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Honors Theses Student Research

2020

Nanoparticles as Alternative Glaze Colorants

Nathan Dinh

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Recommended Citation Dinh, Nathan, " as Alternative Colorants" (2020). Honors Theses. 1447. https://scholarship.richmond.edu/honors-theses/1447

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Nanoparticles as Alternative Ceramic Glaze Colorants

by

Nathan Dinh

Honors Thesis

Submitted to:

Department of Chemistry University of Richmond Richmond, VA

May 1, 2020

Advisor: Dr. Ryan Coppage

This thesis has been accepted as part of the honors requirements in the Program in Biochemistry and Molecular Biology.

______Ryan Coppage______April 28, 2020 (advisor signature) (date)

______Michael Leopold______April 28, 2020 (reader signature) (date)

I. Introduction and Background

Metallic nanoparticles, including gold and silver, have been used in wide-ranging applications due to their versatile characteristics. These include catalysis,1 biosensing,2 imaging,3 cancer treatments,4 and art.5 Due to the phenomenon called surface plasmon , metallic particles change optical properties when sized in the nanoscale based on their composition, shape, diameters, or aspect ratios.6–9 In study, these robust systems are applied as an alternative to traditional bulk colorants in ceramic glazes.

These traditional ceramic glazes consist of high quantities of heavy that are known to be not only toxic and carcinogenic but also environmentally detrimental. including Co, Mn, Cd, and Pb are all commonly used in ceramic glazes, particularly in developing nations.10–13 These metals derive their optical properties through the field theory in which the d and (rarely) f orbitals of these metals are split, allow for absorption of a portion of the striking them, and the remaining is transmitted back to the eye, rebuilt, resulting in a visible color shift.14 This method is highly inefficient, requiring 5-12% by weight of metal colorant to dry .15 Furthermore, these toxic metals are prone to leaching in which the metals are drawn out of the ceramic interface by the food in these containers and are potentially consumed.13 These traditional glaze colorants present significant health concerns in addition to causing environmental harm.

By contrast, gold nanoparticles have been previously found to yield comparable glaze color profiles through surface plasmon resonance with loading masses far lower than traditional metal glazes.16 Prior research by Lambertson et al. found that gold nanoparticles yield noticeable glaze color at loading levels as low as 0.1% to 0.01% by weight. They also found that gold nanoparticles are more resistant to leaching. Because of this, gold nanoparticles are not only noncytotoxic and environmentally friendlier but also are more efficient at coloring as compared to traditional heavy metal colorants. This means that less material would be required and thus increase economic viability of this process. At the time of writing, the price of gold per gram is $54.50, but due to the low loading weight, a standard ceramic would only require roughly 40 to 80 cents of material.17 Due to the efficiency of surface plasmon resonance, gold nanoparticles may be a viable alternative to traditional glaze colorants that are both environmentally friendly and economically accessible.

In this study, the research goals are two-fold: to understand firing mechanism on the gold nanoparticle size and to develop these processes for wider use by regular artisans. In the prior study conducted by Lambertson et al., they found that the average diameters of gold nanoparticles changed from their starting sizes after firing in both reductive and oxidative – two kilns commonly used in art facilities. This effect of firing a range of nanoparticle sizes was not understood, so a multi-size nanoparticle analysis was used to understand how these kilns affect nanoparticle sizes. By elucidating these mechanisms, these processes can be fine-tuned to augment the implementation of nanoparticles as colorants.

In addition to understanding how firing affects nanoparticle sizes, processes to apply both gold and silver nanoparticles as colorants in glazes were developed without the need for laboratory-grade glassware, heating elements, reductants, or specificity which may be unconventional in art studios. Firing kilns were found to be capable of spontaneously promoting nanoparticle formation when bulk gold and/or silver are placed in the dry material. In doing so, artisans can bypass preliminary nanoparticle synthesis entirely while still creating plasmon resonance-generated color. To further reduce waste and analyze greener procedures, nanoparticle waste generated in the lab was applied to explore rescue of plasmon resonance from aggregated nanoparticles.

While there has been extensive research on the optical properties of gold nanoparticles, little research has gone into the mechanisms by which nanoparticle color is applied in ceramic glazes. Application of nano-sized colorants is not novel, however. Metallic colorants have been applied in ancient and medieval and .18–23 One of the most famous examples is the

Lycurgus Cup from the 4th century Roman Empire currently housed in the British Museum in

London.24 The cup consists of blown that changes color based on the light; the cup is a light green when not lit from behind but changes to a shade of deep red when a light is placed within in the cup. Analysis of a sample from the cup revealed that the cup contains both gold and silver nanoparticles, which matches the two color profiles. Despite being widely studied, the process by which Ancient Romans made the cup is unknown. In addition to developing these processes for everyday use, by understanding these mechanisms, proposals can be made on how ancient civilizations were able to develop nanoparticles without knowing about nanoparticles as is known today.

