# Copper substitution in lanthanum-deficient LaFeO3 perovskites for enhanced photoelectrochemical solar hydrogen production

Vincent Guigoz, Stéphanie Bruyère, Sylvie Migot, Raphaël Schneider, Thomas Gries

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Vincent Guigoz, Stéphanie Bruyère, Sylvie Migot, Raphaël Schneider, Thomas Gries. Copper substitution in lanthanum-deficient LaFeO3 perovskites for enhanced photoelectrochemical solar hydrogen production. International Journal of Hydrogen Energy, In press, 148, pp.150072. 10.1016/j.ijhydene.2025.150072. hal-05124597

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# Copper substitution in lanthanum-deficient LaFeO₃ perovskites for enhanced photoelectrochemical solar hydrogen production

Vincent Guigoz<sup>a,b ID</sup>, Stéphanie Bruyère<sup>a ID</sup>, Sylvie Migot<sup>a</sup>, Raphaël Schneider<sup>b ID</sup>, Thomas Gries<sup>a,* ID</sup>

## ARTICLE INFO

Keywords:

Perovskite thin film

La-deficient LaFeO₃

Copper oxide nanoparticles

Photoelectrochemical water splitting

Hydrogen production

## ABSTRACT

Copper substitution in B-site of lanthanum-deficient perovskite thin films led to significant improvements in photocatalytic properties, especially for hydrogen (H₂) production by photoelectrochemical (PEC) water splitting. Thin La₀.₇₈CuₓFe₁₋ₓOᵧ films were synthesized by magnetron co-sputtering, with the composition being finely adjusted from the power applied to the targets. La-deficient perovskites were found to promote surface segregation and agglomeration of copper oxide (CuO) following thermal annealing at 650 °C for 2 h. The growth of highly dispersed CuO nanoparticles with diameters typically between 20 and 50 nm was observed at the top surface, while copper remained present within the film. The impact of varying copper loading in perovskite films on their crystallographic structure, morphology, optical and photoelectrochemical properties was subjected to comprehensive investigation. The La₀.₇₈Cu₀.₁₈Fe₀.₈₂Oᵧ film was found to exhibit optimal performance, demonstrating a significant increase in H₂ production rates in comparison to undoped LaFeO₃. Under AM1.5G illumination (100 mW cm⁻²), the film exhibited a H₂ production rate of 12.3 µmol h⁻¹ cm⁻², which is a 170 % increase compared to undoped LaFeO₃ (4.5 µmol h⁻¹ cm⁻²). Moreover, the copper-doped electrodes were shown to be stable and retained a high activity (approximately 83 %) after the fifth cycle of reuse. These results demonstrate that copper-doped lanthanum-deficient films are a viable approach to enhance the H₂ photoproduction efficiency of LaFeO₃-based PEC electrodes under solar irradiation.

## 1. Introduction

Solar-driven hydrogen (H₂) production from water splitting represents a promising approach to address the global energy crisis of fuel production and its associated environmental challenges, because H₂ has a high energy density (143 MJ kg⁻¹) and this production method does not result in the release of toxic compounds [1]. One of the most promising methods for producing renewable H₂ is photoelectrochemical (PEC) water splitting. Nevertheless, the efficiency of semiconductor photocatalysts used still limits the solar H₂ conversion yield. Indeed, the following properties of the photocatalysts to be used as photoelectrodes are sought: (i) suitable band edge positions for water splitting half reactions to occur; (ii) a light absorption in the visible region of the solar spectrum; (iii) an efficient charge carrier separation and transportation and (iv) a good photochemical stability versus photocorrosion [2]. Moreover, in order to minimise expenditure and environmental impact, photoelectrodes should be composed of abundant and non-toxic

elements. Many materials used for H₂ and O₂ generation under visible light have been successfully synthesized for water splitting, such as carbon nitride, noble metal nanoparticles, and most reported metal oxides/nitrides/sulfides [3]. Many metal oxides behave as semiconductors and exhibit good stability in aqueous environments, making them attractive candidates for water splitting devices, including single metal oxides, oxides with more complex stoichiometry (i.e., ternary and quaternary oxides), spinel oxides or perovskite oxides [4].

