DOI: 10.1002/celc.201402192

Photoelectrochemical Properties of LaFeO3 NanoparticlesVer�nica Celorrio,[a] Kieren Bradley,[b] Oliver J. Weber,[a] Simon R. Hall,[a] andDavid J. Ferm�n*[a]

1. Introduction

Direct generation of H2 from water, powered by solar energyemploying earth-abundant materials, represents a formidablechallenge in materials science. Research in this field is dominat-ed by highly stable photoanodes, such as TiO2, WO3 andFe2O3.[1] By contrast, studies on metal oxides as photocathodesfor hydrogen generation are significantly scarcer. Photogenera-tion of hydrogen has been mostly investigated on p-type sili-con,[2] often loaded with co-catalysts, such as Pt, Ni, or Ni-Mo,to improve the reaction quantum yield.[3] Another promisingmaterial, which can be used as a photocathode in photoelec-trochemical cells (PEC), is cuprous oxide (Cu2O).[4] The main ob-stacle limiting its application in photocathodes is its suscepti-bility to photocorrosion. Research studies have also consideredthe formation of TiO2

[4a, 5] or CuO[4b] protective overlayers, andRuO2 as hydrogen evolving co-catalyst, to improve both thestability and activity.[5] Other important works in the area in-volved p-type GaAs and GaP, elucidating interesting aspects onthe role of hydrogen in recombination losses.[6]

Ferrite materials are attractive candidates for the photoelec-trolysis of water because of their good chemical stability andinertness. There are a number of studies highlighting the pho-tocatalytic properties of ferrites, such as LaFeO3,[7] YFeO3,[8]

BiFeO3,[9] and CuFeO2.[10] Works by Parida et al.[7b] and Tijareet al.[11] have demonstrated the photoactivity of LaFeO3 as sus-pended particles in aqueous solution in the presence of sacrifi-cial electron donors, such as methanol or ethanol and otherco-catalysts like Pt or Ag. However, no systematic work assess-

ing the properties of this material as a photoelectrode hasbeen published so far. Currently, key aspects, such as bandedge energies, kinetics of photoinduced charge transfer, carrierrecombination and collection efficiency, are largely unknown.

The present work elucidates key properties of LaFeO3 nano-structures deposited onto F:SnO2 (fluorine-doped tin oxide,FTO) electrodes, highlighting challenges for their exploitationin direct hydrogen photogeneration. Highly phase-pure LaFeO3

particles with an average diameter of (60�14) nm were syn-thesized by a novel method based in ionic liquids featuringmetallophilic and hydrophilic hard anions to generate dehy-drated precursors of the metal cations. Our studies reveal clearevidences of direct hydrogen photogeneration at pH 12 at po-tentials up to 1 V more positive than the formal hydrogen po-tential and wavelength shorter than 500 nm. Furthermore, wealso show that holes at the valence band are capable of gener-ating oxygen at relatively low overpotentials. Consequently,hole collection at the back contact is in competition withwater oxidation, limiting the apparent photocurrent efficiencyfor H2 generation.

2. Results and Discussion

LaFeO3 was synthesized by the ionic-liquid phase-transfermethod,[12] which involves the dissolution of metal nitrates inthe appropriate stoichiometry to an overall aqueous concen-tration of 0.1 m, followed by addition of 1 mL of 1-ethyl-3-methylimiamidizolium acetate. After stirring the solution for3 h at 80 8C, 100 mg microcrystalline cellulose is added to thedehydrated precursor. The homogeneous gel is calcined for2 h to promote nucleation of the metal oxide phase. Approxi-mately 4 mm-thick films were screen-printed onto the FTO elec-trodes and sintered at 500 8C.

