Preparation of ZnO₂ Nanoparticles Using Organometallic Zinc(II) Isobutylcarbamate in Organic Solvent

Abstract

Zinc peroxide nanoparticles ($ZnO_2$ NPs) were prepared by reacting zinc(II) isobutylcarbamate, as an organometallic precursor, with hydrogen peroxide ($H_2O_2$) at $60^{\circ}C$. Polyethylene glycol and polyvinylpyrrolidone were used as stabilizers, which suppressed aggregation of the $ZnO_2$ NPs. Conditions such as concentrations of $H_2O_2$ and the stabilizer were systemically controlled to determine their effect on the formation of nano-sized $ZnO_2$ NPs. The formation of stable $ZnO_2$ NPs was confirmed by UV-vis, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction. The TEM images revealed that polyvinylpyrrolidone-stabilized $ZnO_2$ NPs (diameter, 10-30 nm) were well dispersed in the organic solvent. Quite pure ZnO NPs were obtained from the peroxide powder by simple heat treatment of $ZnO_2$. The transition temperature of $170^{\circ}C$ was determined by differential scanning calorimetry.

Introduction

Interest in metal oxide nano-materials has increased due to their potential applications. Of these oxides, zinc oxide is a transparent electroconductive and piezoelectric material with applications in electronic, photonic photo cells and laser diodes devices. But, the shape and size of nano zinc oxide govern its properties.1-3 In a wet chemical route, most zinc oxide nanoparticles (NPs) of various morphologies have been synthesized by decomposing zinc compounds through decomposition/oxidation at different temperatures. ZnO2 NPs are a semi-conductor with a wide band gap energy of 4.20 eV.4,5 ZnO2 NPs can be applied to high-tech plastic processing6,7 and the rubber industry.8 They can also be used as an oxidant for explosives and pyrotechnic mixtures.9 ZnO2 NPs can additionally be used as precursors to prepare ZnO NPs.10,11 ZnO2 is an odorless white stable solid at ambient temperature, but it is decomposed by heating at 150 °C to release oxygen.12 New methods of preparing ZnO2 powder use an aqueous system with various zinc salts.12-14 ZnO2 powder is mostly produced by adding one of the followings: ZnO,15 Zn(OH)2,15 ZnR2,4 Zn(NO3)2,16 and ZnCO315 to a solution of H2O2 with an additional source of energy such as light. A variety of ways are available to prepare ZnO2 powder such as sol-gel synthesis, polyol synthesis, autoclaves, hydrothermal method, and laser ablation.12-21 The preparation of stabilized ZnO2 NPs has been reported using zinc oxide as the precursor and phosphate acid or salt derivatives as stabilizers.22 But, most methods require high temperature and vacuum. Furthermore, the products obtained using these methods are poorly crystallized and may contain impure phase precursors or reactants.

In this study, we demonstrate that ZnO2 NPs can be prepared via a thermal reaction of zinc(II) isobutylcarbamate as new organic precursor with H2O2: (Zn(OCONHC4H9)2 + H2O2 → ZnO2 + 2 C4H9NH2 + 2 CO2) at 60 °C within 30 min. The resulting products were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution TEM and selected area electron diffraction (SAED).

 

Experimental

Materials and Instruments.. Isobutylammonium isobutylcarbamate was synthesized from isobutylamine and carbon dioxide. A ZnO2-precursor, zinc(II) isobutylcarbamate (Zn(IBC))2, was prepared by reacting zinc powder with isobutylammonium isobutylcarbamate.23,24 Polyvinylpyrrolidone (PVP K15, Mw = 6,000–15,000) and PVP K30 (Mw = 40,000–80,000) were purchased from Daejung Chem. Metals Co. Ltd. and poly(ethylene glycol) (PEG 6000) was purchased from Yakuri Pure Chem. Co. Ltd. Hydrogen peroxide, used as the oxidizing agent, was obtained from Dongwoo Fine Chem. Co. Ltd. The hydrogen peroxide solution in 2-methoxyethanol was dehydrated with anhydrous MgSO4. TEM (JEM-2000EXII; JEOL, Tokyo, Japan), HR-TEM (JEM-3010; JEOL, Tokyo, Japan) and UV-vis (Shimadzu, UV-1601PC, Tokyo, Japan) spectrometry were employed to characterize the ZnO2 NPs. The XRD patterns of the ZnO2 NPs were measured using a Shimadzu XD-D1 X-ray diffractometer with CuKα radiation (λ = 1.54056 Å) at a scanning rate of 2 °C/second, in the 2θ range from 10–90°. The 2-methoxyethanol solution used for dispersing the ZnO2 NPs was dried slowly on amorphous carbon supported on a TEM copper mesh grid. Raman spectra were taken by Senterra Raman spectroscopy. Surface characterization was performed by XPS. The survey spectrum, Zn 2p, and O 1s lines were recorded with an electron spectrometer (VG Microtech ESCA2000) with an aluminum anode (Kα, 1486.6 eV). Differential scanning calorimetry (DSC) measurements were performed with a Sinco EvoSetaram DSC 131 instrument, and the thermogravimetric analysis (TGA) was conducted on a Shimadzu TGA 50 instrument at a heating rate of 10 °C/min under a nitrogen atmosphere.

