Ultrafine and Well-Dispersed Nickel Nanoparticles with Hierarchical

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Ultrafine and Well-Dispersed Nickel Nanoparticles with Hierarchical Structure for Catalytically Breaking Boron-Hydrogen Bond Lu-Lu Long, Xiao-Yang Liu, Jie-Jie Chen, Jun Jiang, Chen Qian, Gui-Xiang Huang, Qing Rong, Xing Zhang, and Han-Qing Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01588 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 23, 2018

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ACS Applied Nano Materials

Ultrafine and Well-Dispersed Nickel Nanoparticles with Hierarchical Structure for Catalytically Breaking Boron-Hydrogen Bond

Lu-Lu Long, Xiao-Yang Liu, Jie-Jie Chen, Jun Jiang, Chen Qian, Gui-Xiang Huang, Qing Rong, Xing Zhang, Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science and Technology of China, Hefei, 230026, China

*Corresponding

author:

Fax: +86-551-63607592; E-mail: [email protected]

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The catalytic properties of ultrafine metal nanoparticles (NPs) are usually limited by their dispersion of NPs, support is thus needed for ultrafine metal NPs. However, single dimensional supports cannot exhibit superior catalytic performance because of the easy aggregation. In this work, we design a hierarchical structure comprising 0-D ultrafine nickel NPs (Ni NPs), flower-like 3-D MoS2 nanosheets (NSs) and 2-D graphene for superior catalytic performance. In this design, flower-like 3-D MoS2 NSs were the support backbone for the growth of 0-D ultrafine Ni NPs, and 2-D graphene was used to hamper the aggregation and deformation of MoS2 NSs. The well-dispersed hierarchical structures with great structural stability exhibited an excellent catalytic activity for the catalytic hydrolysis of ammonia-borane and catalytic reduction of 4-nitrophenol by sodium borohydride in aqueous solutions. The mechanism behind the catalytic reduction was also elaborated using the in situ Fourier transform infrared spectroscopy technique. This design strategy provides a powerful approach for ultrafine metal NPs to achieve superior catalytic performance as the compositions of the hierarchical structure could be adjusted and replaced. Moreover, the designed 3-D hierarchical structure could be extended to other heterogeneous catalysis systems.

KEYWORDS: Ultrafine, Nickel, MoS2, Graphene, Catalysis, Boron-hydrogen bond


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INTRODUCTION

In catalytic reactions of boron-based compounds, acting B-H bond is one of the important processes, which influence the efficiency of reactions, such as catalytic hydrolysis of ammonia-borane (AB), reduction reaction of sodium borohydride (NaBH4).1,2 However, so far effective catalysts for catalytically breaking boron-hydrogen bond are usually noble metals, e.g., Pt3, Ru4, which are of high costs. As a promising alternative, Ni nanoparticles (NPs) have recently attracted interests because of their low costs and favorable catalytic activity. It is well known that the surface energy of a metal particle increases with its decreasing size and ultrafine metal nanoparticles exhibit unique catalytic properties5. Thus, the large surface area and great numbers of edge and corner atoms of ultrafine Ni NPs are highly beneficial for catalytically breaking B-H bond.6-8 However, the catalytic behavior of ultrafine Ni NPs is usually weakened by their agglomeration in the catalytic processes.6,9 Therefore, reducing the agglomeration of Ni NPs and maintaining their high catalytic efficiency are highly desired. Currently, a commonly adopted solution to this problem is to find out a suitable and multifunctional support to immobilize the Ni NPs. A desirable support should have the several features: 10-13 a large specific surface area, a strong affinity with the catalysts and excellent chemical/mechanical stability in the operating environment. Up to now, various solid supports, such as silica, carbon, metal oxides, and metal chalcogenides, have been used for immobilizing metal NPs to enhance their catalytic



