Trap-State Distributions and Carrier Transport in Pure and Mixed

12786

J. Phys. Chem. C 2008, 112, 12786–12794

Trap-State Distributions and Carrier Transport in Pure and Mixed-Phase TiO2: Influence of Contacting Solvent and Interphasial Electron Transfer Fritz J. Knorr, Candy C. Mercado, and Jeanne L. McHale* Department of Chemistry, Box 644603, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: May 6, 2008; ReVised Manuscript ReceiVed: June 18, 2008

We report the room-temperature photoluminescence spectra of nanocrystalline TiO2 in the anatase and rutile phases and in mixed-phase samples obtained commercially (Degussa P25) and by thermal treatment of nanocrystalline anatase. The photoluminescence spectrum of anatase spans a broad range of visible wavelengths, while the much more intense rutile emission is found in the near-infrared. Photoluminescence spectra as a function of contacting fluid provide insight into the microscopic nature of the luminescence, the basis for its breadth, and the influence of solvent on inter- and intraparticle electron transfer. Anatase photoluminescence results from at least two spatially isolated trap-state distributions, one of which is absent or quenched in P25 and in the presence of hole scavengers. TiO2 nanocrystalline films containing a small amount of rutile show solvent-dependent relative intensities of the anatase and rutile photoluminescence that reveal carrier transport between the two phases. Photoluminescence spectroscopy is shown to be a useful approach for determining the energetic distribution of midband gap states. Introduction Intense interest in the properties of nanocrystalline TiO2 is motivated by a range of applications in photocatalysis and photoelectrochemistry, such as environmental remediation,1 dye-sensitized solar energy conversion,2 and photoelectrolysis of water.3 The performance of TiO2 in these applications depends on the crystalline form of TiO2,4-7 which is most often anatase or rutile, having band gaps Eg of 3.2 and 3.0 eV, respectively. While the rutile phase is thermodynamically more stable in bulk crystalline TiO2, the anatase phase is stabilized over rutile in nanocrystalline samples.8 Commercial sources of TiO2 often contain both phases, and such mixedphase TiO2 photocatalysts can be more effective than singlephase systems owing to the possibility of separating electrons and holes at the interface between phases.5,6 For dyesensitized solar energy conversion, nanocrystalline anatase is preferred over rutile because it provides higher surface area, improved carrier transport, and better dye adsorption.9 However, widely available mixed-phase TiO2 such as Degussa P25, which is about 80% anatase and 20% rutile, is frequently employed in dye-sensitized solar cells. The spatial and energetic distribution of intraband gap trap states exerts much influence on the performance of TiO2 in photocatalysis and solar energy conversion by limiting recombination and carrier transport.10-12 The nature of the physical connection between rutile and anatase crystallites in mixed-phase systems, which may result in unique trap states not found in the pure phases,13-15 can influence interparticle carrier transport. The high surface area to volume ratio of nanoparticle systems highlights the importance of surface defects resulting from undercoordinated atoms.16,17 Oxygen vacancies prevalent on both anatase and rutile surfaces result in electron trap states that lead to formation of Ti3+ when occupied.18 Surface hydroxide, on the other hand, may be associated with a hole trap state that forms hydroxyl radicals when occupied.19 Naturally, surface defects * Corresponding author. E-mail: [email protected].

are susceptible to interaction with adsorbates which can exchange electrons with the semiconductor.20 In this work, we use band gap irradiation (350 nm) to excite photoluminescence (PL) of nanocrystalline films of rutile, anatase, and mixtures thereof in contact with a series of solvents. The PL spectra of anatase and rutile TiO2 have been previously reported for both bulk and nanocrystalline samples. Tang et al.21 and also Ohta et. al22 observed visible PL of bulk anatase crystals and assigned it to radiative recombination of a self-trapped exciton. The near-IR emission of bulk rutile was reported as long ago as 1969 by Ghosh et al.23 and more recently in both bulk and nanocrystalline phases by Nakato et al.24 The latter have assigned the rutile PL, which peaks at about 850 nm, to recombination of conduction band electrons with surface-trapped holes localized on triply coordinated oxygen atoms on (100) and (110) surfaces. Recently, the near-IR emission of rutile and the visible emission of anatase were exploited to follow the phase transformation of nanocrystalline anatase to rutile at elevated temperatures.25 In previous work, we used PL and Raman spectroscopy to investigate the influence of treatment by aqueous TiCl4 on defect luminescence of nanocrystalline anatase and mixedphase (Degussa P25) TiO2.26 The TiCl4 surface treatment is an empirical approach used to improve the performance of TiO2-based dye-sensitized solar cells.27-30 In ref 26, we showed that the visible PL of P25 is completely quenched after exposure to aqueous TiCl4. Despite the ∼20% rutile content of P25, no rutile emission is seen in untreated P25 films, but exposure to aqueous TiCl4 resulted in the appearance of strong near-IR PL characteristic of rutile. Nanocrystalline anatase films, on the other hand, show visible PL similar to that of P25 but extending farther into the red and only partially quenched by TiCl4 treatment. We speculated that the rutile emission is absent in P25 because the luminescent rutile surfaces (i.e., the (100) and (110) planes) are covered by the anatase phase. The disappearance of the visible PL of P25 after TiCl4 treatment and the appearance

