Determination of Midgap State Energy Levels of an Anatase TiO2Nanocrystal Film by Nanosecond Transient Infrared Absorption −Excitation Energy Scanning SpectraMing Zhu, Yang Mi, Gangbei Zhu, Deyong Li, Yunpeng Wang, and Yuxiang Weng*

The Key Laboratory of Softmatter Physics, The Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

*S Supporting Information

ABSTRACT: Midgap levels for wide gap TiO2 have become increasinglyimportant because they can be used to capture solar light efficiently forphotocatalysis as demonstrated by black TiO2 in a recent paper [Chen, X., et al.Science 2011, 331 (6018), 746−750]. However, a method for systematicallycharacterizing the midgap state energy levels is still lacking. We proposed anoptical method, i.e., transient infrared (IR) absorption − excitation energyscanning spectrum, by recording nanosecond time-resolved transient IRabsorption from the excited electrons either in the conduction band or at theexcited localized states below the conduction in combination with midgapexcitation energy scanning. We demonstrate that both the electron trap statesbeneath the Fermi level and those excited localized states below the conduction band as well as the Fermi level of TiO2nanoparticles can unambiguously be determined by this method, which has great potential for characterizing the midgap trapstates of various semiconductor nanomaterials other than TiO2.

■ INTRODUCTION

Wide bandgap oxide semiconductor TiO2 is still considered oneof the best materials for photocatalysis and solar energyconversion,1 while the anatase form is usually considered to bemore active in photocatalytic and photovoltaic applications.The large bandgap of TiO2 ensures the photogeneratedelectrons within the conduction band (CB) have a strongreducing ability and the holes in the valence band have a strongoxidizing ability.2 Either in photocatalysis or in the photovoltaicprocess, nanophase TiO2 has been used for its highly effectivesurface area as well as its large number of surface binding sites.However, the number of surface and interstitial defects knownas trap states also increases substantially in the nanoparticlesversus the number in the single crystals. These trap states withtheir energy levels lying in the bandgap act as carrier traps incompetition with the fast carrier recombination in the bulkduring photoexcitation, which enhances the photoactivity of thenanoparticles. On the other hand, the deep trap states reducethe photocatalytic activities when their chemical potentials areconsidered.3

Because of its wide bandgap, TiO2 absorbs light only in theUV range, which accounts for only 3−5% of the total sunlight;4

this leads to a low light conversion efficiency in the solarspectral region. Therefore, extending the absorption of TiO2 tothe visible range would be an effective means of increasing itsoverall efficiency. One way to increase solar energy absorptionefficiency is to narrow the bandgap by anion doping such asnitrogen doping.5 At present, nitrogen-doped TiO2 exhibits thestrongest response to solar radiation,6 but its absorption in thevisible and infrared region remains insufficient.

In contrast to the cation-doped TiO2, the surface andinterstitial defects usually known in a form of Ti3+ such as self-doping7 act as color centers; these color centers in principlewould not narrow the bandgap but would provide a channel inthe midgap for visible light absorption. Some of the researchershave tested the idea of using surface-mediated midgap states toconduct the visible light-driven photochemical reaction on theTiO2 surface.

8,9 The first example of pure TiO2 showing visiblelight photochemcial activity was repoerted by Iwazawa and co-workers;8 they showed that the photo-oxidation of formate onthe TiO2 (001) surface has a threshold energy between 2.1 and2.3 eV (539−590 nm), apparently much lower than that of thebandgap energy (3.0−3.2 eV). Recently, Chen et al. proposed adifferent approach, i.e., disorder-engineered TiO2 by introduc-tion of disorder into the surface layers of nanophase TiO2

known as black TiO2 to increase the solar absorptionefficiency.10 Unlike defects forming discrete midgap states,large amounts of lattice disorder in semiconductors could yieldmidgap states constituting a continuum extending to andoverlapping with the conduction or valence band edge, leadingto a substantial narrowed band gap. This black TiO2 is capableof producing hydrogen in the presence of sacrificial reagentswith a solar energy conversion efficiency of 24% and todecompose organic substances in water under visible lightillumination. Obviously, knowledge of the location of themidgap energy levels would be important in the design of

