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Review: hydrothermal technology for smart materials

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DOI: 10.1080/17436753.2016.1157131

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Advances in Applied CeramicsStructural, Functional and Bioceramics

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Review: hydrothermal technology for smartmaterials

M. Shandilya, R. Rai & J. Singh

To cite this article: M. Shandilya, R. Rai & J. Singh (2016): Review: hydrothermal technology forsmart materials, Advances in Applied Ceramics, DOI: 10.1080/17436753.2016.1157131

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REVIEW

Review: hydrothermal technology for smartmaterialsM. Shandilya1, R. Rai∗1 and J. Singh2

In broad terms, hydrothermal synthesis is a technology for crystallising materials (chemicalcompounds) directly from aqueous solution by adept control of thermodynamic variables(temperature, pressure and composition). The objective of this review is to introduce the field ofhydrothermal materials synthesis and show how understanding solution thermodynamics of theaqueous medium can be used for engineering hydrothermal crystallisation processes. In thisreview, powder synthesis, and their applications are introduced. In Section ‘Introduction’, we willfocus on the hydrothermal synthesis as a materials synthesis technology by providing history,process definitions, technological merits and comments on its current implementation in thelaboratory. In Section ‘Scope of hydrothermal synthesis in future’, we will describe thedevelopment of hydrothermal technology for materials synthesis, their results and comparisonwith other methods.Keywords: Perovskite, BaTiO3, SrTiO3, BaSrTiO3, Hydrothermal technology

IntroductionThe term ‘hydrothermal’ is purely of geological origin. Itwas first used by the British geologist, Sir RoderickMurchison (1792–1871) to describe the action of waterat elevated temperature and pressure, in bringing aboutchanges in the earth’s crust leading to the formationof various rocks and minerals. It is well known thatthe largest single crystal formed in nature (beryl crystalof >1000 g) and some of the large quantity of singlecrystals created by a man in one experimental run(quartz crystals of several 1000s of grams) are both ofhydrothermal origin. Hydrothermal research wasinitiated in the middle of the nineteenth century by geol-ogists and was aimed at laboratory simulations of natu-ral hydrothermal phenomena. The initial work related tohydrothermal synthesis of materials is attributed toR. W. Bunsen, who grew barium and strontium carbon-ate at temperatures above 200°C and pressures above100 bars in 1839. After that in 1845, E. Schafhautl1

observed the formation of small quartz crystals upontransformation of precipitated silicic acid in a steamdigester, the forerunner of the autoclave. In 1948 Royand Osborn started to study the system Al2O3–SiO2–H2O, it became clear that the hydrothermal conditionwas the only process by which one could make certaincrystalline phases involving the tetra and pentavalentions, the hydroxylated ones.2 In 1956 Roy and Tuttleprovided the first comprehensive survey of mineral syn-thesis under hydrothermal conditions.2,3 In 1963 Roy

pointed out that hydrothermal method was the onlyway by which one could get ordered oxide phaseswhere Al–Si, Mg–Al and various vacancy-disorderingcould approach an equilibrium state.4 In 1950–70 Royand co-workers used the hydrothermal process to syn-thesise systematically for the first time the whole rangeof 7, 10 and 14 Å clay minerals and selected zeolitefamilies with an enormous range of compositions.5

With the availability of high-resolution SEM from1980 onwards hydrothermal researchers started observ-ing such fine products which were earlier discarded asfailures. The hydrothermal research in the 1990s marksthe beginning of the work on the processing of fine toultra-fine particles with a controlled size and mor-phology. The advanced ceramic materials preparedduring that time justify this statement. In the last twodecades, these sub-micrometre to nanosize crystallineproducts have created a revolution in science and tech-nology under a new terminology, ‘Nanotechnology’.Today hydrothermal researchers are able to understandsuch nanosize materials and control their formation pro-cess, which in turn give the desired properties to suchnanomaterials. Thus hydrothermal technology andnanotechnology have a very close link. In the twenty-first century, hydrothermal technology, on the whole,will not be just limited to the crystal growth, or leachingof metals, but it is going to take a very broad shape cov-ering several interdisciplinary branches of science.Therefore, it has to be viewed from a different perspec-tive. Further, the growing interest in enhancing thehydrothermal reaction kinetics using microwave, ultra-sonic, mechanical and electrochemical reactions will bedistinct.6 Also, the duration of experiments is beingreduced, which will in turn make the technique more

1School of Physics, Shoolini University, Solan, HP 173212, India2XRD Lab, SAIF, Panjab University, Chandigarh, India

∗Corresponding author, email [email protected]

© 2016 Institute of Materials, Minerals and MiningPublished by Taylor & Francis on behalf of the InstituteReceived 15 September 2015; accepted 12 February 2016DOI 10.1080/17436753.2016.1157131 Advances in Applied Ceramics 2016 1

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economic. It links all the important technologies likegeo-technology, biotechnology, nanotechnology andadvanced materials technology. Thus it is clear thatthe hydrothermal processing of advanced materials is ahighly interdisciplinary subject and the technique ispopularly used by physicists, chemists, ceramists, hydro-metallurgists, materials scientists, engineers, biologists,geologists and technologists, and the hydrothermal pro-cessing of materials is a part of solution processing andit can be described as superheated aqueous solution pro-cessing. Besides, for processing nanomaterials, thehydrothermal technique offers special advantagesbecause of the highly controlled diffusivity in a strongsolvent medium in a closed system. Nanomaterialsrequire control over their physicochemical character-istics, if they are to be used as functional materials. Asthe size is reduced to the nanometre range, the materialsexhibit peculiar and interesting mechanical and physicalproperties: increased mechanical strength, enhanced dif-fusivity, higher specific heat and electrical resistivitycompared to their conventional coarse-grained counter-parts due to a quantisation effect.7 The use of micro-wave radiation in the hydrothermal system introducedby Komarneni et al.8 promotes the development of anew technique offering reaction kinetic enhancement,formation of materials with different morphologies,low synthesis temperature and reduced processingtimes. Recently, the microwave-assisted hydrothermal(MAH) method has been successfully employed toobtain materials, such as ZnO,9 BaTiO3,

10 CuO11 andCeO2.

12 The main purpose of this study is to reviewsome important aspects related to the hydrothermalcrystallisation process for preparing some selectedoxide and non-oxide compound with electric, piezoelec-tric, ionic conducting and catalytic properties. Figure 1shows various branches of science either emergingfrom the hydrothermal technique or closely linked withthe hydrothermal technique. With an ever-increasingdemand for composite nanostructures, the hydrothermaltechnique offers a unique method for coating of variouscompounds on metals, polymers and ceramics as well asfor the fabrication of powders or bulk ceramic bodies. Ithas now emerged as a front line technology for the pro-cessing of advanced materials for nanotechnology. Onthe whole, hydrothermal technology in the twenty-firstcentury has altogether offered a new perspective whichis illustrated in Fig. 1. It links all the important technol-ogies like geo-technology, biotechnology, nanotechnol-ogy and advanced materials technology. Thus, it isclear that the hydrothermal processing of advancedmaterials is a highly interdisciplinary subject and thetechnique is popularly used by physicists, chemists, cera-mists, hydrometallurgists, materials scientists, engineers,biologists, geologists, technologists, and so on. Nowa-days, the conventional hydrothermal methods as wellas its variants have emerged as a versatile synthesisoption for the preparation of multifunctional ceramicsmaterials including electronic ceramics, bioceramics, cat-alysts, catalyst supports, membranes and ceramics withoptical properties, among others.13

Smart materialsSmart materials are those materials which have one ormore properties that can be significantly changed in a

controlled manner by different fields, such as temperature,pressure, electric flow, magnetic flow, light, mechanical,etc., originating internally or externally.

Properties of smart materials(i) Sensing materials and devices, (ii) actuation materialsand devices, (iii) control devices and techniques, (iv)self-detection, self-diagnostic, (v) self-corrective, self-con-trolled, self-healing and (vi) shock absorbers, damagearrest.

