Fabrication of Hybrid Catalyst ZnO Nanorod / α -Fe 2 O 3 Composites for Hydrogen Evolution Reaction

: This report presents the synthesis of ZnO nanorod / α -Fe 2 O 3 composites by the hydrothermal method with di ﬀ erent weight percentages of α -Fe 2 O 3 nanoparticles. The as-synthesized nanorod composites were characterized by di ﬀ erent techniques, such as X-ray di ﬀ raction (XRD), Fourier transform-infrared (FT-IR), ﬁeld emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). From our results, it was found that the ZnO / α -Fe 2 O 3 (3 wt%) nanorod composites exhibit a higher hydrogen evolution reaction (HER) activity when compared to other composites. The synergetic e ﬀ ect between ZnO and (3 wt%) of α -Fe 2 O 3 nanocomposites resulted in a low onset potential of − 125 mV, which can e ﬀ ectively produce more H 2 than pure ZnO. The H 2 production rate over the composite of ZnO / α -Fe 2 O 3 (3 wt%) clearly shows a signiﬁcant improvement in the photocatalytic activity in the heterojunction of the ZnO nanorods and α -Fe 2 O 3 nanoparticles on nickel foam. ZnO nanorods as shown in Figure 3b,c when the weight percentage is increased to 3 and 5 wt %. In Figure 3d, the formation of more α -Fe 2 O 3 nanoparticles is observed, which may influence the growth of the ZnO nanorods. After the growth of the 5 wt % of α -Fe 2 O 3 nanoparticles on the ZnO nanorods, it is clearly observed that the entire surface of the ZnO nanorods is covered with the α -Fe 2 O 3 nanoparticles. Subsequently, after the hydrothermal growth, the structure of the ZnO nanorods is still well preserved. The magnified TEM image of the ZnO/ α -Fe 2 O 3 (3 wt% α -Fe 2 O 3 ) shows the ZnO nanorods with a 500 nm length and 20 nm width, which are covered by α -Fe 2 O 3 nanoparticles as shown in Figure 3f. The TEM image of Figure 3f shows the needle-shaped ZnO nanorod covered with α -Fe 2 O 3 nanoparticles, and it reveals the close interaction of ZnO with α -Fe 2 O 3 , which is reflected in the synergistic effect between the composites. An energy dispersive X-ray spectroscopy (EDS) analysis was carried out to examine the chemical composition of ZnO/ α -Fe 2 O 3 nanocomposites. The EDS results show the weight ratio of Zn (78.34), Fe (2.67), and O (18.74). As shown in Figure 3g, peaks for the elements of Fe, Zn, and O were detected, and no other impurities can be observed. This result shows the formation of ZnO/ α -Fe 2 O 3 nanocomposites and reveals that Fe, Zn, and O elements exist in the nanocomposites, as shown in Figure 3g.


Introduction
In recent decades, fossil fuels have conventionally been the most important source of fuel, but the consumption of fossil fuels is a serious environmental concern. It is therefore essential to develop renewable energy resources that do not contaminate the environment. To date, many methods have been developed to produce alternative energy sources without the emission of carbon. Of these, solar energy has attracted much attention as a sustainable and clean alternative energy source [1,2]. The conversion of solar energy into chemical energy is a promising technique for producing renewable energy. This technology is an attractive way to directly utilize solar-converted energy in the form of water-splitting systems with high solar-to-hydrogen efficiencies. Photocatalysis and photovoltaic systems are the most important routes for the conversion of solar energy into chemical energy in an environmentally friendly way, so they have been of great interest for several decades. Specifically, the photocatalytic process has attracted enormous interest as an effective method for the treatment of environmental pollutants. In addition, the photogeneration of hydrogen under light irradiation using a photocatalyst has been considered as a potentially significant strategy for hydrogen production [3][4][5].
