Design of ZnO/ZnS/Au sandwich structure photoanode for ...

Nano Res

1

Design of ZnO/ZnS/Au sandwich structure photoanode

for enhanced efficiency of photoelectrochemical water

splitting

Yichong Liu1, Yousong Gu1, Xiaoqin Yan1, Zhuo Kang1, Shengnan Lu1, Yihui Sun1, Yue Zhang1, 2 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0794-y

http://www.thenanoresearch.com on April 17, 2015

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Nano Research

DOI 10.1007/s12274-015-0794-y

Design of ZnO/ZnS/Au Sandwich Structure Photoanode for

Enhanced efficiency of Photoelectrochemical Water Splitting

Design of ZnO/ZnS/Au Sandwich Structure

Photoanode for Enhanced efficiency of

Photoelectrochemical Water Splitting

Yichong Liu1, Yousong Gu1, Xiaoqin Yan1, Zhuo

Kang1, Shengnan Lu1, Yihui Sun1, Yue Zhang1 2

1. State Key Laboratory for Advanced Metals and

Materials, School of Materials Science and Engineering,

University of Science and Technology Beijing, China.

2. Key Laboratory of New Energy Materials and

Technologies, University of Science and Technology

Beijing, China.

We reported a sandwish structure for photoelectrochemical water

splitting photoanode enhanced by the introduction of ZnS interlayer

and Au nanoparticles

Design of ZnO/ZnS/Au Sandwich Structure Photoanode

for Enhanced efficiency of Photoelectrochemical Water

Splitting

Yichong Liu1, Yousong Gu1, Xiaoqin Yan1, Zhuo Kang1, Shengnan Lu1, Yihui Sun1, Yue Zhang1, 2 ()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

ZnO, ZnS, Au,

photoanode,

photoelectrochemical

water splitting

ABSTRACT

We have demonstrated a ZnO/ZnS/Au composite photoanode with significantly

enhanced PEC performance with the ZnS interlayer and Au nanoparticles. The

solar-to-hydrogen conversion efficiency of such ZnO/ZnS/Au heterosturcture

reached 0.21%, which is 3.5 times that of pristine ZnO. The comparison of IPCE

and photoresponse in white and visible light region further verified that such

enhancement is attributed to the contribution of both UV and visible regions. The

modification of Au NPs can improve the PEC performance both in the UV and

visible regions due to the effective surface passivation and surface-plasmon-

resonance effects. And the ZnS interlayer is favorable for photogenerated

electrons under UV light and hot electron under visible light injection into ZnO,

simultaneously suppress the recombination at the photoanode-electrolyte

interface. The optimizing design of an interlayer between the plasmonic

metal/semiconductor composite systems reported here provides a facile and

compatible configuration to enhance the utilization efficiency of the incident light

for photoelectrochemical application.

1 Introduction Hydrogen that is formed by the solar splitting of water

is clearly considered to be the cleanest energy fuel to

meet future demand for energy, and various attempts

have been made in order to develop advanced

processes to produce hydrogen. Among these

different methods[1-3], photoelectrochemical (PEC)

water splitting, considered to be one of the most

promising hydrogen generation, is a highly-efficient

and eco-friendly way. In a PEC system, the selection

and design of the photoelectrode materials are critical

since the capability of the PEC cell for water splitting

is largely determined by the light absorption and

carrier transport of the photoelectrode. Light active

semiconductors or metal oxides such as titanium

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2 Nano Res.

dioxide (TiO2), Zinc oxide (ZnO), hematite (Fe2O3) and

cuprous oxide (Cu2O) have shown great potential for

hydrogen generation[4-13]. As a common and

inexpensive semiconductor, ZnO nanomaterials have

been extensively explored for solar energy conversion

because of their excellent electron mobility, electron-

transfer efficiency (115-155 cm2 V-1 s-1)[14], abundant

morphologies and favorable environmental

compatibility[15-18]. However, wide bandgap in ZnO

limits the photoelectrochemical (PEC) water splitting

performance due to poor absorption of visible light.

