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Constructing high-efficiency photocatalyst for degrading ciprofloxacin: Three-dimensional visible light driven graphene based NiAlFe LDH

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Abstract

A three-dimensional (3D) graphene based photocatalyst, consisting of Fe(III)-doped NiAl layered double hydroxide (NiAlFe LDH), was prepared via a simple one-step hydrothermal approach for the first time. The growth of NiAlFe LDH nanoplates and the reduction of graphene oxide were accomplished simultaneously during hydrothermal process without adding toxic reductant and toxic solvent. No complex and time-consuming preparation steps were needed. The structural characterization shows that NiAlFe LDH nanoplates with a size of 100–150 nm vertically grow on the reduced graphene oxide (RGO) sheet, forming a unique 3D structured graphene based photocatalyst (NiAlFe LDH/RGO). The photocatalyst was applied for the first time to remove ciprofloxacin (CIP) in wastewater under visible-light illumination. Interestingly, after incorporating with an appropriate amount of Fe3+ and RGO, the NiAl LDH shows improved photocatalytic degradation performance in comparison with reported photocatalysts. The reaction rate in the presence of NiAlFe LDH/RGO is respectively 2.4 and 7.3 times faster than that in the presence of NiAl0.85Fe0.15 LDH and NiAl LDH, demonstrating that the addition of Fe3+ and RGO can improve synergistically the photocatalytic performance of the nanocomposite. Moreover, the photocatalytic mechanism of the 3D NiAlFe LDH/RGO photocatalyst was also investigated in detail.

Graphical abstract

Proposed mechanism for the photocatalytic degradation of CIP in the presence of NiAl0.85Fe0.15 LDH/RGO25.

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Introduction

Water pollution has become a serious environmental issue influencing our health [1], [2]. Photocatalysis has aroused extensive attention of researchers for its application in removing organic pollutants in water, such as antibiotic residues, because of its low energy-consumption and environmental friendliness [3], [4], [5].

As dramatic progress being achieved in nanoscience and nanotechnology, various kinds of nanophotocatalysts have been developed, including metal oxide, metal sulfide, layered double hydroxide, etc. [6], [7], [8], [9], [10], [11], [12], [13]. Among them, layered double hydroxide (LDH) is regarded as a promising kind of photocatalyst, owing to its unique layered structure and tunable composition [14], [15]. Lately, series of LDHs used as efficient photocatalysts in the treatment of water pollution have been reported [16], [17]. However, pristine monolayer LDH has poor charge mobility and high density of surface charge, which respectively gives rise to prompt recombination of photogenerated electron-hole pairs and easy aggregation of LDH nanoplates, reducing its photocatalytic activity [18], [19], [20]. Therefore, strategies have been explored to improve the photocatalytic activity of LDH.

Metal elements doping, one of the typical strategies, is extensively used to enhance the photocatalytic activity of LDH [21]. Parida et al. reported a co-precipitation approach to fabricate Cu/Cr LDH doped with Co2+, which showed excellent photocatalytic performance towards the degradation of malachite green under sunlight illumination [22]. Other metal elements, such as Tb, Fe, Ce and Ni, were also doped with LDH to improve its photocatalytic activity [23], [24], [25], [26]. Among those metal elements, Fe has attracted extensive attention due to its low cost, nontoxicity and unique electronic configuration [27]. For example, Mantilla et al. prepared Fe3+ doped ZnAl LDH by co-precipitation method, which exhibited great photocatalytic activity for the degradation of 2, 4-dichlorophenoxyacetic acid [28]. Although metal element doping can inhibit the recombination of photogenerated electron-hole pairs therefore improving the activity of the photocatalysts, the aggregation of LDH nanoplates remains a serious problem.

