Fabrication of Highly Stable Metal Oxide Hollow ... - ACS Publications

May 12, 2017 - Centre for Clean Environment and Energy, Griffith University Gold Coast Campus, Queensland 4222, Australia. •S Supporting Information...
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Fabrication of Highly Stable Metal Oxide Hollow Nanospheres and Their Catalytic Activity toward 4‑Nitrophenol Reduction Guoqing Wu,† Xiaoyu Liang,† Lijuan Zhang,‡ Zhiyong Tang,§ Mohammad Al-Mamun,§ Huijun Zhao,*,§ and Xintai Su*,† †

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Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China ‡ Center for Micro and Nanotechnology, Harbin Institute of Technology, Harbin 150080, China § Centre for Clean Environment and Energy, Griffith University Gold Coast Campus, Queensland 4222, Australia S Supporting Information *

ABSTRACT: In this paper, hollow nanospheres (HNSs) of metal oxides (NiO, CuO, and NiO/CuO) coated with a porous carbon shell (HNSs@C) with good structural stability were successfully prepared on the basis of the nanoscale Kirkendall effect. The formation process was based on a template-free method, and the as-prepared HNSs@C are very clean compared with products of the template process. In addition, the results of N2 adsorption−desorption noted that both the metal oxide HNSs and the coated carbon were mesoporous structures. Therefore, small molecules can access the inner space of the whole HNSs@C, which was expected to increase the active site area and to show better performances in applied fields, such as catalysts and sensors. As an example of the functional properties, the obtained HNSs@C were investigated as the catalyst for the hydrogenation of 4-nitrophenol (4-NP) and manifested highly catalytic activity and excellent stability. This work has opened up a novel route for the development of metal oxide HNSs nanocatalysts. This straightforward method is of significance for development of clean metal oxide HNSs with high stability and multiplied applications. KEYWORDS: hollow nanospheres, hydrogenation, metal oxide, carbon shell, nanocatalyst

1. INTRODUCTION 4-Nitrophenol (4-NP) is a major class of environmental contaminants, while 4-aminophenol (4-AP) is an important intermediate in the preparation of chemical products such as paracetamol (analgesic), antipyretic, and phenacetin. In addition, 4-AP is also widely used as a photographic developer, corrosion inhibitor, anticorrosion lubricant, and hair-dyeing agent.1−3 Although the reaction to convert 4-NP to 4-AP is thermodynamically feasible, it is kinetically restricted in the absence of a catalyst due to the high kinetic barrier between the mutually repelling negative ions 4-NP and BH4−.4 Thus, it is important to develop an environmental friendly, effective, and cheap catalyst for catalytic hydrogenation of 4-NP. As the major catalysts used in catalytic hydrogenation of 4-NP, noble metal nanoparticles (NPs), such as Ag, Au, Pd, and Pt, have been studied for a long time.4−6 Although those metal catalysts display excellent catalytic activity, their greater mobility causes serious stability problems, such as a tendency to aggregate, changes in shape, and damage to their surface states in the catalytic reaction.7−9 For example, Pt NPs dispersed on PVP showed a high activity, but they may easily lose their catalytic activities upon recycling.9 Consequently, great attention has been paid to non-noble metal catalysts with high stability and low cast, such as transition-metal oxides (TMOs).10−12 However, the lack of active sites usually results in poor © 2017 American Chemical Society

catalytic activities. Therefore, the catalytic performance of TMOs has to be improved through structural and compositional engineering for real applications. Hollow nanospheres (HNSs) have attracted a great deal of attention in recent years due to their characteristics such as low density, high surface-to-volume, and low coefficients of thermal expansion and refractivity, as well as their promising applications in catalysis, gas sensors, energy storage, biotechnology, and many others.13−22 The catalytic activity of Pt HNSs was twice that of solid Pt nanoclusters with roughly the same size for methanol oxidation.19 Consequently, various HNSs have been fabricated via self-templated methods, such as those based on the nanoscale Kirkendall effect, Ostwald ripening, galvanic replacement, and surface-protected etching.23−26 Wang and co-workers reported the synthesis of Co3O4 HNSs with sacrificial templates of carbonaceous spheres that exhibited excellent rate capacity, good cycling performance, and an ultrahigh specific capacity.27 Among them, the nanoscale Kirkendall effect has become an interesting synthesis route to HNSs of various material systems.28−31 The Kirkendall effect was initially proposed to describe the formation of voids at the Received: March 3, 2017 Accepted: May 12, 2017 Published: May 12, 2017 18207

