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In the present work Cu2O shape tailored microcrystals were obtained and investigated. The used shape-tailoring approach was based upon the variation of the starting precursor (copper(II) acetate and copper(II) chloride, the latter one being also much more cheaper and easily accessible), the synthesis temperature (60, 70 and 80 °C, respectively) and the shape tailoring agent applied (PVP vs. EDTA). It was found that cubic and polyhedral monodisperse microcrystals were obtained, which showed enhance visible light photocatalytic activity in the degradation of methyl orange. The activity was dependent of the formation of metallic Cu (the formed metallic nanoparticles were obtained when PVP was used), the microcrystals' size and morphology. The band-gap values were directly linkable to the obtained photocatalytic activity, while in the first derivative DRS spectra the electron transition contribution of facet (111) was also found, alongside of the polycrystalline Cu2O electron transition contribution. Also, the surface hydrophylicity played a crucial role in the determination of the photocatalytic properties as evidenced by IR measurements and DLS investigations as well.

Bismuthoxyhalides photocatalysts (BiOX, X = Cl, Br, I) were prepared using solvothermal crystallization method. The influence of the synthesis temperature (120 °C, 140 °C, 160 °C) and duration (3 h, 24 h, 48 h) on the structural (crystal size, phase composition etc.), morphological (crystal shape), optical (bandgap values) parameters and on the resulting photocatalytic activity were investigated. The samples were characterized by the means of X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and diffuse reflectance spectroscopy (DRS). The photocatalytic activity of the synthesized materials was evaluated under visible- (≥ 400 nm) and UV light (≈ 365 nm) by degrading methyl orange and Rhodamine B (RhB). The results pointed out the importance of the crystallization temperature and duration in achieving the highest photocatalytic activity, showing that the lowest temperature and the shortest duration time resulted the highest removal yields. Furthermore, a possible degradation mechanism was elaborated based on a direct hole oxidation approach and the detected degradation intermediates.

TiO2, Al2O3 and ZnO are grown by atomic layer deposition (ALD) at 80 °C on graphene oxide (GO), synthesized by the improved Hummers’ method. The preparation steps and the products are followed by FTIR, Raman, TG/DTA-MS, SEM-EDX, XRD and TEM-ED. Both Al2O3 and TiO2 grown with ALD are amorphous, while ZnO is crystalline. Through decomposing methylene orange by UV irradiation it is revealed that the GO itself is an active photocatalyst. The photocatalytic activity of the amorphous TiO2, deposited by low temperature ALD, is comparable to the crystalline ZnO layer, which is the best photocatalyst among the studied oxides. Al2O3 used as reference suppresses the photocatalytic performance of the GO by blocking its active surface sites.

Orthorhombic molybdenum oxide (α-MoO3) samples were prepared via calcination. During this process, the ratio of the (0 4 0)/(1 1 0) or (0 4 0)/(0 2 1) crystallographic planes was fine-tuned by applying different calcination temperatures. Textural characterization of the samples was carried out to comprehend their differences in photocatalytic and adsorption applications. Except for methyl orange, all the tested dyes (methylene blue, crystal violet, malachite green, and rhodamine B) presented adsorption affinity towards α-MoO3. It was revealed that the presence of α-MoO3 significantly decreased the photocatalytic performance of TiO2 under UV light irradiation. With the growth of the (0 4 0) facet of α-MoO3, the photocatalytic activity increased while the adsorption affinity decreased for cationic dyes. It was found that the solubility of α-MoO3 depends on the proportional presence of the (0 4 0) facet, which determines both the adsorption and photocatalytic behaviors of the oxide. The solubility of α-MoO3 is reflected by the decreasing rate of solution pH, hence the adsorption can be measured accurately, but it varies according to the cationic dye structure. Using cationic dyes to assess the photocatalytic activity of α-MoO3 requires meticulous investigations since adsorption can be mistaken with photocatalytic activity: the adsorption rate depends on both adsorptive and adsorbent structures.

Syntheses of highly ordered carbon nanotube (CNT) forests were studied from the aspect of the quality of CNTs and the properties of Fe-Co catalyst. CNT forests were grown by catalytic chemical vapor deposition. The amount of catalyst was varied in two ways: (1) catalyst layers with different nanostructure were produced with pulsed laser deposition by applying different argon pressures in the deposition chamber, and (2) compact catalyst layers were deposited with different thickness in 1–5 nm range. Our results show that both the nanostructure of the catalyst layer and the layer thickness have a significant role in the formation of catalytic particles, therefore also on the character of CNT growth.

In this work, we have prepared BiOI/CNT composites by hydrothermal method and studied the impact of CNT and hydrothermal treatment conditions (temperature and time) simultaneously on phenomena such as crystallization. The samples were studied using X-ray Diffraction (XRD), nitrogen sorption (Brunauer–Emmett–Teller (BET) specific surface areas), Scanning Electron Microscopy assisted with Energy Dispersive X-Ray spectroscopy (SEM-EDX), High Resolution Transmission Electron Microscopy (HRTEM), UV–vis Diffuse Reflectance spectroscopy (DRS), Raman spectroscopy and photoluminescence spectroscopy (PL). The different heat treatments applied and varied amount of CNT was found to have a profound effect on the structural, morphological, compositional and optical properties of the prepared composites. The distinguishable changes in the above-mentioned properties were clearly visible for the samples prepared at higher temperature and longer duration (150 °C, 6.30 h). Surprisingly, the presence of other phases of BiOI, such as Bi7O9I3, Bi4O5I2, Bi5O7I, also co-existed at the same time, although in low amount. Finally, for the study, the photocatalytic efficiency of the prepared samples was tested for phenol and RhB, as model pollutants, under visible light irradiation for 240 min and 120 min, respectively. The results revealed that the composites prepared at higher temperature with longer duration showed superior performance for the photodegradation of phenol and RhB. Additionally, the composites performed better than their respective reference samples in each case. Interestingly, the introduction of CNT promoted the crystallinity in the composites and promising results for the removal of phenol and RhB were achieved by the use of low power ordinary visible lamps.

