10732
J. Phys. Chem. 1996, 100, 10732-10738
Preparation and Characterization of Transparent Nanocrystalline TiO2 Films Possessing Well-Defined Morphologies Stephen Doherty and Donald Fitzmaurice* Department of Chemistry, UniVersity College Dublin, Dublin 4, Ireland ReceiVed: February 22, 1996; In Final Form: April 16, 1996X
We describe the preparation, characterization, and potential-dependent optical absorption spectroscopy of transparent nanocrystalline TiO2 films possessing well-defined morphologies. Regarding preparation, Langmuir-Blodgett techniques are used to deposit between one and four monolayers of TiO2 nanocrystallites on conducting glass. As the average spacing between the nanocrystallites constituting a deposited monolayer may be controlled, the degree of nanoporosity of the resulting nanocrystalline film can be determined. Subsequent firing fuses the constituent crystallites of the deposited monolayers and ensures an ohmic contact with the conducting substrate. Characterization by electron microscopy and optical absorption spectroscopy has been undertaken. The former reveals the above films to be nanoporous or close-packed arrays of anatase nanocrystallites. The latter has allowed determination of an absorption coefficient for anatase at 326 nm. Measurement of potential-dependent optical absorption spectra has also proved possible. Results obtained suggest that the degree of charge accumulation possible at a given applied potential is dependent on the morphology of the nanocrystalline film.
Introduction
Experimental Section
The recent past has seen increased interest in transparent nanoporous nanocrystalline semiconductor films.1 For example, their use as sensitized photoanodes in regenerative photoelectrochemical cells has attracted considerable attention.2 Recently, other applications have been described. These include lithium-insertion batteries and electrochromic windows.3,4 In short, such films appear to have significant technological potential. Increased awareness of the potential of transparent nanoporous nanocrystalline semiconductor films has led to significant effort being directed toward the elucidation of their properties.5 Consequently, the interfacial and bulk energetic and transport properties of such films have been studied extensively.6,7 Observed differences between such films and the corresponding bulk semiconductor have been attributed, in large part, to the nanoporous structure of the former. To date, no systematic studies of the dependence of the energetic and transport properties of nanoporous nanocrystalline semiconductor films on their morphology have been reported. However, the absence of such studies is not surprising in view of the following: first, the apparently complex morphology of such films; second, the absence of methodologies for systematically controlling the morphology of such films; and third, difficulties that may be encountered in applying conventional characterization techniques to such films. In order to permit systematic studies of the type outlined above, our recent work has been directed toward the preparation and characterization of nanocrystalline films possessing well defined morphologies. Two approaches are being adopted. The first involves use of Langmuir-Blodgett (LB) techniques and is the subject of the present report. The second, involves selfassembly of organized arrays of semiconductor nanocrystallites and will be described elsewhere.8 The ability to prepare nanocrystalline semiconductor films possessing the optimum morphology for a given study or application is a long-term objective.
