Introduction

Renewable energies such as solar, water and wind energies are clean and highly abundant resources which are not depleted by use. Hence, the utility of these resources has received greater concern among researchers. Particularly, the selection of best technology to utilize water/solar energy is an important issue. Among various methods (i) photoelectrochemical (PEC) and (ii) electrochemical water splitting (conventionally known as water electrolysis) are prominent methods to directly utilize renewable energy (water/solar) into chemical energy for the production of hydrogen and oxygen1. Compared with fossil fuels hydrogen has three to four fold higher energy density as well as it never pollutes the environment by producing CO2 gas, hence, PEC and electrochemical water splitting received greater consideration2,3,4,5. Usually, in PEC water splitting, semiconductor materials have been used as photoelectrode. These semiconductors consisting of two energy bands viz., valence and conduction band, and the energy difference between these two bands are known as band gap, the band gap range for semiconductor materials are 1 to 5 eV. The theoretical band energy level for water splitting is 1.23 eV. But in practical, the band gap of the semiconductor should be greater than 1.23 eV to avoid thermodynamic losses (~0.4 eV) and over potentials (~0.3 eV). Therefore, the band gap of semiconductor should be ~1.9 eV for effective water splitting6. Based on these backgrounds, thin film metal oxides such as TiO2, ZnO, Fe2O3, Cu2O and WO3 have already been used extensively as a semiconductor material for PEC water splitting. For example, ZnO/ZnFe2O4 core–shell heterojunction photoelectrode was prepared and the PEC performance of ZnO/ZnFe2O4 was optimized by depositing NiOOH7. Also, one dimensional (1D) Fe2O3 nanorod arrays with dual-axial gradient doping of Zr and Sn facilitate the electron-carrier concentration and charge-separation efficiency across the dual-axial direction of Fe2O3 nanorods8. To enhance the light harvesting and charge separation nature of Cu2O, the spatially separated noble-metal cocatalysts viz., Au and Pt nanolayers have deposited onto Cu2O nanogranules9. Recently, 1D/0D WO3/CdS heterojunction photoanodes was fabricated and modified with dual co-catalysts viz., NiOOH and Co-Pi to enhance the water splitting efficiency, where the heterojunctions prohibit the recombination of photo-generated electron-hole pairs and CdS enhances the absorption of the light10. Moreover, the fabrication of polycrystalline α-Fe2O3 nanowire array through oxidation-assisted stress-induced atomic diffusion under water vapor environment and also single-crystal α-Fe2O3 nanowire array by stress-induced atomic diffusion with surface polishing treatment have been reported11,12. However, the low energy conversion efficiency of PEC water splitting restricts its large scale application.

In contrast, electrochemical water splitting method has high efficiency, excellent adaptability and flexibility, which can efficiently produce hydrogen with high purity. In electrochemical water splitting, anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) are two critical half-cell reactions. The standard oxidation potential for OER is 1.23 V vs. RHE and standard reduction potential for HER is 0 V vs RHE for electrochemical water splitting. But in practice, it should be larger than 1.23 V to avoid some unfavorable factors such as activation energy, ion and gas diffusion, electrolyte concentration, wire and electrode resistances, electrolyte diffusion blockage, bubble formation and thermodynamic losses, which leads additional potential over the standard potential13. Initially, anodic and cathodic reactions were catalyzed by Ru, Ir and their oxides catalysts14,15. However, these noble metals cannot be used for commercial applications due to their high cost and lack of abundance. At this situation transition metals such as Ni, Co, Fe, Mn, W, etc. and their oxides have been extensively used as electrocatalytic materials for effective electrochemical water splitting16,17. Among them iron oxides received greater attention to split water efficiently. In addition, it is highly abundant, non-toxic to the environment, cost effective and also stable in aqueous solution18,19. Moreover, the demerit like slow reaction kinetics can be solved by fabricating an electrode with nanostructured morphology. Based on this phenomenon wide varieties of nanostructures have been reported so far.

