CrystEngComm

PAPER

Cite this: CrystEngComm, 2018, 20,

487

Received 16th October 2017,Accepted 8th December 2017

DOI: 10.1039/c7ce01807j

rsc.li/crystengcomm

Understanding the role of oxygen ion (O2−) activityin 1-D crystal growth of rutile TiO2 in molten salts

Anteneh Marelign Beyene,a Changyeon Baek,a Wook Ki Jung,a

P. Ragupathy ab and Do Kyung Kim *a

Controlled synthesis of nanostructured materials using facile and easily scalable synthesis techniques is

highly attractive for large-scale production of nanomaterials. In this regard, molten salt synthesis is a well-

established technique for large-scale production of nanostructured materials. Few reports have demon-

strated the applicability of the molten salt technique for high-aspect-ratio one-dimensional rutile TiO2 syn-

thesis. However, the crystal growth mechanism of 1-D TiO2 in the molten salt is not well understood. Here,

various sets of experiments have been delivered to investigate 1-D rutile TiO2 crystal growth starting from

anatase TiO2 precursors in molten NaCl with the presence of various inorganic oxy-additives. It was found

that the oxygen ion (O2−) activity of the molten salt matrix, which can be controlled by oxy-additives, is the

decisive factor for the formation of the 1-D structure. The (NaPO3)6 additive, which reduces the O2− activity

of the molten salt matrix, increased the solubility of anatase TiO2. The increased solubility facilitates the

easy mobility of ions to the growth site where the crystallographic surface energy is high. The natural ten-

dency to minimize the total surface energy causes the crystal to grow in a particular direction, which even-

tually leads to 1-D rutile TiO2 nanoparticles.

Introduction

Tailoring the phase and morphology of materials with thehighest precision is highly desirable for various applications.There has been a lot of interest in elucidating the relationshipbetween phase/morphology and material characteristics. Oneintriguing field involves developing one-dimensional (1-D)nanostructures such as nanowires,1–3 nanofibers,4–6

nanobelts,7–9 and nanorods.10–12 The unique 1-D confinedtransport of electrons and phonons, the high surface area-to-volume ratio, and the excellent mechanical properties thatcan be achieved from such structures make them attractive invarious fields.13–17

Solution-based synthesis techniques are the most favor-able approaches to achieve 1-D nanostructures of oxide mate-rials. They offer the flexibility of using various precursors,along with organic additives (surfactants), to direct the crystalgrowth. The most probable mechanism of crystal growth insolution-based synthesis is dissolution and re-precipitation,for which the solubility of the precursors is crucial.14,18 In hy-

drothermal synthesis, high temperature and pressure areemployed to increase the solubility of precursors. In somecases, mineralizers are used to increase the solubility of theprecursors when it is not sufficient under normal hydrother-mal conditions.19–23 However, there is still a large group ofoxides that cannot be solubilized under ambient or hydro-thermal conditions.

Molten salts are capable of solubilizing a wide variety ofmetal oxides owing to their strong ionic nature. Even com-plex oxides that are insoluble under typical hydrothermalconditions can be solubilized in molten salts. Consequently,the molten salt synthesis technique has been widely adoptedto synthesize various complex oxides.14,24,25 Nevertheless, sur-factants cannot be utilized to modulate the morphology inmolten salt synthesis, unlike in solution-based syntheses in-cluding hydrothermal synthesis techniques. The nature ofsurfactants makes them unstable at the very high operatingtemperature of molten salts, resulting in their decompositionor oxidation.14 Hence, the 1-D crystal growth of metal oxidesin molten salts is largely dependent on the natural tendencyof the crystal to grow anisotropically in such environments.

Titanium dioxide (TiO2) is a much explored transitionmetal oxide due to its abundance, unique physicochemicalproperties, moderate cost, and non-toxicity. There are a largenumber of reports on the synthesis of 1-D TiO2 in its differentpolymorphs, using a variety of synthesis techniques for a widevariety of applications.15–17 Among the various techniques,

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aDepartment of Materials Science and Engineering, Korea Advanced Institute of

Science and Technology (KAIST), Yuseong-gu, Daejeon 34141, Republic of Korea.

