Melamine: A New Versatile Reagent for Inorganic Nanomaterials Synthesis

Research Article 

Melamine: A New Versatile Reagent for Inorganic Nanomaterials Synthesis

Corresponding author: Dr. Vladimir K. Ivanov, Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Moscow 119991, Russia, Tel: +7 495 952 12 61; Fax: +7 495 954 12 79; Email:van@igic.ras.ru

Abstract
A novel, versatile technique for nanocrystalline oxides and oxohydroxides synthesis is proposed based on microwave-assisted hydrothermal hydrolysis of melamine. This technique is exemplified by the synthesis of CeO2, ZnO, Eu2(OH)5NO3•xH2O. By varying the temperature of the hydrothermal treatment (in the range of 140-230°C) and melamine concentration, particle sizes and morphology of the solid products can be easily tuned. The proposed method provides an additional control over the microstructure of inorganic materials synthesized by a hydrothermal technique.Keywords: Melamine; Homogeneous Hydrolysis; Microwave-Hydrothermal Synthesis; Layered Rare-Earth Hydroxides; Ceria;
Zinc Oxide

Introduction

Oxides and hydroxides of p-, d- and f-metals are typically synthesized by precipitation from the corresponding salt solutions
upon the action of strong bases (e.g., NH3•H2O, NaOH, KOH, (C4H9)4N+OH, aliphatic amines). The solubility products of these compounds are generally very low and so their precipitation proceeds under substantial supersaturation conditions in non-equilibrium mode. Under these circumstances, the microstructure and structure-sensitive properties of resulting materials are difficult to control.

One good alternative to conventional precipitation is the homogeneous precipitation method [1]. This method uses reagents
that are hydrolyzed slowly (e.g. upon heating). This allows precise control of the degree of supersaturation and, importantly, the avoidance of local concentration gradients that are typical for conventional precipitation and that adversely affect the properties of materials.

Nowadays, the homogeneous precipitation method is widely used for the preparation of metal oxides, hydroxides and chalcogenides (for the synthesis of ZnO whiskers [2], plate-like NiO particles [3], spherical VO2 [4] and ZnS [5] particles, hollow Y2O3 spheres [6], TiO2nanotubes [7] and various composite materials [8,9]).

The homogeneous precipitation technique typically involves the use of urea (thiourea) or hexamethylenetetramine as the slowly hydrolyzing reactants. Reports on the use of other reactants are scarce [10]. One can state that the choice of reactant used in homogeneous precipitation is still very rare.

In this paper, we have focused our efforts on the search for new homogeneous precipitation procedures allowing for the synthesis of well crystallized metal oxides, or hydroxides with a strictly controlled microstructure. We have proposed a new slowly hydrolyzing reactant that has not been used previously in homogeneous precipitation processes, namely melamine (1,3,5-triazine-2,4,6-triamine) [11]. This compound belongs to the triazine family. Because of the presence of amino groups, it is a weak base (pKb ≈ 9). Melamine is barely water soluble at room temperature, while its solubility rises up to ~ 50 g L-1 upon heating [11]. At 25°С, the pH of melamine aqueous solution is 7–8 [12], which is close to the corresponding value for hexamethylenetetramine [13] and is suitable for hydrolysis of a number of metal salts.

Melamine is known to hydrolyze by the substitution of amino groups with hydroxyls. Melamine hydrolysis results in the formation of ammonia, as well as several organic compounds, including ammeline (4,6-diamino-2-hydroxy-1,3,5-triazine), ammelide (6-amino-2,4-dihydroxy-1,3,5-triazine) and cyanuric acid (1,3,5-triazine-2,4,6-triol) [11, 14]. Ammonia can also be formed as a result of melamine molecules’ condensation [11,15,16]; however, these condensation reactions in aqueoussolutions are unlikely to proceed. Gaseous ammonia formation during melamine hydrolysis has previously been used for increasing the surface area and pore size of BiVO4particles [14]. In turn, the use of melamine hydrolysis reaction in the homogeneous precipitation process has not yet been reported.

