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Contents lists available at ScienceDirect

Journal of Materials Science & Technology

journa l homepage: www. jmst .org

ecent progress in molten salt synthesis of low-dimensionalerovskite oxide nanostructures, structural characterization,roperties, and functional applications: A review

iaojie Xue, Heng Wu, Yao Lu, Xinhua Zhu ∗

ational Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China

r t i c l e i n f o

rticle history:eceived 11 April 2017eceived in revised form 30 June 2017ccepted 5 July 2017vailable online xxx

eywords:ow-dimensional perovskiteanostructuresolten salt synthesis

tructural characterizationhysical propertiesunctional applications

a b s t r a c t

Molten salt synthesis (MSS) method has advantages of the simplicity in the process equipment, versatileand large-scale synthesis, and friendly environment, which provides an excellent approach to synthesizehigh pure oxide powders with controllable compositions and morphologies. Among these oxides, per-ovskite oxides with a composition of ABO3 exhibit a broad spectrum of physical properties and functions(e.g. ferroelectric, piezoelectric, magnetic, photovoltaic and photocatalytic properties). The downscalingof the spatial geometry of perovskite oxides into nanometers result in novel properties that are differentfrom the bulk and film counterparts. Recent interest in nanoscience and nanotechnology has led to greatefforts focusing on the synthesis of low-dimensional perovskite oxide nanostructures (PONs) to betterunderstand their novel physical properties at nanoscale. Therefore, the low-dimensional PONs such asperovskite nanoparticles, nanowires, nanorods, nanotubes, nanofibers, nanobelts, and two dimensionaloxide nanostructures, play an important role in developing the next generation of oxide electronics.In the past few years, much effort has been made on the synthesis of PONs by MSS method and theirstructural characterizations. The functional applications of PONs are also explored in the fields of storagememory, energy harvesting, and solar energy conversion. This review summarizes the recent progressin the synthesis of low-dimensional PONs by MSS method and its modified ways. Their structural char-

acterization and physical properties are also scrutinized. The potential applications of low-dimensionalPONs in different fields such as data memory and storage, energy harvesting, solar energy conversion,are highlighted. Perspectives concerning the future research trends and challenges of low-dimensionalPONs are also outlined.

© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science &Technology.

. Introduction

MSS method is a simple, versatile, and environmental-friendlypproach, which has been widely used to synthesize high puritynd nanoscale inorganic oxides with controllable compositions andorphologies. In this approach, inorganic molten salt is served as

he reaction medium to enhance the reaction rate and to reduce theeaction temperature of the reactant oxides. Thanks to the short dif-usion distances and large mobilities of the reactant oxides in the

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olten salts, the whole solid-state reactions are easily carried out atoderate temperatures (600–800 ◦C) in a short soaking time (less

ne hour) [1,2]. Besides the low formation temperature, molten

∗ Corresponding author.E-mail address: xhzhu@nju.edu.cn (X. Zhu).

ttps://doi.org/10.1016/j.jmst.2017.10.005005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of

salts also promote to stabilize the specific morphology of the finalproducts [1–3]. Furthermore, the morphology of the final productscan be well controlled by adjusting the processing parameters (e.g.the types and quantities of the used molten salts, different reac-tant oxides, heating temperature and duration, and heating/coolingrates) in the MSS reactions [4–7]. Up to date, a wide range ofinorganic oxide materials with tunable morphology has been syn-thesized by MSS method [8–11]. For examples, perovskite ceramicpowders of BaTiO3 [12,13], BaZrO3 [14], SrTiO3 [15], Ca1-xSrxTiO3[16,17], Pb(ZrTi)O3 [18], and lead-based relaxors with a A(B′B′′)O3perovskite structure have been synthesized by the MSS method[1,19–21]. In addition, pristine BaTiO3 nanostructures (including

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nanowires) with diameters of 50–80 nm and aspect ratios from 1to over 25, as well as single-crystalline SrTiO3 nanocubes with anaverage edge length of 80 nm have been synthesized [22]. The evo-lution of BaZrO3 particle morphology from predominantly cubes to

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mixture of cubes and spheres and finally to solely spheres is alsoocumented by increasing the annealing/reaction times at selectednnealing temperatures [16,17]. Recently, the MSS method haseen used to synthesize one-dimensional (1D) perovskite MTiO3M = Pb, Ba, Sr elements) nanostripes by using the mixed chloridesNaCl/KCl = 1:1) as molten salt media [23].

Generally, in the process of MSS method, two fundamental reac-ion mechanisms are involved during the formation of the finalroducts. The first one is all the reactant oxides are fully dissolvedithin the molten salt and they diffuse to react in a short time. Oneodel example is the Bi2WO6 system [24], in which Bi2WO6 par-

icles are formed through complete dissolution of the constituentxides (WO3 and Bi2O3) in an equimolar mixed KCl-NaCl chlorideseated at 650 ◦C for 1 h and complete reaction between WO3 andi2O3. The second one is that some reactants are much more sol-ble within the molten salt than the other components, so theyiffuse onto the surfaces of the other components and react withhem, forming the final products with a similar morphology as theess soluble reactants. A model example is template-free fabrica-ion of pure single-crystalline BaTiO3 nanorods by MSS methodt relatively high temperature and large ratios of the salt to therecursors [25]. Since barium oxide has higher solubility in theolten NaCl/KCl salts as compared with the titanium oxide, there-

ore, much amount of barium oxide is dissolved in the molten salthereas the insoluble titanium oxide particles have a chance to

eunite into rods-like particles in the molten salt environment,cting as templates for further growth. As a consequence, the dis-olved barium oxide particles can diffuse to the surfaces of rod-likeitanium oxide and react with it in situ to form BaTiO3 rod-like nano-tructure. Here, the relative dissolution is a key factor in the reactionrocess, and the morphology of the less soluble reactants is finally

nherited during the formation of final products. During the parti-le growth stage, the total surface areas are reduced, thus, finallyhe surfaces with high energy disappear, and the morphology of thenal product is composed of the crystallographic planes with low-st surface energy, reaching the equilibrium state. Details about theundamental reaction mechanisms of MSS reactions can be foundn the previous reviews [1,5,26].

The main processing stages of the MSS method for synthesisf perovskite oxide powders are schematically illustrated in Fig. 1.t stage I, the reactant oxides and/or other appropriate precursorsorresponding to the desired compound are mixed with either theesired salts (e.g., NaCl, KCl) or an eutectic mixture of the saltse.g., NaCl - KCl, NaOH - KOH, NaNO3 - KNO3, Na2SO4 - K2SO4,i2SO4-Na2SO4). At stage II, the mixture is heated at a tempera-ure above the melting point of the salt medium to form a moltenux. At this temperature, precursor molecules disperse, dissociate,earrange, and then diffuse rapidly throughout the salt. At stage III,he product particles start to nucleate and grow up via the solution-recipitation process. The characteristics of the product powder areontrolled by selecting the temperature and duration of the heat-ng. The reacted mass is cooled to room temperature and washed

ith an appropriate solvent (water usually used) to remove thealt. The complex perovskite oxide powders can be obtained afterrying, and they have several unique characteristics as comparedo those obtained by other methods such as solid-state reactions27,28], combustion synthesis [29,30], and sol-gel method [31,32].uch unique features are determined mainly by the chemical andrystallographic constraints given by the salts [33,34], which arespecially distinguishable in the case of strongly anisotropic per-vskite oxides. It is believed that the features are related to theurface and interface energies between the constituents and the

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alts, resulting in a tendency to minimize the energies by forming specific morphology. The environments during the developmentf the morphology of perovskite oxides can be controlled by the

Fig. 1. Schematic illustration of the main processing stages of the MSS method forsynthesis of perovskite oxide powders.

selected salts. Therefore, the selection of the molten salts is criticalin obtaining desirable powder characteristics.

