Hierarchical mesoporous nickel cobaltite nanoneedle/carbon cloth arrays as superior flexible electrodes for supercapacitors
- Deyang Zhang†1, 2,
- Hailong Yan†1, 2,
- Yang Lu†1, 2, 3,
- Kangwen Qiu†1, 2,
- Chunlei Wang1, 2,
- Chengchun Tang3,
- Yihe Zhang4,
- Chuanwei Cheng5 and
- Yongsong Luo1, 2, 6Email author
© Zhang et al.; licensee Springer. 2014
Received: 9 February 2014
Accepted: 8 March 2014
Published: 24 March 2014
Hierarchical mesoporous NiCo2O4 nanoneedle arrays on carbon cloth have been fabricated by a simple hydrothermal approach combined with a post-annealing treatment. Such unique array nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and desirable cycle life at high rates. When evaluated as an electrode material for supercapacitors, the NiCo2O4 nanoneedle arrays supported on carbon cloth was able to deliver high specific capacitance of 660 F g-1 at current densities of 2 A g-1 in 2 M KOH aqueous solution. In addition, the composite electrode shows excellent mechanical behavior and long-term cyclic stability (91.8% capacitance retention after 3,000 cycles). The fabrication method presented here is facile, cost-effective, and scalable, which may open a new pathway for real device applications.
Supercapacitors (SCs), also known as electrochemical capacitors, have attracted significant research attention due to their superior properties like high power density, excellent reversibility, and long cycle life for time-dependent power needs of modern electronics and power systems[1–9]. Especially, with the fast development of portable electronic devices with lightweight and flexible designs, the research on flexible storage devices becomes very important. The key research of supercapacitors is developing novel electrode materials with good specific capacitance and cycling stability plus high power density. It has been well established that nanostructured electrode designs can enhance both the power density (or rate capability) and cycling stability. Although a wide variety of nanostructures have been created and tested, it still represents a grand challenge to enhancing the capacity, maintaining the excellent rate capability and charge-discharge cycling life[10, 11]. Ternary nickel cobaltite (NiCo2O4) has recently been investigated as a high performance electrode material for SCs because of its better electrical conductivity and higher electrochemical activity compared to binary nickel oxide (NiO) and cobalt oxide (Co3O4). Furthermore, NiCo2O4 with a broad range of morphologies were successfully fabricated, including three-dimensional (3D) urchin-like[13, 14], monodisperse NiCo2O4 mesoporous microspheres, 2D nanofilms, mesoporous nanoflakes, nanosheets, 1D nanoneedle, nanowire[20–22], and porous nanotubes. Therefore, NiCo2O4 has been conceived as a promising electrode material for SCs owing to its high specific capacitance, environmental compatibility, and cost-effectiveness.
In this communication, we demonstrate a rapid and facile method to prepare highly ordered 1D nanoneedle-like NiCo2O4 arrays on carbon cloth serving as electrode materials for SCs. Remarkably, the carbon cloth supported NiCo2O4 nanoneedles manifests ultrahigh SCs (660 F g-1 at 2 A g-1) and good cycling stability (91.8% capacitance retention after 3,000 cycles) at high rates in 2 M KOH aqueous electrolyte, making it a promising electrode for SCs. The fabrication method presented here is facile, cost-effective, and scalable, which may open a new pathway for real device applications[24, 25].
Synthesis of NiCo2O4 nanoneedle arrays on carbon cloth
All the reagents were of analytical grade and directly used after purchase without further purification. Prior to deposition, commercial carbon cloths (1.5 × 4 cm in rectangular shape) were cleaned by sonication sequentially in acetone, 1 M HCl solution, deionized water, and ethanol for 15 min each, drying for standby. NiCo2O4 nanoneedle arrays (NCONAs) on carbon cloth were synthesized via a simple one-pot hydrothermal process. Four millimoles (1.1632 g) of Ni(NO3)2.6H2O and 8 mmol (2.3284 g) of Co(NO3)2.6H2O were dissolved into 75 mL of deionized water, followed by the addition of 15 mmol (0.9009 g) of urea at room temperature, and the mixture was stirred to form a clear pink solution. Then, the mixture was transferred in to a 100-mL Teflon-lined stainless autoclave. Then, the well-cleaned carbon cloth was immersed in the mixture, and the autoclave was kept at 120°C for 6 h. After it was cooled down to room temperature, the product supported on the carbon cloth was taken out and washed with deionized water and ethanol several times and cleaned by ultrasonication to remove the loosely attached products on the surface. After that, the sample was dried at 80°C for characterization. Finally, the as-prepared sample was annealed at 400°C in air for 2 h.
