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Giant O-induced photoluminescence modulation in hierarchical titanium dioxide nanostructures Deborah Katia Pallotti, Luca Passoni, Felice Gesuele, Pasqualino Maddalena, Fabio Di Fonzo, and Stefano Lettieri ACS Sens., Just Accepted Manuscript • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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Giant O2-induced photoluminescence modulation in hierarchical titanium dioxide nanostructures

Deborah K. Pallotti1, Luca Passoni2,3, Felice Gesuele4, Pasqualino Maddalena4, Fabio Di Fonzo2* and Stefano Lettieri1* 1

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

2

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

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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

*Corresponding authors.

ABSTRACT. We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO2) nanostructures exposed to molecular oxygen (O2). Optical responsivities up to about 1100% at 20% O2 concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO2 nanopowders (up to a factor of ~7 for response to synthetic air) and significantly improving the ones typically reported in PL-based opto-chemical gas sensing using MOXs. The improved PL response is discussed in terms of the specific morphology of hierarchical structures, characterized by simultaneous presence of small nanoparticles, large surface areas and large voids. These characteristics guarantee an optimal interplay between photogenerated charges, PL-active centers and adsorbed gas molecules. The results highlight the potentialities offered by hierarchical structures based on TiO2 or of other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity.

KEYWORDS: Optical sensing, Oxygen sensing, Photoluminescence, Titanium dioxide, Hierarchical structures, Metal-oxide semiconductors.

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Measuring the content of molecular oxygen (O2) in gaseous and/or in aqueous media is of great importance for many industrial, environmental and biological applications. For example, detection of gaseous O2 provides information useful to improve the efficiency of combustion processes and thus to reduce harmful emission.1 Determining the concentration of O2 dissolved in water (“dissolved oxygen”, DO) allows estimating the pollution levels in wastewaters via measurement of O2 consumption by aerobic microorganisms.2 Moreover, DO measurements in cell culture media give important data on cellular functions (cell growth rate, metabolism, protein synthesis).3–5 Detection of gaseous O2 is often achieved by using electrical sensor devices based on metal oxide (MOX) chemiresistors,6,7 while DO monitoring is frequently performed by means of optical sensors based on photoluminescence (PL) response. The last approach consists in evaluating the O2 concentration by measuring the variations of the PL properties (intensity, spectral shape and/or lifetime) of a luminescent material as it interacts with O2 molecules. Such a method benefits from peculiar advantages of opto-chemical sensing approaches, e.g. electromagnetic immunity, electrical isolation and possibility to engineer compact devices. It is also worth noting that chemiresistors require electrical circuitry and heaters, while PL-based sensors do not need electrical contacts and can operate at room temperature. These characteristics are clearly convenient when the detection of chemicals has to be achieved in liquids or in explosive environments.8–10 PL-based O2 detection is generally performed by using organic dyes whose PL intensity decreases via O2-induced Stern-Volmer quenching mechanism.11 However, it is worth underlining that several light-emitting MOX semiconductors (hereafter MOXs) also exhibit a reversible modification of PL intensity in presence of O2 and of other molecular species.12–21 This not surprising, as the PL in semiconductors originates from elementary processes (electron/hole photogeneration and radiative recombination) that typically occur in a few tenths of nanometers deep subsurface region. As a consequence, PL properties are very sensitive to molecular adsorption and to surface reactions.22


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Unfortunately, exposing luminescent MOXs to gas species usually cause PL intensity modifications smaller (or even much smaller) than those induced in their electrical resistivity. Such a gap between optical and electrical sensitivities hampers the possibility to develop novel MOX-based optochemical sensors of practical use. Conversely, increasing the MOXs PL responsivity to gas adsorption and developing their potentialities as sensitive luminescence-based probes may foster interesting scenarios. For example, some MOXs can be structured in complex architectures having photonic functionalities, such as distributed Bragg reflectors, leaky waveguides, and Fabry-Perot micro-cavities. These structures can be used as platforms for label-free optical biosensors, in which changes in the refractive index accompanied by adsorption of an analyte can be monitored by optical interferometry23–25 of optical waveguide spectroscopy.26–28 Developing an efficient PL-based sensing with MOXs structures may thus pave the way to all-optical double-parametric detection, in which the luminescence plays the role of an additional optical parameters involved in analytes detection. Moreover, the efficient chemoresistive effect of TiO2 coupled with a reliable PL response may also encourage the investigation of “hybrid” (optical and electrical) multiparametric sensing devices. These have been often considered helpful for improving the usually unsatisfactory selectivity of gas sensors12,29–33 but their practical realization is still to come. Titanium dioxide (TiO2) represents a paradigmatic example of a multifunctional MOXs. It can be used for preparing photonic architectures capable to detect analytes through refractometric sensing24,25 and is diffusely used in several applications, e.g. photoelectrolysis of water,34 environmental remediation,35 dye-sensitized solar cells,36 gas sensors,37 self-cleaning surfaces38 and many others. The PL properties of TiO2 are quite particular, as they depend on the crystalline phase. More precisely, the two stable TiO2 polymorphs (anatase and rutile) are characterized by well-separated PL bands39–44 that originate from optical transitions involving different electronic states. For both polymorphs, satisfying results have been observed regarding the PL reactivity of TiO2 toward O2


