Solar photocatalytic and self-cleaning performances of HoVO4 doped ZnO

Article abstract of DOI:10.1166/jnn.2018.14579

In this article we report preparation of 5 wt% HoVO4 doped ZnO via template-free hydrothermal process and investigated its photocatalytic activity against azo dyes Rhodamine-B (Rh-B), Trypan Blue (TB) and Acid Black 1 (AB 1) in solar light irradiation. Th

Full text of DOI:10.1166/jnn.2018.14579

Article
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Nanoscience and Nanotechnology
Vol. 18, 178–187, 2018
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Solar Photocatalytic and Self-Cleaning Performances of
HoVO4 Doped ZnO
Kuppulingam Thirumalai1, Manohar Shanthi1, and Meenakshisundaram Swaminathan1ꢀ2ꢀ∗
1Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India
2Nanomaterials Laborotory, International Research Centre, Kalasalingam University, Krishnankoil 626126, India
In this article we report preparation of 5 wt% HoVO4 doped ZnO via template-free hydrothermal
process and investigated its photocatalytic activity against azo dyes Rhodamine-B (Rh-B), Trypan
Blue (TB) and Acid Black 1 (AB 1) in solar light irradiation. The as prepared HoVO4 doped ZnO was
characterised by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM),
Field emission Transmission electron microscopy (FE-TEM), Brunauer-Emmett-Teller (BET) surface
area measurements, X-ray photoelectron spectroscopy (XPS), Diffused reflectance (DRS) and Pho-
toluminescence (PL) spectroscopy. The SEM images clearly indicate the formation of nanoparticles
in the range of 20–50 nm. BET surface area of the HoVO4–ZnO is 2 times that of ZnO. Higher
activity of HoVO4–ZnO in natural sunlight may be due to higher visible light absorption of HoVO4–
ZnO when compared to undoped ZnO. The results suggested that HoVO4 doping on ZnO has
great influence on the photocatalytic activity. The prepared photocatalyst HoVO4–ZnO possesses
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high stability and reusability without appreciable loss of catalytic activity up to four runs. Significant
Copyright: American Scientific Publishers
hydrophobicity of HoVO4–ZnO reveals its self cleaning property.
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Keywords: Photocatalysis, HoVO4–ZnO, Azo Dyes, Hydrophobicity, Solar Light.
1. INTRODUCTION
usually produces dramatic changes in their electronic prop-
erties by increasing their electron or hole densities.15–19
Rare earth compounds have received extensive attention
because of their potential applications in high-performance
luminescent devices, magnets, catalysts, and other func-
tional materials, on the basis of optoelectronic and chem-
ical properties originating from the 4f shell of rare earth
ions. Particularly metal orthovanadates of rare earth ele-
ments are an important class of inorganic nanomaterials
possessing significant interest as complex oxides due to
their potential application in diverse fields.20–23 Holmium
doping can effectively improve the photocatalytic activity
of TiO2, as a modified semiconductor photocatalyst.24–26
Holmium metal doping has been reported to improve the
photocatalytic efficiency of ZnO.27
Superhydrophobic materials are more suitable for the
absorption of organic pollutants from water and catalytic
supports in reactions including partial oxidation, isomer-
ization, nitroxidation, esterification, epoxidation, hydro-
genation, CO oxidation and as self cleaning materials. The
hydrophobicity of the aerogels was studied for their use as
self cleaning material.19ꢀ28
Oxidation of organic pollutants in wastewater, particularly
azo dyes from the textile industry by utilizing the nat-
ural solar energy in photocatalysis has been regarded as
a very promising environmentally friendly technique to
recover the gradually deteriorating water ecology.1ꢀ2 Pho-
tocatalytic process using novel nanomaterials has been
considered as one of the most promising technique to
address water pollution and the energy crisis.3–8 Heteroge-
neous photocatalytic degradation of industrial organic pol-
lutants from wastewater by semiconductors has attracted
extensive attention in the last decade.9ꢀ10 Among them,
semiconductors, TiO2, ZnO, SnO2, BiVO4 etc., which
drive photocatalysis, have gained a great deal of attention
due to their wide application to environmental remediation,
mainly for the removal of organic pollutants.11–14 Suppres-
sion of the recombination of photogenerated electron–hole
pairs in the semiconductors is essential for improving the
efficiency of a photocatalyst. Introducing foreign atoms
at a small percentage in the regular crystal lattice of
semiconductors or coupling another semiconductor oxide
In continuation of our earlier work on the modification
∗Author to whom correspondence should be addressed.
