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US6592842B2 - Nanocrystalline heterojunction materials - Google Patents
Nanocrystalline heterojunction materials Download PDF
US6592842B2
US6592842B2 US09/859,799 US85979901A US6592842B2 US 6592842 B2 US6592842 B2 US 6592842B2 US 85979901 A US85979901 A US 85979901A US 6592842 B2 US6592842 B2 US 6592842B2
US09/859,799
US20020071970A1 (en
Scott H. Elder
1999-10-01 Priority to US41136099A priority Critical
2001-05-16 Application filed by Battelle Memorial Institute Inc filed Critical Battelle Memorial Institute Inc
2001-05-16 Priority to US09/859,799 priority patent/US6592842B2/en
2002-01-04 Assigned to BATTELLE MEMORIAL INSTITUTE reassignment BATTELLE MEMORIAL INSTITUTE ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HEALD, STEVE M., GAO, YUFEI, SU, YALI, ELDER, SCOTT H.
2002-06-11 Assigned to ENERGY, U.S. DEPARTMENT OF reassignment ENERGY, U.S. DEPARTMENT OF CONFIRMATORY LICENSE (SEE DOCUMENT FOR DETAILS). Assignors: BATTELLE MEMORIAL INSTITUTE, PACIFIC NORTHWEST DIVISION
2002-06-13 Publication of US20020071970A1 publication Critical patent/US20020071970A1/en
2003-07-15 Publication of US6592842B2 publication Critical patent/US6592842B2/en
This is a continuation-in-part of U.S. patent application Ser. No. 09/411,360, filed Oct. 1, 1999, now abadoned, which is incorporated herein by reference.
Anatase nanocrystallites (i.e crystals with a diameter in the range of 20 Å to 100 Å) are of interest because their photophysical and catalytic properties differ from the bulk material (See for example, Brus, J. Phys. Chem., 90: 2555-2560, 1986). Nanocrystallite properties are a direct result of the particle size and dimensionality, making adjustment of crystallite size and architecture an avenue to materials with novel photophysical and catalytic properties. Unfortunately, such small particles are difficult to handle, exhibit poor thermal stability, and exhibit a blue shift (i.e., further away from the ambient solar maximum) in their absorption relative to the bulk material.
Mesoporous silica and aluminosilicate materials with surface areas above 1000 m2 g−1 have been synthesized by surfactant templating (see for example, Kresge et al., Nature, 359: 710-712 and Beck et al. J. Am. Chem. Soc., 114: 10834-10843). Mesoporous titanium doped metal silicates formed in a similar manner are disclosed in Hasenzahl, et al., U.S. Pat. No. 5,919,430. Thermally stable mesoporous materials with metal oxides as the principal wall component have been more elusive.
FIG. 2 shows a plot of the TiO2—(MoO3)x photoabsorption energy (PE) as a function of TiO2—(MoO3)x core-shell diameter (i) TiO2—(MoO3)0.18: 80 Å, PE=2.88 eV; (ii) TiO2—(MoO3)0.54: 60 Å, PE=2.79 eV; (iii) TiO2—(MoO3)1.1: 50 Å, PE=2.68 eV; (iv) TiO2—(MoO3)1.8: 40 Å PE=2.60 eV.
FIG. 3 shows the Mo K-edge XANES data for α-MoO3 (□), TiO2—(MoO3)0.18 (Δ), TiO2—(MoO3)0.54 (x), TiO2—(MoO3)1.1 (O), and TiO2—(MoO3)1.8 (⋄), and MoO2 (solid line).
FIG. 4 shows the Mo—L3-edge XANES data for α-MoO3 (□), TiO2—(MoO3)0.18 (),TiO2—(MoO3)1.8 (x), and Na2MoO4 (O).
FIG. 5 shows Raman Scattering data for the series of TiO2—(MoO3)x core-shell materials, and TiO2 and α-MoO3 for comparison: (A) anatase TiO2 standard, (B) anatase TiO2 with average crystallite size of 100 Å; (C) TiO2—(MoO3)0.18; (D) TiO2—(MoO3)0.54; (E) TiO2—(MoO3)1.1; (F) TiO2—(MoO3)1.8; and (G) α-MoO3.
