Study of oxidation states of Fe

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 5

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Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior Dhayanantha Prabu Jaihindh a, Atul Verma a, Ching-Cheng Chen a, Yu-Cheng Huang b, Chung-Li Dong b, Yen-Pei Fu a,* a b

Department of Materials Science and Engineering, National Dong Hwa University, Hualien, 97401, Taiwan Department of Physics, Tamkang University, Tamsui District, New Taipei City 25137, Taiwan

article info

abstract

Article history:

In this work we propose a study to determine the structure of Fe and Co doped TiO2 by

Received 20 April 2018

using the Fe K-edge, Co K-edge and Ti K- edge X-ray absorption near edge spectroscopy.

Received in revised form

The detailed analysis of Fe and Co-doped TiO2 before and after Methylene blue (MB)

17 July 2018

treatment was examined under the irradiation of 35 W xenon arc lamp for 3 h. The ma-

Accepted 24 July 2018

terials treated with MB were studied by X-ray absorption spectroscopy, EPR and FT-IR

Available online xxx

which revealed that the oxidation state of Co2þ was photo-oxidized to Co3þ and Fe3þ was photo-reduced to Fe2þ or less. Thermodynamic, kinetic properties were studied at different

Keywords:

reaction temperature and the activation energy (Ea), enthalpy (DH), entropy (DS) and free

Photocatalysts

energy (DG) of activation were calculated for the reaction. The activation energy has been

XAS

found for TiO2, FeeTiO2 and CoeTiO2 as 24.771, 11.413 and 15.801 kJ mol1 respectively. The

EPR

structure, morphology and optical properties were studied by XRD, UV-diffuse reflectance

Thermodynamic

spectra, FESEM, TEM and PL. Moreover, electrochemical studies were carried out to

Kinetics

demonstrate the oxygen evolution reaction (OER) activity on TiO2, FeeTiO2 and CoeTiO2 in

Water splitting

1 M of H2SO4 electrolyte, with a scan rate of 50 mV s1 and the as-prepared photocatalysts could act as the promising electrode materials for water splitting. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Recently, the emission of dye waste water from various industries such as textiles, printing, medical, food and cosmetics has become the major environmental problem to human and ecology [1e3]. Some techniques have been used to remove the dye from industrial waste water such as adsorption, flocculation, chemical oxidation and photocatalytic dye degradation. Compared to other methods, photocatalytic dye

degradation has attracted intensive attention due to its capability to destroy organic molecules [4e7]. Another argument, the photocatalytic water splitting to generate oxygen and hydrogen fuel have emerged as potential candidates to encourage the development of clean methods for generating energy [8]. Accordingly, the exploration of abundant, highly efficient, and economic alternatives for water splitting is crucial in terms of solving global energy crisis and environmental challenges. Recently, earth-abundant and environmentally friendly transition-metals-based oxides (Co, Ni, Fe)

* Corresponding author. E-mail address: [email protected] (Y.-P. Fu). https://doi.org/10.1016/j.ijhydene.2018.07.150 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150

