Page 272 - Multifunctional Photocatalytic Materials for Energy

254 Multifunctional Photocatalytic Materials for Energy

11.3.1 Pure TiO 2 nanomaterials for hydrogen production

Since Fujishima and Honda reported the possibility of photocatalytic WS on a TiO 2
electrode years ago [173], people have made many efforts to broaden the photocata-
lytic applications of TiO 2 in photodegradation of pollutants and photogenerated clean
hydrogen fuel from water. Different nanostructured TiO 2 materials containing 0D
NPs, 1D NTAs, NWs, NRs, NFs, 2D NSs, 3D porous, and hierarchical hollow struc-
tures have arisen, increasing interest in their uses in various fields, such as pollution
degradation [174], energy from reduction of CO 2 [175,176], solar cells [177,178],
supercapacitors [179,180], and biosensors [181–183]. Some typical works on photo-
catalytic WS based on TiO 2 -based composites have been broadly reported. Yu's group
utilized P25 for H 2 production from WS in an ethanol/water solution under UV light,
− 1
− 1
but obtained a low generation rate of 13.7 mmol h g [184]. Seeking enhancement,
Janek and coworkers prepared porous TiO 2 NFs via a combination of sol-gel and tem-
plate methods, which led to a 10-fold higher efficiency for WS than crystalline TiO 2
NPs [185]. Compared to 0D TiO 2 NPs, TiO 2 materials with a 1D nanostructure are
more promising for WS because of the superior directional charge transport from the
specific 1D nanostructures [186–188]. D'Ela's group demonstrated the influences of
the morphology of TiO 2 nanostructures on their WS capabilities. Compared to P25,
TiO 2 NTAs obtained from a hydrothermal process were found to be more active un-
der UV irradiation with enhanced charge separation [189]. Park's group successfully
prepared highly aligned, large surface area porous TiO 2 NFs via electrospinning and
calcination processes [190]. The as-prepared TiO 2 NFs were 500 nm in diameter and
a few micrometers in length (Fig. 11.9A and B) and exhibited a much higher photo-
catalytic WS rate than TiO 2 NPs (Fig. 11.9C). The photocatalytic superiority of TiO 2
NFs came from the effects of mesoporosity and the NPs' alignment, which enabled
efficient charge separation through the interparticle charge transfer along the NF
framework. For TiO 2 nanobelts via electrochemical anodization, a photoconversion ef-
ficiency of nearly 4.51% at 0.1 V (vs SCE) was obtained, which was higher than TiO 2
NTAs from the same synthetic method with 2.43% at 0.39 V. Photoelectrochemical
experiments have proved that TiO 2 nanobelts show better separation of photoin-
duced charge carriers than other 1D TiO 2 nanostructures. The production of hydrogen
from TiO 2 nanobelts via an alkali hydrothermal method showed generation rates up
− 1
− 2
to 2.41 m h , which were much higher than those of commercial TiO 2 powder,
− 1
− 2
which had a rate of 1.91 m h . Zheng's group reported a hierarchically branched
TiO 2 NR structure (B-NR) that served as a model architecture for efficient photo-
electrochemical devices because it offered a large contact area with the electrolyte,
excellent light-trapping characteristics, and a highly conductive pathway for charge
carrier collection at the same time [60–62]. Under illumination of an Xe lamp (with
− 2
a spectrum corresponding to AM 1.5G, 88 mW cm ), the B-NRs produced a pho-
− 2
tocurrent density of 0.83 mA cm at 0.8 V vs RHE. The incident photon-to-current
conversion efficiency reached 67% at 380 nm at 0.6 V vs RHE, nearly two times
higher than without branches. We can speculate that the branches improved efficiency
by means of improved charge separation and transport within the branches because of
their small diameters and a fourfold increase in surface area, which facilitated the hole