Peculiar Crystal Growth of the Trivalent Titanium Derived TiO2­SnO2 Precursorunder Hydrothermal Conditions

Go Sakai,*1 Ayano Tanaka,1 Takuma Sueda,1 Tsubasa Ogata,1 Yuji Okuyama,2 and Naoki Matsunaga11Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192

2Organization for Promotion of Tenure Track, University of Miyazaki, Miyazaki 889-2192

(E-mail: sakai-go@cc.miyazaki-u.ac.jp)

TiO2(rut)

As-precipitated

Calcined in air

Hydrothermally treated

0 10 20 30 40 50 60 702θ / degree

SnO2

Calcination at 250 oC(Still amorphous)

Hydrothermal treatmentat 150 oC(Crystal growth)

The crystal growth under the hydrothermal treatment at 150 °C wasobserved for the pre-calcined TiO2­SnO2 precursor obtained by thecalcination at 250 °C. This phenomenon was observed for trivalenttitanium ion (Ti3+) was used as the starting material. The peculiar crystalgrowth phenomenon was thought to be originated from the oxidation oftitanium from trivalent to tetravalent.

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Vol.45 No.3 2016 p.318–320CMLTAG

March 5, 2016

The Chemical Society of Japan

Peculiar Crystal Growth of the Trivalent Titanium Derived TiO2­SnO2 Precursorunder Hydrothermal Conditions

Go Sakai,*1 Ayano Tanaka,1 Takuma Sueda,1 Tsubasa Ogata,1 Yuji Okuyama,2 and Naoki Matsunaga11Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192

2Organization for Promotion of Tenure Track, University of Miyazaki, Miyazaki 889-2192

(E-mail: sakai-go@cc.miyazaki-u.ac.jp)

Crystal growth under hydrothermal treatment at 150 °C wasobserved for the pre-calcined TiO2­SnO2 precursor obtained bythe calcination at 250 °C. This phenomenon was observed for thetrivalent titanium ion (Ti3+) used as the starting material forhydrolysis. The peculiar crystal growth phenomenon was thoughtto be originated from the oxidation of titanium from trivalent totetravalent, because such peculiar crystal growth under hydro-thermal conditions was not observed when tetravalent titaniumwas used.

Keywords: TiO2–SnO2 | Hydrothermal treatment |Trivalent titanium ion (Ti3+)

Titanium dioxide (TiO2) and tin dioxide (SnO2) haveattracted attention as functional materials applicable for variousfields such as semiconductor-type gas sensors, photocatalysts,electrochemical devices, and so on. The mixed solution and/orcomposite of TiO2­SnO2 system has also received much attentionbecause of its unique properties.1­8 Both titanium dioxide andtin dioxide can form the same crystal structure of a rutile-typetetragonal system, so that substitutional solid solutions can besynthesized by various methods.9,10 Theoretical approaches suchas DFT study have also been conducted for TiO2­SnO2 solidsolutions.11 On the other hand, the sintering behavior as well asthe mass transportation (cation diffusion) in TiO2­SnO2 duringannealing have also been conducted intensively due to mainlyscientific interests.12­14 Conventionally, the mixed solution and/or composite of TiO2­SnO2 was derived from tetravalentchlorides (TiCl4 and SnCl4) or alkoxides (e.g., Ti(OC4H9)4).Valence control of the TiO2­SnO2 system has not received muchattention, probably because of its almost stable tetravalent stateafter calcination at elevated temperatures. In contrast, in ourresearch group, control of the valence state of titanium hasrecently been the focus of intense interest. We recently reportedthat metal oxide-based electrocatalysts of Pt-TiOx/C showedenhanced electrocatalytic activity for the oxygen reductionreaction (ORR) with excellent stability for accelerated durabilitycycle testing of electrochemical measurement. It was revealed thatenhanced electrocatalytic activity for the ORR will be realizedwhen TiO2 was reduced and subsequent introduction of Pt0

