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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 *

In order to fulfi ll the multiple requirements for energy production, storage, and utilization in the future, the conventional planar confi guration of current energy conversion/storage devices has to be reformed, since technological evolution has promoted the effi ciency 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 signifi cant 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 effi cient 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.

1. Introduction ............................................. 2

2. Design Principles for Highly Effi cient 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

From the Contents

small 2015, DOI: 10.1002/smll.201500120

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DOI: 10.1002/smll.201500120

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, GermanyE-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

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. Unfortu-

nately, 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 photo-

voltaic cells and water-splitting cells, have received substan-

tial 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 electro-

lyte by the internal fi eld, 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 port-

able strategy, considering the ineffi cient and costly electric

transportation of pushing electrons across power grids as

well as the expanding market for portable electronics and the

electrifi cation of the transportation sector (e.g., battery-pow-

ered 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 gen-

erated by converting chemical energy via redox reactions at

the anode and cathode. As to supercapacitors, by the orienta-

tion 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 confi gura-

tions, and the corresponding converting/storing capabilities

have approached the theoretical values. To break through

this technological bottleneck, one-dimensional (1D) nano-

structure 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 ] Specifi cally, heterogeneous 1D

nanostructure arrays with multiple components could maxi-

mally meet the requirements to improve the performance

of the energy-converting/-storing devices systematically in

various aspects. [ 8 ] Generally, there are two types of heteroge-

neous 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 usu-

ally referred to as ‘longitudinal heterojunction 1D nanostruc-

ture arrays’.

So far, various techniques have shown up for the fabri-

cation 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 fi lling of these template pores with dif-

ferent materials. Particularly, the anodic aluminium oxide

(AAO) template appears as one of the most widely used

nanotemplates, showing advantages including low-cost acces-

sibility, easy scalability, and high structural controllability, and

has been exploited for synthesizing a diverse range of nano-

structure arrays. [ 9 ]

Although there have been some review articles already

regarding the energy device applications of 1D nanostructure

arrays, [ 7c , 10 ] articles that specifi cally summarize heteroge-

neous 1D nanostructure arrays based on AAO templates are

scarce. In this review, we focus on the efforts to develop effi -

cient energy-conversion/-storage devices employing hetero-

geneous 1D nanostructure arrays based on AAO templates.

First, the designing principles for highly effi cient energy-

related devices using heterogeneous 1D nanostructure arrays

are summarized. The technological progress of AAO tem-

plates for realizing such structures is then briefl y 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 super-

capacitors are included. Finally, perspectives of the related

fi elds are given together with a conclusion.

2. Design Principles for Highly Effi cient 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 nanocon-

fi guration, a large-scale array, and a heterogeneous 1D structure.

First, the nanostructure should be confi ned to one dimen-

sion at the nanoscale, yielding a high aspect ratio, indicating

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a large surface-to-volume ratio and the longitudinal conti-

nuity of the material. The large surface-to-volume ratio ena-

bles the relevant structure to have a huge surface area where

the energy-conversion/-storage reactions could occur exten-

sively and rapidly. The one unconfi ned dimension provides

a continuous passway to effi ciently 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 1 a,

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 con-

fi guration to 1D nanoscale, the photogenerated charge car-

riers need to travel just a short distance to the surface, and

the unconfi ned dimension of the structure can provide suf-

fi cient absorption of the incident light, as well as having the

advantage of an absorption-amplifying effect deriving from

the scattering and antirefl ection of 1D nanostructures. Conse-

quently, the external quantum yield (at 500 nm) of GaP pho-

toelectrochemical (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 diffu-

sion length of the photogenerated charge carriers in semi-

conductors, it doesn’t mean that a thin 1D nanostructure

always results in a higher charge-collection effi ciency. The

cross-section of a very thin 1D nanostructure might be too

short to support the full internal electric fi eld 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 recom-

bination at the contact. Only when the radius is bigger than

the thickness of the depletion layer could the internal fi eld

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-sec-

tional profi le of the nanowire. As displayed in the right curve

of Figure 1 a, 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 bat-

teries 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 mini-

mize specifi c power losses originating from slow ionic trans-

portation, the thickness of the insertion electrodes and the

inter-electrode distance should be as small as possible. How-

ever, when the thickness of the electrodes is reduced, the

specifi c energy and operating time of the device will accord-

ingly become smaller and shorter, leading to a dilemma that

limits further performance improvements for the planar

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 nano-

composites, 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 Technol-

ogy in 2011 as a W2 Professor. His group

currently focuses on template-realized func-

tional nanostructures, 3D nanostructuring

and surface nanopatterning, energy-related

and optoelectronic applications.

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 energy-

conversion and -storage devices.

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 sacrifi cing 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 = ( r 2 / L 2 )( 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 conduc-

tivity 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-nano-

structured electrodes and whether the electrode material is

uniformly utilized during cell charging and discharging pro-

cesses. [ 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

small 2015, DOI: 10.1002/smll.201500120



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 elec-

trodes, which probably results in under-utilization of the elec-

trode 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 dis-

tribution of the nanostructure array is well controlled; scat-

tering and antirefl ection effects arise and incident radiation

could be easily collected, enabling it as an advantageous

component in solar energy conversion devices. [ 15 ] To inves-

tigate it more convincingly, we performed fi nite differ-

ence time domain (FDTD) simulations for a conventional

semiconductor Cu 2 O. As shown in the insets of Figure 1 b,

when illuminated by photons at 500 nm, the electric-fi eld dis-

tribution around the planar Cu 2 O fi lm (thickness: 1 µm) is

concentrated on the surface. As to the Cu 2 O nanorod arrays,

the electric fi eld distributing scope is greatly enlarged and a

high electric fi eld surrounds the entire nanowire arrays. As a

result, the Cu 2 O nanorod array exhibits a dramatic improve-

ment in absorption capability in comparison with the Cu 2 O

fi lm 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 1 b.

