Superdurable and fire-retardant structural coloration of carbon nanotubes.

Carbon nanotubes (CNTs) are promising candidates for numerous cutting-edge fields because of their excellent properties. However, the inherent black color of CNTs cannot satisfy the aesthetic/fashion requirement, and the flammability of CNTs severely restricts their application in high-temperature environments with oxygen. Here, we realized a structural coloration of CNTs by coating them with amorphous TiO2 layers. By tuning the TiO2 coating thickness, both CNT fibers and membranes exhibited controllable and brilliant colors, which exhibited remarkable superdurability that could endure 2000 cycles of laundering tests and more than 10 months of high-intensity ultraviolet irradiation. The TiO2-coated CNTs exhibited a notable fire-retardant performance and could endure 8 hours of fire burning. The structural coloration of CNTs with excellent fire retardance substantially improves their performance and broadens their applications.

INTRODUCTION

Nature is full of various brilliant colors, which make it beautiful. To create colorful materials, people developed numerous dyeing and coloring technologies in the long history of civilization. However, there are many materials that are difficult to color using conventional methods because of their chemically inert surfaces. Carbon nanotubes (CNTs) are such types of materials that are difficult to color. During the past 30 years since their discovery in 1991 (), CNTs have attracted worldwide attention and are believed to be ideal candidates for numerous cutting-edge fields such as ultrastrong fibers, next-generation integrated circuit (IC) chips, wearable devices, smart textiles, and conductive wires because of their remarkable mechanical strength, high electrical and thermal conductivity, chemical stability, low coefficient of thermal expansion, lightweight characteristics, etc. (–). However, just like other types of carbon materials, CNTs are inherently black. CNTs are even regarded as one of the super-black or blackest materials ever found because of their extremely strong absorption of light (). The black color of CNTs seriously restricted their applications in many fields such as wearable devices, smart textiles, functional coatings, and optical systems. The coloration of CNTs has always been a huge challenge because of their super-black characteristics, the high degree of surface crystallization due to the sp2-bonded structures of CNTs, and the weak chemical affinity to dyes, pigments, and paints resulting from the highly chemically inert surfaces of CNTs. The dull and tedious black color of CNTs cannot satisfy the aesthetic and fashion requirement and greatly restricts their applications in many areas. Ever since the discovery of CNTs, it has always been a great challenge to render them with various colors. Although single-walled CNTs (SWCNTs) with narrow chirality distribution obtained by separation from mixtures can exhibit some colors, which strongly depend on the chiral structures of SWCNTs (–), this method has serious restrictions and shortcomings. On the one hand, the chirality separation of SWCNTs with narrow distribution is a challenging, sophisticated, and time-consuming technique that is only suitable for short CNTs (CNT powders) and usually exhibits colors in suspensions, but cannot be used for long CNTs and CNT assemblies such as fibers and films. On the other hand, only SWCNTs can exhibit colors after chirality-based purification, while multiwalled CNTs only exhibit black color. To date, there is no feasible and widely applicable technology to color all types of CNTs regardless of their length, chiral structures, wall numbers, and macroscale morphologies. In addition to the black color, another severe weakness of CNTs is that, as a typical carbon material, they are combustible and will easily react with O2 (also including CO2 and H2O) at high temperatures. These shortcomings severely restrict the application of CNTs in high-temperature environments with O2. Therefore, it is of significant importance to develop a fire-retardant technology for CNTs to widen their applications.

In this work, inspired by the beautiful structural colors in nature, which is a unique coloration originated from physical interactions between light and periodic submicrometer architectures of matter (–), we realized a structural coloration of CNTs by coating them with amorphous TiO2 layers via atomic layer deposition (ALD). Both CNT fibers (CNTFs) and CNT membranes (CNTMs) exhibited controllable and brilliant colors by tuning the thickness of TiO2 coatings. Compared with conventional dyes and pigments, which are chemically unstable and cannot be used for coloring CNTs, the TiO2 coating–based structural colors of CNTs exhibited remarkable durability that could endure 2000 cycles of laundering tests and more than 10 months of high-intensity ultraviolet (UV) irradiation tests. The TiO2-coated CNTs exhibited a notable fire-retardant performance. Compared with pristine CNTs that were easily burned down after catching fire, the TiO2-coated CNTs did not catch fire and could remain fully unbroken after 8 hours of fire burning. The superdurable and fire-retardant structural coloration of CNTs significantly improves their performance and broadens their applications.

