ZnO/ZnSxSe1x core/shell nanowire arrays as photoelectrodes with efficient visible lightabsorptionZhenxing Wang, Xueying Zhan, Yajun Wang, Muhammad Safdar, Mutong Niu, Jinping Zhang, Ying Huang, and

Jun He Citation: Applied Physics Letters 101, 073105 (2012); doi: 10.1063/1.4745918 View online: http://dx.doi.org/10.1063/1.4745918 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/7?ver=pdfcov Published by the AIP Publishing

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ZnO/ZnSxSe12x core/shell nanowire arrays as photoelectrodes with efficientvisible light absorption

Zhenxing Wang,1,a) Xueying Zhan,1,2,a) Yajun Wang,1 Muhammad Safdar,1 Mutong Niu,3

Jinping Zhang,3 Ying Huang,2 and Jun He1,b)

1National Center for Nanoscience and Technology, Beijing 100190, China2Wuhan National Laboratory for Optoelectronics and College of Optoelectronic Science and Engineering,Huazhong University of Science and Technology, Wuhan 430074, China3Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China

(Received 18 April 2012; accepted 31 July 2012; published online 13 August 2012)

ZnO/ZnSxSe12x core/shell nanowires have been synthesized on nþ-type silicon substrate via a

two-step chemical vapor deposition method. Transmission electron microscopy reveals that

ZnSxSe12x can be deposited on the entire surface of ZnO nanowire, forming coaxial heterojunction

along ZnO nanowire with very smooth shell surface and high shell thickness uniformity. The

photoelectrode after deposition of the ternary alloy shell significantly improves visible light

absorption efficiency. Electrochemical impedance spectroscopy results explicitly indicate that the

introduction of ZnSxSe12x shell to ZnO nanowires effectively improves the photogenerated charge

separation process. Our finding opens up an efficient means for achieving high efficient energy

conversion devices. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745918]

In recent years, due to their high efficiency charge carrier

transport and surface charge carrier transfer/separation,1–3

core/shell structure nanowires with type II band offsets have

attracted much attention as fundamental building blocks for

the development of next generation solar energy conversion

devices. As one of the most important metal oxides, ZnO

nanowires are widely used as photoanode materials, due to its

obvious advantages of longer carrier lifetime, higher elec-

tronic mobility which is favorable for electron transport, eas-

ier to synthesize than other metal oxide (e.g., TiO2).4

However, its wide bandgap (3.37 eV) prevents efficient

absorption to sunlight in the visible region, thus brings a low

overall solar energy conversion efficiency. To resolve this

problem, constructing the core/shell heterostructure with nar-

rower band gap semiconductor is considered as one promis-

ing way to extend the absorption of sunlight to the visible

region. Meanwhile, core/shell architectures provide a large

interfacial area to ensure the rapid charge separation and con-

sequently increase the carrier collection efficiency.5 Further-

more, the core nanowires afford a direct electrical pathway

for the photogenerated carriers to rapidly transport to the pho-

toelectrode, and the shell allows the holes to flow in the oppo-

site direction.6–8 The core/shell structures can also effectively

prevent the nonradiative recombination of the electrons in

core nanowires with electrolyte and the corrosion of core

nanowires from electrolyte as well.9 Based on above consid-

eration, many kinds of type II radial heterojunctions such

as ZnO/ZnSe,10,11 ZnO/Cl-ZnO,12 ZnO/CdS,13 ZnO/In2S3,14

ZnO/CdTe15 and so on have been fabricated by growing thin

layer of semiconductor materials with different bandgaps on

a ZnO nanowire array. We recently have synthesized ZnO/

C3N4 and TiO2/C3N4 nanoparticle core/shell photocatalyst

and demonstrated that both UV light photocatalytic activity

and visible light photocatalytic activity were greatly

improved due to the enhanced charge separation.16–18

As compared with binary semiconductor shell with fixed

composition and absorption edge, ternary alloy semiconduc-

tors have great advantages of tunable composition and band

gap, for solar cell device application. These merits offer the

opportunities to achieve higher conversion efficiency by hav-

ing a wider range of band gaps that match the solar spectrum.

