5984

J. Phys. Chem. B 1999, 103, 5984-5987

Ultrafast Charge Transfer at TiO2/SCN- (aq) Interfaces Investigated by Femtosecond
Transient Reflecting Grating Method
Tomohiro Morishita, Akihide Hibara, and Tsuguo Sawada*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, JapanIsao Tsuyumoto
Department of EnVironmental Systems Engineering, Kanazawa Institute of Technology, 7-1 Ohgigaoka,
Nonoichi, Ishikawa 921-8501
ReceiVed: December 14, 1998; In Final Form: May 23, 1999

We applied a femtosecond transient reflecting grating (TRG) method to solid /liquid interfaces to successfully
observe the photoinduced ultrafast relaxation dynamics at TiO2(001)/KSCN (aq) interfaces. We found, forthe first time, an ultrafast relaxation component (110-690 fs). The component intensity clearly increases
with increasing concentration of KSCN, and it is strongly dependent on the number of photoexcited carriers.
We concluded that this component is due to the interaction between photoinduced nonequilibrium holes andadsorbed SCN- ions and that ultrafast charge transfer occurs by way of this interaction. These results suggest
a new ultrafast charge transfer process involving nonequilibrium carriers.

Introduction
The charge carrier dynamics at solid/liquid interfaces plays
a crucial role in various processes such as electrode reactions,photocatalysis, photosynthesis, and corrosion.1-5 Understanding
the ultrafast charge transfer dynamics at the interfaces isfundamentally important for designing functional electrodesurfaces and for controlling the interfacial reactions. Over theyears, a number of experimental techniques have been developed
to investigate the static structures and the dynamic properties
of solid/liquid interfaces.6-8 Among these, a transient grating
method is a versatile technique which combines high time
resolution, interfacial selectivity, high sensitivity, and in situ
measurements.9,10 This method utilizes nonradiative relaxation
processes of electrons, while fluorescence (luminescence)
spectroscopy utilizes radiative relaxation processes, and measurements of unmodified interfaces are possible because it does
not require luminescence or fluorescence in situ.
We have extended a transient reflecting grating (TRG) method
to the femtosecond time region and have applied it to solidsurfaces. We successfully monitored the energy dissipation
processes of hot electrons on gold surfaces11 and the carriertransport dynamics on silicon surfaces.12 The TRG method hasan advantage over the conventional transient reflecting (TR)method because it can provide directional dynamic properties
with a much higher S/B (signal to background) ratio.
Recently, the contribution of nonthermalized electrons inphotoexcited reaction systems has attracted much attention. The
electrons which are not in equilibrium with other electrons andphonons are generally called hot electrons. We have observed
an interaction having a relaxation time of 20 fs between
photoexcited electrons and adsorbed solute anions at a platinum/
electrolyte interface using the TR method.13 We presumed that
this ultrafast interaction is due to nonthermalized electrons(NTEs).
* To whom any correspondence should be addressed.

In this study we applied the femtosecond TRG method to
TiO2/KSCN (aq) interfaces. Although a number of researchers
have reported on the dynamics of the photoexcited reaction of
TiO2, their focus has mainly been on electron-hole recombination processes on a time scale of 10 ps to 100 ns.14-17 There
are indeed few reports which deal with photoexcited carriers
within a time scale of 1 ps. Among these, Hannappel et al.18
have reported an ultrafast photoinduced electron transfer fromchemically anchored Ru-dye to a colloidal anatase TiO2 film
within 25 fs. This electron transfer occurred after excitation ofdye molecules, which is of practical importance in a functional
solid surface such as in a solar cell. In the following, we focus
on the behavior of hot carriers after excitation of the TiO2 phaseat TiO2/KSCN (aq) interfaces. Our aim is to elucidate thecontribution of nonequilibrium carriers to the photoexcitedinterfacial reaction of TiO2. We describe an ultrafast relaxation
110-690 fs after the excitation of TiO2 at the interface andpropose a new type of charge transfer which explains the limitsof the applicability of conventional theory.
Experimental Section
The experimental setup for the TRG measurements was thesame as that described in our previous report,19 except for thesample arrangement for the solid/liquid interface measurements.
An outline follows here. The sample surface is irradiated by
two pump pulses of the same wavelength λpump, and an opticalinterference pattern is formed on it. This interference patterntransforms into a periodical refractive index change, which acts
as a transient diffraction grating. A probe pulse is incident
normal to the grating, and diffracted light is observed. The
intensity of the diffracted light is monitored as a function of
the delay time of the probe beam. The TRG signal reflects
dynamic information on surfaces such as refractive index change
by electronic excitation and surface displacement by heatgeneration.

