Self-Assembly of Hierarchically Structured Cellulose@ZnO Composite

Research Article pubs.acs.org/journal/ascecg

Self-Assembly of Hierarchically Structured Cellulose@ZnO Composite in Solid−Liquid Homogeneous Phase: Synthesis, DFT Calculations, and Enhanced Antibacterial Activities Si-Wei Zhao,† Ming Zheng,‡ Xiao-Hang Zou,† Yuanru Guo,*,† and Qing-Jiang Pan*,‡ †

Key Laboratory of Bio-based Material Science & Technology (Ministry of Education), College of Material Science and Engineering, Northeast Forestry University, Hexing Road #26, Harbin, Heilongjiang 150040, China ‡ Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education), School of Chemistry and Materials Science, Heilongjiang University, Xuefu Road #74, Harbin, Heilongjiang 150080, China S Supporting Information *

ABSTRACT: To explore the interactions of nanoparticles and bioresources and elucidate their effects on the morphology of the resulting composite, hierarchically structured cellulose@ZnO composites have been synthesized by an environmentally friendly hydrothermal method in one step. First, self-assembly induces the formation of hierarchical three-level structures, including cellulose/ZnO nanofibers, layers, and microfibers. Then, ZnO microparticles deposit onto the surface of the third-level cellulose/ZnO microfibers and accomplish the fabrication of a cellulose@ZnO composite, which eventually defines the hierarchical morphology of synthesized materials. The self-assembly mechanism was comprehensively examined. The electrostatic attraction between cellulose and ZnO, not hydrogen bonding, was found to be the main driving force for the formation of the first-level structure. A density functional theory study was conducted to support the self-assembly mechanism by optimizing the cellulose/ZnO structures at the molecular level, computing the corresponding thermodynamic energies and examining the spectroscopic properties. A hierarchically structured cellulose@ZnO composite is found to enhance the antibacterial activities. The diameters of the inhibition zone were found to be 48.8 and 45.5 mm against the Gram-positive bacterium Staphylococcus aureus (S. aureus) and the Gram-negative bacterium Escherichia coli (E. coli), respectively. This study is expected to improve food packaging materials while utilizing our newly synthesized cellulose@ZnO composite. KEYWORDS: Cellulose, ZnO crystallites, Self-assembly, Hierarchical structure, Antibacterial activity

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INTRODUCTION Self-assembly is the process by which individual components arrange themselves into an ordered structure through weak forces (e.g., van der Waals, hydrogen bonding).1 Over the past few decades, self-assembly has been used to guide the synthesis of certain structural materials such as ZnO, Co3O4, SiO2, TiO2, fullerenes, and silk.2−7 Diverse morphologies including mesopores, hollow spheres, and nanotubes have been achieved. Interestingly, some of these morphologies have endowed materials with enhanced properties of catalytic activities, antibacterial activities, or electrochemical performance.8−12 A challenging issue that has been posed for chemical and materials scientists is how to properly utilize self-assembly to induce special structures of materials with excellent properties. In this respect, self-assembly mechanisms must be clarified. Alloying-induced self-assembly behaviors and inorganic nanoparticle−solvent interactions and nucleation, for instance, have been proposed for some synthetic reactions.13−15 Therefore, a comprehensive and in-depth understanding of the nature of self-assembly, particularly the interaction behaviors between © 2017 American Chemical Society

reaction precursors in a structurally explicit material, is still desirable. Cellulose is known to be a replenishable source of raw materials in nature. As a biodegradable and biocompatible material,16,17 it is characterized as a high-molecular-weight homopolymer of β-1,4-linked anhydro-D-glucose units. During its synthesis, cellulose chains aggregate into microfibrils that display cross-sectional dimensions ranging from 2 to 20 nm, depending on the source of celluloses. The aggregation phenomenon occurs primarily due to van der Waals forces and intra-/intermolecular hydrogen bonds. Building on this interaction mechanism, many studies have been carried out on applications of cellulose in green and sustainable development. In recent years, researchers have focused on the preparation of cellulose nanocrystals (CNCs) and studied their self-assembly behavior. The self-assembly of cellulose was well summarized in Received: March 19, 2017 Revised: July 2, 2017 Published: July 5, 2017 6585

DOI: 10.1021/acssuschemeng.7b00842 ACS Sustainable Chem. Eng. 2017, 5, 6585−6596

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ACS Sustainable Chemistry & Engineering

