5
36
X. Cheng et al. / Electrochimica Acta 105 (2013) 535–541
(0.1 mol L 1) was served as the supporting electrolyte. Meanwhile,
a 350 W Xenon arc lamp was served as the simulated solar light
source, which was placed 15 cm beside the light source. Photo-
luminescence (PL) spectra were measured at RT on an FP-6500
fluorescence spectrophotometer (Hitachi, Japan).
−
absorb more photons to participate in the PC reaction [6,7]. In addi-
tion, the higher mobility and separation of photogenerated charge
carriers could induce a higher PC efficiency. Among which, CdS
nano-material have been received more focus [8–10].
Although there were many reports about the synthesis of CdS
decorated TiO2 composite catalysts. However, these reports were
mainly related with the pulverous CdS/TiO2 composites. To the
best of our knowledge, it was still lack of comprehensive inves-
tigation about the preparation of CdS NCs decorated TiO2 NTAs
and its enhanced PC mechanism. Thus, in the present study, CdS
nano-crystallite decorated TiO2 NTAs (CdS NCs/TiO2 NTAs) pho-
toelectrodes have been prepared through anodization method,
followed by electrodeposition strategy. Furthermore, the enhanced
photoelectrocatalytic (PEC) performance and mechanism were dis-
2.4. Evaluation of photoelectrocatalysis (PEC) and photocatalysis
(PC)
The PEC and PC degradations were carried out in a self-made
cylindrical quartz photoreactor, which contained 30 mL solution
of Na SO4 (0.1 mol L ) and RhB (5 mg L ). Prior to irradiation,
−
1
−1
2
CdS NCs/TiO2 NTAs electrode was vertically fixed in the reactor
and then magnetic stirred for 30 min in the dark to establish the
equilibrium of adsorption/desorption. Afterwards, 350 W Xenon
light was switched on. At given time intervals, the collected sam-
ples were filtrated and measured at 553 nm using a T6 UV–vis
spectrophotometer. It should be noted that the PEC process was
conducted under the same procedure, while a 2 V external poten-
tial was applied and controlled by a two-channel output DC power
supply (DH1715A-5).
cussed in detail. As a result, CdS NCs/TiO NTAs exhibited higher PC
2
and PEC performances.
2
. Experiments
2.1. Materials
Sodium sulfate (Na SO ), sodium fluoride (NaF), acetone
2
4
(
CH COCH ), absolute ethanol (C H5OH), rhodamine B (RhB), cad-
3. Results and discussion
3
3
2
mium chloride (CdCl ) and thioacetamide (CH CSNH ) were kindly
2
3
2
purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals
used in this study were analytical grade without any further purifi-
cation. Deionized (DI) water was used throughout our experiments.
3.1. SEM and TEM analysis
Fig. 1 showed the typical SEM images of the as-prepared bare
TiO2 NTAs and CdS NCs/TiO2 NTAs photoelectrodes. Clearly, it can
be observed that the average inner diameter of bare TiO2 NTAs was
about 94 nm with a wall thickness of about 19 nm (Fig. 1a). How-
ever, as shown in Fig. 1B, highly ordered nano-tubular structure
could also be observed, which possessed an average inner diame-
ter of around 120 nm and wall thickness of 20 nm. Furthermore, as
shown in Fig. 1b, CdS NCs were uniformly deposited onto the sur-
face of the highly oriented TiO2 NTAs with a uniform size of about
20 nm. In addition, as displayed in Fig. 1c, the shapes and surface
structures of CdS NCs/TiO2 NTAs were further confirmed by TEM
and HRTEM. As shown in inset of Fig. 1c, the perfect lattice fringes of
0.352 nm could be clearly observed, which was in accordance with
the (101) plane of anatase TiO2 [11]. Moreover, as seen from the
TEM bright field image of CdS NCs/TiO2 NTAs sample (as shown in
inset of Fig. 1c), diffraction spots along with diffraction rings could
be obviously observed, revealing the existence of CdS NCs.
