Effect of tetravalent ions dopants and CoOx surface modification on hematite nanorod array for photoelectrochemical degradation of Orange-II dye

Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 305–315

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Effect of tetravalent ions dopants and CoOx surface modification on hematite nanorod array for photoelectrochemical degradation of Orange-II dye Nagsen Meshram1, Mahadeo A. Mahadik1, In-Kwon Jeong, Young Seok Seo, Min Cho∗, Jum Suk Jang∗ Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea

a r t i c l e

i n f o

Article history: Received 20 August 2018 Revised 26 January 2019 Accepted 16 February 2019 Available online 2 March 2019 Keywords: Hematite nanorod Tetravalent doping High temperature annealing Surface passivation

a b s t r a c t The surface modified tetravalent ions doped hematite nanorod photoanodes has been synthesized by successive hydrothermal and spin coating approaches. The effect of tetravalent ion (Sn, Ti, and Zr) doping and high temperature annealing on to the morphological, structural and photoelectrochemical properties has been investigated. Photoelectrochemical analyses indicate tetravalent doping (Sn, Ti, and Zr) and high-temperature annealing (800 °C) showed higher photocurrent and improved activity towards OrangeII dye degradation, compared to the pristine hematite film. Amongst tetravalent dopants, Zr4+ doped hematite can significantly enhance the photocurrent density (1.37 mA cm−2 at 1.23 V vs. RHE) as well as the Orange-II dye degradation activity (93% under one sun illumination in 270 min). Kinetic parameters are also investigated by first-order rate equation. Further, Zr–Fe2 O3 photoanode modified with the appropriate composition of CoOx and specific structural features demonstrated improvement in photocurrent from 1.6 to 1.78 mA cm−2 (at 1.4 V vs. RHE). The efficient charge carrier separation and generated hydroxyl radicals led to enhancements of the Orange-II dye degradation efficiency up to 97% within 270 min. The significant decrease in chemical oxygen demand values suggests the removal of Orange-II dye. This work demonstrates that enhancement in photoelectrochemical activity is due to the combined effect of passivation of surface states and the formation of CoOx /Zr–Fe2 O3 heterojunction. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Recently, the continuous increase in pollution concomitant with rapid industrialization has become a serious and pressing environmental issue. Most of the dye producing and textile manufacturing industries release potentially toxic dye effluents, which are carcinogenic and mutagenic to mammalian, as well as aquatic life. Within the overall category of dyestuffs, Azo dyes are the most stable and are widely used in various pharmaceutical, cosmetic, food additive, textile, and printing industries [1]. The presence of the organic pollutants (pesticides, dyes, and pharmaceuticals) in wastewater has been very serious and pressing the environmental issue, therefore, it is important to remove these toxic dyes from wastewater. Several approaches such as adsorption on high surface area supports, chemical precipitation, sedimentation, biological membranes, and

∗

1

Corresponding authors. E-mail addresses: [email protected] (M. Cho), [email protected] (J.S. Jang). Both authors equally contributed for this work.

ion-exchange processes etc. has been devoted to the removal of toxic dyes [2–5]. The photocatalysis is also one of the attractive processes where semiconductor absorbs the light to produce the photogenerated electron–hole (e–h) pairs which further participate in the redox reaction [6,7]. In recent years, intermediate band gap metal oxides have been intensively investigated as photocatalyst materials [8,9]. Amongst the metal oxide semiconductors, nanostructured hematite (α -Fe2 O3 ) has emerged as an efficient catalyst for water detoxification due to its excellent environmental compatibility, high chemical stability and suitable bandgap energy [10]. Besides its several advantages, Fe2 O3 suffers from low absorption coefficient, low carrier mobility (<1 cm2 V−1 s−1 ), short hole diffusion length (2–4 nm), and shorter excited state lifetime (∼10 ps) [11]. Recent reports demonstrated that the various approaches (elemental doping, heterojunctions and surface modifications) have been attempted to overcome the limitations of hematite [12–18]. Generally, insertion of tetravalent dopants (Zr4+ , Si4+ , Sn4+ , and Ti4+ ) into the hematite lattice (iron substituting site) can attribute to enhancement in photocurrent response. This is due to the extra electron from the dopant increases the

https://doi.org/10.1016/j.jtice.2019.02.025 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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number of charge carriers for the conduction process. Further improvement in the PEC activity of single material and making the process commercially viable, a formation of heterojunctions, such as α -Fe2 O3 /Bi2 WO6 , and TiO2 /Fe2 O3 [19–21], could be one of the outstanding strategy. Although heterostructures enhance the photoelectrochemical performance, they suffer from the surface recombination issues of the photogenerated charge carriers. Hence, the use of co-catalysts on the surface of photoanode is an effective strategy to avoid the photocorrsion of heterostructures by allowing the prompt migration of photogenerated holes to the electrolyte [22]. Lately, several cost-effective metal oxides co-catalysts such as IrO2 [23], RuO2 [24], CoOx [25], Co–Pi [26], WO3 [27] etc. has been exploited. Among them, the cobalt (Co) based co-catalysts have received much attention, due to it lowers the surface states, when coupled with a range of metal oxide photoelectrodes, such as ZnO [28–30], WO3 [27], and α -Fe2 O3 [30,31]. Also, one of the studies by Xia et al. reported that BiVO4 photoanode cocatalyzed with an ultrathin α -Fe2 O3 layer, which facilitates the easy and efficient hole transfer to the surface, and showed greatly enhanced photoelectrochemical performance towards the phenol degradation [32]. Even though some studies have mainly focused on the Fe2 O3 heterojunction for the degradation of organic dyes [2–5,20], however the detailed charge transfer mechanism in CoOx modified Zr4+ doped Fe2 O3 nanorod photoelectrode and its photoelectrocatalytic (PEC) Orange-II dye degradation is not reported yet. In this paper, we prepared tetravalent ion (Zr4+ , Sn4+ , and Ti4+ ) doped hematite nanorod photoanodes for the photoelectrochemical Orange-II dye degradation under solar illumination. The ex-situ doping of Zr4+ , Sn4+ , and Ti4+ was performed by immersion of akaganeite (β -FeOOH) nanorods in a metal precursor solution, followed by high-temperature sintering at 800 °C for the activation of hematite. We have also addressed the effect of CoOx passivation layer on optimum Zr–Fe2 O3 photoanodes. Lastly, we demonstrate a CoOx modified Zr–Fe2 O3 nanorods for the photoelectrochemical Orange-II degradation and compared the efficiency with pristine and the Zr–Fe2 O3 photoanodes. These findings demonstrate that the synergistic effect between tetravalent doping and CoOx co-catalyst results in the advances in degradation efficiency. Additionally, the charge transport mechanism in CoOx modified Zr4+ doped hematite nanorods photoanode is explicitly explained. 2. Experimental 2.1. Synthesis of hematite (α -Fe2 O3 ) based nanorods The α -Fe2 O3 based nanorod array photoanodes were synthesized from akaganeite (β -FeOOH) nanorods grown on fluorine doped tin oxide (FTO) substrate by a hydrothermal method [33]. The FTO substrates (1 cm × 2.5 cm) were cleaned by ultrasonication with deionized water (DI water), ethanol, and acetone for 30 min, and then dried with dry nitrogen gas flow. The cleaned FTO substrates were placed in a 20 ml vial, containing 10 ml aqueous solution of 1 M NaNO3 and 0.15 M FeCl3 ·6H2 O (pH ∼ 1.5, concentrated HCl). Further, vials were kept in autoclave oven at temperature 100 °C for 6 h for the growth of akaganeite nanorods. The as-synthesized samples were washed with DI water several times, and dried with N2 gas. Further, samples were heat treated at 800 °C for 10 min for transformation of the hematite phase from akaganite phase. Tetravalent ion doping (Zr, Ti, and Sn) was achieved by dipping the β -FeOOH electrodes in 9 mM respective dopant solutions (viz, Zirconyl nitrate, Titanium diisopropoxide bis(acetylacetonate), and Tin chloride) for 2 min, dried gently with N2 gas, followed by 800 °C sintering for 10 min. These photoanodes were labeled as pristine Fe2 O3 , Zr–Fe2 O3 , Ti–Fe2 O3 , and Sn–Fe2 O3 , respectively. To prepare the CoOx overlayer, different concentrations (0.5, 1, 3, 5) mM of Co(OAc)2 solution in D.I. water was spin

