Novel sea-urchin-like rutile microstructures synthesizedby the thermal decomposition and oxidation of K2TiF6C.I. VillaVelázquez-Mendoza a,n, J.L. Rodríguez-Mendoza a, R.P. Hodgkins b,V. Ibarra-Galván b, A.L. Leal-Cruz c, A. López-Valdivieso d, M.I. Pech-Canul e

a Universidad de Colima, Facultad de Ingeniería Civil, Laboratorio de NanoMateriales, Carr. Colima-Coquimatlán Km 9, Coquimatlán, Colima 28400, Mexicob Universidad de Colima, Facultad de Ciencias Químicas, Carr. Colima-Coquimatlán Km 9, Coquimatlán, Colima 28400, Mexicoc Universidad de Sonora, Departamento de Investigación en Física, Rosales y Luis Encinas, Hermosillo, Sonora 8300, Mexicod UASLP, Instituto de Metalurgia, A. Sierra Leona 550, SLP 78210, Mexicoe CINVESTAV-Saltillo. Av. Industria Metalúrgica No. 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila 25900, Mexico

a r t i c l e i n f o

Article history:Received 26 December 2013Accepted 27 January 2014Available online 4 February 2014

Keywords:Sea-urchin-like microstructureThermal decompositionRutilePotassium hexafluorotitanateCeramicsThermal analysis

a b s t r a c t

Sea-urchin-like rutile (su-TiO2) microstructures were successfully synthesized by thermal decompositionand oxidation of a solid precursor, K2TiF6. During the synthesis, precursor temperature (640 1C), N2-flow(15 cm3/min) and inner pressure (90 mbar above the atmospheric pressure) remained constant. In orderto evaluate the effect of time on the morphology of su-TiO2, five levels (0.5, 1, 1.5, 2, and 2.5 h) wereconsidered. The thermal decomposition an oxidation of the K2TiF6 precursor was studied by thermalanalyses. It was found that TiO2 was segregated from the matrix, leading to the formation of su-TiO2

during processing. At 640 1C, the precursor underwent thermal decomposition and oxidation thatproduced three different products: K3TiOF5(s), TiO2(s), and F2(g). The su-TiO2 is synthesized only at t¼2 hand t¼2.5 h, presenting defined spikes at the former (100 mm�100 nm) and rougher structures at thelatter (50 mm�10 mm).

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Tailoring the surface morphology and the size of micro andnanoparticles of different materials might yield into new phaseswith outstanding properties. Investigators have developed variousroutes to synthesize micro and nanoparticles with nature-inspiredshapes, such as chrysanthemum [1], snowflake [2], dendritic [3]and sea-urchin-like [4] structures. In the case of sea-urchin-likestructures various compounds have been used to produce theseshapes, being the ZnO the most commonly used [5,4] due to itsgood gas sensitivity [6]. Other oxides also present this shape: NiO[7], MnO2 [8], α-Fe2O3 [9], SnO2 [10], Cu2O [11] and TiO2 [12].Sea-urchin-like shapes are not only characteristic of oxides butalso of Ni [13], Ni0.85Se, NiSe2, Co0.85Se [14], and Au [15]. Hybridsea-urchin-like structures are also found in literatures: CNTs/graphite [16], CNTs/mesoporous carbons [17,18], CNTs/stainless-steel [19], polyaniline composites [20,21] and polystyrene/polyani-line [22]. Most of these structures have been synthesized byhydrothermal reactions, CVD or thermal decomposition routesproducing structures with diameters from 30 nm [5] to 10 mm [21].

The diameter measurement of the sea-urchin-like structures hasnot been well established in the literature due to the fact thatsome structures have solid or hollow core with short spikesprotruding from their surface; while others without solid orhollow core, present longer spikes nucleated and growth fromthe core. This work concerns a novel synthesis of sea-urchin-likerutile microstructures (su-TiO2) with very long spikes (�100 mm)synthesized by the thermal decomposition and oxidation of a solidprecursor, K2TiF6. su-TiO2 might have applications in photovoltaiccells, hydrogen production and storage, sensors, batteries, cancerprevention and treatment, antibacterial and self-cleaning activityand electrocatalysis [23]. The K2TiF6 compound has not beenfound in the literature as a precursor to synthesize su-TiO2

microstructures.