We decided to choose ZnO, CeO2 and Eu2(OH)5NO3•xH2O as the first objects to demonstrate the feasibility of homogeneous precipitation by melamine hydrolysis. Earlier, these compounds had already been synthesized by hexamethylenetetramine, or urea-assisted homogeneous precipitation, but this synthetic approach is rather time-consuming [9,17– 20]. Our primary interest in these compounds is due to their promising functional properties. Zinc oxide is a wide bandgap oxide semiconductor that is extensively used in UV filters, solar cells and photocatalysts [21–23]. CeO2 and ceria-based solid solutions are used as catalysts [24] and UV protectors [25,26], as well as promising inorganic biomaterials [27]. In turn, Eu2(OH)5NO3•xH2O is a member of a recently discovered family of anion-exchange layered rare-earth hydroxides [28], which combine specific properties of rare earth elements (including luminescent properties) and intercalated anions. Layered rare earth hydroxides are believed to be promising sensing materials [29–31], contrast agents for magnetic resonance tomography, materials for drug delivery and neutron-capture therapy [32–34], thin film luminescent materials [35–39], etc.

In this paper, we propose a new synthetic approach, based on melamine-assisted homogeneous precipitation of metal oxides and hydroxides under microwave-hydrothermal treatment. It is generally accepted that microwaves can eliminate kinetic limitation in liquid- and solid-state reactions [40–42]. In particular, under hydrothermal conditions, microwave irradiation allows for faster and more uniform heating of reaction mixtures, and nearly eliminates temperature gradients, thus
allowing for the synthesis of chemically and morphologically uniform products [43–45]. The rate of phase formation is also greatly improved upon the action of microwaves, as exemplified by the microwave-hydrothermal synthesis of layered ra re-earth hydroxides [46–48], zeolites [49,50], etc.

Materials and Methods

Zinc nitrate (99%, Russia), cerium (III) nitrate hexahydrate (99.5%, Alfa Aesar), europium (III) nitrate hexahydrate (99.9%, Alfa Aesar) and melamine (99+%, Sigma-Aldrich) were used as received. All aqueous solutions were prepared with deionized distilled water.

ZnO, CeO2 and Eu2(OH)5NO3•xH2O were prepared by microwave- assisted homogeneous precipitation under microwave- hydrothermal treatment. In a typical experiment, 10 mL of M(NO3)n (M=Zn, Ce, Eu and Y) aqueous solution or deionized water was added to 20 mL of melamine aqueous solution, to give a reaction mixture of a desired concentration (Cmelamine =
0.05 or 0.13 mol•L-1, CM(NO3)n = 0.033 mol•L-1). The solutions obtained were placed in 100 mL polytetrafluoroethylene autoclaves (with a filling coefficient of ~30%) and subjected to microwave–hydrothermal (MWHT) treatment in a Berghof Speedwave MWS 4 setup at 140–230°C for 60 min. Several experiments were also conducted upon conventional heating at 80 and 110°C in glass autoclaves (Ace Glass). After the synthesis was complete, the autoclaves were cooled in air; the solid products were isolated by sedimentation and further decantation, then repeatedly washed with distilled water (until the conductivity of the mother liquor, as measured using a Hanna HI 98312 conductometer, was less than 0.1 mS), followed by drying at 60°C and ~75% relative humidity.

Powder X-ray diffraction (XRD) patterns of the solid samples were collected using a Bruker D8 Advance diffractometer (Bragg–Brentano geometry) with CuKα radiation. The morphology of the samples was investigated using a high resolution scanning electron microscope (Carl Zeiss NVision 40, equipped with an Oxford Instruments X-Max detector). Fluorescence spectra of the melamine solutions were recorded using a Perkin Elmer LS-55 luminescence spectrophotometer at oom temperature with 0.5 nm resolution.

Results and Discussion

Firstly, we have checked that melamine can be hydrolyzed under selected hydrothermal conditions. The melamine hydrolysis and condensation process should lead to the formation of aqueous ammonia and an increase in OH– concentration (i.e. an increase in pH). To show this effect, we have heated melamine aqueous solutions (0.05 M) at various temperatures, (namely 50°C, 80°C, 110°C, 140°C, 170°C, 200°C, 230°C), for 60 min, followed by cooling to ambient temperature. Then pH measurements of these solutions were made (Figure. 1). Upon low temperature treatment (up to 110°C), we observed a negligible decrease in pH, which was probably due to melamine interaction with atmospheric CO2. Starting at 140°C, the pH in creased sharply, which is likely to have been due to ammonium hydroxide formation according to the following scheme:


Figure 1. pH values of the melamine aqueous solutions (0.05 mol•L-1) heated to various temperatures (50–230°C, 60 min).

