Inkjet Color Printing by Interference Nanostructures Aleksandr V. Yakovlev,* Valentin A. Milichko, Vladimir V. Vinogradov, and Alexandr V. Vinogradov* ITMO University, Saint Petersburg 197101, Russia S Supporting Information *
ABSTRACT: Color printing technology is developing rapidly; in less than 40 years, it moved from dot matrix printers with an ink-soaked cloth ribbon to 3D printers used to make three-dimensional color objects. Nevertheless, what remained unchanged over this time is the fact that in each case, dye inks (CMYK or RGB color schemes) were exclusively used for coloring, which inevitably limits the technological possibilities and color reproduction. As a next step in printing color images and storing information, we propose the technology of producing optical nanostructures. In this paper, we report use of inkjet technology to create colored interference layers with high accuracy without the need for high-temperature fixing. This was made possible due to using titania-based colloidal ink yielding monolithic coatings with a high refractive index (2.00 ± 0.08 over the entire visible range) when naturally dried. By controlling the film thickness by using inkjet deposition, we produced images based on controlled interference and implementing color printing with one ink. The lack of dyes in the proposed method has good environmental prospects, because applied systems based on a crystalline anatase sol are nontoxic and biologically inert. The paper explains in detail the principle of producing interference images by the classical inkjet method and shows the advantages of this technique in depositing coatings with uniform thickness, which are required for large-scale interference color imaging even on unprepared polymer films. This article demonstrates the possibility of inkjet printing of nanostructures with a precision in thickness of up to 50 nm, we believe that the proposed approach will be the groundwork for developing interference color printing approach and allow to implement new methods of forming optical nano-objects by widely available techniques. KEYWORDS: interference, inkjet printing, thin film, titanium dioxide, optical nanostructures
I
n recent decades, color printing of text and images has not only retained its popularity but also continued to develop owing to its availability and speed of application. At the same time, few people limit themselves to using only black-andwhite printing, more often resorting to multicolor one. It is hard to imagine your home or office without color printers. However, for the past several decades, color printing technology used the combined interaction of colors, such as RGB and CMYK, reaching more than 16 million (2563) color combinations. In the present work, we use a fundamentally different approach to forming a color image based on the interaction of light with a thin layer structure, Figure 1. We call it interference inkjet printing. The rationale for this approach dates back to the distant times when the phenomenon of interference was observed in thin films for the first time. Indeed, at the interface of materials differing by their refractive index forms a reflected light with a wavelength proportional to the layer thickness for the material with higher refractive index (RI), which is discerned as a monochromatic color by the human eye. Nature reveals many examples of this phenomenon: soap bubbles (air/surfactant in water),1 multilayer pearl iris, © 2016 American Chemical Society
Figure 1. Lighting visualization of interference inkjet printing.
Received: September 26, 2015 Accepted: January 25, 2016 Published: January 25, 2016 3078
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controlling the gelation stage. Moreover, in the case of a high degree of chemical protonation of the surface of particles viscosity will be determined largely by the concentration of solvent, thus excluding sedimentation. It is this advantage of the sol-gel systems that makes them unique for use as a material for interference color printing. Color printed materials obtained using this technology possess unique properties, such as the lack of color change over time, which is promising for the long-term storage of color images, because the base material is extremely stable and does not degrade over time. Besides, inkjet printing of sol-gel high refractive materials allows to use polymeric substrates without preliminary modification and applying linking layers that are commonly used in inkjet printing. Given the ability of many inorganic colloids to resuspend, such printing technology is universal and can be reused upon reapplying the image onto a polymer substrate.17 Among the variety of inorganic colloids that are adapted to inkjet printing and are widely used now, only a few can be considered highly refractive, highly transparent and cheap, for example, ZrO2, TiO2, ZnO, and some mixed oxides. Titania is the most universal in this case, for several reasons: (1) producing crystalline sol-gel inks by soft chemistry is well described; (2) refractive index of TiO2 (anatase) is 2.61; (3) it is completely transparent in the visible region; (4) it is easily crystallized upon thermal dehydration because of always having a crystalline nucleus; (5) high isoelectric point (I.E.P. = 5.9), which allows to