Over the past three decades, hydrogen production through the solar-driven method with the support of semiconductor photocatalysts has been a simple, cost-effective approach for converting solar energy into hydrogen [6,7]. The thermodynamic requirement for water splitting into H 2 and O 2 is 1.23 eV, so the bandgap energy of the photocatalyst should be higher than 1.23 eV [8]. This is the combination of two reactions, namely, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Commonly, three main types of water-splitting methods have been adopted: photocatalysis, photoelectrochemical splitting, and electrocatalysis [9]. Among them, electrocatalytic water splitting is a remarkable, simple, and green process for hydrogen production [10][11][12][13][14]. Generally, a noble metal-based catalyst is used for electrolytic hydrogen production. However, various semiconductor materials have been studied because of their direct bandgap, physicochemical properties, and versatile catalytic activities. Based on the above superior properties, researchers have focused on semiconductor nanomaterials with a high surface area and plasmonic resonance, which are the crucible for novel applications such as energy devices and the removal of organic pollutants [15][16][17]. In addition, various semiconductors have been used for hydrogen generation, especially ZnO, TiO 2 , and α-Fe 2 O 3 , which is one of the most attractive materials for hydrogen production with an n-type semiconductor, and which has excellent optical properties [18,19]. It is the most stable and most abundant material on earth and has been used for various applications, such as photocatalysts, supercapacitors, and sensors. Other than α-Fe 2 O 3 , ZnO is another attractive semiconductor that has many potential applications, such as dye degradation, supercapacitors, and hydrogen evolution [20][21][22]. It can be favorable to study metal oxide photoanodes as they have a direct bandgap with favorable band edges for the water-splitting redox levels. To improve water-splitting performance, utilizing visible light is the most commonly used strategy by fabricating the ZnO heterostructure with other small bandgap materials. Many researchers have used ZnO and α-Fe 2 O 3 as a heterojunction, resulting in hydrogenation with a low bandgap and the ability to stimulate the photoresponse of electrons and holes [23][24][25]. In addition, ZnO and α-Fe 2 O 3 are both earth-abundant photocatalysts with good activity, and a synthesis of these hybrid materials improves photocatalytic performance [26][27][28][29]. Based on this, a coupling of hierarchical α-Fe 2 O 3 with ZnO could be considered as a good strategy for constructing highly effective electrocatalysts for HER.
Recently, much research has focused on developing electrocatalysts for HER, OER, and overall water splitting based on earth-abundant transition metals. Interestingly, Ni-based catalysts, which include Ni2P, NiFeP, NiFe-layered double hydroxide [30][31][32], and Ni/NiO/carbon nanotube, are more attractive for water splitting. It has been shown that the activation of a Ni carbon-based catalyst through the application of electrochemical potential results in HER activity comparable to Pt in an acidic medium. To achieve the high current density with a large producing rate of H 2 /O 2 bubbles, the conductive substrates are essential, such as carbon paper, FTO, nickel foil, iron substrates, and nickel foam [33][34][35]. Among these substrates, nickel foam has the advantages of good electrical conductivity, a high specific surface, and a porous structure. Therefore, using nickel foam in the generation of the hydrogen evolution process has attracted great consideration due to its high current density.
In this present work, we propose a hydrothermal method to fabricate ZnO nanorods with different weight percentages of α-Fe 2 O 3 nanoparticles that are deposited on Ni foam for electrocatalytic studies. This Ni foam has good catalytic activity and acts as a durable electrolyte in the acidic electrolyte. The ZnO/α-Fe 2 O 3 shows excellent morphology, crystal structure, and impedance performance. Systematic investigation of ZnO/α-Fe 2 O 3 nanocomposites was performed, and they were found to be favorable for higher HER activity with better overpotential.

