1. No category

Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers
journal homepage: www.elsevier.com/locate/jtice
Development of efficient chemi-sensor and photo-catalyst based on
wet-chemically prepared ZnO nanorods for environmental
remediation
M. Faisal a, Sher Bahadar Khan b,c, Mohammed M. Rahman b,c,*, Adel A. Ismail a,
Abdullah M. Asiri b,c, S.A. Al-Sayari a
a
Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988,
Najran 11001, Saudi Arabia
b
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, 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 November 2013
Received in revised form 5 May 2014
Accepted 11 May 2014
Available online 29 May 2014
Highly sensitive ammonia chemical sensor has been fabricated by using efficient ZnO nanorods (NRs)
synthesized by low temperature wet-chemical process. The fabricated ammonia sensor showed
excellent sensitivity 5.538 mA/cm2/mM, lower-detection limit (0.11 mM), and large-linear dynamic
range (LDR, 0.5 mM to 0.5 mM) with good linearity (R = 0.7102) in short response time (10.0 s).
Additionally, ZnO NRs displayed excellent performance in degrading acridine orange (AO), methylene
blue (MB), and amido black (AB). The structural and morphological characterizations of the synthesized
ZnO was carried out by field emission scanning electron microscopy (FESEM), powder X-ray diffraction
(XRD), Fourier transform infrared spectrophotometer (FT-IR), and UV–vis spectrophotometer, which
confirmed that the synthesized product is wurtzite type well-crystalline rod-shape structure with an
average diameter of 58.61 5.0 nm.
ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords:
Zinc oxide nanorods
Structural properties
Optical properties
Photo-degradation
Ammonia chemical sensing
1. Introduction
Nanoscience and technology have attracted significant attention due to their potential application in various fields [1]. ZnO a
versatile material, emerges as a challenging prospect in the field of
nanotechnology. Nanosized ZnO has been widely used as a catalyst
[2,3], gas sensor [4,5], active filler for rubber and plastic, UV
absorber in cosmetics and antivirus agent in coating [6–8] and
have more potential application in building functional electronic
devices with special architecture and distinctive optoelectronic
properties. With the development of industry and economy
environmental problem become more and more serious day by
day. Due to certain man made activities numerous hazardous
compounds are introduced into the environment which is a
concerning matter for monitoring agencies and regulation
authorities. Photocatalysis has been regarded as the most viable
* Corresponding author. Tel.: +966 59 642 1830; fax: +966 12 6952292.
E-mail addresses: mmrahmanh@gmail.com, mmrahman@kau.edu.sa (M.M.
Rahman).
technology to solve this problem [9]. Various photocatalysts,
especially metal oxide photocatalyst such as TiO2, tin oxide and
Zinc oxide has attracted extensive attention for the degradation of
organic pollutants in water and air under UV irradiation [10–14].
Although TiO2 is the favorite photocatalyst for the degradation of
majority of organic contaminants but ZnO has recently receiving
much attention because of its simple synthesis procedure, lower
cost and band gap energy around 3.2 eV [15].
Photocatalyst play an important role in the environmental
safety by photocatalytic degradation of pollutants. Environmental
pollutants such as dyes, phenols, pesticides, etc. which discharged
by industries or by certain man made activities have adverse effect
and causes water pollution, environmental contamination and big
threat to human health and aquatic ecosystem due to their
toxicity, carcinogenicity and hazardous effect [16,17]. In the last
years, much research efforts have been focused on finding an
active catalyst suitable for detoxification of these pollutants.
Numerous reports has been well documented in literature
regarding the utilization of ZnO and ZnO based nanocomposites
for the effective treatment of harmful and toxic moieties present
in the waste water. Wang and coworkers successfully synthesized
http://dx.doi.org/10.1016/j.jtice.2014.05.008
1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
ZnO–SnO2 hollow spheres and hierarchical nanosheets using an
aqueous solution containing ZnO rods, SnCl4, and NaOH by using a
simple hydrothermal method. Both hollow spheres and hierarchical nanosheets show higher photocatalytic activities in the
degradation of methyl orange than that of ZnO rods or SnO2 [18].
