(Mn, Co, Cu) doping for visible light

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 4 0 ( 2 0 1 5 ) 1 5 4 2 3 e1 5 4 3 1

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Enhanced charge transport of LaFeO3 via transition metal (Mn, Co, Cu) doping for visible light photoelectrochemical water oxidation Qi Peng a, Bin Shan b, Yanwei Wen b,**, Rong Chen a,c,* a

State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People's Republic of China b State Key Laboratory of Materials Processing and Die &Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People's Republic of China c School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China

article info

abstract

Article history:

A facile transition metals (TM ¼ Mn, Co, Cu) doping has been employed as an effective

Received 4 March 2015

approach to alter the electrical and optical properties of LaFeO3 with substantially improved

Received in revised form

photoelectrochemical (PEC) water oxidation performance under visible light. It is found that

17 September 2015

the highest photocurrent density is obtained with 10% doping concentration, and the

Accepted 19 September 2015

promoted

Available online 20 October 2015

LaFe0.9Co0.1O3 > LaFe0.9Mn0.1O3 > LaFeO3. The 10% Cu dopant yields the highest photocurrent

Keywords:

LaFeO3 under visible light illumination (>420 nm). The Fe3þ discharge is observed in the

LaFeO3

doped samples by cyclic voltammetry study, wherein the discharge capacity exhibits the

Transition metals doping

same trend with the PEC enhancement. The electrochemical impedance spectroscopy

Visible light

shows the enhanced charge transfer on the surface of the TM doped LaFeO3, which is

Photoelectrochemistry

responsible for the improved PEC performance. Furthermore, the increased carrier densities

Water oxidation

upon TM doping revealed by MotteSchottky plots also contribute to the PEC enhancement.

photoconversion

efficiency

follows

the

order

of

LaFe0.9Cu0.1O3

>

density of 0.99 mA/cm2 at 1 V vs. Ag/AgCl, which is three times higher than that of pure

Our results demonstrate that TM doping is a viable way to promote the photoconversion efficiency of LaFeO3-based PEC cells under visible light. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Photocatalytic hydrogen generation by solar energy is a promising clean energy resource and has attracted great

attention over the past decades [1e3]. TiO2 is a common photoelectrode candidate owing to its good photocatalytic activities, abundance, stability and its environmental benignity [4,5]. Nevertheless, due to the large band gap, pure TiO2 cannot absorb the visible light that constitutes more

* Corresponding author. State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People's Republic of China. ** Corresponding author. E-mail addresses: [email protected] (Y. Wen), [email protected] (R. Chen). http://dx.doi.org/10.1016/j.ijhydene.2015.09.072 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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than 30% of the solar spectrum. Some strategies such as chemical ion doping [6,7], cocatalyst coating [6,8], and heterogeneous composites [9,10] have been used to achieve higher photocatalytic efficiency. Chalcogenides like CdS [11,12] and CdSe [13,14], and oxynitrides like TaON [15] and LaTiO2N [16] have also been investigated for their extended visible light adsorption and desired photocatalytic performance. However, structural instability and strict synthesis conditions prevent their wide application in photocatalysis. Some mixture metal oxides, such as Fe2O3/ SrTiO3 [17], NaTaO3eLaCoO3 [18], AgTaO3 [19], AgNbO3 [20], LiNbO3 [21], and CaFe2O4 [22], have also been employed as visible light photocatalysts. Yet their efficiencies still remain low unless sacrificial electron acceptors or donors are added. Due to the good catalytic activity, robust stability in aqueous solution, abundance storage and low cost, perovskites have been applied in many fields such as catalysts [23], gas-sensor [24], etc. Recently, LaFeO3 perovskite with a suitable band gap of ~2.10 eV, was investigated as a promising candidate for visible light water splitting photocatalyst. Parida et al. reported LaFeO3 could generate hydrogen and oxygen under visible light with the apparent quantum efficiency (AQE) as high as 8.07% [25]. Moreover, the composites of perovskite and layered p-conjugated carbon materials were proposed to be applied in photocatalytic hydrogen production and degradation of organic compounds owing to the improved charge transfer [26]. Yet, the photocatalytic activity remains low mainly due to its sluggish oxygen evolution kinetic on the surface and the poor charge transport in the bulk [25,27]. To promote the oxygen evolution kinetics, Tijare et al. loaded Pt on the LaFeO3 and found that the rate of H2 generation was boosted to 3315 umol$g1$h1 [27]. Besides the photocatalytic water splitting, Sora et al. investigated the photoelectrochemical (PEC) properties of LaFeO3 photoanode under simulated solar irradiation and found that Sr and Cu co-doping could improve the photocurrent densities by enhancing the light absorption [28,29]. Transition metal (TM) doping was demonstrated to be an effective way to improve the PEC properties of TiO2 and other photocatalysts [30]. However, to the best of our knowledge, the investigation of TM doping effect on LaFeO3 is still open. Clarifying this point would be helpful to further improve the photocatalytic performance of LaFeO3 for visible light solar utilization. In this work, transition metals (Mn, Co, Cu) are introduced into LaFeO3 and the PEC performance has been investigated. The improvement of the PEC performance under visible light illumination is achieved by substitution of the Fe3þ with TM ions. We find that the highest photocurrent density of 0.99 mA/cm2 is obtained at 1 V vs. Ag/AgCl for 10% Cu doped sample, which is about three times higher than that of LaFeO3. By cyclic voltammetry, the discharge of Fe3þ has been observed in TM doped LaFeO3. The reduction of charge transport resistance and increase of carrier densities are identified to benefit the PEC performance improvement. This work is informative to understand the role of TM dopants on the improved PEC performance of LaFeO3, and will provide guidance to design the LaFeO3-base PEC cells for water splitting under visible light.

