Materials Letters 91 (2013) 5–8

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Materials Letters

0167-57

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weiwei.

peter.la

journal homepage: www.elsevier.com/locate/matlet

Synthesis of single-crystal nanoparticles of indium oxide by ‘‘urea glass’’method and their electrochemical properties

Si Qin, Weiwei Lei n, Dan Liu, Peter Lamb, Ying Chen

Institute for Frontier Materials, Deakin University, Geelong Waurn Ponds Campus, Victoria 3216, Australia

a r t i c l e i n f o

Article history:

Received 30 July 2012

Accepted 16 September 2012Available online 25 September 2012

Keywords:

Indium oxide

Nanoparticles

Nanocrystalline materials

Lithium ion battery

7X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2012.09.050

esponding author. Tel.: þ61 3 52272881.

ail addresses: si.qin@research.deakin.edu.au (

lei@deakin.edu.au (W. Lei), dan.liu@deakin.ed

mb@deakin.edu.au (P. Lamb), ian.chen@deaki

a b s t r a c t

Octahedral nanoparticles of indium oxide have been synthesized using a ‘‘urea glass’’ method, which is

relatively simple, low cost and catalyst-free. Further characterization shows the 40 nm indium oxide

nanoparticles are single crystals with a smooth surface and truncated tips. The electrochemical

properties of the negative electrode using indium oxide nanoparticles as the active material in

lithium-ion batteries were also studied. The nanoparticles show a capacity of 1195.8 mAh g�1 at the

first discharge and 585.3 mAh g�1 at the second discharge, and the reversible capacity is around

107 mAh g�1 after 20 cycles.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Indium oxide (In2O3) is an optically transparent n-type semi-conductor with a wide band gap around 3.6 eV. These transparentconducting oxides (TCOs) are widely used as transparent andelectrically conductive thin films in applications including solarcells [1], solid state gas sensors [2,3], and flat-panel displays [4].Recently, In2O3 films also have been studied as the cathodematerial in lithium-ion batteries (LIBs) [5] and as the activematerial in redox supercapacitors (SCs) [6], indicating potentialapplication in energy storage. In2O3 nanoparticles are potentialanode materials in LIBs because of their high theoreticalcapacity [5].

Different synthesis methods (or routes) including chemicalvapor deposition (CVD) [7,8], hydro-thermal [3,9] and laserablation [10,11] have been reported to produce nano-sizedIn2O3 in structures such as nano-wires [12], nano-rods [3,13]and nanoparticles [14,15]. These methods, however, require ahigh calcination temperature up to ca. 1300 1C [8], toxic surfac-tant/catalyst [16], strong acid [15,17] or special and high costequipment [10,15,18], which limit production quantity andincrease the cost. A simple, low cost and catalyst-free ‘‘urea glass’’method that has previously been used to synthesize nitrides andoxynitrides [19] is demonstrated. Using nitrate as the startingmaterial the method produces relatively pure In2O3 nanoparticlesof 40 nm with a smooth surface morphology and truncated tips.

All rights reserved.

S. Qin),

u.au (D. Liu),

n.edu.au (Y. Chen).

The electrochemical properties of the In2O3 nanoparticles in LIBsare investigated.

2. Experimental methods

Synthesis of In2O3 nanoparticles: Indium nitrate hydrate (In(NO3)3,99.99%) and urea (ACS grade) from Sigma-Aldrich were used asstarting materials. After 300 mg of In(NO3)3 was completely dis-solved in 6 ml of ethanol with the help of magnetic stirring, 300 mgof urea was added to the solution. The mixture was stirred until itwas dry forming a white and translucent glassy gel. The dry gel wasthen loaded into an alumina crucible which was transferred into atube furnace (tube inner diameter 50 mm) for calcination. Thesample was heated at 10 1C min�1 up to 600 1C and kept for 3 hunder nitrogen gas flow of �0.1 L min�1. When the calcination wascomplete, the furnace was cooled slowly to room temperature.About 68 mg (11.3 wt% of the starting material) of a light yellowproduct was collected for further investigations.