Versatility of perovskite oxides (ABO₃) allows establishing semiconductors that exhibit the main aforementioned characteristics [5]. Among the various ABO₃ materials, lanthanum ferrite perovskite (LaFeO₃), a p-type semiconductor with a suitable bandgap of ca. 2.1–2.4 eV was investigated as a promising candidate for solar light water splitting photocathode due to its earth-abundant components, its low toxicity and its high stability in aqueous solution [6,7]. Nevertheless, the photocatalytic activity of the pristine LaFeO₃ is relatively low due to the rapid recombination of photogenerated charge carriers and the poor

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, F-54000, Nancy, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, LRGP, F-54000, Nancy, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: thomas.gries@univ-lorraine.fr (T. Gries).</footnote>
charge transport in the bulk. Recently, a variety of strategies have been applied to improve the PEC performances of LaFeO₃ photoelectrodes, including the construction of heterostructured catalysts [8], the control of morphology [9], defect engineering [10] and the introduction of oxygen vacancies [11] or cocatalysts [12]. Among these approaches, transition metal doping has emerged as a particularly effective method for improving PEC performances.

Indeed, ABO₃ perovskites demonstrate considerable flexibility in accommodating a range of transition metal element substitutions at tetrahedral sites (A sites) or octahedral sites (B sites). This composition versatility (La₁-xAₓFe₁-yBᵧO₃-δ) gives rise to geometrical and charge constraints that modify the textural, structural and electronic properties of the material and consequently tune its redox and catalytic properties [13]. Such doping approach has the potential to introduce defects that modify the bandgap, as well as cause oxygen vacancies, which could inhibit the recombination of electrons and holes on the catalyst surface. Therefore, the incorporation of divalent or trivalent cations as B-site dopants has been investigated, resulting in a notable modification in the photocatalytic activity of LaFeO₃ upon the introduction of transition metal ions into its crystal lattice, including Cr, Co, Ni, Zn, Mn, etc. [14–20]. Despite the lack of consensus on the most appropriate dopant to use on each site [21], Cu is an attractive non-ferromagnetic dopant for the B site offering high electronic conductivity. The introduction of copper as a substituting cation in LaFeO₃ has already been reported to enhance its photocatalytic activity [14,22,23]. For example, Parrino et al. [22] synthesized copper-substituted lanthanum ferrite by the citrate auto-combustion method and observed an enhancement in the photocatalytic degradation of 2-propanol with copper substitution up to 10 mol%. However, further increases in copper content resulted in a slight decrease in degradation efficiency.

On the other hand, previous investigations evidenced a notable improvement in photocatalytic activity for lanthanum deficient LaFeO₃ in comparison to the stoichiometric counterpart [23,24]. Specifically, in order to further improve the activity of perovskite oxide materials, a combination of A-site deficiency associated with transition metal doping of B-site has been attempted. Especially, Wu et al. [25] recently reported that copper substitution in the B site of La-deficient La₀.₇FeO₃ perovskites led to significant improvements in the kinetics of CO and propane oxidation. This was attributed to surface copper segregation leading to more active extra-framework copper oxide species in CO oxidation.

The present study offers a comprehensive and systematic comparative analysis of copper substitution in lanthanum-deficient La₀.₇₈CuₓFe₁ₓOᵧ perovskite oxide photocatalysts with the objective of elucidating the beneficial role of Cu-substituted B-site element on H₂ photoproduction by PEC water splitting. Recently, extensive research has been conducted to synthesize Cu-doped LaFeO₃ primarily through the citrate auto-combustion method of a dry gel obtained from solutions composed of weighted amounts of nitrate salts of Fe, La and Cu which react with citric acid. This approach has been documented in several studies [26–28]. It is widely acknowledged that the synthesis methods employed have a significant impact on the physicochemical and catalytic properties of perovskites. For this reason, this paper presents, for the first time, the growth of Cu-doped La-deficient La₀.₇₈CuₓFe₁ₓOᵧ films synthesized by the magnetron co-sputtering technique. This technique allows the film composition to be finely tuned by adjusting the power applied to each target. The photocurrent density and H₂ evolution rate (HER) measurements demonstrate that the photocatalytic properties are markedly enhanced for an optimal copper content within the film (x = 0.18), resulting in a considerable increase in H₂ production rate (+170 %) by comparison with stoichiometric LaFeO₃ films.

