Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-based Pigments

Abstract

In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi₄O₁₀ and BaCuSi₄O₁₀, the colored components of the historic Egyptian blue and Han blue pigments. The bulk forms of these materials are synthesized by both melt flux and solid-state routes, which provide some control over the crystallite size of the product. The melt flux process is time intensive, but it produces relatively large crystals at lower reaction temperatures. In comparison, the solid-state method is quicker yet requires higher reaction temperatures and yields smaller crystallites. Upon stirring in hot water, CaCuSi₄O₁₀ spontaneously exfoliates into monolayer nanosheets, which are characterized by TEM and PXRD. BaCuSi₄O₁₀ on the other hand requires ultrasonication in organic solvents to achieve exfoliation. Near infrared imaging illustrates that both the bulk and nanosheet forms of CaCuSi₄O₁₀ and BaCuSi₄O₁₀ are strong near infrared emitters. Aqueous CaCuSi₄O₁₀ and BaCuSi₄O₁₀ nanosheet dispersions are useful because they provide a new way to handle, characterize, and process these materials in colloidal form.

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Introduction

Vibrant colors were prized throughout the ancient world. Even today, we can still see the remains of pigments and dyes created by every major culture. Remarkably, two of the most famous synthetic blue pigments share a similar chemical composition and structure, despite having been developed at widely different times and places. The colored components of both Egyptian blue, CaCuSi₄O₁₀, and Han blue, BaCuSi₄O₁₀, belong to the alkali earth copper tetrasilicate series, ACuSi₄O₁₀ (A = Ca, Sr, Ba)¹, as well as the larger gillespite group, ABSi₄O₁₀ (B = Fe, Cu, Cr)^2,3.

Beyond traditional pigment applications, current scientific interest in these materials centers on their strong near infrared (NIR) emission properties. This emission originates from the Cu²⁺ in square planar coordination; these ions are linked by tetrahedral silicate moieties within the three-dimensional crystal structure, and the resulting layers alternate with alkali earth ions^4-6. Recent technical highlights include NIR imaging to identify Egyptian and Han blue pigments on cultural heritage artifacts^7,8, lanthanide doping of ACuSi₄O₁₀ to enhance NIR reflectance properties and open new energy transfer pathways^9,10, the use of ACuSi₄O₁₀ as the active material for optical sensors¹¹, and the exfoliation of CaCuSi₄O₁₀ into monolayer nanosheets¹².

In particular, this last example provides a way to nanostructure CaCuSi₄O₁₀ so that it can be handled as a colloidal dispersion rather than as a particulate solid¹². Because colloidal dispersions are compatible with solution-processing techniques (e.g. spin coating, ink jet printing, layer-by-layer deposition), this advance opens new application areas that range from security inks to biomedical imaging. The experimental protocols illustrated in this contribution will enable researchers from diverse backgrounds to prepare, characterize, and use CaCuSi₄O₁₀ and BaCuSi₄O₁₀ nanosheets in their work.

Protocol

1. Preparation of CaCuSi₄O₁₀

1. Melt Flux Synthesis of CaCuSi₄O₁₀

   1. Weigh out CaCO₃, SiO₂, and Cu₂CO₃(OH)₂ in a 2:8:1 molar ratio: 0.1331 g (1.330 mmol) of CaCO₃, 0.3196 g (5.319 mmol) of SiO₂, 0.1470 g (0.6648 mmol) of Cu₂CO₃(OH)₂. In addition, weigh out the flux components (12.5% by weight): 0.0375 g of Na₂CO₃, 0.0125 g of NaCl, and 0.0250 g of Na₂B₄O₇^.10H₂O. Add these materials to a clean agate mortar.
   2. Hand grind for ~5 min with an agate pestle until the mixture becomes a homogeneous light green powder (Figures 1a and 2a). Transfer this mixture to a clean, dry platinum crucible.
   3. Heat the crucible in a furnace to 875 °C (ramp rate of 2 °C/min), hold at 875 °C for 16 hr, and then cool down to room temperature (rate of 0.8 °C/min).
   4. Remove the crystals from the crucible and gently crush them using a pestle.
   5. Allow the crystals to soak in 50 ml of 1 M aqueous HCl overnight to remove the melt flux.
   6. Filter the crystals and wash with deionized water to fully remove any remaining melt flux. Note: This material should be ground into a finer powder for powder X-ray diffraction (PXRD) analysis (Figure 5). It also can be characterized by optical microscopy (Figure 3), scanning electron microscopy (SEM) (Figure 4), and NIR photography (Figure 8).
2. Solid State Synthesis of CaCuSi₄O₁₀

