Tuning the Iridescence of Chiral Nematic Cellulose Nanocrystal Films

Oct 9, 2014 - ... the direction of the cholesteric helix is the application of a magnetic field. In addition, it has been reported that a shear field ...
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Tuning the Iridescence of Chiral Nematic Cellulose Nanocrystal Films with a Vacuum-Assisted Self-Assembly Technique Qi Chen, Ping Liu, Fuchun Nan, Lijuan Zhou, and Jianming Zhang* Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao City 266042, People’s Republic of China S Supporting Information *

ABSTRACT: Iridescent films composed of the chiral nematic liquid crystal phase of cellulose nanocrystals (CNC) have attracted significant interest due to their fascinating optical properties. However, the current fabrication method, i.e., solution casting with a subsequent evaporation process, has significant limitations and therefore hinders the application of CNC iridescent films. In the present study, we demonstrate, for the first time, that vacuum-assisted self-assembly (VASA) can be used to fabricate highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films. It was found that a long ultrasonic pretreatment is necessary for obtaining CNC iridescent films via VASA. Furthermore, it was also found that the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and degree of vacuum. By combining CD spectroscopy, SEM, and WAXD techniques, the internal structure of CNC iridescent films prepared by VASA has been investigated in detail. Moreover, the origin of the ultrasonic pretreatment effect on the self-assembly behavior of CNCs is also discussed. the substrate.12 Meanwhile, several external processes, such as mechanical (shearing),13 electrical,14 and magnetic fields,15,16 have also been utilized to control the helical orientation of the chiral nematic phase. Among these attempts, the only technique reported thus far to be capable of controlling the direction of the cholesteric helix is the application of a magnetic field. In addition, it has been reported that a shear field will generally induce uniaxial CNC alignment rather than an oriented chiral nematic phase.8 Until now, static solution casting with slow solvent evaporation, which is also termed evaporation-induced selfassembly (EISA) in some literature,17,18 is the most widely used method for obtaining CNC iridescent films. However, there are some limitations to the EISA method. First, it is timeconsuming because it takes up to several days to finish the evaporation of the water in a CNC solution, especially under slow evaporation rate conditions. Second, thus prepared films generally show a polydomain structure with the helical axes of different chiral nematic domains pointing in different directions.19,20 The inhomogeneous structure obtained during the drying process is presumably caused by the so-called “coffee ring effect”.21 Third, as commented by Kelly et al.,22 the films tend to crack during the last stages of EISA into centimetersized pieces due to the significant capillary pressure gradients

1. INTRODUCTION Cellulose nanocrystals (CNCs; nanoscale materials isolated from native cellulose sources via controlled acid hydrolysis)1−3 have inspired vast research enthusiasm because of their remarkable physical and chemical properties as well as because of their abundance, availability, and biocompatibility. The dimensions of rod-shaped CNCs are generally on the order of 5−15 nm in diameter and 100−300 nm in length, depending on the acid hydrolysis conditions and the cellulose feedstock.4 When sulfuric acid is employed, the rod-like CNCs have negatively charged sulfate surface groups that enable them to form stable aqueous dispersions.5,6 Therefore, CNCs obtained from the sulfuric acid-catalyzed hydrolysis have often been used as a well-dispersed nanofiller to reinforce polymers and composites through solution processing. In addition to their mainstream application in nanocomposites, anisotropic CNCs also possess the intriguing ability to self-organize into a chiral nematic liquid crystal phase in concentrated solution, which could result in iridescent solid films after the evaporation of solvents.7,8 In general, the classic helical model of cholesteric liquid crystal has often been used to explain the structural origin of CNC iridescent films.2−4 According to this model, the helical pitch (P) is thought to be the crucial parameter for determining the maximum reflected wavelength of incident light. Previous reports showed that the iridescence of CNC films can be tuned by changing the pitch via either adding electrolytes,9 applying sonication to the CNC dispersion,10 varying the evaporation rate,11 or changing © XXXX American Chemical Society

