Study on the formation of three-dimensionally ordered SiO 2 photonic crystals on polyester fabrics by vertical deposition self-assembly

Abstract

In order to form a well-ordered structure of SiO₂ photonic crystals on polyester fabrics with fewer defects, a series of influential factors such as particle size and monodispersity of colloidal microspheres, evaporation temperature, relative humidity, mass fraction of colloidal microspheres and solvent in vertical deposition assembly were deeply studied, and the complexities of the self-assembly process of colloidal microspheres on polyester fabric substrates were revealed. In different self-assembly conditions, the quality of SiO₂ photonic crystals on polyester fabric substrate was investigated by field emission scanning electron microscopy for the morphology of the crystal structures and by spectrometer measurements for their stop band intensities. Under the conditions of suitable sizes and monodispersity (PDI ≤ 0.08) of colloidal microspheres, the high-quality SiO₂ photonic crystals with face centered cubic (fcc) array on polyester fabrics were produced at a low evaporation rate by adopting relative humidity of about 60% with a medium mass fraction of 1.0–1.5% SiO₂ microspheres at 25℃ with ethanol as the solvent.

Keywords

photonic crystals structural colors polyester fabrics vertical deposition

Different from the ordinary coloration mechanisms of pigments and dyes in dyeing and printing processes,¹ structural colors arise from the physical interaction of light with biological nanostructures,^2,3 such as butterfly wings and feathers of peacocks and other birds.^4–8 In recent decades, structural colors have attracted much attention due to their characteristics of high brightness, high saturation, permanent color and iridescent effect (color changes with the viewing angle).^9,10

Photonic crystals are considered as a kind of dielectric material with highly periodic arrays, which confine and control the propagation of light due to the existence of the photonic band gap, a band of frequency where light propagation in photonic crystals is forbidden.¹¹ It is thought that photonic crystals display vivid colors because the specific wavelengths of the visible light that are not allowed to propagate in the photonic crystal structure will be selectively reflected; however, the dyes and pigments can produce the colors because they selectively absorb and reflect certain wavelengths of visible light. As a typical source of structural colors, photonic crystals are widely used in the application of inkless printing, reflective flat display, gas sensing, photonic crystal sensors, photonic papers and light-emission modulators.^12–18 However, there are few reports of relative research in textile coloration.

As a kind of popular structural unit, silica colloidal microspheres have been applied in fabricating SiO₂ photonic crystals on solid substrates, such as glass, silicon and silicon nitride, by gravitational sedimentation¹⁹ and vertical deposition.^20–23 Very distinct from those smooth and solid substrates, textile fabrics made of the textile fibers and yarns are soft, porous and fluctuant. Therefore, it is considered that the self-assembly processes and possible mechanisms of SiO₂ colloidal microspheres on textile fabrics to construct photonic crystals are impossible, the same as on solid substrates, while there are few reports of related research.

In our previous study, a series of monodispersed polystyrene and silica colloidal microspheres with high sphericity were prepared and the relative photonic crystals on polyester fabrics were formed by gravitational sedimentation and vertical deposition.^24,25 However, it cannot be denied that high-quality photonic crystals on textile substrates required for practical structural colors applications are greatly dependent on proper self-assembly conditions. During the process of colloidal self-assembly, colloidal crystal growth is vastly affected by the temperature, relative humidity (RH), particle size, concentration of the suspension particle, solvent and surface characteristic of substrates. However, the detailed mechanisms and effects of each influential factor mentioned above on the photonic crystal formation on textile substrates still are not clearly understood.

Compared to gravitational sedimentation, the vertical deposition is regarded as a more simple and effective self-assembly method to fabricate three-dimensional (3D) colloidal crystals on textile fabrics. In particular, in view of our previous research experience, vertical deposition is able to achieve a double-sided coloration effect and produce thinner colloidal crystal film than gravitational sedimentation to keep a soft fabric handle.²⁵

In this paper, in order to form a well-ordered structure of photonic crystals on textiles with fewer defects, a series of influential factors in vertical deposition assembly were deeply studied, and the complexities of the self-assembly process of colloidal microspheres on textile substrates were revealed. The woven polyester fabrics not only have smooth fiber appearance, low moisture property, high dimensional stability and good resistance to heat and most chemicals, but also have a relatively flat and compact surface compared to many other textile fabrics, especially natural fiber products. Therefore, to reduce other factors in our experiments, woven polyester fabric was chosen as a preferable textile substrate. In addition, silica colloidal microsphere was selected as a research object to construct photonic crystals attributed to its high hardness, good optical transparency, chemical inertness, biological compatibility, etc.²³

Experimental details

Synthesis of silica particle and its characterization

Monodispersed SiO₂ microspheres with diameter of 200–320 nm were synthesized through the hydrolysis of tetraethoxysilane (TEOS, Si(OC₂H₅)₄, 98%, AR) in water-ethanol mixed solution with ammonia as a catalyst. The relative reactions were carried out in a three-necked round-bottom flask. Firstly, ammonia, water and ethanol were added to the three-necked round-bottom flask. After 10 minutes, accompanied by magnetic stirring (350 r/min), TEOS and ethanol with a volume ratio of 1:4 were put to the three-necked round-bottom flask at 25℃ and the mixture were stirred for 20 h with a magnetic stirrer (350 r/min). Taking the monodispersed SiO₂ microspheres of 280 nm as a sample, the relevant concentrations of ammonia, water and TEOS were 0.31, 6.22 and 0.18 mol/L, respectively. In general, the larger or smaller SiO₂ microspheres can be prepared by increasing or reducing the concentrations of the above reaction materials in a certain range. The prepared SiO₂ microspheres with particle dispersion index (PDI) below 5% were obtained under the strict control of reaction conditions. The size and the size distribution of microspheres were calculated from dynamic light scattering on a Malvern laser particle sizes analyzer (Nano-S, Malvern, England).

