Visible Photoluminescence from Photoinduced Molecular Species in

Article pubs.acs.org/JPCC

Visible Photoluminescence from Photoinduced Molecular Species in Nanometer-Sized Oxides: Crystalline Al2O3 and Amorphous SiO2 Nanoparticles Asako Anjiki and Takashi Uchino* Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: Photoluminescence (PL) characteristics of two structurally and compositionally different oxide nanoparticles, i.e., crystalline γ-alumina and amorphous silica nanoparticles, are investigated. Time-resolved PL measurements have shown that both of these nanoparticles exhibit blue PL emissions characterized by fast (several tens of nanoseconds) and slow (a few seconds) decay constants. The fast PL component is observed over a wide temperature range up to ∼400 K, whereas the slow PL component is observed only at temperatures below ∼200 K. The close similarity between the PL characteristics of γ-alumina and amorphous silica nanoparticles indicates that common molecular-like species are responsible for the observed PL emissions. From the detailed comparison of the present experimental observations with the reported data, it can be concluded that the fast and slow PL components result from O2 molecules and OH radicals, respectively, both of which are photoinduced transient molecular species generated at the surface of these oxide nanoparticles.

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INTRODUCTION Luminescent nanoparticles have attracted a great deal of attention because of their potential application as indicators and photon sources for a variety of biotechnology and information technology.1,2 In particular, amorphous silica-based luminescent nanoparticles have been extensively studied and explored since an elaborate synthesis of size-controlled, monodisperse nanoto micrometer-sized silica particles is possible.3 To obtain luminescent silica particles, inorganic and/or organic emission centers are often incorporated into the particles, yielding stable light-emissive silica-based nanoparticles.4−6 It should further be noted that nondoped amorphous silica-based materials also exhibit a broad photoluminescence (PL) band in the blue region of the visible spectrum (∼400 to ∼450 nm),7−13 which is usually characterized by a PL decay on the time scale of nanoseconds.8−11 The resulting emission quantum yield can exceed ∼20−30% at room temperature by carefully control of the preparation condition.8,14−16 Thus, it has been recognized that silica itself is also promising for an efficient light-emitting material, providing environmentally friendly light emitters.8 Although the blue emission characteristics of amorphous silica and its related silica-based nanostructured materials are well recognized, the origin of the emission is still a matter of discussion.7−16 It is also interesting to note that a number of nanostructured crystalline oxides, e.g., Al2O3,10,17−19 MgO,20−23 and ZnO,10,20 exhibit a blue PL band as well on the time scale of nanoseconds.10,24 Observation of the common blue PL features in these nanostructured amorphous and crystalline oxides allows one to assume that structurally similar emission © 2012 American Chemical Society

centers will exist at the surface of respective oxides. It has often been believed that surface OH groups are responsible for the blue PL emission observed in many different types of oxides.10,12,13,17,21,22,25,26 However, the simple OH-based model cannot be accepted without reservations because of the following reasons. First, the OH groups both in free and hydrogen-bonded states do not practically absorb light in the wavelength region longer than ∼300 nm,27−31 whereas the blue PL emission occurs most effectively by using ∼330 to ∼360 nm light for excitation.7−11,21−23 Second, the blue PL output is normally enhanced after appropriate thermal annealing at ∼300 to ∼600 °C, and annealing at higher temperatures (T ≳ 600 °C) almost eliminates the blue PL emission.11,21−23 However, free and/or H-bonded OH groups at the surface of oxides do not exhibit the corresponding annealing temperature dependence;32−34 rather, a large number of residual OH groups are still present up to ∼800 °C at the surface of alumina,35 silica,36−38 and magnesia.33 It is hence probable that the blue PL emission does not result simply from the surface OH groups as they are but from certain relevant emissive defects, although the true structural origin of the emission center is not unambiguously identified at present.10,31,39 To get further insight into the blue PL emission from nanostructured oxides, we here investigate the PL characteristics of two representative crystalline and amorphous nanoReceived: April 8, 2012 Revised: May 28, 2012 Published: June 27, 2012 15747

dx.doi.org/10.1021/jp303359v | J. Phys. Chem. C 2012, 116, 15747−15755

The Journal of Physical Chemistry C

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Figure 1. Contour plots of the PL intensity as a function of excitation and emission wavelengths for the air-annealed (400 °C) γ-alumina nanoparticles. The PL measurements were carried out (a) under air and (b) vacuum conditions at room temperature.

