Photoluminescence Mechanisms in Anatase and ... - ACS Publications

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Photoluminescence Mechanisms in Anatase and Rutile TiO2 Deborah K. Pallotti,† Luca Passoni,‡,§ Pasqualino Maddalena,∥ Fabio Di Fonzo,*,‡ and Stefano Lettieri*,† †

Institute of Applied Sciences and Intelligent Systems (CNR-ISASI), Via Campi Flegrei 34, I-80078 Pozzuoli, Italy Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli, 70/3, 20133 Milano, Italy § Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, Milano 20133, Italy ∥ Dipartimento di Fisica “E. Pancini”, Università degli Studi di Napoli “Federico II”, Via Cintia I-80126 Napoli, Italy ‡

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S Supporting Information *

ABSTRACT: Photoluminescence (PL) represents a sensitive tool for probing molecular adsorption and surface reactions in photocatalytic materials. Titanium dioxide (TiO2) is one of the most widely used photocatalysis, and clarifying its basic PL mechanism can give important information. However, differently from other electronic and surface processes, the actual PL mechanisms of TiO2 are not extensively studied. In this work, we address the topic by focusing our investigation on which are the different states that trigger the PL activity and on identifying the specific recombination pathways acting in the two stable TiO2 polymorphs (rutile and anatase). On the basis of our experimental results on PL emission, PL excitation, and oxygen-induced and photoinduced PL modifications, we sketch an interpretative scheme for both the polymorphs. Excitation-resolved PL and recombination quenching caused by molecular oxygen evidence distinct contributions to anatase PL, originating from different kinds of hole-trapping and electron-trapping defects that we ascribe to surface and subsurface oxygen vacancies, respectively. Two possible mechanisms are discussed for rutile PL, involving self-trapped holes located at oxygen atoms or trapped electrons occupying midgap states positioned below the Fermi level. We argue that the validity of the former mechanism would imply that self-trapped holes are efficiently formed far from the rutile surface, while the latter mechanism seems more plausible although the very nature of the involved midgap electron state still has to be clarified.

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INTRODUCTION Photoluminescence (PL) spectroscopy analyzes the characteristics of the light spontaneously emitted by a photoexcited material. PL in semiconductors is the macroscopic manifestation of radiative recombination between photogenerated carriers, occurring in tiny regions (few tenths of nanometers wide, typically) below the semiconductor surface.1 Therefore, the PL characteristics (e.g., spectrum, intensity, lifetime) are very sensitive to molecular adsorption and to surface chemical reactions. Moreover, PL intensity quantifies the recombination efficiency of photogenerated charges, which represents the most important limiting factor for photocatalytic efficiency. Hence, since earlier studies by Anpo and co-workers,2,3 PL has been regarded as an important tool for studying photocatalytic materials and surface properties.4−14 TiO2 is one of the most widely used photocatalysts, thanks to its good oxidizing power, chemical stability, low cost, and multifunctionality. Most of the TiO2 applications (e.g., environmental remediation,15,16 dye-sensitized solar cells,17 gas sensors,18,19 self-cleaning surfaces,20 etc.) rest on its photocatalytic properties and on its surface chemistry. Thus, investigating and clarifying the basic PL mechanisms active in this material can give important information. For example, it has been suggested that understanding the origin of the near© 2017 American Chemical Society

infrared (NIR) PL emission of rutile TiO2 can elucidate the mechanisms of its water photooxidation: on the basis of PL studies, some authors proposed21,22 that water splitting is initiated by a nucleophilic attack of H2O molecule to selftrapped holes located on O atoms of rutile surfaces, accompanied by bond breaking. This interpretation is alternative to a more conventional one assuming that the process is initiated by reaction of water with holes trapped on hydroxyl radicals2 and relies on an hypothesis about the nature of the hole states (self-trapped or free) responsible for PL emission of rutile TiO2. TiO2 PL exhibits a peculiar dependence on the crystalline phase, as the two stable TiO2 polymorphs (anatase and rutile) are characterized by well-separated PL bands. These originate from different mechanisms that have not been studied in depth as those controlling other TiO2 electronic and surface properties.17,27,28 In fact, current TiO2 literature does not exhibit a well-established consensus on the involved PL processes nor on the chemical nature of the states involved in the radiative transitions. Several works hypothesize different Received: January 12, 2017 Revised: March 30, 2017 Published: April 11, 2017 9011

DOI: 10.1021/acs.jpcc.7b00321 J. Phys. Chem. C 2017, 121, 9011−9021

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The Journal of Physical Chemistry C

Figure 1. (A) PL spectra of RF (black curve), AF (blue curve), and AP (red curve) under 325 nm excitation, collected in N2. (B, C) Variations of the G-PL intensity (ΦG, black circles) and R-PL intensity (ΦR, red squares) for AF sample (panel B) and AP sample (panel C) caused by O2 desorption. Quantities ΦG and ΦR are defined in eqs 1 and 2. The N2 flow is activated immediately after the acquisition of the data point at t = 0. The values for ΦG and ΦG are normalized by their initial value in synthetic air. (D) PL spectra of samples AP measured in N2 using 325 nm laser excitation (black curve) and 442 nm laser excitation (blue curve). A decomposition of G-PL band in two contribution, according to the work by Tachikawa and coworkers,36 is proposed in the Supporting Information (Figure S1).

of PL-active trap states, and their correlation with the oxygen stoichiometry. In the case of rutile, we review the two possible interpretations of the kind of charge carriers (i.e., hole or electron) trapped in the midgap states that are responsible for the NIR-PL.

