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On-Demand Activation of Photochromic Nanoheaters for High Color Purity 3D Printing

The creation of white and multicoloured 3D-printed objects with high colour fidelity via powder sintering processes is currently limited by discolouration from thermal sensitizers used in the printing process. Here we circumvent this problem by using switchable, photochromic tungsten oxide nanoparticles, which are colourless even at high concentrations. Upon ultraviolet illumination, the tungsten oxide nanoparticles can be reversibly activated making them highly absorbing in the infrared. Their strong infrared absorption upon activation renders them efficient photothermal sensitizers that can act as fusing agents for polymer powders in sintering-based 3D printing. The WO3 nanoparticles show fast activation times, and when mixed with polyamide powders they exhibit a heating-to-colour-change ratio greatly exceeding other sensitizers in the literature. Upon mixing with coloured inks, powders containing WO3 display identical colouration to a pristine powder. This demonstrates the potential of WO3, and photochromic nanoparticles in general as a new class of material for advanced manufacturing.

In recent years a new generation of composite materials with advanced functionalities has arisen, opening up dynamic new frontiers in both conventional and advanced manufacturing by exploiting the unique properties of nanomaterials. The earliest example of this for 3D printed materials was the creation of nanocomposites through blending polymers with ceramic nanoparticles (NPs) such as alumina 1 and nanoclays 2 to improve the mechanical properties of the printed objects. More recently, carbon nanomaterials such as carbon nanorods 3 , graphene 4 , and core-shell structures 5 have been used both to improve mechanical performance of 3D prints via laser sintering and to produce electrically conductive objects 6-810 via a variety of methods Silver nanoparticles have also been used to make antibacterial 3D prints 9 , gold nanoinks have been used to make 3D printed electrochemical reactors for catalysis 10 , and piezoelectric actuators and sensors have been printed with BaTiO3 NP-polymer nanocomposite 11,12 . Another area in which nanomaterials have shown potential to disrupt current additive manufacturing (AM) technologies is in control over the aesthetic properties of the objects.
Traditional 3D prints are printed in a monochrome and must be painted or dyed afterwards. There has been some experimentation with grading powders of different colour to produce a 'two tone' effect 13 but this is a cumbersome process and more recent work has focused on inkjetting coloured inks into the powder during the printing process along with a thermal sensitizer using the 'High Speed Sintering' approach developed by Hopkinson et al 14 . However the colour fidelity of these prints is strongly dependent on the properties of the sensitizers used. Gold nanorods (GNRs) were shown to operate as a photothermal sensitizer to allow the rapid sintering of polymer powders into 3D objects using low-power light sources, and without significantly altering the aesthetic properties 15 . This was achieved by tuning the resonant absorption of the GNRs into the near infrared (NIR), leaving only weak absorption in the visible. Whilst this work showed excellent results, there are some drawbacks to using gold nanorods as a photothermal sensitizer : GNRs are not stable at the very high temperatures required to sinter new high-performance polymers such as PEEK 16 (melting temperature of 343 C 17 ), but crucially GNRs intrinsically have weak absorption within the visible spectrum, which impacts on the colouration of prints when used in high concentrations, imposing intrinsic limits to the amount of GNRs one can use without affecting the color purity of the printed objects. In fact, this can be a problem for almost all plasmonic absorbers used for photothermal studies, in view of their absorption tail in the visible that becomes significant when the NPs are present in large quantities [18][19][20][21][22][23][24] .
In order to reach high color purity in 3D prints, we posited that instead of using permanent absorbers to generate heat, the use of switchable absorbers would allow us to print objects with very high color purity and whiteness. This is important as a strongly coloured powder will limit the colour gamut available for a print, which can impact negatively on both the aesthetics and applications such as barcoding of prints 25 . These would only possess strong optical absorption properties when they are needed to facilitate photothermal sintering, and would otherwise be transparent. Essentially, these switchable absorbers would act as programmable elements, which during the 3D printing process would be programmed, preferably optically, to switch ON and function as photothermal sensitizers, while later on they would switch OFF. To this end, we sought the use of photochromic materials, whose absorption can be altered via an external optical stimulus. In particular, we chose inorganic oxide semiconducting NPs due to their inherent stability at the elevated temperatures required in 3D printing 26 .