Recently, Sharifi et al.20 reported the current density of γ-Fe2O3 modified carbon nanotubes as 1 mA/cm2 at 1.57 V vs RHE in 0.1 M KOH and Tavakkoli et al.21 reported the current density of γ-Fe2O3 nanoparticle decorated carbon nanotubes as 10 mA/cm2 at the potential of 1.61 and 1.57 V vs. RHE in 0.1 and 1 M NaOH, respectively. Moreover, Ashwani Kumar et al.22 reported the current density of NiFe-NC obtained from the composite of NiO and α/γ-Fe2O3 as 10 mA/cm2 at the potential of 1.67 V vs. RHE in 1 M KOH. Recently, Davodi et al.23 reported that MWNTs functionalized with nitrogen-rich emeraldine salt (ES-MWNT) as a promising catalyst support to boost the electrocatalytic activity of magnetic maghemite (γ-Fe2O3) NPs and the current density of the electrocatalyst (Ni@γ-Fe2O3/ES-MWNT) was measured as 10 mA/cm2 at the potential of 1.49 V vs RHE in 1 M NaOH. In addition, Chandrasekaran et al.24 employed γ-Fe2O3 with reduced graphene oxide for PEC water splitting and the photocurrent density of the RGO/γ-Fe2O3 nanocomposite was reported as 6.74 mA/cm2 at 1.80 V vs RHE in 1 M NaOH. Even though these reports are available for electrocatalytic water splitting, still it is challenging to attain greater efficiency at low applied potential through highly available and cost-effective materials.

Usually, highly available and low-cost materials are sluggish in nature which needs some modifications, but the materials and methods used to modify the electrodes are highly expensive. Hence, we intended to use a pure, low cost and highly obtainable material without any additional modification with greater efficiency even at very low applied potential. Moreover, it has been reported that facile thermal oxidation is a simple and effective method to grow nanostructured materials including iron oxides25,26,27. Therefore, in the present study, we fabricated γ-Fe2O3 nanowire array from low cost and easily available Fe plate by adopting simple thermal treatment. The formation of Fe2O3 nanowires and its structural morphology was investigated by microscopic and spectroscopic techniques viz., Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Transmission electron microscopy (TEM), Selected area electron diffraction pattern (SAED) and X-ray photoelectron spectroscopy (XPS). Finally, the current density of the obtained Fe2O3 nanowires was measured by applying the materials as working electrode for electrocatalytic water splitting and the current density has reached as 10 mA/cm2 at 1.88 V vs. RHE. The value obtained from bare Fe2O3 is comparable with previous results, where chemical modifications or doping is necessary20,21,22,23,24.

Results and Discussion

Morphologies and structural characterizations

After thermal treatment, the morphology of Fe plate with scratched surface was compared with unscratched Fe plate using FESEM analysis (Fig. S1, Supporting Information). From the images it is observed that the scratched surface (Fig. S1(a)) showed substantial growth of nanowires than unscratched surface (Fig. S1(b)) and the corresponding mechanisms (Fig. S2) for nanowire formation has drawn on the basis of experimental results and previous reports11,12. A reducing environment or straining the surface by applying external force has already been employed to prepare γ-Fe2O328,29,30. Here, the surface of the Fe plates has been strained by uniform scratching, as a result, the effect of strained surface generates applied stresses. This applied stresses fasten the grain boundaries efficiently from the very beginning of the surface oxidation process. The strained surface encourages the initial compressive stress on the surface of the Fe plate, which clogged the volume expansion of oxide layer. Consequently, the vertical stress gradient occurred and expedites the iron ion diffusion and followed to improve the nanowire growth. Moreover, the strained surface can enhance the surface roughness of the Fe plate and thus expand the volume of Fe2O3 layer. The enhanced volume expansion of Fe2O3 layer can increase the tensile strength of the Fe plate and thus facilitates the driving force of iron ion diffusion. In unscratched Fe2O3, grain boundaries have been created by the effect of internal stresses at a respective temperature. The obtained grain boundaries provides route to iron ion diffusion. However, the inadequate internal stress generated in unscratched Fe2O3 produces lesser grain boundaries for iron ion diffusion and thus indicates fewer growths of nanowires in Figs. S1(b) and S2(b).

