Bilel Chouchene

Thomas Gries

Lavinia Balan

Ghouti Medjahdi

Raphaël Schneider
email: raphael.schneider@univ-lorraine.fr

Graphitic carbon nitride/SmFeO 3 composite Z-scheme photocatalyst with high visible light activity

Keywords: graphitic carbon nitride, SmFeO 3, heterostructured photocatalyst, pollutant photodegradation, Z-scheme mechanism

In this work, novel heterostructured photocatalysts associating graphitic carbon nitride (g-CN) and SmFeO 3 were prepared via a mixing-ultrasonication process. Structural, optical and morphological characterizations demonstrate that the interfacial junction between g-CN and SmFeO3 is well established for all g-CN/SmFeO3 composites prepared with g-CN:SmFeO 3 weight ratio of 20:80, 50:50 and 80:20. The g-CN/SmFeO 3 (80:20) composite exhibits the highest photocatalytic activity for the degradation of pollutants like the Orange II dye and the tetracycline hydrochloride antibiotic under visible light irradiation. This high photocatalytic activity originates from the enhanced light absorption over the whole visible region compared to pure g-CN and from the improved separation and transfer of photogenerated electron/hole pairs as demonstrated by photoluminescence and photocurrent measurements. A Z-scheme charge carrier transfer mechanism was demonstrated for the photocatalytic reactions. The g-CN/SmFeO 3 (80:20) catalyst was also demonstrated to be stable and can be reused up to six times without significant alteration of the activity.

Introduction

Graphitic carbon nitride (g-CN) is a layered π-conjugated polymeric n-type semiconductor that has recently gained considerable attention for various photocatalytic applications like pollutant degradation, hydrogen production and CO 2 reduction [1-4]. g-CN is easily synthesized by the polymerization of nitrogen-rich organic precursors like melamine, urea or dicyandiamide, and exhibits good thermal and chemical stability and a low cytotoxicity.

Moreover, the bandgap energy of g-CN is of ca. 2.7 eV enabling photocatalytic reactions to be conducted in the visible range until ca. 460 nm [1][START_REF] Masih | Graphitic C 3 N 4 based noble-metal-free photocatalyst systems: A review[END_REF][START_REF] Mishra | Graphitic carbon nitride (g-C 3 N 4 )-based metal-free photocatalysts for water splitting: A review[END_REF][START_REF] Fu | g-C 3 N 4 -Based Heterostructured Photocatalysts[END_REF]. The conduction band edge potential of g-CN is located at -1.34 eV vs NHE, which confers to g-CN a marked photocatalytic reducing ability. However, the photocatalytic activity of g-CN is hampered by its low quantum efficiency which originates from the weak charge mobility and from the fast charge carriers recombination [1][START_REF] Masih | Graphitic C 3 N 4 based noble-metal-free photocatalyst systems: A review[END_REF][START_REF] Mishra | Graphitic carbon nitride (g-C 3 N 4 )-based metal-free photocatalysts for water splitting: A review[END_REF][START_REF] Fu | g-C 3 N 4 -Based Heterostructured Photocatalysts[END_REF].

In recent years, various strategies have been developed to overcome these challenges including the building of g-CN-based heterostructured photocatalysts associating g-CN and another semiconductor [START_REF] Zhang | Graphitic carbon nitride (g-C 3 N 4 )-based photocatalysts for water disinfection and microbial control: a review[END_REF][START_REF] Zhang | Recent developments in fabrication and structure regulation of visible-light-driven g-C 3 N 4 -based photocatalysts towards water purification: A critical review Catal[END_REF][START_REF] Darkwah | Photocatalytic Applications of Heterostructure Graphitic Carbon Nitride: Pollutant Degradation, Hydrogen Gas Production (water splitting), and CO2 Reduction[END_REF][START_REF] Mambo | Graphitic carbon nitride (g-C 3 N 4 ) nanocomposites: A new and exciting generation of visible light driven photocatalysts for environmental pollution remediation[END_REF][START_REF] Moussa | Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity[END_REF][START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF], the doping of g-CN with non-metal (P, B, S,…) [11][START_REF] Fu | Hierarchical porous O-doped g-C 3 N 4 with enhanced photocatalytic CO 2 reduction activity[END_REF][START_REF] Huang | An efficient metal-free phosphorus and oxygen co-doped g-C 3 N 4 photocatalyst with enhanced visible light photocatalytic activity for the degradation of fluoroquinolone antibiotics[END_REF] or metal (Mn 2+ , Cu 2+ , Co 3+ ,…) elements [START_REF] Li | Visible photocatalytic water splitting and photocatalytic two-electron oxygen formation over Cu-and Fe-doped g-C 3 N 4[END_REF][START_REF] Hu | In-situ Fe-doped g-C 3 N 4 heterogeneous catalyst via photocatalysis-Fenton reaction with enriched photocatalytic performance for removal of complex wastewater[END_REF][START_REF] Wang | Porous Mn doped g-C 3 N 4 photocatalysts for enhanced synergetic degradation under visible-light illumination[END_REF], the hybridization and sensitization of g-CN with macrocyclic heterocycles [START_REF] Ouedraogo | Copper octacarboxyphthalocyanine as sensitizer of graphitic carbon nitride for efficient dye degradation under visible light irradiation[END_REF] or the modification of g-CN structural and textural properties, for example by intercalation [START_REF] Yang | A surface modification resultant thermally oxidized porous g-C 3 N 4 with enhanced photocatalytic hydrogen production[END_REF][START_REF] Xu | Enhanced visible-light-driven photocatalytic disinfection performance and organic pollutant degradation activity of porous g-C 3 N 4 nanosheets[END_REF][START_REF] Chen | Three-dimensional porous g-C 3 N 4 for highly efficient photocatalytic overall water splitting[END_REF]. When considering the engineering of heterostructured photocatalysts, the use of perovskite type oxides and especially of ABO 3 materials (where A is a rare-earth element or alkaline-earth metal and B is a transition metal element) has received high interest due to their high stability, excellent physico-chemical properties and to their narrow bandgap that allows their activation by visible light [START_REF] Kanhere | review on visible light active perovskite-based photocatalysts[END_REF]. Recently, perovskites like LaMO 3 (M = Fe, Mn, Co, Ni or Ti) or MTiO 3 (M = Sr or Ca) were associated to g-CN to improve the charge carriers delocalization and these photocatalysts were successfully used for water splitting or pollutants degradation [START_REF] Wu | Quasi-polymeric construction of stable perovskite-type LaFeO 3 /g-C 3 N 4 heterostructured photocatalyst for improved Z-scheme photocatalytic activity via solid p-n heterojunction interfacial effect[END_REF][START_REF] Ibarra-Rodriguez L I | Facile synthesis of g-C 3 N 4 /LaMO 3 (M: Co, Mn, Fe) composites for enhanced visible-light-driven photocatalytic water splitting[END_REF][START_REF] Zhou | Construction of LaNiO 3 nanoparticles modified g-C 3 N 4 nanosheets for enhancing visible light photocatalytic activity towards tetracycline degradation[END_REF][START_REF] Rakibuddin | Graphite-like carbon nitride (C 3 N 4 ) modified N-doped LaTiO 3 nanocomposite for higher visible light photocatalytic and photoelectrochemical performance[END_REF][START_REF] Acharya | The enhanced photocatalytic activity of g-C 3 N 4 -LaFeO 3 for the water reduction reaction through a mediator free Z-scheme mechanism[END_REF][START_REF] Konstas | Synthesis, characterization of g-C 3 N 4 /SrTiO 3 heterojunctions and photocatalytic activity for organic pollutants degradation[END_REF][START_REF] Kumar | Perovskite-structured CaTiO 3 coupled with g-C 3 N 4 as a heterojunction photocatalyst for organic pollutant degradation[END_REF][START_REF] Ismael | A facile synthesis method for fabrication of LaFeO 3 /g-C 3 N 4 nanocomposite as efficient visible-light-driven photocatalyst for photodegradation of RhB and 4-CP New[END_REF].

Among rare-earth orthoferrites, the SmFeO 3 p-type semiconductor is of high potential for photocatalytic applications due to its optical bandgap that can be reduced up 2.01 eV allowing the absorption of light up to ca. 600 nm [START_REF] Behzadifard | Novel visible light driven CuO/SmFeO 3 nanocomposite photocatalysts with enhanced photocatalytic activities for degradation of organic pollutants[END_REF][START_REF] Haye | Properties of rare-earth orthoferrites perovskite driven by steric hindrance[END_REF][START_REF] Tang | Microwave synthesis of nanoparticulate SmFeO 3 and Its characterization[END_REF]. However, SmFeO 3 has only scarcely been used in photocatalysis due to its low specific surface area like most perovskite-type oxides [START_REF] Mori | Effect of preparation routes on the catalytic activity over SmFeO 3 oxide[END_REF] and to the fast recombination of photogenerated electron-hole pairs upon light activation [START_REF] Behzadifard | Novel visible light driven CuO/SmFeO 3 nanocomposite photocatalysts with enhanced photocatalytic activities for degradation of organic pollutants[END_REF][START_REF] Tang | Microwave synthesis of nanoparticulate SmFeO 3 and Its characterization[END_REF][START_REF] Niu | Preparation, characterization and photocatalytic properties of REFeO 3 (RE = Sm, Eu[END_REF][START_REF] Maity | Investigation of concentration dependent electrical and photocatalytic properties of Mn doped SmFeO 3[END_REF].

In this paper, a new solvothermal method followed by a calcination was first developed for the preparation of orthorhombic SmFeO 3 nanoparticles with an average diameter of 19 nm using Sm(NO 3 ) 3 and Fe(NO 3 ) 3 as precursors. Next, SmFeO 3 particles were associated to g-CN sheets using a mixing-sonication process to produce g-CN/SmFeO 3 composites. The g-CN:SmFeO 3 ratio was varied and the highest photocatalytic activity under visible light irradiation for the degradation of the Orange II dye and of the tetracycline hydrochloride antibiotic was observed for the g-CN/SmFeO 3 (80:20) composite. Our results demonstrate that the g-CN/SmFeO 3 composite exhibits a significantly improved photocatalytic activity compared to pure g-CN due to the improved transfer and separation of charge carriers and to the enhanced visible light absorption. Based on reactive species trapping experiments, a Z-scheme photogenerated electrons transfer mechanism based on p-n heterojunction was demonstrated for the reactions mediated by the g-CN/SmFeO 3 catalyst.

