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Materials Chemistry and Physics 121 (2010) 235–240

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ynthesis and characterization of rutile TiO2 nano-ellipsoid by water-solubleeroxotitanium complex precursor

ang Zhang, Liangzhuan Wu, Qinghui Zeng, Jinfang Zhi ∗

ey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & Graduateniversity of Chinese Academy of Sciences, No. 2 Bei-yi-tiao, Zhong-guan-cun, Haidian District, Beijing 100190, PR China

r t i c l e i n f o

rticle history:eceived 14 January 2009eceived in revised form 5 January 2010

a b s t r a c t

In this paper, we describe a one-step and solution (water-based) synthesis method for preparing rutilenano-ellipsoids by heating the peroxotitanium complex precursor at 100 ◦C. The size of the nano-ellipsoids could be controlled by changing precursor concentration. Transmission electron microscopy

ccepted 10 January 2010

eywords:iO2

utile nano-ellipsoidynthesis

(TEM) results show that the particles have different length–width ratio depending on the system acidity.The crystal growth mainly follows the oriented attachment process. The resulting samples are also char-acterized with UV–vis absorption and diffuse reflectance spectra. The smaller particles exhibited higherphotocatalytic activity as evaluated in the degradation of methylene blue.

© 2010 Elsevier B.V. All rights reserved.

orphologyeroxotitanium complex precursor

. Introduction

Titanium dioxide (TiO2), an important semiconductor, hasttracted considerable attention for applications in pigments, pho-ovoltaic cells and photocatalysis [1–3]. TiO2 has three polymorphhases: anatase, rutile and brookite [4,5]. Each phase of TiO2isplays different physical and chemical properties with differ-nt functionalities. There are several technological applicationstrongly related to its crystalline structure and morphology [6–8].mong of these, rutile phase is the most thermodynamically sta-le and it has a high refractive index which is especially useful forptical communication devices such as isolators, modulators, andwitches [9–11].

The traditional method for preparing rutile is through high-emperature calcination process of anatase phase. However, thealcination would lead to agglomeration of the nanocrystallinearticles with low surface area. Hence, fabrication of nanocrys-alline rutile TiO2 at low temperature is of paramount importance.urthermore, the synthesis of well-shaped rutile nanocrystals viaydrolysis in solution is much more difficult than the synthesisf anatase nanocrystals. This may be kinetic effect of nano-scale

ystems [12,13]. Due to the high reactivity of the titanium precur-or, control of the reaction rate is a key factor in obtaining rutileanocrystals with the well-dispersed shape. So far, many groupsave explored different methods to prepare rutile TiO2 nanocrys-

∗ Corresponding author. Tel.: +86 10 8254 3537; fax: +86 10 8254 3537.E-mail address: zhi-mail@mail.ipc.ac.cn (J. Zhi).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.01.024

tals at lower temperature. The hydrothermal method was appliedwidely [14–16]. However, this method produced only polydis-persed spherical or irregular shaped particles. Recently, more mildreaction conditions are applied in obtaining well-dispersed rutilenanoparticles. Wang et al. synthesized spindle-like rutile nanocrys-tals by the thermohydrolysis of TiCl4 in hydrochloric acid–alcoholaqueous solutions [17]. Li et al. prepared flowerlike rutile TiO2nanocrystals via stirring an aqueous phase for 24 h [18]. Wang etal. prepared rutile nano-ellipsoids with high photocatalytic activ-ity by hydrolysis of TiCl4 ethanol solution for 24 h [19]. Obviously,the reported methods for the synthesis of well-dispersed rutile TiO2always involved hydrothermal treatments or the addition of strongacid or solvents. Thus, impurities inevitably exist in the final prod-uct. Furthermore, long reaction time (1 day or more) is necessaryfor controlling the reaction rate in these experiments. Therefore,finding a technique to obtain pure rutile easily at low temperatureis still a challenge.

Herein, we report a novel and fast method to prepare well-dispersed rutile nano-ellipsoids by using a peroxotitanium com-plex. Peroxotitanium as precursor to synthesize titania for typicallow-temperature approach, has attracted much attention duringthe last decade [20–23]. Water-soluble peroxotitanium complexis accessible by the addition of H2O2 to an acidic solution ofTi(IV). It has been proved that this system is facile, no toxicity,

and low cost. Peroxotitanium complex as benign and environmentfriendly precursors, would be a preferred choice for preparationof titanium-containing functional materials by solution (water-based) synthesis method. Recently, Gao et al. reported a process ofpreparing rutile TiO2 film from peroxotitanium complex precursor.