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.

small 2015, DOI: 10.1002/smll.201500120

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 fi ve different high-aspect-ratio morphologies all modeled with a uniform doping density of 2 × 10 16 cm −3 and an interfacial equilibrium barrier height of 1 eV. For I, radius r 0 = 300 nm. For II, r 0 = 50 nm. For III, IV, and V, r 0 , bottom = 300 nm and r 0 , top = 150, 100, and 50 nm, respectively. Reproduced with permission. [ 12 ] Copyright 2012, Royal Society of Chemistry. b) FDTD-simulated absorptance spectrum of Cu 2 O nanorod arrays with length L = 600 nm, diameter d = 100 nm on the Cu 2 O substrate with thickness of 400 nm. The absorptance spectrum of a Cu 2 O thin fi lm with thickness of 1 µm is included as a reference. The inset shows the comparison of FDTD-simulated spatial distribution of the electric-fi eld intensity between the planar and 1D nanostructured Cu 2 O in a cross-sectional view. (Left inset: planar Cu 2 O; right inset: Cu 2 O 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 fi eld intensity (bottom) of Cu 2 O nanorod arrays ( L = 600 nm; d = 100 nm), Cu 2 O/InP longitudinal heterojunction nanorod arrays (Cu 2 O at the bottom: L = 300 nm; d = 100 nm; InP on the top: L = 300 nm; d = 100 nm) and Cu 2 O/InP core/shell nanorod arrays (Cu 2 O 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.



The ideal architecture for electrochemical energy-storing

devices is proposed as two sets of interdigitated 1D nanowire

arrays in Figure 1 c. [ 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 addi-

tion, the distance for ion transportation in discharging is dra-

matically reduced by adopting interdigitated 1D nanowires,

and such a structure is signifi cantly less susceptible to Ohmic

losses and other transport limitations. Specifi cally, highly

ordered 1D nanostructure arrays are more performance-

repeatable and more compatible with the adjacent elements

as compared with the irregular counterparts.

Third, according to specifi c requirements, the nanostruc-

tures 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 benefi t for amplifying the absorption capability

of the entire device. As shown in Figure 1 d, we have simu-

lated the absorbing capabilities for Cu 2 O/InP 1D nanowire

arrays with core/shell and longitudinal heterojunction mor-

phologies. Both heterogeneous structures present an opti-

mized electric fi eld distribution at 650 nm and hence an

outstanding enhancement in absorbance, particularly in the

region beyond the absorption limit of Cu 2 O. The absorption

range can be extended to the NIR region with more than

70% of the solar spectrum covered. The internal fi eld at the

interface of Cu 2 O/InP is also profi table for separating the

photogenerated charge carriers and reducing the possibility

of recombination. In electrochemical energy-storage applica-

tions, electrodes are usually constructed by coating the inser-

tion electrodes onto metallic 1D nanostructure arrays. The

metallic core could transport the electric energy effi ciently

and supply a large area for the insertion electrodes.

Based on the above recognitions, three key features are

crucial for realizing highly effi cient energy-conversion and

-storage devices: i) a 1D nano-confi guration: its large surface

area greatly facilitates energy-conversion/-storage reactions,

and the elongated longitudinal dimension provides an effi -

cient passway for transferring charge carriers or ions. ii) The

interaction and/or collecting behavior (e.g., optical scattering

and antirefl ection) 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

scientifi c fashion originates from the seminal work of Masuda

and co-workers, who reported self-ordered AAO templates in

1995. [ 16 ] By choosing appropriate anodization conditions, self-

ordered AAO templates with tunable pore diameters of about

10–400 nm and densities in the range 10 8 –10 10 pores cm −2

can be prepared. In combination with an imprinting pro-

cess, long-range ordering of AAO templates with tunable

pore distributions and profi les 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 ] nanow-

ires, [ 19 ] nanotubes, [ 20 ] and nanopores. [ 16,21 ] Such nanostructure

arrays have already been applied in various fi elds 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 tem-

plates with periodically modulated nanopore diameters along

the pore axes, providing another opportunity for tailoring the

morphological parameters of the as-fabricated 1D nanostruc-

ture arrays. [ 22 ] More importantly, AAO templates are also

very effi cient 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 var-

ious 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 ] Specifi cally, ALD has drawn sub-

stantial attention as a versatile methodology for thin-fi lm

deposition, attributed to the conformal and uniform deposi-

tion of thin fi lms 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

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 infi ltrated in shell nanotubes), III) type C core/shell (nanowires embedded in fi lm), IV) longitudinal heterojunction nanowires.



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 fi lms, conventional techniques for thin-fi lm

preparation qualify to cover 1D nanostructure arrays with a

thin fi lm (Figure 2 , type III). Of course, core/shell architec-

tures are not limited to two components and we can adjust

the number of layers freely according to specifi c applications.

For the growth of longitudinal heterojunction 1D nano-

structures, electrochemical deposition is a popular approach.

In this respect, most of the materials (e.g. , metals, [ 30 ] semi-

conductors [ 31 ] and conducting polymers [ 32 ] ) that could be syn-

thesized electrochemically are able to be deposited into the

template. In a typical process for synthesizing longitudinal

heterojunction nanowires, a thin metal fi lm 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 hetero-

junction 1D nanostructures can be conveniently prepared by

this approach. The interface between multiple components

and the morphological parameters could also be easily engi-

neered during the deposition (Figure 2 , type IV), qualifying

it as a decent methodology to fabricate complicated longitu-

dinal 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 nanow-

ires without evaporation of a thin metal fi lm 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 antirefl ection effect in single-component 1D

nanostructures, core/shell 1D nanostructures can integrate

the advantages of all components. For semiconductor/semi-

conductor 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 nanostruc-

tured 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 benefi cial 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 struc-

tures have already shown merit in improving the overall effi -

ciency. 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 effi ciency over 20%, with minimal short-circuit cur-

rent dependence on the bulk minority carrier diffusion

length and thus the effi cient 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 3 a). After partly removing the AAO, another CVD

process was conducted for growing the CdTe shell. By manip-

ulating the morphological parameters, the solar energy con-

version effi ciency obtained was 6%, displayed in Figure 3 b,c.

The authors also transferred the device to a fl exible substrate,

which is noteworthy. Though the effi ciency was lowered to

4%, the fl exible device could sustain large bending without

degradation in structure and performance (Figure 3 d). Liu

et al. [ 37 ] combined an electrochemical deposition route with

thermal evaporation to grow CdS/CdTe core/shell struc-

tures for photovoltaic applications and the best performance

of the relevant devices was close to 6.5%. Afterwards, the

small 2015, DOI: 10.1002/smll.201500120

Table 1. The development of heterogeneous solar cells based on AAO templates.