obtain highly stable colloids. It is worth noting that there exists a technology of producing films with a high refractive indices.18 Titania formulations were reported by Arin19 and Manga.20 The authors have declared the deposition of conventional sol-gel formulations based on tetraisopropoxy titanate and acetyl acetone by a modified office inkjet printer.21 Bernacka-Wojcik and co-workers recently demonstrated22 a disposable biosensor integrating an inkjet printed photodetector fabricated by printing a commercial dispersion of titania particles with a desktop office printer equipped with a thermal inkjet head. A similar approach was adopted by Yang et al.,23 who used a dispersion of TiO2 printed by a modified office inkjet printer to produce an oxygen demand sensing photoanode. In this paper, we use nanocrystalline TiO2-based ink that can be deposited on a wide variety of surfaces without the need for high-temperature fixing. For this purpose, we used the new low-temperature solgel synthesis technology developed by the authors24 as an alternative to the well-known brick-and-mortar strategy.25 The original titania stock dispersion was further modified for inkjet printing.26 Thin titania coatings were patterned onto fused silica and polyethylene (PET) substrates by inkjet printing, and their material characteristics were carefully investigated. A detailed report on the material properties of the resulting coatings, optical properties, and new color printing technology using highly refractive sol-gel ink are disclosed in this paper. Supporting Information reports the investigations which are not included in the article. The outline of synthesizing a titania sol is given and the major components for its production are described. To determine the compliance of rheological parameters with inkjet properties, we give a table with values of dynamic viscosity and surface tension, as well as the calculated Z numbers. The dependences of the particle size and zeta potential on the sol to ethanol ratio during the preparation of the ink are provided. The description of the printer and its preparation for printing sols is given. A technique for preparing digital files for inkjet printing is described. Detailed information
and so forth. Natural color management occurs even in living objects by changing the optical order of the structure in thin layers (interference), for example, used in camouflaging by squids. Similar mechanisms of color control are found in insects, for example, in manuka beetles.2 Options with specific trapping of visible wavelengths are also known for photonic crystals3−6 and magnetic materials,7 but they are poorly suited for large-scale color printing. The possibility of large-scale monochromatic interference is allowed only if there is a smooth surface or that was formed either by a liquid phase or a solid substrate with minimal roughness. Thus, the artificial production of interfering layers on polished silicon and solid organic polymers was reported previously.8 The use of the last mentioned is preferable, because it imparts flexibility characteristics to such materials. Also, the phenomenon of interference in thin films is fundamental for pearl effect in producing chameleon paints.9 The apparent advantage of interference is natural color reproduction, because the entire sunlight spectrum is used for imaging, including the maximum possible number of colors and shades distinguished by the human eye. However, saturation of color responsible for the contrast of the image will largely depend on the magnitude of the difference between the refractive indices of the applied layer and used substrate.10 Attempts were made to enhance this effect by modifying the polymers using various nanoscale crystalline substances.11 Such approaches allow to obtain a high RI12 for organic polymers, but the optical properties dramatically deteriorate due to the lack of a homogeneous distribution of the components between themselves. Using inorganic polymers instead may be an alternative. In particular, the greatest prospect is the lowtemperature sol-gel synthesis technology, which allows one to obtain crystalline materials at low temperatures and atmospheric pressure. Langlet et al. have shown the use of this technology in producing TiO2 coatings13 and photocatalytic coatings on polymer films.14 In this case, the use of nanocrystalline colloids with minimum amorphous phase content, which are capable of high adhesion even on smooth surfaces, is an optimum for color images. Moreover, such inorganic colloids can be adapted to inkjet printing, because they are already widely used in printing biosensors and electronic objects.15 The thickness of the deposited layers can be easily adjusted from several dozen nanometers to several dozen micrometers by adjusting the volume and composition of droplets leaving the nozzle of the print head. Accordingly, inkjet printing can be used to produce multicolor interference layer printing on polymers, even under conditions when the color of the image varies with a change of a few nanometers in film thickness. Until now, the possibilities of inkjet printing were focused on the microrange. In this paper, we show for the first time that the optical structures can be obtained with much greater accuracy, which in particular opens the prospects in developing controlled interference printing of interference color images. Printing materials by using inkjet is performed by tuning the parameters of viscosity and surface tension of ink or by tuning the printer for a specific composition. In most cases, one has to use additives such as glycerol to increase viscosity and surfactants for decreasing surface tension.16 Using these approaches will inevitably reduce the refractive index due to an increase in the volume fraction of the organic part in the dry residue. However, in contrast to high refractive index polymers, the viscosity parameter in sol-gel systems can be adjusted by 3079