The crystalline structure and phase purity of the products were identified by X-ray diffraction (XRD) using a D8 Advance (Bruker, Karlsruhe, Germany) automated X-ray diffractometer system with Cu-Kα (λ = 1.5406 Å) radiation at 40 kV and 40 mA ranging from 10° to 70° at room temperature. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 microscope (Chiyoda-ku, Japan). Transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-2010, Akishima-shi, Japan, instrument in bright field and on a high-resolution transmission electron microscopy (HRTEM) JEM-2010FEF instrument (operated at 200 kV). Raman spectra were carried out using WITec CRM200 Raman system, Ulm, Germany, equipped with a 532-nm laser source and a × 50 objective lens. The Brunauer-Emmett-Teller (BET) surface area of the NiCo2O4 nanoneedles was determined through nitrogen sorption measurement at 77K.
Electrochemical measurements were carried out by electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai, China) using three-electrode configuration in 2 M KOH aqueous solution. Both the pristine carbon cloth (≈1.5 × 4.0 cm2) and NCONAs (NiCo2O4 mass, ≈5 mg) were directly used as the working electrode. The value of specific capacitance (F g-1) and current rate (A g-1) was calculated based on the total mass of the active materials. The reference and counter electrodes were standard calomel electrode (SCE) and platinum foil, respectively. Cyclic voltammetry (CV) measurements were performed at a scanning rate of 2 to 40 mV s-1 from -0.2 to 0.6 V at room temperature. Galvanostatic charge-discharge measurements were carried out from -0.1 to 0.5 V at a current density of 2 to 16 A g-1, under opens circuit potential. Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternate current (AC) voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz. The specific capacitances were calculated according to equation C = (I Δt)/(ΔV × m), where I is the constant discharge current, Δt is the discharge time, ΔV is the voltage drop upon discharging (excluding the IR drop), and m is the total mass of the active substance of the electrode material.
Results and discussion
Electrode material with a large surface area is highly desirable for electrochemical SCs. The specific surface area and porous nature of the as-prepared nanoneedle-like NiCo2O4 nanostructures were further investigated by nitrogen adsorption-desorption measurements at 77 K. The nitrogen adsorption-desorption isotherm is an IV characteristic with a type H2 hysteresis loop in the range 0.8 to 1.0 p/po (Additional file1: Figure S3), which might appear to be a unique characteristic of mesopores. The inset in the Additional file1: Figure S3 shows the corresponding pore size distribution calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption branch, indicating a narrow pore size distribution (10 to 30 nm) centered at around 12.4 nm. Thus, it can be concluded that the sample is characteristic of mesoporous materials. The specific surface area calculated by the BET method is ca. 44.8 m2 g-1 for the NCONAs.
where I (mA) represents the constant discharge current, m (mg), ΔV (V), and Δt (s) designate the mass of active materials, potential drop during discharge (excluding the IR drop), and total discharge time, respectively. S is the nominal area of CC covered with NCONAs (about 5 cm2). The calculated areal capacitance as a function of the discharge current density is plotted in Figure 6d. On the basis of the above results, the specific capacitance of the NCONAs at 2, 4, 8, 12, and 16 A g-1 is 660, 600, 560, 480, and 384 F g-1, respectively. About 58.2% of specific capacity was retained when the current density increased from 2 to 16 A g-1. It is emphasized that the specific capacitance is calculated according to the mass of the NCONAs, and carbon materials are not included for this calculation. The areal capacitance is as high as 0.660, 0.600, 0.560, 0.480, and 0.384 F cm-2 measured at the discharge current density of 2, 4, 8, 12, and 16 mA cm-2, respectively.
