Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar arrays as 3D binder-free electrode for high performance supercapacitors

Nano Energy (2016) 20, 94–107

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RAPID COMMUNICATION

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar arrays as 3D binder-free electrode for high performance supercapacitors Yuanchuan Zhenga, Zhaoqian Lia, Juan Xua, Tilong Wangb, Xun Liua, Xiaohui Duana, Yongjun Mac, Yong Zhoud,n, Chonghua Peia,nn a

State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials of Sichuan Province, Southwest University of Science and Technology, Mianyang 621010, PR China b School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China c Analytical and Testing Center, Southwest University of Science and Technology, Mianyang 621010, PR China d Eco-materials and Renewable Energy Research Center Nanjing University, 22# Hankou Road, Nanjing 210093, PR China Received 20 August 2015; received in revised form 10 November 2015; accepted 30 November 2015 Available online 15 December 2015

KEYWORDS

Abstract

Multichannel; Hierarchical porous carbon; Nanopillar arrays; Supercapacitors

3D hierarchical carbon-based nanostructures not only create hierarchical porous channels, but also possess high electrical conductivity and maintain excellent structural mechanical stability for highperformance supercapacitors. In this research, hierarchical porous N-doped carbon and Co3O4 nanopillar arrays derived from Morpho butterfly wing scales have been explored. These structures have demonstrated enhanced capacitance, with a maximum specific capacity of 978.9 F g
1 at 0.5 A g
1, and good cycling stability, retaining about 94.5% of their capacitance after 2000 cycles as well as improving supercapacitor energy density without sacrificing power density. The maximum energy density of the carbonized wing scale-cobalt oxide (CWs-Co3O4) composite supercapacitors was found to be 99.11 Wh kg
1. This paper proposes a method for building 3D hierarchical N-doped carbon materials with various morphologies. It also presents a concept and method for designing a carbonbased 3D binder-free electrode with a hierarchical structure. This research provides a vast structural

n

Corresponding author. Corresponding author. Tel./fax: +86 816 241 9280. E-mail addresses: [email protected] (Y. Zhou), [email protected] (C. Pei).

nn

http://dx.doi.org/10.1016/j.nanoen.2015.11.038 2211-2855/& 2015 Elsevier Ltd. All rights reserved.

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar

95

pool for developing novel electrochemical properties based on these materials. & 2015 Elsevier Ltd. All rights reserved.

Introduction The development of high-performance electrical energy storage systems has been an issue of common concern in both academia and industry because electrical energy is extensively used in various parts of society and provides great convenience to our work and life. Supercapacitors, which are high-performance electrical devices with higher power density than Li-ion batteries and higher energy density than conventional dielectric capacitors [1–5], have attracted particular interest from researchers, who have already obtained a wealth of research results. According to their energy storage mechanism, supercapacitors can be divided into two types: electrical double-layer capacitors (EDLCs) (e.g., carbon nanomaterials) [6–9] and pseudocapacitors (e.g., transition metal oxides and conductive polymers) [10,11]. In EDLCs, the ionized electrolyte molecules are accumulated by coulombic attraction at the electrode surface to form a Helmholtz layer and a diffuse layer. These two layers play a key role in determining the electrochemical performance of a capacitor and constitute the charge storage mechanism of EDLCs [12,13]. In pseudocapacitors, the capacitance comes mainly from reversible redox reactions with the electrolyte occurring on the surface of the electrode. In general, pseudocapacitors have inherited many of the virtues of EDLCs, and therefore their capacitance can be significantly higher than the general capacitance of EDLCs because it contains both the non-faradaic and faradaic contributions to the capacitance. Transition metal oxides (TMOs) and conductive polymers are widely used as electroactive materials for pseudocapacitor electrodes, for instance, RuO2, MnO2, V2O5, NiO, SnO2, and Co3O4, and even NiCoO2 and LiCoO2; polypyrrole, polyaniline, and polythiophene are also potential candidates for pseudocapacitors [14–19]. These series appear to be ideally suited electroactive materials for high-performance pseudocapacitors and make the pseudocapacitor a promising replacement for EDLCs. As is well known, however, with the relatively slow redox reaction kinetics and poor conductivity of electroactive materials, the series of materials given above have a relatively low electrical energy density and even worse cycling stability. As a somewhat radical solution to this problem, carbon-based metal-oxide composites for supercapacitor electrodes have been developed to achieve high energy storage and high power output simultaneously. Many carbon materials have been investigated as substrates for supercapacitor electrode materials [20–22]. Among these, carbon materials with various microtextures, such as 0D carbon nanoparticles [7], 1D nanostructures (e.g., carbon nanotubes and carbon nanofibers) [23,24], 2D nanosheets (e.g., graphene and reduced graphene oxides) [25,26], and 3D activated carbon are considered as the main candidates for supercapacitors [27]. Because these nanomaterials have an extraordinarily high specific surface area, they can be a very attractive choice for supercapacitor electrodes.

N-doped porous carbon materials are also promising electrode materials for supercapacitors [24,27]. Consistently, supercapacitor electrodes made of these materials perform better with a larger specific surface area, but a unitary approach with the aim of increasing specific surface area does not necessarily lead to improve supercapacitor performance. Most studies of carbon-based materials have focused on improving these materials to obtain higher effective surface areas, whether graphene, carbon nanotubes, activated carbon, or carbon aerogel [20–27]. However, developing a hierarchical nanostructure of 3D binder-free electrodes as an innovative strategy for improving high-performance energy storage systems is often overlooked, but is very important in practice because it can create hierarchical porous channels while maintaining or even improving ion-transport kinetics. It can also generate higher electrical conductivity and better structural mechanical stability as well as boosting the specific capacitance of carbon-based supercapacitors [28–31]. Hierarchically structured materials have opened up a new route towards realizing advanced energy materials with short iontransport distance, low resistance, and high charge storage density to achieve higher specific capacitance, better cyclic stability, higher energy density, and higher power density for supercapacitors [32–35]. Hierarchical carbon-based nanomaterials are the most advanced form of novel composite electrode materials in that they can synergistically combine the strengths of their constituent nanomaterials. Therefore, capacitive hierarchical hybrid carbon-based metal-oxide nanomaterials are promising electrode materials for nextgeneration supercapacitors. TMOs are considered as ideal electrode materials for next-generation supercapacitors, as they can provide a variety of oxidation states for efficient redox charge transfer. Among them, Co3O4 has been reported as one of the most important functional materials because of its unique application. In particular, Co3O4, with a theoretical specific capacitance as high as 3560 F g
1, has been identified as a promising material for pseudocapacitors. For example, Co3O4 nanowires were deposited onto graphene foam to form a Co3O4–graphene nanofoam electrode, which showed a specific capacitance of 765 F g
1 at 2 mV s
1 [36]. However, poor energy density and rate capacity and very poor cycling stability have limited further development and commercialization of this technology for high-performance electrical energy storage systems. To address these problems, the essential idea is to design hierarchical micro/ nano-structural Co3O4, which can greatly increase specific capacitance by combining the unique properties of their individual constituents, instead of the porous nanowall [37], nanowires [38], nanorods [39], nanosheets [40] and nanoneedles [41] have been synthesized via hydrothermal [42], thermal oxidation [43] and template methods [44,45]. Recent investigations have focused on how to enhance supercapacitor energy performance using hierarchically