of ZnO29ꢀ30 for the enhancement of photocatalytic activity
178
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1533-4880/2018/18/178/010
doi:10.1166/jnn.2018.14579

Thirumalai et al.
Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
we, herein report a facile hydrothermal-thermal decompo-
sition method to synthesize HoVO4–ZnO nanostructure for
photodegradation of three azo dyes such as Rhodamine-B
(Rh-B), Trypan Blue (TB) and Acid Black 1 (AB 1). Fur-
thermore its morphological characteristics and hydropho-
bicity are also discussed.
compared with the standard files to identity the crystalline
phase. The surface morphology of the HoVO4–ZnO was
studied by using a field emission scanning electron micro-
scope (FE-SEM) (Model Carl ZEISS EVO 18). EDS
analysis was performed on gold coated samples using
a FE-SEM (Model ULTRA-55). FE-TEM images were
taken using 200 kV Ultra field emission Transmission
Electron Microscope (JEOL-2010). The specific surface
areas of the samples were determined through nitrogen
adsorption at 77 K on the basis of the BET equation
using a Micrometrics ASAP 2020 V3.00 H. X-Ray pho-
toelectron spectra (XPS) of the catalysts were recorded
in an ESCA-3 Mark II spectrometer (VG Scientific Ltd.,
England) using Al Kꢃ (1486.6 eV) radiation as the source.
The spectra were referenced to the binding energy of C(1s)
(285 eV). A Perkin Elmer LS 55 fluorescence spectrom-
eter was employed to record the photoluminescence (PL)
spectra at room temperature. Diffuse reflectance spectra
were recorded with Shimadzu UV-2450. UV absorbance
measurements were taken using Hitachi-U-2001 spectrom-
eter. The water contact angles were measured using a Drop
Shape Analyzer (DSA) (Kruass GmbH, Germany).
2. MATERIALS
Zinc nitrate hexahydrate [Zn(NO3ꢁ2 · 6H2O], Holmium
nitrate hexahydrate [Ho(NO3ꢁ3 ·6H2O], Ammonium meta-
vanadate [NH4VO3], Oxalic acid dihydrate [C2H2O4 ·
2H2O], Methanol(CH3OH) (HPLC grade) were obtained
from Himedia chemicals. Rhodamine B (Rh-B, molecu-
lar formula C28H31ClN2O3 and molecular weight: 479.01),
Trypan Blue (TB, molecular formula C34H28N6O14S4 and
molecular weight: 872.88), Acid Black 1 (AB 1, molecu-
lar formula: C22H14N6Na2O9S2; molecular weight: 616.57)
from Colour Chem, Pondicherry and ZnO (surface area
5 m2/g, particle size 4.80 ꢂm) from Merck chemicals were
used as received. A gift sample of TiO2–P25 (80% anatase,
20% rutile with BET surface area 50 m2 g−1 and mean
particle size of 30 nm) was supplied by Evonik, Germany.
Deionized distilled water was employed throughout the
experiments.
2.3. Photocatalytic Experiment
All photocatalytic experiments were carried out under sim-
ilar conditions on sunny days of April–May 2015 between
11 am and 2 pm. An open borosilicate glass tube of 40 cm
2.1. Preparation of 5 wt% HoVO4–ZnO
0.584 g of NH4VO3 (0.05 M) was dissolved in 100 ml
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height and 20 mm diameter was used as the reaction ves-
of deionized water. Under vigorous agitation, 0.243 g of
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sel. 50 mL of dye solution with appropriate amount of
HoVO4–ZnO was taken in the reaction vessel and magnet-
ically stirred in the dark for 30 min to attain adsorption–
desorption equilibrium between the dye and HoVO4–ZnO.