   1. Weigh out CaCO₃, SiO₂, and CuO in a 1:4:1 molar ratio: 0.1331 g (1.330 mmol) of CaCO₃, 0.3196 g (5.319 mmol) of SiO_2, and 0.1058 g CuO (1.330 mmol) and add to a clean agate mortar.
   2. Dampen the powder mixture with 1-2 ml acetone and hand grind with an agate pestle for ~5 min. Transfer the resulting light gray powder (Figures 1b and 2b) into a platinum crucible.
   3. Heat the crucible in a box furnace to 1,020 °C at a ramp rate of 5 °C/min, hold for 16 hr, and then cool down to room temperature
   4. Scrape out the loose, light blue-gray powder using a polytetrafluoroethylene (PTFE) spatula. Note: The product can be characterized by optical microscopy (Figure 3), SEM (Figure 4), PXRD (Figure 5), and NIR photography (Figure 8).

2. Synthesis of BaCuSi₄O₁₀

1. Melt Flux Synthesis of BaCuSi₄O₁₀

   1. Weigh out BaCO₃, SiO₂, and CuO in a 1:4:1 molar ratio: 0.2085 g BaCO₃ (1.057 mmol), 0.2539 g SiO₂ (4.226 mmol), and 0.0840 g CuO (1.056 mmol). In addition, weigh out the flux component (12.5% by weight): 0.0765 g of PbO. Add these materials to a clean agate mortar.
   2. Hand grind for ~5 min with an agate pestle until the mixture becomes a homogeneous light gray powder (Figures 1c and 2c). Transfer this mixture to a clean, dry platinum crucible.
   3. Heat the crucible in a furnace to 950 °C (ramp rate of 2 °C/min), hold at 950 °C for 24 hr, then slowly cool down to 700 °C (rate of 0.1 °C/min), and finally cool to room temperature.
   4. Remove the crystals from the crucible and gently crush them using a pestle.
   5. Allow the crystals to soak in 50 ml of 1 M aqueous HNO₃ overnight to remove the melt flux.
   6. Filter the crystals and wash with deionized water to fully remove the remainder of the melt flux. Note: This material should be ground into a finer powder for PXRD analysis (Figure 6). It also can be characterized by optical microscopy (Figure 3) and NIR photography (Figure 8).
2. Solid State Synthesis of BaCuSi₄O₁₀

   1. Weigh out BaCO₃, SiO₂, and CuO in a 1:4:1 molar ratio: 0.2085 g BaCO₃ (1.057 mmol), 0.2539 g SiO₂ (4.226 mmol), and 0.0840 g CuO (1.056 mmol) and add to a clean agate mortar.
   2. Dampen the powder mixture with 1-2 ml acetone and hand grind with an agate pestle for ~5 min. Transfer the resulting light gray powder (Figures 1d and 2d) into a platinum crucible.
   3. Heat the crucible in a box furnace to 960 °C at a ramp rate of 5 °C/min and hold for 16 hr, then cool to room temperature.
   4. Scrape out the loose blue powder using a polytetrafluoroethylene (PTFE) spatula. Note: The product can be characterized by optical microscopy (Figure 3), PXRD (Figure 6), and NIR photography (Figure 8).

