Development of stable and efficient visible-light-driven photocatalysts through heteroatom doping strategy

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.