J. Phys. Chem. 1994,98, 8827-8830
8827
Monoparticulate Layers of Titanium Dioxide Nanocrystallites with Controllable Interparticle Distances Nicholas A. Kotov, Fiona C. Meldrum, and Janos H. Fendler’ Department of Chemistry. Syracuse University, Syracuse, New York 13244-41 00 Received: May 12, 1994; In Final Form: July 20, 1994’
The arrested hydrolysis of titanium tetraisopropoxide by millimolar concentrations of water in a mixture of chloroform and 1-propanol in the presence of hexadecyltrimethylammonium bromide and tetramethylammonium hydroxide resulted in the formation of stable dispersions (sols) of nanocrystalline (18-22 A diameter) TiO2. Spreading the sols on water surfaces in a Langmuir film balance produced monoparticulate Ti02 films. Heat treatment of a given sol prior to dispersion on the aqueous subphase resulted in nanocrystalline monoparticulate Ti02 films with reduced interparticulate distances.
Introduction Size and interparticledistance control is an important criterion in the construction of titanium dioxide nanoparticle-based, transparent ceramic membranes’ and highly efficient thin-film photovoltaic As part of our “membrane-mimeticapproach to advanced materials”? we have recently reported the generation and characterization of monoparticulate layers of cadmium sulfide? silver,* and magnetite (Fe304)9 nanocrystallites on aqueous solution surfaces in a Langmuir film balance. Furthermore, these monoparticulate nanocrystalline metallic and semiconductingfilms could be transferred, layer by layer, to solid substrates by the Langmuir-Blodgett t e ~ h n i q u e . ~The - ~ present a p p r ~ a c h , ~it-should ~ be recognized, is different from forming thin films by the incorporation of oxide precursors between the headgroups of Langmuir-Blodgett (LB) films and subsequently destroying the surfactants (in the LB film) by heat or radiation treatment.10J1 The formation of monoparticulate layers of titanium dioxide (Ti02) nanocrystallites with controllable interparticle distances is the subject of the present communication. Stable dispersions (sols) of surfactant-coated Ti02 nanocrystallites were prepared in an organic solvent mixture. Spreading these sols on water surfaces in a Langmuir balance led to nanocrystalline monoparticulate Ti02 films. Heat treatment of a given sol prior to dispersion on the aqueous subphase resulted in nanocrystalline monoparticulate Ti02 films with reduced interparticulate distances.
Experimental Section Ti02 particles were prepared by the arrested hydrolysis of titanium tetraisopropoxide (0.125 mL, 4.6 X 10-4 mol) by water (20 pL, 1 X 10-3 mol) in a mixture of chloroform (60 mL) and 1-propanol (40 mL) in the presence of a hexadecyltrimethylammonium bromide (CTAB) stabilizer (0.153 g, 4.2 X 10-4 mol) and a tetramethylammonium hydroxide (TMAH) catalyst (0.2 or 0.7 g, 1.1 X 10-3 or 3.6 X 10-3 mol). The dispersions were stable for at least 1 month. Hexane (20%, v/v) was added to the dispersion to improve spreading. The addition of hexane did not induce any change in the absorption spectra of the sols. An appropriate amount (typically 15OpL)oftheTiO2sol (in thechloroform/l-propanol/ *Abstract published in Advance ACS Absrracrs, September 1, 1994.