3. Exfoliation of CaCuSi₄O₁₀

1. Charge a 50 ml round bottom flask with 0.50 g of CaCuSi₄O₁₀, 40 ml of deionized water, and a glass-coated magnetic stir bar.

2. Attach a water-cooled condenser to the flask. Heat the reaction to 85 °C with magnetic stirring at 400 rpm for two weeks.

3. Remove from the heat source, allow the solution to settle undisturbed overnight, and then filter the supernatant through a 0.4 µm membrane filter. Vacuum dry the solids. Note: The product is a light blue powder that can be characterized by optical microscopy (Figure 3), PXRD (Figure 5), transmission electron microscopy (TEM) (Figure 7), and NIR photography (Figure 8).

4. Exfoliation of BaCuSi₄O₁₀

1. Charge a 50 ml plastic centrifuge tube with 0.14 g of BaCuSi₄O₁₀ and 20 ml of N-vinyl pyrrolidone.

2. With the centrifuge tube immersed in an ice/water bath, sonicate with a probe ultrasonicator at 40% amplitude (17 W) for 1 hr.

3. Let the dispersion settle undisturbed overnight, and then decant the supernatant into a new centrifuge tube.

4. Spin down at 10,286 x g using a centrifuge. Decant the supernatant, leaving the nanosheets at the bottom of the centrifuge tube.

5. Resuspend this material in 20 ml of water with a few minutes of bath sonication. To isolate a powder, filter through a 0.4 µm membrane filter and vacuum dry the solids. Note: The product is a light blue powder that can be characterized by optical microscopy (Figure 3), PXRD (Figure 6), TEM (Figure 7), and NIR photography (Figure 8).

5. Ink Preparation

1. Disperse ~0.10 g of CaCuSi₄O₁₀ nanosheets in 5 ml of deionized water using bath sonication for ~10 min. Note: This ink (Figure 9) can be used for painting, printing, etc. See Figure 10 for a representative example where the ink was applied to paper with a brush.

6. Near Infrared Photographic Imaging

1. Irradiate the samples using red light (e.g. with a red light-emitting diode array), taking care to eliminate any other sources of light.

2. Photograph using a camera modified to image in the near infrared region. Use f stop setting f/22 and an exposure time of 0.5 sec.

Representative Results

The described syntheses of CaCuSi₄O₁₀ and BaCuSi₄O₁₀ provide approximately 0.5 g of product per batch. Isolated yields of CaCuSi₄O₁₀ from the melt flux and solid-state syntheses typically range from 70-75% and 90-95%, respectively. For BaCuSi₄O₁₀, theisolated yields from the melt flux and solid-state syntheses typically range from 65-70% and 95-99%, respectively.

The textures of all of the prepared materials, as well as differences in the intensity of their blue color due to varying crystallite sizes, are visible by low magnification optical microscopy (Figures 3a-h). Scanning electron microscopy (SEM) images confirm that the solid-state method of synthesizing CaCuSi₄O₁₀ produces ~1-15 µm primary crystallites (Figure 4b) whereas melt flux conditions lead to ~5-50 µm crystallites (Figure 4a). Powder X-ray diffraction (PXRD) patterns for CaCuSi₄O₁₀ (Figures 5a and 5c) and BaCuSi₄O₁₀ (Figures 6a and 6c) showcase the composition and phase purity of these products.

Representative transmission electron microscopy (TEM) images show the nanosheet morphology of the exfoliated products (Figure 7). In addition, NIR photographic imaging shows the strong luminescence of both the bulk and exfoliated materials (Figure 8). A simple way to illustrate the solution processability of CaCuSi₄O₁₀ nanosheets is to prepare an aqueous ink (Figure 9) suitable for painting (Figure 10).

Figure 1. Photographs of the hand-ground starting materials. (a) CaCuSi₄O₁₀ melt flux, (b) CaCuSi₄O₁₀ solid-state, (c) BaCuSi₄O₁₀ melt flux, and (d) BaCuSi₄O₁₀ solid-state syntheses. Please click here to view a larger version of this figure.

