Polymorphic phase transition among the titania crystal ...· aqueous route and ball milling-induced

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  • Nanoscale

    FEATURE ARTICLE

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    View Article OnlineView Journal | View Issue

    Polymorphic pha

    DKMhC(pUrgDwP

    Physics, I.I.Sc., Bangalore, in the erojunction solar cells. The studyluminescence properties of mixheterojunctions are his present int

    Department of Physics, Indian Institute of

    India. E-mail: ksrkrao@physics.iisc.ernet.in

    Cite this: Nanoscale, 2014, 6, 11574

    Received 26th March 2014Accepted 15th May 2014

    DOI: 10.1039/c4nr01657b

    www.rsc.org/nanoscale

    11574 | Nanoscale, 2014, 6, 115741163

    se transition among the titaniacrystal structures using a solution-based approach:from precursor chemistry to nucleation process

    S. Girish Kumar and K. S. R. Koteswara Rao*

    Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity,

    cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The

    uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic

    properties, which yield different functionalities and thus influence their performances in various green

    energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle

    growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania

    with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based

    approach has paved the way for further exciting areas of research. In this focused review, titania synthesis

    from hydrothermal/solvothermal method, conventional solgel method and solgel-assisted method via

    ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet

    chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry,

    surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/

    time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review

    sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile,

    brookite and TiO2(B) under a variety of reaction conditions. The driving force for crystallization arising from

    complex species in solution coupled with pH of the solution and ion species facilitating the orientation of

    octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the

    nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The non-

    aqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in

    understanding the concepts and future prospects in this exciting field are suggested.

    r S. Girish Kumar is a native ofarnataka (Kolar District,alur Taluk) and has obtainedis M.Sc. degree in Physicalhemistry (2005) and Ph.D.2012) degree in the area ofhotocatalysis from Bangaloreniversity, Bangalore. He is theecipient of India's most presti-ious Dr D. S. Kothari Postoctoral Fellowship (2012) andorks under the supervision ofrof. Rao in the Department ofeld of CdTe/CdS thin lm het-of phase transition and photo-ed phase titania and othererests.

    Dr K. S. R. Koteswara Rao is anassociate professor at theDepartment of Physics, IndianInstitute of Science, Bangalore,India. He works in the eld ofsemiconductors. His researchinterests include understandingthe defects in semiconductormaterials and their hetero-structures by optical methods(such as photoluminescence,and optically induced conduc-tivity modulation) and electrical

    methods. The growth and study of IIIV- and IIVI-based binary,ternary and quaternary compound semiconductor nano- andmicro-structures and their utility for device applications are hiscurrent research studies.

    Science, Bangalore-560012, Karnataka,

    ; raoksrk@gmail.com

    2 This journal is The Royal Society of Chemistry 2014

    http://crossmark.crossref.org/dialog/?doi=10.1039/c4nr01657b&domain=pdf&date_stamp=2014-09-24http://dx.doi.org/10.1039/c4nr01657bhttp://pubs.rsc.org/en/journals/journal/NRhttp://pubs.rsc.org/en/journals/journal/NR?issueid=NR006020

  • Feature Article Nanoscale

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    1 Introduction

    Polymorphic TiO2 is an extensively studied nanomaterialbecause of its specic properties and important roles in allevi-ating environmental and energy crisis by the effective utilizationof solar energy. The landmark report on the photocatalyticsplitting of water using a TiO2 electrode has triggered investi-gation of its applications in diverse areas like photocatalysis,photovoltaics, batteries, photonic crystals, UV blockers, smartcoatings, and lling materials in textiles, paints, papers,cosmetics and biomedical sciences.15 Titania possessesnumerous unique features such as high redox potential, lowcost, non-toxicity, good stability towards adverse environment,hydrophilicity, humidity and gas sensing, dielectric character,favorable band edge positions, and facile preparation withdiverse morphologies.610 Most of the fascinating properties oftitania NPs originate from the size quantization effect. Bydecreasing the particle size, the bandgap widens as reected bythe blue shi in the absorption edge. The shi of CB and VBtowards more negative and positive potentials, respectively,favors the redox process that cannot occur in their bulkcounterparts.11

    Each crystalline form of titania has been highlighted in aspectrum of applications, and the lattice structure differencesamong the polymorphs quite oen lead to multifaceted phys-ico-chemical and opto-electronic properties that control theelectronic structure and bulk diffusion ability of charge carriers.For instance, rutile has a high refractive index, exceptional lightscattering efficiency and UV absorptivity, which enables itsutility as a lter in solar creams, pigments, opaciers andoptical communication devices (isolators, modulators andswitches),12,13 while anatase is largely preferred in photo-catalysis and photovoltaics. Although brookite and TiO2(B) arerare polymorphs in nature and are synthesized only undersensitive conditions, their importance in photocatalysis,photovoltaics and lithium ion insertion has been recentlyrealized.1417 In addition, some structural features of a poly-morph will be vital than others depending on the application:size or surface area to volume ratio play a central role in catal-ysis,18 while controlling their size and shape (exposed facet) isimportant for fabricating photonic crystals, providing moreexibility and options for the design of nanostructures to satisfyunique requirements.19 The mixed crystal framework of titaniais also benecial for green energy technologies, pointing to thepossibility of achieving optimized performances via controllingthe phase composition. The morphology at the polymorphicinterfaces and surface states are critical for such a synergisticeffect, and intimate contact between the phases exhibits magiceffects on the charge carrier transfer dynamics in light-inducedphotoreactions. The different band gaps, as well as the positionof CB and VB edges, result in the formation of a stable hetero-junction that allows the absorption of a wide spectral range andimproves the charge carrier dynamics generation, separationand transfer process.2025

    The study on the phase transition of titania polymorphs is anactive area of research from the view point of ourishing

    This journal is The Royal Society of Chemistry 2014

    scientic interest and technological applications. Since there isno equilibrium temperature between the polymorphs of titania, aspecic or particular temperature regime for the phase transitionto occur is not well dened. It is unambiguously accepted thatintrinsic parameters, such as particle size, purity, nature of tita-nium precursor, surface energy, density of intrinsic and induceddefects, aggregation tendency or particle packing, solutionchemistry, and crystal growth dynamics, coupled with externalfactors, like peptization, addition of modiers/surfactant/chelating agents and annealing ambience, govern the tempera-ture and activation energy of any phase transformation path-ways.2633 Owing to such complexities, it is inevitable to identify asimple method to control the physical features like particle size,shape and the crystal structure that indicates phase. The defectrichness and structural diversity in TiO2 can be manipulated viacareful monitoring of the processing conditions. Among theseveral wet and dry methods, synthesis from a homogeneoussolution viawet chemical approaches seems to be very promisingas multiple reaction parameters like precursor concentration,chelating agents, solvents, mineralizer, pH of the solution, agingtime, reaction temperature, and kinetics of precursor hydrolysis,can be easily adjusted to tailor the intrinsic properties of the nalcrystal structure. The solution approach offers the possibility tocontrol the reaction pathways on a molecular level during thetransformation of precursor species into nal products, enablingthe synthesis of well-dened crystal polymorphs and morphol-ogies without impurities.3437 More importantly, it d