Tetrazole-based infinite coordination polymer for encapsulation of TiO₂ and its potential application for fabrication of ZnO@TiO₂ core–shell structures

Abstract

A new and facile technique for the preparation of ZnO@TiO₂ core–shell nano/micro sphere structures from tetrazolate ligand, (1,3-bis(tetrazol-5-ylmethyl)benzene (btb)), was addressed. Via a solvent-induced precipitation method, nanoparticles of TiO₂ bearing Rhodamine B were encapsulated into the Zn(btb) hollow spheres. The resulting polymeric carriers were characterized by SEM, HRTEM, EDS and XRD analyses and subsequently were used as precursors for the preparation of crystalline zinc oxide mineral spheres containing TiO₂ species using a calcination route.

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Introduction

Molecular encapsulation is an attractive supramolecular strategy because it is inherently flexible and does not necessarily require time-consuming synthetic processes. Indeed, molecular encapsulation is an effective way to recycle familiar dyes that are already well-characterized.¹

Two general supramolecular encapsulation strategies, including robust molecular containers and soft assemblies have been areas of rapid growth.² Coordination compounds with infinite structures have been intensively studied, in particular, compounds with backbones constructed from metal ions as connectors and ligands as linkers, so-called coordination polymers. The phrase, “coordination polymers” appeared in the early 1960s, and the area was first reviewed in 1964.³

Versatile synthetic approaches for the assembly of target structures from molecular building blocks have been developed. The key to success is the design of the molecular building blocks, which directs the formation of the desired architectural, chemical, and physical properties of the resulting solid-state materials. In a surprisingly short time, the structural chemistry has attained a very mature level.²

Recently, micro- and nanoscale amorphous infinite coordination polymers (ICPs) as a subclasses of three generation dimension-based categories of porous coordination compounds,² have attracted great attentions because of their promising useful chemical and physical properties and potential applications as asymmetric catalysis, mixture separations, gas storage, and biosensing.^4–6 Straightforward fabrication of cross linked submicrometer functional metal–organic spheres, via polymerization and fast precipitation with a poor solvent, has been progressed by Oh, Mirkin, Wang and co-workers.^4–6 Maspoch and co-workers⁷ developed a new and versatile method based on the polymerization following a precipitation fabrication process, leading to metal–organic polymeric micro- and nanometer-sized spheres as novel functional encapsulating matrices. Metal–organic hollow spheres by infinite coordination polymerization of Zn^II metal ions and 1,4-bis(imidazol-1-ylmethyl)benzene (bix) have accordingly been synthesized. They showed that these hollow spheres are capable to surround the functional species present in the reaction mixture; leading to formation of encapsulated the desired species. Very recently, we could successfully introduced new metal–organic capsules, Zn(btb), prepared by metal node (Zn²⁺) and a new multidentate organic ligand, 1,3-bis(tetrazol-5-ylmethyl) benzene (btb) ligand. In addition, the potential application of the polymeric carriers was demonstrated in encapsulation of multifunctional guests, comprising quantum dots (QDs), ferromagnetic nanoparticles of iron oxide (Fe₃O₄), and dyes (fluorescein isothiocyanate (FITC), and Rhodamine B). Calcification of the prepared final ICPs exploited crystalline zinc oxide mineral spheres, rods and coral shapes.⁸

We decided to examine the encapsulation talent of tetrazolate ICPs hollow spheres for entrapment of TiO₂ species, expecting to preparation of a new type of ZnO@TiO₂ core–shell structures.⁹ Over the past few decades, some investigations have been carried out for design and preparation of TiO₂ and ZnO based nanocomposites.¹⁰ Besides the use of TiO₂ as a dielectric layer, it has a wide range of applications such as gas sensors, photo and thermal catalysis, photoelectrocatalysis,¹¹ due to its excellent chemical and photochemical stability and being capable of photo-oxidative destruction of most organic pollutants.¹²