## 2. Experimental section

### 2.1. Elaboration of La₀.₇₈CuₓFe₁ₓOᵧ thin films

Metallic La₀.₇₈CuₓFe₁ₓ thin films were deposited on fluorine doped

tin oxide coated glass (FTO, TEC 7 Ω sq⁻¹) and silicon substrates by direct current (DC) magnetron co-sputtering from two metallic targets: a pure lanthanum target (diameter: 50 mm, thickness: 3 mm, purity: 99.9 %) and a pure metallic copper target (diameter: 50 mm, thickness: 0.25 mm, purity: 99.9 %) partially covered with pure iron sheets (thickness: 0.25 mm, purity: 99.5 %). Various Fe coverage of the Cu target surface were investigated and the surface coverage was subsequently correlated with the composition of the film (Fig. S1). The film thickness was set at 500 nm, which was determined to be the optimal value for improving the PEC yield (vide infra). The power applied to each target was adjusted in order to maintain constant lanthanum content and thickness (Table 1). The other deposition parameters were kept constant: the target-to-substrate distance was set at 10 cm, the working pressure was of 0.58 Pa in a 10 vol.% H₂ – 90 vol.% Ar mixture (15 sccm) and the substrate-holder rotation was fixed at 28 rpm to ensure homogeneity.

Prior to deposition, substrates were cleaned in ultrasonic baths of acetone and ethanol successively for a duration of 10 min each. As the samples were not intentionally heated during deposition, a post-annealing treatment was conducted in a tubular furnace at 650 °C for 2 h under air to ensure the oxidation and crystallization of the perovskite structure. Following thermal treatment, samples changed in color from ochre-yellow to reddish-brown hue as the copper concentration within the film increased (Fig. S2).

The La/(Cu+Fe) atomic ratio was set at 0.78 to investigate the impact of substituting Fe by Cu in A-site deficient perovskite material while maintaining a similar photocurrent density to that obtained for pristine LaFeO₃ in absence of copper, as previously reported [24].

### 2.2. Physico-chemical characterization

The crystalline phases of the perovskites were evaluated by X-ray diffraction (XRD) performed at room temperature on a D8 Advance AXS Bruker diffractometer (Cu Kα source, λ = 1.54056 Å). XRD data were collected in a θ/2θ Bragg-Brentano configuration with a step scan of 0.02°. The morphology and elemental composition of the films were estimated by energy dispersive spectroscopy (EDS) using a Zeiss Gemini scanning electron microscope (SEM) 500 equipped with a field emission gun (FEG). The film topography was examined in tapping mode from a Nano-Observer atomic force microscope (AFM) from CSInstruments. Lamella samples from the films were extracted using a focused ion beam (FIB) from a FEI Helios Nanolab 600i SEM dual beam system. Transmission electron microscopy (TEM) and chemical analysis on the extracted lamellae were conducted from a cold FEG JEOL ARM 200 microscope operating at 200 kV and equipped with spherical aberration (Cs) probe and image correctors (point-resolution 0.12 nm in TEM mode). High-angle annular dark field image in scanning TEM (HAADF-STEM) and two-dimensional (2D) elemental mapping using energy dispersive X-ray spectroscopy (EDX) were combined to investigate elemental distribution along the film thickness. The sample thicknesses were measured with a Bruker Dektak XT stylus profilometer. UV-visible absorption spectra of the films were measured from a Cary 7000 UMS

<caption>**Table 1**</caption>

Deposition conditions: power applied on each target, Fe coverage of Cu target surface and composition of La₀.₇₈CuₓFe₁ₓOᵧ thin films synthesized by magnetron co-sputtering and annealed at 650 °C for 2 h in air (thickness = 500 ± 10 nm, deposition duration = 11 min).

<table><thead><tr><th>Power on La target (W)</th><th>Power on Fe-Cu target (W)</th><th>Fe coverage (%)</th><th>Sample composition (x) La₀.₇₈CuₓFe₁ₓOᵧ</th></tr></thead><tbody><tr><td>60</td><td>80</td><td>100</td><td>0</td></tr><tr><td>60</td><td>78</td><td>98</td><td>0.03</td></tr><tr><td>58</td><td>73</td><td>93</td><td>0.10</td></tr><tr><td>58</td><td>67</td><td>87</td><td>0.18</td></tr><tr><td>55</td><td>64</td><td>84</td><td>0.22</td></tr><tr><td>55</td><td>60</td><td>77</td><td>0.30</td></tr></tbody></table>
Agilent spectrophotometer using an integrating sphere.

## 2.3. PEC measurements and $H_2$ photoproduction

PEC measurements were performed at room temperature in a quartz cell filled with 300 mL of 0.1 M $Na_2SO_4$ (pH = 7). The PEC signal was recorded using a potentiostat (BioLogic SP150) connected to a standard three-electrode configuration: thin films deposited on FTO glass substrates acted as the working electrode (1 cm × 1 cm), a Pt coil as the counter electrode and an Ag/AgCl electrode in saturated KCl as the reference electrode. The photocurrent density obtained from chloroamperometric measurements were assessed at -0.3 V vs Ag/AgCl by cyclic exposure to light illumination of 1 sun (100 mW cm⁻²) provided by a xenon lamp (Newport Xe 300 W ozone free) fitted with an air mass filter (AM 1.5G). Prior to measurement, the cell was systematically purged with Ar for 1 h to remove the dissolved oxygen in the electrolyte.

Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range from 500 kHz to 10 mHz using a 10 mV sinusoidal AC amplitude perturbation in dark conditions. Mott Schottky measurements were recorded in the potential range of 0.8–1.6 V vs Ag/AgCl, while Nyquist plots were recorded at open circuit potential (OCP).

Hydrogen and oxygen evolutions from water splitting were carried out in a sealed custom-made metallic cell with a quartz window using the same experimental conditions as described for PEC measurements. The amounts of gases produced were determined during the reaction through the use of a system-connected online quadrupole mass spectrometer (MS, Inficon Transpector MPS 100) that used Ar as carrier gas (Fig. S3). Knowing the Ar flow rate introduced, a quantitative determination of the instantaneous production of $H_2$ and $O_2$ was performed from the composition of the gas mixture measured by MS after calibration. Measurements were taken every hour and averaged over a 2-min period.

## 3. Results and discussion

XRD patterns recorded on $La_{0.78}Cu_xFe_{1-x}O_y$ films deposited on silicon substrates and annealed at 650 °C in air are reported in Fig. 1a. XRD reflections of all samples can be indexed by an orthorhombic unit cell of $LaFeO_3$ perovskite phase with the space group $Pnma$ (PDF file 04-011-7994) without preferential orientations. No additional reflections are discernible such as $CuO$, $Fe_2O_3$ or $La_2O_3$, indicating all doped samples retain the perovskite crystal structure. However, as the Cu content

within the films increases, the intensity of diffraction peaks decreases and becomes slightly broader, which is characteristic of a reduction in crystallinity up to an amorphous pattern for $La_{0.78}Cu_{0.3}Fe_{0.7}O_y$. This result could be attributed to distortions and defects due to Cu doping, which limit the growth of large crystallites in the samples [22].

In addition, a slight shift towards higher $2\theta$ values is observed with increasing Cu concentration (Fig. 1b), which is in good agreement with results reported for copper-doped $LaFeO_3$ powders elaborated by chemical processes [25,28–32]. This contraction effect on the unit cell volume seems to be in contradiction with the expected expansion effect due to the larger ionic radius of $Cu^{2+}$ (0.72 Å) than that of $Fe^{3+}$ (0.64 Å). Nevertheless, this expansion could be counterbalanced by oxygen release with the associated formation of ionic vacancies, which induces a contraction effect on the perovskite structure to maintain the charge balance in copper-doped $LaFeO_3$ [33,34].

Linear sweep voltammetry (LSV) curves under chopped light at 1 sun are reported in Fig. S4. To evaluate the effect of Cu doping on the PEC activity, chloroamperometric measurements were performed under chopped illumination from simulated sunlight source (1 sun). The photocurrent densities of $La_{0.78}Cu_xFe_{1-x}O_y$ samples acting as photocathodes are shown in Fig. 2a for different copper content. The applied potential was set at -0.3 V vs Ag/AgCl (=0.3 V vs RHE) to ensure a dark current close to 0 µA as observed in our previous studies on lanthanum orthoferrite thin films [8,24]. Short spikes and transient photocurrent observed mainly upon illumination can be attributed to charge carrier recombination caused by an effective trapping of photogenerated electrons at the surface. All samples exhibit a cathodic current increase upon illumination, which is characteristic of a p-type semiconductor. It is important to note that the photocurrent density measured for pristine perovskite ($LaFeO_3$) and undoped non-stoichiometric film ($La_{0.78}FeO_y$) are almost similar with values close to -2.7 µA cm⁻² (Fig. S5). Fig. 2b shows the corresponding photocurrent density in absolute value recorded at the end of the light exposure period. These values were obtained after 7 min of measurement (t ≥ 420 s) confirming the high stability of these copper-doped lanthanum-deficient perovskite oxide films. Cu-doped electrodes show much higher photocurrent densities. The steady-state photocurrent increases with copper concentration to reach an optimal value of -11.7 µA cm⁻² for $La_{0.78}Cu_{0.18}Fe_{0.82}O_y$, which is ca. 4.3 times higher than the photocurrent density of undoped $La_{0.78}FeO_y$ film, synthesized under the same experimental conditions. For higher copper content, the photocurrent density decreases sharply. A similar behavior was observed by Peng et al. [14] for transition metal doped $LaFeO_3$ elaborated by the citrate auto-combustion method: excessive

