Spectroelectrochemical Photoluminescence of Trap States of

Article pubs.acs.org/JPCC

Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO2 in Aqueous Media Fritz J. Knorr and Jeanne L. McHale* Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, United States S Supporting Information *

ABSTRACT: Trap state photoluminescence of nanocrystalline TiO2 electrodes is investigated as a function of applied bias and pH in aqueous electrolyte. Films composed of the anatase polymorph reveal an increase in a broad red emission at increasingly negative potentials, with an onset about 200 mV positive of the pH-dependent literature value of the conduction band potential, followed by conversion to the green emission characteristic of hole traps at more negative bias. Green photoluminescence is the only emission seen from mixed-phase (P25, anatase/rutile) films at any applied potential, while red-emitting electron traps in P25 appear to be quenched by electron transfer to rutile hole traps. The influence of surface treatment by TiCl4 is investigated for both anatase and P25 in order to shed light on the mechanism by which this treatment improves the performance of TiO2-based solar cells. Our results reveal the difference between trap state distributions of P25 and anatase nanoparticles and address the molecular basis for red and green emitting traps. The results establish the redox potentials of the traps as a function of pH and reveal the breadth of their energetic distribution.

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INTRODUCTION

potential energies of trap states and the conduction band edge as a function of solvent environment. It is well-known that proton adsorption on the surface of TiO2 and other metal oxides results in pH-dependence of the redox energy of the conduction band edge.14,22−25 In the presence of water at pH 0, the conduction band edge ECB of anatase TiO2 is −0.16 V on the NHE scale (−0.37 V versus the Ag/AgCl reference electrode used here), undergoing a Nernstian shift of ∼0.06 V/pH unit toward more negative potentials as the pH increases, the result of an apparent acid/ base equilibrium at the TiO2 surface. In the presence of aprotic solvents, ECB as reported in ref 12 is considerably more negative than in water and depends on the presence of potentialdetermining ions such as Li+. In nonaqueous media that has not been scrupulously dried, water is preferentially adsorbed16,26 on the surface of nanocrystalline TiO2 and drives the conduction band edge toward more positive potentials. Thus the difficulty in establishing the conduction band edge for nanocrystalline TiO2 stems from two potentially linked phenomena: the existence of intraband gap states from surface traps and interfacial interactions with the nanoparticle environment and surface adsorbates. The present work concerns the conduction band edge and trap state activity of TiO2 electrodes in aqueous media as a function of pH. A future paper will consider the influence of nonaqueous solvents.

Surface trap states of nanocrystalline TiO2 exert tremendous influence on its performance in applications such as solar energy conversion, photocatalysis, and sensing.1−3 Traps which influence carrier transport and recombination in devices may depend on the nanoparticle morphology,4,5 crystallinity,6 surface area,7 contacting solvent,8,9 and grain boundaries.10 The energetic and spatial location of these traps is of considerable interest to optimizing the phase and morphology of nano-TiO2 for a particular application. Intraband gap states, including those leading to an exponential distribution of states seen as the Urbach tail in the optical absorption spectrum,11 cause the TiO2 conduction band edge to be rather ill-defined. Nevertheless, there is a clear dependence of the TiO2 conduction band potential on contacting solvent electrolyte.12−14 In addition, interactions of trap states with solvents and ions influence interfacial electron transfer and recombination in electrochemical and photoelectrochemical devices. The solvent dependencies of trap state activity and conduction band energy influence the efficiency of electron injection and recombination in dye-sensitized solar cells.15−19 Owing to participation of surface states in heterogeneous redox reactions, interfacial electron transfer to acceptors can take place at potentials more positive than the reported conduction band edge.20,21 The participation of trapped versus free carriers in interfacial redox reactions is an important consideration in optimizing metal oxide nanoparticles for photoelectrochemical applications, motivating the need to determine the redox © XXXX American Chemical Society

Received: March 5, 2013 Revised: May 30, 2013

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dx.doi.org/10.1021/jp402264p | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Photoluminescence spectra of nanocrystalline TiO2 at pH 1.8 in 0.2 M NaClO4, excited at 350.7 nm at different applied potentials: (a) anatase and (b) P25. Potentials are vs Ag/AgCl. The sharp features in the vicinity of 700 nm are unfiltered plasma lines from the excitation laser scattered by the sample, and the weak broad feature at 800 nm is a grating artifact arising from the Raman band of water at about 400 nm.