Figure 1. (a−c) XRD patterns of (a) cellulose, cellulose@ZnO composite prepared at pH 10, and sintered ZnO; (b) composites prepared at different pH values; and (c) composites without further washing treatment and (d) (002)/(100) intensity ratios for all types of ZnO-containing samples.

a 2010 review article.18 Different cellulose sources, including wood, plant leaves, and cotton, have been investigated, and the resulting CNCs had 5−10-nm average diameters and average lengths of about 100 nm.19 Generally, cellulose can be formed into cellulose nanofibers by mechanical treatment or pulping processes. The obtained cellulose nanofibers have high aspect ratios, with typical lateral dimensions of 5−20 nm and longitudinal dimensions ranging from tens of nanometers to several micrometers.20 Cellulose nanofibers are much longer, making them undissolvable and hard to form into gels. On the other hand, cellulose nanofibers are easily prepared, for example, by mechanical fibrillation of dilute aqueous suspensions of delignified wood pulp. Because of their aforementioned characteristics and existing/potential applications, cellulose fibers represent a burgeoning area of research. To furnish and/or enhance certain properties of cellulose, metals (e.g., Ag and Cu) and metal oxides (TiO2, ZnO etc.) have been incorporated into cellulose products.21−24 These efforts have brought much attention to the self-assembly behaviors between cellulose and the incorporated materials. As an important semiconductor nanomaterial, zinc oxide (ZnO) has been an intriguing and important subject of research in the past few decades.25−29 Because ZnO exhibits excellent photocatalytic and antibacterial activities,30−33 a great deal of work has dedicated to synthesizing ZnO-doped cellulose composites and developing applications for the resulting materials.24,34−38,21,28−32 Products in the form of filaments, papers, and foams of cellulose/ZnO have been obtained using different sources of cellulose.39−41 Although some studies in the literature have reported the preparation of cellulose/ZnO composites, most have mainly focused on improving certain performance characteristics.15,37,42 Little is known about the details of specific preparation methodologies. Previously, selfassembly through hydrogen bonding has been mentioned to

explain the formation of cellulose/ZnO composites, but no clear and adequate evidence for this process has been presented. Therefore, an unambiguous determination of the mechanism that guides the formation of cellulose/ZnO composites is still needed. In this work, a facile hydrothermal synthesis of cellulose@ ZnO composites has been carried out to explore their structures, interaction natures, and antibacterial activities. A variety of experimental characterizations demonstrated the hierarchical multilevel structures of the newly synthesized composites, which were further complemented by a relativistic density functional theory (DFT) study. The results showed that the self-assembly based on electrostatic attraction drives the formation of the first-level structure of the cellulose/ZnO nanofibers. Then, these basic building blocks stack up in an orderly manner and yield the next-level microfibers, in a process that is induced by self-assembly as a result of the intermolecular hydrogen bonding of cellulose. The unique resulting structures provided our cellulose@ZnO composites with good antibacterial activities for application as paper packaging materials.

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EXPERIMENTAL SECTION

The cellulose@ZnO composites were prepared by a hydrothermal method. Specifically, 3.0 g of zinc acetate was dissolved in 25 mL of distilled water, and then 1.0 g of cellulose was added. The mixture was stirred vigorously for 1 h, and the pH value of the solution was regulated to 10 by the addition of 1 mol·L−1 sodium hydroxide solution. A white suspension of cellulose was obtained after stirring for 30 min. The cellulose suspension was moved into a 100 mL steel autoclave and heated at 100 °C for 10 h. The white turbid liquid was filtered after being cooled to room temperature, and the white product was collected. After the product had been dried at 80 °C, it was used to synthesize cellulose@ZnO composite paper. The details of characterization, antibacterial activity testing, and computations are provided in the Supporting Information. 6586


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Figure 2. SEM images of cellulose@ZnO composites prepared at a variety of pH values.

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RESULTS AND DISCUSSION

composite surface as much as possible. As shown in Figure 1a, the pure cellulose showed characteristic diffraction peaks at 14.82°, 16.31°, and 22.71°, which are identified as the (101), (101̅), and (002) lattice planes of cellulose I. For the cellulose@ZnO composite prepared at pH 10, diffraction

Structural and Morphological Analyses. X-ray diffraction (XRD) analysis was used to identify the crystal structure of the cellulose@ZnO composite. Before testing, the samples were further washed to remove ZnO microparticles on the 6587


