Simultaneous enhanced electrochemical and photoelectrochemical properties of α-Fe2O3/graphene by hydrogen annealing

Two-dimensional (2D) materials have attracted extensive attention in various fields due to their good flexibility, high specific surface area and fast ion transfer rate. Herein, the α-Fe2O3 nanoparticles/graphene composites have been prepared through a hydrothermal method, which is followed by hydrogen annealing to produce oxygen vacancies. Electrochemical properties and photoelectrochemical properties are investigated. The results present a high specific capacitance and the electrochemical properties are improved ascribing to the synergistic effect of graphene’s large specific surface area and excellent electrical conductivity, as well as the pseudocapacitance of transition metal oxides. Moreover, it also promotes the photoelectric response performances in visible light. This work provides a basis for the development and application of transition metal oxide in electrochemical and photoelectric fields.


Introduction
In recent decades, the development and utilization of new energy has become a major research trend. The energy storage devices and photoelectric conversion devices have been widely studied to use for the need of energy storage and conversion [1][2][3][4][5]. Supercapacitors are one of the most promising candidates, which benefit from their high-power density, fast charging rate, long cycle life, environmental protection and safety. The electrode material have attracted extensive attentions as an important component of capacitors. However, it is difficult to apply high-performance matrix materials on a large scale because of low reserves and high production cost. Therefore, it is emergency and important to develop a low-cost and affordable electrode material to meet the need for electrochemical energy storage devices is desired.
Among them, the development of 2D nanomaterials has received a sharp increase in attention [29][30][31][32][33][34]. Represented by graphene is a single layer of carbon atoms, a hexagonal honeycomb structure made up of sp 2 hybrid orbitals [35,36], which has a unique layered structure and excellent physical and chemical properties Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. [37][38][39][40]. Typically, graphene is often used in double-layer capacitors according to the adsorption/desorption mechanism. Unfortunately, several factors have limited its performance gains, such as the fact that the surface of a material is not easy soaked by the electrolyte; and the stronger van der Waals force between sheets is easily causing agglomeration of graphene sheets [41,42]. Therefore, combining graphene with metal oxide particles to form spacers between the layers not only prevents the layers from stacking but also further improves their performance due to synergistic effects [36,[43][44][45]. Before that, it has been reported that different shapes of iron oxide and graphene compound, showing the expected excellent properties. Wang et al [46] prepared singlecrystalline α-Fe 2 O 3 /rGO composite aerogel with a specific capacitance of up to 908 F g −1 at a current density of 2 A g −1 . Wu et al [47] reported Fe 2 O 3 on rGO nanosheets to obtained Fe 2 O 3 /graphene composite by a one-step hydrothermal method with a specific capacitance of 1083 F g −1 at a current density of 2 A g −1 . Bin Xiang et al [48] synthesized 2D nanometer round α-Fe 2 O 3 /rGO composites with an excellent specific capacitance of 621.3 F g −1 at 5 mV s −1 and 533 F g −1 at 1 A g −1 by a simple and inexpensive hydrothermal process. These higher specific capacitance of the samples are due to the fact that the introduction of graphene into metal oxides can accelerate the electron migration rates, increase the active sites and reduces the agglomeration of the α-Fe 2 O 3 nanoparticles [49].
Moreover, the defect structure of metal oxide materials has an important influence on its physical and chemical properties. The formation energy of oxygen vacancy is much lower and easier to achieve than that of metal atom vacancy. In addition, oxygen vacancy can promote electrochemical charge transfer by generating gap electron states, so it is an important condition for impurity diffusion and semiconducting in crystals [50][51][52][53][54]55]. In general, oxygen vacancy formation usually requires reductive reagents, hydrogenation treatment, and hightemperature treatment. The induced defects will affect the geometric structure, electronic structure and chemical properties of metal oxides, thus affecting the properties of materials.
Herein, the synthesis of rGO/α-Fe 2 O 3 composite by hydrothermal method has good properties, which not only solves the agglomeration of α-Fe 2 O 3 but also avoids the interlayer stacking of rGO. Moreover, the nanocomposites with oxygen vacancy were prepared by hydrogen annealing. As expected, these unique features endow rGO/α-Fe 2 O 3 composite a better crystal structure and a faster electron transfer rate, exhibiting an excellent electrochemical and photoelectric property.

