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Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties
Accepted Manuscript Title: Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties Authors: Peizhi Guo, Yutao Sang, Jing Xue, Binghui Xu, Rongyue Wang, Hongliang Li, X.S. Zhao PII: DOI: Reference:
S0927-7757(17)30246-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.03.010 COLSUA 21458
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
28-11-2016 4-3-2017 5-3-2017
Please cite this article as: Peizhi Guo, Yutao Sang, Jing Xue, Binghui Xu, Rongyue Wang, Hongliang Li, X.S.Zhao, Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2017.03.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties
Peizhi Guo,* Yutao Sang, Jing Xue, Binghui Xu, Rongyue Wang, Hongliang Li, X. S. Zhao
Institute of Materials for Energy and Environment, State Key Laboratory Breeding Based of New Fiber Materials and Modern Textile, School of Materials Science and Engineering, Qingdao University, Qingdao, 266071, P. R. China.
*Corresponding Author: [email protected], [email protected]
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Graphical abstract
Highlights
Self-assembly of Cu2O colloidal nanocrystals has been realized. The structure of Cu2O colloidal nanocrystals assembly can be easily adjusted. Magnetic properties of Cu2O colloidal nanocrystal assemblies are related to their assembly structure. The electrocatalytical activity of Cu2O nanocrystal assemblies is determined by their interior primary nanocrystals.
ABSTRACT
Colloidal nanocrystal assemblies (CNAs) of cuprous oxide (Cu2O) are controllably synthesized by in-situ self-assembling primary Cu2O nanocrystals, in which Cu2O CNAs assemblied by nanocrystals of 8~10 nm were about 0.6 μm and Cu2O CNAs assemblied by nanocrystals of 32 nm were about 1.8 μm. From room temperature magnetization measurement results, both of the CNAs showed ferromagnetic behavior with very small saturation magnetization (Ms) values and 10 nm nanocrystal-based CNAs had the largest Ms value of about 9.2 emu/g. Electrochemical experimental results showed that submicron Cu2O CNAs were able to detect dopamine and uric acid quickly while micrometer Cu2O CNAs were not. Cu2O CNAs formed by 8 nm nanocrystals showed the highest electrocatalytic activity in oxygen reduction reaction and displayed the best adsorptive ability to remove Congo red in aqueous solutions, which can be recylced after a simple heat treatment. 2
Furthermore, the formation mechanism and the relationship between crystalline size and assembly of primary nanocrystals as well as physicochemical properties of Cu2O CNAs have been discussed and analyzed.
Keywords: cuprous oxide, colloidal nanocrystal, self-assembly, adsorption, electrocatalysis
1. Introduction
Recent decades have witnessed metal oxides used widely in the fields including catalysis, energy storage, environment protection and biomedicine production [1-11]. Cu2O, one of the important metal oxides and p-type semiconductor materials [12-16], have been investigating as gas sensor [12], heterogeneous catalyst [15,16], high-efficiency
photocatalyst
and
photoelectrocatalyst
[8,9]
for
its
unique
physicochemical properties. With the rapid development of synthetic technologies, fabrication of Cu2O nanomaterials has been improved much recently [12-26]. Many methods such as solution phase synthesis [12,17], replacement-etching [20-21] and solvothermal/hydrothermal synthesis [16,22,26] have been reported to prepare various kinds of Cu2O nanomaterials including polyhedral [17-19], films [20], nanocages [21] hollow spheres [22] and porous structures [23]. Low-temperature synthesis methods are energy efficient and have been applied to prepare many nanomaterials [17-21]. For example, Huang and his co-workers [19] synthesized Cu2O nanocrystals with different morphologies in aqueous solutions in room temperature. However, research on Cu2O
3
particles with designed morphologies, especially formed by self-assembling nanocrystals and study of the structure-property relationship seems to attract few attentions.
Colloidal nanocrystal assemblies (CNAs) formed by ordered self-assembly of inorganic nanocrystals have attracted increasing interest due to their unique properties, especially the effective interactions in adjacent primary nanocrystals [27-33]. Large superstructures can be obtained from in-situ or ex-situ assembly nanoparticles, which typically and continuously occurs and depends on the uniformity of individual particles [27-31]. For example, different inorganic nanoparticles with broad polydispersity can spontaneously assemble and form uniform supraparticles with core-shell structures triggered by the interactive balance between van der Waals attractions and electrostatic repulsions [28]. Recently, the assembly of diverse anisotropic nanoparticles has been realized in the nematic liquid crystals with long-range ordering through the balance of competing interactions [29-31]. Ferrite nanocrystals are able to form ordered assembly structures easily via the in-situ self-assembly partially because of the strong magnetic interactions [32-34]. It is believed that ordered assemblies from smaller nanoparticles possess preferable properties than those from larger ones [28-34]. However, it is still a great challenge to prepare inorganic assembly nanostructures of weak magnetic nanoparticles by a one-pot strategy, hindered by simultaneous controlling the size of nanocrystals and in-situ self-assembly process.
4
Herein, we report a facile solution-phase synthesis method to prepare two types of Cu2O CNAs under mild conditions with the sizes of about 0.6 and 1.8 m respectively. The two CNAs were formed by self-assembly of primary Cu2O nanocrystals with the crystalline sizes of 8~10 and 32 nm, respectively. Results showed the crystalline size and ordered assembly of Cu2O nanocrystals played critical roles in determining the magnetic properties and electrochemical performances of all the CNAs. Furthermore, the assembly formed with smaller Cu2O nanoparticles displayed better adsorption abilities than those formed by larger ones.
2. Experimental
2.1 Materials All chemicals, including CuSO4•5H2O, Cu(CH3COO)2•H2O, CH3COONH4, NH4HCO3, ascorbic acid, uric acid, ethanol and Congo red were of analytical grade (Sinopharm Chemical Reagent Company). Dopamine (99%) was purchased from Aldrich. All chemicals were used without further purification. Double distilled water was used in the experiments.
2.2 Synthesis of MnFe2O4 CNAs In a typical synthesis of CNA-1, 120 mL aqueous CuSO4•5H2O (3 mmol) solution and 30 mL CH3COONH4 (6 mmol) solution were respectively transferred to a 200 mL three-neck flask and held at 50°C for 20 min under stirring. Then, 15 mL ascorbic acid (0.9 mmol) solution was added into the above solution and maintain the 5
temperature at 50°C for 1 h, followed by cooling down to room temperature. The brick red precipitate was collected by centrifugation, washed three times with distilled water, and dried in a vacuum oven at 50°C for 12 h. The synthetic procedure for CNA-2 was similar to that of CNA-1 with the heating time increased to 1.5 h at 50°C, except for the use of Cu(CH3COO)2•H2O (3 mmol) and the content of CH3COONH4 and ascorbic acid increased to 12 mmol and 4.5 mmol, respectively. The synthesis of CNA-3 is almost the same as that of CNA-2 except using NH4HCO3 instead of CH3COONH4.
2.3 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.154 18 nm) from 10° to 80° (2 θ). Scanning electron microscopy (SEM) images were collected on a Hitachi S-4800 FE-SEM and JSM 6390-LV operated at 10 and 20 kV, respectively. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM2010 TEM operated at 200 kV. Magnetic properties were analyzed at room temperature by a LDJ9500 vibrating sample magnetometer (VSM). Electrochemical measurements were performed on a CHI760E electrochemical workstation using a three-electrode cell. The hydrodynamic diameters of the samples were measured on a Malvern Zetasizer (Nano-ZS90).
6
2.4 Electrochemical measurements Electrochemical measurements were performed by using a three-electrode cell with platinum foil as counter electrode and saturated calomel electrode as reference electrode in aqueous phosphate buffer solutions (PBS, 0.1 M, pH 7.4) containing DA, UA and AA with the concentration of 1 mM. Bare glass carbon electrode (GCE, 3 mm in diameter) and Cu2O-modified GCE were used as the working electrodes. A 10 μL homogeneous aliquot suspension (1.5 mg mL−1) was uniformly cast onto the GCE surface. The modified GCEs were dried under ambient conditions before being used. For ORR, electrochemical measurements were conducted using a GCRDE coated with Cu2O catalyst was used as working electrode. An Ag/AgCl electrode and a platinum wire were used respectively as the reference electrode and counter electrode in a 0.1 M aqueous KOH solution. Current densities were normalized to geometric area of the GCE. Cyclic voltammograms were conducted in the potential range -0.8 V to 0 V at a scan rate of 50 mV s-1 with O2-saturated 0.1 M KOH aqueous solution. The linear sweep voltammetry for ORR was obtained with GCRDE at different speeds in an O2-saturated electrolyte from -0.8 to 0 V at a scan rate of 10 mV s-1.
3. Results and discussion
3.1. Crystal structure Three Cu2O CNA samples were synthesized by simply adjusting synthetic systems using solution-phase synthesis method at 50°C. Sample CNA-1 was prepared from aqueous solutions containing CuSO4 and CH3COONH4. Samples CNA-2 and 7
CNA-3 were obtained from Cu(CH3COO)2 systems containing CH3COONH4 and NH4HCO3, respectively. Fig. 1 shows the X-ray diffraction (XRD) patterns of three Cu2O samples. The diffraction peaks are in good accordance with the standard pattern of pure Cu2O phase (JCPDS, No. 05-0667). The sharp peaks appeared at 2 θ degrees of 29.6, 36.5, 42.4, 61.5 and 73.7 can be well-indexed to the (110), (111), (200), (220) and (311) planes of Cu2O phase, respectively. Clearly, the diffraction peaks of CNA-1 (Fig. 1a) were much sharper and narrower than that of CNA-2 and CNA-3, indicating the crystalline size of primary Cu2O nanoparticles in CNA-1 was significantly larger than others. Based on the measurements of the full width at half-maximum (fwhm) of the (111) peaks in the XRD patterns, the average crystallite sizes of primary particles in CNA-1, CNA-2 and CNA-3 were calculated to be about 32, 8 and 10 nm according to the Scherrer equation, respectively.
