Synthesis and Characterization of NiO Nanoparticles by Sol­Gel Method

Lay Gaik Teoh1 and Kun-Dar Li2,+

1Department of Mechanical Engineering, National Pingtung University of Science and Technology,Neipu, Pingtung 912, Taiwan, R. O. China2Department of Materials Science, National University of Tainan, Tainan 700, Taiwan, R. O. China

Due to the outstanding electrical, magnetic and catalytic properties, nickel oxide (NiO) has been received considerable attention during thepast decades. In this study, NiO nanoparticles were prepared by sol­gel method, which is one of the simplest and lowest-cost techniques. Thesynthesis was accomplished by using Poly(alkylene oxide) block copolymer as the surfactant, and Ni(NO3)2·6H2O as the inorganic precursor.The effect of experimental parameters, such as calcination temperatures and H2O concentration on the NiO nanoparticles formation wereinvestigated. TGA, XRD, SEM, TEM and N2 adsorption­desorption isotherms were used to characterize the microstructure and specific surfacearea of the samples. TGA and FTIR analyses demonstrated that copolymers were expelled at 573K. The formation of NiO nanoparticles andtheir structural features were greatly dependent on the calcination temperature. The sample calcined at 923K was composed of pure NiOnanoparticles as shown by XRD. As H2O concentration was increased, the reoxidation process of metallic Ni to form NiO would reduce, but itwould not affect the structural type of NiO nanoparticles. In general, the addition of water would weaken and inhibit oxidation effects. Thetemperature of stable metallic Ni was increased up to 823K. The specific surface area evaluated from the N2 adsorption­desorption indicatedthat the samples consisting of non-porous NiO nanoparticles. Increasing H2O addition resulted in an increase of specific surface area ofnanocrystalline NiO powder. [doi:10.2320/matertrans.M2012244]

(Received July 6, 2012; Accepted September 7, 2012; Published October 24, 2012)

Keywords: NiO nanoparticles, sol­gel method, block copolymer, H2O addition

1. Introduction

In the past decades, nickel oxide (NiO) has been receivedconsiderable attention due to their outstanding electrical,magnetic and catalytic properties.1) Their wide range ofapplications in various fields included the fabrication ofcatalysis,2­4) electrochromic films,5­7) fuel cell electrodes8)

and gas sensors,9­11) battery cathodes,12­16) p­n heterojunc-tions,17) magnetic materials,18­21) photovoltaic devices,22)

electrochemical supercapacitors,23) smart windows24) anddye-sensitized photocathodes.25) Therefore, NiO becameone of the most important transition metal oxides. However,most of these applications require particles with a small sizeand a narrow size distribution. With the volume effect, thequantum size effect and the surface effect, NiO nanoparticlesare expected to possess many improved properties and evenmore attractive applications than those of bulk-sized NiOparticles. For example, NiO nanostructures are p-typesemiconductors with particular magnetic behaviors such assuperparamagnetic, superantiferromagnetic, and ferromag-netic order depending on the particle size, particle shape, andsynthesis route, whereas bulk-sized NiO is an antiferromag-netic insulator with a Neel temperature (TN) of 523K.26,27)

Numerous new techniques have been developed for thepreparation of NiO nanoparticle, such as sol­gel method,ultrasonic radiation, pyrolysis by microwave, hydrothermalsynthesis, precipitation­calcination method, carbonyl meth-od, laser chemical method, mechanochemical processing,microemulsion method, flame spray pyrolysis, solid-statemethod, and so forth.28­32) Among them, the chemicalmethod of sol­gel preparation is one of the simplest andlowest-cost techniques for preparing pure transition metaloxides with relatively high specific surface area at lowtemperature.33­35) By selecting a proper precursor and

surfactant, coupled with a rational calcining procedure, NiOnanoparticle with uniform size and shapes could be obtained.This method also has potential advantages, including opera-tional simplicity, high purity and high yield of product, lowenergy consumption and no special equipment required.

In this study, we had developed a novel synthesis toprepare NiO nanoparticles by using copolymer as thesurfactant, and Ni(NO3)2·6H2O as the inorganic precursor.The findings of our work proved that the surfactant-mediatedmethod is valuable for the preparation of NiO nanoparticles.We also investigated the influence of experimental parame-ters, such as calcination temperatures and H2O concentrationon the formation of NiO nanoparticles. The features ofmicrostructure and specific surface area of the samples werecharacterized by TGA, XRD, SEM, TEM and N2 adsorption­desorption isotherms.

