Thesis Summary
Mechanochemical synthesis of visible-light-active TiO2 photocatalysts: relation between
photocatalytic activities and disorder structures(可視光応答型酸化チタン光触媒のメカノケミカル合成：触媒活性と乱れた構造の相関)
Name: WANG YUFENG
General introduction
Titanium dioxide (TiO 2 ) is the most popular photocatalyst and is used for
self-cleaning and self-sterilization applications, water and air purification, and as awater-splitting catalyst. TiO 2 photocatalyst is activated by UV light due to its wide band gap(3.0~3.2 eV, λ ≤ 387.5 nm). Visible-light activated TiO 2 has attracted much attention using
metal-ion or non-metal doping. However, almost all synthesis methods have required specificexperimental conditions, such as ion-implantation facilities, vacuum, and high temperatures up
to 400-800 °C for several hours.
A mechanochemical synthesis method has emerged as alternatives to modify TiO 2 ,because large amount of particles can be easily obtained by simply grinding solid materials in a
milling vessel with milling balls. High-energy ball milling method is a useful approach toprepare TiO 2 particles, because large amount of particles can be easily obtained by simply
grinding solid materials in a milling vessel with milling balls at room temperature. In fact,mechanochemistry was identified by IUPAC as one of ten world-changing technologies in
2019. In this study, we clarified the crucial factors governing the properties of photocatalysts
and successfully designed high efficient TiO 2 photocatalysts via mechanochemical synthesis.
Chapter 1.

Mechano-synthesized orange TiO 2 shows significant photocatalysis

under visible light
Nitrogen and carbon co-doped TiO 2 particles with a brilliant yellow-orange color were
produced mechanochemically by high-energy ball milling as one-pot synthesis. This facile
synthesis required only grinding TiO 2 with melamine at room temperature. Using
monochoromatic lights with the same intensity, the photocatalytic activity of the TiO 2 samples
was evaluated with respect to the degradation of methylene blue (MB) solution. The activities
under visible light (450 and 500 nm) were, respectively, 4 and 2 times higher than that of thepristine TiO 2 under UV light (377 nm). The properties and structure of the co-doped TiO 2
particles before and after milling were analyzed using eight experimental methods. As a result,it was found that the nitrogen replaced as an oxygen site in milled TiO 2 has the highest
concertation (2.3%) in the past studies and the structure of milled TiO2 is composed of apolymorphism of four different solid phases of TiO 2 . A good repeatability of the photocatalyst
was investigated by the number of cycles for the decomposition reaction of the aqueous dyesolution.

Chapter 2.

Mechanochemical Synthesis of Red-Light-Active TiO 2 Photocatalysts:

Four Colors, Defect-Rich, Polymorphism, and No Metal Loading
Herein we report the syntheses of four colored TiO 2 photocatalysts, green, gray, orange, andyellow, via a mechanochemical method (Table). All four colored TiO 2 samples, which had
absorptions in the UV-vis-NIR regions. The densities of the defects, oxygen vacancies, and Ti 3+
in TiO 2 were quantified and were used to evaluate the photocatalytic activity in conjunction
with the contents of the four polymorphic phases, i.e. anatase, rutile, TiO 2 Ⅱ, and amorphousphases. Green and orange TiO 2 , which contained all four phases and had high densities of
dopants and oxygen vacancies, exhibited a 5-fold enhancement of the photocatalytic activity
with respect to that of P25. According to the action spectrum of the reaction rate constant, redlight from around the NIR region was vital in enhancing the photocatalytic reaction.
Furthermore, all the prepared TiO 2 photocatalysts did not need promoters nor metal-loading in
the reaction.
Chapter 3.

Two-stepmechanochemical

synthesisof

disorderedgreen

TiO 2-x

photocatalyst
To produce TiO 2 photocatalyst with rich and stable oxygen vacancies and Ti 3+ , it is better toseparate one-pot milling to two steps milling. The first step is TiO 2 ball milling in inert
ambient to produce the Vo and Ti 3+ earlier. Then, the second step is mechanochemical doping Nor C elements on the disordered surface of TiO 2 , so that the doping level is deeper and morestable. The present study investigated the photocatalytic activity of the TiO 2 particles preparedby two steps milling. The size, morphology, crystal structure, electronic structure of bonding
and spin density of TiO 2 particles as functions of milling time compared to one-pot millingwere evaluated by five different measurements.
General conclusions
1) The significant enhancement of TiO 2 photocatalytic activity was observed by adjusting theamount of added melamine in ball-milling. The electronic structure, bandgap energy (Eg) andthe wavelength of light absorption, polymorphism, and disorder of the TiO 2 particles could becontrolled in specific milling condition, which are also the important factors of photocatalytic
activity enhancement.
2) It is the first time to design and synthesize the full UV-vis region active TiO 2 photocatalyst
without metal loading, by milling P25 with melamine in argon ambient, which shown
significant enhancement of photoactivity than the pristine TiO 2 . The photocatalyst with colorof green is the most efficient sample. High photocatalytic activity is present in samples with adefect density > 4×10 18 mol -1 and an absorption area > 300 (Art. unit).
3) Based on the one-pot ball-milling method, a two-step milling method was used for green
TiO 2 synthesis. The synthesized disordered TiO 2 photocatalyst was more stable that keeping Voand Ti 3+ for over 2 years.