Similarly in this series Zhang et al. reported the fabrication of Onedimensional ZnO–SnO2 nanofibers by a combination method of
sol–gel process and electrospinning technique. The photocatalytic
activity of the ZnO–SnO2 nanofibers has been carried out for the
degradation of rhodamine B (RB) and ZnO–SnO2 nanofibers
showed higher photocatalytic activity than that of electrospun
ZnO and SnO2 nanofibers [19]. Uddin et al. prepared nanoporous
SnO(2)–ZnO heterojunction nanocatalyst and found that heterostructure SnO(2)–ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than
those of individual SnO(2) and ZnO nanomaterials [20]. Taking
into account the potential applications of ZnO and its nanocomposites we successfully synthesized the low dimension ZnO
nanostructured. The main aim of the present study is to explore
the multi remediation effect of these nanostructures for environmental application. Our results indicates that synthesized ZnO
utilized as redox mediator and photocatalyst for the fabrication of
efficient chemi-sensor and for the degradation of various dyes
molecules.
Rapid industrialization and urbanization also increases the
number of reports of accidental leakage of harmful gases and
liquids which is also a matter of great concern for scientific society.
Recently considerable attention has been focused to develop
semiconducting metal oxide based chemical sensors for safety of
environment by environmentalist and scientific community
[21,22]. Ammonia gas in liquid form i.e. ammonium hydroxide
is one of the important industrial gas as it is toxic and has polluting
nature. Because of its broad range of applications like in fertilizers,
nitrogen containing chemicals, industrial refrigerant, etc. its world
wide production excess to 100 million tons per annum. Concentrated form is highly fatal and can cause severe burn to skin, throat,
eyes and have bad impact on lungs also. Ammonium hydroxide
aerosols are also corrosive, sun blocking function and their fumes
reduces the temperature also. Thus it is very important to develop
sensors and devices for early detection and quantification of
ammonium hydroxide in a wide range of industrial application.
Significant attention has been focused on solution phase ammonia
sensors based on electrical, optical and chemical detection because
it can be operated at room temperature, low level concentration
can be detected with fast response time and its useful application
in order to quantify trace level ammonia solution in ecosystem.
Lots of efforts have been made to develop chemical sensor for the
detection of NH4OH such electrochemical sensor and chemiresistive sensor [23], SAW sensors [24] and optical sensors [25,26].
In general, metal oxide due to their unique surface activities
imparted by huge surface areas, are supposed to be the ideal
sensing elements as chemi-sensors. Chemical sensing technique
using thin film of metal oxide mainly utilizes the properties of that
thin film formed by physi-sorption and chemisorption technique.
Detection of chemical is based on change in current value of
fabricated thin film caused by the chemical components of the
systems reacting in aqueous medium [27,28].Here, the main target
is to detect the minimum ammonium hydroxide concentration
required for electrochemical detection.
Taking into account their potential applications an attempt
have been made to develop a photocatalyst and chemi-sensor
and for this purpose zinc oxide nanostructures were synthesized.
These nanostructures were applied for catalytic and sensing
application and demonstrated good degradation and sensing
properties. To best of our knowledge, this is the first report for
detection of ammonium hydroxide (in liquid phase) with ZnO
nanorods using simple and reliable I–V technique in short
response time. These chemical sensing and photocatalytic
properties of ZnO nanorods are of great importance for the
utilization of ZnO as a chemical sensor and photocatalyst
[26–35].
2. Experimental
2.1. Materials and methods
Analytical grade zinc chloride, urea, ammonium hydroxide,
butyl carbitol acetate, ethyl acetate, monosodium phosphate,
disodium phosphate, acridine orange (AO), methylene blue (MB),
amido black (AB) and all other chemicals used were purchased
from Sigma–Aldrich and used as received. The solution was made
in double distilled water before performing any reaction. ZnO
nanorods were synthesized by wet-chemical method in which
ZnCl2 (1.36 g) and urea (2.4 g) were dissolved in distilled water
(100.0 ml) with a constant stirring for about 30 min at room
temperature and were then pH was adjusted at 10.2 by drop wise
addition of NH4OH solution. The resultant solutions were then
stirred at 80.0 8C for 15 h. After terminating the reaction, white
precipitates were obtained which were washed with water and
ethanol several times and dried at room-temperature. The
resulting white powders were calcined at 400.0 8C for 5 h.