The schematic diagram for nanowire formation is shown in Fig. 1. The element distributions of the fabricated Fe2O3 electrodes were quantitatively measured by EDS analysis. The top view FESEM image of Fe2O3-1 and the corresponding EDS mapping of Fe2O3, Fe and O are shown in Fig. S3, Supporting Information. Moreover, the corresponding EDS spectrum is shown in Fig. S4, Supporting Information and the weight percentage of Fe and O elements in Fe2O3-1 were found as 71.14 and 28.86%, respectively. Similarly, the EDS mapping of Fe2O3, Fe and O in Fe2O3-2, and the corresponding EDS spectrum are shown in Figs. S5 and S6, Supporting Information, the weight percentage of Fe and O elements in Fe2O3-2 were found as 70.02 and 29.98%, respectively. Therefore, the mapping images obtained from Fe2O3-1 and Fe2O3-2 clearly indicates the abundant and uniform distributions of Fe and O elements in Fe2O3 layer and the appropriate weight percentage of Fe and O are further evidence for the formation of Fe2O3. The side view of nanowires growth was captured from low to high magnifications as shown in Figs. 2, 3 and S7, and the corresponding cross-section FESEM images are shown in Fig. S8. In Fig. 2, the obtained images of Fe2O3-1 showed highly dense and ordered growth of Fe2O3 nanowires on iron plate than Fe2O3-2. The specific reason for the growth of nanowire is atomic diffusion of Fe atom due to the existence of compressive stress. Also, the continuous supply of Fe and O at higher temperature for prolonged duration enhances the growth of nanowires31,32.

Figure 1
figure 1

Schematic diagram for fabrication of Fe2O3 nanowires.

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Figure 2
figure 2

FESEM images of Fe2O3-1 nanowires at (a) low and (b) high magnification.

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Figure 3
figure 3

FESEM images of Fe2O3-2 nanowires at (a) low and (b) high magnification.

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In briefly, the as-received Fe-sheet was subjected to pretreatment using distilled water and ethanol and then individually employed for annealing process at 480 and 550 °C for 2 h and thus obtained Fe2O3-1 and Fe2O3-2, respectively. The thickness of the as-prepared samples viz., Fe2O3-1 and Fe2O3-2 was measured by FESEM analysis. Moreover, to measure the thickness of each layer obtained after thermal treatment, the side view of the cross-sectional FESEM images were taken by cutting the samples as shown in Fig. S9, Supporting Information. The average thickness of Fe2O3 and Fe layers in Fe2O3-1 were measured as 3.61 and 90.76 μm, respectively. Similarly, in Fe2O3-2, the average thickness of Fe2O3 and Fe layers were measured as 8.32 and 82.84 μm, respectively. On comparing the results, the thickness of Fe2O3 layer increased on increasing the temperature and the remained Fe layer decreased due to the promoted oxidation. Moreover, the cross-sectional EDS image of Fe2O3-1 and the elemental mapping of Fe and O are shown in Fig. S10, Supporting Information. The obtained results supported the uniform distribution of elements with two different layers such as Fe2O3 and Fe which indicates the formation of Fe2O3 nanowires onto Fe plate. Similar observation was also noticed for Fe2O3-2 as shown in Fig. S11, Supporting Information. Then, the phase transitions of Fe2O3 nanostructures were investigated by FTIR analysis. Fig. S12 (Supporting Information) shows the characteristic peaks for γ-Fe2O3 at 440, 558 and 636 cm−1, α-Fe2O3 at 471 cm−1 and Fe3O4 at 565 cm−1 which indicates the existence of different phase in the obtained materials. Papadas et al.33 reported the similar peaks for γ-Fe2O3 at 442, 558 and 636 cm−1, α- Fe2O3 at 471 cm−1 and Fe3O4 at 565 cm−1. Zhang et al.34 also reported the same observations for γ, α-Fe2O3 and Fe3O4.