coated (2500 rpm for 25 s) over the sintered Zr–Fe2 O3 followed by heat treatment at 250 °C for 15 min. Schematic representation of experimental procedure for pristine and CoOx modified Zr–Fe2 O3 photoelectrodes is shown in Fig. 1. 2.2. Characterizations Structural properties of the synthesized nanorod photoanodes were studied by X-ray diffractometer XRD (Rigaku RINT 2500) with a Cu Kα radiation source. The morphology of the photoanodes was analyzed using field emission scanning electron microscope (FESEM, ZEISS SUPRA 40VP) equipped with X-ray energy dispersive spectrometry (EDS). Optical absorption study of samples was performed using UV−Vis-DRS spectra by dual-beam spectrophotometer (Shimadzu, UV−2600 series) in the wavelength range 30 0−80 0 nm. Photo-electrochemical characterizations of the prepared photoanodes were performed by potentiostat (CompactStat, Ivium) in 1 M NaOH electrolyte solution (pH ∼ 13.6). Threeelectrode cell configuration was used in this experiment with the prepared photoanode as the working electrode, with Ag/AgCl (3 M NaCl) reference electrode, and platinum coil as the counter electrode. All the potentials were reported against the reversible hydrogen electrode (RHE) [34].

ERHE =EAgCl +0.059pH+E o AgCl , with E o AgCl = 0.1976V at 25◦ C

(1)

A portable potentiostat (CompactStat, Ivium, Netherlands) equipped with an electrochemical interface and impedance analyzer was employed for EIS measurements. The experimental EIS data were fitted using suitable equivalent circuit model using the Z View (Scribner Associates Inc.) software. Orange-II dye degradation experiments were performed with the specially designed photoelectrochemical reactor with the hematite based photoanodes, reference and counter electrodes. The PEC reactor was filled with 70 ml of 10 μM of Orange-II dye solution prepared in 1 M NaOH supporting electrolyte. The photoelctrochemical reaction was performed by applying a potential to the working electrode vs. RHE (i.e. 1.1 and 1.2 V) under solar simulator light source (AM1.5 G). A photograph of the photoelectrochemical reactor arrangement is shown in Fig. S1 (Supporting Information (SI)). Absorption spectra of freshly prepared Orange-II dye solution during the photoelectrochemical reactions were recorded by UV–vis spectrophotometry (Shimadzu, model-UV mini 1240). The starting point of the reaction was defined when the PEC reactor exposed to the solar simulator light. Further, as the reaction proceeds, after every 30 min, 1.2 ml of dye solution was taken out of the PEC reactor, and its absorption spectra analyzed by spectrophotometry for 5 h. Similarly, photochemical (PC; without bias) and electrochemical (EC; with bias in dark) dye degradation studies were performed, according to the similar manner. The degradation efficiency is calculated by the following equation [35]:

Degradation efficiency (Deg. Efficiency ) =(C0 −Ct )/C0 ×100,

(2)

where, C0 is the initial concentration of Orange-II dye solution, and Ct is the concentration of Orange-II dye at the time t. 3. Results and discussion The morphologies of the hydrothermally grown pristine and Zr, Ti, and Sn-doped α -Fe2 O3 samples were investigated by FESEM. Figure S2 shows the top view images of the pristine and Zr, Ti, and Sn-doped α -Fe2 O3 photoanodes. Pristine hematite nanorods are observed to be uniformly grown on FTO substrate with diameter of 80–100 nm and length of ∼350 nm. After Zr, Sn, and Ti doping, the morphology of the nanorods array changed slightly; some nanorods are observed to be merged together. Such dense

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Fig. 1. Schematic representations of experimental work for pristine and CoOx modified Zr–Fe2 O3 photoelectrodes.

Fig. 2. (a) photocurrent density vs. potential curves under the dark and simulated AM 1.5 illuminations conditions for the pristine and tetravalent ions doped hematite nanorods array photoanode, (b) first order derivative curve of photocurrent density, and (c) photoconversion efficiency (PCE) of pristine, Ti, Sn, and Zr doped hematite photoanode, (Electrolyte-1M NaOH aqueous solution with pH = 13.6).

structure could also be helpful to increase the photoanode conductivity, and thereby its activity. Fig. S3 shows the XRD pattern of the pristine, and Zr, Ti, and Sn-doped Fe2 O3 photoanodes. All diffraction peaks are matched with the hematite (α –Fe2 O3 ) (denoted as ‘H’, JCPDS 33–0664), along with FTO substrate (denoted as ‘F’, JCPDS 41–1445). However, the small variation in the 2θ position and width of the hematite (110) peak is due to the variation in crystallite size and lattice strain of the hematite phase, which might be affected by doping by the various dopant. Fig. 2(a) illustrates the PEC analysis of the prepared pristine, Sn, Ti, and Zr-doped hematite nanorod photoanodes. The photocurrent density of hematite based photoanodes was dramatically improved

after tetravalent doping. Photocurrent densities of 1.17, 1.4, 1.6, and 1.71 mA cm−2 are observed for the pristine, Ti, Sn, and Zr doped hematite nanorods at 1.4 V vs. RHE, respectively. Interestingly, in the case of Sn and Zr-doped photoanodes, the increment is higher compared to Ti-doped hematite sample. The highest photocurrent density of 1.71 mA cm−2 was achieved for Zr–Fe2 O3 sample, indicating the incorporation of Zr4+ dopants into the Fe2 O3 lattice. The trivalent Fe3+ ions of Fe2 O3 can be replaced by the tetravalent dopant M4+ ions, which can increase the electron carriers for conduction. This enhances the redox reaction rate and electron transport properties during photoelectrochemical analyses. In addition to the enhancement in photocurrent density, the onset potentials

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Fig. 3. (a) Nyquist plots of the pristine and doped α -Fe2 O3 photoanodes at 1.4 V vs. RHE in 1M NaOH electrolyte under AM 1.5 illuminations. Inset shows the equivalent circuit model, and (b) Mott–Schottky plot of the pristine and doped α -Fe2 O3 photoanodes.

