Ultradispersed Cobalt Ferrite Nanoparticles Assembled in Graphene Aerogel for Continuous Photo-Fenton Reaction and Enhanced Lithium Storage Performance

The Photo-Fenton reaction is an advanced technology to eliminate organic pollutants in environmental chemistry. Moreover, the conversion rate of Fe3+/Fe2+ and utilization rate of H2O2 are significant factors in Photo-Fenton reaction. In this work, we reported three dimensional (3D) hierarchical cobalt ferrite/graphene aerogels (CoFe2O4/GAs) composites by the in situ growing CoFe2O4 crystal seeds on the graphene oxide (GO) followed by the hydrothermal process. The resulting CoFe2O4/GAs composites demonstrated 3D hierarchical pore structure with mesopores (14~18 nm), macropores (50~125 nm), and a remarkable surface area (177.8 m2 g−1). These properties endowed this hybrid with the high and recyclable Photo-Fenton activity for methyl orange pollutant degradation. More importantly, the CoFe2O4/GAs composites can keep high Photo-Fenton activity in a wide pH. Besides, the CoFe2O4/GAs composites also exhibited excellent cyclic performance and good rate capability. The 3D framework can not only effectively prevent the volume expansion and aggregation of CoFe2O4 nanoparticles during the charge/discharge processes for Lithium-ion batteries (LIBs), but also shorten lithium ions and electron diffusion length in 3D pathways. These results indicated a broaden application prospect of 3D-graphene based hybrids in wastewater treatment and energy storage.


Results
The overall fabrication procedure of CoFe 2 O 4 /GAs is illustrated in Fig. 1. Firstly, iron nitrate hydrate (Fe(NO 3 ) 3 • 9H 2 O) and cobalt nitrate hydrate (Co(NO 3 ) 2 • 6H 2 O) are dissolved in the graphene oxide (GO) suspension at room temperature. During the process, positively charged Fe 3+ and Co 2+ can be absorbed to the hydroxyl and carboxyl groups on the surface of the negatively charged GO sheet by electrostatic attraction. The controllable nucleation site of CoFe 2 O 4 on the GO sheet can be realized by the addition of sodium hydroxide (NaOH) solution. That is, upon the addition of NaOH solution, the hydrolysis of Fe 3+ and Co 2+ leads to the formation of CoFe 2 O 4 crystal seeds deposited on the surface of GO sheets. This result can be confirmed by the HRTEM images of CoFe 2 O 4 / GO. As shown in Figure S1a,b, a large number of CoFe 2 O 4 crystal seeds with a size of ~3 nm are highly dispersed on the GO sheets. Thereafter, the 2D GO sheets with a uniform decoration of CoFe 2 O 4 crystal seeds self-assemble into the 3D monolithic networks during hydrothermal treatment, where reduction of GO sheets and crystallization and growth of CoFe 2 O 4 crystal seeds are simultaneously realized. Finally, the CoFe 2 O 4 /GAs composites are obtained through the lyophilization. As a control experiment, the two-dimensional (2D) CoFe 2 O 4 /reduced graphene oxide (RGO) composites are prepared by physically mixing CoFe 2 O 4 and RGO, denoted as CoFe 2 O 4 / RGO.
The morphology and microstructure of the resulting CoFe 2 O 4 /GAs composites were elucidated by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and nitrogen adsorption/ desorption analysis. As shown in Fig. 2a,b, the CoFe 2 O 4 /GAs composites show macroporous structure with well-defined interconnected pores at micrometer order. The partial overlapping or coalescence of the graphene sheet led to the physically cross-linked sites in the CoFe 2 O 4 /GAs composites. The driving force for assembly of 3D porous interconnected framework in CoFe 2 O 4 /GAs through the hydrothermal process should be ascribed to π -π interaction between graphene sheets. The FESEM images of CoFe 2 O 4 /GAs (Fig. 2c,d)   nanoparticles with a size of around 9 nm are highly dispersed on the surface of RGO sheets. It is noteworthy that some CoFe 2 O 4 nanoparticles can be encapsulated within the RGO sheets (Fig. 2d), which can effectively prevent the layer-by-layer stacking of GO sheets during the reduction process and avoid direct connect between CoFe 2 O 4 and electrolyte. The mesoporous nature of the CoFe 2 O 4 /GAs composites was confirmed by nitrogen adsorption/ desorption analysis. The adsorption data reveal a remarkably high specific surface area of 177.8 m 2 g −1 (Fig. 2e), and the pore size distribution curve indicates the presence of hierarchical porous structure (Fig. 2f). The mesoporous size is in the range of 14~18 nm, and the macroporous size is in a wide range of 50~125 nm. This result highlights that the building up of 3D-GAs by hydrothermal method is an effective way to achieve a high surface area and hierarchical porous structure for 3D graphene-based materials.
