Carbon dots modifying sphere-flower CdIn2S4 on N-rGO sheet muti-dimensional photocatalyst for efficient visible degradation of 2,4-dichlorophenol
Introduction
Phenol and phenolic compounds (like chlorophenols (CPs), 2,4-dichlorophenol (2,4-DCP)), mainly originating from chemical plants, coal industry, oil refineries, and their like, are wildly employed in may aspects in our daily life [1]. However, they are toxic pollutants hard to decompose for their stable structure that they mainly contain chlorine atoms and the ring of benzene [2]. Besides, they are easy to biological accumulation from circulatory of the ecosystem, denaturing protein of organism, even causing death [3], [4], [5]. 2,4-DCP, on behalf of the products for characteristic herbicide, are utilized in the filed of agriculture and manufacturing plants [6], [7]. At the same time, the higher concentration of herbicide is infiltrating the ground water, and pouring into the river, further resulting in wastewater [8], [9]. As a consequence, it is necessary to eliminate CPs from wastewater. Some methods has been springing up to solve this problem, such as liquid phase micro-extraction [10], [11], electrochemical treatments [12], [13], adsorption [14], [15] and et al. However, these methods assume more energy and some more organic solvents. While the photocatalysis, an alternative and promising technique, could degrade organic pollutants, and capture people's attention in recent years [16], [17], [18], especially the photocatalyst could degrade CPs from waster under the sunlight irradiation, like C/ZnO/CdS nanocomposite [19], Ag/CDots/BiOBr ternary photocatalyst [20], 2D porous graphitic C3N4 nanosheets/Ag3PO4 [21].
Among the semiconductor photocatalyst, metal sulfides for their suitable band gap and good coefficient have caught people attention [22], [23], [24]. Binary metal sulfides (CdS, CuS) are susceptible to photocorression in the progress that photogenerated holes created by electrons of valence band excited transfer to conductive band have impact on them. There are several methods to handle it, such as constructing heterojunction, loading on supporting materials, adulterating with other compounds, mainly by incorporation into their interlayers. However, that incorporating steps could lead to poor photocatalytic performance [25]. Compared with binary metal sulfides, ternary ones with strong visible light absorption and narrow forbidden band are becoming an focus. CdIn2S4, as a member of family that belong to AB2C4 ternary compounds, has a direct band gap (2.0–2.2 eV) [26], exhibiting a good photocatalytic activity under the visible light. Although CdIn2S4 nanomaterials are employed in many aspects, like lighting-emitting diodes, photocatalyst, solar cells, photoelectrical application and so on [27], [28], [29], the further application is still confined by high recombination of photocarries. Therefore, several methods have been springing up to handle it. One way is that change the morphology of CdIn2S4 to tune the band gap. Huang et al. made it that CdIn2S4 microsphere could effectively degrade the Azo Dye [30]. Another way is that construct heterostructure to reduce the recombination rate of photocarries. It is reported that Sun et al. had been successful in synthesizing hierarchical ZnIn2S4/CdIn2S4 microspheres that they could degrade 99.7% organic dyes after 90 min [31]. The other way is loading on supporting materials. Xiao et al. prepared RGO/CdIn2S4/g-C3N4 ternary hybrid photocatalysts that they could show significantly efficient photocatalytic activities (10 mg/L tetracycline hydrochloride degradation rate up to 74% for 3 h) [32].
Among numerous photocatalyst, carbon materials have drawn people's attention for their excellent conductivity, good physicochemical properties, great absorption ability and et al. Therefore, it is significant to design heterostructure to reduce the recombination of photocarries and enlarge the visible light absorption range. The zero-dimensional CQDs with prominent morphology of amorphous or turbostratic carbon, as an emerging novel nano-carbon material, are composed of zero-dimensional structures within typical 10 nm size in diameter [33]. CQDs get more studies due to its applications in bioimaging for low toxicity and good biocompatibility [34], optics for outstanding electrochemical properties [35], selective oxidation for excellent PL and photochemical abilities [36], and its like. It is noticeable that CQDs have the up-converted capability [37], thus making full use of sunlight spectrum ascribed to the conjugated π-domains [38], and PL emission associated with surface state, conjugated structures, and special structure sites [39], which contribute to improvement of photocatalytic performance. There are many experiments that the introduction of CQDs into other photocatalyst could effectively improve the photocatalytic ability, which are ascribed to photoinduced electron-accepting properties [40]. What iss more, CQDs could convert long wavelength into short wavelength [41], [42]. It is effective that CQDs could combine themselves with other composites, thus constructing heterostructure, which is beneficial to improve the whole photocatalytic performance for these CQDs could function as photoinduced electron transfer and acceptor [43]. For example, Pan et al. synthesized that Z-scheme CdS/CQDs/BiOCl heterojunction could efficiently decompose phenol that the 42% 10 ppm phenol was photodegraded under UV light irradiation for 2 h [44]. Liang et al. made it that carbon quantum dots modifying BiOBr microspheres could degrade 94.3% rhodamine under visible irradiation for 30 min [45]. Li et al. could successfully prepare Bi/BiOCl-TiO2CQDs quaternary photocatalyst by which about 94% of MO and 82% of PNP were degraded under visible-light irradiation for 100 min and 240 min, respectively [46].
