interlinked by sharing corner oxygen atoms to form a three-dimensional lattice.
Contrarily in anatase, the three-dimensional lattice is formed only through sharing
edge bonding among TiO 6 octahedra. It means that the octahedra in anatase share
four edges and are arranged in zigzag chains while the octahedra in brookite share
both edges and corners and form an orthorhombic structure [66, 67].
Researchers usually use the X-ray diffraction (XRD) experimental method to
determine these crystal structures and estimate the crystal grain size of anatase,
rutile, and brookite. Anatase peaks in X-ray diffraction are found at θ ¼ 12.65
,
18.9
, and 24.054
; the rutile peaks occur at θ ¼ 13.75
, 18.1
, and 27.2
, while
brookite peaks emerge at θ ¼ 12.65
, 12.85
, 15.4
, and 18.1
(θ represents the
X-ray diffraction angle) [69, 70].
6.2.2 Stability and Phase Transformation
Compared to the other two types of phases, rutile is the most stable phase. It is almost
impossible for rutile to decompose or undergo a phase transformation even at very
high temperatures. However, metastable anatase and brookite can be transformed
into thermodynamically stable rutile when they are calcined to a certain temperature.
There has been extensive research on the phase transfer mechanism of TiO 2 during
the calcination process. Shannon [71] proposed that the transformation of anatase to
rutile from crystallography includes a nucleation and growth process. At first, rutile
nucleates on the surface of anatase and then expands to the bulk. Due to the great
diversities between anatase and rutile, the transformation involving the breaking and
reforming of bond processes can be reconstructed [72]. During the course of anatase
transforming to rutile, the {112} planes in anatase are persisted as the {100} planes
in the newly generated rutile. And Ti and O atoms synergistically rearrange in these
planes by moving Ti atoms to a new location to form rutile via the breaking of two
Ti–O bonds in the TiO 6 octahedron. As a result, the newly formed oxygen vacancies
accelerate the transformation; meanwhile, Ti interstitials inhibit the phase transformation. The transformation from anatase to rutile is a nonequilibrium phase transition, which usually occurs at a certain range of temperature (400~1000
C). During
the transformation process, the calcination temperature has a great influence on the
impurities, particle size, and surface area of products. Because impurities and
processing atmosphere can result in different defect structure, they also strongly
influence the temperature and rate of phase transition. Generally, impurities such as
the oxides of Li, K, Na, Fe, Ce, and Mn usually promote the phase transformation via
increasing the oxygen vacancies; on the contrary, impurities like S, P, and W usually
restrain the phase transformation. A reductive atmosphere such as H 2 and Cl 2 can
accelerate the transformation, while a conducive atmosphere can inhibit the phase
transformation via the formation of Ti interstitials.
Artificial synthesis conduces to the preparation of anatase nanoparticles, especially the synthesis of TiO 2 in aqueous solution [73] for the reason that the energy of
the three kinds of TiO 2 phases is quite close. On condition that the nanoparticles are
136
6 Phase Control of TiO 2 Photocatalyst
Contrarily in anatase, the three-dimensional lattice is formed only through sharing
edge bonding among TiO 6 octahedra. It means that the octahedra in anatase share
four edges and are arranged in zigzag chains while the octahedra in brookite share
both edges and corners and form an orthorhombic structure [66, 67].
Researchers usually use the X-ray diffraction (XRD) experimental method to
determine these crystal structures and estimate the crystal grain size of anatase,
rutile, and brookite. Anatase peaks in X-ray diffraction are found at θ ¼ 12.65
,
18.9
, and 24.054
; the rutile peaks occur at θ ¼ 13.75
, 18.1
, and 27.2
, while
brookite peaks emerge at θ ¼ 12.65
, 12.85
, 15.4
, and 18.1
(θ represents the
X-ray diffraction angle) [69, 70].
6.2.2 Stability and Phase Transformation
Compared to the other two types of phases, rutile is the most stable phase. It is almost
impossible for rutile to decompose or undergo a phase transformation even at very
high temperatures. However, metastable anatase and brookite can be transformed
into thermodynamically stable rutile when they are calcined to a certain temperature.
There has been extensive research on the phase transfer mechanism of TiO 2 during
the calcination process. Shannon [71] proposed that the transformation of anatase to
rutile from crystallography includes a nucleation and growth process. At first, rutile
nucleates on the surface of anatase and then expands to the bulk. Due to the great
diversities between anatase and rutile, the transformation involving the breaking and
reforming of bond processes can be reconstructed [72]. During the course of anatase
transforming to rutile, the {112} planes in anatase are persisted as the {100} planes
in the newly generated rutile. And Ti and O atoms synergistically rearrange in these
planes by moving Ti atoms to a new location to form rutile via the breaking of two
Ti–O bonds in the TiO 6 octahedron. As a result, the newly formed oxygen vacancies
accelerate the transformation; meanwhile, Ti interstitials inhibit the phase transformation. The transformation from anatase to rutile is a nonequilibrium phase transition, which usually occurs at a certain range of temperature (400~1000
C). During
the transformation process, the calcination temperature has a great influence on the
impurities, particle size, and surface area of products. Because impurities and
processing atmosphere can result in different defect structure, they also strongly
influence the temperature and rate of phase transition. Generally, impurities such as
the oxides of Li, K, Na, Fe, Ce, and Mn usually promote the phase transformation via
increasing the oxygen vacancies; on the contrary, impurities like S, P, and W usually
restrain the phase transformation. A reductive atmosphere such as H 2 and Cl 2 can
accelerate the transformation, while a conducive atmosphere can inhibit the phase
transformation via the formation of Ti interstitials.
Artificial synthesis conduces to the preparation of anatase nanoparticles, especially the synthesis of TiO 2 in aqueous solution [73] for the reason that the energy of
the three kinds of TiO 2 phases is quite close. On condition that the nanoparticles are
136
6 Phase Control of TiO 2 Photocatalyst