Fabrication of SiO₂ photonic crystals on polyester fabrics by vertical deposition self-assembly

In our study, black plain woven polyester fabrics were used as substrate material to construct SiO₂ photonic crystals by vertical deposition self-assembly. Compared to other base colors, black fabrics had remarkable ability to absorb the transmitted light and scattered light outside the photonic band gap, and enhance the chroma of structural colors markedly from photonic crystals.²⁶ Before self-assembly, the black plain woven polyester fabrics were pre-treated by ultrasonication in deionized water to ensure a clean surface. Different concentrations of SiO₂ colloidal microsphere suspensions were diluted with ethanol, water or their mixtures in various ratios and the polyester woven fabrics were put in the above diluted microsphere suspensions. Finally, the polyester fabrics with the diluted microsphere suspensions were located in a vacuum drying oven at different temperatures of 25℃, 35℃, 45℃ with RH of 30%, 45%, 60% for more than 72 h, dependent on various deposition rates of colloidal microspheres.²⁷ With the evaporation of the solvent, a solid structure with well-ordered SiO₂ photonic crystals on the polyester fabrics was obtained.

Surface morphology and structural colors

The surface morphology of the SiO₂ photonic crystal structure on polyester fabrics was observed by field emission scanning electron microscopy (FESEM, ALTRA55, Germany) and a 3D video microscope (KH-7700, HIROX, Japan). All FESEM images were collected with an electron gun with accelerating voltage of 1.0 kV. The structural colors of SiO₂ photonic crystals on fabrics were observed by a digital camera (EOS600D, Canon, Japan) at normal incidence. The reflectance spectra of structural colors on fabrics were recorded in the range of 400–700 nm on an ultraviolet-visible (UV-Vis) spectrometer (UV-2600, SHIMADZU, Japan) at normal incidence.

Results and discussion

Effects of colloidal microsphere sizes

Figure 1 shows the different structural colors of SiO₂ photonic crystals on polyester fabrics with various colloidal microsphere sizes by vertical deposition. In view of our previous papers, it is confirmed that the specific structural colors could be explained by Bragg’s law taking into account Snell’s law of refraction:^28–30

m λ_{max} = 2 d_{hkl} \sqrt{n_{avg}^{2} - \sin θ^{2}}

(1)

where λ_max is the wavelength of the reflectance peak maximum (i.e. the position of the photonic band gap), d_hkl is the interplanar spacing between hkl planes, m is the order of the Bragg diffraction, n_avg is the average refractive index of the photonic structure and θ is the angle between the incident light and the surface normal of the sample (the same as the viewing angle).

Figure 1.

The structural colors of SiO₂ photonic crystals on polyester fabrics fabricated by vertical deposition with different sizes of SiO₂ microspheres: (a) 312 nm; (b) 287 nm; (c) 255 nm; (d) 240 nm; (e) 223 nm; (f) 215 nm; (g) 200 nm.

Based on the above law, it is calculated that SiO₂ colloidal microspheres with different diameters in the range of 180–380 nm could have been used to fabricate SiO₂ photonic crystals, displaying variable structural colors at the visible light range. However, in our experiments, it is actually found that when the diameters of SiO₂ colloidal microspheres are larger than 330 nm, the fabricated SiO₂ photonic crystals on polyester fabrics became extremely irregular due to rapid depositions of SiO₂ microspheres in colloidal suspension, which inevitably leads to pale structural colors, even the absence of bright red structural colors.³¹ However, that does not mean that the smaller the diameters of SiO₂ colloidal microspheres, the brighter the structural colors of SiO₂ photonic crystals on polyester fabrics. It is noticed that when the diameters of SiO₂ colloidal microspheres are less than 200 nm, such as 180 nm, although the well-ordered SiO₂ photonic crystal structure on polyester fabrics can be fabricated as well, the corresponding structural colors is difficult to capture with the naked eye, because the relevant photonic band gap positions are located near the UV region. In particular, the diameters of SiO₂ microspheres calculated directly from the Bragg’s law are smaller than those determined by Dynamic Light Scattering by approximately 10–15% due to the shrinkage of SiO₂ microspheres on drying in the colloidal crystals.^32,33 Therefore, in order to obtain bright structural colors on polyester fabrics, it is vital to accurately control the size of SiO₂ colloidal microspheres in the range of 200–320 nm.