Sample characterization: X-ray diffraction (XRD) analysis wasperformed with a Panalytical X’Pert Pro X-ray diffractometerusing Cu Ka radiation source with 2y ranging 201–901 with astep size of 0.021. Scanning electron microscopy (SEM) wasconducted using a Carl Zeiss LEO 1530 instrument operating at5.00 kV. The chemical composition of these nanoparticles wasfurther determined by energy-dispersive X-ray spectroscopy (EDS).Transmission electron microscopy (TEM) images were taken on aJEOL JEM-2100 at an acceleration voltage of 200 kV. The thermalbehavior of the dry gel was analyzed using thermogravimetricanalysis (TGA) and differential scanning calorimetry (DSC) on aNetzsch STA 409 PC/PG thermal analyzer at a heating rate of

S. Qin et al. / Materials Letters 91 (2013) 5–86

10 K min�1 from room temperature to 1000 1C in nitrogengas flow.

Electrochemical measurement: The electrochemical properties ofthe In2O3 nanoparticles as the negative electrode in LIBs weretested in 2032 type coin cells, which were assembled in an argon-filled glovebox with both oxygen and moisture level below 2 ppm.Active material (In2O3 nanoparticles), conductivity agent (Super-Pcarbon black, Timcal) and binder (polyvinylidine difluoride,Aldrich) at a weight ratio of 80:10:10 were mixed in N-methyl-pyrrolidone (NMP, Aldrich) solvent to produce a slurry. The slurrywas coated onto a copper foil current collector and then dried in avacuum oven to completely remove the solvent and water andform the working electrode. Lithium foil was used as the counterand reference electrode and 1 M LiPF6 in 1:1 w/w ethylenecarbonate: diethyl carbonate as electrolyte. Galvanostatic tests

Fig. 1. XRD pattern of In2O3 nanoparticles.

Fig. 2. (a) SEM micrograph of In2O3 nanoparticles; (b) magnified SEM image of the

nanoparticles and (d) HRTEM image of a single In2O3 particle.

were carried out on a Solartron 1470E potentiostat/galvanostatand the working electrodes were cycled between 3 V and 0.01 V vs.Liþ/Li with a current density of 0.03 A g�1.

3. Results and discussion

The XRD pattern of the sample after calcination (Fig. 1) showsa full set of diffraction peaks of body-centred cubic (bcc) indiumoxide (JCPDS no. 06-0416, space group: Ia-3 (206)). The sharpdiffraction peaks suggest a well crystallized structure. The calcu-lated lattice parameter (a¼1.0119 nm) for the In2O3 sample isconsistent with the reference value (a¼1.0118 nm from JCPDS no.06-0416).

SEM and TEM micrographs of the In2O3 sample are displayedin Fig. 2. The SEM images in Fig. 2(a) and (b) show a large numberof octahedral nanoparticles with a very smooth surface andtruncated tips. The size of these particles is around 40 nm. Thebright-field TEM images in Fig. 2(c) and (d) suggest each particleis a single crystal. The lattice fringe of the (110) planes can beseen from the HRTEM image (Fig. 2(d)) and the spacing of theplane is about 0.72 nm.

The chemical composition of the sample was further deter-mined by energy-dispersive X-ray spectroscopy (EDS). It wasfound that the sample contained a small amount of amorphouscarbon (1.60 wt%) which is invisible in the XRD patterns and theSEM micrographs [19]. No nitrogen was found in the samplesuggesting that no nitride or oxynitride is formed in the synthesisprocess.

The possible formation mechanism of these In2O3 nanoparti-cles can be summarized as follows. After the addition of urea tothe In(NO3)3 ethanol solution, urea coordinated with In3þ cationsand formed an indium-urea complex. The urea ligand group in theinner coordination sphere and nitrate anion in the outer sphereare possibly formed in the solution [20]. After the ethanolevaporates, the mixture condenses to a translucent glassy gel inwhich the ligation of indium ion and urea provide a coordinated

In2O3 nanoparticles of the position marked in (a); (c) TEM micrograph of In2O3

Fig. 3. (a) TG/DSC curves of the dry gel and (b) scheme for the synthesis of In2O3 nanoparticles.