3.2. Microstructure and Evolution of the intermediates Fig. 2 shows the scanning electron microscope (SEM) images of Cu2O CNAs. Obviously, CNA-1 displayed a irregular flower-like shape. CNA-2 had a rather smooth surface compared with that of CNA-3 and both were dispersed spherical structures. The size of CNA-1 samples was about 1.8±0.3 m, which was the largest among the three samples. CNA-2 and CNA-3 were submicrometer spheres with the sizes of about 0.7±0.2 and 0.6±0.2 m, respectively. Enlarged SEM images (Fig. 2d-2f) confirmed that all of the Cu2O CNAs were supposed to be formed from the assembly of primary Cu2O nanocrystals. Clearly, CNA-1 was composed of larger nanoparticles while CNA-2 and CNA-3 were formed by smaller Cu2O nanoparticles. 8
These observations were essentially consistent with the XRD measurement results. In the meantime, the hydrodynamic diameters of CNA-2 and CNA-3 measured in aqueous solutions were about 0.89 and 0.85 m, respectively, while sample CNA-1 cannot show a reasonable value due to its large polydispersity index, consistent with those of SEM observations.
Transmission electron microscopy (TEM) images of Cu2O CNAs confirmed the formation of spherical assembly structures (Fig. 3). It is observed that CNA-1 was about 2 m (Fig. 3a) and was composed of large well–crystalline nanoparticles based on the selected area electron diffraction (SAED) pattern (the inset in Fig. 3a), in which bright diffraction spots were observed. When copper acetate is introduced into the synthetic systems, smaller spheres with a much smooth surface could be obtained than that of CNA-1 (Fig. 3b and 3c). Furthermore, bright circles composed of arc-like diffraction spots were observed in the SAED patterns of CNA-2 and CNA-3 (the insets in Fig. 3b and 3c), which were rather different from that of CNA-1. CNA-2 and CNA-3 should be formed from the in-situ self-assembly of small nanoparticles with slight misalignments among primary nanocrystals, similar to those of assembly structures of magnetite and ferrites [29,30]. Furthermore, it can also be derived that the self-assembly of large Cu2O nanoparticles cannot obviously influence the properties of primary nanoparticles, similar to those of large manganese ferrite CNAs [34].
The formation mechanisms of Cu2O CNAs were investigated based on the characterizations of intermediate products during the synthesis of the targeted samples. 9
It was observed that only precipitates were formed before ascorbic acid (AA) was added into the synthetic systems of CNA-1 and CNA-3, and the XRD results (Fig. S1) indicated the precipitates were mainly copper basic carbonate (JCPDS, No. 13-0131). According to the XRD and SEM results (Fig. S1 and S2), sphere precursors for CNA-3 were composed of small nanoparticles, while flower-like spheres for CNA-1 were formed by large particles. When ascorbic acid was introduced into the solution, redox reactions between the reductive agent and the precipitates could be triggered to form CNA-1 and CNA-3, which was confirmed by the SEM results of the corresponding intermediates (Fig. S3 and S4).
However, no precipitates were formed for CNA-2 before adding ascorbic acid. Similar spherical assembly structures of CNA-2 were obtained directly from the reduction reaction between copper ions and ascorbic acid, which were formed via the assembly of small Cu2O nanocrystals. Compared with those similar structures of magnetite and ferrite, the assembly structures of Cu 2O are formed very quickly in this work during the synthetic processes (Fig. S3-S5) [20,32]. It was speculated that the formation of CNAs should be closely related to the coordinative and oxidative ability of copper ions and the reductive ability of ascorbic acid as well as the trend to reduce the surface energy of the products (Fig. 3d). The structural differences between CNA-1 and CNA-3 should be ascribed to the different structures of their precursors. It can also be concluded that the dynamic control was responsible for the structure of CNAs because the precipitate disappeared when the reaction time was up to 10 h and reformed again 1 h later during the synthesis of CNA-2. 10
3.3. BJH measurement As Cu2O CNAs were composed of small nanocrystals, the structural features of these samples were further investigated by nitrogen gas sorption technique. Fig. 4 shows the N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution curves of three CNA samples. It can be seen that the type IV physisorption nature of three isotherms was obtained according to the IUPAC classification, which showed a typical H1-type hysteresis loop, especially for CNA-2 and CNA-3, indicating that abundant mesoporous structures are existed in these samples. The specific surface areas and total pole volumes were respectively calculated about 3.0 m2 g-1 and 0.018 cm3 g-1 for CNA-1, 26.8 m2 g-1 and 0.104 cm3 g-1 for CNA-2, and 20.0 m2 g-1 and 0.066 cm3 g-1 for CNA-3. These results further indicated the self-assembly nature of small nanoparticles in the CNA samples.
3.4. Magnetic characterization Magnetic properties of Cu2O samples were measured under room temperature. Fig. 5A shows magnetization curves of Cu2O CNAs at the full scale (H=1.4×104 Oe). It was cleared that the saturation magnetization (Ms) values was about 3.2, 4.7 and 9.2 emu/g for CNA-1, CNA-2 and CNA-3, respectively, although these Ms values were very small compared with that of iron series materials.27 An obvious hysteresis loop could be observed for CNA-3 (Fig. 5A), while CNA-1 and CNA-2 also showed very small hysteresis loops in the magnified image, indicating all the CNAs samples displayed the ferromagnetic behavior. It was found that the values of remnant 11
magnetization (Mr) and coercivity (Hc) were respectively measured to be 0.29 emu/g and 88.8 Oe for CNA-1, 0.55 emu/g and 115.3 Oe for CNA-2, and 0.95 emu/g and 117.5 Oe for CNA-3.
Usually, magnetic nanoparticles have a large Ms value when their crystalline size is large. In the meantime, large ferromagnets nanoparticles have large Mr and Hc values. Based on the magnetic results of our Cu2O CNAs, it can be concluded that the values of the Ms, Mr and Hc of Cu2O CNAs were determined not only by the crystalline size but also by the self-assembly of primary nanoparticles. The critical size of Cu2O for the superpara magnetic-ferromagnetic transition was slight larger than 32 nm, which was much larger than that of ferrite nanoparticles. On the other hand, CNA-2 displayed much higher values of the Ms, Mr and Hc than CNA-1, indicating the unique assembly nature of CNA-2. These should be ascribed to the template synthesis of CNA-1 while CNA-2 was formed directly from the in-situ self-assembly of the as-formed 8 nm Cu2O nanoparticles with the well-crystalline nature.
3.5. Electrochemical sensing performance To disclose the structure-property relationship of Cu2O CNAs, three types of experiments
were
conducted,
namely
electrocatalysis
of
small
molecules,
electrocatalysis for oxygen reduction reaction and adsorption of organic pollutants. It has been reported that Cu2O has the catalytic activity toward the reduction or degradation of specific molecules. In this work, Cu2O CNAs were further explored for the electrocatalysis of soluble molecules,10-14 which would be expected to distinguish 12
the structural effect based on their electrochemical performances. The glass carbon electrodes (GCEs) modified by Cu2O CNAs are abbreviated as CNA-1/GCE, CNA-2/GCE and CNA-3/GCE. Fig. 6A shows the cyclic voltammogram (CV) curves of the modified GCEs in PBS solutions containing ascobic acid, dopamine (DA) or urea acid (UA), in which the redox peaks can be observed, respectively. It can be seen that AA and UA showed obviously oxidation peaks at all of the three modified GCEs, and the redox peaks for AA are very broad (Fig. 6A and S6). While, one pair of redox peaks can be observed clearly for the detection of DA. These observations should be ascribed to the differences in the molecular structures of these molecules. In addition, the separation of anodic and cathodic peak potential of DA is 0.104, 0.090 and 0.096 V for CNA-1, CNA-2 and CNA-3 modified GCEs, respectively, indicating that the electrochemical reaction on CNA-2/GCE was more reversible than that of any other samples. Furthermore, the potential of the anodic peaks of DA were at 0.196, 0.186 and 0.191 V at CNA-1/GCE, CNA-2/GCE and CNA-3/GCE, respectively. The most negative peak potential for CAN-2/GCE indicated that DA could react at a relative lower potential compared with others.
As shown in Figure 6B, the catalytic peak currents in the differential pulse voltammetry (DPV) curves of CNA-2/GCE were linearly increased with the concentration of DA with a correlation coefficient of r = 0.998 in the experimental range. It is shown that the response of DA was also apparent even if the concentration was 0.01 mM. Therefore, the lowest detection limit of DA at Cu 2O-2/GCE was evaluated to be 0.01 mM. 13
Furthermore, the detection of DA was almost unaffected in the presence of AA and UA although the peak currents of UA were slightly influenced by DA (Fig. 7). It could be seen from Fig. 7A that CNA-2/GCE and CNA-3/GCE could simultaneously detect DA and UA in ternary electrolytes containing AA, DA and UA and CNA-2/GCE showed better catalytic performances than CNA-3/GCE (Fig. 7B-7D). However, CNA-1/GCE can only discern DA under the same electrochemical conditions. It is suggested that the crystalline sizes of nanoparticles in CNAs should be responsible for their electrocatalytic performances with adjustable electron transfer rate [35] and the quicker electro-transfer kinetics should also be expected for DA than that of UA [36,37].