Nanostructured oxide materials exhibit novel phenomenabecause of the size effects that appear during the downscaling ofthe spatial geometry of oxide materials. Compared with the bulkcounterparts, nanostructured oxide materials possess some spe-cific surfaces and have lots of surface atoms and higher levels ofsurface energy. Up to date, nanostructured oxide materials includ-ing zero-dimensional (0D), 1D, and two-dimensional (2D) oxidenanostructures, have attracted much attention due to their uniquegeometries and novel properties, which make them as key buildingblocks to construct various oxide nanodevices [35–37]. However,it is found that the synthesis of 1D or 2D oxide nanostructures bythe traditional MSS method is much difficult due to the nature ofequiaxial growth for the oxide particles. To achieve the anisotropicgrowth of oxide particles in MSS process, selection of suitabletemplates with specific morphology is crucible to promote the pro-cess [39–41]. The template method is an efficient and mild wayto prepare pure 1D or 2D oxide nanostructures with controllablemorphology at moderate reaction conditions [40–42].

Among the inorganic oxides, perovskite oxides with a compo-sition of ABO3 are one of the most important functional materialsdue to their rich physical properties (e.g. dielectric, ferroelectric,piezoelectric, magnetic, multiferroic properties) [35]. Generally, inthe ABO3 perovskite structure A-site is occupied by alkaline earthions or rare-earth ions, and B-site is occupied by transition metalions. Many physical properties of perovskite oxides are resultedfrom B-site cations while tuned by A-site cations [43,44]. Overthe past few years, many low-dimensional PONs (e.g. nanoparti-

2017), https://doi.org/10.1016/j.jmst.2017.10.005

cles, nanowires, nanorods, nanotubes, nanofibers, nanobelts, and2D-nanostructures) have been synthesized by MSS method. Theirprocess-structure-property relations are intensively investigated.

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Fig. 2. Structural characterizations of the as-prepared BaZrO3 nanoparticles with cubic or spherical morphology synthesized by MSS method. SEM images of BaZrO3 nanopar-ticles with (a) cubic and (b) spherical morphology. (c) and (d) XRD and EDS patterns of the BaZrO3 nanoparticles, respectively. The lower pattern in panel (c) correspondsto a database standard (JCPDS 06-0399) for the cubic phase of BaZrO3. The carbon peak in (d) originates from the conductive carbon tape. (e) and (f) TEM, HRTEM, andSAED pattern (inset) of a cubic BaZrO3 nanoparticle. (g) and (h) TEM, HRTEM, and SAED pattern (inset) of a spherical BaZrO3 nanoparticle. Reproduced with permission [48].Copyright 2007, American Chemical Society.

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he functional applications of PONs are also explored in the fields

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f storage memory, energy harvesting, and solar energy conversion.This review summarizes the recent advances and progress in

he molten salt synthesis of PONs. The structural characterization,

H-KOH and (b) NaCl-KCl based salt systems under different temperatures. (c) andolten salt systems. Reproduced with permission [67]. Copyright 2010, Gordon and

physical properties, and potential applications of low-dimensional

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PONs in the fields of storage memory, energy harvesting, andsolar energy conversion, are also addressed. The prospects and

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Fig. 4. SEM images of the KNbO3 nanomaterials synthesized with different precursors. (a) KNb3O8 nanowires, (b) H3ONb3O8 nanorods, and (c) Nb2O5 nanowires. Reproducedwith permission [69]. Copyright 2009, American Chemical Society.

Fig. 5. XRD patterns of the KNbO3 nanomaterials synthesized with different precursors: (a) KNb3O8 nanowires, (b) H3ONb3O8 nanorods, and (c) Nb2O5 nanowires. (d) TEMimage of a single KNbO3 nanorod synthesized with the precursor of Nb2O5 nanowires and its corresponding SEAD pattern (inset). Reproduced with permission [69]. Copyright2009, American Chemical Society.

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Fig. 6. Structural characterizations of the single-crystalline BaTiO3 nanowires syn-thesized by MSS method. (a) Low-magnification TEM image of the BaTiO3 nanowires.(b) and (c) Enlarged TEM images of the parts B and A marked in Fig. a, respectively.(oE

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d) HRTEM image of a single BaTiO3 nanowire. Inset in Fig. c is the SAED patternf the single BaTiO3 nanowire. Reproduced with permission [25]. Copyright 2014,lsevier Ltd.

hallenges of future researches of low-dimensional PONs are alsoutlined.

. Low-dimensional PONs synthesized by MSS

.1. Perovskite oxide nanoparticles

Perovskite oxide nanoparticles exhibit some special physicalroperties that are ascribed to the surface effect, small size effect,uantum confinement effect, and etc. [45,46]. Up to date, manyerovskite oxide nanoparticles are synthesized by MSS method. Forxample, Yang et al. [47] reported the perovskite SrFeO3 nanoparti-les synthesized by MSS method. They investigated systematicallyhe effects of processing parameters of MSS method (e.g. typesf metal precursors, salt medium, annealing temperature, oxidiz-

ng properties of the melt salts) on the structural characteristicsf the final products (e.g. phase compositions, crystallite sizes,nd morphology). It was found that the formation of the per-vskite SrFeO3 phase was mainly dependent upon the nature ofhe metal precursors and the used salt medium. Metal nitrates wereound to be the suitable precursors and NaNO3-KNO3 eutectic witha2O2 was the suitable salt medium for the synthesis of pure per-vskite SrFeO3 nanoparticles. Li et al. [15] also synthesized cubichase SrTiO3 nanoparticles by MSS method, where the eutecticaCl - KCl molten salts were used as reaction medium, TiO2 andr(NO3)2 were used as the reactant powders. The results demon-trated that the sizes of the as-synthesized SrTiO3 nanoparticlesere controlled the types of TiO2 precursors, indicating that the

ormation of SrTiO3 nanoparticles in the NaCl - KCl molten salts wasainly controlled via a template formation mechanism. Zhou et al.

