Designing Heterogeneous 1D Nanostructure Arrays Based on AAO Templates for Energy Applications.

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Nanostructure Arrays

Designing Heterogeneous 1D Nanostructure Arrays Based on AAO Templates for Energy Applications Liaoyong Wen, Zhijie Wang, Yan Mi, Rui Xu, Shu-Hong Yu,* and Yong Lei*

From the Contents 1. Introduction ............................................. 2 2. Design Principles for Highly Efficient Energy-Conversion and -Storage Devices Using Heterogeneous 1D Nanostructure Arrays ........................................................ 2 3. Technological Progress of AAO Templates for Heterogeneous 1D Nanostructure Arrays ........................................................5 4. Heterogeneous 1D Nanostructure Arrays for Solar Energy Conversion Applications .............................................. 6 5. Heterogeneous 1D Nanostructure Arrays for Energy-Storage Applications .....................11 6. Conclusions and Perspectives...................17

small 2015, DOI: 10.1002/smll.201500120

In order to fulfill the multiple requirements for energy production, storage, and utilization in the future, the conventional planar configuration of current energy conversion/storage devices has to be reformed, since technological evolution has promoted the efficiency of the corresponding devices to be close to the theoretical values. One promising strategy is to construct multifunctional 1D nanostructure arrays to replace their planar counterparts for device fabrication, ascribing to the significant superiorities of such 1D nanostructure arrays. In the last three decades, technologies based on anodic aluminium oxide (AAO) templates have turned out to be valuable meaning for the realization of 1D nanostructures and have attracted tremendous interest. In this review, recent progress in energy-related devices equipped with heterogeneous 1D nanostructure arrays that fabricated through the assistance of AAO templates is highlighted. Particular emphasis is given on how to develop efficient devices via optimizing the componential and morphological parameters of the 1D nanostructure arrays. Finally, aspects relevant to the further improvement of device performance are discussed.

© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1. Introduction To date, the worldwide electric generation capacity has been estimated to exceed 20 terawatt hours and more than 70% of that electrical energy supply is from fossil fuels. Unfortunately, the production of fossil energies like oil and natural gas is predicted to peak, and it cannot meet the increasing global energy demand in the near future.[2] So far, several solutions have been carried out to address this issue. With the magnitude of the available power striking the surface of earth at any one instant equal to 130 million 500 MW power plants, solar energy is deemed an ideal source for generating electricity.[3] Devices for converting solar energy, like photovoltaic cells and water-splitting cells, have received substantial interest over that last half a century. The key component of solar energy conversion devices is a semiconductor, which absorbs the photons by exciting electrons from the valence band to the conduction band. The photogenerated charge carriers can be extracted to an external circuit or an electrolyte by the internal field, which is induced by the junctions with semiconductors, metals, and electrolytes.[4] Meanwhile, storing and utilizing electric energy in an electrochemical way turns out to be an economical and portable strategy, considering the inefficient and costly electric transportation of pushing electrons across power grids as well as the expanding market for portable electronics and the electrification of the transportation sector (e.g., battery-powered vehicles).[5] Systems for electrochemical energy storage and utilization include batteries, supercapacitors, and fuel cells, which have attracted substantial attention in the past decades.[6] In batteries and fuel cells, electrical energy is generated by converting chemical energy via redox reactions at the anode and cathode. As to supercapacitors, by the orientation of electrolyte ions at the electrode/electrolyte interface, so-called electrical double layers are formed and released, yielding a parallel movement of electrons in the external wire. Moreover, the electrodes of supercapacitors could be electrochemically active, to store and release chemical energy during the charging and discharging processes. Present structures of commercial energy-conversion and -storage devices are dominated by planar configurations, and the corresponding converting/storing capabilities have approached the theoretical values. To break through this technological bottleneck, one-dimensional (1D) nanostructure arrays have been explored as energy-conversion and -storage structures, and exhibit a great superiority over their conventional counterparts in improving the relevant performance dramatically.[7] Specifically, heterogeneous 1D nanostructure arrays with multiple components could maximally meet the requirements to improve the performance of the energy-converting/-storing devices systematically in various aspects.[8] Generally, there are two types of heterogeneous 1D nanostructure arrays. One is fabricated by growing the complementary materials in a radial direction, and such complex structures are termed ‘core/shell 1D nanostructure arrays’. The other is created by growing the complementary materials longitudinally, and the resultant structures are usually referred to as ‘longitudinal heterojunction 1D nanostructure arrays’.

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So far, various techniques have shown up for the fabrication of heterogeneous 1D nanostructure arrays, including a series of ‘top down’ and ‘bottom up’ processes.[8a,b,d,e] Among these techniques, hard nanoporous templates provide a straightforward scaffold for fabricating 1D nanostructure arrays, by the direct filling of these template pores with different materials. Particularly, the anodic aluminium oxide (AAO) template appears as one of the most widely used nanotemplates, showing advantages including low-cost accessibility, easy scalability, and high structural controllability, and has been exploited for synthesizing a diverse range of nanostructure arrays.[9] Although there have been some review articles already regarding the energy device applications of 1D nanostructure arrays,[7c,10] articles that specifically summarize heterogeneous 1D nanostructure arrays based on AAO templates are scarce. In this review, we focus on the efforts to develop efficient energy-conversion/-storage devices employing heterogeneous 1D nanostructure arrays based on AAO templates. First, the designing principles for highly efficient energyrelated devices using heterogeneous 1D nanostructure arrays are summarized. The technological progress of AAO templates for realizing such structures is then briefly reviewed. A series of heterogeneous 1D nanostructure arrays that have recently been considered for solar energy conversion (e.g., solar cells and solar water-splitting cells) is subsequently described, and aspects relevant to applications of the these arrays for energy-storage devices like batteries and supercapacitors are included. Finally, perspectives of the related fields are given together with a conclusion.