Received: September 11, 2014 Revised: October 8, 2014

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(KQ3200DE Kunshan, Shumei Ultrasonic Instrument Co. Ltd., China, 150 W, 40 kHz) as the ultrasonic processor with full energy output. The ultrasonic pretreatment was carried out in an ice bath to avoid overheating, which might cause desulfation of the sulfate groups on the surface of the CNCs. Long-duration ultrasonic treatments were performed in 30 min segments, with intermediate cooling intervals. Vacuum filtration was generally performed under 0.09 MPa vacuum except where otherwise indicated. The filtration was usually completed within 3 h. After the remaining water in the filter cake evaporated naturally, the film was peeled off from the filter paper for characterization. 2.4. Characterization of CNC Solid Films. UV−vis reflection spectra of the CNC solid films were collected with a UV−vis spectrophotometer (Shimadzu UV-2550). Scanning electron microscopy (SEM) images were obtained with a JSM-7500F field emission SEM at an accelerating voltage of 3 kV on samples sputter-coated with gold. 2D-WAXD measurements for the iridescent CNC films along different directions were performed to test the orientation of the nanocrystals using a RIGAKU R-axis VII X-ray diffractometer in transmission mode with a graphite-monochromatized Mo Kα X-ray beam (λ = 0.71 Å) and a flat imaging plate as detector. Circular dichroism (CD) spectroscopy measurements, used to confirm the chirality of the CNC iridescent solid films, were performed with a CD J-815 spectropolarimeter on the as-prepared film directly or on tablets of powder samples ground with KBr in transmission mode.

generated during evaporation, which is a major barrier to exploiting these materials in applications that require large, homogeneous films. Therefore, an alternative strategy for preparing CNC iridescent films, which can overcome the above pitfalls of the EISA method, is strongly desired. It is well-known that vacuum filtration, which is also named vacuum-assisted self-assembly (VASA), is a simple and fast way to prepare large-scale free-standing films, provided that the material is of high aspect ratio and dispersible in the solvent.23,24 Herein, we demonstrate that highly oriented, centimeter-sized, and structurally homogeneous CNC iridescent films can be quickly obtained by VASA. Moreover, it was found that ultrasonic treatment of the CNCs solution prior to vacuum filtration is a critical factor to tune the self-assembly structure of thus prepared films. To the best of our knowledge, this is the first time that the VASA technique has been successfully used to fabricate an iridescent film of CNCs. This finding will open a new way to prepare CNC-based functional materials with a highly ordered nanostructure under a flow field and will also enhance our understanding of the internal liquid crystal structure of CNC iridescent films and its self-assembly behavior.

2. EXPERIMENTAL SECTION

3. RESULTS 3.1. Preparation of CNC Solid Films via VSVA. The preparation process of the CNC iridescent film is illustrated as Scheme 1a. As mentioned in the Introduction, the VASA method provides a simple and easy way to prepare large-scale free-standing films. However, this method requires materials