of the Sn–Fe2 O3 , Ti–Fe2 O3 , and Zr–Fe2 O3 doped photoanodes are shifted after the tetravalent doping. The onset potential appears at 0.91, 0.92, and 0.80 V vs. RHE for the Sn–Fe2 O3 , Ti–Fe2 O3 , and Zr– Fe2 O3 , respectively, and exhibited an anodic shift of 0.25, 0.26, and 0.14 V for Sn, Ti and Zr doped Fe2 O3 than the pristine Fe2 O3 . This potential shift of the photoanodes can also be evaluated by the first order derivative curves of photocurrent density with respect to potential [36,37]. The dJ/dV curve gives a similar onset value, which we observed in the photo response curve given in Fig. 2(b). A small anodic shift in the onset potential and the higher photocurrent of the Zr doped Fe2 O3 photoanodes suggest Zr4+ is an appropriate dopant for the hematite. The photo-conversion efficiency (PCE) of all the prepared photoelectrodes is calculated using the following equation:

 





0 PCE (% )= Jph Erev − |Eb | /Pin × 100

(3)

where, Jph is the photocurrent density (mA cm−2 ); Pin is the incident light intensity (100 mW cm−2 ); E0 rev is the standard statereversible potential for water splitting (1.23 eV). Eb is the applied bias potential, which is the difference between the applied potential measured vs. Ag/AgCl, and the electrode open-circuit potential [38]. Fig. 2(c) shows that the pristine hematite attains maximum PCE of 0.47% at 1.23 V vs. RHE, whereas Ti–Fe2 O3 and Sn–Fe2 O3 nanorods showed 0.53% and 0.71% PCE, respectively. However, the maximum PCE of 0.75% at 1.23 V vs. RHE was achieved for Zr–Fe2 O3 nanorods. To gain further insights into the influence of tetravalent dopants on the charge-transfer processes and charge carrier densities of hematite photoelectrode the EIS and MS analyses were conducted. Fig. 3(a) shows the Nyquist plots for the pristine and tetravalent doped Fe2 O3 measured under light illumination at 1.4 V vs. RHE. The experimental data were fitted to an equivalent circuit model (inset of Fig. 3(a)). The circuit model consists of (i) the series resistance Rs (resistance of the conducting substrate and the electrolyte), (ii) Rtrap and Rct are representing the charge transfer resistances of the hematite nanorod photoelectrodes and resistance between photoelectrode to the electrolyte solution, respectively. The CPE1 and CPE2 are the space charge capacitance for hematite nanorods and the surface state capacitance, respectively. Table 1 shows that, after the Ti treatment, the resistance value (Rtrap ) of the hematite nanorods is reduced by a factor of half from that of the pristine (i.e. from 777 to 423 ). This further supports the Ti incorporation into the bulk of the hematite film [39]. Also, the Rct decreased from 199 to 117 , indicating the enhanced electrical conductivity upon Ti4+ doping and reduced electron transport resistance in Ti doped Fe2 O3 compared to pristine Fe2 O3 photoanodes. Moreover, after Sn doping,

the lowest arc of the EIS semicircle resulted due to reduced photogenerated charge recombination in hematite photoanodes. The decrease in Rtrap values to 370  is due to the Sn4+ ions diffusing into the hematite lattice from surface of the film by substitution, after high-temperature annealing [40]. Additionally, during annealing at 800 °C, partial release of Sn4+ ions from FTO substrate occurs, which readily diffuse into the Fe2 O3 by migrating toward the crystallizing Fe2 O3 lattice. Thus, Sn dopants in the hematite lattice act as the electron donor, which improves the electrical conductivity of hematite. Further, the charge balance could be maintained by cation vacancies or partial reduction of Fe3+ into Fe2+ [41]. The decreased value of Rct of Sn doped Fe2 O3 from 199 to 64  with higher trap state capacitance (CPE2 ), as shown in Table 1, is due to the effective and fast photogenerated charge carrier separation and interfacial charge transfer to the electron donor or acceptor [42]. However, compared to the Sn and Ti dopant ions, the diffusion rate of Zr ions is relatively smaller, and gives shallow doping of Zr into hematite. This led to the higher Rtrap value of the Zr doped Fe2 O3 than the Sn and Ti doped samples [43]. However, the observed lower value of Rtrap in Zr doped Fe2 O3 to that of the pristine sample is due to the n-type lattice substitution of Fe3+ by Zr4+ [44]. The low charge transfer resistance (Rct ) in Zr–Fe2 O3 compared to the other samples is due to the efficient doping as well as surface passivation by Zr ions, which decreases surface trap sites, and can increase the charge transport at the electrode/electrolyte interface [45]. Thus, the enhancement in photocurrent density of Zr–Fe2 O3 can be attributed to the combined effect of shallow Zr4+ ion doping as well as surface passivation by Zr ions. The negative shift in the onset potential indicates the improvement in surface kinetics of the Zr doped Fe2 O3 compared with the Sn and Ti doped samples [46]. In order to clarify the enhanced charge density upon dopant incorporation in the doped Fe2 O3 , Mott–Schottky analysis was conducted. The positive slopes of the Mott–Schottky plot of the pristine and doped Fe2 O3 photoanodes suggested the tetravalent atoms act as n-type dopants in hematite (Fig. 3(b)). Table 1 shows calculated values of carrier densities from the slopes of the Mott– Schottky plots for the pristine and doped Fe2 O3 photoanodes. The noticeable increase of carrier density in the Ti doped Fe2 O3 compared to the other samples clearly indicates the effective Ti incorporation in Fe2 O3 , and hence the improved electrical conductivity [47]. After successfully synthesis and optimization of photoelectrode the photocatalytic ability of pristine and doped hematite photoanode was investigated using the photocatalytic degradation of Orange-II dye. The UV–vis absorption peaks of Orange-II dye solution were measured and analyzed before and after exposure

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Table 1 Photocurrent density, parameters determined from EIS fitting, and Carrier densities of pristine and doped hematite photoanode. Sample

Jph (μA cm−2 )

R s ( )

Rtrap ()

Rct ()

CPE1 (μF)

CPE2 (μF)

Carrier density (cm−3 )

Pristine Fe2 O3 Ti–Fe2 O3 Sn–Fe2 O3 Zr–Fe2 O3

922 961 1120 1370

48 50 32 31

777 423 370 666

199 117 64 45

13.66 24.5 20.92 24.39

212 1511.6 279.1 163.8

1.47616 × 1020 4.80255 × 1020 3.29446 × 1020 2.17061 × 1020

Fig. 4. (a) UV–vis absorption spectra of Orange-II dye in the supporting electrolyte 1M NaOH solution after PEC degradation at potential 1.2 V (b) corresponding photocurrent profile with respect to time, (c) PEC dye degradation kinetics of Orange-II by pristine Fe2 O3 and Sn, Ti, and Zr doped Fe2 O3 photoanode, and (d) degradation efficiency of Orange-II dye on various doped hematite photoanode.

of light to explore the visible light PEC dye degradation activity of the doped α -Fe2 O3 nanostructures photoanode up to 270 min (Figs. 4(a) and S4(a)–(c)). The absorption peak intensity of dye solution was found to be decreased with increasing light exposure time, indicating the photo-electrochemical degradation of Orange-II dye. Figs. S4(a)–(c) shows the UV–vis absorption spectra of Orange-II dye solution during PEC reaction for pristine Sn, Ti, and Zr doped hematite electrode, respectively. Fig. 4(b) shows the stability of the pristine and Sn, Ti, and Zr doped hematite photoanodes at 1.2 V vs. RHE under the one sun illumination conditions during dye degradation. As seen in the PEC measurement (Fig. 2(a)), Zr–Fe2 O3 shows the highest photocurrent compared to the pristine, Sn, and Ti photoanode, a similar trend observed during dye degradation analysis. The slight increment in the photocurrent density with time is possibly due to decolorization of solution which increases the light absorption by the photoanode and generates the maximum charge carrier after illumination. Also, the prolonged illumination promotes the production of H2 O2 at the cathode surface, which acts as a more powerful electrontrapper than oxygen molecule. Both electron capture by chemical H2 O2 and electron transfer via external circuit can work together to promote electron–hole pair separation in hematite photoanode.

