A facile self-template and carbonization strategy to fabricate nickel nanoparticle supporting N-doped carbon microtubes

Jianping Wanga, Min Zhang*a, Teng Miaoa, Yang Linga, Qiong Wena, Jing Zheng*a, Jingli Xua, Tasawar Hayatbc and Njud S. Alharbid
aCollege of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China. E-mail: zhangmin@sues.edu.cn; kkzhengjing707@163.com
bDepartment of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan
cNAAM Research Group, King Abdulaziz University, Jeddah, Saudi Arabia
dBiotechnology Research Group, Department of Biological Sciences, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

Received 17th January 2018 , Accepted 29th January 2018

First published on 29th January 2018

The efficient preparation of non-precious metal nanocatalysts embedded in carbon nano/microtubes remains a considerable challenge. Herein, we report a facile self-template and carbonization strategy to fabricate nickel nanoparticle (NP) supporting N-doped carbon microtubes. Firstly, molybdenum trioxide (a-MoO3) microrods are coated with polypyrrole (MoO3@PPy) by a simple in situ polymerization route. Then, PPy@PDA-Ni2+ microtubes with a hollow structure are obtained by further coating the PDA-Ni2+ complex in ammonia solution. In contrast, the direct coating of MoO3 microrods with PDA-Ni2+ under the same experimental conditions only leads to aggregated PDA-Ni2+ spheres. Moreover, nickel/N-doped carbon microtubes (Ni/NCMTs) can be obtained by a further heating treatment and highly graphitic carbon microtubes are achieved by etching the nickel nanoparticles. The hollow Ni/NCMTs exhibit excellent reduction activity on 4-nitrophenol (4-NP). Moreover, the size of Ni nanoparticles (Ni NPs) which can effectively control the catalytic performance of the as-prepared nanocomposites is facilely adjusted via changing the roasting temperature. Benefitting from the highly exposed surface, short diffusion distance, and homogeneous Ni NP dispersion, the Ni/NCMT catalyst exhibits an improved activity on 4-NP. This novel structure is helpful for further applications in hydrogen evolution reaction, supercapacitors and batteries.


Heterogeneous catalysts play important roles in diverse catalytic fields, including organic synthesis, energy conversion, fuel cells and bio/gas sensors.1–4 In general, catalysts include active sites and a support which is used to carry metal nanoparticles.5 Hence, the overall catalytic activity is determined by the morphology of metal nanoparticles and the characteristics of the support. Therefore, extensive research efforts have been devoted to constructing metal nanostructures and selecting effective catalyst supports. Various catalyst supports such as activated carbon materials, metal oxides, and zeolites have been applied to stabilize metal nanoparticles for improved dispersibility and catalytic performance.6,7 Among these supports, nitrogen-containing carbon nano/microtubes have been proven to be an excellent support for metal nanoparticles (NPs) by improving the metal–carbon nanotube binding through the coordination ability of nitrogen. The incorporation of nitrogen with carbon nano/microtubes can enhance the chemical reactivity of carbon nano/microtubes, thus it is favorable for the nucleation of fine metal catalyst particles.

Recent studies demonstrated that nitrogen atoms in carbon materials can serve as coordination sites for transition metal ions8 and form an N-doped carbon–metal heterojunction.9 Therefore, it is an effective route to enhance the catalytic performance by introducing uniform N dopants into carbonaceous supports because it is beneficial for anchoring uniformly dispersed metal NPs as well as decreasing their size distribution. Thus, extensive attention has been focused on the research of N-doped carbon materials. Li et al. demonstrated that palladium nanoparticles supported on mesoporous N–doped carbon nanotubes show high activity in promoting biomass refining during biofuel upgradation.2 Zhou et al. developed an efficient method for synthesizing ultrasmall cobalt NPs supported on nitrogen-doped porous carbon nanowires and they exhibited high performance in hydrogen evolution from ammonia borane.10 Lv et al. utilized a hierarchical N-doped carbon nanotube-graphene hybrid nanostructure (NCNT-GHN) to immobilize noble metal (e.g., Pt Ru) nanoparticles, which exhibited excellent performance in the methanol electrooxidation reaction.11 More recently, Duan and his coworkers reported palladium NP supported nitrogen-doped carbon microtubes and the organocatalyst was applied toward the reduction of 4-nitrophenol.12 However, most previous reports about nitrogen-based carbon materials involved the deposition of noble metal NPs such as Pd, Pt and Ru. Although these catalysts exhibit high catalytic activities, the lack of facile and economically controllable strategies, high price and scarce supply greatly limit their practical application. Therefore, it is essential and desirable to find more economical, effective and environmentally friendly alternatives such as metal free and/or earth-abundant non-precious metal-based catalysts.13