DOI: 10.1021/acsami.7b03120 ACS Appl. Mater. Interfaces 2017, 9, 18207−18214

Research Article

ACS Applied Materials & Interfaces

quartz cuvette and monitored using UV−vis spectroscopy at room temperature. Aqueous 4-NP solution (3.0 mL, 0.10 mM) was mixed with 0.25 mmol of NaBH4 (9.5 mg), resulting in the formation of a deep yellow solution. Subsequently, metal oxide HNSs dispersed in deionized water (50 μL, 1 mg/mL) were added to the yellow solution. The progress of the reaction was determined by UV−vis absorption spectroscopy (Shimadzu UV-2450PC) performed at intervals in a scanning range of 250−500 nm. As the reaction proceeded, it could be observed that the color of the solution vanished gradually. The percent conversion of 4-NP can be calculated by eq 1

interface of two bulk materials due to their different interdiffusion rates.32 In this approach, outward diffusion of the metal cations is much faster than the inward diffusion of the anions, and then an inward flux of vacancies accompanies the outward metal cation flux to balance the diffusivity difference, leading to a continuous evacuation of the core materials and the formation of the hollow structure. Some metal oxide hollow structures have been obtained via the Kirkendall effect, such as NiO,28,33 CuxO,33,34 CoO,35 FexOy,33 ZnO,36 and Al2O3.34,36 Despite the above successes, study on the controlled synthesis of metal oxide HNSs with high stability based on the nanoscale Kirkendall effect is still ascendant. Meanwhile, carbon materials have been considered as excellent supports due to their intrinsic properties, such as high surface area, unique electronic properties, and chemical inertness, as well as thermal stability and high mechanical strength.37−43 Amorphous carbon is an interesting and unique coating material for nanostructures.44 Coating HNSs with a thin layer of amorphous carbon may be an effective method to stabilize the hollow nanostructure. Recently, we have prepared a series of metal/metal oxide NPs/carbon nanocomposites via a molten salt−calcination process.45−49 In this work, highly stable metal oxides HNSs@C were fabricated from metal NPs/carbon nanocomposite based on the nanoscale Kirkendall effect. First, Cu/C, Ni/C, and CuNi alloy/C nanocomposites were prepared with a molten salt−calcination process. Second, the as-synthesized metal/C nanocomposites were calcinated at 400 °C for 2 h to obtain HNSs@C nanocomposites. Toward 4-NP reduction, the prepared HNSs@C showed highly catalytic property and high structure stability. This work has enriched the synthesis methodology of the HNSs and will open up a potential for the development of high-performance nanocatalysts.

conversion (%) = (C0 − Ct )/C0 × 100%

(1)

where C0 is the initial concentration of 4-NP in solution and Ct is the concentration of 4-NP in solution at the time t. In the recycle test, 10 recycles of the activity were measured for NiO/CuO HNSs. After the first run, 0.25 mmol of NaBH4 (9.5 mg) and 10 μL of 30 mM 4-NP were directly added into the reaction system for the second run, and the same process occurred for the next eight runs. 2.4. Characterization. The crystal structure of the samples was determined by a Rigaku D/max-ga X-ray diffractometer (XRD) at a scanning rate of 6°·min−1 with Cu Ka radiation (λ = 1.541 78 Å). The morphology and microstructure of the samples were characterized by transmission electron microscopy (TEM) (Hitachi H-600). The morphology was observed by field-emission scanning electron microscopy (FESEM) on a Hitachi S-8010 field emission electron microscope operating at 20 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo XPS ESCALAB 250Xi instrument with an Al Kα (1486.8 eV) X-ray source. Highresolution TEM measurements were taken on a Tecnai F20 electron microscope. Nitrogen adsorption−desorption measurements were performed at 77 K using a Micromeritics ASAP 2020 physisorption instrument. The UV−visible absorption spectra were recorded using a Hitachi U-3010 spectrophotometer equipped with an integral sphere assembly. The weight percentage was determined using thermal gravimetric analysis (TGA) at an increasing temperature rate of 15 °C min−1 from 50 to 1000 °C, under an O2 atmosphere rate of 10 °C min−1.

