COVER ARTICLEFan et al.Hydrogen evolution via sunlight water splitting on an artifi cial butterfl y wing architecture

HOT ARTICLEMoortgat et al.The gas-phase ozonolysis of α-humulene

ISSN 1463-9076

Physical Chemistry Chemical Physics

www.rsc.org/pccp Volume 13 | Number 23 | 21 June 2011 | Pages 10825–11452

10872 Phys. Chem. Chem. Phys., 2011, 13, 10872–10876 This journal is c the Owner Societies 2011

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 10872–10876

Hydrogen evolution via sunlight water splitting on an artificial butterfly

wing architecturew

Huihui Liu,za QibinZhao,za Han Zhou,aJian Ding,

aDi Zhang,

aHanxing Zhu

band

Tongxiang Fan*a

Received 16th March 2011, Accepted 25th March 2011

DOI: 10.1039/c1cp20787c

A prototype of nature’s butterfly wing architecture using Pt

loaded TiO2 is provided and demonstrated to be able to enhance

the hydrogen evolution rate by 2.3 times in sunlight water

splitting. This is due to advantages brought about by the

hierarchical architecture in both meso scope and nano scope.

The global energy crisis and environmental problems have

become increasingly prominent as a consequence of industrial

civilization, and the situation is unlikely to improve in decades

to come. Given the right way to utilize it, solar energy could

provide a promising solution. The real challenge is how to

convert solar energy into high-energy chemicals and how to

collect such energy products efficiently. Using sunlight to split

water into hydrogen and oxygen seems to be a promising way

as its product, hydrogen, is a clean and sustainable energy

source.1,2 To further develop this strategy we need to focus on

enhancing the performance of the photocatalyst used in this

reaction. The reaction of sunlight water splitting consists of

three steps: light harvesting, energy conversion, hydrogen

production and evacuation.3 Here, we put forward a general

strategy of using nature’s butterfly wing hierarchical architecture

to get better photocatalytic performance of TiO2, a typical

sunlight water splitting photocatalyst, in all three steps. We

believe nature has provided hidden solutions in many forms

and butterfly wing architecture may be one of them, but not

the only one. This concept could be extended to many other

relevant species, thus give a broad scope of building proto-

types to exploit solar energy for sustainable energy resources.

As a high efficiency photocatalyst under ultraviolet light,

titanium dioxide (TiO2) has been widely investigated in

dye-sensitized solar cells,4,5 water and air detoxification,6,7

self-cleaning,8 water splitting,9 and so forth. To enhance its

photocatalytic performance in sunlight water splitting,

investigators have been conducting research with an emphasis

on modification of TiO2 by means of metal loading,10,11 metal

ion doping,12 dye sensitization,13 composite semiconductor,14

anion doping,15 and metal ion-implantation.16,17 Recently more

and more researchers have recognized the effect of micro

architectures, and have reported significantly enhanced photo-

catalytic properties on TiO2 nanotube arrays,18 ordered meso-

porous TiO2 thin films,19 improvement of light harvesting

with interconnected web-like architecture TiO2 films, and so

forth. However, present techniques are very limited in taking

the challenge of designing and producing effective micro

architectures, while nature has plenty of exquisite architectures.

Butterfly wings exhibit a variety of beautiful colors that

are not only the result of pigments but are also more

importantly due to the presence of periodical submicrometer

architectures.20,21 In the nanometre to micron scale, butterfly

wing scales show an extremely delicate and complex morphology

consisting of aligned lamellas, which are in turn arranged into

highly ordered architectures forming pores and layers.22 These

exquisite architectures offered by Mother Nature could be

borrowed as bio-templates to get TiO2 with a hierarchical

architecture, in order to help promote the photocatalytic

efficiency and thus enhance the energy transfer system.

Vaviaty methods like chemical vapor deposition, sol–gel,

atomic layer deposition have been reported on using photonic

structures from insects to create inorganic replicas.23–29 Here

we learn from these works and make a modulated prototype of

efficient photocatalyst after studying the original architecture.

We tried this idea with the male Papilio helenus Linnaeus

(Fig. 1a) which belongs to the Papilionidae family. It’s

basically all black only with a pair of white spots on the dorsal

wings. Field emission scanning electron microscopy (FESEM)

images show that the wing is covered with layers of scales

arranged orderly like roof tiles (Fig. 1b). A typical scale is

about 130 mm long and 60 mm wide, consisting of a simple

bottom layer (about 220 nm thick) and an upper layer with

hierarchical architecture (Fig. 1c). In this case, the upper layer

is comprised of a continuous bottom open hollow wedge like

ridges, resembling the structural microwave absorbers in

microwave chambers,30 and adjacent ridges are separated by

hole arrays with a kind of quasi-honeycomb arrangement

(Fig. 1d). The ridges are about 640 nm high with a space of

about 2.9 mm between neighboring ridges, and the average

hole diameter in the hole arrays is about 700 nm.