The preparation of Ni-based nanocatalysts is indeed a good choice and also arouses particular interest since this metal is not only abundant and economical, but also highly active and environmental friendly. Xia reported a carbon black (CB) supported nano-Ni catalyst as the reducing agent of nitrophenol.14 Krishna developed a Ni nanoparticle intercalated LTA-type nanozeolite (KZ) which was supported on reduced graphene oxide to reduce 4-nitrophenol.15 Yang et al. developed a one-pot approach to construct a nickel-ion-polydopamine complex thin coating on graphene oxide, which can be carbonized to produce hybrid nanosheets with metallic nickel NPs embedded in a PDA-derived thin graphic carbon layer.16 Despite these successes, substantial efforts are still needed to exploit high-performance Ni based catalysts by using N-doped nano/microtubes as the support. Meanwhile, the fabrication of hollow carbon nanomaterials is of great interest because of their outstanding properties and promising applications,17,18 especially hollow graphitic nano/microtubes. However, the carbon materials obtained by direct carbonization of PDA are usually non-graphitizable,19 while the presence of a transition metal could promote the graphitization and lead to a graphitic carbon shell. It was found that nickel NPs could improve the graphitization of amorphous carbon and hollow-tunneled graphitic carbon nanofibers were produced by chemical activation and acid treatment.20 Therefore, it is of great interest to explore and develop a facile strategy for the preparation of N-doped porous highly graphitic carbon microtubes.

Herein, we established a mild strategy to synthesize hollow PPy@PDA-Ni2+ microtubes from MoO3@PPy nanocables, involving in situ polymerization of dopamine with the aid of nickel ions and the removal of MoO3 cores in alkaline solution with a one-step reaction. The hollow PPy@PDA-Ni2+ microtubes can be further transformed into hollow carbon microtubes consisting of Ni NPs entrapped in the N-doped microtubes. The composite shows the following several advantages: (1) the hollow microtubes and pores from the removal of MoO3, which can provide a large specific surface area; (2) the removal of MoO3 cores and the coating of the PDA-Ni2+ complex could be simultaneously achieved within one step, which not only saves time and improves the efficiency, but also does not use some harmful solvents (such as HF); (3) the easy to operate, cost-effective synthetic method enables the large-scale production of Ni/NCMTs; (4) a high degree of graphitic carbon-tubes can also be obtained through changing the roasting temperature and acid treatment, which may be used in other applications such as batteries or supercapacitors. As expected, the as-prepared Ni/NCMTs served as a good catalyst for the 4-nitrophenol reduction and they exhibit superior catalytic activity and cycling stability.


Chemicals and reagents

Pyrrole and dopamine hydrochloride were obtained from the Alfa Aesar Chemical Company. Nickel chloride six hydrate (NiCl2·6H2O), ammonia solution (28–30%), deionized water and absolute ethanol were used for all analytical grade experiments. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), concentrated nitric acid, sodium borohydride (NaBH4) and ammonium persulfate ((NH4)2S2O8) were purchased from Sinopharm Chemical Reagent co. Ltd. Other chemical reagents were purchased from Shanghai chemical reagent company. All reagents were used directly without further purification.

Synthesis of MoO3 microrods

The synthesis of MoO3 microrods is based on a reported hydrothermal method.21 In a typical synthesis, 1 g of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) was dissolved in a mixed solution of 65% HNO3 and deionized water with a volume ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]5. After being fully dissolved, this reaction solution was transferred into a Teflon-lined stainless steel autoclave (50 mL capacity) and heated at 180 °C in a preheated electric oven for 20 h. The products were washed several times with deionized water and ethanol after it cooled down to room temperature and then dried at 60 °C for one night.

Synthesis of MoO3@PPy

The MoO3@PPy composites were synthesized through in situ oxidative polymerization according to a previous report with minor modification.22 Freshly prepared MoO3 (0.1 g) microrods were homogenously dispersed into a mixed solution of 40 mL deionized water and 5 mL anhydrous ethanol. With vigorous stirring, 0.1 mL of pyrrole was added to the white suspension and stirring was continued for 0.5 h. Then 0.329 g of (NH4)2S2O8 (dissolved in 5 mL of deionized water) was added dropwise into the above solution, and the polymerization was performed for 4 h. The black solution was filtered and then dried at 60 °C to obtain the MoO3@PPy nanocomposites.

