Quantum-size effects in the titanosilicate molecular sieveEnzo Borello, Carlo Lamberti, Silvia Bordiga, Adriano Zecchina, and Carlos Otero Areán Citation: Applied Physics Letters 71, 2319 (1997); doi: 10.1063/1.120060 View online: http://dx.doi.org/10.1063/1.120060 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quantum-size effect on the electronic and optical properties of hybrid TiO2/Au clusters J. Chem. Phys. 141, 054301 (2014); 10.1063/1.4891241 Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying Appl. Phys. Lett. 104, 121910 (2014); 10.1063/1.4866901 Size-effects on the optical properties of zirconium oxide thin films Appl. Phys. Lett. 95, 231905 (2009); 10.1063/1.3271697 Rapid synthesis of stable ZnO quantum dots J. Appl. Phys. 92, 6537 (2002); 10.1063/1.1518132 Stoichiometric and sodium-doped titanium silicate molecular sieve containing atomically defined –OTiOTiO–chains: Quantum ab initio calculations, spectroscopic properties, and reactivity J. Chem. Phys. 112, 3859 (2000); 10.1063/1.480533

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Quantum-size effects in the titanosilicate molecular sieveEnzo Borello, Carlo Lamberti, Silvia Bordiga, and Adriano Zecchinaa)

Dipartimento di Chimica IFM, Universita` di Torino, I-10125 Via P. Giuria 7, Torino, Italy

Carlos Otero AreanDepartamento de Quı´mica, UIB, 07071 Palma de Mallorca, Spain

~Received 10 June 1997; accepted for publication 20 August 1997!

The recently synthesized Engelhard titanosilicate~ETS-10! represents a material which contains inthe structure well defined atomic•••O–Ti–O–Ti–O••• quantum wires embedded in a highlyinsulating siliceous matrix. We report and discuss the UV-Vis spectrum of this material andcompare the experimentally determined optical band gap with the results predicted by simplemodeling of a titanium oxide semiconductor wire unidimensionally confined by an infinite potentialbarrier. © 1997 American Institute of Physics.@S0003-6951~97!02542-4#

The electronic properties of semiconductor materialsstart to change drastically when at least one dimension be-comes smaller than a few tens of nanometers, and very small~e.g.,<5 – 10 nm! semiconductor particles possess electronicproperties which markedly differ from those of both mol-ecules and bulk materials. The first manifestation of this so-called quantum size effect1 is a blue shift of the optical bandgap, i.e., as the semiconductor particle size decreases theoptical absorption edge shifts to shorter wavelength. Suchblue shifts have been observed in many two- and zero-dimensional semiconductor materials~termed quantum wellsand quantum dots, respectively!, and the phenomenon hasgreat potential in the area of optoelectronics. One-dimensional quantum confined structures~termed quantum-well wires or quantum wires! have been studied to a lesserextent,2 mainly due to difficulties involving their manufac-ture. Quantum wires can be fabricated by conventional meth-ods of lithography and pattern transfer,3 a process which isnow applicable only down to a size approaching 10 nm. An-other method involves chemical vapor deposition inside aV-shaped groove etched into a suitable substrate,4 but itmeets too with severe limitations on ultimate size. A differ-ent approach involves the preparation of quantum chainsemiconductors inside the channels of zeolite hosts, whichhave a diameter in the nanometer range. Examples are thereported synthesis of selenium chains in mordenite5~a! andother zeolites,5~b! and the parallel synthesis of semiconductorclusters in faujasites and sodalites.5~c–e! The recentlysynthesized6 Engelhard titanosilicate~ETS-10! constitutes amaterial which directly contains in the structure a well de-fined quantum wire which enables quantum size effects to bestudied on unprecedented geometrically well defined•••O–Ti–O–Ti–O••• wires. We report and discuss the UV-Visspectrum of this material and compare the experimentallydetermined optical band gap with the results predicted bysimple theoretical calculations.