Effects of microsphere monodispersity

Monodispersity can be used to define the state of uniformity in molecular weight of all molecules of a substance or of a polymer system that is homogeneous in molecular weight. In our study, the PDI characterized by dynamic light scattering on a Malvern laser particle size analyzer was taken as an evaluation index to describe monodispersity of the prepared SiO₂ colloidal microspheres. In our study, the prepared SiO₂ colloidal microspheres with similar diameters of different PDI values varying from 0.02 to 0.40 were used to fabricate photonic crystals on polyester fabrics. It is found that the lower the PDI values, the brighter the resultant structural colors, as show in Figure 2.

Figure 2.

Different structural colors on polyester fabrics with SiO₂ colloidal microspheres of the similar sizes in different monodispersity values: (a) 256 nm, PDI = 0.02; (b) 250 nm, PDI = 0.07; (c) 246 nm, PDI = 0.15; (d) 244 nm, PDI = 0.40.

In the same way, as shown in Figure 3, the higher the PDI values, the more inhomogeneous the molecular weight of SiO₂ colloidal microspheres, or to say, the more nonuniform sizes of SiO₂ colloidal microspheres they have, and the more irregular photonic crystals on polyester fabrics might be obtained, as shown in Figures 3(c) and (d). In general, the fabrication procedure of colloidal self-assembly starts with the synthesis of monodisperse colloidal microspheres, followed by microsphere suspension, self-assembling by sedimentation, and then thermal treatment at the end. It is thought that at far higher PDI values, the SiO₂ microspheres in colloidal suspension easily become instable and inclined to aggregate together in the process of self-assembly, which might highly affect the ultimate arrangement of the photonic crystal structure on fabric substrates. So, it is convincing that the monodispersity of SiO₂ colloidal microspheres have substantial influences on the resultant photonic crystals and the relevant structural colors on polyester fabrics. In our study, the optimal PDI values of SiO₂ colloidal microspheres are considered to be less than 0.08, even 0.05, which is in accordance with the previous study.^34,35

Figure 3.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics with SiO₂ colloidal microspheres of similar sizes with different monodispersity : (a) 256 nm, PDI = 0.02; (b) 250 nm, PDI = 0.07; (c) 246 nm, PDI = 0.15; (d) 244 nm, PDI = 0.40.

Effects of mass fractions of colloidal microspheres

It has to be admitted that both the diameters and monodispersity of SiO₂ colloidal microspheres are mainly determined by the synthesis parameters, such as the concentrations of TEOS, NH₃ and H₂O in the Stöber method. In addition to the synthesis process, the following self-assembly of SiO₂ colloidal microspheres has vital influences on the quality of photonic crystals as well. To make an analogy, the synthesis process could determine the characteristics of the prepared SiO₂ colloidal microspheres, which is regarded as the core factor to fabricate high-quality SiO₂ photonic crystals on textile fabrics, and the self-assembly parameters (such as RH, evaporation temperature, mass fraction of colloidal microspheres, solvent, etc.) are considered to be the external factors on the fabrication of SiO₂ photonic crystals on fabric substrates.

Figure 4 shows the assembled polyester fabric samples in SiO₂ colloidal suspensions with different mass fractions of microspheres under the same RH and evaporation temperature conditions. As shown in Figure 4, it is easily found that compared to (a) and (d), the fabric samples of (b) and (c) display the brighter and more uniform structural colors, especially sample (c), which also can be identified by their corresponding reflection spectra data in Figure 4(e) and Table 1. In previous studies, there were many indexes originated from reflectance spectra that could be used to evaluate the quality photonic crystals, such as peak width,³⁶ peak area and peak height.³⁷ In Figure 4(e) and Table 1, it can be seen that there are some differences in the reflectance spectra of those four fabric samples assembled with various mass fractions of SiO₂ colloidal microspheres, especially in their peak locations, peak areas and heights of reflection peak, which are inconsistent with the different structural color appearances in Figure 4(a)–(d). From a spectral viewpoint, the intensity of a spectrum is defined as a peak height or a peak area for a spectrum analysis. In our study, the photonic band gap intensity is defined as either a peak height, that is, between points A and B for the 1.5% case, or a peak area for 0.5% case. As shown in Table 1, it is noticed that both the peak area and peak height of sample (c) produced at the mass fraction of 1.5% SiO₂ colloidal microspheres are far higher than others, namely, this sample has the strongest intensity of the photonic band gap.

Figure 4.

Influence of the mass fractions of SiO₂ microspheres, (a) 0.5%; (b) 1.0%; (c) 1.5%; (d) 2.5%, on SiO₂ photonic crystals, and the corresponding reflectance spectra of SiO₂ photonic crystals (e).

Table 1.