Classification of smart materials(i) Piezoelectric materials: When subjected to an elec-tric charge or a variation in voltage, piezoelectricmaterial will undergo some mechanical change, andvice versa. These events are called the direct and con-verse effects. Piezoelectric effect in ferroelectricmaterials is a basis for a large number of devices includ-ing pressure, force and vibration sensors, acceler-ometers, displacement actuators, force generators,transformers, gyroscopes and high-frequency transdu-cers. Piezoelectric materials are among the most prom-ising candidates for active components in the newgeneration of so-called micro-electromechanical sys-tems. In many cases, properties of piezoelectricelements must be tuned to better satisfy requirementsof high-performance devices.(ii) Electrostrictive materials: This material has thesame properties as piezoelectric material, but the mech-anical change is proportional to the square of the elec-tric field. This characteristic will always producedisplacements in the same direction.(iii) Magnetostrictive materials: When subjected to amagnetic field, and vice versa (direct and converseeffects), this material will undergo an induced mechan-ical strain. Consequently, it can be used as sensors and/or actuators.(iv) Rheological materials: These are in liquid phasewhich can change state instantly through the appli-cation of an electric or magnetic charge. These fluidsmay find applications in brakes, shock absorbers anddampers for vehicle seats.(v) Thermo-responsive materials: Thermo-responsive isthe ability of a material to change properties in responseto changes in temperature. They are useful in thermo-stats and in parts of automobiles and air vehicles.(vi) Electrochromic materials: Electrochromic is theability of a material to change its optical properties(e.g., colour) when a voltage is applied across it. Theyare used in LCDs and cathodes in lithium batteries.(vii) Fullerenes: These are spherically caged moleculeswith carbon atoms at the corner of a polyhedral struc-ture consisting of pentagons and hexagons. These areusually used in polymeric matrices for use in smart sys-tems. They are used in electronic and microelectronicdevices, super-conductors, optical devices, etc.(viii) Biomimetic materials: The materials and struc-tures involved in natural systems have the capabilityto sense their environment, process the data andrespond instantly. For example: to allow leaf surfacesto follow the direction of sunlight and essentially areal-time change in the load path through the structureto avoid overload of a damaged region. The field of

Shandilya et al. Hydrothermal technology for smart materials

2 Advances in Applied Ceramics 2016

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biomimetic materials explores the possibility of engin-eering materials and structures.(ix) Smart gels: These are gels that can shrink or swellby several orders of magnitude. Some of these can alsobe programmed to absorb or release fluids in responseto a chemical or physical stimulus. These gels areused in areas such as food, drug delivery, organ replace-ment and chemical processing.

Scope of hydrothermal synthesis infutureScope of hydrothermal synthesis in future depends uponthe advantages and disadvantages of hydrothermal pro-cess. To weigh the advantages and disadvantages ofhydrothermal powder processing, the following aspectsmay be considered. Hydrothermal synthesis offers many

advantages over conventional and non-conventional cer-amic synthetic methods. All forms of ceramics can be pre-pared by hydrothermal synthesis, namely powders, fibresand single crystals, monolithic ceramic bodies, and coat-ings on metals, polymers and ceramics. From the stand-point of ceramic powder production, there are far fewertime- and energy-consuming processing steps since high-temperature calcinations, mixing and milling steps areeither not necessary or minimised. Moreover, the abilityto precipitate already crystallised powders directly fromsolution regulates the rate and uniformity of nucleation,growth and ageing, which results in improved control ofsize and morphology of crystallites and significantlyreduced aggregation levels, that is not possible withmany other synthesis processes.14 The elimination/reduction of aggregates combined with narrow particlesize distributions in the starting powders leads to opti-mised and reproducible properties of ceramics because

1 Hydrothermal flow chart with different branches of science and technology


Advances in Applied Ceramics 2016 3

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of better microstructure control. Precise control of pow-der morphology can also be significant. For instance,powders with crystallites having well-developed shapescorresponding to particular crystallographic directions,such as whiskers, plates or cubes, can be oriented toform materials with single crystal-like properties, suchas polymer–ceramic composites or textured ceramic–cer-amic composites with anisotropic properties. Hydrother-mal processing can take place in a wide variety ofcombinations of aqueous and solvent mixture-based sys-tems. In general, processing with liquids allows for auto-mation of a wide range of unit operations such ascharging, transportation and mixing and product separ-ation. Moreover, relative to solid-state processes, liquidsgive a possibility for acceleration of diffusion, adsorption,reaction rate and crystallisation, especially under hydro-thermal conditions. However, unlike many advancedmethods that can prepare a large variety of forms andchemical compounds, such as chemical vapour-basedmethods, the respective costs for instrumentation, energyand precursors are far less for hydrothermal methods.From Table 1 it follows that the hydrothermal synthesisof ceramic powders provides two major advantages: (i)the elimination or at least minimisation of any high-temp-erature calcinations or conditioning steps and (ii) the util-isation of relatively inexpensive precursor materials.While generally not limited to oxide compounds, the tech-nique is particularly suitable for preparing not onlyadvanced functional ceramic powders such as PZT orBNT as well as a wide range of magnetic ferrite oxideswith magneto plumbite or spinel structures but alsoadvanced structural oxide ceramics such as alumina andzirconia.Consequently, hydrothermal synthesis will find its com-

mercial niche precisely in the area of producing advancedelectronic ceramics if process-related and economicobstacles can be successfully overcome in the future.Some advantages are:(i) The process utilises comparatively inexpensive pre-cursor chemicals such as oxides, hydroxides, chlorides,acetates and nitrates rather than the expensive alkox-ides required for sol–gel processing.(ii) Reactants that are normally volatile at the requiredreaction temperature tend to condense during thehydrothermal process and thus maintain the reactionstoichiometry. Consequently, highly pure, multicompo-nent anhydrous ferroelectric powders can be obtained.(iii) Hydrothermal synthesis is a low-temperature pro-cess with many effects achievable even below 300°C.The relatively low temperature can break down stableprecursors under pressure, thus avoiding the extensiveagglomeration that solid-state reactions usually causeat high sintering temperature. The low reaction temp-eratures also avoid other problems encountered withhigh-temperature processes, for example, poor stoichi-ometry control due to volatilisation of components (e.g., Pb volatilisation in Pb-based ceramics).(iv) The process is amenable to produce solid solutionparticles with controlled size distribution, shape andcomplex chemical composition. Multi-doped perovs-kite ABO3 ceramic powders, for example, can begrown down to sub-micrometre or even nanometresize by close control of the nucleation and growth steps.(v) Hydrothermal synthesis is that the purity of hydro-thermally synthesised powders significantly exceeds the

purity of the starting materials. It is because the hydro-thermal crystallisation is a self-purifying process,during which the growing crystals/crystallites tend toreject impurities present in the growth environment.The impurities are subsequently removed from the sys-tem together with the crystallising solution, which doesnot take place during other synthesis routes, such ashigh-temperature calcination.(vi) Powders grown by the hydrothermal process rarelyrequire pre-sintering or calcination steps. This is par-ticularly important for synthesising high-quality PZTpowders since lead oxide is quite volatile at conven-tional calcination or sintering temperatures.16

(vii) Synthesis is accomplished in a closed system fromwhich different chemicals can be recovered andrecycled, thus making it an environmentally benigntechnology.(viii) The process can be easily scaled up to industrialdemand since hydrothermal synthesis in principlelends the opportunity for cost-effective and reproduci-ble production of high-quality ceramic powders on alarge industrial scale. Hydrothermal methods aremore environmentally benign than many other syn-thesis methods, which can be attributed in part toenergy-conserving low processing temperatures,absence of milling, ability to recycle waste and safeand convenient disposal of waste that cannot berecycled.14

Materials synthesised under hydrothermal conditionsshow improvement in point defects when compared tomaterials prepared by high-temperature synthesismethods. For instance, in barium titanate (BT), hydroxya-patite or α-quartz, water-related lattice defects are amongthe most common impurities and their concentrationdetermines essential properties of these materials. Theproblem of water incorporation can be overcome by eitherproperly adjusting the synthesis conditions or by use ofnon-aqueous solvents (solvothermal processing). Anotherimportant technological advantage of the hydrothermaltechnique is its capability for continuous materials pro-duction, which can be particularly useful in continuousfabrication of ceramic powders.17

These decisive advantages have to be judged againstsome disadvantages of the hydrothermal processing tech-nology. Thus investigated numerous unary, binary andternary oxide syntheses using the modified hydrothermal(microwave hydrothermal (MH)) process, which wasshown to (a) lead to rapid heating to temperature ofheat treatment, (b) increase the kinetics of reaction byone to two orders of magnitude, (c) lead to the formationof some novel phases, and (d) lead to selective crystalliza-tion of phases in the chemical system used. Materials syn-thesised under hydrothermal conditions often exhibitdifferences in point defects when compared to materialsprepared by high-temperature synthesis methods. Forinstance, tungstates of Ca, Ba and Sr synthesised atroom temperature by an hydrothermal–electrochemicalmethod do not contain Schottky defects usually presentin similar materials prepared at high temperatures,18

which results in improved luminescent properties. Othertypes of defects, such as hydroxyl ions substituted for oxy-gen ions in BT generate barium ion vacancies, arebelieved to degrade the dielectric properties.19 Synthesesare usually conducted at autogenesis pressure, which cor-responds to the vapour pressure above the solution at the



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specified temperature and composition of the hydrother-mal solution. However, in the case of hydrothermalgrowth of single crystals, additional pressure adjustmentis done to control solubility and growth rate. Reactantsused in hydrothermal synthesis are generally called pre-cursors, which are administered in the form of solutions,gels and suspensions. Mineralisers are either inorganicor organic additives that are often used to control pH,but are used at excessively high concentrations (e.g., 10mol) to also promote solubility. Other additives, alsoorganic or inorganic, are used to serve other functionssuch as promote particle dispersion or control crystalmorphology (Table 2).