The powder X-ray diffraction (XRD) pattern of LaFeO3 (Fig-ure 1 A) displayed peaks that are consistent with a cubic perov-skite, with a certain degree of broadening resulting from thenanocrystalline nature of the material. The very close matchbetween the corresponding JCPDS-ICDD (International Center

The photoelectrochemical properties of LaFeO3 nanoparticlesare discussed for the first time. Highly phase-pure LaFeO3 parti-cles prepared by a novel ionic-liquid-based method are charac-terized by a (60�14) nm mean size and a band gap of (2.56�0.07) eV. Thin films deposited by screen printing exhibit photo-current responses associated with hydrogen generation at po-

tential up to 1 V more positive than the formal hydrogen po-tential. Analysis of the photocurrent responses as a function ofphoton flux and potential bias suggest that the hole-collectionefficiency at the back contact is in competition with interfacialwater oxidation.

[a] Dr. V. Celorrio, O. J. Weber, Dr. S. R. Hall, Dr. D. J. Ferm�nSchool of ChemistryUniversity of BristolCantocks Close, Bristol BS8 1TS (UK)E-mail : David.Fermin@bristol.ac.uk

[b] K. BradleyBristol Centre for Functional NanomaterialsUniversity of BristolNanoscience and Quantum Information BuildingTyndall Avenue, Bristol, BS8 1FD (UK)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/celc.201402192.

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for Diffraction Data, file No. 01-075-0541) pattern and the ex-perimental data demonstrates a high degree of phase purity.Rather weak diffraction peaks observed at 25, 39, 47 and 548reveal the presence of secondary phases, most probably asso-ciated with La oxides. The absorption spectrum of LaFeO3

powders were measured by diffuse reflectance spectroscopy(DRS, see the Supporting Information, SI, Figure S1). The Taucplot in Figure 1 B was obtained by analyzing the DRS spectrumusing the Kubelka–Munk function. The linear regression fit il-lustrated in Figure 1 B suggests a direct band gap transition of(2.56�0.07) eV, in agreement with recent studies of LaFeO3 ep-itaxial films grown on SrTiO3.[13] The transmission electron mi-croscopy (TEM) image in Figure 1 C reveals the distribution ofparticle sizes obtained after calcination of the gel precursor.Statistical analysis of over a hundred TEM features yielda mean particle size of (60�14) nm. The high-resolution TEM(HR-TEM) image in Figure 1 D confirms the excellent degree ofcrystallinity, featuring lattice fringes with a d-spacing of0.27 nm, which is consistent with the 110 plane.

Figure 2 A illustrates the (photo)electrochemical responses ofa 4 mm LaFeO3 film screen-printed onto FTO electrodes ina 0.1 m Na2SO4 solution at pH 12. Cyclic voltammograms re-corded at 5 mV s�1 in the dark (red line) and under square-wave light perturbation (black line) at 404 nm are contrastedin Figure 2 A. The film was illuminated from the back (FTO/LaFeO3 boundary) with a photon flux of 1.4 � 1016 cm�2 s�1.Clear cathodic photocurrents are observed from about 1.10 V(vs. the reversible hydrogen electrode, RHE), which increases asthe potential is swept towards more negative values. Experi-ments were carried out under Ar-saturated solutions, indicat-ing that the photocurrent responses are associated with the

generation of hydrogen fromwater. As the electrode potentialis swept beyond 1.10 V (vs. RHE),the dark current significantly in-creases suggesting the transferof holes to the electrolyte solu-tion. This behavior not only re-veals that the potential associat-ed with the valence band edgeof LaFeO3 overlaps with the po-tential for oxygen evolution, butalso the overpotential for the re-action is relatively low. As dis-cussed further below, this hasimportant implications on thequantum efficiency for hydrogenevolution.

Figure 2 B shows photocur-rent–voltage measurementsunder square-wave perturbationof light (2 Hz, 1.4 � 1016 cm�2 s�1

peak-to-peak) and lock-in detec-tion. The real component of thephotocurrent becomes morenegative with decreasing poten-tial bias, consistent with an in-

crease in the cathodic photocurrent magnitude. The imaginarycomponent is rather small throughout large portions of thepotential range, indicating that the response is practically in-phase in the sub-second timescale. An interesting feature isthe change in photoresponse sign at potentials above 1.20 V.This trend suggests that at sufficiently positive bias, hole trans-fer to the electrolyte solution and electron collection at theback contact can be promoted. However, the magnitude ofthe anodic photoresponse is significantly smaller than thecathodic one.