Figure 2. Contour plots of the PL intensity as a function of excitation and emission wavelengths for the air-annealed (400 °C) amorphous silica nanoparticles. The PL measurements were carried out (a) under air and (b) vacuum conditions at room temperature.

at temperatures from 200 to 600 °C for 1 h in air. In this work, unless otherwise indicated, the results of the air-annealed samples at 400 °C are shown since the thus annealed samples exhibit the maximum PL output for both the alumina and silica nanoparticles. XRD patterns were obtained with a diffractometer (Rigaku, SmartLab) using Cu Kα radiation. We confirmed from XRD measurements that the main crystalline phase of the alumina nanoparticles annealed at 200−600 °C is still cubic γ-alumina (see Figure S1 in the Supporting Information). Steady-state PL and PL excitation (PLE) spectra and time-resolved PL signals in the millisecond to second time region were measured using a Xe lamp, a single monochromator, a photomultiplier tube, a mechanical shutter, and appropriate sharp cut filters. We further carried out timeresolved PL measurements in the nanosecond to microsecond time region using two pulsed laser sources: one is the second harmonic (350−360 nm) of a mode-lock Ti:sapphire laser (Spectra Physics, Tsunami) with a pulse duration of ∼200 fs operating at 1 MHz and an average power density of ∼3 mW/ cm2, and the other is the fourth harmonic (266 nm) of a pulsed Nd:yttrium aluminum garnet (YAG) laser (Spectra Physics, INDI 40) with a pulse duration of 8 ns operating at 10 Hz and an average power density of ∼10 mW/cm2. Temperaturedependent PL measurements were carried out under static

particles, namely, crystalline alumina nanoparticles and amorphous silica nanoparticles. A detailed comparison of these two compositionally and structurally different nanoparticles will highlight the PL characteristics common to nanostructured oxides. It has been revealed that, in addition to the well-recognized fast (∼nanoseconds) PL component, a slow PL emission process in the time scale of several seconds is observed at temperatures below ∼200 K in both the nanoparticles. The present observations have also elucidated that photoinduced molecular species are responsible for these fast and slow PL components. We then discuss the mechanism of the slow and fast PL processes on the basis of the present observations and the previously reported data.

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EXPERIMENTAL SECTION We used commercial γ-alumina (Evonic Degussa, Aeroxide Alu C) and amorphous silica (Evonic Degussa, Aerosil 380) particles, both of which are produced by the vapor-phase hydrolysis of the corresponding metal chlorides in a hydrogen/ oxygen flame. Average primary particle sizes of γ-alumina and amorphous silica particles are 13 and 7 nm, respectively (product specification). The sample powders were pressed into free-standing pellets using a uniaxial press at a pressure of 530 MPa. Thermal treatment of the pressed samples was carried out 15748



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Figure 3. Room temperature PL spectra of the air-annealed (400 °C) (a) alumina and (b) silica nanoparticles before and after evacuation for the designated time. The excitation wavelengths are 340 and 337 nm for the alumina and silica nanoparticles, respectively. For the evacuated samples, the PL measurements were carried out under vacuum (∼10 Pa) atmosphere. The peak positions, numbered as 1, 2, and 3, are shown in nanometers and the observed spacing (Δν) between the adjacent peaks is indicated in wavenumber. The dashed lines indicate the room temperature PL spectra of the fully evacuated samples, followed by air exposure for the designated time, and the PL measurements were performed under ambient air atmosphere.

Figure 4. PL spectra of the fully evacuated (a) alumina and (b) silica nanoparticles measured at designated temperatures under vacuum (∼10 Pa) atmosphere. The excitation wavelengths are 340 and 337 nm for the alumina and silica nanoparticles, respectively.

increasing evacuation time, accompanied by the appearance of at least three sharp peaks at ∼370, ∼390, and ∼415 nm. The structured spectra show a nearly equal spacing of ∼1350 cm−1, implying vibrational progressions of the relevant emission centers. It is also interesting to note that, when these evacuated samples were re-exposed to ambient air conditions, the structured PL features were substantially diminished, almost returning to the original broad structureless PL spectra, as also shown in Figure 3. These reversible spectral changes upon airexposure/vacuum cycles indicate that removal of some atmospheric molecules such as N2, O2, and H2O from the respective nanostructured oxides induces the emission state relating to the structured PL spectra. Among other atmospheric species, H2O is the most common surface contaminant adsorbed on oxide surfaces. We hence suggest that the adsorption/desorption of water molecules is responsible for the disappearance/appearance of the structured PL features. Adsorbed water molecules presumably perturb the vibronic states of the emission centers, resulting in a broad structureless PL band. When the adsorbed water molecules are removed partly by evacuation, the inherent vibronic spectral features are revealed accordingly. This will result in the structured PL features in addition to the original broad PL band. Previously, similar structured PL features have been observed (even without evacuation) in silica based organic−inorganic hybrid

vacuum using a closed-cycle N2 cryostat in the temperature region from 77 to 400 K.