TEM and HRTEM characterizations were conducted to obtain a closer morphology and structure of the CoFe 2 O 4 /GAs composites. The low-resolution TEM image (Fig. 3a) (Fig. 3b, inset). The HRTEM image (Fig. 3c) demonstrates that the highly crystalline CoFe 2 O 4 nanoparticles are randomly distributed on two sides of RGO sheets with different contrasts. Moreover, the edge of RGO sheets can be clearly observed as indicated by the arrow (Fig. 3c)   Elemental mapping analysis of the CoFe 2 O 4 /GAs composites is performed to illustrate the distribution of carbon, cobalt, iron, and oxygen components in the composites ( Figure S2). Apparently, the carbon, cobalt, iron, and oxygen components are uniformly distributed on RGO sheets, further verifying the ultradispersed distribution of CoFe 2 O 4 nanoparticles on the surface of RGO sheets.
The XRD patterns of the as-prepared CoFe 2 O 4 /GAs depicted in Fig. 4a show diffraction peaks at 2θ = 30.1°, 35.4°, 43.1°, 57.1°, 62.7°, which correspond to the crystal indexes of (220), (311), (400), (511), and (440) plane, respectively. All the diffraction peaks are completely consistent with the peaks of commercial CoFe 2 O 4 , indicating that the CoFe 2 O 4 nanoparticles grown on the RGO sheets are well crystallized after the hydrothermal treatment. The presence of characteristic peaks in Raman spectra (Fig. 4b) also confirm the generation of highly crystallized CoFe 2 O 4 on the RGO sheets. Moreover, the diffraction (001) reflection at 2θ = 11.7° of the initial GO sheet can be observed, but no corresponding diffraction peak can be observed in the XRD patterns of CoFe 2 O 4 /GAs, indicating the reduction of GO under the hydrothermal treatment. These results suggest the reduction of GO sheets and the crystallization of CoFe 2 O 4 nanoparticles are proceed simultaneously. In addition, the obvious increasement of the intensity ratio of D/G bands through the hydrothermal process in the Raman spectra further confirms the reduction of GO (D/G ratio increases from 0.96 to 1.03, Fig. 4b). TGA measurement carried out in the air was used to determine the mass fraction of CoFe 2 O 4 in the composites. As shown in Fig. 4c, the TGA curve displays a significant loss weight at approximately 450 °C. The miniscule weight loss (< 3%) that appeared below 300 °C is most likely attributed to the evaporation of water molecules adsorbed into the 3D interconnected networks. The major weight loss from 300 to 500 °C was about 20%, indicating the combustion of RGO. Therefore, the CoFe 2 O 4 / GAs composites contained about 72% (w/w) of CoFe 2 O 4 .

Discussion
The Fenton processes for waste water treatment have attracted more attention because of the formation of hydroxyl radicals (• OH) during degradation 39 . Actually, the generated • OH radicals are highly active and nonselective, and they are able to decompose many non-biolodegradable and persistent organic compounds 40 . Iron-containing materials 41 , other transitional metals 42 , or nonmetallic materials exhibit catalytic activity for the Fenton reaction. In addition, electro-, sono-, photo-assisted Fenton reaction, or to say, an integration technology, have been widely studied as well 43 .
