Evolution of a Cu2O Cube to a Hollow Truncated Octahedron and

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Evolution of Cu2O Cube to Hollow Truncated Octahedron and Their Photocatalytic and Electrocatalytic Activity Shuibo Liu, Ziqi Yang, Xuehua Liu, Ruirui Liu, Guoqiang Wang, Qianbin Wang, Hongliang Li, and Peizhi Guo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01084 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Applied Nano Materials

Evolution of Cu2O Cube to Hollow Truncated Octahedron and Their Photocatalytic and Electrocatalytic Activity Shuibo Liu‡, Ziqi Yang‡, Xuehua Liu, Ruirui Liu, Guoqiang Wang, Qianbin Wang, Hongliang Li and Peizhi Guo*

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. ‡: Equal contribution *Corresponding Author: [email protected]; [email protected]

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ABSTRACT: The structure regulation of cuprous oxide (Cu2O) has been realized from submicrometric cube to irregular hollow polyhedrons and hollow truncated octahedron. It is suggested that the evolution of Cu2O exotic morphology and hollow structure are caused by an inside–out Ostwald ripening mechanism in the synthetic system, followed by the induced formation of eight (111) and six (100) planes of hollow truncated octahedron, while Cu2O cube with six (100) planes is formed quickly via the directed attachment in a weak reduction environment. Photocatalysis of methyl orange indicated that the (111) planes of hollow truncated octahedron showed higher catalytic activities than the (100) planes of both structures. However, both the (100) and (111) planes in Cu2O cube or hollow truncated octahedron display high electrocatalytic activities towards the reduction of nitrobenzene. The glassy carbon electrode modified by Cu2O hollow truncated octahedron showed that the highest catalytic current was 401.8µA with the most negative potential at -0.82V due to the small size and hollow nature among these Cu2O particles. The formation of Cu2O cube and hollow polyhedrons and their structure-property relationship have been studied based on the experimental data.

KEYWORDS: Cuprous oxide; Hollow polyhedron; Assembly; Photocatalysis; Electrocatalysis;


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INTRODUCTION

The development of synthesis of small-sized structured materials has made a great progress during the last few decades1-3, which in turn promoted the elaboration of the relationship between the structure and the properties of the materials4,5. Accordingly, the precise synthesis of targeted materials with specific morphologies is of great importance. Spherical particles usually represent the aggregation properties due to the anisotropy of crystal growth while nanoparticles with specific facets can provide an excellent platform to understand the structure-related physicochemical properties6-9. For cuprous oxide (Cu2O), various morphologies including those with well-defined facets10-12, such as cube13, octahedron14, dodecahedron15 and truncated octahedron16, have been realized by solution phase strategies due to the vast applications in photocatalysis17-19, electrocatalysis20,21, secondary batteries22, solar energy conversion23, etc. Among the diverse structures, hollow structures24,25 including nanotubes26, hollow spheres27, hollow polyhedrons28,29, nano frames30 and nanocage structures31 are one kind of important materials, providing plenty of surface areas both inside and outside. The synthesis of hollow structures can usually be divided into two strategies, template or templateless synthesis. Template method needs the synthesis of template as a core first, and then the continuous growth of the desired shells, followed by the removal of the template32,33. Templateless approach usually displays the self-construction of hollow spaces by growth and aggregation of nanocrystallites following the principle of the lowest energy, balancing the


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thermal dynamic and kinetic control, or by the selectively etching to achieve the desired structures34-37. For example, Cu2O-CuO-TiO2 hollow nanocages have successfully prepared using Cu2O as the self-template to realize the morphology of hollow nanocages38. Hierarchical Prussian white hollow structures with kinked surfaces have been realized through a selfaggregation and etching strategy39. Lately, the chemical bonding theory has also been developed for the crystal growth including Cu2O structures40,41. Despite the recent progress, fabrication of well-defined hollow Cu2O polyhedrons as well as their forming process of exotic morphologies have rarely been demonstrated in a simple manner42,43. In this study, truncated octahedron of Cu2O with interior hollows and their structural evolution with ends to solid cube has been synthesized by varying the volume ratios of N2H4·H2O and water in the solutions containing surfactant and NaOH. The systematic characterizations of intermediates collected after different reaction time and concurrent color change have been involved to clarify the formation mechanism of Cu2O particles. The as-made Cu2O submicrometric structures can be used as catalysts of photocatalytic degradation of methyl orange and electrocatalytic reduction nitrobenzene. Hollow truncated octahedron of Cu2O showed an obviously better catalytic activity than cubes or intermediates, indicating the significance of the unique structural features and size effect of Cu2O nanostructures.