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have exhibited superior performance in OER catalysis due to their reasonable reactivity and stability [9,10]. The various oxide based semiconductor photocatalysts being widely examined (e.g., TiO2, ZnO, SrTiO3, Fe2O3, WO3, and Cu2O) [11e13]. So far, many of materials have been developed as OECs, including the traditional noble-metal based catalysts (such as IrOx, and RuOx), and 3d transition metals-based catalysts (such as Co-Pi, (Fe, Co, Ni)-based hydroxides) [14], and so forth. Therefore, the development of robust and economic routes to achieve highly active transition-metal-based catalysts will contribute greatly towards various OER relevant fields. Among us, TiO2 based materials have become the most agreeable photocatalysts because of their numerous benefits such as structural stability, abundance, eco-friendly, and lowcost [15]. Anatase TiO2 can degrade many pollutants under ultraviolet (UV) irradiation by the formation of free radicals; electron/hole pairs are generated from anatase TiO2 which react with H2O and/or O2 to produce OHC free radicals, which is a strong oxidant. Modification of TiO2 with metal ions could enhance photocatalytic activity and the photoresponse could be brought close to visible region [16]. The incorporation of 3d-transition metals in TiO2 is an effective approach to decrease the band gap and to improve visible-light photocatalytic activity [17]. Band gap reduction is the result of the creation of states in the band gap of the matrix, as systematically investigated theoretically by Umebayashi et al. [18]. They found that V, Cr, Mn, Fe, and Co create donor states, their position shifting toward the valence band with the increase of atomic number. Iron and cobalt have been considered a good candidate for TiO2 doping due to the Fe3þ, Co2þand Ti4þ ionic radii similarity; and the enhanced crystallization by Fe3þ, Co2þ addition due to the increased amounts of oxygen vacancies in the TiO2 lattice created by replacing the Ti4þ sites of the TiO2 lattice [19,20]. Recent years many literature report available on cobalt and Fe doped TiO2 was studied in various applications like oxidation of NO and CO [18], degradation of the methylene blue [21], methyl orange [22], 2- chlorophenol [23] 4-chlorophenol [24], oxidation of cyclohexane [25] and watersplitting [26,27]. Moreover, the photocatalytic dye degradation is a more complex process which is crucially linked to dopant sites. In fact, dopants may induce charge carrier recombination, or scattering or may trap charge carriers for long time intervals near the surface of nano-particles, making them available for oxidation and reduction processes, leading to improved efficiency for photodegradation. Therefore, a detailed atomistic knowledge of the charge dynamics involving defect sites is of most importance for a physical understanding of the material's function and may lead to knowledge-based device engineering [28]. Xray absorption spectroscopy (XAS) is a powerful tool to examine the local atomic and electronic structure of materials. Detailed analysis of the fine structure oscillations in an extended energy range (EXAFS: extended X-ray absorption fine structure) based on the real-space multiple scattering formalism [29,30] is able to provide a quantitative determination of the composition of the first few coordination shells around the excited atom, the interatomic distances, and their spread around the average value. Moreover, analysis of the line shape of the spectral region near the absorption edge

(XANES: X-ray absorption near-edge structure) can provide important information about the oxidation state, valence, atomic geometry, and site/symmetry selected density of states (DOS) of unoccupied electronic states. In the present work, we investigated the effect of methylene blue (MB) solution treatment over Fe- and Co-doped TiO2 and studied (i) X-ray absorption spectroscopy, which is sensitive to oxidation state and local crystal geometry, (ii) X-ray photoelectron spectroscopy (XPS), which is primarily sensitive to oxidation state, and (iii) Electron Paramagnetic Resonance (EPR) used for the measurement of species that contain iron and cobalt ions in the nanocomposites as well as found in other related studies. The thermodynamic and kinetic parameters at different reaction temperature have been studied. Also, we have been studied oxygen evolution reaction in the acidic electrolyte under dark and 35 W light irradiation.

Experimental section Sample preparation Iron and cobalt doped titanium dioxide (FeeTiO2 and CoeTiO2) photocatalysts were synthesized by a conventional solid-state reaction method with high purity anatase TiO2, Fe2O3 and CoO3 powders (>99%) as starting materials. In this study, the nominal molar ratio of Fe/Ti and Co/Ti is 2/100. These powders were mixed with ethanol (99.5%) and milled for 12 h with zirconia balls. These ball-milled mixtures were dried in oven at 70 C and ground into powder with mortar, pestle and then calcined at 600 C for 4 h in air.

Characterization A computerized X-ray powder diffractometer (XRD) with Cu Ka radiation (l ¼ 0.15406 nm) (Rigaku D/Max-Ⅱ) was used to identify the crystalline phase of Fe- and Co doped TiO2. The particle size, morphology, and composition were observed and analyzed by scanning electron microscopy (SEM; Hitachi 3400N) equipped with energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS; VGS Thermo K-Alpha) with an Al Ka radiation as the exciting source was adapted to assist us to figure out the surface chemical bonding of the Fe and Co doped TiO2. All the binding energies were referenced to the C 1s peak at 284.9 eV. The ultraviolet-visible spectra were obtained using a UV-VIS spectrometer with an integrating sphere (V-600, Jasco). X-band electron paramagnetic resonance (EPR) spectra were recorded by a Bruker EMX spectrometer under 77 K. XAS measurements were performed at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The Ti, Fe, and Co K edges were performed at the beamline BL17C using the transmission mode.

