J. Phys. Chem. 1995, 99, 8954-8958
8954
Preparation, Characterization, and Potential-Dependent Optical Absorption Spectroscopy of Unsupported Large-Area Transparent Nanocrystalline Ti02 Membranes Robert U. Flood and Donald Fitzmaurice" Department of Chemistry, University College Dublin, Dublin 4, Ireland Received: January 26, 1995; In Final Form: March 30, 1995@
We describe preparation, characterization, and the potential-dependent optical absorption spectroscopy of an unsupported large-area transparent nanocrystalline Ti02 membrane. Regarding preparation, an aqueous sol of 10 nm nanocrystallites is used to deposit a Ti02 gel film on an ammonium chloride pellet. This salt pellet is then sublimed by heating under reduced pressure to yield an unsupported film. Firing of this film, at temperatures sufficiently high to fuse the constituent crystallites, leads to formation of a mechanically robust 2 x 2 cm transparent membrane. Characterization by electron microscopy and electron diffraction reveals the above to be a 2 pm thick nanoporous-nanocrystalline anatase structure. Measurement of potentialdependent optical absorption spectra has proved possible, following formation of an ohmic contact using silver epoxy, for both underivitized membranes and membranes derivitized by adsorption of a redox dye. Results obtained are consistent with those for transparent nanocrystalline anatase films supported on conducting glass. Some implications of these findings are discussed, as are possible applications for such membranes.
Introduction The recent past has seen increased interest in transparent nanocrystalline semiconductor films.' For example, their use as sensitized photoanodes in regenerative photoelectrochemical cells has attracted considerable attention.2 Recently, other applications have been described. These include lithiuminsertion batteries and electrochromic window^.^.^ In short, such films appear to have significant technological potential. Increased awareness of the potential of transparent nanocrystalline semiconductor films has led to significant effort being directed toward elucidation of their proper tie^.^ Specifically, it is desirable that such films be characterized in terms of their morphology, bulk and interfacial defects, bulk and interfacial energetics, and carrier properties. Of particular interest is the mechanism of carrier transport in such films.6 Conceming techniques that have been applied to the study of such films, they may be classified as follows: first, those that would normally be applied to the study of crystalline semiconductor materials and, second, those that have been applied to address the limitations of conventional techniques in the face of the increased complexity of nanocrystalline semiconductor materials. Among the latter have been a range of optoelectrochemical techniques which have exploited the transparency of the such films and the fact that they can be prepared on conducting glass substrate^.^ However, despite recent advances, it is clearly the case that a greater range of techniques could usefully be applied to the study of such films. To facilitate application of a wider range of spectroscopic and other characterization techniques, we have prepared and ohmically contacted large-area unsupported transparent nanocrystalline semiconductor films, hereafter referred to as membranes. Specifically, we describe preparation, characterization, and the potential-dependent optical absorption spectroscopy of unsupported 2 x 2 cm transparent nanocrystalline Ti02 membranes (2 pm thick) ohmically contacted using silver epoxy. Also described are similar experiments for membranes derivitized by adsorption of a redox dye. We note that unsupported
* To whom correspondence
should be addressed.
'Abstract published in Advance ACS Absrracrs, May 1, 1995.
