DOI: 10.1002/cssc.201300423

Continuous Mesoporous Titania Nanocrystals: TheirGrowth in Confined Space and Scope for ApplicationSaikat Dutta*[a] and Asim Bhaumik*[b]

Among the tools used to control the growth of nanocrystalsduring synthesis towards particular shapes and facets, one ispreferential adsorption of molecular species to specific planesof atoms exposed on crystal surfaces.[1] A variety of chemicalspecies (surfactants, polymers, biomolecules, small molecules,anions, or metal ions) can induce shape control in the growthof metal-oxide nanocrystals to desired sizes and shapes, for ex-ample to offer superior electron mobility with high photocata-lytic efficiency.[2] In addition, the shape of certain nanocrystals(NCs) (e.g. , Cu2O) can be controlled by varying the ratio ofchloride ions to sodium dodecylsulphate ions in the solutionfrom which those crystals form, because the chloride and do-decyl sulphate ions stabilize different planes ({100} and {111},respectively).[3] The high energy barrier associated with homo-geneous nucleation has been a longstanding problem for at-taining the controlled growth and desired shape of NCs.[4]

“Soft” materials (e.g. , self-assembly of block copolymers) withgyroid morphology have been applied as sacrificial templatesto developing functional ceramics with controlled internal sur-faces (Scheme 1), and these have been utilized in hybrid solar

cells.[5] Accomplishing a continuous network of an electronical-ly active phase connected to solar cell electrodes is particularlychallenging, and when using a gyroid network phase based onblock copolymer templates, only in-plane continuity can beachieved in films owing to the columnar or lamellar morpholo-gies.

A recent report from the Clarendon Laboratory emphasizedthe growth of NCs in the confined spaces of a guiding tem-plate in a dilute regime.[6] The researchers employed a strategyof “seeding” microscopic nucleation sites for crystal growth,with the aim to overwhelm the energy barrier associated withhomogeneous nucleation in the absence of a template. By pre-seeding the template, a near-unity yield of clearly facetedsingle crystals of anatase TiO2 was accessible, containing inter-nal mesopores orders of magnitude smaller than the externaldimensions of the NCs (Scheme 2).

The superiority of this strategy for the fabrication of meso-porous single crystals (MSCs) is that instead of producinga porous assembly of nanoscale crystallites with common ori-entations, the route offers mesocrystals with a single coherentatomic domain. However, for a templating strategy, a templatevolume that encompasses the entire reaction volume is critical,because otherwise only a small number of crystals nucleateinside the template while most will nucleate in the bulk solu-tion.[7] The seeded growth in a confined space demonstratedthe formation of faceted, truncated bipyramidal crystals withan external symmetry matching the homogeneously nucleatedbulk crystals, and with a mesoscale structure that was a nega-tive replica of the mesoporous silica template. When a crystalgrew from a seed located on or close to an external surface ofthe template, it was directed outwards into the solution result-ing in a compact, faceted crystal.

It is essential to justify the role of the seeded growth strat-egy in the fabrication of MSCs. A higher seeded growth rate

Scheme 1. Gyroid network replication from block copolymer (P-b-P) tem-plates for fabrication of TiO2 semiconductor array. PFS: poly(4-fluoro styrene),PLA: poly(lactic acid).

Scheme 2. Nucleation and growth of TiO2 nanocrystals within a mesoporoussilica template.

[a] Dr. S. DuttaDepartment of ChemistryUniversity of FloridaGainesville, FL 32603 (USA)Fax: (+ 1) 352-392-8758E-mail : saikatdutta@ufl.edu

[b] Prof. Dr. A. BhaumikDepartment of Materials ScienceIndian Association for the Cultivation of ScienceJadavpur, Kolkata—700032 (India)Fax: (+ 91) 33-24732805E-mail : msab@iacs.res.inHomepage: http ://www.iacs.res.in/matsc/msab/

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compared to the nonseeded rate inside the template demon-strates how a template-directed seed-mediated growth is ca-pable of minimizing the energy barrier of homogeneous nucle-ation of the NCs inside the pores of the template, comparedto the same in bulk solution. Obviously, the seeding densityplays a major role in the relative number of crystals and theirsize in the template. In fact, by varying the concentration ofTiCl4 (seeding density), the average crystal volume varies,which effectively decouples the MSC domain size from the in-ternal pore dimensions of the silica template and thus thismethod is agnostic of the pore size (20–250 nm) of the tem-plate. Essential requirements for accessing TiO2 MSCs are:(1) adjusting the internal mesopore size by replicating silicabead templates of different diameters, such that the surfacearea of the template is inversely proportional to the d spacing;(2) control over the mesopore dimensions of the MSCs; (3) in-dependent tuning of the crystal and pore size of the MSCs.

