Synthesis of nanostructured NiO and its application in laser-induced photocatalytic reduction of Cr(VI) from water

Article abstract of DOI:10.1016/j.molcata.2011.03.029

Owing to its high toxicity, chromium(VI) is considered as top priority pollutant for many countries in the world and removal of this pollutant from waste water is highly desirable. The present investigation addresses the synthesis of NiO nanoparticles usi

Full text of DOI:10.1016/j.molcata.2011.03.029

Journal of Molecular Catalysis A: Chemical 341 (2011) 83–88
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of nanostructured NiO and its application in laser-induced
photocatalytic reduction of Cr(VI) from water
M. Qamara, M.A. Gondala,b,∗, Z.H. Yamania,b
a Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, KFUPM Box 741, Dhahran 31261, Saudi Arabia
b Laser Research Group, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2010
Received in revised form 28 March 2011
Accepted 30 March 2011
Available online 9 April 2011
Owing to its high toxicity, chromium(VI) is considered as top priority pollutant for many countries in
the world and removal of this pollutant from waste water is highly desirable. The present investigation
addresses the synthesis of NiO nanoparticles using sol–gel method and their application in heterogeneous
photocatalytic reduction of Cr(VI) using a 355 nm laser radiation generated from Nd:YAG laser. The study
showed that ∼90% Cr(VI) was removed within short laser exposure time (75 min) in presence of nanos-
tructured NiO without the use of any additive as reported in other studies. Effect of critical parameters,
such as calcination temperature, calcination time, laser energy, catalyst amount, chromium concentra-
tion, and added electron donor and acceptor on the photocatalytic reduction process was investigated.
The photocatalytic reduction of metal was found to be strongly dependant on the above mentioned
parameters.
Keywords:
Photocatalysis
Chromium reduction
Nanostructured NiO
Water purification
Laser applications
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
alytic processes using semiconductors have demonstrated the need
to overcome the limitations in achieving higher photonic efficien-
Chromium has been used extensively in several industries, such
as alloys and steel manufacturing, metal plating, military purposes,
and tanning of leather, as well as in the pigment and refractory
industries [1,2]. The increasing level of chromium(VI) metal ion
in the environment is a great concern to the societies and regu-
lation authorities around the world. The toxicity and carcinogenic
properties of chromium(VI) have been known for a long time [3]
and it is in the list of priority pollutants of many countries in the
world [4]. Conventionally, industrial waste treatments for heavy
metal removal include techniques such as biological treatment, ion
exchange, liquid–liquid extraction, precipitation, reverse osmosis,
and activated carbon adsorption [5]. However, these techniques
often utilize potentially hazardous or polluting materials and can
only transform the pollutants from one phase to another [6]. More-
over, they may require pretreatment, their by-products are often
considered hazardous, and their disposal is costly.
cies. The fundamentals of heterogeneous photocatalytic oxidation
processes have been discussed extensively in the literature [15,16].
Briefly, when a photocatalyst absorbs a photon of energy equal to or
greater than its band gap, an electron is promoted from the valence
band to the conduction band (e−cb) leaving behind an electron
vacancy or “hole” in the valence band (h+vb). If the charge separa-
tion is maintained, the electron and hole may migrate to the catalyst
surface where they participate in redox reactions with adsorbed
species. It has been reported that the photocatalytic reduction of
highly toxic Cr(VI) into non-toxic Cr(III) can be achieved with the
help of semiconductors and light under certain conditions [17–21].
Although the efforts have been made to investigate the
reduction of metal using semiconductor-mediated photocatalytic
process, most of the studies have been carried out using some
organic additives such as salicylic acid, oxalate, phenols, dye,
and humic acid [22–25], as well as some electron donor such as
methanol, formate ions, etc. [5,26]. It is worth mentioning that
all the previous work on reduction of heavy metals from waste
water using heterogeneous photocatalysis has been carried out
with broad spectral radiation sources such as lamps and TiO2 as
a photocatalyst where the reduction and/or precipitation of metal
ions was found to be negligible in the absence of organic additives.