semiconductor films, also referred to as membranes, have previously been prepared by Anderson and co-workers.* Some implications of our findings are discussed, as are possible applications for such membranes. Experimental Section Preparation of Ti02 Membranes. A colloidal Ti02 dispersion was prepared by hydrolysis of titanium i~opropoxide.~ The average diameter of the initially formed crystallites was 7 nm. Autoclaving the above dispersion at 200 "C for 12 h increased the average crystallite diameter to 10 nm. Concentrating this sol to 160 g/L and adding Carbowax 20000 (40% wt equiv of TiOz) yielded a white viscous sol. This sol was used to deposit a 2 x 2 cm gel film on a 2.5 cm diameter ammonium chloride pellet. We note that the above pellet was previously treated by application of a 5% solution of Triton X-100 in 2-propanol. After drying in air for 30 min, the gel-coated pellet was placed in a vacuum tube and the salt substrate sublimed during 15 h under reduced pressure at 120 OC.Io The unsupported Ti02 film obtained was placed in a fumace and fired in air at 450 "C for 12 h. At this temperature the constituent crystallites are fused, forming a mechanically robust and transparent 2 x 2 cm nanocrystalline Ti02 membrane. We note that addition of Carbowax prevents collapse of the gel's nanoporous structure during firing. Characterization Techniques. All optical absorption spectra were recorded using a Hewlett-Packard 8452A diode array spectrophotometer. Transmission electron micrographs (TEMs) and electron diffraction patterns were recorded using a JEOL 2000 FX TEMSCAN. Membranes were either mounted directly on a Formvar-coated copper grid or embedded in Epon 812 epoxy resin and sectioned as would be a biological membrane. The latter approach allows TEMs of a transverse section to be obtained and is, therefore, useful for determining membrane thicknesses.' Scanning electron micrographs (SEMs) were recorded using a JEOL 35 TEM. Membranes were mounted directly on a conducting stub using silver DAG. Potential-Dependent Optical Absorption Spectroscopy. Membranes prepared as described above are sufficiently robust that they may be mounted on a platinum support using silver
0022-3654/95/2099-8954$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 22, 1995 8955
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SCHEME 1: Transparent Nanocrystalline Ti02 Membrane Mounted on a Platinum Support Using Silver Epoxy
SCHEME 3: 1-Ethyl- 1’-[(4-carboxy-3-hydroxyphenyl)methyl]-4,4’bipyridinium Perchlorate
Platinum Support
H
0
sv
continuous bubbling with nitrogen. The above cell was incorporated in a diode array spectrometer. Derivitized membranes were prepared by first mounting them as shown in Scheme 1 and then placing them in a saturated ethanolic solution of 1-ethyl- 1’-[(4-carboxy-3-hydroxyphenyl)methyl]-4,4’-bipyridinium perchlorate (SV) for 24 h, see Scheme 3. Preparation of SV and its adsorption at nanocrystalline Ti02 films supported on a conducting glass substrate have been described elsewherea! Transparent Anatase Membrane
SCHEME 2: Transparent Nanocrystalline Ti02 Membrane Incorporated in a Closed Three-Electrode Electrochemical Cell; Working Electrode Is as in Scheme 1 Counter Electrode
Working Electr.de
Reference Electrode
Nitrogen
Transparent Anatase Membrane
Electrolyte Solution
Reference Electrode
A.
-
Electrolyte Solution
epoxy; see Scheme 1. We note that silver epoxy is known to form an ohmic contact with Ti02.12 Membranes mounted in such a manner formed the working electrode (0.3 cm2 surface area) of a closed three-electrode, single-compartment electrochemical cell; see Scheme 2. The counter electrode was a platinum flat and the reference electrode a saturated calomel electrode (SCE). Potential control was provided by a Thompson Electrochem Ministat precision potentiostat. The electrolyte solution, 0.2 mol dm-3 LiClO4 in distilled deionized water acidified or basified by addition of HC104 or KOH, respectively, was thoroughly deaerated by