2. Experimental

su-TiO2 was synthesized in a horizontal tube furnace. Prior toloading the precursor K2TiF6 (Sigma-Aldrich, analytic grade) intothe tube furnace (Carbolite MTF 12/38/400), scanning electronmicroscopy with energy dispersive spectrometry (SEM-EDS,Hitachi Tabletop-3000) and X-ray diffractometry (XRD, PhilipsX'Pert Pro-PANalytical diffractometer applying Cu-Kα radiation)

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.01.1650167-577X & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ52 312 3161167.E-mail address: carlos.ivm@gmail.com (C.I. VillaVelázquez-Mendoza).

Materials Letters 121 (2014) 191–193

were performed to K2TiF6, along with thermal analyses (NetzschSTA 449 F1 Jupiter): differential (DTA) and thermogravimetric(TGA). Thermogravimetric and differential thermal analyses withultra-high purity nitrogen atmosphere indicate that the appropri-ate temperature for the thermal decomposition and oxidation ofK2TiF6 begins at 600 1C, which is an endothermic reaction, accordingto the reaction depicted in Eq. (1). However, in this work 640 1C isused mainly due to the geometry of the solid precursor pellet and theslope of the temperature distribution of the tube furnace in order toassure that the whole pellet is over 600 1C. At 640 1C K2TiF6 decom-poses and oxidizes into three new products (Fig. 1):

3K2TiF6ðsÞ þ2O2ðgÞ ⟹T ¼ 640 1C

2K3TiOF5ðsÞ þTiO2ðsÞ þ4F2ðgÞ ð1ÞK2TiF6 powders were uniaxially compacted applying a 160 kg

force with a home-made manual press in order to form porouspellets. Since the complete pellet must be over 600 1C, the frontface of the pellet was placed at 640 1C.

Later, the pellet was strategically placed into the reaction chamber(tube furnace) where 640 1C is present. Since the chamber was notexhausted prior to processing, remaining oxygen reacts with theproduct during the process, obtaining K3TiOF5 and TiO2. N2 at a15 cm3/min flow rate was introduced into the chamber to displace the

TiO2 structures from within the pellet to the surface of its inner face.The inner pressure is set at 90 mbar controlled by awater column. Theheating rate of the furnace was 10 1C/min until 640 1C. At thistemperature, five different levels were considered (0.5, 1, 1.5, 2, and2.5 h) to evaluate their influence on the morphology of su-TiO2. Thecrystallinity and chemical properties are analyzed by XRD and EDS.While the su-TiO2 and the spikes morphologies are studied by SEM.

3. Results and discussion

The growth mechanism of the su-TiO2 originates from the thermaldecomposition of K2TiF6 at 640 1C. At room temperature, K2TiF6 is astable salt with particle diameters from 20 to 100 mm, Fig. 2a. Duringits thermal decomposition and oxidation three products are obtained,two solid: K3TiOF5 and TiO2 (Fig. 2c) and 1 volatile phase: F2. Duringthe synthesis, F2 is released from K2TiF6 and exhausted from thereactor, solid TiO2 is segregated from the matrix by the N2 flux effectproducing various structures with different geometries (Fig. 2d), whichare affected by the reaction time; the remaining solid is K3TiOF5,Fig. 2b. The su-TiO2 microstructures are found at the inner surface ofthe K3TiOF5 pellet, Fig. 2c and d.

At short times (0.5 and 1 h) TiO2 tend to form bar-shape TiO2

microstructures mixed with K3TiOF5 and K2TiF6 (Fig. 3a and b); at1.5 h su-TiO2 appear erratically but containing K3TiOF5 at theircore (Fig. 3c); at 2 h, clean, sharp and well defined su-TiO2 coverthe surface of the pellet (Fig. 3d); and finally at 2.5 h the su-TiO2

microstructures gain more mass increasing the diameter of theirspikes (Fig. 3e). Consequently, time does have a deep effect on thesynthesis of the su-TiO2.

4. Conclusions

su-TiO2 (rutile) microstructures were successfully synthesizedby thermal decomposition and oxidation of a solid precursor,

Fig. 1. XRD of the solid products (K3TiOF5 and TiO2-rutile) of the thermaldecomposition and oxidation of K2TiF6 and EDS of su-TiO2 in the inset.