TiO₂ based nanocomposite thin films such as ZnO/TiO₂, Fe₂O₃/ZnO/TiO₂ and Fe₂O₃/TiO₂ were developed by Yang et al.¹³ on titanium substrates by applying a dip-coating technique using the sol–gel method. Wang et al.¹⁴ prepared three-dimensional ZnO@TiO₂ hierarchical structures via a combination of an electrospinning and a hydrothermal technique. By adjusting the experimental parameters, two different morphologies of ZnO@TiO₂ heteroarchitectures were achieved. Zn-doped TiO₂ nanofibers shelled with ZnO hierarchical nanoarchitectures have been fabricated by combining electrospinning of TiO₂ nanofibers and metal–organic chemical vapor deposition (MOCVD) of ZnO.¹⁵ Uyar et al. have fabricated core–shell heterojunction (CSHJ) nanofibers from ZnO and TiO₂ core–shell structures in two combinations via electrospinning and atomic layer deposition, respectively which were then subjected to calcination.¹⁶

Herein, we describe that typically solvent-induced precipitation method following by calcination process, revealed a new facile way for preparation of ZnO@TiO₂ core–shell nano/micro sphere structures.

Experimental section

Synthesis of btb ligand

To a 250 mL round-bottomed flask was added the nitrile (20 mmol), sodium azide (60 mmol), zinc bromide (20 mmol), and 40 mL of water. The reaction mixture was refluxed for 48 h with vigorous stirring. After cooling to room temperature, HCl (3 N, 30 mL) were added, and vigorous stirring was continued until the aqueous layer had a pH of 1. Subsequently, 200 mL of 0.25 N NaOH was added, and the mixture stirred for 30 min, until the original precipitate was dissolved and a suspension of zinc hydroxide was formed. The suspension was filtered, and the solid washed with 20 mL of 1 N NaOH. 40 mL of 3 N HCl was added to the filtrate with vigorous stirring causing the tetrazole to precipitate. The tetrazole was filtered and washed with HCl and dried in a drying oven to furnish the tetrazole as a white or slightly colored powder. The product had the following data: mp 269–270 °C.

¹H NMR (d-DMSO): 7.27 (t, 1H), 7.25 (b, 1H), 7.24 (b, 1H), 7.17 (m, 3H), 4.2 (s, 4H).

¹³C NMR: 173, 155, 136.5, 129.16, 127.9, 29.60.

Anal. calcd for btb: C, 49.57; H, 4.13; N, 46.28. Found: C, 49.12; H, 4.04; N, 43.86.

Encapsulation of TiO₂ linked RB in hollow Zn(btb) spheres (RB/TiO₂@Zn(btb))

In a typical experiment, a mixture of btb (242 mg, 1 mmol) in DMF (40 mL), Rhodamine B (20 μL, 200 ppm) and TiO₂ (1 mg, Degussa P25) was prepared. The mixture was homogenized by sonication and stirred for specific times (30 min to 12 h) at room temperature. After overnight stirring, a methanolic solution (10 mL) of Zn(NO₃)₂·6H₂O (296 mg, 1 mmol) was added, to generate the nano-micro spheres encapsulate TiO₂ containing-dye nanoparticles. The resulting encapsulated metal–organic systems in all these cases were purified by centrifugation and washed three times with DMF, and redispersed in DMF to obtain the corresponding colloidal solutions was named RB/TiO₂@Zn(btb).

IR for RB/TiO₂@Zn(btb) (KBr pellet, cm⁻¹): 596 (w), 698 (w), 761 (m), 1105 (m), 1182 (w), 1253 (w), 1432 (w), 1487 (s), 1655 (s) 2931 and 3405 (m).

Preparation of ZnO@TiO₂ core–shell spheres

The RB/TiO₂@Zn(btb) hallow spheres was placed in a furnace and calcined at three temperatures, 400, 500 and 550 °C for 3 h in air. After 3 h holding, the generated precipitates were cooled to room temperature.

