ZnO2 Nanoparticles Using Zinc(II) Isobutylcarbamate Bull. Korean Chem. Soc. 2014, Vol. 35, No. 2 431http://dx.doi.org/10.5012/bkcs.2014.35.2.431

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

Kyung-A Kim,† Jae-Ryung Cha,† Myoung-Seon Gong,† and Jong-Gyu Kim*

Department of Chemistry, Dankook University, Cheonan, Chungnam 330-714, Korea. *E-mail: jkim16@dankook.ac.kr†Deparment of Nanobiomedical Science and WCU Research Center of Nanobiomedical Science,

Dankook University, Cheonan, Chungnam 330-714, KoreaReceived September 16, 2013, Accepted November 12, 2013

Zinc peroxide nanoparticles (ZnO2 NPs) were prepared by reacting zinc(II) isobutylcarbamate, as an organo-metallic precursor, with hydrogen peroxide (H2O2) at 60 °C. Polyethylene glycol and polyvinylpyrrolidonewere used as stabilizers, which suppressed aggregation of the ZnO2 NPs. Conditions such as concentrations ofH2O2 and the stabilizer were systemically controlled to determine their effect on the formation of nano-sizedZnO2 NPs. The formation of stable ZnO2 NPs was confirmed by UV-vis, Raman spectroscopy, X-rayphotoelectron spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM),and X-ray diffraction. The TEM images revealed that polyvinylpyrrolidone-stabilized ZnO2 NPs (diameter,10–30 nm) were well dispersed in the organic solvent. Quite pure ZnO NPs were obtained from the peroxidepowder by simple heat treatment of ZnO2. The transition temperature of 170 oC was determined by differentialscanning calorimetry.

Key Words : Zinc peroxide, Zinc(II) isobutylcarbamate, ZnO-precursor, Hydrogen peroxide

Introduction

Interest in metal oxide nano-materials has increased due totheir potential applications. Of these oxides, zinc oxide is atransparent electroconductive and piezoelectric materialwith applications in electronic, photonic photo cells andlaser diodes devices. But, the shape and size of nano zincoxide govern its properties.1-3 In a wet chemical route, mostzinc oxide nanoparticles (NPs) of various morphologieshave been synthesized by decomposing zinc compoundsthrough decomposition/oxidation at different temperatures.ZnO2 NPs are a semi-conductor with a wide band gapenergy of 4.20 eV.4,5 ZnO2 NPs can be applied to high-techplastic processing6,7 and the rubber industry.8 They can alsobe used as an oxidant for explosives and pyrotechnic mix-tures.9 ZnO2 NPs can additionally be used as precursors toprepare ZnO NPs.10,11 ZnO2 is an odorless white stable solidat ambient temperature, but it is decomposed by heating at150 °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 thefollowings: ZnO,15 Zn(OH)2,15 ZnR2,4 Zn(NO3)2,16 andZnCO3

15 to a solution of H2O2 with an additional source ofenergy such as light. A variety of ways are available to pre-pare ZnO2 powder such as sol-gel synthesis, polyol syn-thesis, autoclaves, hydrothermal method, and laser ablation.12-21

The preparation of stabilized ZnO2 NPs has been reportedusing zinc oxide as the precursor and phosphate acid or saltderivatives as stabilizers.22 But, most methods require hightemperature and vacuum. Furthermore, the products obtain-ed using these methods are poorly crystallized and maycontain impure phase precursors or reactants.

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

Experimental

Materials and Instruments. Isobutylammonium iso-butylcarbamate was synthesized from isobutylamine andcarbon dioxide. A ZnO2-precursor, zinc(II) isobutylcarba-mate (Zn(IBC))2, was prepared by reacting zinc powder withisobutylammonium isobutylcarbamate.23,24 Polyvinylpyrro-lidone (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) waspurchased from Yakuri Pure Chem. Co. Ltd. Hydrogen per-oxide, used as the oxidizing agent, was obtained from Dong-woo Fine Chem. Co. Ltd. The hydrogen peroxide solution in2-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 tocharacterize the ZnO2 NPs. The XRD patterns of the ZnO2

