Effect of Adsorption on the Photoluminescence of Zinc Oxide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effect of Adsorption on the Photoluminescence of Zinc Oxide Nanoparticles Sol Kim, R.M. Dulanga S. Somaratne, and James Edward Whitten J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04715 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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The Journal of Physical Chemistry

Effect of Adsorption on the Photoluminescence of Zinc Oxide Nanoparticles Sol Kim, R.M. Dulanga S. Somaratne, James E. Whitten* Department of Chemistry, University of Massachusetts Lowell Lowell, Massachusetts 01854, USA

*Corresponding author: Professor James E. Whitten Phone: (978) 934-3666 Fax: (978) 934-3013 Email: [email protected]

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ABSTRACT Photoluminescence (PL) changes of ZnO nanospheres at room temperature have been measured during exposure to gases and vapors using a traditional fluorometer and a portable, ultraviolet (UV) light-emitting diode based instrument. Thermal gravimetric analysis indicates that the nanospheres are essentially fully hydroxylated, with OH groups and H atoms attached to surface Zn and O sites, respectively. The PL spectrum has both a UV excitonic emission peak and a visible, defect-related one. Exposure to the gases and vapors studied, whether they physisorb or chemisorb, causes a decrease in the intensity of the visible emission peak relative to pure nitrogen, although to different degrees. Electron donating molecules, such as hydrogen and methanol, cause a reversible increase in the UV emission peak intensity due to formation of an electron-rich accumulation layer around the nanoparticles. Electron withdrawing molecules, such as oxygen and water, cause a corresponding decrease due to a depletion layer. For reactive adsorption, such as by sulfur dioxide and methanethiol (MT), surface hydroxyl groups play an important role in reactivity and PL changes. X-ray photoelectron spectroscopy, coupled with density functional theory calculations, confirms that MT adsorption occurs by replacement of hydroxyl groups adsorbed on Zn sites, while SO2 adsorption leads to sulfite formation and removal of H atoms attached to O sites. The latter process causes a more dramatic decrease in visible emission, and it is postulated that hydroxyl groups formed by adsorbed H on O sites act as efficient charge traps that enhance visible PL. Their removal decreases visible PL, partially shutting down this energy pathway and causing an increase in UV emission. While SO2 and MT adsorption occur mainly by replacement of surface hydroxyls, benzene adsorbs at defect sites, such as oxygen vacancies. HCl and Cl2 decrease the visible and UV emission peaks by transforming the surface of the ZnO to ZnCl2, while H2S causes the emergence of a PL peak at 422 nm, which is postulated to arise from the formation of Zn interstitial defects. 2


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INTRODUCTION Semiconducting nanoparticulate metal oxides have a number of potential applications, including catalysis, optoelectronic devices and gas sensing. In the latter case, the most common type of sensor operates by measuring the electrical resistance of heated films (e.g., 200-500oC) during exposure to gases and vapors.1-3

In terms of gas sensing, a relatively

small number of studies have explored the possibility of monitoring changes in the photoluminescence (PL) of metal oxide nanoparticles.4-13 There may be some advantages to so-called "optochemical sensing", which may be performed at room temperature, in terms of lower power consumption and reduced hazards for flammable gas detection. Portable instrumentation is possible by combining ultraviolet (UV) light-emitting diodes and miniature light detectors, as we recently demonstrated for applications in gas filtration monitoring.14 Additionally, how gases and vapors affect the emissive properties of metal oxide nanoparticles may be important for the performance of future optoelectronic devices, such as light-emitting diodes and injection lasers. Nanoparticulate zinc oxide (nano-ZnO), which was first proposed as a resistive gas sensor15 in 1962, has a strong bimodal PL spectrum consisting of a relatively sharp UV emission peak and a broad visible peak. The former originates from excitonic electron-hole pair recombination, and the latter arises from surface defects that include oxygen vacancies.16,17 It is the only common metal oxide that exhibits a room temperature excitonic peak, which occurs due to its high exciton binding energy (ca. 60 meV).18 The origin of the visible emission is still somewhat controversial,18 with different types of surface defects likely responsible for the green, yellow and orange emissions. Green emission is commonly attributed to singly ionized oxygen vacancies (Vo+)16 and/or deeply trapped holes (Vo++)19; interstitial Zn (Zni) is thought to be responsible for orange and red emission.19 The presence of surface hydroxyl groups on ZnO has also been proposed as responsible for at least a 3



portion of the green and yellow emission.20-22 Because the PL spectrum depends at least partly on surface states, it should be possible to monitor adsorption by measuring changes in the PL spectrum of nanoparticulate (i.e., high surface area) metal oxides. Nano-ZnO has the advantage, compared to other photoluminescent nanoscale metal oxides, that it is relatively strongly fluorescent at room temperature, and the ratio of the UV and visible emission peaks may serve as an indicator of adsorption, independent of the excitation light intensity. Our previous studies14,23 were aimed at demonstrating that chemisorption of sulfur dioxide on PL nanoparticulate metal oxides (and hydroxides) could be monitored using both a traditional and custom-built, two wavelength UV LED fluorometer. In the case of exposure of ZnO to sulfur dioxide, we previously showed that strongly bound sulfite forms.23 However, the mechanisms of the PL changes were not investigated. In the present study, we have studied the PL changes that result when nano-ZnO is exposed to a variety of gases and vapors, including ones that chemisorb and physisorb, toward the goal of correlating adsorption with PL changes and elucidating the mechanisms of the PL changes. The experimental results are accompanied by density functional theory (DFT) calculations of Zn36O36 clusters, with and without co-adsorbed hydroxyl groups. It is found that desorption and replacement of adsorbed hydroxyl groups play a profound role in the observed PL changes.