It is well-known that an efficient oxygen reduction reaction (ORR) activity is essential to develop advanced fuel cells and metal-air batteries. Recently, it has attracted great interest to explore the ORR electrocatalysts with low cost and excellent performance [15,16,38,39]. However, the electrocatalytic properties of Cu2O nanostructures for ORR had not yet been fully understood [15,16]. As for the working mechanism of Cu2O as an ORR catalyst, in charge control potential ranges, there should be a rate-limiting first electron transferring to adsorbed O2 to form superoxide on Cu2O along with the first-order dependence on O2 partial pressure (reaction 1), possibly involving a concurrent reaction with water (reaction2) [15,16,40]. Fig. 8A depicts the CV curves of the modified glass carbon rotating disk electrodes (GCRDEs) in aqueous N2 or O2-saturated 0.1 M KOH solutions under a scan rate of 50 mV s-1 at room temperature. Obviously, the CV curve of the modified electrode in N 2-saturated 14
electrolytes was essentially featureless while a notable cathodic peak appeared at -0.46 V for CNA-2 and CNA-3 in O2-saturated electrolytes, indicating that these two samples had potentially electrocatalytic activity for ORR [41]. Futhermore, the onset reduction potentials for Cu2O are shift positively with the order of CNA-2, CNA-3 and CNA-1. Cu2O-O2+e-→Cu2O-O2-
(1)
Cu2O-O2+H2O+e-→Cu2O-HO2+OH-
(2)
The inset in Fig. 8A shows the onset potentials and half-wave potentials were different for three samples, and positively shifted with the decrease of crystalline sizes of Cu2O nanoparticles in the CNAs, suggesting that CNA-2 should have a superior electrocatalytic activity compared to CNA-3 and CNA-1. The ORR kinetics of CNAs was conducted by varying the rotation speed to qualitatively analyze the catalytic activity. Taking CNA-2 as an example (Fig. 8B), the current density increases smoothly with the rotation speeds, probably caused by the faster oxygen flux to the electrode surface. The linearity of the Koutecky-Levich plots and near parallelism of the fitting lines indicated the first order reaction kinetics toward oxygen gas on CNA-2 within the potential window ranging from -0.6 to -0.8 V. The transferred electron number (n) for CNA-2, which can be obtained from the slope and intercept of the Koutecky-Levich plots [42], was about 2.10, 2.31 and 2.58 at the potential of -0.6, -0.7 and -0.8 V, respectively, indicating that CAN-2 sample had the electrochemical activity for the ORR and O2 can be reduced to OH- ions through the 2-electron 15
reduction mechanism (reaction 3 and 4). The n values of CAN-1 and CNA-3 was much smaller than that of CAN-2, suggesting again that CNA-2 had a better electrocatalytic activity which was consistent with the CV and LSV data in Fig. 8. It is proposed that the ORR kinetics of Cu2O-based electrocatalysts should be related to the microstructure and crystalline size of Cu2O structures as well as the assembly interactions/nature among nanoparticles in the CNAs [43]. It also should be pointed out that the catalytic current densities of the samples were not large although CNA-2 displays the best electrocatalytic activity for ORR among these Cu 2O samples. O2+H2O+2e-→HO2-+OH-
(3)
HO2-+H2O+2e-→3OH-
(4)
3.6. Adsorption of Congo red Generally, transition metal oxides can be used to remove organic complex from water through adsorption or subsequent catalysis [44,45]. Cu2O CNAs showed adsorption ability for the typical organic waste, Congo red, under dark conditions (Fig. 9 and 10). The adsorption of Congo red onto the surfaces of CNAs was suggested because of the same experimental results under daytime, visible light (by using a 300 W Xe lamp) or dark conditions. The variation of adsorption of Congo red with time could be confirmed by the UV-vis absorption data (Fig. 9). Clearly, only absorption strength, not the absorption peak position, was decreased with the processing time. CNA-2 showed the best absorption ability among three samples with the removal rate of about 0.27 gram Congo red for CNA-2 per gram(Fig.10A). The removal ability of 16
CNA-3 was better than that of CNA-1. This should be ascribed to the structural nature of CNA-2, which had the smallest crystalline size, the largest specific surface area and highest surface energy among three samples. CNAs could be reused for several cycles after simply heating treatment. However, further prolonging the cycling runs led to the decrease of the absorption performance, probably due to the structural change of the sample based on the TEM image (Fig. 10B).
4. Conclusions
In summary, Cu2O CNAs with different sizes were synthesized by self-assembly of primary nanoparticles under mild conditions. The CNAs composed of 8 nm nanoparticles showed the best capability to remove Congo red in aqueous solutions and was able to detect dopamine and uric acid quickly as electrocatalysts. Besides, CNAs assemblied by 8 nm nanoparticles possess a the higher electrocatalytic activity in oxygen reduction reaction than any of other samples. All the three Cu2O CNAs display weak ferrimagnetic behavior with the staturation magnetization values of 3.2, 4.7 and 9.2 emu/g for the CNAs formed by 8, 10 and 32 nm nanoparticles, respectively. These results indicate that the crystalline size and self-assembly of primary nanoparticles play the key roles in determining the performances of colloidal nanocrystal assemblies.
17
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. U1232104), the National High Technology Research and Development Program of China (2012AA110407), National college students innovation and entrepreneurship training program (201611065003) and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province, P. R. China.
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References
[1] J. C. Park, J. Kim, H. Kwon and H. Song, Gram-Scale synthesis of Cu2O nanocubes and subsequent oxidation to CuO hollow nanostructures for Lithium-Ion battery anode materials, Adv. Mater. 21 (2009) 803-807.
[2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496-499.
[3] J. Zeng, L. Jing, Y. Hou, M. Jiao, R. Qiao, Q. Jia, C. Liu, F. Fang, H. Lei and M. Gao, Anchoring group effects of surface ligands on magnetic properties of Fe3O4Nanoparticles: towards high performance MRI contrast agents, Adv. Mater. 26 (2014) 2694-2698.
[4] X. Hu, J. Yu, J. Gong, Q. Li and G. Li, α-Fe2O3 nanorings prepared by a microwave-assisted hydrothermal process and their sensing properties, Adv. Mater. 19 (2007) 2324-2329.
[5] X. Lang, X. Chen and J. Zhao, Heterogeneous visible light photocatalysis for selective organic transformations, Chem. Soc. Rev. 43 (2014) 473-486.
[6] M. Tsai, R. Hadt, P. Vanelderen, B. Sels, R. Schoonheydt and E. Solomon, [Cu2O]2+ active site formation in Cu–ZSM-5: geometric and electronic structure requirements for N2O activation, J. Am. Chem. Soc. 136 (2014) 3522-3529. 19
[7] C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie and J. Ma, One-Pot synthesis of octahedral Cu2O nanocages via a catalytic solution route, Adv. Mater. 17 (2005) 2562-2567.
[8] S. Shinde and K. Nanda, Facile synthesis of large area porous Cu2O as super hydrophobic yellow-red phosphors, RSC Adv. 2 (2012) 3647-3650.
[9] Y. Sang, Y. Cui, Z. Li, W. Ye, H. Li, X.S. Zhao and P. Guo, Electrochemical reaction of nitrobenzene and its derivatives on glassy carbon electrode modified with MnFe2O4 colloid nanocrystal assemblies, Sensors Actuat. B 234 (2016) 46-52.
[10] G.-Z. Yuan, C.-F. Hsia, Z.-W. Lin, C. Chiang, Y.-W. Chiang and M. H. Huang, Highly facet-dependent photocatalytic properties of Cu2O crystals established through the formation of Au-decorated Cu2O heterostructures, Chem. Eur. J. 22 (2016) 12548-12556.
[11] C.-S. Tan, S.-C. Hsu, W.-H. Ke, L.-J. Chen and M. H. Huang, Facet-dependent electrical conductivity properties of Cu2O crystals, Nano Lett. 15 (2015) 2155-2160.
[12] J. Zhang, J. Liu, Q. Peng, X. Wang and Y. Li, Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867-871.
[13] A. Marin, D. Muñoz-Rojas, D. Iza, T. Gershon, K. Musselman and J. MacManus-Driscoll, Novel atmospheric growth technique to improve both light 20
absorption and charge collection in ZnO/Cu2O thin film solar cells, Adv. Funct. Mater. 23 (2013) 3413-3419.
[14] Y. Sui, Y. Zheng, L. Fu, W. Zheng, D. Li, B. Lin and B. Zou, Low-temperature synthesis of porous hollow structured Cu2O for photocatalytic activity and gas sensor application, RSC Adv. 3 (2013) 18651-18660.
[15] Q. Li, P. Xu, B. Zhang, H. Tsai, S. Zheng, G. Wu and H. Wang, Structure-dependent electrocatalytic properties of Cu2O nanocrystals for oxygen reduction reaction, J. Phys. Chem. C 117 (2013) 13872-13878
[16] X. Yan, X. Tong, Y. Zhang, X. Han, Y. Wang, G. Jin, Y. Qin and X. Guo, Cuprous oxide nanoparticles dispersed on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, Chem. Commun. 48 (2012) 1892-1894.
[17] L. Gou and C. Murphy, Solution-phase synthesis of Cu2O nanocubes, Nano Lett. 3 (2003) 231-234.
[18] W. Huang, L. Lyu, Y. Yang and M. Huang, Synthesis of Cu2O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity, J. Am. Chem. Soc. 134 (2012) 1261-1267.
[19] C. Li and M. Kanan, CO2 reduction at low overpotential on Cu electrodes resulting from the reduction of thick Cu2O films, J. Am. Chem. Soc. 134 (2012) 7231-7234.
21
[20] C. Kuo, Y. Chu, Y. Song and M.H. Hong, Cu2O nanocrystal-templated growth of Cu2S nanocages with encapsulated Au nanoparticles and In-Situ transmission X-ray microscopy study, Adv. Funct. Mater. 21 (2011) 792-797.
[21] Y. Sui, W. Fu, Y. Zeng, H. Yang, Y. Zhang and H. Chen, Synthesis of Cu2O nanoframes and nanocages by selective oxidative etching at room temperature, Angew. Chem. Int. Ed. 49 (2010) 4282-4285.
[22] J. J. Teo, Y. Chang and H. C. Zeng, Fabrications of hollow nanocubes of Cu2O and Cu via reductive Self-Assembly of CuO nanocrystals, Langmuir 22 (2006) 7369-7377.
[23] Y. Yu, F. Du, J. Yu, Y. Zhuang and P. Wong, One-dimensional shape-controlled preparation of porous Cu2O nano-whiskers by using CTAB as a template, J. Solid State Chem. 177 (2004) 4640-4647.