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EXPERIMENTAL SECTION

Materials and reagents

All chemicals, including sodium dodecyl sulfate (SDS), Cu(CH3COO)2·H2O, ethanol, NaOH, N2H4·H2O (50%), K3[Fe(CN)6], KCl, methyl orange, Na2SO4, KBr, NaH2PO4, p-nitrotoluene, mdinitrobenzen, K4[Fe(CN)6]·3H2O and nitrobenzene (NB) were of analytical grade (SCRC), and used directly. Synthesis of Cu2O particles

In a typical synthesis, 0.87 g SDS was dissolved in deionized water (69.0, 79.0 and 89.0 mL) in sample vials under vigorous stirring. 5.0 mL of 0.1 M Cu(CH3COO)2 solution was then added. The vials were set in a water bath at 34±1 ºC, followed by the adition of 2.0 mL of 1.0 M NaOH solution. The resulting solution turned light blue quickly. Finally, 24.0, 14.0 and 4.0 mL of 0.1 M N2H4·H2O solution were quickly injected respectively and then stirred for 30s. The final solution volume in each vial is 100 mL. After 1h, the solutions were kept in the water bath for 1 h and then centrifuged at 4500 rpm to get hollow truncated octahedron (Cu2O-HTO), hollow irregular polyhedron (Cu2O-HIP), and cubes (Cu2O-Cube), respectively. Characterization

The morphology and structure as well as crystallographic information of the Cu2O particles were examined by scanning electron microscope (SEM), transmission electron microscope


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(TEM), X-ray diffractometer (XRD) and Raman spectroscopy described in detail in our recent reports44,45. UV-visible absorption spectra were obtained according to the literature46. Electrochemical measurements

The detailed description of the electrochemical measurements was shown in our recent report45 except for the electrocatalyst (Cu2O particles) solution (1 mg/mL in water). RESULTS AND DISCUSSION

The microstructure and morphology of Cu2O polyhedrons were preliminarily studied by SEM (Figure 1 and Figure S1) and XRD (Figure S2) measurements. In fact, before the formation of Cu2O, nanorods or nanowires of Cu(OH)2 were formulated first (Figure S3). Based on the SEM images shown in Figure 1 and Figure S1, the morphology of Cu2O-Cube, Cu2O-HIP and Cu2OHTO changed greatly, which was related to the concentration of reduction reagent, N2H4·H2O, in the synthesis system. When small amount of N2H4·H2O was added to the solution, the assynthesized Cu2O-Cube showed a regular cubic shape with the size of about 600±150 nm (Figure 1A). Increasing the content of N2H4·H2O in the system led to the formation of Cu2O precipitates with irregular shape. For example, Cu2O-HIP displayed an irregular polyhedron with rough surface under a middle content of reductant, as shown in Figure 1B, containing tortuous edges with the size scales of 250±80 nm. Interestingly, with the further increase of N2H4·H2O content in the system, truncated octahedrons of Cu2O were obtained with a size distribution (400±50 nm) (Figure 1C), although the edge sizes of squares and hexagons were slightly different from each


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other. According to previous reports, Cu2O-Cube is composed of six (100) faces47. There are six hexagonal (111) faces and eight square (100) faces for Cu2O-HTO48,49.

Figure 1 SEM images of Cu2O-Cube (A), Cu2O-HIP (B) and Cu2O-HTO (C).