In order to get the relationship between the blackness

of its wings and the architecture, 3D FDTD (finite-difference

a State Key Lab of Composites, Shanghai Jiaotong University,200240, Shanghai, P.R. China. E-mail: txfan@sjtu.edu.cn;Fax: +86-21-34202497; Tel: +86-21-54747779

b School of Engineering, Cardiff University, Cardiff, CF24 3AA, UKw Electronic supplementary information (ESI) available: See DOI:10.1039/c1cp20787cz These authors contributed equally to this work.

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time-domain) simulations are performed to investigate the

light harvesting capacity of the black dorsal wing. To compare

the optical properties between the original nanoarchitectured

scales and their nonarchitectured counterparts, simulation

model 1 was built according to the average size of original

wing scales obtained by FESEM observation, and a planar

scale model was also built by squeezing model 1 into a slab of

the same bottom area and the same volume (Fig. 1i).

Parameters of the 3D FDTD simulations are shown in the

ESIw. Fig. 1e and g present the comparison between the

simulation results of model 1 and the planar scale model, plus

the measured reflection/transmission spectra of original scales.

Compared with the result of the planar scale model (about

14% in reflection), architecture of the scales exhibits a significant

antireflection effect (about 1–4% in reflection). In order to

demonstrate how light interacts with the architecture, Poynting

vector maps are presented to show the energy distribution of

the reflected/transmitted light by the architecture. According

to Fig. 1f and h, the hole arrays, especially the side holes, have

the smallest reflection and transmission, suggesting the strongest

light absorption, while the ridges have both stronger reflection

and transmission. Based on FESEM observation, there are

three basic elements in the wing scale architecture, that’s why

models 2, 3 and 4 were built based on model 1 by varying the

parameters of one of the three basic elements: hole arrays

(model 2), bottom layer (model 3), and the ridges (model 4), to

show the optical effect of each element (Table S1 & Fig. S1.1w).Simulation results show that increasing the hole array

depth brings about the increase in reflection and decrease in

transmission (Fig. S1.2w). When the bottom layer is removed,

reflection decreases very little while transmission increases

quite a lot. Reducing the gradient of the ridges basically does

not affect the transmission spectrum but slightly decreases

reflection (Fig. S1.2w). According to these results, we can

assume that the ridges on the upper layer reflect and guide

the light into the quasi-honeycomb hole arrays, where multiple

scattering happens, while light escaping from the hole arrays is

reflected back by the bottom layer. A high rate of light

absorption is achieved through cyclic reflection and scattering.

In this way, the three basic elements with different and

supporting functions in the hierarchical architecture work

together and thus achieve an enhanced light harvesting

efficiency.

The architecture discussed above is not the only one in

butterfly wings. Looking more closely into the ribs that

separate the holes in nano scope, it can be seen that ultra-thin

chitin layers are arranged together to form the matrix (Fig. 1j,

Fig. S2w). For butterflies this nano architecture helps use less

material to form the wings while maintaining high strength

and durability, thus reduce the whole body weight without

Fig. 1 Structural and optical properties of butterfly Papilio helenus Linnaeus (male). (a), An overall view of the butterfly. (b), FESEM image

showing the scales of the black area. (c), FESEM image showing the cross section of a scale. (d), FESEM image showing the architecture of the

upper layer. (e), Measured reflection spectrum of the original scale, simulated results of the original scale (model 1) and the planar scale model. (f),

Poynting vector distribution map of model 1 for reflection by Gaussian modulated continuous wave. (g), measured transmission spectrum of the

original scale, simulated results of the original scale (model 1) and the planar scale model. Dashed rectangles in f and g indicate the middle holes,

and brown rectangles indicate the side holes. (h), Poynting vector distribution map of model 1 for transmission by Gaussian modulated continuous

wave. (i), The planar scale model is obtained by squeezing the 3D original model into a slab of the same bottom area and volume. (j), TEM image

showing the ultra thin chitin layers.

10874 Phys. Chem. Chem. Phys., 2011, 13, 10872–10876 This journal is c the Owner Societies 2011

affecting the function. For photocatalytic purposes this is an

optimistic prospect because photocatalytic reactions prefer

smaller grains, shorter charge transfer pathways and larger

outermost layer surface.

Based on the prototype of the hierarchical architecture of

butterfly wings, artificial butterfly wing architecture TiO2

(ABWA-TiO2) were produced by a simple immersion-calcination

method using the original wings as template and TiCl3 solution as

precursor. TiO2 nanoparticles without template (NT–TiO2)

were also synthesized under the same conditions for comparison.

