Novel Strategy for Facile Synthesis of C-Shaped CeO2 Nanotubes

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Novel Strategy for Facile Synthesis of C-Shaped CeO Nanotubes with Enhanced Catalytic Properties 2

Nan Lv, Ji-Lin Zhang, Guangming Li, Xun Wang, and Jiazuan Ni J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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The Journal of Physical Chemistry

Novel Strategy for Facile Synthesis of C-shaped CeO2 Nanotubes with Enhanced Catalytic Properties Nan Lv,†,‡ Jilin Zhang,†,* Guangming Li,†,‡ Xun Wang,†,‡ and Jiazuan Ni†,§ †State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Changchun 130022, P. R. China. ‡University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

§College of Life Science, Shenzhen University, Shenzhen 518060, P. R. China.

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ABSTRACT: Despite synthesis progress of one dimensional (1D) nanomaterials, the fabrication of nanotubes with exposed internal space remains a major challenge. Herein, we report a novel strategy to synthesize C-shaped CeO2 nanotubes (NTs) via electrospinning technique utilizing two immiscible polymers as template and Kirkendall effect for the first time. The C-shaped nanotubes with the half-wall-like tublar structure possess exposed internal space and large surface area, just like the letter “C”. The formation mechanism of C-shaped CeO2 nanotubes has been rationally explained and their exceptional performance and practicability have been befittingly evaluated. This facile synthesis strategy is applicable to various rare earth oxides. Our work opens a new route for the construction of 1D C-shaped nanotubes structure. With their unique structure, the C-shaped nanotubes exhibit fantastic properties, which are extremely intriguing for their potential applications in catalysis, sensor and microreactor.

Keywords: nanostructures, rare earth oxides, electrospinning, catalysis


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1. INTRODCUTION Metal oxide nanotubes with high aspect ratio and large surface area (e.g. TiO2, WO3, CeO2, etc.) have attracted extensive research interest due to their outstanding performance in catalysis, absorption, chemical sensor and so on.1-6 However, the internal surface of nanotubes is not easily accessible due to their long and thin channels, and utilization of internal structures and surface characters of nanotubes is frequently limited, which severely hinders their value in use. Although, many efforts have been made to solve this problem by fabricating various new nanostructures (e.g. biomimic hierarchical multichannel microtubes7) and improving surface properties (e.g. nanoparticles deposited on both sides of nanotubes),8-11 the internal space and interior surface functional properties are still not fully utilized. Thus, it is very necessary to develop a kind of nanostructure with exposed internal space and interior surface. C-shaped nanotubes with the half-wall-like tubular structure, just like the letter “C”, nearly enjoy the structural characteristics and physical and chemical properties of one-dimensional (1D) nanostructured counterparts. More importantly, compared with nanotubes, the C-shaped nanotubes possess exposed internal surface which is easier to be modified and used, and molecules are easier to diffuse and transport in the inner space. Recently, Choi et al. synthesized thin-walled WO3 hemitubes and the Pt-functionalized WO3 hemitubes which were used for detection of H2S and CH3COCH3 respectively from respiratory gas with high sensitivity and low detection limits. But then, the synthesis of WO3 hemitubes requires expensive sputtering equipment and complex fabrication process.12 Thus, there is a deep interest in exploring a simple and cost effective strategy to fabricate the C-shaped nanotubes for potential applications.


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Electrospinning has become one of the most straightforward and versatile technique to generate 1D nanostructures.13-20 Here, we report a general and facile synthesis strategy towards novel C-shaped rare earth oxide nanotubes for the first time by employing two parallel spinnerets and dual immiscible polymer templates for electrospinning and subsequent calcination treatment. The C-shaped nanotubes with the half-wall-like tubular structure possess exposed internal space and large surface area. To understand the formation mechanism, we studied the evolution of morphology and structure of the C-shaped CeO2 NTs. To evaluate their catalytic and adsorptive properties, the C-shaped CeO2 NTs were used for dephosphorylation of phosphorylated molecules. The applicability and generality of the C-shaped nanotubes fabrication have been demonstrated by successful synthesis of various rare earth oxides such as La2O3, Nd2O3 and Y2O3. In this strategy, the two immiscible polymers assembled side-by-side are a prerequisite for the fabrication of 1D C-shaped nanotubes and a possible formation mechanism is proposed based on the Kirkendall effect. It is believed that this work may suggest a new route for devising the large-scale production of various C-shaped nanotubes materials and the resultant C-shaped nanotubes may expand future applications in catalysis, adsorption and electrochemical devices. 2. EXPERIMENTAL SECTION Material Synthesis. The synthesis of C-shaped CeO2 nanotubes, as a typical example of the Cshaped rare earth oxide nanotubes, was described in detail below. Two spinning solutions were used to fabricate C-shaped CeO2 nanotubes. The spinning solution I for fabricating PAN/Ce(NO3)3 nanofibers was prepared as below: 1.0 g of Ce(NO3)3∙6H2O was dissolved in 10 g of DMF and then 1.1 g of PAN was added into the solution with magnetic stirring for 8 h. The spinning solution II for PVP nanofibers was synthesized as follows: 1 g of PVP was added into 7 g of DMF with magnetic stirring for 8 h. The [PAN/Ce(NO3)3∙6H2O]//PVP Janus fibers were