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EXPERIMENTAL SECTION Nanocrystalline TiO2 films were prepared on FTO glass (TEC 15) by the doctor blade method, using Scotch brand Magic tape to define the thickness of the casting. The pastes were prepared by extended stirring of 1 g of 20 nm anatase (SigmaAldrich) powder or P25 (Evonik) powder in 5 g of surfactantfree ethanol for two weeks, as previously described.37 This method has been shown to produce TiO 2 films of approximately 10 μm thickness. The TiO2 films on FTO glass were sintered at 400 °C for 2 h. The coated glass electrodes were broken into roughly 1 cm2 pieces that had one edge of uncoated FTO where electrical connection was made with an alligator clip. This TiO2 electrode was partially immersed in the electrolyte in a quartz cuvette, leaving the connection edge above the solution. The aqueous electrolyte solutions were prepared from ultrapure water degassed by extensive purging with Ar. The supporting electrolyte was 0.2 M NaClO4. The pH 1.8 solution was prepared by addition of HClO4, pH 12 was prepared by addition of NaOH, pH 6.4 was prepared by addition of 0.1 M phosphate buffer, and pH 9.6 was prepared by addition of 0.1 M carbonate buffer. The solutions were stored under Ar and sparged with Ar again prior to the measurements. Acetate buffer was avoided as it converts the red PL to green PL at open circuit as a result of hole scavenging.29 The voltages for cyclic voltammetry and spectroelectrochemistry were controlled by a BAS 100A potentiostat. The working electrode was the TiO2 film, the counter electrode was a platinum wire, and the reference electrode was Ag/AgCl (3 M), for which the standard potential is 0.210 V on the NHE scale. The cyclic voltammograms were linear sweeps at 100 mV/s. The TiO2 electrodes were illuminated with the 350.7 nm line of a krypton ion laser. The laser output was filtered with a bandpass filter to attenuate plasma lines, and focused with a cylindrical lens to a line on the front surface of the TiO2 electrode. The bandpass filter passes a small window of plasma lines around 700 nm. The power density was approximately 1 W/cm2, corresponding to 2 mW focused onto a 0.1 mm × 2 mm area. The power density is of concern because higher powers result in a blue-shifted PL spectrum resulting from the Burstein−Moss effect. The emission was collected in back-

In recent work, we have investigated the spatial and energetic distributions of surface traps on TiO2 nanoparticles using photoluminescence (PL) spectroscopy as a function of particle morphology and chemical environment.4−8,27−30 To summarize our results, we have assigned the broad visible PL arising from ultraviolet excitation of nanocrystalline anatase TiO2 to contributions from two spatially isolated distributions of traps: Type 1:

− eCB + htr+ → hνgreen

(1)

Type 2:

+ hVB + etr− → hνred

(2)

Type 1 and Type 2 PL refer to emission associated with hole and electron traps, respectively, which recombine with mobile carriers in the conduction or valence band. The latter type of PL dominates the emission spectrum of conventional anatase nanocrystals, which show a broad visible PL extending into the red with a peak at about 650 nm, hereafter referred to as “red PL”. We have recently assigned this red PL to the radiative recombination of trapped electrons, on 5-fold coordinated Ti on minority {001} facets, with valence band holes.4 In the presence of hole scavengers such as ethanol and acetic acid, this red PL is quenched leaving the Type 2 “green PL” characteristic of radiative recombination of mobile electrons with trapped holes hypothesized to be located on the majority {101} facets. The assignment of the red PL to electrons trapped on undercoordinated Ti3+ (when occupied) is consistent with spectroelectrochemical studies using optical absorption.2,31−33 We have speculated that the hole traps which prevail on {101} facets are the result of oxygen vacancies, since vacuum annealing of TiO2 films results in a conversion of red PL to green PL.29 We also previously concluded27 that TiCl4 treatment of TiO2,34−36 frequently used to improve the performance of TiO2 films in dye-sensitized solar cells, was capable of healing these green-emitting traps. The present work examines trap state luminescence under Fermi level control to provide further insight and some modification of these previous conclusions. Our results provide the position and width of electron trap distributions on an absolute energy scale and reveal a fundamental difference in the activity of trap state distributions of anatase and P25 nanoparticles. B



