Development of stable and efficient visible-light-driven photocatalysts through heteroatom doping strategy
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Date
31/07/2021Author
Cao, Mengyu
Metadata
Abstract
The photocatalysis is one of the most promising sustainable technologies to tackle the
challenges of environmental pollutions. However, traditional photocatalysts such as TiO2
exhibit the narrow light absorption range and low quantum efficiency. These drawbacks
seriously limit their practical applications. The development of high-efficiency photocatalysts
with large specific surface area and high photocatalytic activity has become the key to the
photocatalysis technology. Doping heteroatoms into the crystal of photocatalyst is an effective
way to improve its photocatalytic activity. With appropriate photocatalyst design, the dopants
in moderate doping concentration can optimise the catalysts in the following multiple aspects:
(i) with extra dopant energy level in the bandgap of semiconductor, dopants can reduce the
bandgap to broaden the light absorption of the photocatalysts; (ii) dopants can intentionally
shift the valence band position to improve the photooxidation capability of the catalysts; (iii)
dopants can suppress the photo-excited electrons and holes recombination, which results in an
enhanced quantum efficiency; (iv) in plasmonic photocatalysts, dopants can modify the
electronic structure of the plasmonic crystal to enhance the photo-excited charge carrier
generation and increase the energy of the excited charge carriers.
In this thesis, the heteroatom doping strategy has been used to enhance the dye
photodegradation performance of photocatalysts. With the help of molecular and electronic
structure analyses, the mechanisms underpinning the enhancement of photocatalytic
performance are elucidated. In chapter 3, the Zn doped C3N4 has been successfully synthesized
in eutectic ZnCl2-KCl salts mixture for the first time. The low melting temperature of ZnCl2-
KCl promotes the dispersion of the organic precursors, therefore creating a specific surface
area at least ~7.4 times larger than the bulk C3N4 synthesized via the conventional thermal
polymerization method in air (C3N4-M-Air). The significant improvement in the photocatalytic
activity is achieved through locating the melting point of the salt mixture within the
temperature window between dicyandiamide and melamine oligomer formation steps in the
polycondensation process. Using dicyandiamide as the precursor shifts the valence band
maximum (VBM) of the prepared C3N4 (C3N4-D) positively, therefore enhancing the oxidation
capability of the photocatalysts. The Zn dopants at the interstitial site of C3N4 in an appropriate
concentration suppress the photo-excited electron-hole recombination, which significantly
contributes to the high photocatalytic activity. The optimal sample C3N4-D shows ~4.2 times
larger photocurrent density and ~1.46 times longer carrier lifetime than the C3N4-M-Air. In photocatalytic methyl orange (MO) degradation, the pseudo-first reaction rate constant of
C3N4-D is ~4.15 times higher than that of the C3N4-M-Air control group.
In chapter 4, the combined effects of Cl doping and agitation are used for the first time to
improve the photocatalytic performance of C3N4 synthesized via solvothermal method. The
enhanced photocatalytic RhB degradation activity is attributed to the optimized electronic
structure, enlarged specific surface area and balanced interstitial/substitutional Cl doping.
More importantly, it is found that the preferred doping site for Cl dopants is strongly controlled
by the agitation rate. The atomic ratio of interstitial over substitutional Cl dopants shows a U shape correlation with the agitation rate. Furthermore, the different effects of interstitial and
substitutional Cl dopants on the photocatalytic activity are distinguished and elucidated. The
optimal synthesis condition for Cl-doped C3N4 is associated with a moderate agitation rate of
60 rpm (60-C3N4). Under 60 rpm agitation during the synthesis, the 60-C3N4 exhibits
remarkably larger specific surface area, stronger photo-oxidation capability, reduced bandgap
and suppressed electron-hole recombination comparing with the control group g-C3N4
synthesized via conventional thermal polycondensation method. An outstanding
photocatalytic RhB degradation performance is therefore observed for 60-C3N4 with ~37-fold
higher pseudo-first reaction rate constant than the control group conventional g-C3N4 sample.
In chapter 5, the C doped TiN/ultrathin carbon layer has been synthesized via the calcination
of TiCl4/urea mixture and shows the prominent plasmonic photocatalytic RhB degradation
performance under visible light irradiation. Based on the systematic investigations on the
preparation conditions, it is found that the urea amount and calcination temperature are the
two critical factors determining the chemical composition and crystal size of TiN nanoparticles.
In the optimal condition with 3.0g urea and 1100 o
C synthesis temperature, the TiN
nanocrystals with the mean size of ~37 nm are formed and well-dispersed on N doped ultrathin
carbon layer layers. The larger amount of urea and higher synthesis temperature result in the
increase of TiN nanoparticle size. Moreover, it is proven that the appropriate amount of C
doping can enhance the plasmonic photocatalytic activity of TiN. Based on DFT calculation,
the C sp band introduced into TiN band structure can enhance the interband excitation of
electrons, which results in the excited holes with higher quantity and energy. In visible light
driven RhB photodegradation, the optimal C doped TiN/ultrathin carbon layer sample shows
the higher first-order reaction rate constant than the benchmark rutile TiO2 and C3N4/TiO2 by
~34.2 and 6.5 times, respectively.