Types of hydrothermal methodAmajor advantage of hydrothermal synthesis is the myr-iad of ways the technology can be hybridised with otherprocesses to gain advantages such as enhancement ofreaction kinetics or the ability to make new materials.A great amount of work has been done to enhancehydrothermal synthesis, by hybridising this methodwith microwaves, electrochemistry, ultrasound, mechan-ochemistry, optical radiation, hot-pressing and manyother processes.

Microwave–hydrothermal processingThis method is used mostly for synthesis of ceramic pow-ders. It enhances crystallisation kinetics by l− 2 orders ofmagnitude with respect to standard hydrothermal proces-sing. Additional advantages of this method are very highheating rates and the synthesis of novel phases. Thehydrothermal–microwave technique has been used to syn-thesise different ceramic powders with controlled size andmorphology, such as TiO2, ZrO2, Fe2O3, BaTiO3, hydro-xyapatite, etc. Among the chemical methods, hydrother-mal synthesis is often used due to its simplicity, allowingthe control of grain size, morphology and degree of crys-tallinity by easy changes in the experimental procedure. Avariation of this method, MAH synthesis, has the advan-tage of lower processing temperature and time, and a uni-form nucleation of the powders in suspension.22,23 TheMAH method is successfully used to evaluate the influ-ence of the order–disorder degree of self-assembled nano-particles as function of cation exchange on the photoluminescent properties. The greater local disorder forthe samples oriented crystal growth and contributes toan increase in photoluminescence (PL) emission. Thisbehaviour indicates that, although the material has well-defined crystal structure and morphology, these proper-ties are governed by the intrinsic defects of each clusterand how they are arranged to form the crystal. Anotherinteresting feature of microwave heating is its selectiveheating characteristic. Since different materials showdifferent abilities to absorb microwave energy and to con-vert it into heat (loss factor), this phenomenon could beuseful to select a suitable reaction system. Therefore, areaction medium with high microwave absorbing proper-ties can be chosen looking forward to efficient and rapidheating. The use of ionic liquids as solvents for hydrother-mal reactions is a good example of this. In the same sense,the loss factor phenomenon is the reason why some ferritematerials can be synthesised at low temperatures onlywhen microwave heating is used.24 Tadjarodi et al.25Ta

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≥10

>99

.9Mod

erate

Yes

Yes

Exce

llent

Hyd

rothermal

Exce

llent

Goo

d,high

Goo

d,high

≥10

>99

.5Po

orNo

No

Mod

erate



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prepared successfully ZnFe2O4 nanoparticles, ironhydroxyl phosphates (NH4Fe2(PO4)2OH·2H2O) nanos-tructures26 as well as α-FeOOH hollow spheres and α-Fe2O3 nanoparticles

27 via MAH ionic liquid synthesis.Other synthesised materials are Fe3O4 nanorods

27 andSnO2 microspheres.

28

Sol–gel–hydrothermal methodThe structural and morphological characteristics of sol–gel–hydrothermal prepared ceramic powders often varyas a function of the synthesis parameters, such as reactiontime, reaction temperature and composition. Thus thestudy of how these parameters influence the ceramic com-position is considered important, first to explain the fun-damental crystallisation process and second to optimisethe material properties for potential technological appli-cations. Recent studies indicate that the chlorides are auseful starting material at 180°C temperature in an oxy-gen atmosphere for sol-gel hydrothermal method.29

Then effect to the synthesis conditions (presence of chlor-ide ions and alkali metal ions) and the different moleratios and on the characteristics of the resulting com-pound is analysed. Finally, theoretical calculations werecarried out to help with the interpretation of the exper-imental results. From TEM analysis spherical particlesof 120–300 nm were observed to gradually convert intoaggregates of very fine particles. Notably, this type of dis-integration is usually related to the hydroxylation in thesol–gel process in the early stage of hydrolysis. The for-mation of BaxSr1−xTiO3 (BST) crystallites then occurs

inside the segregates because the diffusion distance isdecreased and the driving force is locally increased bythe presence of particles with a lower radius of curvature.It is found that this method to be simple, without usingexpensive apparatus. The typical experimental proceduresinclude:Sol–gel preparation: According to the stoichiometricratio of composition, the raw materials of elements inthe form of nitrate or acetate were weighted and thenintroduced into the solution of tetrabutyl titanate inethanol. In this step, the control of pH value is impor-tant to obtain homogeneous sol. Then, the sol washeated at 80°C to produce a dry gel.Hydrothermal treatments: The obtained gel precursorwas poured into a Teflon vessel with surfactantaddition, and then subjected to hydrothermal treatmentat an appropriate temperature under auto-generatedpressure. After cooling, the product was filtered,washed with distilled water and dried at the ambienttemperature. The main difference between sol–gel–hydrothermal method and sol–gel method is that insol–gel method calcination is performed after dryingthe gel.

Hydrothermal–electrochemical synthesisThis method combines the hydrothermal method withelectrochemical treatment and involves deposition ofpolycrystalline oxide films on reactive metal electrodesubstrates. It is particularly important when crystallineoxide products cannot precipitate from solution in the

Table 2 Comparison of different advanced powder synthesis routes with hydrothermal methods13,20,21

S.No. Hydrothermal methods Other methods

1. Acetates, isopropoxides, oxides, nitrates and calorids withlow purity. It is because the hydrothermal crystallisation is aself-purifying process.

In sol–gel solid state and other methods chemicals used areoxides, acetates, isopropoxides and oxy-nitrates with highpurity.

2. The process utilises comparatively inexpensive precursorchemicals. Another important advantage of the hydrothermalsynthesis is that the purity of hydrothermally synthesisedpowders significantly exceeds the purity of the startingmaterials.

High-purity chemicals are expensive alkoxides required forsol–gel solid state and other methods processing.

3. There is no need of calcination, only hydrothermal treatmentis required at very low temperature. The low reactiontemperatures also avoid other problems, for example, poorstoichiometry control due to volatilisation of components.Calcination, mixing and milling steps are either notnecessary or minimised. However, in addition to precursorphase preparation, pure and well-crystallised materials canbe synthesised directly by hydrothermal reactions, thusavoiding further thermal treatments.

In sol–gel and conventional method the dried powder wascalcined at high temperature for a couple of hours thereforesafe and costly furnace is required. From the standpoint ofceramic powder production, there are far fewer time- andenergy-consuming processing steps since high-temperature calcination, mixing, milling steps, etc., areeither not necessary or minimised.

4. The conventional hydrothermal method as well as its variantshas emerged as a versatile synthesis option for thepreparation of multifunctional ceramic materials, includingelectronic ceramics, bioceramics, catalysts, catalystsupports, membranes and ceramics with optical properties,among others.

Other methods are not so versatile. Hydrothermal methodset-up is easy and safe for performing because it is anenvironment friendly method compared to others.

5. The process can be easily scaled up to industrial demandsince hydrothermal synthesis in principle lends theopportunity for cost-effective and reproducible production ofhigh-quality ceramic powders on a large industrial scale.

Sol–gel and co-precipitation techniques cannot be easilyscaled up to industrial demand because low-quality ceramicpowders.

6 Hydrothermal crystallisation also shows some advantagesover other non-conventional processes of soft chemistrybecause in hydrothermal crystallisation the reaction timesare shorter with a good control of the crystallisation, crystalsize, purity and even morphology of the products.

On the other hand sol–gel and co-precipitation techniqueshave less control of the crystallisation and even morphologyof the products.



