General Surface Modification Method for Nanospheres via Tannic Acid

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A General Surface Modification Method for Nanospheres via Tannic AcidFe Layer-by-Layer Deposition: Preparation of a Magnetic Nanocatalyst Dongdong Li, Xun Xu, Xingwei Wang, Rui Li, Chao Cai, Tongbing Sun, Yiping Zhao, Li Chen, Jian Xu, and Ning Zhao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00477 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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ACS Applied Nano Materials

A General Surface Modification Method for Nanospheres via Tannic Acid-Fe Layer-by-Layer Deposition: Preparation of a Magnetic Nanocatalyst Dongdong Li,a,

d

Xun Xu,b Xingwei Wang,b Rui Li,a,

d

Chao Cai,a Tongbing Sun,a

Yiping Zhao,*c Li Chen,c Jian Xua, d and Ning Zhao*a a Beijing

National Laboratory for Molecular Sciences, Laboratory of Polymer Physics

and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. b

State Key Laboratory of Polyolefins and Catalysis, Shanghai Key Laboratory of

Catalysis Technology for Polyolefins, Shanghai Research Institute of Chemical Industry Co., Ltd., Shanghai, 200062, P. R. China. c

School of Materials Science and Engineering, Tianjin Polytechnic University,

Tianjin 300387, P. R. China. d University

of Chinese Academy of Sciences, Beijing 100049, P. R. China.

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ABSTRACT: In this paper, a tannic acid-Fe layer-by-layer (TA-Fe LbL) modification process was developed to modify the highly curved surfaces of nanospheres. In contrast to the traditional TA-Fe one-step assembly method, TA-Fe coordination complexes were uniformly coated onto the surface of Fe3O4@SiO2 nanospheres by the TA-Fe LbL modification process, with no TA-Fe aggregates forming on the nanospheres or in the reaction solution. By virtue of the reduction capability and effective immobilizing effect of TA, Ag nanoparticles (NPs) with small sizes (2~10 nm in diameter) and a narrow size distribution were formed in situ on the surface of the modified nanospheres. The resultant nanocomposites exhibited excellent catalytic performance and good stability for the reduction of 4-nitrophenol. This strategy can be extended to modify various highly curved surfaces to fabricate a variety of functional nanocomposites. KEYWORD: catalyst, coordination, LbL, Ag nanoparticles, tannic acid


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INTRODUCTION The development of nanotechnology has enabled the application of nanomaterials in catalytic fields. Among these nanomaterial catalysts, noble metal nanoparticles (NMPs), such as gold, silver, platinum, and palladium, have displayed unexpectedly high catalytic activities due to their large specific surface areas and high surface atomic activity1-6. However, the aggregation of NMPs and the difficulty of separating NMPs from reaction systems have greatly hinder their practical application. A worthwhile synthetic challenge, therefore, is the fabrication of catalysts with uniformly dispersed and easily recoverable NMPs. Loading NMPs onto solid substrates is an effective way to solve these problems, and a suitable substrate would have the following properties: a strong ability to immobilize NMPs, excellent chemical and mechanical stabilities to improve the durability of the catalyst in the reaction environment, and a high specific surface area to provide more space for NMP loading. Recently, magnetically separable substrates, such as Fe3O47-10, Ni11, Fe12 and FexOy13, have received extensive interest. Immobilizing NMPs on magnetic substrates can not only improve their dispersity but also enable magnetically separating the catalyst, thereby facilitating the recycling and reuse of NMPs. However, the lack of functional groups and the instability of magnetic substrates limit their practical application. To solve these problems, magnetic substrates have been encapsulated within inorganic or organic layers or functionalized with active functional groups to improve their comprehensive performance14-16. Tannic acid (TA) is a polyphenolic compound that is widely distributed in nature. The abundant phenolic hydroxyl groups endow TA with anti-oxidation and metal ion chelating ability17-18. TA also exhibits strong reducing capabilities under alkaline conditions, which makes it a competitive green reducing agent for fabricating NMPs19-21. Caruso and coworkers17 developed a one-step assembly strategy for the preparation of robust coatings comprising TA-Fe coordination complexes on a range of substrates. However, forming homogeneous TA-Fe coatings on the highly curved surface of micro/nano particles by a one-step method was difficult. Large coordination complex aggregates were formed and adhered to the particle surfaces,



