J. Phys. Chem. B 2005, 109, 20485-20492
20485
Electron Injection into the Surface States of ZrO2 Nanoparticles from Photoexcited Quinizarin and Its Derivatives: Effect of Surface Modification Madhab C. Rath, G. Ramakrishna, Tulsi Mukherjee, and Hirendra N. Ghosh* Radiation Chemistry & Photo Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India ReceiVed: June 23, 2005; In Final Form: August 28, 2005
The effect of surface modification on interfacial electron transfer (IET) dynamics into the surface states of ZrO2 nanoparticles sensitized by quinizarin (Qz) and its derivatives has been carried out using time-resolved emission spectroscopy. The surface of ZrO2 nanoparticles has been modified by sodium dodecyl benzyl sulfonate . We have observed that Qz’s can form a strong charge-transfer (CT) complex with both unmodified and surface-modified (SM) ZrO2 nanoparticles. We have confirmed electron injection into the surface states of ZrO2 nanoparticles from the photoexcited Qz molecule in our earlier work (J. Phys. Chem. B 2004, 108, 4775; Langmuir 2004, 20, 7342). In the present investigation, we have observed electron injection from photoexcited Qz derivatives into the surface states of both unmodified and SM ZrO2 nanoparticles and also detected CT emission. Monitoring CT emission, we have determined back electron transfer (BET) dynamics of the dye-nanoparticle systems. We have found that the BET rate for the QZs/ZrO2 systems decreases as the relative driving force increases following Marcus inverted region kinetic behavior for an IET process. BET dynamics was found to be faster on SM ZrO2 nanoparticles as compared to that of the unmodified (bare) one. Our time-resolved emission data indicates that upon surface modification the majority of the deeper trap states of ZrO2 nanoparticles can be removed with the formation of some new shallower trap states in the band gap region.
1. Introduction The dynamics of interfacial electron transfer (IET) between molecular adsorbates and semiconductor materials has been a subject of intense research interest in recent years.1-17 One of the most important applications is the efficient conversion of sun light into electricity in a dye-sensitized solar cell (DSSC). In a DSSC, electron transfer takes place from the photoexcited dye to the conduction band of nanoparticles. The overall efficiency of DSSC depends on the dynamics of electron injection, recombination, electron transport, neutralization of the cation radical by the electrolyte, and so forth. Among them, the density of surface states , which lies below the conduction band edge (energetically lower compared to the conduction band), can really affect the IET dynamics and the overall performance of a DSSC.18-22 However, not many reports are available in the literature where the involvement of strictly surface states in IET processes has been discussed, except for a few reports by Huber et al.,23 Hao et al.,24 and Olsen et al.25 These groups have reported electron injection into the ZrO2 nanoparticles, a wide band gap (Eg ) 5.0 eV)26 semiconductor, where the energy level of the photoexcited sensitizer molecule lies below the conduction band of the nanoparticle. Recently,13,15 we have also reported direct electron injection into the surface states of ZrO2 nanoparticles from photoexcited quinizarin (Qz). We have reported that the formation of the charge transfer (CT) complex between the dye and the semiconductor facilitates the electron injection into the surface states. Upon excitation of this dye-semiconductor CT complex, CT emission has been * To whom correspondence should be addressed. E-mail: hnghosh@ magnum.barc.ernet.in. Fax: 00-91-22-25505151.
detected. Back electron transfer (BET) dynamics has been determined15 by monitoring the CT emission. Surface states play an important role in their optical and physicochemical properties in nanoparticles. Modification of these surface states can alter the physical, chemical, and photocatalytic properties of nanoparticles.27 Surface modification of nanoparticles can lead to the following effects: (i) it may enhance their excitonic and defect emission by blocking nonradiative electron/hole (e-/h+) recombination at the defect sites (traps) on the surface of the semiconductor nanoparticles,28 (ii) it may increase the photostability of semiconductor nanoparticles2, (iii) it may create new traps on the surface of the nanoparticles leading to the appearance of new emission bands,29 and (iv) it may boost the selectivity and efficiency of lightinduced reactions occurring on the surface of semiconductor nanoparticles.28 One can modify the surface states of nanoparticles by chemical treatments, which interact with the surface molecules, and eventually manipulate the desired property of the modified particles. Rajh et al.30 have shown that the optical response of TiO2 nanoparticles can be improved tremendously by using suitable modifier molecules such as ascorbate, mercaptocarboxylic acid, and enediol ligand. Surface chelation with a Ti atom with an electron-donating bidentate ligand changes the electronic properties of the nanoparticles. Recently,11,12 we have shown that surface modification can play a major role in IET dynamics, particularly in BET dynamics of dye-sensitized TiO2 nanoparticles. We have modified the surface of TiO2 nanoparticles with sodium dodecyl benzene sulfonate (DBS), and the IET dynamics of dibromo fluorescein (DBF)11 and alizarin (Alz)-sensitized12 surface-modified nanoparticles has been studied using fast and ultrafast spectroscopy. We have shown that the rate of the BET reaction can be slowed by