Structural characterizations of the nano-materials were investigated using field emission scanning electron microscope (FE-SEM;
JSM-7600F, Japan), X-ray diffraction (XRD; X’Pert Explorer,
PANalytical diffractometer) data was executed with Cu-Ka
radiation. Fourier transforms infrared spectrometer (FT-IR; Perkin
Elmer) spectrum was recorded in KBr dispersion in the range of
400–4000 cm1. UV/visible spectrum were recorded in the range
of 300–800 nm (Perkin Elmer-Lambda 950-UV–visible spectrometer). The morphologies, sizes, and structures of calcined ZnO NRs
were executed by HR-TEM (TEM; JEM-2100F, Japan). HR-TEM
sample was prepared as follows: the synthesized calcined ZnO
NRs were dispersed into ethanol under ultrasonic vibration for
2 min, and then the HR-TEM film is dipped in the solution and
dried for investigation. Brunauer–Emmett–Teller (BET) measurements were investigated on nitrogen gas sorption system
(Autosorb, Quantachrome Instruments) using nitrogen as the
adsorbate. ZnO nanorods were degassed for 12.0 h at 400.0 8C
prior to measurement. The nitrogen sorption curve was taken as
60/60 pts adsorption/desorption (equal timeout: 240/240 ads/
des), with the BET surface-area calculated using a multi-point BET
analysis.
2.2. Photo-catalytic experimental set-up
The photocatalytic reaction were carried out in a 250.0 ml
beaker, which contain 150.0 ml of each dye solution (0.03 mM) and
150.0 mg of ZnO nanomaterial. Prior to irradiation, the solution
was stirred and bubbled with oxygen for at least 15 min in the dark
to allow equilibrium of the system so that loss of compound due to
the adsorption can be taken into account. The suspension was
continuously purged with oxygen bubbling throughout the
experiment. Irradiation was carried out using 250 W high pressure
Mercury lamps. Samples (5.0 ml) were collected before and at
regular intervals during the irradiation and dye solution were
separated from the photocatalyst by centrifugation before analysis.
The degradation of the dye derivatives was followed by measuring
the change in absorption intensity at their lmax 491 nm (Acridine
orange), 663 nm (Methylene blue), and 618 nm (Amido black) as a
function of irradiation time using UV–vis spectrophotometer
(Perkin Elmer Lambda 950 model).
M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
2.3. Gold electrode fabrication and ammonium hydroxide detection
by ZnO NRs
Glassy carbon electrode (surface area, 0.0316 cm2) is coated
with prepared ZnO using butyl carbitol acetate (BCA) and ethyl
acetate (EA) as a coating agent. Then it is kept in the oven at 60 8C
for 3 h until the film is completely dried. 0.1 M phosphate buffer
solution at pH 7.0 is made by mixing 0.2 M Na2HPO4 and 0.2 M
NaH2PO4 solution in 100.0 mL de-ionize water. A cell is constructed consisting of nanomaterial coated glassy carbon electrode
as a working electrode and Pd wire is used as counter electrode.
Ammonium hydroxide solution is diluted at different concentrations in DI water and used as a target chemical. Amount of 0.1 M
phosphate buffer solution was kept constant as 20.0 mL for all
measurement. Solution was prepared with various concentrations
of ammonium hydroxide in DI water. The ratio of voltage and
current (slope of calibration curve) is used as a measure of
ammonium hydroxide sensitivity. Electrometer is used as a voltage
sources for I–V measurement in simple two electrode system.
3. Results and discussions
3.1. Morphological, structural and optical characterization
FESEM was used for the general structural characterization of
the calcined products and demonstrated in Fig. 1. It is clear from
the images that the synthesized products are grown in very highdensity and possess rods shape structure (Fig. 1(a)–(c)). The
average diameter of the grown nanorods were in the range of
58.61 5 nm.