E-mail: dkkim@kaist.ac.kr; Fax: +82 42 350 3310; Tel: +82 42 350 4118b Electrochemical Power Sources Division, Fuel Cells Section, Central

Electrochemical Research Institute, Karaikudi-630 003, India

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molten salt synthesis is widely accepted as a promising routefor TiO2 synthesis. Few studies have reported the synthesis ofmicrometer-sized, high-aspect-ratio, single-crystal rutile TiO2

from metastable anatase TiO2 or amorphous TiIJOH)4 (rutileseed in most cases) using the molten salt synthesistechnique.26–31 In the reports, NaCl was used as the moltenmedia, either alone or with other salts that provide a eutecticcomposition. Dibasic sodium phosphate (Na2HPO4) is theother salt that has commonly been used together with NaCl inthe majority of the reports.26,27,29,31 In some of the reports,the 1-D rutile TiO2 growth is attributed to the formation of anintermediate tetrasodium titanium nona-oxodiphosphate(Na4TiP2O9) phase (from the precursor) and Na2HPO4.

27,29 It isa well-known strategy to grow 1-D nanostructures using natu-rally anisotropic intermediates that can further be treated togive the desired material.32 However, to the best of our knowl-edge, the intermediate Na4TiP2O9 in these reports is notknown for its natural tendency to grow as a 1-D structure.Moreover, other reports demonstrated the synthesis of 1-D ru-tile TiO2 in molten NaCl without Na2HPO4.

28,30 This suggeststhat the role of Na2HPO4 might not be as depicted by thereports.

In this study, the synthesis of a high-aspect-ratio 1-D rutileTiO2 from an anatase TiO2 precursor, rutile TiO2 seed, andNaCl salt with sodium hexametaphosphate additive[(NaPO3)6] has been demonstrated. A more plausible explana-tion is herein proposed for the roles of phosphates in a mol-ten NaCl salt by analyzing the chemical activity of differentinorganic additives. The advantage of using (NaPO3)6 specifi-cally, instead of other phosphates, for the synthesis of high-aspect-ratio 1-D TiO2 is also explained. The growth trend hasalso been shown by tracing the evolution of the phase andmorphology during the synthesis.

Experimental procedure

Rutile TiO2 (99.995% trace metal basis), anatase TiO2

(≥99%), sodium chloride (NaCl, ≥99%), sodium nitrate(NaNO3), sodium carbonate (Na2CO3), sodium sulfate(Na2SO4), sodium hydroxide (NaOH), and sodium hexameta-phosphate ((NaPO3)6, 96%), all of which were from SigmaAldrich, were used for the experiments. TiO2 (1 : 10, rutile-to-anatase weight ratio), NaCl, and additives were mixed in a 4 :8 : 1 weight ratio using wet milling for 24 h, where ethanolwas used as liquid medium and zirconia balls were used formilling. A control sample was also made from TiO2 (1 : 10weight ratio, rutile to anatase) and NaCl in a 1 : 2 weight ratiowithout using any additive. The mixed powder ethanol sus-pension was then evaporated at ~80 °C while being stirred ona hot plate. The semi-dried powder was then placed in a dry-ing oven at 80 °C overnight to obtain the final dried powdermixture. The mixture was then formed into pellets and heat-treated in a box furnace at 800 °C for 4 h. For the growthtrend analysis, the samples were treated at 785–800 °C for0–30 min. The heat-treated pellets were then boiled in de-ionized (DI) water to leach out the NaCl and additives. The

final powder was obtained after drying the leached outpellets in an oven at 80 °C overnight and grinding the driedpellets using a mortar and pestle.

