Shape-Controlled Hematite: An Efficient Photoanode for

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Shape-Controlled Hematite: An Efficient Photoanode for Photoelectrochemical Water Splitting Mamta Devi Sharma, Chavi Mahala, and Mrinmoyee Basu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00739 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Industrial & Engineering Chemistry Research

Shape-Controlled Photoanode for Splitting

Hematite: An Efficient Photoelectrochemical Water

Mamta Devi Sharma,1 Chavi Mahala,1 Mrinmoyee Basu1* 1Department

of Chemistry, BITS Pilani, Pilani Campus, Rajasthan-333031

Abstract

Photoelectrochemical water splitting has gained considerable interest in the last few decades because of its potential for harvesting solar light for H2 production. For harvesting solar light, the design of a semiconductor photoelectrode is the critical parameter to control performance. In this regard, vertically aligned, interconnected 2D nanosheets of α-Fe2O3 show the most efficient activity for PEC water splitting as compared to other morphologies like thick sheets and nanorods as the former absorb more light, provide less path length for photon penetration, and a short minority carrier (hole) diffusion length. Compared to thick sheets and nanorods, the separation efficiency of Fe2O3 nanosheet is 7.3, which is higher than the structures as mentioned above, at 1.23 V vs. RHE. To further legitimize the efficacy of α-Fe2O3 nanosheet vis-à-vis the thick sheets and nanorods, Mott-Schottky analysis is carried out to calculate the carrier density that is 8.68×1020 cm-3, 8.68×1019 cm-3, and 2.89×1020 cm-3 respectively.

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Keywords: Thin nanosheets, α-Fe2O3, nanorod, photoelectrochemical water splitting, surface modification. Introduction

With the continuous demand for green, sustainable energy sources, water splitting is being intensively studied as the potential alternative route to generate H2. Hydrogen has a high energy density, is clean, and at the same time does not emit any greenhouse gases. Water splitting in any form like photocatalytic, electrochemical or photoelectrochemical are hence being studied extensively for last few decades.1-7 Solar water splitting is a sustainable process which can generate hydrogen by a simple reduction reaction. The aim for the scientists from the last few decades is to develop/design a suitable semiconductor which can function as an efficient photoelectrode. Ideally, this will be able to generate high photocurrent density; at the same time, it should work for more extended period to have a practical industrial application. Honda and Fujishima have done the pioneering work on photoelectrochemical water splitting in 1972 utilizing TiO2 as the photoanode under irradiation of UV-light.8 After that, enormous efforts have been put for the development of efficient photoelectrodes focusing on greater ‘solar to hydrogen’ efficiency. Scientists have also developed different active semiconductors like ZnO, Fe2O3, TiO2, WO3 and BiVO4 as photoanodes.9-20 In the field of PEC water splitting, among all the studied catalysts, hematite (α-Fe2O3) has attracted much attention because of its unique properties, namely, high stability in neutral and alkaline condition, nontoxicity, low cost and suitable band gap (2.0-2.2 eV)21-27 that satisfies the necessities for efficient visible light absorption. The valence band energy level of α-Fe2O3 is lower than O2/OH- redox potential, as the holes produced by the α-Fe2O3 upon photo-irradiation are capable of oxidizing water to O2. However, due to the conduction band position with respect to hydrogen evolution, α-Fe2O3 requires more external bias for water splitting. Limitations of αFe2O3 in PEC water splitting are the low absorptivity (photon penetration depth ~118 nm at λ = 550 nm), short minority carrier (hole) diffusion length (2-4 nm), and the slow water oxidation kinetics.28,29


