Incorporation of Nano-Zinc Oxide in Lamellar Zirconium Phosphate: Synthesis and Characterization

Danielle Mattos Mariano, Daniela França Silva Freitas, Gerson Alberto Valencia Albitres, Luis Claudio Mendes^*, Maria Ines Bruno Tavares

Instituto de Macromoléculas Professora Eloisa Mano, Centro de Tecnologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.

DOI: 10.4236/msa.2023.146022                

Abstract

In order to provide ultraviolet barrier, antifungal and antibacterial properties, nano-zinc oxide (ZnO) was added to lamellar zirconium phosphate (ZrP). The phosphate was synthesized via reaction of zirconium oxychloride octahydrate and phosphoric acid following its chemical modification with Jeffamine and nano-ZnO. Diffractometric, morphological, thermal, structural and relaxometric evaluations were conducted. Fourier transform infrared spectroscopy (FTIR) revealed increase of the area between 4000 - 3000 cm^-1 due to the formation of ionic specie PO? ⁺NH₃-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)] and nano-ZnO particles. Wide-angle X-ray diffraction indicated that intercalation of Jeffamine was successful. Thermogravimetry confirmed that nano-ZnO particle forced the expulsion of Jeffamine outside ZrP galleries. Scanning electron microscopy evidenced the Jeffamine intercalation and sample heterogeneity. Hydrogen molecular relaxation indicated the increase of molecular rigidity owing to the formation of ionic specie and the addition of nano-ZnO particles. It was postulated that a multifunctional and miscellaneous material constituted by as prepared ZrP, some delaminated ZrP platelets and nano-ZnO particles was achieved. The material has potential for usage as filler in polymeric composites.

Keywords

Lamellar Zirconium Phosphate, Jeffamine, Nano-Zinc Oxide, Intercalation

1. Introduction

Nano-zinc oxide as nanoparticle possesses many attributes. Prominent physical and chemical properties make it a suitable candidate for applications in the electronic, pharmaceutical, cosmetic, personal care, civil construction and many other industries. Interest has been given to applications related to its bactericidal properties. The investigation of ZnO nanofiller as shielding against ultraviolet radiation in polymer blend of recycled poly (ethylene terephthalate)/polycarbonate (rPET/PC) was carried out by Pires and co-authors. The authors concluded that in some extent the nZnO attenuated the matrix damage being a promising ultraviolet (UV) barrier agent [1] . Synergic action of ZnO nanoparticle and gamma-radiation in recycled polycarbonate as barrier against ultraviolet radiation was studied. The authors considered that the combined action of nanofiller and irradiation as barrier to UV was satisfactory [2] . The action of ZnO nanoparticles antibacterial agent was reviewed. The mechanisms and its interaction with a diversity of microbes were approached [3] . Kavitha et al. published a review with detailed approach on antibacterial capacity of ZnO nanoparticles [4] . Zirconium and titanium phosphate (ZrP and TiP) are the most studied in the field of lamellar metal transition phosphates. They presented combination of tetrahedral and octahedral as lamellar structural arrangement. This permits a wide range of physical and chemical modifications [5] . Copper-containing titanium phosphate was developed by Yerga et al. with electrocatalytic effect towards the oxidation of glucose [6] . Searching tribological application and dispersion stability in lubricating oil, Jiang et al. reported the expansion of zirconium phosphate galleries with amines. The well-dispersed nanosheets decreased the coefficient of friction and pin volume loss resulting in protection of the rubbing surface [7] . Gentamicin was intercalated inside the zirconium phosphate. The filler was embedded in nanocomposites based on polymer blend of poly(ethylene oxide terephthalate)/poly(butylene terephthalate). The system was studied as functional biocomposites as drug carrier [8] . Ramos-Garcés and Colón presented an overview about of the advancements of zirconium phosphate as inorganic support for the electrocatalysis in oxygen evolution reaction [9] . Layered nanocrystalline zirconium (IV) phosphonate-phosphate was synthesized searching an ion exchange material with selectivity for terbium³⁺ [10] . Hexagonal, platelets, rods, cubes and spheres of ZrP were synthesized following their modification with Co (II) and Ni (II) cations in order to investigate their electrochemical oxygen evolution reaction behavior. Maximum ion exchange capacity varied according to ZrP morphology and kind of cation chosen [11] . Synthesis and amine intercalation of lamellar zirconium phosphate (ZrP) was experienced by Mariano et al. using ether-amine oligomer (EA) as intercalator agent at different EA:ZrP ratios. The material was thought for using in the controlling release of drugs [12] . Aiming application in the textile industry, Albitres et al. synthesized α-titanium phosphate (α-TiP). Following, they performed its intercalation with two different long chain amines with the aid of short amine. Raman spectroscopy revealed chemical modification of α-TiP [13] . To develop a nanocarrier material, Carvalho et al. synthesized and modified α-zirconium phosphate (α-ZrP) with zinc gluconate (ZnG) as control drug delivery. The authors stated that the strong interaction between ZrP and ZnG occurred [14] . Octadecylamine (Oct) was experienced as intercalating agent for α-zirconium phosphate (α-ZrP) by ethanol/water at different amine:phosphate ratios (0.5:1, 1:1, and 2:1). The intercalation revealed the presence of free and bonded amine besides the influence on the amine melting temperature [15] .