Results Electron Microscopy. Membranes, prepared as described above, were examined first by electron microscopy. Specifically, low-resolution SEMs show such membranes to be nonporous on a macroscopic scale; see Figure la. Pinholes of the order of 10 pm in diameter are apparent. We note that failure to treat the salt substrate with surfactant yields membranes that are highly porous on the same length scale. An SEM of a membrane cleaved along one edge shows it to be nanoporous and about 2 pm thick; see Figure lb. These findings are confirmed and extended by TEM studies; see Figure 2a. Shown is the surface of a membrane similar to that in Figure l a at high resolution. Readily apparent are the constituent crystallites of the membrane. Also apparent are the lattice patterns of individual crystallites. The electron diffraction pattern measured for the membrane in Figure 2a forms the insert. From this we determine the following d/,k/spacings: 3.6 (3.52), 2.5 (2.43), 1.9 (1.89), 1.7 (1.70), 1.6 (1.67), and 1.4 (1.48). These values are in good agreement with those expected for anatase and given in parentheses.I3 Shown in Figure 2b is a transverse section, prepared using a microtome, of a similar membrane previously embedded in resin. We see that such membranes are indeed nanoporous. Further, evidence for the nanoporous nature of these membranes comes from the extent of dye uptake from solution. Such studies indicate the effective surface area of a membrane is many times that of the geometric surface area.I4 Potential-Dependent Optical Spectroscopy. Membranes prepared as described above are sufficiently robust that they may be easily manipulated once attached to a platinum support using silver epoxide as shown in Scheme 1. This platinum support facilitates measurement of potential-dependent optical absorption spectra as shown in Scheme 2. Shown in Figure 3a is an absorbance spectrum of a membrane in aqueous LiC104 (0.2 mol dm-j) solution at pH 2.0 (added HC104). There is some scattering by the sample; however, an absorption onset at 380 nm is clearly seen. This spectroscopy is, therefore, consistent with that expected for Ti02 (anatase). Shown in Figure 3b are difference spectra of the same membrane measured at the indicated potentials. The above are measured with respect to a background spectrum recorded at 0.00 V. Similar experiments have been performed at pH 3.0 and pH 11.0. In each case the absorbance change at 780 nm, the wavelength at which absorption by trapped (localized) electrons is a minimum, is plotted against applied potential; see Figure
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I
40 nm 200 pm
1 um Figure 1. (a, top) SEM of a Ti02 membrane mounted on conducting stub using silver DAG. (b, bottom) SEM of the cleaved edge of a membrane, similar to that in (a), also mounted on a conducting stub using silver
DAG.
4a. The potential for the onset of absorbance at 780 nm, as determined from Figure 4a, is plotted against pH in Figure 4b. The spectra shown in Figure 5a were measured at pH 2.0 and the indicated potentials for membranes derivitized by adsorption of SV. The absorbance changes observed at negative applied potentials are assigned to accumulated electrons (see Figure 3b) and reduced viologen. Absorbance at 550 and 780 nm is plotted against applied potential in Figure 5b. At potentials more negative than about -0.70 V no additional reduction of viologen is detected.
Figure 2. (a, top) TEM of a Ti02 membrane mounted on a Formvarcoated copper grid. The insert is the electron diffraction pattern measured for the same sample. (b. bottom) TEM of a transverse section of a Ti02 membrane mounted on a Formvar-coated copper grid. The sample was prepared by previously embedding a membrane in resin and sectioning using a microtome.
Discussion On the basis of the above and many similar electron micrographs, we conclude that large-area unsupported transparent membranes, formed by fusing IO nm anatase crystallites at elevated temperatures, are largely homogeneous and continuous on the micrometer scale. We also conclude they are porous on the nanometer scale. Further, it is apparent from the potential dependence of their optical spectroscopy that silver epoxide may be used to form an ohmic contact to such membranes. Below we discuss the potential-dependent spectroscopy of such membranes.
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U.UU
--t
3*00
--C
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- 1 .o
-1.5
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I 0.0
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(b) -0.6Slope 68 mVipH unit
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-0.lOP 350
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Wavelength (nm) Figure 3. (a) Absorbance spectrum, in aqueous electrolyte solution (0.2 mol dm-3 LiC104 acidified to pH 2.0 by addition of HClO4), of a Ti02 membrane mounted on a platinum support using silver epoxy. (b) Difference absorbance spectra of membrane in (a) measured at the indicated potentials with respect to a background spectrum recorded at 0.00 V.