and the modified particles easily agglomerated.22-26 Herein, we describe an effective encapsulation-functionalization strategy for the preparation of high-performance magnetic Ag nanocomposite catalysts. A new tannic acid-Fe layer-by-layer (TA-Fe LbL) modification process was developed. We find that the layer-by-layer deposition of Fe3+ and TA can effectively eliminate the formation of coordination-complex aggregates and reduce the agglomeration of the modified nanospheres. In addition, the resultant coordination-complex coatings on the highly curved surfaces of the nanospheres are extremely uniform, which is beneficial for the formation of Ag nanoparticles (NPs) with small particle sizes and a narrow size distribution. The as-synthesized magnetic Ag nanocomposite catalysts exhibit excellent catalytic activity toward the reduction of 4-nitrophenol due to their elaborate structure and the enrichment of 4-nitrophenol by TA. Furthermore, the magnetic core enables the convenient recovery of the catalyst, and the catalytic activity is well maintained after five reuse cycles. This method for the continuous layer-by-layer deposition of TA and Fe3+ has never been reported before, and it provides a versatile approach for uniformly modifying the highly curved surfaces of micro/nanoscale particles. Furthermore, the facile and green characteristics of the TA modification in water at room temperature and the reducing strength, metal ion chelating ability and general surface binding affinity of TA simplify the reduction of NMPs and improve the stability of the loaded nanoparticles.

EXPERIMENTAL SECTION Materials. Tannic acid (ACS grade), AgNO3 (ACS grade), 4-nitrophenol (99%) and sodium borohydride (98%) were purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS, 98%) and FeCl3 (97%) were purchased from Sigma Aldrich. Urea (analytical grade) and sodium citrate (analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd. Aqua ammonia (25 wt%) and ethylene glycol were obtained from Beijing Chemical Works. All of the materials were used as received. Deionized water was used throughout. Preparation of the Fe3O4 nanospheres. The magnetic nanospheres were obtained


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via a modified solvothermal method. Typically, 2.025 g of FeCl3 and 4.5 g of urea were dissolved in 60 mL of ethylene glycol under vigorous sonication for 30 min at room temperature until the solids had dissolved. Then, 0.075 g of sodium citrate was added, and the solution was sonicated for an additional 30 min. The obtained solution was transferred to and sealed in a 100 mL Teflon-lined stainless-steel autoclave and then heated to 200 °C for 10 h. After cooling to room temperature, the black powder was collected via a magnet and washed twice each with water and ethanol. The Fe3O4 nanospheres were obtained after drying at 60 °C for 5 h. Synthesis of the Fe3O4@SiO2 nanospheres. The Fe3O4 nanospheres (100 mg) were dispersed in a mixture of 10 mL of water, 172 mL of ethanol and 8 mL of an aqueous ammonia solution under vigorous sonication. After 30 min, 2 mL of TEOS and 8 mL of ethanol were mixed and added dropwise to the reaction under strong mechanical stirring. After stirring for 6 h, the final products were collected with a magnet, washed with water several times, and dried at 60 °C for 3 h. Modification of the Fe3O4@SiO2 nanospheres. Layer-by-layer deposition of Fe3+ and TA on the Fe3O4@SiO2 surface: Fe3O4@SiO2 nanospheres (10 mg) were dispersed in 20 mL of water. Then, 200 μL of an FeCl3 solution (10 mg/mL in water) was added, and the solution was vigorously stirred for 1 min. The products were collected with a magnet, washed twice with water, and redispersed in 20 mL of water. Then, 200 μL of a TA solution (40 mg/mL in water) was added and vigorously stirred for 1 min. After being collected with a magnet and washed twice with water, the product, labeled Fe3O4@SiO2@(TA-Fe)1, was obtained. Repeating the above modification process with Fe3O4@SiO2@(TA-Fe)1 as the starting material would lead to the formation of Fe3O4@SiO2@(TA-Fe)n. After each deposition step, the cleaned samples were redispersed in deionized water (0.1 mg/mL) for zeta potential testing. One-step modification of Fe3O4@SiO2 nanospheres: Fe3O4@SiO2 nanospheres (10 mg) were dispersed in 20 mL of water; then, 200 μL of an FeCl3 solution (10 mg/mL in water) was added and vigorously stirred for 1 min. Then, 200 μL of a TA solution (40 mg/mL in water) was added and vigorously stirred for an additional 1 min. The