[24] Q. Zheng, M. Morimoto and T. Takanohashi, A novel process for the production of aromatic hydrocarbons from brown coal in water medium by hydrothermal oxidation and catalytic hydrothermal decarboxylation, Fuel 182 (2016) 437-445.
[25] S. Cao, H. Chen, T. Han, C. Zhao and L. Peng, Rose-like Cu2O nanoflowers via hydrothermal synthesis and their gas sensing properties, Mater. Lett. 180 (2016) 135-139
22
[26] X. Luo, M. Wang, D. Yang, J. Yang and Y. Chen, Hydrothermal synthesis of morphology controllable Cu2O and their catalysis in thermal decomposition of ammonium perchlorate, J. Ind. Eng. Chem. 32 (2015) 313-318.
[27] Z. Lu and Y. Yin, Colloidal nanoparticle clusters: functional materials by design, Chem. Soc. Rev. 41 (2012) 6874-6887.
[28] Y. Xia, T. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. Glotzer and N. Kotov, Self-assembly of self-limiting monodisperse supraparticles from polydisperse nanoparticles, Nat. Nanotech. 6 (2011) 580-587.
[29] H. Mundoor, B. Senyuk and I. I. Smalyukh,Triclinic nematic colloidal crystals from competing elastic and electrostatic interactions, Science 352 (2016) 69-73.
[30] H. Mundoor and I. I. Smalyukh, Mesostructured composite materials with electrically tunable upconverting properties, Small 11 (2015) 5572–5580.
[31] Q. Liu, Y. Yuan and I. I. Smalyukh, Electrically and optically tunable plasmonic guest–host liquid crystals with long-range ordered nanoparticles, Nano Lett. 14 (2014) 4071–4077.
[32] J. Ge, Y. Hu, M. Biasini, W. Beyermann and Y. Yin, Superparamagnetic magnetite colloidal nanocrystal clusters, Angew. Chem. Int. Ed. 46 (2007) 4342-4345.
23
[33] P. Guo, Z. Li, S. Liu, J. Xue, G. Wu, H. Li and X. S. Zhao, Electrochemical properties of colloidal nanocrystal assemblies of manganese ferrite as the electrode materials for supercapacitors. J. Mater. Sci. 52 (2017) 5359-5365.
[34] R. Liu, M. Lv, Q. Wang, H. Li, P. Guo and X. S. Zhao, Solvothermal synthesis of size-tunable ZnFe2O4 colloidal nanocrystal assemblies and their electrocatalytic activity towards hydrogen peroxide, J. Mag. Mag. Mater. 424 (2017) 155-160.
[35] P. Wang, Y. Li, X. Huang and L. Wang, Fabrication of layer-by-layer modified multilayer films containing choline and gold nanoparticles and its sensing application for electrochemical determination of dopamine and uric acid, Talanta 73 (2007) 431-437.
[36] J. Huang, Y. Liu, H. Hou and T. You, Simultaneous electrochemical determination of dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbon nanofibers modified electrode, Biosens. Bioelectron. 24 (2008) 632-637.
[37] H. Bi, Y. Li, S. Liu, P. Guo, Z. Wei, C. Lv, J. Zhang and X. S. Zhao, Carbon-nanotube-modified glassy carbon electrode for simultaneous determination of dopamine, ascorbic acid and uric acid: The effect of functional groups, Sensors Actuat. B 171-172 (2012) 1132-1140.
[38] J. Wu and H. Yang, Platinum-based oxygen reduction electrocatalysts, Acc. Chem. Res. 46 (2013) 1848-1857.
24
[39] H. Zhu, S. Zhang, Y. Huang, L. Wu and S. Sun, Monodisperse MxFe3–xO4 (M = Fe, Cu, Co, Mn) nanoparticles and their electrocatalysis for oxygen reduction reaction, Nano Lett. 13 (2013) 2947-2951.
[40] D. B. Sepa, M. V. Vojnovicm and A. Damjanovic, Kinetics and mechanism of O2 reduction at Pt IN alkaline solutions, Electrochim. Acta 25 (1980) 1491-1496. [41] W. Chen, D. Ny and S. Chen, SnO2–Au hybrid nanoparticles as effective catalysts for oxygen electroreduction in alkaline media, J. Power Sources 195 (2010) 412-418.
[42]
W.
Xiao,
D.
Wang
and
X.
W.
Lou,
Shape-controlled
synthesis
of
MnO2 nanostructures with enhanced electrocatalytic activity for oxygen reduction, J. Phys. Chem. C 114 (2009) 1694-1700.
[43] M. Shao, A. Peles and K. Shoemaker, Electrocatalysis on platinum nanoparticles: particle size effect on oxygen reduction reaction activity, Nano Lett. 11 (2011) 3714-3719.
[44] L. Zhong, J. Hu, H. Liang, A. Cao, W. Song and L. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Adv. Mater. 18 (2006) 2426-2431.
[45] P. Guo, M. Lv, G. Han, C. Wen, Q. Wang, H. Li and X. S. Zhao, Solvothermal synthesis of hierarchical colloidal nanocrystal assemblies of ZnFe2O4 and their application in water treatment, Materials 9(2016) 806.
25
Figure Captions： Fig. 1 XRD patterns of CNA-1 (a), CNA-2 (b) and CNA-3 (c) Fig. 2 SEM images of CNA-1 (a, d), CNA-2 (b, e) and CNA-3 (c, f). Fig. 3 TEM images of CNA-1 (a), CNA-2 (b) and CNA-3 (c) and schematic illustration of the formation mechanism of CNAs (d). Fig. 4 Nitrogen adsorption/desorption isotherms (A) and BJH pore size distributions (B) of CNAs samples. Fig. 5 Magnetization hysteresis of Cu2O CNAs (A) and the magnified graph of (B). Fig. 6 (A) CV curves of the CNA-3/GCE in PBS solutions containing AA (0.8 mM), DA (0.8 mM) or UA (0.8 mM). (B) DPV curves of CNA-2/GCE in PBS solutions containing different concentrations of DA: (a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.10, (f) 0.15, (g) 0.20, (h) 0.30, (i) 0.40, (j) 0.50, (k) 0.60, (l) 0.70 and (m) 0.80 mM. Fig. 7 (A) DPV curves of CNAs-modified GCEs in PBS solutions containing AA (1 mM), DA (0.8 mM) and UA (1 mM). DPV curves of CNA-1/GCE (B), CNA-2/GCE (C) and CNA-3/GCE (D) in PBS solutions containing AA (1 mM), UA (1 mM) and different DA concentrations: (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, (e) 0.5, (f) 0.6, (g) 0.7 and (h) 0.8 mM. Fig. 8 (A) CVs curves of CNAs modified GCRDEs obtained in N2 or O2-saturated KOH solutions. The inset is the linear sweep voltammetry (LSV) curves of NAs-GCRDEs in O2-saturated electrolytes at 1600 rpm. (B) The LSV curves of CNA-2 obtained at different rpms (300-2000) in O2-saturated 0.1 M KOH electrolytes. The inset shows the Koutecky-Levich plots (J-1 versus ω-0.5) of CNA-2 modified GCE. Fig. 9 UV-vis absorption spectra of aqeuous Congo red solutions in the presence of CNA-2 at different time intervals after diluting 5 times.
26
Fig. 10 (A) Adsorption rates of Congo red on CNA-1 (a), CNA-2 (b) and CNA-3 (c) under dark conditions. (B) Cycling runs in the adsorption of Congo red for CNA-2. The inset is the TEM image of CNA-2 after 5 cycling test.
27
Fig. 1 XRD patterns of CNA-1 (a), CNA-2 (b) and CNA-3 (c).
28
Fig. 2 SEM images of CNA-1 (a, d), CNA-2 (b, e) and CNA-3 (c, f).
29
Fig. 3 TEM images of CNA-1 (a), CNA-2 (b) and CNA-3 (c) and schematic illustration of the formation mechanism of CNAs (d).
30
Fig. 4 Nitrogen adsorption/desorption isotherms (A) and BJH pore size distributions (B) of CNAs samples.
31
Fig. 5 Magnetization hysteresis of Cu2O CNAs (A) and the magnified graph of (B).
32
Fig. 6 (A) CV curves of the CNA-3/GCE in PBS solutions containing AA (0.8 mM), DA (0.8 mM) or UA (0.8 mM). (B) DPV curves of CNA-2/GCE in PBS solutions containing different concentrations of DA: (a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.10, (f) 0.15, (g) 0.20, (h) 0.30, (i) 0.40, (j) 0.50, (k) 0.60, (l) 0.70 and (m) 0.80 mM.
33
Fig. 7 (A) DPV curves of CNAs-modified GCEs in PBS solutions containing AA (1 mM), DA (0.8 mM) and UA (1 mM). DPV curves of CNA-1/GCE (B), CNA-2/GCE (C) and CNA-3/GCE (D) in PBS solutions containing AA (1 mM), UA (1 mM) and different DA concentrations: (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, (e) 0.5, (f) 0.6, (g) 0.7 and (h) 0.8 mM.
34
Fig. 8 (A) CVs curves of CNAs modified GCRDEs obtained in N2 or O2-saturated KOH solutions. The inset is the linear sweep voltammetry (LSV) curves of NAs-GCRDEs in O2-saturated electrolytes at 1600 rpm. (B) The LSV curves of CNA-2 obtained at different rpms (300-2000) in O2-saturated 0.1 M KOH electrolytes. The inset shows the Koutecky-Levich plots (J-1 versus ω-0.5) of CNA-2 modified GCE.
35
Fig. 9 UV-vis absorption spectra of aqeuous Congo red solutions in the presence of CNA-2 at different time intervals after diluting 5 times.
36
Fig. 10 (A) Adsorption rates of Congo red on CNA-1 (a), CNA-2 (b) and CNA-3 (c) under dark conditions. (B) Cycling runs in the adsorption of Congo red for CNA-2. The inset is the TEM image of CNA-2 after 5 cycling test.