attributions, often contradicting one each other. Literature works report both free holes6,25 and trapped holes21,22 as responsible for rutile PL. Similarly, different hypothesis have been proposed for anatase PL, including donor-to-acceptor transition,26 superposition of optical transitions from band states to electron and hole defect states,4,27 intrinsic selftrapped excitons,28,29 or combinations of the above contributions.26,30,31 The difference in PL mechanisms of the two TiO 2 polymorphs also reveals an intriguing feature: interaction of anatase and rutile TiO2 with molecular oxygen (O2) induces opposite PL intensity changes (PL decrease in anatase and a PL enhancement in rutile).32 Such an “anticorrelated” PL modulation in the two polymorphs potentially represents an effective basis to enhance the sensitivity of PL-based chemical detection.33 Hence, a clear understanding of PL emission may also open interesting scenarios in the field of optical sensing of O2. In this work, we address the PL mechanisms in anatase and rutile TiO2, focusing on which are the different states triggering the PL activity and the specific radiative recombination pathways acting in the two polymorphs. To this aim, an experimental study of PL emission, PL excitation (photoluminescence excitation spectroscopy or “PLE”), and oxygeninduced and photoinduced PL modifications has been performed on different TiO2 films and nanoparticle samples. By reviewing the experimental outcomes, we sketch an interpretative scheme for both the polymorphs. Regarding anatase TiO2, we evidence the presence of two physically distinct contribution to PL spectra and highlight the separated recombination pathways for free and trapped charges, the role

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EXPERIMENTAL DETAILS Samples Preparation and Characterization. Nanostructured and quasi-one-dimensional TiO 2 films were deposited on silicon substrates by pulsed laser deposition (PLD) through the procedure described in the Supporting Information. The as-deposited samples were subjected to thermal annealing in air at different temperatures, producing their crystallization in anatase and rutile phases. Anatase and rutile PLD-prepared films used in this work are hereafter named “AF” (anatase film) and “RF” (rutile film). They were prepared at an annealing temperature of T = 500 and 800 °C, respectively, and their crystal phase was proved by X-ray diffraction and Raman spectroscopy. Film morphology and thickness was assessed by cross-section SEM analysis, determining a sample thickness of 1.4 and 1.3 μm for AF and RF, respectively. Experiments were also carried out on bulk pellets obtained by cold-pressing commercial TiO2 nanopowders through a hydraulic press. Sigma-Aldrich 232033 nanopowder was employed to prepare anatase pellets, hereafter named “AP”. Mixed-phase TiO2 pellets (“MP” hereafter) were also prepared for the PLE experiment. For MP samples preparation, rutile nanopowder was prepared by annealing Aeroxide P25 (SigmaAldrich 718467) at 900 °C in ambient air for 1 h, guaranteeing its full crystallization in the rutile phase. Next, the rutile and 9012


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physically distinct components, centered in the green (peak at ∼2.35−2.45 eV photon energy, corresponding to λ ≈ 530−510 nm) and in the red (peak at ∼1.9 eV photon energy, corresponding to λ ≈ 650 nm) regions of the optical spectrum, hence hereafter named as “G-PL” and “R-PL”, respectively. Interactions with external chemical species changes the relative weight of the G-PL vs R-PL contribution, determining a variation in the overall spectral shape of visible PL.4,25,27 In this section, we show that the physical nonequivalence of these two PL components manifests itself in (i) anatase/O2 interaction and (ii) PL excitation mechanisms. Concerning the first point, Figures 1B and 1C display the changes in the total PL intensity ΦG and ΦR associated with GPL and R-PL bands occurring during the desorption of O2 from sample surfaces. The quantities ΦG and ΦR are defined as

anatase nanopowders were then mixed (relative mass percentages ∼10% rutile and 90% anatase), and the resulting composite powder was cold-pressed obtaining pellets containing both TiO2 crystal phases. PL and PLE Measurements in Controlled Gas Environment. Room temperature PL measurements in controlled ambient gas were performed using as excitation source a HeCd laser having two available emission wavelengths: λ = 325 nm (corresponding to 3.82 eV photon energy) and λ = 442 nm (corresponding to 2.8 eV photon energy). The power of the two laser lines was 5 and 40 mW, respectively. Excitationresolved photoluminescence (PLE) measurements were performed by using as optical excitation source a white-light broadband Xe lamp coupled with a computer-controlled double-grating monochromator and providing tunable and spectrally filtered light (spectral full width at half-maximum of about 2 nm). PL and PLE measurements were carried out by placing the samples in a home-built sealed stainless steel chamber equipped with a quartz window. A computer-driven mass flow control system was used to manage the gaseous composition inside the test chamber. Further details on the experimental setup are reported in the Supporting Information.

ΦG =

ΦR =

∫Δλ

φ (λ ) d λ

(1)

G

∫Δλ

φ (λ ) d λ

(2)

R

where φ(λ) is the experimental PL spectrum and the wavelength intervals are chosen as ΔλG = 465−540 nm (∼2.3−2.6 eV photon energy) and ΔλG = 590−730 nm (∼1.7−2.1 eV photon energy). The PL intensity was initially measured in synthetic dry air (80% N2, 20% O2). After the acquisition at time t = 0 the flow composition switched to 100% dry N2 (i.e., O2 was removed) while continuing measuring the PL spectra. The data points in Figures 1B and 1C are normalized with respect to their value at t = 0 (measured in synthetic air) to allow comparing the relative increase during O2 desorption. The results show that both PL bands of anatase are enhanced as O2 desorbs from the surface, but the G-PL emission is more enhanced than the R-PL one. The difference was particularly marked for AF films (Figure 1B) but we observed in all the investigated anatase samples (see for example Figure 1C, referring to an AP sample). The larger reactivity of G-PL occurs also for O2 adsorption and not only for desorption, as shown in the Supporting Information (Figure S6). The second feature we highlight is that the anatase G-PL and R-PL components exhibit different excitation mechanisms. In Figure 1D, we show the PL spectra (measured in N2) of AP obtained by excitation at 325 nm (black curve) and at 442 nm (blue curve). It is observed that the G-PL is dominant for above-bandgap excitation at 325 nm and much weaker for below-bandgap excitation at 442 nm (its photon energy of about 2.80 eV is 0.4 eV below the bandgap edge of bulk anatase TiO2). The same result was obtained for PLD-prepared AP films (see Figure S2). To better understand the excitation mechanisms, we performed a photoluminescence excitation (PLE) analysis, consisting in measuring the PL spectra for various values of the wavelength of the excitation light. The data provided by PLE are summarized in the “PLE map” φ(λexc,λPL) expressing the PL intensity φ measured at wavelength λPL when the sample is excited by a monochromatic source of wavelength λexc. The PLE amplitude IPLE Δλ (λexc) associated with a PL band positioned in the wavelength interval Δλ is defined as the quantity