At negative applied potentials spectral changes consistent with charge accumulation are observed.I5 Specifically,an absorbance increase at wavelengths longer than about 400 nm is assigned to electrons localized at bulk and surface states and to free electrons occupying available states of the conduction band. Consistent with the results of previous studies on supported films, it is suggested that the measured absorbance at 780 nm is principally due to free electrons present in the conduction band, i.e., band filling.I6 Also observed at wavelengths shorter than about 400 nm is an absorbance loss assigned to a BursteinMoss shift.I7 Specifically, band filling results in a shift to higher energies of the lowest energy allowed transition. These assignments are further supported by the pH dependence of spectra measured at a given applied potential. Specifically, the onset of absorption at 780 nm, assigned principally to free conduction band electrons, is observed only at increasingly negative potentials as the pH of the electrolyte solution increases; see Figure 4a. This is consistent with the expected shift in the potential the conduction band edge (Vcb)of a metal oxide semiconductor at the semiconductor-liquid electrolyte solution interface (SLI) to more negative potentials at higher pHs. Specifically, if we assume that Vcb may be estimated from the potential at which absorption by the membrane is first detected, then we may plot Vcb against pH as in Figure 4b. The resulting slope of this plot, 68 mV/pH unit, is in reasonable agreement with the expected value of 59 mV/ pH unit.'* Further, the estimated V& at pH 0.00of -0.52 V is close to the
0
2
4
6
8
1 0 1 2 1 4
PH Figure 4. (a) Absorbance at 780 nm plotted against applied potential for membrane in 3b. Also plotted are the results of similar experiments at pH 3.0 and pH 11.0. (b) Onset of absorbance at 780 nm for membranes in (a) plotted against pH of electrolyte solution. value of -0.40 V measured for similar nanocrystalline films prepared on a conducting glass substrate.I6 Absorbance by the SV derivitized membrane in Figure 5a is assigned to reduced viologen and accumulated electrons.4a No further increase in absorption by reduced viologen is observed at potentials more negative than about -0.70 V, and we assume all adsorbed SV has been reduced. From the known extinction coefficient of the reduced viologen, 4240 mol-' cm-' at 550 nm,I9 we calculate a surface concentration for SV of 6 x lOI5 cm-2. However, it is established that such molecules are adsorbed at surface Ti4+ states and that the surface density of such states is 2 x lOI3 cm-*.*O This, therefore, suggests a surface roughness factor of greater than 300 for the nanoporous membranes in question. Further, we note from Figure 5b that reduction of SV is at potentials more negative than that of the conduction band edge (-0.52 V at pH 2.0) and is consistent with assignment of the underlying absorbance to electrons accumulated in available states of the conduction band. The immediate motivation for the work reported above is to extend the techniques of optoelectrochemistry to the infrared region of the spectrum. This has been possible in the absence of an entirely contiguous conducting substrate, and initial results are encouraging.*' It is hoped these studies will lead to an improved understanding of the chemical nature of electrons trapped at bulk and surface sites of the constituent crystallites of a membrane. This, in turn, would be expected to contribute to an improved understanding of carrier transport within nanocrystalline semiconductor films and membranes. Wider applications for transparent nanocrystalline-nanoporous semiconductor membranes are also foreseen. For
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Acknowledgment. This work was funded by the Commission of the European Union under the Joule I1 Programme (Contract JOU2-CT93-0356).
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References and Notes
0.15
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0.000,-0.200,.O 400 V
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-
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Applied Potential (V, SCE) Figure 5. (a) Difference absorbance spectra in aqueous electrolyte solution (0.2 mol dm-3 LiC104 acidified to pH 2.0 by addition of HC104) of a Ti02 membrane mounted on a platinum support using silver epoxy and derivitized by adsorption of SV. Spectra were measured at the indicated potentials with respect to a background spectrum recorded at 0.00 V. (b) Absorbance at 550 and 780 nm plotted against applied potential for membrane in (a).
example, the rectifying properties of such membranes may be exploited in artificial biological systems. Other possible applications include those in the area of information storage.
Conclusions We have prepared large-area transparent nanocrystallinenanoporous Ti02 (anatase) membranes. These membranes are mechanically robust and may be attached, forming an ohmic contact, to a platinum support using silver epoxy. The potentialdependent optical spectroscopy of these membranes is similar to that observed for nanocrystalline films prepared on a conducting glass substrate. Specifically, accumulation of electrons in the available states of the conduction band can be potentiostatically controlled and corresponding changes in the optical absorption spectrum monitored. However, the fact that the degree of accumulation may be controlled without the need for an entirely contiguous conducting substrate offers the prospect of applying a wider range of spectroscopic and other techniques to the study the properties of nanocrystalline semiconductor materials. Other applications for such membranes in the areas of energy and information storage may readily be envisaged.
