# Facile Preparation of Graphene Oxide-MIL-101(Fe) Composite for the Efficient Capture of Uranium
## 3 Results and Discussion  
### Characterization of GO-MIL-101(Fe) Composite  
As shown in Figure 1a, pure GO presented a two-dimensional morphology with lateral size up to tens of micrometers. GO-MIL-101(Fe) composites were fabricated through a facile one-pot in situ growth method. The component ratio was adjusted by tuning the addition amount of GO in the synthetic precursor solution. Figure 1b-d confirmed the successful growth of nanosized MIL-101(Fe) octahedrons on GO nanosheets. The negligible dispersed MIL-101(Fe) indicated that most MIL-101(Fe) nanoparticles formed on the GO matrix. Moreover, MIL-101 octahedrons were well distributed on both sides of the GO nanoplatelets, which guaranteed the synergistic interaction between individual compositions. Notably, the dimension of MIL-101(Fe) octahedrons in the composite decreased with the amount of GO increasing (460 nm for GO10-MIL-101(Fe), 350 nm for GO15-MIL-101(Fe), and 150 nm for GO20-MIL-101(Fe)). It was reported that low concentrations of a monocarboxylic acid with different pKa values (1.60-10.15) could modulate the crystal growth of MIL-101(Cr), with higher pKa leading to smaller nanoparticles [45]. GO synthesized from Hummers method possessed many oxygenated groups like carboxylic (pKa = 4.3 and 6.6) and phenolic groups (pKa = 9.8) [46]. Therefore, both carboxylic groups and phenolic groups gave rise to the controlling of the size of MIL-101 in GO15-MIL-101(Fe) composite. Considering that most MIL-101 octahedrons nucleated on the surface of GO, phenolic groups with especially high pKa played a dominant role. We postulated that GO interfered with the nucleation of MIL-101(Fe) which led to smaller sized nanoparticles with the addition amount of GO increasing. As shown in Figure S1, all the XRD patterns of the composite materials showed peaks at 20 = 5.88\({}^{\circ}\) and 9.08\({}^{\circ}\), which can be assigned to typical (531), (911) diffractions of MIL-101(Fe), confirming the successful synthesis of the composite sample [47; 48]. However, the decreased peak intensity in GO20-MIL-101(Fe) indicated the reduction of MOF crystallinity, which was also reflected from the loss of the octahedron morphology. ATR-FTIR spectroscopy was used to identify the functional groups of the GO-MIL-101(Fe) composite. The broad band in the range of 3000 cm\({}^{-1}\)\(\sim\)3500 cm\({}^{-1}\) originated from the hydroxyl groups of absorbed water molecules. The IR signals at 3475 cm\({}^{-1}\) and 3345 cm\({}^{-1}\) in Figure 2a corresponded to the asymmetric and symmetric N-H stretching vibrations in amino groups of MIL-101(Fe) [49]. Because of the low concentration of MIL-101(Fe) in the composite, these peaks became negligible in Figure 2b,c. For the sample MIL-101, FT-IR signals at 1575 cm\({}^{-1}\) and 1429 cm\({}^{-1}\) were asymmetric and symmetric stretching of carboxylic groups in the organic ligand NH\({}_{2}\)BDC, respectively [50; 51]. The peak at \(\sim\)1379 cm\({}^{-1}\) obviously indicated the effect of the amino groups on the symmetric stretching of carboxylic groups. C-N-stretching vibrations were clearly recognized at 1257 cm\({}^{-1}\) in IR spectrum of MIL-101(Fe) [40]. The spectrum peaks at 1501 cm\({}^{-1}\) and 767 cm\({}^{-1}\) can be attributed to the stretching vibration (C=C) and deformation vibration (C-H) of benzene, respectively [52]. As shown in Figure 2b, the asymmetric stretching of O=C-O, C-N stretching, and C-H deformation vibration were suppressed, which was more obvious with the GO increasing. This result implied the modulation of GO towards the growth of MIL-101(Fe). The comparison of FTIR spectra between GO15-MIL-101(Fe) and that loaded with uranyl ions can be used to deduce the uranyl ions-composite interaction. As shown in Figure 2c, the peak at \(\sim\)920 cm\({}^{-1}\) can be assigned to the stretching vibration of the linear structure of the UO\({}_{2}\)\({}^{2+}\) group in the U(VI)-loaded materials [53].