In this study, Photo-Fenton reactions are conducted for methyl orange (MO 10 mg/L) degradation to test the activity of CoFe 2 O 4 /GAs. The hydrochloric acid (HCl 0.1 M) is used to adjust the pH value of the reaction system. The reaction is proceeded under the illumination of a 300 W Xenon lamp by an AM 1.5 G solar simulator. It is noteworthy, on the other hand, to highlight the fact that the CoFe 2 O 4 /GAs composites were grinded to powders in order to increase their contact area with the H 2 O 2 molecules during the Photo-Fenton reaction, thereby improving the utilization efficiency of H 2 O 2 . As shown in Fig. 5a, the CoFe 2 O 4 /GAs composites in the dark show superior adsorption capacity in the first cycle test and all the MO molecules are absorbed in 1 min. Thereafter, the adsorption capacity gradually decreased after 5 cycles, but 65% of the MO molecules can still be adsorbed in 30 min, which reveals the good adsorption capacity of CoFe 2 O 4 /GAs. With the addition of H 2 O 2 in the dark, the decrement of MO content is caused by the adsorption and Fenton-like reaction. However, the Fenton-like reaction activity still decreased after 5 cycles, which suggests that the conversion efficiency of Fe 3+ /Fe 2+ in the Fenton-like reaction without the aid of light is very low. So we introduce light into the Fenton-like reaction. As shown in Fig. 5a, the activity with photo-assisted has been improved greatly. Importantly, the activity keeps almost unchanged after 5 cycles, indicating the high conversion efficiency of Fe 3+ /Fe 2+ . For comparsion, pure CoFe 2 O 4 nanoparticles are prepared and keep a good dispersed state ( Figure S3). Seen from Fig. 4a, pure CoFe 2 O 4 shows decreased Photo-Fenton activity after 5 cycles due to low conversion efficiency of Fe 3+ /Fe 2+ and leaching of Fe 2+ . Furthermore, we used 1, 10-phenanthroline monohydrate (Phen) as a testing Fe 2+ reagent to detect the leaching of Fe 2+ ( Figure S4). The Fe 2+ ions can react with the Phen to generate a strong visible absorption signal. After adding with Phen, the reaction solution of the CoFe 2 O 4 powders gives a strong visible absorption signal, but the reaction solution of CoFe 2 O 4 /GAs gives a very low visible absorption signal, which indicates the leaching of Fe 2+ ions in the aqueous solution is low. To further highlight the structure stability of CoFe 2 O 4 /GAs, we observe the morphology of the catalyst after 5 cycles. As shown in Figure S5, all the CoFe 2 O 4 particles are still ultra-dispersed on the surface of RGO sheets ( Figure S5a,b) and the 3D porous structure can be observed clearly ( Figure S5c,d), which further reveals the high stability of structures. Figure S6 shows ferromagnetic property of the as-prepared CoFe 2 O 4 /GAs composites, suggesting that such composites might be easily separated from solution phase through inducing an external magnetic field.
The pH of the solution plays a key role in Photo-Fenton degradation of pollutants 44 . The MO solution can be degraded with CoFe 2 O 4 /GAs within pH 3.5-9 (Fig. 5b). In order to excluding the strong adsorption of MO (Fig. 5a), we conducted cycle tests and selected the data of the third cycle test of CoFe 2 O 4 /GAs under different pH. It can be observed that the degradation rate decreases a little when pH is increased from 3.5 to 9, which is in good agreement with the previous reports 30,45 . When pH is adjusted to 9, the Photo-Fenton degradation rate is up to 78% in 30 min. In addition, the H 2 O 2 concentration on the rate of degradation of MO was also investigated by varying the H 2 O 2 concentration from 25 to 150 mM (Fig. 5c). We also conducted cycle tests and selected the data of the third cycle test of CoFe 2 O 4 /GAs under different H 2 O 2 concentration. Figure 5c   The photogenerated electrons are quickly trapped by graphene (Eq. (2)), limiting the recombination of holes and electrons. At the same time, the photogenerated holes (h + ) are subsequently trapped by OH-to produce ▪ OH radicals. The electrons trapped by graphene can be used to reduce Fe 3+ to form Fe 2+ (Eq. (3)). The Fe 2+ can react with H 2 O 2 to form ⋅OH radical and Fe 3+ (Eq. (4)) 30 . The generated Fe 3+ can be reduced to Fe 2+ again by the electron concentrated on the surface of RGO sheets to keep the cycle of Fe 3+ /Fe 2+ , thus achieving the high Photo-Fenton activity. On the other hand, the lithium-insertion/extraction properties of the CoFe 2 O 4 /GAs composites as anode material were investigated by galvanostatic charge/discharge measurements over a voltage range of 0.01-3.0 V. Figure 6a shows the charge/discharge curve of CoFe 2 O 4 /GAs at a current density of 0.1 A g −1 . In the first discharge step, the CoFe 2 O 4 /GAs composites present an extended/long voltage plateau at about 0.8 V, followed by a sloping curve down to the cut off voltage of 0.01 V, which is a typical characteristic of voltage trend for the CoFe 2 O 4 electrode 31,46 . A high initial reversible capacity of 1905 mA h g −1 can be derived in the first discharge step, with a corresponding charge capacity of 1037 mA h g −1 based on the weight of the CoFe 2 O 4 /GAs composites. The initial capacity loss can