Coal tar pitch as natural carbon quantum dots decorated on TiO2 for visible light photodegradation of rhodamine B
Graphical abstract
Introduction
Human life and production activities have brought more and more environmental pollution. Many processes have been used to degrade pollutants, such as reclamation, burying, and incineration, which have practical significance for ecological and social development. Solar-driven photocatalysis is expected to play an important role in environmental purification and pollution control, and therefore, the development of highly efficient photocatalysts has received increasing attention [[1], [2], [3], [4]].
As the most promising photocatalyst [5], TiO2 materials provide a simple and inexpensive method to addressing the abovementioned issues due to their strong oxidizing power, chemical stability, long-term durability, non-toxicity and low cost [6]. It is well known that TiO2 can only be effectively excited by light having an energy greater than or equal to its band gap, which has high reactivity and stability under ultraviolet light (UV, λ < 387 nm), and the band gap of the anatase phase is ∼3.2 eV [7]. However, the sunlight incident on the surface of earth contains only 4% of the UV light [1], so TiO2 based photocatalysts have limited applications due to their wide band gap, which hinders the efficient use of sunlight. Hence, the development of highly active photocatalysts in the visible range (λ > 400 nm) has become a severe challenge [7,8]. For effective use of visible light and/or achievement of higher photocatalytic efficiency, a variety of strategies have been used to modify TiO2, including metal doping, non-metal doping, mental decorating, coupling with other semiconductors, dye sensitization and controlling morphology [[9], [10], [11], [12], [13], [14]]. Nowadays, the modification of TiO2 with carbon nanomaterials (such as carbon fibers [15], activated carbon [16], carbon nanotubes [17] and graphene [18]) is being increasingly investigated to improve photocatalytic performance in the visible range.
Among many carbon-based nanomaterials, carbon quantum dots (CQDs) are usually divided into carbon dots (CDs) and graphene quantum dots (GQDs) [19], which can become a promising alternative to traditional semiconductor quantum dots. CQDs are usually synthesized from carbonaceous precursors (carbon fibers, graphite, graphene oxide, glucose and citric acid, etc.) by acid oxidation, hydrothermal, electrochemical oxidation and microwave-assisted methods [20,21]. Recently, CQDs have received considerable attention because of their unique properties [22], such as chemical inertness, stable luminescence, low toxicity and excellent biocompatibility. Therefore, they have been widely used in the fields of sensors, bioimaging, electrocatalysis, photocatalysis, photovoltaic devices, nanomedicine and among many others [20,[22], [23], [24], [25]]. In recent years, CQDs have significant potential to enhance the photocatalytic activity of semiconductor photocatalysts, such as CQDs/TiO2, CQDs/ZnO, CQDs/Fe2O3, CQDs/Bi2O3, CQDs/g-C3N4 composites, etc [[26], [27], [28], [29], [30]]. Pan et al. [31] prepared CQDs/TiO2 nanotubes composites, which exhibited enhanced photocatalytic activity in Rhodamine B (RhB) degradation. Wang et al. [32] employed CQDs to modify TiO2 to enhance light absorption and restrain the recombination of photogenerated electron-hole pairs, thereby improving the photocatalytic activity of CQDs/TiO2 under UV and visible light irradiation. Ming et al. [33] synthesized CQDs/TiO2 photocatalysts by a one-step hydrothermal route, and the H2 evolution reaction possessed higher activity compared with pure TiO2 nanoparticles. Zhang et al. [34] reported that TiO2 mesocrystals coupled with CQDs were used for photocatalytic reduction of Cr(VI) under UV irradiation, and the prepared CQDs/TiO2 mesocrystals displayed higher activity than pure TiO2 mesocrystals. Xu et al. [35] prepared CQDs by a hydrothermal method, and then decorated CQDs on TiO2 nanoparticles by a hydrothermal-calcination method, which showed a marked performance of Cr(VI) reduction under visible light irradiation. In the above researches of CQDs modified TiO2, it is generally recognized that CQDs play an essential role in improving the photocatalytic activity of CQDs/TiO2, which is attributed to the fact that photoinduced CQDs are excellent electron acceptors and can effectively promote the separation of photogenerated electron-hole pairs. However, the preparation of such hybrid photocatalysts usually requires first preparing CQDs from relatively expensive precursors, then performing cumbersome post-processing purification procedure of CQDs, and finally combining with TiO2 to form CQDs/TiO2 composites [31,[34], [35], [36], [37], [38]]. Therefore, it is still highly desirable to develop CQDs/TiO2 composite photocatalysts that can be prepared on a large scale in a simple one-step process using cheap and readily available precursors or materials.