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absence of an applied electrical potential. Highly crystal-lised ceramic thin films, such as BaTiO3, SrTiO3, LiNiO2,PbTiO3 and CaWO4, can be deposited on metallic sub-strates from aqueous solutions at low temperatures(100–200°C) within several hours by the hydrothermal–electrochemical method.30,31

The hydrothermal–electrochemical technique alsoenables fabrication of ceramic super lattices. Alternatelayers of thallium (RI) oxide films (7 nm thick) withdifferent defect structures were deposited using thismethod.32 Figure 2 shows the schematic illustration ofthe electrochemical cell and circuit arrangements underhydrothermal condition. Monomolecular layers of semi-conductors, such as GaAs, CdTe, CdSe and CdS, aremade by electrochemical atomic layer epitaxial, which isanalogous to molecular beam epitaxial, but instead usesaqueous solutions instead of a vapour phase for transportof growth species.33,34

Mechanochemical–hydrothermal synthesisMechanochemical hybridises hydrothermal synthesis andthe classical mechanochemical powder synthesis, which isa solid-state synthesis method that takes advantage of theperturbation of surface-bonded species by pressure toenhance thermodynamic and kinetic reactions between

solids.35 Materials such as PbTiO3 and hydroxyapatitehave been made with this approach.35,36 It is knownthat high pressures in excess of 1 GPa catalyse low-temp-erature solid-state reactions in ceramic materials byorders of magnitude.37 Mechanochemical–hydrothermalsynthesis utilises the solvency of an aqueous solution,which capitalises on using the pressure environment pro-vided by the mechanochemical reactor to accelerate oneor more the rate-determining steps that limit the lowertemperature for hydrothermal reactions such as inter-facial reaction, crystal dissolution or dehydroxylation.The mechanochemical activation of slurries can generatelocalised zones of high temperature and high pressure dueto friction and adiabatic heating of gas bubbles (if presentin the slung), while maintaining the average temperatureclose to the room temperature.38 Since any type of millor comminuting equipment can be used, the mechano-chemical–hydrothermal route hydrothermal crystallisa-tion of ceramics offers the potential for process scale-upyet eliminates the need for use of a pressure vessel or exter-nal heating (Fig. 3).However, control of particle size, morphology and

aggregation is a challenge for this method since currentmethods fail to regulate rates of nucleation, growth andageing, as well as conventional hydrothermal technologiesare able to. To address this issue, recent work40 has incor-porated the use of emulsions in mechanochemical–hydro-thermal reactors to better regulate nucleation and growth,which resulted in hydroxyapatite that is far less aggre-gated. A mechanochemical–hydrothermal reactionmethod has been used to promote reaction of the silicondioxide in water. This method has previously beenshown to work in theMgO–SiO2 andMg(OH)2–SiO2 sys-tem where amorphous products were obtained throughhydrothermal reactions.41 Temuujin et al. was able toshow that hydrated oxides react faster mechano-chemicalthan for anhydrous oxide mixtures.42 The mechanochem-ical reactions may involve mechanical stress. Mechanicaltreatment to increase the reactivity of solids has beenknown in the ceramics industry for a long time, as amethod for generating new surfaces and various defects.43

Although the mechanochemical method appears promis-ing for the synthesis of ceramic precursors, it has somepossible disadvantages: (i) high energy consumption; (ii)possible contaminations from the milling media.

2 Schematic illustration of the electrochemical cell and cir-cuit arrangements for anodic oxidation of Ti metal plateunder hydrothermal condition. https://www.google.co.in/search?q=images+of+Hydrothermal-electrochemical+synthesis

3 A commercially available mechanochemical–hydrothermal reactor39



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https://www.google.co.in/search?q=images+of+Hydrothermal-electrochemical+synthesishttps://www.google.co.in/search?q=images+of+Hydrothermal-electrochemical+synthesishttps://www.google.co.in/search?q=images+of+Hydrothermal-electrochemical+synthesis

Ultrasound-assisted hydrothermalcrystallisationThis method hybridises hydrothermal synthesis with ultra-sound (acoustic 20 kHz–10 MHz). Ultrasound is knownto accelerate the reaction kinetics by as much as two ordersof magnitude.7 This has been attributed to sharp tempera-ture gradients with localised peak temperature zones thatare speculated to be as high as 5000 K and localised peakpressure zones of up to 180 MPa, while maintaining theaverage temperature close to room temperature.44 Thesegradients create cavitation in bubble collapse events thatinhibit the formation of agglomerates or aggregates duringcrystallisation. The sonochemical environment is also con-sidered to alter molecular chemistry (chemical bond scis-sion, generate excited states and accelerate electrontransfer steps in chemical reactions), and enhance masstransport and crystallisation kinetics.44 In sonochemicalsynthesis the sonication of solution is performed by a sonicsuntil the completely precipitated product was reached. As-obtained intermediate products were then loaded into aTeflon-lined stainless steel autoclave for hydrothermal pro-cess. Fine nanoparticles of anatase TiO2 with size less than10 nm can be obtained by single sonochemical process for30 min. The amelioration in their shape regularity and crys-tallite size can be achieved by the incorporation of hydro-thermal process for short reaction time.45 A photo of thetransducer and the schematic vibration mode are shownin Fig. 4a and b, respectively. The piezoelectric parts areoutside the reaction vessel, and the irradiation surface isinside. Such a design enables the solution to be directlyexposed to the high-power ultrasonic irradiation.The performance degradation of the previous ultrasonic

transducer under a high-temperature condition was thoughtto be reduced because of the lower pre-stress to the piezoelec-tric devices caused by the thermal expansion of the bolt-tigh-tening metal parts and piezoelectric parts. Therefore, toimprove the performance under a high-temperature con-dition, we clamped the piezoelectric device between duralu-min washers with a high thermal expansion coefficient.Such a structure can prevent a degradation of the pre-stressto the piezoelectric device due to the temperature rise. Inaddition, to magnify the vibration speed at the tip, wedesigned a horn structure. The transducer was held tightlyat the nodal position, as shown in Fig. 4. This metal partfor holding is also utilised as one part of the container lid.46,47

Hydrothermal–photochemical synthesisThis method utilises laser irradiation to increase growthrates by an order of magnitude for ceramics and three

orders of magnitude for metals.48,49 Moreover, it enablesprecise patterning of thin films with a resolution of 1pm, which is essential in the integration of hydrothermaltechniques with other device fabrication technologies.Enhancement of the reaction rate can be attributed totemperature, diffusional enhancement due to light-induced thermal gradients that micro-stir the solutionand/or photo-chemistry.19 Examples of ceramics syn-thesised by the hydrothermal–photochemical includethin films of Ni-, Zn-, Co- and Mn-ferrites, Tl2O3 andFe3O4.

Hydrothermal hot-pressingIt is a simple and effective fabrication technique forshaped ceramics under mild conditions (T= l00–350°C,PC 25 MPa), within a short reaction time below 1 h,often in only one processing step (reactive hydrothermalsintering or hot-pressing). This technique is very usefulfor solidification of radioactive wastes and sludgeashes.50 The process involves compacting a ceramic pow-der or its precursor under hydrothermal conditions eitherin a special hot-pressing apparatus where uniaxialpressure can be applied or simply in a metal capsule.51

Another possibility is direct hydrothermal sintering of apressed pellet of powder. During the hydrothermal treat-ment, mass transport leading to densification occursmostly by a dissolution precipitation mechanism. Theresulting materials are usually very porous, but exhibitfairly good mechanical properties. Nevertheless, relativedensities as high as 94% have been reported.49 Low proces-sing temperatures enable the incorporation of organiccomponents that can improve mechanical strength of theporous ceramics. Examples of ceramics synthesised and/or densified by this method include zirconia, titania, silica,calcium carbonate, strontium carbonate, magnesium car-bonate, hydroxyapatite, glass and mica (Fig. 5).

Hydrothermal synthesis for simpleoxidesThe literature on the hydrothermal crystallisation of oxideand non-oxide powders is vast; a particular attempt ismade to broaden the traditional concepts of processingperovskite powders with controlled chemical compositionand particle morphology, those aspects will be discussedbased on the chemical reactivity of the precursor reactants(gels). Furthermore, an additional approach that takesinto account the solubility of the solid species (mineralreagents) that are employed as a precursor in the

4 a Ultrasonic transducer and reaction container. b Schematic of the ultrasonic assist system46



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hydrothermal systems on controlling the crystallisationprocess of oxide particles was further investigated for pre-paring titanates oxide ceramic. The specific reaction path-ways and kinetic aspects are discussed and illustrated byexperimental set-ups for the solution of selected problemsin hydrothermal crystallisation. Therefore, this methodcould be attractive for preparing net-shaped materialswith controlled porosity, with optimised functional prop-erties, because can be used as gas sensor, substrates forporous catalytic materials, filters, among other potentialapplications. Most common are oxide materials, bothsimple oxides, such as ZrO2, TiO2, SiO2, ZnO, Fe2O3,Al2O3, CeO2, SnO2, Sb2O5, Co3O4, HfO2, etc.

