Properties of TiO2 catalyst

Since Fujishima et al.
discovered in 1972 that the TiO ₂ electrode can electrolyze water under ultraviolet light, it has many potential applications in the fields of photovoltaics, photocatalysis, photoelectrochemistry, and photoelectric sensors, which has caused worldwide research on TiO ₂ Boom
.

In the past few decades, nanoscience and technology have experienced rapid development

Titanium (Ti) is the elemental metal of TiO ₂ , and it is the fourth in the earth's metal reserves, next to aluminum, iron, and magnesium
.

Titanium is a typical transition element.

There are two main crystal types of TiO ₂ used for photocatalysis —anatase and rutile.
Among them, the anatase has higher catalytic activity.
Both crystal structures can be represented by interconnected TiO ₂ octahedra.
The difference between the two lies in the degree of distortion of the octahedrons and the way the octahedrons are connected to each other
.

These structural differences result in different mass densities and electronic band structures of the two crystal types

The recombination rate of electron-hole pairs generated by rutile TiO ₂ after being excited by light is fast.
Compared with anatase TiO ₂ , the recombination rate is slower.
Therefore, anatase TiO ₂ has a better catalytic oxidation effect
.

The electronic properties of solids are usually described by energy band models.
When isolated atoms represented by full or empty atomic orbitals converge, new molecular orbitals are formed.
The energy levels of these orbitals are so close that they form a continuous energy band, and full bonds are formed.
The orbitals form the valence band (VB), and the empty antibond orbitals form the conduction band (CB), as shown in Figure 4-1
.

Figure 4-1 Energy band model of a solid

TiO ₂ is a wide band gap n-type semiconductor
.

The main feature of a semiconductor is the existence of a band gap, and its energy band structure is usually composed of a low energy valence band (Valence Band, VB) full of electrons and an empty high energy conduction band (Conduction Band, CB), the valence band and the conduction band The area between is called the forbidden band, and the size of the area is called the forbidden band width

Under sufficiently high energy excitation, the semiconductor's valence band electrons undergo an inter-band transition, from the valence band to the conduction band, and at the same time generate the same number of positive charge vacancies in the valence band, called "holes"
.

Electrons in the valence band can migrate to this hole to form new holes.

Because TiO ₂ is non-toxic and odorless, it has the characteristics of high photocatalytic activity, high chemical stability, low price, and safe use.

It has attracted unprecedented attention, involving its catalytic activity and service life preparation methods, modification and fixation.

The size of semiconductor particles strongly affects the photocatalyst activity.
Nano-TiO ₂ particles have higher photocatalytic activity than ordinary particles, because nanoparticles have a significant quantum size effect, which is mainly manifested in the conduction band and valence band becoming discrete energy levels , The energy gap is widened, the photogenerated electrons and holes have stronger redox ability, and the activity of semiconductor photocatalytic oxidation is improved; at the same time, the surface area of ​​the nanoparticles is large, the adsorption capacity is strong, and due to the surface effect, there are a lot of particles on the surface Oxygen vacancies increase the reaction activity significantly
.

At present, TiO ₂ photocatalytic oxidation technology is still in the laboratory and theoretical exploration stage as a whole.