One of the most promising of these materials is tungsten (VI) oxide, WO3, which has a strong absorption in the NIR that can be efficiently switched on and off using electricity (electrochromic) 21 or ultraviolet radiation (photochromic) 27 . Moreover, WO3 is stable at high temperatures well above 400 C 26,28 , and can be readily synthesized as nanoparticles using cheap reactants 29,30 . In this work, we utilize for the first time the photochromic properties of WO3 NPs to develop a 3D printing method that offers very high color purity and achieves white 3D printed objects via photothermal sintering with low cost NIR light sources.
To achieve this we synthesized WO3 uncoated nanoparticles and mixed them with commercial AM powders to create a photochromic nanocomposite powder. We characterized the photochromic properties of these particles in ways not before applied to WO3 NPs, and their behavior is found to agree with the general model of excitation seen in previous studies. We demonstrate the strong, rapid photochromic response of the WO3 particles, and compare the photothermal heating and colouration of the powders to equivalent composite powders made with the industry standard carbon black (CB) as well as recently reported plasmonic nano-heaters based on GNRs. We demonstrate the sintering of these powders via a low powder laser (a 1 W diode laser, compared to the ~40 W CO2 lasers in traditional SLS machines 31 ) to make 3D printed objects in order to demonstrate the colour contrast between the three sensitizers. Finally, we compare effect of the NP heat-sources on the hues of powders when mixed with coloured inks, highlighting the excellent utility of WO3 for 3D printing white and finely coloured objects.
The photochromic effect of Tungsten oxide is due to the formation and decay of a hydrogen tungsten bronze (HWO3) created by photo-induced interactions with the surrounding environment 32 . Briefly, the photo-generated electrons induced by UV-illumination decompose the water molecules on the surface of the NCs while reducing tungsten to W +5 oxidation state forming 5 HWO3 near the surface, creating a doped hydrogen bronze 27,32,33 . Upon illumination, these extra electrons in a WO3 nanoparticle can interact with light in one of two ways. (i) Charge hopping between lattice sites of different W oxidation levels (the small polaron model) 32,34,35 , (ii) absorption by free electrons and the excitation of surface plasmons [34][35][36] . The predominance of each mechanism is dependent on the substoichiometry of the particle and the level of hydrogen doping at any given time 34,37 . This process has been shown to be highly reversible and stable over thousands of cycles 38 . There are two distinct peaks present at 640 nm and 974 nm. Several previous studies have observed this double peak feature and have attributed the long wavelength peak to a plasmonic resonance in the nanoparticle, and the shorter wavelength peak to polaron hopping, although the exact position of peaks has been found to be highly dependent on the size and composition of the nanoparticles 34,35 . Thus the potential of this material is already clear, if the strong absorption peak shown in Fig 1a can be switched on and off, then it is possible to achieve strong heating and sintering of powders via IR absorption, and later the absorption will switch off to leave the printed object uncoloured. transmission is reduced to 25% of its start point in, < 2 seconds and saturates at < 5% after 30s. This demonstrates that the photochromic effect is excited very rapidly in the particles, and is a process compatible with 3D printing manufacturing. The inset of Fig 1b shows a TEM image of the WO3 NPs. Several images were taken and a survey of NPs found their average size to be 6 ± 2 nm (See Fig. S2).
The origin of photochromism in WO3 as described earlier is associated to its crystal structure and chemical composition being altered upon UV illumination in the presence of a hydrogen donor.