activity and stability.10-17 Among them, MoS2, especially the material with a three-dimensional (3-D) superstructure, has been recognized as a promising support because of its large specific surface area, unique electrical conductivity, and exposed sulfur edge sites.10,11 For example, MoS2 nanosheets were reported to be a good template for the epitaxial growth of noble metal nanostructures.18 The 3-D hierarchical MoS2@Ni/CC nanocomposites, when used as an electrocatalyst, exhibited a higher activity and better stability than the pure ultrafine Ni without MoS2.19 However, these studies have focused on the formation process and the performance of the ultrafine materials that were synthesized on the MoS2 supports, but failed to explore the deformation and aggregation of the MoS2 supports themselves due to their ultrathin-layer structure. Once the support substrate becomes self-deformed or self-aggregated, the catalytic activity and stability of the whole catalyst would decline inevitably. Therefore, developing an effective strategy to further improve the performance of the ultrafine Ni NPs and simultaneously maintain the structural stability of the MoS2 support is of great significance. Recently, 3-D hierarchical structures, which incorporate materials of different dimensions, have shown unique and high performance in many applications.20,21 In these structures, the design of the assembled materials with hierarchical structures is particularly important as it helps to enlarge the specific surface area and enhance the structural stability. Inspired by these reports, in this work we developed a novel hierarchical structure comprising 0-D ultrafine and well dispersed Ni NPs, MoS2 nanospheres and 2-D graphene (Ni/MoS2@graphene) using a hydrothermal method. Graphene, a typical


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2-D material with a large loading capacity and strong affinity toward metal ions and organo-function groups,22 was introduced as the growing substrate for MoS2 NS to reduce its aggregation and improve its stability. Additionally, the phase, composition and structure of these products were characterized, and the crucial factors for the formation of the hierarchical structure were explored. Furthermore, the catalytic performances of the as-prepared Ni/MoS2@graphene towards the catalytic hydrolysis of AB and reduction of 4-nitrophenol (4-NP) by NaBH4 were evaluated. The catalytic mechanism of the catalytic reduction was also elucidated using the in situ Fourier transform infrared spectroscopy (FTIR) technique. In this way, an optimized strategy based on the hierarchical structure design was developed, and an efficient and cost-effective catalytic material towards catalytically breaking boron-hydrogen bond became available.

EXPERIMENTAL SECTION

Reagents. Nickel chloride (NiCl2·6H2O), sodium molybdate (Na2MoO4·2H2O), sulfourea, ammonia, hydrazine hydrate and ethanol were purchased from Sinopharm Chemical Co., China. All the chemicals were used as purchased without further purification. Deionized (DI) water was used throughout the experiments. Preparation of MoS2@Graphene. Graphene oxide (GO) was synthesized using the Hummers method.23 Typically, 120 mg of Na2MoO4·2H2O, 240 mg of sulfourea, and 0.5 mL GO (11.75 mg) were mixed with 30 mL of DI water. The mixed solution



was treated by sonication for several minutes and then transferred into a Teflon-lined stainless steel autoclave, heated to 200 °C for 24 h, and then air-cooled to room temperature. The obtained black powders were centrifuged and washed thoroughly with DI water and freeze-dried for 24 h. Preparation of Ni/MoS2@Graphene. The hybrids were synthesized using a one-pot hydrothermal method, in which Ni NPs were in situ reduced from nickel ammonium ions by hydrazine. Briefly, 20 mg of the as-prepared MoS2@graphene powder was dispersed in a solution containing 0.3 mL of NiCl2·6H2O (2 M) and 5 mL of ammonia and treated by sonication for 10 min. Then, 20 mL of ethanol and 5 mL of hydrazine were added, respectively. The mixture was transferred into the Teflon-lined stainless steel autoclave and heated at 160 °C for 12 h. The product was centrifuged and washed twice with ethanol and DI water. Characterizations. The Ni/MoS2@graphene catalysts were examined by X-ray diffraction (XRD) (Rigaku TTR-III, PHILIPS Co., the Netherlands) with Cu Kα radiation in the 2theta range from 10 to 70°. X-ray photoelectron spectral (XPS) analysis was carried out on an ESCALAB 250 instrument (VG Instrument Ltd., USA) with a monochromatic Mg Kα X-ray source to determine the surface composition of the sample. The morphology of the samples was characterized by a transmission electron microscope (TEM, JEM-2011, JEOL Co., Japan) and scanning electron microscope (FE-SEM, SIRION200, FEI Co., the Netherlands). High-resolution transmission electron microscope (HRTEM) images, selected-area electron diffraction (SAED), elemental mapping and energy-dispersive X-ray spectroscopy (EDS) data


