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Preparation and Characterization of Nanocrystalline CeO2-Tb2O3 Films Obtained by Electrochemical Deposition Method Dunlin Qu,† Fangyan Xie,‡ Hui Meng,§ Li Gong,‡ Weihong Zhang,‡ Jian Chen,*,‡ Gaoren Li,† Peng Liu,† and Yexiang Tong*,† MOE of Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Institute of Optoelectronic and Functional Composite Materials, Instrumental Analysis & Research Center, and State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China ReceiVed: August 7, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
Nanocrystalline CeO2-Tb2O3 composite oxides films with various morphologies were prepared by electrodeposition under different conditions. These samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS). The magnetic properties of the samples were also studied. By increasing the concentration of Tb(NO3)3 or current density, the CeO2-Tb2O3 films with nanobelt structure changed into a blossom-shaped structure with lots of wrinkles and large surface area. XRD and Raman spectroscopy showed the crystal sizes of the samples to be 4-7 nm. XPS analysis indicated that higher current density was favorable to the deposition of CeO2 and inhibited the formation of Tb2O3. The morphology and magnetic property of the samples were determined by the composition. Only the sample mainly composed of Tb2O3 showed ferromagnetism. 1. Introduction Recently, rare earth oxides have attracted much attention for their wide applications.1 Among them, Ceria (CeO2) is one of the most important functional materials with high mechanical strength, thermal stability, optical property, oxygen ion conductivity, and oxygen storage capacity,2,3 which gives it a wide range of applications in mechanical polishing of microelectronics, catalysts for three-way automobile exhaust systems, additives in ceramics, phosphor, magnetic field, etc.4,5 However, CeO2 used alone is sometimes not satisfactory because of its poor thermostability and oxygen vacancies.6-8 Transition metal or rare earth metal is doped into CeO2 to improve thermal stability, store and release oxygen, and compensate oxygen vacancies.9-13 Thus, a kind of mixed oxide solid compound is formed. The doped compound usually has enhanced catalytic properties and electrical properties, which depend on a series of factors such as the particle size, the structural characteristics, and the morphology, etc. Many methods, such as forced hydrolysis,14 hydrothermal synthesis, sol-gel,15 solid-state synthesis,16 combustion synthesis,17 etc., have been used for the preparation of diverse CeO2based nanomaterials. However, these methods are always complicated and usually require relatively high temperature and high pressure. Therefore, it is necessary to search for a lowcost and simple preparation method. A microemulsion method is reported in which the Ce(III) and Tb(III) nitrates were introduced in a reverse microemulsion with proper surfactant, and after being dried the mixture was calcined to get the final product.18 This is a relatively simple way to prepare the mixed oxide compound; however, there is still space to improve. For example, in this method there is no sufficient * Corresponding author. Tel.: +86 20 84113215. Fax: +86 20 84114050. E-mail:
[email protected] (J.C.);
[email protected] (Y.T.). † MOE of Key Laboratory of Bioinorganic and Synthetic Chemistry. ‡ Instrumental Analysis & Research Center. § State Key Laboratory of Optoelectronic Materials and Technologies.
