photoanodes for photoelectrochemical water splitting

Renewable Energy 133 (2019) 566e574

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Renewable Energy journal homepage: www.elsevier.com/locate/renene

Sn doped a-Fe2O3 (Sn¼0,10,20,30 wt%) photoanodes for photoelectrochemical water splitting applications B. Jansi Rani a, G. Ravi a, R. Yuvakkumar a, *, S. Ravichandran b, Fuad Ameen c, **, S. AlNadhary d a

Nanomaterials Laboratory, Department of Physics, Alagappa University, Karaikudi - 630 003, Tamil Nadu, India Electro Inorganic Division, CSIReCentral Electrochemical Research Institute (CSIReCECRI), Karaikudi e 630003, Tamil Nadu, India Department of Botany & Microbiology, College of Science, King Saud University, Riyadh 11451, Saudi Arabia d Department of Plant Protection, College of Agriculture, King Saud University, Riyadh 11451, Saudi Arabia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 January 2018 Received in revised form 18 June 2018 Accepted 12 October 2018 Available online 12 October 2018

One pot hydrothermal route was adapted to synthesis pristine and Sn doped a-Fe2O3 nanospheres successfully. Sharp high intense diffraction peaks obtained from XRD confirmed crystalline nature of rhombohedral hematite. The secondary SnO2 face formation was due to increasing Sn dopant concentration. Raman spectra confirmed intrinsic phonon vibration modes [Eg(1)þEg(2)þEu] of hematite nanospheres. 2P3/2(1) / 2P1/2 transition by emission peak at 549 nm confirmed hematite phase formation. Metal oxygen vibration (FeeO stretching) was confirmed by absorption band situated at 539 cm1. The noticeable variation in band gap of pristine hematite nanospheres was due to tetravalent Sn4þ dopant concentration. The lowest band gap energy 1.90 eV was found for 10 wt% Sn4þ doped hematite. Highest photocurrent 2.34 mA/cm2 at 0.098 V V RHE was obtained for 10% Sn doped hematite nanospheres. The EIS exposed the charge transferring mechanism of synthesized pristine and Sn doped a-Fe2O3 nanospheres. M-S plot evidenced that the lower shift of flat band potential for 10 wt% Sn4þ doped hematite was as 0.35 V. CA study proved the good stability over 4 h of the best performed photoanodes. Sn4þ doping and its dopant concentration on pristine hematite had dominant effect on photocatalytic activity of hematite nanospheres. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Sn doped hematite nanospheres Dopant concentration Water splitting

1. Introduction Nature always have a wonderful solution for all kind of problems in such a way that water undergoes splitting process into hydrogen and oxygen by sunlight in which cyanobacteria involved [1]. Photosynthesis is a key to the existence of living organisms in earth. Mankind is attempting to reproduce the natural photosynthesis process for the alternatives of non-renewable resources. Sunlight is the unconditional energy source to achieve artificial photosynthesis interms of photoelectric and photovoltaics [2,3]. One of the most predominant considerable clean energy criteria is

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (R. Yuvakkumar), [email protected] (F. Ameen). https://doi.org/10.1016/j.renene.2018.10.067 0960-1481/© 2018 Elsevier Ltd. All rights reserved.

pure hydrogen. Solar water splitting is such an unavoidable route to produce sustainable energy [4]. There are two ways in solar water splitting process. One is to use coupled solar cell-water electrolysis system and another one is direct photoelectrochemical (PEC) cell setup. The former one has the disadvantage of more efficiency loss and high cost but the later one is more economic as well as it consists of both solar cell and electrolyzer in a single system [5,6]. Since 1976, the world attention have been turned towards the revolutionary demonstration of PEC water splitting by two famous researchers Fujishima and Honda [7]. Nowadays, researchers focus directly on PEC water splitting. Semiconductor materials are of great interest to utilize as a photoanode for solar water splitting process due to its favorable band position and peculiar physicochemical properties. Among other semiconductor materials, hematite (a-Fe2O3) hold a sincere promise for high efficient solar water splitting due to its band gap (2.0e2.3 eV) which is close enough to the optimum band gap required for solar water splitting