2
.2. Preparation of CdS NCs/TiO2 NTAs photoelectrode
Firstly, titanium foils (99.6%, 0.2 mm thickness) were ultrasoni-
cally cleaned in acetone and absolute ethanol, followed by rinsing
with DI water and drying for 1 h at 80 C. Typically, anodization pro-
cess was conducted in a traditional two electrode configurations,
in which Ti foils and Pt electrode were served as anode and cath-
ode, respectively. At room temperature (RT), the pre-treated Ti foils
◦
2
(
with an efficient electrode area of 2 cm ) were anodized at 20 V
for 2 h in a mixed electrolyte which containing 0.5 wt% NH F and
1
ples were rinsed with DI water and dried for 24 h at 343 K. Finally,
the obtained samples were annealed for 2 h at 723 K in a muffle
furnace with a heating rate of 3 C min
In this study, CdS NCs/TiO2 NTAs photoanode have been pre-
pared through electrodeposition strategy, in which Pt electrode
and TiO2 NTAs were served as the anode and cathode electrodes,
respectively. The electrolyte solution was CdCl (0.1 mol L ) and
CH CSNH (0.06 mol L ). Electrodeposited process was performed
4
−
.0 mol L Na SO . Subsequently, the as-anodized TiO NTAs sam-
2 4 2
1
◦
−1
.
3.2. XRD and DRS analysis
−
1
2
−1
XRD patterns of TiO2 NTAs and CdS NCs/TiO2 NTAs samples were
performed and shown in Fig. 2A. Clearly, as shown in Fig. 2A, all
the crystallite phase could be indexed from their corresponding
characteristic peaks using the anatase phase (JCPDS No. 21-1272)
and Ti metal phase (JCPDS NO. 44-1294) [12], which were marked
distinctly with A and T in the XRD patterns, respectively. How-
ever, it should be noted that after decoration with CdS NCs, TiO2
NTAs sample exhibited three additional diffraction peaks located
3
2
at 0.8 V for 0.5 h. After electrodeposition, the as-obtained samples
were rinsed with DI water and annealed at 623 K for 2 h in a muffle
◦
−1
.
furnace with a heating rate of 3 C min
.3. Characterization
The morphologies of samples were observed by using a field
2
◦
◦
◦
emission scanning electron microscope (Quanta 200F). X-ray
diffraction (XRD) has been performed on a D8 Advance (Bruker)
diffractometer with Cu K␣ radiation. The accelerating voltage
and applied current were held at 40 kV and 30 mA, respectively.
Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS)
was recorded on a TU-1901 spectrophotometer equipped with an
integrating sphere, in which BaSO4 was used as the reflectance
sample. The photoelectrochemical (PECH) measurements were
measured on a LK 3200 electrochemical workstation in a three-
cell configuration with the as-prepared TiO2 NTAs, Pt electrode
and saturated calomel electrode (SCE) as the photoanode, counter
at 26.5 , 43.8 and 51.4 , corresponding to the hexagonal phase
(1 1 1), (2 2 0), (3 1 1) of CdS NCs [13]. Thus, it was reasonable that
CdS NCs were successfully deposited onto the surface of TiO NTAs.
2
In order to investigate the light absorption ability of the as-
prepared TiO2 NTAs samples, UV–vis diffuse reflectance spectra
(DRS) were recorded. As shown in Fig. 2B, both TiO2 NTAs samples
exhibited typical onset absorption edge at about 390 nm, corre-
2−
sponding to the electronic transition from O anti-bonding orbital
to the lowest empty orbital of Ti4+ (O2p → Ti3d) [14]. Nevertheless,
compared with bare TiO2 NTAs, the light absorbance edge of CdS
NCs/TiO2 NTAs photoelectrode was significantly red-shifted to
visible region with the strongest peak located at 450 nm between
electrode and reference electrode, respectively. Na SO4 solution
2