Preparation of the ZnO2 NPs. Nanometer-sized ZnO2 NPs were synthesized using an organometallic zinc carbamate precursor. Zn(IBC)2 (2.05 g) and PEG 6000 (2 g) were dissolved in 2-methoxyethanol (30 mL) under vacuum at room temperature for 30 min. Then, H2O2: 2-methoxyethanol (3:97) (10 mL) was slowly injected into the Zn(IBC)2 solution, which was heated to 60 °C for 30 min. The reaction mixture gradually formed a white dispersion to produce the ZnO2 nanocolloid solution. The ZnO2 NPs were separated by centrifugation and washed with distilled water and alcohol several times. The precipitate was dried to a powder in air. Other ZnO2 NPs were prepared in the presence of PVP stabilizer using a similar procedure.

 

Results and Discussion

Zinc(II) isobutylcarbamate (Zn(IBC)2) is prepared by the reaction of isobutylammonium isobutylcarbamate with zinc metal powders. After 24 h, the reaction mixtures changed from gray slurry to transparent solution, indicating formation of Zn(IBC)2. The formation of Zn(IBC)2 was confirmed by FT-IR, 1H NMR, 13C NMR and Raman spectra (Figure S1-S4). The TGA curve showed no heat flow up to 200 °C and the endothermic peak appeared at 125 °C, confirming that the secession of decomposition of Zn(IBC)2 occurred in succession, and that 2-methoxyethanol evaporated while ZnO was formed, which matched well with the heat flow in the DSC curve (Figure S5).

Zn(IBC)2 and H2O2 were used as the zinc precursor and oxidizing agent, respectively, to prepare the ZnO2 nanocolloids. This is the first example of preparing a ZnO2 nanocolloidal solution in an organic solvent using a Zn(IBC)2 solution.

Several stabilizers were tested and the results indicated that the nature and concentration of the stabilizers are crucial parameters for yield of the colloidal solution and stability of the particles. PVP was selected as the stabilizer to disperse the nano-sized zinc peroxide in organic solvent. When the oxidizing agent solution was poured into the Zn(IBC)2 solution at room temperature, the reaction did not proceed. As the temperature was increased to 60 °C, the color of the solution changed from transparent to white colloid. At this time, the precursor was rapidly oxidized to produce ZnO2 NPs. The oxidation reaction of Zn(IBC)2 with H2O2 may be represented as follows: Zn(OCONHC4H9)2 + H2O2 → ZnO2 + 2 C4H9NH2 + 2 CO2.

Experiments were carried out to determine the optimum conditions for preparing stable ZnO2 colloids of different sizes, shapes, and particle size distributions, while varying the concentrations of Zn(IBC)2, oxidizing agent, and stabilizer in the mixture.

Figure 1 shows the UV-vis spectra of zinc peroxide colloidal solutions under different concentrations of oxidizing agent (3%, 5%, and 7%), which was prepared by dispersing ZnO2 NPs in 2-methoxyethanol. Maximum absorption occurred at 234 nm when the H2O2 concentration was 3% or 5%. ZnO2 NPs prepared using 7% H2O2 exhibited a slight red shifted absorption from 234 to 243 nm. Abrupt oxidation and decomposition of Zn(IBC)2 at high concentration of H2O2 caused aggregation of the ZnO2 NPs.

TEM studies were conducted to confirm the shape and size of the ZnO2 NPs formed by reacting Zn(IBC)2 (1.5% in 2-methoxyethanol) and hydrogen peroxide (3% H2O2 in 10 mL of 2-methoxyethanol) without a stabilizer. The formation of ZnO2 NPs was monitored by the concentration (3%, 5%, and 7%) of H2O2 (Figure 2). No detectable difference in size was observed when the H2O2 concentration was less than 5% (Figure 2(a) and (b)). The particles were particularly multi-dispersed at higher H2O2 concentrations, with concomitant aggregation of the particles (Figure 2(c)).