96 structured porous carbon-based composite nanomaterials. Among the most investigated materials, the carbide-derived carbon derived from Morpho butterfly wings has emerged as the most competitive candidate for manufacturing hierarchically structured porous carbon materials. This is the case because butterfly wings contain photonic crystal structures [46–48], which are three-dimensional periodic structures, and a 3D binder-free electrode can be effectively constructed using a combination of macro-, meso-, and micro-pores and multiple channels. This kind of structure can certainly improve effective specific surface area, reduce production cost, and shorten iondiffusion and transport pathways. In addition, the wings can be directly carbonized to form N-doped carbon materials, supplying more active sites to increase the interaction between carbon and adsorbents. All these features can result in highperformance electrode materials. Simultaneously, nanostructured transition metal oxides are widely considered as a promising material for advanced electrodes because of their high capacity, low cost, and environmental friendliness [49,50]. This paper reports on a simple and effective strategy to design and construct hierarchical hybrid carbon-based Co3O4 nanopillar arrays as a 3D binder-free electrode highperformance supercapacitor. These novel hierarchically periodic structural electrode materials have been prepared using Morpho butterfly wing scales as bio-templates. This kind of structure can certainly improve the energy density and longterm cycling stability of a pseudocapacitor and dope nitrogen into the carbon skeleton with no additional nitrogen source. The unique architecture of the electrode materials, which offer high specific surface area and possess abundant nitrogen functional groups by using wing scales as the carbon source, have contributed to enhancing the deposition of Co3O4 and providing large numbers of active sites in the reversible electrode reaction. Moreover, the interconnected micro-, mesoand macroporous structural features facilitate rapid diffusion of electrolyte ions. Furthermore, the numerous ion channels shorten the ion-diffusion and transport distance and even improve ion-transport kinetics. As a result, these novel hierarchically periodic structured electrode materials have been endowed with high capacitance (978.9 F g
1 at 0.5 A g
1) as well as outstanding long-term cycling stability, retaining about 94.5% of their capacitance after 2000 cycles.

Experimental section Preparation of carbonized wing scales (CWs) The natural Morpho butterfly wing scales were added to 5 wt% HCl solution and 10 wt% NaOH solution successively for dipping treatment. Then the wing scales were extensively washed with ultrapure water and ethyl alcohol and dried at 45 1C overnight under vacuum. Finally, the obtained wing scales were pyrolyzed under flowing Ar atmosphere at 950 1C for 4.0 h to generate the N-doped CWs.

Y. Zheng et al. 0.5 h. After this, the reaction mixture from the first step was transferred to a 40 mL Teflon autoclave for hydrothermal reaction at 180 1C for 20 h. The resulting product was collected by centrifugation, washed with ethyl alcohol and water successively, and then freeze-dried in a bulk tray dryer.

Preparation of carbonized wing scale-cobalt oxide composite (CWs-Co3O4) Typically, the N-doped CWs were collected and dispersed in absolute ethyl alcohol. The concentration of the final CWsabsolute ethyl alcohol suspension was 0.33 mg mL
1 (concentration of the CWs stock suspension was determined by measuring the mass of CWs lyophilized from a certain volume of suspension). Similarly, as the first step in the preparation of the composite, 1.2 mL of 0.4 M Co(Ac)2 aqueous solution was added to 24 mL of CWs-absolute ethyl alcohol suspension, followed by the addition of 1.2 mL of water at room temperature. The reaction was kept at 80 1C with constant stirring for 0.5 h. After this, the reaction mixture from the first step was transferred to a 40 mL Teflon autoclave for hydrothermal reaction at 180 1C for 20 h. The resulting product was collected by centrifugation, washed with ethyl alcohol and water successively, and then freezedried in a bulk tray dryer. Approximately 20 mg of CWsCo3O4 composite was obtained after lyophilization.

Characterization Scanning electron microscopy (SEM) was performed with a field emission scanning electron microanalyzer (Carl Zeiss Ultra 55). Transmission electron microscope (TEM) images and High-resolution transmission electron microscope (HRTEM) were performed on a Libra200FE (Carl Zeiss) transmission electron microscope with CCD imaging system. Scanning TEM images (STEM) and elemental mapping were obtained on an Oxford IETEM100 energy dispersive spectrometer of the Libra200FE transmission electron microscope. X-ray diffraction (XRD) data were characterized on a PANalytical X’Pert Pro X-ray diffractometer equipped. Raman scattering spectra were obtained on a Renishaw System inVia spectrometer. FTIR spectra were characterized on Spectrum One infrared spectrometer. Thermal analysis data was performed with TA SDT Q600 simultaneous thermal analyzer. X-ray photoelectron spectra (XPS) were determined on an X-ray photoelectron spectrometer (Thermo SCIENTIFIC ESCALAB 250) with an excitation source of Al Kα radiation (1486.8 eV). N2 sorption analysis was recorded from a JW-BK112 specific surface area analyzers, at 77 K using Barrett–Emmett–Teller (BET) calculations for the surface area. The pore size distribution plot was conducted on the adsorption branch of the isotherm based on Barrett– Joyner–Halenda (BJH) model.

Synthesis of free Co3O4 nanoparticles.