Photocatalytic activity The catalytic reaction for the degradation of methylene blue (MB) solution was carried out in a 200 mL Pyrex glass vessel under constant magnetic stirring. The initial concentration of MB solution was set to 20 mg/L (20 ppm). 10 mg of photocatalysts were taken with 50 mL of MB solution. The

Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150

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photocatalytic activities were evaluated by measuring the degradation of MB using 35 W Xe arc lamp with color temperature of 6000 K as visible light source. Illumination was implemented after the suspension was ultrasonicated for 10 min and placed in the dark room for 20 min to reach adsorption-desorption equilibrium. At specific time intervals (every 30 min), 5 mL of the sample was taken from the suspensions and centrifuged to remove photocatalyst particles prior to spectral measurement. MB was monitored by measuring the absorbance at a wavelength of 664 nm. The MB degradation was carried out at different temperatures such as 20, 30 and 40 C and the corresponding kinetics parameters were calculated.

Methylene blue treatment The photocatalytic materials, TiO2, FeeTiO2 and CoeTiO2 were taken in 50 mL of MB solution (10 ppm) and kept under stirring for 3 h under the irradiation of 35 W Xe arc lamp. After 3 h, the photocatalytic materials were washed with DI water and ethanol few times and dried in an oven at 70 C. Methylene blue treated and untreated photocatalytic materials were further characterized with XAS, FTIR and electrochemical studies.

Photoelectrochemical properties Photoelectrochemical properties was determined by using a three electrode cell consisting of a working electrode (WE), Pt as the counter electrode (CE), and Ag/AgCl (in 3 M KCl) as the reference electrode (RE). H2SO4 solution (1 M) was used as an electrolyte. The electrochemical measurements were performed using a potentiostat/galvanostat (CHI, 6273D) at room temperature. The catalysts inks were prepared by ultrasonication of a turbid solution containing 20 mg of photocatalytic materials with 300 mL of deionized water and 30 mL of 5% Nafion for 20 min. A known amount of the catalyst ink was taken and placed on a glassy carbon electrode (GCE) with an active surface area of 0.071 cm2, which acted as the working electrode in the three electrode cell system. The oxygen evolution reaction (OER) was carried out using a 35 W Xe arc lamp with color temperature 6000 K and emissions in the range of 360e1000 nm were used to irradiate the samples.

Results and discussions Composition and morphology characterization Fig. 1 exhibits the XRD patterns for annealed TiO2, FeeTiO2 and CoeTiO2 powders; obviously crystalline samples were obtained consisting of Anatase (JCPDS 711166) and minor Rutile (JCPDS78-1508) phases. It is noteworthy that no iron oxide peak was observed in the XRD pattern. It is assumed that the Fe ions totally incorporated into the structures of TiO2 by replacing the titanium ions or located at the interstitial sites. On the basis of the (200) and (004) diffraction peaks, the lattice constants a ¼ b and c of the Fe-doped TiO2 were estimated as 3.782 and 9.515 A and Co-doped TiO2 were estimated to be 3.779 and 9.491 A respectively, which are slightly larger

Fig. 1 e XRD characterization of TiO2, FeeTiO2 and CoeTiO2.

than the theoretical values of pure TiO2 with anatase phase (a ¼ b ¼ 3.747 A, c ¼ 9.334 A) due to the Fe and Co ion substitution [31]. The differences of the lattice constants suggest that there are lattice distortions in the anatase phase. The phase composition and lattice parameters of prepared materials are tabulated in Table 1. The morphology and the particles size of NPs were analyzed by a field emission scanning electron microscope (FESEM). As shown in Fig. 2b and c, elevation of the Fe and Co impurities decreases the homogeneity of the Fe and Co doped TiO2 NPs in comparison with pure TiO2 (Fig. 2(a)), which is due to the reduction of the particle size. Evidently, with the reduction of size of the particles, interatomic attraction and molecular forces grow, thereby the tendency of the NPs is enhanced to approach each other, which causes agglomeration. The EDX analysis has been used to determine the elemental percentage in the sample. Fig. 2(aec) demonstrates the x-ray energy diffraction spectrum for TiO2, Fe-doped TiO2 and Co-doped TiO2. Clearly, only O, Ti, Fe and Co elements exist in the sample, indicating the purity of the sample. In the samples, the atomic percentage of Fe and Co have been measured as 0.46% and 0.12%, which is less than the 2% molar percentage used in the experiment. This difference in the percentages is due to the fact that in the EDX analysis, the surface of the sample is analyzed and few other factors such as beam parameters, topography of sample, several acquisition settings, electronics and external field noises, atomic number of element, detector type, and so on.

Table 1 e Phase composition and lattice parameters estimated from XRD data. a ( A) b ( A) c ( A) Volume Density ( A3) (g/cm3) TiO2 FeeTiO2 CoeTiO2

3.78268 3.78281 3.77929

3.78268 3.78281 3.77929

9.52469 9.51568 9.49183

136.2856 136.1661 135.5721

3.8940 3.8974 3.9147


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Fig. 2 e SEM characterization of (a) TiO2, (b) FeeTiO2 and (c) CoeTiO2.