Therefore, it’s urgent to search for a strategy to

simultaneously increase the light capture.

Recently, plasmonic nanostructures of noble metal

especially gold (Au) have been demonstrated to be

promising for photocatalytic and solar energy

conversion due to its tunable interactions with light in

visible and infrared regions through a strong light

scattering and localized surface-plasmon resonance

(SPR) [19-22]. Also, Au nanoparticles are stable

enough for preventing corrosion during the

photoreaction[23]. Many efforts have been attempted

to design Au-ZnO based plasmonic nanostructures

for solar water splitting. The enhanced photoactivity

of Au NPs-decorated ZnO in the UV region is ascribed

to effective surface passivation as well as electric-field

amplification effect, while the photoactivity

enhancement in the visible region is mainly caused by

hot electron generation upon SPR excitation[24-28].

Therefore, it is highly desirable to design the ZnO/Au

photoanode for improving the efficiency of energy

conversion.

A suitable choice of interlayer material not only

reduces the recombination losses, but also enhances

the electron collection efficiency. The formation of ZnS

layer at the ZnO surface has been proved to be a

promising platform for photoelectrochemical water

splitting, due to fast photogenerated electron-hole

separation and enhanced injection efficiency[29-31].

The ZnO/ZnS/Au photoanode offers an optimal

configuration for PEC water splitting, as it

incorporates the merits of each composition.

In this paper, we demonstrated that the PEC

performance of ZnO/ZnS/Au photoanode can be

enhanced compared with pristine ZnO and ZnO/Au.

The PEC performance of different photoanodes were

systematically investigated with the illumination of

white light or visible light, respectively. The

ZnO/ZnS/Au sandwich structure possessed superior

photocurrent density compared with pristine ZnO

and ZnO/Au both in the UV and visible region, which

can be explained by EIS measurement and energy

band structure.

2 Experimental The fabrication sequence of ZnO/ZnS/Au nanorods

array via hydrothermal process[32], chemical

conversion process[33] and photochemical deposition

method[34] is shown in Scheme 1.

Scheme 1 Schematic Diagram of the Fabrication Procedure for

ZnO, ZnO/Au, and ZnO/ZnS/Au photoanodes

2.1 Synthesis of ZnO nanorods array. The samples

were prepared by the hydrothermal growth method.

A colloidal solution of zinc acetate [Zn(CH3COO)2

2H2O] (0.5 M) was spin-coated onto the FTO substrate

to form the ZnO seed layer, and subsequently

annealed at 350 °C in air for 30 min. In the

hydrothermal process, substrates were placed in the

aqueous solution containing 50 mM zinc nitrate

hexahydrate [Zn(NO3)2 6H2O] and

hexamethylenetetramine (HMTA) [(CH2)6N4] at 90 °C

for 12 h, with the seeded side facing down. Finally, the

substrate with product was repeatedly rinsed with

deionized water for several times and annealed in air

at 450 °C for 3h to remove residual surface salts.

2.2 Synthesis of ZnS interlayer. A simple chemical

conversion approach was adopted to fabricate

ZnO/ZnS core/shell nanostructures. The substrate

with ZnO nanorods array was immersed in

thiacetamide (TAA) aqueous solution (0.2 M) and the

sulfidation process was carried out at 95 °C for 15

minutes. The final product was also washed with

deionized water and ethanol repeatedly to remove the

impurities and dried at room temperature.

2.3 Synthesis of Au nanoparticles. The ZnO/ZnS and

ZnO nanorods array were immersed in a 10 ml

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

chloroauric acid (HAuCl4, 0.01%) solution with a

pH=7 adjusted by 0.1M NaOH solution, 1 ml 1%

polyvinyl alcohol （ PVA, [C2H4O]n ） and 0.5 ml

methanol (CH3OH, 99.5%). Under UV irradiation for

10 minutes, followed by calcination at 350℃ in N2 for

30 minutes, the Au ions (Au3+) were reduced to neutral

atoms attached to the surfaces.

