Morphological Control over ZnO Nanostructures from Self-Emulsion

Article pubs.acs.org/crystal

Morphological Control over ZnO Nanostructures from Self-Emulsion Polymerization Santosh Kumar,† Hong-Joon Lee,† Tae-Ho Yoon,† C. N. Murthy,†,‡ and Jae-Suk Lee*,† †

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Korea ‡ Macromolecular Materials Laboratory, Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda, Vadodara 390001, India S Supporting Information *

ABSTRACT: Three different morphologies of ZnO nanostructures, such as nanospheres, nanorods, and nanoribbons, were controlled by tuning the ratio of the Zn2+ precursor to the 4VP monomer when polymerized in aqueous medium utilizing self-emulsion polymerization. The amphiphilic homopolymer (P4VP) acts as a template to form the ZnO/ P4VP nanocomposite. The aspect ratio of the nanostructures is strongly dependent on the molar concentration of the Zn2+ precursor and becomes higher as its concentration increases. This results in different morphologies that are consistently repeatable. Pure ZnO was obtained from the ZnO/P4VP nanocomposites by calcination at 400 °C or by solvent washing. The calcination of the nanocomposties resulted in different morphologies, such as spherical, corolla shaped, and nanosheets. In addition, hexagonal nanoblocks, nanorods, and nanoribbons were observed when the polymer was removed from the nanocomposites by washing with chloroform. Removing polymer by solvent washing is a very easy, cost-effective method and has the potential for mass production of pure and highly crystalline ZnO nanostructures with known and controllable morphologies. The nanocomposites and pure ZnO nanostructures obtained after polymer removal were characterized by transmission electron microscopy, high resolution transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses, which confirmed the crystalline nature of the ZnO.

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piezotronic transistors,27 UV (ultraviolet) nanolasers,28,29 UV protection properties, 30 photodetectors, 31 textiles, as a fungistat, sensors and actuators, corrosion inhibition, cosmetics, medical and dental, rubber, ceramics and concrete32 etc. Also, it is the base material for transparent conducting oxide.33 Shape controlled synthesis of ZnO in terms of morphology and size is highly desirable for researchers because the properties of the material can be controlled by these factors. To employ the applications of these unique nanostructure materials, the crystalline morphology, orientation, and surface architecture of nanostructures must be well controlled. The methods listed above, which have been widely used to synthesize ZnO nanostructures, utilize complex processes, sophisticated equipment, and poisonous and expensive reagents, tend to bring together the impurities in the final products when templates and catalysts are used in the reaction system and are usually multistep. High temperatures make them very difficult for the large-scale production for

evelopment of a methodology for synthesizing zinc oxide (ZnO) with predetermined morphology that can be used on an industrial scale has become a subject of growing interest in science as well as in industry. Various chemical and physical processes, such as chemical vapor deposition, chemical bath deposition, metal−organic chemical vapor deposition, physical vapor deposition, wet chemical synthesis, molecular beam epitaxy, electrospinning, sputtering, flux methods, and even topdown approaches by etching, have been developed to synthesize zinc oxide.1,2 Additionally, the synthesis of metal nanoparticles and semiconducting metal oxides within microphase-separated diblock copolymers based on diffusing them into the copolymer matrix have been widely reported3−12 as ZnO is an important group II−VI semiconductor with different morphologies and nanostructures, which include nanowires,13−15 nanorods,16 quantum dots,17 nanotubes,18,19 nanobelts,20 nanohelixes,21 and nanodiscs.22 These structures show different properties due to the morphological differences. These different nanostructures have stimulated research interest regarding their fundamental properties and their potential applications in numerous fields, such as gas sensors,23 piezoelectric nanogenerators,24 photocatalysis,25 solar cells,26 © XXXX American Chemical Society