2. Design Principles for Highly Efﬁcient Energy-Conversion and -Storage Devices Using Heterogeneous 1D Nanostructure Arrays The term ‘heterogeneous 1D nanostructure array’ generally refers to structures with three distinctive features: a 1D nanoconfiguration, a large-scale array, and a heterogeneous 1D structure. First, the nanostructure should be confined to one dimension at the nanoscale, yielding a high aspect ratio, indicating

L. Wen, Z. Wang, Y. Mi, R. Xu, Prof. Y. Lei Institute of Physics & IMNMacro Nanos (ZIK) Ilmenau University of Technology Ilmenau, Prof. Schmidt-Str.26 98693, Germany E-mail: [email protected] Prof. S.-H. Yu Department of Chemistry University of Science and Technology of China Hefei 230026, PR China E-mail: [email protected] Prof. Y. Lei Institute of Nanochemistry and Nanobiology School of Environmental and Chemical Engineering Shanghai University Shanghai 200444, PR China DOI: 10.1002/smll.201500120


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a large surface-to-volume ratio and the longitudinal continuity of the material. The large surface-to-volume ratio enables the relevant structure to have a huge surface area where the energy-conversion/-storage reactions could occur extensively and rapidly. The one unconfined dimension provides a continuous passway to efficiently transfer charge carriers. In this case, the key disadvantage of a short carrier diffusion length (DL) relative to the depth of visible-light absorption in semiconductors could be well resolved (see Figure 1a, left). For instance, the optical absorptivity of GaP near the bandgap absorption is small and requires a minimal thickness of 28 µm for the bulk material to capture light at 540 nm. The minority-carrier DLs in single-crystalline GaP wafers are usually observed in the range of 10–100 nm, indicating a high possibility for photogenerated charge carriers in the bulk of GaP to recombine uselessly before travelling to the surface and being collected.[11] When we transform the material configuration to 1D nanoscale, the photogenerated charge carriers need to travel just a short distance to the surface, and the unconfined dimension of the structure can provide sufficient absorption of the incident light, as well as having the advantage of an absorption-amplifying effect deriving from the scattering and antireflection of 1D nanostructures. Consequently, the external quantum yield (at 500 nm) of GaP photoelectrochemical (PEC) electrodes is dramatically enhanced from approximately 0 to 1, when a 1D GaP nanostructure replaces the planar counterpart. Although 1D nanostructures could reduce the diffusion length of the photogenerated charge carriers in semiconductors, it doesn’t mean that a thin 1D nanostructure always results in a higher charge-collection efficiency. The cross-section of a very thin 1D nanostructure might be too short to support the full internal electric field dictated by the interfacial equilibrium contact, hence, the driving force for separating the photogenerated electrons and holes is greatly crippled, which induces substantial majority-carrier recombination at the contact. Only when the radius is bigger than the thickness of the depletion layer could the internal field be large enough to separate the photogenerated charges and conduct them to the electrodes. The quantum yield of the PEC electrodes featuring 1D nanostructure arrays has been illustrated to be largely determined by the cross-sectional profile of the nanowire. As displayed in the right curve of Figure 1a, the three tapered GaP nanowires III, IV, and V show lower attainable quantum yield values than that of nanowire I due to the morphology difference, and nanowire II presents the lowest quantum yield, attributed to its having the thinnest section.[12] With the term “rocking chair” battery, lithium ion batteries employ insertion processes for both the anodes and cathodes.[13] The transport of Li ions between the electrodes that are always arranged in parallel is 1D in nature. To minimize specific power losses originating from slow ionic transportation, the thickness of the insertion electrodes and the inter-electrode distance should be as small as possible. However, when the thickness of the electrodes is reduced, the specific energy and operating time of the device will accordingly become smaller and shorter, leading to a dilemma that limits further performance improvements for the planar small 2015, DOI: 10.1002/smll.201500120

Liaoyong Wen received his BS degree in 2006 from the Department of Chemical Engineering of Zhengzhou University and is currently pursuing his PhD at Ilmenau University of Technology (Germany) in the group of Prof. Y. Lei. His research interests include the fabrication of 3D nanostructures using anodic aluminium oxide templates for applications in high-performance energyconversion and -storage devices.

Shu-Hong Yu is the Cheung Kong Chair Professor of Chemistry in the Department of Chemistry at the University of Science and Technology of China (USTC), PR China. He is the Director of the Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale. His research interests include the bioinspired synthesis and self-assembly of new nanostructured materials and nanocomposites, and their related properties and applications.

Yong Lei received his PhD from the Chinese Academy of Sciences in 2001. After two years as a Singapore-MIT Alliance postdoc, he worked as an Alexander von Humboldt Fellow at Karlsruhe Research Center in 2003–2006. He was a group leader at the University of Muenster from 2006 and was promoted to a W1 Professor in 2009. He joined the Ilmenau University of Technology in 2011 as a W2 Professor. His group currently focuses on template-realized functional nanostructures, 3D nanostructuring and surface nanopatterning, energy-related and optoelectronic applications.

batteries. To break this morphologic limit, 1D nanostructures prove to be a good solution. In this regard, large areal energy capacities can be realized without thickening the insertion electrodes and sacrificing the power density. The low power density and substantial Ohmic potential losses associated with the small area-to-volume ratio and long ion-transport distances in planar devices could be improved fundamentally. The dimensionless number U = (r2/L2)(µ/σ)(1/C)—where r and L are the radius and length of a nanowire, respectively, µ is the ionic mobility of cations, σ is the electronic conductivity of electrode materials, and C is the volumetric energy capacity (C cm−3)—could be used to quantitatively estimate the uniformity of current across the surfaces of 1D-nanostructured electrodes and whether the electrode material is uniformly utilized during cell charging and discharging processes.[14] A decreasing U corresponds to a more uniform current distribution around the 1D nanostructured electrode. This scenario is a desired one, since the decrease in U usually



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Figure 1. a) Left: comparison of photogenerated charge-carrier collection at planar and high-aspect-ratio photoelectrodes. The gray bottom represents the back contact for majority carrier collection. Right: comparison of the simulated quantum yield–potential photoresponse under AM 1.5 illumination from five different high-aspect-ratio morphologies all modeled with a uniform doping density of 2 × 1016 cm−3 and an interfacial equilibrium barrier height of 1 eV. For I, radius r0 = 300 nm. For II, r0 = 50 nm. For III, IV, and V, r0,bottom = 300 nm and r0,top = 150, 100, and 50 nm, respectively. Reproduced with permission.[12] Copyright 2012, Royal Society of Chemistry. b) FDTD-simulated absorptance spectrum of Cu2O nanorod arrays with length L = 600 nm, diameter d = 100 nm on the Cu2O substrate with thickness of 400 nm. The absorptance spectrum of a Cu2O thin film with thickness of 1 µm is included as a reference. The inset shows the comparison of FDTD-simulated spatial distribution of the electric-field intensity between the planar and 1D nanostructured Cu2O in a cross-sectional view. (Left inset: planar Cu2O; right inset: Cu2O nanorod arrays) illuminated by photons at 500 nm. c) Left: array of interdigitated cylindrical cathodes and anodes. Right: dependence of electrode utilization on electrode conductivity σ and ion diffusivity D. Reproduced with permission.[13] Copyright 2004, American Chemical Society. d) FDTD-simulated absorptance spectra (top) and spatial distribution of the electric field intensity (bottom) of Cu2O nanorod arrays (L = 600 nm; d = 100 nm), Cu2O/InP longitudinal heterojunction nanorod arrays (Cu2O at the bottom: L = 300 nm; d = 100 nm; InP on the top: L = 300 nm; d = 100 nm) and Cu2O/InP core/shell nanorod arrays (Cu2O as the core: L = 600 nm; d = 50 nm; InP as the shell: L = 600 nm; thickness T = 25 nm) illuminated by photons at 650 nm.

