Harvesting Photons in the Infrared. Electron Injection from Excited

TiO2 and Ag@TiO2 core shell nanoparticles have been modified with a carbocyanine dye (IR-125) to extend the photoresponse in the near-infrared...
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J. Phys. Chem. C 2007, 111, 488-494

Harvesting Photons in the Infrared. Electron Injection from Excited Tricarbocyanine Dye (IR-125) into TiO2 and Ag@TiO2 Core-Shell Nanoparticles P. K. Sudeep,† K. Takechi,† and Prashant V. Kamat*,†,‡ Radiation Laboratory and Departments of Chemistry and Biochemistry and Chemical & Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5674 ReceiVed: October 3, 2006

TiO2 and Ag@TiO2 core shell nanoparticles have been modified with a carbocyanine dye (IR-125) to extend the photoresponse in the near-infrared. Upon binding dye molecules to TiO2, we observe a sharp decrease in the fluorescence yield. The electron injection into TiO2 was found to dominate the deactivation of the excited singlet state. The rate constant for the charge injection process as determined from the decay of the excited singlet is ∼1011 s-1. In the case of Ag@TiO2, the electrons injected into the TiO2 layer are quickly transferred to the Ag core. The metal core in Ag@TiO2 did not alter the forward charge-transfer kinetics, but it influenced the back electron transfer. The regeneration of the dye involving the reaction between the oxidized dye and injected electron was a factor of 2 slower for Ag@TiO2 than the TiO2 system. Use of composite nanoparticles comprised of a metal core semiconductor shell may provide new ways to modulate charge recombination processes in dye-sensitized solar cells.

Introduction The interfacial electron transfer between the semiconductor nanoparticles and organic dyes has been the topic of interest for the past few years.1-8 The electron-transfer process at the semiconductor-dye interface has been successfully utilized in the development of solar cells, electronic devices, heterogeneous photocatalysis, and wastewater treatment.1,2,9,10 The efficiency of these devices depends on the properties of the dye and the semiconductor and their interaction under photoexcitation. A higher rate of electron injection from the excited dye into semiconductor and a slower recombination rate are important in designing efficient solar cells. To date, many sensitizer molecules absorbing in the visible region have been tested in dye-sensitized solar cells (DSSC). Ruthenium(II) polypyridyl complexes are widely used as a sensitizer due to their strong metal to ligand charge transfer absorption band covering a wide spectral range from near UV to visible.11,12 Dyes absorbing in the IR region have not been explored extensively.13-17 Recently, the merocyanine dyes and chlorophyll and porphyrins which absorb at 600 nm have been used for harvesting light in the red region.18-20 Carbocyanine dyes are an important class of dyes that exhibit strong absorption in the near IR region. In the present study, we have employed a tricarbocyanine dye 2-[7[1,3-dihydro-1,1-dimethyl-3-(4-sulfobutyl)-2H-benz[e]indol-2ylidene]-1,3,5-heptatrienyl]-1,1-dimethyl-3-(4-sulfobutyl)-1Hbenz[e]indolium hydroxide, inner salt, sodium salt (product name: IR-125) as a sensitizer. This dye consists of heteroaromatic ring structures linked through a flexible polymethine chain and binds with the TiO2 surface through electrostatic interactions (Scheme 1). Another possible method for improving the efficiency of the electron-transfer dynamics is through the modification of * Address correspondence to this author. E-mail: [email protected]. Internet: http://www.nd.edu/∼pkamat. † Radiation Laboratory. ‡ Departments of Chemistry and Biochemistry and Chemical & Biomolecular Engineering.

SCHEME 1: Photoinduced Charge Injection and Charge Separation in a Ag Core/TiO2 Shell Particle