10.1021/jp0533980 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/29/2005
20486 J. Phys. Chem. B, Vol. 109, No. 43, 2005 modifying the TiO2 nanoparticle surface. Upon surface modification, Fermi level pinning of the nanoparticle takes place, pushing up the energy levels of the conduction band edge and Fermi level11,12 and thereby increasing the overall free energy of reaction (-∆G°) for BET and eventually slowing down the rate of the reaction. We have also shown that the energy level of the surface states can be affected upon surface modification. We have reported13,15 along with others23-25 that it is possible to monitor IET dynamics only into the surface states of the ZrO2 nanoparticles. However, no report is available in the literature on the effect of surface modification of these surface states on ET dynamics. Upon surface modification, the trap depth of the surface states will be changed, which may in turn affect the IET dynamics into the surface states. In the present investigation, we have reported and compared the ET dynamics of quinizarin (Qz) and its derivatives sodium quinizarin 2-sulphonate (Qz-2S)- and sodium quinizarin 6-sulphonate (Qz-6S)-sensitized ZrO2 nanoparticles. Both the Qz derivatives can form complexes with ZrO2 nanoparticles through the quinoid moiety like Qz molecules.13 It has been observed that electron injection takes place from the photoexcited dye molecules into the surface states of the ZrO2 nanoparticles. Charge transfer (CT) emission has been detected from these dye-nanoparticle systems. BET dynamics of the above dyenanoparticle systems has been determined by monitoring the CT emission. We have also modified the ZrO2 nanoparticle surface using sodium dodecyl benzene sulfonate (DBS) as modifier molecules and carried out ET dynamics between Qz derivatives and surface-modified nanoparticles. Both Qz-2S and Qz-6S form a CT complex with the modified ZrO2 nanoparticles, and CT emission has been detected from these complexes. BET dynamics has been determined for the surface-modified particles by monitoring the CT emission and compared with that of the bare one. 2. Materials and Methods (a) Materials. Quinizarin (Qz), sodium quinizarin 2-sulfonate (Qz-2S), and sodium quinizarin 6-sulfonate (Qz-6S) were obtained from Fluka and were purified by crystallization. The zirconium(IV) isopropoxide 2-propanol complex Zr[OCH(CH3)2]4‚(CH3)2CHOH (Aldrich, 99.9%) was used without further purification. Nanopure water was used for making aqueous solutions. (b) Sample Preparation. Nanometer-size ZrO2 was prepared by the controlled hydrolysis of the zirconium(IV) isopropoxide 2-propanol complex, and it has been described in detail in our earlier work.13 We have synthesized DBS-capped ZrO2 nanoparticles following the method similar to that of DBS-capped TiO2 nanoparticles.31 In 500 mL of freshly prepared ZrO2 colloids in water synthesized as reported earlier,13 250 mL of toluene was added in a round-bottomed flask. The resulting mixture was stirred slowly for 15-20 min. In the stirred solution, 100 mL of 0.2 M DBS (C12H25C6H4SO3Na) was added, and the final mixture was stirred slowly for 3 h. DBS can be dissolved only in water, because it is ionic in nature. As the surface of the ZrO2 nanoparticles is positively charged, DBS molecules can easily bind through a sulfonic group (SO3-) with the nanoparticles. The newly capped ZrO2 nanoparticles can be dissolved in many organic solvents. In this situation, ZrO2 nanoparticles migrate from water to the organic phase (toluene). The organic phase was separated out with the help of a separating funnel. At this stage, the organic phase looked a little cloudy. The organic phase was dried in CaCl2 and transformed to an optically clear solution in toluene. The organic phase was
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Figure 1. Optical absorption spectra of Qz-2S and Qz-6S in water and on an unmodified ZrO2 nanoparticle surface: (a) Qz-2S (s) and (b) Qz-6S (-‚-) in water, (c) Qz-2S (s) and (d) Qz-6S (-‚-) on a ZrO2 nanoparticle surface at pH 2.5., (e) monoanion of free Qz-2S at pH 11, and (f) monoanion of free Qz-6S at pH 11 (conditions, [Qz2S] ) 1.4 × 10-4 mol dm-3, [Qz-6S] ) 1.4 × 10-4 mol dm-3, and [ZrO2] ) 20 g/L (1.0 × 10-4 mol dm-3), 6.4 × 10-4 M).
then refluxed for 2 h, and the solvent was taken out with the help of a rotary evaporator in N2 atmosphere. Dry ZrO2 nanoparticles, capped by DBS, which were left in the flask, could be dissolved in many nonaqueous solvents to obtain a colloidal solution in that solvent. (d) Picosecond Time-Resolved Fluorimeter. Time-resolved fluorescence measurements were carried out using a diode laser based spectrofluorometer from IBH (U.K.). The instrument works on the principle of time-correlated single-photon counting (TCSPC).32 In the present work, 455 nm (1 ns, 1 MHz), 490, and 560 nm light emitting diodes were used as the excitation light sources, and a TBX4 detection module (IBH) coupled with a special Hamamatsu PMT was used for fluorescence detection. (e) Cyclic Voltammetry. Voltammetric experiments were performed with Auto Lab PGSTAT 20 (Manufactured by EcoChemie, Netharlands) coupled to a Metrohm 663 VA stand electrode system comprised of glassy carbon (GC)/Pt/Ag/AgCl. The PG STAT was driven by Autolab software. The temperature of the solution was maintained at 25 ( 0.1 °C. Measurements were done in an acetonitrile solution with TEAP (triethylammonium perchlorate) as a supporting electrolyte in N2 atmosphere. Oxidation redox potentials of Qz, Qz-2S, and Qz-6S molecules have been determined to be 1.63, 1.36, and 1.3 V, respectively, against the Ag/AgCl electrode.
