Ž .Materials Science and Engineering C 19 2002 323–326www.elsevier.comrlocatermsec

Phase transformation and grain growth of doped nanosized titania

Yu-Hong Zhang a,b,), Armin Reller a

a Solid State Chemistry, Institute of Physics, UniÕersity of Augsburg, 86135 Augsburg, Germanyb Department of Chemistry, Zhejiang UniÕersity, Hangzhou 310027, China

Abstract

The influence of altervalent cation doping of TiO on its phase transition and grain growth has been investigated. It is shown that2

dopants like Fe3q, Si4q, V5q, Ru3q and Ni2q affect the phase transition temperature of the titania host, and that significant variation isobserved for silicon doping. Iron titanium oxide and nickel titanium oxide phases were found in the iron-doped and nickel-doped system,respectively, at elevated calcination temperatures, while other doped systems only show the modifications of anatase and rutile at theobserved range of calcination temperature and dopant content. Compared with the pure TiO , grain growth is arrested for the iron-doped2

and silicon-doped systems, and this tendency is especially distinct in the silicon-doped system. q 2002 Elsevier Science B.V. All rightsreserved.

Keywords: Titania; Doping; Phase transition

1. Introduction

Titania is to date the most suitable photocatalyst semi-conducting material due to its high stability toward photo-corrosion and its relatively favorable band-gap energy.Numerous investigations are in progress for the improve-

w xment of the catalytic activity of TiO 1–6 . The attempts2

to enhance the photoactivity and broaden the absorption tothe solar spectrum are concentrated in introducing foreignspecies into TiO . Co-precipitation, ion exchange and wet2

impregnation are conventionally used for preparing dopedw xtitania 7,8 . By employing these methods, however, it is

not only difficult to prepare homogeneously doped TiO ,2

but also hard to control the grain growth. The homoge-neous distribution and the integration of the dopants in thestructural framework play the key role for the optimizationof specific photoelectrochemical and catalytic propertiesw x9 . The high-temperature diffusion technique used to formhomogeneously doped TiO easily causes the complete2

structural transformation of anatase to rutile modificationw x10 .

Titania adopts three crystal forms: brookite, anatase andrutile. Rutile is the stable phase while anatase and brookiteare metastable. It has been widely demonstrated that some

) Corresponding author. Solid State Chemistry, Institute of Physics,University of Augsburg, 86135 Augsburg, Germany. Tel.: q49-821-598-3004; fax: q49-821-598-3002.

Ž .E-mail address: zhangyuh@physik.uni-augsburg.de Y.-H. Zhang .

properties of pure and doped TiO , especially its photocat-2

alytic properties, are very sensible to the crystal modifica-w xtion 11 . Together with the crystal structure, the size of

crystalline particles and the surface area strongly influencew xthe catalytic activity as well 12 .

In this paper, we present the influence of homogeneousaltervalent cation doping of the titania matrix on its phasetransition and grain growth. Homogeneous doping impliesuniform incorporation of the foreign cation into the TiO2

matrix, which is substantially different from surface pro-motion.

2. Experimental

Ž . Ž .Titanium propoxide Merck, 98% , Ni NO P6H O,3 2 2Ž . Ž . Ž .Fe NO P 9H O, Si OEt , Ru C H O and2 3 3 2 4 5 7 2 3Ž .VO acac were used as precursors. The precursors were2

mixed rapidly by stirring in glycol. The mixture was agedat ambient temperature for a few days. During these days,the liquid became progressively viscous and eventually adry gel formed. Crystallization was achieved by subse-quent calcination of the dry gel in air at different tempera-tures by using the same heat treatment program for thesame doping element samples. The crystallinity and thestructurermodification of the products were studied on aPhillips Xpert X-ray diffractometer using Cu Ka radia-tion. The investigation of the particle morphology andnanostructure was performed on a CM-30 transmission

Ž .electron microscope TEM and the micrographs were

0928-4931r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.Ž .PII: S0928-4931 01 00409-X

( )Y.-H. Zhang, A. RellerrMaterials Science and Engineering C 19 2002 323–326324

Fig. 1. Phase transformation temperatures of iron-doped titania. ab—theemergence of anatase; rb—the emergence of rutile; ae—the end oftransformation of anatase to rutile; pbb—emergence of pseudobrookite.

recorded at 300 kV. The samples were prepared by dis-persing the powder products as a slurry in heptane, whichwas then deposited and dried on a holey carbon filmmounted on a Cu grid. The iron distribution was deter-

Ž .mined by an energy dispersive X-ray detector EDXwhich was mounted in a LEO-982 scanning electron mi-

Ž .croscope SEM . Nitrogen sorption data were collected ona micromeritics ASAP2000 Sorption Analyzer. Pore sizedistribution was modeled using the BJH method from thedesorption data.