Experimental details are provided in the ESIw.The FESEM images of the mesoporous architecture of the

ABWA-TiO2 are shown in Fig. 2a, b and c. According to the

FDTD simulations, three basic elements in the hierarchical

architecture work together in different but related functions to

enhance light harvesting efficiency. To inherit this light

harvesting property from nature, ABWA-TiO2 must inherit

the hierarchical architecture from the wing templates.

Compared with the original wing scale, the bottom layer were

preserved, ridges were well copied, and the hole arrays

were kept not only with a proper depth but also with the

quasi-honeycomb arrangement. However, the hole size has

shrunk by about 50%, i.e. the average hole size has reduced

from about 700 nm to about 350 nm.

Fig. 2d (see also Fig. S3w) is a transmission electron

microscope (TEM) image of the nanoscopic architecture of

the ABWA-TiO2. Fig. 2d and c show the homogeneous size

distribution of grains and nano pores. The average grain size

obtained from the TEM image is about 11 nm. The porous

architecture on the nanoscale was also characterized by

nitrogen-adsorption measurements. Fig. 2e displays the

adsorption-desorption isotherm and the pore size distribution

curve deduced from BET isotherms and the Kelvin equation.

For this sample the pore size is about 8.6 nm and the size

distribution is very narrow, which agrees with the FESEM and

TEM observation.

These meso scope architectures and nano pores in our

ABWA-TiO2 are actually not separated, but work as a system

to form a kind of hierarchical micro-mesoporous catalyst with

microporosity embedded in mesoporous walls. The meso

scope hole arrays and the nano pores make a highly organized

hierarchical evacuating pathway for the photocatalysis

products, which are produced on the outermost layer surface

and transferred into the nanopores first, then absorbed by the

meso scope hole arrays into the solution. These functions of

micro and mesoporosity are similar to those of the substantial

quantity of pores, enabling effective diffusion of nutrients

and metabolites to and from the cells in latest work about

photobioreactors.31,32

Powder X-ray diffractograms of ABWA-TiO2 are identified

as being due to the anatase phase of TiO2 (Fig. S4w). In order

to distinguish the crystalline-size broadening and microstrain

broadening, the method proposed by W. H. Hall was

employed.33 An average grain size of 11.1 nm was estimated

from the integral breadth of the (101), (200) and (204) diffraction

lines. This result is consistent with TEM observation (Fig. 2d).

NT–TiO2 was also identified to be the anatase phase of TiO2,

only with a larger average grain size of 20.5 nm (Fig. S5w).According to classical nucleation theory, the existence of

impurity phase, i.e. the ultra thin chitin layers of bio-template

in this case, can slow down nucleus growth by providing a

large number of nuclei, and thus result in smaller grains.

Smaller grains possess larger outermost layer surface and

smaller density of grain boundaries, which is associated slower

recombination rates for charge carriers.34,35 Considering

that quantum size effects occur for semiconductor particles

Fig. 2 (a), (b), (c), FESEM image of the architecture of ABWA-TiO2. (d), TEM image of ABWA-TiO2. (e), Nitrogen-adsorption-measurement

results of ABWA-TiO2, with the blue and black curves showing the adsorption-desorption isotherms of the sample, and the red one showing the

pore-size distribution of the sample. (f), Light harvesting efficiency of ABWA-TiO2 (red) and NT–TiO2 (blue).

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 10872–10876 10875

of 1–10 nm size, it is safe to say that the size of both systems

are in the same level and this effect does not play the key role

in photocatalysis efficiency compared with factors like light

harvesting.36 Surface areas of both samples were tested by

nitrogen-adsorption measurements. Results show the surface

area of ABWA-TiO2 is 51.2 m2 g�1, with micropore area being

13.8 m2 g�1 and external surface area being 37.4 m2 g�1.

Corresponding data for NT–TiO2 is 74.0 m2 g�1, 6.3 m2 g�1

and 67.8 m2 g�1. This also demonstrated the multi-porous but

hierarchically connected architecture of ABWA-TiO2.

Fig. 2f shows the light harvesting efficiency of the

ABWA–TiO2 and NT–TiO2 in comparison. The exact pattern

between the hole size and light absorption still needs further

investigation, and latest work implies that optical properties

do show a difference when mesopore size changes.37 Here as

for ABWA–TiO2, the overall size shrank with the architecture

preserved, while the required wavelength shifted from the

visible range for butterflies to the UV range for photocatalyst.