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RESULTS Emission and Excitation Properties of Anatase and Rutile TiO2. Sample analyses performed by XRD and SEM are reported in the Supporting Information and confirm previously reported results.19,34,35 The samples are highly crystalline and consistin the case of anatase PLD-prepared films (“AF”)of quasi-one-dimensional hyperbranched structures (“nanotrees”), whose branches are single-crystal with preferential (011)-type surface termination and [004] direction growth.34 The transition from anatase to rutile caused by thermal annealing leads to grain growth, with coalescence of the nanoscale branches (Figure S2) and the transformation of the quasi-one-dimensional hyperbranched structures in polycrystalline nanostone piles.19 The loss of the nanoscale texture of anatase nanotrees determines a decrease of the surface-tovolume ratio19 after the annealing at 800 °C and accounts for the shrinkage of the line width of XRD peaks (compared to the XRD peaks of anatase) noticeable in Figure S1, associated with the presence of larger crystallites. The PL spectra of rutile and anatase TiO2 extend in the nearinfrared (NIR) and visible ranges, respectively. NIR-PL of rutile is typically centered at about 1.48−1.51 eV (820−840 nm wavelength) with a full width at half-maximum of about 0.2 eV. The visible PL emission of anatase is much broader (full width at half-maximum ∼0.8−0.9 eV) and is typically centered at about 2.3−2.5 eV, even if the actual peak position and spectral shape strongly depend on the surrounding gaseous environment and on the excitation conditions (as shown later). Representative NIR-PL and visible PL spectra obtained from RF, AF, and AP samples, measured in flowing N2 at abovebandgap excitation (325 nm HeCd laser excitation), are reported Figure 1A. It is noted that AF luminescence displays Fabry−Perot fringes, caused by interference between the backscattered PL light and the forward-scattered PL light reflected at the TiO2/substrate interface. The presence of Fabry−Perot fringes indicates a high degree of homogeneity and optical quality for the PLD-prepared anatase films. Previous works4,27,31 evidenced that the broad VIS-PL exhibits two

IΔPLE λ (λexc) =

∫λ

λ2

1

9013

φ(λexc , λPL) dλPL /Δλ

(3)


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The Journal of Physical Chemistry C In the above expression, λ1 and λ2 are the extremes of the wavelength interval Δλ = λ2 − λ1. The excitation spectrum of each specific PL band can thus be extracted by proper choice of the integration interval in eq 3. As the spectral PL shape of AF samples is partially hidden by Fabry−Perot interference fringes, we used bulk powdered MP samples for a more precise analysis of the spectral positions. The simultaneous presence of anatase and rutile crystals in the MP sample allowed determining the excitation spectra all the three investigated components (G-PL, R-PL, NIR-PL) in a single experimental run. The related PLE map and the excitation spectra are reported in Figures 2A and 2B, respectively. The excitation spectra have

The R-PL emission (red open circles) also increases when excitation photon energy crosses the onset of interband transitions but remains substantial also at below-bandgap excitation conditions in which it is the dominant band (in agreement with Figures 1D and 1E). We underline that the PLE amplitude of R-PL emission only halves from its maximum value to its minimum value at below-bandgap excitation (2.8 eV), while the G-PL emission decreases by a factor of about 20. Finally, the PLE amplitude data for NIR-PL emission (open blue squares) indicate that this band is activated by interband transition (i.e., by free carriers), with an excitation onset at about 3.0 eV that agrees with the literature bandgap edge of rutile. PLE experiment on PLD-prepared films, reported in the Supporting Information, led to equivalent results. Reversible and Irreversible O2-Induced Photoluminescence Modifications. Previous works32,33,36 indicate that O2 adsorption of on TiO2 surfaces modifies the PL intensity of the two polymorphs toward opposite direction (intensity enhancement in utile, intensity quenching in anatase). We show here that the O2-induced changes in PL intensity contain both a reversible and an irreversible (Δφirr) component. For “reversibility”, we refer here to the ability of the material to recover the PL intensity measured in inert environment (e.g., N2) after exposure to O2. This phenomenon is relevant to elucidate the PL mechanisms, as shown in the next section. The relative “weight” of the irreversible component Δφirr/φ0 (where φ0 indicates the “intrinsic” PL intensity, i.e., the steadystate PL intensity measured in N2) was found to depend on the overall exposure to O2 and UV laser light experienced by the sample. In fact, larger Δφirr was observed in unexposed samples and became less relevant as the samples were repeatedly exposed to the HeCd laser light in the presence of O2. Representative examples of this behavior are shown in Figures 3A−C. In Figure 3A we report the G-PL intensity of AF sample during the exposure to two consecutive O2 + N2 flows having the same O2 concentration (1%, yellow-colored time interval in the figure), followed by exposure to pure N2 flow. It is seen that the initial PL intensity in N2 was not recovered after the first 1% O2 exposure, while instead the next steady-state PL intensity was reobtained after a successive O2 and N2 exposure. An analogous result is obtained for NIR-PL intensity (RF sample), shown in Figure 3B: the PL baseline in N2 is in this case higher after the first exposure to UV light and O2. The irreversible modifications in the PL efficiency experienced by the two samples cannot be attributed to a simple long-term oxidation in ambient air. In fact, the data reported in Figure 3B (rutile film) were collected 2 weeks after those in Figure 3A, during which the RF sample was stored in the dark and in ambient air. Then, the experiment in Figure 3B was performed on the R-TiO2 sample, observing even so the shift of the PL level. Very importantly, we observed that the irreversible PL modification caused by O2 + UV treatment in anatase TiO2 involves the G-PL emission more than the R-PL one. We prepared an AP sample and placed an in the test chamber under a constant N2 flow (400 sccm) for 16 h. Next, an O2 + UV treatment was performed by exposing for 2 h the sample to a 400 sccm flow of synthetic dry air (20% O2, 80% N2) and to continued illumination from the UV laser. Successively, the sample was exposed again to a 100% N2 flow until a PL equilibrium value was restored. PL emission spectra were