Fig. 4. (a) charge–discharge profiles of In2O3 nanoparticles and (b) cycle performance and coulombic efficiency of In2O3 nanoparticles with a current density of 0.03 A g�1

in LIB.

S. Qin et al. / Materials Letters 91 (2013) 5–8 7

environment for the subsequent reactions. The calcinationprocess was studied using TGA/DSC, as shown in Fig. 3(a).As the temperature increases, a small endothermic peak is seenaround 147 1C which corresponds to the melting of residual ureawith no mass loss. The major mass loss occurs between 180 1C and550 1C and three small peaks can be observed. This corresponds tothe decomposition of the indium-urea complex and the pyrolysis ofthe ligands, releasing a number of gases including HNCO, NH3, NO2.With the presence of the vigorous oxidizing group (NO3

�), theresidue turns into nanosized oxide particles. Normally, chloridesare used in the urea route for synthesizing nitrides [19,21]. However,nitrate was used in this case. The result indicates that NO3

- anionsfrom the nitrate play an important role in the formation of the oxide[20]. The proposed main reactions are

CO(NH2)2-HNCOþNH3 (1)

[In(CO(NH2)2)6](NO3)3-In(NO3)3þHNCOþNH3 (2)

In(NO3)3þCO(NH2)2-In2O3þNO2þNOþNH3þCO2 (3)

The single crystal structure is formed during the extendedheating above 550 1C when there is no mass loss. The completesynthesis process of the In2O3 nanoparticles is illustrated inFig. 3(b).

The electrochemical behavior of lithium ion insertion/extrac-tion of the In2O3 nanoparticles has been investigated. The first,second and fifth discharge/charge curves of the electrode com-posed of In2O3 nanoparticles at a current density of 0.03 A g�1 areshown in Fig. 4(a). It can be seen that the first and seconddischarge curves are significantly different from the fifth. Theelectrode shows an irreversible capacity loss in the first discharge

above 0.65 V, possibly because of the formation of an In metalphase in the electrode. The reversible capacity below 0.65 V is dueto the alloying of indium with lithium forming a LixIn alloy phase,as previously observed by Ho et al. [5]. As shown in Fig. 4(b),the first discharge and charge capacities of the electrode are1195.8 mAh g�1 and 491.7 mAh g�1, respectively. The reversiblecapacity of the electrode reduces to 107.9 mAh g�1 and thecoulombic efficiency rises to 98.8% after 20 cycles. The massivecapacity reduction after the first several cycles may be due toincomplete Li alloying/de-alloying in the discharge/charge pro-cess as a consequence of low diffusion rates of Liþ ions in In andLixIn phases. The performance of In2O3 electrodes in a LIB cell issimilar to other semiconductor active materials.

4. Conclusions

A simple, low cost and catalyst-free synthesis method of singlecrystal octahedral nanoparticles of In2O3 has been demonstrated. SEManalysis shows that the octahedral nanoparticles have a smoothsurface and truncated tips with the size of about 40 nm. TEM analysisshows that each particle is a single crystal. The electrochemicalproperties of the In2O3 nanoparticles as the active material in thenegative electrode of a lithium-ion battery cell were studied. Thereversible capacity after 20 cycles was around 107 mAh g�1.

Acknowledgments

Authors acknowledge the support from the Alfred Deakin Post-doc Fellowship from Deakin University, Central Research Grant

S. Qin et al. / Materials Letters 91 (2013) 5–88

Scheme (CRGS) from Deakin University and Australia ResearchCouncil (ARC) Centers of Excellence scheme from the AustralianResearch Council.

References

[1] Nakazawa T, Takamizawa K, Ito K. High efficiency indium oxide/cadmiumtelluride solar cells. Appl Phys Lett 1987;50:279–80.

[2] Seetha M, Mangalaraj D. Nano-porous indium oxide transistor sensor for thedetection of ethanol vapours at room temperature. Appl Phys A: Mater2012;106:137–43.