48] reported the synthesis of single-crystalline perovskite BaZrO3anoparticles with cubic or spherical morphology by MSS method,here BaC2O4 and ZrO2 were used as precursors, and NaOH -OH was used as the molten salt. Recently, single-phase perovskiteb(Fe0.5Nb0.5)O3 (PFN) [49], Ca1-xSrxTiO3 (0 ≤ x ≤ 1) nanoparticles16], and multiferroic BiFeO3 nanoparticles [50–54] have been syn-hesized via MSS method. In order to understand the reaction

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echanism in MSS reactions, the growth behavior of lead zirconateitanate (PZT) nanoparticles in MSS process was investigated byortolani and Dorey [55]. They found that lead was first dissolved

n the molten salts and then reacted with insoluble TiO2 to form

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an intermediate phase of PbTiO3, and then the zirconium diffusedinto and reacted with PbTiO3 to form PZT nanoparticles. The dif-fusion of zirconium through the PbTiO3 follows a rate-limitinglaw, and simultaneously, the particle coarsening takes place basedon a challenge to create small particles where two low solubilityphases are present. Besides the ferroelectric perovskite nanopar-ticles, magnetic perovskite nanoparticles such as La1-xSrxMnO3(LSMO) (x = 0.18 − 0.37) with a rhombohedral perovskite structure,are also synthesized by MSS method [56]. The synthesized LSMOnanoparticles with mean size of ∼50 nm have a lower magnetiza-tion and reduced Curie temperature as compared with their bulkcounterpart.

2.2. Perovskite oxide 1D nanostructures (nanowires, nanorods,and nanotubes)

1D PONs include nanowires, nanotubes, nanorods, nanorib-bons, and etc., which are not only used as the building blocks forconstructing future nanodevices, but also provide with the possi-bilities for fundamentally investigating the intrinsic size effects ofphysical properties [36–38]. For instance, perovskite ferroelectricoxide nanowires have excellent piezoelectric properties, and thisenables them used as active cantilevers in micro-electromechanicaland nanoelectromechanical systems. Deng et al. [23] first reportedon the general surface-free synthesis of MTiO3 (M = Pb, Ba, Sr)perovskite nanostrips by MSS method. The nanostrips had awidth of 50–200 nm, thickness of 20–50 nm, and length up totens of micrometers. Giant dielectric calcium copper titanate(CaCu3Ti4O12) perovskite nanorods were also synthesized by MSSmethod [57], indicating that single-crystalline quaternary per-ovskite nanostructures can be formed. Besides perovskite oxidenanowires, perovskite nanotubes are also synthesized by MSSmethod. Wang et al. [58] also reported on the synthesis of single-crystalline LaFeO3 nanotubes by co-precipitation followed by MSSmethod. In this process, La(NO3)3 and Fe(NO3)3·9H2O were firstadded drop-wise to the co-precipitation reagent of NaOH and NP10,and then the precipitates of Fe(OH)3 and La(OH)3 were mixed withNaI and the mixture was annealed at 680 ◦C for 2.5 h. Finally, tube-like LaFeO3 nanostructures with a diameter of 40–200 nm andlength up to 3 �m were obtained. Lead-free (K,Na)NbO3 (KNN)single-crystalline nanorods with a high piezoelectric coefficientwere also obtained by MSS method [59,60]. Their diameters werein the range from 200 nm to 700 nm with a length from 10 �mto 20 �m. Similarly, perovskite cubic phase of BaTiO3 nanowireswith different diameters and aspect ratios were also synthesized byMSS method [25,61]. Recently, much longer (up to 80 �m) single-crystalline perovskite BaTiO3 nanowires with a tetragonal phasestructure have also been synthesized [62]. They exhibit uniformcylindrical structure with diameters in the range of 100 nm to 1 �m.

2.3. Perovskite oxide 2D nanostructures

2D materials are described as the materials having a fewnanometers or less in the thickness direction. Therefore, elec-trons can move freely in the two-dimensional plane normal to thethickness direction, however, their motion is limited in the thick-ness direction and governed by quantum mechanics. Typical 2Dmaterials include quantum wells, nanoplates and nanofilms, andgraphene. As one typical example of 2D materials, graphene hasreceived much attention due to its fascinating electronic, mag-netic, optical, and mechanical properties [63]. Recently, the focusedpoint has switched to the low-dimensional PONs because of their

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unusual electron transport and magnetic properties, which areinvaluable for developing multi-functional microelectronic, mag-netic, and spintronic devices based on 2D PONs. However, it isnot easy to fabricate 2D PONs due to the equiaxed growth nature

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f perovskite oxides. The MSS method provides an approach tobtain the 2D PONs [64–66]. For instance, Su et al. [64] fabricatedhe micrometer-sized (001)-oriented BaTiO3 (BT) platelets with aiameter of 10–20 �m and thickness of 0.5–1.0 �m by a modifiedwo-step MSS method. Plate-like NaNbO3 with high aspect ratiothe ratio of length to thickness from 15:1 to 50:1) was synthe-ized by Zhang et al. via topochemical MSS method [65]. In thisrocess, plate-like bismuth layered perovskite Bi2.5Na3.5Nb5O18recursors were first synthesized by MSS method at 1100 ◦C witholten NaCl salt as reaction medium, and then they reacted with

he complementary reactant Na2CO3 in NaCl flux at 950 ◦C. Dur-ng the transformation of plate-like Bi2.5Na3.5Nb5O18 particles intohe plate-like perovskite NaNbO3 particles, the morphology ofhe Bi2.5Na3.5Nb5O18 precursors was inherited. The transformation

echanism of the above reaction can be expressed as follows [65]:

i2.5Na3.5Nb5O18 + Na2CO3NaClflux→ NaNbO3 + Bi2O3 + CO2 (1)

Similarly, needle-like or plate-like KNbO3 (KN) and

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1-xNaxNbO3 (KNN) nanostructures with high aspect ratiosere also synthesized, which grew with preferred orientation

long the [011] or [100] directions through a dissolution - precipi-ation mechanism [66]. It is expected that with the development

ig. 7. (a) TEM image of the as-prepared K0.5Bi0.5TiO3 nanowires with random orienta0.5Bi0.5TiO3 nanowire. (d) SAED pattern taken from an individual nanowire. (e) High-res

71]. Copyright 2007, The American Institute of Physics.

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of oxide electronics, more and more 2D PONs will be synthesizedby MSS method and its improved ways.

3. Structural characterization of low-dimensional PONssynthesized by MSS method

3.1. Introduction

The structural characterizations of low-dimensional PONsare conducted to investigate their crystal structures, chemicalcompositions, and morphologies. The crystal structures are usu-ally characterized by X-ray diffraction (XRD), Raman spectrum,Fourier transformed infrared spectroscopy, transmission electronmicroscopy (TEM), high-resolution TEM (HRTEM), and selectedarea electron diffraction (SAED). The chemical compositions areusually examined by energy dispersive X-ray detector (EDX), elec-tronic energy loss spectroscopy (EELS), and X-ray photoelectronspectroscopy (XPS). The morphologies are usually characterizedby atomic force microscopy (AFM), scanning electron microscopy

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(SEM) and TEM. In this section, the structural characterizations oflow-dimensional PONs are described to provide a brief review ofthe microstructures of low-dimensional PONS synthesized by MSSmethod.

tion. (b) XRD pattern of the corresponding nanowires. (c) TEM image of a singleolution TEM image of a single K0.5Bi0.5TiO3 nanowire. Reproduced with permission

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Fig. 8. (a) TEM image and (b) EDS spectrum of the BaMnO3 nanorods synthesized by MSS method at 200 ◦C for 120 h. (c) TEM image of a single BaMnO3 nanorod and (d) itsdiffraction pattern, and (e) HRTEM image taken from the square box marked in Fig. c. Reproduced with permission [72]. Copyright 2006, The American Chemical Society.

