Design of Carbon Nitride-Based Materials for Highly Efficient Visible-Light Driven Photocatalysis
Creators: name: Li, Xuying
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340305
subject: Physical properties of materials
classificationCode: 340303
subject: Nanochemistry
classificationCode: 401605
subject: Functional materials

description: Since the discovery of photocatalytic water splitting in 1971, there have been significant efforts to investigate photocatalysts. One promising candidate is polymeric carbon nitride (referred to as CN, commonly mislabelled as g-C3N4 in the literature), which has several advantages such as visible light absorption (with a band gap of 2.7 eV), low-cost, easy synthesis, appealing electronic structure and high stability. In addition, a lead-free all-inorganic perovskite, Cs2AgBiBr6, has attracted significant interest as an efficient photocatalyst due to its favourable optoelectronic properties including a long charge carrier lifetime, low effective carrier mass and good carrier mobility.
Although CN has shown promise in the field of photocatalysis, its overall performance is hindered by several factors including rapid charge recombination, sluggish charge transport and a relatively wide band gap. In this thesis, defect-engineering and self-assembly approaches were developed to modify the visible-light absorption, electronic structure, and surface properties of CN for efficient photodegradation reactions. First, CN with robust visible-light photoactivity was designed through defect engineering by the use of carbon doping and surface vacancy modification. The additional carbon inhibits charge recombination and broadens the absorbance wavelength range, while surface cyano functionality promotes charge separation via its electron-withdrawing effect. The controlled introduction of the C dopant and surface cyano groups by varying the alkali volume and introducing citric acid during synthesis optimized the band structure of CN. This resulted in a high production of active oxidation radicals for visible-light driven photocatalytic degradation towards the organic pollutants rhodamine B and tetracycline, indicating the potential of this material for visible-light driven water purification.
Further, a unique spherical cauliflower-like supramolecular CN precursor was prepared using the solvothermal treatment of melem in the presence of citric acid and alkali. The 3D spherical architecture resulted from time-dependent hydrolysis and breaking of H-bonds in the framework, with control over the morphological evolution occurring during the solvothermal treatment of melem and self-assembly of melamine-cyanuric acid aggregate with alkali and citric acid. The alkali promoted hydrolysis and breakage of the H-bonds during solvothermal treatment, while the citrate controlled particle size and surface morphology. The resulting 3D porous CN had an increased surface area, enhanced light utilization and charge carrier transport, leading to more effective photodegradation of tetracycline hydrochloride and rhodamine B under visible light irradiation.
In addition to modifying CN itself, integrating it with another semiconductor is a promising approach to improve the charge separation. Sb-doped Cs2AgBiBr6 was prepared to improve the light absorption and charge separation in Cs2AgBiBr6. The composite Sb-doped Cs2AgBiBr6/CN was constructed using a simple mechanical grinding method. The addition of the Sb dopant increased visible light absorbance up to 650 nm and prolonged the Cs2AgBiBr6 lifetime. A type II heterojunction established in the composite leads to reduced electron-hole recombination, which was attributed to strong interface interactions between the CN surface NHx groups and perovskite bromine anions through N-Br chemical bonds. The resulting composite demonstrated over 96% selectivity and 2.4-fold and 72.5-fold higher visible light photocatalytic toluene oxidation rates compared to Sb-substituted Cs2AgBiBr6 and CN alone. This all-inorganic, lead-free halide perovskite-based heterostructure has the potential for broad applications in visible light photocatalytic organic transformations.
Doping with transition metals is a promising approach to enhance the photoactivity of lead-free all-inorganic halide perovskites. Using density functional theory (DFT) calculations, the impact of 4th period 3d metal doping on the photoactivity of Cs2AgBiBr6 was investigated utilizing effective mass as an indicator. This study found that 3d metals with fully occupied d orbitals, typically Zn2+, can significantly improve the charge mobility of Cs2AgBiBr6. This was experimentally validated, resulting in significantly enhanced toluene photooxidation conversion rates in Zn-doped Cs2AgBiBr6. The calculations revealed that the introduction of shallow trap states below the conduction band minimum in Zn-doped Cs2AgBiBr6 effectively suppresses charge recombination and prolongs the lifetime of charge carriers. Furthermore, the theoretical calculation emphasized the utilization of B-site metal d-band and X-site atom p-band center as well as p-d band center as indicators to predict the photoactivity of perovskites, which showed the facilitated C(sp3)−H bonds dissociation and desorption process of intermediates in Zn doped Cs2AgBiBr6. Additionally, the introduction of s orbitals in CBM shifts it up, enhancing the reduction ability towards O2 molecules. All these factors contribute to the significant promotion in toluene oxidation, highlighting the effectiveness of employing computational approaches in identifying doping elements for enhancing the photoactivity of perovskite materials.