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All the limitations as mentioned earlier result in high recombination of photogenerated electron-hole pairs that further restrict α-Fe2O3 for practical application in water splitting. It is, therefore, necessary to design an efficient nanostructure of α-Fe2O3 that overcome such limitations. The efficiency of α-Fe2O3 can be improved by tuning the morphology which enables high light absorption and improved charge carrier separation. However, not much attention was paid to determine how morphology controlling can enhance PEC activity.30 However, Zhou et al., have summarized the morphology-dependent PEC activity of BiVO4 as the photoanode.31 In terms of morphology, among 0D, 1D, 2D and 3D architectures, 2D nanostructures have advantages like enhancing light absorption through multiple reflection and scattering, decoupling the direction of light absorption and charge-carrier transportation, and finally, providing a large number for surface active atoms. In PEC water splitting, light harvesting and photogenerated charge carrier transportation are the two most essential parts, and α-Fe2O3 suffers majorly due to low absorptivity and short minority carrier diffusion length. These limitations of Fe2O3 can be overcome if thin nanosheets are developed. Here in comes the utility of 2D nanostructure. One observation regarding this is corroborated by Zheng et al., who studied the morphologydependent property of α-Fe2O3 varying from nanoparticle to sparse dendrites where they observed that α-Fe2O3 with short length dendrites could show highest photocurrent density of 66.8 µA/cm2 at +0.22 V vs. Ag/AgCl.32 Similarly, Mei et al., tuned nanosheet of α-Fe2O3 decorated with C3N4 and CoOx that can function as an efficient photoanode. It can generate twice higher photocurrent density compared to bare α-Fe2O3 at 1.23 V vs. RHE.33 Keeping in mind that 2D thin sheets are supposed to outperform other nanostructures, the morphology of Fe2O3 is tuned. Thin 2D sheets dismiss the limitation of a decreased photon penetration depth of hematite, and at the same time, shorten the minority carrier (hole) diffusion length. Here, we have developed a simple methodology for the growth of α-Fe2O3 vertically aligned thin sheets on fluorine doped tin oxide coated glass (FTO) following an electrodeposition technique. Initially, electrodeposited nanosheets of iron are grown on FTO which are further calcined for the development of α-Fe2O3 thin sheets. PEC activity of α-Fe2O3 nanosheets is compared with some other morphologies like thick sheets of Fe2O3 and nanorods of Fe2O3. Stability of α-Fe2O3 is checked for 1000 seconds, and it is apparent that α-Fe2O3 thin sheets and nanorods generate unaltered photocurrent density under illumination.


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Experimental section Preparation of Fe2O3 electrodes Three different morphologies like nanosheets, thick sheets and nanorods of Fe2O3 were grown on a fluorine doped tin oxide (FTO) coated glass substrate following simple electrodeposition, hydrothermal and drop casting methodologies followed by calcination in air. Fe2O3 from the electrodeposition method Fe2O3 nanostructure was fabricated on FTO through electro-deposition technique. The FTO glass slides were cleaned ultrasonically in soap water, ethanol, and acetone respectively, for more extended period, to remove the thin layer of adhesive. For Fe2O3 deposition, 100 mL of 1 M ferrous sulfate aqueous solution was prepared first, and 0.5 M aqueous solution of KCl was added to it. For electrodeposition, a three-electrode system was used, where Ag/AgCl was used as the reference electrode, Pt wire was used as the counter electrode, and FTO was used as the working electrode. All three electrodes were dipped into the prepared solution of Fe2+, and a chronoamperometry study was carried out at an applied potential -1.0 V for 10 minutes. The black color thin film was deposited on FTO which was carefully rinsed with DI water. Finally, the film deposited FTO was annealed for variable periods like 2, 3, 4 and 5 hours at 400 ºC and cooled naturally at room temperature. A dark red color film deposited FTO was obtained as a result. The sample deposited FTO was used for further characterization and PEC. The synthesis of Fe2O3 thin sheets on FTO is represented as Scheme 1. Fe2O3 from the hydrothermal method Fe2O3 nanomaterial was also synthesized following the hydrothermal method. First, 2 mmol of ferric chloride was taken as a precursor of iron and 5 mmol urea [CO(NH2)2] as a hydrolyzing agent. Both the compounds were dispersed in 15 mL of DI water by stirring. The cleaned FTO was dipped in this solution, and the reaction mixture was taken in a Teflon vessel to carry out the hydrothermal reaction at 120 ºC for 12 hours. After completion of the reaction, the sample deposited FTO was washed with DI water and ethanol 2-3 times and dried overnight at 50 ºC. Then, the sample coated FTO was calcined for 3 hours at 400 ºC and further used for characterization and application purpose.