FIG. 7 shows the arrangement of the TiO2 core and MoO3 shell valence bands (VB) and conduction bands (CB) for TiO2—MoO3)1.8 after heterojunction formation.
EXAMPLE 1 Preparation of TiO2—(MoO3)x Core-Shell Materials: a Heterojunction Material
(1−y)(NH4)2Ti(OH)2(C3H4O2)2(aq)+(y/8) Na4Mo8O26(aq)+CTAC(aq) (1)
As an example, for the synthesis of TiO2—(MoO3)0.18 4.8 g (y=0.10) of (NH4)2Ti(OH)2(C3H4O2)2 (Tyzor LA,; Dupont) were combined with 4.9 g of cetyltrimethylammonium chloride surfactant (CTAC, 29 wt % aqueous solution, Lonza). To this solution, 30 ml of a 1.8 mM Na4Mo8O26 aqueous solution was added with vigorous stirring, which produced a voluminous white precipitate. The Na4Mo8O26 aqueous solution was made by dissolving Na2MoO4.2H2O in H2O, and adjusting the pH to 3.5 with concentrated HCl.
The reaction was stirred at room temperature overnight, at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor (hydrothermal aging step). The precipitate was isolated by washing and centrifuging several times with water, and the CTAC was removed by calcining in air at 450° C. for 2 h. The synthesis of TiO2—(MoO3)0.54 (y=0.25), TiO2—(MoO3)1.1 (y=0.50), and TiO2—(MoO3)1.8 (y=0.57) were accomplished in an analogous manner, and the chemical compositions were determined by elemental analysis. The quantity y (equation 1) can be continuously varied from 0.01 to 0.57, but the four core-shell compositions described above adequately represent the range of structural and electronic properties displayed by the TiO2—(MoO3)x compounds. It is important to note that if no CTAC was included, or if the CTAC was substituted with NH4Cl, no precipitation reaction occurred at any point in the reaction steps. Furthermore, if no Mo8O26 4− (aq) was included in the reaction, only a white solid was produced, and the same observation was made if only Mo8O26 4− (aq) was used in reaction 1. Only bulk crystalline TiO2 and α-MoO3 can be prepared for y>0.57, which is indicative of macroscopic phase separation.
The variable x in the TiO2—(MoO3)x nomenclature was calculated as follows. The MoO3 shell thickness was calculated by considering the elemental analysis data, the surface area of the powders (Table 1 below), and the crystallographic structure of α-MoO3. The Mo surface density on the (010) plane of α-MoO3 is 6.8×1018 m−2, and this Mo surface density, combined with the measured surface area, was used to define the MoO3 monolayer coverage for the shell. For example, one monolayer is one layer of comer sharing MoO6 octahedra, and two monolayers has the thickness of one of the slabs oriented perpendicular to the b-axis of α-MoO3.
Elemental analysis data (mol % Mo) and surface area for the
TiO2-(MoO3)x materials that were used to calculate the number
of MoO3 monolayers (x) in the shell (y refers to the reaction
stoichiometry in equation 1).
Core-shell Material Y Mol % Mo (m2/g)
TiO2—(MoO3)0.18 0.10 2 125
TiO2—(MoO3)0.55 0.25 10 200
TiO2—(MoO3)1.1 0.50 25 205
TiO2—(MoO3)1.8 0.57 30 150
EXAMPLE 2 Characterization of TiO2—(MoO3)x Core-Shell Materials
The color of the calcined TiO2—(MoO3)x powders was expected to be white, or possibly very light yellow, because the transition metals are in their fully oxidized state (i.e. d°). In contrast, they surprisingly displayed a variety of colors ranging from gray-green to green as a function of MoO3 content. XRPD (X-ray powder diffraction) studies were conducted to ascertain how the crystallographic structures of the TiO2—(MoO3)x compounds correlated with these colors. The XRPD data for a series of TiO2—(MoO3)x core-shell compounds (FIG. 1) exhibited diffraction peaks that could be indexed on the TiO2 (anatase) unit cell. In FIG. 1, the lowest set of stick-figure data is that reported for pure anatase TiO2. No crystalline molybdenum oxide phase was observed in the XRPD data. Based on the peak broadening, the TiO2 was determined to be nanocrystalline, with the crystallite size decreasing as the MoO3 shell thickness increased. The average TiO2 crystallite size was determined using the Scherrer equation and confirmed with high-resolution transmission electron microscopy. The crystallite diameters are: TiO2—(MoO3)0.18: 80 Å, TiO2—(MoO3)0.54: 60 Å, TiO2—(MoO3)1.1: 50 Å, TiO2—(MoO3)1.8: 40 Å.