Preparation of TiO2 Nanocrystallites. TiO2 nanocrystallites were prepared as described by Kotov et al.,9 that is, by arrested hydrolysis of titanium tetraisopropoxide. Briefly, 1.125 mL of a 1:9 (v/v) mixture of titanium tetraisopropoxide and 1-propanol was added to chloroform (60 mL) and 1-propanol (40 mL) containing excess water (150 µL present in added catalyst). The above addition was made in the presence of cetyltrimethylammonium bromide (CTAB) stabilizer (0.612 g) and tetramethylammonium hydroxide (TMAH) catalyst (0.2 g, 75 wt % water). The resulting CTAB sol was characterized by UVvisible absorption spectroscopy and by transmission electron microscopy (TEM). The observed onset for absorption and average crystallite diameter of 360 nm and 22 ( 2 Å, respectively, are in good agreement with reported values. The above preparation was repeated and the resulting sol, hereafter referred to as a heat-treated CTAB sol, refluxed at 90 °C for 120 min. The observed onset for absorption and average crystallite diameter were found to be 370 nm and 23 ( 2 Å, respectively. Deposition of Nanocrystalline TiO2 Films. Monolayers of TiO2 nanocrystallites were deposited on conducting glass (Glastron) using a JL Automation Langmuir minitrough. Briefly, 20 mL of hexane was added to 100 mL of the CTAB sol prepared as described above. A 100 µL aliquot of this stock was spread, using a precision syringe, on a water subphase and 30 min allowed for solvent evaporation. The resulting monolayer was conditioned by successive compression-expansion cycles to 25 mN m-1. Maintaining the above monolayer at 25 mN m-1 for 30 min resulted in an additional 25% decrease in surface area. Subsequent compression-expansion cycles to 25 mN m-1 were significantly more reversible, yielding a final area per particle of 1200 Å2 at 25 mN m-1. More highly compressed monolayers were prepared by spreading 150 µL of the above CTAB sol, following addition of hexane, on a water subphase and allowing 30 min for solvent evaporation. The resulting monolayer was conditioned by successive compression-expansion cycles to 25 mN m-1. As above, maintaining the monolayer at 25 mN m-1 for 30 min
X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
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© 1996 American Chemical Society
Transparent Nanocrystalline TiO2 Films resulted in an additional 25% decrease in surface area. Subsequent compression-expansion cycles to 40 mN m-1 were significantly more reversible, yielding a final area per particle of 950 Å2 at 40 mN m-1. Still more highly compressed monolayers were prepared by adding 20 mL of hexane to 100 mL of the heat-treated CTAB sol prepared as described above. A 200 µL sample of this stock was spread, using a precision syringe, on a water subphase and 30 min allowed for solvent evaporation. The resulting monolayer was conditioned by successive compression-expansion cycles to 25 mN m-1. As above, maintaining the monolayer at 25 mN m-1 for 30 min resulted in an additional 25% decrease in surface area. Subsequent, compression-expansion cycles to 40 mN m-1 were highly reversible and correspond to a final area per particle of 550 Å2 at 40 mN m-1. During deposition, fluorine-doped tin oxide conducting glass (Glastron) was dipped, at a rate of 20 mm min-1, through a monolayer of TiO2 nanocrystallites compressed to 25 or 40 mN m-1. Deposition, which occurs on both sides of the glass, is observed only on the upstroke. For deposition of multilayers, the conducting glass substrate was dipped, with a wait time of 30 s between cycles, the required number of times. Finally, monolayer or multilayer films were fired in air at 450 °C for 2 h. Potential-Dependent Optical Absorption Spectroscopy. Nanocrystalline TiO2 film, prepared as described above, formed the working electrode (2 cm2 surface area) of a closed threeelectrode single-compartment cell, the counter electrode being platinum and the reference electrode a saturated calomel electrode (SCE). Aqueous electrolyte solutions, degassed by bubbling with Ar for 30 min, contained LiClO4 (0.2 mol dm-3) at pH 2.0 (added HClO4) and pH 12.0 (added LiOH). Potential control was provided by a Thompson Electrochem Ministat potentiostat and a Hewlett-Packard 3310B function generator. The above cell was incorporated into the sample compartment of a diode array spectrometer. All reported difference absorbance spectra were recorded with respect to a background measured at 0.00 V. All stated potentials are versus SCE. Characterization Techniques and Instrumentation. All optical absorption spectra were recorded using a HewlettPackard 8452A diode array spectrophotometer. TEM and microelectron diffraction (MED) patterns were recorded using a JEOL 2000 FX Temscan. Films were deposited on carboncoated nickel grids and fired at 450 °C for 2 h.10 Results and Discussion Deposition of Nanocrystalline TiO2 Films. As stated, colloidal dispersions of TiO2 in chloroform were prepared following the method of Kotov et al.9 Use of CTAB as a stabilizer ensures the resulting nanocrystallites are sufficiently hydrophobic that they may be deposited on conducting glass using LB techniques. The surface pressure (Π) versus area (A) isotherms measured during conditioning of a monolayer of CTAB sol are shown in Figure 1a. Successive compression-expansion cycles to 25 mN m-1 result in increasingly reversible isotherms. Further compression and still greater reversibility results from maintaining the above monolayer at a surface pressure of 25 mN m-1 for 30 min; see also Figure 1a. This behavior has been attributed to a gradual loss of surfactant molecules to the aqueous subphase.9 Monolayers were deposited at a fixed surface pressure of 25 mN m-1 corresponding to an area of 1200 Å2 per particle; see Figure 1b. Up to four monolayers have been deposited in this manner.