Preparation of GO
All the chemical reagents are analytical grade in the experiment and without further purification. The graphene (rGO) was exfoliated from natural graphite using a modified hummer method in this experiment. The experimental process is as follows: graphite powder (1.5 g) was immersed in a beaker with cold concentrated sulfuric acid (H 2 SO 4 ) (50.7 ml). Then, sodium nitrate (1.14 g) was added to the mixture solution while stirring for 30 min KMnO 4 (6 g) was added slowly while stirring for 2 h. After 3 days, 5% H 2 SO 4 solution (150 ml) was slowly added and the reaction was finally terminated by the addition of 30% H 2 O 2 (4.5 ml). After which the color of the solution change to bright yellow. 10% HCl (200 ml) was added to the mixture to remove the metal ions. Finally, the mixture was washed with plenty of deionized water until PH=6, and then freeze-dry the precipitate to obtain GO.

Preparation of α-Fe 2 O 3 /rGO
The α-Fe 2 O 3 /graphene was synthesized by a one-step hydrothermal method. As shown in figure 1, GO (40 mg) was first added to ethanol (40 ml) and dispersed by ultrasound for 2 h. Then, 1.352 g FeCl 3 ·6H 2 O was slowly added to the above GO solution, and a trace amount of deionized water was added and vigorously stirred for 30 min. Subsequently, add 4 g of sodium acetate while stirring and continue to stir for 1 h. After that, the mixed solution was transferred to the reaction kettle (100 mL) and the hydrothermal reaction was conducted for 24 h at 180°C. After the solution was cooled to room temperature, it was centrifuge washed for 3-4 times with deionized water and ethanol, respectively. The collected precipitation was dried in vacuum at 60°C for 10 h to obtain nanopowder of composite materials. The obtained sample was α-Fe 2 O 3 /rGO composite, denoted as GFC4. Other samples were prepared by the same process, with no GO added and with GO of different content (20 mg, 60 mg, 80 mg) denoted as Fe 2 O 3 , GFC2, GFC6, GFC8. In addition, GFC4 was treated by hydrogenated annealed for 3 h at 300°C with a hydrogen ratio of 2%.

Characterization
The structure and morphology of the prepared α-Fe 2 O 3 and its composites were characterized by x-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM, Quanta 250 FEG). XRD was used for the Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.5418 Å). The microstructure of the samples was further characterized by transmission electron microscopy (TEM, JEOL JEM-2100). Nitrogen adsorption-desorption isotherms were obtained at 77 K using a micromeritics ASAP 2020 analyzer. The specific surface area (SBET) was evaluated using the Brunauer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA) was performed on SDT Q600 under N 2 from room temperature to 900°C at 10°C min −1 . X-ray photoelectron spectroscopic (XPS) was measured using a Thermo Scientific ESCALAB 250 Xi using Al Kα radiation (USA). The UV-visible diffuse reflectance spectrums were measured by a UV-visible spectrophotometer.

Electrochemical performance test
The measuring system is a three-electrode measuring system, and the instrument used is an electrochemical workstation (Shanghai chenhua). The sample electrode was mixed evenly according to the proportion of active material 80%, conductive acetylene 10% and polytetrafluoroethylene (PTFE) 10%, and then anhydrous ethanol was added as a solvent to form a paste. Finally, it was uniformly applied on conductive foam nickel with an area of 1 cm 2 . The electrode was dried in a vacuum drying box at 50°C for 6 h and then press tightly with the tablet press to prevent the sample from falling off during the test. The electrode was immersed in the electrolyte for 8 h before the test. The prepared electrode was used as the working electrode, the reference electrode was a saturated calomel electrode, and the counter electrode was platinum plate. Electrochemical tests were performed in 6 mol KOH electrolyte solution, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD, voltage window from 0.2-0.6), and impedance spectra (EIS, 100 kHz to 0.01 Hz). The mass-specific capacitance is calculated by formula: where I is the charge-discharge current, Δt is the discharge time, ΔV is voltage window, m is the mass of the active material.