Experimental section

Materials

Samarium (III) nitrate hexahydrate (99.9%, Sigma), iron (III) nitrate nonahydrate, (99%, Sigma), citric acid (99.5%, Sigma), ammonium hydroxide solution (28%, Alpha Aesar), melamine (99.9%, Sigma) and anhydrous ethanol were of analytical grade and were used without further purification. All solutions were prepared using Milli-Q water (18.2 MΩ.cm, Millipore) as solvent.

Synthesis of SmFeO 3 nanoparticles

SmFeO 3 was synthesized by a solvothermal method followed by a calcination. Briefly, Sm(NO 3 ) 3 (5 mmol, 2.222 g), Fe(NO 3 ) 3 (5 mmol, 2.020 g) and citric acid (10 mmol, 1.920 g) were dissolved in an ethanol/water (2:1, 30 mL) mixture and the pH of the solution was adjusted to 7 using ammonium hydroxide. The solution was stirred for 30 min at room temperature, subsequently transferred into a 100 mL Teflon-sealed autoclave and heated at 180°C for 12 h. After cooling, the SmFeO 3 precursor was recovered by centrifugation (15 min, 4000 rpm), washed with water (2 x 40 mL), with absolute ethanol (2 x 40 mL) and dried at 70°C overnight. Finally, the green precursor was calcined under air at 600°C for 2 h using a heating rate of 10°C.min -1 to obtain SmFeO 3 nanoparticles.

Synthesis of bulk g-CN

Bulk g-CN was prepared by thermal condensation of melamine. In a typical experiment, melamine (5 g) was placed in a crucible with a cover and heated at 500°C for 2 h using a heating rate of 10 °C.min -1 . The yellow powder obtained was grounded using an agate mortar before use.

Synthesis of g-CN/SmFeO 3 catalysts

The g-CN/SmFeO 3 catalysts were synthesized through a mixing-ultrasonic method. In a typical experiment, appropriate amounts of SmFeO 3 and g-CN were separately dispersed in 50 mL of distilled water under sonication for 60 min. Then, the solutions were mixed and further sonicated for 90 min. The solid was recovered by centrifugation (4000 rpm for 15 min), washed with water (2 x 40 mL), with absolute ethanol (2 x 40 mL) and dried for 12 h at 70°C. The loading in g-CN was varied to prepare catalysts containing 20, 50, and 80 wt% g-CN relative to SmFeO 3 .

Visible light photocatalytic activity

The photocatalytic activity of the g-CN/SmFeO 3 catalysts was evaluated in the photodegradation of the Orange II dye and of tetracycline hydrochloride, both used at a 10 mg.L -1 concentration. Experiments were conducted under visible light irradiation using a 300 W Osram lamp equipped with UV cut-off filter (λ > 400 nm). The light intensity used was 15 mW.cm -2 . In a typical experiment, 30 mg of photocatalyst were dispersed in 30 mL of the pollutant aqueous solution and the suspension was stirred in the dark for 30 min to reach the adsorption-desorption equilibrium between the photocatalyst and the pollutant. Then the mixture was exposed to light irradiation. At a given irradiation time interval, 2 mL of the mixture were removed, centrifuged for 2 min at 15000 rpm to remove the photocatalyst and the photodegradation was monitored by UV-visible spectroscopy at the wavelength of 485 nm for Orange II and 356 nm for tetracycline hydrochloride.

Characterization

The crystal structures of the synthesized powders were determined by X-ray diffraction (XRD) using a Panalytical X'Pert pro MPD diffractometer using Cu Kα radiation (wavelength of 1.5405 Å). The morphology and the microstructure of g-CN/SmFeO 3 catalysts were investigated by scanning electron microscopy (SEM, JEOL, JSM-6490 LV) and transmission electron microscopy (TEM) (Philips CM200 instrument operating at 200 KV).

X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Gammadata Scienta SES 200-2 spectrometer. The CASAXPS software (Casa Software Ltd, Teignmouth, UK, www.casaxps.com) was used for all peak fitting procedures and the areas of each component were determined according to classical Scofield sensitivity factors.

Thermogravimetric analyses (TGA) were conducted using the TGA/DSC STARe (Mettler Toledo) equipment from 35°C to 1000°C at a heating rate of 10 °C.min -1 under O 2 atmosphere with a flow rate of 50 mL.min -1 . N2 adsorption-desorption isotherms were measured at -196°C on a Micromeritics ASAP 2420 equipment. Before analysis, the samples were degassed at 150°C for 12 h. The value of the specific surface area was determined by the Brunauer-Emmett-Teller (BET) method.

The optical measurements were performed at room temperature under ambient conditions.

A Bruker ALPHA spectrometer was used to record the Fourier transform infrared (FT-IR) spectra in the range of 400-4000 cm -1 . UV-visible absorption spectra were recorded using a Thermo Scientific Evolution 220 UV-visible spectrometer. UV-visible diffuse reflectance spectra were recorded using a Shimadzu 2600-2700 UV-visible spectrometer.

Photoluminescence (PL) emission spectra were recorded on a Horiba Fluoromax-4 Jobin Yvon spectrofluorimeter.

The surface charge of the prepared materials was obtained using Zetasizer Nano Zs (Malvern Instruments). Typically, 20 mg of the sample was dispersed in 50 mL of distilled water under sonication at room temperature. The pH of the solutions was adjusted with 0.1 M HCl and 0.1 M NaOH solutions before measurement.

The electrochemical measurements were carried out in a quartz beaker using SP150 Biologic potentiostat with a standard mode with three electrodes. An 0.1 M aqueous solution of Na 2 SO 4 was used as electrolyte. For the working electrode, 2 mg of photocatalyst was dispersed in 250 μl of DMF and then deposited on a FTO glass with an area of 2 cm 2 . After being heated for 2 h at 150°C to remove DMF, a homogeneous film was obtained. A platinum wire and an Ag/AgCl electrode were used as counter electrode and reference electrode, respectively.

Results and discussion

Synthesis and characterization of g-CN/SmFeO 3 composites

SmFeO 3 nanoparticles were prepared using a solvothermal process using Sm(NO 3 ) 3 and Fe(NO 3 ) 3 as precursors followed by calcination at 600°C. Next, SmFeO 3 and g-CN particles were mixed and ultrasonicated for 90 min to prepare g-CN/SmFeO 3 photocatalysts (Fig. 1).

The composites were recovered by centrifugation and dried at 70°C before use. Composites with g-CN:SmFeO 3 weight ratios of 20:80, 50:50 and 80:20 were prepared. The thermal stability and the weight percentage of g-CN in g-CN/SmFeO 3 composites were determined by TGA from room temperature to 1000°C at a heating rate of 10°C.min -1 (Fig. 2). For pure SmFeO 3 , the weak weight loss (ca. 1.5%) originates from the removal of physisorbed water molecules. Pure g-CN is stable until ca. 550°C and shows a fast weight loss from 585 to 722°C due to the combustion of g-CN under air atmosphere. The thermal stability of g-CN/SmFeO 3 composites is lower than that of pure g-CN and the burning of g-CN is observed at ca. 500°C, indicating that SmFeO 3 particles catalyze the decomposition of g-CN. The actual content in SmFeO 3 was determined from the remaining mass at 1000°C and was found to be of 78.6, 46.0 and 14.2% for g-CN/SmFeO 3 composites prepared with g-CN:SmFeO 3 ratios of 20:80, (Fig. S1) and no impurities like Sm 2 O 3 or Fe 2 O 3 could be detected [START_REF] Xu | Generalized low-temperature synthesis of nanocrystalline rare-earth orthoferrites LnFeO[END_REF]. For bulk g-CN, two diffraction peaks located at 27.48 and 13.19° corresponding to the (002) plane (inter-layer stacking of the conjugated aromatic system) and to the (100) plane (in-plane structural packing motif of tri-s-triazine units), respectively, were observed [START_REF] Moussa | Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity[END_REF][START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF][START_REF] Yan | Photodegradation performance of g-C 3 N 4 fabricated by directly heating melamine[END_REF]. For the g-CN/SmFeO 3 composites, the coexistence of SmFeO 3 and g-CN phases can be clearly seen and the intensities of the signals related to g-CN increase with the loading in g-CN. The crystal phases of SmFeO 3 and g-CN were not affected by the mixing-ultrasonication method used to assemble these materials.

The FT-IR spectrum of SmFeO 3 shows two strong absorption bands located at 412 cm -1 and 540 cm -1 attributed to the octahedral stretching mode of Fe-O and to the deformation vibration mode of O-Fe-O bonds, respectively (Fig. 3b) [START_REF] Rao | Infrared and electronic spectra of rare earth perovskites: Ortho-Chromites, -Manganites and -Ferrites[END_REF]. The FT-IR spectrum further confirms the phase purity of SmFeO 3 . For pure g-CN, the peaks located at 1227, 1390, 1464 and 1627 cm -1 originate from C-N stretchings [START_REF] Chai | Graphitic carbon nitride (g-C 3 N 4 )-Pt-TiO 2 nanocomposite as an efficient photocatalyst for hydrogen production under visible light irradiation[END_REF][START_REF] Wang | Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C 3 N 4[END_REF], while the signal at 808 cm -1 is associated to the characteristic breathing mode of s-triazine units [START_REF] Wang | Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C 3 N 4[END_REF]. The weak band between 3100 and 3300 cm -1 corresponds to the N-H stretching vibration. The peaks of g-CN and SmFeO 3 appear in all the spectra of the g-CN/SmFeO 3 composites and further confirm their successful association. To investigate the surface composition and chemical state of the elements present in the photocatalysts, XPS measurements were conducted (Fig. 4). As can be seen from the survey spectrum (Fig. 4a), only the C, N, Sm, Fe and O elements were detected. The high resolution XPS spectrum of C 1s can be deconvoluted into three signals. The main peak located at 288.17 eV can be assigned to sp 2 -hybridized C atoms in the N-C=N bonds of the aromatic system [START_REF] Lin | Efficient synthesis of monolayer carbon nitride 2D nanosheet with tunable concentration and enhanced visible-light photocatalytic activities[END_REF]. The weaker signals at 284.68 and 286.08 eV originate from adventitious carbon (C-C and C=C bonds) and from C-O bonds, respectively (Fig. 4b). The N 1s XPS signal shows three components located at 398.77, 400.32 and 401.38 eV corresponding to sp 2 -hybridized nitrogen (C-N=C), to tertiary nitrogen N-(C) 3 and to free amino functions, respectively (Fig. 4c) [START_REF] Elbanna | g-C 3 N 4 /TiO 2 Mesocrystals composite for H 2 evolution under visible-light irradiation and its charge carrier dynamics[END_REF]. The Sm 3d 5/2 peak and the Fe 2p 3/2 peak are located at 1084.12 and 711.83 eV, respectively, and confirm that Sm and Fe are both present in the +3 oxidation state (Fig. 4de). Both signals are shifted to higher binding energies compared to pure SmFeO 3 , which suggests a strong association between g-CN and SmFeO 3 [START_REF] Bukhari | Effect of cobalt substitution on thermal stability and electrical conductivity of Sm 0.95 Ce 0.05 FeO 3 -δ in oxidizing and reducing conditions Solid State Ion[END_REF][START_REF] Asamoto | Catalytic activity of multi-metallic perovskite-type oxide prepared by the thermal decomposition of heteronuclear cyano complex, Sm[Fe x Co 1-x (CN) 6 ] • nH 2 O Top[END_REF]. Finally, the O 1s high resolution spectrum can be fitted with two components located at 530.77 and 533.02 eV corresponding to Sm-O and Fe-O bonds in the SmFeO 3 lattice and to carbonate species adsorbed at the surface of the photocatalyst, respectively (Fig. 4f) [START_REF] Mori | Effect of preparation routes on the catalytic activity over SmFeO 3 oxide[END_REF]. The specific surface area of bulk and exfoliated g-CN, of SmFeO 3 and of the g-CN/SmFeO 3 (80:20) composite were determined via N 2 adsorption-desorption experiments conducted at 77 K (Fig. 7). All samples exhibit type IV isotherms according to the IUPAC classification. The BET specific surface area measured for pure SmFeO 3 is 25.4 m 2 .g -1 . After association with g-CN, the specific surface area of g-CN/SmFeO 3 (80:20) composite decreased to 18.84 m 2 .g -1 , likely due to the blocking of some active sites of SmFeO 3 by g-CN. Noteworthy is that these values are significantly higher than those determined for bulk and exfoliated g-CN (5.80 and 11.17 m 2 .g -1 , respectively). The H3 type hysteresis loops observed in the 0.9 to 1.0 relative pressure range indicate the presence of slit-like pores, which should provide more surfaceactive sites and thus improve the photocatalytic activity.