10.1021/jp8039934 CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

Trap-State Distributions and Carrier Transport in TiO2 of near-IR emission characteristic of rutile were interpreted as resulting from the healing of anatase oxygen vacancies and the formation of a thin shell of rutile on the surface of nanoparticles, which are mainly anatase. This previous work left unanswered why the visible emission spectra and the results of TiCl4 treatment differ for P25 and anatase and why the near-IR emission of rutile is absent in P25. In the present study, emission spectra of nanocrystalline TiO2 films are obtained as a function of contacting solvents and gases to address questions about the nature of the TiO2 emission (i.e., its assignment to surface versus bulk defects, the basis for the breadth of the spectrum, and the possible influence of solvent on inter- and intraparticle carrier transport). As will be shown, the association of anatase and rutile phases with emission in the visible and near-IR, respectively, provides a convenient handle on migration of carriers between rutile and anatase in mixed-phase systems. In addition, the energetic distribution of luminescent traps in anatase and P25 is revealed. Our results also address long-standing questions concerning the physical nature of mixed-phase TiO2 and the basis for its improved photocatalytic activity compared to that of pure anatase. Bickley et al.31 reported TEM evidence for particles of P25 containing both phases as well as separate anatase and rutile particles, and further speculated that in the case of mixed-phase P25 particles the rutile phase is on the surface. Ohno et al.32 compared the diffuse reflectance spectrum of a physical mixture of anatase and rutile to that of P25 and concluded that the latter consists of separate nanocrystallites of rutile and anatase in close physical contact, in harmony with the results of Datye et al.,33 who observed individual particles of anatase and rutile using electron microscopy. Whatever the microscopic nature of P25, the absence of the strong rutile near-IR emission is an important question, the answer to which may shed light on the basis for improved catalytic efficiency. The nature of P25 may be quite complex, and indeed, Hurum et al.6 found increasing relative amounts of rutile in larger aggregates of P25 nanoparticles. P25 is produced from flame hydrolysis of TiCl4 and may differ in structure from mixed-phase systems created by thermal treatment of anatase, which as shown here and in ref 25 results in PL from both the anatase and rutile components. We compare PL spectra of P25 to that of a twophase mixture with small rutile content obtained from thermal treatment of an anatase film. We then use the emission spectrum of the latter sample, exhibiting both anatase and rutile PL, to investigate interphasial electron transfer as a function of contacting fluid. The present study will also shed light on why TiCl4 treatment, as previously reported,26 completely quenches the PL of P25 but only partially quenches that of anatase. It will be shown here that the intensity and in some cases also the shape of the photoluminescence spectrum of TiO2 are strongly dependent on solvent environment. As an indirect semiconductor, band gap emission from recombination of conduction band electrons with valence band holes is very weak at room temperature, but relaxed selection rules for localized defect states permit radiative recombination of trapped electrons and holes.34 Arguments will be presented here for the association of the broad visible anatase PL with two types of radiative recombination: that of electrons in deep trap states with holes in the valence band, and recombination of conduction band and shallow trap electrons with trapped holes. Deep trap states in anatase have been associated with

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12787 surface oxygen vacancies.35-39 Consistent with this assignment, we showed40 that the intensity of visible emission of P25 is decreased by sintering in air, increased by annealing in vacuum (shown in this work, Figure S1, Supporting Information), and quenched in the presence of ambient oxygen, which scavenges electrons from Ti3+ trap sites.41 Nakato et al. presented compelling evidence for assigning the near-IR rutile emission to radiative recombination of conduction band electrons with holes trapped at normal surface oxygen atoms.24 On the other hand, Montocello et al.42 assigned the rutile PL to emission of self-trapped excitons associated with oxygen vacancies and attributed the breadth of the spectrum to a progression in a ∼110 cm-1 phonon mode coupled to the transition. Others assigned the near-IR emission of rutile to bulk defects.43,44 The rutile nearIR PL is much stronger than the visible emission of anatase, and electron-hole recombination rates are faster in rutile.45 As will be shown in this work, minor amounts of rutile can result in emission comparable in intensity to that of the majority anatase phase. In both phases of TiO2, there exists a range of defects that can serve as luminescence centers or as nonradiative recombination centers. Serpone and Kuznetsov46-48 examined the broad visible absorption spectrum of P25 that results from UV irradiation in the presence of hole scavengers and assigned it to at least two different color centers resulting from trapping of electrons at pre-existing defects. Such sites may lead to photoinduced absorption spectra via promotion of trapped electrons to the conduction band or transitions of Ti3+ to higher excited states. The formation of oxygen vacancies leads to neutral, singly charged, and doubly charged F-centers.47,49 Other studies identified two kinds of Ti3+ trap sites50 and at least two kinds of hole traps.51 Thus, in addition to the contribution of phonon mode progressions, the emission spectrum may be inhomogeneously broadened owing to a range of luminescence centers. The solvent-dependent PL spectra reported here address questions about this distribution and the nature of mixed-phase TiO2. Experimental Section The solvents methoxypropionitrile (MPN), dimethylsulfoxide (DMSO), dimethylformamide (DMF), and acetonitrile (ACN), were purchased from Acros Organics and were all extra-dry grade except for acetonitrile, which was anhydrous grade. Ethanol (EtOH, 100%) was used as received from AAPER. Deionized (18 MΩ) water was used for experiments on films in contact with water and for preparing 0.1 M HCl and 5% w/w H2O2 in water. Rutile nanoparticles were purchased from Aldrich and were nominally 10 nm × 40 nm in size. Commercial 5-nm anatase nanoparticles were also from Aldrich. Degussa P25, consisting of ∼80% anatase and 20% rutile with approximately 25-30-nm particle size, was a generous gift from Degussa. Films were cast on quartz microscope slides (Chemglass) from ethanol dispersion and sintered in air at 450 °C as previously described.26,40 Films were prepared from surfactant-free ethanolic dispersions of TiO2 that had been subjected to prolonged magnetic stirring, permitting the investigation of unsintered films without interference from organic additives. Sintered films were subjected to heating in air at 450 °C for 30 min unless otherwise noted. Films were stored in a desiccator. Solvents were purged of air by bubbling with argon and were stored under argon. As received, the rutile nanoparticles displayed broad Raman bands that evidence poor crystallinity52 and

