Hydrothermal fabrication of natural sun light active Dy2WO6 doped ZnO and its enhanced photo-electrocatalytic activity and self-cleaning properties

Article abstract of DOI:10.1039/c6ra24843h

In this article we report the fabrication of 3 wt% Dy₂WO₆ doped ZnO via a template-free hydrothermal process and its photocatalytic activity against azo dyes Rhodamine-B (Rh-B) and Trypan Blue (TB) in solar light irradiation. The as prepared Dy₂WO₆ doped ZnO was characterised by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution scanning electron microscopy (HR-SEM) field emission transmission electron microscopy (FE-TEM), X-ray photoelectron spectroscopy (XPS), diffused reflectance (DRS) and photoluminescence (PL) spectroscopy. The results suggested that rare earth tungstate doping, with Dy₂WO₆, on ZnO has a great influence on the photocatalytic activity. Dy₂WO₆-ZnO possesses high reusability without appreciable loss of catalytic activity up to four runs and exhibits higher electrocatalytic activity than the prepared ZnO for methanol electrooxidation in alkaline medium, revealing its promising potential as the anode catalyst in direct methanol fuel cells. Hydrophobicity of ZnO increases on doping with Dy₂WO₆.

Full text of DOI:10.1039/c6ra24843h

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PAPER
Hydrothermal fabrication of natural sun light active
Dy WO doped ZnO and its enhanced photo-
2
6
Cite this: RSC Adv., 2017, 7, 7509
electrocatalytic activity and self-cleaning
properties†
a
a
Kuppulingam Thirumalai, Manohar Shanthi
ab
and Meenakshisundaram Swaminathan*
In this article we report the fabrication of 3 wt% Dy
2
WO
6
doped ZnO via a template-free hydrothermal process
and its photocatalytic activity against azo dyes Rhodamine-B (Rh-B) and Trypan Blue (TB) in solar light irradiation.
The as prepared Dy WO doped ZnO was characterised by X-ray diffraction (XRD), field emission scanning
2
6
electron microscopy (FE-SEM), high resolution scanning electron microscopy (HR-SEM) field emission
transmission electron microscopy (FE-TEM), X-ray photoelectron spectroscopy (XPS), diffused reflectance
(
DRS) and photoluminescence (PL) spectroscopy. The results suggested that rare earth tungstate doping, with
Received 7th October 2016
Accepted 11th November 2016
Dy WO , on ZnO has a great influence on the photocatalytic activity. Dy WO -ZnO possesses high reusability
2
6
2
6
without appreciable loss of catalytic activity up to four runs and exhibits higher electrocatalytic activity than
the prepared ZnO for methanol electrooxidation in alkaline medium, revealing its promising potential as the
DOI: 10.1039/c6ra24843h
www.rsc.org/advances
2 6
anode catalyst in direct methanol fuel cells. Hydrophobicity of ZnO increases on doping with Dy WO .
5
6
7
8
9
10
11
12
Cu, Co, Mn, Sr, Fe, V and W, Mg, Pd etc. and metal oxide
Introduction
13
14
15
16
such as CdO, WO
3
,
Bi
2
O
3
,
BiVO
4
.
Metal tungstates as
The semiconductor photocatalytic process has been widely ancestors of multicomponent metal oxide compounds, have
applied as an eco-friendly technique for the destruction of been well thought-out largely due to their fascinating struc-
organic pollutants in wastewater because of its advantages of tures, interesting physic–chemical behavior, as well as their
1,2
photocatalytic activity in both UV and solar light. Among extensive range of applications in various elds specically in
17–22
various semiconductor nanomaterials, the II–VI semiconductor photocatalysis.
Nowadays research is focussed on doping of
ZnO possesses a special place due to its several interesting ZnO with rare earth metals and their oxides for the enhance-
23–27
properties and broad applications. The metal oxide, ZnO, ment of photocatalytic activity of ZnO.
a wideband-gap semiconductor with a large exciton binding
In this connection here we introduced a rare earth metal
energy of 60 meV at room temperature, is rationally expected to tungstate, Dy WO , doped ZnO nanoparticles, synthesised by
2
6
be a promising candidate for heterogeneous catalysis under simple hydrothermal method and analysed its photocatalytic
solar light and optoelectronics due to its long-term stability and activity under natural sun light irradiation in the degradation of
3
biocompatibility. However, it's large band gap energy (3.2 eV), two azo dyes (Rhodamine-B and Trypan Blue).
hampers its widespread practical applications. Thus, the pho-
tocatalytic activity of ZnO requires to be further improved.