2 istry and Physics 121 (2010) 235–240

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uring the preparation, HCl or HNO3 was utilized [20,21]. Ribeiro etl. also reported a synthesis method of the rutile TiO2 and anataseomposite phase by hydrothermal treatment of the peroxotitaniumomplex solution [22]. Recently, we have reported the controllablereparing of different TiO2 phases from peroxotitanium complex23]. We used a low pH value and the appropriate ratio of H2O2/Ti30:1 is the optimal value) solution to obtain pure rutile nanopar-icles. Since rutile is formed directly in solution, no further thermalreatment is required. Herein, we will give a detailed report on thereparation of spindle-like rutile TiO2 nanoparticles from the per-xotitanium complex solution at 100 ◦C without addition of anycid or other organic at relatively short reaction time. The affects ofariety of the precursor, concentration and reaction time on particleize, crystallite phase and specific area were investigated.

. Experimental

.1. Chemicals

Titanium tetrachloride (TiCl4, 99.0%), hydrogen peroxide (H2O2, 30%), were pur-hased from Beijing Chemical Co. Methylene blue was obtained from Tokyo Kasei.ll chemicals were chemical grade and used without further purification. Ultra-pureater was obtained from a Millipore Milli-Q purification system (>18 M� cm).

.2. Samples preparation

In a typical synthesis, TiCl4 was used as a starting reagent for the synthesis ofiO2 nanoparticles. H2O2 aqueous solution (9%, w/w) was used for dissolution ofiCl4 at ice-cooled temperatures with vigorous stirring. 1 mL (0.5 mL, 0.25 mL) ofure TiCl4 liquid was added carefully drop wise into 100 mL of H2O2 aqueous solu-ion and transparent deep red solutions with [Ti] = 0.1 M, (0.05 and 0.025 M) wererepared. The molar ratio of [H2O2]/[Ti] was kept to 20:1 which is beneficial forutile preparation based our previous reported results [23]. These solutions are sta-le at ambient environment for several days. After being stirred for several minutes,esulting solution was maintained in a reflux apparatus which allowed releasing2 produced by the decomposition of excess H2O2. The solution was kept refluxedt 100 ◦C for 6 h. The red-color of the solution gradually disappeared and a whiterecipitate would form at the end of reaction. The precipitate was washed withltra-pure water for several times and dried at 100 ◦C for later characterization. Theroducts prepared by different precursor concentration were denoted as T1 (0.1 M),2 (0.05 M), T3 (0.025 M), respectively. Furthermore, in order to study the effect ofrecursor, the TiCl4 solution (0.1 M) without addition of H2O2 (denoted as T4), orith the addition of hydrochloric acid ([HCl]/[Ti] = 20:1, denoted as T5) are refluxed

t the same procedure for comparison.

.3. Characterization

The phases of various TiO2 powders obtained by drying the as-preparedol at 100 ◦C were identified by X-ray diffraction (XRD) which was conductedn a XD-2 diffractometer (Purkinje General Instrument Co., Ltd), using graphiteonochromatic Cu K� radiation at 36 kV, 30 mA. The morphology of the particlesas observed by transmission electron microscopy (TEM; JEOL 2010 microscope).

elected area electron diffraction pattern (SAED) and high-resolution TEM (HRTEM)ere carried out on a Philips Tacnai 20 electron microscopy. Optical properties

f the nano-ellipsoids were investigated by a UVIKON-XL UV–vis spectropho-ometer (Secoman). The UV–vis absorption spectra are obtained by scanning theispersion solutions of rutile nano-ellipsoids at a concentration of approximately.1 mg mL−1. The specific surface area of the powder is analyzed by Ankersmidolland Co., Ltd.

