CoFe Amorphous Double Hydroxides Modiﬁed Hematite Photoanode with the Synergism of Co and Fe for Enhanced Photoelectrochemical Water Oxidation

: In this study, a hematite photoanode with a CoFe-ADH co-catalyst loaded on the surface was successfully prepared through hydrothermal treatment, annealing, and electrodeposition. We investigated the inﬂuence of the deposition time and Co/Fe molar ratio for CoFe-ADH on the performance of α -Fe 2 O 3 in photoelectrochemical (PEC) water oxidation. With an optimized condition of CoFe-ADH, the prepared CoFe-ADH/ α -Fe 2 O 3 photoanode exhibited a high photocurrent density of 1.58 mA/cm 2 at 1.23 V RHE , which is about 2.5 times as high as that of α -Fe 2 O 3 . A series of characterization results and detailed mechanism studies reveal that surface modiﬁcation of hematite by introducing CoFe-ADH can boost the surface charge transfer efﬁciency, which can be attributed to the good optical transparency, the amorphous structure of CoFe-ADH, and the synergism of Co and Fe in CoFe-ADH. The good optical transparency contributes to decreasing the loss of light absorption by the photoanodes; the amorphous structure could prevent the formation of grain boundaries and provide more active catalytic sites for PEC water oxidation; and the synergism of Co and Fe in CoFe-ADH enhances photogenerated carriers effective separation and the hole’s injection. This work provides valuable insights into the inﬂuence of bimetallic co-catalysts on the PEC performance of photoanodes, offering guidance for photoanodes to achieve excellent performance


Introduction
Photoelectrochemical (PEC) water splitting has received widespread attention due to the urgent demand for hydrogen energy as a sustainable, clean energy source that can address economic and environmental problems associated with fossil fuels [1][2][3]. PEC decomposition of water into hydrogen and oxygen is a complex multi-electron reaction process powered by electrons and holes generated in a semiconductor catalyst under light irradiation [4]. There are two half-reactions in PEC water splitting, including the oxygen evolution reaction (OER) on the photoanode and the hydrogen evolution reaction (HER) on the photocathode. Compared with the two-electron HER, OER requires four electrons to finish the reaction from two water molecules to one oxygen molecule [5,6]. Therefore, developing efficient photoanode materials for PEC water splitting is a key challenge. To date, hematite has become one of the most promising photoanode materials because of its narrow band gap of about 2.1 eV, good chemical stability, superior theoretical solar-tohydrogen efficiency (15.3%), earth abundance, and nontoxicity [7,8]. However, the low charge separation and mobility, as well as the sluggish oxygen evolution kinetics, have significantly reduced the PEC performance of hematite, resulting in a large gap between theoretical and practical efficiency [9][10][11].
The introduction of oxygen evolution co-catalysts (OECs) on the surface of hematite is an effective approach to address the above problems regarding charge transfer kinetics [12][13][14]. Among various OECs, bimetallic hydroxides, including layered double hydroxides (LDHs) and amorphous double hydroxides (ADHs), have been considered ideal candidates due to their unique structure and versatility in composition. For instance, a series of bimetallic hydroxides have been used to couple with the semiconductor photoanodes to obtain excellent PEC performance, such as NiFe-LDHs [15,16], NiCo-LDHs [17], ZnCo-LDHs [18], CoAl-LDHs [19,20], and CoMn-LDHs [21]. Recently, CoFe bimetallic hydroxides have attracted research interest for (photo)electrocatalytic water oxidation because of the fact that the interaction between Fe and Co sites could favor the enhancement of OER activity [22,23]. It has been demonstrated that the medication with CoFe hydroxides could improve the PEC activities of TiO 2 [24], BiVO 4 [25][26][27], and g-C 3 N 4 [28,29]. In addition, Tang et al. [30] developed the interfacial heterojunction-engineered Fe 2 O 3 /CoFe-LDH catalyst for electrocatalytic oxygen evolution reactions, proving that the surface coupling Fe 2 O 3 could further enhance the electrocatalytic activities of CoFe-LDH catalysts. Despite the merits of using CoFe hydroxides as OECs, there are few reports of CoFe-LDHs or CoFe-ADHs modified hematite for PEC water oxidation. The underlying influences of Co and Fe in the CoFe hydroxide co-catalyst on the photoanode for PEC water oxidation are unclear.
