Optical Sensing of Molecular Oxygen (O₂) via Metal Oxide Photoluminescence: A Comparative Study of TiO₂, SnO₂ and ZnO

Abstract

A comparative study is presented on the photoluminescence (PL) response toward molecular oxygen (O₂) in tin dioxide (SnO₂), zinc oxide (ZnO) and titanium dioxide (TiO₂) nanoparticles. The findings show that both PL enhancement and PL quenching can be observed on different materials, arguably depending on the spatial localization of the defects responsible for the PL emission in each different oxide. No significant results are evidenced for SnO₂ nanoparticles. ZnO with red/orange emission shown an O₂-induced PL enhancement, suggesting that the radiative emission involves holes trapped in surface vacancy oxygen centers. While the ZnO results are scientifically interesting, its performances are inferior to the ones shown by TiO₂, which exhibits the most interesting response in terms of sensitivity and versatility of the response. In particular, O₂ concentrations in the range of few percent and in the range of a few tenths of a part per million are both detectable through the same mixed-phase TiO₂ sample, whose rutile phase gives a reversible and fast response to larger (0.4–2%) O₂ concentration while its anatase phase is usable for detection in the 25–75 ppm range. The data for rutile TiO₂ suggest that its surfaces host deeply trapped electrons at large densities, allowing good sensitivities and, more notably, a relatively unsaturated response at large concentrations. Future work is expected to improve the understanding and modeling of the photophysical framework that lies behind the observations.

1. Introduction

In this work, we investigate the interplay between molecular oxygen (O₂) adsorption and photoluminescence (PL) emission of selected metal oxide (MOX) nanoparticles, aiming to shed light on the potentialities vs. limitations in employing such materials as optical O₂ sensors and to discuss the photophysical framework that lies behind the observations.

It is well-known that many MOXs, including those considered in this work, are of great importance not only in the field of chemosensors but also for photocatalytic applications. This point is relevant in relation to the motivations of the present study, which can be summarized by two arguments. The first relates to the applicative interests in developing O₂ sensors, while the second underlines the scientific importance of elucidating the role of O₂ in heterogeneous photocatalysts.

Oxygen sensing can be implemented in terms of detection of O₂ in gas phase or of dissolved oxygen (DO) in water. Measurements of concentration for the latter are of great importance for monitoring the health of marine environments and of water pollution in general [1]. Furthermore, the control of DO content is a relevant issue in biology: to mention a few examples, hyperoxia has been related to formation of reactive oxygen species and subsequent cell death [2,3]. Additionally, hypoxia and re-oxygenation enhance the proliferation, invasiveness and metastatic potential of tumor cells [4,5].

While electrochemical cells (i.e., Clark electrodes) [6] have been used for several years for DO analyses, approaches and technologies relying on optical measurements are also largely used thanks to peculiar advantages such as the fact that they do not consume oxygen and can be relatively portable and easy to miniaturize [7,8,9,10]. The optical sensing of O₂ typically relies on the use of photoluminescent organo-metallic coordination complexes, whose intensity is dependent on the local O₂ concentration because of “static quenching” phenomena. As a result, the O₂ concentration is correlated to the PL intensity of the optical sensing material (or “oxygen indicator”) by the Stern–Volmer quenching relation:

$$I_0/I=1+k_{SV}\left[O_2\right]$$

(1)

where $I_0$ and $I$ are the luminescence intensity in absence and presence of oxygen (or, more generally, in presence of the quencher), $\left[O_2\right]$ is the oxygen concentration and $k_{SV}$ is the Stern–Volmer constant related to the luminescence lifetime and the probability of quenching interaction [11].

Metal–ligand coordination complexes are widely used for this purpose. However, it is important to highlight that inorganic semiconductors can also exhibit oxygen-dependent (and—more generally—gas-dependent properties) that can often be exploited in either optical and/or electrical responses [12,13,14,15,16,17,18,19,20]. Among inorganic semiconductors exploitable for chemical sensing there are of course several MOXs which are not only the primary materials of choice for realization of chemoresistive sensors but are also suitable for optical sensing. Optochemical sensing by MOXs includes PL-based detection [21,22,23,24,25] but also other techniques: for example, MOXs can be used to prepare photonic crystals or metasurfaces [26,27,28,29] finding applications in chemical and/or biological sensing [27,30] and whose “slow light” effect can be advantageous for increasing photocatalytic efficiencies [31,32].

