Effect of Surface Modification on Back Electron Transfer Dynamics of ...

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free energy of reaction, BET rate decreases on modified surface. However ... is more in bare particles, so the BET reaction in longer time domain is slow.
Effect of Surface Modification on Back Electron Transfer Dynamics of Di-bromo Fluorescein (DBF) Sensitized TiO 2 Nanoparticles G. Ramakrishna1, Amit Das 2 and Hirendra N. Ghosh* 1 1

Radiation Chemistry and Chemical Dynamics Division, 2 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai – 400 085, India

Abstract: Electron injection and back electron transfer (BET) dynamics has been carried out for di-bromo fluorescein (DBF) sensitized TiO 2 nanoparticles capped (modified) with sodium dodecyl benzene sulphonate (DBS) using transient absorption techniques in picosecond and microsecond time domain. Electron injection has been confirmed by direct detection of electron in the conduction band, cation radical of the adsorbed dye and a bleach of the dye in real time as monitored by transient absorption spectroscopy in the visible and near-IR region. The dynamics of BET from TiO 2 to the parent cation has been measured by monitoring the recovery kinetics of the bleach of the adsorbed dye and it is found to be multiexponential. BET dynamics have been compared with bare (unmodified) nanoparticles for the same DBF/TiO 2 system. It has been observed that BET reaction is slow on the modified surface compared to on the bare surface in earlier time domain (picosecond). This observation has been explained by the fact that on surface modification the energy levels of the semiconductor nanoparticles shifts towards more negative. As a result, the free energy of reaction (-∆G0 ) for BET reaction of dye/SM-TiO 2 system increases as compared to the dye/bare-TiO 2 system. High exoergic BET reaction in dyesensitized TiO 2 nanoparticles surfaces fall in the Marcus inverted regime, so with increasing free energy of reaction, BET rate decreases on modified surface. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond). In longer time domain BET reaction is faster on the modified surface as compared to on the bare surface. Surface modification removes many of the deep trap states. Recombination dynamics between deep-trapped electron and parent cation is slow due to low coupling strength of BET reaction. As the number of deep-trapped electrons is more in bare particles, so the BET reaction in longer time domain is slow. *

Corresponding author. Email: [email protected]

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Introduction: Semiconductor nanoparticles have attracted significant interest in the past 15 years due to their potential applications in solar energy conversion, nonlinear optics and heterogeneous photo-catalysis1 . Surface modification of semiconductor nanoparticles can change their optical, chemical and photo-catalytic properties significantly1c. Surface modification of nanoparticles 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

nanoparticles2 , ii) it may increase the photo-stability of semiconductor

nanoparticles2 , iii) it may create new traps on the surface of the nanoparticles leading to the appearance of new emission bands3 , iv) it may boost the selectivity and efficiency of lightinduced reactions occurring on the surface of semiconductor nanoparticles1c,4. The performance of most semiconductor-containing devices is critically dependent on the electronic properties of the semiconductor surface band bending (Vs) and the surface recombination velocity (SRV)5 . These properties in turn depend on the density and energy distribution of surface states. As the surface state properties are controlled by the chemistry of the surface, much effort has been devoted to modify the surface states by chemical treatments6 . The use of suitable organic or organometallic molecules as surface treatments holds great promise for fine-tuning the desired surface electronic properties. Thus, a group that optimizes molecular binding to the surface, can be augmented with auxiliary groups, can provide control over molecular dipole moments, frontier orbital energy levels, light sensitization properties, hydrophilic/hydrophobic character, etc. Rajh et.al.7 have reported a new route to improve the optical response of nanocrystalline TiO 2 in the visible region. The approach involves direct electron transfer from ascorbate7b, mercaptocarboxylic 7c and enediol ligand7e modifier of TiO 2 into the conduction band of nanocrystalline TiO 2 particles. Chelation of surface Ti atoms with electron donating bidentate ligands in these systems changes the electronic properties of the nanocrystalline particles. Photo electrochemical solar cells based on dye-sensitized TiO 2 films have received much attention in recent years because of their potential applications as a cost-effective alternative to silicon-based cells 1a. Since the report by Gratzel’s group that solar cells based on Ru(dcbpy) 2 (NCS)2

[dcbpy

(4,4′-dicarboxy-2,2′-bipyridine)]