semiconductor nanoparticles with a noble metal deposit. Semiconductor-metal composites have been widely used in photocatalysis.21-23 The deposition of the metal on semiconductor particles enhances the efficiency of the photocatalytic redox process.24,25 The composite films based on TiO2 and metal particles have shown higher photocurrent and photovoltage because of improved charge separation.26-28 Au-TiO2 and AgTiO2 nanocomposites exhibit enhanced charge-transfer efficiency by shifting the Fermi level of the composite to more negative potentials.29-31 Metal core-semiconductor shell structures are another attractive class of composites that can facilitate charge separation (Scheme 1). Using this strategy, we were able to transfer electrons from the excited TiO2 shell into the Ag core.32,33 The flow of electrons from the conduction band of TiO2 (ECB ) -0.5 V vs NHE) into the silver core with low lying Fermi level (∼0.45 V) is energetically favored. Electrons stored in the silver core (40-50 electrons per particle) can be readily discharged on-demand to an electron acceptor. Such core-shell structures provide an interesting strategy to improve the charge separation in a dye sensitized semiconductor system. Since the electrons injected into the TiO2 shell are quickly transferred to the Ag core, it should be possible to suppress the back electron transfer (Scheme 1).

10.1021/jp0665022 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2006

Harvesting Photons in the Infrared

J. Phys. Chem. C, Vol. 111, No. 1, 2007 489

In a preliminary communication, we have shown the ability of carbocyanine dye cluster film to exhibit a p-type semiconductor property.34 In the present paper, we have compared the electron injection and the recombination rate of IR-125 sensitized TiO2 and the TiO2 protected silver colloids using the spectroscopic techniques. The excited-state dynamics of the dye in the absence and presence of the colloids are studied using the femtosecond laser flash photolysis experiments. Experimental Section Materials. The IR-125 dye (2-[7-[1,3-dihydro-1,1-dimethyl3-(4-sulfobutyl)-2H-benz[e]indol-2-ylidene]-1,3,5-heptatrienyl]1,1-dimethyl-3-(4-sulfobutyl)-1H-benz[e]indolium hydroxide, inner salt, sodium salt) is obtained from Exciton and was used asreceived.Titanium-(triethanolaminato)isopropoxide(N((CH2)2O)3TiOCH(CH3)2) (TTEAIP) and dimethyl formamide, DMF, AgNO3 were obtained from Aldrich. Preparation of a Colloidal Solution of Ag@TiO2. The colloidal solution of TiO2 coated silver particles was prepared as per the reported procedure.35-37 A solution of TTEAIP (8.3 mM, 18 mL) prepared in 2-propanol was stirred with AgNO3 solution (15 mM, 2 mL). Dimethylformamide (10 mL) was then added as a reducing agent into the TTEAIP-Ag solution. The concentrations of silver ions and TTEAIP in the reaction mixture were 1 and 5 mM, respectively. The solution was stirred for 15 min at room temperature and then refluxed with continued stirring. With continued heating of the solution, the color slowly changed from colorless to light brown. After 90 min, the color of the suspension turned to dark brown. At this point, the heating was stopped and the suspension was stirred until it cooled to room temperature. The colloids were precipitated by adding toluene (10 mL) to the reaction mixture and centrifuged to obtain the residue. It was washed three times with toluene to remove the unreacted reagents and then resuspended in acetonitrile. Colloidal TiO2 was prepared following the same methodology except for the addition of silver nitrate. This ensured the TiO2 surface to be similar to that of Ag@TiO2 for investigating the interaction with the dye molecules. Methods. The absorption spectra were recorded using a Varian CARY50 Bio UV-vis spectrophotometer. Emission spectra were recorded on a Horiba Jobin Yvon-Fluorolog equipped with a Hamamatsu R928 photomultiplier tube. Pulse radiolysis experiments were performed with a 8-MeV Titan Beta model TBS-8/16-1S linear accelerator at the Notre Dame Radiation Laboratory. Femtosecond transient absorption experiments were conducted using a Clark-MXR 2010 laser system and an optical detection system provided by Ultrafast Systems (Helios). The source for the pump and probe pulses was the fundamental of the Clark laser system (775 nm, 1 mJ/pulse, FWHM ) 150 fs, 1 kHz repetition rate). The pump beam was attenuated to 5 µJ/ pulse with a spot size of 2 mm (diameter) at the sample where it was merged with the white light incident on the sample cell with an angle 300 µs. Radiolysis of N2 saturated methanol produced a reducing methoxy radical which in turn is capable of facilitating one electron reduction of the dye molecules. The transient absorption spectrum recorded 50 µs after the electron pulse shows a sharp absorption band at 500 nm. This absorption spectral feature is similar to the one observed for the dye anion radical generated by the reaction between the solvated electron and the dye.47 Excited State Interaction of Carbocyanine Dye with TiO2 and Ag@TiO2 Core-Shell Nanoparticles. The addition of the TiO2 or Ag@TiO2 colloids to the IR-125 dye solution in methylene chloride resulted in the adsorption of the dye on the TiO2 surface. The dye adsorbed particles were isolated by centrifugation and were re-suspended in acetonitrile. The dye