Preparation of ZnO/α-Fe 2 O 3 Nanorod Composites
All the reagents and chemicals used for the preparation of the nanorod composites are analytical grade. In this study, 500 mg of zinc acetate and 5, 10, and 15 mg of ferric nitrate with sodium hydroxide (0.5M) were mixed with 20 ml of DI water. These solutions were transferred to 20 ml of Teflon-lined autoclave, followed by a hydrothermal treatment at 200 • C for 24 hours. After being cooled to ambient temperature, the brown solid was collected from the autoclave and thoroughly washed with ethanol to remove the impurities, followed by annealing at 450 • C for 4 h. The α-Fe 2 O 3 nanoparticles were grown on ZnO nanorods through a simple co-precipitation method using the hydrothermal approach as illustrated in Scheme 1. In the beginning, both metals exist in the form of ions in the alkaline medium. The hydrothermal process initiates the co-precipitation of metal ions into hydroxides, followed by nucleation and growth of α-Fe 2 O 3 nanoparticles decorated ZnO nanorods. A further annealing process leads to the formation of highly crystalline nanocomposites without impurities. cooled to ambient temperature, the brown solid was collected from the autoclave and thoroughly washed with ethanol to remove the impurities, followed by annealing at 450 °C for 4 h. The α-Fe2O3 nanoparticles were grown on ZnO nanorods through a simple co-precipitation method using the hydrothermal approach as illustrated in Scheme 1. In the beginning, both metals exist in the form of ions in the alkaline medium. The hydrothermal process initiates the co-precipitation of metal ions into hydroxides, followed by nucleation and growth of α-Fe2O3 nanoparticles decorated ZnO nanorods. A further annealing process leads to the formation of highly crystalline nanocomposites without impurities. Scheme 1. Synthesis of ZnO/α-Fe2O3 nanocomposites by the hydrothermal method.

Hydrogen Evolution Reaction (HER)
The three-electrode electrochemical workstation (CHI 614 C, Austin, TX, USA) was used to measure the linear sweep voltammetry at room temperature. Platinum is the counter electrode, nickel foam is the working electrode with 0.5 M H2SO4, and Ag/AgCl is the reference electrode. The working electrode was prepared by mixing ZnO/α-Fe2O3 with 1 ml of 10 μl Nafion, 10 μl ethanol, and 80 μl distilled water under ultrasonication for 45 min. The linear sweep voltammetry (LSV) was measured between −0.8 and −1.6 eV vs. Ag/AgCl at 50 mVs −1 . The onset overpotential generally denotes the applied potential, and thus, a current starts to appear. To improve the catalysts quantitatively, the overpotential at 10 mA cm −2 was taken as a reference. Before the electrocatalytic H2 evolution, the solution was purged with N2 for 30 min. In this measurement, the Ag/AgCl was used as the reference electrode and was converted with respect to a reversible hydrogen electrode (RHE): The current density of the electrode was measured by dispersing the working electrode, then uniformly coated onto bare nickel foam with the mass loading of 20 mg and dried in an electric oven at 50 °C for 1 hr. Before each electrochemical test, the electrode was activated by 5~10 CV cycles in the corresponding testing electrolyte.

Characterization
The X-ray diffraction spectra were analyzed (Analytical X'Pert 246 PRO, Almelo, Netherlands) using filtered CuKα radiation (λ = 1.5418 Å). The chemical bonds were studied using a PerkinElmer (Seer Green, UK), an FT-IR spectrometer. The structural morphologies of the catalysts were studied using field emission scanning electron microscopy (FE-SEM) with a JEOL JSM-7100F (Tokyo, Japan). The binding energy of the nanocomposites was observed from the X-ray photoelectron spectra (XPS) obtained using a JEOL JPS-9030 (Tokyo, Japan). Transmission electron microscopy (TEM) was analyzed by a JEOL JEM-2100F (MA, USA). The electrochemical impedance measurement was carried out using a Zive Potentiostate, SP100. Electrochemical impedance spectroscopy (EIS) was used to observe the interfacial properties of the ZnO/α-Fe2O3 with various ratios of α-Fe2O3 with a modified glassy carbon electrode (GCE) in 0.1M KCl containing 5mM of Fe(CN) 6 , wherein bias potential = 0 V, amplitude = 5 mV, and frequency = 0.1Hz-100 kHz were applied.