RESULTS AND DISCUSSION

Figure 1A illustrates the process for preparing colorful CNTs by coating TiO2 through ALD (for details, see fig. S1 and Materials and Methods). The pristine CNT samples (CNTFs and CNTMs) with pure black color are shown in Fig. 1 (B to D and K and L, respectively) (also see figs. S2 to S8 for more samples). The CNT samples were first pretreated with O2 plasma for 10 min to produce some oxygen-containing functional groups, such as ─OH and ─COOH. These plasma-pretreated samples were then transferred to an ALD chamber where newly formed active sites can react with the titanium isopropoxide (TIP) precursor molecule to form a ─OCH(CH3)2 surface by chemical bonding. Then, H2O molecule was subsequently pulsed onto the reactor chamber and reacted with the ─OCH(CH3)2 that formed a ─OH surface. TiO2 was deposited on the surface of samples layer by layer at 150°C for a predetermined duration (depending on the required thickness of TiO2 coatings) by repeating the above process. The theoretical thickness of one layer of TiO2 through one ALD cycle is ca. 0.1 nm (fig. S9). Thus, the TiO2 coating thickness can be well controlled by varying the ALD cycling numbers. The CNTF and CNTM samples coated with different TiO2 layer thicknesses were denoted as CNTF-n and CNTM-n, respectively, where n refers to the number of ALD cycles. Compared with the diameter of CNTFs (60 to 150 μm; Fig. 1, G and H, and fig. S5) and the thickness of CNTMs (10 to 50 μm; Fig. 1, N and O, and fig. S8), the TiO2 coatings were much thinner (usually 5 to 300 nm, depending on the number of ALD cycles) and relatively uniform (Fig. 1, F and M, and figs. S4 and S7), which, thus, did not affect the flexibility of CNTFs and CNTMs. Besides, the TiO2 coatings also exhibited many nanoscale wrinkles and particle-like humps on the surface, which were very important for the generation of brilliant structural colors of CNT samples due to the synergistic effect of light reflection, refraction, and scattering (which will be discussed later). To show the crystal structures of TiO2 coatings, we also deposited a very thin layer of TiO2 (3 to 5 nm) on single CNTs (Fig. 1I) and CNT bundles (Fig. 1J), and the corresponding TEM images showed the characteristics of the interface between CNTs and TiO2 coatings. No obvious cracks or gaps were found on the interfaces. Moreover, the continuous and uniform TiO2 coatings were also confirmed by the HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) images and the corresponding element mappings of C, O, and Ti elements (Fig. 1, P to S).

Fig. 1.

Fabrication and structural characterization of colorful CNTs.

(A) Schematic illustration for preparing colorful CNTFs by coating TiO2 through ALD. (B) Photo of pristine CNTFs. (C and D) Scanning electron microscopy (SEM) images showing the surface structure of a pristine CNTF. (E and F) SEM images showing the surface of a TiO2-coated CNTF. (G) SEM image showing the cross section of a TiO2-coated CNTF. (H) An enlarged view of the TiO2 coating around a CNTF shown in (G). (I and J) Transmission electron microscopy (TEM) images of a TiO2-coated single CNT and a TiO2-coated CNT bundle, exhibiting an amorphous structure of TiO2 coating. (K and L) Photo and SEM images of a CNTM. (M) SEM image showing the surface of a TiO2-coated CNTM. (N) SEM image showing the cross section of a TiO2-coated CNTM. (O) An enlarged view of the TiO2 coating on the surface of a CNTM shown in (N). (P to S) TEM image and corresponding element mappings of a TiO2-coated CNT showing the conformal coverage of amorphous TiO2. (T) X-ray photoelectron spectroscopy (XPS) spectrum of a pristine CNTF and a TiO2-coated CNTF. (U) XPS spectrum of C 1s for a pristine CNTF. (V and W) XPS spectra of C 1s and Ti 2p for a TiO2-coated CNTF. (X) Raman spectra of a pristine CNTF, a plasma-treated CNTF (CNTF-P), and a TiO2-coated CNTF.

