A schematic representation of (A) (i) powdered samples of the NTiO2¬–oxCNT nanohybrids, (ii) a basic unit of the NTiO2¬–oxCNT nanohybrid, (iii) the elemental bonding details of the repeating unit that forms an NTiO2 nanocrystal and (iv) microscopic images of the nanohybrids and (B) results for ultra violet-visible spectrophotometer scans and a demonstration of the degradation of a model textile dye.
A schematic representation of (A) (i) powdered samples of the NTiO2¬–oxCNT nanohybrids, (ii) a basic unit of the NTiO2¬–oxCNT nanohybrid, (iii) the elemental bonding details of the repeating unit that forms an NTiO2 nanocrystal and (iv) microscopic images of the nanohybrids and (B) results for ultra violet-visible spectrophotometer scans and a demonstration of the degradation of a model textile dye.

Photocatalysts use light to power chemical reactions in a whole host of applications from fuel cells to water remediation. There are many ways to improve the performance of titania (TiO2) photocatalysts from using catalyst support materials, which reduce recombination rates, increase photosensitization, and help break down organic pollutants, to doping or co-doping with other atoms to expand the absorption range. Since using metal atoms such as dopants is problematic for water treatment if they leach into the environment, non-metallic dopants like nitrogen (N) are considered safer.

Researchers from the University of South Africa have used these two strategies in conjunction to produce N-doped TiO2 photocatalysts on a carbon nanotube (CNT) support and systematically compared the effects of using different synthesis routes [Zikalala et al., Materials Today Chemistry 10 (2018) 1-18]. Edward N. Nxumalo and his team find that photocatalysts with distinctly different properties and performance are produced depending on whether a hydrothermal or solgel synthetic route is used.

The nanohybrid catalyst comprises N-doped TiO2 nanocrystals embedded onto the outer surface of functionalized or oxidized CNTs (oxCNTs). Alternatively, CNTs can intertwine around larger N-TiO2 nanoparticles.

Both synthesis routes start with a colloidal mixture of oxCNTs and TiO2 nanocrystals, which have been exposed to a nitrogen source. In the hydrothermal approach, the mixture is heated in a sealed autoclave for 24 hours. By contrast, the solgel process requires the colloidal mixture to be stirred until a thick gel is formed, which is then calcined in an oven for 2 hours. The solgel method yields a yellow-colored powder, confirming the presence of N-doping. The hydrothermal process, however, yields a black/grey powder, indicating an interaction between TiO2 and oxCNTs instead.

The hydrothermally synthesized nanohybrids have higher oxCNT content and, therefore, increased photosensitization in the visible range and a greater propensity for faster photocatalytic reactions. Solgel produced nanohybrids, on the other hand, show a correlation between the oxCNT content and the surface area available for adsorption.

“The hydrothermal method offers better control of the optical properties of nanohybrids by varying the oxCNT content, producing a photocatalyst with a higher propensity to harness visible light than solgel-synthesized material,” explains Nxumalo. “On the other hand, the solgel method offers better control over the active surface area and produces denser attachment of NTiO2 nanoparticles onto the oxCNTs.”

The researchers believe that the choice of synthesis method enables photocatalysts’ ability to absorb a broader spectrum of light to be tailored. Both nanohybrids could be used to harness sunlight for environmental remediation of organic pollutants in water.

“We are now in the process of evaluating the photocatalytic performance of these nanohybrids under actual sunlight for the degradation of dyes in real textile waste,” Nxumalo told Materials Today.