This effective separation ultimately contributes the hydroxyl radical generation, as well as the degradation reaction of dye. Fig. S4d shows the pseudo first order degradation kinetics of dye using pristine and doped hematite photoanodes. The degradation rates of the pristine, Sn, and Ti doped films show almost similar rate and lie in the range from 5–6 × 10−3 /s (Fig. 4(c)). However, Zr-doped film shows a higher degradation rate (9 × 10−3 /s), compared to the pristine and Ti and Sn dopant. Moreover, Fig. 4(d) shows that Zr doped photoanode exhibited the highest degradation efficiency (93%) compared with the Ti (75%) and Sn (80%) doped and pristine films. This indicates that a high degradation rate is achieved by Zr-doped hematite photoanode for the Orange-II dye degradation. The catalytic activity of the Zr–Fe2 O3 film for Orange-II dye degradation was also investigated in various configuration with (a) photoelectrocatalytic (PEC; at a bias potential of 1.2 V vs. RHE), (b) photocatalytic (PC; PEC without bias potential), and (c) electrocatalytic (EC; at a bias potential of 1.2 V vs. RHE in dark condition). Fig. S5 (a)–(c) shows the UV–vis absorption spectra of OrangeII dye solution after PC, EC, and PEC reaction on the Zr–Fe2 O3 photoanodes for 270 min. The characteristic peak of Orange-II at 550 nm was found to be decrease continuously with reaction time. Figs. S5 and 5(a) show the degradation in PEC reaction is more

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Fig. 5. (a) Degradation kinetics of Orange-II dye by the pristine under photoelectrocatalytic (PEC), photocatalytic (PC), and electrocatalytic (EC) conditions (for PEC and EC at the applied potential of 1.2 V vs. RHE), and (b) degradation efficiency of Orange-II dye at PEC, PC, and EC conditions.

rapid than the EC and PC reactions. The degradation performance of the Zr-Fe2 O3 film in the different processes was in the order PEC > PC > EC. In the PEC process, at bias potential of 1.2 V RHE, 93% of Orange-II was degraded after 270 min. However, Fig. 5(b) shows that only 42.0% was removed in the photocatalytic and 3.0% in electrochemical processes, respectively, in the same duration. The highest rate constant of 0.0091 min−1 was obtained for PEC, which is ∼5 times greater than that for PC (0.002 min−1 ) and 10 times EC (0.0 0 011 min−1 ), as shown in Fig. S5(d). The lower rate constant in PC is due to the poor transport properties of photogenerated charges without an applied bias. This reveals that under the visible light and applied positive bias the Zr–Fe2 O3 photoanode can improve the charge separation and significantly promotes the degradation of Orange-II dye. The effect of the bias potential on the PEC degradation rate of Orange-II dye was investigated under one sun illumination, as shown Fig. 6(a). Higher degradation rate is achieved at the applied potential 1.2 V vs. RHE than 1.15, 1.3, and 1.4 V, indicates effective photoelectrochemical degradation occurs at potential 1.2 V vs. RHE under one sun illumination. Furthermore, up to 60 min, the degradation efficiency of Orange-II dye was observed to be similar, but after 60 min only with potential 1.2 V vs. RHE, the dye solution degraded rapidly compared to at other potentials. The maximum degradation efficiency obtained is 93% for the Zr–Fe2 O3 photoanode at potential 1.2 V vs. RHE, as shown in Fig. 6(b). The photocurrent stability profile of the Zr–Fe2 O3 photoanode measured at various applied potentials (Fig. 6(c)), show that the higher photocurrent obtained for 1.4 V vs. RHE. The limited electrochemical degradation of dye could take place when the applied potential is well below the redox potential of the dye. However, when the bias potential is higher than the dye potential, the electro-oxidation reaction may take place. The influence of applied bias potential on Orange-II dye degradation was more significant at a higher potential than at lower potential [48]. In the present study, the photo-induced carrier reached sufficient separation at the applied potential of 1.2 V vs. RHE. Once the photocurrent exceeded 1.2 V, the degradation efficiency did not increase with the increasing the applied bias potential. A similar phenomenon was observed in previous reports; the slow degradation was attributed to the mechanism of dye oxidation due to the simultaneous oxygen evolution at higher potential [48,49]. At an applied potential, the photocatalytic degradation rate of Orange-II is promoted by separation and transfer of photogenerated holes and electrons. Also, the applied bias potential not only separates the holes and electrons, but can also directly electrolyze the dye. Therefore, in the PEC system, the dye degradation was through both the direct electro-oxidation and photocatalysis. At a potential greater than

1.23 V vs. RHE, the water discharges on the Zr–Fe2 O3 electrode surface may lead to the form of • OH by Eqs. (5) and 6, and •O2 − by Eq. (8) [49,50].

Zr − Fe2 O3 + hv → Zr − Fe2 O3 + h+ + e

(4)

H2 O + h+ → OH− + H+

(5)

OH− + h+ →• OH

(6)

Zr − Fe2 O3 [• OH] + Or − II →→ CO2 + by products

(7)

where, Or-II stand for target dye (Orange-II).

O2 + e− →• O− 2

(8)

+ • O•− 2 +H → OOH

(9)

OOH• + H+ + e− → H2 O2

(10)

H2 O2 + e− → • OH + OH−

(11)

O2 + 2H+ + 2e− →→ H2 O2

(12)

−)

The superoxides (•O2 formed during the photochemical process can produce additional amounts of hydroxyl radicals (Eqs. (8)– (11)). However, at higher potential, the increase of the dissolved oxygen in the solution (electrochemical evolution of oxygen at the anode) may promote the production of H2 O2 at the cathode surface by Eq. (12). According to Masui et al., the H2 O2 formed at the cathode surface has a lower activity than • OH radical, and does not degrade dye efficiently. In order to check the effect of CoOx loading on the photocurrent density and stability of Zr–Fe2 O3 photoanodes, the surface of photoanodes was spin-coated with different concentration 0.5, 1, 3, and 5 mM of Co(OAc)2 precursor. Fig. 7(a) shows the photocurrent density of Zr–Fe2 O3 photoanode was observed to increase after CoOx modification. After surface modification with 0.5 mM Co(OAc)2 solution, the photocurrent density was increased from 1.65 to 1.72 mA cm−2 at 1.4 V vs. RHE with slight shift in the onset potential. However, further, increase in the Co(OAc)2 concentration to 1 mM the photocurrent density value reached at 1.78 mA cm−2 . After, 1 mM Co(OAc)2 precursor concentration, the photocurrent density was observed to be decreased to 1.67 and 1.58 mA cm-2 ,

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Fig. 6. Orange-II dye degradation kinetics of pristine Zr–Fe2 O3 photoanode by PEC at various potentials, (b) Orange-II dye degradation efficiency, and (c) photocurrent profile of Zr–Fe2 O3 photoanode with respect to degradation time for different applied potentials (1.15, 1.2, 1.3, and 1.4) V).