J. Phys. Chem., Vol. 100, No. 25, 1996 10733
Figure 1. (a) Surface pressure (Π) versus area (A) isotherms for conditioning of a monolayer of CTAB sol prior to deposition. The initially measured isotherms up to 25 mN m-1 are shown, as is a typical isotherm measured up to 25 mN m-1 following conditioning for 30 min at a constant surface pressure of 25 mN m-1. (b) Change in surface area (∆A) versus time during deposition of the compressed monolayer in (a) at 25 mN m-1.
A more compressed monolayer may be conditioned by successive compression-expansion cycles to 40 mN m-1. Initially, however, the monolayer is conditioned by successive compression-expansion cycles to 25 mN m-1. The above monolayer is then maintained at a surface pressure of 25 mN m-1 for 30 min during which time additional monolayer compression is observed; see Figure 2a. As above, this behavior has been attributed to a gradual loss of surfactant molecules to the aqueous subphase. Subsequent compression-expansion cycles to 40 mN m-1 were more reversible and resulted in further monolayer compression; see also Figure 2a. Monolayers were deposited at a fixed surface pressure of 40 mN m-1 corresponding to an area of 950 Å2 per particle. Up to four monolayers have been deposited in this manner. The most highly compressed monolayers were prepared using the heat-treated CTAB sol. Initially monolayers are conditioned by successive compression-expansion cycles to 25 mN m-1. Further compression results from maintaining the above monolayer at a surface pressure of 25 mN m-1 for 30 min. Subsequent, compression-expansion cycles to 40 mN m-1 resulted in further monolayer compression. Monolayers were deposited at a fixed surface pressure of 40 mN m-1, corresponding to an area of 550 Å2 per particle. This degree of compression, corresponding to an essentially close-packed array of nanocrystallites, is possibly due to thinning of the surfactant layer upon heat treatment.9 Up to four monolayers have been deposited in this manner. Shown in Figure 2b, for comparison, are isotherms obtained prior to deposition of films compressed to 550, 950, and 1200 Å2 per particle.
10734 J. Phys. Chem., Vol. 100, No. 25, 1996
Figure 2. (a) Surface pressure (Π) versus area (A) isotherms for conditioning of a monolayer of CTAB sol prior to deposition. The initially measured isotherms up to 25 mN m-1 are shown, as is a typical isotherm measured up to 40 mN m-1 following conditioning for 30 min at a constant surface pressure of 25 mN m-1. (b) Surface pressure (Π) versus area (A) isotherm for a monolayer of heat-treated CTAB sol, following conditioning as in (a) and prior to deposition (1). Also shown are isotherms for monolayers of CTAB sol conditioned to 25 (2) and 40 mN m-1 (3) prior to deposition.