Experimental Preparation of pure and TM doped LaFeO3 Various methods have been used to synthesize pure as well as TM doped LaFeO3 such as reactive grinding [31,32], a glycine nitrate process [33], and a citrate auto-combustion method [34]. A solegel method was chosen because it is easy to control and makes product with good crystallinity [35]. All chemicals used were the analytical-grade reagents without any further purification. La(NO3)3$6H2O and Fe(NO3)3$9H2O and the dopants (Mn(NO3)2 (50wt% aq), Co(NO3)2$6H2O and Cu(NO3)2$3H2O) were dissolved in deionized water (18.20 MU) with stoichiometric ratios and kept stirring until the salts were dissolved completely. Then citric acid was added and the resulting solution was concentrated to obtain the sol. After that, the sol was thermally treated overnight to obtain the dry ground gel, which was calcined at 500 C for 2 h to obtain the crystalline nanoparticles. More details could be found in the supplementary information (SI).

Characterizations of materials The X-ray diffraction (XRD) patterns were recorded on an analytical X'pert PRO diffractometer operating at 40 kV and 40 mA, using Cu Ka radiation with l ¼ 1.5406 A in the 2q ranging from 10 to 80 . Transmission electron microscopy (TEM) images were obtained by using a JEM 2100 microscope operated at 200 kV. X-ray Photoelectron Spectroscopy (XPS) studies were characterized on VG Multilab 2000 X-ray Photoelectron Spectroscope with an Al Ka radiation. The starting angle of the photoelectron was set at 90 . UVeVis absorbance spectra were characterized on LabRAM HR800 UVeVis spectrophotometer with scanning wavelength ranging from 200 nm to 900 nm using BaSO4 as the reference.

Photoelectrochemical performance measurement The photocurrent densities were evaluated by measuring the current density with and without 100 mW/cm2 visible light illumination from 500 W Xenon lamps (CHF-XM500, Trusttech) with UV filters (>420 nm) via linear voltammetry sweeping. The electrochemical signal was recorded by Autolab Electrochemical Workstation in a standard three electrode electrochemical system. The slurry electrodes acted as the working electrode, Pt as the counter electrode, and the saturated Ag/AgCl as the reference electrode. A 0.1 M KOH solution was used as the electrolyte. All the potential values mentioned below were against Ag/AgCl. The photoconversion efficiency is calculated according to Eq. (1), h% ¼ E0rev Eapp J 100%=Iin E0rev

(1)

where equals 1.23 V representing the Gibbs free energy change per photon in the water splitting reaction. Eapp ¼ Emean Eaoc , where Emean is the potential of the working electrode and Eaoc is the potential of the same working electrode at open-circuit condition with the same reaction system.


