ZnO Oxygen Vacancies Formation and Filling Followed by in Situ

Article pubs.acs.org/JPCC

ZnO Oxygen Vacancies Formation and Filling Followed by in Situ Photoluminescence and in Situ EPR Charlotte Drouilly,†,‡ Jean-Marc Krafft,†,‡ Frédéric Averseng,†,‡ Sandra Casale,†,‡ Delphine Bazer-Bachi,§ Céline Chizallet,§ Vincent Lecocq,§ Hervé Vezin,∥ Hélène Lauron-Pernot,†,‡ and Guylène Costentin*,†,‡ †

UPMC, University Paris 06, UMR 7197, Laboratoire Réactivité de Surface, F-75005 Paris, France CNRS, UMR 7197, Laboratoire Réactivité de Surface, F-75005 Paris, France § IFP Energies Nouvelles, Rond-Point de l’échangeur de Solaize, BP3, 69360 Solaize, France ∥ Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq cedex, France ‡

S Supporting Information *

ABSTRACT: Oxygen vacancies of zinc oxide were followed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies. The green PL emission was associated with oxygen vacancies: its intensity is enhanced upon static thermal treatment under inert or under vacuum, whereas it decreases upon oxygen treatment. A unique EPR signal at g = 1.96 was measured at room temperature after thermal in situ treatment under flow of inert or oxygenated atmospheres, its double integration follows the same trends than the green PL emission and its evolution was shown to probe the oxygen vacancy concentrations. The relative concentration of the related paramagnetic species would be increased/decreased upon trapping/release of the electron associated to the formation/filling of oxygen vacancy. The influence of Ti impurities on the PL and RPE signals was investigated. Finally, it is concluded that the EPR signal is related to oxygen vacancies and its position shift could be explained by the involvement of some mixing orbitals. Thanks to static (PL and EPR) and dynamic (EPR) in situ characterizations, the conditions of formation or filling of oxygen vacancies are discussed depending of the atmosphere and temperature of the pretreatment of kadox and ex-carbonate zinc oxide. High temperature treatments, inert atmospheres, and vacuum lead to the formation of new oxygen vacancies. This process is reversible upon oxygenated atmospheres. The efficiency of such filling up depends on the temperature and starts to prevail on the oxygen vacancy formation below 500 K. It is also shown that few native oxygen vacancies can also be filled up.

1. INTRODUCTION Zinc oxide is a very common oxide used in many fields for its pigment, antiseptic, or vulcanization promoter properties but also, more recently, for its semiconducting, optical, or even catalytic properties.1−4Oxygen vacancies are expected to play a determining role in these latter properties.3−6 Indeed, in low oxygen partial pressure conditions, reduction of zinc oxide results in VO oxygen vacancies formation, usually noted VO●●, VO●, and VO⌀ depending on whether they have trapped zero, one, or two electrons, respectively. Recently, the temperature and atmosphere of pretreatment were shown to play a direct role in the control of the conversion level of zinc oxide toward alcohol transformation which is of particular importance in the context of valorization of bioethanol and was assigned to a modification of oxygen vacancies concentrations.3 Moreover, the deactivation of the ZnO catalytic system was also shown to be closely related to the possible filling up process occurring during the reaction in relation to the temperature and atmosphere conditions.4 However, direct relationships between physicochemical properties and the latter vacancies are not straightforward since (1) zinc oxide is very sensitive to the © 2012 American Chemical Society

conditions of analyses and (2) the characterization techniques that are claimed to be sensitive to oxygen vacancies in zinc oxide, mainly photoluminescence and EPR, led to quite controversial assignments of the spectra. Photoluminescence features of zinc oxide are widely studied in relation to its optoelectronic properties. The photoluminescence emission spectrum of zinc oxide is composed of two main bands. The first band is a quite narrow one at 375 nm (3.31 eV), usually referred to as the UV band or NBE band (near band edge), relative to the recombination of the photoinduced electron from the conduction band to the valence band; indeed, its position at 375 nm (3.31 eV) is in good agreement with the band gap value (3.37 eV).7−9 The second one is called the green band. It is broad and centered at 550 nm (2.3 eV) and would be associated with defects whose nature is much more discussed.10 Actually, this band origin is controversially ascribed either to (1) surface defects11 due to its Received: February 17, 2012 Revised: September 13, 2012 Published: September 13, 2012 21297

dx.doi.org/10.1021/jp307693y | J. Phys. Chem. C 2012, 116, 21297−21307

The Journal of Physical Chemistry C

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beam. The samples were suspended in ethanol, dispersed with microwaves and then rapidly supported on a copper grid. As shown by TEM pictures in Figure 1, the kadox sample exhibits