3. Results and Discussion Charge-Transfer Interaction between ZrO2 Nanoparticles and Quinizarin Derivatives. We have observed in our earlier studies that for the study of IET in the surface states of nanoparticles, it is important to have a strong CT interaction between dye molecules and nanoparticles.13 Figure 1 shows the absorption of free Qz-2S and Qz-6S in water and also those adsorbed on the ZrO2 nanoparticles. The optical absorption spectra of Qz-2S and Qz-6S in water at pH 2.5 show peaks at 465 and 485 nm, respectively. However, upon addition of ZrO2 nanoparticles, the absorption spectra of both Qz-2S and Qz-6S are shifted to longer wavelengths, with a peak at 543 nm and a shoulder at 585 nm for Qz-2S and to 540 nm and a shoulder at 580 nm for Qz-6S. In our earlier studies,13 we have observed
ZrO2 Nanoparticles that Qz forms a strong CT complex; similarly, it is seen from optical absorption spectra that both Qz-2S and Qz-6S form strong CT complexes with ZrO2 nanoparticles. Both these Qz’s can form chelate-type six-membered complexes with ZrO2 nanoparticles, such as the Qz/ZrO2 system. Theoretical calculation on quinizarin molecules adsorbed on R-Al2O333 and TiO234 surfaces with the formation of a chelate complex suggests that the π-π* character of Qz is conserved in the excited state. Now, the question is whether complexation between the Qz and ZrO2 nanoparticles will have ligand-tometal charge transfer (LMCT) character or not. In our earlier studies,13 we have observed that the interaction between Qz and ZrO2 is much stronger than that of Qz and TiO2. We explained that the cubic structure of ZrO2 has a better geometry to form a six-membered ring with Qz than that of the hexagonal structure of TiO2.13 A six-membered ring has larger bond angles that can accommodate the cubic structure. As a result, Qz-ZrO2 interaction is much stronger than that of Qz-TiO2. Similarly, Al2O3 has a rhombohedral (very close to hexagonal)35 crystal structure, which also cannot allow the formation of a strong complex with Qz. According to Rajh et al.,30c probe molecules, which can form a six-membered chelate complex with ZrO2, can give LMCT transitions. Similarly, in the present investigation, we can tell that in Qz-sensitized ZrO2 nanoparticles LMCT transition takes place. To reconfirm that those optical absorption spectra of the dyeZrO2 systems are not due to the anion form of the dye molecules, we have carried out steady-state absorption experiments of free Qz derivatives at higher pH values (pH ∼ 11). The optical absorption spectra of the monoanion form of QZ-2S (Figure 1e) and Qz-6S (Figure 1f) are shown in Figure 1. It has been observed that, in the monoanionic form, the dye molecules are red shifted as compared to the neutral form and even the CT complex of the corresponding dye-nanoparticle systems. However, the monoanionic form of the Qz derivatives is very unstable and decomposes very fast, within a couple of hours. On the other hand, the dye-nanoparticle complex is stable for weeks together. We have carried out measurements on the ZrO2 nanoparticle surface at low pH (2.5) where the formation of the monoanionic form in solution is impossible because the monoanion form can exist only at very high pH values. So, from the above observations, we can conclude that the redshifted absorption spectra of the dye-nanoparticles systems are due to CT complexes rather than the monoanionic form of the dye molecules. Experiments for surface-modified nanoparticles were also carried out (Figure 2) only in nonaqueous solvents (like chloroform and toluene), as surface-modified (SM) ZrO2 particles cannot be dispersed in water. As the solubility of Qz molecules in these nonaqueous solvents is very high, it is not a good candidate for the sensitization of SM ZrO2 nanoparticles either in chloroform or in toluene. On the other hand, Qz-2S and Qz-6S are insoluble in chloroform and toluene, so sensitization for SM ZrO2 nanoparticles by these molecules will be ideal. Under this condition, we can see only CT interaction between dye and nanoparticles. Figure 2 shows the steady-state absorption spectra of free Qz-2S in water, DBS/water, and Qz-2Ssensitized unmodified ZrO2 nanoparticles in water as well as SM ZrO2 nanoparticles in chloroform. It is quite evident from the optical absorption spectra that Qz-2S interacts very strongly with ZrO2 nanoparticles even on the modified surface. To find out ground-state interactions between the sensitizer, that is, Qz2S, and the modifier molecule (DBS), we have carried out optical absorption measurements of Qz-2S in the presence and
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20487
Figure 2. Optical absorption spectra of Qz-2S in (a) water (s) and (b) sodium dodecyl benzene sulfonate (DBS)/water solution (‚‚‚), and Qz-2S sensitized on (c) unmodified ZrO2 nanoparticles (s) and (d) surface-modified ZrO2 nanoparticles (-‚-) in chloroform.