10.1021/jp984729u CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/02/1999

Ultrafast Charge Transfer at TiO2/SCN- (aq) Interfaces

J. Phys. Chem. B, Vol. 103, No. 29, 1999 5985
TABLE 1: Time Constants and Pre-exponential Factors
Calculated as Fitting Parameters for the TRG Signalsa
pump power
intensity (mW)t1 (fs)t2 (ps)C1

C24578110200
450690

173
137
130
150

0.54
0.38
0.22
0.17

0.46
0.62
0.780.83

aFigure 1. Typical TRG signals from TiO2/SCN- (aq) interfaces withvarious SCN- concentrations. The concentrations of SCN- are (a) 20;(b) 15; (c) 10, and (d) 0 mM. For clarity, the signals are verticallydisplaced.

The light source was a regeneratively amplified Ti:sapphirelaser system which had an autocorrelation of 150 fs, awavelength of 800 nm, and a repetition rate of 1 kHz. The output
beam of the 800 nm wavelength was partly reflected by a half
mirror to generate a probe beam passing through a computercontrolled optical delay line. The residual beam was frequencydoubled by a BBO crystal to generate a 400 nm beam which
was split into two parts. Each pump pulse had a spot size of
600 µm, and the fluency was changed from 2.16 to 4.57 mJ/
cm2. The pump pulse had p-polarization. We wanted to study
how the photoexcited carrier density affects the interface
reaction. The fringe spacing was set to 2.1 µm by setting theincident angle to 5°. Polarizations of the pump and probe beams
were perpendicular to each other. The diffracted light was
detected by a PIN photodiode. The output of the photodiodewas fed into a fast-gated integrator and its average signal wasrecorded as a TRG response.
We fabricated a sample cell for solid/liquid interface measurements using an optical cell. Its optical window was madeof quartz glass with optically flat surfaces, and the thickness of
the liquid phase could be controlled by changing the spacers.
Controlling the thickness of the liquid phase was indispensable
to improvement of the S/N ratio because fluctuation of the liquid
phase greatly disturbs the measurement. A solid/liquid interface
was prepared by pouring a KSCN (aq) solution into the samplecell. A TiO2 single crystal was attached to the sample cell. TheTiO2 single crystal with rutile structure (001) surfaces waspurchased (K & R Creation Co., Inc.) and used without anymodification. KSCN was also purchased (Wako Chemical Co.,Inc.; special grade, 99.5% purity) and used without furtherpurification. The concentrations of KSCN (aq) solution werein the range 0-20 mM, with a pump beam fluency of 4.5 mJ/
cm2. We intended to study how the population of adsorbate on
TiO2 affects the behavior of photoexcited carriers.
Results and Discussion
TRG responses from the interfaces between TiO2 and KSCN(aq) in the range of 0-20 mM are shown in Figure 1. The S/Nratio of the TRG signals was dramatically improved bydecreasing the thickness of the liquid phase which prevented
its fluctuation. We empirically found that the best S/N ratio wasobtained when the thickness was 0.5 mm. In this study, allmeasurements were performed with 0.5 mm thick liquid phases.
The intensity of the TRG signals in the experiments below wasConcentration of KSCN is fixed at 15 mM.

