Microwave Hydrothermal Synthesis of Terbium Ions Complexed with Porous Graphene for Effective Absorbent for Organic Dye

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. Electronic supplementary material The online version of this article (doi:10.1186/s11671-017-1962-7) contains supplementary material, which is available to authorized users.


Background
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][2][3][4] As the "graphene gold rush" go on, tremendous niche applications have been developed, including sensors catalysts, energy conversion, and energy storage [5][6][7]. Especially in the functionalized graphene material field, sundrymodified 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][13][14][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][17][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][22][23]. Based on these features, different properties of the product may be generated during microwave synthesis progress.
Herein ,Tb (III)/reduced graphene oxide was prepared, for the first time, through the microwave hydrothermal reaction as the enhanced adsorbent for organic dyes. Nevertheless, the compound owned a regular graphene lattice with relatively small size (3-5 nm), charming graphene quantum dots (GQDs) like photoluminescence emissions, induced by the interaction between Tb ions and oxygen functional groups on RGO. The fluorescence quenching test indicated that the adsorption process was accorded with the pseudo-first-order model and the asprepared complex had ultrahigh removal rate. Hence, this compound we addressed can be applied in optoelectronics, quality adsorbence, and biosensing ( Fig. 1).

Materials
Tb 4 O 7 (99.9%) was purchased from Shanghai company. Terbium chloride was obtained by dissolving Tb 4 O 7 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 [19]. 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 TbCl 3 (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 TbCl 3 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.