Material Confi guration Synthetic approach

Effi ciency [%]

Material Confi guration Synthetic approach

Effi ciency [%]

CdS/CdTe [36] CS a) PN junction CVD b) and TE c) 6 P3HT/C60 [46] CS polymer

junction

Annealing and

spin coating

1.12

CdS/CdTe [38] CS PN junction ED d) 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

TiO 2 /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

TiO 2 / P3OT [43] CS hybrid PN

junction

ED and

spin-coating

1.06 ITO/TiO 2 [51] CS photoanode of

DSSCe)

ALDf) 1.1

CdSe/P3HT [44] CS hybrid PN

junction

ED and

spin-coating

1.38 Au/TiO 2 [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.



same group developed a modifi ed strategy, in which the CdS

nanowires and the CdS/CdSe PN junctions were further

treated by multistep CdCl 2 soaking and annealing proce-

dures. [ 38 ] With graphite paste as the electrode, the device pre-

sented a power-conversion effi ciency of 9.8%, which was the

highest value for nanowire-based solar cells. The above struc-

tures are based on the deposition of active materials into

AAO templates to form 1D nanostructures like nanorods

and nanotubes, however, through manipulation of the ano-

dizing 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/a-

Si:H/ITO structures for solar-energy conversion. Due to the

purposely prepared antirefl ection structure, the absorption

effi ciency was improved and thus a high energy-conversion

effi ciency of 7.6% was attained.

Regarding the semiconductor/polymer hybrid solar

cells, semiconductor 1D nanostructure arrays are usually

embedded in polymer fi lms. As a renowned electron acceptor,

TiO 2 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 effi -

ciency of the relevant device was around 0.51%, almost fi ve-f

old larger than the device using planar TiO 2 . Alternatively,

TiO 2 nanotube arrays were synthesized by the technique inte-

grating 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 nano-

structures. Attributed to the large bandgap of TiO 2 (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

effi ciency obtained was 1.06%. [ 43 ] An improvement for this

structure was achieved by building CdSe/P3HT core/shell

nanorod arrays, and an effi ciency 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 effi ciency 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 4 a). The chain alignment in

the P3HT nanorods could facilitate the charge transfer in

small 2015, DOI: 10.1002/smll.201500120

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 effi ciency of the solar cells as a function of the embedded nanopillar height. d) Performance characterization of a fl exible solar cell based on a PDMS substrate. The inset shows a picture of the set-up for bending the fl exible modules. Reproduced with permission. [ 36 ] Copyright 2009, Nature Publishing Group.



each nanorod (Figure 4 b,c). Compared with planar devices,

the resultant P3HT nanorod/C60 polymer solar cells showed

a great improvement in energy-conversion effi ciency up to

1.12%. Actually, C60 is not a good candidate as the elec-

tron 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 effi ciency 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 con-

ditions. 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 effi -

ciency 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 con-

structing the cross-linkable fullerene material [6,6]-phenyl-

C61-butyric styryl dendron ester (PCBSD) nanorod arrays

to collaborate with P3HT:IBCA (indene–C60 bisadduct), as

shown in Figure 4 d. [ 50 ] This novel structure integrates the

advantages of both bulk heterojunction and core/shell 1D

nanostructure, enabling the relevant devices with an unprece-

dented overall effi ciency of 7.3% in the area of polymer solar

cells (Figure 4 e).

The huge surface area and effi cient charge conduc-

tion in 1D nanostructure arrays also provide a good plat-

form 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/

TiO 2 core/shell structure as the photoanode, where AAO

behaved as the scaffold, ITO nanoshell arrays were used as

the electron collectors, and TiO 2 was the active material for

PEC reactions. In this design, electrons can diffuse radially

through the walls of semiconducting tubes and be effi ciently

collected by the adjacent and concentric ITO tubes. The

yielded conversion effi ciency 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 effi ciency was gauged as 5.4% by soaking the

relevant Au/TiO 2 core/shell photoanode in TiCl 4 solution.

small 2015, DOI: 10.1002/smll.201500120

Figure 4. a) SEM images of P3HT nanorods prepared using the AAO template, after removal of the Al/Al 2 O 3 layer. b) Out-of-plane grazing incidence angle X-ray diffraction intensities as a function of scattering angle 2 θ for planar P3HT fi lms. c) Schematic representation of P3HT chain conformation in nanorods and thin fi lms. 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.



Maijenburg et al. [ 53 ] adopted sol–gel and electrodeposition

methods for synthesizing Ag/TiO 2 isolated core/shell nanow-

ires and applied it as the photoanode for water splitting. It

was demonstrated that the as-prepared device showed a

higher effi ciency than bare TiO 2 nanotubes and TiO 2 nano-

tubes with attached Ag nanoparticles. A high H 2 generation

rate of 1.23 × 10 −3 mol g −1 h −1 was achieved. In considera-

tion of the large bandgap of conventional photoanode mate-

rials like ZnO and TiO 2 , 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 TiO 2 , forming a Schottky metal/semi-

conductor 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 5 a–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-Fe 2 O 3 core/shell arrays on this peculiar architecture

(Figure 5 d). The absorption effi ciency 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 5 e,f). Mean-

while, the short hole diffusion length in α-Fe 2 O 3 cannot limit

the active layer thickness by this design.

As to photocathodes in water splitting, core/shell 1D

nanostructure arrays still exhibit impressive merits. Huang

et al. [ 56 ] fabricated photocathodes with a complex Cu 2 O/

CuO/TiO 2 core/shell structure based on AAO techniques.

In this case, the Cu 2 O suffers from signifi cant 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 Cu 2 O nanowires with protecting

layers of CuO and TiO 2 . The photocathodes with Cu 2 O/CuO/

TiO 2 were found to gain a 74% higher photocurrent and

4.5-times higher stability compared to that of a bare Cu 2 O

nanowires array.