The seed-mediated growth of crystals in micrometer-sizemesoporous single-domain semiconductor materials as sacrifi-cial template is an exciting novelty; however, attempts togrow nanocrystals of uniform size by a seeding approach isa widely explored method to access noble metal NCs.[8] A keyprerequisite to exclusively obtain particular-selective formationof NCs is to ensure tight control over the population of seedsor on the mechanism to prevent the spontaneous nucleationof NCs when not using seed nanoparticles. In many cases, a dis-tribution of seeds with different structures is present in a syn-thesis mixture, due to differences in the reaction environmentand structural fluctuations of the seeds. These factors make itextremely difficult to achieve shape selectivity. Despite this,preseeded mesoporous-silica-template directed growth of TiO2

NCs offer MSCs of one particular shape, which is close to theshape obtained via homogeneous nucleation in the absence ofa template.[6] Thus, preseeding of the sacrificial template isa key prerequisite for the fabrication of MSCs. Essentially,a comprehensive understanding of the seed-mediated crystalgrowth is still lacking, which presents a significant challenge inrealizing the full potential of this method. However, significantadvances have been made in the mechanistic investigation ofNC growth by using ex situ and in situ techniques.[9]

The MSCs reported by Snaith et al. have long-range crystalorder and demonstrate electron conductivity maxima threeorders of magnitude higher than those of a TiO2 nanoparticlefilm, confirming high charge density and electron mobility.Moreover, the effective electron mobility of MSC films with theinfluence of interparticle boundaries was over an order of mag-nitude higher than that of the nanoparticle film, highlightingits potential application in low-temperature solid-state solarcells (SSCs) with power conversion efficiencies over 7 % largerthan conventional SSCs.[10] However, several options remainopen to investigation at this stage; for example: (1) kinetic in-formation on the growth process; (2) access to anisotropic NCs(e.g. , bars, rods, multipods) with mesopores; (3) the potentialof this method to access MSCs of noble metals. Nevertheless,the report is a fascinating new strategy to fabricate semicon-ductor NCs with mesoscopic void spaces that enhance theelectron mobility of the material for a wide range of applica-

tions,[10] and offers a perspective for the synthesis of othermetal oxides.

Furthermore, the growth of these MSCs as demonstrated in-dicates that the growth process can be dependent on the hy-drolysis rate of the Ti precursor, which offers scope for furtherstudies. In this context, biocompatible precursors for homoge-neous nucleation of TiO2 anatase nanocrystals[11] hold promisefor growth at room temperature and also to enable growth onpolymeric substrates for solar cell applications. For example, ti-tanium lactate (TiBALDH) in aqueous medium equilibrates withammonium tris-lactato-titanate and crystalline TiO2 nanoparti-cles, and upon minimizing the polarity of the medium theequilibrium can be shifted to nano-TiO2.[11b,c] Control of thephase development in TiO2 would be critical for improved per-formance in photocatalytic and photovoltaic applications.

The new approach for fabricating semiconductor MSCsholds promise for fabricating an enormous range of templatemorphology-based functional materials from both solution andvapor phase, with broad scope in the area of energy storageand generation. TiO2 semiconductor MSCs will become theobject of a wide range of further studies, including applicationas support materials for metal catalysts, ion-diffusion compo-nent in batteries, solid-state hole conductors, dye-sensitizedsolar cells, and biosensors for detection and diagnostics. Takentogether, the preseeding of the template followed by the fabri-cation of MSCs at the nucleation sites, materials characteriza-tion, and large-scale production involves a transitional researchprogram, which could have great potential in chemical andmaterials science, and nanotechnology in general.

Acknowledgements

We acknowledge the United States Department of State for theirgenerous support through a Fulbright exchange scholar program,and the Department of Science and Technology (DST), New Delhifor a Fast Track Grant on the Photocatalytic Applications ofNanocrystals.

Keywords: crystal growth · mesoporous materials ·semiconductors · template synthesis · titania

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Received: May 5, 2013Published online on && &&, 0000

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HIGHLIGHTS

S. Dutta,* A. Bhaumik*

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Continuous Mesoporous TitaniaNanocrystals: Their Growth inConfined Space and Scope forApplication

Enjoying the single lifestyle: With anoverwhelming efficiency compared tothermally sintered preformed nanocrys-tals, mesoporous single crystals (MSCs)of TiO2 constitute a new class of semi-conductor materials for low-cost solarpower, solar fuel, photocatalysis, and

energy storage applications. This High-light explores the benefits of template-directed seed-mediated growth in theconfined space of a preseeded mesopo-rous template, and possible researchavenues for further improvements.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 0000, 00, 1 – 4 &4&

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