be probably associated with the formation of solid electrolyte interphase (SEI) layer on the surface of electrode in the first discharge step 47 . After 20 charge/discharge cycles, a high capacity of 830 mA h g −1 can still be retained. For comparsion, the mechanically mixed CoFe 2 O 4 /RGO composites were prepared ( Figure S7). The mechanically mixed CoFe 2 O 4 /RGO composites demonstrate a relatively low capacity of 1772 mA h g −1 , and the capacity decreases rapidly to 366 mA h g −1 after 20 charge/discharge cycles (Fig. 6b). In addition, the cycling performance of the CoFe 2 O 4 /GAs composites is greatly superior to that of the mechanically mixed CoFe 2 O 4 /RGO (Fig. 6c). The capacity of CoFe 2 O 4 /GAs is very stable at the current density of 0.1 A g −1 and the high reversible capacity of 830 mA h g −1 is still retained after 50 cycles, while the capacity of CoFe 2 O 4 /RGO rapidly decays from 1424 to 350 mA h g −1 . The rate performances of CoFe 2 O 4 /GAs at the current rates of 0.1~2.0 A g −1 are depicted in Fig. 6d. Reversible capacity are retained at 602 mA h g −1 and 500 mA h g −1 at 0.5 A g −1 and 1.0 A g −1 , respectively. In order to further highlight advantage of CoFe 2 O 4 /GAs, we synthesized pure CoFe 2 O 4 ( Figure S3) and GAs ( Figure S8) and tested their LIBs performance, respectively ( Figure S9). The cycle stability of these three materials is given in Figure S9a. It can be observed that pure CoFe 2 O 4 showed the low Li + storage ability and bad stability due to the volume expansion and contraction associated with Li + insertion/extraction during the charge/discharge processes. The GAs electrode gives an initial charge capacity of only 307 mA h g −1 , much lower than that of CoFe 2 O 4 /GAs at the same current density and also lower than its theoretical value (372 mA h g −1 ). The rate capability of CoFe 2 O 4 /GAs, pure CoFe 2 O 4 and GAs is compared in Figure S9b. Compared with pure CoFe 2 O 4 and GAs, the CoFe 2 O 4 /GAs composites demonstrate a remarkably improved rate capability. The charge capacities of CoFe 2 O 4 /GAs at 0.1, 0.2, 0.5, 1.0, 2.0 A g −1 are 830, 710, 602, 500 and 340 mA h g −1 , respectively, greatly higher than those of bare pure CoFe 2 O 4 and GAs.
The outstanding electrochemical behavior of CoFe 2 O 4 /GAs with high capacity, stable cycle performance and excellent rate capacity, can be assigned to the following factors: (1) the unique 3D interconnected structure of CoFe 2 O 4 /GAs, which consists of macro-and mesopores on the graphene network, can effectively reduce the diffusion length for both electron and Li + ions and provide multidimensional routes to facilitate the transport of electrons in the bulk electrode. (2) The large surface area of CoFe 2 O 4 /GAs can greatly improve ion adsorption for Li + ions insertion/extraction during the charge/discharge process. (3) The strong coupling effect between CoFe 2 O 4 and GAs can prevent large volume expansion/contraction and aggregation of CoFe 2 O 4 nanoparticles associated with Li + ions insertion/extraction during the discharge/charge process.
In conclusion, we have fabricated the CoFe 2 O 4 /GAs composites through a facile and cost-efficient hydrothermal self-assembly and freeze-drying two-step strategy. The generation of CoFe 2 O 4 nanoparticles is accompanied with the reduction of GO under the hydrothermal condition and the obtained CoFe 2 O 4 nanoparticles with diameters focused on around 9 nm are ultra-dispersed on the surface of RGO sheets. The CoFe 2 O 4 /GAs composites exhibit the superior Photo-Fenton activity for the degradation of MO in an aqueous system due to improved adsorption toward pollutants and high conversion efficiency of Fe 3+ /Fe 2+ . In addition, the magnetic recyclable usability of the CoFe 2 O 4 /GAs composites demonstrates over many successive reaction cycles. Besides of the promising application in Photo-Fenton reaction, the composites show excellent lithium storage performance with high reversible capacity and remarkable cyclic retention at each current density when used the anode material in LIBs. We believe that such multifunctional composites will have many potential practical applications in the environmental protection and energy development. It is also expected that the involved preparation method can be easily adapted and extended as a general approach to other systems for the preparation of highly dispersed nanoparticles on graphene aerogels.

Synthesis of Graphene Oxide (GO).
Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers method 48 . Typically, 2 g graphite powders were added into a mixture of 50 mL H 2 SO 4 and 1 g NaNO 3 . The solution was kept at 5 °C in an ice bath under vigorous stirring for 2 h. Thereafter, 6 g KMnO 4 was added slowly into the mixture while the temperature was kept from exceeding 5 °C, then the temperature of the system was heated up to 35 °C and maintained for 2 h. Afterwards, 80 mL of water was slowly added and then the mixture was heated to 98 °C for 1 h. 280 mL of water and 80 mL of 30% H 2 O 2 were added to end the reaction, followed by 5% HCl and filtration. Finally, the wet graphene oxide was freeze-dried at − 60 °C for 24 h.
