TiO 2 in a powder form possesses some several drawbacks such as poor porosity, low
adsorption capacity and its poor recovery from water (Bedia et al. 2019). In
addition, it was seen that the photocatalytic activity of TiO 2 anatase, possesses a
band gap energy of 3.2 eV (λ ! 387 nm) which is high and needs to be activated by
UV radiation (Kumar and Rao 2017; Lee et al. 2016). Therefore, it is necessary to
modify the surface of TiO 2 with materials such as carbon, graphene or metal
deposition (Bedia et al. 2019; Kumar and Rao 2017; Gao et al. 2014). However,
it is always difficult to prepare TiO 2 based photocatalysts for wastewater treatment
using visible and solar light. Consequently, it is imperative to explore competent,
robust and cost-effective photocatalyst for replacement of the traditional ones. In the
past few years, a type of crystalline materials named metal organic organic frameworks (MOFs) have received consideration in photocatalysis. These type of materials offer a wide spectrum of applications due to their structural arrangement of
coordination bonds between unsaturated metal core/node and multidentate organic
linkers (catalytically active) (Kojtari and Ji 2015). Furthermore, MOFs possess large
surface area and well-ordered porous structures that can contribute significantly in
many fields. In HP, the use of MOFs photocatalysts in HP is mainly based due to the
following important factors such as encapsulation of chromophores in the internal
structure of MOF, promotion of e
À /h
+ separation in the metal core and preparation of
MOFs using materials with absorption bands at visible region (Bedia et al. 2019;
Llabrés i Xamena et al. 2008). Moreover, some MOFs such as MOF-5 (Llabrés i
Xamena et al. 2007), NTU-9 (Gao et al. 2014) and UiO-66 (Shen et al. 2013) can act
as semiconductors. In these MOFs, the energy transfer takes place from the organic
linker to the metal-oxo cluster (Qiu et al. 2018). Nevertheless, most MOF
photocatalysts possess large band gap values caused by an insulating character of
the organic linker which results in poor conductivity (Ramohlola et al. 2017a, b, c,
2018; Monama et al. 2018, 2019; Mashao et al. 2019). The large band gap limits
their further application (Qiu et al. 2018). and this could be enhanced using dye
sensitized materials (Yuan et al. 2015), decoration of linker or metal center (Silva
et al. 2010; Fu et al. 2012) and also to combine with other semiconductors (Majedi
et al. 2016). Hence, surface modification and functionalization of MOFs are required
for their application as suitable photocatalytic materials.
10.4 Metal Organic Frameworks
By a careful inspection of the structure of MOFs (as shown in Fig. 10.2), one can see
that the metal nodes function as both the joining points and the organic linkers whose
sole purpose is to connects the ligands (Loera-Serna et al. 2012; Llabrés i Xamena
et al. 2007). Due to the distinct crystallinity of MOFs, identification of the exact
positions of all atoms in the framework can be observed. To properly identify a
porous solid, the structure should contain permanent channels or pores which
permeate through it and have dimensions huge enough to permit solvent or other
molecules to migrate into the structure (Kumar et al. 2013). The other great features
266
T. C. Maponya et al.
adsorption capacity and its poor recovery from water (Bedia et al. 2019). In
addition, it was seen that the photocatalytic activity of TiO 2 anatase, possesses a
band gap energy of 3.2 eV (λ ! 387 nm) which is high and needs to be activated by
UV radiation (Kumar and Rao 2017; Lee et al. 2016). Therefore, it is necessary to
modify the surface of TiO 2 with materials such as carbon, graphene or metal
deposition (Bedia et al. 2019; Kumar and Rao 2017; Gao et al. 2014). However,
it is always difficult to prepare TiO 2 based photocatalysts for wastewater treatment
using visible and solar light. Consequently, it is imperative to explore competent,
robust and cost-effective photocatalyst for replacement of the traditional ones. In the
past few years, a type of crystalline materials named metal organic organic frameworks (MOFs) have received consideration in photocatalysis. These type of materials offer a wide spectrum of applications due to their structural arrangement of
coordination bonds between unsaturated metal core/node and multidentate organic
linkers (catalytically active) (Kojtari and Ji 2015). Furthermore, MOFs possess large
surface area and well-ordered porous structures that can contribute significantly in
many fields. In HP, the use of MOFs photocatalysts in HP is mainly based due to the
following important factors such as encapsulation of chromophores in the internal
structure of MOF, promotion of e
À /h
+ separation in the metal core and preparation of
MOFs using materials with absorption bands at visible region (Bedia et al. 2019;
Llabrés i Xamena et al. 2008). Moreover, some MOFs such as MOF-5 (Llabrés i
Xamena et al. 2007), NTU-9 (Gao et al. 2014) and UiO-66 (Shen et al. 2013) can act
as semiconductors. In these MOFs, the energy transfer takes place from the organic
linker to the metal-oxo cluster (Qiu et al. 2018). Nevertheless, most MOF
photocatalysts possess large band gap values caused by an insulating character of
the organic linker which results in poor conductivity (Ramohlola et al. 2017a, b, c,
2018; Monama et al. 2018, 2019; Mashao et al. 2019). The large band gap limits
their further application (Qiu et al. 2018). and this could be enhanced using dye
sensitized materials (Yuan et al. 2015), decoration of linker or metal center (Silva
et al. 2010; Fu et al. 2012) and also to combine with other semiconductors (Majedi
et al. 2016). Hence, surface modification and functionalization of MOFs are required
for their application as suitable photocatalytic materials.
10.4 Metal Organic Frameworks
By a careful inspection of the structure of MOFs (as shown in Fig. 10.2), one can see
that the metal nodes function as both the joining points and the organic linkers whose
sole purpose is to connects the ligands (Loera-Serna et al. 2012; Llabrés i Xamena
et al. 2007). Due to the distinct crystallinity of MOFs, identification of the exact
positions of all atoms in the framework can be observed. To properly identify a
porous solid, the structure should contain permanent channels or pores which
permeate through it and have dimensions huge enough to permit solvent or other
molecules to migrate into the structure (Kumar et al. 2013). The other great features
266
T. C. Maponya et al.