NPs were measured using a Shimadzu XD-D1 X-ray diffr-actometer with CuK radiation ( = 1.54056 Å) at a scann-ing rate of 2 °C/second, in the 2 range from 10–90°. The 2-

432 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 2 Kyung-A Kim et al.

methoxyethanol solution used for dispersing the ZnO2 NPswas dried slowly on amorphous carbon supported on a TEMcopper mesh grid. Raman spectra were taken by SenterraRaman spectroscopy. Surface characterization was perform-ed by XPS. The survey spectrum, Zn 2p, and O 1s lines wererecorded with an electron spectrometer (VG MicrotechESCA2000) with an aluminum anode (K, 1486.6 eV).Differential scanning calorimetry (DSC) measurements wereperformed with a Sinco EvoSetaram DSC 131 instrument,and the thermogravimetric analysis (TGA) was conductedon a Shimadzu TGA 50 instrument at a heating rate of 10 oC/min under a nitrogen atmosphere.

Preparation of the ZnO2 NPs. Nanometer-sized ZnO2

NPs were synthesized using an organometallic zinc carba-mate precursor. Zn(IBC)2 (2.05 g) and PEG 6000 (2 g) weredissolved in 2-methoxyethanol (30 mL) under vacuum atroom 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 oC for 30 min. The reactionmixture gradually formed a white dispersion to produce theZnO2 nanocolloid solution. The ZnO2 NPs were separated bycentrifugation and washed with distilled water and alcoholseveral times. The precipitate was dried to a powder in air.Other ZnO2 NPs were prepared in the presence of PVPstabilizer using a similar procedure.

Results and Discussion

Zinc(II) isobutylcarbamate (Zn(IBC)2) is prepared by thereaction of isobutylammonium isobutylcarbamate with zincmetal powders. After 24 h, the reaction mixtures changedfrom gray slurry to transparent solution, indicating formationof Zn(IBC)2. The formation of Zn(IBC)2 was confirmed byFT-IR, 1H NMR, 13C NMR and Raman spectra (Figure S1-S4). The TGA curve showed no heat flow up to 200 oC andthe endothermic peak appeared at 125 oC, confirming thatthe secession of decomposition of Zn(IBC)2 occurred insuccession, and that 2-methoxyethanol evaporated whileZnO was formed, which matched well with the heat flow inthe DSC curve (Figure S5).

Zn(IBC)2 and H2O2 were used as the zinc precursor andoxidizing agent, respectively, to prepare the ZnO2 nano-colloids. This is the first example of preparing a ZnO2 nano-colloidal 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 crucialparameters for yield of the colloidal solution and stability ofthe particles. PVP was selected as the stabilizer to dispersethe nano-sized zinc peroxide in organic solvent. When theoxidizing 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 oC, the color of thesolution changed from transparent to white colloid. At thistime, the precursor was rapidly oxidized to produce ZnO2

NPs. The oxidation reaction of Zn(IBC)2 with H2O2 may berepresented 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 differentsizes, shapes, and particle size distributions, while varyingthe concentrations of Zn(IBC)2, oxidizing agent, and stabilizerin the mixture.

Figure 1 shows the UV-vis spectra of zinc peroxide colloi-dal solutions under different concentrations of oxidizingagent (3%, 5%, and 7%), which was prepared by dispersingZnO2 NPs in 2-methoxyethanol. Maximum absorption occurr-ed at 234 nm when the H2O2 concentration was 3% or 5%.ZnO2 NPs prepared using 7% H2O2 exhibited a slight redshifted absorption from 234 to 243 nm. Abrupt oxidationand decomposition of Zn(IBC)2 at high concentration ofH2O2 caused aggregation of the ZnO2 NPs.

TEM studies were conducted to confirm the shape andsize of the ZnO2 NPs formed by reacting Zn(IBC)2 (1.5% in2-methoxyethanol) and hydrogen peroxide (3% H2O2 in 10mL of 2-methoxyethanol) without a stabilizer. The formationof ZnO2 NPs was monitored by the concentration (3%, 5%,and 7%) of H2O2 (Figure 2). No detectable difference in sizewas observed when the H2O2 concentration was less than5% (Figure 2(a) and (b)). The particles were particularlymulti-dispersed at higher H2O2 concentrations, with con-comitant aggregation of the particles (Figure 2(c)).