The polycrystalline ETS-10 sample~sodium form! usedin this study was supplied by Engelhard~Iselin, NJ, USA!.Chemical analysis confirmed the expected stoichiometry andpowder x-ray diffraction showed good crystallinity. The ide-alized structure of ETS-10, which is topologically similar to

that of zeoliteb, is shown in Fig. 1. The structure containscorner-sharing TiO6 octahedra and corner-sharing SiO4

tetrahedra.7 In the framework, every titanium atom is associ-ated with a22 electrical charge which is compensated byextra framework alkali-metal cations~not drawn for clarity!.The corner-sharing TiO6 octahedra form chains of infinitelength which are isolated from one another and which have adiameter D50.67 nm ~i.e., the diagonal of the TiO6octahedron!.8 In such an octahedron, the titanium atom issurrounded by two types~I and II! of oxygen atoms. OxygenI bridges Ti atoms along straight•••Ti–OI–Ti••• chains ofmacroscopic length,L. Oxygen II belongs to Ti–OII –Sigroups; four of such oxygens surround every Ti in a planenormal to the chain axis. Note that, in the absence of latticedefects, the chain would have the same length as the micro-crystals: typically L52210mm, which implies (TiO)nchains characterized by a numbern of the order of one thou-

a!Electronic mail: zecchina@silver.ch.unito.it

FIG. 1. Upper part: the framework structure of ETS-10, showing chains ofcorner-sharing TiO6 octahedra which run along two perpendicular direc-tions, and which are isolated by corner-sharing SiO4 tetrahedra. For clarity,extra framework~charge-balancing! cations were omitted; Lower part: detailof an •••O–Ti–O–Ti–O••• chain ~stick representation!.

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sand. However, real crystals often have some Ti vacancies sothat the chains are broken at random points. It is thus morecorrect to consider a gaussian distribution of chain lengthshaving an average valueL& with a standard deviations.High resolution electron microscopy studies7 showed that(TiO)n chains having n&5150 are a reasonable model; thisimplies that^L&22s is greater than 25 nm, which can betaken to represent an infinite length.

Figure 2 reports the room temperature diffuse reflectanceUV-Vis spectrum of Na-ETS-10, obtained after outgassingthe sample at 473 K under a dynamic vacuum~residual pres-sure,1022 Pa!. This thermal treatment was given in orderto remove adsorbed gases~mainly water vapor! from thetitanosilicate channels. However, it is worth mentioning thatindependent experiments have shown that filling of the chan-nels with water vapor or some other adsorbates~hydrocar-bons! had no significant effect on the UV-Vis spectrum ofNa-ETS-10. Also, equivalent spectra were obtained from thepotassium form of the titanosilicate. The UV-Vis spectrumof Na-ETS-10 shows two absorption bands with maximaaround 4.4 and 5.8 eV~282 and 214 nm!, respectively. Con-sidering the octahedral TiO6 unit, these bands can be as-signed in terms of ligand-to-metal charge transfer~LMCT!

transitions: OI,II22Ti41→

hv

OI,II2 Ti31, where two types of oxygen

ligands are involved. The LMCT involving OII anions affectsO–Ti bonds normal to the•••O–Ti–O–Ti–O••• chain and

will be hereafter termedT' , while that involving OI ionswill be denoted asTi . It is clear that whileT' transitionsoccur inside individual TiO6 units,Ti transitions affect adja-cent units along the oxygen-titanium chain.

Besides the optical spectrum of Na-ETS-10, Fig. 2 alsoreports the spectrum~obtained in identical conditions! of ti-tanium silicalite. This is an MFI-type zeolite containing asmall amount~about 1 wt %! of Ti41 ions which substitutefor tetrahedrally coordinated Si41 ions in the siliceous frame-work of silicalite, thus forming isolated tetrahedral TiO4

units.9 The UV-Vis spectrum of this material shows a singleabsorption band at 6.1 eV~202 nm!,9 which is not far fromthe 5.8 eV~214 nm! band observed for ETS-10; the smalldifference can be accounted for in terms of different coordi-nation symmetry of the Ti41 ion. This fact, and the foregoingconsideration about the isolated~molecular! nature of T'

strongly suggest that the 5.8 eV band of ETS-10 can beassigned to this LMCT transition. The energy,ELMCT , in-volved in molecular LMCT transitions depends upon the op-tical electronegativity,xopt, of both the ligand and the metalion, following the phenomenological relationship:10

ELMCT53.72 @xopt~ligand!2xopt~metal!#, ~1!

where the empirical constant 3.72 is given in eV, and wherexopt has been estimated to be in the range of 2.0–2.15 forTi41 ions; the extreme values corresponding to tetrahedraland to octahedral coordination, respectively.10 For O22 ionsin silicatesxopt takes the approximate value10 of 3.6. Usingthese values, Eq.~1! predicts an energy of 5.39–5.95 eV forlocalized O22→Ti41 charge transfer transitions. The 5.8 eVband observed in the optical spectrum of ETS-10 falls withinthis range, thus lending further support to the foregoing as-signment of this band to theT' transition.