Synthesis of PPy@PDA-Ni2+ and nickel/N-doped carbon microtubes (Ni/NCMTs)

Typically, 50 mg of MoO3@PPy composites were uniformly dispersed in a mixture solution of 25 mL absolute ethanol and 15 mL deionized H2O by ultrasonication. Then, 5 mL of ammonia aqueous solution was added and stirred for 5 min; after this, 45 mg of dopamine and 113 mg of NiCl2·6H2O were added into the above solution and stirred for 16 h at room temperature. Subsequently, the product was collected by centrifugation and washed with deionized water and ethanol several times and dried at 60 °C. Lastly, the obtained PPy@PDA-Ni2+ were burned in a tube furnace under an N2 atmosphere at 500 °C, 700 °C, and 900 °C respectively for 5 h to obtain Ni/NCMTs-500, Ni/NCMTs-700 and Ni/NCMTs-900.


The morphology and microstructures of the samples were characterized by scanning electron microscopy (SEM, JEOL-4800) and transmission electron microscopy (TEM, JEOL-1011). The crystal structure of samples was determined with an X-ray diffractometer (XRD). The XRD pattern of each sample was recorded with a Shimadzu (Japan) D/Max-2500 diffractometer, using a monochromatized X-ray beam with nickel-filtered Cu Kα radiation. Thermogravimetric analysis was conducted from room temperature to 800 °C in air with a ramping rate of 10 °C min−1 using a STA449 F3 Jupiter thermogravimetric analyzer (NETZSCH). XPS of the PPy@PDA-Ni2+ and its calcined sample were obtained using a Physical Electronics PHI 5600 XPS spectrophotometer with a monochromatic Al K_ (1486.6 eV) excitation source. The data of UV-vis adsorption were obtained by using a UV-2450 spectrophotometer (Shimadzu, Japan).

Catalytic reduction of 4-nitrophenol

The catalytic hydrogenation reaction of 4-nitrophenols was chosen as the model reaction to value the performance of Ni/NCMTs. It was conducted in the presence of excess NaBH4 in aqueous solution at room temperature with different catalysts prepared at different temperatures. Typically, 10 mg NaBH4 was mixed with freshly prepared 4-nitrophenol aqueous solution (0.1 mM, 10 mL). Afterwards, the as-prepared Ni/NCMT nanocatalysts (1 mg) were added into the reaction mixture. During the reaction process, the bright yellow color became more and more shallow and eventually colorless which was monitored by UV-vis spectroscopy. The catalyst was collected from the reaction solution with an external magnetic field. And then the spent catalyst was washed several times with distilled water, and utilized under the same experimental conditions to test the catalytic activity.

Results and discussion

Fig. 1 shows a general procedure for the preparation of nickel/N-doped carbon microtubes (Ni/NCMTs). Firstly, MoO3@PPy microrods were synthesized through the in situ polymerization of pyrrole using MoO3 microrods as the template and ammonium persulfate as the oxidization initiator. Secondly, a one-step transformation reaction was initiated to form PPy@PDA-Ni2+ microtubes with the retention of their microscopic one-dimensional morphology by adding dopamine and nickel salt into a suspension of MoO3@PPy in a mixture solution of ammonia, ethanol, and deionized water. Notably, in the formation of PPy@PDA-Ni2+ microtubes, ammonia played dual roles in both the polymerization of dopamine and removal of MoO3 cores at the same time. Finally, the precursor PPy@PDA-Ni2+ microtubes were calcined under an inert atmosphere and Ni/NCMTs were obtained.
image file: c8qi00039e-f1.tif
Fig. 1 Schematic illustration of the preparation process of the nickel/N-doped carbon microtube (Ni/NCMTs) composite and the corresponding TEM images of products.