(or

Ru

N3)-sensitized

nanocrystalline TiO 2 thin films can achieve a solar to electric power conversion efficiency of

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about 10%,1a electron injection and recombination properties of dye-sensitized semiconductors have been studied by us 8 and many groups9 . It is necessary to establish conditions of both fast electron injection and slow recombination for an efficient solar energy conversion. However, the detailed mechanisms, the nature and the rate of electron injection into the semiconductor, and factors determining the rate of back electron transfer are not well understood. Interfacial electron transfer kinetics for TiO 2 nanoparticle has been monitored mostly in aqueous medium8, 9a, 9d

and in thin film9b,

9c, 9e

. Only few examples of electron transfer processes in ethanol10 and

acetonitrile10 medium for dye-sensitized TiO 2 nanoparticles are reported in the literature. Effect of solvents and solvent polarity on electron transfer dynamics in dye sensitized TiO 2 nanoparticles has not been reported in the literature. In order to explore the above said effects, there is a need to synthesize TiO 2 nanoparticles, which can be dispersed in many organic solvents. In the present investigation we have studied electron injection and back electron transfer (BET) reaction of di-bromo fluorescein (DBF) sensitized bare and sodium dodecyl benzene sulphonate (DBS) modified TiO 2 nanoparticles using transient absorption techniques detecting in the picosecond and microsecond time domain. BET dynamics of both the systems have been compared in both the time domains. The electron injection to both bare and surface modified TiO 2 nanoparticles by DBF have been confirmed by transient absorption studies, where conduction band electron in the nanoparticles have been detected in the visible region and near IR region. BET rate constants have been measured from the bleach recovery kinetics. It has been observed that BET rate is slower for DBF sensitized surface modified TiO 2 nanoparticles compared to the bare one. BET rates have been found to be multi exponential and can be fitted with time constants of 170 ps (28.4%) and > 5 ns (71.6%) for the surface modified TiO 2 nanoparticles in chloroform and 72 ps (13.9 %), 1.54 ns (46.5%) and > 5 ns (39.6 %) for the bare TiO 2 nanoparticles in water. Slow BET for the case of surface-modified TiO 2 can be explained on the fact that free energy (-∆G0 ) is increased on surface modification compared to that of bare one for DBF/TiO 2 systems. On surface modification pinning of the Fermi level of semiconductor takes place, where the energy levels (conduction band edge, Fermi level, shallow and deep trap state) of semiconductor nanoparticles moves towards more negative direction. As a result, the free energy (-∆G0 ) of BET reaction increases. BET reaction for the DBF/TiO 2 system comes under the Marcus inverted regime. Hence, BET reaction becomes slow on the

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modified TiO 2 surface compared to the bare one. To follow BET dynamics in longer time domain we have carried out transient absorption experiments in microsecond time domain. BET dynamics in the longer time scale can also be fitted with multiexponential time constants 1.55 µs (68.6%) and > 20 µs (31.4%) for the modified TiO 2 and found to be faster compared to the bare TiO 2 which recombines with time constants 3.29 µs (66.5%) and > 20 µs (32.5%). The reversible trend in BET has been explained in the following way. In case of bare TiO 2 density of deep trap state are much higher compared to that of modified TiO 2 due to the pinning effect of surface modification12 . The HOMO of the surface modifier molecules interacts with the unfilled deep surface states. The density of deep trap states will be decreased in the case of modified TiO 2 nanoparticles due to surface passivation. Recombination reaction (BET) is much slower for the electrons in deep trap state and the parent cation due to lower coupling matrix for BET reaction. As a result, BET reaction is slower in the longer time domain for DBF-sensitized bare TiO 2 nanoparticles compare to the modified one.