EXPERIMENTAL Materials and Experimental Methods Micron-size granulated ZnO was prepared by a previously described process.24 Briefly, a ZnO wafer was formed with a Carver press from powder having an average particle size of 30 nm, purchased from Nanostructured & Amorphous Materials, Inc. (stock no. 5811HT). The wafers were then crushed in a laboratory-scale jaw crusher and sieved into 20 x 40 mesh 4


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granules. No binder was used. The density of the resulting ZnO granules was 1.6 g/cm3. This granulation is necessary to allow gas to flow through the packed material with minimal pressure buildup. In-situ PL monitoring of ZnO at room temperature during gas or vapor exposure was performed using either a commercial, double-pass diffraction grating fluorometer (JY Horiba Spex Fluorolog 2) optimized for front-face optics or a custom-built, two-wavelength UV LED based fluorometer, which has been described in detail.14 For measurements using the commercial fluorometer, 3 mm excitation and emission slit widths were used. An excitation wavelength of 340 nm was employed, and a 345 nm long-pass optical filter (Newport CGA345) was placed in front of the emission monochromator to prevent scattered excitation light from entering the detection system. The second method of following PL changes employed the custom-built instrument. This has the advantage that a large number of measurements can be taken rapidly with minimal UV light exposure of the sample. It also demonstrates the potential use of portable instrumentation for optochemical sensing. Briefly, it consists of a 340 nm LED, monochromatized by passing its output light through a 340 nm bandpass filter (Thorlabs, Inc., FB340-10), as the excitation source, with the excitation light passing through a stainless steeljacketed bifurcated fiber optic reflection probe (Avantes, Inc., FCR-7UVIR 200-45-BX). This probe contains six 200 µm fibers for the excitation leg and one 200 µm fiber for the emission leg and is terminated in a 45o sapphire window. Depending on its wavelength, the emission light either passes through or is reflected by a dichroic element (Thorlabs, Inc. MD416) and into two miniature photomultiplier tubes (Hamamatsu Photonics, A10037)) equipped with 380 ± 10 and 550 ± 10 nm bandpass filters (Thorlabs FB380-10 and FB550-10, respectively). The measurements (and turning on/off of the LED) are under the control of a microprocessor, which sends the data to a laptop computer. Graphs of the 380 and 550 nm signals for the 5



figures in this paper were prepared by maintaining the relative PMT output voltages, but normalizing the 550 nm output voltage to 1.0 V once steady state was achieved due to flowing nitrogen gas through the ZnO granules and prior to exposure to the analyte. In essence, this means that the 550 nm voltage is set to 1.0 V prior to analyte exposure. This provides a convenient way of comparing the different plots regardless of excitation light intensity and positioning of the fiber optic probe. For the commercial fluorometer measurements, a Suprasil quartz tube (4 mm and 6 mm inner and outer diameters, respectively) was packed with 0.35 g of ZnO granules, held in place using quartz wool. For the UV LED-based fluorometer, 0.5-0.6 g of the ZnO granules were placed in a stainless steel, 1/4 inch Swagelok tee apparatus, with the granules held in place by glass wool at both ends; the sapphire window of the fiber optic probe rested directly against the ZnO granules. The gas flow was maintained constant during the experiments with mass flow controllers (MKS, Inc.), with the appropriate correction factors applied for the gas or carrier gas. All gases were purchased from Airgas, Inc., unless otherwise noted. In the case of organic vapors, nitrogen carrier was bubbled through a saturator cell filled with the liquid, at approximately 23oC. The concentration in parts-per-thousand (ppt) could then be calculated based on the vapor pressure of the organic liquid and assuming the nitrogen is at a total pressure of 1 atm. Field-emission scanning electron microscopy (FESEM) was performed using a JEOL JSM 7401F instrument, with an accelerating voltage of 15 kV, to examine the morphology of the nanoparticles and the engineered granules. For the nanoparticles, gold sputter coating was performed (Denton, Vacuum, Desk IV) with a power of 15 mA for 180 secs at 60-70 mTorr. The specific surface area of the nanoparticles and granules was determined using an Autosorb-3B multi-point BET method by Quantachrome Instruments. Samples were degassed at 25oC for 120 hours followed by N2 adsorption/desorption isotherm measurement 6


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at 77 K. Thermal gravimetric analysis (TGA) of ZnO samples was performed under N2 atmosphere (in a platinum pan) with a Thermal Analysis Q50 instrument at a heating rate of 10oC min-1. X-ray photoelectron spectroscopy (XPS) was carried out using a Sigma Probe (Thermo VG Scientific) instrument equipped with a monochromatized Alkα X-ray source. ZnO granules were adhered to vacuum compatible, double-sided adhesive carbon tape on a sample plate and transferred into the XPS instrument. The kinetic energies of the ejected photoelectrons were measured in fixed analyzer transmission mode with a pass energy of 20 eV and an energy step size of 0.05 eV. XPS spectra were corrected for surface charging, which occurs due to incomplete compensation of ejected photoelectrons, by shifting all spectral regions by the energy necessary to align the Zn 2p3/2 peak to 1021.7 eV, the known binding energy of ZnO.25 In the case of spectra from reactions with HCl, Cl2, or H2S, which convert ZnO to ZnCl2 or ZnS, Zn 2p3/2 binding energies of 1023.1 and 1021.7 eV, respectively were used, which are typical values for these compounds.26 Density Functional Theory Calculations Density functional theory (DFT) calculations were performed using quantum chemical software package DMol3 (Biovia, Inc.).27, 28 The generalized gradient approximation (GGA) utilizing the exchange- correlation functional by Perdew and Wang (PW91) was employed for geometry optimizations.29 A double numerical plus d-functions basis set (3.5) was utilized in the expansion of Khon-Sham orbital during the spin-polarized self-consistent field (SCF) calculations. Geometry optimizations were performed with a convergence of 0.001 a.u. for the maximum force, 1×10-6 a.u. for the energy and 0.005 Å for the maximum displacement. A Zn36O36 cluster was used, with the the adsorbates binding to a specified site on the cluster. The adsorption energy ( ) for different adsorbates on the cluster is defined as:

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= [ − + ]

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(1)

where is the total energy of the molecules adsorbed on the metal oxide cluster, and and are the total energies of the free cluster and isolated adsorbates, respectively. The Zn36O36 Wurtzite cluster consists of three types of Zn atoms, grouped by their coordination numbers. We will refer to the ones that are coordinated to three, four and five oxygen atoms as “alpha (α)”, “beta (β)” and “gamma (γ)” Zn atoms, respectively. Calculations were performed on the cluster with and without the presence of hydroxyl groups to determine how they affect reactivity. Because of computing limitations, the cluster was not fully hydroxylated, but the role of hydroxyl groups was assessed by using a Zn36O36(OH) cluster, with the OH attached to an α Zn, referred to in our numbering scheme as “Zn#25” (see Figure 1). This cluster was used for all Zn36O36(OH), unless otherwise noted. For some calculations, a hydrogen atom was attached to a lattice oxygen adjacent to the hydroxylated Zn#25; this lattice oxygen is referred to as “O#25”, and bonding of the hydrogen atom to this lattice oxygen yields a bridging hydroxyl group. In this way, adsorption on a terminal or bridging hydroxyl group may be compared. Several calculations were also performed on Zn36O35, which is a cluster containing an oxygen vacancy; this was created by removing O#25. A few comparative calculations were also performed on β and γ Zn sites, as will be discussed.

RESULTS Water and Atmospheric Gases FE-SEM images of the primary nanoparticles and the granulated ZnO are shown in Figure 2. The average primary particle size is ca. 30 nm, and the average granule size is ca. 0.9 mm. Granulation does not significantly affect the available surface area, with measured values (from BET) of 16.53 and 17.43 m2/g, respectively, for the granules and primary 8


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nanoparticles. Figure 3 shows PL spectra of granulated ZnO before and after flowing dry N2 for 2 h. The effect of flowing nitrogen is an increase in the overall intensity of the PL spectrum and an increase in the relative intensity of the UV peak compared to the visible one. Figure 4a displays PL intensity changes of ZnO at 380 and 550 nm (and their ratio) versus time during exposure to dry and then humidified nitrogen. These data were acquired using the custombuilt two wavelength fluorometer, which has the advantage of being able to acquire a large number of data points with minimal UV light dose. Exposure to water vapor causes both the UV and visible emission peaks to decrease in intensity, with the 380-to-550 nm ratio also decreasing. However, the changes are reversible upon re-exposure to dry nitrogen. The main reason for these changes is removal of physisorbed water. Exclusion of oxygen also contributes to a small extent, as will be demonstrated soon. Figure 4b displays TGA data for a ZnO sample immediately following nitrogen purging, along with a sample that was not dried. Both samples exhibit mass losses at ca. 70-80, 130, 215-230, 360-400, and 550oC. Morishige, et al.30 studied thermal desorption from water-exposed ZnO particles and combined this with infrared absorption measurements and adsorption isotherms. They found the main aspects of the desorption spectrum to be independent of the preparation method of the ZnO. Strongly bound physisorbed water (e.g., water in direct contact with the hydroxyl groups on the surface) was found to desorb at ca. 80oC, and surface hydroxyl groups had desorption peaks at ca. 220 and 270oC. Slight mass loss above 400oC was not believed to be due to hydroxyls but to “decomposition of ZnO”, although this was not explained further. Dierre et al.31 performed temperature programmed desorption (TPD) mass spectrometry from O- and Zn-terminated single crystal ZnO surfaces. They found similar results for both surfaces; OH and H2O desorption peaks were measured at ca. 200oC while H2 desorption occurred at ca. 520oC. Their TPD spectra correlate well with the TGA derivative spectrum in 9



Figure 4. In the case of Figure 4b, the peak in the range of 70-80oC is ascribed to water physisorbed on top of hydroxyl groups. The “as-received” sample that was not nitrogen-dried shows an additional feature at ca. 40oC, which undoubtedly arises from desorption of loosely bound, trapped water. The 130, 215-230 and 360-400oC peaks are assigned to hydroxyl groups, with the different desorption temperatures related to their different environments. Based on ref. 31, the peak at ca. 560oC is attributed to H2 desorption. As expected, purging with nitrogen does not remove chemically bound hydroxyls. These results indicate that loosely adsorbed water is mainly responsible for the decreased PL intensity observed in Figure 4a. The surface density of hydroxyls, estimated from the BET surface area and the weight loss from TGA (see details in the supporting material) is 4.8 × 10 OH/g of ZnO. This may be compared with the maximum possible adsorption sites for hydroxyl groups, 3.8 × 10 OH/g, which was found by counting the number of surface Zn and O atoms in a ZnO sphere having a radius of 10-12 nm. While there is some error in this estimation, this indicates that the ZnO nanoparticles are essentially fully hydroxylated. Hydroxyl groups contribute to the visible PL of ZnO nanoparticles. To demonstrate this, the PL spectrum of ZnO nanoparticle powder pressed into a cavity in a small aluminum block was measured as a function of temperature; the aluminum block was heated with cartridge heaters, and its temperature was monitored with a thermocouple. The results are shown in Figure 5. The spectra were acquired after heating the aluminum block to the indicated temperature (which took approximately 30 min.) and then allowing it to cool to around 35oC. The procedure was followed to eliminate possible effects of a hot sample on the PL spectrum, and the PL spectra were measured immediately after the block cooled to minimize reaction with water from air. As shown in the figure, heating the sample from 32oC to 260oC causes 10