2. MATERIALS AND METHODS

3. RESULTS AND DISCUSSION 3.1. Preparation of HNSs@C. As illustrated in Figure 1, the synthesis of metal oxide HNSs@C was simple and

2.1. Starting Materials. All chemicals were of analytical grade and used as received without any further purification. Nickel(II) nitrate hexahydrate [Ni(NO3)2·6H2O, 98.0%] was purchased from Tianjin Guangfu Chemical Reagents Co., Ltd. Copper nitrate trihydrate [Cu(NO3)2·3H2O, 98.0%] and hexahydrate [Co(NO3)2·6H2O, 99.0%] were purchased from Tianjin Shengao Chemical Reagents Co., Ltd. Sodium oleate (C18H35COONa or NaOA, 98%) was purchased from TCI. Absolute ethanol and hexane were purchased from Tianjin Baishi Chemical Reagents Co., Ltd. 2.2. Preparation of Metal Oxide Nanospheres. In a typical synthesis process, 1 mmol of Ni(NO3)2·6H2O (0.29 g), 1 mmol of Cu(NO3)2·3H2O (0.12 g), and 4 mmol of NaOA (0.61 g) were added to a solution containing 10 mL of deionized water, 20 mL of absolute ethanol, and 30 mL of hexane. The mixed solution was stirred at 70 °C for about 30 min. Then 4 mmol of NaOH (0.16 g) was added to the solution and the mixture was stirred for 4 h. After it cooled down, the upper organic phase was dried at 70 °C for 12 h and a green precipitate was obtained. Then the precipitate mixed with 10 g of Na2SO4 was ground for 0.5 h to form a homogeneous powder. The mixture was heated to 500 °C at a heating rate of 10 °C·min−1 under an argon atmosphere and then kept at that temperature for 3 h. After cooling to room temperature, the product was washed with deionized water and ethanol and dried at 80 °C for 6 h, and the metal NPs dispersed on the carbon (NPs/C) nanocomposites were obtained. Then heat treatment was performed in air at 400 °C for 2 h and NiO/ CuO HNSs@C was formed. NiO HNSs@C and CuO HNSs@C were prepared similarly via this designated procedure. 2.3. Catalytic Reduction of 4-NP. The reduction of 4-NP by NaBH4 was chosen as a model reaction to test the catalytic activity of the prepared samples. Typically, the reaction was carried out in a

Figure 1. Schematic illustration of the synthesis and the sturcture of metal oxide HNSs@C.

straightforward. First, metal salt and sodium oleate were dissolved in aqueous and hexane solutions, respectively. Metal ions were transferred to hexane and formed metal-oleate due to the self-assembly of oleate reverse micelles. Second, heated under Ar atmosphere, metal NPs/C precursors were obtained and metal NPs were dispersed evenly with the carbon nanosheets (see TEM images and XRD results of obtained 18208

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Figure 2. (a) XRD patterns of metal NPs/C precursors and (b) XRD patterns of metal oxide HNSs@C.

Figure 3. TEM images of (a) CuO, (b) NiO, and (c) NiO/CuO HNSs@C and their relative particle size distribution (d, e, f, respectively).

metal NPs/C shown in Figure S1 in the Supporting Information). Afterward, the nanocomposites were heated in air; during the procedure, the carbon of the metal NPs/carbon nanosheets was partly oxidized and metal NPs were transferred to metal oxide HNSs on the basis of the nanoscale Kirkendall effect. 3.2. Characterization of HNSs. As seen in Figure 2, the Ni/C, Cu/C, and CuNi/C precursors and NiO, CuO, and NiO/CuO HNSs@C showed differing XRD patterns. The peaks of the pink and yellow lines in Figure 2a were indexed to cubic Ni (JCPDS 87-0712) and Cu (JCPDS 85-1326). The peaks of the green line in Figure 2a were indexed to the mixed cubic Ni and Cu, respectively. The peaks of the pink and yellow lines in Figure 2b were indexed to cubic NiO (JCPDS 73-1352) and CuO (JCPDS 80-0076), respectively. The green line in Figure 2b indicated that the NiO/CuO HNSs composed of NiO and CuO were obtained, which was further confirmed by