Figure 2 and Figure S4 show the TEM and HRTEM images of Cu2O-Cube (A), Cu2O-HIP (B) and Cu2O-HTO (C). It is clear that the sizes of Cu2O-Cube, Cu2O-HIP and Cu2O-HTO are essentially the same as those observed from the SEM measurements. It is interesting that only Cu2O-Cube has a solid structure. After adding a few reductants into the system, hollow polyhedrons appeared (Figure S5A). Only hollow structures are observed in other as-made Cu2O materials including Cu2O-HIP and Cu2O-HTO, as shown in Figure 2B, 2C and Figure S4B. As depicted in Figure 2A, square shape of Cu2O-Cube in the TEM image indicates the Cu2O cubes are located on the copper net with one of six planes contacted. Well crystalline nature of Cu2OCube can be further demonstrated by the Fast Fourier Transform (FFT) patterns (inset in Figure 2A) and the HRTEM image of the edge of one face of Cu2O-Cube, near the vertices (Figure 2D). The lattice spacing calculated is around 2.17-2.20 Å ascribed to the (200) faces of the (111) faces. These observations are in good accordance with the XRD results (Figure S2), which show


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the crystallite size is about 32.4 nm according to the strongest diffraction peak in the XRD pattern47. As for Cu2O-HIP, small hollow interiors are formed in the core of all the polyhedrons with the calculated crystallite size of around 26.5 nm. All the corresponding FFT pattern, HRTEM images as well as XRD measurements indicate the good crystalline feature for Cu2OHIP. At this stage, Cu2O-HIP does not have a regular exotic shape, the observed lattice spacing in Figure 2E is around 2.48-2.50 Å attributed to the (111) plane of cube or other Cu2O phase.

Figure 2 TEM (A-C) and HRTEM (D-F) images of Cu2O-Cube (A, D), Cu2O-HIP (B, E) and Cu2O-HTO (C, F).

Figure 2C shows the TEM image of Cu2O-HTO and large hollow interiors of each particle can be clearly observed with a relatively thin shell compared with Cu2O-HIP. Two series of lattice spacing is measured at 2.16 Å and 2.50 Å, as shown in HRTEM image of Cu2O-HTO (Figure 2F), ascribed to the (100) and (111) planes, respectively. The shape variation can also be


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observed from the intermediates during the synthesis of Cu2O-Cube and Cu2O-HTO. Cu2O Cube was formed at a very short reaction time of 5 min, while Cu2O irregular intermediates were formed, followed by the formation of a decahedron structure. From the Raman spectra (Figure S5), that the peaks at 100 cm-1 and 400 cm-1 of Cu2O-HIP are significantly different from those of Cu2O-Cube and Cu2O-HTO, probably due to the irregular morphology feature of Cu2O-HIP47. These are also consistent with XRD, SEM and TEM data, however, the measured FTIR spectra of Cu2O polyhedrons are pretty similar (Figure S6)

Scheme 1 The schematic illustration of formation processes of Cu2O polyhedrons. These observations clearly show that the increased content of N2H4·H2O in the system leads to the formation of hollow structures (Scheme 1), accompanied by the shape evolution from solid cube to hollow truncated octahedron with irregular intermediates. It is also found that Cu2OCube can be formed in a few minutes while Cu2O-HTO is gradually obtained via a time-involved formation process (Figure S7). The selection of surfactants can also affect the morphology of the


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Cu2O particles. When CTAB and other surfactants were used, a variety of Cu2O structures with different shapes were formed. It is suggested that the “directed attachment” and “inside–out Ostwald ripening” mechanisms can be acceptable for the explanation of shaped nanoparticles. Usually, inside–out Ostwald ripening means the process of coarsening and recrystallization in an almost saturated solution phase system. This process may involve the redeposition of small particles on the surface of larger crystals, which ultimately leads to a reduction in the total Gibbs energy. Therefore, the crystallites in the central parts are dissolved and relocated on the outer surfaces, leading to the appearance of an empty interior space. Directed attachment often emerges before the Ostwald ripening to form a hollow structure. As for the directed attachment process, a single crystal grain as the main subunit can be directly combined with its neighboring crystal grains to gain a mesostructure via a principle to decline interface energy. Based on the SEM (Figure 1A and S3C) and TEM (Figure 2A and 2D) images, Cu2O-Cube is suggested to be formed via a “directed attachment" process to form a well-defined geometry with the least Gibbs energy (Scheme 1). With the increase of the reductant, the voids are later created in the interior part of the shaped particles, resulting in the facet structures. Meantime, the growth rates of different planes are suggested to be determined by the chemical bonding theory40, leading to the formation of hollow truncated octahedron with the (111) and (100) facets.

