White and Brightly Colored 3D Printing Based on Resonant

Jul 10, 2018 - To create white or colorful prints with this method, visibly transparent equivalents are needed. Here, we address this problem by desig...
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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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White and Brightly Colored 3D Printing Based on Resonant Photothermal Sensitizers Alexander W. Powell,†,§ Alexandros Stavrinadis,†,§ Ignacio de Miguel,† Gerasimos Konstantatos,*,†,‡ and Romain Quidant*,† †

ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, 08860 Barcelona, Spain ICREA-Institució Catalana de Recerca i Estudis Avançats, 08810 Barcelona, Spain

‡

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S Supporting Information *

ABSTRACT: The use of photothermal sensitizers to facilitate the sintering of polymer powders is rapidly becoming a pivotal additive manufacturing technology, impacting multiple sectors of industry. However, conventional carbon-based sensitizers can only produce black or gray objects. To create white or colorful prints with this method, visibly transparent equivalents are needed. Here, we address this problem by designing resonant photothermal sensitizers made of plasmonic nanoparticles that strongly absorb in the near-infrared, while only minimally interacting with visible light. Gold nanorods were coated with silica before being mixed with polyamide powders to create stable colorful nanocomposite powders. At resonance, these composites showed greatly improved light-to-heat conversion compared with equivalent composites using the industry standard carbon black as a sensitizer and could be sintered using lowpower light sources. Furthermore, they appear much whiter and can produce brightly colored 3D objects when mixed with dyes. Our results open a new route to utilize plasmonic nanoparticles to produce colorful and functional 3D-printed objects. KEYWORDS: 3D printing, plasmon, photothermal, nanotechnology, additive manufacturing

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However, due to the broad absorption spectrum of PTS varieties reported thus far, all of which are carbon-based, printed objects turn out black or gray in color. This makes it impossible to produce brightly colored parts without extensive postprocessing steps, which significantly limits the application in fields such as functional prototyping,17,18 medical modeling,19,20 and sportswear and fashion.21 Although other techniques such as fused-deposition modeling, binder jetting, material jetting, and sheet lamination are able to produce multicolour 3D prints,22 none of these techniques combines the speed, surface quality, and mechanical properties found with powder sintering methods. To allow coloring in sinteringbased printing, a photothermal sensitizer with low absorption in the visible range, yet supporting a strong resonant absorption in the near-to-mid infrared range, is required to produce white or colored 3D objects using this method. Additionally, a resonant absorber that could be tuned to the spectrum of the optical source would allow the photothermal conversion to be maximized. The localized surface plasmon resonances present in metal nanoparticles produce strong absorption peaks which can be tuned from the UV to the nearinfrared by engineering the size, shape, and chemistry of the nanoparticles. As metals are not ideal conductors at optical

electively sintering a powdered material via optically induced heating is one of the most important approaches to 3D printing (3DP);1 however, this technique typically requires the use of high-power CO2 lasers and remains slow and costly compared to conventional processing methods. To overcome this, the polymer powders used in selective sintering are often mixed with strongly optically absorbing additives, which act as photothermal sensitizers (PTS) by heating significantly more than the surrounding polymer upon illumination. This allows for faster powder fusion or sintering and has led to the development of several low-cost selective laser sintering (SLS) printers, which use low-power, solid-state lasers operating in the visible and near-infrared, where many polymers used for SLS are weak absorbers.2 Materials such as carbon nanotubes,3−6 carbon black (CB),7−9 graphene,10 and reduced graphene oxide flakes11 have been utilized as PTS additives and have demonstrated a drastic reduction in the laser power required for effective sintering,12 paving the way for the replacement of bulky CO2 lasers by cheap, compact solid-state semiconductor laser diodes. PTS have also led to new 3DP approaches, such as high-speed sintering (HSS), where a PTS ink is selectively applied to an area of powder that is then heated and sintered using a broad-area light source instead of a focused laser.13−15 This technique demonstrated a significant increase in speed compared to other additive manufacturing technologies such as SLS or stereolithography16 and has the potential to rival the small-batch production speed of conventional industrial methods. © XXXX American Chemical Society

