アナターゼ型チタニア固溶体ナノ粒子の水熱条件下における創製

第9号 2007年

アナターゼ型チタニア固溶体ナノ粒子の水熱条件下における創製

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Abstract Anatase同typetitania (Ti02) solid solution nanoparticles doped with transition metal cations

were directly synthesized from aqueous solutions under hydrothermal conditions. Scandium was doped into anatase nanoparticles up to 10 mol% by hydrothermal crystallization企omamorphous co同preclpItates

Niobium-doped anatase-type titania nanoparticles were also directly formed under mild hydrothermal conditions using the hydrolysis of urea. The prominent effects of forming solid solutions with niobium oxide for anatase-type Ti02 on the enhancement of photo-decomposition of M B under UV irradiation and adsorptivity in the dark were confirmed. The anatase司typeTil_2XNbxAlX02 solid solutionsョthatwere

directly formed under mild hydrothermal conditions using the hydrolysis of urea， showed improved photocatalytic activity 1. Introduction Tit加cr.a (Ti02) can effectively photo司oxidize a vari巴:ty of hazardous organic chemicals at room temperaωre under ne紅 UV-light or sunlight irradiation [1，2]. The properties of photocatalyst can be modified by doping various components， and its performance is also in:fluenced by synthesis technique and preparatton

∞

ndition.Intensive attention has been devoted to wet che

m

c

r

.

cal routes to obtain nanometer-sized particles of inorganic materialsコrecently[3]. Well-develop巴:ddefect-free single crystals [4] and also homogeneous nanometer-siz巴dmetal-oxide solid solutions [5] have been synthesized from aqueous precursor solutions through hydrothermal佐eatment.We have directly synthesized crystalline metal-oxide nanoparticles企omaqueous prωursor solutions using precipitation and hydrothermal techr

明

ues at relatively low temperaωres [6-15]. Tit削 asolid solution nanoparticles doped叩th iron [16]， zirconium [17-19]， scandium [20]， and niobium [21，22]， and co-doped with niobium and alu

m

c

r

.

num [23] and titania/silica

∞

mposite nanoparticles [24-28] have been叩lthesizedthrough so合 solution routes As many transition metal cations having sirnilar ion radiuses ofTi4_{+ are in some degr巳esoluble to Ti02， there紅emany reports on}

the prepatぽionand prop出iesof titania solid solutions doped with those cations [29，30]. The preparation ofhomogeneous anata印刷type solid solutions with dopant by the solid-state reaction and to clarif

シ

the extent of solubility into anatasec_勿pe蛇uc加reare not so easy， because pure anatase phase is metastable and easily changes to stable rutile one by heat treatment above 635 oC from the T 愛 知 工 業 大 学 工 学 部 応 用 化 学 科 (豊田市) re乱lltofthe kinetic study [31，32]. This article is a surnmaty ofthe study on the direct formation of anatase-type solid solution nanoparticles under

m

c

r

.

ld hydrothermal conditions 2. Exper並E品目tal Reagent-grade TiOS04， Sc(N03h Al(N03)3コ and巴thanol solution ofNbCls wer巴usedas the starting materials. A Wxture of an aqueous solution of the starting materials according to the

∞

mposition ofthe samples was prepared in a Teflon container. The solution mix旬rewas controlled to have a wealc basic condition afler hydrother百laltreatment by the hydrolysis of urea or the addition of aqueous antmonia. This solution

m

c

r

.

x加rein the Teflon container was then placed in a stainless-steel vesse A.lfter the vessel was tightly sealedヲ辻washeated at 1800C for 5 h under rotation at 1.5 rpm After hydrothermal treatment， the precipitates were washed with distilled water until the pH value of白erinsed water became 7.0， separated from the solution by centrifitgatio

，

n

and dried in an oven at 60oC. The powders thus prepared were heated in an alu

m

c

r

.

