Microwave Hydrothermal Synthesis of Terbium Ions Complexed with Porous Graphene for Effective Absorbent for Organic Dye
© The Author(s). 2017
Received: 1 December 2016
Accepted: 27 February 2017
Published: 20 March 2017
A luminescent terbium ions/reduced graphene oxide complex (Tb-RGO) was successfully and rapidly synthesized by the microwave hydrothermal reaction via the interactions between terbium ions and the active oxygen functional groups of graphene oxide. The as-prepared material was porous stacked by multilayer graphene in all directions. Thus, the resulting product owed the high specific surface area, high adsorption capacity and ultra-fast adsorption rate. Combined with the characteristic photoluminescence derived from terbium ions, the material has potential applications in biosensing and environmental protection.
KeywordsMicrowave Absorbent Rare earth ions Graphene
Graphene with a hexagonal packed lattice structure is widely proposed to be a star material, boosted by its appealing properties, including superb mechanical property, high superficial area, eco-friendliness, etc. [1–4] As the “graphene gold rush” go on, tremendous niche applications have been developed, including sensors catalysts, energy conversion, and energy storage [5–7]. Especially in the functionalized graphene material field, sundry-modified materials emerge in an unending flow, deriving countless technologies and applications. It is versatile for the aforementioned functional application to produce reduced graphene oxide-based material, for its rich active oxygen functional groups can be ornamented before the reduced process [8, 9]. Generally, for these materials, the adsorption is a practical and common application and it has been reported that reduced graphene oxid with its derivative could effectively absorb manifold organic dyes, such as methylene blue (MB), Rhodamine B (RhB), through π-π stacking, hydrogen bonding, the electrostatic force, etc. [10, 11].
As another outstanding material, lanthanide ions are usually regarded as excellent dopants in nanoparticles and powders due to their characteristic features, including fine luminescence stability, high fluorescence quantum efficiency, low toxicity, and long luminescence lifetimes [12–15]. Among these elements, terbium (Tb) ions with the favorable green emissions have been the typical representative for doping into the matrix. Otherwise, terbium ions combined with graphene-based materials have been reported to be utilized for detecting mercury ions, monitoring hypochlorite, and enhancing the electrical conductivity, which indicated that wide applications were developed in the fields of optoelectronics and biosensing [16–18].
Based on logical speculation, amazing features will be generated for the compound-combined graphene and lanthanide ions. To obtain this imaginary material, seeking for the right preparation remains an obstacle. The composite’s composition, structure, and property are mainly affected by the method. Until now, there are many synthesis methods for graphene-based materials, including chemical vapor deposition, hydrothermal method, etc. [19, 20]. It is well known that the former method tends to require high synthesizing temperature while the latter will need relatively long time. Microwave hydrothermal method has been a popular method in synthesizing various nanoparticle-based materials with great superiority, such as high yield and contained cost. In the meantime, it owns the superior characteristic features containing fast heating, uniform heating throughout the whole material, mild reaction condition, and high energy utilization rate [21–23]. Based on these features, different properties of the product may be generated during microwave synthesis progress.
Tb4O7 (99.9%) was purchased from Shanghai company. Terbium chloride was obtained by dissolving Tb4O7 in concentrated hydrochloric acid then matched to the concentration of 2 mg/mL.
Preparation of Tb-RGO Hydrogels and Xerogels
Graphene oxide (GO) was prepared following the improved Hummers method . For studying the time of reaching the adsorption equilibrium, considering the change of the color could be observed in time, we chose the concentration of 2.5 mg/mL for RhB to observe the adsorption process.
Twenty milliliters GO (3 mg/mL) and 2.4 mL TbCl3 (2 mg/mL) solutions were introduced into a Teflon-lined autoclave and heated at 170 °C for 20 min by microwave. The control group was performed like the above product except for replacing TbCl3 solution with distilled water. After being filtered, cylinder-like micro assemblies (hydrogel) were finally formed for Tb-complexed graphene and pristine graphene, respectively.
To preserve their original structures and get the xerogel, the as-prepared samples were subject to freeze-drying treatment. For short, the obtained Tb-complexed graphene and pristine graphene are called terbium ions/reduced graphene oxide complex (Tb-RGO) and reduced graphene oxide complex (RGO), respectively. The following experiments were designed for studying some best parameters and the results were applied for the best absorption experiment.
Considering the influence of the composition of materials for adsorption of RhB, eight kinds of Tb-RGO with different ratios of TbCl3 and RGO (1:2.5, 1:3.1, 1:4.1, 1:6.25, 1:12.5, 1:18.8, 1:25, 1:50) were prepared. For keeping the same amount of water, the concentrations of TbCl3 were separately 10, 8, 6, 4, 2, 1.5, 1, and 0.5 mg/mL. Then, the absorption experiment for the Tb-RGO hydrogels with different concentration was carried out.