Figure 2. (A) PLE map of MP TiO2 sample, measured in N2. (B) Corresponding excitation spectra obtained by numerical integration of the PLE intensity map the photon energy intervals 2.3−2.6 eV (G-PL, black circles), 1.7−2.1 eV (R-PL, open red circles), and 1.4−1.55 eV (NIR-PL, open blue squares).

been obtained by numerical integration of the PLE map in the following wavelength intervals (photon energy intervals): GPL: 465−540 nm (∼2.3−2.6 eV); R-PL: 590−730 nm (∼1.7− 2.1 eV); NIR-PL: 800−885 nm (∼1.40−1.55 eV). The G-PL emission spectrum (black circles) resembles that of an interband absorption plus a sub-bandgap band tail. In fact, the PLE amplitude is large for excitation energy greater than 3.4 eV but decreases markedly as the excitation energy lowers and approaches the onset of interband transition (extrapolated at about 3.1 eV by a Tauc plot; see Supporting Information). The subgap region shows a Urbach-like tail, indicating a distribution of shallow states. An estimation of the Urbach energy led to the value of 0.27 eV. 9014


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Figure 3. (A, B) G-PL intensity of AF sample (panel A) and NIR-PL intensity of RF sample (panel B) monitored during two consecutive exposures to N2 + O2 flows at the same O2 concentrations ([O2] = 1%). The gray regions indicate the time interval in which the O2 was flown in the test chamber, alternating with pure N2 flows (uncolored regions). (C) Peak-normalized PL curves of AP sample, measured in N2 before (curve a, black) and after (curve b, red) a 2 h O2 + UV treatment followed by the restoral of the equilibrium PL in N2. The spectra are normalized to their maximum value at the G-PL central position. Hence, the increase in the R-PL component highlighted in the inset evidence that the irreversible quenching affects the G-PL component more than the R-PL one.

in materials having a large specific surface area, such as nanostructured systems. Our PL and PLE results of Figures 1 and 2 indeed indicate that such nonexcitonic states contribute to anatase PL, as the red-centered PL emission occurs for below-bandgap excitation when excitons cannot be formed. Concerning the G-PL excitation mechanisms, the PL (Figures 1D,E) and PLE (Figures 2A,B) results show that the G-PL intensity follows an interband excitation behavior, indicating that conduction band (CB) electrons eCB− are responsible for this emission band. Therefore, the O2-induced quenching of G-PL can be interpreted in a natural way as due to the scavenging of photogenerated eCB− by O2 molecules. This process is competitive with the relaxation and/or radiative recombination of the photogenerated electron and thus quenches the G-PL emission. This same mechanism allows explaining the PL quenching caused by oxidizing molecules observed in many n-type MOX semiconductors.32,42−47 We consider now the R-PL band. It can be inferred from the experimental results that this band cannot be activated only by eCB−, as this would conflict with its lesser (compared to G-PL) sensitivity to O2. Moreover, the PLE data indicate that R-PL is significantly efficient under sub-bandgap optical absorption, differently from the G-PL one. Putting together these two observation, we can conclude that the R-PL is likely to start through the promotion of electrons from the valence band to shallow subgap states. After being promoted to such states, electrons can further relax by occupying trap states positioned

monitored during the whole process. We observed that this treatment quenched the G-PL emission more than the R-PL one, resulting in a modification of the intrinsic PL shape. In Figure 3C, we compare the PL shapes of the sample measured both in N2, before the start of the O2 + UV treatment (curve a, black) and after the restoral of the equilibrium (curve b, red). The spectra are normalized to the G-PL peak value, and the inset evidences the relative increase in the R-PL/G-PL ratio. In other words, it indicates that the UV irradiation in O2 environment irreversibly quenches the G-PL component more than the R-PL one.

