adopted the stepwise deposition–precipitation method to prepare Au–Cu alloy/TiO 2
photocatalyst, using 0.2 M NaOH to tune the pH value of TiO 2 -HAuCl 4 and
Au/TiO 2 -Cu(NO 3 ) 2 aqueous slurry to 8.5 and annealing in air and H 2 atmosphere,
respectively. The Au–Cu alloy NPs were characterized by HR-TEM which constitute Au–Cu lattice fringes; meanwhile, the redshift of Au SPR adsorption peak in
UV-vis DRS spectra also suggests the Au and Cu formed alloy status. It should be
noted that, using this synthesis method, the unalloyed Au and Cu NPs also could be
detected.
12.3.1.2 Semiconductor Heterojunctions and Z-Scheme Composites
In order to develop the economic noble metal-free photocatalysts with high efficiency in CO2PR, some semiconductor junction composites have been developed;
the common methods include hydrothermal/solvothermal method, impregnation
method, self-template method, ALD method, etc. (Table 12.1).
Jin et al. [38] reported a hierarchical-structured Z-scheme CdS-WO 3
photocatalyst and applied it in CO2PR. The hierarchical hollow WO 3 spheres
were formed by immersing SrWO 4 in HNO 3 at first, and then the precipitate was
washed and calcined at 500
C in air. The as-prepared WO 3 spheres were negatively
charged at pH ¼ 7; therefore, stepwise adding Cd
2+ and S
À source slowly will
generate heterostructure CdS–WO 3 composite. Wang et al. [34] developed a porous
ZnO@Co 3 O 4 composite by using ZIF-8 and ZIF-67 as precursor templates
(Fig. 12.2). First, the ZIF-8@ZIF-67 core–shell structure was synthesized through
a solvothermal process and then followed by a N 2 -400
C 2-h calcination and
air-400
C 2 h calcination treatment. The two-step calcination process was determined by the TG/DTA analysis, while one-step calcination under air atmosphere
will lead to nonporous ZnO NPs.
In et al. [37] designed a novel CuO–TiO 2 – x N x hybrid hollow nanocubes with the
use of CuN 3 nanocubes as reactive templates (Fig. 12.3a). After slow hydrolysis of
titanium-n-butoxide on the surface of CuN 3 , the calcination treatment at 450
C was
carried out. During the calcination process, the CuN 3 reacts with the oxygen and
form hollow CuO nanocube; meanwhile, the nitrogen diffuses outward and reacts
with the crystalline TiO 2 to form TiO 2 – x N x . Park and coworkers [57] once proposed
a novel Cu x O–TiO 2 p–n heterojunction (Fig. 12.3b). The Cu/Cu 2 O nanoparticles
were first synthesized through a thermal decomposition method, and then the TiCl 4
was mixed with the Cu/Cu 2 O NPs in argon; after calcination in air, the TiCl 4
crystallized to TiO 2, and the Cu/Cu 2 O NPs were oxidized to Cu x O with organic
ligands removed at the same time; at last, the mesoporous Cu x O–TiO 2 composites
were obtained.
286
12 Roles and Properties of Cocatalysts in Semiconductor-Based Materials. . .
photocatalyst, using 0.2 M NaOH to tune the pH value of TiO 2 -HAuCl 4 and
Au/TiO 2 -Cu(NO 3 ) 2 aqueous slurry to 8.5 and annealing in air and H 2 atmosphere,
respectively. The Au–Cu alloy NPs were characterized by HR-TEM which constitute Au–Cu lattice fringes; meanwhile, the redshift of Au SPR adsorption peak in
UV-vis DRS spectra also suggests the Au and Cu formed alloy status. It should be
noted that, using this synthesis method, the unalloyed Au and Cu NPs also could be
detected.
12.3.1.2 Semiconductor Heterojunctions and Z-Scheme Composites
In order to develop the economic noble metal-free photocatalysts with high efficiency in CO2PR, some semiconductor junction composites have been developed;
the common methods include hydrothermal/solvothermal method, impregnation
method, self-template method, ALD method, etc. (Table 12.1).
Jin et al. [38] reported a hierarchical-structured Z-scheme CdS-WO 3
photocatalyst and applied it in CO2PR. The hierarchical hollow WO 3 spheres
were formed by immersing SrWO 4 in HNO 3 at first, and then the precipitate was
washed and calcined at 500
C in air. The as-prepared WO 3 spheres were negatively
charged at pH ¼ 7; therefore, stepwise adding Cd
2+ and S
À source slowly will
generate heterostructure CdS–WO 3 composite. Wang et al. [34] developed a porous
ZnO@Co 3 O 4 composite by using ZIF-8 and ZIF-67 as precursor templates
(Fig. 12.2). First, the ZIF-8@ZIF-67 core–shell structure was synthesized through
a solvothermal process and then followed by a N 2 -400
C 2-h calcination and
air-400
C 2 h calcination treatment. The two-step calcination process was determined by the TG/DTA analysis, while one-step calcination under air atmosphere
will lead to nonporous ZnO NPs.
In et al. [37] designed a novel CuO–TiO 2 – x N x hybrid hollow nanocubes with the
use of CuN 3 nanocubes as reactive templates (Fig. 12.3a). After slow hydrolysis of
titanium-n-butoxide on the surface of CuN 3 , the calcination treatment at 450
C was
carried out. During the calcination process, the CuN 3 reacts with the oxygen and
form hollow CuO nanocube; meanwhile, the nitrogen diffuses outward and reacts
with the crystalline TiO 2 to form TiO 2 – x N x . Park and coworkers [57] once proposed
a novel Cu x O–TiO 2 p–n heterojunction (Fig. 12.3b). The Cu/Cu 2 O nanoparticles
were first synthesized through a thermal decomposition method, and then the TiCl 4
was mixed with the Cu/Cu 2 O NPs in argon; after calcination in air, the TiCl 4
crystallized to TiO 2, and the Cu/Cu 2 O NPs were oxidized to Cu x O with organic
ligands removed at the same time; at last, the mesoporous Cu x O–TiO 2 composites
were obtained.
286
12 Roles and Properties of Cocatalysts in Semiconductor-Based Materials. . .