Synthesis And Optical Properties Of TiO₂Based Mirco-nano Material With Special Morphology

As one of the most promising semiconductor materials, titanium dioxide hasaroused keen interest of researchers in the fields of solar cells, photocatalyticdegradation of pollutants and hydrogen production by water-splitting. In the microand nano scales, titanium dioxide shows a variety of morphology （such asnanoparticles, nanowires, nanorods, nanotubes, micro-nano-balls, etc.） and crystalstructure （such as amorphous, rutile, anatase, mixed crystal type）. Many studies haveindicated that the microscopic morphologies and crystal structures of titanium dioxidebased micro-nano material have significant impacts on the UV-Vis light absorptionand photocatalytic properties; however, there are very limited researches investigatingthe morphology diversity, formation mechanism or the optical properties of titaniumdioxide based micro-nano material with complex morphologies. Such researches havepositive practical significance in promoting the development of new energy andenvironmental governance.In this thesis, a series of titanium dioxide based micro-nano materials areprepared by hydrothermal or solvothermal method, and the morphology, crystalstructure, optical absorption and photocatalytic properties are studied exhaustively.The results are helpful to analyse the influencing factors of morphology evolution oftitanium dioxide within the range of the micro-nano scale, to synthetize titaniumdioxide with good visible light response for new energy and environmental protectionapplications, and to further study the relationship between the complex morphology oftitanium dioxide and its performance. The thermodynamic calculations by CASTEPmodule from Materials Studio are used to futher elaborate the mechanism of crystalgrowth of titanium dioxide. The main study contents and results of this paper aredescribed in detail as follows: （1）By improving the traditional hydrothermal method for the synthesis oftitanium dioxide nanowires or nanotubes, a solvothermal process for the synthesis oftitanium dioxide mesoporous nanobelts is developed. Different volume ratio mixturesof ethanol and10mol/L sodium hydroxide solution are used as the reaction mediumto prepare titanium dioxide nanobelts with different morphologies, when the volumeratio is1:1, the nanobelts with structured and uniform geometric morphology isobtained. When soaked in hydrochloric acid solution （pH <3） for24hours andcalcined at400℃for2hours, the titanium dioxide nanobelts will change intomesoporous structure, the size of the pore is about5-7nm and the number of thesepores has a slight increase when pH decreases. The experimental results show that thereaction pressure has great impact on the morphology of the titanium dioxide: thegreater the pressure, it is the more beneficial to the formation of nanobelts. Comparedwith other systhesis methods for nanobelt, the solvothermal process can significantlyreduce the amount of sodium hydroxide and hydrothermal reaction time, and thuslower costs and energy consumption. The specific surface area of nano titaniumdioxide is further improved by maintaining the lamellar Titanate in this solvothermalprocess.The visible light photocatalytic activity of mesoporous Li-doped titaniumdioxide nanobelts is studied. The doping of Li⁺ion does not change the crystallinephase but decompose the lattice of titanium dioxide nanobelts and enhance theelectronegativity by replacing Ti⁴⁺ion. The lattice breaking and the electronegativityhave important but contradictory influences on the photocatalytic activity of themesoporous Li-doped TiO₂nanobelts.（2） Compared to the one-dimensional nanowire, nanotube and nanobeltstructures, three-dimensional titanium dioxide micro-nano structures are morecomplex, while their synthesis technology, crystal growth mechanism or opticalproperties are lack of in-depth research.Titanium dioxide mixed crystals, havingcomplex morphologies and responsing to the whole visible region, are prepared under strong acidic conditions by hydrothermal method. The potassium sulfate is believed tochelate with the octahedral growth unit [TiO₆], which will change the crystal growthdirection and the growth rate of titanium dioxide. As a result, morphology control canbe achieved through the concentration control of potassium sulfate. Rutile with atypical radial prismatic structure, anatase with an octahedral structure, as well asrutile-anatase coexistence structures is synthesized. Both the morphology and grainsize of the samples have significant effects on the optical absorption properties oftitanium dioxide. These materials with visible light response have importantapplications prospects in solar cells, photocatalysis and other fields.（3） The semiconductor coupling is an effective means to extend the visible lightresponse range; however, there are few researches involving the coupling methods forthere-dimensional titanium dioxide based mirco-nano materials. Based on the thornyrod-like CdS, CdS/titanate composite nanostructure, consisting of nanoroses andnanowires, is prepared by hydrothermal method. The amount of P25is vital for theformation of rose-like nanostructure: a sufficient amount of titanium dioxide caneffectively compete with S2-ion in all related reactions, inhibit the free growth of CdSand make the transition from nanoparticle to nanoroses; inadequate amount oftitanium dioxide, however, gaves rise to diverse morphologies, such as particles,irregular columns and fibres. The formation and release rate of S^2-ion determines thenucleation and growth rate of CdS dynamically, which has a decisive role on themorphology of the final products. From the kinetic point of view, the rate controllingstep of chemical reactions is the key to prepare nanomaterials with specificmorphologies and properties.（4） The interaction of sulfuric acid with rutile titanium dioxide （110） surfacefrom density functional theory calculations is carried on the CASTEP module ofMaterials Studio to determine the major adsorption mode from a thermodynamicpoint of view. The results show that, under strong acidic conditions, the chelationeffect between the potassium sulfate and [TiO₆] is mainly achieved by H+HSO₄ molecular fragment, partly by H₂SO₄molecular, but unlikely by2H+SO₄; the resultsfurther elaborate the crystal growth mechanism of titanium dioxide mixed crystalsfrom the energy point of view.