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RESULTS Figures 1 and 2 show contour plots of the PL intensity as a function of excitation and emission wavelengths for the airannealed (400 °C) alumina and silica nanoparticles, respectively. The PL measurements were performed at room temperature under air (see Figures 1a and 2a) and continuous evacuation (∼10 Pa, see Figures 1b and 2b) conditions. One sees from Figures 1a and 2a that these two samples show PL and PL excitation (PLE) signals in similar wavelength regions under ambient air; that is, the PL signals are observed over a wavelength region from ∼400 to ∼450 nm under excitation of light with wavelengths of ∼250 and ∼330 to ∼360 nm. We are hence reassured that there exists a close similarity of the PL spectral features between the alumina and silica nanoparticles. When the PL measurements are carried out under vacuum, however, additional PL features are developed especially under excitation of light with wavelength of ∼340 nm, as indicated by arrows in Figures 1b and 2b. To get further information on the vacuum induced PL signals under ∼340 nm excitation, we compare in Figure 3 the corresponding PL spectra of the airannealed alumina and silica nanoparticles before and after evacuation. The intensity of the PL signals increases with 15749



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Figure 5. PL spectra of the fully evacuated (a) alumina and (b) silica nanoparticles measured at designated temperatures under vacuum (∼10 Pa) atmosphere. The excitation wavelength is 250 nm.

Figure 6. PL decay profiles of the fully evacuated (a) alumina and (b) silica nanoparticles in the 1−10 s time region measured at designated temperatures under vacuum (∼10 Pa) atmosphere. The excitation and monitor wavelengths are 250 and 440 nm, respectively. Solid lines represent best fits of the data with eq 1.

Figure 7. Temperature dependence of the fitted pre-exponential factors (Ai) and decay constants (τi) (inset) in eq 1 obtained for the PL decay data of the (a) alumina and (b) silica nanoparticles.

gels8,14,16 (see, for example, Figure S2 in the Supporting Information). It has previously been suggested that carbonrelated vibrational modes are responsible for the structured PL bands.8 According to our scheme, however, the appearance of the structured PL spectra of silica based organic−inorganic hybrids can be interpreted in term of a hydrophobic nature of the constituent organic groups by which adsorbed water is repelled from the surface. We next show the results of temperature-dependent PL measurements performed under static vacuum; Figures 4 and 5

show the PL spectra of the air-annealed alumina and silica nanoparticles under excitation of ∼340 and ∼250 nm lights, respectively, obtained at different temperatures. Under excitation of ∼340 nm light, as shown in Figure 4, the structured features shown in Figure 3 become more conspicuous with decreasing temperature, demonstrating typical characteristics of vibrational progressions. Under excitation of ∼250 nm light, however, a broad structureless PL band at ∼440 nm is developed as the measurement temperature is lowered below ∼200 K (see Figure 5). This 15750



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implies that the ∼250 nm excitation can induce an emission process which is different from that realized under the ∼340 nm excitation especially at temperatures below ∼200 K. We further carried out time-resolved PL measurements using different excitation wavelengths. In addition to the wellrecognized nanosecond decay component observed in the 250−360 nm excitation wavelength range (see Figure S3 in the Supporting Information), we found that the excitation by light in the 240−270 nm wavelength region gives rise to a slow decay component in the time scale of several seconds. Figure 6 shows the PL decay profiles at ∼440 nm in the millisecond to second time region obtained under ∼250 nm light excitation. We found that the slow decay component can be well fitted with a double-exponential function, characterized by two decay time constants τ1 and τ2 (τ1 < τ2): I(t ) = A1 exp( − t /τ1) + A 2 exp( − t /τ2)