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Fe2O3 from the drop-casting method Fe2O3 was also synthesized by a simple drop-casting method. First, an ethanolic solution of 0.1 M ferrous sulfate was prepared in 10 mL of ethanol. 100 µL of Fe2+ solution was dropped on clean FTO and dried in air. After that, the FTO was calcined at 500 ºC for 3 hours. The sample coated FTO was kept for further characterization and application. The α-Fe2O3, that was prepared from the electrodeposition method, hydrothermal method, and drop-casting method are noted as Fe2O3-NS, Fe2O3-NR, and Fe2O3-TS throughout the MS. Results and Discussion Crystal structure, phase purity and the crystallinity of the synthesized materials are determined with the help of XRD analysis and shown in Figure 1. XRD pattern of the initial electrodeposited material is shown in Figure 1ai, which indicates that Fe has the body-centered cubic crystal structure and its XRD pattern matches with the JCPDS card No. 87-0721.34 Pure phase synthesis of Fe nanostructure on FTO is claimed from the XRD analysis. The XRD pattern of α-Fe2O3-NS synthesized from electrodeposition method is shown in Figure 1aii, which indicates that it has a rhombohedral crystal structure and it matches with the JCPDS card No. 240072.35 The XRD analysis indicates the synthesis of the pure phase Fe nanostructure through electrodeposition and its complete conversion to α-Fe2O3 followed by the calcination method. XRD pattern of the synthesized Fe2O3 by hydrothermal and drop casting method are shown in Figure 1b. The XRD analysis indicates the formation of pure phase, highly crystalline, rhombohedral α-Fe2O3. XRD pattern is well matched with the JCPDS card No. 24-0072, which shows the creation of the pure phase and crystalline α-Fe2O3 nanostructure. Synthesis of Fe nanostructure and the formation of α-Fe2O3 is reconfirmed through Raman analysis. Observed Raman spectra are shown in Figure 1c, which perfectly matches with the reported Raman spectra of α-Fe2O3.36,37 The Raman analysis indicates the formation of crystalline, and pure α-Fe2O3. Our aim was to develop 2D thin sheets of Fe2O3. Thin sheets of Fe2O3 is obtained after calcination of the sheet-like ‘Fe’ nanostructure at 400 °C for 3 hours (Figure S1a). To obtain the optimum condition for thin sheet formation, time-dependent FESEM images are carried out and shown in Figure S1. High magnification FESEM images (top view) show that α-Fe2O3 have thin sheets which are interconnected to each other (Figure 2a-c). These interconnected, vertically


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aligned sheets have more exposed surface area. A cross-sectional view of the same α-Fe2O3 on FTO is shown in Figure 2d, which indicates the growth of the vertically aligned α-Fe2O3 on an FTO. An EDS mapping analysis of FTO@ α-Fe2O3 indicates the uniform distribution of ‘Fe’, ‘O’ and ‘Sn’ as elements (Figure S2). To establish the phenomena of superior activity in PEC water splitting of thin sheets of α-Fe2O3, two other different morphologies are also developed. FESEM analysis confirms that hydrothermally synthesized α-Fe2O3 have highly dense nanorod like morphology (Figure S3a,b). EDS analysis confirms the uniform distribution of Fe, O as the elements and the EDS spectra show the corresponding signals (Figure S4). FESEM image of αFe2O3 synthesized by the drop-casting method is shown in Figure S3c, d, which indicate the formation of aggregated thick sheets. The EDS analysis shows the presence of ‘Fe’, ‘O’ and ‘Sn’ as elements (Figure S5). Optical properties of the resultant-synthesized materials are determined with the help of UV-visible absorbance spectroscopy. Figure 1d shows the optical property of the Fe2O3-TS, Fe2O3-NS and Fe2O3-NR sample. Fe2O3-NS clearly shows higher absorption of visible light compared to that of Fe2O3-NR as well as Fe2O3-TS, which is presumably due to the thin sheet morphology. Very thin sheets of Fe2O3 allow more light to penetrate the material, whereas these phenomena do not occur in the case of thick sheets and nanorods.38 Vertically aligned thin sheets also favor more light absorption by lowering the backscattering through multiple reflections and scattering of light inside it that leads to more light and matter interaction in the case of the αFe2O3 thin sheets. Absorption tail of both Fe2O3-NR, TS, and Fe2O3-NS is extended up to 750800 nm. The observed optical property of the synthesized α-Fe2O3 is verified by the existing literature.39-42 Photoelectrochemical Activity The photoelectrochemical activity of the synthesized Fe2O3-NS, TS and NR were checked with the help of LSV techniques, and the i-t curves, with chopped and continuous illumination conditions. PEC performances of the already-obtained electrodes were studied in 1 M NaOH as an electrolyte, and photocurrent density was observed with the application of potential under irradiation of 100 mW/cm2 illumination. Three electrode system was used to check PEC activity, where Ag/AgCl, Pt wire, and sample deposited FTO were used as the reference, counter, and working electrode respectively. All the electrochemical measurement


