Homogeneous Ag Nanoparticles Decorated TiO₂Nanotube Arrays And Their Photoelectrochemical Properties

TiO₂nanotube arrays （TNTA） fabricated by anodization method show greatpotential applications in photocatalytic degradation of organic pollutants,photolysis of water into hydrogen and so on, due to their high surface area, stability,high catalytic activity and its highly regular array channel for electrons transition.However, pure TNTA only absorb UV light of solar energy, which restricts theapplication of pure TNTA in photoelectrochemistry. In this paper, based on theorthogonal experiment design, we carried out the investigation of anodic oxidationparameters on the morphologies of TNTA. The Ag nanoparticles （NPs） wereintroduced onto the surface of TNTA, and the photoelectrochemical properties offabricated Ag/TNTA composites were also investigated. The results indicated thatthe TNTA with Ag NPs showed enhanced properties in photocurrent response andphotoelectro-degradation of methyl orange （MO）. The main contents of the paperare showed as following:（1） Anodic oxidation parameters showed great influence on the morphologiesof TNTA. The orthogonal test was arranged as per L25（56） orthogonal test table,with water content, anodization voltage and concentration of NH₄F as three factors.The length, inner diameter and wall thickness of prepared TNTA were measuredand calculated. The range analysis revealed that the anodic potential had the largesteffect on the experiment results. With the anodic potential increasing, the length,inner diameter and wall thickness of TNTA showed different degree of increase. Italso revealed the water content and concentration of NH₄F were less importantfactors to the experiment results. The change of water content and concentration ofNH₄F caused a fluctuation of nanotube length, inner diameter and wall thicknesswithin the limits of experiment.（2） For the successful loading of Ag NPs, the in-situ photoreduction methodwas introduced. The well-prepared TNTA was immersed in precursor solution for awhile, and then taken out for the reduction of absorption silver ions to metallic Ag0under UV light. The silver particle size and its content could be controlled just bychanging the immersion time. The photocurrent response and thephotoelectrocatalytic degradation of MO were used to evaluate the photoelectrochemical properties of the fabricated samples. The results showed thatthe photocurrent response and photoelectrocatalytic activity largely depended onthe loaded Ag particle size and content. TNTA films with a diameter of17.92nmand silver content of1.15at.%showed the highest photocurrent response anddegradation rate of MO. The enhanced properties could be attributed to thesynergistic effect between Ag NPs and TiO₂. Neither smaller nor larger Ag NPswill cause decreased photoelectrical properties and low usage of solar light.（3） To improve the location of Ag NPs in the TNTA, a new kind offlow-through TNTA （f-TNTA） supporter was prepared for the loading of Ag. Ahigh potential （140V） was immediately exerted at the end of anodization, then alow voltage was applied to improve the bottom morphologies, and finally themodified f-TNTA was obtained. Compared with the conventional TNTA（c-TNTAs）, the modified f-TNTA showed a better ability for well dispersion of Agnanoparticles. The Ag NPs could evenly load in the different region （upper, centraland bottom region） of f-TNTA, while only sporadic Ag NPs loaded in the centraland bottom region of c-TNTA. The photocatalytic properties of these two differentsupporters with different silver content were investigated. The results indicated thephotocatalytic activities of Ag/TNTA composite films relied on the loaded silvercontent and the distribution of Ag NPs in the TNTA. With the same loaded silvercontent, the Ag NPs in f-TNTA showed a higher photodegradation rate of MO thanc-TNTA. This is due to the inhomogeneous dispersion of Ag NPs in c-TNTA andthe decreased Ag-TiO₂active sites.