To investigate the structures, the scanning emission electron microscopy (SEM) and transmission electron microscopy (TEM) images of MoO3, MoO3@PPy, PPy@PDA-Ni2+ and Ni/NCMTs-500 are shown in Fig. 2. The SEM (Fig. 2a) and TEM images (Fig. 2b) show that the MoO3 template possesses a rod-like structure with a uniform diameter of ∼300 nm and a length of ∼10 μm. The crystal phase of MoO3 was further confirmed by X-ray diffraction in Fig. S1, which suggested that all the diffraction peaks of MoO3 were in good agreement with the literature values (JCPDS file no. 05-0508),23 so it can be indexed as the orthorhombic MoO3 phase. Without using any surfactant, the MoO3 microrods were successfully coated with the PPy shell. As shown in Fig. 2c, the surface of MoO3@PPy is rougher than that of MoO3 and the thickness of PPy is average 80 nm according to the TEM image (Fig. 2d). The XRD pattern (Fig. S2a) also shows that the peak intensity of the MoO3@PPy composites is obviously weaker than that of the virginal MoO3 microbelts. This can be mainly attributed to the semi-crystalline nature of the PPy coating.24,25 The one-step transformation reaction was initiated by adding dopamine and nickel salt into a suspension of MoO3@PPy in a mixture solution of ammonia, ethanol and water. The PPy@PDA-Ni2+ microtubes were formed by both PPy shell and coordination assembly between Ni2+ ions and PDA. Fig. 2d shows the hollow structure of PPy@PDA-Ni2+, proving that MoO3 was completely etched by ammonia which was also in accordance with XRD analysis (Fig. S2b). In our previous work, the PDA-Ni2+ complex was coated on various supports including Fe3O4, SiO2, MnO2 and carbon nanofibers in ethanol, ammonia and water solution.26,27 Meanwhile, the MoO3 template can also be removed in an aqueous ammonia solution.28 Herein in this work, the ammonia solution played dual roles in the formation of PPy@PDA-Ni2+ microtubes. Firstly, it provided an alkaline environment to facilitate the polymerization of dopamine. Second, the ammonia solution etched the MoO3 cores to generate hollow microtubes within a one step reaction. Followed by calcination under an inert atmosphere at 500 °C, Ni/NCMTs-500 could be easily obtained and it offered excellent performance in the catalysis of 4-NP. As shown in Fig. 2g, h, Ni/NCMTs-500 became much rougher after carbonization and the inner and outer surfaces were decorated with tiny metallic Ni NPs. Wide-angle X-ray diffraction further confirmed that peaks at 44.5° and 51.6° are associated with the (111) and (200) planes of the face-centered-cubic structure Ni (PDF-04-0850).14

image file: c8qi00039e-f2.tif
Fig. 2 SEM and TEM images of MoO3 (a, b), MoO3@PPy (c, d), PPy@PDA-Ni2+ (e, f) and Ni/NCMTs-500 (g, h).

To further understand the influencing factors on the transformation from MoO3@PPy microcables to PPy@PDA-Ni2+ microtubes in ammonia solution, different amounts of ammonia solution were also evaluated on the morphology of the resulting PPy@PDA-Ni2+ microtubes. Control experiments were conducted in which all the parameters in the above experiment were systematically investigated and only the amount of the ammonia solution was changed. It can be found that the amount of ammonia can well modulate the morphology of PPy@PDA-Ni2+ microtubes. The PPy@PDA-Ni2+ exhibited the hollow microtube morphology with different amounts of ammonia, while many PDA-Ni2+ nanospheres were observed in the presence of 0.5 mL ammonia (Fig. 3a and b). As the ammonia was increased to 2 mL, a few PDA-Ni2+ nanospheres were attached onto the PPy@PDA-Ni2+ microtubes (Fig. 3c and d). When the excess ammonia was 5 mL, no PDA-Ni2+ nanospheres were found on the final products (Fig. 2g and h), and PPy@PDA-Ni2+ microtubes with a smooth surface were obtained, which further proved that the PDA-Ni2+ shell layer was coated on the surface of PPy microtubes. Meanwhile, the resulting products synthesized in 0.5 mL and 2 mL ammonia solution were collected and determined by XRD. As expected, they were amorphous and no peaks of MoO3 were observed in the XRD pattern (Fig. S3), suggesting the removal of MoO3 cores during the PDA-Ni2+ coating. SEM and TEM images (Fig. S4) show that the direct calcination treatment at 500 °C on PPy@PDA-Ni2+ prepared in 0.5 mL and 2 mL ammonia produced hollow Ni/NCMTs (named Ni/NCMTs-0.5 and Ni/NCMTs-2 respectively) as well as the C–Ni nanospheres derived from the aggregation around the microtubes. XRD patterns of Ni/NCMTs-0.5 and Ni/NCMTs-2 are also consistent with standard card JCPDs 04-0850. However, the diffraction peaks of Ni/NCNTs-0.5 are sharper and more intense than Ni/NCMTs-2 (Fig. S4), revealing that the aggregated Ni NPs becomes bulky and bigger in size. Obviously, the strategy for constructing hollow microtubes described here proceeds spontaneously without the removal of micro/nanobeads with harsh oxidizing agents or an applied potential involved in the Kirkendall effect and the galvanic displacement methods in the coating of the PDA-Ni2+ complex. Based on the above investigations, we suggested that the formation of the PPy@PDA-Ni2+ hollow microtubes went through a transformation from the solid MoO3@PPy to PPy@PDA-Ni2+ hollow microtubes and should include a core dissolution and PDA-Ni2+ shell deposition process with a one-step reaction, typical of the “kill two birds with one stone” process.

image file: c8qi00039e-f3.tif
Fig. 3 SEM and TEM images of PPy@PDA-Ni2+ prepared in 0.5 mL (a. b) and 2 mL (c, d) ammonia solution.