Irradiation was carried out in the open air condition. The
suspension was continuously aerated by a pump to provide
oxygen and for the complete mixing of reaction solution.
During the illumination time no volatility of the solvent
was observed. Heat produced in the experiment was dis-
sipated by natural convection using a cooling fan. After
dark adsorption the first sample was taken. At specific time
intervals, 2 mL of the sample was withdrawn and cen-
trifuged to separate the catalyst. 1 mL of the centrifugate
was diluted to 10 mL and its absorbance was measured at
259, 236 and 320 for Rh-B, TB and AB 1 dyes respec-
tively. The absorbance at 259, 236 and 320 nm represent
the aromatic content of Rh-B, TB and AB 1 respectively
and their decrease indicate the degradation of dye. Solar
light intensity was measured for every 30 min and the
average light intensity over the duration of each experi-
ment 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 × 100 100 lux. The inten-
sity was nearly constant during the experiments. Heber
multilamp photoreactor model HML-MP 88 was used for
degradation dyes under UV-A light irradiation. The irradi-
ation was carried out using four parallel medium pressure
Ho(NO3ꢁ3 · 6H2O solution (0.05 M) was added into the
NH4VO3 solution at room temperature. The pH of the
solution was adjusted to 10 with NaOH for the com-
plete precipitation of HoVO4. The HoVO4 suspension was
mixed with 100 mL 0.4 M Zn(NO3ꢁ2 · 6H2O (11.90 g)
solution and stirred for 30 min. 100 mL of oxalic acid in
distilled water (0.6 M) was introduced to the above solu-
tion drop wise and stirred for 4 h to ensure complete pre-
cipitation of zinc oxalate. The mixed precipitate of zinc
oxalate and HoVO4 was treated hydroꢀthermally in a Teflon
lined stainless steel autoclave at 115 C for 12 h with the
pressure of 18 psi.ꢀHydrothermally treated precipitaꢀte was
dried in air at 90 C for 12 h and calcined at 450 C for
12 h in a muffle furnace to obtain 5 wt% of HoVO4 in
ZnO. Catalysts with 3 wt%, 9 wt% and 12 wt% of HoVO4
were prepared by the addition of appropriate amounts of
Ho(NO3ꢁ3 · 6H2O and NH4VO3 using the above proce-
dure. Among these four catalysts, 5 wt% HoVO4–ZnO was
found to be more efficient for the degradation of Rh-B,
TB and AB 1 dyes, on primary analysis. Hence this cat-
alyst was characterized using various surface analytical
techniques.
2.2. Catalyst Characterization
X-ray diffraction (XRD) patterns were recorded with
a Siemens D5005 diffractometer using Cu Kꢃ (k =
0ꢄ151 418 nm) radiation. Maximum peak positions were
J. Nanosci. Nanotechnol. 18, 178–187, 2018
179

Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
Thirumalai et al.
mercury lamps (4 × 8 W = 32 W) emitting at 365 nm
(IUV = 1ꢄ381 ×10−6 einstein L−1 s−1ꢁ.
Where ꢆ is the crystallite size, ꢇ is the wavelength
of X-ray used; K is the shape factor, ꢈ is the full line
width at the half-maximum height of the peak, and ꢅ is the
Bragg angle. Particles sizes were calculated using all the
intense peaks. The average crystallite size of HoVO4–ZnO
is found to be 27 nm.
3. RESULTS AND DISCUSSION
3.1. XRD Analysis
XRD can be used as an effective non destructive tool for
qualitative and quantitative analysis of the phase structure.