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Fig. 10. Dielectric constant and dielectric loss of the BiFeO3 ceramics measuredas a function of the frequency. The BiFeO3 ceramics were prepared from thecorresponding BiFeO3 micro- and nano-crystals synthesized by MSS method at

ig. 9. SEM image of the platelet-like BaTiO3 particles heated at 1100 ◦C for 1 h.eproduced with permission [75]. Copyright 2007, The American Chemical Society.

.2. Perovskite oxide nanoparticles

Zhou et al. [48] reported the microstructural characterizations ofingle-crystalline perovskite BaZrO3 nanoparticles with cubic andpherical morphologies synthesized by MSS method. The resultsncluding the SEM images, XRD and EDS patterns, TEM and HRTEMmages, are shown in Fig. 2. As it can be seen that the lattice fringesn the HRTEM images and the SAED patterns (Fig. (f)–(h)) clearlyeveal the single-crystalline nature of the BaZrO3 particles. The XRDattern (Fig. 2(c)) demonstrated that the BaZrO3 particles crystal-

ized in a cubic phase with a lattice constant of a = 0.4183 nm, whichas in good agreement with literature data (a = 0.4181 nm, JCPDS

o. 06-0399). Sahoo and Mazumder [67] reported the microstruc-ural evolutions of perovskite BaTiO3 nanoparticles during theirormation process in MSS reaction. The XRD patterns revealed thature perovskite BaTiO3 phase appeared at 175 ◦C in the NaOH -OH based-salt system (Fig. 3(a)), whereas pure perovskite BaTiO3hase appeared at much higher temperature (e.g. 700 ◦C) in theaCl - KCl based-salt system (Fig. 3(b)). That demonstrates that the

election of different molten salt systems play an important role inffecting the formation temperature of perovskite phase [68]. TEMmages demonstrated that the average particle size of the BaTiO3articles synthesized in the NaOH - KOH based-salt system wasbout 70–100 nm with cube shape (Fig. 3(c)), whereas in the NaCl

KCl based-salt system the BaTiO3 particles exhibited cube shapeixed with some irregular shaped ones (Fig. 3(d)).

.3. Perovskite oxide 1D nanostructures (nanowires, nanorodsnd nanotubes)

In spite of the effectiveness of MSS method for perovskite oxideanoparticles, the reliable synthesis of 1D PONs is restricted tonly a few types of perovskite oxides such as niobates [69,70],itanates [21,23,71], as well as some manganate systems [72,73].i et al. [69] reported on the perovskite niobate ANbO3 (A= K, Na,r Na/K) nanorods, which were prepared by topochemical MSSethod, where the Nb2O5 nanorods were used as precursors. They

ound that only KNbO3 nanoparticles rather than nanorods werebtained as either KNb3O8 nanowires or H3ONb3O8 nanorods weresed as precursors, indicating the morphologies of the precursorsre not inherited. That was confirmed by the SEM images (Fig. 4).n the contrary, when the Nb2O5 nanorods were used as the pre-ursors, their morphology were inherited, thus, the rods of KNbO3ere obtained, as shown in Fig. 4(c). The above results show that

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uitable selection of the template (1D Nb2O5 precursor) is cru-ible for the formation of perovskite KNbO3 nanowires becauseuring the molten flux reaction the layer-structured rod-like pre-ursors (e.g. KNb3O8 or H3ONb3O8) are easily disrupted by the

different (a) annealing temperatures, (b) soaking time, and (c) molar ratios ofFe2O3:Bi2O3:NaCl:KCl. Reproduced with permission [81]. Copyright 2011, ElsevierLtd.

transport of alkali ions, and thus, their initial rod-like morphologyis not able to be inherited. By contrast, Nb2O5 precursors main-tain a more compact and stable crystal structure, which can easilyaccommodate the alkali ions without destroying its initial rod-likemorphology, therefore, perovskite KNbO3 nanorods were formedby inheriting the rod-like morphology of the Nb2O5 precursors.XRD patterns (Fig. 5(a)-(c)) demonstrated that all the synthesizedKNbO3 nanorods crystallized in an orthorhombic structure (JCPDS

71-946). Their growth direction was along the [01−1] direction, as

marked by an arrow in Fig. 5(d). By using a modified multi-step MSSprocedure, Wong’s group synthesized potassium and sodium nio-bate nanorods and LiNbO3 nanowires [70]. Recently, much efforthas been made to synthesize 1D MTiO3 (M = Pb, Ba, Sr) perovskitesdue to their promising applications in oxide microelectronics. Donget al. [23] first reported on the general synthesis of MTiO3 nanos-trips (M = Pb, Ba, Sr) by a surfactant-free approach in non-aqueous

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molten salt media. The as-synthesized MTiO3 nanostrips had awidth of 50–200 nm, a thickness of 20–50 nm, and a length up totens of micrometers. They all grew in the [001] direction. In this MSSprocess, the corresponding metal oxalate (barium oxalate, stron-

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ium oxalate and lead oxalate) and TiO2 were used as precursors in NaCl - KCl flux. It was found that high reaction temperature, longeaction time as well as small-sized TiO2 precursors were benefitor growing the titanate nanostrips. The possible growth mecha-ism of the MTiO3 nanostrips can be expressed in the followingeaction [23]:

C2O4 (s) + TiO2NaCl-KCl flux→ MTiO3 + CO2 (g) (2)

Similarly, single-crystalline perovskite BaTiO3 nanowires werelso synthesized by MSS method [64]. As shown in Fig. 6(a), theaTiO3 nanowires had a width of 30–100 nm and length up to sev-ral micrometers. The enlarged TEM images collected from theip points A and B (Fig. 6(b)-(c)) revealed that the tips of theanowires exhibited triangle or circular arc morphologies. Fig. 6(d)as a representative HRTEM image taken from the tip of a BaTiO3

anowire with a triangle shape. Clearly, the two-dimensional lat-ice fringes with spacings of 0.4013 nm and 0.1795 nm are resolved,hich correspond to the (100) and (210) lattice planes of the

ubic BaTiO3 nanoparticle, respectively. The SAED pattern takenrom a single BaTiO3 nanowire was shown as an inset in Fig. 6(c),

hich demonstrated the growth direction of the nanowire along001] direction. Yang et al. [71] also fabricated single-crystalline0.5Bi0.5TiO3 nanowires by a large scale and facile molten salt syn-

hetic method. Analytical grade K2CO3, Bi2O3 and TiO2 were useds the starting materials, and KCl used as molten salt. A represen-ative TEM and HRTEM image of the as-prepared (K0.5Bi0.5)TiO3anowires (Fig. 7) reveal that the nanowires have a tetragonal per-vskite structure with a diameter about 40 nm and lengths over