na cruιible at heating rate 200oCh/ヲheldat 800回 10500Cfor 1 h in air， and then cooled to room tempera加rein a furnace. Commercially

available pure Ti02 powder for phoもocatalysts(ST-O 1ラanatase-匂rpe 佑uctぽ久BETspecific surface area: 302 m2/g，αystallite size: 7

n

m

，

Ishihara Sangyo Kaisha Ltd.， Osalcaヲ Japan)was used asせle

reference sample for estimation of ilie photocatalytic activity. The phases of the as同preparedand heated powders were

ex組曲led by X-ray di缶actome町 (XRD; modelRlNT-2000ラ

Rig北u，Tokyo， Japan) using CuKαradiation. The morphology of the as旬preparedsamples was observed by回ns

m

c

r

.

ssionelectron

m

c

r

.

croscopy (TEM; model JEM-2010， JEOL， Tol守0，Japan). The crystallite size of anatase was estimated企omthe line broadening of

78 愛知工業大学総合技術研究所研究報告，第9号， 2007年 101 and 200 diJfraction peaks， according to the Scherrer equatio民 D双 D二 KYscosB，wh巴:reθisthe Bragg angle of di岳actionIines;K is a shape factor (K = 0.9 in this work)ラλisthe wavelength of incident X-raysヲandβisthe corrected half-width given by

;

J

=βf -s/ヲwheresm is the measured half-width and品 目 白ehalιwidth of a standard sample. The lattice p紅ameterswere measured using silicon as the intemal standard. The chemical composition of the resultant powders was analyzed using an inductivity-coupled plasma emission spectrometer(lC

，

p

model; ICP575，I1Nippon Jarrell-Ashヲ Japan). The specific surface area of the prep紅ed samples was calculated丘omthe adsorption isotherm of nitrogen at 77 K based on the Brunau巴r-Emmett-Tell巴rmethod 但ET，model;

NOVA 1200， Yuasa IOl吐ωコOsak

，

a

Japan). The diffuse ref1ectance

spec回 measurementsfor powder samples have been made. The optical absorption ofthese pr巴:paredpowd巴:rswas measured using an ul廿aviolet-visiblespectrophotometer (V-560， Nihon Bunko， To勾10， Japan) The photocatalytic activity and adsorptivity of these prepared powders were separately estimated from the change in the concentration of methylene blue (guaranteed reagent grade， C1JI18N3S，お侶)both under ultraviolet ray代凡うirradiation企om black lightラandin the伽lらrespectively.To aqueous M B solutio民 sample powders were dispersed via ultrasonic stirring for 5 min and maintained in the dark for 24 h with stirring. After completion ofthe adsorption ofMB by maintenance ofthe solution in the dark for 24 h under stirring， the solution was maintained under irradiation of ultraviolet ray under stirring for 0-5 h. Thus， the UV-light irradiation time d巴pendenceof M B decomposition was巴strmat吋 丘omthe absorbance change with the spectrophotomete.r 3. Res1.I.lts and DisClJ.ssion 3.1. Scandium-doped tit倒1αsolidsolutions Figur巴1shows)改Dpa抗emsof solid precipitates formed und巴f hydrothermal conditions at various temperatures from the

∞

-precipitates in the presence of aqueous ammonia， which w巴:re obtained at the composition of 5 mol% scandium from the precursOI solutions ofTiOS04 and Sc別03)3.The XRD pa町mof the sample before hydrothermal treatment shows it is amorphous. The di缶actionpealcs due to anatses-type Ti02 were not observed in the samples hydrothermally treated below150 oC for5 h. The precipitates obtained by hydrothermal仕巴atmentat180and240 oC おr5 h were detected as single-phase anatase-type s飢 川 町e，and no trace ofdiJfraction peaks due to another phase were detected. Itis suggested that the hydrothermal treatme凶 attempera:旬remorethan 180 oC is necess創yfor the nucleation of anatase andせlecrystal growth of their nuclei企omthe amorphous co-precipitates (t祉制um hydroxide or hydrated titanium oxide with scandium hydroxide) XRD pa仕巴:ms of solid precipitates obtained企om the