under vacuum (see Figure 5). It is hence most likely that the structural origin of the slow PL component is in principle different from that of the fast PL component. It is interesting to point out that at temperatures below ∼200 K systems containing hydroxyl groups, for example, H2O ice and alkali halide crystals doped with the corresponding alkali hydroxide, generally exhibit a broad PL emission at ∼420 nm under ultraviolet (UV) excitation in the range 240−260 nm.42−44 This blue PL emission has a typical decay time on the order of several seconds and is attributed to the spinforbidden 4Σ− → X2Π transition of photoinduced OH radicals.42,43 It has also been shown that the PL decay profile can be well fitted with a double-exponential function, yielding two characteristic decay times of 0.8 ± 0.1 and 2.5 ± 0.4 s, which vary very little with temperature.42 Thus, the reported PL characteristics of OH containing systems are almost identical to those of the slow PL component observed in this work and in refs 40 and 41. It would hence be reasonable to assume that the slow (a few seconds) low-temperature (T ≲ 200 K) PL component observed from nanostructured oxides result from photoinduced OH radicals derived from residual surface OH groups. It should be worth mentioning, however, that the systems simply containing hydroxyl groups, e.g., H2O ice and OH-doped alkali halides, do not exhibit the nanosecond PL decay in the blue spectral region under irradiation of ∼350 nm light.43,44 This allows us to ensure that the residual OH groups in nanostructured oxides will not be responsible for the fast (∼nanosecond) PL band. We will next turn our attention to point 2 in terms of the structural origin of the emission center related to the fast PL component. As mentioned earlier, the structured PL spectra have a nearly equal spacing of ∼1350 cm−1. This also rules out the simple OH-based model because of disagreement between the observed spacing (∼1350 cm−1) and the vibrational frequency expected from the stretching motion of OH groups (∼3600 cm−1). It should be stressed once again that both the alumina and silica nanoparticles yield similar structured PL spectra with nearly the same positions and spacing under excitation of light in a limited wavelength range near 340 nm (see Figures 1−4). This strongly suggests that the observed vibrational progressions result not from the vibrational species related to the oxide matrices but from certain photoinduced molecular species present at the surface of the alumina and silica nanoparticles. We should note, however, that the peak positions of the structured PL spectra show a slight material dependence, suggesting that the expected molecular species are not released completely from the matrices but are still trapped inside of the matrices; that is, the energy states of the photoinduced molecular species are, in principle, affected by the surrounding matrix environment. Among other molecular species, we propose that O2 is the most likely candidate to be responsible for the structured PL spectra. It has been well documented that O2 molecules exhibit a series of fluorescence peaks in the ultraviolet to visible region under excitation in the 250−300 nm region. This emission is attributed to the A′3Δu → X3Σ−g progressions.45,46 It is quite interesting to point out that some of the A′ → X emissions are in good accordance with those seen in the structured spectra observed for the present alumina and silica nanoparticles (see Table 1). The decay time of the A′ → X emissions is ∼200 μs at 4.4 K;46 however, the decay time shows an abrupt decrease with increasing temperature, e.g., 25 μs at 13 K. Although the A′ → X emission decay time at temperatures higher than ∼30 K has

(1)

where Ai is the PL intensity of the ith component at t = 0. The fitted values of τi and Ai are shown in Figure 7. As shown in the insets of Figure 7, τ1 and τ2 are rather temperature independent, yielding the values of 0.5−1 and 2−4 s, respectively. We also see from the temperature dependence of Ai that the decay components in the 1−10 s time scale are observed only in the temperature region below ∼200 K. It is hence most likely that this slow decay component is responsible for the low-temperature structureless PL band at ∼440 nm shown in panels (a) and (b) of Figure 5.

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DISCUSSION The present observations have elucidated the following points as for the spectral characteristics of the blue PL emissions common to crystalline alumina and amorphous silica nanoparticles. (1) The slow PL decay component in the 1−10 s time scale is observed under ∼250 nm light excitation as the measurement temperature is lowered below ∼200 K. (2) Highly structured PL features are developed when the PL measurements are carried out under vacuum for ∼340 nm light excitation. We will first discuss point 1. Thus far, only a few studies40,41 have recognized the existence of the slow (a few seconds) PL decay component in nanostructured oxides. It has been reported that strongly oxidized porous silicon40 and silica nanotubes41 exhibit a slowly decaying blue PL component with a typical time constant of several seconds at low temperatures (T < ∼200 K) under excitation in the wavelength region below ∼300 nm. The reported PL features are basically in agreement with those of the slow PL component observed in this work. Kux et al.40 assumed that both the fast and slow decay components result from the same emission center and also suggested that a certain trapping process slows down the usually fast (∼nanosecond) PL decay by 9 orders of magnitude. We, however, propose that the slow and fast PL components originate from different emission centers because of the two observations presented in this work. First, the fast PL component is observed rather efficiently under excitation of ∼340 nm light over a wide temperature region up to ∼400 K, whereas the slow PL component is observed only under ∼250 nm light excitation at temperatures below ∼200 K. Second, the PL spectra of the fast PL component show the vibronic structure under vacuum conditions, whereas those of the slow PL component do not show such vibronic progressions even 15751