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Monodispersed nanoparticles without any aggregation are crucial to attain high dispersion stability during the printing process. Protonation of the surface of particles substantially shifts the critical point of gelation, preventing the development of coagulation contact between the particles. As a result, the formation of the structure according the mechanism of polycondensation begins to occur at an interaction force of 10−11 to 10−10 N/contact, when the distance between the particles is reduced to 10−9 m. Such an approach can significantly reduce the nozzle diameter while maintaining high stability of inkjet printing. Thus, to prepare stable TiO2 nanocrystalline inks, extremely alkaline environments should be avoided and a pH range between 2 and 5 is favorable (I.E.P. = 5.9). In this case, the zeta potential of TiO2 nanoparticles is +36 mV, which provides a stable state of the particles (Figure S2). In addition to high stability, it is also necessary to increase the degree of crystallinity of the sol particles to increase the refractive index of the solid phase. This can be achieved by increasing the ionic strength of the solution27 upon adding compounds with a high dissociation constant, such as inorganic acids. In our case, we have used nitric acid (Ka = 24). Given the fact that the protonated agent is capable of affecting the phase composition of TiO2 in the process of crystallization of amorphous particles of the sol, this selection was due to the ability of nitric acid to promote the most active optic phase of anatase. Given the fact that the most popular polymorphic TiO2 modifications, rutile, anatase, and brookite, possess similar refractive index values, the advantage of obtaining anatase is using a pH value closer to neutral, thus minimizing the impact of corrosion processes in the printing head. Another major component is a volatile solvent. Ethanol acts as a transmitting fluid by carrying suspended titania particles. Ethanol also plays a very important role because it is the main factor influencing the solvent evaporation rate. Lower ethanol concentration results in slow drying, mottling, and dust accumulation, while higher concentration lead to premature evaporation and banding pattern formation. Given the fact that peripheral ring deposit (known as the coffee-ring effect) forms because of a radial capillary flow carrying particles, high solvent evaporation rate will help minimize this phenomenon, because the high surface charge of TiO2 particles will prevent increasing viscosity and nonuniform accumulation of solid phase on the perimeter of the formed structure. Moreover, this approach promotes applying thin highly refractive layers onto three-dimensional surfaces, maintaining the thickness of the layer. Proposed mechanism of solid sol-gel phase formation from inkjet printing is presented in Figure 3. Table S1 summarizes the dilution ratios and corresponding densities of the printing formulations. Figure 4 depicts the viscosity and surface tension of the tested formulations. Formulation containing 30 vol % of the stock TiO2 sol in ethanol was identified as optimal for providing a balanced compromise. The drop dynamics and film formation can be predicted to a certain degree by theoretical descriptors such as the Weber number (We = δ·v2·d/σ), Ohnesorge number (Oh = ν / (d ·σ ·δ) ), Z number (Z = (d ·σ ·δ) /ν ), and Reynolds number (Re = σ·v2·d/ν), where δ is the density, v is the drop velocity, d is the diameter of the nozzle, σ is the surface tension, and ν is the viscosity of the reliability and sufficient dry mass content. Table S1 also gives the values of the Z numbers that were used as an indicator of drop formation and printability (e.g., capillary break-off length and time, droplet volume, and satellite formation). Although some
on selected area electron diffraction (SAED) pattern and interpretation of the data are provided. Characterization of the crystal structure of titania nanoparticles is given and powder diffraction pattern is demonstrated. Microphotographs of the film surface obtained using high-resolution SEM are shown. Contrasting sections of films are measured by energy-dispersive spectroscopy (EDS) using SEM and the elemental composition of the films with different thickness is quantified. A technique for investigating adhesion of the layers and calculating adhesion characteristics obtained by inkjet printing is described in more detail. Transmittance spectra for films onto fused silica are shown within the UV and visible region. A setup for measuring the refractive properties of these films is described.