.4. Photocatalytic activity

The photocatalytic activities of the prepared rutile TiO2 particles were evaluatedy the methylene blue degradation in an aqueous solution under 365 nm black-light

amp irradiation. Glass slides were used as substrates. TiO2 film was deposited onhe substrates by dip-coating process at room temperature with the speed aboutmm s−1 in 15 mg mL−1 particle dispersed ethanol solution. Then the films wereried for 30 min at 100 ◦C. Several pieces of 25 mm × 39 mm glass slides coated with

−3

iO2 film were firstly dipped in 2 × 10 M methylene blue for 8 h to obtain thequilibrium adsorption of methylene blue, then put into a watch glass containing0 mL of 1 × 10−3 M methylene blue solution and exposed to UV-irradiation. The UV

ntensity on the film was about 0.5–1 mW cm−2 by a UV intensity monitor. UV–visbsorption spectra of the irradiated methylene blue samples were measured everyh on a UV–vis spectrophotometer (SECOMAN UVIKONxs). The concentration ofethylene blue aqueous solution was determined by measuring its adsorption at

65 nm.

Fig. 1. XRD patterns of the products obtained under different experimental con-ditions (peroxotitanium complex precursor: T1, 0.1 M; T2, 0.05 M; T3, 0.025 M,respectively; T4, TiCl4 0.1 M only; TiCl4 and HCl, 0.1 M, respectively).

3. Results and discussion

3.1. XRD patterns

The XRD patterns of the as-prepared samples obtained at dif-ferent experimental conditions were shown in Fig. 1. It is noticedthat the patterns from heating the different concentration perox-otitanium complex solutions (Fig. 1, T1, T2, and T3) can be wellindexed to rutile phase (JCPDS No. 21-1276), the intense diffractionpeaks appearing at about 2� = 27.3, 35.9, 41.2, 54.4 correspond tothose from (1 1 0), (1 0 1), (1 1 1) and (2 1 1), respectively. However,anatase phase with low crystallinity was obtained when a freshTiCl4 dilute solution was refluxed directly (Fig. 1, T4), while thesample prepared from refluxing hydrochloric acid and TiCl4 mix-ture solution exhibits a composite phase, i.e., more anatase, lessrutile and brookite were observed (Fig. 1, T5). These results revealthat the type of precursor affects on crystal-phases of resulted prod-ucts, the peroxotitanium complex is the key factor for obtainingrutile phase. Furthermore, from T1 to T3 of Fig. 1, it is noticed thatthe rutile particles are all well-crystallization, meanwhile, no peaksof anatase or brookite phase can be observed, indicating a highpurity of the products. By applying the Debye–Scherrer formulaon the rutile (1 1 0) diffraction peaks, the average crystallite sizesof the products obtained can be calculated to be 24.8, 20.4, and16.7 nm for T1, T2, and T3, respectively. The results suggested thatthe crystal sizes of the resulting rutile nanoparticles decreased withthe decrease of the Ti concentration.

Furthermore, with decreasing of Ti concentration, the specificsurface area determined by nitrogen sorption (BET) increased. TheBET results show the specific surface areas are 43.90 m2 g−1 for T1,50.22 m2 g−1 for T2 and 53.25 m2 g−1 for T3, respectively.

3.2. TEM images

The size and morphology of the products were further ana-lyzed by TEM measurements. As shown in Fig. 2, it can be observedthat the morphologies of the products obtained varied with dif-ferent reaction systems. Well-dispersed rutile nano-ellipsoids canbe obtained when the peroxotitanium was used as the precursor(Fig. 2a, obtained from T1, T2 and T3 samples has similar nano-ellipsoid morphology which the TEM images will be shown below).

According to the HRTEM image (Fig. 2b), the lattice spacing is about0.322 nm which is corresponding to the distance between (1 1 0)crystal planes of rutile phase. It can be concluded that the rutilenano-ellipsoids grow along the [1 1 0] direction in the present con-dition. While in the other two situations, which using TiCl4 or TiCl4

Y. Zhang et al. / Materials Chemistry and Physics 121 (2010) 235–240 237

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Fig. 2. TEM images of T1 (a and b) T4 (c) and T

nd HCl as precursor, the agglomerate particles were observed inhe final products (Fig. 2b and c). The SAED images (insert graphs)f the different samples in Fig. 2a, c and d were well consistent withhe XRD patterns.