To remove the organic material present at the surface of the deposited nanocrystallites the above monolayers or multilayers were fired at 450 °C for 2 h. Characterization of Nanocrystalline TiO2 Films by Transmission Electron Microscopy. Shown in Figure 3a is a TEM of a fired monolayer whose average area per particle, as determined from the isotherms shown in Figure 1a, is 1200 Å2. The average area per particle as determined from the above TEM is in excellent agreement with this value, i.e., 1200 Å2. This image also confirms that there is no significant increase in particle size or disruption of the initially deposited monolayer during firing. The corresponding MED pattern (not shown) confirms that the constituent crystallites of the fired film are anatase. Specifically, diffractions corresponding to the following lattice spacings (in Å) are observed: 3.5 (3.52); 2.4 (2.43); 1.9 (1.89); 1.7 (1.70); 1.6 (1.67); 1.4 (1.48).11 Shown in Figure 3b is a TEM of a fired monolayer whose average area per particle, as determined from the isotherm shown in Figure 2b, is 550 Å2. The average area per particle as determined from the above TEM is in good agreement with this value, i.e., 550 Å2. Examination of this TEM confirms there is no significant increase in particle size or disruption of the initially deposited monolayer during firing. As above, the corresponding MED pattern (not shown) confirms that the constituent crystallites of the fired films are anatase. Specifically, diffractions corresponding to the following lattice spacings (in Å) are observed: 3.5 (3.52); 2.4 (2.43); 1.9 (1.89); 1.7 (1.70); 1.6 (1.67); 1.4 (1.48).11
Doherty and Fitzmaurice
Figure 3. (a) TEM of a monolayer of CTAB sol, conditioned and deposited as shown in Figure 1a, following firing in air at 450 °C for 2 h. The average area per particle, as determined from the TEM, is 1200 Å2. (b) TEM of a monolayer of heat-treated CTAB sol, conditioned as shown in Figure 2b, following firing in air at 450 °C for 2 h. The average area per particle, as determined from the TEM, is 550 Å2.
SCHEME 1. Morphology of Nanoporous and Close-Packed Nanocrystalline Films
Finally, we represent the TiO2 films whose preparation and characterization are described above as shown in Scheme 1. Specifically, we may prepare either close-packed or nanoporous nanocrystalline TiO2 films of a known thickness. Further, in the case of nanoporous nanocrystalline films, it is possible to
Transparent Nanocrystalline TiO2 Films
Figure 4. (a) Optical absorption spectra of unfired nanoporous nanocrystalline films consisting of the indicated number of deposited monolayers whose average area per particle is 1200 Å2. (b) As in (a) following firing in air at 450 °C for 2 h.
control the degree of porosity of such films by controlling the average spacing between the nanocrystallites constituting a deposited monolayer. Hereafter, films prepared from a CTAB sol that has not been heat-treated, and which consist of monolayers whose average area per particle is either 1200 or 950 Å2, are referred to a nanoporous nanocrystalline films. Those prepared from heat-treated CTAB sols, and which consist of monolayers whose average area per particle is 550 Å2, are referred to as close-packed nanocrystalline films. Characterization of Nanocrystalline TiO2 Films by Optical Absorption Spectroscopy. Nanoporous and close-packed nanocrystalline TiO2 films were characterised by UV-visible absorption spectroscopy. Shown in Figure 4a are the absorbance spectra of one- to four-monolayer (1200 Å2 per particle) nanoporous nanocrystalline films following deposition. Shown in Figure 4b are spectra of these films following firing. As expected, the measured absorbance increases linearly with the number of deposited monolayers for both fired and unfired films. Also as expected, absorbance by a fired film is greater than by the corresponding unfired film, this latter observation being accounted for by fusing of the constituent crystallites upon firing.12 Accounting for deposition of monolayers on both sides of the conducting glass, the average absorbance increase per monolayer at 326 nm is 0.006 in the case of the fired films above. Assuming an optical path length that is an integral multiple, equal to the number of deposited monolayers (four), of the average diameter of the nanocrystallites constituting a given monolayer (22 ( 2 Å), we calculate an absorption coefficient of 6 × 104 cm-1. This value is smaller than that previously reported for polycrystalline rutile at the same wavelength,13 5 × 105 cm-1, and that estimated for anatase at 340 nm,14 2 × 105 cm-1. However, examination of the
J. Phys. Chem., Vol. 100, No. 25, 1996 10735
Figure 5. (a) Optical absorption spectra of four-monolayer nanoporous nanocrystalline films, following firing in air at 450 °C for 2 h, whose average areas per particle are (1) 1200 and (2) 950 Å2. (b) As in (a) for four-monolayer nanoporous and close-packed nanocrystalline films whose average areas per particle are (1) 1200 and (2) 550 Å2, respectively.