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exposure,21 in spite of the fact that samples preparation and morphologies were not optimized. Encouraged by these results, here we face the issue of improving the optical responsivity of the TiO2 nanomaterials by focusing on sample morphology as a key factor. To guarantee an optimal modulation of PL emission intensity, gas molecules have to reach the highest possible number of surface sites belonging to PL-active (i.e. photoexcited) nanoparticles. Hence, large sensitivities result from a favorable interplay between the inter-particle permeability to gas molecules and the spatial distribution of photogenerated charges. A possible route to optimize such interplay is to take advantage from the peculiar characteristics of the hierarchical architectures. These consist of low-dimensional nano-building blocks assembled on a three-dimensional interpenetrated network of semi-1D hierarchical nanostructures and specular large voids. The nanoparticle-based morphology guarantees large specific surface areas and close contact between photogenerated charges and adsorbed molecules, while the large voids act as a manifold with parallel distributing nano-channels for the gas molecules. For these reason, hierarchical organization can exhibit optimal gas sensing performances.45 In the present work, we show that exceptionally large PL modulations induced by O2 exposure can be obtained in hierarchical hyperbranched TiO2 nanostructures prepared by pulsed laser deposition (PLD). The PL intensity response toward O2 of both anatase and rutile PLD-deposited samples was tested under controlled flow of dry O2 at variable concentrations. The results were compared with those obtained through the same experimental procedure in reference TiO2 nanoparticle samples, consisting of aggregates of commercial crystalline nanopowders. Giant and uncommonly high responsivities to O2 exposure were observed in hierarchical anatase, up to about 1100% at O2 concentration of 20%. Such a value was seven times larger than the one measured at the same O2 concentration in reference anatase nanoparticles. An enhancement of the responsivity (compared to the one measured in reference samples) was also observed for PLD-prepared rutile samples, despite


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a reduction of the available surface area caused by the morphology changes that accompany the anatase-to-rutile transition. Overall, the resulting responsivities outperform those obtained by simple nanoparticle aggregates and significantly arise over the ones typically reported in the MOXs literature. The reasons for these giant improvements in the gas response are discussed in terms of an outcome of the hierarchical architecture of the nanoparticle assembly.

RESULTS AND DISCUSSION Amorphous hierarchical aggregates of TiO2 nanoparticles were prepared by PLD using the procedure described in the Supporting Information (“Methods” section). As shown in a previous work,46 our choice for the PLD parameters (laser fluence, target-substrate distance, background oxygen pressure) allows the formation of quasi-1D hyperbranched nanostructures, exhibiting a hierarchical “forest-like” morphology. Crystallization of the as-deposited amorphous samples is obtained by thermal annealing that converts amorphous nano-trees in quasi-1D hyperbranched anatase structures. The annealing temperature correlates with average crystallite sizes, determined by XRD analysis and Debye-Scherrer formula, and with the specific surface area.47 Two series of TiO2 samples of different crystalline phase were prepared: thermal treatment in ambient air for two hours at 500°C produced anatase-phase TiO2 (hereafter “A500” samples), while thermal treatment in ambient air for two hours at 800°C led to formation of rutile-phase TiO2 (hereafter “R800” samples). Samples consisting of anatase and rutile nanoparticle aggregates (hereafter “A-np” and “R-np” samples, respectively) were also prepared to set up a comparison with the results obtained in A500 and R800. The formation of the desired crystalline phase in thermally annealed nanostructures was verified by Raman measurements.


