OrganicâInorganic Hybrid Thin Films Fabricated by Layer-by-Layer

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Organic-Inorganic Hybrid Thin Film Fabricated by Layer-by-Layer Assembly of Phosphorylated Cellulose Nanocrystal and Imogolite Nanotubes Linlin Li, Wei Ma, Yuji Higaki, Kazutaka Kamitani, and Atsushi Takahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03107 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Organic-Inorganic Hybrid Thin Film Fabricated by Layer-by-Layer Assembly of Phosphorylated Cellulose Nanocrystal and Imogolite Nanotubes

Linlin Li, 1 Wei Ma, 2 Yuji Higaki, 1, 2, 3 Kazutaka Kamitani,3 Atsushi Takahara*1, 2, 3

1Graduate

2International

School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan

3Institute

for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395,

Japan

Abstract Phosphorylated

cellulose

nanocrystal

(P-CNC)/imogolite

nanotube

(natural

aluminosilicate nanotube) hybrid thin films were fabricated by spin-assisted layer-by-layer (LBL) assembly. Phosphorylation of CNC with diammonium hydrogen phosphate ((NH4)2HPO4) was carried out to introduce phosphate groups on CNC surface for the enhanced interaction with imogolite. Structure of the P-CNC/imogolite thin film was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and grazing incidence wide angle X-ray diffraction (GIWAXD). The film thickness increased linearly with the increment of the P-CNC/imogolite bilayer. Benefitting from the strong affinity between the phosphate group of P-CNC and the AlOH group of imogolite, the P-CNC/imogolite thin films were quite stable in water within a wide range of pH values, compared with the deterioration of the CNC/imogolite film

ACS Paragon Plus Environment

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under the same soaking conditions. Introduction Since the layer-by-layer (LBL) assembly was firstly proposed by Iler in 1966,1 this technique has become a powerful tool for the fabrication of multilayered films with tunable compositions and properties.2-3 LBL is available for a variety of materials including polyelectrolytes and colloidal particles.4-7 Recently, cellulose nanocrystal (CNC) has emerged as a promising building block for the fabrication of multicomponent materials. CNC can be easily prepared from a variety of natural resources and has a rigid rod-shaped morphology. It is mechanically strong due to the unique crystal structure and numerous hydrogen bonds within the crystals. Generally, during the preparation of CNC, oxidation of the hydroxyl group to carboxylic acid group in the hydrolysis procedure will result in the negatively charged surface of CNC.8 Based on this property, CNC can be applied to prepare LBL films with positively charged polymers or nanostructures through electrostatic interaction.8-10 The LBL films of CNC with chitosan and poly(allylamine hydrochloride) (PAH), as well as gibbsite nanoplatelets have been reported.11-12 Nevertheless, the hybrid films produced by the electrostatic interaction as the driving force are generally not stable and are prone to be damaged in harsh conditions, for instance, buffer solutions of different pH.13 In recent years, due to the excellent mechanical property and other functions, such as gas barriers and flame retardancy, the nanocomposites of nano-fibrillated cellulose and


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clays, have attracted a great deal of research interest.14-19 Tubular clays, such as halloysite and imogolite, have been studied extensively due to the unique structure and properties.2021

As shown in scheme 1c, imogolite is one of the naturally occurring tubular clay with a

formula of (OH)3Al2O3SiOH.22 It has an outer diameter of ca. 2.0 nm and length of hundred nanometers. The aluminol (Al-OH) groups that exposed on the outer surface can be protonated under acidic conditions, forming positively charged Al-OH2+ groups.23 With this property, imogolite has been employed to fabricate thin films with polyelectrolytes by LBL assembly.23-24 Through the electrostatic interaction of positively charged Geimogolite and negatively charged CNC, Cathala et al. reported a CNC/Ge-imogolite nanotube multilayered thin film.25 Our previous studies indicated that imogolite nanotubes can interact strongly with phosphonic acid groups in aqueous solution through the interaction between Al-OH groups and phosphonic acid.26-28 Based on those studies, it is believed that the interaction between CNC and imogolite could be improved if phosphate group is introduced onto the CNC surface. In this study, we reported an LBL assembly thin film composed of phosphorylated CNC and imogolite nanotubes. CNC was phosphorylated by an ammonium salt of phosphoric acid ((NH4)2HPO4) in urea. The P-CNC and imogolite nanotubes were alternatively assembled onto substrate by spin-assisted LBL procedure. The assembly process of the hybrid thin films was tracked by Fourier transform infrared (FT-IR) and AFM. The stability of the films was evaluated by soaking in aqueous solutions of different pH. Surface morphology observation indicated that the layer was densely stacked, and nano-cavities