Graphene, a two-dimensional carbon material, has been widely served as an ideal carrier for nanoplates, owing to its superior electron mobility as well as large specific surface areas [29], [30]. The coupling of LDHs with graphene was reported to be able to effectively facilitate the separation of photogenerated electron-hole pairs in nanocomposites and suppress the aggregation of LDH nanoplates [31], [32], [33]. Hwang et al. reported a ZnCr LDH-graphene nanohybrid, which was prepared by electrostatically derived self-assembly and exhibited great photocatalytic activity on O2-generation [34]. Li et al. spent 24 h fabricating a hybrid nanocomposite, ZnCr LDH/RGO, using co-precipitation method, which displayed a promoted photocatalytic activity toward the degradation of RhB [35]. Nevertheless, the preparation of graphene based LDH (LDH/graphene) photocatalysts through electrostatic self-assembly or co-precipitation is complex. Multiple preparation steps and long synthetic time are required. Hence, exploiting a facile method to prepare LDH/graphene photocatalysts is meaningful. Apart from that, a structural change might also be meaningful in terms of enhancing the catalytic activity of the nanocomposite. LDH/graphene photocatalysts with LDH nanoplates horizontally deposited on the graphene sheets have been reported [33], [34], [35]. If the LDH nanoplates can be vertically deposited onto graphene sheets to form a three-dimensional (3D) LDH/graphene photocatalysts, a larger specific surface area, more adsorption and active sites, and less aggregation of LDH can definitely be obtained, thereby an improved photocatalytic activity.

In our previous work, we have investigated in detail the preparation of graphene and dedicated to developing a variety of methods to fabricate graphene-based photocatalysts [19], [36], [37], [38], [39], [40]. Based on our research results, we prepared, for the first time, a 3D NiAlFe LDH/RGO photocatalysts via a simple one-step hydrothermal method. Neither complex nor time-consuming preparation steps are needed. It costs only half of the time that Li et al. spent with just one preparation step required [35]. Moreover, no toxic chemical reductants or additional toxic solvents were used. Structural characterization proves that both the vertical growth of NiAlFe LDH nanoplates on the surface of graphene and the reduction of graphene oxide are accomplished simultaneously during hydrothermal process. Moreover, the as-prepared 3D NiAl0.85Fe0.15 LDH/RGO25 was applied, also for the first time, in degrading CIP under visible light illumination, where remarkable photocatalytic performance with good stability was observed. The degradation rate constant in the presence of NiAl0.85Fe0.15 LDH/RGO25 is 2.4 and 7.3 times higher than that in the presence of NiAl0.85Fe0.15 LDH and pristine NiAl LDH, respectively. More importantly, the performance of NiAl0.85Fe0.15 LDH/RGO25 is superior to that of some photocatalysts previously reported [19].

Section snippets

Experimental reagents

Natural graphite powder (99.85%, 500 mesh), Ni(NO3)2·6H2O, Al(NO3)3·9H2O, Fe(NO3)3·9H2O and CON2H4 were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were analytical reagent (AR) and used as received without further purification. Graphite oxide (GO) was fabricated by Hummers method [41].

Preparation of NiAl1-XFeX LDH/RGO nanocomposites

NiAl1-XFeX LDH nanocomposites with a varying molar ratio of Fe (X = 0, 0.05, 0.1, 0.15, 0.2 and 0.3, respectively) and NiAl0.85Fe0.15 LDH/RGOY with a diverse mass ratio of RGO (Y = 0, 5, 15,

Characterization of NiAl0.85Fe0.15 LDH/RGO nanocomposites

The XRD patterns of NiAl LDH, NiAl0.85Fe0.15 LDH and NiAl0.85Fe0.15 LDH/RGO25 are shown in Fig. 1A. NiAl LDH, NiAl0.85Fe0.15 LDH and NiAl0.85Fe0.15 LDH/RGO25 all display characteristic peaks of (0 0 3), (0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes, which are similar to standard NiAl hydrotalcite (JCPDS No. 15-0087), demonstrating that the hydrotalcite structure is successfully prepared by the hydrothermal method [42]. However, NiAl0.85Fe0.15 LDH shows weaker characteristic

Conclusions

In conclusion, a 3D NiAlFe LDH/RGO photocatalyst was successfully synthesized for the first time, through an environmentally benign and simple one-step hydrothermal method. Structural characterization indicates that NiAlFe LDH nanoplates grow perpendicularly on the RGO sheets, forming a 3D structured photocatalyst with large specific surface area. The 3D NiAl0.85Fe0.15 LDH/RGO25 photocatalyst exhibits impressive photocatalytic performance and good stability toward the photodegradation of CIP.

Acknowledgments

The authors are grateful for the financial support from National Natural Science Foundation of China (No. 51572036, 51472035), Changzhou Key Laboratory of Graphene-Based Materials for Environment and Safety (CM20153006, CE20185043), and PAPD of Jiangsu Higher Education Institution.

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