absence of the DBS molecule. As the solubility of the surfactant molecule in nonaqueous solvents is lower, we have carried out the above-mentioned experiments in water. It is seen from Figure 2 that the absorption maximum of Qz-2S in DBS/water is around 465 nm (Figure 2a), which is very similar to that in water (Figure 2b) indicating a negligible ground-state interaction between the sensitizer and surfactant molecules. It is seen in Figure 2c that Qz-2S forms a strong CT complex with unmodified ZrO2 nanoparticles absorbing at 543 nm with a shoulder at 585 nm (also mentioned in Figure 1). Figure 2d shows that Qz-2S forms a CT complex with SM ZrO2 nanoparticles in chloroform. The CT complex of the Qz-2S/ZrO2 system on a modified surface absorbs at 526 nm with a shoulder at 563 nm in toluene. It is interesting to observe that the CT complex of the QZ-2S/ZrO2 system absorbs in the blue side of the spectrum on the modified surface as compared to the unmodified one. (b) Emission Spectroscopy of Qz and Its Derivatives on a ZrO2 Nanoparticles Surface. In our earlier studies,15 we have clearly demonstrated the emission of Qz on ZrO2 nanoparticles and also clarified the emissive nature of the CT complex of the Qz/ZrO2 system. Figure 3 shows the normalized emission spectra of Qz-2S and Qz-6S in water and on the ZrO2 nanoparticles surface. The emission spectrum of Qz-2S in water shows a peak at 592 nm with a shoulder at 553 nm at pH 2.5 (Figure 3a). It has been observed that, upon addition of ZrO2 nanoparticles, the original emission gets quenched with the appearance of a red-shifted emission with a maximum at 613 nm and a shoulder at 648 nm (Figure 3c). Again, the emission spectrum of Qz-6S in water shows a very broad band at around 558 nm at pH 2.5 (Figure 3b). However, upon addition of ZrO2 nanoparticles, the original emission gets quenched completely with the appearance of a red-shifted emission band at 602 nm with a shoulder at 643 nm (Figure 3d). We have also carried out emission measurements of the monoanionic form of Qz-2S and Qz-6S that shows they are red shifted as compared to the CT emission spectra of the dye-nanoparticle systems (Figure 3). However, emission yield is a lot less (1 order of magnitude or less) when compared to the neutral form of the dye molecules and CT complex under similar experimental conditions. Also, the emission intensity decreases with time, confirming the decomposition of the monoanionic forms. The red-shifted and increased emission in the dye-ZrO2 nanoparticle systems has been attributed to the CT emission. Quenching of original emission of the quinizarin derivatives on the ZrO2 nanoparticles
20488 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Figure 3. Normalized emission spectra spectra of Qz-2S and Qz-6S in water and on an unmodified ZrO2 nanoparticle surface: (a) Qz-2S (s) (λex ) 465 nm) and (b) Qz-6S (-‚-) (λex ) 470 nm) in water, (c) Qz-2S (s) (λex ) 585 nm) and (d) Qz-6S (-‚-) (λex ) 580 nm) on a ZrO2 nanoparticle surface at pH 2.5, (e) monoanion of free Qz-2S at pH 11, and (f) monoanion of free Qz-6S at pH 11 (conditions, [QZ2S] ) 1.4 × 10-4 mol dm-3, [QZ-6S] ) 1.4 × 10-4 mol dm-3, and [ZrO2] ) 20 g/L (1.0 × 10-4 mol dm-3), 6.4 × 10-4 M).
SCHEME 1: Mechanistic Scheme of Electron Transfer (ET) into the Surface States of ZrO2 Nanoparticles and Effect of Surface Modification on ET in Qz- and Its Derivatives-Sensitized Unmodified (left) and Modified (right) ZrO2 Nanoparticlesa
a Fermi level pinning is seen due to surface modification on ZrO2 nanoparticles (right). Surface-modification density of deep trap states decreases with the formation of some new shallower trap states. Groundand excited-state energy levels of the Qz’s are depicted in the scheme. Ground-state redox potentials (ES/S+) were measured from cyclic voltametry, and excited-state potentials (ES*/S+) were determined after adding E00 with ES/S+. E00 has been determined from the crossing point of excitation and emission spectrum of the Qz’s.
surface can be attributed to the electron injection into the surface states of the ZrO2 nanoparticles. As the energy levels of the excited states of the Qz-2S (ES*/S+ ) -0.97 eV) and Qz-6S (ES*/S+ ) -1.03 eV) lie much below the conduction band edge of ZrO2 nanoparticles (ECB ) -1.8 eV vs Ag/AgCl)30c (Scheme 1), electron injection in the conduction band is not possible. We detected CT emission on the nanoparticle surface for the first time in coumarin 343 (C-343)/TiO27 and xanthene/TiO28 systems. Later on, Hupp and co-workers36 reconfirmed the CT emission in C-343-sensitized TiO2 nanoparticles using stark emission spectroscopy. We have also detected CT emission in Qz-sensitized ZrO2 nanoparticles.15 We have observed excitation of the CT complex in the dye-nanoparticle system; electron
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Figure 4. Normalized emission spectra of Qz-2S in (a) water (s) (λex ) 465 nm) and (b) sodium dodecyl benzene sulfonate (DBS)/water solution (‚‚‚) (λex ) 465 nm), and Qz-2S sensitized on (c) unmodified ZrO2 nanoparticles (s) (λex ) 585 nm) and (d) surface-modified ZrO2 nanoparticles (-‚-) in chloroform (λex ) 565 nm).