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were collected on a JEM-ARM200F (JEOL Co., Japan). The surface area was measured by using the Brunauer–Emmett–Teller method with a Builder 4200 instrument (Tristar II 3020M, Micromeritics Co., USA) at liquid nitrogen temperature. The time-resolved online ATR-FTIR spectra and spectra of solutions of 4-NP and 4-aminophenol (4-AP) were recorded on a Vertex 70 spectrometer (Bruker Co., Germany) equipped with an MCT detector. Each spectrum was obtained after 32 scans with 4 cm-1 resolution. The obtained time-resolved FTIR spectra were smoothed and calculated by principal component analysis (PCA) with the software MATLAB r2012b (Mathworks., USA) Catalytic Hydrolysis of AB. Five mg of the as-prepared catalyst was dispersed into 3-mL deionized (DI) water in a test tube with a serum bottle mouth stopper. After that, 0.5 mmol AB dissolved in 2-mL DI water was quickly injected into the catalyst suspension with a syringe under vigorous stirring. The reaction was carried out at room temperature and under N2 atmosphere. The gas evolution was monitored using a gas chromatograph (Agilent GC7890, using N2 as carrier gas). The turn over frequency (TOF) value was calculated using the following method24: NH3BH3 + 2H2O = NH4+ + BO2- + 3H2 TOF = nH2 / (nmetal × t). In which, nH2 is the mole number of generated H2. t is the completed reaction time (min). nmetal is the total mole number of metal (Ni). The same procedure was repeated 10 times. Catalytic Reduction of 4-NP. The aqueous solutions of 4-NP (0.125 M) and NaBH4 (190 g/L) were freshly prepared. Typically, 1 mg of Ni/MoS2@graphene



catalyst was added into 4 mL of 4-NP aqueous solution. Subsequently, 10 μL of NaBH4 solution was mixed with the 4-NP solution containing the Ni/MoS2@graphene nanocomposites. The reaction was monitored in situ using UV/vis spectroscopy (UV-2401PC, Shimadzu Co., Japan). A solution of the prepared catalyst (2 mg in 1 mL ethanol) was added dropwise onto a trapezoidal ZnSe internal reflection element and evaporated at 45 ºC to form a thin catalyst film. After adding the 4-NP and NaBH4 solutions, each spectrum was collected immediately at intervals of 30 s using the spectra after adding 4-NP as the background.

RESULTS AND DISCUSSION

Structural, Morphological and Chemical Characteristics of the Catalyst. The synthetic route for the hierarchical structure is illustrated in Figure 1. In a typical synthesis process, the GO solution and adequate molybdenum source were first mixed at an appropriate weight ratio. GO was used as the substrate for the growth of MoS2. Table S1 reveals that an increase in the amount of GO influenced the generation of MoS2 spheres. The highest yield of MoS2 could be obtained for an appropriate proportion (3:6) of MoO4- and GO (MoGo-3). After a hydrothermal process, the MoS2@graphene was obtained (Figure S1). Then, nickel ammonia ions were used as the Ni precursor. After a further hydrothermal process at 160 ºC, most of the nickel ammonia ions were reduced to Ni NPs anchored on the MoS2 NSs by hydrazine hydrate.