The second point to underline is that O₂ affects the photophysical behavior of photogenerated charge carriers in photocatalytically active MOX materials. This is another field in which the PL analysis—especially when conducted in controlled environment conditions—can play a central role. In fact, PL monitors the recombination of charge carriers, while O₂ acts as an efficient electron scavenger, adsorbing on MOXs and producing charged species at the surface (mainly superoxide ions). These ions trigger the production of reactive oxide species and, importantly for the present discussion, modify (i.e., increase) the depth of the depletion region where the built-in electric field spatially separates the opposite charge carriers.

A schematic is proposed in Figure 1. Charge neutrality imposes the equality between the surface charge (negative in the example of Figure 1) and the bulk excess charge of opposite sign (represented as valence band holes in Figure 1). The latter is proportional to the depth $D_{scr}$ of the space charge region, in which the radiative recombination of electrons and holes is hampered by the spatial separation of the opposite charges. PL experiments are hence sensitive to variation of $D_{scr}$ in a range close to the optical penetration length, which can be as low as a few tenths of nanometers for PL experiments using above-bandgap excitation. In this sense, PL spectroscopy is not a surface-specific technique such as second harmonic generation [33,34], but is specifically sensitive to modifications of charge density occurring in the sub-surface region affected by the band bending [35,36,37].

The spatial extension of the depletion region decisively affects the migration of photogenerated charge carriers toward the catalyst surface. Hence, elucidating the effect of O₂ on the PL activity of photocatalytic MOXs also provides important information on how O₂ can affect the efficiency of photocatalytic reactions.

Previous studies highlighted the peculiar position of TiO₂ in the field of gas-sensitive luminescent MOXs. In particular, it is established that both PL enhancement and PL quenching can be induced by O₂, depending on the TiO₂ crystal phase [38,39]. This property appears to be a peculiar for TiO₂ and suggests that this material is the most interesting one among typical gas-sensitive MOXs for O₂ optochemical detection. However, the literature on chemosensors lacks comparative studies involving different MOX nanomaterials. Motivated by this consideration, we propose here a comparative study of TiO₂, ZnO and SnO₂ nanoparticles in relation to their ability to sense O₂ via PL modulation. These materials are chosen based on their importance in the field of chemosensors and on the fact that—to the best of the author’s knowledge—the possibility of detecting O₂ by ZnO and SnO₂ has not been explored much.

While different physical methods can be employed to prepare gas-sensitive MOX nanoparticles [40,41,42,43], this work investigates nanoparticulated powders prepared by the same chemical route which is used to prepare thick film chemo-resistors, having the advantage of good production yields and well-established preparative procedures for the realization of stable devices [44,45,46].

2. Materials and Methods

2.1. Nanoparticles Preparations and Characterizations

Nanopowders of TiO₂, SnO₂ and ZnO were obtained through wet-chemical synthesis from reagent-grade starting materials used as received by the Merck Group supplier (Milan, Italy) mainly following the procedure described in References [47,48,49], respectively.

In summary, for TiO₂ preparation a solution of Ti(IV) n-butoxide in absolute ethanol was slowly added under stirring to an ethanol/water solution with a volume ratio of 1. After 10 min of stirring, the suspension was filtered by gravity, washed several times with water and diethyl ether, then dried at 100 °C overnight. The powders were calcined for 2 h at a temperature of 650 °C. This temperature was chosen on the basis of previous characterizations [50] with the aim to obtain both anatase and rutile phases though maintaining a small crystallite size.

SnO₂ powders were prepared by adding drop-wise a small amount of deionized water to a 0.7 M n-butanol solution of Sn(II)2–ethylexanoate. By introducing HNO₃, the solution pH was found to be 1. Finally, the mixture was stirred for 3 h at room temperature. The resulting gel was filtered by gravity, washed several times with diethyl ether, dried at 100 °C overnight and calcined at 650 °C for 2 h.

ZnO synthesis started by preparing a 0.05 M water solution of Zn(NO₃)₂*6H₂O to which was added ammonia hydroxide solution to reach a pH of 10. The solution was aged for 24 h at room temperature, then the white precipitate was filtered by gravity, washed with water and diethyl ether, and then it was dried in air at 100 °C overnight, and finally calcined for 2 h at 450 °C.

