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Excitation-Dependent Ultrafast Carrier Dynamics of Colloidal TiO2 Nanorods in Organic Solvent Leonardo Triggiani,†,‡ Adalberto Brunetti,§ Antonio Aloi,§,∥ Roberto Comparelli,‡ M. Lucia Curri,‡ Angela Agostiano,†,‡ Marinella Striccoli,*,‡ and Raffaele Tommasi*,§,‡ †

Department of Chemistry, University of Bari Aldo Moro, Via E. Orabona 4, 70125 Bari, Italy CNR-IPCF, Bari Division, c/o Department of Chemistry, University of Bari Aldo Moro, Via E. Orabona 4, 70125 Bari, Italy § Department of Basic Medical Sciences, Neuroscience and Sense Organs, University of Bari Aldo Moro, Piazza G. Cesare 11, 70124 Bari, Italy ‡

ABSTRACT: The relaxation dynamics of charge carriers of organic capped TiO2 nanorods dispersed in chloroform was investigated by femtosecond transient absorption in a weakexcitation regime. Anisotropic TiO2 nanocrystals were excited in the UV−vis range, using different pump wavelengths, namely above (300 nm), close to (350 nm), and below (430 nm) the direct band gap of anatase TiO2. We show that the ultrafast dynamics strongly depends on excitation wavelength and determine the time constants of all the processes entering the relaxation. Moreover, we demonstrate that two transient absorption bands at 500 and 700 nm, typically attributed to trapped h+ and e−, respectively, are accessible only when TiO2 is photoexcited well above the band gap, while there is no evidence of such bands when TiO2 is photoexcited close to or below its band gap. In such cases the observed dynamics are attributed to trapped excitons.

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INTRODUCTION Titanium dioxide (TiO2) is a very interesting material, widely used for energy conversion devices thanks to the opportunity to generate electron−hole (e−−h+) pairs under proper optical irradiation. Indeed, TiO2 nanocrystals are successfully exploited in applications related to energy conversion, such as photocatalysis1−6 and photovoltaics.7−12 The efficiency of TiO2 in such processes is strongly correlated to the lifetime of the photogenerated species before their recombination, which can occur either in the bulk of the material or at surface sites. Once charge carriers are photoexcited in a nanoscale semiconductor, they may undergo a number of processes competing with the more common two-particle recombination. For instance, e− (h+) can migrate toward the surface of the nanoparticle where, if they have appropriate energy, they can reduce (oxidize) surrounding species.2,3,5,13 This circumstance represents the conditio sine qua non for exploitation of the photogenerated charges in photocatalysis. Moreover, despite its indirect band gap, TiO2 shows a relatively weak broadband photoluminescence in the visible range, with this characteristic attributed to the formation of excitons localized on TiO6 octahedra or on defects. In TiO2, distortion of the crystalline structure influences the coordination number of octahedra and, in turn, the nature of excitons: in rutile the octahedra are almost undistorted and the excitons are considered f ree; in anatase, instead, the distortion is much higher, and charge carrier pairs are localized in the octahedral sites, originating the so-called self-trapped excitons.14−17 Defect states, as surface and subsurface oxygen vacancies or Ti3+ states, © 2014 American Chemical Society

can also serve as traps for electrons and holes, hosting excitons themselves.18−22 Although the major evidence of exciton formation in anatase arose from TiO2 luminescence measurements,23−27 some authors also related transient absorption features to trapped carriers in bound states.28−30 In nanoscale systems it is usually very difficult to discern between the two kinds of excitons, so here the bound states will be referred to as trapped excitons. Hence, after excitation, charge carriers can follow, in principle, different pathways. The route toward equilibrium is influenced by several factors, with excitation energy playing a major role, as it defines the initial potential of the carriers. In this perspective a great interest in elucidating the relaxation dynamics of charge carriers photogenerated with different excess energies can be envisaged. We report here a comprehensive ultrafast spectroscopic investigation of organic-capped TiO2 rod-like nanocrystals dispersion in chloroform, synthesized by a colloidal chemistry route able to tailor size/shape and crystalline phase of the nanoparticles. TiO2 nanorods (NRs) were synthesized in anatase form with size of about 3 nm in diameter and 22 nm in length, with their surface coordinated by a layer of oleic acid (OLEA) molecules, that make them promptly dispersible in a range of organic solvents, providing optically clear samples. Such type of nanomaterial is particularly relevant as, due to the specific surface chemistry, it can be placed in virtually any Received: July 23, 2014 Revised: October 1, 2014 Published: October 8, 2014 25215