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The XRD pattern of the as-prepared sample is shown in Figure S2a. Apart from the diffraction peaks of the MoS2 (JCPDS 37-1492) and rGO, an obvious peak at 44.3° can be assigned to the reflection of the (111) planes of the Ni NPs (JCPDS 65-0380). XPS was used to investigate the chemical state of the as-prepared sample. Figure S2b-f shows the survey XPS spectra of Ni 2p, Mo 3d, C 1s and N 1s, respectively. The measured binding energies of Ni 2p are equal to 856.3 eV, 858.5 eV, 873.9 eV and 878.4 eV, suggesting that the surface Ni species were mainly composed of Ni(II) and NiOOH.25,26 The oxidized Ni species in the sample did not appear in the XRD pattern (Figure S2a), as the amount of oxidized species was too low to be detected by XRD. Although a weak peak of metallic Ni was also measured (Figure S2c), the oxidized Ni species were the dominant state of Ni on the surface. In addition, Ni(II) might arise from the residual nickel ammonia ions after reduction. As shown in Figure S2d, the Mo 3d signal consisted of two doublets. The doublet (Mo 3d5/2= 229.3 eV, Mo 3d3/2= 232.4 eV) at a relatively low binding energy could be assigned to the Mo ion in the +4 oxidation state. Furthermore, the binding energy of 226.6 eV could be assigned to S 2p.27 The C 1s XPS spectra for the sample in Figure S2e could be identified as C groups, i.e., C=C, C-N, and C-O bonds, with binding energies of 284.5, 285.1, and 286.1 eV, respectively.28 The N 1s spectrum of the sample has three resolved peaks, i.e., pyrrolic N (399.9 eV), pyridinic-N (398.3 eV) and doped N (395.6 eV). These results indicate that the GO of the sample was doped with nitrogen.29 The XRD and XPS results demonstrate that the Ni/MoS2@graphene samples were successfully fabricated.



After the reduction of the nickel ammonia ions, 3-D MoS2 nanoflowers were still closely wrapped in the 2-D grapheme (Figure 2a), because GO could not only provide more nucleation sites for the growth of MoS2, but also improve the stability and dispensability. Figure 2b shows the lattice fringes perpendicular of the ultrafine NPs with a spacing of 0.201 nm, which is equal to the lattice parameter of the Ni (111) facet. The lattice spacings, 0.338 nm and 0.648 nm, matched well with the graphene and the (002) planes of hexagonal MoS2, respectively. Numerous NPs of ca. 2-3 nm in diameter were uniformly dispersed on the support (Figure 2c, d). These images clearly indicate the successful synthesis of the ultrafine and highly dispersed Ni NPs on the substrate of MoS2@graphene. To further confirm the presence of Mo, S, Ni, C, EDS mapping analyses were performed (Figure 2e). The mass ratios of Ni, Mo, S and C were 16%, 42%, 30%, and 12%, respectively (Figure S3). The ICP method was also used to determine the nickel content in the samples (Ni: 0.259 mg/mg). Compared to the uniform distribution of Mo and S, the mapping of Ni exhibited a similar distribution, indicating that a large amount of Ni NPs grew on the 3-D MoS2 nanoflowers. This result suggests that the 3-D MoS2 nanoflowers acted as a good support for the ultrafine Ni NPs and accordingly improved the loading amount of Ni. This observation is also supported by the EDS analysis spectrum results. The loading amount of Ni NPs on the MoS2 spheres was larger than that on the graphene (Figure S4). The 3-D hierarchical nanostructure comprising 0-D ultrafine Ni NPs, 3-D MoS2 nanoflowers, and 2-D graphene was successfully fabricated. The aforementioned