37
S0927-7757(17)30246-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.03.010 COLSUA 21458
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
28-11-2016 4-3-2017 5-3-2017
Please cite this article as: Peizhi Guo, Yutao Sang, Jing Xue, Binghui Xu, Rongyue Wang, Hongliang Li, X.S.Zhao, Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2017.03.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Assembly of colloidal cuprous oxide nanocrystals and study of its magnetic and electrocatalytic properties
Peizhi Guo,* Yutao Sang, Jing Xue, Binghui Xu, Rongyue Wang, Hongliang Li, X. S. Zhao
Institute of Materials for Energy and Environment, State Key Laboratory Breeding Based of New Fiber Materials and Modern Textile, School of Materials Science and Engineering, Qingdao University, Qingdao, 266071, P. R. China.
*Corresponding Author: [email protected], [email protected]
1
Graphical abstract
Highlights
Self-assembly of Cu2O colloidal nanocrystals has been realized. The structure of Cu2O colloidal nanocrystals assembly can be easily adjusted. Magnetic properties of Cu2O colloidal nanocrystal assemblies are related to their assembly structure. The electrocatalytical activity of Cu2O nanocrystal assemblies is determined by their interior primary nanocrystals.
ABSTRACT
Colloidal nanocrystal assemblies (CNAs) of cuprous oxide (Cu2O) are controllably synthesized by in-situ self-assembling primary Cu2O nanocrystals, in which Cu2O CNAs assemblied by nanocrystals of 8~10 nm were about 0.6 μm and Cu2O CNAs assemblied by nanocrystals of 32 nm were about 1.8 μm. From room temperature magnetization measurement results, both of the CNAs showed ferromagnetic behavior with very small saturation magnetization (Ms) values and 10 nm nanocrystal-based CNAs had the largest Ms value of about 9.2 emu/g. Electrochemical experimental results showed that submicron Cu2O CNAs were able to detect dopamine and uric acid quickly while micrometer Cu2O CNAs were not. Cu2O CNAs formed by 8 nm nanocrystals showed the highest electrocatalytic activity in oxygen reduction reaction and displayed the best adsorptive ability to remove Congo red in aqueous solutions, which can be recylced after a simple heat treatment. 2
Furthermore, the formation mechanism and the relationship between crystalline size and assembly of primary nanocrystals as well as physicochemical properties of Cu2O CNAs have been discussed and analyzed.
Keywords: cuprous oxide, colloidal nanocrystal, self-assembly, adsorption, electrocatalysis
1. Introduction
Recent decades have witnessed metal oxides used widely in the fields including catalysis, energy storage, environment protection and biomedicine production [1-11]. Cu2O, one of the important metal oxides and p-type semiconductor materials [12-16], have been investigating as gas sensor [12], heterogeneous catalyst [15,16], high-efficiency
photocatalyst
and
photoelectrocatalyst
[8,9]
for
its
unique
physicochemical properties. With the rapid development of synthetic technologies, fabrication of Cu2O nanomaterials has been improved much recently [12-26]. Many methods such as solution phase synthesis [12,17], replacement-etching [20-21] and solvothermal/hydrothermal synthesis [16,22,26] have been reported to prepare various kinds of Cu2O nanomaterials including polyhedral [17-19], films [20], nanocages [21] hollow spheres [22] and porous structures [23]. Low-temperature synthesis methods are energy efficient and have been applied to prepare many nanomaterials [17-21]. For example, Huang and his co-workers [19] synthesized Cu2O nanocrystals with different morphologies in aqueous solutions in room temperature. However, research on Cu2O
3
particles with designed morphologies, especially formed by self-assembling nanocrystals and study of the structure-property relationship seems to attract few attentions.
Colloidal nanocrystal assemblies (CNAs) formed by ordered self-assembly of inorganic nanocrystals have attracted increasing interest due to their unique properties, especially the effective interactions in adjacent primary nanocrystals [27-33]. Large superstructures can be obtained from in-situ or ex-situ assembly nanoparticles, which typically and continuously occurs and depends on the uniformity of individual particles [27-31]. For example, different inorganic nanoparticles with broad polydispersity can spontaneously assemble and form uniform supraparticles with core-shell structures triggered by the interactive balance between van der Waals attractions and electrostatic repulsions [28]. Recently, the assembly of diverse anisotropic nanoparticles has been realized in the nematic liquid crystals with long-range ordering through the balance of competing interactions [29-31]. Ferrite nanocrystals are able to form ordered assembly structures easily via the in-situ self-assembly partially because of the strong magnetic interactions [32-34]. It is believed that ordered assemblies from smaller nanoparticles possess preferable properties than those from larger ones [28-34]. However, it is still a great challenge to prepare inorganic assembly nanostructures of weak magnetic nanoparticles by a one-pot strategy, hindered by simultaneous controlling the size of nanocrystals and in-situ self-assembly process.
4
Herein, we report a facile solution-phase synthesis method to prepare two types of Cu2O CNAs under mild conditions with the sizes of about 0.6 and 1.8 m respectively. The two CNAs were formed by self-assembly of primary Cu2O nanocrystals with the crystalline sizes of 8~10 and 32 nm, respectively. Results showed the crystalline size and ordered assembly of Cu2O nanocrystals played critical roles in determining the magnetic properties and electrochemical performances of all the CNAs. Furthermore, the assembly formed with smaller Cu2O nanoparticles displayed better adsorption abilities than those formed by larger ones.
2. Experimental
2.1 Materials All chemicals, including CuSO4•5H2O, Cu(CH3COO)2•H2O, CH3COONH4, NH4HCO3, ascorbic acid, uric acid, ethanol and Congo red were of analytical grade (Sinopharm Chemical Reagent Company). Dopamine (99%) was purchased from Aldrich. All chemicals were used without further purification. Double distilled water was used in the experiments.
2.2 Synthesis of MnFe2O4 CNAs In a typical synthesis of CNA-1, 120 mL aqueous CuSO4•5H2O (3 mmol) solution and 30 mL CH3COONH4 (6 mmol) solution were respectively transferred to a 200 mL three-neck flask and held at 50°C for 20 min under stirring. Then, 15 mL ascorbic acid (0.9 mmol) solution was added into the above solution and maintain the 5
temperature at 50°C for 1 h, followed by cooling down to room temperature. The brick red precipitate was collected by centrifugation, washed three times with distilled water, and dried in a vacuum oven at 50°C for 12 h. The synthetic procedure for CNA-2 was similar to that of CNA-1 with the heating time increased to 1.5 h at 50°C, except for the use of Cu(CH3COO)2•H2O (3 mmol) and the content of CH3COONH4 and ascorbic acid increased to 12 mmol and 4.5 mmol, respectively. The synthesis of CNA-3 is almost the same as that of CNA-2 except using NH4HCO3 instead of CH3COONH4.
2.3 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.154 18 nm) from 10° to 80° (2 θ). Scanning electron microscopy (SEM) images were collected on a Hitachi S-4800 FE-SEM and JSM 6390-LV operated at 10 and 20 kV, respectively. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM2010 TEM operated at 200 kV. Magnetic properties were analyzed at room temperature by a LDJ9500 vibrating sample magnetometer (VSM). Electrochemical measurements were performed on a CHI760E electrochemical workstation using a three-electrode cell. The hydrodynamic diameters of the samples were measured on a Malvern Zetasizer (Nano-ZS90).
6
2.4 Electrochemical measurements Electrochemical measurements were performed by using a three-electrode cell with platinum foil as counter electrode and saturated calomel electrode as reference electrode in aqueous phosphate buffer solutions (PBS, 0.1 M, pH 7.4) containing DA, UA and AA with the concentration of 1 mM. Bare glass carbon electrode (GCE, 3 mm in diameter) and Cu2O-modified GCE were used as the working electrodes. A 10 μL homogeneous aliquot suspension (1.5 mg mL−1) was uniformly cast onto the GCE surface. The modified GCEs were dried under ambient conditions before being used. For ORR, electrochemical measurements were conducted using a GCRDE coated with Cu2O catalyst was used as working electrode. An Ag/AgCl electrode and a platinum wire were used respectively as the reference electrode and counter electrode in a 0.1 M aqueous KOH solution. Current densities were normalized to geometric area of the GCE. Cyclic voltammograms were conducted in the potential range -0.8 V to 0 V at a scan rate of 50 mV s-1 with O2-saturated 0.1 M KOH aqueous solution. The linear sweep voltammetry for ORR was obtained with GCRDE at different speeds in an O2-saturated electrolyte from -0.8 to 0 V at a scan rate of 10 mV s-1.
3. Results and discussion
3.1. Crystal structure Three Cu2O CNA samples were synthesized by simply adjusting synthetic systems using solution-phase synthesis method at 50°C. Sample CNA-1 was prepared from aqueous solutions containing CuSO4 and CH3COONH4. Samples CNA-2 and 7
CNA-3 were obtained from Cu(CH3COO)2 systems containing CH3COONH4 and NH4HCO3, respectively. Fig. 1 shows the X-ray diffraction (XRD) patterns of three Cu2O samples. The diffraction peaks are in good accordance with the standard pattern of pure Cu2O phase (JCPDS, No. 05-0667). The sharp peaks appeared at 2 θ degrees of 29.6, 36.5, 42.4, 61.5 and 73.7 can be well-indexed to the (110), (111), (200), (220) and (311) planes of Cu2O phase, respectively. Clearly, the diffraction peaks of CNA-1 (Fig. 1a) were much sharper and narrower than that of CNA-2 and CNA-3, indicating the crystalline size of primary Cu2O nanoparticles in CNA-1 was significantly larger than others. Based on the measurements of the full width at half-maximum (fwhm) of the (111) peaks in the XRD patterns, the average crystallite sizes of primary particles in CNA-1, CNA-2 and CNA-3 were calculated to be about 32, 8 and 10 nm according to the Scherrer equation, respectively.
