Photoluminescence Mechanisms in Anatase and ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Article 2

Photoluminescence Mechanisms in Anatase and Rutile TiO

Deborah Katia Pallotti, Luca Passoni, Pasqualino Maddalena, Fabio Di Fonzo, and Stefano Lettieri J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00321 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photoluminescence Mechanisms in Anatase and Rutile TiO2 Deborah K. Pallotti1, Luca Passoni2,3, Pasqualino Maddalena4, Fabio Di Fonzo*,3 and Stefano Lettieri*,1

1

Institute of Applied Sciences and Intelligent Systems (CNR-ISASI), Via Campi Flegrei 34, I80078 Pozzuoli, Italy 2

Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli, 70/3, 20133 Milano, Italy

3

Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, Milano 20133

4

Dipartimento di Fisica “E. Pancini”, Università degli Studi di Napoli “Federico II”, Via Cintia I80126 Napoli, Italy

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

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). Based on our experimental results on PL emission, PL excitation, 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 sub-surface oxygen vacancies, respectively. Two possible mechanisms are discussed for rutile PL, involving self-trapped holes located at oxygen atoms or trapped electrons occupying mid-gap 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 mid-gap electron state still has to be clarified.


Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Photoluminescence (PL) spectroscopy analyzes the characteristics of the light spontaneously emitted by a photo-excited material. PL in semiconductors is the macroscopic manifestation of radiative recombination between photo-generated 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, that represents the most important limiting factor for photocatalytic efficiency. Hence, since earlier studies by Anpo and coworkers,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-infrared (NIR) PL emission of rutile TiO2 can elucidate the mechanisms of its water photooxidation: based on 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 radicals,2 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 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

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 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 self-trapped excitons28,29 or combinations of the above contributions.26,30,31 The difference in PL mechanisms of the two TiO2 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 “anti-correlated” 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”), oxygen-induced 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 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 mid-gap states that are responsible for the NIR-PL. 4 ACS Paragon Plus Environment

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL DETAILS Samples preparation and characterization. Nanostructured and quasi-one-dimensional TiO2 films were deposited on silicon substrates by pulsed laser deposition (PLD) through the procedure described in Supporting Information section. 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°C 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 µm 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 pellet (“MP” hereafter) were also prepared for the PLE experiment. For MP samples preparation, rutile nanopowder was prepared by annealing Aeroxide® P25 (Sigma Aldrich 718467) at 900°C in ambient air for 1 hour, guaranteeing its full crystallization in the rutile phase. Next, the rutile and 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 mW and 40 mW, respectively. Excitation-resolved photoluminescence (PLE) measurements were performed by using as optical excitation source a white-light broadband Xe lamp coupled with a computer 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

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 Supporting Information.

RESULTS A. Emission and Excitation Properties of Anatase and Rutile TiO2. Sample analyses performed by XRD and SEM are reported in Supporting Information and confirm previously reported results19,34,35. 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 nano-scale branches (Fig S2) and the transformation of the quasione-dimensional hyper-branched structures in polycrystalline nano-stone piles.19 The loss of the nanoscale texture of anatase nano-trees determines a decrease of the surface-to-volume ratio19 after the annealing at 800°C and accounts for the shrinkage of the linewidth of XRD peaks (compared to the XRD peaks of anatase) noticeable in Fig. S1, associated to the presence of larger crystallites. The PL spectra of rutile and anatase TiO2 extend in the near-infrared (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, 6 ACS Paragon Plus Environment

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIG 1. (A): PL spectra of RF (black curve), AF (blue curve) and AP (red curve) under 325 nm excitation, collected in N2. (B) and (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 ad 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 Supporting Information (Fig. S1).

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 above-bandgap excitation (325 nm HeCd laser excitation), are reported Fig. 1A. It is noted that AF luminescence displays Fabry-Perot fringes, caused by interference between the back7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

scattered 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 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 non-equivalence of these two PL components manifests itself in i): Anatase/O2 interaction, and ii): PL excitation mechanisms. Concerning the first point, Figs. 1B and 1C display the changes in the total PL intensity ΦG and ΦR associated to G-PL and R-PL bands occurring during the desorption of O2 from sample surfaces. The quantities ΦG and ΦR are defined as:

ΦG = ∫ ϕ (λ)d λ

(1)

ΦR = ∫ ϕ(λ )d λ

(2)

∆λG

∆λR

where ϕ (λ ) is the experimental PL spectrum and the wavelength intervals are chosen as ∆λG = 465540 nm (~2.3-2.6 eV photon energy) and ∆λG = 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 Figs 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.


Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 (Fig. 1B) but we observed in all the investigated anatase samples (see for example Fig. 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 Supporting Information (Fig. S6). The second feature we highlight is that the anatase G-PL and R-PL components exhibit different excitation mechanisms. In Fig. 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 Supporting Information, Fig. 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 ( λ exc , λ P L ) expressing the PL intensity ϕ measured at wavelength λPL when the sample is excited by a monochromatic source of wavelength λexc. The PLE amplitude I ∆PLE λ (λexc ) associated to a PL band positioned in the wavelength interval ∆λ is defined as the quantity: λ2

I ∆PLE ∆λ λ ( λexc ) = ∫ ϕ ( λexc , λPL ) d λ PL

(3)

λ1

In the above expression, λ1 and λ 2 are the extremes of the wavelength interval ∆λ = λ2 − λ1 . The excitation spectrum of each specific PL band can thus be extracted by proper choice of the integration interval in Equation (3). 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

FIG 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 (GPL, black circles), 1.7-2.1 eV (R-PL, open red circles) and 1.4-1.55 eV (NIR-PL, open blue squares).

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 Fig. 2A and 2B. respectively. The excitation spectra have been obtained by numerical integration of the PLE map in the following


Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wavelength intervals (photon energy intervals): G-PL: 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 subbandgap 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 sub-gap 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. 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 Figs. 1D and 1E). We underline that the PLE amplitude of R-PL emission only halves from its maximum value to its minimum value at belowbandgap 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 Supporting Information, led to equivalent results.

B. 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 towards 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 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

FIG 3. (A) and (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 same O2 concentrations ([O2] = 1%). The grey 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 hours 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 evidences that the irreversible quenching affects the G-PL component more than the R-PL one.

an irreversible (∆ϕirr ) 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 next section.


Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The relative “weight” of the irreversible component ∆ϕirr ϕ0 (where ϕ0 indicates the “intrinsic” PL intensity, i.e. the steady-state 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 ∆ϕirr were observed in unexposed samples, and became less relevant as the samples were repeatedly exposed to the HeCd laser light in presence of O2. Representative examples of this behavior are shown in Figs. 3A, 3B and 3C. In Fig. 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 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 re-obtained 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 Fig. 3B (rutile film) were collected two weeks after those in Fig. 3A, during which the RF sample were stored in dark and in ambient air. Then, the experiment in Fig. 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 hours. Next, an O2 + UV treatment was performed by exposing for two hours 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 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

emission spectra were 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 Fig. 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.

DISCUSSION A. Photoluminescence Mechanisms in Anatase TiO2 Earlier studies28 suggested that the anatase PL originates from radiative recombination of selftrapped 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 in materials having a large specific surface area, such as nanostructured systems. Our PL and PLE results of Figs. 1 and 2 indeed indicate that such non-excitonic 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 (Figs. 1D, 1E) and PLE (Fig. 2A and 2B) results show that the G-PL intensity follows an interband excitation behavior, indicating that


Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


− 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 subbandgap 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 sub-gap states. After being promoted to such states, electrons can further relax by occupying trap states positioned well below the CB edge. The works by Tang and coworkers evidencing Urbach tails (i.e. continuum of shallow sub-gap states) in optical absorption of anatase TiO2 (and not present in rutile) support this picture.29 In the light of these considerations, we summarize in Fig. 4 the deduced PL mechanisms of anatase TiO2. In this scheme, above-bandgap excitation populates the conduction band by creating hot carriers ((a) in Fig. 4) that next relax non-radiatively (b) toward the lowest unoccupied CB states, while below-bandgap excitation close to the CB edge can populate sub-gap shallow states (c). Next, relaxed photo-generated 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 Fig. 4): − eCB + O2 ( g )

O2− ( g )

(4)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

In the above equation O −2 ( ads ) 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 states (trapped holes) that determines the O2-quenchable G-PL (see (e) in Fig. 4): − eCB + htr+ → G-PL

(5)

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 Fig. 4 involving electron trap states. The R-PL is in fact assigned to the process: + etr− + hVB → R-PL

(6)

involving he 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 Fig. 4) occupied through below-bandgap optical excitation (see (c) in Fig. 4) of from CB through nonradiative relaxation of free electrons ((g) in Fig. 4). Thus, optical transitions in Eq. (6) can be started by photogenerated free electrons relaxing from the conduction band (see (h) in Fig. 4) and from electrons already present at equilibrium ((i) in Fig. 4). While the 16 ACS Paragon Plus Environment

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIG 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) and (g): population of available electron trap states through relaxation of shallow states (below-bandgap excitation) and conduction states (above-bandgap states), respectively. (h) and (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) beforehand the laser excitation.