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Figure 2. Photoluminescence intensity of (a) anatase at 610 nm and (b) P25 at 530 nm, as a function of applied potential and at pH 1.8 (back squares), 6.4 (red circles), 9.6 (green triangles), and 12.0 (blue inverted triangles). The excitation wavelength is 350.7 nm and the electrolyte is 0.2 M NaClO4. The arrows denote the literature values of the conduction band potential from ref 14.

scatter geometry, and filtered with a 385 nm long pass filter. The spectra were measured with a 1/3 m imaging monochromator with a thermoelectrically cooled CCD. The TEC 15 glass is highly fluorescent when illuminated with 350 nm light; however, the absorption of the TiO2 film limits the penetration depth of the excitation to less than 1 μm. There was no evidence that the UV excitation was able to excite the glass substrate. The PL spectrum of a bare FTO glass is shown in Figure S1 of the Supporting Information. The TiO2 film thickness is a factor of 20 larger than the 1/e penetration depth of the incident light, so little incident light reaches the conductive support. For the emission spectra, the potential of the TiO2 electrode was adjusted with the potentiostat, and allowed to equilibrate for 15 s. The exposure time for each spectrum was 4 s, but each entire spectrum took approximately 60 s to gather since it is a step and glue composite of 12 exposure windows.

Figure 2a shows the intensity of the SEPL of anatase at 610 nm as a function of applied potential in different pH solutions. The emission intensity increase follows a proton-coupled Nernstian behavior with an onset that shifts to more negative potential as the pH increases. Note that the emission intensity increase begins ∼200 to 300 mV below the literature potential of the conduction band,14,38 depending on pH. For the anatase film, the decrease in the emission at 610 nm at more negative potentials is the result of the blue shift in the spectrum. Figure 2b shows similar data for the P25 films using the PL intensity at 530 nm. Lacking a literature value for the P25 conduction band (CB) potential, the PL onsets for P25 have been compared to the CB potential of anatase. Similar to the data for anatase, the onset potential for green PL of P25 is less negative than the nominal conduction band by a few hundred millivolts. Figure 3 compares the cyclic voltammogram (CV) of anatase to the voltage-dependent SEPL intensity of anatase at 530 and

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RESULTS Figure 1 shows the spectroelectrochemical photoluminescence (SEPL) of anatase and P25 films under band gap illumination (350.7 nm) and in contact with aqueous NaClO4 at pH 1.8. At potentials more positive than about −0.2 V, the SEPL of anatase (Figure 1a) shows the characteristic broad red PL seen previously8 for anatase at open circuit in the absence of hole scavengers. This is the Type 2 PL of eq 2. As the potential is made more negative, the PL intensity increases and the spectrum shifts to shorter wavelengths, eventually resembling the green PL characteristic of P25 and of anatase in the presence of hole scavengers, for which the maximum emission is at 530 nm (Type 1 PL, eq 1). The PL of P25 (Figure 1b) shows only the green PL characteristic of hole traps. This PL increases in intensity as the potential is decreased from −0.3 V. At more positive potentials, the PL of P25 remains weak but similar in shape. There is no evidence of red PL from P25 at any accessible potential, demonstrating that the red-emitting traps found in anatase are absent or quenched in P25. The intensity of the green PL increases by a factor of about 60 as the Fermi level is raised above the conduction band, consistent with our previous assignment of green PL to the recombination of conduction band electrons with trapped holes. For both P25 and anatase films, there is a decrease in PL intensity and further blue shift at potentials more negative than about −0.7 V. Over the range of potentials reported in Figure 1, the changes in SEPL were found to be reversible.