dependence upon either the morphology of the sample (an enhanced relative intensity of the green emission is reported in presence of high surface/volume particles)12 or the extinction of the band in the presence of adsorbates13,14 or (2) the involvement in the radiative recombination process of energy level associated with oxygen vacancies. This latter assignment is supported by the dependence of the green emission on thermal treatments15 and by the reported correlation between green emission and VO● oxygen vacancies detected by EPR.15,16 Note that several models were proposed making compatible these two assignments.8,16−18 However, there is a lack of photoluminescence data dealing with the influence on the spectra of recording conditions that may impact the energy transfers process, and their precise control should be important to help rationalizing the modifications of shape and intensities observed depending on the pretreatment conditions. As far as EPR is concerned, besides signals associated with zinc vacancies, interstitial oxygen,19−23 or chemisorbed oxygen,15,20 many papers refer to signals possibly linked to paramagnetic oxygen vacancies.15,16,24−30 Depending on the authors, the latter are associated to a signal at either g = 2.0125,26 or 1.96.31 The latter signal that is increased upon UV irradiation28 is probably the most widely observed and its assignment to VO● oxygen vacancies is supported by its sensitivity to oxidative or reductive atmosphere.29 However, alternative assignments, based on similar evolution of this EPR with conduction properties, proposed the involvement of a mobile electron either in the conduction band and/or shallow donor bands,28,32,33 possibly hydrogen,34,35 or simply of chemical impurities.16,36,37 In the present work, photoluminescence and EPR spectroscopies are complementarily used to study the occurrence or filling up of oxygen vacancies in ZnO. In particular, attempts to establish relationships between the modification of the photoluminescence and EPR spectra depending on the nature of the pretreatment (atmosphere/temperature) and the appearance and disappearance of oxygen vacancies are presented. This point is of particular importance since it could allow one to adapt the conditions of activation of zinc oxide used as catalysts3 and to rationalize the deactivation processes that are particularly sensitive to the stability of oxygen vacancies.4 For this purpose, the two spectroscopies were carried out under in situ conditions, that is to say, allowing both thermal treatment under different gaseous or vacuum atmospheres directly followed, without any contact with the ambient atmosphere, by recording spectra in a controlled gaseous atmosphere. Furthermore, in order to get closer to the dynamic treatment involved in real catalytic applications, EPR treatments were not only carried out under gaseous flows before in situ experiments, but dynamic in situ EPR was also implemented to record the spectra under dynamic gaseous flows at various temperatures. Two powdered ZnO samples, selected for their catalytic interest,3,4,38−42 namely kadox and ex-carbonate ZnO samples, were investigated.

Figure 1. TEM pictures of (a) kadox hexagonal prims particles and (b) ex carbonate badly defined small particles.

well-defined particles with length ranging from 50 to 300 nm, whereas the ex carbonate is made of smaller and less shaped defined particles (30−60 nm). Neither morphological, neither textural properties are impacted by the different thermal treatments applied in the present study. From X-ray diffraction diagrams (Figure 2) recorded on a Siemens diffractometer equipped with a copper anode (λ = 1.5418 Å), both samples exhibit the crystalline wurzite zincite structure (JCPDS 01-0897102). The yellow color of the ex-carbonate sample could be

2. EXPERIMENTAL SECTION 2.1. Samples. Two ZnO samples were studied, a commercial one (Kadox 911, Horsehead Corporation, 99,995%), 9 m2 g−1, called kadox in what follows, and an excarbonate one, 22 m2 g−1, which was obtained by thermal decomposition of zinc carbonate (Fluka, 98.8%) at 773 K for 2 h under air in a muffle furnace. TEM characterization was performed with a JEOL JEM 1100 using a 100 keV electron

Figure 2. X-ray diffraction patterns of kadox (a) and ex carbonate (b) samples. 21298



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spectra were found quite irreproducible, probably due to non radiative deexcitation processes favored by the higher amount of residual adsorbates, namely carbonates, compared to the kadox sample (Supporting Information S1). 2.3. EPR. In Situ Experiments. EPR spectra were recorded on a JEOL FA300 computerized spectrometer at 9.3 GHz (X band) using a 100 kHz field modulation and a 0.5 G standard modulation width (104 G = 1 T). The presented spectra were recorded at 298 or 77 K using a microwave power of 0.5 mW and accumulated 5 times. Relative concentrations of radicals were determined by double integration taking as a reference the value obtained for as prepared kadox or ex-carbonate. For this purpose, the volumes of samples introduced in the EPR tube were rigorously identical to that of the corresponding reference. The error on the relative concentration measurements was estimated to be of 10%, on the basis of reproducibility experiments. A U-shaped quartz flow reactor equipped with a side arm EPR tube (Suprasil grade) allowed thermal treatment and the transfer of the sample into the EPR arm to be performed in situ under the same controlled atmosphere. This reactor was equipped with valves to isolate the pretreated sample from the air before positioning it in the EPR cavity. It was checked that the nature of the gas present in the reactor during spectra recording did not impact the signal. To analyze the influence of conditions of treatments on the EPR spectra, the temperature profile represented on Figure 3b was achieved: the sample was heated (5 K min−1) up to 773 K, maintained at this temperature for 2 h., then cooled (3 K min−1) to 403 K (sequence 1), and maintained at this temperature for 1 h (sequence 2). The same gas nature (N2 or O2) and flow (20 mL.min−1), were used for this complete pretreatment. An additional pretreatment was achieved, using N2 for sequence 1 and O2 for sequence 2. Pulsed EPR Experiments. Echo field sweep experiments was performed on a Brüker ELEXYS 580 FT spectrometer. These experiments were done at 4.2 K using a standard Hahn echo sequence π/2−τ−π−echo. The τ value was set to 300 ns and π/2 and π pulses were respectively 12 and 28 ns. The echo was fully integrated using 16 step phase cycling. Post processing with pseudo modulation algorithm was performed to recover conventional CW EPR spectrum. Dynamic in Situ EPR Experiments Recorded during the Cooling Step of the Pretreatment. An additional set of experiments were carried out on a X band Brüker ELEXSYS E500 spectrometer operating at 9.58 GHz with 100 kHz modulation frequency and a 1 G modulation amplitude. Microwave power was set to 5 mW. The standard thermal treatments of the sample (heating up to 773 K for 2 h) were achieved in a dedicated cell44 under gas flow (20 mL min−1). EPR spectra were recorded during the cooling step from 613 to 338 K. To ensure accuracy of the EPR measurements in relation to a stable temperature, the cooling rate was set to 10 °C min−1 and a hold was maintained for 15 min at each measurement temperature before the spectrum was recorded. In fact, due to the drastic limitation of the intensity measured compared to the first set of EPR experiments (recorded under static conditions at room temperature) that are related both to the lower quality properties of the TE102 cavity dedicated to dynamic EPR experiments and to the temperature effect on the intensity of the signal, only spectra related to the ex-carbonate sample (that are more intense than those of the kadox sample)