Figure 6. (A) Absorption and (B) emission spectra of IR-125 (1 µM) (a) in neat solvent, (b) bound to TiO2, and (c) bound to Ag@TiO2 nanoparticles. The inset shows the normalized absorption spectra of (d) TiO2 and (e) Ag@TiO2 colloids in acetonitrile. The emission spectra were recorded using 770 nm excitation and absorbance at 770 nm was maintained at 0.1.

bound colloids remained suspended in acetonitrile, and these suspensions were directly employed in all spectroscopic measurements. The absorption spectrum of the dye molecules bound to the surface of TiO2 and Ag@TiO2 are shown in Figure 6. The absorption band of the TiO2-bound dye in the IR is similar to that observed in neat solvents (Figure 1). This indicates that the surface aggregation effects are minimal and the dye exists mainly in the monomer form when bound to the TiO2 surface. These results also indicate that the dye coverage on the TiO2 surface is mono- or submonolayer. Slight broadening of the monomer band seen in the absorption spectra can be attributed to the interaction of the dye with the TiO2 surface. Such absorption changes arising from the dye-TiO2 interactions have been extensively investigated in earlier studies.49-53 The absorption spectra of TiO2 and Ag@TiO2 particles are also shown in the inset for comparison. TiO2 being a large band gap semiconductor absorbs in the UV region (λ < 360 nm). Ag@TiO2 on the other hand exhibits an additional absorption band with a broad maximum around 460 nm. The plasmon absorption of the Ag particles is red-shifted because of the high dielectric constant of the TiO2 capping. Details on the absorption and electron storage properties of Ag@TiO2 colloids have been discussed in an earlier work.32,33 The influence of the Ag core on the excited-state interaction between the dye and TiO2 was probed by comparing the singlet excited-state properties of the dye bound to TiO2 and Ag@TiO2 particles. The emission spectra were recorded using 770 nm excitation wavelength. (The absorbance at the excitation wavelength was kept constant for comparing the emission yields.) The fluorescence emission of the dye is quenched when it is

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bound to the TiO2 surface. The emission yield of the dye is slightly lower when bound to Ag@TiO2 than when bound to TiO2 particles. (Fluorescence quenching measurements at different concentrations of TiO2 and Ag@TiO2 are included in the Supporting Information, see Figures S3 and S4.) The fluorescence quenching seen in Figure 6B suggests that an additional pathway dominates the deactivation of the excited singlet of the dye on the TiO2 surface. As shown earlier, dyes with excited-state energy more negative than the conduction band of TiO2 are capable of injecting electrons into TiO2 particles (reaction 6 and 7)9

dye* + TiO2 f dye+• + TiO2(e)

(6)

dye* + Ag@TiO2 f dye+• + Ag@TiO2(e)

(7)

