1.   Introduction

Chemical fertilizers are commonly used to provide nutrients in plants for optimal growth and production. However, the available plant production process is insufficient to support the rapid growth of food demand [1]. Hence, nano-fertilizers have been applied in plants to enhance crop yields by increasing seed germination, seedling growth rate, nitrogen metabolism, protein and carbohydrate synthesis, and durability. Nano-fertilizers can also be used in small quantity to increase the ease of use and reduce the economic cost compared with conventional fertilizers [2].

In recent years, slow-release ammonium metal phosphate (NH₄M(II)PO₄H₂O)-based fertilizers, where M is a metal atom, have been used [3]. Such fertilizers are environmental friendly because their negligible toxicity. Recently, many efforts have been devoted to improve the performance of these fertilizers by modifying their morphology, particle size, and doping [4]. Zinc is an essential micronutrient required for the normal growth and development of plants. This micronutrient promotes protein metabolism and reduces plant resistibility in unsuitable environments. For example, ammonium zinc phosphate (NH₄ZnPO₄) is as a good source of phosphorus, nitrogen, and zinc for plant production [5–6]. NH₄ZnPO₄ fertilizer can be synthesized hydrothermally under autogenous pressure at temperatures ranging from 100 to 250°C. It is also a good flame retardant in gas and condensed phases, and is safe while storing [7–8].

Until now, a method for the preparation of NH₄ZnPO₄ nanofertilizer is lacking. Hence, a suitable method to prepare nano-NH₄ZnPO₄ is important to improve crop yield. Recent studies have synthesized nanostructures by using various techniques, such as sol–gel, hydrothermal, and co-precipitation methods. The changes in size and morphology can be determined through the intermolecular interactions of atoms in the compound, such as solvent type, compound permanence, material type, and supersaturation degree [9–10]. The co-precipitation method is commonly used to prepare nanomaterials; in this method, the size and shape can be controlled by varying the amount and type of solvent. For example, NH₄CoPO₄∙H₂O nano–micro-particles with sizes ranging from 200 to 4000 nm can be achieved by using a simple co-precipitation method in ethylene glycol solvent [9].

In this work, we report the synthesis of ammonium zinc phosphate fertilizer by using a simple precipitation method at room temperature with various solution templates, including polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), glucose, and hexadecyltrimethylammonium bromide (CTAB). The structure, morphology, local structure, and surface electronic structure properties of the NH₄ZnPO₄ products synthesized with different precursors were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), inductively coupled plasma atomic emission spectroscopy (ICP-OES), X-ray photoelectron spectrometry (XPS), and X-ray absorption spectroscopy (XAS).

2.   Experimental

2.1   Material preparation

The samples were prepared using a simple precipitation method consisting of starting chemicals of zinc(II) chloride (ZnCl₂) used as the metal sources and polymer solutions. In this work, 5% by weight of the polymer was mixed with 100 mL of deionized water under vigorous stirring at 27°C until a homogeneous solution was obtained. Water, PVP, PVA, glucose, and CTAB were used as the polymer sources. The metal sources (20 mmol) and phosphoric acid solution (H₃PO₄, 70wt%, 3 mL) were dissolved in deionized water and polymer solution (50 mL) to form a clear solution. The mixed solution was precipitated by adding ammonium solution (25wt%) with vigorous stirring until pH = 7. The precipitates were washed several times with deionized water and ethanol. Then, the samples were dried at 70°C in air. A schematic of the preparation of the samples is shown in Fig. 1. Finally, the samples were stored in a desiccator for further characterization.

2.2   Materials characterization

The prepared products were characterized using XRD, SEM, XPS, and XAS. The XRD patterns were examined on a Bruker D8 Advance (USA) with Cu K_α radiation (λ = 0.154184 nm) to confirm the phase information of the samples. Scanning electron microscopy and energy dispersive spectrometry were performed using Zeiss AURIGA FIB-SEM (Germany) with FE-SEM at 7 kV to reveal the morphology of the prepared samples. Transmission electron microscopy was performed with a FEI Tecnai G2 F20 field emission transmission electron microscope (USA) operated at 200 kV. X-ray absorption near-edge structure (XANES) spectra at Zn K-edge were collected at the SUT–NANOTEC–SLRI XAS beamline (BL 5.2) at the Synchrotron Light Research Institute of Thailand to obtain the oxidation states of the samples. The XANES spectra were obtained in transmission mode using Ge (220) double crystal monochromators with an energy resolution of 2 × 10⁻⁴ eV.

Figure  1.  Schematic of the formation of NH₄ZnPO₄ powder synthesized by a chemical precipitation method using water, PVP, PVA, sucrose, and CTAB solutions.