High resolution transmission electron microscopy (HRTEM) studies on these samples supported the anatase crystallite size calculated from the XPD data, and also confirmed there was no crystalline or large (>20 Å) amorphous molybdenum oxide phases. Images from high-resolution transmission electron microscopy (HRTEM) studies on these samples exhibited particles with well-defined lattice fringes. The lattice spacing of the crystallites with non-crossed fringes measured 3.5±0.05 Å, which corresponds to the distance between the (101) planes in anatase TiO2. The TiO2 crystallite sizes measured in the HRTEM images were similar to those calculated from the XRPD data. Finally, there were no crystalline or large (≧10 Å) amorphous molybdenum oxide domains evident in HRTEM data.
As a clarifying note, despite the increase in MoO3 shell thickness when going from TiO2—(MoO3)0.18 to TiO2—(MoO3)1.8, the overall particle size (core+shell) decreases since the TiO2 core size decreases rapidly in this series, but the shell is never more than ˜6 Å thick. The plot of PE vs. particle size (FIG. 2) clearly shows that the PE becomes more red-shifted with decreasing particle size. Such behavior is unexpected for nanocrystalline materials.
The electronic bandgap transitions for the TiO2—(MoO3)x compounds are fundamentally different than those previously reported for II-VI and III-V core-shell systems. Theoretical and experimental work on II-VI and III-V core-shell nanoparticle systems indicate that PE is a function of both size quantization effects and the relative composition of the core-shell particle (i.e. relative thickness of the core and shell). In the limiting case it is logical to expect the PE of a core-shell nanoparticle system to be greater than or equal to the smallest band gap material comprising the core-shell system. In addition to this, a PE blue-shift, relative to the band gap energies of the bulk materials, is expected when the core-shell particle size is in the quantum regime (i.e., core diameter or shell thickness equal to or smaller than the Bohr radius of the valence/conduction band electron). Indeed, previous work demonstrates these two effects. For these reasons the PE for the TiO2—(MoO3)x core-shell materials was expected to be greater than 2.9 eV (PE for MoO3), and likely greater than 3.2 eV (Eg for TiO2) due to the dominant size quantization effects, especially for TiO2—(MoO3)1.8 where the core-shell size is ˜40 Å. For example, a band gap energy blue-shift is observed for PNNL-1 (Eg=3.32 eV)x which contains nanocrystalline TiO2 with an average crystallite size of 25-30 Å. In contrast, the TiO2—(MoO3)x PE's range from 2.88 to 2.60 eV, approximately equal to or lower in energy than bulk MoO3, which places the PE of TiO2—(MoO3)1.8 in the most intense region of the solar spectrum. The charge-transfer absorption properties exhibited by the TiO2—(MoO3)x compounds appear to be fundamentally different than previously reported for the II-VI and III-V core-shell systems. Conversely, the TiO2—(MoC)3)x materials did not fluoresce when they were photoexcited at energies above their PE edge, as opposed to a sample of pure TiO2 that gave a characteristic fluorescence spectrum. The lack of fluorescence is readily understood considering that the TiO2—(MoO3)x materials exhibit photochromic properties: they become blue/black in color when exposed to light under ambient conditions. This photochromism was studied by irradiating each of the powders with monochromatic light, and it was found that all of the samples turned blue/black when excited with light having 420 nm (TiO2—(MoO3)0.18)≦λ≦460 nm (TiO2—(MoO3)1.8). For comparison, bulk TiO2 and MoO3 exhibit photochromism, but only when irradiated with ultraviolet light (˜300 nm). The photochromic behavior of the materials is believed to be first reported visible-light induced photochromism of TiO2 or MoO3 without prior bluing by cathodic polarization.