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prepared by electrospinning technique using the specially designed parallel spinneret, as illustrated in Figure 1f. Two spinning solutions were injected into the two parallel spinnerets, respectively. A sheet of aluminium foil was used as a collector and placed about 16 cm away from the top of the spinneret. A positive direct current (DC) voltage of 20 kV was set between the spinneret and the collector. The as-spun product was heated to 700 °C at a heating rate of ca. 0.7 °C min-1 and the reaction was then maintained at 700 °C for 2 h in air in a furnace. Finally, C-shaped CeO2 nanotubes were obtained. Under the same experimental conditions, other Cshaped rare earth oxide nanotubes such as La2O3, Nd2O3 and Y2O3 can also be prepared using other rare earth nitrate instead of cerium nitrate. Evaluation of the adsorption and catalysis performance of C-shaped CeO2 nanotubes. The adsorption and catalyzed dephosphorylation of p-nitrophenyl disodium orthophosphate (p-NPP) by C-shaped CeO2 nanotubes in aqueous solutions were appraised by detecting the UV-Vis absorbance of the supernatant solution. For comparison, CeO2 nanofibers (NFs) and CeO2 nanotubes (NTs) were also used to control catalytic experiments. CeO2 materials (20 mg) were first ultrasonically dispersed in 9.5 mL of water, and then 0.5 mL of HNO3 (1 M) and 10 mL of p-NPP aqueous solution (0.8 mg mL−1) were added in succession. The final volumes and pH values of the mixtures are 20 mL and 3, respectively. The mixtures were then magnetically stirred at 25 °C. At certain time intervals, about 0.6 mL aliquots were sampled. After centrifugation, the collected supernatants were used for the UV-Vis measurement. All UV-Vis spectra of the samples are measured under alkaline conditions because the p-NPP has a UV-Vis adsorption peak near 311 nm and p-nitrophenol (p-NP) has a UV-Vis adsorption peak near 400 nm under alkaline conditions.


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3. RESULTS AND DISCUSSION The morphology, structure and elemental analysis of the C-shaped CeO2 NTs obtained by calcination of [PAN/Ce(NO3)3]//PVP Janus fibers at 700 °C were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersion spectrum (EDS), as shown in Figure 1. Obviously, the C-like shape of products is clearly visible, with a diameter of around 150-300 nm (Figure 1a and 1b). TEM image in Figure 1c further reveals that the C-shaped CeO2 nanotube is composed of many CeO2 crystalline grain with lattice spacing of 0.312 nm (Figure 1d), which is in good agreement with that of the (111) plane of the cubic CeO2 (JCPDS card No. 43-1002), as illustrated in X-ray diffraction (XRD) analysis (Figure S1, ESI†). Figure 1e indicates the presence of C, O, Ce and Cu elements. Among them, the O and Ce stem from the CeO2, while the C and Cu derive from the sample carrier. The BET specific surface area and the total pore volume of C-shaped CeO2 NTs are 137.86 m2 g-1 and 0.19 cm3 g-1, respectively (Figure S2a, ESI†).