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very little change in the UV intensity. The visible intensity, however, decreases dramatically. Further heating to 344oC leads to a decrease in the UV peak and to a much greater decrease in the visible one. Taken together with the TGA data in Figure 4, these results show that desorption of hydroxyl groups dramatically decreases photoluminescence of the ZnO nanoparticles. Figure 6 shows PL changes that occur upon exposing the ZnO granules to pure hydrogen, oxygen and carbon monoxide (in that order), with dry nitrogen purging between the runs. The first observation from these data is that hydrogen exposure causes the intensity of the visible emission peak to approximately stay the same while that of the UV peak increases relative to nitrogen exposure. In contrast, oxygen causes the UV peak to decrease; in this case, the visible peak also decreases. The ratio of the 380-to-550 nm intensities tracks the UV changes. The hydrogen- and oxygen-induced changes are reversible, which is reasonable since the ZnO granules were previously exposed to air, and any irreversible reaction with these gases would have occurred previously. Yadav and coworkers4 studied PL changes of ZnO nanowires upon exposure to O2 and H2. These authors observed similar results to ours except that in the case of H2 exposure, the intensity of the visible peak increased. Collision of oxygen molecules with ZnO nanoparticles is known to lead to O2- adsorption, band bending and formation of a so-called surface “depletion layer” which is deficient in electrons. The electric field that is formed in the case of adsorption of high ionization potential molecules facilitates spatial separation of electrons and holes, decreases their recombination rate and decreases the PL intensity.32 In the case of O2, the following reaction occurs O2 (g)＋ZnO (lattice) → O2- (ads) + ZnO+ (lattice).

(2)

Therefore, for oxygen exposure, removal of electrons from the conduction band leads to fewer radiative excitonic recombinations, weaker 380 nm emission intensity and fewer 11



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electrons to transfer to lower lying defect states that give rise to visible emission. In the case of H2 exposure, the sample previously was exposed to air. Nitrogen purging may remove physisorbates, but not strongly bound ones. Reaction with molecular hydrogen removes some of the depletion layer via the formation of either adsorbed or gas phase water, which is swept away by the hydrogen gas flow. H2 (g)＋O2- (ads) → H2O (l)

(3)

However, rather quickly, the entire depletion layer is removed, since the hydrogen concentration is 100%, and there is no source of new oxygen present. At this point, hydrogen molecules collide with the ZnO nanoparticles and transiently donate electrons to the conduction band, forming an accumulation layer. Both of these processes (i.e., removal of the depletion layer and formation of an accumulation layer) are expected to lead to an increase in the UV emission intensity because more electrons enter the semiconductor conduction band and increase the length of the ZnO nanoparticle that is active with respect to photoluminescence.32 The visible emission in progressing from nitrogen gas (which we consider inert) to hydrogen gas is essentially unchanged or decreases slightly. Somewhat surprisingly, carbon monoxide exposure causes an irreversible decrease in both the UV and visible peaks. These decreased intensities are in accord with previous experiments by Idriss and Barteau7 in which PL changes of nanoparticulate ZnO were monitored when gases, including CO, were leaked into a vacuum chamber; however, in their experiments, the PL signals due to a partial pressure of CO slowly recovered as the gas was pumped out. In the present case, flowing oxygen over the ZnO has undoubtedly resulted in O2- adsorption, as per eqtn. (2), prior to 100% carbon monoxide exposure. It is likely that this adsorbed O2- reacts with impinging carbon monoxide to form CO2. However, the details were not explored further.

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Methanethiol, Methanol, Sulfur Dioxide and Benzene Exposure of ZnO to methanethiol (MT) gas causes the UV peak to increase and the visible one to decrease in intensity, as shown in Figure 7. To study the time dependence of the changes and demonstrate portable detection ability, the experiment shown in Figure 8a was performed using 1.0 vol. % CH3SH gas in nitrogen. The same trend is observed in Figure 8a as in Figure 7. In previous studies33,34 by our group, it was shown that thiolate adsorption from gas phase and in solution occurs on ZnO single crystals and nanorods exposed to thiols, via the formation of Zn-S bonds. Figure 8b shows XPS of the S2p region of the ZnO nanoparticles before and after MT exposure. The peak at a binding energy of 162.9 eV is consistent with thiolate adsorption35 and in accord with known adsorption of thiolates on ZnO surfaces.33-36 Figure 8c displays TGA data of the powder before and after MT exposure. The TGA spectra of the unexposed and MT-exposed granules are very different, and the large mass loss between 150 and 300oC in the exposed sample indicates desorption of strongly bound thiolate. Table 1 includes results of DFT calculations of the adsorption energy of MT on Zn36O36, with several different binding modes explored using Zn#25 and O#25 as the adsorption sites. More details are presented in the supplementary materials. These results indicate that for clean ZnO, bonding occurs via formation of Zn-S bonds and OH formation. This is consistent with a previous theoretical study in which it was concluded that dissociative adsorption of CH3SH, forming surface hydroxyl groups on a Zn22O22 cluster, was more favored than molecular adsorption on the same cluster.37