Li et al. prepared a composition-graded ZnxCd12xSe@ZnO

core/shell nanowire array photoelectrodes via a temperature-

gradient chemical vapor deposition (CVD) method,19 which

yield a continuous absorption edge from 460 nm to 700 nm

across the sample surface. Wu et al. also synthesized a type

II ZnO/ZnxCd12xSe core/shell nanocables with tunable shell

composition and band gaps to cover almost the entire visible

spectrum using an ion-exchange route.20 Additionally, TiO2/

CdSSe ternary semiconductor system also have been detail-

edly investigated.21 ZnSxSe12x alloy semiconductors have

significant importance in photonics and electronics, since

their absorption edge (Eg) can be tuned over a range of from

340 nm to 460 nm (0� x� 1),22–24 partially in visible light

region. However, no attempts have been made to synthesize

ZnSxSe12x-sheathed ZnO nanowires using CVD method. In

this study, we report our effort in exploring low-cost and

high efficient photoelectrodes made of ZnSxSe12x ternary

semiconductor compound sheathed ZnO nanowires on a

heavily doped n-type silicon substrate. We synthesized the

ZnO/ZnSxSe12x core/shell nanowires via a two-step CVD

method. ZnO nanowires were grown directly on a silicon

substrate. Then, a continuous ZnSxSe12x layer was deposited

on the pre-grown ZnO nanowires. By optimizing the deposi-

tion parameters, we achieved ZnO/ZnSxSe12x coaxial heter-

ojunction with very smooth shell surface and high shell

thickness uniformity. The core/shell structure photoelectrode

demonstrates a great visible light photoresponse. The charge

transfer and recombination processes were also investigated

by electrochemical impedance spectroscopy (EIS).

a)Zhenxing Wang and Xueying Zhan contributed equally to the work.b)Author to whom correspondence should be addressed. Electronic mail:

hej@nanoctr.cn.

0003-6951/2012/101(7)/073105/5/$30.00 VC 2012 American Institute of Physics101, 073105-1

APPLIED PHYSICS LETTERS 101, 073105 (2012)

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The core/shell heterojunction was prepared by a two-

step CVD method. For ZnO nanowires growth, nominal

5 nm thick gold layer were deposited on the heavily doped

n-type Si(100) substrate as the vapor-liquid-solid (VLS)

growth catalyst. A mixture of ZnO powder and graphite

powder (200 mesh size from Alfa Aesar) with a molar ratio

of 1:1.2 ZnO/C was loaded at the end of a quartz cuvette as

the source. And then the substrate was placed in the

upstream 6 cm away from the source. With 20 sccm of argon,

the furnace temperature was raised to 1000 �C at a rate of

50 �C/min. During the growth, 20 sccm of oxygen was used

as the reaction gas under the same carrier gas flow. After

reaction for 40 min, the furnace was allowed to cool to room

temperature under the argon flow. For the ZnSxSe12x coating

on ZnO nanowires, the source materials were prepared by

physically mixing the high-purity ZnS and ZnSe powders

(Alfa Aesar, 99.99%) with the molar ratio of ZnS to ZnSe of

3:7. The mixture of ZnS and ZnSe powders placed in a

quartz boat. The substrates with pre-grown ZnO nanowires

were placed in the downstream in the boat 8 cm away from

the source. The furnace was heated to 1100 �C with a heating

rate of 75 �C/min and then cooled to room temperature with

20 sccm of argon carrier gas.