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DISCUSSION Photoluminescence Mechanisms in Anatase TiO2. Earlier studies28 suggested that the anatase PL originates from radiative recombination of self-trapped excitons (STE). STEs result from the spontaneous self-localization of photogenerated charges occurring in polar semiconductors with strong electron−phonon coupling (e.g., SrTiO3, BaTiO3, and MgTiO3).37,38 Compared to rutile TiO2, anatase TiO2 is characterized by longer Ti−Ti interionic distances, lower TiO6 octahedral coordination, and poorer symmetry. These characteristics favor STE formation, according to the Toyozawa theory of self-localization.39−41 However, it has been often argued that recombination processes involving surface states and trap states4,25,30,31 also contribute to anatase PL. These additional contributions are expected to be particularly relevant 9015


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states (trapped holes) that determines the O2-quenchable G-PL (see (e) in Figure 4):

well below the CB edge. The works by Tang and co-workers evidencing Urbach tails (i.e., continuum of shallow subgap states) in optical absorption of anatase TiO2 (and not present in rutile) support this picture.29 In light of these considerations, we summarize in Figure 4 the deduced PL mechanisms of anatase TiO2. In this scheme,

eCB− + h tr + → G‐PL

As mentioned previously, the combination of the two competitive processes 4 and 5 justifies the O2-induced quenching of PL intensity. To explain the activation at below-bandgap excitation of the R-PL and its lesser sensitivity to electrons scavenging by O2, we depict an additional radiative channel in Figure 4 involving electron trap states. The R-PL is in fact assigned to the process

e tr − + hVB+ → R‐PL

(6)

involving the recombination of electrons localized in trap states with photogenerated valence band holes. Part of the electron traps involved in eq 6 can be energetically positioned below the Fermi level and thus occupied beforehand the optical excitation. Others, initially empty, can be populated from band-tail states (see (f) in Figure 4) occupied through below-bandgap optical excitation (see (c) in Figure 4) of from CB through nonradiative relaxation of free electrons ((g) in Figure 4). Thus, optical transitions in eq 6 can be started by photogenerated free electrons relaxing from the conduction band (see (h) in Figure 4) and from electrons already present at equilibrium ((i) in Figure 4). While the latter process is expected to be weakly influenced by the adsorption of O2, the former (process (h)) is instead clearly affected by O2 adsorption, as the quasi-equilibrium dynamics between occupied CB states and trap states dictates a proportionality between their occupation densities under stationary optical excitation. The participation of these two processes can explain the lesser (compared to the green PL) but still present O2induced quenching of the red component of anatase PL. Our scheme for anatase PL does not support the involvement of self-trapped excitons or of donor−acceptor pair recombination, while instead it strongly supports the works by McHale and co-workers,4,25,27 who, on the basis of a series of studies on PL modification of anatase TiO2 in various gaseous and aqueous solutions with different solvents, also correlated the green and the red emission of anatase to hole and electron traps (as in eqs 5 and 6). Photoluminescence Mechanisms in Rutile TiO2. PL of rutile TiO2 is more difficult to interpret than the anatase one. Recombination of self-trapped excitons cannot be invoked for rutile, while the energy position of its PL spectrum (about 1.5 eV) implies that a deep midgap state of unknown chemical origin is necessarily involved. Our PLE results indicate that rutile NIR-PL emission is efficiently excited only by photons whose energy exceeds the bandgap energy. The optical process leading to NIR-PL thus necessarily involves the recombination between a free carrier (CB electron or VB hole) and a trapped carrier occupying a midgap state. Therefore, the discussion necessarily splits in two possible interpretations, which are (1) trapped holes recombine radiatively with free electrons or (2) free holes recombine radiatively with trapped electrons. We review them separately. Case 1: The “Trapped Holes” Mechanism. Studies conducted by Nakato and co-workers at Osaka University identified the light-emitting state of NIR-PL with an intermediate of the water photooxidation reactions in anodic bias.21,22,49−53 In particular, authors of these studies proposed that photogenerated free holes coming to the surface can be

Figure 4. Summary of anatase PL mechanisms: (a) excitation of hot carriers by above-bandgap optical absorption; (b) relaxation of hot electrons in the conduction band; (c) excitation of band-tail states by below-bandgap optical absorption; (d) scavenging of free electrons by O2; (e) radiative recombination of conduction band electrons with trapped holes, with emission of green PL peaked at about 2.5 eV; (f, g) population of available electron trap states through relaxation of shallow states (below-bandgap excitation) and conduction states (above-bandgap states), respectively; (h, i) radiative recombination between trapped electrons and valence band holes, with emission of red PL peaked at about 1.9 eV. The R-PL emission can be caused by recombination of electron in states populated via relaxation (h) and by recombination of photogenerated holes with electron occupying states below the Fermi level (μF) before the laser excitation.

above-bandgap excitation populates the conduction band by creating hot carriers ((a) in Figure 4) that next relax nonradiatively (b) toward the lowest unoccupied CB states, while below-bandgap excitation close to the CB edge can populate subgap shallow states (c). Next, relaxed photogenerated electrons in conduction band can follow different recombination paths. In an O2 environment, they can be scavenged by chemisorbed O2 with formation of superoxide species (see (d) in Figure 4): eCB− + O2 (g) ⇌ O2−(ads)

(5)

(4)

O2−(ads)

In the above equation, is a chemisorbed superoxide species and O2(g) is an oxygen molecule in gas phase. In addition to the subtraction of photoelectrons, it has also to be noted that the prolonged accumulation of chemisorbed superoxide further increase the TiO2 upward band bending, enlarging the depth of the depletion layer and shrinking the region in which photoelectrons can recombine with PL-active defects.48 These effects lead to the PL intensity decrease upon the adsorption of O2. Another possible route, dominant for above-bandgap excitation, is the radiative recombination with deep empty 9016


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The Journal of Physical Chemistry C trapped at 3-fold coordinated (normal) O atoms at (or close to) the (110) or (100) rutile surfaces,21,22,53 resulting in the formation of a self-trapped hole (STH). Next, the STH can recombine with the conduction band electrons, leading to NIRPL emission: eCB− + STH → ℏω NIR