To check the crystallinity of the synthesized ZnO nanorods, Xray diffraction technique was used and results are shown in
Fig. 2(a). A series of characteristic peaks were obtained which are
well-matched with that of bulk wurtzite hexagonal well-crystalline ZnO. No other impure diffraction peaks were detected within
2735
the detection limit of the X-ray diffraction which confirms that the
obtained nanomaterial are pure ZnO with wurtzite hexagonal
phase [36,37].
Fig. 2(b) shows the typical FTIR spectra of the ZnO nanomaterial
measured in the range of 420–4000 cm1. The appearance of a
sharp band at 495.18 cm1 in the FTIR spectra confirms the
synthesis of ZnO because it is the characteristic absorption band for
the Zn–O stretching vibration [37]. Additionally broad absorption
peaks centered at around 3477 cm1 and 1612 cm1 are caused by
the O–H stretching of the absorbed water molecules and carbon
dioxide because the nanocrystalline materials exhibit a high
surface-to-volume ratio. A very small band at 904 cm1 may be
due to the carbonate moieties which generally appears when FTIR
samples are measured in air [38].
Optical property is one of the most important properties of any
material for evaluation of its photocatalytic activity. The wavelength (lmax) of ZnO nanorods was measured by using UV/visible
spectrophotometer and presented in Fig. 2(c). It displays a welldefined and strong absorption peak at 377.1 nm, a characteristic
and corresponding peak to the wurtzite hexagonal phase bulk ZnO
[39]. No other peak related with impurities and structural defects
were found in the spectrum which confirms that the prepared
material contain only crystalline ZnO.
By using UV/visible spectrum, the optical band gap of
nanomaterials was calculated by Tauc’s formula, which shows
the relationship among absorption coefficient and the incident
photon energy of ZnO nanostructures. The Tauc’s equation is
presented in below as,
ahn ¼ Aðhn Eg Þn
where a is the absorption coefficient, A is constant, and n is equal to
1/2 for a indirect transition semiconductor and 2 for direct
transition semiconductors. Accordingly, to calculate the optical
Fig. 1. Typical (a) low-magnification, (b) and (c) high-resolution FESEM images of the synthesized ZnO nanorods synthesized via simple thermal stirring process.
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M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
Fig. 2. Typical (a) XRD pattern, (b) FTIR spectrum, (c) UV–vis spectrum (d) tauc plot (band-gap energy calculation) of the synthesized ZnO nanorods synthesized via simple
thermal stirring process.
band gap (Ebg) for the calcined ZnO, a plot of (ahn)2 against incident
photon energy (hn) has been displayed in Fig. 2(d). The direct band
gap energy was estimated by extrapolating the straight-line
segment of the plot ‘‘(ahn)2’’ vs ‘‘(hn)’’ to zero absorption
coefficient value. From the Fig. 2(d), the value of Ebg was found
to be 3.00 eV [40].
3.2. HR-TEM analysis
Further structural characterization of the calcined hexagonalshaped ZnO nanostructures grown in rod-shaped structures was
investigated by the transmission electron microscopy (TEM) and
presented in Fig. 3(a and b). The low-magnification TEM
observation shows the exact morphology of the rod-shaped ZnO
nanostructures assembled in rod-like structures as was seen in
FESEM and reveals the full consistency in terms of shape and
dimension (Fig. 3 (a and b)). The typical diameter of the observed
nanorod is consistence with the FESEM investigation, which
possessing a very clean and smooth surfaces with the uniform
diameter pass throughout their lengths. Fig. 3 shows the TEM
image which confirm that the calcined structure is single
crystalline with the lattice d-spacing of [0 0 0 1] crystal planes
of the wurtzite hexagonal ZnO and preferentially grown along
[0 0 0 1] direction.
3.3. BET analysis
Brunauer–Emmett–Teller (BET) theory aims to explain the
physical adsorption of gas molecules (especially nitrogen gas) on a
solid surface (undoped metal oxides) and serves as the basis for an
important analysis technique for the measurement of the specific
surface area of prepared undoped ZnO nanomaterials. The average
pore diameter and specific surface area (BET: surface and pore
volume) were measured for ZnO nanorods using a Quantochrome
NOVA 1000 (Boynton Beach, FL, USA). In order to estimate if any
Fig. 3. Typical (a) low-magnification and (b) high-resolution of the calcined highaspect-ratio ZnO rod-like structures composed of perfectly hexagonal-shaped ZnO
nanorods.