X-ray powder diffraction (XRD) measurements were performed using a X'pert diffractometer manufactured by Philips with monochromatized CuKα radiation. The samples were characterized with a scanning electron microscope (SEM) (Philips XL 30) with a gold coating. Transmission electron microscopy (TEM) images were obtained with a Hitachi H-9500 apparatus. NMR data were collected by a BRUKER DRX500 AVANCE. An ultrasonic bath (Tecna 6; 50–60 Hz and 0.138 kW) was used for the ultrasonic irradiation. PL spectra were measured by means of a spectrofluorimeter manufactured by Cary eclipse FL0912M014.

Results and discussion

As described in experimental section, RB/TiO₂@Zn(btb) hallow spheres were prepared by addition of a methanolic solution of Zn(NO₃)₂·6H₂O to a TiO₂ suspension containing a dye in DMF solvent. For more precise structural characterization of enclosing the TiO₂ species into the Zn(btb) hollow spheres, ascribed to our previous report⁸ as well directing their multifunctional ability, RB was used in the mother reaction mixture. Strong interaction mode between the functional groups of dyes and the surface sites of TiO₂ by a chemical bond lead to anchoring of dyes onto the TiO₂ surface. Whereas, RB is known to be bound on the TiO₂ surface via esterification between the carboxylic group of the dye and the surface hydroxyl of TiO₂ (Scheme 1), it was also chosen for the mentioned monitoring purpose.¹⁷ Moreover, the dye encapsulation talent of this procedure, had been proved before.⁸

Scheme 1 Schematic path for encapsulation of TiO₂ containing RB, within Zn(btb) nano and micro spheres; btb (242 mg, 1 mmol) in DMF (40 mL), Rhodamine B (20 μL, 200 ppm) and TiO₂ (1 mg, Degussa P25); conditions (a) 5 min sonication, (b) stirring at rt, 12 h (overnight), (c) a methanolic solution (10 mL) of Zn(NO₃)₂·6H₂O (296 mg, 1 mmol) was added, stirring at rt (5 h).

Therefore, it was a commensurate examinee to contribute encapsulation process of TiO₂ within the Zn(btb) spheres. Actually, it was found that in the presence of 3 mg of TiO₂, i.e. more than optimized amount (1 mg), some uncaptured TiO₂ species remain intact. SEM images of this sample represented in Fig. 1, clearly indicate the constructed coral shape capsules besides the unentrapped TiO₂ particles. Furthermore, calcinations of the precipitate sample at 400 °C lead to formation of some segregated ZnO crystals and other aggregated TiO₂ particles. During the optimization process of concentration and time, it was understood that in the presence of 1 mmol of tetrazolate ligand, 1 mmol of zinc salt, 1 mg of the commercial TiO₂, and 20 μL of 200 ppm RB solution, best result of encapsulated TiO₂ containing dye spheres were obtained. Whole the following evidences descried bellow let us to deduce the overall encapsulation process as demonstrated in Scheme 1. In fact, in the mixture containing suitable amount of the commercial TiO₂ and in the presence of RB, the dye molecules are anchored to the titania surface. During overnight stirring, formation of these linked species becomes more completed. Once entrance of the btb ligand and the zinc salt, construction of the hallow polymeric structures of Zn(btb) is started and coincidently, encapsulation of the present titania bearing RB is occurred.

Fig. 1 Representative SEM images of (a) and (b) RB/TiO₂@Zn(btb) prepared in the presence of 3 mg of commercial TiO₂, some unentrapped species can be seen. (c) and (d) after calcination at 400 °C for 3 h; segregated ZnO crystals and other aggregated TiO₂ particles are clear.

Fig. 2 shows FESEM images of the as-synthesized ICPs of these new particles. These images revealed the formation of spherical particles, RB/TiO₂@Zn(btb), with approximate dimensions of 225 nm. In the first investigations, encapsulation of TiO₂ species into the spheres of the metal–organic polymer of Zn(btb) was proved by the high-resolution transmission electron microscopy (HRTEM) images of the final precipitate. As it shows in Fig. 3 spherical particles with diameters in the range 100–2500 nm have been formed. Comparison of the infrared spectrum taken of the bare Zn(btb) and RB/TiO₂@Zn(btb), not only showed that the ligand has been coordinated to metal ions, as evidenced by two strong peaks at 1655 and 1432 cm⁻¹ represented the presence of deprotonated tetrazolate groups,¹⁸ but also indicated nearly identical intact patterns (Fig. S5 and S6 in ESI†). Moreover, upper scales of TEM images, clearly confirmed surrounding the TiO₂ species with the Zn(btb) spherical polymers (Fig. 3b). Indeed, control experiments performed by adding a methanolic solution of TiO₂ particles to a presynthesized colloidal solution of Zn(btb) spheres did not show encapsulation of this particles, while physisorption onto the external surface of the metal–organic spheres and some self titania aggregation occurred. Hence, we were encouraged to elucidate their character precisely with more characterization of these new coordination polymers.