The spectra data of SiO₂ photonic crystals on polyester fabrics with various mass fractions of SiO₂ microspheres

Index	Mass fractions of SiO₂ colloidal microspheres (%)
Index	0.5	1.0	1.5	2.5
Reflection peak (nm)	568.0	583.0	578.0	572.0
Peak width (nm)	185.0	179.0	182.0	184.0
Peak area	1121.2	1284.6	1605.2	1283.5
Peak height	12.0	18.2	22.0	12.1

In order to further study the effects of mass fractions of SiO₂ colloidal microspheres on fabricating the photonic crystal structure, the relevant FESEM images of those assembled fabric samples are shown in Figure 5. It is found that the lower the mass fractions of SiO₂ colloidal microspheres, the more gaps on the assembled polyester fabrics can be observed. Specifically, when the polyester fabric is vertically inserted into SiO₂ microsphere emulsion, by active Brownian motion, a great deal of the SiO₂ colloidal microspheres would be absorbed on the surface of the fabric substrate, as same as on other solid substrates. However, textile fabrics are characterized by their porous, rough and fluctuant surfaces due to weaving ways and basic characteristic of the fibers, which are destined to have different self-assembly behaviors from solid substrates; therefore, most of the SiO₂ colloidal microspheres actually enter those gaps between fibers and yarns.

Figure 5.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics fabricated with different mass fractions of SiO₂ microspheres: (a) 0.5%; (b) 1.0%; (c) 1.5%; (d) 2.5%.

In Figure 5(a), it is easily noticed that there is a mass of defects on the surface of SiO₂ photonic crystals, such as vacancies and dislocations, most of which are obviously formed during the growth of the crystals. Comparing Figure 5(a) with Figures 5(b) and (c), it is noticed that after self-assembly, well-ordered photonic crystals on the polyester fabrics seem to be more easily formed at the relative higher mass fractions of SiO₂ microspheres in colloidal suspension. To verify this opinion, a series of experiments are designed and the corresponding results are illustrated in Figure 6.

Figure 6.

Photographs of SiO₂ photonic crystals on polyester fabrics with different mass fractions of SiO₂ microspheres, (a₁) 0.3%; (a₂) 0.5%; (a₃) 0.8%; (a₄) 1.1%; (a₅) 1.3%; (a₆) 1.5%; (a₇) 1.8%; (a₈) 2.0%, and the corresponding reflectance spectra of SiO₂ photonic crystals (b).

In our experiments, SiO₂ colloidal suspensions of eight different mass fractions in the range of 0.3–2.0% were chosen to assemble on polyester fabrics by vertical deposition; the images were taken by a digital camera and FESEM, and the relevant reflection spectra of each resultant fabric sample are shown in Figures 6 and 7. As shown in Figure 6(a1), it is observed that at the lowest mass fractions of SiO₂ colloidal microspheres in emulsion, after vertical deposition assembly, the lower half of the fabric substrate remained black but the top half displays uneven structural colors, which indicates that SiO₂ colloidal suspensions of 0.3% mass fractions is insufficient to complete the assembly on the whole fabric substrate. The similar result is also presented in the FESEM image of Figure 7(a1). As shown in Figure 7(a1), many gaps on the surface of assembled fabrics could be easily observed, especially at the crossing points between the filling yarns and wrap yarns, which means that during the assembly process SiO₂ colloidal microspheres are far less to fill in all of the gaps on polyester fabrics, seriously hindering the fabrication of SiO₂ photonic crystals on the fabric substrate. More importantly, from Figure 6(a1), it is also implied that in the vertical deposition the formation of SiO₂ photonic crystal structure is mainly due to capillary force with the evaporation of solvent in colloidal suspension from top to bottom on the fabric substrate, absolutely distinct from gravitational sedimentation self-assembly.³⁸

Figure 7.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics with different mass fraction of SiO₂ microspheres: (a₀) 0%; (a₁) 0.3%; (a₂) 0.5%; (a₃) 0.8%; (a₄) 1.1%; (a₅) 1.3%; (a₆) 1.5%; (a₇) 1.8%; (a₈) 2.0%.

With the increase of mass fractions from 0.3% to 1.5%, as shown in Figures 6(a1)–(a6), it is found that more and more fabric surface areas display structural colors, more uniform and brighter. To further observe the relevant FESEM images in Figure 7, just as expected, along with an increasing mass fractions of SiO₂ microspheres, fewer and fewer gaps occur on the surface of the assembled fabric samples and the criss-cross filling yarns and wrap yarns become less clear; in other words, in the early stage of self-assembly, SiO₂ colloidal microspheres gradually fill the gaps between fibers and yarns, slowly forming a relatively flat surface for the subsequent self-assembly. It is convincing that SiO₂ microspheres in colloidal suspension have to fill the gaps of the fabric substrate firstly and then stack photonic crystals on fabric substrates. Accordingly, as shown in Figure 5(c), more well-ordered photonic crystals on polyester fabrics were constructed at the mass fraction of 1.5% than other smaller mass fractions in Figures 5(a) and (b), which demonstrates that the mass fraction of 1.5% in colloidal suspension might be enough to complete the fabrication of SiO₂ photonic crystals on polyester fabrics in our study.

