Materials Letters 93 (2013) 377–379

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Materials Letters

0167-57

http://d

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

Controlled synthesis of large-sized Cr2O3 via hydrothermal reduction

Zhenzhao Pei n, Xia Zhang

Department of Chemistry, Hebei University of Engineering, Handan 056038, PR China

a r t i c l e i n f o

Article history:

Received 24 June 2012

Accepted 28 November 2012Available online 7 December 2012

Keywords:

Hydrothermal synthesis

Cr2O3

Large-sized

Microstructure

X-ray techniques

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

x.doi.org/10.1016/j.matlet.2012.11.127

esponding author. Tel./fax: þ86 310 2078796

ail address: peizhenzhao@tsinghua.org.cn (Z.

a b s t r a c t

In this paper, large sized polyhedron-like and plate-like Cr2O3 were firstly obtained via hydrothermal

synthesis under the assistance of the surfactant citric acid. The raw materials were K2Cr2O7 and HCHO.

With the increase of citric acid from 0.02, 0.05, 0.1, 0.12 to 0.16 g in sequence, morphologies of products

changed accordingly. When it was 0.12 g and reaction temperature was 180 1C, polyhedrons with the

size of 9–18 mm formed. With the increase of reaction temperature and keeping other reaction

conditions the same, morphologies of products also changed obviously. When it was 240 1C, products

were different sized plates from 5 mm to 106 mm made up of tiny particles. Influences of spatial effects

and viscosity from amount of the added surfactant and reaction temperatures on the nucleation and

diffusion rate lead to differences of stacking manners of precursor particles, and then lead to different

morphologies of the products. Microstructures of materials were observed by a field emission scanning

electron microscope and composition of products was investigated by X-ray techniques.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Cr2O3 is an important inorganic material. It is widely appliedin fields such as catalysis [1], advanced colorants [2], hydrogensorption materials [3] and wear resistance materials [4]. Largesized Cr2O3 can be used as spraying material that has someoutstanding properties such as thermal resistance, corrosionresistance, wear resistance and so on [5,6].

There have been a lot of methods to prepare Cr2O3, includingsol–gel method [7], gas condensation [8], sonochemical reaction [9],microwave plasma [10], decomposition of chromium(III) nitratesolution [11], laser-induced deposition [12], hydrothermal synthesis[13,14], and the morphologies of obtained products are nanoparti-cles [14], nanotubes [15], thin film [16], hollow nanospheres [17]and so on. But until now, very few investigations on the synthesis oflarge sized Cr2O3 have been reported. There is no literature aboutlarge sized Cr2O3 synthesized via the hydrothermal process.

In this research, big sized plates from 5 mm to 106 mm and bigsized polyhedrons with the size of about 9–18 mm were obtainedby changing the amount of surfactant and reaction temperature inhydrothermal system. Results of this research provide a new ideato synthesize big sized materials.

2. Experimental procedure

All the reactants are of AR grade and are used without furtherpurification. Distilled water was home-made.

All rights reserved.

.

Pei).

The typical process is as follows. 2.94 g K2Cr2O7, 1.65 ml HCHO(13.55 mol/L) and 0.12 g citric acid were added to a 50 ml teflon-lined bomb filled with 35 ml distilled water and magneticallystirred to a homogeneous solution. The sealed bomb was heatedat 180 1C for 1 h and then it was brought out and cooled to roomtemperature. After the hydrothermal procedure, a brown gel wasobtained. It was filtrated and watered several times until thefiltrate turned transparent. The filter cake was dried at 105 1C for10 h and changed to black particles with size in the grade ofseveral mm to mm, which were called precursor. The precursorwas calcinated at 1000 1C for 2 h and the products were obtained.The precursor was a black powder but the latter was green. Theother experiments were done as above and the only differencewas in the amount of citric acid and reaction time.

3. Analysis

FT-IR (Avatar 370, Nicolet, powers were diluted in KBr) wasused to investigate the precursor. An X-ray powder diffractometer(D/Max-2200, Rigaku, Japan) using Cu Ka radiation (l¼1.5408)was used to verify the precursor and products. And field-emissionscanning electron microscopy (JSM6360LV, Electron Company,Japan) was performed to investigate the morphology and sizes,and high-quality photomicrographs were obtained.

4. Results and discussion

IR spectra of the precursor are shown in Fig. 1. It shows thatthe characteristic bands are 3420, 1630, 1610, 1440, 1150, 1100,879, 843 and 793 cm�1. Among them, 3420 cm�1 is due to the

Fig. 1. IR spectra of the precursor.

Fig. 2. XRD patterns of the precursor and products obtained after calcinating at

1000 1C for 2 h. (a) the precursor and (b) products.

Z. Pei, X. Zhang / Materials Letters 93 (2013) 377–379378

OH stretching of nondissociated water molecules and surfacehydroxyls of dissociative chemisorption [18]. 1630 and1610 cm�1 are due to the bending modes of nondissociated watermolecules. 1440 cm�1 is derived from the adsorption of bidentatecarbonates that emerged during the reaction process [18]. 1150and 1100 cm�1 are assigned to the coupling of the stretchingvibration of C–O single bond and Cr–O–Cr vibrations. 879, 843and 793 cm�1 are due to the torsional oscillations of trappedwater molecules [18]. From the above messages, it can bededuced that the formula of Material 1 is Cr(OH)3 �xH2O.