Fig. 2 Representative FESEM images of (a) and (b) RB/TiO₂@Zn(btb) spheres prepared in the presence of 1 mg of commercial TiO₂, (c) and (d) calcined RB/TiO₂@Zn(btb) spheres at 400 °C, 3 h, (e) and (f) calcined RB/TiO₂@Zn(btb) spheres at 500 °C, 3 h, (g) calcined RB/TiO₂@Zn(btb) spheres at 550 °C, 3 h and (h) broken RB/TiO₂@Zn(btb) spheres calcined at 500 °C, in an ultrasonic bath (138 W, 30 min).

Fig. 3 HRTEM images of RB/TiO₂ encapsulated within the spherical Zn(btb) denoted as RB/TiO₂@Zn(btb).

Comparison of chemical composition of the Zn(btb) spheres with the RB/TiO₂@Zn(btb) spheres, which was determined by and energy dispersive X-ray (RX-EDS) microanalysis (Fig. 4), right confirming HRTEM images (Fig. 4, left), showed that the later component contains titania species that do not exist in the bare spheres.

Fig. 4 Comparison of the original spheres of Zn(btb) with those containing RB/TiO₂ species the HRTEM image of (a) the bare spherical Zn(btb) capsules and (b) the RB/TiO₂@Zn(btb). The EDX of (c) the bare Zn(btb) capsules and (d) the RB/TiO₂@Zn(btb).

In addition, the recent reported multifunctional capability of these micro and nanometer size metal–organic capsules^7,8 provides the other opportunity for them to be monitored via choosing an appropriate strategy. As, it was mentioned above, we applied RB, as a probe during the preparation step, not only to illustrate the multifunctional capability of this new carrier capsules, but also to arrange a way for distinguishing the resultant products. Fluorescence optical microscope images and the corresponding fluorescence spectrum of the bare Zn(btb) and encapsulated spheres of RB/TiO₂@Zn(btb) have been presented in Fig. 5. These images evidently indicated the existence of the dye species within the capsules, which means that encapsulation process has performed successfully. In the previous work, we indicated the potential application of tetrazole-based infinite coordination polymers for fabrication of mineral zinc oxide nano and micro structures.⁸ Regarding to this capability of our simple method as well importance of zinc oxide as a versatile inorganic oxide in advanced materials and industrial chemicals, such as field effect transistors,¹⁹ solar cells,²⁰ luminescent materials,²¹ photocatalysts,²² chemical sensors,²³ construction of a new type of ZnO@TiO₂ core–shell structures seemed to be achievable. Among various morphology of ZnO micro and nanostructures, those structures with hollow cavities have attracted a great deal of attention owing to their unique architecture such as low density, high surface area, good surface permeability,^24,25 and broad applications in biomimetic studies, drug-delivery,²⁶ catalysis, inks and pigments,^27,28 chemical storage,²⁹ microcapsule reactors,³⁰ sensors, etc.³¹

Fig. 5 Representative fluorescence optical microscope images (up); and the fluorescence spectrum of (a) Zn(btb) and (b) RB/TiO₂@Zn(btb).