into chemically induced defect site of TiO2 was achieved.15 Thisfinding allowed novel strategies in designing cathode electro-catalysts for proton-exchange membrane fuel cells (PEMFCs).The formation of direct interaction between platinum and thetransition metal ion in metal oxides (Pt0­Mn+), in which electrontransfer from metal ion (Mn+) to platinum (Pt0) is achieved, isconsidered to be essential for enhanced electrocatalytic activityfor the ORR. For the application of electrocatalysts in PEMFCs,high stability under strong acidic conditions is required. In thissense, TiO2 and SnO2 are candidates for oxide supports for

electrocatalysts operable in severe acidic as well as oxidativeatmospheres such as the cathode material of PEMFCs. Theresistance to such severe conditions is one of the most importantfactors to choose materials for practical applications. Thus,valence control of the TiO2­SnO2 solid solution is valuable notonly for our strategies of enhanced activity for the ORR, but alsofor the preparation of functional materials with high stability forvarious fields. In the present study, TiO2­SnO2 solid solution wasprepared by hydrolysis from the low valence state of titanium ion(Ti3+) and tin ion (Sn2+) to create the reduced state of metal ionsin solid. In the preparation process, peculiar crystal growth, i.e.,the precursor did not show the crystal growth by the calcinationprocess at 250 °C; on the other hand, the pre-calcined sampleobtained at 250 °C showed significant crystal growth by thefollowing process of hydrothermal treatment at 150 °C. It wasfound that the peculiar crystal growth phenomenon originatedfrom the characteristics of the trivalent titanium (Ti3+), i.e., thecrystallization of the precursor is accompanied by the oxidation oftitanium species from trivalent (Ti3+) to tetravalent (Ti4+) towardthe formation of TiO2­SnO2 solid solution or rutile TiO2 phaseunder the hydrothermal conditions.

TiCl3 solution was purchased from Wako Chem., Japan. Allchemicals were used without further purification. 50mL of 0.1MTiCl3 solution was prepared by mixing the commercially availableTiCl3 solution with 1-propanol. SnCl2 powder was dissolved in1-propanol to prepare 50mL of 0.1M SnCl2 solution. The twoprepared solutions were mixed to prepare a 100mL of mixedsolution containing Ti3+ and Sn2+ ions. The mixed solution wasadded stepwise to 500mL of 0.1M NH4HCO3 for hydrolysis toobtain precipitates. Conventionally, an alkaline solution is addedto a metal salt containing acid solution in order for hydrolysis totake place. However, in the present study, the metal salt containingacid solution was added stepwise to an alkaline buffer solutionto keep the pH for steady hydrolysis, resulting in uniformprecipitates. The obtained deep-yellow precipitates were washedand filtrated several times to remove chloride ions and dried at60 °C in air for 1 day. The dried precipitates were calcined at250 °C for 5 h in air, argon, or 3% H2 balanced with N2.

For the hydrothermal treatment, 0.4 g of the 250 °C-calcinedprecursors (pre-calcined samples) was suspended in 200mL of0.25% ammonia solution, and the suspension was transferred toa Teflon-lined autoclave (300mL) and heated at 150 °C for 24 h.The pre-calcined sample in air was hydrothermally treated atvarious temperatures (80, 100, 120, and 150 °C) for 24 h in orderto clarify the crystallization behavior of the pre-calcined samples.The resulting hydrothermally treated hydroxides were collectedby filtration and washed with deionized water several times. X-raydiffraction (XRD) patterns were recorded on a Rigaku diffrac-tometer (RINT UltimaMI) using Cu Kα radiation. N2 adsorptionand desorption isotherms were determined on a Micromeritics

CL-151106 Received: December 1, 2015 | Accepted: January 4, 2016 | Web Released: March 5, 2016

318 | Chem. Lett. 2016, 45, 318–320 | doi:10.1246/cl.151106 © 2016 The Chemical Society of Japan

Tristar 3000 system at liquid nitrogen temperature. Before themeasurement, the samples were degassed at 120 °C for 3 h andthen under vacuum. The specific surface areas of the sampleswere calculated with the BET method. The mean pore diametersand pore size distributions were determined with the BJH methodusing the corresponding desorption branches of the isotherms.The chemical composition and valence state of the samples wereverified by XPS (Kratos AXIS-HS, Shimadzu) measurements.