Synthesis of the CoFe 2 O 4 /GAs composites. In a typical experiment, 75 mg GO powders were dispersed a mixed solvent containing 75 mL ethanol and 25 mL acetonitrile in an ultrasound bath for 90 min. Thereafter, 0.48 g Fe(NO 3 ) 3 ·9H 2 O and 0.173 g Co(NO 3 ) 2 ·6H 2 O were added into the solution under the stirring for 1 h, then 1 mL of NaOH (0.1 M) solution was added into the above solution while stirring. After stirring for 1 h, the suspension was centrifuged and washed with ethanol and water. The as-prepared product was re-dispersed in 25 mL of water followed by an ultrasonic treatment, which was then transferred into a 50 mL autoclave, and kept at 180 °C for 12 h. The aerogels was treated by freeze-drying to obtain a three-dimensional CoFe 2 O 4 /GAs composites. As a control experiment, two-dimensional (2D) CoFe 2 O 4 /reduced graphene oxide (RGO) composites were prepared by physically mixing CoFe 2 O 4 and RGO. With the absence of GO, the pure CoFe 2 O 4 nanoparticles were prepared by the similar method of preparation of CoFe 2 O 4 /GAs. Pure GAs were prepared by hydrothermal treatment of GO solution.
Characterization. X-ray diffraction (XRD) patterns of all samples were collected in the range 10-80° (2θ ) using a RigakuD/MAX 2550 diffract meter (Cu K radiation, λ = 1.5406 Å), operated at 40 kV and 100 mA. The morphologies were characterized by transmission electron microscopy (TEM, JEM2000EX). The particle size distribution curve was derived from 100 CoFe 2 O 4 nanoparticles. The surface morphologies were observed by scanning electron microscopy (TESCAN nova Ш) and field emission scanning electron microscopy (FESEM, NOVA NanoSEM450). Raman measurements were performed at room temperature using Raman microscopes (Renishaw, UK) under the excitation wavelength of 532 nm. BET surface area measurements were carried out by N 2 adsorption at 77 K using an ASAP2020 instrument. Thermogravimetric and differential thermal analyses were conducted on a Pyris Diamond TG/DTA (PerkinElmer) apparatus at a heating rate of 20 K min −1 from 40 to 800 °C in air flow.
Photo-Fenton Reaction. The photocatalytic activity of each catalyst was evaluated by in terms of the degradation of methyl-orange (MO, 10 mg/L). The CoFe 2 O 4 /GAs powders were added into a 100 mL quartz reactor containing 75 mL MO solution. Prior to reaction, the initial pH value of the MO solution was adjusted to a certain pH value with 0.1 M HCl or 0.1 M NH 3 . Fenton reaction was initiated by adding a known concentration of H 2 O 2 (a certain volume value, 30 wt %) to the solution. A 300 W Xe lamp (with AM 1.5 air mass filter) was used as a simulated solar light source. At the given time intervals, the analytical samples were taken from the mixture and immediately centrifuged before filtration through a 0.22 μ m millipore filter to remove the photocatalysts. The filtrates were analyzed by recording variations in the absorption in UV-vis spectra of MO using a Cary 100 ultraviolet visible spectrometer. The leaching of Fe ions during reaction was analyzed using a Cary 100 ultraviolet visible spectrometer. In detail, a certain amount of solution was taken from the Photo-Fenton system. Next, a centrifuge separated the supernatant from the solution. And then, 1 mL 1, 10-phenanthroline monohydrate (0.5 wt%) as a testing Fe 2+ reagent were added into 3 mL supernatant. After 15 minutes' standing, the levels of ferrous iron were examined by using a Cary 100 ultraviolet visible spectrometer. Electrochemical Measurements. The electrochemical experiments were performed in coin-type cells.
The working electrodes were prepared by mixing the hybrids, carbon black (Super-P), and poly-(vinyl difluoride) (PVDF) at a weight ratio of 80:10:10 to form slurry in N-methyl-2-pyrrolidinone (NMP), which was coated onto a copper foil (99.6%). Pure lithium foils were used as counter and reference electrodes. The electrolyte was consisted of a solution of LiPF 6 (1 M) in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in weight percent). The cells were assembled in an Ar-filled glove box with the concentrations of moisture and oxygen below 1 ppm. The electrochemical performance was tested on a LAND CT2001A battery test system in the voltage range of 0.01-3.00 V versus Li + /Li at room temperature.