# 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
Figure 1: The scanning electron microscopy (SEM) images of (**a**) GO, (**b,f**) GO10-MIL-101(Fe) composite, (**c,g**) GO15-MIL-101(Fe) composite, (**d,h**) GO20-MIL-101(Fe) composite and (**e**) MIL-101(Fe). Figure 2: The Attenuated total internal reflection Fourier (ATR-FTIR) spectra of (a) MIL-101(Fe), (b) GO10-MIL-101(Fe), (c) GO15-MIL-101(Fe), (d) GO20-MIL-101(Fe) and (e) GO-15-MIL-101(Fe) loaded with U(VI). The XPS measurement provided the information of elemental composition and surface binding condition of GO-MIL-101(Fe) composite. XPS survey spectra showed the typical peaks of Fe2p, C1s, O1s, and N1s in GO-MIL-101(Fe) composites and additional U4f peaks upon the uptake of uranyl ions (Figure 3a). The O auger signal (976 eV) was observed [54]. The Fe auger signals (893 eV, 841 eV, and 786 eV) and U4d\({}_{5/2}\) signal (740 eV) [55; 56] were also recognized. XPS signals at 311 eV and 554.9 eV can be assigned to the C1s and O1s satellite peaks [57; 58; 59]. The negligible observation of the characteristic signal of DMF molecules (C-N symmetric stretching at 866 cm\({}^{-1}\) and the O=C-N stretching mode at 659 cm\({}^{-1}\)) indicated the removal of residual DMF molecules in the system. The C1s spectra of the composite materials can be deconvoluted into three peaks including C-C, C-O, and C=O with the binding energy increasing. O1s spectra were composed of oxygen anion (OH\({}^{-1}\)) at 530.9 eV, C=O (carboxylic group) at 531.5 eV and C-O in aromatic rings, phenols, and ethers at 533.4 eV [60]. The analysis of the XPS data for GO-MIL-101(Fe) samples before and after the uranyl ions adsorption demonstrated the interaction between the adsorbents and U(VI). As shown in Figure 3b, GO-MIL-101(Fe)-U(VI) exhibited typical U4f XPS peak which consisted of two doublet-peaks at 382 eV (U4f\({}_{7/2}\)) and 392 eV (U4f\({}_{5/2}\)) [6]. The peak at 399.97 eV and 403.13 eV corresponded to the N1s XPS signal with the former corresponding to the amine groups on MIL-101(Fe) and the latter peak indicated the presence of N doping in the form of pyridinic N-oxide on GO. Such N doping may originate from the DMF treatment under the solvothermal condition [61]. The decreased intensity of the N1s peak in GO-MIL-101(Fe)-U in comparison with the raw sample indicated that the amino group of NH\({}_{2}\)BDC ligand contributed partial adsorption performance. The decreased intensity of C-O and C=O peaks of C1s as well as the decreased area of O1s peaks indicated the interaction between the uranyl ions and different sort of functionalized oxygen groups on the composite. The XPS analysis eVidered the synergistic effect of GO and MIL-101(Fe) in the removal of U(VI), which agrees well with the FT-IR results. Brunauer-Emmett-Teller (BET) analysis further provided the surface area and pore structure information. MIL-101(Fe) showed a mixture of type I and IV adsorption-desorption isotherms with the sharp N\({}_{2}\) uptakes at low relative pressure (P/P\({}_{0}\) < 0.1), indicating its micro/mesoporous structure (Figure S2a) [62]. The pore size distribution result (Figure S2b) indicated the existence of 2.15 nm and 2.52 nm mesocages and 1.26 nm microages in MIL-101(Fe). In contrast, GO-15-MIL-101(Fe) also exhibited characteristics of type II isotherms, which revealed that the introduction of macropores in the composite (Figure S2c). The H\({}_{3}\)-type hysteresis loops implied that the sandwich structures formed in the 2-D GO nanosheets-MIL-101 octahedrons composite gave rise to slit mesopores [47; 63]. Figure S2d further showed enlarged mesocages of \(\sim\)2.75 nm. Both isotherm observation and statistic data (Table S2) showed that GO15-MIL-101(Fe) had a lower specific surface area and larger total pore volume than that of pure MIL-101(Fe), which resulted from the formation of meso-macropores in the composite structure. The pore texture analysis illustrated that the GO15-MIL-101(Fe) composite synthesized from the in situ growth method may modulate the pore structure of both individual components, which may optimize the adsorption performance.