Photoelectric performance test
The photoelectric properties of α-Fe 2 O 3 and other composite sample electrodes were tested on an electrochemical workstation using a xenon lamp (CHF-XM-500W) as a simulate of sunlight. The electrolyte was 0.1 M Na 2 SO 4 aqueous solution. Each sample was sonically dissolved in an ethanol solvent at a concentration of 1 mg ml −1 , and then the same amount of solution was dripped through the pipette onto conductive glass (Indium tin oxide ITO) with an area of 1 cm 2 . Finally, they were dried in a vacuum oven at 50°C for 6 h, and the I-T curves and linear sweep voltammetry (LSV) were tested. The prepared electrode, platinum foil, and Hg/HgO were used as working, counter and reference electrodes, respectively.

Structure and morphology analysis
The hydrothermal process of one-step preparation of α-Fe 2 O 3 /rGO composite materials is illustrated in figure 1. In the hydrothermal process, the Fe 3+ cations from FeCl 3 react in solution to form FeOOH, which is attached to the surface of GO. Then, the hydrophilic functional groups on the surface of GO are partially removed during the reduction process, and GO is converted to rGO. After that, the FeOOH on the surface of rGO gradually decomposes into Fe 2 O 3 [46]. Detailed steps are in the experiment section. XRD patterns of the structure and phase characteristics of 2D GFC, rGO and Fe 2 O 3 are shown in figure 2(a). The result shows that no extra diffraction peaks of possible impurity compared with the pristine sample. All the diffraction peaks are consistent with the initial phase of α-Fe 2 O 3 (space group: R3c), which are corresponds well with the standard card (JCPDS no. 33-0664). This happens when the graphene lamellar structure collapses or is regularly distributed [56]. According to the analysis of the morphology of the corresponding samples, this phenomenon is caused by the high crystallization of α-Fe 2 O 3 and the uniform dispersion on the surface of graphene. It is noteworthy that the peak (002) of graphene at the position of 26°indicates that GO can be reduced to rGO by hydrothermal reaction [57]. The XRD pattern from GO to rGO exhibits the GO has been successfully reduced (the right in figure 2(b)).
To determine the loading amount of Fe 2 O 3 in the Fe 2 O 3 /rGO composites, TGA measurement of GO and GFC4 was carried out in air conditions (figure 2(c)). After the product had been calcined at 900°C in air, the graphene content is shown in the illustration to be almost zero. It can be seen that the curve decline of the sample can be divided into two stages. The first stage is due to the absorbed water and impurities, and the second stage is the mass loss of graphene. The content of Fe 2 O 3 in Fe 2 O 3 /rGO composites was calculated to be 83.02%. In addition, the specific surface area of the GFC4 samples are characterized by nitrogen adsorption-desorption isotherms measurements. As shown in figure 2(d), the typical type-IV isotherm of GFC4 indicates the presence of mesoporous structures. The BET specific surface area of GFC4 is larger than Fe 2 O 3 powder values in the references [46,58,59]. This result demonstrates the advantages of the present rGO decorated route to produce large accessible surface area and prevent the restacking of graphene sheets. Figure 3 shows the morphologies and sizes of the α-Fe 2 O 3 and its composites characterized by FESEM. It can be seen from figure 3(a) that α-Fe 2 O 3 shows a hexagonal structure of uniform size. The high magnification image of α-Fe 2 O 3 /rGO composite indicates that the α-Fe 2 O 3 is firmly confined to the rGO nanosheet, with an average size of about 50-60 nm and evenly distributed. This prevents the agglomeration of α-Fe 2 O 3 itself and the stacking between the rGO layers. In addition, the dispersion of α-Fe 2 O 3 nanoparticles on rGO becomes more and more sparse with the increases of rGO content (figures 3(b)-(e)), which may have a certain influence on the performance of the electrode. In figure 4(a), the microstructure of the GFC4 after hydrogen annealing shows no significantly change. Furthermore, the microstructure of the GFC 4 after hydrogenation is employed by TEM. Typical regular lattice fringes in the selected region are shown in figure 4(b), and the d-spacing is 0.57 nm, corresponding to the d spacing of (110) planes [46]. The oxygen vacancies in semiconductor play key roles in promoting photoelectricity activity. X-ray photoelectron spectroscopy (XPS) characterization was applied to reveal the valence of cations and the existence of oxygen vacancies on GFC 4 before and after hydrogen annealing, respectively. The Fe atom has a doublet spectrum corresponding to Fe 2p 3/2 and Fe 2p 1/2 states as shown in figures 5(a), (c). The 2p 3/2 spectra of Fe atom can be described as the superposition of two peaks by Gaussian distribution, which refers to the co-existence of both Fe 3+ and Fe 2+ ions of binding energies 711.4 and 710.7 eV, respectively [52,60]. Meanwhile, the O 1 s spectra (figures 5(b), (d)) can be divided into two characteristic peaks of Fe-O (530.2 eV O I ), defect sites with a low oxygen coordination (531.6 eV O II ) [61]. The higher O II value of GFC4 after hydrogen annealing indicated that the composite possessed more oxygen vacancies [54].
Fe e Fe 4 3 2 Equations (1) and (2) show that the oxygen vacancies are responsible to create conduction electrons, and equation (3) shows the multivalent Fe-ions also could be key the factor to promote conduction process [60].