Figure 1(a) shows XRD patterns of the prepared ZnO. The
2ꢅ values of ZnO at 31.78, 34.47, 36.27, 47.71, 56.46,
63.20, 66.34, 67.90, 69.03, 72.39, and 76.87ꢀ correspond
to (100), (002), (101), (101), (110), (103), (220), (112),
(201), (004) and (202) planes of Wurtzite ZnO (JCPDS
file no. 36–1451).31 The 2ꢅ values at 25.04, 28.86, 43.44
and 52.65 for the planes (200), (211), (103) and (213) in
Figure 1(b) are due to the tetragonal body centered struc-
ture of HoVO4 (JCPDS Card No. 82–1973).32 It should
be noted in Figure 1(b) that 2ꢅ angles of (100), (002)
and (101) planes for pure ZnO was gradually shifted to
the lower diffraction values indicating the appropriate dop-
ing of Ho3+ ions into the ZnO lattice. This is because of
expansion of ZnO lattice caused by the larger radius of
Ho3+ (1.015 Å) than that of Zn2+ (0.704 Å). This struc-
tural analysis clearly expose the shifting of lower angle
peak position on Ho3+ doping, which has a strong evi-
dence of the incorporation of Ho3+ into ZnO lattices.33 The
Scherrer formula (Eq. (1)) was used for the calculation of
3.2. Structural and Morphological Analysis
The morphology, size and microstructure details of the
HoVO4–ZnO were investigated by SEM and TEM tech-
niques. Figure 2 depicts typical information about mor-
phological features of the HoVO4–ZnO through FE-SEM
images. Figures 2(a), (b) clearly show that the pure
HoVO4–ZnO has large number of uniform spherical shape
particles with sizes in the range of 20–50 nm. The incor-
poration of HoVO4 into the crystal structure of ZnO can
reduce the aggregation and crystal size and enhance the
shape uniformity of the doped nanoparticles. The SEM
images of HoVO4–ZnO further confirm the presence of
many of the cavities in its lattice structure (Fig. 2(a)).
These different morphological structures of HoVO4–ZnO
help to increase the photocatalytic activity.
(a)
the average crystallite size of HoVO –ZnO
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4
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Kꢇ
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ꢆ =
(1)
ꢈcosꢅ
(b) HoVO4-ZnO
100 nm
JCPDS File No.82-1973
(b)
(a) Prepared ZnO
JCPDS File No.36-1451
200 nm
2 theta (degree)
Figure 1. XRD patterns of (a) Prepared ZnO and (b)
5
wt%
Figure 2. FE-SEM images of HoVO4–ZnO at different magnifications
HoVO4–ZnO.
(a) 100 nm and (b) 200 nm.
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Thirumalai et al.
Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
(a)
20 µm
(c)
(b)
20 µm
20 µm
(d)
(e)
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20 µm
20 µm
Figure 3. Elemental colour mapping images of HoVO4–ZnO (a) HoVO4–ZnO composite, (b) Zn, (c) Ho, (d) V and (e) O.
Figure 3(a) clearly depicts the distribution of Ho, Zn,
V and O in the surface of the catalyst by FE-SEM ele-
mental colour mapping. Figures 3(b)–(e) show individual
elemental colour mapping of Zn, Ho, V and O. It implies
from Figures 3(b) and (e) that Zn and O are present in
higher density and there is a homogenous distribution of
elements. This analysis also shows the purity of the pre-
pared HoVO4–ZnO nanomaterial. The energy dispersive
spectroscopy (EDS) results confirm the presence of Ho,
Zn, V and O in HoVO4–ZnO.
Furthermore the internal morphology and microstruc-
ture of as prepared HoVO4–ZnO were revealed by FE-
TEM images, shown in Figure 4. The low-magnification
TEM image (0.2 ꢂm) reveals the presence of HoVO4 (dark
shade) on the ZnO lattice (gray) (Fig. 4(a)). Figure 4(b)
also confirms that the particles are present in the range
of 20–50 nm. As can be seen from Figure 4(c) prepared
catalyst has particles with hexagonal structure. From the
Figure 4(d), tube-like particles are seen at a particular
location.
AFM images of HoVO4–ZnO with 5 ꢂm × 5 ꢂm
regions, shown in Figure 5, clearly indicate the surface
roughness and porosity of the catalyst. The average surface
roughness was found as 31.20 nm. The existence of nano-
sized particles in the HoVO4–ZnO nanomaterial is more
clearly reflected in its 2D AFM image (Fig. 5(a)). The sur-
face roughness of the prepared HoVO4–ZnO is shown in
3D AFM image (Fig. 5(b)). Higher surface roughness and
lower particle size reveals that the surface is highly porous
in nature. The surface porosity and roughness in the pho-
tocatalyst permits the adsorption of dye molecules onto the
surface, thereby increasing the photodegradation rate.