The purpose of this work led in consideration the nano-ZnO characteristics as ultraviolet barrier, antifungal and antibacterial. In this context, we synthesized ZrP following its chemical modification with Jeffamine and nano-ZnO. Structural, crystallographic, thermal, morphologic, and molecular mobility characteristics were assessed. A multifunctional and miscellaneous material was reached potentially applicable for polymeric composites.

2. Experimental

Materials

Phosphoric acid (H₃PO₄), zirconium (IV) oxide chloride 8-hydrate (ZrOCl₂∙ 8H₂O), Jeffamine^® M-600 (≈600 g/mol, backbone propylene oxide:ethylene oxide, 9:1) ethyl alcohol, zinc oxide (ZnO, <100 nm) by Sigma-Aldrich Co. Jeffamine^® was designated as ether-amine oligomer and its acronym was E-A.

Synthesis of layered zirconium phosphate

By direct precipitation, phosphoric acid solution equal 12 M was mixed in zirconium oxychloride at ratio Zr/P = 18, kept under reflux at 110˚C, under stirring, for 24 hours. After centrifugation, the solid was washed successively until attaining pH around 6 [15] .

Chemical modification of layered zirconium phosphate

Chemical modification was performed in two steps. In the first step, Jeffamine was added in different proportions of amine:phosphate (0.5:1, 1:1, 2:1). The description of the experimental procedure was reported in elsewhere [16] . It was decided to use the material with 0.5:1 ratio which was designated as E-A/ZrP. Following, this sample was chemically modified with nano-ZnO at proportion 1:1 ratio. Nano-ZnO was dispersed in ethanol and slowly drip in E-A/ZrP ethanol dispersion, at 25˚C, under stirring, for 24 hours. Subsequently, washed with water and dried at 110˚C, until constant weight. A sketch of the experimental procedure is showed in Figure 1.

Wide-angle X-ray diffraction (WAXD)

Rigaku equipment, model Ultima IV was used at CuKα radiation with wavelength (1.5418 Å) Ni filter, 30 kV voltage and current of 15 mA, 2θ between 2˚ - 50˚ and resolution of 0.05˚. Interlayer spacing was evaluated by Bragg equation n = 2d_hkl sen θ (n—diffraction order, d_hkl—interlayer spacing, θ—diffraction angle). Debye-Scherrer equation $\tau =K*\lambda /\beta *\cos \theta$ (K = proportionality constant, 0.9; λ = mean value of the wavelength of Cu radiation in the apparatus, 1.5418 Å; β = peak width at half height (in radians); θ = half of the 2θ value from crystalline peak) was applied for determining crystal size.

Field emission scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

SEM analysis was performed at Tescan field emission microscope, model

Figure 1. Sketch of the ZrP synthesis and chemical modification.