3.2. Microstructure and Evolution of the intermediates Fig. 2 shows the scanning electron microscope (SEM) images of Cu2O CNAs. Obviously, CNA-1 displayed a irregular flower-like shape. CNA-2 had a rather smooth surface compared with that of CNA-3 and both were dispersed spherical structures. The size of CNA-1 samples was about 1.8±0.3 m, which was the largest among the three samples. CNA-2 and CNA-3 were submicrometer spheres with the sizes of about 0.7±0.2 and 0.6±0.2 m, respectively. Enlarged SEM images (Fig. 2d-2f) confirmed that all of the Cu2O CNAs were supposed to be formed from the assembly of primary Cu2O nanocrystals. Clearly, CNA-1 was composed of larger nanoparticles while CNA-2 and CNA-3 were formed by smaller Cu2O nanoparticles. 8
These observations were essentially consistent with the XRD measurement results. In the meantime, the hydrodynamic diameters of CNA-2 and CNA-3 measured in aqueous solutions were about 0.89 and 0.85 m, respectively, while sample CNA-1 cannot show a reasonable value due to its large polydispersity index, consistent with those of SEM observations.
Transmission electron microscopy (TEM) images of Cu2O CNAs confirmed the formation of spherical assembly structures (Fig. 3). It is observed that CNA-1 was about 2 m (Fig. 3a) and was composed of large well–crystalline nanoparticles based on the selected area electron diffraction (SAED) pattern (the inset in Fig. 3a), in which bright diffraction spots were observed. When copper acetate is introduced into the synthetic systems, smaller spheres with a much smooth surface could be obtained than that of CNA-1 (Fig. 3b and 3c). Furthermore, bright circles composed of arc-like diffraction spots were observed in the SAED patterns of CNA-2 and CNA-3 (the insets in Fig. 3b and 3c), which were rather different from that of CNA-1. CNA-2 and CNA-3 should be formed from the in-situ self-assembly of small nanoparticles with slight misalignments among primary nanocrystals, similar to those of assembly structures of magnetite and ferrites [29,30]. Furthermore, it can also be derived that the self-assembly of large Cu2O nanoparticles cannot obviously influence the properties of primary nanoparticles, similar to those of large manganese ferrite CNAs [34].
The formation mechanisms of Cu2O CNAs were investigated based on the characterizations of intermediate products during the synthesis of the targeted samples. 9
It was observed that only precipitates were formed before ascorbic acid (AA) was added into the synthetic systems of CNA-1 and CNA-3, and the XRD results (Fig. S1) indicated the precipitates were mainly copper basic carbonate (JCPDS, No. 13-0131). According to the XRD and SEM results (Fig. S1 and S2), sphere precursors for CNA-3 were composed of small nanoparticles, while flower-like spheres for CNA-1 were formed by large particles. When ascorbic acid was introduced into the solution, redox reactions between the reductive agent and the precipitates could be triggered to form CNA-1 and CNA-3, which was confirmed by the SEM results of the corresponding intermediates (Fig. S3 and S4).
However, no precipitates were formed for CNA-2 before adding ascorbic acid. Similar spherical assembly structures of CNA-2 were obtained directly from the reduction reaction between copper ions and ascorbic acid, which were formed via the assembly of small Cu2O nanocrystals. Compared with those similar structures of magnetite and ferrite, the assembly structures of Cu 2O are formed very quickly in this work during the synthetic processes (Fig. S3-S5) [20,32]. It was speculated that the formation of CNAs should be closely related to the coordinative and oxidative ability of copper ions and the reductive ability of ascorbic acid as well as the trend to reduce the surface energy of the products (Fig. 3d). The structural differences between CNA-1 and CNA-3 should be ascribed to the different structures of their precursors. It can also be concluded that the dynamic control was responsible for the structure of CNAs because the precipitate disappeared when the reaction time was up to 10 h and reformed again 1 h later during the synthesis of CNA-2. 10
3.3. BJH measurement As Cu2O CNAs were composed of small nanocrystals, the structural features of these samples were further investigated by nitrogen gas sorption technique. Fig. 4 shows the N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution curves of three CNA samples. It can be seen that the type IV physisorption nature of three isotherms was obtained according to the IUPAC classification, which showed a typical H1-type hysteresis loop, especially for CNA-2 and CNA-3, indicating that abundant mesoporous structures are existed in these samples. The specific surface areas and total pole volumes were respectively calculated about 3.0 m2 g-1 and 0.018 cm3 g-1 for CNA-1, 26.8 m2 g-1 and 0.104 cm3 g-1 for CNA-2, and 20.0 m2 g-1 and 0.066 cm3 g-1 for CNA-3. These results further indicated the self-assembly nature of small nanoparticles in the CNA samples.
3.4. Magnetic characterization Magnetic properties of Cu2O samples were measured under room temperature. Fig. 5A shows magnetization curves of Cu2O CNAs at the full scale (H=1.4×104 Oe). It was cleared that the saturation magnetization (Ms) values was about 3.2, 4.7 and 9.2 emu/g for CNA-1, CNA-2 and CNA-3, respectively, although these Ms values were very small compared with that of iron series materials.27 An obvious hysteresis loop could be observed for CNA-3 (Fig. 5A), while CNA-1 and CNA-2 also showed very small hysteresis loops in the magnified image, indicating all the CNAs samples displayed the ferromagnetic behavior. It was found that the values of remnant 11
magnetization (Mr) and coercivity (Hc) were respectively measured to be 0.29 emu/g and 88.8 Oe for CNA-1, 0.55 emu/g and 115.3 Oe for CNA-2, and 0.95 emu/g and 117.5 Oe for CNA-3.
Usually, magnetic nanoparticles have a large Ms value when their crystalline size is large. In the meantime, large ferromagnets nanoparticles have large Mr and Hc values. Based on the magnetic results of our Cu2O CNAs, it can be concluded that the values of the Ms, Mr and Hc of Cu2O CNAs were determined not only by the crystalline size but also by the self-assembly of primary nanoparticles. The critical size of Cu2O for the superpara magnetic-ferromagnetic transition was slight larger than 32 nm, which was much larger than that of ferrite nanoparticles. On the other hand, CNA-2 displayed much higher values of the Ms, Mr and Hc than CNA-1, indicating the unique assembly nature of CNA-2. These should be ascribed to the template synthesis of CNA-1 while CNA-2 was formed directly from the in-situ self-assembly of the as-formed 8 nm Cu2O nanoparticles with the well-crystalline nature.
3.5. Electrochemical sensing performance To disclose the structure-property relationship of Cu2O CNAs, three types of experiments
were
conducted,
namely
electrocatalysis
of
small
molecules,
electrocatalysis for oxygen reduction reaction and adsorption of organic pollutants. It has been reported that Cu2O has the catalytic activity toward the reduction or degradation of specific molecules. In this work, Cu2O CNAs were further explored for the electrocatalysis of soluble molecules,10-14 which would be expected to distinguish 12
the structural effect based on their electrochemical performances. The glass carbon electrodes (GCEs) modified by Cu2O CNAs are abbreviated as CNA-1/GCE, CNA-2/GCE and CNA-3/GCE. Fig. 6A shows the cyclic voltammogram (CV) curves of the modified GCEs in PBS solutions containing ascobic acid, dopamine (DA) or urea acid (UA), in which the redox peaks can be observed, respectively. It can be seen that AA and UA showed obviously oxidation peaks at all of the three modified GCEs, and the redox peaks for AA are very broad (Fig. 6A and S6). While, one pair of redox peaks can be observed clearly for the detection of DA. These observations should be ascribed to the differences in the molecular structures of these molecules. In addition, the separation of anodic and cathodic peak potential of DA is 0.104, 0.090 and 0.096 V for CNA-1, CNA-2 and CNA-3 modified GCEs, respectively, indicating that the electrochemical reaction on CNA-2/GCE was more reversible than that of any other samples. Furthermore, the potential of the anodic peaks of DA were at 0.196, 0.186 and 0.191 V at CNA-1/GCE, CNA-2/GCE and CNA-3/GCE, respectively. The most negative peak potential for CAN-2/GCE indicated that DA could react at a relative lower potential compared with others.
As shown in Figure 6B, the catalytic peak currents in the differential pulse voltammetry (DPV) curves of CNA-2/GCE were linearly increased with the concentration of DA with a correlation coefficient of r = 0.998 in the experimental range. It is shown that the response of DA was also apparent even if the concentration was 0.01 mM. Therefore, the lowest detection limit of DA at Cu 2O-2/GCE was evaluated to be 0.01 mM. 13
Furthermore, the detection of DA was almost unaffected in the presence of AA and UA although the peak currents of UA were slightly influenced by DA (Fig. 7). It could be seen from Fig. 7A that CNA-2/GCE and CNA-3/GCE could simultaneously detect DA and UA in ternary electrolytes containing AA, DA and UA and CNA-2/GCE showed better catalytic performances than CNA-3/GCE (Fig. 7B-7D). However, CNA-1/GCE can only discern DA under the same electrochemical conditions. It is suggested that the crystalline sizes of nanoparticles in CNAs should be responsible for their electrocatalytic performances with adjustable electron transfer rate [35] and the quicker electro-transfer kinetics should also be expected for DA than that of UA [36,37].