results from high electronic conductivity in the electrodes instead of low ionic conductivity in the electrolyte. Increasing U is related to a more nonuniform discharge of the electrodes, which probably results in under-utilization of the electrode materials during rapid discharge and increasing stress along the length of the electrodes. Second, 1D nanostructures should be assembled into large areas, extending millions of nano-units to large-scale arrays for macroscopic energy applications. After the distribution of the nanostructure array is well controlled; scattering and antireflection effects arise and incident radiation could be easily collected, enabling it as an advantageous component in solar energy conversion devices.[15] To investigate it more convincingly, we performed finite difference time domain (FDTD) simulations for a conventional


semiconductor Cu2O. As shown in the insets of Figure 1b, when illuminated by photons at 500 nm, the electric-field distribution around the planar Cu2O film (thickness: 1 µm) is concentrated on the surface. As to the Cu2O nanorod arrays, the electric field distributing scope is greatly enlarged and a high electric field surrounds the entire nanowire arrays. As a result, the Cu2O nanorod array exhibits a dramatic improvement in absorption capability in comparison with the Cu2O film below 480 nm, the absorbance of the 1D nanostructured specimen is nearly 1, while the value is only 0.7 for the planar sample, as displayed in Figure 1b. For energy-storage applications, the available void volume between adjacent nanostructures results in fast ion transportation, easy electrolyte accessibility to the electrode and tolerance for volume expansion of the active materials.


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The ideal architecture for electrochemical energy-storing devices is proposed as two sets of interdigitated 1D nanowire arrays in Figure 1c.[13] The spacing between the anodic and cathodic nanowires that accommodates a large volume change during the charging/discharging process shall be well controlled. Consequently, the deterioration problems in the planar batteries with increasing charge/discharge cycles could be largely alleviated by using 1D nanowire arrays. In addition, the distance for ion transportation in discharging is dramatically reduced by adopting interdigitated 1D nanowires, and such a structure is significantly less susceptible to Ohmic losses and other transport limitations. Specifically, highly ordered 1D nanostructure arrays are more performancerepeatable and more compatible with the adjacent elements as compared with the irregular counterparts. Third, according to specific requirements, the nanostructures could be constructed by multiple materials to overcome the shortcomings of a single component. For solar energy absorption, the complementary usage of semiconductors with different bandgaps in heterogeneous 1D nanostructure is of great benefit for amplifying the absorption capability of the entire device. As shown in Figure 1d, we have simulated the absorbing capabilities for Cu2O/InP 1D nanowire arrays with core/shell and longitudinal heterojunction morphologies. Both heterogeneous structures present an optimized electric field distribution at 650 nm and hence an outstanding enhancement in absorbance, particularly in the region beyond the absorption limit of Cu2O. The absorption range can be extended to the NIR region with more than 70% of the solar spectrum covered. The internal field at the interface of Cu2O/InP is also profitable for separating the photogenerated charge carriers and reducing the possibility of recombination. In electrochemical energy-storage applications, electrodes are usually constructed by coating the insertion electrodes onto metallic 1D nanostructure arrays. The metallic core could transport the electric energy efficiently and supply a large area for the insertion electrodes. Based on the above recognitions, three key features are crucial for realizing highly efficient energy-conversion and -storage devices: i) a 1D nano-configuration: its large surface area greatly facilitates energy-conversion/-storage reactions, and the elongated longitudinal dimension provides an efficient passway for transferring charge carriers or ions. ii) The interaction and/or collecting behavior (e.g., optical scattering and antireflection) of large-scale arrayed 1D nanostructures could improve the overall energy-conversion and -storage performance; iii) heterogeneous 1D nanostructures combine the advantages of different materials, resulting in a ‘1+1 > 2’ property optimization.

3. Technological Progress of AAO Templates for Heterogeneous 1D Nanostructure Arrays AAO templates that can be attained by anodizing aluminium metal in acid electrolytes has inspired a wave of fanaticism for the fabrication of highly ordered nanostructure arrays. This scientific fashion originates from the seminal work of Masuda and co-workers, who reported self-ordered AAO templates in small 2015, DOI: 10.1002/smll.201500120

Figure 2. Strategies for realizing heterogeneous 1D nanostructure arrays: I) type A core/shell (shell coated on core nanowires), II) type B core/shell (core infiltrated in shell nanotubes), III) type C core/ shell (nanowires embedded in film), IV) longitudinal heterojunction nanowires.

1995.[16] By choosing appropriate anodization conditions, selfordered AAO templates with tunable pore diameters of about 10–400 nm and densities in the range 108–1010 pores cm−2 can be prepared. In combination with an imprinting process, long-range ordering of AAO templates with tunable pore distributions and profiles could be easily achieved.[17] Thus, during the last three decades, AAO templates have been intensively exploited for synthesizing a large diversity of nanostructure arrays in the forms of nanodots,[18] nanowires,[19] nanotubes,[20] and nanopores.[16,21] Such nanostructure arrays have already been applied in various fields including optics, electronics, optoelectronics, and sensing. Meanwhile, the developed pulse anodization procedure as well as the alternative usage of mild and hard anodization procedures for one specimen could enable the fabrication of AAO templates with periodically modulated nanopore diameters along the pore axes, providing another opportunity for tailoring the morphological parameters of the as-fabricated 1D nanostructure arrays.[22] More importantly, AAO templates are also very efficient for realizing heterogeneous 1D nanostructure arrays combined with other techniques by growing different 1D nano-units sequentially. For the growth of core/shell nanostructures, the shell materials should be coated homogeneously onto the surface of the core nanostructures (Figure 2, type I). Accordingly, such core/shell architectures could be fabricated through various approaches such as sol–gel procedures,[23] pyrolysis,[24] surface thermal oxidation,[25] core shrinkage,[26] pore etching,[27] selective electrochemical growth,[28] and atomic layer deposition (ALD).[29] Specifically, ALD has drawn substantial attention as a versatile methodology for thin-film deposition, attributed to the conformal and uniform deposition of thin films on substrates using this technique, no matter how rough the substrates are. Alternatively, a core/shell architecture could also be realized by depositing the core