2. Experimental Section: a) Materials: Di-bromo fluorescein was obtained from Aldrich and was used without further purification. Titanium (IV) tetraisopropoxide {Ti[OCH(CH3 )2 ]4 } (Aldrich, 97%) and isopropyl alcohol (Aldrich) were purified by distillation. Dodecyl Benzyl sulphonic acid (DBS) was obtained from Aldrich. Chloroform (CHCl3 ), di-methyl formamide (DMF), pyridine were obtained from Spectrochem India Ltd. and were used without further purification. b) Nanoparticle preparation: We have synthesized DBS-capped TiO 2 nanoparticles, and reported earlier13 . Briefly, in a 500mL of freshly prepared TiO 2 colloids in water synthesized as reported earlier8,14, 250 mL of toluene was added in a round bottom flask. The resulting mixture was stirred slowly for 15-20 minutes. In the stirred solution, 100 mL 0.2M DBS (C 12 H25 C6 H4 SO3 Na, sodium dodecyl benzene sulphonate) was added and the final mixture had been stirred slowly for 3 hours. DBS can be dissolved only in water, because of its ionic nature. As the surface of the TiO 2 nanoparticles is positively charged, DBS molecules can easily bind through the sulphonic group (SO3 -) with the nanoparticles. The newly capped TiO 2 nanoparticles looks like a reverse micelle and can be dispersed in many organic solvents. In this situation, TiO 2 nanoparticles migrate from water to the organic phase (toluene). With the help of a separating funnel the organic phase was separated out. At this stage the organic phase looked

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little cloudy. The organic phase was dried in CaCl2 and transformed to an optically clear solution. The organic phase was then refluxed for 2 hours and the solvent was taken out with the help of a rotary evaporator in N2 atmosphere. Dry TiO 2 particles capped by DBS, which were left in the flask, could be dissolved in many non-aqueous solvents to get colloidal solution in that particular solvent. To prepare sensitized nanoparticles, DBF was added to TiO 2 colloid and sonicated for 1 minute. For all the measurements the sample solutions were de-oxygenated by continuously bubbling high purity nitrogen (99.95 Iolar grade from Indian Oxygen Co. Ltd., India) through the solutions. The solutions were flowed through a 1cm X 1cm quartz cell during all the measurements. c) Picosecond visible Spectrometer: Picosecond laser flash photolysis experiments were carried out using a pump-probe spectrometer, described elsewhere.15 Briefly, the second harmonic output (532 nm, 8 mJ, 35 ps) of an active-passive mode locked Nd:YAG laser (Continuum, USA, model 501-C-10) was used for the excitation of the samples. The transients produced in the irradiated samples were detected by their optical absorption. A white light continuum (~400 to 950 nm) produced by focusing the residual fundamental (1064 nm) of the Nd:YAG laser onto a 10 cm length quartz cell containing 50:50 (v/v) H2O-D2O mixture was used as the monitoring light source. The probe light was passed through a variable optical delay line (1 m long) and then split into two parts using a 50:50 beam splitter. One part of the monitoring light was used as the reference beam and the other was used as the analyzing beam (passing through the irradiated sample). Both the reference and the analyzing beams were dispersed through a spectrograph and monitored using a dual diode array based optical multi channel analyzer, which is interfaced to a personal computer to process the data. Time-resolved experiments in the microsecond time domain were carried out using the same picosecond laser for excitation, a tungsten lamp as analyzing light source, a Bausch & Lomb monochromator (0.20 m), a PMT (Hamamatsu R-928) and a digital oscilloscope (Tektronics TDS-500, 500 MHz bandwidth).

3. Results and Discussion: a) Dye – Nanoparticles Interaction:

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To study the dye sensitized electron transfer reaction in the excited state, it is very important to know the type of interaction between the dye and the nanoparticles. Reports are available on interfacial ET dynamics for dye-sensitized TiO 2 nanoparticles mainly in aqueous solution8 and on thin film9b,

c, e

. In the present investigation, we have carried out dye sensitization experiments

in some non-aqueous solvents for the surface-modified (SM) TiO 2 nanoparticles. The steadystate optical absorption spectra of di-bromo fluorescein (DBF) in chloroform and in SM TiO 2 nanoparticles colloidal solution are shown in Figure 1. DBF molecule shows optical absorption up to 560 nm with a peak at 480 nm in chloroform (Fig 1a). On addition of SM TiO 2 nanoparticles, the optical density of DBF increases and is red shifted. At high concentration of the SM TiO 2 nanoparticles (10 g/L) the optical absorbance goes beyond 640 nm with a peak at 500 nm (Fig 1b). We have also compared the dye-nanoparticles interaction for the bare TiO 2 nanoparticles in aqueous solution. The optical absorption spectra of DBF in water show optical absorption up to 560 nm with a peak at 480 nm. Here also dye-nanoparticles interaction is quite strong. The optical absorbance of DBF goes beyond 640 nm with a peak at 500 nm at high TiO 2 nanoparticles concentration (not shown in Fig 1). In aqueous solution measurements, we have kept the pH at 2.8. We have already explained that xanthene dyes8b (including DBF) have good interaction with TiO 2 nanoparticles. To find out the dye-nanoparticles interaction in the excited state we have carried out steady-state fluorescence experiments for the above systems. We have observed in our earlier studies8b that DBF gives a broad (450-650 nm) emission with a peak around 560 nm. In the presence of TiO 2 nanoparticles the emission quantum yield of DBF was drastically reduced. This was attributed to the electron injection from the excited-state of the dyes to the conduction band of the nanopareticle. We have also carried out similar measurements for SM TiO 2 nanoparticles in chloroform. On excitation, DBF molecules in chloroform give an emission band (450-650 nm) with a peak at 550 nm. But in the presence of SM TiO 2 nanoparticles in chloroform the emission quantum yield was drastically reduced. This reduction in emission quantum yield has been attributed to the electron injection from DBF to SM TiO 2 nanoparticles.

