Engineering Angle‑Dependent Structural Color Patterns with Aligned ZnO Nanofibers

Background

Structural color offers advantages over conventional pigments, including eco‑friendliness, resistance to photochemical degradation, and the ability to generate angle‑dependent patterns that are impossible with traditional dyes. These properties have attracted interest across textiles, paints, cosmetics, security printing, and sensing technologies. Recent work shows that zinc oxide (ZnO) nanostructures exhibit color through quasi‑ordered scattering, which is governed by the size and spacing of the nanostructures.

Quasi‑ordered scattering requires a seed layer that facilitates hydrothermal growth. The seed layer defines the pattern geometry, and because hydrothermal synthesis occurs in aqueous solution at 40–80 °C, the growth region is dictated by the seed layer’s layout. To fabricate optical switching patterns, an aligned nanofiber seed layer is essential. Electrospinning, the most common method for producing nanofibers, can be adapted to align fibers in a floating state between parallel electrodes, enabling the transfer of an oriented seed layer to a substrate. This approach avoids the complexity and cost of photolithography‑based patterning.

ZnO is ideal for such patterns due to its high refractive index (n = 2.0034) and versatile synthesis routes. The method presented here can generate visually striking patterns or be adapted for gas‑sensing and anti‑tamper applications.

Experimental Methods

Materials

Polyvinylpyrrolidone (PVP, MW = 1,300,000) from Alfa Aesar; ammonia (28–30 % v/v), zinc chloride, zinc nitrate hexahydrate from Junsei Chemical; hydrochloric acid and N,N‑dimethylformamide (DMF) from Sigma‑Aldrich. All reagents were used as‑received.

Electrospinning Conditions

Electrospinning was performed at 25 °C and 15–20 % relative humidity. A DMF solution containing 500 mM Zn(NO₃)₂ and 0.2 g mL⁻¹ PVP was used. A 50 mm tip‑collector gap and 6.5 kV applied voltage produced aligned fibers between parallel aluminum electrodes (3 cm × 2 cm). The aligned fibers were transferred onto glass or silicon substrates.

ZnO Nanostructure Fabrication

The electrospun nanofibers were heat‑treated at 500 °C to decompose PVP and form a ZnO seed layer. Hydrothermal growth was conducted in 10 mM ZnCl₂ solution, heated to 40–80 °C. Ammonia (5 µL mL⁻¹) was added to raise the pH above 10, triggering Zn²⁺ precipitation and ZnO nucleation. Growth time was varied to control nanostructure dimensions.

Patterning of ZnO Microwires

Masking tape patterned with a paper cutter (Silhouette Cameo) defined the hydrothermal growth area. The tape was removed after growth to reveal the structural color pattern.

Characterization

Scanning electron microscopy (SEM) was performed with a TESCAN LYRA 3 XMH. Optical microscopy (Nikon D800) with a white LED illuminated the samples.

Replication Using PDMS

PDMS (base:curing agent = 10:1) was degassed in a vacuum chamber for 1 h, poured over the ZnO master, and cured at 65 °C for 1 h. The resulting PDMS replica retained the nanostructure morphology while remaining flexible and transparent.

Results and Discussion

Aligned nanofibers were collected in a vertical orientation between parallel electrodes (Fig. 1a), transferred to the substrate, and heat‑treated to form a ZnO seed layer (Fig. 1b). Hydrothermal growth proceeded only within the masked region (Fig. 1c), producing the desired structural color pattern. Subsequent removal of the tape yielded the final design (Fig. 1d). Additional masking and growth steps enable the creation of complex patterns.

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Schematic illustration of the aligned ZnO structural color fabrication process. a Electrospun nanofibers collected between parallel electrodes and transferred to the substrate. b Heat treatment at 500 °C removes the polymer and forms the seed layer. c Masking and hydrothermal growth create the pattern. d Tape removal reveals the final structure. (Additional masking and growth steps allow more intricate designs)

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Figure 2 shows how the structural color evolves with hydrothermal growth time. Longer growth increases microwire thickness, shifting the optical response. The pattern remained reproducible for each synthesis time, and the masked region confined the growth precisely. Randomly bright patterns were achieved by immersing the seed layer in the growth solution without masking, producing a uniform distribution of colors (Fig. 2b). SEM images confirm the diverse microwire dimensions corresponding to the color variation.

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a Change of structural color as a function of synthesis time. b Optical and SEM images of nanofibers illustrating the color variation achieved by random growth times.

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Figure 3 demonstrates the scalability of the approach. A ZnO pattern on glass was replicated in PDMS (Fig. 3A and 3A’). The PDMS replica preserved the nanostructure geometry, and the pattern survived repeated (10×) replication cycles, confirming robustness. Additional hydrothermal growth on the patterned area produced distinct colors (Fig. 3B and 3B’), showing that post‑growth modifications are feasible. SEM confirms that increased microwire dimensions alter the quasi‑ordered scattering and thus the color.

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a Structural color pattern of an angel and its duplication after 1× (A’) and 10× (A”) PDMS replication. b Pattern with two distinct colors obtained by varying synthesis time (b’) and its optical micrograph. c, d SEM images of the outer (yellow) and inner (green) regions of (b’).

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The structural color’s angle dependence was verified on both reflecting (silicon wafer) and transparent (glass) substrates (Fig. 4a,b). The color shifted with the incidence angle, and the aligned nanofibers produced an optical switching effect: when light strikes perpendicular to the fiber alignment, the pattern is bright; when light is parallel, the pattern becomes nearly invisible. This directional visibility arises from the limited scattering directions for parallel incidence versus the broad scattering for perpendicular incidence.

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Change of color of a structural pattern as a function of incidence angle on a a reflecting substrate and b transparent substrate. c Effect of light orientation relative to the nanofiber alignment: perpendicular (left) vs parallel (right).

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Conclusion

We successfully fabricated optical switching patterns by aligning ZnO nanofibers and controlling hydrothermal growth time to tune structural color. The method leverages electrospinning for precise seed layer orientation and allows independent adjustment of fiber alignment and nanostructure size. PDMS replication confirms the process’s scalability, enabling large‑area, angle‑dependent patterns suitable for sensors, anti‑tamper tags, and decorative applications.

Abbreviations

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DI

Deionized water

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PDMS

Polydimethylsiloxane

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PVP

Polyvinylpyrrolidone

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SEM

Scanning electron microscopy

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ZnO

Zinc oxide

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