Received: June 17, 2013Revised: August 20, 2013Published: August 22, 2013

Article

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visible light active TiO2-based new photocatalytic nanomateri-als.Electron paramagnetic resonance (EPR) can characterize the

trapped electrons (Ti3+) and holes (O− and O2−) on or in

TiO2.9,11,12 The EPR technique cannot detect those delocalized

electrons in the CB; therefore, it cannot be used to determinethe transition to the CB. Optical spectroscopies have been usedextensively for detection of the CB electrons, trapped electrons,holes, and transition energy levels. In an earlier work, Ghosh etal. conducted a comprehensive study of the midgap energylevels of the rutile single crystal.13 A large number of defectenergy levels, including at least eight shallow trap levels (<1eV), were detected on the basis of various kinds of experiments,including photoconductivity and photoluminescence excitationspectra, thermoluminescence, thermally stimulated current,electroreflectance spectra, the kinetic response of photo-conductivity, and optical and thermal bleaching of traps.However, some of the trap levels are difficult to determineunambiguously from their data.14 Later different methods havebeen employed to determine the energy levels of the midgapstates as well as the correlation to the various defects, e.g.,cathodeluminescence that reveals whether the luminescence isfrom the bulk defects or surface defects by tuning thepenetration depth of the electron beam;15 and surface-sensitivephotoluminescence either by surface modification of TiO2nanoparticles with loading of platinum16 or by treatment with

TiCl417 giving a consensus of surface or bulk trap state

luminescence. Usually, the luminescence spectrum gives abroadband spectral profile, which can hardly reveal the discretemidgap energy levels. The optical spectroscopies mentionedabove could not provide information about the positions ofboth the initial and final electronic states, only the energydifference between the initial and final levels. Therefore, it is achallenge to identify the excited state level of an opticaltransition.Excitation of a trapped electron could occur via a polaronic

excited state (localized) or into a delocalized state (or CB).18

The IR absorption spectra can be used to identify delocalizedelectrons within the CB and the trapped electron at thelocalized states based on their difference in the spectralprofile.19,20 If the electrons in the trap states (initial levels) canbe photoexcited into the bottom of the CB or to the excitedstate of the localized states (final levels), then the initial state ofthe midgap levels can be precisely determined by the excitationenergy. In this work, we employed the time-resolved transientmid-IR absorption spectra combined with midgap excitationenergy scanning, with which midgap energy levels canunambiguously be determined.

■ EXPERIMENTAL SECTION

Anatase TiO2 nanoparticles were commercially available andexamined by XRD. The average size of the nanoparticles was

Figure 1. Two kinds of different transient IR absorption decay kinetics and time-resolved difference spectra together with different oxygenquenching properties for a 7.9 nm TiO2 nanocrystal film placed in a chamber with a vacuum of 1.0 × 10−6 mbar. (a) Normalized kinetics probed at2100 cm−1 excited at various wavelengths from the visible to near-IR region as indicated. (b) Normalized kinetics probed at 2100 cm−1 excited in afurther near-IR region as indicated: excitation power of 1.02 mJ/pulse and beam size diameter of 5 mm. (c) Transient IR difference spectra of thecorresponding sample excited at 532 nm (5.5 mJ/pulse) and 1064 nm (1.2 mJ/pulse) with a delay time of 250 ns after the excitation pulse. (d) Plotsof the transient IR absorbance signals probed at 2100 cm−1 excited at 532 nm (delayed at 350 ns, excitation energy of 0.4 mJ/pulse, beam sizediameter of 3 mm) and 1064 nm (delayed at 250 ns, 0.7 mJ/pulse, beam size diameter of 5 mm) vs the oxygen pressure inside the chamber.