b). Electron Injection and Charge Recombination: It has been demonstrated by us 8 and many workers9 that on optical excitation of xanthene dye molecules adsorbed on colloidal and thin film TiO 2 surface, electrons are injected

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into the conduction band of the nanoparticles. Electron injection in TiO 2 colloidal solution, sensitized by the eosin dye excited at 532 nm laser light was carried out by Moser et.al. 16 . Ultrafast electron injection and recombination dynamics in fluorescein-27 sensitized TiO 2 nanoparticles has been observed by Sundstrom el.al9c,

17

. Recently Pelet et.al18 have studied

ultrafast ET dynamics of Eosin-sensitized metal oxide nanoparticles (TiO 2 , ZrO 2 and Al2 O3 ). In the present investigation, we have carried out picosecond laser flash photolysis experiments exciting at 532 nm for DBF sensitized surface modified TiO 2 nanoparticles in different nonaqueous solvents and bare TiO 2 nanoparticles in water to follow the interfacial electron transfer dynamics on the semiconductor surface. Figure 2 shows the transient absorption spectra of dibromo fluorescein (DBF) sensitized SM TiO 2 nanoparticles in chloroform from 470 – 700 nm wavelength region at different time delay. A bleach (negative absorption) in the spectral region 510-560 nm centered around 530 nm and a broad positive feature in the spectral region 570-700 nm have been observed. The negative absorption change has been attributed to the bleaching of the ground state of the dye upon excitation by the laser pulse. The broad spectral absorption in the 570-700 nm region is attributed to the conduction band electrons in the nanoparticles. It has already been shown by many workers that the conduction band electrons can be detected both by visible8,

9e

and infrared absorption9a, b. However, a small positive absorption appears below

510 nm with a peak at 500 nm can be unambiguously assigned to the oxidized state of DBF. The optical density of the cation peak is small due to the ground state absorption of the dye molecules in that region. To confirm the transient absorption peak for the cation radical we have carried out the pulse radiolysis experiment of DBF in water saturated with N2 O in the presence of NaN 3 19 . Under such experimental conditions, only the cation radical of DBF is produced following pulse radiolysis. Transient absorption spectra obtained from the above solution show a strong absorption peak at around 500 nm and a small peak at 400 nm. To ensure that the observed transient absorption spectra are due to the photo-excitation of dye-sensitized surface modified (SM) TiO 2 colloid, experiments with unsensitized SM TiO 2 /chloroform and dye/chloroform were performed. We would like to point out at this point that the solubility of DBF molecule (since it is a sodium salt) in chloroform is negligible in the absence of SM nanoparticles. So we can conclude that in the experimental solution all the dye molecules are adsorbed quantitatively on the nanoparticles surface. It should be pointed out that the ground-

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state UV/Vis spectra of SM TiO 2 colloid in chloroform show no absorbance at 532 nm. In the present investigation electron injection process can be given by the following scheme. hν ν

TiO2 + DBF ⇔ [TiO2 –DBF]Adsorb

e -cb(TiO2 ) + DBF +

(1)

Electron injection

Where, DBF + is the cation radical of di-bromo fluorescein dye and e-cb is the conduction band electron in TiO 2 nanoparticles. In the present investigation we could not monitor the cation radical properly due to low intensity of the probe light in the region of cation absorption and also due to the huge ground-state absorption of the dye molecule. However, the bleach at 530 nm recovers (Fig 2 Inset) and is due to the back reaction involving recapture of the conduction band electrons by DBF cation radical. e -cb(TiO2 ) + DBF +