Considering the relative ease in which reduced molybdenum oxides are formed, generally called Magneli phases, EPR data was collected on the TiO2—(MoO3)x compounds to determine if paramagnetic molybdenum species played a role in the observed optical properties. Both CW-EPR spectra and profiles of the electron spin-echo intensity as a function of magnetic field were recorded between room temperature and 5 K for each sample. There was a single dominant EPR signal exhibiting roughly axial symmetry with g∥=1.883 and g⊥=1.93. The relative ordering of the g-values is typical for Ti(III) in oxides, while the opposite is usually observed for Mo(V) in oxides. In addition, Ti(III) at the surface of an aqueous colloid of TiO2 has g∥=1.88 and g⊥=1.925, making it likely the EPR signal from the samples is due to Ti(III) either at an exposed TiO2 surface or at the Ti/Mo interface. Assuming equal packing densities for each sample, the double integrals of the CW-spectra indicated that the number of spins was directly proportional to the Mo content. However, because the packing density of the samples in the EPR tubes was not known very accurately, double integration was not a precise method for determining absolute spin concentrations in the sample. Yet, the possibility that these centers might be responsible for the PE shifts made such data important. We therefore turned to measurements of the electron spin-spin relaxation to set upper limits on the absolute spin concentration. Interaction with nearby paramagnetic centers is one reason for decay of the two-pulse electron spin-echo and adds to the decay rate from other sources. The decay rate caused by nearby spins is well understood and for these samples is expected to be equal to αC·Cloc where αc˜(0.3-0.9)×10−13 cm3/s and Cloc is the local concentration of paramagnetic species. The electron spin-echo decay rates were not obviously related to Mo content and varied between 0.4×106 and 1.25×106 s−1 with little, if any, temperature dependence below 150 K. Taking the fastest decay as an absolute upper bound on local paramagnetic concentration, which occurs in the sample with the highest Mo content (TiO2—(MoO3)1.8), gives a local concentration of paramagnetic centers of 4×105 Å−3. This corresponds to approximately one paramagnetic center per particle. The CW EPR measurements showed that the number of centers is 10 times less in TiO2—(MoO3)0.18, suggesting a local concentration 10 times lower in a particle with roughly 10 times larger volume, again giving an upper limit of approximately one paramagnetic center per particle. The actual local concentration is probably at least an order of magnitude lower than this upper bound.
Raman scattering data (FIG. 5) were collected on the TiO2—(MoO3)x materials to gain further evidence for the core-shell arrangement, since these measurements are quite sensitive to the atomic connectivity. The data labeled as TiO2 (std.) (curve A) in FIG. 5 are from powder that is 99% anatase (estimated from XRPD data) with average crystallite size greater than 0.5 μm. These data match the literature data for anatase TiO2. The next set of data labeled 100 Å TiO2 (curve B) are for a powder that resulted when no Mo8O26 4− (aq) was included in the reaction described in Example 3 above. These data are quite similar to the TiO2 (std.) data except there is a slight shift of the two eg bands to higher wavenumber due to the quantum confinement of the phonon states in the TiO2 nanocrystallites. The data between 100 and 700 cm−1 for the TiO2—(MoO3)x materials are quite similar to those of 100 Å TiO2 except for the progressively greater shift to higher energy of the eg bands and an increase in band broadening. This is expected since the nanocrystalline TiO2 size decreases as MoO3 coverage increases. The other feature is the appearance of a broad peak at ˜820 cm−1 and a second peak at ˜1000 cm−1. Both of these peaks become more prominent with increasing MoO3 content, and their origin can be