The morphology, the crystalline structure and the specific surface area (SSA) of the powders were studied by: (i) Field Scanning Electron Microscopy (FE-SEM) using a Carl Zeiss Sigma microscope operating at 3.00 kV, (ii) X-ray diffraction (XRD) analysis with a Philips PW 1830 vertical diffractometer in Bragg–Brentano geometry (PANalytical, formerly Philips Analytical, Almelo, The Netherland) (Cu Kα radiation, 40 kV, 30 mA) performing the Rietveld analysis with FullProf program (release 2011) and calculating the average crystallite size using the Scherrer’s formula [a], and (iii) the Brunauer–Emmett–Teller (BET) method to the adsorption/desorption isotherms of N₂ at 77 K obtained with a Micromeritics ASAP 2010 physisorption analyzer.

2.2. Photoluminescence Measurements

PL measurements of MOX nanoparticulate at a tunable excitation wavelength in a controlled environment were performed by placing the samples in a home-built sealed stainless-steel test chamber equipped with a fused silica entrance optical window. Mixtures of dry N₂ (99.9995% purity) and dry air (20% O₂ and 80% N₂, 99.9995% purity) were flown in the test chamber during the measurements by mass flow control system. Optical excitation (~2 nm for the FWHM of the samples) was provided by a Xe lamp (450 W power) coupled with a double-grating excitation monochromator (Horiba Gemini, effective focal length = 180 mm) equipped with two 1200 grooves/mm gratings. The slit widths of the excitation monochromator were set up to obtain light centered at a desired wavelength with a spectral width (full width at half maximum) of about 3 nm. The monochromatized light was collected by a liquid waveguide and focused on the sample, which consisted of about 100 mg of nanoparticle powders lightly pressed into a steel washer to form a circular film of 0.7 cm diameter.

A system of converging lens and optical filters was employed to image the illuminated part of the sample on the input slit of a monochromator (Horiba Jobin-Yvon Triax 320, effective focal length 320 mm) collecting the PL emission and cutting off the excitation light. The PL spectra were obtained using a cooled CCD camera. The PL intensities are reported in counts per second (cps) units, which are the total counts of the digitally processed PL intensity as obtained by the CCD camera divided by the integration time. The entire system was controlled via a computer using a home-made LabView program.

3. Results

3.1. Structural and Morphological Characterizations

FE-SEM images (Figure 2) show a homogeneous distribution in particle size and a somewhat round shape for all materials. In particular, the grain dimension ranges from 20 to 40 nm for TiO₂ and SnO₂ and from 40 to 60 nm for ZnO. XRD patterns of the tree powders (Figure 3) confirm a pure single phase with the exceptions of TiO₂, for which there are both anatase (95%) and rutile (5%) phases, as expected. The crystalline phases (obtained from Rietveld refinement), the average crystallite sizes (evaluated by Scherrer’s formula) and the calculated specific surface areas are summarized and reported in

Table 1. Calcination temperatures, crystal size, specific surface area (SSA) and the composition of the TiO₂ powders (expressed in atomic percentages).

Sample	Calcination Temperature (°C)	Crystalline Phase	Composition (at. %)	Crystallite Sizes (nm)	SSA (m2/g)
TiO2	650	Rutile	5	61	29
		Anatase	95	34
ZnO	450	Wurtzite	100	33	12
SnO2	650	Cassiterite	100	21	28

.

3.2. Photoluminescence Response towards O₂

The figures shown in this Section express the PL intensity of the samples measured vs. time as the sample is exposed to pure N₂ or to flowing N₂–air mixtures.

We start by presenting the results obtained for TiO₂ nanoparticles, which are by far the most interesting ones (as evidenced next).

A representative example of the results obtained using TiO₂ is shown in Figure 4, which reports the intensity map for the PL spectra acquired at excitation wavelength of 370 nm during exposures to pure N₂ (from t = 0 to t = 1200 s) and followed by exposures to air (and hence O₂) at different concentrations. Both visible and near-infrared PL emission (VIS-PL and NIR-PL, respectively) are observed. The two PL emission bands, centered at about 550 nm and 840 nm, originate from anatase and rutile phases of TiO₂, respectively [51].

Air–N₂ mixtures at different (increasing) amounts of air concentration in Figure 4 have been inserted in the test chamber at times t = 1200 s (0.4% O₂ concentration), 1500 s (0.8% O₂ concentration) and 1800 s (2% O₂ concentration), preceded and followed by exposure to pure N₂ (i.e., in absence of O₂). The anti-correlated response to O₂ mentioned in the introduction is clearly evidenced.

