A Facile Synthesis of La2O3/GO Nanocomposites in N,N-Dimethylformamide with High Dye Degradation Efficiency

La2O3/graphene oxide (GO) nanocomposites were simply synthesized from commercially available LaCl3⋅7H2O and multilayer GO (5–10 layers) in N,N-dimethylformamide (DMF) under reflux condition without extra stabilizer. The characterization results of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectrum (XPS) spectroscopies showed that the crystalline La2O3 nanoparticles were successfully attached on the surface of GO. Moreover, the as-prepared nanocomposites greatly enhanced the degradation efficiency of organic dyes after 15min under ultrasound irradiation in pure water. The degradation efficiency of the nanocomposites for methylene blue could be over 99%.


Introduction
Owing to their great catalytic, mechanical, magnetic, and optoelectronic properties, the synthesis of graphene oxide-(GO-) based nanocomposites has received great attention in recent decade [1].Therefore, many kinds of GO-nanoparticle composites have been prepared by decorating GO with the nanoparticles such as Au [2], Pt [3], Ag [4], Ni [5], Fe 3 O 4 [6], MnO 2 [7], NiO [8], and TiO 2 [9].Up to now, however, GO nanocomposites based on the hybridization with rare earth oxides are relatively rare.Lanthanum oxide nanoparticles, one of the rare earth oxides, have been exploited for application in many interesting fields [10][11][12][13][14].And more recently Chen group [15] reported removal of phosphate with graphene-lanthanum composite.However, the composite was obtained in two steps: synthesizing La 2 O 3 particles and then mixing with GO to produce the composite.Hence, it is still necessary to develop new method for synthesis of La 2 O 3 /GO nanocomposites.
In this work, La 2 O 3 /GO nanocomposites were simply prepared from the commercially available LaCl 3 ⋅7H 2 O and GO in DMF without extra stabilizer in one step.And the as-prepared products were characterized by SEM, TEM, and XPS.Moreover, the nanocomposites showed excellent efficiency for methylene blue (MB), rhodamine B (Rh), and methyl orange (MO) degradation in water under ultrasound for 15 min.

Materials and Methods
Multilayer GO (5-10 layers) was purchased from Hengqiu Graphene Technology (Suzhou, China) and all other chemical reagents were of analytical grade and used as received without further purification.SEM images were obtained on Hitachi JSM-6360LV equipped with an energy dispersive Xray spectroscopy.TEM investigations were performed on JEOL JEM 1200EX.XPS were obtained on Thermo Escalab 250Xi.UV-Vis spectrum was recorded on Beijing Persee TU-1810.Sonication was performed on ShenZhen JieMeng JP-010T ultrasonic cleaner with a frequency of 40 KHz and a nominal power of 100 W.
Preparation procedure: 0.050 g multilayer GO was added to 50 mL DMF and sonicated for 1 h.Then different amount of LaCl 3 ⋅7H 2 O (0.050 g, 0.100 g, and 0.150 g) was added and the whole mixtures were stirred under reflux condition for 1 h.The synthesized nanocomposites were collected by centrifugation and washed three times with distilled water   and ethanol and then dried in a vacuum oven at 60 ∘ C overnight and labeled as GL1, GL2, and GL3, respectively.Dye degradation procedure: the GL1, GL2 and GL3 nanocomposites were dispersed in distilled water and sonicated for 30 min to give 1 mg/mL suspension, respectively.The 0.2 mL as-prepared suspensions were added to rhodamine B, methylene blue, and methyl orange solutions in water (50 mL, 10 −6 mol/L), respectively, and the all mixtures irradiated under ultrasound for 15 min.After centrifugation, the UV-vis spectra of the supernatants were recorded for calculation of dye degradation efficiency.

