improve the CO2PR efficiency. Shi et al. [56] developed an electrostatic selfassembly strategy to fabricate the UiO-66/C 3 N 4 composite. Firstly, the C 3 N 4
nanosheet (CNNS) was prepared by liquid-state ultrasound exfoliation method;
after the centrifuge to remove the large bulk C 3 N 4 , the CNNS was obtained.
Secondly, the as-prepared UiO-66 and CNNS were mixed in water, because the
CNNS is negatively charged in water with À35.91 mV Zeta potential and + 7.71 mV
for UiO-66; that is the reason why electronic statistic self-assembly happened. Li
et al. [53] developed a Cu 3 (BTC) 2 (HKUST-1)@TiO 2 core–shell structured composite; the solid Cu precursor and the involvement of PVP are key to coat TiO 2 on
the Cu 3 (BTC) 2 nanocrystals uniformly. During the control experiments, using Cu
(OH) 2 as Cu precursor and in the absence of PVP, the TiO 2 cannot be coated on the
Cu 3 (BTC) 2 uniformly; besides, using unsolid Cu(NO 3 ) 2 as precursor, the thermal
stability of Cu 3 (BTC) 2 is low; in this case, it will decompose at the 180
C coating
process and also cannot get desirable result.
Carbon nanodots, including carbon quantum dots, carbon dots, and graphene
quantum dots, which are a new class of zero-dimensional (0D) carbon materials have
attracted people’s attention over the past few decades; the unique properties of
carbon dots such as superior up-conversion and size-dependent photoluminescence,
high stability, low cytotoxicity, earth abundance, etc., thus made it a plausible
candidate in many fields. The synthesis of carbon nanodots can be roughly classified
into two approaches: bottom-up approach and top-down approach. Kang et al. [59]
reported a facile electrochemical approach to synthesize large-scale high-quality
carbon dots. The authors used two graphite rods as the counter electrode and
ultrapure water as the electrolyte; statistic potential with 15–60 V was applied to
the two electrodes; after 120-h electrolysis, a dark-yellow solution was formed, and
the water-soluble carbon dots were obtained after filter and centrifuge. Ong et al.
[55] adopted glucose as the carbon source using alkali-assisted ultrasonication
method to prepare carbon nanodots (CND). Briefly, glucose and NaOH solution
was mixed together and sonicated for 2 h and resulted in a dark-brown solution; after
neutralization and filter, a brown carbon dot solution was obtained. Owing to the
natural properties of same negative polarity of C 3 N 4 and carbon dots, the coupled
CND/C 3 N 4 in this paper was obtained by protonation C 3 N 4 in HCl solution in
advance; after that, the C 3 N 4 is positively charged thus can attract the CND by
electrostatic force.
12.4 Roles and Properties of Different Cocatalysts
12.4.1 Promote the Charge Separation and Transfer
It is well-known that the noble metal NPs such as Pt, Pd, Au, Ag, Ru, etc. loaded
onto the semiconductors could trap the photo-generated electrons and promote the
separation of charge carriers. The reason could be attributed to the Fermi level of
metal NPs which lies energetically below the conduction band level of its
290
12 Roles and Properties of Cocatalysts in Semiconductor-Based Materials. . .
nanosheet (CNNS) was prepared by liquid-state ultrasound exfoliation method;
after the centrifuge to remove the large bulk C 3 N 4 , the CNNS was obtained.
Secondly, the as-prepared UiO-66 and CNNS were mixed in water, because the
CNNS is negatively charged in water with À35.91 mV Zeta potential and + 7.71 mV
for UiO-66; that is the reason why electronic statistic self-assembly happened. Li
et al. [53] developed a Cu 3 (BTC) 2 (HKUST-1)@TiO 2 core–shell structured composite; the solid Cu precursor and the involvement of PVP are key to coat TiO 2 on
the Cu 3 (BTC) 2 nanocrystals uniformly. During the control experiments, using Cu
(OH) 2 as Cu precursor and in the absence of PVP, the TiO 2 cannot be coated on the
Cu 3 (BTC) 2 uniformly; besides, using unsolid Cu(NO 3 ) 2 as precursor, the thermal
stability of Cu 3 (BTC) 2 is low; in this case, it will decompose at the 180
C coating
process and also cannot get desirable result.
Carbon nanodots, including carbon quantum dots, carbon dots, and graphene
quantum dots, which are a new class of zero-dimensional (0D) carbon materials have
attracted people’s attention over the past few decades; the unique properties of
carbon dots such as superior up-conversion and size-dependent photoluminescence,
high stability, low cytotoxicity, earth abundance, etc., thus made it a plausible
candidate in many fields. The synthesis of carbon nanodots can be roughly classified
into two approaches: bottom-up approach and top-down approach. Kang et al. [59]
reported a facile electrochemical approach to synthesize large-scale high-quality
carbon dots. The authors used two graphite rods as the counter electrode and
ultrapure water as the electrolyte; statistic potential with 15–60 V was applied to
the two electrodes; after 120-h electrolysis, a dark-yellow solution was formed, and
the water-soluble carbon dots were obtained after filter and centrifuge. Ong et al.
[55] adopted glucose as the carbon source using alkali-assisted ultrasonication
method to prepare carbon nanodots (CND). Briefly, glucose and NaOH solution
was mixed together and sonicated for 2 h and resulted in a dark-brown solution; after
neutralization and filter, a brown carbon dot solution was obtained. Owing to the
natural properties of same negative polarity of C 3 N 4 and carbon dots, the coupled
CND/C 3 N 4 in this paper was obtained by protonation C 3 N 4 in HCl solution in
advance; after that, the C 3 N 4 is positively charged thus can attract the CND by
electrostatic force.
12.4 Roles and Properties of Different Cocatalysts
12.4.1 Promote the Charge Separation and Transfer
It is well-known that the noble metal NPs such as Pt, Pd, Au, Ag, Ru, etc. loaded
onto the semiconductors could trap the photo-generated electrons and promote the
separation of charge carriers. The reason could be attributed to the Fermi level of
metal NPs which lies energetically below the conduction band level of its
290
12 Roles and Properties of Cocatalysts in Semiconductor-Based Materials. . .