12788 J. Phys. Chem. C, Vol. 112, No. 33, 2008 no photoluminescence. After being sintered at 900 °C for 48 h, the rutile Raman bands became sharp and the near-IR PL characteristic of rutile was observed (Figure S2, Supporting Information). Mixed-phase TiO2 films containing small amounts of rutile were prepared by sintering anatase films at 900 °C for periods up to 60 h. These are referred to below as thermally treated anatase films. For PL measurements, the sample was excited at 350 nm with 1 mW incident power from a Kr ion laser, the elastically scattered light was reduced with a long-pass filter, and the emission was measured with a single monochromator and thermoelectrically cooled CCD detection using backscatter geometry. Films were mounted in a quartz cuvette with the TiO2 facing inward such that the excitation beam impinged on the sample through the quartz substrate. Spectra of a given film were recorded in room air, as well as while purging with Ar, and in contact with a series of Ar-purged solvents as indicated in the text. For a given film, photoluminescence measurements were made first with air and Ar, followed by solvents in order of decreasing volatility, and the film was subjected to mild vacuum at room temperature for 30 min between solvent exchanges, except for measurements in contact with water and aqueous solution which were made after flushing three times with the final aqueous solution. The films were allowed to equilibrate for several minutes after each solvent change. The solvent-dependent intensity changes for a given film were determined to be reversible and reproducible. The reported spectra were not corrected for instrumental response. It should be noted that the PL of TiO2 at room temperature and ambient air is very weak (on the order of Raman scattering), so care must be taken to avoid instrumental artifacts arising from the strongly scattering samples as well as background fluorescence from optics. Raman spectra of TiO2 films were recorded in air using 413-nm excitation and a double monochromator equipped with photomultiplier tube detection. All spectra were recorded at room temperature. Anatase films annealed at 900 °C were monitored by X-ray diffraction, Raman, and photoluminescence to follow the anatase-to-rutile phase transition. Samples were analyzed on a Siemens D5000 X-ray diffractometer utilizing Cu KR radiation and were scanned from 2 to 80° 2θ with a 0.02° step size. Scans were compared to anatase and rutile reference scans from the powder diffraction file database. Owing to the much larger quantum yield of rutile PL as compared to that of anatase, annealing times on the order of a few hours resulted in easily measurable near-IR luminescence characteristic of rutile despite the appearance of only weak rutile features in the Raman and X-ray data. Results P25. The solvents used in this study were EtOH, DMF, MPN, DMSO, ACN, and water, selected because of their widespread use in electrochemistry and in TiO2-based photoelectrochemical cells. Figure 1a shows the PL spectrum of an unsintered nanocrystalline TiO2-P25 film, excited at 350 nm, in contact with this series of solvents and with Ar and air. Figure 1b shows the same series for a sintered film. Despite large and reproducible changes in the intensity of the PL spectrum, its shape is relatively independent of environment and sintering, with a maximum at about 525 nm and a shoulder at 550 nm. A weaker shoulder at around 420-440 nm varies in intensity for different samples and was assigned to a bulk exciton in previous work on the basis of its insensitivity to the TiCl4 surface treatment.26 In

Knorr et al.

Figure 1. Photoluminescence spectra excited at 350 nm of (a) an unsintered and (b) a sintered film of P25 in contact with EtOH, DMF, MPN, DMSO, ACN, argon, water, and air, in order of decreasing intensity in (a).

