Structural Color Patterns on Paper Fabricated by Inkjet Printer and

(17) Song et al. fabricated the anticounterfeiting quick response code (QR code) ... have pure cubic phase (JCPDS card number: 80-0019) as shown in Fi...
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Structural Color Patterns on Paper Fabricated by Inkjet Printer and Their Application in Anticounterfeiting Suli Wu,* Baoqi Liu, Xin Su, and Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, PR China S Supporting Information *

ABSTRACT: Inkjet-printed structural color patterns have attracted great attention in recent years because of their broadly promising applications. However, the patterns are usually fabricated on pretreated plastic substrates. Herein a convenient inkjet printing method was developed to fabricate large-scale computer-designed structural color patterns on photo paper without any treatment using inks containing monodisperse CdS spheres. By this strategy, not only were the single-color and multicolor structural color patterns on paper successfully obtained, but also invisible photonic anticounterfeiting was achieved without any external stimuli. The key point of this anticounterfeiting technique is printing patterns and the background with inks containing uniformed CdS spheres with different diameters but similar intrinsic colors, so that the invisible patterns can be observed clearly by simply changing the viewing angle. The invisible and visible can be realized without the change of intrinsic structure, and the patterns are all solids. The patterns will have long lifetime and good durability, which is beneficial for their practical usage.

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invisible photonic prints shown by water.21 The invisible photonic patterns were prepared on a siloxane-containing responsive photonic paper. Through controlling the crosslinking degree with lithographical methods, the invisible photonic patterns can be observed when they were soaked in water because of the different swelling speed and lattice expansion of the patterns and the background. The same group also developed invisible photonic prints shown by deformation.22 The patterns and background were constructed with very close photonic structures but different mechanochromic capabilities through controlling the cross-linking degree by lithographical method, leading to invisible photonic patterns that can be revealed under deformation because of the nonuniform change in the photonic structure. Chen’s group23−26 fabricated magnetically responsive anticounterfeiting photonic prints using magnetic-responsive photonic display film. When the photonic film was put into a vertical magnetic field, only the prints with magnetic-responsive photonic activities were changed and the prehidden patterns were observed because of greatly different reflection wavelength contrast between the background and the print. To date, most anticounterfeiting applications of structural color patterns were constructed on responsive photonic films such as PDMS by lithographical methods, and the patterns can changed from invisible by structure change under external stimuli such as solvent, mechanochromic deformation, and magnetic field. Nam et al. fabricated monolayered PCs on glass,

nkjet printing is a widely applied and attractive method for fabricating large-scale and complex patterns because of its direct-writing, low-cost, mask-free, and high-throughput features.1−3 Accordingly, inkjet printing has also been applied to create structural colors by forming photonic crystals (PCs)4,5 in very recent years because patterned structural colors have attracted great attention because of their broadly promising applications in optical devices,6−8 displays,9−12 sensors,13 detection,14 etc. Song and co-workers successfully fabricated PCs with structural colors on different plasma-treated plastic substrates by inkjet printing method.15 They also inkjet-printed colloidal spheres into highly ordered arrays with structural colored patterns by controlling the sliding of the three-phase contact line on octadecyl trichlorosilane-treated Al foil or polydimthylsiloxane (PDMS).16 The obtained dome-shaped structures exhibit angle-independent colors due to their highly symmetric geometry. Kim and co-workers prepared monolayered PCs with structural colors on glass, Si-wafer, PDMS, and polypropylene (PP) by inkjet printing.17 Although the above methods are effective in fabricating structural colored materials, the used substrates are usually plastic, and sometimes special pretreatment and special devices are required for the inkjet printing process. Among various applications of structural color patterns, anticounterfeiting techniques have been intensively investigated in the past few years.18−20 When responsive materials are employed to build PC patterns, their colors could transform between appearance and disappearance from the background. The invisible photonic prints were designed to be unseen under normal circumstances, but can be recognized under external stimuli according to the dependence of the reflected structural color on the photonic structure. Ge’s group has reported © 2017 American Chemical Society

Received: June 1, 2017 Accepted: June 9, 2017 Published: June 9, 2017 2835

DOI: 10.1021/acs.jpclett.7b01372 J. Phys. Chem. Lett. 2017, 8, 2835−2841

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Scheme 1. Schematic Illustration of the Inkjet Printing Process of CdS Invisible Photonic Pattern on Paper-Based Substrates for Anticounterfeiting Applications

Figure 1. Characterization of the CdS spheres. (a−d) SEM images of CdS spheres with different diameters: (a) 270 nm, (b) 290 nm, (c) 315 nm, and (d) 335 nm; (e) XRD patterns of CdS spheres with different diameters.

