Effects of different reaction parameters in the hydrothermal synthesis of molybdenum oxides (MoO₃) were investigated and monoclinic (β-) MoO₃ was prepared hydrothermally for the first time. Various temperatures (90/210 °C, and as a novelty 240 °C) and durations (3/6 h) were used. At 240 °C, cetyltrimethylammonium bromide (CTAB) and CrCl₃ additives were also tested. Both the reaction temperatures and durations played a significant role in the formation of the products. At 90 °C, h-MoO₃ was obtained, while at 240 °C the orthorhombic (α-) MoO₃ formed with hexagonal rod-like and nanofibrous morphology, respectively. The phase transformation between these two phases was observed at 210 °C. At this temperature, the 3 h reaction time resulted in the mixture of h- and α-MoO₃, but 6 h led to pure α-MoO₃. With CTAB the product was bare o-MoO₃, however, when CrCl₃ was applied, pure metastable m-MoO₃ formed with the well-crystallized nanosheet morphology. The gas sensing of the MoO₃ polymorphs was tested to H₂, which was the first such gas sensing study in the case of m-WO₃. Monoclinic MoO₃ was found to be more sensitive in H₂ sensing than o-MoO₃. This initial gas sensing study indicates that m-MoO₃ has promising gas sensing properties and this MoO₃ polymorph is promising to be studied in detail in the future.

In this study, Bi₂WO₆ was prepared by the hydrothermal method. The effects of reaction temperature (150/170/200 °C) and reaction time (6/12/24 h) were investigated. The role of strongly acidic pH (1 >) and the full range between 0.3 and 13.5 were studied first. Every sample was studied by XRD and SEM; furthermore, the Bi₂WO₆ samples prepared at different temperatures were examined in detail by EDX and TEM, as well as FT-IR, Raman and UV-vis spectroscopies. It was found that changing the temperature and time slightly influenced the crystallinity and morphology of the products. The most crystallized product formed at 200 °C, 24 h. The pure, sheet-like Bi₂WO₆, prepared at 200 °C, 24 h, and 0.3 pH, gradually transformed into a mixture of Bi₂WO₆ and Bi_3.84W_0.16O_6.24 with increasing pH. The nanosheets turned into a morphology of mixed shapes in the acidic range (fibers, sheets, irregular forms), and became homogenous cube- and octahedral-like shapes in the alkaline range. Their band gaps were calculated and were found to vary between 2.66 and 2.59 eV as the temperature increased. The specific surface area measurements revealed that reducing the temperature favors the formation of a larger surface area (35.8/26/21.6 m²/g belonging to 150/170/200 °C, respectively).

In this study, we prepared hydroxyapatite (HAP) samples using hydrothermal method. We investigated the effect of reaction conditions such as phosphate excess applying 1.49 and 1.67 (as stoichiometric) Ca/P ratio, pH (9/ 10/ 11/ 12) and time (4/ 8/ 12/ 24 h). Sample characterization was carried out by XRD and SEM. The results showed, all samples had HAP structure, however, lower Ca/P ratio, larger reaction time and setting the pH to 10 increased the crystallinity. Then, we synthetized Sr-doped HAP samples, varying the Sr concentration using 2, 4, 6, 8 and 10 % Sr/ (Ca+Sr). The Sr content was revealed by EDX. Sr-incorporation did not change the obtained crystalline HAP phase but the unit cell parameters increased. We calculated lattice constants and found that a, b changed from 9.4310 Å to 9.4700 Å, c from 6.8819 Å to 6.9227 Å and the unit cell volume from 530.0951 ų to 537.6556 ų due to the larger ionic radius of Sr compared to Ca. The pure and doped samples had uniform, mostly needle-like morphology with 100-300 nm length and 25-100 nm. In vitro cytotoxicity tests revealed evident antibacterial activity in the case of doped samples compared to pure HAP against E. coli.