Porous Cu(Mn)-Doped ZnO-MgO Nanocomposites for Photocatalytic and Antibacterial Applications

3.1. XRD Analysis Figure 1 shows XRD patterns of obtained composites. Intensive peaks of wurtzite hexagonal ZnO crystals are observed in both diffractograms. The calculations carried out by using Scherrer’s formula showed that the average size of ZnO crystals was less than 20 nm in both ZnMgCu and ZnMgMn composites. Small peaks of cubic MgO crystals (periclase) (ICSD 166273) are also observed in these patterns. XRD patterns showed the absence of any peaks of Mn or Cu compounds that can be related to the small content of these components to the embedding of Mn and Cu ions into the structure of ZnO crystals. The experimental data showed that the peak positions in XRD patterns of obtained composites are slightly different (Table 3). The peaks in the XRD pattern of ZnMgMn powder are located at smaller 2θ values than in the diffractogram of ZnMgCu sample. The variation of the peaks position indicates the difference of the parameters of ZnO crystal cells in obtained composites. Zn²⁺ ions in wurtzite hexagonal ZnO crystals are in tetrahedral coordination and the ionic radius of Zn²⁺(IV) is 0.6 Å. Mg²⁺(IV), Mn²⁺(IV) and Cu²⁺(IV) ions have ionic radii 0.57; 0.66 and 0.57 Å, correspondingly [47] and can substitute zinc ions in the lattice of ZnO crystals [31,35,42,48]. The replacement of Zn²⁺ ions by bigger ones can be accompanied by some deformation of ZnO crystal lattice. So, some expansion of ZnO crystal cells at the embedding of Ag⁺ ions having ionic radii bigger than that Zn²⁺ ions were observed earlier in [10,49]. The embedding of Cu ions into ZnO crystal lattices also leads to some of their distortion [50]. Table 4 shows the calculated ZnO crystal cell parameters in the composites and different literature data on these values in pure ZnO. The volumes of crystal cells of ZnO crystals in Mn-doped composites are bigger than in pure ZnO. This fact suggests embedding Mn²⁺ ions into the structure of ZnO crystals in ZnMgMn composite. This assumption agrees with the results reported previously in [51]. The scheme illustrating the embedding of Mn²⁺ ions into the ZnO crystals is shown in Figure 2a. Also, it is worth noting that the comparison of ZnO crystal cell parameters in composites obtained in this study with the data given in [51] for composite Zn_0.91Mg_0.09Ag_0.01O shows that their values are similar. The volume of ZnO crystal cells in the composite ZnMgCu is less than in pure ZnO, that can be explained by the replacement of Zn²⁺ by the smaller ions Cu²⁺. This phenomenon was observed earlier in [39]. Mg²⁺ ions in cubic MgO crystals are in octahedral coordination and ionic radius Mg²⁺(VI) is 0.72 Å [47]. Zn²⁺(VI), Mn²⁺(VI) and Cu²⁺(VI) ions have ionic radii 0.74; 0.83 and 0.73 Å, correspondingly [47] and can substitute Mg²⁺ ions in the structure of cubic MgO crystals. Table 5 shows the positions of peaks of MgO crystals in XRD patterns of obtained composites and calculated crystal cell parameters. MgO peaks are shifted into the smaller 2θ values in XRD patterns of composites compared with standard values (ICSD code 166273). This fact suggests that Mg²⁺ ions could be substituted by the bigger ions (Zn²⁺, Mn²⁺ or Cu²⁺) and MgO crystal cell was expanded. The scheme illustrating the embedding of Mn²⁺ ions into the MgO crystals is shown in Figure 2b. Clearly, that the absence of peaks of Mn and Cu compounds and observed peaks positions shifts are not the strong evidence of the embedding of doping ions into ZnO and MgO crystal lattices. However, the obtained data from XRD analysis allows the assumption that the doping additions of Cu²⁺ and Mn²⁺ modify and deform the structure of ZnO and MgO crystals. 3.2. Morphology of Composites Figure 3 demonstrates SEM photos of ZnMgMn (a) and ZnMgCu (b) composites. Both composites have porous structures related to the use of PVP additions, providing gas product generation during the calcination stage during composites synthesis. Composites structures are composed of aggregates having a size < 100 nm. This porous materials morphology provides them with high specific surface areas and effective contact with the surrounding media. The data of BET method (argon adsorption) show that the values of specific surface areas of powders are 5.6 m²/g and 7.8 m²/g for ZnMgMn and ZnMgCu samples, correspondingly. 3.3. Spectral Properties of Composites Figure 4 shows diffuse reflection spectra of composites ZnMgMn (curve 1) and ZnMgCu (curve 2). The observed significant decrease of the reflection at λ < 400 nm is related with the absorption edge of ZnO crystals. The dependencies (F_KM·hν)² = f (hν) obtained for composites ZnMgCu (a) and ZnMgMn (b) are shown in Figure 5. Obtained E_g values are 3.42 and 3.41 eV for composites ZnMgCu and ZnMgMn, correspondingly. These values are higher than the band gap values of ZnO nanocrystals (3.10 ÷ 3.37 eV [8,40]) and ZnO-MgO nanocomposites (3.20 ÷ 3.28 eV [33]). In spite of these values being estimated only, obtained results suggest that UV irradiation is preferable for the effective excitation of composites. 