RESULTS Ink Characterization. Anatase nanoparticles formed during the preparation of the ink possess a structure with clear crystalline lattice fringes, with an average size of crystalline formations of about 5 nm, Figure 2, which correspond to the
Figure 2. (a) HRTEM image with SAED pattern (as insert) and (b) SEM with scanning transmission electron microscopy (STEM as insert) of as-synthesized titania xerogel after drying in air.
(101) planes of the body-centered tetragonal structure of anatase. The attached SAED pattern (shown in Figure 2, with details in Supporting Information section F, Figure S5) could be indexed as the (101) zone axis, which confirmed the single crystalline anatase structure. The nanocrystallization of the TiO2 nanoparticle framework is very important for applying it in interference solid thin films as already mentioned before. The high-resolution transmission electron microscopy (HRTEM) images, Figure 2a, confirm the presence of a single-phase ultradisperse material which possesses a high degree of crystallinity. X-ray diffraction (XRD) patterns of the as-synthesized TiO2 are shown in Figures S6 and S7. The diffraction peaks at about 25.411 (101), 37.911 (004), 48.011 (200), 54.011 (105), 54.911 (211), and 62.811 (204) can be also indexed to anatase phase TiO2 with a Scherrer crystallite size of about 5 nm, which is in full agreement with HRTEM analysis and SAED pattern. Film uniformity is crucial to reliability for printed thin interference solid layers, so an ink with well-suspended nanoparticles is needed for liquid thin film deposition. For oxide particles, surfactant or capping agents are regularly used in commercial inks to facilitate nanoparticle suspension. However, the addition of surfactants or capping agents may result in a significant loss in optical properties and stability for the printed TiO2 films. Therefore, in this study, a simple pH adjustment approach is used to increase the particle suspension stability and to achieve high homogeneity of the formed layers. 3080
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fully complies with the conditions of drop formation and its separation from the nozzle head, in accordance with the theoretical models of inkjet printing. Characterization of Inkjet Titania Films. To control the color of the image produced by inkjet refractive TiO2 layers, one has to form smooth homogeneous layers to achieve the effect of interference in solid thin films. To explore the relief of deposited layers onto fused silica (or PET), we used atomic force microscopy for scanning the surface topography, Figure 5. According to the profilogram of AFM images, Figure 5, roughness is less than 20 nm and does not depend on film thickness. Such a surface fully complies with the conditions of interference in thin films and can be used for colorful inkjet printing optical nanostructures. The change in the surface relief is due to the small size of the particles, which does not exceed 10−15 nm by HRTEM and SEM data. These data allow us to assume that printing inkjet TiO2 nanostructures is easily achievable with high accuracy required to construct interference layers. It is known that producing images by inkjet printing on PET implies that the film should be preheated to 70 °C to increase the rate of ink drying.31 In our case, we use the volatile ethanol additive, despite the fact that evaporation rate is much slower than that at 70 °C. The gradual evaporation of solvents allows TiO2 nanoparticles to settle and form a compact thin film (Figures 5−7). From a SEM image (Figure 2 and Figure S8), it is seen that TiO2 nanoparticles with a 5−20 nm diameter were closely stacked to form a compact thin film. The results of surface profilometry also show that the film had a curved surface with little roughness, indicating the compact aggregation of nanoparticles in the slow evaporating process. A more detailed view of the surface texture is provided by AFM for different layers (Figure 5). Individual grains making up larger flakes can be resolved. Superposition of layers does not result in a change in the structure of the surface due to “healing” the previous layer defects by the refilled sol. Continuity of layers confirms the absence of surface cracking that can occur upon fast drying and nonuniform application of material layers