Improving The Photocatalytic Activity Of TiO₂ Nanowires:Homojunctions Construction And Substrates Selection

Titanium oxide(TiO2)photocatalysis technology has been widely applied in green energy fields and environmental remediation.One-dimension(1D)nanowire is considered to be an appropriate nanostructure guaranteeing a high photocatalytic efficiency because of its large specific surface area and easy separation of photogenerated electron-hole pairs.In this thesis,both semiconductor homojunctions and the option,of substrate with excellent adsorption performance are adopted to further improve the efficiency of TiO2 nanowires to assist photodegradation of organic pollutants in water.The main research results are as follows:(1)P25/TiO2 nanowire hierarchical structure:Commercial P25 nanoparticles were introduced into the Ti-H2O2-melamine reaction system to serve as heterogeneous nucleation centers,which thus induced precipitations of core/shell hierarchical structure P25/TiO2 nanowire powders.The P25 content in the composite powders can be easily adjusted by varying their amounts in the reactants.It is found that the composite powder consisted of 17.7 wt.%P25 exhibited the best photocatalytic activity.Under the UV light illumination,its ability to assist photodegradation of phenol in water is slightly higher than the commercial P25 nanoparticles,3.1 times that of TiO2 nanowire powders,and 1.5 times that of a mechanical mixture of nanowire and P25 powders with the same composition.The improvement in the photodegradation efficiency can be attributed to a synergetic effect arising from phase junctions.(2)TiO2 nanostructured composite thin films on Ti substrates:Quasi-aligned TiO2 nanorod,nanowire and nanoflower arrays were precipitated on Ti substrates via a Ti-H2O2 interaction.A sol-gel dip-coating process was then adopted to fill 3-7 wt.%anatase TiO2 nanoparticles among the free space of TiO2 nanostructured arrays.After a final calcination in air,0D/1D and 0D/3D structured TiO2 composite thin films were achieved.When utilized to assist photodegradations of phenol in water,the photocatalytic activity of TiO2 nanorod,nanowire and nanoflower arrays on Ti substrates embedded with a small amount of anatase TiO2 nanoparticles improved 76%,136%and 26%,respectively.The enhancement in the photocatalytic activity can be attributed to a synergetic effect arising from phase junctions,as well as the increasing loading of photocatalyst on thin films arising from filling the nanoparticles into the free space among nanostructured arrays.(3)TiO2 nanowires on carbon fiber cloth:The carbon fiber cloth was utilized as a substrate to deposit anatase TiO2 nanowires using the Ti-H2O2-melamine reaction system.The thickness(1.5-4.5 ?m)of thin films can be tuned by a multiple deposition process.The adsorption characteristic towards rhodamine B(RhB),sulfosalicylic acid(SSA)and phenol in water was studied in detail.The results indicate that the adsorption behavior of the TiO2 nanowires on the carbon fiber cloth substrates conforms to the quasi-second order dynamics model,which means that the adsorption kinetic is controlled by chemical interactions.The adsorption towards RhB and phenol molecules in water accords with the L-type isothermal adsorption;whilst the adsorption of SSA accords with S-type isothermal adsorption.It thus suggests that various factors influence the adsorption behavior,including pore structure of thin films and surface charge characteristic.During the deposition of TiO2 nanowires on carbon cloth substrates,the H2O2 treatment followed by the subsequent thermal treatment remarkably enhanced the adsorption capacity of the carbon cloth substrate itself.The saturated adsorption capacity(qe)towards RhB,phenol and SSA is 0.007,1.018 and 0.027 mmol/g,respectively,which is 2.3,102 and 27 times that of the pristine carbon cloth.The TiO2 nanowires coverage reduced the adsorption capacity.The results of photocatalytic tests suggest that the film that is 4.5 ?m in thickness exhibited the highest total removal efficiency of RhB and SSA in water.Compared with similar TiO2 nanowire arrays on metallic Ti plate substrates,photocatalytic efficiency of TiO2 nanowire arrays on carbon cloth evidently improved,which can be contributed to a synergetic effect arising from the great adsorption capacity by carbon cloth substrate and photodegradation by TiO2 nanowires:1)carbon cloth offers a great deal of adsorption sites,resulting in the enrichment of solute molecules around the surface of thin film;2)the adsorbed molecules near photocatalytic active sites diffuse to the photocatalytic active sites to be degraded;3)photogenerated electrons transfer from TiO2 to the conductive carbon cloth,promoting the separation of photogenerated electron-hole pairs,and 4)fibrous structure of the substrate greatly increases the effective contacting area and avoids the nanowires piling up.The results achieved in the current investigation suggest that both the semiconductor homoj unctions and the substrates with enhanced adsorption capacity can realize a“1+1>2" synergetic effect,which are effective strategies to promote the industrial application of TiO2 photocatalysis in wastewater treatments.