A Core-Shell Strategy for Improving Alloy Catalyst Activity for

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A Core-Shell Strategy for Improving Alloy Catalyst Activity for Continual Growth of Hollow Carbon Onions Chenguang Zhang, Ke Ma, Naiqin Zhao, and Zhihao Yuan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01249 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Crystal Growth & Design

A Core-Shell Strategy for Improving Alloy Catalyst Activity for Continual Growth of Hollow Carbon Onions Chenguang Zhang†‡, Ke Ma†‡, Naiqin Zhao*∆, Zhihao Yuan*†‡ † School

of Materials Science and Engineering, Tianjin University of Technology, Tianjin,

300384, PR China ‡

Tianjin Key Laboratory for Photoelectric Materials & Devices, Tianjin University of

Technology, Tianjin, 300384, PR China ∆

School of Materials Science and Engineering, Collaborative Innovation Center of Chemical

Science and Engineering, Tianjin University, Tianjin, 300072, PR China

*To whom correspondence should be addressed: Naiqin Zhao, [email protected]; Zhihao Yuan, [email protected]

KEYWORDS: Hollow carbon onion, Core-shell catalyst, Continual growth, Chemical vapor deposition

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ABSTRACT: Herein, we report the controlled synthesis of hollow carbon nano-onions (HCNOs) through using core-shell Fe-Ni alloy catalysts. The core-shell structure is formed through the phase transition during the thermal treatment under hydrogen. The inner γ-Fe-Ni core shows the catalytic activity in growing HCNOs, whereas the outer shell is catalytically inert and acts as the protective layer, which can also be converted to the inner core during the growth. This continuous supply of active metallic atoms from the outer shell to the inner catalyst can in-situ maintain the catalytic activity, avoiding the use of additional oxygen-containing oxidative agents. In addition, the shell-to-core conversion can be well controlled by tuning hydrogen flow rates during the chemical vapor deposition process to enable a continual growth of HCNOs from one catalyst. Furthermore, the size-controlled growth can be realized by controlling the size of the catalyst nanoparticles. This controlled shell-to-core conversion allows for the improvement of catalytic activity for the continual growth of HCNOs before deactivation. To demonstrate the advantage of the hollow structure in energy storage, the performance of HCNOs as the supercapacitor electrode material has been tested. This study offers a possibility for catalyst engineering by core-shell strategy towards lifetime extending in the structure-controlled and scalable growth of desirable carbon nanostructures.

1. INTRODUCTION Carbon nano-onion (CNO) is a quasi-zero-dimensional member in the nanocarbon family with a spherical onion-like structure and concentric graphitic layers. CNOs cover a wide range of nanostructures, including nested fullerenes, metal or diamond encapsulated CNOs (M@CNOs) and hollow CNOs (HCNOs).1-4 Owing to the novel structure, CNOs promise potential


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applications in drug delivery,2 point electron sources,3 and energy storage.4 Recently, CNOs have been demonstrated to be the candidates for electrode materials in electrochemical energy storage.5-9 For energy storage in the previous studies, CNOs with the inner solid core but without structure control were usually employed. It has been demonstrated in our previous research that the HCNOs have higher energy storage capacity than the [email protected] Moreover, it has been reported that creating the inner hollow space in spherical carbon structures is beneficial for enhancing the energy storage capacity.11 Therefore, HCNO structures are desirable for the performance enhancement in energy storage applications. However, up until now, there still remains uninvestigated on the controllable and scalable synthesis of the HCNOs. Various methods have been employed for the synthesis of HCNOs, including the high temperature annealing,4 chemical vapor deposition (CVD), chlorination of carbon-containing precursors, heat treatment of bio-precursors,12 metal-organic framework13 and pulsed plasma in liquid.14 Among them, the CVD method shows the advantages in achieving structure- or sizecontrolled growth and high graphitic crystallinity of the product through tuning the growth conditions, such as growth temperature, gas flow rate, catalyst type and size.10,15,16 For example, CVD growth at higher temperature preferentially yields HCNO structures, whereas M@CNOs tend to be grown at lower temperature;10 Fe-Ni alloy catalyst exclusively yields CNO structures independent of catalyst size, whereas the catalyst size affects the grown graphitic structures when using other types of catalysts.15 Various catalysts have also been used for the synthesis of CNOs, including Fe, Ni and Co catalysts. It has been demonstrated that Fe-Ni alloy catalyst is more efficient for large-yield growth of carbon products,17 and more favorable for growing pure CNO structures or HCNO structures than Fe or Ni catalyst.10,15 However, during the CVD growth of CNOs, the catalyst is easily deactivated to form the M@CNOs as the main structure. Hence, the


