and carbon quantum dots–citric acid/TiO 2 by using citric acid as precursor. Among
these two heterojunctions, carbon quantum dots–glucose/TiO 2 has better crystalline
property which is responsible for facile charge carrier migration and hence responsible for higher photocatalytic activity. Sarkar et al. (2016) prepared water-soluble
carbon quantum dots from sucrose by using cost-effective and environment-friendly
hydrothermal technique. Mixture of sucrose solution and ethanol was taken in Teflon
autoclave and heated to 175–180
C for 2 h and then cooled. The yellow-colored
solution is obtained which is centrifuged at 16,000 rpm. Moreover, graphene
quantum dots were also synthesized by a hydrothermal method using graphene
sheets which are obtained by thermal oxidation of graphene oxide sheets at 200
C
(Pan et al. 2010a). The synthesized graphene quantum dots were used in organic
photovoltaic devices.
Mehta et al. (2014) devised a highly cost-effective and green hydrothermal
technique to synthesize fluorescent quantum dots using plant-based precursor
Saccharum officinarum. These quantum dots were used in cellular imaging of
bacteria and yeast. Several green techniques have been devised for the preparation
of carbon dots by utilizing inexpensive renewable precursor. Researchers developed
green synthetic approach for the synthesis of carbon dots from watermelon peel and
pomelo peel using hydrothermal method (Zhou et al. 2012a; Lu et al. 2012).
Prasannan and Imae (2013) also reported the synthesis of fluorescent quantum
dots by hydrothermal method at 180
C using orange peel. Orange peel used mainly
consists of various carbohydrates, glucose, fructose, sucrose, and cellulose, which
are used as carbon sources. The morphology and chemical composition were
characterized by various spectroscopic techniques. The carbon dots prepared were
amorphous in nature with large quantity of functional group. Typically, orange
waste was firstly washed with water and then in H 2 SO 4 solution and again rinsed
with water followed by drying in hot air oven at 150
C for 10 h. The secured
product was further treated with sodium hypochlorite solution and kept for 4 h at
room temperature followed by washing with water several times till pH 7 is attained.
Lastly, the oxidized orange peel was kept in autoclave for heating, followed by
washing with dichloromethane to remove extra organic species followed by centrifugation. Hydrothermal carbonization is considered as a green and effective method
avoiding the usage of strong toxic chemical.
Kapitonov et al. (2018) present a new method for carbon dot synthesis using
hydrothermal method by choosing different precursors: berry juice, birch bark soot,
glucose, and citric acid which are reported earlier. Various functional groups present
on the surface of carbon dots are epoxy, carbonyl, hydroxyl, and carboxyl providing
hydrophilic character to carbon dots. The carbon dots can be functionalized by
doping with heteroatom to improve the surface property and also the quantum
yield. Thus, nitrogen-doped carbon quantum dots were synthesized by hydrothermal
method used for the selective sensor for the detection of Hg
2+ (Liu et al. 2015), Fe
3+
(Wu et al. 2013), and Ag
+ (Li et al. 2017). Wang et al. (2018b) prepared nitrogendoped carbon dots utilizing mandelic acid together with ethylenediamine as carbon
and nitrogen source, respectively. The solution of mandelic acid and
ethylenediamine were mixed ultrasonically at room temperature with subsequent
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
95
these two heterojunctions, carbon quantum dots–glucose/TiO 2 has better crystalline
property which is responsible for facile charge carrier migration and hence responsible for higher photocatalytic activity. Sarkar et al. (2016) prepared water-soluble
carbon quantum dots from sucrose by using cost-effective and environment-friendly
hydrothermal technique. Mixture of sucrose solution and ethanol was taken in Teflon
autoclave and heated to 175–180
C for 2 h and then cooled. The yellow-colored
solution is obtained which is centrifuged at 16,000 rpm. Moreover, graphene
quantum dots were also synthesized by a hydrothermal method using graphene
sheets which are obtained by thermal oxidation of graphene oxide sheets at 200
C
(Pan et al. 2010a). The synthesized graphene quantum dots were used in organic
photovoltaic devices.
Mehta et al. (2014) devised a highly cost-effective and green hydrothermal
technique to synthesize fluorescent quantum dots using plant-based precursor
Saccharum officinarum. These quantum dots were used in cellular imaging of
bacteria and yeast. Several green techniques have been devised for the preparation
of carbon dots by utilizing inexpensive renewable precursor. Researchers developed
green synthetic approach for the synthesis of carbon dots from watermelon peel and
pomelo peel using hydrothermal method (Zhou et al. 2012a; Lu et al. 2012).
Prasannan and Imae (2013) also reported the synthesis of fluorescent quantum
dots by hydrothermal method at 180
C using orange peel. Orange peel used mainly
consists of various carbohydrates, glucose, fructose, sucrose, and cellulose, which
are used as carbon sources. The morphology and chemical composition were
characterized by various spectroscopic techniques. The carbon dots prepared were
amorphous in nature with large quantity of functional group. Typically, orange
waste was firstly washed with water and then in H 2 SO 4 solution and again rinsed
with water followed by drying in hot air oven at 150
C for 10 h. The secured
product was further treated with sodium hypochlorite solution and kept for 4 h at
room temperature followed by washing with water several times till pH 7 is attained.
Lastly, the oxidized orange peel was kept in autoclave for heating, followed by
washing with dichloromethane to remove extra organic species followed by centrifugation. Hydrothermal carbonization is considered as a green and effective method
avoiding the usage of strong toxic chemical.
Kapitonov et al. (2018) present a new method for carbon dot synthesis using
hydrothermal method by choosing different precursors: berry juice, birch bark soot,
glucose, and citric acid which are reported earlier. Various functional groups present
on the surface of carbon dots are epoxy, carbonyl, hydroxyl, and carboxyl providing
hydrophilic character to carbon dots. The carbon dots can be functionalized by
doping with heteroatom to improve the surface property and also the quantum
yield. Thus, nitrogen-doped carbon quantum dots were synthesized by hydrothermal
method used for the selective sensor for the detection of Hg
2+ (Liu et al. 2015), Fe
3+
(Wu et al. 2013), and Ag
+ (Li et al. 2017). Wang et al. (2018b) prepared nitrogendoped carbon dots utilizing mandelic acid together with ethylenediamine as carbon
and nitrogen source, respectively. The solution of mandelic acid and
ethylenediamine were mixed ultrasonically at room temperature with subsequent
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
95