Hydrogen Evolution Reaction (HER)
The three-electrode electrochemical workstation (CHI 614 C, Austin, TX, USA) was used to measure the linear sweep voltammetry at room temperature. Platinum is the counter electrode, nickel foam is the working electrode with 0.5 M H 2 SO 4 , and Ag/AgCl is the reference electrode. The working electrode was prepared by mixing ZnO/α-Fe 2 O 3 with 1 ml of 10 µl Nafion, 10 µl ethanol, and 80 µl distilled water under ultrasonication for 45 min. The linear sweep voltammetry (LSV) was measured between −0.8 and −1.6 eV vs. Ag/AgCl at 50 mVs −1 . The onset overpotential generally denotes the applied potential, and thus, a current starts to appear. To improve the catalysts quantitatively, the overpotential at 10 mA cm −2 was taken as a reference. Before the electrocatalytic H 2 evolution, the solution was purged with N 2 for 30 min. In this measurement, the Ag/AgCl was used as the reference electrode and was converted with respect to a reversible hydrogen electrode (RHE): The current density of the electrode was measured by dispersing the working electrode, then uniformly coated onto bare nickel foam with the mass loading of 20 mg and dried in an electric oven at 50 • C for 1 hr. Before each electrochemical test, the electrode was activated by 5~10 CV cycles in the corresponding testing electrolyte.

Characterization
The X-ray diffraction spectra were analyzed (Analytical X'Pert 246 PRO, Almelo, Netherlands) using filtered CuKα radiation (λ = 1.5418 Å). The chemical bonds were studied using a PerkinElmer (Seer Green, UK), an FT-IR spectrometer. The structural morphologies of the catalysts were studied using field emission scanning electron microscopy (FE-SEM) with a JEOL JSM-7100F (Tokyo, Japan). The binding energy of the nanocomposites was observed from the X-ray photoelectron spectra (XPS) obtained using a JEOL JPS-9030 (Tokyo, Japan). Transmission electron microscopy (TEM) was analyzed by a JEOL JEM-2100F (MA, USA). The electrochemical impedance measurement was carried out using a Zive Potentiostate, SP100. Electrochemical impedance spectroscopy (EIS) was used to observe the interfacial properties of the ZnO/α-Fe 2 O 3 with various ratios of α-Fe 2 O 3 with a modified glassy carbon electrode (GCE) in 0.1M KCl containing 5mM of Fe(CN) 6 , wherein bias potential = 0 V, amplitude = 5 mV, and frequency = 0.1Hz-100 kHz were applied.  110) can be readily ascribed to the characteristic peaks of the ZnO phase, which is confirmed with a JCPDS card 79-0206 as shown in Figure 1 [22]. The distinct 2θ peaks observed at 32.2, 35.7, 40.9, and 49.4 • confirmed the presence of the α phase of α-Fe 2 O 3 nanoparticles according to the JCPDS map 79-0007 as shown in the inset Figure 1 [36]. After the addition of α-Fe 2 O 3 nanoparticles, the small relative intensity peaks started to broaden at 32.2 and 35.7 • with the increasing weight ratio of α-Fe 2 O 3 nanoparticles as seen in Figure 1. The crystallite size of the ZnO and ZnO/α-Fe 2 O 3 with different weight percentages of α-Fe 2 O 3 nanoparticles was also calculated from the XRD spectra using Scherrer's formula:

Results and Discussion
where λ, θ, and B are the X-ray wavelength (1.54 Å), Bragg diffraction angle, and line of full width at half maximum, respectively. .99 nm, respectively. The decreasing particle size is observed due to increasing the α-Fe 2 O 3 concentration and decreasing the growth of the ZnO nanorods. However, the observed particle size of the nanocomposites is~500 nm, which differs from the calculated crystallite size. This is because the nanocomposite particles are composed of several different crystallites, which results in changes in the size of the particle.   