It is known that recombination rate of photogenerated electron-hole pairs and electrons transfer are two key factors that estimate photocatalytic performance. Another carbon material is graphene, a two-dimensional nano-sheet material. In order to overcome the shortcomings (like zero band gap, weak interlayer force) of graphene, adulterating other heteroatoms (such as N, P, Cl) into carbon lattice of graphene could effectively tailor itself, thus bringing in physicochemical properties. Nitrogen atom, comparable atomic size as carbon atoms, contributes to forming strong bonds [47], [48]. There are many reports that show that N-rGO could change the spin density and charge distribution, thus creating active regions and influencing the lattice alignment, so as to improve the photocatalytic activity [49], [50], [51]. What is more, synergistic function between two components takes place. In addition, N-rGO could function as support material, which is beneficial to the dispersion of semiconductors [52], [53]. In recent years, that combine N-rGO with other semiconductors has gained extensive attention, like Co3O4/N-RGO [54], Pd50Ag50/Fe3O4/N-rGO [55], Cu–Zn/N-rGO [56], and et al.
In this paper, zero dimensional CQDs modifying three-dimensional sphere-flower CdIn2S4 loading on amorphous two dimensional N-rGO nano-sheet composite photocatalyst was successfully prepared via a facile hydrothermal method and solvothermal method. We construct the semiconductor heterojunction between CQDs and CdIn2S4, further changing the path of photogenerated electron and hole pairs and prolonging the lifetime of carries. N-rGO could not only benefit the electron transfer, but also it gives more active sites, further improving photodegradation of 2,4-DCP. And the mechanism of CQDs/CdIn2S4/N-rGO is studied through free radicals trapping experiments.
Section snippets
Synthesis of N-rGO, sphere-flower CdIn2S4 and CQDs
According to our previous research [57], the preparation of N-rGO was listed in supporting information. The sphere-flower CdIn2S4 photocatalysts were got by the hydrothermal method. Thiourea (4 mmol), cadmium chloride (1 mmol) and Indium(III) chloride tetrahydrate (2 mmol) were added into 30 mL deionized water under ultrasonic treatment for 0.5 h, and then transferred into Teflon-lined autoclave. Then the samples were washed by deionized water for several times before heated at 180°C for 48 h,
Structures and surface elements
In a bid to investigate the crystallographic structures of the samples we obtained, the XRD patterns are shown in Fig. 1. It is apparent that the characteristic sharp peak of GO is at 10.8°, which has an interlayer spacing of 0.7 nm, bettering the hydration and exfoliation of layers and layers. When the hydrated hydrazine is added, as a reducing agent, the full width at half maxima, the peak at around 24.5°corresponding to 002 plane of graphite, becomes larger rather than sharp peak, because
Conclusions
In summary, the zero-dimensional CQDs modifying three-dimensional sphere-flower CdIn2S4 on two-dimensional N-rGO sheet composites are prepared by means of simple hydrothermal method and solvothermal method. When the mass fraction of N-rGO is 5% and the volume of CQDs is 0.5 mL, the best photocatalytic performance of CQDs/CdIn2S4/N-rGO on photodegrading 2,4-DCP (50 mg/L) could reach 80%. There improvement could be attributed to the defects of N-rGO and its good conductive properties and unique
Acknowledgments
We are grateful to acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 21576125 and 21776117), China Postdoctoral Science Founsation (2017M611716 and 2017M611734), Six talent peaks project in Jiangsu Province (Grant No. 2016-XCL-014), Zhenjiang Science & Technology Program (Grant No. SH2016012).
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