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overcome the mechanical force of extra washing. The other type is regular ZnO particles with quite intense (002) diffraction peaks that are formed separately in solution without directly contacting cellulose. The latter particles should be physically deposited on the surface of the cellulose/ZnO composite, and thus, they can be removed by careful washing. Some studies have proposed that hydrogen bonding is the driving force for self-assembly between ZnO and cellulose.15,37,42 We argue that a mechanism driven only by hydrogen bonding is not applicable to our case. The presence of two types of ZnO particles suggests that two different formation mechanisms occur simultaneously in one system. More importantly, hydrogen bonding between the oxygen of the ZnO crystals and the hydrogens of cellulose cannot produce ZnO particles with a diminished peak for the (002) plane. In brief, another driving force other than hydrogen bonding must exist in this system. Moreover, this driving force would be predominant to guide the self-assembly process. Scanning electron microscopy (SEM) was used to characterize the morphologies of samples that were prepared at different pH values. From Figure 2, one can see that there were two types of ZnO particles in the composites. Fine and uniform ZnO nanoparticles with sizes of 10 or 30 nm “stand” on the cellulose fibers and together form a linear architecture (see images on right side of Figure 2). The second type of ZnO particles have sizes of a few micrometers and are randomly dispersed on the cellulose surface. The SEM results confirm the XRD analyses about the presence of two types of ZnO particles and further specify them as nano- and microscale. More importantly, these SEM images intuitively support our assumption based on the XRD results, namely, that there is another driving force that dominates the self-assembly process, other than hydrogen bonding. For the cellulose@ZnO composites prepared at pH 8 and 9, the ZnO microparticles were loosely dispersed on cellulose, as can be seen in Figure 2a,c. These ZnO microparticles were about 2.5 μm long and 1 μm wide. The random distribution of particles suggests no definite interaction between the ZnO microparticles and the cellulose. In contrast, in the highresolution image (Figure 2b,d), 30-nm ZnO nanoparticles growing along the cellulose fibers can be observed, and these nanocrystals are arranged as linearly ordered structure. When the pH value of the reaction solution was regulated to 10, ZnO nanoparticles were clearly observed to grow along the surface of the cellulose fibers (Figure 2f). ZnO nanoparticles with a size of about 30 nm exhibited linearly ordered array matrix on the cellulose fiber surface. This image further confirms that the self-assembly took place between the ZnO crystal nuclei and the cellulose. Concomitantly formed ZnO microparticles still randomly distributed on the surface of the cellulose (Figure 2e), indicating no interaction between the microscale ZnO particles and the cellulose. As the solution basicity was increased further to pH 11 and 12, 1-μm shuttle and needlelike ZnO particles were formed, as can be seen in panels g and i, respectively, of Figure 2. The production of these long microparticles indicates that ZnO prepared under highly basic conditions favors growth along the c axis to stabilize the products. ZnO nanoparticles growing along the cellulose fibers were also observed (Figure 2h,j) but were smaller (10 nm) and fewer compared to those prepared at pH 10 and below. To fully decipher the self-assembly of ZnO and cellulose, SEM images of the cross-sectional surface of the cellulose@