Electrochemical measurement

In the first step, 1.2 ml of 0.4 M Co(Ac)2 aqueous solution was added to 24 mL of absolute ethyl alcohol, followed by the addition of 1.2 mL of ultrapure water at room temperature. The reaction was kept at 80 1C with constant stirring for

Synthesis of CWs-Co3O4 composite as a 3D binder-free electrode: the stainless-steel wire mesh was first cleaned using 0.05 M diluted HCl aqueous solution for 10 min, then washed with acetone, ethyl alcohol, and water successively,

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar and finally dried. Then CWs-Co3O4 composite were dispersed in ethanol, and the working electrodes were made by a heat roll pressing machine which deposited the as-prepared CWsCo3O4 composite onto the preprocessed stainless-steel wire mesh by applying suitable compressive force. The mass of active materials in each electrode was about 5 mg. To ensure that the electrode materials were thoroughly wetted with the 6.0 M KOH electrolyte solution, the working electrode was vacuum-impregnated with the electrolyte for over 2 h before the electrochemical tests. All electrochemical measurements were carried out in a threeelectrode system with a Pt counter-electrode and a standard mercuric oxide reference electrode on a CHI 760C electrochemical workstation, with 6.0 M KOH aqueous solution as the electrolyte. The specific capacitance (C) can be calculated from the galvanostatic charge–discharge (GCD) curve using Eq. (1): C ¼ IΔt=V;

ð1Þ
1

where I is the response current density (A g ), V is the voltage range of one sweep segment (V), and Δt is the time required for a sweep segment (s). The energy density (E) and power density (P) were calculated according to the following equations: E ¼ 1=2CV 2 ;

ð2Þ

P ¼ E=Δt

ð3Þ

where V is the voltage range of one sweep segment (V) and Δt is the time required for a sweep segment (s).

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Results and discussion The morphologies of natural Morpho butterfly wings were first characterized from the macro- to the nanoscale. As shown in Figure 1a. The beautiful blue colors exhibited by the butterfly wings can be observed. Previous research revealed that the blue color of the wings results from periodic submicrometer structures [46–48]. To provide a better view of the fundamental structures, a large-scale structure was built that mimics the framework of the wing scales, as shown in the overview image of part of a photograph of a Morpho butterfly wing (Figure 1a). In the schematic illustration of the wing scales, it is clear that the periodic submicrometer structures mentioned above are arrays of vertically aligned net-like skeleton structures. These wing scales were removed from the wing surface and investigated by scanning electron microscopy (SEM). An image of an original Morpho butterfly wing, shown in Figure 1b, shows that the scales on the wing are well laid out, highly ordered in some areas, and arranged very close to each other with a slight gap in between. The typical dimensions of a scale are 300 μm in length and 80 μm in width. A higher-magnification view of the scales is shown in Figure 1c. The scales consist of many ridge lamellae located 150 nm from each other and concatenated by abundant cross-ribs in the vertical direction, which dictate the interlamellar spacing. With the cross-ribs well sprung out from the ridges, dividing them into multitudinous consecutive rectangles, the most common specific dimensions of this

Figure 1 Characterization of natural Morpho butterfly wings from the macro- to nanoscale. (a) Photograph of a Morpho butterfly and schematic illustration of the wing scales. (b) SEM image of original Morpho butterfly wings. (c) High magnification SEM images of the natural wing scales. (d) SEM image of cross-section of the natural wing scales.

98 shape are 150 nm wide and 60 nm high. Through observation, it was determined that the wing scales consist of regular multilayer stacks with alternating chitin and chitin-air layers, as shown in the SEM image of a crosssection of natural wing scales in Figure 1d. The wing scales provide an abundant selection of micro- to nano-structures that can be used as templates for fabricating 3D hierarchically periodic structured carbon nanomaterials (HPCs) as electrode materials for advanced supercapacitor devices. Because of their 3D hierarchically periodic structure, HPCs not only offer hierarchical porous channels, but also provide higher electrical conductivity and better structural mechanical stability than other electrode materials. The wing scales have been chosen as templates for fabricating 3D HPCs because of their structure and properties. Hierarchical carbon structures with multi-scale organization have been successfully fabricated covering the full range of structure sizes from nano- to micro- to macro-features. The resulting carbonized wing scales (CWs), which closely resemble the structure of natural Morpho butterfly wing scales, have been characterized by SEM. Like natural Morpho butterfly wings, the CWs are composed of overlapping scales arranged in a number of layers above the wing membrane, much like roof tiles (Figure 2a). Each overlapping scale is quasi-rectangular, its tip is serrated, and its dimensions are 300 μm in length and 80 μm in width. There are two types of structures at different scales: “type I”, the basal scale, and “type II”, the cover scale. The distinctions between the two types of scale structures are shown in Figure 2b and c. The type-I structure (Figure 2b), which forms the bottom layer, reveals that the dorsal

Y. Zheng et al. surface of the CWs consists of a network of parallel longitudinal ridges concatenated together by abundant crossribs, which form rectangular windows into the interior of the CWs. Figure 2c, representing the scale surface and showing the structure of the lamellae, reveals the typical dimensions of a type-II scale; the carbonized ridge lamellae are located 150 nm from each other and have serrated edges. Between one ridge and the next, in either the closer or the farther space, each ridge extends to both ends, and the two ridges are parallel with each other, creating periodic nanostructures on the scales. Furthermore, the CWs consist of regular multilayer stacks which form alternating layers of carbonized cuticles and air, as shown in Figure 2d, which represents a cross-section of the CWs. Because of the nature of the type-I and type-II structures, the parallel window structures run the full length of the ridges, and the serrated parallel edge ridges form arrays of vertically aligned net-like skeleton structures incorporating meso- and macro-pores. This structure is expected to facilitate electrolyte penetration when used as an electrode material, and the 3D hierarchically periodic structures make it a unique substrate for constructing 3D supercapacitor electrodes. Further characterization of the CWs was carried out by transmission electron microscopy (TEM). Because the CWs possess high strength and flexibility, they can be transferred to and manipulated on TEM grids without destroying the fine structures. Figure 3a shows a low-magnification TEM image of a CWs. The results show that the CWs structure is composed of many parallel transparent ridge structures, with a range of supporting cross-ribs between each pair. Details of the ridge

Figure 2 SEM image of the CWs, showing two different types of wing scale (type-I, type-II) on the surface. (a) Low-magnification image of the CWs. (b) High magnification SEM images of type-I wing scales. (c) High magnification SEM images of type-II wing scales. (d) SEM image of cross-section of CWs.