way to control the morphology of the product. The electrochemical deposition is an economical and efficient route for the preparation of CeO2-based nanomaterials at room temperature. The morphology, size, and thickness of deposits can be well controlled by changing potentials, current density, salt concentrations, or additive. In this Article, CeO2-Tb2O3 nanobelts were synthesized by electrochemical depostion. By increasing the concentration of Tb(NO3)3 or current density, CeO2-Tb2O3 nanobelts were changed into blossom-shaped structures with lots of wrinkles and high surface area. The samples were characterized by XRD, RS, and XPS; finally, their magnetic properties were studied, and it was found that different content of Tb2O3 caused a big change in the magnetic property. 2. Experimental Section All chemical reagents used were analytically pure. The dehydrated Ce(NO3)3 was obtained by dehydration of Ce(NO3)3 · 6H2O in a vacuum at 343 K, and the dehydrated Tb(NO3)3 was obtained from Tb(NO3)3 · 6H2O in a vacuum at 383 K for more than 10 h. The electrochemical deposition was carried out in an aqueous solution of 0.01 mol L-1 Ce(NO3)3 and three different concentrations of Tb(NO3)3, which were 0.001, 0.005, and 0.01 mol L-1. To control the morphology of deposits, NH4NO3 (0.1 mol L-1) was added to the deposition baths as additive. All electrochemical measurements were performed on a CHI700C electrochemical workstation (CHI Instrument, Inc., USA). A simple three-electrode cell was used in this experiment. A spectral grade graphite rod with surface area of 2.0 cm2, a saturated calomel electrode (SCE), and a Cu (99.99 wt %) plate with surface area of 4 cm2 were used as auxiliary electrode, reference electrode, and working electrode, respectively. All of the electrochemical deposition experiments were carried out in a configured glass cell with a galvanostatic method, with current
10.1021/jp907628g 2010 American Chemical Society Published on Web 01/06/2010
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TABLE 1: Electrodeposition Conditions of CeO2-Tb2O3 Composite Oxides Films sample 1 2 3 4 5 6
electrolyte -1
applied current density -1
0.01 mol L Ce(NO3)3-0.001 mol L Tb(NO3)3 0.01 mol L-1 Ce(NO3)3-0.001 mol L-1 Tb(NO3)3 0.01 mol L-1 Ce(NO3)3-0.001 mol L-1 Tb(NO3)3 0.01 mol L-1 Ce(NO3)3-0.001 mol L-1 Tb(NO3)3 0.01 mol L-1 Ce(NO3)3-0.005 mol L-1 Tb(NO3)3 0.01 mol L-1 Ce(NO3)3-0.01 mol L-1 Tb(NO3)3
densities between 0.5 and 2.0 mA/cm2 at room temperature. Before electrodeposition, Cu substrates were cleaned in 0.1 mol/L HCl, distilled water, and acetone, and then rinsed with distilled water again. The detailed preparation parameters for samples 1-6 are shown in Table 1. The surface morphology of the deposits was observed by field emission SEM (FE-SEM, JSM 6330F JEOL). Powder XRD patterns of the as-synthesized samples were recorded by a D/MAX 2200 VPC diffractometer equipped with Cu KR (λ ) 1.541 Å) radiation operating at 40 kV and 40 mA for 2θ angle ranging from 25° to 80°. The crystal size of ceria oxide was determined from the peak broadening by the Scherrer equation. The Raman spectra were measured with a laser microRaman spectrometer (Renishaw inVia) at room temperature. An Ar+ laser with 514.5 nm excitation in a backscattering configuration was used to irradiate the samples. The spectral resolution was 2 cm-1. The XPS was used to determine the chemical bonding state and surface composition of the deposits. XPS spectra were acquired using an ESCA Lab 250 (Thermo VG) with 200 W Al KR radiation in twin anode, and the distance between X-ray gun and sample was about 1 cm. The analysis chamber pressure was about 2 × 10-7 Pa, and the pass energy was 20 eV for high resolution scans. The charging shifts of the spectra were calibrated using the Ce3d3/2 peak at 916.8 eV from ceria. The magnetic property measurement system (MPMS XL-7) was used to study the magnetic property of CeO2-Tb2O3 composites. 3. Results and Discussion 3.1. Surface Morphology Characterization. In Figure 1, the six samples prepared under different electrodeposition parameters show distinct morphology. The difference in morphology is caused by two factors: the concentration of Tb(NO3)3 and current density. Table 1 shows the different electrodeposition parameters of the film. Samples 1-4 are of the same concentration but of different current density, 5 is of the same current density as 1, while at 5 times concentration of Tb(NO3)3, and 6 is of the same current density as 4, while at 10 times concentration of Tb(NO3)3. Sample 1 has the structure of nanobelts, with the increase of applied current density, the size of branch-shaped structure in sample 2 decreases as compared to sample 1 until becoming twig-shaped in sample 3, and eventually changes into a tendril-shaped structure