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structure control technique for the preferential growth of HCNOs as the majority is still lack and remains uninvestigated. Moreover, the continual growth of HCNOs is still difficult to realize. As mentioned, the catalyst can be rapidly deactivated during the growth of CNOs. Thus, the further nucleation and growth of additional CNOs are terminated, making it impossible for the continual growth of HCNOs. Although the inner metallic catalyst of the M@CNOs can be removed by the acid purification,18-21 only one HCNO particle can be obtained from one catalyst nanoparticle, limiting the yield of HCNOs. Previously, we have demonstrated that the deactivated catalyst nanoparticle can be reactivated by the high-temperature annealing to catalyze the growth of the pre-stored carbon into HCNOs, and more than one HCNOs can be generated from one catalyst nanoparticle.10 However, since no exterior carbon is supplied during the annealing, HCNOs cannot be continually grown. Therefore, it is required to develop the catalyst to increase the catalytic activity for growing more HCNOs frkom one catalyst nanoparticle. Unfortunately, no investigations have been reported yet. In fact, the continuous growth of graphitic structure has been realized in the growth of carbon nanotubes (CNTs). The carbon species dissolved in the catalyst nanoparticle can be precipitated into the graphitic structure in the growth region to supplement the growth of the CNT. If the catalytic activity is high enough, the side-walled graphene layers can be grown into a considerable length, as illustrated in Scheme 1a. Many efforts have been devoted to maintaining or increasing the catalyst activity and avoiding the Ostwald ripening22 for growing CNTs, such as introducing oxygen-containing species during the CVD growth,22-24 coating alumina onto catalyst or doping Al atoms into catalysts,25,26 and applying the Trojan catalyst strategy.27 Inspired by the growth of CNT, we expect that if the structure of catalyst is properly designed


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with high activity, the HCNOs can be nucleated periodically and grown continually from the catalyst, instead of being deactivated to form M@CNOs, as illustrated in Scheme 1b and c.

Scheme 1. Schematic of the growth process of the CNT (a), M@CNOs (b), and HCNOs (c) Herein, we developed a core-shell-structured Fe-Ni alloy catalyst for the controlled and continual growth of HCNOs. In this core-shell-structured catalyst, the inner γ-Fe-Ni core shows real catalytic activity in growing HCNOs, whereas the outer shell structure keeps catalytically


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inert, which can be continuously converted into the inner γ-Fe-Ni core during the CVD growth, providing the active metallic atoms to the inner catalyst until fully converted. It also serves as the buffering pathway to control the dissolving speed of carbon atoms into the inner catalyst and protects the inner catalyst nanoparticles from aggregation. This core-shell strategy enables the insitu constant maintaining of the catalytic activity, which allows for the maximum utilization of the catalytic activity before the deactivation and avoids using additional oxidizing agents. A controlled shell-to-core conversion was achieved by controlling the hydrogen flow rate to realize the continual growth of HCNOs from one catalyst nanoparticle. The hollow space size of the HCNOs is closely correlated to the size of catalyst nanoparticles, and the amount of HCNOs is controlled by the shell-to-core conversion rate. Therefore, the controlled and continual growth of HCNOs has been realized by our core-shell catalyst strategy. Further, the application of HCNOs as the high-rate supercapacitor electrode material has been demonstrated. This study offers a reference for designing the catalyst structure to in-situ constantly maintain the catalytic activity towards the scalable growth of hollow carbon nanostructures for desired applications.

2. RESULTS AND DISCUSSION 2.1. Formation of Core-Shell Alloy Catalysts. The formation of core-shell-structured catalysts was obtained from the products reduced by hydrogen. Figure 1 shows the transmission electron microscope (TEM) images of the corresponding products reduced at different temperatures. As observed in Figure 1a-c, the catalyst nanoparticles located on the MgO substrate have an average size of ~10 nm. The insets in Figure 1a and b show the Fast Fourier Transform (FFT)-derived diffraction patterns of the nanoparticles, which can be indexed from [110] zone axis of face-centered (fcc)-structured γ-Fe-Ni. The interplanar distances of 0.21 and 0.18 nm, correspond to the (111) and (002) facets of γ-Fe-Ni, respectively. Thus, it can be


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concluded that the nucleation of γ-Fe-Ni phase can be induced by hydrogen reduction at elevated temperatures. The primary dots in the FFT-derived diffraction pattern in the nanoparticles reduced at 650 °C are clearer than those reduced at 550 °C, indicating the formation of a higher phase crystallinity of the γ-Fe-Ni at the higher temperature. From Figure 1b and c, the appearance of inner dark regions can be observed in the catalyst nanoparticles reduced at 650 °C, while the size grew into a larger one at 750 °C. At 850 °C, as shown in Figure 1d, after reducing for only 3 min, the interfacial region between the catalyst nanoparticle and the MgO substrate became darker. When increasing the reducing time to 10 min, the core-shell structures were formed (Figure 1e). Further increasing the reducing time to 30 min led to the increased size of the core nanoparticle to ~12 nm but decreased the thickness of the shell (Figure 1f). The lattice measurements in Figure 1f are consistent with that in Figure 1b and c, further confirming the formation of the γ-Fe-Ni phase in the core structure. X-ray diffraction (XRD) patterns in Figure S1 show that the γ-Fe-Ni phase emerged at 750 °C and grew into a bigger size at 850 °C. To identify the role of hydrogen in the catalyst formation, we carried out the heat treatment without hydrogen. Figure S2 shows that there is no formation of core-shell structures when heat-treated at 850 °C under the Ar flow only. From these observations, it can be concluded that the hydrogen reducing and the high temperature can promote the growth of γ-Fe-Ni and the evolvement of catalyst into core-shell structure. Moreover, increasing the hydrogen reducing temperature and time can result in the larger core region and better crystallinity. In contrast, the shell is always in a short-range ordered structure.