The crystalline properties and crystalline phase of the obtained Fe2O3 was investigated by XRD analysis. In Fig. 4(a), Fe2O3-1 showed 2θ peaks at 29.9°, 35.3°, 42.9°, 56.8° and 62.3° which are relevant to (220), (311), (400), (511) and (440) crystal planes of γ-Fe2O3, respectively. Similarly, in Fig. 4(b), Fe2O3-2 showed the peaks at 30.0°, 35.4°, 43.0°, 56.9° and 62.5° due to (220), (311), (400), (511) and (440) crystal planes of γ-Fe2O3, respectively. In addition, the peak observed at 44.3° for the crystal plane of (110) in Fe2O3-1 belongs to iron plate. In Fe2O3-2, the new peak observed at 65.0° was relevant to the crystal plane of (300) of α-Fe2O3. These peaks and its planes are agreed well with the standard XRD database of γ- and α- Fe2O3 (JCPDS no: 39-1356 and 33-0664)34. Hence, the disappearance of Fe peak and the appearance of new peak for α-phase clearly indicate the phase transition of iron oxide (γ to α) from lower temperature to higher temperature. Usually, Fe has various oxidation states but the common oxidation state of Fe is +2 and +3. Initially Fe donate 2e− to oxygen to attain Fe2+ oxidation state and which produce FeO, when increasing the temperature and duration, FeO replaced by Fe3O4 in such case the oxidation state of Fe in Fe3O4 (Magnetite) is +2 and +3, and it is ferrimagnetic in nature, further heating leads β/γ- Fe2O3, but β and γ phases are intermediate phases. Since β-phase is highly unstable, not easy to separate, but there is some possibility of getting γ-Fe2O3 (Maghemite), and it is also ferrimagnetic in nature but the oxidation state is +3. At higher temperature, γ-phase is obviously replaced by the stable α-phase (hematite). The phase transition temperature can vary from materials to materials based on the thickness and purity of the Fe-precursor which have been chosen for analysis and the equipment (oven/furnace/ceramic heater/CVD) used for annealing process.

Figure 4
figure 4

XRD patterns of (a) Fe2O3-1 and (b) Fe2O3-2 nanowires.

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The morphology, size and shape of single nanowire were investigated by TEM analysis. As shown in Fig. 5(a,c) the morphology of the nanowires were found smooth. The diameter and length of Fe2O3-1 was measured as 397 nm and 1.14 µm (Fig. 5(a)). Whereas, the TEM image of Fe2O3-2 exhibit different sized nanowires, which are due to the fact that not all nanowires were nucleated at the same time. In Fig. 5(c), the diameter and length of the longer one was found as 10 nm and 1.21 µm, respectively, and the shorter one showed the diameter and length of 19 and 640 nm, respectively. It is suggest that the thinner nanowires are longer than thicker one at a specific oxidation temperature and time35. On comparing the TEM results of Fe2O3-1 and Fe2O3-2, the nanowires found in Fe2O3-2 was thinner and longer than Fe2O3-1 nanowire. The reason is considered to be due to the increased compressive stress with the increase of temperature from 480 to 550 °C which influenced the growth of longer nanowires observed in Fe2O3-2 than that of Fe2O3-131,32. The crystal planes and crystalline structure of single Fe2O3 nanowire was further confirmed by SAED pattern. As shown in Fig. 5(b,d), the diffraction spots obtained from Fe2O3-1 and Fe2O3-2 were indexed as (220), (311) and (440) planes by measuring the distance of the spots using its appropriate d-spacing, which are corresponding to the cubic crystal cells of γ-Fe2O3. Further, the obtained results clearly indicate that the fabricated γ-Fe2O3 nanowires were existed in single crystalline structure and the results are agreed well with previous reports36,37.

Figure 5
figure 5

TEM images and SAED patterns of Fe2O3-1 (a,b) and Fe2O3-2 (c,d).