in sample 4 (Figure 1a-d). Obviously with the increase of current density the belts become thinner and smaller until they turn into a tendril. The mechanism of the formation of CeO2-Tb2O3 mixed oxide solid compound is proposed as follows. NO3- ions are first electroreduced to get OH- ions, which will react with Ce3+ and O2 to form CeO2. Because of the Ce(III) oxidation state being unstable as compared to the Ce(IV) oxidation state in the presence of O2, Ce3+/Ce4+ oxidation is favored in neutral and alkaline solution, and conversion from Ce(OH)3 to Ce(OH)4 (CeO2) can be realized facilely. Finally, the Ce(OH)4 is dehydrated to form CeO2. The whole process for the formation of CeO2 can be described as eq 2. At the same time, Tb3+ also
-2
0.5 mA cm 1.0 mA cm-2 1.5 mA cm-2 2.0 mA cm-2 0.5 mA cm-2 2.0 mA cm-2
pH (before and after) 4.71/7.38 4.71/7.09 4.71/7.06 4.71/7.11 4.70/7.15 4.62/7.43
reacts with OH- to get Tb2O3. Generally, the Tb(OH)3 could be further oxidized to obtain TbO2 in the presence of O2. This conversion via electrochemical deposition has been realized by our previous study.19 However, in this case, the oxidation of Ce3+ is more favorable than that of Tb3+, because the Ce3+ has a lower oxidation energy with respect to Tb3+ while they are of cubic structure.20 On the other hand, the concentration of Tb3+ is equal to or smaller than that of Ce3+. These processes can be expressed as follows: NO3 + H2O + 2e f NO2 + 2OH
(1)
4Ce3+ + 12OH- + O2 f 4CeO2 + 6H2O
(2)
2Tb3+ + 6OH- f Tb2O3 + 3H2O
(3)
To better understand the reaction process, another experiment was carried out. The details are described as follows: the electrolyte was purged with nitrogen (99.999%) for 1 h before electrodeposition. The sample was kept in absolute ethanol (purged with nitrogen for 1 h) after electrodeposition. From the XRD pattern of this sample (Figure S1), it is noted that a spot of Ce(OH)3 existed in the sample. This result may be attributed to the reduction of dissolved oxygen. In our case, O2 will generate continuously at the anode via eq 4.
Figure 1. SEM images of the nanocrystalline CeO2 films of samples 1-6 (a-f).
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(4)
The produced O2 will increase the concentration of dissolved oxygen of the electrolyte. It is difficult to totally eliminate the dissolved oxygen. As a result, the conversion from Ce(OH)3 to Ce(OH)4 (CeO2) is still favorable, although the electrolyte was purged with nitrogen, and the Ce(OH)3 and CeO2 will be obtained together when the dissolved oxygen is not enough. When the concentration of Tb(NO3)3 is increased to 0.005 and 0.01 M, the morphology of the prepared film is very different from that with lower concentration of 0.001 M. The blossom-shaped structures with round-edge (Figure 1e) or sharpedge (Figure 1f) are observed. From the SEM results, one conclusion can be reached: the concentration of Tb(NO3)3 and current density are two main factors that control the morphology of the CeO2-Tb2O3 composites. Increasing current density or the concentration of Tb(NO3)3, the structure of nanobelts changed into blossom-shaped structures. According to the proposed mechanism, OH- plays a very important role in the process. So this reaction is pH sensitive.Yet in the reaction, OH- is produced by the electroreduction of NO-3 , and then it is consumed in the following reaction where CeO2 and Tb2O3 are formed. The formation of Tb2O3 is especially sensitive to OH-, and the reaction will continue until the depletion of OH-, which means the starting solution should not be alkaline. According to our test, the pH value (Table 1) of the starting solution, which is the mixture of Ce(NO3)3, Tb(NO3)3, and NH4NO3, is about 4.7. After the electrodeposition, the pH of the solution changed into 7.06-7.43, which is caused by the unreacted OH-. The tested pH values are in well accordance with our proposed mechanism. 3.2. XRD Analysis. Figure 2 shows the XRD patterns of the prepared samples. The distinct fluorite oxide diffraction pattern of CeO2 (JCPDS 34-0394) and the strong diffraction peaks of Cu substrate (JCPDS 89-2838) were detected in all six samples. The positions of the strong diffraction peaks of Tb2O3 (JCPDS 86-2478, 2θ: 28.8°, 47.9°, 56.88°) are similar to those of CeO2 (2θ: 28.55°, 47.48°, 56.34°), so it is difficult to distinguish CeO2 and Tb2O3 in the CeO2-Tb2O3 composite oxide by analyzing XRD data. However, the broad and asymmetric XRD peaks indicate the nanosized structure of the films. According to the Scherrer equation, the crystalline sizes of the six samples 1-6 are 4.4, 4.7, 5.6, 6.1, 7.2, and 6.4 nm, respectively. Studying samples 1-4, according to SEM results with the increase of the current density applied, the products
Figure 2. XRD patterns of the nanocrystalline CeO2 films.