indicative of a more defective material. Note also that, from Xray fluorescence analysis performed on a SPECTRO XEPOS apparatus, titanium impurity was detected (40 ppm for kadox and ex-carbonate samples). 2.2. Photoluminescence. Photoluminescence experiments were performed using a spectrofluorophotometer Spex Fluorolog II from Jobin-Yvon (equipped with a 450 W Xe lamp as an excitation source, and color filters to eliminate scattered light). The excitation and emission band-passes were both set to 1.1 nm. Emission photoluminescence spectra were obtained fixing the excitation wavelength at 300 nm. Note that a corresponding harmonic at 610 nm was detected on the emission spectra. Due to the low radius of curvature of the cell, described earlier,43 its relative intensity was slightly affected by the positioning of the cell in the incident beam. This homemade cell was connected to a vacuum system allowing not only thermal treatment to be performed in situ in a controlled static atmosphere but also spectra to be recorded in a perfectly controlled atmosphere. The kadox sample (about 30 mg) was introduced in the cell and treated following the sequence described in Figure 3a.

Figure 3. (a) Temperature profile (heating rate 5 K min−1, periods at 773 K maintained for 2 h, rapid cooling to room temperature) and successive atmospheres (primary vacuum, O2, PO2 =0.25 bar; Ar, PAr = 1.2 bar) in the photoluminescence cell and labeling of the corresponding recorded emission spectra and (b) temperature profile achieved for in situ static and dynamic EPR measurements, with spectra recorded after coming back to room temperature or during the cooling step, respectively.

Between the successive thermal treatments at 773 K (5 K min−1 for 2 h) performed either under oxygen or argon (PO2 = 0.25 bar, PAr = 1.2 bar), the sample was cooled to room temperature. Intermediate vacuum steps were performed either at room temperature, to process to the modification of the atmosphere in the cell, or 15 min before the end of the period at 773 K, to avoid the molecules desorbed upon thermal treatments to readsorb during the further cooling step. No PL results related to the ex-carbonate sample are presented in the paper since, although qualitatively similar to those observed with the kadox sample, the intensity of the 21299



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Figure 4. Phololuminescence emission spectra of kadox sample recorded at room temperature during the sequence described in Figure 1. (a) Ari series: spectra recorded under argon (PAr = 1.2 bar) and (b) O2j series: spectra recorded under oxygen (PO2 = 0.25 bar).

recorded under nitrogen and air flow (oxygen flow resulted in too low intensities) were considered.

that the higher quenching efficiency of gaseous oxygen compared to that of Ar43 has been balanced by its lower partial pressure, PO2 = 0.25 bar, against PAr = 1.2 bar for the Ari series. Intensities and Desription of the Bands. To discuss the evolution in the two series of spectra, it should also be underlined that the reproducibility of the results was checked. Moreover, the examination of the behavior of the band at 410 nm, which is very commonly assigned to surface OH groups for many oxides,49−55 also attests the reliability of the system. Indeed, infrared studies reported in the Supporting Information S1 (Figure S1) show that the nature of the atmosphere (O2 or inert gas) during the pretreatment of the ZnO sample at 773 K does not impact the amount of the residual OH groups, which is in agreement with the high stability of the 410 nm band upon subsequent treatments (series Ari and O2j). Thus, it can also be concluded to almost no variability occurs in the compacting of the powder in the cell due to successive evacuation-filling up cycles that may have influenced at second order the intensities of the spectra. As far as the band at 610 nm is concerned, it is associated to a harmonic contribution of the excitation wavelength and its evolution that was explained in the Experimental Section will not be further commented on. Besides the two contributions mentioned above, the general shape of the spectra is in agreement with those reported in the literature,11,12 involving a narrow band at 375 nm (UV band) and a broad one centered around 550 nm (green band). The first one corresponds to the recombination of a photoelectron from the conduction band to a hole of the valence band,56 whereas the second one is controversially associated with surface defects57,58 and/or the involvement of oxygen vacancies.7,8,15,56 The main modification of the spectra occurring upon the subsequent treatments concerns the relative intensities of these two bands. The influence of the different treatments on the spectra was analyzed on the basis of the evolution of the ratio of the intensities (taken at the maximum of each band) of the green/UV bands (R = Igreen/IUV) bands reported in Figure 5, panels a and b, for Ari and O2j series, respectively. Influence of the Thermal Treatments and of the Nature of the Atmosphere on Green/UV Ratios. Starting from R = 0.67