Figure 3. Cyclic voltammogram of anatase film, compared to the voltage-dependent PL intensity at 610 (red) and 530 nm (green) for anatase and at 530 nm (blue) for P25. The P25 intensity was normalized to bring it to the same scale. pH is 1.8. The arrow indicates the CB potential from ref 14.

610 nm and that of P25 at 530 nm, at pH 1.8 in all cases. The CV is in good agreement with literature reports.10,39,40The CV of P25, shown in Figure S2 of the Supporting Information, is similar to that of anatase at the same pH for films of similar thickness and area. A small peak at about −0.2 V was C



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Figure 4. SEPL spectra of (a) anatase (black) and TiCl4-treated anatase (red) and (b) P25 (black) and TiCl4-treated P25 (red), at an applied potential of −0.60 V and pH 6.4. (c) SEPL intensity at 610 nm of anatase (black) and TiCl4-treated anatase (red) and (d) SEPL intensity at 530 nm of P25 (black) and TiCl4-treated P25 (red), at different applied potentials at pH 6.4. The sharp features around 700 nm in parts a and b are plasma lines from the excitation laser.

As shown previously from measurements at open circuit,27 when P25 is treated with TiCl4, the near-infrared emission of rutile43 develops, centered at 820 nm, and the 530 nm PL decreases. Under Fermi level control, a much weaker rutile PL is observed in TiCl4-treated P25, as compared to what was seen in ref 27. Figure 5 shows the behavior of the 820 nm emission of P25 before and after TiCl4 treatment as a function of applied

sometimes observed as has been previously noted in the literature and variously assigned to traps at the surface41 or at grain boundaries.42 This small peak does not appear to be related to the luminescent traps observed here. On the other hand, the capacitive current seen at increasingly negative potentials has been associated with the filling of trap states accompanied by proton intercalation. The coincidence of this feature with the onset of PL and the pH-dependence of this onset shows that the luminescent traps are related to those seen in the CV experiment. Figure 4 presents a reexamination of our previous study27 of the effect of TiCl4 treatment on the PL of anatase and P25. At open circuit in Ar atmosphere, we had found that the green PL was quenched by TiCl4 treatment for both anatase and P25, and for the latter, the quenching is accompanied by the appearance of rutile PL at near-IR wavelengths. We concluded at that time that the traps responsible for green PL were healed by the surface treatment. It is apparent from the data in Figure 4 that this is not the case. Panels a and b of Figure 4 show the PL of anatase and P25, respectively, before and after TiCl4 treatment, at an applied potential of −0.60 V. Panels c and d of Figure 4 show how the peak PL intensity varies with potential for anatase and P25. Under Fermi level control, there is a slight decrease in both anatase and P25 PL after TiCl4 treatment, more so at more negative applied potentials, but the green PL does not vanish. The present data do not support the conclusion that TiCl4 treatment heals traps. Rather, diminished PL intensity at all applied voltages points to improved transport after surface treatment, which decreases the probability that nascent electron−hole pairs will radiatively recombine.

Figure 5. SEPL intensities as a function of applied potential. Black and red lines are the PL intensity at 530 nm of P25 and TiCl4-treated P25, respectively. Blue and pink lines are the intensity at 820 nm of P25 and TiCl4-treated P25, respectively. The electrolyte is 0.2 M NaClO4, pH 6.4. D