Figure 3. Raman spectra of samples 1-6.
changed from nanobelts into blossom structure, and the thickness and size of the nanobelts became thinner and smaller. XRD results revealed that the crystalline sizes of the sample were increasing with the increase of the current density. Combining these two points, a conclusion is reached: at high current density, the product tends to have smaller macroscopical structure than with bigger crystalline size. This may be explained by the mechanism of the electrodeposition. According to eqs 1-3, during the deposition, electron transfers between the substrate and the potentiostat, and the transfer rate and amount are determined by the current density applied. At high current density, there will be excessive electrons available, while the concentration of the precursor is quite low. So at high current density, the reaction takes place faster and more intense, leading to bigger crystalline size. 3.3. Raman Spectra and Analysis. Raman spectroscopy has been demonstrated in many cases to be an effective tool for investigating structural changes in solids depending on external parameters. As for cerium dioxide nanomaterial, Weber et al.,21 Kosacki et al.,22 and Spanier et al.23 have reported on the Raman scattering correlation with the change in particle size. Rekhi et al.24 studied high-pressure Raman spectroscopy on nanocrystalline CeO2 up to 36 GPa and found that the transition pressure was less than that for the bulk. CeO2 has a fluorite-structure with Oh5 space group. It gives one first-order Raman line F2g and possible 45 s order Raman modes.21 Typical Raman spectra of CeOx are shown in Figure 3. Their Raman shifts are 268, 460, 553, 616, and 739 cm-1, respectively. Besides a few second-order peaks of 268 cm-1 (2TA), 553 cm-1 (2LA), and 616 cm-1 (2TO), and a peak of 739 cm-1, which is attributed to precursor nitrate, the F2g mode that is assigned to the Ce-O8 symmetry stretching mode is shown at 460 cm-1. However, for sample 5, there is no Raman band in the range from 200 to 1000 cm-1, indicating no CeOx exists in the sample. Sample 5 was electrodeposited at 0.5 mA cm-2 current density in 0.01 mol L-1 Ce(NO3)3 and 0.005 mol L-1 Tb(NO3)3 electrolyte. For this sample, the concentration of the Tb precursor is very high and was deposited at lower current density. The nonexistence of Raman signal of CeOx for this sample indicates that in this sample the percent of CeOx is too low to be detected by Raman. This is in accordance with the XPS results in the following discussion. As compared to that in bulk, the peak shifts 5 cm-1 to lower frequency and broadens asymmetrically with a low-frequency tail. The change of the 460 cm-1 peak indicates phonon confinement effect of the nanocrystalline size, inhomogeneous strain by dispersion in lattice constants, and the presence of oxygen vacancies corre-
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Figure 4. Ce 3d XPS spectra of samples 1-6 and CeO2 powder. Figure 5. Tb 4d XPS spectra of films 1-6, Tb(NO3)3, and Tb4O7.