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estimated to be 7.9 nm from the XRD pattern using Sherrer’sequation (Supporting Information). TiO2 nanocrystal filmswere prepared followed the reported procedure.21 Details of thesetup for time-resolved IR transient absorption measurementhave been described previously.20 Briefly, three quantumcascade lasers [4.69−4.88 μm, TLC-21045; 5.9−6.5 μm, EC-QCL TLS-11088; 6.2−7.1 μm, TLC-41068 (Daylight Sol-utions)] and a liquid N2-cooled continuous wave CO laser[5.0−6.5 μm, with a spectral spacing of 4 cm−1 (DalianUniversity of Technology, Dalian, China)] were used as thetunable mid-IR probe light covering different spectral regions;355 nm laser pulses from a Nd:YAG laser (Quanta Ray, SpectraPhysics) with a pulse duration of 10 ns and a repetition rate of10 Hz were used to pump an optical parametric oscillator(GWU premiScan-ULD/240, Spectra Physics) acting as awavelength-scanning excitation source (signal beam of 410−709 nm, idler beam of 710−2630 nm) to excite the midgapstates, with a scanning step of 5 nm. The transient IRabsorption was performed in a vacuum chamber describedpreviously.22

■ RESULTS AND DISCUSSION

We identify whether the optical excitation brings the trappedelectron to the CB or the localized excited states by measuringboth the transient IR absorption spectrum and the correspond-ing decay kinetics after the midgap excitation. We find that thetransient IR absorption decay kinetics excited at higher energy(λ < 880 nm) and a lower energy (λ > 900 nm) (Figure 1a,b)and the corresponding time-resolved transient IR absorbancedifference spectra excited at a given wavelength (Figure 1c) arequite different. The transient absorbance difference spectrum ofthe former exhibits a typical feature of CB electron absorptionwith its spectral profile that can be described by the formulaΔA(λ) ∝ λp, where p is a fitted value of λ = 1.6 slightly differentfrom the theoretical value of 1.5 (Figure 1c). While that of thelatter absorbs at the blue side (4.7−4.9 μm) with the positionof its absorption maximum being out of reach of the probelaser, the spectral profile matches the red side of the IRabsorption for the trapped electrons located ∼0.42 eV belowthe CB (3380 cm−1).19 Therefore, we assign the blue sideabsorption excited at a lower energy to that of the shallowtrapped electrons. Another much lower absorption has alsobeen observed in the spectral region of 6.1−7.0 μm, which hasthe same fast decay time constant as that observed for theshallow trapped electrons when excited with a laser pulse with alower energy (λ > 900 nm) and is assigned to the absorption ofthe shallow trapped electron accordingly. It should be notedthat no detectable transient absorption signal has beenobserved in the spectral region of the CO laser (5.1−6.0μm). Obviously, the transient absorbance of the CB electrons(Figure 1a) decays slower than that of the shallow trappedelectrons; this is due to the thermally activated migration of theelectrons in the CB with a larger electron−hole separationdistance, which gives rise to a slower electron−holerecombination process.20 We further performed the O2quenching experiments with these two different electrons toconfirm the assignments. The flat band potential of TiO2 is−0.53 V (vs the normal hydrogen electrode),23 while thereduction potential of O2 adsorbed on the TiO2 surface is−0.28 V (O2/O2

−).24 Therefore, the CB electrons would bequenched by the O2 more efficiently than the trapped electronsas shown in Figure 1d.