Received: March 22, 2018 Revised: May 17, 2018 Published: July 10, 2018 A

DOI: 10.1021/acs.nanolett.8b01164 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters frequencies, the free electron oscillations forming the plasmons experience damping, leading to significant Joule heating, and PNPs have been shown to be highly efficient light-to-heat converters.23 This makes plasmonic nanoparticles ideal candidates for use as nanoengineered photothermal sensitizers (NEPS), enabling the heating and sintering of powdered materials with low optical powers with minimal disruption to the aesthetic properties of the material. In the present work, we demonstrate the use of gold nanorods (GNRs) as NEPS, allowing the production of white or brightly colored 3D parts via selective sintering with lowpower diode lasers. Whereas promising photothermal behavior has been observed in plasmonic nanoparticles of a variety of geometries,24−26 we found that for ease of large-volume fabrication, spectral tunability, and high photothermal efficicency,26 GNRs were an excellent choice to demonstrate this principle. It is worth emphasizing here that GNRs are compatible with large batch production at moderate cost. We highlight the importance of the surface coating of the GNRs and the mixing conditions with the polymer powder in forming a homogeneous, thermally stable nanocomposite. The efficacy of GNRs as a photothermal sensitizer is shown to be far superior to that of carbon black, the current industrial PTS, demonstrating greatly improved light-to-heat conversion and requiring only a third of the concentration to reach a given temperature on illumination. Aesthetically, GNR nanocomposites are shown to be far whiter than those made using CB, allowing for the production of vivid, multiply colored 3D objects. NEPS offer a whole new class of materials to be used in a variety of additive manufacturing techniques, giving the benefits of conventional photothermal sensitizers, along with near complete control over the aesthetic and other optical properties of the finished shape. To create the nanocomposite powder, photothermal sensitizer solutions, either of gold nanorods or carbon black, were mechanically mixed with polyamide (PA12) powder to produce a nanocomposite for 3D printing. Objects were then formed using the SLS method, as shown in Figure 1a, where the top layer of a powder bed is selectively sintered by a 1 W 808 nm laser to give a solid cross section of the final object. A roller or wiper then takes new powder from a feeder bed and coats a thin layer onto the print bed, and the process repeats to create a 3D object. This process could, in principle, be applied to any polymer powder available for SLS, but PA12 was chosen here as the industry standard. Two identical complex shapes sintered in the printer under identical conditions using a nanocomposite GNR powder with 0.01 wt % GNRs (Figure 1b) and a commercially available black PA12 powder27 (Figure 1c) are shown. Both shapes appear well-defined, but the GNR object can be seen to be significantly whiter than the CB shape. This demonstrates the ability of the GNRs to act as effective photothermal sensitizers and enable the creation of complex, white 3D objects with low-power diode lasers. The plasmon resonance of the GNRs is strongly dependent on the size and shape of the nanoparticle,28 and so it is crucial that these properties are not significantly altered by either the mixing process or the sintering of the polymer powder.29 Figure 2 shows an optical (a) and two-photon luminescence (b) image of a cross section of a sintered sample, where the GNRs (shown in green) tend to be concentrated along lines and intersections that can be seen to correspond to the grain boundaries between the original powder particulates (c). The concentration of the NEPS at the surfaces of the powder is

Figure 1. 3D printing with nanoengineered photothermal sensitizers. (a) Left to right: Base components which are mixed to form the nanocomposite powders and the SLS powder sintering process, highlighting the heating effect of the GNRs in the inset. (b,c) Sintered objects made with PA12 powders sensitized with GNRs and CB, respectively, and sintered using a 1 W 808 nm diode laser.