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SEM images of the four samples are reported in Figure 1. Fig. 1a shows a SEM image of the anatase reference sample A-np, while Figures 1b, 1c and 1d refer to A500 (1b: top-view image, 1c and 1d: cross-section images at different magnifications). Fig. 1e displays an image of rutile reference sample R-np, while Figs. 1f, 1g and 1h show top-view (Fig. 1f) and cross-section (Figs. 1g and 1h) images of R800. The A500 sample exhibits a multiscale hierarchical organization of TiO2 structures, with a “forest-like” morphology characterized by an array of elongated and branched structures (“trees”) whose growth axis is perpendicular to the Si substrate.48 The lateral branches of these quasi-1D hierarchical nanostructures have an elongated shape and comprise anisotropic single crystals with preferential (001) growth42 (see Fig. 1c). Hence, the overall structure presents a broad distribution of pores and channels, resulting from the simultaneous presence of nanometer-sized pores (inner core of the “trees”), larger pores in between adjacent branches and vertical channels among the elongated structures.42,48 This results in large values of the roughness factor per unit thickness46 (RF, i.e. the ratio between effective surface and geometric volume of the films) and, at the same time, large permeability to external molecules. The morphology of R800 is quite different from the A500 one. In fact, the transition from anatase to rutile lead to an evident grain growth (please compare Figs. 1b and 1f), with the transformation of the quasi-1D hierarchical nanostructures in 1D polycrystalline nano-stone piles (see Fig. 1f). The loss of the nanoscale features determined a significant decrease of the surface-to-volume ratio and, hence, of the RF values, as measured by dye desorption method,46 from about 80 µm-1 (500°C annealing) to about 20 µm-1 (800°C annealing).49 Moreover, the densification of the nanostructures increased the width of vertical channels separating the quasi-1D elongated structures. This can be seen by comparing Figs. 1b and 1e and by observing the large grooves in the top-view SEM image of R800 (Fig. 1e), significantly larger and more evident than the ones observed in A500 (Fig. 1a). Due to its smoother morphology, the A500 films were optically homogeneous in the visible range, even exhibiting a strong iridescence at visual inspection in reflected light, due to Fabry-Perot


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interference fringes. On another hand, the R800 samples appeared whitish and exhibited no iridescence, likely because of its larger surface roughness caused by the irregular grooves. SEM images of anatase and rutile nanoparticles used as reference samples are shown in Figures (1d) and (1h), respectively, using the same magnification for the sake of comparison. The reference anatase sample (A-np) consists of unprocessed anatase nanoparticles whose average size is about 25 nm (Fig. 1a), while R-np (Fig. 1e) presents much wider grains (∼200-300 nm size) because of the thermal annealing required to convert the mixed-phase Aeroxide (Sigma Aldrich 718467) to rutile phase. The inter-grain voids are also significantly larger in R-np, while A-np presents a much more compact structure with pores of the order of few tenths of nm. The PL spectra of PLD-prepared samples are reported in Supporting Information (Figure S1). The PL spectrum of A500 sample (blue curve in Fig. S1) extended in the visible range (VIS-PL) and exhibited Fabry-Perot interference fringes. R800 showed the expected near-infrared PL (NIR-PL) of rutile centered at about 840 nm, and exhibited no interference fringes. The total VIS-PL and NIR-PL intensities (indicated by ϕ v and by ϕ n respectively) used in this work were extracted from the experimental data by numerical integration of the PL spectra as follows:

ϕv =

∫ ϕ (λ ) d λ

(1)

∫ ϕ (λ ) d λ

(2)

∆λv

ϕn =

∆λn

where the subscripts “v” and “n” stand for “visible” and “near-infrared” (respectively) and ϕ ( λ ) is the experimental spectral PL intensity. The numerical integration was performed over the wavelength intervals ∆λv = 450 − 580 nm and ∆λn = 800 − 900 nm , also highlighted in Figure S1 (see Supporting Information).