CuO flower-nanostructureHydrothermal method has gained space as a versatilemethod for preparation of copper oxide in temperaturesranging from 373 to 473 K for different times.52–54 CuOflower-nanostructures were synthesised by a hydrother-mal microwave method in the presence of polyethyleneglycol (PEG). The experimental details were as follows:5 × 10−3 mol L−1 of CuCO3·Cu(OH)2 (99% purity,Aldrich) and 0.1 g of PEG (Mw 400) (99.9% purity,Acros Organics) were added to 100 mL of deionisedH2O. The solution was stirred for 15 min and later 5mL of NH4OH (30% in NH3, Acros Organics) wasadded under constant stirring, producing an intenseblue precipitate of [Cu(NH3)4]

2+. The resulting solutionwas transferred into a sealed Teflon autoclave and placedin a domestic microwave. Finally, the reactional systemwas heat-treated at 393 K for 1 h and washed with deio-nised water several times and then dried at 353 K in ahot plate.

CuO flower-nanostructures were synthesised by a dom-estic hydrothermal microwave method after thermal treat-ment at 393 K for 1 h. The CuO flower-nanostructureswith single phase were identified by XRD and micro-Raman techniques and present monoclinic lattice. Theaverage diameter is 1.3 nm and the fast formation ofCuO flower-nanostructures is caused by the microwaveirradiation in the presence of PEG and NH4OH. TEMmicrographics showed that the thorn presents an averagediameter of 13.28 nm. The obtained CuO flower-nanos-tructures are a promising candidate for potential appli-cation in catalysis (Fig. 6).11

Zinc oxideZinc oxide, famous for their properties, such as highchemical stability, high electrochemical coupling coeffi-cient, broad range of radiation absorption and highphoto stability, is a multifunctional material.55,56 Inmaterials science, zinc oxide is classified as a semiconduc-tor, whose covalence is on the boundary between ionic andcovalent semiconductors. A broad energy band (3.37 eV),high bond energy (60 meV) and high thermal and mech-anical stability at room temperature make it attractivefor potential use in electronics, optoelectronics and lasertechnology.57–59 The piezo- and pyroelectric propertiesof ZnO mean that it can be used as a sensor, converter,energy generator and photocatalyst in hydrogen pro-duction.60–62 Because of its hardness, rigidity and piezo-electric constant it is an important material in theceramics industry, while its low toxicity, biocompatibilityand biodegradability make it a material of interest for bio-medicine and in pro-ecological systems (Fig. 7).63–65

Zinc oxide is a multifunctional material because of itsmany interesting properties (piezo- and pyroelectric), awide range ofUVabsorption, high photostability, biocom-patibility and biodegradability. ZnO can also be obtainedwith avarietyof particle structures, which determine its usein newmaterials and potential applications in awide rangeof fields of technology. Therefore the development of amethod of synthesising crystalline zinc oxide which can

5 Processing procedure used for preparing ferroelectricpowders by hydrothermal method

6 Mechanism of the CuO flower-nanostructures formation incomparison to a flower with thorns11



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be used on an industrial scale has become a subject ofgrowing interest in science as well as industry (Fig. 8).

ZrO2 nanoparticlesNanocrystalline particles play an important role in optoe-lectronic applications and microscopic physics research.These nanocrystalline particles exhibit size-dependentproperties, and novel optical, electronic, magnetic andmechanical properties that cannot be achieved usingtheir bulk counterparts.66–68 Zirconium oxide is one ofthe most intensively studied materials owing to its techno-logically important applications in oxygen sensors, fuelcell electrolytes, catalysts and catalytic supports, metaloxide semiconductor devices, superior thermal andchemical stability, etc.69 Many different methods of pro-ducing nanosize zirconium oxide powder were describedin the literature, such as sol–gel processing, hydrothermalprocessing and precipitation method.69,70 Among themethods listed above, hydrothermal synthesis meets theincreasing demand for the direct preparation of crystal-line ceramic powders and offers a low temperaturealternative to conventional powder synthesis techniquein the production of anhydrous oxide powders. This tech-nique can produce fine, high purity and stoichiometricparticles of single and multi-component metal oxides.Furthermore, if the process conditions such as solutionpH, solute concentration, reaction temperature, reactiontime, seed materials and the type of solvent are carefullycontrolled, the zirconium oxide particles with desiredshape and size can be produced.71 A simple low-tempera-ture hydrothermal process was used to prepare thesmall-sized tetragonal ZrO2 nanocrystallites. The spheri-cal-shaped particles with average sizes of 5 and 7 nmwere obtained, which was confirmed by SEM, XRDand TEM studies. The room temperature PL emissionspectra revealed that the obtained emission bands wereattributed to the surface defects and oxygen vacancies inthe material. This work would be meaningful to providea methodology to synthesise ultrafine nanomaterial.This method can be applied to a wide range of materialsfor various branched nanostructures, which may serveas potential building blocks in different advanced nanodevices.

TiO2 nanoparticlesTiO2, an exceptionally important material for applicationin photocatalysis, solar-photovoltaic, ceramic material,filler, coating, pigment and cosmetics, has been attractingattention in both fundamental research and practicaldevelopment work. Titanium dioxide occurs as twoimportant polymorphs, the stable rutile and metastableanatase. These polymorphs exhibit different propertiesand consequently different photocatalytic perform-ances.72 Reports of TiO2 with different shapes such asnanoparticles, thin films, nanorods, nanowires and nano-tubes have spurred a great interest in studies on TiO2nanostructure synthesis and their application.73 Nanoma-terials with different shape and structure usually have var-ied chemical, optical and electrical properties. Shapecontrol has been a significant concern in nanotechnology.Properties also vary as the shapes of the shrinking nano-materials change.74 Many excellent reviews and reportson the preparation and properties of nanomaterials havebeen published recently. The specific surface area and sur-face-to-volume ratio increase dramatically as the size of amaterial decreases. The performance of TiO2-baseddevices is largely influenced by the sizes of the TiO2 build-ing units, apparently at the nanometre scale.

Comparison of TiO2 nanoparticles synthesised by sol–geland hydrothermal method

The microstructure of the TiO2 nanoparticles synthesisedby sol–gel and hydrothermal method in the present studywas observed by FESEM. As it can be seen in the mor-phologies of TiO2 nanoparticles, the as-prepared (sol–gel)sample shows particle with great aggregation. The size ofthe particle is around 50 nm. The shape of the particle isnot uniform and it looks like spherical in shape. The micro-structure of the sol–gel sample calcined at 300°C showsreduction in the agglomeration. The formed nanoparticlesare visible clearly. Here also the shape of the particle wasalmost sphere-like morphology with different size. Furtherincrease of the calcination temperature to 600°C did notshow much difference in the morphology of the product.But the visibility of the separate nanoparticles is increased.Also the size distribution is almost uniform compared toother particles. But the samples treated hydrothermally

7 Examples of zinc oxide structure: flower a; rods b; wires c,d57



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show much difference compared to sol–gel method. Thedistribution of the particle is very uniform and the size ofthe particle is almost same. It is observed that the particlepossesses clear spherical shape.73

Titania applications

The primary application of titanium dioxide is as a whitepigment in paints, food colouring, cosmetics, toothpastes,polymers and other instances in which white colorationsare desired.75 The reason for this is the high refractiveindices of rutile and anatase, which result in high reflectiv-ity from the surfaces. Consequently, titania of small par-ticle size and correspondingly high surface areas areused owing to the resultant opacifying power and bright-ness. However, paints utilise polymeric binders to fix thepigment and, when in contact with titania, the polymermay oxidise when exposed to sunlight. This effect isknown as chalking and, in addition to the direct degrad-ing effect of ultraviolet (UV) radiation, is accelerated bythe photocatalytic activity of TiO2, which also isenhanced by the high surface area of this material76

(Table 3).

CeO2 nanoparticlesRare earth oxide nanoparticles have exceptional lumines-cence, magnetic and electronic properties due to theirunfilled 4f electronic structure. As such, rare earth-basedphosphors, magnetic materials, hydrogen storage materialand high surface area support catalyst are being widelydeveloped. Most of the applications require the use ofnon-agglomerated nanoparticles, as aggregated nanopar-ticles lead to inhomogeneous mixing, poor sinterability

and compromised quantum properties.87 However, nano-crystallites with a primary particle size

situation, has the ability to donate its oxygen for theremoval of CO and hydrocarbons during the O2-deficientpart of the cycle, while absorbing and storing oxygen fromO2, NO and water during the excess O2 environment.