To further distinguish the chemical valence states of the atoms in pristine CNTs and TiO2-coated CNTs, wide-scanning x-ray photoelectron spectroscopy (XPS) characterization was conducted. Consistent with the energy-dispersive x-ray spectra (fig. S10), only C and O elements were observed in the XPS of pristine CNTFs (Fig. 1, T and U). In comparison, after the ALD treatment, the O content increased, and the Ti element apparently appeared in TiO2-coated CNTFs (Fig. 1T), which exhibited four deconvoluted C 1s peaks at 284.78, 285.17, 285.16, and 288.72 eV, attributed to C─C, C─O, C═O, and ─C─O─Ti─ bonding (Fig. 1V), respectively (). The formation of C─O─Ti bonding in TiO2-coated CNTFs proved the self-limiting chemical bonding reaction of ALD at the interface between the TiO2 coatings and CNTs. In addition, two typical Ti 2p peaks that appeared at 458.5 and 464.5 eV with a peak gap of 6.0 eV could be attributed to Ti 2p3/2 and Ti 2p1/2 (Fig. 1W), respectively, which is in agreement with the XPS spectra of Ti4+ (–), confirming the formation of TiO2. The chemical bonding (─C─O─Ti─) between TiO2 and CNTs resulted in a strong adhesion and durability of TiO2 coatings (which will be discussed later). Besides, the phase constitution of TiO2-coated CNTFs was characterized using x-ray diffraction (XRD). Pristine CNTs exhibited two strong peaks of crystal planes (, ). However, the typical diffraction peaks of TiO2, such as anatase and rutile crystal phases (–), were not observed in TiO2-coated CNTs (fig. S11), indicating that the coated TiO2 exhibited an amorphous phase. Raman spectrum was also used to reveal the structural variation between pristine CNTFs, plasma-pretreated CNTFs (CNTF-P), and TiO2-coated CNTFs (CNTF-TiO2). As shown in Fig. 1X, the CNTFs exhibit two characteristic peaks at 1360 and 1580 cm−1, corresponding to the D-band and G-band, respectively (, ). The strength ratio of D/G in CNTF-P was smaller than that in CNTFs due to the fact that plasma decreased the defect sites, while the strength ratio of D/G in TiO2-coated CNTFs was close to that in CNTFs, confirming that the ALD process had little impact on the intrinsic structures of CNTFs (see detailed discussion below).

The TiO2-coated CNTs exhibited brilliant colors (Fig. 2). As shown in Fig. 2 (A to E), with the increase of TiO2 thickness (i.e., the increase of ALD cycle numbers), the CNTFs exhibited indigo, yellow-brown, blue, purple, and green colors, respectively. It should be pointed out that the structural colors shown by the TiO2-coated CNTFs were not monochromatic colors but composite colors formed by the combination of two or more primary colors. As is known, visible light is composed of seven different colors in nature, i.e., red, orange, yellow, green, blue, indigo, and purple. Among them, red, green, and blue are the three primary colors that cannot be decomposed into other colors. The combination of any two colors can produce a new composite color. The brilliant structural colors of TiO2-coated CNTFs were generated by the synergistic effect of light reflection, refraction, and scattering on the surface of TiO2 coatings as well as the interfaces between the TiO2 coatings and CNTFs. This is the first time that CNTFs have shown such brilliant structural colors. We also characterized the fine structures of colorful TiO2-coated CNTFs under an optical microscope and found that even the thinner CNT bundles could exhibit brilliant colors (Fig. 2, F to J). Besides, various beautiful patterns could also be fabricated using these colorful CNTFs (Fig. 2, K to O), which greatly broadens the application of CNTFs in numerous areas such as wearable electronics (, ), smart textiles (, ), and functional coatings (, ). In addition to colorful CNTFs, we also realized the fabrication of colorful CNTMs with brilliant colors. As shown in Fig. 2 (P to T), with the increase of ALD cycling numbers, the color variation of TiO2-coated CNTMs exhibited similar but slightly different characteristics with TiO2-coated CNTFs. It was found that, even with the same ALD cycle numbers, the colors of TiO2-coated CNTFs and TiO2-coated CNTMs were not exactly the same, but with a slight difference. The reason lies in the morphological differences between the surfaces of CNTFs and CNTMs (Fig. 1, D and L, and figs. S3 and S7). As is known, the structural colors are generated by the synergistic effect of light reflection, refraction, and scattering on the surface of subjects, which are very sensitive to the surface morphologies and structures. Therefore, different surfaces generated different structural colors. Besides, the TiO2 coating–assisted structural coloration can also be realized on any other types of CNTFs, such as the CNT/PVA (polyvinyl alcohol) composite fibers, which also exhibited various brilliant colors (fig. S12).