Doping of ZnO nanomaterials signicantly increases the Dy
As heterostructured nanocomposites show good hydropho-
bicity and electrocatalytic activity, these characteristics of
2
6
WO -ZnO were analysed. We believe that our results can
surface defects which can presumably shi the absorption open a new and effective avenue to further improve the solar
towards visible region. The enhancement in optical absorption photocatalytic activity, self cleaning and electrocatalytic appli-
due to surface defects by doping has been studied by several cations of rare earth metal tungstate coupled ZnO systems.
4
research groups using different transition metals such as Cd,
Experimental procedure
a
Photocatalysis Laboratory, Department of Chemistry, Annamalai University,
Annamalai Nagar–608 002, Tamil Nadu, India. E-mail: chemres50@gmail.com; m. Materials
swaminathan@klu.ac.in; Fax: +91-4144-220572; Tel: +91-4144-220572
Zinc nitrate hexahydrate (Zn(NO ) $6H O), dysprosium nitrate
b
3 2
2
Nanomaterials Laboratory, International Research Centre, Kalasalingam University,
Krishnan Kovil–626126, India
hexahydrate Dy(NO ) $6H O, sodium tungstate dihydrate (Na -
3 3
2
2
WO
(CH
4
$2H
2
O), oxalic acid dihydrate (C
2
H
2
O
4
$2H
2
O), methanol
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra24843h
1
3
OH) (HPLC grade) were obtained from Himedia chemicals.
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Rhodamine-B (Rh-B, molecular formula C28
H31ClN O
2 3
2 6
and solution with Dy WO -ZnO was continuously aerated by a pump
molecular weight: 479.01), Trypan Blue (TB, molecular formula to provide oxygen and for the complete mixing of reaction solu-
C H N O S and molecular weight: 872.88) from Colour tion. During the illumination time no volatility of the solvent was
3
4
28 6 14 4
2
ꢀ1
Chem, Pondicherry and ZnO (surface area 5 m g , particle size observed. Heat produced in the experiment was dissipated by
.80 mm) from Merck chemicals were used as received. Deionized natural convection using a cooling fan. Aer dark adsorption the
4
distilled water was employed throughout the experiments.
rst sample was taken. At specic time intervals, 2 mL of the
sample was withdrawn and centrifuged to separate the catalyst. 1
mL of the centrifugate was diluted to 10 mL and its absorbance
was measured at 259 and 236 nm for Rh-B and TB dyes respec-
tively. The absorbance at 259 and 236 nm represents the aromatic
content of Rh-B and TB respectively and the decrease indicates
the degradation of dye. Solar light intensity was measured for
every 30 min and the average light intensity over the duration of
each experiment was calculated. The sensor was always set in the
position of maximum intensity. The intensity of solar light was
measured using LT Lutron LX-10/A Digital Lux meter and the
intensity was 1250 ꢂ 100 ꢃ 100 lux. The intensity was nearly
constant during the experiments.
Preparation of Dy
.649 g of Na WO
deionized water. Under vigorous agitation, 0.243 g of Dy
NO $6H O solution (0.05 M) was added into the Na WO
2
WO
6
-ZnO
1
2
4 2
$2H O (0.05 M) was dissolved in 100 mL of
(
)
3 3
2
2
4
solution at room temperature. The pH of the solution was
adjusted to 10 with NaOH for the complete precipitation of
Dy WO . The Dy WO suspension was mixed with 100 mL of
2
6
2
6
0.4 M Zn(NO ) $6H O (11.90 g) solution and stirred for 30 min.
3 2 2
100 mL of oxalic acid in distilled water (0.6 M) was introduced to
the above solution drop wise and stirred for 4 h to ensure
complete precipitation of zinc oxalate. The mixed precipitate of
zinc oxalate and Dy
2
WO
6
was treated hydrothermally in a Teon
ꢁ
lined stainless steel autoclave at 115 C for 12 h with the pressure
of 18 psi. Hydrothermally treated precipitate was dried in air at
Contact angle measurements
Coatings with catalysts and tetra ethoxyorthosilane (TEOS) were
successfully fabricated on a glass substrates using spin coating
method at room temperature. Catalysts coated substrates were
sintered at 125 C for 3 h with heating rate of 5 C min in
programmed furnace to ensure densication of the gel network.
A water droplet of 4 mL was placed on the coating and its water
contact angle was measured. The average of 5 measurements is
reported as the water contact angle (WCA) on the substrate.
ꢁ
ꢁ
9
0 C for 12 h and calcined at 450 C for 12 h in a muffle furnace
to obtain 3 wt% of Dy WO in ZnO. In the above procedure,
2
6
3 3 2 2 4 2
appropriate amounts of Dy(NO ) $6H O and Na WO $2H O were
ꢁ
ꢁ
ꢀ1
used to get 1 wt% and 5 wt% of Dy
2
WO
6
in ZnO.