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data were obtained with respect to Ag/AgCl and reported here with respect to RHE using the Nernst equation. Comparative LSV curve of the Fe2O3 NS, NR, and TS are shown in Figure 3a. Optimum condition (calcination time) for the synthesis of α-Fe2O3 is chosen as 400 °C for 3 hours depending on the observed PEC activity of the synthesized α-Fe2O3 in different calcination conditions (brief description and the LSV curves are shown in Figure S7). In the case of Fe2O3NS, onset potential was 0.568 V vs. RHE (Figure 3b, 4a), whereas, onset potential for Fe2O3-NR and Fe2O3-TS are 0.655 V and 0.642 V vs. RHE respectively. The Cathodic shift in the onset potential indicates that Fe2O3-NS is photo-chemically more active compared to Fe2O3-NR or Fe2O3-TS. Vertically aligned thin sheets can absorb more visible light due to the active and more exposed surface atoms; at the same time, these interconnected sheets allow more electrolyte to penetrate inside so that the photogenerated charge carriers can quickly react with the electrolyte.43 Fe2O3-NS can generate 0.868 mA/cm2 current density upon application of 1.625 V vs. RHE. However, Fe2O3-TS and NR can generate 0.19 and 0.05 mA/cm2 current density respectively at 1.625 V vs. RHE. The transient photoactivity of Fe2O3-NS, Fe2O3-TS and Fe2O3NR are shown in Figure 6b under chopped illumination condition which maps the switch off-on behavior with light for all the electrodes. The onset potential and current density at 1.625 V vs. RHE follow a volcano type plot, that shows the highest observed current density in the case of Fe2O3-NS (calcined at 400 °C for 3 hours) with the lowest onset potential (Figure 4b). In the chopped-light voltammogram after turning light on, very sharp photocurrent spikes are observed that denotes a high density of undesirable surface state acting as recombination centers for photogenerated carriers.44 The anodic current spikes are due to the accumulation of holes at the electrode/electrolyte interface that are not injected to the electrolyte due to slow reaction kinetics; rather, they oxidize the trap states. On the other hand, cathodic spikes are generated at the time of light off that indicates the recombination of the accumulated holes in the interface with the electrons diffused from the external circuit. Here, we have made a detailed comparison where Fe2O3 is used as a photoanode in the existing literature and shown in Table S1. It can be clearly understood that bare Fe2O3-NS can generate a much higher photocurrent density in low applied potential compared to what is being stated in the existing literature. To establish the superiority of the Fe2O3-NS, the electronic properties of the developed Fe2O3-NS, Fe2O3-TS and Fe2O3-NR, along with their carrier concentrations, were studied based on Mott-Schottky plots. The observed data is presented in Figure 5a, which dictates the positive