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appeared on the films because of both the high aspect ratios and stiffness of imogolite and CNC.

Experimental Section Materials Milli-Q water with a resistance above 18.2 MΩ cm was used throughout the study. Diammonium hydrogen phosphate ((NH4)2HPO4) (≥ 98 %) and urea (≥ 98 %) were purchased from Sigma-Aldrich. Hydrochloric acid (35 %), acetic acid (≥ 93 %), sodium hydroxide (≥ 97 %), sodium dihydrogen phosphate (NaH2PO4) (≥ 99 %) and sodium hydrogen phosphate (Na2HPO4) (≥ 99 %) were purchased from Wako Pure Chemical Industries, Ltd. CNC ((C6O5H10)22-28SO3Na) was kindly supplied by CelluForce, Montreal, Canada.


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Scheme 1. (a) The phosphorylation process of CNC, (b) fabrication of P-CNC/imogolite film by spin-assisted LBL assembly procedure and (c) the structure of imogolite nanotube.

Imogolite was synthesized according to a previously reported method.29 After synthesis, the suspension was dialyzed using cellulose dialysis membranes (molecular weight cut-off of 5000–8000 Da from Spectrum Labs) for 5 weeks. The concentration of the final imogolite suspensions were 0.33 mg L-1. Phosphorylation of CNC. Phosphorylation used in this study was based on the procedure for preparation of phosphorylated polyvinyl alcohol.30-31 The phosphorylation of CNC was carried out by mixture of urea (5 mol) and (NH4)2HPO4 (1 mol), followed by added to a 1 wt % CNC dispersion under continuous stirring. The mixture was dried in an oven at 70 oC,

then cured at 150 °C for 60 min under N2 atmosphere. After that, the cured solid was


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re-dispersed into water and dialyzed for 5 weeks to remove any unreacted chemicals (see scheme 1a). Preparation of P-CNC/Imogolite hybrid film. Hybrid films were assembled on silicon wafers or glass plates. The silicon wafers (Osaka Special Alloy Co., Ltd.) and glass plates (Matsunami Glass) were firstly sonicated in methanol and acetone for 30 min, respectively, then exposed to vacuum ultraviolet light (VUV, λ = 172 nm) for 10 min. The multilayers were prepared immediately after VUV treatment. Spin coating was conducted with a spincoater (Kyowa Riken Co., Ltd., K-359S1). 0.5 mL of imogolite suspension (0.3 mg/mL) was dropped on the substrate. After 5 s, the substrate was spun at 1500 rpm for 10 s, subsequently at 3000 rpm for 40 s. The film was then rinsed with 2.0 mL of water and spun at 1500 rpm for 10 s, subsequently at 3000 rpm for 40 s. After the assembly of imogolite, 0.5 mL of P-CNC suspension (0.3 mg/mL) was assembled under the same protocol. One bilayer is defined as an imogolite single layer and a P-CNC single layer. Multilayered LBL films were assembled by repeating the cycle (scheme 1b). The films obtained were dried under vacuum and stored at 25 °C in vacuum. CNC/imogolite hybrid films were prepared through the same method with P-CNC/imogolite films. Characterization FT-IR spectroscopic measurement was carried out with a Spectrum One (PerkinElmer Japan Co., Ltd.) with a resolution of 1.0 cm-1 at room temperature. AFM observation (tapping-mode) was carried out using an SPA400 with a SPI 3800 Probe Station (SII Nano