The morphology of the synthesized powders was analyzedusing a field-emission scanning electron microscope (FE-SEM; XL 30, Philips, The Netherlands), and their correspond-ing phases were analyzed using high-resolution X-ray diffrac-tion (XRD; Rigaku D/Max-RB, 12 kW, Japan) with Cu Kα radi-ation (λ = 1.5148 Å) operating at 40 kV and 300 mA. TG-DSCanalysis was performed using thermogravimetric analysis anddifferential scanning calorimetry. For the TG-DSC analyses,the mixed powder was placed into the instruments andheated to 800 °C from room temperature at a rate of 10 °Cmin−1. High-resolution TEM (HR-TEM) analysis was carriedout using a transmission electron microscope (TEM; model:Tecnai G2 F30 S-Twin, FEI, Eindhoven, The Netherlands) with300 kV accelerating voltage.

Results and discussion

The phase purity analysis of the samples synthesized usingdifferent oxy-additives was carried out using the powder XRDanalysis method. Accordingly, Fig. 1 shows the XRD patternsof products obtained in molten NaCl from an anatase TiO2

precursor using different oxy-additives. All the XRD diffrac-tion peaks of the samples obtained from using sodium ni-trate (NaNO3), sodium carbonate (Na2CO3), and sodium hy-droxide (NaOH) additives match with the sodium titanate(Na2Ti6O13) phase (JCPDS #01-080-5525) as shown inFig. 1(a–c). The use of a sodium sulfate (Na2SO4) additive,on the other hand, results in a powder with mixed phases(major rutile TiO2, JCPDS #21-1276, with minor anataseTiO2 impurities, JCPDS #21-1272, and Na2Ti6O13) as shownin Fig. 1(d). Pure rutile TiO2 is obtained only in the cases ofusing a sodium hexametaphosphate [(NaPO3)6] additive andusing no additive [Fig. 1(e) and (f)].

Fig. 1 XRD patterns of the samples synthesized in molten NaCl froman anatase TiO2 precursor using different additives: a) NaNO3, b)Na2CO3, c) NaOH, d) Na2SO4, e) (NaPO3)6, and f) no additive (controlsample).

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The morphologies of the samples synthesized using thedifferent oxy-additives were analyzed by using SEM with theaim of determining the additives' effect on the morphology.Accordingly, Fig. 2(a–f) show the resulting micrographs fromthe SEM analysis. Powders obtained from using sodium ni-trate (NaNO3), sodium carbonate (Na2CO3), and sodium hy-droxide (NaOH) additives (all in the Na2Ti6O13 phase) exhibit1-D morphology as shown in Fig. 2(a–c). However, the sizeand thickness of the particles in each case are different fromeach other. Small nano-sized rods together with fewmicrometer-sized larger rods and some broken particles areobservable in the case of the powder obtained from using theNaNO3 additive (Fig. 2a). The powder obtained using theNa2CO3 additive shows a relatively homogeneous, fewmicrometer-sized rod morphology (Fig. 2b), whereas mainlyshorter and thicker rods are observable in the case of usingthe NaOH additive. On the other hand, the powder obtainedfrom using the Na2SO4 additive is composed of a mixed mor-phology (its phase is also mixed). The majority of the parti-cles are few micrometer-sized equiaxed particles with fewrods and nanoparticles being together. In the case of usingthe (NaPO3)6 additive, high-aspect-ratio and relatively longer

rods are obtained as can be observed from Fig. 2e. The mor-phology of the powder has become a mixture of submicron-sized equiaxed particles and few low-aspect-ratio rods for thesample synthesized without an additive (Fig. 2f).