Melamine has an electron-rich π system, capable of allowing π*-π transitions (π-antibonding to π-bonding orbitals), and it is favourable to the phenomenon of fluorescence [51,52]. Figure 2 presents the emission spectra of the melamine aqueous solutions, which have been treated under microwave-hydrothermal conditions at 170°C (Figure 2a), 200°C (Figure 2b) and230°C (Figure 2c). for 60 min. The decrease in the melamine solution fluorescence emission intensity when its hydrothermal treatment temperature increases indicates that melamine is actually hydrolyzed under hydrothermal conditions, and that the rate of hydrolysis increases with temperature.

Figure 2. Emission spectra of the 0.05 M melamine aqueous solutionsexposed to microwave-assisted hydrothermal treatment at 170°C (a), 200°C (b) and 230°C (c). Excitation wavelength was set to 260 nm, theexcitation slit was set to 15 nm and the emission slit was set to 2.5 nm.

 Synthesis of CeO2

The possibility of the use of melamine for the homogeneous precipitation of nanocrystalline metal oxides was first assessed using CeO2. This material has an extremely low solubility product constant (<10-48) [53], and so it can be readily obtained using various techniques, including hydrothermal treatment [54,55] and solvothermal treatment [56–58], as well as by homogeneous precipitation using hexamethylenetetramine as the slowly hydrolyzing reagent [44].

Our data indicate (Figure 3) that homogeneous precipitation of CeO2 in the presence of melamine proceeds even at relatively low temperatures (e.g. 80°C), where the rate of melamine hydrolysis is nearly negligible, as shown by the data presented in Figure. 1. One possible reason for this could have been the shift of the melamine hydrolysis equilibrium according to the following equation:

where n is an integer number from 0 to 3.

The increase in microwave-hydrothermal treatment temperature caused the CeO2 yield to increase (Figure 3). When the melamine concentration was increased (up to 0.13 mol•L-1), the CeO2 yield also increased, by up to 100%.

Figure 3. CeO2 yields upon microwave-hydrothermal treatment of the mixed solutions of cerium (III) nitrate and melamine (melamine concentration: square labels – 0.05 mol•L-1; circle label – 0.13 mol•L-1).

The increase in the melamine hydrolysis temperature influenced not only yield, but also the particle size of CeO2, as illustrated by XRD (Figures 4 and 5). One should note that the powder prepared at 200°C (Figure 4e) contained a small admixture of unidentified phase, which was probably a product of melamine hydrolysis or condensation processes. However, this admixture phase can be easily removed by washing with a 0.1 mol•L-1 NaOH solution.

Figure 4. X-ray diffractograms of the powders synthesized by treatment of melamine (0.05 mol•L-1) and cerium nitrate (0.033 mol•L-1) aqueous solutions at 80°C (a), 110°C (b), 140°C(c), 170°C (d) and 200°C(e) for 60 min.

Figure 5. CeO2 crystallite sizes of the powders synthesized by treatment of melamine (0.05 mol•L-1) and cerium nitrate (0.033 mol•L-1) aqueous solutions at 80-200°C for 60 min.

The dependence of CeO2 crystallite sites (as calculated from XRD data) on the temperature of the synthesis is shown in Figure 5. The size of CeO2 can be easily tuned in the range from 7 to 25 nm, simply by changing temperature. Data obtained were in good agreement with the TEM results (Figure 6). It is worth noting that samples synthesized at high temperatures (170°C, 200°C) consisted of relatively large nanoparticles that are hard to obtain using a conventional precipitation method [59].

Figure 6. TEM images of CeO2 samples obtained by microwave-assisted hydrothermal treatment of the powders synthesized by treatment of melamine (0.05 mol•L-1) and cerium nitrate (0.033 mol•L-1) aqueous solutions at 140°C (a), 170°C (b) and 200°C (c) for 60 min.

Synthesis of ZnO

ZnO can be easily synthesized using homogeneous precipitation with melamine (Figure 7). As in the case of ceria, increasing
the temperature of the microwave-hydrothermal treatment to 200°C resulted in the formation of the same admixture phase. Therefore, the following ZnO syntheses (as well as the synthesis of Eu2(OH)5NO3•xH2O, see below) were conducted at temperatures of 170°C or lower.