transfer takes place from the dye to the nanoparticle. Finally, upon recombination of the charge-separated CT complex, CT emission can be seen. We have also carried out emission measurements of Qz derivatives on a modified ZrO2 nanoparticles surface. In the previous section, we have discussed the CT absorption behavior of the Qz-2S/ZrO2 system and we have observed that the CT absorption spectra is blue shifted on the modified surface as compared to the unmodified surface (Figure 2). Now, it will be interesting to monitor the CT emission behavior on the modified surface as compared to the unmodified surface. Figure 4 shows the emission spectra of Qz-2S in water, in DBS/water, on the unmodified ZrO2 surface, and on the modified ZrO2 surface (in chloroform). We have chosen chloroform to study the emission behavior of Qz-2S/ZrO2 system on a modified surface because the contribution from the free dye toward the observed emission is very negligible in that solvent. Interestingly, here we have observed that CT emission in the Qz-2S/ZrO2 system is blue shifted on the modified surface as compared to the unmodified surface as well. (c) Determination of Back Electron Transfer from CT Emission. We have also shown in our earlier studies7,8 that, by monitoring the time-resolved CT emission, the exact BET time constants can be determined. It has been observed that CT emission decay can be fitted with a multiexponential function as compared to the free dye emission decay, which fits to a single-exponential function. In our earlier studies, the contribution of free dye emission has always been observed along with the CT. As a result, some difficulties have been faced to extract pure CT emission out of these analyses. This problem arose due to the excitation of the free dye and CT complex together. Upon excitation of free dyes on the nanoparticle surface, some of them inject an electron and some do not. The dyes that do not inject an electron into the nanoparticle surface contribute to the free emission component in the multiexponential decay kinetics of dye-nanoparticle systems. This problem would not have arisen if we could excite only the CT complex in the dyenanoparticle systems. In our earlier investigations of C-343/ TiO2,7 xanthene dyes/TiO2,8 and Qz/ZrO215 systems, the samples for emission time decay analysis were excited at 444 nm. At this wavelength, both the free dye and the CT complex will get excited together in all of the above systems. In the present investigation for time-resolved emission studies, we have excited
ZrO2 Nanoparticles
Figure 5. Time-correlated single-photon-counting traces of Qz, Qz2S, and Qz-6S in water and on an unmodified ZrO2 nanoparticle surface at pH 2.5. Emission wavelengths were kept at their respective emission peak positions. Excitation wavelengths were chosen at 455 and 490 nm for the free dyes and 560 nm for the dye-nanoparticle systems.
Qz- and its derivatives-sensitized ZrO2 nanoparticles at that wavelength, where the CT complex of the dye-nanoparticle system has maximum absorption. Figure 5 shows the decay kinetics at emission peaks of Qz, Qz-2S, and Qz-6S in water and on a ZrO2 nanoparticles surface after excitation at 490 nm for the free dyes and after excitation at 560 nm for the dye-nanoparticle systems. At this wavelength, the free dye contribution on ZrO2 surface is negligible. So, upon excitation of the dye-nanoparticle systems, we mostly have excited the CT complex (Figure 1) and hence one can expect pure CT emission from these systems. Decay kinetics in Figure 5 for the free dyes in water has been fit to a single-exponential function (Table 1). It has been observed that the excited singlet state lifetime of the free dye molecules in water are 1.48 ns, 915 ps, and 556 ps for Qz, Qz-6S, and Qz-2S, respectively, exciting at 455 nm for Qz and at 490 nm for Qz-2S and Qz-6S. However, on the ZrO2 nanoparticle surface, the emission decay kinetics has been fitted to a biexponential function (Table 1). Interestingly, it has been observed that, in the decay analysis for dye-nanoparticle systems, a contribution due to the free dyes is absent. The emission decay in the Qz/ZrO2 system has been fitted with time constants of 4 ns (6.5%) and 12.5 ns (93.5%). In our earlier investigation15 exciting the Qz/ZrO2 system at 444 nm, we have observed that the decay kinetics fitted triexponentially with time constants of 1.48, 3.9, and 11 ns. The 1.48 ns component was due to the free dye contribution, and longer components were attributed to the CT emission. So, in the current investigation, we can attribute the emission decay kinetics for the Qz/ZrO2 system in Figure 5 purely to CT emission. CT emission decays for the other two systems can be fitted biexponentially with time constants of 2.5 ns (20%) and 9.6 ns (80%) for Qz-6S/ZrO2 and 3 ns (22.3%) and 12.5 ns (77.7%) for Qz-2S/ZrO2. To reconfirm the appearance of CT emission in Qz-sensitized ZrO2 nanoparticles, we have also carried time-resolved emission studies for free dye (Qz’s) and Qz-sensitized ZrO2 nanoparticles exciting at different wavelengths. It is interesting to observe that the emission lifetimes of free dye molecules remain unchanged by exciting at different wavelengths. We have also carried out time-resolved emission studies exciting the Qz/ZrO2 systems at a blue wavelength (λex ) 444 and 490 nm) in the