agreement with that conclusion, the intensity of the PL peak near 420 nm is found to be fairly insensitive to environment in the present study, while the intensity of the broad emission that peaks in the green is quite dependent on contacting solvent or gas. For the most part, the relative intensities in the presence of different solvents are the same for the sintered and unsintered films: the PL intensity is weakest in the presence of air and strongest in contact with EtOH, followed closely by DMF, and MPN. The same intensity was observed in neat ethanol as in a dilute solution of EtOH in ACN (Figure S3, Supporting Information), and the intensity was observed to be independent of isotopic substitution of ethanol (not shown). Note the weaker PL in the sintered compared to that in unsintered film in the case of argon, air, and water. In the presence of a given organic solvent, however, the PL intensity of the sintered film is similar to that of the unsintered one. When normalized to the same maximum intensity, the spectra are found to have similar shapes in all environments, with minor variations on the red edge (Figure S4, Supporting Information,). There is a rough trend to lower relative intensity in the red in solvents for which the overall PL is more intense. We previously showed26 that when P25 is sintered at 900 °C and converted to rutile, as evidenced by Raman spectroscopy, the emission at visible wavelengths vanishes and is replaced by emission centered at 840 nm which is about 2 orders of magnitude more intense than the original visible PL. Note that this near-IR emission is absent in both sintered and unsintered P25 despite the ∼20% rutile content. Anatase. Figure 2 shows the PL spectrum of a sintered anatase film in contact with different solvents and with air and argon. Also included are the PL spectra in contact with aqueous HCl and aqueous H2O2. As for the P25 film, the PL of anatase is greatly reduced in the presence of air. In contact with aqueous H2O2, the broad PL is completely quenched, leaving only the band at about 420 nm. The PL spectrum, including peak intensity, is similar in the presence of water and 0.5 M HCl. In contrast to the case for the P25 film, both the shape and the intensity of the anatase PL spectrum vary strongly with environment. In contact with DMF, MPN, and EtOH, the shape of the anatase emission peaks at about 540 nm and is very similar to that of P25 in all environments but slightly red-shifted. The PL spectrum of anatase is identical in argon and ACN and extends farther into the red than the spectrum of P25 or that of anatase in contact with MPN, DMF, and EtOH. Water and dilute HCl result in similar

Trap-State Distributions and Carrier Transport in TiO2

Figure 2. Photoluminescence of a sintered anatase film, excited at 350 nm, in contact with (in order of decreasing intensity) DMF, EtOH ≈ MPN, ACN ≈ argon, aqueous HCl, H2O, air, and aqueous H2O2. For comparison, the scaled spectrum of a sintered P25 film in contact with EtOH is shown.

Figure 3. Photoluminescence of rutile excited at 350 nm in contact with various fluids, in order of increasing intensity: aqueous H2O2, air, argon, EtOH ≈ H2O ≈ aqueous HCl, and ACN ≈ DMF ≈ MPN.

partial quenching of the PL relative to that of argon. The change in the spectrum from “anatase-like” to “P25-like” in the presence of EtOH was determined to be reversible: drying in argon restored the original broad visible PL. Rutile. The PL spectrum of a nanocrystalline rutile film, sintered at 900 °C, is shown in Figure 3. Note the much greater intensity of this near-IR emission centered at about 840 nm compared to the visible PL of P25 and anatase. In repeat measurements with different films, the PL intensity was unchanged or slightly decreased in the presence of air (relative to that of argon) but never significantly quenched. This is in contrast to the effect of air on anatase and P25 PL, which always resulted in significant quenching. As previously mentioned, the as-received rutile nanoparticles have a poor degree of crystallinity and showed no luminescence. After being sintered at 900 °C, the Raman lines become sharper, suggesting improved crystallinity (Figure S2, Supporting Information) and the near-IR PL characteristic of rutile appears. The PL intensity of rutile does not vary as strongly with solvent as does that of anatase and P25. Mixed-Phase TiO2. Figure 4 shows the emission spectrum of an anatase film that was annealed in air at 900 °C for 60 h, resulting in approximately 5% rutile content. The anatase-to-rutile phase transition temperature is dependent on particle size;53 hence, we used X-ray and Raman scattering to determine the rutile content as a function of annealing time. The emission in the visible region resembles that of P25 rather than pure anatase, despite the low concentration

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Figure 4. Photoluminescence excited at 350 nm of a thermally treated anatase film containing a small amount of rutile, in contact with (in order of increasing intensity in the near-IR) EtOH, argon, MPN ≈ DMF, H2O, aqueous HCl, air, ACN, and aqueous H2O2. The inset shows the visible region on an expanded scale.

of rutile. The intensity variations in the visible PL with different contacting solvents, however, are unlike those of P25, though the maximum intensity at about 530 nm is strongest in the presence of EtOH in both cases. There is a clear trend toward increasing intensity of the visible PL as the near-IR PL decreases. Note the similar intensity of the visible PL in air and water, though air results in slightly less intensity on the red edge of the anatase PL. It is striking that the intensities of both the visible and near-IR bands are identical in the presence of MPN and DMF, in contrast to what is seen for anatase and P25 where the intensities in the presence of MPN and DMF differ. Relative to argon, these two solvents result in an increase in the rutile PL and a decrease in the anatase PL in the mixed-phase sample, the latter in contrast to results for pure anatase. Also surprising is the failure of air to quench the anatase PL as is observed for pure anatase. We note also a minor decrease in the intensity on the red edge of the visible PL in contact with air compared to the otherwise similar spectrum in contact with water. Discussion Excitation of TiO2 at 350 nm creates electrons in the conduction band and holes in the valence band which are rapidly trapped at defect sites.18,54,55 For discussion purposes, it is convenient to define two types of photoluminescence associated with radiative recombination involving trapped electrons and trapped holes. Radiative transitions in which mobile electrons (i.e., those in the conduction band or in shallow bulk traps) recombine with trapped holes will be referred to as type 1 PL. Type 2 PL is defined as recombination of trapped electrons with holes in the valence band. These type 1 and type 2 transitions are the reverse of transient absorption spectra assigned, respectively, to electrons and holes.56-58 There also exist radiative transitions that are excitonic in nature (e.g., recombination of bound electron-hole pairs). Numerous experiments revealed surface electron traps of anatase associated with oxygen vacancies.35-39 Boschloo and Goossens deduced trap depths from 0.5 to 0.7 eV in anatase and in the range 0.5 to 0.6 eV in P25.38 In several articles, Serpone and Kuznetsov pointed out the common features of photoinduced absorption spectra of doped TiO2 and concluded that trapping by pre-existing defects explains the lack of