Moreover, certain problems are associated with the use of lamps
emitting over broad spectral wavelength range such as the long
term power stability due to over heating of lamps during the oper-
ation, low photonic efficiency, etc. for complete mineralization of
Heterogeneous photocatalysis involving semiconductors has
attracted considerable attention in recent years for the remedia-
tion of undesirable pollutants both in aqueous [7–11] and gaseous
phase [12–14] using solar or artificial light sources. The photocat-
∗
Corresponding author at: Laser Research Group, Physics Department, King Fahd
University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia.
Tel.: +966 38602351; fax: +966 38602293.
E-mail address: magondal@kfupm.edu.sa (M.A. Gondal).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.029

84
M. Qamar et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 83–88
1800
1600
1400
1200
1000
800
600
400
200
0
pollutants. Keeping in view the long term stability of lamps, it is of
interest to use laser radiations as an excitation source to study the
activity of photocatalysts because laser light has unique properties
like mono-chromaticity, high intensity, and low beam divergence.
However, the initial cost of laser based systems will be much higher
than lamp based systems. The main purpose of using laser in this
work is to study the performance of the NiO catalyst in short span of
time and other experimental parameters as compared to lamps for
effective reduction of Cr(VI) from water. However for commercial
scale applications of water purification, lasers are not suitable and
one can use the solar radiations. Furthermore, we are not claiming
in this work that lasers are the best alternatives for other irradiation
sources.
To the best of our knowledge, complete and efficient photocat-
alytic reduction of Cr(VI) in the presence of nanostructured NiO
without the use of any additive has not been reported so far. The
present study describes the synthesis of NiO nanoparticles follow-
ing sol–gel method and their application in reduction of Cr(VI) from
water system using a 355 nm laser irradiation. The dependence of
the metal reduction from water on calcination temperature, calci-
nation time, laser irradiation energy, catalyst amount, chromium
concentration, electron donor and acceptor was investigated. The
data concerning temporal behavior of metal reduction was fitted
to first-order kinetic and reaction rate was estimated.
202
400
30
225
026
004
022
316
604
510
332
60
136
10
20
40
50
70
80
2θ (deg.)
Fig. 1. XRD patterns of nickel acetate dihydrate (NiC2O4·2H2O) obtained after drying
the gel-product at 110 ◦C for 12 h.
amount of photocatalyst was added and the solution was stirred
for at least 15 min in the dark. The pH of the reaction mixture
was adjusted to ∼7 by adding a dilute aqueous solution of HNO3
or NaOH. The zero time reading was obtained from a blank solu-
tion kept in the dark but otherwise treated similar to the irradiated
solution. Irradiations were carried out using a 355 nm wavelength
high power laser beam generated from the third harmonic of the
Spectra Physics Nd:YAG laser (Model GCR 250), with a pulse width
of ∼8 ns. In order to avoid the destructive effect of radiation, laser
beam diameter was expanded to 1.0 cm. A schematic of the experi-
mental set-up was documented in our earlier publications [27,28].
Samples (3 mL) were collected before and at regular time intervals
during the irradiation. The catalyst was removed through filtration
before the analysis.
2. Experimental
2.1. Chemicals
Potassium dichromate (K2Cr2O7) was obtained from
Sigma–Aldrich. Nickel acetate was obtained from Fluka and
oxalic acid was purchased from Fisher Scientific Company. The
other chemicals H2O2 and alcohols (ethanol and methanol) were
obtained from Merck.
2.2. Synthesis of NiO nanoparticles
The synthesis of NiO nanoparticles was carried out adopt-
ing sol–gel method. For this purpose, nickel acetate tetrahydrate
((CH3COO)2–Ni·4H2O) was dissolved in ethanol under constant
stirring for 2 h at 40–45 ◦C to obtain a clear and light greenish sol.