products were collected with a magnet, washed several times with water, dried at 60 °C for 3 h and labeled as Fe3O4@SiO2@TA-Fe. In situ formation of the Ag NPs. Different amounts of a freshly prepared Ag(NH3)2OH solution (10, 20, 30 and 40 μL) were added to 10 mL dispersions of the nanospheres (0.1 mg/mL in water), and the suspension was stirred for 1 h. The resultant nanocomposites, labeled Ag-10, Ag-20, Ag-30 and Ag-40 according to the corresponding Ag(NH3)2OH solution that was used, were collected with a magnet and washed several times with water. Catalytic performance. A NaBH4 solution (0.75 mL, 0.4 M in water) and a 4-nitrophenol solution (0.75 mL, 0.4 mM in water) were added to 1.47 mL of deionized water in a quartz cuvette. Then, 30 μL of a nanocomposite dispersion (0.3 mg/mL in water) was added. A UV-Vis spectrophotometer was used to monitor the variation in the 4-nitrophenol concentration every 2 min. Characterization. Scanning electron microscopy (SEM) and the energy dispersive X-ray spectrometry (EDS) measurements were performed on JSM-7500F (JEOL, Japan) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) measurements were performed on HT7700 and JEM-2100F. Powder X-ray diffraction (XRD) patterns were collected on an Empyrean X-ray diffractometer using Cu Kα radiation. Zeta potential values were measured by using an nanoparticle size and zeta potential analyzer (Malvern Zetasizer Nano series). The magnetic properties were obtained on a physical property measurement system (PPMS-9, Quantum Design, USA) at 298 K in a magnetic field from -1.0 T to +1.0 T. UV-vis absorption spectra were measured on a TU1901 spectrophotometer. The inductively coupled plasma mass spectrometry (ICP-MS) was detected by using 7700 series ICP-MS (Agilent Technologies). Size distribution of Ag NPs was obtained by particle size analysis software (Nano Measurer), and 100 particles were counted.

RESULTS AND DISCUSSION The fabrication process is schematically shown in Figure 1. The prepared Fe3O4 nanospheres were coated with SiO2 by a modified Stöber method. The SiO2 shell not


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only increased the chemical stability and acid resistance of the magnetic core but also resulted in the nanospheres being negatively charged. Therefore, Fe3+ could be adsorbed onto the Fe3O4@SiO2 nanospheres when Fe3+ was added to the dispersion. Excess Fe3+ could be removed by washing the product with water. Then, TA was chelated by the Fe3+ layer when TA was added to the solution. Repeating this process led to the formation of a uniform TA-Fe coordination-complex coating on the SiO2 shell. Finally, when Ag(NH3)2OH was added to the solution, Ag(NH3)2+ was adsorbed onto the TA layer through static and coordination interactions and then in situ reduced into Ag NPs on the surface of the nanospheres due to the reduction ability of the catechol moieties in TA. The homogeneous reduction environment provided by the uniform TA-Fe coating and the effective immobilizing effect of TA favored the formation of small Ag NPs with a narrow size distribution19. Because of their higher specific surface area and more negative redox potential, smaller Ag NPs are more beneficial than larger Ag NPs for electron transfer from the Ag surface to the catalytic reactants27-28.

Figure 1. Synthetic strategy for the core-shell magnetic nanocatalyst.



As shown in Figure 2a, the Fe3O4 nanospheres possessed a uniform size of approximately 420 nm in diameter and a rough surface decorated with granular protrusions (inset in Figure 2a). After SiO2 encapsulation, a uniform layer of SiO2 (approximately 20 nm thick) coated the surface of the Fe3O4 nanospheres, and the obtained Fe3O4@SiO2 nanospheres had a smooth surface (Figure 2b). Figure 2c shows the change in zeta potential during the modification process. Because of the negatively charged Si-OH groups in the shell, the zeta potential of the Fe3O4@SiO2 nanospheres was -26.3 mV. This value increased to -11.9 mV after the adsorption of Fe3+. After chelation with TA, the zeta potential decreased to -28.2 mV due to the acidic nature of the galloyl groups in TA17. The zeta potential increased to -10.7 mV after the second layer of Fe3+ was deposited. The zeta potential changes clearly showed the layer-by-layer assembly of the TA-Fe coating. The surface morphology of the nanospheres did not significantly change after being modified with TA-Fe and the final size of the obtained Fe3O4@SiO2@(TA-Fe)4 nanospheres was approximately 470 nm (Figure 2d). The TEM image, inset in Figure 2d, shows the existence of a thin coating layer of TA-Fe.


