Morphology Control Of TiO₂ And Its Photocatalytic CO₂ Reduction Activity

With the rapid increasing of enormous challenges in energy demands and environmental pollution ignited by fossil fuels consumption, developing of renewable and green technologies for energy production has aroused widespread concern in the past few decades. Among various proposed technologies, semiconductor-based photocatalysis for CO2 reduction have been known as one of the most perspective strategies because of its potential in renewable energy creation and environmental remedies. In details, photocatalysis CO2 reduction can transform the green-house gas(CO2) into the valuable solar fuels such as CH4, HCO2 H, CH2 O and CH3 OH. This outstanding ability of the photocatalysis CO2 reduction is the main motivation for the searching of efficient and visible-light active photocatalyst. After years of research and development, many semiconductors have been assayed and explored to be used as an effective photocatalyst for photocatalysis CO2 reduction, such as Ti O2, Cd S, g-C3N4, Zn O and Bi2WO6. However, the practical application of photocatalysis CO2 reduction is still limited by its low CO2 conversion efficiency due to the fast charge carriers recombination and low light utilization. Among these semiconductors, Ti O2 has regarded as the most promising photocatalyst because of its low cost, environmentally friendliness and excellent photostability. However, Ti O2 owns low photoconversion efficiency even in the UV-region due to its large bandgap, less photocatalytic reaction site, low electron mobility and short minority carrier diffusion length. Several strategies have been carried out to improve the photoconversion efficiency of the Ti O2, such as extending the Ti O2 absorption to visible light, boosting electron and hole lifetimes through doping and loading metals, and increasing the surface area by building a porous structure.Morphology tuning is an effective way to enhance the photocatalytic activity of the photocatalyst. Nowadays, there is an enormous interest in controlling Ti O2 morphology due to their fascinating shape-dependent physicochemical properties The advancements of the nano-sized semiconductor has arisen much attention from the scientific community. Due to the rapid growth of the nano-materials, researches on the nano-sized Ti O2 for photocatalysis applications are also under fast-paced evolution, and a lot of interesting and stunning finding have been made in the past few years. It is apparent that the smaller size of the Ti O2 can lead to the enhancement of specific surface area, which is beneficial for creating more reaction site for the photocatalysis reaction. Moreover, with the proper morphological tuning, the photoinduced charge carriers can be migrated to the surface of the Ti O2 rapidly, resulting in fast photocatalytic reaction. Therefore, much studies and researches have been carried out in the shape-controlled synthesis of Ti O2 for enhancing photocatalytic activity. In addition, synthesis of complex and hierarchical hetero-nanostructures has been introduced to boost the photoinduced electron-hole pair's separation and enhance light absorption efficiency of the Ti O2 for enhancing photocatalytic activity.Herein, we report for the first time the photocatalytic reduction of CO2 over anatase Ti O2 with truncated octahedral bi-pyramid morphology prepared by hydrothermal method were examined. Moreover, the effect of the ratio of co-exposed {001} and {101} facets on the aforementioned reduction process. Further, a new “surface heterojunction” concept is proposed on the basis of the DFT calculations to explain the difference in the photocatalytic activity of anatase Ti O2 with co-exposed {001} and {101} facets.Thereafter, Ag loaded Ti O2 nanotube arrays(TNTAs) were prepared by simple electrochemical anodization of Ti foil in fluoride-based electrolytes, followed by electrochemical deposition of Ag into the TNTAs, overcoming the serious limitations of the conventional powder nanomaterials, the need for post-treatment separation in a slurry system. Moreover, the electrochemical deposition of Ag is expected to overcome the capillary effect of TNTAs and Ag NPs can be directly deposited into the interior of Ti O2 nanotubes. The evenly distribution of Ag NPs can enhance the SPR effect, which is beneficial for enhancing the photocatalytic activity of the photocatalyst.Then, a simple and novel anodization and calcination method was implemented to prepare reusable Ti O2 photonic crystals. Then, a novel method was prepared to evaluate the photocatalytic CO2 reduction activity of the samples. Through this method, the PBG of the Ti O2 photonic crystal could be varied either into or out of the electronic absorption band of the Ti O2 during the photocatalytic CO2 reduction. In order to determine the superior properties of the Ti O2 PCs, Ti O2 nanotube array(Ti O2 NTA), which does not exhibit any photonic crystals properties was prepared as a comparison.