Furthermore, for a better understanding of the synergistic effect in this electrode design, the cycling performance of the NCONAs at progressively increased current density was recorded in Figure 8c. During the first 100 cycles with a charge discharge density of 2 A g-1, the hybrid structure shows a cycle stability performance and the specific capacity as high as 658 F g-1. In the following cycles, the charge/discharge rate changes successively; the hybrid structure always demonstrates stable capacitance even suffering from sudden change of the current delivery. With the current rate back to 2 A g-1 for the rest of cycles, a capacitance of approximately 656 F g-1 can be recovered and without noticeable decrease, which demonstrates the hybrid structure has excellent rate performance and cyclability. The loss of specific capacitance may result from ineffective contacts between part of the unstable NCONAs and the following deterioration of the electron transfer and ion diffusion.
To further show the merits of the NCONAs and CC composite material as the electrode material, EIS provided beneficial tools to reveal the electronic conductivity during the redox process. Impedance spectra of the NCONAs electrode material were measured at open circuit potential with an AC perturbation of 5 mV in the frequency range from 0.1 Hz to 103 KHz. Nyquist plots in Figure 8d were composed of an arc in the high-frequency region and a nearly straight line in the low-frequency region. Herein, the high-frequency intercept with the X-axis represented the equivalent series resistance (Rs), associated with the sum of the electrolyte solution resistance, the intrinsic resistance of active material, and the contact resistance at the electrode-electrolyte interface. The charge transfer resistance of electrode (Rct) was calculated from the diameter of the semicircle in the high-frequency region, while the straight line at lower frequencies presented the diffusion behavior of ions in the electrode pores. The steeper shape of the sloped line represented an ideal capacitive behavior with the faster diffusion of ions in electrolyte. The measured impedance spectra were analyzed using the complex nonlinear least-squares fitting method on the basis of the equivalent circuit, which is given in the inset of Figure 8d. From the magnified high-frequency regions in the inset of Figure 8d, the NCONAs electrodes after 1st and 3,000th cycles show the charge transfer resistances (Rct), respectively. The Rct value increases only slightly from 1st and 3,000th cycles owing to good contact between the current collector and nanoneedle arrays. These analyses revealed that the good electrical conductivity and ion diffusion behavior resulted in the high performance of NCONAs carbon cloth composite as electrode material for SCs.
Based on abundant electrochemical analysis, owing to the synergistic effects between nanoneedle arrays and carbon cloth, the flexible NCONAs and carbon cloth composite electrode material exhibit high specific capacitance. The improved electrochemical performance could be related to the following structural features. Firstly, large surface areas facilitate ion diffusion from the electrolyte to each NCONA, making full use of the active materials, which undoubtedly contributes to the high capacitance. Secondly, carbon cloth in the hybrid materials could provide not only double layer capacitance to the overall energy storage but also fast electronic transfer channels to improve the electrochemical performances. Third, the direct growth of NCONAs on a conductive substrate could ensure good mechanical adhesion, and more importantly, good electrical connection with the conductive substrate that also serves as the current collector in such binder-free electrodes[35, 37]. In this way, the decreased ion diffusion and charge transfer resistances lead to the improved specific capacitance. Meanwhile, the synergistic effects result in the better cycling stability of the NCONAs and carbon cloth composite electrode. NCONAs in a vertical array and carbon cloth as the platform for sustaining nanoneedles arrays withstand the strain relaxation and mechanical deformation, preventing the electrode materials from seriously swelling and shrinking during the insertion-deinsertion process of the counter ions[38, 39].
In summary, we have presented a facile and high-efficiency hydrothermal method for the direct growth of NCONAs on flexible substrates. The synthesis route presented here is robust and may be extended to fabricate other nanostructures for various applications in electrochemical energy storage and optical devices. The NCONAs supported on carbon cloth were tested as highly flexible SCs, and they have demonstrated excellent electrochemical performance; also, they have superior cycling stability that can maintain good performance over 3,000 cycles. Our as-fabricated SCs electrode material demonstrate their feasibility as efficient energy storage devices. Our work here opens up opportunities for flexible energy storage devices in future wearable devices area and many other flexible, lightweight, and high-performance functional nanoscale devices.