∞

-precipitates with various compositions up to 10 mol% scandium by hydrothermal往eatmentat180 oC showed that the precipitates were all detected as singleゃhaseanatas巴-typestmctu民 間dnotrace ofdi世actionpealcs due to another crystalline phas巴weredetected A gradual shift of the di飴actionpeaks of anatase匂rpeTi02 to a lower diffraction angle in the)侭Dp副emsofthe samples is clearly observed when the scandium content was increased. The chang巴

m

the lattice parameters

a

o

and c口ofthete甘agonalanatase-type Ti02， relative to the scandium content showed that the la仕iceparameterCo is noticed to increase with increase of scandium contentフalthough noticeable change in

a

o

is not observed. A metastable crystalline phase， anatase， which was hydrothermally crystallized at 180 oC 企omthe co-precipitates formed企omthe preα江sorsolutions of TiOS04 and SC(N03)3 in the presen印 ofaqu巴ousanJ.TIlom弘was confirmed to be solid solutions doped with scandium 2 9 ⑧TiOianatase) (@同国園田両)

金

判

明

君

富

岡

1

0

2

0

3

0

4

0

5

0

6

0

29?CuK

α

， Fig.1.XRD pa杭巴:msof precipitates obt組ledfrom the solutions of TiOS04 and SC(N03)3 in the presence of aqu回usammonia at the starting composition of 5 mol% scandium before and after hydrothermal treatment at various temperatures for 5 h. Fig. 2. TEM micrographs of precipitates cont加古19(a) 0， and (b) 5 mol% scandium obtained under hydroth巴:rmalconditions at 180 oC for5 h Figure 2 shows τEM micrographs of the ana旬se勾peTi02 precrp託atescontaining 0 and 5mol% scandium. The precipitates

show the morphology of spindle-like particle. The particle size was observed to increase when the scandium content was increased

The crystallite size was almost constant value of 23-25 um ag♂nst mcreas巴dscandium conten.tThe crystallite sizes of anatase

巴stimated合omthe line broadening of the XRD pe北werenoticed to be small in comparison with the p副iclesizes observed企om TEM micrographs. The spindle-like particles seem to be made of small anatase crystallites

3.2. Niobium-dopedit回 liasolid solutions The e島ctof the niobium content on由 巳 紺uctureof solid precipitates is shown as XRD p胡emsinFig. 3. The c巧Tstalline phase detected in all the as-prepared solid precipitates is only anatase-句'peTi02. No di飴actionpeaks due to other crystalline phases were detected H C H O

。

Anatase 口 。 司 ) H H 門 A O

m

-- o

c c N O

h H

a d

g s

10 20 30 40 50 60 28/" CuKα F

氾

3.XRD pa抗emsof precip削 esobtained under hydrothermal conditions at 1800C for 5 h Fig. 4. TEM micrograph of precipitates containing 10 mol% niobium The crystallite size of anatase in the sarnples estimated企omthe XRD line broadening of th巴101 and 200 pealcs gradually increased

企om8.5 to 17-19

nm

when the niobium content was increased TEM image of the as-prepared anatase sarnple doped with niobium is shown in Fig. 4. The p副iclesize of anatase increased when the

niobium content was increased The lattice pararnetersaoandCo gradually increased when the niobium content was increased up to around 20 mol% niobium in the samples. A metastable crystalline phase， anatase that was formed directly企omthe precursor solution of TiOS04 andNbCls in the presence of urea is considered to be making solid solutions with niobium.As the anatase-type solid solutions were fonned by substituting for tit削 叩1site by niobium with a slightly larger ion radius than that of titaniumヲ expected increase in the lattice pararneters of anほasewas observed According to the gradual increase of the crystall白 sizeof anatase， the specific surface area decreased企om142 to 100 m2_/g with increased niobium content up to 20 mol%，乱lthoughits value remained around 140-150 m2_{/g and a significant change in the} specific surface area was not observed up to 7.5 mol% niobium.By con甘ast，the specific surface area ofthe sarnple containing 30 mol% niobium increased to be about 160 m2_{/g regardless of increased} crystallite size of anatase， which may suggest that the possibility of the presence of very fine substances with high specific surface area coexisted with crystalline anatase p紅ticles.The high speci五csurface area for the sarnple containing 30 mol% niobium is considered to be due to the niobium