Optical properties

The UV-visible absorption spectra of SmFeO 3 , g-CN and the g-CN/SmFeO 3 photocatalysts are given in Fig. 8a. The bandgap energy (Eg) of the samples was determined by the equation αhʋ = B(hʋ -Eg) 2 , where α, h, ʋ, Eg and B are the absorption coefficient, the Planck constant, the light frequency, the bandgap energy and a constant, respectively (Fig. 8b).

SmFeO 3 nanoparticles absorb light over the whole visible region due to their low bandgap energy (2.01 eV). The absorption of perovskite-type oxides like SmFeO 3 at high wavelengths originates from the electronic transition from the valence band to the conduction band (O 2p → Fe 3d) [START_REF] Parida | Fabrication of nanocrystalline LaFeO 3 : an efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition[END_REF]. The absorption edge of pure g-CN is at ca. 450 nm, which corresponds to a bandgap of 2.75 eV, value in good accordance with previous reports [START_REF] Moussa | Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity[END_REF][START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF][START_REF] Xu | Fast and facile preparation of metaldoped g-C 3 N 4 composites for catalytic synthesis of dimethyl carbonate[END_REF]. The UVvisible absorption in the 300-600 nm region of g-CN/SmFeO 3 composites decreases with the g-CN content due to its lower bandgap. Simultaneously, the bandgap energy increases from 2.01 to 2.65 eV. All g-CN/SmFeO 3 composites exhibit a significantly higher absorption in the visible range than g-CN which should improve their photocatalytic activity under visible light irradiation.

Photocatalytic activity

The surface charge of g-CN, SmFeO 3 and g-CN/SmFeO 3 (80:20) photocatalysts was determined by zeta potential measurements (Fig. S4). The point of zero charge (pzc) for g-CN is of 4.21, value in good agreement with previous reports [START_REF] Zhu | Isoelectric point and adsorption activity of porous g-C 3 N 4[END_REF]. The pzc of the g-CN/SmFeO 3 min -1 and further confirm that the g-CN/SmFeO 3 (80:20) composite exhibits the highest performance for the degradation of Orange II (Fig. S5). The physical mixing of g-CN and SmFeO 3 (wt ratio of 80/20) yields a photocatalyst exhibiting an activity slightly lower than that of pure g-CN (k = 1.45 min -1 ), which further accounts for the enhanced charge separation when g-CN and SmFeO 3 are brought in contact by ultrasonication. The high activity observed for g-CN/SmFeO 3 (80:20) composite likely originates from its weak bandgap of 2.65 eV that allows to produce more photo-generated charge carriers than g-CN under visible light irradiation, from the p-n junction that promotes the separation of the charge carriers (vide infra) and from its relatively high specific surface area (18.84 m 2 .g -1 )

that affords more active sites for the degradation of Orange II.

The photodegradation of tetracycline hydrochloride (TC), an antibiotic widely used against bacteria-related diseases, was also investigated [START_REF] Palominos | Photocatalytic oxidation of the antibiotic tetracycline on TiO 2 and ZnO suspensions[END_REF]. TC is a refractory pollutant and only poorly decomposed via conventional oxidative or biological processes [START_REF] Wei | Synthesis of tetragonal prismatic γ-In 2 Se 3 nanostructures with predominantly {110} facets and photocatalytic degradation of tetracycline[END_REF]. Since TC exists in its zwitterionic form at neutral pH [START_REF] Hsu | Adsorption of tetracycline on Fe (hydr)oxides: effects of pH and metal cation (Cu 2+ , Zn 2+ and Al 3+ ) addition in various molar ratios[END_REF], no adsorption of TC was observed at the surface of g-CN or g-CN/SmFeO 3 (80:20) catalysts during the 30 min of stirring in the dark (Fig. 9b). After 180 min of visible light irradiation, 97% of TC was degraded using g-CN/SmFeO 3 (80:20) or (50:50) composites while only 77% of TC was decomposed using pure g-CN or the g-CN/SmFeO 3 (20:80) composite. As previously, the photocatalytic activity of SmFeO 3 is modest and only 18% degradation could be achieved. The photodegradation data were fitted to the pseudo-first-order model and the k values determined for g-CN and g-CN/SmFeO 3 (80:20), (50:50) and (20:80) catalysts are 0.85×10 -2 , 1.88×10 -2 min -1 , 1.83×10 -2 min -1 and 0.81 ×10 -2 min -1 , respectively (Fig. S6).

The performances of the g-CN/SmFeO 3 (80:20) composite are better than those of g-CNbased catalysts recently developed for the degradation of TC (P-and S-doped g-CN, C-doped g-CN, WO 3 /g-CN/Bi 2 O 3 or Ag 3 PO 4 /AgBr/g-CN) [START_REF] Jiang | Phosphorus-and Sulfur-Codoped g C 3 N 4 : Facile Preparation, Mechanism Insight, and Application as Efficient Photocatalyst for Tetracycline and Methyl Orange Degradation under Visible Light Irradiation ACS Sustainable[END_REF][START_REF] Panneri | Bifunctional Granules of Carbon-Doped g C 3 N 4 as Adsorptive Photocatalyst for the Efficient Removal of Tetracycline Antibiotic ACS Sustainable[END_REF][START_REF] Jiang | In-situ synthesis of direct solid-state dual Z-scheme WO 3 /g-C 3 N 4 /Bi 2 O 3 photocatalyst for the degradation of refractory pollutant[END_REF][START_REF] Yu | Enhanced photocatalytic degradation of tetracycline under visible light by using a ternary photocatalyst of Ag 3 PO 4 /AgBr/g-C 3 N 4 with dual Z-scheme heterojunction Sep[END_REF]. The photodegradation yield is of 70-85% after ca. 1 h irradiation using a light intensity of 100 mW.cm -2 and the irradiation time could even be reduced to 25 min when using a light intensity of 160 mW.cm -2 . The lower rate observed in this study for the degradation of TC is related to much lower intensity irradiation used (25 mW.cm -2 ) .

The stability of the g-CN/SmFeO 3 (80:20) photocatalyst was evaluated in six consecutive experiments using Orange II and TC as pollutants (see Fig. 9c and Fig. S7 for the results obtained with Orange II and TC, respectively). After each reaction, the photocatalyst was recovered by centrifugation and reused without any washing or drying in the next run. As can be seen, a slight decrease of the photocatalytic performance for Orange II removal from 97 to 82% was observed after the six cycles, indicating that the g-CN/SmFeO 3 composite exhibits a good stability and thus a high potential for practical applications. The slight decrease in activity observed after six reuses may originate from the adsorption of byproducts formed during the degradation of the dye and remaining at the surface of the catalyst. Similar results were obtained using TC (decrease in activity from 97 to 88% after the 6 th reuse). The high stability of the g-CN/SmFeO 3 (80/20) composite was further confirmed by SEM and XRD experiments in which no significant differences were observed between the as-prepared and the reused photocatalyst (Fig. S8).

Influence of pH on the photodegradation

As the surface charge of the photocatalyst, the band edge position of its components and the charge of the pollutant depend on pH, it is of high importance to evaluate its influence on the catalytic activity [START_REF] Moussa | Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity[END_REF]. A series of experiments were conducted by adjusting the pH of the Orange II solution to 4.5, 6, 7.5, 9 and 11 using 0.1 M HCl or NaOH before the adsorption phase. Fig. S9a presents the kinetics of the photodegradation vs pH using the g-CN/SmFeO 3 (80:20) catalyst. The adsorption of the dye on the catalyst surface is more pronounced at pH 4.5 (ca. 18%) due to the strong electrostatic interactions between the positively-charged catalyst surface (pzc = 5.02) and the negatively-charged dye. The dark-adsorption of the dye decreases with pH increase and almost no adsorption was observed at pH 11 due to the strong electrostatic repulsion between the dye (pK a naphthol = 10.6) with the catalyst surface. As can be seen in Fig. S9b, the highest photocatalytic activity was observed at pH 4.5

(k = 2.1 10 -2 min -1 ) and Orange II was completely degraded in 150 min under visible irradiation. The degradation rates significantly decrease at pH 6, 7.5 and 9 (k = 1.9×10 -2 , 1.3×10 -2 and 10 -2 min -1 , respectively) and the lowest kinetic was observed at pH 11 (k = 0.02×10 -2 min -1 ).