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Ti3+ versus the Drude absorption of CB electrons. However, correlation of this absorption with EPR spectra of Ti3+ (refs 33, 45, and 46) argues for the former assignment. Capture of electrons by Ti4+, whether associated with undercoordinated Ti or Ti adjacent to oxygen vacancies, has been shown to be accompanied by proton intercalation.24,39,47,48 The activity of these intraband gap states is seen in cyclic voltammetry and is frequently interpreted39,42,49 to result from electron traps within an exponential density of states.50,51 The coincidence of the increase in red PL with features in the CV and the Nernstian behavior of the PL onset at different pH values strongly suggest that the red emission can be assigned to the recombination of trapped electrons with valence band holes. In previous work using TiO2 nanosheets with (001) texture,4 which are rich in 5-fold coordinated titanium, we assigned the red PL to the recombination of electrons trapped at surface titanium ions with valence band holes: Ti5c3+ + h+VB → Ti5c4+ + hυ. We propose that this radiative transition is the reverse of the one responsible for the Urbach tail in the absorption spectrum. Indeed, we have previously seen that the intensities of the absorption band tail and the photoluminescence are correlated in that both are larger for nanocrystalline films prepared from aqueous rather than ethanolic dispersion.37 The pH-dependence of the voltage at which the red PL increases is accounted for by proton intercalation or adsorption: e.g. Ti4+− OH + e− + H+ → Ti3+−OH(H+)ads.38 It is reasonable to assume that in aqueous solution, water completes the coordination sphere of Ti5c. Comparison of the red PL onset potential to the reported flat-band potential reveals that redemitting electron traps are located about 0.2 eV below the conduction band edge. We previously interpreted the large breadth of the anatase PL to a broad distribution of trap depths; however, in this work we find that the increase in red PL with potential takes place over a rather narrow range of about 200 to 300 mV, which is much less than the breadth of the PL spectrum. The present results suggest that the potential range over which the red PL increases is associated with the reorganization energy for reducing Ti4+ to Ti3+. The breadth of the red PL, on the other hand, results from the reorganization energy for recombination of trapped electrons (Ti3+) with valence band holes. Since oxygen 2p states comprise the valence band, this recombination has the character of a Ti to O charge transfer transition, and a large Franck−Condon progression in Ti−O stretching vibrations is expected. In addition, the Jahn−Teller splitting expected for Ti3+ could also contribute to the observed spectral width.52 Thus the breadth of the optical spectrum associated with Ti3+ reflects contributions beyond those resulting from a distribution of trap depths. In the case of anatase, the conversion of the red PL characteristic of trapped electrons to the green PL associated with mobile electrons is responsible for the eventual decrease in intensity at 610 nm at more negative potentials as seen in Figure 1. However, if these changes in the PL spectrum result from raising the Fermi level through the traps states and then above the CB, then one must ask why the red PL decreases when the green PL turns on. We considered whether the answer may be that at potentials more negative than Vfb, an accumulation layer forms,14 and the valence band holes required for red PL are swept away from the surface. However, the small size of the nanoparticles and screening by the electrolyte minimize the importance of a space charge-layer, and a more likely scenario is a change in the surface charge as

potential. For comparison, the SEPL behavior of the 530 nm emission of P25 and TiCl4-treated P25 is included in Figure 5. We have shown27 using Raman spectroscopy that TiCl4 treatment of P25 increases the rutile content. Under Fermi level control, the NIR rutile emission decreases sharply at the same potential that the green 530 nm emission begins to increase, at about −0.6 V for pH 6.4, for both treated and untreated P25. The apparent increase in emission at 820 nm at potentials more negative than −0.6 V just results from the growth of the green PL which tails well into the red. Representative spectra of TiCl4-treated P25 at various potentials are shown in Figure S3 of the Supporting Information. The 820 nm PL of rutile is assigned to recombination of CB electrons with trapped holes,43 a Type 1 PL in our notation. As the mutual Fermi level of the anatase and rutile phases is raised, the rutile PL must eventually diminish as the midgap hole trap level becomes occupied by electrons, as seen in Figure 5.