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reviews www.MaterialsViews.com Table 1. The development of heterogeneous solar cells based on AAO templates. Material

Configuration

Synthetic approach

Efficiency [%]

Material

Configuration

Synthetic approach

Efficiency [%]

CdS/CdTe[36]

CSa) PN junction

CVDb) and TEc)

6

P3HT/C60[46]

CS polymer junction

Annealing and spin coating

1.12

CdS/CdTe[38]

CS PN junction

EDd) and TE

9.8

P3HT/PCBM[47]

CS polymer junction

Annealing and spin coating

2.4

AZO/a-Si:H/ITO[39]

CS PN junction

Sputtering and CVD

7.6

P3HT/PCBM[49]

CS polymer junction

Mixture and Annealing

3.6

TiO2/P3HT[40]

CS hybrid PN junction

Sol-gel and spin-coating

0.51

PCBSD/P3HT:IBCA[50]

CS polymer junction

Spin coating and Annealing

7.3

TiO2/ P3OT[43]


ED and spin-coating

1.06

ITO/TiO2[51]

CS photoanode of DSSCe)

ALDf)

1.1

CdSe/P3HT[44]


ED and spin-coating

1.38

Au/TiO2[52]

CS photoanode of DSSC

ED and sol–gel

5.4

CdSe/PEDOT:PSS[45]

CS Schottky junction

ED and spin-coating

3.22

a)

core/shell; b)chemical vapor deposition; c)thermal evaporation; d)electrochemical deposition; e)dye-sensitized solar cell; f)atomic layer deposition.

materials into the pores of the shell nanotube arrays and the deposition techniques mentioned above could work for this procedure in principle as well (Figure 2, type II). As to the structure formed by embedding the nanowire or nanotube arrays in the thin films, conventional techniques for thin-film preparation qualify to cover 1D nanostructure arrays with a thin film (Figure 2, type III). Of course, core/shell architectures are not limited to two components and we can adjust the number of layers freely according to specific applications. For the growth of longitudinal heterojunction 1D nanostructures, electrochemical deposition is a popular approach. In this respect, most of the materials (e.g., metals,[30] semiconductors[31] and conducting polymers[32]) that could be synthesized electrochemically are able to be deposited into the template. In a typical process for synthesizing longitudinal heterojunction nanowires, a thin metal film is evaporated on one side of the template as the working electrode for the subsequent electrodeposition process, followed by sequential deposition of the desired components. Longitudinal heterojunction 1D nanostructures can be conveniently prepared by this approach. The interface between multiple components and the morphological parameters could also be easily engineered during the deposition (Figure 2, type IV), qualifying it as a decent methodology to fabricate complicated longitudinal heterojunction 1D nanostructures.[33] Other methods, such as vacuum impregnation, electroless plating, chemical polymerization, and sol–gel procedures, have been proposed to prepare more kinds of longitudinal heterojunction nanowires without evaporation of a thin metal film on one side of the AAO template as the working electrode.[34]

4. Heterogeneous 1D Nanostructure Arrays for Solar Energy Conversion Applications 4.1. Core/Shell 1D Nanostructures Despite the antireflection effect in single-component 1D nanostructures, core/shell 1D nanostructures can integrate


the advantages of all components. For semiconductor/semiconductor core/shell nanostructures, the formation of a PN junction on the nanoscale supplies a large contact area for photogenerated charge separation and collection. In the case of semiconductor/metal core/shell contacts, the nanostructured metal cores or shells could introduce the well-known surface plasmon resonance effect to the relevant devices, offering another opportunity to enhance the harvesting of solar energy and the collection of the generated charge carriers. With regard to semiconductor/polymer contacts, the nanostructure provides a good platform to modulate the polymer morphology and make it beneficial for charge transport and collection. Thus, these advanced core/shell 1D nanostructure arrays have been applied in most categories of photovoltaic and PEC cells. As to the traditional inorganic solar cells, core/shell structures have already shown merit in improving the overall efficiency. Results of selected studies are presented in Table 1. Through simulation, Kapadia et al.[35] claimed that core/shell CdS/CdTe nanopillar array solar cells could get an optimal overall efficiency over 20%, with minimal short-circuit current dependence on the bulk minority carrier diffusion length and thus the efficient collection of photogenerated carriers. Fan et al.[36] proposed such solar cells based on CdS/ CdTe nanopillar arrays experimentally. The core CdS was prepared by a chemical vapor deposition (CVD) procedure (Figure 3a). After partly removing the AAO, another CVD process was conducted for growing the CdTe shell. By manipulating the morphological parameters, the solar energy conversion efficiency obtained was 6%, displayed in Figure 3b,c. The authors also transferred the device to a flexible substrate, which is noteworthy. Though the efficiency was lowered to 4%, the flexible device could sustain large bending without degradation in structure and performance (Figure 3d). Liu et al.[37] combined an electrochemical deposition route with thermal evaporation to grow CdS/CdTe core/shell structures for photovoltaic applications and the best performance of the relevant devices was close to 6.5%. Afterwards, the


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Figure 3. a) Scanning electron microscope (SEM) images of a CdS nanopillar array after partial etching of the AAO. b) Current–voltage (J–V) characteristics at different illumination intensities. c) Experimentally obtained efficiency of the solar cells as a function of the embedded nanopillar height. d) Performance characterization of a flexible solar cell based on a PDMS substrate. The inset shows a picture of the set-up for bending the flexible modules. Reproduced with permission.[36] Copyright 2009, Nature Publishing Group.