12790 J. Phys. Chem. C, Vol. 112, No. 33, 2008 dependence of the absorption spectrum on the chemical nature of the dopant.46-48 While there is general agreement that there exists a range of trap depths, the precise energetic distribution varies in literature reports, and trap depths may be highly dependent on sample preparation and history. In the case of P25 and anatase, the strong dependence of TiO2 PL on contacting solvent strongly suggests that surface trap states are associated with this luminescence. The exception to this statement is the sharp PL at about 420-440 nm that appears not to vary with solvent, nor is it quenched by TiCl4 treatment,26 air, or H2O2. This emission is observed in all samples, and its assignment to a bulk self-trapped exciton is consistent with the contribution of TiO6 octahedra to both the rutile and anatase crystal structures. We consider the intensity of PL in the presence of argon to represent the intrinsic luminescence of the TiO2 sample, which can be perturbed by contacting media by a variety of mechanisms. In the case of anatase and P25, the very low luminescence quantum yield, less than 0.01,35 implies significant competition from nonradiative recombination. Spatial overlap of an electron and a hole is required for radiative recombination to take place, such that enhanced separation of geminate electron-hole pairs is expected to diminish the PL intensity. The contacting medium could thus influence the intensity of PL by altering carrier transport, by scavenging electrons or holes, or by passivating defect sites associated with trap states. Type 1 PL is expected to be quenched by efficient electron scavengers, such as oxygen, while type 2 PL would be quenched by efficient hole scavengers, such as ethanol, as well as by scavengers of conduction band electrons. Hole scavenging from TiO2 by ethanol has been shown to be 100% efficient,59 and O2 scavenges conduction band electrons and electrons trapped at Ti3+ sites.41,58,59 The observed solvent trends in PL intensity do not appear to be correlated with the solvent-dependent conduction band edge, which has been reported for nanocrystalline anatase to be -2.04 V SCE in contact with DMF and ACN, compared to -1.39 V in EtOH (in the absence of Li+).60 The weak influence of added HCl on the anatase PL, compared to that of water, also argues against the notion that the observed PL intensity changes are the result of band edge movement. Further, the comparison of P25 PL in ACN and in dilute EtOH/ACN shows that the intensity variations are not the result of a trivial change in bulk optical properties (refractive index) while invariance to solvent isotopic substitution argues against associating the solvent trends with variations in nonradiative relaxation through energy transfer to solvent. Thus, we conclude that solvents perturb the intensity of TiO2 PL by exchanging charge and/or by interacting with traps that control transport and recombination. In the case of anatase in the presence of EtOH, MPN, and DMF, changes in the shape of the PL suggest direct interaction of the solvents with luminescent traps. The strong near-IR PL of rutile has been more well-studied than the visible PL of anatase. In this work, we find the rutile PL to be narrower and less dependent on environment than that of anatase and P25. This is consistent with the assignment24 of rutile PL to recombination of conduction band electrons with holes trapped on triply coordinated oxygen atoms located at the bottom of grooves on the rutile (100) plane. These oxygen atoms are not undercoordinated, and the trapped holes are stabilized by steric hindrance, possibly accounting for their weaker tendency to interact with contacting solvents. The failure of air to quench rutile PL seems at odds with its assignment by Nakato et al.24 to radiative recombination of conduction band electrons with trapped holes. However, oxygen adsorbs less

Knorr et al. strongly on rutile than on anatase and the O2-/O2 redox couple is nearly isoenergetic with the rutile TiO2 conduction band edge.20,59 Our results suggest that O2 is a less efficient electron scavenger for rutile than for anatase. The solvent trends that are seen in the case of pure rutile are suggestive of the importance of Lewis acid-base chemistry, with the strongest PL observed in contact with Lewis bases ACN, DMF, and MPN, followed by slightly lower intensity in the presence of Lewis acids EtOH, water, and aqueous HCl. Nakato et al.24 pointed out the role of Lewis acid-base chemistry in the mechanism for water oxidation by rutile, and thus the observed correlation between rutile PL intensity and the electron donor-acceptor properties of solvents is especially intriguing, since the hole traps assigned to the 840-nm PL are thought to be sites that undergo nucleophilic attack by water. Slight reduction of the rutile PL intensity in the presence of air and H2O2 is assumed to result from electron scavenging. Comparison of PL spectra of sintered and unsintered films (Figure 1) addresses the question of whether intensity variations spring from the influence of solvent on the rate of interparticle electron transfer. In the presence of argon, the unsintered P25 film shows more intense emission than the sintered one. This could reasonably result from decrease in the number of luminescent traps on annealing in air, but sintering also improves interparticle carrier transport and would result in less PL through improved separation of electron-hole pairs. The similar PL intensity of sintered and unsintered films in the presence of organic solvents suggests that intra- rather than interparticle dynamics control the radiative recombination of electron-hole pairs in this case. The PL intensity of both sintered and unsintered P25 is always lower in the presence of ACN than in DMSO, EtOH, DMF, and MPN. ACN is widely used in nonaqueous electrochemistry because it is stable to oxidation or reduction over a wide potential range: 1.8 to -1.5 V versus SCE.61 DMSO, EtOH, and DMF, on the other hand, are known to undergo oxidation more readily. Our results suggest that the relative intensities of P25 PL in the presence of solvents are dominated by differences in their tendency to scavenge holes from TiO2. For ease of discussion, we will refer to the P25 emission that peaks at about 540 nm as “green PL”, which we assign to type 1 PL, while the broad anatase PL is assigned to a combination of this same green type 1 PL along with red type 2 PL. In previous work, we found the putative green PL of anatase to be quenched by TiCl4 treatment, leaving a broad PL extending well into the red. On the other hand, the green PL of P25 is completely quenched by the same treatment. Our present results can be explained by a model in which there are two spatially separated distributions of electron (red PL) and hole (green PL) traps, perhaps associated with different crystal planes as depicted in Figure 5a. Both type 1 and type 2 PL are quenched by O2, which scavenges electrons from the conduction band and shallow traps (Figure 5b). Similarly, water62 scavenges electrons, resulting in a decrease in both types of PL in anatase and the green PL of P25. H2O2 has the capacity to efficiently scavenge holes as well as electrons.24d,63,64 In agreement with this, as shown in Figure 2, H2O2 quenches both type 1 and type 2 emission. EtOH is an efficient hole scavenger and would inhibit type 2 PL (Figure 5c), resulting in the observed similarity of P25 PL (in all solvents) to anatase PL in the presence of EtOH. We assume that DMF and MPN are similarly capable of scavenging the valence band holes required for type 2 radiative recombination. The similar intensity of P25 PL in the presence of neat ACN and dilute EtOH in ACN (Figure S3, Supporting