B. Jansi Rani et al. / Renewable Energy 133 (2019) 566e574

[8,9]. Despite of these advantages to adapt itself as a good photoanode, it has some of the disadvantages in its intrinsic properties like short charge diffusion distance which restricts the collection of photogenerated charge carriers [10]. To overcome these difficulties, researchers made efforts like doping of foreign elements and synthesis of different morphologies at nanoscale range such as 1D structures like nanorods, nanowires, nanotubes and 2D structures like nanoplatelets, nanosheets etc [11e16] which provided the ideal platform for charge migration issues of hematite photoanodes. The hot trend to improve the performance of hematite is to introduce the metallic ion doping to increase the carrier concentration [17]. Moreover, innumerous research activities are going on all around the world for resolving the challenge of increasing the depletion layer thickness by doping the foreign cations on to the host hematite. Recently, Mingyang Li and his team investigated morphology and doping engineering of Sn-doped hematite nanowire photoanodes for water splitting applications [18]. Ruan and his group reported the improved photoelectrochemical performance of simultaneous doping and growth of Sn-doped hematite nanocrystalline films [19]. Arunprabaharan Subramanian and his research team examined and reported the effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting applications [20]. Wand and his team worked on boosting the photoelectrochemical performance of hematite photoanode with TiO2 under layer by extremely rapid high temperature annealing [21]. Rong Zhang and his co-workers reported enhanced photoelectrochemical water oxidation performance of Fe2O3 nanorods array employing S doping [22]. Kirtiman Deo Malviya and group reported the influence of Ti doping levels on the photoelectrochemical properties of thin-film hematite (aFe2O3) photoanodes [23]. Hossein Bemana and Sahar RashidNadimi jointly worked to investigate the effect of sulfur doping on photoelectrochemical performance of hematite [24]. Peng Yi Tang and his co-team successfully reported the enhanced photoelectrochemical water splitting of hematite multilayer nanowires photoanode with tuning surface state via bottom-up interfacial engineering [25]. Wei-Hsuan Hunga and his research squad demonstrated the enhanced carrier transport, charge separation and long term stability for photocatalytic water splitting by a rapid hot press process [26]. Venkatkarthick and his team fabricated aFe2O3/TiO2 heterostructured photoanode on titanium substrate for photoelectrochemical water electrolysis [27]. Rong Zhang and research group fruitfully analyzed and proposed Se doping and an effective strategy toward Fe2O3 nanorod array for greatly enhanced solar water oxidation [28]. Kaushik Natarajan and team examined visible-light-induced water splitting based on a novel a-Fe2O3/CdS heterostructure photoanode [29]. Lei Wang and his team fabricated Au/FeOOH sandwiched single crystalline Fe2O3 nanoflake photoelectrodes for enhanced solar water splitting by swift charge separation [30]. In the present study, pristine and tetravalent Sn4þ doped hematite has been successfully synthesized by employing one pot hydrothermal route. The main perspective of this work is to investigate the photoelectrochemical performance of hematite with respect to dopant concentration and to reveal the surfactant assisted synthesis mechanism as a vital role in obtaining particle shape and size. The structural, optical, morphological studies have been examined. The dopant concentration effect of pristine hematite nanospheres on photoelectrochemical behavior has been investigated by employing LSV, EIS, M-S plot and CA studies. The nanosphere morphology with various particle sizes along with modified band positions influenced the PEC performance in addition to the dopant concentration. The highest photocurrent has been observed for hematite doped with 10 wt % of Sn.

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2. Experimental section 2.1. Materials Ferric (III) chloride anhydrous (FeCl3), Tin (IV) chloride anhydrous (SnCl2), Cetyltrimethylammonium bromide-CTAB (C19H42BrN), Sodium nitrate (NaNO3), Nitric acid (HNO3), Ammonium hydroxide (NH3OH), Ethanol (C2H5OH) and deionized water were purchased from Sigma Aldrich and were utilized as such without further purification. 2.2. Preparation of pristine and Sn doped a-Fe2O3 photoanodes To synthesis hematite nanostructures by using ferric chloride precursor, the possible path way follows the formation of FeOOH which has been reported by many literature [31]. It purely depends on concentration of reagents and pH level maintained during the synthesis. In general hematite synthesis, the nucleation takes place by discrete dissolution re-precipitation mechanism [32]. Mainly, the synthesis starts with FeOOH is due to the role of FeOOH phase served as a template for the growth of hematite nucleation and resulted the desired compact morphology. This FeOOH phase is apparent to the final morphology of the hematite nanostructures. For pristine a-Fe2O3 nanosphere synthesis, initially 0.1 M FeCl3 and 0.1 g CTAB was dissolved in 25 and 10 ml of deionized water respectively. Both were mixed together under constant stirring speed of 600 rpm for 10 min. 1 M NaNO3 and 1 M HNO3 were then added step by step into the above solution and maintained the stirring for further 30 min until the pale orange color solution turned into dark orange color which was indicated the successful completion of chemical reaction between precursor and NaNO3þHNO3 solution. Then, the pH was adjusted with ammonium hydroxide to 12. This solution was then transferred into 100 ml Teflon autoclave and maintained at 100 C for 15 h. After naturally cooled down to room temperature, the powder was washed with deionized water and absolute ethanol for 5 times respectively to get rid of other chemical impurities. The cleaned powder was then calcinated at 700 C and named as A0. For Sn doped a-Fe2O3 nanospheres, initially, 0.09 M FeCl3 þ 0.01 M SnCl2, 0.08 M FeCl3 þ 0.02 M SnCl2 and 0.07 M FeCl3 þ 0.03 M SnCl2 was dissolved in 50 ml of deionized water respectively. Other procedures were same as that of the pristine hematite synthesis as mentioned above. These samples were named as A1, A2 and A3 respectively. Hence, the pristine and Sn doped hematite nanospheres were obtained. Preparation of photoanode has been explained in following steps. Initially, the fluorine doped tin oxide (FTO) glass plate (7 U/sq) was cleaned with deionized water, absolute ethanol and acetone under 30 min ultrasonication respectively. Then, the cleaned FTO plate was dried in hot air oven for 6 h at 80 C. This was then taken out to coat the active materials on it. For A0 photoanode fabrication, 40 mg of A0 sample was ultrasonicated with 0.5 ml ethanolþ 0.5 ml deionized water mixed solution for 45 min to get slurry which was coated on the dried FTO plate by using 10 ml pipettes in the active area 1 1 cm. The active material coated FTO plate was then naturally dried for 24 h to obtain the photoanode ready to take PEC measurements. Similarly, A1, A2 and A3 photoanodes were fabricated to investigate its PEC performance. 2.3. Formation mechanism The CTAB surfactant in precursor solution formed the micelles more polar and rough surface porous cluster which allows the water to penetrate it greater. Hence, the micelles created the active entities through condensation reaction for the growth of spherical particles. The exact formation mechanism of Sn4þ doped hematite