∞

mponent that could not form solid solutions with Ti02， which is suggested from the saturation of increase in the

la社lcep訂 釦letersat the compos託ionof30 mol% niobium

回 心 ︿ Nb Omol% Nb 2.5mol% Nb 5mol% Nb 7.5mol% Nb 10mol% Nb 20mol% Nb 30mol% 300 400 500 600 700 800 Wavelength

/

n

m

Fig. 5. Di血Jsereflectance spectmm of as-prepared anatase-type Ti02 containing various amounts of niobium Figure 5 shows diffuse reflectance spec加m of as回prepared 釦atase-typ巴Ti02containing various arnounts of niobium.V¥なlen the niobium content was increasedヲonsetof absorption in the di伍lse reflectance spectra of the as回prep訂edniobium-doped Ti02 slightly shifted to longer wavelengths. The band-gap value of anatase slightly decreas巴dwhen above 2.5 mol% niobium was doped. It

hardly changed overthe compositional range 2.5-30 mol% niobium In the present sωdy， the adsorptivity and photoca凶yticactivi守 of the sarnples were evaluated separatel

y

.

A食ercompletion of the adsorption ofl¥侶bythe sarnple powders during the maintenance of the solution in the必rkfor 24 h under stirring， UV light was irradiated. The degree of change in the concentration ofl¥侶with time in theぬrkshows that the adsorption of M B by the samples increased when the niobium content increased. The behavior of d巴crease m 恥侶 concen仕ation by photooxidation under UV irradiation suggests that photocatalytic activity is gradually increased with increase in niobiumιontent in the anatase-type powde

.

r

Figure 6 shows the plots of -hl(C/Co) versus irradiation time， 組d they demonstrate that l¥侶 photodegradation follows the psudo-first-order kinetics with resp倒 toM B concentration. The slope of the straight lines is observed to be gradually steep with increase in niobium content up to 15 mol%. The rate constant

80 愛知工業大学総合技術研究所研究報告，第 9号， 2007年 app紅巳ntlyincr巴asedwith increased niobium content in the sample up to 15 mol% niobium 3.5 3町O ーぐ)..NbO mol% 合 一Nb2.5mol%

_

_

，

，

_

_

Nb 5 mol%

。

Nb7.5 mol% 一 畳 一Nb10 mol% B -Nb 15 mol% 一 世 「 一ST】01 / / 園 ..O 2.5 k

=

0.79 q v ヴ f k υ A ﹃ nunu

- - 一

-じ K L K 圏 / O ' 口 U 2.0 ち にJ ) E 1.5 / 口/ 1.0 ! k

=

0.31 0.50 0.0輩丘三五

。

3 2 4 5 6 Irradialion lime / h Fig. 6. The photocata1ytic d巴gradationof M B as aおnctionofUV irradiation time and日rst-ord巴r r:ate constants for as-prepared anatase-typ巴Ti02containing niobium Inthe prese凶 study，the combination of photoactiv町 with adsorptivity was achieved in a single anatase material by the formation of solid solutions with niobium 0沼 化 釦dby the direct formation tecln

明

u巴 usingthe mild hydroth巴:rmalmethod. The increase in the photocata1ytic activity of th巴 sarnples may be attributed to the increased surface acidity that is based on the solid solutions in the system Ti02-Nb20s [33] 3.3.同研1iasolid solutions co-dopedwith niobium andαluminum A mixture of an aqueous solution of TiOS0，4A1別03，3)and ethanol solution of NbC15 in different ratios of Tのめ/A1was prepared in a Te宜oncontainer. This solution mi双urein different ratios ofTの.fu/A1with tota1meta1cation concentrations (Ti

+

Nb

+

A1)ofO.5 mol/dm3 _{added with suitable arnount ofthe urea solution} in the Teflon container was heated at 1800C for 5 h Fig.7τEM image of precipitates with composition of X = 0.15 in Til_2XNbxA1x02

