Zirconium and silver co-doped TiO2 nanoparticles as visible light catalyst for reduction of 4-nitrophenol, degradation of methyl orange and methylene blue

Article abstract of DOI:10.1016/j.saa.2014.07.070

Catalytic activity of Zr and Ag co-doped TiO₂ nanoparticles on the reduction of 4-nitrophenol, degradation of methylene blue and methyl orange was studied using sodium borohydride as reducing agent. The nanoparticles were characterized using X-ray diffraction, energy dispersive X-ray, high resolution transmission electron microscopy, selected area electron diffraction and UV-Vis spectroscopy. The rate of the reduction/degradation was found to increase with increasing amount of the photocatalyst which could be attributed to higher dispersity and small size of the nanoparticles. The catalytic activity of Zr and Ag co-doped TiO₂ nanoparticles showed no significant difference even after recycling the catalyst four times indicating a promising potential for industrial application of the prepared photocatalyst.

Full text of DOI:10.1016/j.saa.2014.07.070

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 814–819
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Zirconium and silver co-doped TiO2 nanoparticles as visible light catalyst
for reduction of 4-nitrophenol, degradation of methyl orange and
methylene blue
Saraschandra Naraginti a, Finian Bernard Stephen b, Adhithya Radhakrishnan b, A. Sivakumar a,
⇑
a Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, India
b School of Mechanical and Building Sciences, VIT University, Vellore 632 014, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Simple sol–gel synthesis of Zr and Ag
co-doped TiO2 nanoparticles is
described.
Effective photocatalysis for the
reduction of nitrophenol and
degradation of dyes shown.
Recycling of photocatalyst up to 4
cycles discussed as promising
industrial application.
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic activity of Zr and Ag co-doped TiO2 nanoparticles on the reduction of 4-nitrophenol, degradation
of methylene blue and methyl orange was studied using sodium borohydride as reducing agent. The
nanoparticles were characterized using X-ray diffraction, energy dispersive X-ray, high resolution trans-
mission electron microscopy, selected area electron diffraction and UV–Vis spectroscopy. The rate of the
reduction/degradation was found to increase with increasing amount of the photocatalyst which could be
attributed to higher dispersity and small size of the nanoparticles. The catalytic activity of Zr and Ag
co-doped TiO2 nanoparticles showed no significant difference even after recycling the catalyst four times
indicating a promising potential for industrial application of the prepared photocatalyst.
Ó 2014 Elsevier B.V. All rights reserved.
Received 3 February 2014
Received in revised form 29 June 2014
Accepted 28 July 2014
Available online 7 August 2014
Keywords:
Photocatalyst
Visible light
4-Nitrophenol
Methyl orange
Methylene blue
Introduction
large band gap (ꢁ3.2 eV). This restricts the use of TiO2 in practical
applications because of the limited availability of UV radiation
Titanium dioxide (TiO2) is a promising photocatalytic material
due to its high stability, non-toxic nature & high performance in
the mineralization of organic pollutants in air and water media
and affordable cost [1–4]. However, photocatalytic processes with
TiO2 catalysts are mostly favoured under UV irradiation due to its
(only 5% of solar light). Hence, the focus of researchers has been
on introducing dopants into TiO2 to make the material absorb radi-
ation visible region of the electromagnetic spectrum for its use in
several applications. This approach of doping metals into TiO2
has been proved to be a very useful in making visible light sensitive
catalysts [5]. Similar approach has been reported to yield good
results while two different metals were doped into ZnO or TiO2
resulting in enhanced photocatalytic activity and unique
⇑
Corresponding author. Tel.: +91 416 2202339.
E-mail address: svkmr7627@gmail.com (A. Sivakumar).
http://dx.doi.org/10.1016/j.saa.2014.07.070
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

S. Naraginti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 814–819
815
characteristics compared to doping of a single element [6–14].