Inspired by the above discussion, in this study, a CoFe-ADH/α-Fe 2 O 3 photoanode was prepared via hydrothermal treatment, annealing, and electrodeposition. The surface modification of the hematite photoanode by introducing CoFe-ADH was demonstrated to be an efficient strategy for enhancing the PEC performance of water oxidation. The influences of deposition time and Co/Fe molar ratio on CoFe-ADH were investigated to clarify its co-catalytic effect. This work contributes to a deeper understanding of the promotion mechanism of surface modification on the photoanodes for photoelectrochemical (PEC) water oxidation.

Results and Discussion
The synthesis process of CoFe-ADH/α-Fe 2 O 3 photoanode films is depicted in Figure 1a. First, the yellow β-FeOOH nanorod layer was prepared on the surface of the FTO glass substrate through a hydrothermal method. After that, the yellow layer of β-FeOOH transformed into the red layer of α-Fe 2 O 3 by annealing. Then, CoFe-ADH nanosheets were decorated on the surface of α-Fe 2 O 3 using a simple electrodeposition method. Through the cathodic reduction of NO 3 − to form hydroxide (NO 3 − + 7H 2 O + 8e → NH 4 + + 10OH − ), CoFe bimetallic hydroxide can be obtained at the α- [27,31]. The morphology and nanostructure of as-synthesized α-Fe 2 O 3 , CoFe-ADH(80)/α-Fe 2 O 3 , and CoFe-ADH(1200)/α-Fe 2 O 3 samples were characterized by SEM and TEM, and the obtained images are shown in Figure 1b-f. It is shown that α-Fe 2 O 3 is made up of numerous vertically aligned nanorods with average diameters of 70~100 nm (Figure 1a). After CoFe-ADH nanosheet deposition for 80 s, the nanorod-like structure is still well preserved (Figure 1b), and a slightly rougher surface can be observed for CoFe-ADH/α-Fe 2 O 3 compared with α-Fe 2 O 3 ( Figure 1c). As the deposition time increases for CoFe-ADH, the CoFe-ADH sheets can be seen clearly in Figure 1d, and the nanorods of α-Fe 2 O 3 are completely covered. The TEM and HRTEM images in Figure 1e,f confirm the morphology of CoFe-ADH(80)/α-Fe 2 O 3 . The α-Fe 2 O 3 nanorods are coated by CoFe-ADH nanosheets, forming a core-shell-like structure. In addition, the HRTEM image of the interface between α-Fe 2 O 3 and CoFe-ADH in the CoFe-ADH/α-Fe 2 O 3 sample is shown in Figure 1f. A lattice spacing of 0.252 nm is observed in one region, corresponding to interface between α-Fe2O3 and CoFe-ADH in the CoFe-ADH/α-Fe2O3 sample is shown in Figure 1f. A lattice spacing of 0.252 nm is observed in one region, corresponding to the (110) plane of α-Fe2O3. Meanwhile, no lattice fringes are observed in another region, corresponding to the amorphous state of CoFe-ADH.   peaks of hematite (JCPDS No. 86-0550). After depositing CoFe-ADH, the diffraction peaks ascribed to the hematite phase are still clearly identified. Although increasing the deposition time of CoFe-ADH even completely covers α-Fe 2 O 3 , we can only observe that the diffraction peak intensity of α-Fe 2 O 3 clearly reduces, but there is still no detectable peak of CoFe-ADH. These results could be attributed to its amorphous structure, which is in accordance with the HRTEM results. The XRD patterns of α-Fe2O3, CoFe-ADH(80)/α-Fe2O3, and CoFe-ADH(1200)/α-Fe2O3 are shown in Figure 2a. After deducting the diffraction peaks of the FTO substrate, the data from three kinds of photoanode samples can all be indexed to the characteristic peaks of hematite (JCPDS No. 86-0550). After depositing CoFe-ADH, the diffraction peaks ascribed to the hematite phase are still clearly identified. Although increasing the deposition time of CoFe-ADH even completely covers α-Fe2O3, we can only observe that the diffraction peak intensity of α-Fe2O3 clearly reduces, but there is still no detectable peak of CoFe-ADH. These results could be attributed to its amorphous structure, which is in accordance with the HRTEM results.   