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20489 optical absorption spectra. The emission decay traces in those systems contain both free dye emission and CT emission (Table 1). However, if we excite at the red wavelength of the Qz/ZrO2 systems where only the CT complex absorbs, then we get an emission profile only due to CT emission (Table 1). In that case, free dye emission is completely absent. These measurements clearly indicate that in Qz/ZrO2 systems there is a strong interaction between the dyes and nanoparticles that eventually forms the CT complex. And, by exciting these CT complexes, we get pure CT emission. We have also carried out time-resolved emission spectroscopic studies of CT emission of Qz-2S/ZrO2 and Qz-6S/ZrO2 systems on a modified surface and compared the results to those on an unmodified surface exciting at 560 nm (Table 2). Here, we have chosen the solvents to study time-resolved CT emission behavior on a modified surface as well, where the dye molecules (Qz-2S and Qz-6S) do not have any solubility. As a result, the contribution from the free dye will be completely absent. In Table 2, we have shown and compared the lifetime analysis of Qz-2S- and Qz-6S-sensitized ZrO2 nanoparticles on both unmodified and modified surfaces in different solvents exciting at 560 nm. It is seen in Table 2 that CT emission of the Qz2S/ZrO2 system on the unmodified surface has two components of 3 ns (22%) and 7.8 ns (78%); however, on the modified surface in chloroform solvent, CT emission decay can be fitted with three components with time constants 1.09 ns (6%), 3.45 ns (65%), and 7.48 ns (29%). It is interesting to see that upon surface modification, a new shorter component arises which was absent on the unmodified surface. Another important observation we made was that the contribution of the longer component (∼7.5 ns) decreases drastically on the modified surface (29%) as compared to the unmodified surface (78%). We also made a similar observation in toluene. Similarly, in the Qz-6S/ZrO2 system on the modified surface in chloroform, we observed an extra faster component (230 ps) in CT emission decay kinetics, which was absent on the unmodified surface (Table 2). Another interesting point to be observed in Table 2 is that the Qz-2S can form a complex with SM ZrO2 both in toluene and chloroform, however, Qz-6S form a complex with SM ZrO2 only in chloroform. This can be explained as follows. We have already explained in the paper that to sensitize the surfacemodified ZrO2 two conditions have to satisfy: (i) the solubility of the sensitizer in the solvent has to be very poor (insoluble) and (ii) the interaction between the dye and nanoparticle system should be strong enough. Now, the sulfonic group resides close (two position) to the quinone moiety in Qz-2S (Chart 1), so it will have an added interaction with the ZrO2 nanoparticles with the quinone moiety as compared to Qz-6S (where the sulfonic group resides in the six position, Chart 1). So, the Qz-2S molecule can form a better complex as compared to Qz-6S in both chloroform and toluene. However, in Table 2, we have observed that Qz-6S forms complex with the SM ZrO2 only in chloroform not in toluene. It is well-known that complexation in this system can be favored by increasing the polarity of the medium. As chloroform is more polar when compared to toluene, so complexation between the Qz-6S and SM ZrO2 nanoparticles will be favorable in chloroform. As a result, we have observed complexation in the Qz-6S/SM ZrO2 system only in chloroform. (d) Back Electron Transfer Reaction on ZrO2 Nanoparticle Surface and Effect of Free Energy (-∆G°). In the present investigation, we have studied systematically IET dynamics in the surface states of ZrO2 nanoparticles sensitized by Qz and its derivatives. It would be interesting to compare the BET
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TABLE 1: Emission Lifetimes of Qz, Qz-2S, and Qz-6S in Water and on a ZrO2 Nanoparticle Surface τ (ns) material systems
free dyea
Qz/ZrO2
1.48 ( 0.1
Qz-6S/ZrO2
0.915 ( 0.1
Qz-2S/ZrO2
0.556 ( 0.08
a
complex (λex ) 490 nm) 1.48 ( 0.1 (47.4%)b 3.9 ( 0.2 (46.2%)b 11.0 ( 0.6 (6.4%)b 0.8 ( 0.08 (11.7%) 3.1 ( 0.2 (34.7%) 9.7 ( 0.5 (53.6%) 0.6 ( 0.09 (10%) 3.2 ( 0.3 (35%) 8.0 ( 0.6 (55%)
complex (λex ) 560 nm) 4.2 ( 0.2 (6%) 12.5 ( 0.7 (94%)
τave (ns) (for λex ) 560 nm)
(1/τave) x108 (s-1)
E00 (-∆G0) (eV)
12.0
0.83
2.135
2.2 ( 0.1 (13.2%) 9.1 ( 0.5 (88.6%)
8.19
1.22
2.085
3.0 ( 0.1 (22%) 7.8 ( 0.4 (78%)
6.74
1.48
2.061
Excitation wavelength was 455 nm for Qz and Qz-2S and 490 nm for Qz-6S. b Data taken from ref 15 (excitation wavelength is 444 nm).
TABLE 2: Fluorescence Lifetimes of the Free Dyes and Their Corresponding CT Complexes with ZrO2a and SM ZrO2a Nanoparticles in Different Solvents sample
τ (ns) (water)
Qz-2S free dye ZrO2
0.556 ( 0.080 3.0 ( 0.1 (22%) 7.8 ( 0.4 (78%) 〈τ〉 ) 6.74
SM ZrO2
Qz-6S free dye ZrO2 SM ZrO2
b
0.915 ( 0.10 2.5 ( 0.1 (20%) 9.6 ( 0.5 (80%) 〈τ〉 ) 8.18 b
τ (ns) (chloroform)
τ (ns) (toluene)
b
b
b
b
1.09 ( 0.14 (6%) 1.1 ( 0.23 (14%) 3.45 ( 0.2 (65%) 3.4 ( 0.2 (63%) 7.48 ( 1.0 (29%) 7.77 ( 0.77 (23%) 〈τ〉 ) 4.48 〈τ〉 ) 4.10 b
b
b
b
0.23 ( 0.15 (8.5%) 3.45 ( 0.2 (71%) 9.21 ( 1.3 (20.5%) 〈τ〉 ) 4.35
c
a ZrO is the unmodified nanoparticles, which are soluble only in 2 water. SM ZrO2 is the surface-modified nanoparticles, which are insoluble in water. b Not soluble. c No CT complex found.