Synthesis of Zr and Ag co-doped TiO2 nanoparticles
4-nitrophenol (4-NP), listed by the US EPA as a major pollutant,
may induce blood disorders, eye, skin irritation, kidney and liver
damage as well as poisoning of the central nervous system in
humans and animals [15]. 4-NP has been used extensively as a
raw material in chemical industry for manufacture of pesticides,
herbicides, synthetic dyes, pharmaceuticals, for treatment of
leather and in several military applications [16]. Several silver
nanoparticle based catalysts have been reported for the reduction
reaction of 4-NP. Naik et al. have reported the formation of Ag
nanoparticles within the pores of mesoporous silica [17,18]. Zhang
et al. have reported the preparation of tubular nanocomposites of
Ag nanoparticles and silica with fairly uniform diameters in the
range of 250–350 nm, by combining the single capillary electros-
pinning technique (for silica nanotubes as the supports) and an
in situ reduction approach (for Ag nanoparticles) [19]. Liu et al.
have modified halloysite (Al2Si2O5(OH)4ꢂ2H2O,1:1 layer alumino-
silicate) nanotubes by mercaptoacetic acid, ethylene glycol to
encapsulate Ag nanoparticles and utilized it for the catalytic reduc-
tion of 4-NP [20].
A mixture of 5 mL of titanium (IV) isopropoxide in 50 mL
isopropanol was added drop wise to 200 mL of distilled water
maintained at pH 1.5 while the solution was continuously stirred.
To this solution, required amount of aqueous solutions of AgNO3
and ZrO(NO3)2 (0.2–0.8 mol%) were added drop wise and stirring
continued for an additional 45 min. Then, a small aliquot of
distilled water and 0.05 M hydrazine hydrate were added to it
followed by 5 mL of tween 20 (capping agent to prevent agglomer-
ation of particles) and the stirring was continued for an additional
30 min. The resultant sol was sonicated at 80 MHz for 90 min and
then dried at 100 °C in a hot air oven for 24 h to get the dry gel. The
gel was then calcinated at 450 °C to obtain required nanoparticle
powder.
Characterization of co-doped TiO2 nanoparticles
Powder XRD pattern was recorded using X-ray BRUKER D8
Advance X-ray diffractometer with Cu K
a source (k = 1.5406 Å).
Textile industries discharge effluents containing fairly large
amounts dyes into water, some of which are mutagenic and carcin-
ogenic to humans [21]. These effluents are characterized by their
fluctuating pH with suspended particles, high oxygen demand,
non-biodegradability and resistance to oxidation [22,23]. The com-
plex structures of these dyes and their high refractoriness to deg-
The crystalline phase of the nanoparticles was identified by com-
paring the major peak positions with standard JCPDS files. JEOL
JEM 2100 high resolution transmission electron microscope
(HRTEM) was used for imaging, SAED pattern and energy disper-
sive X-ray with an accelerating voltage of 200 kV at different mag-
nifications. Diffuse reflectance spectra were recorded using JASCO
V-670 UV–Vis spectrophotometer. Specific surface area of the sam-
ple was arrived at through nitrogen adsorption at 77 K using BET,
Micromeritics ASAP2020 V4.01 (V4.01 H).
radation poses
a big challenge in their decolorisation and
complete mineralization. It has, therefore, become necessary to
develop new promising materials for dye removal. Methylene blue
(MB) causes nausea, hypertension, haemolysis and respiratory dis-
tress. To overcome the problems posed by different conventional
dye removal agents, there has been a good amount of interest in
inorganic composites of nano-scale dimensions. It may be seen
that doping of ZnO or TiO2 with transition elements induces crystal
defects which can also change their photocatalytic properties. By
electron trapping, Zr-doping possibly conquers the recombination
of electrons and positive holes [24]. Furthermore, Zr is an isoelec-
tric impurity belonging to deep energy level doping elements [25].
Methylene Blue (MB), Methyl Orange (MO), nitrobenzene and tri-
chloroethylene have also been used as model pollutants to deter-
mine the activity of the photocatalyst [26–29].
Catalysis
Reduction of 4-NP, degradation of MB and MO catalyzed by Zr
and Ag co-doped TiO2 nanoparticles was carried out at room tem-
perature (30 °C). The reduction process was initiated by adding the
0.01 g doped nanoparticles to a solution containing 50 mL of 4-NP
(0.1 mmol Lꢃ1) and 5 mL of freshly prepared 0.05 M NABH4 in a
beaker while constantly stirring. The resultant suspension was
irradiated with
a
200 W Philips tungsten filament lamp
(k > 400 nm) which was placed at a distance of 5 cm from the
suspension.