Optical characterization Optical properties of the specimens are tested via using UV-VIS diffuse reflectance spectra (DRS) as shown in Fig. 3(a). For TiO2, a white powder, the absorption onset is observed at ca. 380 nm and the onset for Fe and Co-doped TiO2 absorption shifted towards the visible-light region; the red shift in the absorption spectra can be associated to the band-gap (Eg) narrowing of these semiconductors. The band gap can be estimated from the variation of semiconductor absorption (a) with the incident photon energy (hn), employing a modified KubelkaMunk function [32]. The Eg for TiO2 was estimated as 3.22 eV, which

is in good agreement with the reported values for anatase phase [33]. When Fe3þ ions are doped with TiO2, the absorption edge spread in the visible region and the absorption edge corresponds to the electron transfer from the valence band (VB) to the conduction band (CB). Since Fe3þ in the 3d orbital is half filled, as Fe3þ doped into TiO2, the empty Eg state is near the bottom of the conduction band, while the occupied t2g state of Fe locates at the top of the valence band. There are multiple electronic transitions in Fe-doped TiO2, and multiple energy levels between VB and CB are shown in Fig. 3(b). In anatase phase of TiO2, Ti4þ is surrounded by six oxygen atoms in an octahedral coordination, forming TiO6 octahedra.



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Fig. 3 e (a) Absorbance spectra of TiO2, FeeTiO2 and CoeTiO2, (b) Deconvoluted FeeTiO2 absorbance spectrum and (c) PL spectra of TiO2, FeeTiO2 and CoeTiO2.

When Co2þ substitutes Ti4þ, it forms chemical bonds with six oxygen atoms. Now, according to crystal field theory, the electrons in the d-orbital of Co2þ will undergo repulsion by the electrons of the six surrounding oxygen atoms. This results in the splitting of d-orbital of Co2þ, showing the ded transition. Appearance of these transitions in the spectra of doped TiO2 nanoparticles is an indication of incorporation of Co2þ ions in TiO2 lattice. In our UV-DRS results, undoped TiO2 has an absorption edge around 380 nm, this wavelength corresponds to the band gap of 3.2 eV. When TiO2 is doped with Co2þ, the absorption edge slightly shifts into the visible region. The absorption edge corresponds to the electron transfer from the valence band (VB) to the conduction band (CB). As observed Co2þ doping leads to band gap narrowing. This may be due to the upward shift in the VB edge or downward shift in TiO2 CB edge or both depending on Co2þ content. So, the red shift is due to band gap narrowing because of Co2þ doping or carrier concentration effects. Photoluminescence (PL) signals and their intensities are closely related to the activities of photocatalysts [34]. The PL study could be an effective tool to study the lattice defects in the metal oxides coupled and doped TiO2 samples. The PL spectra of TiO2 based materials are assigned to three kinds of physical

origins: self-trapped excitons, oxygen vacancies and surface states (defects) [35]. Photoemission spectra (lexc ¼ 320 nm) for TiO2, FeeTiO2 and CoeTiO2 samples are depicted in Fig. 3(c). The strong emission peak at 389 nm and the emission spectra around 435e470 nm is corresponding to the charge transfer transition of oxygen vacancy trapped electrons of TiO2. PL signal at 485 nm is related to the surface oxygen vacancies or defects in the specimen and the PL signal at 527 nm, may originate from the Fþ center on the surface of the TiO2. Notably, the luminescence intensities of the Fe doped TiO2 were lower than other photocatalysts, which confirmed the lower electronehole recombination probability for the Fe doped TiO2 compared with others. This revealed that transition metal doping in TiO2 slowed down the electronehole pair recombination in the photocatalyst which may increase the photocatalytic activity. Photoluminescence is a surface phenomenon, and a change in the surface environment would have a significant effect on the photoluminescence process. It is reported that the visible luminescence band originates from the oxygen vacancies associated with Ti3þ in anatase TiO2 [36]. Upon the loss of an O atom in TiO2 lattice, the electron pair that remains trapped in the vacancy cavity Vo left behind a pair of electrons which give rise to an F center [37,38]. The basic assumption is


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that one of the electron in F center tends to occupy the neighboring Ti4þ ion and yield Ti3þ center and Fþ center forming a shallow and deep trap states, respectively which is explained in the following equations. Vo þ 2e /F