Figure 3 shows the UV-vis absorption peak of ZnO2

colloids at 213 nm when PEG 6000, PVP K15 and PVP K30

Figure 1. UV-vis absorption spectra of ZnO2 colloid solutionsobtained from Zn(IBC)2 (1.5% in 2-methoxyethanol) at variousconcentration of H2O2.

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

ZnO2 Nanoparticles Using Zinc(II) Isobutylcarbamate Bull. Korean Chem. Soc. 2014, Vol. 35, No. 2 433

were used as stabilizers. A blue shift of the maximum ab-sorption peak was observed by adding stabilizer. However, abroad absorption peak in the spectrum extended from 300–400 nm, indicating the formation of larger NPs due to moreaggregation of the ZnO2 NPs. Moreover, the ZnO2 NPs ex-hibited a slight sharp absorption peak at 250–400 nm whenthe PVP concentration was increased.

Stabilized particles of various sizes and shapes formed enmasse at concentrations < 1.5% Zn(IBC)2, as shown in

Figure 4. Nevertheless, the ZnO2 NPs retained nano-rangesizes. All ZnO2 NP samples were spherically shaped withsizes of 50–200 nm. This result shows that the surfaces ofthe ZnO2 NPs were hydrophobic due to the absorbed PEG orPVP. However, the results shown in Figure 4 indicate thatstabilizer was required to disperse the ZnO2 NPs. Use ofPEG 6000 resulted in a large difference between agglome-ration shapes, as shown in Figure 4. ZnO2–PEG 6000 reveal-ed quite dense polygon-type agglomerates with sizes of 30–100 nm, whereas ZnO2–PEG 200 created large typical sphericalNPs (inset figure). Figure 4(b) shows a TEM image of theZnO2 NPs synthesized using PVP K15; the product obtainedwas well-dispersed spherical NPs with sizes of 50–200 nm.The SEM image shows that nanospheres with a large sizedistribution (50–220 nm in diameter) were formed (Figure4(c)). The SEM image revealed that the nanospheres werestructurally solid and their diameters were consistent withthe TEM result.

PVP K30, with a larger molecular weight, was employedto obtain ZnO2 NPs < 100 nm. Spherical ZnO2 NPs withparticle sizes of 60–150 nm were randomly formed in thepresence of PVP K30 (2 g) in Figure 5(a). When the amountof PVP K30 was increased, spherical ZnO2 NPs withparticle sizes of 10–30 nm were observed (Figure 5(b)). As aresult, the dispersion ability of PVP K30 was better than thatof PVP K15. Therefore, high molecular weight PVP K30was useful to obtain nano-sized, spherical ZnO2 NPs. This isbecause PVP K30 adsorbed on ZnO2 surfaces plays animportant role sterically stabilizing ZnO2 colloidal disper-sions. Figure 5(c) shows a high resolution TEM image of

Figure 3. UV-vis absorption spectra of ZnO2 colloid solutionsobtained from Zn(IBC)2 in the presence of PEG and PVPstabilizers.

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) SEMimage of sample obtained from (b).

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

434 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 2 Kyung-A Kim et al.

ZnO2 NPs with lattice spacing of about 0.28 nm for the (111)plane in the cubic ZnO2. Figure 5(d) shows the correspond-ing selected area electron diffraction (SAED) pattern, con-firming crystallinity of the product. These interval distancesof 0.28 nm correlate to the distances of (111) of ZnO2, whichhas a cubic crystal.

Figure 6 presents the XPS survey spectrum of the ZnONPs. Note that no impurities, other than adventitious carbon,were detected. The inset figure shows the O 1s regions of theXPS spectrum. The highest peak in the O 1s spectrum was531.23 eV, which is characteristic of oxygen species integ-rated in the material as hydroxide (OH) or peroxide (O2

2).The oxygen in zinc peroxide compared with the oxygen inzinc oxide (529.9 eV) was located at a higher binding energyof 531.23 eV. The other broad peak at 528.87 eV wasattributed to poorly bound oxygen ions.