Fig. 2.

Photographs of colorful TiO2-coated CNTFs and CNTMs.

(A to E) Colorful TiO2-coated CNTFs with different ALD cycle numbers. The corresponding samples were denoted as CNTF-n, where n refers to the number of ALD cycles. (F to J) Optical microscopy images of TiO2-coated CNTFs with different ALD cycles shown in (A) to (E). (K to O) Various patterns of colorful CNTFs. (P to T) Colorful TiO2-coated CNTMs with different ALD cycle numbers.

We investigated the mechanism for the generation of structural colors of TiO2-coated CNTs (Fig. 3A). Briefly speaking, the mechanism for structural coloration of TiO2-coated CNTs is mainly based on thin-film interference (Fig. 3B; for detailed discussion, see text S1), and the apparent color shows a strong dependence on the thickness of TiO2 coatings. When an incident light reaches the surface of a TiO2-coated CNTF, in addition to the absorption of a small portion of light, most of the incident light will also be reflected, scattered, or refracted by the surface of TiO2 coating (Fig. 3C). We characterized the variation of the corresponding reflection spectra of CNTs before and after coating TiO2 by an optical spectrometer. As shown in Fig. 3D, for pristine CNTFs, the reflection spectrum was almost a straight line, thus rendering pristine CNTFs with a pure black color. For TiO2-coated CNTFs with different TiO2 thickness, the spectrum presented different reflection peaks with different ranges, which varied with the TiO2 thickness. We also calculated all the predicted reflection distribution of all TiO2-coated CNTFs with different TiO2 thicknesses (Fig. 3E). Note here that the measured reflection spectrum did not match well with the predicted spectrum obtained by a finite element method on COMSOL software (Multiphysics 5.5) using a planar surface interference model, which was attributed to the hierarchical structure of the CNTFs and the rough surfaces of TiO2 coatings. Briefly speaking, as shown in Fig. 1 (C to F) and figs. S2 to S5, CNTFs were formed by the random intertwisting of countless single CNTs and CNT bundles, exhibiting hierarchical surfaces with obvious roughness. Besides, the coated TiO2 layer on CNTFs also exhibited a rough surface with many nanoscale wrinkles and particle-like humps. The apparent colors of TiO2-coated CNTFs are actually generated by the synergistic effect of the thin-film interference between the TiO2 coatings and the surface of CNTFs, as well as the combination effect of the reflection, refraction, and scattering from the wrinkles and humps on the surfaces of TiO2 coatings. Therefore, the dependence of peak positions on these factors did not exhibit a simple linear relationship with the thickness of TiO2 layers, but a complicated one (for detailed discussion, see texts S2 and S3). Besides, we also found that it was very difficult to achieve a pure structural red color, which has been discussed before (). In addition, we also measured the angle-resolved reflective spectra of CNTF-2500 with the incident angles of 0°, 10°, 20°, 30°, 40°, 50°, and 60°. We found that the colors of TiO2-coated CNTFs exhibited only very little change from different view angles. As shown in Fig. 3F, with the increase of incident angles from 0° to 60°, the reflection peaks around 545 and 778 nm only showed a slight blue shift because of the effect of Bragg diffraction (). The main reason was that the incident light shifted from the axis, and the light was averaged by adjacent fibers.

Fig. 3.

Schematic illustration of structural coloration and the reflection of TiO2-coated CNTFs.

(A) Schematic illustration of the generation of structural colors on the surface of TiO2-coated CNTFs. (B) Schematic illustration of thin-film interference on the surface of TiO2-coated CNTFs. (C) Schematic illustration of the light refraction (mainly thin-film interference), scattering, reflection, and their synergistic effect on the surface of TiO2-coated CNTFs. (D) Reflection spectra of CNTFs and TiO2-coated CNTFs. (E) Reflection spectra of TiO2-coated CNTFs with a continuous TiO2 thickness variation from planar surface interference model. (F) Angle-resolved reflection spectra of a TiO2-coated CNTF (CNTF-2500).