Extending from the concept of Ag/TiO 2 isolated core/

shell nanowires, [ 53 ] an alternative architecture of core/shell

nanostructures, a core and a shell with discontinuously dis-

tributed 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 6 a–c, Au

nanorods prepared using AAO templates have a purposely

deposited shell consisting of a thin layer of Pt-decorated TiO 2

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 TiO 2 . The Pt nanoparticles on the sur-

face of TiO 2 facilitate the injected electrons transferring to

the interface with the electrolyte to drive the water-reduction

reactions therein for H 2 generation, producing 5 × 10 13 H 2

molecules cm −2 s −1 under 1 sun illumination, with long-term

small 2015, DOI: 10.1002/smll.201500120

Figure 5. a) Schematic of core/shell 1D nanostructures of Au/TiO 2 /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 H 2 . c) Faradaic effi ciency of the process. Reproduced with permission. [ 54 ] Copyright 2012, American Chemical Society. d) SEM image of the Ti/Pt/FTO/Fe 2 O 3 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.



operational stability (Figure 6 d–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 O 2 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 signifi cance

for improving the energy-conversion effi ciency and reducing

fabrication costs, since the relevant nanorod arrays can be

dispersed in the electrolyte and generate H 2 and O 2 synchro-

nously without any circuit connection or external bias.

4.2. Longitudinal Heterojunction 1D Nanostructures

Rather than core/shell nanostructures that extend com-

ponents cross-sectionally, longitudinal heterojunction 1D

nanostructures contain multiple segments in the longi-

tudinal 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 inor-

ganic/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 photo-

voltaic performance, and a power-conversion effi ciency 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 com-

plicated structure gained a 1.1% power-conversion effi ciency,

almost 100-fold larger than that of the CdS/PPy devices.

An energy-conversion effi ciency 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 signifi -

cant 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-fi lm photodetectors.

Macroscopically, the longitudinal heterojunction 1D

nanostructure arrays could be utilized as signifi cant com-

ponents 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 6. a) Schematic of a cross-section of an individual photosynthetic unit. b) The corresponding transmission electron micrograph (TEM). c) Magnifi ed TEM views of the platinum/TiO 2 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 O 2 and H 2 photoproducts as a function of time. Reproduced with permission. [ 57 ] Copyright 2013, Nature Publishing Group.



heterostructure nanowires, as displayed in Figure 7 a,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-fl oating device for water splitting. When

illuminated by light, the devices were demonstrated to pro-

duce hydrogen at a stable rate for over 24 h in corrosive

hydroiodic acid electrolyte without applying any external

bias (Figure 7 c). The quantum effi ciency for absorbed photo-

ns-to-hydrogen conversion was 7.4% and solar-to-hydrogen

energy effi ciency of incident light was 0.9%. Figure 7 d shows

another impressive example proposed by Wang et al., [ 64 ]

in which multisegmented CdS–Au nanowire arrays with a

sequential and highly tunable confi guration were employed

specifi cally 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 forward-

bias 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 effi ciency of the photo-

anode, the photocurrent at 0 V vs Ag/AgCl was as high as

10.5 mA cm −2 .

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 appli-

cation 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 LiCoO 2 to LiFePO 4 as well as to more

complex compounds like LiCo 1/3 Ni 1/3 Mn 1/3 O 2 , 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, sil-

icon, germanium, tin, titanium dioxide, and other transition-

metal oxides. [ 66 ] Though extensive efforts have been devoted

to enhancing the overall performance from a material aspect,

it seems that the specifi c capacity is approaching the theoret-

ical 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

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-magnifi cation SEM image of the multiple component nanowires. Reproduced with permission. [ 63 ] Copyright 2013, American Chemical Society. c) Artifi cial 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.



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 multicom-

ponent 1D nanostructure arrays become promising and each

component can be designed to improve specifi c 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 sub-

sequently 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 com-

monly used as the current-collecting material for the nega-

tive electrode, as the potential of such an electrode is close

to 0 V vs. Li/Li + and there is no signifi cant 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/Fe 3 O 4 core/shell composite electrodes as the cathode of

lithium ion batteries using a two-step electrochemical syn-

thesis process via AAO templating ( Figure 8 a). They dem-

onstrated 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

small 2015, DOI: 10.1002/smll.201500120

Figure 8. a) Cross-sectional views of a Cu nanostructured current collector before (left) and after (right) Fe 3 O 4 deposition. b) Rate capability plots for the fi ve Fe 3 O 4 -deposited Cu nanostructured electrodes. Reproduced with permission. [ 68 ] Copyright 2006, Nature Publishing Group. c) SEM images of straight Ni/TiO 2 nanowire arrays (left) and 3D Ni/TiO 2 nanowire networks (right). d) Areal discharging capacity of 3D Ni/TiO 2 nanowire networks, straight Ni/TiO 2 nanowire arrays, and TiO 2 -coated plain Ni foil. Reproduced with permission. [ 77 ] Copyright 2012, American Chemical Society.

Table 2. The heterogeneous 1D electrode arrays based on AAO templates for batteries.

Material Confi guration Synthetic approach Specifi c capacity Capacitance retention

Cu/Fe 3 O 4 [68] CS a) cathode of Li-ion Two-step ED b) 0.655 mA h cm −2 (1C) ≈100% (100 cycles at 8C)

Cu/ Cu 2 O [71] CS cathode of Li-ion ED and oxygen plasma 455 mA h g −1 (1C) ≈100% (94 cycles at 1C)

Cu/Ni 3 Sn 4 [72] CS cathode of Li-ion Two-step ED 500 mA h g −1 (0.8C) ≈100% (100 cycles at 0.8C)

Al/LiCoO 2 [73] 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/MnO 2 [78] CS cathode of Li-ion Annealing and CVD c) 2170 mA h g −1 (50 mA g −1 ) 30% (16 cycles at 50 mA g −1 )

CNT/Au/V 2 O 5 [79] CS cathode of Li-ion CVD and sputtering 473.7 mA h g −1 (1C) N/A

Sulfur/CNT [84] CS cathode of Li-sulfur CVD and infusion 1283 mA h g −1 (0.5C) ≈100% (1000 cycles at 0.5C)