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slope of three samples claiming the n-type property of Fe2O3. The carrier density and the flat band potential at the electrode/electrolyte potential can be determined from the equation:45-49 1/C2 = (2/eεε0NdA2) [(V-VFB-kT/e)] where, C is the specific capacitance, Nd is the carrier density, e is the electron charge, ε0 is the electric permittivity of vacuum, ε is the dielectric constant of the semiconductor, VFB is the flat band potential, A is the area of the sample, T is the temperature, and k is the Boltzmann constant. The slope of Fe2O3-NS is smallest compared to the other three, and this indicates higher carrier concentration in the Fe2O3-NS. The carrier density of Fe2O3-NS is 8.68×1020 cm-3, whereas, for Fe2O3-TS and NR, it is 8.68×1019 cm-3 and 2.89×1020 cm-3 respectively. The higher carrier density in Fe2O3-NS enables it to generate high photocurrent density at a fixed applied potential in comparison to Fe2O3-TS and Fe2O3-NR. The flat band potential of Fe2O3-NS is more negative, and the value is 0.137 V vs. RHE and, thus, indicates that the Fermi level is proximal to its conduction band and higher than the Fe2O3-TS (0.168 V) and Fe2O3-NR (0.176 V). Therefore, by controlling the morphology, three different Fermi levels can be successfully generated. More negative flat band potential justifies the observed most anodic shift in onset potential in the case of Fe2O3-NS. To understand the superior photoelectrochemical activity of the Fe2O3-NS in comparison to the Fe2O3-NR and Fe2O3-TS, photoelectrochemical impedance measurement was carried out at the respective onset potentials upon illumination of 100 mW/cm2 light. The radius of the observed semicircle of the Nyquist plot in the case of Fe2O3-NS reflects the charge transfer resistance and is much lower compared to Fe2O3-TS and Fe2O3-NR, which further signifies the ease of charge transport in Fe2O3 nanosheets. The lower charge transfer resistance indicates an increase in conductivity due to a higher number of charge carriers in the bands (proved from MS plot and charge separation efficiency), that results in higher PEC activity (Figure 6a). Along with the easy charge transfer, diffusion of ions occurred in the case of Fe2O3-NS, that endorses the higher activity of Fe2O3-NS (inset of Figure 6a) compared to Fe2O3-TS and Fe2O3-NR. In the case of Fe2O3-NS, there was a gradual increase in the photocurrent density, with increased applied potential due to the migration of ions. It is associated with the diffusional extraction of electrons from Fe2O3 thin nanosheets. Due to the formation of such very thin sheets of Fe2O3, the


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plateau in the photocurrent can be linked to successful extraction or migration of ions throughout the film.50 An impedance spectroscopy supported this observed phenomenon. The PEC performance of the developed photoanodes was further evaluated by quantitative calculation of carrier separation efficiency as per existing literature reports.50,51 Carrier separation efficiency was obtained by using the equation ηsep = (Jabs × ηox)/JPEC, where Jabs is the observed photocurrent density, JPEC is the theoretical maximum photocurrent density and ηox is the yield of the surface reaching holes that participate in the oxidation process. To determine the separation efficiency, sodium sulfite was used as the electrolyte where surface recombination is suppressed completely, and the ideal ηox is expected to be 100%. At a fixed potential of 1.23 V vs. RHE, the separation efficiency of Fe2O3-NS is 7.3, which is 7.3 times higher than Fe2O3-TS, and 3.65 times higher than that of Fe2O3-NR respectively (Figure 5b). Higher carrier separation efficiency in thin nanosheets of α-Fe2O3 indicates the creation of additional band bending that helps to improve carrier separation. Higher the band bending is, the lesser applied potential is required to generate minimum photocurrent and can generate more photocurrent density under an applied potential which is clear from LSV curves and observed onset potential. As a result, in case the of α-Fe2O3-NS, we observe maximum cathodic shift in the onset potential. Increased charge separation efficiency also indicates less surface recombination, and this reflects low charge transfer resistance evidenced from EIS analysis. The observed result means that Fe2O3-NS can generate more photocurrent density compared to Fe2O3-TS and Fe2O3-NR. The photoelectrochemical stability of Fe2O3-NS, Fe2O3-TS, and Fe2O3-NR were determined at a fixed potential of 1.625 V vs. RHE for 1000 seconds, which indicates that αFe2O3-NS, Fe2O3-TS and Fe2O3-NR are stable and can generate unaltered photocurrent for a long time (Figure 6b). To know the structural stability of Fe2O3-NS, we performed the FESEM analysis after long term stability that indicates that the nanosheets can withstand the corrosive environment with little enhancement in the surface porosity of the thin sheets (Figure 7). An EDS analysis was also carried out to verify the retention of elemental composition and is shown in Figure S6. Water Oxidation Catalysis by Cobalt (II) modified α-Fe2O3-NS