It is also found that excessive mass fractions of SiO₂ microspheres in suspension amount to nothing but to fabricate disordered photonic crystals and display dark and uneven structural colors on polyester fabrics, as shown in Figures 4(d), 5(d), 6(a7) and 6(a8). Compared to Figure 5(c), obviously, there are many defects, that is, vacancies, dislocations, grain boundaries and cracks formed during the growth of the crystals in Figure 5(d). It is speculated that much higher mass fractions of SiO₂ microspheres in suspension easily bring about the following problems during the self-assembly process. Firstly, the higher the mass fractions of SiO₂ microspheres in suspension, the more SiO₂ microspheres are migrated from suspension system to the surface of the fabric substrate. Therefore, it is nothing out of the ordinary that amounts of SiO₂ colloidal microspheres can aggregate on the surface of fabric substrate at much higher mass fractions of SiO₂ microspheres. However, in a short period of time, before those SiO₂ colloidal microspheres have a chance to form a compact crystalline structure of high order on the surface of the fabric substrate, other SiO₂ colloidal microspheres in suspension have transferred near to the fabric substrate. Therefore, the solvent evaporation time becomes so short due to the lower mass fractions of solvent in suspension that is not enough to make a dynamic balancing between the aggregation of SiO₂ microspheres on the substrate and their subsequent crystallization, giving rise to more defects on the fabricated photonic crystals. Then, during the assembly process, multiple SiO₂ microspheres tend to take up the same location in the photonic crystal structure, which readily induces vacancies and dislocations in some places. Finally, many sio₂ microspheres are inevitably inclined to increase the thickness of the photonic crystals on polyester fabrics, which easily generates more cracks on the surface of photonic crystals.

Effects of relative humidity

In our study, different RH conditions were adopted to fabricate SiO₂ photonic crystals on polyester fabric substrate in the closed chamber. Figure 8 shows the structural colors and their reflectance spectra of SiO₂ photonic crystals on polyester fabrics. It is known that RH has great impact on the evaporation rate of the solvent and the colloidal crystal formation.^39,40 At a higher RH in the drying chamber, the evaporation rate of solvent in suspension solution becomes slower, so the shrinkage process might be slowed, and the drying/shrinkage stresses were reduced and, hence, the number of cracks in photonic crystals could decrease. When controlling the RH at 60%, even up to 75%, the fabricated SiO₂ photonic crystals on polyester fabrics exhibit vivid structural colors, as shown in Figures 8(c) and (d), it is easy to distinguish the warp yarns and filling yarns on assembled fabrics through the photonic crystals of regular arrangement. On the contrary, a lower RH might promote a higher array growth rate with additional internal stress, which could lower the quality of the photonic crystals.^41,42 Figure 8(e) shows the reflectance spectra of the SiO₂ photonic crystals on polyester fabrics produced under different RH environments. The photonic band gaps of those samples appear around 535 nm for the cases of the particle sizes at 255 nm. Apparently, at RH of 60%, the related reflectance spectrum has a higher peak area and peak height than the others, which are verified in Table 2 as well. If we increase the RH up to 75%, although well-ordered SiO₂ photonic crystals on polyester fabrics are fabricated as well, the self-assembly time has to be extended to 7 days, or even 10 days, due to the low evaporation rate.

Figure 8.

Influence of relative humidity, (a) 30%; (b) 45%; (c) 60%; (d) 75%, on SiO₂ photonic crystals and the corresponding reflectance spectra of SiO₂ photonic crystals (e).

Table 2.

The spectra data of SiO₂ photonic crystals with variable relative humidity

Index	Relative humidity (%)
Index	30	45	60	75
Reflection peak (nm)	524.0	533.0	538.0	536
Peak width (nm)	212.0	219.0	206.0	201
Peak value	51.5	46.5	41.8	43.1
Peak area	1360.7	1883.8	1895.6	1831.5
Peak height	12.1	19.6	29.1	29.1

Figure 9 shows the morphology images of SiO₂ photonic crystals in different RH conditions. At lower RH, the arrangements of SiO₂ photonic crystals on polyester fabrics appear random and void, whereas the photonic crystal structure seems much ordered and dense with fewer defects produced in the higher RH-controlled environment, which is in accordance with the results of structural colors and their reflectance spectra presented in Figure 8. Therefore, it is confirmed that the evaporation rate of the solvent is an essential factor to vary the quality of the SiO₂ photonic crystals produced in different RH conditions.

Figure 9.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics with different relative humidities: (a) 30%; (b) 45%; (c) 60%; (d) 75%.

Effects of evaporation temperature

The solvent evaporation, further the capillary force and the transfer of SiO₂ colloidal microspheres through solvent flux, are directly dominated by the evaporation temperatures, which are of great importance in crystal growth.⁴³ In Figure 10, with the increasing evaporation temperatures, the produced structural colors on polyester fabrics become visibly dark and uneven. Although the higher temperatures can effectively promote both solvent evaporation and Brownian diffusion of the microspheres in suspension, it would not necessarily be good for the ordered arrangement of photonic crystal structure on polyester fabrics.⁴² The FESEM images in Figure 11 also verify our opinion. As shown in Figure 11, it is found that at higher evaporation temperatures, the arrangement of SiO₂ photonic crystals becomes more disordered, which directly has an adverse impact on the photonic band gap and the corresponding structural colors.⁴⁴ Similarly, the relevant reflection spectra data in Figure 10 and Table 3 demonstrate that the fabric sample assembled at 25℃ has overwhelming advantages in the peak area and peak height in the three reflection spectra, indicating a stronger photonic crystal band and better structural colors, which further confirm the above view.³⁷ In a word, it is preferable to control a relatively lower evaporation temperature at about 25℃ in vertical deposition self-assembly in our study.