Mechanism

To evaluate the contributions of the various active species in the photodegradation of Orange II, trapping experiments were conducted using 1,4-benzoquinone (p-BQ), tertbutanol (t-BuOH), EDTA and DMSO as scavengers of superoxide (O 2 •-) radicals, hydroxyl ( • OH) radicals, holes and electrons, respectively (all experiments were performed at pH 6.7

which corresponds to the pH of the Orange II solution without any additives) [START_REF] Moussa | Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity[END_REF][START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF][START_REF] Ouedraogo | Copper octacarboxyphthalocyanine as sensitizer of graphitic carbon nitride for efficient dye degradation under visible light irradiation[END_REF]. As can be seen from Fig. 10a and S10, O 2 •-radicals and holes play a major role in the decomposition of the dye (only 46 and 50% degradation after 150 min irradiation when adding p-BQ and EDTA, respectively) while the addition of electrons and • OH radicals scavengers only slightly restrained the photodegradation. Similar experiments were also conducted using TC as pollutant and at pH 4.4 which corresponds to the pH of the starting TC solution (Fig. S11). The same active species were found to be involved in the degradation of TC but holes have the major role (43 and 73% degradation after 150 min irradiation when adding EDTA and p-BQ, respectively). These experiments suggest that photogenerated electrons quickly react with O 2 molecules adsorbed at the surface of the photocatalyst to generate O 2 •-radicals, and thus preventing electrons to recombine with holes.

To further explore the photocatalytic mechanism, the separation efficiency of photogenerated electron-hole pairs was investigated by PL spectroscopy. The energy bandgap of SmFeO 3 was calculated to be 2.01 eV (Fig. 8b). The band edge position of the conduction band (E CB ) and of the valence band (E VB ) of SmFeO 3 at the point of zero charge can be determined using the Mulliken electronegativity theory : couple (-0.18 eV vs NHE) (Fig. 11a). These data are inconsistent with the scavenging experiments previously described which show that O 2 •-radicals play an important role in the degradation of Orange II and TC. Therefore, reactions mediated by the g-CN/SmFeO 3 photocatalyst likely occur via a Z-scheme mechanism (Fig. 11b) [START_REF] Li | Constructing Fe-MOF-Derived Z-scheme Photocatalysts with Enhanced Charge Transport: Nanointerface and Carbon Sheath Synergistic Effect[END_REF]. The interfacial junction

E CB = χ -E e -0.

Conclusion

A series of heterostructured g-CN/SmFeO 3 photocatalysts was prepared via a mixingultrasonication process using perovskite SmFeO 3 nanoparticles with an average diameter of •-radicals and holes play the major role and a Z-scheme charge carrier transfer pathway is proposed for the mechanism. We expect that the g-CN/SmFeO 3 composites developed in this work can be of high interest for wastewater treatment, but also for other photocatalytic applications like hydrogen production.

Fig. 1 .

 1 Fig. 1. Schematic representation of g-CN/SmFeO 3 composites synthesis.

Fig. 2 .

 2 Fig. 2. TGA plots of bulk g-CN, SmFeO 3 and g-CN/SmFeO 3 composites.

Fig. 3 .

 3 Fig. 3. (a) XRD patterns and (b) FT-IR spectra of bulk g-CN, SmFeO 3 and g-CN/SmFeO 3 photocatalysts.

Fig. 4 .

 4 Fig. 4. (a) XPS survey spectrum of the g-CN/SmFeO 3 (80/20) composite and high resolution XPS spectra of (b) C 1s, (c) N 1s, (d) Sm 3d, (e) Fe 2p and (f) O 1s.

Fig. 5 .

 5 Fig. 5. SEM images of (a) SmFeO 3 and (b-d) g-CN/SmFeO 3 composites prepared with g-CN:SmFeO 3 ratios of 20:80, 50:50 and 80:20, respectively.

Fig. 6 .

 6 Fig. 6. TEM images of (a) SmFeO 3 and (b-d) g-CN/SmFeO 3 composites prepared with g-CN:SmFeO 3 ratios of 20:80, 50:50 and 80:20, respectively. The top inset of a is the size distribution and the bottom insets are the SAED patterns. (e-f) HR-TEM images of g-CN/SmFeO 3 composites.

Fig. 7 .

 7 Fig. 7. N 2 adsorption-desorption isotherms of bulk and exfoliated g-CN, SmFeO 3 and of the g-CN/SmFeO 3 (80:20) composite.

Fig. 8 .

 8 Fig. 8. (a) UV-visible absorption spectra of bulk g-CN, SmFeO 3 and g-CN/SmFeO 3 photocatalysts and (b) the corresponding [αhν] 2 vs photon energy plots.

(80: 20 )

 20 catalyst increased to 5.02 due to the high pzc value of pure SmFeO 3(8.82). These results indicate that the g-CN/SmFeO 3 (80:20) composite is positively-charged at neutral pH, which should facilitate the degradation of anionic pollutants that will strongly adsorb at its surface via electrostatic interactions.The photocatalytic activity of g-CN/SmFeO 3 composites was first evaluated in the degradation of the negatively-charged Orange II dye (10 mg.L -1 ) under visible light irradiation (intensity of 15 mW.cm -2 ) and at neutral pH. Before irradiation, the solution was stirred in the dark for 30 min to reach the physisorption equilibrium of the dye. The percentage of Orange II adsorbed increases with the loading in g-CN in g-CN/SmFeO 3 composites (from 8 to 15%) (Fig.9a), which agrees with zeta potential measurements. Preliminary experiments show that the photolysis of Orange II in the absence of catalyst is negligible. As can be seen from Fig.9a, pure SmFeO 3 exhibits no photocatalytic activity, likely due to the high recombination rate of photogenerated electron-hole pairs, while ca. 92% of the dye was bleached after 150 min irradiation using g-CN. The photocatalytic activity of g-CN/SmFeO 3 composites increases with the loading in g-CN. The highest activity was obtained for the g-CN/SmFeO 3 (80:20) catalyst with Orange II being decomposed at 97% after 150 min irradiation. The photodegradation was found to be a pseudo-first-order kinetic process expressed by ln(C 0 /C) = kt where C 0 and C are the concentrations of Orange II at time 0 and t, respectively, and k is the apparent first-order rate constant. The rate constants determined for g-CN and g-CN/SmFeO 3 20:80, 50:50 and 80:20 catalysts are 1.6, 0.5, 1.3 and 1.9×10-2

Fig. 9 .

 9 Fig. 9. Concentrations changes of (a) Orange II and (b) tetracycline hydrochloride under visible light irradiation (15 mW.cm -2 ) using g-CN, SmFeO 3 or g-CN/SmFeO 3 composites as

  photogenerated electron-hole pairs was investigated by PL spectroscopy. Fig.10bshows the PL emission spectra of g-CN, SmFeO 3 and of the g-CN/SmFeO 3 composites after excitation at 350 nm (the PL intensities g-CN and g-CN/SmFeO 3 were normalized to the actual g-CN weight in the photocatalysts). Two strong PL emissions centered at 438 and 460 nm can be observed for g-CN after excitation at 350 nm indicating a high recombination rate of photogenerated electron-hole pairs while SmFeO 3 is non-fluorescent. A marked decrease of the PL intensity is observed with the increase of the SmFeO 3 loading in the photocatalysts, indicating that the electron-hole recombination in g-CN is effectively impeded after coupling with SmFeO 3 .

Fig. 10 .

 10 Fig. 10. (a) Photodegradations efficiencies of Orange II after 150 min irradiation over the g-CN/SmFeO 3 (80:20) catalyst when adding p-BQ, DMSO, EDTA and t-BuOH as scavengers (all

  photodegradation mechanism.

Fig. 11 .

 11 Fig. 11. (a) Band structure of the g-CN/SmFeO 3 photocatalyst and (b) schematic illustration of the delocalization of charge carriers and of the formation of reactive species involved in the photodegradation.

  photodegradation shows that O 2•-radicals and holes play the major role and a Z-scheme

  [START_REF] Zhang | Graphitic carbon nitride (g-C 3 N 4 )-based photocatalysts for water disinfection and microbial control: a review[END_REF] Eg where χ is the absolute electronegativity of SmFeO 3 (5.56 eV), E e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV vs NHE) and Eg is the bandgap energy[START_REF] Xu | The absolute energy positions of conduction and valence bands of selected semiconducting minerals[END_REF]. For SmFeO 3 , the calculated ECB and EVB values are 0.05 and 2.06 eV, respectively. The χ value for g-CN is 4.42 eV and E CB and E VB values were determined to be -1.34 and 1.41 eV,

	respectively.
	Under visible light irradiation, both SmFeO 3 and g-CN can be photo-activated to generate
	electrons and holes. As depicted in Fig. 11a, E CB (g-CN) is more negative that E CB (SmFeO 3 ) and
	photoexcited electrons in the CB of g-CN can easily transfer to the lower energy CB of
	SmFeO 3 due to the p-n junction established between SmFeO 3 and g-CN and the inner electric
	field generated. However, these electrons cannot reduce O 2 into O 2	•-radicals since the CB
	edge potential of SmFeO 3 (0.05 eV) is more positive than the redox potential of the O 2 /O 2	•-

Acknowledgments

This work has received the financial support from the Institut Carnot ICEEL.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/.


Vincent Guigoz

Lavinia Balan

Abdelhay Aboulaich

Raphaël Schneider
email: raphael.schneider@univ-lorraine.fr

Thomas Gries
email: thomas.gries@univ-lorraine.fr

Heterostructured thin

Keywords: Lanthanum ferrite, graphitic carbon nitride, photoelectrochemical water splitting, hydrogen evolution

published or not. The documents may come

Introduction

Hydrogen (H 2 ) is a clean and carbon-free burning molecule and has become a promising candidate fuel as future energy carrier due to high energy density [START_REF] Mazloomi | Hydrogen as an energy carrier: prospects and challenges[END_REF][START_REF] Züttel | Hydrogen: the future energy carrier[END_REF]. However, 96% of H 2 worldwide production originates from fossil fuels [START_REF] Ewan | A figure of merit assessment of the routes to hydrogen[END_REF], which poses several environmental concerns such as the emission of a large amount of greenhouse gases such as CO 2 , SOx, NOx and CO as by-products, which are mainly responsible of global warming issue. In addition to its environmental issues, most of H 2 production methods are highly energy consuming, which limits their application at very large scale. A big achievement would be to produce H 2 via an environmental-friendly and cost-effective method. In this regard, solar energy can be converted into H 2 by splitting water into oxygen (O 2 ) and H 2 , storing the solar energy in the H-H chemical bond (artificial photosynthesis). Hydrogen photoproduction can be achieved by two means, photocatalytically via the irradiation of a catalyst in water or photoelectrochemically at the surface of a semiconductor electrode in three-electrodes cells in which the illuminated semiconductor electrode either oxidizes water into O 2 (photoanode) or reduces it into H 2 (photocathode) [START_REF] Acar | Review of photocatalytic water-splitting methods for sustainable hydrogen production[END_REF][START_REF] Fajrina | A critical review in strategies to improve photocatalytic water splitting towards hydrogen production[END_REF]. This photoelectrode is connected to a counter electrode that electrocatalyzes the respective half redox reaction.