3. Results and discussion

The samples of as-prepared doped titanium dioxideŽ .were examined by energy-dispersive X-ray analysis EDX

which was mounted on the scanning electron microscope

Ž .SEM . The results show that the dopant is homogeneouslydistributed in the TiO matrix, which exists either in2

anatase or rutile modification.The dependence of the phase transformation tempera-

tures of pure and iron-doped nanocrystalline TiO is pre-2

sented in Fig. 1. The phase transition of pure TiO from2

amorphous to anatase is observed at lower temperatures asthe one of iron-doped titania. It is important to note thatfor pure nanocrystalline TiO , the transformation from2

anatase to rutile takes place at 500 8C, i.e. at much lowerŽtemperatures than the one of iron-doped TiO transition2

.temperature: 600 8C . It is evident that the remarkable shiftof the crystallization and the phase transition from anataseto rutile to higher temperatures is caused by the structuraliron doping, i.e. the substitution of titanium ions by ironions within the given structural framework. It is notewor-thy that at the end of the anatase to rutile transformation ofiron-doped nanocrystalline TiO with an iron doping of2

more than 3 at.%, the iron titanium oxide pseudobrookite,Fe Ti O , forms. By exploring the X-ray diffraction pat-2 2 5

terns, no indication at all could be found for the formationŽ .of binary iron oxide s . These findings are consistent with

the results obtained by EDX: in all doped materials, iron ishomogeneously distributed in the titania matrix.

ŽTypical electron micrographs of the samples TF6 6.at.% iron calcined at 600 8C for 1 h in air are presented in

Fig. 2a and b. The comparison with pure titania exposed toŽ .the same calcination procedure Fig. 2c shows that the

particle size of the iron-doped titania is much more uni-form and much smaller with an average diameter of 14nm. It is evident that the particle growth becomes re-strained by the iron doping, which is decisive not only forthe design of the surface properties and surface area butalso for the electronic structure.

Particle growth is observed at high calcination tempera-tures due to the high surface energy of the nanocrystallineparticles. In contrast to the pure titania, the iron-dopedtitania is somehow stable against particle growth at hightemperatures. Using Scherrer’s equation, average particlesizes of different samples calcined at 600 8C for 1 h have

Ž . Ž .Fig. 2. HRTEM micrographs of pure and iron-doped titania products calcined in air at 600 8C for 1 h. a, b HRTEM micrographs of TF06. c HRTEMmicrograph of TF0.

( )Y.-H. Zhang, A. RellerrMaterials Science and Engineering C 19 2002 323–326 325

Table 1Surface area, pore size and particle size from specimens of different ironcontent at various calcination temperature

Iron Temperature Time Surface Average Averagew x w xcontent 8C h area pore particle

2w x w xat.% m rg radius diameterw x w xnm nm

0 400 3 79 3.7 10.16 400 3 96 3.7 9.30 500 1.5 7.03 3.7 20.31 500 1.5 28.04 3.7 13.56 500 1.5 48.9 3.7 12.10 600 1 1.9 – 306 600 1 23.3 3.7 14.4

Ž .been determined as shown in Table 1 . Pure titania samplecalcined at 600 8C for 1 h reaches a particle size around 30nm, while the one of the 6 at.% iron-doped titania samplescalcined at the same temperature is maintained at around14 nm. By increasing the iron content, the tendency ofarresting the grain growth is more evident. The averageparticle sizes calculated from XRD peak broadening areconsistent with those determined by TEM. The resultsshow that iron doping decreases grain growth rate. Thistrend is further confirmed by nitrogen adsorption–desorp-tion analysis. Fig. 3 presents the nitrogen sorption isothermsand pore size distribution curves of the samples withvarious iron content calcined at 500 8C. The isotherms as

Ž .shown in Fig. 3a are type IV isotherms IUPAC, 1985

Ž . Ž .Fig. 3. a N2 adsorption–desorption isotherm and b pore size distribu-tion of TF0, TF1 and TF6 calcined at 500 8C for 1.5 h.

with an H hysteresis loop, which is a typical indication2Ž .for a network of mesopores. The BJH analysis Fig. 3b

yields a corresponding peak centered at the pore radius of3.71 nm, indicating a very narrow distribution of themesopore dimensions. However, the surface area obvi-ously decreases when the calcination temperature in-

Ž .creased Table 1 . At calcination temperatures close to 6008C, pure TiO underwent drastic densification leading to a2

surface area reduction to 1.9 m2rg. The remarkable obser-vation is that the surface area of iron-doped samples is

Žalways larger than that of pure TiO for the same heating2.program . For example, the mesoporous structure and large

Ž 2 .surface area 23.3 m rg is still maintained for TF6 after itwas calcined at 600 8C for 1 h. There is no doubt that theincreasing of surface area of iron-doped samples is due tothe decreasing of the grain size, which is consistent withthe results of XRD and TEM.