(1)

F þ Ti4þ /Fþ þ Ti3þ

(2)

Vo þ e /Fþ

(3)

Ti4þ þ e / Ti3þ

(4)

thus, Ti3þ center and Fþ center (single electron associated with oxygen vacancies) are formed due to the absence of an oxygen atom. From the above explanation, we can conclude the PL emission at 527 nm is due to deep trap state which is associated with the Fþ center. The XPS spectra of TiO2-based specimens are shown in Fig. 4 respectively. The full range survey spectra of 2 mol% Fe and Co-doped TiO2 powder is shown in Fig. 4(a). The main peaks are C 1s, Ti 2p, O 1s, Fe 2p and Co 2p, centered at 284.9 eV, 458.8 eV, 530.0 eV, 710.5 eV, and 780.35 eV, respectively. The carbon peak C 1s at 284.9 eV is due to the surface adventitious carbon as shown in Fig. 4(b). Fig. 4(c) shows the Ti

Fig. 4 e XPS spectra of the TiO2, FeeTiO2 and CoeTiO2: (a) wide-survey spectrum; the results of curve-fitting of the XPS spectra for (b) C 1s Spectra, (c) Ti 2p region, (d) O 1s region (e) Fe 2p spectrum for FeeTiO2 and (f) Co 2p spectrum for CoeTiO2. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150


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Fig. 5 e (a) XANES spectra of fresh and after MB treated TiO2, FeeTiO2 and CoeTiO2 at Ti K-edge (b) Pre-edge region of XANES spectra of Fresh TiO2, FeeTiO2 and CoeTiO2.

2p XPS region spectra. The Ti 2p3/2 and 2p1/2 photoelectron peaks of undoped TiO2 are symmetric in nature and their binding energy (BE) positions are at 457.46 and 463.20 eV, respectively, revealing the Ti4þ state [39]. The Ti 2p peaks of Ti in Fee and Co-doped TiO2 specimens are also identical with the undoped TiO2 except for a slight positive shift in BE, suggesting the substitution of Ti(IV) by Fe and Co [40e43]. This BE shift is due to the movement of electron cloud toward electronegative transition metals of Fe and Co. A similar positive shift in BE was found in the case of O 1s for Fe- and Co-doped TiO2. Fig. 4(d) shows the O 1s XPS region spectrum of undoped TiO2. The O 1s feature is deconvoluted to three peaks, at binding energies of 529.05 eV, 530.18 eV and 531.26 eV, which can be assigned to the TieO, OeH and CeO respectively. In Fig. 4(e) the binding energies at 709.16 eV and 723.78 eV should be assigned to 2p3/2 and 2p1/2 of Fe3þ, respectively. These data are very close to those in Fe2O3 (710.7 eV for 2p3/2 and 724.3 eV for 2p1/2) [44]. Therefore, Fe is trivalent existed in Fe-doped TiO2 powder and in Fig. 4(f) Co 2p XPS spectra of corresponding Co doped TiO2 are 780.35 eV (2p3/2) and 797.76 eV (2p1/2), which suggests the Co (II) chemical state in CoeTiO2 [45].

hexa-coordinated Ti atoms. Due to the relatively low iron and cobalt content, no appreciable changes influenced the Ti Kedge energy region. Fig. 6 shows a Ti K-edge spectrum of TiO2 before and after MB dye treatment, there are no any significant changes in the intensities of the pre-edge spectrum. From pre-edge spectra energies and intensities, in anatase the Ti atoms are 6 coordinated to oxygen to form a distorted octahedron in such a way that the Ti site symmetry group is D2d. The pre edge peaks correspond to transitions of the core electron to Ti 3d4p4s hybridized states [50]. After dye treatment, a small change has been observed towards lower energy in the edge spectrum, possibility of oxidation state of Ti could be reduced. The XAS and XANES spectra of Co-doped TiO2 at Co K-edge is shown in Fig. 7. The information revealed the Co oxidation state, pre-edge energy and the geometry of cobalt sites. Fig. 7(b) shows a pre-edge peak located at 7709.7 eV, followed