The PL signal curves vs. gas exposure shown in the rest of the work have all been obtained from intensity data, as shown in Figure 4, by integrating the PL spectra over specified wavelength intervals $\mathsf{\Delta }\lambda$, i.e.,

$$\mathrm{PL}\mathrm{signal}: {\mathsf{\Phi }}_{PL}\left({\lambda }_{exc},\mathsf{\Delta }\lambda \right)={{\displaystyle \int }}_{\mathsf{\Delta }\lambda }I\left({\lambda }_{exc},{\lambda }_{PL}\right)d{\lambda }_{PL}$$

(2)

where $I\left({\lambda }_{exc},{\lambda }_{PL}\right)$ is the PL intensity measured at wavelength ${\lambda }_{PL}$ as the sample is excited at wavelength ${\lambda }_{exc}$. The different values used for the integration interval and the excitation wavelength are reported in the text and in the figure captions.

The data in Figure 5 show the wavelength-integrated PL intensity spectra for the TiO₂ sample (integrated over different wavelength intervals, namely $\mathsf{\Delta }\lambda =500\mathrm{nm}-600\mathrm{nm}$ for the VIS-PL (Figure 4A) and $\mathsf{\Delta }\lambda =800\mathrm{nm}-900\mathrm{nm}$ for the NIR-PL (Figure 4B).

During the first 20 min (“stabilization phase”) the sample was kept under continuous illumination at a wavelength excitation of 370 nm and remained exposed to nitrogen flow. During this “stabilization” process in nitrogen, an O₂ desorption likely occurs as the sample was previously kept in natural air. Next, the gaseous flow was modified by introducing different amounts of dry air (namely 2%, 4% and 10%) leading to O₂ concentrations of 0.4%, 0.8% and 2%. The total flow was kept constant at a rate of 300 sccm during the entire measurement.

The stabilization phase in Figure 5A shows a VIS-PL enhancement during the O₂ desorption, i.e., indicates that O₂ quenches the VIS-PL at relatively high O₂ concentrations. It is to be noted that the PL variation is sub-linear vs. the O₂ concentration, i.e., the response saturates. This occurrence is not uncommon and can be explained in terms of saturation of a Langmuir-type saturation of available adsorption sites.

Considering then the NIR-PL results reported in Figure 5B, we see that the behavior is quite opposite to the one exhibited by the VIS-PL (as clearly visualized by the intensity map in Figure 4). In fact, the data obtained in the stabilization phase show a NIR-PL quenching during the O₂ desorption, thus indicating that O₂ enhances it. This is confirmed by the data obtained during the exposure to air. Interestingly, the NIR-PL responses are much more linear than the one observed previously for VIS-PL.

Additionally, it is important to note that (i): a complete stabilization is reached in a few minutes, which does not occur for the VIS-PL PL intensity, and (ii): as the 100% nitrogen flow is restored the PL signal almost completely reverts to the initial level. Both these elements clearly evidence that the NIR-PL signal shows the better performance as an O₂-sensing parameter.

Given the interesting results obtained in Figure 5, the sample was tested also for exposure to low O₂ concentrations. The results are reported in Figure 6 for O₂ concentrations of 25, 50 and 75 ppm.

The data in Figure 6B indicate that the NIR-PL does not sense the O₂ concentrations in the range 25–75 ppm, except for a small response to the first O₂ step which is not replied at successive O₂ exposures. Additionally, for the VIS-PL (Figure 6A), the response to the first O₂ pulse is peculiar, as it produces a sudden decrease in the PL intensity which however recovers during the rest of the measurement, until the O₂ concentration is increased. When O₂ is changed to 50 ppm, a stable decrease is then observed, which is produced also for the successive change at 75 ppm of O₂ concentration.

Two elements can be underlined and discussed here: (i) the anomalous behavior immediately after the exposure to the first O₂ step (25 ppm) and (ii) the possibility to sense the O₂ at 50–75 ppm level using the VIS-PL. Instead, these concentrations have to be considered too low for detection via the rutile PL.

The comparative study was continued by testing nanoparticulated powders of SnO₂ and ZnO produced as previously described. These metal oxides are well established as the sensitive element of chemoresistive sensor devices and their PL response to NO₂ has also been studied in some works [52,53,54,55]. However, only very limited investigations of the potentialities of their PL response to O₂ have been published (to the best of our knowledge) [24,56]. The experimental results are summarized in Figure 7.

The SnO₂ powder was characterized by a very weak PL intensity (approximately two orders of magnitudes lower than the one measured on ZnO sample in comparable conditions). The excitation spectra of SnO₂ and of ZnO are also significantly different, as the maximum PL intensity was obtained at a wavelength excitation of 320 nm for SnO₂ and 370 nm for ZnO. Such a difference correlates with the larger bandgap energy of SnO₂ (compared to the ZnO one).