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar

99

Figure 3 TEM characterizations of the CWs. (a) Low magnification image of the CWs corresponding to the typical structure of natural Morpho butterfly wing scales. (b) High magnification TEM image showing the hollow periodic tubular structure. (c) HRTEM image of the CWs. (d) Select area electron diffraction (SAED) pattern of the CWs.

structures on the scales are shown in the closer view in Figure 3b, which shows that each ridge has a hollow body, forming a channel which is interconnected through the crossribs and thus generating a complex multidirectional tubular structure. These channels are parallel with each other and run through the wing scales. As indicated by the white dashed box, the channel is  400 nm wide, and the thickness of the channel wall is  30 nm. A high-resolution TEM (HRTEM) image of the CWs (Figure 3c) reveals that the channel walls and cross-ribs include a large number of micro-pores, most of them might be from the carbonization process, which increased the micropores and decreased the mesopores because the partial pyrolysis of the wings give rise to micropores while the mesopores to some extent collpase due to the shrinkage during the carbonization at high temperature. This configuration would enable further electrolyte infiltration, which is promising for enhancing the double-layer capacitance of an electrode. The crystal structure of the CWs was characterized by selected-area electron

diffraction (SAED), and their amorphous structure was confirmed by the halos, as shown in Figure 3d. The morphologies of the carbonized wing scale-cobalt oxide (CWs-Co3O4) composite were first characterized using SEM (Figure 4), which revealed the two types of morphology (type I and type II). Both type-I (Figure 4a) and type-II CWsCo3O4 structures (Figure 4b and c) maintain the overall scale shape, the parallel ridge lamellae, and the cross-ribs of the CWs. An overview image of part of a type-I structure (Figure 4a) presents a large number of relatively uniform, closely packed Co3O4 nanoparticles with a diameter of 20 nm, which attach a coated layer to the CWs carrier in a close arrangement, thereby forming a coated overlay composed of nanoparticles. For the type-I CWs-Co3O4 composite structure, the morphology is similar to the type-I structure of CWs, with parallel window structures running along the ridges. The window structures are  100 nm in length and  20 nm in width, but instead of having a quasirectangular shape, some window structures are restricted

100

Figure 4 SEM image of the CWs-Co3O4, showing two different types of morphology (type-I, type-II). (a) High magnification image of the type-I CWs-Co3O4. (b,c) High magnification image of the type-II CWs-Co3O4.

and distorted in a cramped space and have become triangular in certain regions. As the number of Co3O4 nanoparticles increased in certain regions of the coated layer, the nanopillar constituents of the nanoparticles interwove together in all directions. The type-II CWsCo3O4 composite structure is shown in Figure 4b (side view) and Figure 4c (top view), the diameter of each Co3O4 nanopillar is 20 nm, and its height is 150 nm. For the CWs-Co3O4 composite, the arrays of vertically aligned netlike skeleton CWs structures are fully and uniformly covered by Co3O4 nanoparticles and Co3O4 nanopillar arrays. Significantly for use of the CWs-Co3O4 composite as a 3D electrode architecture, it possesses a 3D hierarchically periodic structure consisting of carbon nanomaterial and

Y. Zheng et al. transitional metal oxide, potentially providing considerably enhanced capacitance compared to that of conventional carbon-based supercapacitor electrode materials. Figure 5 provides further insight into the morphologies and microstructures of the CWs-Co3O4 composite and the pure Co3O4. As can be seen in Figure 5a, the Co3O4 nanoparticles and the Co3O4 nanopillar arrays are well incorporated within the carbon skeleton. One part of the Co3O4 nanoparticles is intercalated into the interstices of the parallel window structures, and another part is deposited onto the surface of the ridges in an orderly stack of parallel tubes and mutually interlocking Co3O4 nanopillar arrays (the dark rows, perpendicular to the projection screen), forming a net-like appearance. The CWs-Co3O4 composite retains the overall scale morphology of the CWs, and the Co3O4 nanoparticles, the Co3O4 nanopillar arrays, and the carbon skeleton differentiate from and interact with each other, forming an inseparable whole. Figure 5c indicates that the constitutional unit of the CWsCo3O4 composite is polycrystalline and that the diffraction rings are indexed to the (200), (220), (311), (511), and (400) planes. Lattice fringes with interplanar distance d(311) = 0.244 nm are obviously discernible in Figure 5b. Moreover, the results show an obvious boundary between the crystalline region (the Co3O4) and the amorphous region (the carbon skeleton). A low-magnification image of the pure Co3O4 nanoparticles (prepared by a control) is shown in Figure 5d. The results indicate that the dispersion of the nanoparticles is good, the crystal size is 25 nm, and the nanoparticles are well-proportioned and without agglomeration. The distribution of elements in the CWs-Co3O4 composite was also characterized by scanning transmission electron microscopy (STEM) and X-ray elemental mapping images (Figure 5e). These images reveal that carbon, nitrogen, oxygen, and cobalt are uniformly distributed in the CWs-Co3O4 composite. The phase and structure of the CWs, the CWs-Co3O4 composite, and the pure Co3O4 were verified by powder Xray diffraction (XRD) spectra and the JCPDS Card no. 00-421467 reference pattern, as shown in Figure S1a. The diffraction peaks of the CWs-Co3O4 composite and the pure Co3O4 can be well indexed with cubic-phase Co3O4 (JCPDS Card no. 00-42-1467); 2θ at 19.01, 31.31, 36.81, 44.81, 59.41, and 65.21 can be assigned to the (111), (220), (311), (400), (511), and (440) diffraction planes. However, two broad low-intensity peaks from 2θ at 101 to 301, which are observed in the CWs pattern, belong to the amorphous phase of carbon. The diffraction peaks locate 2θ at 15.01 and 24.51, corresponding to the characteristic peak of graphite oxide and the (002) plane of graphite respectively, which could be attributed to formation of graphite layers in the CWs. The XRD spectral results show that thermal and electrical conductivity were improved during carbonation of the Morpho butterfly wing scales because the proportion of graphite is the main factor affecting thermal and electrical properties. The FTIR spectra of the CWs and the CWs-Co3O4 composite are presented in Figure S1b. In the CWs curve, a broad absorption band at 3434 cm
1 is present because of the v(N– H) and v(O–H) bonds. The absorption bands at 1574, 1405, and 1227 cm
1 are assigned to the absorption peaks which overlap v(C= C), v(C= N), and v(O–H) bond stretching

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar

101

Figure 5 TEM characterizations of the CWs-Co3O4 and the pure Co3O4. (a) Low magnification image of the CWs-Co3O4 shows two different types of morphology. (b) HRTEM image of the CWs-Co3O4. (c) SAED pattern of the CWs-Co3O4, calculation of the plane distance demonstrated that the Cobalt oxide is the Cobalt oxide (II, III). (d) Low magnification image of the pure Co3O4 nanoparticles. (e) STEM image of the CWs-Co3O4 with corresponding elemental mapping images of C (Carbon), N (Nitrogen), O (Oxygen), and Co (Cobalt).

