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The Crystallization of Anatase and Rutile from Amorphous Titanium

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The Crystallization of Anatase and Rutile from Amorphous Titanium AmericanMineralogist, Volume61, pages419424, 1976 The crystallizationof anataseand rutile from amorphoustitanium dioxideunder hydrothermal conditions Ar-eNMerrsEws Departmentof Geology,The Hebrew Uniuersity Jerusalem.Israel Abstract The hydrothermal crystallizationof amorphous titanium dioxide at 300-600'C, I kbar pressure,follows the reaction path: amorphous reactant - anatase- rutile. The transforma- tion is acceleratedby increasingtemperature and decreasingsolution pH. Two mechanism stagescan be envisaged:(1) the formation of anatase by the dehydration and gradual structural rearrangementof the reactant titanium-oxygen lattice, and (2) solution of the anataseintermediate and direct precipitationof rutile. The metastabledevelopment of anatase is in accordancewith a modern formulation of the Ostwald Step Rule. Introduction ride until the pH of the solution was in the range l-4. The kinetics and mechanismsof the solid state The resulting dense white precipitate was washed transformation anatase+ rutile have beenwell stud- thoroughly and dried under infrared heat. X-ray dif- ied (Czandernaet al., 1958;Rao, l96l; Shannonand fraction indicated the material to be completely Pask, 1965;Heald and Weiss,1972). However, less amorphous. Experimentswere performed at I kbar attention has been paid to the relationshipsexisting pressurein cold sealhydrothermal bombs, with reac- betweenthe two polymorphs in hydrothermal aque- tants sealedinside silver capsules.Charges consisted ous conditions.Osborn (1953)found that high tem- of 50 mg solid and 500-600 mg solution. Products perature, and to a lesser extent pressure increase, were examinedby X-ray diffraction, and where pos- favor the conversionof anataseinto rutile. Dachille sible (at 600"C only) anatase-rutileproportions were et al. (1968) hydrothermally crystallized titanium estimatedby the method of Spurr and Myers (1957), dioxide gel, obtaining anataseat lower temperatures Solutions used were sodium carbonate, water, and and rutile at higher temperatures. Mineralizing hydrochloric acid. Tests with pHydrion papers agents, such as alkali metal fluorides, catalyze the showedthat the solutionsunderwent a slight increase formation of rutile from gel or anatase (Osborn, in alkalinity during reaction.This presumablyarises 1953;Anikinand Rumyantseva,1964). Jamieson and from the releaseduring crystallization of ammonra Olinger (1969)showed that anataseis thermodynam- trapped in the amorphous reactant during its prepa- ically always unstablewith respectto rutile. Thus the ration. No evidencecould be seen of reaction be- persistenceor formation of anatase under hydro- tween the silver capsulesand the acid solutions.The thermal conditions should be regarded as a con- solution conditionsgiven in Table I detail both initial sequenceof sluggishreaction kinetics or metastable concentrationsand final pH values. crystallization.Correspondingly, the formation of ru- tile from gels or anataseis catalyzedby increasing Experimental results temperature, pressure,and the presenceof miner- The resultsof crystallizationexperiments are given alizing agents.The purpose of this paper is to in- in Table l. It can be seenthat neutral, alkaline, and vestigatethe conditions and mechanismsof anatase mildly acidic conditions favor anatase formation, and rutile formation during the hydrothermal crystal- whereasmore strongly acid environmentsfavor ru- lizationof amorphoustitanium dioxide. tile. This is clearly indicated by the results of long experimentsperformed at 500'C. In sodium carbon- Experimental methods ate, water, and 0.01 M HCl, anatase is formed, Amorphous titanium dioxide was preparedby the whereasin 0.1 M and 0.5M HCI the productis rutile. addition of 0.88 sp gr ammonia to titanium tetrachlo- Increasingtemperature also enhancesrutile forma- 419 420 ALAN MATTHEWS Tesr-s l. Resultsof crystallizationexperiments ments.These observations are consistentwith the transformation: Solutions Toc** t. h"r* products Initial Final pH amorphoustitanium dioxide + anatase+ rutile The datagiven in TableI indicatethat therate of the 1.0M.HCI 0.0 304 68 Rutile transformationis acceleratedby increasingtemper- 406 0.4 Rutil e atureand decreasingsolution pH. 0.5 M.HCt 0.4 500 0.4 Rutile,smalIanatase The proposedreaction mechanism is analogousto 502 204 Ruti I e that observedfor the hydrothermalcrystallization of 0.I M.HC] 305 0.3 Anatasedevelopinq 295 1.5 Anatasedevelobin; amorphoussilica, in whichthe polymorphs cristobal- av2 3 Rut'ile,pooranitaie ite and occasionallysilica K precedethe formation z9a 14 Rutile,smallanatase? of 304 68 Rutil e the thermodynamicallystable phase, quartz. It is 404 0.3 Pooranatase,small ruti'le