The EIS technique was used to disclose the variation of electron transfer resistance with particle morphology. Figure 3 shows the EIS results of Cu2O-Cube/GCE, Cu2O-HIP/GCE and


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Cu2O-HTO/GCE in KOH solutions containing [Fe(CN)6]3−/[Fe(CN)6]4− ions (5 mM). In the high frequency region all the Cu2O modified GCEs display a semicircle related to the interfacial charge-transfer resistance between Cu2O polyhedrons and electrolyte45. Obviously, the resistances of the Cu2O polyhedrons followed the order of Cu2O-HTO/GCE < Cu2O-Cube/GCE < Cu2O-HIP/GCE. Meantime, the knee frequencies in the EIS data representing a lower limit of the high frequency are around 4.64, 6.81 and 2.15 Hz for Cu2O-Cube/GCE, Cu2O-HIP/GCE and Cu2O-HTO/GCE, respectively, denoting the best electrochemical response and the fastest charge transfer rate of Cu2O-HTO/GCE among all the modified GCEs.

Figure 3 EIS spectra of the electrodes modified by Cu2O polyhedrons. It is well accepted that the surface structure of nanomaterials can affect their properties50,51. It is reported that Cu2O nanostructures show catalytic properties related to morphology or structure52,53. In this study, photocatalytic and electrocatalytic behavior has been selected to find out the catalytic characteristics of the structural nature of different Cu2O particles. Figure 4 shows the UV-vis absorption spectra of aqueous solutions containing methyl orange before and


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after addition of Cu2O-Cube, Cu2O-HIP and Cu2O-HTO. The photocatalytic tests were done under normal illumination. The photocatalytic efficiency η is measured from the absorbance peak values according to the equation, η=

A0 − A ×100% A0

where A0 and A are the absorbance peak values without and with natural light for 5 h irradiation, respectively. It is seen that the peak position of methyl orange is almost unaltered while the peak intensities are decreased with the elongation of time during the photocatalytic experiments. Seen from Figure 4, the photocatalytic efficiency of Cu2O-Cube, Cu2O-HIP and Cu2O-HTO is calculated to be about 12%, 48% and 76%, respectively. Considering the size effect of these three samples, Cu2O-HTO exhibits the best ability to photodegrade methyl orange compared with Cu2O-Cube and Cu2O-HIP. In previous studies, Cu2O nanomaterials demonstrated weak ability to photodegrade methyl orange17-19. These data indicate that the photocatalytic activity of Cu2O nanocrystals should be mainly ascribed to the (111) faces of Cu2O polyhedrons, which shows a higher catalytic activity than the (100) faces. These results are also consistent with the higher surface energy of (111) face than (100) face, and methyl orange molecules may be apt to adsorb more easily on the surface with high surface energies17-19. Even if Cu2O-HIP does not have a regular outer shape, it shows much better photocatalytic activity than Cu2O-Cube, possibly because small irregular Cu2O polyhedron has a much higher specific surface areas than Cu2O cube.


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Figure 4 UV-vis absorption spectra of aqueous methyl orange solutions under normal illumination for Cu2O-Cube (A), Cu2O-HIP (B) and Cu2O-HTO (C) after 0, 10, 20, 45, 90, 120, 180, 240 and 300 min, (D) The variation of photocatalytic degradation of methyl orange with time for Cu2O polyhedrons and Cu2O-cube under dark condition.