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



the merit of 1D nanostructure arrays (Figure 8 b). Further-

more, 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 ] Cu 2 O [ 71 ] and

Ni 3 Sn 4 intermetallic, [ 72 ] were also deposited on the surface

of Cu nanowire arrays to construct core/shell 1D nanostruc-

tured negative electrodes. Positive electrodes with core/

shell 1D nanostructure arrays have also attracted extensive

interest. The typical structure has been fabricated by spin

coating LiCoO 2 precursors onto electrodeposited Al nanorod

arrays [ 73 ] and then annealing at 650 °C or by ALD coating

of TiO 2 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 LiCoO 2 positive elec-

trode illustrated a capacity of around 100 µA h cm −2 and the

conformal layer of TiO 2 deposited by ALD on Al nanorods

indicated that an area gain larger than 10 could be attained

during cycling. Moreover, a positive electrode confi gured 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 specifi c 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 reten-

tion 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 electro-

lyte 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/

TiO 2 nanowire networks could be more advantageous than

the straight Ni/TiO 2 nanowire array as a battery electrode. A

Ni/TiO 2 nanowire network was obtained using 3D AAO tem-

plate-assisted Ni electrochemical deposition followed by ALD

coating of TiO 2 . [ 77 ] Due to the interconnected network, areal

energy densities were shown to be higher than those from the

straight Ni/TiO 2 nanowire arrays. The volumetric energy den-

sity 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 8 c,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 nano-

structure arrays. Through the AAO approach combined

with simple vacuum infi ltration and CVD techniques, highly

conducting CNT was attained to construct core/shell 1D

nanotube arrays with a high-capacity metal oxide, MnO 2 . [ 78 ]

The nanosized and porous nature of the MnO 2 shell allowed

fast ion diffusion and the highly conductive CNT core helped

an effi cient electron transport to the MnO 2 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 fi rst

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 MnO 2 nanotubes or CNTs solely. Other

CNT-based core/shell composites like V 2 O 5 /CNT, [ 79 ] SnO 2 /

CNT, [ 80 ] Fe 2 O 3 /CNT, [ 81 ] and Co 3 O 4 /CNT [ 82 ] have also been

investigated extensively. For instance, CNT/Au/V 2 O 5 core/

shell nanorods were tested as lithium battery electrodes by

Kim et al. [ 79 ] The electrode consisted of a high density of ver-

tically 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 mechan-

ical stability. Additionally, SnO 2 /amorphous CNT core/shell

nanowire arrays were fabricated by fi lling the AAO template

with SnO 2 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 SnO 2 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 SnO 2

volume expansion upon lithium insertion.

Besides Li ion batteries, CNT-based core/shell 1D struc-

tures also offer promising features to address the drawbacks

of Li–sulfur batteries, such as signifi cant 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–

polysulfi des within the hollow CNTs and reduce the deposi-

tion of sulfur on the external carbon surfaces, thus minimizing

polysulfi de 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 signifi cant improve-

ment, 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 confi guration, sulfur nanowires were

well aligned and completely covered by a minimal amount

of carbon ( Figure 9 a–c). The electrode design in conjunction

with the monoclinic crystal sulfur structure addressed all of

the aforementioned issues and resulted in an excellent elec-

trical performance: a specifi c capacity reaching the theoret-

ical value, substantial capacity retention over 1000 cycles, and

rate capability with <1 min discharge time were realized. It is

to be noted that this battery performance was attained under

the highest sulfur content to date: about 81 wt% in the active

material (Figure 9 d).

small 2015, DOI: 10.1002/smll.201500120



Though the above-mentioned core/shell 1D nanostruc-

ture arrays could be used to build highly effi cient electrodes

for batteries due to an enlarged energy per unit area (elec-

trode surface area) without compromising the rate capa-

bility, one challenge in the research associated with this

design resides in obtaining complete control over the spacing

between the 1D nanostructures to accommodate the subse-

quent electrolyte and electrode layers. One feasible attempt

was made by drop-coating a conformal layer of 25 nm thick

poly(methyl methacralate) (PMMA) (separator/gel electro-

lyte) on 1D nanostructured Ni–Sn electrode. [ 85 ] In this case,

the nanostructured confi guration of the gel electrolyte led

to a two-orders-of-magnitude improvement in the reversible

discharge capacity, compared to its planar counterpart. Sub-

sequently, the authors proposed an integrated electrochem-

ical energy-storage device, in which both anode (Ni–Sn) and

cathode (polyaniline, PANI) were fabricated into nanowire

arrays and the two electrodes were packaged by wrapping

a conformal polyethylene oxide (PEO) polymer on the two

kinds of nanowires as the separator (Figure 9 e–h). [ 86 ] Such a

device was built as a Li-free electrode but still showed good

charge/discharge characteristics with a discharge capacity of

3 µA h cm −2 at a current rate of 0.03 mA cm −2 , meaningful to

better understand Li ion electrochemistry on the nanoscale.

5.2. Core/Shell 1D Nanostructure Arrays for Supercapacitors

Electrochemical supercapacitors (ES), owing to their high

power density and long cycle life, play important roles in

various energy-storage devices and have been applied par-

ticularly in hybrid vehicles and backup power supplies. Con-

sidering the constraints from low energy densities in the

current supercapacitors, it is extremely crucial to develop

supercapacitors with high energy densities. [ 87 ] Generally,

there are two types of ES in development. One is the elec-

trical double-layer supercapacitor (EDLS), in which the

electrode material (like carbon) is electrochemically inert.

In this scenario, there are no electrochemical reactions hap-

pening on the electrode material during the charging and

discharging processes, and pure physical charge accumulation

occurs at the electrode/electrolyte interface. The other type

is the faradaic supercapacitor (FS), in which the electrode

materials (e.g., metal oxides and electronically conducting

polymers) are electrochemically active. These electrode

materials can directly store and release charges during the

charging and discharging processes. For both types, confi g-

uring the electrodes into 1D nanostructure arrays is currently

considered one of the most promising plans. In this section,

thus, we mainly concentrate on the recent progresses in core/

shell 1D nanostructure arrays based on AAO templates for

supercapacitor applications.

The general procedures for fabricating 1D nanostructured

electrodes for supercapacitors are similar to that for the elec-

trodes of batteries, and a series of core/shell nanostructure

arrays, like Ni/V 2 O 5 , [ 88 ] MnO 2 /PEDOT, [ 89 ] RuO 2 /PEDOT, [ 90 ]

Pt/RuO 2 , [ 91 ] Au-MnO 2 /CNT, [ 92 ] MnO 2 /TiN, [ 93 ] and Ni/

NiO [ 94 ] was realized. Results of selected studies are listed in

Table 3 . Typically, MnO 2 /PEDOT core/shell nanowire arrays

were synthesized by a one-step electrochemical deposition

small 2015, DOI: 10.1002/smll.201500120

Figure 9. a) The fi nal electrode structure after removal of the membrane by a wet etching step, alongside a schematic of each S/C core/shell nanowire with the dimensions of the key components denoted. b) SEM images of the S/C core/shell nanowire array. c) An HRTEM image of a single S/C core/shell nanowire showing its dimensions and thin carbon layer. d) Capacity retention with a discharge rate of 5C and a charge rate of 2C for 1000 cycles. Reproduced with permission. [ 84 ] Copyright 2013 Wiley-VCH. e) Schematic representation of alumina template-based nanowire energy-storage device. f) TEM image of the anode/electrolyte (Ni-Sn/PEO) core/shell nanowire. g) Schematic representation of the nanowire energy storage device. ih) TEM image of the cathode/electrolyte (PANI/PEO) core/shell nanowire. Reproduced with permission. [ 86 ] Copyright 2011, American Chemical Society.