It is apparent from the results that the use of an appropri-ate additive is important to obtain the anticipated phase andmorphology. High-aspect-ratio 1-D rutile TiO2 was onlyachieved with the use of the (NaPO3)6 additive. Table 1 showsthe comparison of other crystallographic surfaces with re-spect to the most abundant (110) surface of rutile TiO2. The(110) crystallographic surface is much more abundant in thecase of the sample synthesized with the (NaPO3)6 additivethan those of the control sample and the sample synthesizedwith the Na2SO4 additive. This clearly indicates that (NaPO3)6

Fig. 2 SEM micrographs of samples synthesized in molten NaCl from an anatase TiO2 precursor using different additives: a) NaNO3, b) Na2CO3, c)NaOH, d) Na2SO4, e) (NaPO3)6, and f) no additive (control sample).

Table 1 Relative intensity of major crystallographic surfaces of rutileTiO2 synthesized using (NaPO3)6, Na2SO4 and no additives

Additive I(101)/I(110) I(111)/I(110) I(211)/I(110)

— 0.59 0.26 0.60(NaPO3)6 0.16 0.10 0.34Na2SO4 0.58 0.30 0.70

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has a decisive role in obtaining the high-aspect-ratio 1-Dmorphology and the rutile phase.

Understanding the fundamental mechanism that resultsin different phases and morphologies of the final powderdepending on the nature of additives used is crucial to theability to fine-tune the phase and morphology as desired. Ithas been reported that the solubility of the precursor in mol-ten salts is one of the most important factors that determinethe phase and morphology of the final powder.14 Just as thesolubility of oxides under hydrothermal conditions can be en-hanced using mineralizers (strong acids, strong bases, orcomplexing agents), the solubility of oxides in molten salts isalso enhanced by the Lux–Flood acid–base interaction.25

Table 2 shows the reaction behavior of the different additivesin the molten environment and the corresponding effect onthe phase and morphology of the final powder. NaNO3 andNa2CO3 decompose at a relatively lower temperature and re-sults in a strong Lux–Flood base (Na2O), which readily pro-vides O2− ions to the system, thereby increasing its O2− ionactivity.14,33 Similarly, NaOH dissociates in the molten salt togive a strong Lux–Flood base (OH−) that also increases theO2− activity in the system. The increment of O2− activity inthe system results in the solubility of anatase TiO2 in the

form of TiO32−, which eventually yields stable Na2Ti6O13. On

the other hand, the PO3− ion, which is a strong Lux–Flood

acid,34 depletes the O2− ions and reduces their activity in thesystem. The decrement of oxygen activity in the system cre-ates a reducing atmosphere which causes the removal of oxy-gen from the TiO2 crystal lattice. Consequently, the metasta-ble anatase TiO2 nanoparticles become unstable and dissolvein the molten salt. Since the anatase-to-rutile phase transfor-mation is a reconstructive type (which involves bond break-age and rearrangement), the removal of oxygen from the crys-tal lattice of TiO2 is favourable for this phase transformationas it facilitates the restructuring of atoms. The rutile seeds re-main stable in the molten salt and serve as nucleation sites,thereby avoiding the high activation barrier of homogeneousnucleation.26 The PO3

− ion converts to stable PO43− after tak-

ing up O2− from the surroundings. Na2SO4, on the otherhand, has no effect on the O2− ion activity because it is stableunder the synthesis conditions. Thus, the resulting powderbecomes similar (in phase and morphology) to the controlsample, except that the particles are larger and that there areanatase TiO2 (nanoparticles) and titanate (nanorods) impuri-ties. The anatase TiO2 impurities could be the result of theretarding effect of the SO4

2− ion on the anatase-to-rutile

Table 2 Expected reaction with different additives in the molten salt and their effect on the phase and morphology of the final powder

Sodium oxo-saltadditives Expected reaction

O2−

activityLux–Flooddefinition Aspectratio Phase

NaNO3 Increase Base 6.2 Na2Ti6O13

Na2CO3 Na2CO3 → Na2O + CO2 Increase Base 8.1 Na2Ti6O13

NaOH NaOH → Na+ + OH− Increase Base 4.1 Na2Ti6O13

Na2SO4 Na2SO4 → 2Na+ + SO42− Neutral Neutral — TiO2 (rutile), Na2Ti6O13, TiO2 (anatase)

(NaPO3)6 2PO3− + 2O2

2− → 2PO43− + O2 Decrease Acid 15.3 TiO2 (rutile)

Scheme 1 A simplified representation of the role of oxygen ion (O2−) activity in the molten media in the phase and morphology of the finalnanocrystal.