It is also consistent with a theoretical study by

Jena et al.38 that showed chemisorption of methanethiolate on a Zn site of ZnO nanoclusters. However, as discussed previously, the ZnO nanoparticles contain surface hydroxyl groups from reaction with ambient water. To investigate the effect that they have on adsorption, similar calculations were performed on a Zn36O36(OH) cluster, with the OH group 13



attached to Zn#25. The results are also included in Table 1 and show that, as expected, the presence of the hydroxyl group makes both molecular or dissociative adsorption less favorable. For hydroxylated ZnO, the most favorable adsorption scenario is attachment of methanethiolate to a Zn site and release of water via a condensation reaction of the liberated proton with the hydroxyl group, with an adsorption energy of -0.74 eV. This is consistent with our observations of moisture droplet formation and heat release when 100% MT gas is flowed through a tube containing ZnO nanoparticles. With respect to the observed PL changes, the irreversible UV emission intensity increase and visible emission intensity decrease suggest that replacement of hydroxyl groups attached to Zn sites by methanethiolates partially turns off the visible emission pathway. The electrons in the conduction band that would have followed this radiative path are now available to return to the valence band via excitonic recombination and UV emission, accounting for the increase in the 380 nm photoluminescence. Methanol provides a contrasting example. The time dependence of ZnO exposure to 150 ppt of methanol in nitrogen carrier gas, with subsequent nitrogen purging, is shown in Figure 9a. During exposure, the UV emission increases, and the visible emission decreases, similar to MT. However, unlike for MT, the effects are completely reversible upon removal of the organic vapor. The TGA spectra in Figure 9b are similar for the unexposed and methanolexposed samples. These results indicate that methanol does not react with ZnO at room temperature, at least for hydroxylated nanoparticles. DFT calculations for several cases of methanol adsorption are included in Table 1. For a clean Zn36O36, cluster, the most favorable case is dissociative chemisorption, with methoxy attachment to a Zn site and attachment of the liberated hydrogen to a nearby O site, with an adsorption energy of -1.19 eV. For Zn36O36(OH), dissociative adsorption of methoxy on Zn#25, with water formation from reaction of the liberated proton with the hydroxyl group, 14


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has an adsorption energy of +0.05 eV and essentially zero for the cases of without and with a proton attached to O#25, respectively. This case was thermodynamically favorable for methanethiol; however, this is not the case for methanol. This is consistent with the TGA data and observed reversible PL changes. It is interesting, however, that the presence of methanol (even though it does not irreversibly bind to the ZnO particles) affects the PL spectrum while it is flowing through the ZnO granules. This indicates that nonreactive collisions bring about reversible changes in which the UV emission peak increases and the visible one decreases. This is similar the trend that was observed for hydrogen gas. To test whether there is variation in the DFT results if adsorption occurs on sites other than Zn#25, comparative calculations were performed for both CH3SH and CH3OH adsorption on hydroxylated Zn#23 and Zn#27, which are respectively γ and β sites (see Figure 1). The particular reactions investigated methanethiolate and methoxy adsorption on the Zn sites, with water liberation. For methanethiol, the adsorption energies on Zn#23, Zn#25, and Zn#27 were -0.74, -0.74 and -0.76 eV, respectively. The corresponding methanol values were 0.05, 0.05 and -0.12 eV, respectively. These results suggest that the choice of the particular site does not affect the qualitative conclusions of this paper. Figure 10a shows PL changes of ZnO versus time upon exposure to 1.0% SO2 in N2. The changes are irreversible, similar to what is observed for MT exposure, except that the decrease in the visible emission peak intensity is even more dramatic. This is consistent with the PL spectra shown in Figure 7. Figure 10b displays S2p XPS data of an unexposed and exposed samples. The S2p binding energy of 168.4 eV is consistent with adsorbed sulfite. The TGA data in Figure 10c show a decomposition/desorption peak at ca. 166oC, consistent with a strongly bound adsorbate. The adsorption of SO3 upon exposing clean and water-dosed ZnO to sulfur dioxide is in accord with existing literature.39,40 DFT calculations for several different binding modes of SO2 on Zn36O36 were examined 15



(included in Table 1). Comparison of the adsorption energies for SO2 via attachment of its sulfur to Zn#25 or O#25 gives adsorption energies of -1.32 and -2.09 eV, respectively. In the case of reactivity with Zn36O36(OH), with OH attached to Zn#25, binding of the sulfur atom to Zn#25 or O#25 has adsorption energies of -2.05 or -2.12 eV, respectively. For reaction with the cluster containing a hydroxyl group on Zn#25 and a proton on O#25, molecular, nondissociative adsorption on O#25 has an adsorption energy of -1.08 eV. Bonding to O#25, with water liberation by reaction of the liberated proton from O#25 with the hydroxyl group on Zn#25, has an adsorption energy of -0.89 eV. However, the most energetically preferred binding occurs by sulfur attachment to O#25, forming sulfite, with concomitant H atom cleavage from O#25; the hydroxyl group on Zn#25 is undisturbed. Two liberated hydrogen atoms would then combine to form H2(g). This occurs with an adsorption energy of -1.39 eV per adsorbed sulfur dioxide, making it the most thermodynamically favored adsorption scenario. An experiment was performed in which 100% sulfur dioxide was flowed through a pyrex tube containing ZnO nanopowder; the tube became warm to the touch. However, unlike the case of MT reaction, water droplets were not observed. These observations are consistent with sulfite formation via the liberation of hydrogen gas, in accord with the DFT calculations. Experimental and theoretical results therefore support the schematic of adsorption shown in Figure 11. MT adsorption occurs by attachment of the sulfur to a lattice Zn site, with water formation. On the other hand, sulfur dioxide adsorption occurs by attachment of the sulfur to a lattice O site, with hydrogen gas evolution. It is interesting that the visible emission peak is quenched to a much greater extent in the case of sulfur dioxide compared to methanethiol exposure (see Figures 8a and 10a). For the former case, the decrease is 60%, while it is ca. 10% for the latter. This appears to be related to the different adsorption modes (i.e., SO2 binding to an O site and MT binding to a Zn site) 16