Catalyst characterization
X-ray diffraction (XRD) patterns were recorded with a Siemens
D5005 diffractometer using Cu Ka (k ¼ 0.151418 nm) radiation.
Maximum peak positions were compared with the standard les
to identity the crystalline phase. The surface morphology of the
2 6
Dy WO -ZnO was studied by using a eld emission scanning
electron microscope (FE-SEM) and high resolution scanning
electron microscope (HR-SEM) (Model ULTRA-55). EDS analysis
was performed on gold coated samples using a FE-SEM (Model
ULTRA-55). HR-TEM images were taken from 200 kV ultra high
resolution transmission electron microscope (JEOL-2010). X-Ray
photoelectron spectra (XPS) of the catalysts were recorded in an
ESCA-3 Mark II spectrometer (VG Scientic Ltd, England) using Al
Ka (1486.6 eV) radiation as the source. The spectra were refer-
enced to the binding energy of C (1s) (285 eV). A Perkin Elmer LS
55 uorescence spectrometer was employed to record the photo-
luminescence (PL) spectra at room temperature. Diffuse reec-
tance spectra were recorded with Shimadzu UV-2450. UV
absorbance measurements were taken using Hitachi-U-2001
spectrometer. The water contact angles were measured using
a Drop Shape Analyzer (DSA) (Kruass GmbH, Germany).
Photocatalytic experiment
Photocatalytic experiments were performed under similar
conditions on sunny days of April–May 2015 between 11 am and
2
pm. An open borosilicate glass tube of 40 cm height and 20 mm
diameter was used as the reaction vessel. The suspensions were
magnetically stirred in the dark for 30 min to attain adsorption–
2 6
desorption equilibrium between the dye and Dy WO -ZnO. Irra-
2 6
Fig. 1 XRD patterns of (a) prepared ZnO, (b) 1 wt% Dy WO -ZnO, (c) 3
diation was carried out in the open air condition. 50 mL of dye wt% Dy
2
WO -ZnO and (d) 5 wt% Dy WO -ZnO.
6
2
6
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Electrochemical studies
Results and discussion
Cyclic voltammetry (CV) was performed with a CHI660 electro-
chemical workstation (CH Instruments, USA). Cyclic voltam-
metry has three-electrodes, Ag/AgCl electrode as the reference
electrode (saturated KCl reference electrode), a platinum wire as
a counter electrode and the modied glassy carbon electrode
Preliminary study on the degradation of Rh-B and TB with 1
wt%, 3 wt% and 5 wt% of Dy WO in ZnO catalysts was carried
out. The percentages of Rh-B degradation with 1, 3 and 5 wt% of
Dy WO in ZnO were found to be 68, 98 and 75 respectively for
5 minutes irradiation. The maximum efficiency (98%) was
observed with 3 wt% Dy WO in ZnO. Similar trend was
2
6
2
6
7
(GCE) as the working electrode. GCEs were polished before the
2
6
experiments in sequence with 1, 0.30 and 0.05 micron
aluminium/water slurry on micro cloth pads, followed by care-
ful cleaning in 1 : 1 HNO –H O (v/v), ethanol, and water via
observed for the degradation TB. Hence this 3 wt% Dy WO in
ZnO was characterized and used for further experiments.
XRD can be used as an effective non destructive tool for
qualitative and quantitative analysis of the phase structure.
Fig. 1a shows XRD pattern of the prepared ZnO. The diffraction
2
6
3
2
ultrasonication. The working electrode was prepared by spray-
ing the catalyst dispersed in iso-propyl alcohol. Cyclic voltam-
mograms were obtained between +1.0 and ꢀ0.1 V at a scan rate
ꢀ
1
peaks of ZnO at 31.82, 34.55, 36.55, 47.84, 56.75, 62.74, 66.77,
of 50 mV s . All potentials were reported with respect to the
reversible hydrogen electrode. The CO stripping voltammetry
curves were obtained aer adsorption of CO on catalysts at
ꢁ
6
7.85, 69.50, 72.77, and 77.32 correspond to (100), (002), (101),
(101), (110), (103), (220), (112), (201), (004) and (202) planes of
28
wurtzite ZnO (JCPDS card no. 36-1451). The new peaks
appeared in the XRD patterns of 1, 3 and 5 wt% of Dy WO -ZnO
ꢀ
0.2 V. For methanol electro-oxidation, 0.5 M methanol solu-
tion in 0.5 M NaOH solution was used as the electrolyte.