onto the surface. These data are in good agreement with the classical methods of deposition.32 SEM images were also used to obtain film thickness by cross section, Figures 6 and S9−S10. Thickness is the most important parameter influencing the physicochemical properties of optical thin layers. Multiple techniques were used to measure and verify the thickness of the printed samples, including optical investigation and SEM cross section as well, Figure 7. Because inkjet printing may not necessarily produce even-thickness layers because of the banding phenomenon or clogged nozzles, measuring the samples several times with different methods provides a means to verify the thickness uniformity. To determine compliance of the values with the thickness of the films of the reflection spectra, we used ultrahigh resolution scanning electron microscopy (SEM). The cut was directed perpendicular to the direction of the print head movement. In this way, we were able to observe the entire length of the cut and check for printing artifacts and general thickness uniformity. To distinguish the TiO2/fused silica interface and to determine the true thickness of the film, EDS analysis with an image mapping function was used. For the first and second layers of TiO2, SEM cross sections are shown in Figures S9− S10, respectively. Excellent evenness without any significant variations was observed, indicating the flawless printability of the used formulation and well-managed merging of the printed bands. The sample thickness was measured at 15 randomly
Figure 3. Schematic presentation of particle migration upon depositing colloidal TiO2 ink with a high surface charge.
Figure 4. Dependence of the viscosity and surface tension of the tested formulations on the ethanol dilution of the as-prepared titania hydrosol.
theories predict a stable drop formation in drop-on-demand systems when Z > 2,28 the other imply that a printable fluid should have a Z value between 1 and 10.29 It is also known that the viscosity of the fluid and its printing ability governs the lower limit, whereas the upper limit is determined by the point at which multiple drops are formed instead of a single droplet.30 In our study, the Z values were in the range from 4 to 9, which is in full agreement with the current theories. These data allow us to conclude that the concentration of selected printable ink 3081
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Figure 5. AFM images of an inkjet TiO2 film with several layers.
Figure 6. SEM cross section images of the studied TiO2 samples with EDS image mapping visualization (red line, Ti; blue line , Si; green line, O).
selected spots along the cut. According to the data obtained, Figure 7, high uniformity of film thickness and consistency between various dimensions is achieved. Besides, Figures 6 and S9−S10 clearly demonstrate that the film thickness has the same perimeter regardless of the number of applications. It is also clearly visible that the resulting films have a firm contact with the substrate surface. This is due to the sol-gel transition of TiO2 ink during drying and condensation of the sol into a dense layer, Figure 10. The size of the particles not exceeding 20 nm allows to perform deposition of inks with high penetrating ability. Mechanical Hardness. The study on mechanical properties of TiO2 inkjet coatings on the PET film surface was performed using a texture analyzer (see Mechanical Hardness in Supporting Information section I, Figure S11, and Figure 8). It is known that microhardness of films deposited from solutions is a key indicator because the use of soft chemistry usually leads to loss of mechanical hardness due to the formation of a high porosity layer. Furthermore, inkjet printing on nonporous surfaces, such as glass and polymers, causes a number of difficulties, not only due to the coalescence of droplets, but also due to the low adhesion of the dry film to the substrate. In our case, a 2-fold decrease in the mechanical hardness (for the seventh layer relative to the first one) is due to the porousity of each layer of the particles themselves and, as
Figure 7. Comparative thickness analysis by refractive measurements data (blue line) and SEM cross section measurements (green line). The average thickness of a single layer is estimated to be 85 nm.