Figure 2 C contrasts the responses of LaFeO3 electrodes inAr- and O2-saturated 0.1 m Na2SO4 solutions at pH 12. Experi-ments were carried out employing a 2.5 mV s�1 linear-sweeppotential superimposed to a 0.05 Hz squared-wave light per-turbation (404 nm). Cathodic photocurrents are readily ob-served in Ar-saturated solutions from potentials as positive as1.1 V. However, no clear photoresponses are observed underidentical conditions in O2-saturated solutions. The current re-sponses in the presence and absence of O2 converge at poten-tials above 1.2 V, confirming that the oxygen evolution reac-tion dominates in this potential range. Considering the quasi-steady-state nature of the current–potential relationship in Fig-ure 2 C (slow potential scan rate), it is clear that this responseis essentially faradaic, as opposed to the capacitive responsecommonly linked to filling trap states.[14] Towards potentialvalues below 0.8 V, an increase in the dark cathodic current isobserved in the case of O2-saturated solutions. This response islinked to the oxygen reduction reaction involving valence-band electrons. The kinetic and mechanistic aspects of thisprocess are currently being investigated. However, the ensem-ble of the data demonstrates that the density of states of the

Figure 1. A) Powder X-ray diffraction pattern of the LaFeO3 nanoparticles. The close match with the standard pat-tern JCPDS-ICDD File No. 01-075-0541 (red bars) indicates a very high degree of phase purity. B) Tauc plot forLaFeO3 obtained from diffuse reflectance spectroscopy (DRS), showing an optical band gap of 2.56�0.07 eV.C) TEM image of an ensemble of particles showing particles sizes in the range of 50 to 70 nm. D) HRTEM imagehighlighting the lattice fringes associated with the 110 plane of LaFeO3.

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valence band overlap with the redox energy levels associatedwith oxygen reduction. This is an important observation as itprecludes oxygen reduction reaction as the process involved inthe photocurrent responses recorded in Ar-saturated solutions.

Figure 3 A displays a set of photocurrent transients at differ-ent electrode potentials in 0.1 m Na2SO4 solution at pH 12. This

experiment allows examining the dynamic photoresponse inthe second timescale. The photocurrent transients exhibita fast response upon illumination, followed by a relaxation toa quasi-steady-state value. An anodic photocurrent spike isalso observed upon interruption of illumination. As the elec-trode potential becomes more negative, the magnitude of thephotocurrent responses increases, in agreement with the datashown in Figure 2 B. From a phenomenological point of view,the photocurrent decay and overshoot upon light interruptionresemble the behavior observed for single-crystal photoelectr-odes in the presence of interfacial recombination.[6a, 15] Howev-er, a closer examination of the photocurrent transient revealsthat the magnitude of the photocurrent decay in the on-transi-ent is significantly larger than photoanodic responses in theoff-transient.

Figure 3 B contrasts the initial (F0, top panel) and photosta-tionary (Fss, bottom panel) external quantum yield as a func-tion of the photon flux (I0) at 0.30 V. In this figure, the openand filled symbols correspond to front and back illumination,respectively. While F0 shows a slight increase with increasing I0

under both conditions, Fss exhibits a contrasting behavior be-tween front and back illumination. A key observation is thatthe photoresponses obtained upon back illumination are

Figure 2. A) Cyclic voltammogram (recorded at 5 mV s�1) of LaFeO3 photo-electrodes in the dark (red) and under square-wave light perturbation(black) with a frequency of 0.05 Hz. B) Real and imaginary components ofthe photocurrent responses as a function of the potential biased measuredunder square-wave photon flux perturbation (2 Hz) and lock-in detection.C) Linear potential scans recorded at 2.5 mV s�1 of LaFeO3 in Ar (red line) orO2 (black line) saturated solutions under square-wave light perturbation(0.05 Hz). Measurements were taken with a photon flux of 1.4 � 1016 cm�2 s�1

and a 404 nm wavelength, in a 0.1 m Na2SO4 solution at pH 12.