dx.doi.org/10.1021/jp507383w | J. Phys. Chem. C 2014, 118, 25215−25222

The Journal of Physical Chemistry C

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Furthermore, elucidation of the relaxation dynamics in such anisotropic TiO2 nanocrystalline structures can be extremely useful to understand their reactivity and, thus, to further rationalize their behavior in photocatalytic and energy conversion applications.

chemical and physical environment and promptly processed for different technological applications.31−33 In addition NRs of TiO2 were already studied as photocatalytic materials, showing an extraordinarily enhanced efficiency when compared with their spherical counterparts, as demonstrated in photocatalytic metal reduction processes,34−37 due to their larger surface/ volume ratio and the greater availability of active surface sites. TiO2 NRs were characterized by steady state absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The e−−h+ pair recombination dynamics of the nanocrystals was extensively investigated by means of ultrafast transient absorption (TA) spectroscopy. Femtosecond TA spectroscopy was demonstrated to be an extremely effective tool with unique capabilities for elucidating the e−−h+ recombination dynamics of nanocrystals. It has been usefully applied in many studies on charge carrier dynamics in nanocrystalline TiO2,28,29,38−41 as it allows for very high temporal resolution and for identification of chemical species generated in short transients. Recently, the influence of metal doping on the dynamics of carriers in TiO2 was also investigated by means of TA spectroscopic techniques.42 However, most of these studies were carried out on thin nanocrystalline films rather than on clear, stable dispersions, thus leaving still uncovered different aspects of singlenanoparticle ultrafast dynamics. TA spectra of electrons and holes photogenerated in anatase TiO2 were reported to be very broad, extending over the whole visible and in the near-infrared regions,29,43,44 and relaxation dynamics of charge carriers were studied through the temporal evolution of TA. It is worth noting that insights relevant for photocatalytic and energy conversion applications, which exploit sunlight illumination, can be achieved only carrying out the investigation in a weak-excitation regime, i.e., using pulse intensity low enough to avoid second order hole− electron recombination.29,45−47 Indeed, weak excitation conditions allow the generation of one single electron−hole pair in each particle at most, and either geminate recombination can occur or the carriers can migrate at the surface; otherwise, under more intense excitation, multiple pairs are generated in each particle, and recombination is likely to follow secondorder kinetics. The latter case gives rise to more complicated TA dynamics at the picosecond time scale, and actual mechanisms behind these processes are not fully understood.40 Furthermore, when several carriers per particle are generated, the occupancy degree of deep traps grows, and the natural equilibrium between the population of deep and shallow traps is biased toward the former (trap-filling effect).48−51 As a consequence, in such an unbalanced system, the untrapped carriers show an enhanced effective mobility; therefore, the observed dynamics is faster and hardly predictable. In this work, the excitation energy dependence of carrier relaxation was investigated by selecting three pump wavelengths in UV−vis range, namely 300, 350, and 430 nm, corresponding to excitation above, close to, and below, respectively, the direct band gap of bulk anatase TiO2 (359 nm).52 This kind of study, to the best of our knowledge, has never been performed on TiO2 nanocrystals in solution, yet it is expected to provide major information on charge carrier dynamics. Remarkably, unlike most of the literature reports in the field, here all TA measurements were done on clear, stable colloidal dispersions at room temperature and under atmospheric pressure, without deareation, thus accessing the dynamics of the nanocrystals in the most general conditions.