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Figure 1. TEM images of the catalyst nanoparticles reduced under hydrogen at (a) 550 °C, (b) 650 °C, and (c) 750 °C for 30 min and at 850 °C for different time of (d) 3 min, (e) 10 min, and (f) 30 min. Insets show the diffraction patterns of the nanoparticles, indexed from [-110] zone axis of γ-Fe-Ni phase. 2.2. Formation of Graphitic Structures from Core-Shell Alloy Catalysts. To further investigate the nucleation and formation of the graphitic structures from the core-shell alloy catalysts, CVD growth at different temperatures without the hydrogen flowing was carried out. Figure S3a shows the photographs of the products grown from the Fe-Ni/MgO catalysts at different temperatures for 30 min. With the temperature increasing from 550 to 1000 °C, the products became darker in color. The color change suggests the low yield of the carbon product during the CVD synthesis at the temperature below 750 °C, consistent with the TEM results. As


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shown in Figure S3b-d, nanoparticles with core-shell structures were formed as the main products with the rare nucleation of graphitic layers. When growing at higher temperatures, the outer shell structure disappeared and the graphitic layers were formed over the surface of the nanoparticles, forming into M@CNOs at 850 °C, single-walled CNTs at 950 °C, and M@CNOs at 1000 °C, respectively (Figure S3e-g), which are in agreement with our previous work.15 Usually, the growth temperature below 850 °C provides sufficient kinetic energy to initiate the nucleation of carbon nanostructures,28 but in our case, no carbon was formed below 850 °C. We therefore supposed that the outer shell structure of the catalyst may have an influence on the growth of carbon products. To examine the structural evolution of the graphitic structures from the core-shell catalysts, the CVD growth for different time at different temperatures was conducted. As shown in Figure 2a, after growing at 850 °C for 3 min, the nanoparticles are still in the core-shell structure without the nucleation of graphitic layers. The inset in Figure 2a shows the diffraction pattern of the core region indexed from [-110] zone axis of γ-Fe-Ni. The inter-planar distances were measured to be 0.21 and 0.18 nm, corresponding to the (111) and (002) facets of γ-Fe-Ni, respectively. Thus, the inner core is demonstrated to be the γ-Fe-Ni phase. After growing for 5 min, the nucleation of graphene layers started. As shown in Figure 2b, the graphitic layers emerged in the core/shell interface, partially encapsulating the core nanoparticle. It can be inferred that during this process, the graphitic layers were precipitated from the inner core nanoparticle, taking place of the core/shell interface region. This also indicates that the core catalytically grew the graphitic structure, whereas the outer shell is catalytically inert. When the growth time was prolonged to 10 min, the core nanoparticles were entirely encapsulated by the graphitic layers (Figure 2c). To verify this phenomenon, multi-walled CNTs with larger


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graphitic features were grown at 950 °C by using a higher loading amount of catalysts (20wt%). Figure S4 shows that by increasing the metal loading on the substrate, the products were dominated by multi-walled CNTs. Figure 2d shows 4~5 graphitic layers occupied the core-shell boundary region (indicated by the white arrow), pushing out the outer shell. Hence, the inner core is catalytically active for growing graphitic layers. Another evidence can be found in Figure 2e, in which the white arrow indicates a residual amorphous shell structure outside the inner core surrounding by the graphitic layers. The product was dominated by the bamboo-structured CNTs. The amorphous shell structure attached on the surface of the CNTs can also be observed (indicated by the white arrow in Figure 2f), further confirming that the inner core is catalytically active and the CNTs were nucleated from the inner core nanoparticle.


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Figure 2. TEM images of the nucleation and growth of CNOs over Fe-Ni/MgO catalysts at 850 °C at different stages (3 min, 5 min and 10 min) and MWNTs at 950 °C for 10 min. Inset in (a) shows the diffraction pattern of the nanoparticle, indexed from [-110] zone axis of γ-Fe-Ni phase. To understand the formation mechanism of the core-shell catalysts and the graphitic structures, we carried out energy dispersive spectrometer (EDS) characterization. Figure S5 shows the TEM images and the corresponding EDS profiles of two core-shell catalysts after the CVD growth at 850 °C for 3 min, during which the graphitic structures were still not nucleated. The holes and defects generated by the electron beam bombardment shown in the insets indicate the precise focus and data collection in the interested regions. According to the quantitative analysis of the EDS results, the atomic mole ratio of Fe to Ni in the inner core was measured to be ~1.68 in average, close to that in γ-Fe-Ni (Fe0.64Ni0.36). The atomic mole ratio of Fe to Ni in the shell structure was measured to be ~0.83, much lower than that in α-Fe-Ni (~9:1), indicating that the formation of core-shell structure cannot be ascribed to the phase separation between γand α-Fe-Ni.29, 30 The atomic mole ratio of Fe to Ni in the catalyst precursor is 1:1. According to the Fe-Ni phase diagram presented in the references,31,32 the Fe-Ni alloy with Fe to Ni atomic mole ratio of 1:1 will enter a single phase fcc-structured γ-Fe-Ni region at and above the reduce temperature of 550 °C. Thus, the phase transition will happen, and the γ-Fe-Ni crystal phase will be nucleated and grown in the initial amorphous Fe-Ni structure, forming the inner γ-Fe-Ni core. In addition, the correct stoichiometric number is necessary to achieve a particular degree of longrange ordering. The formation of γ-Fe-Ni (Fe0.64Ni0.36) requires an atomic mole ratio of Fe to Ni to be around 1.77 (initial ratio is 1:1 in catalyst precursor), making the remaining compositions of Fe-Ni non-stoichiometric to form any Fe-Ni crystalline phases. Therefore, the non-converted