0022-3654/94/2098-8827%04.50f 0
hexane solvent mixture) was spread on the water surface in a Lauda Langmuir film balance by using a Hamilton syringe, taking care not to place the tip of the syringe further than 4 mm above the water surface. Water was purified by a Millipore Milli-Q system to a resistivity of 18 MQ-cm. Surface pressure (n) us surface area ( A ) isotherms were taken 30 min subsequent to the introduction of the Ti02 sol onto the water surface at a moving barrier rate of 1 cm/min. The concentrations of Ti02 nanoparticles in the spreading solution were calculated by assuming a complete hydrolysis of their precursors and 3.8 g/cm3 for the density of Ti02 (3.8 g/cm3 = density of anatase). Absorption spectra of the Ti02 sols were taken on a HewlettPackard 8452A diode-array spectrophotometer. Examination of the Ti02 monoparticulate layers by Brewster angle microscopy (BAM) was carried out in a homemade trough by using a IO-pW He/Ne laser as the illumination source.7 The thickness of the monoparticulate layers on the water subphase was determined by in situ reflectivity measurements. A 5-mW Hughes He-Ne laser was rigidly mounted on a rotator such that it could be precisely turned through a known angle. The intensity of the reflected light, I , was measured in 0.5’ incident angle (with f0.02’ precision) intervals by a Spectra Physics 404 laser power meter. The intensity of the incident light was measured for several hours at a distance of 60 cm from the source (to avoid interferencefrom noncoherent light). The light intensity remained stable (within 3%) during the entire set of measurements. The obtained data were treated in terms of the classical Fresnel equations, and the reflectivity curve ( I l l 0 us incident angle) was fitted by a commercially available software program (Sigmaplot 5.OWIN) to yield values for the thickness and the refractive index.12 Due care was taken in setting the rotator precisely and simultaneously recording the intensities of the incident and reflected lights at each angle. Transmission electron microscopic (TEM) images were taken on a JEOL 2000FX electron microscope operating at 200 kV. Ti02 monoparticulate films were picked up on Formvar-covered, carbon-coated, 400-mesh copper grids by horizontal lifting from the top surface of the films. Results and Discussion The absorption spectrum of a Ti02 sol, prepared by the hydrolysis of 0.125 mL of titanium tetraisopropoxide by 20 pL 0 1994 American Chemical Society
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8828 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994
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Figure 2. Surface pressure, n, us surface area, A, isotherms for Ti02 monoparticulate films (spread from the sol whose absorption spectrum is shown as curve a in Figure 1) obtained under continuous compression (dotted line) and under consecutivecompression and expansioncycles in which each compression step had a final surface pressure which was 5 mN/m greater than the previous one (curves 1-7).
Figure 1. Absorption spectra of Ti02 sols, prepared by the hydrolysisof
titanium tetraisopropoxide(0.125 mL, 4.6 X l w mol) by water (20 pL, 1.0 X lO-' mol) in a mixture of chloroform (60 mL) and 1-propanol(40 mL) in the presenceof CTAB (0.153 g, 4.2 X 1 Pmol) and TMAH (0.2 g, 1.1 X 10-3mol), in the absence of heat treatment (a) and subsequent to heating at 70 O C for 90 min (b) and to heating at 90 O C for 60 min (c) and 120 min (d). of water in the presence of 0.153 g of CTAB and 0.20 g of TMAH in a mixture of 60 mL of chloroform and 40 mL of 1-propanol, displayed an absorption threshold of 320 nm (see curve a in Figure 1). Heating this sol a t 70 OC for 90 min resulted in a shift of the absorption threshold to 322 nm (see curve b in Figure 1); heating at 90 OC for 60 and 120 min caused further shifts to 324 and 326 nm, respectively (see curves c and d in Figure 1). Importantly, the absorption maximum (Figure 1) and, thus, the concentration of Ti02 in the sol did not change as a consequence of the heat treatment. The diameters of the Ti02 nanoparticles in the sols, 2R, were evaluated from the measured absorption thresholds, A,, by the semiempirical relation~hipl~
A , = A,
+ C,/R2 + C2/R
where Ab is the absorption threshold of the bulk Ti02 semiconductor and CIand C2 are constants, determined as 82.8 A2 eV and 2.58 A eV from data derived from high-resolution transmission electron microscopic images of Ti02 particles and their corresponding absorption thresholds? Substitution of the absorption thresholds of 320, 322, 324, and 326 nm into eq 1 yielded 18.5, 18.8,19.4,and 19.6Afor themeandiametersoftheTiOzparticles whose absorption spectra are shown by curves a, b, c, and d in Figure 1. Increasing the concentration of TMAH to 0.7 g (while keeping the concentrations of all other components the same as that described for the sol in Figure 1) produced Ti02 particles with diameters of 22 A (as calculated from the adsorption edge). Therefore, all of the sols examined contained size-quantized Ti02 particles, and neither heat treatment nor changing the concentration of TMAH appreciably altered the size of the nanocrystallites. The nanocrystallite Ti02 particles were formed by thearrested hydrolysis of titanium tetraisopropoxide in a basic organic medium. In this environment, the surface of the incipient Ti02 is negatively charged and attracts the cationic CTAB molecules. The CTAB-coated Ti02 particles, thus, became hydrophobic. The behavior of the Ti02 particulate film markedly depended on the rate and the extent of compression. The surface pressure (II) us surface area (A) isotherm, obtained after the introduction of theTiO2 sol onto the water surface a t a continuous compression, rose gradually until the film collapsed at ca. 50 mN/m (dotted
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A, A2/PARTICLE Figure 3. Reversible surface pressure, n,us surface area, A, isotherms for well-conditioned Ti02 monoparticulatefilms. The solid lines represent compression,and the broken lines correspondto the subsequentexpansion cycles. Isotherms 2 were obtained on spreading sols whose adsorption spectrumisshownascwea in Figure 1(aftereightrepeatedcompressionexpansion cycles). Isotherms 3 were obtained on spreading sols whose absorption spectrum is shown as curve d in Figure 1 during the first compression-expansion cycle. The nonreversible isotherm 1 represents an early compression-expansion cycleduring the conditioningof the film whose final n-A curve is shown as isotherm 2.