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EXPERIMENTAL SECTION Materials. All chemicals were used as received without further purification. Titanium tetraisopropoxide (TTIP, 98.9%) and trimethylamine N-oxide dihydrate (TMAO, 98%) were purchased from Aldrich, and oleic acid vegetable (OLEA, 78%) from Merck. Chloroform, methanol, and ethanol were of the highest purity available and also purchased from Aldrich. Synthesis of TiO2 NRs. Fast Hydrolysis Route. Rod-like anatase TiO2 nanocrystals were obtained by reacting TTIP with an excess of water, according to the protocol described by Cozzoli et al.53 Briefly, 35 g of OLEA was dried at 100 °C for 15 min under vigorous stirring. Under nitrogen flow, 8 mmol of TTIP was added, and the solution turned from colorless to pale yellow. At this stage, 2.5 mL of a 2 M aqueous base (TMAO) solution was rapidly injected into the reaction mixture. The solution was maintained in a closed system at 100 °C and stirred under mild reflux with water over 5 days to promote further hydrolysis and crystallization of the product. The reaction was stopped by removing the flask from the heating source, and a TiO2 NR powder was recovered by adding ethanol or methanol. The as-prepared TiO2 NRs were isolated by centrifugation and washed several times with ethanol, in order to remove excess surfactant, and then dispersed in chloroform for further characterization. Characterization. Transmission Electron Microscopy. TEM investigation was performed with a JEOL 1011 microscope operated at 100 kV. Samples were prepared by dropping a dilute dispersion of the nanocrystals in chloroform onto carbon-coated copper grids and then letting the solvent evaporate. Statistical analysis of nanocrystal size was performed by counting at least 200 nanoparticles for each sample. UV−vis Absorption Spectroscopy. UV−vis absorption measurements were carried out with a Varian Cary 5000 double grating spectrophotometer, equipped with a deuterium lamp for UV excitation and a tungsten halogen lamp for the visible region. Light detection was achieved thanks to a Hamamatsu R928 photomultiplier. FTIR-ATR Spectroscopy. Midinfrared spectra were acquired with a Varian 670-IR spectrometer equipped with a DTGS (deuterated tryglicine sulfate) detector. The spectral resolution for all experiments was 4 cm−1. For Attenuated Total Reflection (ATR) measurements the internal reflection element used was a single-bounce 2 mm diameter diamond microprism. Films were directly cast onto the internal reflection element, by depositing the dispersion of interest (3−5 μL) on the upper face of the diamond crystal and allowing the solvent to evaporate. Transient Absorption. Femtosecond pump and probe experiments were carried out by the usual noncollinear configuration at room temperature. A commercial diodepumped Ti:sapphire femtosecond oscillator (Mai Tai, Spectra Physics) produces 100 fs pulses at 78 MHz repetition rate. These pulses constitute the seed of a regenerative Ti:sapphire amplifier (Spitfire Pro, Spectra Physics), pumped by a Qswitched Nd3+:YLF laser (Empower, Spectra Physics) at a 1 kHz repetition rate, which emits 110 fs pulses of up to 3.2 mJ per pulse at 798 nm. The pulse duration can be measured by an 25216



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Figure 1. (A) Steady state absorption spectrum and (B) TEM micrograph of TiO2 NRs dispersed in chloroform.