�m. The single-crystalline nature of the (K0.5Bi0.5)TiO3 nanowiresre also confirmed by the SAED pattern and the lattice fringesesolved in the HRTEM image. In addition, manganese perovskitexide nanowires are also synthesized by MSS method. For exam-le, Hu et al. [72,73] reported on BaMnO3 and Ba(Ti0.5Mn0.5)O3anorods/nanowires synthesized under a mixed KOH and NaOHith a K/Na ratio of 48.5:51.5. The synthesis temperature was as

ow as 170 ◦C. Typical BaMnO3 nanorods had a width of 50–100 nm,nd their lengths were easily controlled by processing parame-ers of the MSS process (e.g. annealing temperature, soaking time).ig. 8 demonstrated the TEM characterizations of the BaMnO3anorods synthesized at 200 ◦C for 120 h. The SAED pattern andRTEM (Fig. 8(d)-(e)) revealed the single-crystalline nature of theanorods, which grew along the direction of [001]. The formationechanism of perovskite BaMnO3 nanorods is described as the

ollowing reactions [72]:

aCl2 + 2NaOH → Ba(OH)2 + 2NaCl (3)

nO2 + 2NaOH → Na2MnO3 + H2O (4)

a(OH)2 + Na2MnO3 → BaMnO3 + 2NaOH (5)

.4. Perovskite oxide 2D nanostructures

Perovskite oxide 2D nanostructures such as perovskite oxideanosheets, exhibit interesting optical and electrical properties dueo the electron and phonon confinement, large surface-to-volumeatios and tap density, high surface reaction activity, and high cat-lytic efficiency. They have promising applications in the fields ofigh-density memory and storage, solar cells, and photocatalysis.ecently, Arney et al. [74] reported the synthesis of rectangularlatelet-like perovskite La2Ti2O7 photocatalyst by MSS method,here the mixed Na2SO4-K2SO4 flux was used as molten salt and

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nalytical grade TiO2 and La2O3 oxides were used as the startingaterials. SEM images revealed that rectangular platelet-like mor-

hology was obtained with maximal dimensions of 500–5000 nmnd thicknesses below 100 nm. The products with smaller sizes

measured at room temperature and at 10 Hz for the NaNbO3 ceramics (a) beforeand (b) after polarization treatment. Reproduced with permission [83]. Copyright2014, The American Ceramic Society.

can be synthesized under lager molar ratio of molten salt to thestarting oxides and the extent of aggregation is increased withincreasing the reaction time. It is found that the exposed crystal-lite edges and the (010) and (001) crystal faces in the platelet-likeperovskite La2Ti2O7 sheets play a key role in enhancing their photo-catalytic activities. Meanwhile, Liu et al. [75] reported micron-scaleplatelet-like BaTiO3 particles with an average size of 5–10 �m anda thickness of 0.5 �m. Fig. 9 shows the SEM image of platelet-likeBaTiO3 particles heated at 1100 ◦C for 1 h, whereas the morphologyof these platelets are still kept after high-temperature treatment.However, up to date perovskite oxide 2D nanostructures syn-thesized by MSS method are rarely reported, their researchesand developments are still at the embryonic stage. Therefore, theintegration of perovskite oxide 2D nanostructures into oxide micro-electronic devices faces great challenges. Much work lies ahead inthis direction.

4. Physical characterizations of low-dimensional PONssynthesized by MSS method

4.1. Introduction

Perovskite oxides exhibit excellent ferroelectric, piezoelectric,pyroelectric, ferroelastic, and other unique physical properties suchas ferromagnetism, magnetoresistance, and multiferroics [76–78].Up to date, the most widely investigated perovskite oxides include

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BaTiO3, PbTiO3, SrTiO3, Pb(Zr,Ti)O3, (Ba,Sr)TiO3, KTaO3, and multi-ferroic oxides such as BiFeO3 [79,80]. Recent interest in nanoscienceand nanotechnology has led to great efforts focusing on the syn-thesis of low-dimensional PONs to better understand their novel

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ig. 12. (a) Schematic diagram for measuring the effective piezoelectric coefficientf an individual (K,Na)NbO3 nanorod and three measured points. (c) - (e) Displaceoints 1, 2, and 3, respectively. Reproduced with permission [86]. Copyright 2011, T

hysical properties at nanoscale. In this section, some importanthysical properties of low-dimensional PONs synthesized by MSSethod, are addressed and discussed.

.2. Electrical properties

Perovskite oxide nanocrystals and nanowires can be used to fab-icate ferroelectric ceramics at low sintering temperatures, whichxhibit highly uniform nanostructured texture and grain sizes.ecently, Liu et al. [81] prepared BiFeO3 ceramics, which are made

rom the BiFeO3 micro- and nano-crystals synthesized by MSSethod in the NaCl − KCl salt system. They found the BiFeO3 ceram-

cs exhibiting relatively high dielectric constants (the maximalalue ∼250 measured @103 Hz and room temperature). The corre-ponding dielectric losses were in the range of 0.05–0.2, which wereependent upon the processing parameters of the MSS method.he effects of MSS processing parameters (e.g. annealing temper-

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ture, soaking time, and the molar ratio of the starting oxides toolten salts) on the dielectric properties of the BiFeO3 ceram-

cs are shown in Fig. 10. Thus, the dielectric properties of BiFeO3eramics can be effectively controlled by adjusting the processing

ong the vertical direction of an individual (K,Na)NbO3 nanorod. (b) Top view image - voltage curve (black line) and d33 - voltage curve (blue line) measured at three

erican Ceramic Society.

parameters of the MSS method. In another example, Zhao et al. [82]fabricated 0.67Pb(Mg1/3Nb2/3)O3 - 0.33PbTiO3 (0.67PMN- 0.33PT)perovskite ceramics based on the corresponding ceramic powderssynthesized by MSS method in sulphate and chloride molten salts.Large grain sizes and better electrical properties were observed inthe 0.67PMN- 0.33PT ceramics prepared in chloride molten saltsin comparison with that prepared in sulphate molten salts. Themaximal dielectric constant of the 0.67PMN- 0.33PT ceramics was29385, and its piezoelectric constant d33 was about 660 pC/N.High-density NaNbO3 piezoelectric ceramics with an orthorhombicstructure were also prepared by MSS method [83]. Their ferro-electric properties before and after the polarization treatment areshown in Fig. 11, which are closely related to the polarizationtreatment. More specifically, the polarized specimens have smallercoercive electric field (Ec) than that of the un-polarized speci-mens, however, they have two times larger remanent polarizationthan that of the un-polarized specimens. The piezoelectric constant

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d33 of the NaNbO3 ceramics was evaluated to be 28 pC/N afteraging for 24 h, which is comparable to the values reported previ-ously [84,85]. Recently, the piezoelectric properties of an individual(K,Na)NbO3 nanobar synthesized by MSS method, is also reported

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Fig. 13. Plots of (a) MR ratio measured at 5 T for LSMO calcined at different tem-peratures, and (b) their resistance (@ 0 T and 5 T) as a function of the temperature.