Coal tar pitch (CTP) is a by-product of coking industry. Due to its low cost, high carbon content and easy to graphitize, it has become a very promising candidate for the production of new carbon materials [39] such as graphene nanocapsules [40], porous carbon nanosheets [41], carbon nanotubes [42], carbon foams [43,44] and GQDs [45]. However, the open literature just considered CTP as a precursor of carbon materials [[40], [41], [42], [43], [44]], in fact, CTP has a unique molecular structure and should be used more subtly. CTP molecules are usually made up of polycyclic aromatic hydrocarbon nuclei and alkyl side chains or heteroatom functional groups attached to the edge of the aromatic nuclei [46], which are very similar to the structure of CQDs described in the literature [20]. Hence, CTP molecules can be considered as “natural CQDs” and are possible to substitute for some functions of CQDs in specific fields.
In this work, we first report the use of CTP as a natural alternative for CQDs to prepare an effective CTP/TiO2 composite photocatalyst via a simple and green one-step solvothermal route, in which CTP is directly deposited on the surface of the commercial TiO2 nanoparticles (P25) under mild solvothermal conditions to replace the effect of CQDs in CQDs/semiconductor photocatalysts. The CTP content in CTP/TiO2 composites can be adjusted by simply changing the amount of CTP added in the preparation process. The photocatalytic activity of the CTP/TiO2 composite photocatalyst with optimal CTP content for degrading RhB under visible light can be 23 times higher than that of pure TiO2. Moreover, the CTP/TiO2 composite photocatalysts possess excellent reusability and stability. This research is important for the development of excellent TiO2-based photocatalysts for environmental purification and remediation through simple large-scale and green inexpensive approaches.
Section snippets
Materials
The raw CTP was provided by Henan Zhonghong Coal Chemical Co., Ltd. (Pingdingshan, China) and heated at 360 °C to drive off the light components. Dimethyl formamide (DMF) and absolute ethanol were purchased from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Microporous membranes (0.22 μm) were obtained from Jinteng Experimental Equipment Co., Ltd. (Tianjin, China). TiO2 nanoparticles (P25) were purchased from Degussa (Germany). RhB, ammonium oxalate (AO), isopropyl alcohol (IPA) and
Characterization of the photocatalysts
Using CTP as a similar alternative for CQDs, CTP/TiO2 photocatalysts were prepared by a simple one-step solvothermal reaction in DMF, as illustrated in Fig. 1. The TiO2 nanoparticles were evenly dispersed into DMF containing CTP by means of ultrasonication before the solvothermal reaction. Under the solvothermal “sintering” conditions, the DMF-soluble CTP was stably deposited on the surface of TiO2 nanoparticles, resulting in the formation of strongly coupled CTP/TiO2 composite photocatalysts.
Conclusions
In conclusion, using coal tar pitch as an alternative for CQDs directly, an effective CTP/TiO2 composite photocatalyst with good reusability and stability has been successfully prepared by a simple and environmentally friendly one-step solvothermal method, and its photocatalytic degradation ability of RhB under visible light irradiation was studied. Compared with pure TiO2, CTP/TiO2 composite photocatalysts have higher photocatalytic degradation activity of RhB, and the degradation rate
Acknowledgements
This work was funded by the National Natural Science Foundation of China, China (U1361119) and the Fundamental Research Funds for the Universities of Henan Province, China (NSFRF170805).
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