diagrammatic representation of this nanoporous nanocrystalline film in Scheme 1 indicates a void fraction of 0.79. Correcting the effective path length appropriately yields an absorption coefficient of 3 × 105 cm-1, close to the expected value. Shown in Figure 5a are the spectra of four monolayer nanoporous nanocrystalline films following firing whose average areas per particle are 1200 and 950 Å2. These films are estimated to have void fractions of 0.79 and 0.73 respectively. Consequently, the more compressed film is predicted to have a measured absorbance that is 1.3 times that of the less compressed film. This expectation is consistent with the spectra shown in Figure 5a. We note also, the excellent agreement between the spectrum for the less compressed film (1200 Å2 per particle) and a similar film in Figure 4b. The average absorbance increase per monolayer at 326 nm for the more compressed film, taking into account deposition of both sides of the glass substrate, is 0.007. Assuming a particle diameter of 22 ( 2 Å and a void fraction of 0.73, the absorption coefficient at 326 nm is calculated to be 3 × 105 cm-1. Shown in Figure 5b are the spectra of four-monolayer fired nanocrystalline films whose average areas per particle are 1200 and 550 Å2. These films are estimated to have void fractions of 0.79 and 0.50, respectively. Consequently, the close-packed film is predicted to have a measured absorbance that is 2.4 times that of the nanoporous film. This expectation is consistent with the absorbance spectra shown in Figure 5b. We note also, the excellent agreement between the spectrum for the less compressed film (1200 Å2 per particle) and a similar film in Figure 4b. The average absorbance increase per monolayer at 326 nm for the close-packed film, taking into account deposition of both sides of the glass substrate, is 0.014. Assuming a particle
10736 J. Phys. Chem., Vol. 100, No. 25, 1996
Figure 6. (a) Difference absorbance spectra at pH 2.0 of a onemonolayer nanoporous nanocrystalline film (1200 Å2 per particle), following firing in air at 450 °C for 2 h, measured at the indicated applied potentials (V, SCE). (b) As in (a) for a four-monolayer nanoporous nanocrystalline film (1200 Å2 per particle).
diameter of 23 ( 2 Å and a void fraction of 0.50, the absorption coefficient is calculated to be 3 × 105 cm-1. The absorption coefficients calculated above for anatase at 326 nm are in good agreement. These values also compare well with those previously reported for polycrystalline rutile at the same wavelength,13 5 × 105 cm-1, and anatase at 340 nm,14 2 × 105 cm-1. The observed agreement between the calculated values is a consequence of correction of the initially determined values for the void fraction of the film in each case. The void fraction is determined from the area occupied per particle in the monolayers constituting a given film and assumes deposited monolayers are stacked. This, in turn, supports the diagrammatic representation of the above films in Scheme 1. This appears to be the first experimental determination of an absorption coefficient for the anatase form of TiO2.15 Potential-Dependent Spectroscopy of Nanocrystalline TiO2 Films. Following firing, the constituent crystallites of a nanoporous nanocrystalline TiO2 film prepared using sol-gel techniques are in ohmic contact with each other and with the conducting glass.16 As a consequence, potentiostatic control of the Fermi potential (Vf) within such films is possible.17 That is, at applied potentials (Vapp) more negative than the potential of the conduction band edge (Vcb) at the semiconductor-liquid electrolyte interface (SLI), electrons occupy the available states of the conduction band. Accumulation is accompanied by increased absorbance in the visible and near-infrared spectral regions assigned to trapped and free electrons.18 Accumulation is also accompanied by an absorbance loss at wavelengths shorter than that corresponding to the band-gap energy assigned to the Burstein shift that accompanies band filling.18,19 Similar potential control would be expected, following firing, in nanoporous or close-packed nanocrystalline films.