Fujishima and Honda first demonstrated photoelectrochemical (PEC) water splitting for hydrogen production with a biased photoanode (titanium oxide, TiO 2 ) in 1972 [START_REF] Fushijima | Electrochemical photolysis of water at a semiconductor electrode[END_REF]. To produce H 2 using PEC, the semiconductor material (photoelectrode) should have a suitable bandgap for water splitting half reactions to occur, absorb in the visible region of the solar spectrum, a high quantum efficiency (i.e. produce reactive charge carriers per absorbed photon) and a good photochemical stability (i.e. withstand photocorrosion) [START_REF] Grätzel | Photoelectrochemical cells[END_REF][START_REF] Lin | Semiconductor nanostructure-based photoelectrochemical water splitting: a brief review[END_REF][START_REF] Wang | Facile synthesis of interlocking g-C 3 N 4 /CdS photoanode for stable photoelectrochemical hydrogen production[END_REF][START_REF] Zhang | Electrodeposition of Cu 2 O/g-C 3 N 4 heterojunction film on an FTO substrate for enhancing visible light photoelectrochemical water splitting[END_REF]. To reduce costs and environmental impact, the chosen material should also be abundant and nontoxic.

Most water splitting studies were conducted using wide bandgap semiconductors like TiO 2 or ZnO that can only be activated by UV light [START_REF] Yu | Enhanced photoelectrochemical efficiency and stability using a conformal TiO 2 film on a black silicon photoanode[END_REF][START_REF] Wolcott | Photoelectrochemical study of nanostructured ZnO thin films for hydrogen generation from water splitting[END_REF]. In recent years, intensive efforts were devoted to the extension of TiO 2 or ZnO visible light absorption and to the improvement of their photocatalytic properties via doping or building heterostructured catalysts in which the separation of charge carriers is improved [START_REF] Acar | Review of photocatalytic water-splitting methods for sustainable hydrogen production[END_REF][START_REF] Kumaravel | Photocatalytic hydrogen production using metal doped TiO 2 : a review of recent advances[END_REF][START_REF] Cai | Light-driven sustainable hydrogen production utilizing TiO 2 nanostructures: a review[END_REF] Recently, lanthanum ferrite perovskite (LaFeO 3 ) has attracted a great attention [START_REF] Wheeler | Photoelectrochemical properties and stability of nanoporous ptype LaFeO 3 photoelectrodes prepared by electrodeposition[END_REF][START_REF] Parida | Fabrication of nanocrystalline LaFeO 3 : an efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition[END_REF][START_REF] Celorrio | Photoelectrochemical properties of LaFeO 3 nanoparticles[END_REF][START_REF] Díez-García | Metal doping to enhance the photoelectrochemical behavior of LaFeO 3 photocathodes[END_REF][START_REF] Pawar | Unbiased spontaneous solar fuel production using stable LaFeO 3 photoelectrode[END_REF][START_REF] Pawar | Enhanced photoactivity and hydrogen generation of LaFeO 3 photocathode by plasmonic silver nanoparticle incorporation[END_REF], with applications in sensors [START_REF] Jaouali | LaFeO 3 ceramics as selective oxygen sensors at mild temperature[END_REF], catalysts [START_REF] Barbero | Synthesis and characterisation of La 1-x Ca x FeO 3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds[END_REF] and monitoring [START_REF] Anajafi | Optical, magnetic and gas sensing properties of LaFeO 3 nanoparticles synthesized by different chemical methods[END_REF] due to its good chemical and thermal stability and its low toxicity. Moreover, LaFeO 3 has an advantageous optical bandgap, up to 2.1 eV, allowing the absorption of visible light until 590 nm. However, the photocatalytic activity of pure LaFeO 3 and its PEC performances are modest due to the fast recombination of photogenerated electron/hole pairs, to its weak charge collection, and to its low specific surface area [START_REF] Wheeler | Photoelectrochemical properties and stability of nanoporous ptype LaFeO 3 photoelectrodes prepared by electrodeposition[END_REF][START_REF] Peng | Surface modification of LaFeO 3 by Co-Pi electrochemical deposition as an efficient photoanode under visible light[END_REF][START_REF] Li | Photoinduced charge property of nanosized perovskite-type LaFeO 3 and its relationships with photocatalytic activity under visible irradiation[END_REF][START_REF] Hu | Photocatalytic activities of LaFeO 3 and La 2 FeTiO 6 in p-chlorophenol degradation under visible light[END_REF][START_REF] Thirumalairajan | Shape evolution of perovskite LaFeO 3 nanostructures: a systematic investigation of growth mechanism, properties and morphology dependent photocatalytic activities[END_REF][START_REF] Li | Synergetic effect of copper species as cocatalyst on LaFeO 3 for enhanced visible-light photocatalytic hydrogen evolution[END_REF][START_REF] Ismael | Perovskite-type LaFeO 3 : photoelectrochemical properties and photocatalytic degradation of organic pollutants under visible light irradiation[END_REF]. Noteworthy is also that LaFeO 3 exhibits poor or no activity for H 2 photoproduction because its conduction band (CB) edge is close and too positive compared to that of the H + /H 2 couple (0 V), which does not allow photogenerated electrons to reduce H + into H 2 .

Heterostructured catalysts associating LaFeO 3 to reduced graphene oxide [START_REF] Ren | Controlled growth of LaFeO 3 nanoparticles on reduced graphene oxide for highly efficient photocatalysis[END_REF], TiO 2 [START_REF] Humayun | Exceptional visible-light activities of TiO 2 -coupled N-doped porous perovskite LaFeO 3 for 2,4-dichlorophenol decomposition and CO 2 conversion[END_REF][START_REF] Gong | Electrochemical synthesis of perovskite LaFeO 3 nanoparticle-modified TiO 2 nanotube arrays for enhanced visiblelight photocatalytic activity[END_REF] or graphitic carbon nitride (g-C 3 N 4 ) [START_REF] Xu | Photocatalytic hydrogen evolution performance of NiS cocatalyst modified LaFeO 3 /g-C 3 N 4 heterojunctions[END_REF][START_REF] Rakibuddin | Graphite-like carbon nitride (C 3 N 4 ) modified Ndoped LaTiO 3 nanocomposite for higher visible light photocatalytic and photoelectrochemical performance[END_REF][START_REF] Jin | Boosting visible light photocatalytic performance of g-C 3 N 4 nanosheets by combining with LaFeO 3 nanoparticles[END_REF][START_REF] Wu | Quasi-polymeric construction of stable perovskite-type LaFeO 3 /g-C 3 N 4 heterostructured photocatalyst for improved Zscheme photocatalytic activity via solid p-n heterojunction interfacial effect[END_REF][START_REF] Ye | Photocatalytic, fenton and photo-fenton degradation of RhB over Z-scheme g-C 3 N 4 /LaFeO 3 heterojunction photocatalysts[END_REF][START_REF] Gao | A plasmonic Z-scheme three-component photocatalyst g-C 3 N 4 /Ag/LaFeO 3 with enhanced visible-light photocatalytic activities[END_REF][START_REF] Ibarra-Rodriguez | Facile synthesis of g-C 3 N 4 / LaMO 3 (M: Co, Mn, Fe) composites for enhanced visible-light-driven photocatalytic water splitting[END_REF][START_REF] Acharya | The enhanced photocatalytic activity of g-C 3 N 4 -LaFeO 3 for the water reduction reaction through a mediator free Z-scheme mechanism[END_REF] have received a much higher interest due to the improved separation of charge carriers. The organic g-C 3 N 4 semiconductor is a highly promising material for photocatalytic applications like H 2 production, environmental remediation or CO 2 reduction into fuel molecules due to its low cost, its high thermal and chemical stability and its relatively low bandgap of ca. 2.7 eV allowing the absorption of visible light up to 460 nm [START_REF] Ye | A review on g-C 3 N 4 for photocatalytic water splitting and CO 2 reduction[END_REF][START_REF] Wen | A review on g-C 3 N 4 -based photocatalysts[END_REF][START_REF] Fu | g-C 3 N 4 -based heterostructured photocatalysts[END_REF][START_REF] Mishra | Graphitic carbon nitride (g-C 3 N 4 )-based metal-free photocatalysts for water splitting: a review[END_REF][START_REF] Moussa | Growth of ZnO nanorods on graphitic carbon nitride gCN sheets for the preparation of photocatalysts with high visible-light activity[END_REF]. Moreover, the energy positions of g-C 3 N 4 valence band (VB) and of its CB are located at ca. 1.4 and -1.3 eV vs normal hydrogen electrode (NHE), making this semiconductor suitable for water splitting [START_REF] Ye | A review on g-C 3 N 4 for photocatalytic water splitting and CO 2 reduction[END_REF][START_REF] Wen | A review on g-C 3 N 4 -based photocatalysts[END_REF][START_REF] Fu | g-C 3 N 4 -based heterostructured photocatalysts[END_REF][START_REF] Mishra | Graphitic carbon nitride (g-C 3 N 4 )-based metal-free photocatalysts for water splitting: a review[END_REF]. As LaFeO 3 , g-C 3 N 4 suffers from various drawbacks like a high recombination rate of charge carriers and a low charge mobility. Due to the adequate band edge positions of g-C 3 N 4 and LaFeO 3 , the coupling of these materials affords a Z-scheme charge transfer system which allows to promote the separation of electron/hole pairs and to extend the visible light absorption [START_REF] Ismael | Perovskite-type LaFeO 3 : photoelectrochemical properties and photocatalytic degradation of organic pollutants under visible light irradiation[END_REF][START_REF] Xu | Photocatalytic hydrogen evolution performance of NiS cocatalyst modified LaFeO 3 /g-C 3 N 4 heterojunctions[END_REF][START_REF] Rakibuddin | Graphite-like carbon nitride (C 3 N 4 ) modified Ndoped LaTiO 3 nanocomposite for higher visible light photocatalytic and photoelectrochemical performance[END_REF][START_REF] Jin | Boosting visible light photocatalytic performance of g-C 3 N 4 nanosheets by combining with LaFeO 3 nanoparticles[END_REF][START_REF] Wu | Quasi-polymeric construction of stable perovskite-type LaFeO 3 /g-C 3 N 4 heterostructured photocatalyst for improved Zscheme photocatalytic activity via solid p-n heterojunction interfacial effect[END_REF][START_REF] Ye | Photocatalytic, fenton and photo-fenton degradation of RhB over Z-scheme g-C 3 N 4 /LaFeO 3 heterojunction photocatalysts[END_REF][START_REF] Gao | A plasmonic Z-scheme three-component photocatalyst g-C 3 N 4 /Ag/LaFeO 3 with enhanced visible-light photocatalytic activities[END_REF][START_REF] Ibarra-Rodriguez | Facile synthesis of g-C 3 N 4 / LaMO 3 (M: Co, Mn, Fe) composites for enhanced visible-light-driven photocatalytic water splitting[END_REF][START_REF] Acharya | The enhanced photocatalytic activity of g-C 3 N 4 -LaFeO 3 for the water reduction reaction through a mediator free Z-scheme mechanism[END_REF]. Xu et al. reported the preparation of a visible light active LaFeO 3 (5%)/g-C 3 N 4 photocatalyst via a calcination process [START_REF] Xu | Photocatalytic hydrogen evolution performance of NiS cocatalyst modified LaFeO 3 /g-C 3 N 4 heterojunctions[END_REF]. After association to 2% NiS used as co-catalyst and using triethanolamine as sacrificial substrate, a H 2 evolution rate (HER) of 121 µmol.g -1 .h -1 was obtained. Acharya et al. also described the mechanical synthesis of a LaFeO 3 (80%)/g-C 3 N 4 catalyst exhibiting a HER of 1152 µmol.g -1 .h -1 using methanol as sacrificial substrate and 1% Pt as co-catalyst [START_REF] Acharya | The enhanced photocatalytic activity of g-C 3 N 4 -LaFeO 3 for the water reduction reaction through a mediator free Z-scheme mechanism[END_REF].