To extend this synthetic strategy, RF-Ni2+ was used as another demonstration for the synthesis of the hollow Ni-based microtubes. The previous work reported that resins-formaldehyde (RF) chelated with the transition metal ion during the polymerization reaction and it showed a similar effect like PDA on the coating process.29 Therefore, it is interesting to note that this strategy can be easily extended to the synthesis of the hollow Ni-based microtubes by simply replacing PDA-Ni2+ with RF-Ni2+, while the price of RF is lower than PDA. Using this extended method, we were able to coat the RF-Ni2+ complex on MoO3@PPy (Fig. S5). It must be mentioned that after the calcination of the PPy@RF-Ni2+, the resultant products possessed well-defined shells and uniform Ni NPs, certifying the possibility of using the same strategy for synthesizing other Ni-based carbon microtubes.

Furthermore, a similar procedure was conducted according to the above synthesis of hollow PPy@PDA-Ni2+ using MoO3 microrods instead of MoO3@PPy. The SEM image (Fig. S7a) showed that aggregated PDA-Ni2+ nanospheres were observed in 2 mL ammonia rather than hollow microtubes. The TEM image (Fig. S7b) demonstrated that the MoO3 template was completely etched by the ammonia solution. Moreover, similar results were also achieved using 0.5 mL and 5 mL ammonia (Fig. S7c and d). The PDA-Ni2+ powder was calcined by the same annealing process to generate Ni@C spheres. As shown in Fig. S7e, f, uniform Ni NPs were embedded in the final aggregated carbon spheres. These results further indicated that the MoO3@PPy template played a vital role in both coating the PDA-Ni2+ shells and maintaining tubular structures to form PPy@PDA-Ni2+ microtubes.

In-depth analyses into the chemical composition and the nature of chemical bonds for PPy@PDA-Ni2+ (a) and Ni/NCMTs-500 (b) composites were conducted by XPS. The XPS survey spectrum (Fig. 4A) confirmed the presence of C, N, O and Ni. The Ni 2p (Fig. 4B(a)) XPS spectrum of PPy@PDA-Ni2+ showed peaks at 855.7 eV and 873.4 eV in a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 area ratio, readily assigned to the Ni 2p3/2 and Ni 2p1/2 transitions, respectively.30 As for the Ni 2p spectrum of Ni/NCMTs-500 (Fig. 4B(b)) composites, two peaks containing a high energy band at 855.3 eV and a low energy band at 872.8 eV were observed, which must be attributed to Ni 2p3/2 and Ni 2p1/2 respectively. Meanwhile, the peak of metallic Ni could be observed at 852.7 eV.31,32 A deconvoluted Ni 2p spectrum (Fig. S8) exhibited a complicated structure with intense satellite signals of high binding energy adjacent to the main peaks due to multi-electron excitation.33 The existence of Ni2+ was reasonable on account of that the Ni NPs on the surface could be easily oxidized to form Ni oxide and hydroxide when exposed to air in the presence of water.34 The C 1s spectra were identified as graphitic sp2-bonded amorphous carbon at 284.6 eV (Fig. 4C). In Fig. 4D, the N 1s signal revealed two components centered at (398.4 eV), and (400.8 eV), which were attributed to pyridinic N and graphitic N species.35

image file: c8qi00039e-f4.tif
Fig. 4 XPS of PPy@PDA-Ni2+ (a) and Ni/NCMTs-500 (b): (A) survey spectra. (B) Ni 2p, (C) C 1s and (D) deconvoluted N 1s of Ni/NCMTs-500.