process using AAO templates, as portrayed in Figure 10 a. [ 89a ]

The composition of the nanowires was able to be adjusted

by the different deposition potentials (Figure 10 b). In this

architecture, the MnO 2 core was utilized for achieving a high

energy density and the PEDOT shell was employed for its

highly conductive, porous, and fl exible nature. As a conse-

quence, the corresponding devices yielded high specifi c capac-

itances (180 F g −1 ) at high current densities (25 mA cm −2 ),

much higher than the devices containing pure MnO 2 nanow-

ires, PEDOT nanowires, and MnO 2 thin fi lms. Afterwards,

by carefully tuning the electrochemical deposition potential,

it was found that the MnO 2 /PEDOT core/shell nanowires

small 2015, DOI: 10.1002/smll.201500120

Table 3. Heterogeneous 1D electrode arrays based on AAO templates for supercapacitors.

Material Synthetic approach Specifi c capacity [F g −1 ] Capacitance retention Energy and Power densities [Wh kg −1 ],[kW kg −1 ]

MnO 2 /PEDOT [89a] One-step ED a) 180 (25 mA cm −2 ) N/A N/A

RuO 2 / PEDOT [90] Two-step ED 1217 (5 mA cm −2 ) N/A 28 and 20

Pt/RuO 2 [91] Two-step ED 1585 (2 mV s −1 ) ≈100% (200 cycles at

25 mV s −1 )

N/A

Au-MnO 2 /CNT [92] CVD b) and ED 68 (6.6 A g −1 ) ≈100% (1000 cycles at

6.6 A g −1 )

4.5 and 33 d)

MnO 2 /TiN [93] ALD c) and ED 662 (45 A g −1 ) 80% (350 cycles at 15 A g −1 ) N/A

Ni/NiO [94] Two-step ED 179 (10 mV s −1 ) 80% (2000 cycles at

2 mA cm −2 )

N/A

Pt/MnO 2 [95] ALD and ED 810 (10 mV s −1 ) ≈100% (8000 cycles at

20 A g −1 )

61 and 56

SnO 2 /MnO 2 [96] ALD and ED 910 (1 A g −1 ) 80% (2600 cycles at 5 A g −1 ) N/A

SnO 2 /ppy-SnO 2 /MnO 2 [97] ALD and ED 260 and 910 (1 A g −1 ) 80% (2000 cycles at 5 A g −1 ) 27.2 and 24.8 d)

Au/MnO x [98] Sputtering and ED 840.3 (1 A g −1 ) N/A 46.8 and 11.83 d)

Ni/MnO 2 [99] Two-step ED 672 (2 mV s −1 ) 83% (3000 cycles at 5 mV s −1 ) N/A

a) electrochemical deposition; b) chemical vapor deposition; c) atomic layer deposition; d) The energy is calculated based on the total weight of two electrodes, while the others are only based on

the weight of a single electrode.

Figure 10. a) One-step synthesis of MnO 2 /PEDOT core/shell nanowires. b) PEDOT shell thickness variation with applied potential from 0.6 to 0.9 V. Reproduced with permission. [ 89a ] Copyright 2008, American Chemical Society. c) SEM images of Au segmented CNT/MnO 2 (left) and CNT/MnO 2 core/shell structures (right). d) Specifi c capacitance versus cycle number plots of supercapacitors with symmetric assembly of CNT/MnO 2 and Au–CNT/MnO 2 electrodes. Reproduced with permission. [ 92 ] Copyright 2010, American Chemical Society.


16 www.small-journal.com © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2015, DOI: 10.1002/smll.201500120

can be prepared between 0.6 and 0.9 V. An optimal spe-

cifi c capacitance of 270 F g −1 was obtained from the MnO 2 /

PEDOT core/shell nanowires that prepared from 0.7 V. [ 89c ] In

other efforts, the same authors loaded fi nely dispersed MnO 2

nanoparticles into PEDOT nanowires by simply soaking

them in a potassium permanganate solution, which yielded

an enhancement of up to 4 times in the energy storage

capacity and caused only a minimal volume expansion in the

polymer. [ 89c ] Alternatively, RuO 2 /PEDOT core/shell nano-

tube arrays were approved as a good example. Such nanotu-

bular composites were synthesized by a step-wise method. [ 90 ]

The PEDOT nanotubes were grown in an AAO template

by electropolymerization and then RuO 2 was deposited into

this porous PEDOT nanotube matrix electrochemically. The

relevant electrode presented a specifi c capacitance of up

to 640 F g −1 even at a high power demand of 20 kW kg −1 .

In other attempts, the multisegmented Au–MnO 2 /CNT

core/shell arrays were fabricated in AAO templates using a

method integrating electrochemical deposition, infi ltration,

and CVD. [ 92 ] In this structure, CNTs served as an additive

for improving the electrical conductivity of the manganese

oxide electrodes and could also be a host for charge storage

due to the active electrode characteristics. The well-adhered

interface between Au and MnO 2 /CNT core/shell segments

led to nanoscale electrical contacts between the electrode

and current collectors, favorable for reducing the Ohmic

loss (Figure 10 c). Noteworthily, integrating CNTs and metal

oxide nanowires, the electrode possessed a dual storage

mechanism of EDLC and FS and exhibited a maximum spe-

cifi c capacitance of 68 F g −1 , a power density of 33 kW kg −1 ,

and an energy density of 4.5 W h kg −1 (Figure 10 d).

In order to accomplish an ideally nanostructured elec-

trode and ameliorate the capacitance approaching the theo-

retical value, several efforts were also made by our group.

Based on AAO nanotechnology, a binder-free and highly

fl exible supercapacitor featuring a Pt/MnO 2 core/shell nano-

tube array was fabricated by the assistance of ALD and elec-

trochemical deposition processes ( Figure 11 a,b). [ 95 ] In this

peculiar process, the length, thickness and inter-tube volume

of the nanotubes were able to be precisely manipulated.