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 O2-induced quenching of the red component of anatase PL. 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

Our scheme for anatase PL does not support the involvement of self-trapped excitons or of donoracceptor pair recombination, while instead it strongly supports the works by McHale and coworkers,4,25,27 who, based on 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 Eq. (5) and (6)).

B. 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 mid-gap 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 mid-gap 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 coworkers 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 photo-generated free holes coming to the surface can be trapped at three-fold coordinated (normal) O atoms at (or close to) the (110) or (100) rutile surfaces,21,22,53 resulting in the formation of a selftrapped hole (STH). Next, the STH can recombine with the conduction band electrons, leading to NIR-PL emission: − eCB + STH → hωNIR

(7) 18

ACS Paragon Plus Environment

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or initiate other reactions that ultimately lead to oxygen photo-evolution.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 works6,32,33,36 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 Fig. 5A. It assumes that the STHs are formed not only at the surface but also in the sub-surface regions, outside the “scavenging radius” of adsorbed O2. The recombination might then occur between photogenerated sub-surface electrons not captured by adsorbed O2 and self-trapped holes formed in the sub-surface 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 O−2 occurs, leading to an upward band bending at TiO2 surface and to the accumulation of holes in the sub-surface region. The effect of such a process on STH-related PL efficiency would be ambivalent. On one side, oxygen would scavenge part of the photo-generated 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 sub-surface 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

indicate that most of the photogenerated conduction band electrons are not captured by O2 and will radiatively recombine with trapped holes that accumulates in the sub-surface regions.


Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIG 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 sub-surface 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 mid-gap states, positioned below the Fermi level µF and already occupied in dark conditions. In absence of O2, the photogenerated valence band holes can also recombine non-radiatively with electrons via non-radiative 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 mid-gap 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 mid-gap electrons closer to the hole-rich surface.

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 literature and recent spectroelectrochemical



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

photoluminescence (SEPL) measurement,6 next discussed, introduce new evidences 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 mid-gap trapped electron and a free (valence band) hole, as schematically depicted in Fig. 5B (bottom figure). The elementary process shown in Fig. 5B is compatible with the NIR-PL enhancement in O2 under the very reasonable hypothesis that the deep mid-gap states involved are well below the Fermi level. Hence, they are already occupied beforehand the light absorption, and the capture of the associated electrons by O2 molecules is unlikely. When O2 is then present to scavenge photogenerated electrons density of free photogenerated holes available for radiative recombination) and the PL increases. Very relevantly, recent investigations conducted by Rex and coworkers6 through SEPL analysis of rutile photoanodes evidenced the presence of an electronic mid-gap 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 free holes are involved (considering a bandgap of rutile of 3.0 eV). In the light of these SEPL results and of the uncertainties accompanying the hypothesis based on recombination of self-trapped holes, we consider the scheme depicted in Fig. 5B involving electrons trapped in mid-gap states below the Fermi level and free holes as most plausible one for rutile NIRPL.


Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C. 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 (Figs. 3A, 3B and 3C) 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 (Fig. 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 Fig. (4)) are localized at VO sites (F-centers) or at Ti sites adjacent to VO, interpreting the results in Figs. 3A and 3C as caused by a surface photo-assisted 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 (Fig. 3C). We suggest that the electron traps contributing to R-PL may be associated on oxygen 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

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 mid-gap electrons of rutile are trapped at VO sites. To this regard, Rex et al6 also observed that annealing rutile nanowires in hydrogen (H2) atmosphere, thought to increase O vacancies, did not increase the NIR-PL intensity. Indeed, their result 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 mid-gap electron states responsible for NIR-PL and surface re-oxidation of O vacancies seems therefore 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 mid-gap 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 to the Ti interstitial are calculated to be close to the middle of the rutile bandgap, they are possible candidates for the NIRPL active species. In this sense, the slight and irreversible increase of rutile PL during concomitant exposure to UV light and O2 (see Fig. 3B) may be interpreted as the result of the improved spatial


Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.

