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This study is focused on developing visible-light sensitive anatase TiO 2 photocatalysts rather than the conventional UV-sensitive photocatalysts. We have adopted a surface modification technique using sodium borohydride (NaBH 4 ) chemical treatment. Thermal treatment under remarkably, favourable ambient air conditions were employed in this work instead of the previously reported inert gas or vacuum conditions, making it noteworthy. The synthesized surface-modified anatase TiO 2 is yellow in colour and has a crystalline core and disordered shell structure. The surface-modified sample was enriched with photocatalytically active shallow-traps due to the presence of Ti 3+ ions and singly ionized oxygen vacancies (V o + ). This surface-modified TiO 2 sample showed enhanced photodegradation of organic pollutants under visible light. The photocatalytic performances of the pristine and surface-modified samples were compared with the commercially available standard photocatalyst, Degussa P25. Notably, the surface-modified sample showed the highest degradation of Rhodamine B (RhB) dye, with a rate constant of 6.11 × 10 –3 min −1 . In contrast, the corresponding values for the pristine sample and Degussa P25 catalysts were only 4.15 × 10 –3 and 1.92 × 10 –3 min −1 , respectively. Our study summarises that surface modification is an effective strategy for developing efficient visible-light-sensitive TiO 2 photocatalysts. Graphical Abstract

Pristine and modified/doped titania are still some of the most widely investigated photocatalysts due to its high activity, stability, abundance and proper redox properties to carry out various reactions. However, modifiers and/or dopants resulting in visible-light activity might be expensive or work as recombination centers under UV irradiation. It seems that defective titania, known as “self-doped” TiO2, might be the best solution since it can be obtained under mild conditions without the addition of expensive materials and methods. This review discusses various methods of defective titania preparation, characterization of defect types, their localization (surface vs. bulk) and their function, as well as proposed mechanisms of photocatalytic reactions in the presence of self-doped titania. Although many kinds of defective titania samples have already been prepared with different colors, color intensities and defect kinds (mainly Ti3+ and oxygen vacancies), it is difficult to conclude which of them are the most recommended as the preparation conditions and activity testing used by authors differ. Furthermore, activity testing under solar radiation and for dyes does not clarify the mechanism since bare titania can also be excited and sensitized, respectively, in these conditions. In many reports, authors have not considered the possible influence of some impurities originated from the synthesis method (e.g., H, Al, Zn, Cl, F) that could co-participate in the overall mechanism of photocatalytic reactions. Moreover, some reports indicate that defective titania, especially black ones, might decrease activity since the defects might work as recombination centers. Despite some unproven/unclear findings and unanswered questions, there are many well-conducted studies confirmed by both experimental and theoretical studies that defective titania might be a promising material for various photocatalytic reactions under both UV and visible-light irradiation. Based on available literature, it could be proposed that optimal defects’ concentration, the preferential role of surface defects, a higher surface-to-bulk ratio of defects in rutile than in anatase, and the beneficial impact of disordered surface are the most important aspects to be considered during the preparation of defective titania.

The recent observation of spectacular photocatalytic activity enhancements generated tremendous interest in the synthesis, properties, and potential applications of black titania. Most black titania are core–shell structures consisting of a perfect crystalline core surrounded by a defective surface shell. Because the properties are attributed to the defective shell, it is particularly important, but very challenging, to obtain atomic structure information of the core, the shell, and the core–shell relationship on a single particle level. While the role of various synthesis approaches for producing black titania with different properties has been extensively reviewed, this review focuses on understanding the structure–functionality relationship in black titania on a single particle level. We start by introducing the crystal and electronic band structure of different TiO2 phases, followed by the discussion of particle size effects, the origin of lattice distortions, and phase control by synthesis, and concluding with the discussion of crystalline order formation and evolution creating the defective shell.

Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO2 and NaBH4 mixture at 350 °C successfully introduced Ti3+ defects and oxygen vacancies at the surface of TiO2, with an increase in the photocatalytic degradation of amoxicillin—an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO2 samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti3+ defects in a high concentration occurred mainly in the anatase sample annealed at 400 °C, contributing to the bandgap reduction from 3.32 eV to 2.92 eV. The reduced band gap enhances visible light absorption and the efficiency of photocatalysis. The nanoparticles of ~90 m2/g specific surface area and 12 nm average size exhibit ~100% efficiency in the degradation of amoxicillin under simulated solar irradiation compared with pristine TiO2. Mineralization of amoxicillin and by-products was over 75% after 48 h irradiation for the anatase sample, where the Ti3+ defects were present in a higher concentration at the catalyst’s surface.

Doping inorganic acid ions represents a promising pathway to improving the photocatalytic activity of TiO2, and oxygen vacancy has been regarded as the determinant factor for photocatalytic activity. A series of samples doped with Cl−, NO3−, and SO42− was prepared via a simple sol–gel method. Two different oxygen vacancies in the crystal layer of NO3−/TiO2 and Cl−/TiO2 were found, and those are [Ti3+]-V0-[Ti3+] and [Ti3+]-Cl, respectively. The photocurrent of NO3−/TiO2 with [Ti3+]-V0-[Ti3+] is significantly greater than that of Cl−/TiO2 with [Ti3+]-Cl. The least oxygen vacancy is in the gel layer of SO42−/TiO2, and the negligible photocurrent is due to difficulty in forming a stable sol. Furthermore, the process conditions for the application of TiO2 were investigated in this work. The optimal process parameters are to adjust the solution to pH = 3 during sol–gel preparation, to adopt 550 °C as the calcination temperature, and to use an alkaline electrolyte, while the rest of the preparation conditions remain unchanged. This work reveals a new avenue for designing efficient photocatalysts for air pollutant degradation.