2.1. Materials. Cotton linter pulp (CLP) provided by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China) was used as starting material. The degree of polymerization (DP), measured by the viscosity method, was 700 for CLP. Deionized water was used for all experiments. Sulfuric acid (H2SO4, 98 wt %, analytically pure, Yantai, China) and sodium hydroxide (NaOH over 96.0% purity, analytically pure, Tianjin, China) were of reagent grade and used without further purification. 2.2. Preparation and Characterization of the CNC Suspension. An aqueous CNC suspension was prepared from CLP mainly by following the sulfuric acid hydrolysis method reported in previous literature.3 However, two modifications to the traditional method were used to generate repeatable and optimal hydrolysis conditions. First, alkalization pretreatment was used. In a typical procedure, 5 g of CLP milled with a food processor was first soaked in a 4 wt % aqueous NaOH solution for 24 h at room temperature. The obtained slurry was filtered, thoroughly washed with distilled water until a neutral pH was reached, and then dried at 60 °C in a vacuum oven for 24 h. Previous studies25,26 suggested that such alkali pretreatment is effective for obtaining CNCs having a Form I crystal, which is the native crystal structure of cellulose. Second, in order to accelerate the acid hydrolysis reaction and improve the yield of CNCs, CuSO4 (CLP/CuSO4 = 100:1) was added to a 64 wt % sulfuric acid solution.27 Then, CLP was dispersed in this acid solution at room temperature (CLP/acid= 1 g/ 16 mL). The mixture was stirred vigorously at 50 °C for 50 min, and the acid hydrolysis was subsequently stopped by diluting the mixture with cold water. The resulting suspension was centrifuged and washed repeatedly until the pH was close to 2. Finally, an ivory-white suspension with a concentration around 3.5 wt % was obtained in this manner. Prior to preparing solid films via vacuum filtration, the thus obtained CNC suspension was diluted to 1 wt % and used directly without dialysis. The particle size of the CNCs was analyzed by atomic force microscropy (AFM, Bruker Instrument Nanoscope V) and dynamic light scattering (DLS, Malvern Nano ZS90). The AFM sample was prepared by drying a drop of a dilute suspension (0.01% w/v) on a silicon wafer. 2.3. Preparation of CNC Solid Films via VASA. CNC solid films were prepared by a vacuum filtration method from a 1.0 wt % CNC suspension using poly(tetrafluoroethene) (PTFE) filter paper (50 mm diameter, 0.22 μm pore size) located on a porous ceramic support of a filtration funnel. Prior to vacuum filtration, the CNC suspension was sonicated for various amounts of time using an ultrasonic cleaning bath

Scheme 1. (a) Illustration of the Fabrication Process of CNC Iridescent Solid Films via VASA, (b) AFM Image of CNC Nanorods Spin-Coated on a Mica Substrate, and (c) Photograph of an Iridescent CNC Solid Film Overlaid on Paper with Texta

In (c), the film size is 4 cm, as indicated by the ruler; the logo is reproduced by permission of Qingdao University of Science and Technology. a

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with a high aspect ratio that also need to be dispersible in the solvent and rigid enough to be stable for suction filtration.23 Therefore, the particle size and colloidal stability of our prepared CNC suspension were first examined. As shown in Scheme 1b, the rod-like nanocrystals deposited on a clean silicon wafer possess dimensions ranging mostly from 100 to 300 nm in length and around 8 nm in width. The particle size and colloidal nature of the CNC suspension were also determined by dynamic light scattering (DLS). According to the distribution curve of the rh for CNCs found by DLS measurements, the average particle size of the prepared CNCs is about 110 nm, which is consistent with AFM observations. The average zeta potential of the aqueous CNCs was −46.8 eV, which is a value similar to the one in the literature.4 In general, it is thought that an aqueous suspension can remain stable when the zeta potential is above 30 eV.28 Therefore, the prepared CNC suspension presents a fairly stable colloidal state and is suitable for the VASA method. Prior to vacuum filtration, a 1 wt % CNC suspension was sonicated for different amounts of time, and a certain volume of the solution was then vacuum-filtered. When the solution underwent suction filtration to completely remove water from the suspension, the filter cake could be peeled off from the filtration membrane as an intact, free-standing film. The thickness of the free-standing film depends on the volume of the filtered solution and the size of the filtration funnel. In our system, 14 mL of the CNC suspension gave a thickness of about 76 μm by using a funnel with inner diameter of 4 cm. The lateral size of the as-prepared films is equal to the inner diameter of the filtration funnel, which indicates that the films do not shrink before and after peeling them from the filtration membrane. Of particular interest, as displayed by the optical photos in Scheme 1a, is that ultrasonic pretreatment of the CNC suspension for a short period (10 h) results in an iridescent film. The solid film obtained via VASA is smooth and crack-free, which is rather difficult to achieve via the traditional EISA technique. Moreover, the thus prepared large-area freestanding film with a thickness of 76 μm shows good transparency. As demonstrated by the optical photo in Scheme 1c, the film’s transparency is vividly demonstrated by the ability to discern the text on the paper placed below the iridescent film. The surface and cross-sectional morphology of colorless and iridescent films whose CNC particles underwent short and long durations of sonication were observed through SEM images, as shown in Figure 1. As seen from the surface morphology images (Figure 1a,b), the colorless film demonstrates a rough surface compared with that of the iridescent film. When viewed perpendicular to the cross-section of the film (Figure 1c,d), it is easy to see that the CNC nanorods are arranged according to a certain rule to form a periodic helical structure in the iridescent film, whereas the colorless film has randomly oriented CNCs. According to the structural observations in Figure 1, it is reasonable to conclude that ultrasonic pretreatment of CNC suspensions strongly affects the VASA structure of the resulting films, which should be responsible for the color variations discerned by the naked eye as shown in Scheme 1. It is generally well-accepted that the iridescence of a CNC film originates from an underlying chiral nematic liquid crystal structure in the material. We will discuss the nature of the internal structure of the iridescent CNC films prepared by VASA in detail later.