It is well-known that an efficient oxygen reduction reaction (ORR) activity is essential to develop advanced fuel cells and metal-air batteries. Recently, it has attracted great interest to explore the ORR electrocatalysts with low cost and excellent performance [15,16,38,39]. However, the electrocatalytic properties of Cu2O nanostructures for ORR had not yet been fully understood [15,16]. As for the working mechanism of Cu2O as an ORR catalyst, in charge control potential ranges, there should be a rate-limiting first electron transferring to adsorbed O2 to form superoxide on Cu2O along with the first-order dependence on O2 partial pressure (reaction 1), possibly involving a concurrent reaction with water (reaction2) [15,16,40]. Fig. 8A depicts the CV curves of the modified glass carbon rotating disk electrodes (GCRDEs) in aqueous N2 or O2-saturated 0.1 M KOH solutions under a scan rate of 50 mV s-1 at room temperature. Obviously, the CV curve of the modified electrode in N 2-saturated 14
electrolytes was essentially featureless while a notable cathodic peak appeared at -0.46 V for CNA-2 and CNA-3 in O2-saturated electrolytes, indicating that these two samples had potentially electrocatalytic activity for ORR [41]. Futhermore, the onset reduction potentials for Cu2O are shift positively with the order of CNA-2, CNA-3 and CNA-1. Cu2O-O2+e-→Cu2O-O2-
(1)
Cu2O-O2+H2O+e-→Cu2O-HO2+OH-
(2)
The inset in Fig. 8A shows the onset potentials and half-wave potentials were different for three samples, and positively shifted with the decrease of crystalline sizes of Cu2O nanoparticles in the CNAs, suggesting that CNA-2 should have a superior electrocatalytic activity compared to CNA-3 and CNA-1. The ORR kinetics of CNAs was conducted by varying the rotation speed to qualitatively analyze the catalytic activity. Taking CNA-2 as an example (Fig. 8B), the current density increases smoothly with the rotation speeds, probably caused by the faster oxygen flux to the electrode surface. The linearity of the Koutecky-Levich plots and near parallelism of the fitting lines indicated the first order reaction kinetics toward oxygen gas on CNA-2 within the potential window ranging from -0.6 to -0.8 V. The transferred electron number (n) for CNA-2, which can be obtained from the slope and intercept of the Koutecky-Levich plots [42], was about 2.10, 2.31 and 2.58 at the potential of -0.6, -0.7 and -0.8 V, respectively, indicating that CAN-2 sample had the electrochemical activity for the ORR and O2 can be reduced to OH- ions through the 2-electron 15
reduction mechanism (reaction 3 and 4). The n values of CAN-1 and CNA-3 was much smaller than that of CAN-2, suggesting again that CNA-2 had a better electrocatalytic activity which was consistent with the CV and LSV data in Fig. 8. It is proposed that the ORR kinetics of Cu2O-based electrocatalysts should be related to the microstructure and crystalline size of Cu2O structures as well as the assembly interactions/nature among nanoparticles in the CNAs [43]. It also should be pointed out that the catalytic current densities of the samples were not large although CNA-2 displays the best electrocatalytic activity for ORR among these Cu 2O samples. O2+H2O+2e-→HO2-+OH-
(3)
HO2-+H2O+2e-→3OH-
(4)
3.6. Adsorption of Congo red Generally, transition metal oxides can be used to remove organic complex from water through adsorption or subsequent catalysis [44,45]. Cu2O CNAs showed adsorption ability for the typical organic waste, Congo red, under dark conditions (Fig. 9 and 10). The adsorption of Congo red onto the surfaces of CNAs was suggested because of the same experimental results under daytime, visible light (by using a 300 W Xe lamp) or dark conditions. The variation of adsorption of Congo red with time could be confirmed by the UV-vis absorption data (Fig. 9). Clearly, only absorption strength, not the absorption peak position, was decreased with the processing time. CNA-2 showed the best absorption ability among three samples with the removal rate of about 0.27 gram Congo red for CNA-2 per gram(Fig.10A). The removal ability of 16
CNA-3 was better than that of CNA-1. This should be ascribed to the structural nature of CNA-2, which had the smallest crystalline size, the largest specific surface area and highest surface energy among three samples. CNAs could be reused for several cycles after simply heating treatment. However, further prolonging the cycling runs led to the decrease of the absorption performance, probably due to the structural change of the sample based on the TEM image (Fig. 10B).
4. Conclusions
In summary, Cu2O CNAs with different sizes were synthesized by self-assembly of primary nanoparticles under mild conditions. The CNAs composed of 8 nm nanoparticles showed the best capability to remove Congo red in aqueous solutions and was able to detect dopamine and uric acid quickly as electrocatalysts. Besides, CNAs assemblied by 8 nm nanoparticles possess a the higher electrocatalytic activity in oxygen reduction reaction than any of other samples. All the three Cu2O CNAs display weak ferrimagnetic behavior with the staturation magnetization values of 3.2, 4.7 and 9.2 emu/g for the CNAs formed by 8, 10 and 32 nm nanoparticles, respectively. These results indicate that the crystalline size and self-assembly of primary nanoparticles play the key roles in determining the performances of colloidal nanocrystal assemblies.
17
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. U1232104), the National High Technology Research and Development Program of China (2012AA110407), National college students innovation and entrepreneurship training program (201611065003) and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province, P. R. China.
18
References
[1] J. C. Park, J. Kim, H. Kwon and H. Song, Gram-Scale synthesis of Cu2O nanocubes and subsequent oxidation to CuO hollow nanostructures for Lithium-Ion battery anode materials, Adv. Mater. 21 (2009) 803-807.
[2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496-499.
[3] J. Zeng, L. Jing, Y. Hou, M. Jiao, R. Qiao, Q. Jia, C. Liu, F. Fang, H. Lei and M. Gao, Anchoring group effects of surface ligands on magnetic properties of Fe3O4Nanoparticles: towards high performance MRI contrast agents, Adv. Mater. 26 (2014) 2694-2698.
[4] X. Hu, J. Yu, J. Gong, Q. Li and G. Li, α-Fe2O3 nanorings prepared by a microwave-assisted hydrothermal process and their sensing properties, Adv. Mater. 19 (2007) 2324-2329.
[5] X. Lang, X. Chen and J. Zhao, Heterogeneous visible light photocatalysis for selective organic transformations, Chem. Soc. Rev. 43 (2014) 473-486.
[6] M. Tsai, R. Hadt, P. Vanelderen, B. Sels, R. Schoonheydt and E. Solomon, [Cu2O]2+ active site formation in Cu–ZSM-5: geometric and electronic structure requirements for N2O activation, J. Am. Chem. Soc. 136 (2014) 3522-3529. 19
[7] C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie and J. Ma, One-Pot synthesis of octahedral Cu2O nanocages via a catalytic solution route, Adv. Mater. 17 (2005) 2562-2567.
[8] S. Shinde and K. Nanda, Facile synthesis of large area porous Cu2O as super hydrophobic yellow-red phosphors, RSC Adv. 2 (2012) 3647-3650.
[9] Y. Sang, Y. Cui, Z. Li, W. Ye, H. Li, X.S. Zhao and P. Guo, Electrochemical reaction of nitrobenzene and its derivatives on glassy carbon electrode modified with MnFe2O4 colloid nanocrystal assemblies, Sensors Actuat. B 234 (2016) 46-52.
[10] G.-Z. Yuan, C.-F. Hsia, Z.-W. Lin, C. Chiang, Y.-W. Chiang and M. H. Huang, Highly facet-dependent photocatalytic properties of Cu2O crystals established through the formation of Au-decorated Cu2O heterostructures, Chem. Eur. J. 22 (2016) 12548-12556.
[11] C.-S. Tan, S.-C. Hsu, W.-H. Ke, L.-J. Chen and M. H. Huang, Facet-dependent electrical conductivity properties of Cu2O crystals, Nano Lett. 15 (2015) 2155-2160.
[12] J. Zhang, J. Liu, Q. Peng, X. Wang and Y. Li, Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867-871.
[13] A. Marin, D. Muñoz-Rojas, D. Iza, T. Gershon, K. Musselman and J. MacManus-Driscoll, Novel atmospheric growth technique to improve both light 20
absorption and charge collection in ZnO/Cu2O thin film solar cells, Adv. Funct. Mater. 23 (2013) 3413-3419.
[14] Y. Sui, Y. Zheng, L. Fu, W. Zheng, D. Li, B. Lin and B. Zou, Low-temperature synthesis of porous hollow structured Cu2O for photocatalytic activity and gas sensor application, RSC Adv. 3 (2013) 18651-18660.
[15] Q. Li, P. Xu, B. Zhang, H. Tsai, S. Zheng, G. Wu and H. Wang, Structure-dependent electrocatalytic properties of Cu2O nanocrystals for oxygen reduction reaction, J. Phys. Chem. C 117 (2013) 13872-13878
[16] X. Yan, X. Tong, Y. Zhang, X. Han, Y. Wang, G. Jin, Y. Qin and X. Guo, Cuprous oxide nanoparticles dispersed on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, Chem. Commun. 48 (2012) 1892-1894.
[17] L. Gou and C. Murphy, Solution-phase synthesis of Cu2O nanocubes, Nano Lett. 3 (2003) 231-234.
[18] W. Huang, L. Lyu, Y. Yang and M. Huang, Synthesis of Cu2O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity, J. Am. Chem. Soc. 134 (2012) 1261-1267.
[19] C. Li and M. Kanan, CO2 reduction at low overpotential on Cu electrodes resulting from the reduction of thick Cu2O films, J. Am. Chem. Soc. 134 (2012) 7231-7234.
21
[20] C. Kuo, Y. Chu, Y. Song and M.H. Hong, Cu2O nanocrystal-templated growth of Cu2S nanocages with encapsulated Au nanoparticles and In-Situ transmission X-ray microscopy study, Adv. Funct. Mater. 21 (2011) 792-797.
[21] Y. Sui, W. Fu, Y. Zeng, H. Yang, Y. Zhang and H. Chen, Synthesis of Cu2O nanoframes and nanocages by selective oxidative etching at room temperature, Angew. Chem. Int. Ed. 49 (2010) 4282-4285.
[22] J. J. Teo, Y. Chang and H. C. Zeng, Fabrications of hollow nanocubes of Cu2O and Cu via reductive Self-Assembly of CuO nanocrystals, Langmuir 22 (2006) 7369-7377.
[23] Y. Yu, F. Du, J. Yu, Y. Zhuang and P. Wong, One-dimensional shape-controlled preparation of porous Cu2O nano-whiskers by using CTAB as a template, J. Solid State Chem. 177 (2004) 4640-4647.