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and type and number of hydroxyl groups that are removed by adsorption. To compare the adsorption of these molecules to an aromatic one, experiments were carried out using benzene (Bz). Figure 12a shows PL changes of ZnO nanoparticles upon exposure to 114 ppt of Bz in nitrogen carrier gas, followed by subsequent nitrogen purging. Exposure to Bz vapor causes an irreversible 15% decrease in the visible emission intensity and a slight increase in the UV emission intensity, which almost recovers to its pre-exposure value upon purging. Figure 12b displays corresponding TGA spectra. For this particular figure, TGA of the Bz-exposed sample was acquired several weeks after exposure, accounting for return of the loosely bound water peak at ca. 47oC. The peak present at ca. 470oC is indicative of chemically bound Bz or Bz fragments. This result is somewhat surprising since a fully hydroxylated ZnO particle is expected to only weakly bind aromatic molecules. Nagao and Matsuoka41 coupled infrared spectroscopy with adsorption isotherms to measure the interaction between ZnO particles and aromatic molecules, including Bz, as a function of the degree of hydroxylation. The strength of interaction with ZnO decreases as the hydroxyl content increases. While it was found that hydroxyl groups on ZnO interact with the π electrons of the aromatic molecules, this interaction is relatively weak. Recent work related to hydroxyl groups on other surfaces, including silica, indicate that the interaction may be considered hydrogen bonding between the hydroxyl groups and the aromatic rings, but desorption occurs at less than room temperature.42 The most likely explanation for observed the Bz-induced PL changes and the TGA derivative peak at ca. 470oC is binding of Bz to defect sites, such as oxygen vacancies. In the case of the other reactive molecules studied (MT and SO2), it was not necessary to invoke interaction with defect sites since adsorption occurs by replacement/removal of abundant hydroxyl groups. For these adsorbates, there may indeed be interactions with surface defects, but they are secondary to reactions with the hydroxyl groups. To evaluate how adsorption on 17



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oxygen defects may affect benzene adsorption, several cases were compared with DFT calculations. Figure 13 shows the optimized geometry for benzene bonding to Zn36O36, with the π electrons interacting with the Zn#25 site. Included for comparison are cases for Bz bonding to a Zn36(OH)O36(H) cluster, which has a hydroxyl group adsorbed on Zn#25 and a hydrogen adsorbed on O#25. These have adsorption energies of -0.78 and -0.995 eV, respectively. If a +2 oxygen vacancy is introduced to the cluster by removing O#25, benzene adsorption on the clean cluster (i.e., Zn36O35) at Zn#25 has an adsorption energy of -1.42 eV. For adsorption on Zn#25 with a hydroxyl group attached to it and one attached to a nearby Zn atom (see Figure 13) the adsorption energy is -1.72 eV. In all cases, bonding is through the π electrons. These calculations support the conclusion that oxygen vacancies increase reactivity of benzene with both clean and hydroxylated ZnO clusters.

Hydrogen chloride, chlorine and hydrogen sulfide The reactions of ZnO nanoparticles with HCl, Cl2 and H2S fall into a different class than those previously discussed because of their aggressive nature. Exposure to high doses of these gases potentially results in transformation of ZnO into ZnCl2 (for HCl and Cl2) and ZnS (for H2S). ZnO (s) + 2 HCl (g) → ZnCl2 (s) + H2O (l)

(4)

ZnO (s) + Cl2 (g) → ZnCl2 (s) + ½ O2 (g)

(5)

ZnO (s) + H2S (g) → ZnS (s) + H2O (l)

(6)

All of these reactions are thermodynamically favored, with ∆Go values of -192.6, -146.1 and 75.5 kJ per mole of ZnO, respectively, as calculated using thermodynamic tables.43 Figure 14 shows PL data for exposure of the ZnO granules to 10% HCl and 500 ppm Cl2 in nitrogen carrier gas, with the changes monitored using the custom-built two wavelength fluorometer. The figure also includes Cl2p XPS data of the granules following exposure. A 18


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trace amount of Cl is present in the unexposed samples. However, reaction with hydrogen chloride and chlorine gases causes transformation of the surfaces of the ZnO granules to ZnCl2, as confirmed by the appearance of strong peaks at a binding energy of 199.1 eV, in general agreement with the known value.25 Exposure to 10% H2S in nitrogen carrier gas was studied by measuring the fluorescence spectrum using a traditional fluorometer, before and after exposure to the gas, as shown in Figure 15. All of these aggressive gases cause irrecoverable changes in PL intensity of both the UV and visible emission peaks. However, there is only partial loss of PL intensity, since the reaction takes place in a limited depth of the granule. XPS of the S2p region of the granules after extensive exposure to 10% H2S has a peak binding energy of 161.3 eV, consistent with ZnS.44 However, it is also consistent with previous studies of ultrahigh vacuum, room temperature dosing of polycrystalline ZnO with H2S which gave a S2p3/2 peak binding energy of 161.2 eV45 and indicated adsorption of atomic sulfur and/or HS species. As shown in Figure 15, H2S exposure causes emergence of a PL emission peak at ca. 422 nm. This is likely due to the formation of Zn interstitial defects on ZnO, which have been shown to give rise to emission in this range.46 Since the PL spectrum of ZnO does not disappear (but is changed by the emergence of the strong peak at 422 nm), these data suggest that reaction with H2S induces interstitial defects but does not transform the surface of the ZnO nanoparticles to ZnS. This was also confirmed by Raman spectroscopy of the H2S-exposed ZnO granules.