3. RESULTS AND DISCUSSION 3.1. Photoluminescence. Zinc oxide being a semiconductor, the excitons are expected to be mobile and thus to be easily transferred to adsorbates or to gaseous molecules present in the cell, by collision or adsorption processes.45−48 These phenomena have to be carefully controlled since they are expected to influence both the shape and the intensity of the emission spectra (due to competitive nonradiative deexcitation processes), as confirmed by the few studies reporting the influence of organic adsorbates resulting from synthesis precursors on the photoluminescence properties of ZnO.13,14 However, whereas ZnO properties are expected to be sensitive to the environmental conditions,1 there is a lack of photoluminescence data dealing with the influence on the spectra of pretreatment conditions of ZnO and of recording atmospheres. The experimental set up described in the Experimental Section allows us to control both the influence of the adsorbates and of the gas phase, providing that the two following precautions are taken. First, a prior thermal treatment at 773 K under oxygen was achieved before recording the first PL spectrum in order to clean up the surface from adsorbates. Indeed, from infrared studies (Supporting Information S1) this temperature was required to ensure desorption of most of the natural adsorbates (as CO2 and H2O). Moreover, from these data, the adsorbate content is not impacted by the nature of the pretreatment atmosphere, which means that their eventual contribution to nonradiative deexcitation processes is not modified from one spectrum to the other. Second, taking into account that the transfer processes to gas phase are impacted by both the partial pressure and the nature of the gas, only spectra recorded under the same atmosphere are compared. The spectra obtained with the kadox sample are thus reported in Figure 4, panels a and b, as two different series, Ari and O2j, respectively, depending on the nature of the gas present in the cell and following their order of recording, as sequenced in Figure 3a. Nevertheless, from the similar intensities of both series of spectra in Figure 4, it can be seen 21300



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involved in green emission, explaining hence that the intensity of the two main UV and green bands are expected to compete. The presence of structural defects associated with oxygen vacancies results in an enhanced contribution of the green band at the expense of the UV one. 3.2. EPR. Influence of the Pretreatment. A unique EPR signal at g = 1.959 was observed for both samples on all of the spectra (Figure 6a,b). The signal is however about 20 times

Figure 5. Green/UV ratio of the intensity of the photoluminescence bands centered at 550 and 380 nm of spectra recorded during the sequence described in figure 1. (a) Under Ar (PAr = 1.2 bar) and (b) under O2 (PO2 = 0.25 bar).

for the reference O21 spectrum, the impact of subsequent thermal treatments at 773 K depends on the nature of the gaseous atmosphere: if thermal treatment under Ar resulted in an increase of the green/UV ratios (from R = 0.95 for Ar2 to R = 1.15 for Ar3 or from R = 0.67 for O21 to R = 0.92 for O22), the one performed under oxygen in reverse resulted in a decrease of this ratio (from R = 1.01 for O23 to R = 0.75 for O24). Moreover, all of the intermediate steps performed under primary vacuum at room temperature also led to an increase of the ratios (from R = 0.75 for Ar1 to R = 0.95 for Ar2; from R = 0.92 for O22 to R = 1.01 for O23; or from R = 0.75 for O24 to R = 0.89 for O25). Assignment. As it was checked that the successive steps of the sequence presented in Figure 3a modified neither the texture nor the morphology of the samples, nor their adsorbates content, the modification of the green/UV ratios presented in Figure 5 could only be ascribed to the modification of vacancies defect concentration. Considering that treatment under Ar at 773 K and under vacuum are both expected to favor oxygen release from zinc oxide and that they enhance the relative weight of the green band, it can thus be concluded that oxygen vacancies are involved in the green PL emission. This result is consistent with the model first proposed by Vanheusden.16 Note however that the precise nature of the defect involved in the green emission process was further discussed due to the discrepancy between the expected transition from an oxygen vacancy (where the energetic level is only 0.19 eV below that of the conduction band) to the valence band that would rather lead to a 3.18 eV transition, whereas the observed green emission lies at ∼2.4 eV. It was thus proposed that the green emission could result from other transitions involving for instance donor−acceptor complexes associating an oxygen vacancy to a zinc vacancy.17,18 Van Dijken further proposed a transition from the conduction band to an oxygen vacancy.59 In all of these processes, the photoelectron from the conduction band may be transferred to the intermediate levels