FT-IR spectra of MoO3, MoO3@PPy, PPy@PDA-Ni2+ and Ni/NCMTs-500 are also identified in the range of 400–4000 cm−1. As can be seen from Fig. 5A, the characteristic peaks of MoO3 appeared at about 552 cm−1, 871 cm−1, and 997 cm−1, which agreed well with those in the literature.36 By contrast, in the spectrum of MoO3@PPy coaxial microcables, the peaks at 997, 871 and 552 cm−1 shifted to 940, 864 and 545 cm−1, respectively, and the intensity of these peaks was also decreased, indicating the mutual interaction between the PPy and α-MoO3. Furthermore, the characteristic peaks of PPy also appeared in the MoO3@PPy coaxial microcables. The peak at 1568 cm−1 is assigned to the C[double bond, length as m-dash]C stretching vibration in the PPy rings.37 In addition, the peak at about 1189 cm−1 is assigned to the symmetric stretching vibration of the C–C bond in the PPy rings.38 For the PPy@PDA-Ni2+ microtubes, the adsorption peaks of MoO3 were lost, reflecting that the MoO3 cores were successfully etched by ammonia solution in the fabrication of PPy@PDA-Ni2+ microtubes. After carbonization, most of the characteristic peaks of organic groups disappeared, confirming that PPy and PDA groups were successfully carbonized. The bands at 3453 cm−1 and 1628 cm−1 corresponded to the stretching vibration of the O–H and water molecules respectively, which may be due to the absorption of moisture on the material surface.22 As depicted in Fig. 5B, the thermogravimetric trace curve showed a weight loss of 8.6% between 20 °C–120 °C, which was due to the removal of the adsorbed water and residual solvent. A further weight loss of 51.7% occurred in the range of 120 °C–800 °C, pertaining to the thermal decomposition of PPy and PDA. Fig. 5C shows the Raman spectrum of Ni/NCMTs-500 composites with two instinct absorption peaks. The peaks at ∼1350 cm−1 and ∼1570 cm−1 were D (disordered carbon) and G (graphitic carbon) band respectively.39 The ID/IG ratio was 0.92 for Ni/NCMTs-500, suggesting a good graphitized nature, which was corresponding with the XPS results. The surface area and pore size distributions of the as-synthesized Ni/NCMTs-500 were characterized using the nitrogen adsorption/desorption isotherm. The general shape of the N2 adsorption isotherm presented in Fig. 5D can be classified as type IV according to the IUPAC nomenclature, which was characteristic of the appearance of the hysteresis loop at relatively high pressure. Besides, the specific surface area of Ni/NCMTs-500 was calculated to be 66.3 m2 g−1 and the average pore size was about 4 nm. The mesoporous nature of the as-formed nanocomposite made it suitable for catalysis and electron transport. The magnetic properties of Ni/NCMTs-500 were determined using a vibrating sample magnetometer (VSM). As illustrated in Fig. S9A, the specific saturation magnetization of Ni/NCMTs-500 was 1.22 emu g−1. As shown in Fig. S9B, the Ni/NCMTs-500 could be lifted with a magnet even inside a glass vial, indicating the strong interaction between the hybrids and external magnetic field, which was of benefit to the recycling catalysis.

image file: c8qi00039e-f5.tif
Fig. 5 (A): FT-IR absorption spectra of MoO3 (a), MoO3@PPy (b), PPy@PDA-Ni2+ (c) and Ni/NCMTs-500 (d); (B): TGA curves of PPy@PDA-Ni2+ microtubes. (C): Raman spectrum of Ni/NCMTs-500; (D): nitrogen adsorption–desorption isotherms of Ni/NCMTs-500; the inset shows the corresponding pore-size distributions, derived from the adsorption branch of the isotherms by applying the BJH method.

The effect of carbonization temperature on the structure was also evaluated in the pyrolysis of PPy@PDA-Ni2+ at 700 °C and 900 °C and their morphologies were characterized by SEM and TEM in Fig. 6. For Ni/NCMTs-500 (Fig. 1g and h), several Ni NPs with a diameter of 5–10 nm were distributed uniformly in the inner and outer part of the N-doping carbon microtubes. With the temperature increase to 700 °C, the diameter of Ni NPs covering on the support materials increased to 15–25 nm, which was attributed to the sintering effect of nickel NP aggregation at high temperature. Moreover, as shown in Fig. 6a, small pores can be observed in the surface of carbon tubes which resulted from the inserting of nickel into the inside of the support. On further increasing the calcination temperature to 900 °C, Ni/NCMTs-900 still maintained a tubular structure, which successfully illustrated the stability of samples. However, the aggregation of Ni NPs was serious and the average size of Ni particles was ∼70 nm (Fig. 6c and d). Meanwhile, it is notable that some of the nickel NPs were embedded or encapsulated in the N-doped carbon tubes.20

image file: c8qi00039e-f6.tif
Fig. 6 SEM and TEM images of Ni/NCMTs-700 (a, b) and Ni/NCMTs-900 (c, d) which were prepared by annealing PPy@PDA-Ni2+ under an N2 atmosphere at 700 °C and 900 °C respectively. Inset: Corresponding high multiple SEM and TEM images.