Compared with the colors of previously reported SWCNT suspensions or films with narrow chirality distribution (–), the structural colors of TiO2-coated CNTFs and CNTMs were more beautiful and brilliant. Besides, this ALD coating–assisted structural coloration method can be easily conducted on any type of CNT samples, regardless of their chirality, lengths, and morphologies. In addition to TiO2, other oxides such as Al2O3 and ZnO can also be used to realize the structural coloration of CNTs using the same ALD methods (fig. S13). Therefore, this ALD TiO2 coating–assisted structural coloration method shows obvious advantages. In addition, we also tested the coloration performance of CNTFs using conventional dyeing methods (red dye solution, red ink, and red paint were selected as the dyeing materials). Compared with the easily dyed textiles such as cotton fabric (fig. S14), the CNTFs still exhibited black color after the red dye solution and red ink dyeing; no visible color change could be observed (fig. S15, A to D). Although the surface of CNTFs could be partially covered by the red paint with a very thick and nonuniform layer, the red paint could be easily removed after water washing due to the weak affinity of paint to CNTFs (fig. S15, E and F). Therefore, conventional dyeing materials and methods are incapable of coloring CNTs due to their high chemically inert surfaces and the weak chemical affinity to dyes, pigments, and paints, further confirming the advantages of ALD coating–assisted structural coloration for CNTs.

To test the stability and durability of the structural colors of TiO2-coated CNTs, we tested their laundering durability and UV irradiation resistance, both of which are very important factors in determining the durability, applicability, and lifetime of colored materials in various harsh circumstances. First, the laundering durability test was conducted on CNTF-2000 according to the American Association of Textile Chemists and Colorists (AATCC) 61-2009 standard. It was found that the morphology and color of CNTF-2000 did not change even after washing 2000 times with water (Fig. 4, A to C). To further investigate the effect of washing on the colored CNTF, the reflection spectra before and after the laundering test were also compared (Fig. 4K). No changes were found among the three reflection spectra for CNTF-2000 before washing, after washing 1000 times, and after washing 2000 times, respectively, showing the stability and durability of the structural colors of TiO2-coated CNTFs. In addition, we also tested the UV resistance of the structural colors of TiO2-coated CNTFs. As is known, for colored products, especially those colored with organic dyes, pigments, and paints, UV light has a great influence on color stability and lifetime. For conventional organic dyes, pigments, and paints, their molecules will gradually decompose during the long-term UV irradiation process, resulting in the fading or degradation of colors. In comparison, the inorganic TiO2 coatings on the surface of CNTs exhibited a notable stability and durability after long-term UV irradiation. We conducted an accelerated aging test using a UV irradiation with an intensity of 407.5 W/m2 at 50°C. As shown in Fig. 4 (D to F), the TiO2-coated CNTFs (CNTF-2500) could endure more than 10 months of high-intensity UV irradiation without any visible color degradation (which equals to about 227 years in ambient conditions; see text S4). Moreover, we also compared the reflection spectra of CNTF-2500 before and after the UV irradiation, and no visible changes were found among the three reflection spectra (Fig. 4L).

Fig. 4.

Superdurability and fire retardance of the structural colors and their influence on the electrical and mechanical properties of TiO2-coated CNTFs.

(A to C) Photographs of CNTF-2000 before the laundering durability test, after washing 1000 times with water, and after washing 2000 times with water, respectively. (D to F) Photographs of CNTF-2500 before UV irradiation, after 5 months of intense UV irradiation, and after 10 months of intense UV irradiation, respectively. The intensity of the UV irradiation was 407.6 W/m2 and the temperature was 50°C. (G to J) Fire retardance test of pristine CNTF and CNTF-2500. (K) The reflection spectra of CNTF-2000 before and after washing at different times. (L) The reflection spectra of CNTFs-2500 before and after UV irradiation at different durations. (M) Current-voltage curves of CNTFs and TiO2-coated CNTFs. (N) Stress-strain curves of CNTFs and TiO2-coated CNTFs.