The CoFe-ADH(80)/α-Fe 2 O 3 samples were analyzed by XPS characterization to determine the surface elemental composition and prove the successful decoration of CoFe-ADH. As shown in Figure 2b, the peaks of C, O, Fe, and Co elements for CoFe-ADH(80)/α-Fe 2 O 3 can be clearly observed from the survey spectrum. The high-resolution XPS spectra of Fe 2p, O1s, and Co 2p are shown in Figure 2c-e. It can be seen from the Fe 2p XPS spectrum ( Figure 3b) that there are two major peaks at 711.0 eV and 724.5 eV, corresponding to Fe 2p 3/2 and Fe 2p 1/2 . After fitting deconvolution for Fe 2p 3/2 and Fe 2p 1/2 , the two peaks at 709.5 eV and 720.5 eV are attributed to Fe 2+ of α-Fe 2 O 3 . The two peaks next to Fe 2+ of α-Fe 2 O 3 at 710.9 eV and 723.9 eV are assigned to Fe 3+ of α-Fe 2 O 3 . The other two peaks appear at 713.4 eV and 726.0 eV, which are assigned to Fe 3+ in CoFe-ADH [32][33][34]. The peaks of O1s (Figure 3c) can be fitted into three peaks with binding energies centered at 529.9 eV, 531.3 eV, and 532.4 eV, which correspond to lattice oxygen species in hematite, hydroxyl groups bonded with metal cations of CoFe-ADH, and absorbed H 2 O, respectively [35]. The Co 2p XPS spectrum is shown in Figure 3d, revealing two major peaks at binding energies of Co 2p 3/2 and Co 2p 1/2 , along with satellite peaks located at 787.9 eV and 803.3 eV. The Co 2p 3/2 and Co 2p 1/2 peaks are located at 781.2 eV and 797.3 eV, suggesting the presence of Co 2+ in CoFe-ADH [36,37]. These results confirm the successful deposition of CoFe-ADL onto the surface of α-Fe 2 O 3 .
Catalysts 2023, 13, x FOR PEER REVIEW 5 of 12 The CoFe-ADH(80)/α-Fe2O3 samples were analyzed by XPS characterization to determine the surface elemental composition and prove the successful decoration of CoFe-ADH. As shown in Figure 2b, the peaks of C, O, Fe, and Co elements for CoFe-ADH(80)/α-Fe2O3 can be clearly observed from the survey spectrum. The high-resolution XPS spectra of Fe 2p, O1s, and Co 2p are shown in Figure 2c-e. It can be seen from the Fe 2p XPS spectrum ( Figure 3b) that there are two major peaks at 711.0 eV and 724.5 eV, corresponding to Fe 2p3/2 and Fe 2p1/2. After fitting deconvolution for Fe 2p3/2 and Fe 2p1/2, the two peaks at 709.5 eV and 720.5 eV are attributed to Fe 2+ of α-Fe2O3. The two peaks next to Fe 2+ of α-Fe2O3 at 710.9 eV and 723.9 eV are assigned to Fe 3+ of α-Fe2O3. The other two peaks appear at 713.4 eV and 726.0 eV, which are assigned to Fe 3+ in CoFe-ADH [32][33][34]. The peaks of O1s ( Figure 3c) can be fitted into three peaks with binding energies centered at 529.9 eV, 531.3 eV, and 532.4 eV, which correspond to lattice oxygen species in hematite, hydroxyl groups bonded with metal cations of CoFe-ADH, and absorbed H2O, respectively [35]. The Co 2p XPS spectrum is shown in Figure 3d, revealing two major peaks at binding energies of Co 2p3/2 and Co 2p1/2, along with satellite peaks located at 787.9 eV and 803.3 eV. The Co 2p3/2 and Co 2p1/2 peaks are located at 781.2 eV and 797.3 eV, suggesting the presence of Co 2+ in CoFe-ADH [36,37]. These results confirm the successful deposition of CoFe-ADL onto the surface of α-Fe2O3.  (d) EIS spectra at a potential of CoFe-ADH/α-Fe 2 O 3 with different Co/Fe molar ratio photoanodes (R s , R bulk , R ct , and CPE refer to the series resistance, the charge trapping resistance in bulk, the charge transfer resistance across the interface of photoanode and electrolyte and the constant phase element, respectively).