Trap-State Distributions and Carrier Transport in TiO2

Figure 5. (a) Cartoon depicting proposed spatial separation of green emitting hole traps (type 1 PL) and red emitting electron traps (type 2 PL) in nanocrystalline anatase. (b) Type 1 PL is quenched by O2 acting as an electron scavenger, while (c) type 2 PL is quenched by EtOH acting as a hole scavenger.

Information) strongly suggests that EtOH is preferentially adsorbed on the surface of P25. Ethanol and related molecules are well-known to adsorb at oxygen vacancies of TiO2.65,66 Thus, in addition to hole scavenging, EtOH and by inference also MPN and DMF might be capable of passivating the red emitting traps of anatase by adsorbing at the associated defect sites, resulting in P25-like PL. To understand why the same three solvents (EtOH, MPN, and DMF), which result in the conversion of anatase PL from “anatase-like” to “P25-like”, also result in the largest PL intensity of P25, we must consider the role of nonradiative recombination as well. The low intensity of anatase and P25 PL implies that most electron-hole pairs recombine nonradiatively, such that hole scavengers would also inhibit nonradiative recombination, resulting in greater type 1 PL intensity. Again, such scavenging requires that solvent molecules be adsorbed at the surface. In contrast, the similar PL intensity of both anatase and P25 in the presence of argon and ACN provides evidence that ACN does not interact strongly with the TiO2 surface. With this simple model, we can derive the energetic distribution of radiative electron and hole traps of anatase, shown in Figure 6. Our approach is similar to that used to separate the contributions of hole and electron transitions to the transient absorption spectra of nanocrystalline anatase in the presence of electron and hole scavengers, respectively.51,56,58 Transitions that are spectroscopically allowed in transient absorption should have equally allowed emissive counterparts, but the widths of PL and transient absorption spectra are not directly comparable. This is because the width of transient absorption spectra of electrons reflects the electron trap distribution as well as the density of final states in the conduction band, while that of holes depends on the energetic distribution of hole traps and the density of states in the valence band. As a result of rapid thermalization of mobile carriers, type 1 PL does not depend on the width of the conduction band, only on the distribution of final hole trap states. Similarly, the breadth of type 2 PL directly reveals the energetic spread of electron traps. The shaded regions of Figure 6 represent the approximate range of trap depths as determined from the wavelengths spanned by the photoluminescence spectra. The approximately 500-700-nm range of P25 PL is consistent with hole traps about 1.8-2.5 eV below the conduction band edge, below the Fermi level (in the dark) as expected. Similarly, the type 2 PL in anatase, stretching from about 500-800 nm, correlates with electron

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Figure 6. Model for trap state photoluminescence in anatase (left) and rutile (right). Wavy and straight lines indicate nonradiative and radiative transitions, respectively. The photoluminescence of anatase is considered to be a combination of both type 1 and type 2 PL involving spatially separated hole and electron traps, respectively, while the rutile PL at ∼840 nm is type 1.24 The shaded regions represent the energetic distributions of traps estimated from the wavelengths spanned by the PL spectra. EF is the Fermi level in the dark in the separate phases and is shown 0.1 eV lower for rutile.