The relative high refractive index, good water dispersity, and the intrinsic absorption of CdS spheres make them potential candidates to generate brilliant structure color on paper by inkjet printing. As illustrated in Scheme 1, large-scale computerdesigned structural color patterns on paper can be obtained using inks containing monodisperse CdS spheres with different diameters via a common inkjet printer. The inks containing CdS spheres with different diameters have similar intrinsic color, but their inkjet printed structures could show different structural colors according to Bragg’s equation. Furthermore, the brightness of the structural colors is angle-dependent. Hence, the structural color patterns constructed by CdS spheres with different diameters can be selectively hidden and visible compared with the intrinsic yellow color when changing the viewing angle, which makes the obtained structural color patterns highly desirable for anticounterfeiting applications. This technique is capable of creating anticounterfeiting patterns directly by inkjet printing on paper in macroscale, and the patterns can appear and disappear simply by changing the

Si-wafer, PDMS, and PP by using an inkjet printer, and the patterns transform from weak color to brilliant color under strong illumination.17 Song et al. fabricated the anticounterfeiting quick response code (QR code) by printing three kinds of PC dots, shaped as bumps, plates and coffee rings on the PDMS substrate, and the QR code could display different images by changing the lighting conditions.27 The anticounterfeiting structural color patterns without responsive substrate fabricated by commercially used inkjet printer on common paper have been rarely reported because of the challenging in creating brilliant structure color on common paper and making the “pattern” responsive because the responsive pattern and the nonresponsive background are usually needed to be realized by lithographical method. Herein we developed a very simple method similar to the process we usually use to print in our daily life with inkjet printing structural color patterns on commercially available paper (premium waterproof photo paper with contact angle > 50°) without any pretreatment. The essence of this strategy is the ink, which is prepared using monodisperse CdS spheres. 2836

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Figure 2. (a−d) Photographs of the structural colors of CdS films (taken by a digital camera, Sony ILCE-5100) constructed by inkjet printing using inks containing CdS spheres with diameters of 270, 290, 315, and and 335 nm; (e−h) SEM images of CdS sphere arrays formed by inkjet printing with different diameters (from left to right: 270, 290, 315, and 335 nm); (i−l) reflection spectra of different colored films with the incident and detection angle of 45°.

1 h to achieve complete dispersion of CdS spheres. The structural color patterns with different concentrations of CdS dispersions were inkjet printed to investigate the effects of layer thickness and surface morphology on the structural color patterns. The results demonstrated that the optimum concentration of the suspension was 15 wt % for inkjet printing, and the thickness of pattern was about 1.25 μm (Figures S1 and S2 in the Supporting Information). The viscosity of the ink was about 2.21 mPa·s, low enough for the smooth ejection of ink droplets, and its surface tension was about 42.6 mN·m−1, which was suitable for spreading of the ink droplet. According to the study of Song’s group,16 the high contact angle of ink on substrate will improve the arrangement of spheres into ordered structure and therefore increase the reflectivity of the prepared PC film. Here we selected some kinds of papers as substrates for inkjet printing CdS spheres, and the contact angles of our ink on them were measured. The results demonstrated that when commercially available premium waterproof photo paper with contact angle greater than 50° is used as substrate (Figure S3), CdS can self-assemble into short-range ordered structures (Figure 2e−h). The CdS inks were printed via a multiorifice Canon PIXMA ip2780 inkjet printer at room temperature. Figure 2a−d shows the structural colors of CdS films (taken by a digital camera, Sony ILCE-5100) constructed by inkjet printing using inks containing CdS spheres with different diameters. Excitingly, the brilliant green, orange, and red colors were achieved using inks containing 270, 315, and 335 nm CdS spheres, respectively. In other words, the color hues are tuned by changing the diameter of CdS spheres. It is found that the reflection peak position is in a linear relationship with the diameter of CdS spheres at the specular angle (Figure S4), which is consistent with the reported structural colors seen in

