We
developed low-temperature (up to 200 °C) synthesis of TiO2 nanocomposites, which allows precise tuning of the anatase/rutile
ratio. The materials were synthesized hydrothermally from strongly
acidic solutions of Ti(III) precursors and hydrogen peroxide. Two
series of materials (A and B series) have been synthesized; the A
series is characterized by smaller anatase particles, lower crystallinity,
and higher specific surface area. The synthesized materials were examined
in water oxidation, hydrogen peroxide reduction, photocurrent generation,
and methanol-assisted water reduction. The optimal phase composition
depends on the reaction typehigh contents of rutile or anatase
should be considered for H2O2 reduction and
water oxidation, respectively, while water reduction requires moderate
contents of both polymorphs. The A series appeared more active in
oxidation-controlled reactions, while the B series facilitated reduction-controlled
processes. The DOS analysis confirmed that rutile is a significantly
better reducer, whereas anatase is a better oxidant. Presented data
show that a synergistic effect observed usually for anatase/rutile
nanocomposites can result from both intrinsic and extrinsic factors.
The intrinsic factors are responsible for the improved photocatalytic
activity of the material related to its physicochemical properties, e.g., efficiency of charge separation, lifetime of photogenerated
charges, absorption properties, redox characteristics, porosity, specific
surface area, etc. The extrinsic factors are strongly related to the
redox reactions to be performed at the photocatalyst surface, both
oxidation and reduction. Therefore, we confirm hereby with presented
experimental evidence that the design of the optimal photocatalyst
must take into account both intrinsic and extrinsic factors.
The C 3 N 4 /ZnO composite photocatalysts were synthesized by mechanical milling combined with a calcination process. Various ratios of melamine and ZnO powders were milled by a planetary ball mill for 10 h. After heating at 540 • C for 3 h in air, melamine was converted to C 3 N 4 but the formation of C 3 N 4 depended on the ratios of the melamine and ZnO (M/Z) powders. From the experimental results, the conversion of melamine to C 3 N 4 could be inhibited by ZnO particles; as there was no detectable C 3 N 4 in the sample at low M/Z values or high ZnO contents. The photocatalytic activities of prepared samples were investigated under the illumination of blacklight and fluorescent lamps as the low wattage light source. The C 3 N 4 /ZnO showed a better photocatalytic activity than ZnO to degrade a methylene blue (MB) dye solution using blacklight lamps, but there is no significant difference in photocatalytic activities between ZnO and prepared C 3 N 4 /ZnO under visible light by the fluorescent lamps. However, the prepared C 3 N 4 /ZnO can well function under illumination by Xe lamp as the high power light source. Ecotoxicities of MB solutions before and after photocatalytic process were also studied through growth inhibition of the alga Chlorella vulgaris.
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