The peaks at 3436 and 1629 cm -1 are attributed to the stretching vibration of the hydroxyl groups of ZnO and α-Fe 2 O 3 . This confirms the presence of the adsorption site, i.e., the hydroxyl group that helps to increase the photocatalytic efficiency of the nanocomposites. A significant peak observed from 500 to 600 cm −1 was attributed to the stretching vibration of the Zn-O and Fe-O bonds in the ZnO nanorods and α-Fe 2 O 3 nanoparticles [23,37,38]. The peaks at about 1326 and 1629 cm −1 are assigned to the symmetric and asymmetric stretching vibration of the carboxylate band, due to the presence of the acetate group, which may be formed as a surface species chelated to Zn(II) [39,40]. Hence, the FT-IR results agreed well with those of the XRD profiles, indicating the formation of α-Fe 2 O 3 nanoparticles on the surface of the ZnO nanorods. The FE-SEM images of the ZnO nanorods and ZnO/α-Fe2O3 with different weight percentages of α-Fe2O3 nanoparticles are illustrated in Figure 3. In Figure 3a, the ZnO nanorods with a length size of 500 nm appeared with a sharp end. After the introduction of 1 wt % α-Fe2O3, α-Fe2O3 nanoparticles with sizes of 20 nm are grown on the ZnO nanorods. More α-Fe2O3 nanoparticles are observed on the ZnO nanorods as shown in Figure 3b,c when the weight percentage is increased to 3 and 5 wt %. In Figure 3d, the formation of more α-Fe2O3 nanoparticles is observed, which may influence the growth of the ZnO nanorods. After the growth of the 5 wt % of α-Fe2O3 nanoparticles on the ZnO nanorods, it is clearly observed that the entire surface of the ZnO nanorods is covered with the α-Fe2O3 nanoparticles. Subsequently, after the hydrothermal growth, the structure of the ZnO nanorods is still well preserved. The magnified TEM image of the ZnO/α-Fe2O3 (3 wt% α-Fe2O3) shows the ZnO nanorods with a 500 nm length and 20 nm width, which are covered by α-Fe2O3 nanoparticles as shown in Figure 3f. The TEM image of Figure 3f shows the needle-shaped ZnO nanorod covered with α-Fe2O3 nanoparticles, and it reveals the close interaction of ZnO with α-Fe2O3, which is reflected in the synergistic effect between the composites. An energy dispersive X-ray spectroscopy (EDS) analysis was carried out to examine the chemical composition of ZnO/α-Fe2O3 nanocomposites. The The FE-SEM images of the ZnO nanorods and ZnO/α-Fe 2 O 3 with different weight percentages of α-Fe 2 O 3 nanoparticles are illustrated in Figure 3. In Figure 3a, the ZnO nanorods with a length size of 500 nm appeared with a sharp end. After the introduction of 1 wt % α-Fe 2 O 3 , α-Fe 2 O 3 nanoparticles with sizes of 20 nm are grown on the ZnO nanorods. More α-Fe 2 O 3 nanoparticles are observed on the ZnO nanorods as shown in Figure 3b,c, when the weight percentage is increased to 3 and 5 wt %. In Figure 3d, the formation of more α-Fe 2 O 3 nanoparticles is observed, which may influence the growth of the ZnO nanorods. After the growth of the 5 wt % of α-Fe 2 O 3 nanoparticles on the ZnO nanorods, it is clearly observed that the entire surface of the ZnO nanorods is covered with the α-Fe 2 O 3 nanoparticles. Subsequently, after the hydrothermal growth, the structure of the ZnO nanorods is still well preserved. The magnified TEM image of the ZnO/α-Fe 2 O 3 (3 wt% α-Fe 2 O 3 ) shows the ZnO nanorods with a 500 nm length and 20 nm width, which are covered by α-Fe 2 O 3 nanoparticles as shown in Figure 3f. The TEM image of Figure 3f shows the needle-shaped ZnO nanorod covered with α-Fe 2 O 3 nanoparticles, and it reveals the close interaction of ZnO with α-Fe 2 O 3 , which is reflected in the synergistic effect between the composites. An energy dispersive X-ray spectroscopy (EDS) analysis was carried out to examine the chemical composition of ZnO/α-  The optical absorption spectra of the ZnO and ZnO/α-Fe2O3 nanocomposites with different weight percentages are shown in Figure 4, which shows the pure ZnO absorption peak at 345 nm exhibited in the UV region [41,42]. Further, it extends to the visible region after incorporating α-Fe2O3 with different weight percentages. The absorption spectra of α-Fe2O3 are included as the inset of Figure 4 and reveal the absorption edge at around 580 nm, which is the characteristic of α-Fe2O3 pristine [43,44]. Apparently, the ZnO and ZnO/α-Fe2O3 (1 and 3 wt%) nanocomposites show similar absorption edges. When adding more α-Fe2O3 (5 wt%) with ZnO nanorods, the absorption characteristics start to dominate, which show high absorption