sponding to a nonstoichiometric CeO2-x. According to the relationship between the half-width (Γ) of the Raman line and the grain size (dg) described as formula Γ (cm-1) ) 10 + 124.7/ dg (nm),22 we obtained the CeO2-x nanograin size of 4.3, 3.8, 4.1, 5.2, -, and 2.8 nm for samples 1-6, respectively. 3.4. XPS Spectra and Analysis. XPS was employed to characterize the chemical states of Ce and Tb in the deposited oxides. In Figure 4, there are six peaks labeled as vo, v1, v2 (3d5/2), v′o, v′1, and v′2 (3d3/2), referring to three pairs of spin-orbit doublets in the Ce 3d XPS spectrum of CeO2 powder, which indicate that the valence state of Ce is Ce4+. The high binding energy doublet v2/v′2 at 898.2 and 916.8 eV corresponds to the final state of Ce(IV)3d94f°O2p6, doublet v1/v′1 at 888.9 and 907.5 eV is related to the state of Ce(IV)3d94f1O2p5, and doublet v0/v′0 at 882.3 and 901.0 eV originated from the final state of Ce(IV)3d94f2O2p4.25 The Ce 3d spectra of samples 1-6 are similar to that of CeO2, indicating the chemical state of Ce is CeO2. For terbium, the most intense core level is Tb 3d at the high binding energy between 1230 and 1290 eV. For Tb 3d, it is difficult to distinguish accurately the chemical states of terbium with the commercial X-ray Al KR source (1486.6 eV). This is because the photoemitted electrons with very low kinetic energies are easily influenced by the inelastic scattering in the sample. So the next most intense core level Tb 4d is used to study the terbium oxidation states using the commercial XPS spectrometer. Although the Tb 4d spectra are quite complex, the spectra of Tb(III)(NO3)3 and Tb4O7 (Tb(III) and Tb(IV) coexist in Tb4O7) are different as shown in Figure 5. In addition, the peak at 164 eV in the spectrum of Tb4O7 is attributed to the Tb(IV). The shape of Tb 4d XPS spectrum of sample 5 containing the highest amount of Tb is similar to that of Tb(NO3)3. However, the peak position at 149 eV of sample 5 is more negative than the peak at 151 eV of Tb(NO3)3 due to the stronger electronegativity of nitrate ion than O atom. Furthermore, there is no peak at 164 eV, indicating no Tb(IV) exists in sample 5. From the above analysis, the chemical state of sample 5 is Tb2O3. The Tb 4d XPS spectra of the other deposited films except sample 2 are the same as that of sample 5, demonstrating the chemical state of Tb in the most films is Tb2O3. To compare the chemical components in different samples, the Tb:Ce ratio is shown in Table 2. For samples 1-4, the concentrations of Ce(NO3)3 and Tb(NO3)3 in electrolyte are 0.01 and 0.001 mol L-1, respectively, and the used current density is from 0.5 to 2.0 mA cm-2. The main composition of samples 1-4 is CeO2 due to the lower percentage content of Tb in the
TABLE 2: Tb:Ce Ratio in Different Films sample Tb:Ce ratio
1 0.27
2 0.13
3 0.11
4 0.09
5 5.56
6 0.17
mixed oxide. When the current density is increased, the ratio of Tb:Ce in the mixed oxide decreases, which indicates that the large current density facilitates the deposition of CeO2 and the small current density is suitable for the deposition of Tb2O3. Sample 5 was electrodeposited at 0.5 mA cm-2 current density in 0.01 mol L-1 Ce(NO3)3 and 0.005 mol L-1 Tb(NO3)3 electrolyte. The major constituent of sample 5 is Tb2O3 due to the high Tb:Ce ratio (5.56). Sample 6 is deposited in 0.01 mol L-1 Ce(NO3)3 and 0.01 mol L-1 Tb(NO3)3 solution at 2.0 mA cm-2 current density. As compared to sample 4, the ratio of Tb:Ce in sample 6 slightly grows from 0.09 to 0.17, although the content of Tb(NO3)3 is increased by 10 times. This again proves the large current density is unsuitable for the electrodeposition of Tb2O3. From the above analysis, we can conclude that Tb2O3 is easily electrodeposited at low current density with high concentration of Tb(NO3)3. As shown in SEM, the morphologies of samples 1-4 and 6 are nanobelts; nevertheless, that of sample 5 is a blossom-shaped structure. The difference in morphology between sample 5 and other samples may be caused by the difference in composition in the samples. 3.5. Magnetic Property Measurements. The results of magnetic measurements: magnetization (M) versus magnetic field (H) measured at 5 K for nanocrystalline CeO2-Tb2O3 mixed oxides are shown in Figure 6. As is evident from Figure 6, samples 1-3 demonstrate paramagnetism, while sample 5 shows ferromagnetism. A clear hysteresis loop for H > 25 kOe is observed in sample 5 with a coercive field HC ≈ 1 kOe. The saturation magnetization of this film is ∼0.04 emu/g measured from the M-H curve. Experimental data display significant coercivity and remanence. The results demonstrate that room temperature ferromagnetic ordering exists in sample 5. Generally speaking, the stoichiometric CeO2 should be nonmagnetic; however, the mixed oxide solid compound prepared here showed paramagnetism or ferromagnetism. In the electrodeposition process, the incomplete crystallization may generate various structural defects, such as oxygen vacancies, Ce vacancies, or interstitials. These defects may bring changes to the electronic structures or quantum confinement effects. This may be the origin of the magnetic property of the mixed oxide. The different magnetic property between samples 1-3 and sample 5 is due to the main composition of the samples. As
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Figure 6. The magnetization (M) versus magnetic field (H) at 5 K for nanocrystalline CeO2-Tb2O3 films.