Figure 2. Scanning Electron Microscopy. Images of the hand-ground starting materials for the (a) CaCuSi₄O₁₀ melt flux, (b) CaCuSi₄O₁₀ solid-state, (c) BaCuSi₄O₁₀ melt flux, and (d) BaCuSi₄O₁₀ solid-state syntheses. All samples were coated with gold prior to imaging. Please click here to view a larger version of this figure.

Figure 3. Optical Microscopy. Bulk CaCuSi₄O₁₀ prepared by melt flux (a) and solid state (b) procedures. Bulk BaCuSi₄O₁₀ prepared by melt flux (c) and solid state (d) procedures. Exfoliated products (e-h) of (a-d), respectively. All images share the 1 mm scale bar show in panel (a). Please click here to view a larger version of this figure.

Figure 4. Scanning Electron Microscopy. Images of bulk CaCuSi₄O₁₀ made by melt flux (a) and solid state (b) methods. Samples were coated with gold prior to imaging. Please click here to view a larger version of this figure.

Figure 5. Powder X-Ray Diffraction: CaCuSi₄O₁₀. Patterns for bulk CaCuSi₄O₁₀ prepared by melt flux (a) and solid state (c) methods. Asterisks denote a silica impurity. Patterns for exfoliated CaCuSi₄O₁₀, (b) and (d), prepared from (a) and (c), respectively. Please click here to view a larger version of this figure.

Figure 6. Powder X-Ray Diffraction: BaCuSi₄O₁₀. Patterns for bulk BaCuSi₄O₁₀ prepared by melt flux (a) and solid state (c) methods. Asterisk denotes a silica impurity. Patterns for exfoliated BaCuSi₄O₁₀, (b) and (d), prepared from (a) and (c), respectively. Please click here to view a larger version of this figure.

Figure 7. Transmission Electron Microscopy. Representative images of exfoliated CaCuSi₄O₁₀ derived from bulk CaCuSi₄O₁₀ made by melt flux (a) or solid state (b) methods. Representative images of exfoliated BaCuSi₄O₁₀ derived from bulk BaCuSi₄O₁₀ made by melt flux (c) or solid state (d) methods. Please click here to view a larger version of this figure.

Figure 8. Near Infrared Imaging. Luminescence of bulk CaCuSi₄O₁₀ prepared by melt flux (a) and solid state (b) procedures. Luminescence of bulk BaCuSi₄O₁₀ prepared by melt flux (c) and solid state (d) procedures. Luminescence of the exfoliated products (e-h) of (a-d), respectively. Powder samples are contained within glass vials, and the entire set of samples was imaged at once. Please click here to view a larger version of this figure.

Figure 9. Photograph of a CaCuSi₄O₁₀ nanosheet ink in a vial.

Figure 10. Near Infrared Imaging. A rudimentary painting with the CaCuSi₄O₁₀ nanosheet ink that illustrates both its simple application and its luminescence properties.

Discussion

The preparation of Egyptian blue pigment, a mixture of mostly CaCuSi₄O₁₀ and SiO₂, is a well-studied process^4,13-21. The numerous reported procedures may be categorized as either melt flux or solid-state reactions. Two major advantages of the melt flux approach are that it permits lower reaction temperatures (<900 °C) and allows CaCuSi₄O₁₀ crystals to nucleate and grow from a molten glass phase²⁰. The flux component is typically an alkali salt (e.g. Na₂CO₃) or borate compound (e.g. borax). In comparison, the solid-state syntheses omit the flux but require higher temperatures (~1,000 °C) for the reaction between Ca, CuO, and SiO₂ sources to reach completion.