2
6
2 6
Fig. 2 FESEM and HR-SEM images of Dy WO -ZnO (a) FE-SEM image at 2 mm, (b) 10 mm and 2 (c–e) HR-SEM images at 100 nm.
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as seen in Fig. 1b–d respectively. The 2q values at 22.89, 23.74, used for the calculation of the average crystallite size of
2
4.21, 28.70, 30.52, 38.26, 41.28, 53.85, 64.73 and 74.17 for the Dy WO -ZnO.
2 6
planes (011), (211), (130), (321), (231), (320), (132), (741), (441)
and (622) in Fig. 1d (5 wt% Dy WO -ZnO) are due to the
Kl
F ¼
(1)
2
6
b cos q
monoclinic body cantered structure of Dy
conrmed by JCPDS card no. 26-0595 and 01-079-1722.
should be noted in Fig. 1b–d that 2q angles of (100), (002) and
101) planes for pure ZnO were gradually shied to the lower
diffraction values with increasing Dy WO concentration. This
is because of expansion of ZnO lattice caused by the larger
2 6
WO which is
29,30
where F is the crystallite size, l is the wavelength of X-ray used;
K is the shape factor, b is the full line width at the half-
maximum height of the peak, and q is the Bragg angle. Parti-
cles sizes were calculated using all the intense peaks. The
It
(
2
6
2 6
average crystallite size of Dy WO -ZnO is found to be 18.5 nm.
3
+
2+
Electronic microscopy is a technique widely used in structural
and chemical characterizations of materials. It provides infor-
mation about morphology, grain size, chemical composition,
crystallinity, and identication of the phases in the coated mate-
rials. Fig. 2 displays typical information about morphological
˚
˚
radius of Dy (0.91 A) than that of Zn (0.74 A). The increase in
lattice parameter and the shi to lower angle of the XRD peaks
with increasing in Dy
2 6
WO concentration were expected to have
the inuence on the lattice deformation and strain resulting
31,32
from Dy
2
WO
6
doping.
The Scherrer formula (eqn (1)) was
Fig. 3 Elemental colour mapping images of Dy
WO
2 6
-ZnO (a) Dy
2 6
WO -ZnO composition (b) Dy (c) Zn (d) W and (e) O.
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Fig. 4 FE-TEM images of Dy WO -ZnO (a and b) 20 nm and (c) 100 nm.
features of the Dy WO -ZnO through FE-SEM and HR-SEM
AFM images of Dy WO -ZnO with 5 mm ꢂ 5 mm regions are
2
6
2
6
images. FE-SEM images of Dy WO -ZnO, shown in Fig. 2a and shown Fig. 5. They clearly indicate the surface roughness and
2
6
b, clearly prove that the pure Dy WO -ZnO has a highly ordered porosity of the catalyst. The observation from AFM analyses
2
6
porous structure in three dimensions over a range of microme- conrms that the particles are nanosized. The existence of
ters. The HR-SEM images (Fig. 2c) further conrm the prepared nanosized particles in the Dy
photocatalyst Dy WO
nanochain (Fig. 2d) and sandwich-like mesoporous microstruc- roughness of the prepared Dy
ture clustered together (Fig. 2e). The size of Dy WO
is around 100 nm. These different morphologies of Dy WO -ZnO found as 87.4 nm. Higher surface roughness and lower size of
2
WO
6
doped ZnO nanomaterial is
2
6
-ZnO present in the form of combination of more clearly reected in its 2D AFM image (Fig. 5a). The surface
2
WO doped ZnO is also shown in
6
2
6
-ZnO clusters 3D AFM image (Fig. 5b). The average surface roughness was
2
6
may enhance photoactivity of the catalyst.
the particle imply that the surface is highly porous in nature.
Elemental composition of prepared Dy WO -ZnO was The surface porosity and roughness in the photocatalysts
2
6
determined using energy dispersive X-ray (EDX) spectroscopy. permit the adsorption of dye molecules on to the surface,
FE-SEM elemental colour mapping (Fig. 3a) clearly depicts the thereby increasing the photodegradation rate.
distribution of Dy, Zn, W and O on the surface of the catalyst.