We therefore performed Raman and X-ray Photoelectron spectroscopy to verify the mechanism at play in our NPs. Raman analysis in Fig. 1c shows two sharp peaks around 712 cm −1 and 809 cm −1 , both resulting from O-W 6+ -O stretching modes 32 . Upon UV illumination the peaks become significantly broader, which points to a decrease in WO3 crystallinity, which is typical of the formation of the defect W 5+ states in WO3 and has been observed previously for electrochromic 39 and plasma-based excitation 40 of WO3. We further confirm the photo-induced reduction of the tungsten by probing the W 4f core levels of the WO3 NCs with X-ray photoelectron spectroscopy.
In Fig. 1 d & e we show the W 4f core level spectra before and after UV illumination. The XPS spectra of the pristine WO3 consisting of doublet peak due to spin-orbit coupling assigned as W 4f7/2 and W 4f5/2 with binding energies of 35.9 eV and 38.1 eV respectively, which corresponds to W +6 . Furthermore, the XPS analysis of the W4f spectra after UV illumination suggest the reduction of the W +6 to W +5 which appears as an additional doublet peak shifted 1eV at lower energies in line with the previous reports studying plasma excitation. 29 We have not observed a complete reduction of the tungsten into W +5 oxidation state after UV-treatment which confirms that the photo-reduction mechanism takes place near the surface of the NCs as previously reported and agrees with the model of the formation of tungsten bronzes being responsible for the photochromic effect. 30  Fig. 2b, which shows that the WO3-PA12 powder remains white in its relaxed state for all concentrations used. Fig. S3 shows that as with the solutions in Fig. 1a, the WO3-PA12 powders and solid melted samples also take on a bluish colour after excitation with UV light, which then decays back to white after relaxation. GNR's have a smaller impact on the colouration than CB, but significantly more than WO3and the ΔCol values lie almost at the midpoint of those of CB and WO3 for all concentrations measured.
When illuminated at resonance with a 100 W LED array, the CB and GNRs heat more effectively than the WO3 (in its active state after UV photoexcitation) at all concentrations, although the difference is less marked at higher concentrations, as shown in Fig. 2c. The GNRs have the strongest heating out of the three for all measured values. However the most important figure of merit is the heating that can be achieved for a change in colouration. The ideal photothermal sensitizer for colour 3DP will maximize heating and without altering the colouration. We therefore propose a new parameter, the heating-to-colour-change ratio, defined as : χTC=ΔT/ΔCol . Fig. 2d shows that for CB, with increasing concentration, the change in colouration is stronger than the increase in heating, whereas for WO3, the change in colouration is negligible and so the increase in heating dominates. The benefit of the photochromicity of WO3 is clear here, as in the excited state they show a significant photothermal heating, but once this state has decayed they are almost colourless, producing the high χTC oserved. The GNR's follow a similar trend to the CB, although with much higher χTC values. Very low concentrations of GNRs appear white and their very high heating allows for them to outperform low concentrations of WO3, but as seen in Fig. 2c the powder quickly darkens and takes on a pinkish tinge with increasing concentrations, and its heating-to-colour-change ratio is quickly overtaken by WO3 for concentrations above 0.08 %wt.
Powders with concentrations of GNRs greater than 0.02 %wt were found to have a pinkish tinge, especially notable when sintered, as seen in Fig. 3c. We can therefore state that WO3 is a more effective sensitizer than CB and GNRs in terms of the χTC parameter. Figure 3 illustrates the importance of this effect. A custom-made laser sintering bed, using a 1W 808 nm laser was used to sinter WO3-PA12, CB-PA12 and GNR-PA12 nanocomposite powders in exactly the same method that would be used in an SLS 3D printer (Fig.   S9). The WO3-PA12 samples were illuminated via a UV lamp during sintering in order to maintain the highly absorbing excited state. The powders used to make these samples were mixed to have the same photothermal response at 808 nm, so in Fig. 3a the WO3 has a concentration of 1 % wt, the CB has a concentration of 0.08% wt. and the GNRs have a concentration of 0.03 % wt in the powder. This shows the importance of considering both heating and colouration together. As the photothermal properties of the samples at the laser wavelength are identical, they will show the same ΔT for a given illumination, but the colour contrast between them is starkthe CB sample appears almost completely black and the GNR sample appears pink, whilst the WO3 sample remains white.