DISCUSSION A summary of the observed changes in PL of the ZnO nanospheres upon exposure to the different gases and vapors is shown in Table 2. Key observations are that collisions and transient adsorption of electron donating molecules (e.g., H2 or CH3OH) with the ZnO lead to a reversible increase in the 380 nm UV emission intensity relative to nitrogen gas. A 19



corresponding decrease in intensity is observed for electron withdrawing molecules (e.g, O2 or H2O). This results from formation of either an electron accumulation or depletion layer around the ZnO particles and occurs even though the particles are hydroxylated and, in some cases, surrounded by chemisorbed molecules. For example, in the case of CH3SH (Figure 8), the intensity of the UV peak decreases slightly (back to its pre-CH3SH exposure value) once excess CH3SH is purged by flowing nitrogen. Interestingly, the visible emission intensity, which decreased due to methanethiolate adsorption, does not change upon purging with nitrogen. The combination of these changes is reflected in the ratio of the 380-to-550 nm intensities. A similar situation is observed for benzene (Figure 12). The UV peak (which increased due to benzene exposure), decreases almost back to its pre-exposure value upon purging with N2. However, the visible emission does not recover. ZnO exhibits irreversible changes in both its 380 and 550 nm emission peaks upon exposure to sulfur dioxide, with both decreasing due to sulfite formation, which is known to occur for room temperature dosing of ZnO with this gas.39,40 Every gas/vapor investigated in this study causes a decrease in the visible emission intensity relative to pure nitrogen gas. However, the magnitude varies, with large but reversible decreases in visible emission occurring even for molecules that do not chemisorb, such as water and methanol. When this radiative channel is partially turned off, however, the intensity of the UV emission does not necessarily increase, indicating that the energy is dissipated non-radiatively when adsorption occurs on defects (in the case of benzene) or by removal of hydroxyl groups (for CH3SH or SO2). A growing body of literature shows that hydroxyl groups act as charge traps on ZnO21,47-50 and other metal oxide nanoparticles.51,52 Electron spin resonance spectroscopy of UVirradiated ZnO confirms that hydroxyl species cause the formation of hole trap states.53 These hole traps are most likely the origin of enhanced visible PL that occurs due to the presence of 20


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adsorbed hydroxyl groups and that diminishes upon replacement by other adsorbates. Furthermore, it appears that hydroxyl groups formed by protonation of O sites may play a particularly prominent role in the PL of metal oxide nanoparticles. Müller and colleagues54 have shown that protonation of 3-coordinated oxygen sites of MgO nanocrystals shifts the occupied and unoccupied states to lower energies, especially affecting the 2p states of the protonated oxygen. The effect decreases as distance from the protonated oxygen increases. The authors claim their result to have similar implications for other metal oxide nanoclusters. It therefore appears that hydrogen adsorbed on surface oxygen sites is unique because of the ability of the hydrogen atoms to donate electrons to the O 2p orbitals and form efficient hole traps. Replacement of the H by SO2 (or other) adsorbates removes these trap states and leads to a decrease in visible emission.

CONCLUSIONS The present work has investigated the origin of PL changes of ZnO nanoparticles upon exposure to various gases and vapors and laid the groundwork for optochemical sensing using this material. ZnO is particularly attractive as a sensor because of its room temperature bimodal PL spectrum and relatively high reactivity compared to many metal oxides. All of the gases/vapors studied cause a decrease in the visible emission peak, but the decrease is reversible in the case of physisorption. In the case of species that do not bind to the surface at room temperature (i.e., physisorption), this decrease likely is due to collisions with defects that decrease the lifetime of the defect-related visible emission. Collisions of electronwithdrawing molecules (e.g., O2) may also cause band-bending that removes electron population from the conduction band, with fewer electrons available to decay to defectrelated states. Band-bending certainly also affects the intensity of the UV emission peak, which may increase in the case of collisions by electron-donating molecules (e.g., H2). 21



Methanethiol, sulfur dioxide and benzene all react irreversibly with ZnO. The reactivity of CH3SH and SO2 is largely driven by the presence of hydroxyl groups, with the former forming a Zn-S bond via a condensation reaction and the latter bonding to a surface O site with the formation and desorption of H2(g). Both reactions lead to removal of hydroxyl groups from the surface, but the removal of bridging hydroxyl groups (originally formed by H adsorption on O sites) causes a more dramatic decrease in the visible emission peak. Benzene appears to react not with hydroxyl groups but with oxygen vacancies. While this could also occur in the case of SO2 or CH3SH adsorption, it is likely a minority reaction pathway for those adsorbates. Gases such as HCl, Cl2 and H2S react aggressively with the ZnO nanoparticles, forming ZnCl2 or zinc interstitial defects, respectively. While it was not the goal of this work to develop a gas sensor system, a few comments on the potential of optochemical sensing using metal oxide nanoparticles are in order. As for any other sensor system, orthogonality between sensing nodes is ultimately required in order to discriminate between different analytes. Clearly, for optochemical sensing and analyte identification to be feasible, nanoparticles that respond very differently to target analytes are required. As such, similar studies as presented in the paper with other photoluminescent metal oxide nanoparticles, such as ceria and zirconia, are required. These studies are planned in our laboratory. ACKKNOWLEDGEMENTS The authors acknowledge partial support for this work from the Army Research Office under Defense Threat Reduction Agency, project number BA07PRO105. The authors also acknowledge Dr. Joseph Rossin at Guild Associates, Inc. for preparing the ZnO granules.

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FIGURE CAPTIONS Figure 1. Optimized geometry of the Zn36O36 cluster used for the DFT calculations presented in this work. Gray and red spheres represent Zn and O atoms, respectively. Most of the simulations involve the Zn and O sites denoted as “Zn#25” and “O#25”, and their locations on the cluster are shown. Also shown are “Zn#23” and “Zn#27”, which were used for some comparison calculations. Figure 2. FE-SEM images of a) the ZnO nanoparticle powder used for this study and b) a granule prepared from the nanoparticle powder. Figure 3. Photoluminescence spectra of the ZnO granules before and after drying by flowing nitrogen gas through the granules for 2 h at a flow rate of 150 ml min-1. Figure 4. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure and humidified nitrogen through the granules at a flow rate of 1.0 L min-1. b) TGA and derivative TGA spectra of as-prepared and nitrogen-dried ZnO granules using a heating rate of 10oC min-1. Figure 5. PL spectra of ZnO powder after heating to the indicated temperature, as described in the text. The spectra were acquired immediately after the samples cooled to ca. 35oC. Figure 6. PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right y-axis) measured using the UV LED-based instrument while successively flowing nitrogen, hydrogen, nitrogen, oxygen, nitrogen, carbon monoxide and nitrogen gases thorough a Suprasil tube containing the ZnO granules. A flow rate of 1.0 L min-1 was used. Figure 7. PL spectra of ZnO granules after nitrogen drying and after exposing nitrogen-dried granules to 1% SO2 or 1% CH3SH in nitrogen carrier gas. The granules were purged with