This work was financially supported by the National Natural Science Foundation of China (Nos. U1304108, U1204501, and 11272274) and the Science and Technology Key Projects of Education Department Henan Province (No. 13A430758). The authors are indebted to Dr D. L. Xu and Y. X. Liu for their technical assistances and kind help.
- Zhou C, Zhang YW, Li YY, Liu JP: Construction of high-capacitance 3D CoO @ polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor. Nano Lett 2013, 13: 2078–2085. 10.1021/nl400378jView ArticleGoogle Scholar
- Dar FI, Moonooswamy KR, Es-Souni M: Morphology and property control of NiO nanostructures for supercapacitor applications. Nanoscale Res Lett 2013, 8: 363. 10.1186/1556-276X-8-363View ArticleGoogle Scholar
- Marcinauskas L, Kavaliauskas Z, Valincius V: Carbon and nickel oxide carbon composites as electrodes for supercapacitors. J. Mater. Sci. Technol 2012, 28: 931–936. 10.1016/S1005-0302(12)60153-4View ArticleGoogle Scholar
- Gao Y, Pandey GP, Turner J, Westgate CR, Sammakia B: Chemical vapor deposited carbon nanofibers on carbon fabric for supercapacitor electrode applications. Nanoscale Res Lett 2012, 7: 651. 10.1186/1556-276X-7-651View ArticleGoogle Scholar
- Shi C, Zhitomirsky I: Electrodeposition and capacitive behavior of films for electrodes of electrochemical supercapacitors. Nanoscale Res Lett 2010, 5: 518–523. 10.1007/s11671-009-9519-zView ArticleGoogle Scholar
- Liu JP, Jiang J, Cheng CW, Li HX, Zhang JX, Gong H, Fan HJ: Co3O4 nanowire @ MnO2 ultrathin nanosheet core/shell arrays: a new class of high-performance pseudocapacitive materials. Adv Mater 2011, 23: 2076–2081. 10.1002/adma.201100058View ArticleGoogle Scholar
- Meng FH, Yan XL, Zhu Y, Si PC: Controllable synthesis of MnO2 polyaniline nanocomposite and its electrochemical capacitive property. Nanoscale Res Lett 2013, 8: 179. 10.1186/1556-276X-8-179View ArticleGoogle Scholar
- Jiang J, Li YY, Liu JP, Huang XT, Yuan CZ, Lou XW: Recent advances in metal oxide based electrode architecture design for electrochemical energy storage. Adv Mater 2012, 24: 5166–5180. 10.1002/adma.201202146View ArticleGoogle Scholar
- Jiang J, Li YY, Liu JP, Huang XT: Building one-dimensional oxide nanostructure arrays on conductive metal substrates for lithium-ion battery anodes. Nanoscale 2011, 3: 45. 10.1039/c0nr00472cView ArticleGoogle Scholar
- Luo YS, Luo JS, Jiang J, Zhou WW, Yang HP, Qi XY, Zhang H, Fan HJ, Denis YWY, Li CM, Yu T: Seed-assisted synthesis of highly ordered TiO2@α-Fe2O3 core/shell arrays on carbon textiles for lithium-ion battery applications. Energy Environ Sci 2012, 5: 6559–6566. 10.1039/c2ee03396hView ArticleGoogle Scholar
- Li DW, Meng FH, Yan XL, Yang LH, Heng H, Zhu Y: One-pot hydrothermal synthesis of Mn3O4 nanorods grown on Ni foam for high performance supercapacitor applications. Nanoscale Res Lett 2013, 8: 535. 10.1186/1556-276X-8-535View ArticleGoogle Scholar
- Wei TY, Chen CH, Chien HC, Lu SY, Hu CC: A cost-effective supercapacitor material of ultrahigh specific capacitances: spinel nickel cobaltite aerogels from an epoxide driven sol–gel process. Adv Mater 2010, 22: 347–351. 10.1002/adma.200902175View ArticleGoogle Scholar