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observation shows that most of the ultrafine Ni NPs were uniformly dispersed on the MoS2 NSs (Figure S5a). The formation of well-dispersed ultrafine Ni NPs could be attributed to two key factors (Figure 3). First, MoS2 was an important support for the synthesis of Ni particles. For comparison, we used pure rGO without MoS2 as the support for Ni reduction (Figure S5b), but fewer Ni NPs with a broad size distribution were generated. This result was attributed to the defect-rich structure of MoS2, which could offer abundant unsaturated sulfur atoms to generate active sites and lead to the enhancement of the intrinsic conductivity.15 These features are helpful for adsorption, nucleation, and the growth of metal ions.30 Abundant of sulfur atoms provided many uniform nucleation sites, which resulted in the good dispersion of Ni NPs, while pure reduced graphene oxide provided active groups on the surface as the nucleation sites, which were fewer in number and non-uniform in distribution. Hence, MoS2 played a positive role in enhancing the catalytic performance of this elaborately established hierarchical nanostructure. Second, the Ni precursor (nickel ammonium ions) was essential to prepare the ultrafine Ni NPs. For comparison, we tried to use Ni ions as the Ni source; the result suggests that we could not obtain ultrafine NPs through the same synthetic process via Ni ions (Figure S5c). This is because the special structure of the nickel ammonium complex influences the reaction rate of the Ni source with the same amount of reductant in a limited reaction time and hampers the contact between the Ni atoms and exposed sulfur sites on MoS2. The growth rate of Ni NPs was limited by many ineffective collisions, which are due to steric hindrance. The ultrafine catalyst is known to be easily aggregated or deformed in many chemical



reactions, which will influence the catalytic performance. Hence, graphene was introduced as a part of the hierarchical nanostructure for maintaining the morphology. As shown in Figure S5d, in the absence of graphene, the MoS2 NSs could not maintain their initial morphology and exhibited obvious agglomeration after loading with Ni NPs. This indicates that graphene could act as a growth substrate for the MoS2 NSs, effectively preventing the agglomeration of the MoS2 NSs after loading with magnetic Ni particles and improving the stability. Catalytic Hydrolysis of AB. To evaluate the catalytic performance of the prepared catalyst, the hydrolysis of AB catalyzed by Ni/MoS2@graphene was tested. The hydrogen evolution curves are displayed in Figure 4a. Ni/MoS2@graphene completed the hydrolysis process within 10 min and exhibited a much higher catalytic activity than Ni/MoS2, Ni/rGO and pure materials. In the absence of MoS2, Ni/graphene did not perform better than the prepared hierarchical nanostructure (Figure S5b), although the presence of graphene enhanced the dispersion of the Ni NPs. This might be attributed to the reduced loading amount of Ni NPs, which was the main contributor to the catalytic hydrolysis of AB. The presence of MoS2 helped to form the well-dispersed and ultrafine Ni NPs and increase the loading amount of Ni. Without graphene, Ni/MoS2 also exhibited a poor performance for catalytic hydrolysis of AB compared to the hierarchical nanostructure (Figure S5d). These results indicate that the prepared hierarchical nanostructure had a potential to maximize the catalytic acting of B-H bond. Other catalysts, including the MoS2@graphene, pure MoS2 and pure graphene, did not exhibit favorable catalytic


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activities. Therefore, the Ni NPs acted as the main catalytic species for the catalytic hydrolysis of AB. The optimized Ni/MoS2@graphene achieved the highest TOF value of 6.82 (H2)/(Cat.)mol·min in the hydrolysis of AB. Figure 4b shows that the activity of the Ni/MoS2@graphene had no substantial decline after 5 runs and declined slightly after 10 runs, suggesting its good stability. The morphologies of the different catalyst samples are shown in Figure S6. As reported previously,31 the whole AB hydrolysis process occurrs on the surface of Ni NPs. The Ni NPs will interact with AB and H2O and activate them. In the H2-generating process, the splitting step of water is the rate-governing step, which leads to the reaction of –H with the activated AB through breaking B-H bond to form H2. A schematic diagram to show the catalytic hydrolysis of AB on the Ni/MoS2@graphene is illustrated in Figure 4c. Catalytic Reduction Reaction of NaBH4. The as-synthesized materials were also tested in terms of the reduction of 4-NP with NaBH4 in aqueous solution. Figure 5a shows the successive UV-Vis spectra of 4-NP in the presence of hierarchical nanostructures and NaBH4 in aqueous solution. The intensity of the absorbance peak at 400 nm, which could be assigned to 4-NP, gradually decreased along with the increase in a new absorption peak at 300 nm, indicating the formation of 4-AP. The plots of ln(C/C0) vs. time (t) for the reduction of 4-NP in the presence of 10 and 100 μL equivalents of NaBH4 with varying amounts (0.1 mg and 1.0 mg) of the prepared catalysts are shown in Figure 5b, and the values for the rate constant were calculated from the slope of the straight line. With the increase in the rate constant value, there