Figure 3. A) Transient photocurrent responses at different electrode poten-tials and a photon flux of 1.1 � 1016 cm�2 s�1. B) External quantum yield re-corded upon illumination (F0) and at photostationary conditions (Fss) asfunction of the photon flux (I0). Responses were recorded with front (opensymbols) and back (filled symbols) illumination. Measurements were taken inan Ar-saturated 0.1 m Na2SO4 solution at pH 12.

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larger than those obtained in the case of front illumination(see the Supporting Information, Figure S2), indicating that theprobability of hole collection increases the closer the carriergeneration occurs to the back contact.

In view of the nanoscopic crystalline domains, carrier trans-port across the film is mainly determined by diffusion. Conse-quently, a number of parameters affect the photocurrent quan-tum yield for hydrogen evolution, including charge-carrier life-times, effective diffusion coefficient and quasi-Fermi levels. Asthe bias potential is decreased, the gradient of the quasi-Fermilevel of holes (he*f) is such that hole diffusion is promoted to-wards the back contact. Augmenting photon flux under frontillumination increases the he*f gradient towards the back con-tact, leading to an increase in Fss. Under back illumination, themaximum of he*f will be located closer to the back contact,promoting hole diffusion away from the FTO/LaFeO3 boundary.Similar considerations have been postulated for carrier genera-tion and collection at dye-sensitized solar cells.[16] However, thefact that electrons and holes are involved in the photocurrentgeneration introduced further complexity as recently discussedby Bisquert and co-workers.[17] It should also be consideredthat the hole lifetime is significantly decreased by waterphoto-oxidation at the LaFeO3/electrolyte boundary.

The photocurrent spectrum of the LaFeO3 film at 0.30 V (vs.RHE) is displayed in Figure 4. The photocurrent onset is located

at 500 nm, increasing to a maximum value of 0.65 % at375 nm. It should be mentioned that quantum yields for hy-drogen generation at pH 12 are commonly rather low, as re-ported for GaP or SiC single-crystal electrodes.[18] Ida et al. re-ported values of 6 % for a high-quality CaFe2O4 film in 0.1 m

NaOH electrolyte at �0.8 V (vs. Ag/AgCl).[19] Given the complextransport properties associated with the nanostructured natureof the films, it is expected that the quantum yield would bestrongly dependent on the film thickness. We have not ex-plored this issue thoroughly yet.

3. Conclusions

This report unveils key physical parameters underpinning theproperties of nanostructured LaFeO3 electrodes for water split-ting. The novel synthetic approach highlighted herein allowsthe preparation of particles with a high degree of phase purityand a mean diameter of (60�14) nm. Photoelectrochemicalstudies carried out at pH 12 indicate that the valence-bandedge potential overlaps with the reversible potential foroxygen evolution and significant currents can be observedwith a relatively low overpotential. The external quantum yieldexhibits a complex dependence on photon flux, electrode po-tential, and orientation of illumination (front versus back illumi-nation), revealing a rather restricted mobility of the photogen-erated carriers, in particular the holes. The fact that LaFeO3 ex-hibits a low overpotential for oxygen evolution suggests thatthis reaction pathway is in competition with hole collection atthe back contact, limiting the photocurrent conversion efficien-cy for hydrogen generation.

Experimental Section

Experimental Details: 500 mL of 0.1 m La(NO3)3 (aq) and 500 mL of0.1 m Fe(NO3)3 (aq), were added to a vial containing 1 mL 1-ethyl-3-methylimiamidizolium acetate (97 %, Sigma). After dehydration at80 8C for 3 h and addition of 100 mg of cellulose, the gel was cal-cined at 900 8C for 2 h to generate LaFeO3.