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 non-equivalent 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 RPL 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. Based on 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 sub-gap states to deep defect 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

states. The trapped holes responsible for G-PL are found to correlate with photo-stimulated 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 to sub-surface 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 coworkers4,25,27 and, in addition, suggests a correlation between the O 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, another involving electrons occupying mid-gap 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 evidences reported in 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information include: 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 PLD-prepared 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 above-bandgap vs. below-bandgap excitation.

AUTHOR INFORMATION Corresponding authors *S.L.:E-mail: [email protected] *F.D.F.:E-mail: [email protected]

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

REFERENCES (1) (2)

(3) (4)

Gfroerer, T. H. Photoluminescence in Analysis of Surfaces and Interfaces. Encycl. Anal. Chem. 2000. Anpo, M.; Shima, T.; Kubokawa, Y. ESR AND PHOTOLUMINESCENCE EVIDENCE FOR THE PHOTOCATALYTIC FORMATION OF HYDROXYL RADICALS ON SMALL TiO 2 PARTICLES. Chem. Lett. 1985, 14 (12), 1799–1802. Anpo, M.; Tomonari, M.; Fox, M. A. In Situ Photoluminescence of Titania as a Probe of Photocatalytic Reactions. J. Phys. Chem. 1989, 93 (21), 7300–7302. Mercado, C. C.; Knorr, F. J.; McHale, J. L.; Usmani, S. M.; Ichimura, A. S.; Saraf, L. V. Location of Hole and Electron Traps on Nanocrystalline Anatase TiO2. J. Phys. Chem. C 2012, 116 (19), 10796–10804. 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5)

(6)

(7)

(8) (9)

(10)

(11)

(12)

(13) (14)

(15) (16)

(17) (18) (19)

(20) (21)

(22)

(23) (24)

Page 28 of 37

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. Rex, R. E.; Yang, Y.; Knorr, F. J.; Zhang, J. Z.; Li, Y.; McHale, J. L. Spectroelectrochemical Photoluminescence of Trap States in H-Treated Rutile TiO 2 Nanowires: Implications for Photooxidation of Water. J. Phys. Chem. C 2016, 120 (6), 3530–3541. Dozzi, M. V.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E. Fluorine-Doped TiO 2 Materials: Photocatalytic Activity vs Time-Resolved Photoluminescence. J. Phys. Chem. C 2013, 117 (48), 25586–25595. 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. 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. Ruiz Peralta, M. D. L.; Pal, U.; Zeferino, R. S. Photoluminescence (PL) Quenching and Enhanced Photocatalytic Activity of Au-Decorated ZnO Nanorods Fabricated through Microwave-Assisted Chemical Synthesis. ACS Appl. Mater. Interfaces 2012, 4 (9), 4807–4816. 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. Nasr, M.; Abou Chaaya, A.; Abboud, N.; Bechelany, M.; Viter, R.; Eid, C.; Khoury, A.; Miele, P. Photoluminescence: A Very Sensitive Tool to Detect the Presence of Anatase in Rutile Phase Electrospun TiO2 Nanofibers. Superlattices Microstruct. 2015, 77, 18–24. 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. Ma, S.; Reish, M. E.; Zhang, Z.; Harrison, I.; Yates, J. T. Anatase-Selective Photoluminescence Spectroscopy of P25 TiO 2 Nanoparticles: Different Effects of Oxygen Adsorption on the Band Bending of Anatase. J. Phys. Chem. C 2017. Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63 (12), 515–582. 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. Hagfeldt, A.; Graetzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95 (1), 49–68. 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. 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. Ganesh, V. A.; Raut, H. K.; Nair, A. S.; Ramakrishna, S. A Review on Self-Cleaning Coatings. J. Mater. Chem. 2011, 21 (41), 16304. 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. 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. Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48 (5–8), 53–229. Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66 (6– 7), 185–297. 28 ACS Paragon Plus Environment

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25)

(26)

(27)

(28) (29) (30) (31)

(32) (33) (34)

(35) (36) (37) (38) (39) (40) (41) (42) (43)

(44)

(45) (46)

(47)