X-ray photoelectron spectroscopy (see Figure S2 in the Supporting Information for the XPS spectra of NiO/CuO HNSs@C). No other peaks were observed, indicating the high purity of the obtained metal/C nanocomposites and metal oxide HNSs@C. The morphology and structure of the as-prepared metal oxides HNSs were investigated. As shown in Figure 3a−c, three samples of CuO, NiO, and NiO/CuO HNSs@C were successfully obtained. These HNSs were arranged closely together, without being aggregated. The average sizes of CuO, NiO, and NiO/CuO HNSs@C were 87.3 nm (Figure 3d), 33.6 nm (Figure 3e), and 18.2 nm (Figure 3f), which had uniform size distributions with standard deviations as little as 12.7, 4.77, and 2.81 nm, respectively. The order of size for the three HNSs@C was NiO/CuO < NiO < CuO. The smaller sizes of the NiO/CuO hollow nanospheres might result from the barrier of the surface between NiO and CuO. 18209

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Figure 4. HRTEM images (a, b) and EDS mapping results (c−h) of NiO/CuO HNSs@C.

Figure 5. N2 adsorption−desorption isotherms and pore size distribution of (a) NiO, (b) CuO, and (c) NiO/CuO HNSs@C.

2.52%, 2.94%, and 2.37%, respectively (see Figure S4 in the Supporting Information). To determine the specific surface area and pore size distribution of the structure, N2 adsorption−desorption analysis was carried out. As shown in Figure 5, the results indicated that the adsorption−desorption isotherm of the prepared HNSs was a typical type IV isotherm (according to the IUPAC classification) with a hysteresis loop at relatively high P/P0, indicating the presence of mesopores. The BET surface areas of NiO, CuO, and NiO/CuO HNSs@C were 8.09, 3.90, and 38.81 m2/g, respectively. It was found that the surface area of NiO/CuO HNSs@C was higher those of NiO HNSs@C and CuO HNSs@C, which might be attributed to the smaller size and high surface roughness of NiO/CuO HNSs@C. Mesoporous pores were produced both on the NiO/CuO HNSs and on the coated carbon (inset of Figure 5) during annealing, since both NiO/CuO HNSs and the coated carbon easily produced nanosized pores via high-temperature calcination.50,51 Due to the existence of these mesoporous pores, small molecules will be able to access the inner space of the whole HNSs@C, which facilitates their catalytic applications.52 3.3. Catalytic Reduction of 4-NP with Different HNSs. The catalytic reduction of 4-NP to 4-AP by NaBH4 was chosen to investigate the catalytic activity of the prepared HNSs. For all the experiments, the initial concentrations of 4-NP and NaBH4 were 0.1 mM and 0.08 M, respectively. Figure 6 showed the

In order to further characterize the morphology and composition of the NiO/CuO HNSs@C, HRTEM mapping and EDS were performed. Figure 4a shows a typical HRTEM image of NiO/CuO HNS@C, which exhibited an average diameter of 25 nm. Figure 4b shows a part of a HNS and the corresponding morphology sketch (inset of Figure 4b). It was observed that the shell of a thin layer of NiO/CuO HNSs and coated carbon were 3−5 and 1−2 nm in thickness, respectively. The EDS spectrum of the NiO/CuO HNSs@C (Figure 4c) exhibited the presence of C, Cu, Ni, and O elements (the scanned region from the Figure 4d area). The mapping results of the Figure 4d zone are presented in Figure 4, parts e, f, g, and h, respectively, in which the distributions of C, Cu, Ni, and O elements are the displayed. The zone of the distribution of C element was larger than that of Cu, Ni, and O, which was consistent with the result that carbon coated the surface of NiO/CuO HNSs. Meanwhile, these results indicated that the Cu and Ni elements were evenly dispersed among O element in the inner HNSs layer and carbon was coated on the NiO/CuO HNSs. Selected area electron diffraction (SAED) measurements (see Figure S3 in the Supporting Information for the SAED patterns of the NiO/CuO HNSs@C) suggested that the prepared sample was polycrystalline. In addition, the molar ratio of Ni to Cu was ∼1, which was consistent with that of the raw materials added. The weight ratios of carbon coated on CuO, NiO, and NiO/CuO HNSs determined by TGA were 18210