Composite systems were prepared by doping NaYF4 (NYF) with lanthanide cations (Yb3+, Er3+, Tm3+), TiO2, and gold nanoparticles to exploit the light conversion properties of NYF and its possible impact on the photocatalytic activity of the composites. The composites were synthesized via solvothermal crystallization and the obtained structures were anatase TiO2, hexagonal NYF, and gold nanoparticles of 5–7 nm. The presence of NYF enhanced the photoactivity of TiO2 towards Rhodamine B degradation under UV irradiation, while it decreased its performance under visible light. The presence of Au was beneficial when visible light was applied for the degradation experiments. The reason behind the enhanced activity was the fluorescence of NYF at 400 nm that was the most intense for the composites that did not contain gold. The NYF-based samples also showed signs of up-conversion when 900 nm was the excitation light source, highlighting the potential of this material for photocatalytic applications that utilize the full (UV–IR) light spectrum.

Titanium dioxide hollow spheres (TiO2-HSs) were fabricated by applying carbon spheres (CSs) as templates, which were eliminated by calcination. The most suitable TiO2 was selected and an attempt was made to further increase its photocatalytic activity via noble metal (Au and Pt, at 0.25 wt%) deposition. The photocatalytic efficiency was determined by the decomposition of phenol and oxalic acid under both UV-A and visible light irradiation. It was established, that both the unique morphology and the presence of noble metals contributed to the photocatalytic activity gain compared to the solid spherical reference. For the elucidation of the observed phenol degradation performance under UV-A light irradiation, new insights were proposed: within the TiO2 samples the ratio of HSs with enhanced light trapping properties were demonstrated, and the data was compared to the observed photocatalytic activities and a direct correlation was found.

Bismuth oxybromide samples were synthesized using solvothermal crystallization and different additives (CTAB-cetyltrimethylammonium bromide, PVP–polyvinylpyrrolidone, SDS–sodium dodecyl sulfate, U–urea and TU–thiourea). The effect of the mentioned compounds was investigated through structural (crystallite size, crystal phase composition, etc.), morphological (crystal shape), optical (band gap energy) parameters, surface properties (surface oxidation states), and the resulting photocatalytic activity. It was found that the ratio of the (1 0 2)/(1 1 0) crystallographic planes, the presence of oxidized Bi4+ and Bi5+ species were responsible for the obtained photocatalytic activity. Moreover, a strong dependency was revealed between the surface tension of the shape tailoring agents and the overall morpho-structural parameters, pointing out the fact that the properties of the semiconductor can be more easily tuned using the surface tension modification as a tool.

Carbon nanospheres were used as templates for the first time to prepare titania inverse opal photonic crystal. From the spheres, opal colloid crystals were made by vertical deposition on microscope slides, and TiO2 was grown on them using atomic layer deposition (ALD). For this technique, the relatively high thermal stability and the presence of oxygen containing functional groups on the surface of the carbon spheres are beneficial. Subsequent annealing burned out the template spheres, leaving behind the inverse opal structures. The upper solid TiO2 layer was removed with argon ion sputtering. The samples were characterized with SEM, Raman spectroscopy, XRD, EDX, UV–Vis diffuse reflectance spectroscopy and their photocatalytic activity was investigated in decomposing organic dyes under UV and visible illumination. A new approach was used to test photocatalysis on the surface by utilizing UV–Vis reflectance and Raman spectroscopy in conjunction.

Bismuth oxybromide (BiOBr) is a novel visible light photocatalyst and when combined with multiwalled carbon nanotubes (CNT), it could lead to low electron/hole recombination rates, thus enhancing its photocatalytic activity. In this work, hierarchal BiOBr and BiOBr/CNT composites were successfully synthesized via hydrothermal method. The samples prepared were characterized for structural, morphological and optical properties via XRD, SEM, TEM and DRS. The influence of carbon nanotubes on various properties of BiOBr were studied and correlated. The impact of synthesis parameters (time and temperature) on structural properties was also studied. The photocatalytic degradation of phenol as model pollutant was carried out under visible light source to determine its photocatalytic activity. It was found that the presence of CNTs induced a growth in the crystallite size of the particles, which somewhat lowered the photocatalytic activity. As the CNT content increased in the sample so does the activity, due to the CNTs' visible light absorption capacity. Furthermore, a crystal orientation changes (crystallographic plane of (003)) were induced by varying several parameters, which were found to be influencing the activity as well, while the formation of Bi6O6(OH)3(NO3)3·1.5H2O was also observed. Its presence enhanced the photocatalytic activity but induced an instability problem.