For n-type TiO2, the Fermi level would be pinned at thehighest level of the trapped electron populated midgap states. Ifthe excitation energy is scanned, it is expected that two minimaltransition energies exist, i.e., (1) from the Fermi level to thebottom of the CB and (2) from the Fermi level to the lowestenergy level of the localized states. This can be realized bymonitoring the transient IR decay kinetics at a givenwavelength with a complete scan of the excitation wavelengthfrom the visible to the near-IR region. The transient IRabsorption−midgap excitation energy scanning spectrum isshown in Figure 2a. In a search of these two minimal excitationenergies, a phase transition in the kinetic decay curves fromthose of the CB electrons to the localized electrons wouldoccur, and we do observe this kinetic phase transition as shownin Figure 2b. In a comparison of the IR absorption decaykinetics excited at 880 and 890 nm, the former exhibits only aslow decay component that can be attributed to that of CBelectrons while the latter clearly contains a fast decaycomponent similar to those of electrons at the localized statesand a slower decay component almost identical to that at 880nm. The biphasic feature at 890 nm indicates that the trappedelectrons at localized states are in thermal equilibrium with theCB electrons.As shown in Figure 2a, the excitation of the trapped electrons

to the excited localized state starts at 1450 nm (0.86 eV). Theending wavelength for the excitation of the trapped electron tothe localized state can be set at 890 nm, which is the phasetransition wavelength of transient kinetics (inset of Figure 2a).Accordingly, we assign 880 nm as the starting excitationwavelength of the trapped electrons to the CB. Basically, thesetwo typical wavelengths are merged around 880 nm, indicatingthe shallow trap state is continuously distributed below thebottom of the CB. Therefore, the Fermi level (EF) of anataseTiO2 nanoparticles (7.9 nm) is determined to be 1.41 eV belowthe CB, while the lowest trap state level is 0.86 eV above theFermi level or 0.55 eV below the CB. In addition, we haveacquired the transient IR absorption − excitation energyscanning spectrum probed at 1600 cm−1 and searched theexcitation wavelength for the transition of the CB electrondecay to the trapped electron decay probed at 1600 cm−1,which is red-shifted with respect to 2100 cm−1 as suggested byone of the reviewers. The corresponding results are similar tothose probed at 2100 cm−1 (data not shown), which led to aconsistent value in the determination of the Fermi level.As shown in Figure 2a, the envelopes of the excitation

spectrum for the trapped electrons to the CB and to thelocalized trap states are different; i.e., the IR absorptionintensity in the former monotonically increases with excitationenergy, while that of the latter does not, indicating thedifference underlying the corresponding mechanisms for thesetwo types of excitations. The energy levels within the CB arealmost continuously distributed; relaxation of the photoexcitedelectrons at the higher energy levels to the bottom of the CB isexpected to be on a time scale of ≤100 fs, while the electronsrelax to the shallow traps on a time scale of typically a fewpicoseconds to tens of picoseconds. Further trapping fromshallow trap states to deep trap states can take place on timescales of tens of picoseconds to hundreds of picoseconds.25

Therefore, the energy levels within the CB have enoughvacancies to accept the photoexcited electrons. When theexcitation energy within the midgap is being scanned, ifexcitation energy Eex exceeds the energy gap between the Fermilevel and the bottom of the CB (denoted ΔEF), all the trap

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states in the range from the Fermi level to an energy level witha magnitude Eex − ΔEF lower than the Fermi level would beexcited into CB if the absorption of the excitation light issaturated. Thus, we denote this excitation mode as an integralexcitation, i.e., A(Eex) ∝ ∫ ΔEF

Eex α(E) dE, where A(Eex) is theexperimentally measured IR absorbance at a given excitationenergy and α(E) is the absorption intensity of the trappedelectrons excited into the CB at the excitation energy, andα(Eex) can be derived from the equation α(Eex) = d/dEexA(Eex). Therefore, the absorption peaks on the integral

excitation envelope reflect the transition from the trap statesbelow the Fermi level to the bottom of the CB. The discretemidgap energy levels for the trapped electrons beneath theFermi level (read from the excitation or its derivative spectrumshown in Figure 2c) are listed in Table 1, which shows that thetrapped electron populated states can be found from the Fermilevel (1.41 eV) down to 2.99 eV with an increasing intensity(Figure 2c).