peaks of both ZnO and cellulose were observed. The peaks perfectly matched those of the hexagonal wurtzite crystal structure of ZnO (JCPDS no. 89-1397), which indicates that ZnO was well crystallized in the composite. Compared to those of ZnO, the diffraction peaks of cellulose in the composite were much weaker. This might be caused by ZnO particles covering the cellulose surface, which make it hard to collect the diffraction data of cellulose during the XRD examination. For comparison, the sintered product of the composite was also measured. The obtained XRD pattern displayed only the ZnO diffraction peaks, indicating that calcination gave the pure phase of ZnO. Next, we studied the effect of the pH value (pH 8−12) on the synthesized cellulose@ZnO composites. The XRD spectra of the composites prepared at different pH values are shown in Figure 1b. One can see that all of the samples showed characteristic peaks of both cellulose I and wurtzite ZnO. However, careful inspection revealed impurity diffraction peaks of zinc acetate in the composites prepared at pH 8 and 9. Increasing the pH to 10 and above made the phase of zinc acetate (impurity) disappear. These results demonstrate that composites containing pure ZnO phase and cellulose can be obtained at high pH values (pH 10 and above). Interestingly, an unusual phenomenon was observed for the (002) planes in all cellulose@ZnO composites: greatly decreased diffraction intensity relative to that of bulk ZnO (JCPDS no. 89-1397). To study the shrinkage of the (002) diffraction peaks, the (002)/(100) intensity ratios of the ZnO particles were calculated, as shown in Figure 1d. Generally, the (001) plane of ZnO is believed to have a higher surface energy than the other planes.43 To decrease the system energy, ZnO crystals prefer to grow along the c axis and to form a needlelike morphology to avoid exposing high-energy (001) surfaces. Thus, the (002) plane usually has a relatively high diffraction intensity, yielding a ratio to (100) of 0.75 in bulk ZnO (JCPDS no. 89-1397), for instance. Our samples exhibited different behavior. As shown in Figure 1d, the (002)/ (100) intensity ratios of the cellulose@ZnO composites were calculated in the range from 0.27 to 0.45. We deduce that cellulose likely interacts with the (001) plane of ZnO and inhibits ZnO crystallites from growing along the c axis, eventually leading to a low intensity of (002) diffraction in the XRD patterns. Upon calcination of our sample, the highenergy state of ZnO would undergo grain regrowth to minimize its system energy and reach a metastable state as in standard bulk ZnO. Notably, different XRD patterns were obtained when the samples were not subjected to an extra washing before the test, as can be seen in Figure 1c. Lack of this extra washing resulted in composites with the typical character of bulk ZnO, namely, featuring strong diffraction peaks of (002) and high (002)/ (100) ratios within the range of 0.6−0.8. When high-pH synthetic conditions were applied, the ZnO nuclei favored growth along the c axis and contributed to high values of the (002)/(100) ratio, which were even higher than those of sintered ZnO and standard ZnO (JCPDS no. 89-1397). The two different XRD patterns in Figure 1b,c provide evidence that two types of ZnO particles were present in the composites that differed in terms of whether they interacted with cellulose. First are ZnO particles that self-assemble to cellulose through their (001) plane anchoring and thus have low-intensity peaks for their (002) planes. The anchored interaction between ZnO and cellulose is sufficiently strong to 6588


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Figure 3. SEM images of cellulose@ZnO prepared at pH 10: (a−c) cross section and (d) surface.

Figure 4. TEM of images cellulose@ZnO prepared at pH 10 with ultrasound treatments of (a) 20 and (b) 50 min.

Figure 5. (a) TG and (b) DTG curves of mechanically mixed cellulose and ZnO and cellulose@ZnO composite prepared at pH 10, together with the cellulose treated by the same procedure as the composite.

ZnO composites were taken, as shown in Figure 3a−c. Inspection of Figure 3a,b indicates that the microparticles irregularly covered the cellulose microfibers and formed coating layers. We hypothesize that the ZnO microparticles formed independently without direct contact with cellulose during the reaction process. Because no inducing interaction between ZnO particles and cellulose was found, one can deduce that the microparticles would physically deposit (e.g., driven by gravity) onto the surface of the cellulose, which is reflected by the random distribution of particles. When the microfibers formed the paper network, both the interior and the exterior of the composites had ZnO microparticles. At high magnification, ZnO nanoparticles with a size of 30 nm can be seen on the cross-sectional surface in Figure 3c. Moreover, these nano-

particles appear to be arranged in a layered structure on the cross section of microfibers. Further enlargement clearly shows that the ZnO nanoparticles form linear array architectures on the surface of the composite (Figure 3d). The high-resolution SEM image of raw cellulose without zinc acetate provides evidence that the diameters of these fibers were about 30 nm (Figure S1), which falls well within the range of our measured separation values. This supports our assumption that cellulose nanofibers were prepared in our hydrothermal process. The growth of ZnO along the linear cellulose fibers must be driven by a certain specific force (Figure 3d). The details of selfassembly will be fully addressed later in this article. Furthermore, transmission electron microscopy (TEM) images of cellulose@ZnO prepared with ultrasound treatments 6589


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Figure 6. XPS spectra of cellulose@ZnO prepared at pH 10: (a) wide-range spectrum, (b) C 1s spectrum, (c) O 1s spectrum, and (d) Zn 2p spectrum.