line in Figure 2). Subsequent and consecutive multiple compression-expansion cycles, to progressively larger II values, were found to be irreversible (see curves 1-7 in Figure 2). After each compression, the expansion isotherm rapidly collapsed to IT = 0 mN/m and the ensuing compression curve rose at a smaller A value and had an inflection at a surface pressure corresponding to the maximum II value measured for the previous compression. Subsequent to this conditioning (Le., after performing several compression-expansion cycles such as shown in Figure 2), the II us A isotherm of the Ti02 nanocrystalline sol became completely reversible (see curve 2 in Figure 3). This behavior may be attributed to the gradual dissolution of excess surfactant molecules into the aqueous subphase. The U-A isotherms corresponding to heat-treated sols were reversible a t any II value smaller than 45 mN/m (curve 3, Figure 3). The uniformity of the Ti02 nanocrystalline film, during all stages of its compression, was demonstrated by Brewster angle microscopy (Figure 4). The presence of the film in static pictures only became apparent when contrast was introduced by cracks
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The Journal of Physical Chemistry, Vol. 98. No. 36. 1994 8829
Figure 4. Brewster angle microscopic images of a Ti02 monoparticulate film (spread from the sol whose absorption spectrum is shown as curve a in Figure I ) al n = 5 (a). IS (b), 50 (c). and 0 mN/m; decompression after collapse (d).
in theimages,caused by dust particles. Anincreaseofthesurface pressure to 2OmN/m brightened theimagesomewhat andclosed some of the cracks (Figure 4b). The solid state of the Ti02 nanocrystalline film at Il = 50 mN/m could be observed as a featureless bright field (Figure 4c), whereas after collapse and expansion the image revealed a seriesof stripeswhich corresponded to the disruption of the film by internal stress (Figure 4d). Measurements of angle-dependent reflectance of ppolarized light have been fruitfully employed for the determination of the reflective index ( n ) and thickness (d)of monoparticulate films on water surfaces.12 The measured reflectivity of the Ti02film was found tobesignificantlydifferent from the bare water surface only at high surface pressures. The obtained mean thickness of 32 IO A and reflective index of 2.2 f 0.2 are compatible with the presence of a monoparticulate thick nanocrystalline Ti02 film on the water surface. This value should be compared to the diameteroltheTiO2particles(22A), assessedfrom theabsorption edge of the sol. Thermal fluctuations and the presence of an organic surfactant shell are believed to be responsible for the larger diameters, observed by reflectivity, than those assessed from absorption spectrophotometry for the Ti02 particles. Transmission electron microscopic images substantiated the formation of monoparticulate films. In spite of the very low contrast (due to the small particle size and thinness of the Ti02 film), uniformity and monoparticulate coverage are discernible in the images of nanocrystalline Ti02 films (Figure 5 ) . We estimate the diameters of the Ti02 particles to be 20 f 5 A.
*
Heat treatment of the Ti02sols dramatically decreased the interparticle distances in the spread monoparticulate layers. Il us A isothermsofwell-conditioned(i.e.,subsequent to consecutive multiple compression-expansion cycles until the Il-A isotherms became reversible), monoparticulate Ti02 films shifted to progressively smaller surface areas upon using sols which were subjected to increasingly long periods of heat treatment (Figure 6). Importantly, concentrations and sizes of the Ti02 particles wereessentiallyunaffected by heat treatment (Le., they remained in the 18.5-19.6-A range). A T i 0 2 particle prepared from sols in the absence of any prior heat treatment (see curve 1 in Figure 6) occupied a mean area of 1150 A.2 The monoparticulate layers prepared from Ti02 sols which had been heated for 90 min at 70 OC (curve 2 in Figure 6). for 60 min at 90 "C (curve 3 in Figure 6). and for 120 min at 90 OC (curve 4 in Figure 6 and curve 3 in Figure3)resultedinmeanareasof 1000,650,450A20ccupied by a Ti02 particle. Decrease of the interparticulate distances in the monoparticulate films is the consequence of thinning of the Ti02surface coating upon heating the sols. Heating alters the surface structure of the Ti02 nanocrystallites, which renders the binding of CTAB to be less favorable; heat-treated Ti02 nanocrystalliteshavethinnersurfactantcoatsthanuntreatedones (Figure 7). Thus, assuming hexagonal close packing, mean areas of 1150, 1000, 650, and 450 A in the monoparticulate layers correspond to 9 5 , 8-, 4-, and 2-A average thicknesses of the surfactants covering the Ti02 particles. Note that the length of the CTAB molecule is ca. 15 A and its hydrocarbon cross section
8830 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994
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"7. SEhematicsofTiO~monopartieulatefilms.viewed from above, withalonger,A(isotherm I in Figure6),andwithashorter,B(isotherm 4 in Figure 6). interparticle distance.