Doherty and Fitzmaurice
Figure 7. (a) Difference absorbance spectra at pH 2.0 of a fourmonolayer nanoporous nanocrystalline film (950 Å2 per particle), following firing in air at 450 °C for 2 h, measured at the indicated applied potentials (V, SCE). (b) As in (a) for a four-monolayer closepacked nanocrystalline film (550 Å2 per particle).
The potential-dependent spectra measured at pH 2.0 for fired one and four monolayer nanoporous (1200 Å2 per particle) nanocrystalline films are shown in Figure 6. We note the following: First, spectra measured for one- and four-monolayer films are in excellent qualitative agreement; second, the magnitude of the absorbance assigned to accumulated electrons in a film consisting of four monolayers is 4 times that measured for a film constituted from a single monolayer; and finally, the onset of the absorbance loss assigned to the Burstein shift is at 380 nm, the value expected for fused nanocrystallites. The potential-dependent spectra, measured at pH 2.0 for fired four-monolayer nanoporous (950 Å2 per particle) and closepacked (550 Å2 per particle) nanocrystalline films are also shown; see Figure 7. However, while these spectra are in excellent qualitative agreement with those measured for fired four-monolayer nanoporous (1200 Å2 per particle) films (see Figure 6b), we note that the magnitude of the measured absorbance at a given Vapp is progressively reduced as a film becomes less nanoporous. This observation is noteworthy when one considers the following: First, that the relative densities of films prepared from monolayers compressed to 1200, 950, and 550 Å2 per particle are 1.0, 1.3, and 2.4, respectively; and second, that the corresponding absorbance spectra show the expected increases in measured absorbance at 326 nm; see Figures 4 and 5. An explanation consistent with the above observations is that the amount of charge that may be accumulated at a given Vapp more negative than Vcb is dependent on the morphology of the nanocrystalline film. That is, as a film becomes less nanoporous and eventually close-packed, the capacity to compensate accumulated charge is reduced. The proposed explanation is consistent with the emerging view that charge accumulation in nanocrystalline semiconductor electrodes is accompanied by
Transparent Nanocrystalline TiO2 Films
Figure 8. (a) Absorbance at 320 and 780 nm of a one-monolayer nanoporous nanocrystalline film (1200 Å2 per particle), following firing in air at 450 °C for 2 h, plotted against applied potential (V, SCE) at pH 2.0 and 12.0. (b) As in (a) for a four-monolayer nanoporous nanocrystalline film.
significant adsorption-intercalation of compensating charged species.4b,6g,17b,c Specifically, in aprotic solvents such as acetonitrile, charge accumulation at Vapp more negative than Vcb is accompanied by adsorption- intercalation of cations such as Li+. However, in protic solvents such as water, it appears proton adsorption-intercalation is important. In short, as a nanocrystalline film becomes less nanoporous and eventually closepacked, the surface area available at which to adsorb-intercalate compensating charge decreases as does the same film’s capacity to accumulate charge. For all of the above films the potential-dependent spectra measured at pH 12.0 are in excellent qualitative agreement with those measured at pH 2.0. However, we note, that at a given Vapp the visible and near-infrared absorbance assigned to accumulated electrons is smaller at pH 12.0 than at pH 2.0. This observation is consistent with a shift of Vcb to more negative potentials at higher pHs, a property characteristic of metal oxide electrodes in aqueous electrolyte solutions.15,20 Consequently, another possible explanation for the observation that the visible and near-infrared absorbance assigned to accumulated electrons is smaller at a given Vapp as a film becomes less nanoporous and eventually close-packed is that Vcb is shifted to more negative potentials. To exclude this possibility, the potential applied to a nanoporous (1200 Å2 per particle) one and four monolayer films was swept to more negative values at pH 2.0 and 12.0. The absorbances at 320 and 780 nm, assigned to the Burstein shift and accumulated conduction band electrons, respectively, were monitored in each case. Clearly, accumulation of electrons is observed at more negative potentials at pH 12.0 than at pH 2.0; see Figure 8. In fact, the measured shift in the onset for absorption is 63 mV per pH unit in good agreement with the predicted value of 59 mV per pH unit.15,20 From the traces
J. Phys. Chem., Vol. 100, No. 25, 1996 10737
Figure 9. (a) Absorbance at 320 and 780 nm of a four-monolayer nanoporous nanocrystalline film (950 Å2 per particle), following firing in air at 450 °C for 2 h, plotted against applied potential (V, SCE) at pH 2.0 and pH 12.0. (b) As in (a) for a four-monolayer close-packed nanocrystalline film (550 Å2 per particle).