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The biding energy, oxidation state and phase transition of Fe2O3-1 and 2 were further verified by XPS analysis. In Fig. 6(a), the survey spectra of Fe2O3-1 and Fe2O3-2 showed the characteristic peaks at 530.3, 710.3 and 724.3 eV corresponding to O 1s, Fe 2p3/2 and Fe 2p1/2, respectively. The deconvoluted XPS spectra of both Fe2O3-1 and 2 showed the major peaks for Fe 2p3/2 at 709.9 eV and Fe 2p1/2 at 723.3 eV (Fig. 6(b)). Further, the charge transfer satellite peak for Fe 2p3/2 and Fe 2p1/2 was observed at 717.8 eV and 731.8 eV which are strong evidence for the oxidation state of Fe3+ in γ-Fe2O3. This observation was agreed well with previous literatures38,39. Also, Wang et al.40 and Pereira et al.41 reported similar binding energies for Fe 2p and the charge transfer satellite peaks for +3 oxidation state of γ-Fe2O3 phase. The deconvoluted peaks for O 1s observed at 529.0 eV indicates the biding energy of lattice oxygen in Fe2O3 and a shoulder at 530.8 eV indicates the presence of chemisorbed oxygen (OH−) species on sample surface (Fig. 6(c)). This interference was matched with the reported results35,42,43. On comparing the results, the nanowires obtained from Fe2O3-1 and Fe2O3-2 showed similar characteristic peaks in XPS spectra which indicate that the two samples were exist in same form and also in same phase35.

Figure 6
figure 6

(a) XPS survey spectra of (i) Fe2O3-1 and (ii) Fe2O3-2. Deconvoluted XPS spectra of (b) Fe 2p and (c) O 1s of (i) Fe2O3-1 and (ii) Fe2O3-2.

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Electrochemical measurements

The electrochemical water splitting measurements were performed in 1 M NaOH electrolyte solution at pH 13.5 with the help of conventional three electrode system. The fabricated Fe2O3-1 and Fe2O3-2 nanowires were applied directly as working electrode without any chemical modification or doping, which is highly advantageous for large scale application in near future and also it can reduce the cost of expensive chemical substances/dopants to modify the electrode surface. As shown in Fig. 7, the linear sweep voltammograms (LSV) of Fe2O3-1 and Fe2O3-2 illustrate the enriched current density of 10 mA/cm2 at the potentials of 1.88 and 1.91 V vs. RHE with 50 mV/sec scan rate. Even though the applied potential is higher than previous results obtained from chemically modified or doped γ-Fe2O3 nanostructures, no one has reported improved performance for bare Fe2O3 electrode (Table 1)20,21,22,23,24. Specifically, three important factors such as, (i) highly dense and ordered growth of nanowires, (ii) more abundant of γ-phase and (iii) single crystalline structure played essential role to improve the performance of the as-prepared Fe2O3 electrodes. In detail, the highly dense and ordered growth of Fe2O3 nanowires (obtained after scratching the electrode surface, Figs. S1(a) and S2(a), Supporting Information) produced high surface area. The more abundant γ-phase on Fe2O3 electrode acts as an efficient electrocatalyst for oxygen evolution reaction due to its greater electrical conductivity for rapid electron transfer. Also, γ-Fe2O3 provided substantial and synergetic effect on improving catalysis by producing more catalytic active sites. Therefore, the enormous surface area and the enhanced electrocatalytic active sites offered more capability to adsorb OH− ions (presented in the electrolyte solution) which leads the enhancement in catalysis and fast electron transfer towards oxygen evolution reaction, as a result of the improved performance in Fe2O3 electrode (Fig. 7). Moreover, the sharp edges of the Fe2O3 nanowires highly supported to the charge transfer process of electrode/electrolyte interface i.e. electrons obtained from oxygen evolution reaction were easily moved from electrolyte solution to Fe2O3 electrode. In addition, the single crystalline structure of Fe2O3 electrode (Fig. 5) played vital role to enhance the electron movements from Fe2O3 to Fe layer and which also improved the performance of the as-prepared Fe2O3 electrode.