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Table 1. Emission Wavelengths (in nm) for A′ → X Emission of 16O2 in Gas Phase and in Ar and N2 Matrices along with the Analogous Peak Positions of the Structured Spectra shown in Figure 3

interstitial O2 molecules just physically trapped at interstitial sites in amorphous silica do not absorb light with a wavelength longer than ∼250 nm.47 It is hence probable that in these nanoparticles an oxygen molecule is chemically bonded to a certain specific site, forming a certain O2-related emission center. One of the possible candidates for the O2-related emission center is a peroxy radical, for example, Si−O−O•, which has been suggested to exist in some luminescent silica-based materials.15 Indeed, the peroxy radical is one of the commonly observed defect centers in silica glass, and their optical and paramagnetic properties have been extensively explored using experimental48−50 and theoretical techniques.51 However, the observed and calculated optical absorption/excitation wavelengths48−51 (630, 260, 230, and 163 nm) of the peroxy radical at the surface and/or in the bulk of silica glass do not match with the excitation wavelength (∼340 nm) for the present structured PL band. Furthermore, the peroxy radical in silica glass has been shown to exhibit a structureless PL band at ∼550 nm characterized by a decay time of ∼0.3 and ∼0.8 μs at room temperature and 20 K, respectively.50 Thus, the reported spectral features of the peroxy radical are quite different from those observed from alumina and silica nanoparticles. This strongly suggests that the peroxy radical is not responsible for the O2-related emission. In our previous papers,11,16,52,53 we have proposed a model of the blue PL emission center in amorphous silica on the basis of the annealing induced dehydroxylation reaction of adjacent geminal silanol groups. The proposed reaction is shown in Scheme 1. This dehydroxylation reaction results in a defect pair

peaks in Figure 3

a

band (ν′, ν″)

gas phase

(0, 3) (0, 4) (0, 5) (0, 6) (0, 7)

337.43 355.24 374.71 396.10 419.66

a

in Ar

b

338.59 356.42 376.01 397.43 421.05

in

N2b

− 357.40 377.11 398.71 422.56

alumina

silica

374 394 416

372 392 414

Reference 45. bReference 46a.

not been reported,46 it can be assumed that the observed decay time falls in the nanosecond region at temperatures higher than 77 K, as observed in this work. To further rationalize the O2based model, however, we have to answer the following questions: (1) Why does the PL intensity increase after appropriate annealing? (2) Why are the fast PL signals observed under excitation of ∼340 nm light? (The A′ → X emissions of O2 are observed under excitation below ∼300 nm light.) (3) Why are the vibrational progressions observed only under vacuum conditions? Question 1 can be interpreted in terms of the O2-based model by assuming that the environmental oxygen gets trapped at the surface of the nanoparticles during the annealing process. If this assumption is valid, the PL intensity will depend on the annealing environment. To evaluate the possible effect of environmental gas on the PL characteristics, we annealed the alumina and silica particles under flowing Ar and O2 conditions at 400 °C and measured the corresponding PL spectra in air atmosphere at room temperature (see Figure 8). We found that the PL intensity of the O2 annealed sample is almost comparable to that of the air-annealed sample, whereas the Ar annealing substantially suppresses the PL output. This demonstrates that the environmental oxygen is required, directly or indirectly, to attain the annealing induced PL emission in these nanoparticles. It should be noted, however, that the O2 annealing does not appreciably enhance the PL output, implying that oxygen molecules are not trapped at every interstitial site but are attached to a limited preferential site. This consideration also gives a clue to answer question 2 since

Scheme 1. Possible Formation Route to Emission Defect Pair at Silica Surface

consisting of a dioxasilirane, Si(O2), and a silylene, Si:, center. We11,53 have demonstrated from a series of density functional theory (DFT) calculations that the above defect pair can exist as a metastable equilibrium structure, yielding the singlet-to-singlet transition energies that agree well with the two principal PLE peak wavelengths (∼340 and ∼250 nm) of the blue PL band. It should be noted that chemical synthesis of a dioxasilirane is indeed possible, as reported by Bornemann and Sander.54