Received: March 28, 2016 Revised: May 21, 2016

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section of Supporting Information and the self-assembly behavior in Figure S1 of the Supporting Information. In the present work, three different morphologies of ZnO/P4VP nanocomposites were obtained when we varied the zinc precursor molar ratio with respect to the 4VP monomer, as shown in Scheme 1. We observed that when the molar concentration of the zinc precursor was very low (0.25:1) compared to the monomer (Table 1, VPZ-1) spherical particles containing ZnO were

commercial applications. Therefore, the synthesis of ZnO nanostructures with controlled morphology, size, and orientation on a large scale from existing techniques remains a significant technological challenge. Our route detailed here may open up new opportunities to fabricate these materials and extend their applications based on a low-cost, environmentally friendly, and high-yield mass production strategy. Polymers with pyridine units are always an area of interest in the field of materials science due to their reactive nitrogen heteroatom.34−36 In addition, the properties of 4-vinylpyridine (4VP), such as its basicity and hydrophilic−hydrophobic balance, make it of great importance. The homopolymers of 4VP have many applications, such as sensors, actuators, antimicrobial materials, host/ligand of metal-containing chromophores, etc.37 In our previous article38 we reported the in situ synthesis of ZnO nanoribbons by the polymer assisted growth mechanism utilizing the amphiphilic homopolymer (P4VP) as a template via self-emulsion polymerization (SEP). We showed that P4VP plays an important role in the formation of ZnO nanoribbons with high aspect ratios. It has been reported previously that P4VP makes coordination bonds to form complexes with metal oxides and metals.39−41 Chandra et al. has shown that a Schiff-base amine is a good template for growing mesoporous ZnO, although the role of the amine group has not been discussed.42 Herein, we report the synthesis and morphology control of ZnO nanostructures employing SEP and using this homopolymer as a template instead of using a surfactant or conventional amphiphilic block copolymer, methods that have been reported previously by others mentioned above. The morphology of the polymer/ZnO nanocomposite was controlled by varying the zinc precursor concentration with respect to the monomer during the synthesis, as shown in Scheme 1. Thus, SEP brings forward a new idea of using a homopolymer as a template to synthesize shape-controlled ZnO nanostructures.

Table 1. Preparation of Poly (4-Vinylpyridine): ZnO (ZnO/ P4VP) Nanocomposite sample

monomer 4VP (mmol)

precursor Zn2+ (mmol)

initiator VA044 (mmol)

solvent H2O (mL)

time/temp (min/°C)

VPZ-1 VPZ-2 VPZ-3

4.75 4.75 4.75

1.18 2.37 4.75

0.047 0.047 0.047

60 60 60

120/75 120/75 120/75

obtained. However, when the concentration of the precursor was (0.5:1) with respect to the monomer (Table 1, VPZ-2), we could obtain elongated hexagonal prismatic and rod-shaped blocks. Nanoribbons were obtained when an equimolar concentration of the zinc precursor (1:1) was used with respect to the monomer (Table 1, VPZ-3). Removing the polymer coating on the nanoparticles by solvent washing or by calcination resulted in a morphology change, depending on the method of polymer removal. The SEM and TEM micrographs of sample VPZ-1 are shown in Figure 1, panels a and d, respectively. Spherical ZnO nanostructures (∼300 nm) were observed, which signifies that the spherical polymeric micelle confines the growth of the ZnO crystals in all directions, resulting in the spherical shape of the nanocomposite. Figure 1b,e shows the micrographs of sample VPZ-2. Here, the elongated hexagonal rod-like morphology of the nanocomposite (VPZ-2) with a length of ∼400 nm and a width of ∼200 nm was obtained. In the case of VPZ-3, we obtained long nanoribbons of ZnO (length >10 μm and width ∼450 nm), as shown in Figure 1c,f. The growing homopolymer seems to play a very critical role in the formation of these nanostructures. In classical emulsion polymerization, the surfactant micelle behaves like a reservoir for the monomer, which diffuses into the aqueous bulk, where the locus of polymerization is situated. Thus, the growing polymer particles are stabilized by the surfactant.43,44 However, when there is no added surfactant, as in our case (SEP), an amphiphilic monomer growing into an oligomer tends to form vesicular structures, as we have demonstrated previously.38 This is similar to the structures of phospholipids present in living organisms forming barriers in cell walls.45 These vesicular structures can be spherical or lamellar, depending on various conditions. A similar situation seems to exist in SEP in the presence of a Zn precursor. Because of the variation of the ratio of the zinc precursor compared to the monomer, the role of the growing polymer is 2-fold. One, the homopolymer confines the growth of ZnO nanoparticles. and two, it directs the growth of the ZnO crystal face along a different orientation, possibly by inhibiting the growth of other crystal faces, as we proposed previously.37 This preferential growth of the crystal faces is reflected in the X-ray diffraction patterns. Generally, the peak positions of polymorphs of a crystalline material are identical, although the peak intensities are different. This peak intensity