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nanosphere in the presence of CTAB is given in schematic diagram 1. 2.4. Characterization of pristine and Sn doped hematite nanospheres Crystalline phase and structure of synthesized pristine and Sn doped hematite nanospheres were identified by X-ray powder diffraction (X'pert PRO analytical diffractometer) using CuKa as radiation (1.541 Å) source. The spherical morphology of the synthesized samples was examined by Scanning Electron Microscope (ZESIS-V80). The particle size and shape further confirmed employing transmission electron microscope - TEM (Philips Tecnai 12 G2 TEM, at 120 kV). The phonon active modes and secondary phase impurities were detected by Imaging Spectrograph STR 500 nm focal length laser micro Raman spectrometer SEKI, Japan with resolution: 1/0.6 cm1/pixel and Flat Field: 27 mm (W) 14 mm (H). The luminescent behavior of pristine and Sn doped hematite were analyzed by Varian Cary Eclipse Photoluminescence Spectrometer with Oxford low temperature LN2 77 K setup. The metal oxygen vibration and the functional groups present in the sample were confirmed by Infrared (IR) spectra and recorded using Fourier transform infrared spectrophotometer employing Thermo Nicolet 380 with resolution 0.5 cm1 and S/N ratio: 2000:1 ppm for 1 min scan. The band gap estimation of the samples was observed by Varian Cary 5E UV/VIS-NIR UVevisible spectrophotometer. The elements present in the material and its oxidation states was confirmed by X-ray photoelectron spectroscopy (XPS) carried out on a PHI-5500 (PHI Company, U.S.A). 2.5. Photoelectrochemical measurement Photoelectrochemical studies such as linear sweep voltammogram (LSV), electrochemical impedance spectroscopy (EIS), MottSchottky (M-S) plot and chronoamperometry (CA) study were carried out in conventional three electrode photoelectrochemical (PEC) cell set-up consisted of platinum wire as counter electrode, Hg/HgO as reference electrode and active material (pristine and Sn doped hematite nanospheres) coated on fluorine doped tin oxide (FTO) glass plate (7 U/sq) in 1 1 cm as working electrode. 1 M KOH was used as electrolyte. All the photoelectrochemical measurements were carried out by employing solar simulator (Orion SOL3A) calibrated with silicon diode equipped with 100 mW/cm2 xenon lamp as slight source. The power was adjusted to 1 SUN and the stimulator was coupled with UV cutoff >420 nm filters which

Scheme 1. Formation mechanism of Sn4þ doped a-Fe2O3 nanospheres.