The aesthetic impact of the photothermal sensitizers is also key for printing fully coloured objects, as the colour of the powder will obviously affect the colour of the finished prints. It is important to be able to print the full gamut of colours, as well as just in monochrome. This is investigated in In conclusion we have demonstrated that the use of photochromic tungsten oxide nanoparticles as a photothermal sensitizer has the potential to revolutionize high color quality 3D printing. WO3 particles are easy and cheap to manufacture, show fast activation times, and demonstrate a heatingto-colour-change ratio far superior to other available sensitizers. When added to coloured inks, WO3 reproduces even very pale shades perfectly in comparison to pristine powders. This work opens the door to a whole new class of nanomaterials for advanced manufacturing.

METHODS
Gold nanorods were synthesized in-house via the seed-mediated method of Nikoobackht & El-Sayed 42 , and coated in silica following Gorelikov and Matsuura 43 . Carbon black powder was purchased from PlasmaChem. WO3 was synthesized following the procedure of Brutsch et al 29 and dispersed in a concentrated form in deionized water. This method was selected as it produces small, uncoated nanoparticles with a high surface-to-volume ratio, which has been shown to be advantageous for maximizing the photothermal heating efficiency of nanoparticles [44][45][46] , and will also aid the rate of photochromic excitation, which largely occurs at the particle surface 32,34,37 .
The small particle size also introduces quantum confinement effects which blueshift the absorption band edge out of the visible, leading to more transparent NPs (in their unexcited state). All of these effects will lead to a greater heating-to-colour-change ratio for the WO3-PA12 composites, which is highly desirable Absorption measurements were obtained from particles in solution using a Cary 5000 spectrometer. TEM measurements were carried out in a JEOL JEM-2100 LaB6 transmission electron microscope operating at 200kV. Raman spectra were collected using a Renishaw InVia MicroRaman spectrometer, exciting the samples with a 100 mW diode laser at λ=532 nm, using a × 50 magnification microscope objective. XPS measurements were performed with a Phoibos 150 analyzer (SPECS GmbH) in ultra-high vacuum conditions using a monochromatic Kalpha x-ray source (1486.74eV). The photochromic excitation speed tests were carried out using a custom setup defined in Figure S7.
The NPs were diluted in ethanol then mixed with pristine PA12 powder from Advanc3Dmaterials at differing wt % concentrations then dried in an oven at 60 C overnight. After drying powders were sieved prior to printing. For measuring the effects of the NPs on the colour of powders, various concentrations of commercially available inks were drop-cast onto the powders and the colouration measured with a colorimeter.
To perform the comparative tests between the different sensitizers, the powder, small samples of powder in a shallow aluminium holder were illuminated by a 100W 810 nm or 100W 940 nm LED array from Shenzhen Hanhua Opto Co. The temperature of all samples was measured using a FLIR ax5 thermal camera, and the colour tests were performed using a PCE instruments PCE-CSM 4 colorimeter.
To sinter the printed shapes in Fig. 3, a custom made laser writing setup was used, as described previously 15 . Samples were preheated by a hotplate below and IR lamps from above in order to mimic the conditions in a SLS printer. The bulk of the powder was heated to 150C using the hotplate and then the top layer heated to 172.5 C using the lamps. A Dymax Bluewave 200 UV lightsource was used to illuminate the WO3-PA12 sample during this process to maintain the nanoparticles in their photoexcited state. A 1W 808 nm diode laser from Thorlabs was used to sinter the powder and define the shape of the prints.

Supporting Information
Showing illustrations of the experiments used and additional results on the colouration of powders after the addition of the inks.