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DISCUSSION In the present work, we compare our potential-dependent SEPL to the reported pH-dependent conduction band potential Vfb of nanocrystalline anatase.14,24 In both refs 14 and 24, the determination of Vfb was based on the assumption that CB electrons lead to optical absorption in the vicinity of 780 nm. Since the absorption of CB electrons overlaps that from trapped electrons, there could be some ambiguity in this assumption. However, the good agreement of the pHdependent flatband potential from refs 14 and 24 with that determined from a Mott−Schottky analysis for single-crystal anatase44 validates the determination of Vfb based on optical absorption. As we show below, the similarity of the pHdependence of the onset potential for red PL and the pHdependence of the conduction band suggests close communication between the conduction band and the trap-site redox couple, as has been previously noted.24,33 By controlling the Fermi level in the presence of band gap illumination, a more clear picture of the nature of luminescent traps of TiO2 emerges, albeit with some remaining puzzles. The present data present us with the challenge to understand pHdependence of the onset of red PL in anatase or green PL in P25, the apparent absence of red-emitting traps in P25, and the decrease in red PL of anatase at more negative potentials where the green PL associated with CB electrons is seen. Comparing our SEPL data for anatase to literature reports of the spectroelectrochemical absorption,2,33,38,44 we find comparable onsets for red PL and visible absorption that are in both cases several hundred millivolts less negative than the reported flatband potential, Vfb. This is as expected for population of subband gap states and is in accord with our previous assignment of red PL to trapped electrons. For example, Boschloo and Fitzmaurice2 observed a broad visible absorption that increased in intensity at more negative potentials, and noted that the onset of the absorption increase is about 0.5 V more positive than the Vfb. They assigned the absorption to conduction band electrons and attributed the onset of absorption at energies below the CB to the existence of band tails. Berger at al.39 working at pH ∼1 observed a broad absorption from ultraviolet to near-IR wavelengths which increased in intensity over a range of potentials similar to what is reported here for the increase in red PL in our sample at pH 1.8. As noted in ref 39, there is some controversy over the association of this spectroelectrochemical absorption with a d-d transition of E



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the Fermi level is raised above the conduction band edge. The well-known coloration of TiO2 under negative bias, resulting from the trapped and mobile electrons, causes increasing selfabsorption of the emitted light at longer wavelengths, and could increasingly attenuate the red PL at more negative potentials. However, the backscattering geometry and low-penetration depth of the incident light (about 0.5 μm) used in this work should minimize the effects of self-absorption. Weak selfabsorption is highly unlikely to be the reason for the evolution of the SEPL with potential. For example, the conversion of the red PL of anatase in the presence of argon, water, or acetonitrile to green PL in the presence of ethanol as a hole scavenger8 takes place at open circuit in the absence of any film coloration. Also, the green PL seen for anatase at negative potentials is identical in shape to the PL of P25 at open circuit. Alternatively, the eventual diminution of the red PL as the Fermi level is raised above the CB may result from the activity of another trap or substrate, invisible by PL measurements, which is capable of oxidizing Ti3+ to Ti4+. For example, desorption of O2 at more negative potentials could make O 2 available for this oxidation.53,54 Yet another possibility invokes the rate at which Ti(IV) centers are reduced as the Fermi level is raised through the Gerischer distribution of the traps in their oxidized form, the width of which is determined by the reorganization energy for the reduction of Ti4+ to Ti3+. Since the PL is weak, there is a significant rate at which trapped electrons combine nonradiatively with valence band holes created by band gap excitation. As the Fermi level is raised well above the Gerischer distribution, the rate at which traps are filled slows down and might not be able to compete with the nonradiative decay of trapped electrons. In addition, hole-trapping and recombination with CB electrons could be fast compared to recombination of trapped electrons with valence band holes such that emergence of green PL is accompanied by diminution in red PL. The data of Figure 2 can be used to estimate the trap state distribution using the Gerischer55 model by associating the traps states with Ti4+/3+. The density of trap states Nt(E) is taken to be a Gaussian distribution for which the standard deviation is (2kBTλ)1/2: Nt(E) =

NL (4πkBTλ)1/2

2⎤ ⎡ − (E − E F,redox − λ) ⎥ exp⎢ ⎢⎣ ⎥⎦ 4kBT

Figure 6. Density of states (DOS) for conduction band (black) and trapped (red) electrons, estimated by using the data from Figure 2a for the pH 9.6 sample as explained in the text.

