Mapping the phase-, size- and shape-controlled hydrothermal synthesis of materials with principal component analysis Peter Dunne 1 , Andrew S. Bathe 1 , Adrián Sanz Arjona 1,2 , Annie Regan 1,3 1. School of Chemistry, Trinity College Dublin, College Green, Dublin 2, Ireland 2. Department of Chemistry, University of Copenhagen,Universitetsparken 5, DK-2100 Copenhagen O 3. CDT ACM, Trinity College Dublin, College Green, Dublin 2, Ireland As a simple, versatile, and green synthetic method hydrothermal (and solvothermal) techniques have seen a surge in interest in recent years, being applied to a wide variety of functional materials and offering control over composition, phase, size and shape. 1 Despite its growing importance the technique remains largely a “black box” as it requires sealed pressurised vessels, such that in-situ monitoring is restricted to synchrotron-based experiments. 2 As a result, gaining insights into hydrothermal reaction mechanisms, and thus design of reaction systems to achieve desired product characteristics commonly requires exhaustive empirical testing with the concomitant demands on characterising obtained products ex-situ . This can represent a significant time sink and obstacle to screening of reaction conditions for materials discovery or optimisation. Here we report on the development of a novel hydrothermal injection reactor which offers more precise control over reaction conditions within the hydrothermal space, akin to that of the well-established hot-injection method. This technique has been applied to the synthesis of polymorphic cadmium sulfide and calcium carbonate, and is shown to offer a high degree of control over product phase, size, and shape, as revealed by powder X-ray diffraction and electron microscopy. Furthermore, we show here that the statistical method of principal component analysis (PCA) may be readily applied to the large library of laboratory powder X-ray diffraction data acquired from the extensive screening of reaction conditions under both conventional batch and injection hydrothermal methods for these polymorphic materials. The use of PCA quickly and easily reveals trends emerging from the impact of reaction conditions such as precursor choice, temperature, and time, which may be readily correlated to key material properties such as phase and size, by use of predicted diffraction patterns and more selective use of more thorough analysis by Rietveld refinement or microscopy methods. Taken together these approaches may allow for the faster and easier mapping of the hydrothermal synthesis landscape, enhancing material discovery and process optimisation.
References 1. R. I. Walton, Chemistry – A European Journal , 2020, 26 , 9041-9069. 2. P. Afanasiev, Comptes Rendus Chimie , 2008, 11 , 159-182. 3. T. Jolliffe and J. Cadima, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences , 2016, 374 , 20150202.
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