In addition to notable laundering durability and UV irradiation resistance, the TiO2 coatings also imparted the CNTs with an excellent fire retardance and improved electrical and mechanical performance. As is known, as a typical carbon material, pristine CNTs are easy to catch fire in air at high temperatures. Therefore, it is of significant importance to improve the thermal stability and fire retardance of CNTs. As shown in Fig. 4 (G to J), pristine CNTFs easily caught fire after making contact with fire for only several seconds and will be gradually burned down after several minutes. However, the TiO2-coated CNTFs (CNTF-2500) exhibited an excellent thermal stability and fire retardance. As shown in Fig. 4I, even after being continuously heated in a fire for as long as 8 hours, the TiO2-coated CNTF-2500 remained unbroken. This is of significant importance for the applications of CNTs in many high-temperature environments with oxygen.

Moreover, we also compared the effect of TiO2 coatings on the electrical and mechanical properties of CNTFs. Figure 4M shows that, with the increase of thickness of TiO2 coatings, the current-voltage curves of TiO2-coated CNTFs even became better than that of pristine CNTFs. The reason can be attributed to the fact that, with the TiO2 coating, the packing of CNTs became denser than pristine CNTs, thus decreasing the electrical contact resistance inside the CNTFs. The TiO2 coatings exhibited a complicated influence on the mechanical performance of CNTFs. As shown in Fig. 4N, compared with pristine CNTFs, both the tensile strength and the breaking strain of CNTF-500 that had a thin TiO2 coating only showed a very slight decrease. With the increase of TiO2 coating thickness, the CNTF-1000 and CNTF-1500 exhibited an even higher tensile strength but a slightly lower breaking strain than pristine CNTFs. However, with the further increase of TiO2 coating thickness, the CNTF-2000 and CNTF-2500 showed both decreased tensile strength and lower breaking strain. The reason was attributed to the fact that many chemical bonds between the TiO2 coatings and the functional group on the surface of CNTFs were formed during the ALD process, which increased the internal interaction between CNTs inside the fibers and, thus, increased the overall mechanical strength for a thin TiO2 coating thickness. However, because of the brittleness of TiO2 coatings, a thick layer of TiO2 coating will cause a decrease of tensile strength and breaking strains of TiO2-coated CNTFs. In all, the above performance confirmed the multiple functions and advantages of TiO2 coatings on the surface of CNTs. Such an efficient and simple method to fabricate colorful CNTs by coating them with amorphous TiO2 layers through ALD can be very useful, which not only increases the structural and functional diversity of the CNTs but also exhibits great promise in developing emerging optical devices for advanced applications such as colorimetric sensors, anti-counterfeiting devices, information encryption, multicolor passive photonic displays, optical fiber, and lasers, to broaden the application of CNTs.

MATERIALS AND METHODS

Chemicals and materials

CNTFs and CNTMs were purchased from Tanrand Technology (Beijing, China), Nanjing XFNANO Materials Tech Co. Ltd., and Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Science (CAS), respectively. First, CNTFs and CNTMs were soaked in absolute ethanol, acetone, and deionized water with ultrasonic washing for 60 min. After that, the cleaned CNTFs and CNTMs were dried in an oven at 105°C for 20 min. Titanium (IV) isopropoxide (TIP; 99.999% metals basis) purchased from Aladdin Industrial Co. Ltd. was used as the titanium-based precursor material. Other chemicals including absolute ethanol (CH3CH2OH, analytical grade) and acetone (CH3COCH3, analytical grade) were provided by Sinopharm Chemical Reagent Co. Ltd. Deionized water was supplied from a Milli-Q Plus 185 ultrapure water purification system with a resistivity of 18.2 megohm·cm.