Recovery of the catalyst is an important step in heterogeneous
catalysis and to facilitate this, metal nanoparticles are usually dis-
persed onto solid matrices while preparing the heterogeneous cata-
lysts. The supporting matrices used include carbon nanotubes, silica,
titania, ceria and alumina[30–32]. Shi et al. [33,34] have synthesized
stable gold nanoparticles using natural materials such as tannin or
collagen. However, TiO2 is still one of the most favorable supports
for metal nanoparticles due to its thermal and chemical stability,
non-toxic nature and relatively low cost. The present study reports
the synthesis of Zr and Ag co-doped TiO2 nanoparticles and their
application as photocatalyst for the reduction of 4-NP, degradation
of MB and MO using NaBH4 as the reducing agent.
The catalytic degradation of MB and MO was carried out by add-
ing1 mL of 0.01 M NABH4 solution to 30 mL of 10ꢃ3 M MB and MO
solutions respectively while stirring. After 5 min, 0.01 g of catalyst
powder was added and the stirring continued. The kinetics of deg-
radation of dyes was studied by taking a small aliquot of the sam-
ple and measuring the absorbance at specific kmax at regular time
intervals. Degradation was visualized by the disappearance of color
of the dye solutions. In the case of MB, the blue color which was
initially seen in an oxidizing environment turned colorless in the
presence of reducing agent (NaBH4) indicating the degradation of
MB to leuco MB (LMB) [35].
The catalytic activity was verified by varying the amount of cat-
alyst (0.01, 0.05 and 0.1 g) while monitoring the degradation pro-
cesses through absorbance measurements on the UV–Vis
spectrophotometer.
Materials and methods
Materials
Results and discussion
Titanium (IV) isopropoxide from Sigma Aldrich, commercial azo
dyes methylene blue, methyl orange, 4-nitrophenol and sodium
borohydride (NaBH4) from SD-Fine chemicals, hydrazine hydrate
and zirconyl nitrate [ZrO(NO3)2] from SRL chemicals, tween 20
from Himedia were used in the present study. Milli-Q water was
used in all the experiments.
Optical properties
Fig. 1(a) shows the UV–Vis absorption spectra of pure TiO2, Ag
doped TiO2 & Zr and Ag co-doped TiO2. A definite band edge in
the UV region at 300–350 nm seen in the spectra which could be
assigned to photo excitation from valence band to conduction

816
S. Naraginti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 814–819
Fig. 2. X-ray diffraction pattern of Zr and Ag co-doped TiO2 nanoparticles.
to their low concentration levels. Doping of Ag and Zr did not result
in significant changes in crystallinity. A slight shift in the d-spacing
with discernible change in cell volume indicated the incorporation
of dopants into TiO2. Since Ag and Zr have not shown any phase
change, aggregates may have been formed on crystal borders and
on the surface of the photocatalyst, thus facilitating visible light
absorption as discussed earlier.
TEM and EDX analysis
The morphologies of the nanocrystals were analyzed using TEM.
Fig. S1(a) and (b) (Supplementary data) show highly uniform nano-
crystalline microstructure images of Zr and Ag co-doped TiO2
nanoparticles confirming uniform grain sizes as estimated by
XRD. Uniform distribution of particles ranging from 8 to 20 nm,
with irregular sized spherical morphologies and slight agglomera-
tion can also be seen from the images. The SAED pattern of the
sample, shown in Fig. S1(c), has dark rings on the right correspond-
ing to the standard polycrystalline diffraction rings for the anatase
phase (indexed). In spite of great care, no sign of diffraction rings
related to other phases were observed. EDX mapping (Fig. S1(d))
of the photocatalyst showed uniform dispersion of the doped met-
als onto the support, which is in agreement with the XRD results.
