C. Cai et al. / Electrochimica Acta 274 (2018) 298e305
299
ꢁ
determining the catalytic activity. A large specific surface area is
essential to expose more activity sites. Besides, a monolithic cata-
lyst photoelectrode with catalyst grown on transparent conductive
substrate is another essential factor worthy to be considered.
Moreover, nanostructure electrode could produce local high elec-
tric field that concentrate alkali metal cations, which in turn leads
immerged into the solution at 90 C for 5 h, 10 h and 20 h, respec-
tively. After the reaction, the substrate was washed several times
ꢁ
with deionized water and calcined at 500 C for 30 min.
2.3. Synthesis of ZnO@ZnSe heterogeneous nanosheet (NS) arrays
to high local concentration of CO
2
close to the active CO
2
reduction
An aqueous solution containing 12.6 mM Se powder and
ꢁ
reaction surface [18]. Compared with the common-used electrode
made by sticking pre-prepared catalyst powders onto conductive
substrate, the monolithic electrode can provide stronger durability,
better electrical contact and much more active sites [19]. However,
it cannot be realized in some cases depending on the synthesis
conditions.
88.2 mM NaBH
4
was prepared at 50 C under string. The above-
mentioned ZnO NS array film was immersed in the solution for
1 h, 3 h and 6 h, respectively. The substrate was washed several
times with ethanol and dried in vacuum.
2.4. Structural and electronic property characterization
Zinc selenide (ZnSe), with a direct band gap of 2.67 eV [20], is a
very attractive II-IV semiconductor for optoelectronic devices, such
as blue-green light-emitting diodes [21], electrooptical detectors
The morphology and composition of the samples were
measured using a field-emission scanning electron microscope (FE-
SEM, Hitachi SU8010), high-resolution transmission electron mi-
croscopy (HRTEM, JEOL 2010HR), X-ray photoelectron spectroscopy
(XPS, ESCALAB 250) and X-ray diffractometer (XRD, Rigaku Smar-
tlab). A UVeviseNIR spectrophotometer (Shimadzu UV-3600) was
employed to measure the UVevis absorption spectra. Mott-
Schottky plots were recorded on a Zennium electrochemical
workstation (Zahner, German) operating with the Thales 2.29
software.
[
22], and solar cells [23]. Benefiting from the visible light response,
ZnSe has been used as efficient photocatalyst for degradation of
organic pollution [24,25] and PEC splitting of water [26]. For
example, a ZnSe@CdS@CdSe triple-sensitized ZnO NW arrays ma-
terial was reported to exhibit enhanced oxygen evolution activity
through the effective synergistic light absorption and the multi-
type-II graded bandgap level between composite nanostructures
[
27]. Interestingly, the conduction band edge of ZnSe is negative
enough to fulfil the driving force for CO reduction kinetics. Natu-
2
rally, we consider it as a very promising photocathode. Surprisingly,
to the best of our knowledge, neither nanostructured ZnSe powder
2.5. PEC reduction of CO
2
nor photocathode had been developed for CO
porous ZnO@ZnSe nanosheet array on fluorine doped-tin oxide
FTO) glass is synthesized via dissolution-recrystallization process
by using ZnO nanosheet array as the template, and is used for the
first time as the photocathode for PEC reduction of CO . Under the
irradiation of visible light, the as-prepared photocathode exhibits
onset potentials at 0.39 V (vs. RHE) in CO -purged 0.5 M NaHCO
solution and the selectivity for CO versus H generation is
increased with enlarging the applied negative bias. A faradaic ef-
2
reduction. Herein,
All the electrochemical characterizations were performed on a
Zennium electrochemical workstation (Zahner, German) operating
with the Thales 2.29 software. Unless otherwise stated, all the
potentials were reported relative to the reversible hydrogen elec-
(
2
2
trode (RHE). The solar-driven PEC reduction of CO was performed
in 0.5 M NaHCO electrolyte (pH 7.5), a standard three-electrode
3
2
3
configuration (Fig. S1) with the as-prepared film as the photo-
cathode, a Pt mesh as the counter electrode and an Ag/AgCl (in
saturated KCl) electrode as the reference electrode. The ZnO@ZnSe
film was directly fabricated on FTO glass. Thus, it was feasible for
this film be used as photocathode directly. To simulate the sunlight
illumination, a 150 W Xe lamp (LSXS-150, Zolix, China) coupled
with an AM 1.5G filter (BCF-AM 1.5G-050, Zolix, China) was
2
2
ficiency of 52.9% for CO and selectivity of 96.6% for CO
2
reaction is
obtained at ꢀ0.4 V (vs. RHE).
2
. Experimental section
ꢀ2
equipped and the light density was adjusted to 100 mW cm by
calibrating with a standard Si solar cell. Before the reduction
2.1. Materials
experiment, the electrolyte solution was bubbled with CO
2
gas
2 2
Zinc acetate dehydrate (Zn(Ac) $2H O) was purchased from
(99.99%) for 30 min.
Sinopharm Chemical Reagent Co., Ltd.; Zinc nitrate hexahydrate
Zn(NO $6H O) was purchased from Guangzhou Chemical Re-
agent Factory; Urea (H NCONH ) was purchased from Tianjin Baishi
A Fuli Model GC9790 II gas chromatograph equipped with a
TDX-01 packed column, a TCD and a FID detectors was used for the
catalytic product analysis. The gaseous samples were tested for
(
3
)
3
2
2
2
Chemical Industry Co., Ltd; Selenium (Se) powder was purchased
from Aladdin Industrial Inc. (Shanghai, PR China); Sodium boro-
2
potential products of CO reduction such as CO, methane, ethylene
and ethane by injecting into the gas chromatograph. A SE-30 col-
umn was employed for testing the liquid products of methanol and
ethanol with an FID detector. An attempt to measure the quantity of
HCOOH was made by ion chromatography (IC) (Metrohm 882)
equipped with a Mestrosep A sup 5 column.
4
hydride (NaBH ) was purchased from Aladdin Industrial Inc.
(
Shanghai, PR China). All the reagents were of analytical grade and
used as received without further purification. Deionized water was
used throughout the experiments.
2.2. Synthesis of ZnO nanosheet (NS) arrays
3. Results and discussion
Fluorine doped-tin oxide (FTO) glasses were cleaned with
The preparation procedure of ZnO@ZnSe nanosheet (NS) array
electrode was illustrated in Scheme 1. It started with the growth of
ZnO NS array on FTO glass coated with ZnO seed layer, which was
used as template subsequently to grow ZnSe by anion exchange
ethanol, acetone, and deionized water (DI water) in sequence, and
then dried in air. In a typical experimental procedure, 1.5 mL
diethanolamine was added to an ethanol solution containing 0.6 M
2
ꢀ
Zn(AC)
2
$2H
2
O. After storing overnight, the solution was spin coated
reaction with a Se source. The ion exchange reactions were based
ꢁ
onto a FTO glass substrate and thermally cured at 300 C for 30 min
on the difference of solubility product constant (Ksp) for ZnO
ꢀ17
ꢀ26
to obtain the ZnO seeded FTO glass. Then 0.1 M Zn(NO
.1 M urea were dissolved in 50 mL DI water. The solution was
transferred to a glass bottle. The ZnO seeded FTO glass was
3
)
3
$6H
2
O and
(6.8 ꢂ 10 ) and ZnSe (3.6 ꢂ 10 ) [13]. As shown in Fig. 1a, the X-
ray diffraction (XRD) pattern of the ZnO NS array showed a hex-
agonal wurtzite structure (JCPDS no.36-1451) [27,28]. After ion
0