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Fe-Ni precursor was formed into the shell structure with the disordered or short-range ordered Fe-Ni solid solution. Previous research has indicated that the enhanced atomic diffusion induced by the high-temperature treatment can lead to the development of longer-range ordering phase.33 The thermal reduction in hydrogen is also critical in accelerating the formation of orderedstructured Fe-Ni alloy.34 Thus, the disordered or short-range ordered Fe-Ni alloy precursor can serve as a matrix to nucleate the inner fcc-structured γ-Fe-Ni with long-range ordering once the hydrogen reducing starts. With an increase of the temperature and time of the thermal treatment, hydrogen reducing in both the reduction and growth process will promote the phase transition into the γ-Fe-Ni to grow into a larger size, resulting in a continuous conversion of the amorphous shell structure into the γ-Fe-Ni core structure. Furthermore, the EDS characterization (Figure S4c-d) show that the carbon content in the core structure is significantly higher than that in the shell structure. This suggests that the core nanoparticle is able to dissolve carbon atoms and catalytically active for growing graphitic structure, whereas the solubility of carbon atoms is low in the shell structure. The catalytic activity of the core-shell catalysts was tested under the electron beam irradiation during the TEM characterization. It has been reported that the amorphous carbon film on the copper grid can be graphitized by the catalyst nanoparticles,35 as observed during our TEM characterization. Hydrogen reducing at 850 °C for 1 h was conducted to ensure the γ-Fe-Ni core to be fully exposed without shell structures. During the electron beam focusing at an acceleration voltage of 200 keV, the γ-Fe-Ni nanoparticles moved fast in random directions on the surface of the lacy carbon film. As shown in Figure S6a and b, a number of individual HCNOs or connected CNO chains were generated and aligned in a line along the path of the catalyst nanoparticles moved through. Since the high energy from the electron beam irradiation can lower


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the conversion energy barrier for sp3-carbon to sp2-carbon,35 the catalytically active γ-Fe-Ni nanoparticles continuously absorbed the carbon atoms from amorphous carbon and graphitized them into the HCNO structure. The same characterization was carried out on the γ-Fe-Ni nanoparticles with the shell structure obtained by hydrogen reducing at 850 °C for 0.5 h. The shell structure cannot be clearly observed, which is attributed to the low contrast of the thin thickness in the background of the amorphous carbon film. The core-shell nanoparticle stayed still on the lacy carbon film during the electron beam focusing, and no graphitic structures were formed around the nanoparticle (Figure S6c-d). The result strongly confirmed that the inner γFe-Ni core nanoparticle is catalytically active for converting carbon atoms into sp2-carbon, whereas the shell structure is catalytically inert. In this manner, the outer shell structure is capable to function as the protective shell and the real catalyst for growing graphitic structures is protected in the inner region. Moreover, Figure S7 clearly shows the graphene layers nucleated from the inner core, further demonstrating that the inner γ-Fe-Ni is the catalytically active component that is responsible for nucleating graphitic structures. It also indicates that the shell structure can be the pathway for carbon atoms to dissolve into the inner γ-Fe-Ni instead of blocking the carbon access and the graphitic formation. Combined with the results in Figure 1df, it is believed that the shell structure can be continuously converted into the inner γ-Fe-Ni core with increasing the temperature or reducing the period. We therefore conclude that the hydrogen thermal treatment can promote the growth of γ-Fe-Ni phase as the inner core, and the shell-tocore conversion can be controlled by the parameters of the thermal treatment. 2.3. Formation Mechanism of HCNOs from Core-Shell Alloy Catalysts. The above results remind us that the catalytic activity of γ-Fe-Ni can be in-situ dynamically controlled and maintained by the shell-to-core conversion, which can possibly be controlled by the thermal and


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hydrogen treatment. Following these findings, we carried out the CVD growth with hydrogen flowing through. It was found that the catalysts can yield a large amount of HCNOs at a hydrogen flow rate (Vhydrogen) of 30~60 sccm (growth temperature of 850 °C, methane flow rate (Vmethane) = 60 sccm). Figure 3a and b show the TEM images of the HCNOs with diameters around 40~50 nm, which have been grown by catalysts with an average size of 20~30 nm. To synthesize CNOs with smaller diameters, we made the catalyst nanoparticles with an average size as small as 5 nm. Figure 3c-d show that a large number of HCNOs with an average size below 10 nm were synthesized by these smaller-sized catalysts at the same condition. This result demonstrates that the size-controlled growth of HCNOs can be realized by tuning the size of the core-shell alloy catalysts. We used the catalyst nanoparticles with the smaller size of ~5 nm to carry out the growth of HCNOs in the following study. To investigate the influence of growth parameters on the synthesis of HCNOs, we carried out CVD growth with different Vhydrogen and Vmethane. As shown in Figure 4a-c, the majority of the products are M@CNOs with few HCNOs when there is no hydrogen flowing, while Vhydrogen with the higher amount (80 and 120 sccm) promotes the growth of short CNTs. Therefore, the growth with medium Vhydrogen of 30~60 sccm preferentially yielded HCNOs. Vmethane also has an influence on the growth of HCNOs. As observed in Figure 4d-f, no carbon structures were formed when Vmethane is 10 sccm. When Vmethane increased to 20 sccm, the HCNOs were grown with a small quantity, while large amounts of HCNOs were formed when increasing the Vmethane to 40 sccm. The above results indicate that the medium Vhydrogen and sufficient Vmethane are essential for the continuous growth of HCNOs.


