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Abstract
Quantum dots (QDs) are a class of semiconductor nanomaterials that have a tunable electronic band gap through quantum confinement. QD materials have a variety of technological applications including photocatalytic water splitting, energy generation via solar cells, light emission in display technologies and luminescent biomarkers. However, a challenge in sustainably producing these materials is to move away from the high synthesis temperatures and toxic nature of the precursors used in current preparation routes. In this work, low temperature, aqueous phase, bioinspired synthesis routes have been used to prepare a variety of chalcogenide-based QD systems. Multiple characterization techniques have been used to evaluate the structure and properties of the synthesized QD material. In particular, aberration corrected scanning transmission electron microscopy (AC-STEM) has been used to show how different synthesis conditions affect size, morphology, crystal structure and composition of the particles.
The room temperature, aqueous biosynthesis of CdS QDs using the cystathionine γ-lyase enzyme (CSE) was performed utilizing L-cysteine as a capping ligand and sulfur source. The nucleation and growth mechanisms for this synthesis route were investigated and compared with syntheses involving controlled additions of sulfur via NaHS. AC-STEM characterization of the CdS QD materials produced showed them to have controllable particle size distributions (PSDs) and comprise a mixture of wurtzite/zincblende type structures. In addition, a new bio-inspired non-enzymatic aqueous preparation route with L-cysteine, pyridoxal phosphate (PLP), and FeCl3 showed that removing the enzyme expands the effective range of temperature and pH over which the wurtzite/zincblende type CdS QDs can be controllably synthesized. The quality of the CdS QDs produced by both these methods compared favorably with commercially produced ‘organic phase’ CdS particles that had undergone ligand exchange to stabilize them in water. Studies were performed to see if the biomineralization synthesis procedure could be adapted to grow CdS QDs directly onto a pre-existing TiO2 substrate. After confirming that the CSE enzyme retained its activity after immobilization, it was used to directly synthesize CdS QDs onto the TiO2 support particles. It was also found that even though the enzyme activity decreased after multiple QD synthesis cycles, it’s activity for QD formation was recoverable by addition of a PLP co-enzyme.
In a separate series of experiments, attempts were made to attach pre-formed biomineralized CdS QDs to the surface of partially reduced graphene oxide (rGO) in order to improve charge separation during the water splitting reaction. This goal was partially accomplished via the addition of poly-L-lysine (PLL) as a linker molecule between the L-cysteine capping ligand and the rGO surface, which allowed for particle attachment, but unfortunately reduced the photocatalytic performance of the QDs. To reduce the length of the linkage between CdS QDs and the rGO support, a ligand exchange reaction was performed whereby cystathionine (CA) replaced L-cysteine. This modification promoted the electrostatic attachment of CdS to rGO and simultaneously provided the desired charge separation effect required for improved photocatalytic action.
In order to use untethered biomineralized CdS colloids as photocatalysts for the water splitting reaction, modifications had to be made stabilize and improve their catalytic activity. A sequential biomineralization process was used to form CdS/ZnS particles. For these materials, no secondary populations of ZnS were produced, the structure of the core CdS particles was unchanged, and zinc association with the CdS particles was confirmed by XEDS. The CdS/ZnS particles exhibited a greater structural stability compared to CdS QDs alone, but unfortunately a lower activity. Hence an optimal compromise Zn loading was identified which significantly improved particle stability without blocking all the catalytically active surface sites.
Another way of improving the photocatalytic performance of CdS QD systems is to introduce a co-catalyst such as NixSy or Pt. NixSy was found to form a complex with L-cysteine under normal biomineralization conditions, and so has to be produced separately under low pH conditions. When added a co-catalyst, the relatively large amorphous NixSy particles generated became atomically dispersed and incorporated into/onto the CdS QDs. The photocatalytic performance of the resultant CdS/NixSy material was shown to be much higher than that of the base CdS QDs, and comparable to that of CdS/Pt catalysts prepared via photodeposition.
Copper zinc tin sulfide (CZTS) QDs were also biosynthesized and tested for application in a quantum dot sensitized solar cells device. These QDs are attractive due to the relative abundance of the constituent elements and their low toxicity compared to CdS. Firstly, CuS, ZnS and SnS were all confirmed to be compatible with the biomineralization route, and then a one-pot synthesis method was devised for preparing CZTS particles. Detailed structural and compositional analyses of a series of CZTS samples with differing Cu:Zn:Sn ratios was required in order to optimize the optoelectronic performance of the material. In addition, the fabrication process by which the QDs were incorporated in the solar cell structure heavily impacted the device’s performance, with drop cast CZTS layers showing inferior performance to films having CZTS particles grown in-situ within the TiO2 active layer.
Finally, aqueous phase AgInS QDs with Zn additions were biosynthesized and investigated as materials for cell biomarker applications due to their inherent low toxicity and biocompatibility. This was achieved by biosynthesizing InS QDs, and then subsequently converting them to AgInS QDs, and then finally adding Zn to form AgInS/ZnS structured materials. The optoelectronic performance of these materials was correlated with AC-STEM compositional and structural studies. The optimum ratio of Ag-In was found to be 1:5, the particles exhibited a AgIn5S8 type structure. The final Zn addition did not change the base AgInS particle structure or form separate ZnS particles, but rather is believed to form a surface stabilizing ZnS species. Cell viability studies, and conjugation of the AgInS/ZnS particles to macrophage cells were successfully performed, which allowed for the targeted attachment of these fluorescent QDs to the outer membrane of THP-1 monocytes and macrophages.
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