XRD patterns of Ni/NCMTs-700 and Ni/NCMTs-900 were further characterized (Fig. S10). From Ni/NCMTs-700 to Ni/NCMTs-900, the characteristic diffraction peaks of Ni at 44.5°, 51.8° and 76.4° were obvious and the reflection peaks of nickel were enhanced with the increase of temperature. This phenomenon could be attributed to a high degree of crystallinity and the aggregation of Ni NPs at high temperature, which agreed with the above SEM and TEM analysis. What's more, it is interesting to note that the broad peak at 26.3° in Fig. S10b is owing to the graphitized carbon, indicating severe graphitization at 900 °C, which is ascribed to that Ni promotes the graphitization at high temperature. To prove this view, N-doped carbon microtubes (NCMTs-900) and N-doped graphitized carbon microtubes (NGCMTs-900) were synthesized with minor modification according to the preparation of Ni/NCMTs-900 (experimental detail is shown in the ESI). The microstructure of the NCMTs-900 and NGCMTs-900 was investigated by XRD, SEM, and TEM (Fig. 7). The XRD patterns in Fig. 7A(a) showed that the NCMTs exhibited a broad band typical of amorphous carbon at around the 2θ = 20–30° region, while the XRD pattern of the NGCMTs-900 exhibited a sharp peak at around 2θ = 26.3°, which can be assigned to the (002) diffraction of graphitic carbon. The three peaks at 43.8°, 51.2° and 75.3° were in accordance with the standard card of carbon (JCPDs#43-1104). Fig. 7B shows that NCMTs-900 was simple macrotubes, while NGCMTs-900 showed a loose, porous and typical lamellar structure, suggesting the high degree of graphitization.

image file: c8qi00039e-f7.tif
Fig. 7 A: XRD pattern of NCMTs-900 (a) and NGCMTs-900 (b); B: SEM and TEM images of NGCMTs-900 (a, b) and NCMTs-900 (c, d).

Catalytic function of Ni/NCMTs

4-Nitrophenol has been widely used in evaluating the catalysis performance of various catalysts.40–42 Herein, the catalytic activities of Ni/NCMTs were investigated by the reduction of 4-nitrophenol to 4-aminophenol with excess NaBH4 and UV-vis spectroscopy was used to monitor this reaction. Fig. 8a shows the UV-vis spectrum changes of the reaction mixture in the catalytic progress. It was observed that the absorption peak of pure 4-NP solution was at 317 nm; after mixing with NaBH4, an absorption peak at about 400 nm could be observed.43 Furthermore, the peak at about 400 nm decreased after adding Ni/NCMTs, whereas the absorption peak at about 295 nm corresponding to 4-AP44 increased simultaneously. After a short time, the reaction mixture turned from bright yellow into colorless gradually (right in the insert picture), indicating that 4-NP was completely reduced. The catalytic performance of Ni/NCMTs pyrolyzed at different carbonization temperatures was monitored under the same conditions and the catalytic results are shown in Fig. 8b, c and d which correspond to Ni/NCMTs-500, Ni/NCMTs-700 and Ni/NCMTs-900 respectively. Notably, in the mixture resolution, the amount of NaBH4 was excess. Hence, the reduction rate can be assumed as a constant in the whole reduction process. A pseudo first-order kinetic equation which is written as ln(C/C0) = ln(A/A0) = −kt can be applied to evaluate the catalytic rate of different catalysts, where C0 is the initial concentration, C is the concentration of 4-NP at time t and k is the apparent rate constant. The ln(C/C0) vs. time plot is displayed in Fig. 8e, the rate constant k is calculated to be 5.7 × 10−3 s−1 (Ni/NCMTs-500), 8.7 × 10−3 s−1 (Ni/NCMTs-700) and 4.2 × 10−3 s−1 (Ni/NCMTs-900) respectively. Nevertheless, it is not entirely reasonable to compare different supported catalysts according to rate constant k for the loading of different nickel amounts. Therefore, the activity parameter κ which was defined as the ratio of k (the rate constant) to the loading amounts of catalysts was introduced.45 According to the ICP data which are shown in Table S1, the activity parameter κ is 0.139 mg s−1, 0.071 mg s−1 and 0.032 mg s−1 for Ni/NCMTs-500, Ni/NCMTs-700 and Ni/NCMTs-900 respectively. In addition, the catalytic performance of NGCMTs-900 was also monitored under the same conditions. Fig. S11a demonstrates that the NGCMTs-900 was also active for the 4-NP reduction as the intensity of the absorption peaks decreased obviously compared to the original sample after 60 min and the rate constant k was calculated to be 5.8 × 10−4 s−1 according to Fig. S11b. This should be attributed to the doped nitrogen atom in carbon microtubes and the existing Ni NPs protected by graphitized carbon.46 Based on the above results, the Ni/NCMTs-500 exhibits the highest catalytic activity with a turnover frequency (TOF) of 12.5 min−1. Furthermore, the comparison of catalytic activities in our work and other previous reports are presented at Table S2. Hence, it can be observed that the catalytic performance of Ni/NCMTs-500 was higher than most catalysts. In addition, the cycle test of the representative Ni/NCMTs-500 was carried out to check the reusability of nickel catalysts under the same conditions as for the first run. As displayed in Fig. 8f, it can be found that the reaction also occurred successfully and conversion remained higher than 85% even when Ni/NCMTs-500 was reused 5 times. And the SEM image of Ni/NCMTs-500 was observed after recycling, as shown as Fig. S12, which shows that the spent Ni/NCMTs-500 catalyst well maintains the morphology and structure of one dimensional microtubes and the size of Ni NPs does not change significantly. Therefore, Ni/NCMTs-500 exhibited excellent catalytic stability.
image file: c8qi00039e-f8.tif
Fig. 8 (a) UV-vis spectrum changes of the reaction mixture in the catalytic progress; (b) UV-vis spectra of the Ni/NCMTs-500 sample catalyzed 4-NP to 4-AP developed at different reaction times; (c) UV-vis spectra of Ni/NCMTs-700 sample catalyzed 4-NP to 4-AP developed at different reaction times; (d) UV-vis spectra of Ni/NCMTs-900 sample catalyzed 4-NP to 4-AP developed at different reaction times; (e) plot of ln(C/C0) against the reaction time of 4-NP reduction over 1.0 mg Ni/NCMTs-500 sample (black line), 1.0 mg Ni/NCMTs-700 sample (red line) and 1.0 mg Ni/NCMTs-900 sample (blue line) respectively; (f) The recyclability of the Ni/NCMTs-500 as the catalyst for p-nitrophenol.