Knorr, F. J.; Mercado, C. C.; McHale, J. L. Trap-State Distributions and Carrier Transport in Pure and Mixed-Phase TiO2: Influence of Contacting Solvent and Interphasial Electron Transfer. J. Phys. Chem. C 2008, 112 (33), 12786–12794. Wang, X.; Feng, Z.; Shi, J.; Jia, G.; Shen, S.; Zhou, J.; Li, C. Trap States and Carrier Dynamics of TiO2 Studied by Photoluminescence Spectroscopy under Weak Excitation Condition. Phys. Chem. Chem. Phys. 2010, 12 (26), 7083. Mercado, C.; Seeley, Z.; Bandyopadhyay, A.; Bose, S.; McHale, J. L. Photoluminescence of Dense Nanocrystalline Titanium Dioxide Thin Films: Effect of Doping and Thickness and Relation to Gas Sensing. ACS Appl. Mater. Interfaces 2011, 3 (7), 2281–2288. Tang, H.; Berger, H.; Schmid, P. E.; Lévy, F.; Burri, G. Photoluminescence in TiO2 Anatase Single Crystals. Solid State Commun. 1993, 87 (9), 847–850. Tang, H.; Lévy, F.; Berger, H.; Schmid, P. E. Urbach Tail of Anatase TiO_{2}. Phys. Rev. B 1995, 52 (11), 7771–7774. Cavigli, L.; Bogani, F.; Vinattieri, A.; Faso, V.; Baldi, G. Volume versus Surface-Mediated Recombination in Anatase TiO2 Nanoparticles. J. Appl. Phys. 2009, 106 (5), 053516-053516-8. Pallotti, D. K.; Orabona, E.; Amoruso, S.; Aruta, C.; Bruzzese, R.; Chiarella, F.; Tuzi, S.; Maddalena, P.; Lettieri, S. Multi-Band Photoluminescence in TiO2 Nanoparticles-Assembled Films Produced by Femtosecond Pulsed Laser Deposition. J. Appl. Phys. 2013, 114 (4), 043503. Pallotti, D.; Orabona, E.; Amoruso, S.; Maddalena, P.; Lettieri, S. Modulation of Mixed-Phase Titania Photoluminescence by Oxygen Adsorption. Appl. Phys. Lett. 2014, 105 (3), 031903. Lettieri, S.; Pallotti, D. K.; Gesuele, F.; Maddalena, P. Unconventional Ratiometric-Enhanced Optical Sensing of Oxygen by Mixed-Phase TiO2. Appl. Phys. Lett. 2016, 109 (3), 031905. Passoni, L.; Ghods, F.; Docampo, P.; Abrusci, A.; Martí-Rujas, J.; Ghidelli, M.; Divitini, G.; Ducati, C.; Binda, M.; Guarnera, S.; et al. Hyperbranched Quasi-1D Nanostructures for Solid-State DyeSensitized Solar Cells. ACS Nano 2013, 7 (11), 10023–10031. Passoni, L.; Giordano, F.; Zakeeruddin, S. M.; Grätzel, M.; Di Fonzo, F. Hyperbranched Self-Assembled Photoanode for High Efficiency Dye-Sensitized Solar Cells. RSC Adv 2015, 5 (113), 93180–93186. Pallotti, D. K.; Amoruso, S.; Orabona, E.; Maddalena, P.; Lettieri, S. Biparametric Optical Sensing of Oxygen by Titanium Dioxide. Sens. Actuators B Chem. 2015, 221, 515–520. Leonelli, R.; Brebner, J. L. Time-Resolved Spectroscopy of the Visible Emission Band in Strontium Titanate. Phys. Rev. B 1986, 33 (12), 8649–8656. De Haart, L. G. J.; De Vries, A. J.; Blasse, G. On the Photoluminescence of Semiconducting Titanates Applied in Photoelectrochemical Cells. J. Solid State Chem. 1985, 59 (3), 291–300. Toyozawa, Y. Phonon Structures in the Spectra of Solids. J. Lumin. 1970, 1–2, 732–746. Toyozawa, Y. Elementary Processes in Luminescence. J. Lumin. 1976, 12–13, 13–21. Toyozawa, Y. Dynamics of Excitons in Deformable Lattice. J. Lumin. 1981, 24–25, Part 1, 23–30. Faglia, G.; Baratto, C.; Sberveglieri, G.; Zha, M.; Zappettini, A. Adsorption Effects of NO2 at Ppm Level on Visible Photoluminescence Response of SnO2 Nanobelts. Appl. Phys. Lett. 2005, 86 (1), 011923. Bismuto, A.; Lettieri, S.; Maddalena, P.; Baratto, C.; Comini, E.; Faglia, G.; Sberveglieri, G.; Zanotti, L. Room-Temperature Gas Sensing Based on Visible Photoluminescence Properties of Metal Oxide Nanobelts. J. Opt. Pure Appl. Opt. 2006, 8 (7), S585–S588. Lettieri, S.; Causà, M.; Setaro, A.; Trani, F.; Barone, V.; Ninno, D.; Maddalena, P. Direct Role of Surface Oxygen Vacancies in Visible Light Emission of Tin Dioxide Nanowires. J. Chem. Phys. 2008, 129 (24), 244710. Setaro, A.; Lettieri, S.; Maddalena, P.; De Stefano, L. Highly Sensitive Optochemical Gas Detection by Luminescent Marine Diatoms. Appl. Phys. Lett. 2007, 91 (5), 051921-051921-3. Lettieri, S.; Setaro, A.; Baratto, C.; Comini, E.; Faglia, G.; Sberveglieri, G.; Maddalena, P. On the Mechanism of Photoluminescence Quenching in Tin Dioxide Nanowires by NO 2 Adsorption. New J. Phys. 2008, 10 (4), 043013. Valerini, D.; Cretì, A.; Caricato, A. P.; Lomascolo, M.; Rella, R.; Martino, M. Optical Gas Sensing through Nanostructured ZnO Films with Different Morphologies. Sens. Actuators B Chem. 2010, 145 (1), 167–173. 29 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(48) (49)