J. Nanosci. Nanotechnol. 18, 178–187, 2018
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Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
(a) (b)
Thirumalai et al.
0.2 µm
20 nm
(d)
(c)
20 nm
100 nm
Figure 4. FETEM images of HoVO4–ZnO (a) 0.2 ꢂm (b, c) 20 nm and (d) 100 nm.
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The pore structure of the HoVO –ZnO photocata- mainly consists of Holmium (Ho), Zinc (Zn), Oxygen (O)
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lyst was investigated by nitrogen adsorption–desorption
and Vanadium (V). Peaks of other elements are not
observed. Figure 7(b) shows Zn 2p signal at 1045.6
and 1022.5 eV corresponding to Zn 2p1/2 and Zn 2p3/2
orbitals.35 In Figure 7(c), the peak centred at 159.7 eV is
ascribed to Ho 4d and is in good agreement with the Ho
4d5/2ꢄ36 Therefore, this peak is related to the Ho3+ ions
in the ZnO lattice, which confirms the proper incorpora-
tion of Ho3+ into the ZnO lattice. Binding energy peak of
O 1s (Fig. 7(d)) is asymmetric and can be fitted to two
symmetrical peaks (locating at 531.1 and 532.7 eV), for
two different kinds of O species in the sample. These two
peaks should be associated with the lattice oxygen (OL)
of Ho–O–Zn and chemisorbed oxygen (OH) caused by the
surface hydroxyl groups which is helpful to improve the
photocatalytic activity of HoVO4–ZnO.37 The V 2p1/2 and
V 2p3/2 peaks (Fig. 7(e)) centred at 525.6 and 518.1 eV
are assigned to V4+ of HoVO4 and it is confirmed with the
reported values.38
isotherms and the pore size distribution was calcu-
lated by Barrett-Joyner-Halenda (BJH) method. The
N2 adsorption/desorption isotherms of the synthesized
HoVO4–ZnO (Fig. 6(a)) exhibit a type III isotherm with
a H3 hysteresis loop according to the classification of
IUPAC. A sharp increase in the adsorption volume of N2
was observed and located in the P/P0 range of 0.80–0.99,
which can be attributed to the capillary condensation, indi-
cating the good homogeneity of the sample and macro pore
size since the P/P0 position of the inflection point is related
to the pore size. The average pore radius of HoVO4–ZnO
is found to be 215 Å (Fig. 6(b)). The pore size distribution
of the HoVO4–ZnO catalyst thus shows the macroporous
structure. Surface area measurements, made by the BET
method, provide the specific surface area of HoVO4–ZnO
as 20.5 m2g−1, which is 2 times that of prepared ZnO
(10.2 m2g−1ꢁ. It is to be emphasized that this type of
isotherm indicates the presence of macroporous structure in
HoVO4–ZnO. This porous structure with high surface area
(20.5 m2g−1ꢁ leads to higher photocatalytic activity.34
3.4. Optical Properties of HoVO4–ZnO
To find out the optical properties, UV-vis diffuse
reflectance and photoluminescence spectra for the ZnO
and HoVO4–ZnO catalysts were recorded. Figure 8(a)
shows the diffuse reflectance spectrum of HoVO4–ZnO.
Any impurity in the semiconductor oxide can form inter-
mittent band energy levels and this leads to the decrease
of bandgap energy, which increases the UV-visible
3.3. X-ray Photoelectron Spectral Analysis
The elements present and their valence states can be
found with the help of X-ray photoelectron spectroscopy.
Figure 7(a) shows the survey spectrum of HoVO4–ZnO
photocatalyst and it proves that prepared nanomaterial
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Thirumalai et al.
Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
(a)
that prepared HoVO4–ZnO catalyst can be used as UV
and visible active material. UV-Vis spectra in the diffuse
reflectance mode (R) were transformed to the Kubelka-
Munk function F ꢉRꢁ to separate the extent of light absorp-
tion from scattering. The band gap energy (2.82 eV) was
obtained from the plot of the modified Kubelka-Munk
function ꢉF ꢉRꢁEꢁ1/2 versus the energy of the absorbed
light E (Eq. (2)) (Fig. 8(b))
ꢀ
ꢁ
1/2
ꢉ1 −Rꢁ2
2R
F ꢉRꢁE1/2
=
×hꢊ
(2)
Photocatalysts generate electrons and holes on irradia-
tion by light, and the recombination of electrons and holes
release energy in the form of fluorescence emission. There
is a strong correlation between PL intensity and photo-
catalytic activity as reported earlier.39 Lower fluorescence
emission intensity implies lower electron–hole recombi-
nation rate. The emission spectrum of prepared ZnO and
HoVO4–ZnO are shown in Figures 9(a) and (b) respec-
tively. Both prepared ZnO and HoVO4–ZnO exhibit pre-
dominantly blue emission with peaks centered at 418, 446
and 530 nm. In particularly, PL emission peak at 418, 446
(b)
5
5
and 530 nm are assigned to the 5I8 → G5; 5I8 → G6, 3K8;
5
and 5I8 → S2, 5F4 transitions, respectively. These PL peaks
result from sharp f –f absorption lines typical for holmium
ions.40 It can be seen that the positions of the peaks are
similar, while PL intensities are different. The PL intensity
of prepared ZnO at 418 nm due to electron–hole recombi-
AFM images of the surface ImPo:rp1h9ol1og.1y0o1f .555ꢂ.m44× O5 ꢂnm: Sun, 05 Aug 2018 00:26:28
Figure 5.
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nation is higher than that of HoVO4–ZnO. The decreased
region of the HoVO4–ZnO. (a) 2D image and (b) 3D image of HoVO4–
ZnO.
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emission intensity in HoVO4–ZnO, implies that the recom-
bination of charge carriers is suppressed by doped HoVO4
on ZnO, which leads to the enhanced photoactivity.
absorption. As seen in Figure 8(a), prepared nanomate-
rial HoVO4–ZnO (Fig. 8a(i)) shows reduced reflectance
spectrum (increased absorption) both in UV and visible
region compared to prepared ZnO (Fig. 8a(ii)). Hence the
band gap energy is less than prepared ZnO. This reveals
3.5. Photodegradation Studies
Photocatalytic degradation of Rh-B under different con-
ditions with increasing irradiation times is displayed in
Figure 10. Dye is resistant to self photolysis and for the
same experiment with HoVO4–ZnO in the dark, a decrease
(12%) in dye concentration was observed. This is because
of the adsorption of dye on the catalyst. Rh-B undergoes
96% degradation in the presence of HoVO4–ZnO under
natural sunlight in 75 min. But, prepared ZnO, TiO2–P25,
and undoped HoVO4 shows 64%, 62% and 54% degrada-
tions, respectively in 75 min. This results shows prepared
5 wt% HoVO4–ZnO nanomaterial is most efficient for
degradation of Rh-B dye relative to other photocatalysts
(Fig. 10). We also compared the efficiency of prepared
catalyst 5 wt% HoVO4–ZnO with solar and UV irradia-
tion, it is found that HoVO4–ZnO shows higher activity in
solar irradiation (96%) than in UV light of 365 nm (86%).
To test the activity of the catalyst on the degradations of
other azo dyes, we had carried out the experiments on
the degradation of TB and AB 1 under the same condi-
tions. Degradations of Rh-B, TB and AB 1 using HoVO4–
ZnO at different irradiation times were compared. Rh-B,
TB and AB 1 undergo 96%, 95% and 97% degradations
80
(a)
(b)
60
40
20
0
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Figure 6. (a) N2 adsorption–desorption isotherm of HoVO4–ZnO and
(b) Pore volume/Pore radius of HoVO4–ZnO.
J. Nanosci. Nanotechnol. 18, 178–187, 2018
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Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
Thirumalai et al.