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2.3   Ion release properties

In addition to crystal size and morphology, ion release is an important property that determines the potential use of materials as fast- or slow-release fertilizers. In this study, the NH₄ZnPO₄ samples prepared using water, PVP, PVA, sucrose, and CTAB solutions were chosen to study the ion release. In this experiment, the aqueous ion release at room temperature was examined in the prepared solution (0.5 g of the dried sample dissolved in 50 mL of deionized water), which was stirred continuously for 24 h. The sample was collected by filtering with a 0.22 µm filter membrane and subjecting to high-speed centrifugation at 8000 r/min. Finally, the Zn²⁺ and P⁵⁺ concentrations in the supernatant were determined using ICP-OES (Optima 2100 DV ICP-OES, Perkin Elmer Instruments, USA).

3.   Results and discussion

3.1   Structural and morphological characterization

Fig. 2 shows the XRD patterns of the prepared samples. The XRD patterns of the prepared NH₄ZnPO₄ samples show the formation of a hexagonal structure within space group P63 [11]. The diffraction peaks of the NH₄ZnPO₄ sample mainly consist of (100), (101), (200), (002), (201), (210), (211), (202), (103), (220), (222), and (123) planes, which agree with the standard data (JCPDS NO.89-6315). In addition, the PVP and sucrose conditions present the phase impurities of NH₄ZnPO₄ with monoclinic structure and Zn₃(PO₄)₂4H₂O. The CTAB condition shows only NH₄ZnPO₄ monoclinic structure, which is in good agreement with the standard data (JCPDS NO.88-1126) and is consistent with the literatures [12–13].

Figure  2.  XRD patterns of the NH₄ZnPO₄ powder prepared by using (a) water, (b) PVP, (c) PVA, (d) sucrose, and (e) CTAB.

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The unit cell parameters (a, b, and c), cell volume (V), expected profile residual (R_exp), profile residual (R_p), weighted profile residual (R_wp), and goodness-of-fit (GOF) values of all NH₄ZnPO₄ samples were calculated using Rietveld refinement unit cell analysis (TOPAS software), and the results are summarized in Table 1. Using the medium solution of PVP, sucrose, and CTAB can change the phase structure from hexagonal to NH₄ZnPO₄ monoclinic. Moreover, the CTAB condition showed an XRD pattern of only NH₄ZnPO₄ monoclinic structure. The growth process of NH₄ZnPO₄ by adding CTAB is different. The surfactant reduces the surface tension of the solution and then lowers the energy needed to form a new phase of NH₄ZnPO₄ monoclinic structure at a low supersaturation [12]. In another case, CTAB could also be considered to influence the disintegration of zinc and the growth of NH₄ZnPO₄ through electrostatic and stereochemical effects [14]. The hexagonal and monoclinic structures of NH₄ZnPO₄ can be used as fertilizers [15].

Table  1.  Lattice parameters (a, b, c, and β angle), cell volume (V), Rietveld refinement parameters (R_p, R_exp, R_wp, and GOF), phase composition, and ion releases of the NH₄ZnPO₄ powder prepared using different media (water, PVP, PVA, sucrose, and CTAB)

Media	a / nm	b / nm	c / nm	β / (°)	V / (106 pm3)	Space group	Rp / %	Rexp / %
Water	1.0692	1.0692	0.8709	90	862.13	P63	4.67	2.46
PVP	1.0691	1.0691	0.8707	90	861.96	P63	3.84	2.27
PVA	1.0693	1.0693	0.8708	90	862.40	P63	3.77	2.77
Sucrose	1.0690	1.0690	0.8706	90	861.76	P63	4.39	2.28
CTAB	0.8790	0.5450	0.8960	90.34	429.16	P21	3.03	2.56
Media	Rwp / %	GOF / %	Phase composition / %			Zn release / (mg·L−1)	P release / (mg·L−1)
			Hexagonal	Monoclinic	Hopeite
Water	6.66	2.70	100	0	0	0.34 ± 0.01	33.57 ± 3.15
PVP	5.94	2.62	13.88	69.86	16.26	1.65 ± 0.02	41.97 ± 5.90
PVA	5.03	1.82	100	0	0	0.66 ± 0.01	34.02 ± 4.10
Sucrose	6.02	2.64	20.44	56.30	23.26	4.21 ± 0.02	48.29 ± 3.17
CTAB	3.88	1.51	0	100	0	0	26.57 ± 3.32

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Fig. 3 shows the FE-SEM images of the NH₄ZnPO₄ powders prepared using water and polymer sources of PVP, PVA, sucrose, and CTAB solutions. As shown in Fig. 3, the microparticles are about 0.4–10 µm in size. The results confirm that the ammonium zinc phosphate provides different morphologies and sizes. During precipitation, morphological properties such as particle size depend not only on the chemical reaction of the compound but also on the other experimental conditions, such as surfactants and solvents. Herein, NH₄ZnPO₄ powders were synthesized in water solution as a reference, and results showed that the product has normal morphology. As shown in Fig. 3(a), the microrods obtained using non-surfactant of water formed through self-assembly [11]. The ion concentration profiles in water are defined by the speciation diagrams of phosphoric acid (H₃PO₄) and ammonia solution. The chemical reaction of ammonium zinc phosphate can be expressed by the following equation:

Figure  3.  SEM images of the NH₄ZnPO₄ powders prepared using a chemical precipitation method with (a) water, (b) PVP, (c) PVA, (d) sucrose, and (e) CTAB; (f) corresponding average particle sizes of NH₄ZnPO₄ powders.