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Figure 1. Scanning electron microscopy (SEM) images (a, b), transmission electron microscopy (TEM) image (c), high resolution transmission electron microscopy (HRTEM) image (d) and energy dispersion spectrum (EDS) (e) of C-shaped CeO2 nanotubes and schematic illustration (f) of the synthesis strategy of C-shaped CeO2 nanotubes. To understand the formation mechanism of C-shaped CeO2 NTs, we studied the morphological and structural evolution of the Janus fibers calcined at different temperatures (200 °C, 300 °C, 350 °C, 500 °C and 700 °C). The [PAN/Ce(NO3)3]//PVP Janus fiber is composed of two strandaligned semi-fibers with smooth surface and the mean diameter of each semi-fiber is ca. 300 nm (Figure 2a1). The two semi-fibers have different Ce content distribution (Figure 2a2), the Ce


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content in the PAN/Ce(NO3)3 semi-fiber is much higher than that of PVP (inset of Figure 2a2). After calcination at 200 °C, a thin central hollow (in red box) is clearly distinguishable in PAN/Ce(NO3)3 semi-fiber apart from the diameter of the Janus fibers becomes small (Figure 2b1 and 2b2). When the temperature rises to 300 °C, the [PAN/Ce(NO3)3]//PVP Janus fibers obviously contract, the surface becomes rough and the shape takes place change (Figure 2c1) due to partial thermal decomposition of polymers and cerium nitrate, which is confirmed by the weight loss (ca. 40%) of the Janus fibers in TG-DSC data (Figure S3, ESI†). TEM image in Figure 2c2 clearly discloses that the hollow structure rooted in PAN/Ce(NO3)3 semi-fiber (left side) arises due to Kirkendall effect.21 The EDS line scanning (inset of Figure 2c2) further reveals that the Ce content in the fringe area of the PAN/Ce(NO3)3 semi-fiber (left side) is much higher than that in the interface (center area). The corresponding XRD pattern reveals that after calcination at 300 °C there is no CeO2 crystalline phase to be detected (Figure 3a). After calcined at 350 °C, the Janus fibers begin to form incompleted-wall tubular structure with the diameter of around 150-300 nm (Figure 2d1 and 2d2) owing to thermal decomposition with a weight loss (ca. 70%) (Figure S3, ESI†). All intermediate products and the polymer residues can be removed after calcination at 500 °C or 700 °C for 2 h and C-shaped CeO2 NTs composed of many ceria nanocrystals are obtained, which has been verified by the SEM, TEM (Figure 2e and 2f), XRD (Figure 3a), FTIR (Figure 3b) and TG-DSC (Figure S3, ESI†).


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Figure 2. SEM and TEM images showing the morphological evolution of the [PAN/Ce(NO3)3]//PVP Janus fibers before calcination (a1 and a2) and after calcination at 200 °C (b1 and b2); 300 °C (c1 and c2); 350 °C (d1 and d2); 500 °C (e1 and e2); 700 °C (f1 and f2); The formation illustration (g) of C-shaped CeO2 nanotubes by calcination of [PAN/Ce(NO3)3]//PVP Janus fibers. The insets in Figure 2a2 and 2c2 show EDS line scanning of elemental Ce.


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Figure 3. XRD patterns (a) and FTIR spectra (b) of [PAN/Ce(NO3)3]//PVP Janus fibers calcined in air at different temperature. Based on the aforementioned experiments and observations, a possible formation mechanism of C-shaped CeO2 NTs is proposed (Figure 1f and 2g). During electrospinning, the [PAN/Ce(NO3)3]/PVP Janus fiber composed of PAN/Ce(NO3)3 semi-fiber and the PVP semifiber assembled side-by-side is fabricated (Figure 1f). Meanwhile, with the solvent gradually evaporating, tiny amount of Ce3+ ions in PAN/Ce(NO3)3 semi-fiber could radially diffuse into the PVP semi-fiber due to the concentration gradient, as confirmed by the Ce content distribution (insets of Figure 2a2). With further elevatory calcination temperature, the radial diffusion accelerates outwards, which results in the formation and incessant development of the thin central hollow (i.e. Kirkendall voids) in PAN/Ce(NO3)3 semi-fiber, as shown in Figure 2b2.22 A noticeable phenomenon is that only a few cerium ions can diffuse into the PVP semi-fiber after calcination due to existence of the interface between two immiscible polymers, which is corroborated by the Ce content distribution in the EDS line scanning (inset of Figure 2c2). Furthermore, ceria nanocrystals prefer to form and develop in the outer surface of PAN/Ce(NO3)3 semi-fiber owing to thermal decomposition of cerous nitrate under the existence