Figure 1. SEM images of CNC solid films with different durations of sonication, 0 h (S0) and 14 h (S1): SEM surface (a, b) and crosssectional images (c, d).

In nature, the formation of a chiral nematic liquid crystal phase of CNCs in solution or of the dried film involves a phase separation process. It has been reported that the key variables governing the phase separation of rod-like CNCs are the axial ratio (length to diameter), surface charge, and polydispersity of the CNC rods. That is, the chiral nematic ordering in the suspensions of CNCs was found to be highly dependent on the hydrolysis and preparation conditions. Moreover, it is also reported that the initial concentration, pH value, etc. also affect the self-assembly behavior of CNC rods during the evaporation process. Therefore, in the following sections, we used the same batch of CNC solution with a fixed concentration (1 wt %) and pH value (2) to examine the influence of the processing conditions, i.e., ultrasonic time, suspension volume, and degree of vacuum, on the self-assembly structure of CNC films prepared by the vacuum filtration technique. 3.2. Tuning the Iridescence of CNC Solid Films by Sonication Time. In the laboratory, ultrasonic treatment is normally used as a final step to obtain a well-dispersed colloidal CNC suspension. The effects of sonication on tuning the iridescent color of CNC films have been studied by Beck and co-workers.10 They reported that the ultrasonic energy provided to the CNC suspension prior to film casting increases the pitch of the chiral nematic phase and results in a red-shift in the reflection wavelength of the final solid iridescent films. Herein, we performed increasing durations of sonication (7, 10, 12, and 15 h, with the corresponding samples named S1− S4, respectively) on a 28 mL CNC suspension to investigate the influence of sonication on the optical properties of films obtained by VASA. It should be pointed out that, for those CNC iridescent films prepared via the traditional casting method, a short sonication time, usually no more than 1 h, was applied to the CNC suspension before casting. However, in our case, we find that a long sonication time is necessary to obtain the CNC iridescent film via VASA. As shown in Figure 2a, the films demonstrate an obvious iridescent color when the sonication duration is above 10 h. Moreover, the resulting film’s color becomes brighter when the CNC suspensions experience prolonged sonication. Figure 2b shows UV−vis reflection spectra of the CNC films prepared after various ultrasonic pretreatment times. The C

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Figure 3. SEM cross-sectional images of CNC iridescent films sonicated for different durations: 7 h (a), 10 h (b), 12 h (c), and 15 h (d). White dotted lines show the interlayer separation corresponding to the half pitch of the CNCs liquid crystal self-assembling structure. Figure 2. (a) Digital photographs of CNCs solid films obtained with different sonication pretreatment times. (b) UV−vis spectra of the CNC solid films.