actually beneficial for sintering as heating is generated primarily at the surfaces of the powders, facilitating rapid melting and sintering of these surfaces for very low volumes of sensitizer material.4 It is worth mentioning that spectroscopy measurements on the sintered material display plasmon resonances very similar to those of GNRs in solution (Figure 2d). It was previously documented that GNRs heated in solution30 or by high-energy pulsed lasers31 would reshape and become less rod-like in order to conserve free surface energy,32 leading to a blue shift and reduction in intensity of the resonance peak. However, there have been no studies so far into the long-term thermal stabilities of GNRs with different coatings, in varied environments such as the nanocomposites created here. Figure 2e plots the resonance wavelength of GNRs mixed in a PA12 powder, stored in an oven at 150 °C, to approximate the thermal conditions inside a printer.27 GNRs coated in poly(ethylene glycol) (PEG) and silica are examined, with PEG as a standard coating for GNRs and silica as it has been shown to improve thermal stability.31 The resonance peak of the PEG can be seen to blue shift significantly with initial heating, a trend which continues steadily throughout the time in the oven, with a total change over the measured time of 38 nm, demonstrating that the shape of the particle is not stable under general printing conditions. Coating the GNRs with a silica layer of a few tens of nanometers (see Figure S2 for scanning electron microscopy images) was found to stabilize the surface and drastically limit the reshaping effect due to the thermal environment required for 3DP, maintaining a stable resonance observed over 55 h (see Figure S3 for extended plot and spectra). Therefore, our method provides a nanocomposite that retains the thermooptical stability required for use across multiple print runs. The standard PTS in commercial processes is carbon black,33 and so any new material must be benchmarked against this. Nanocomposite powders containing GNRs were found to heat significantly more efficiently than CB, and three times B

DOI: 10.1021/acs.nanolett.8b01164 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

relation between the photothermal ability and chromacity is investigated here for the first time and stands as a key factor for determining the efficacy of a PTS. A concentration of 0.01 wt % GNRs was chosen for these tests in order to balance heating ability and coloration. To investigate the color of printed objects, composite powders of CB and GNRs were placed in the test bed shown in Figure S7 and sintered at varying laser scan speeds with a 1 W 808 nm diode laser, under conditions closely simulating those inside the 3D printer. The coloration of sintered rectangles composed of the two powders can be compared quantitatively with the CIELAB scale,35 measured using a colorimeter. Here, the visual color of an object as seen by the human eye is divided into three dimensions: L* defines the object lightness, and a* and b* define the coloration (for a more complete description of the CIE scale, see the Supporting Information). This scale has been used before to investigate the chromacity of 2D-ordered plasmonic arrays and is now applied here to macroscale objects containing colloidal NPs. The color of the prints is somewhat dictated by the writing speed of the laser, as can be seen in Figure 3, as this defines the degree of sintering. Therefore, a variety of sinter speeds was used to achieve a good comparison, from the limit where no visible sintering occurs, above 400 mm/s to below 40 mm/s, the limit where the powder is melted to the bottom of the holder. The results shown in Figure 3 indicate that printing with Au GNRs instead of CB will produce far whiter prints, so the GNR nanocomposites can be said to both heat more effectively and have superior optical properties to those using carbon. Figure 3b shows the evolution of L*, the lightness value of the prints, with different sintering times, and for all scan speeds, the GNR powder is found to be significantly lighter than the CB powder. The a*,b* color plot (Figure 3a) shows that both materials have very weak coloration (hue), although there is a slight pinkness to the GNR powder due to the transversal plasmon mode, which is more prominent for lower scan speeds. Still, the pigmentation of the prints made using Au GNRs is minimal and visibly very weak (Figure 1), and far whiter prints can be achieved compared to when using CB, which allows for combining GNRs with bright color low-cost fabric dyes for achieving prints with similarly bright striking colors, as shown in Figure 4. A key frontier in additive manufacturing with polymers is the ability to print in 3D with full color or to be able to easily dye or stain printed objects. This is not readily compatible with carbon PTS due to their broad absorption spectrum. Here, we mixed GNR solutions with commercially available fabric dyes and PA12 powder to make a selection of brightly colored powders, as shown in Figure 4a, which were then sintered to form complex, brightly colored objects such as the dragon in Figure 4b,c via SLS. This demonstrates that the use of photothermal sensitizers and color are no longer mutually exclusive, and using NEPS, it is possible to achieve wellsintered, brightly colored 3D objects with any polymer powder sintering technology. The technology reported here paves the way for future advancements on the optimal use of NEPS in 3D printing. The colloidal character of plasmonic nanoparticles could be leveraged through mixing with colored inks, which would enable full-gamut, color-on-demand printing via rapid, inkjetbased methods such as HSS, allowing the production of detailed, multicolor prints. This would eliminate the complexity and costs currently associated with production time and