(7)

or initiate other reactions that ultimately lead to oxygen photoevolution.21,54,55 A major issue for this interpretation is the observed enhancement of NIR-PL intensity in O2. This phenomenon, still unobserved at the time of the first proposal of STH mechanism but unambiguously confirmed in several successive works,6,32,33,36 implies that the participation of conduction band electrons in the NIR-PL radiative transitions is unlikely. Otherwise, we should expect an O2-induced quenching of NIR-PL intensity (exactly as in anatase). The O2-induced enhancement of NIR-PL intensity indicates instead that the electron states participating to NIR-PL are not depleted by O2 adsorption. A possible way to maintain the “trapped holes” interpretation is depicted in Figure 5A. It assumes that the STHs are formed not only at the surface but also in the subsurface regions, outside the “scavenging radius” of adsorbed O 2. The recombination might then occur between photogenerated subsurface electrons not captured by adsorbed O2 and selftrapped holes formed in the subsurface region. In the above situation, the O2-induced enhancement of PL may be explained as follows: As the rutile is exposed to O2, formation of superoxide species O2− occurs, leading to an upward band bending at TiO2 surface and to the accumulation of holes in the subsurface region. The effect of such a process on STH-related PL efficiency would be ambivalent. On one side, oxygen would scavenge part of the photogenerated carriers in the conduction band, preventing their radiative recombination with trapped holes and decreasing the PL. On the other hand, it would favor the accumulation of holes in the subsurface region, increasing the concentration of STHs and thus the PL intensity. The net O2 effect on NIR-PL intensity may result from the balance of these two processes. If we maintain the STH-based interpretation, the observation of net increase in the PL intensity might indicate that most of the photogenerated conduction band electrons are not captured by O2 and will radiatively recombine with trapped holes that accumulate in the subsurface regions. It is to be underlined that such an interpretation still has issues: the spatial nature of the hypothesized STHs states is not well determined in the literature, and recent spectroelectrochemical photoluminescence (SEPL) measurement,6 next discussed, introduces new evidence pointing toward the “free holes” interpretation. Case 2: The “Free Holes” Mechanism. It is quite natural to suppose that the free carriers involved in NIR-PL are holes, discarding the participation of CB electrons. In fact, these eCB− would be scavenged by O2, leading to a PL quenching in rutile (as it happens in anatase). As this is not the case, the NIR-PL emission is likely to be caused by radiative recombination between a midgap trapped electron and a free (valence band) hole, as schematically depicted in Figure 5B (bottom figure). The elementary process shown in Figure 5B is compatible with the NIR-PL enhancement in O2 under the very reasonable hypothesis that the deep midgap states involved are well below

Figure 5. Possible interpretations for rutile NIR-PL. Model 1 (“trapped hole recombination”) involves the trapping of a photogenerated VB hole at a three-coordinated oxygen site close to the surface (STH formation, (a)) followed by the radiative recombination between STH and CB electrons. O2 adsorption and formation of superoxide ions leads to a larger band bending at the surface, accompanied by the formation of a space charge layer. Consequently, holes accumulate in the subsurface region, enhancing the local concentration of STHs (as depicted in (b)) and thus increasing the NIR-PL intensity. In model 2 (“free hole recombination”), the conduction band electrons do not participate to the radiative recombination. The NIR-PL process ((a) in bottom left figure) instead involves free holes and electrons trapped in midgap states, positioned below the Fermi level μF and already occupied in dark conditions. In the absence of O2, the photogenerated valence band holes can also recombine nonradiatively with electrons via nonradiative centers (see (b)). Free and shallow electrons scavenged by O2 are no more competitive for recombination with valence band holes, thus O2 adsorption enhances the PL emission. Based on Wendt et al.,60 assigning the midgap electrons to interstitial Ti allows also to hypothesize that NIR-PL is further enhancement by O2 because of an improved spatial superposition between trapped electrons and free holes. This is represented in bottom right figure by depicting the midgap electrons closer to the hole-rich surface.

the Fermi level. Hence, they are already occupied before the light absorption, and the capture of the associated electrons by O2 molecules is unlikely. If O2 is instead present, it scavenges photogenerated electrons. As a result, the density of free holes available for radiative recombination is increased, and the NIRPL is enhanced. Very relevantly, recent investigations conducted by Rex and co-workers6 through SEPL analysis of rutile photoanodes evidenced the presence of an electronic midgap state positioned at about 1.5 eV below the conduction band edge and participating to the NIR-PL optical transition. The existence of such a state can justify the NIR-PL spectral position only if 9017


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to exist. However, caution is needed before asserting a surface nature for these defects. In fact, the previously cited SEPL measurements6 also evidenced that the NIR-PL spectrum does not shift in energy vs pH changes in aqueous media. These considerations give some clue on the chemical origin of midgap rutile states in the light of the results by Wendt and coworkers,65 who presented theoretical calculations and scanning tunnel microscopy studies supporting the assignment of a defect state, positioned at about 0.85 eV below the Fermi level of rutile (110), to interstitial titanium. In particular, they reported that the adsorption of O2 at reduced (110) rutile surface determines an energetically favorable pathway for an interstitial Ti atom to diffuse toward the surface.65 As the energy level of electrons associated with the Ti interstitial are calculated to be close to the middle of the rutile bandgap, they are possible candidates for the NIR-PL active species. In this sense, the slight and irreversible increase of rutile PL during concomitant exposure to UV light and O2 (see Figure 3B) may be interpreted as the result of the improved spatial overlap between Ti interstitials diffusing toward the surface and free holes accumulating in the subsurface region because of the surface band bending induced by adsorbed superoxide species.