were controlled by Autolab PGSTAT 12 Eco Chemie (Netherlands). 3. Results and discussion XRD shows the diffraction patterns of annealed X doped a-Fe2O3 (X ¼ 0, 10, 20 and 30 wt% of Sn) as shown in Fig. 1aed. All the diffraction patterns revealed rhombohedral centered hematite structure. The exact rhombohedral hematite diffraction peaks centered at 2q values 24.01, 33.18, 35.60, 40.73, 49.38, 54.01, 62.32 and 63.98 corresponds to the respectively crystal planes (012), (104), (110), (113), (024), (116), (214) and (300) and are perfectly matched with the standard JCPDS card no #33-0664 of rhombohedral hematite phase [33]. When compared to pristine hematite diffraction peaks, as the tetravalent dopant Sn4þ concentration increases from 10 to 30 wt%, the sharp peaks tend to broaden. For sample A2, the formation of SnO2 phase may appear to start while increasing the dopant concentration from 10 to 20 wt% and the highest dopant concentration of 30 wt% Sn4þ with hematite may induce the secondary SnO2 phase formation in addition to the pure hematite phase which clearly demonstrated by the sharp diffraction peaks of SnO2 (Fig. 1c) which stressed out the secondary phase formation abundantly reduced photo catalytic activity of the sample A3 [34e38]. Hence, from the XRD result, the dopant effect and concentration of Sn4þ effect on pristine hematite predominantly influence the crystalline nature and phase formation of hematite. The spherical morphology of the pristine and Sn4þ doped hematite was examined by SEM in 200 nm scale. Generally, the CTAB played a vital role in nanoparticle synthesis and acts like a linking agent which helps the nanosphere formation. Fig. 2aed noticeably illustrated the particle formation and its growth. Fig. 2a represents the larger agglomeration of bare hematite nanoparticles which may due to the superior interaction between the magnetic nanoparticles during the synthesis process. Fig. 2b represents the well defined evenly distributed nanospheres with lesser aggregation and smaller particle size (~60e80 nm) which supports the higher photocatalytic activity of the sample. Fig. 2c and d represent the increasing particle size as compared to A0 and A1 samples which may due to the substitution of higher atomic weight (Sn4þ) tetravalent dopant concentration on pristine hematite nanospheres. The addition of abundant tetravalent dopant during the nanosphere

Fig. 1. XRD pattern a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ aFe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.


Fig. 2. SEM images a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ aFe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.

formation may kindle the chemical reaction kinetics inside the reaction medium in addition to the surface effect [39] which results the larger sized (~100e250 nm) nanospheres. Moreover, SEM images of the pristine and Sn4þ doped hematite revealed the spherical morphology nature. Fig. 3aed represents the transmission electron microscopy (TEM) images of 10 wt% of Sn4þ doped a-Fe2O3 nanospheres at different nanometer scale. Fig. 3aec clearly illustrated the nanosphere morphology of 10 wt% of Sn4þ doped hematite with particle size 60e80 nm which exactly coincided with the approximate particle size from SEM images. The selected area electron diffraction (SAED) pattern of 10 wt% Sn4þ doped hematite precisely illustrates the poly crystalline nature of the sample. TEM images strongly confirmed the formation of nanosphere morphology with the particle size in between 60 and 80 nm which effectively windup the discussion of SEM images. Further, in Fig. 3, the hematite nanospheres capped with the small dot like nanoparticles (mentioned in dotted red color line) occurred in TEM images is nothing but the formation of small SnO2 nanoparticles due to the Sn atom relatively has a larger size than that of Fe atom. Hence, it is obviously difficult to diffuse the Sn dopant into the hematite host

Fig. 3. TEM images of 10 wt% Sn4þ doped a-Fe2O3 a) 100 nm b) 50 nm c) 20 nm scale d) SAED pattern.

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[40]. The presence of small boundary around the nanospheres in 20 nm scale range caused by the earlier growth of hematite particles by FeOOH phase was already reported in literature [41]. Raman spectra of the pristine and Sn4þ doped hematite samples A0, A1, A2 and A3 are respectively shown in Fig. 4aed. There are five well defined Raman active modes such as Eg(1), Eg(2), A1g, Eu and 2Eu situated at ~273, 389, 463, 641 and 1287 cm1 respectively which are coincided with the exact characteristic phonon vibration modes of a-Fe2O3 [42]. The Raman spectra (Fig. 4aed) showed the minimum reduction in over all peak intensity compared to pristine hematite. This may cause from the substitution of dopant in pure hematite phase [43,44]. Moreover, the peak situated at ~641 and ~1287 cm1 deliberately reduced with respect to the Sn4þ dopant concentration as 0, 10, 20 and 30 wt% respectively. The reducing intensity effect in Raman spectra of hematite nanospheres may due to the disorder induced by the substitution of dopant in high concentration [38]. Hence, the Raman spectra of the synthesized pristine and Sn4þ doped hematite nanospheres undoubtedly illustrated the influence of dopant concentration which supports the XRD discussion. Fig. 5aed represents the photoluminescence spectra of pristine and X doped a-Fe2O3 (X ¼ 0, 10, 20 and 30 wt% Sn) annealed at 700 C respectively. There are four emission peaks observed respectively around the wavelength 484, 495, 521 and 552 nm by exciting the samples at 450 nm. The entire PL spectra observed in the visible region shows the samples possess oxygen deficiency. The peaks located at 484 and 495 nm could be attributed to ded transition of overcoming forbidden rule due to the quantum confinement effect and delocalization states [45]. The band at 521 nm could be attributed to 3d5 / 3d4 4s of Fe3þ cation [46]. The final band situated at 545 nm attributed to the 2P3/2 (1) / 2P1/2 transition of hematite confirms the formation of hematite phase [47]. From Fig. 5aed, the luminescent intensity slightly reduced with increase of Sn4þ dopant concentration which may due to the increase of particle size. The increase in particle size probably reduced the recombination rate which cause for the lower surface and lower trapping density which results reduction in PL intensity. Fig. 6aed precisely demonstrated the FTIR spectra of pristine and Sn4þ doped hematite nanospheres which evidently confirmed the metal oxygen vibrations at 469 and 557 cm1 and corresponds

Fig. 4. Raman spectra a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ aFe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.