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Figure 2. SEM and TEM images of (a) Fe3O4 and (b) Fe3O4@SiO2. (c) Variation of the zeta potential values for each experimental step. (d) SEM image of Fe3O4@SiO2@(TA-Fe)4. The inset images are high-magnification TEM images.

Typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Ag-30 are shown in Figure 3a and b. The Ag NPs formed in situ on the surface of the Fe3O4@SiO2 nanospheres were uniform, compact and nearly spherical with diameters of 2~10 nm (Figure 3e). The high-resolution TEM (HRTEM) image (Figure 3c) indicates that the interlattice spacing of the Ag (111) facet was approximately 0.23 nm. The irregular diffraction points in the selected area electron diffraction (SAED) pattern imply that the reduced Ag NPs possessed a polycrystalline structure (Figure 3d).

Figure 3. (a) SEM, (b) TEM and (c) HRTEM images of Ag-30; (d) SAED pattern of Ag-30; and (e) particle size distribution of the Ag NPs of Ag-30.

We found that the morphology of the Ag NPs could be adjusted by changing the



concentration of Ag(NH3)2OH in the reaction system. Both the number and size of Ag NPs increased with increasing Ag(NH3)2OH concentration (Figure 4a~c). When the concentration was too high, many large Ag NPs formed on the surface of Ag-40, which decreased the specific surface area of the Ag NPs (Figure 4d).

Figure 4. SEM and TEM images of (a) Ag-10, (b) Ag-20, (c) Ag-30 and (d) Ag-40.

Figure 5a shows the wide-angle powder X-ray diffraction (XRD) patterns of Fe3O4, Fe3O4@SiO2 and Ag-30. Typical diffraction peaks appeared at 18.1, 30.1, 35.5, 43.1, 56.9, 62.7 and 74.6° corresponding to the (111), (220), (311), (400), (511), (440) and (533) Bragg diffraction peaks of the cubic lattice of the Fe3O4 nanospheres (JCPDS no. 19-0629). For the Fe3O4@SiO2 sample, a new broad peak at approximately 24.0° indicated the presence of the SiO2 shells. Following the formation of the Ag NPs, four new peaks appeared at 38.1, 44.5, 64.7 and 77.8°, which were assigned to the (111), (200), (220) and (311) crystal facets of the face-centered cubic (fcc) Ag structure (JCPDS no. 04-0783). The composition and distribution of the elements in Ag-30 were surveyed by energy dispersive X-ray spectrometry (EDS). As shown in Figure S1, Fe, O, Si, C, and Ag were detected, and the uniform yellow color illustrated the


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good distribution of the abundant Ag NPs on the nanosphere surface. The XPS spectra (Figure 5b) of Fe3O4@SiO2@(TA-Fe)4 and Ag-30 contained Fe 2p, O 1s, C 1s and Si 2p signals. In the high-resolution Fe 2p spectrum (Figure S2a), the three peaks at approximately 710.5, 712.5 and 723.5 eV were assigned to Fe 2p3/2, Fe3+ and Fe 2p1/2, respectively12. Since the thickness of the SiO2 layer was approximately 20 nm, the signal of Fe 2p could mainly be attributed to the TA-Fe coordination complexes on the nanoparticle surface, confirming the occurrence of a complexation reaction. A strong Ag 3d signal at approximately 368 eV was visible in the Ag-30 spectrum (Figure 5b). In the high-resolution Ag 3d spectrum, the two peaks at approximately 368 and 374 eV were assigned to Ag 3d5/2 and Ag 3d3/2, respectively, indicating the presence of Ag in the metallic state21. The inductively coupled plasma mass spectrometry (ICP-MS) results showed that the mass content of Ag in Ag-30 was 11.0 ± 0.5 wt% (1.02 ± 0.05 mmol/g).

Figure 5. (a) XRD patterns of Fe3O4, Fe3O4@SiO2 and Ag-30. (b) XPS spectra of Fe3O4@SiO2@(TA-Fe)4 and Ag-30 and the high-resolution Ag 3d spectrum of Ag-30.