Figure 7. X-ray diffractograms of ZnO powders obtained by microwave-hydrothermal treatment of mixed aqueous solutions of zinc nitrate (0.033 mol•L-1) and melamine (0.05 mol•L-1 (a), 0.013 mol•L-1 (b)) at 170°C for 60 min.

Figure 7. Shows X-ray diffractograms of ZnO powders obtained by the microwave-hydrothermal treatment of mixed aqueous
solutions of zinc nitrate (0.033 mol•L-1) and melamine (0.05 mol•L-1 and 0.013 mol•L-1). The crystallite sites along the (100) direction, as calculated from XRD data, were 59 and 43 nm for the samples prepared using 0.05 mol•L-1 and 0.013 mol•L-1 melamine solutions, respectively. These values are close to those of the particles for ZnO samples synthesized by hexamethylenetetramine-assisted homogeneous precipitation [60], but the melamine-assisted nanoparticles were less aggregated, as shown using SEM (Figure 8).

Figure 8. SEM images of ZnO powders obtained by microwave-hydrothermal treatment of mixed aqueous solutions of zinc nitrate (0.033 mol•L-1) and melamine (0.05 mol•L-1 (a), 0.013 mol•L-1 (b)) at 170°C for 60 min.

Synthesis of Eu2(OH)5NO3•xH2O

X-ray diffraction data (Figure 9) indicate that microwave-hydrothermal treatment of mixed aqueous solutions of europium nitrate and melamine resulted in the formation of layered europium hydroxynitrate, Eu2(OH)5NO3•xH2O. Besides a set of diffraction maxima typical for layered rare earth hydroxynitrates [48,61,62], several unidentified reflexes can be clearly seen.

Figure 9. X-raydiffractograms of Eu2(OH)5NO3•xH2O powders obtained by microwave- hydrothermal treatment of mixed aqueous solutions of europium nitrate (0.033 mol•L-1) and melamine (0.05 mol•L-1 (a), 0.013 mol•L-1 (b)) at 170°C for 60 min.

Figure 10. SEM images of Eu2(OH)5NO3•xH2O powders obtained by microwave-hydrothermal treatment of mixed aqueous solutions of europium nitrate (0.033 mol•L-1) and melamine (0.05 mol•L-1 (a), 0.013 mol•L-1 (b)) at 170°C for 60 min.

The nature of these reflexes is not clear yet, while similar reflexes were observed for layered rare earth hydroxychlorides [63]. Unlike the previously reported method of layered rare earth hydroxides synthesis by microwave-assisted homogeneous
hydrolysis in the presence of hexamethylenetetramine [47,48], our new technique allows the tuning of the morphology of the products simply by changing the composition of reaction mixtures, namely the melamine / Eu(NO3)3•6H2O ratio. Figure. 10 shows SEM images of layered europium hydroxynitrates, which formed spherical aggregates upon synthesis witha 0.05 mol•L-1 melamine solution (Figure. 10a), and nanoaggregate lamellar particles upon synthesis with a 0.013 mol•L-1 melamine solution (Figure. 10b).

It is worth noting that the synthesis of both types of particle can be performed by other methods, but the duration of synthesis is typically very long [19–21,64]. Therefore, melamine-assisted homogeneous precipitation is an effective means of shortening the duration of layered rare earth hydroxides’ synthesis in comparison with widely used procedures based on homogeneous precipitation in the presence of hexamethylenetetramine, or precipitation by alkalis followed by hydrothermal
treatment [29,65].

Conclusions

Melamine has been demonstrated to be a promising reagent for the homogeneous precipitation of inorganic materials under microwave-hydrothermal conditions, as exemplified by the syntheses of cerium dioxide, zinc oxide and layered europium hydroxynitrate. The microstructure of materials synthesized by melamine-assisted homogeneous precipitation can be easily tuned by changing either the concentrations of the  initial solutions or the temperature of the microwave-hydrothermaltreatment. Thus, the method proposed will provide an additional co ntrol over the microstructure of inorganic materials.

Acknowledgements

The work was supported by the Russian Foundation for Basic Research (project 14-03-00907).

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