The Preparation And Modification Of TiO₂Nanotube Arrays And Their Photocatalytic Properties

Over the past decade, because of its high regulation, large surface areas, high recycleefficiency and excellent controllability, highly ordered TiO₂nanotube arrays (TNA) byanodization have attracted great attention on photocatalysis, gas sensing, biomedicine,environmental monitoring, and solar energy conversion. However, the band gap of TiO₂isvery wide (3.0-3.2eV) leading to its absorption only in UV light, otherwise, catalyst surfacephotoinduced charges recombination and lower IPCE limit the applicability of TNA. In orderto promote its visible light photocatalytic activity, many researches on the improved methodfor preparing TNA, reasonable modification were developed, which is great significance onthe wide range of practical applications. In this paper, a new hierarchically structured TiO₂(HST) film was prepared. The resultant HST films as photoanodes were evaluated byphotocatalytic oxidation of water or organics. The formation mechanism was investigated.Highly dispersed and nano-size polyhedral Cu₂O loaded on TiO₂nanotube arrays (Cu₂O/TNA)were successfully prepared by an electrodeposition method, which exhibited excellentphotocatalytic and good bactericidal activity under visible light. Ag nanoparticles loadedN-doped TiO₂nanotube arrays (Ag/N-TNA) were successfully synthesized, improved visiblelight photocatalytic activities were observed. The main contents are as follow:Transparent TiO₂nanostructured films composed of porous TiO₂nanoparticle top layer andhighly ordered TiO₂nanotube bottom layer were successfully fabricated onto metal titaniumdeposited FTO conducting substrates by anodization technique. For the hierarchicallystructured TiO₂(HST) film, the top porous nanoparticle film had a thickness of ca.300nm,and the bottom nanotubes have a mean length of1.7μm and a mean outer diameter of150nm.The total thickness of the hierarchically structured TiO₂film was around2.0μm. XRDanalysis revealed that the fabricated hierarchically structured TiO₂film after calcination at500°C was predominantly consisted of anatase (76%). The resultant HST films asphotoanodes were evaluated by photocatalytic oxidation of water or organics (i.e., glucose).The experimental results indicated that the HST film photoanodes exhibited higherphotocatalytic activity than the TNA film fabricated on titanium foil substrates as the samelength, which is due to larger photocatalytic activity area supplied by the top nanoparticleporous layer and excellent photoelectron transport in bottom nanotube layer of the HST filmcompared to the single nanotube film (TNA). Photoelectron transport property (reflected byconstant resistance, R0) in two nanostructured films were characterized by a simplephotoelectrochemical method. Although the constant resistance of HST film (R0=85.5Ω) is larger than that of TNA film (R0=65.5Ω), the porous nanoparticle top layer for the HST filmcould effectively enhance the photocatalytic activity area, and thus significantly improvingthe resultant photocatalytic activity compared to the TNA film.The highly ordered TiO₂nanotube array (TNA) films were fabricated on titanium foilsubstrates by anodization, and then annealed at500℃for4h under ammonia, N-TNA wasprepared. Ag loaded N-doped TiO₂nanotube arrays (Ag/N-TNA) were fabricated by anelectrodeposition method. The Ag/N-TNA was characterized by Field-emission scanningelectron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), UV-vis diffusion refection spectroscopy (DRS). The effect of differentelectrochemical deposition time on photocatalytic activity was investigated. The resultsshowed the best depositon of time was5s. The average photocurrent density andphotocatalytic degradation efficiency of the Ag/N-TNA obtained from the degradation ofAcid Orange II(AO-II)weree6.0and6.8times higher than those of pure TNA, respectively.This bestows the Ag/N-TNA as a promising photocatalytic material for organic pollutantdegradation under visible light.Highly dispersed and nano-size polyhedral Cu₂O loaded on TiO₂nanotube arrays(Cu₂O/TNA) were successfully fabricated by an electrodeposition method. The obtainedsamples were characterized by FESEM, XRD, XPS and UV-vis diffusion refectionspectroscopy (DRS). The results showed nanotubes had a mean length of900nm, a meaninter diameter of100nm and the wall thickness of approximately20nm. The content of Cu₂Oloaded on the arrays was controlled by changing the deposition coulombs. The results showedthe photocurrent density and photocatalytic degradation efficiency of Cu₂O/TNA obtainedwhen the deposition coulombs arrived500mC were about35.5and18.2times as that of pureTiO₂nanotube arrays under visible light, respectively. The polycrystalline Cu₂O surfacemodified TNA will be a promising visible-light photocatalytic material in solar energyconversion and degradation of organic pollution.Using a non-toxic nanocrystal Cu₂O-loaded TiO₂nanotube array (Cu₂O/TNA) film washigh performance visible light bactericidal photocatalyst. This Cu₂O/TNA film photocatalystwas capable of complete inactivation of high concentration E. coli (5×10⁷CFU/mL) within arecord short disinfection time of20min under visible light irradiation. The averagebactericidal percentage of the Cu₂O/TNA for E. coli under visible light irradiation were20times and6.6times higher than those of TNA under the same conditions and Cu₂O/TNAwithout light, respectively. This superior bacterial performance is mainly attributed to the high ability to produce OH radicals by both photogenerated electron and hole of the preparedphotocatalyst under visible light.