Recombination

[TiO2 –DBF]Adsorb

(2)

The recovery of the observed bleach can be fitted by a multi-exponential function with time constants of 172 ps (28.4%) and > 5 ns (71.6%), although upto 6 ns (our maximum time limit) only 70 % of the bleach recovers. In our earlier reports8b we have observed the recombination dynamics of DBF sensitized bare TiO 2 nanoparticles in water is multiexponential with typical time constants of 72 ps (13.9%), 1.54 ns (46.5%) and >5 ns (39.6%) as measured by time-resolved picosecond absorption spectrometer. It has been observed above that more time constants are required to fit the decay in the case of bare particles compared to the modified one in the same time domain (up to 6 ns). This may be due to more heterogeneity of bare nanoparticles due to more number of surface states. We have compared the recombination dynamics in Fig 3 for bare and modified TiO 2 nanoparticles sensitizing by DBF molecules. It is interesting to see that recombination reaction is slow for DBF/TiO 2 nanoparticles system on the modified surface. We have also carried out sensitization experiments for the DBF and SM-TiO 2 by dispersing the nanoparticles in different non-aqueous solvents by using picosecond flash photolysis. Picosecond time-resolved spectra have been obtained for all the above systems in 500-800 nm wavelength region. In all the above cases bleach around 500-570nm and a broad absorption band in 570-800 nm wavelength region for the electron have been observed. Bleach recovery dynamics has been determined at the corresponding bleach peak and has been compared in Table 1.

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To compare the recombination dynamics in longer time scale for DBF/TiO 2 systems both for bare and SM particles we have carried out flash photolysis experiments in the microsecond time domain. The bleach recovery kinetics of the above systems has been shown in Fig 4 monitoring at 510 nm. We have observed that the recombination reaction can be fitted multi-exponentially with typical time constants of 1.55 µs (58.6%) and 10 µs (41.4%) for surface modified nanoparticles in chloroform (Table 1). We have also carried out recovery kinetics for DBF/TiO 2 system on bare surface. Here also we have observed that the recombination kinetics can be fitted multi-exponentially (Table 1). It is interesting to see that the recombination reaction is slow for the DBF/TiO 2 system on bare surfaces compared to the modified one (Fig 4). We can observe a reversible trend in recombination dynamics for the above systems in picosecond and microsecond time domains. We have carried out bleach recovery kinetics on modified surface by changing the solvent and observed that the recombination dynamics were very similar (Fig 4, Table 1). The detailed mechanisms of back electron transfer reaction for the above systems have been discussed in the following section.

c). Mechanism of Charge Recombination Reaction: According to the semi classical theory20 formulation for the back electron transfer (BET) rate constant k BET is given in equation 3 as

k BET where

 2π  2 = [H AB ]  h 

 ( ∆G 0 + Λ ) 2  exp −  4ΛkT 4πΛkT   1

(3)

Λ is the total reorganization energy, HAB is the coupling element, ∆G0 is the overall free

energy of reaction = (EC – ES/S+), EC is the potential of electrons in the conduction band of the semiconductor (-0.49V)21 , ES/S+ is the redox potential of the adsorbed dye (Scheme I). In the present investigation we have compared BET dynamics for the DBF/TiO 2 system for bare nanoparticles with the modified one. As they are same dye-TiO 2 nanoparticle systems, we can imagine that

Λ, the total reorganization energy and HAB, the coupling element will be same for

the systems studied. In such a case, kBET will depend on the overall free energy of reaction (∆G0 ). We have observed that recombination reaction (BET) between the injected electron and the parent cation is faster on the bare nanoparticle surface compared to the modified one. The results can be explained by invoking a model in which the energy levels in the semiconductor