- Wang QF, Liu B, Wang XF, Ran SH, Wang LM, Chen D, Shen GZ: Morphology evolution of urchin-like NiCo2O4 nanostructures and their applications as psuedocapacitors and photoelectrochemical cells. J Mater Chem 2012, 22: 21647–21653. 10.1039/c2jm34705aView ArticleGoogle Scholar
- Xiao JW, Yang SH: Sequential crystallization of sea urchin-like bimetallic (Ni, Co) carbonate hydroxide and its morphology conserved conversion to porous NiCo2O4 spinel for pseudocapacitors. RSC Adv 2011, 1: 588–595. 10.1039/c1ra00342aView ArticleGoogle Scholar
- Li JF, Xiong SL, Liu YR, Ju ZC, Qian YT: High electrochemical performance of monodisperse NiCo2O4 mesoporous microspheres as an anode material for li-ion batteries. ACS Appl Mater Interfaces 2013, 5: 981–988. 10.1021/am3026294View ArticleGoogle Scholar
- Hu LF, Wu LM, Liao MY, Fang XS: High performance NiCo2O4 nanofilm photo detectors fabricated by an interfacial self-assembly strategy. Adv Mater 2011, 23: 1988–1992. 10.1002/adma.201004109View ArticleGoogle Scholar
- Zhang LX, Zhang SL, Zhang KJ, Xu GJ, He X, Dong SM, Liu ZH, Huang CS, Cui GL: Mesoporous NiCo2O4 nanoflakes as electrocatalysts for rechargeable Li-O2 batteries. Chem Commun 2013, 49: 3540–3542. 10.1039/c3cc40393aView ArticleGoogle Scholar
- Lu XH, Huang X, Xie SL, Zhai T, Wang CS, Zhang P, Yu MH, Li W, Liang CL, Tong YX: Controllable synthesis of porous nickel-cobalt oxide nanosheets for supercapacitors. J Mater Chem 2012, 22: 13357–13364. 10.1039/c2jm30927kView ArticleGoogle Scholar
- Zhang GQ, Wu HB, Hoster HE, Park MBC, Lou XW: Single-crystalline NiCo2O4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high performance supercapacitors. Energy Environ Sci 2012, 5: 9453–9456. 10.1039/c2ee22572gView ArticleGoogle Scholar
- Wang QF, Wang XF, Liu B, Yu G, Hou XJ, Chen D, Shen GZ: NiCo2O4 nanowire arrays supported on Ni foam for high-performance flexible all-solid-state supercapacitors. J Mater Chem A 2013, 1: 2468–2473. 10.1039/c2ta01283aView ArticleGoogle Scholar
- Wang HL, Gao QM, Jiang L: Facile approach to prepare nickel cobaltite nanowire materials for supercapacitors. Small 2011, 7: 2454–2459.Google Scholar
- Jiang H, Ma J, Li CZ: Hierarchical porous NiCo2O4 nanowires for high rate supercapacitors. Chem Comm 2012, 48: 4465–4467. 10.1039/c2cc31418eView ArticleGoogle Scholar
- Li LL, Peng SJ, Cheah YL, Teh PF, Wang J, Wee G, Ko YW, Wong CL, Srinivasan M: Electrospun porous NiCo2O4 nanotubes as advanced electrodes for electro- chemical capacitors. Chem Eur J 2013, 19: 5892–5898. 10.1002/chem.201204153View ArticleGoogle Scholar
- Fang XS, Zhai TY, Gautam UK, Li L, Wu LM, Bando Y, Golberg D: ZnS nanostructures: from synthesis to applications. Prog Mater Sci 2011, 56: 175–287. 10.1016/j.pmatsci.2010.10.001View ArticleGoogle Scholar
- Fang XS, Hu LF, Huo KF, Gao B, Zhao LJ, Liao MY, Chu PK, Bando Y, Golberg D: New ultraviolet photodetector based on individual Nb2O5 nanobelts. Adv Funct Mater 2011, 21: 3907–3915. 10.1002/adfm.201100743View ArticleGoogle Scholar
- Hu LF, Wu LM, Liao MY, Hu XH, Fang XS, Hu L, Wu L, Liao M: Electrical transport properties of large, individual NiCo2O4 nanoplates. Adv Funct Mater 2012, 22: 998–1004. 10.1002/adfm.201102155View ArticleGoogle Scholar