ZnO−cellulose interaction is present in the composite that changes its thermal properties. This interaction was further confirmed by DFT thermodynamic calculations, as discussed below. On the basis of the XRD and SEM analyses, we deduce that the special hierarchical structure would further accelerate the decomposition of the composite. It is known that the thermal conductivity of cellulose is very low (usually less than 1 W m−1 K−1),46 whereas ZnO has thermal conductivity as high as 60 W m−1 K−1.47,48 In the composite, the building blocks (cellulose/ZnO nanofibers) are formed by 30-nm ZnO particles anchored on the cellulose nanofibers. This ordered and contiguous structural feature and the direct interaction make heat transport much easier and, consequently, result in the cellulose@ZnO being decomposed at a much lower temperature. In previous reports,42,49−51 the thermal stability of cellulose and ZnO composites did not change after ZnO coating or doping treatment, which is obviously different from our results. Three reasons might explain the difference: (1) the low ZnO contents in the previously synthesized materials, (2) the lack of nanoscale ZnO particles, and (3) the lack of an ordered architecture of the composites constructed by self-assembly. Thus, the small ZnO nanoparticles, ordered hierarchical structure, and relatively strong interaction between the ZnO and cellulose subunits are the key factors leading to the low decomposition temperature of our composites. To explore the interactions between ZnO and cellulose, Fourier transform infrared (FT-IR) and Raman spectra of the cellulose@ZnO composite and its precursors were obtained (Figure S2). From the IR spectra, all of the bands of the cellulose@ZnO composite exhibit a slight displacement compared to those of its precursors, which implies that there exists an interaction between the ZnO and the cellulose in the composite. From the Raman spectra, it is difficult to distinguish the Zn− O vibrations contributed by the ZnO particles from those

of different durations are shown in Figure 4. For 20 min of ultrasound, ZnO particles of about 30 nm in size attached to the cellulose nanofibers (Figure 4a), and these nanoparticles were formed by even smaller ZnO particles. For comparison, the TEM image of a sample prepared with a 50-min ultrasound treatment was obtained (Figure 4b). One can see ZnO particles of about 5 nm were bound to the cellulose nanofibers. The results further evidence that (1) ZnO nanoparticles can selfassemble on cellulose nanofibers and form the level 1 structure and (2) only nanosize ZnO can self-assemble on cellulose nanofibers. Thermogravimetric (TG) analysis was also carried out to understand the thermal stabilities of the cellulose@ZnO composite. Comparison was made with a mixture of ZnO and cellulose as well as with cellulose treated by the same experimental procedure as the composite. Notably, the mixture was mechanically combined in the same mole ratio as the synthesized cellulose@ZnO composite. Generally, cellulose can be decomposed into H2O and CO2 at 300−400 °C in one step. As shown in Figure 5, the treated cellulose, the mechanical mixture (ZnO and cellulose), and the cellulose@ZnO composite all exhibited one-step degradation, which is consistent with previous reports.44,45 The results indicate there were no impurities such as zinc acetate or zinc hydroxide in the composite when prepared at pH10. Differential thermogravimetry (DTG) plots were prepared according to the TG curves and are shown in Figure 5b. The decomposition temperature of the treated cellulose was found to be 359 °C, in agreement with the reported value.44,45 A slightly lower thermal decomposition temperature (350 °C) was measured for the mechanical mixture of ZnO and cellulose. In contrast, a significant difference was found for our synthesized cellulose@ZnO composite. The thermal decomposition temperature was determined to be as low as 267 °C, 92 and 83 °C lower than cellulose and mechanical mixture, respectively. This dramatic reduction suggests that a strong 6590


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Figure 7. Optimized structures of the dimeric glucose (cellulose), (ZnO)12 nanoparticle cluster (ZnO), and various isomers of the cellulose−ZnO composite. Three types of oxygen atoms (side Os, terminal Ot, and endocyclic Oe) are labeled in the cellulose. The resulting ss′- and s-type isomers are shown in the second and third rows, respectively, compared with more extended models (ss′-1)-ZnO and (ss′-1)2 (fourth row). The structures with the Ot-involved (t-type) isomers are presented in Figure S3 of the Supporting Information.

exothermic process (Table S2), suggesting strong bonding interactions between the cellulose and ZnO. These findings agree with the above structural characterizations and support the conclusion that the hydrogen bonding is not the dominant driving force for the formation of the cellulose/ZnO nanofibers in the self-assembly synthetic process. The dominant driving force would be the force that makes the zinc atom bind with side oxygen atoms (denoted as Os), namely, the electrostatic attraction between the (001) planes of the ZnO nanoparticles and the cellulose. In the following discussion, only the ss′-1 isomer is considered because is it energetically favored. Inspection of the optimized ss′-1 isomer shows that the (ZnO)12 cluster is chelated by two Os atoms of the dimeric glucose; moreover, the two side oxygen Os atoms come from different glucose monomers. The Zn−Os distances were calculated to be 2.13 and 2.14 Å, slightly longer than Zn−O distances (1.91−2.02 Å) in the ZnO cluster (Table S3). It is known that the cellulose is a linear chain that is polymerized by several thousands of glucose units and linked by their terminal Ot atoms. Therefore, one can imagine that the ZnO nanoparticles would grow on the sides of the cellulose fibers, eventually forming an overall linear array. This assumption was confirmed by further optimizations of the (ss′-1)−ZnO and