MIRA 4 LMU, cannon FEG Schottky (Field Emission Scanning Electron Microscope). The SEM has as an accessory an EDS detector with a 30 mm² Si₃N₄ window and a resolution lower than 129 eV for the MnKα emission line, used to mapping test.

Thermogravimetry (TGA)

Thermogravimetry was carried out in TA instrument, model Q500. The analysis was conducted from 10˚C to 700˚C, at 10˚C/min, with nitrogen as carrying gas. Mass loss and derivative curves were assessed.

Fourier transform infrared spectroscopy (FTIR)

In Perkin Elmer equipment, model Frontier the infrared spectroscopy was performed at range of 4000 - 400 cm⁻¹ with 50 scans and 4 cm⁻¹ of resolution, by using KBr disk.

Hydrogen nuclear magnetic resonance in time domain (¹HNMRTD)

¹HNMRTD was performed in a MARAN Ultra spectrometer with an electromagnet operating at 0.54 T (Larmor frequency corresponding to 23.4 MHz for the proton), probe with an internal diameter of 18 mm, at 27˚C. The 90˚ excitation pulse was automatically calibrated to 7.5 μs. The longitudinal (T₁) relaxation time was fitted by inversion-recovery (IR). Inversion-recovery (T₁) was accomplished in the time intervals, considering forty points spaced logarithmically at a range of 0.1 to 5000 ms, with a recycle interval of 3 s and 8 accumulations [17] .

3. Results and Discussion

Infrared spectroscopy (FTIR)

For better understanding, all spectra were divided at different spectral regions (Figure 2). ZrP bands at 3595, 3511 and 1617 cm⁻¹ (asymmetric/symmetric bending of water in the interlayer region), 3153 cm⁻¹ (stretching of the hydrogen bonding between H-O-H and P-OH group), 1250 cm⁻¹ (P-O-H bending), 1112, 1074 and 1050 (PO₄ asymmetric stretching) and 980 and 968 cm⁻¹ (PO₄ symmetric stretching) were registered. These attributions agreed with reported by Smith et al. [18] . Bands at 599 and 522 cm⁻¹ were attributed to the Zr-O vibrations coinciding with published by Ran and co-authors [19] . Bands at 2972, 2931 and 2873 cm⁻¹ (CH and CH₂ stretching), 1460 cm⁻¹ (CH bending), 1108 and 1016 cm⁻¹ (C-O-C stretching) were associated to the main absorptions of Jeffamine [12] .

Figure 2. Infrared spectra of the Jeffamine and phosphates.