Although the synthesis of Han blue pigment is not as well studied as that of Egyptian blue^4,22-25, the preparation of BaCuSi₄O₁₀ follows similar melt flux and solid-state routes with two differences: (1) a PbO flux should be used, and (2) the reaction temperatures must be more closely controlled because of alternative Ba-Cu-Si-O phases that can form (e.g. BaCuSi₂O₆).

These points are illustrated by the detailed procedures and results described in this paper. First, for all methods, the starting materials should be ground to a smooth powder (Figures 1a-d) consisting of 5-20 µm particles (characterized by SEM; Figures 2a-d). Next, the use of a significant amount of flux (12.5% by weight) in the preparation of CaCuSi₄O₁₀ and BaCuSi₄O₁₀ leads to highly crystalline products, which are characterized by intense blue coloration (Figures 3a and 3c), relatively large particle sizes (Figure 4a), and strong PXRD patterns (Figures 5a and 6a). The diminished isolated yields (~70%) from these preparations are caused by adhesion of the melted reaction mixtures to the crucible. In comparison, CaCuSi₄O₁₀ and BaCuSi₄O₁₀ prepared by the solid-state route exhibit less intense coloration (Figures 3b and 3d) and smaller particle sizes (Figure 4b). As synthesized, these products are powders that can be isolated in near-quantitative yields. Thus, for both CaCuSi₄O₁₀ and BaCuSi₄O₁₀, the advantages of flux and the importance of reaction temperature cannot be overstated.

Remarkably, the exfoliation of CaCuSi₄O₁₀ and BaCuSi₄O₁₀ occurs under simple aqueous conditions. In the case of CaCuSi₄O₁₀, this reaction is quite slow at room temperature (≥6 weeks to see any appreciable exfoliation), but it becomes synthetically useful at 80 °C (substantial exfoliation after 2 weeks). In comparison, the exfoliation of BaCuSi₄O₁₀ is sluggish even at 80 °C, and so we apply an even greater energy input in the form of ultrasonication. These reactions are highly reliable with two caveats. For CaCuSi₄O₁₀,it is important to use a glass-coated stir bar; if a standard PTFE-coated stir bar is used, we find that PTFE byproducts contaminate the CaCuSi₄O₁₀ nanosheet product. For BaCuSi₄O₁₀, it is important to control the ultrasonication power and time so that the reaction is stopped before the nanosheets become degraded.

Transmission electron microscopy (TEM) of the nanosheet products shows that these very thin materials have lateral dimensions ranging from hundreds of nanometers to several microns. In general these lateral dimensions correlate with the crystallite size of the three-dimensional starting material. In prior work, atomic force microscopy provided topographic mapping that demonstrated the single-layer thicknesses (~1.2 nm) of these nanosheets¹². Photographs of powder CaCuSi₄O₁₀ and BaCuSi₄O₁₀ nanosheet samples (Figures 3e-h) show that their color is less intense than that of the starting materials, a direct result of nanostructuring.

Additional information is provided by PXRD (Figures 5 and 6), which reveals basal cleavage along the (001) plane and preferred orientation along the {00l} series for all nanosheet samples. These features reflect the stacked alignment of these highly anisotropic nanomaterials when drop-cast onto a substrate. Furthermore, the characteristic NIR emission of CaCuSi₄O₁₀ at ~910 nm and BaCuSi₄O₁₀ at ~950 nm is illustrated in a NIR photograph of all eight samples (Figure 8).

The solution processing of CaCuSi₄O₁₀ can be accomplished by simply preparing a colloidal dispersion of CaCuSi₄O₁₀ nanosheets (Figure 9) to use as an ink. This ink then can be applied to a substrate via spin coating, spray coating, ink jet printing¹², or simply brushing (Figure 10). Importantly, the NIR emission properties of CaCuSi₄O₁₀ are retained at all stages of this process. These new possibilities highlight the contrast between CaCuSi₄O₁₀ nanosheets and the traditional use of Egyptian blue pigment, a highly granular material that is challenging to incorporate into a smooth paint.

Disclosures

The authors have no competing financial interests.