The morphology and microstructures of the as-prepared

ZnO/ZnSxSe12x core/shell nanowires were examined using

Hitachi S-4800 field-emission scanning electron microscopy

(FE-SEM) and FEI Tecnai F20 high resolution transmission

electron microscopy (HRTEM). The photoluminescence

(PL) measurement of the ZnO/ZnSxSe12x core/shell nano-

wires was performed using a microzone confocal Raman

spectroscope (HORIBA Jobin Yvon, LabRam HR 800)

equipped with a color charge-coupled device camera. The

325 nm He-Cd laser (Kimmon, ik3301R-G) was used as the

excitation source. The photoelectrochemical measurements

of the ZnO/ZnSxSe12x core/shell photoelectrodes were car-

ried out using an electrochemical system (CHI660B)

equipped with a conventional three-electrode system. The

ZnO/ZnSxSe12x photoelectrodes served as the working elec-

trode, a Pt wire as the counter electrode, and saturated calo-

mel electrode (SCE) as the reference electrode. The

photocurrents with light on and off were measured in electro-

lytes containing 0.1 M Na2SO4 at 0.0 V versus SCE. UV light

(k¼ 254 nm) and visible light (k> 420 nm) were used as the

light sources with the intensity of 714 lW/cm2 and 377 mW/

cm2, respectively. Visible irradiation was obtained from a

500 W Xenon lamp (Beijing China Education Au-light Co.

Ltd.) with a 420 nm cutoff filter.

Fig. 1(a) shows a typical SEM image of the high-density

ZnO/ZnSxSe12x core/shell nanowires on a heavily doped

n-type silicon substrate. Due to the higher secondary electron

emission efficiency of ZnO than ZnSxSe12x under electron

beam of SEM, we can clearly see the brighter ZnO nanwires

surrounded by ZnSxSe12x shells. The diameters of ZnO

nanowires are in the range of 50–150 nm in the inset of Fig.

1(a) and increase up to the range of 100–250 nm after coating

the ZnSxSe12x alloy shell, as shown in Fig. 1(a). An x-ray

energy-dispersive spectrum (EDS) acquired from the ZnO/

ZnSxSe12x sample clearly exhibits S and Se peaks with an

approximate atomic ratio of 0.87:2.49, close to 1:3, as shown

in Fig. 1(b). The microstructure and crystallinity of the ZnO/

ZnSxSe12x nanostructure is further investigated by TEM and

HRTEM. Fig. 1(c) shows the sample has typical core/shell

structures. It can be seen that, under the optimized deposition

condition, the alloy shell is deposited on the entire surface of

the ZnO naowire. Moreover, the alloy shell has uniform

thickness along the length of ZnO nanowire and exhibits a

smooth and dense surface. It is worth noting the shell in the

head end has same thickness with that in the body, as shown

in the inset of Fig. 1(c). In particular, the ZnSxSe12x shell

thickness can be controlled by deposition time. These

characteristics mentioned above will be quite beneficial to

surface charge separation and transfer for the photoelectro-

chemical applications.19 The TEM image Fig. 1(d) explicitly

reveals the diameter of a typical single ZnO nanowire is

�70 nm, and the thickness of the ZnSxSe12x shell is

�70 nm. The HRTEM image of the core/shell structure

FIG. 1. (a) SEM image of ZnO/

ZnSxSe12x core/shell nanowires. Inset:

Side-view SEM image of ZnO nanowires

before coating. (b) An x-ray EDS of

ZnO/ZnSxSe12x core/shell nanowires. (c)

TEM image of ZnO/ZnSxSe12x core/

shell nanowires. Inset: Enlarged the head

ends of ZnO/ZnSxSe12x core/shell nano-

wires. (d) Magnified TEM and (e)

HRTEM images of a single ZnO/

ZnSxSe12x core/shell nanowire. Inset: the

SAED pattern of the core/shell nanowire.

073105-2 Wang et al. Appl. Phys. Lett. 101, 073105 (2012)

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further reveals that ZnO nanowire is grown with a preferen-

tial [0001] orientation as indicated by the white arrow in

Fig. 1(e). The crystal structure was further confirmed by

selected area electron diffraction (SAED) pattern, shown in

the inset of Fig. 1(e). Meanwhile, we notice the shell exhibits

an amorphous structure. These characteristics demonstrate

the two-step CVD method is an effective fabrication

approach for core/shell heterostructure.