XPS is a selective and sensitive surface characterization tech-
From Fig. 3a, it is clear that Zn and O are present in higher nique to determine the chemical compositions of materials, and
density and there is a homogenous distribution of Dy, Zn, W it is also effective in investigating the characteristics (valence) of
and O. Thus elemental mapping reveals that catalyst is the constituent atoms (ions) by monitoring their binding ener-
composed of Dy, Zn, W and O. Individual element contribution gies. Fig. 6a reveals the survey spectrum of Dy WO -ZnO photo-
2
6
is displayed in Fig. 3b–e for Dy, Zn, W and O respectively. This catalyst and it mainly consists of dysprosium (Dy), zinc (Zn),
also indicates the purity of the prepared catalyst Dy WO -ZnO. oxygen (O), tungsten (W) along with weak carbon (C) peak and no
The EDX analysis (Fig. S2†) conrmed the constituents of peaks of other elements are observed. Fig. 6b shows the XPS
Dy WO -ZnO as Dy, Zn, W and O with appropriate ratio. binding energy peak at 158.37 eV for Dy 4d5/2. This indicates
The FE-TEM images (Fig. 4a–c) reveal that the core and the dysprosium (Dy) ion is present in +3 oxidation state. XPS spectra
shell are separated, and there is a strong contrast between the of Zn 2p is shown in Fig. 6c and the peak positions of Zn 2p1/2 and
2
6
2
6
30
33
34
core (Dy WO -dark) and shell (ZnO-bright). As can be seen Zn 2p3/2 orbitals locate at 1044.74 and 1022.41 eV respectively.
2
6
from Fig. 4a, prepared catalyst has particles with hexagonal From these peaks, we can conclude that Zn is in the oxidation
2
+
structure. Fig. 4b clearly displays a large number of regular- state of Zn . From the Fig. 6d we can observe that one set of lines
shaped hollow spherical-like particles with a diameter of for W (4f7/2) and W (4f5/2) at binding energies of 31.15 and
6
+
2
0 nm. It further proves that the lattice structure of prepared 38.74 eV, respectively. The peak energies are consistent with W
35
2 6
photocatalyst Dy WO -ZnO is highly ordered. The sizes particles standard peaks. Binding energy peak of O 1s is asymmetric and
are in the range of 15 to 20 nm.
can be tted to two symmetrical peaks (locating at 528.86 and
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efficiency of the photodegradation of the organic dye molecule
by the prepared photocatalyst. UV-Vis spectra in the diffuse
reectance mode (R) were transformed to the Kubelka–Munk
function F(R) to separate the extent of light absorption from
scattering. The band gap energy was obtained from the plot of
1
/2
the modied Kubelka–Munk function (F(R)E)
energy of the absorbed light E (eqn (2))
versus the
"
#
1=2
2
ð1 ꢀ RÞ
1=2
FðRÞE
¼
ꢂ hn
(2)
2
R
Photocatalysts generate electrons and holes aer being acti-
vated by light, and recombination of some electrons and holes
can release energy in the form of uorescence emission. Lower
uorescence emission intensity implies lower electron–hole
recombination rate. The uorescence emission is mostly from
the recombination of the photogenerated electron–hole pairs,
and a strong correlation between PL intensity and photocatalytic
37
activity has been previously reported. Fig. 7 presents the pho-
toluminescence spectra of the prepared ZnO (a), and Dy WO -
2
6
ZnO (b). The excitation wavelength is 380 nm and two emission
bands occur at 419 and 489 nm. It can be seen that the positions
of the peaks are similar, while PL intensities are different. The PL
intensity of prepared ZnO at 419 nm, which is due to electron–
2 6
hole recombination, is higher than Dy WO -ZnO. The electron
capture by the loaded Dy WO will decrease the electron–hole
2
6
recombination and cause a decrease in emission intensity of
23
Dy WO -ZnO as reported for Pr O -ZnO. Higher photocatalytic
2
6
6 11
2 6
activity of Dy WO -ZnO reveals that the decrease in emission
intensity is due to suppression of electron–hole recombination by
15,38
2 6
the loaded Dy WO .
Reduction in electron–hole recombina-
tion leads to the increased availability of electrons and holes for
the production of radicals for degradation.
Fig. 5 AFM images of the surface morphology of 5 mm ꢂ 5 mm region
2 6 2 6
of the Dy WO -ZnO (a) 2D image and (b) 3D image of Dy WO -ZnO.
Photocatalytic activity of Dy WO doped ZnO
2
6
Photocatalytic degradation of Rh-B under different conditions
with increasing irradiation times is displayed in Fig. 8. Dye is
resistant to self photolysis and for the same experiment with
5
(
31.2 eV), for two different kinds of O species in the sample
Fig. 6e). These two peaks should be associated with the lattice
oxygen (OL) of Dy WO -ZnO and chemisorbed oxygen (OH)
2
6
Dy
observed due to the adsorption of dye on the catalyst. Rh-B
undergoes 98% degradation in the presence of Dy WO -ZnO
under natural sunlight in 75 min. But, prepared ZnO, TiO , TiO
P25, and undoped Dy WO shows 72%, 57%, 68% and 64%
2 6
WO in the dark, a decrease (8%) in dye concentration was
36
caused by the surface hydroxyl groups.