For the localized states below the CB, once the excitationenergy exceeds the energy gap between the Fermi level and thelowest level of the localized states, in principle, the electrons atthe Fermi level would be excited into the localized states andthose beneath the Fermi level would also be excited into anenergy level lower than that excited from the Fermi level.Therefore, the excitation spectrum would consist of both thetransition from the Fermi level to the various localized statesand those beneath the Fermi level. If electrons at the Fermilevel are excited, excitation of the electrons beneath the Fermilevel would be suppressed because of the fast relaxation of theelectrons excited from the Fermi level to the lower energylevels. Therefore, excitation to the localized states would mainlyfollow the one-level to one-level excitation mode.

Figure 2. (a) Typical transient IR absorption − excitation energyscanning spectrum for the TiO2 nanocrystal film probed at 2100 cm−1

acquired with a delay time of 250 ns after the excitation pulse:excitation energy of 0.8 mJ/pulse, beam size diameter of 5 mm,vacuum of chamber of 1.0 × 10−6 mbar. The IR absorbance has beenscaled by the excitation intensity in terms of the number of photons(1013 per pulse). The inset shows an expanded view of the location ofthe kinetic phase transition wavelength. (b) Two typical decay kineticsexcited at 880 and 890 nm. (c) Two typical sets of derivative spectraderived from the corresponding transient IR absorption − excitationenergy scanning spectra.

Table 1. Experimentally Observed Optical Transitions ofTrapped Electrons at and beneath the Fermi Level to theBottom of the Conduction Band

opticaltransition(nm)

locationbelow the CB

(eV) literature values and attributionb

415 2.99 2.99 eV (PL), X2b/X1a → Γ1b, 2.1 nm colloid(anatase)26

420 2.95 420 nm (PC), Ghosh et al.13

2.95 eV (PL), X2b/X1a → Γ1b (indirect), 2.5 nmcolloid (anatase)14

440 2.82 2.81 eV (CL)15

455 2.72 450 nm (PC), Ghosh et al.13

450 nm, anatase polycrystalline annealed inO2

27

470 2.64 2.67 eV (PL), F center;4 2.62 eV (CL), Ovac15

480 2.58 2.59 eV, Ovac14

2.55 eV, Ovac4

490 2.53 2.54 eV, photoinduced Ti3+-doped4

510 2.43 520−550 nm (PL), anatase polycrystalline27

530 2.34 2.34 eV (PL), F+ center4

540 2.30560a

590a 580 nm (PL), anatase polycrystalline27

600a 2.07615a 2.02 2.01 eV, Ovac

14

630a 1.97 630 nm (PC), Ghosh et al.13

645a 1.92670a 1.87 1.87 eV (CL)15

730 1.70800 1.55 800 nm (OI), Ghosh et al.;13 1.54 eV (CL),

rutile15

830 1.49 820 nm (TL), Ghosh et al.;13 1.51 eV (PL),anatase polycrystalline, Ti3+ interstitial ions.27

860 1.44 850 nm (TL), Ghosh et al.13

aRead from derivative spectra (Figure 2c); other unmarked valueswere read from Figure 2a. bAbbreviations: PC, photoconductivity; PL,photoluminescence; CL, cathodeluminescence; OI, optical ionization;TL, thermoluminescence.

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The experimentally observed transitions to the localizedstates are listed in Table 2. There are at least 13 transition

peaks; however, only seven of them correspond to thetransition from the Fermi level to the localized states, andthe other six most likely arise from the transition to the existinglocalized levels from the trapped electrons beneath the Fermilevel based on the following accounts.Upon excitation of the trapped electrons to the localized

states, two kinds of transitions can be expected as described inFigure 3. The first consists of transitions from the Fermi level