Jeffamine-modifying ZrP presented the ZrP characteristic bands in the 4000 - 3000 cm⁻¹ spectral region. Jeffamine bands at 3000 - 2500 cm⁻¹ and between 1500 - 1300 cm⁻¹ also appeared. The ZrP band at 1250 cm⁻¹ remained. At 1200 - 900 cm⁻¹, P-O (phosphate) and C-O-C (Jeffamine) bands overlapped but discrete shift was noticed when compared to the precursors. New band appeared at 600 - 700 cm⁻¹. Albitres et al. registered new bands at 1548, 1206, 1092 and 665 cm⁻¹ after intercalation of α titanium phosphate with Jeffamine and ethylamine [13] . Weak vibration at 596 cm⁻¹ (P-OH stretching) was reported after intercalation of layered ZrP [13] [20] . After exfoliation, sulfonic group was anchored onto ZrP. The authors registered the superposition of the S=O and P-O bands [21] . ZnO showed a broad and intense band at 3436 cm⁻¹ along with a series of weak bands (1563 cm⁻¹, 1534 cm⁻¹, 2925 cm⁻¹, 2855 cm⁻¹, 1456 cm⁻¹, 1379 cm⁻¹, 1316 cm⁻¹, 1161 cm⁻¹, 1053 cm⁻¹, 831 cm⁻¹, 496 cm⁻¹ and 437 cm⁻¹). The searching for works to support our results revealed that bands attribution was dependent on the starting reagent, preparation method, temperature, sample size and geometry, and mainly the lack of efficient purification of the final ZnO [22] [23] [24] [25] . The spectrum of the phosphate modified with ZnO exhibited ZrP bands at 3595; 3511; 3153; 1617 cm⁻¹ already described. Bands at 1250  cm⁻¹; 1112; 1074; 1050; 980 and 968 cm⁻¹ associated with P-O-H and PO₄ groups were consistent with the starting ZrP. Bands at 496 and 437 cm⁻¹ related to the Zn-O stretching sharply appeared. The peak area at spectral region of 4000 - 3000 cm⁻¹ (absorptions at 3595; 3511; 3153 cm⁻¹ associated to the hydrogen bond of P-OH and water) and 3000 - 2500 cm⁻¹ (absorptions at 2972; 2931; 2873 cm⁻¹ related to the vibration of C-H and H-C-H) were measured. Table 1 depicts the calculation. Significant variations were noticed. After the addition of Jeffamine, the peak area at region 4000 - 3000 cm⁻¹ showed a highlighted increase. We supposed that the variation is associated with the formation of ionic specie PO^{− +}NH₃-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)] by Brϕnsted acid-base reaction between P-O-H and H-N-H group. The addition of nano-ZnO generated high increment possibly due to OH groups on its surface. Respect to another spectral region, we postulated that its reduction would be due to the disruption of E-A-ZrP sites by nano-ZnO particles below 30 nm. Its action inducted the ejection Jeffamine free molecules and some delamination of the E-A-ZrP platelets.

Wide-angle X-ray diffraction (WAXD)

Table 1. Area of specified spectral region.

Figure 3 illustrates the WAXD diffraction patterns of the samples and a sketch of the assumed changes after each chemical modification. Table 2 summarizes the diffraction angle, d_spacing and crystal size of the samples. Jeffamine^TM diffractogram (black curve) indicated an amorphous matter. ZrP (red curve) showed diffraction angles around 2θ = 11.75 (d_spacing = 7.52 Å), 19.89˚ and 25.04˚

Figure 3. Diffraction pattern of the samples and a sketch of the assumed changes after ZrP chemical modifications.

Table 2. Diffraction angle, d_spacing and crystal size of as prepared and modified ZrP.

*Sequentially, ZrP and Jeffamine-modifying ZrP d_spacing values.

corresponding to crystalline plane (002), (110) and (112) [Garcia et al. and Han et al.] [26] [27] . At E-A/ZrP diffraction pattern (light blue curve), hkl plane of ZrP remained but new diffraction angles appeared at 6.32˚, 4.70˚ and 3.15˚ (magnified laterally). These mixing of intercalated phases mean that more probably in each one there is variation in Jeffamine alignment. Similar result was published by Kale et al. using ethylenediamine as intercalator [28] . ZnO diffraction pattern (dark blue curve) presented 2θ around 31.78˚, 34.5˚, 36.2˚ and 47.6˚ corresponding crystallographic plane (100), (002), (101) e (102), respectively, as published by Mallakpour and Javadpour [29] . In the E-A/ZrP/ZnO diffraction pattern (pink curve), the absence of the diffraction angles below 10˚, the diffraction angles of ZrP and zinc oxide were depicted. The presence Jeffamine increased the ZrP d_spacing up to fourfold. Crystal size of ZrP decreased dimension by consecutive addition of Jeffamine and nano-ZnO. With respect to the ZnO, the supplier stated that ZnO particle size is below to 100 nm but the experimental technique for its determination and range of size were not released. The addition of induced the disappearance of diffraction angles below 10˚. A reasonable explanation would be that the E-A-ZrP intercalated sites were undone by nano-ZnO particles below 30 nm ejecting Jeffamine molecules outside. The fragmentation of E-A-ZrP structure was evidenced by the reduction of crystal size. At frame 1, the arrangement of as prepared zirconium phosphate can be seen. Frame 2 represents the entrance of the Jeffamine molecules into ZrP galleries. The lamellar spacing was increased by formation of the ionic specie PO^{− +}N₃H-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)]. Frame 3 depicts the action of nano-ZnO particle below 30 nm provoking the expulsion of E-A molecules outside and the delamination of E-A-ZrP sites. Then, we assumed that the final product (frame 4) is a miscellaneous material constituted by as prepared ZrP, some delaminated ZrP platelets and ZnO particles. The result is in accordance with the FTIR analysis.