The model of isolated LMCT is not well suited for theTi

transition, since charge transfer can now be delocalizedalong the•••O–Ti–O–Ti–O••• chain. Therefore, the 4.4 eV~282 nm! absorption band present in the UV-Vis spectrum ofETS-10~Fig. 2! can better be explained in terms of the bandtheory of solids, with the corresponding electronic transitiontaking place from the top of the filled valence band~formedby oxide anion orbitals! to the bottom of the empty conduc-tion band ~free orbitals of Ti41 ions!. Note however thatquantum confinement in two directions~normal to the chainaxis! turns the•••O–Ti–O–Ti–O••• chain into a quantum-well wire surrounded by a highly insulating medium~SiO2,Eg>12 eV!; this would bring about a blue shift of the opticalband gap (Eg), as compared to bulk TiO2. Reportedvalues11–13~e! of Eg for this bulk semiconductor are 3.02 eV~410 nm! for rutile and 3.18 eV~390 nm! for anatase; both ofwhich TiO2 polymorphs contain octahedrally coordinatedTi41 ions, as in the case of ETS-10.

Neglecting minor terms~vide infra!, the blue shift ex-pected for a semiconductor quantum wire having a diameterD ~and a lengthL.10 nm! is given by13

DEg5h2

4mD2 1h2

8mL2 >h2

4mD2 for L→1`, ~2!

whereh is Planck’s constant andm is the reduced effectivemass of the electron-hole couple. The validity of the adopted

FIG. 2. Room temperature diffuse reflectance UV-Vis spectrum of A!: ETS-10, and B!: titanium silicalite~reported for comparison!. Full and dotted anddashed arrows indicate the optical energy gap of bulk rutile and anatase andfor ETS-10 respectively.

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L→1` limit is justified by the sharpness of the absorptionedge of ETS-10 in Fig. 1. This means that the density of Tivacancies is not high enough to determine a gaussian distri-bution of L able to influence the blue shift described in Eq.~2! i.e., ^L&22s.10 nm. In fact, the neglected term is lessthan 2 meV forL510 nm, while the average chain length, asdetermined by TEM, is much grater: about 25 nm~vide su-pra!. Equation~2! has been written by considering infinitethe potential outside the wire and neglecting the excitonbinding energy. Both approximations are well acceptablesince theEg of the host SiO2 matrix (>12 eV) is muchgreater than that of TiO2 (>3 eV), and because of the highdielectric constant of TiO2 (e>180). The validity of theformer approximation is confirmed by the fact that ion ex-change,~substitution of Na1 with K1! and presence of ad-sorbates in the channels doesn’t affect at all the UV-Vis ad-sorption profile~not shown for brevity!: this means thatTi isnot affected by a variation of the potential outside the wire,which in turn represents a further proof that, due to the highinsulating character of the siliceous host, the assumption thatthe potential outside the (TiO)n wire is very large is valid.Equation~2! correctly predicts a shift ofEg to higher ener-gies asD decreases. However, quantitative agreement withexperimental values is not always found.14 The main diffi-culty involves choosing the correct value ofm which is usu-ally taken from bulk semiconductors, while strictly the effec-tive masses of electron and hole are related to the band gap.15

For bulk TiO2 reported values13~c–e!,16 of m vary from 1.4 to2.7me , whereme is the free electron mass.

A final point to be debated is related with the fact thatboth crystal structure and effective mass of ...-Ti–O–Ti–O-... wires in ETS-10 are not the same as that in bulk TiO2,however these factors play a secondary role in determiningthe energy of the absorption edges, as can be easily provedby considering that anatase and rutile, having a differentstructure and slightly different masses, exhibit two energygaps separated only by 0.16 eV, while the energy shift ob-served in ETS-10 is nearly one order of magnitude greater.This means that the main effect is due to the quantum con-finement, as correctly predicted by our simple model, whichso proves that the very peculiar optical characteristic of thismaterial is associated with the presence of a wire and that theeffects due to the different crystal structures and effectivemasses are second order corrections.

Taking as a first approximationm52me and D50.67 nm, Eq.~2! yields DEg50.84 eV. This leads to anoptical band gap for the•••O–Ti–O–Ti–O••• chains ofETS-10 in the range of 3.86–4.02 eV, depending on whetherrutile or anatase are taken as the reference material. Since theevaluation of theEg for bulk anatase and rutile is made at theinflection point, the comparison of the computedEg 3.86 to

4.02 eV values must be done with the corresponding inflec-tion point in the optical spectrum of ETS-10~see arrow inFig. 2!: 4.03 eV. The experimentalEg value matches verywell with those obtained with our simple model, particularlywhen due account is taken of all the approximations made.We thus conclude that•••O–Ti–O–Ti–O••• chains inETS-10 behave as very thin quantum wires of uniform andwell defined geometry which can be used as model systemsfor further studies on quantum-size effects. To the best of ourknowledge, this very peculiar optical property of ETS-10 hasnever been reported before.