Herein, we report the first preparation of a thin LaFeO 3 film deposited on a g-C 3 N 4 film for PEC water splitting. Spin-coating and magnetron sputtering, which are facile and scalable methods, were used to deposit g-C 3 N 4 and LaFeO 3 , respectively, to obtain heterostructured films. Photocurrent and HER measurements revealed that photocatalytic properties of LaFeO 3 were significantly improved after coupling with g-C 3 N 4 . A high HER value of 10.8 µmol -1 .h -1 .cm -2 was determined after 7 h under visible light irradiation. Moreover, 70% of the photocatalytic activity of the LaFeO 3 /g-C 3 N 4 were retained after the third cycle, indicating its good stability.

Experimental section

Synthesis and deposition of g-C 3 N 4 on FTO

The g-C 3 N 4 powder was synthesized by heat treatment of urea. Briefly, urea was placed in a crucible and stepwise heated at 250, 350 and 450°C for 1 h each with a heating rate of 10°C.min -1 . After cooling to room temperature, the pale yellow solid was grounded into powder.

Prior to any deposition, a transparent conductive oxide substrate, Fluorine doped Tin Oxide coated glass (FTO, TEC 7 Ω/sq, Solems) was successively washed with acetone and ethanol for ten minutes in an ultrasonic bath before drying with compressed air. g-C 3 N 4 was dispersed in methanol at the concentration of 0.5 mg•mL -1 and ultrasonicated for 20 min.

The dispersion was then left to rest for at least 4 days to let the largest particles to sediment.

The concentration of the remaining lightweight exfoliated g-C 3 N 4 in methanol was determined to be of ca. 0.10 mg•mL -1 . The dispersion was stable for months. g-C 3 N 4 nanosheets were deposited by dropping 10 µL of the previously prepared dispersion solution onto cleaned FTO and rotated at 500 rpm for 10 s. This step was repeated several times in order to control the g-C 3 N 4 film thickness. Spin-coating was followed by a thermal treatment at 115°C on a hot plate for 30 min in air to evaporate methanol and assure good electrical contact.

Elaboration of LaFeO 3 thin film

LaFe films were deposited on pre-cleaned FTO or on g-C 3 N 4 deposited on FTO by DC magnetron co-sputtering from two metallic targets: iron target (diameter: 50 mm, thickness: 0.25 mm, purity: 99.5%) and lanthanum target (diameter: 50 mm, thickness: 3 mm, purity: 99.9%). Two independent magnetrons focused at the centre of the substrate-holder and inclined by 30° with respect to the substrate normal were used with a target to substrate distance set equal to 10 cm. The base pressure in the sputtering chamber during deposition was 0.58 Pa in an atmosphere containing 10 vol.% H 2 -90 vol.% Ar. The compositions of the thin films were controlled by the adjustment of the power applied to metallic target and stoichiometric LaFe films were obtained at 80 W on Fe target and 65 W on La target, respectively. Before deposition, a shutter was placed in front of the target to isolate the substrate during 5 min in order to remove native oxide layers. Thickness was controlled by adjusting the sputtering time. The substrate-holder rotated at 28 rpm during deposition to ensure homogeneity. Thin films were deposited without external heating and the deposition temperatures were below 50°C. At these temperatures, films remained amorphous and a post-annealing was performed to synthesize LaFeO 3 perovskite structure in a tubular furnace at 500°C for 2 h in air.

Characterization

The surface morphology of the films was imaged by a Philips XL30S-FEG scanning electron microscope (SEM). Film thicknesses were measured by SEM cross-section observation. The La/Fe atomic ratio was determined by energy dispersive spectroscopy (EDS). The crystallographic structure of the films was determined by X-ray diffraction (XRD) performed at room temperature using Bragg-Brentano θ/2θ measurements on a D8 Advance Bruker diffractometer (Cu Kα, λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra were recorded on an Agilent Cary 680 equipped with an attenuated total reflexion (ATR) element (resolution = 4 cm -1 ). UV-visible absorption spectra were recorded on Cary 7000 UMS Agilent UV-visible spectrophotometer. Raman analyses were conducted using a Horiba Labram HR800 equipment with a 633 nm laser. All the optical measurements were performed at room temperature under ambient conditions. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Gammadata Scienta (Uppsala, Sweden) SES 200-2 spectrometer under ultra-high vacuum (P < 10 -9 mbar). The measurements were performed at normal incidence (the sample plane is perpendicular to the emission angle). The spectrometer resolution at the Fermi level is about 0.4 eV. The depth analyzed extends up to about 8 nm. The monochromatized Al Kα source (1486.6 eV) was operated at a power of 420 W (30 mA and 14 kV) and the spectra were acquired at a take-off angle of 90° (angle between the sample surface and photoemission direction).

During acquisition, the pass energy was set to 500 eV for wide scans and to 100 eV for highresolution spectra. CASAXPS software (Casa Software Ltd, Teignmouth, UK, www.casaxps.com) was used for all peak fitting procedures and the areas of each component were modified according to classical Scofield sensitivity factors.

The photoelectrochemical performance of as-prepared samples was measured in a threeelectrode cell with Pt wire as the counter electrode, a saturated Ag/AgCl electrode as reference electrode and FTO glass coated with LaFeO 3 or LaFeO 3 /g-C 3 N 4 materials with a surface area of 1 cm² as the working electrode. A 0.1 M Na 2 SO 4 (pH = 7) aqueous solution was used as electrolyte. All measurements were carried out under light illumination of 1 sun (100 mW•cm -2 ) provided by a 300 W Xe arc lamp fitted with an AM 1.5G filter. Linear sweep voltammetry (LSV) measurements were performed by scanning the potential from -0.8 V to 0 V vs Ag/AgCl at a scan rate of 5 mV.s -1 . Chronoamperometric measurements were recorded at a constant potential of -0.30 V vs the reference electrode by using a SP150 BioLogic potentiostat, cycling between dark and light illuminations of 30 s each for 10 min for evaluating photocurrent density.

Electrochemical Impedance spectroscopy (EIS) was performed for Nyquist measurements.

Frequency range of 500 kHz -1 mHz and magnitude of the modulation signal of 10 mV were used. Applied potential was -0.3 V vs Ag/AgCl under 1 sun illumination. Mott-Schottky spectra were acquired at frequencies from 6 kHz to 10 kHz in the potential range of 0.6 V to 1.6 V vs the reference electrode in the dark.

Hydrogen photoproduction

The photocatalytic hydrogen production test was performed in a sealed custom-made metallic photoelectrochemical cell with an attached quartz viewport using the irradiation conditions previously described. The whole system was filled with 0.1 M Na 2 SO 4 (pH = 7) and purged with argon for 1 h before measurements. During the reaction, the amount of produced H 2 was determined with online quadrupole mass spectrometer (MS, Inficon Transpector MPS 100) operating with Ar as carrier gas. Measurements were realized at an interval of 1 h and averaged over 2 min.

Results and discussion

Materials characterization

LaFeO 3 /g-C 3 N 4 films on FTO were prepared using a three steps process depicted in Fig. 1.