2.4 Characterization of Materials. The structure and

morphology of the electrodes were characterized by

X-ray diffraction (XRD) (Rigaku DMAX-RB, CuKα),

field emission scanning electron microscopy (FE-SEM)

(FEI QUANTA 3D). The formation of ZnS and Au on

the ZnO nanorods and the bonding characteristics

were determined by X-ray photoelectron

spectroscopy (XPS, ESCALAB 250).

2.5 Eletrochemical Characterization. The photoanode

was fabricated by securing a copper wire on the

exposed electric conductive parts of the FTO with a

silver conducting paint. The substrate was

subsequently sealed on all edge with epoxy resin

except the active working area. All electrochemical

measurements were performed in a three electrode

mode with a photoanode as the working electrode, a

coiled Pt wire as counter electrode and a Ag/AgCl

reference electrode, and 0.5M Na2SO4 aqueous

solution (with pH buffered to ~7.0) as the electrolyte.

Electrochemical measurements were performed on an

electrochemical workstation (Solartron SI 1287/SI

1260). All photoanodes were illuminated from the

front side under AM 1.5G illumination provided by a

solar simulator (Oriel, 91159A). Visible light

illumination was obtained by passing the AM 1.5 light

through a 420 nm long-wave-passfilter. The white and

visible light intensity was 50 mW/cm2 and 45 mW/cm2

measured by a Si diode (Newport).

3 Results and discussion Figure 1(a) and (b) showed a typical SEM image of the

as-synthesized ZnO nanorods array with a smooth

surface. The side view (Figure 1(a)) clearly showed

that ZnO nanorods were aligned with a height of

about 5µm. The top view (Figure 1(b)) showed a dense

and oriented array of ZnO nanorods with the

diameter around 200 nm on the FTO substrate.

After sulfuration and Au-decoration, the morphology

changes of the nanorods array are observed in SEM

images. Figure 1(c) showed that Au nanoparticles

were uniformly distributed on the surface of each ZnO

nanorod and the nanorods still maintained their

vertically-oriented characteristics. The Au NPs

decoration of ZnO nanorods array was achieved by

the following reaction under UV illumination:

ZnO→ZnO(e+h)

ZnO(e+h)→e-+h+

AuCl4-+3e-→Au+4Cl-

As shown in Figure 1(d), different from the as-

synthesized ZnO/Au, the surface morphology after

the sulfuration became coarse, while the surface was

still hexagonal in shape and the nanorods were still

well-ordered, suggesting that sulfuration process did

not change the shape and size of the nanorods. During

the sulfuration, ZnO on the surface of nanowire was

etched and converted into ZnS, resulting in formation

of the ZnS shell:

CH3CSNH2+H2O=CH3CONH2+H2S

H2S=2H++S2-

ZnO+S2-+H2O=ZnS+2OH-

Figure 1 (a) Cross-sectional-view SEM images of the as-

synthesized ZnO nanorods array. Top view of pristine ZnO (b),

ZnO/Au(c) and ZnO/ZnS/Au (d) on an FTO substrate. (e) and (f)

TEM images of the ZnO/ZnS/Au.

The TEM image shown in Figure 1(e) (f) confirmed the

sandwich structure for ZnO/ZnS/Au. As can be seen,

the ZnS shell was uniform in width with the thickness


4 Nano Res.

about 4 nm on the surface of ZnO nanorods, and the

surface-deposited Au nanoparticles had an average

diameter of 10~30 nm distributed on the surface of

ZnS. The composition of the as-prepared photoanodes

was further examined by energy-dispersive X-

ray(EDX) measurement, and the atomic ratios were

calculated from the EDX spectrum, as shown in Figure

S1 in the ESM.