traps about 0.7-1.6 eV below the conduction band. Also shown in Figure 6 is the more narrow hole trap distribution of rutile associated with the previously assigned type 1 PL. Figure 6 takes into account the reported 0.2 eV lower conduction band edge of rutile.27 Next, we examine whether the trap distributions shown in Figure 6 can account for the difference in the P25 and anatase PL spectra, and the absence of rutile PL in P25 but not the thermally treated anatase sample. The absence of red PL in P25 could be the result of passivation of the relevant electron traps at the anatase-rutile interface or healing of these traps in the P25 manufacturing process, which uses flame hydrolysis of TiCl4. Indeed, Hurum et. al6 found EPR evidence for the existence of two types of electron traps in anatase, only one of which is observed in P25. Note that the minor differences in the red edge of the normalized P25 spectra shown in Figure S4 suggest that there are residual red emitting traps in P25 that can be further quenched in the presence of EtOH, MPN, and DMF. It is possible that in P25 the traps that would otherwise emit in the red are in intimate contact with the luminescent centers of rutile, permitting trapped electrons in anatase to recombine with trapped holes in rutile, quenching both the anatase red PL and rutile near-IR PL. This might explain the absence of near-IR emission in P25. A simpler explanation for the absence of rutile PL in P25 is that the luminescent (110) and (100) crystal planes are buried, but this explanation appears to conflict with results from electron microscopy, which reveal separate particles of anatase and rutile.31,33 It is unlikely that these relatively stable crystal planes would be absent from isolated rutile nanoparticles. However, the composition of P25 may be complex and heterogeneous on length scales much larger than those sampled by electron microscopy.6 Our PL results reveal ensemble average properties of P25 that need not be consistent with the limited regions sampled by TEM. Experiments on sintered and unsintered films (Figure 1) address the importance of inter- and intraparticle transport. In the presence of argon, water, and air, the emission intensity of a sintered P25 film is much lower than that of an unsintered one, while the intensities of sintered and unsintered films in a given organic solvent environment are similar. The reduced PL

12792 J. Phys. Chem. C, Vol. 112, No. 33, 2008 intensity of sintered compared to unsintered P25 in inert atmosphere could derive from two reasons: the reduced density of surface defects and the greater probability of interparticle electron-hole separation in the sintered film. We found that vacuum annealing of a P25 film results in enhanced green PL (Figure S1, Supporting Information), but such enhancement might also result from increase in the number of nonradiative traps which limit electron transport rather than an increase in the number of luminescent centers. Oxygen vacancies, which are increased by vacuum annealing, are known to be associated with deep electron traps that impede transport.37 The solvents EtOH, DMF, and MPN may impede interparticle electron transport by virtue of strong interaction with the nanoparticle surface, increasing the activation energy for interparticle electron transport through increased solvent reorganization energy. Perhaps in the presence of these solvents intraparticle recombination dominates and similar PL intensities are observed in sintered and unsintered P25 films. This interpretation hinges on the assumption that the observed decrease in the PL after sintering for P25 in contact with air and argon is the result of more facile interparticle carrier transport rather than decrease in the number of luminescent defects. With the assignment of green and red PL of anatase to transitions involving hole and electron traps, respectively, we can revisit our earlier results on the influence of TiCl4 treatment on PL of anatase and P2526 and ask why the rutile emission is quenched in P25 but not in thermally treated anatase with much lower rutile content. Our previous results would be explained if TiCl4 treatment could passivate hole traps responsible for type 1 PL but not the electron traps associated with type 2 PL, resulting in complete quenching of P25 but not anatase emission. The selective passivation of hole but not electron traps could result from location of hole traps on the surface and electron traps in the bulk, or from the isolation of the two types of traps on different anatase crystal planes with different reactivity toward TiCl4. It has been shown by electron microscopy that TiCl4 treatment of anatase results in nonuniform deposition of TiO2,30,67 lending credence to the second hypothesis. The absence of red type 2 PL in P25 could result from energetically favorable electron transfer from anatase electron traps to rutile hole traps, which would quench the rutile PL as well. Facile electron transport between rutile and anatase in mixed-phase P25 would require intimate contact between otherwise luminescent crystal planes. Theory68 has suggested that the relatively abundant rutile (110) plane binds most strongly to anatase (101) and that these and other rutile-anatase interfaces are stabilized by an increase in the number of 6-fold coordinated Ti atoms. Thus, mutual quenching of the near-IR and red PL in P25 could be the result of removal of anatase oxygen vacancies and coverage of luminescent rutile planes, in which case one would not have to invoke interphasial electron transport to explain the observations. As shown in Figure 3, the PL intensity of rutile is moderately dependent on solvent environment. In the thermally treated anatase films, however, the intensities of the anatase and rutile PL bands vary strongly with environment, and the trend toward one PL band increasing at the expense of the other is highly suggestive of interphasial electron transfer. The higher conduction band edge in anatase than that in rutile is expected to permit conduction band electrons created in the anatase phase to transfer to the rutile conduction band, as has been inferred in previous studies.69 In contrast, Hurum et al.6 found EPR evidence for electron transfer from rutile to anatase trap states in P25 excited with wavelengths longer than 400 nm. Such a mechanism could