Oxalic acid solution (0.23 M in ethanol) is then added slowly to
the warm sol to yield a greenish thick gel. The thick gel was then
refluxed at 80–90 ◦C for 24 h under vigorous stirring in the air atmo-
sphere. It was further dried in an oven by heating at 110 ◦C for
whole night. Dried gel/product was grinded into fine powders and
calcined in a tube furnace from 300 ◦C to 700 ◦C for different time
periods to obtain nanocrystalline NiO. This process produced a fine
black powder.
2.5. Analysis
The reduction of Cr(VI) was monitored by measuring the
absorbance on a UV–Vis spectrophotometer. The absorption max-
ima of the potassium dichromate was found to be 372 nm. The
reduction of the metal, therefore, was estimated at this wavelength
as a function of irradiation time. For each experiment, the rate for
the reduction of the model pollutant was calculated from the initial
slope obtained by linear regression from a plot of the natural loga-
rithm of absorbance of the metal as a function of irradiation time,
i.e. first order reduction kinetics.
2.3. Characterization
3. Results and discussion
Characterization of the synthesized nanostructure NiO was car-
ried out employing Field Emission Electron Microscope, Transmis-
sion Electron Microscope (JEOL, JEM-2100F), X-ray Diffractometer
(Shimadzu 6000 using Cu–K radiation, with operating voltage of
40 kV and current of 30 mA), Particle Size Analyzer (Microtrac,
S3500). The absorbance of the samples was monitored by measur-
ing the absorbance on a UV–Vis spectrophotometer (JASCO).
3.1. Structural and morphological characterization of NiO
nanoparticles
The XRD pattern of the dried gel obtained after drying at 110 ◦C
for 12 h has been depicted in Fig. 1. The XRD patterns corresponded
to the formation of nickel oxalate dihydrate (NiC2O4·2H2O) hav-
˚
ing an orthorhombic structure with lattice parameters a = 11.84 A,
˚
˚
b = 5.345 A, c = 15.716 A, Z = 8, and space group Cccm [29]. This result
contradicts the result obtained by Wang et al. [30] where the forma-
tion of nickel oxalate hydrate was observed following the similar
sol–gel method with only difference that nickel nitrate instead of
nickel acetate was used as precursor. Fig. 2 illustrates the FESEM
and TEM micrographs of nanostructured nickel oxide calcined at
400 ◦C for 2 h. The microscopic study indicated that all the particles
2.4. Evaluation of photocatalytic activity
Stock solutions of the potassium dichromate containing desired
concentrations were prepared in distilled water. For irradiation
experiments, 100 mL solution of desired concentration of the model
pollutant was taken into the reaction vessel and the required

M. Qamar et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 83–88
85
Fig. 2. FESEM and TEM images of nanostructured NiO calcined at 400 ◦C for 2 h. Figure inset: particle size distribution.
Table 1
Effect of calcination temperature, heating time on the photocatalytic reduction of Cr(VI) in the presence of NiO nanoparticles.
Calcination temperature (◦C)
Heating time (h)
Average particle
size (nm)
Rate constant
Experimental conditions
(min−1
)
300
2
07.0
0.009
Laser energy = 240 mJ,
chromium
concentration = 50 mg/L, NiO
concentration = 1.0 g/L,
irradiation time = 75 min.
400
500
600
700
400
400
400
400
2
2
2
2
1
3
4
5
09.5
20.0
55.0
125.0
07.5
12.5
17.0
24.0
0.026
0.021
0.016
0.010
0.015
0.023
0.018
0.014
were spherical in shape and were in the range 8–15 nm. The average
particle size of the calcined NiO samples at different temperatures
and times were determined using particle size analyzer and the
obtained values are listed in Table 1. A representative example of
particle size analysis of the NiO sample calcined at 400 ◦C for 2 h is
presented as inset figure in Fig. 2. It can be seen that the average par-
ticle size determined by the particle size analysis is almost similar
to that obtained by TEM. The structural evolution of nickel oxalate
dihydrate as a function of calcination temperatures from 300 to
700 ◦C for 2 h was recorded through XRD and obtained results are
illustrated in Fig. 3. The patterns corresponded to the formation
NiO with fcc structure and improvement in its crystallinity with
increasing temperature.