[24] Q. Zheng, M. Morimoto and T. Takanohashi, A novel process for the production of aromatic hydrocarbons from brown coal in water medium by hydrothermal oxidation and catalytic hydrothermal decarboxylation, Fuel 182 (2016) 437-445.
[25] S. Cao, H. Chen, T. Han, C. Zhao and L. Peng, Rose-like Cu2O nanoflowers via hydrothermal synthesis and their gas sensing properties, Mater. Lett. 180 (2016) 135-139
22
[26] X. Luo, M. Wang, D. Yang, J. Yang and Y. Chen, Hydrothermal synthesis of morphology controllable Cu2O and their catalysis in thermal decomposition of ammonium perchlorate, J. Ind. Eng. Chem. 32 (2015) 313-318.
[27] Z. Lu and Y. Yin, Colloidal nanoparticle clusters: functional materials by design, Chem. Soc. Rev. 41 (2012) 6874-6887.
[28] Y. Xia, T. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. Glotzer and N. Kotov, Self-assembly of self-limiting monodisperse supraparticles from polydisperse nanoparticles, Nat. Nanotech. 6 (2011) 580-587.
[29] H. Mundoor, B. Senyuk and I. I. Smalyukh,Triclinic nematic colloidal crystals from competing elastic and electrostatic interactions, Science 352 (2016) 69-73.
[30] H. Mundoor and I. I. Smalyukh, Mesostructured composite materials with electrically tunable upconverting properties, Small 11 (2015) 5572–5580.
[31] Q. Liu, Y. Yuan and I. I. Smalyukh, Electrically and optically tunable plasmonic guest–host liquid crystals with long-range ordered nanoparticles, Nano Lett. 14 (2014) 4071–4077.
[32] J. Ge, Y. Hu, M. Biasini, W. Beyermann and Y. Yin, Superparamagnetic magnetite colloidal nanocrystal clusters, Angew. Chem. Int. Ed. 46 (2007) 4342-4345.
23
[33] P. Guo, Z. Li, S. Liu, J. Xue, G. Wu, H. Li and X. S. Zhao, Electrochemical properties of colloidal nanocrystal assemblies of manganese ferrite as the electrode materials for supercapacitors. J. Mater. Sci. 52 (2017) 5359-5365.
[34] R. Liu, M. Lv, Q. Wang, H. Li, P. Guo and X. S. Zhao, Solvothermal synthesis of size-tunable ZnFe2O4 colloidal nanocrystal assemblies and their electrocatalytic activity towards hydrogen peroxide, J. Mag. Mag. Mater. 424 (2017) 155-160.
[35] P. Wang, Y. Li, X. Huang and L. Wang, Fabrication of layer-by-layer modified multilayer films containing choline and gold nanoparticles and its sensing application for electrochemical determination of dopamine and uric acid, Talanta 73 (2007) 431-437.
[36] J. Huang, Y. Liu, H. Hou and T. You, Simultaneous electrochemical determination of dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbon nanofibers modified electrode, Biosens. Bioelectron. 24 (2008) 632-637.
[37] H. Bi, Y. Li, S. Liu, P. Guo, Z. Wei, C. Lv, J. Zhang and X. S. Zhao, Carbon-nanotube-modified glassy carbon electrode for simultaneous determination of dopamine, ascorbic acid and uric acid: The effect of functional groups, Sensors Actuat. B 171-172 (2012) 1132-1140.
[38] J. Wu and H. Yang, Platinum-based oxygen reduction electrocatalysts, Acc. Chem. Res. 46 (2013) 1848-1857.
24
[39] H. Zhu, S. Zhang, Y. Huang, L. Wu and S. Sun, Monodisperse MxFe3–xO4 (M = Fe, Cu, Co, Mn) nanoparticles and their electrocatalysis for oxygen reduction reaction, Nano Lett. 13 (2013) 2947-2951.
[40] D. B. Sepa, M. V. Vojnovicm and A. Damjanovic, Kinetics and mechanism of O2 reduction at Pt IN alkaline solutions, Electrochim. Acta 25 (1980) 1491-1496. [41] W. Chen, D. Ny and S. Chen, SnO2–Au hybrid nanoparticles as effective catalysts for oxygen electroreduction in alkaline media, J. Power Sources 195 (2010) 412-418.
[42]
W.
Xiao,
D.
Wang
and
X.
W.
Lou,
Shape-controlled
synthesis
of
MnO2 nanostructures with enhanced electrocatalytic activity for oxygen reduction, J. Phys. Chem. C 114 (2009) 1694-1700.
[43] M. Shao, A. Peles and K. Shoemaker, Electrocatalysis on platinum nanoparticles: particle size effect on oxygen reduction reaction activity, Nano Lett. 11 (2011) 3714-3719.
[44] L. Zhong, J. Hu, H. Liang, A. Cao, W. Song and L. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Adv. Mater. 18 (2006) 2426-2431.
[45] P. Guo, M. Lv, G. Han, C. Wen, Q. Wang, H. Li and X. S. Zhao, Solvothermal synthesis of hierarchical colloidal nanocrystal assemblies of ZnFe2O4 and their application in water treatment, Materials 9(2016) 806.
25
Figure Captions： Fig. 1 XRD patterns of CNA-1 (a), CNA-2 (b) and CNA-3 (c) Fig. 2 SEM images of CNA-1 (a, d), CNA-2 (b, e) and CNA-3 (c, f). Fig. 3 TEM images of CNA-1 (a), CNA-2 (b) and CNA-3 (c) and schematic illustration of the formation mechanism of CNAs (d). Fig. 4 Nitrogen adsorption/desorption isotherms (A) and BJH pore size distributions (B) of CNAs samples. Fig. 5 Magnetization hysteresis of Cu2O CNAs (A) and the magnified graph of (B). Fig. 6 (A) CV curves of the CNA-3/GCE in PBS solutions containing AA (0.8 mM), DA (0.8 mM) or UA (0.8 mM). (B) DPV curves of CNA-2/GCE in PBS solutions containing different concentrations of DA: (a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.10, (f) 0.15, (g) 0.20, (h) 0.30, (i) 0.40, (j) 0.50, (k) 0.60, (l) 0.70 and (m) 0.80 mM. Fig. 7 (A) DPV curves of CNAs-modified GCEs in PBS solutions containing AA (1 mM), DA (0.8 mM) and UA (1 mM). DPV curves of CNA-1/GCE (B), CNA-2/GCE (C) and CNA-3/GCE (D) in PBS solutions containing AA (1 mM), UA (1 mM) and different DA concentrations: (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, (e) 0.5, (f) 0.6, (g) 0.7 and (h) 0.8 mM. Fig. 8 (A) CVs curves of CNAs modified GCRDEs obtained in N2 or O2-saturated KOH solutions. The inset is the linear sweep voltammetry (LSV) curves of NAs-GCRDEs in O2-saturated electrolytes at 1600 rpm. (B) The LSV curves of CNA-2 obtained at different rpms (300-2000) in O2-saturated 0.1 M KOH electrolytes. The inset shows the Koutecky-Levich plots (J-1 versus ω-0.5) of CNA-2 modified GCE. Fig. 9 UV-vis absorption spectra of aqeuous Congo red solutions in the presence of CNA-2 at different time intervals after diluting 5 times.
26
Fig. 10 (A) Adsorption rates of Congo red on CNA-1 (a), CNA-2 (b) and CNA-3 (c) under dark conditions. (B) Cycling runs in the adsorption of Congo red for CNA-2. The inset is the TEM image of CNA-2 after 5 cycling test.
27
Fig. 1 XRD patterns of CNA-1 (a), CNA-2 (b) and CNA-3 (c).
28
Fig. 2 SEM images of CNA-1 (a, d), CNA-2 (b, e) and CNA-3 (c, f).
29
Fig. 3 TEM images of CNA-1 (a), CNA-2 (b) and CNA-3 (c) and schematic illustration of the formation mechanism of CNAs (d).
30
Fig. 4 Nitrogen adsorption/desorption isotherms (A) and BJH pore size distributions (B) of CNAs samples.
31
Fig. 5 Magnetization hysteresis of Cu2O CNAs (A) and the magnified graph of (B).
32
Fig. 6 (A) CV curves of the CNA-3/GCE in PBS solutions containing AA (0.8 mM), DA (0.8 mM) or UA (0.8 mM). (B) DPV curves of CNA-2/GCE in PBS solutions containing different concentrations of DA: (a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.10, (f) 0.15, (g) 0.20, (h) 0.30, (i) 0.40, (j) 0.50, (k) 0.60, (l) 0.70 and (m) 0.80 mM.
33
Fig. 7 (A) DPV curves of CNAs-modified GCEs in PBS solutions containing AA (1 mM), DA (0.8 mM) and UA (1 mM). DPV curves of CNA-1/GCE (B), CNA-2/GCE (C) and CNA-3/GCE (D) in PBS solutions containing AA (1 mM), UA (1 mM) and different DA concentrations: (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, (e) 0.5, (f) 0.6, (g) 0.7 and (h) 0.8 mM.
34
Fig. 8 (A) CVs curves of CNAs modified GCRDEs obtained in N2 or O2-saturated KOH solutions. The inset is the linear sweep voltammetry (LSV) curves of NAs-GCRDEs in O2-saturated electrolytes at 1600 rpm. (B) The LSV curves of CNA-2 obtained at different rpms (300-2000) in O2-saturated 0.1 M KOH electrolytes. The inset shows the Koutecky-Levich plots (J-1 versus ω-0.5) of CNA-2 modified GCE.
35
Fig. 9 UV-vis absorption spectra of aqeuous Congo red solutions in the presence of CNA-2 at different time intervals after diluting 5 times.
36
Fig. 10 (A) Adsorption rates of Congo red on CNA-1 (a), CNA-2 (b) and CNA-3 (c) under dark conditions. (B) Cycling runs in the adsorption of Congo red for CNA-2. The inset is the TEM image of CNA-2 after 5 cycling test.
37