X-ray absorption spectroscopy (XAS) Fig. 5(a) shows the Ti K-edge of pre- and main-edge spectra of fresh and after MB treatment of TiO2, FeeTiO2, CoeTiO2. By examination of these data and comparison with the literature [29,46e48], we may make the following subjective comments. In Fig. 5(b) describes the pre-edge spectra of TiO2 based specimens, the four characteristic features denoted as A1, A2, A3 and Ttet in the pre-edge structure of the Ti K-edge spectra at 4969.2 eV, 4972.2 eV, 4974.6 eV and 4971 eV. The pre-edge Ti K-edge XANES structure arises from a 1s / 3d dipole electronic transition, being the peak intensities affected by the local geometry and the particular medium range structure of the sample around central Ti atoms [49]. Anatase type structure can be clearly observed from the spectra is shown in Fig. 5(b). Moreover, pre-edge features, i.e. the height and position of the pre-peaks, reveal that these samples contain

Fig. 6 e Ti K-edge spectrum of TiO2 fresh and after MB treated.


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Fig. 7 e (a) XAS spectra of CoeTiO2 at Co K-edge (b) XANES spectra CoeTiO2 at Co K-edge fresh and after MB solution treatment.

by a shoulder at 7723.3 eV and white line occurred at 7729 eV. Interestingly, despite the octahedral environment of Co (II) ions in CoO, the energy position of this peak in the CoO spectrum is substantially the same [51]. From literature survey, there is no consistency in the occurrence of white line at particular energy [52e54]. After MB treatment, the edge energy shows a red shift which may be due to the increase in oxidation state of Co2þ.

The XAS spectra of fresh Fe-doped TiO2, and after dye degradation are shown in Fig. 8(a) and (b) for comparison. The three low-intensity pre-edge peaks (A1, A2, A3) observed for all samples can be assigned to the transition from the 1s core level of Ti to three different kinds of molecular orbitals (1t1g, 2t2g and 3eg), and they are characteristic of the octahedral coordination of Ti in the TiO2 lattice [55,56]. The presence of tetrahedrally coordinated Ti would be evidenced by a small

Fig. 8 e (a) and (b) shows XASand Pre-edge spectra at Ti K-edge, (c) and (d) shows XAS and Pre-edge spectra at Fe K-edge. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150


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Fig. 9 e EPR study at 77 K of fresh and after MB-treated (a) TiO2, (b) FeeTiO2 and (c) Full spectrum of FeeTiO2.

Fig. 10 e (a) FT-IR spectrum of FeeTiO2 fresh and MB treated (b) FT-IR spectrum of CoeTiO2 fresh and MB treated. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150

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shoulder peak between the A1 and A2 peaks, the shoulder peak intensity between A1 and A2 (marked Ttet) is indeed observed for Fe doped TiO2 material [57]. This suggests that the small concentration of oxygen vacancies in Fe-doped TiO2 may lead to the formation of tetrahedrally coordinated Ti4þ species. The Fe K-edge XANES spectrum of Fe-doped TiO2 is shown in Fig. 8(c) and (d). A single pre-edge peak is observed in fresh Fe doped TiO2 at 7114 eV and a strong white line above the absorption edge exist, which are also typical of Fe ions in the octahedral coordination of oxygen atoms. Comparison of

ar et al. [58]. For these spectra to those reported by Vrac SrFexTi1xO3d (STF) shows that our FeeTiO2 spectra are similar to reduced STF patterns, in which Fe is known to be in the þ3 state. If Fe4þ was present, the main peak would have been at slightly higher energies. This information confirms our earlier conclusion from the XPS data that Fe is present in the þ3 oxidation state in FeeTiO2 material. After dye degradation, the pre edge energy is little reduced and broadened, it is similar to Fe metal XANES spectrum illustrated in the paper by Yang et al. [59].

Fig. 11 e Photocatalytic dye degradation of MB at 20 C (a), (b) First order kinetics plots at 20 C, (c) 30 C MB degradation, (d) First order kinetics plots at 30 C, (e) 40 C MB degradation and (f) First order kinetics plots at 40 C. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150

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Table 2 e MB degradation in percentage at different reaction temperatures. Temperature ( C)

20 30 40

Degradation (%) TiO2

FeeTiO2

CoeTiO2

70.38 86.49 90.80

85.86 90.68 94.93

82.95 90.17 90.92

and Ti metal leaching. In Fig. 10(b), in the range of 450e900 cm1 the pattern changes might be some TieOeTi bond changes. From FT-IR studies, it was revealed that there are no possibilities of Fe and Co metal leaching, could be concluded from XAS and EPR results that the oxidation states changed from Fe3þ to Fe2þ or less and Co2þ to Co3þ.