a consequence, a decrease in the gradient of mechanical hardness with increasing layer thickness. In addition, layer-bylayer deposition can promote deformation of the previous layers, complicating the process of producing hierarchical structures. To increase the mechanical hardness, heat treatment at 300−500 °C is most commonly used, increasing the degree of crystallinity and initiating the interfacial interactions with subsequent sintering. Obviously, the effect of such temperatures renders deposition on a polymeric substrate impossible. In our case, using the value of the friction coefficient depending on the number of deposited TiO2 layers, figreffig:friction, one can establish a correlation between the obtained values and other known substances,33 thereby classifying the mechanical properties of the material. The maximum value of the stiction coefficient corresponds to the first coated layer, reaching a value of 0.933 and 2-fold exceeding that for a pure PET film.34 However, when three or more layers are applied, mechanical 3082
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ings. At certain radiation wavelengths, a minimum in the reflection spectra is observed. This corresponds to destructive interference of light reflected from the air−film and film-fused silica interfaces. In this case, regardless of the film thickness, reflectance is determined only by the refractive indices of air and fused silica. The coefficient of reflection from the air-fused silica interface is shown in Figure 9 as a green curve and plotted using the expression R(λ) = [(n1 − n3(λ))/(n1 + n3(λ))]2
(1)
where n1 = 1 is the refractive index of air and n3(λ) is the refractive index of fused glass. Reflectance maxima in Figure 9 correspond to positive interference of the reflected light with certain wavelengths. It makes a film multicolor, and the number of colors depends on film thickness and angle of observation (see Figure S14). In this case, regardless of the film thickness, the reflection coefficients at these wavelengths are determined by the expression
Figure 8. Shear strength vs load time for TiO2 films with different thickness on PET (see more inSupporting Information section I).
hardness exceeds the critical value, complicating the further deposition. In our case, this complicates the process of producing interference solid films with high thickness. That is why production of green shades (>500 nm) is a very difficult task whose solution would be the use of a single-stage technology employing an ink with a higher TiO2 concentration, but such inks are characterized by low sedimentation stability and coalesce on the surface of a nonporous substrate via low hydrophilicity and high surface tension. Optical Properties. Despite the structural peculiarities of the films we produced, their optical properties appeared to be similar to those for calcined materials.35 In particular, assynthesized TiO2 ink nanoproducts may be classified as promising highly refractive dielectric coatings. Figure 9 shows
⎡ (n ·n (λ) − n 2(λ)) ⎤2 2 ⎥ R (λ ) = ⎢ 1 3 2 ⎣ (n1·n3(λ) + n2 (λ)) ⎦
(2)
where n2(λ) is the refractive index of the film at the same wavelengths. The experimental data on the reflection coefficients allow to plot the dispersion of the refractive index of the films, Figure 9. In case of black substrate for TiO2 films, one can achieve more contrast and bright for images, as we mentioned above. Indeed, for black substrate (not absolutely blackbody) the reflection R of light tends to zero. It means that the real part of refractive index of substrate approaches to that of air (n3 = n1, see eq 1). Therefore, if we put TiO2 film onto the black substrate, the reflection will increase from 0.2 up to 0.36 (see eq 2) for entire spectral region (see Figure S14). It makes the image brighter. On the other hand, minimum in reflection goes down from 0.04 up to zero; that supports the increase of contrast. Using the maximum reflection condition 2dn(λ) − λ/2 = 2m(λ/2), one can estimate the minimum film thickness d using the formula d = λ1λ2/2(λ1·n(λ2) − λ2·n(λ1)), where λ1(λ2) are the wavelengths corresponding to the reflectance maxima, and n(λ1) and n(λ2) are the refractive indices at these wavelengths for the thin film. The calculated values of film thickness depending on the number of sample are shown in Figure 7. Samples with the number of applications 1−2 have a thickness of less than 160, so it cannot be determined by this method. The refractive indices for TiO2 film obtained with use of eq 2 are presented in Figure 9. As can be seen from the data, nanocrystalline titania sol possesses a high RI, comparable to those for the calcined samples.35 In the entire visible range, this index does not fall below 1.85, indicating the prospects of its use as a substitute for organic refractive polymers,36 given the high uniformity of deposition in the sol-gel transition process. Inkjet Printing of Interference Solid Thin Films. Printing was repeated several times to obtain different overall thicknesses of titania layers. Each layer was completely dried after printing so that the following layer was printed in the “wet-to-dry” manner. The formation of the color image occurred during the condensation of the ink on the substrate surface, Figures 3 and 11. Evaporation of the solvent and gelation of the nanocrystalline TiO2 sol with a preset layer thickness promoted the control of light by interference. This process is schematically shown in Figure 10. As seen from the figures, the lack of heating for fixing the ink positively affected
Figure 9. Dispersion of refractive index and reflectance at normal incidence of the optical radiation onto a TiO2 film on fused silica vs wavelength.
