*4.2. Structural and Functional Characterization of the Obtained Materials* 4.2.1. SEM—Scanning Electron Microscopy

In order to identify the morphological properties of the materials of interest, scanning electron microscopy (SEM) was used with a high-resolution microscope, FEI Quanta3 DFEG model, Brno, Czech Republic, working at 5 kV voltage, in high-vacuum mode with Everhart–Thornley secondary electron (SE) detector, coupled with energy-dispersive X-ray (EDAX) spectrometer.

#### 4.2.2. X-ray Diffraction (XRD) and X-ray Fluorescence (XRF)

Elemental analysis of the samples was carried out using a Rigaku ZSX Primus II spectrometer, Rigaku Corp., Tokyo, Japan with wavelength dispersion in vacuum atmosphere. The spectrometer was equipped with 4.0 kW X-ray Rh tube. The XRF results were analysed using EZ-scan combined with Rigaku SQX fundamental parameters software (standard less), which is capable of automatically correcting all matrix effects, including line overlaps.

Powder X-ray diffraction patterns were recorded using Rigaku's Ultima IV multipurpose diffraction system, Rigaku Corp., Tokyo, Japan, a Cu target tube (λ = 1.54060 Å) and a graphite (002) monochromator, with working conditions of 30 mA and 40 kV. The data were collected at room temperature between 3 and 75◦ in 2θ, with a 0.02◦ step size and a scanning rate of 1◦/min. Phase identification was performed using Rigaku's PDXL software, connected to the ICDD PDF-2 database. The lattice constants were refined using diffraction line position.

#### 4.2.3. UV–Vis Spectrophotometry

In order to investigate the optical properties of the investigated samples, the diffuse reflectance (DR) spectra were recorded with a UV–Vis spectrophotometer, Analytik Jena Specord 200 Plus, Jena, Germany. The diffuse reflectance UV–Vis data were converted into absorbance using the Kubelka–Munk function.

#### 4.2.4. Photoluminescence (PL) Measurements

Photoluminescence (PL) data were obtained using a Carry Eclipse fluorescence spectrometer, Agilent Technologies, Kuala Lumpur, Malaysia, for the following working parameters: 120 nm min−<sup>1</sup> scan rate, 0.5 nm spectral resolution, and 10 nm slits in excitation and emission. Powder samples (0.001 g) were suspended in ultrapure water. The measurements were performed at room temperature for λexc = 260 nm.

#### 4.2.5. Identification of Reactive Oxygen Species (ROS) Generation under Solar Irradiation

For ·OH radicals trapping, 1 mg of each catalyst was suspended in 10 mM coumarin (Merck) solution and exposed to simulated solar light irradiation for fluorescent umbelliferone formation. This was conducted with Carry Eclipse fluorescence spectrometer for λexc = 330 nm.

For O<sup>2</sup> − formation, 3 mg of each sample was suspended in 3 mM solution of XTT sodium salt (2, 3-bis(2-methoxi-4-nitro-5sulfophenyl)-2H-tetrazolium-5-carboxanilide) (Sigma-Aldrich) and exposed to simulated solar light, following XTT reduction by the photogenerated O<sup>2</sup> −. These results in the formation of XTT formazan complex were identified with a UV–Vis spectrophotometer, Analytik Jena Specord 200 Plus, a broad peak located at 470 nm being indicative.

#### 4.2.6. Electrokinetic Potential Measurements

Electrokinetic potential measurements were carried out on a Malvern Nano ZS Zetasizer, Model ZEN 3600, Malvern, UK, at room temperature. The electrokinetic potential values were calculated using Helmholtz–Smoluchowski equation. The tests were conducted in triplicate.

#### 4.2.7. Photoelectrochemical (PEC) Tests

Photoelectrochemical measurements were carried out in an electrochemical cell equipped with a quartz window using a two-electrode configuration by means of a Zahner IM6 potentiostat, Zahner-Elektrik GmbH, Kronach—Gundelsdorf, Germany. The reference and counter electrode leads were connected to a platinum wire, and working electrode lead was attached to prepared samples (geometric area of ~3 cm<sup>2</sup> ). An aqueous suspension of interest powder (0.01 g in 2 mL ultrapure water) was sonicated and coated on TCO conductive glass (Solaronix, Aubonne, Switzerland) by successive depositions and subsequent drying. The as-obtained coatings were heat treated at 300 ◦C in air for 1 h. The experiments were conducted in 0.5 M Na2SO<sup>4</sup> electrolyte, pure or containing NH4OH (12 µL/120 mL) under simulated solar light irradiation (AM 1.5, Peccell-L01, Yokohama, Japan). Photocurrent– voltage curves were recorded by linear sweep voltammetry at a scan rate of 10 mV/s.