Nitrobenzene is selected as a typical molecule to investigate the electrocatalytic performance of crystalline faces of Cu2O-Cube, Cu2O-HIP and Cu2O-HTO. Figure 5 compares the CV curves of three modified GCEs in aqueous PBS (pH = 7.4), Na2SO4 (0.2 M) and NaOH (0.2 M) solutions containing NB (0.4 mM). One peak at about -0.75 V to -0.82 V (referenced to saturated calomel electrode (SCE)) appears. Clearly, the catalytic peak of NB on Cu2O-HTO/GCE is sharper and shows a larger current than that of Cu2O-Cube/GCE or Cu2O-HIP/GCE. As depicted in Figure 5A, the reductive peak currents of NB in PBS solutions are 124.4, 141.8 and 114.6 µA for Cu2O-Cube/GCE, Cu2O-HTO/GCE and Cu2O-HIP/GCE at -0.75, -0.78 and -0.76 V,


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respectively. Aqueous Na2SO4 solution is used as the electrolyte, which enhances the peak currents to 259.7, 401.8 and 148.4 µA for Cu2O-Cube/GCE, Cu2O-HTO/GCE and Cu2OHIP/GCE in Figure 5B, at -0.79 V, -0.82 V and -0.77 V, respectively. However, the currents are decreased to 94.1, 112.7 and 57.9 µA at -0.78, -0.79 and -0.77 V in Figure 5C, respectively.

Figure 5 CV curves of the Cu2O modified GCEs in PBS (A), Na2SO4 (B) and NaOH (C) solutions containing NB. (D) CV curves of the electrocatalysis of NB on Cu2O-HTO/GCE in Na2SO4 solution.

It can be concluded that sample Cu2O-HTO shows the largest catalytic currents among all the used aqueous electrolytes. In the meantime, the catalytic peak position of Cu2O-HTO displays the most negative potential value in the three electrolytes. These observations indicate that both the (111) and (100) faces of Cu2O particles in Cu2O-Cube and Cu2O-HTO can catalyse the electrochemical reduction of NB. A slight higher catalytic peak current of Cu2O-HTO than


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Cu2O-Cube should be attributed to a smaller size and hollow nature of the former. Compared with the above photocatalytic results, the (100) face may be more apt to electrochemical catalysis of NB than (111) face of Cu2O nanostructured materials. Cu2O-HIP has the smallest size among the samples and the absence of catalytic active surfaces should be responsible for its worst electrocatalytic activity toward the electrocatalysis of NB. This is obviously different from the photocatalytic behavior of Cu2O-HIP. There is no peak appeared in the first scan from -0.3 to -0.6 V in Na2SO4 solution (Figure 5D), indicating no reaction occurs during this process. However, a reductive peak appears at -0.78 V in the second segment. The shape of CV curves is almost identical in the used solutions, confirming that a similar pathway for the electroreduction of NB occurs. It should be pointed out that several intermediates may be involved in this process such as aniline, phenylhydroxylamine, and azoxybenzene, depending upon the potential window (-0.3 – -0.9 V) and electrocatalysts37. According to the reported results, the catalytic peak should be related to the 4e reaction of NB to phenylhydroxylamine (Figure S8). The catalytic stability of Cu2O-HTO/GCE is not good, similar to those ferrite electrocatalysts54, but obviously lower than chemically modified carbon nanotubes37. These results suggest that the active sites on the outer surface of oxide nanostructures may be deactivated after the electrocatalytical process or the adsorption of products of electroreduction may exist on the oxide surface that inhibit the subsequent catalytic reactions.


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Figure 6A and Figure S9 show the effect of scan rate on the electroreduction of NB in Na2SO4 solutions for Cu2O-Cube/GCE, Cu2O-HIP/GCE and Cu2O-HTO/GCE. The reduction peak potential is shifted negatively for all the three modified GCEs with the catalytic currents increasing simultaneously. As shown in the insets in Figure 6B, a linear variation of the currents with the square root of scan rates is observed, rather than the function of currents with scan rate (Figure 6C), indicating the existence of a diffusion controlled process.

Figure 6 (A) CV curves of the NB electroreduction on Cu2O-HTO/GCE (A) in Na2SO4 solutions at various scan rates (mV/s): 20 (a), 40 (b), 60 (c), 80 (d), 100 (e), 120 (f) and 160 (g). The variation of current in CV curves with the square root of scan rate (B) or with scan rate (C). CV curves of Cu2O-HTO/GCE (D) and Cu2O-HIP/GCE (E) in Na2SO4 solutions containing NB. From (a) to (g): 50, 100, 150, 200, 250, 400 and 600 µM. (F) The variation of peak current in CV curves with the concentration of NB on the modified electrodes.