Photocatalytic activity test

EPR studies Fig. 9 shows EPR spectra of fresh TiO2 and FeeTiO2 and after MB solution treatment under the irradiation of 35 W for 3 h. EPR measurements were conducted at 77 K under visible light irradiation. In Fig. 9(a), the EPR spectra of the TiO2, a signal at g ¼ 1.96 suggested that the electrons trapped in the lattice defect sites lie a few electron volts below the CB edge. The value of g ¼ 2.07 and 2.05 are attributed to the photogenerated holes trapped on the surface or sub-surface bridging oxygen atom sites of the TiO2. In Fig. 9(b) and (c), sharp peak indicates the presence of an isolated octahedral Fe3þ cation in anatase (single-site doping), which is surrounded by Ti4þ cations at g ¼ 4.27 is assigned to the presence of oxygen vacancies by isolated rhombic Fe3þ cation [60e62]. After MB-treatment, in TiO2-based materials, the relative sharp peak intensities decreases with decreasing Fe3þ content, and also the oxygen vacancies at g ¼ 4.27 disappeared, indicating the Fe3þ might be reduced in to Fe2þ or less. From XAS and EPR, we could understand that after MB treatment of TiO2-based materials, Fe3þ oxidation state might be reduced.

FT-IR studies The molecular structures of the TiO2, FeeTiO2 and CoeTiO2 were characterized by FTIR spectroscopy as shown in Fig. 10. The peak around 3400 cm1 is due to physically adsorbed water molecules (OH) on the specimens [63]. Since Fe was doped into TiO2, the characteristic peak located at 575 cm1 is attributed to the stretching vibration of the FeeO bond [32,64]. Typically, TieOeTi bond were revealed at low frequency bands in the range of 450e900 cm1. The absorption band appeared at 1637 cm1, indicating that OeH band in the infrared region which causes the increase in surface hydroxylation with the doping of Fe and Co into TiO2. After MB treatment of specimens, in Fig. 10(a), for FeeTiO2 there are no particular changes at 575 cm1 indicating no possibility of Fe

To examine the effects of the photocatalytic performance of the TiO2, FeeTiO2 and CoeTiO2 photocatalysts, a degradation test was carried out at 20 ppm of 50 mL of MB under the irradiation of 35 W xenon arc lamp with different temperatures such 20, 30 and 40 C. Fig. 11(a) and (b) shows the dye degradation of MB and first order kinetic plots at 20 C, Fig. 11(c, d, e and f) correspond to the degradation of MB and first order kinetic plots at 30 and 40 C are shown. From these results Fe doped TiO2 has given highest photocatalytic activity compared with CoeTiO2 and TiO2 and the percentage of photocatalytic dye degradation are tabulated in Table 2.

Activation parameters The positive values of enthalpy and the increasing value of kapp with increasing temperature indicate that the photodegradation is an endothermic process [65]. The large positive DG at the higher temperature indicates no spontaneous processes and weak adsorption of organic pollutants on TiO2, Fee TiO2 and CoeTiO2. An increase in temperature facilitates the photocatalytic reactions to compete more efficiently with electronehole pair recombination. In addition, an increase in temperature would facilitate the electron transfers in the valence band to higher energy levels. The maximum kapp at temperature of 40 C confirmed the above explanation. The rate constants and the activation parameters for the MB dye degradation of TiO2, FeeTiO2 and CoeTiO2 as shown in Table 3. In Fig. 12(a) and (b) thermodynamic activation parameters, enthalpy (DH) and entropy (DS) of the reaction evaluated based on activation complex theory (ACT) showed the following values of 22.253 kJ mol1 for TiO2, 8.895 kJ mol1 for FeeTiO2 and 13.283 kJ mol1 for CoeTiO2 respectively. Gibb's free energy of activation (DG) average was 93.584 kJ mol1 for TiO2, 80.195 kJ mol1 for FeeTiO2 and 84.038 kJ mol1 for Coe TiO2. This indicated nonspontaneous, endergonic, and endothermic characteristics, because the values of DG and DH were positive, and DS was negative [66,67]. From Table 3 Values of

Table 3 e Rate constant and activation parameters of the MB dye degradation at different temperatures. Sample TiO2

FeeTiO2

CoeTiO2

Temperature (ºC)

K

Ea (KJ/mol)

DH (KJ/mol)

DS (KJ/mol.K)

DG (KJ/mol)

20 30 40 20 30 40 20 30 40

0.00557 0.00885 0.01063 0.00874 0.01095 0.01322 0.00796 0.01032 0.01071

24.771

22.253

0.2353

11.413

8.895

0.2352

15.801

13.283

0.2334

91.231 93.584 95.937 77.8438 80.195 82.547 81.704 84.038 86.372


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Fig. 12 e (a) Plot of ln k versus 1/T for the degradation of MB and (b) Plot of ln K/T versus 1/T for the degradation of MB.