Figure 10.

Influence of evaporation temperature, (a) 25℃; (b) 35℃; (c) 45℃, on SiO₂ photonic crystals and the corresponding reflectance spectra of SiO₂ photonic crystals (d).

Figure 11.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics with different evaporation temperatures (a) 25℃; (b) 35℃; (c) 45℃.

Table 3.

The spectra data of SiO₂ photonic crystals with the variation of evaporation temperatures

Index	Evaporation temperature (℃)
Index	25	35	45
Reflection peak (nm)	570	557	560
Peak width (nm)	190	191	192
Peak value	40.274	38.4	45.333
Peak area	1825.115	1295.118	1210.662
Peak height	29.922	11.829	12.441

Effects of solvents

It is pointed out that the solvent flow by the external forces is one of the major effects on the crystal growth system. Therefore, it is of interest to study the solvent effects on the crystal formation to improve the quality of the produced SiO₂ photonic crystal on polyester fabrics. In our study, ethanol, water and their mixtures of different ratios by volume were used as solvents in SiO₂ colloidal emulsion with tunable surface tension (γ), viscosity (η) and volatility.⁴⁵

As shown in Figure 12, it is noticed that both the resultant fabric samples fabricated by ethanol and water as the solvents, especially ethanol, present brighter and more uniform structural colors than other fabric samples using water/ethanol mixtures as the solvents, which is also in accordance with the morphology images in Figure 13, the former two in Figures 13(a) and (g) exhibiting much more compact and regular hexagonal array with fewer vacancies and dislocations than the others in Figures 13(b)–(f). Therefore, it is thought that water and ethanol appear to be suitable solvents to form high-quality SiO₂ photonic crystals on polyester fabrics. Among those mixture solvents, the SiO₂ photonic crystals on polyester fabrics fabricated from ethanol-rich solvents (like water/ethanol = 1:4) and the water-rich solvents (like water/ethanol = 4:1) show better structural colors and less cracks than others, especially 1:1 water/ethanol solvent.

Figure 12.

Water, ethanol and their mixtures of different ratios by volume (a) water; (b) water/ethanol = 4:1; (c) water/ethanol = 2:1; (d) water/ethanol = 1:1; (e) water/ethanol = 1:2; (f) water/ethanol = 1:4; (g) ethanol on the effects of SiO₂ photonic crystals and the corresponding reflectance spectra of SiO₂ photonic crystals (h).

Figure 13.

Field emission scanning electron microscopy images of SiO₂ photonic crystals on polyester fabrics with different solvents (a) water; (b) water/ethanol = 4:1; (c) water/ethanol = 2:1; (d) water/ethanol = 1:1; (e) water/ethanol = 1:2; (f) water/ethanol = 1:4; (g).

As we well know, water has long been regarded as a good solvent for almost all kinds of self-assembly processes. Compared with water, as a kind of organic solvent, ethanol has a much lower surface tension (γ), higher viscosity (η) and volatility. Specifically, at 25℃, the surface tension of water (71.97 mN m⁻¹) is over three times higher than that of ethanol (21.96 mN m⁻¹), and the viscosity of water (0.8937 mPa·s) is a little lower than that of ethanol (1.06 mPa·s).^41,46,47 It is thought that in vertical deposition self-assembly the solvent evaporation occurs in two stages: the first stage takes place immediately after SiO₂ colloidal suspension is spread on the fabric substrate to induce the formation of the SiO₂ colloidal crystal, during which the arrangement of the microspheres requires time for perfecting the order, and the second stage occurs in the drying process, during which the bulk residual solvent between the colloidal microspheres is removed and the capillary forces between microspheres compress the arrangement of the microspheres locally. In the first stage, the colloidal microspheres organize themselves to form a structure; the formation of stacking faults, amorphous structure and dislocations is possible in this stage. Thus, it can be expected that a good balance between the gravity, buoyancy and surface tension of the solvent may benefit the formation of high-quality SiO₂ colloidal crystals on polyester fabrics. In the second stage, the structure has already formed, although residual solvent is still present in the interstices of the microspheres. Crack formation often occurs during the second stage because of the loss of solvent from the interstitial spaces due to capillary stress.⁴⁸ Therefore, it is thought that ethanol has more advantages as a solvent in fabricating SiO₂ colloidal crystals due to its relatively fast evaporation and low surface tension, which is also evidenced by Table 4. In our opinion, due to the great differences in surface tension, viscosity and volatility, it is inevitable to increase the instability of the self-assembly process when using ethanol/water mixture suspensions as the solvent, such as the evaporation rates, interaction forces among colloidal microspheres, solvent and substrates, and even the assembly behaviors of colloidal microspheres. Therefore, it is easy to understand that the colloidal crystals derived from the water and ethanol suspensions are more homogeneous than those obtained from ethanol/water mixture suspensions.

Table 4.