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86]. A schematic diagram used for measuring the effective piezo-lectric coefficient d33 in the vertical direction by scanning probeicroscopy is shown in Fig. 12(a), and (b) shows its top view image,here three measured points are marked by 1, 2 and 3, respectively.

ig. 12(c)–(e) displays the displacement and d33 as a function of thepplied voltage, which are measured at points of 1, 2, and 3, respec-ively. Based on the experimental data collected from the abovehree measured points, an average effective piezoelectric coeffi-ient d33 was calculated to be 113 pm/V, which was higher than thatf the bulk ceramics prepared by conventional solid-state reaction87,88].

.3. Magnetic properties

Perovskite manganites with a composition of R1-xAxMnO3 (R = Land rare earths, A = Ca, Sr, Ba, etc.) exhibit complex interactionsmong the freedoms of spin, charge, orbital, and lattice degrees,eading to novel physical phenomena such as colossal magnetore-istance (CMR) effect, metal - insulator (M - I) transition, electronichase separation [89–93]. At nanoscale the properties of perovskiteanganites are quite different from their corresponding bulk coun-

erparts. Numerous special physical characters such as surfacepin glass behavior, high magnetic coercivity, and small remanentagnetization are reported in the low-dimensional PONs [94–97].

or instance, Kacenka et al. [56] found that the single-phaseerovskite La1-xSrxMnO3 (LSMO, x = 0.18 − 0.37) nanoparticles syn-hesized by molten salt method, exhibited much lower remanent

agnetization and Curie temperature in comparison with sam-les derived from a sol-gel process. Such anomalous behaviorsould be partially ascribed to the presence of magnetically deadurface layer in the perovskite magnetic nanoparticles. In addi-ion, neutron diffraction data collected from the as-prepared LSMOanoparticles indicate that the anomalous behaviors of the LSMOanoparticles is probably originated from an over-doped outerhell, which is resulted from both the surface oxygen chemisorptionnd an increased Sr concentration in the shell. Luo et al. [98] alsoound that the nanocrystalline La0.7Sr0.3MnO3 particles synthe-ized by MSS method, exhibited an enhanced magnetoresistanceMR). Their temperature dependence of the MR ratio (defined asR0T − R5T)/R5T) of the products and the resistance are shown inig. 13. As it can be seen that the MR ratio of the samples annealedt 700 ◦C is much larger than that of the samples annealed at 500nd 600 ◦C, and the MR ratio reaches 1050% at 10 K for the sam-les annealed at 700 ◦C (Fig. 13(a)). This value is much higher thanhat of the LSMO samples annealed at 800 ◦C derived from theol − gel process [99]. The enhanced MR effect is mainly ascribedo the enhanced spin polarized tunneling effect caused by themproved crystallinity of the nanosized LSMO grains [100,101].n Fig. 13(b), it was observed that the resistivity of LSMO sam-les was increased as the annealed temperature was increased,hich was due to the reduction of the LSMO particle size. Gen-

rally, as the particle size is decreased, the resistance between therain boundary is increased, which is benefit for the spin polarizedunneling. It is known that the crystallization degree of the LSMOrains becomes improved with increasing the annealed tempera-ure, resulting in the disappearance of surface spin-glass behavior.s a consequence, the blocking temperature (TB) that corresponds

o the super-paramagnetic transformation, moves towards higheremperature as increasing the annealed temperature, as markedy arrows in Fig. 13(c). The inset of Fig. 13(c) demonstrates thathe magnetization is increased with increasing the annealed tem-erature whereas it is not saturated even at the external magnetic

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eld up to 5 T. Overall, the above results indicate that the surfacerystallization degree of the manganese particles determines theirpin-glass behavior and enhances the MR effect at low tempera-ures.

(c) Temperature dependence of the ZFC and FC magnetizations (@ H = 200 Oe) of theLSMO particles calcined at different temperatures. The inset shows typical M - Hcurves at 5 K. Reproduced with permission [98]. Copyright 2003, Elsevier Ltd.

4.4. Optical properties

The optical properties of perovskite oxide nanomaterials (i.e.,spontaneous light emission of nanocrystals due to the quantumsize confinement effect), have received much attention in the pastfew years [102–104]. Particularly, as a powerful tool the photo-luminescence (PL) us used to probe the electronic structures andimpurities of perovskite oxides [105]. For example, a broad lumi-nescence band near 500 nm was observed at low temperatures inthe perovskite ATiO3 (A = Ba, Sr, Ca, Pb, etc.) oxides, which wasascribed to the presence of imperfections or defects [106]. Recently,such behavior is renewed the interest in the luminescent propertiesof rare earth-doped perovskite oxide powders synthesized by MSS

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method, such as Er- or Eu-doped LaAlO3 nanocrystals [107], andEu- or Pr-doped CaTiO3 porous micron-scale spheres [108]. Fig. 14shows the PL spectra and excitation spectra of the Eu-doped LaAlO3nanocrystals. As shown in Fig. 14(a), the PL spectra exhibits the typ-

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F d LaAf er Ltd

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ig. 14. (a) Photoluminescence spectrum and (b) excitation spectra of the Eu-doperom 550 nm to 590 nm. Reproduced with permission [107]. Copyright 2012, Elsevi

cal Eu3+-emission, where the optical absorption bands from 5D0o 7FJ (J = 0 − 5) are clearly resolved. Three strong optical absorp-ion bands are contributed from the 5D0 -7F1,2,4 transitions can beeen, i.e. the dipole - dipole type and hypersensitive 5D0 -7F2 (ateast 3 peaks) dominate the magnetic type 5D0 -7F1 transition (3eaks). The structure of 5D0 -7F4 transition is very complex and

t demonstrates at least 8 bands. In addition, the forbidden tran-ition from 5D0 to 7F0 was also observed, which was composedf at least 3 bands, implying that the Eu3+ ions are located threeifferent local sites in the Eu-doped LaAlO3 nanocrystals. Gener-lly, such non-degenerate transition is often used as a probe toetect structural information on the local crystallographic sym-etry, number of sites and site distribution of Eu3+cations in the

rystalline matrix. The excitation spectra shown in Fig. 14(b) areomposed of one broad (FWHM ∼ 50 nm) and intense band as wells less intense and narrow (FWHM ∼ 7 nm) bands. The former oneith maximum is located at around 325 nm that was ascribed to

he O2− → Eu3+ charge transfer transition, while the latter narrowxcitation bands are originated from the f - f transitions of Eu3+

ons. These bands can be assigned to the charge transfer transitionsrom 7F0 to highest manifolds (e.g. 7F0 → 5GJ, 5LJ at 373–382 nm

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ange, 5F0→ 5L6 at 393 nm, and 7F0 → 5D3, 5D2, 5D1 at 413, 463

nd 520–540 nm), respectively. The PL spectra of the CaTiO3 phos-hors doped by Eu with different doping concentrations (i.e. 0, 2,, and 6 mol%) is shown in Fig. 15(a) [108]. As it can be seen, the PL

ig. 15. (a) Photoluminescence spectra of the Eu-doped CaTiO3 prepared by MSS method wpectra obtained from 6 mol% Eu-doped CaTiO3, BaTiO3, and SrTiO3, prepared by MSS metermission [108]. Copyright 2016, Royal Society of Chemistry.

lO3 nanocrystals. The inset in Fig. a shows the local photoluminescence spectrum.

intensity was increased with increasing the Eu-doped concentra-tions, and a maximum value was reached at the doping level of 6mol%, yielding the highest PL red emission intensity. As the optimalEu-doping level of 6 mol% is kept as a standard, the optical prop-erties of the Eu-doped other perovskites such as SrTiO3, BaTiO3,and CaTiO3, are comparatively investigated, details are shown inFig. 15(b). Clearly, the expected Eu3+- emission bands correspond-ing to the f - f transitions, are observed for all of the examinedperovskite titanate structures. Among them, the Eu-doped CaTiO3nanoparticles exhibit the highest intensity of the PL red emission[108].