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After checking the PEC activity of the already-obtained α-Fe2O3-NS, it was rinsed with DI water and dried at 60 °C for 2 hours. It was then dipped into 10 mL of 5 mM Co2+ solution for 200 seconds that allow absorbing Co(II) on α-Fe2O3-NS. Cobalt treatment resulted in a 100 mV cathodic shift in onset potential and continuous increase in the observed current density (Figure 8a). The Co(II) modified α-Fe2O3-NS can generate a photocurrent density of 0.332 mA/cm2 upon application of 1.025 V vs. RHE, whereas, bare Fe2O3-NS can generate 0.114 V. The stability of the Co(II) adsorbed α-Fe2O3-NS was determined under an applied potential of 1.025 V vs. RHE for 90 seconds, that reflected the robust nature of the electrodes (Figure 8c). Transient photo activities of both the electrodes were checked under an applied potential of 0.925 V vs. RHE under chopped light illumination for 300 seconds (Figure 8b). All the observed results indicated an enhancement in PEC performance after Co(II) decoration. Existing literature indicates that such kind of enhancement occurs due to the trapping of the hole by Co(II) that successively oxidizes to Co(III) and may further be converted to Co(IV).52 Hence, prompt charge transport through Co(II) reduces the recombination rate of the photo-generated charges that result in a cathodic shift of onset potential, and at the same time, enhances photocurrent density. Conclusion An innovative method for the fabrication of α-Fe2O3-NS has been developed and explained in this paper. The synthesis of α-Fe2O3 nanosheets is carried out via electrodeposition method followed by calcination at 400 ºC for 3 hours. Fe2O3-NR and Fe2O3-TS are synthesized using hydrothermal and drop casting method respectively. Among all the integrated Fe2O3 materials, Fe2O3-NS demonstrated excellent photoelectrochemical performance. α-Fe2O3-NS have higher charge transport due to the more active exposed surface atoms. Fe2O3-NS can absorb more visible light and electrolyte can penetrate the thin sheets easily as compared to Fe2O3-NR and Fe2O3-TS. α-Fe2O3-NS can generate 0.868 mA/cm2 photocurrent density at an onset potential of 0.568 V vs. RHE, and this is superior to other Fe2O3 nanomaterials. The separation efficiency of Fe2O3-NS is 7.3, is 7.3 times higher than Fe2O3-TS and 3.65 times higher than Fe2O3-NR, respectively at 1.23 V vs. RHE. Using a Mott-Schottky plot, the carrier density of Fe2O3-NS, TS and NR are calculated, and the values are 8.68×1020 cm-3, 8.68×1019 cm-3, and 2.89×1020 cm-3, respectively. α-Fe2O3-NS demonstrates the stability up to 1000 seconds and can generate unaltered photocurrent efficiently. Further, the surface of α-Fe2O3-NS is modified with Co(II) to increase the lifetime of the photogenerated charge-carrier. Co(II) modified α-Fe2O3-NS can ACS Paragon Plus Environment

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generate a photocurrent density of 0.332 mA/cm2 upon application of 1.025 V vs. RHE, whereas, bare Fe2O3-NS can generate a photocurrent density of only 0.114 mA/cm2. In this present condition, Co(II) works as a hole trapper on the surface of Fe2O3-NS.