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P-nitrotoluene and m-dinitrobenzen are selected to distinguish the effect of additional functional groups on the electrocatalytic process. It can be seen from Figure S10 that the electroreduction of p-nitrotoluene on Cu2O-HTO/GCE is similar to NB, indicating that the reduction mechanism for both compounds is similar. Two peaks appear at −0.60 and −0.78 V for m-dinitrobenzen. The peak at −0.78 V should be related to the reduction of NB to phenylhydroxylamine. These data are similar to other catalysts45. The possible schematic electrochemical reduction mechanism is shown in Figure S11. It is proposed that the transformation of inner molecular structures of NB derivatives on the surface of Cu2OHTO/GCE should be sensitive to the nature and position of functional groups, similar to the conditions in general organic reactions. The effect of NB concentration in Na2SO4 solution is shown in Figure 6 (D, E, F) and S10. It is clear to find that the peak potentials of both Cu2O-Cube/GCE and Cu2O-HTO/GCE change slightly, while the peak position during the electrocatalysis on Cu2O-HTP/GCE is varied randomly with a larger potential shift than Cu2O-Cube/GCE or Cu2O-HTO/GCE, possibly due to the irregular morphology nature of Cu2O-HIP/GCE. The catalytic current of Cu2O-HTO/GCE reached 480µA at 600 mM (NB), much higher than that of Cu2O-Cube/GCE (294µA) or Cu2OHIP/GCE (175µA). So the electrocatalytic and photocatalytic features of Cu2O-cube, Cu2O-HTP and Cu2O-HTO are mainly caused by the particle size and their surface structure of polyhedrons. For the photocatalytic of methyl orange, the bare surface of Cu2O particles including well-defined


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crystal planes, especially for the (111) plane, and irregular particle surface can contribute to the photocatalytic ability. However, as shown in Figure 7, for electrocatalysis of NB, the (100) and (111) planes of outer surface of Cu2O particles display higher electrocatalytic activities than irregular surfaces. On the contrary to photocatalytic results, the (100) planes of Cu2O polyhedrons show a slightly better performance in electrocatalysis of NB than the (111) planes. These observations are essentially consistent with those report results55. Namely, the photocatalytic or electrocatalytic performances of Cu2O particles should be closely related to the size, surface structure and shape of the materials56.

Figure 7 The crystal models showing the (A) (100) and (B) (111) faces of Cu2O.

CONCLUSION

A series of submicrometric Cu2O particles with morphology gradually changing from cube to hollow truncated octahedron with intermediate irregular polyhedron, have been synthesized via a low-temperature solution synthesis strategy. Cu2O cubes covered by six (100) faces can be easily formed even at the early stage with a small amount of reductant while hollow truncated


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octahedron with six (100) and eight (111) faces outside is gradually formulated via continuous evolving process under a high reductant content in the synthesis system. Photocatalysis of methyl orange shows that the (111) faces in hollow truncated octahedron display a better photocatalytic activity than the (100) faces in polyhedrons or irregular surface structure and the latter two structures have a similar photocatalytic activity. Electrochemical results toward electroreduction of nitrobenzene reveal that the (111) faces in truncated octahedron exhibit a higher electrocatalytic performance than the (100) faces of Cu2O particles and both of the well-defined crystal planes show better electrochemical catalytic activity than irregular bare surface of Cu2O polyhedrons. In short, both particle size and surface nature of Cu2O particles play important roles in determining their catalytic properties. These data would help the design of shaped nanomaterials with targeted functionalities.

ASSOCIATED CONTENT

Supporting Information.

Additional supporting figures (Figure S1-S10) for the morphology of Cu2O particles as noted in the text, XRD patterns and SEM images of Cu(OH)2 precursors, XRD patterns, Raman spectra and SEM images of the Cu2O nanocrystals, schematic diagrams of electrocatalytic mechanisms as well as CV curves Cu2O nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21773133), College student innovation and entrepreneurship training program and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province.

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Table of Contents


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Evolution of a Cu2O Cube to a Hollow Truncated Octahedron and

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