intensity autocorrelator (Pulse Scout, Spectra Physics). A beam splitter divides the amplified laser beam in two parts (90% transmitted and 10% reflected). The transmitted pulses are sent to an optical parametric amplifier OPA (TOPAS, Spectra Physics) that provides 120 fs pulses tunable in a broad spectral range (290−2600 nm). The output of the OPA constitutes the pump beam sent into a broadband ultrafast transient absorption spectrometer (Helios, Ultrafast Systems). The pump beam passes through a depolarizer to cancel out orientational effects in the measured decay curves. Then, after crossing an optical chopper the pump beam is focused onto the sample on a spot of ≈1 mm in diameter. The weaker beam reflected by the beam splitter after the Ti:sapphire regenerative amplifier enters the transient absorption spectrometer where it is time delayed by using a variable optical delay line. Then it is focused onto a CaF2 crystal to generate a white light continuum (WLC) in a broad spectral range (450−750 nm). WLC is used as a probe and is focused within the pump spot at the sample. After passing the sample, the WLC is detected by using a fibercoupled CCD spectrometer. The measured pump−probe cross correlation was ∼160 fs full-width half-maximum. The samples were stirred using a magnetic microstirrer to prevent their photodegradation. Due to the chirp of the white light, the temporal overlap between pump and probe, that is time zero, is wavelength-dependent. To obtain the transient spectra the data have been corrected for the chirp; i.e., matrix data have been rescaled accounting for the bare solvent dynamics. The average signal-to-noise ratio was 103.

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Figure 2. FTIR spectra of TiO2 NRs and OLEA recorded in ATR mode in the range 4000−600 cm−1, showing typical stretching (ν) and bending (δ) vibrations of the samples. The spectra are shifted on the vertical axis for the sake of clarity.

to aliphatic C−H stretch vibrations (both antisymmetric and symmetric, νC−H) between 2920 and 2850 cm−1 and the presence of typical olefinic stretch (νC−H). As expected, in the spectrum of pure OLEA, the stretching vibration of CO moiety in carboxylic acid dimers stands out at 1710 cm−1 (νCO), while the broad OH stretching band (νO−H) is superimposed to the CH stretching region. The analogous νO−H band in TiO2 NRs is associated with surface titanol groups and correspondingly shifted toward higher wavenumbers. The absence of any CO stretching feature in the IR spectrum of TiO2 NRs, along with the intensity of the bands at 1523 and 1427 cm−1 (ascribable to antisymmetric and symmetric vibrations of carboxylate anions, νCOO−, respectively), suggests that OLEA molecules do coordinate surface Ti centers. Furthermore, the distance between the two νCOO− bands (Δν = 96 cm −1 ) is characteristic of a signal corresponding to bidentate ligands.57,58 Finally, a band associated with TiO2 lattice vibrations is clearly visible in the region below 1000 cm−1 of the NRs spectrum. Thus, the FTIR spectrum of TiO2 NRs reasonably accounts for the presence of oleate ions coordinated as bidentate ligands at nanocrystal surfaces. Ultrafast TA spectroscopy enables the investigation of temporal dynamics of absorption changes occurring upon femtosecond photoexcitation.59,60 The samples were excited using 300, 350, or 430 nm fs-laser pulses and, for each excitation wavelength, transient absorption changes were probed in the visible region of the electromagnetic spectrum,

RESULTS AND DISCUSSION

The reported colloidal chemistry route was used to prepare pure anatase TiO2 NRs surface-coordinated with OLEA molecules, which make them easily dispersible in organic solvents and provide optically clear samples. Figure 1A shows the steady-state absorption spectrum of the TiO2 NRs dispersed in chloroform. The typical indirect band-edge absorption is responsible for the sharp rise below 375 nm. The absorption tail extending in the visible region is mostly due to absorption from surface defect states54,55 and concentrationdependent scattering effects.56 Figure 1B reports a representative TEM image of the sample. Statistical analysis showed that TiO2 NRs are sized 22 ± 6 nm in length and 3.6 ± 0.5 nm in diameter. Figure 2 reports a comparison between the FTIR spectrum of colloidal NRs and that of the pure capping agent. Both the FTIR spectra of TiO2 NRs and of pure OLEA reveal signals due 25217