J is the photocurrent density and Iin is the intensity of incident light.

Results and discussion Structural characterization of LaFeO3 and LaFe1xTMxO3 (TM ¼ Mn, Co and Cu) Pure and TM (Mn, Co and Cu) doped LaFeO3 with 10% concentration are prepared and the XRD patterns of the powders are conducted to examine the crystal structures in Fig. 1. The crystal structures are nearly independent on the TM dopants, since all peaks are identical to that of pure LaFeO3. The main diffraction peaks in the XRD patterns at 2q angles of 22.65 , 32.16 , 39.80 , 46.31 , 57.55 , 67.43 and 76.67 can be indexed as the (101), (002), (022), (202), (232), (410) and (204) crystal planes, indicating that the fabricated LaFeO3 is well crystalized with an orthorhombic structure (JCPDS No. 88e0641 (a ¼ 5.556, b ¼ 5.565 and c ¼ 7.862)). No impurity peak appears after TM doping, indicating all doped samples retain the perovskite crystal structure. As shown in Fig. 2A, the fabricated LaFeO3 appears to be yellow, and the TM doped oxide films are brown black for Mn and Co and dark black for Cu. The color change after doping implies that the incorporation of the heterogeneous TM atoms may induce effective light adsorption. Polygonal shaped regular nanoparticles are observed from TEM (Fig. 2B) and the particle size is around 20e50 nm. HRTEMs of pure and TM doped LaFeO3 are characterized in Fig. 2CeF. Lattices with periodic fringes spacing of 2.78 A (Fig. 2C) correspond to the (002) planes in LaFeO3 (JCPDS No. 88e0641). Very little change is observed in the microstructures after doping as seen from the similar interplanar spacing. Fig. 3A shows the high resolution XPS of Fe ion in pure and doped LaFeO3 and the two peaks at 710.22 eV and 723.79 eV are identified as Fe3þ 2p3/2 and 2p1/2 [36,37]. Slight shifting of the Fe3þ peak is observed for the TM doped LaFeO3 and attributed to the affected FeeO bond by introducing the TM dopant. The

Fig. 1 e X-ray diffraction spectra of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3.

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high resolution XPS spectra of Mn, Co and Cu 2p are shown in Fig. 3BeD, respectively. The binding energy of Mn 2p XPS spectrum with Mn 2p1/2 peak at 653.36 eV and 2p3/2 at 641.84 eV in Fig. 3B, are consistent with that of Mn3þ/Mn4þ [38,39]. As shown in Fig. 3C, the two peaks are identified as Co 2p1/2 peak at 795.91 eV and 2p3/2 at 780.96 eV, which indicates the existence of Co as Co2þ/Co3þ state [40,41]. In Fig. 3D, the XPS spectrum of Cu is identified as 2p1/2 peak at 953.60 eV and 2p3/2 at 933.84 eV. Together with the satellite peaks at 961.39 eV and 942.30 eV, it confirms the existence of Cu2þ state [42,43]. The XPS spectra of the TM doped samples clearly indicate that Mn, Co and Cu have been successfully incorporated into LaFeO3 without forming other phases.