to the localized states, where the excitation energy meets a realtransition as described in panel a. In this case, the trappedelectrons below the Fermi level can also be excited in principle;however, because of the excited electron relaxation to thelower-energy states and the possible mismatch of the excitationenergy from these states to the localized states, transitions fromthose trap states beneath the Fermi level would be suppressed.Therefore, the observed transitions correspond to genuinelocalized energy levels.However, when the excitation energy of the trapped

electrons at the Fermi level falls in the gap between the

localized states, this energy coincidentally promotes trappedelectrons beneath the Fermi level to a true localized state belowthe false energy level expected from the transition from theFermi level at the given excitation energy as described in panelb. Apparently, these kinds of transitions should be excludedfrom the observed excitation energy scanning spectrum.We start such an exclusion process as follows. From Table 1,

we know a number of discrete energy levels below the Fermilevel (880 nm), i.e., 830, 800, 775, and 730 nm appearing in thetransient IR absorption − excitation energy scanning spectrum,which are 0.085, 0.141, 0.19, and 0.291 eV below the Fermilevel, respectively. The observed minimal transition energy inTable 2 (1450 nm or 0.86 eV) must come from that from theFermi level to the lowest energy level of the localized states,which constitutes the starting level of our deduction. We thenconstruct a look-up table as shown in Table 3, where the first

column lists all the observed excitation peak wavelengthsrelated to the transition to the localized states. Once a genuinelocalized energy level is fixed, the corresponding transitionenergy from above four trap levels beneath the Fermi level tothis determined localized energy level can be calculated asshown in the corresponding rows of the table. Then we go tothe next observed excitation wavelength expected of a transitionfrom the Fermi level to the next higher localized level. Weexamine the look-up table, and if this excitation energy equals(within the experimental error, scanning step of 5 nm) thosetransition energies from the lower energy levels beneath theFermi level to a genuine lower localized state already scanned,then this transition peak would most probably not belong to anew higher genuine localized state. The correlation is denotedby the same letter in the two corresponding boxes in Table 3.We repeat the process described above until the highestscanning transition energy is reached. All the genuine localized

Table 2. Experimentally Observed Optical Transitionsa ofTrapped Electrons at the Fermi Level to the Localized States

opticaltransition(nm)

opticaltransition (eV)

location belowthe CB

compared to literatureassignmentb

1450 0.86 0.55 0.56 eV (TL&TSC),Ghosh et al.13

1390 0.89 0.521360 0.91 0.50 0.48 eV (TL&TSC),

Ghosh et al.13

0.48 eV (TL&TSC),shallow trap13

1160 1.07 0.34 0.3 eV (calcd), Ti6c3+28

0.32 eV (TL&TSC),Ghosh et al.13

1040 1.19 0.22 0.2 eV, F2+29

1000 1.24 0.17 0.2 eV, F2+29

950 1.31 0.10aTransitions at 1310, 1270, 1240, 1130, 1080, and 970 nm arecoincident with those from the trapped levels beneath the Fermi levelabsorbing at 830, 800, 775, and 730 nm. bTL&TSC, thermolumi-nescence and stimulated thermal current.

Figure 3. Energy diagram for the transitions of the trapped electronsto the localized states and the successive relaxations. (a) Trappedelectrons at the Fermi level with a true transition. (b) Trappedelectrons beneath the Fermi level with a true transition while thetransition from the Fermi level fails.

Table 3. Observed Transitions and Correlation with theExpected Transitions of the Trapped Electrons at andbeneath Fermi Levelsa

aThe letters in parentheses indicate the possible transition from thetrap states below the Fermi level. The correlation is denoted by thesame letter in the two corresponding boxes.

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energy levels and the corresponding possible transitions fromthe lower trap levels to these true localized levels are marked inthe shaded boxes in Table 3. For the sake of brevity, all theobserved midgap energy levels are summarized in Figure 4.