To determine the influence of temperature, 1.0 g Tb-RGO hydrogels prepared under different temperatures (140, 150, 160, 170, and 180 °C) were mixed with 20 mL RhB dye solutions, respectively.
Adsorption of Rhodamine B by Tb-RGO Hydrogels
The mixtures mixing the as-prepared hydrogels and RhB dye solutions (15 and 2.5 mg/L) were placed at the room temperature. When it became colorless as time went on, it is then filtered. As we know, the adsorption procedure is reflected by the chroma, which is determined by the concentration of RhB. Due to the luminescence intensity of RhB being proportional to the concentration, the concentrations of RhB could be speculated through the relative luminescence intensity detected by fluorescence spectra. So, we could conjecture the concentration by the luminescence intensity to describe the adsorption progress. For RhB solutions, the concentration was determined by the intensity at 581 nm.
Where C 0 and C e are the initial and equilibrium concentration of RhB solutions, respectively.
To measure the concentration of RhB correctly, the internal standard method was carried out. The standard RhB liquids of different concentration were detected by fluorescence spectrum.
The Influence of Contact Time
Where q e and q t are the amounts of dyes adsorbed on the hydrogel (mg/g) at equilibrium time and t minutes, respectively. And k 1 and k 2 are the rate constants of pseudo-first-order and pseudo-second-order.
Results and Discussion
The RGO and RGO-Tb had also been characterized using various physicochemical techniques (FTIR, Raman, SEM and HRTEM spectroscopy). Additional file 1: Figure S1(a) showed the typical features in the FTIR spectrum of RGO, such as the O–H stretching vibration (3429 cm−1), C = C from unoxidized sp2 C = C bands (1550-1620 cm−1), C–O vibration (1151 cm−1), and HO–C = O stretching vibration band (1718 cm−1). While in the FTIR spectrum of Tb-RGO, there were some differences, including arisen absorption band at 667 cm−1 of Tb–O, increasing band of C–O, subdued band of O−H. It can be inferred that Tb ions joined the molecules by replacing H atom of –OH, O = C–OH or breaking the C–O–C band. Raman spectra of the Tb-RGO and RGO were included in Additional file 1: Figure S1(b). In the Raman spectrum of RGO, the G peak was at 1590 cm−1 and D peak was at 1345 cm−1. The prominent D band was associated with the vacancies, grain boundaries, amorphous carbon species and the residual oxygen functionalities. The ratio of the D peak and the G peak slightly descended from 1.047 to 1.004, indicating the Tb-RGO had less introduced defects.
The adsorption progress could be explained that the concentration of luminophore was decrease due to the bonding between the hydrogel and RhB, which called fluorescence quenching. It took only 1 h for completely quenching of the fluorescence of Rhodamine B for Tb-RGO. While for RGO, a longer time (~8 h) was needed seen in Fig. 6b and Additional file 1: Figure S3. Thus, the absorption efficiency had been greatly improved by the introduction of Tb ions. It had been reported that the fluorescence quenching greatly depended on the electronic energy transfer and charge transfer. Thus, the introduction of Tb ions would inevitably tune the microstructure, electronic states of the RGO sheets and enhance the quenching ability for Rhodamine B. And we proposed two ways for Tb ions enhancing the adsorption based on its ability of central coordination shown at the bottom of Fig. 6.
Kinetic parameters for the adsorption of RhB by Tb-RGO at the equilibrium time (60 min)
In conclusion, we found a facile time-saving approach to synthesize Tb-RGO hydrogels at relatively low temperature (170 °C). The as-obtained hydrogels exhibited the high surface area with a porous structure. And the Tb-RGO hydrogels were applied as adsorbents for RhB from the aqueous solutions. The results indicated that there were high removal efficiencies for RhB due to the strong π-π stacking and special structure. The equilibrium time of adsorption was 60 min for RhB and the solution could be decolorized to nearly colorless. The concentration of Tb and the reaction temperature were further investigated for removal process. And the kinetics of removal process was researched. The results demonstrated that Tb-RGO hydrogels would have great potential for the practical application in adsorption material.
Terbium ions/reduced graphene oxide complex
This work was financially supported by the National Science Foundation of China (51402140, U1632129), the Fundamental Research Funds for the Central Universities (lzujbky-2016-128).
Availability of Data and Materials
All data are fully available without restriction.
KC and HG designed the experiments and analyzed the data; KS and BB carried out the experiments; WL provided the apparatus and the standard operating procedure; KC wrote the manuscript; HG and XL assisted with the writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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