(50)

(51)

(52)

(53)

(54)

(55)

(56) (57)

(58)

(59) (60)

(61)

(62)

(63)

(64)

Page 30 of 37

Stevanovic, A.; Büttner, M.; Zhang, Z.; Yates, J. T. Photoluminescence of TiO2: Effect of UV Light and Adsorbed Molecules on Surface Band Structure. J. Am. Chem. Soc. 2012, 134 (1), 324–332. Nakato, Y.; Tsumura, A.; Tsubomura, H. Photo- and Electroluminescence Spectra from an N-Titanium Dioxide Semiconductor Electrode as Related to the Intermediates of the Photooxidation Reaction of Water. J. Phys. Chem. 1983, 87 (13), 2402–2405. Nakato, Y.; Akanuma, H.; Magari, Y.; Yae, S.; Shimizu, J.-I.; Mori, H. Photoluminescence from a Bulk Defect near the Surface of an N-TiO2 (Rutile) Electrode in Relation to an Intermediate of Photooxidation Reaction of Water. J. Phys. Chem. B 1997, 101 (25), 4934–4939. Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. Selective Formation of Nanoholes with (100)-Face Walls by Photoetching of N-TiO2 (Rutile) Electrodes, Accompanied by Increases in WaterOxidation Photocurrent. J. Phys. Chem. B 2000, 104 (20), 4873–4879. Kisumi, T.; Tsujiko, A.; Murakoshi, K.; Nakato, Y. Crystal-Face and Illumination Intensity Dependences of the Quantum Efficiency of Photoelectrochemical Etching, in Relation to Those of Water Photooxidation, at N-TiO2 (Rutile) Semiconductor Electrodes. J. Electroanal. Chem. 2003, 545, 99– 107. Imanishi, A.; Okamura, T.; Ohashi, N.; Nakamura, R.; Nakato, Y. Mechanism of Water Photooxidation Reaction at Atomically Flat TiO2 (Rutile) (110) and (100) Surfaces: Dependence on Solution pH. J. Am. Chem. Soc. 2007, 129 (37), 11569–11578. Nakamura, R.; Ohashi, N.; Imanishi, A.; Osawa, T.; Matsumoto, Y.; Koinuma, H.; Nakato, Y. CrystalFace Dependences of Surface Band Edges and Hole Reactivity, Revealed by Preparation of Essentially Atomically Smooth and Stable (110) and (100) N-TiO2 (Rutile) Surfaces. J. Phys. Chem. B 2005, 109 (5), 1648–1651. Nakamura, R.; Ohashi, N.; Imanishi, A.; Osawa, T.; Matsumoto, Y.; Koinuma, H.; Nakato, Y. CrystalFace Dependences of Surface Band Edges and Hole Reactivity, Revealed by Preparation of Essentially Atomically Smooth and Stable (110) and (100) N-TiO2 (Rutile) Surfaces. J. Phys. Chem. B 2005, 109 (5), 1648–1651. Göpel, W.; Rocker, G.; Feierabend, R. Intrinsic Defects of TiO2(110): Interaction with Chemisorbed O2, H2, CO, and CO2. Phys. Rev. B 1983, 28 (6), 3427–3438. Pan, J.-M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. Interaction of Water, Oxygen, and Hydrogen with TiO2(110) Surfaces Having Different Defect Densities. J. Vac. Sci. Technol. A 1992, 10 (4), 2470– 2476. Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Intrinsic Defects on a TiO2(110)(1×1) Surface and Their Reaction with Oxygen: A Scanning Tunneling Microscopy Study. Surf. Sci. 1998, 411 (1–2), 137–153. Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Evidence for Oxygen Adatoms on TiO2(110) Resulting from O2 Dissociation at Vacancy Sites. Surf. Sci. 1998, 412–413, 333–343. Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E. Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals—An Electron Paramagnetic Resonance Study. ChemPhysChem 2005, 6 (10), 2104–2112. Lira, E.; Hansen, J. Ø.; Huo, P.; Bechstein, R.; Galliker, P.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Dissociative and Molecular Oxygen Chemisorption Channels on Reduced Rutile TiO2(110): An STM and TPD Study. Surf. Sci. 2010, 604 (21–22), 1945–1960. Scheiber, P.; Riss, A.; Schmid, M.; Varga, P.; Diebold, U. Observation and Destruction of an Elusive Adsorbate with STM: ${\mathbf{O}}_{2}/{\mathrm{TiO}}_{2}(110)$. Phys. Rev. Lett. 2010, 105 (21), 216101. Knorr, F. J.; Zhang, D.; McHale, J. L. Influence of TiCl4 Treatment on Surface Defect Photoluminescence in Pure and Mixed-Phase Nanocrystalline TiO2. Langmuir 2007, 23 (17), 8686– 8690. Knorr, F. J.; McHale, J. L. Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO 2 in Aqueous Media. J. Phys. Chem. C 2013, 117 (26), 13654–13662.


Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(65)

Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. O.; Matthiesen, J.; BlekingeRasmussen, A.; Laegsgaard, E.; Hammer, B.; et al. The Role of Interstitial Sites in the Ti3d Defect State in the Band Gap of Titania. Science 2008, 320 (5884), 1755–1759.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O2

Page 32 of 37

O2Conduction band

Shallow states ࢋି ࢚࢘

G-PL ࢎା ࢚࢘

Fermi level R-PL

Valence band

TOC Graphics


Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIG 1. (A): PL spectra of RF (black curve), AF (blue curve) and AP (red curve) under 325 nm excitation, collected in N2. (B) and (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 ad 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 Supporting Information (Fig. S1). 160x111mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(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). 114x160mm (300 x 300 DPI)


Page 34 of 37

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIG 3. (A) and (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 same O2 concentrations ([O2] = 1%). The grey 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 hours 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 evidences that the irreversible quenching affects the G-PL component more than the R-PL one. 159x138mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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) and (g): population of available electron trap states through relaxation of shallow states (below-bandgap excitation) and conduction states (above-bandgap states), respectively. (h) and (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) beforehand the laser excitation. 79x76mm (300 x 300 DPI)


Page 36 of 37

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Possible interpretations for rutile NIR-PL. The first scheme (“trapped hole recombination”, upper figure) involves the trapping of a photogenerated valence-band hole at a three-coordinated oxygen site close to the surface (STH formation, (a)) followed by the radiative recombination between STH and conduction band electrons. Oxygen 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 NIRPL intensity. Per the second scheme (“free hole recombination”, lower figure), 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 mid-gap states, positioned below the Fermi level µF and already occupied in dark conditions. In absence of oxygen, the photogenerated valence band holes can also recombine non-radiatively with electrons via non-radiative 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 the results by Wendt and coworkers,64 assigning the mid-gap electrons



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 mid-gap electrons closer to the hole-rich surface. 108x143mm (300 x 300 DPI)


Page 38 of 37

Photoluminescence Mechanisms in Anatase and ... - ACS Publications

Recommend Documents