Topics in Current Chemistry (2019) 377:27
1 3
4 Photocatalytic Systems Based on C 3 N 4
Graphitic carbon nitride (g-C 3 N 4 )—a polymeric semiconductor—has elicited
great attention in the context of the search for robust and visible-light-active
semiconductor photocatalysts. It is formed by ordered tri-s-triazine subunits
connected through planar tertiary amino groups within layers and weak van der
Waals force between layers. It shows appealing electronic properties, high physicochemical stability, and easy synthesis from N-containing precursors (i.e.
urea, cyanamide, dicyandiamide, melamine, etc.). It has a moderate band gap
(2.4–2.8 eV) that results in an onset visible light absorption of ~ 450 nm [96, 97].
Although the history of C 3 N 4 -based polymers started in the nineteenth century by
the investigation of Berzelius and Liebig [98], its use in heterogeneous catalysis
started much later, in 2006 [99], and its properties as a metal-free conjugated
semiconductor photocatalyst for the evolution of H 2 was pioneered by Wang just
10 years ago [100].
Since then, g-C 3 N 4 has been used widely in numerous photocatalytic applications, such as water splitting, CO 2 reduction, and degradation of pollutants, etc.
[101–103]. Recently, g-C 3 N 4 has also received attention as a catalyst for H 2 production from hydrogen carrier molecules [20, 44, 50, 104]. g-C 3 N 4 -photocatalysts used for decomposition of FA contain metal nanoparticles so as to construct
Mott–Schottky photocatalysts, in which charge separation is enhanced. This was
the case in the study reported by Chen et al. [105], in which Mott–Schottky type
Pd-C 3 N 4 photocatalysts, with mesoporous C 3 N 4 (mpg-C 3 N 4 ) as support, were
prepared by wet impregnation; the resulting material was denoted Pd@CN. For
this, mpg-C 3 N 4 was first synthetized from cyanamide and a Ludox HS40 solution,
and subsequently impregnated with PdCl 2 to give a final metal loading of 8%.
Furthermore, reference samples with N-doped layered carbon and carbon black
(Pd@N-LC and Pd@CB, respectively) were also synthetized. The average nanoparticle size was determined to be 3–5 nm for Pd@CN and Pd@N-LC, and 10 nm
for Pd@CB. In order to assess the performance of the samples, catalytic and
photocatalytic tests were monitored. It was observed that, in dark conditions (i.e.
catalytic test at 15 °C), Pd@CN displayed the highest TOF number among investigated (49.8 mol H 2 mol
−1
Pd h
−1
), which was attributed to the Mott–Schottky
effect (i.e. support effect). This effect was confirmed by the decreased intensity
of the photoluminescence spectra after loading of Pd nanoparticles on mpg-C 3 N 4 .
In addition, the good catalytic activity found was increased further upon visiblelight irradiation (71.0 mol H 2 mol
−1
Pd h
−1
). It was claimed that the enhancement
observed in the catalytic activity under light irradiation conditions was due to the
electron enrichment of Pd nanoparticles, which, in turn, was strongly dependent
on the wavelength of the light used (see Fig. 9).
Yu et al. also reported on the application of Mott–Schottky heterojunctions [106]. In that case, PdAg nanowires (NWs) with various Pd/Ag compositions (Pd 7 Ag 3 NWs@g-C 3 N 4 , Pd 5 Ag 5 NWs@g-C 3 N 4 , Pd 3 Ag 7 NWs@g-C 3 N 4 ,
and Pd NWs@g-C 3 N 4 ) were formed in situ on g-C 3 N 4 . XPS analysis confirmed
the electron transfer from Ag and g-C 3 N 4 to Pd of the Pd 5 Ag 5 NWs@g-C 3 N 4
210
Reprinted from the journal
1 3
4 Photocatalytic Systems Based on C 3 N 4
Graphitic carbon nitride (g-C 3 N 4 )—a polymeric semiconductor—has elicited
great attention in the context of the search for robust and visible-light-active
semiconductor photocatalysts. It is formed by ordered tri-s-triazine subunits
connected through planar tertiary amino groups within layers and weak van der
Waals force between layers. It shows appealing electronic properties, high physicochemical stability, and easy synthesis from N-containing precursors (i.e.
urea, cyanamide, dicyandiamide, melamine, etc.). It has a moderate band gap
(2.4–2.8 eV) that results in an onset visible light absorption of ~ 450 nm [96, 97].
Although the history of C 3 N 4 -based polymers started in the nineteenth century by
the investigation of Berzelius and Liebig [98], its use in heterogeneous catalysis
started much later, in 2006 [99], and its properties as a metal-free conjugated
semiconductor photocatalyst for the evolution of H 2 was pioneered by Wang just
10 years ago [100].
Since then, g-C 3 N 4 has been used widely in numerous photocatalytic applications, such as water splitting, CO 2 reduction, and degradation of pollutants, etc.
[101–103]. Recently, g-C 3 N 4 has also received attention as a catalyst for H 2 production from hydrogen carrier molecules [20, 44, 50, 104]. g-C 3 N 4 -photocatalysts used for decomposition of FA contain metal nanoparticles so as to construct
Mott–Schottky photocatalysts, in which charge separation is enhanced. This was
the case in the study reported by Chen et al. [105], in which Mott–Schottky type
Pd-C 3 N 4 photocatalysts, with mesoporous C 3 N 4 (mpg-C 3 N 4 ) as support, were
prepared by wet impregnation; the resulting material was denoted Pd@CN. For
this, mpg-C 3 N 4 was first synthetized from cyanamide and a Ludox HS40 solution,
and subsequently impregnated with PdCl 2 to give a final metal loading of 8%.
Furthermore, reference samples with N-doped layered carbon and carbon black
(Pd@N-LC and Pd@CB, respectively) were also synthetized. The average nanoparticle size was determined to be 3–5 nm for Pd@CN and Pd@N-LC, and 10 nm
for Pd@CB. In order to assess the performance of the samples, catalytic and
photocatalytic tests were monitored. It was observed that, in dark conditions (i.e.
catalytic test at 15 °C), Pd@CN displayed the highest TOF number among investigated (49.8 mol H 2 mol
−1
Pd h
−1
), which was attributed to the Mott–Schottky
effect (i.e. support effect). This effect was confirmed by the decreased intensity
of the photoluminescence spectra after loading of Pd nanoparticles on mpg-C 3 N 4 .
In addition, the good catalytic activity found was increased further upon visiblelight irradiation (71.0 mol H 2 mol
−1
Pd h
−1
). It was claimed that the enhancement
observed in the catalytic activity under light irradiation conditions was due to the
electron enrichment of Pd nanoparticles, which, in turn, was strongly dependent
on the wavelength of the light used (see Fig. 9).
Yu et al. also reported on the application of Mott–Schottky heterojunctions [106]. In that case, PdAg nanowires (NWs) with various Pd/Ag compositions (Pd 7 Ag 3 NWs@g-C 3 N 4 , Pd 5 Ag 5 NWs@g-C 3 N 4 , Pd 3 Ag 7 NWs@g-C 3 N 4 ,
and Pd NWs@g-C 3 N 4 ) were formed in situ on g-C 3 N 4 . XPS analysis confirmed
the electron transfer from Ag and g-C 3 N 4 to Pd of the Pd 5 Ag 5 NWs@g-C 3 N 4
210
Reprinted from the journal