4.5. Photocatalytic properties

Recently, perovskite oxide nanomaterials have received muchattention in solar energy conversion due to their high conductionband gap position, high stability, solar absorption band gap. Espe-cially, the controllable physicochemical properties enable them asenergetic materials for catalysis. For example, BiFeO3 nanowiresand nanocubes exhibit photocatalytic abilities to produce O2 underUV irradiation [109,110]. It is also found that BiFeO3 nanoparticles

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coated with SrTiO3 can produce H2 under visible-light irradiationalthough the pure BiFeO3 nanoparticles are not able to producehydrogen [111]. Previous reports demonstrate that among theABO3-type perovskites, tantalates (e.g. NaTaO3), titanates (e.g.

ith different doping concentrations (i.e. 0, 2, 4, and 6%), and (b) Photoluminescencehod. Data are collected with an excitation wavelength of 399 nm. Reproduced with

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Fig. 16. Response characteristics of a gas sensor fabricated with single-crystallineLaFeO3 nanotubes. (a) Response rate of the gas sensor to five different gases. (b)The response rate as a function of the chlorine gas concentration measured at room

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rTiO3) as well as niobates (e.g. KNbO3, NaNbO3) exhibit excel-ent hydrogen production under UV-light irradiation [112–114]. Inddition, under UV-light irradiation perovskite NaTaO3 and KTaO3anomaterials also exhibit high photocatalytic activities for waterplitting [113,115]. However, most perovskite oxide nanomateri-ls primarily absorb UV light due to their wide bandgap (Eg) (e.g.aTaO3 Eg = 4.0 eV, KTaO3 Eg = 3.6 eV). Unfortunately, the UV lightnly accounts for 3.5% of the solar spectrum. On the contrary, theisible and near infrared (NIR) lights account for 48% and 44% of theolar spectrum, respectively. Therefore, to improve the water split-ing efficiency under sunlight irradiation, the development of newerovskite oxide nanomaterials that are sensitive to UV, visible, andIR lights at the same time become crucial issue [113]. It is alsooticed that the small absorption coefficients of perovskite oxideanomaterials further weakens their light harvesting capabilities.

n addition, to realize efficient charge separation and migration,erovskite oxides nanostructures with small size and high crys-allinity are also highly required because the small particle sizerovides higher diffusion length and electrons or holes can be eas-

ly transported to the catalyst surface for reaction. The high degreef crystalline structure also avoids electron’s trapping inside thearticles.

In recent years, besides the ideal and the modified ABO3-typeerovskite oxides, numerous layered perovskite oxides are alsoeported to be used as photocatalysts. For instance, the (110)-layertructured perovskites such as M2Nb2O7 (M = Ca, Sr) and R2Ti2O7R = Y, La-Yb) exhibit the highest photocatalytic rates, and theiruantum yields are reported to be as high as ∼20–50% under UV

rradiation [116,117]. In the past decade, the photocatalytic activ-ties of R2Ti2O7 for H2 and/or O2 have been studied as a functionf the different rare-earth elements (i.e. R = Y, La, Pr, Nd, Sm, Gd)118–120], transition-metal dopants for its sensitization to visibleight [121,122], as well as for the decomposition of organic pollu-ants [123,124]. Among the layer-structured R2Ti2O7 perovskites,a2Ti2O7 (R = La) has received the most attention due to its highhotocatalytic activity under sacrificial reagents or pure water.ecently, Arney et al. [74] reported the photocatalytic properties oferovskite platelet-like La2Ti2O7 particles with thicknesses below00 nm, which were synthesized by a modified MSS method. Thehotocatalytic properties of the platelet-like La2Ti2O7 productseasured under UV irradiation in aqueous methanol solutions,ere about 55–140 �mol H2 h−1g−1. Such photocatalytic rates are

early two times higher than that of the La2Ti2O7 powders preparedy solid-state reaction method. That is ascribed to the exposedrystallite edges in the platelet-like La2Ti2O7 products and their010) and (001) crystal faces play a key role in enhancing the pho-ocatalytic activity. To extend the application of layer-structureda2Ti2O7 perovskite into the visible-light region, doping approachith different metals (e.g. Ba, Sr, Ca, Cr, Fe and Rh) has been used

112,113,125,126]. Recently, Rh3+-doped La2Ti2O7 powders with alate-like morphology (prepared by MSS method) were reported toxhibit photocatalytic activity for H2 evolution under visible lightrradiation up to 540 nm in an aqueous methanol solution [126].he visible light activity was ascribed to the electron transitionrom the Rh3+ donor states to the conduction band, and the oxi-ized Rh4+ species went back to their original states by oxidizationf CH3OH. However, up to date the reported layered perovskiteso far have wide band gaps, thus, to extend their photocatalyticerformance into the visible light region band gap engineering isighly needed to shift their band gaps into visible light region, andven to the NIR region. As compared to the simple binary metalxides with d0, d10, and f0 metal ions (e.g. TiO2, Nb2O5, WO3, Ga2O3,

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nd CeO2), perovskite and perovskite related structures with threer more compositions, can provide a very wide platform to adjustheir physicochemical properties by controlling the compositionsnd stoichiometry. To rationally design and synthesize perovskite

temperature. (c) The response transients of the gas sensor to 5 ppm and 20 ppmchlorine at room temperature. Reproduced with permission [58]. Copyright 2006,IOP Publishing LTD.

oxide photocatalysts with the optimal photocatalytic activity, it is

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necessary to better understand the relationships between the com-positions, structures and photocatalytic performance of perovskiteoxide nanomaterials. In this direction, there is a long way to walk onbefore the commercialization of perovskite oxide nanomaterials.