Knorr et al. be tied to the mutual quenching of red and near-IR PL in P25. However, in the presence of smaller amounts of rutile, as in our thermally treated anatase films, the rutile PL is not quenched. We observed that longer annealing times resulting in higher rutile content simply led to increasing rutile PL, so that the difference in PL of P25 and thermally treated anatase is not simply the result of different proportions of rutile and anatase. This suggests that the microsopic structures are quite different for P25 and mixed-phase samples created by thermal treatment of anatase. The data of Figure 4 lend support to the idea of bidirectional interphasial electron transfer in the mixed-phase samples prepared in this work. A nearly perfect trend toward increasing rutile PL with decreasing anatase PL is observed in the solvent series shown in Figure 4. The existence of a crossing point reminiscent of an isosbestic point suggests a competition for carriers between the two phases. As shown in Figure 6, the simultaneous transfer of electrons and trapped holes from anatase to rutile is thermodynamically favorable. Note that the visible emission in Figure 4 is more similar to that of P25 than that of pure anatase. In some of our thermally annealed anatase samples (not shown), the visible emission remained more similar to that of pure anatase than that of P25 even in the presence of rutile emission, which increased with annealing time at the expense of the visible PL. In these samples, we did not see the behavior of Figure 4, only weak solvent dependence of the rutile PL as in the pure rutile sample. We suppose that in these samples the rutile phase did not form in intimate contact with the anatase particles. We suggest that, in P25 and in the sample shown in Figure 4, the quenching of red-emitting traps of anatase in mixed-phase TiO2 is associated with intimate contact between the anatase and rutile components, which facilitates interphasial electron transfer. It is interesting that, for the thermally treated anatase film of Figure 4, the relative intensities of the anatase and rutile PL bands in different environments are in some cases different from what is seen in the pure phases. For example, aqueous H2O2 results in the strongest rutile PL in Figure 4, even though it only causes slight quenching of the emission in pure rutile. As shown in Figure 2, H2O2 effectively quenches anatase PL. If this is the result of scavenging electrons by H2O2, then the increase in rutile PL shown in Figure 4 could result from hole transfer from anatase to rutile, enhancing the radiative recombination responsible for the 840-nm emission. At the other extreme, EtOH results in maximum anatase PL and minimum rutile PL in the mixed-phase sample. It is possible that the scavenging of anatase valence band hole by EtOH facilitates the transfer of trapped electrons to trapped hole sites in rutile, decreasing the near-IR PL. Again, the validity of this speculation depends on the spatial proximity of anatase electron traps and rutile hole traps. However, though EtOH, MPN, and DMF have similar effects on the PL of pure anatase and P25, the latter two solvents result in changes to the anatase and rutile bands in the thermally treated anatase sample that are different from those of EtOH. These and other differences in the solvent trends in the PL intensity between pure and mixed-phase samples suggest a rather complicated interplay of inter- and intraparticle carrier dynamics. Though a complete interpretation of the results of Figure 4 is difficult, the evidence for bidirectional electron transfer between the rutile and anatase phases is strong. Further study is needed to determine the possible role of preferential adsorption of water70 in the presence of different nonaqueous solvents on the PL intensity trends reported here. In studies using P25, water was shown to deplete conduction

Trap-State Distributions and Carrier Transport in TiO2 band electrons and result in the formation of surface Ti-OH.62 Scavenging of conduction band electrons by water explains the diminished intensity of P25 and anatase emission in contact with water observed here. Though great care was used to employ dry organic solvents in this study, we did not attempt to remove strongly adsorbed (chemisorbed) water from our TiO2 films, because it was our desire to work with films that are relevant to conditions in photocatalytic and photoelectrochemical applications. Since the intensity trends with different solvents reported here were verified in numerous repeat experiments, it does not seem likely that differing degrees of preferential adsorption of residual water in the presence of different organic solvents are the root cause of these trends. In the case of P25 and anatase, solvent trends in PL intensity suggest varying tendencies of solvents to adsorb on the TiO2 surface. The similar PL intensity in the presence of argon and acetonitrile suggests that the latter does not interact strongly with the anatase or P25 surface. Solvents that do adsorb strongly on the anatase surface (i.e., methoxypropionitrile, ethanol, and dimethylformamide) may lead to a higher barrier to interparticle electron transfer and enhanced radiative recombination. Conclusions Photoluminescence spectroscopy of TiO2 nanocrystalline films are reported as a function of contacting solvent and reveal detailed information about the energetic distribution of electron and hole traps in pure and mixed-phase samples. The total photoluminescence of anatase is shown to be a superposition of transitions involving spatially separated trapped electrons and trapped holes, which are, respectively, about 0.7-1.6 eV and 1.8-2.5 eV below the conduction band edge. The former are largely quenched in mixed-phase samples including P25. Direct evidence is presented for bidirectional interphasial electron transport between anatase and rutile components of a mixedphase sample, with solvents exerting a strong influence on the competition for electrons between the two phases. Contacting solvents can influence photoluminescence intensities by competing for charge carriers involved in radiative and nonradiative recombination and possibly also by influencing the rate of interparticle electron transport. Supporting Information Available: Figures S1-S4. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr Chem. ReV. 1995, 95, 735. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) (a) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (b) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (3) (a) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (b) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141. (4) Bacsa, R. R.; Kiwi, J. Appl. Catal. 1998, 16, 19. (5) Miyagi, T.; Kamei, M.; Mitsuhashi, T.; Ishigaki, T.; Yamazaki, A. Chem. Phys. Lett. 2004, 390, 399. (6) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 4545. (7) Ding, Z.; Lu, C.-Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104, 4815. (8) (a) Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2000, 104, 3481. (b) Ferna´ndez-Garcı´a, M.; Wang, X.; Belver, C.; Hanson, J. C.; Rodriguez, J. A. J. Phys. Chem. C 2007, 111, 674. (9) (a) Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 8989. (b) Koelsch, M.; Cassaignon, S.; Minh, C. T. T.; Guillemoles, J.-F.; Jolivet, J.-P Thin Solid Films 2004, 451-452, 86. (10) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 11307.

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