3.3. Tuning the Iridescence of CNC Solid Films by Varying the CNC Suspension Volume and Degree of Vacuum. In order to determine how the amount of the CNC suspension influences the chiral nematic structure in the film, we prepared iridescent CNC films with different thicknesses by simply varying the volume of the CNC suspension. Here, the applied CNC suspension maintains a constant concentration; therefore, the CNC amount is merely related to the volume adopted. By varying just the CNC suspension volume and applying a 12 h sonication to the suspension sample, different colors of CNC solid films were obtained via VASA, as shown in Figure 4a, and the thicknesses of these samples, determined by SEM images, ranged from 163 to 76 μm, as depicted in Figure 4b. Figure 4c demonstrates that there is a linear relationship between the film thickness and CNC suspension volume, suggesting that the thickness of the iridescent film obtained by VASA can be precisely controlled by the volume of the CNC suspension. The differences in the color of the films indicate that the helical pitch of the chiral nematic structure in the CNC films is variable with the film thickness. This speculation is further confirmed by the SEM observation on the cross-section of these films, as displayed in Figure 5. It can be seen that all four samples show a highly ordered layered structure, whereas their periodic distances differ from each other. As shown in Table S1, the helical pitches of these samples are in the range of 400−800 nm, which is comparable to the wavelength of visible light. The degree of vacuum was also controlled to investigate the relationship between the vacuum conditions during the VASA process and the resulting color of the iridescent films. As shown in Figure 6, the iridescent color changes as the degree of vacuum was varied from 0.07 to 0.04 MPa. This result suggests that the iridescent color is also dependent on the degree of vacuum applied. However, there is no systemic trend in the UV−vis reflection spectra of these samples as the degree of vacuum changes (data not shown). Recently, several groups32,33 studied the formation kinetic of CNC chiral liquid crystals in solution and their resultant film. Their results showed that the self-assembly of CNCs is considerably more subtle than previously suspected, with complicated kinetics involved in determining the helical pitch. Herein, it is thought that the

results show that the samples pretreated with short durations of sonication (7 and 10 h, denoted S1 and S2, respectively) did not present any obvious reflection peaks in the wavelength range of 300−800 nm, whereas those with longer durations of sonication (12 and 15 h, denoted S3 and S4, respectively) demonstrate clear reflection peaks, which are blue-shifting with increasing sonication time. For a chiral nematic structure, light at a normal incidence is selectively reflected according to λmax = nP, where λmax, n, and P are the maximum reflected wavelength, the average refractive index of the film, and the helical pitch length, respectively.29 Because the refractive index of CNC is 1.54 and λmax of S3 and S4 corresponds, respectively, to 526 and 460 nm in the UV−vis spectra, P values of these two samples are therefore calculated to be 342 and 300 nm. The internal structure of the CNC solid films was further examined by SEM. As shown in Figure 3, at a fracture perpendicular to the surface of S1 and S2, there seems to be no ordered arrangement of CNC rods. However, when observing the cross-section of S3 and S4, a highly ordered layered structure at the submicrometer level could be identified clearly. The periodic layered structure is present throughout the entire thickness. In the iridescent CNC film, the periodic band is generally believed to be related with a 180° rotation of the chiral nematic director (i.e., a half helical pitch), that is, to P/ 2.30,31 With such an assumption, the helical pitch of samples S3 and S4 was 354 and 316 nm, respectively, according to the SEM images, which were roughly in agreement with the previously values determined from the peak position of the UV−vis spectra of these films. Meanwhile, the larger helical pitch of S3 (P = 354 nm) compared with that of S4 (P = 316 nm) indicates that the increasing duration of the ultrasonic treatment causes a slight decrease in the helical pitch, which, in turn, causes a blue shift of the reflection wavelength. This finding is different from Beck’s observation on samples obtained with the EISA method.10 Such inconsistent results may be caused by the different sampling methods. On the other hand, a much longer sonication time is used in our case, as mentioned above. D

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solution volume and the degree of vacuum influence the flow rate of the filtered water. Then, a change in the kinetic of the CNC self-assembly process will affect the helical pitch of the resultant film. However, to clarify this phenomenon, more detailed work is required to investigate the formation kinetic of CNC chiral liquid crystals under conditions of vacuum filtration.

