Langmuir 1998, 14, 3153-3156
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Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots A. Zaban, O. I. Mic´ic´,* B. A. Gregg, and A. J. Nozik* National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401 Received December 17, 1997. In Final Form: March 30, 1998 Quantum dots (QDs) of InP strongly adsorb onto transparent, porous, nanocrystalline TiO2 electrodes prepared by sintering 200-250 Å diameter TiO2 colloidal particles. The interparticle space of the TiO2 electrodes is large enough to permit deep penetration of 65-Å InP QDs into the porous TiO2 film. The absorption of light increases linearly with the thickness of the TiO2 film indicating that the InP QDs are adsorbed homogeneously on the TiO2 surface. We found that large particles adsorb better than smaller ones probably due to less hindrance by the stabilizer. The solid films exhibit strong photoconductivity in the visible region indicating photosensitization of TiO2 by InP QDs. The photocurrent action spectrum of the TiO2/InP QD film at a potential of +1 V is consistent with the absorption spectrum of the InP QDs. A photoelectrochemical cell was formed that consisted of p-type InP QDs loaded on TiO2, which was immersed in a I-/I3- or hydroquinone/quinone acetonitrile solution, and a Pt counter electrode. These photoelectrochemical experiments show that electron transfer from InP QD into TiO2 nanoparticles occurs. p-Type InP/TiO2 electrodes are stable during illumination while n-type photocorrodes in an electrochemical cell.
Introduction Photosensitization of nanocrystalline TiO2 semiconductor electrodes by absorbed dyes has been extensively studied recently because of its potential application in a new type of solar cell.1 The key feature of this system is the use of nanocrystalline TiO2 films that have an extremely large surface-to-volume ratio. This allows for greatly increased dye coverage in the TiO2 film and produces very high quantum yields for photon-to-electronic current flow (above 80%). Recently, sensitization of TiO2 using CdS, CdSe, and PbS metal chalcogenide quantum dots (QDs) has been reported.2-8 In these previous experiments, only very small sized QDs were prepared and deposited in situ on the TiO2 particles in a single step. In this work, we report a different approach. We first synthesized InP QDs separately with the desired size and then subsequently adsorbed them onto the TiO2 nanoparticle surfaces. We have recently successfully synthesized InP QDs with a well-crystallized zinc blende structure and with diameters ranging from 20 to 80 Å.9,10 The conduction band offset between InP QDs and TiO2 allows for efficient photoinduced electron transfer from InP to TiO2, as shown in Figure 1. Bulk InP has a conduction band potential of -0.6 V and a valence band potential of 0.75 V vs NHE at pH 7. The band gap of the QDs increases as the QD size decreases. The conduction band of TiO2 is at about -0.45 V vs NHE at pH 7. Thus, as seen in Figure 1, InP QDs have a large driving force for injecting photogenerated electrons into TiO2. (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Vogel, R.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (3) Vogel, R.; Klaus, P.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (4) Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1361. (5) Hotchandani, S.; Kamat, V. Chem. Phys. Lett. 1992, 191, 320. (6) Liu, D.; Kamat, P. V. J. Phys. Chem. 1993, 97, 10769. (7) Frang, J.; Wu, J.; Lu, X.; Shen, Y.; Lu, Z. Chem. Phys. Lett. 1997, 270, 145. (8) Hoyer, P.; Ko¨nenkamp, R. Appl. Phys. Lett. 1995, 66, 349. (9) Mic´ic´, O. I.; Sprague, J. R.; Curtis, C. J.; Jones, K. M.; Machol, J. L.; Nozik, A. J.; Giessen, H.; Fluegel, B.; Mohs, G.; Peyghambarian, N. J. Phys. Chem. 1995, 99, 7754. (10) Mic´ic´, O. I.; Cheong, H. M.; Fu, H.; Zunger, A.; Sprague, J. R.; Mascarenhas A.; Nozik, A. J. J. Phys. Chem. 1997, 101, 4904.
Figure 1. Schematic band diagram of InP QD/TiO2 interface and bulk InP. The electron affinity of TiO2 is about -4.05 eV.