Fig. 7. J–V curves of CoOx modified Zr–Fe2 O3 nanorod photoanodes at various Co–concentrations, inset shows the variation of photocurrent density vs. Co– concentrations at an applied potential of 1.4 V vs. RHE, and (b) transient photocurrent profile with cut-off cycle at 1.4 V vs. RHE for CoOx modified Zr–Fe2 O3 photoanode.

respectively, for 3 and 5 mM Co(OAc)2 concentration. A maximum photocurrent density was obtained for the 1 mM Co(OAc)2 solution with the cathodic shift of 200 mV (inset of Fig. 7(a)). Thus, the increase of the current density with increasing the amount of cobalt solution can be attributed to passivation of the surface traps, as well as the oxidative nature of CoOx in newly formed CoOx /Zr– Fe2 O3 heterojunction. However, a further increase in the amount of cobalt leads to aggregation of the CoOx nanoclusters into larger CoOx particles, which decreases the kinetics of the reaction and absorption efficiency of the hematite. Fig. 7(b) shows transient photocurrent response of surfacemodified Zr–Fe2 O3 photoanode. The highest photocurrent density was obtained for the 1 mM Co(OAc)2 treated Zr–Fe2 O3

photoelectrode in 1 M NaOH electrolyte (A similar trend was obtained as seen in Fig. 7(a)). CoOx modification of Zr–Fe2 O3 photoanodes showing the absence of any spike, which indicates CoOx on the Zr–Fe2 O3 surface promotes water oxidation without any side reaction. After successful optimization, the CoOx , modified Zr–Fe2 O3 photoanode with high photocurrent density was employed for the dye degradation. Fig. 8 shows the FESEM images of the CoOx modified Zr–Fe2 O3 nanorods prepared by spin coating at various concentrations (0.5, 1, 3, and 5) of Co(OAc)2 . It is observed that shape and size of the Zr doped Fe2 O3 nanorod retained after the surface modification with CoOx . Also, with increasing the concentration of spin-coated Co(OAc)2 (Fig. 8(a)–(d)), the surface of nanorods is slightly changed than the pristine (Fig. S2). It

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Fig. 8. FESEM images of surface modified Zr–Fe2 O3 nanorod arrays prepared by spin coating of Co(OAc)2 at various concentrations of (a) 0.5 mM, (b) 1 mM, (c) 3 mM and (d) 5 mM, respectively, (e) schematic of formation of CoOx layer on the Zr–Fe2 O3 nanorod arrays.

suggests the presence of CoOx layer on the surface of Zr–Fe2 O3 nanorod arrays. The schematic of the formation of CoOx layer on the Zr–Fe2 O3 nanorod arrays are shown in Fig. 8(e). X-ray photoelectron spectroscopy (XPS) measurements were performed to identify the chemical state of Co species in the 1 mM CoOx modified Zr–Fe2 O3 sample. Fig. S6(a)–(d) shows the XPS high-resolution spectra of (a) Fe 2p, (b) O 1s, (c) Sn 3d, (d) Zr 3d and (d) Co 2p, respectively. Fig. S6e further shows the Co 2p-peaks at the binding energies (BE) of 780.6 and 796.6 eV correspond to the Co 2p3/2 and Co 2p1/2 peaks, respectively [51]. The 2p1/2 to 2p3/2 separation of 16 eV and the very flat, weak satellite structure found in the high binding energy side of 2p3/2 and 2pl/2 transitions indicate the existence of Co2+ on the surface of the Zr– Fe2 O3 [52]. Zhou et al. was also reported similar Co peaks for the Co3 O4 nanoparticles [53]. For the Zr-Fe2 O3 photoanodes, the Fe 2p region shows the peaks at the energy of 711 eV and 724 eV for Fe 2p3/2 and Fe 2p1/2 which were assigned to Fe3+ , and the O1s peak observed at 530.1 eV was assigned to lattice oxygen, indicating the oxygen i.e., O2− in the Fe2 O3 [54]. The high-resolution XPS spectra of Zr 3d of the doped hematite photoanodes indicate the Zr 3d5/2 and Zr 3d3/2 peaks observed at 182.41 and 184.86 eV, respectively, which is in agreement with previous reports [55–57]. Also, XPS signal of Sn observed at BE of 486.4 and 494.9 eV, respectively, were assigned to Sn 3d5/2 and Sn 3d3/2 peaks. This suggests that the high-temperature sintering (800 °C) process induces the Sn doping from the FTO substrate into the hematite nanorod crystal structure. The above results clearly indicate that the surface of Zr–Fe2 O3 is modified with the Co3 O4 . Fig. 9 shows the photoelectrochemical degradation of the Orange-II dye under one sun light irradiation by using CoOx modified Zr–Fe2 O3 photoanode. A significant enhancement in the degradation was observed after modification with 1 mM CoOx . Also,

Fig. S7 shows the faster decolorization of the dye occurring in CoOx modified Zr–Fe2 O3 sample compared to tetravalent doped hematite photoanode, demonstrating the higher photoelectrocatalytic degradation activity of the photoelectrode. The stability of the CoOx modified Zr–Fe2 O3 photoanodes during dye degradation at 1.4 V vs. RHE under the standard illumination conditions over a period of 270 min is shown in Fig. 9(a). There was no significant loss of photocurrent activity of CoOx modified photoelectrode, demonstrating the good stability and reusability of the CoOx modified Zr–Fe2 O3 photoelectrodes. The slight increment in the photocurrent was observed after 90 min, which is possibly due to the decolorization of dye solution during degradation experiments. This increases the light absorption by the photoanode which lead to the additional generation of charge carrier. The degradation rate was almost similar up to 90 min for both with and without CoOx modification; after that, the degradation rate is 1.5 times faster for the CoOx modified hematite sample (Fig. 9(a)). Therefore, ∼88% of dye degraded within 150 min; however, 210 min was required for the same amount of degradation without CoOx modified sample. The degradation efficiencies of the Orange-II dye with and without CoOx after 270 min are 97 and 93%, respectively. Fig. 9(b) shows the pseudo first-order degradation kinetics of the Orange-II dye on the CoOx modified Zr–Fe2 O3 and Zr–Fe2 O3 photoelectrode. The degradation rate of the CoOx modified Zr–Fe2 O3 showed a higher degradation rate i.e. 14 × 10−3 /s compared to Zr–Fe2 O3 . Therefore, the higher photoelectrocatalytic activity of the CoOx modified Zr– Fe2 O3 film could be attributed to the formation of direct charge transfer pathway of the one-dimensional nanostructure (Zr–Fe2 O3 ), the passivation of surface states of the uniform Zr–Fe2 O3 arrays, and the well-formed heterojunction of CoOx /Zr–Fe2 O3 . Chemical Oxygen Demand (COD) analysis was carried out using the OrangeII dye sample of before and after (240 min) photoelectrochemical

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Fig. 9. (a) Photocurrent stability of 1 mm CoOx modified Zr–Fe2 O3 photoanode and PEC dye degradation kinetics of Orange-II from the UV–vis absorption spectra for Zr– Fe2 O3 and CoOx modified Zr–Fe2 O3 photoanode in the supporting electrolyte 1 M NaOH solution (PEC degradation at potential 1.2 V vs. RHE), (b) first order rate kinetics of Orange-II dye degradation by Zr–Fe2 O3 and CoOx modified Zr–Fe2 O3 photoanode.