found that the rate of quartzformation is accelerated 398 72 RutiI e by increasingtemperature and pressure(Carr 500 0.4 Ruti1e,sma11anatase and 501 276 Ruti l e Fyfe,1958), but in contrastto theabove observations 597 264 Ruti l e the reactionis catalyzedby alkalinesolution species 0.01M.HCI 2.8 404 168 Anatase (Campbelland Fyfe, 1960;Fyfe and McKay, 1962; s05 235 Anatase Matthews, 1972).Campbell and Fyfe relatedthe al- 597 264 Anatase(75%),rutile(25%)kali catalysisto the natureof the silicatesolution Water 8.8 302 0.2 Anatasedevelopinq species,and presumablythe differencesnoted reflect 295 3 Anatasedevelobini 304 68 Anatase 410 0.3 AnatasedeveloDinq 401 188 Anatase A s02 0.4 Anatase 1o 501 216 Anatase 597 264 Anatase(90%),ruti'le(10%) 0.1 M.Na2C03 500 0.4 Anatase 505 235 Anatase * Timeat temperature,ie. after completionof run up **Temperature t5oc tion; crystallization of the amorphous reactant in water and 0.01 M HCI at 400.C and 500.C yields anataseonly, but at 600oCrutile appearsin the prod- ucts. A third naturally occurring polymorph, brook- ite, is not found in any experimentalproducts. 1c R The mechanismof reaction is illustrated by X-ray R diffraction patterns obtained from the products of reactionsat 300'C using 0.1 M HCI (Fig. l). After l7z hours reaction anataseonly is developing (Fig. la), but after 3 hours reaction the poorly developed anatase has diminished and",rutile is forming (Fig. lb). Fourteenhours reactionyields rutile, with per- haps a small amount of anataseremaining (Fig. lc), and 68 hours reaction gives rutile only. Similarly, 10 35 32 28 21 other short experiments at 400'C and 500.C using 20 C" (Ka) 0.1 M and 0.5 M HCI indicate that anataseprecedes Ftc. I X-ray diffractionpatterns of products the formation of rutile. Reactionsin water at 300.C, from crystalliza- tionexperiments at 300oCusing 0.1 M HCl. (a) 1.5hours reaction. 400oC,and 500oC,in which anatase is the only prod- (b) 3 hoursreaction. (c) l4 hoursreaction. Legend: A=Anatase, uct, also show anatase developing in short experi- R:Rutile CRYSTALLIZATION OF ANATASE AND RUTILE FROM AMORPHOUS TITANIUM DIOXIDE that titanium-oxygen speciesare acid- rather than alkali- stabilized. The developmentof phasesas indicated by X-ray diffraction revealsa dissimilarityin the specificmech- anismsof anataseand rutile formation. The develop- ment of anataseis first indicatedby the appearanceof 'hump' a broad in the 20 region of the anatase l0l peak. For reactionsin water (and presumablyalso 0. I M Na,CO, and 0.01 M HCI) this hump gradually developsand sharpens(see Fig. la) untilfully crystal- line anataseis indicated.For reactionsin 0.1 M and 0.5 M HCl, rutile growth commencesbefore full crys- talline development of the anatasecan occur (Fig. lb). However, the rutile gives well defined X-ray peaks,even when presentin small amounts;i.e. in the earlystages of development(Figs. I b and lc). Similar observationscan be made for the silicic acid crystalli- zation (cf. Carr and Fyfe, 1958,Fig. l). The appear- ance of well-orderedrutile is consistentwith its for- matlon occurring through a solution and Ftc. 3. SEM micrograph showing gross morphology of an precipitation mechanism.However, anataseforma- anatasegrain. Product from 235 hours reaction at 505'C using 0.01 M HCl. tlon may occur by the gradual structural rearrange- ment of the titanium-oxygenlattice of the amorphous vestigatethese mechanism proposals. Representative reactant, with accompanying dehydration. Dehy- micrographsare given in Figs.2-6, and the following dration may precede the structural rearrangement. observationscan be made: The isotopic studiesof Clayton et al. (1972)on silicic (l) The gross morphologiesof the reactantgrains acid crystallizationshowed that dehydration occurs are preservedin the product anatase(Figs. 2 and 3). before any loss of amorphous character can be de- (2) The large,fully crystalline,anatase grains con- tected in the reactant. sist of a mass of reasonablyequidimensional small Scanning electron microscopy was used to in- crystals(Fig. a). (3) The grossmorphologies of rutile grains do not possessthe similaritiesto the reactantthat are shown by anatasegrains (Figs. 2,3, and 5). (a) The rutile grains consist of a mass of small crystals.However, thesecrystals are much more in- tergrown than anatasecrystals (Figs. 4 and 6). Observations(l) and (2) are consistentwith the idea that anataseforms by the dehydration and grad- ual structural rearrangementof the reactant lattice. Small volumes of reactant appear to be forming into discrete fine anatasecrystals, with the gross mor- phology of the reactant being preservedin the prod- uct. The loss of water from the reactantduring dehy- 'space' dration potentially provides in which the lattice rearrangementscan occur. The role of the aqueous solution may be to catalyze the rearrange- ment by aiding the disproportionation
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