in the visible light region. This indicates that there is an increase in the optical absorption coefficient for ZnO/α-Fe2O3 at longer wavelengths compared to ZnO due to the fact that a semiconductor with a smaller bandgap, α-Fe2O3, is present in the nanocomposite material. The optical absorption spectra of the ZnO and ZnO/α-Fe 2 O 3 nanocomposites with different weight percentages are shown in Figure 4, which shows the pure ZnO absorption peak at 345 nm exhibited in the UV region [41,42]. Further, it extends to the visible region after incorporating α-Fe 2 O 3 with different weight percentages. The absorption spectra of α-Fe 2 O 3 are included as the inset of Figure 4 and reveal the absorption edge at around 580 nm, which is the characteristic of α-Fe 2 O 3 pristine [43,44]. Apparently, the ZnO and ZnO/α-Fe 2 O 3 (1 and 3 wt%) nanocomposites show similar absorption edges. When adding more α-Fe 2 O 3 (5 wt%) with ZnO nanorods, the absorption characteristics start to dominate, which show high absorption in the visible light region. This indicates that there is an increase in the optical absorption coefficient for ZnO/α-Fe 2 O 3 at longer wavelengths compared to ZnO due to the fact that a semiconductor with a smaller bandgap, α-Fe 2 O 3 , is present in the nanocomposite material.  The XPS survey spectra revealed the elemental composition of the ZnO, α-Fe2O3, and ZnO/α-Fe2O3 as illustrated in Figure 5a. At the survey spectra of α-Fe2O3, the peak is observed at 711.18 eV, which corresponds to the 2p3/2 of Fe2O3. [45]. Moreover, peaks at 1022.3 and 1045.3 eV are present in The XPS survey spectra revealed the elemental composition of the ZnO, α-Fe 2 O 3 , and ZnO/α-Fe 2 O 3 as illustrated in Figure 5a. At the survey spectra of α-Fe 2 O 3 , the peak is observed at 711.18 eV, which corresponds to the 2p 3/2 of Fe 2 O 3 . [45]. Moreover, peaks at 1022.3 and 1045.3 eV are present in the survey spectra of ZnO, corresponding to Zn 2p 3/2 and Zn 2p 1/2 , which is attributed to Zn-O, thus confirming that Zn exists in the oxidation state [46,47]. In Figure 5b, the four peaks with different binding energies are fitted at 527.82, 530.85, and 532.7 eV for the ZnO/α-Fe 2 O 3 nanocomposites. The peak at 529.95 arises from the Zn-O band. The peak which emerged at 530.85 eV and is observed from the surface lattice oxygen and at 532.7 eV is attributed to the surface-adsorbed oxygen species of O 2- [47]. It should be noted that after adding α-Fe 2 O 3 , it leads to the energy levels of oxygen vacancies, which confirm the charge separation of photogenerated electrons and holes. It is noted that the peaks at 710.39, 711.75, and 713.57 eV are attributed to Fe 3+ , confirming the α-Fe 2 O 3 nanoparticles on the ZnO nanorods as shown in Figure 5c [48]. The Fe 3+ ion observed in the nanocomposites is evidence of the synergetic effect, which improves the photocatalytic process. From the analysis spectra of ZnO and the ZnO/α-Fe 2 O 3 nanocomposites, the exhibited peaks at 1021.5 and 1045 eV correspond to Zn2p 3/2 and Zn2p 1/2 respectively, as shown in Figure 5d [47,49]. The high-intensity major peak at 1022.3 eV represents the Zn 2+ of the deposited ZnO, whereas the low-intensity peak at 1021.5 eV of the ZnO/α-Fe 2 O 3 nanocomposites indicates the changed chemical environment. The binding of Zn 2+ with the surface oxygen attached to Fe 3+ probably results in the shifting of binding energy as shown in Figure 5d. These results indicate the formation of the ZnO/α-Fe 2 O 3 containing Zn, Fe, O, and C elements with their binding energies.  