Electrochemical properties
The electrochemical properties of sample electrodes with α-Fe 2 O 3 and its different graphene composite proportions are measured in the three-electrode system. The comparison of CV curves at a scan rate of 50 mV s −1 are plotted in figure 6(a). It can be seen that all the samples show a pair of obvious redox peaks, indicating that the charge storage mechanism of the electrode belongs to the Faraday battery generated by the valence state change of Fe 2+ /Fe 3+ . Among them, the area enclosed by the CV curve of electrode GFC4 is largest, which indicates that the electrode has the highest specific capacitance. It reveals that adding the appropriate amount of graphene can increase the storage performance of the material, which takes advantage of the synergistic effect between the pseudo-capacitance property of α-Fe 2 O 3 and the double-layer properties of rGO. The GCD curves of the different samples electrode at the current density of 1 A g −1 are shown in figure 6(b). According to the calculation of formula (1), the specific capacitance of the electrode are 420, 537.4, 551.5, 436.6 and 276.1 F g −1 (corresponding to α-Fe 2 O 3 , GFC2, GFC4, GFC6, GFC8), respectively. Meanwhile, the rate properties of the electrode at different current densities (1, 2, 3, 5, 10 A g −1 ) indicate that the composite materials have better overall specific capacitance when doped with a small amount of graphene, but the performance will decline when the excess is added. This is because excess graphene can lead to a re-stacking between the sheets, reducing the pore structure and surface area. The results show that the high concentration of rGO will affect the nanostructures of the material, which does not favor the diffusion of electrolyte in the hole and cannot provide a good channel for electrolyte ion conduction, and reduces the storage space of ions. In addition, EIS characterization was used to deeper understand the electrodynamics of the electrode material, and the Nyquist plots of each sample are shown in figure 6(d). The illustration is an electrochemical equivalent circuit and a local magnification of the high frequency region. Where the distance of the curve from the origin on the X-axis represents the intrinsic resistance of the active material (R s ), and a smaller the intercept reveals a smaller resistance. The arc radius of the high-frequency region represents the sum of the contact resistance between the active material and the collector and the charge transfer contact resistance (R ct ) between the electrode and electrolyte. The low-frequency region is the resistance of ion movement from the electrolyte to the electrode/electrolyte interface, which is the Warburg impedance (W 0 ) caused by ion diffusion [62], and a larger the slope of the line represents a smaller W 0 . Therefore, the GFC4 electrode has relatively small resistance and high conductivity. The results demonstrates that adding appropriate amount of graphene can promote the utilization rate and reaction kinetics of the active substances, and is beneficial to improve the performance of electrode material. Figure 7 shows the comparison of electrochemical properties of α-Fe 2 O 3 , GFC4 and GFC4 after hydrogen annealing. It is worth noting that hydrotreating does not significantly promote the discharge time of the electrode at low current density. (figures 7(a)-(b)). However, the specific capacitance of GFC4 at high current density is improved compared with that before hydrogenation (figure 7(c)), and the clearer values are shown in table 1. Meanwhile, the cyclic stability tests of 8000 times at 10 A g −1 also indicates that hydrogenation annealing improved the stability of the material and had superior performance ( figure 7(d)). This result is attributed to the faster diffusion rate of the ion after hydrogen annealing, due to its higher crystallinity, and the formation of oxygen vacancy. Figures 7(e)-(f) shows the CV and GCD curves of GFC4 after hydrogen annealing electrode at different scanning rates. The shapes change of the curves in the figure is not obvious, which indicates that the electrode material has good stability and reversibility. Therefore, the α-Fe 2 O 3 /rGO composite can provide superior electrochemical performance. It can be inferred that the photoelectric response performance of the sample will increase after hydrogen annealing.