We found that strong adsorption of InP QDs on nanocrystalline TiO2 electrode films occurs and that the InP particles can be utilized for photosensitization of TiO2. InP is generally more stable than metal chalcogenides because of the presence of an oxide layer formed in air on the InP surface.11,12 InP QDs also have a high absorption (11) Heller, A.; Miller, B.; Lewerenz, H. J.; Bachmann, K. J. J. Am. Chem. Soc. 1980, 102, 6555. (12) Heller, A.; Aharon-Shalom, A.; Bonner W. A.; Miller, B. J. Am. Chem. Soc. 1982, 104, 6942.
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coefficient over much of the visible spectral region. The absorption coefficient of the QDs increases with the inverse cube power of the QD diameter, while the oscillator strength per particle is independent of particle size;13 the absorption edge of the QD shifts to the blue with decreasing QD diameter. Experimental Section Synthesis of Colloidal InP QDs. Colloidal InP QDs were synthesized by colloidal chemistry methods using indium oxalate and tris(trimethylsilyl)phosphine (P(SiMe3)3) as starting reactants; the reactants are heated for 3 days at 260-280 °C in the presence of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP). Details of the preparation are given in ref 14. During the synthesis, the QDs are capped with TOPO/TOP, which produce a compact hydrophobic shell on the QD surface.15 p-Type (InP:Zn), n-type (InP:S), and undoped QDs of InP were synthesized as well-crystallized zinc blende nanocrystals. For synthesis of p-type InP QDs, indium oxalate was mixed with zinc acetate in a ratio such that the resulting QDs contain on average one Zn2+ ion per particle. For example, for 40-Å QDs, the ratio In:Zn is 103:1. For n-type InP QDs, P(SiMe3)3 was mixed with (bis(trimethylsilyl) sulfide) and then the indium salt was added. The TOPO/TOP caps on the InP surface are replaced with 4-tertbutylpyridine (TBP) ligands by dispersing QDs in toluene solution containing an excess of TBP, followed by precipitation and washing with methanol; this ligand exchange process was repeated several times to ensure complete capping with TBP. Preparation of (InP)/TiO2 Electrodes. The nanocrystalline TiO2 films were prepared using standard sol-gel techniques.16,17 First, the colloidal TiO2 solution was coated onto conducting glass (F-doped SnO2, Libby Owens Ford, 8 Ω/ square).18,19 The porous space between TiO2 nanoparticles was about 200 Å, and the total thickness of the electrode was either 2, 4, or 8 µm. Then, the InP QDs are adsorbed onto TiO2 particles by immersing the porous nanocrystalline TiO2 electrodes into a colloidal solution of InP QDs in hexane or toluene. The adsorption rate depends on the size of the TiO2 pores and the thickness of the film. For optimum adsorption of QDs on the TiO2 surface, it is important to eliminate the presence of free, excess stabilizer. Hence, the QDs were washed carefully with methanol and then dissolved in toluene or hexane. Adsorption is faster in toluene than in hexane, but the final degree of adsorption is the same. It was found that adsorption is favored when InP QDs are capped with TBP, probably because its shorter ligand length allows easier penetration of InP-TBP through TiO2 pores. The adsorption process can be enhanced if the TiO2 electrode is first immersed in a toluene solution of thiolactic acid for several hours, and then in the InP QD solution. Thiolactic acid is a bifunctional molecule containing CO2- and SH hydrophilic headgroups. The CO2- can be covalently bound to the porous TiO2 surface forming a selfassembled monolayer, while the free SH group has great affinity toward the InP surface.20 After adsorption, the resulting InP/ TiO2 electrode is colored but optically nonscattering and transparent. The samples were stored in the dark. Apparatus. Photocurrent-voltage measurements of dry InP/ TiO2 electrodes, or in a photoelectrochemical cell, were made at white light excitation with wavelengths between 400 and 800 nm. A xenon arc lamp in conjunction with a monochromator was used for photocurrent action spectra. (13) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (14) Mic´ic´, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J. J. Phys. Chem. 1994, 98, 4966. (15) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (16) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (17) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (18) Zaban, A.; Ferrere, S.; Sprague, J.; Gregg, B. A. J. Phys. Chem. 1997, 101, 55. (19) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. 1997, 101, 4490. (20) Gu, Y.; Lin, Z.; Smetkowski, V.; Butera, R.; Waldeck, D. H. Langmuir 1995, 11, 1849.
