| Abstract |
Highly efficient and cost-effective photocatalysts are among the most prominent targets
in the field of clean energy production and environmental remediation. Τhe
understanding of photochemical charge transfer mechanisms at the nanoscale is
essential to develop effective catalysts for energy conversion and environmental
remediation applications. Photocatalytic hydrogen generation through water splitting is
regarded as a promising solution to future energy demands. This approach utilizes a
semiconductor-based catalyst that absorbs sunlight and splits water, producing
hydrogen. Alongside, water pollution is on top of the most permeative threats
worldwide, risking human health and quality life. This is because voluminous amounts
of toxic metals, such as hexavalent chromium, Cr(VI), are released directly or
incidentally to the environment. Therefore, finding an effective way for remediation of
Cr(VI)-contaminated solutions is undoubted of high priority in the field of
environmental and health protection.
In this dissertation, new and cost-effective synthetic strategies for preparing 2D/2D
layered nano-heterostructures of transition metal dichalcogenides (MS2, M = Mo, Sn)
and graphitic carbon nitride (g-C3N4) have been successfully developed and the
resulting materials have been tested against photocatalytic production of hydrogen and
reduction of toxic Cr(VI). A controllable synthesis method and a combination of
electron microscopy, optical absorption, photoluminescence, and electrochemical
impedance spectroscopic studies have been utilized to investigate the effect of
MoS2 nanosheet lateral dimension and edge length size on the photochemical behavior
of MoS2-modified g-C3N4 heterojunctions. These nano-heterostructures, which
comprise interlayer junctions with variable area, i.e., MoS2 lateral size ranges from 18
nm to 52 nm, provide a size-tunable interfacial charge transfer through the MoS2/g-
C3N4 contacts, while exposing a large fraction of surface MoS2 edge sites available for
the hydrogen evolution reaction. Importantly, modification of g-C3N4 with MoS2 layers
of 39±5 nm lateral size (20 wt % loading) creates interfacial contacts with relatively
large number of MoS2 edge sites and efficient electronic transport phenomena, yielding
a high photocatalytic H2-production activity of 1497 μmol h−1 gcat−1 and an apparent
QY of 3.3 % at 410 nm light irradiation. This study offers a design strategy to improve
light energy conversion efficiency of catalysts by engineering interfaces at the
nanoscale in 2D-layered heterojunction materials.
10
By modifying the above MoS2 layers with nickel, a novel series of 2D/2D layer
heterostructures composed of exfoliated Ni-doped MoS2 nanosheets and g-C3N4 layers
have been prepared. These hybrid materials can carry out photocatalytic Cr(VI)
reduction in aqueous solutions with outstanding activity, exhibiting apparent QYs as
high as 29.6 % and 23.7 % at 375 and 410 nm. Ni doping of MoS2 markedly increases
the photochemical activity, which, together with electrochemical spectroscopy and
theoretical DFT studies, arises from the enhanced carrier density and mobility at the
Ni-MoS2/g-C3N4 interface. In addition to the favorable charge transport properties,
delineation of the photoinduced oxidation reactions by control catalytic experiments
and gas monitoring techniques reveals that the high efficiency also arises from fast
water oxidation kinetics. Due to the efficient dissociation and transport of free excitons,
surface-reaching holes effectively oxidize water to form molecular oxygen. The results
of this work mark an important step forward in understanding and designing low-cost
and earth-abundant catalysts for detoxification of Cr(VI)-contaminated industrial
effluents.
Additional subject of the present research work is the synthesis of 2D/2D SnS2/g-C3N4
layered heterostructures with reduced interfacial resistance and improved charge
transfer kinetics. The realization of these materials was accomplished by using a
photochemical deposition method. These newly developed catalysts, which consist of
exfoliated g-C3N4 flakes and SnS2 nanosheets (~25–30 nm in lateral diameter),
demonstrate outstanding photocatalytic Cr(VI) reduction (with a 21.2 μmol h–1
conversion rate) and water oxidation (with a 15.1 μmol h–1 O2 evolution rate) activity.
The SnS2/g-C3N4 heterostructures reach energy conversion efficiencies of up to 16.4%
and 12.1% at 375 nm and 410 nm, respectively, that is among the best known Cr(VI)
reduction catalysts reported to date. Based on X-ray photoelectron, UV–vis optical
absorption, and electrochemical and photoelectrochemical measurements, we provide
detailed mechanistic insight into the photochemical redox reactions and charge
transport dynamics in this catalytic system. The results demonstrate the great potential
of the SnS2-decorated g-C3N4 nano-heterostructures as viable photocatalysts for
environmental protection, including remediation of Cr(VI)-contaminated industrial
effluents.
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