inert atmosphere with addition of citric acid. The obtained product is purified with
acetone several times. The oil-soluble carbon dots are appreciably soluble in organic
solvents like toluene, hexane, and chloroform. Wang et al. (2011) synthesize amorphous carbon dots via pyrolytic method using anhydrous citric acid in the presence
organosilicane as the coordinating solvent at 240
C for 1 min. The ultrasmall carbon
dot with a size of 0.9 nm was prepared with high quantum yield of 47%. Simple
heating is required for the preparation of carbon dots from organosilicane as
compared to other method where addition of polymer or inorganic compound is
needed.
Template Method and Substance Oxidation
Mesoporous templates have been employed to confine the carbon dots in pores to
gain a narrow size distribution, but effectively restricting the pore size of templates is
challenging. In template method, carbon quantum dots are prepared by calcination
using mesoporous template like silicon sphere or by etching to obtain nanosized
carbon quantum dots. The uniform, spherical mesoporous silica was prepared using
tetra-ethoxysilane as the precursor, N-hexadecylamine as the surfactant, and ammonia as the catalyst with particle size of 1.3 mm in diameter and pore size of 3.60 nm
(Grun et al. 2000). Further, the nanosized hydrophilic carbon dots are synthesized by
impregnated method by performing calcination of mesoporous silica with mixture of
complex salts and citric acid solution (Zong et al. 2011). The mesoporous silica was
used as support to prevent the aggregation of carbon dots, and also the small pore
size of silica enables synthesis of nanosized carbon dots with sizes ranging from
1.5 nm to 2.5 nm.
Yang et al. (2013a) reported combination of copolymer pluronic and mesoporous
silica as soft–hard template for the synthesis of mono-dispersed photoluminescence
carbon dots. Soft–hard template method overcomes the disadvantages of using
mesoporous silica alone and synthesizes carbon dots with narrow size distribution
and well-defined morphology. The organic molecules with different aromatic framework like diaminebenzene, pyrene, 1,3,5-trimethylbenzene, and phenanthroline are
used as carbon precursors. The use of different organic precursors is beneficial to
modify the size, structure, composition, and photoluminescent property of carbon
dots. Briefly, organic precursor was enwrapped into micelles of soft template with
mesoporous silica followed by carbonization, template removal through etching, and
passivation. Here, a soft template provides nano-space for the formation of
nanosized carbon dots and mesoporous silica, while a hard template prevents
aggregation of carbon dot particle. In another method free from catalysts,
mesoporous silica was blended with polyethylene glycol and glycerol as carbon
source (Lai et al. 2012). The majority of top-down techniques involve tedious
methods and usage of extensively harsh chemicals which generate more chemical
waste and high temperature for a prolonged time. On the other hand, bottom-up
approach is a time-consuming process and complicated synthetic scheme and
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
101
acetone several times. The oil-soluble carbon dots are appreciably soluble in organic
solvents like toluene, hexane, and chloroform. Wang et al. (2011) synthesize amorphous carbon dots via pyrolytic method using anhydrous citric acid in the presence
organosilicane as the coordinating solvent at 240
C for 1 min. The ultrasmall carbon
dot with a size of 0.9 nm was prepared with high quantum yield of 47%. Simple
heating is required for the preparation of carbon dots from organosilicane as
compared to other method where addition of polymer or inorganic compound is
needed.
Template Method and Substance Oxidation
Mesoporous templates have been employed to confine the carbon dots in pores to
gain a narrow size distribution, but effectively restricting the pore size of templates is
challenging. In template method, carbon quantum dots are prepared by calcination
using mesoporous template like silicon sphere or by etching to obtain nanosized
carbon quantum dots. The uniform, spherical mesoporous silica was prepared using
tetra-ethoxysilane as the precursor, N-hexadecylamine as the surfactant, and ammonia as the catalyst with particle size of 1.3 mm in diameter and pore size of 3.60 nm
(Grun et al. 2000). Further, the nanosized hydrophilic carbon dots are synthesized by
impregnated method by performing calcination of mesoporous silica with mixture of
complex salts and citric acid solution (Zong et al. 2011). The mesoporous silica was
used as support to prevent the aggregation of carbon dots, and also the small pore
size of silica enables synthesis of nanosized carbon dots with sizes ranging from
1.5 nm to 2.5 nm.
Yang et al. (2013a) reported combination of copolymer pluronic and mesoporous
silica as soft–hard template for the synthesis of mono-dispersed photoluminescence
carbon dots. Soft–hard template method overcomes the disadvantages of using
mesoporous silica alone and synthesizes carbon dots with narrow size distribution
and well-defined morphology. The organic molecules with different aromatic framework like diaminebenzene, pyrene, 1,3,5-trimethylbenzene, and phenanthroline are
used as carbon precursors. The use of different organic precursors is beneficial to
modify the size, structure, composition, and photoluminescent property of carbon
dots. Briefly, organic precursor was enwrapped into micelles of soft template with
mesoporous silica followed by carbonization, template removal through etching, and
passivation. Here, a soft template provides nano-space for the formation of
nanosized carbon dots and mesoporous silica, while a hard template prevents
aggregation of carbon dot particle. In another method free from catalysts,
mesoporous silica was blended with polyethylene glycol and glycerol as carbon
source (Lai et al. 2012). The majority of top-down techniques involve tedious
methods and usage of extensively harsh chemicals which generate more chemical
waste and high temperature for a prolonged time. On the other hand, bottom-up
approach is a time-consuming process and complicated synthetic scheme and
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
101