Fig. 2 shows the normalized room temperature PL spec-

tra of pristine ZnO nanowires, pristine ZnSxSe12x nanowires

with same growth condition to that of the alloy shell coating

ZnO, and ZnO/ZnSxSe12x core/shell nanowires, respec-

tively. The ZnO/ZnSxSe12x core/shell nanowire exhibited ef-

ficient and broad PL emissions in the visible light region

(green curve in Fig. 2). The ZnO/ZnSxSe12x nanowires show

an obvious shoulder peak at �440 nm, which can be ascribed

to the near band edge (NBE) emission of ZnSxSe12x. The

ZnO/ZnSxSe12x nanowires also yield other emission peaks

at the range >460 nm (460 nm, corresponding to the peak of

ZnSe when x¼ 0), which can be attributed to the defect-

assisted recombination in ZnSxSe12x alloy shell.25 It should

be noted that the strong NBE emission peak of ZnO nano-

wire at �380 nm (red curve in Fig. 2) is not observed in

ZnO/ZnSxSe12x core/shell nanowires, which may be attrib-

uted to three reasons.10 First, ZnO/ZnSxSe12x heterojunction

has type II band offsets between ZnO and ZnSxSe12x and the

shell has a narrower band gap than ZnO, as shown in the

inset of Fig. 2. The charge separation between the core/shell

of type-II band alignment results in a low hole concentration

and recombination rate in ZnO. Second, the alloy shell

absorbs the excitation laser and prevents ZnO efficient

absorption to excitation laser. Third, even if the ZnO yields

emission, the shell will absorb the emission light from ZnO,

keeping the emission from been detected.

Alloying of binary II–VI semiconductors is an important

method to obtain tunable bandgap emissions through compo-

sition modulation. For the ZnSxSe12x system, the nonlinear

variation of the energy gaps with x can be fitted by a quad-

ratic function as follows:

EgðxÞ ¼ xEgðZnSÞ þ ð1� xÞEgðZnSeÞ � xð1� xÞb; (1)

where Eg(x), Eg(ZnS), and Eg(ZnSe) are the energy gaps of

ZnSxSe1�x, ZnS, and ZnSe, respectively, and b is the bowing

parameter, which was found to be in the range from 0.40 to

0.65 eV.26 To get the value of Eg(x), we performed PL of the

pristine ZnSxSe12x nanowires (blue curve in Fig. 2) and the

peak is 442 nm. Here, we set b¼ 0.6,22 Eg(x), Eg(ZnS), and

Eg(ZnSe) are 2.80 eV, 3.66 eV, and 2.67 eV, respectively.

Then x is calculated to be 0.24, i.e., the composition of

ZnSxSe1�x is ZnS0.24Se0.76, which is consistent with the

measured value by EDS, as shown in Fig. 1(b).

Considering the importance of photoelectrochemical cells

(PEC) as solar energy conversion devices, PEC measurements

of the pristine ZnO and ZnO/ZnSxSe12x core/shell nanowires

as photoanodes were carried out. In order to ensure rigorous

control experiments of before and after coating ZnSxSe12x

shell, we performed the PEC measurements using the same

ZnO nanowire substrate. Fig. 3 shows amperometric I-t curves

of the ZnO/ZnSxSe12x core/shell nanowires and pristine ZnO

nanowires photoelectrodes at a 0.0 V versus SCE with the illu-

mination on and off. The changes of both “on” and “off”

FIG. 2. Normalized room-temperature PL spectra of the pre-grown ZnO,

ZnSxSe12x nanowires, and the as-grown ZnO/ZnSxSe12x core/shell nano-

wires. The inset is the scheme of the charge transfer process from ZnSxSe12x

to ZnO.

FIG. 3. Amperometric I-t curves of the ZnO/ZnSxSe12x core/shell nano-

wires and pre-grown ZnO nanowires photoelectrodes at a zero versus SCE

voltage under (a) 714 lW/cm2 UV (k¼ 254 nm) and (b) 377 mW/cm2 visi-

ble light (k > 420 nm) with on/off cycles.