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The variation of room temperature PL emission of the A500 and R800 samples caused by changing the test cell atmosphere from synthetic dry air (20% O2, 80% dry N2) to 100% dry N2 is reported in Figure 2. In particular, black curves in Figures 2A and 2C represent the PL spectra of A500 and R800 samples acquired in synthetic air (i.e. 80% N2, 20% O2), while the red curves are the PL spectra of the same samples measured after 30 minute exposures to flowing N2. The time dynamics of the PL intensities are reported in Figures 2B (A500) and 2D (R800) as a function of the time duration of the N2 flow. It is observed that the gradual removal of O2 leads to a very large enhancement of the VIS-PL intensity. Vice versa, the efficiency of rutile PL emission decreases under the same conditions, evidencing the phenomenon dependence on crystalline phase. To quantify the PL response towards O2 and compare the results presented here with those reported in literature, we use here dimensionless responsivities rQ and rE , associated to a quenched and to an enhanced PL response (respectively), as:

rQ =

ϕ v0 ; ϕv

rE =

ϕn ϕ n0

(3)

In the above expressions, ϕv and ϕn are the steady-state intensity of VIS-PL and NIR-PL (respectively) in presence of the gaseous analyte (O2), while ϕ v0 and ϕ n0 are the steady-state intensity of VIS-PL and NIR-PL in inert ambient (N2). In the case of hierarchical A500 sample, the transition from synthetic air (80% N2+ 20%O2) to 100% N2 (see Fig. 2B) was associated to a change in the measured VIS-PL intensity (expressed in arb. units) from ϕ v = 0.40 × 105

to ϕ v0 = 4.85 × 105 , corresponding to a responsivity of

rQ = ϕ v0 ϕ v = 12.1 = 1210% (see Eq. (3)). Such a value is unusually large for PL responses based on

MOXs materials, as will be discussed in more detail later. The same experiment performed on the


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R800 sample led to ϕ n = 2.75 × 10 6 and ϕ n0 = 1.25 × 10 6 , corresponding to a responsivity of rE = ϕ n ϕ n0 = 2.2 = 220% .

In order to further compare the PL reactivity of the PLD-prepared samples with that of the reference samples prepared by TiO2 commercial nanopowders, we carried out an experiment consisting in measuring the PL spectra during four exposures to mixtures of O2/N2 flows of different O2 concentration ([O2] = 1%, 2%, 5% and 20%), alternated to exposure to pure nitrogen. The same experimental procedure was carried out on four samples, i.e. the two PLD-prepared samples (A500, R800) and the two reference samples (R-np, A-np) prepared starting from commercial TiO2 nanopowders. The PL intensities ϕv (t ) and ϕ n (t ) measured during the experiment and normalized by the stabilized values ϕ v0 and ϕ n0 (measured in N2) are reported in Supporting Information (Figure S3). Using these experimental data, the time-dependent O2 responsivities rQ (t ) = ϕ v0 ϕ v (t ) and rE (t ) = ϕ n (t ) ϕ n0 were calculated for each sample. The corresponding results are shown in Figures

3A and 3B, respectively. The exposure to O2 occurred in the yellow-colored regions, alternating to exposures to flowing N2 (white regions). From the steady-state (i.e. equilibrium) values of rQ (t ) and rE (t ) we then calculated the PL responsivity at equilibrium to the tested O2 concentrations. The results for each of the investigated sample are reported in Figure 3C. The equilibrium responsivities obtained by A500 range from about 3.7 (1% O2 concentration) to about 10.9 (20% O2 concentration), while the corresponding values measured for A-np are significantly lesser (1.42 at 1% O2, 1.60 at 20% O2). This corresponds to an increment of a factor of 6.8 for the A500 responsivity in synthetic air, compared with that of A-np. Such a significant difference is noticeable also by the fact that the responsivities in Fig. 3A are reported in logarithmic scale. Another point to underline is that the responsivity values of A-np saturate as the O2 concentration is increased. Such a saturation indicates that the maximum achievable superposition between ACS Paragon Plus Environment