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nanoparticles are shifted due to surface modification (Scheme 1). Dimitrijevic et.al.22 have observed that the position of the Fermi level in modified TiO 2 colloids shifted by at least negative 0.1 V compared to the unmodified one. It has been reported previously by Ellis et.al. 6ad

and Natan et.al. 6e that strong adsorption of negative counter ions on the electrode surface

shifted the flat band potentials (Vfb) to more negative values. Yan and Hupp23 have reported that with increasing pH on the semiconductor electrode surface, the flat band potentials (Vfb) move towards more negative values. In the present investigation, DBS molecule (modifier) adsorbed strongly to the nanoparticles through sulphonic acid group (SO3 H-, negatively charged). So on surface modification the different energy levels of the semiconductor nanoparticles like deep trap states, shallow trap states, Fermi level and conduction band edge will be changed and will move towards more negative due to interaction with the modifier molecule. As the conduction band edge of the modified colloids shift towards more negative, the overall free energy of BET reaction of DBF/SM-TiO 2 will be increased compared to the DBF/TiO 2 system (-∆G2 0 >-∆G1 0 Scheme I). According to Marcus electron transfer theory ET rates ultimately decrease with the increasing thermodynamic driving force (-∆G0 )20 . This high exoergic region is often termed as “inverted regime”. Back electron transfer processes in dye-sensitized TiO 2 nanoparticles surfaces fall in the Marcus inverted regime for its high free energy of reaction8b,

9d, 24

. In this

region, with increasing driving force (-∆G0 ) of reaction, the rate of BET decreases. As a result, BET rate on the modified surface is slower compared to the bare one. We have carried out dye sensitization experiments on surface modified nanoparticles in different solvents to see the effect of di-electric constant of the medium on BET kinetics. However we have not observed much difference in BET rate constants (Table 1) on the modified surface with changing the polarity of the solvents. On the modified surface, the surfactant molecules are strongly adsorbed on the surface13 . As a result, the solvent molecules cannot approach the nanoparticles surface quantitatively. So the effective polarity on the surface is different from that of the bulk. The actual polarity on the nanoparticles surface might not be changing very much with solvent polarity. As a result, the total reorganization energy Λ may change marginally. So kBET on the modified surface will depend mostly on free energy (-∆G0 ) of the reaction. In the present investigation we have observed little difference in recombination dynamics (BET) in different polar solvents on modified nanoparticles surface both in pico and microsecond time domains (Table 1).

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It is interesting to follow the recombination dynamics of DBF/TiO 2 system in longer time domain (microsecond) for the bare and surface-modified nanoparticles. BET dynamics was found to be multiexponential with time constants of 3.29 µs (66.5%) and > 20 µs (32.5%) for bare TiO 2 in water and 1.55 µs (68.6%) and > 20 µs (31.4%) for surface modified nanoparticles in chloroform. On a modified surface, the injected electron and the parent cation recombine faster than that on a bare surface in longer time domain (microsecond). We have already observed (previous paragraph) that the recombination dynamics of DBF/TiO 2 system is faster on the bare surface compared to that on the modified surface in shorter time scale (picosecond). This reversible trend in BET dynamic has been explained in the following way. On surface modification the orbital interaction takes place between HOMO of the surface modifier molecules and the unfilled deep surface states (act as LUMO)6c. On interaction, energy level of LUMO goes up and HOMO goes down. So energy levels of the surface states move toward more negative. As a result the density of deep trap states decreases (Scheme I). Energetically shallow trap states are higher in energy compared to the deeper one. So, on surfacemodification, interaction between the modifier molecular orbital and the deep trap states is much more compared to that with the shallower one. As a result, the density of the deep trap states decreases dramatically compared to that of the shallow trap states. So, after injection, the injected electrons are relaxed to shallow trap states in longer time scale in the case of modified surface. However, in the case of bare particles, more number of the injected electrons will reside in the deep trap states. It has been observed by us 8c,

8e

and Moser et.al.25 that the recombination

reaction (BET) is much slower for the deep trap state electron and the parent cation due to low coupling matrix for BET. As a result BET reaction is slower for the case of DBF-sensitized bare TiO 2 nanoparticle compared to the modified one in microsecond time domain. Although free energy (-∆G0 ) of BET reaction on modified particles favors slow recombination as we have observed in early time domain, but weak coupling of deep trapped state electrons slow down the recombination dynamics on the bare nanoparticles surface. The results can also be explained by adopting the model as suggested by Durrant et.al26 and Tachiya et.al.27 for slow recombination reaction in dye-sensitized electron transfer process. According to their model, before recombination with the parent cations, the electrons can diffuse out some distance on the surface of the nanoparticles before getting trapped in a particular trap state. Under that condition, the electrons are apart from the parent cations so that the electronic