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stimulated emission (ΔA < 0), and photoluminescence (ΔA < 0).64 Under our experimental conditions, we do expect that neither stimulated emission nor ground state bleaching play any significant role. However, a small negative contribution to ΔA coming from photoluminescence of TiO2 nanocrystals is likely to occur when exciting at 300 nm, because of the strong optical absorption of anatase in the near-UV range.20,65 The signal observed at very long time delays can thus be ascribed to such a weak luminescence. The experimental data of TA dynamics can be fitted by using a convolution function C(t) between the instrumental response function R(t) and the system (TiO2 NRs) response function S(t):

from 450 to 750 nm. In order to avoid trap-filling and secondorder charge recombination, and thus reproduce sunlight irradiation conditions, it was necessary to operate under weak excitation conditions. Some authors determined the intensity threshold for the second order effects to take place at 20, 160, and 280 μJ/cm2 (refs 40, 29, and 46, respectively) for 20 nmdiameter particles. Therefore, here, all the measurements were run under conservative conditions with respect to the reported intensity threshold, by keeping the pulse intensity constantly lower than 20 μJ/cm2. Figure 3A illustrates TA spectra measured on TiO2 NRs chloroform dispersion, photoexcited using 300 nm radiation and recorded at different pump−probe time delays.

C(t ) = R(t ) ⊗ S(t ) =

∫0

+∞

R(t ′) ·S(t − t ′) dt ′ = ΔA(t ) (1)

The instrumental response R(t) is assumed to be a Gaussian function R (t ) =

⎡ t2 ⎤ 1 exp⎢ − 2 ⎥ 2πσ ⎣ 2σ ⎦

(2)

where σ is related to the experimental full width at half maximum (FWHM) of the cross correlation between pump and probe pulses according to the following equation: FWHM =2(2 ln 2)1/2σ. The sample response S(t) is a 3-exponential decay function with time constants τdi and amplitudes Ai (i = 1, 2, 3):

S(t ) =

∑ i = 1,3

Ai ·e−t / τdi (3)

Figure 3B reports the temporal dynamics of TA in the first 50 ps after photoexcitation at 300 nm, measured for the main bands at 500 and 700 nm. Initial trapping of free carriers photogenerated in anatase was reported to occur very rapidly, in a time scale of less than about 200 fs.28,38,40 Furthermore, Tamaki and co-workers found slightly different trapping times for holes (within 220 fs) with respect to electrons ( 10 ns). The little, negative ΔA observed at τ ≤ 0 ps when λprobe = 700 nm has been attributed to a coherent artifact.66 The fast component (τd1) of the decay dynamics was thought to be due to a rapid redistribution on nonequilibrium carriers among shallow, surface trap states that are close in energy to the photoexcited states, and attribute the slower component (τd2) to the relaxation of carriers toward deeper bulk trap levels at lower energy. Finally, the third component (τd3) is related to the long-lasting processes leading to recombination of the trapped carriers. Yang and Tamai obtained similar decay times in nanosized anatase TiO2 colloidal systems28 and ascribed their temporal profiles to second-order electron and hole recombination

Figure 3. Transient absorption spectra at different time delays between pump and probe pulses (A) and selected temporal dynamics (B) of TiO2 NRs dispersion in chloroform upon excitation at 300 nm.

At very early time delays, two broad absorption bands can be clearly identified. The former, centered at 500 nm, is ascribed to holes trapped in defect states, while the latter, at 700 nm, is attributed to trapped electrons.28,41,44,61 Defect states for electrons and holes have been demonstrated to be mainly located on particle surfaces, respectively, on oxygen vacancies and hydroxyl groups.62,63 Free or quasifree electrons lie in bulk or in shallow trap sites, respectively, and their contribution to TA is confined to the near and middle infrared regions.29 These two TA bands rapidly broaden, and in ≈20 ps they lose their structure, resulting in a large and featureless continuum, which extends over the entire investigated spectral range. The relaxation proceeds quickly, and after 100 ps only a faint signal is detectable over the whole spectrum. Then, the signal completely recovers in about 500 ps, and finally, for longer time delays, a nearly negligible, negative transient absorption is measured. It is worth noticing that TA signal is typically given by the sum of several, different terms: excited state absorption (ΔA > 0), ground state bleaching (ΔA < 0), 25218