contributed by the interaction of zinc atoms of ZnO and oxygen atoms of cellulose.52,53 Figure 6 shows X-ray photoelectron spectroscopy (XPS) spectra of the cellulose@ZnO composite prepared at pH 10. The sample exhibits three distinct peaks (C 1s, O 1s, and Zn 2p) in the wide-range spectrum of Figure 6a, which indicates the presence of ZnO and cellulose in the composite. The highresolution XPS spectrum of the O 1s region is shown in Figure 6c. The peak at 532.2 eV is characteristic of the oxygen of alcoholic C−OH groups. Simultaneously, the peak due to the O atom of ZnO can be observed at 531.4 eV.42 Density Functional Theory Calculations. To rationalize the experimental synthesis and structural characterizations and to provide deeper insight into the driving force behind the selfassembly between cellulose and ZnO, we carried out relativistic DFT calculations. Optimizations found three types of energetically stable isomers, as can be seen in Figures 7 and S3. The ss′1 isomer was computed to be the most stable among all 16 isomers (Table S1). The ss′-2 and ss′-3 isomers are 15 and 20 kJ/mol (ΔE, total energy) less stable, respectively. The s- and ttype isomers (hydrogen binding type) are 9−61 kJ/mol higher in energy than ss′-1. The calculations of the formation reactions of the cellulose/ZnO nanofibers revealed an 6591


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Figure 8. Inhibition zones against S. aureus and E. coli of cellulose@ZnO composites prepared (a) at different pH values and (b,c) with different amounts of zinc acetate.

better antibacterial activities than those prepared at other pH values (8, 11, and 12). To further investigate the influence of ZnO, we also tested the antibacterial activities of cellulose@ZnO composites prepared with different zinc acetate contents, and the results are shown in Figure 8b,c. As one can see, the agar medium containing only cellulose was fully covered by bacteria, which means that cellulose itself does not have any antibacterial activity. When cellulose@ZnO composites were applied, all agar disks showed inhibition zones around the cellulose@ZnO wafers. Although the cellulose@ZnO composite prepared with 5 g of zinc acetate showed a slightly larger inhibition zone, it contained zinc acetate impurities in the cellulose@ZnO, as evidenced by the TG and DTG results (Figure S9). Hence, use of 3 g of zinc acetate and pH 9 or 10 represents optimal conditions for the preparation of cellulose@ZnO composites with excellent antibacterial activities. The MIC was also applied to quantitatively analyze the antibacterial activities of both cellulose@ZnO and ZnO. The results of MIC testing at the 100% antibacterial rate are presented in Table 1. Notably, the lower the MIC value, the higher the antibacterial activity. Samples prepared at pH 9 and 10 had smallest MIC values and provided the strongest antibacterial activities, in agreement with the results of the disk diffusion tests. The MIC values for commercial ZnO were measured to be greater than 20 mg·mL−1 for E. coli, which indicates a lower antibacterial ability. In brief, our cellulose@

(ss′-1)2 structures shown in Figure 7, as well as the SEM images in Figure 3. Good agreement was obtained between the experimental and calculated infrared spectra of the cellulose/ZnO composite (Figures S2 and S4). The electronic structures of the ss′-1 isomer were calculated and are shown in Figures S5−S7, along with the simulated absorption spectra of the (ZnO)12−cellulose complex in Figure S8. As shown in Figure S8, the environmental medium has a slight effect on the absorption of the composite. The low-energy absorptions ranging from 350 to 450 nm have been assigned to charge transfer from the O 2p orbital to the Zn 4s orbital localized around the ZnO nanocrystallites, in agreement with previous experimental observations. Antibacterial Activities. The antibacterial activities of cellulose@ZnO and ZnO were examined by the disk diffusion method and the minimum inhibitory concentration (MIC) method. These two methods correspond to qualitative and quantitative analyses, respectively. All samples were washed to neutral before being tested. First, the antibacterial activities of cellulose@ZnO composites prepared at different pH value were investigated against S. aureus and E. coli by the disk diffusion method, and the results are shown in Figure 8a. The results indicate that all prepared composites had antibacterial activities, although the cellulose@ ZnO composites prepared at pH 9 and 10 had the largest inhibition zones against S. aureus and E. coli. These results might be because antibacterial activity is related to a composite’s structure as well as its ZnO percentage content. In solutions of high pH, ZnO can easily nucleate and grow, leading to greater production of ZnO. This supports better antibacterial activities of cellulose@ZnO composites prepared in high-pH systems. On the other hand, the self-assembly process is also influenced by pH. In high-pH solutions, ZnO nuclei grow too fast to assemble onto the cellulose, which results in less ZnO content in the composite. Consequently, the antibacterial activity decreases. Because of these two factors, the cellulose@ZnO composites prepared at pH 9 and 10 exhibited