the dependence of reflectance at normal incidence of the optical radiation onto the homogeneous film placed between two dielectric media (air and a 2 mm plate of fused silica) vs wavelength. The optical properties of the produced samples (Figure S12) were measured on the installation shown in Figure S13 (see highly refractive TiO2 coatings characterization in Supporting Information section J). Basically, color for films is going from light interference, which is provided by the thickness of film and different refractive indices of surround3083
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corresponds to the thickness of the applied layer, which in this case was regulated by the number of deposited layers, Figure 11. In this work, we managed to produce for the first time a color image using colorless ink and an inkjet printer. Using this method, one can produce any image colored in the entire visible range. Some examples are shown in Figure 12. The uniqueness of the described technology, used materials, and images produced by a new technique is due to the fact that a conventional and inexpensive desktop office printer was used for deposition. The possibility to control the rheological properties of TiO2 colloids and management of the sol-gel transition are provided by a wide range of adjusted properties required for the production of ink (see Methods section). Nanocrystalline TiO2 sol with an amorphous phase content of less than 5% with respect to the crystalline one (Figure S7) promotes relevant optical properties. Condensation and physical cross-linking to the surface of unmodified PET paper facilitated perfect adhesion of a TiO2 film to the surface. This allowed us to obtain unique color images with adjustable coloristics, molded on flexible surfaces without heat treatment, Figure 12. The uniqueness of the demonstrated results consists in the fact that we have learned to control the inkjet printing to form nanostructures with high accuracy. Achieving this was made possible by using a fundamentally different approach to fixing the “ink” on a nonporous substrate, as well as by optimizing the synthesis of titania-based colloids. Protonation of the surface of particles contributed to an increase in the stability of the ink and a shift in the phase sol-gel transition upon drying layers with high RI. As a result, inkjet large-scale optical nanostructures are produced for the first time. These data are the basis for using soft chemistry to create quantum communication objects and an effective platform for the transport of photons in the future. High-precision coating and optical characteristics of TiO2 layers can be the basis for planar waveguides, camouflaging microembossed paper, as well as the formation of wide-angle photon-induced panel as a basis for the creation of a supercomputer operating by the photon/signal principle. Enhancing the possibilities of inkjet printing, we open a new scientific field which has huge practical importance.
Figure 10. Visualization of producing a high RI thin solid film after depositing colloidal ink on PET.
the preservation of the morphology of the applied layer. Given the low concentration of solids in the ink, we were able to carry out high-precision positioning of titania nanostructures with high RI. After printing, an even wet layer formed on the film surface Figure 10. After completely drying the layer, the second application, in a “wet-to-dry” manner, was carried out in several stages to yield the specified color, Figures 11 and 12. The figure
Figure 11. Scheme of layer-by-layer inkjet printing to produce a full color image. Strips and discoloration caused in a single layer depend on inkjet characteristics into nonporous surfaces associated with drying and subsequent application of a small deviation from the place printing.