3.2. Photolysis of NiO suspensions containing potassium
dichromate
Fig. 4a shows the typical UV absorption spectrum illustrating
the trend of reduction of Cr(VI) in presence of NiO with as a func-
tion of time under laser irradiation. It can be seen from the figure
that about 90% Cr(VI) was removed within short laser exposure
time (75 min) using 355 nm laser radiations. In order to determine
the role of photocatalyst in the reduction process, blank experi-
ment was also carried out by irradiating the aqueous solution of
the potassium dichromate under laser light in the absence of NiO
where about 5% reduction was observed.
For each experiment, the rate constant was calculated from the
plot of natural logarithm of pollutant concentration as a function of
irradiation time. A typical plot of −ln A/A0 (for Fig. 4a) versus time is
depicted in Fig. 4b for Cr(VI) reduction. The least square fit R = 0.97.
200
3.3. Effect of calcination temperature and time on the
photocatalytic reduction process
111
220
The photocatalytic activity of NiO samples obtained after heat-
ing at different calcination temperatures from 300 to 700 ◦C as
well as different heating times was also evaluated and obtained
values are listed in Table 1. It can be noticed from Table 1 that
the photocatalytic activity of samples increases with increasing
calcination temperature presumably due to the improvement of
crystallinity, and the samples calcined at 400 ◦C showed the best
activity for the reduction of Cr(VI) followed by a decrease at higher
temperatures. The best activity shown by the sample calcined at
400 ◦C may be mainly attributed to the generation and separation
of charge carriers (e− and h+) which is affected by various prop-
erties, such as crystallinity, surface area, and particle size, of the
materials (given the fact that with increasing calcination tempera-
ture and time, the particle size increases and surface area decreases)
[7,31]. Briefly, in the photocatalytic process, the separated charge
311
222
700 0C
600 0C
500 0C
400 0C
300 0C
10
20
30
40
50
60
70
80
2θ (deg.)
Fig. 3. XRD patterns of the NiO samples obtained after calcination at different tem-
peratures for 2 h.

86
M. Qamar et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 83–88
Table 2
a
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Laser-induced photocatalytic reduction of Cr(VI) in aqueous suspensions of NiO
under different experimental conditions.
0 min
10 min
20 min
Parameter
Rate constant (min−1
)
Experimental conditions
30 min
Laser energy (mJ)
70.00
0.0002
Chromium
concentration = 50 mg/L, NiO
concentration = 1.0 g/L,
irradiation time = 75 min.
45 min
110.0
150.0
200.0
240.0
0.0005
0.0013
0.0019
0.0026
60 min
75 min
Cr(VI) concentration
25.00
250
300
350
400
450
500
0.0017
Laser energy = 240 mJ, NiO
concentration = 1.0 g/L,
irradiation time = 75 min.
Wavelngth (nm)
50.00
75.00
100.0
0.0026
0.0021
0.0011
b
y = -0.22432 + 0.026222x R= 0.97177
2
1.5
1
Catalysts amount (g L−1
0.5
)
0.0009
Laser energy = 240 mJ,
chromium
concentration = 50 mg/L,
irradiation time = 75 min.
0.5
0
1.0
2.0
3.0
4.0
5.0
0.0026
0.0039
0.0045
0.0048
0.0043
-0.5
0
10
20
30
40
50
60
70
80
Irradiation time (min)
Fig. 4. (a) Typical UV–vis spectra showing the change in absorption intensity as a
function of laser irradiation time for an aqueous solution of K2Cr2O7 in the presence
of NiO. (b) A plot of −ln A/A0 versus the reaction time (for (a)) for Cr(VI) reduction
showing the first-order kinetics.