Optical and photocatalytic properties of LaFeO3 and LaFe1xTMxO3 (TM ¼ Mn, Co and Cu) The UVeVis absorbance spectra of pure and TM doped LaFeO3 have been conducted to characterize the light absorbance in the visible region (See Fig. S1). These samples demonstrate strong light absorption in UVeVis spectra and the adsorption wavelength extends to ~550 nm. The optical band gaps have been calculated by KubelkaeMunk equation [45] and shown in the inset of Fig. S1. The deduced optical band gap is 2.08 eV for LaFeO3, which agrees well with previous results [26,27]. We note the gaps of TM doped LaFeO3 nearly keep constant at ~2.08 eV. To evaluate the doping effect on the PEC performance, the photocurrent densities of pure and TM doped LaFeO3 acted as photoanodes are measured. It is observed that the onset photopotential of LaFeO3 (0.48 eV) exhibits obviously cathodic shift after TM doping with 0.27 V for LaFe0.9Cu0.1O3 and LaFe0.9Co0.1O3, and 0.34 V for LaFe0.9Mn0.1O3 as shown in Fig. 4A. The photocurrent density increases with the bias potential and reaches to 0.99 mA/cm2, 0.85 mA/cm2 and 0.52 mA/ cm2 for Cu, Co, Mn doped samples at 1 V, respectively. The Cu doped LaFeO3 yields the maximum photocurrent density among all the doped samples. The chronoamperometric results at 0.5 V of these samples with 30 s cyclic visible light illumination are shown in Fig. 4B. The current densities are observed to be photoactive and exhibit sensitive response with light on and off, indicating the rapid photocurrent generation. The photocurrent density of LaFeO3 is obtained to be 0.01 mA/cm2 and they are promoted to be 0.06 mA/cm2 for Mn doping, and 0.21 mA/cm2 for Co and Cu doping, which are in good agreement with the linear sweep voltammetry results. Upon the continuous electrolysis with light on and off, no significant decrease of the photocurrent density is observed owing to the preferable stability of the fabricated perovskite samples in the aqueous solution. We notice that the 10% TM doping treatment could improve the photocurrent of LaFeO3. However, excessive dopants may act as recombination centers, which would inhibit electrons and holes transfer to the surface. Fig. 4C shows the photocurrent density of TM doped LaFeO3 with different concentrations of dopants. It is observed in all TM doped samples that the 10% dopants yield the highest photocurrent density comparing with 5% and 20% TM doped ones. Photoconversion efficiency of light to chemical energy is calculated according to Eq. (1). It is found that the

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Fig. 2 e A, Images of the pure and TM doped LaFeO3 photoanodes, B, TEM images of LaFeO3 nanoparticles, C, D, E and F are the HRTEM images of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3, respectively.

photoconversion efficiency of LaFeO3 reaches to 0.22% at 1.10 V as shown in Fig. 4D. LaFe0.9Cu0.1O3 yields the highest efficiency of 0.51% at 0.98 V, while the other two doped samples also get improved photoconversion efficiency of 0.31% for LaFe0.9Mn0.1O3 at 1.11 V and 0.37% for LaFe0.9Co0.1O3 at 0.92 V. The sequence of the highest photoconversion efficiency of these samples is noted to be in good agreement with their order of photocurrent densities in Fig. 4A.

PEC mechanism discussion of LaFeO3 and LaFe1xTMxO3 (TM ¼ Mn, Co and Cu) To study the PEC performance of the samples under visible light, cyclic voltammetry (CV) at low scan rate of 5 mV s1 is shown in Fig. 5. Neither oxidation nor coupled reduction peak is observed in the negative potential region for pure LaFeO3. In contrast, obvious peaks emerge at 0.17 V and 0.55 V for the TM doped samples, which could be attributed to Fe2þ/Fe3þ redox peaks [46]. This is reasonable since Fe2þ or Fe4þ is introduced as a result of the charge compensation when Mn, Co and Cu are doped. In the positive potential region, anodic

peak at 0.47 V and cathodic peak of 0.32 V are observed after TM doping, which may correspond to the process of Fe3þ/Fe4þ discharging [47e49] and Fe4þ/Fe3þ charging [47e51]. The appeared peaks of Cu and Mn doped LaFeO3 are all attributed to the Fe3þ discharging since the Cu2þ cannot be oxidized further in the electrochemical process and the Mn3þ/Mn4þ discharging could not happen in this potential region [52]. It is possible that the anodic peak of the Co2þ/Co3þ discharging appears in this range (0.3e0.5 V) [52,53], but the low doping concentration makes it to be negligible compared with that of Fe3þ. Thus the main difference of CV curves between the pure and TM doped LaFeO3 indicates the Fe ion discharging is responsible for the increased current after doping. To further identify the current difference between the doped samples, the CV measurements are conducted repeatedly from 0 V to 0.6 V to eliminate the hysteresis effect. The results are shown in Fig. 5B and the intensities of the peaks (0.47 V/0.32 V) are in the order of LaFe0.9Cu0.1O3 > LaFe0.9Co0.1O3 > LaFe0.9Mn0.1O3, which is consistent with that of the PEC. Since no obvious change of Fe chemical states is observed in the TM doped LaFeO3 in Fig. 3A, we attribute the distinct order of Fe