X-ray diffraction patterns were recorded using a Bruker AXS D8 Ad-vance diffractometer with a q–q configuration and using CuKa ra-diation (l= 0.154 nm). HR-TEM measurements were performed ona JEOL JEM 2010 instrument. UV/Vis diffuse reflectance spectrawere recorded using a Shimadzu UV-2600 UV/Vis spectrophoto-meter.

The photoelectrodes were prepared by screen-printing on FTO-coated glass.[20] Ethyl cellulose and terpineol additives were usedfor obtaining uniform, porous and adhesive films. The final compo-sition of the paste was 16 % wt LaFeO3, 9 % wt ethyl cellulose, and73 % wt terpineol. The films were heat-treated at 500 8C for 15 minto remove the additives.

Photoelectrochemical measurements were carried out using anIvium CompactStat equipment. The studies were performed inaqueous 0.1 m Na2SO4 solutions at pH 12, purged with argon. A Ptcounter electrode and an Ag/AgCl (KCl saturated) reference elec-trode were used. To facilitate the discussion, all potentials hereinare quoted against the RHE. Measurements under illuminationwere performed using a 404 nm purple LED (Thorlabs) driven bya waveform generator (Stanford Research Systems). Photocurrentspectra were obtained using a 150 W Arc Xe lamp fitted toa single-pass monochromator. Photon fluxes were calibrated usinga standard silicon photodiode (Newport Corporation).

Acknowledgements

V.C. gratefully acknowledges the UK National Academy for sup-port though the Newton International Fellows program.

Figure 4. External quantum yield (instantaneous response) as a function ofthe illumination wavelength for nanostructured LaFeO3 thin films at 0.30 Vin a 0.1 m Na2SO4 solution at pH 12.

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Keywords: hydrogen evolution · LaFeO3 · nanoparticles ·perovskite electrodes · photoelectrochemistry

[1] a) A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38; b) M. Woodhouse,B. A. Parkinson, Chem. Soc. Rev. 2009, 38, 197 – 210; c) A. Kudo, Y. Miseki,Chem. Soc. Rev. 2009, 38, 253 – 278; d) K. Sivula, F. Le Formal, M. Gr�tzel,ChemSusChem 2011, 4, 432 – 449.

[2] a) J. Oh, T. G. Deutsch, H.-C. Yuan, H. M. Branz, Energy Environ. Sci. 2011,4, 1690 – 1694; b) S. W. Boettcher, E. L. Warren, M. C. Putnam, E. A. San-tori, D. Turner-Evans, M. D. Kelzenberg, M. G. Walter, J. R. McKone, B. S.Brunschwig, H. A. Atwater, N. S. Lewis, J. Am. Chem. Soc. 2011, 133,1216 – 1219.

[3] J. R. McKone, E. L. Warren, M. J. Bierman, S. W. Boettcher, B. S. Brunsch-wig, N. S. Lewis, H. B. Gray, Energy Environ. Sci. 2011, 4, 3573 – 3583.

[4] a) A. Paracchino, N. Mathews, T. Hisatomi, M. Stefik, S. D. Tilley, M. Grat-zel, Energy Environ. Sci. 2012, 5, 8673 – 8681; b) Z. Zhang, P. Wang, J.Mater. Chem. 2012, 22, 2456 – 2464.

[5] S. D. Tilley, M. Schreier, J. Azevedo, M. Stefik, M. Graetzel, Adv. Funct.Mater. 2014, 24, 303 – 311.

[6] a) J. Li, R. Peat, L. M. Peter, J. Electroanal. Chem. 1984, 165, 41 – 59; b) B.Rastegar, J. F. Wager, Semicond. Sci. Technol. 1986, 1, 207 – 212.