Figure 7
figure 7

LSV measurements of (a) Fe2O3-1 and (b) Fe2O3-2 (inset shows the potentials of (a) Fe2O3-1 and (b) Fe2O3-2 at 10 mA/cm2).

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Table 1 Comparison of current density of recently reported electrode materials.
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On comparing the results obtained from electrochemical measurements, a small potential difference has been noticed between Fe2O3-1 and Fe2O3-2 (Fig. 7). This is due to non-uniform and lack of nanowires growth, and increased thickness of Fe2O3 layer found in Fe2O3-2. The non-uniform and lack of nanowires growth slightly reduced the surface area and catalytic active sites of the electrode, and the thickness slowdown the electron movement from Fe2O3 to Fe layer during water splitting. Therefore, Fe2O3-2 electrode showed slight increment in applied potentials than Fe2O3-1. Moreover, stability measurements were performed for both Fe2O3-1 and Fe2O3-2 against current density vs. time at a constant potential of 1.7 V vs. RHE with the scan rate of 50 mV/sec as shown in Figs. 8 and 9. From the result it is observed that there is no substantial change in stability of the electrode up to 3275 sec and the current density was observed as 9.6 mA/cm2 for Fe2O3-1 and 9.5 mA/cm2 for Fe2O3-2, respectively. The vigorous formation of bubbles during stability measurement (see Supplementary Movie 1) is directly corresponding to the oxygen and hydrogen evolution reactions on both anode and cathode surfaces, respectively44,45, and the observation was explained by the following possible reaction mechanism13. The OER reaction on γ-Fe2O3 was also supported by previous report21. Specifically, catalytic reactions involved in four steps such as (a) adsorption of reactant molecules onto catalyst surface, (b) diffusion of reactants onto catalyst surface, (c) reaction takes place from reactants and which produce products, and finally (d) desorption of products from catalyst surface. The electrocatalytic measurement of the present investigation has been performed under alkaline condition, therefore the OER reaction proceeds by the following reaction steps:

  1. (1)

    Adsorption of ‘OH−’ (present in alkaline electrolyte solution) onto γ-Fe2O3 catalyst surface.

    $${{\rm{OH}}}^{-}+{{\rm{M}}}^{\ast }\to {{\rm{M}}}^{\ast }-{\rm{OH}}+{{\rm{e}}}^{-}$$
    (i)
  2. (2)

    Where, M*- represents the active sites of γ-Fe2O3 metal catalyst. The obtained M*-OH was further reacted with OH− and formed M*-O.

    $${{\rm{M}}}^{\ast }-{\rm{OH}}+{{\rm{OH}}}^{-}\to {{\rm{M}}}^{\ast }-{\rm{O}}+{{\rm{H}}}_{2}{\rm{O}}+{{\rm{e}}}^{-}$$
    (ii)
  3. (3)

    (The formation of O2 can take place in two pathways, one is direct coupling of two M*-O, but the thermodynamic barrier of this reaction is high.

    $${{\rm{M}}}^{\ast }-{\rm{O}}+{{\rm{M}}}^{\ast }-{\rm{O}}\to {{\rm{O}}}_{2}$$
    (iii)
  4. (4)

    In second pathway, M*-O reacts with OH− and produced intermediate molecule (M*-OOH). This intermediate consequently reacted with another OH− and produced O2.

$${{\rm{M}}}^{\ast }-{\rm{O}}+{{\rm{OH}}}^{-}\to {{\rm{M}}}^{\ast }-{\rm{OOH}}+{{\rm{e}}}^{-}$$
(iv)
$${{\rm{M}}}^{\ast }-{\rm{OOH}}+{{\rm{OH}}}^{-}\to {{\rm{O}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}+{{\rm{e}}}^{-}$$
(v)
Figure 8
figure 8

Stability measurement of Fe2O3-1 at 1.7 V vs. RHE for 3275 sec at 50 mV/sec scan rate.