The PEC performance of the as-prepared photoanodes containing α-Fe 2 O 3 , CoFe-ADH/α-Fe 2 O 3 with different deposition times (40 s, 80 s, 160 s, and 320 s) was evaluated by the linear sweep voltammetry (LSV) curves under light irradiation. As shown in Figure 3a, the α-Fe 2 O 3 photoanode exhibits a very low PEC performance with a photocurrent density of 0.62 mA/cm 2 at 1.23 V RHE . By electrodepositing CoFe-ADH on the surface of α-Fe 2 O 3 , as the deposition time increases from 40 s to 80 s, the photocurrent density obviously increases from 0.87 mA/cm 2 at 1.23 V RHE to 1.58 mA/cm 2 at 1.23 V RHE , which indicates the more efficient photogenerated carrier transfer with increasing the CoFe-ADH deposition amount. However, with a further increase in the deposition time of CoFe-ADH from 80 s to 320 s, the photocurrent density decreases from 1.58 mA/cm 2 at 1.23 V RHE to 1.12 mA/cm 2 at 1.23 V RHE , which could be attributed to the fact that the light absorption of α-Fe 2 O 3 was hindered because of the excessive surface coverage of CoFe-ADH. To confirm this effect of CoFe-ADH on the light absorption of α-Fe 2 O 3 , the UV-vis DRS spectra were explored. As shown in Figure 3b, the CoFe-ADH(80)/α-Fe 2 O 3 and α-Fe 2 O 3 photoanodes present very similar absorbance curves, indicating the amorphous CoFe-ADH has little light absorption or scattering in the wavelength range of 300~800 nm. While the CoFe-ADH(320)/α-Fe 2 O 3 has an obviously enhanced light absorption in the visible range due to the deterioration of optical transparency for CoFe-ADH compared with the CoFe-ADH(80)/α-Fe 2 O 3 and α-Fe 2 O 3 photoanodes [26,38]. These results demonstrate that strong light absorption or scattering could be avoided for CoFe-ADH with a deposition time of 80 s. In this case, the majority of visible light could be transmitted to hematite, so CoFe-ADH(80)/α-Fe 2 O 3 shows the best PEC performance.
In addition, to study the effect between Co and Fe in the CoFe-ADH co-catalyst, the PEC performance of CoFe-ADH/α-Fe 2 O 3 with different molar ratios of Co/Fe was also investigated under AM1.5 sunlight irradiation to obtain the influence of the Co/Fe ratio on the PEC performance of the photoanode. The CoFe-ADH(80-1:3)/α-Fe 2 O 3 and CoFe-ADH(80-3:1)/α-Fe 2 O 3 photoanodes were prepared through the same synthesis method as CoFe-ADH(80)/α-Fe 2 O 3 , except that the ratio of Co and Fe in the precursor solution from 2:2 was replaced by 1:3 or 3:1. As shown in Figure 3c, the highest photocurrent density of 1.58 mA/cm 2 at 1.23 V RHE could be obtained when the Co/Fe molar ratio was 2:2, that is, CoFe-ADH(80)/α-Fe 2 O 3 . The CoFe-ADH(80-3:1)/α-Fe 2 O 3 photoanode shows a moderate photocurrent density of 1.32 mA/cm 2 at 1.23 V RHE , while the photocurrent density of CoFe-ADH(80-1:3)/α-Fe 2 O 3 is the lowest of these three photoanodes. It is noteworthy that an apparent positive shift of onset potential can be observed from 0.72 V RHE of CoFe-ADH(80)/α-Fe 2 O 3 to 0.83 V RHE of CoFe-ADH(80-1:3)/α-Fe 2 O 3 . It could be attributed to the insufficient active sites due to the low concentration of Co [19,39], which leads to low PEC performance. Although there is a higher Co concentration in the CoFe-ADH(80-3:1)/α-Fe 2 O 3 photoanode, its PEC performance is lower than that of the CoFe-ADH(80)/α-Fe 2 O 3 , implying the existence of the synergy effect of Co and Fe in CoFe-ADH to influence the PEC performance of the photoanode.