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Figure 3. TEM images of the HCNOs synthesized by catalysts with an average diameter of 20~30 nm (a-b) and 5 nm (c-d) at the Vhydrogen of 30~60 sccm. We also carried out the CVD growth at different temperatures to examine its effect on the growth of HCNOs. It is shown in Figure S8a-e that no carbon nanostructures were formed when growing at 750 °C, while the growth at 775 °C yielded long CNT structures. The products grown at 800 °C and 825 °C contained mainly short CNT structures, while that grown at 850 °C


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exhibited a majority of HCNOs. These results indicate that the growth temperature of 850 °C is the most suitable one for enhancing the catalytic activity to promote the growth of HCNOs. To study the effect of the catalyst particle size on the growth of HCNOs, CVD growth was also carried out over the catalysts with different metal loading amounts (0.5%, 1%, and 2%). As shown in Figure S8f-h, with an increase of the loading amount of metal catalysts on the substrate from 0.5% to 2%, the density of catalysts increased, and the average size increased from ~5 nm to ~10 nm. It was observed that the nucleation density of the HCNOs grown by M/MgO (2%wt) was significantly less than those grown by and M/MgO with the loading of 0.5%wt and 1%wt. This result suggests that the large-sized catalyst particles have the lower catalytic activity for growing HCNOs.


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Figure 4. TEM images of the carbon nanostructures synthesized by temperature-increasing CVD growth at 850 °C with different growth durations, hydrogen (Vhydrogen) and methane (Vmethane) flow rates. (a-c) Vhydrogen =0, 80, and 120 sccm (Vmethane=60 sccm); (d-f) Vmethane=10, 20, and 40 sccm (Vhydrogen=80 sccm). Figure 5 shows the TEM images of HCNOs with various structures, including quasi-spherical structures, polyhedron structures, interconnected polyhedron structures, few-walled structures and multi-walled structures. The HCNOs generally have sizes below 10 nm, while part of which are around 5 nm.

Figure 5. TEM images of the small-sized HCNOs with (a) quasi-spherical structures, (b) polyhedron structures, (c) interconnected polyhedron structures, (d)-(e) few-walled structures, and (f) multi-walled structures.


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To reveal the origin of the γ-Fe-Ni in the initial stage, we carried out the EDS mapping characterization of the core-shell catalysts formed by hydrogen reducing. As shown in Figure S9, besides the catalyst particle, Fe and Ni atoms are also presented on the surface of MgO substrate around the catalyst regions, which can also serve as the source atoms for forming γ-FeNi in the core nanoparticles. Based on the above results, the HCNO growth mechanism from core-shell alloy catalysts is proposed in Scheme 2. During the hydrogen reducing process in thermal treatment, the phase transition in single γ-Fe-Ni phase region will take place, resulting in the formation of the fccstructured γ-Fe-Ni phase crystal as the inner core in the disordered Fe-Ni catalyst (Figure 1c). Since the atomic ratio of Fe/Ni in the precursor is not equal to that in the γ-Fe-Ni, the formation of γ-Fe-Ni will make the remaining compositions of precursor non-stoichiometric to form of any Fe-Ni alloy phases. Thus, the excess metallic atoms will constitute the disordered or short-range ordered shell structure. Eventually, the core-shell-structured Fe-Ni alloy catalysts are formed (Scheme 2a left). The transitional metal nanoparticles with the good crystallinity have a higher carbon solubility than those with disordered or short-range ordered structures. Besides, the graphitic layers tend to be precipitated and nucleated along the well-defined crystalline planes instead of the disordered ones. To this regard, the inner γ-Fe-Ni nanoparticle is catalytically active for growing graphitic structures whereas the shell structure is catalytically inert. Hence, during the CVD growth process, the carbon atoms generated from the methane decomposition are preferentially dissolved in the inner γ-Fe-Ni nanoparticle instead of the shell structure. The inert shell structure acts as the protective layer for the inner real catalyst and serves as the pathway for exterior carbon atoms to enter the inner γ-Fe-Ni. Consequently, the graphene layers