Fig. 10. A proposed schematic model of efficient charge separation during the photoelectrochemical Orange-II degradation over CoOx modified Zr–Fe2 O3 photoelectrodes.

degradation by standard method (CODMn) [APHA, 1989]. Fig. S8 (a) shows the reduction in COD values with respect to PEC degradation time. The continuous decay in the COD values with time is indicating the breakdown of Orange-II dye and its formation and destruction of secondary compounds during the degradation experiments. The almost 54% COD removal efficiency of Orange-II dye was achieved in 240 min using 1 mM CoOx modified Zr–Fe2 O3 photoelectrodes. On the other hand, only 40% COD removal efficiency of Orange-II dye was achieved using Zr–Fe2 O3 (Fig. S8(b)). The rate of COD removal was slightly higher in the initial period however it slowed down at the later stage, indicating the generation of few long- lived intermediates. To study and confirm the vital role of hydroxyl radicals during the photoelectrochemical degradation, the reactive species trapping experiment were carried out. In order to determine the role of • OH radicals during Orange-II dye, various concentration of tert-butanol (TBA) were used as scavenger. Fig. S9(a) and (b) shows the UV-DRS absorbance spectra and first-order rate kinetics of Orange-II dye degradation after adding the various amount of tert-butanol in to Orange-II dye degradation reaction. The Orange-II dye degradation efficiency was decreased from 97 to 59% and 31% for addition of 1.5 and 5 ml tert–butanol, respectively. The result of these reactive species trapping experiments suggesting that the photoelectrochemical degradation process might be regulated by • OH radicals during Orange-II degradation by Zr– Fe2 O3 based photoelectrodes. Photocatalytic activity and structural characteristics of the CoOx modified Zr–Fe2 O3 photoanodes before

and after the Orange-II dye degradation were studied to evaluate the reusability and stability of the photocatalysts (Fig. S10). Photoelectrochemical performance of the 1 mM CoOx modified Zr–Fe2 O3 nanorod electroctrode was detected using J–V characteristics of the photocatalyst before and after the Orange-II dye degradation. Fig. S10a depicts that the only 5% loss in the activity of the CoOx modified Zr-Fe2 O3 nanorod during the degradation of Orange-II dye. Additionally, the stability of CoOx modified Zr–Fe2 O3 before and after the photoelectrochemical degradation (after 270 min) was further studied using FESEM and XRD analyses. Figure S10b suggests that there is no change in the XRD pattern of CoOx modified Zr–Fe2 O3 nanorod before and after degradation. Also, there are no additional peaks related to FeOOH or FeO species, implying that the CoOx modified Zr–Fe2 O3 are photostable. Fig. S10(c) and (d) also shows that there is no change in the morphology after the degradation experiments. Therefore, the CoOx modified Zr–Fe2 O3 photoelectrodes may serve as one of the capable candidates for dye degradation, or bacterial application in the near future. Furthermore, the degradation pathway of Orange-II is also proposed on the basis of available literature and our experimental results (Fig. S11). On the basis of the above analysis, a model of efficient charge separation during the photoelectrochemical Orange-II dye degradation by CoOx modified Zr–Fe2 O3 is proposed and shown in Fig. 10. Under the illumination, the photogenerated electron–hole pairs are formed in CoOx layer and Zr–Fe2 O3 . The photogenerated

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electrons in CoOx layer and Zr–Fe2 O3 will be transported through the Zr–Fe2 O3 nanorods, and reach to the Pt (counter electrode) to generate •O2 − radicals. However, the photogenerated holes will meanwhile travel toward the CoOx layers, and subsequently oxidate OH− or H2 O to create • OH radicals. Therefore, such migration of photogenerated carriers can also suppress the recombination of electron–hole, which greatly enhances the photocatalytic reaction. Herein, CoOx acts as an oxidation cocatalyst, accelerating dye oxidation reaction and reducing surface charge recombination [58], which could be considered another reason for the improved PEC performances of the Zr–Fe2 O3 modified films. Consequently, the photogenerated electrons and holes were separated at the CoOx /ZrFe2 O3 nanorods array to form the hydroxyl radicals (• OH). The superoxide radicals (•O2 − ) will be formed by the combination of electrons with the absorbed O2 on the surfaces of Zr–Fe2 O3 . The superoxide radicals might be transformed to the hydroxyl radicals [59]. As powerful oxidants, the superoxide radicals and hydroxyl radicals can effectively decompose the organic species, such as Orange-II dye. Therefore, combining the above-mentioned factors, CoOx modified Zr–Fe2 O3 electrode holds the admirable ability to produce energetic •O2 − and • OH radicals, and showed a powerful and durable photoelectrochemical dye degradation performance.

4. Conclusions The ex-situ tetravalent ion Sn, Ti and Zr doped Fe2 O3 nanorods were fabricated via hydrothermal method, followed by dipping and a high-temperature annealing. Photoelectrochemical analyses of the doped hematite nanorod samples show enhancement in photocurrent and onset potential. The Zr–Fe2 O3 displayed higher photocurrent (i.e., 1.4 mA cm−2 at 1.23 V vs. RHE), as compared to the Sn and Ti doped Fe2 O3 , due to the better electron transport in the former, without damage to the morphology of Fe2 O3 . An optimized Zr–Fe2 O3 leads to 66% increase in photocurrent performance than the pristine hematite photoanode. The effect of tetravalent doping on Fe2 O3 nanorods was assayed in the Orange-II degradation in the presence of one sun illumination. The improved carrier concentration and passivation of surface states ultimately increases the activity for the Orange-II dye degradation after tetravalent doping. Zr–Fe2 O3 photoanode showed ∼93% Orange-II dye degradation efficiency under one sun illumination in 270 min, which is higher compare to other doped (Sn and Ti) photoelectrodes. Improved photocurrent density of 1.78 mA–cm−2 at 1.4 V vs. RHE was achieved for Zr–Fe2 O3 photoanode after modification with CoOx . Also, the oxidative degradation of Orange-II reached 97% using CoOx modified Zr–Fe2 O3 photoelectrodes under one sun illumination. The enhanced photocurrent density and Orange-II dye degradation efficiency after modification was achieved by effective charge separation and surface passivation. We conclude that the Zr–Fe2 O3 photoanode modified with CoOx cocatalyst is a good candidate for the PEC oxidation of azo dyes, and has potential application in wastewater treatment in the near future.