2. Experimental

NiO nanoparticles were prepared by a surfactant-mediatedmethod using nonionic copolymer F108 as an organictemplate material. Poly(alkylene oxide) block copolymerF108 with 1 g was dissolved in 10ml of anhydrous ethanol.Different amounts of water (H2O) (0, 10, 20, 40mass%) wereadded to dilute the ethanol solution. 0.01 mole of nickelnitrate (Ni(NO3)2·6H2O) was then added to the F108 ethanolsolution and stirred vigorously for 1 h. The role of the blockcopolymer in the as-made sample was used to control thegrowth of nanoparticles and coat the nanoparticles to preventthem from further oxidation and aggregation. These nano-particles were easily dispersed in ethanol to form ahomogeneous colloidal solution. The resulting sol solutionswere aged and dried at 343K in an oven for 48 h. The as-made sample was then calcined at various temperatures (623,723, 823, 923K) for 3 h to remove the copolymer. Thenanostructure of NiO nanoparticles was then investigated by+Corresponding author, E-mail: kundar@mail.nutn.edu.tw

Materials Transactions, Vol. 53, No. 12 (2012) pp. 2135 to 2140©2012 The Japan Institute of Metals

thermogravimetric analysis (SETSYS Evolution TGA, Ther-mal Analysis System). Powder XRD data was measured witha Rigaku D/max-IV diffractometer with Cu K¡ radiation(­ = 0.15418 nm). The sample was scanned from 20 to 80°(2ª) in steps of 4°/min. A SEM image was obtained using aHitachi 3000N, and the sample was prepared by dispersingthe final powder in conductive glue, and this dispersion wasthen sprayed with carbon. The TEM micrographs were madewith a FEI E.O Tecnai F20 G2 MAT S-TWIN transmissionelectron microscope operated at 200 keV. The sample forTEM was prepared by dispersing the final powder in ethanol,and this dispersion was then dropped on carbon­copper grids.The N2 adsorption­desorption isotherm was recorded on aMicromeritics ASAP 2010 automated sorption analyser. Thesample was outgassed for 7 h at 423K before the analysis.The Barrett­Joyner­Halenda (BJH) model and the Brunauer­Emmett­Teller (BET) methods were applied to determine thepore size and BET surface area, respectively. FTIR spectra,in the range of 4000­400 cm¹1, were recorded on a Perkin­Elmer Spectrum GX infrared spectrophotometer.

3. Results and Discussions

Figure 1 shows the result of TGA curve for the as-synthesized sample, which was carried out from the roomtemperature to 1073K in the air. Two steps of weight loss arepresented during this process. Below 373K, the gravimetricloss of about 5% is attributed to the evaporation of residualwater in the dried sample. As shown in Fig. 1, between423 and 573K the weight loss is about 65%, which iscorresponding to the total amount of F108 and NO2 in themixture of F108 and nickel nitrate (Ni(NO3)2). Accordingly,the combustion of organic compounds is the main reactionfor the removal of the copolymer template in this step.According to the analysis of TGA data, most of thecopolymer in the as-made powders is eliminated at about573K, and the calcinating of the NiO powders in air has beenperformed above 573K.

Figure 2 presents the FTIR spectra in the range 4000­400 cm¹1 of the copolymer and the sample calcined at 623Kfor 3 h. Due to the vibrations of ­CH2­ and ­CH3

36) ofcopolymer, some bands around 2950 and 1480 cm¹1 areclearly seen in Fig. 2(a). After calcination at 623K, there areno bands of organic species present, which implied that thecopolymer template was completely removed by calcination,as shown in Fig. 2(b). The bands due to the oxide structureappeared in the region between 400 and 850 cm¹1.37) The twopeaks at 3400 and 1650 cm¹1 corresponded to the surfaceadsorbed water and the hydroxyl groups, respectively.38)

From the FTIR result, calcination at 623K for 3 h wasconducted to form nickel oxide structure and remove themajority of the copolymer template from the as-madepowder.

Figure 3 illustrates the influence of temperature and H2Oconcentration on the structural characterizations of NiOnanoparticles, which were performed by XRD on samplescalcinated at 623 to 923K for 3 h with different amounts ofwater. From Fig. 3(a), it can clearly be seen only puremetallic Ni nanoparticles were presented at 623K. Threecharacteristic peaks, indexed as the reflections from {110},