The EIS spectra of α-Fe 2 O 3 photoanode and three CoFe-ADH/α-Fe 2 O 3 photoanodes with different Co/Fe molar ratios at a potential of 1.23 V RHE under light irradiation were given in Figure 3d to analyze the influence of Co/Fe ratio on the charge transfer behaviors, and the EIS data were fitted through an equivalent circuit inset in Figure 3d, and the fitting values are listed in Table S1. The Nyquist plots of four photoanodes show a similar double semicircular shape. The two semicircles could be attributed to the charge trapping resistance of bulk and the charge transfer resistance from photoanode to electrolyte [40,41].  Furthermore, the value of R ct for photoanodes of CoFe-ADH(80)/α-Fe 2 O 3 (283.7 Ω) is lower than that of CoFe-ADH(80-3:1)/α-Fe 2 O 3 (417.9 Ω) and CoFe-ADH(80-1:3)/α-Fe 2 O 3 (550.8 Ω), demonstrating that Co and Fe in CoFe-ADH have a synergy effect on promoting the photogenerated carriers to be injected into the electrolyte. Thus, the CoFe-ADH(80)/α-Fe 2 O 3 photoanode with a Co/Fe molar ratio of 2:2 shows higher charge separation and transfer efficiencies.
Consequently, the optimized electrodeposition conditions for CoFe-ADH on α-Fe 2 O 3 were obtained, with an electrodeposition time of 80 s and a molar ratio of 2:2 for Co(NO 3 ) 2 to Fe(NO 3 ) 3 . The corresponding highest photocurrent density achieved under these conditions is 1.58 mA/cm 2 at 1.23 V RHE , which is 2.5 times higher than that of the pristine α-Fe 2 O 3 .
To further clarify the co-catalytic effect of CoFe-ADH on the α-Fe 2 O 3 photoanode and gain insight into the promoting mechanism of CoFe-ADH/α-Fe 2 O 3 for water oxidation, the transient photocurrent density curves of the pristine α-Fe 2 O 3 and CoFe-ADH/α-Fe 2 O 3 photoanodes were obtained and shown in Figure 4a. The transient photocurrent spikes for α-Fe 2 O 3 and CoFe-ADH/α-Fe 2 O 3 photoanodes can be observed under illumination. The α-Fe 2 O 3 presents a very strong photocurrent spike, which is much higher than that of CoFe-ADH/α-Fe 2 O 3 . It suggests that hematite suffers from severe charge recombination at the surface states, but this phenomenon could be alleviated by decorating CoFe-ADH, which might be attributed to the amorphous structure of CoFe-ADH (Figures 1g and 2a), implying that CoFe-ADH depositing reduces photocurrent loss of α-Fe 2 O 3 induced by surface and interfacial charge recombination. The amorphous structure could inhibit the generation of grain boundaries, provide more active catalytic sites [25,42], facilitate the extended lifetime of the charge carriers, and enhance water oxidation kinetics. However, the photocurrent spike of CoFe-ADH(320)/α-Fe 2 O 3 becomes high compared with that of CoFe-ADH(80)/α-Fe 2 O 3 , which might be attributed to the excessive deposition of CoFe-ADH, leading to the formation of new recombination centers to trap electrons and holes. Furthermore, the influence of CoFe-ADH on the transient decay time of the photogenerated holes was also investigated. As shown in Figure 4b, the transient decay time could be obtained according to the following Equation (1): where I t , I m , and I s represent the photocurrent at time t, the stabilized photocurrent, and the photocurrent spike, respectively. For the qualitative comparison, the transient decay time (τ) is defined as the time at which lnD = −1 [43,44]. Apparently, the τ value of CoFe-ADH(80)/α-Fe 2 O 3 is longer than that of pure α-Fe 2 O 3 and CoFe-ADH(320)/α-Fe 2 O 3 . It demonstrates the decrease in the recombination efficiency of surface and interfacial charge in CoFe-ADH(80)/α-Fe 2 O 3 , but the excess of CoFe-ADH loading is unfavorable. In order to understand the photogenerated