easily understood by comparing them to the Raman data for α-MoO3 (curve G). The peak at ˜820 cm−1, most apparent in the TiO2—(MoO3)1.8 data (curve F), is attributed to the Mo—O—Mo stretching mode of the comer sharing MoO6 octahedra. This Raman band should be most evident in TiO2—(MoO3)1.1 and TiO2—(MoO3)1.8 since these two have at least one complete MoO3 shell. The 820 cm−1 band is the strongest band in the α-MoO3 data (a1g/b1g mode), but is rather ill defined in the TiO2—(MoO3)x data due to the highly distorted comer sharing octahedral arrangement in the shell. The better-defined Raman band at ˜1000 cm−1 matches the symmetric Mo═O stretch (b3u mode) in α-MoO3. The Mo═O groups in the α-MoO3 structure are located at the surface of the (010) slabs, pointing out. However, there are no other distinct α-MoO3-like bands in the TiO2—(MoO3)x Raman data, which indicates the shell does not possess any long-range crystalline order. Previous Raman studies on dispersed molybdenum oxide compounds have shown the presence of polymolybdate and pentacoordinate molybdate surface species. However, in contrast to our materials, the structural nature of the surface molybdenum oxide species were found to be highly dependent on the concentration of molybdenum oxide present and whether the samples were hydrated (i.e. exposed to ambient air) or dehydrated. The 998 cm−1 band position in α-MoO3 (M═O stretching mode) is not influenced by hydration, and this is what is observed for TiO2—(MoO3)x materials (all samples were stored and measured under ambient conditions). This strongly indicates that our new compounds have a molybdenum oxide shell structurally similar to α-MoO3, and not like MoO4 2−, Mo7O24 6−, Mo8O26 4−, or pentacoordinate species.
Catalyst Light Source/Filter % Conversion
Degussa TiO2 Hg lamp/quartz 18
Xe lamp/quartz 63
Xe lamp/Pyrex 22
Xe lamp/Pyrex and 420 nm 0.0
TiO2—(MoO3)1.8 Hg lamp/quartz 0.0
Xe lamp/quartz 25
Xe lamp/Pyrex 15
TiO2—(MoO3)0.54 Hg lamp/quartz 0.0
Xe lamp/quartz 20
Xe lamp/Pyrex 11
EXAMPLE 3 Preparation and Characterization of Mesoporous Vanadium Oxide/Nanocrystalline Titanium Dioxide Heterojunction Materials
Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O1,O2]-, ammonium salt (Tyzor LA from Dupont, 2.23M in Ti), (NH4 )6(V10O28), and ceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.) were combined in a 3 Ti:1 V:2 CTAC molar ratio. The resulting mixture was stirred while slowly adding water until irreversible precipitation was complete. The precipitate was stirred overnight at room temperature, at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor. The aged precipitate was isolated by washing and centrifuging several times with fresh aliquots of water and the CTAC was removed by calcining in air at 450° C. for 2 h. Materials with Ti:V initial ratios of 9:1, 1:1, and 0.75:1 were also be prepared in an analogous fashion.
The polyoxometallate cluster (NH4)6(V10O28) used in this synthesis was prepared by dissolving V2O5 in water while simultaneously adjusting the pH to approximately 9 with concentrated NH4OH. The pH was adjusted back to 5.5-5.6 with concentrated HCl to stablize the cluster.
EXAMPLE 4 Preparation of Mesoporous Aluminum Oxide/Nanocrystalline Titanium Dioxide Heterojunction Materials
Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O1,O2]-, ammonium salt (Tyzor LA from Dupont, 2.23M in Ti), Al13O4(OH)24Cl17, and ceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.) were combined in a 3 Ti: 1 Al:2 CTAC molar ratio. The resulting mixture was stirred while slowly adding water until irreversible precipitation was complete. The precipitate was stirred overnight at room temperature, at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor. The aged precipitate was isolated by washing and centrifuging several times with fresh aliquots of water and the CTAC was removed by calcining in air at 450° C. for 2 h. Materials with Ti:Al ratios of 9:1, 1:1, and 0.75:1 were prepared in an analogous fashion.