Furthermore, we also found that the concentration of the per-xotitanium affects the sizes and shapes of the rutile particlesbtained. As shown in Fig. 3, the rutile nanoparticles obtainedrom peroxotitanium complex precursors are all spindle-like struc-ures, however, the length–diameter ratio is increasing with theoncentration of Ti concentration decreasing (T1: 290 nm × 64 nm,2: 230 nm × 67 nm and T3: 180 nm × 80 nm, respectively), i.e.,igher precursor concentration leads to bigger length–widthatio. Moreover, the particles of T1 are spindly (Fig. 3a) andhanged to pyriform when the concentration of TiCl is 0.025 M

4Fig. 3c).

In general, the growth mechanism of nano-ellipsoids can benderstood on the basis of oriented aggregation by polar forces [24].he primary nanoparticles can transfer into rutile nano-ellipsoids

Fig. 3. TEM images of T1 (a), T2 (

respectively, insert graphs is the SAED images.

by oriented aggregation. Jiggling of nanoparticles by the drivingforce may allow adjacent particles to construct the low energystructures, represented by a coherent particle–particle interface.In this work, it seems that, the growth of rutile nanocrystals ismainly perpendicular to the (1 1 0) direction due to the smallersurface energy of the (1 1 0) surface [16]. While, it is known thatthe surface energy of the rutile particles may vary depending onthe acidity of the solution. Accurate energy value of rutile (1 1 0)surface has been evaluated for 1.60 J m−2 on hydrogen-rich surfaceand 1.08 J m−2 on hydrated surface according to Ref. [25]. Therefore,different surface energy leads to different grow rate and particlesobtained with different length–width ratio. In our experiments,solution acidity became a little weaker with the concentration ofthe peroxotitanium decreased, as measured were T1, pH 0.79, T2,

pH 0.99, T3, pH 1.10, respectively, and the surface state of the rutileparticles is changed from hydrogen-rich surface to little hydratedsurface. Therefore, it has been expected that the hydrogen-rich sur-face exhibits the higher length–diameter ratio than the hydrated

b) and T3 (c), respectively.

2 istry and Physics 121 (2010) 235–240

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urface. As a result, the rutile shape was spindly in T1 sample (0.1 M)nd changed to pyriform in T3 sample (0.025 M).

.3. Possible formation mechanism

We have studied the formation pathway, through which theutile nano-ellipsoids are achieved, by carrying out a time-esolved experiment by heating the peroxotitanium complex withTi] = 0.1 M. From the XRD patterns in Fig. 4, it is obviously shownhat the crystal phase changed from amorphous to rutile after 2 h,he prolonged heating of peroxotitanium complex solution doesot yield any detection changes. This is not consistent with thenes reported by Ribeiro [22], it is said the crystallization path toutile will necessarily pass through amorphous to anatase and toutile at last. While in present reaction condition, no anatase phaseas observed. This result allows us to infer that the rutile-seed

ould originate via direct decomposing of the appropriate perox-titanium complex, following which further growth continuationould occur exclusively in the rutile phase.

In the present reaction environment, the Ti4+ hydrolyzesnder the H2O2 solution to form peroxotitanium complex. The

ydrolyzed species is the precursor for the final condensationroduct. As reported, generally, the peroxotitanium complexresent dinuclear structure in acid solution, there could be sev-ral ligands for Ti4+, including O2

−, OOH−, OH, H2O, and oneitanium atom is only cooperate to one peroxo bond based on

Fig. 5. Possible mechanism

Fig. 6. TEM images of the sample heating at different reaction times: (a) 1 h

Fig. 4. XRD patterns of the sample heating at different reaction times: (a) 1 h, (b)2 h, (c) 4 h and (d) 6 h, the concentration of the peroxotitanium is 0.1 M.

the stoichiometric analyze, therefore, the detail dinuclear struc-ture could be Ti2(O2)2(OH)a(H2O)b, Ti2(OOH)2(OH)a(H2O)b orTi2(O2)(OOH)(OH)a(H2O)b, which changes with the concentration

of the H2O, H2O2 and pH. A lower pH (high acidity) benefit for bring-ing OOH− ligand, and hence the complex Ti2(OOH)2(OH)a(H2O)bseems to exist in the beginning hydrolysis step, i.e., the two Tiatoms will be bridged by two peroxy groups to form an octahe-

of rutile formation.