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Technology, Inc.). SI-DF40 rectangular cantilever with a spring constant of 33 N m-1, and a resonant frequency around 303 kHz was used. Hybrid film thickness was measured from the height gap of the scratch track, and the averaged value of 3 different points was obtained. SEM images of the films were observed using a JEOL-JSM-7401F SEM (USA, Inc.) at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) was carried out on an XPS-APEX (Physical Electronics Co., Ltd.) at 2.0 ×10-9 Pa using a monochromatic AlKα X-ray source of 100 W. The thermal gravimetric analysis (TGA) was performed on a thermo-balance, SII-EXSTAR 6000 DSC 6200 (Hitachi High-Tech Science Corporation, Tokyo, Japan), with a heating rate of 10 °C min-1 under nitrogen atmosphere from 25 to 700 °C. Zeta potential of the CNC and P-CNC in Na2HPO4/NaH2PO4 buffer at different pH were measured on ELSZ-2000 (Otsuka Electronics Co., Ltd) at 25 °C and each sample was performed 5 times. Powdered X-ray Diffraction of CNC and P-CNC was carried out by X-ray diffractometer (Rigaku SmartLab) with Cu target (λ = 0.15406 nm) in reflection mode with acceleration voltage 45 kV, and anode current 200 mA. Scattering vector q(nm-1) is defined as q = (4π/λ) sinθ, where λ and θ are the wavelengths of the X-ray and scattering angle, respectively. Grazing-incidence wide angle X-ray diffraction (GI-WAXD) test was carried out at BL40B2 beamline in SPring-8. The wavelength of X-ray beam was set at 0.1 nm. A 1475 × 1579 pixels imaging plate detector (PILATUS3, DECTRIS, Switzerland) was used to record the scattering patterns. The pixel resolution of the detector was 172 μm. The camera length was set at 592.917 mm. The scattering vector was calibrated using the peak positions of silver behenate.


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Results and Discussion Phosphorylation of cellulose using phosphorylating agents such as phosphorus oxychloride (POCl3), phosphorus pentoxide (P2O5) has been extensively studied for the fabrication of flame-retardant materials and control of biomineralization processes during last decades.32-34 Phosphorylation of cellulose nanofibrils by diammonium hydrogen phosphate ((NH4)2HPO4) in the presence of urea was reported recently.35 Excess amounts of urea were used to prevent the degradation of cellulose caused by the released phosphoric acid at elevated temperature.34-35 Compared with other phosphorylating agent, (NH4)2HPO4 has the advantages of less toxicity and inducing lower level of cellulose hydrolysis during the whole reaction process.35 Therefore, in this work, we choose this chemical to phosphorylate CNC. The XPS spectrum of CNC before and after phosphorylation treatment were shown in Figure 1a, in which the C1s and O1s signals were observed in both neat CNC and P-CNC. Two minor peaks of sulphur group (S2s and S2p) and one minor peak of silicon (Si2p) were observed for the neat CNC. Those elements are possibly due to the residual sulfuric acid during the preparation of CNC and surface contamination, respectively. P-CNC exhibited three new peaks at 134.4, 190.5, and 401.4 eV, which were assigned to P2p, P2s, and N1s, respectively.32-34, 36 The appearance of those peaks indicates that CNC was successfully phosphorylated. The relative surface concentrations of C, O, N and P atoms were calculated by integrating the peak area (Table S1). After phosphorylation,


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the chemical composition of carbon decreased from 63.3 % to 54.3 % because of the surface modification by (NH4)2HPO4.

Figure 1. (a) XPS spectra and (b) FT-IR spectra of neat CNC and P-CNC.

The success in phosphorylation was also verified by FT-IR spectroscopy (Figure 1 b). Both the CNC and P-CNC showed characteristic bands of C−O−C at 1055 and 1075 cm−1 in the glycosidic units.37 Absorption bands appeared at 1250 cm−1 and 925 cm−1 in the PCNC are assigned to the P = O stretching and P-OH stretching 32, 37-38, respectively. The PCNC also exhibited new band at 3156 cm−1 which are attributed to the N-H stretching. TGA curves (Figure S1) showed that neat CNC and P-CNC were both thermally stable before 185 oC. After phosphorylation, the cellulose decomposition temperature reduced significantly, because of an earlier dehydration to produce char.39-41 While for P-CNC, there was more carbonized residue remained at 700 oC. Zeta potential analysis indicated that both of the CNC and P-CNC were negatively charged, while imogolite was positively charged in water of different pH (Figure S2). The