XRD data of ZnO nanorods (blue curve in Fig. 2)

revealed that all diffraction peaks of the pristine

photoelectrodes were indexed to the planes of highly

crystalline ZnO (JCPDS file no. 36-1451) with no

impurities. Moreover, the main phase of all the

samples can be identified as the self-supported ZnO

nanoarchitectures with wurtzite (002) nanorods array,

which is consistent with SEM images. After the

decoration of Au nanoparticles, another two new XRD

peaks appeared at 2θ of 38.1° and 44.4°, which can be

identified as the characterisitic peaks of (111) and (200)

(JCPDS file no. 04-0784). For ZnO/ZnS/Au

photoanodes(red curve in Figure 2), a weak peak

belonging to ZnS (111) diffraction was detected at 2θ

~ 28.5°, resulting from a very thin ZnS layer coating on

ZnO (JCPDS file no. 05-0566).

Figure 2 XRD patterns of the ZnO, ZnO/Au and ZnO/ZnS/Au

photoanodes on the FTO substrate.

The XPS measurement of ZnO/ZnS/Au sample

demonstrated the presence of Zn, O, S and Au without

other unexpected elements (Fig. 3(a)). The high

resolution scans of Zn2p, S2p and Au peaks were

shown in Fig. 3(b), (c) and (d). The XPS data were

analyzed by fitting to a Gaussian−Lorentzian function

and using a Shirley background subtraction. The

Zn2p3/2 peak at 1021.8 eV and Zn2p1/2 peak at 1044.9

eV were shown in Fig. 3(b). In the S2p XPS spectrum

of Figure 3(c), the S2p3/2 and S2p1/2 peaks centered at

161.8 eV and 163.0 eV, respectively, are related to Zn−S

bondings, indicating that ZnS was successfully

synthesized by the chemical conversion process in our

experiments. XPS was further employed to identify

the chemical states of Au element. For Fig. 3(d), the

spectrum can be decomposed into two components,

which can be assigned to the peaks of Au04f7/2 and

Au04f5/2. It can be derived from the XPS results that the

Au constituent is in the metallic state.

Figure 3 XPS spectra of ZnO/ZnS/Au composite heterostructure:

(a) wide scan, (b) Zn2p, (c) S2p and (d) Au4f.

The PEC performance of various as-synthesized

different photoanodes were characterized through

photoinduced I-V curves. As can be seen, with the

introduction of ZnS interlayer, the ZnO/ZnS/Au

photoanode exhibited larger photocurrent density in

the whole potential window compared to ZnO/Au

and pristine ZnO photoanode. Under illumination as

shown in Figure 4(a), the photocurrent densities of the

prinstine ZnO, ZnO/Au and ZnO/ZnS/Au were 0.18

mA/cm2, 0.37 mA/cm2 and 0.58 mA/cm2 at 1V versus

Ag/AgCl. Sweeping the potential anodically, the I-V

curve of the ZnO/ZnS/Au exhibited a steeper increase

in current with respect to potential and achieved a

prompt saturation of the photocurrent density at 0.4 V

versus Ag/AgCl, suggesting much more efficient

charge separation and collection than in the prinstine

ZnO and ZnO/Au photoanode.

Furthermore, the photoconversion efficiencies (η) for

PEC water splitting of such photoanodes were

estimated using the following equation[35]:

η=I(1.23-Vapp)/Plight (1)


5 Nano Res.

Vapp is the applied external potential versus reversible

hydrogen electrode (RHE). I is the externally

measured current density at Vapp. Plight is the power

density of the incident light. The potentials were

measured versus the Ag/AgCl reference electrode and

converted to the reversible hydrogen electrode (RHE)

scale using the Nernst function:

ERHE=EAg/AgCl+E°

Ag/AgCl+0.059pH (2)

ERHE is the converted potential versus RHE. EAg/AgCl is

the external potential measured against the Ag/AgCl

reference electrode. E°

Ag/AgCl is the standard electrode

potential of Ag/AgCl reference electrode (0.1976 V

versus RHE at 25 ℃). Figure 4(b) presented the plots

Figure 4 Comparisons of PEC properties of three different

photoanodes: prinstine ZnO (black), ZnO/Au (blue) and

ZnO/ZnS/Au (red): (a) Linear sweep voltammetry measurements

under simulated sunlight illumination with a 20 mV/s scan rate. (b)