Scheme 1. Different Morphologies of the ZnO/P4VP Nanocomposite and Pure ZnO Obtained after Removing the Polymer by Solvent Washing and Calcination

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RESULTS AND DISCUSSION In our previous report on SEP,38 we showed that P4VP, when polymerized alone in water with a hydrophilic initiator, forms spherical nanoparticles with a uniform size in the absence of any additional surfactant. In addition, the mechanism of the growth of the polymer was explained. The typical homopolymerization procedure can be found in the experimental B

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morphologies of ZnO. Figure 3a−c represents samples after removing the polymer by solvent washing, whereas Figure 3d−f are for the samples after calcination. The relative peak intensities for the , , and orientations differ as the morphology changes, which has also been highlighted by others.48−50 All of the XRD patterns can be indexed as the pure hexagonal wurtzite ZnO nanostructure. Figure 4a−c displays the SEM micrographs of pure ZnO when measured in the powder state after removing the polymer from VPZ-1, VPZ-2, and VPZ-3, respectively, by solvent washing. We observed different morphologies of the ZnO nanostructures after removing the polymer cover by solvent washing. ZnO nanostructures are composed of tetrahedrally coordinated Zn2+ and O2− ions which are stacked alternatively along the c-axis in a number of alternating planes. The growing nanostructure of ZnO has both polar and nonpolar faces that are unstable. Because of different electrostatic energies, the morphology of the final structure is determined by the thermodynamic and kinetic factors that influence the growth of these faces.51−55 This is the reason why after removing the polymer, ZnO acquires different morphologies. Thus, our technique of removing the polymer by solvent washing presents a choice for the mass production of ZnO with three different morphologies, i.e., hexagonal nanoblocks (VPZ-1) with an average diameter and length of ∼250 nm, elongated hexagonal nanorods (VPZ-2) having an average diameter of ∼250 nm and an average length of ∼400 nm, and nanoribbon-like morphology (VPZ-3) having an average diameter of ∼200 nm and an average length of ∼4 μm. In the case of VPZ-1 when the molar concentration of the zinc precursor was very low (0.25:1) compared to the monomer, during the growth process of the seed crystal, the polymer encapsulates it and inhibits its growth in all directions which results in spherical nanoparticles containing ZnO. When the polymer was removed by solvent washing followed by annealing the ZnO particles acquired nanoblock-like morphology with a exposed (0001) facet and became more stable due to its surface reconstruction as shown in Figure 4a. In case VPZ-2, when the molar concentration of the zinc precursor was half compared to the monomer, we obtained rodlike morphology. The seed crystals were not covered fully by the growing polymer to suppress the growth of polar crystal facets resulting in small rodlike morphology as shown in Figure 4b. In the case of VPZ-3, when equimolar concentration of the zinc precursor and monomer was used we obtained long nanoribbon-like morphology of ZnO as shown in Figure 4c. In this case, the polymer is not enough, and subsequently its capping effect is least so; the homopolymer stimulates the growth of seed crystals letting them to grow in its most energetically preferred crystal growth facet (0001). It is also known that ZnO nanostructures are more likely to grow so as to minimize the surface energy along the polar facets and increase the surface area along the nonpolar facets, as can be seen in Figure 4c. To explore the mechanism for the formation of long ribbonlike morphology (VPZ-3), we studied the growth kinetics of the nanocomposite as a function of time by SEM, taking this sample as a representative example. The synthesis procedure can be found in the Experimental Section, and SEM micrographs are shown as Figure S2 in Supporting Information. After 15 min of polymerization, the ZnO nanoribbons are completely wrapped with the polymer. As the polymerization progresses, the morphologies of the ZnO structures do not

Figure 1. FE-SEM micrographs of ZnO/P4VP nanocomposite samples showing three different morphologies when the zinc precursor molar ratio is varied compared to the monomer (a) 0.25:1 (VPZ-1), (b) 0.5:1 (VPZ-2), and (c) 1:1 (VPZ-3) and their corresponding TEM micrographs, (d), (e), and (f), respectively.