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zeta potential value of P-CNC (-35 mV) was obviously higher than that of CNC (-15 mV). This was because a large amount of negatively charged phosphoric acid groups were introduced on the CNC surface. Due to the existence of opposite charges on the surfaces, imogolite and P-CNC (or CNC) was assembled into multilayer films through LBL assembly. Based on the high-resolution XPS spectra of C1s, O1s and P2p in P-CNC (Figure S3), the degree of surface phosphorylation on the CNC was estimated by ratio of the integrating peak area of P-O-C and C-OH. The surface phosphorylation percentage is 3.4 %, similar to the previously reported value.35 The powdered X-ray diffraction was also performed (Figure S4). Both the CNC and P-CNC showed the peaks at q = 10.5, 15.8, and 22.6 nm-1, suggesting the Cellulose-I structure. 42-44 These peaks are assigned to the 110, 200 and 004 planes, respectively. 43-44 The crystallinity index of CNC and P-CNC could be roughly calculated by the height ratio between the intensity of the crystalline peak (I200 - Iam) and total intensity (I200) after subtraction of the background signal measured without cellulose sample,45 where the Iam is the intensity of the background scatter. After the calculation, the crystallinity index of CNC was 71.5%, while the P-CNC was 50.2%, indicating the crystallinity decreased during the phosphorylation process. Similar phenomenon was also reported in other work.46 The morphologies of CNC, P-CNC, and imogolite were also investigated by AFM, as shown in Figure 2. From these images, it could be confirmed that before and after phosphorylation, the morphology and size of CNC did not change obviously (Figure 2a, b). This is because the phosphorylation reaction happened only on the surface of CNC.35


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The rod-shaped morphology and size of imogolite (Figure 2c) were similar to those of CNC and P-CNC.

Figure 2. AFM topographic images of (a) neat CNC，(b) P-CNC, and (c) imogolite. The dialyzed samples diluted, followed by dip on silicon wafer.

Figure 3. Bilayer number dependence of (a) FT-IR absorbance intensity at 3340 cm-1 and (b) thickness determined by AFM at each stage of the P-CNC/imogolite and CNC/imogolite LBL assembly.


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Figure 4. AFM height images on a 2 μm × 2 μm area of (a) single imogolite layer, (b) one bilayer of CNC/ imogolite film, and (c) one bilayer of P-CNC/ imogolite film. All the layers were spin-coated on the silicon wafers. The height profiles were in accordance with the black lines in the AFM images.

Hybrid film was fabricated by using P-CNC and imogolite via LBL assembly. Compared with dip-assisted LBL assembly, spin-assisted LBL features the advantages of time-saving and well-control of the thickness. In this work, spin-assisted LBL assembly was employed to prepare the multilayered films. The growth of the multilayered film prepared by repeating LBL assembly was tracked by FT-IR absorbance at 3340 cm-1 (Figure S5), and the absorbance intensity was plotted as a function of a bilayer number (Figure 3a). For this purpose, the P-CNC and imogolite solutions were alternatively spincoated on a CaF2 plate, which is transparent at 3340 cm-1. As shown in Figure 3a, for P-