shown in Figures 8, it is possible to estimate a value of Vcb. In each case at pH 2.0 this is about -0.55 V, in good agreement with the value determined under similar conditions for nanoporous nanocrystalline films prepared by sol-gel methods, i.e., -0.52 V.18 Finally, we note that the absorbance change for a four monolayer film is about 4 times that observed for a one monolayer film, as would be expected. Similar experiments were performed for more compressed nanoporous (950 Å2 per particle) and close-packed nanocrystalline films (550 Å2 per particle). Specifically, the potential applied to fired four-monolayer films was swept to more negative values at pH 2.0 and 12.0; see Figure 9. As above, the absorbances at 320 and 780 nm, assigned to the Burstein shift and accumulated conduction band electrons, respectively, were monitored in each case. Clearly, accumulation of electrons is observed at more negative potentials at pH 12.0 than at pH 2.0. In fact, the measured shift in the onset for absorption is 60 mV per pH unit, in good agreement with the value of 59 mV per pH unit predicted.15,20 From the traces shown in Figure 9 it is possible to estimate a value of Vcb. In each case at pH 2.0 this is about -0.55 V, in good agreement with the value determined for less compressed nanoporous films in Figure 8, that is, -0.52 V. We conclude that the decrease in the absorbance change assigned to accumulated electrons at a given Vapp more negative than Vcb observed for close-packed nanocrystalline films is a consequence of a reduced capacity to compensate accumulated charge and not a shift in Vcb to more negative potentials. Conclusions We have described the preparation and characterization of transparent nanoporous and close-packed nanocrystalline TiO2
10738 J. Phys. Chem., Vol. 100, No. 25, 1996 films. Specifically, using LB techniques it has proved possible to prepare nanocrystalline films of a specified thickness and possessing the required degree of porosity. The accumulation of charge at Vapp more negative than Vcb in these films was monitored spectroscopically. On the basis of these studies, it was concluded that as a film becomes less nanoporous and eventually close-packed its capacity to compensate accumulated charge by adsorption-intercalation of cations or protons is significantly reduced. This, in turn, emphasises the need to control morphology in such films in order to facilitate the study of their properties and to ensure optimal performance of devices in which they are incorporated. Acknowledgment. This work was supported by a grant from the Commission of the European Union under the Joule II program (Contract JOU2-CT93-0356). References and Notes (1) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (2) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Huang, S.