Nanoscience of Metal Silicate-Based Pigments
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Nanoscience of Metal Silicate-Based Pigments Tina T. Salguero, Darrah Johnson-McDaniel, Christopher A. Barrett, Asma Sharafi, Richard Weimar and Terra Blevins Department of Chemistry, The University of Georgia, Athens, GA, 30602-2556, USA. e-mail: [email protected] ABSTRACT The colored component of several important ancient pigments, including Egyptian blue and Han blue, are based on alkali earth copper tetrasilicate materials. In recent work, we have found that these layered materials can be chemically exfoliated into their constituent monolayers to provide alkali earth copper tetrasilicate nanosheets—defined by nanometer thickness and lateral dimensions that are on the order of several microns. The facile exfoliation of these materials into nanosheets is especially surprising in view of their long history on artifacts under a variety of environmental conditions, and we have examined the issue of whether archaeological samples are affected by this exfoliation mechanism. We have characterized the properties of these nanosheets by an array of analytical techniques, including powder x-ray diffraction, photoluminescence measurements, and Raman spectroscopy. In all cases, we observe differences between nanosheet and bulk samples that originate from the loss of coupling between layers when going from three-dimensional to two- dimensional structures. Both CaCuSi4O10 nanosheets (derived from Egyptian blue) and BaCuSi4O10 nanosheets (derived from Han blue) have strong near-infrared luminescence properties like their bulk counterparts, yet they are amenable to modern solution processing methods. We have demonstrated ink jet printing with CaCuSi4O10 nanosheet inks, as well as the fabrication of nanosheet-based papers. Potential applications for these materials include NIR-based biomedical imaging and security inks. INTRODUCTION The quest for a material that is strongly luminescent in the near-infrared (NIR) region of the electromagnetic spectrum, stable with respect to environmental conditions, and that can be processed by solution-based methods has led us to the ancient past and to the beautiful pigments Egyptian blue and Han blue. Dating to antiquity, these inorganic materials are two of the first synthetically produced pigments, and they dazzled the ancient world with vibrant shades of blue in paintings and sculptures. Egyptian blue (CaCuSi4O10) and Han blue (BaCuSi4O10) belong to the gillespite series ABSi4O10 (where A = Ca, Ba, Sr and B = Fe, Cu, Cr), which takes its name from the iron tetrasilicate mineral. Although archaeological interest in Egyptian blue has been strong for some time [1], this field of study is enjoying a resurgence due to the convenient imaging of artifacts by NIR photography [2]. Recently we found that Egyptian blue exfoliates under surprisingly mild conditions: hot water and agitation [3]. The resulting nanosheets are stable and maintain the useful and interesting NIR emission of the bulk material. Therefore, we have turned our attention to the other members of the copper tetrasilicate series (BaCuS
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