First, exfoliated g-C 3 N 4 was coated on FTO via spin-coating. Next, La and Fe were deposited at the surface of g-C 3 N 4 by DC magnetron co-sputtering. Finally, crystalline LaFeO 3 was produced by air oxidation of La-Fe films at 500°C for 2 h. In order to reveal the structure, the composition and the morphology of the prepared samples, SEM, FT-IR, XRD and XPS analyses were conducted. Due to the high roughness of the FTO support, g-C 3 N 4 and LaFeO 3 layers could not be observed by SEM (Fig. S1 and2). The associated EDS mappings (Fig. 2) show that g-C 3 N 4 and LaFeO 3 /g-C 3 N 4 films are homogeneously distributed at the surface of FTO. The La/Fe ratio in La-Fe and LaFeO 3 films is of ca. 1.0 and the La/Fe/O ratio in the LaFeO 3 film is of 1.0/1.0/3.0 (Fig. S2), which indicates the successful deposition of the LaFeO 3 film at the surface of g-C 3 N 4 . When the same deposition process was conducted on Si instead of FTO, thicknesses of 50 and 100 nm could be determined by SEM for g-C 3 N 4 and LaFeO 3 layers (Fig. S3), respectively, which further confirms the efficiency of the magnetron co-sputtering method for the preparation of hierarchically structured films. Fig. 3a shows the FT-IR spectra of pure g-C 3 N 4 and of LaFeO 3 /g-C 3 N 4 on the FTO surface. For g-C 3 N 4 , the bands observed between 1625 and 1200 cm -1 can be ascribed to C-N and C=N bonds stretching vibrations while signals appearing at 1250 and 1205 cm -1 correspond to the C-NH-C stretching vibration between heptazine units [START_REF] Fu | g-C 3 N 4 -based heterostructured photocatalysts[END_REF][START_REF] Li | Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine[END_REF]. The sharp signal located at 810 cm -1 corresponds to the bending vibration of triazine units [START_REF] Moussa | Growth of ZnO nanorods on graphitic carbon nitride gCN sheets for the preparation of photocatalysts with high visible-light activity[END_REF][START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF][START_REF] Ouedraogo | Copper octacarboxyphthalocyanine as sensitizer of graphitic carbon nitride for efficient dye degradation under visible light irradiation[END_REF][START_REF] Li | Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine[END_REF][START_REF] Xiang | Preparation and enhanced visible-light photocatalytic H 2production activity of graphene/C 3 N 4 composites[END_REF]. The peaks of g-C 3 N 4 disappear after deposition of LaFeO 3 . The signal at 541 cm -1 is the characteristic stretching vibration of the Fe-O bond in the octahedral FeO 6 of crystalline LaFeO 3 [START_REF] Tang | Microwave-assisted synthesis of nanoparticulate perovskite LaFeO 3 as a high active visible-light photocatalyst[END_REF]. The two signals observed at 1469 and 1406 cm -1 can be attributed to the splitting of the ν 3 asymmetric stretching of La or Fe carbonates present at the surface of the film [START_REF] Kumar | Facile hydrothermal synthesis and characterization of LaFeO 3 nanospheres for visible light photocatalytic applications[END_REF]. The powdered g-C 3 N 4 shows a characteristic peak at 2Ɵ = 26.5° corresponding to the (002) plane and associated to the interlayer stacking of the conjugated aromatic systems (interlayer distance (d) of 0.32 nm) (Fig. 3b) [START_REF] Wang | A metal-free polymeric photocatalyst for hydrogen production from water under visible light[END_REF]. The intensity of the g-C 3 N 4 peak after deposition via spin coating on FTO is low compared to the signals of the substrate due to the weak amount of g-C 3 N 4 present at the FTO surface (estimated to be ca. 1 µg per cm²). The thickness of the g-C 3 N 4 layer is of ca. 50 nm compared to the 600 nm thickness of FTO. The signal of g-C 3 N 4 can be better observed after deposition on a Si substrate (Fig. S4). Finally, the peaks of LaFeO 3 deposited on g-C 3 N 4 can be indexed to the orthorhombic structure (JCPDS No 00-037-1493) [START_REF] Geller | Crystallographic studies of perovskite-like compounds. I. Rare earth orthoferrites and YFeO 3 , YCrO 3 , YAlO 3[END_REF]. The co-existence of the peaks related to LaFeO 3 and g-C 3 N 4 confirms the presence of these two materials on the FTO substrate and indicates that the g-C 3 N 4 layer is stable after the oxidative treatment following the deposition of La and Fe by magnetron co-sputtering.

Raman spectroscopy was also used to examine the structural phase of the materials prepared (Fig. 3c). For pure LaFeO 3 , La vibrations can be observed below 200 cm -1 while signals observed at 301 and 433 cm -1 correspond to A g and B 3 g modes, respectively. These values are in good agreement with those described in the literature for the orthorhombic structure of LaFeO 3 [START_REF] Li | Photoinduced charge property of nanosized perovskite-type LaFeO 3 and its relationships with photocatalytic activity under visible irradiation[END_REF]. The other peaks observed may originate from structural defects (cation or oxygen vacancies). The strong signal appearing at 625 cm -1 likely originates from two-phonon scattering and/or impurities as previously reported [START_REF] Venugopalan | Raman scattering study of LuFeO 3[END_REF]. After deposition of LaFeO 3 on g-C 3 N 4 , signals corresponding to CN heterocycles appear in the Raman spectrum [START_REF] Wang | Raman spectroscopy on the β-C 3 N 4 structure formed by low-energy nitrogen ion implantation into a diamond surface[END_REF], further confirming the successful association of g-C 3 N 4 with LaFeO 3 .

Optical and electrochemical measurements

The UV-visible absorption spectrum of the pure 50 nm-thick LaFeO 3 film is presented in Fig. 5a. The optical bandgap (Eg) was determined using the Tauc/David-Mott model, by plotting (αhv) 1/n vs. hv, where h is Planck's constant, α is the absorption coefficient γ is the photon frequency and n is 1/2 for direct allowed transitions or 2 for indirect allowed transition [START_REF] Davis | Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors[END_REF].

The Eg value was estimated by extrapolating the linear region of the plot onto energy axis (insets of Fig. 5a). The absorption spectrum clearly shows two absorption edges, located at approximately λ 1 = 350 nm and λ 2 = 480 nm. The optical properties of LaFeO 3 were studied by Scafetta et al. using the density functional theory (DFT) [START_REF] Scafetta | Band structure and optical transitions in LaFeO 3 : theory and experiment[END_REF]. The lowest bandgap was demonstrated to originate from a direct forbidden transition. Applying the same model to the present experimental data, a bandgap of ca. 2.28 eV was determined. This value is in good agreement with those described in the literature for nanocristalline LaFeO 3 [START_REF] Scafetta | Band structure and optical transitions in LaFeO 3 : theory and experiment[END_REF][START_REF] Köferstein | Magnetic and optical investigations on LaFeO 3 powders with different particle sizes and corresponding ceramics[END_REF]. A similar approach was also been used to determine the higher indirect transition at ca. 3 eV.

The high absorption in the visible region originating from the electronic transition from the valence band to the conduction band (O 2p → Fe 3d) [START_REF] Parida | Fabrication of nanocrystalline LaFeO 3 : an efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition[END_REF] should be beneficial for the photocatalytic activity under simulated solar light irradiation. Similar optical features were observed for the LaFeO 3 film deposited on g-C 3 N 4 (Fig. 5b). (

) 1
where C is the capacitance, e is the electronic charge, ε r is the relative permittivity of materials, ε 0 is the permittivity of vacuum, N D is the carrier concentration, k B is the Boltzmann's constant, T is the absolute temperature, A is the area of electrode, V is the applied potential and V fb is the flatband potential. Therefore, the flatband potential can be obtained by extrapoling the linear portion of the Mott-Schottky plot to V axis. A similar potential of +1.47 V vs Ag/AgCl was obtained for each applied frequency. The measured flatband potential can be converted to the RHE scale using the Nernst equation:

(

and was determined to be 2.08 V vs NHE. Assuming in first approximation that the flatband potential of a p-type semiconductor is very close to the valence band (VB) edge position [START_REF] Kumar | Facile hydrothermal synthesis and characterization of LaFeO 3 nanospheres for visible light photocatalytic applications[END_REF], the valence band maximum (VBM) of LaFeO 3 was estimated to be 2.08 V vs RHE. Combined with the bandgap of 2.28 eV, the conduction band minimum (CBM) of the LaFeO 3 film was estimated to be -0.2 V vs RHE. As the accuracy of the flatband value determined from Mott-Schottky plot may be debatable due to a possible influence of the surface roughness and the electrolyte [START_REF] Hamann | Measurement of the dependence of interfacial charge-transfer rate constants on the reorganization energy of redox species at n-ZnO/H 2 O interfaces[END_REF], a semiempirical method was also used to confirm the band positions. CBM and VBM values were calculated from Mulliken's electronegativity at point of zero charge using the following equation [START_REF] Lu | Novel CaCO 3 /g-C 3 N 4 composites with enhanced charge separation and photocatalytic activity[END_REF]:

(3) (4)
where E VB and E CB are the VB edge potential and CB edge potential respectively, χ is the absolute electronegativity of the semiconductor on Mulliken's scale (χ = 5.54 for LaFeO 3 ), E e is the energy of free electrons on the hydrogen scale (4.5 eV vs RHE) and E g is the bandgap energy estimated from UV-Vis absorption spectra. VBM and CBM positions were estimated

Potential (V vs RHE)

1.4 to be 2.18 and -0.1 V vs RHE, respectively. These theoretical calculations are in good agreement with Mott-Schottky plots.

VBM and CBM positions of g-C 3 N 4 were determined in [START_REF] Abdelaziz | One pot synthesis of bismuth oxide/graphitic carbon nitride composites with high photocatalytic activity[END_REF]. The corresponding energy diagram of LaFeO 3 and g-C 3 N 4 before contact is reported in Fig. 7a. LaFeO 3 exhibits VBM and CBM that straddle the redox potential of water. This suggest that LaFeO 3 is able to generate hydrogen using visible part of the spectrum without applying any external bias voltage as observed by Pawar et al. [START_REF] Pawar | Unbiased spontaneous solar fuel production using stable LaFeO 3 photoelectrode[END_REF]. However, the kinetics of production is very low due to the very close position of CB in regards to reduction potential of hydrogen. LaFeO 3 associated to g-C 3 N 4 will form a p-n heterojunction. PEC efficiency can be greatly influenced by the recombination rate of photo-generated charge carriers and p-n junctions generally promote the effective separation of light-induced electrons and holes. Before contact, CB and Fermi level positions of LaFeO 3 are lower than those of g-C 3 N 4 . At the junction, the Fermi level after contact, under equilibrium conditions, is equal and constant in g-C 3 N 4 and LaFeO 3 , causing an energy band bending. Indeed, after contact between p-type LaFeO 3 and n-type g-C 3 N 4 , electrons diffuse from g-C 3 N 4 to LaFeO 3 due to the initial positions of CB, leading to an accumulation of negative charges in LaFeO 3 close to the junction. At the same time, a positive section in g-C 3 N 4 is generated near the junction. As a consequence, the Fermi level and energy band positions of g-C 3 N 4 shift downward (to more positive potentials), while those of LaFeO 3 shift upward (to more negative potentials) to form an equilibrium state, where an inner electric field is generated and the CB of LaFeO 3 is higher than that of g-C 3 N 4 (Fig. 7b). Thus, photogenerated electrons on the CB of LaFeO 3 will transfer to that of g-C 3 N 4 .

Migration of electrons and holes are promoted by the internal field. Subsequently, the interface of the heterojunction will improve the separation of electron-holes pairs and reduce their recombination probability [START_REF] Liang | Constructing a novel p-n heterojunction photocatalyst LaFeO 3 /g-C 3 N 4 with enhanced visible-light-driven photocatalytic activity[END_REF].

Photoelectrochemical (PEC) analysis

Linear sweep voltammetry (J-V) curves of LaFeO 3 /g-C 3 N 4 were recorded under simulated sunlight illumination (100 mW•cm -2 , AM 1.5G), where a chopped beam was used for acquiring both dark current and photocurrent in a single scan at a rate of 5 mV.s -1 (Fig. 8a).

Each cycle lasts 25 mV. The steady state photocurrent onset potential was around 0.6 V vs RHE and the dark current remained stable until 0.25 V vs RHE.

To determine the LaFeO 3 on g-C 3 N 4 ratio giving the highest performance, the amount of each material was optimised from chronoamperometric tests under chopped illumination (30 s).