Photoconversion efficiency as a function of applied potential

versus RHE.

of the photoconversion efficiencies vs RHE. The

ZnO/ZnS/Au exhibited an optimal photoconversion

efficiency of 0.21% at potential of 0.928 V vs RHE, 2

times and 3.5 times higher that of ZnO/Au (0.10% at

0.924 V vs RHE) and pristine ZnO (0.06% at 0.884 V vs

RHE) photoelectrode, respectively. The above

electrochemical results convincingly concluded that

the ZnS interlayer played positive roles in enhancing

the PEC water splitting efficiency of ZnO/Au

heterostructure.

For better revealing the influence of the ZnS interlayer

on the PEC performance of Au-ZnO based plasmonic

photoanode, chronoamperometic I-t curves were

collected at 0.5V vs Ag/AgCl with repetitive on/off

cycles under white light and visible light. As shown in

Figure 5 (a), the recorded photocurrent densities were

consistent with the values obtained from the linear

sweep voltammograms, indicating the photocurrents

are stable without photoinduced charging effect[36].

From the Diffuse reflectance UV−vis absorption

spectra, the absorption edge was extended to the

visible-light region due to the modification of Au NPs

(Figure S2 in the ESM). For visible light illumination,

the photocurrent density of ZnO /Au photoanode was

1.6 μA/cm2, while there was barely no photocurrent

for pristine ZnO photoanode. The enhanced

photocurrent in the visible region is caused by the SPR

effect of the modified Au NPs. With the introduction

of ZnS interlayer, the photocurrent density reached 3.9

μA/cm2, 1.6 times higher that of ZnO/Au photoanode,

demonstrating that the adjunction of ZnS layer could

enhance the visible-light photoactivity.

Furthermore, incident-photon-to-current-conversion

efficiency (IPCE) tests at 0.9 V versus RHE were

utilized to investigate the specific PEC active spectra

of as-systheised three different photoanodes. From

Figure 5(c), the ZnO/Au showed improved IPCE

values both in UV and VIS region due to Au NPs

modification. With the introduction of ZnS interlayer,

the IPCE values in the UV and VIS region were

improved compared to ZnO/Au photoanode, which

was consistent with its exclusive photocurrent

enhancement observed under white and visible light

illumination in Figure 5(a) and (b).


6 Nano Res.

Figure 5 Chronoamperomertic I-t curves collected at 0.5V versus

Ag/AgCl for three different photoanodes under (a) white-light (50

mW/cm2) and (b) visible-light (45 mW/cm2). (c) IPCE curves for

three different photoanodes measured in the wavelength range

from 370 to 700 nm at an applied voltage of 0.9 V vs RHE.

Electrochemical impedance spectroscopy (EIS)

measurements were performed to further explore the

influence of the ZnS layer on the charge

recombination at an applied bias voltage of 0.1V vs

Ag/AgCl under white light and the results were fitted

by the equivant circuit in Figure 6. In the Figure 6, the

arch for ZnO/ZnS/Au photoanode was much smaller

than that of pristine ZnO and ZnO/Au photoanode,

implying that simultaneous introduction of the ZnS

and Au significantly reduced the resistance on the

movement of charge carriers at the interface. In this

work, the obtained results were fitted into the models

of a RC circuit inserted in Fig. 6. In this equivalent

circuit, Rs is the resistance of the device and Rct is the

interfacial charge-transfer resistance of the

photoanode-electrolyte interface. According to the

fitting data from the EIS results, the Rct were 3.93, 2.30

and 0.97 kΩ cm2 coupled for pristine ZnO, ZnO/Au

and ZnO/ZnS/Au photoanode, respectively. The Rct of

ZnO/Au was smaller than that of prinstine ZnO

indicated Au NPs enhanced the electron mobility by

surface passivation to reduce the recombination of

photoexcited electrons and holes[20]. With the

introduction of ZnS interlayer, surface traps states

such an oxygen vacancies /adsorb oxygen were

further reduced, resulting in the highest separation

efficiency of photogenerated electron-hole pairs and

fastest charge transfer of ZnO/ZnS/Au composite

structure[26]. Therefore, such results positively

supported the highest acquired water splitting

efficiency of ZnO/ZnS/Au in Figure 4(b).

