ArticlePDF Available

Abstract and Figures

To produce titanium dioxide (TiO2) nanoparticle coatings, it is desirable that the nanoparticles are dispersed into a liquid solution and remain stable for a certain period of time. Controlling the dispersion and aggregation of the nanoparticles is crucial to exploit the advantages of the nanometer-sized TiO2 particles. In this work, TiO2 nanoparticles were dispersed and stabilized in aqueous suspensions using two common dispersants which were polyacrylic acid (PAA) and ammonium polymethacrylate (Darvan C). The effect of parameters such as ultrasonication amplitude and type and amount of dispersants on the dispersibility and stability of the TiO2 aqueous suspensions were examined. Rupture followed by erosion was determined to be the main break up mechanisms when ultrasonication was employed. The addition of dispersant was found to produce more dispersed and more stabilized aqueous suspension. 3 wt.% of PAA with average molecular weight (Mw) of 2000 g/mol (PAA 2000) was determined to produce the best and most stable dispersion. The suspensions were then coated on quartz glass, whereby the photocatalytic activity of the coatings was studied via the degradation of formaldehyde gas under UV light. The coatings were demonstrated to be photocatalytically active.
Content may be subject to copyright.
Hindawi Publishing Corporation
Journal of Nanomaterials
Volume 2012, Article ID 718214, 10 pages
doi:10.1155/2012/718214
Research Article
Dispersion and Stabilization of Photocatalytic TiO2
Nanoparticles in Aqueous Suspension for Coatings Applications
Siti Hajar Othman,1, 2 Suraya Abdul Rashid,1, 3
Tinia Idaty Mohd Ghazi,1and Norhafizah Abdullah1
1Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia
2Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia
3Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia
Correspondence should be addressed to Suraya Abdul Rashid, suraya@eng.upm.edu.my
Received 21 March 2012; Accepted 8 May 2012
Academic Editor: Wenhong Fan
Copyright © 2012 Siti Hajar Othman et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
To produce titanium dioxide (TiO2) nanoparticle coatings, it is desirable that the nanoparticles are dispersed into a liquid solution
and remain stable for a certain period of time. Controlling the dispersion and aggregation of the nanoparticles is crucial to exploit
the advantages of the nanometer-sized TiO2particles. In this work, TiO2nanoparticles were dispersed and stabilized in aqueous
suspensions using two common dispersants which were polyacrylic acid (PAA) and ammonium polymethacrylate (Darvan C). The
eect of parameters such as ultrasonication amplitude and type and amount of dispersants on the dispersibility and stability of
the TiO2aqueous suspensions were examined. Rupture followed by erosion was determined to be the main break up mechanisms
when ultrasonication was employed. The addition of dispersant was found to produce more dispersed and more stabilized aqueous
suspension. 3 wt.% of PAA with average molecular weight (Mw) of 2000 g/mol (PAA 2000) was determined to produce the best and
most stable dispersion. The suspensions were then coated on quartz glass, whereby the photocatalytic activity of the coatings was
studied via the degradation of formaldehyde gas under UV light. The coatings were demonstrated to be photocatalytically active.
1. Introduction
TiO2nanoparticles can be used for a variety of applications
including self-cleaning [1], water treatment [2], antibacterial
[3], and air purification [4] due to their eective photo-
catalytic activity. For such applications, TiO2nanoparticles
are generally coated on suitable substrates. TiO2coatings
can be prepared by various ways including chemical vapour
deposition (CVD), physical vapour deposition, sol-gel, as
well as spraying. Among them, the coating process using
TiO2suspensions by spraying is the most cost eective
method besides being compatible for large areas. Producing
TiO2suspensions necessitates the incorporation of TiO2
nanoparticles in the liquid phase, the break up and disper-
sion of nanoparticle clusters, and subsequently stabilization.
To achieve uniform coatings, the degree of particle
agglomeration should be minimized especially when work-
ing with nanoparticles. Nanoparticles have strong tendency
to agglomerate compared to submicrometer particles due to
their large surface area. The agglomeration is generally due
to the Van der Waals attraction forces between nanoparticles
which can be counterbalanced by electrostatic and steric
stabilization. Electrostatic stabilization can be achieved by
the addition of charge to the nanoparticles so that they can
repel one another especially under the influence of the pH.
