Typical Synthesis and Environmental Application of Novel Ti Nanoparticles - Advanced Materials for Agriculture, Food and Environmental Safety - page 424

O

CH(CH 3 ) 2

CH 3

H 3 C

C OH

CH(CH 3 ) 2

SO 3 H

Figure 15.4 h e chemical structure of h ymol blue.

15.2.4 Bromocresol Green (BG)

Bromocresol green (3,3,5,5'-tetrabromo-m-cresolsulfonphthalein) is a
dye of the triphenylmethane family (triarylmethane dyes), which is used
as a pH indicator and as a tracking dye for DNA agarose gel electrophore-
sis. In aqueous solution, both solids ionize to give the monoanionic form
(yellow), that further deprotonates at higher pH to give the dianionic form
(blue), which is stabilized by resonance. h e absorbance of Bromocresol
green is 616 nm, which was measured by a UV-vis spectrophotometer.
h e chemical structure and resonance form of Bromocresol green are pre-
sented in Figure 15.5.
h erefore, TiO 2 is an interesting material for photocatalytic applica-
tions and is regarded as the most ei cient and environmentally benign
photocatalyst, and it has been most widely used for photodegradation of
various pollutants such as Rhodamine B (RhB) [73-76], Rhodamine 6G
(Rh6G) [77-85], methyl orange (MO) [78, 79], methylene blue (MB) [78,
80-83], thymol blue [84], Bromocresol green [41] and phenol [75-83].
h e principle of the semiconductor photocatalytic reaction is straightfor-
ward. When photons with energies > 3.2 eV, i.e., exceeding the band gap
energy of TiO 2 , are absorbed by the anatase particles in the mesoporous
TiO 2 photocatalysts, electrons are rapidly promoted from the valence band
to the conduction band leaving holes behind in the valence band [86]. h e
thus formed electrons and holes participate in redox processes at the semi-
conductor/water interface. h e valance band holes migrate to the surface
of the particles where they react with adsorbed hydroxide ions (or water
molecules), generating adsorbed ∙ OH radicals. h is photodecomposition
process usually involves one or more radicals or intermediate species such
as HO - , O .- , H 2 O 2 or O 2 .- , which play important roles in the photocatalytic