To nd out the optical characteristics, UV-Vis diffuse
reectance and photoluminescence spectra for the ZnO and
2
6
2
2
–
Dy
2
WO
6
-ZnO catalysts were recorded. Fig. S3† shows the DRS
WO -ZnO. Any impurity in the semiconductor
2
6
spectrum of Dy
2
6
degradations, respectively in 75 min. These results show that
oxide can form intermittent band energy levels and this leads to
the decrease of bandgap energy, which increases the UV-visible
absorption. The prepared ZnO presents a sharp absorption edge
at 380 nm. From the Fig. S3,† prepared photocatalyst shows an
increased absorption over the undoped ZnO material both in
the visible and the ultraviolet regions. Hence the band gap
energy is reduced relative to prepared ZnO and it serves as
a support to resist the recombination process together with the f
orbitals of dysprosium tungstate, which traps the electrons
excited during the photodegradation process. These would
facilitate the reaction of the separated hole with the water
molecules to produce OH free radicals, thus enhancing the
prepared 3 wt% Dy WO -ZnO nanomaterial is most efficient for
degradation of Rh-B dye than other photocatalysts (Fig. 8). 3 wt%
2
6
Dy
Dy
2
WO
WO
6
-ZnO shows higher activity (98%) than other 5 wt%
-ZnO (80%). It is obvious that the higher photocatalytic
2
6
2 6 2 6
activity of Dy WO -ZnO is due to the support of Dy WO . To test
the effectiveness of the catalyst on the degradations of other azo
dyes, we had carried out the experiments on the degradation of
TB under the same conditions. Fig. S4† shows the comparative
studies of degradations of Rh-B and TB using Dy WO -ZnO at
2
6
different irradiation times. Rh-B and TB undergo 98% and 94%
degradation respectively in 75 min, revealing that this catalyst is
efficient in the degradation of azo dyes.
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2 6
Fig. 6 XPS analysis of Dy WO -ZnO (a) survey spectrum, (b) Dy 4d, (c) Zn 2p, (d) W 4f and (e) O 1s.
The solution pH plays a crucial role in the photocatalytic than undoped ZnO in the degradation of Rh-B because it has
degradation. The effect of pH on the photodegradation of Rh-B maximum efficiency at the neutral pH 7.
was studied in the pH range 3–11 and the results are shown in
The reusability of Dy WO -ZnO was tested for the degrada-
2 6
Fig. S5.† The degradation efficiency is high at pH 7 and it tion of Rh-B dye under the same reaction environment. Aer
decreases when the pH is above or below 7. Low removal effi- complete degradation, the catalyst was separated and washed
ciency at acidic pH range may be due to the dissolution of ZnO in with deionized water. The recovered catalyst was dried in hot air
ꢁ
Dy
2
WO
6
-ZnO. It shows prepared catalyst is more advantageous oven at 100 C for 3 h and used for a second run. Fig. S6† shows
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Scheme 1 Degradation mechanism.
2 6
Fig. 7 Photoluminescence spectrum of Dy WO -ZnO (a) prepared
2 6
ZnO and (b) Dy WO doped ZnO.
2
7
in comparison with that of pure ZnO. The mechanism of
electron capture and transfer is displayed in Scheme 1. The
the result of Rh-B degradation for four successive runs.
Dy WO -ZnO gives enhanced activity in Rh-B dye degradation
decreased PL intensity in Dy
recombination of electron is suppressed by Dy
indicating that an appropriate amount of Dy
2
WO
6
-ZnO, reveals that the
WO particles,
WO could
2
6
2
6
percentages as 98.0, 97.4, 96.0 and 96.0 for 75 min in the rst,
second, third and fourth run respectively. There is no signi-
cant change in the degradation efficiency of Dy WO -ZnO aer
2 6
2
6
signicantly reduce the irradiative recombination rate of pho-
togenerated electrons and holes in ZnO. The band gap energy
obtained by Kubelka–Munk function is comparatively lower
than pure undoped ZnO. AFM studies also prove that the
surface of the synthesized photocatalyst is very rough and
porous in nature and this may assist the absorption of organic
molecules on the surface of the catalyst.
second run.