This work has been partially supported by CNR andMURST.

1For an overview, see: a! R. M. Kolbas and N. Holonyak, Jr., Am. J. Phys.52, 431~1984!; b! W. P. Halperin, Rev. Mod. Phys.58, 533~1986!; c! K.Ploog, Angew. Chem. Int. Ed. Engl.27, 593 ~1988!.

2~a! H. Sasaki, Jpn. J. Appl. Phys.19, L735 ~1980!; ~b! D. Campi, M.Meliga, and A. Pisoni, IEEE J. Quantum Electron.QE-30, 2001~1994!.

3H. I. Schmith and H. G. Craighead, Phys. Today24 ~1990!.4R. Bhat, E. Kapon, S. Simhony, E. Colas, D. M. Hwang, N. G. Stoffel,and M. A. Koza, J. Cryst. Growth107, 716 ~1991!.

5~a! V. N. Bogomolov, S. V. Kholodkevich, S. G. Romanov, and L. S.Agroskin, Solid State Commun.47, 181 ~1983!; ~b! J. B. Parise, J. E.MacDougall, N. Herron, R. Farlee, A. W. Sleight, Y. Wang, T. Bein, K.Moller, and L. M. Moroney, Inorg. Chem.27, 221 ~1988!; ~c! Y. Wangand N. Herron, J. Phys. Chem.91, 257 ~1987!; ~d! N. Herron, Y. Wang,M. M. Eddy, G. D. Stucky, D. E. Cox, K. Moller, and T. Bein, J. Am.Chem. Soc.111, 530 ~1989!; ~e! G. A. Ozin, A. Kuperman, and A. Stein,Angew. Chem. Int. Ed. Engl.28, 359 ~1989!.

6~a! S. M. Kuznicki, U.S. Patent 4853202~1989!; ~b! S. M. Kuznicki andA. K. Thrush, European Patent 0405978A1~1990!.

7~a! M. W. Anderson, O. Terasaki, T. Ohsuna, P. J. O. Malley, A. Philip-pou, P. MacKay, A. Ferreira, J. Rocha, and S. Lidin, Philos. Mag. B71,813 ~1995!; ~b! M. W. Anderson, O. Terasaki, T. Ohsuna, A. Phylippou,S. P. MacKay, A. Ferreira, J. Rocha, and S. Lidin, Nature~London! 367,347 ~1994!; ~c! T. Ohsuna, O. Terasaki, D. Watanabe, M. W. Anderson,and S. Lidin, Stud. Surf. Sci. Catal.84, 413 ~1994!.

8R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor.Gen. Crystallogr.32, 751 ~1976!.

9~a! S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F.Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, and G. Vlaic, J.Phys. Chem.98, 4125~1994!; ~b! F. Geobaldo, S. Bordiga, A. Zecchina,E. Giamello, G. Leofanti, and G. Petrini, Catal. Lett.16, 109 ~1992!.

10C. K. Jørgensen, Prog. Inorg. Chem.12, 101 ~1970!.11T. F. Towey, A. Khan-Lohdi, and B. M. Robinson, J. Chem. Soc. Faraday

Trans.86, 3757~1990!; L. Motte, C. Petit, L. Boulanger, P. Lixon, and M.P. Pileni, Langmuir8, 1049~1992!.

12M. Anpo, T. Shima, S. Kodama and Y. Kubokawa, J. Phys. Chem.91,4305 ~1987!.

13~a! C. J. Sandroff, D. M. Hwang, and W. M. Chung, Phys. Rev. B33,5953 ~1986!; ~b! M. L. Steigerwald and L. E. Brus, Acc. Chem. Res.23,183 ~1990!; ~c! C. Kormann, D. W. Bahnemann, and M. R. Hoffmann, J.Phys. Chem.92, 5196 ~1988!; ~d! L. Kavan, T. Stoto, M. Gra¨tzel, D.Fitzmaurice, V. Shklover,ibid. 97, 9493~1993!; ~e! W. Choi, A. Terminand M. R. Hoffmann,ibid. 98, 13669~1994!.

14R. Rossetti, J. L. Ellison, J. M. Gibson, and L. E. Brus, J. Chem. Phys.80,4464 ~1984!.

15K. Seeger,Semiconductor Physics~Springer-Verlag, New York, 1973!.16N. Serpone, D. Lawless, and R. Khairutdinov, J. Chem. Phys.99, 16 646

~1995!.

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