Figure 8. Room temperature PL spectra of the (a) alumina and (b) silica nanoparticles annealed at 400 °C for 1 h under air, Ar, and O2 atmospheres. The excitation wavelengths are 340 and 360 nm for the alumina and silica nanoparticles, respectively. 15752



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These authors54 have also demonstrated that methyl(phenyl)dioxasilirane is photolabile; irradiation with blue light (∼400 nm) results in the cleavage of the O−O bond and the structural rearrangement accompanied by migration of the phenyl or methyl group. In the case of the dioxasilirane center at the surface of a solid oxide, however, a substantial structural rearrangement will be difficult to occur because of a rigid oxide matrix. We rather suggest that photoabsorption of ∼340 nm light will induce a transient cleavage of the dioxasilirane center into O2 and silylene in their excited states, resulting in the blue emission from the excited O2 state. This can explain why O2derived emissions are observed even under excitation of ∼340 nm light (question 2). According to scheme 1, however, the source of O2 is the oxygen atoms in the surface OH groups. This does not necessarily mean that the environmental oxygen is not required to form the relevant defect pair. Since the association energy of silylene and O2 is estimated to be rather low (∼80 kal/mol),54 it is likely that O2 is removed under thermal annealing in a low oxygen partial pressure, e.g., in Ar. In other words, the environmental O2 will be needed to retain the supposed defect pair configuration consisting of dioxasilirane and silylene. The above defect pair model can also provide a reasonable answer for question 3. If the proposed defect pair is present at the silica surface, it would be reasonable to expect that adsorbed water molecules can interact with the dioxasilirane center via hydrogen-bond interactions. As a result, the vibronic states of the defect pair in the ground and excited states will be strongly affected and hence will be smeared out, resulting in the inhomogeneously broadened PL features. It hence follows that the vibrational progressions are developed only under vacuum conditions where the absorbed water will be removed partially from the surface. We consider that the above scheme can also be applied to alumina nanoparticles since their surface are covered with a number of OH groups.35,55 The corresponding defect formation reaction can be described as in Scheme 2 if we

not from any defective state related to the oxide matrix but from certain photoinduced molecular-like species located at their surfaces. The PL features can be classified into two groups depending on the characteristics decay time. One is the fast (∼nanosecond) PL component, which is responsible for the PL signal at room temperature. The other is the slow (a few seconds) PL component, which is observed only at temperatures below ∼200 K. The PL spectra related to the fast PL component tend to exhibit vibrational progressions when the PL measurements are carried out under vacuum. The resulting vibrational progressions are in reasonable agreement with those attributed to the A′3Δu → X3Σ−g transitions of O2 molecules. Furthermore, the intensity of the fast PL component increases after appropriate thermal annealing in air and O2 but not in Ar. These results allow us to conclude that the fast PL component along with the relevant vibrational progressions results from the photoinduced excited state of O2. Although the structural origin of the O2-related emission center has not been unambiguously determined at the moment, the defect pair consisting of dioxasilirane and silylene is a likely candidate. On the other hand, the PL characteristics of the lowtemperature slow component are quite comparable to those observed from systems containing hydroxyl groups. It can hence be concluded that the residual OH groups at the surface of oxide nanoparticles are responsible for the low-temperature slow PL component but do not contribute to the blue PL signals at room temperature. We believe that the present emission model based on the photoinduced formation of the molecular species will shed new light on the intriguing but challenging PL phenomena observed commonly from nanostructured oxides.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction patterns of the air-annealed alumina nanoparticles, PL spectrum of a silica-based organic−inorganic hybrid, and PL decay profiles in the nanosecond time region. This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 2. Possible Formation Route to Emission Defect Pair at Alumina Surface

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

simply assume that the coordination number of the Al atoms with OH groups is six. Although the lone pair electrons at the undercoordinated Al site on the right-hand side of the scheme may not be well-defined as compared with the case of a silylene, the dehydroxylation reaction of the surface alanol groups on alumina nanoparticles will result in a similar defect pair as that proposed for silica nanoparticles. We, however, admit that the above consideration is still in a speculative stage at the moment, and further experimental and theoretical investigations will be required to confirm the applicability of the model to systems other than silica.

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS We have shown from the temperature- and time-dependent PL measurements of alumina and silica nanoparticles that the PL characteristics observed from these structurally and compositionally different nanoparticles are very similar to each other. This strongly suggests that the observed PL emissions result 15753



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