Mechanism for dye degradation
The dysprosium is a rare earth metal belongs to lanthanide
group and the element possessing f shells and they are capable
of trapping the electrons generated due to presence of f–f
39
transition at visible light by the photocatalyst. The transitions
of 4f electrons of lanthanides lead to the implementation of the
optical adsorption of catalysts and support the separation of
photogenerated electron–hole pairs. In the case of dysprosium
Electrochemical methanol oxidation
The electronic structure of the electrode greatly inuences its
electrocatalytic activity. It has been proved that the electro-
oxidation of methanol is a surface-sensitive reaction. Pt and
Ag/AgCl electrodes are normally used as the counter and refer-
ence electrodes, respectively. Fig. 9a and b shows the cyclic
voltammograms (CVs) of glassy carbon electrode coated with
prepared ZnO and Dy WO -ZnO, taken in N saturated 0.5 M
NaOH solution at a scan rate of 50 mV s . Typical potential
regions for hydrogen adsorption/desorption and the formation/
reduction of surfaces of metal oxide or metal–OHads of ZnO, and
3
+
4+
3+
dopant, it can exist as Dy and Dy . Thus, Dy may give an
electron to O adsorbed on the surface of Dy WO -doped ZnO to
2
2
6
ꢀ
4+
form cO2 by transforming into Dy , favouring a charged
migration to O and an enhancement of the photoreaction rate
2
2
6
2
ꢀ
1
Dy
2
WO
6
-ZnO are clearly seen in Fig. 9a and b.
Anodic and cathodic reactions of methanol fuel cell are given
below. Water is consumed at the anode and generated at the
cathode. Electrons pass through externally from anode to
cathode, producing the current.
+
ꢀ
3 2 2
Anode: CH OH + H O / CO + 6H + 6e
+
ꢀ
Cathode: O + 4H + 4e / 2H O
2
2
Overall: CH
3
OH + 3/2O
2
/ CO
2
2
+ 2H O
Fig. 9a shows the typical cyclic voltammograms of prepared
ZnO for methanol electrochemical oxidation up to 20 segments.
The current cathodic oxidation peak at ꢀ0.27 V (vs. Ag/AgCl) in
the forward scan and the reduction peak appearing at ꢀ0.037 V
correspond to the methanol oxidation and removal of the CO
ꢀ
ꢀ1
4
Fig. 8 Primary analysis: Rh-B dye concentration ¼ 3 ꢂ 10
M,
ꢀ1
catalyst suspended ¼ 3 g L , pH ¼ 7, airflow rate ¼ 8.1 mL s and
irradiation time 75 min, Isolar ¼ 1250 ꢂ 100 lux ꢃ 100.
7516 | RSC Adv., 2017, 7, 7509–7518
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Fig. 10 Water contact angle measurements (a) uncoated glass slide,
(b) TEOS coated glass slide, (c) TEOS + ZnO coated glass slide and (d)
TEOS + Dy WO -ZnO coated glass slide.
2
6
nanomaterial is signicantly less (4.8 mA to 2.64 mA) than that
on the prepared ZnO (Fig. S7c,† 4.8 mA to 1.5 mA) and Dy WO6
2
(Fig. S7b,† 4.8 mA to 2.1 mA) and the steady current is reached
at 2.64 mA for Dy WO -ZnO. This indicates that Dy WO -ZnO
2
6
2
6
attained stability earlier than other catalysts and the improve-
ment in the electrochemical constancy for methanol electro-
oxidation in alkaline medium.
Contact angle measurements
Surface non-wettability or the hydrophobicity of the catalyst is
exposed by water contact angle. If a surface has a contact angle
ꢁ
Fig. 9 Cyclic voltammogram of (a) prepared ZnO (b) and Dy
2
WO
6
-
with water that is greater than 90 , then the surface is classed as
ZnO in N
2
and saturated 0.5 M NaOH + 0.5 M CH
3
OH solution at
ꢁ
hydrophobic and if the contact angle is less than 90 , the
surface is hydrophilic. Water contact angles were determined
using glass slides coated with TEOS, TEOS + ZnO and TEOS +
ꢀ1
ꢁ
a scan rate of 50 mV s at 25 C.
2 6
Dy WO -ZnO to investigate the hydrophobicity of the catalysts.
species adsorbed on the catalyst surface respectively. Therefore,
the peak potential and current density of the forward anodic
oxidation peak can be used to evaluate the catalytic activity of
the electrocatalyst. Fig. 9b depicts cyclic voltammogram of
Dy WO -ZnO, and it clearly explains the effect of Dy WO on
ZnO in methanol oxidation. The sharp anodic peak at ꢀ0.13 V
due to oxidation of methanol and the current efficiency reached
around 2.64 mA, which is two times higher than prepared ZnO
Fig. 10 shows the images of water drops on coated and uncoated
ꢁ
glass slides. Water contact angle (WCA) of 28.5 on uncoated
glass slide shows the hydrophilicity and this WCA increases
ꢁ
gradually on glass slides coated with TEOS (50.2 ), TEOS + ZnO
2
6
2
6
ꢁ ꢁ
73.6 ) and TEOS + Dy WO -ZnO (98.9 ). Surface coated with
2 6
(
TEOS + Dy WO -ZnO has more hydrophobic character. In TEOS,
2
6
2 6
the O–Si–O groups are modied by Dy WO -ZnO to make the
surface rougher, stable and non-wettable. Hence the contact
(1.32 mA). This enhanced electrochemical activity of Dy
2
WO
6
-
ꢁ
angle increases above 90 exhibiting the hydrophobicity of the
ZnO reveals that the catalyst will be useful in fuel cell
application.