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photoexcited nanoparticles and molecular adsorption has been reached.17 This is not observed in A500, suggesting that a larger number of PL-active (i.e. photoexcited) TiO2 nanoparticles is available in the sample. Moreover, it is also worth noting that the reference samples do not exhibit relevant differences in their responsivities. These observations can be interpreted by considering the main characteristics of homogeneous vs. hierarchical distributions of nanoparticles. As mentioned previously, overcoming the problem of low optical (PL) sensitivity to gas adsorption in multifunctional MOXs can represent a major advance toward realization of efficient all-optical or hybrid (electrical/optical) multi-parametric devices that may avail selective analysis of chemical species in mixtures.29–32,50 This underlines the importance of the results reported here. In particular, we point out that the PL variations caused by O2 in PLD-prepared samples are not only significantly larger than the ones we observed in reference nanopowder samples, but are uncommonly large even when compared to those reported in the literature of PL-based gas sensing. To evidence the last point, Table S1 in Supporting Information summarizes the PL responsivities obtained here and in other works in which MOXs and other materials (e.g. ZnO, SnO2, TiO2, WO3, SiO2, Ag nanoparticles, carbon nanotubes) have been employed to detect various chemical species (including NO2, O2, CO, water, ethanol, ammonia, etc.). The responsivities span on a range of about 1-3 for different sensitive materials and various analytes, underlining that the results here reported represent outstanding performances. To discuss the origin of these uncommon PL responses, we focus on the connection between morphological properties and optical response. It is know that when the nanoparticles are consolidated into sensing materials forming large and dense aggregates, only the particles close to the geometrical surface of the film contribute to the gas sensing reaction, while the inner part remains much less active.51 In such a condition, most of the gas-induced phenomena will occur only near the geometrical surface region, (i.e. in a region much smaller than the specific surface area) and a high gas response cannot be achieved. Moreover, compact aggregation of nanoparticles hamper the gas diffusion, slowing the response speed.51,52


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As seen in Figure 1a, A-np is composed of closely aggregated anatase nanoparticles whose average size is of about 25 nm. Such a structure guarantees two major advantages, namely (i) a large specific area, and (ii) possible contribution to overall PL emission coming from underneath layers. In fact, the average size of nanoparticles is lesser than the optical extinction length Λ for TiO2 at λ =325 nm ( Λ = λ 4π k

52 nm , using for the extinction coefficient k the value k ∼ 0.5 measured at

λ =325 nm by Jellison and coworkers53). Therefore, UV laser light can photo-excite particles located below the topmost layer. However, their participation to the PL response is hindered by the compact structure of A-np, exhibiting inter-particle pores of the order of few tenths of nm. On another hand, R-np (Figure 2b) presents much larger and more separated crystallites, due to the thermal annealing required to convert the mixed-phase Aeroxide (Sigma Aldrich 718467) to rutile phase. The inter-grain voids are significantly larger in R-np, thus favoring gas circulation. To this regard, it is worth noting that R-np exhibit a response time to O2 faster than the A-np one: this is an indication of the fact that the gas circulation in A-np is hampered with respect to the one occurring in R-np. Another thing to note is that the average size of individual grains in R-np (200-300 nm) is more than three times larger than the attenuation length of TiO2 at 325 nm radiation. This indicates that the layer of grains underneath the first one is scarcely PL-active. In a sense, this is an ideal configuration for an optimal overlap between the gas coverage and the PL-active part of the material. Notwithstanding this, R-np has a major drawback in its low surface area and large crystallites size. Summarizing, the overall balance of morphological properties (favoring R-np) and crystallite sizes (favoring A-np) lead to optical responsivities comparable for the two reference samples, while the peculiar properties of PLD-prepared samples (A500 in particular) favor their gas response, as they combine more efficient O2 diffusion, small nanoparticles and high surface areas. The morphology of PLD-prepared samples is very different from the reference samples ones. These samples exhibit a quasi-1D hierarchical morphology, also described in terms of a “nano-forest”48


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characterized by the simultaneous presence of small nanoparticles, branched and elongated (“quasi1D”) structures (“nano-trees”) and large channels. The nanometer-scale size of nanoparticles is an important factor, as it not only guarantees large surface areas, but also assures a closer contact between photogenerated charges and adsorbed molecule. It is in fact to be noted that the photogenerated charges (or excitons) in large nanostructures can migrate toward the bulk core of the crystallite, becoming less sensitive to the presence of surface adsorbates and decreasing the optical responsivity to gas.20 In addition, the large voids separating the quasi-1D structures (present in both A500 and R800 but more evident in the latter) can act as a manifold with parallel distributing nanochannels, providing high permeability to the gas molecules. These peculiarities favor the optimal superposition between photogenerated charges, radiative (PL-emitting) centers and adsorbed gas molecules, and are therefore likely to explain the giant response to O2 observed in A500 morphology. Another interesting result to note is that “baseline” PL intensity measured in N2 is not fully recovered after the first O2 exposure (see Fig. 3A and 3B), while it becomes more stable after the successive exposures to O2. This interesting phenomenon is not dominant and saturates as the O2 exposures proceed, thus does not affect significantly the possibility to TiO2 nanostructures for PLbased O2 sensing. Nevertheless, it deserves attention and we are currently investigating on it. The findings suggest that a stable modification occurs to some degree in the materials during the exposure to UV light in O2 environment. Even if conclusive statements are still to come, we suggest that the phenomenon is a consequence of a partial photo-assisted oxidation of pristine TiO2 surfaces, occurring during the combined exposure to UV light and O2 molecules. Finally, we comment on the fact that the responses towards oxygen are opposite and anti-correlated in the two crystalline phases of TiO2. The O2-induced quenching of VIS-PL can be explained quite naturally by assuming that the initial states of the optical transitions leading to VIS-PL are free (i.e. − conduction band) electrons eCB and/or electrons occupying trap states which are populated from the