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Fig. 3 e XPS spectra of A, Fe2p of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3, B, Mn2p of LaFe0.9Mn0.1O3, C, Co2p of LaFe0.9Co0.1O3, D, Cu2p of LaFe0.9Cu0.1O3.

discharge to the changes of the oxygen environment in these doped samples. It could be inferred that the oxygen vacancy is more likely to be formed, especially for Co and Cu doped samples owing to the charge compensation. To clarify this point, the O1s XPS in both pure and TM doped LaFeO3 are presented in SI (Fig. S2A). The peak at 528.9 eV is indexed as the lattice oxygen (Olatt) which is connected with metal atoms [42,44]. The position of this peak is found to shift slightly due to the introducing of the heteroatoms in the LaFeO3 lattice. The other peak at 531.0 eV is associated with oxygen on the surface (Oads) [44]. The area ratios of Oads: Otot (sum of Olatt and Oads) are shown in Fig. S2B and it is clear that the Oads for Co and Cu doped samples increase a lot, which would benefit the photoelectrochemical reaction on the surface. To further understand the role of Fe ion discharge in the PEC process, the whole oxygen evolution reaction (OER) based on the LaFeO3 photocatalysts is discussed. For pure LaFeO3, Fe(III) has been directly involved in the OER by chemisorbing the hydroxyl and transferring it to O2 (See SI) according to previous studies [54e56]. However, it is different after TM atoms doping due to the involvement of Fe3þ discharging. Similar to the case of TM doped NiCo2O4 and LaCoO3 [55,56], the actual OER of TM doped LaFeO3 samples should follow five steps as described in Eqs. (2)e(6). Compared to the OER of pure LaFeO3, the observation of Fe3þ/Fe4þ peak in CV testing

indicates the formation of Fe4þ and the hydroxyl is expected to preferably be chemisorbed on Fe4þ due to the stronger charge interaction (Eqs. (2),(4)). In the fourth step, the formation of Feþþ(V)eO (Eq. (5)) would enhance the charge transfer and transport in the electrochemical process [54e56], which benefits the photocurrent densities. The full process is illustrated in Fig. S3 and the shadow part highlights the steps of the acceleration of charge transfer and transport in TM doped LaFeO3 samples.

Fe(III) / Feþ(IV) þ e

(2)

Feþ(IV) þ OH / Feþ(IV)eOH þ e

(3)

Feþ(IV)eOH þ OH / Feþ(IV)eO þ H2O

(4)

Feþ(IV)eO / Feþþ(V)eO þ 2e

(5)

2Feþþ(V)eO / 2Fe(III) þ O2

(6)

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Fig. 4 e A, Photocurrent density vs. voltage and B, transient photocurrent response of pure and 10% Mn-, Co-, Cu-doping LaFeO3 samples, C, Photocurrent density with different doping concentrations, D, Photoconversion efficiency of these samples.

Electrochemical impedance spectroscopy (EIS) measurement is conducted with frequency ranging from 0.01 Hz to 100 KHz to verify the accelerated charge transport in the TM doped LaFeO3. The Nyquist plots of these samples in the dark condition are shown in Fig. 6A. The equivalent circuit of EIS Nyquist plots could be described by the model of a thin layer semiconductor [57] shown in Fig. 6B. In this model, Rs is the series resistance, including the FTO resistance, the resistance

related to the ionic conductivity in the electrolyte and the external contact resistance (e.g. wire connections), Rct is the charge transport resistance occurring in the semiconductor bulk, Cct is the capacitance of the space charge region, Rce is the semiconductor/electrolyte charge transfer resistance and Cce is the Helmholtz capacitance. The fitting data of the parameters in the doped samples is listed in Table 1. Rct and Rce both decrease after TM doping and they follow the order of

Fig. 5 e Cyclic voltammetry measurement of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3 in 1 M KOH, A, ranged from ¡0.8 V to 0.6 V, B, ranged from 0 V to 0.6 V.