The relationship between the phase transformation tem-perature of doped specimens and the silicon content ispresented in Fig. 4. It can be seen that the anatase phaseforms at a temperature of 330 8C for the pure titanium 88C,indicating the beginning of the transformation from anataseto rutile. The anatase peaks in the XRD pattern disappearafter the heat treatment of pure titanium dioxide at 700 8Cfor 1 h, suggesting the end of the transformation of anataseto rutile. The evolutions of both anatase and rutile start athigher temperature for silicon-doped samples. It should benoted that only 1 at.% silicon dopant is able to cause a bigenlargement of coexistence temperature of anatase andrutile, between 640 and 830 8C. The transition temperatureof anatase to rutile progressively elevates when the silicon

Fig. 4. The phase transformation of silicon-doped titanium dioxide.ab—the emergence of anatase phase; rb—the emergence of rutile phase;ae—the end of transition of anatase to rutile.

( )Y.-H. Zhang, A. RellerrMaterials Science and Engineering C 19 2002 323–326326

dopant is increased. When the silicon content approachesto 10 at.%, the temperature for single phase of anataseextends to 800 8C and the range of coexistence of anataseand rutile enlarges from 800 to 1100 8C. The temperaturerange for single-phase anatase is dramatically increasedwhen the silicon contents are 50, 60 and 70 at.%, andlocated above 1000 8C. On the other hand, the upper levelof the phase transformation temperature from anatase torutile is significantly elevated as well, leading to a remark-able increasing of the coexistence range of anatase andrutile. Neither silicon dioxide nor silicon titanium oxidepeaks were found in the XRD patterns of all the samples inour experiments, suggesting that silicon is dispersed uni-formly in the titanium dioxide matrix.

The trend of grain growth arresting in the silicon-dopedsystem is even more significant. For example, the puretitanium dioxide calcined at 600 8C for 1 h sinters togetherheavily and a reduction to the surface area of 1.9 m2rg isobserved. However, just 1 at.% silicon dopant causes asignificant increase of the surface area to 17 m2rg , andthe surface area of the sample containing 6 at.% silicondopant achieves 87 m2rg after calcination at 600 8C for 1h, which is apparently a subsequence of smaller particlesizes.

Compared to the iron- and silicon-doped systems, whichshow a significant grain growth hampering trend, thevanadium, nickel, and ruthenium dopants exhibit verysmall effects on the retardation of the grain growth. Thisindicates that different cations have different effects on thegrain growth mechanism of the TiO host during heat2

treatment processes. However, these dopants still influencethe phase transformation temperature of the titania. All ofthe ruthenium, nickel and vanadium dopants catalyze thetransition of anatase to rutile, and this phenomenon isespecially manifested at high dopant concentrations. Theformation of anatase from amorphous phase needs highertemperatures for either nickel- or vanadium-doped samplesthan that of pure titania. Oppositely, the amorphous-to-anatase transformation is observed at lower temperaturesfor ruthenium-doped samples, and just 0.2 at.% rutheniumdopant demonstrates an evident influence on the phasetransition of titania.

In conclusion, the altervalent cation dopants exert atremendous influence on the phase transformation andgrain growth of the titania host during the heat treatment.

v Silicon doping strongly restrains the phase transitionof anatase to rutile and significantly arrests the graingrowth of the titania host. The sequence of arresting graingrowth is: Si4q)Fe3q)V5q)Ru3q)Ni2q.

v Iron doping shows the analogous effect of siliconŽ .doping; however, iron titanium oxide pseudobrookite was

observed in samples with high iron content calcined athigh temperature.

v On the contrary, nickel, vanadium and rutheniumdoping promote the transition of anatase to rutile and causea little variation to the grain-growth rate. In the vanadium-and ruthenium-doped systems, only anatase and rutile areobserved. Nickel titanium oxide was found for the nickel-doped samples heated at high temperature.

Acknowledgements

The authors acknowledge Dr. A. Weidenkaff for inspir-ing discussions and Dr. O. Becker for recording HRTEMimages.

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