The electrochemical impedance spectra were also recorded for ZnO and ZnO/α-Fe2O3 with different weight percentages of α-Fe2O3 (1, 3, and 5 wt%) as illustrated in Figure 6. EIS was carried out to analyze the photoinduced charge-transfer resistance on the interfacial layers of the different modified electrodes [50]. This method is used to measure the amplitude of the alternating current (AC) signal with different frequencies that are able to observe the ratio of the AC signal voltage to the current with the variation of sinusoidal frequency (ω) or the phase angle of impedance. Further, this method is used to determine the frequency-dependent impedance of the electrochemical system by applying equivalent circuit modeling to observe the electrical properties of the system. Nyquist The electrochemical impedance spectra were also recorded for ZnO and ZnO/α-Fe 2 O 3 with different weight percentages of α-Fe 2 O 3 (1, 3, and 5 wt%) as illustrated in Figure 6. EIS was carried out to analyze the photoinduced charge-transfer resistance on the interfacial layers of the different modified electrodes [50]. This method is used to measure the amplitude of the alternating current (AC) signal with different frequencies that are able to observe the ratio of the AC signal voltage to the current with the variation of sinusoidal frequency (ω) or the phase angle of impedance. Further, this method is used to determine the frequency-dependent impedance of the electrochemical system by applying equivalent circuit modeling to observe the electrical properties of the system. Nyquist plots indicate the decrease in charge-transfer resistance, and they lead to a fast interfacial charge-transfer process as well as an effective separation of photogenerated electron-hole pairs [51]. Moreover, they give information about the nanomaterial properties, such as conductivity and the dielectric constant and interfacial region of capacitance and derived quantities. The semicircle of all the materials normally consists of charge-transfer resistance (R ct ) [52] and interfacial resistance (R int ) [53]. Nevertheless, R ct and R int are not easily separated from the depressed semicircle because their time constants are near to each other. Here, we consider the total resistance (R tot ), which is generally reflected in the electrical conductivity of all the materials. The low R ct suggests that these electrodes have a lower interfacial resistance with a better charge transfer. The smallest semicircle radius was observed in ZnO/α-Fe 2 O 3 (3 wt%), which has low impedance when compared with the other weight percentages of α-Fe 2 O 3 (1 and 5 wt%). This low impedance is due to the synergistic effect between ZnO and α-Fe 2 O 3 and thus improves the electron transfer charge. In addition, the importing interfacial charge separation between ZnO and α-Fe 2 O 3 also reduces exciton quenching and energy dissipation.

Hydrogen Evolution Reaction (HER)
The electrocatalytic activity of hydrogen evolution was measured for ZnO and the ZnO/α-Fe2O3 nanocomposites with different weight percentages of the α-Fe2O3 nanoparticles as shown in Figure  7. A HER study for all the catalysts was evaluated under identical conditions. The ZnO/α-Fe2O3 (3 wt%) nanocomposite shows a low onset potential of −125 mV to reach the current density of 10 mA cm −2 . However, the required onset potential for pure ZnO is −210 mV, which is higher than the hybrid nanocomposites to attain the benchmark current density of 10 mA cm −2 . This onset potential of ZnO/α-Fe2O3 (3 wt%) is shifted negatively compared with that of ZnO/α-Fe2O3 (1 and 5 wt%) and thus enhances the catalytic activity due to the superior electron transfer efficiency caused by the electronic conductivity, which is confirmed by EIS. Consequently, the ZnO/α-Fe2O3 (3 wt%) is perfect for hydrogen adsorption and enhances HER activity. The particular combination of α-Fe2O3 helps to improve the redox sites between the ZnO and α-Fe2O3 heterojunction to increase the conductivity. Further, the incorporation of Fe could enhance the adsorption of water on the surface of the nanocomposites. This leads to the transfer of electrons to the water molecules, thereby accelerating the formation of Hads. Thus, the increased HER activity of ZnO/α-Fe2O3 (3 wt%) probably originates in the facile charge transfer between the catalyst and the water molecules. From these results, we observed that the HER performance from a negative electrode arose from the reduction of the protons

Hydrogen Evolution Reaction (HER)