102 vibrations respectively. The peaks at 1179 and 1083 cm
1 could represent the vibrational motion characteristic of the v(C–O) bond. However, a broad band at 3432 cm
1 in the CWs-Co3O4 composite curve is assigned to the v(N–H) and v (O–H) bonds. In addition, the absorption peaks at 1631, 1421, and 1090 cm
1 are assigned to the absorption peaks which overlap the v(O–C = O) and v(C =N) bond stretching vibrations as well as the v(C–O) bond stretching vibration. In the low-frequency region, there are two strong absorption peaks at 667 and 573 cm
1, which are assigned to the stretching vibration of Co3O4. From the FTIR spectral results, it can be concluded that the CWs skeleton has connected with and coordinated the metal ions with azomethine nitrogen, hydroxyl oxygen, and carboxylic oxygen atoms in the composite. The information in the framework and constitution of the CWs and the CWs-Co3O4 composite was characterized using Raman shift and TG-DSC analysis. Figure S1c shows Raman spectra obtained from the CWs and the CWs-Co3O4 composite. The CWs spectrum exhibits bands characteristic of the C sp2 atom, which are located at 1600 cm
1 (the G band). Moreover, the D band, located at around 1315 cm
1, can be ascribed to defects like edges and disordered carbon atoms, which describe the atomic state of the C sp3 atom. For the CWs-Co3O4 composite, the G-band and D-band centers appear at 1585 and 1316 cm
1 respectively. The intensity ratio of the D and G bands (ID/IG) is 0.83, which is greater than the ID/IG value of the CWs. However, further evidence indicates that the CWs accumulated various defects during the fabrication process. Metal oxide can be seen to have bonded onto the CWs skeleton, increasing the disorder density of the carbon materials. Therefore, clearly defined peaks were observed at 190, 480, 617, and 688 cm
1, which can be assigned respectively to the T2g, Eg, T2g, and A1g active modes of Co3O4 [49,50,53,54]. Figure S1d shows the TG-DSC curve of the CWs-Co3O4 composite from room temperature to 800 1C. Thermal decomposition of the composite occurs in a two-step process. The weight loss before 105 1C can be attributed mainly to water loss, whereas the second weight loss between approximately 105 1C and 630 1C can be attributed to oxidation of the carbon skeleton; meanwhile, the DSC curve shows a strong exothermal peak at 374 1C. The remnants after 630 1C should be attributed to Co3O4. The thermal analysis results show that the carbon skeleton in the composite is about 28.5 wt% and that the weight percentage of Co3O4 is about 67.0 wt%. The presence and chemical state of the elements in the CWs were identified by X-ray photoelectron spectroscopy (XPS). As shown in Figure S2d, the survey spectrum of the CWs confirms the coexistence of carbon, nitrogen, and oxygen; the sharp peaks at 285.2, 402.1, and 532.2 eV correspond to the characteristic peaks of C1s, N1s, and O1s respectively. As for the core-leveled high-resolution spectrum of C1s (Figure S2a), a distinct peak with binding energy at 284.8 eV can be ascribed to the C =C and C = N bonds, the peak with binding energy at 285.6 eV can be ascribed to the C–N and C–O bonds, and the peak at 286.5 eV is assigned to the C= O bond. The core-leveled spectrum of N1s shown in Figure S2b shows two peaks with binding energy at 398.3 eV (N–C) and 401.6 eV (N = C) in the curve. The O1s core-leveled spectrum of the CWs (Figure S2c) suggests that oxygen exists in the CWs because the feature

Y. Zheng et al. peaks of O–H (531.6 eV), O–C (532.6 eV), and O= C (533.6 eV) are present [51,52]. The results reveal that carbon, nitrogen, and oxygen exist in the CWs; however, the doped nitrogen elements were derived from the protein and chitin of the natural Morpho butterfly wing scales and the carbonation process with no additional nitrogen source. XPS was used to confirm the formation of Co3O4 in the CWs-Co3O4 composite, with the results shown in Figure S3. Figure S3a shows that the C1s core-leveled spectrum of the CWs-Co3O4 composite is similar to that of the CWs, with the exception of the peak located at 288.7 eV, which is assigned to the O–C = O bond. The N1s core-leveled spectrum of the CWs-Co3O4 composite is shown in Figure S3b and demonstrates that nitrogen has two different chemical states in the composite: the peak with binding energy located at 398.9 eV is assigned to the N–C bond, whereas the 400.7 eV peak can be assigned to N-Co and N= C. The O1s core-leveled spectrum of the CWs-Co3O4 composite (Figure S3c) shows a strong peak at 523.0 eV, can be assigned to the Co3O4. The two weak peaks at 531.5 and 532.8 eV can be assigned to O–H (531.5 eV), O–C (532.8 eV), and O= C (532.8 eV). In the survey spectrum of the CWsCo3O4 composite (Figure S3d), the sharp peaks with binding energy at about 285.1, 398.2, 532.2, 780.2, and 795.5 eV correspond to the feature peaks of C1s, N1s, O1s, Co2p3/2, and Co2p1/2 respectively, which reveal the presence of carbon, nitrogen, oxygen, and cobalt in the composite. The Co2p photoelectrons have also been investigated, as shown in Figure S3e. The peaks located at 779.9, 786.2, and 795.2 eV can be associated with Co2O3; in addition, the peaks at 789.2 and 797.2 eV can be assigned to CoO; and in particular, the distinct peak with binding energy at 781.8 eV can be ascribed to N-Co. The high-resolution XPS spectrum of Co2p is shown in Figure S3f; in the Co2p1/2 and Co2p3/2 spectra, two peaks at 795.6 and 780.5 eV can be observed, which are assignable to the Co2p/2 and Co2p3/2 spin–orbit peaks of Co3O4. Moreover, there are two shakeup satellite peaks located at about 10 eV above the main peak, which confirms that cobalt exists in the form of Co3O4 [53,54]. The doped N and O atoms can coordinate with Co2 + , which is favorable for uniform formation of Co3O4 nanoparticles and nanopillar arrays because the doped N and O atoms are not only the ligand, but also the anchoring sites for Co3O4. More remarkably, the doped N can provide more active sites, which are conducive to interaction between the carbon-based electrode and the electrolyte. This property shows promise for enhancing supercapacitor power density and cycling stability, because doping of the N atom in the carbon skeleton could introduce donor states near the Fermi level to generate n-type conductive materials by changing the cell parameters and increasing dislocation, which expands the tunnels for ion diffusion and also enhances electrical conductivity. In addition, the XPS results show that the CWs have been doped with 4.19 at% of nitrogen and that the CWs-Co3O4 composite has been doped with 3.39 at% of nitrogen, which will enhance electrochemical activity, lower charge-transfer resistance, and improve conductivity [55–57]. Figure S4 shows the nitrogen adsorption–desorption isotherms and pore-size distribution curves for the CWs and the CWs-Co3O4 composite. The CWs isotherm is of type II, and the isotherm of the CWs-Co3O4 composite with an H1 hysteresis