was a corresponding increase in the activity of the catalyst. The rate constant values for the hierarchical nanostructures under different reaction conditions were found to be 0.00296, 0.00454, 0.005 and 0.01429 s-1. For comparison, the rate value of 0.00454 s-1 was chosen as a reference. The rate constant values for the different catalysts and the corresponding curves are, respectively, shown in Figures 4c and S7. These results suggest that, after adding the prepared catalysts, the absorbance at 400 nm decreased rapidly within 210 s, which was markedly shorter than this process for the other catalysts. The curve of the blank (Figure S7b) indicates that NaBH4 itself, in the absence of the catalysts, could not reduce 4-NP. Moreover, commercial Pt/C powders were also used for this catalytic reduction reaction under the same conditions mentioned above.32 The prepared hierarchical nanostructures had better performance than the commercial Pt/C. Furthermore, Table 1 shows that the rate constant (k) values and activity parameters of the hierarchical structures were also higher than those reported for other catalysts. Since the catalytic activity is also affected by the size of the Ni NPs,9 the Ni NPs with different diameters were synthesized via controlling the reaction time with hydrazine hydrate in the autoclave. Figure S8 shows the TEM images, size distribution, and the corresponding reduction spectra of Ni NPs with different diameters grown on the MoS2@graphene. Clearly, with an increase in the average size of the Ni NPs, the catalytic activity of the hierarchical structures decreased. This could be ascribed to the decrease in the active surface sites of the Ni NPs with the increasing size of the NPs. Thus, all these results clearly indicate that the excellent catalytic performance of the hierarchical structures


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should be attributed to the ultrasmall particle size, good dispersion of the Ni NPs and good adsorption ability of MoS2 on the support. Apart from the excellent performance for the catalytic reduction of 4-NP, another advantage of the hierarchical structures was its easy separation with an external magnetic field. The visual demonstration experiment shows that, in the presence of an external magnet, the catalysts could be readily separated from the mixture after the reaction was completed (inset of Figure 5d). As shown in Figure 5d, in the initial three cycles, the catalytic activity exhibited a slight decrease. Then, after five cycles, the activity gradually became stable. After nine cycles, the reaction time was prolonged to 410 s for the complete conversion of 4-NP. The substantial decrease in the reusable activity of the hierarchical structures should be attributed to the catalyst loss during the repeated tests. The TEM image of the used samples after 10 cycles illustrates that the morphology of the hierarchical structures changed (Figure S9). Most of the nickel nanoparticles grown on graphene disappeared, while the nickel nanoparticles grown on MoS2 flowers maintained ultrafine size and clear shape. This result indicates the importance of MoS2 in our system, and further demonstrates that MoS2 was helpful to the activity and stability. With the above results and the previous studies of 4-NP reduction,42,43 a possible mechanism for the catalytic 4-NP reduction by the hierarchical structures is proposed and illustrated in Figure 6a. The BH4- ions are adsorbed on materials surface and then catalytically hydrolyzed by Ni NPs. Then, the generated active hydrogen atoms through the hydrolysis process are transferred to the nitro group of 4-NP, which goes