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) XRD patterns of $La_{0.78}Cu_xFe_{1-x}O_y$ thin films deposited on silicon substrates and annealed at 650 °C in air for 2 h and (b) magnification of the peak corresponding to plane (112) in the 32.0–32.8° 2θ range (x = 0, 0.03, 0.1, 0.18, 0.22 and 0.3). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Transient current density photo-response for different La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> thin electrodes under chopped illumination (1 sun) and (b) stationary photo-current densities in absolute value as a function of x for La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> electrodes (thickness = 500 nm) at a fixed potential of 0.3 V vs RHE in 0.1 M Na<sub>2</sub>SO<sub>4</sub> aqueous solution.</figcaption></figure>

dopants could act as recombination centers, which would inhibit the transfer of electrons and holes to the surface.

In addition, photocurrent measurements carried out on FTO elec-
trodes coated with La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> thin films of different thick-
nesses show an optimal photocatalytic activity for a 500 nm-thick film
(Fig. S6). A crucial parameter in PEC applications is the photoelectrode
thickness, since increasing the thickness of the photocatalyst increases
light absorption and thus the photocurrent. However, when its thickness
becomes comparable to the carrier diffusion length, bulk recombination
starts to dominate and the photocurrent decreases [6,35]. Thus, a
thickness of 500 nm seems to be optimized for this copper-doped lan-
thanum-deficient perovskite oxide film to further enhance the photo-
electronic properties.

The change in electron transfer kinetics of La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> films was investigated by EIS measurements as a function of the copper content using Nyquist plots (Fig. 3). The impedance spectra were fitted by a Randles equivalent circuit (inset in Fig. 3) including a solution resistance R<sub>s</sub>, a charge transfer resistance R<sub>CT</sub>, a constant phase element CPE and a Warburg impedance W. The corresponding parameters are given in Table S1. The evolution of R<sub>CT</sub> with copper content is shown in Fig. S7. As expected from the decrease of the semicircle diameter with copper on Nyquist plots, lowest values of R<sub>CT</sub> are obtained for La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> electrodes with x typically higher or equal to 0.18. Similar results were obtained by Peng et al. [14] with lower impedance values for LaFeO<sub>3</sub> after doping with Cu, suggesting an improved effective charge transfer efficiency at the photocatalyst interfaces. Both photocurrent and EIS measurements show that copper-doped lanthanum orthoferrite films

exhibit more efficient electron-hole separation and transport within the
photoelectrode than undoped perovskite photoelectrodes.

As expected from the magnetron sputtering process, EDS-SEM
elemental mappings confirm that Fe, La and Cu are homogeneously
distributed on the surface of FTO at micrometric scale (Fig. S8). To
gather more details on the phase distribution within the photo-
electrodes, cross-sectional TEM investigations were carried out on a thin
lamella extracted by FIB from the optimized La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> film
deposited on silicon substrate and oxidized under air at 650 °C for 2 h
(Fig. 4). The corresponding EDS-STEM elemental maps show a depth-
dependent evolution of the composition. A copper segregation is
observed at the film-substrate interface with the formation of a 15 nm-
thick copper oxide layer. A thin layer (~5 nm) with an excess of Fe is
also observed on the top of this copper oxide layer. Furthermore,
segregation of copper also occurs on the upper surface of the film with
the growth of copper oxide nanoparticles with diameters typically be-
tween 20 and 50 nm (Fig. 4 and S9), while copper remains present
within the film. All these nanoparticles on the surface are composed
solely of the CuO phase, which is consistent with the post-treatment
conditions, achieved by annealing in air at 650 °C for 2 h. The surface
topography recorded by AFM images confirms the roughness due to the
formation of nanoparticles at the upper surface of the film (Fig. S10).

HRTEM measurements were carried out to investigate a single
nanoparticle grown *in situ* during the thermal treatment process and its
interface with the film (Fig. 5). Fig. 5a shows a nanoparticle with a near
spherical shape and an estimated diameter around 25 nm. The interface
with the film shows neither obvious porosity nor a high density of de-
fects, suggesting a firm anchorage of the nanoparticle, promoting high
mechanical resistance and the formation of heterojunctions, which are
expected to improve photogenerated charge separation and transfer.
High magnification image of a particle shows the crystal lattices all in
the same orientation with a constant period, indicating a mono-crystal
growth (Fig. 5b). As shown in the intensity histogram (Fig. 5c) extrac-
ted from the line profile in Fig. 5a, a periodic spacing between atomic
planes of 2.34 Å was measured, which is in good agreement with the
(111) plane of the CuO monoclinic structure (PDF file 04-007-1375,
space group C2/c).