-Y.; Kavan, L.; Kay, A.; Gra¨tzel, M. J. Act. PassiVe Electron. Compon., paper submitted to a special issue entitled AdVances in Lithium Rechargeable Battery Research. We thank the authors for sending us a preprint of this paper. (4) (a) Marguerettaz, X.; O’Neill, R.; Fitzmaurice, D. J. Am. Chem. Soc. 1994, 116, 2629. (b) Hagfeldt, A.; Vlachopoulos, N.; Gra¨tzel, M. J. Electrochem. Soc. 1994, 141, L82. (5) For examples of recent studies directed toward the preparation and characterization of close-packed and nanoporous nanocrystalline semiconductor films, see ref 1 and a special issue of Sol. Energy Mater. Sol. Cells 1994, 32, 221. Also see refs 6 and 7 below. (6) (a) O’Regan, B.; Gra¨tzel, M.; Fitzmaurice, D. J. Phys. Chem. 1991, 95, 10525. (b) Redmond, G.; Gra¨tzel, M.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 6951. (c) Hotchandani, S.; Kamat, P. J. Phys. Chem. 1992, 96, 6834. (d) Liu, D.; Kamat, P. J. Phys. Chem. 1993, 97, 10769. (e) Hotchandani, S.; Kamat, P. J. Electrochem. Soc. 1992, 139, 1630. (f) Hodes,
Doherty and Fitzmaurice G.; Albu-Yaron, A. Proc. Electrochem. Soc. 1988, 88, 298. (g) Hodes, G.; Howell, I.; Peter, L. J. Electrochem. Soc. 1992, 139, 3136. (h) Lyons, A.; Hupp, J. J. Phys. Chem. 1995, 99, 15718. (7) (a) Schwarzburg, K.; Willig, F. Appl. Phys. Lett. 1991, 58, 2520. (b) Willig, F; Kietzmann, R.; Schwarzburg, K. In Photochemical and Photoelectrochemical ConVersion and Storage of Solar Energy; Tian, Z., Cao, Y., Eds.; International Academic Publishers: Beijing, 1993; p 129. (c) Lindquist, S.-E.; Finnstrom, B.; Tegner, L. J. Electrochem. Soc. 1983, 130, 351. (d) Hagfeldt, A.; Bjorksten, U.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cell 1992, 27, 293. (e) Sodergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 94, 5552. (8) Rizza, R.; Fitzmaurice, D., paper in preparation. (9) Kotov, N.; Meldrum, F.; Fendler, J. J. Phys. Chem. 1994, 98, 8827. (10) The authors thank the staff of the Electron Microscopy Unit at UCD for their advice and assistance in the preparation of these samples. (11) Holzer, J.; Mc Carthy, G. North Dakota State University, Fargo, ND. JCPDS Grant-in-Aid Report, 1990. (12) (a) Kavan, L.; Stoto, T.; Gra¨tzel, M.; Fitzmaurice, D.; Shklover, V. J. Phys. Chem. 1993, 97, 9493. (b) Kavan, L.; O’Regan, B.; Kay, A.; Gra¨tzel, M. J. Electroanal. Chem. 1993, 346, 291. (13) Cronemeyer, D. Phys. ReV. 1952, 87, 876. (14) Salvadore, P. Sol. Energy Mater. 1982, 6, 241. (15) Finklea, H. Semiconductor Electrodes; Elsevier: New York, 1988; pp 65-66. (16) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (17) (a) Liu, C.-Y.; Bard, A. J. Phys. Chem. 1989, 93, 7749. (b) Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 1426. (c) Enright, B.; Redmond, G.; Fitzmaurice, D. 1994, 98, 6195. (d) Fitzmaurice, D. Sol. Energy Mater. Sol. Cells 1994, 32, 289. (e) Hoyer, P.; Eichenberger, R.; Weller, H. Ber. Bunsenges. Phys. Chem. 1993, 97, 630. (f) Redmond, G.; O’Keeffe, A.; Burgess, C.; MacHale, C.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 11081. (g) Bedja, I.; Hotchandani, S.; Kamat, P. J. Phys. Chem. 1993, 97, 11064. (18) (a) O’Regan, B.; Gra¨tzel, M.; Fitzmaurice, D. Chem. Phys. Lett. 1991, 183, 89. (b) Rothenberger, G.; Fitzmaurice, D.; Gra¨tzel, M. J. Phys. Chem. 1992, 96, 5983. (19) (a) Burstein, E. Phys. ReV. 1969, 184, 733. (b) Moss, T. J. Appl. Phys. 1961, 32, 2136. (20) Hunter, R. Zeta Potential In Colloidal Science; Academic Press: London, 1981.
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