Steric stabilization can be achieved by the adsorption of a
thin layer of polymer onto the nanoparticle surface to phys-
ically prevent the nanoparticles from coming close enough
to each other and cause agglomeration [5]. Generally, the
combination of both electrostatic and steric stabilization
2Journal of Nanomaterials
is termed electrosteric stabilization and can be achieved
using polyelectrolytes. A polyelectrolyte is a polymer chain
with numerous dissociable groups. Polyacrylic acid (PAA)
is one example of polyelectrolyte dispersants, which is fully
dissociated at pH 8.5 to 9 [5,6].
Ultrasonication has been proven as a useful tool to
disperse nanoparticles and to eliminate agglomeration in
aqueous suspensions [7]. Shock waves caused by collapsing
cavitations under ultrasonic irradiation will lead to collisions
among particles, whereby the agglomerated particles are
eroded and split apart by the collisions [7,8]. Basically,
there are three dierent mechanisms of nanoparticle cluster
break up which are rupture, erosion, and shattering. Rupture
occurs when large agglomerate is broken up into numerous
agglomerates of either the same or dierent size which
can be subsequently broken up further. Erosion, on the
other hand, occurs when small fragments are gradually
sheared oand detached from the outer surface of large
agglomerates. The smaller fragments are either primary
particles or aggregates that cannot be broken up further
under the eect of hydrodynamic stresses. Finally, shattering
occurs when the energy level provided is very high, in which
the agglomerate disintegrate into numerous small fragments
of either aggregates or primary particles in a single event. The
details of the break up modes are reported in the work by
¨
Ozcan-Tas¸kin et al. [9].
Although there are numerous studies incorporating TiO2
nanoparticles into thin films [10] or polymer composites
[11], the dispersion and stabilization of photocatalytic TiO2
nanoparticles for coatings application have not been studied
extensively. Greenwood and Kendall [5] studied the eect
of adding various commercially available polyelectrolyte
dispersants into micron-sized TiO2powder aqueous suspen-
sions. Ran et al. [12] examined the eects of their synthesized
PAA on rheological and dispersion properties of also micron-
sized aqueous TiO2suspensions. Liufu et al. [13]investigated
the adsorption of PAA in aqueous suspension onto the
surface of TiO2nanoparticles and discussed the major factors
influencing the adsorption of PAA. Sato et al. [7]focused
on the eects of ultrasonic irradiation on slurry viscosity
and aggregate size of commercial nanocrystalline TiO2
aqueous suspensions containing polyelectrolyte dispersants
and compared the results with those obtained via ball milling
and bead milling. Both Liufu et al. [13]andSatoetal.[7]
studied high-concentration TiO2aqueous suspensions. Fazio
et al. [14], on the other hand, studied the eect of dispersant
nature, concentration, and the eect of ultrasonication time
on colloidal behaviour of dilute commercial and synthesized
(sol-gel)-nanosized TiO2powder aqueous suspensions.
To the best of our knowledge, no study has been done on
the dispersion and stability of TiO2nanoparticles produced
via metal organic chemical vapour deposition (MOCVD)
technique. Besides, no report has yet been published on the
eect of ultrasonication amplitude towards the dispersion
and stability of TiO2nanoparticle aqueous suspension
although amplitude is one of the important parameters of the
ultrasonic equipment. In addition, all of these studies mainly
focused on preparing disperse and stable TiO2aqueous
suspensions. There is very inadequate information about the
photocatalytic application of the TiO2aqueous suspensions
prepared. The knowledge on the dispersion and stabilization
of photocatalytic TiO2nanoparticles for coatings application
is much needed since a more thorough understanding could
bring improvement to its industrial applications.
