Page 243 - Multifunctional Photocatalytic Materials for Energy

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production [23]. According to this result, this heterojunction enhanced the separation
efficiency for electrons and holes generated. These results showed that 6.7 wt.% BBT/
TiO 2 /Pt exhibited a robust H 2 production ability in a 7 h test, and up to ∼1246 μmol H 2
was detected, which corresponds to a H 2 production rate of ∼5933 μmol/h/g, ∼18.0
times higher than that of pure BBT. Their results indicate that the composite of BBT
and Pt/TiO 2 as well as the tight interaction between BBT and Pt/TiO 2 are important
in enhancing the photogenerated carriers’ separation and then for photocatalytic H 2
production. Another approach in enhancing the lifetime of charge carriers and sup-
pressing the electron-hole recombination is to use a p-type semiconductor along with
TiO 2 , i.e., an n-type semiconductor. For example, the p-n junction of NiO/TiO 2 ex-
hibited higher photocatalytic reduction activity than pristine TiO 2 but lower oxidation
activity [24].
Porphyrins, which are a group of heterocyclic organic compounds, are good ab-
sorbers of visible light [25]. The excited states formed with the help of porphyrin
aid in charge transfer. Porphyrin-based dyes have several intrinsic advantages, such
as their rigid molecular structures with large absorption coefficients in the visible
region and many reaction sites, i.e., four meso and eight β positions that are readily
available for functionalization, which enables fine-tuning of the optical and physical
properties of porphyrins. Porphyrins exhibit excellent photophysical, photochemi-
cal, electrochemical, and structural properties [26]. Many metalated porphyrins
have also been studied. Photocatalytic properties of porphyrins can be controlled
by inserting metal cations into the porphyrin ring. Al- and Zn-metalated meso-
tetrakis(4- carboxylphenyl)porphyrin H 2 TCPP metal organic frameworks (MOFs)
were synthesized by Fateeva et al. for hydrogen generation [27]. The BET surface
2 −1
area of the aluminum-porphyrin MOF was observed to be 1400 m g , which pre-
sented a large surface area for reaction sites. The metalated porphyrins were found
to generate H 2 that could be enhanced by using platinum [27]. Note that the addi-
tion of Pt aids in easier hydrogen dissociation and has been investigated in detail
[28–30]. The role of porphyrin-modified TiO 2 in the degradation of 4-nitrophenol
using visible ultraviolet light was investigated by Duan et al., who found that the
photoactivities of porphyrin-based TiO 2 structures were greater than that of pristine
TiO 2 [31]. Dichloro and dihydroxo tin porphyrins were investigated, and it was es-
tablished that a meso-site peripheral substituent influenced the photodegradation
activity. MOFs have also been employed for photocatalysis owing to the former’s
porous coordination networks. MOFs are a class of advanced materials comprising
metal ions/clusters linked with organic molecules that provide additional support for
photocatalytic materials because of MOFs’ high surface area, structural versatility,
and chemical stability. These MOF materials are constructed by joining metal or
metal oxides (secondary building units, SBU) with organic linkers using strong bonds
to create an open crystalline framework with permanent porosity—for example,
6+
MOF-5 with inorganic [Zn 4 O] groups connected to an octahedral array of [O 2 C–
C 6 H 4 –CO 2 ] 2 (1,4-benzenedicarboxylate) groups. Many different kinds of MOFs can
be prepared by changing the linker and the inorganic part, and MOFs exist in both
semiconducting and conducting states [32]. Direct use of metal organic frameworks
for photocatalytic hydrogen production was first reported by Silva et al. They found