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Kirsten Jensen1 Troels Christiansen1

1, Department of Chemistry, University of Copenhagen, Copenhagen Ø, , Denmark

Despite decades of research into nucleation processes, very little is currently known on how nanocluster formation takes places on the atomic scale. We have developed methods which allows using X-ray Total Scattering and Pair Distribution Function analysis to follow nanoparticle nucleation and growth in situ.1-3 In contrast to conventional crystallographic studies, PDF analysis gives structural information from non-crystalline species, allowing obtaining structural information on the atomic scale, all the way from precursor to the final nanoclusters during synthesis. We have furthermore shown how PDF allows obtaining detailed information on nanocluster structures with no long range order on nanoclusters fundamentally different from bulk materials.4
Here, we use in situ PDF to study the formation of anionic bismuth based clusters. Bismuth oxido clusters in solution have recently attracted much attention for a wide range of applications in catalysis, medicine, and as precursors for advanced materials.5 Despite the very broad range of applications, the chemical processes involved in the formation of the clusters in solution are not well understood. While the cluster structures have been studied by single crystal diffraction after crystallization to the solid form, it is much more challenging to study the clusters directly in dilute solutions using conventional techniques.
Large bismuth oxido clusters of the type [Bi38O45L24] (where L = ligand, e.g. NO3-,) (here abbreviated {Bi38}), form in DMSO through a hexanuclear bismuth oxido cluster, {Bi6}. We have applied X-ray total scattering studies to follow the formation of ionic bismuth oxido clusters in solution. Using PDF, we can follow the transformation of cluster structures from {Bi6} units to stable {Bi38} clusters. We see that a disordered cluster containing a {Bi18} motif form as an intermediate structure in the cluster growth, also inducing disorder in the {Bi38} cluster, which orders on ligand exchange. By combining PDF with small angle scattering and ESI-MS, we obtain a unified view of the cluster growth, getting closer to an atomistic understanding of the cluster transformation processes.

1. Jensen, K. M. O.; Christensen, M.; Juhas, P.; Tyrsted, C.; Bojesen, E. D.; Lock, N.; Billinge, S. J. L.; Iversen, B. B. J. Am. Chem. Soc. 2012, 134, (15), 6785-6792.
2. Jensen, K. M. Ø.; Tyrsted, C.; Bremholm, M.; Iversen, B. B. ChemSusChem 2014, 7, (6), 1594-1611.
3. Jensen, K. M. O.; Andersen, H. L.; Tyrsted, C.; Bojesen, E. D.; Dippel, A. C.; Lock, N.; Billinge, S. J. L.; Iversen, B. B.; Christensen, M. ACS Nano 2014, 8, (10), 10704-10714.
4. Jensen, K. M. O.; Juhas, P.; Tofanelli, M. A.; Heinecke, C. L.; Vaughan, G.; Ackerson, C. J.; Billinge, S. J. L. Nat Commun 2016, 7.
5. Schlesinger, M.; Weber, M.; Ruffer, T.; Lang, H.; Mehring, M. Eur. J. Inorg. Chem. 2014, 2014, (2), 302-309.

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