free holes are involved (considering a bandgap of rutile of 3.0 eV). In light of these SEPL results and of the uncertainties accompanying the hypothesis based on recombination of selftrapped holes, we consider the scheme depicted in Figure 5B involving electrons trapped in midgap states below the Fermi level and free holes as the most plausible one for rutile NIR-PL. Physical Nature of the PL-Active States. Clues on the possible nature of the PL-active defect states in anatase and rutile can be extracted by examining the irreversible modifications of intrinsic PL intensities observed during the UV+O2 exposures (Figures 3A−C) in the light of the PL mechanisms illustrated in the previous sections. As indicated in several studies,56−58 oxygen vacancies (VO) can be healed by dosing mildly reduced TiO2 with molecular oxygen. Such healing occurs, at room temperature, via a dissociative adsorption of oxygen59 in which one O atom of the O2 molecule fills the vacancy, leaving the remaining oxygen adatoms on the TiO2 surface. Hence, this process increases the surface O/Ti stoichiometric ratio. EPR analyses60 indicate that free electrons generated by UV illumination favor the adsorption of O2 molecules via formation of superoxide species, in turn favoring the successive dissociative adsorption.61,62 These considerations suggest that the stable decrease of PL intensity in anatase samples observed after prolonged exposure to O2 and UV light (Figure 3A) can be a consequence of surface oxidation, underlining in turn a role of VO in the defective PL emission. Recalling that the G-PL emission is the one prone to irreversible quenching after UV+O2 treatment, we propose that the hole traps related to the G-PL emission (see Figure 4) are localized at VO sites (F-centers) or at Ti sites adjacent to VO, interpreting the results in Figures 3A and 3C as caused by a surface photoassisted oxidation. This interpretation again supports the already mentioned one by McHale and coworkers,4,27,63,64 whose studies also suggested that that the green component results from recombination of free electrons with holes trapped on Ti3+ surface atoms adjacent to bridging O vacancies. Like the G-PL band, the anatase R-PL band also quenches during UV+O2 exposure and is partially degraded by the treatment. However, its modification is lesser than the one experienced by G-PL (Figure 3C). We suggest that the electron traps contributing to R-PL may be associated on oxygen vacancies positioned below the anatase surface, whose photoinduced filling is supposed to be less likely than that of surface bridging O vacancies. Concerning the NIR-PL emission, our experimental results indicate that more reduced (O-vacant) rutile surfaces correspond to less intense NIR-PL emission, while the intensity increases as the VO are healed and the rutile surface is oxidized. The increase of the intrinsic PL intensity during photooxidation rules out the possibility that the PL-active midgap electrons of rutile are trapped at VO sites. To this regard, Rex et al.6 also observed that annealing rutile nanowires in hydrogen (H2) atmosphere, thought to increase O vacancies, did not increase the NIR-PL intensity. Indeed, their results indicate that the H2 treatment decreased the intrinsic PL intensity (measured in argon), even if they did not observe any precise correlation between H2 treatment temperatures and NIR-PL intensity measured in air. A relation between midgap electron states responsible for NIR-PL and surface reoxidation of O vacancies seems therefore

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CONCLUSIONS In this work, we investigate the basic PL mechanisms of anatase and rutile phases of TiO2. Our experimental results can be summarized as follows: (i) Rutile PL is initiated by free carriers. (ii) Physically nonequivalent recombination processes occur in anatase PL, leading to two spectrally separated contributions to PL centered at about 2.5 eV (G-PL) and 1.9 eV (R-PL). Both PL bands experience an intensity quenching when TiO2 is exposed to O2, but the former (G-PL) is more sensitive than the latter. (iii) The excitation spectrum of the G-PL band follows the interband absorption curve, while the R-PL emission is relevantly present also at sub-bandgap excitation, indicating that it is not ruled by recombination of self-trapping excitons. (iv) Irreversible changes of the intrinsic PL intensity were observed in samples not previously exposed to UV light and O2, opposite for the two polymorphs (increase in rutile, decrease in anatase), saturating after ∼30 min of simultaneous exposure to O2 and UV light. In the case of anatase, this change is more relevant for the G-PL band than for the R-PL one. On the basis of these outcomes and on related considerations, we propose an interpretation scheme for the PL mechanisms in anatase and rutile. We ascribe G-PL of anatase to radiative recombination of free electrons with holes in defect states (trapped holes) and R-PL to radiative recombination between free holes and electrons that relax from conduction band and shallow subgap states to deep defect states. The trapped holes responsible for G-PL are found to correlate with photostimulated oxidation treatment, suggesting that the chemical defect to be associated with them are oxygen vacancies, i.e., that these photogenerated holes are trapped on surface F-centers (i.e., are localized at O vacancy sites) or on Ti3+ atoms adjacent to O vacancies. Concerning the electron states involved in R-PL emission, we suggest they may be associated with subsurface O vacancies, owing to their lesser sensitivity to prolonged exposure to UV illumination in oxygen environment. This picture provides an independent support to the interpretations elaborated by McHale and co-workers4,25,27 and, in addition, suggests a correlation between the O 9018