92

Hydrothermal processing studies on the synthesis ofnanoparticles have focused on particle size, morphologyand crystal polymorph. The pH of the reaction mediumis a significant parameter affecting the nature and crystal-linity of the nanoparticles. Wu et al.93 reported on theeffects of pH of the reaction medium on the crystallisationof ceria grains under hydrothermal conditions when cer-ium hydroxide was used as the precursor. The synthesismechanism was thought to be by Ostwald ripening,where in an acidic medium and with the dissolution ofthe precursor, grain growth is faster in contrast to abasic medium.The ceria synthesised from cerium (IV) hydroxide in

Fig. 9a after 24-h hydrothermal treatment exhibitedvery fine particles, which were agglomerated. Crystallinitycould be observed based on the particles and its corre-sponding electron diffraction pattern. Its crystallite sizeis about 5–6 nm as estimated from the TEMmicrographs.The particles generally show rounded edges but they arenot well-defined due to its small size. For the ceria syn-thesised from the acetate system after 24 h in Fig. 9b, par-ticles are very well-defined and relatively dispersed. Goodcrystalline faces and crystallinity state could be observed.The particle sizes, at about 10–15 nm, are slightly biggercompared to the cerium (IV) hydroxide system. Theceria acetate system appears to be less agglomeratedthan the cerium (IV) hydroxide system. However,agglomeration of the particles still appears to be aproblem.94

Fe2O3 nanocubesIron oxide nanocubes were prepared by a hydrolysis reac-tion of Fe3+ in triethylamine at the temperature of 160°Cand the triethylamine provides OH− to form the Fe(OH)3deposition. After reacting in hydrothermal environment,the Fe(OH)3 translated into α-FeOOH through heatdecomposition at first, and then the α-FeOOH translatedinto α-Fe2O3 through heat decomposition (Fig. 10). Withthe Fe2O3 crystal particles growing, the cubes wereformed because triethylamine influenced the growth rateof some crystal faces. Furthermore, the iron oxide

cubes’ formation was influenced by reaction time. Theformation mechanism and influenced factors of ironoxide cubes will be discussed thoroughly in our furtherinvestigation. The iron oxide nanocubes were successfullysynthesised via hydrothermal synthetic route under mildconditions. It is expected that the iron oxide of uniformnanocrystalline may be promoted to some importantapplications in fields, for example, sensors, magneticmedia, catalytic, and so forth. This synthetic approachprovided a simple and economical route to synthesisenanocrystals. We have also discovered that many of oursynthesis techniques could be utilised in the preparationof other nanostructured metal oxides, which will bereported in later synthesis. In a typical procedure, 0.404g Fe(NO3)3·9H2O and 3 mL triethylamine were dissolvedin deionised water (10 mL) to form a homogeneous sol-ution and then the solution was stirred vigorously for 5min. After that, the solution was sealed in a 50-mLTeflon-lined autoclave filled with deionised water up to80% of the total volume, and the container was main-tained at 160°C for 1–24 h without shaking or stirring.The resulting products were filtered and then washed suc-cessively with deionised water and anhydrous ethanol forseveral times, and finally, the product was dried for 5 hunder vacuum at a temperature of 50°C.95

Hydrothermal synthesis for complexoxidesA variety of ceramic powders have been synthesised byhydrothermal methods in which complex oxides, such asBaTiO3, BaCaTiO3, BaSrTiO3, PZT, PbTiO3, KNbO3,KTaO3, LiNbO3, ferrites, apatites, tungstates, vanadates,molybdates and zeolites, are very famous in ceramicworld. Some of which are metastable compounds, whichcannot be obtained using classical synthesis methods athigh temperatures. Hydrothermal synthesis of a varietyof oxide solid solutions and doped compositions is com-mon. The hydrothermal technique is also well suited fornon-oxides, such as pure elements (for example Si, Ge,Te, Ni, diamond and carbon nanotubes), selenides(CdSe, HgSe, CoSe2, NiSe2 and CsCuSe4), tellurides(CdTe, Bi2Te3, CuxTey and AgxTey), sulphides (CuS,ZnS, CdS, PbS and PbSnS3), fluorides, nitrides (cubicBN, hexagonal BN), arsenides (InAs, GaAs), etc.

9 TEM and electron diffraction pattern of CeO2 from cerium (IV) hydroxide a and ceria acetate b after 24-h hydrothermaltreatment94



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Hydrothermal synthesis of BaTiO3 undervarious conditionsBT is one of the most important ferroelectric materials usedin electronics ceramic industry.96 The hydrothermal method,a low-temperature process, has enjoyed success in prep-aration of high-purity homogeneous and ultrafine powderof BaTiO3 under an environmental friendly condition.

97,98

This method belongs to the category of liquid phase reac-tions, characteristically produces extremely fine particleswith a narrow size distribution maintaining a spherical mor-phology. This technique utilises heating an aqueous suspen-sion of insoluble salts in an autoclave at a moderatetemperature and pressure so that the crystallisation of adesired phase will take place. The advantages of hydrother-mal crystallisation are the reduced energy costs due to themodest temperatures sufficient for the reaction, less pol-lution, simplicity in the process equipment and the enhancedrate of the precipitation reaction.99 The particle size ofhydrothermally synthesised BaTiO3 is usually less than200 nm which is adequate for the thin layer application.

The chemicals and apparatus

Nano-BT can easily be produced using the hydrothermalmethod with barium acetate (Ba(CH3COO)2) and

titanium isopropoxide (Ti(OPri)4). Tetrabutyl-ammonium hydroxide (TBAH, 40% solution in H2O)and i-Propanol can be used as a solvent. The solventwas dried with molecular sieve (Fluka, 3 Å XL8) beforeuse. Deionised water was used for the hydrolysis of Ti(OPri)4.

100 The fine BT powders are also synthesisedthrough the hydrothermal method by using reagents ofBa(OH)2·8H2O and TiCl4 as barium and titaniumsources, respectively. The hydrous titanium oxide was pre-pared by adding the TiCl4 aqueous solution, prepared byslowly adding TiCl4 into chilled water, into ∼2 mol dm−3NH3 aqueous solution under vigorous stirring. The pre-cipitate was centrifuged and washed thoroughly until noCl− could be tested. The resulting hydrous titaniumoxide and Ba(OH)2·8H2Owere put into the autoclave, fol-lowed by distilled water until the total volume reached to∼70% of the capacity of the autoclave.96 BT powders arealso synthesised by mixture of BaCl2·2H2O, and TiO2.For some samples, an organic surfactant of 1.5-g dolapixET85 was charged in the solution.101

Tetragonality of BT powder for application

The cubic phase shows paraelectric properties with a tri-vial dielectric constant, while the tetragonal phaseshows ferroelectric properties which are more interestingfor dielectric applications due to its high dielectric con-stant. In the temperature range for the cubic phase, i.e.,above the curie point, the ideal perovskite structure of acubic and symmetrical unit cell is stable. BaTiO3 particleslarger than 0.5 µm usually show a tetragonal-to-cubicphase transition at a curie temperature which is generallylocated at 120–130°C.102 The tetragonal phase is the onlythermodynamically stable phase of bulk BT material atroom temperature. The TiO6 structure has to be comple-tely distorted in the tetragonal BTwith a displacement ofthe Ti ion in its oxygen coordination octahedron for 0.12and an oxygen displacement about 0.03.103

However, in BT nanocrystals, the distortion of TiO6structure leading to a cubic-to-tetragonal phase transitionby cooling the sample through the curie point is not poss-ible. This is because the nanocrystals are so small thatstructural defects in the particles prevent the completionof the structural transition, leading to high strains withinthe crystals. These internal stains are due to the cubic-to-tetragonal deformation, representing a small amount oftetragonality within the nanocrystals. Since there are

10 a SEM images of the Fe2O3 nanocubes. b TEM image of Fe2O3 nanocubes prepared at 160°C for 24 h with 3 mL triethylamine,and the inset is the SAED image95

Table 4 Range of nanoparticles developed by differentresearchers

S. No. Researcher ResultsMeasurement method

1 Uchino et al.106 ≤120 nmcubic

XRD, BET

2 Hennings andSchreinemacher107

≤400 nmcubic

XRD, SEM

3 Begg et al.108 ≤190 nmcubic

XRD, SEM

≤270 nmtetragonal

4 Lu et al.97 ≤80 nmmainly cubic

XRD, DSCRaman

5 Yoon100 ≤70 nm 80%tetragonal

XRD, DSC

6 Kim et al.109 ≤100 nm 82%cubic

NMR, Raman

7 Lu et al.97 77.8 ± 23.5nm cubic

XRD



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high strains inside the nanocrystals, the distortion of thecubic structure does occur, yet do not lead to an entire dis-tortion to the tetragonal structure. These strains are fromdefects of BT nanoparticles formed during the hydrother-mal synthesis, primarily in the form of lattice OH− ionsand their compensation by cation vacancies.104,105 Vive-kanandan and Kutty99 suggested that strains in the crys-tallites are related to the point defects in the lattice.Compensation of the residual hydroxyl ions in the oxygensublattice by cation vacancies results in strains leading tothe presence of metastable cubic phase at room tempera-ture.97 Clark and Sinclair104 observed that as-preparedpowders contain many defects, primarily in the form oflattice OH− ions. Ohara et al.105 also reported that thestabilisation of BT cubic phase by hydrothermal methodis caused by surface defects that include OH− defectsand barium vacancies (Figs. 11 and 12).