The potential was kept constant at -0.3 V versus Ag/AgCl (0.3 V vs RHE) to ensure the minimal impact of dark current in the 0.1 M Na 2 SO 4 (pH = 7) electrolyte solution. The photocurrent density varied with the amount of g-C 3 N 4 underneath 50 nm-thick LaFeO 3 (Fig. 8b). The photoelectrode elaborated using 1 µg of g-C 3 N 4 exhibits the highest photocurrent density (4 µA.cm -2 ). The thickness of the LaFeO 3 film deposited on 1 µg of g-C 3 N 4 was also found to affect the photocurrent density (Fig. 8c).

Optimal quantities for both materials were thus determined to be a 50 nm thick layer of LaFeO 3 deposited on top of 1 µg of g-C 3 N 4 with a maximum photocurrent density of 4 µA.cm - 2 . Spikes and transient photocurrent observed during each period can be attributed to charge carriers recombination. A large number of photoinduced electrons cannot reach the electrolyte before recombining with holes. The photocurrent density becomes stable only when the balance between charge generation, separation, transport and recombination processes is achieved. As expected, thickness of photoelectrode is major parameter that directly impacts the photocurrent density. The electron transport within LaFeO 3 is low and charge carriers recombination probability increases with thickness. By contrast, photoactive materials have to be thick enough to ensure light absorption leading to the formation of a high density of electron/hole pairs [START_REF] May | Thickness-dependent photoelectrochemical water splitting on ultrathin LaFeO 3 films grown on Nb:SrTiO 3[END_REF]. The observed higher photocurrent density of LaFeO 3 /g-C 3 N 4 by comparison with LaFeO 3 suggests an enhanced charge carriers separation in the heterostructured photoelectrode. Fig. 8d shows the Nyquist plots of EIS spectra for LaFeO 3 (50 nm) and optimised LaFeO 3 /g-C 3 N 4 under illumination. The semicircle diameter is related to the charge transfer resistance at the electrode/electrolyte interface. LaFeO 3 /g-C 3 N 4 has a smaller arc diameter than that of LaFeO 3 , which means a smaller charge transfer resistance. Both photocurrent and EIS results indicate that heterostructured LaFeO 3 /g-C 3 N 4 film exhibits more efficient electron-hole separation under simulated sunlight illumination, mainly due to the electrostatic field in the junction [START_REF] Yang | A novel p-LaFeO 3 /n-Ag 3 PO 4 heterojunction photocatalyst for phenol degradation under visible light irradiation[END_REF]. The stability is a crucial parameter to determine if a system is suitable for practical utilisation. In order to test the stability, a chronoamperometry measurement was conducted for LaFeO 3 /g-C 3 N 4 heterostructured photocathode at a constant potential of -0.3 V vs Ag/AgCl over a 24 h period under a continuous simulated sunlight illumination in a 0.1 M Na 2 SO 4 electrolyte solution (Fig. 9). After a slight drop in the first 2 h, the current density remained almost constant over the following 22 h (ca. 2.9 µA.cm -2 ), confirming excellent stability of the photoelectrode. The slight decrease of photocurrent at the beginning of the reaction is generally attributed to the accumulation of gas bubbles at the electrode/electrolyte interface leading to a reduced effective surface area [START_REF] Hernández | Considerations on oxygen bubble formation and evolution on BiVO 4 porous anodes used in water splitting photoelectrochemical cells[END_REF]. LaFeO 3 and g-C 3 N 4 are promising photoelectrode materials mainly due to their chemical and thermal stability. Moreover, LaFeO 3 thin films synthesized by magnetron sputtering have a dense columnar structure with no voids and pores. As a consequence, the electrolyte cannot deeply penetrate in the film and the effective surface area is low, leading to a relatively low photocurrent density. However, the compact nature of the films ensures a good stability of the photoelectrodes by limiting their degradation during a long period of operation. LaFeO 3 /g-C 3 N 4 heterostructured films can thus be used as photocathode for hydrogen production performed over several hours.

Hydrogen evolution measurement

Fig. 10a shows the hydrogen production rates of electrodes composed of g-C 3 N 4 , LaFeO 3 and optimized LaFeO 3 /g-C 3 N 4 in an aqueous Na 2 SO 4 (pH = 7) electrolyte under a continuous simulated sunlight illumination and at a constant potential of -0.3 V vs Ag/AgCl. The hydrogen production rate was monitored each hour (over 7 h) by mass spectrometry. The cell was first purged with argon for 1 h to ensure that oxygen from air was not involved when the reaction starts. It is worth noting that hydrogen production rates generated by these different photocathodes are in good agreement with current-potential curves.

Samples showed very similar hydrogen evolution rates with an initial activation period of about 3 h, followed by a stable and maximum production period. H 2 production rates of g-C 3 N 4 , LaFeO 3 and LaFeO 3 /g-C 3 N 4 in the steady states are 5.5, 6.2 and 10.8 µmol.h -1 .cm -2 , respectively. As a consequence, an increase of 74% in H 2 production rate is observed by depositing a g-C 3 N 4 sublayer underneath LaFeO 3 thin film. To determine the stability LaFeO 3 /g-C 3 N 4 heterostructured film, sample underwent 3 cycles of water splitting test (Fig. 10b). These results provide additional evidence that the film is reusable although a decrease of 30% is observed after the third cycle.

Based on the previously described experimental results, a possible mechanism can be proposed for the photogenerated charges separation and the PEC water splitting process (Fig. 10c). Under visible light irradiation, both g-C 3 N 4 and LaFeO 3 absorb photons and electron-hole pairs are generated. In the heterostructured film, photo-excited electrons in the CB of g-C 3 N 4 can transfer to the CB of LaFeO 3 while holes in the VB of LaFeO 3 move to the VB of g-C 3 N 4 , thus facilitating the separation of electron/hole pairs. In the overall PEC process, the reaction of hydroxyl anions OH -(or H 2 O) at the Pt counter electrode will produce electrons that can combine with holes in the VB of g-C 3 N 4 . Due to their more positive potential than the H + /H 2 couple, the electrons in the CB of LaFeO 3 can reduce H + into H 2 . The LaFeO 3 /g-C 3 N 4 photocathode benefits from the efficient charge carrier separation and exhibits a high PEC water splitting efficiency.

Conclusion

For the first time, a thin LaFeO 3 film was deposited using magnetron sputtering and associated with g-C 3 N 4 for PEC hydrogen production. g-C 3 N 4 was deposited directly on the substrate, prior to LaFeO 3 , using a simple spin-coating method. Deposition of a sublayer of g-C 3 N 4 improves the separation of charge carriers at the LaFeO 3 /g-C 3 N 4 interface, while optical properties of LaFeO 3 still allows it to absorb a large range of the visible spectra, increasing its efficiency. Optimal quantities of both materials were determined using chronoamperometry. Subsequently, water splitting test was performed to estimate the photocatalytic properties of the g-C 3 N 4 /LaFeO 3 heterojunction to produce hydrogen. Associating LaFeO 3 with g-C 3 N 4 led to hydrogen production rate of 10.8 µmol•h -1 •cm -2 and to an increased amount of produced hydrogen by 74% compared to pure LaFeO 3 . The hydrogen production remains high after three reuses, demonstrating the stability of the g-C 3 N 4 /LaFeO 3 film after further cycles of production and its ability to harvest light for days. Our work thus demonstrates a simple route to obtain well performant heterostructured materials systems for hydrogen production from low-cost, non-toxic and abundant components.

Fig. 1 .

 1 Fig. 1. Schematic illustration of the LaFeO 3 /g-C 3 N 4 film synthesis on FTO.

Fig. 2 .

 2 Fig. 2. (a) SEM image of LaFeO 3 /g-C 3 N 4 on FTO and EDS elemental mapping of (b) C, (c) N, (d) La, (e) Fe and (f) O.

Fig. 3 .

 3 Fig. 3. (a) FT-IR spectra of g-C 3 N 4 and of LaFeO 3 /g-C 3 N 4 deposited on FTO, (b) XRD patterns of g-C 3 N 4 , FTO, g-C 3 N 4 on FTO and LaFeO 3 /g-C 3 N 4 on FTO and (c) Raman

Fig. 4 .

 4 Fig. 4. High resolution XPS spectra of (a) Fe 2p, (b) La 3d, (c) O 1s and (d) C 1s for the LaFeO 3 /g-C 3 N 4 film.

Fig. 5 .

 5 Fig. 5. UV-visible optical absorption spectrum of (a) a 50 nm-thick LaFeO 3 film on FTO (the insets show the Tauc plots for a direct-forbidden and indirect-allowed model for the first and second transitions, respectively). (b) LaFeO 3 /g-C 3 N 4 film.

Fig. 6 .

 6 Fig. 6. Mott-Schottky plots for LaFeO 3 film (50 nm) at different frequencies determined from EIS measurements in the dark, in Na 2 SO 4 (pH = 7).

Fig. 7 .

 7 Fig. 7. Schematic diagrams of band energy positions of g-C 3 N 4 and LaFeO 3 (a) before contact and (b) the proposed electrons/holes transfer and separation processes after contact.

Fig. 8 .

 8 Fig. 8. Current-potential curve (a) and current-time curves of LaFeO 3 /g-C 3 N 4 heterojunctions at -0.3 V vs Ag/AgCl as a function of (b) the quantity of g-C 3 N 4 spin-coated on FTO (LaFeO 3 thickness = 50 nm) and (c) the LaFeO 3 film thickness (amount of g-C 3 N 4 = 1 µg) under chopped illumination. (d) EIS Nyquist plots of 50 nm thick LaFeO 3 and optimised LaFeO 3 /g-C 3 N 4 heterostructure (electrolyte: 0.1 M Na 2 SO 4 (pH = 7)).

Fig. 9 .

 9 Fig. 9. J-t plot at -0.3V vs Ag/AgCl of 50 nm LaFeO 3 deposited over 1 µg of g-C 3 N 4, insert is a detail of the first hour.

Fig. 10 .

 10 Fig. 10. (a) Hydrogen production rate of LaFeO 3 , g-C 3 N 4 and LaFeO 3 /g-C 3 N 4 photoelectrodes and (b) stability performances of LaFeO 3 /g-C 3 N 4 heterostructured film over 3 cycles (-0.3 V vs Ag/AgCl, 0.1 M Na 2 SO 4 , 100 mW.cm -2 -AM1.5G) and (c) Schematic illustration of the electron transfer pathway for visible light PEC hydrogen production using LaFeO 3 /g-C 3 N 4 films.

Acknowledgements

This work was supported partly by the French PIA project « Lorraine Université d'Excellence », reference ANR-15-IDEX-04-LUE.

Appendix A. Supplementary data

Supplementary data to this article can be found at http://doi.org/