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CNC/imogolite thin films, the absorbance intensity increased linearly with the increase in bilayer number (R2 = 0.998), indicating uniform assembly of P-CNC/imogolite layers. By contrast, the CNC/imogolite thin film exhibited less regular growth (R2 = 0.977). Meanwhile, the absorbance intensity of P-CNC/imogolite increased much faster than that of the CNC/imogolite film, indicating the thickness of per P-CNC/imogolite bilayer was larger than that of the CNC/imogolite film. Thickness of the multilayered films was measured by AFM (Figure S6). The thickness growth of the first 30 bilayers coated on a silicon wafer was shown in Figure 3 b. For the P-CNC/imogolite films (red squares), the thickness of each bilayer is ca. 4 nm. However, for the CNC/imogolite film, the thickness of each bilayer is less than 2 nm. The thickness of each bilayer in this study is not only much less than that of dip-assisted LBL assembly of polyelectrolyte/CNC films but also less than clay/CNC films, both of which are around tens of nanometers.8-12, 25 In our previous work, the thickness of each polymer/imogolite bilayer prepared by dip-assisted LBL assembly was also larger than that of spin-assisted method.24 This is because the spin-assembly process is non-equilibrium and the process can be influenced by factors including the interaction between the single layers, centrifugal force and air shear force.24-25 In this study, both centrifugal force and air shear force were same for P-CNC/imogolite and CNC/imogolite film under the same parameters of assembly. Therefore, the main influence factor is the interaction between the layers. The thickness growth rate of CNC/imogolite is much lower than that of P-CNC/imogolite films in despite of the similar diameters of CNC and P-CNC (Figure 2). This is possibly because


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the interaction between CNC and imogolite is much weaker than that of P-CNC and imogolite. In addition, both CNC and imogolites were loosely charged nanomaterials compared with polyelectrolytes.25 In the LBL assembly process of CNC/imogolite, loosely layered materials have high possibility to be washed away when rinsed, leading them to be sparsely dispersed onto the substrate. The CNC/imogolite thickness thus increased slowly. Conversely, the stronger interaction between P-CNC and imogolite could make the top layer uniformly dispersed onto the bottom layer, resulting in a rapid thickness growth. As it has been known, the assembly of initial layers has a big influence on the whole assembly process. AFM measurement for the initial single imogolite layer and one bilayer of the two kinds of thin films was thus carried out, as shown in Figure 4. For the single imogolite layer (Figure 4a), the height profile of the cross section indicated the layer exhibited plenty of nanotubes, which uniformly absorbed on the silicon wafer surface, forming a dense and homogeneous layer. This layer was much better than the layer prepared by dip-assisted LBL assembly, in which, the imogolites were agglomerated, forming a well-networked structure.23-25 In this imogolite layer, the nanotubes were rarely aggregated. The CNC single layer was clearly appeared when spin-coated on the imogolite layer (Figure 4b). Moreover, unlike the CNC absorbed onto the PAH layer, forming a dense CNC layer,11 CNC loosely adsorbed on the imogolite layer, some of them were agglomerated and the surface appeared rough (Figure 4b). This may be due to the weak surface charge of both CNC and imogolite, resulting in weak adsorption between them. Consequently, the thickness of CNC/imogolite film was significantly less than prediction.


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When the P-CNC was assembled on the imogolite layer, a dense P-CNC layer was observed (Figure 4c). In addition ，the surface of the P-CNC layer was much smoother than that of the CNC layer. The P-CNC had a better package not only because it had better dispersion in the solution due to the larger zeta potential values, but also because of the stronger affinity with imogolite.27 Therefore, it could be predicted that as the next imogolite layer was spin-coated on the CNC or P-CNC layer, the amount of imogolite should be more in P-CNC/imogolite film than that of CNC/imogolite film, resulting in the higher growth rate and larger thickness in P-CNC/imogolite film.

Figure 5. (a) 2D GI-WAXD pattern of 20 bilayers P-CNC/imogolite film and (b) the corresponding 1 D plot of the out-of-plane and the in plane diffractions.

Figure 5 shows the 2D GI-WAXD pattern of the hybrid film with 20 bilayers and the corresponding 1 D plot of out-of-plane and in plane diffractions. The diffraction arcs were observed only in the out-of-plane direction. In the 1D plot of the out-of-plane diffraction,


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the peaks at around q = 2.7 and 6.9 nm-1 indicated the parallel bundles of imogolite nanotubes.47-50 The GI-WAXD result indicated that imogolite was assembled parallel to the substrate. The diffractions due to CNC were not observed in the q range of 2 to 15 nm-1. This is because the diffractions of CNC crystals are located at the region above 15 nm-1. Moreover, CNC should not form bundles, since no signal of bundle structure was observed.