[7] a) S. Li, L. Jing, W. Fu, L. Yang, B. Xin, H. Fu, Mater. Res. Bull. 2007, 42,203 – 212; b) K. M. Parida, K. H. Reddy, S. Martha, D. P. Das, N. Biswal, Int.J. Hydrogen Energy 2010, 35, 12161 – 12168.

[8] a) X. L�, J. Xie, H. Shu, J. Liu, C. Yin, J. Lin, Mater. Sci. Eng. B 2007, 138,289 – 292; b) M. A. Butler, D. S. Ginley, M. Eibschutz, J. Appl. Phys. 1977,48, 3070 – 3072.

[9] a) F. Gao, X. Y. Chen, K. B. Yin, S. Dong, Z. F. Ren, F. Yuan, T. Yu, Z. G. Zou,J. M. Liu, Adv. Mater. 2007, 19, 2889 – 2892; b) F.-t. Li, Y. Liu, R.-h. Liu, Z.-

m. Sun, D.-s. Zhao, C.-g. Kou, Mater. Lett. 2010, 64, 223 – 225; c) C. M.Cho, J. H. Noh, I. S. Cho, J. S. An, K. S. Hong, J. Y. Kim, J. Am. Ceram. Soc.2008, 91, 3753 – 3755.

[10] C. G. Read, Y. Park, K.-S. Choi, J. Phys. Chem. Lett. 2012, 3, 1872 – 1876.[11] S. N. Tijare, M. V. Joshi, P. S. Padole, P. A. Mangrulkar, S. S. Rayalu, N. K.

Labhsetwar, Int. J. Hydrogen Energy 2012, 37, 10451 – 10456.[12] D. C. Green, S. Glatzel, A. M. Collins, A. J. Patil, S. R. Hall, Adv. Mater.

2012, 24, 5767 – 5772.[13] M. D. Scafetta, Y. J. Xie, M. Torres, J. E. Spanier, S. J. May, Appl. Phys. Lett.

2013, 102, 0819041 – 0819045.[14] Q. Zhang, V. Celorrio, K. Bradley, F. Eisner, D. Cherns, W. Yan, D. J.

Ferm�n, J. Phys. Chem. C 2014, 118, 18207.[15] a) L. M. Peter, Chem. Rev. 1990, 90, 753 – 769; b) L. M. Abrantes, L. M.

Peter, J. Electroanal. Chem. 1983, 150, 593 – 601.[16] a) H. K. Dunn, L. M. Peter, S. J. Bingham, E. Maluta, A. B. Walker, J. Phys.

Chem. C 2012, 116, 22063 – 22072; b) H. Wang, L. M. Peter, J. Phys.Chem. C 2012, 116, 10468 – 10475; c) E. Guill�n, L. M. Peter, J. A. Anta, J.Phys. Chem. C 2011, 115, 22622 – 22632.

[17] L. Bertoluzzi, J. Bisquert, J. Phys. Chem. Lett. 2012, 3, 2517 – 2522.[18] J. O. M. Bockris, K. Uosaki, J. Electrochem. Soc. 1977, 124, 1348 – 1355.[19] S. Ida, K. Yamada, T. Matsunaga, H. Hagiwara, Y. Matsumoto, T. Ishihara,

J. Am. Chem. Soc. 2010, 132, 17343 – 17345.[20] S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Gr�tzel, M. K. Nazeeruddin,

M. Gr�tzel, Thin Solid Films 2008, 516, 4613 – 4619.

Received: June 16, 2014

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ARTICLES

V. Celorrio, K. Bradley, O. J. Weber,S. R. Hall, D. J. Ferm�n*

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Photoelectrochemical Properties ofLaFeO3 Nanoparticles

LaFeO3 nanostructures are synthesizedby a novel solution-processable routebased on ionic liquid/cellulose mixtures.Photoelectrochemical studies in Ar-satu-rated solutions at pH 12 provide clearevidence of hydrogen photogenerationat potentials up to 1 V more positivethan the formal hydrogen potential.

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