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Figure 9
figure 9

Stability measurement of Fe2O3-2 at 1.7 V vs. RHE for 3275 sec at 50 mV/sec scan rate.

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Moreover, it is value to mention here that α-Fe2O3 (hematite) offers a favorable combination of good visible light absorption up to 590 nm hence it has been widely used as photoactive material for solar water splitting46, but recently few reports are demonstrated the excellent electrocatalytic activity of chemically modified γ-Fe2O3 in electrocatalytic water splitting20,21,22,23,24. The morphology of γ-Fe2O3 electrode used for stability measurements was observed by FESEM analysis and the images are shown in Fig. S16 (Supporting Information). This image shows that there was no substantial change in the electrode morphology and the result reveals the best stability of γ-Fe2O3 electrode in alkaline solution and the resistivity against corrosion. The samples after stability measurements were also measured by XPS analysis and the results are shown in Fig. S17 (Supporting Information). These results suggested that γ-Fe2O3 NWs are highly efficient material to achieve excellent current density and stability. Moreover, the single crystalline structure of γ-Fe2O3 is also an important factor to increase the efficiency of electrocatalytic water splitting, however, the preparation of γ-Fe2O3 with single crystalline structure is still a challenging so far36,37.

In addition, the electrochemical capacitances have been calculated through double layer capacitance of the fabricated electrode material in 1 M NaOH22,47,48. To measure double layer capacitance, cyclic voltammetry measurements were performed at non-faradaic region (i.e., −0.090 to 0.110 V vs. Ag/AgCl is converted as 0.904 to 1.104 V vs. RHE) with different scan rates viz., 5, 10, 20, 50, 100, 200, 500 and 1000 mV/sec as shown in Fig. 10. All measured current at this non-faradaic region was assumed to as double-layer charging current. Then the cathodic and anodic capacitance current was measured at 0.026 V vs Ag/AgCl (i.e. 1.020 V vs. RHE) from each scan rate. The obtained capacitance currents were plotted against scan rates (Fig. 11). Then the double-layer capacitance was calculated from the average absolute value of the both cathodic and anodic slope of the linear fitting of the plot, i.e. double layer capacitance (CDL) was measured from the slope of charging currents (ic) as a function of scan rate (υ) as shown in Eq. 1.

$${{\rm{i}}}_{{\rm{c}}}=\upsilon {{\rm{C}}}_{{\rm{DL}}}$$
(1)
Figure 10
figure 10

Cyclic voltammogram measured for Fe2O3-1 at non-faradaic region (−0.090 V to 0.110 V vs. Ag/AgCl) with different scan rates (5, 10, 20, 50, 100, 200, 500 and 1000 mV/sec).

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Figure 11
figure 11

Anodic and cathodic capacitance currents measured for Fe2O3-1 at 0.026 V vs. Ag/AgCl and plotted as a function of scan rate.

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The electrochemical double-layer capacitance measured from the scan-rate study for the Fe2O3-1 catalyst was CDL = 0.119 mF. The electrochemically active surface area (ECAS) of Fe2O3-1 was measured from the obtained electrochemical double-layer capacitance of the catalytic surface by using Eq. 2.

$${\rm{ECAS}}={{\rm{C}}}_{{\rm{DL}}}/{{\rm{C}}}_{{\rm{s}}}$$
(2)

Where Cs is the specific capacitance of the sample or the capacitance of a planar surface per unit area under identical electrolyte conditions. The average specific capacitance of 1 M NaOH was reported as 0.040 mF cm−2 21,47,48. Then, the electrochemically active surface area of Fe2O3-1 was measured as ECAS = 3 cm2 by applying the Cs value of 1 M NaOH in Eq. 2. Further, the roughness factor (RF) of the Fe2O3-1 electrode has been calculated by using Eq. 3.

$${\rm{RF}}={\rm{ECSA}}/{\rm{GSA}}$$
(3)

where GSA is geometric surface area, the geometric surface area of the catalytic material used in this study was 0.25 cm2. Then, the roughness factor (RF) of the working electrode Fe2O3-1 was measured as RF = 12. Therefore, it has been cleared that the measured electrochemically active surface area (3 cm2) and surface roughness (12) of Fe2O3-1 played vital role to contribute to the high activity of the fabricated material. The higher activity of the fabricated Fe2O3 electrocatalyst was corresponding to the existence of γ-phase in Fe2O3-1. Tavakkoli et al.21 also reported that γ-Fe2O3 nanoparticles showed higher activity than α-Fe2O3.