[3] Xu JQ, Chen YP, Pan QY, Xiang Q, Cheng ZX, Dong XW. A new route forpreparing corundum-type In2O3 nanorods used as gas-sensing materials.Nanotechnology 2007:18.

[4] Murali A, Barve A, Leppert VJ, Risbud SH, Kennedy IM, Lee HWH. Synthesisand characterization of indium oxide nanoparticles. Nano Lett 2001;1:287–9.

[5] Ho WH, Li CF, Liu HC, Yen SK. Electrochemical performance of In2O3 thin filmelectrode in lithium cell. J Power Sour 2008;175:897–902.

[6] Prasad KR, Koga K, Miura N. Electrochemical deposition of nanostructuredindium oxide: high-performance electrode material for redox supercapaci-tors. Chem Mater 2004;16:1845–7.

[7] Wang QY, Yu K, Xu F, Wu J, Xu Y, Zhu ZQ. Synthesis and field-emissionproperties of In2O3 nanostructures. Mater Lett 2008;62:2710–3.

[8] Hao YF, Meng GW, Ye CH, Zhang LD. Controlled synthesis of In2O3 octahe-drons and nanowires. Cryst Growth Des 2005;5:1617–21.

[9] Bagheri-Mohagheghi MM, Shahtahmasebi N, Mozafari E, Shokooh-Saremi M.Effect of the synthesis route on the structural properties and shape of theindium oxide (In(2)O(3)) nano-particles. Physica E 2009;41:1757–62.

[10] Li C, Zhang D, Han S, Liu X, Tang T, Lei B, et al. Synthesis, electronicproperties, and applications of indium oxide nanowires. Ann NY Acad Sci2003;1006:104–21.

[11] Beena D, Lethy KJ, Vinodkumar R, Mahadevan Pillai VP, Ganesan V, Phase DM,et al. Effect of substrate temperature on structural, optical and electricalproperties of pulsed laser ablated nanostructured indium oxide films.

Appl Surf Sci 2009;255:8334–42.[12] Yu D, Wang D, Yu W, Qian Y. Synthesis of ITO nanowires and nanorods with

corundum structure by a co-precipitation-anneal method. Mater Lett2004;58:84–7.

[13] Limmer SJ, Takahashi K, Cao G. Electrochromic and transparent conductingoxide nanorods. In: Cao G, Xia Y, Braun PV, editors. SPIE; 2003. p. 25–32.

[14] Gurlo A, Lauterbach S, Miehe G, Kleebe H-J, Riedel R. Nanocubes or

Nanorhombohedra? Unusual crystal shapes of corundum-type indium oxideJ Phys Chem C 2008;112:9209–13.

[15] Yu J, Kang S, Kim J, Kim J, Han J, Yoo J, et al. Nano-sized indium oxide powdersynthesized by spray pyrolysis process. Mater Trans 2006;47:1838–46.

[16] Souza CC, Rey JFQ, Muccillo ENS. Synthesis and characterization of sphericaland narrow size distribution indium oxide nanoparticles. Appl Surf Sci

2009;255:3779–83.[17] Rey JFQ, Plivelic TS, Rocha RA, Tadokoro SK, Torriani I, Muccillo ENS.

Synthesis of In2O3 nanoparticles by thermal decomposition of a citrate gel

precursor. J Nanopart Res 2005;7:203–8.[18] Zhou YN, Zhang H, Xue MZ, Wu CL, Wu XJ, Fu ZW. The electrochemistry of

nanostructured In2O3 with lithium (thin film). J Power Sour 2006;162:1373–8.

[19] Giordano C, Erpen C, Yao WT, Antonietti M. Synthesis of Mo and W carbideand nitride nanoparticles via a simple urea glass route. Nano Lett 2008;8:4659–63.

[20] Qiu Y, Gao L. Metal-urea complex—a precursor to metal nitrides. J Am CeramSoc 2004;87:352–7.

[21] Giordano C, Erpen C, Yao WT, Milke B, Antonietti M. Metal nitride and metalcarbide nanoparticles by a soft urea pathway. Chem Mater 2009;21:5136–44.