XRD patterns of the precursor and products obtained aftercalcinating at 1000 1C for 2 h are shown in Fig. 2(a) and (b).Fig. 2(a) reveals that the precursor is amorphous and noncrystal-line. Fig. 2(b) reveals that the products are well crystallized, andall diffraction peaks in Fig. 2(b) can be indexed to those of pureEskolaite crystallographic phase Cr2O3 (JCPDS no. 38-1479).

Then the possible reactions during the process were concludedas follows:

K2Cr2O7þHCHOþH2O-Cr(OH)3 �xH2OþKOHþK2CO3þCO2 (1)

Cr(OH)3 �xH2OþO2-Cr2O3þH2O (2)

Fig. 3 shows the FE-SEM photographs of the precursor beforecalcination and Cr2O3 particles obtained at different reaction con-ditions. Fig. 3(a) is the image of the precursor. It shows that the

precursor is made up of loosely agglomerated particles withdifferent sizes of less than 1 mm. Fig. 3(b)–(f) are images of Cr2O3

particles obtained after calcination at 1000 1C for 2 h with citric acidof 0.02, 0.05, 0.1, 0.12 and 0.16 g, correspondingly. When citric acidwas 0.02 g, the products were agglomerated tiny particles withsizes less than 1 mm. In comparison with the precursor, the sizechange was not obvious. When citric acid was 0.05 g, relativelylarge particles with the size of about 10 mm among tiny particlesemerged. 0.1 g corresponded to products that were different sizedblocks made up of tiny particles and 0.12 g corresponded to thepolyhedron-like products in sizes of about 9–18 mm. And whencitric acid was 0.16 g, products changed to different sized blocksmade up of tiny particles. The experimental phenomena as abovedisclose that the amount of surfactant is the key factor to influencethe morphologies of products.

When only the reaction temperatures were changed and theamounts of citric acid, HCHO, reaction time were kept respec-tively 0.1 g, 1.65 ml and 1 h, morphologies of products changedwas obvious. When the reaction temperature was 200 1C, pro-ducts were different sized blocks from mm grade to tens of mmmade up of tiny particles. And some tiny particles were looselyattached on the surface of blocks and the strength among tinyparticles and the attached blocks is weak. So the products showloose aggregates made up of tiny particles. But when the reactiontemperature was 240 1C, products changed to huge plates from5 mm to 106 mm. By comparison of Fig. 3(d), (g) and (h), it wasfound that the products crystallized better in sequence with theincrease of reaction temperature.

According to the above discussion, amount of the added surfac-tant and reaction temperatures have important influences on themorphologies of the final products. The addition of surfactant canchange the dielectric constant, surface tension and viscosity. Citricacid can react with Cr3þ to form coordination compound, anddifferent spatial effects and different solution viscosity changes [13]due to the addition of different amounts of surfactant lead todifferent nucleation and diffusion rates of the precursor. Moresurfactants promote changes from 2-dimensional to 3-dimensional morphologies. Likewise, with the increase of reactiontemperatures, viscosity of the reaction system decreases in degree.It makes the reaction speed more rapid, and nucleation rate anddiffusion rate of intermediate particles also speed up. Interaction ofnucleation and diffusion rates leads to differences of stackingmanners of the precursors particles, thus the precursor obtainedunder different reaction temperatures or different amounts ofsurfactant have different surface energies. Through calcinations,the precursors having different stacking manners and differentsurface energies decompose into Cr2O3 with different morpholo-gies. From above, the conclusion can be drawn that spatial effectand solution viscosity are key factors to control the productsmorphologies.

5. Conclusion

In this research, Cr2O3 with different morphologies and sizeswere successfully synthesized in a hydrothermal system by chan-ging the amount of the added surfactants and reaction tempera-tures. When the amount of citric acid changed from 0.02, 0.05, 0.1,0.12 to 0.16 g in sequence, morphologies of products changedaccordingly. And the increase of reaction temperatures also led toobvious changes of morphologies of products. Large sizedpolyhedron-like and plate-like Cr2O3 of about several mm were firstobtained. Amount of the added surfactant and reaction tempera-tures affect the morphologies of the final products. Their influenceson the nucleation and diffusion rates lead to differences of stackingmanners of the precursor particles, thus precursors having different

Fig. 3. FE-SEM photographs of the precursor and chromic oxide particles under different amounts of citric acid and different reaction temperatures: (a) the precursor;

(b) 0.02 g, 180 1C; (c) 0.05 g, 180 1C; (d) 0.1 g, 180 1C; (e) 0.12 g, 180 1C; (f) 0.16 g, 180 1C; (g) 0.1 g, 200 1C and (h) 0.1 g, 240 1C.

Z. Pei, X. Zhang / Materials Letters 93 (2013) 377–379 379

stacking manners and different surface energies decompose intoCr2O3 with different morphologies after calcinations.

Acknowledgments

The research was financially supported by the supports fromDoctoral Foundation of Hebei University of Engineering, OpenFoundation of Key Laboratory of Photochemical Conversion andOptoelectronic Materials, TIPC, Chinese Academy of Sciences,Open Foundation of Key Lab of Power Material & AdvancedCeramics, School of Materials Science and Engineering, NorthernUniversity For Nationalities, China, and Key Program of Educa-tional Commission of Hebei Province, China.

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