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phase transformation,26 whereas the titanate impuritiescould be the result of the decomposition of few Na2SO4 mole-cules (which can acquire the necessary activation energy fordecomposition) to the Lux–Flood base (Na2O). A simplified il-lustration of the role of oxygen ion (O2−) activity in the mol-ten salt in the phase and morphology of the final powder ispresented in Scheme 1.

To confirm whether the reaction has proceeded asdepicted by the overall reaction (Table 2) or not, TG-DSCanalysis was carried out for the sample with the (NaPO3)6 ad-ditive. The result of the TG-DSC analysis is presented inFig. 3a. As can be seen in the DSC curve, a major endother-mic peak is observed at 792.9 °C, which corresponds to themelting of the salt matrix. Two minor endothermic peaks ataround 295 and 50 °C are also observed and they correspondto 0.53 and 0.51% mass losses. Because (NaPO3)6 is a hygro-

scopic material, the probable reason for the minor endother-mic peaks could be the desorption of physically adsorbed wa-ter molecules and evaporation of trace amounts of water,which could then incorporate into the crystal lattices of thepowder. From 295 °C to the onset of melting, no significantendothermic or exothermic peaks are observed except for asmall endothermic hump and corresponding 1.3% mass loss.From the lowest point of the endothermic hump to the pointcorresponding to the total melting of the salts, the mass lossis ~1.26%. According to the overall reaction (Table 2), the oxy-gen (O2) molecule is the only matter that leaves the systemand is thus responsible for the mass loss. The mass percentof O2 molecules leaving the system based on calculationsfrom the stoichiometric equation is ~1.2%. The calculatedmass loss is in good agreement with that obtained from theTG-DSC curve. In addition to that, Na3PO4 peaks are clearlyobservable from the XRD patterns of the final unwashed sam-ple synthesized with the (NaPO3)6 additive (Fig. 3b), furthersupporting the validity of the overall reaction.

Other phosphates have also been analyzed for their effecton the anisotropic crystal growth of rutile TiO2. Theirstrength as a Lux–Flood acid, which is proportional to theirreactivity with oxygen ions, and the corresponding effect onthe aspect ratio of the final powder are represented inTable 3 and Fig. 4. The result shows that PO3

−, which is thestrongest Lux–Flood acid among the phosphates, gives veryhigh-aspect-ratio particles; whereas, P2O7

4− and HPO42−, with

relatively lower reactivity, result in particles with a lower as-pect ratio. The reactivity of P2O7

4− and HPO42− is comparable;

correspondingly, the aspect ratios of the final powders forthe two cases are also comparable. These results indicate thatthe degree to which the oxygen activity decreases (the reactiv-ity of phosphates) can be related to the degree of the aniso-tropy of the particles, which affects the aspect ratio. In thecase with a highly reactive phosphate, the solubility of theprecursor (anatase TiO2) is much higher than with the lessreactive phosphates. The higher solubility leads to the easymobility of the growth unit to the most active growth siteswhere the crystallographic surface energy is high [(001) crys-tallographic surface of rutile TiO2].

35 The natural tendency tominimize this high crystallographic surface energy leads tothe 1-D crystal growth of the resulting rutile TiO2 nano-particles in the [001] crystallographic direction. For the lessreactive phosphates, however, the solid-state phase transfor-mation becomes a significant part of the phase

Fig. 3 a) Thermogravimetric (TG) and differential scanning calorimetry(DSC) curves of the mixed powder undergoing heat treatment at 800°C for 1 h; b) XRD pattern of an unwashed sample synthesized usingthe (NaPO3)6 additive.