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Table 1. Fitting Parameters for the TA Temporal Profiles of the TiO2 NR Dispersion in Chloroforma λpump (nm)

λprobe (nm)

τd1 (ps)

A1 × 103

τd2 (ps)

A2 × 103

τd3 (ps)

A3 × 103

300 300 350 350 430 430

500 700 500 700 500 700

3 4 5 5 0.1 0.2

2.5 4.0 2.1 3.0 8.5 4.8

60 90 85 85 5 5

1.8 1.4 1.8 2.0 0.5 0.4

>10 000 >10 000 850 850 350 450

−0.3 −0.3 1.8 1.4 0.6 0.6

The relative uncertainty for both decay times and amplitudes is ≈20%. Slower components of the decay have been obtained by fitting full 3 ns TA temporal evolution (not reported).

a

specific, well-defined, resonance usually associated with separate e− (700 nm) trapped at surface was detected, even at very short time delays. The TA spectrum, broad and structureless since the early stages, reaches the maximum amplitude for τ ≤ 0.5 ps and then decays with time constants that are almost independent of probe wavelengths. Furthermore, when pumping occurs close to the band gap, a positive ΔA is measured in the whole spectrum, even at the longest time delay investigable with our setup (3 ns). A possible explanation for these findings can be given by considering that λpump = 350 nm is almost resonant with the direct transition at the X point of the anatase band structure. After the photogenerated carriers get trapped very rapidly, their energy allows them to form excitons, which are presumably responsible for the broad, nearly featureless TA band in the visible range (Figure 4A), in agreement with the observations of Yang and Tamai.28 The formation of trapped excitons can also justify the measured long-living ΔA > 0 contribution to TA temporal dynamics (Figure 4B). Indeed, due to the featureless appearance of the spectra, no significant variation of temporal dynamics as a function of probe wavelength is expected to occur. TA for λpump = 350 nm at the specific probe wavelengths of 500 and 700 nm was fitted in order to make a direct comparison with the results obtained when pumping above the gap (λpump = 300 nm). The decay of ΔA is again 3-exponential with time constants of ≈5, ≈85, and ≈850 ps that are almost independent of probe wavelength, supporting the hypothesis of the formation of a bound excitonic state. The fast relaxation time constants τd1, at both 500 and 700 nm, previously attributed to redistribution among trap states close in energy, are comparable to the ones obtained when exciting at 300 nm (data reported in Table 1). Also, τd2 values, ascribed to relaxation toward deeper trap states, are of the same order of magnitude at both detection wavelengths. These results indicate that, even upon excitation with λpump = 350 nm, carriers are trapped in surface states almost instantaneously, as in the case of λpump = 300 nm. However, for λpump = 300 nm such states have energies significantly higher than those required to form trapped excitons. Consequently, the two well-defined bands at 500 and 700 nm observed in Figure 3A can be definitely ascribed to the single trapped carriers. Such energetic carriers are allowed to undergo a number of processes while relaxing, like hopping and trapping in deep bulk defect states or radiative recombination, as previously discussed, thus accounting for lacking any long-living TA dynamics after 300 nm excitation. To get further insight into the recombination mechanism, we performed ultrafast TA measurements by photoexciting at 430 nm, i.e., well below the band gap of TiO2 NRs. In this case the observed dynamics can exclusively be attributed to TiO2 defect

kinetics. However, we performed several sets of measurements at different pump intensities in the range 2−20 μJ/cm2 (data not reported here), and comparable decay times were systematically obtained, within the experimental uncertainty (20%). Therefore, second-order effects do not play any role in our experimental conditions. Interestingly, in the same work Yang and Tamai measured a transient absorption dynamics independent of the probe wavelength for long time delays and suggested that, once holes and electrons are trapped, the broad transient absorption band arises from the electron−hole pair. Also, they tentatively assumed that the absorption of trapped holes and trapped electrons could not be distinguished from the absorption band in the 430−700 nm region on longer time scales. In order to get a deeper understanding of this issue, additional measurements were performed by photoexciting the samples using 350 nm radiation, i.e., very close to the direct band gap of TiO2. Figure 4A presents the TA spectra at different time delays obtained for the NRs pumped at 350 nm. Also in this case several sets of measurements at different pump intensities were performed to check whether second order effects may influence the observed dynamics. Here no