II. and Methods

a. Multi-size Nanoparticle Synthesis

For the multi-size analysis, gold nanoparticles were synthesized that increased in size by increments of ~20 nm according to procedures by Jana et al.25 A 300 mL growth solution of 1.0 mM HAuCl4 was combined with 7.5 g of CTAB as the capping agent. The solution was heated until the CTAB was fully dissolved and the solution turning a dark, translucent orange. This growth solution was then cooled to room temperature. While cooling, a 10 mL solution of 0.1 M ascorbic acid was prepared for use as a reductant. A 20 mL seed solution was then made with 1.0 mM HAuCl4 and 1.0 mM trisodium citrate as the reductant in an Erlenmeyer flask. After the growth solution cooled, a 1.0 mM solution of NaBH4 was made using 1.0 mL of water that had been chilled on . The NaBH4 was then immediately but slowly added to the seed solution, which changed from a light yellow to a dark purple, indicating nanoparticle formation.

Once the seed solution was prepared, three different solutions were synthesized to yield nanoparticles of incremental sizes (Batches A-C). For batch A, 99 mL of the growth solution was transferred to an Erlenmeyer flask with 1.5 mL of the ascorbic acid solution while stirring. The solution changed from a dark orange to a light yellow, indication of gold atom reduction. Then 11 mL of the seed solution was added to batch A while vigorously stirring for 10 minutes. The solution color changed from light yellow to clear to wine red. The solution was then allowed to rest for 20 minutes. After resting, batch B was prepared the same way as batch B except for 11 mL of batch

A being used rather than the original seed solution. The final color of batch B was purple-brown.

Again, the solution was allowed to rest before being used in batch C, which was prepared the same way as batches A and B but with an aliquot of batch B as the seed solution. The final color of batch

C was an opaque medium brown. All of the flasks were completely covered in aluminum foil until size confirmation via TEM imaging.

b. Multi-Size Nanoparticle Glaze Synthesis

After the sizes of the three batches were confirmed, each solution was centrifuged at 10,000 rpm for 90 minutes. Roughly three quarters of the supernatant was removed to create more concentrated solutions. Each solution was then added to separate 80.0 g glaze batches, which consisted of 20% Kaolin EPK, 19% silica, 6% talc, 20% 3134 (of which is 22.8% B2O3 content), 15% wollastonite, and 20% G-200 . Water was added to the solution until a consistent with traditional glaze practices was achieved. Ceramic sample were then dipped into the glaze solution, held for 3 seconds, and removed before allowing to dry before firing. Duplicate tiles were made for each solution, and the tiles were fired in either a reductive or oxidative environment to cones 10 (2345 F, 1286 C) and 6 (2200 F, 1200 C), respectively. By applying these glazes using modern glazing and firing techniques, these results demonstrate the viability of these processes for normal artisans.

c. Precursor Glaze Synthesis

As the second part of the research focuses on developing these techniques for everyday artists, gold salt (HAuCl4) and silver salt (AgNO3) were added directly within the glaze solution, bypassing the previous nanoparticle synthesis entirely. 39.38 mg of HAuCl4 and 100 mg of AgNO3 were dissolved in 18.0 MΩ•cm nanopure water in separate 20 g scintillation vials. Prior to adding the salt, the aluminum foil cap seals were removed. These solutions were then added into 160 g and 200g glaze batches, respectively, which had the same composition as for the multi-size nanoparticle glazes. Purchased silver nanoparticles with diameter sizes of 21.5 ± 9 nm was also added to a glaze as a stock reference. For this glaze, 5 mL of silver nanoparticle solution was added to an 80 g glaze batch. In order to further develop greener glaze profiles, nanoparticle waste from the lab was added to glaze samples. The black precipitate was allowed to settle and the supernatant poured off. Due to severe aggregation, no visible plasmon band is visible in a UV- vis spectrum (Figure 1). Roughly 10 mL of the precipitated solution was added to a 200 g glaze batch. Duplicate sample tiles were made for all four solution and fired using the same procedure as above.

d. Post-Firing Analysis

After all of the samples were fired, the samples were grinded using a Dremel and the dust collected in separate scintillation vials. The dust was further grinded down using the mortar and pestle for roughly 5 minutes before being suspended in ethanol. The mortar and pestle were rinsed using acetone and water before the next sample. After processing, a 5 µL sample of each solution was placed on separate 400-grid, -coated, mesh TEM grids and imaged. Nanoparticle diameters were then measured by hand using Adobe Photoshop. At least 100 nanoparticles were analyzed to find averages and standard deviations. Glaze color profiles were also quantified via reflectance spectra using an Ocean Halogen lamp (HL-2000-FHSA) and Flame miniature spectrometer (FLAME-S-VIS-NIR-ES, 350-1000 nm).