Thermogravimetry (TGA)

Figure 4 shows the mass loss and derivative curves of samples. Table 3 discloses the degradation temperature and the amount of mass loss for each step. Jeffamine displayed one step of mass loss with maximum around 299˚C. Three thermal degradation decays were revealed by α-ZrP. The first one was located between 100˚C - 200˚C (T_max = 135˚C) attributed to the release of adsorbed water. The two others corresponded to the dehydroxylation reaction (T_max = 466˚C) of the hydroxyl groups attached to the phosphorus atoms and the chemical

Figure 4. TGA mass loss and derivative curves of precursors and modified phosphates: (A) mass loss; (B) derivative; (C) amplified derivative (100˚C - 300˚C); (D) amplified derivative (400˚C - 700˚C) and (E) amplified mass loss (0˚C - 350˚C)

Table 3. TGA data of the precursor and modified phosphates.

(in parentheses, the mass loss percentage).

transformation of phosphate to pyrophosphate (T_max = 555˚C), respectively. Similar result was reported by Mariano and co-authors [12] . Jeffamine-modifying ZrP presented a decay around 300˚C (T_max = 329˚C) ascribed to the thermal degradation amine molecules (free and bonded). We understood that the absence of dehydroxylation step showed the effectiveness of amine intercalation as pointed out elsewhere [12] . The result is agreement with Kale et al. reporting intercalation of ZrP with ethylenediamine [28] . The second thermal degradation decay was related to the conversion of phosphate to pyrophosphate shifted to higher temperature (T_max = 577˚C). ZnO presented a unique decay (T_max = 268˚C) being imputed to the release of water from crystalline lattice. As seen in infrared evaluation, herein the ZnO degradation steps revealed disagreement. Routes of synthesis, precursors and purification could be the reasons for this inconsistency [23] [30] [31] [32] . E-A/ZrP/ZnO showed 3 steps of degradation. The first one around 170˚C was attributed to ZrP water releasing. Around 300˚C, we supposed that the 8% of mass loss jointly represented the water from ZnO crystalline lattice (6%) plus the amine portion of ionic specie PO^{− +}NH₃-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)] (2%). The third step was related to the conversion of phosphate to pyrophosphate. The result agrees with the postulated at FTIR and WAXD evaluations. The final product is a multifunctional material encompassing as prepared ZrP, some delaminated ZrP platelets and ZnO particles.

Field emission scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

Figure 5(A), Figure 5(E) depict the SEM images of ZrP and EDS evaluation. As prepared ZrP (Figure 5(A)) presents as a series of hexagonal structures with some agglomeration and then without homogeneity in diameter and thickness. Similarly, Ramos-Garcés et al. synthesized and modified ZrP with metal attained agglomerated ZrP with varied morphology—hexagonal, rod-like, cube-like, and spherical [11] . EDS evaluation (Figure 5(E)) revealed its elemental composition. The percentage of zirconium, oxygen and phosphorus are in good agreement with the chemical formula [Zr(HPO₄)₂∙2H₂O] representing the phosphate structure reported by Silva et al. in the work related to zirconium phosphate organically intercalated/exfoliated with octadecylamine [15] . Figure 5(B), Figure 5(F) show the SEM images and EDS of E-A/ZrP. The image clearly confirms the hexagonal structure of ZrP with variation in diameter and thickness. Higher magnified image confirmed that Jeffamine was partially intercalated into ZrP galleries. The highlighted region shows the separation of the lamellae caused by the intercalator. This in good agreement with WAXD analysis in which partial interposition of Jeffamine along ZrP lamellae revealing new diffraction angles lesser than 10˚. The content of nitrogen was approximately to one detected in TGA analysis. SEM images and EDS of ZnO were depicted in Figure 5(C), Figure 5(G). Its morphology seems to be like agglomeration of fragmented rounded rod shape particles. Tymoszuk and Wojnarowicz reported heterogeneous shapes (rods, bodies with hexagonal base and irregular bodies) of ZnO submicron particles [33] . EDS evaluation of ZnO revealed its atomic composition. E-A/ZrP/ZnO images and EDS are shown Figure 5(D), Figure 5(H). Heterogeneity is sharply revealed. As prepared layered ZrP appeared as aggregate of hexagonal structures without homogeneity in diameter and thickness. Also, isolated platelets with lower thickness can be seen and could be associated with delaminated lamella from the action of ZnO. Small particle of ZnO was easily identified. These evidences corroborate the supposition in which a mixing of as prepared ZrP, some delaminated ZrP platelets and ZnO particles was attained.