Figure 6. EPR spectra of (a) kadox and (b) ex-carbonate samples.

more intense in the case of the ex-carbonate sample (Figure 6b). Moreover, for both samples, if the nontreated samples are considered as references, the intensity of the signal is modified by the pretreatment at 773 K and the variation observed depends on the nature of the atmosphere of pretreatment: the double integration of the related signals recorded after coming back to room temperature is increased upon nitrogen pretreatment by +140% and +115% for kadox and excarbonate, respectively, whereas it is decreased upon oxygen pretreatment by −20% and −60% for kadox and ex-carbonate, respectively. Assignment. Oxygen Vacancy versus Chemical Impurity. This qualitative trend is thus very similar to that already described for the green/UV ratio in photoluminescence for thermal treatments performed under inert (Ar) and oxygen atmospheres. From this behavior, the EPR signal also appears to be closely related to oxygen vacancies concentration. Besides, in many papers, it is directly assigned to VO● paramagnetic oxygen vacancies.15,29,31,60−62 Consistently with what is assumed in all of these EPR papers, the possible involvement of two trapped electrons defects (VO⌀) is not considered here since we did not observed any doublet (fine structure) typical S = 1 system in the 0−9000 G range at X band. Such an investigation of S = 1 would require a high spectrometer if D ≫ hv, which is out of the scope of the present paper. 21301



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However, this VO● assignment is very controversial since such a g value classically corresponds to the signature of a transition element better than that an electron trapped in oxygen vacancy (VO●) expected around g = 2.01.22,63−65 Thus, on this basis, several authors consider that the signal originates from chemical impurity which nature is scarcely discussed or even skipped.16,36,37 However, considering its very general observation, its dependence of the signal intensity to the pretreatment conditions described above, as well as its behavior closely related to reported catalytic features3, it appears crucial to clarify if its origins is related to oxygen vacancies or/and impurities. Taking into account both the g value and the abovedescribed influence of pre-treatments on its intensity, the corresponding impurity, assuming it is a transition metal, should fit several conditions schematized in Figure 7: (1)

calcination which associated electrons finally increase the Ti3+ fraction. In order to check the hypothesis of the direct involvement of Ti3+ in the EPR signal, the kadox ZnO sample was doped with TiO2 (5 wt %). The two oxides were finely ground, and the resulting mechanical mixture was calcined at 1273 K in a muffle furnace. If no EPR signal could be detected at room temperature (R.T.) for the chemical mixture, the corresponding spectrum recorded at 77 K reported in Figure 8 exhibits a signal

Figure 8. EPR spectra recorded at 77 K of kadox sample: reference (black curve), after calcination at 1273 K (blue curve), and of a mechanical mixture of kadox and TiO2 (5 wt %) calcined at 1273 K (purple curve).

Figure 7. Evolution of the proportion of the reduced (paramagnetic species) and oxidized form of the chemical impurity upon inert or oxygen thermal treatment.

at g = 1.96. This signal is more intense and qualitatively very similar (shape and position) to those observed in the same recording conditions for pure ex-carbonate samples, the reference one and that calcined at 1273 K. Before considering the similarity of the reported spectra as a proof of the Ti3+ involvement in the EPR signal recorded for our ZnO samples, the temperature effect on the signal has to be discussed. First, the concentration of titanium in the chemical mixture is much higher than in the ZnO samples and it is known that EPR Ti3+ signal in a TiO2 matrix is only detectable at low temperature.62,64,67,68 It could be thus inferred that the observation of Ti3+ at R.T. in ZnO samples could be assigned to a modification of relaxation time induced by the different environment and lattice interactions of the paramagnetic species. However, another questionable point concerning the assignment of the EPR signal observed in the ZnO sample to Ti3+ is the smaller width of the signal compared to that usually reported for Ti3+.68 Electron in Oxygen Vacancies Close to Titanium Centers. An alternative assignment for the g = 1.96 signal in ZnO samples could be proposed by analogy with the assignment proposed for the similar signal (g = 1.96) observed in ceria,69,70 which is also a oxygen deficient semiconductor.70−74 Moreover, as in the case of ZnO, the signal is detected at relatively high temperature70 and its width is small. The origin of this signal was also very controversial, being directly compatible neither with Ce3+ ions nor directly to VO● (for the same reasons than those mentioned above for ZnO). It was proposed that the deviation observed for the g value from that expected for an electron trapped in oxygen vacancy VO● (∼ge) would be due to some orbital mixing with the empty states of nearby Ce4+ cations.70 Thus, in the case of the ZnO, and taking into account the presence of the empty d orbitals of titanium present as traces, it could be inferred that the electron released upon oxygen vacancy formation could be trapped as quasi free