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In comparison, the use of surfactant as a template provides various morphologies of NH₄ZnPO₄. The morphologies of the prepared NH₄ZnPO₄ particles are controlled by the relative growth rate of different crystal facets [16]. Moreover, the rare earth orthophosphate with hexagonal phase shows a high anisotropic structure, which strongly influences their morphological evolution [12]. The phosphates with monoclinic structure have less anisotropy and low tendency to grow along a random direction [17]. Thus, the use of PVP, PVA, sucrose, and CTAB solutions can change the morphology of NH₄ZnPO₄ to the nanoparticle size.

The possible formation mechanisms of NH₄ZnPO₄ microstructures by using water and polymer sources of PVP, PVA, sucrose and CTAB are proposed in Fig. 4. In general, the formation of particles is related to nucleation and crystal growth [18–19]. Nucleation consists of the formation of stable nuclei and the growth of critical nuclei, resulting in the generation of large crystals in crystal growth [20–22]. In the absence of a surfactant as shown in Fig. 4, the secondary particles resulting from the aggregation of the primary particles self-assemble into the microrods. Particle growth is related to the adsorption of surfactants onto particular crystallographic facets of the growing crystal [23]. The addition of surfactants such as PVP, PVA, sucrose, and CTAB into the nucleation process can affect the surface energy, and thus control the nucleation rate [24]. Slower nucleation rate corresponds to larger particle size. The particles agglomerated with a microrod-like shape when the nonionic surfactant of PVP was added. These results may be attributed to the low PVP concentration. The explanations are as follows [23]: (i) the adsorption of PVP was insufficient for the effective coverage of the NH₄ZnPO₄ phase, resulting in particle agglomeration; (ii) the particles underwent continuous self-assembly, resulting in a microrod-like shape. For PVA, a polymer chain of PVA adsorbed onto the particle surface through non-covalent bonding of hydroxyl groups. The adsorbed PVA formed a multilayer coating around the particles as a water-protective agent, decreasing the particle size and preventing agglomeration [25]. In the case of sucrose, the multiple hydroxyl groups of a large molecular size of non-reducing sucrose adsorbed onto certain crystal planes as a coating agent to form steric hindrance. Thus, sucrose may possibly be transformed to glucose and gluconic acid that were coated on the particle surface, decreasing the particle size [26–27]. The hydrophobic tail of CTAB is adsorbed onto the particle surface, and the adsorbed CTAB hinders other nutrients to attach to the surface [28]. Then, the growth rate slows down, resulting in the agglomeration of particles forming as bulk products of sphere [29].

Figure  4.  Possible formation mechanisms of NH₄ZnPO₄ microstuctures prepared by using water and polymer sources of PVP, PVA, sucrose, and CTAB.

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3.2   Iron release properties

The ion release measurement was used to clarify the type of fertilizer of all NH₄ZnPO₄ samples prepared using water, PVP, PVA, sucrose, and CTAB solutions. The ion release properties are summarized in Table 1. The P ion releases of (33.57 ± 3.15), (41.97 ± 5.90), (34.02 ± 4.10), (48.29 ± 3.17), and (26.57 ± 3.32) mg·L⁻¹ were observed in the NH₄ZnPO₄ prepared in water, PVP, PVA, sucrose, and CTAB solutions, respectively. The Zn ion releases of (0.34 ± 0.01), (1.65 ± 0.02), (0.66 ± 0.01), (4.21 ± 0.02), and 0 mg·L⁻¹ were observed in the NH₄ZnPO₄ prepared in water, PVP, PVA, sucrose, and CTAB solutions, respectively. These results indicate that more Zn and P ions with hexagonal structure were released than those with monoclinic structure. In addition, Zn and P ions were released with a higher proportion of Hopeite. Considering the molar mass ratio of NH₄ZnPO₄ prepared in sucrose solution, the mass of P and Zn ions was calculated to be around 540 mg in 1 g of this compound. After combining the value of P and Zn ion release (52.5 mg·L⁻¹), approximately 5 mg out of 540 mg of P and Zn ions corresponding to around 1% of the total mass was released in 24 h. On the basis of the obtained results, our NH₄ZnPO₄ powders can be classified as slow-release fertilizers where less than 15% of the ions were released in 24 h [30]. In specific, the Zn ion release of 4.17 mg·L⁻¹ is comparable with previous reports on Zn(OH)₂ (9.62 mg·L⁻¹) [31], the core–shell ZnSO₄ (3.8 mg·L⁻¹) [32], the zeolite-based ZnSO₄ (4.5 mg·L⁻¹) [33], and the commercial ZnO-based fertilizer (1–2 mg·L⁻¹) [31]. These results suggest that our simple facile route can be used to synthesize a NH₄ZnPO₄-based slow-release fertilizer.