The aim of this study was to investigate the eect of
ultrasonication amplitude (0–40%), type of polyelectrolyte
dispersants (PAA with average molecular weight (Mw)
of 2000 (PAA 2000) and 5000 g/mol (PAA 5000), and
Darvan C with Mwof 13 000 g/mol), and amount of
dispersant (0–3 wt.%) on the dispersion and stability of
photocatalytic TiO2nanoparticle aqueous suspensions. The
dispersion and stability of the aqueous suspensions were
characterized using particle size analyzer, zeta potential,
and transmission electron microscope (TEM), and stability
test. The fourier transform-infrared (FTIR) analysis was
carried out to confirm the adsorption of dispersant onto
the TiO2nanoparticles’ surface. To demonstrate that the
TiO2nanoparticle suspensions can be used for coatings
application, the suspensions were then coated on quartz
glass. The energy dispersive X-ray (EDX) analysis was done
on the surface of the coatings to study the uniformity of
the elemental distribution. Finally, the photocatalytic activity
of the coatings was determined via the degradation of
formaldehyde gas under UV light.
2. Experimental
2.1. Preparation and Characterization of TiO2Nanoparticles.
TiO2nanoparticles were synthesized via a custom-built metal
organic chemical vapour deposition (MOCVD) reactor. The
dimensions of the custom-built MOCVD system and the
method used to produce TiO2nanoparticles have been
described in detail elsewhere [15]. However, in this work,
oxygen feed (100 mL/min) was introduced inside the reactor
along with the nitrogen carrier gas that carried the precursor.
This was done to reduce carbon impurities that might
originate from the precursor, and thus postdeposition heat
treatment was not performed for this work. The deposition
temperature was set at a fixed temperature of 700C.
The size and morphology of the TiO2nanoparticles were
studied by TEM (Hitachi H7100). The surface areas were
determined by the N2adsorption using Brunauer-Emmett-
Teller (BET) method (Bel Japan Inc., Belsorp II). Before
any measurement, all the samples were exposed to vacuum
condition at 150C overnight to remove any remaining
moisture. The mean particle size can be calculated from the
BET data by applying the following equation [16]:
D=6
SA ×ρ×1000, (1)
where Dis the mean particle size, SA is the surface area
from BET data, and ρis the density. Finally, the crystalline
phase of the sample was determined using X-ray diraction
(XRD, Philips X’pert Pro PW3040) with a Cu Kαradiation
source (λ=0.15406 nm) operated at 40 kV and 30 mA.
The isoelectric point (iep) of the TiO2nanoparticles was
foundtobeatpH5.2.
Journal of Nanomaterials 3
2.2. Preparation and Characterization of Aqueous Suspen-
sions. All the suspensions were ultrasonically irradiated
using an ultrasonic homogenizer equipped with temperature
controller (Cole-Parmer; frequency =20 kHz; Power =
500 Watt with amplitude range; probe diameter =13 mm).
The ultrasonic irradiation period was set to 30 minutes
because large aggregates were dispersed almost completely
by ultrasonic irradiation for 30 minutes [7]. The suspensions
were irradiated with a 10-second pulse with every 10 seconds
of irradiation period to prevent undesirable increase in
the suspension temperatures. Note that all the prepared
suspensions were aged overnight at room temperature using
an orbital shaker to ensure that the nanoparticles were kept
dispersed in the suspensions as well as to maintain the
homogeneity of the suspensions. The pH of the suspensions
was adjusted and fixed at pH 8.5 using ammonia solution
because PAA and Darvan C (a type of PAA) is fully associated
at this pH value. Ammonia solution was chosen to adjust the
pH because it has a buering eect in which the pH of the
suspensions can be kept at nearly constant value for a wide
variety of chemical applications.
To study the eect of ultrasonication, 0.05 g TiO2
nanoparticle suspensions were prepared in 100 mL of dis-
tilled water (pH adjusted to pH of 8.5) without and with
ultrasonication. Based on initial preliminary studies, 0.05 g
was chosen as an ideal amount of loading for further
dispersion and application studies because it produced glass
coatings with an optimum value of around 80% light trans-
mittance. For the suspensions prepared with ultrasonication,
the amplitude was varied from 0 to 40%. Ultrasonication
amplitude is one of the important sonication parameters.