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of oxygen-rich area near the surface of fibers. The Kirkendall voids in the PAN/Ce(NO3)3 semifiber can continuously expand during outward diffusion of Ce3+ ions and eventually form an asymmetric CeO2 tubular distribution, as verified in Figure 2c2.23 With the decomposition of PVP and PAN, and the [PAN/Ce(NO3)3]//PVP Janus fiber subsequently evolves into a halfbaked tube structure as seen in Figure 2d2. Finally, C-shaped CeO2 NTs can be fabricated when the polymers are completely removed after calcination at 500 °C or above (Figure 2e2 and Figure 2f2). Clearly, the initial side-by-side structure of electrospun nanofiber is a prerequisite for the final formation of C-shaped nanotubes. Thereinto, the interface in [PAN/Ce(NO3)3]//PVP Janus fiber functions as “insulating layer” and hence prevents the more cerium ions from diffusing towards the PVP semi-fiber. To demonstrate our guess, the Ce(NO3)3/PAN/PVP composite nanofibers were prepared as a control. After calcination at 700 °C for 2h (Figure S4, ESI†), the products possess multiform structures (e.g. wire-in-tubes, collapsed nanofibers, hollow nanofibers and multi-channel nanofibers), which is contributed to the phase separation of PAN and PVP and the Kirkendall effect. That is to say, it is hard to obtain the C-shaped nanotube structure without the “insulating layer” assembled side-by-side. In addition, the obtained multiform structures also reveal inhomogeneous diffusion of cerium ions in PAN/PVP composite medium. To prove the necessity using two immiscibility polymer templates (i.e. PAN and PVP) for the formation of Cshaped CeO2 NTs, we also used the same polymer template to fabricate [PAN/Ce(NO3)3]//PAN and [PVP/Ce(NO3)3]//PVP Janus fibers (Figure S5, ESI†). As depicted in Figure S5b and S5d (ESI†), the hollow nanofibers can be obtained owing to equably radial outdiffusion of cerium ions from center towards fringe area of the fiber without interface interference between the same template. The content of Ce(NO3)3 in the spinning solution also plays an important role in


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determining the wall thickness of C-shaped nanotubes. In general, the wall thickness of the Cshaped nanotubes increased as the content of Ce(NO3)3 was enhanced, as shown in Figure S6. The simple and straightforward fabrication method is available to prepare other C-shaped rare earth oxide nanotubes. As shown in Figure 4, C-shaped La2O3 (JCPDS No. 74-2430) (Figure 4a), Nd2O3 (JCPDS No. 21-0579) (Figure 4b) and Y2O3 (JCPDS No. 79-1257) (Figure 4c) nanotubes can be synthetized under the same experimental condition as the C-shaped CeO2 nanotubes. Thus, this approach may be extended to prepare other C-shaped oxide nanotubes.

Figure 4. SEM images, XRD patterns and EDS spectra of C-shaped La2O3 (a, a1, a2), Nd2O3 (b, b1, b2) and Y2O3 (c, c1, c2) nanotubes. To examine the exceptional performance and practicability of C-shaped CeO2 NTs, we evaluate their catalytic activity for the dephosphorylation of p-nitrophenyl disodium orthophosphate (p-NPP).24,25 For comparison, CeO2 nanofibers (NFs) and CeO2 nanotubes (NTs)


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fabricated by electrospinning were also used to control catalytic experiments (Figure S2b and S2c, ESI†). Figure 5a gives a schematic illustration of C-shaped CeO2 NTs for catalytic dephosphorylation of the model p-NPP. Figure 5b and 5c show the time-dependent changes in pNPP and p-NP (one product of the p-NPP hydrolysis) concentrations, which are calculated according to the time-dependent UV-Vis adsorption spectra results (Figure S7, ESI†) and the standard curves of p-NPP and p-NP (Figure S8, ESI†). Figure 5b and 5c reveal that the hydrolysis of p-NPP in pure water (i.e., the blank test) is very weak. In contrast, the p-NPP can be fully catalytically dephosphorylated into p-NP by CeO2 NFs in 12 h, by CeO2 NTs in 3 h and by C-shaped CeO2 NTs in 30 min, which indicates that C-shaped CeO2 NTs can dramatically enhance the dephosphorylation of p-NPP compared to CeO2 NFs and CeO2 NTs. To compare their catalytic efficiency for the p-NPP dephosphorylation, the dephosphorylation percentages of p-NPP are calculated after 30 min and they are about 48.83%, 56.50% and 98.06% for CeO2 NFs, CeO2 NTs and C-shaped CeO2 NTs, respectively. Thus, it is clear that C-shaped CeO2 NTs catalyze the dephosphorylation of p-NPP much more rapidly than CeO2 NFs and CeO2 NTs.