Figure 1.UV-vis absorption spectra of ZnO2 colloid solutions obtained from Zn(IBC)2 (1.5% in 2-methoxyethanol) at various concentration of H2O2.

Figure 2.TEM images of ZnO2 NPs obtained from Zn(IBC)2 at a concentration of (a) 3%, (b) 5% and (c) 7% hydrogen peroxide.

Figure 3 shows the UV-vis absorption peak of ZnO2 colloids at 213 nm when PEG 6000, PVP K15 and PVP K30 were used as stabilizers. A blue shift of the maximum absorption peak was observed by adding stabilizer. However, a broad absorption peak in the spectrum extended from 300–400 nm, indicating the formation of larger NPs due to more aggregation of the ZnO2 NPs. Moreover, the ZnO2 NPs exhibited a slight sharp absorption peak at 250–400 nm when the PVP concentration was increased.

Figure 3.UV-vis absorption spectra of ZnO2 colloid solutions obtained from Zn(IBC)2 in the presence of PEG and PVP stabilizers.

Stabilized particles of various sizes and shapes formed en masse at concentrations < 1.5% Zn(IBC)2, as shown in Figure 4. Nevertheless, the ZnO2 NPs retained nano-range sizes. All ZnO2 NP samples were spherically shaped with sizes of 50–200 nm. This result shows that the surfaces of the ZnO2 NPs were hydrophobic due to the absorbed PEG or PVP. However, the results shown in Figure 4 indicate that stabilizer was required to disperse the ZnO2 NPs. Use of PEG 6000 resulted in a large difference between agglomeration shapes, as shown in Figure 4. ZnO2–PEG 6000 revealed quite dense polygon-type agglomerates with sizes of 30–100 nm, whereas ZnO2–PEG 200 created large typical spherical NPs (inset figure). Figure 4(b) shows a TEM image of the ZnO2 NPs synthesized using PVP K15; the product obtained was well-dispersed spherical NPs with sizes of 50–200 nm. The SEM image shows that nanospheres with a large size distribution (50–220 nm in diameter) were formed (Figure 4(c)). The SEM image revealed that the nanospheres were structurally solid and their diameters were consistent with the TEM result.

Figure 4.TEM images of ZnO2 NPs obtained from Zn(IBC)2 oxidized with 3% H2O2 in the presence of 2 g of (a) PEG 6000; inset figure, PEG 200, (b) PVP K15 as a stabilizer and (c) SEM image of sample obtained from (b).

PVP K30, with a larger molecular weight, was employed to obtain ZnO2 NPs < 100 nm. Spherical ZnO2 NPs with particle sizes of 60–150 nm were randomly formed in the presence of PVP K30 (2 g) in Figure 5(a). When the amount of PVP K30 was increased, spherical ZnO2 NPs with particle sizes of 10-30 nm were observed (Figure 5(b)). As a result, the dispersion ability of PVP K30 was better than that of PVP K15. Therefore, high molecular weight PVP K30 was useful to obtain nano-sized, spherical ZnO2 NPs. This is because PVP K30 adsorbed on ZnO2 surfaces plays an important role sterically stabilizing ZnO2 colloidal dispersions. Figure 5(c) shows a high resolution TEM image of ZnO2 NPs with lattice spacing of about 0.28 nm for the (111) plane in the cubic ZnO2. Figure 5(d) shows the corresponding selected area electron diffraction (SAED) pattern, confirming crystallinity of the product. These interval distances of 0.28 nm correlate to the distances of (111) of ZnO2, which has a cubic crystal.

Figure 5.TEM images of ZnO2 NPs obtained from Zn(IBC)2 in the presence of (a) 2 g and (b) 4 g of PVP K30, (c) HRTEM image of ZnO2 sample of (a) or (b), and (d) the corresponding SAED pattern.

Figure 6 presents the XPS survey spectrum of the ZnO NPs. Note that no impurities, other than adventitious carbon, were detected. The inset figure shows the O 1s regions of the XPS spectrum. The highest peak in the O 1s spectrum was 531.23 eV, which is characteristic of oxygen species integrated in the material as hydroxide (OH−) or peroxide (O22−). The oxygen in zinc peroxide compared with the oxygen in zinc oxide (529.9 eV) was located at a higher binding energy of 531.23 eV. The other broad peak at 528.87 eV was attributed to poorly bound oxygen ions.