Table 1. MICs (mg·mL−1) of Cellulose@ZnO and Commercial ZnO S. aureus

E. coli

cellulose@ZnO prepared at different pH values pH 8 1.25 pH 9 0.44 pH 10 0.63 pH 11 1.25 pH 12 2.50 commercial ZnO 20

1.68 0.63 0.83 1.67 4.17 >20

sample

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Figure 9. Schematic model illustrating the formation of the cellulose@ZnO composite.

the cellulose and results in different self-assembly efficiencies. As long as a ZnO nucleus exists, it will endeavor to selfassemble on cellulose as a result of electronic attraction and form the level 1 structure; simultaneously, this leads to the small size of the ZnO nanoparticles. On the other hand, the ZnO nucleus will independently grow into a large microparticle if it does not interact with cellulose. At relatively low pH (8− 10), ZnO nuclei grow slowly, which allows enough time for self-assembly. Consequently, most of the nanofibers are anchored by uniform ZnO nanoparticles with sizes of 30 nm. In contrast, under high-pH conditions (pH 11 and 12), ZnO nuclei grow rapidly in solution and produce more microscale ZnO particles. In this case, the electrostatic attraction is too weak to efficiently drive the self-assembly between large ZnO particles and cellulose nanofibers to form an ordered structure. As a result, fewer and smaller ZnO nanoparticles (10 nm) are produced on the cellulose nanofibers. Therefore, pH can be used to tune the self-assembly extent of the synthesized composite and control its final morphology. pH 10 provides the optimal conditions for the self-assembly of ZnO and cellulose. According to the above analysis, a self-assembly mechanism is proposed as shown in Figure 9. There are two types of driving forces in the fabrication of hierarchically structured cellulose@ZnO composites. One is electrostatic attraction. In the reaction system, Zn2+ ions will form first Zn(OH)42− ion and then ZnO crystal nuclei. Upon forming the ZnO crystal nuclei, the Zn atoms on the (001) plane self-assemble to the Os atoms of cellulose to stabilize the system, under the driving force of electrostatic attraction. The interaction between ZnO and cellulose suppresses the ZnO growth along the c axis and fabricates the first-level structure: linear cellulose/ZnO nanofibers. These are the basic building blocks (see the top part of Figure 9). The second driving force is intermolecular hydrogen bonding, which is well-known in cellulose- containing materials.18 Once formed, the cellulose/ZnO nanofibers (level 1) self-assemble into a layered structure (level 2) through the intermolecular hydrogen bonding of cellulose. The level 3 structure of the cellulose/ZnO microfibers is built by the orderly stacking of cellulose/ZnO layers, for which the driving force is still considered to be hydrogen bonding. Thus, the self-

ZnO composite exhibits an enhanced antibacterial activity, superior to that of commercial ZnO. Self-Assembly Mechanism. It is known that cellulose exists as assemblies of individual cellulose chain-forming fibers. These fibers can fibrillate into micro-/nanofibers called microfibrils. These rodlike microfibrils are well dispersed in solution, display partial negative charges on their surface, and produce an electric double layer. As polar crystals, ZnO has positively charged (001) planes on Zn terminus and negatively charged (001̅) planes on the O terminus in ZnO structural cells.54 As evidenced by our XRD and SEM results, the growth of ZnO along the c axis is suppressed. In conjunction with our DFT study, we determined that the presence of Zn−Os bonding energetically stabilizes the ZnO−cellulose molecular structure, which derives from an exothermic formation reaction (energies ranging from −85 to −152 kJ/mol for our model complex). On basis of these results, we conclude that it is the electrostatic attraction between the Zn atoms on the (001) plane (positively charged) and the Os atoms of cellulose (negatively charged) that drives the selfassembly of ZnO nanoparticles and cellulose in the reaction system. Consequently, the first-level structure, cellulose/ZnO nanofibers (level 1), is produced and is significantly stabilized by the formation of Zn−Os bonding. Thus, the dominant driving force for self-assembly to build level 1 structure is electrostatic attraction. To further evidence the electrostatic attraction between ZnO and cellulose, zeta potentials were measured (Figure S10). The zeta potentials were found to be negative, ranging from −146.8 to −34.8 mV for raw cellulose, whereas in the cellulose@ZnO solution, the zeta potentials were significantly shifted in the positive direction, falling between −2.0 and 1.0 mV. Together with the XRD analysis, the zeta potentials near zero provide evidence that the negative cellulose nanofibers bind to ZnO by means of Zn atoms [on the (001) plane], which makes ZnO stand and grow on the cellulose fibers. The pH value of the reaction solution was found not to affect the nature of the self-assembly between the ZnO and cellulose, at least in our studied basic range of pH 8−12. pH does have an effect, however, on the possibility of ZnO crystals contacting 6593