Table 3 Expected reaction of different phosphates, their corresponding oxygen activity in the molten salt, and the corresponding aspect ratio of the fi-nal powder

Phosphate Expected reaction Equilibrium constant (k)34 Aspectratio

(NaPO3)6 2PO3− + 2O2

2− → 2PO43− + O2 2.05 × 1020 15.3

Na4P2O7·10H2O 6.6 × 105 5.01

Na2HPO4 2.8 × 105 4.5

Na3PO4·12H2O No reaction with O2− ions

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transformation. This solid-state phase transformation leadsto the formation of equiaxed rutile TiO2 particles,28 whichlater combine with 1-D particles by Ostwald ripening to givethicker (lower aspect ratio) rods.

To see how the phase and morphology evolved to give 1-Drutile TiO2, (NaPO3)6-containing samples were collected at dif-ferent stages during the course of the synthesis. The resultsof the phase and morphology analysis of the samples at dif-ferent stages are presented in Fig. 5 and 6, respectively. As

observed from the XRD patterns [Fig. 5(a–h)], when the saltmixture just starts melting at 785 °C, the intensity of the100% peak of rutile TiO2 (at 2θ ~27°) (Fig. 5a) was muchlower than that of the corresponding 100% peak of anataseTiO2 (at 2θ ~25°) indicating that the major portion of thesample is in the anatase phase. Progressively, with an in-crease in temperature and time, the intensity of the 100% ru-tile peak increases and that of the anatase peaks diminishesimplying a progressive rutile phase proportion increment inthe samples. After 10 min of holding at 800 °C, the rutilephase fraction has become >92%. The SEM micrographs ofthe corresponding samples [Fig. 6(a–h)] show that there is astrong relationship between the phase and morphology. Thesample at 785 °C (Fig. 6a) is composed of mainly particulatemorphology with only a few rods. However, as the rutilephase proportion in the samples increases, the rod morphol-ogy becomes more dominant in the samples. For the finalsample, of which the rutile phase fraction is greater than92%, almost all the particles in the sample have become rods(Fig. 6h).

It is evident from the trend in the evolution of the phaseand morphology that the particulate forms of the samples arein the anatase phase and the rods are in the rutile phase. Forfurther confirmation, high-resolution transmission electronmicroscopy (HR-TEM) and selected area diffraction pattern(SAD) analysis were performed on samples of the particulateand rod morphologies [Fig. 7a–d)]. The d-spacing of the latticefringes for the particulate morphology is ~2.43 Å (Fig. 7c),which corresponds to the spacing of {103} parallel planes ofanatase TiO2. The SAD pattern (Fig. 7d) also corresponds tothe diffraction pattern of anatase TiO2 for the <010> zoneaxis. In contrast, the d-spacing for crystallographic planes

Fig. 4 SEM micrographs of the powders synthesized using different phosphate additives: a) (NaPO3)6, b) Na4P2O7·10H2O, c) Na2HPO4, and d)Na3PO4·12H2O.

Fig. 5 XRD patterns of samples taken at different stages duringsynthesis at a) 785 °C, b) 790 °C, c) 795 °C, d) 800 °C, e) after 2.5minutes of holding at 800 °C, f) 5 minutes of holding at 800 °C, g) 7.5minutes of holding at 800 °C, and h) 10 minutes of holding at 800 °C.

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parallel to the growth direction of the rod is ~1.48 Å (Fig. 7a),which corresponds to the lattice spacing between the {002}crystallographic parallel planes of rutile TiO2. Similarly, theSAD pattern of the rod (Fig. 7b) corresponds to the diffractionpattern of rutile TiO2 with a <010> zone axis. This phasemorphology evolution trend also asserts that the formation of1-D rutile TiO2 follows the dissolution/re-precipitation mecha-nism where anatase particles dissolve in the molten salt andsubsequently precipitate out to give rutile rods.