The TEM image of as-pr巴:paredsarnple is shown in Figure 7 XRD pa:社emsof as-prepared s但l1plesthatWI巴reprecipitated at various compositions(X = 0 -0.25 in Til-2X

N

b

x

A

1

x02) showed that the cl)'sta11ine phase detected in the as-prepared precipitates (Xく0.20)was anatase. The cl)'sta1lite size of anatase formed at the compositions X = 0 - 0.15 was 12 -20 nm. The ana1ytical

∞

mpositions of the as-prepared sarnples estimated using an ICP emission spectrometer relatively well coincided with the starting compositions ofthe sarnples The corresponding change in the lattice pararneters

a

o

and

c

o

of the as-prepa吋 te仕agonalanatase-type Ti02ラ asdetermined via XRD using silicon as the intema1stand昌rdヲrelativeto the va1ue ofX， is shown in Figure 8. 0

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1

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5 0.20 X Fig. 8. Lattice pararneters of anatase-type Ti1_:0"NbxA1X02 prepared at 180 oC for 5 h Replacement of Ti4+ by Nb5十andAl3₊ can maintain the charge balance of Ti02 solid solutionsヲwhichis the key factor for the difference between niobium doped titania and a1uminum and niobium co-doped one.Inthe present s旬dy，印刷Y15-20 mol% niobium anda1uminum were con:firmed to be co-doped into anatase-type structure under hydrothermal conditions at 1800C In th巴dark 0.060 UVi汀adiation

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0.000 0 5 10 15 20 25 30 Time/h Fig. 9. Change in l¥侶concentrationof the solution with time in the dark and under UV irradiation InFig. 9， the effect of the composition of anatase-勿pe Th-2X

N

b

x

A

1

x02 solid solutions on the adsorptivi勿 and photo叫a1yticactiv町isshown for the sarnples and reference pure Ti02 powder ST-Ol as changes in the concentration of l¥在Bwith time in the dark and under UV irradiation. The l¥侶 adso叩tlOn

increased when the value ofX increased toX~.20 exc巴:ptthe sampleX~.05. Especially high adsorption appeared in the sample X=0.20 is considered to relate to its high specific surface紅ea Anatas巴-typeTi02 nanoparticles co-doped with niobium and aluminum were directly formed under mild hydrothermal conditions using the hydrolysis ofurea. The anatase-type Til-2XNbxAlx02 solid solutions (X = 0.10-0.20) showed improved photocatalytic activity 4. Summary Direct formation of anatase phase of scandium-doped and niobium-doped titania and Til_2XNもχAlX02solid solutions匹 = 0.10-0.20) was performed as nanosized-particles from the prec附 or solutionslIDder mild hydrothermal conditions. The spindle-like particles of scandium-doped titania with small anatase crystallit巴B W巴re formed by hydroth巴:rmal crystallization at 180oC. Th巴 formation of solid solutions with niobium0刻 化 foran削 除type Ti02 was e能ctivefor the巴 出ancementof photo-d巴compositionof M B lIDd巴rUV irradiation and adsorptiv町 intheぬrk.The anatase-type Ti02 nanop征ticles co-doped with niobium and

aluminum σ11_2XNbXAlX02 (X = 0.10-0.20))showed irnproved

photocatalytic activity Aclrnowledgement The author thanks ProfM. Inagald and D

.

r

H.Iwata of Aichi Institute of Technology for useful discussions and TEM observationsヲrespectively References 1. A Fuiishima andK.HondaヲNatur官(Lo政わF抗238(1972) 37-38 2. M. A Fox andM. T.D叫ay，Chem. Rev.，見(1993) 341-357 3. M. HiranoヲRecentRes.‘Devel司MatSci.， 3， Edited匂S G P叩dalai，Reseαrch Signpos，t Trivandrum， Indiα(2002) 563-598 4. W J.DawsonヲAm.Ceram. Soc. Bull. 67 (1988) 1673-1678 5. M 回ranoand K 回rai，J Nanoparticle Res.， 5 (2003) 147-156 6. M団rano，JMa的 :Chem. 10 (2001) 469-472 7. M. 回ranoフS.Okumur

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