{200} and {220} planes (JCPDS card No. 87-0712),represents a typical face-centered cubic (fcc) structure ofNi. Thus, the metallic Ni nanoparticles can be prepared bysurfactant-mediated method using copolymer as the surfac-tant. The thermal decomposition of the copolymer derivedthe formation of hydrogen, CO and CO2 as the sample wascalcinated around 573K.39) Interacting with the generated H2

and CO, the Ni2+ in the inorganic precursor was reduced tometallic nickel particles. While the calcination temperaturewas increased to 723K, NiO nanoparticles with an fccstructure were formed, as shown in Fig. 3(a). The presenceof peaks, corresponding to {111}, {200}, {220}, {311} and{222} planes, confirmed the structure of cubic-NiO (JCPDScards No. 47-1049). The formation of NiO could beattributed to the reoxidation of the previously formed nickelnanoparticles39) due to the completion of thermal decom-position of the inorganic precursor, which causes the decreaseof hydrogen and creates the oxidation environment. Con-sequently, at higher temperatures metallic Ni nanoparticles

Fig. 1 TGA curve of the as-synthesized sample NiO prepared with20mass% H2O.

Fig. 2 FTIR spectra of (a) copolymer, and (b) NiO prepared with40mass% H2O and calcined at 623K for 3 h.

L. G. Teoh and K.-D. Li2136

would be fully oxidized to NiO. For example, the XRDanalysis shown in Fig. 3(a) indicated that calcination at923K was needed to completely oxidize metallic Ni.Accordingly, the formation of Ni and NiO nanoparticlesand their structural features were greatly dependent onthe calcination temperature. With increasing calcinationtemperature, the intensities of the diffraction peaks of NiOincreased, and the degree of crystallinity would be improved.To investigate the effects of H2O on the synthesis, 10, 20 and40mass% H2O were applied into the systems, and nano-particles of Ni and NiO were obtained again. Considering tothe effects of H2O concentration on the formation of NiOnanoparticles, the metallic Ni nanoparticles are presentedwith few amount of NiO at 623K (Fig. 3(a)). While H2Oconcentration was increased at this calcination temperature(623K), the diffraction peaks of NiO became moreobviously, as shown in Figs. 3(b)­3(d). It is clear that byappropriate addition of H2O, the phase and phase composi-tion in the case of a mixed phase can be readily controlled.Similar trends of the influence of H2O concentration couldbe found at 723K. When the calcination temperature isincreased to 823K, comparing to Figs. 3(a) and 3(b) the

amount of metallic Ni become increased as adding moreH2O, as shown in Figs. 3(c) and 3(d). This implies thereoxidation process of metallic Ni would reduce with theincrease of H2O concentration. However, it should be noticedthat the behaviors are almost the same for the 20mass% H2Oaddition and the 40mass% H2O addition in Figs. 3(c) and3(d). That is rationally presumed that the optimal conditionfor controlling the synthesis process is achieved, since thesimilar results of N2 adsorption­desorption isotherm for the20mass% and 40mass% H2O additions are also observed inthe latter discussions. According to XRD in Fig. 3, the fullwidth at half-maximum (FWHM) intensity of NiO diffractionpeaks becomes narrow and more close to perfect fcc structurewith the increase of temperature. The H2O concentration hasno influences on the NiO phase formation. That indicates theinorganic precursor with different amount of water would notaffect the type of NiO structural formation, but reduce thetransformation rate of metallic Ni into NiO.

To obtain the insight information about the surfacemorphology and particle size of the NiO powders, bothSEM and TEM analyses were performed. Figure 4 shows theSEM images of the NiO nanoparticles morphology calcined

Fig. 3 XRD patterns of NiO prepared with (a) 0mass%, (b) 10mass%, (c) 20mass%, (d) 40mass% H2O and calcined at 623­923Kfor 3 h.

Synthesis and Characterization of NiO Nanoparticles by Sol­Gel Method 2137

at 723K for 3 h with different H2O concentration. As shownin Fig. 4, the particles do not seem to be sintered, and theaggregated nanocrystals were produced after calcination at723K for 3 h. The agglomeration of nanocrystalline particlescould be attributed to their extremely small dimensions withhigh surface energy during the polycondensation and dryingsteps of sol­gel process.40) However, there is no obviousdifference on the morphology of NiO for different H2Oconcentrations by SEM observations. Limited information of

the microstructure of NiO nanoparticles could be revealedfrom Fig. 4. With the study of TEM investigations, as shownin Fig. 5, the nanoscale size of NiO powder with 10mass%H2O calcined at 723K for 3 h are confirmed. From Fig. 5(a),it could be seen that uniform size with similar shape of NiOnanoparticles are observed. In addition to the individualparticles, some aggregated particles are also present. Theparticle sizes are measured in the range of 20 to 30 nm. Thecorresponding selected area electron diffraction (SAED)

Fig. 4 SEM images of NiO prepared with (a) 0mass% H2O, (b) 10mass% H2O, (c) 20mass% H2O, (d) 40mass% H2O and calcined at723K for 3 h.