M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
change of the physical structure occurs of undoped ZnO nanorods,
the pore size distribution and the specific surface area were
investigated using multipoint BET analysis by considering the
adsorption/desorption of N2. As shown in Fig. 4, it is calculated the
pore size (pore width), pore volume specific, and surface area of
4.8 nm (Fig. 4a), 0.016 cc/g, and 11.4 m2/g (Fig. 4b) respectively
for the ZnO nanorods by considering the adsorption/desorption of
N2 (0.100/0.100:ads/des). Therefore, the observed surface area
results from a formation of nanopores accessible for the nitrogengas, resulting in an increase of the capacitance of the semiconductor metal oxides (ZnO nanorods). The experimental data exhibited
that varying the synthetic conditions, extensively affected the pore
size distribution and specific surface area of obtained ZnO
nanorods as presented in Fig. 4.
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4. Potential applications
4.1. Photocatalytic degradation of dyes by ZnO NRs
Fig. 5(a–c) exhibits the change in absorption spectra for the
photo catalytic degradation of acridine orange (AO), methylene
blue (MB) and amido black (AB) with exposure time. It can be seen
that irradiation of aqueous suspension of dyes under consideration
in the presence of ZnO nanorods leads to decrease in absorption
intensity. Maximum absorbance at 491.0 nm for AO, 663 nm for
MB and 618 nm for AB gradually decreases with increase in
irradiation time. Fig. 5(d and e) shows the plots of change in
absorption intensity and % degradation vs irradiation time (min)
for the oxygen saturated aqueous suspension of all the three dye
derivatives in the presence and absence the synthesized metal
oxide nanostructures. It could be seen from the Fig. 5(e) that 87.5%
of AO,79.5% of MB and 37.22% of AB dye derivatives degraded after
170.0 min of irradiation time(in the presence of synthesized ZnO
nanostructures) whereas in the absence of photo catalyst no
observable loss of dye could be seen.
The degradation rate for each dye derivatives was also
calculated. Degradation rate constant for each experiment was
calculated from the initial slope obtained by linear regression from
a plot of the natural logarithm (ln) of absorbance of the AO, MB and
AB as a function of exposure time, i.e., first-order degradation
kinetics. This rate constant is used to calculate the degradation
rates for the decomposition of AO, MB and AB using the formula
appended Eq. (1) below:
d½A
¼ kcn
dt
(1)
where k is the rate constant, c the concentration of the dye and n
the order of reaction.
The degradation rates for the decomposition of AO, MB and AB
in the presence of ZnO nanomaterial is shown in Fig. 5(f). It is
interesting to note that synthesized nanomaterial shows highest
efficiency for the degradation of AO while for MB proved to be a
good photocatalyst whereas in case of AB showing pronounced
effect for the degradation.
4.2. Reaction kinetics of photo-degradation
In order to realize the degradation behaviors we studied the
degradation pattern of dyes by Langmuir–Hinshelwood (L–H)
model. L–H model well defines the relationship among the rate of
degradation and the initial concentration of dyes in photo-catalytic
reaction [41]. The rate of photo-degradation is calculated by using
Eq. (2):
r¼
dC
¼ K r KC ¼ Ka p pC
dt
(2)
where r represents degradation rate of dye, Kr reflects reaction rate
constant, K represents equilibrium constant, C represents reactants
concentration. When C is very low, then KC is negligible; so that
Eq. (2) become first order kinetic. Setting Eq. (2) under initial
conditions of photo-catalytic procedure, (t = 0, C = C0), it become
Eq. (3).