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Fig. 17. (a) Schematic illustration for the measurement setup of random BiFeO3 nanofiber-based photovoltaic devices. (b) I - V curve for BiFeO3 nanofibers in dark andu ound

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nder illumination. Inset (i) shows expanded view of current density behavior arifferent deposition time from 1 to 4 h. Reproduced with permission [151]. Copyrigh

llumination. Inset shows enlarged I - V curve in the portion of dotted circle. Reprod

. Applications of low-dimensional PONs

Low-dimensional PONs are of great attraction and show poten-ial applications in different fields such as data memory storage,olar energy conversion, and so on. The ferroelectric random-ccess memories (FeRAM) are one of the main applications ofow-dimensional PONs in data memory storage, which are superioro the electrically erasable and programmable read-only mem-ries (EEPROM’s) and Flash memories in terms of write-accessime and overall power consumption. Their operational principlesre based on the reversible polarization of a perovskite ferro-lectric materials used in nonvolatile memory, and the directionf polarization switching representing the binary ‘1’ and ‘0’ inata storage [127–130]. Since the density of FeRAM nonvolatile-emory can be increased thousands-fold by reading and writing

n nanoparticle-based FeRAM, thus, low-dimensional PONs (e.g.anowires [131,132], nanotubes [133–135], and nanoparticles136–138]) have been widely investigated for potential applica-ion in storage memory. Owing to the high dielectric permittivityerovskite ferroelectric nanoparticles (e.g., BaTiO ) can be also

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3sed as the dielectric layer for constructing multilayered ceramicapacitors (MLCCs). Besides the practical applications in the MLCCs,erovskite ferroelectric nanoparticles are also use as a filling

zero-bias. Inset (ii) shows averaged photocurrent after several measurements for, American Chemical Society. (c) I - V curve for BiFeO3 nanowires in dark and under

with permission [152]. Copyright 2015, Elsevier Ltd.

dielectric material in the polymer-based nanocomposite struc-tures [139,140], which have promising applications in the fieldsof dielectric energy storage, flexible thin-film dielectric capaci-tors [141–143], and flexible electrical energy generators [144].For the applications of the potential energy harvesting from col-lective mechanical movements, piezoelectric oxide nanowires areinvestigated extensively [145–147]. For example, electrical energycollected from piezoelectric nanostructures has already been uti-lized effectively to power nanoelectronic devices [148] and sensors[149]. 1D perovskite piezoelectric nanostructures, especially piezo-electric oxide nanowires and rods, have mostly been used forpiezotronics applications due to their large mechanical straintolerance [145]. Particularly, vertically or horizontally alignedarrays of piezoelectric nanowires are in an ideal configuration forenergy harvesting applications due to their enhanced anisotropicpiezoresponse [148,149]. Besides energy harvesting application,perovskite oxide nanowires are also used for fabricating humid-ity sensors. For example, Li et al. [25] reported the response time(humidification from 11% to 95% humidity) and recovery time (des-iccation from 95% to 11% humidity) of the humidity sensors based

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on perovskite BaTiO3 nanowires, which were about 14 and 20 s,respectively. Such fast response and recovery behaviors demon-strate that BaTiO3 nanowires are suitable for high-performance

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umidity sensors. Similarly, perovskite oxide nanotubes are alsoidely investigated as potential building blocks for gas sensors.ecently, Wang et al. [58] synthesized single-crystalline LaFeO3anotubes with diameters of 40 − 200 nm and length up to 3 �my a co-precipitation method followed by MSS. Gas sensors fabri-ated from the as-prepared LaFeO3 nanotubes exhibit fast responseharacteristics (Fig. 16), and the detection limit of the sensorschieves 1 ppm Cl2 at room temperature. These sensors also exhibitood selectivity and stability (Fig. 16(c)). The (110)-layered per-vskite La2Ti2O7 photocatalysts synthesized via MSS method alsoxhibit a promising prospect in the photocatalytic area [74]. Theirhotocatalytic rates are nearly two times larger than that of theounterpart prepared by the solid-state method. As compared withiO2 photocatalysts, perovskite oxide photocatalysts have theirwn unique features such as tunable compositions, crystalline andlectronic/energy band structures, which in some cases lead toore effective photo-induced reduction and oxidation efficien-

ies. In addition to the photocatalytic properties, perovskite oxideslso exhibit photovoltaic behavior, which has important appli-ations in photovoltaic devices such as solar cells. The typicalxample is perovskite 1D BiFeO3 nanostructures with band gap of.5 eV [109,150]. Fig. 17(a) is schematic illustration for the mea-urement setup of random BiFeO3 nanofiber-based photovoltaicevices [151]. The I - V curves of BiFeO3 nanofibers and nanowiresnder dark and illumination condition, are shown in Fig. 17(b) andc), respectively [151,152]. It is found that much higher currentsre generated under the illumination as compared with the darkondition, demonstrating the excellent photovoltaic behavior ofD BiFeO3 nanostructures. It is reported that the current densityf BiFeO3 nanofibers (∼200 nm) in response to the applied voltage

s about 1 mA/cm2, which is 2 −10 times larger than that of BiFeO3hin films [151]. The enhanced photovoltaic properties of BiFeO3anofibers are ascribed to the free-standing nanofibers with ferro-lectric domains to be switched easily and more photons trappedy nanofibers due to their geometric confinement. In addition,he depolarization field may be helpful for driving electrons andoles in opposite directions, hampering their combination and thusnhancing the photovoltaic performance [152]. To date, perovskitexide photocatalysts exhibit much better activities in environmen-al remediation and solar fuel generation fields. It is believed thathe applications of low-dimensional PONs in photocatalytic andhotovoltaic devices have a bright future.

. Conclusions

Over the past decade, many low-dimensional perovskite oxideanostructures have been fabricated successfully by MSS methodnd its modified version. Extensive researches on the effects of dif-erent precursors, annealing temperature, soaking time, etc. in MSS

ethod have been conducted in depth. Despite the recent tremen-ous progresses are very inspiring but there are still some issues

or further researches. Apart from the common zero-dimensionalanoparticles, much more attention has been switched to 1D orD PONs and other special nanostructures. However, in the growthf 1D or 2D PONs by traditional MSS method, some problems arergent to be solved. For instance, the growth mechanisms of low-imensional PONs in MSS process are not well understood, andore improvements on synthesis of low-dimensional PONs with

ontrollable morphologies, sizes and crystallinity, require furtheresearches. In the applications of low-dimensional PONs in pho-ocatalysis, the fundamental understanding of the photocatalysis

Please cite this article in press as: P. Xue, et al., J. Mater. Sci. Technol. (

rocess and the relationships between the compositions, struc-ures and photocatalytic performance of low-dimensional PONshould be well understood. Therefore, ongoing and future effortshould pay attention to the bandgap engineering of perovskite

PRESS & Technology xxx (2017) xxx–xxx 15

oxide nanomaterials, making them be sensitive to UV, visible, andNIR lights and enhancing their photocatalytic performance. Over-all, we still have a relatively long journey to achieve this ambition.In this report, we provide a comprehensive review of the state-of-the-art research activities in the field of molten salt synthesisof PONs, microstructural characterization, physical properties, andpotential applications in the fields of storage memory, energy har-vesting, and solar energy conversion. This review will be a goodtutorial material for non-expert readers for understanding moltensalt synthesis of low-dimensional PONs. Also, it provides impor-tant updates to experts in these research areas. It is believed thatlow-dimensional PONs have important applications not only in thearea of perovskite oxide nanoelectronics, but also in energy fieldsincluding solar energy and energy storage. Much work remains tobe done in this area.

Acknowledgements

The authors acknowledge the financial support from theNational Natural Science Foundation of China (Grant Nos.11674161, 11174122 and 11134004), the Six Big Talent Peak Projectfrom Jiangsu Province (Grant No. XCL-004), and open project ofNational Laboratory of Solid State Microstructures, Nanjing Uni-versity (Grant No. M28026).

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