Figure 1 shows the XRD patterns of the as-precipitatedprecursor and calcined precursors in given atmospheres at 250 °C.The as-precipitate and the calcined precursors showed typicalamorphous patterns, i.e., obvious peaks could not be recognizedfor these samples. However, by applying hydrothermal treatmentin ammonia solution at 150 °C for the pre-calcined, a few sharppeaks appeared, as shown in Figure 2. These sharp peaksobviously revealed the crystal growth of these samples even at150 °C, although these precursors were pre-calcined at 250 °C.

Commonly, the crystal growth of oxide ceramics depends oncalcination temperature; however, the trivalent titanium-derivedamorphous precursor of TiO2­SnO2 obtained by calcination at250 °C showed crystallization by the following hydrothermaltreatment even at 150 °C, which is 100 °C lower than thecalcination temperature. This phenomenon was thought tooriginate from the crystal growth behavior of constituent metaloxides, so that Ti3+, Ti4+, Sn2+, and Sn4+ were examined asthe starting materials individually for the present preparationprocesses to obtain single oxides. Among these starting materials,the precipitate obtained from Ti3+ showed the same behavior asthe above-mentioned trivalent titanium-derived amorphous pre-cursor of TiO2­SnO2, i.e., the crystal growth under hydrothermaltreatment at 150 °C in addition to the amorphous state of the as-precipitated precursor and pre-calcined precursor at 250 °C. It wasalso obviously revealed that diffraction peaks shown in Figure 2were positioned almost at the center between the rutile phase ofTiO2 and SnO2. These metal oxides were essentially the samesystem of rutile, so that the main crystal phase of the obtainedsamples was considered to be the solid solution of TiO2­SnO2.Thus, the as-mentioned phenomenon, i.e., the crystallization ofthe pre-calcined precursor is thought to be accompanied by the

oxidation of titanium species from trivalent (Ti3+) to tetravalent(Ti4+) toward the formation of TiO2­SnO2 solid solution underthe hydrothermal conditions. Furthermore, it was found thatcrystallization under the hydrothermal conditions occurred evenat 80 °C, as shown in Figure 3, although the peaks of these lowertemperature-treated samples were rather broad as compared withthose of the 150 °C-treated one. Generally, crystallization takesplace by applying heat treatment due to the need for sufficientenergy for diffusion, replacement, and arrangement of theconstituent elements in solid. Thus, the crystallinity is readilydetermined by the calcination temperature. However, in thepresent study, the trivalent titanium-derived TiO2­SnO2 precur-sors pre-calcined at 250 °C showed clear peaks after the hydro-thermal treatment at 150 °C, which is 100 °C, lower than the justprevious calcination process.

The physicochemical sorption properties and pore parametersof the obtained samples were examined by nitrogen adsorption­desorption measurements. Table 1 summarizes the BET surfaceareas and textural properties of the as-precipitated precursor aswell as the pre-calcined precursors at 250 °C. The as-precipitatedprecursor showed a fairly high surface area of 258m2 g¹1. Byconducting calcination process, the surface areas decreased to

(a)

(b)

(c)

(d)

0 10 20 30 40 50 60 702θ / degree

Figure 1. XRD patterns of (a) the as-precipitated precursor preparedfrom Ti­Sn mixed solution by hydrolysis, and the precursors calcinedat 250 °C in (b) air, (c) argon, and (d) 3% hydrogen balanced with N2

atmosphere for 5 h, respectively.

0 10 20 30 40 50 60 70

2θ / degree

TiO2(rut)SnO2

(b)

(a)

(c)

Figure 2. XRD patterns of the hydrothermally treated samples at150 °C for the pre-calcined precursors at 250 °C in (a) air, (b) argon,and (c) 3% hydrogen balanced with N2 atmosphere, respectively.

TiO2(rut)

0 10 20 30 40 50 60 702θ / degree

SnO2

(b)

(a)

(c)

Figure 3. XRD patterns of the hydrothermally treated samples at (a)80, (b) 100, and (c) 120 °C for 24 h of the pre-calcined precursors at250 °C for 5 h in air.