viewing angle without any external stimuli, which is highly beneficial for their practical applications. The most commonly used building blocks of PC structure in inkjet printing are polystyrene (PS), poly(methyl methacrylate) (PMMA), and SiO2 spheres with relatively low refractive index (1.59 for PS, 1.49 for PMMA, and 1.46 for SiO2).28 According to previous reports, the films constructed by the abovementioned spheres with short-range ordered structure showed weak structure color visibility.29−32 However, it is difficult to obtain long-range ordered structure on paper-based substrates by inkjet printing because of their rough surface and low contact angle with ink. Hence, in order to obtain bright structure color, the most commonly used substrates for printing PC structure are plastics. In this work, we suppose that using spheres with relative high refractive index will realize the bright structural color on paper. Monodisperse spheres of CdS with a theoretical refractive index of 2.5 were selected to generate structural color by the inkjet printing method.33 Except the relatively high refractive index, CdS also has intrinsic color (yellow) because of its absorption in the visible region. First, CdS spheres with diameters of 270, 290, 315, and 335 nm were synthesized by our recently reported hot-injection and heating up method. The obtained spheres were characterized by SEM (Figure 1a−d). Clearly, the CdS spheres displayed narrow size distribution, which guarantees their self-assembly to form ordered structure. Because of the existence of PVP on the surface of CdS spheres, they can be well dispersed in water or ethanol. The X-ray diffraction (XRD) patterns reveal that the spheres have pure cubic phase (JCPDS card number: 80-0019) as shown in Figure 1e. (The spectra were vertically offset for clarity.) The CdS inks were prepared by dispersing the CdS particles into a mixed solvent of deionized water, ethylene glycol, glycerol, and ethanol. The mixture was treated ultrasonically for 2837

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Figure 3. (a−f) Digital photographs of our school badge (upper) and double-colored QR code pattern (bottom) at different viewing angles; (g) angle dependence of the reflectance spectra of CdS film (patterns fabricated by CdS with diameter of 270 nm); (h) digital photographs of QR code patterns on the photo paper: (left) curved pattern; (right) the pattern (from left-hand panel) after relaxing.

Figure 4. (a, b) Digital photographs of an anticounterfeiting pattern. The pattern is invisible at the viewing angle of 70°, but it is clearly visible at the viewing angle of 10° (the incident angle of light is 10°). (c, d) Color-filled contour map of red and green regions.

Moreover, the letter “A” at micro scale has been inkjet printed by our method. As shown in Figure S7, the letter “A” is clearly observable in the micro scale. However, because of the resolution limit of the used printer, the ink drop diameter is about 46.3 μm, and the smaller pattern in the micro scale is not as clear as that in large scale. Based on the above method, single- and double-colored structural color patterns as shown in Figure 3 were designed on computer and printed. The upper pattern was printed using one ink cartridge containing 315 nm CdS spheres, while the bottom QR code was printed using two ink cartridges containing 270 and 335 nm CdS spheres. Because of the angle dependence of the structural color, the patterns display different colors at different viewing angles. The color of the upper pattern changes from yellow to orange as the viewing angle changes from 70° to 10°. As for the double-colored pattern, the pattern changes from single yellow color to green and red double colors as the viewing angle changes from 70° to 10°.

amorphous photonic structures originating from constructive interference of scattered light.34−36 The SEM images in Figure 2e−h indicate that the CdS spheres self-assemble into short-range ordered structure. The reflection spectra of these printed films were also measured and are shown in Figure 2i−l. The reflection peak signal was consistent with the color in Figure 2a−d. For clarity, the measured reflectance spectra of these colors were converted into Commission International de L’Eclairage (CIE) chromaticity values and are shown in Figure S5. Although the printed CdS films are not long-range ordered photonic crystal structures, they have bright structural colors and obvious reflection peaks as we expected. As a control experiment, PS spheres were also used to prepare inks for inkjet printing. In sharp contrast, although PS spheres also formed short-range ordered structure, the structural color is very weak, as shown in Figure S6. All these results indicate that the brilliant structure colors on paper can be achieved by inkjet printing using inks containing CdS spheres because of their high refractive index. 2838

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angle, the brightness of the structural color is nearly zero, which leads to the hidden structural color pattern. To further confirm the practical application of the newly developed strategy, invisible patterns of QR codes were also designed and printed by the inkjet printing method (Figure 5).