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar loop is of type IV, which has the characteristics of a drastic increase in adsorption capacity caused by capillary condensation after multilayer adsorption. It can therefore be concluded that the samples include an ideal pore shape, a rigid structure, and an oversimplified model, for instance, capillary condensation and micropore filling. Moreover, the results reveal the presence of micropores (less than 2 nm), mesopores (between 2 and 50 nm), and macropores (larger than 50 nm). In addition, in CWs with a large number of mesopores mainly distributed from 2.0 to 13.0 nm, the volume ratio of micropores to all pores is 15.2%, and the volume ratio of macropores to all pores is 24.8%. The pore-size distribution curve of the CWs-Co3O4 composite is similar to that of the CWs and is typical of mesoporous materials; the mesopores are distributed from 2.0 to 25.0 nm. The proportion of micropores is 8.0%, and the proportion of macropores is 20.2%. As 3D hierarchically periodic structured carbon nanomaterials, CWs can provide an abundant selection of micro- to nano-structures, which contribute to the pore hierarchy. Simultaneously, the Brunauer–Emmett–Teller (BET) surface area was characterized by nitrogen adsorption–desorption, with the results shown in Table S1. The results show that the specific surface areas are 143.9 and 263.4 m2 g
1 for the CWs and the CWsCo3O4 composite respectively and reveal that the carbon skeleton has incorporated the Co3O4, increasing the specific surface area of the composite.

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The potential for using the CWs-Co3O4 composite as an electrode material for supercapacitors was tested by cyclic voltammogram (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). In the following step, the capacitive performance of the CWs, the CWs-Co3O4 composite, and the prepared pure Co3O4 was evaluated using a three-electrode system with a Pt electrode and a standard mercuric oxide electrode used as the counterelectrode and the reference electrode respectively in 6.0 M KOH aqueous electrolyte. Figure 6a shows the CV curves of the CWs electrode within the potential range from 0 to 0.7 V at a scan rate from 10 to 100 mV s
1. Obviously, these curves are similar to quasi-rectangles and show near-mirror-image symmetry. In addition, the shape of the CV curves was not significantly influenced by variations in scan rate, and the area surrounded by the CV curves gradually increased, which suggest good charge–discharge properties and rate capability of the carbon material electrode. This material can be used to develop a composite electrode containing both carbon and transition metal oxide as the supercapacitor electrode. Figure 6b shows the CV curve of the CWs electrode during continuous growth by potential cycling in 6.0 M KOH aqueous electrolyte at a scan rate of 50 mV s
1 with 200 cycles, which indicates that the electrode possesses excellent stability. These results indicate that the CWs electrode possesses ideal capacitive behavior, excellent reversibility, and good charge

Figure 6 Electrochemical properties of the materials measured using a three-electrode system in 6.0 M KOH aqueous electrolyte. (a) Cyclic voltammograms of the CWs. (b) Cyclic voltammograms a CWs during continuous growth by cycling potential scan of 200 iterations. (c) Cyclic voltammograms of the CWs-Co3O4. (d) Cyclic voltammograms of the pure Co3O4.

104 propagation at the electrode surface following the electric double-layer charging mechanism, showing a specific capacitance of 370.0 F g
1 at 5 mV s
1. All these properties can be attributed to the 3D hierarchically periodic structure and the hierarchically micro-, meso-, and macro-porous structure of CWs, which provide a high specific surface area, a large number of active sites, and fast ion-diffusion and transport channels. The rate capabilities of the CWs-Co3O4 composite and of the pure Co3O4 were also investigated by measuring the CV curve at different scan rates. Figure 6c shows the curve for the CWs-Co3O4 composite. Clearly, the CV curves of the CWs-Co3O4 composite reveal a good linear relationship between the redox currents and the square root of the scan rates, as well as a pair of redox peaks. As the scan rate speeds up, the peak shapes remain the same. Similar results were obtained for the CV curves of pure Co3O4 (Figure 6d); however, a better-defined redox peak than for the CWs-Co3O4 composite appears at the same scan rate because the interaction of the carbon skeleton and the Co3O4 in the peak representing the composite becomes smoother. For the CWs-Co3O4 composite and for pure Co3O4, the CV curves show a pair of redox peaks, which can be associated with the charge storage reaction as follows: Co3 O4 þOH
þ H2 O ¼ 3CoOOH þ e


Y. Zheng et al. CoOOH þOH
¼ CoO2 þ H2 Oþ e
Comparing the CV curves of CWs-Co3O4 composite and of pure Co3O4, it is obvious that the area under the CV curve of the CWsCo3O4 composite is much larger than for pure Co3O4 at the same scan rate. Similar results were obtained for the CWs-Co3O4 composite and the CWs. The current peak intensity and the area enclosed by the CV curve of the CWs-Co3O4 composite are much greater than those of the pure Co3O4 or the CWs because of the increased surface area and the hybrid structural effect arising from the 3D hierarchically periodic structure of the carbon skeleton and the Co3O4 nanopillar arrays. These results show that the CWs-Co3O4 composite has higher capacitance than the pure Co3O4 and the CWs, as shown in Figure S5. Figure 7a–c shows the galvanostatic discharge curves of the CWs, the CWs-Co3O4 composite, and the pure Co3O4 electrode respectively. The discharge curves of the CWs tested at current densities of 0.5–5.0 A g
1 are shown in Figure 7a. Clearly, the curves have good symmetry and fairly linear slopes (Figure S6). The specific capacitance is calculated as C = (I  Δt) / (m  ΔV), where I is the discharge current, Δt is the discharge time, m is the mass of the CWs, and ΔV is the voltage drop upon discharging. The specific capacitance values calculated from the discharge