Sol–gel–hydrothermal method to prepare BaTiO3powder

A novel sol–gel–hydrothermal method has been carriedout to prepare BaTiO3 powder successfully. Well-crystal-lised, pure BaTiO3 particles with cubic structure havebeen obtained at only 120°C for 12 h as the KOH concen-tration was over 1.0 M. Although the crystallinity andmorphology are less affected by further increase in thereaction temperature and time, the KOH concentration

has an important effect on the crystallinity and particlessize of samples. The average size of BaTiO3 particles var-ies from 370 to100 nm when the KOH concentrationincreases from 1.0 to 8.0 M. Being a gentle environmentmethod, the sol–gel–hydrothermal method provides asimple route to produce the highly pure and uniformBaTiO3 powder products, and exhibits a wide applicationprospect.In MH process, the formation of tetragonal BaTiO3

was strongly enhanced. At reaction temperature of 240°C, the tetragonal phase with c/a= 1.0063 could form inBaTiO3 with synthesis time as low as 3 h. The extent oftetragonality and particle size increased quickly with reac-tion time, whereas the content of lattice hydroxyl groupsdecreased. Tetragonal BaTiO3 of nearly full tetragonality(c/a ratio = 1.010) was obtained in 20 h (Fig. 13).As reaction temperature lowered down to 220°C, the

formation of tetragonal structure and the growth of par-ticles slowed down substantially, showing a critical effectof the reaction temperature on the MH processing oftetragonal BaTiO3.Higher Ba(OH)2/Ti mole ratio enhanced the formation

of tetragonal BaTiO3 and so did higher initial concen-tration of Ti with fixed Ba(OH)2/Ti ratio. It was found,in this study, that the excess barium cations in hydrother-mal reaction solutions played an important role on theformation of tetragonal BaTiO3 (Table 4).

11 Processing procedure used for preparing Ti-based perovskite ferroelectric powders by hydrothermal method

Table 5 Characteristic of different samples used toinvestigate different conditions101

Sample (Id) pH Surfactant Crystallite size/nm

BT3 12.5 Dolapix ET85 42BT4 13 Dolapix ET85 39BT6 14 Dolapix ET85 28

Table 6 Average grain size of the Ba1−xCaxTiO3 samples110

Samples Band gap/eV Crystallite size/nm

BaTiO3 3.36 39Ba0.75Ca0.25TiO3 3.24 34Ba0.50Ca0.50TiO3 3.44 17Ba0.25Ca0.75TiO3 3.78 N.ACaTiO3 3.51 45



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pH factor

Samples of BaTiO3 synthesised in the different conditionsare shown in Table 5. With variation of the amount ofdifferent surfactants, a structural change does not occur.Also, different pH employed has no significant effect onthe spinel structure and do not change the crystallo-graphic structure (although some peaks of impurity aredetected in the sample and BT4). However, it is obviousthat the degree of crystallinity is different for variouscases. This issue can be well understood from the powdermorphologies. With the increase the synthetic pH (BT6:pH = 14), crystalline size is further reduced.From these SEM results it is clear that nanomaterials

obtained at higher pH values exhibit very broad size distri-bution, which are also encountered for compoundsobtained by the hydrothermal synthesis. Furthermore,the usage of a high basic condition and surfactant are thetwo crucial keys in ensuring the formation of BaTiO3nanostructure under the hydrothermal condition (Fig. 14).

BCT as prominent candidate for PL

BaTiO3 nanoparticles can be prepared by severalmethods. Among the chemical methods, the hydrother-mal is one of the promising routes for production ofoxides with extremely fine particles, with different mor-phology and homogeneous distribution of grain sizes.10

In a recent publication it was observed that CaTiO3

powders synthesised using the MAH method show thatintermediate energy states within the band gap are mainlyresponsible for PL emission.110

The band gap determined from UV–Vis absorption forall samples is between 3.24 and 3.78 eV (Table 6). The PLexcitation energy used in the present work (3.52 eV corre-sponding to 350 nm) is of the same order as these bandgaps, so it is difficult for an electron in the valence bandto be directly excited to the conduction band, mainlydue to thermal effects. Therefore, it is likely that the elec-trons were excited first to the localised levels within theforbidden gap and, then the observed broad bands inPL emissions do not result from the free-exciton recombi-nation. In this work, we observed poly- and nanocrystal-line oxide where oxygen vacancies are known to be themost common defects which usually act as radiativecentres in luminescence processes.111–113 The broad lumi-nescent band usually observed at low temperatures in per-ovskite-type crystals is associated with the presence ofimperfections or defects and is typical of multiphononand multilevel process. The literature includes severalpapers explaining favourable conditions for PL emissionin materials presenting a degree of order–disorder. Theauthors attributed the radiative decay process to distortedoctahedral, self-trapped exciton, oxygen vacancies, sur-faces states and a charge transfer via intrinsic defectsinside an oxygen octahedron.112,114 Table 6 shows bandgap energies obtained from UV–Vis spectra of the BCT

12 TEM bright-field and dark-field images of two individual BT nanocrystals a equal bright–dark colour contrast with nearlyhomogeneous distribution; b bright colour dominant contrast97

13 a SEM photographs of BaTiO3 powders sol–gel–hydrothermal synthesised at 120°C with a KOH concentration of 2.0 M fordifferent reaction times: (I) 6; (II) 12; (III) 24 and (IV) 48 h. b SEM micrographs of BaTiO3 powders prepared at 240°C (MHmethod) for: (I) 3, (II) 12, (III) 20 and (IV) 96 h96



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with different compositions. These values show that for x= 0.25, the band gap to BCT phase is smaller than to BT(x= 0) phase, suggesting that the BCT phase has a higherdegree of order–disorder than the BT phase. This occursbecause the BT tetragonal structure is disordered by theCaO12 clusters making available additional electroniclevels in the forbidden band gap. For x= 0.50, the highstructural distortion can result in a higher band gapvalue than in the prior phases. For x= 0.75, the bandgap has the highest calculated value which could beassociated with the structural threshold among tetragonaland orthorhombic phases. For the intermediate phases(0.25–0.75), it is observed that the band gap energy riseswith the Ca concentration.115

BST prepared by MAH methodWithin the ABO3 perovskite class of compounds, SrTiO3(ST) and BaTiO3 in their crystalline form display a semi-conductor behaviour with many interesting properties. BTand ST are materials that show PL emission at roomtemperature in the form of amorphous powders andnanocrystals. This PL behaviour is explained as resultingfrom either quantum confinement or structural defects.116