electronic configuration 0 < d < 5 (for g < 2.00), (2) no hyperfine structure, hence, no nuclear spin (I = 0), (3) be likely to be reduced or oxidized in the pretreatment conditions (heating under inert or oxygenated flow, respectively), and (4) be paramagnetic in its reduced form only. As the transition metal can not be easily reduced only upon inert gas treatment, the electron source for the related reduction/oxidation process described in Figure 7 may be linked to the formation or filling of oxygen vacancies. Note that in this case, despite the fact that the EPR signal would not directly originate from oxygen vacancies themselves and provided that the metal supply is larger than the amount of oxygen vacancy, it could even be an indirect probe for their formation/filling up. Involvement of Ti3+ in the EPR Signal? From X-ray fluorescence analyses, titanium traces were detected in both samples (40 and 30 ppm for kadox and ex-carbonate samples, respectively). This element could be a good candidate for the g = 1.96 signal (d1/d0 electronic configuration for the Ti3+/Ti4+ redox couple). Consistently, even if the EPR signal of Ti3+ is known to be sensitive to its local environment,66 some papers report an EPR signal of Ti3+ at g = 1.96.62,64,67,68 Note that in ref 64 the signal is assigned to Ti3+ having anion vacancies in its coordination sphere. Such assignment of the EPR signal to Ti3+ impurities would imply that the carbonate precursor, used in the synthesis of the ex-carbonate sample, involves the same chemical impurity. Consistently, the presence of the EPR signal at g = 1.96 in the zinc carbonate sample was confirmed (Figure 2S). The higher intensity of the signal of the ZnO ex-carbonate sample compared to its carbonate precursor could be explained by its higher density (metal atom per volume of solid) compared to that of zinc carbonate and by the formation of an increased amount of oxygen vacancies in the ZnO formed upon 21302



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the electron in the oxygen vacancy leading to some orbital mixing with nearby empty d orbitals of Ti4+. EPR: toward a Quantitative Probe of Oxygen Vacancies? To evaluate if the signal observed at room temperature can be taken as a quantitative tool for oxygen vacancies formation detection, it should be checked if the amount of titanium species (responsible for the position of the signal at room temperature) is large enough to trap all of the electrons associated with oxygen vacancies formation. One should note that no direct signature of VO● close to ge was observed upon nitrogen high temperature treatment nor upon continuous irradiation by UV/visible light of the sample. On the contrary, such an irradiation that continuously fills up oxygen vacancies with photoelectrons led to an increase of the signal at g = 1.96 of about 10% (results not shown), indicating that, in the conditions of treatments applied here, the reservoir in titanium is high enough to quantitatively follow the oxygen vacancies formation. Consistently, taking into account the Ti content (∼ 40 ppm) and the spin concentrations reported in several quantitative EPR studies15,35 (∼ 1016 cm−3) and the geometry of our cell, it could be estimated that the reservoir in Ti species is about 25 times larger than the amount of the detected paramagnetic species. Thus, considering that this value is 1 order of magnitude higher than the maximum variation of the EPR signal observed (+115% for the ex-carbonate sample after nitrogen treatment), it can be concluded that quantification of oxygen vacancies could be estimated via the measurement of the EPR signal. 3.3. EPR versus Photoluminescence. Finally, coming back to the emission process involved in the green photoluminescence, the results presented here for the EPR process make questionable the possible role played by titanium in the photoluminescence process. The photoluminescence spectrum of the kadox sample enriched in titanium following the protocol described above was registered. Compared to the same kadox sample treated in similar conditions in the absence of titanium, the relative weight of the green photoluminescence emission was decreased upon doping by titanium (Figure 3S). Thus, doping by titanium is not involved in the exaltation of the green band. The decrease of the green band could be ascribed either to an enhanced nonradiative deexcitation induced by titanium impurities or to a trapping of electrons by Ti4+ as stable Ti3+ species. Hence, these electrons would become unavailable for electronic transitions involving oxygen vacancies. This result supports the direct implication of oxygen vacancies in the green emission process. Even though the nature of the electron donor oxygen vacancies involved in the green photoluminescence process has been scarcely discussed, Vanheusden proposed the involvement of both VO● and VO⌀ at room temperature.15 Regarding EPR, from the proposed assignment, direct observations of oxygen vacancies would be limited to the detection of the lone formation of VO●; the only one to be paramagnetic. This could explain the very close qualitative relationships between the features observed in the present paper by EPR and photoluminescence spectroscopies, but as underlined by Vanheusden, the correlation he invoked was more qualitative than really quantitative.16 This feature could also be ascribed to more difficult quantification of photoluminescence yield compared to EPR: as mentioned above in most of the photoluminescence studies, the lack of experimental details appears very detrimental to the control of deexciation processes that might occur at the expense of the emission process.

electron at particular oxygen vacancies in close interaction with d orbital of titanium impurities. Note that such assignment is consistent with the evolution of the concentration in paramagnetic centers upon the different pretreatments described above. At this stage in order to state about the nature of the signal observed, pulsed EPR experiments were performed on the excarbonate sample. Figure 9 displays the electron spin echo

Figure 9. Echo field sweep spectrum of the ex-carbonate sample recorded at 4.2 K. The onset zoom is the pseudomodulated spectrum.