The attribution of the midgap energy levels has beeninvestigated both experimentally and theoretically. For thereduced TiO2, the surface trap states have been determinedmainly as Ti3+ surface defects and oxygen vacancies; the latterconsists of two Ti3+ ions adjacent to each other generated uponthe loss of an O atom in the TiO2 crystal phase while theelectron pair still remains trapped in the cavity.12 The oxygenvacancies are grouped into three types of color centers, F2+

(without an electron), F+ (with one electron), and F (with twoelectrons), depending on the number of the captured electrons.Ti3+ exhibits a broad absorption band in the visible to near-IRregion,30 while the optical transition of the oxygen vacanciesdepends on the type of color center.4 Currently, we can hardlyattribute the origins of all the observed discrete midgap energylevels; however, we compare them with literature values fromboth experimental observations and theoretical calculationtogether with their possible attributions as listed in Tables 1and 2. We find that many of the observed midgap energy levelsfor the trapped electrons below the Fermi level or those of thelocalized states below the CB can find their counterparts in theliterature, though the samples could be varied. It should benoted that in a number of reported works, there is a localizedlevel lying 0.86 eV below the CB. In this work, we observedonly a 0.86 eV transition from the Fermi level to the lowestlocalized state (0.55 eV below the CB); we even scanned anexcitation wavelength as high as 2300 nm (0.54 eV), and notransient absorption signal was observed beyond 1450 nm,indicating that there is no localized level lower than 0.55 eV ortheir density of state is beyond our detection limit (1.0 × 10−5

ΔOD). This can be attributed to the difference in samples usedor the fact that in the previous works, the 0.86 eV transitioncould be to the localized state rather than to the CB.

■ CONCLUSIONSOne major limitation of the optical excitation method fordetermining the midgap energy levels of semiconductormaterials is the difficulty in simultaneously defining the initialand final energy levels of the observed optical transitions,providing that one of the energy levels is already known. Weproposed a novel method capable of systematically character-

izing the midgap energy level of TiO2 nanoparticles thatemploys time-resolved transient IR difference absorbancedetection combined with a midgap excitation energy scanningspectrum. The time-resolved transient IR absorption spectrumcan identify the final energy levels, i.e., electrons at the bottomof the CB or at the excited localized levels below the CB, whilethe midgap excitation energy scanning spectrum defines thedifference in energy between the initial and final states.Furthermore, because of the different relaxation modes forthe photoexcited electrons in the CB and at the localized levels,the excitation modes are also different for the excitation of thetrapped electrons to the CB and to the localized states. Wedetermine that the excitation mode for the former is integral;i.e., all the trapped electrons above this scanned level would beexcited into the CB. The excitation mode of the latter isbasically one-level to one-level. This gives rise to an apparentfeature in the excitation spectral envelope distinguishing thetransitions to the CB from those to the localized states. Withthis method, more than 20 trap states beneath the Fermi leveland seven excited localized states below the CB for an 8 nmTiO2 nanoparticle film of anatase have been determined, and arelative density of the state for the trap states below the Fermilevel has also been derived. In addition, the Fermi level of theTiO2 nanoparticles has been determined to be 1.41 eV belowthe CB. Obviously, this method has a great potential inmeasuring the midgap levels of semiconductor materials otherthan TiO2, which is under further investigation.

■ ASSOCIATED CONTENT*S Supporting InformationXRD pattern of TiO2 nanoparticles. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*CAS Key Laboratory of Soft Matter Physics, Institute ofPhysics, Chinese Academy of Sciences (CAS), Beijing 100190,China. Telephone: +86-10-82648118. Fax: +86-10-82648118.E-mail: yxweng@aphy.iphy.ac.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the Natural Science Foundation ofChina (Grants 20925313 and 21090342), the National BasicResearch Program of China (Grant 2009CB930700), and theChinese Academy of Sciences Innovation Program (KJCX2-YW-W25).

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Figure 4. Schematic diagram summarizing the experimentallyobserved midgap energy levels for the trapped electrons beneath theFermi level and those for the localized states (shallow trap states)under the conduction band.

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The Journal of Physical Chemistry C Article

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