The electrochemical stability of the catalysts for methanol
electro-oxidation was investigated by chronoamperometric
catalyst. This surface non-wettability leads to a self cleaning
property of the catalyst.
Conclusion
experiments at ꢀ0.1 V in 0.5 M NaOH + 0.5 M CH
3
OH solution
and the curves are shown in Fig. S7a–c.† All these catalysts
initially show a rapid decrease in current for the oxidation of
methanol, and then a relatively steady current is achieved. The
rapid current decay is due to the poisoning of intermediate and
various poisoning species formed during methanol oxidation
2 6
In conclusion, a new sunlight active Dy WO -ZnO photocatalyst
was synthesized by a simple hydrothermal-thermal decompo-
sition method and characterized by the various analytical
techniques. The XRD pattern conrms that ZnO has wurtzite
structure with good crystallinity and Dy WO has body-centered
2
6
40,41
reaction in alkaline medium.
It is clear from the Fig. S7a†
monoclinic structure with crystallite size of 18.5 nm. The
that the current decay for the reaction on the Dy WO -ZnO
2
6
HRSEM images of Dy
2
WO
6
-ZnO indicate a sandwich-like
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microstructure loosely clustered together, and the average size 17 E. T. Deva Kumar, K. Thirumalai, R. Aravindhan,
of Dy WO -ZnO particles is about 100 nm. FETEM further
M. Swaminathan, J. Raghava Rao and B. U. Nair, RSC Adv.,
2015, 5, 60926–60937.
2
6
proves the morphology of prepared Dy WO doped ZnO as
2
6
hexagonal and regular sphere shaped particles. Elemental 18 G. Colon, S. Murcia Lopez, M. C. Hidalgo and J. A. Navio,
colour mapping studies reveal the homogeneous distribution of Chem. Commun., 2010, 46, 4809–4811.
Dy, Zn, W and O in the catalyst. Dy WO -ZnO shows enhanced 19 D. Li, R. Shi, C. Pan, Y. Zhu and H. Zhao, CrystEngComm,
activity in the degradation of azo dyes Rh-B and TB compared to 2011, 13, 4695–4700.
the prepared ZnO, Dy WO , TiO , and TiO –P25. This catalyst 20 B. Hu, L. H. Wu, S. J. Liu, H. B. Yao, H. Y. Shi, G. P. Li and
was found to be stable and reusable. Electrocatalytic activity of S. H. Yu, Chem. Commun., 2010, 46, 2277–2279.
Dy WO -ZnO exhibited enhanced current production by elec- 21 N. Tian, Y. Zhang, H. Huang, Y. He and Y. Guo, J. Phys. Chem.
2
6
2
6
2
2
2
6
trochemical methanol oxidation under room temperature.
C, 2014, 118, 15640–15648.
Hydrophobicity of Dy WO -ZnO makes it useful for industrial 22 Z. Zhang and W. Wang, Dalton Trans., 2013, 42, 12072–12074.
2
6
self-cleaning material.
23 S. Balachandran, K. Thirumalai and M. Swaminathan, RSC
Adv., 2014, 4, 27642–27653.
2
4 L. Honglin, L. Yingbo, L. Jinzhu and Y. Ke, J. Alloys Compd.,
014, 617, 102–107.
Authors are thankful to the CSIR, New Delhi, for the nancial 25 M. Khatamian, A. A. Khandar, B. Divband, M. Haghighi and
support through research Grant no. 02 (0144)/13/EMR-II.
S. Ebrahimiasl, J. Mol. Catal. A: Chem., 2012, 365, 120–127.
Authors are grateful to International Research Centre, Kalasa- 26 P. V. Korake, A. N. Kadam and K. M. Garadkar, J. Rare
Acknowledgements
2
lingam University for FESEM facility.
Earths, 2014, 32(4), 306–313.
27 A. Khataee, R. D. C. Soltani, Y. Hanifehpour, M. Safarpour,
H. G. Ranjbar and S. W. Joo, Ind. Eng. Chem. Res., 2014,
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