250
Nano-Photocatalyst Optimization
TiO 2 is the most widely used semiconductor photocatalyst in water/wastewater
treatment owing to its low toxicity, chemical stability, low cost, and abundance as
raw material. It generates an electron/hole (e
−
/h
+
) pair upon absorbing a UV photon,
which later either migrates to the surface and forms reactive oxygen species (ROS)
or undergoes undesired recombination. The photoactivity of nano-TiO 2 can be
improved by optimizing particle size and shape, reducing e
−
/h
+
recombination by
noble metal doping, maximizing reactive facets, and surface treatment to enhance
contaminant adsorption.
The size of TiO 2 plays an important role in its solid-phase transformation, sorption, and e
−
/h
+
dynamics. Among the crystalline structures of TiO 2 , rutile is the most
stable for particles larger than 35 nm, while anatase, which is more efficient in producing ROS, is the most stable for particles smaller than 11 nm [99, 352]. A major
cause for the slow reaction kinetics of TiO 2 photocatalysis is the fast recombination
of e
−
and h
+
. Decreasing TiO 2 particle size lowers volume recombination of e
−
/h
+
,
and enhances interfacial charge carrier transfer [356]. However, when particle size
is reduced to several nanometers, surface recombination dominates, decreasing
photocatalytic activity. Therefore, the photocatalytic activity of TiO 2 is maximum
due to the interplay of the aforementioned mechanisms, which lies in the nanometer
Table 13.2 TiO 2 photocatalyst optimization
Optimization
objectives
Optimization
approaches
Optimization mechanisms
Water treatment
applications
Enhance
photocatalytic
reaction kinetics
Size
More surface-reactive sites,
higher reactant adsorption,
lower electron/hole
recombination
High-performance
UV-activated
photocatalytic
Nanotube
morphology
Shorter carrier-diffusion
paths in the tube walls, higher
reactant mass transfer rate
toward tube surface
Reactors
Noble metal
doping
Better electron/hole
separation, lower electron/
hole recombination
Reactive
crystallographic
facets
Higher reactant sorption,
better electron/hole
separation, lower electron/
hole recombination
Expand
photoactivity
range
Metal impurity
doping
Anion doping
Dye sensitizer
doping
Narrow bandgap
Semiconductor
doping
Impurity energy levels
Bandgap narrowing
Electron injection
Electron injection
Low-energy-cost solar/
visible light-activated
photocatalytic reactors
13 Wastewater
Nano-Photocatalyst Optimization
TiO 2 is the most widely used semiconductor photocatalyst in water/wastewater
treatment owing to its low toxicity, chemical stability, low cost, and abundance as
raw material. It generates an electron/hole (e
−
/h
+
) pair upon absorbing a UV photon,
which later either migrates to the surface and forms reactive oxygen species (ROS)
or undergoes undesired recombination. The photoactivity of nano-TiO 2 can be
improved by optimizing particle size and shape, reducing e
−
/h
+
recombination by
noble metal doping, maximizing reactive facets, and surface treatment to enhance
contaminant adsorption.
The size of TiO 2 plays an important role in its solid-phase transformation, sorption, and e
−
/h
+
dynamics. Among the crystalline structures of TiO 2 , rutile is the most
stable for particles larger than 35 nm, while anatase, which is more efficient in producing ROS, is the most stable for particles smaller than 11 nm [99, 352]. A major
cause for the slow reaction kinetics of TiO 2 photocatalysis is the fast recombination
of e
−
and h
+
. Decreasing TiO 2 particle size lowers volume recombination of e
−
/h
+
,
and enhances interfacial charge carrier transfer [356]. However, when particle size
is reduced to several nanometers, surface recombination dominates, decreasing
photocatalytic activity. Therefore, the photocatalytic activity of TiO 2 is maximum
due to the interplay of the aforementioned mechanisms, which lies in the nanometer
Table 13.2 TiO 2 photocatalyst optimization
Optimization
objectives
Optimization
approaches
Optimization mechanisms
Water treatment
applications
Enhance
photocatalytic
reaction kinetics
Size
More surface-reactive sites,
higher reactant adsorption,
lower electron/hole
recombination
High-performance
UV-activated
photocatalytic
Nanotube
morphology
Shorter carrier-diffusion
paths in the tube walls, higher
reactant mass transfer rate
toward tube surface
Reactors
Noble metal
doping
Better electron/hole
separation, lower electron/
hole recombination
Reactive
crystallographic
facets
Higher reactant sorption,
better electron/hole
separation, lower electron/
hole recombination
Expand
photoactivity
range
Metal impurity
doping
Anion doping
Dye sensitizer
doping
Narrow bandgap
Semiconductor
doping
Impurity energy levels
Bandgap narrowing
Electron injection
Electron injection
Low-energy-cost solar/
visible light-activated
photocatalytic reactors
13 Wastewater