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conduction band through fast electronic relaxation. In such a case, the scavenging of free electrons −

by adsorbed oxygen forming superoxide species O2 will decrease the radiative recombination rate and quench the anatase PL emission. Such a mechanism is not radically different from that observed in other n-type MOX exposed to oxidizing agents, e.g. SnO2 exposed to nitrogen dioxide.54 In the present work, the green component of anatase PL peaked at about 520-540 nm (see Fig. S2 in Supporting Information) represents the dominant contribution to VIS-PL. A series of studies by McHale and coworkers41–43,55,56 indicate that this PL component is ascribable to the radiative − recombination of eCB with trapped holes. Consequently, the O2-induced electron scavenging can

explain the green PL quenching observed in anatase TiO2. On another hand, the same mechanism cannot be invoked to explain the NIR-PL enhancement in rutile. It hence looks natural associating NIR-PL emission with recombination between free or trapped holes and electrons occupying states + which are not scavenged by adsorbed O2. Indeed, both trapped57–59 and free41–43,55,56 holes (hVB )

have been invoked as participating to the NIR-PL in rutile. In several studies by Nakato and coworkers the rutile NIR-PL was assigned to recombination of conduction band electrons with selftrapped holes localized at three-fold coordinated O atoms. However, the supposed participation of conduction band electrons makes this assignment quite problematic because of its incompatibility with NIR-PL enhancement. On another hand, recombination between valence band (and not trapped) holes and trapped electrons can explain the NIR-PL once we assume that the density of these PL-active trapped electrons is not affected by O2 adsorption, i.e. that they recombine + radiatively with hVB without participating to superoxide formation.

Even if the interpretation based on trapped holes may be still somehow maintained by assuming that the NIR-PL transitions occur in the sub-surface regions and not at the surface,21 a recent spectroelectrochemical photoluminescence (SEPL) analysis in rutile TiO2 nanowires conducted by Rex and coworkers60 strongly supports the idea that the NIR-PL does not involve trapped holes, but is instead caused by the radiative recombination of free holes with electron traps which are deep ACS Paragon Plus Environment

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both in energetic sense (i.e. mid-gap states) and spatial sense (i.e. located in subsurface layers). These electron traps are quite elusive, as the presence of water in the contact electrolyte leads to an overpotential in the potentiometric occupation of these trap states. Such an effect has been evidenced in SEPL measurement for both rutile60 and anatase.61 However, once SEPL analysis are conducted in an appropriate solvent (dry acetonitrile), Rex and coworkers evidenced electron trap states located about 1.5 eV below the edge of the conduction band edge.60 The energy position of such states is compatible with NIR-PL once it is assumed that they recombine radiatively with free holes. As these mid-gap electron trap states are already occupied at equilibrium (their energy is below the Fermi level), their density will not decrease due to O2 adsorption. Instead, O2 will scavenge the free electrons, decreasing the competition for the recombination with free holes and thus determining the NIR-PL intensity enhancement in O2.

CONCLUSIONS Extremely large PL intensity modulation induced by O2 exposure is demonstrated in PLD-prepared hierarchical hyperbranched TiO2 nanostructures. Equilibrium responsivities up to 1100% and of 220% are obtained in response to 20% O2 diluted in N2 for anatase and rutile TiO2 nanostructures, respectively. The results observed in hierarchical anatase structures outperform those measured on anatase nanoparticles aggregates by a factor rQ (hierarchical) rQ (nanoparticle) spanning from 2.6 at 1% O2 concentration up to 6.8 at 20% O2 concentration. Even more importantly, the experimental responsivities are uncommonly large (in some cases even for a factor of one order of magnitude, see the reference data reported in Supporting Information section, Table S1) also when compared to those reported in the literature of gas sensing based on MOXs photoluminescence. Such a giant optical responsivity is attributed to the optimal interplay occurring between photogenerated charges, PL-emitting centers and adsorbed gas molecules, favored by the hierarchical organization of the TiO2 structures. In particular, we argue that the presence of small ACS Paragon Plus Environment