EXAMPLE 5 Preparation of Mesoporous Tungsten Oxide/Nanocrystalline Titanium Dioxide Heterojunction Materials
Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O1,O2]-, ammonium salt (Tyzor LA from Dupont, 2.23M in Ti), (NH4 )6(W12O39)(H2O), and ceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.) were combined in a 3 Ti: 1 W:2 CTAC molar ratio. The resulting mixture was stirred while slowly adding water until irreversible precipitation was complete. The precipitate was stirred overnight at room temperature, at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor. The aged precipitate was isolated by washing and centrifuging several times with fresh aliquots of water and the CTAC was removed by calcining in air at 450° C. for 2 h. Materials with Ti:W ratios of 9:1, 1:1, and 0.75:1 were prepared in an analogous fashion.
EXAMPLE 6 Preparation of Mesoporous Niobium Oxide/Nanocrystalline Titanium Dioxide Heterojunction Materials
Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O1,O2]-, ammonium salt (Tyzor LA from Dupont, 2.23M in Ti), K8Nb6O19, and ceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.) were combined in a 3 Ti: 1 Nb:2 CTAC molar ratio. The resulting mixture was stirred while slowly adding water until irreversible precipitation was complete. The precipitate was stirred overnight at room temperature, at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor. The aged precipitate was isolated by washing and centrifuging several times with fresh aliquots of water and the CTAC was removed by calcining in air at 450° C. for 2 h. Materials with Ti:Nb ratios of 9:1, 1:1, and 0.75:1 were prepared in an analogous fashion.
1. A heterojunction material, comprising:
a nanocrystalline titanium dioxide phase; and
a second metal oxide phase, wherein the nanocrystalline titanium dioxide phase is chemically bonded to the second metal oxide phase through a heterojunction formed at an interface between the nanocrystalline titanium dioxide phase and the second metal oxide phase.
2. The heterojunction material of claim 1, wherein the nanocrystalline titanium dioxide phase is anatase.
3. The heterojunction material of claim 1, wherein the material exhibits a photoabsorption energy lower than 3.2 eV.
4. The heterojunction material of claim 1, wherein the second metal oxide phase forms a shell around a core comprising the nanocrystalline titanium dioxide phase, the heterojunction formed at the interface between the shell and the core.
5. The heterojunction material of claim 4, wherein the shell comprises from less than a complete monolayer to one or more monolayers of the second metal oxide.
6. The heterojunction material of claim 4, wherein the shell is molybdenum oxide and the core is nanocrystalline anatase.
7. The heterojunction material of claim 6, wherein the molybdenum oxide phase has a structure substantially similar to bulk α-MoO3.
8. The heterojunction material of claim 6, wherein Mo L3-edge data for the material has a low-energy peak of greater intensity than a high-energy peak.
9. The heterojunction material of claim 6, wherein a Raman scattering signal corresponding to the M═ symmetric stretching mode is not influenced by hydration.
10. The heterojunction material of claim 1, wherein the metal oxide comprises a metal selected from the group consisting of V, W, Nb, Mo, Al and combinations thereof.
11. The heterojunction material of claim 10, wherein the metal is V, Al, W or Nb.
US09/859,799 1999-10-01 2001-05-16 Nanocrystalline heterojunction materials Expired - Fee Related US6592842B2 (en)
US41136099A true 1999-10-01 1999-10-01
US09/859,799 US6592842B2 (en) 1999-10-01 2001-05-16 Nanocrystalline heterojunction materials
US10/429,106 US6685909B2 (en) 1999-10-01 2003-05-01 Nanocrystalline heterojunction materials
US41136099A Continuation-In-Part 1999-10-01 1999-10-01
US10/429,106 Division US6685909B2 (en) 1999-10-01 2003-05-01 Nanocrystalline heterojunction materials
US20020071970A1 US20020071970A1 (en) 2002-06-13
US6592842B2 true US6592842B2 (en) 2003-07-15
ID=29254706
US09/859,799 Expired - Fee Related US6592842B2 (en) 1999-10-01 2001-05-16 Nanocrystalline heterojunction materials
US10/429,106 Expired - Fee Related US6685909B2 (en) 1999-10-01 2003-05-01 Nanocrystalline heterojunction materials
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