charge carrier behavior, the PL spectra of α-Fe 2 O 3 , CoFe-ADH(80)/α-Fe 2 O 3 , and CoFe-ADH(320)/α-Fe 2 O 3 photoanodes are given in Figure S1. It can be seen that CoFe-ADH(80)/α-Fe 2 O 3 exhibits the lowest intensity in the PL emission spectra, implying the obviously high separation efficiency of photogenerated electron-hole pairs. The results of photoluminescence measurements further confirm the decreased carrier recombination efficiency of CoFe-ADH(80)/α-Fe 2 O 3 . In addition, the applied bias photo-conversion efficiency (ABPE) as a function for α-Fe 2 O 3 and CoFe-ADH/α-Fe 2 O 3 photoanodes is depicted in Figure 4c. The ABPE value of CoFe-ADH(80)/α-Fe 2 O 3 reaches up to 0.23% at 0.96 V RHE , which is about 2.6 times higher than that of α-Fe 2 O 3 without deposition of co-catalysts (0.09% at 1.01 V RHE ) and CoFe-ADH(320)/α-Fe 2 O 3 (0.07% at 1.05 V RHE ). Furthermore, the potential located at the maximum ABPE for the CoFe-ADH(80)/α-Fe 2 O 3 photoanode is negatively shifted by 50 mV compared with that of α-Fe 2 O 3 , which can be ascribed to the reduced surface charge recombination. The PEC stability of the CoFe-ADH(80)/α-Fe 2 O 3 photoanode was evaluated by applying a constant bias potential of 1.23 V RHE . It can be observed from  On the basis of the above discussions, a graphic mechanism for the PEC water oxidation process using the prepared CoFe-ADH/α-Fe2O3 photoanode is illustrated in Figure 5. Under light irradiation, hematite can be excited to generate electron-hole pairs. After CoFe-ADH deposits on the surface of hematite, Firstly, because there is good optical transparency for CoFe-ADH, it has little influence on the light absorption by the α-Fe2O3 photoanode, which facilitates the formation of photogenerated holes and electrons. Secondly, the amorphous structure of CoFe-ADH obtained by electrodeposition can prevent the formation of grain boundaries and provide more active catalytic sites for PEC water oxidation. Finally, in CoFe-ADH as co-catalysts, Co ions play the role of active sites to receive holes from hematite, and Fe ions contribute to decreasing surface trapping sites on hematite. The synergy effect of Co and Fe reduces surface charge recombination and improves the photogenerated injection efficiency of holes. Thus, it demonstrates the vital role of CoFe-ADH in enhancing PEC performance by increasing photogenerated electron and hole separation and transfer as well as decreasing recombination. On the basis of the above discussions, a graphic mechanism for the PEC water oxidation process using the prepared CoFe-ADH/α-Fe 2 O 3 photoanode is illustrated in Figure 5. Under light irradiation, hematite can be excited to generate electron-hole pairs. After CoFe-ADH deposits on the surface of hematite, Firstly, because there is good optical transparency for CoFe-ADH, it has little influence on the light absorption by the α-Fe 2 O 3 photoanode, which facilitates the formation of photogenerated holes and electrons. Secondly, the amorphous structure of CoFe-ADH obtained by electrodeposition can prevent the formation of grain boundaries and provide more active catalytic sites for PEC water oxidation. Finally, in CoFe-ADH as co-catalysts, Co ions play the role of active sites to receive holes from hematite, and Fe ions contribute to decreasing surface trapping sites on hematite. The synergy effect of Co and Fe reduces surface charge recombination and improves the photogenerated injection efficiency of holes. Thus, it demonstrates the vital role of CoFe-ADH in enhancing PEC performance by increasing photogenerated electron and hole separation and transfer as well as decreasing recombination.