same group developed a modified strategy, in which the CdS nanowires and the CdS/CdSe PN junctions were further treated by multistep CdCl2 soaking and annealing procedures.[38] With graphite paste as the electrode, the device presented a power-conversion efficiency of 9.8%, which was the highest value for nanowire-based solar cells. The above structures are based on the deposition of active materials into AAO templates to form 1D nanostructures like nanorods and nanotubes, however, through manipulation of the anodizing and pore openings, inverted nanocone structures could also be interesting. Using such a novel architecture, Lin et al.[39] fabricated multiple core/shell AAO/Ag/AZO/aSi:H/ITO structures for solar-energy conversion. Due to the purposely prepared antireflection structure, the absorption efficiency was improved and thus a high energy-conversion efficiency of 7.6% was attained. Regarding the semiconductor/polymer hybrid solar cells, semiconductor 1D nanostructure arrays are usually embedded in polymer films. As a renowned electron acceptor, TiO2 nanorod arrays were fabricated by AAO templates with a sol–gel technique and P3HT was prepared by spin coating to form the hybrid cell.[40] The resultant energy-conversion efficiency of the relevant device was around 0.51%, almost five-f old larger than the device using planar TiO2. Alternatively, small 2015, DOI: 10.1002/smll.201500120

TiO2 nanotube arrays were synthesized by the technique integrating AAO templates and ALD[41] (or liquid ALD[42]) to construct photovoltaic devices with P3HT. Similar results were gained, indicative of the superiority of 1D core/shell nanostructures. Attributed to the large bandgap of TiO2 (3.2 eV) that could inhibit absorption in the visible range, CdTe nanorod arrays synthesized via electrochemical deposition procedures with AAO templates were adopted to obtain core/shell structures with P3OT and the energy conversion efficiency obtained was 1.06%.[43] An improvement for this structure was achieved by building CdSe/P3HT core/shell nanorod arrays, and an efficiency of 1.38% was realized by increasing the length of the CdSe nanorods to 612 nm.[44] Replacing the P3HT with PEDOT:PSS to form Schottky junctions instead of PN junctions with CdSe, a further enhancement in efficiency was claimed as 3.22%.[45] Alternatively, AAO templates can be used as the scaffold to regulate the growth of polymer nanorods. Kim et al.[46] placed an AAO template on a P3HT-coated ITO substrate. By annealing at 250 °C under vacuum, the molten P3HT chains readily entered the nanopores by capillary action. Well-ordered polymer nanorod arrays were achieved after removing the template (Figure 4a). The chain alignment in the P3HT nanorods could facilitate the charge transfer in



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Figure 4. a) SEM images of P3HT nanorods prepared using the AAO template, after removal of the Al/Al2O3 layer. b) Out-of-plane grazing incidence angle X-ray diffraction intensities as a function of scattering angle 2θ for planar P3HT films. c) Schematic representation of P3HT chain conformation in nanorods and thin films. Reproduced with permission.[46] Copyright 2010 Wiley-VCH. d) Chemical structure of C-PCBSD and ICBA. Schematic representation of the nanostructured device architecture. e) J–V characteristics of the as-fabricated C-PCBSD/P3HT/ICBA devices: A: P3HT/ICBA without C-PCBSD nanorods; B: P3HT/ICBA with 170 nm C-PCBSD nanorods; C: P3HT/ICBA with 360 nm C-PCBSD nanorods. Reproduced with permission.[50] Copyright 2011 Wiley-VCH.

each nanorod (Figure 4b,c). Compared with planar devices, the resultant P3HT nanorod/C60 polymer solar cells showed a great improvement in energy-conversion efficiency up to 1.12%. Actually, C60 is not a good candidate as the electron acceptor in polymer solar cells. Chen et al.[47] made a further improvement by using spin-coated PCBM to replace the C60 and the overall efficiency was enhanced to 2.4%. Instead of spin coating PCBM into the P3HT nanorods, in another work, PCBM was mixed with P3HT in the solution and then spun on the ITO substrate.[48] After placing AAO on the top, the sample was annealed at a temperature higher than the melting points of both materials under vacuum conditions. Owing to the different diffusion rates into the AAO pores, P3HT/PCBM core/shell structure formed naturally in the annealing procedure.[49] The optimal conversion efficiency was around 3.6% for the devices with nanorods of about 200 nm in length. This unique procedure for realizing core/shell polymer heterojunction structures was proven to be advantageous over the procedure of spin-coating PCBM into P3HT nanorods arrays. In that case, the morphology of the architecture was not optimized for exciton separation and charge conduction. Further progress was made by constructing the cross-linkable fullerene material [6,6]-phenylC61-butyric styryl dendron ester (PCBSD) nanorod arrays


to collaborate with P3HT:IBCA (indene–C60 bisadduct), as shown in Figure 4d.[50] This novel structure integrates the advantages of both bulk heterojunction and core/shell 1D nanostructure, enabling the relevant devices with an unprecedented overall efficiency of 7.3% in the area of polymer solar cells (Figure 4e). The huge surface area and efficient charge conduction in 1D nanostructure arrays also provide a good platform for PEC reactions. According to their roles in the PEC reactions, different core/shell 1D nanostructures have been fabricated particularly as PEC electrodes. Based on AAO templates, Martinson et al.[51] prepared an AAO/ITO/ TiO2 core/shell structure as the photoanode, where AAO behaved as the scaffold, ITO nanoshell arrays were used as the electron collectors, and TiO2 was the active material for PEC reactions. In this design, electrons can diffuse radially through the walls of semiconducting tubes and be efficiently collected by the adjacent and concentric ITO tubes. The yielded conversion efficiency was dramatically enhanced to 1.1%, much higher than that of the planar devices. Similar structures were acquired by using Au nanowires as electron collectors to replace the ITO in the above design.[52] The conversion efficiency was gauged as 5.4% by soaking the relevant Au/TiO2 core/shell photoanode in TiCl4 solution.


small 2015, DOI: 10.1002/smll.201500120


Figure 5. a) Schematic of core/shell 1D nanostructures of Au/TiO2/OEC. b) The quantity of evolved hydrogen as a function of time, the photocurrent simultaneously recorded at 1 V vs RHE with visible-light illumination and the photocurrent calculated from the evolved H2. c) Faradaic efficiency of the process. Reproduced with permission.[54] Copyright 2012, American Chemical Society. d) SEM image of the Ti/Pt/FTO/Fe2O3 nanospike array. e) UV−vis optical absorption spectra of device on nanospike substrate with different pitches. f) Comparison of the J−V curves between electrodes with and without CoPI. Reproduced with permission.[55] Copyright 2014, American Chemical Society.