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are nucleated from the inner γ-Fe-Ni nanoparticle (Scheme 2a middle), evolving into a graphitic cap structure for the subsequent structural growth (Scheme 2a right). According to our previous report,10 the growth of the HCNOs or CNT structures is dependent on the interplay between the supplying rate of carbon atoms and the growth rate of the graphitic layers. The graphitic cap structure will be nucleated once the dissolved carbon atoms are saturated in the catalyst nanoparticle. As shown in Scheme 2, if the dissolved carbon atoms can diffuse to the graphene growth region during or before the nucleation of the graphitic cap structure (Scheme 2c, VG < VP, VG and VP represent the velocity of graphene growth and carbon atom precipitation, respectively), the CNT structure can be formed due to the timely carbon supply. Otherwise (Scheme 2b, VG > VP), the graphitic cap structure will be detached from the catalyst surface, followed by the closure of graphitic layers to form the HCNOs with more defects, as indicated by the lower IG/ID ratio of the HCNOs than that of the M@CNOs in Figure S10. Furthermore, to grow more HCNOs from one catalyst nanoparticle, the catalytically active metallic atoms have to be continuously supplied into the catalyst to maintain its activity. Therefore, it is concluded that two requirements have to be fulfilled to realize the continual growth of HCNOs, including the lower rate of carbon atom supply than that of graphitic cap growth (VG > VP) and the continuous supply of catalytically active metallic atoms into the catalyst. Herein, the shell structure plays three roles during the growth of graphitic structure: including i) shell conversion: providing the metallic Fe and Ni atoms continuously to convert into the inner γ-Fe-Ni core to in-situ maintain the catalyst activity (black atoms in Scheme 2); ii) pathway: serving as the pathway for carbon atoms to dissolve into the inner γ-Fe-Ni core and buffering their dissolving rate (grey atoms in Scheme 2); iii) protection: protecting the inner γ-Fe-Ni cores


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from aggregation and confining them for in-situ maintaining the catalyst activity. The shell-tocore conversion also avoids the using of oxidizing agents which may easily introduce defects in the graphene structure.

Scheme 2. Schematic of the growth mechanism of the graphitic structures from the core-shell catalyst. (a) Process of the core-shell catalyst formation and the graphitic cap nucleation; (b)


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Formation process of HCNOs from the core-shell catalyst; (c) Formation process of CNTs from the core-shell catalyst. Once the conversion is completed, the inner γ-Fe-Ni catalyst nanoparticle is fully exposed in the atmosphere of carbon atoms, followed by encapsulating by carbon layers and deactivation. To make full use of the core-shell catalysts to grow more HCNOs, it is essential to in-situ maintain the catalyst activity dynamically and constantly before the completion of the shell-tocore conversion. Therefore, we have to make the shell-to-core conversion occur at a medium rate instead of a rather low or high rate. If the shell-to-core conversion is extremely low, the inner γFe-Ni cannot get enough active metallic atoms in time from the shell to maintain its catalytic activity. In this case, only the first requirement VG > VP is fulfilled, and the catalyst will be easily deactivated to form the M@CNO structure. In contrast, if the shell-to-core conversion is extremely high, the high dissolving and diffusion rate of the carbon atoms will result in VG < VP, leading to the formation of CNT structure in the initial stage. The fast shell-to-core conversion makes the shell-to-core conversion completed in a short time, leading to the fast exposure of γFe-Ni to the carbon atmosphere. Thus, in this case, neither of the requirements for the continual growth of HCNOs is satisfied, and the catalyst will be deactivated soon, forming short CNT structures. These discussions help us to understand the above experimental results. According to the results of Figure S8a-e, graphitic structures were nucleated in a temperature ranging from 750 to 775 °C. The shell-to-core conversion can be controlled by adjusting the hydrogen flow rate. A high flow rate (80~120 sccm) will result in a fast shell-to-core conversion, which consequently formed short CNT structures are formed (Figure 4b and c). In case of no hydrogen, the shell-tocore conversion is rather slow, and only the first requirement VG > VP is satisfied, leading to the


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formation of M@CNOs as the main product (Figure 4a). Only in case of a medium hydrogen flow rate (60~80 sccm), both of the requirements are satisfied and the HCNOs can be continually nucleated from the core-shell catalysts. To this end, tuning the hydrogen flow rate is essential for shell-to-core conversion and the catalytic activity controlling. The shell structure can buffer the dissolving rate of the carbon atoms approaching the inner γ-Fe-Ni, winning time for itself to provide metallic atoms to the inner catalyst core to in-situ maintain the catalytic activity. The dissolving of the carbon atoms into the inner γ-Fe-Ni also becomes easier with reducing the thickness of the shell structure. To verify of the conversion of the shell structure and to make a theoretical understanding of the crystalline growth, we carried out molecular dynamics (MD) simulations using LAMMPS program. The γ-Fe-Ni core region with the periodic atom arrangement and the shell structure with the disordered atom arrangement were established within a single nanoparticle. The simulation was conducted by maintaining a constant pressure of 0.1 MPa and a temperature of 1100 K with a time step of 1 fs. A clear trend of growing into crystallization is observed for the outer shell structure, as illustrated in the simulation process in Figure 6a-d. Figure 6a presents the side view of the core sliced in half, and FeNi atoms outside the core were randomly distributed for the initial configuration of the simulation. The vacuum forms on all corners of the simulation box as the ambient pressure is loaded. As the simulation undergoes, Fe and Ni atoms in the outer shell becomes periodically and regularly arranged, and starts merging with the core, as shown in Figure 6c. Finally, the fcc-structured γ-Fe-Ni substitutional solid solution is formed throughout the whole simulated structure (Figure 6d). It is thus believed that the crystallized conversion of the shell structure into the inner core is facilitated by the existed γ-Fe-Ni core with the fcc structure.