Adsorption of Rhodamine B by Tb-RGO Hydrogels
To measure the adsorption capacity for as-prepared material, the swelling degree (SD) [24] related to the mass of the specific hydrogel (this material got by soaking in the deionized water with xerogel for 24 h) and xerogel, which were recorded as m 1 and m 0 , respectively, was calculated using Eq. (1): 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.
The removal efficiency of the dyes was calculated according to Eq.(2) 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
To investigate the kinetics of the adsorption, 1.0 g Tb-RGO hydrogels with the best adsorption property were mixed with 15 mL RhB dye solutions, respectively. At predetermined time intervals (0, 10, 20, 30, 40, 50, 60 min), the concentrations of pollutant solutions were detected by the fluorescence spectra. The adsorption capacity as a function of time (q t ) was calculated according to the following equation: Where C 0 is the concentration of the dye (mg/L) at 0 min. C t is the concentration of the dye (mg/L) at t min. V is the volume of the solution and m is the mass of the xerogel. Two conventional kinetic models (pseudo-first-order and pseudo-second-order) as followed were used to simulate the experimental data.
In q e -q t ð Þ¼ ln q e -k 1 t ð4Þ 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
To obtain the composition and structure of the asprepared material, a series of characterization were carried out as followed description. The XPS survey spectra, indicating the valence state and chemical bonding of the Tb-RGO, shown in Fig. 2a. The analysis showed the distinctive presence of carbon (C) at ca. 284.6 eV, oxygen (O) at ca. 532.7 eV and Tb atoms at ca. 148~152 and 168 eV. There were three apparent spectral components at 284.5, 285.5, and 287.8 eV in the C 1 s peak (Fig. 2b). The main peak at a binding energy of 284.5 eV was assigned to sp 2 hybridized C atoms in graphene, indicating the restoration of the C = C bonds after the hydrothermal reduction (~50%), whereas the other two peaks should be corresponded to sp 2 C and sp 3 C atoms bonded to O: epoxy/hydroxyls (C-O, 285.5 eV) and carbonyl (C = O, 287.8 eV) [25]. In addition, the O 1 s core level spectrum showed in Fig. 2c. The O 1 s peak could be divided into two peaks at 530.8 eV and 533.1 eV. As reported, the signal at lowest binding energy (529.0-531.1 eV) was assigned to the surface lattice oxygen and O-H, while the peak at higher energy (~533.0 eV) would involve various functional oxygen groups in RGO, such as C-OH, C = O, and O = C-OH [26,27]. Through the Tb 4d core level spectrum shown in Fig. 2d, two regions were observed in this system suggesting the existence of different valence states [28,29]. The 168.4 eV peak would be related to Tb 4+ in Tb(III,IV) oxide, differing from Tb 4 O 7 and Tb 2 O 3 . In addition, some peaks appeared in the low binding energy region due to the multiple splitting, indicating the interaction between Tb ions and RGO. Calculated by the formula of Tb/(Tb + C), in which the sign of atom means the peak area, the atomic concentration of Tb ions were 0.55%. The above XPS results indicated that the residual oxygen functionalities would be the bridge between the  (Fig. 1).
The RGO and RGO-Tb had also been characterized using various physicochemical techniques (FTIR, Raman, SEM and HRTEM spectroscopy). Additional file 1: Figure S1 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 morphology of as-prepared materials shown in Fig. 3 was investigated by SEM and HRTEM, respectively. It could be observed that the structure of RGO were twisty and reunited together as the 3D porous between multilayer graphene sheets within the procedure, which was beneficial for adsorption process (shown in Fig. 3d). Compared with RGO, the sheets of Tb-RGO composite (shown in Fig. 3a) seemed more smooth and better-ordered, which was consistent with the analysis of the Raman results. What is more, a large number of dark dots distracted on the sheet of the composite uniformly while no dots but a very thin sheet appeared in RGO (shown in Fig. 3b, e). Through the high resolution TEM, it could be observed that the dots were formed with slightly fluctuant lattice and the interplanar crystal spacing was 0.24 nm (shown in Fig. 3c and inset graph). This was similar to the phenomenon of graphene quantum dots. The EDX image shown in Fig. 3f indicated the existence of Tb ions.
The corresponding photoluminescence (PL) emission spectra of Tb-RGO excited from 250 to 335 nm were shown in Fig. 4 (top). It was immediately clear that some relatively narrow PL peaks centered at 489, 544, 585, and 621 nm were observed, which actually were attributed to  was faded, which could be attributed to the absorption of the matrix (shown in Fig. 4 (bottom)). What is more, the intensity of the excited peak (260-280 nm) improved, due to the enhanced transition rate after the incorporation of RGO. It is correlated with the adsorption spectra of GO in this region (3-5 eV), attributed to the interactions between the valence band and O 2p orbital, which indicated that the oxygen functionalities destroyed the symmetry of the π-π* system to widen the gap [30]. Hence, we proposed this model that the energy motivating Tb was from GO. And it could be observed that a broad peak appeared (400-550 nm), which attributed to the luminescence of graphene. Furthermore, it could be linked to the quantum dot effect. Likewise, the above results could be inferred that the bonding between Tb ions and RGO.
Except for PL and PLE analysis, the decay lifetime curve of Tb-RGO was shown in Fig. 5d for supporting the evidence the coupling between Tb ions and RGO. With excitation at 265 nm and recorded at the emission wavelength of 544 nm for Tb ion, the curve was roughly fitted to a single-exponential function, described with I(t) = A*exp(-t/τ). While the intensity of RGO was weak for measuring, which was attributed to the reduction progress of GO. As we all known, the decay life for composite with Tb were τ = 1~2000 μs according to different matrix, which supported more approach to transfer energy and descend the lifetime. The lifetime of the composite was τ = 667.57 μs, and the weighting parameter A was 23.54. The lifetime of Tb ions indicated the transition type is 4f-4f transition because the radiation probability to this parity-forbidden transition roughly is 10 3 s -1 for crystals doped by rare earth ions (III) [31].
Using Eq. (1), swelling degree of Tb-RGO xerogel was calculated to be about 80 mg/g, which was the average of five experiments. The date stated that the degree of crosslinking for the composite was high, which corresponding to the above morphology discuss. The result demonstrated that the Tb-RGO had strong adsorption potential. Here, we had used the organic fluorophore, Rhodamine B, to monitor the progress for its superb observability. As shown in Fig. 6a, b and Additional file 1: Figure S3, it was obvious that the color fades after mixing RGO or Tb-RGO hydrogel with Rhodamine B. The adsorption efficiency was related to the reaction temperature and the ratio of precursors (TbCl 3 and GO), which was reflected by the concentration of TbCl 3 . It was clearly seen that the optimum reaction temperature and concentration of Tb to get the best removal efficiency for RhB are 170°C and 2 mg/mL. So we used the material prepared under these conditions to study the adsorption property.
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.
Before research on the kinetics of adsorption process, the concentration of RhB should be measured correctly. The internal standard method curve was prepared ( Fig. 4(top)) monitored at the summit of curve, indicating nearly proportional relation (the error might be caused by the background). From the kinetics of adsorption process (Fig. 4b), it was shown that the equilibrium time for RhB was about 60 min. Table 1 presents the kinetic parameters for the removal of dyes by the asprepared hydrogels. For RhB, it was clearly seen that the correlation coefficient R 2 value of the pseudo-first-order model reached 0.99373. It was much higher than that of the pseudo-second-order model (0.98547). It was also found that the calculated adsorption capacity (q e,cal ) obtained from the pseudo-first-order model, were 4.74120 mg/g. And obtained from the pseudo-secondorder model were 5.70613 mg/g, which was close to the experimental adsorption capacity (q e,exp ). The result indicated that, compared with the pseudo-second-order model, the pseudo-first-order model provided a better correlation for the adsorption of dyes on Tb-RGO. That was to say, the chemical reaction rate was directly proportional to the concentration of RhB.

Conclusions
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.