Results and Discussion
The La 2 O 3 /GO nanocomposites were synthesized by using LaCl 3 ⋅7H 2 O and GO in DMF under reflux condition in one step.As shown in Figure 1(a), the SEM image of GL1 showed that only a few of spherical nanoparticles could be observed on GO surface.In addition, more and larger nanoparticles could be observed in Figures 1(b) and 1(c) for GL2 and GL3, respectively, because of increasing the concentration of LaCl 3 ⋅7H 2 O.The above results indicated that particle size of La 2 O 3 on GO surface could be enlarged as increasing the concentration of La 3+ in the mixture.In addition, EDX spectra were also studied in Figure 1(d), which confirmed that lanthanum could be successfully attached on the surface of nanocomposites.
TEM images of GL2 in Figure 2 showed the morphology of the La 2 O 3 /GO nanocomposites.As shown in Figure 2(a), a few of the La 2 O 3 nanoparticles were aggregated which could be also observed in SEM images.And the average size of the most La 2 O 3 nanoparticles on GO sheets was estimated to be approximately 8 nm with a narrow size distribution of 4-19 nm.Moreover, most of the La 2 O 3 nanoparticles were found on the surface of GO sheets and almost no free nanoparticles were observed, which indicated the strong interaction between La 2 O 3 nanoparticles and GO sheets.Similarly, the same phenomena could be found in TEM images of GL1 and GL3, respectively.And in order to study the function of GO in the formation of the nanocomposites, the controlled experiment was carried out in absence of GO (Supplementary Materials (available here)).Some white solid could also be obtained in DMF without GO; however, most of the particles were aggregated according to the TEM images (Figure S1).The result suggested that GO could not only be used as a template or substrate with a large surface area for anchoring nanomaterials, but also prohibit the aggregation of nanoparticles [4].Moreover, the component of the solid was studied by XRD.According to the XRD spectra (Figure S1C), the solid may contain some La(OH) 3 , but most of the solid was complex of lanthanum with DMF.All the results indicated that the GO played a significant role in the synthesis of La 2 O 3 nanoparticles, and moreover using LaCl 3 in some organic solvent should be evaluated.
Further evidence for the La 2 O 3 /GO nanocomposites was illuminated by the XPS of the GL2.The survey XPS spectra of GL2 (Figure 3  showed the binding energy of La3d 5/2 was 838.15 eV.All the above results suggested that La 2 O 3 /GO nanocomposites were successfully synthesized in DMF.The XRD spectra of La 2 O 3 / GO nanocomposites were also studied; unfortunately, only a broad peak was observed. To investigate the application of as-synthesized La 2 O 3 / GO nanocomposites, the degradation ability for MB, Rh, and MO was studied in pure water.Figure 4 showed the UV-vis spectra of MB, Rh, and MO solutions in presence of GL1, GL2, and GL3 under ultrasound irradiation, respectively.As shown in Figures 4(a), 4(b), and 4(c), the absorbance of MB, Rh, and MO would be decreased in presence of La 2 O 3 /GO nanocomposites under ultrasound irradiation for 15 min.The decomposition may be due to the reaction of dye molecules with hydroxyl radical which is formed under the ultrasonic irradiation in the presence of La 2 O 3 /GO nanocomposites [16].And the degradation efficiency for MB, Rh, and MO was shown in Figure 4(d).The degradation efficiency for MB was about 99.6%, 98.4%, and 99.8% in presence of GL1, GL2, and GL3 after sonication for 15 min, respectively.And degradation efficiency for MO was a little bit lower (76.2%,79.7%, and 82.5%, resp.), whereas the degradation efficiency for Rh was 49.6%, 63.2%, and 59.6%, respectively.
For comparison, the degradation for MB, Rh, and MO in presence of GO was also studied.There was no obvious change in UV-vis spectra for MO (5.6% efficiency) in presence of GO under ultrasound for 15 min and the degradation efficiency for MB and Rh was only 61.7% and 21.3% in presence of GO, respectively.The above results indicated the as-synthesized La 2 O 3 /GO nanocomposites were excellent degradation material for some organic dye molecules.

Conclusions
In summary, a facile method for the synthesis of La 2 O 3 /GO nanocomposites has been developed.The La 2 O 3 /GO nanocomposites could be obtained by simply mixing LaCl 3 ⋅7H 2 O and GO in DMF under reflux condition.The method could be applied to synthesize other rare earth oxides/GO nanocomposites with various morphologies and novel properties.Furthermore, the dye degradation properties were also studied, and the as-synthesized La 2 O 3 /GO nanocomposites showed excellent degradation efficiency for some organic dye molecules.And, the explicit degradation mechanism is still being investigated.