Gerischer distribution for the traps peaks well-above the conduction band edge, the effect is a trap density of states similar to an exponential tail on the conduction band. In addition, overlap of the distributions for trapped and CB electrons explains the facile occupation of traps as the Fermi level is raised, and the occurrence of photocurrents at potentials below the conduction band edge.59 We have previously assigned hole traps associated with the green PL of anatase and P25 to oxygen vacancies based on open circuit measurements on vacuum-annealed films.29 However, creation of oxygen vacancies also raises the Fermi level which could, on its own, account for our previous observation that vacuum treatment converts red PL to green PL. In favor of our previous assignment of the hole traps to oxygen vacancies, we note the well-known occurrence of color centers associated with oxygen vacancies which lead to absorption at visible wavelengths.53,60−62 Experiments and calculations point to the existence of an F center consisting of two electrons trapped at an oxygen vacancy, located just above the valence band edge. The UV-induced absorption of TiO2 at blue wavelengths is assigned to the promotion of an electron from this trap to the conduction band.59,60 We suggest that green PL is the reverse of this transition, in which the Stokes shift from blue to green is reasonable in light of the breadth of both the induced absorption and the PL. This brings us to the consideration of the difference in the SEPL of anatase and P25, including the absence of red PL from P25 at any potential. It has been previously noted that fewer defects are present in P25 than in anatase nanoparticles,63 so if the red-emitting traps are absent in P25, what is the reason? We have speculated that the red- and green-emitting traps of anatase nanocrystals are sequestered on different facets, and that one of these is passivated in P25 by the rutile−anatase interface. However, there is little consensus in the literature concerning the morphology of P25 nanoparticles. TEM studies of P25 find anatase nanocrystals exposing (100), (001), and (101) facets,64,65 and little evidence for a rutile−anatase interface.66 However, a thin rutile overlayer would be difficult to detect, and any model for the juxtaposition of the anatase and rutile phases must explain the extremely weak near-IR rutile emission associated with the (110) and (100) facets.27,43 (The near-IR PL of pure rutile nanocrystals is quite strong.) A difference in the proportion of (001) and (101) facets in anatase and in the anatase component of P25 might be reflected in electrochemistry experiments as a result of their

(3)

Here, λ is the reorganization energy for the reduction of Ti4+ to Ti3+, EF,redox is the Fermi energy of the redox couple, and NL is the total density of electron trap states. EF,redox was estimated from the voltage at which the intensity of red PL reaches half its maximum value, about 0.26 eV below the conduction band for the anatase sample at pH 9.6. From the width at half-height of the graph of PL intensity versus voltage, we estimate λ ≈ 0.44 eV. The total density of trap states was taken as NL = 2 × 1020 cm−3, in accord with several literature values.1,56,57 Since the center of the trap state distribution is at EF,redox + λ, this moves the peak of Nt(E) well into the range of the CB density of states, NCB(E). For the comparison shown in Figure 6, the latter was taken to be NCB(E) =

(2m*)3/2 1/2 E 2π 2ℏ3

(4)

where the CB edge is taken to be zero, and the effective mass m* was taken to be equal to the electron mass.58 Figure 6 sheds light on a number of experimental observations. Since the F



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applied potentials, results from improved interparticle carrier transport36 which competes with radiative recombination.