difference arises due to the growing crystal face area and is dependent on the seed crystal environment.46,47 In most of the reported literature, the peak intensities of the , , and planes in the XRD pattern of wurtzite ZnO are found to vary depending on the morphology. This observation has not been fully explained yet. The variation in XRD peak intensities and orientations depending upon the morphologies is shown in Figure 2a−c for sphere, rod, and ribbon-like nanostructures, respectively. Figure 2d−f shows the HRTEM images taken from the TEM micrographs, as shown in Figure 1, for the ZnO/P4VP nanocomposites (a) VPZ-1, (b) VPZ-2, (c) 1:1 VPZ-3 and their corresponding inverse fast Fourier transform (IFFT) and processed fast Fourier transform (FFT) images measured along the , , and directions, respectively. The HRTEM images (Figure 2d−f) reveal that they are polycrystalline structures with nanosized grains, and this is confirmed by the corresponding FFTs (upper right corners). The nanosized grains are highly crystalline, with a lattice spacing of 0.28 nm, which corresponds to the distance between the planes in the ZnO crystal lattice in the cases of VPZ-1 and VPZ-2. In the case of VPZ-3, the lattice spacing is 0.26 nm, which corresponds to the distance between the planes in the ZnO crystal lattice, as shown in Figure 2d−f (lower right corners). This shows that the polymer behaves as a template for the controlled growth of the faces and varies with the concentration of the Zn precursor trapped in the simultaneously growing polymer micelle/vesicle. Figure 3 shows the Xray diffraction pattern of the samples with different C

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Figure 2. X-ray diffraction patterns of ZnO/P4VP nanocomposites (a) 0.25:1 (VPZ-1), (b) 0.5:1 (VPZ-2), and (c) 1:1 (VPZ-3) and corresponding HRTEM images recorded from the TEM micrographs of the ZnO/P4VP nanocomposites (d), (e), and (f), respectively. The HRTEM images also show IFFT and processed FFT measured along the , , and directions.

the polymer was removed by solvent washing or calcinations, the exposed surfaces of the ZnO crystallites undergo a change in surface energy, which is a function of crystal face stability. Therefore, the face of that particular plane forms the most stable equilibrium structure. Thus, the crystallites show different ZnO nanostructures in all cases. The energy dispersive spectroscopy (EDS) analyses from the SEM micrographs of all nine samples shown in Scheme 1 were also investigated and are shown in Figure S3 (for the as-prepared VPZ-1, VPZ-2, and VPZ-3), Figure S4 (for samples after solvent washing) and Figure S5 (for samples after calcination) in the Supporting Information. In the case of prepared samples the EDS patterns indicate that the ZnO nanostructures are composed of C and N (from the polymer), O, Zn, and Si (from the substrate). In the case of the samples after solvent washing and calcination, no signal from C or N is shown, which signifies that the polymer matrix is substantially reduced. These data also confirm the high purity of the ZnO structures synthesized by SEP. To further confirm whether solvent washing can remove all of the polymer to obtain pure ZnO, thermogravimetric analysis (TGA) of the representative sample VPZ-3 (as prepared and

change. The nanoribbons gradually grow along the orientation (c-axis), and no change in morphology is observed until the polymerization ends at 120 min. Because the polymer can also be removed by calcination, all three samples were prepared by making a film on a silica substrate by drop casting and subjected to calcination in a furnace at a temperature of 400 °C for 1 h in an oxidizing environment. Figure 5a−c shows the SEM micrographs when measured in the film state of the calcined VPZ-1, VPZ-2, and VPZ-3, respectively. ZnO with three different morphologies, i.e., spherical nanoparticles (VPZ-1) with an average diameter of ∼250 nm and corolla shaped (VPZ-2) structures having an average petal thickness with a conical head of ∼50 nm and an average length of ∼400 nm, were obtained. Figure 5c displays ZnO with nanosheet-like morphology (VPZ-3), having an average diameter of ∼200 nm and an average length of ∼4 μm. As we have mentioned before, the growth of the ZnO crystal faces depends on the microenvironment that exists in the polymer micelle/vesicle. The high affinity of the Zn2+ and the pyridine for the growing polymer and the polarity of the 4VP facilitate the controlled growth of these nanostructures. After D

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Figure 3. X-ray diffraction pattern of the samples with different morphologies of ZnO after removing the polymer by solvent washing (a−c) and for samples after calcination (d−f).