almost the same, regardless of the thickness of the liquid phase.
This indicates that the TRG signals were not from the bulk
solutions but from the interface.
First, we discuss the TRG response. An increase in the TRG
signal corresponds to the change in the refractive index of thesample surfaces induced by the generation of photoexcitedcarriers. We assume that the change in the extinction coefficientis negligible at the probe wavelength. The change in therefractive index upon the photoexcitation of electron-hole pairs
can be approximated by the Drude model for the opticalproperties of quasi-free carriers. Therefore, the change in therefractive index ∆n is given by∆n )-Ne22nmehωp20(1)where N is the photoexcited carrier density, e is the fundamental
electron charge, meh is the effective reduced mass of theelectron-hole pair, and ωp is the radial probe frequency. A
noticeable point in this experiment is that the change in therefractive index due to the holes is expected to be about 1000
times greater than that from the electrons because the effective
masses of holes and electrons are 0.02me and 30me,20 respectively. Therefore, the TRG responses are mainly due to thechange in the number of the photoexcited holes as a functionof the delay time. Based on this assumption, the TRG responses
were analyzed below.
The change in the TRG signal reflects the changes in both
the real part (n) and the imaginary part (k) of the refractiveindex. The TRG method detects the phenomena which causechanges in the complex dielectric constant. For simplicity, weinterpret the change in TRG signals as that in n in this paper,
because n and k are associated with each other by KramersKronig relations and the change in n is considered to reflect
the same phenomena as that in k.
In Figure 1, around time zero each signal shows an abrupt
rise due to generation of holes followed by an ultrafast small
fall and then a gradual decrease. This gradual decrease is due
to electron-hole recombination which occurs on a nanosecond
time scale.21 Details of the recombination process remain acontroversial issue, but they are not the topic of this paper.
We focus on the ultrafast small fall within 1 ps. In Figure 1,
the small fall after the peak at the TiO2/SCN- interfaces clearly
becomes greater with increasing SCN- concentration and thesmall fall is never observed at the TiO2/H2O interface. The time
constant of this decay component is summarized in Table 1 (seebelow). The adsorption of SCN- ion is not saturated in thisconcentration region, and the number of adsorbate (SCN-)
increases with increasing concentration of SCN-.17 Therefore,
the increase in the intensity of the small fall reflects thephenomena involving the adsorbate (SCN-). The maximum
height of the TRG signal is almost constant in all experiments,
which suggests that there is no change in the density of pumpbeam fluency due to self-focusing induced by the SCN- (aq).

5986 J. Phys. Chem. B, Vol. 103, No. 29, 1999

Figure 2. Intensity of the ultrafast decay component vs the concentration of SCN-. The intensity of the component increases with increasing
concentration of SCN-.

Figure 3. Typical TRG signals from the TiO2/SCN- (aq) interfaces
with various pump beam powers. The intensities of the pump beam
power are (a) 4; (b) 5; (c) 7; (d) 8 mW. For clarity, the signals arevertically displaced.

These facts indicate that an ultrafast relaxation process of thephotoexcited holes within 1 ps occurs at the TiO2/SCN- (aq)interfaces.
For a detailed analysis of the ultrafast relaxation process, theintensity of the ultrafast decay component vs the concentrationof SCN- is shown in Figure 2. As we show the next experiment,
the relative intensity of the ultrafast relaxation component
decreases with increasing pump beam power. Therefore, there
is little possibility that this ultrafast relaxation component
corresponds to a nonlinear effect induced by SCN- ion solution.
In Figure 2, the intensity of the ultrafast decay component
increases with the increase of the SCN- concentration; that is,
the ultrafast relaxation of the photoexcited holes becomesdominant with the increase in the number of adsorbed SCNions. Thus, we can presume that the observed ultrafast decay ismainly due to ultrafast hole transfer at the interfaces 110-690
fs.
Next, we consider the photoexcited hole dynamics at several
pump beam powers. The dependence of the TRG responses fromthe TiO2/SCN- interfaces on the number of the photoexcited
carriers is shown in Figure 3. The concentration of the SCN(aq) is all 15 mM in this experiment. The atomic density of theTiO2 is 100 times higher than that of the photoexcited carriers.
Therefore, the absorption of the solid sample is not saturated
even at high power, and the excitations are all localized in theMorishita et al.