on the TiO, particles (Figure 7B). The present report demonstrates that interparticle distance may be controlled in a well-defined semiconductor monoparticulate layer by colloid chemical techniques. Determination of interparticle-distance-dependentelectrical, optical, and el=trooptical properties of this and related systems is the subject of our intense current investigations. of surfactants indicates a tightly wrapped mantle
Figure 5. Transmission electron microscopic imager of a Ti02 m a n e particulate film, formed from Ti02 sols prepared by the hydrolysis of titanium tetrairapropaxide (0.125 mL. 4.6 X IW mol) by water (2OpL. 1.0 X 10-3 mol) in a mixture of chloroform (60mL) and I-propanol (40 mL)inthepresenceofCTAB(0.153g.4.2X 1Wmol)andTMAH (0.7
g. 3.6 x IC-' mol). in the absence of heat treatment. The scale bars are 400 (a) and 40 nm (b). A Ti01 particle is shown by the arrow.
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. References and Notes ( I ) Brinka, C.J.; Schcm, G. W.Sol-Gel Science. The Physics and Chemistry of Sol-Gel Prcxessing; Academic Pres: New York 1990. (2) ORcgan. B.: Moosr. I.; Anderson. M.: Gdtlcl. M.J. Phyr. Chem.
c'
20
.'.,
10
.,
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1990.94,8720. (3) Gr6trcl. M.MRS Bull. 1993, oerobcr 61. (4) Liu, D.;Kamat. P. V . 1. Phys. Chem. 1993. 97, 10769. ( 5 ) Vogel, R.: Pohl. K.: Wdler. H. Om.P h p kn,1990. J74.241. (6) Fcndler. I. H. Membronc-MimelieApproachloAdvanced Martrlds;
.-
Springer-Verlag: Berlin, 1994. (7) Kotov, N.A,; Meldrum, F. C.: Fendler, J. H.1. Phys. Chcm. 1994,
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A. A'IPARTICLE F b m 6. Surface pmure, II. US surface area, A, isotherm for wellconditioned Ti02 monoparticulate films. The sols used were prepared by the hydrolysis of titanium tetraisopropoxidc (0.125 mL, 4.6 x lW mol) by water (20 pL. 1.0 X lV3mol) in a mixture of chloroform (60 mL) and I-propanol (40mL) in the presence of CTAB (0.153 8.4.2 x Io-'mol) and TMAH (0.2. g. 1.1 X lV3mol), in the absence of heat treatment (isotherm I), and subsequent to heating at 70 OC for 90 min (isotherm 2) and to heating a t 90 OC for 60 min (isotherm 3) and 120 min (isotherm 4).
is ca. 2 A; hence, a 9.5-A-thick 'coat" corresponds to fairly stretched surfactants (see Figure 7A), whereas a 2-A-thick layer
98,2735. ( 8 ) Meldrum. F. C.: Kotov.
N. A,; Fendlcr, J. H.Lonmuir, in pras. (9) Meldrum, F. C.;Kot~v.N. A,; Fendler, J. H.1. Phys. Chem., in
pnss.
(IO) Paranjaw, D. V.; Sastry, M.: Ganguly, P. Appl. Phys. .!.ell. 1993. 6 .3 .. ,I...X ~ (11) Amm. D.T.; Johnson. D.J.: Launcn,T.; Gupta, S.K. Appl. Phys. k l l . 1992.61 (5). 522. (12) Zhao. X.; Xu,S.; Fcndlcr. J. H.J. Phw. Chem. 1990.94.2573. (13) Equation I is a generalized form of the thmrctical relationship E, + I/mr)/2Rl+ 1.78W/t where E. and E, are the band gaps of the sizequantmd particles and the bulk semiconductor. R io the radius of the semiconductor particla. me and mr are the effective mmcS of electrons and holes, and e is thsdielectricconstan1.J~Uncertainties in meand mr vslues for sirequantized Ti02 precluded the use of the theoretical relationship. (14) Bahnmann, D.W.Is,. J . Chrm. 1993. 33, 115.
= E, + h ' r Y l / s
