(200g) Scalable Biomineralization of AgInZnS Quantum Dots for Photocatalytic Hydrogen Generation

Cadmium-based metal chalcogenide quantum dots are promising semiconductors capable of meeting the demands for photocatalytic H₂ generation, yet their intrinsic toxicity has motivated the search for high-performance, non-toxic alternatives.^1–3 The low toxicity, suitable band gaps, high extinction coefficients, and long photoluminescence lifetimes characteristic of ternary I-III-VI QDs makes them promising candidates especially when Zn is also incorporated to improve quantum yield (QY) and passivate the surface. One particularly promising composition is AgInZnS, owing to the tunability of its band gap (1.8-2.4 eV) to within the range suitable for photocatalytic H₂ generation.⁴ However, the low compositional toxicity of AgInZnS is undercut by the high-temperatures and toxic solvents conventionally employed for its synthesis.^4,5

Biomineralization-based routes to size-controlled QDs offer an alternative, ‘green’, and potentially scalable synthesis strategy, characterized by low temperatures and aqueous phase processing. Biomineralization of size-tunable CdS QDs, for example, is possible by the aqueous-phase, low-temperature enzymatic (cystathionine gamma lyase) turnover of the amino acid L-cysteine to H₂S in buffered solutions of cadmium acetate.⁶ In this study, we expand the scalability and versatility of QD biomineralization by 1) establishing strategies for cyclic QD synthesis with facile enzyme recovery and reuse, and 2) elucidating mechanistics insight into single-enzyme synthesis of compositionally complex QDs. Specifically, we establish facile enzyme immobilization strategies on inexpensive, easily recoverable supports that offer sufficient enzyme activity to support continuous cyclic synthesis of QDs with narrow particle size distribution. This facile strategy for immobilized enzyme-mediated QD synthesis opens exciting possibilities for the scalable, ‘green’, biosynthesis of consistently sized QDs.

We then expand the compositional diversity of the single-enzyme biomineralization approach to AgInZnS QDs. In order to elucidate the AgInZnS formation mechanism, we carry out both one pot and sequential syntheses that identify critical In/Cys ratios affecting photoluminescence, offer insight into the role of Ag content in nucleation and growth, and begin to elucidate In-S complexation and the role of Ag ion exchange in tuning photoluminescence emission wavelength. We will demonstrate synthesis-structure-function relations in the context of photocatalytic hydrogen generation rates. Ultimately, this study advances the scalability and versatility of single-enzyme biomineralization as a means for ‘green’ synthesis of various non-toxic semiconductors.

References

1. Rao, V. N. et al. Sustainable hydrogen production for the greener environment by quantum dots-based efficient photocatalysts: A review. Journal of Environmental Management (2019) doi:10.1016/j.jenvman.2019.07.017.
2. Liu, Y. et al. One-step aqueous synthesis of highly luminescent hydrophilic AgInZnS quantum dots. J. Lumin.(2018) doi:10.1016/j.jlumin.2018.05.040.
3. Zang, Z. et al. Tunable photoluminescence of water-soluble AgInZnS–graphene oxide (GO) nanocomposites and their application in-vivo bioimaging. Sensors Actuators, B Chem. (2017) doi:10.1016/j.snb.2017.07.144.
4. Tang, X. et al. Nanocomposites of AgInZnS and graphene nanosheets as efficient photocatalysts for hydrogen evolution. Nanoscale (2015) doi:10.1039/c5nr05145b.
5. Lin, P. C., Wang, P. Y., Li, Y. Y., Hua, C. C. & Lee, T. C. Enhanced photocatalytic hydrogen production over In-rich (Ag-In-Zn)S particles. Int. J. Hydrogen Energy (2013) doi:10.1016/j.ijhydene.2013.04.125.
6. Spangler, L. C., Cline, J. P., Kiely, C. J. & McIntosh, S. Low temperature aqueous synthesis of size-controlled nanocrystals through size focusing: a quantum dot biomineralization case study. Nanoscale 10, 20785–20795 (2018).