The Preparation Of TiO₂-based Nanomaterials And Their Photoelectrocatalytic Performance

Titanium dioxide(Ti O2) has attracted great attention in heterogeneous photocatalysis applications due to its high reactivity, strong oxidizing power, long-term stability and non-toxicity. It has been found that the photocatalytic performance of Ti O2 is mostly governed by their microstructure, polymorph composition and optical properties, etc. However, how the physical properties of the Ti O2-based nanomaterials influence their photocatalytic performances are not fully understood yet. Here, we investigated the influence of crystallite size and crystallinity of Ti O2 nanoparticles on the photodegredation of phenol systematically, and also the microstructure of Ti O2 nanotubes. The Ti O2 nanotubes was further doped by Ni Mo Zn alloy nanoparticles to prepare an earth-abundant, long-term stability and high reactivity photocatalyst for water splitting hydrogen evolution. Such studies are essential for the design of efficient photocatalyst toward large-scale application of photocatalysis.Two series of crystallite size and crystallinity controlled pristine anatase nanoparticles have been synthesized under supercritical water-isopropanol conditions. Photocatalytic degradation of phenol was carried out to evaluate the influence of crystallite size and crystallinity on the performance of anatase. We discovered that the reactivity of anatase nanoparticles is independent of the crystallinity. In contrast, increasing the crystallite size from 6.6 nm to 26.6 nm resulted in an enhancement of the photocatalytic performance. By tracking the evolution of phenolic intermediates and analyzing the reaction kinetics we revealed how the crystallite size influences the photo-decomposition of phenol. When small anatase nanoparticles were used as photocatalysts, phenol was selectively oxidized to benzoquinone, and then it subsequently underwent a rapid redox reaction wherein photo-generated readicals were inefficiently consumed. Conversely, hydroqu inone was found to be the dominant intermediate product in the phenol decomposition when using large anatase nanoparticles. In this case, the rates of hydroquinone-benzoquinone redox reactions were significantly suppressed, which facilitated the full decomposition of phenol and phenolic compounds.The self-organized Ti O2 nanotubes, which was characterized by high specific surface area and excellent electronic property, has attracted considerable interest over the past decade. Here we prepared a series of Ti O2 nanotubes by varying the concentration of F- in the electrolyte, the oxidation potential, temperature and oxidation time. The micromorphology of the Ti O2 nanotubes and the corresponding photocatalytic and photoelectrocatalytic performance were further studied. The results showed that, the presence of F- in the electrolyte was essential for the formation of Ti O2 nanotubes. Both the length and the diameter of the Ti O2 nanotubes increased as the increasing of F- concentration. However, excessive F- in the electrolyte will over-etch the Ti O2 nanotube surface, which resulted in shorter and irregular arrays. Meanwhile, the potential, temperature and oxidation time varied the length, tube diameter and the regularity of the Ti O2 nanotubes. The optimized conditions for preparing Ti O2 nanotubes was the F- concentration of 150 mmol/L, the potential of 50 V, the temperature of 15 ℃, the time of 120 min. By investigating the micromorphology of Ti O2 nanotubes on their photocatalytic and photoelectrocatalytic performance, we found that the increasing of the tube length, diameter and regularity improve the UV absorption of Ti O2 nanotubes and the charge transfer efficiency. While, the surface impurities will act as the recombination center of photo-generated electron and hole pairs, which will depress the photocatalytic reactivity of Ti O2 nanotubes.To further improve the photocatalytic performance of Ti O2 nanotubes for water splitting hydrogen evolution reaction, the Ni Mo Zn alloy nanoparticles was deposited onto the Ti O2 nanotube surface using the UV-assistant electro-deposition method. We found that, the catalytic performance of Ni Mo Zn alloy nanoparticles was mostly governed by its Zn concentration. The addition of small amount of Zn(1~3 at%) will significantly enhance its electrocatlaytic performance by improving the charge transfer process during the water splitting reaction. However, excessive Zn in the alloy nanoparticles not only depresses the charge transfer process, but also inhibits the proton adsorption process. The UV light irradiation remarkably improves the wetability of Ti O2 nanotubes, which is beneficial for the electro-deposition of Ni Mo Zn alloy nanoparticles. After deposition of 15 s, the Ni Mo Zn alloy nanoparticles loaded onto the Ti O2 nanotubes surface uniformly. And the photon to current conversion efficiency of the doped Ti O2 nanotubes was ~8% higher than previously.