1

1

In the present case, the excited dye has an oxidation potential of -1.0 vs NHE (ground state oxidation potential was measured from cyclic voltammetry) and is energetic enough to inject electrons into TiO2 (ECB ) -0.5 V vs NHE). The greater extent of the fluorescence quenching observed for the Ag@TiO2 system indirectly suggests the possible influence of Ag core on the efficiency of net charge injection event. Ultrafast Electron Injection from the IR-125 Dye into TiO2 and Ag@TiO2 Nanoparticles. The charge transfer interaction between the dye and the TiO2 was further probed using femtosecond pump-probe spectroscopy. The transient absorption spectrum recorded immediately after the laser pulse shows the characteristics of the singlet excited dye. The close resemblance with the spectrum in Figure 2A indicates that the primary event involved in the excitation of the dye is the formation of the singlet excited-state on the TiO2 surface. As the singlet excited dye decays, a difference in the absorption feature emerges. The spectra recorded 5 ps after the laser pulse excitation in the absence and presence of TiO2 are compared in Figure 7A. The spectrum b shows the formation of an additional product with broad absorption in the 600-650 nm region. Based on our radiolytic oxidation of the dye, we can assign this transient to the cation radical of the dye formed as a result of the charge injection process (reaction 6). A similar electron-transfer product was also observed when the dye bound to the Ag@TiO2 nanoparticle was excited with a 775 nm laser pulse. These experiments confirmed that the deactivation of the singlet excited dye on TiO2 and Ag@TiO2 particles occurred via a similar electron injection process. We followed the decay of the transient absorption at 550 nm and bleaching recovery at 730 nm (Figure 7B). Two distinct decay components were evident in these absorption-time profiles. The fast decay corresponds to the deactivation of singlet excited-state and the long-lived component corresponds to the dye cation radical. Absorption-time profiles recorded at 515 nm were analyzed with a biexponential decay kinetics. The kinetic fitting of these two traces gave lifetimes of 10 ( 1 and 300 ( 5 ps, respectively. We attribute the fast decay to the quick deactivation of the singlet excited-state on the colloidal TiO2 surface arising from the electron injection process (reaction 6). The same experiments were repeated with the Ag@TiO2 system. Both singlet excited-state decay and the formation of the radical cation of the dye were observed following 775 nm laser pulse excitation. The transient absorption spectra and absorption-time profiles are shown in Figure 8. The singlet excited-state decay and the bleaching recovery were analyzed using biexponential decay kinetics. The short and long time components of the two traces had a lifetime of 12 ( 1 and 600

Figure 7. (A) Transient absorption spectra of IR-125 in (a) absence and (b) presence of colloidal TiO2. The spectra were recorded 5ps after the laser pulse excitation. (B) The absorption-time profiles recorded at 550 and 730 nm in presence of TiO2.

Figure 8. (A) Transient absorption spectra of IR-125 in (a) absence and (b) presence of Ag@TiO2 recorded 5 ps after the laser pulse excitation. (B) The absorption-time profile of IR-125 recorded at 515 nm (a) in absence and (b) presence of Ag@TiO2.

( 10 ps, respectively. Comparison of the short component corresponding to the singlet decay in TiO2 and Ag@TiO2 systems indicates that the excited deactivation in both cases occurs with a similar rate constant. Based on the lifetimes of the singlet excited-state, we obtain a charge injection rate constant in the range of 0.8-1.0 × 10-11 s-1. The similar values

Harvesting Photons in the Infrared

J. Phys. Chem. C, Vol. 111, No. 1, 2007 493 product. These results demonstrate a simple way of suppressing charge recombination in the semiconductor-dye system. By utilizing the metal nanocore and a semiconductor shell, we were able to suppress the charge recombination by at least a factor of 2. By constructing a network of interlinked metal particles capped with a layer of TiO2 and dye on an electrode surface, it should be possible to collect and transport photogenerated electrons quite efficiently. Photoelectrochemical measurements are underway to address this issue. Conclusions

Figure 9. Comparison of the charge recombination between the oxidized dye and the TiO2 in (a) TiO2 and (b) Ag@TiO2 colloids. The absorption-time profiles were recorded at 730 nm following the 775 nm femtosecond laser.

of the charge injection rate constants are also indicative of the fact that the Ag core has little influence on the charge injection kinetics in Ag@TiO2 systems. Thus, the electron-transfer process is dominated by the interaction between the excited dye and the oxide support, viz.,TiO2. As demonstrated in our earlier study,32,33electrons accumulated within the TiO2 layer get quickly transferred to Ag core (reaction 8)

Ag@TiO2 (e) f Ag(e)@TiO2

(8)

The lower lying Fermi level of Ag (+0.45 V vs NHE) energetically favors the transfer of electrons from TiO2 until the two systems equilibrate. For example, upon UV irradiation of Ag@TiO2 colloids electrons are quickly transferred from TiO2 into the Ag core.32,33 The accumulation of electrons in the Ag core can be monitored from the blue-shift in plasmon absorption from 460 to 420 nm. In a similar fashion, we expect the electrons injected from the excited dye into TiO2 to get transferred to the Ag core. Because of the weak monitoring probe intensity in the 420 nm region, we were not able to directly monitor the changes in the plasmon absorption. If indeed such electron accumulation should occur, we should be able to see its influence on the charge recombination (or back electron transfer) process (reaction 9). Charge Recombination in TiO2 and Ag@TiO2 Systems. The spectral and kinetic traces recorded in Figure 8 show the formation of dye cation radical as the singlet excited-state is quenched within few picoseconds. The dye cation undergoes recombination as it reacts with the injected electrons.