Fig. 2. (a) Particles of the K2TiF6 precursor, (b) solid product of the thermal decomposition and oxidation, K3TiOF5, (c) K3TiOF5 plus su-TiO2 microstructures, and (d) su-TiO2

microstructures.

C.I. VillaVelázquez-Mendoza et al. / Materials Letters 121 (2014) 191–193192

K2TiF6. The appropriate decomposition temperature established bythermal analyses (TG and DTA) was 640 1C. Time was crucial forthe formation of su-TiO2 microstructures. Only at 2 and 2.5 h, thesu-TiO2 microstructures were synthesized, while at the formertime longer spikes were formed (100 mm in length�100 nm indiameter), at the latter time thicker spikes were produced (50 mmin length�10 mm in diameter).

Acknowledgments

This work was financially supported by CONACyT CienciasBásicas 2010-155444.

References

[1] Jiang J, Gao G, Yu R, Qiu G, Liu X. Solid State Sci 2011;13:356–60.[2] Zhu L, Xie Y, Zheng X, Liu X, Zhou G. J Cryst Growth 2004;260:494–9.[3] Zhu Y, Zheng H, Yang Q, Pan A, Yang Z, Qian Y. J Cryst Growth 2004;260:

427–34.[4] Umar A, Kim SH, Hahn YB. Superlattices Microstruct 2006;39:145–52.[5] Sekar A, Kim SH, Umar A, Hahn YB. J. Cryst Growth 2005;277:471–8.[6] Jia X, Fan H. Mater Lett 2010;64:1574–6.

[7] Bai L, Yuan F, Hu P, Yan S, Wang X, Li S. Mater Lett 2007;61:1698–700.[8] Wang H-E, Qian D. Mater Chem Phys 2008;109:399–403.[9] Zhang F, Yang H, Xie X, Li L, Zhang L, Yu J, et al. Sens Actuators, B

2009;141:381–9.[10] Liu S, Xie M, Li Y, Guo X, Ji W, Ding W, et al. Sens Actuators, B

2010;151:229–35.[11] Duan X, Gao R, Zhang Y, Jian Z. Mater Lett 2011;65:3625–8.[12] Fan K, Peng T, Chen J, Zhang X, Li R. J Power Sources 2013;222:38–44.[13] Liu X, Liang X, Zhang N, Qiu G, Yi R. Mater Sci Eng B 2006;132:272–7.[14] Liu X, Zhang N, Yi R, Qiu G, Yan A, Wu H, et al. Mater Sci Eng B

2007;140:38–43.[15] Fang J, Du S, Lebedkin S, Li Z, Robert K, Kappes M, et al. Nano Lett

2010;10:5006–13.[16] Zhang H-L, Zhang Y, Zhang X-G, Li F, Liu C, Tan J, et al. Carbon

2006;44:2778–84.[17] Chuang C-M, Sharma SP, Ting J-M, Lin H-P, Teng H, Huang C-W. Diam Relat

Mater 2008;17:606–10.[18] Kuo P-L, Hsu C-H, Ting W-T, Jhan J-YJ, Chen W-F. J Power Sources

2010;195:7983–90.[19] Nguyen XH, Lee YB, Lee CH, Lim D-S. Carbon 2010;48:2910–6.[20] Yang M, Yao X, Wang G, Ding H. Colloids Surf, A 2008;324:113–6.[21] Wang J, Wang J, Wang Z, Zhang F. Macromol Rapid Commun 2009;30:604–8.[22] Tan L, Cao L, Yang M, Wang G, Sun D. Polymer 2011;52:4770–6.[23] Khataee A, Ali Mansoori G. Nanostructured Titanium Dioxide Materials.

Properties. Preparation and Applications. 1st ed.. Singapore: World ScientificPublishing; 2012.

Fig. 3. su-TiO2 geometrical differences in function with the reaction time. (a) TiO2 bar-shapes, t¼0.5 h; (b) TiO2 bar-shapes, t¼1.0 h; (c) erratic su-TiO2 with bar-shapes,t¼1.5 h; (d) well defined su-TiO2, t¼2.0 h; and (e) thicker su-TiO2, t¼2.5 h.

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