(a)
Holmium
Zn 2p3/2
1022.5 eV
Zinc
(b)
(c)
Ho 4d
159.7 eV
Zn 2p1/2
1045.6 eV
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(d)
(e)
Oxygen
Vanadium
O 1s
V 2p3/2
518.1 eV
531.1 eV
V 2p1/2
O 1s
525.6 eV
532.7 eV
Figure 7. XPS analysis of HoVO4–ZnO (a) Survey spectrum (b) Zn 2p (c) Ho 4d (d) O 1s and (e) V 2p.
respectively in 75 min. The results reveal that this catalyst
is efficient in the degradation of azo dyes.
catalyst is more advantageous than undoped ZnO in the
degradation of Rh-B because it has maximum efficiency
at the neutral pH 7.
The solution pH plays a crucial role in the photocatalytic
degradation. The effect of pH on the photodegradation of
Rh-B was studied in the pH range 3–11 and the results
reveal 44, 62, 96, 77 and 60% degradations at pH 3, 5, 7,
9 and 11 respectively. The degradation efficiency is high
at pH 7 and it decreases when the pH is above or below 7.
Low removal efficiency at acidic pH range may be due to
the dissolution of ZnO in HoVO4–ZnO. It shows prepared
The reusability of HoVO4–ZnO was tested for the degra-
dation of Rh-B, TB and AB 1 dyes under the same reaction
environment. After complete degradation, the catalyst was
separated and washed with deionized water. The recov-
ꢀ
ered catalyst was dried in hot air oven at 100 C for 3 h
and used for a second run. Figure 11 shows the results of
Rh-B, TB and AB 1 degradation for four successive runs.
184
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Thirumalai et al.
Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
(a)
(ii)
(i)
Figure 10. Primary analysis: Rh-B dye concentration = 3 × 10−4 M,
catalyst suspended = 3 g L−1, pH = 7, airflow rate = 8.1 mL s−1 and
irradiation time 75 min, Isolar = 1250 × 100 Lux 100, IUV = 1ꢄ381 ×
(b)
10−6 einstein L−1 s−1
.
3.5.1. Mechanism of Photodegradation
The presence of f electrons is acknowledged as essential
for an optimal photocatalytic performance. This is because
rare earth metal ions with incompletely occupied 4f and
empty 5d orbitals often serve as a catalyst or promotes
catalysis.41 Furthermore, better photocatalytic activity in
orthovanadates is observed to be due to the presence of
regular VO4 tetrahedra, which is beneficial for the charge
transfer to surface of material.42 DRS spectrum of HoVO4
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Figure 8. (a) Diffused reflectance spectrum of HoVO4–ZnO
(b) K-M bandgap plot of HoVO4–ZnO.
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shows increased absorption profile compared to undoped
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ZnO. The absorption edge shifts to a longer wavelength
depending on the content of HoVO4. The improved pho-
tocatalytic activity for the HoVO4–ZnO is attributed to
efficient electron trapping followed by electron transfer.
The dopant ion (Ho3+ꢁ is reduced (Ho2+ꢁ upon trapping
an electron, which is then followed by the transfer of
the electron to O2 to form a radical species (Oꢁ2−ꢁ with
Dye degradation percentages of Rh-B are 96, 95, 94 and
94 in 75 min for the first, second, third and fourth run
respectively. In TB dye degradation HoVO4–ZnO shows
95, 94, 93 and 93% degradations in 75 min for the first
four cycles. HoVO4–ZnO gives 97, 96, 95 and 95% degra-
dations in AB 1 dye for first, second, third and fourth run
respectively in 75 minutes. There is no significant change
in the degradation efficiency of HoVO4–ZnO after second
run for all the three azo dyes.
(a)
(b)
Figure 11. Reusability of HoVO4–ZnO on degradation of dyes Rh-B,
TB and AB 1: dye concentration: Rh-B = 3 × 10−4 M, TB = 1 ×
10−4 M and AB 1 = 1 × 10−4 M catalyst suspended = 3 g L−1
,
Figure 9. Photoluminescence spectrum of HoVO4–ZnO (a) prepared
ZnO and (b) HoVO4–ZnO.
pH = 7, airflow rate = 8ꢄ1 mL s−1 and irradiation time 75 min,
Isolar = 1250 ×100 Lux 100.