Figure 5. SEM images of the samples (A)-(D); EDS of the samples: ZrP (E), EA-ZrP (F), Zn (G), EA-ZrP-ZnO (H).

Hydrogen nuclear magnetic resonance in time domain (¹HNMRTD)

Figure 6 and Table 4 show the domain distribution curves and relaxation times of the samples. Domain distribution curve of Jeffamine showed one domain between 10⁵ - 10⁶ ms. ZrP presented two domains—flexible (lowest relaxation time) and rigid (highest relaxation time). The first one was related to the hydrogen relaxation of adsorbed water and free P-OH. The second one was associated to the hydrogen motion due to hydrogen bond between P-OH and water inside of its crystalline lattice. Bakhmutov and Clearfield studied the relaxation and molecular mobility of layered α-zirconium phosphate, at variable experiments. They reported two types of motions meaning low-frequency proton transfer and high-frequency rotation in P-OH groups [34] . The domain distribution curve of E-A/ZrP showed enlargement indicating heterogeneity as reported by Ribeiro et al. in article on molecular dynamics of poly (hydroxybutyrate) filled with microcrystalline cellulose [35] . The T₁H of the lowest domain decreased drastically. The entrance of Jeffamine inside the ZrP galleries increases the lamellar spacing favoring the hydrogen molecular mobility. The highest

Figure 6. Domain distribution curves of the Jeffamine, ZrP and modified phosphates.

Table 4. T₁H of the Jeffamine, ZrP and modified phosphates.

domain presented T₁H slightly higher. The formation of the ionic specie PO^{− +}NH₃-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)] hampered the hydrogen mobility in the rigid domains. The domain distribution curve of E-A/ZrP/ZnO also enlarged indicating heterogeneity. The higher T₁H value was associated to the ZnO particle which restrained the hydrogen mobility. Molecular relaxation endorsed the evidences showed in FTIR, WAXD, TGA and SEM evaluations.

4. Conclusion

This work proposed to synthesize a new filler through the combination of the inorganic and organic matters to improve ZrP properties by addition of nano-ZnO. ZrP was synthesized and chemically modified with Jeffamine and nano-ZnO. FTIR indicated the Brϕnsted acid-base reaction between P-O-H and H-N-H groups inducing the formation of ionic specie PO^{− +}NH₃-[C-(H)(CH₃)-CH₂-O-(C-(H)(CH₃)-CH₂-O)₈-(CH₂-CH₂-O-CH₃)]. WAXD evidenced the amine intercalation and its breakdown by action of nano-ZnO which was endorsed by TGA analysis. SEM described the intercalation and the collapse of the Jeffamine-modifying ZrP. ZrP molecular relaxation was altered by the presence of Jeffamine and nano-ZnO. Multifuncional and heterogeneous material was reached.

Aknowledgements

The authors thank Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and the Universidade Federal do Rio de Janeiro staff for supporting this research.

Author’s Contribution

D. M. Mariano—Conceptualization, Writing; D. F. S. Freitas—Visualization; G. A. V. Albitres—Visualization; L. C. Mendes—Conceptualization, Resources, Supervision, Writing; M. I. B. Tavares—Resources.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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