through several steps of hydro-deoxygenation reactions to form 4-AP. Thus, the ability of catalytically breaking the B-H bond of NaBH4 is essential in the whole reduction process. In this catalytic reduction process, 4-(hydroxyamino) phenol was an important intermediate product, which can be detected by FTIR. Thus, the in-situ FTIR was used to explore the possible reaction mechanism. Figure 6b and 6d shows the time resolved online IR spectra of the initial 4000 s of the reduction process catalyzed individually by the Ni/graphene and Ni/MoS2@graphene. Compared to the IR spectra of pure 4-NP and 4-AP shown in Figure S9, both 4-NP and 4-AP could be found in the spectra in the 4000-s reaction. The characteristic peaks at 1497 cm-1, 1587 cm-1 and 1660 cm-1 indicate the presence of hydroxylamine. As shown in Figure 6b and 6d, the peaks of hydroxylamine increased initially but decreased later.44 Considering the superposition in the fingerprint region, principal component analysis (PCA) was used to analyze the FTIR spectra of the reduction process. The loading plots are exhibited in Figure S10, showing that the first principal component (PC1) should arise from the contribution of the product 4-AP and the second principal component (PC2) from the combining contributions of 4-(hydroxyamino) phenol and reactants. The scatter plots of the scores of PC1 and PC2 are shown in Figure 6c and 6e, respectively, and the numbers labeled above the points represent the sequence of the sampling time. In Figure 6c, the PC1 captured 90% of the spectral variation, while the PC2 covered 9% of the spectra variation with the Ni/graphene catalyst, and the vertex was at the 15th time point. The scores of PC1 increased quickly with time until the 15th time point, and then the increasing rate decreased. The PC2 scores initially


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increased with time, but decreased after the 15th time point. The changing tendency of PC2 indicates that the reactants were initially absorbed onto the catalyst surface, forming the intermediate. Afterwards, since the consumption rate was faster than the absorption or the intermediate production rate, the PC2 scores decreased. Figure 6e shows that 86% and 13% of the spectral variation with the Ni/MoS2@graphene catalyst could be explained by PC1 and PC2, respectively. Compared to Figure 6c, the vertex was at the 7th time point, suggesting that 4-(hydroxyamino) phenol was readily generated and transformed to 4-AP quickly in the presence of MoS2. This result reveals that the catalysts with MoS2 had a faster variation tendency than those without MoS2, which further proves the importance of MoS2 in the enhanced performance of the Ni NPs. Two possible reasons might be responsible for this observation45,46: First, the MoS2@graphene, as a substrate of Ni NPs, had a better adsorption capacity than the pure MoS2 or pure graphene, as evidenced by the BET data in Figure S11. They could capture the negatively charged BH4- and 4-NP molecules through the Mo4+ and unsaturated sulfur atoms. Secondly, MoS2 might be beneficial to the transformation of active hydrogen atoms among the reduction steps.

CONCLUSIONS

In summary, we designed and fabricated a hierarchical structure comprising 0-D ultrafine Ni NPs in situ reduced on 3-D MoS2 nanoflowers and 2-D graphene for enhancing catalytic performance. The prepared catalyst had a great structural stability,



and exhibited excellent performance towards catalytically breaking B-H bond. In addition, the nickel ammonium ions and MoS2 nanoflowers were found to be two key factors to prepare a large amount of ultrafine and well-dispersed Ni NPs, and 2-D graphene further played an important role in maintaining superior catalytic performance. The designed hierarchical structure exhibited the best activity as expected, which would provide sufficient justifications and momentum for its further development. Therefore, our design gave an effective case for reaching superior catalytic performance and offered reasonable guidance to design high-performance catalysts for heterogeneous catalysis applications.