Figure 6. SEM images of (a) 7 P-CNC/imogolite bilayers, (b) 20 P-CNC/imogolite bilayers, (c) 7 CNC/imogolite bilayers, and (d) 20 CNC/imogolite bilayers. All the scale bars are 200 nm.

Surface morphology of 7 and 20 bilayers of the two kinds of films were observed by SEM (Figure 6). Densely stacked nanotubes and nanorods produced thick layers and noticeable nano-cavities appeared in the P-CNC/imogolite films (Figure 6 a, b). Those


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cavities were regarded as the defect of the films, they were formed because of both the strong stiffness and high aspect ratio of nanotubes and nanorods, limiting the rearrangement of the layer structure.24,51 Specially, for the CNC/imogolite films, the nanotubes and nanorods agglomerated and tangled together, resulting in the lower dense stacked nanostructures (Figure 6c) compared with corresponding films of the PCNC/imogolite (Figure 6a), leading to the less thickness of CNC/imogolite films. The agglomeration was more clearly displayed in the 20 bilayers, and the cavities with much larger diameter also appeared (Figure 6d). In addition, from the top layers of the two kinds of films, it seems that the nanotubes in CNC/imogolite film were anisotropic and loosely stacked, forming the random-array coating, while the components in P-CNC/imogolite film have a superior array than that of CNC/imogolite film. The array was formed because imogolite has a strong tendency to form bundle structure during drying process.49-50 In the case of other tubular clay, for instance, halloysite nanotubes, though they could selfalignment in the polyelectrolyte solution,52 this array structure in the LBL film has not yet been reported.53 This is presumably because the external surface of halloysite has more

defects compared with that of imogolite.

Figure 7. Intensity evolution of FT-IR absorbance of P-CNC/imogolite (red squares) and


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CNC/imogolite (black dots) multilayers after being immersed in the (a) pH = 4, (b) pH = 6, and (c) pH = 8 Na2HPO4/NaH2PO4 buffer solutions for varied time. The stability of the multilayered films in water of different pH was investigated. Thin films with 15 bilayers were immersed in buffer solutions for varied period followed by rinsing with water, then the IR absorbance intensity at 3340 cm-1 was measured (Figure 7). In the weak acidic water (pH = 4), the absorbance of CNC/imogolite was almost zero after being immersed for 3 h, indicating that the CNC/imogolite film was completely dissociated. The CNC/imogolite films became a little bit more stable when the pH is higher (pH = 8), as the absorbance kept as high as 0.007 after being immersed for 5 h. However, it is still less stable compared with the P-CNC/imogolite film, whose IR absorbance only slightly dropped after being immersed for 5 h in all solutions. The difference in film stability is because the main attraction force between CNC and imogolite was the electrostatic and hydrogen bonding interactions. While for P-CNC and imogolite, in addition to the electrostatic and hydrogen bonding interactions, the special interaction between phosphate group of P-CNC and Al-OH group of imogolite also existed,26-28 leading to the superior stability of the P-CNC/imogolite hybrid film.

Conclusions P-CNC/imogolite and CNC/imogolite hybrid thin films were prepared by spin-assisted LBL assembly. Phosphorylation of CNC was achieved and confirmed by XPS, FT-IR and


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TGA analysis. FT-IR measurement and AFM observation revealed that P-CNC/imogolite thin film formed uniformly and the thickness increased linearly through the assembly process, which were better than the CNC/imogolite hybrid thin film. Moreover, the thickness growth rate of P-CNC/imogolite films was much more rapid than that of CNC/imogolite films. The stability of the P-CNC/imogolite and CNC/imogolite films in different pH buffer solutions indicated the inferior stability of CNC/imogolite multilayers, while that the P-CNC/imogolite thin film was stable in both the acid and alkaline conditions.

Acknowledgments The authors acknowledge the financial support of JSPS Grant-in-aid for Scientific Research (A) (Grant No. 26248053, 17H01221). Linlin Li and Wei Ma contributed equally to this paper. GI-WAXD measurements were conducted on the BL40B2 beamline in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal No. 2018A1177). We thank Dr. Noboru Ohta for experimental assistance in BL40B2. The Supporting Information is available free of charge on the ACS Publications website.

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