In summary, we report an efficient and universal strategy to fabricatemetallic nickel NPs decorated on both the inner and outer parts of carbon microtubes using MoO3@PPy as the template, which involves an extended Stöber method and a subsequent carbonization treatment under a nitrogen atmosphere. Notably, the MoO3 cores can be completely and rapidly removed without perturbing the integrity of the PPy shells in the process of PDA-Ni2+ coating. Moreover, the morphology of the resultant hollow PPy@PDA-Ni2+ microtubes can be controlled by the amount of ammonia solution. Followed by carbonization under an inert atmosphere, the metallic nickel NPs were well dispersed onto the inner and outer parts of carbon microtubes, and the corresponding size and density of nickel NPs could be easily tuned by changing the calcination temperature. Meanwhile, a formation mechanism was also proposed for the generation of the hollow-structured metal decorated N-doped microtubes. Furthermore, the resultant materials were further treated by acid treatment to develop N-doped graphitic carbon microtubes. Such highly graphitic hollow microtubes with a large surface area endowed the materials with exceptional properties and performances. Because of the accessible mesoporous structure of the N-doped microtube support and the uniform small size of the nickel NPs, Ni/NCMTs performed efficiently as a recyclable heterogeneous catalyst for the efficient reduction of 4-NP at ambient temperature. In particular, the Ni/NCMTs could be easily separated from the solution due to the magnetic properties. Considering the easily-operable synthetic procedures and the dramatic catalytic activity of Ni/NCMTs, this simple synthetic process can be extended to fabricate other kinds of catalysts, or porous graphitic carbon materials.

Conflicts of interest

There are no conflicts to declare.


The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21305086 and 21371108), the Research Innovation Program of Shanghai Municipal Education Commission (14YZ138), the Special for Outstanding Young Teachers for Outstanding Young Teachers in Shanghai Higher Education Institutions (ZZGJD13016), and the Start-up Funding of Shanghai University of Engineering and Science (2013td08).

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Electronic supplementary information (ESI) available: X-Ray diffraction patterns of MoO3. XRD patterns of MoO3@PPy (a), PPy@PDA-Ni2+ (b) and Ni/NCMTs-500 (c). XRD patterns of PPy@PDA-Ni2+ synthesized in 0.5 mL ammonia (a) and 2 mL ammonia (b). SEM and TEM images of Ni/NCMTs-0.5 (a, b) and Ni/NCMTs-2 (c, d). XRD patterns of Ni/NCMTs-0.5 (a) and Ni/NCMTs-2 (b). Magnetic hysteresis curves of Ni/NCMTs-500. SEM and TEM images of PPy@RF-Ni2+ (a) and the carbonized product (b) SEM and TEM images of PDA-Ni2+ (a, b) prepared in 2 mL ammonia, SEM and TEM images of the carbonized product (e, f); SEM images of PDA-Ni2+ prepared in 0.5 mL (c) and 5 mL (d) ammonia. XPS spectra of Ni 2p monitored in Ni/NCMTs-500 etc. See DOI: 10.1039/c8qi00039e

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