Figure 6.XPS spectrum of ZnO2 NPs obtained from Zn(IBC)2 and H2O2; inset figure is the O 1s changes between ZnO and ZnO2.

Figure 7 depicts the XRD spectra of ZnO (JCPDS card No. 36-1451) and ZnO2 (JCPDS card No. 13-0351) synthesized in powder form by simple heating Zn(IBC)2 at 125 °C and heating at 60 °C in the presence of H2O2, respectively. The XRD spectra clearly show the crystalline structure and the various peaks of the ZnO2 NPs. The dominant peaks of ZnO2 were identified at 2θ = 32.8, 37.18, 53.48, and 63.48 (ZnO for 2θ = 31.38, 34.88, 36.48, 48.28, 57.08, 63.28, and 68.38), which was in agreement with the literature.25

Figure 7.X-ray diffraction patterns of ZnO2 and ZnO prepared by reacting Zn(IBC)2 with H2O2 at 60 °C and simple heating at 125 °C, respectively.

Figure 8.TGA and DSC curves of the ZnO2 powders obtained from Zn(IBC)2.

ZnO2 can be used as a precursor to prepare ZnO. The thermal behavior of the ZnO2 powder was investigated by TGA and DSC analyses to confirm the transformation of ZnO2 to ZnO. The TGA curve in Figure 8 shows that the weight losses below 100 °C were about 4.8%, indicating that the powder samples adsorbed water.4 The small amount of adsorbed water in the powder samples was possible to avoid due to the preparation method using organic solvent such as 2-methoxyethanol. But the fine particle size and instability of the material retains moisture. Weight losses were about 9.2% up to 170 °C in the TGA curve. It drops from 91.8% to 76.8% in the temperature range from 170 to 210 °C and then remains almost constant up to 600 °C. Based on the reaction of 2 ZnO2 → 2 ZnO + O2, the calculated weight loss was 16.4%. The DSC curve in Figure 8 shows a sharp exothermic peak starting at 170 °C, which is coincident with the abrupt weight loss in TGA curve. It is known that the transition temperature of ZnO2 is about 200 °C.26 The result of isothermal aging (annealing) at 170 °C for 30 min was not coincident with that of the sample annealed with a 10 °C/min. Overall, the TGA/DSC results confirmed that the second weight loss at around 170 °C is indeed due to the decomposition of the ZnO2 cubic phase into the hexagonal ZnO.

Figure 9.X-ray diffraction patterns of zinc peroxide (ZnO2) heated at various temperatures for 30 min.

Figure 10.The Raman spectra of ZnO prepared from annealing of ZnO2 at 170 °C for 30 min and ZnO2 nanoparticles.

ZnO2 powder was prepared by adding 3% H2O2 during the thermal decomposition process, and the solution was heated to various annealing temperatures of 120–170 °C for 30 min to investigate the effect of heating on the transformation of ZnO2 into ZnO.

Zn(IBC)2 was directly transformed to ZnO by heating Zn(IBC)2 in an H2O2 solution. The transparent solution of Zn(IBC)2 and H2O2 changed into a white colored small grain. Nanodimensional ZnO powders were obtained through thermal decomposition via the formation of ZnO2. It appeared that the XRD patterns of the decomposed samples belonged to a single pure ZnO crystalline phase.25

Figure 10 shows the Raman spectra of the ZnO2 powder after heating at 170 °C for 30 min, and the result was the same as that of bulk ZnO sample. Four peaks were observed at 331, 385, 438, and 578 cm−1. The peaks at about 438 cm1 were assigned to the two nonpolar Raman active E2 vibrational modes of ZnO and are associated with the E2-low mode and E2-high mode, respectively. The peaks at about 385 and 578 cm−1 were attributed to the A1(TO) and A1(LO) modes, and the 331 cm−1 band was due to the 2-E2(M) mode of ZnO, which is related to the multiphonon process.27

 

Conclusions

In summary, pure stabilized ZnO2 NPs were prepared by reacting new organometallic precursor, zinc(II) isobutylcarbamate and hydrogen peroxide at 60 °C. Spherical and nano-sized ZnO2 NPs with sizes of 10–30 nm were obtained using high molecular weight PVP K30 as the stabilizer. The size and shape of the ZnO2 NPs were confirmed by UV-vis, SEM, TEM and HRTEM. The ZnO2 NPs were reduced to ZnO by heating at 170 °C within 30 min. According to these characteristics, this Zinc(II) isobutylcarbamate precursor has sufficient potential for the preparation of ZnO2 and ZnO at low temperature.