By optimizing the morphological parameters, a remarkable

high gravimetric and areal specifi c capacitance (810 F g −1

and 75 mF cm −2 at 5 mV s −1 ) as well as an excellent rate

capability (68% capacitance retention from 2 to 100 A g −1 )

were realized. In addition, a negligible capacitance loss was

observed after 8000 random charge/discharge cycles from 2 to

100 A g −1 (Figure 11 c). Such good results were attributed to

the high accessible surface area, a suitable inter-tube volume

for fast and reversible Faradic reactions, and short ion diffu-

sion paths of the well-defi ned Pt nanotube arrays. A similar

strategy was employed for fabricating open-end and closed-

end SnO 2 /MnO 2 core/shell nanotube arrays, where the SnO 2

nanotubes served as current collectors. [ 96 ] The extensive data

demonstrated that the open-end electrode presented a higher

specifi c capacitance than the closed-end electrode, owing to

the large available surface area of the open-ended electrode

(Figure 11 d–f). Afterwards, a complete asymmetric superca-

pacitor with a high operating voltage window of 1.7 V was

prepared on the basis of two separate core/shell nanotube

arrays, PPy/SnO 2 and MnO 2 /SnO 2 , which were used as nega-

tive and positive electrodes, respectively. The as-fabricated

device exhibited a high specifi c energy of 27.2 Wh kg −1 and a

high specifi c power of 24.8 kW kg −1 (Figure 11 g–i). [ 97 ] Mean-

while, a Au/MnO x core/shell nanocone array was tried by

Qiu et al. [ 98 ] The Au/MnO x nanocone array demonstrated an

outstanding specifi c mass (areal) capacitance of 840.3 F g −1

(88.2 mF cm −2 ) at a current density of 2 A g −1 . In addition,

asymmetric supercapacitors adopting the Au/MnO x nano-

cone array as the positive electrode and a carbon-based

material as the negative electrode could achieve a capaci-

tance of 108.5 F g −1 at a current density of 1 A g −1 , corre-

sponding to an energy density of 46.8 Wh kg −1 at a power

density of 0.72 kW kg −1 .

Though many efforts were made to increase the length of

the 1D nanostructures to load more active materials, practi-

cally, the length cannot be prolonged infi nitely. As the length

of 1D nanostructures increases, agglomeration of the 1D

nanostructures becomes more serious and this will jeopardize

the performance and stability of electrodes. On the other

hand, the AAO template can sustain the depth of the pore

to a large scale. Thus, it is wise to duplicate such structures

from insulative AAO templates to other functional materials.

Recently, our group copied the insulative AAO template to

a Ni template precisely through two replication processes. [ 99 ]

In the fi rst step, an ordered array of PMMA nanopillars was

obtained after removing the AAO template that acted as

the initial template. Then the PMMA nanopillar array was

adopted as the template for the electrochemical deposition

of a Ni nanopore array with a gold fi lm as the working elec-

trode in the second step ( Figure 12 a). The resulting material

was used to form a Ni/MnO 2 core/shell electrode for superca-

pacitor (Figure 12 b,c). Accordingly, the electrode presented

a symmetrically rectangular shape of CV curves without any

signifi cant distortion, even when a scan rate of 1000 mV s −1

was applied (Figure 12 b). The specifi c capacitance was meas-

ured as 672 F g −1 at 2 mV s −1 with a MnO 2 mass loading of

80 µg cm −2 and it could still reach up to 382 F g −1 at 2 mV s −1

when the loading was increased to 400 µg cm −2 (Figure 12 d–f).

Moreover, such a metallic nanopore array with tunable struc-

tural parameters can be further exploited as an alternative

1D nanostructure array for various other applications, such

as electrodes for batteries, water-splitting devices, and so on.

Apart from electrochemical supercapacitors, the AAO

template also has great potential for fabricating conven-

tional metal/insulator/metal (MIM) electrostatic capacitors.

One of the characteristic features for electrostatic capacitors

is a high power density, which is largely limited by surface

charge. To achieve a dense packing of the active interfaces

over a large surface area, 1D nanostructure arrays that offer

a huge internal surface turn out to be a good choice. Using

CVD, composite MIM nanotubes with alternating conduc-

tive (carbon) and insulating (boron nitride) layers were cre-

ated within 200 nm AAO templates by Shelimov et al. [ 100 ]

For 50 µm-thick templates, specifi c capacitances as high as

2.5 µF cm −2 were attained. Additionally, Al/Al 2 O 3 barrier

layer/CNT MIM capacitors were fabricated by growing CNTs

through a thermal CVD process in an AAO template with

30 nm pore size. The results yielded a specifi c capacitance of


17© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2015, DOI: 10.1002/smll.201500120

174 nF cm −2 at 100 kHz. [ 101 ] A further signifi cant improve-

ment in specifi c capacitance was achieved by Banerjee

et al., [ 102 ] who fabricated TiN/Al 2 O 3 /TiN MIM structures

through a three-step continuous ALD process ( Figure 13 a)

with the assistance of an AAO template with a 100 nm pore

size. These highly regular arrays possessed a specifi c capaci-

tance of 10 µF cm −2 for a 1 µm-thick AAO template and

100 µF cm −2 for a 10 µm-thick AAO template, signifi cantly

exceeding previously reported values for MIM capacitors in

porous templates. Afterwards, the same group carried out a

careful chemical pore-widening treatment on the AAO tem-

plate, which resulted in a sharp and smooth AAO nanotopog-

raphy (Figure 13 d,e). [ 103 ] The according breakdown fi eld for

the MIM layer was then increased from about 4 MV cm −1

to about 10 MV cm −1 . The expected energy densities were

1.5 Wh kg −1 , the highest ever reported for electrostatic MIM

nanocapacitors and two times larger than that of their pre-

vious work.