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ACS Sustainable Chemistry & Engineering assembly of a hierarchical structure is accomplished in the hydrothermal process. The final morphology of the cellulose@ ZnO composite is achieved in such a manner that the large ZnO microparticles, which are independently formed without contact with cellulose, randomly cover the hierarchical cellulose/ZnO microfibers, more likely as a result of physical forces (e.g., gravity). Thus, the mechanism of self-assembly is not applicable to ZnO microparticles.

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AUTHOR INFORMATION

Corresponding Authors

CONCLUSIONS A hierarchical cellulose@ZnO composite has been prepared by a facile hydrothermal method. Two sizes of ZnO crystallites in the nano- and micrometer ranges were found, where only the smaller-size nanoparticles take part in the self-assembly process. The ZnO nanoparticles uniformly grow along the linear chains of the cellulose nanofibers, producing a linear array composite nanofiber structure. The newly synthesized composites exhibit a hierarchical three-level structure: cellulose/ZnO nanofibers (level 1), cellulose/ZnO layers (level 2), and cellulose/ZnO microfibers (level 3). Then, the large ZnO microparticles coat the microfibers and contribute to the final morphology of the cellulose@ZnO composite. It was found that ZnO and cellulose trigger the self-assembly and accomplish the fabrication of the first-level structure. The driving force for this process is the electrostatic attraction between the Zn atoms on the (001) plane of ZnO (positively charged) and the Os atoms in cellulose (negatively charged). DFT calculations revealed a bonding interaction between ZnO and cellulose in the formed composite. Structural optimizations, energy calculations, and spectroscopic analyses demonstrated the growth of ZnO nanoparticles following the linear chain of the cellulose fibers. The formations of the level 2 and level 3 structures are driven by the intermolecular hydrogen bonding of cellulose, as indicated in previous reports. In the final morphology of the cellulose@ZnO composites, we did not find any obvious interaction between the level 3 structure and the coating of ZnO microparticles. This suggests that a physical force such as gravity might be responsible. In brief, selfassembly based on electrostatic attraction was unambiguously established between ZnO and cellulose in their hierarchical structure. The current study demonstrates that electrostatic attraction plays a key role in fabricating the first-level structure, which differs from the simple conjecture in previous reports. The prepared cellulose@ZnO composites exhibited excellent antibacterial activities against S. aureus and E. coli. Our method for preparing cellulose@ZnO composites with a specific structure is simple, low-cost, and environment-friendly. The newly synthesized composite is expected to be applied in food packaging materials.

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diagrams of frontier orbitals; TG curves of the cellulose@ZnO composite; and plots of zeta potentials. Tables listing calculated relative energies of various isomers of the cellulose−zinc oxide complex, formation reaction energies, and optimized geometry parameters (PDF)

*E-mail: [email protected] (Y.R.G.). *E-mail: [email protected] (Q.J.P.). ORCID

Yuanru Guo: 0000-0002-9015-4577 Qing-Jiang Pan: 0000-0003-2763-6976 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2572017EB07). The Natural Science Foundations of Chinese Heilongjiang Province (B201318) and China (21273063, 30901136) are also gratefully acknowledged. Q.J.P. is grateful to Dr. Dimitri Laikov for providing us with the Priroda code.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00842. Experimental and computational details. SEM images, FT-IR spectra, and Raman spectra of synthesized materials; optimized structures of various isomers of the cellulose−zinc oxide complex; simulated vibrational spectra, absorption spectra, and density of states (DOS) of the (ZnO)12−cellulose complex, together with 6594


Research Article

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Self-Assembly of Hierarchically Structured Cellulose@ZnO Composite

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