073105-3 Wang et al. Appl. Phys. Lett. 101, 073105 (2012)

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currents are quite sharp, which indicates that charge transport in

these as-prepared sample proceeds very quickly. Upon UV illu-

mination with a density of 714 lW/cm2, pristine ZnO and ZnO/

ZnSxSe12x core/shell nanowires have similar photorecurrent

density �0.075 lA/cm2. Under dark condition, they have simi-

lar dark current �0.006 lA/cm2 as well. This indicates the

ZnSxSe12x shell does not reduce the UV light absorption effi-

ciency at a same UV density as shown Fig. 3(a). From Fig.

3(b), one can see that ZnO/ZnSxSe12x core/shell nanowires

have great improvement to the adsorption of visible light

(k> 420 nm). The photocurrent density of ZnO nanowire pho-

toanode has no clear response, but that of ZnO/ZnSxSe12x

nanowire photoanode vertically changes from �0.01 to

�0.19 lA/cm2 with visible light of 377 mW/cm2 on. These

results suggest that the photoelectrode after deposition of the

alloy shell significantly improved visible light absorption effi-

ciency, meanwhile does not obviously reduce the absorption ef-

ficiency of UV light. In ZnO and ZnSxSe12x type II band

alignment system, when the ZnSxSe12x shells absorb the visi-

ble light to generate the electron-hole pair in the shell, the pho-

toelectrons can be transferred to the conduction band of ZnO

nanowires, which facilitates the charge separation and transfer

processes.

It is well known that EIS is a powerful tool to study

charge separation/transfer and recombination processes in

solar cells. Here, EIS was performed to investigate the pho-

togenerated charge separation processes of ZnO and ZnO/

ZnSxSe12x core/shell nanowires under UV and visible light

irradiation as shown in Fig. 4. In the two cases, the arc EIS

planes clearly suggest charge transfer happens on the photo-

electrodes. The radius of the arc on the EIS spectra repre-

sents the electron transfer resistance at the surface of

electrodes.18,27 In Fig. 4, the arc radius on EIS Nynquist plot

of the ZnO/ZnSxSe12x core/shell nanowires is smaller than

that of the ZnO nanowires under both UV and visible light

irradiation, which suggest that ZnSxSe12x shell leads to a

more effective charge separation and a faster interfacial

charge transfer.28 It is worth noting that the arc radius on

EIS Nynquist plot of the ZnO/ZnSxSe12x core/shell nano-

wires is also smaller than that of the ZnO nanowires without

light irradiation, which imply that the ZnSxSe12x shell could

change the charge distribution of ZnO nanowires and result

in an easier charge transfer process as shown in Fig. 4(c).

These EIS results clearly indicate that the introduction of

ZnSxSe12x shell to ZnO nanowires can effectively improve

the photogenerated charge separation process.

In summary, we demonstrated a facile, low-cost two-

step CVD method to synthesize ZnO/ZnSxSe12x core/shell

nanostructures as photoelectrodes. TEM results explicitly

reveal that ZnSxSe12x is deposited on the entire surface of

ZnO nanowire to form the heterojunction along the whole

nanowire with very smooth shell surface and high shell

thickness uniformity. The PEC measurements suggest

that the photoelectrode after deposition of the ternary alloy

shell significantly improved visible light absorption

efficiency. EIS results clearly indicate that the introduction

of ZnSxSe12x shell to ZnO nanowires can effectively

improve the photogenerated charge separation process.

These results make ZnO/ZnSxSe12x core/shell heterostruc-

tures promising for various photovoltaic and photodoncuc-

tive applications.

This work at National Center for Nanoscience and Tech-

nology is supported by 973 Program of the Ministry of Sci-

ence and Technology of China (No. 2012CB934103) and the

100-Talents Program of the Chinese Academy of Sciences

(No. Y1172911ZX).

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