HRTEM shown in Fig. S1(b) was used to obtain more discernible
microstructure information to enable analyze the single grains
and grain boundaries accurately. The BET (Brunauer–Emmet–
Teller) surface area of the Zr and Ag co-doped TiO2 has been mea-
sured as 309.2 m2/g.
Fig. 1. (a) UV–Vis absorption spectra of nanoparticles and (b) optical energy gap
(Eg) of nanoparticles.
band. It is interesting to note that the sample doped with silver (Ag
doped TiO2) has shown lower shift compared to Zr and Ag co-
doped TiO2 (0.8 mol%), which has shown a significant red shift to
higher wave length. This shift was observed to increase with
increasing levels of Zr and Ag, indicating enhancement in the opti-
cal activity of the samples. The energy gap (Eg) was calculated from
the equation [36]
2
n
ð
aht
Þ ¼ Aðh
t
ꢃ EgÞ
where
a is the absorption coefficient, A is a constant, and n = 2 for
direct transition; n = 1/2 for indirect transition. The optical energy
gap (Eg) was arrived at by extrapolation of Kubelka Munk plot of
Catalysis
2
ht
vs. (a
ht)
as shown in Fig. 1(b).
The energy gap values of the pure TiO2, Ag doped TiO2 & Zr and
Catalytic reduction of 4-nitrophenol
Ag co-doped TiO2 nanoparticles were calculated to be approxi-
mately 3.21 eV, 2.87 eV and 2.71 eV respectively. Doping TiO2 with
transition metals (zirconium and silver) is usually accompanied
with a decrease in band energy and increase in the wave length
value (red shift). These results indicate that both Zr and Ag
co-doped in the TiO2 nanoparticles probably make the material
exhibit a high photocatalytic activity in the visible region.
The reduction of 4-NP using the metal nanocomposites in the
presence of NaBH4 was previously investigated by researchers for
the efficient production of 4-aminophenol (4-AP) [37–40]. In addi-
tion, in the present study, this reaction was used as a model reac-
tion to examine the effect of visible light on the catalytic activity
of Zr and Ag co-doped TiO2. From the results, it is clear that the
prepared catalysts have efficiently assisted the reduction of 4-NP.
Furthermore, no reduction of 4-NP was observed without the cata-
lyst. After addition of NaBH4 to 4-NP, intense yellow color was
observed due to the formation of highly stable 4-nitrophenolate
intermediate. Interestingly, after the addition of photocatalyst to
the solution, there was a continuous decrease in the intensity of
color till it finally turned colorless. This phenomenon was con-
firmed by the decrease in intensity of the absorption at 400 nm,
X-ray diffraction
Fig. 2 shows the X-ray diffraction pattern of the doped nanopar-
ticles. In all the samples, anatase phase of the TiO2 was confirmed
by the 2h peaks at 25.3°, 37.9°, 47.9°, 55.0° and 62.8°, while phases
related to Ag and Zr were not identified in any of the samples due

S. Naraginti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 814–819
817
Catalytic degradation of methyl orange
Today, environmental protection has become extremely impor-
tant for human beings and some toxic and stable dye molecules
like MO show potential danger to the environment [42]. Hence,
the present study attempts to use Zr and Ag co-doped TiO2 nano-
particles for catalytic degradation of MO. Figs. 4(a) and S3(b) and
(c) show the visible light catalytic degradation of MO. From the
graphs, it can be seen that the MO absorption band at 463 nm dis-
appeared and a new absorption band appeared at 247 nm because
of formation of hydrazine derivative [43]. Maximum absorbance at
463 nm gradually has been observed to decrease with respect to
time and the solution finally became colorless, indicating complete
degradation of the dye. However when the same experiment was
performed in the absence of photocatalyst, only 7% degradation
was observed even after irradiation with tungsten lamp for 5 h.
In the presence of Zr and Ag co-doped TiO2 nanoparticles, but in
the absence of NaBH4, 18% degradation was observed after 5 h,
which could be due to the adsorption of dye on the surface of
doped nanoparticles. This observation confirmed that both catalyst
and visible light are essential for effective degradation of dye.