Figure 2. Absorption spectra of InP QDs adsorbed on transparent porous TiO2 electrode. The QD particle diameters were (a) 45 ( 3 Å and (b) 65 ( 14 Å. The conductivity across dry solid InP/TiO2 films was measured between the conductive glass substrate and a back contact that was simply pressed against the TiO2 film. The back contact consisted of Pt evaporated on conductive glass. Clamps were used to press the back contact against the InP/TiO2 film. The cell area was 0.54 cm2 and the thickness of the film was 8 µm. The actual contact area between the Pt back contact and the InP/TiO2 film is not known. In the electrochemical cell configuration, InP/TiO2 films on conducting glass were employed in a sandwich-type cell. The counter electrode was platinum-coated conductive glass. Electrolyte solution was placed in the space between the electrodes, which were separated by 50 µm with a Teflon spacer. The standard electrolyte consisted of 0.3 M LiI and 0.03 M I2 or 0.5 M hydroquinone and 0.05 M quinone in acetonitrile solution. Absorption spectra were measured on a Hewlett-Packard 8453 diode array spectrophotometer. All experiments have been done at room temperature.
Results and Discussion Adsorption of InP QDs on Porous TiO2 Matrix. Figure 2 shows the absorption spectra of InP QDs with two different particle sizes that are adsorbed on a TiO2 electrode. The absorption spectra were measured through the optically transparent thin layer electrode. Curve a in Figure 2 shows the absorption spectrum of InP QDs with a diameter of 42 Å and a particle size distribution of 10%; an excitonic transition can be seen at 650 nm. Curve b shows the absorption spectrum of 65-Å InP QDs with a broader size distribution of about 15%. These absorption spectra cover the visible spectral region from red to ultraviolet. Below 660 nm, both films absorb more than 60% of the incident light. For 65-Å InP QDs in the wavelength region 660-800 nm, there is an absorption tail with about 15-60% light absorption. We found that large InP particles adsorb better onto TiO2 nanoparticles than small ones. This can be explained by the fact that larger QDs have a lower concentration of stabilizer on its
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Langmuir, Vol. 14, No. 12, 1998 3155
Figure 4. Photocurrent (µA) vs wavelength of 8 µm thick 65-Å InP/TiO2 film at +1 V. Insert: Current response to light (500 nm) at a potential of +1 V. The illumination area is 1 cm2.
Figure 3. Current-voltage characteristic of solid 8-µm p-InP/ TiO2/Pt (c, a) and bare TiO2 (d, b) electrodes during illumination with white light (>400 nm) and in dark. Voltage is relative to the Pt back electrode.
surfaces; it has been shown that the fraction of TOPO ligand on the InP QD surface decreases as the diameter increases from 25 to 40 Å due to steric effects. TOPO ligands are better able to access the surface of the smaller particles because of their higher curvature.21 The average interparticle space (∼200 Å) in the TiO2 nanocrystalline electrode is large enough that QDs with a diameter of 65 Å can penetrate deeply into the pores of the TiO2 electrode and become adsorbed there. Adsorption of InP QDs on a 2 µm thickness TiO2 film is much faster than that on 4 or 8 µm thick films. However, when saturation of adsorption is reached, then the absorption increases linearly with increasing film thickness. This indicates that InP QDs are homogeneously distributed on the TiO2 surface. For a 8 µm thick TiO2 film, the absorption spectrum corresponds to that formed by about 100 layers of close-packed InP QDs (absorbance of a monolayer of 65-Å InP QDs is 4 × 10-3 at the maximum excitonic peak). For this film the porous volume is 4 × 10-4 cm3 per 1 cm2 at TiO2 films, and about 8 × 10-5 cm3 of QD volume is loaded into the pores (∼20% of the total volume). If a TiO2 electrode is used with a larger pore size, then even slightly larger QDs can be loaded which can absorb light up to 900 nm. Photoconduction of InP/TiO2 Films. The photoconductivity in porous TiO2 is strongly enhanced by the presence of InP QDs. The conductivity of InP/TiO2 solid films under illumination increases by a factor of 50 to 100 compared to that of bare TiO2 or InP/TiO2 films in the dark (Figure 3). Figure 4 shows the photocurrent per incident photon in the 8 µm thick InP/TiO2/Pt film vs wavelength at a potential of +1 V relative to the Pt back electrode. In the bare TiO2 film, the photoconduction onset (21) Guzelian, A. A.; Katari, J. E. B.; Kadavanich, A. V.; Banin, U.; Hamad, K.; Juban, E.; Alivisatos, A. P.; Wolters, R. H.; Arnold, C. C.; Heath, J. R. J. Phys. Chem. 1996, 100, 7212.