Figure 4. Intensity of the TRG signals and the relative intensity of
the fast relaxation component vs pump beam power.

same volume. The intensity of the TRG signals and the relative
number of the photoexcited carriers which contribute to theinterfacial reaction vs the pump beam power is plotted in Figure
4. Both the intensity of the TRG signals and the relative numberof the photoexcited carriers increase with the increase of thepump beam power. However, the coefficients of the increase
are not as expected. This means that the ratio of the ultrafast
charge transfer decreases with an increasing number of photoexcited carriers. This is probably because the path for theultrafast charge transfer is controlled by not only the density ofthe photoexcited carriers but also other factors, such as thenumber of adsorbate molecules and the number of lattice defects
at the interface.
The observed TRG signals are fitted with a double exponential decay using a least-squares fitting method. The signals aredeconvolved using an instrumental function with fwhm of 250
fs. As shown in Table 1, the time constant increases withincreasing the pump beam power (110-690 fs), whereas theintensity decreases with increasing the pump beam power. The
increase in the time constants is probably due to the change inpath for the ultrafast charge transfer as discussed above.
It is known that photoexcited holes exist as nonequilibrium
holes on a time scale of 100 fs after which they undergo anenergy dissipation process due to the carrier-carrier scattering
process. Our experimental results suggest a role for nonequilibrium holes in the reaction at the TiO2/SCN- interfaces. Withthe increase in the number of photoexcited carriers, the energy
dissipation process becomes dominant and the interfacial
reactive process occurs less, because the rate of the carriercarrier scattering process increases, and vice versa. We believethat this is the first observation of the contribution of nonequilibrium holes to the reaction at TiO2/SCN- interfaces.
Generally, the interfacial charge transfer rate has been
explained by Marcus theory,22 which has been adopted to TiO2/
SCN- interfaces and has well explained the charge transfer
there.14,17 In this theory, the charge transfer rate is determinedby the difference of the redox potential at the interfaces andthe solvent reorganization energy. Reorganization of the solvent
molecules always accompanies the molecular vibration; therefore, the maximum of the charge transfer rate is restricted by
the molecular vibration rate.23 For TiO2/SCN- interfaces, theestimated charge transfer time by Marcus theory is 1.8 ns, andBahnemann et al.17 reported that the time constant for the holetransfer process there is 5 µs. Our experimental value of 110690 fs is much faster than the estimated and reported values,
which suggests that the observed hole transfer process is a new-Ultrafast Charge Transfer at TiO2/SCN- (aq) Interfaces
type transfer process beyond Marcus theory. One possible
explanation of the new-type process is nonequilibrium holetransfer before thermalization of the photoexcited energy. It isvery exciting to note that the ultrafast nonequilibrium holetransfer in 110-690 fs was observed here because it has been
believed that nonequilibrium hole transfer does not occur andonly equilibrated holes can be transferred across an interface.
This work indicates that the nonequilibrium holes should betaken into account in interface reactions. We believe this result
will provide valuable knowledge for designing electrode surfaces
and controlling interfacial reactions.
Acknowledgment. The authors are grateful to Prof. A.
Harata for valuable discussions. This work was supported by
Grant-in-Aids for Specially Promoted Research (No. 07102004)
and for Scientific Research on Priority Area of Electrochemistryof Ordered Interfaces (No. 09237219) from the Ministry ofEducation, Science and Culture of Japan.
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