4. DISCUSSION 4.1. The Nature of the Internal Structure of the Iridescent Films Prepared by VASA. Until now, cellulose films obtained via VASA have been typically described as freestanding and transparent, and no mention of their structural color patterns has been made.34,35 The iridescent color presented in this work reminds us of the classic left-handed helical model of cholesteric liquid crystals,36 which has often been used to explain the structural origin of CNC iridescent films; therefore, we also use it to account for the layered structure observed in the SEM cross-sectional images in Figure 3c,d. However, the flow field may alter the liquid crystal’s selfassembled structure during the VASA process. Therefore, it is necessary to examine the internal structure of the iridescent films obtained via VASA in more detail. To confirm the nature of the liquid crystal structure in the resulting iridescent films via VASA, we characterized the morphology of their fracture structure, including both vertical and oblique sectional SEM images. As denoted by the red cycles in Figure 7a, the picture taken by the optical microscope

Figure 4. (a) Optical photographs of CNC iridescent solid films prepared with various CNC suspension volumes; (b) SEM images of the cross-sections of the films showing the thickness of the corresponding films in panel a; (c) plot of film thickness as a function of CNC suspension volume.

Figure 5. SEM cross-sectional images of CNC iridescent films produced from different CNC solution volumes: 28 mL (a), 21 mL (b), 17.5 mL (c), and 14 mL (d).

Figure 7. (a) Microscope image of a fracture of a CNC iridescent film; (b, c) SEM images of a cross-section perpendicular to the film’s surface and an oblique section of the film, as indicated by the red cycle in panel a, respectively; (d) CD spectra of an iridescent CNC film (sonicated for 14 h) in transmission mode. The black and red lines are measurements of the original film and that gound with KBr powder, respectively.

shows that there are different micro domains in the broken fracture surface of the film, which are caused by inhomogeneous bending forces when preparing sample by hand. The corresponding SEM picture (Figure 7c) taken perpendicular to the film surface shows a twisted layered structure, with the spacing between the layers on the order of the submicrometer level, whereas the one taken from an uneven oblique section (Figure 7b) demonstrates that the CNC nanorods are tightly

Figure 6. Digital photographs of CNC iridescent films prepared with various degrees of vacuum.

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the pitch of the chiral nematic phase and results in a red-shift in the reflection wavelength of the final solid iridescent films. Meanwhile, it was shown that the effects of sonication on treated CNC suspensions are permanent and cumulative. They proposed that ultrasonic treatment may involve the expulsion of ions from the bound-water layer and screen the electrostatic repulsions between the CNC particles. Herein, it was found that a long sonication time is a necessary condition to obtain the iridescent CNC solid films via VASA. Moreover, it was also found that the color of CNC solid films via VASA could be tuned by changing the ultrasonic pretreatment duration. To investigate the origin of this phenomenon, we first investigated the effect of ultrasonic pretreatment on the crystal structure and crystallinity of CNC nanorods. Figure 9a,b shows the typical 2D WAXD patterns of the CNC iridescent film taken from two measuring directions, i.e.,

arranged in pseudolayers, with their long axes parallel to the plane of the layers. These observations suggest that the nematic liquid crystal structure of CNCs is indeed formed in the iridescent CNC films. To clarify the chirality of the VASA structure, CD measurements were performed. Unexpectedly, when the CD measurement of the film was done directly, the iridescent film with the thinnest thickness (ca. 76 μm) in our samples showed very strong chirality, so the CD signal is easily saturated, like the black line in Figure 7d. So, we also performed a CD experiment with the KBr squash technique to decrease the ellipticity signal, as depicted by the red line in Figure 7d. In both cases, positive ellipticity signals are observed (Figure 7d), which suggests a left-handed character of the nematic liquid crystal structure of the CNCs in the iridescent film prepared via VASA. The chirality arrangement of the CNC nanorods could be also identified in a representative SEM picture of a smoothly oblique section of a CNC iridescent film. As depicted in Figure 8a, the fan-like arcing pattern could be observed if the cross-