recorded as function of magnetic field at 4.2 K. A broad band with hyperfine structure can be observed. For better accuracy in term of spectrum analysis the echo field sweep spectrum was first derivative pseudo modulated to recover standard CW spectrum shape. This spectrum displays nine lines. These hyperfine couplings can arise from interaction from electron spin with nucleus in the vicinity. 47Ti (I = 5/2) or 67Zn (I = 5/ 2) isotopes could be considered. For both nuclei six hyperfine lines are expected in isotropic regime. The fact that more than 6 lines are observed assumes anisotropy of g factor. In fact, g factor exhibits two different values, that can be assigned to axial symmetry with g⊥ > g∥ and their respective value of 2.084 and 1.841. In the g∥ region the hyperfine structure is overlapped with the perpendicular region. As g⊥and g∥ are respectively doubly degenerated and nondegenerated values, we can calculate a giso value of 2.003, close to the free electron one. This result supports the assignment of the EPR signal to oxygen vacancy and confirms, as proposed above, that the assignment to Ti3+ (d1) has to be rejected. Moreover three pulse ESEEM have been performed and multi quanta nuclear frequencies (ΔmI = 2 and 3) are observed and centered at Zn Larmor frequencies. These results will be published elsewhere in a more detailed paper. From the obvious absence of interaction with 49Ti (I = 7/2), coupling with Ti does not take place at very low temperature due to electron localization effect. The hyperfine coupling to the 67Zn isotope is more likely in these conditions. A coupling of about 210 MHz was measured. Thus, considering the isotropic coupling for 67Zn is 2087 MHz,75 this experimental coupling value indicates a contribution of 10% of Zn orbital to the unpaired electron spin density. This experiment confirms that the EPR signal is associated with electron in oxygen vacancies. However, from the g value measured at room temperature, it could be inferred that increasing the temperature could enhance the delocalization of 21303



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Figure 10. Difference between the areas of the EPR signals (dynamic experiments) measured under nitrogen and air flow during the cooling step (from 613 to 338 K) performed at the end of the thermal treatment achieved at 773 K.

not as efficient toward filling of oxygen vacancies as that entirely achieved under oxygen flow. As mentioned above, the kinetics seems to be quite slow at room temperature. Moreover, under oxygenated thermal treatment, thermodynamics state a competition between formation and filling of oxygen vacancies depending on the temperature and of the oxygen partial pressure PO2. Dynamic in situ EPR measurements were thus achieved to evaluate the critical temperature at which the filling of oxygen vacancies becomes more efficient than the oxygen release during the cooling step of the thermal treatment (measurements done in the 613−338 K temperature range). To get rid of the complex intrinsic influence of temperature on the intensity of the spectra (Curie’s law, modification of relaxation times, ...), for each temperature, the intensity measured for EPR spectra recorded under air flow was subtracted from that measured under nitrogen (Figure 10). Thus considering that for a given temperature the intrinsic temperature effect on the EPR intensity does not depend on the atmosphere a positive value accounts for a higher amount of oxygen vacancies detected under nitrogen flow as compared to air flow. Down to 498 K, quite similar EPR signals are measured on both atmospheres, indicating that, at high temperature, the amount of oxygen vacancies is controlled by the temperature rather than by the nature of the atmosphere and that in these conditions, air treatment (as N2 treatment) results in oxygen vacancies formation. From 473 K (by decreasing values of T), the influence of the nature of the atmosphere becomes significant. The difference between the detected amount of oxygen vacancies formed under high temperature nitrogen flow and that remaining after a similar treatment under air flow becomes globally larger and larger as the temperature decreases. Thus a critical temperature of 473 K can be considered to indicate, in these conditions, the beginning of the filling of the vacancies. Note also that, in the conditions of the experiment, the filling of oxygen vacancies seems to intensify from 398 K. 3.5. Synopsis: Conditions of Occurrence/Filling of O Vacancies. The scheme presented in Figure 11 summarizes

3.4. Reversibility. Photoluminescence. Considering the first O21 spectrum as a reference (R = 0.67), the oxygen vacancies formed upon thermal treatment under Ar evaluated by the O22 spectrum (R = 0.92) and the following vacuum at room temperature (O23 spectrum, R = 1.01) were partially filled up by the second thermal treatment under oxygen and the cooling step to room temperature (see spectrum O24, R = 0.75). Moreover, the complete oxygen vacancies filling up process went on at room temperature, but quite slowly, being only achieved after 24 h (see spectrum O26, R = 0.67). Thus, the oxygen vacancies formation process occurring upon the successive reducing treatments is completely reversible under oxygen (PO2 = 0.25 bar), though quite slow at room temperature. This reversible behavior confirms that, as already mentioned in section 2.1 and in the Supporting Information S1, no other modification of physicochemical properties (such as morphology or textural properties or even adsorbate content) that might have impacted the non radiative deactivation processes have occurred during the subsequent treatments. EPR. To check if such filling of oxygen vacancies observed in static conditions by photoluminescence also occurs under dynamic flow, an additional two step treatment was achieved in the EPR cell using N2 flow for sequence 1 and O2 for sequence 2 (switch from N2 to O2 at 403 K) as illustrated in Figure 3b. As shown in Figure 6 reporting the results obtained for kadox and ex-carbonate samples, the increased intensity of the signal after sequence 1 under N2 (+140 and 115% respectively compared to the respective reference spectra) is completely lost after sequence 2 (O2 flow at 403 K) (−15% and −5% respectively compared to the respective reference spectra). Thus not only the oxygen vacancies formed upon thermal treatment under nitrogen flow have been filled up upon the oxygen treatment at 403 K but few native oxygen vacancies (pre-existing in the reference sample) can also be filled up. However, the different intensity of the signal measured after O2 (−20 and −60% for kadox and ex-carbonate respectively) and N2/O2 treatments (−15 and −5% for kadox and excarbonate respectively) shows that the N2/O2 treatment was 21304