Figure 4. Transient absorption spectra at different time delays between pump and probe pulses (A) and selected temporal dynamics (B) of TiO2 NRs dispersions in chloroform upon excitation at 350 nm. 25219



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such organic capped TiO2 NRs by pumping at 300, 350, and 430 nm and probing in a broadband spectral range extending from 450 to 750 nm. The temporal evolution of photoinduced absorption changes, probed with subpicosecond time resolution, was found to be strongly dependent on the excitation conditions, both at short and long time delays. The reported results demonstrate that TA represents a highvalue advanced spectroscopic tool to provide original insights and relevant indications on the relaxation dynamics in anisotropic TiO2 colloidal nanocrystals. The obtained results will be useful, also, to better elucidate the TiO2 specific reactivity in photocatalytic application.

states, as the laser pump intensity is too low to induce any twophoton absorption process. As for excitation at λpump = 350 nm, even at λpump = 430 nm there is no spectroscopic evidence of the two bands at 500 and 700 nm (see Figure 5A), typical of separate h+ and e− dynamics, respectively.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: raff[email protected] Present Address ∥

Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MD Eindhoven, Netherlands.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the EC-funded 7th FP projects ORION (CP-IP 229036-2) and LIMPID (G.A. 310177), by the Apulian Regional Projects “Progetti Reti di Laboratori Pubblici di Ricerca” RELA-VALBIOR POR FSE 2007-2013, SENS&MICROLAB, and Laboratorio Regionale di Sintesi e Caratterizzazione di Nuovi Materiali Organici e Nanostrutturati per Elettronica, Fotonica e Tecnologie Avanzate, by Regione Puglia (Cod. 20) and by Italian FIRBFuturo in Ricerca 2012 Project RBFR122HFZ.

Figure 5. Transient absorption spectra at increasing time delays between pump and probe pulses (A) and selected temporal dynamics (B) of TiO2 NRs dispersions in chloroform upon excitation at 430 nm.

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This supports the hypothesis that the charge carriers follow distinct dynamics only when they are photoexcited well above the gap, while excitonic dynamics is observed for photoexcitation close to or below that energy. After an ultrafast transient (Figure 5B), characterized by a coherent artifact at λprobe = 500 nm, the system quickly reaches a quasiequilibrium condition. In this case τd1 ≈ 0.1−0.2 ps; i.e., this ultrafast transient closely follows the cross correlation between pump and probe pulses, which means that it is nearly resolutionlimited. The second relaxation component, with τd2 ≈ 5 ps, is now related to redistribution of excitons among states close in energy and not to relaxation toward deeper trap states, because now the system is photoexcited resonantly with deep traps. The third component (τd3 ≈ 350−450 ps), which describes the ultimate relaxation, is, in this case, faster than for λpump = 350 nm, in agreement with the fact that excitons are generated at lower energy in defect states. Table 1 summarizes all the parameters obtained by fitting eq 1 to experimental temporal dynamics.

REFERENCES

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CONCLUSIONS TiO2 NRs were synthesized by colloidal chemistry routes and dispersed in organic solvents. The FTIR measurements confirmed the presence of oleate ions coordinating as bidentate ligands nanocrystal surface. Femtosecond TA experiments were performed on optically clear colloidal dispersions of anisotropic TiO2 nanocrystals in different excitation conditions. In detail, we performed nondegenerate pump−probe experiments in 25220



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