- Tarasevich MR, Efremov BN: Electrodes of Conductive Metallic Oxides Part A. USA: Elsevier; 1982:227.Google Scholar
- Luo YS, Jiang J, Zhou WW, Yang HP, Luo JS, Qi XY, Zhang H, Yu DYW, Li CM, Yu T: Self-assembly of well-ordered whisker-like manganese oxide arrays on carbon fiber paper and its application as electrode material for supercapacitors. J Mater Chem 2012, 22: 8634–8640. 10.1039/c2jm16419aView ArticleGoogle Scholar
- Hu ZA, Xie YL, Wang YX, Xie LJ, Fu GR, Jin XQ, Wu HY: Synthesis of α-cobalt hydroxides with different intercalated anions and effects of intercalated anions on their morphology, basal plane spacing, and capacitive property. J Phys Chem C 2009, 113: 12502–12508. 10.1021/jp8106809View ArticleGoogle Scholar
- Zhong JH, Wang AL, Li GR, Wang JW, Ou YN, Tong YX: Co3O4/Ni (OH)2 composite mesoporous nanosheet networks as a promising electrode for supercapacitor applications. J Mater Chem 2012, 22: 5656–5665. 10.1039/c2jm15863aView ArticleGoogle Scholar
- Liu B, Zhang J, Wang XF, Chen G, Chen D, Zhou CW, Shen GZ: Hierarchical three dimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion batteries. Nano Lett 2012, 12: 3005–3011. 10.1021/nl300794fView ArticleGoogle Scholar
- Wang X, Han XD, Lim MF, Singh N, Gan CL, Ma J, Lee PS: Nickel cobalt oxide-single wall carbon nanotube composite material for superior cycling stability and high-performance supercapacitor application. J Phys Chem C 2012, 116: 12448–12454. 10.1021/jp3028353View ArticleGoogle Scholar
- Gupta V, Gupta S, Miura N: Potentiostatically deposited nanostructured CoxNi1-x layered double hydroxides as electrode materials for redox-supercapacitors. J Power Source 2008, 175: 680–685. 10.1016/j.jpowsour.2007.09.004View ArticleGoogle Scholar
- Hu CC, Cheng CY: Ideally pseudocapacitive behavior of amorphous hydrous cobalt nickel oxide prepared by anodic deposition. J Electrochem Solid-State Lett 2002, 5: A43-A46. 10.1149/1.1448184View ArticleGoogle Scholar
- Luo YS, Luo JS, Zhou WW, Qi XY, Zhang H, Denis YWY, Li CM, Fan HJ, Yu T: Controlled synthesis of hierarchical graphene-wrapped TiO2@Co3O4 coaxial nanobelt arrays for high-performance lithium storage. J Mater Chem A 2013, 1: 273–28. 10.1039/c2ta00064dView ArticleGoogle Scholar
- Liu S, Liu XH, Li ZP, Yang SR, Wang JQ: Fabrication of free-standing grapheme polyaniline nanofibers composite paper via electrostatic adsorption for electrochemical supercapacitors. New J Chem 2011, 35: 369–374. 10.1039/c0nj00718hView ArticleGoogle Scholar
- Luo YS, Kong DZ, Luo JS, Chen S, Zhang DY, Qiu KW, Qi XY, Zhang H, Li CM, Yu T: Hierarchical TiO2 nanobelts@MnO2 ultrathin nanoflakes core–shell array electrode materials for supercapacitors. RSC Adv 2013, 3: 14413–14422. 10.1039/c3ra42229aView ArticleGoogle Scholar
- Xu J, Wang K, Zu SZ, Han BH, Wei Z: Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010, 4: 5019–5026. 10.1021/nn1006539View ArticleGoogle Scholar
- Zhang J, Zhao XS: Conducting polymers directly coated on reduced graphene oxide sheets as high-performance supercapacitor electrodes. J Phys Chem C 2012, 116: 5420–5426.View ArticleGoogle Scholar
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