3.4. Singlet Oxygen Photogeneration It is known that singlet oxygen is one of the most chemically active oxygen species. It is generated by photocatalytic materials and actively taking part in their antibacterial activity and the photodecomposition of organic contaminations. Figure 6a shows photoluminescence spectrum of ZnMgMn composite in NIR spectral range. Characteristic luminescence band with λ_max = 1270 nm attributed to the singlet oxygen is observed in the spectrum. It is worth to notice that this luminescence is observed at the excitation of the composite by the blue light (λ_ex = 405 nm) with the photon energy 3.06 eV that is less than the band gap value of ZnMgMn composite (E_g = 3.41 eV). This phenomenon can be attributed to the effect of structural defects which are often present in highly dispersive materials and play a key role in the generation of ROS and in the photocatalytic processes [4,7,9,52]. The dependencies of the luminescence intensities of singlet oxygen photogenerated by the powders ZnMgCu (curve 1) and ZnMgMn (curve 2) from the excited light (λ = 405 nm) power density are shown in Figure 6b. The growth of power density of the excited light increases the intensity of the singlet oxygen emission, and the dependencies shown in Figure 6b are almost linear. Similar results were obtained earlier for the singlet oxygen generation by the porous ZnO powder [8]. Also, it is worth noticing that the linear dependencies of the constant rate k of organics photocatalytic degradation from the visible light intensity I_ex were observed in [12]. The deviations from the linearity of these dependencies were observed at the high I_ex values [1,12]. 3.5. Kinetics of Dye Photocatalytic Decomposition The experiments showed that obtained composites demonstrate high photocatalytic activity for the CSB decomposition. The kinetics of CSB dye photolysis under UV and blue light irradiation in solution without any additions is shown in Figure 7a,b. The decreases of CSB content in solution by the dye photolysis without photocatalytic additions are negligible at the application of both UV and blue light irradiations. Figure 7 also demonstrates the kinetics dependencies of photocatalytic decomposition of CSB dye in solutions in the presence of powders of ZnMgCu (curve 2) ZnMgMn (curve 3) and under UV (Figure 7a) and visible (Figure 7b) irradiations. UV irradiation of dye solutions with ZnMgCu and ZnMgMn composites additions for 30 min leads to the decomposition of about 50% of CSB molecules. Observed rates of photocatalytic dye decomposition are remarkably higher at the application of blue LED as the light source, that is explained by its significantly higher power density of radiation (see Section 2). The kinetic equation of pseudo-first order, which is often used in photocatalysis in integral form, can be written as [8]: where C₀ and C—initial and current dye concentrations (mM); t—duration of UV irradiation (min); and k—reaction rate constant (min⁻¹). The calculations showed that the Equation (4) successfully describe experimental results (R² > 0.9). The comparison of obtained values of constant rates of CSB decomposition k with the literature data related to the photocatalytic decomposition of this dye is given in Table 6. The rates of CSB photocatalytic decomposition observed in the presence of prepared Cu- and Mn-doped ZnO-MgO composites are similar to the values for the grainy ZnO powder and ZnO 80.16 mol.% + ZnAl₂O₄ 19.83 mol.% + CuO 0.04 mol. % composite, given in [8,18]. These k values are significantly higher than the rate constant observed in [18] for composite ZnO 20.81 mol. % + ZnAl₂O₄ 79.18 mol.% + CuO 0.01 mol. %. However, the rate constants of dye decomposition obtained in this study are significantly lower than k values reported for porous ZnO powder ([8]) and ZnO nanoflowers ([48]). This comparison demonstrates the important roles of high ZnO content and optimized morphology (porous structure; nanoflowers) in the formation of highly effective ZnO-based photocatalysts. It is worth noticing that this conclusion was made for the photocatalytic decomposition of CSB under UV irradiation. Figure 7b shows the kinetic dependencies of CSB photocatalytic decomposition in the presence of obtained composites under blue light irradiation. The rates of dye photocatalytic decomposition under blue light irradiation are remarkably higher than that observed under UV radiation. This fact is related to the higher power density of the radiation when the LED is applied. 3.6. Antibacterial Activity Figure 8 demonstrates photos of Petri dishes with agar containing gram-positive bacteria Staphylococcus aureus ATCC 209P (a) and gram-negative bacteria Escherichia coli ATCC 25922 (b) and powder samples ZnMgCu (samples 1) and ZnMgMn (samples 2). Bacteria-free zones around all powder samples are observed in these photos. The data of the average thicknesses of observed zones free from bacteria are given in Table 7. The thicknesses of the zones free from the bacteria Staphylococcus aureus ATCC 209P (Figure 7a) are significantly bigger than that observed for the bacteria Escherichia coli ATCC 25922 (Figure 7b). This fact can be related to the difference in the structure of these bacteria. The obtained data show that prepared composites demonstrate high antibacterial activity against gram-positive bacteria Staphylococcus aureus ATCC 209P. However, their activity against gram-negative bacteria Escherichia coli ATCC 25922 is relatively weak.