2.4. Sample Testing Methods

The synthesized samples were characterized by X-ray diffraction (XRD), low-temperature nitrogen adsorption, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR TEM), and XANES/EXAFS X-ray absorption spectroscopy.

The phase composition of the photocatalysts was determined by XRD with a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Ettlingen, Germany) using monochromatized Cu Kα radiation with a wavelength of 1.5418 Å. The crystal size (CS) was estimated as the coherent scattering domain size using the Scherrer formula. The specific surface area (SSA) and pore volume of the catalysts were obtained by low-temperature N₂ adsorption–desorption (N₂ adsorption at 77 K) using an ASAP 2400 apparatus (ASAP Industries Manufacturing, Houma, LA, USA). Their optical properties were studied by the DRS method. Diffuse reflection spectra in UV and visible regions were obtained using a Shimadzu UV-2501 PC spectrophotometer (Shimadzu, Kyoto, Japan) with an ISR-240A diffuse reflection attachment.

The chemical composition of the catalyst surface was studied by XPS with a photoelectron spectrometer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) using non-monochromatized Al Kα radiation (hυ = 1486.6 eV). The spectrometer was equipped with a PHOIBOS-150 hemispherical analyzer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) and an XR-50 X-ray source with a double Al/Mg anode. The charging effect was corrected using the binding energy of the Ti2p3/2 peak at 459.0 eV.

The chemical state of copper in the bulk of the catalysts was studied using XANES X-ray absorption spectroscopy at the station of the Kurchatov Synchrotron Radiation Source (Moscow, Russia). The electron energy in the storage ring was 2.5 GeV at a beam current of 50–150 mA. To monochromatize synchrotron radiation, we used a silicon single crystal with (111) orientation in the form of a cut-out monoblock mounted on a goniometric head. The energy resolution achieved was ΔE/E = 2 × 10−4. The X-ray absorption spectra of the catalysts were obtained in fluorescence geometry (a sample with 20% Cu was tested in the transmission mode). The X-ray beam intensity before and after passing through the sample was measured using ionization chambers equipped with Keithley 6487 digital picoammeters.

The microstructure of the photocatalysts was studied by HRTEM using a ThemisZ electron microscope (Thermo Fisher Scientific, Waltham, MA, USA) at an accelerating voltage of 200 kV. The microscope was equipped with a SuperX energy-dispersive spectrometer and a spherical aberration corrector. The maximum resolution of the microscope was 0.06 nm. For the HR TEM analysis, the samples were ultrasonically dispersed onto perforated carbon substrates attached to aluminum grids.

The photocatalytic activity of synthesized samples was determined using the setup shown in Figure 1. The setup consisted of a glass reactor with a quartz window (S_window = 22 cm²), an LED source of irradiation, and a magnetic stirrer.

(a) Spectra of LEDs with the wavelength at maximum intensity; (b) scheme of reaction set-up for photocatalytic study. 1—LED, 2—quartz window, 3—sampler, 4—reactor, 5—reaction mixture, 6—stir bar, 7—magnetic stirrer.

The reaction mixture consisted of a photocatalyst (50 mg) and a 2.8% aqueous solution of glycerol. The total volume of the suspension was 100 mL. Before the experiment, the reaction mixture was purged with Ar for 15 min to remove atmospheric oxygen. After the purge, either an LED-381 nm (for UV irradiation) or an LED-427 nm (for visible light irradiation) was turned on (Figure 1). Under irradiation of the photocatalyst, the reaction mixture evolved hydrogen. During the experiment, the gas phase (250 μL) was sampled with a gas syringe (Hamilton) every 15 min. The experiment lasted from 90 to 150 min, depending on the photocatalyst activity. The amount of hydrogen was determined using a Chromos GC-1000 chromatograph (Chromos, Moscow, Russia) with a thermal conductivity detector and with Ar as the carrier gas.