The rutile phase fraction (XR) was further calculated fromthe 100% intensity peaks of the rutile (IR) and anatase (IA)phases using eqn (1)36 for the samples taken at different

stages. The phase fraction was plotted with respect to timeand temperature (Fig. 8) to see how the anatase-to-rutilephase transformation proceeded with temperature and time.

(1)

Three clearly distinctive stages can be seen from the trendin the plot. The first stage corresponds to the temperatureat which the endothermic peak of the DSC curve startsforming (~785 °C). At this stage, only a small portion of the

Fig. 6 SEM micrographs of samples taken at different stages during synthesis at a) 785 °C, b) 790 °C, c) 795 °C, d) 800 °C, e) after 2.5 minutes ofholding at 800 °C, f) 5 minutes of holding at 800 °C, g) 7.5 minutes of holding at 800 °C, and h) 10 minutes of holding at 800 °C.

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salt is molten; consequently, very little of the anatase coulddissolve and then precipitate to form rutile rods. Next comesa relatively flat profile of the rutile phase fraction (~68–69%)vs. the temperature curve (790–800 °C). This stage probablycorresponds to the melting of the salt, during which the saltundergoes phase transition from solid to liquid at constanttemperature (~793 °C, the minimum point of the meltingpeak on the DSC curve). The temperature range in the plotcorresponding to this stage (~790–800 °C) is probably the sur-rounding temperature, not the temperature at the actualsample. During this period, the salt completely transforms

into the molten state, but because it is still just at the melt-ing point, there will be no temperature increment until allthe salt is melted. Consequently, the viscosity of the melt re-mains constant. The slight increment in the phase fractionobserved during this period is probably due to the incrementof the amount of melt with time, which results in dissolutionof more anatase TiO2 and some re-precipitation to form rutilerods. After the salt is completely melted, the temperaturestarts to rise again (sensible heat) resulting in a consequentdecrement in the viscosity of the melt. The dramatic incre-ment of the rutile phase fraction during this stage could bethe direct consequence of the lower viscosity of the melt andthe consequent higher solubility of anatase TiO2.

Conclusions

The 1-D crystal growth of rutile TiO2 in molten NaCl was ana-lyzed systematically. The role of additives in the phase andmorphology of the final powder synthesized from the anataseTiO2 precursor and rutile TiO2 seed in molten NaCl was in-vestigated in detail. Depending on their effect on the oxygenion (O2−) activity of the system, the additives were found toresult in different phases and particle morphologies.(NaPO3)6, which reduces the O2− activity of the system (i.e., itis a strong Lux–Flood acid), was found to be the appropriateadditive to obtain high-aspect-ratio 1-D rutile TiO2. The trendin the evolution of the morphology and phase from anatase

Fig. 7 a) HR-TEM image of the rutile TiO2 rod shown in the inset, b) indexed SAD pattern of the rutile TiO2 rod for the [010] zone axis, c) HR-TEMimage of the anatase TiO2 particle shown in the inset, and d) indexed SAD pattern of the anatase TiO2 particle for the [010] zone axis.

Fig. 8 Phase fractions of rutile and anatase in the samples at differentstages of synthesis vs. time and temperature.

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TiO2 nanoparticles to rutile TiO2 rods was also analyzed. Itwas observed that dissolution/re-precipitation phase transfor-mation is responsible for 1-D crystal growth. This work pro-vides better insight into the mechanism by which the 1-Dcrystal growth of rutile TiO2 occurs in molten NaCl. We be-lieve that this work may open new avenues for controlled syn-thesis of various oxides in molten salt environments.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to express their gratitude to Dr. KyoungHoon Kang of KCC for his valuable suggestions and supportduring this study. Ragupathy is grateful to the Korean Federa-tion of Science and Technology Societies for financial supportthrough the Brain Pool Program and Dr. V. K. Pillai, Directorof CSIR-CECRI, for his support and granting him sabbaticalleave.

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