Under natural light, by tuning the viewing angle, the obtained patterns can display different colors, as shown in Figure 3. To further understand the color change of the patterns with viewing angle, the reflectance spectra of CdS patterns fabricated with 270 nm CdS spheres at different viewing angle were measured and are summarized in Figure 3g. It is obvious that when the incident angle was fixed, although the reflection wavelength shifts slightly with the change of viewing angle, the reflectivity is dramatically increased when the viewing angle matches well with the incident angle, and the reflective spectrum is very weak when the viewing angle far from the incident angle, which means that the structural color is obviously observable and unobservable in some special viewing angles. When the viewing angle is far from the incident angle, the observed yellow color originates from the absorption of CdS (Figure S8), while when the viewing angle is close to the incident angle, the observed color is dominated by the structural color created by the short-range ordered structure. In addition, the pattern is flexible because of the flexibility of the photo paper, and it remains stable during the curving and relaxing cycles as shown in Figure 3h. The QR code can still be identified by using a mobile phone easily and fast after 10 curving and relaxing cycles, which indicates the structure was unbroken when the substrate was curved. According to above phenomena, we think it is possible to create invisible structural color patterns for anticounterfeiting applications. Because the structural color of CdS is dependent on the diameters of CdS spheres and their intrinsic color originated from absorption is very similar, the key point of fabricating the invisible patterns is printing patterns and background with the inks containing uniform CdS spheres with different diameters. When the observed colors are dominated by the intrinsic color, the PC patterns are hidden, and when the observed colors are dominated by the reflective structural color (in special viewing angle), the patterns are displayed clearly. That is to say the patterns can be made to appear and disappear by simply changing the viewing angle. On the basis of this strategy, we designed a pattern composed of a red butterfly and a green butterfly in a yellow background, and the pattern was printed using three inks containing CdS spheres with different diameters (Figure 4). One butterfly is printed by ink containing 270 nm CdS spheres, and the other butterfly is printed by ink containing 335 nm CdS spheres; the background is printed by ink containing 290 nm CdS spheres. According to their ultraviolet−visible absorption spectra (Figure S8), all the CdS spheres have yellow intrinsic color. As revealed in Figure 4, in certain viewing angles, the structural colors of the two butterflies are hidden because of the angle dependence of the structural color. However, when the viewing angle matches well with the incident angle, the butterflies display brilliant red and green structure colors, while the background still shows a yellow color, which forms a large contrast in color immediately. Thus, the invisible patterns can be changed to visible simply by changing viewing angle without any external stimuli. To confirm the visual observation and explain the mechanism of invisible patterns, the evolution of reflection spectra of two butterflies under different viewing angles is recorded in Figure 4c,d. It is clear that when the incident angle is fixed at 10°, the brightness of the structural color is highest during the viewing angle from 8° to 15°, nearly the specular angle. However, when the viewing angle is far from the specular

Figure 5. Digital photographs of QR codes for anticounterfeiting application. The totally invisible images can be obtained using CdS inks with similar intrinsic colors. The background was obtained with 290 nm CdS spheres; the left red QR code was obtained with 335 nm CdS spheres, and the right green QR code pattern was obtained with 270 nm CdS spheres.

The left one was printed using 290 nm CdS spheres as background and 335 nm CdS spheres as pattern, while the right one was printed using 290 nm CdS spheres as background and 270 nm CdS spheres as pattern. Under natural light, the QR code patterns are invisible in some viewing angles because of the similar intrinsic yellow color and clearly visible in certain viewing angles because of the brilliant structural color. As revealed by the video in the Supporting Information, in some viewing angles, the invisible QR code can be scanned, resulting in a connection to the homepage of Dalian University of Technology. This results suggests the potential of their practical application in anticounterfeiting. Importantly, switching between the invisible and visible patterns of the anticounterfeiting QR codes requires no external stimuli, and the printed structures are maintained their original form. This will overcome the challenges in durability and lifetime. To measure the durability of the inkjet printing color, the structural color and reflective spectrum of samples printed 6 months apart were measured and compared. As shown in Figure S9, the structural color is still present and brilliant after the pattern has been in place for six months, and the reflectivity is nearly unchanged. In addition, poly(vinyl alcohol) (PVA) was added in the inks to improve the adhesion of the inkjet printed patterns. As depicted in Figure S9, the structural color of the pattern is not affected by adding PVA. A video in the Supporting Information demonstrates the adhesion of the inkjet printed patterns. The video indicates that the structural color pattern is unchanged by rubbing, indicating its strong adhesion. In conclusion, a convenient method has been developed to fabricate large-scale computer-designed structural color patterns on paper by inkjet printing methods, and invisible photonic anticounterfeiting patterns have also been achieved. Using 2839