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nanoparticles distributed on elongated (“quasi-1D”) structures separated by large channels represent a key element to assure an optimal gas response. In fact, nanometer-scale size of nanoparticles guarantees large surface areas and close contact between photogenerated charges and adsorbed gas molecules, while in addition the large channels and voids percolating the whole structure provide high permeability to the O2 molecules. These elements result in an optimal spatial overlap between molecular adsorbates and PL-active nanoparticles carrying photogenerated charges, significantly enhancing the optical response to gas respect to standard aggregates of nanoparticles. For future developments, we consider beneficial deepening the study of the correlations between morphology and PL responsivity for each of the two TiO2 allotropic forms. The results encourage exploring the use hierarchical architectures of nanostructures based on TiO2 (or on other MOXs) as a strategy for overtaking the problem of low PL sensitivity to gas adsorption in MOXs, possibly paving the way toward the use of oxide nanostructured films as all-optical or hybrid (opto/electrical) multi-parametric sensor devices.

AUTHOR INFORMATION Corresponding Authors Stefano Lettieri ([email protected]); Fabio Di Fonzo ([email protected]) Author Contributions S.L. conceived the experiment. L.P. and F.D.F. developed the PLD-prepared samples and performed their SEM and Raman characterizations. D.K.P., S.L. and F.G. prepared the reference samples and performed the PL experiments. S.L., F.D.F and P.M. discussed the results. S.L. and F.D.F wrote the manuscript with contributions by L.P., D.K.P. and P.M. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT ACS Paragon Plus Environment

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S.L. and D.K.P. acknowledge the Italian Ministry for University and Research (FIRB Project No. RBAP115AYN “Oxide at the nanoscale: multifunctionality and applications”) and the Italian Ministry of Foreign Affairs and International Cooperation (NANOGRAPH Project) for financial support.

SUPPORTING INFORMATION The following file is available free of charge: “Pallotti et al_SI.pdf”. It includes: Peak-normalized photoluminescence spectra of A500 and R800 measured in dry N2; Photoluminescence intensity curves for R800, A500, R-np and A-np samples during exposure to O2 flows of different concentrations; Experimental methods section and Table S1 reporting the PL responsivities toward gas species of various metal-oxide semiconductors and inorganic composites.

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FIGURE CAPTIONS Figure 1. a): SEM image of anatase reference sample A-np. b): Top-view SEM image of A500 surface. c) and d): Cross-section SEM images of A500 sample at different magnifications. e): SEM image of rutile reference sample R-np. f): Top-view SEM image of R800 surface. g) and h): Crosssection SEM images of R800 sample at different magnifications.


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Figure 2. (A): PL spectrum of A500 sample in synthetic air (black curve) and after 30 minutes in N2 flow (red curve). (B): Time evolution of A500 PL intensity (integrated in the 450-580 nm wavelength range) during exposure to N2. (C): PL spectrum of R800 sample in synthetic air (black curve) and after 30 minutes in N2 flow (red curve). (D): Time evolution of R800 PL intensity (integrated in the 800-900 nm wavelength range) during exposure to N2. The dashed arrows in (A) and (C) indicate the direction of PL intensity variations occurring during O2 desorption. The full arrows in (B) and (D) indicate the switch-off of the O2 flow in the test chamber after the completion of the first spectral acquisition in synthetic air.

Figure 3. (A): Time-dependent quenching responsivity rQ(t) for anatase samples A500 (black curve) and A-np (blue curve), reported in log scale. (B): Time-dependent enhancement responsivity rE(t) for rutile samples R00 (black curve) and R-np (blue curve). In both figures (A) and (B) the exposure to O2 occurs in the yellow-colored regions, followed by exposure to flowing nitrogen (white regions). Tested O2 concentrations are 1%, 2%, 5% and 20%. (C): Steady state maximum responsivities of the tested samples measured for different O2 concentrations. The experimental data used to calculate the responsivities are shown in Supporting Information (Figure S2).


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Figure 1


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(B)

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900

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(A)

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