A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in the School of Chemistry Faculty of Science, University of Witwatersrand, 2017Advancements in nanotechnology and nanosystems promise to extend limits of sustainable
development and environment remediation in an attempt to address some of the world most
challenging problems. Specifically, nanotechnology has played an important role in the design,
synthesis, and characterization of various new and novel functional nanomaterials possessing
extremely unique properties. For example, low dimensional nanostructures such as
semiconductor nanocrystals with well controlled sizes, shapes, porosities, crystalline phases, and
structures have been prepared via various synthetic methods. In addition these semiconductor
nanocrystals have attracted research attention because of their fundamental role in the
comprehension of the quantum size effect and great potential applications to save resources and
improve the environment.
Tremendous studies have established that morphological, optical, catalytic and electronic
properties of semiconductor nanocrystals can be manipulated during synthesis by simply varying
the growth parameters. Herein we establish the effect of different synthetic methods and several
growth parameters on the properties of the as-synthesized semiconducting metal selenides
nanocrystals (NixSey and InxSey) including structural, optical, electronic and catalytic properties.
For example, reducing coordinating solvent oleylamine was seen to favour a particular
morphologies and stoichiometries despite the duration of synthesis. In the case of InxSey
nanocrystals, oleylamine favoured indium monoselenide (InSe) nanosheet formation while
addition of 1-DDT as a co-surfactant to oleylamine produces In2Se3 nanowires. For NixSey
nanocrystals, TOP as a co-surfactant to different ligands favoured the formation Ni3Se2 with
different shapes including dots, plates, rods and wires in different solvents. Other parameters
studied included the reaction time and temperature.
Besides the properties, we probe the potential applications of these materials in dye sensitized
solar cells as counter electrodes as well in chemical sensor as the sensing material. NixSey
nanocrystals were employed as CE in DSSCs in an attempt to replace the noble expensive
platinum conventionally used as CE in most DSSCs. It was established that different
stoichiometry played a significant role in the catalytic reduction of I3-. Thus, different
photovoltaic performance parameters were obtained with NiSe2 giving a higher PCE of 1.5 %
followed Ni3Se4 then Ni3Se2. These values were however very low compared to the ones
reported in literature, something that was attributed to low electron mobility, enhanced
recombination and reduced catalytic performance as a result of poor device assembly and the
organic ligand layer encapsulating the nanocrystal.
In another scenerio, indium monoselenide nanocrystals were employed in chemiresistive sensors
to detect the presence of a number of VOCs including formaldehyde, methanol, chloroform and
acetone in the ambient. Indeed despite the well-known electrical, optical and structural properties
previously reported in literature, metal selenides such as CdSe, PbSe and ZnSe among others
present lack of investigation for gas sensing. The experimental results showed that different
morphologies of InSe nanostructures interacted differently to the analyte gas suggesting
difference in the electronic properties of different morphologies. The InSe nanoparticle based
sensors gave a good response to HCHO and MeOH fumes and were more selective to HCHO
fumes than chloroform and acetone. While those fabricated using the InSe nanosheets though
responding well to HCHO recovered half way when exposed back in air and resulted in
relatively high noise to signal ratio when exposed to MeOH. The operating temperature range for
the InSe sensor devices were determined to be near room temperature. The sensors response was
observed to decrease with increasing temperature from 30 °C to 90 °C. Evident from the results, the surface capping molecule (oleylamine) employed to stabilize the nanostructures during
synthesis was responsible for the poor sensing abilities of the nanostructures.XL201