Figure 7A,B show the PL intensity maps vs. time obtained for the SnO₂ and the ZnO sample, respectively. It can be immediately seen that the SnO₂ sample showed a much weaker PL emission, which was almost insensitive to O₂. Air exposures for Figure 7A,B occurred at different concentrations: higher air concentrations (10%, 20% and 50%) were tested for SnO₂ due to the very weak response of the material. Other measurements at air concentrations of 2%, 4% and 10% were also performed on the sample (not shown here) and exhibited even less significant variations of the PL intensity.

The data in Figure 7B refer to ZnO. Compared to SnO₂, the latter shows a significant O₂-dependent PL intensity in the visible range. Here, the same air concentrations used in Figure 4 (i.e., 2%, 4% and 10%) were employed to favor a comparison with TiO₂.

Figure 7C,D show the stabilized PL spectra measured for SnO₂ and ZnO (respectively) in N₂ and in air (50% air for SnO₂, 10% air for ZnO). While any difference in the former is hardly recognizable, the latter exhibits a PL increase in 10% air that leaves the overall PL spectral shape unaltered, as also occurred for TiO₂. The result is interesting and somehow unexpected, as discussed in the next section.

The time dynamics of PL intensity for SnO₂ and ZnO upon air exposure are reported in Figure 7E,F for PL spectra integrated with wavelength intervals $\mathsf{\Delta }\lambda =500\mathrm{nm}-750\mathrm{nm}$ and $\mathsf{\Delta }\lambda =600\mathrm{nm}-700\mathrm{nm}$, respectively. The latter evidences the O₂-related PL enhancement with a saturating (sub-linear) response.

4. Discussion

The experimental results indicate that TiO₂ can be considered the most interesting material among the ones considered in this study for possible application as an O₂ optical sensor. The other oxides are less performing, but the experimental findings are nevertheless interesting from a scientific point of view. In fact, both SnO₂ and ZnO exhibit an O₂-induced enhancement, which in TiO₂ is observed only for the NIR-PL. This finding appears to be puzzling, as the mechanism schematized in Figure 1 would suggest for both SnO₂ and ZnO a PL quenching caused by the formation of surface charged species such as $O_2^{•}{}^{-}$ (superoxide ion), in accordance with the results obtained for anatase TiO₂. Therefore, the data lead us to discuss how the O₂ adsorption can cause either PL enhancement or PL quenching.

In this context, it is worth mentioning an interesting investigation by Ma and coworkers [57] who conducted PL measurements in controlled low-pressure O₂ conditions at low temperature, showing that the intensity of anatase PL can be either enhanced or quenched by exposure to molecular oxygen (O₂) depending on the O₂ dose. More precisely, they evidenced PL enhancement at low O₂ dosages and PL quenching at higher O₂ dosages and interpreted the data as a result of two O₂ adsorption mechanisms. At low O₂ pressures, a dissociative adsorption occurs at surface O vacancy sites, shrinking the depletion region (or, equivalently, decreasing the upward band bending). This increases the electron-hole spatial overlap and hence the PL efficiency. On the other hand, at higher O₂ partial pressures the relative amount of oxygen molecules with respect to available surface O vacancy sites also increases. Additional O₂ molecules are therefore adsorbed in the molecular form, forming superoxide species (e.g.,) enlarging the depletion region as shown in Figure 1 and hence quenching the PL intensity in a reversible way.

Even though the mechanism discussed by Ma and coworkers is sound, it appears to be incomplete. In fact, it describes the PL enhancement as caused by an irreversible bonding of O atoms with the (formerly reduced) lattice, while the only reversible mechanism (chemisorbed molecules) is associated with the PL quenching. In other words, it does not envisage the possibility of a reversible PL enhancement, which is indeed observed for rutile TiO₂. Furthermore, our data do not show O₂-induced enhancements of VIS-PL. Therefore, we conclude that additional explanations have to be proposed here.

The basic observation we propose here is that any PL emission which is enhanced by O₂ exposure shall arise from recombination of deeply trapped electrons characterized by a scarce mobility and not from free (conduction band) electrons. In fact, free electrons would be scavenged by O₂, leading to PL quenching according to the mechanism depicted in Figure 1. This basic observation is valid regardless of the chemical nature of the deeply trapped electron.