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coupling is negligibly small and recombination through tunneling cannot take place. But the trapped electron can move assisted by thermal energy, namely they can move through trap and de trap process. So the diffusion of the injected electrons from trap to trap may take place. As we know that the density of trap states in the surface modified nanoparticles are less compared to the bare one, so the probability of diffusion from one trap to another trap will be less. As a result the recombination will be faster on modified particle surface. In the present investigation we have observed the presence of electrons at longer time (>20 µs) in surface modified nanoparticles. This indicates the presence of some deep trap states even in the surface modified nanoparticles. This is because through surface modification we could not remove the deep trap states quantitatively. We have also carried out bleach recovery kinetics in the longer time domain for the modified particles in different solvents and observed marginal difference in recombination dynamics (Table 1). Recently we are carrying out dyesensitized electron transfer on modified nanoparticles surface by changing the modifier molecule. We are on the process to find a suitable modifier molecule, which can remove the surface states of a nanoparticles more efficiently.

5. Conclusion: Pico and microsecond transient absorption spectroscopy have been carried out to study the effect of surface modification on electron injection and back electron transfer (BET) dynamics of di-bromo fluorescein (DBF)-sensitized TiO 2 nanoparticles capped (modified) with sodium dodecyl benzene sulphonate (DBS). Electron injection has been confirmed by direct detection of electron in the conduction band, cation radical of the adsorbed dye and a bleach of the dye in real time as monitored by transient absorption spectroscopy in the visible and near-IR region. Charge recombination (BET) dynamics have been measured by monitoring the bleach recovery kinetics of the adsorbed dye at 530 nm and found to be multi-exponential. BET dynamics have been found to be slow for modified particles compared to the bare one in the earlier time domain (pico and nanosecond). On surface-modification, the flat band potential of the nanoparticles shifts toward more negative. As a result, the free energy (-∆G0 ) of reaction increases. BET reaction in DBF/TiO 2 system falls in the inverted regime of ET reaction, where with increasing free energy, the BET rate decreases. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond).

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Where BET reaction is faster for modified particles compared to the bare one. 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 BET reaction. So BET reaction is found to be slow on bare particle surface compared to the modified one.

Acknowledgment: We are thankful to Dr. T. Mukherjee and Dr. J.P. Mittal for encouragement.

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X.; Hupp, J.T. J. Am. Chem. Soc. 1999, 121, 8399. (e) Hannapel, T.; Burfeindt, B.; Storck, W.; Willig, F. J.Phys. Chem. B 1997, 101, 6799. (10) Kamat, P.V.; Fox, M.A. Chem. Phys. Lett. 1983, 102, 379. (12) Bard, A.J.; Bocarsly, A.B.; Fan, F.F.; Walton, E.J.; Wrighton, M.S. J. Am. Chem. Soc. 1980, 102, 3671. (13) Ramakrishna, G.; Ghosh, H.N. Langmuir. 2003, 19, 505. (14) Ghosh, H.N.; Adhikari, S. Langmuir 2001, 17, 4129. (15) Ghosh, H.N.; Pal, H.; Sapre, A.V.; Mittal, J.P. J. Am. Chem. Soc. 1993, 115, 11722. (16) Moser, J.E.; Gratzel, M.; Sharma, D.K.; Serpone, N. Helv.Chim.Acta. 1985, 68, 1686. (17) Hilgendroff, M.; Sundstrom, V. J. Phys. Chem. B. 1998, 102, 10505. (18) Pelet, S.; Gratzel, M.; Moser, J.E. J.Phys.Chem. B 2003, 107, 3215 (19) Ramakrishna, G.; Ghosh, H.N. Supporting Data. (20) Marcus, R.A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (21) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (22) Dimitrijevic, N.M.; Savic, D.; Micic, O.I.; Nozik, A.J. J.Phys.Chem. 1984, 88, 4278. (23) Yan, S.G.; Hupp, J.T.; J.Phys.Chem. 1996, 100, 6867. (24) Lu, H.; Prieskorn, J.N.; Hupp, J.T.; J. Am. Chem. Soc. 1993, 115, 4927. (25) Moser, J. E.; Gratzel, M. Chem. Phys. 1993, 176, 493. (26) (a) Haque, S.A.; Tachibana, Y.; Willis, R.L.; Moser, J.E.; Gratzel, M.; Klug, D.R.; Durrant, J.R. J. Phys. Chem. B. 2000, 104, 538. (b) Nelson, J.; Haque, S.A.; Klug, D.R.; Durrant, J.R. Phys. Rev. B. 2001, 63, 205321. (27) Barzykin, A.V. Tachiya, M. J. Phys. Chem. B . 2002, 106, 4356.