which in turn increases the photocatalytic reduction process. It is
pertinent to mention here that the maximum energy we can get
with the laser we have is about 240 mJ and hence the effect of laser
energy more than 240 mJ could not be investigated in the present
study. In contrast to the conventional photocatalytic systems where
one has to change the lamp to control the energy, the laser equip-
ment furnishes very easy access as well as a range of energy to
control the photonic efficiencies, according to the requirements,
during the photocatalytic reactions. It has been observed in this
study that the reduction of chromium metal can be significantly
enhanced (almost complete reduction within the 75 min of irra-
diation time) by simply increasing the laser energy. The observed
improvement in the reduction trend of Cr(VI) as a function of laser
energy is more significant than that obtained using different con-
ventional UV lamps [32].
carriers escape to the particle surface (surface-trapped charge car-
riers) where they participate in redox reactions with the adsorbed
species (interfacial-charge transfer process). When the particle size
of the sample is very small, surface-trapped charge carriers may get
annihilated by the subsequent photo-generated e−/h+ pair before
the interfacial-charge transfer process takes place which in turn
leads the lower performance of the catalyst. When, on the other
hand, the crystallite size is larger, the photo-induced e−/h+ have to
travel much distance to reach the surface and this extended jour-
ney increases the possibility of recombination within the particle
volume. When the size of the particles are neither very small nor
too large, the generated e−/h+ pair may move to the surface of par-
ticles and get trapped before they undergo volume charge–carrier
recombination process (surface charge carrier trapping). For the
optimum photocatalytic performance, therefore, the rate of volume
and surface charge carrier recombination process should be mini-
mum, while that of the interfacial-charge transfer process should
be maximum. Similar explanation can also be offered for the best
activity shown by the NiO sample heated at 400 ◦C for 2 h as com-
pared to those heated for 1, 3, 4, and 5 h.
3.5. Effect of Cr(VI) concentration on the photocatalytic reduction
process
It is important both from mechanistic and application points of
view to study the dependence of the photocatalytic reaction rate
on the substrate (pollutant) concentrations. Hence, the influence
of substrate concentration varying from 25 mg/L to 130 mg/L on
the photocatalytic reduction process was studied and the obtained
results are presented in Table 2. It can be seen from Table 2 that
the reduction increased with the increase in substrate concentra-
tion from 25 to 50 mg/L and a further increase in the concentration
of the metal led to a decrease in the reaction rate. The decrease in
reduction process at higher metal concentration may be rational-
ized in terms of the fact that after the reduction of certain amount of
metal on the surface of catalyst, the catalyst’s surface might get sat-
urated and a further increase in chromium concentration remain in
the aqueous solution. Another possible explanation for this behav-
ior may be ascribed to the fact that as the pollutant concentration
increases, the color of the irradiating mixture becomes more and
more intense (given the fact that the aqueous solution of potassium
dichromate produces greenish color) which prevents the penetra-
3.4. Effect of incident laser energy on the photocatalytic
reduction process
The effect of incident laser energy on the reduction of Cr(VI)
was investigated and obtained results are presented in Table 2. It is
obvious from Table 2 that the reduction of metal was significantly
influenced by the laser energy and reduction was found to increase
almost linearly with the increase in laser energy within the range
studied. This phenomenon may be explained in terms of the fact
that when higher laser energy is employed, incident photon flux
increases in the solution exciting more and more catalyst particles

M. Qamar et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 83–88
87
tion of light to the surface of the catalyst. Hence, the generation
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
of relative amount of reactive species on the surface of the cata-
lyst does not increase as the other parameters such as intensity of
light, illumination time and concentration of the catalyst are kept
constant. Consequently, the reduction of metal decreases as the
concentration exceeds the optimum limit.
3.6. Effect of NiO amount on the photocatalytic reduction process
The dependence of the reduction of metal on the NiO amount
is included in Table 2. NiO amount was optimized by exposing
the aqueous suspensions of potassium dichromate containing NiO
ranging from 50 mg to 500 mg by keeping other parameters con-
stant. As the amount of NiO was increased from 50 to 300 mg, the
reaction rate was much faster and a further increase in catalyst
loading (from 300 to 400 mg) was not found to be much benefi-
cial for the photocatalytic reduction of metal. When the catalyst
amount was increased above 400 mg, the reduction rate of the
metal was decreased. As the other parameters, such as exposure to
a constant photon flux, beam diameter, stirring rate, metal concen-
tration, irradiation time, etc. were kept constant, further increase
in particle density beyond optimum density fails to contribute sig-
nificantly to the reduction process. When the catalyst amount is
very high, after traversing a certain optical path, turbidity impedes
further penetration of laser light in the reactor (incidence of the
combined phenomena of particle masking and scattering) lowering
the efficiency of the catalytic process.