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Figure 6. Catalytic performance of the obtained metal oxide HNSs@C. (a) UV−vis spectra of 4-NP before and after addition of NaBH4 solution. (b) Catalytic reduction of 4-NP by NaBH4 using CuO HNSs@C as the catalyst. (c) Catalytic reduction of 4-NP by NaBH4 using NiO HNSs@C as the catalyst. (d) Catalytic reduction of 4-NP by NaBH4 using NiO/CuO HNSs@C as the catalyst. (e) Plot of C/C0 versus reaction time and (f) plot of ln(C/C0) versus reaction time for the reduction of 4-NP.

typical UV−vis spectra of absorbance change of 4-NP with NaBH4 in the presence of the prepared HNSs. The color of 4NP changed from light yellow to dark yellow after adding NaBH4 (Figure 6a). The absorption peak at 400 nm remained unaltered for 50 min without the introduction of metal oxide HNSs, indicating that 4-NP was not reduced by aqueous NaBH4 (see Figure S5 in the Supporting Information for the reduction of 4-NP by NaBH4 in the absence of metal oxide HNSs catalysts). As shown in Figure 6b, when a small amount of NiO HNSs@C catalyst was added to the solution, the dark yellow color of the 4-NP solution vanished quickly, indicated by the fast decrease in the absorbance peak at 400 nm. Meanwhile, a new peak appeared at 300 nm, which was assigned to 4-AP. The same spectral changes of reducing 4-NP were also observed for CuO and NiO/CuO HNSs@C, as shown in Figure 6c,d. These results indicated that the obtained samples play an important role in this reaction. The NiO/CuO HNSs exhibited better catalytic performance compared to that of pure CuO and NiO HNSs (Figure 6e,f). Both the synergistic effect between NiO and CuO components and the small size of the NiO/CuO@C hollow nanospheres were beneficial to the enhancement of the catalytic activity of NiO/CuO@C hollow nanospheres. The order of the catalytic activity for the three samples was NiO/CuO > CuO > NiO (Figure 6e,f). In the experiments, excess NaBH4 was used so that the catalytic reduction of 4-NP was zero-order with respect to NaBH4 concentration and followed a first-order rate with respect to the concentration of 4-NP.53 To compare the catalytic activity of the obtained metal oxide HNSs@C with other catalysts reported in the literature, the apparent rate constant (kapp) values were calculated from eq 2 and are shown in Table 1: −dc /dt = kappC

Table 1. Summary of the Catalytic Activity for the Reduction of 4-NP Catalyzed by Metal Oxide HNSs of This Work entry

sample

catalyst concn (mg/mL)

kapp (min−1)

1 2 3

CuO NiO NiO/CuO

0.0017 0.0017 0.0017

0.5984 0.4118 1.5032

(see Table S1 in the Supporting Information for a comparison for the catalytic reduction with some noble-metal catalysts). Meanwhile, the nontoxic and low cost properties of the prepared samples make it a good substitute for noble metals in catalytic hydrogenation. The apparent activation energy (Ea) is a very important parameter to evaluate the catalytic performance of a catalyst. Therefore, the catalytic reduction of 4-NP in the presence of the prepared NiO/CuO HNSs was studied at different temperatures of 293.15, 298.15, 303.15, 308.15, and 313.15 K. As shown in Figure 7a, the reduction rate of 4-NP increased with an increase in the temperature. We obtained Ea on the basis of the linear fitting of ln k vs 1/T and the Arrhenius equation (eq 3). Calculated from Figure 7b, the Ea was 33.56 kJ mol−1, which was approximated to Ea values for several recently reported nanocatalysts, such as Au nanocages (28 kJ mol−1),54 PVPh-Ni3Co nanochains (31.9 kJ mol−1),55 and silver NPs immobilized on Fe3O4@C (40.92 kJ mol−1).56 ln k = ln A − ln Ea /RT

(3)