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Figure 6. MD simulation of the conversion of the disordered FeNi outer shell structure into the crystalline structure. (a) A simulation box filled with FeNi fcc-structured core and disordered FeNi atoms outside the core region; (b) Vacuum forms as the simulation starts; (c) Outer shell structure becomes partially crystallized; (d) FeNi substitutional solid solution structure forms throughout the whole simulated region. 2.4. Supercapacitor Performance of the HCNOs. The advantage of the HCNO structure in energy storage was evaluated by investigating the supercapacitor performance of the HCNOs and M@CNOs. Supercapacitor devices were assembled in coin cells using the CNOs as the symmetric electrode materials and [HMIm]NTf2 as the electrolyte. The CV measurements at scan rates up to 2 V s-1 were tested on both structures with a potential window of 0~2 V. Figure 7a and b show the CV curves at low scan rates from 0.2 to 2 V s-1. The CV curve of the HCNOsbased supercapacitor keeps a near-rectangular shape at the scan rate up to 2 V s-1, while that of the M@HCNOs-based supercapacitor keeps the near-rectangular shape only up to 1 V s-1. The result indicates that HCNOs have higher rate capability, which is beneficial for fast chargingdischarging applications. Figure 7c shows the Nyquist plots of the devices. The HCNOs-based supercapacitor has much lower charge transfer resistance than the M@CNOs-based supercapacitor. The inset in Figure 7c shows the Bode plots of the devices. The HCNOs- and M@CNOs-based supercapacitors have nearly overlapped curves in a wide frequency range, suggesting that they have similar phase angles. Combined with the Nyquist plots, it can be


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concluded that the HCNOs-based supercapacitor has lower real and imaginary resistances than the M@CNOs-based supercapacitor when the frequency is the same. Hence, the presence of the inner metallic core will increase the interface and charge transfer resistance. The rate capability performances of the supercapacitors were also evaluated by the galvanostatic charge-discharge measurements. Figure 7d shows that the device based on HCNOs has the specific capacitance of 22.1 F g-1 at a current density of 50 mAg-1, nearly twice of that based on M@CNOs (11.5 F g-1). It is worth noting that the capacitance value of the HCNOs-based supercapacitor here is higher than those of the reported supercapacitors based on CNOs36 and MWNT arrays.9 Moreover, when the current density is increased from 50 mA g-1 to 5 A g-1, the capacitance retention of the HCNOs-based supercapacitor is higher than 82%, superior to that of the M@CNOs-based supercapacitor (54%), indicating a better rate capability of the HCNOs structure. The better energy storage performance of HCNOs can be ascribed to the following aspects: i) the inner hollow space provides additional space for accommodating more ion absorption and thus achieve a higher capacity; ii) HCNOs contain more defects than M@CNOs, facilitating the access of the ions to the inner space; iii) both the outer and inner spaces of HCNOs provide double-layer capacitive surfaces, enabling the fast absorption-desorption transport, which also results in the lower charge transfer resistance and better rate capability. Therefore, the HCNOs are the promising structure for the applications in supercapacitors.


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Figure 7. CV curves of HCNOs (a) and M@CNOs (b); (c) Nyquist plots of HCNOs and M@CNOs, inset shows the corresponding Bode plots; (d) Gravimetric specific capacitance of HCNOs and M@CNOs-based supercapacitors versus discharge current density.

3. CONCLUSIONS In summary, a controlled and continual growth of HCNOs was realized by using a core-shellstructured catalyst in the CVD method. The inner core γ-Fe-Ni catalyst is catalytically active for growing HCNOs, whereas the outer shell structure is catalytically inert and acts as three roles: including the conversion into the inner γ-Fe-Ni, the pathway for dissolving carbon atoms, and the protection of the inner γ-Fe-Ni nanoparticles from aggregation. The shell conversion can continuously provide active metallic atoms to the inner γ-Fe-Ni catalyst to in-situ dynamically


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and constantly maintain the catalyst activity. Moreover, the controlled shell-to-core conversion can be realized by tuning the hydrogen flow rate in a range of 60~80 sccm to continually synthesize HCNOs, and the size-controlled growth of HCNOs can also be realized by tuning the catalyst size. Finally, the performance of the supercapacitors was studied to demonstrate the advantage of the hollow structure in energy storage. The controlled shell conversion developed by the core-shell catalyst strategy provides possibilities in extending the catalyst lifetime through self-maintaining the catalytic activity dynamically and constantly. This study also initiates a new way for the controlled and high-quality growth of different carbon nanostructures for various applications.

4. EXPERIMENTAL SECTION 4.1. Preparation of the Core-Shell Alloy Catalyst Precursor. For preparing the core-shell alloy catalyst precursor, Fe(NO3)3·9H2O and Ni(NO3)2·6H2O, given by the Fe/Ni mol ratio of 1/1 and the total metal loading of 0.5%, 1% and 2%, were dissolved in ethanol. Then, MgO powder was added into the above solution to form the mixture, which was stirred at 70 °C until the ethanol was totally evaporated, ensuring the homogeneous dispersion of the metallic cations on the MgO powder. The Fe-Ni/MgO, as the alloy catalyst precursor, was dried in an oven at 90 °C, and then grounded into fine powder. 4.2. Growth of HCNOs from Core-Shell Alloy Catalysts. The catalyst precursor was loaded in the reaction zone in the CVD reactor. The powder was reduced by the H2/Ar flow (60 sccm/100 sccm) at 550 °C for 1h. For growing HCNOs, the CVD growth was conducted at the temperature range of 750~850 °C under the flow of CH4/H2/Ar (60 sccm/80 sccm/100 sccm). During the CVD growth, the core-shell-structured alloy catalysts were formed, which were