Figure 6 (a) Nyquist plots measured at an applied potential 0.1 V

versus Ag/AgCl at white light for ZnO, ZnO/Au and ZnO/ZnS/Au,

respectively. The insert is a equivalent circuit employed to fit the

Nyquist plots.

For better understanding the electron-transfer

mechanism, we displayed the energy level diagram.

As illustrated in the Figure 7(b), surface plamons in

the Au NPs are photoexcited by visible light


7 Nano Res.

illumination and hot electron-hole pairs are generated.

Subsequently, the hot electrons transiently occupying

Figure 7 Schematic for (a) the proposed mechanism of PEC water

splitting and (b) the energy band structure of the ZnO, ZnO/Au and

ZnO/ZnS/Au photoanodes.

the surface plasmon states (SP states) were transferred

to the conduction band of ZnO to improve

photocurrent in the visible region[25-26]. With the

introduction of ZnS in the Figure 7(c), the higher

conduction band position of the ZnS shell than ZnO

leads to faster electron transfer to conduction band[29].

And, when ZnO is stacked with ZnS, the lattice strain

existed along the interface of ZnO/ZnS

heterostructure can lead to an increase in the oscillator

strength, where the individual band gap could be

smaller than the initial value[37-39]. The band gap of

ZnO can be compressed resulting from the interface

strain from such a staggered II band alignment for

ZnO/ZnS heterostructure. From the point of view of

thermodynamic, the free energy change (ΔG) for the

plasmon-induced hot-electron transfer from the

surface plasmon resonance level to the CB of ZnS-

coated ZnO (ΔGtransfer) is more positive than that of

uncoated ZnO. As a result, the increased ΔGtransfer is

beneficial for electron transfer from excited Au to ZnO

via tunneling effect across a thin layer of ZnS, which

finally lead to the improved photocurrent density

with the introduction of ZnS layer under visible light.

4 Conclusions In summary, ZnO/ZnS/Au nanorods array have been

successfully fabricated to serve as the photoanode of

photoelectrochemical water splitting. The

photoconversion efficiency of the ZnO/ZnS/Au

photoanode reached to 0.21%, 2 times and 3.5 times

higher that of ZnO/Au and pristine ZnO photoanodes,

respectively. The ZnO/ZnS/Au sandwich structure

possessed superior photocurrent density compared

with pristine ZnO nanorods array photoanodes both

in the UV and visible region, which can be explained

by EIS measurement and energy band structure. The

surface passivation and surface-plasmon-resonance

effect of Au NPs can enhance the photocurrent under

illumination, while the introduction of ZnS interlayer

is favorable for electrons injection into ZnO and

reduces the recombination at the photoanode/

electrolyte interface. The optimizing design confirms

a new avenue of preparing efficient photoanode for

PEC water splitting.

Acknowledgements This work was supported by the National Major

Research Program of China (2013CB932601), the Major

Project of International Cooperation and Exchanges

(2012DFA50990), the Program of Introducing Talents of

Discipline to Universities(B14003), the National Natural

Science Foundation of China (51232001, 51172022,

51372023, 31371203), the Research Fund of Co-

construction Program from Beijing Municipal

Commission of Education, the Fundamental Research

Funds for the Central Universities, the Program for

Changjiang Scholars and Innovative Research Team in

University.

Electronic Supplementary Material: EDX and Diffuse

reflectance UV−vis absorption spectra is available in

the online version of this article at

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