For modifying nanospheres, forming a homogeneous TA-Fe coating on the highly curved surface by the previously reported common assembly method is difficult22-26. As a control experiment, the TA-Fe coating formed on Fe3O4@SiO2 nanospheres by the one-step assembly method was very rough, with large coordination-complex aggregates adhering to the particle surface (Figure 6a). When using these nanospheres, labeled Fe3O4@SiO2@TA-Fe, irregular Ag NPs with large sizes and severe aggregation were obtained under the same reaction conditions (Figure 6b). Compared with the results in Figure 3b, the results here further indicated the importance of a uniform TA-Fe coating for the formation of small Ag NPs with a narrow size distribution. Moreover, massive TA-Fe aggregates were found in the reaction system after the one-step assembly method. Many large Ag NPs were also formed on the surface of these aggregates during the subsequent reduction reaction (Figure 6c). Previous study found that for multiple TA-Fe one-step modification reactions, smoother films can be obtained by adding TA first in each modification step rather than adding Fe3+ first26. However, coordination-complex aggregates are still inevitably produced. Based on the abovementioned results, the difference between the TA-Fe one-step assembly and LbL assembly modification methods is schematically illustrated in Figure 6d. In the TA-Fe one-step assembly process, the coordination reaction occurs not only on the template surface but also in the reaction solution. The rapid reaction rate (completed within seconds) results in the coordination-complex aggregates easily forming within the system. The adhesion of the aggregates to the nanospheres impairs the homogeneity of the coating. In contrast, the continuous monolayer adsorption of Fe3+ and TA ensures the uniformity of the adsorbed layers and avoids the formation of aggregates. In addition, for each deposition step, the surface composition of the modified particles is the same, which ensures the uniform dispersion of the particles and prevents their aggregation. A previous study confirmed that multistep TA-Fe coatings exhibited higher stabilities than that of one-step coatings29, which is preferable for improving the stabilities of the modified materials.


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Figure 6. TEM images of (a) the Fe3O4@SiO2@TA-Fe prepared by the one-step assembly method and (b) the corresponding Fe3O4@SiO2@TA-Fe@Ag NPs. (c) TEM image of the in situ reduced Ag NPs on the TA-Fe aggregates that formed in the reaction system. (d) Illustration showing the one-step and layer-by-layer coating processes.

The application of the Fe3O4@SiO2@(TA-Fe)4@Ag NPs to the reduction of 4-nitrophenol was demonstrated. The reduction process was monitored by using a UV-Vis spectrophotometer. During the reaction, the absorption peak of 4-nitrophenol at 400 nm rapidly disappeared, and the absorption peak of 4-aminophenol appeared at approximately 300 nm (Figure 7a). The reduction reaction could be completed within 32, 19, 6 and 14 min by using Ag-10, Ag-20, Ag-30 and Ag-40, respectively, as the catalysts (Figure 7c). The reason for the gradual increase in the catalytic activity was the increased loading of the Ag NPs (Figure 4a, b and c); similar observations have been reported previously20, 30. For Ag-40, the catalytic activity decreased because the size of the formed Ag NPs was too large, decreasing the total specific surface area.5



As the concentration of NaBH4 was substantially greater than the concentration of 4-nitrophenol (1000:1), the reduction rate could be assumed to be independent of the NaBH4 concentration. Thus, the reduction could be considered a pseudo-first-order reaction with regard to only the 4-nitrophenol concentration and can be described by the following equation: Ln(Ct/C0) = -kappt Here, the apparent rate constant kapp was determined from a linear plot of Ln(Ct/C0) versus the reaction time t, where Ct and C0 were the 4-nitrophenol concentrations at time t and time 0, respectively. As shown in Figure 7b, kapp was calculated to be 0.058, 0.113, 0.431 and 0.149 min-1 for the Ag-10, Ag-20, Ag-30 and Ag-40 catalysts, respectively, revealing that Ag-30 had the best catalytic activity.

Figure 7. (a) UV-Vis absorption spectra for the reduction of 4-nitrophenol with Ag-30 as the catalyst. (b) Plots of Ln(Ct/C0) versus reaction time for the reduction of 4-nitrophenol with Ag-10, Ag-20, Ag-30 and Ag-40 as the catalysts. (c) The change in absorbance at 400 nm as a function of the reaction time using the different catalysts. (d) Magnetization curves of Fe3O4 and Ag-30. Inset photos show the magnetic separation of Ag-30 with an external magnetic field. (e) Successive reduction reactions of 4-nitrophenol using Ag-30 as the catalyst.