Acknowledgements This work was also supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2017H1D3A1A02014020). Also this work was supported by the Korea Environment Industry and Technology institute Program (KEITI) through Public Technology Program based on Environmental Policy funded by Korea Ministry of Environment (MOE) (20180 0 020 0 0 01).

Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.02.025.

References [1] Subash B, Krishnakumar B, Swaminathan M, Shanthi M. Highly efficient, solar active, and reusable photocatalyst: Zr-modified Ag−ZnO for reactive red 120 dye degradation with synergistic effect and dye-sensitized mechanism. Langmuir 2013;29:939–49. [2] Ahmed MA, El-Katori EE, Gharni ZH. Photocatalytic degradation of methylene blue dye using Fe2 O3 /TiO2 nanoparticles prepared by sol–gel method. J Alloys Compd 2013;553:19–29. [3] Hepel M, Luo J. Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO3 electrodes. Electrochim Acta 2001;47:729–40. [4] Lopez C, Valade AG, Combourieu B, Mielgo I, Bouchon B, Lema JM. Mechanism of enzymatic degradation of the azo dye Orange- II determined by ex situ 1H nuclear magnetic resonance and electrospray ionization-ion trap mass spectrometry. Anal Biochem 2004;335:135–49. [5] Lucas MS, Peres JA. Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation. Dyes Pigm 2006;1:236–44. [6] Ran J, Jaroniec M, Qiao SZ. Cocatalysts in semiconductor-based photocatalytic CO2 reduction: achievements, challenges, and opportunities. Adv. Mater. 2018;30:1704649. [7] She X, Wu J, Xu H, Mo Z, Lian J, Song Y, Liu L, Du D, Li H. Enhancing charge density and steering charge unidirectional flow in 2D non-metallic semiconductor-CNTs-metal coupled photocatalyst for solar energy conversion. Appl Catal B Environ 2017;202:112–17. [8] Bai S, Chu H, Xiang X, Luo R, He J, Chen A. Fabricating of Fe2 O3 /BiVO4 heterojunction based photoanode modified with NiFe-LDH nanosheets for efficient solar water splitting. Chem Eng J 2018;350:148–56. [9] Chen F, Xie Y, Zhao J, Lu G. Photocatalytic degradation of dyes on a magnetically separated photocatalyst under visible and UV irradiation. Chemosphere 2011;44:1159–68. [10] Wheeler DA, Wang G, Ling Y, Li Y, Zhang JZ. Nanostructured hematite: synthesis, characterization, charge carrier dynamics, and photoelectrochemical properties. Energy Environ Sci 2012;5:6682–702. [11] Sivula K, zboril r, formal fl, robert r, weidenkaff a, tucek j, frydrych j, grätzel m. photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach. J Am Chem Soc 2010;132:7436–44. [12] Kleiman-Shwarsctein A, Huda MN, Walsh A, Yan Y, Stucky GD, Hu YS, Al-Jassim MM, McFarland EW. Electrodeposited aluminum-doped α -Fe2 O3 photoelectrodes: experiment and theory. Chem Mater 2010;22:510–17. [13] Dunkle SS, Helmich RJ, Suslick KS. BiVO4 as a visible-light photocatalyst prepared by ultrasonic spray pyrolysis. J Phys Chem C 2009;13:11980–3. [14] Kleiman-Shwarsctein A, Hu YS, Forman AJ, Stucky GD, McFarland EW. Electrodeposition of α -Fe2 O3 doped with Mo or Cr as photoanodes for photocatalytic water splitting. J Phys Chem C 20 08;112:1590 0–7. [15] Ling Y, Wang G, Wheeler DA, Zhang JZ, Li Y. Sn-doped hematite nanostructures for photoelectrochemical water splitting. Nano Lett 2011;11:2119–25. [16] Zhang P, Kleiman-Shwarsctein A, Hu YS, Lefton J, Sharma S, Forman AJ, McFarland EW. Oriented Ti doped hematite thin film as active photoanodes synthesized by facile APCVD. Energy Environ Sci 2011;4:1020–8. [17] Mirbagheri N, Wang D, Peng C, Wang J, Huang Q, Fan C, Ferapontova EE. Visible Light Driven Photoelectrochemical Water Oxidation by Zn- and Ti-Doped Hematite Nanostructures. Acs Catalysis 2014;4:2006–15. [18] Shen S, Guo P, Wheeler DA, Jiang J, Lindley SA, Kronawitter CX, Zhang JZ, Guo L, Mao SS. Physical and photoelectrochemical properties of Zr-doped hematite nanorod arrays. Nanoscale 2013;5:9867–74. [19] Abe R, Shinmei K, Hara K, Ohtani B. Robust dye-sensitized overall water splitting system with two-step photoexcitation of coumarin dyes and metal oxide semiconductors. Chem Commun 2009;24:3577–9. [20] Barbosa IA, Zanatta LD, Spimpolo DME, Silva DL, Nascimento LF, Zanardi FB, de Sousa Filho PC, Serra OA, Iamamoto Y. Magnetic diatomite(Kieselguhr)/Fe2 O3 /TiO2 composite as an efficient photo-Fenton system for dye degradation. Solid State Sci 2017;72:14–20. [21] Banisharif A, AliKhodadadi A, Mortazavi Y, Firooz AA, Beheshtian J, Agah S, Menbari S. Highly active Fe2 O3 -doped TiO2 photocatalyst for degradation of trichloroethylene in air under UV and visible light irradiation: experimental and computational studies. Appl Catal B:Environ 2015;165:209–21. [22] Tsyganok A, Klotz D, Malviya KD, Rothschild A, Grave DA. Different roles of Fe1– x Nix OOH cocatalyst on hematite (α -Fe2 O3 ) photoanodes with different dopants. ACS Catal 2018;8:2754–9. [23] Wang Z, Liu G, Ding C, Chen Z, Zhang F, Shi J, Li C. Synergetic effect of conjugated Ni(OH)2 /IrO2 cocatalyst on titanium-doped hematite photoanode for solar water splitting. J Phys Chem C 2015;119:19607–12. [24] Maeda K. Photocatalytic water splitting using semiconductor particles: history and recent developments. J Photochem Photobiol C 2011;12:237–68. [25] Steinmiller EPM, Choi KS. Photochemical deposition of cobalt-based oxygen evolving catalyst on a semiconductor photoanode for solar oxygen production. Proc Natl Acad Sci USA 2009;106:20633–6.