Figure 4. FESEM micrographs of ZnO (a), (b), and (c), after removing the polymer by solvent washing from VPZ-1, VPZ-2, and VPZ-3, respectively.

Figure 5. FESEM micrographs of ZnO, (a), (b), and (c), after removing the polymer by calcination from VPZ-1, VPZ-2, and VPZ-3, respectively.

after solvent washing) was performed, as shown in Figure S6 in the Supporting Information. Major weight losses are observed in the range of ∼150−400 °C, which corresponds to the initial water loss up to 150 °C and then the structural decomposition of the polymer until 400 °C, as has previously been reported for P4VP by others.56,57 A careful examination of the TGA thermograms indicates the presence of ∼39% ZnO. Almost no polymer was left, indicating that the solvent washing method for removing the polymer from the nanocomposites is a versatile, easy, and scalable way to obtain crystalline inorganic materials synthesized by SEP.

report where an amphiphilic monomer has been used to alter and control the morphology of ZnO in a reproducible manner, which arises due to the templated growth. The nanostructures, such as the nanospheres, nanorods, and nanoribbons of the ZnO/P4VP nanocomposite, can be varied by varying the molar ratio of the zinc precursor with respect to monomer. In addition, the morphology of ZnO can also be changed/ controlled by removing the polymer covering, either by washing with solvent to create hexagonal nanoblocks, nanorods, and nanoribbons or by calcination to form spherical nanoparticles, corolla-shaped structures, and nanosheets. Our methodology has the potential for mass production of highly pure and crystalline materials of not only ZnO but also other semiconducting metal oxides and metal nanoparticles. This method can also be utilized with other polymeric templates with sufficient hydrophilic and hydrophobic balance. Studies

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CONCLUSIONS We demonstrate a cost-effective, environmentally friendly aqueous solution route for the synthesis of controlled ZnO nanostructures via self-emulsion polymerization. This is the first E

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are underway to understand the correlation between the preferred templated growth of the ZnO nanostructures and the polymer micelle/vesicle dimensions.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

The details of the materials used, instrumentation, and homopolymerization (P4VP) procedure are given in the Supporting Information. Preparation of ZnO/P4VP Nanocomposite. In brief, for the synthesis of ZnO/P4VP nanocomposite (VPZ-3), 4-VP (4.75 mmol), and an equimolar amount of ZnAc, as mentioned in Table 1 (VPZ-3), was taken in a two necked flask containing 60 mL of deionized water and stirred using a magnetic bar. The mixture was homogenized under continuous flow of argon for 30 min. The water-soluble radical initiator, VA-044 (0.047 mmol), was dissolved in deoxygenated water and then added through a syringe. The polymerization was continued at 75 °C for 120 min under an argon atmosphere. Aliquots were withdrawn from the flask at various time intervals using a syringe under a counterflow of argon gas. A similar procedure was followed for VPZ-1 and VPZ-2, except the molar ratios between the monomer and the zinc precursor were varied. For SEM measurements, the samples were drop cast on previously cleaned glass substrate and annealed at 150 °C for 12 h. The SEM images of aliquots withdrawn at different times are shown in the Supporting Information as Figure S2. To obtain the powder sample, the nanocomposites were subjected to extensive solvent washing with CHCl3 (4−5 times). After the polymer was removed, the pure ZnO obtained was dried at 150 °C before TGA was performed and the effect on morphology was determined by FE-SEM and the crystalline character was determined by XRD analysis. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00479. Materials used, instrumentation, homopolymerization (P4VP) procedure, SEM and TEM micrographs of the P4VP homopolymer, the growth mechanism shown by SEM images, EDS profiles of all of the samples and TGA of VPZ-3 (as prepared and after solvent washing) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82627152306. Fax: +82629702304. E-mail: jslee@ gist.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2014H1C1A1067014); by Research Program To Solve Social Issues of the National Research Foundation of Korea (NRF) (PM 2.5 Research Center) funded by the Ministry of Science, ICT & Future Planning (NRF2014M3C8A5030613); and by the Mid-career Researcher Program (NRF-2015R1A2A1A01002493) through the NRF grant funded by the MEST. CNM has joined GIST under the Brain Pool Program (151S-4-3-1296).

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REFERENCES

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.6b00479 Cryst. Growth Des. XXXX, XXX, XXX−XXX