To study the effect of amount of catalyst loading on the photo-
degradation of MO, experiments were carried out by varying its
amount from 0.01 to 0.1 g for a fixed concentration of the dye. It
was observed that time to degrade MO has decreased with increase
in amount of catalyst from 0.01 to 0.1 g and when the amount of
catalyst was 0.01 g, total degradation of MO has been seen in
30 min while 0.05 g and 0.1 g of photocatalyst have been able to
complete the degradation in 12 min and 7 min respectively.
Fig. 4(d) shows the plot of Ct/C0 vs. time for different amounts of
catalyst used during the reduction of MO. It can be seen that the
Fig. 3. UV–Vis absorption spectra for the reduction of 4-nitro phenol by NaBH4 in
the presence of (a) 0.01 g of photocatalyst and (d) plot of Ct/C0 vs. time.
which is characteristic of 4-nitrophenolate (Figs. 3(a) and S2(b) and
(c)). Also, appearance of two new absorbance peaks at 295 and
230 nm which showed increase with progress of time indicated
the increase in concentration of 4-AP. It was noticed that, intensity
of the 4-AP peak was not proportional to decrease in the 4-NP
concentration, probably due to the difference in the molar
extinction co-efficient of 4-nitrophenolate and 4-AP [41].
The results of the visible light photocatalytic reduction of 4-NP
in aqueous solution showed that doping of Zr and Ag greatly
improved the efficiency of TiO2 nanocrystallites. The improvement
of visible light photocatalytic activity could be due to the modifica-
tion of TiO2 with Ag and Zr, which could increase the separation
efficiency of photogenerated electrons and holes in TiO2, and the
improvement of photo stability of TiO2 is attributed to a consider-
able decrease in the surface defect sites of TiO2 after doping of Zr
and Ag.
The effect of the amount of Zr and Ag co-doped TiO2 nanoparti-
cles on the rate of reduction of 4-NP to 4-AP was investigated using
different amounts of the catalyst ranging from 0.01 to 0.1 g, while
keeping the other parameters unchanged. Fig. 3(d) shows plot Ct/C0
vs. time for different amounts of catalyst used for reduction of
4-NP in the presence of NaBH4 at room temperature. As expected,
the catalytic efficiency and reduction percentage (Fig. S5(a), in
Supplementary data) of the photocatalyst have been observed to
increase with increasing amount of the catalyst. When the catalyst
added was 0.01 g, the reduction time was observed to be 28 min
for conversion of 4-NP to 4-AP and the same got shortened to
16 min and 8 min when 0.05 g and 0.1 g of photocatalyst was used.
Fig. 4. UV–Vis absorption spectra for the degradation of methyl orange by NaBH4 in
the presence of (a) 0.01 g of photocatalyst and (d) plot of Ct/C0 vs. time.

818
S. Naraginti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 814–819
efficiency of the photocatalyst increased with increase in catalyst
loading (Fig. S5(b), see Supplementary data). The increase in rate
constant with increase in amount of photocatalyst could be attrib-
uted to an increase in number of active sites on photocatalyst sur-
face which resulted in an increase in generation of hydroxyl
radicals and thus led to an increase in the number of dye molecules
degraded.
irradiated with the visible light, electrons get promoted from the
valence band to the conduction band of the semiconducting oxide
to give electron–hole pairs. The potential of the valence band (hVB
)
is positive enough to generate hydroxyl radicals at the surface and
the potential of the conduction band (eCB) is negative enough to
reduce molecular oxygen. The hydroxyl radical is a powerful
oxidizing agent and attacks organic pollutants present at or near
the surface of photocatalyst (Zr and Ag co-doped TiO2).
In order to determine the effect of amount of photocatalyst on
photodegradation of MB, experiments were performed by varying
the amount of catalyst from 0.01 to 0.1 g. The rate of degradation
was observed to increase as the amount of catalyst was increased
from 0.01 to 0.1 g (Fig. S5(c), see Supplementary data). This could
be due to increase in the number of photons with increase in the
number of Zr and Ag co-doped TiO2 nanoparticles which the dye
molecules absorb. Fig. 5(d) shows the plot of Ct/C0 vs. time for
0.1 g of catalyst. It can be seen that rate of degradation increased
with increase in catalyst loading. The time for complete degrada-
tion of MB was observed to come down from 30 min to 9 min with
increase in amount of catalyst from 0.01 to 0.1 g.