Fabrication of colorful CNTFs, CNTMs, and CNTF/PVA

For ALD, the fabrication process often uses alternating precursor vapor exposures in a vacuum condition. Such a processing procedure can create a sequence of self-limiting surface chemisorption reactions and help deposit a series of inorganic and polymeric thin films with high conformity and precise nanoscale thickness on complex geometries. Although the self-limiting surface reactions characteristic of ALD allow uniform coatings on nonplanar, porous, and other tortuous substrates, the successful deposition highly relies on the functional groups of a substrate surface, including hydroxyl (─OH), carboxyl (─COOH), and lactone (─OCO─). Therefore, it is necessary for CNTFs and CNTMs to induce the formation of new functional groups that can initiate the ALD reactions through other techniques, such as plasma treatment and high-energy x-ray radiation treatment. In this work, cleaned CNTFs and CNTMs were first treated with O2 plasma for 10 min before the ALD process. Then, the activated CNTFs and CNTMs were transferred to a commercial hot-wall closed chamber–type ALD reactor (D100-4882, Nuotu, Chongqing) with a temperature at 150°C in a 0.5-torr environment for 5 min to 20 hours, depending on the required thickness of TiO2 coatings. During the whole ALD process, high-purity nitrogen (N2) with a purity of 99.999% was used as purging and carrying gas at a steady flow rate of 50 standard cubic centimeters per minute. Precursors, TIP, and deionized water were used as the titanium (Ti) source and oxygen (O) source for depositing the conformal TiO2 film. A typical ALD TiO2 growth cycle was performed according to the following procedure, as shown in fig. S1. First, the precursor TIP molecules were introduced in the vapor phase to the ALD reaction chamber until all active sites were occupied and the surface was fully saturated. TIP during this procedure could react with the active oxygen groups on the substrate surface and create the terminal titanium isopropoxide ─O─Ti─OCH(CH3)2 on the fiber surface. Then, the superfluous precursor TIP and reaction by-products (CH3)2CHOH were rinsed away by high-purity N2. Third, another precursor, H2O molecules, was fed in the vapor phase to the ALD reaction chamber and reacted with surface molecules ─O─Ti─OCH(CH3)2, thus forming a solid ─O─Ti─(OH)3 layer. The residues, including excess precursor H2O and reaction by-products (CH3)2CHOH, were further rinsed away by high-purity N2. To produce sufficient vapor pressures and prevent the condensation of vapor precursors in the pipeline, TIP was heated to 80°C, while H2O was still maintained at 30°C. Each precursor gas was pulsed into the reactor chamber for 0.2, 8, 25, 0.1, 8, and 25 s for TIP pulse, TIP exposure, N2 purge, H2O pulse, H2O exposure, and N2 purge, respectively. The estimated theoretical thickness of ALD TiO2 films for a monomolecular layer was about 0.1 nm. Therefore, the desired film thickness could be achieved by repeating the ALD cycles. Preset samples with various film thicknesses were obtained and denoted as CNTF-n and CNTM-n, where n refers to the number of cycles. The procurer of the colored CNTF/PVA was similar to the above process.

Characterization

The surface morphologies and structures of samples were characterized by field-emission scanning electron microscopy (JSM 7401, JEOL Co. Ltd., Japan), high-resolution transmission electron microscopy (JEOL JEM-2010), and double spherical aberration-corrected transmission electron microscopy (FEI Titan Cubic Themis G2 300) equipped with energy-dispersive x-ray spectroscopy. The chemical valence state of the atoms was investigated by using XPS (SPM-9700, SHIMADZU Co.) with an Al-Kα radiation source (1486.6 eV) under ultrahigh vacuum (1 × 10−9 mbar). The phase composition and crystalline structure were revealed by an XRD spectrometer (Bruker D8 Advance Diffractometer, Germany) equipped with a Cu-Kα radiation source at the scanning rate of 5°/min in the 2θ range from 5° to 90°. Raman spectra were obtained to analyze the defects of as-prepared samples by using a Raman spectrometer (LabRam HR800, Horiba Jobin-Yvon, France) with a charge-coupled device detection. Optical images of samples were taken using a digital camera (Nikon DSLR D5100) and an optical microscope (Olympus BX53) under ordinary white light. The corresponding reflective spectra of samples were recorded using a PG2000-Pro spectrometer (Idea Optics Co. Ltd., China). Angle-resolved reflective spectra were collected by an angle-resolved microspectroscopy system (ARM160, Ideaoptics, China). The intense UV stabilities were evaluated by the color change of colored CNTF, which were exposed in intense UV irradiation with an intensity of 407.5 W/m2 at 50°C. The accelerated laundering durability test of the colored CNTF was also investigated on the basis of the AATCC 61-2009 standard. The color stability was explored by the color change before and after accelerated water washing and the corresponding reflective spectra.