Scheme 1: Schematic representation for the development of vertically aligned thin sheets of α-Fe2O3 using electrodeposition technique.


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Figure 2: FESEM image of α-Fe2O3-NS (Top view) in different magnifications (a) low, (b) medium and (c) high, (d) cross section FESEM image of α-Fe2O3-NS.


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NS-2h

NS-3h

NS-4h

NS-5h

1000

TS

NR

NS-2h

NS-3h

NS-4h

Limiting Current Density (Acm2)

Page 15 of 34

0

NS-5h

Material

Figure

4:

(a)

Comparative

onset

potential

of

various

synthesized

α-Fe2O3

nanomaterials, (b) observed current density at 1.625 V vs. RHE and onset potentials with respect to Fe2O3 synthesized by various conditions.


15


18

14

14

Fe2O3-TS

12

Seperation Efficieny (%)

Fe2O3-TS

b

Fe2O3-NS

a

Fe2O3-NR

16

1/C2/109 F-2cm4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

Fe2O3-NS

12 10 8 6 4

Fe2O3-NR 10 8 6 4 2

2 0

0 0.0

0.2

0.4

0.6


0.8

1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4


Figure 5: (a) Mott-Schottky plots, (b) Carrier separation efficiencies of Fe2O3-NS, NR and TS and the inset shows the values of flat band potential and carrier density.


16

Page 17 of 34

-400

1.0 Fe2O3-NS Fe2O3-NR

-280

Fe2O3-TS

-80 -70

-240

Fe2O3-NR Fe2O3-TS

-50 -40 -30 -20 -10

-200

0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

-160

b

Fe2O3-TS

Fe2O3-NS

-60

Z" (ohm)

-320

Fe2O3-NS

a

Z' (ohm)

-120 -80

Current Density (mA/cm2)

-360

Z" (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe2O3-NR

0.8

0.6

0.4

0.2

-40 0 0

40

80

120

160

200

0.0 0

200

Z' (ohm)

400

600

800

1000

Time (sec)

Figure 6: (a) Nyquist impedance plots of α-Fe2O3-NS, α-Fe2O3-TS and α-Fe2O3-NR, (b) Chronoamperometric study for the determination of stability up to 1000 sec.


17


Page 18 of 34

Figure 7: FESEM images of α-Fe2O3-NS after stability checking.


18

Page 19 of 34

0.6

0.20 Fe2O3-NS

0.4

a


Fe2O3-NS-Co(II)


0.2

0.0

-0.2

b

0.15

Fe2O3-NS-Co(II) Fe2O3-NS

0.10

0.05

0.00

-0.05

-0.10 0.5

0.6

0.7

0.8

0.9

1.0

1.1

0

50

100

Potential (V) vs. RHE 0.30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

150

Time (Sec)

200

250

300

Fe2O3-NS Fe2O3-NS-Co(II)

0.15

0.00

-0.15 0

30

Time (Sec) 60

90

Figure 8: (a) LSV plot under chopped illumination, chronoamperometric study (b) for 300 seconds at an applied potential of 0.925 V vs. RHE, (c) for 90 seconds of α-Fe2O3NS and α-Fe2O3-NS-Co(II).

ASSOCIATED CONTENT


19


Page 20 of 34

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website. Material characterization data are shown here. The following files are available free of charge.

AUTHOR INFORMATION

Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Department of Science and Technology (DST) Inspire (DST/ INSPIRE/04/2015/000239) program, and DST Science and Engineering Research Board (SERB) (YSS/2015/000100)

Conflict of Interest The authors declare no conflict of interest.

ACKNOWLEDGMENT


20

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We are thankful to Department of Science and Technology (DST) Inspire (DST/ INSPIRE/04/2015/000239) program, and DST Science and Engineering Research Board (SERB) (YSS/2015/000100), Govt. of India. We are thankful to the central instrument facility of BITS Pilani for FESEM and Raman Facility. We also thank to the Department of Physics, BITS Pilani for assistance with powder x-ray diffraction studies.


21


Page 22 of 34

TOC


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Shape-Controlled Hematite: An Efficient Photoanode for

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