N. Meshram, M.A. Mahadik and I.-K. Jeong et al. / Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 305–315 [26] Maitani MM, Yamada T, Mashiko H, Yoshimatsu K, Oshima T, Ohtomo A, Wada Y. Microwave effects on Co–Pi cocatalysts deposited on α -Fe2 O3 for application to photocatalytic oxygen evolution. ACS Appl Mater Interfaces 2017;9:10349–54. [27] Seabold JA, Choi KS. Effect of a cobalt-based oxygen evolution catalyst on the stability and the selectivity of photo-oxidation reactions of a WO3 photoanode. Chem Mater 2011;23:1105–12. [28] Xiong S, Li P, Jin Z, Gao T, Wang Y, Guo Y, Xiao D. Enhanced catalytic performance of ZnO–CoOx electrode generated from electrochemical corrosion of Co–Zn alloy for oxygen evolution reaction. Electrochim Acta 2016;222:999–1006. [29] Irshad A, Munichandraiah N. Photochemical deposition of Co-Ac catalyst on ZnO nanorods for solar water oxidation, 2015, 162: H235–H243. [30] Jaramillo-Páez C, Navío JA, Hidalgo MC, Bouziani A, ElAzzouzi M. Mixed α -Fe2 O3 /Bi2 WO6 oxides for photoassisted hetero-Fenton degradation of Methyl Orange and Phenol. J Photochem Photobiol A 2017;332:521–33. [31] Barrosoa M, Cowan AJ, Pendlebury SR, Grätzel M, Klug DR, Durrant JR. The role of cobalt phosphate in enhancing the photocatalyticactivity of α -Fe2 O3 toward water oxidation. J Am Chem Soc 2011;133:14868–71. [32] Xia L, Bai J, Li J, Zeng Q, Li L, Zhou B. High-performance BiVO4 photoanodes cocatalyzed with an ultrathin α -Fe2 O3 layer for photoelectrochemical application. Appl Catal B Environ 2017;204:127–33. [33] Mahadik MA, Subramanian A, Chung HS, Cho M, Jang JS. CdS/Zr:Fe2 O3 nanorod arrays with Al2 O3 passivation layer for photoelectrochemical solar hydrogen generation. ChemSusChem 2017;10:2030–9. [34] Lopes T, Andrade L, Formal FL, Grätzel M, Sivula K, Mendes A. Hematite photoelectrodes for water splitting: evaluation of the role of film thickness by impedance spectroscopy. Phys Chem Chem Phys 2014;16:16515–23. [35] Ghasemi S, Rahimnejad S, Setayesh SR, Rohani S, Gholami MR. Transition Metal Ions Effect on the properties and photocatalytic activity of nanocrystalline TiO2 prepared in an ionic liquid. J Hazard Mater 2009;172:1573–8. [36] Deng J, Lv X, Zhang H, Zhao B, Sun X, Zhong J. Loading the FeNiOOH cocatalyst on Pt-modified hematite nanostructures for efficient solar water oxidation. Phys Chem Chem Phys 2016;18:10453–8. [37] Du C, Yang X, Mayer MT, Hoyt H, Xie J, McMahon G, Bischoping G, Wang D. Hematite-based water splitting with low turn-On voltages. Angew Chem Int Ed 2013;52:12692–5. [38] Jia H, Hu Y, Tang Y, Zhang L. Synthesis and photoelectrochemical behavior of nanocrystalline CdS film electrodes. Electron Commun 2006;8:1381–5. [39] Taffa DH, Hamm I, Dunkel C, Sinev I, Bahnemann D, Wark M. Electrochemical deposition of Fe2 O3 in the presence of organic additives: a route to enhanced photoactivity. RSC Adv 2015;5:103512–22. [40] Cai J, Li S, Li Z, Wang J, Ren Y, Qin G. Electrodeposition of Sn-doped hollow α -Fe2 O3 nanostructures for photoelectrochemical water splitting. J Alloys Compd 2013;574:421–6. [41] Spray RL, McDonald KJ, Choi KS. Enhancing photoresponse of nanoparticulate α -Fe2 O3 electrodes by surface composition tuning. J Phys Chem C 2011;115:3497–506. [42] Miao C, Shi T, Xu G, Ji S, Ye C. Photocurrent enhancement for Ti-doped Fe2 O3 thin film photoanodes by an in situ solid-state reaction method. ACS Appl Mater Interfaces 2013;5:1310–16.

315

[43] Shinde PS, Lee SY, Choi SH, Lee HH, Ryu J, Jang JS. A synergisitic effect of surfactant and ZrO2 underlayer on photocurrent enhancement and cathodic shift of nanoporous Fe2 O3 photoanode. Sci Rep 2016;6:3243601–14. [44] Cesar I, Sivula K, Kay A, Zboril R, Grätzel M. Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting. J Phys Chem C 2009;113:772–82. [45] Han J, Zong X, Wang Z, Li C. A hematite photoanode with gradient structure shows an unprecedentedly low onset potential for photoelectrochemical water oxidation. Phys Chem Chem Phys 2014;16:23544–8. [46] Wang L, Nguyen NT, Schmuki P. A facile surface passivation of hematite photoanodes with iron titanate cocatalyst for enhanced water splitting. Chem Sustain Chem 2016;9:1–7. [47] Luo Z, Li C, Liu S, Wang T, Gong J. Gradient doping of phosphorus in Fe2 O3 nanoarray photoanodes for enhanced charge separation. Chem Sci 2017;8:91–100. [48] Candal RJ, Zeltner WA, Anderson MA. Effects of pH and applied potential on photocurrent and oxidation rate of saline solutions of formic acid in a photoelectrocatalytic reaction. Environ Sci Technol 20 0 0;34:3443–51. [49] Chen H, Motuzas J, Martens W, Costa JCD. Degradation of azo dye Orange II under dark ambient conditions by calcium strontium copper perovskite. Appl Catal B Environ 2018;221:691–700. [50] Monteagudo JM, Durán A, Martín IS, Aguirre M. Catalytic degradation of Orange II in a ferrioxalate-assisted photo-Fenton process using a combined UV-A/C–solar pilot-plant system. Appl Catal B Environ 2010;95:120–9. [51] Petitto SC, Marsh EM, Carson GA, Langell MA. Cobalt oxide surface chemistry: the interaction of CoO(100), Co3 O4 (110) and Co3 O4 (111) with oxygen and water. J Mol Catal A Chem 2008;281:49–58. [52] Feng Y, Li L, Niu S, Qu Y, Zhang Q, Li Y, Zhao W, Li H, Shi . Controlled synthesis of highly active mesoporous Co3 O4 polycrystals for low temperature CO oxidation. Appl Catal B Environ 2012;111:461–6. [53] Zhou LP, Xu J, Li XQ, Wang F. Metal oxide nanoparticles from inorganic sources via a simple and general method. Mater Chem Phys 2006;97:137–42. [54] Liu YT, Yuan QB, Duan DH, Zhang ZL, Hao XG, Wei GQ, Liu SB. Electrochemical activity and stability of core–shell Fe2 O3 /Pt nanoparticles for methanol oxidation. J Power Sources 2013;243:622–9. [55] McIntyre NS, Zetaruk DG. X-Ray photoelectron spectroscopic studies of iron-oxides. Anal Chem 1977;49:1521–9. [56] Annamalai A, Shinde PS, Jeon TH, Lee HH, Kim HG, Choi W, Jang JS. Fabrication of superior α -Fe2 O3 nanorod photoanodes through ex-situ Sn-doping for solar water splitting. Sol Energy Mater Sol Cells 2016;144:247–55. [57] Subramanian A, Annamalai A, Lee HH, Choi SH, Ryu J, Park JH, Jang JS. ACS Appl Mater Interfaces 2016;8:19428–37. [58] Shen S, Mao SS. Nanostructure designs for effective solar-to-hydrogen conversion. Nanophotonics 2012;1:31–50. [59] Yu XX, Liu SW, Yu JG. Superparamagnetic γ -Fe2 O3 @SiO2 @TiO2 composite microspheres with superior photocatalytic properties. Appl Catal B Environ 2011;104:12–20.