Fig. 4. BET for (a) plotting of pore width and pore volume (pore size distribution),
and (b) plotting and calculation of multi-point BET analysis regarding surface area
of ZnO nanorods for different nitrogen gas loadings. Parameters: sample weight:
0.1993 g; out-gas time: 12.0 h; analysis gas: N2; pressure tolerance: 0.100/0.100
(ads/des); analysis time: 776.5 min; sample volume: 0.06643 cc; outgas temp:
400.0 8C; bath temp: 77.3 K; equal time:60/60 (ads/des); sample density: 3.0 g/cc,
equal timeout: 240/240 s (ads/des). DFT: N2 at 77.0 K on carbon (slit pore, NLDFT
equilibrium model); Rel. press. range: 0.0000–1.0000. Adsorbate: gas: nitrogen:
cross section: 16.200; liquid density: 0.806 g/cc.
ln
C
¼ kt
C0
(3)
Half-life, t1/2 (in min) is
t 1=2 ¼
0:693
k
(4)
Fig. 4(c) showed that the degradation of AO, MB and AB
followed first-order kinetics (plots of ln (C/C0) vs time showed
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M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
Fig. 5. Photocatalytic degradation of AO, MB and AB using ZnO nanorods. (a) Spectrum of AO, (b) spectrum of MB, (c) spectrum of AB at different time interval, (d) comparison
of absorbance and (e) % degradation in different time intervals of AO, MB and AB in presence and absence of ZnO nanorods. (f) Comparision of degradation rate for
decomposition of AO, MB and AB in the presence of ZnO nanorods synthesized via simple thermal stirring process.
linear relationship). First-order rate constants, evaluated from the
slopes of the ln (C/C0) vs time plots and the half-life of the degraded
AO, MB and AB can then be easily calculated by Eq. (4) [41]. The
rate constant for AO, MB and AB in the presence of ZnO nanorods
were found to be 0.01243 min1 (t1/2 = 55.75 min), 0.00889 min1
(t1/2 = 77.95 min), and 0.00248 min1 (t1/2 = 279.43 min). Thus the
kinetic study revealed that ZnO nanorods is a proficient photocatalyst for degradation of organic pollutants.
The degradation rate for each dye was also calculated by using
rate constant for the decomposition of AO, MB and AB. The
degradation rates for the decomposition of AO, MB and AB in the
presence of ZnO nanomaterial is shown in Fig. 5(d). It is interesting
to note that synthesized nanomaterial shows highest efficiency for
the degradation of AO while for MB proved to be a good
photocatalyst whereas in case of AB showing pronounced effect
for the degradation. On comparing the photo activity of prepared
ZnO nanomaterial with commercially available Degussa P25
(TiO2), the commercially available material was found to be more
effective. The rate constant for AO, MB, and AB in the presence of
Degussa P25 (TiO2) were found to be 0.0201 min1 (t1/
1
(t1/2 = 39.92 min), and 0.0072 min1
2 = 34.47 min), 0.0174 min
1 (t1/2 = 96.25 min) respectively. One important feature of prepared ZnO nanomaterial is the reusability, it can give almost same
performance after 2–3 times of recycling.
A very important parameter in the photocatalytic reactions
taking place on the particulate surfaces is the pH of the solution,
since it dictates the surface charge properties of the photocatalyst
and size of aggregates it forms. Earlier studies have shown that in
M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
case of AO the reaction rate increases as the reaction pH increases
from 3 to 7 and becomes almost doubled at pH 7. The maximum
degradation achieved at this pH and with further increase in pH,
rate of photodegradation decreases [42]. In case of MB, the extent
of photocatalysis increases with increase in pH value with
maximum destruction of dye in alkaline pH [43]. For AB
degradation was found to increase with increase in pH from
acidic to basic range and maximum degradation was observed at
pH 9.5 [44].
4.3. Mechanism of heterogeneous photocatalysis
Mechanism of heterogeneous photocatalysis for the degradation of various pollutants has been discussed extensively in
literature. Formation of electron and hole pair will take place on
absorbing photon of energy equal to or greater than its band gap
by a semiconductor or metal oxide (MOx). Generated electron
and hole migrate to the surface of semiconductor or metal oxide
if charge separation is maintained where they participate in
redox reaction with organic substrate dissolved in water in
presence of oxygen. Hydroxyl radicals (OH) and superoxide
radical anions (O2) are supposed to be the main oxidizing
species and these oxidative reaction results in the oxidation of
the pollutants, often upto complete mineralization. The whole
mechanism of photoactivity of synthesized ZnO nanomaterial is
depicted in Fig. 6. Above results clearly indicate that prepared
nano-material showing good photo catalytic activity, has very
simple synthesis procedure and low cost, so it can be a beneficial
photo catalyst for wastewater treatment beside other metal
oxide.