Fig. 5 (a) is the bright field and dark field of the TEM images, and (b) is HRTEM of NiO prepared with 10mass% H2O and calcined at723K for 3 h.

L. G. Teoh and K.-D. Li2138

pattern proves the well crystallinity of the NiO framework(Fig. 5(a) inset), which verifies the XRD results. TheHRTEM image of the sample with 10mass% H2O calcinedat 723K for 3 h is shown in Fig. 5(b). This figure displaysobviously the microstructure of NiO nanoparticle. Themeasured lattice distances in the HRTEM image, 0.42 nmfor the nanoparticle (d(200) = 0.21 nm), is quite consistentwith the known parameters for NiO.41,42) This observation isin good agreement with the XRD results.

Figure 6 demonstrates the N2 adsorption­desorption iso-therm and BJH pore size distribution curve of the samplecalcined at 723K for 3 h with different H2O concentration,and the experimental results are summarized in Table 1.According to the desorption branch of the isotherm shown inFig. 6 with the BJH model, the powder sample exhibits acharacteristic of non-porous NiO nanoparticles.43) The BETsurface area of nanocrystalline NiO powder calcined at 723Kwith 0mass% H2O is found to be 7.5m2/g and the porevolume is 0.022 cm3/g. The calculated broad pore diameterdistribution is at approximately 25.24 nm (Fig. 6(a) inset).

With the increase of H2O concenteration, the pore diameterdistribution is decreased to 19.68 nm and the specific surfacearea is increased to 10.4m2/g, respectively. The increase ofspecific surface area could be attributed to the reduction ofpolycondensation rate for the inorganic precursor with H2Oaddition.44) During the micelle formation of sol­gel process,the inorganic precursor would aggregate to form a phaseseparation between the inorganic and organic phases beforethe liquid phase of micelle formation. With an increased H2Oaddition, the polycondensation reaction of the inorganicprecursor would be inhibited, and the micelle formationcould be conducive to gain a high BET surface area ofNiO nanoparticles. However, more detailed investigationsneed to be conducted to confirm the mechanism. For alower calcination temperature, it would be also helpful toobtain a high specific surface area under the same H2Oconcentration. The specific surface area increases from 12.9to 25.5m2/g while the calcination temperature is decreasedfrom 723 to 623K for 10mass% H2O concentration, asshown in Table 1.

Fig. 6 N2 adsorption­desorption isotherms of NiO prepared with (a) 0mass% H2O, (b) 10mass% H2O, (c) 20mass% H2O, and(d) 40mass% H2O and calcined at 723K.

Synthesis and Characterization of NiO Nanoparticles by Sol­Gel Method 2139

4. Conclusions

In this paper, we have investigated the effect of thermalstability on the NiO nanoparticles formation prepared by sol­gel method, which was accomplished by using copolymer asthe surfactant. According to TGA and FTIR analyses, it wasshown that the copolymers were expelled at 573K. Thethermal stability of NiO nanoparticles was examined byperforming various calcination temperatures. The samplecalcined at 923K was composed of pure NiO nanoparticles,as shown by XRD, while both metallic Ni and NiO wereformed as the calcining temperature between 623 and 823K.The formation of Ni and NiO nanoparticles and theirstructural features were greatly dependent on the calcinationtemperature. With increasing of H2O concentration, thereoxidation process of metallic Ni would reduce, but it wouldnot affect the structural type of NiO nanoparticles. In generalthe addition of water would weaken and inhibit oxidationeffects. The temperature of stable metallic Ni was increasedfrom 723 to 823K. Based on the results of XRD and TEM,the formation of NiO nanoparticles was clearly confirmed.The specific surface area evaluated from the N2 adsorption­desorption indicated that the samples consisting of non-porous NiO nanoparticles. Increasing H2O addition resultedin an increase of specific surface area of nanocrystalline NiOpowder. A low calcination temperature would be useful togain a high specific surface area of NiO nanoparticles.

Acknowledgements

This work is supported by National Science Council ofTaiwan, Republic of China under grant NSC 101-2221-E-020-015.

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Table 1 BET surface area, average pore size and pore volume of NiOprepared with various water contents.

TemperatureT/K

Contentsof H2OC/mass%

Surface areaA/m2/g

Averagepore sizeD/nm

Pore volumeVp/cm3/g

723 0 7.5 25.24 0.022

723 10 12.9 24.42 0.028

723 40 10.4 19.68 0.018

623 10 25.5 12.17 0.050

L. G. Teoh and K.-D. Li2140