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Fig. 7. UV vis graph a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ aFe2O3 d) 30 wt% Sn4þ þ a-Fe2O3 a-d) Tauc plot from UV plot.

Fig. 5. PL spectra a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ a-Fe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.

Tauc plot results as the variation in band gap. The calculated band gap from absorption spectra by Tauc plot is given by the formula, ahy ¼ A (hy-Eg)n where, a is absorption co-efficient, h incident photon energy, A constant, Eg band gap energy and n ½ and 2 for direct and indirect band gap materials. Tauc plot extrapolated the band gap energy of pristine and Sn4þ doped hematite photoanodes (Fig. 7aed). The respective band gaps found as 2.23, 1.90, 2.12 and 2.20 eV for 0 wt% doped a-Fe2O3, 10 wt% Sn4þ doped a-Fe2O3, 20 wt % Sn4þ doped a-Fe2O3 and 30 wt% Sn4þ doped a-Fe2O3 respectively. This shows that there is a visible reduction in band gap energy from 2.23 to 1.90 eV due to dopant effect in the hematite lattice. The lowest band gap observed for A1 sample further supports the enhanced photocurrent response. Further the increment in dopant concentration changed the band position which results the variation in band gap of the samples. It is clear that the doped samples slightly possessed improved light absorption property than bare hematite samples. 10% Sn doped hematite showed the lower optical band gap than other samples [52]. This reduction in band gap may be due to the formation of sub band gap states formed by Sn doping. Further increasing the dopant concentration varies the band gap values may which caused by the disappearance of direct hematite transitions [53]. Hence, 10% Sn doping has been considered to give the higher PEC performance under illumination. XPS spectra was carried out to confirm the chemical states of the best performed product 10 wt% Sn doped a-Fe2O3 nanospheres. Fig. 8A represents the high resolution full survey scan of A1 photoanode which revealed the presence of dopant and host elements and its oxidation states. Fig. 8B illustrates the dopant Sn 3d peaks

Fig. 6. FTIR spectra a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ aFe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.

to the t mode and FeeO stretching vibration mode respectively [48]. Further, the bands at 1052 and 2321 cm1 corresponds to CO and CO2 stretching vibration modes respectively [49]. The band observed at 2931 cm1 corresponds to CH symmetric stretching vibration and the band at 3438 cm1 corresponds to OH stretching vibration mode [50]. Hence, FTIR analysis deliberately confirmed the metal oxygen vibration of hematite nanospheres. The light absorption properties of the samples have been characterized by employing UV spectra as shown in Fig. 7. The band gap value and optical absorption characteristics of synthesized pristine and Sn4þ doped hematite photoanodes are shown in Fig. 7aed. The absorption spectra of all the four samples demonstrated the optical property observed in between the range 200e800 nm which are reliable to the hematite photoanodes [51]. There is a noticeable longer wavelength shift observed in the absorption spectra for pristine and Sn4þ doped samples which directly reflected in the

Fig. 8. (A) High resolution XPS survey scan (B) Sn 3d (C) Fe 2p (D) O 1s XPS spectra of 10 wt% Sn doped a-Fe2O3.