The spectra data of SiO₂ photonic crystals on polyester fabrics with the variation of solvents

Solvent	Reflection peak (nm)	Peak width (nm)	Peak value	Peak area	Peak height
Water	537	198	37.596	1668.104	23.554
Water/ethanol = 4:1	537	174	69.023	777.283	11.734
Water/ethanol = 2:1	535	200	62.836	856.003	12.068
Water/ethanol = 1:1	539	201	60.335	567.411	6.63
Water/ethanol = 1:2	535	185	33.167	661.109	4.995
Water/ethanol = 1:4	541	187	43.749	1708.502	19.45
Ethanol	540	177	38.047	1785.892	25.674

Conclusions

In summary, the vertical deposition self-assembly was applied to form SiO₂ photonic crystals on polyester fabrics to endow uniform and brilliant structural colors. The formation of high-quality SiO₂ photonic crystals is closely correlated with many self-assembly factors, such as the particle sizes and monodispersity of the prepared colloidal microsphere and the mass fractions of the colloidal microsphere and solvents in suspension and the RH and evaporation temperatures in the assembly process. The size of SiO₂ colloidal microspheres can largely determine the structural colors of the fabricated photonic crystals, and sizes higher than 320 nm or lower than 200 nm are proved to be inappropriate. The monodispersity of the colloidal microsphere plays an important role in the surface morphology and inner structure of the resultant colloidal crystals, and the optimal PDI values of SiO₂ colloidal microspheres are considered to be less than 0.08, or even 0.05. In the process of assembly, the mass fraction of SiO₂ microspheres is in the range of 1.0–1.5%, RH about 60%, evaporation temperature near 25℃ and using ethanol as a solvent were though be beneficial to control the deposition rate, reduce the crystal defects and construct well-ordered photonic crystal on polyester fabrics. It is believed that our study will broaden the applications of silica in textile fields, especially in developing anti-counterfeit textile labels and color-changing printed fabrics. However, it is still in its infancy with many challenges, such as color fastness and fabric handle, to be met.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant Nos. 51403188 and 51073142); Zhejiang Provincial Natural Science Foundation of China (Grant No. LY13E030004); Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20123318120005); Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Fund for the Excellent Doctoral Thesis Program of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology in Zhejiang Sci-Tech University (Grant No. 11110132271412); the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (ZYG2015011) and “521” Talent Project of Zhejiang Sci-Tech University.

References

McPhedran

Nicorovici

McKenzie

. Structural colors through photonic crystals. Physica B 2003; 338: 182–185.

Zhang

. Structural colored fibers based on photonic crystal structures by colloidal assembly. J Fiber Bioeng Inform 2010; 2: 222–226.

Kinoshita

Yoshioka

. Structural colors in nature: the role of regularity and irregularity in the structure. ChemPhysChem 2005; 6: 1442–1459.

von Freymann

Kitaev

Lotsch

. Bottom-up assembly of photonic crystals. Chem Soc Rev 2013; 42: 2528–2554.

Yoshioka

Kinoshita

. Effect of macroscopic structure in iridescent color of the peacock feathers. Forma-Tokyo 2002; 17: 169–181.

Liu

Wang

Jiang

. Structural coloration and optical effects in the wings of Papilio peranthus. J Optics-UK 2010; 12: 065301–065301.

Lee

Smith

. Detailed electromagnetic simulation for the structural color of butterfly wings. Appl Optics 2009; 48: 4177–4190.

Yin

Dong

Liu

. Amorphous diamond-structured photonic crystal in the feather barbs of the scarlet macaw. Proc Natl Acad Sci 2012; 109: 10798–10801.

Sato

Kubo

. Structural color films with lotus effects, superhydrophilicity, and tunable stop-bands. Accounts Chem Res 2008; 42: 1–10.

10.

Vukusic

Sambles

. Photonic structures in biology. Nature 2003; 424: 852–855.

11.

Sinitskii

Klimovsky

Garshev

. Synthesis and microstructure of silica photonic crystals. Mendeleev Commun 2004; 14: 165–167.

12.

Singh

Haverinen

Dhagat

. Inkjet printing—process and its applications. Adv Mater 2010; 22: 673–685.

13.

O'Brien

Puzzo

Chutinan

. Selectively transparent and conducting photonic crystals. Adv Mater 2010; 22: 611–616.

14.

Bonifacio

Puzzo

Breslav

. Towards the photonic nose: a novel platform for molecule and bacteria identification. Adv Mater 2010; 22: 1351–1354.

15.

Lee

Asher

. Photonic crystal chemical sensors: pH and ionic strength. J Am Chem Soc 2000; 122: 9534–9537.

16.

Fudouzi

Xia

. Photonic papers and inks: color writing with colorless materials. Adv Mater 2003; 15: 892–896.

17.

Fudouzi

Xia

. Colloidal crystals with tunable colors and their use as photonic papers. Langmuir 2003; 19: 9653–9660.

18.

Shkunov

Vardeny

DeLong

. Tunable, gap-state lasing in switchable directions for opal photonic crystals. Adv Funct Mater 2002; 12: 21–26.

19.

Lee

Chan

Bevan

. Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates. Langmuir 2004; 20: 5262–5270.

20.

Huang

Pemberton

. Fabrication of colloidal arrays by self-assembly of sub-100nm silica particles. Colloids Surface A 2011; 377: 76–86.

21.

Luan

Huang

. Colloidal crystal heterostructures by a two-step vertical deposition method. Colloids Surface A 2007; 295: 107–112.