CONCLUSIONS In this paper, we report the first application of nanocrystalline sol-gel systems for the production of controlled interference in thin films using colorless ink based on titanium dioxide colloids. Adjustable multilayer printing of highly refractive layers allowed us to create optical nanostructures on the polymer surface. These nanostructures provide the light interference, which results in visual coloration. The presence of a highly refractive
clearly shows that the image printed by this method has multiple colors with a high degree of detail. The coloration is caused by the emergence of reflected light waves formed at the interface of two materials with different refractive indices, TiO2−air and TiO2−PET. Interfering with each other, they form a light wave complementary to the layer thickness, which caused this phenomenon, Figure 1. Thus, the shade
Figure 12. Examples of color images produced using interference in thin solid films on fused silica or PET. (a), (b), (c) Printed with fresh TiO2 ink and (d) 100th copy of image. 3084
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ACS Nano layer enclosed between the air and polymeric substrate, both with low RI, allows to extract the light reflection complementary in wavelength to the thickness of this layer. Given the number of colors in the visible spectrum, the number of shades of the structures applied by an inkjet printer is virtually unlimited. However, at this stage of the research, no clear red colors have been obtained with this interference approach and the color prints produced by this method retain the intended coloration only within a narrow range of viewing angles. Nevertheless, unique optical, morphological, and textural properties of the TiO2 interference thin films allowed one to put these advantages into practice. In this paper, we showed the possibility of printing color text and color images of any shape. Furthermore, we reported the first “green” ink, which is safe for the ecosystem and does not fade from exposure to UV. In this regard, we expected that these data will not only lead to the creation of new color printing technology but will also help preserve our planet. We hope that the presented technology will enhance the application of inkjet printing for the deposition of optical structures and nanostructures.
Materials, calculations of Z numbers and rheological properties, DLS measurements, inkjet printing information, inkjet printing fabrication, SAED pattern, XRD, SEM images, mechanical hardness, characterization of coatings, transmittance spectra, and additional characterization. (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Russian Government, Ministry of Education (research was made possible due to financing provided to the customer from the federal budget aimed at maximizing customer’s competitive advantage among world’s leading educational centers) and internal ITMO University grant for young scientists. The authors are grateful to David Avnir for comprehensive support and essential advice, to Noam Ralbag and Nano Center Huji for research assistance and carrying out high resolution SEM measurement.
METHODS Synthesis of a TiO2 Sol. To produce a sol, two solutions were prepared. For the first solution, 16 mL of titanium isopropoxide and 12 mL of 2-propanol were taken. For the preparation of the second solution, 0.7 mL of nitric acid was added to 100 mL of water, and the mixture was heated to 70 °C, after which the first solution was slowly added to the second one under stirring. The resulting mixture was maintained for 1 h at 80 °C and then hermetically covered by a film and kept for 1−2 weeks at room temperature and under stirring. The complete outline for synthesis is shown in Figure S1. Synthesis of TiO2 Inks. The synthesized sol solution was at first evaporated in a rotary evaporator under reduced pressure at 50 °C to bring pure TiO2 to a solid phase concentration of no less than 8 wt %. To change the surface tension and viscosity, the aqueous solution of the sol was mixed with ethanol, and a series of printing formulation candidates was prepared and denoted, Figure 4. The resulting solution was homogenized for at least 12 days. The main rheological properties of the ink are shown in Table S1. Analyzing the data obtained, inkjetready colloidal systems with the highest stability were identified. According to a plot of hydrodynamic radius of the particles and zeta potential vs ethanol concentration in the original TiO2 sol, Figure S2, the ink containing 70% of ethanol was considered an optimal colloid for inkjet printing. It should be noted that the stability of the colloidal solution is greatly reduced by adding more than 70% of ethanol. This is due to the fact that ethanol modifies the structure of the electrical double layer of TiO2 particles, sharply decreasing their stability. Interference Inkjet Printing. For inkjet printing of color images, a polyethylene (PET) film with A4 size and a thickness of 1.5 μm was used. Contrasting of the image was achieved by using a black film (black color polyethylene terephthalate film, 105 μm thickness, A4 size). Printing was carried out using the black cartridge, which was preliminarily filled with 8 mL of TiO2 ink (Figure S3). To print color images by a three-stage procedure (see information in Supporting Information section E, Inkjet Printing Fabrication), we used a high print quality setting that provides a 2-fold increase in the application of ink. To analyze the structure of the layers, inkjet printing with medium quality was employed, providing application of highly refractive layers with increased precision, but with a smaller amount of ink. Specially prepared files with camouflaged areas were used for printing, as shown in Figure S4.
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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/acsnano.5b06074. 3085
DOI: 10.1021/acsnano.5b06074 ACS Nano 2016, 10, 3078−3086
Article
ACS Nano
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DOI: 10.1021/acsnano.5b06074 ACS Nano 2016, 10, 3078−3086