CHART 1: Molecular Structure of Qz, Qz-2S, and Qz6S
dynamics on a nanoparticle surface for different sensitizers. Electron transfer (ET) rates should ultimately decrease with an increasing thermodynamic driving force (-∆G°) according to classical37 and quantum mechanical38 theories of ET. The prediction of “inverted” rate behavior has also been demonstrated by many authors39,40 and by us8 on dye-sensitized TiO239,8 and SnO240 nanoparticle surfaces, where electron injection took place in the conduction band. It would be interesting to monitor the ET rate behavior by changing the free energy of the dyenanoparticle systems while ET takes place in the surface states of the nanoparticles. It is seen from Table 1 that the average BET rate (1/τav) is different in different dye-nanoparticle systems. The free energy (-∆G°) can be calculated from E00 of the CT complex on the nanoparticle surface. E00 is the energy of A f A*, where A is the CT complex. Farid and co-workers41 also have determined the free energy (-∆G°) for the BET reaction from the CT emission spectra. In the present investigation, we have calculated E00 values from the crossing point of
charge-transfer excitation and emission spectra and they are shown in Table 1, which is a linear function of the driving force (-∆G°). We have observed in Table 1 that with increasing -∆G°, the rate of BET (1/τav) has been decreased. Since all the quinizarin dyes (Scheme 1) coupled similarly with ZrO2 nanoparticles upon the formation of a six-membered ring, sulfonate derivatives will not alter the electron coupling. The observed trend of decreasing BET with increasing driving force is consistent with Marcus inverted region behavior. Our observation shows another Marcus inverted behavior of ET reaction on the nanoparticle surface. (e) Effect of Surface Modification on Interfacial Electron Transfer Dynamics. The main aim of this investigation is to see the effect of surface modification on the IET dynamics in Qz-sensitized ZrO2 nanoparticles, where electron injection takes place directly into the surface states. Quinizarin derivatives interact with ZrO2 nanoparticles very strongly in the absence or presence of surface-modifier molecules (DBS) as observed from the optical absorption studies. On the other hand, the sensitizers do not interact with modifier molecules (DBS) both in the ground and in the excited state. This is an ideal system to study the effect of surface modification on ET dynamics in the surface states of semiconductors. However, in the absorption and emission spectroscopic measurements for the Qz-2S/ZrO2 system in Figures 2 and 4 we have observed that both absorption and emission spectra of the dye-nanoparticle system move toward the blue region of the spectra upon surface modification. It is interesting to observe that the CT emission of the dyenanoparticle system can be fitted triexponentially on a modified surface as compared to biexponentially on an unmodified surface (Table 2). Again, the average lifetime of CT emission in chloroform is 4.48 ns on a modified surface as compared to 6.74 ns on an unmodified surface for the Qz-2S/ZrO2 system (Table 2), thus indicating that BET rate from the nanoparticles to the dye molecule increases upon surface modification. In our earlier investigation, we have demonstrated the effect of surface modification in alizarin (Alz)-12 and dibromo fluorescein (DBF)-11 sensitized TiO2 nanoparticles, where electron injection took place in the conduction band of the semiconductor nanoparticles. We have observed in those systems that BET rate decreases upon surface modification. We explained the mechanism as follows: Upon surface modification, the Fermi level of the modified colloids shifts more negative. As a result, the overall free energy of reaction increases (-∆G°).11,12 Upon modification, both the flat band potential and the valence band shifted more negative in the same amount.12 As a result, the band gap of nanoparticles does not change with modification but the conduction band edge energy level shifts toward higher energies. According to Marcus ET theory, with increasing thermodynamic driving force (-∆G°),
ZrO2 Nanoparticles the ET rate initially increases to reach a maximum value and then starts decreasing. This high exoergic region is often termed the “inverted regime”. BET processes in dye-sensitized TiO2 nanoparticles surfaces fall in the Marcus inverted regime for its high free energy of reaction.8,39,40 In this region with increasing driving force (-∆G°) of the reaction, the rate of BET decreases. As the free energy for BET for the case of SM TiO2 is higher than that of the unmodified TiO2 nanoparticles, the BET rate on the modified surface is slower as compared to the unmodified surface. In the present investigation, BET reaction dynamics on the modified surface is faster as compared to the unmodified surface. As the electron injection takes place into the surface states, pinning of the Fermi level of the conduction band will not make any difference on ET dynamics in ZrO2 nanoparticles. However, upon surface modification, the orbital interaction takes place between the HOMO of the surface-modifier molecules and the unfilled deep surface states (act as the LUMO).42 Upon interaction, the energy level of LUMO goes up and that of the HOMO goes down. So, energy levels of the surface states move toward a more negative value. As a result, the density of the deep trap states decreases as depicted in Scheme 1. Energetically shallow trap states are higher in energy as compared to the deeper ones. So, upon surface modification, interaction between the modifier molecular orbital and the deep trap states is much more when compared to that with the shallower one. As a result, the density of the deep trap states decreases more than that of the shallow trap states. Again, during the surface modification