The photoelectrochemical performance
Considering that the semiconductor properties of Fe 2 O 3 are more sensitive to light, the photoelectric properties of samples were studied under the light in 0.5 M Na 2 SO 4 . Figure 8(a) presents a typical transient current diagram (I-T) under chopped illumination, which shows a good switching behavior and uniform response when the lights are on and off. It shows the photocurrent of the GFC4 is highest than the pristine α-Fe 2 O 3 , and other composites. Furthermore, the linear sweep voltammetry (LSV) curves at voltage from −1.75 to 0 V for the rGO/α-Fe 2 O 3 composites and that of the pristine α-Fe 2 O 3 are shown in figure 8(b). The results show that the electrical rectification behavior of samples is obviously, and the performance improvement is consistent with the trend of I-T. Figure 9(a) shows the impedance spectrum of the samples before and after hydrogen annealing. The resistance of the sample is decreased after hydrotreatment. This provides a good basis for the improvement of photoelectric performance. The test results in figures 9(c)-(d) verify the improvement of conductivity and confirm the relationship between defect and photoelectric performance. The UV-vis diffuse reflectance spectra show the absorption of light from α-Fe 2 O 3 , GFC4 and GFC4 samples after hydrogen annealing. It is well known that semiconductors have a wide band, and the threshold is set according to its bandgap. The absorption threshold of α-Fe 2 O 3 is about 590 nm, and the absorption threshold of the sample formed after recombination with graphene in figure 9(b) is about 600 nm, which is consistent with the bandgap of α-Fe 2 O 3 . However, due to the zero-band gap of graphene, the absorption intensity of composite in the visible region increases significantly. This is because the electron transfer between the energy bands of the composite nanocrystals and the oxygen  vacancy generated internally change the electron density in the crystals, ensuring good electron and ion transport mechanics. The complex theoretical mechanism of the electron-hole migration theory of rGO/α-Fe 2 O 3 complex under illumination is discussed in figure 10. According to early reports, the conduction band-edge (E c ) and valance band-edge (E v ) of Fe 2 O 3 are −4.78 eV and −6.88 eV, respectively, and the band-gap width is 2.1 eV [6,8]. Thus, charge separation cannot be carried out by the formula if the wavelength is lower than the 590 nm threshold ( /l = E hc , g max h=4.13567×10 -15 eV s, c=3×10 17 nm s −1 ). The electron-hole recombination rate of single Fe 2 O 3 is high, while the graphene with high conductivity in the composite material can rapidly separate charge carriers and reduce the recombination rate. This is of great significance to the application of visible light in photoelectricity and energy production.

Conclusions
In summary, a series of the rGO/α-Fe 2 O 3 composite electrode materials are successfully prepared by controlling the mass of rGO in the hydrothermal process. The GFC4 sample exhibits an optimal electrochemical performance with a specific capacitance of 551.5 Fg −1 and has an excellent retention rate after 8,000 cycles with a current density of 10 A g −1 in a three-electrode system. What's more, the oxygen vacancy generated by hydrogen annealing greatly improves the photoelectric response of the electrode. The improvement of these properties is mainly attributed to the combination of the large specific surface area and high conductivity of rGO, as well as the pseudo-capacitance of α-Fe 2 O 3 , and the generation of oxygen vacancies further facilitates the conductance process. This study provides a promising alternative strategy to develop excellent electrochemical and photoelectrochemical electrodes for energy storage devices.