J. Nanosci. Nanotechnol. 18, 178–187, 2018
185

Solar Photocatalytic and Self-Cleaning Performances of HoVO4 Doped ZnO
Thirumalai et al.
simultaneous reoxidation of the dopant ion. The radical is
able to efficiently undergo other photocatalytic processes
leading to the degradation of the organic species. The
results may be attributed to the charge transfer between
4f electrons of Ho ion and the conduction (or valence)
band of undoped material.43 As self-sensitive dyes, the
excited Rh-B∗ molecules after irradiation, chemisorbed on
the surface of photocatalysts may inject electrons into the
conduction band of ZnO, which enhance the visible pho-
toactivity of catalyst.44 The decreased emission intensity
in HoVO4–ZnO, also confirm the suppression of recombi-
nation of electron–hole by HoVO4 particles, and this leads
to enhanced phtocatalytic activity. AFM studies also prove
that the surface of the synthesized photocatalyst is very
rough and porous in nature and this may assist the trapping
of the electrons generated during photo process and subse-
quently generate radicals to degrade the organic molecules
on the surface of the catalyst.
increases above 90ꢀ exhibiting the hydrophobicity of the
catalyst. This surface non-wettability leads to a self clean-
ing property of the catalyst.
4. CONCLUSIONS
In summary, a new solar light active HoVO4–ZnO photo-
catalyst was synthesized by a simple hydrothermal-thermal
decomposition method and characterized by the various
analytical techniques. The XRD pattern confirmes ZnO
having hexagonal wurtzite structure and HoVO4 tetrago-
nal body-centered structure with average crystallite size
of 27 nm. FESEM and FETEM images further prove the
morphology of prepared HoVO4 doped ZnO as hexagonal
and sphere shaped particles in regular arrangement. Ele-
mental colour mapping studies reveals the presence and
homogeneous distribution of Ho, Zn, V and O in HoVO4–
ZnO. AFM images reveal the higher surface roughness of
HoVO4–ZnO, leading to large surface area. BET surface
area of the HoVO4–ZnO is 2 times higher than that of
prepared ZnO. During photocatalysis, Ho3+ ions acted as
electron scavengers and suppressed electron–hole recombi-
nation, which enhanced the activity of HoVO4–ZnO in the
degradation of azo dyes Rh-B, TB and AB 1. HoVO4–ZnO
was found to be a stable and reusable catalyst. HoVO4–
ZnO forms WCA of 109.5ꢀ showing its self cleaning prop-
erty. HoVO4–ZnO will be very useful for industries for
3.6. Contact Angle Measurements
Surface non-wettability or the hydrophobicity of the cat-
alyst is exposed by water contact angle. If a surface
has a contact angle with water greater than 90ꢀ, then
the surface is classed as hydrophobic and if the con-
tact angle is less than 90ꢀ, the surface is hydrophilic.
Water contact angles were determined using glass slides
coated with TEOS, TEOS + ZnO and TEOS + HoVO4–
ZnO to determine the hydrophobicity of the catalysts.
Figure 12 shows the images of water drops on coated
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effective treatment of effluents containing dyes and organic
pollutants and also as a self-cleaning material.
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and uncoated glass slides. Water contact angle (WCA) of
21.5ꢀ on uncoated glass slide shows the hydrophilicity and
this WCA increases gradually on glass slides coated with
TEOS (50.2ꢀ), TEOS+ZnO (73.4ꢀ) and TEOS+HoVO4–
ZnO (109.5ꢀ). Surface coated with TEOS + HoVO4–ZnO
has more hydrophobic character. In TEOS, the O–Si–O
groups are modified by HoVO4–ZnO to make the surface
rougher, stable and non-wettable. Hence the contact angle
Acknowledgment: Authors are thankful to the CSIR,
New Delhi, for the financial support through research
Grant no. 02 (0144)/13/EMR-II. Authors are grateful to
International Research Centre, Kalasalingam University,
Krishnankoil, Tamil Nadu for availing FE-SEM facility.
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Received: 25 November 2016. Accepted: 23 December 2016.
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187