Graphene oxide supported ternary metal sulphides as photocatalyst for the degradation of emerging contaminants
Abstract
The development of effective ternary metal sulfides supported graphene oxide composites as catalytic materials have been explored in this thesis. The Single source precursor method employing dithiocarbamate metal complexes were employed as precursor in a heat-up method in the synthesis of the copper-based ternary metal sulphides, which were composited with graphene derivatives through facile techniques.
The suitability of the metal complexes as single source precursors was firstly evaluated by exploring the obtained metal complexes as precursors for the synthesis of their respective binary metal sulphides. Particularly, the ability to obtain different CuxS phases, which is important for the selective synthesis of different stoichiometric phases of Cu-M-S was explored. Thermolysis of copper bis dithiocarbamate metal complex in a mixture of oleylamine (OLA) and dodecanethiol (DDT) showed a mixture of phases were obtained at relatively low temperature with pure phase of Cu5S9 only obtainable at high temperature. This study showed the significance of solvent property in influencing phase selectivity in the synthesis of Cu2-xS.
The thermolysis of bismuth(III) tris (N-methyl-N-phenyl dithiocarbamate) complex was also explore in a solvent mixture oleylamine, dodecanethiol and octadecene. At temperature range of 150 -250 °C Bi2S3 nanoparticles with varying microstructural properties was obtained. The microstructural properties of the nanoparticles were explored using the Scherrer equation, Williamson-Hall plot and Rietveld analysis. A close agreement between the models was observed with the particle size ranging between 20.1 – 45.9, 20.9 -26.6 and 23.0-29.7 nm for the Scherrer equation, Williamson-Hall’s plot and Rietveld analysis respectively. Similarly, antimony(III) tris (N-methyl-N-phenyl dithiocarbamate) complex was thermolyzed in hexadecylamine solvent and the microstructural property was studied. In addition to the microstructural studies, other crystal properties and empirical parameters such as lattice parameters, cell volume, bulk density, X-ray density, surface area and porosity were evaluated. The particle size for the Sb2S3 nanoparticles obtained ranged between 85.1-111.3 nm for the explored models, while all the models affirmed the strain in the strain to be in the lattice to be due to lattice contraction. The empirical analysis showed the presence of Sb vacancies, while the lattice parameters and other cell parameters agreed significantly with values in literature.
After establishing the thermal decomposition products of these complexes, they were co-thermolyzed with other precursor compounds which have comparable decomposition profiles for the synthesis of ternary Cu-M-S. Firstly, the selective synthesis of two phases of copper tin sulphides: petrukite (Cu3SnS4) and kuramite (Cu2SnS3), was achieved by the solvothermal decomposition of diphenyl tin(II) dithiocarbamate and Cu (II) bis N-methyl-N-phenyl dithiocarbamate complex by the heat-up process in high boiling solvents. Cu2SnS3 was obtained in oleylamine at 220 °C, while Cu3SnS4 was obtained in a mixture of oleylamine
and dodecanethiol at 180 °C. The particle size distribution was 13.7 ± 2.4 for Cu2SnS3 and 11.3 ± 1.5 nm for Cu3SnS4, with the band gap energy being 1.16 and 1.57 for Cu2SnS3 and Cu3SnS4 respectively. The photoluminescence spectroscopy measurement showed a lower charge carrier recombination in Cu2SnS3 compared to Cu3SnS4, which account for the 94.0% degradation efficiency towards Tetracycline degradation compare to 73.0% recorded for Cu3SnS4. Electrochemical measurements also showed that Cu2SnS3 exhibited lower resistance, getter charge transport and conducting properties compare to Cu3SnS4.
Subsequently, supported Cu2SnS3 and Cu3SnS4 nanoparticles were synthesized through facile solvothermal processes. Cu2SnS3 supported by reduced graphene oxide (rGO) were prepared at different rGO ratio 0-20%. The photocatalytic activity of the nanoparticles for tetracycline degradation was influenced by the weight fraction of rGO, with composite with 20% rGO composition showing the maximum degradation efficiency of 93% compared to compared to 64, 65.5 and 68.8% degradation recorded for 0, 5, and 10% rGO composition respectively. rGO was observed to improve the degradation efficiency by enhancing the adsorption capacity of the nanoparticle and also reducing charge carrier recombination. For the Cu3SnS4 phase, the influence of graphene oxide (GO), protonated graphitic carbon nitride (PCN) and composite of GO and PCN (GO/PCN) on the degradation of tetracycline was explored. The study showed that the composite GO/PCN showed the highest degradation efficiency of 96.5% compare to 85.2 and 90.9% recorded for composites with PCN and GO respectively. The enhanced activity of the GO/PCN based composite was found to be due to exciton generation and enhanced adsorption property.
Afterwards, selective synthesis of copper antimony sulphides (CAS) was explored. Three different phases of CAS were obtained: chalcostibite (CuSbS2), famantinite (Cu3SbS4) and tetrahedrite (Cu12Sb4S13) using copper(II) dithiocarbamate and antimony(III) dithiocarbamate complexes in a solvothermal heat-up process. The catalytic activity of the three phases for persulfate activation of tetracycline degradation was explored with degradation efficiencies of 26.1, 54.6 and 55.6% achieved by CuSbS2, Cu12Sb4S13 and Cu3SbS4 respectively. The study of the influence of pH on the persulfate activation process showed a higher efficiency of 85.5% was achieved under neutral condition. Radical scavenging experiment showed the process was radical based process, with the initiation being a one electron transfer process. the Cu3SbS4 phase with tetrahedrite phase exhibiting the highest activity. Further study on improving the activity of CuSbS2 was explored by supporting the nanoparticle on graphene oxide. The supported CuSbS2 was then explored in the degradation of tetracycline in persulfate activation and photocatalytic processes. The percentage degradation was 94 and 74% for the photocatalytic and persulfate activation process respectively. Mechanism study showed that the photocatalytic process proceeded significantly through the activity of photogenerated holes while the persulfate activation process, was a non-radical process.
Finally, the selective synthesis of copper bismuth sulphide (CBS) phase was attempted using Bi(III) bis N-methyl-N-phenyl dithiocarbamate and Cu(III) bis N-methyl-N-phenyl dithiocarbamate as single source precursors. However, only the wittichenite phase (Cu3BiS3) was obtained in pure phase. The Cu3BiS3
nanoparticles was agglomerated and oval-shaped with a particle size of 60.1 nm. The obtained semiconductor nanoparticle was further supported on reduced graphene oxide and explored as a catalyst in UV-light assisted persulfate activation process and in a photocatalytic process for the degradation of diclofenac. A degradation efficiency of 74% was achieved in the photocatalytic process, while the UV-light assisted persulfate activation process attained 84% degradation efficiency. The composites showed great capacity for degradation for the degradation of a wide variety of pollutant with 100% degradation efficiency achieved for methyl orange, tetracycline and methylene blue, while bisphenol reached 81% efficiency. The study showed that the nanocomposite exhibited a higher activity in the UV-light assisted process, with the process showing lower energy consumption when compared to other reported process.
The results of this study confirm the potential of dithiocarbamate complexes as precursor compounds for achieving phase selectivity synthesis of Cu-M-S semiconductors and also, the potential of these materials as versatile catalysts for advanced oxidation processes. In addition, the study has shown that graphene oxide/reduced graphene oxide is a suitable support material for enhancing the catalytic activity of these semiconductors.