which consist of two predominant peaks located at 486.5 and 496 eV and could be characterized to 3d5/2 and 3d3/2 Sn states [54] which is in agreement with fully oxidized Sn4þ state of Sn dopant reported in literature [55]. In addition, the peak at 486.5 eV may corresponds to the mixture of SnO2 phase caused by small amount of Sn atoms which may precipitate on the surface due to high calcinations temperature [56]. Fig. 8C represents the XPS spectra of Fe 2p in 10 wt% Sn doped a-Fe2O3. The peaks located at the binding energies 711.7 and 725.3 eV could be characterized to Fe 2p3/2 and Fe 2p1/2 states of hematite host respectively [57]. Fig. 8D represents the XPS spectra of O 1s peak at the binding energy 530.5 eV. The dopant Sn4þ is an n- type semiconductor which has been confirmed by literature before starting this work. Further, the n type Sn doping is confirmed by intensity broadening effect observed from XRD and Raman studies. In addition, the Sn4þ doping is inferred by the change in optical band gap values of bare hematite in UV which is explained in detail in the UV result. The most common significant concern of doping with Sn reflects in the improved charge transfer kinetics [58] which is also deduced from EIS spectra which results the improved photocurrent response. Hence, the presence of elements and state of chemical environment such as Sn4þ doping of hematite has been confirmed by XPS spectra and discussed in detail. Fig. 9aed revealed the photoresponse of all the synthesized samples. The current density-voltage curve was recorded under illumination with scan rate of 10 mV/s. Compared to pristine hematite, Sn4þ doped hematite photoanodes showed better photoresponse. The photocurrent density is observed as 0.44, 2.34, 1.46 and 0.71 mA/cm2 at 0.098 V V RHE for pristine a-Fe2O3, 10 wt% Sn4þ doped a-Fe2O3, 20 wt% Sn4þ doped a-Fe2O3 and 30 wt% Sn4þ doped a-Fe2O3 respectively. The highest photocurrent of 2.34 mA/cm2 at 0.098 V V RHE is obtained for 10 wt% of Sn4þ doped with the bare hematite sample which is nearer to the already reported values [59]. The A1 photoanode illustrated the considerable enhancement in photocurrent density than the other samples. This may due to the improvement in majority carriers to conduct which would lead the high electric field across space charge layer and enhancement in transformation of charge photogenerated charge carriers and reduces the electron-hole recombination rate [60] occurred in

Fig. 9. LSV graph (JV curve) under illumination a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ aFe2O3 c) 20 wt% Sn4þ þ a-Fe2O3 d) 30 wt% Sn4þ þ a-Fe2O3.

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pristine hematite by Sn4þ dopant. Further increment in dopant (Sn4þ) concentration in pristine hematite causes the minimization of space charge layer width which reduces the photocurrent density from 2.23 to 1.46 mA/cm2 Vs RHE. Sample A3 (30 wt% Sn4þ doped a-Fe2O3) undoubtfully affected the recombination nature which completely cancels out the separation efficiency in addition to the secondary phase formation confirmed by XRD results that highly reduces the photocurrent density as 0.71 mA/cm2 Vs RHE [61]. From our investigation, we reported that 10 wt% of Sn4þ doping on pristine hematite is the optimum doping concentration that possessed the highest current density of 2.3 mA/cm2 Vs RHE which is approximately five times greater than the current observed for pristine hematite which may due to the evenly distributed smaller size nanosphere formation and favorable band gap position of 10 wt% Sn4þ doped hematite for photolectrochemical water splitting application. Unconditionally, the enhancement in photocurrent density for 10 wt% of Sn4þ doped hematite is due to the occurrence of optimization between increasing conductivity and decreasing space charge width which facilitated the accumulation of photogenerated charge carriers [62]. We can clearly see the lowered shift of onset potential from 0.31 to 0.23 V (Vs Hg/HgO) for A0 and A1 samples respectively. Fig. 10A(a-d) demonstrated the Nyquist plot of electrochemical impedance spectra (EIS) of synthesized four samples. The Nyquist plot revealed the solution resistance (Rs), charge transfer resistance (Rct) and separation efficiency of photogenerated carriers for the entire samples elaborately. The solution resistance (Rs) values is observed as 249, 74, 131 and 149 U for the samples pristine a-Fe2O3, 10 wt% Sn4þ doped a-Fe2O3, 20 wt% Sn4þ doped a-Fe2O3 and 30 wt% Sn4þ doped a-Fe2O3 respectively. The semicircle region represented the interface layer resistance occurred at the surface of the photoanodes and the lowering semiconductor diameter demonstrated the higher separation efficiency of photoanodes towards the photogenerated charge carriers [55]. This evidence moreover supports the highest photocurrent observed for 10 wt% Sn4þ doped a-Fe2O3 sample. Fig. 10B represented the depressed semicircle region governed by the samples which also represented the lowest charge transferring resistance possessed by 10% Sn doped hematite photoanode. Further, Mott-Schottky plot is one of the most important techniques to bring out the photoelectrochemical properties of synthesized samples. Mott-Schottky (M-S) plot gives the values of both ND and Efb from the relationship, 1/C2sc ¼ 2/eεεoNDA2s [(E-Efb-kT)/e] where, e is the electron charge, ε the dielectric constant of semiconductor, εo the permittivity of vacuum, As the surface area of working electrode, k the Boltzmann's constant and T the temperature. The positive slop of M-S plot specified that both pristine and Sn4þ doped hematite are n type semiconductors. The subsequent donar concentration on hematite nanospheres definitely made an impact on its photoelectrochemical properties once again

Fig. 10. (A) Nyquist plot of EIS measurement a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ aFe2O3 c) 20 wt% Sn4þ þ a-Fe2O3 d) 30 wt% Sn4þ þ a-Fe2O3 photoanodes under illumination in 1 M KOH electrolyte (B) Zoomed image of semicircle region.