Maijenburg et al.[53] adopted sol–gel and electrodeposition methods for synthesizing Ag/TiO2 isolated core/shell nanowires and applied it as the photoanode for water splitting. It was demonstrated that the as-prepared device showed a higher efficiency than bare TiO2 nanotubes and TiO2 nanotubes with attached Ag nanoparticles. A high H2 generation rate of 1.23 × 10−3 mol g−1 h−1 was achieved. In consideration of the large bandgap of conventional photoanode materials like ZnO and TiO2, other materials with absorption capabilities in the visible region have to be employed. Lee et al.[54] designed a plasmonic water-splitting cell. The cell functioned by illuminating a dense array of aligned gold nanorods capped with TiO2, forming a Schottky metal/semiconductor interface which collected and conducted the hot electrons to an unilluminated platinum counter-electrode, where hydrogen gas evolved. The result demonstrated that 95% of the effective charge carriers derived from surface plasmon decay to hot electrons (Figure 5a–c). Furthermore, by removing the squarely ordered AAO template to leave an Al nanospike array behind, Qiu et al.[55] fabricated Al/Ti/Pt/ FTO/α-Fe2O3 core/shell arrays on this peculiar architecture (Figure 5d). The absorption efficiency of the corresponding photoanode approached 100% at 700 nm. After ED of CoPI onto the nanostructures, the photocurrent was measured to be as high as 4.36 mA cm−2 at 1.60 V vs RHE, about three times of that for a planar photoelectrode (Figure 5e,f). Meanwhile, the short hole diffusion length in α-Fe2O3 cannot limit the active layer thickness by this design. small 2015, DOI: 10.1002/smll.201500120

As to photocathodes in water splitting, core/shell 1D nanostructure arrays still exhibit impressive merits. Huang et al.[56] fabricated photocathodes with a complex Cu2O/ CuO/TiO2 core/shell structure based on AAO techniques. In this case, the Cu2O suffers from significant photo-induced reductive decomposition, but the electronic state of copper transferred from Cu(I) to Cu(0) could be well alleviated by modifying the surface of the Cu2O nanowires with protecting layers of CuO and TiO2. The photocathodes with Cu2O/CuO/ TiO2 were found to gain a 74% higher photocurrent and 4.5-times higher stability compared to that of a bare Cu2O nanowires array. Extending from the concept of Ag/TiO2 isolated core/ shell nanowires,[53] an alternative architecture of core/shell nanostructures, a core and a shell with discontinuously distributed multiple components, has been designed. Each shell component is in charge of an individual function. As such, the core/shell nanostructure has great potential to accomplish an integrated application. As portrayed in Figure 6a–c, Au nanorods prepared using AAO templates have a purposely deposited shell consisting of a thin layer of Pt-decorated TiO2 on the top and Co-OEC on the sides.[57] In this regard, the hot electrons in the stimulated Au nanorods could inject to the conduction band of TiO2. The Pt nanoparticles on the surface of TiO2 facilitate the injected electrons transferring to the interface with the electrolyte to drive the water-reduction reactions therein for H2 generation, producing 5 × 1013 H2 molecules cm−2 s−1 under 1 sun illumination, with long-term



9


Figure 6. a) Schematic of a cross-section of an individual photosynthetic unit. b) The corresponding transmission electron micrograph (TEM). c) Magnified TEM views of the platinum/TiO2 cap (top right) and the Co-OEC (bottom right). d) Hydrogen evolution under visible-light illumination (λ > 410 nm) as a function of time. e) Hydrogen produced per hour with various illumination wavelengths. f) Measured O2 and H2 photoproducts as a function of time. Reproduced with permission.[57] Copyright 2013, Nature Publishing Group.

operational stability (Figure 6d–f). The remaining holes in the Au nanorods transfer to the contact zone of Au/Co-OEC. The Co-OEC acts as the oxidation catalyst and initiates water oxidation reactions for O2 generation utilizing the holes therein. Accordingly, such unique core/shell nanorods integrate both a photocathode and a photoanode. Getting rid of external wires, this architecture is of great significance for improving the energy-conversion efficiency and reducing fabrication costs, since the relevant nanorod arrays can be dispersed in the electrolyte and generate H2 and O2 synchronously without any circuit connection or external bias.

4.2. Longitudinal Heterojunction 1D Nanostructures Rather than core/shell nanostructures that extend components cross-sectionally, longitudinal heterojunction 1D nanostructures contain multiple segments in the longitudinal direction, meaning that each 1D nanostructured unit could serve as an individual photovoltaic device. Such a device would be an advantage as the power supply for microelectronic circuits that do not need a large driving current. To be exciting, the elongated straight pores in AAO template provide an irreplaceable mold for growing the multiple segmental nanostructure arrays. For the inorganic/organic heterostructures, CdS/PPy nanowires show a


strong photodependent rectifying effect and the conducting property of the organic/inorganic PN junction nanowires can be tuned by changing the intensity of the incident light.[58] Meanwhile, the structure still possesses an obvious photovoltaic performance, and a power-conversion efficiency of 0.018% under an illumination intensity of 6.05 mW cm−2 was detected.[59] Yoo et al.[60] employed a Au/CdSe/PPy structure as a segment buried in the porous Au nanowires. The complicated structure gained a 1.1% power-conversion efficiency, almost 100-fold larger than that of the CdS/PPy devices. An energy-conversion efficiency of 0.14% was acquired by using P3HT:PCBM heterojunction composite nanowire.[61] In addition, nanowires with even more segments like Ni/ Au/PEDOT/CdSe:P3HT/Au could be realized by AAO templates,[62] and the current–voltage characterization of a single nanowire exhibited PN diode behavior with a significant photoconductive effect under illumination. Such single nanowire devices maintain a remarkable 280-fold on/off ratio under 70 mW cm−2 excitation at 3.75 V potential, which at that time was comparable to that for the best P3HT/CdSe thin-film photodetectors. Macroscopically, the longitudinal heterojunction 1D nanostructure arrays could be utilized as significant components in large-scale energy-conversion devices. Mubeen et al.[63] reported a scalable design and molecular-level fabrication strategy to create PEDOT/CdSe/Ni/Au/Pt


small 2015, DOI: 10.1002/smll.201500120


Figure 7. a) Schematic of an individual nanowire unit with a semiconductor absorber layer protected inside a nonabsorbing insulating AAO pore. b) Cross-sectional SEM image and high-magnification SEM image of the multiple component nanowires. Reproduced with permission.[63] Copyright 2013, American Chemical Society. c) Artificial photosynthetic performance of the structures. d) SEM images of the multisegmented CdS–Au nanorod arrays. e) Linear sweep voltammogram curves of the multisegmented CdS–Au nanorod arrays with different numbers of segments under simulated AM 1.5 G illumination. f) Measured IPCE (external quanum yield) spectra of the multisegmented CdS–Au and pure CdS nanorod arrays collected at the incident wavelength range of 400 to 800 nm at a potential of 0 V vs Ag/AgCl. Reproduced with permission.[64] Copyright 2014, Wiley-VCH.