Figure 8. (a) Illustration of the fan-like arcing appearance generated from the oblique section through the chiral nematic structure. (b) SEM image of an oblique section of a CNC iridescent film prepared via VASA.

section is made at an oblique angle to the left-handed helicoidal arrangement of the CNCs. The arcing pattern, denoted by the white line in Figure 8b, highly resembles the morphological characterization of CNC solid films obtained via the EISA technique in previous literature.37 This observation further confirms that the structural origin of the CNC iridescent films obtained via VASA could be ascribed to the classic left-handed helical model of cholesteric liquid crystals. The evidence presented above suggest that, during the VASA process, the flow field does not destroy the chiral liquid crystal self-assembly structure of CNCs. The internal structure of iridescent films obtained via VASA is essentially the same as that via the EISA process. However, it is worth mentioning that the helical axis orientation of chiral nematic structures in the CNC iridescent films prepared via the VASA technique is more uniform than that obtained via the EISA process, as evidenced by the highly ordered layered structure across the entire film thickness. The remarkably uniform chiral nematic structure, including the helical orientation and the pitch, determines the apparent single color of the resulting film obtained via VASA, as demonstrated in previous optical photos (Figures 2, 4, and 6). 4.2. Origin of the Ultrasonic Pretreatment Effect on the Assembly Behavior of CNCs. The effects of ultrasound treatment on the iridescence10 and self-assembled structure38 of CNC solid films via the EISA process have been explored in previous work. Beck et al.10 reported that the ultrasonic energy provided to the NCC suspension prior to film casting increases

Figure 9. Typical 2D WAXD patterns of a CNCs solid film measured with edge (a) and through modes (b); corresponding 1D integrated WAXD profiles of CNC films prepared with various sonication times (c, d).

parallel to and through the film’s surface, respectively. The results revealed that CNC nanorods take the isotropic orientation in the plane of film (Figure 9a), whereas they have a highly oriented structure in the plane that is perpendicular to the surface of the film, as evidenced by the discrete diffraction spots in Figure 9b. The corresponding 1D integrated WAXD profiles of CNC films prepared with various sonication times are depicted in Figure 9c,d. It should be mentioned that the Mo Kα (λ = 0.71 Å) X-ray beam was used in our measurement. For comparing our WAXD result with that in the literature, the 2θ angle was recalculated with the normally used wavelength of 1.54 Å (Cu Kα). As shown in Figure 9c,d, there is no discernible change in both WAXD profiles with the increasing sonication time. This observation suggests that the input energy of the ultrasonic pretreatment did not change the inner crystal structure or even the degree of crystallinity of the CNC nanorods. Then, we further investigated the effect of prolonged ultrasonic time on the particle size and zeta potential of CNC suspensions. DLS measurements show that the CNC F

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nanorods obtained have an average length of 134 ± 68 nm, which is comparable to the values of 180 ± 75 nm in the literature.39 The plot of the rh and zeta potential versus sonication duration are shown in Figure 10. It was found that

Therefore, the decrease of the negatively charged sulfate surface group caused by prolonged sonication time, which, in turn, reduces electrostatic repulsion of the CNC nanoparticles, may be responsible for the reflection wavelength blue-shifting of the CNC iridescent films obtained via VASA. Our EDX data also show that Cu ions used to accelerate the acid hydrolysis reaction and improve the yield of CNCs could not be detected in the resulting CNC films (see Figure S1 in the Supporting Information). This indicates that the residual impurities, especially the free ions and small molecules, in the as-prepared CNC suspensions could be effectively removed by vacuum filtration. Therefore, another advantage of the VASA method to prepare CNC iridescent films is that it circumvents the time-consuming dialysis, which is normally required for the EISA process.

