Controllable Synthesis And Noble Metal-modified TiO₂-based Photocatalysts With Enhanced Photocatalytic Activities

Recently, titanium oxide(Ti O2), as one of the most important transition metal semiconductors oxides, has attracted much more attention due to its superior physical and chemical performance, such as cheap, easy acceptance, non-toxicity, photo-stability, etc, and are widely applied in the fields of photocatalysis, water splitting for hydrogen production, biological, biomedical sensing and photovoltaic. However, the large bandgap(anatase~3.2e V) and the rapid combination of photoreduced charge mainly limit its further applications. Many efforts have been attempted to enhance the charge separation efficiency and the absorption in the visible spectrum by tuning of the band gap of Ti O2.Herein, monodispersed and uniform Ti O2 microspheres have been synthesized via a controlled hydrolysis of tetrabutyl titanate in ethanol combined with hydrothermal method, using tetrabutyl titanate, enthanol and KCl solution as precursors; silver nanoparticles-modified Ti O2 microspheres have been synthesized through soaking Ag-loading combined hydrothermal method, using Ti O2 microspheres as basic materials and Ag NO3 as silver source; Then, C, Ag co-modified Ti O2 microspheres have been synthesized via soaking Ag-loading combined post-calcinations, in which the Ag NO3 and citric acid was used as Ag and C source, respectively. Finally, silver-modified Ti O2 microspheres have been synthesized through a novel in-situ Ag-loading method, in which tetrabutyl titanate(TBOT) is hydrolyzed in a mixed solution of ethanol and Ag NO3. The structures, compositions and optical properties of the as-prepared products were characterized by XRD, FESEM, HRTEM, FT-IR, PL, Raman, XPS, photocurrent responses(I-t curves) and UV-vis diffuse reflectance techniques, and the photocatalytic properties of the product was estimated for the photocatalytic of simulated pollutants under visible light irradiation. And then, we observed the effect of synthesis processes on compositions, microstructure and photocatalytic properties of as-prepared Ti O2-based photocatalysts, and proposed a possible mechanism for relative photocatalytic reactions under visible light irradiation.The results show that:(1) monodispersed and uniform Ti O2 microspheres have been synthesized via a controlled hydrolysis of tetrabutyl titanate in ethanol, in which the optimal synthesis processes are: volume of KCl solution(0.4 m L) and hydrothermal temperature(160℃). The as-prepared products reveal spherical structures with the smooth surface and a narrow distribution(with the diameters of about 650-700nm), which consist of large amount of fairly uniform Ti O2 grains;(2) With the silver species introduced into hybrid system, the photo-response and absorption would greatly enhance. The increasing content of silver species would change the Ti O2 surface in turn. And the as-prepared silver-modified Ti O2 photocatalysts could attach large numbers of micro-pores on its surface(The optimal addition volume of Ag NO3 solution 2.5m L), which would be beneficial to adsorb enough pollutant, and thus accelerating photocatalytic activites.(3) Citric acid, as one of the common reducing agents, could not only largely increase the density of silver nanoparticles attached on C, Ag co-modified Ti O2 microspheres ascribed to its reductive property; Moreover, citric acid will produce newly carbonlike species in heat treatment, which might coat on the surface of Ti O2 microspheres and interact with silver nanoparticles to co-enhance the surface plasmonic resonance(SPR) effect so as to increase of their absorption and light response in ultraviolet and visible-light region.(4) Silver-modified Ti O2 microspheres have been synthesized via a novel in-situ Ag-loading method that control the hydrolysis of tetrabutyltitanate(TBOT) in a mixed solution of ethanol and Ag NO3, in which the bonding growth of silver nanoparticles on the surface of Ti O2 is isolated and prevents their migration and coalescence during the catalytic reactions, thus improving the physical and chemical stability of the unique hybrid system. The monodispersed Ti O2-based microspheres with a loose structure and the diameter of about 650-700nm; they are composed of large amounts of fairly uniform Ti O2 nanoparticles with a diameter of about 20-30 nm, and lots of bright silver grains and the presence of micro-pores on the surface, which can be attributed to the phase separation caused by the addition of silver species. This unique structure gathering abundant micro-pores on the surface is able to adsorb enough pollutants. On the other hand, the as-prepared Ti O2-based microspheres could obtain abundant photo-response, absorption and relatively narrow band gap due to the surface plasmon resonance(SPR) effect of the silver species, all of which could enhance the photocatalytic activities in the visible light region.