3.3   X-ray absorption study

The XANES spectra of NH₄ZnPO₄ of Zn K-edge XANES were obtained at the energy range of 9600–9760 eV to examine the oxidation states of the iron element in the samples. Fig. 5 shows the normalized Zn K-edge XANES spectra of the NH₄ZnPO₄ H₂O prepared using water, PVP, PVA, sucrose, and CTAB solutions, in comparison with the standard compounds of Zn⁰⁺ and Zn²⁺. The edge energy positions of the NH₄ZnPO₄ H₂O samples were 9662.60, 9662.62, 9662.27, 9662.54, and 9662.50 eV, which lied on the edge energy positions of ZnO (E₀ = 9662.69 eV) standard sample. These results imply that the oxidation state of Zn ions in all the prepared samples is 2+.

Figure  5.  Zn K-edge XANES spectra of NH₄ZnPO₄ powder prepared by chemical precipitation.

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Fig. 6 shows the normalized P K-edge XANES spectra of the NH₄ZnPO₄ prepared with water, PVP, PVA, sucrose, and CTAB conditions, in comparison with the standard compounds of P⁰⁺ and P⁵⁺. The results indicate that the spectra of all samples show similar features. The edge energy positions of the samples match with that of the KH₂PO₄ standard sample. These results suggest that the oxidation state of P ions in all the prepared samples is 5+.

Figure  6.  P K-edge XANES spectra of NH₄ZnPO₄ powder prepared by chemical precipitation.

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3.4   X-ray photoelectron spectroscopy

The chemical compositions of all the prepared samples were examined by XPS. The XPS spectra of ammonium zinc phosphate are shown in Fig. 7, and these spectra were calibrated using C 1s (284.8 eV) as the reference. The XPS spectra of the Zn 2p region of NH₄ZnPO₄ show Zn²⁺ at binding energy (BE) of ~1022 eV (2p_3/2), indicating that all samples have Zn²⁺ in the structure. The XPS spectra of the P 2p region of the NH₄ZnPO₄ samples prepared using water, PVP, PVA, sucrose, and CTAB solutions are shown in Fig. 8. All of the NH₄ZnPO₄ samples exhibited the only peak of P 2p at BE of ~134 eV (2p_3/2). The XPS results for other phosphates also showed that the products were completely formed in the stoichiometric ratio and that P existed in the P⁵⁺ state. These results indicate that the presence of P⁵⁺ (NH₄ZnPO₄) in the sample corresponds to the XRD and XAS results.

Figure  7.  Curve-fitting for Zn 2p and XPS spectra of NH₄ZnPO₄ samples prepared with (a) water, (b) PVP, (c) PVA, (d) sucrose, and (e) CTAB.

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Figure  8.  Curve-fitting for P 2p and XPS spectra of NH₄ZnPO₄ samples prepared with (a) water, (b) PVP, (c) PVA, (d) sucrose, and (e) CTAB.

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4.   Conclusion

NH₄ZnPO₄ powders with particle sizes of ~0.1–10 µm used as slow-release fertilizers were successfully prepared using a simple and polymers-assisted co-precipitation method. The use of PVP, PVA, sucrose, and CTAB solutions affects the morphology of the prepared samples. Depending on the type of polymer used, XRD results indicate that the hexagonal structure changes to monoclinic structure in the absence of polymer solution when CTAB is added. XANES and XPS results confirm that the oxidation states of Zn and P ions in all the prepared samples are 2+ and 5+, respectively. The simple co-precipitation method using only water solution can also produce NH₄ZnPO₄ particles. This simple method can be extended to prepare other metal-oxide or complex-oxide micro–nano-particles. In addition, the resulting NH₄ZnPO₄ shows great potential applications in slow-release fertilizers to reduce the amount of chemicals used in plants.

Acknowledgements

The authors would like to thank the Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, Thailand for XPS and XANES facilities. This work was supported by Suranaree University of Technology (SUT)-PhD Fund from Suranaree University of Technology. This work was also supported by the SUT and by the Office of the Higher Education Commission under NRU Project of Thailand, Suranaree University of Technology, Nakhon Ratchasima, Thailand.

Conflict of Interest

The authors declare no conflict of interest.