For the practical application of heterogeneous systems, the recyclability of the catalyst is an important factor. The stability and reusability of the NiO/CuO HNSs nanocatalysts were examined by catalyzing the reduction of 4-NP. As shown in Figure 8, 10 recycles of the activity were measured for prepared NiO/CuO metal oxides HNSs. The catalytic process was achieved within 3.3 min for the 1st run and 6.8 min for the 10th run. The NiO/CuO HNSs exhibited similar catalytic perform-

(2)

The catalytic activity of the prepared samples is much higher than that of some noble-metal NPs for the reduction of 4-NP 18211

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Figure 7. (a) Plots of the conversion of 4-NP in the presence of the prepared NiO/CuO HNSs@C vs reduction time at different temperatures. (b) Arrehenius plots of ln k vs 1/T for the reduction of 4-NP at different temperatures.

via the mesoporous pores; (2) adsorption of 4-NP on the surface of HNSs; (3) formation of 4-AP by the hydrogenation reduction of p-nitrophenol on the surface of HNSs in the presence of highly injecting electrons from BH4−,11 where HNSs act as an “electrical” connection between the particles, which play a key role as the electrons’ transport to the reduction site from the oxidation site;57 (4) desorption of 4-NP from the surface of HNSs; and (5) exit of 4-AP from the HNSs@C via the mesoporous pores. The electron transfer on the HNSs surface might be the essential problem to influence the kinetic constant or catalytic activity of catalyst.11 It must be noted that, besides enhanced electron transfer, the coated carbon can also supply an advantageous environment for the hydrogenation reduction reaction.12 On the basis of the above analysis, the higher catalytic activity of NiO/CuO HNSs observed here can probably be attributed as follows: on the one hand, bimetallic oxides HNSs@C with mesoporous structure could provide high surface area and surface roughness, leading to highly active sites. On the other hand, bimetallic oxides HNSs@C with abundant interconnected nanobranches could provide facile pathways for electron transfer by reducing the interface resistance.

Figure 8. Reusability of the NiO/CuO HNSs@C nanocatalysts for the reduction of 4-NP by NaBH4.

ance without significant reduction, revealing that the asprepared NiO/CuO HNSs catalysts were stable. 3.4. Proposed Mechanism. To investigate the hydrogenation of 4-NP to 4-AP, a proposal mechanism was proposed (Figure 9). There were five stages during the whole reaction: (1) penetration of 4-NP into the inner space of the HNSs@C

4. CONCLUSION In summary, we have rationally designed a functional nanoarchitecture, coaxial hollow nanospheres, for high stability and catalytic activity. In the nanoarchitecture, the metal oxide HNSs were coated with a porous carbon shell. HNSs@C of NiO, CuO, and NiO/CuO were successfully obtained via the designed procedure. The as-prepared HNSs exhibited an excellent catalytic activity toward 4-nitrophenol reduction. In addition, the proof-of-concept structural design presented in this work should be valuable for the preparation of other highly stable metal oxide HNS materials for future application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03120. TEM image, size distribution, and XRD pattern of obtained CuNi/C HNSs; XPS spectra of NiO/CuO HNSs@C; SAED patterns of the NiO/CuO HNSs@C; TGA curves for CuO, NiO, and CuO/NiO HNS@C under an O2 atmosphere rate of 10 °C min−1; UV−vis absorption spectra of the reaction of 4-NP with NaBH4

Figure 9. Schematic diagram of the catalytic mechanism of metal oxide HNSs@C. 18212

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in the absence of metal oxide HNSs catalysts; and a comparison of Kapp for the catalytic reduction of 4-NP by some noble-metal nanoparticles and metal oxides HNSs of this work (PDF)

AUTHOR INFORMATION

Corresponding Authors

*H.Z.: e-mail, h.zhao@griffith.edu.au; tel, +86 991 8582807; fax, +86 991 8581006. *X.S.: e-mail, [email protected]. ORCID

Huijun Zhao: 0000-0003-3794-4497 Xintai Su: 0000-0002-2832-7406 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Australian Research Council (ARC) Discovery Project and the National Natural Science Foundation of China (Grant Nos. U1503391, 21566037, 51372248, and 51432009).

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DOI: 10.1021/acsami.7b03120 ACS Appl. Mater. Interfaces 2017, 9, 18207−18214