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meanwhile subjected to the carbon growth. For growing M@CNOs, the CVD growth condition is the same except that no hydrogen was introduced. Finally, the samples were cooled down to room temperature under the protection of the Ar atmosphere. 4.3. Preparation of Supercapacitor Electrodes. The produced CNOs were purified by acid to remove the substrate and the residual metallic catalysts. HCNOs were thus dried and obtained, which were mixed with carbon black and polyvinylidene fluoride in 1-methyl-2-pyrrolidinone solvent in a weight ratio of 8:1:1. The slurry was coated on the titanium foil to get an even surface with a thickness of 100 μm as the working electrode, which was dried overnight at 80 °C in a vacuum oven. Two electrodes were assembled into a coin cell with 1-hexyl-3methyllimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm]NTf2) as the electrolyte. By comparison, the electrodes made of M@CNOs were also prepared and assembled into coin cells. 4.4. Characterization. X-ray diffraction (XRD) measurement was conducted using a Rigaku D/max 2500 V/pc diffractometer with a Cu Kα radiation at a wavelength of 1.54 Å to determine the

phase

composition

and

crystallinity.

Transmission

electron

microscope

(TEM)

characterization was conducted on a JEOL 2100F ﬁeld emission gun TEM to observe the microstructure. EDS characterization on the TEM equipment was carried out to detect the elements in the desired regions of the object. Fast Fourier Transform (FFT) on a high-resolution transmission electron microscope (HRTEM) was conducted using DigitalMicrograph software. Electrochemical tests were conducted in a three-electrode system (Pt as the counter electrode and Ag/AgCl as the reference electrode) using ZAHNER-Thales electrochemical working station. 4.5. Simulation Methods. Molecular Dynamic (MD) simulations were carried out using the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) program.37 EAM/alloy


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potential for iron-copper-nickel (Fe-Cu-Ni) compounds38 is chosen for the interactions between Fe and Ni atoms in the model. As an attempt to resemble the core lattice, a sphere with the radius of 1.5 nm is formed by Fe and Ni atoms, at the center of the 3.0 nm × 3.0 nm × 3.0 nm cubic simulation box that was periodic in three dimensions. Positions of constituent Fe and Ni atoms of the spherical structure followed the face-centered cubic (fcc) γ-Fe-Ni lattice with a fixed lattice parameter of 0.35 nm throughout the simulation. The rest of the simulation box was filled with additional randomly distributed 5000 atoms each for Fe and Ni atoms. MD simulation ran over 2 ns as NPT ensemble by maintaining a constant pressure of 0.1 MPa and a temperature of 1100 K with a time step of 1 fs.

ASSOCIATED CONTENT Supporting Information. XRD patterns, Photographs, TEM images, EDS spectra and mapping of the catalyst, TEM images and Raman spectra of the carbon products. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors Naiqin Zhao, Email: [email protected], Tel: +86-22-27891371; Zhihao Yuan, Email: [email protected], Tel: +86-22-60214006 Author Contributions Chenguang Zhang proposed the idea, designed the experiment and wrote the manuscript. Ke Ma provided the assistance in computational simulation. Naiqin Zhao and Zhihao Yuan overviewed the manuscript and provided helpful suggestions. All authors have given approval to the final version of the manuscript.


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Funding Sources National Natural Science Foundation of China (No. 51702233) and Natural Science Foundation of Tianjin City (No. 16JCYBJC41000)

ACKNOWLEDGMENT The authors acknowledge the finance support by National Natural Science Foundation of China (No. 51702233), Natural Science Foundation of Tianjin City (No. 16JCYBJC41000) and support by Tianjin Key Subject for Materials Physics and Chemistry.

ABBREVIATIONS CNOs, carbon nano-onions; HCNOs, hollow carbon nano-onions; M@CNOs, metalencapsulated carbon nano-onions; CVD, chemical vapor deposition; CNT, carbon nanotube; Vhydrogen, hydrogen flow rate; Vmethane, methane flow rate; M/MgO, metal/MgO; VG, the velocity of graphene growth; VP, the velocity of the carbon atom precipitation; XRD, X-ray diffraction; SEM, scanning electron microscope; TEM, transmission electron microscope; FFT, Fast Fourier Transform; HRTEM, high-resolution transmission electron microscope; EDS, Energy Dispersive Spectrometer; MD, molecular dynamics.

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For Table of Contents Use Only

A Core-Shell Strategy for Improving Alloy Catalyst Activity for Continual Growth of Hollow Carbon Onions Chenguang Zhang, Ke Ma, Naiqin Zhao*, Zhihao Yuan*

Table of Contents Graphic:

Synopsis: A controlled and continual growth of hollow carbon nano-onions is realized by using a core-shell Fe-Ni alloy catalyst during CVD process. The inner core γ-Fe-Ni catalyst is catalytically active, whereas the outer shell structure is catalytically inert and continuously supplies active metallic atoms, achieving self-maintaining of the catalytic activity. The core-shell strategy provides the possibilities for extending catalyst lifetime.


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A Core-Shell Strategy for Improving Alloy Catalyst Activity for

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