III. Results and Discussion

a. Multi-Size Nanoparticle Pre-Fire Synthesis Analysis

The sizes of the three multi-size gold nanoparticle batches were verified using transmission microscopy and at least 100 nanoparticles were measured to calculate averages and standard deviations (Figure 2). For batches A, B, and C, the nanoparticles were 21.0 ± 3.0 nm,

38.7 ± 3.4 nm, and 57.5 ± 10.5 nm, respectively. Batches B and C contained some nanorod formations, which were excluded from measurement. These nanorod formations are consistent with prior literature. The synthesis produced nanoparticles that incremented in size by ~20 nm, allowing us to observe how different firing mechanisms affect gold nanoparticle sizes and color profile based on preliminary sizes. b. Multi-Size Nanoparticle Glazes

The 20, 40, and 60 nm gold nanoparticles yielded vibrant glazes with color profiles comparable to traditional glazes and those previously studied by Lambertson et al.16 Despite containing the same glaze composition, the reductive and oxidative samples differed greatly depending on the firing environment (Figure 3). A reduction kiln firing yielded a glaze that was a deep purple-red whereas an oxidative kiln firing produced a light pink hue. In addition to these vastly different shades, these samples yielded interesting trends in sizing after both reductive and oxidative kiln firings.

For all three groups, the final average nanoparticle size was smaller than the pre-fired size.

For the batch A sample (starting size of 20 nm), the reduction sample had larger particles with an average of 16.6 ± 13.4 nm while the oxidative sample had smaller particles with an average of 5.5

± 0.7 nm. In batch B (starting size of 40 nm), the reductive and oxidative samples had comparable nanoparticle sizes of 14.9 ± 4.3 nm and 15.1 ± 4.8 nm, respectively. Finally, the nanoparticles of the batch C samples (starting size of 60 nm) had an opposite trend from batch A. The reduction samples now have the smaller average size of 13.4 ± 5.4 nm, and the oxidative sample has the larger sized nanoparticles at 24.2 ± 8.2 nm. This data suggests that as starting nanoparticle size increases, there is a flip in difference in sizes that are created in reductive and oxidative kilns. With smaller starting nanoparticles, the reduction kiln produces smaller particles than in the oxidative kiln. As the starting nanoparticles become larger, this relationship equalizes until they reach a potential threshold where the reduction kiln produces smaller final nanoparticles and the oxidative kiln creates larger nanoparticles. Potential mechanisms of firing on nanoparticles is discussed in further detail in the following sections.

c. Salt Precursor Glazes

Glazes with salt precursors yielded diverse and vibrant glazes in all eight variations (Figure 4). Glazes with pure HAuCl4 added to the glaze mixture in both firing environments were comparable to the glazes containing pre-synthesized nanoparticles, suggestive of spontaneous nucleation and growth of gold nanoparticles during firing.

Glazes with pure AgNO3 also produced distinct glazes. The reductive sample produced a burnt laurel green shade, and the oxidative sample produced a very light green glaze. The aggregated nanoparticle waste also produced samples that were comparable to the pure HAuCl4 tiles although the oxidative sample is a darker orange shade, likely due to the other non-gold impurities that was in the original solutions. The stock silver nanoparticle solution tiles produced a jade green shade in the reduction kiln while the oxidative sample had a colorless, white glaze.

Further, cross-sections of these glazes demonstrate that the colorant is dispersed throughout the entire glaze layer down to the ceramic surface rather than just a topical application as often used in lusterware

(Figure 5). These samples demonstrate the viability of directly adding raw but pure metallic and even gold laboratory waste can be added to glazes to yield vibrant glazes that are comparable in tone to traditional glazes.

d. Firing Mechanisms on Nanoparticle-Doped Glazes

In addition to these intense optical profiles, TEM analysis of these samples shows that there is nanoparticle nucleation and growth. Consistent with the multi-size nanoparticle study, similar trends were found in post-firing sizes in reductive and oxidative kilns. For all four samples, the reductive samples contained nanoparticles smaller than the oxidative sample. Because of this previously proposed mechanisms can be applied for kiln firing.26 In a reductive kiln, natural gas is combusted in an -limited environment by controlling the flue (Scheme 1), resulting in incomplete combustion and production of carbon monoxide. CO then strikes the surface of the glaze during firing, extracting oxygen and leaving behind free . Disassociated, positively charged gold atoms bind to these electrons and are reduced to their neutral state. Once neutral, these atoms are able to nucleate into nanoparticles and grow.