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Figure 11. Schematic representation of the relative oxygen concentration in kadox and ex-carbonate ZnO as prepared samples and after the treatments applied before recording PL or EPR spectra at room temperature.

(4) Besides the filling of oxygen vacancies formed upon thermal treatment, it is also shown that a few native oxygen vacancies can also be filled up.

the comparative balance of oxygen vacancies presence in kadox and ex-carbonate samples after the different treatments followed by photoluminescence and EPR and subsequent measurement performed after coming back to room temperature. (1) Both kadox and ex-carbonate reference samples (without post synthesis treatment) exhibit native oxygen vacancies, with a larger concentration in the case of the excarbonate sample, which is consistent with its yellowish color. (2) From both EPR and photoluminescence spectroscopies performed at room temperature but after in situ static thermal treatments, new oxygen vacancies are formed upon inert treatments at 773 K (N2 or Ar for EPR and photoluminescence, respectively) but also upon vacuum treatment at RT (photoluminescence). If similar treatments performed under oxygen, and after coming back to room temperature, globally result in a decrease of signal related to oxygen vacancies, dynamic in situ EPR has evidenced that oxygen vacancies are also formed under oxygenated atmosphere (air) at high temperature since, above 473 K, the areas of EPR signals recorded under nitrogen or air flow are quite comparable. (3) As mentioned above, the formation of these oxygen vacancies resulting from high temperature oxygen treatments is reversibly fillable as illustrated by the decrease of the related signals recorded at room temperature. Moreover, the oxygen vacancies formed upon nitrogen treatments or vacuum treatments are also reversible, being filled up under subsequent oxygen atmosphere (comparison of N2 and N2/O2 treatments followed by EPR). This evidences the opposite effects of high temperature and partial pressure of oxygen favoring the formation/filling of oxygen vacancies, respectively. From dynamic in situ EPR study, the critical temperature, below which the filling of oxygen vacancies is thermodynamically favored under air flow, was estimated to be ∼473 K.

4. CONCLUSION In situ photoluminescence and EPR spectroscopies were complementary used to characterize ZnO samples after thermal treatments under different atmospheres. Respective PL and EPR signals, green emission evaluated thanks to Green/UV signals ratios and EPR signal at g = 1.96 are both enhanced after inert treatment performed at 773 K (or under vacuum) whereas they are decreased after oxygen or air treatments. Both signals were shown to be associated to oxygen vacancies: the paramagnetic species observed would probe the electron formed upon oxygen release in close interaction with the d orbitals of Ti impurities, whereas, the photoluminescence emission process may involve VO● and VO⌀ as electron donor center. Both techniques were used to semiquantitatively follow the increase or decrease of oxygen vacancies concentration. However, EPR exhibits several advantages: (1) it can be implemented under dynamic in situ conditions, thus in conditions directly related to those involved in catalytic applications, and (2) the quantification is easier to achieve: only the fine control of the experimental set up (and complementary characterization of surface adsorbate content) can really ensure the control of deexcitation processes. In these conditions, starting from a higher amount of native oxygen vacancies in the ex-carbonate ZnO sample compared to that for the kadox sample, it could be evidenced that new oxygen vacancies are formed upon inert treatments at 773 K but also upon vacuum treatment (photoluminescence). Moreover, the process is reversible upon oxygenated atmosphere. In fact, oxygen is able to fill up not only these created oxygen vacancies but also few native ones. From dynamic in situ EPR experiments performed under nitrogen and air flows, it is evidenced that under oxygenated atmosphere, formation and filling of oxygen vacancies are competing processes: under air flow, the former one prevails on the latter one at temperatures higher than 473 K, whereas a filling up process is favored below this critical temperature. 21305



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ASSOCIATED CONTENT

S Supporting Information *

Stability of hydroxyl and carbonates groups upon temperature and influence of the atmosphere of the pretreatment (Figure S1). EPR spectrum of zinc carbonate precursor evidencing a signal at g = 1.96 (Figure S2). Influence of titanium doping on the photoluminescence spectrum of kadox sample (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 1 44 27 36 32. Fax: +33 1 44 27 60 33. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to Nadia Touati and E. Berrier from Lille and O. Delpoux from IFPen for their technical support for dynamic in situ EPR experiments, C. P. Lienemann from IFPen for chemical analysis, and J. Védrine from the LRS for fruitful discussions.

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ZnO Oxygen Vacancies Formation and Filling Followed by in Situ

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