To clarify this aptitude, RB/TiO₂@Zn(btb) hallow spheres was placed in a furnace and calcined at two temperatures, 400 and 500 °C for 3 h in air. As evidenced by SEM analysis, in both temperatures, spherical shape of spheres was retained. However, the original shapes of nano and micro spheres remain approximately intact after calcination at 500 °C. The adhesive spherical structures obtained at 400 °C, whereas separated spheres were prepared during calcination at 500 °C (Fig. 2c–f). Interestingly, calcination of the spheres at 550 °C led to stick some of them to the others (Fig. 2g). Although the reason of these observations was not determined yet, it may ascribe to different removal rate of the solvent from inside the spherical polymers at these two various temperatures. On the other hand, irradiation of the final RB/TiO₂@Zn(btb) suspension calcined at 500 °C, in an ultrasonic bath (138 W, 30 min) made some broken spheres (Fig. 2). As evidenced by FESEM, represented in Fig. 2h, some microsize capsules have been exploded and their inside components obviously become clear. With more precise vision into the exploded formed spheres, not only the hollow space interior the ZnO spheres can be observed, but also surrounded TiO₂ species with a shell of ZnO structure can be clearly monitored from the SEM image.

The powder XRD pattern recorded for the RB/TiO₂@Zn(btb) hallow spheres and its calcination sample at 500 °C, denoted as RB/TiO₂@Zn(btb)-500 in comparison with PXRD spectrum of ZnO and TiO₂ (P25) are represented in Fig. 6. As might be expected, no distinguished diffraction peaks identify in XRD analysis of the RB/TiO₂@Zn(btb) hallow spheres which is a sign of their amorphous character (Fig. 6a). After calcination process, PXRD pattern of RB/TiO₂@Zn(btb)-500, contains both ZnO and TiO₂ diffraction peaks. This observation, combined with EDS and SEM analyses, accordingly confirm TiO₂ particles embedded into ZnO structure. The zinc containing polymeric capsules of RB/TiO₂@Zn(btb) crystallize in the wurtzite ZnO structure which is the most stable phase in ambient conditions, and the titania phase PXRD pattern includes the anatase and rutile TiO₂ phases that is identically recognized the Degussa P25 crystalline phase. Diffuse reflectance UV-vis spectroscopy of the bare Zn(btb), RB/TiO₂@Zn(btb), Degussa P25 and ZnO represented in Fig. 7. As it shown, the red shift assigned in RB/TiO₂@Zn(btb) spectrum obviously confirmed RB/TiO₂ entrance into the Zn(btb) structure. Additionally, after calcination at 500 °C, while evaporation of whole organic content, the representative reflectance pattern of RB/TiO₂@Zn(btb) completely alters and confirming the PXRD analysis, become resemble with crystalline ZnO structure (Figure dotted red and violet patterns). More efforts for characterization and finding the electronic potential of this kind of nanocomposites are currently in progress in our laboratory.

Fig. 6 PXRD diffraction patterns of (a) RB/TiO₂@Zn(btb), (b) calcined RB/TiO₂@Zn(btb) at 500 °C, 3 h, (c) crystalline ZnO and (d) TiO₂ Degussa P25.

Fig. 7 Comparison of the diffuse reflectance UV-vis spectroscopies of the bare Zn(btb) and RB/TiO₂@Zn(btb), with Degussa P25 and ZnO as well calcinated RB/TiO₂@Zn(btb) at 500 °C.

Conclusions

In summary, we have demonstrated a new simple method for construction of micro/nano size spherical Zn(btb) polymers containing dye anchored TiO₂ species. Encapsulation of the commercial Degussa P25 linked to the Rhodamine B, within the hallow space of Zn(btb) was accordingly investigated by SEM, HRTEM, XRD, EDS, Diffuse reflectance and Fluorescence spectroscopy. Subsequently, it has been indicated that using a solvent-induced precipitation method followed by a heating step, a type of TiO₂@ZnO core–shell micro/nano spheres can be designed.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details anf IR spectroscopy of btb, Zn(btb) and RB/TiO₂@Zn(btb) compounds as well more HRTEM images of Zn(btb) and RB/TiO₂@Zn(btb), and EDX analysis (of SEM images) of RB/TiO₂@Zn(btb), and calcined RB/TiO₂@Zn(btb) spheres at 400 °C, 3 h are available. See DOI: 10.1039/c5ra06093a

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