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To compare the catalytic performance of Ag-30 with that of previously reported noble metal hybrid catalysts used for 4-nitrophenol reduction, kapp was normalized to the mass concentration (M/V) of catalyst (k’, k’ = kapp·M-1·V). As shown in Table 120, 31-39,

k’ of Ag-30 was calculated to be 143 mL/mg·min, showing a competitive

catalytic activity. This excellent catalytic performance could be ascribed to the following: (1) the nanospheres can hinder the aggregation of the Ag NPs, and the dense packing of the small Ag NPs (2~10 nm) offers a greater number of active reaction sites40; and (2) the abundance of TA on the surface can adsorb 4-nitrophenol via π-π stacking interactions, resulting in an effective enrichment of 4-nitrophenol near the catalytic nanospheres32, 41.

Table 1. Comparison of the apparent rate constants normalized to the mass concentration of the catalyst (k’) for the reduction of 4-nitrophenol with catalysts reported in the literature.

The magnetic performance of the sample was investigated by a Physical Property Measurement System (PPMS) at 298 K (Figure 7d). The saturation magnetization values for Fe3O4 and Ag-30 were 80 and 63 emu/g, respectively. This strong magnetic



response enables efficient magnetic recovery of the catalysts from the reaction solution. Using an external magnet, the Ag-30 catalysts could be easily separated from the reaction mixture within 1 min without significant loss (inset photo in Figure 7d). Recycling tests indicated that the Ag-30 catalyst had a favorable stability. After the 5 successive separation-reuse cycles, the conversion of 4-nitrophenol within 8 min remained at nearly 100 % of the initial conversion rate (Figure 7e). The Ag-30 catalyst was characterized after 5 successive uses. TEM images (Figure S3) show that several of the Ag NPs with larger particle sizes had fallen off of the substrate surface after the 5 repeated uses, but there were still a large number of small-sized Ag NPs loaded onto the surface of the nanospheres. The ICP-MS results showed that the mass content of Ag was 5.4 ± 0.2 wt% after the catalytic reactions, and the (111), (200), (220) and (311) crystal facets of the face-centered cubic (fcc) Ag structure (JCPDS no. 04-0783) could still be seen in the XRD spectrum of Ag-30 after the catalytic reactions (Figure S4). The XPS analysis (Figure S2b, c and d) indicated that the valence states of Fe and Ag after the catalytic reactions had not changed, demonstrating the excellent stability of the TA-Fe coordination-complex layer. Clearly, the presence of TA could stabilize the Ag NPs and prevent their removal and aggregation, endowing the catalysts with excellent cyclic stability and durability.

CONCLUSIONS In summary, the highly curved surfaces of Fe3O4@SiO2 nanospheres were successfully modified with a uniform thin layer of TA-Fe coordination complexes by a TA-Fe LbL modification process. Ag NPs with small particle sizes and a narrow size distribution were uniformly affixed to the surface of the nanospheres. Because of the dense arrangement of the small Ag NPs and the enrichment of the catalytic reactants by TA, the nanocomposites exhibited excellent catalytic performance and good stability toward the reduction of 4-nitrophenol. This strategy for the continuous layer-by-layer deposition of TA and Fe3+ provides a versatile approach for uniformly modifying the highly curved surfaces of micro/nanoscale particles to prepare functional nanocomposites for catalytic, biomedical and other applications.


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ASSOCIATED CONTENT Supporting Information Additional characterization data, including EDS mappings and high-resolution Fe 2p spectrum of Ag-30 and EDS spectra of Fe3O4 and Ag-30; TEM images, XRD pattern, XPS spectrum and high-resolution Fe 2p and Ag 3d spectra of Ag-30 after catalytic reaction.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (N.Z.). *E-mail: [email protected] (Y.-P.Z.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS N.Z. is thankful for the financial support by National Natural Science Foundation of China (nos. 51522308 and 21421061), Key Research Program of Frontier Sciences of CAS (QYZDB-SSW-SLH025) and Shanghai Research Institute of Chemical Industry Co., Ltd. (SKL-LCTP-201801). REFERENCES (1) Chen, T.; Chen, S.; Zhang, Y. W.; Qi, Y. F.; Zhao, Y. Z.; Xu, W. L.; Zeng, J. Catalytic Kinetics of Different Types of Surface Atoms on Shaped Pd Nanocrystals. Angew Chem Int

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X.

M.

Enhanced

Catalytic

Activity

of

Ag

Nanoparticles

Supported

on

Polyacrylamide/polypyrrole/graphene Oxide Nanosheets for the Reduction of 4-Nitrophenol.


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TOC:



TA-Fe LbL modification of nanospheres with extraordinary uniformity and its application in preparation of high-performance catalysts.


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General Surface Modification Method for Nanospheres via Tannic Acid

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