removal of CR under irradiation of visible light. However, in prior works, morphology control of PANi on BiOCl substrate has not been addressed, which may reduce the synergistic effects of two components. Hence, the construction of specially structured composites of BiOCl and PANi, which allows highly efficient utilization of PANi, remains a challenge.

In the present work, as shown in Scheme 1, we employed a novel two-step approach for the fabrication of PANi/BiOCl photocatalyst. For this means, firstly, 2D BiOCl sheets were synthesized by a hydrothermal approach then the in-situ oxidative polymerization of aniline monomer was performed in the existence of them to create PANi nanoarrays on their surface. The obtained PANi/BiOCl nanocomposite showed well-arranged nanoarrays of PANi on 2D BiOCl sheets surface and showed remarkable photocatalytic activity compared to previously reported work [39] in the degradation of CR under visible light irradiation.

Section snippets

Materials

The bismuth(III) nitrate pentahydrate (Bi(NO3)3.5H2O), sodium chloride (NaCl), ammonium persulfate ((NH4)2S2O8), hydrochloric acid (HCl), aniline (C6H5NH2), ethanol (C2H5OH), and KCl were all in analytical grade and purchased from Merck. The reagents were utilized as received without further purification. The distilled and deionized water was used during experiments.

Synthesis of 2D BiOCl sheets

The 2D BiOCl layers were synthesized as described elsewhere [42]. In brief, 1.94 g Bi(NO3)3·5 H2O was dissolved in 100 mL

Characterization of PANi/BiOCl photocatalysts

Fig. 1a indicates the XRD patterns of BiOCl and a set of PANi/BiOCl photocatalysts with 1, 5, 7, and 9 wt% of PANi. The XRD pattern of hydrothermally synthesized BiOCl is inconsistent with standard tetragonal structure (JCPDS 01-073-2060). The key diffraction peaks appear at 2θ’s of 11.98°, 24.10°, 25.40°, 33.45°, 34.40°, 36.50°, 41.51°, 46.52°, 49.6°, 55.70°, 58.41°, 61.33°, 75.43°, and 77.32° which correspond to the (0 0 1), (0 0 2), (1 0 1), (1 1 0), (1 1 1), (0 0 3), (1 1 2), (2 0 0), (1 1

Conclusions

In conclusion, we developed a new approach to synthesis 2D BiOCl sheets decorated by PANi nanoarrays as an efficient visible light driven photocatalyst for the removal of CR. The characterization techniques showed the effective growth and attachment of PANi nanoarrays on 2D BiOCl sheets which enhanced the active surface area and light-absorbing behavior, especially in the visible region. Almost 83.5% of CR at 10 mg L–1 was removed by 5 mg of 5 wt% loading of PANi/BiOCl within 60 min were the

Conflict of interest

There are no conflicts to declare.

Acknowledgments

The authors would like to acknowledge the University of Tabriz for support of this research.

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