different work functions and different photoredox activities.67−70 However, as shown in the Supporting Information (Figure S2) the cyclic voltammograms of anatase and P25 films of comparable thickness and area, and at the same pH, are similar, though not identical. If the electron traps responsible for red PL are major contributors to the capacitive CV response, then Figure S2 argues against the conclusion that they are absent in P25. This suggests a scenario we have previously considered in trying to account for the absence of red PL and the very weak near-IR rutile PL in P25 at open circuit: that the mutual quenching of PL from red-emitting anatase traps and near-IR rutile traps results from interphasial electron transfer from anatase to rutile. The similar CV data for anatase and P25 suggests similar electron trap distributions below the CB edge despite their very different PL spectra. In our SEPL experiments, in contrast to experiments at open circuit, the Fermi levels of the anatase and rutile components of P25 are equilibrated, but the 0.2 eV higher CB energy of anatase compared to rutile44 could still permit transfer of photogenerated electrons from the anatase to rutile phase which, if fast enough, could compete with the rate of red PL emission, which occurs on a microsecond time scale.71 If indeed the red traps are present but their PL inactive in P25, it is also necessary to explain why these putative Ti4+/3+ traps give rise to spectroelectrochemical absorption (d-d transitions of Ti3+) but not emission. The time scale difference for absorption (instantaneous) and emission (microsecond time scale) of these electron traps could explain the presence of spectroelectrochemical absorption associated with Ti3+ centers in P2533 despite their inactivity in the photoluminescence spectrum. The present results support the conclusion that the Ti4+/3+ electron traps exist in anatase and in P25, but their emission is quenched in the latter, perhaps as a result of recombination of trapped electrons on anatase with holes in the rutile phase. Such a scenario implies that the facets on which the anatase electron traps reside are in intimate contact with the rutile facets on which the hole traps reside. After TiCl4 treatment of P25, rutile PL appears at positive potentials and green PL is diminished at all applied voltages, as shown in Figure 5 for pH 6.4. At more positive potentials, we see rutile PL that results from photogenerated electrons combining with holes trapped in the rutile phase. As the potential is made more negative, these hole traps become occupied by electrons, and the rutile emission is quenched as the green PL of the anatase phase increases. Given that the TiCl4 treatment results in formation of additional rutile phase, these results suggest formation of new (110) and (100) facets on which rutile hole traps are known to reside.43 The band gap of rutile is 3.0 eV, thus 350 nm excitation also creates electron− hole pairs in the rutile component of P25. As the potential is made more negative and green PL from the anatase phase increases, the near-IR PL of rutile decreases greatly, as shown in Figure 5. (The apparent increase in 820 nm PL below −0.6 V results from the tail of the increasing green PL. See Figure S3 of the Supporting Information.) The potential of the CB edge of anatase is about 0.2 V more negative than that of rutile,34 hence conduction band electrons produced in the anatase phase would be expected to transfer to the rutile phase and lead to near-IR PL. The rapid (nanosecond71) time scale for decay of the green PL may explain the low intensity of rutile PL at potentials more negative than the conduction band edge of anatase. It is speculated that the small decrease in the entire PL spectrum of both anatase and P25 after TiCl4 treatment, at all

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CONCLUSIONS In this work, we have used Fermi level control of a nanocrystalline TiO2 electrode while illuminating with band gap radiation to probe the pH-dependent redox energies of surface electron traps. A Nernstian shift in the trap state distribution with pH is attributed to a proton-coupled redox process in which under-coordinated Ti4+ centers are increasingly reduced to Ti3+ as the Fermi level is raised. The radiative recombination of electrons trapped at these centers with valence band holes results in broad visible photoluminescence with a peak in the red. This red photoluminescence is concluded to be the emission counterpart to the Urbach tail observed in absorption. The dependence of the intensity of this PL on applied voltage enables the reorganization energy for reduction of Ti4+ to Ti3+ to be determined, and reveals that the Gerischer distribution overlaps the density of states of the conduction band, emulating an exponential tail below the band edge. In agreement with previous work assigning the green photoluminescence of anatase, observed in the presence of hole scavengers, to the recombination of conduction band electrons with trapped holes, the red PL of anatase converts to green PL when the Fermi level is raised above the conduction band. We associate the emissive hole traps with oxygen vacancy color centers known to result in absorption at blue wavelengths. In mixed phase (P25) TiO2 this green PL dominates the emission at all applied voltages owing to the quenching of red-emitting electron traps. A practical conclusion of this study is that the photoluminescence intensity of nanocrystalline TiO2 is quite sensitive to environment through its effect on the Fermi level, perhaps explaining some of the discrepancies in reported spectra taken at open circuit. (See, for example, ref 30 for a literature review.) We suggest that spectroelectrochemical photoluminescence (SEPL) can be advantageously applied to probe the trap state distributions of nanoparticles and the influence of contacting solvents and adsorbates on these distributions. In future work, we will consider the influence of nonaqueous media on TiO2 traps using solvents of interest in photoelectrochemical and photovoltaic applications.

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ASSOCIATED CONTENT

* Supporting Information S

PL of bare FTO support (Figure S1), CV data for P25 and anatase at different pH values(Figure S2), and SEPL spectra of TiCl4-treated P25 as a function of applied bias (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 509 335 4063. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the National Science foundation (CHE1149013) is gratefully acknowledged. We thank Scot Wherland for the generous loan of his potentiostat. We are grateful for G



Article

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helpful discussions about electrochemistry with our recently departed and sorely missed colleague Jim Schenk.

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Spectroelectrochemical Photoluminescence of Trap States of

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