ASSOCIATED CONTENT Supporting Information The quality of molybdenum in MoS2@graphene (Table S1), TEM images of the MoS2@graphene (Figure S1), XRD and XPS Characterizations (Figure S2), EDS spectrum of the Ni/MoS2@graphene (Figure S3), TEM images and EDS spectrum of Ni on different places (Figure S4), TEM images of the different catalysts (Figure S5) Time-dependent adsorption spectra of the reaction (Figure S6), Morphology of the different supports (Figure S7), TEM images and the corresponding catalytic activity of Ni NPs with different sizes (Figure S8), Morphology of Ni/MoS2@graphene after several cycles (Figure S9), Morphology of Ni/MoS2@graphene after several cycles (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org/.


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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21590812, 51538011 and 51821006), and the Program for Changjiang Scholars and Innovative Research Team in University of the Ministry of Education of China and Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF) for the partial support for supporting this work.

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Table 1 Comparison of Kinetic Constant (k) Values and Activity Parameters for the 4-NP Reduction by Various Catalysts catalyst (25 °C)

initial molar

catalyst

rate

activity

quantity

concentration

constant

factor

(k)/s-1

(k)/s-1mg-1

(mM)

(g/L)

ref.

Ni nanoparticle

0.0003

1 g/L

0.0027

0.0009

33

Ni2Co1-xFe2O4

0.72

2.5 g/L

0.172

0.00344

34

Ni/C

0.25

0.1 g/L

0.00626

0.02087

13

Ni/SiO2

0.0017

1.33 g/L

0.084

0.021

35

Ni@Pd

0.0003

0.13 g/L

0.0204

0.039

36

CPSA@MATP/Ag

0.25

0.33 g/L

0.014

0.00028

37

Fe@Au/Go

0.025

1.8 g/L

0.0014

0.0002

38

PtAu/CeO2

0.0018

1.6 g/L

0.1087

0.002174

39

Pt nanoparticle

0.008

0.9116 g/L

0.00075

0.00021

40

Pt-Au

0.1

0.002 g/L

0.00958

0.016

41

Ni/MoS2@NRGO

0.125

0.25 g/L

0.01429

0.01429

this work

0.005

0.05

0.00454

0.00454

(100 μL) 0.025 g/L (100 μL) 0.25 g/L (10μL)



Figure Legends

Figure 1. Illustration of the synthetic route for the hybrid of ultrafine Ni NPs, MoS2 NSs and graphene hierarchical nanostructure.

Figure 2. (a-c) Low- and high-magnification TEM images of the obtained samples; inset: SEM image; (d) Size distribution of Ni NPs and (e) Element mapping of Ni/MoS2@graphene.

Figure 3. Schematic illustration of the formation factors of ultrafine Ni NPs on MoS2@graphene.

Figure 4. (a) Hydrogen evolution curves of the hydrolysis of AB aqueous solution catalyzed by different samples; (b) The plots of volume of H2 vs. time for hydrolysis of AB catalyzed by Ni/MoS2@graphene catalyst during a 10-cycle reusability test.

Figure 5. (a) The time-dependent adsorption spectra of the reaction solution in the presence of the hierarchical nanostructures; (b) plots of ln(C/C0) versus time for the reduction of different amounts of catalysts and NaBH4; (c) the plot of the ln(C/C0) versus time for the reduction of the different catalysts; and (d) the reusability of hierarchical nanostructures and inset: images of the hierarchical nanostructures magnetically separating from the 4-NP after completion of the reaction

Figure 6. (a) Possible reaction mechanism for the reduction of 4-NP by NaBH4 with the hierarchical structures as catalysts; (b) Time-resolved online IR spectra of Ni/graphene; (c) Scatter plots of the first two principal components calculated from PCA on mean-cantered online IR spectra of Ni/graphene; and (d-e) Time-resolved online IR spectra and Scatter scores plot of Ni/MoS2@graphene. 28 ACS Paragon Plus Environment

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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Table of contents (TOC) art


Ultrafine and Well-Dispersed Nickel Nanoparticles with Hierarchical

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