Synthesis of Photoanodes
The hematite materials were fabricated on the FTO glass substrates using a hydrothermal-annealing method. To be specific, firstly, a precursor solution containing 0.15 M FeCl3·6H2O and 1 M NaNO3 was prepared. After that, four pieces of cleaned FTO were placed conductively face-down against the wall of the Teflon-lined autoclave and immersed in the above precursor solution. The autoclave was heated to 100 °C for 4 h, then cooled to room temperature. The resulting yellow materials attached to the surface of FTO are β-FeOOH. Finally, the obtained samples were washed with deionized water and subsequently annealed under air at 550 °C for 2 h in a muffle furnace and at 700 °C for 20 min in a muffle furnace.
CoFe-ADH materials were electrodeposited on the surface of the hematite photoanode substrates, and the synthesis procedure was as follows: The electrodeposition of CoFe-ADH samples on the substrate was performed in a three-electrode configuration with the substrate as the working electrode, a platinum plate as the counter electrode, and an Ag/AgCl reference electrode. The electrolyte consists of Co(NO3)2 aqueous solutions (concentration 0.006 M) and Fe(NO3)3 aqueous solutions (concentration 0.006 M). It was carried out at a constant potential (Edep = −0.8 V vs. Ag/AgCl). After deposition for a certain period of time, the working electrodes were carefully washed with copious amounts of water. The obtained samples with different deposition times were designated as CoFe-ADH (the deposition time)/α-Fe2O3. For example, the obtained samples with a deposition time of 80 s were designated as CoFe-ADH(80)/α-Fe2O3.

Characterization
The morphology and nanostructure of photoanode materials were characterized with a field-emission scanning electron microscope (FE-SEM, GeminiSEM 300, ZEISS, Oberkochen, Germany) and a high-resolution transmission electron microscope (TEM, FEI Tecnai G2 F20, FEI, Hillsboro, American). The crystal structure of all the photoanodes prepared was measured by XRD patterns using a SmartLab X-ray (Rigaku, Tokyo, Japan) diffractometer with Cu Kα radiation. The optical properties of photoanodes were evaluated via diffuse UV-vis absorption spectra on a Shimadzu (Kyoto, Japan) spectrophotometer with the integral sphere in the range of 300-800 nm. The surface elemental composition and chemical states were recorded through the X-ray photoelectron spectra (Al Kα Xray source, PHI Quantera II spectrometer, ULVAC-PHI, Kanagawa, Japan). The shifts of binding energies were calibrated using the C1s core level at 284.8 eV. The photoluminescence (PL) spectra were conducted through an Edinburgh Instrument FLS 920P (Edinburgh Instruments, Livingston, UK) fluorescence spectrophotometer.

Synthesis of Photoanodes
The hematite materials were fabricated on the FTO glass substrates using a hydrothermal-annealing method. To be specific, firstly, a precursor solution containing 0.15 M FeCl 3 ·6H 2 O and 1 M NaNO 3 was prepared. After that, four pieces of cleaned FTO were placed conductively face-down against the wall of the Teflon-lined autoclave and immersed in the above precursor solution. The autoclave was heated to 100 • C for 4 h, then cooled to room temperature. The resulting yellow materials attached to the surface of FTO are β-FeOOH. Finally, the obtained samples were washed with deionized water and subsequently annealed under air at 550 • C for 2 h in a muffle furnace and at 700 • C for 20 min in a muffle furnace.