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Fig. 11. Mott-Schottky plot a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ a-Fe2O3 d) 30 wt% Sn4þ þ a-Fe2O3 photoanodes under illumination in 1 M KOH electrolyte at 1 kHz frequency.

Fig. 12. (A) Chronoamperometry a) 0 wt% a-Fe2O3 b) 10 wt% Sn4þ þ a-Fe2O3 c) 20 wt% Sn4þ þ a-Fe2O3 d) 30 wt% Sn4þ þ a-Fe2O3 photoanodes under light-chopping conditions (AM 1.5100 mW/cm2) (B) Chronoamperometry of 10 wt% Sn4þ þ a-Fe2O3 for 4 h under light.

evidenced by M-S plot shown in Fig. 10aed. The overall improved photoelectric response than pristine hematite may due to the majority carriers for the conductance by the substitution of Sn4þ ion on Fe3þ lattice results good separation efficiency of photogenerated charge carriers. Fig. 11aed clearly demonstrated the estimated flat band potential shift observed from 0.26 to 0.36 V. In this case, the X axis intercept of linear fit to M-S plot, the predicted flat band potentials are 0.26, 0.35, 0.34 and 0.28 V for 0 wt% doped aFe2O3, 10 wt% Sn4þ doped a-Fe2O3, 20 wt% Sn4þ doped a-Fe2O3 and 30 wt% Sn4þ doped a-Fe2O3 respectively. This kind of negative shift observed in flat band potential designated that the improved photoelectrochemical ability of photoanodes abruptly supports the separation efficiency of photogenerated charge separation efficiency [63]. In order to investigate the detailed photoelectric response of

pristine and Sn4þ doped hematite nanospheres, chronoamperometry (CA) study was carried out for all the four synthesized samples (Fig. 12A(a-d)). The synthesized photoanodes are tested from 0 to 1400 s in CA study. The good photostability was found from this study for all the four photoanodes. The photocurrent observed in CA under light on off state indicated that the transient effect in excitation while light is on and the fast return action while light is off observed from CA graph. The result of pristine hematite photoanode indicated that the faster recombination of photogenerated charge carriers on the surface of the photoanode due to the inadequate life time of electron-hole pair and poor minority carriers [64]. The best performed A1 photoanode was subjected to CA study under illumination for 4 h to explore its stability where the gradual reduction in photocurrent observed over 2 h. After that the photocurrent was same around 0.35 mA/ cm2 throughout the experiment up to 4 h is as shown in Fig. 12B. Table 1 demonstrated the comparative performance of our synthesized Sn doped hematite photoanodes with the reported literature. It expressed the observed photocurrent value for the optimum dopant concentration of 10% in the present work is superior to the compared literature. Further, the structural and morphological exploration if investigated, the highest PEC response would be expected in our future works. In case of doped samples Sn4þ showed the better photoelectric response due to the synergistic dopant effect. This termination further evident from the results and discussion of LSV, EIS and M-S plot of 0 wt% doped aFe2O3, 10 wt% Sn4þ doped a-Fe2O3, 20 wt% Sn4þ doped a-Fe2O3 and 30 wt% Sn4þ doped a-Fe2O3 respectively. Hence, the enhanced photocurrent response observed for 10 wt% Sn4þ doped hematite nanospheres is definitely due to the synergetic effect of optimized dopant concentration, uniform smaller size nano sphere formation and favorable band position (1.90 eV) of the sample confirmed by above all characterization studies to improve the overall increment in photocurrent (2.34 mA/cm2 vs RHE), good electrical conductivity, lowered flat band potential (0.35 V) and good photostability around 1400 s than other samples. As already explained, 10% Sn doped Fe2O3 photoanode exposed the higher photocurrent density than the other samples. The aim of the present work is to investigate which percentage of dopant concentration yield the superior PEC performance. Hence, while doping the 10% Sn provided the improvement in majority carriers and results the higher electric field produced across the space charge layer and faster charge transfer kinetics evidenced by EIS spectra. Further, the reduction in recombination rate evidenced by PL spectra for the 10% Sn doped sample additionally supports the higher PEC performance [51]. In addition, the optical band gap of the photoanode widely influenced the PEC response of the photoanode. Hence, the shrinkage of optical band gap for 10% Sn doped hematite may abundantly contribute for the best performance. This narrower band gap helps to utilize the solar spectrum effectively than other samples [69]. Moreover, with an increase in the dopant concentration as 20 and 30%, the gradual reduction in PEC performance is observed which may due to the excess dopant concentration in host hematite created more recombination centers and results higher recombination rate than the 10% Sn doped sample

Table 1 Comparative study of photoelectrochemical performance of previously reported photoanodes with our current results. Photoanode Sn Sn Sn Sn Sn

a-Fe2O3 nanorods doped a-Fe2O3 doped a-Fe2O3 nanoflakes doped a-Fe2O3 nanorods doped a-Fe2O3 nanospheres