Among the perovskite-type compounds, crystalline BSThas received much attention due to attractive ferroelectric,pyroelectric and piezoelectric properties and high dielec-tric constants coupled with good thermal stability.117 Par-tial substitution of network atoms by another isovalentcation (Ba, Mg, Ca, Sr) modifies the dielectric propertiesand the ferroelectric transition, and also significantlybroadens the phase transition.118 Several chemicalmethods have been used to prepare nanoparticles ofBST. Deshpande et al.119 and Pązik et al.120 have pre-pared BST nanoparticles using the MAH method. Thisrapid synthetic method has been used efficiently in thepreparation of ceramic powders as perovskite-type com-pounds.115 This synthetic method is recognised as anemerging, rapid and environmentally friendly procedure.The MAHmethod has been also introducing the differentcrystal growth ways to perovskite compounds.121,122 Toexplain these interesting morphologies many approachescan be evaluated, as mesocrystal growth and/or mesoscaletransformations,123 self-assembly associated to orientedattachments and reversed crystallisations.124 Amongthese crystallisations/crystal growth process the self-assembly followed by mesoscale transformations havebeen the most generally useful to describe the MAH crys-tal growth. Figure 15 shows the three different types ofcrystallisation mechanisms for BST. For x= 0, poly,meso and single crystals are observed at the micro- and

nanoscale (Fig. 15a). In the left image, a faceted grain(upper-right) can be observed, with rounded polycrystalmorphology which is transforming into a mesocrystalwith dodecahedral morphology, as that observed in themiddle image. Figure 15b (x= 0.25) shows three imageswith single and mesocrystal at the microscale. For x= 1(Fig. 15c), there is a predominance of rounded nanoparti-cles, and although they do not show faceted surfaces, thegrowth of the particles by agglomeration can be observed.We believe that the growth of particles is influenced by theintrinsic structure of the crystals, although the methods orconditions for preparation, the precursors used and theconcentration of ions in solution also influence theobserved morphology of the material. Also pH influ-ences126 nanoparticle morphology, but in this work itwas always kept high (14). As discussed, the perovskitestructure of BST is formed.Therefore, different types of aggregates are initially

formed; consequently, they will grow into differentshapes generating different morphologies. The additionof 25% Ba induces the formation of a greater amountof cluster pairs of [TiO5]–[TiO6], resulting in an orientedaggregation process and crystallites with smaller dimen-sions, as shown in Fig. 15a. These crystallites, in turn,tend to grow neatly to form single and mesocrystalmicrostructures with approximately cubic morphology,with well-defined edges and smooth surfaces. On theother hand, even a random substitution of O−2 byOH− can occur, resulting in [TiO6H]

− complex clusters.In these cases, the OH− groups may cause an electro-static repulsion between the planes of the crystal, givingan anisotropic growth with different sizes andmorphologies.19

It is possible that the samples of BST consist of signifi-cant amounts of agglomerates of [TiO5]–[TiO6] pairs, aswell as complex clusters of the [TiO6H]

− type, resultingin different types of crystal growth, crystal sizes anddifferent morphologies. The samples with x= 0.25 shouldhave a higher concentration of [TiO5]–[TiO6] clusters,leading to a more growth-oriented and pronounced mor-phology and resulting in a higher intensity in PL emission,as shown in Fig. 15. Crystalline nanoparticles (cubic andtetragonal) of perovskite-type barium strontium titanatewere successfully obtained using the MAH method. Thegreater local disorder for the samples with 25% Ba fostersoriented crystal growth and contributes to an increase inPL emission. This behaviour indicates that, althoughthe material has well-defined crystal structure and mor-phology, these properties are governed by the intrinsicdefects of each cluster and how they are arranged toform the crystal.

14 SEM images of the BaTiO3 prepared under different conditions a BT3, b BT4 and c BT6101



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BST prepared by sol–gel–hydrothermalsynthesisSeveral methods have been considered for the preparationof BST in powder form, including the sol–gel–hydrother-mal synthesis. The latter method is advantageous due tolow temperature processing, the non-vacuum requirementand a low cost compared with others.In ferroelectric fine particles, ferroelectricity decreases

with decreasing particle grain size and disappears belowa certain critical size.127 Other physicochemical factors,such as the density, shape, presence of impurities andstructural defects, also affect this property. Moreover,Komarneni et al.23 have proposed that relationship existsbetween the tetragonality in BT and the type of counter-ion used in the synthesis. Although the exact mechanismof this process is not yet known, the authors suggestedthat most soluble salt scan promote the dissolution ofBT, disturbing the dissolution–recrystallisation process.Thus, a better understanding of the structure–propertyrelationship of perovskite nanocrystals is highly desirable,which is also necessary for developing high-performanceelectronic devices (Fig. 16).29

BST prepared by hydrothermal synthesisHigh-temperature BST powder shows some drawbacks,such as larger particles, higher impurity content as a resultof repetitive calcinations and grinding treatments andlower chemical activity; therefore they are not suitablefor the preparation of the BST ceramics with fine grainsize, etc. Sol–gel method at a lower temperature presentssome particular advantages in obtaining the BST powderwith high purity and homogeneity.128 Among the solutionprocessing routes, the hydrothermal (solvothermal) pro-cess has been proposed to be an effective method forsynthesising well-crystallised fine ceramic powder withuniform shape and narrow particle size distribution with-out high-temperature calcinations.129 Recently, the syn-thesis of nanoparticles with controlled size and shapewas of fundamental and technological interest. It is wellknown that the shape of powder as well as its size isvery important to the properties of materials. The syn-thesis of powder with regular shape has been a challengeto material chemist in recent years. Herein, we used low-cost raw materials to synthesise single-crystalline BSTnanocubes via a simple solvothermal route in the absence

15 FESEM images of the BST samples: a x = 0; b x = 0.25; c x = 1125

16 TEM images of BST samples for both methods: Ba0.2Sr0.8T29



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of surfactant. This route requires no further high-temp-erature crystallisation treatment to the product.Hou et al.130 reported TEM micrograph of single BST

nanocubes with square shape (the sample S-0.25 onlyshown here in Fig. 17). The TEM observation showsthe absence of domain boundaries within each nanoparti-cle, suggesting that each nanoparticle consists of a singlecrystal. However, for the solvothermal pure BaTiO3,although it shows single-crystalline nature,130 the particlesize from TEM is more than 60 nm, which is markedlymore than the crystal size from XRD. Figure 17b showsa high-resolution TEM image of a single BST nanotube,in which clear lattice fringes suggest its single-crystallinestructure. The spacing of the fringes was measured to beabout 0.28 nm, which corresponds well with the spacingof (110) planes. These fringes make an angle of 45° withthe edge of the nanocube. It is found that the mixed sol-vents, the higher reaction temperature (≥240°C) andhigher Ba/Ti molar ratio in reactant (≥1.6) are necessaryto obtain pure BaTiO3 particles with cubic shape. On theother hand, different source materials, such as BaCl2 andTiO2, have no obvious influence on the particle shape but

result in different particle size. The growth of BT and bar-ium strontium titanate particles under our experimentalconditions is possibly subjected to dissolution–crystallisa-tion mechanism model.131–133 Due to the equilibriumbetween crystallographic habit growth and preferentialdissolution of high-energy faceted edges, for the fullygrown BT particles, the dominant morphology is spheri-cal and only a few particles still retain cubic. In this syn-thesis conditions, the equilibrium breaks and morespherical nuclei develop into cubic particles. In addition,the particle growth after nucleation in hydrothermal syn-thesis is often reported via either solute addition reactionor aggregation.134

Ceramics prepared by microwave sintering

The solid-state method is a universal way to prepareBaZrxTi1−xO3 ceramics, but it shows some disadvantagessuch as long processing time, low purity and non-uniformgrain size (10 μm), which always lead to poor dielectricproperties. Hydrothermal method is a low-temperatureprocess to synthesis high-purity, homogeneous and ultra-fine powders under an environmentally friendlycondition.Figure 18 shows the SEM micrographs of BaZrO3 a

and BaTiO3 b powders synthesised by conventionalhydrothermal method. Both crystals are well grownand have uniform grain size. It can be seen that theBaZrO3 microcrystal with a grain size of ∼1 μm has ashape of rhombic dodecahedron and the BaTiO3 micro-crystal with a grain size of ∼0.1 μm has a shape ofsphere.

18 SEM micrographs of a BaZrO3 and b BaTiO3135

17 HRTEM images of the sample B0.75S0.25T130

19 Hydrothermal method to obtain lead-free piezoelectric powders and the sintering process138



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(K,Na)NbO3 by hydrothermal methodLead-free piezoelectric ceramics have been widely studiedas replacements for PZT ceramics. Alkaline niobate-based piezoelectric ceramics have good piezoelectricproperties and high curie temperatures. Among these,(K,Na)NbO3 is considered as a promising candidate forlead-free piezoelectric ceramics. When synthesising (K,Na)NbO3 ceramics by using the conventional solid-statereaction, the hygroscopic property of the startingmaterials, such as K2CO3 or Na2CO3, causes a severe pro-blem in precise weighing, and high-purity organic solventsshould be utilised for the milling process. Moreover, Na2Ois easily evaporated.136,137 In contrast, the hydrothermal

method is proposed to obtain source powders for theseceramics, and it has been verified that this method enablesthe production of high-quality powders.138 Crystallisationfrom the solution was achieved with the hydrothermalmethod, so that pure crystal powder could be obtainedwithout difficulty. In addition, the potassium to niobium

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