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Fig. 7 e MotteSchottky plots of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3 measured in 1 M KOH solution at the given bias potential with the frequency of 1 KHz under the dark condition.

MotteSchottky equation is shown in Eq. (7), where c is the space charge capacitance in the semiconductor, E is the applied potential, EFB is the flat band potential, T is the temperature, k is the Boltzmann constant, e is the elemental charge, Nd is the charge carrier density, ε0 and ε are the vacuum permittivity and the relative permittivity respectively. Thus the charge carrier densities (Nd) of the pure and TM doped LaFeO3 can be calculated by the Eq. (8).

Fig. 6 e A, EIS Nyquist plots of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3, and B, corresponding equivalent circuit of these samples. LaFe0.9Cu0.1O3 < LaFe0.9Co0.1O3 < LaFe0.9Mn0.1O3 < LaFeO3. This order is in good agreement with that of Fe3þ discharge capacity as well as the PEC performance. Thus the PEC improvement of TM doped LaFeO3 could be primarily attributed to the accelerated charge transfer and transport caused by the Fe3þ discharge. On the other hand, the TM doping changes the carrier density of LaFeO3, which also affects the PEC performance. The MotteSchottky plots of pure and TM doped LaFeO3 are displayed in Fig. 7 and all the samples exhibit positive slopes, which implies the n-type semiconductors phase. The

1 c2 ¼ ðE EFB kT=eÞ=Nd eε0 ε

(7)

eε0 ε Nd ¼ 2 dE=d 1 c2

(8)

Taking the relative permittivity ε of LaFeO3 as 220 [58], the slopes of the curves are fitted to be to be 1.14 1017, 1.95 1017, 2.27 1017, 2.80 1017 cm3 for LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3, respectively. It is clear that the carrier density increases after TM doping and the increased degree follows the order of LaFe0.9Cu0.1O3 > LaFe0.9Co0.1O3 > LaFe0.9Mn0.1O3 > LaFeO3. This indicates that the increased carrier density by TM doping also contributes to the enhanced PEC performance of LaFeO3 under visible light.

Conclusion Table 1 e Equivalent circuit-fitting results of LaFeO3, LaFe0.9Mn0.1O3, LaFe0.9Co0.1O3 and LaFe0.9Cu0.1O3.

LaFeO3 MnCoCu-

Rs (MU$cm2)

Rct (MU$cm2)

Cct (F)

Rce (MU$cm2)

Cce (F)

50.21 50.12 50.14 50.08

386.54 243.24 198.21 171.56

1.71 1.80 1.75 1.86

121.36 118.21 109.53 103.21

1.56 1.62 1.58 1.87

The pure and TM (Mn, Co and Cu) doped LaFeO3 have been synthesized. It is found the TM doping can effectively enhance the PEC performance of LaFeO3 under visible light. The photoconversion efficiency promoted by TM doping follows the order of LaFe0.9Cu0.1O3 > LaFe0.9Co0.1O3 > LaFe0.9Mn0.1O3 > LaFeO3. The 10% Cu doped LaFeO3 demonstrates the highest photocurrent density about three times higher than that of pure LaFeO3. The charge transport resistance is observed to be reduced by the electrochemical impedance spectroscopy

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measurements. Furthermore, the MotteSchottky plots imply that the TM doping induces the increase of carrier densities, which also contributes to the PEC performance enhancement. This work depicts the charge behavior of TM atoms doped LaFeO3 and shall be instructive for the design of high efficient LaFeO3-based PEC cells under visible light.

Acknowledgment This work is supported by National Basic Research Program of China (2013CB934800), National Natural Science Foundation of China (51575217, 51572097 and 51302094), the Hubei Province Funds for Distinguished Young Scientists (2015CFA034 and 2014CFA018), Fundamental Research Funds for the Central Universities, HUST (2015QN009 and 2014TS037), the State Key Laboratory of Digital Manufacturing Equipment and Technology Funding (DMET2015A01). Rong Chen acknowledges the Thousand Young Talents Plan, the Recruitment Program of Global Experts and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13017) and National Engineering Research Center for Nanomedicine. The authors also would like to acknowledge equipment supports from AMETEK lab and the technology support from the Analytic Testing Center of HUST.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.09.072.

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