Recovery and reuse of catalyst in catalytic processes is of major
importance for industry, since the use of catalytic process signifi-
cantly contributes to lowering the operational cost and wastewater
treatment cost. To explore the advantage of Zr and Ag-doped TiO2
photocatalyst and the applicability and reuse of catalysts was ver-
ified for the reduction of 4-NP, degradation of MO and MB (Fig. S6,
see Supplementary data). Experiments were performed where the
photocatalyst was recovered (0.1 g) and reused by keeping all
other parameters constant. The results revealed that Zr and Ag
co-doped TiO2 photocatalyst has shown very good activity for four
successive catalytic runs without any significant loss in catalytic
activity. The yield for the conversion 4-NP, MO and MB was 94%,
93% and 95% respectively for the four runs respectively. It could
be concluded that Zr and Ag co-doped TiO2 nanoparticles have
good stability and they can be reused for at least 4 runs, thus show-
ing a promising potential for practical applications.
Catalytic degradation of methylene blue
The oxidized and reduced forms of MB have different absorp-
tion bands in the UV–Vis spectrum. Hence, the progress of decolor-
ation or reduction reaction from MB to leuco MB (LMB) can be
monitored by measuring the decrease in absorption of MB on
UV–Visible spectrum at a kmax of 664 nm. The time dependent elec-
tronic absorption spectrum of MB during visible light photo irradi-
ation is presented in Figs. 5(a) and S4(b) and (c). After 30 min of
irradiation under visible light in the presence of Zr and Ag co-
doped TiO2 suspension, more than 95% of dye got degraded and
the solution became colorless. Also, no new bands appeared in
the UV–Vis spectrum, confirming the absence of reaction interme-
diates during the degradation process. The effect of amount of pho-
tocatalyst on degradation of MB was also studied. In the presence
of mixed photocatalyst and visible radiation, more than 95% of dye
got degraded after 30 min of irradiation while only 32% degrada-
tion was observed in the presence of photocatalyst without irradi-
ation and only 11% degradation of the dye was observed in the
absence of photocatalyst even after 3 h of irradiation. These exper-
iments demonstrate that both visible light and a photocatalyst are
essential for effective degradation of MB. This could be attributed
to the fact that when Zr and Ag co-doped TiO2 nanoparticles are
Possible degradation mechanism of pollutants
Based on the above results and the available literature, the pos-
sible mechanism of photocatalytic degradation of three organic
pollutants by Zr and Ag co-doped TiO2 is shown in Fig. S7 (see Sup-
plementary data). When the semiconductor is irradiated by visible
light, an electron from valance band (VB) goes to conduction band
(CB) leaving a hole in valance band. Generally, these electron–holes
recombine to reduce the photocatalytic activity of semiconductors.
But ‘Ag’ and ‘Zr’ present in the material, trap the electron from CB
of TiO2 which inhibits the electron–hole recombination. It is well
established that ‘Ag’ traps the electrons from CB of TiO2. The ‘Zr’
doping also suppresses the electron–hole recombination by elec-
tron trapping [24,44]. The electrons trapped by Ag and Zr produce
more number of superoxide radical anions and at the same time VB
holes of TiO2 react with water to produce highly reactive hydroxyl
(ÅOH) radical. The superoxide radical anion and hydroxyl radical
facilitate the degradation of dye.
Conclusion
In this work, Zr and Ag co-doped TiO2 nanoparticles prepared
through sol–gel process were used as effective photocatalyst for
reduction of 4-NP, degradation of MO and MB under visible light
irradiation. The effective loading of Zr and Ag decreased the optical
band gap of TiO2 significantly, making them suitable for visible
light photocatalysis resulting in the better utilization of a much
broader spectrum of solar radiation. The results confirm that both
Fig. 5. UV–Visible absorption spectra for the degradation of methylene blue by
NaBH4 in the presence of (a) 0.01 g of photocatalyst and (d) plot of Ct/C0 vs. time.

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