Synthesis and supporting of carbon dots onto titania, carbonized chitosan, silica spheres, hollow and solid carbon spheres for the removal of methyl orange from wastewater

Abstract

Since the discovery of carbon dots (Cdots) in 2004, there has been an exponential increase in the number of research publications reporting on their synthesis, characterization, and potential applications. This is attributed to their excellent fluorescence properties which are central to potential applications in various scientific fields such as catalysis, nanomedicine, sensing and bioimaging, among others. However, issues pertaining to Cdots such as the origin of their fluorescent emissions, fluorescent quenching when in the solid-state, purification procedures (due to ultra-particle size ≤ 10 nm), and exploration of Cdots/support composites, among others still need to be addressed. In this project, Cdots and nitrogen-doped Cdots (NCdots) derived from chitosan, glycerol, and urea were synthesized using a hydrothermal carbonization method in an autoclave (180 and 200 ℃) and a microwave-assisted method (700 W). To obtain the NCdots in the solid-state, a systematic approach was established for the incorporation of the NCdots on the surface of a solid support such as silica spheres (SSs), hollow carbon spheres (HCSs), solid carbon spheres (SCSs), carbonized chitosan (CHIT-xh), and TiO2 particles. A mixture of ethanol, water, and ammonia solution provided a medium for the good dispersion and incorporation of NCdots on the surface of the solid supports. The obtained NCdots were characterized using various techniques and were observed to have a typical size below 10 nm, spherical shapes, abundant functional groups, and enhanced fluorescent emission. The fluorescent emissions of the unsupported and supported Cdots were observed to be dependent on the excitation wavelength and exhibited a typical redshift as the wavelength was increased. In addition, the fluorescent emissions were influenced by the nature and interaction between NCdots and the support matrix. For example, the photoluminescence quantum yield (PLQY) of bare NCdots derived from chitosan was 34 %, while the supported NCdots PLQY values were dependent on the support with values in the order SSs (35.3 %) > HCSs (13 %) > SCSs (7.5 %) at 20 wt. % loading. In addition, a “ship in a bottle” approach was used to contain the Cdots and NCdots nanoparticles derived from glycerol and urea inside the hollow cavity of HCSs via a microwave assisted method. The solvatochromism of the encapsulated Cdots and NCdots revealed solvent dependent characteristics when dispersed in water, hexane, and acetone. This suggests that the ii surface functional groups of Cdots and NCdots are the prominent constituents responsible for fluorescent emission and interacted with the solvent via hydrogen bonding and/or dipole-dipole interactions, resulting in the tuning of the fluorescent emissions. The composite materials consisting of NCdots, carbonized chitosan (CHIT-xh), and TiO2 were explored for potential application in wastewater treatment via adsorption and photodegradation processes. For this, NCdots/CHIT-xh and NCdots/TiO2 nanocomposites were prepared and used for the adsorption and photodegradation of methyl orange (MO) in an aqueous solution, respectively. These composites exhibited high MO adsorption and photodegradation efficiency which was attributed to the presence of NCdots on their surfaces. For example, the incorporation of 10 wt. % NCdots on the surface of TiO2 was an optimum concentration that exhibited higher MO photodegradation efficiency compared to pristine TiO2. This was attributed to photosensitization (photo-induced electron transfer process) and a synergetic effect between TiO2 and NCdots. In both the adsorption and photodegradation studies, the MO removal/degradation efficiency was dependant on the initial pH, the concentration of the solution, and catalyst dosage. These composites have demonstrated a potential for application in the removal of MO from wastewater via adsorption and photodegradation processes.