Within the film, the measured lattice spacings correspond to planes
of orthorhombic LaFeO3, which is in good agreement with XRD results
(Fig. S11). The fast Fourier transform (FFT) patterns recorded from
HRTEM micrographs further confirm the high crystallinity of the
La0.78Cu0.18Fe0.82Oy film after annealing in air at 650 °C for 2 h.

The phenomenon of cation enrichment or segregation near surfaces or interfaces has been widely studied in recent years, especially for perovskite oxide materials. These researches have contributed to the development of rational design for a diverse range of applications including PEC water splitting systems [36]. Li et al. [12] provided an overview of the recent progress made in the understanding of segregation mechanisms in several widely studied perovskite-based material

<figure><img src="image_6.png" /><figcaption>Fig. 3. EIS Nyquist plots of La<sub>0.78</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>y</sub> coated FTO electrodes as a function of Cu content and corresponding equivalent circuit. (in 0.1 M Na<sub>2</sub>SO<sub>4</sub> at OCP under dark conditions).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Cross-section STEM-HAADF micrograph of La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> film deposited on Si substrate and oxidized in air at 650 °C for 2 h and the corresponding EDS maps of (b) copper, (c) iron, (d) oxygen and (e) lanthanum. (The interface between the silicon substrate and the film is indicated by white dotted lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) TEM cross-sectional micrograph recorded at the upper surface of the La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> film, (b) micrograph at higher magnification in the region delimited by a dashed red frame and (c) intensity histogram showing a periodic spacing between atomic planes extracted along dashed blue line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

This heterogeneous process is influenced by combined factors including the nature of the materials (e.g. cation non-stoichiometry, dopant species, crystallinity, lattice strain, etc.) and environmental conditions (e.g. gas environment, temperature, polarization, etc.). In the case of lanthanum orthoferrite, Wu et al. [25] reported that La-deficient La<sub>0.7</sub>Cu<sub>x</sub>Fe<sub>1-x</sub>O<sub>3</sub> perovskite were found more prone to surface copper segregation, leading to the formation of well-dispersed extra-framework CuO aggregates than LaCu<sub>x</sub>Fe<sub>1-x</sub>O<sub>3</sub> after calcination in air at 600 °C for 8 h. The segregation and agglomeration processes of copper occur much faster on La-deficient perovskites. Similar findings were reported by Nandi et al. [27]. The atomic diffusion mechanism during the final thermal treatment indicates that cation segregation is thermodynamically favourable. This may be attributed to higher co-segregation energy of Cu cations compared to Fe cations, as well as the thermodynamic stability of LaFeO<sub>3</sub> at high temperature.

To further examine the photocatalytic properties of copper-doped lanthanum-deficient perovskite synthesized by magnetron sputtering technique, the hydrogen production efficiency was measured on a

La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> electrode and on a pristine LaFeO<sub>3</sub> perovskite electrode elaborated under similar conditions for comparison (Fig. 6a). In the absence of photocatalysts or light irradiation, no H<sub>2</sub> was detected in either case at 0.3 V against RHE in 0.1 M Na<sub>2</sub>SO<sub>4</sub>. Similar results were obtained with catalyst and light, in unbiased conditions. The LaFeO<sub>3</sub> photoelectrode exhibited an average H<sub>2</sub> production rate of 4.5 µmol h<sup>-1</sup> cm<sup>-2</sup> under solar irradiation of 1 sun, whereas the La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> photoelectrode reached an average value of 12.3 µmol h<sup>-1</sup> cm<sup>-2</sup>. It is noteworthy that the H<sub>2</sub> production rates are 2.7 times higher (+170 %) simply by modifying the composition of the photocatalyst, which can be readily achieved through magnetron sputtering processes. These findings are also in agreement with the results of the photocurrent density measurements (Fig. 2).