heterostructure nanowires, as displayed in Figure 7a,b. Without removing the AAO template, each nanowire was isolated from its neighbor by the transparent electrically insulating oxide cellular enclosure and the architecture could be used as a free-floating device for water splitting. When illuminated by light, the devices were demonstrated to produce hydrogen at a stable rate for over 24 h in corrosive hydroiodic acid electrolyte without applying any external bias (Figure 7c). The quantum efficiency for absorbed photons-to-hydrogen conversion was 7.4% and solar-to-hydrogen energy efficiency of incident light was 0.9%. Figure 7d shows another impressive example proposed by Wang et al.,[64] in which multisegmented CdS–Au nanowire arrays with a sequential and highly tunable configuration were employed specifically as photoanodes. When the multisegmented CdS– Au nanowires were implemented as a photoanode in the PEC cell and a positive bias was applied, a series of forwardbias and reverse-bias Schottky barriers was generated at the Au/CdS and CdS/Au interfaces, respectively. In combination with the surface plasmon effects of each Au nano-unit that could greatly enhance the absorption efficiency of the photoanode, the photocurrent at 0 V vs Ag/AgCl was as high as 10.5 mA cm−2. small 2015, DOI: 10.1002/smll.201500120

5. Heterogeneous 1D Nanostructure Arrays for Energy-Storage Applications 5.1. Core/Shell 1D Nanostructure Arrays for Batteries Among various rechargeable batteries, rechargeable lithium batteries have gained considerable attention and wide application in both portable electronics and electric vehicles, due to their advantageous volumetric and gravimetric energy densities. In the last several decades, the cathode materials have evolved from LiCoO2 to LiFePO4 as well as to more complex compounds like LiCo1/3Ni1/3Mn1/3O2, etc., with the increasing requirements of cost reduction, improving the energy and power density, safe operation, and environmental benignity.[65] The anode materials usually include carbon, silicon, germanium, tin, titanium dioxide, and other transitionmetal oxides.[66] Though extensive efforts have been devoted to enhancing the overall performance from a material aspect, it seems that the specific capacity is approaching the theoretical value and the planar devices still suffer from poor cyclic stability induced by the volume expansion during the lithium uptake/release processes. Additionally, these materials always have a poor electronic conductivity and thus conductive



11

reviews www.MaterialsViews.com Table 2. The heterogeneous 1D electrode arrays based on AAO templates for batteries. Material

Configuration

Cu/Fe3O4[68]

CS cathode of Li-ion

Cu/ Cu2O[71]


[72]

Cu/Ni3Sn4

Synthetic approach

a)

Specific capacity

b)

Two-step ED

0.655 mA h cm

ED and oxygen plasma

−2

(1C)

455 mA h g−1 (1C) −1

≈100% (94 cycles at 1C) ≈100% (100 cycles at 0.8C)


Two-step ED

CS anode of Li-ion

Two-step ED

0.12 mA h cm−2 (0.2C)

70% (100 cycles at 0.2C)

Ni/Si[75]

CS anode of Li-ion

ED and sol-gel

1900 mA h g−1 (0.05 C)

88% (100 cycles at 0.5C)

CNT/MnO2[78]


Annealing and CVDc)

2170 mA h g−1 (50 mA g−1)

30% (16 cycles at 50 mA g−1)

CNT/Au/V2O5[79]


CVD and sputtering

473.7 mA h g−1 (1C)

N/A

Sulfur/CNT

CS cathode of Li-sulfur

CVD and infusion

(0.8C)

≈100% (100 cycles at 8C)

Al/LiCoO2[73]

[84]

500 mA h g

Capacitance retention

−1

1283 mA h g

(0.5C)

≈100% (1000 cycles at 0.5C)

a)

core/shell; b)electrochemical deposition; c)chemical vapor deposition.

carbons and other conductive additives have to be included to reduce the Ohmic loss, implanting another unstable factor into the system. Alternatively, with a good controllability over size, morphology, crystallinity, and chemical composition, 1D nanostructure arrays have been proposed to break the limits imposed by the planar structure. Considering the fact that single-component 1D nanostructure arrays cannot resolve all the problems above, architectures composed of multicomponent 1D nanostructure arrays become promising and each component can be designed to improve specific properties of batteries.[67] In this section, we mainly highlight the recent progress in heterogeneous 1D nanostructure arrays based on AAO templates for battery applications. When the current collector is transformed into a 1D nanowire or nanotube array, the active material can be subsequently coated onto these nanowires or loaded into the nanotubes to form core/shell nanowires or tube-in-tube com-

posite electrodes. Results of selected studies are summarized in Table 2. In Li ion battery applications, copper is commonly used as the current-collecting material for the negative electrode, as the potential of such an electrode is close to 0 V vs. Li/Li+ and there is no significant reaction between lithium and copper. For the positive electrode, aluminium is one of the few metals that does not corrode at potentials close to +4.5 V vs. Li/Li+ and is regarded as a good candidate for the anode in Li-ion batteries. Taberna et al.[68] prepared Cu/Fe3O4 core/shell composite electrodes as the cathode of lithium ion batteries using a two-step electrochemical synthesis process via AAO templating (Figure 8a). They demonstrated that the power density was improved by a factor of six when the electrode featuring composite nanostructures was employed to replace the planar electrodes. The capacity at the 8C rate was 80% of the total capacity and was shown to increase as the loading of magnetite increased, evidencing

Figure 8. a) Cross-sectional views of a Cu nanostructured current collector before (left) and after (right) Fe3O4 deposition. b) Rate capability plots for the five Fe3O4-deposited Cu nanostructured electrodes. Reproduced with permission.[68] Copyright 2006, Nature Publishing Group. c) SEM images of straight Ni/TiO2 nanowire arrays (left) and 3D Ni/TiO2 nanowire networks (right). d) Areal discharging capacity of 3D Ni/TiO2 nanowire networks, straight Ni/TiO2 nanowire arrays, and TiO2-coated plain Ni foil. Reproduced with permission.[77] Copyright 2012, American Chemical Society.