NiO/H2O2
NiO/methanol
NiO
Fig. 5. Effect of electron donor and acceptor on the photocatalytic reduction of
Cr(VI).
ing the reduction process slow that was clearly indicated by the
retardation of chromium reduction rate.
4. Conclusions
The present investigation showed that the nanostructured NiO
synthesized by sol–gel technique can be successfully applied for
the complete and efficient reduction of Cr(VI) from aqueous solu-
tion using a novel laser-induced photocatalytic process. The study
showed that ∼90% Cr(VI) was removed within short time (75 min)
of laser exposure without the use of any additive. A linear depen-
dence of chromium reduction was found on the incident laser
energy. The most appropriate metal concentration for the max-
imum reduction was found to be about 50 mg/L in this study.
The maximum reduction of chromium was achieved using laser
energy = 240 mJ and catalyst concentration = 4 g L−1. The addition
of electron donor such as methanol enhanced the metal reduction
process. This study clearly demonstrated that the NiO-mediated
laser-induced photocatalytic process could be applied as an effec-
tive method to remove the heavy metals present in waste water in
shorter time duration without incorporation of any additives. The
optimization of various operational parameters demonstrates the
significance of selection of the optimum experimental conditions
to obtain a high reduction rate.
3.7. Effect of added electron donor and acceptor on the
photocatalytic reduction process
One practical problem in photocatalytic reactions using semi-
conductors is the undesired electron/hole recombination, which,
in the absence of proper electron acceptor or donor, is extremely
efficient and hence represents the major energy-wasting step thus
limiting the achievable quantum yield. One strategy to inhibit
electron-hole pair recombination is to add other (irreversible) elec-
tron acceptors or donor to the reaction. They could have several
different effects such as (1) to increase the number of trapped
electrons and, consequently, avoid recombination, (2) to gener-
ate more radicals and other oxidizing species, and (3) to increase
the reduction rate of metals. It is pertinent to mention here that
in highly toxic wastewater where the reduction of heavy metal is
the major concern, the addition of additives to enhance the pho-
tocatalytic process may often be justified. In this connection, we
studied the effect of electron acceptor and donor such as hydrogen
peroxide and methanol respectively on the photocatalytic reduc-
tion of the model pollutant under investigation and the obtained
results are shown in Fig. 5. The enhancement in reduction rate of
Cr(VI) in the presence of methanol may be ascribed to current-
doubling effect [33]. The methanol scavenges the generated holes
and get oxidized producing the electron-donating species •CH2OH
(E0 (•CH2OH/CH2O) = −0.95 V). Due to its large negative potential,
the methanol radical then injects an electron into the semiconduc-
tor particles thereby increasing the total number of electrons on the
catalyst’s surface. The effect of methanol on the metal reduction
process, in the absence of photocatalyst, under identical condi-
tions was also investigated. The metal reduction was found to be
more or less similar as observed in the case of blank experiment
which indicates that the presence of methanol in the absence of
catalyst is not effective for the reduction of metal. We also investi-
gated the reduction of metal in presence of NiO and methanol but
in the absence of light. The analysis did not indicate any change
in chromium concentration. Conversely, the addition of hydrogen
peroxide affected the reduction process negatively as evident from
Fig. 5. Hydrogen peroxide can pick up the excited electrons mak-
Acknowledgements
The support by Center of Excellence in Nanotechnology (CENT)
and King Fahd University of Petroleum and Minerals (KFUPM) is
gratefully acknowledged under project # 08-NAN-93-4.
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