On the other hand, an oxidative kiln fires the glaze in an oxygen rich environment. Because of this, complete combustion is able to occur and CO2 is produced. CO2 is unable to extract oxygen and thus is unable to leave free electrons to reduce the gold atoms (Scheme 2). However, there is still nanoparticle formation as shown in the TEM imaging. This may be occurring because gold and silver atoms are able to be reduced although to a lesser degree compared to a reductive environment due to getter reduction. During getter reduction, impurities within the glaze or in the kiln are able to act as sacrificial reducing agents in contributing free electrons to the metallic atoms.7 As getter reduction is a weaker reductive force than a CO-rich environment, less atoms are able to be reduced. This produces fewer but larger nanoparticles as there are less initial nucleation sites for free atoms to attach to. On the other hand, aggressive reduction in the reduction kiln yields smaller but more individual nanoparticles.

This reverse relationship of size and quantity between oxidative and reductive kilns may explain the stark differences in color profiles between the two sets of glazes. A greater concentration of individual nanoparticles, through plasmon resonance, would produce a deeper, more vibrant glaze color as seen in the reductive samples. A lesser concentration of nanoparticles would yield paler but still vivid hues that augment the color range of from precious metal as colorants. Using a miniature spectrometer, reflectance spectra were obtained for each of the glaze samples for both multi-size and salt precursor studies (Figure 6). Reflectance spectra has a direct relationship with color intensity and profile: darker glazes have lower reflectance bands in absorbed whereas lighter glazes have higher reflectance bands in reflected, and thus observed, wavelengths. These spectra are directly related to the colorant concentration within the glaze matrix and can be used to measure the gold nanoparticle concentration due to the plasmon resonance.28 The reduction samples have overall lower reflectance spectra relative to the oxidation samples, indicative of a higher number of individual gold nanoparticles absorbing more light. On the other hand, the oxidation samples have higher reflectance spectra and thus lighter glazes, which demonstrates a lower concentration of individual nanoparticles.

This relationship between nanoparticle size and concentration is also consistent with the sizing trends for both sets of experiments. For the multi-size analysis, the glazes with the 20 nm gold nanoparticle precursor have smaller particle sizes in the reduction sample and larger particle sizes in the oxidation sample. The particle sizes equalize with the 40 nm precursor in both samples until the trend switches in the 60 nm precursor samples

(Figure 7) These results suggest that at a certain starting size, a reductive environment will degrade the gold nanoparticles into smaller nucleation sites yielding smaller but more plentiful nanoparticles; an oxidative environment will yield bigger but less numerous nanoparticles. For the salt precursor study, the average sizes of the reduction sample nanoparticles for all four glaze types were smaller than their respective oxidative samples (Figure 8).

These mechanisms are consistent with the multi-size gold nanoparticle study. With these processes, spontaneous formation of gold and silver nanoparticles from their salt forms is possible without the need for preliminary nanoparticle synthesis. These processes are accessible, economically viable, and environmentally friendlier than traditional glazes.

IV. Conclusion

In this study, the feasibility of metallic nanoparticles is explored as an alternative to traditional metal colorants in ceramic glazes via processes that are accessible to everyday artisans.

Through a multi-size nanoparticle analysis, changes in nanoparticle sizes as a function of different firing mechanisms are outlined. Because oxygen is limited in a reduction kiln, the nanoparticles are smaller but more plentiful, producing darker and more vibrant glazes. In the oxygen-rich environment of an oxidative kiln, the nanoparticles are larger but less concentrating, yielding lighter but still applicable hues.

In an effort to develop these processes for wider use by everyday artisans, additional research was undertaken to investigate the spontaneous formation and growth of nanoparticles to produce glaze coloring. These methods bypass the need for preliminary nanoparticle synthesis which consists of acids, heating elements, and glassware that is atypical in everyday art studios.

By applying widely accessible raw gold and silver salt precursors as colorants, artisans can utilize the efficiency of plasmon resonance coloring in order to produce vibrant ceramic glazes that are biologically inert, economically viable, and environmentally friendlier.

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