, (b) 2 h and (c) 4 h the concentration of the peroxotitanium is 0.1 M.

Y. Zhang et al. / Materials Chemistry and Physics 121 (2010) 235–240 239

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ig. 7. Plots of the (˛E)2 vs. the energy of absorbed light and UV–vis absorptionpectra (inset) of T1, T2, and T3.

ron, as shown in Fig. 5, then two edges of opposing octahedralre connected to build linear chains which linked via corners, theutile-seed can be obtained directly from condensation of the smallhain without recrystallization process. The rutile lattice eventuallyevelops along its unique axis of symmetry.

On the other hand, as observed above, that the particles are wellrystalline in 2 h in the present reaction condition which is differ-nt from more than 24 h of mostly reports [18,19]. It is mainly dueo the higher reaction rate of peroxo bond decomposing than theehydration rate in the acid or alcohol system at 100 ◦C. As a result,he high crystallinity rutile nanocrystals were easily formed in ahort time. More detail crystal growth process was evaluated by theEM images in Fig. 6. It is found that the smaller needle nanocrys-als gradually aggregated together to form nano-ellipsoid with theime increasing. This is an obvious character oriented attachment

echanism [26].

.4. UV–vis diffuse reflectance spectra

It is well known that the rutile TiO2 is a good inorganic UVbsorbent in cosmetics, pigments and plastics because of its highbsorbance for ultraviolet rays. Fig. 7 shows the diffuse reflectancepectra of the different products obtained. The value of the directransition optical gap can be evaluated by plots of the (˛E)2 vs.he energy of absorbed light. According the results calculated, themaller particles show narrower optical gap (T1 3.10 eV, T2 3.09 eVnd T3 3.02 eV, respectively). Inserted curve displays the UV–visbsorption spectra of the nano-ellipsoids dispersion solutions of1, T2 and T3. It can be obviously observed that a characteristicbsorption of rutile TiO2 nanoparticles appeared at about 350 nm27,28], while a tiny blue shift of the maximum absorption of theifferent size particles from T1 to T3 (420 nm for T1, 366 nm for T2nd 344 nm for T3, respectively) can be observed.

The different optical spectra are reflection of quantum-sizeffect for semiconductors. The electronic properties of small semi-onductor particles dependent upon the crystalline size due touantized motion of the electron and hole in a confined space29]. The smaller nanoparticles usually exhibit blue shifts in thebsorption spectra and narrower band gap.

.5. Photocatalytic activity

It has been reported that the rutile phase also has somehotocatalytic activity for decomposing methylene blue, so thehotocatalytic activity of our rutile sample was also evaluated fromethylene blue decomposition. Fig. 8 shows the photocatalytic

egradation of different sample. It is shown that the activity of T2[

Fig. 8. Photocatalytic degradation activity of different samples (T1, T2 and T3).

and T3 sample which are the smaller particles show higher degra-dation ratio than T1. This result also proved the T2 and T3 samplehave better quantum-size effect, which could shorten the route foran electron to migrates from the conduction band to its surface andenhance the activities of electrons and holes.

4. Conclusions

In this paper, we have described a simple method for preparingrutile nano-ellipsoids with tunable aspect ratios from the perox-otitanium complex precursor at mild ambient environment and ashort time. The peroxotitanium complex precursor is a key factoron the rutile nanocrystals formation. It was found that the stabilityof the peroxotitanium complex and the fast decomposing rate ofthe peroxo bond made a synergistic effect in the present system.The photodegradation experiments proved that the smaller par-ticles exhibited higher photocatalytic activity in the degradationof methylene blue. Our investigations have allowed us to infer adirectly rutile-seed formation by the decomposition of an appropri-ate peroxotitanium complex precursor, followed by further growthof rutile-seed. The advantage of this new synthesis approach isthe possibility to produce pure rutile nanoparticles without anyadditive and calcinations step, i.e., no thermal treatment for crys-tallization is necessary. This can be very interesting and useful forcatalyst supports, industrial high refractive index film and opticalcoatings.

Acknowledgment

Financial support by knowledge innovation project of ChineseAcademy of Science (No. KJCX2.YW.H04) is gratefully acknowl-edged.

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