Figure 7 Capacitive performances of the supercapacitor. (a) Galvanostatic discharge curves of the CWs in 6 M KOH solution. (b) Galvanostatic discharge curves of the CWs-Co3O4 in 6 M KOH solution. (c) Galvanostatic discharge curves of the pure Co3O4 in 6 M KOH solution. (d) Nyquist plots of CWs, CWs-Co3O4 and pure Co3O4 electrodes.

Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar curves are 237.4 and 184.4 F g
1 at current densities of 0.5 and 1 A g
1 respectively. For instance, average values for reduced graphene oxide are in the range of 300– 1000 m2 g
1, resulting in a lower practical gravimetric capacitance, 100–270 F g–1 and 70–120 F g–1 with aqueous and organic electrolytes respectively [58]. Moreover, the CWs electrode possesses excellent cyclic stability, as shown in Figure S8b; the CWs keep 97.6% of their initial capacitance without significant decrease after 2000 cycles, indicating that CWs have outstanding electrochemical activity, which is in agreement with the CV results. The relationship between specific capacitance and current density is shown in Figure S8a. The CWs-Co3O4 composite electrode possesses higher specific capacitance than the pure Co3O4 electrode throughout the entire current density range. The specific capacitance values calculated from the discharge curves of the CWs-Co3O4 composite are 978.9, 940.9, 812.0, 628.9, and 444.4 F g
1 at current densities of 0.5, 1, 2, 5, and 10 A g
1 respectively. Even at 15 A g
1, the specific capacitance is still 303.3 F g
1. Moreover, the specific capacitance of the CWs-Co3O4 composite is also much higher than that of the other supercapacitors (Table S2). The results demonstrate that the CWs-Co3O4 composite has been endowed with excellent electrochemical capacitance. Moreover, there are voltage plateaus at about 0.35 V, which are consistent with the CV curves. The discharge curves are approximately symmetric with their corresponding charge counterparts in the whole potential region, indicating a good electrochemical capacitive characteristic and a superior reversible redox reaction (Figure S7). On the other hand, the pure Co3O4 electrode exhibits much lower capacitance values of 430.9, 394.7, 405, 354.4, 277.78, and 220.0 F g
1 at 0.5, 1, 2, 5, and 10, and 15 A g
1 respectively. Furthermore, the long cycling performance for the GCD of the CWs-Co3O4 composite and the pure Co3O4 supercapacitors at a current density of 2.0 A g
1 is illustrated in Figure S8b. In the case of the CWs-Co3O4 composite, the electrode still retains about 94.5% of its original capacitance even after 2000 cycles. In contrast, although pure Co3O4 shows higher specific capacitance stability with increasing current density (Figure S8a), the cyclic property is worse, and it only retains about 67.1% of its original capacitance after 2000 cycles. The maximum energy density (99.11 Wh kg
1) of the CWs-Co3O4 composite is much higher than for the CWs and the pure Co3O4 supercapacitor. EIS was carried out to understand the mechanism behind the performance of the supercapacitor electrode materials. Nyquist plots of the CWs, the CWs-Co3O4 composite, and the pure Co3O4 electrode are shown in Figure 7d. As is well known, the Nyquist curve can be differentiated into two parts. One part is the semicircle located in the high-frequency region; the intersection of the real axis and the curve is the electrolyte resistance (Rs), which is made up of the ohmic resistance of the electrode and the solution resistance. In the lowfrequency region of the semicircle, the intercept of the real axis and the semicircle represents the contact resistance. The other part of the Nyquist curve, in the low-frequency region, is the Warburg impedance, represented by a straight line inclined at about 451 to the real axis. Comparison and analysis of the Nyquist plots of the electrode materials reveals that the CWs electrode has minimum electrolyte resistance, whereas that of the pure Co3O4 electrode is the maximum. Conversely, the contact resistance of the pure Co3O4 electrode is less than that of the two other electrode materials. Moreover, among

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the electrode materials, the CWs-Co3O4 composite has the highest diffusion coefficient, which provides more capacitance. These results provide important clues that pure Co3O4 possesses higher specific capacitance stability with increasing current density, whereas the CWs-Co3O4 composite is characterized by a more stable cyclic property and an optimal specific capacitance. The equivalent circuit used to fit the impedance spectra of the CWs-Co3O4 composite is shown in the inset of Figure 7d, where Rs refers to the electrolyte resistance; Rct and CPE1 are the double-layer charge-transfer resistance and capacitance; Zw is the Warburg impedance, which relates to electrolyte and proton diffusion in active materials; and CPE2 is the capacitance of the transitional metal-oxide electrodes. The improved capacitances can be attributed to rational incorporation of the multichannel carbon skeleton and the Co3O4 nanopillar arrays into the 3D hierarchically periodic structure, which readily allows iondiffusion and transport across the interface between the carbon skeleton and the Co3O4 nanopillar arrays, resulting in superior electrochemical activity and stability. This outstanding electrochemical performance might be attributed to a synergistic effect between the 3D hierarchically periodic structure of the CWs and the Co3O4 nanopillar arrays. This effect ensures that the electrode material possesses abundant nitrogen functional groups and high specific surface area, which provide the ions with a more flexible response to the active sites. It might also be attributed to the interconnected micro-, meso-, and macro-porous structure, which can improve electrochemical performance. As a potential electrolyte storage medium, a macroporous structure can shorten iondiffusion and transport distance. A mesopore structure is an important component in ion transport because it can provide especially rapid transport channels. Moreover, a microporous structure with a high electrostatic adsorption capacity can make the electrode materials more electroactive and ensure a higher electrochemical energy storage capability. The hierarchically porous structure can also effectively increase the electrochemical active surface area and the number of electrochemically active sites, significantly reducing the potential for polarization caused by a large current, which will improve supercapacitor electrochemical performance by exploiting synergy among the features [59–61]. In addition, the channels, which are interconnected through the cross-ribs, generate complex multidirectional tubular structures. The channels shorten the ion-diffusion and transport distance by more than half from the outer surface to the core of a ridge, forming hierarchical pathways for effective ion transport, making presumably inaccessible pores and surfaces available to electric double-layer charges, and even improving iontransport kinetics at the same time. Moreover, the CWs-Co3O4 composite can be directly used as a binder-free working electrode, with its various advantages that can enhance supercapacitor performance.