dye+• + TiO2(e) f dye + TiO2

(9)

Since the charge recombination step involves recovery of the dye, we followed the kinetics of this process from the bleaching recovery at 730 nm. Figure 9 compares the recovery of the dye in the TiO2 and Ag@TiO2 systems. A fast and a slow component is seen during the bleaching recovery. As discussed in the previous section, the short component decay that occurs with a lifetime of 10-12 ps arises from the decay of the singlet excited state. The long time component of the recovery corresponds to regeneration of the dye (reaction 9). Distinct differences are seen in the time scale with which dye regenerates on the TiO2 and Ag@TiO2 surface. Whereas the dye regenerates with a lifetime of ∼300 ps in the TiO2 system, it regenerates with a lifetime of ∼600 ps in the Ag@TiO2 system. At the end of the 1 ns time scale, we could still observe residual bleaching in the Ag@TiO2 system suggesting the existence of charge separated

Charge injection from the excited carbocyanine dye into TiO2 is a useful approach to extend the photoresponse of the large band gap semiconductor into the infra red region. The charge injection from excited IR-125 into TiO2 occurs with a rate constant of 1011 s-1. The formation of electron-transfer product has been confirmed from the spectral characterization of transients in the subnanosecond time scale. The transient absorption studies carried out with dye modified TiO2 and Ag@TiO2 systems indicate that the metal core has little effect on the charge injection from the excited dye into TiO2. On the other hand, the presence of the metal core suppresses the charge recombination (or dye regeneration) rate by a factor of 2. Electron accumulation within the metal core is likely to influence the overall charge separation in the composite system. Implications of the metal core-semiconductor shell in improving the performance of dye-sensitized solar cells are currently being pursued. Acknowledgment. The research described herein is supported by the Office of Basic Energy Science of the Department of the Energy. We also thank Toyota Central R&D Labs, Aichi, Japan for the generous research grant for enabling T.K. stay at Notre Dame. This is contribution No. NDRL 4678 from the Notre Dame Radiation Laboratory. Supporting Information Available: Time-resolved absorption spectra of IR-125 dye in glycerol (S1), absorption-time profiles at early times (S2), and quenching of IR125 dye emission by Ag@TiO2 (S3) and TiO2 particles (S4) are presented. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hagfeldt, A.; Graetzel, M. Chem. ReV. 1995, 95, 49. (2) Kamat, P. V. Chem. ReV. 1993, 93, 267. (3) Anderson, N. A.; Lian, T. Q. Annu. ReV. Phys. Chem. 2005, 56, 491. (4) Anderson, N. A.; Lian, T. Coord. Chem. ReV. 2004, 248, 1231. (5) Galoppini, E. Coord. Chem. ReV. 2004, 248, 1283. (6) Piotrowiak, P.; Deshayes, K.; Romanova, Z. S.; Pagba, C.; Hore, S.; Zordan, G.; Place, I.; Farran, A. Pure. Appl. Chem. 2003, 75, 1061. (7) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. B 2001, 63, 205321. (8) Hilgendorff, M.; Sundstro¨m, V. J. Phys. Chem. B 1998, 102, 10505. (9) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (10) Alivisatos, P. J. Phys. Chem. 1996, 100, 13226. (11) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (12) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (13) He, J. J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.; Akermark, B.; Sun, L. C.; Hagfeldt, A.; Sundstrom, V. J. Am. Chem. Soc. 2002, 124, 4922. (14) Spitler, M.; Parkinson, B. A. Langmuir 1986, 2, 549. (15) Wang, X. J.; Perzon, E.; Delgado, J. L.; de la Cruz, P.; Zhang, F. L.; Langa, F.; Andersson, M.; Inganas, O. Appl. Phys. Lett. 2004, 85, 5081.

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