Figure 7 depicts the XRD spectra of ZnO (JCPDS cardNo. 36-1451) and ZnO2 (JCPDS card No. 13-0351) synthe-sized in powder form by simple heating Zn(IBC)2 at 125 °Cand heating at 60 °C in the presence of H2O2, respectively.The XRD spectra clearly show the crystalline structure andthe various peaks of the ZnO2 NPs. The dominant peaks of

ZnO2 were identified at 2 = 32.8, 37.18, 53.48, and 63.48(ZnO for 2 = 31.38, 34.88, 36.48, 48.28, 57.08, 63.28, and68.38), which was in agreement with the literature.25

ZnO2 can be used as a precursor to prepare ZnO. Thethermal behavior of the ZnO2 powder was investigated byTGA and DSC analyses to confirm the transformation ofZnO2 to ZnO. The TGA curve in Figure 8 shows that theweight losses below 100 °C were about 4.8%, indicating thatthe powder samples adsorbed water.4 The small amount ofadsorbed water in the powder samples was possible to avoiddue to the preparation method using organic solvent such as2-methoxyethanol. But the fine particle size and instabilityof the material retains moisture. Weight losses were about9.2% up to 170 °C in the TGA curve. It drops from 91.8% to76.8% in the temperature range from 170 to 210 °C and thenremains almost constant up to 600 °C. Based on the reactionof 2 ZnO2 2 ZnO + O2, the calculated weight loss was16.4%. The DSC curve in Figure 8 shows a sharp exo-thermic peak starting at 170 °C, which is coincident with theabrupt weight loss in TGA curve. It is known that the

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. X-ray diffraction patterns of ZnO2 and ZnO prepared byreacting Zn(IBC)2 with H2O2 at 60 oC and simple heating at 125oC, respectively.

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

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

ZnO2 Nanoparticles Using Zinc(II) Isobutylcarbamate Bull. Korean Chem. Soc. 2014, Vol. 35, No. 2 435

transition temperature of ZnO2 is about 200 °C.26 The resultof isothermal aging (annealing) at 170 °C for 30 min was notcoincident with that of the sample annealed with a 10 °C/min. Overall, the TGA/DSC results confirmed that the secondweight loss at around 170 °C is indeed due to the decom-position of the ZnO2 cubic phase into the hexagonal ZnO.

ZnO2 powder was prepared by adding 3% H2O2 during thethermal decomposition process, and the solution was heatedto various annealing temperatures of 120–170 oC for 30 minto investigate the effect of heating on the transformation ofZnO2 into ZnO.

Zn(IBC)2 was directly transformed to ZnO by heatingZn(IBC)2 in an H2O2 solution. The transparent solution ofZn(IBC)2 and H2O2 changed into a white colored smallgrain. Nanodimensional ZnO powders were obtained throughthermal decomposition via the formation of ZnO2. It ap-peared that the XRD patterns of the decomposed samplesbelonged to a single pure ZnO crystalline phase.25

Figure 10 shows the Raman spectra of the ZnO2 powderafter heating at 170 oC for 30 min, and the result was thesame as that of bulk ZnO sample. Four peaks were observedat 331, 385, 438, and 578 cm1. The peaks at about 438 cm1

were assigned to the two nonpolar Raman active E2 vibra-tional modes of ZnO and are associated with the E2-lowmode and E2-high mode, respectively. The peaks at about385 and 578 cm1 were attributed to the A1(TO) and A1(LO)modes, and the 331 cm1 band was due to the 2-E2(M) modeof ZnO, which is related to the multiphonon process.27

Conclusions

In summary, pure stabilized ZnO2 NPs were prepared byreacting new organometallic precursor, zinc(II) isobutyl-carbamate and hydrogen peroxide at 60 oC. Spherical andnano-sized ZnO2 NPs with sizes of 10–30 nm were obtainedusing 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 toZnO by heating at 170 oC within 30 min. According to thesecharacteristics, this Zinc(II) isobutylcarbamate precursor hassufficient potential for the preparation of ZnO2 and ZnO atlow temperature.

Acknowledgments. The present research was conductedby the research fund of Dankook University in 2012.

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Figure 10. The Raman spectra of ZnO prepared from annealing ofZnO2 at 170 oC for 30 min and ZnO2 nanoparticles.