CoFe-ADH materials were electrodeposited on the surface of the hematite photoanode substrates, and the synthesis procedure was as follows: The electrodeposition of CoFe-ADH samples on the substrate was performed in a three-electrode configuration with the substrate as the working electrode, a platinum plate as the counter electrode, and an Ag/AgCl reference electrode. The electrolyte consists of Co(NO 3 ) 2 aqueous solutions (concentration 0.006 M) and Fe(NO 3 ) 3 aqueous solutions (concentration 0.006 M). It was carried out at a constant potential (Edep = −0.8 V vs. Ag/AgCl). After deposition for a certain period of time, the working electrodes were carefully washed with copious amounts of water. The obtained samples with different deposition times were designated as CoFe-ADH (the deposition time)/α-Fe 2 O 3 . For example, the obtained samples with a deposition time of 80 s were designated as CoFe-ADH(80)/α-Fe 2 O 3 .

Characterization
The morphology and nanostructure of photoanode materials were characterized with a field-emission scanning electron microscope (FE-SEM, GeminiSEM 300, ZEISS, Oberkochen, Germany) and a high-resolution transmission electron microscope (TEM, FEI Tecnai G2 F20, FEI, Hillsboro, American). The crystal structure of all the photoanodes prepared was measured by XRD patterns using a SmartLab X-ray (Rigaku, Tokyo, Japan) diffractometer with Cu Kα radiation. The optical properties of photoanodes were evaluated via diffuse UV-vis absorption spectra on a Shimadzu (Kyoto, Japan) spectrophotometer with the integral sphere in the range of 300-800 nm. The surface elemental composition and chemical states were recorded through the X-ray photoelectron spectra (Al Kα X-ray source, PHI Quantera II spectrometer, ULVAC-PHI, Kanagawa, Japan). The shifts of binding energies were calibrated using the C1s core level at 284.8 eV. The photoluminescence (PL) spectra were conducted through an Edinburgh Instrument FLS 920P (Edinburgh Instruments, Livingston, UK) fluorescence spectrophotometer.

Photoelectrochemical Measurements
The photoelectrochemical (PEC) experiments were carried out on an electro-chemical workstation (CHI660E, CHI, Shanghai, China) using a typical three-electrode-cell system. This system contained a platinum plate (1 × 1 cm 2 ) as the counter electrode, an Ag/AgCl electrode as the reference electrode, and the prepared photoanodes as the working electrode. The electrolyte was 1 M NaOH, and the surface area of photoanodes that were contacted with the electrolyte was 0.283 cm 2 . The light source was obtained from a 300 W Xe lamp with an AM 1.5 filter (100 mW/cm 2 ). All the measured potentials vs. Ag/AgCl could be converted to the potentials vs. RHE using the following Equation (2): where E RHE is the converted potential vs. RHE. E Ag/AgCl is the measured result vs. Ag/AgCl. E 0 Ag/AgCl = 0.197 V at 25 • C. To obtain the photocurrent-potential curves, linear sweep voltammetry (LSV) measurements were performed using a voltage range of 0.5 V to 1.5 V vs. RHE with a scanning rate of 10 mV/s. The transient photocurrent densities of the photoanodes were measured by recording the photocurrent response at a potential of 1.23 V vs. RHE. ABPE can be calculated using the following Equation (3): where J represents the photocurrent density (mA/cm 2 ), V b refers to the applied bias vs. RHE, and P total is the total light intensity of AM 1.5 G (100 mW/cm 2 ).

Conclusions
In summary, we developed a hematite photoanode with CoFe-ADH deposited on the surface for PEC water oxidation. The resulting CoFe-ADH/α-Fe 2 O 3 photoanode showed an excellent photocurrent density of 1.58 mA/cm 2 at 1.23 V RHE , which was approximately 2.5 times higher than that of pristine hematite. The enhancement of PEC performance can be attributed to three aspects. Firstly, the good optical transparency of CoFe-ADH can reduce the loss of light absorption by the photoanodes, which promotes the generation of photo-excited holes and electrons. Secondly, the CoFe-ADH grown via electrodeposition is amorphous, which may prevent the formation of grain boundaries. Last but not least, the synergy effect of Co and Fe in CoFe-ADH can enhance the photogenerated electron and hole separation and transfer, reduce surface charge recombination of α-Fe 2 O 3 , and facilitate the rapid injection of photogenerated holes at the interface of photoanode and electrolyte.

Conflicts of Interest:
In this paper, all the authors declare that there is no conflict of interest.