shown in the XPS analysis, the main components of samples 1-3 are CeO2, while sample 5 mainly contains Tb2O3. As we know, Ce(IV) in a compound shows nonparamagnetism because there is no unpaired electron. For CeO2, there is interaction of multielectrons between Ce 4f and O 2p such as the final states Ce(IV)3d94f2O2p4, Ce(IV)3d94f1O2p5 indicated by XPS spectra in Figure 4. Thus, the unpaired electrons exist in CeO2, resulting in paramagnetism. For Tb2O3, because the electronic configuration of Tb(III) is 4f8, there are six unpaired electrons, which lead to ferromagnetism. In summary, in the electrodeposition of the CeO2-Tb2O3 composite oxides films, different parameters are studied: for samples 1-4, the ratio of precursors remains constant and the current density is gradually increased, while sample 5 is prepared at the same current density as 1 except that the concentration of Tb(NO3)3 is 5 times larger, and sample 6 is at the same current density as samples 1-4, the width and thickness of the belt structure decreased with the increase of current density, and the morphology of sample 5 was very different from that of 1-4. With XRD one cannot distinguish CeO2 and Tb2O3 in the film, but the broad and asymmetric XRD peaks indicate the nanosized structure of the films. Judging from the Raman shift of 460 cm-1, there is no CeOx in sample 5, while for other samples there exists nonstoichiometric CeO2-x. The XPS spectrum shows the chemical state of Ce in the film is CeO2, and the chemical state of Tb in the most films is Tb2O3. Further analyzing the XPS data gives the Tb:Ce ratio for samples 1-6, which leads to another conclusion: the large current density facilitates the deposition of CeO2, and the small current density is suitable for the deposition of Tb2O3. Magnetization study
shows that sample 5 is ferromagnetic, while samples 1-3 demonstrate paramagnetism; this is because the main component of samples 1-3 is CeO2, while sample 5 is mainly Tb2O3. 4. Conclusions By controlling different electrochemical deposition conditions, six CeO2-Tb2O3 composite oxides samples were electrodeposited; the nanocrystalline sizes of the prepared samples are 4-7 nm as indicated by XRD and Raman analysis. Increasing the current density is favorable to the deposition of CeO2 and inhibits the formation of Tb2O3. The different composition in the samples determines the morphology and magnetic property. CeO2 is the main component in samples 1-4 and 6, resulting in nanobelt-morphology and paramagetism property. However, sample 5 has a blossom-shaped structure, showing ferromagnetism due to the main composition of Tb2O3. Acknowledgment. We appreciate the financial support from the National Natural Science Foundation of China (Grant Nos. 20573136, J0730420, and 50572123, 20873184 and 90923008), and the Natural Science Foundation of Guangdong Province (Grant No. 9251027501000002) and Guangdong Province Science and Technology Development Program (Grant No.2008B010600040). Supporting Information Available: XRD pattern of the sample prepared in the electrolyte purged with nitrogen for 1 h. This material is available free of charge via the Internet at http:// pubs.acs.org.
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