. Conclusions
Titania has been successfully doped with I, N and Fe and also deposited with Ag and Au nanoparticles. It has been shown that the RHSE technique enhances the UV and VIS activity of the sol–gel synthesized samples particularly for nitrogen and iron doped photocatalysts. The results of UV and VIS photodegradation of phenol and oxalic acid can be summarized:
(i)
Phenol can be efficiently degraded with hydrophilic polyhedral titania particles with relatively large particle size (D = 30–50 nm) and low specific surface area (SBET = 20–50 m2/g). This type of photocatalyst can be prepared by the flame synthesis method and also by the sol–gel technique using the RHSE method. The most efficient photocatalysts were the TiO2-FH > TiO2-P25 > TiO2-RHSE samples in this study for phenol under UV irradiation.
(ii)
Under visible light (λ > 400 nm) irradiation, phenol was efficiently degraded by doped photocatalysts with efficient light absorption and small average crystal size (D = 6.5–9.0 nm). Undoped reference titanium dioxide with some rutile content was also efficient. The activity order for this substrate under VIS irradiation is TiO2-N > TiO2-P25 ≈ TiO2-I.
(iii)
Oxalic acid substrate was efficiently degraded under UV irradiation on noble metal deposited titanium dioxide and also on nitrogen doped titania with high specific surface area (SBET = 139 m2/g): TiO2-P25-Ag ≈ TiO2-P25-Au > TiO2-N.
(iv)
Under visible light irradiation, oxalic acid decomposition rate was the highest when iron doped titanium dioxide was applied due to specific interactions with the iron(III) ions in the photocatalyst. High specific surface area and nitrogen doping also sensitized titanium dioxide toward the mineralization of oxalic acid. The activity order was TiO2-Fe ≫ TiO2-N > TiO2-P25.
Different structural characteristics are beneficial for achieving the optimal UV and visible light activity. Silver and gold deposition enhanced significantly the photocatalytic activity for oxalic acid decomposition under UV irradiation; this modification with the photodeposition method was detrimental for the visible light activity. Our best photocatalysts exhibited significantly higher activity than that determined for Aeroxide P25 TiO2 for both substrates and irradiation conditions. Phenol degradation resulted in dihydroxy benzene intermediates, such as pyrocatechol and hydroquinone both under UV and visible light irradiation with our TiO2-N photocatalyst. The results of this comparative study could help to explore the optimal synthesis conditions for a given organic substrate.