Fig. 13 e Electrocatalytic property of TiO2, FeeTiO2 and CoeTiO2 electrodes for oxygen evolution reaction in 1 M H2SO4 at 50 mV s¡1 scan rate (V vs. RHE) (a) and (b) CV curves under dark and light, (c) and (d) polarization curves under dark and light, (e) and (f) EIS spectra under dark and light. Please cite this article in press as: Jaihindh DP, et al., Study of oxidation states of Fe- and Co-doped TiO2 photocatalytic energy materials and their visible-light-driven photocatalytic behavior, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.07.150


Table 4 e Current density values in dark and light irradiation. Photocatalysts

TiO2 CoeTiO2 FeeTiO2

Current density (mA cm2) In dark

Current density (mA cm2) In light

10.31 11.08 12.01

12.00 12.76 13.26

DS (DS < 0) may arise as result of association mechanism, which means that the reactant species joined to each other to form over transition states along the reaction. Thus, the transition state structure is more ordered than the reactants in the ground state; therefore, there is a negative activation entropy. Meanwhile, DH values (DH>0) indicate that the process is endothermic; that is, an external power source is needed to raise the energy level and transform the reactants to their transition state. Values of DG (DG > 0) can be attributed to high energy level in the transition state rather than to the reacting species.

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parameters reveals that the degradations are endothermic in nature on the Fe and Co doped TiO2, following pseudo first order kinetics. Further, Fe-doped TiO2 at 40 C showed good MB degradation reaction percentage of 94.93%. XPS, PL, XANES, EPR were employed to study the samples in detail which provided crucial information about the materials such as bond formation, changes in the oxidation state during dye degradation and also the percentage of degradation. The photoelectrochemical studies of OER were carried out, and the overpotential h was 0. 83 V for Fe-doped TiO2 and the corresponding current density was 13. 26 mA cm2 under the light illumination.

Acknowledgements The authors would like to thank Ministry of Science and Technology of Taiwan for financially supporting this research under contract number: MOST 106-2113-M-259-011.

references

Photoelectrochemical properties The photoelectrochemical properties of TiO2, Fe-doped TiO2 and Co-doped TiO2 were studied in 1 M H2SO4 electrolyte. Fig. 13(a) and (b) show cyclic voltammograms (under dark conditions and under 35 W light irradiation in 1 M H2SO4 solutions). The current densities were observed at the anodic vertex of 2.6 V for dark and light irradiation; Fe-doped TiO2 has the highest current density of 13.266 mA cm2 under the light [68]. Table 4 shows the current densities of all materials under dark and light conditions. Fig. 13(c) and (d) shows linear sweep voltammetry curves for the photocatalysts with dark and light irradiation, respectively. The oxygen evolution reaction (OER) begins as an onset potential of 2.06 V for TiO2, Fe-doped TiO2 and CoeTiO2 and the corresponding overpotential is h ¼ 0.83 V with respect to the reversible hydrogen electrode (RHE) but there is no expected onset potential shift for Fe-doped TiO2 and CoeTiO2. Fig. 13(e) and (f) reveals the Nyquist plot; the electron transfers resistance (Ret) for TiO2, FeeTiO2 and Coe TiO2 were measured under dark and 35 W light irradiation. The results suggest that the FeeTiO2 shows little better current densities and less electron transfer resistance for OER activity compared with TiO2 and CoeTiO2.

Conclusion The modification of TiO2 with Fe3þ and Co2þ can inhibit recombination and hence allow the accumulation of trapped electrons as Ti3þ. There is clear evidence for this from EPR studies. FeeTiO2 and CoeTiO2 have been elucidated by x-ray absorption spectroscopy. XANES spectra indicated that the fresh catalyst comprised of well-dispersed Fe3þ and Co2þ. Upon exposure with 35 W xenon arc lamp for 3 h, the Fe3þ was reduced to the Fe2þ or Fe0 and Co2þ partially oxidized into Co3þ. The reduced catalyst was composed of electron-rich Fe metal and it has high photocatalytic activity in the environmental pollutant removal applications. The study on activation

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