Compared to NT–TiO2, light harvesting efficiency was

enhanced by about 30 percent in wave range from about

220 nm to 380 nm. Considering the fact that TiO2 is a high

efficiency photocatalyst under UV light, this not only proved

our idea of borrowing nature’s architecture to enhance the

light harvesting efficiency of an artificial energy transfer

system, but also shifted the absorption range from nature’s

requirement (longer wavelength) to our requirement (shorter

wavelength).

Fig. 3 shows the H2 producing performance by every gram

of different samples in sunlight water splitting. 80 mg of

samples were used each time. The activity of ABWA–TiO2

in pure water is low which might be attributed to the high

recombination of the photogenerated electrons and holes as

well as the back reaction between the produced H2 and O2.

When the reaction was performed in 10% aqueous methanol,

a known sacrificial electron donor,38 the hydrogen evolution

rate is about 500 times higher than that in pure water

(Fig. S6w). Fig. 3a shows the comparison between ABWA–TiO2

and NT–TiO2. Compared to NT–TiO2, ABWA–TiO2 has got a

hydrogen production rate 7 times higher, producing a total H2

amount of 22.87 mmol after 5 h, with the evolution rate of

1.003 mmol min�1 per gram sample. As discussed above, in meso

scope, the hierarchical wing architecture enables ABWA-TiO2

enhanced light-harvesting capacity, capturing more photons for

the photocatalytic reactions. In nano scope, ultra-thin chitin

layers that construct the wings work as an impurity phase in

TiO2 crystallization and ensure a size control of grains and pores.

In both scopes, the micron hole arrays and the nano pores make a

highly organized hierarchical evacuating pathway for the photo-

catalysis products, hence increase the photocatalytic efficiency

dynamically. On the whole, the hierarchical architecture

borrowed from butterfly wing template works on several levels,

and enhances the catalytic activity of ABWA–TiO2.

Pt nano particles were loaded on ABWA-TiO2 in order to

promote charge transfer and create hydrogen desorption

sites.39 According to Fig. 3b, Pt loaded samples show a

significant improvement in hydrogen yield. We tried a series

of different Pt loading amounts in our samples and found that

H2 yield increased initially with increasing Pt content, reached

a maximum and then started to decrease once the Pt content

exceeded a level around 1.5 wt%. The reason for this trend

could be that the deposition of small amounts of Pt is

indispensable for the removal of photogenerated electrons

from TiO2, their storage and for the reduction reaction.

However, as more and more Pt is added to the TiO2

surface and beyond a certain optimal value, blockage of the

photosensitive TiO2 surface occurs and consequently reduces

the surface concentration of electrons and holes available for

reaction. Therefore, H2 yield starts to decrease after this

optimal point.39 The 1.5 wt% Pt loaded ABWA-TiO2

produced a total H2 amount of 882.85 mmol after 5 h

(Fig. 3b), with an evolution rate of 41.32 mmol min�1 per

gram sample, which is 41 times higher than that of 0 wt% Pt

loaded ABWA-TiO2, and 2.3 times higher than that of

1.5 wt% Pt loaded NT–TiO2 (Fig. 3c).

Conclusions

In conclusion, as a typical prototype, artificial butterfly wing

architecture TiO2 (ABWA-TiO2) has been produced using the

original butterfly wings as templates and enhanced photo-

catalytic efficiency has been achieved. The hierarchical

architecture borrowed from butterfly wing template works

on several levels to enhance catalytic activities of Pt loaded

TiO2 and thus is able to enhance the hydrogen evolution rate

by 2.3 times. These results demonstrate a new strategy for

mimicking Mother Nature’s elaborate creations in making

materials for renewable energy. Currently existing artificial

energy transfer systems and techniques are very limited in

design and production of effective micro architectures, while

nature has tens of thousands of kinds of butterflies and other

Fig. 3 Hydrogen evolution of the samples in 10% aqueous methanol under UV and visible light irradiation. (a) Comparison between

ABWA–TiO2 and NT–TiO2. (b) Comparison between ABWA–TiO2 series loaded with different amount of Pt particles. (c) Hydrogen evolution

rates of six typical samples.

10876 Phys. Chem. Chem. Phys., 2011, 13, 10872–10876 This journal is c the Owner Societies 2011

related species with innumerable elaborate hierarchical

architectures evolved over millions of years to meet specific

biological purposes. The concept of learning from nature

could be extended broadly, thus give a broad scope of building

technologically unrealized hierarchical architecture and design

blueprints to exploit solar energy for sustainable energy

resources.

Acknowledgements

This work is supported by National Natural Science

Foundation of China (No. 50972090), National Basic Research

Program of China (No. 2011CB610300), Dawn program of

Shanghai Education Commission (No. 08SG15), Shanghai

Rising-star Program (No. 10QH1401300) and Research

Fund for the Dectoral Program of Higher Education

(No. 20100073110065).

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