1.4.3 Surface Reaction
Upon obtaining the charge transfer kinetics and band structures, an in-depth insight
of charge dynamics in the photocatalytic reaction on the semiconductor surface is
vital for completing the photocatalyst design (Osgood 2006). To measure the surface
reaction, several characterization techniques have been established as powerful tools
to detect the reaction pathways, products, and intermediate, as well as the activation
and adsorption sites of the redox reaction. Infrared (IR) spectroscopy is widely used
to check surface reactions. Explicitly, the Fourier transform IR spectroscopy is used
to investigate the nature of adsorbed species and the surface properties of
photocatalyst systems (Chen et al. 2007). Separately, time-resolved infrared spectroscopy can precisely measure the charge kinetics of the adsorbed species (Chen
et al. 2007). Thermal desorption spectroscopy (TDS) is used to measure the thermodynamic and kinetic reaction of desorption and adsorption processes on the
photocatalyst surface (Yuan et al. 2013). The defect sites on the photocatalyst
surface can be measured by electron spin resonance spectroscopy, X-ray absorption
spectroscopy, X-ray absorption fine structure, and positron annihilation spectroscopy (Bai et al. 2015).
1.5 Synthesis of Nanomaterials
Up to now, several semiconductor heterostructures have been successfully designed
and synthesized. Mostly, the three-dimensional nanoÀ/microcomposites are synthesized via the self-assembly of nanosized building blocks, such as nanowires,
nanosheets, and nanoparticles (Li et al. 2016). Nonetheless, it is extremely challenging to design and synthesize several types of photocatalyst materials with controlled
morphologies. Therefore, it is essential to design a simple and low-cost technique to
prepare photocatalyst materials with hierarchical and high crystallinity
nanostructures. Up to now, hierarchical semiconductor photocatalysts are prepared
via template-free methods (chemically induced self-transformation, self-template
strategy, and in situ template-free assembly), template method (in situ templatesacrificial dissolution and two-step template method), and post-synthetic treatment
method (Li et al. 2016). The template method is one of the most often used technique
to fabricate hierarchical nanostructured photocatalysts due to its good reproducibility, abundant types of physical templates, and large-scale synthesis (Liu et al.
2013b). The in situ template-sacrificial dissolution technique has drawbacks, such
as the high cost of template, the presence of heterogeneous impurities, tedious
synthetic procedures, and sudden morphological changes during template removal
(Li et al. 2016). All these limit the large-scale synthesis of photocatalyst materials
(Lou et al. 2006). The self-template method is among the most effective and simple
techniques to synthesize several photocatalyst materials (Cai et al. 2009). The
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
9
Upon obtaining the charge transfer kinetics and band structures, an in-depth insight
of charge dynamics in the photocatalytic reaction on the semiconductor surface is
vital for completing the photocatalyst design (Osgood 2006). To measure the surface
reaction, several characterization techniques have been established as powerful tools
to detect the reaction pathways, products, and intermediate, as well as the activation
and adsorption sites of the redox reaction. Infrared (IR) spectroscopy is widely used
to check surface reactions. Explicitly, the Fourier transform IR spectroscopy is used
to investigate the nature of adsorbed species and the surface properties of
photocatalyst systems (Chen et al. 2007). Separately, time-resolved infrared spectroscopy can precisely measure the charge kinetics of the adsorbed species (Chen
et al. 2007). Thermal desorption spectroscopy (TDS) is used to measure the thermodynamic and kinetic reaction of desorption and adsorption processes on the
photocatalyst surface (Yuan et al. 2013). The defect sites on the photocatalyst
surface can be measured by electron spin resonance spectroscopy, X-ray absorption
spectroscopy, X-ray absorption fine structure, and positron annihilation spectroscopy (Bai et al. 2015).
1.5 Synthesis of Nanomaterials
Up to now, several semiconductor heterostructures have been successfully designed
and synthesized. Mostly, the three-dimensional nanoÀ/microcomposites are synthesized via the self-assembly of nanosized building blocks, such as nanowires,
nanosheets, and nanoparticles (Li et al. 2016). Nonetheless, it is extremely challenging to design and synthesize several types of photocatalyst materials with controlled
morphologies. Therefore, it is essential to design a simple and low-cost technique to
prepare photocatalyst materials with hierarchical and high crystallinity
nanostructures. Up to now, hierarchical semiconductor photocatalysts are prepared
via template-free methods (chemically induced self-transformation, self-template
strategy, and in situ template-free assembly), template method (in situ templatesacrificial dissolution and two-step template method), and post-synthetic treatment
method (Li et al. 2016). The template method is one of the most often used technique
to fabricate hierarchical nanostructured photocatalysts due to its good reproducibility, abundant types of physical templates, and large-scale synthesis (Liu et al.
2013b). The in situ template-sacrificial dissolution technique has drawbacks, such
as the high cost of template, the presence of heterogeneous impurities, tedious
synthetic procedures, and sudden morphological changes during template removal
(Li et al. 2016). All these limit the large-scale synthesis of photocatalyst materials
(Lou et al. 2006). The self-template method is among the most effective and simple
techniques to synthesize several photocatalyst materials (Cai et al. 2009). The
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
9