process, we end up creating some more new, shallower trap states, which also take part in the electron-transfer process (Scheme 1). We can see from Scheme 1 that a higher number of injected electrons will reside in shallow trap states for modified particles and in deep trap states for the unmodified particles. We have observed9 that the recombination reaction (BET) for the deep trap state electron and the parent cation is much slower due to a low coupling matrix for BET. As a result, BET reaction is slower on the unmodified nanoparticle surface as compared to the modified one. This mechanistic scheme can also explain the appearance of a new shorter component (1.1 ns for Qz-2S/ZrO2, Table 2) in the CT emission on the modified nanoparticles surface. It is seen from Scheme 1 that, upon surface modification, a new set of shallower trap states appeared from where recombination of the injected electron and the parent cation will be faster due to stronger coupling as compared to the deeper trap states. 4. Conclusion Time-resolved emission spectroscopy has been used to study the electron-transfer dynamics of quinizarin (Qz) and its derivatives (Qz-2S and Qz-6S) adsorbed onto unmodified and surface-modified ZrO2 nanoparticles. Surface modification of the nanoparticles was carried out using DBS as the modifier molecule. The quinone moiety of Qz and its derivatives form a CT complex with ZrO2 nanoparticles. Excitation of this CT complex gives direct electron injection into the surface states of ZrO2 nanoparticles. Recombination of this charge-separated CT complex can take place through a radiative pathway. As a result, a red-shifted CT emission has been observed from dyenanoparticle systems. Monitoring the CT emission back ET time constants has been determined and found to be multiexponential. It has been also found that the BET rate for Qz- and its derivatives-sensitized ZrO2 nanoparticle systems decreases as the relative driving force increases. Assuming a negligible change in the electronic coupling, our results provide the
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20491 evidence for the Marcus inverted region kinetics behavior for an IET process where electron injection takes place into the surface states of the nanoparticles. We have also carried out sensitization experiments for surface-modified ZrO2 nanoparticles using Qz-2S and Qz-6S in toluene and chloroform solvents. Both CT absorption and emission spectra of the dye-nanoparticle systems have been observed to be blue shifted on the modified surface as compared to the unmodified one. It is interesting to see that, upon surface modification, the average BET time decreases with the formation of new shallower trap states, which are responsible for faster BET time constant in the kinetic decay trace. Our time-resolved data indicates that surface modification removes many of the deeper trap states, which are responsible for the long time recombination dynamics of injected electrons (deep trapped) and the parent cation due to low coupling strength of the BET reaction. As a result, the BET reaction is found to be slower on the unmodified particle surface compared to the modified one. In conclusion, we have shown that strongly coupled sensitizers molecules can directly inject the electron into the surface states (defective states) of nanoparticles. Electron-transfer processes in these surface states of nanoparticles is still a gray area. We have also shown that by surface modification using suitable modifier molecules it is possible to remove most of the lowerlying defective states. This observation can, in turn, help the researchers who build devices made of nanocrystalline materials, where defect states of the materials play an important role. Supporting Information Available: Graphs showing the optical absorbance spectra of Qz-Zr, Qz-2S-Zr, and Qz-6SZr complexes, the emission spectroscopy of Qz and Qz-Zr(IV) complex in isopropanol, and the transient absorption decay kinetics for the injected electron in Qz-2S-sensitized ZrO2 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Miller, R. D. J.; McLendon, G. L.; Nozik, A. J.; Schmickler, W.; Willig, F. Surface Electron-Transfer Processes; VCH Publishers, Inc.: New York, 1995. (2) Kamat, P. V.; Meisel, D. Semiconductor Nanoclusters - Physical Chemical, and Catalytic Aspects; Elsvier: Amsterdam, The Netherlands, 1997; Vol. 103. (3) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (4) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061. (5) Oregan, B.; Gratzel, M. Nature 1991, 353, 737; 2001, 414, 338. (6) Hannapel, T.; Burfeindt, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (7) Ghosh, H. N. J. Phys. Chem. B 1999, 103, 10382. (8) Ramakrishna, G.; Ghosh, H. N. J. Phys. Chem. B 2001, 105, 7000. (9) Ramakrishna, G.; Ghosh, H. N.; Singh, A. K.; Palit, D. K.; Mittal, J. P. J. Phys. Chem. B 2001, 105, 12786. (10) Ramakrishna, G.; Ghosh, H. N. J. Phys. Chem. A 2002, 106, 2545. (11) Ramakrishna, G.; Das, A.; Ghosh, H. N. Langmuir 2004, 20, 1430. (12) Ramakrishna, G.; Singh, A. K.; Palit, D. K.; Ghosh, H. N. J. Phys. Chem. B 2004, 108, 1701. (13) Ramakrishna, G.; Singh, A. K.; Palit, D. K.; Ghosh, H. N. J. Phys. Chem. B 2004, 108, 4775. (14) Ramakrishna, G.; Singh, A. K.; Palit, D. K.; Ghosh, H. N. J. Phys. Chem. B 2004, 108, 12489. (15) Ramakrishna, G.; Ghosh, H. N. Langmuir 2004, 20, 7342. (16) Ghosh, H. N.; Asbury, J. B.; Lian, T. J. Phys. Chem. B 1998, 102, 6482. (17) Asbury, J. B.; Hao, E.; Wang, Y. Q.; Ghosh H. N.; Lian, T. J. Phys. Chem. B 2001, 105, 4545. (18) Fermin, D. J.; Jensen, H.; Moser, J. E.; Girault, H. H. ChemPhysChem 2003, 1, 85. (19) Rothenberger, G.; Fitzmaurice, D.; Gra¨tzel, M. J. Phys. Chem. 1992, 96, 5983. (20) Haque, S. A.; Tachibana, Y.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 1998, 102, 1745.
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