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(5) Rex, R. E.; Knorr, F. J.; McHale, J. L. Surface Traps of TiO 2 Nanosheets and Nanoparticles as Illuminated by Spectroelectrochemical Photoluminescence. J. Phys. Chem. C 2014, 118 (30), 16831− 16841. (6) Rex, R. E.; Yang, Y.; Knorr, F. J.; Zhang, J. Z.; Li, Y.; McHale, J. L. Spectroelectrochemical Photoluminescence of Trap States in HTreated Rutile TiO 2 Nanowires: Implications for Photooxidation of Water. J. Phys. Chem. C 2016, 120 (6), 3530−3541. (7) Dozzi, M. V.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E. Fluorine-Doped TiO 2 Materials: Photocatalytic Activity vs TimeResolved Photoluminescence. J. Phys. Chem. C 2013, 117 (48), 25586−25595. (8) Lettieri, S.; Santamaria Amato, L.; Maddalena, P.; Comini, E.; Baratto, C.; Todros, S. Recombination Dynamics of Deep Defect States in Zinc Oxide Nanowires. Nanotechnology 2009, 20 (17), 175706. (9) Bohle, D. S.; Spina, C. J. Cationic and Anionic Surface Binding Sites on Nanocrystalline Zinc Oxide: Surface Influence on Photoluminescence and Photocatalysis. J. Am. Chem. Soc. 2009, 131 (12), 4397−4404. (10) Ruiz Peralta, M. D. L.; Pal, U.; Zeferino, R. S. Photoluminescence (PL) Quenching and Enhanced Photocatalytic Activity of Au-Decorated ZnO Nanorods Fabricated through MicrowaveAssisted Chemical Synthesis. ACS Appl. Mater. Interfaces 2012, 4 (9), 4807−4816. (11) Iatsunskyi, I.; Pavlenko, M.; Viter, R.; Jancelewicz, M.; Nowaczyk, G.; Baleviciute, I.; Załęski, K.; Jurga, S.; Ramanavicius, A.; Smyntyna, V. Tailoring the Structural, Optical, and Photoluminescence Properties of Porous Silicon/TiO 2 Nanostructures. J. Phys. Chem. C 2015, 119 (13), 7164−7171. (12) Nasr, M.; Abou Chaaya, A.; Abboud, N.; Bechelany, M.; Viter, R.; Eid, C.; Khoury, A.; Miele, P. hotoluminescence: A Very Sensitive Tool to Detect the Presence of Anatase in Rutile Phase Electrospun TiO2 Nanofibers. Superlattices Microstruct. 2015, 77, 18−24. (13) Stevanovic, A.; Yates, J. T. Electron Hopping through TiO 2 Powder: A Study by Photoluminescence Spectroscopy. J. Phys. Chem. C 2013, 117 (46), 24189−24195. (14) Ma, S.; Reish, M. E.; Zhang, Z.; Harrison, I.; Yates, J. T. AnataseSelective Photoluminescence Spectroscopy of P25 TiO 2 Nanoparticles: Different Effects of Oxygen Adsorption on the Band Bending of Anatase. J. Phys. Chem. C 2017, 121, 1263. (15) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63 (12), 515−582. (16) Rajeshwar, K.; Chenthamarakshan, C. R.; Goeringer, S.; Djukic, M. Titania-Based Heterogeneous Photocatalysis. Materials, Mechanistic Issues, and Implications for Environmental Remediation. Pure Appl. Chem. 2001, 73 (12), 1849−1860. (17) Hagfeldt, A.; Graetzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95 (1), 49−68. (18) Arafat, M. M.; Dinan, B.; Akbar, S. A.; Haseeb, A. S. M. A. Gas Sensors Based on One Dimensional Nanostructured Metal-Oxides: A Review. Sensors 2012, 12 (12), 7207−7258. (19) Pallotti, D. K.; Passoni, L.; Gesuele, F.; Maddalena, P.; Di Fonzo, F.; Lettieri, S. Giant O 2 -Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures. ACS Sens. 2017, 2 (1), 61−68. (20) Ganesh, V. A.; Raut, H. K.; Nair, A. S.; Ramakrishna, S. A Review on Self-Cleaning Coatings. J. Mater. Chem. 2011, 21 (41), 16304. (21) Nakamura, R.; Nakato, Y. Primary Intermediates of Oxygen Photoevolution Reaction on TiO2 (Rutile) Particles, Revealed by in Situ FTIR Absorption and Photoluminescence Measurements. J. Am. Chem. Soc. 2004, 126 (4), 1290−1298. (22) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. Molecular Mechanisms of Photoinduced Oxygen Evolution, PL Emission, and Surface Roughening at Atomically Smooth (110) and (100) N-TiO2 (Rutile) Surfaces in Aqueous Acidic Solutions. J. Am. Chem. Soc. 2005, 127 (37), 12975−12983.

deficiency and the efficiency of intrinsic NIR-PL that was not evidenced by the same authors.6 In the case of rutile, two possible PL mechanisms are discussed: one involving self-trapped holes located at O atoms and recombining with conduction band electrons and another involving electrons occupying midgap states positioned below the Fermi level at equilibrium and recombining with valence band holes. We argue that the former mechanism shall require that self-trapped holes are efficiently formed far from the rutile surface, while the latter seems to be the most plausible one, in view of the PL enhancement caused by O2 adsorption and of SEPL evidence reported in the literature. The positive correlation between UV-stimulated surface oxidation and NIR-PL intensity in rutile rules out the possibility that the latter arises from O vacancy states. Instead, the findings are discussed in terms of Ti interstitials as possible responsible for the NIR-PL activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00321. Additional experimental details on samples preparation and PL/PLE analyses; XRD and SEM measurements; Gaussian deconvolution of the anatase G-PL and R-PL; Tauc plot-based extrapolation of anatase bandgap energy and additional optical data; PLE results for PLDprepared TiO2 films; PL spectra of mixed-phase pellets (“MP” sample) at different excitation wavelength; PL quenching dynamics of G-PL and R-PL emission under O2 adsorption; PL spectra of the AF sample at abovebandgap vs below-bandgap excitation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.L.) E-mail [email protected]. *(F.D.F.) E-mail [email protected]. ORCID

Stefano Lettieri: 0000-0002-4471-3024 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.L. and D.K.P. acknowledge the Regione Campania (INTERFET Project, L.R. n.5/02), INFN-CNR national project “PREMIALE 2012-EOS”, and the Italian Ministry of Foreign Affairs and International Cooperation (NANOGRAPH Project) for financial support.

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REFERENCES

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