DOI: 10.1021/acs.jpclett.7b01372 J. Phys. Chem. Lett. 2017, 8, 2835−2841

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The Journal of Physical Chemistry Letters solutions containing monodisperse CdS spheres with different diameters as inks and premium waterproof photo paper as substrates, CdS films with various brilliant structural colors were fabricated by an inkjet printer. It is worth noting that by this method, invisible anticounterfeiting patterns can be realized by printing patterns and background using the inks containing uniformed CdS spheres with different diameters but the same intrinsic color. The prepared patterns can appear clearly and disappear by simply changing the viewing angle. Switching between the invisible and visible patterns of the anticounterfeiting structural color requires no external stimuli, and the printed structures maintain their original form. This method will overcome the challenges in lifetime and durability of externally stimulated patterns and therefore benefits their practical usages in antifraud labels and identity recognition. Furthermore, the convenient, controllable and scalable structural color-creating method may also have a significant impact on other practical applications of structural color.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suli Wu: 0000-0002-3956-1855 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21476040, 21276040, 21536002) and the Program for Changjiang Scholars and Innovative Research Team in the University (IRT0711), fund for innovative research groups of the National Natural Science Fund Committee of Science (21421005), and the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT2016TB12).

EXPERIMENTAL SECTION Synthesis of Monodisperse CdS Spheres. The monodisperse CdS spheres were prepared with a modified procedure according to our previous work. A 4.13 g sample of Cd(NO3)2·4H2O, 1.14 g of thiourea (TU), and 5 g of poly(vinylpyrrolidone) (PVP) were dissolved in 150 mL of diethylene glycol. The solution was transferred into a three-necked flask, and then the mixture was slowly heated to 160 °C, maintained for 5 h with vigorous magnetic stirring, and then cooled naturally to room temperature. The reaction product was harvested by centrifugation and washed with deionized water and ethanol three times. Finally, the product was dried in a vacuum oven for further use. The molar ratio of Cd and TU precursor was kept to 1:1 in the preparation process. To obtain CdS spheres with diameters of 270, 290, 315, and 335 nm, the amount of Cd precursor added was 15, 18, 20, and 22 mmol, respectively. Preparation of CdS Inks. The CdS inks were prepared by dispersing the CdS particles into a mixed solvent of deionized water (62 wt %), ethanol (10 wt %), glycerol (1.1 wt %), ethylene glycol (8 wt %), poly(vinyl alcohol) (PVA) (0.5 wt %), and a small amount of defoamer. The mixture was treated ultrasonically for 1 h to achieve complete dispersion of CdS spheres. Characterization. Powder X-ray diffraction measurements were performed on a Rigaku D/MAX-2400 diffractometer with Cu Kα radiation. The printing was performed with Canon PIXMA ip2780 inkjet printer. The morphology of the prepared samples was obtained by using a Nova Nanosem 450 field emission scanning electron microscope (FE-SEM). Digital photographs and optical microscopy images of the patterns were obtained with a digital camera (Sony ILCE-5100) and an optical microscope (Optec MIT 500), respectively. The reflection spectra of the structural colored patterns were measured using a Hitachi U-4100 spetrophotometer. The contact angles were measured using a contact-angle system (Powereach, China). The surface tension was measured with surface tension-meter k100c, and the dynamic viscosity of ink was measured using an All in One rheometer.

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Photographs of CdS films, contact angles of drops, CIE chromaticity diagram, SEM images of PS sphere arrays, and absorption spectra (PDF) Video demonstrating an invisible QR code being scanned (AVI) Video showing thatt the structural color pattern is unchanged by rubbing (AVI)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01372. 2840

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