Electrolyte solution

Photocurrent density (mA/cm2 Vs RHE) at 10 mV

References

1M 1M 1M 1M 1M

0.94 1.2 1.6 1.6 2.34

[65] [66] [67] [68] Present work

KOH NaOH KOH NaOH KOH


[70]. Hence, the 10% Sn doping has been concluded as optimum dopant concentration for our synthesized hematite nanosphere photoanodes. Moreover, the present work debated that the dopant concentration strongly influences the physical, morphological and photoelectric performance of hematite nanostructures. 4. Conclusions One pot hydrothermal synthesis of Sn4þ doped a-Fe2O3 nanosphere was successfully prepared. The dopant and the concentration effect of Sn4þ with pristine hematite highly discussed from LSV result exemplified the photocurrent of 2.34 mA/cm2 at 0.098 V V RHE for 10 wt% Sn4þ doped hematite nanosphere. EIS and M-S plot supported the enhanced photocurrent response by its higher conductivity and lower flat band potential (0.35 V). Good photostability of 10 wt% Sn4þ doped hematite nanosphere was reported for 4 h from chronoamperometry study. This work left a big deal of synthesizing the complex morphological effect with the current optimized dopant concentration with hematite which will be carried out in future works. The results obtained strongly suggested that the Sn4þ dopant and its concentration evidently made an impact on photocurrent response of pristine hematite nanostructure and hence Sn doped hematite photoanode is a promising photoanode for solar water splitting applications. Acknowledgements This work was supported by UGC Start-Up Research Grant No.F.30-326/2016 (BSR) and the Deanship of Scientific Research at King Saud University (Research group no. RGP-1438-029). References [1] D.J. Des Marais, When did photosynthesis emerge on earth? Science 289 (2000) 1703e1705. [2] A. Kay, I. Cesar, M. Gratzel, New benchmark for water photooxidation by nanostructured a-Fe2O3 films, J. Am. Chem. Soc. 128 (2006) 15714e15721. [3] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to hydrogen and oxygen, Acc. Chem. Res. 28 (1995) 141e145. [4] T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, Photo-electrochemical hydrogen generation from water using solar energy Materials-related aspects, Int. J. Hydrogen Energy 27 (2002) 991e1022. [5] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338e344. [6] J. Young Kim, G. Magesh, D. Hyun Youn, J.W. Jang, J. Kubota, K. Domen, J. Sung Lee, Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2013) 2681. [7] A. Fujishima, k. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37. [8] T.J. LaTempa, X. Feng, M. Paulose, C.A. Grimes, Temperature-dependent growth of self-assembled hematite (a-Fe2O3) nanotube Arrays: rapid electrochemical synthesis and photoelectrochemical properties, J. Phys. Chem. C 113 (2009) 16293e16298. [9] C.Y. Cummings, F. Marken, L.M. Peter, K.G. Upul Wijayantha, A.A. Tahir, New insights into water splitting at mesoporous a-Fe2O3 films: a study by modulated transmittance and impedance spectroscopies, J. Am. Chem. Soc. 134 (2012) 1228e1234. [10] Y. Lin, S. Zhou, S.W. Sheehan, D. Wang, Nanonet-based hematite heteronanostructures for efficient solar water splitting, J. Am. Chem. Soc. 133 (2011) 2398e2401. [11] N. Beermann, L. Vayssieres, S.E. Lindquist, A. Hagfeldt, Photoelectrochemical studies of oriented nanorod thin films of hematite, J. Electrochem. Soc. 147 (2000) 2456e2461. [12] T. Lindgren, H. Wang, N. Beermann, L. Vayssieres, A. Hagfeldt, S.E. Lindquist, Aqueous photoelectrochemistry of hematite nanorod array, Sol. Energy Mater. Sol. Cells 71 (2002) 231e243. [13] A. Mao, G.Y. Han, J.H. Park, Synthesis and photoelectrochemical cell properties of vertically grown a-Fe2O3 nanorod arrays on a goldnanorod substrate, J. Mater. Chem. 20 (2010) 2247e2250. [14] K.X. Wang, Z. Yu, V. Liu, M.L. Brongersma, T.F. Jaramillo, S. Fan, Nearly total solar absorption in ultrathin nanostructured iron oxide for efficient photoelectrochemical water splitting, ACS Photonics 1 (2014) 235e240. [15] J. Li, Y. Qiu, Z. Wei, Q. Lin, Q. Zhang, K. Yan, H. Chen, S. Xiao, Z. Fan, S. Yang, A three-dimensional hexagonal fluorine-doped tin oxide nanocone array: a superior light harvesting electrode for high performance photoelectrochemical water splitting, Energy Environ. Sci. 7 (2014) 3651e3658.

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