Figure 5. Intensity dependence of the photocurrent for 8-µm InP/TiO2 film at 500-nm excitation wavelength and at a potential of +1 V.
is below 400 nm and cannot be seen in Figure 4. The photoconductivity action spectrum correlates with the absorption spectrum of InP QDs (Figure 2, curve b). This implies that upon the illumination, it is the photogenerated carriers from InP that give rise to the conductance of TiO2. This effect was also observed previously for PbS QDs adsorbed on a TiO2 film.8 The insert in Figure 4 shows the photocurrent response at 1 V when the light source (2.3 mW/cm3) was turned on for 2 min and then turned off; the current completely decayed to zero in 30 s. This long time is related to the long lifetime of injected electrons in the TiO2 nanoparticles. We also observed (see Figure 5) that the photocurrent increases linearly with light intensity. This also indicates that traps do not play a significant role in carrier transport through the nanocrystalline film, since traps would shorten the carrier lifetime. Photoelectrochemical Cells. Electron transfer from InP QDs into TiO2 was also observed in a liquid photoelectrochemical cell; here, the transport of holes from the photoexcited InP QDs to the counter electrode occurred through a redox electrolyte solution. The photoelectrochemical cell consisted of InP QDs loaded onto the TiO2 nanocrystalline electrode, which was immersed in a I-/I3or hydroquinone/quinone acetonitrile solution, and a Pt
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counter electrode. We added 3 mM TBP to adsorb onto the TiO2 electrode in order to suppress the deleterious recombination of injected electrons with I3-. We found that p-type InP/TiO2 electrodes are stable during illumination while n-type InP/TiO2 electrodes photocorrode in the electrochemical cell. The reason for this behavior is not understood. Perhaps the p-type InP produces photocathodic protection at its surface as occurs in bulk p-InP photoelectrodes.22 The current-voltage curve under illumination is presented in Figure 6; a fill factor f ) 0.685 was obtained. Under monochromatic illumination with incident light intensity of ∼7 µW/cm2, very low values for ICPE were obtained (Figure 6, insert). Since the photoconductivity of the InP/TiO2 solid film is very high, this suggests that the transfer and/or transport of holes may be the major limiting factor in the sensitization efficiency in these photoelectrochemical cells. Further improvement of the photoelectrochemical cell requires optimization of (1) the hole transport from the InP QDs through the liquid electrolyte to the counter electrode, (2) the quantum yield of electron injection per InP QD, and (3) electron transport through the TiO2 nanocrystalline film. Conclusions. Our results show that InP quantum dots strongly adsorb on porous nanocrystalline TiO2 electrodes. InP QDs have the ability to sensitize TiO2 and extend its photoresponse over the visible and near-IR spectral region. Photoconductivity spectra of the InP/TiO2/Pt solid films are governed by InP light absorption. The sensitivity for (22) Heller, A.; Miller, B.; Thiel, F. A. Appl. Phys. Lett. 1981, 38, 282.
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Figure 6. Current-voltage characteristic of the p-InP/TiO2I-/I3--ACN-Pt electrochemical cell during illumination with white light. Insert: Photocurrent action spectrum for 65-Å InP QDs adsorbed on 2-µm TiO2 electrode in I-/I3- acetonitrile solution at low light intensity (∼7 µW/cm2).
photoconductivity of TiO2 using InP QDs is large. Photoelectrochemical solar cells based on photosensitization of TiO2 nanocrystalline films with InP QDs have been demonstrated. LA9713863