Figure 2. Dispersion and stability of GNRs in a sintered PA12 matrix. Optical transmission image showing sintered nanocomposite powders (a), two-photon luminescence image of the same, showing the distribution of GNRs in green (b), and a superposition of the two showing that the distribution of GNRs is principally along the borders between sintered polymer grains (c). The scale bar shows 30 μm. (d) Absorption spectra of the silica GNRs both in solution and in a sintered sample. (e) Resonance wavelength of GNR−PA12 composite powders with GNRs coated in poly(ethylene glycol) and silica after being heated in an oven at 150 C.

more carbon was required to achieve equivalent heating at any given concentration (see Figure S4). This significant enhancement can be attributed to both the strong, resonant absorption of the GNRs and their extremely high photothermal conversion efficiency. GNRs show very large absorption cross sections about their resonance wavelength due to the intense electric field associated with the longitudinal surface plasmons, and in Figure S1 it can be seen that this translates into an absorption 2.4 times that of CB at the laser wavelength of 808 nm. The nanorods have been engineered to maximize the internal electric field and hence Joule heating within the particle,34 resulting in a photothermal efficiency of η = 0.86 ± 0.6 (see Supporting Information). The coloration and the heating ability of GNR powders as a function of gold wt % were also compared (Figures S4 and S5), both displaying a highly nonlinear response to the GNR concentration. This C

DOI: 10.1021/acs.nanolett.8b01164 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. Coloration of GNR and CB samples using the CIELAB scale. Here, the color of an object is separated into the hue, defined by a* and b*, and the lightness, defined by L*. (a) CIELAB plots showing the hue (a* and b*) and brightness (L*) of CB (black) and GNR (red) samples sintered at different scan speeds. The arrows represent increasing scan speed. (b) L* of measured samples as a function of scan speed, showing that the GNR samples are significantly whiter than the CB samples for all scan speeds. Photos of samples are shown next to data points for clarity. The dotted lines represent the unsintered powder values.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b01164.

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Description of the experimental methods used, absorption spectra and scanning electron microscopy images of particles, description of the relationship between GNR concentration, heating and coloration in GNR-PA12 powder mixes, description of the method to calculate the photothermal conversion efficiency, and description of the CIELAB color space (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] ORCID

Alexander W. Powell: 0000-0001-6357-5408 Alexandros Stavrinadis: 0000-0001-9066-2478 Gerasimos Konstantatos: 0000-0001-7701-8127 Romain Quidant: 0000-0001-8995-8976

Figure 4. (a) Colored powders used to make the colorful objects, (b) visualization of the 3D design (“Hollow Draudi” created by Fundació CIM UPC), and (c) finished brightly colored 3D-printed object.

Author Contributions §

A.W.P. and A.S. contributed equally to this work.

Notes

postprocessing of multicolor 3D-printed objects. Beyond aesthetic functionality, the use of NEPS allows for customization of the photothermal and spectral response of both the composites and the final printed objects, and this could lead to improved efficiency and opportunities to use new types of light sources for additive manufacturing. Additionally, this permits the creation of objects that exhibit spatially and spectrally tuned photothermal functionality from UV to infrared. Overall, the utilization of resonant nanoparticles in additive manufacturing allows for near complete control over the visual and photothermal properties of 3DP materials and opens the door to color-on-demand powder-based 3D printing.

The authors declare the following competing financial interest(s): A.W.P., A.S., G.K., and R.Q. of the Institute of Photonic Sciences (ICFO) have filed a patent application related to the use of plasmonic nanoparticles for 3D printing.

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ACKNOWLEDGMENTS The authors acknowledge support from the European Research Council through Grant QnanoMECA (CoG-64790), Fundació Privada Cellex, CERCA programme/Generalitat de Catalunya and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0522), and Grant FIS201680293-R. The authors also acknowledge Pr. Luis Torner for fruitful discussions and support from the ICFO KTT unit. D

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DOI: 10.1021/acs.nanolett.8b01164 Nano Lett. XXXX, XXX, XXX−XXX