Conclusions This paper has presented a simple, cost-effective, and green approach to constructing a 3D binder-free electrode by growing Co3O4 nano-arrays on carbonized Morpho butterfly wing scales, creating an N-doped, multichannel, hierarchically porous, and 3D hierarchically periodic structure

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Y. Zheng et al.

for high-performance electrochemical capacitors. The results show that the obtained CWs-Co3O4 composite can be used as an electrode material for high-performance supercapacitors with a maximum specific capacity of 978.9 F g
1 at a current density of 0.5 A g
1. This material still retained about 94.5% of its capacitance after 2000 cycles, and its maximum energy density was 99.1 Wh kg
1, calculated based on the total mass of the composite materials. The obtained CWs-Co3O4 composite not only exhibited high specific capacitance, but also a remarkable rate capability and excellent cycling ability. The encouraging results of this research open up a new pathway to high-performance supercapacitors, advanced catalysts for energy conversion, and other energy storage devices.

Acknowledgment This work is financially supported by the Open Fund of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials of Sichuan Province (No. 13ZXFK08 and No. 13ZXFK21).

Appendix A.

Supplementary material

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. nanoen.2015.11.038

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Multi-channeled hierarchical porous carbon incorporated Co3O4 nanopillar [53] Y. Dong, K. He, L. Yin, A. Zhang, Nanotechnology 18 (2007) 435602. [54] D. Cai, D. Wang, B. Liu, L. Wang, Y. Liu, H. Li, Y. Wang, Q. Li, T. Wang, ACS Appl. Mater. Interfaces 6 (2014) 5050–5055. [55] X. Wang, G. Sun, P. Routh, D. Kim, W. Huang, P. Chen, Chem. Soc. Rev. 43 (2014) 7067–7098. [56] U. Maiti, W. Lee, J. Lee, Y. Oh, J. Kim, J. Shim, T. Han, S. Kim, Adv. Mater. 26 (2014) 40–66. [57] X. Li, M. Antonietti, Chem. Soc. Rev. 42 (2013) 6593–6604. [58] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 14 (2015) 271–279. [59] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Energy Environ. Sci. 6 (2013) 2497–2504. [60] Y. Li, Z. Fu, B. Su, Adv. Funct. Mater. 22 (2012) 4634–4667. [61] P. Hao, Z. Zhao, J. Tian, H. Li, Y. Sang, G. Yu, H. Cai, H. Liu, C. Wong, A. Umar, Nanoscale 6 (2014) 12120–12129. Yuanchuan Zheng received his master's degree at State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology (SWUST) in 2015. Since then, he joined as an assistant researcher in College of Chemistry, Chemical Engineering and Materials Science of Soochow University. His research interests focus on nanoscience for energy research: energy storage, electrocatalysis and selfpowered nanosystems. Zhaoqian Li received his doctoral degree in Advanced Materials and Manufacturing Technology in 2010 from the East China University of Science and Technology. He is currently working in State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials at the Southwest University of Science and Technology (SWUST), Mianyang, China. Now he joined as a visiting scholar in School of Chemical and Biomedical Engineering of Nanyang Technological University (NTU) funded by China Scholarship Council. His scientific interests focus on organic–inorganic hybrid nanomaterials, polymer composites and energetic materials. Juan Xu is currently pursuing her Master degree under the supervision of Prof. Chonghua Pei at the Chemistry Department of Southwest University of Science and Technology (SWUST). Her research interests mainly focus on nanocomposite materials templated by carbonated bacterical cellulose.

Tilong Wang received his master's degree in School of Materials Science and Engineering of Southwest University of Science and Technology in 2015. He is now a research associate in the Sichuan Non-Ferrous Metallurgy Research institute. His current research is focused on the development of lithium-ion batteries and supercapacitors for high-performance energy storage.

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Xun Liu studied inorganic materials at Sichuan University (SCU), and received his master's degree there in 2003. Since then, he joined as an assistant professor in State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology (SWUST), Mianyang, China. In June 2015, he received his Ph. D. degree in biomedical engineering. Biomimetic mineral materials and biomedical materials are now his major research directions. Xiaohui Duan studied chemistry and physics at the Sichuan University (SCU) and received her Ph.D. degree there in 2005. From 2005 to now, she has worked as a full professor in State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology (SWUST), Mianyang, China. Now, her research mainly focuses on the energetic materials and theoretical simulations and calculations. Yongjun Ma studied Condensed Matter Physics at institute of physics, Chinese Academy of Science (CAS), and received his Ph.D. degree there in 2005. From 2005, he joined as an Associate Professor in Analytical and Testing Center, Southwest University of Science and Technology (SWUST), Mianyang, China. Now, nanostructured materials, electron microscopic analysis and energetic materials are his main research fields. Yong Zhou studied chemistry and physics at the University of Science and Technology of China (USTC), and received his Ph.D. degree there in 2000. After working in Kyoto University in 2000–2001, the Max Planck Institute of Colloids and Interfaces in 2002–2003, the National Institute of Materials Science (NIMS) (Japan) in 2003–2004, the National Institute of Advanced Industrial Science and Technology (AIST) (Japan) in 2004–2008, and the National University of Singapore (NUS) in 2008–2009, he joined as a full professor in the Eco-materials and Renewable Energy Research Center (ERERC), School of Physics, National aboratory of Solid State Microstructures, Nanjing University, China. His research now focuses on the design and fabrication of solar-light-driven clean energy materials for photocatalysis and flexible solar cells. Chonghua Pei received his Ph.D. degree from Nanjing University of Science and Technology (NUST) in 1996. Between 19962003, he focused on superfine powders and bacterial cellulose in Hainan University (HNU). From 2004, he joined as a full professor in State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology (SWUST), Mianyang, China. Now, nanostructured materials, biomimetic mineral materials and energetic materials are his main research fields.