Furthermore, a simultaneous O<sub>2</sub> evolution rate was also measured in the cell from the La<sub>0.78</sub>Cu<sub>0.18</sub>Fe<sub>0.82</sub>O<sub>y</sub> electrode, with a mean value of about 5.8 µmol h<sup>-1</sup> cm<sup>-2</sup> (Fig. 6b). This average value is in close alignment with the half of the H<sub>2</sub> evolution rate (O<sub>2</sub>/H<sub>2</sub> ratio of approximately 0.47) as would be expected from the overall water
<figure><img src="image_9.png" /><figcaption>Fig. 6. (a) H₂ production rates of FTO electrodes coated by LaFeO₃ and La₀.78Cu₀.18Fe₀.82Oᵧ thin films by light induced water splitting and (b) H₂ and O₂ evolution rates for La₀.78Cu₀.18Fe₀.82Oᵧ electrode (at 0.3 V vs RHE in 0.1 M Na₂SO₄ under solar illumination of 1 sun).</figcaption></figure>

splitting reaction. This result was confirmed by the pH stability of the electrolyte during H₂ and O₂ photoproduction tests, with only a slight decrease over an extended illumination period of 8 h (from 7.3 to 6.7). Consequently, the concentrations of HO⁻ and H₃O⁺ ions in the electrolyte remain almost constant, thereby reducing the likelihood of photoelectrode degradation throughout the process.

The stability and reusability of La₀.78Cu₀.18Fe₀.82Oᵧ photocatalytic films were investigated for H₂ evolution in the same experimental conditions. As illustrated in Fig. 7, H₂ production rate exhibits a slight decline during the initial three cycles (from 12.3 to 10.8 µmol h⁻¹ cm⁻²), followed by a stabilization (~10.2 µmol h⁻¹ cm⁻²), indicative of a high photostability. The photocatalytic stability of the perovskite oxide material is not affected by the A-site deficiency and the copper doping effect, which lead to diffusion and segregation mechanisms. This is a crucial parameter for their practical application as photoelectrodes for PEC water splitting.

The recorded UV-Visible absorption spectra of LaFeO₃ and La₀.78CuₓFe₁₋ₓOᵧ (x = 0 and 0.18) are displayed in Fig. 8a. As observed in previous studies [22,28], an increase in absorption is observed in the visible region for the copper-substituted sample. This can be connected with the evolution of film colour with copper content (Fig. S2). Optical bandgap energies were estimated using the Tauc plot method (Fig. 8b), whereby the function (αhν)¹/ⁿ was plotted against hν and the linear portion of the curve was extrapolated to zero absorption, where h is Planck's constant, α is the absorption coefficient, ν is the photon frequency and n denotes the nature of the transition [37]. As expected, the absorption spectrum of LaFeO₃ displays two distinct transition edges, occurring at about 350 nm and 480 nm. Scafetta et al. [38] demonstrated from density functional theory (DFT) calculations and experimental measurements that the lower energy transition is consistent with a direct forbidden transition (n = 3/2), the other transition being attributed to an indirect allowed transition. A bandgap value of ca. 2.27 eV was determined for LaFeO₃, which is in good agreement with the data reported in the literature for nanocrystalline LaFeO₃. The same model

was applied to lanthanum-deficient films, yielding comparable values for La₀.78FeOᵧ and La₀.78Cu₀.18Fe₀.82Oᵧ (2.22 eV and 2.21 eV, respectively). A slight evolution of bandgap energy was previously observed for copper-doped LaFeO₃ powder, attributed to lattice contraction caused by Cu substitution by Pan et al. [31], whereas Nekkach et al. [39] explained this evolvement by considering the Burstein-Möss effect. Nevertheless, there is a discrepancy in the literature regarding the nature of the electronic transition and several articles report a direct allowed transition (n = 1/2) for rare-earth orthoferrites RFeO₃ (R=La, Pr, Nd or Sm) [40]. However, the nature of the transition has no significant influence on the difference in bandgap value between La₀.78FeOᵧ and La₀.78Cu₀.18Fe₀.82Oᵧ and therefore cannot explain the large improvement of photocatalytic properties observed for the copper-doped perovskite film.

Similarly, Mott-Schottky plots for La₀.78Cu₀.18Fe₀.82Oᵧ (Fig. 8b) and La₀.78FeOᵧ (Fig. S12) exhibit a comparable behaviour at different frequencies in the dark. The negative slope implies a p-type semiconductor in both cases. The flatband potential can be estimated by identifying the point of intersection between the linear extrapolation of the Mott-Schottky curve and the x-axis. The flatband potentials of these samples were found to be at ca. 1.46 V and 1.42 V vs Ag/AgCl electrode for La₀.78Cu₀.18Fe₀.82Oᵧ and La₀.78FeOᵧ, respectively (or 2.07 V and 2.03 V vs RHE). The outcomes are in close alignment with those obtained for pristine LaFeO₃ thin film deposited on FTO electrodes (2.08 V vs RHE) as reported in Ref. [8]. Typically, the flatband potential of p-type semiconductors is negative by 0.1-0.3 eV, compared to the valence band edge potential. Given that the differences in bandgaps and band edge positions of La₀.78Cu₀.18Fe₀.82Oᵧ and La₀.78FeOᵧ were minor, the large improvement in photocatalytic properties and hydrogen production yield cannot be attributed to an evolution of the electronic band structure. Nevertheless, the substitution of Fe by Cu in the lanthanum-deficient perovskite can promote different mechanisms that reduce electron/hole recombination:

<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) H₂ production rate from La₀.78Cu₀.18Fe₀.82Oᵧ photoelectrode over 5 cycles (at 0.3 V vs RHE in 0.1 M Na₂SO₄ under solar illumination of 1 sun) and (b) corresponding average value recorded for each cycle.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** (a) UV–Visible absorption spectra of LaFeO₃, La₀.78FeO₃ and La₀.78Cu₀.18Fe₀.82O₃ films. The inset shows the corresponding Tauc plots used to estimate the bandgap. (b) Mott-Schottky plots for La₀.78Cu₀.18Fe₀.82O₃ films (500 nm) at different frequencies from EIS measurements in the dark (in 0.1 M Na₂SO₄, pH = 7).</figcaption></figure>

(1) The presence of copper within the film as well as the presence of oxygen vacancies resulting from the incorporation of a dopant with a different oxidation state in perovskite material, may act as electron traps, thereby reducing charge carrier recombination and consequently enhancing the photocatalytic activity [22,41].

(2) The presence of highly-dispersed extra-framework CuO nanoparticles on the perovskite film may also reduce the recombination by the development of an efficient electric field driving electron-hole separation at the interface of the two semiconductors [42]. Significant enhancements in photocatalytic activity have been previously reported for CuO nanoparticles dispersed on the surface of different semiconductors, attributed to this cocatalytic effect [25,43–46].

Additionally, enhanced effective charge transfer efficiency at the photocatalyst interfaces was observed from Nyquist plots for copper-doped samples, leading also to an optimal photocatalytic activity. The improved photocatalytic activity observed for lanthanum-deficient copper-doped perovskite could also be attributed, to a lesser extent, to the slightly higher surface area resulting from the growth of CuO NPs, which is beneficial for any catalytic reactions [47].

## 4. Conclusion

In this study, copper-doped lanthanum-deficient perovskite (La₀.78CuₓFe₁-xOᵧ) thin films were deposited by magnetron co-sputtering for H₂ production by PEC processes. The film composition can be precisely adjusted by modifying the power applied to each target. As expected for La-deficient perovskite, HRTEM revealed that copper segregation and agglomeration occur on the surface following thermal annealing. These mechanisms were supported by the presence of highly-dispersed extra-framework CuO nanoparticles (diameter: 20–50 nm), while copper remained present within the film. An optimal Cu-doping of x = 0.18 is associated with the highest photocatalytic properties, as confirmed by a significant increase in both the photocurrent density and the H₂ production rate when compared to an undoped LaFeO₃ film. Under solar irradiation, the H₂ production rate increased from 4.5 µmol h⁻¹ cm⁻²–12.3 µmol h⁻¹ cm⁻² (+170 %), while the photocurrent density increased from –2.7 µA cm⁻² to –11.7 µA cm⁻² (+333 %). Furthermore, the photocatalyst displays remarkable stability over multiple recycling tests, exhibiting only a slight decline after 5 cycles. EIS measurements indicate an enhanced charge transfer at the surface of the Cu-doped perovskite. The substitution of Fe by Cu within the film and the growth of CuO nanoparticles on the surface of the film may also reduce the charge carrier recombination. This study demonstrates that the synthesis of copper-doped lanthanum deficient perovskites (La₀.78CuₓFe₁-xOᵧ) by magnetron co-sputtering represents a promising approach for the development of high-efficiency and stable

photoelectrodes for H₂ production by PEC.

## CRediT authorship contribution statement

**Vincent Guigoz:** Writing – original draft, Methodology, Investigation, Conceptualization. **Stéphanie Bruyère:** Writing – review & editing, Investigation. **Sylvie Migot:** Writing – review & editing, Investigation. **Raphaël Schneider:** Writing – review & editing, Validation, Project administration, Funding acquisition, Conceptualization. **Thomas Gries:** Writing – review & editing, Validation, Project administration, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work was supported partly by the French PIA project « Lorraine Université d'Excellence », reference ANR-15-IDEX-04-LUE and by the project HyPE (FRCR program funded by Région Grand-Est).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2025.150072.

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