4.4. Detection of ammonia using NRs by I–V technique
The thin film of synthesized nanomaterial (rod shape ZnO) was
made using conducting binder and embedded on the glassy carbon
electrode. Fabricated electrode was kept in oven at low temperature (60.0 8C) for 3 h until the film is completely dried and uniform
the film completely. Ammonium hydroxide was used as a target
molecule for the measurement in liquid phase. The electrical
2739
response of target molecule has been measured using I–V
electroanalytical technique. The sensing properties of I–V sensors
(two electrodes system) having ZnO thin film has been studied. I–
V responses sensor having ZnO thin film as a function of time for
the ammonia is shown in Fig. 7(a and b). A significant increase in
the current value with applied potential is clearly demonstrated.
The gray-solid and dark-solid dotted curves indicate the response
of the film before and after injecting 100.0 mL chemicals in bulk
solution. Significant increase in the sample current is measured
after injection of target component. 0.5 mM concentration of
ammonia was initially taken in the cell and added the higher
concentration (every step, 10 time) is in each injection from the
stock concentration of ammonia, which was added to the 20.0 mL
bulk buffer solution. Each I–V response to varying concentration
of chemicals from 0.5 mM to 5.0 mM on thin rod shape ZnO
coatings for 10 s (delay of time) was presented in Fig. 7(c). It shows
current of ZnO as a function of target concentration at room
temperature. It is observed that at lower to higher concentration
of target compound, the current increases gradually. The
calibration curve was plotted from the variation of ammonia
concentration, which is shown in Fig. 7(d). The sensitivity is
calculated from the calibration curve, which is closed to 5.538 mA/
cm2/mM. The linear dynamic range of this sensor exhibits from
0.5 mM to 0.5 mM and the detection limit was found 0.11 mM
(3 N/S). The response time was around 10.0 s for the nanomaterial
coated electrode to acquire saturated steady state current. The
reason for high sensitivity of film may be due to the good
absorption ability (porous surfaces fabricated with coating) and
adsorption property, good biocompatibility and high catalytic
activity and of the zinc oxide nanomaterial. The sensitivity
obtained for the fabricated sensor is relatively higher than
previously reported cases of ammonium hydroxide sensors based
on other composite or materials modified electrodes [32,41,45–
50]. Due to large surface area, the nanorods of zinc oxide provide a
favorable nano-atmosphere for the detection of chemical with
good quantity. The high sensitivity of NRs provide high electron
communication features which improve the direct transfer of
electron between the active sites of NRs and GCE [51–56]. The
fabricated thin film had a good stability.
Fig. 6. Mechanism of ZnO nanorods under UV light photo-excitation of colored AO dye.
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M. Faisal et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2733–2741
Fig. 7. I–V curves of (a) with and without coating of ZnO (b) with and without ammonium hydroxide (c) concentration variation of ammonium hydroxide; and (d) calibration
curve of ammonium hydroxide sensors.
5. Conclusions
The rod-shaped ZnO has been successfully synthesized by wetchemical technique in the alkaline medium. The composition and
detail structural characterization have been studied by UV, FT-IR,
XRD and FESEM which revealed that the synthesized nanostructures are well-crystalline, possessing wurtzite hexagonal shape.
The potential applications on catalytic behavior and chemical
sensing were carried out with synthesized nanomaterial. The
photo-catalytic performance of ZnO nanorods were evaluated by
degradation of acridine orange, methylene blue and amido black
which efficiently degraded the dyes. By applying to the detection
and quantification of ammonium hydroxide, the performance of
the developed ammonium hydroxide sensor is excellent in terms
of sensitivity, detection limit, linear dynamic ranges and response
time.
Acknowledgement
This work was funded by King Abdulaziz University, under
Grant no. (D-001/431). The authors, therefore, acknowledge
technical and financial support of KAU.
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