Chem. Lett. 2016, 45, 318–320 | doi:10.1246/cl.151106 © 2016 The Chemical Society of Japan | 319

around 160­190m2 g¹1. There was no significant difference in thedecreased values of the surface area for calcination atmospheres;i.e., the calcination atmospheres hardly affect the change in grainsduring the calcination. Furthermore, even at 250 °C, the surfaceareas were kept high without vanishing mesopores. The BETsurface areas of the hydrothermally obtained TiO2­SnO2 samplesare also shown in Table 1. In accordance with crystallization,the surface areas of these hydrothermally obtained samplesdecreased to around 110­120m2 g¹1. This decrease in surfaceareas indicates the change in grains during the hydrothermalprocess paradoxically, leading to evidence for crystallization evenat lower temperature hydrothermal treatment than the previouslyconducted process of calcination at 250 °C. As a matter of course,the crystallinity cannot be determined only by the treatmenttemperature. In particular, the inside pressure of the Teflon-linedautoclave during the hydrothermal treatment performed in thisstudy might be an important factor; however, 80 °C-hydrothermaltreated sample also showed crystallization behavior. The temper-ature of 80 °C is apparently under the boiling point of water,so that the pressure inside the Teflon-lined autoclave becomesslightly higher according to the vapor pressure of ammonia andwater. It is hardly considered such a little increase in pressureproceed the crystallization of the amorphous phase of TiO2­SnO2.Thus, the crystallization mechanism could not be explained by theincrease in the inside pressure of the autoclave.

X-ray photoelectron spectroscopy (XPS) was conducted toinvestigate the crystal growth mechanism under the hydrothermaltreatment. Figure 4 reveals the XP spectra of Ti2p for the as-precipitated precursor as well as the hydrothermally treated one.The obtained peaks in the XPS measurements were correctedrelated to the C1s signal centered at 285.0 eV. It was revealed thatthe binding energy (BE) of Ti2p for the as-precipitated precursorwas 456.2 eV. The obtained BE was fairly close to the reportedvalues for trivalent titanium (Ti3+).16 Contrary to the reduced stateof titanium in the as-precipitated precursor, the hydrothermallytreated sample revealed a typical XP spectrum of the tetravalent(Ti4+) of the titanium oxides (TiO2).17 On the other hand, thevalence state of tin was almost unchanged before and after thehydrothermal treatment. This result implies that the startingmaterial of divalent tin(II) was oxidized during hydrolysis to thetetravalent state (IV) before the hydrothermal treatment. Thus,the proceeded crystallization of the present TiO2­SnO2 underhydrothermal treatment was thought to be originated from theoxidation of titanium species from trivalent (Ti3+) to tetravalent

(Ti4+) toward the formation of the TiO2­SnO2 solid solution. Themechanism of crystal growth during the hydrothermal treatmentis under investigation. A possible explanation is the effect ofthe medium (water) for the oxidation of the unstable trivalenttitanium to the stable tetravalent state during the hydrothermaltreatment.

The trivalent titanium-derived amorphous precursor of TiO2­

SnO2 pre-calcined at 250 °C showed crystallization obviouslyunder following hydrothermal treatment at a lower temperatureof 150 °C. This finding would promote the valence control of theTiO2­SnO2 solid solution applicable for various fields.

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Table 1. BET surface areas and textural properties of the as-precipitated and pre-calcined precursors at 250 °C precipitate as wellas the hydrothermally treated samples at 150 °C

Surface area/m2 g¹1

Pore volume/cm3 g¹1

Pore diameter/nm

As-precipitated 258 0.321 5.7Pre-calcined at 250 °C in air 175 0.306 6.5Pre-calcined at 250 °C in argon 169 0.251 6.0Pre-calcined at 250 °C in 3% H2 190 0.281 5.7Hydrothermally treated forpre-calcined in air

121 0.347 9.9

Hydrothermally treated forpre-calcined in argon

118 0.287 8.1

Hydrothermally treated forpre-calcined in 3% H2

111 0.317 9.6

456.2 eV

Binding Energy / eV

As-precipitated

Hydrothermally treated sample

458.2 eV

471 451461 456466

Figure 4. X-ray photoelectron spectrum of the as-precipitatedprecursor and the hydrothermally treated sample.

320 | Chem. Lett. 2016, 45, 318–320 | doi:10.1246/cl.151106 © 2016 The Chemical Society of Japan