*3.2. Synthesis of LaFeO3 by the Citric Acid Assisted Sol-gel Method*

The lanthanum ferrite perovskite was synthesized by the citric acid assisted sol-gel method [23]. In a typical synthesis, Fe(NO3)3·9H2O (0.0041 moles), La(NO3)3·6H2O (0.0041 moles) and citric acid were separately dissolved in deionized water under continuous stirring in a 1:1:4 molar ratio. A high surplus of citric acid was required to chelate the metal cations and to prevent aggregation. The obtained dark yellow, clear and transparent solution was obtained after the powders were completely dissolved in the solution. From that, the viscous gel was formed by heating at 300 ◦C for 2–3 h under continuous magnetic stirring. Subsequently, the solvent was evaporated by combustion in a pyrolysis setup at 400 ◦C for 1 h. The resulting fluffy powder, which was used as a precursor for LaFeO3, was crushed to a fine powder and subsequently calcined at different temperatures for 4 h in air. After cooling, the obtained samples were characterized and tested in the photocatalytic degradation. They were named S-700, S-750, S-850 and S-900 according to the calcination temperature applied.

#### *3.3. Characterization*

The crystalline phase and size of the obtained LaFeO3 nanoparticles were checked with a Empyrean theta-theta X-ray diffraction system (PANalytical, Almelo, The Netherlands) operating with Cu Kα radiation (λ = 1.540598 nm) at 40 kV and 40 mA in the 2θ range of 20–80◦. The Brunauer-Emmett-Teller (BET) surface areas were calculated from nitrogen adsorption-desorption isotherms measured on a Tri Star II (Micromeritics GmbH, Aachen, Germany). All the samples were degassed at 150 ◦C overnight prior to the adsorption measurements.. The diffuse reflectance spectra (DRS) and UV-Vis absorption spectra of the dye solutions were measured with a Varian Cary 4000 (Mulgrave, Australia). This spectrometer could also be equipped with an Ulbricht sphere allowing the recording of UV-Vis diffuse reflectance spectra (DRS) in the region 200–800 nm using the white standard MgO as a reference. The photoluminescence (PL) spectra were recorded using a Varian Cary Eclipse fluorescence spectrophotometer (Mulgrave, Australia) at room temperature, with excitation by incident light of 380 nm. The Fourier transform infrared spectra (FT-IR) were recorded using a Bruker FT-IR Tensor 27 Spectrometer with a platinum ATR unit. The morphology of the prepared samples and the content of the elements were studied using scanning electron microscopy (Hitachi, S-3200N, Krefeld, Germany) and energy-dispersive X-ray spectroscopy (EDX Oxford INCAx-act, Abingdon, UK), respectively. Since the photocatalytic properties of the LaFeO3 strongly depend on the surface chemical state of the samples this was analyzed by X-ray photoelectron spectroscopy (XPS) with an ESCALAB 250 Xi (Thermo Fisher, East Grinstead, UK) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) as the excitation source under ultrahigh vacuum conditions. The high-resolution spectra for the C 1s, O 1s, La 3d and Fe 2p photoelectron lines were recorded with a bandpass energy of 20 eV and a step size of 0.1 eV. The spectra were analyzed using Avantage software (version 5.951). Binding energies in the high-resolution spectra were calibrated by setting the C 1s signal to 284.8 eV. Electrochemical impedance spectroscopy (EIS), photocurrent measurement and Mott Schottky plots were performed on an IM6e potentiostat (Zahner Elektrik, GmbH, Kronach, Germany) and evaluated with the Thales (version 4.12) software. A standard three-electrode cell configuration with Ag/AgCl (saturated KCl), coiled Pt-wire and LaFeO3 coated FTO were used as the reference, counter and working electrodes, respectively. The working electrodes were prepared on the conductive fluorine-doped tin oxide (FTO) glass slides (Pilkington, Weiherhammer, Germany) of 2 × 6 cm size, which were sonicated before by applying in a sequence 0.1 M HCl, 0.1 M NaOH, acetone and ethanol in an ultrasonic bath, then rinsed with deionized water and dried in an air stream. Then 0.1 g of the respective LaFeO3 sample was dispersed in 500 μl of ethanol in an ultrasonic bath for 30 min and 200 μl of the obtained suspension were coated on the FTO glass substrate using the doctor blade method. Finally, the FTO substrate was dried in a furnace for 30 min at 353 K and calcined for 3 h at 873 K. An electrolyte 0.1 M Na2SO4 aqueous solution was used for the Mott-Schottky measurements and a 0.1 M Na2SO4/0.1 M Na2SO3 aqueous solution was used as the supporting electrolyte for photocurrent and electrochemical impedance measurements, respectively.