5. CONCLUSIONS In summary, we demonstrate that the VASA technique can be used to fabricate highly oriented and large area CNC iridescent films with uniformly layered helical structures. Our results revealed that the internal structure of the iridescent films obtained via VASA resembles the chiral nematic liquid crystal structure used to describe the self-assembled structure of the iridescent film via EISA. It was found that a long ultrasonic pretreatment is necessary to obtain the CNC iridescent film via the VASA process. Nevertheless, long periods of ultrasonic treatment had no detrimental effect on the inner crystal structure and crystallinity of CNC nanorods. In addition, we show that the iridescent color of the CNC film produced via VASA could be tuned by sonication time, suspension volume, and degree of vacuum. Moreover, the thickness of the resulting iridescent film could be easily controlled by the suspension volume. Compared to the EISA method, the VASA technique could be used to fabricate colorful CNC films with more homogeneous internal chiral nematic ordering along the entire film. This new method, which could overcome the limitations of the EISA process, should be impactful toward the production of advanced optical materials by utilizing the self-assembly behavior of CNCs.

Figure 10. Plot of the particle radius and zeta potential of CNC suspensions measured with DLS as a function of sonication time.

the particle size decreases with sonication time in the first 5 h and that the sonication level does not significantly change the particle size in the following period. In a recent work, Liu et al.40 also reported that sonication treatments markedly shortened the mean particle length and width of CNC nanorods. They proposed that CNCs could be broken into smaller nanocytstals due to inherent defects or cracks caused by sulfuric acid hydrolysis. Different from the change in the particle size, the zeta potential of the CNC suspension always shows a subtle linear increase with sonication time. However, it is noted that, even with prolonged sonication pretreatment, the value of the zeta potential was still below −30 mV, which is considered to be a critical value to retain the stable dispersive state of the nanoparticles in water via electrostatic repulsion. According to previous reports, CNC particles of a longer average length have reflection bands shifted toward the red end of the visible spectrum, whereas shorter CNC particles produce films with the reflection wavelength shifted toward the blue end.41 Nevertheless, the particle size of the CNC nanorods shows no change with sonication time above 5 h. Therefore, the blue shifting displayed in Figure 2b induced after long (>10 h) ultrasonic treatment should be related to the change in surface properties rather than a size change of the CNC nanorods. It is well-known that the content of negatively charged sulfate surface groups determines the surface repulsion of CNC particles produced by sulfur acid hydrolysis. In general, long sonication times of CNC suspensions were performed at intervals, and the surrounding bath temperature was kept below 40 °C. Therefore, it was expected that the desulfation of the CNC suspension should not occur.42 However, according to the EDX data (Table 1) of solid CNC films, the decrease of sulfur content indicates that desulfation takes place as the sonication time increases, which also provides justification for the subtle increase in the zeta potential during sonication.

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Helical pitches of the iridescent films as a function of CNC suspension volume and a typical EDX spectrum of a CNC iridescent film obtained by VASA. This material is available free of charge via the Internet at http://pubs.acs.org.

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0

7

10

12

15

content of sulfur element (%)

9.87

8.13

7.64

6.50

5.37

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-532-84022791. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Taishan Mountain Scholar Foundation (TS20081120) and Natural Science Fund for Distinguished Young Scholars of Shandong Province (JQ200905) is greatly appreciated.

Table 1. Weight Percentage of Sulfur from EDX Measurements of CNC Films Sonicated for Different Amounts of Time sonication time (h)

ASSOCIATED CONTENT

S Supporting Information *

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