## 2 Materials and Methods
### Sample Characterization  
To be able to characterize the obtained materials, the powders were first ground. The first analysis performed was X-ray diffraction (XRD). The purpose of this analysis was to determine the crystal structure and the component phases. Also, the width and shape of the diffraction maxima give us information about the size of the crystallites. X-ray diffraction analysis was performed using the PANalytical Empyrean equipment (Malvern PANalytical in Bruno, The Netherlands) in Bragg-Brentano geometry equipped with an X-ray tube with Cu anode (\(\lambda\)CuK\(\alpha\) = 1.541874 A) with in-line focusing, programmable divergent slit on the incident side, and anti-slot-programmable spreader mounted on a PIXcel3D detector on the diffracted side. X-ray diffractograms were recorded in the 10-80\({}^{\circ}\) angle range. The morphological and elemental composition investigation of the samples was performed using the Quanta Inspect F50 scanning electron microscope (Thermo Fisher, Eindhoven, The Netherlands) equipped with a field emission gun (FEG) with a resolution of 1.2 nm and coupled with an energy-dispersive X-ray spectrometer (EDX) with MnK\(\alpha\) resolution of 133 eV. Bright-field transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) images were taken using G2 F30 S-TWIN HRTEM equipment purchased from the FEI (Hillsboro, OR, USA), equipped with STEM with HAADF detector, EDX, EELS, energy filter, and GIF operated at 300 kV. The sample preparation was performed as follows: a small amount of sample was added to water in \begin{table}
\begin{tabular}{c c c} \hline \multirow{2}{*}{**Sample Name**} & **Zn2+ Precursor** & \multirow{2}{*}{**Reaction Time (min)**} \\  & **Concentration (mol/L)** & \\ \hline S1\_15 & 0.3488 & 15 \\ \hline S1\_30 & 0.3488 & 30 \\ \hline S1\_60 & 0.3488 & 60 \\ \hline S2\_15 & 0.1744 & 15 \\ \hline S2\_30 & 0.1744 & 30 \\ \hline S2\_60 & 0.1744 & 60 \\ \hline S3\_15 & 0.0872 & 15 \\ \hline S4\_15 & 0.0436 & 15 \\ \hline S5\_15 & 0.0218 & 15 \\ \hline \end{tabular}
\end{table} 
Table 1: The resulting powders and main synthesis parameters. Figure 1: Schematic representation of the ZnO powder microwave-assisted hydrothermal synthesis. an Eppendorf tube and sonicated for 15 min. Then, 10 \(\upmu\)L of the dispersed sample was placed on a 400 mesh Cu-covered carbon grid and left to dry until analysis. This technique provides qualitative information regarding surface morphology and particle size. The volumetric nitrogen adsorption (Brunauer-Emmett-Teller; BET) analysis was conducted using a Gemini V2 model 2380 (Micromerities Instruments Corporation, Norcross, GA, USA) surface area and pore size analyzer. This analysis aimed to determine the specific surface area (SSA) and pore volume of the powders. Adsorption isotherms were recorded by measuring the amount of gas adsorbed at different relative pressures and a constant temperature (77 K) with pressure ranging from 780 to 7.8 mmHg. Conversely, desorption isotherms were obtained by quantifying the amount of gas released as the pressure was decreased. The powders were pre-treated under a vacuum for 24 h.  
The true density of the ceramic powders, defined as the ratio between the mass of the dried sample and its actual measured volume, was determined using Pycnomatic ATC equipment (Thermo Fisher Scientific, Waltham, MA, USA), with helium as the testing gas of analytical purity. Measurements were conducted in triplicate, and the standard deviation was provided for each sample. Data are represented as mean \(\pm\) standard deviation (S.D.). Graphs and statistical analysis were performed using MS Excel software for Microsoft 365 (Version 2404). Data comparisons were made using a one-way analysis of variance (ANOVA), followed by a two-tailed \(t\)-test. Probability (\(p\)) values \(<\) 0.05 were considered statistically significant. In statistical analysis, the \(p\)-value is a measure used to determine the significance of the results obtained from a statistical test.