22.

Joannopoulos

. Photonics: self-assembly lights up. Nature 2001; 414: 257–258.

23.

Norris

Arlinghaus

Meng

. Opaline photonic crystals: how does self–assembly work. Adv Mater 2004; 16: 1393–1399.

24.

Shao

Zhang

. Preparation of monodispersed polystyrene microspheres and self-assembly of photonic crystals for structural colors on polyester fabrics. J Text I 2014; 105: 938–943.

25.

Liu

Zhou

. The fabrication of full color P (St-MAA) photonic crystal structure on polyester fabrics by vertical deposition self-assembly. J Appl Polym Sci 2015; 132: 4385–4393.

26.

Liu

Zhou

. Optical properties of three-dimensional P (St-MAA) photonic crystals on polyester fabrics. Opt Mater 2015; 42: 72–79.

27.

Dimitrov

Miwa

Nagayama

. A comparison between the optical properties of amorphous and crystalline monolayers of silica particles. Langmuir 1999; 15: 5257–5264.

28.

Sinitskii

Knot'ko

Tretyakov

. Silica photonic crystals: synthesis and optical properties. Solid State Ionics 2004; 172: 477–479.

29.

Mayoral

Requena

Moya

. 3D long–range ordering in an SiO₂ submicrometer–sphere sintered superstructure. Adv Mater 1997; 9: 257–260.

30.

Chiappini

Armellini

Chiasera

. Design of photonic structures by sol–gel-derived silica nanospheres. J Non-Cryst Solids 2007; 353: 674–678.

31.

. Relationship between the size of SiO₂ nanospheres and the structure colour. Micro Nano Lett 2011; 6: 527–529.

32.

Wang

Liang

. Fabrication of near-infrared photonic crystals using highly-monodispersed submicrometer SiO₂ spheres. J Phys Chem B 2003; 107: 12113–12117.

33.

Dong

Cheng

. Synthetic SiO₂ opals. Adv Mater 2001; 13: 437–441.

34.

Jiang

Bertone

Hwang

. Single-crystal colloidal multilayers of controlled thickness. Chem Mater 1999; 11: 2132–2140.

35.

Dimitrov

Nagayama

. Continuous convective assembling of fine particles into two-dimensional arrays on solid surfaces. Langmuir 1996; 12: 1303–1311.

36.

Cheng

Jin

. More direct evidence of the fcc arrangement for artificial opal. Opt Commun 1999; 170: 41–46.

37.

Liau

LCK

Huang

. Effects of influential factors on sedimentation self-assembly processing of photonic band gap crystals by relative humidity-controlled environments. Chem Eng Process 2008; 47: 1578–1584.

38.

Hartsuiker

Vos

. Structural properties of opals grown with vertical controlled drying. Langmuir 2008; 24: 4670–4675.

39.

Zhang

Sun

Yang

. Self-assembly of photonic crystals from polymer colloids. Curr Opin Colloid In 2009; 14: 103–114.

40.

Liu

Wang

. Influence of growth parameters on the fabrication of high-quality colloidal crystals via a controlled evaporation self-assembly method. Thin Solid Films 2010; 518: 5083–5090.

41.

Zhang

Wang

Zhao

. Silica photonic crystals with quasi-full band gap in the visible region prepared in ethanol. Prog Nat Sci 2003; 13: 717–720.

42.

McLachlan

Johnson

Richard

. Thin film photonic crystals: synthesis and characterisation. J Mater Chem 2004; 14: 144–150.

43.

Kuai

Hache

. High-quality colloidal photonic crystals obtained by optimizing growth parameters in a vertical deposition technique. J Cryst Growth 2004; 267: 317–324.

44.

LeBlanc

Haché

. Self-assembling three-dimensional colloidal photonic crystal structure with high crystalline quality. Appl Phys Lett 2001; 78: 52–54.

45.

Weast

. Handbook of chemistry and physics. Am J Med Sci 1969; 257: 355–437.

46.

Zhao

Zhang

Tang

. Self-assemble mechanism and defect analysis of colloidal silica photonic crystals. J Process Eng 2002; 2: 431–434.

47.

Duan

Peng

Tang

. The effect of gravity sedimentation medium on monodisperse SiO₂ microspheres assembled opal template. Laser Part Beams 2008; 20: 784–788.

48.

Cai

Teng

Yan

. Solvent effect on the self-assembly of colloidal microspheres via a horizontal deposition method. Colloid Surface A 2012; 402: 37–44.

Study on the formation of three-dimensionally ordered SiO 2 photonic crystals on polyester fabrics by vertical deposition self-assembly

Abstract

Keywords

Experimental details

Synthesis of silica particle and its characterization

Fabrication of SiO2 photonic crystals on polyester fabrics by vertical deposition self-assembly

Surface morphology and structural colors

Results and discussion

Effects of colloidal microsphere sizes

Effects of microsphere monodispersity

Effects of mass fractions of colloidal microspheres

Effects of relative humidity

Effects of evaporation temperature

Effects of solvents

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References

Fabrication of SiO₂ photonic crystals on polyester fabrics by vertical deposition self-assembly