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Legends of Schemes and Figures Scheme I. Mechanistic scheme of electron transfer (ET) and effect of surface modification on ET in di-bromo fluorescein (DBF) sensitized bare (left) and modified (right) TiO 2 nanoparticles. Fermi level pinning is seen due to surface modification on TiO 2 nanoparticles (right). The energy levels of the modified nanoparticles shifts toward more negative and effective free energy (-∆G0 ) increases. On surface modification, deep trap states are passivated.

Figure 1: Optical absorption spectra of (a) di-bromo fluorescein (DBF) dye in sodium dodecyl benzene sulphonate (DBS)/chloroform solution and (b) DBF sensitized surface-modified TiO 2 nanoparticles (10gm/L) in chloroform.

Figure 2: Transient absorption spectra of di-bromo fluorescein (DBF) sensitized surface modified (SM) TiO 2 nanoparticles in chloroform at (a) 0, (b) 66, (c) 132, (d) 330ps (e) 1.32, (f) 2.31, (g) 3.63 and (h) 5.28ns after excitation at 532 nm. The spectrum at each time delay consists of a small positive peak at 500 nm, a bleach at 510-560 nm region centered around 530 nm and a broad positive absorption feature in the whole spectral region (570 - 700 nm). These features are assigned to the cation radical and the ground state bleach of di-bromo fluorescein dye and injected electron in the nanoparticles. [Inset: Kinetic trace of the bleach recovery at 530 nm].

Figure 3: Comparison of bleach recovery kinetics of DBF-sensitized TiO 2 nanoparticles at 530 nm a) bare b) modified particles as measured by transient picosecond flash photolysis.

Figure 4: Comparison of bleach recovery kinetics of DBF-sensitized TiO 2 nanoparticles at 510 nm a) bare b) modified particles as measured by transient microsecond flash photolysis.

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Table 1:Bleach recovery kinetics of DBF sensitized bare and surface modified (SM) TiO2 nanoparticles in different solvents after exciting with 532 nm laser light. System/Solvent Bare TiO2 /Water

SM-TiO2 / CHCl3 SM-TiO2 / Pyridine SM-TiO2 /DMF

Life time of the transient in Life time of the transient in shorter time (ns) longer time (µ µ s) τ 1 = 0.072(13.9%) τ 2 = 1.54 (46.5%) τ 3 > 5.00 (39.6%) τ 1 = 0.172 (28.4%) τ 2 > 5.00 (71.6%) τ 1 = 0.156 (68.3%) τ 2 >5.00 (21.7%) τ 1 = 0.221 (62.3%) τ 2 > 5.00 (37.7%)

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τ 1 = 3.29 (66.5%) τ 2 > 20 (33.5%) τ 1 = 1.54(68.8%) τ 2 > 20 (31.2%) τ 1 = 1.94(75.8%) τ 2 > 20 (24.2%) τ 1 = 1.76(74.1%) τ 2 > 20 (25.9%)

SM TiO2

TiO 2 e-

e-

ES*/S+

C.B.

C.B. Ef

Shallow SS

Shallow SS Deep SS

kBET

kBET

−∆G1

Ef

−∆G2

ES/S+ V.B.

V.B.

Scheme 1: Ramakrishna, G. et.al.

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Absorbance (a.u)

0.4

b

0.3

a

0.2

0.1

0.0 400

500

600

700

Wavelength (nm)

Figure 1: Ramakrishna, G. et.al.

19

0.1

0.00 -0.05

-0.1

h

∆A

∆A

0.0

-0.10

τ1=172 ps τ2> 5 ns

-0.15

-0.2

a

-0.20 0

1500

3000

4500

Delay time (ps) 500

550

600

650

700

Wavelength (nm)

Figure 2: Ramakrishna, G. et.al.

20

0.00 -0.05

a

∆A

-0.10

b

-0.15 -0.20 -0.25 0

1

2

3

4

5

time (ns)

Figure 3: Ramakrishna, G. et.al.

21

∆ A (m O.D.)

0.0

b

-1.0 -2.0

a

-3.0 -4.0 -5.0 0

5

10

15

Delay Time (µs)

Figure 4: Ramakrishna, G. et.al.

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