Conclusions

In conclusion, we have successfully fabricated highly effective, easily available and low-cost γ-Fe2O3 nanowires by adopting a simple thermal oxidation method. The current density of the newly fabricated nanowires array was measured as 10 mA/cm2 at 1.88 V vs. RHE with the scan rate of 50 mV/sec in 1 M NaOH solution and the stability of the working electrode were maintained up to 3275 sec and the current density was observed as 9.6 mA/cm2. This result is in a similar or higher order with that of other reported chemically modified/doped γ-Fe2O3 nanostructured electrodes. The unique electrocatalytic nature, γ-phase, high density and single crystalline structure of the fabricated γ-Fe2O3 nanowires played a vital role to obtain higher current density and long durability. The obtained simple bare electrode can definitely possess cost effectiveness, high availability, higher energy conversion, compatibility and activity than existing modified electrodes. Hence, the newly fabricated Fe2O3 nanowires obtained without using any co-catalyst, doping agents or any other modification onto electrode materials are expected to be a prominent candidate for electrochemical water splitting in near future compared with existing modified/doped nanostructured materials.

Methods

The schematic representation for the fabrication of γ-Fe2O3 was shown in Fig. 1. Initially, the commercially available Fe plate with 99.5% purity was cut into small pieces (5 mm × 10 mm), and the surface of the Fe plate (front side) which is used for electrolysis was scratched by knife. Then the plate was cleaned by dispersing with ethanol and distilled water separately and dried under room temperature. Afterwards it was introduced for thermal treatment i.e. the pretreated Fe plate was placed on ceramic boat and heated for 2 h at 480 °C by using electric furnace under air. After thermal treatment the steel blue color of the Fe plate changed into dark brown color, and which has been named as Fe2O3-1. Similarly, Fe2O3-2 has been prepared by treating the Fe plate for 2 h at 550 °C. The obtained samples were analyzed by spectroscopic and microscopic techniques. All the time three set of samples have been prepared in both experimental condition and employed for spectroscopic and microscopic analysis. The morphology and elemental mapping of the samples were monitored by Field Emission Scanning Electron Microscopy (FESEM) combined with Energy-dispersive X-ray spectroscopy (EDS) using JSM-7200F. Then the different iron oxides group present in the sample was observed by Fourier-transform infrared spectroscopy (JASCO FT/IR-4100). The crystalline properties of the Fe2O3-1 and Fe2O3-2 were investigated by X-ray diffraction analysis (XRD, ATX-G, RIGAKU with Cu Kα1). Then, the morphology, structure, size and the corresponding Selected Area Electron Diffraction pattern (SAED) of a single nanostructure was investigated by Transmission Electron Microscopy (TEM) analysis using JEM-2100. The binding affinity of iron and oxygen present in γ-Fe2O3 and the oxidation state of Fe in γ-Fe2O3 was further verified by X-ray Photoelectron Spectroscopic (XPS) analysis (ESCALAB 250). Electrochemical water splitting measurements were measured by using potentiostat (ALS/DY2325 BI-POTENTIOSTAT) in 1 M of NaOH electrolyte solution with the pH of 13.5. In this measurement, Platinum wire was used as counter electrode, Ag/AgCl as reference electrode and the fabricated Fe2O3 nanostructures (0.25 cm2) as working electrode. The obtained results were converted to the theoretical water splitting potential of 1.23 V vs. Reversible Hydrogen Electrode (RHE) with the help of Nernst equation.