needed for the activation process (Chang 2014). To effectively use the natural
sunlight to handle water pollution problems, it is of significant interest to discover
new visible light-driven photocatalysts.
Since the initial study by Fujishima and Honda (Fujishima and Honda 1972),
several semiconductor materials, such as metal-free semiconductors (Cao et al.
2015), plasmonic metals (Sun et al. 2015), metal oxides (Li et al. 2015), metal
sulfides (Zhang and Guo 2013), and metal (oxy)nitrides (Hitoki et al. 2002), are
available for multifunctional and diversified applications (Wang et al. 2013d).
Nevertheless, some of these semiconductor photocatalysts cannot completely satisfy
the practical applications due to their short lifetime of photogenerated charge carriers
and wide band gap (Li et al. 2015). Therefore, it is significant to design highly stable
and efficient photocatalysts with visible light-driven activity by optimizing the
existing approaches by incorporating them with nanocarbon (Zhou et al. 2014b).
Recently, activated carbons with a high surface area are considered as a conventional
adsorbent for water purification (Bhatnagar et al. 2013). Nonetheless, the column
fouling and high production cost are the major disadvantages of using activated
carbons as an adsorbent. In the last decade, graphene has been promising adsorbents
for water treatment owing to its fast adsorption kinetics, excellent electronic property, large surface area, and low-cost compared to the well-developed activated
carbons (McAllister et al. 2007; Xu et al. 2013b; Zhu et al. 2011). In this book
chapter, much effort has been made to comprehensively review the properties,
synthesis, characterization techniques, and application of graphene/semiconductor
composites for the removal of water pollutants, such as organic molecules, waterborne pathogens, and heavy metal ions. The theoretical outcomes in water treatment
using graphene/semiconductor composites are also summarized. A brief outlook on
the challenges and new strategies is provided for developing effective water/wastewater treatment techniques using graphene-based materials.
1.2 Semiconductors Overview
A semiconductor is a crystalline structure, which has a few free electrons at room
temperature (Rahman 2015). Semiconductors are the basic building block of modern
electronics, such as transistors, solar cells, light-emitting diodes, analog, and digital
integrated circuits (Rahman 2015). The semiconductor properties depend on the
quantum physics to explain the migration of electron–hole pairs in a lattice and
crystal structure (Feynman et al. 2013). A semiconductor has an electrical conductivity property between that of an insulator (e.g., glass) and a conductor (e.g., copper)
(Feynman et al. 2013). Semiconductor devices show beneficial properties, such as
sensitivity to heat/light and variable resistance (Rahman 2015). Semiconductors can
be used for energy conversion, switching, and amplification since their electrical
properties can be tuned by applying electrical fields or doping. An n-type semiconductor is a doped semiconductor containing free electrons, while p-type is a semiconductor containing free holes. Semiconductor heterogeneous photocatalysis is
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
3
sunlight to handle water pollution problems, it is of significant interest to discover
new visible light-driven photocatalysts.
Since the initial study by Fujishima and Honda (Fujishima and Honda 1972),
several semiconductor materials, such as metal-free semiconductors (Cao et al.
2015), plasmonic metals (Sun et al. 2015), metal oxides (Li et al. 2015), metal
sulfides (Zhang and Guo 2013), and metal (oxy)nitrides (Hitoki et al. 2002), are
available for multifunctional and diversified applications (Wang et al. 2013d).
Nevertheless, some of these semiconductor photocatalysts cannot completely satisfy
the practical applications due to their short lifetime of photogenerated charge carriers
and wide band gap (Li et al. 2015). Therefore, it is significant to design highly stable
and efficient photocatalysts with visible light-driven activity by optimizing the
existing approaches by incorporating them with nanocarbon (Zhou et al. 2014b).
Recently, activated carbons with a high surface area are considered as a conventional
adsorbent for water purification (Bhatnagar et al. 2013). Nonetheless, the column
fouling and high production cost are the major disadvantages of using activated
carbons as an adsorbent. In the last decade, graphene has been promising adsorbents
for water treatment owing to its fast adsorption kinetics, excellent electronic property, large surface area, and low-cost compared to the well-developed activated
carbons (McAllister et al. 2007; Xu et al. 2013b; Zhu et al. 2011). In this book
chapter, much effort has been made to comprehensively review the properties,
synthesis, characterization techniques, and application of graphene/semiconductor
composites for the removal of water pollutants, such as organic molecules, waterborne pathogens, and heavy metal ions. The theoretical outcomes in water treatment
using graphene/semiconductor composites are also summarized. A brief outlook on
the challenges and new strategies is provided for developing effective water/wastewater treatment techniques using graphene-based materials.
1.2 Semiconductors Overview
A semiconductor is a crystalline structure, which has a few free electrons at room
temperature (Rahman 2015). Semiconductors are the basic building block of modern
electronics, such as transistors, solar cells, light-emitting diodes, analog, and digital
integrated circuits (Rahman 2015). The semiconductor properties depend on the
quantum physics to explain the migration of electron–hole pairs in a lattice and
crystal structure (Feynman et al. 2013). A semiconductor has an electrical conductivity property between that of an insulator (e.g., glass) and a conductor (e.g., copper)
(Feynman et al. 2013). Semiconductor devices show beneficial properties, such as
sensitivity to heat/light and variable resistance (Rahman 2015). Semiconductors can
be used for energy conversion, switching, and amplification since their electrical
properties can be tuned by applying electrical fields or doping. An n-type semiconductor is a doped semiconductor containing free electrons, while p-type is a semiconductor containing free holes. Semiconductor heterogeneous photocatalysis is
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
3