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Figure 5. Schematic illustration of the C-shaped CeO2 NTs for catalytic dephosphorylation of the model p-NPP (a); The time-dependent changes of the p-NPP (b) and p-NP (c) concentrations in an aqueous mixture in the presence/absence of CeO2. Reaction conditions: c initial (p-NPP) = 0.4 mg mL−1, c initial (p-NP) = 0, the concentration of CeO2 nanofibers (NFs), CeO2 nanotubes (NTs) and C-shaped CeO2 nanotubes (C-shaped NTs) is 1 mg mL-1, respectively. All the volumes of the mixtures are 20 mL, pH initial = 3, T = 25 °C. The adsorption capacity of C-shaped CeO2 NTs are also calculated. It can be inferred that the dephosphorylation of p-NPP caused by CeO2 should undergo adsorption and hydrolysis two-step processes (Figure 5a). Furthermore, the adsorption of the p-NPP on the surface of CeO2 is a precondition for the hydrolysis. Thus, the p-NPP dephosphorylation products contain phosphate besides p-NP. The amounts of phosphate (calculated in P element) in the three final supernatants are 21.66, 20.98 and 19.91 P μg mL−1, respectively (Figure S9, ESI†). According to the mass conservation of P element, the surface adsorption amount of molecules (including phosphate and


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p-NPP on CeO2 NFs, CeO2 NTs and C-shaped CeO2 NTs) are 346.945 μmol g-1, 362.675 μmol g-1 and 428.955 μmol g-1, respectively.26 Thus, C-shaped CeO2 NTs with more active sites on the exposed surface (Figure S10, ESI†) exhibit the enhanced adsorption and catalytic hydrolysis capacities, indicating its higher activity than CeO2 NFs and CeO2 NTs. Based on the above experimental results, we can expect that C-shaped CeO2 nanotubes have great practical application potential in catalysis and adsorption. 4. CONCLUSIONS In summary, we reported a novel strategy to synthesize C-shaped CeO2 nanotubes via electrospinning technique utilizing two immiscible polymers as template and Kirkendall effect for the first time. The capability and feasibility of this strategy have been demonstrated by the successful fabrication of various C-shaped rare earth oxide nanotubes. The formation mechanism has been rationally explained and their exceptional performance and practicability have been befittingly evaluated. It is believed that the method described here can be extended to many other C-shaped nanotubes materials with potential applications (e.g. catalysis, sensor, microreactor, etc.). ASSOCIATED CONTENT Supporting Information. Sample preparation, Characterization, SEM images, XRD patterns, TG-DSC curve, BET surface area, UV-Vis spectra, standard curves and structure illustration (PDF) This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected] * Tel: 86 0431 85262346 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NNSFC) (Grant Nos. 21171161 and 21671186). REFERENCES (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353-389. (2) Sarauli, D.; Riedel, M.; Wettstein, C.; Hahn, R.; Stiba, K.; Wollenberger, U.; Leimkuhler, S.; Schmuki, P.; Lisdat, F. Semimetallic TiO2 Nanotubes: New Interfaces for Bioelectrochemical Enzymatic Catalysis. J. Mater. Chem. 2012, 22, 4615-4618. (3) Yada, M.; Mihara, M.; Mouri, S.; Kuroki, M.; Kijima, T. Rare Earth (Er, Tm, Yb, Lu) Oxide Nanotubes Templated by Dodecylsulfate Assemblies. Adv. Mater. 2002, 14, 309-313. (4) An, S.; Park, S.; Ko, H.; Lee, C. Fabrication of WO3 Nanotube Sensors and Their Gas Sensing Properties. Ceram. Int. 2014, 40, 1423-1429. (5) Haase, M. F.; Sharifi-Mood, N.; Lee, D.; Stebe, K. J. In Situ Mechanical Testing of Nanostructured Bijel Fibers. ACS Nano 2016, 10, 6338-6344.


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(26) Wang, Z. G.; Bi, W. Z.; Ma, S. C.; Lv, N.; Zhang, J. L.; Sun, D. H.; Ni, J. Z. FacetDependent Effect of Well-Defined CeO2 Nanocrystals on the Adsorption and Dephosphorylation of Phosphorylated Molecules. Part. Part. Syst. Char. 2015, 32, 652-660.


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Novel Strategy for Facile Synthesis of C-Shaped CeO2 Nanotubes

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