It has also to be observed that the O₂-induced PL enhancement strongly suggests that the energy of the involved trapped electrons is below the Fermi level (E_F), i.e., that trapped electrons are present at equilibrium, beforehand the UV illumination. The other possibility—i.e., energy levels of trapped electrons above E_F—conflicts with O₂-induced PL enhancement. In fact, an electron state with energy above E_F is empty at initial equilibrium and becomes occupied only during UV illumination via the relaxation of a photogenerated electron in conduction band. However, such a relaxation is clearly in competition against the scavenging of conduction band electrons by O₂, so that the net effect of oxygen adsorption would be to quench the PL intensity and not to enhance it.

As the O₂ adsorption favors both the migration of free (valence band) holes toward the surface (due to the upward band bending) and the PL emission, it is reasonable to propose that the trapped electrons causing the NIR-PL emission in rutile TiO₂ and the “red/orange” PL emission in ZnO are prevalently localized at the surface or sub-surface region. The resulting mechanism can be represented as in Figure 8.

Clearly, the chemical identification of the electron-occupied defects of Figure 8 in the oxides that show O₂-enhanced PL is a subject that goes beyond the aim of this work. Here, only some suggestions can be discussed on that matter. Regarding the NIR-PL from rutile TiO₂, some works indeed argued that it shall be related to surface or sub-surface interstitial Ti³⁺ [58].

In regard to ZnO, Marotti and coworkers [59] performed a specific study on red-PL in ZnO, gathering evidence to suggest that it may be caused by the recombination of an electron in an intrinsic shallow state with a hole in an intrinsic deep state. They also proposed that the intrinsic deep state is also involved in the frequently observed green PL, which might be caused by the electronic transition from the conduction band to the mentioned deep state. Such a picture has been supported by an extensive study by Choi and coworkers [60], who indeed recognized (a): recombination between conduction electron and holes in singly ionized surface vacancy oxygen centers (indicated as $V_{O,S}^{+}$ in their manuscript) and (b): recombination between shallow donors and $V_{O,S}^{+}$ as the origin of red PL [60]. This process in schematized in Figure 9.

As discussed before, any surface accumulation of holes caused by O₂ adsorption is expected to favor the hole trapping at the deep defect, so that the recombination represented in the Figure 9 shall be favored in the presence of O₂. This possible mechanism suggests a surface nature of the deep defect: in this sense, it agrees with the work by Choi et al. However, its identification with $V_{O,S}^{+}$ cannot be stated with certainty only on the basis of our experimental findings.

5. Conclusions

To summarize, this work dealt with the effect exerted by molecular oxygen (O₂) adsorption on the photoluminescence (PL) of some metal oxides commonly used as sensitive elements of chemoresistive devices. We presented a comparative study of the PL response of SnO₂, ZnO and TiO₂ nanoparticles toward O₂. The metal-oxide nanoparticles were all prepared via sol–gel routes which are already established for the preparation of thick film chemosensors.

By subjecting the investigated samples to the same experimental procedures, very different results have been evidenced. SnO₂ exhibited virtually no response, while its PL efficiency was also found to be much lower than for the other oxides. ZnO nanoparticles exhibiting strong red/orange emission shown, surprisingly, an O₂-induced PL enhancement. TiO₂ exhibited by far the most interesting results, both in terms of sensitivity and versatility of the response. In particular, rutile TiO₂ is found to give a reversible and fast response for O₂ concentration in the range of relatively high O₂ concentrations (0.4–2%) in terms of its peculiar near-infrared PL emission. Anatase TiO₂ is instead found to be employable in lower concentrations ranges, manifesting sensitivity down to 25 ppm.

We argued that O₂-induced PL quenching is observed when the luminescence originates from free electrons, due to the enlargement of the depletion region and shrinking of the PL-active volume. On another hand, PL enhancement likely originates from the recombination of surface-trapped electrons with free (e.g., Figure 8) or trapped (e.g., Figure 9) holes.

The experimental findings for NIR-PL of rutile TiO₂ suggest that its surfaces host deeply trapped electrons at large densities, so that a good sensitivity and, importantly, a relatively unsaturated response is obtained. The results thus indicate that a proper maximization of rutile TiO₂ surface area might well complement the anatase phase, so that a mixed phase nanoparticle system can be used to probe a wide range of O₂ concentrations.

We finally suggest that future work attempting to identify the chemical nature of deep defect states involved in the O₂-enhanced PL could proceed by computational quantum chemistry and by experiments on nanostructures with variable particle sizes and specific surface areas.