The electrocatalytic activity of hydrogen evolution was measured for ZnO and the ZnO/α-Fe 2 O 3 nanocomposites with different weight percentages of the α-Fe 2 O 3 nanoparticles as shown in Figure 7. A HER study for all the catalysts was evaluated under identical conditions. The ZnO/α-Fe 2 O 3 (3 wt%) nanocomposite shows a low onset potential of −125 mV to reach the current density of 10 mA cm −2 . However, the required onset potential for pure ZnO is −210 mV, which is higher than the hybrid nanocomposites to attain the benchmark current density of 10 mA cm −2 . This onset potential of ZnO/α-Fe 2 O 3 (3 wt%) is shifted negatively compared with that of ZnO/α-Fe 2 O 3 (1 and 5 wt%) and thus enhances the catalytic activity due to the superior electron transfer efficiency caused by the electronic conductivity, which is confirmed by EIS. Consequently, the ZnO/α-Fe 2 O 3 (3 wt%) is perfect for hydrogen adsorption and enhances HER activity. The particular combination of α-Fe 2 O 3 helps to improve the redox sites between the ZnO and α-Fe 2 O 3 heterojunction to increase the conductivity. Further, the incorporation of Fe could enhance the adsorption of water on the surface of the nanocomposites. This leads to the transfer of electrons to the water molecules, thereby accelerating the formation of H ads . Thus, the increased HER activity of ZnO/α-Fe 2 O 3 (3 wt%) probably originates in the facile charge transfer between the catalyst and the water molecules. From these results, we observed that the HER performance from a negative electrode arose from the reduction of the protons to H atoms on the ZnO/α-Fe 2 O 3 nanorod composites and produced hydrogen gas with the recombination of two H atoms. Moreover, acidic electrolytes of 0.5 M H 2 SO 4 were used, which is beneficial for the production of hydrogen for the large supply of H + ions in the solution.

Conclusions
The fabrication of ZnO nanorods/α-Fe2O3 composites was effectively prepared by the hydrothermal method. The synthesized photocatalyst demonstrates superior photocatalytic efficiency with moderate hydrogen evolution. The ZnO/α-Fe2O3 nanocomposites show the best HER results when compared with other ZnO and α-Fe2O3 materials. The photogenerated electrons were successfully separated and transferred, and the recombination of electron-hole pairs was avoided; thus, ZnO/α-Fe2O3 significantly improves the photocatalytic hydrogen production performance of the nanocomposites. The main advantages of this unique architecture, the ZnO/α-Fe2O3 nanocomposites, can provide a large number of active sites, and the highly conductive Ni foam can promote the transfer of electrons. The three-dimensional-networked structure can facilitate the diffusion and penetration of the electrolyte. Electrochemical measurements reveal that ZnO/α-Fe2O3 on Ni foam electrodes exhibits greatly improved performance for HER with a low onset overpotential (−125 mV). The achieved results will open up the fabrication of ZnO-based hybrid devices with lowcost materials.

Conclusions
The fabrication of ZnO nanorods/α-Fe 2 O 3 composites was effectively prepared by the hydrothermal method. The synthesized photocatalyst demonstrates superior photocatalytic efficiency with moderate hydrogen evolution. The ZnO/α-Fe 2 O 3 nanocomposites show the best HER results when compared with other ZnO and α-Fe 2 O 3 materials. The photogenerated electrons were successfully separated and transferred, and the recombination of electron-hole pairs was avoided; thus, ZnO/α-Fe 2 O 3 significantly improves the photocatalytic hydrogen production performance of the nanocomposites. The main advantages of this unique architecture, the ZnO/α-Fe 2 O 3 nanocomposites, can provide a large number of active sites, and the highly conductive Ni foam can promote the transfer of electrons. The three-dimensional-networked structure can facilitate the diffusion and penetration of the electrolyte. Electrochemical measurements reveal that ZnO/α-Fe 2 O 3 on Ni foam electrodes exhibits greatly improved performance for HER with a low onset overpotential (−125 mV). The achieved results will open up the fabrication of ZnO-based hybrid devices with low-cost materials.