small 2015, DOI: 10.1002/smll.201500120


the merit of 1D nanostructure arrays (Figure 8b). Furthermore, this electrode was able to retain its full capacity after many cycles, even at a high rate. Utilizing similar strategies, other negative materials, such as Sn,[69] Bi,[70] Cu2O[71] and Ni3Sn4 intermetallic,[72] were also deposited on the surface of Cu nanowire arrays to construct core/shell 1D nanostructured negative electrodes. Positive electrodes with core/ shell 1D nanostructure arrays have also attracted extensive interest. The typical structure has been fabricated by spin coating LiCoO2 precursors onto electrodeposited Al nanorod arrays[73] and then annealing at 650 °C or by ALD coating of TiO2 on the identical Al nanorod arrays.[74] All these core/ shell electrodes showed good rate capabilities owing to the intimate contact of the electrode material with the current collector. The conformally deposited LiCoO2 positive electrode illustrated a capacity of around 100 µA h cm−2 and the conformal layer of TiO2 deposited by ALD on Al nanorods indicated that an area gain larger than 10 could be attained during cycling. Moreover, a positive electrode configured by a Ni/Si core/shell nanorod array was prepared.[75] The anode presented a good capability to overcome the severe volume change problem of Si during charging/discharging process and a high specific capacity of 1900 mA h g−1 at 0.05 C was achieved. After 100 cycles at 0.5 C, 88% of the initial capacity (1300 mA h g−1) remained, suggesting a good capacity retention ability. In addition, other strategies were developed to improve the energy and power density of the battery based on 1D nanostructured metal current collectors. Instead of using solid nanorod arrays, a nanoporous Au nanorod array was applied as a current collector by Gowda et al.,[76] which provided an increasing surface area for electrode deposition arising from the porosity of each nanorod, yet keeping ordered spacing between nanorods for the deposition of subsequent electrolyte and electrode layers. The yielded capacity was measured as high as 32 µA h cm−2, even though the charge/discharge process was running up to 75 cycles at a current rate of 0.04 mA cm−2. At high current rates like 0.8 mA cm−2, the device still exhibited a good rate capability. Meanwhile, Ni/ TiO2 nanowire networks could be more advantageous than the straight Ni/TiO2 nanowire array as a battery electrode. A Ni/TiO2 nanowire network was obtained using 3D AAO template-assisted Ni electrochemical deposition followed by ALD coating of TiO2.[77] Due to the interconnected network, areal energy densities were shown to be higher than those from the straight Ni/TiO2 nanowire arrays. The volumetric energy density was maintained even when the thickness of the nanowire network was extended to 32 µm, which was not the case for the 28 µm thick straight nanowire arrays concerning the increased agglomeration as the aspect ratio became larger (Figure 8c,d). Except for collectors consisting of metallic nanowire arrays, carbon nanotubes (CNTs) have also been considered an attractive electrode material in energy-storage devices, due to the outstanding electrical properties, chemical stability, and mechanical strength. Generally, there are several methods that are suitable to fabricate CNTs into core/shell 1D nanostructure arrays. Through the AAO approach combined with simple vacuum infiltration and CVD techniques, highly conducting CNT was attained to construct core/shell 1D small 2015, DOI: 10.1002/smll.201500120

nanotube arrays with a high-capacity metal oxide, MnO2.[78] The nanosized and porous nature of the MnO2 shell allowed fast ion diffusion and the highly conductive CNT core helped an efficient electron transport to the MnO2 shell. Moreover, the CNT can also act as an additional site for lithium ion storage, leading to a dual mechanism of lithium storage and thereby resulting in an improved reversible capacity (a first discharge capacity of 2170 mA h g−1 and a reversible capacity of 500 mA h g−1 after 15 cycles of charge/discharge). Such capacities were an order of magnitude higher than those from the device using MnO2 nanotubes or CNTs solely. Other CNT-based core/shell composites like V2O5/CNT,[79] SnO2/ CNT,[80] Fe2O3/CNT,[81] and Co3O4/CNT[82] have also been investigated extensively. For instance, CNT/Au/V2O5 core/ shell nanorods were tested as lithium battery electrodes by Kim et al.[79] The electrode consisted of a high density of vertically standing core/shell nanorods that were geometrically isolated but electrically connected by the conducting core, resulting in a high capacity (473.7 mA h g−1 at 1C rate) and excellent rate performance (379.2 mA h g−1) at a 10 C rate, due to the facilitated charge transport and improved mechanical stability. Additionally, SnO2/amorphous CNT core/shell nanowire arrays were fabricated by filling the AAO template with SnO2 sol capped by a citric acid chelating agent and followed by drying and annealing treatments.[80b] This core/ shell electrode exhibited a better reversible capacity than the electrode with SnO2 powder after 30 cycles. Moreover, the CNT shell in this structure could not only supply electronic conductive channels for the electrode, but also limit the SnO2 volume expansion upon lithium insertion. Besides Li ion batteries, CNT-based core/shell 1D structures also offer promising features to address the drawbacks of Li–sulfur batteries, such as significant morphological changes of sulfur during the charge/discharge process and the loss of electrical contact between the active materials and current collector. Recently, 1D sulfur/CNT core/shell nanostructures were obtained with the assistance of AAO templates. In those structures, the CNTs could trap the sulfur– polysulfides within the hollow CNTs and reduce the deposition of sulfur on the external carbon surfaces, thus minimizing polysulfide dissolution. Electrochemical testing demonstrated that the sulfur cathode delivered a high initial discharge capacity of around 1400 mA h g−1 with around 75% sulfur loading in the electrode and 1 mg cm−2 of sulfur content. The cycling capacity retention showed a significant improvement, with a reversible capacity of about 730 mA h g−1 after 150 charge/discharge cycles. To reduce the amount of exposed sulfur in the electrode, further work was done by Moon et al.[84] In this configuration, sulfur nanowires were well aligned and completely covered by a minimal amount of carbon (Figure 9a–c). The electrode design in conjunction with the monoclinic crystal sulfur structure addressed all of the aforementioned issues and resulted in an excellent electrical performance: a specific capacity reaching the theoretical value, substantial capacity retention over 1000 cycles, and rate capability with

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