(1) Hagfeldt, A,: Graetzel, M. Chem. Rev. 1995, 95, 49. We thank the authors for sending us a preprint of this review. (2) (a) O’Regan, B.: Graetzel, M. Nature 1991, 353, 737. (b) Nazeeruddin, M. K.: Kay, A,; Rodicio, I.: Humphry-Baker, R.: Muller. E.: Liska, P.; Vlachopoulos, N.; Graetzel, M. J. Am. Chem. SOC.1993, 115, 6382. (3) Huang, S.-Y.; Kavan, L.: Kay, A,; Graetzel, M. J. Act. Passive Electron. Components. 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) Marguerettaz, X.; O’Neill, R.; Fitzmaurice. D. J. Am. Chem. SOC. 1994, 116, 2629. (b) Hagfeldt, A,; Vlachopoulos, N.; Graetzel, M. J. Electrochem. SOC.1994, 141, L82. ( 5 ) For examples of recent studizs directed toward the preparation and characterization of nanocrystalline semiconductor films, see ref 1 and a special issue of Sol. Energy Mater. Sol. Cells 1994, 32 (171. Also, see refs 6 and 7. (6) (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, %-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. (0O’Regan, B.; Graetzel, M.; Fitzmaurice. D. J. Phys. Chem. 1991, 95, 10525. (g) Redmond, G.; Graetzel, M.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 6951. (h) Hotchandani, S.: Kamat, P. J. Phys. Chem. 1992, 96, 6834. (i) Liu, D.: Kamat, P. J. Phys. Chem. 1993, 97, 10769. (i) Hotchandani, S.: Kamat, P. J. Electrochem. SOC.1992, 139, 1630. (k) Hodes, G.; Albu-Yaron, A. Proc. Elecrrochem. SOC. 1988, 88, 298. (I) Hodes. G.; Howell, I.; Peter, L. J. Electrochem. Soc. 1992, 139, 3136. (7) (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. J. Phys. Chem. 1994, 98, 6195. (d) Fitzmaurice, D. Sol. Energy Mater. Sol. Cells 1994, 32, 289. (0Hoyer, P.; Eichenberger, R.: Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1993,97, 630. (g) Redmond. G.; O’Keeffe, A,; Burgess. C.: MacHale. C.: Fitzmaurice, D. J. Phys. Chem. 1993, 97, 11081. (h) Bedja, I.; Hotchandani, S.; Kamat, P. J. Phys. Chem. 1993, 97, 11064. (e) Kavan, L.; O’Regan, B.: Kay, A.: Graetzel, M. J. Elecrroanal. Chem. 1993, 346, 291. (f) Kavan, L.: Stoto. T.; Graetzel, M.; Fitzmaurice, D.; Shklover, V. J. Phys. Chem. 1993. 97. 9493. (g) Lantz, J.; Com, R. .I. Phys. Chem 1994, 98, 4899. (8) Anderson, M.; Gieselmann, M.: Qunyin Xu J. Membr. Sci. 1988, 39, 243. (9) O’Regan, B.: Moser, J.: Anderson, M.: Graetzel, M. J. Phys. Chem. 1990, 94, 8720. (10) (a) Conlon, D. Studies in Ultraviolet Absorption Spectroscopy. Thesis, National University of Ireland. Dublin. 1962. (b) Conlon, D.; Doyle, W. J. Chem. Phys. 1961, 35, 752. (1 1) Flood. R.: Cottell, D.: Fitzmaurice, D. Manuscript in preparation. (12) Finklea, H. Semiconductor Electrodes: Elsevier: New York, 1988: pp 65-66. (13) Holzer, 3.; McCarthy, G. JCPDS Grant-in-Aid Report, North Dakota State University, 1990. (14) Flood. R.: Fitzmaurice, D. Manuscript in preparation. (15) O’Regan. B.: Graetzel, M.: Fitzmaurice. D. Chem. Phys. Lett. 1991. 183, 89. (16) Rothenberger, G.; Fitrmaurice, D.; Graetzel, M. J. Phys. Chem. 1992, 96, 5983. (17) (a) Burstein, E. Phys. Rev. 1969, 184, 733. (b) Moss, T. S . J. Appl. Phys. 1961, 32, 2136. (18) Hunter, R. Zeta Potential in Colloidal Science; Academic Press: London, 1981. (19) Kok, B.: Rurainski, H. J.; Owens, 0. Biochem. Biophys. Acta 1965, 109, 347. (20) Sinpala, W.; Tomkievicz, M. J. Elecrrochem. SOC.1982, 129, 1240. (21) Flood, R.; Wenger, J.; Fitzmaurice. D. Manuscript in preparation.
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