6. Conclusions and Perspectives

We have provided a summary of the recent progress of heter-

ogeneous 1D nanostructure arrays based on AAO templates

for energy-conversion and -storage applications. Specifi -

cally, 1D nanostructures combining multiple components

are of great importance, and the resultant devices can per-

form multiple tasks simultaneously, benefi cial for enhancing

Figure 11. a) Schematic illustration of a typical fl exible supercapacitor electrode based on a Pt/MnO 2 nanotube core/shell array. b) SEM image of the Pt nanotube array after 30 s MnO 2 deposition. c) Cycling stability of the electrode in (b) at random current densities up to 8000 cycles. Reproduced with permission. [ 95 ] Copyright 2014, Wiley-VCH. d) SEM images of open and closed SnO 2 nanotube arrays. e) TEM image and EDX mapping (O K, Mn K, and Sn L spectra) of a single core/shell nanotube, showing a well conformal coating of MnO 2 across the SnO 2 nanotube. f) Rate capability of open and closed SnO 2 /MnO 2 nanotube arrays. Reproduced with permission. [ 96 ] Copyright 2014, Elsevier. g) Schematic diagram of the fabrication and assembly of asymmetric supercapacitors based on SnO 2 /MnO 2 and SnO 2 /PPY core/shell nanotube arrays. h) Cyclic voltammetry (CV) curves of single SnO 2 /PPy and SnO 2 /MnO 2 core/shell nanotube arrays measured at 20 mV s −1 in a three-electrode cell, showing the complementary voltage windows of both materials. i) CV curves of PPy/MnO 2 asymmetric supercapacitor measured at different potential windows from 0.9–1.7 V in a two-electrode cell confi guration at 20 mV s −1 , exhibiting a mirror-image current response on voltage reversal until 1.7 V. Reproduced with permission. [ 97 ] Copyright 2014, Elsevier.



the overall performance systematically. Meanwhile, the 1D

nanostructured architecture offers a versatile platform to

investigate energy-conversion/-storage processes microscopi-

cally, and the results could be utilized to further optimize the

effi ciency. The nano-units of these complicated heterogeneous

structure arrays, however, are identical, and it is still chal-

lenging to manipulate the spatial component of these nano-

units to fabricate spatially controllable binary or complex 1D

Figure 13. a) Schematic of a unit cell of AAO MIM capacitor. b) Cross-sectional SEM of the sample in (a). c) 1 inch wafer with capacitors of different areas defi ned on the surface. Reproduced with permission. [ 102 ] Copyright 2009, Nature Publishing Group. d) Cross-sectional SEM for MIM capacitor fabricated without a BAA intermediate step, resulting in sharp interpore asperities. e) SEM for MIM capacitor fabricated with barrier anodic alumina intermediate step, resulting in rounded interpore peaks. Reproduced with permission. [ 103 ] Copyright 2012, American Chemical Society.

Figure 12. a) Cross-sectional SEM image of a bare Ni nanopore array and (inset) a photograph of the relevant Ni nanopores. b,c) Ni nanopore arrays after loading with 80 µg cm −2 and 400 µg cm −2 MnO 2 , repectively. d) CV curves of the sample in (b) at different scan rates. e) Galvanostatic charge/discharge profi les of the sample in (b). f) Cycling stability of the sample in (b) as a function of cycle number with a scan rate of 50 mV s −1 . Reproduced with permission. [ 99 ] Copyright 2014 Wiley-VCH.



nanostructure arrays interdigitated with two or more sets of

nanostructure arrays. In this case, each set of sub-nanostruc-

ture arrays can perform a series of tasks individually or intro-

duce novel functions that originate from the coupling effect

between each sub-nanostructure array. With the develop-

ments in experimental techniques for precisely controlling

the AAO anodization and etching, we are confi dent that this

constraint will be eventually surpassed and even more appli-

cations will emerge. Thus, a promising future prospect for

AAO nanotechnology is appreciable.

Though signifi cant achievements have been obtained in

the application of heterogeneous 1D nanostructure arrays to

energy conversion, there is still a large space to improve the

overall effi ciency, regarding the existing challenges that need

to be addressed in the system. First, due to the large surface-

to-volume ratio, surface defects are increasing dramatically,

which act as charge carrier trap sites that deteriorate the

performance of energy-conversion devices. More delicate

engineering of surface modifi cations or surface passivation

for the 1D nanostructures will be more important. Accord-

ingly, the varied components shall work more effi ciently. To

thoroughly understand the transfer dynamics of photogen-

erated charge carriers in the complicated surface surround-

ings and to explore a more effective means for harnessing

solar energy, comprehensive and systematic investigations

are still required.

The second challenge is the choice of different functional

materials and organizing them to an integrated device. Large-

bandgap semiconductors can generate a large open-circuit

potential and drive a broad range of PEC reactions, while

semiconductors with a small bandgap can absorb a wide

region of solar energy. How to manipulate these materials

on the nanoscale and to engineer bandgap alignments ben-

efi cial for charge generation, separation, and transportation is

worthy of intensive attention in the future. In particular, the

association of effi cient charge transfer with a built-in poten-

tial in the depletion layer deserves more focus, given that the

built-in potential is relevant to the morphological and doping

conditions of the 1D nanostructures.

Third, the popular methods to in-fi ll or cover the tem-

plate with active materials are electrochemical deposition,

ALD, physical vapor deposition, spin coating, as well as

sol–gel infi ltration, and the resulting materials fabricated by

these methods are not usually single crystalline. Thus, how to

improve the crystal quality becomes crucial to reducing the

charge-carrier loss in the crystal defects.

As to the energy-storage applications, many different

heterogeneous 1D nanostructure arrays have been proposed

as good candidates for cathodes or anodes, and substan-

tial complete devices with both electrodes equipped with

heterogeneous 1D nanostructure arrays have been devel-

oped, accompanied by a remarkable progress in the perfor-

mance. However, interdigitated confi gurations consisting

of two sets of heterogeneous 1D nanostructure arrays with

each set as the anode and cathode, respectively, have not

been realized. Developing future techniques, we believe that

this ideal design can be realized and it should even be pos-

sible to control the profi le and distribution of interdigitated

confi gurations for different applications. By modulating the

morphological parameters of the interdigitated nano-anode

and nano-cathode, promising device performances are highly

anticipated.

Acknowledgements

L. Wen and Z. Wang contributed equally to this work. Financial support from the European Research Council (ThreeD-surface: 240144), BMBF (ZIK-3DNanoDevice: 03Z1MN11), Volkswagen Stiftung (Herstellung funktionaler Oberfl ächen: I/83 984) is gratefully acknowledged. The funding support from the National Basic Research Program of China (Grants 2014CB931800, 2013CB933900), the National Natural Science Foundation of China (Grants 21431006, 91227103) is appreciated.

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