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nitrogen gas to remove excess, physisorbed sulfur dioxide or MT prior to taking the spectra. The excitation wavelength was 340 nm. Figure 8. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure nitrogen, 1% MT in nitrogen, and then nitrogen through the granules at a flow rate of 1.0 L min-1; b) AlKα XPS of the S2p region of unexposed and MT-exposed ZnO granules; c) TGA and derivative TGA spectra of as-prepared and methanethiol-exposed ZnO granules using a heating rate of 10oC min-1. Figure 9. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure nitrogen, 150 ppt methanol in nitrogen, and then nitrogen through the granules at a flow rate of 1.0 L min-1; b) TGA and derivative TGA spectra of as-prepared and methanol-exposed ZnO granules using a heating rate of 10oC min-1. Figure 10. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure nitrogen, 1% SO2 in nitrogen, and then nitrogen through the granules at a flow rate of 1.0 L min-1; b) AlKα XPS of the S2p region of unexposed and MT-exposed ZnO granules; c) TGA and derivative TGA spectra of as-prepared and sulfur dioxide-exposed ZnO granules using a heating rate of 10oC min-1. Figure 11. a) Schematic of the proposed reactions that takes place between the hydroxylated ZnO nanoparticle surface and a) methanethiol (MT) that results in water liberation and b) SO2 that results in hydrogen gas formation.

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Figure 12. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure nitrogen, 114 ppt benzene in nitrogen, and then nitrogen through the granules at a flow rate of 1.0 L min-1; b) TGA and derivative TGA spectra of as-prepared and benzene-exposed ZnO granules using a heating rate of 10oC min-1. Figure 13. Optimized interaction (from DFT calculations) of benzene with a) the Zn36O36 cluster near Zn#25 and b) a Zn36O35 cluster that has a +2 charge at the oxygen vacancy and hydroxyl groups attached to Zn#25 and Zn#27. Figure 14. a) PL intensities of the 380 and 550 nm peaks (left y-axis) and their ratio (right yaxis) measured using the UV LED-based instrument while flowing pure nitrogen, 500 ppm Cl2 in nitrogen, and then nitrogen through the granules at a flow rate of 1.0 L min-1; b) A similar experiment for 10% HCl; c) AlKα XPS of the Cl 2p region of unexposed, Cl2exposed and HCl-exposed ZnO granules. Figure 15. a) PL spectra of ZnO granules after nitrogen drying and after exposure to 10% H2S in nitrogen. The granules were purged with nitrogen gas to remove excess, physisorbed hydrogen sulfide prior to taking the spectrum. The excitation wavelength was 340 nm. b) AlKα XPS of the S 2p region of unexposed and H2S-exposed ZnO granules.

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Table 1: Calculated adsorption energies (Eads) for various cases of methanethiol, methanol and sulfur dioxide adsorption on clean and hydroxylated zinc oxide clusters. Adsorbate system CH3SH O36Zn36-CH3SH - Molecular O36Zn36-CH3SH – Water liberation O36Zn36-CH3SH - H2 liberation Zn36O36- CH3SH - Molecular Zn36O36- CH3SH - H2 liberation O36Zn36-(SCH3)(H) - Dissociative O36Zn36-(SCH3)(H) - H2 liberation O36Zn36-SCH3 - Water liberation CH3OH O36Zn36-CH3OH - Molecular O36Zn36-(OCH3)(H) - Dissociative O36Zn36-(OCH3)(H) – H2 liberation O36Zn36-OCH3 - Water liberation SO2 O36Zn36-SO2 - Molecular Zn36O36-SO2 - Molecular Zn36O36-SO2 - Water liberation Zn36O36-SO2 – H2 liberation O36Zn36-(OSO2)(H)

Clean

Eads (eV) Zn36O36(OH)

Zn36(OH)O36(H)

-1.07 -0.63 -1.82 -

-0.72 -0.12 -1.59 -0.74

0.13 0.01 0.60 1.46 -0.62

-0.86 -1.19 -

-0.59 -1.04 0.05

-0.31 -0.001

-1.32 -2.09 -

-2.05 -2.12 -1.15

-1.08 -0.89 -1.39 -

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Table 2: Summary of the effects of the different gases/vapors on hydroxylated ZnO nanoparticle photoluminescence relative to pure nitrogen gas. Gas/Vapor

Reversible

UV Emission

Visible Emission

UV/Vis Ratio

(Yes/No)

(Increase/Decrease)

(Increase/Decrease)

(Increase/Decrease)

H2 O

Yes

Decrease

Decrease

Decrease

H2

Yes

Increase

Decrease

Increase

O2

Yes

Decrease

Decrease

Decrease

CO

No

Decrease

Decrease

Decrease

CH3SH

No

Increase

Decrease

Increase

CH3OH

Yes

Increase

Decrease

Increase

SO2

No

Decrease

Decrease

Increase

C 6 H6

No

Increase

Decrease

Increase

34


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Figure 1

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Figure 2

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Figure 3

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Figure 4 38


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Figure 5

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Figure 6

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Figure 7

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Figure 8 42


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Figure 9

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Figure 10 44


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Figure 11

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Figure 12

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Figure 13

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Figure 14

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Figure 15

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TOC Graphic

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Effect of Adsorption on the Photoluminescence of Zinc Oxide

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