It indicates the amplitude of vibration at the probe tip
and directly represents the output power of ultrasonic
equipment. Greater amplification creates more intense and
greater disruption.
To study the eect of type and amount of dispersant
on the suspensions, two types of PAA with Mwof 2000 and
5000 g/mol (Acros Organics) along with Darvan C (R.T.
Vanderbilt Company, Inc.) with Mwof 13000 g/mol were
used as dispersants. 0.05 g TiO2nanoparticles were mixed
with 100 mL of distilled water (pH adjusted to pH 8.5), and
the required amount of dispersant (0 to 3 % based on the
TiO2weight (wt.%)) was added followed by ultrasonication
at 40% amplitude.
The particle size distribution of the suspensions was
characterized using a particle size analyzer (Zetasizer Nano S,
Malvern Instruments). The potential stability of the suspen-
sions was studied using a zeta potential analyzer (Zetasizer
Nano Z, Malvern Instruments), around 24 hours after
ultrasonication. Moreover, a stability test was performed to
determine the ability of the TiO2nanoparticle suspensions
to remain dispersed over a period of time. Photos of the
suspensions prepared under various experiment conditions
inside clear glass containers were captured at certain inter-
vals over the course of 8 weeks. To study the eect of
ultrasonication and dispersant addition on the morphology
of the TiO2nanoparticles, TEM images of nanoparticles
extracted from all suspension samples were captured and
compared.
2.3. FTIR Analysis. The FTIR (Nicolet, UK) analysis was
carried out to confirm the adsorption of dispersant onto
the TiO2nanoparticles’ surface. The FTIR spectrum of the
dispersant was taken in liquid state. The FTIR spectrums
of TiO2nanoparticles before adsorption of dispersant were
taken in solid state after they had been dried overnight at
60C to remove any moisture. For the FTIR spectrums of
the TiO2nanoparticles after adsorption, the nanoparticles
were prepared by centrifuging the TiO2suspensions with
optimum dispersant. The nanoparticles were then collected,
washed with deionized water, and dried overnight at 60Cto
remove any moisture.
2.4. Preparation and Characterization of Glass Coatings. The
suspensions were coated on both sides of 8 cm ×8cm
quartz glass using an air brush (AB931, Ingersoll Rand). The
EDX (NORAN System SIX, Thermo Scientific) spectrum
imaging was performed on the coated glass to characterize
the elemental uniformity of the coating.
2.5. Photocatalytic Study. The photocatalytic activity of the
coatings was determined using a simple qualitative analysis
via the degradation of formaldehyde gas under UV light.
A coated glass slide was positioned inside a 1-litre glass-
beaker. Then, the beaker was sealed with a parafilm. A
known amount of formaldehyde liquid was introduced
inside the beaker using a pipette. The initial concentration
of formaldehyde was kept at 2 ppm (short-term exposure
limit (STEL) of formaldehyde gas is 2 ppm in 15 minutes
average) for all experiments. Then, the beaker was quickly
and carefully sealed with 2 layers of parafilm and aluminium
foil to prevent any gas leakage. The beaker was placed inside
a dark box overnight to allow the formaldehyde liquid to
completely evaporate and reach equilibrium. After that, the
photocatalytic degradation of formaldehyde gas was carried
out under the illumination of an 8 W UV light (365 nm peak
wavelength) for a designated period of time.
The final concentration of the gas was then estimated
using a colorimetric method [17] where a known amount
of gas inside the beaker was withdrawn using a syringe and
mixed with a mixture of 2 M ammonium acetate, 0.05 M
acetic acid, and 0.02 M acetylacetone. Then the solution
was heated for 30 minutes in a water bath at around
40C to complete the reaction. A yellow colour gradually
developed owing to the formation of diacetyldihydrolutidine
(DDL). A UV-vis spectrophotometer (Shimadzu UV-1650
PC) was used to determine the absorbance value at 413 nm
(absorbance peak of DDL). The percentage degradation of
formaldehyde gas at a certain period of time was estimated
based on the absorbance value obtained from UV-vis spec-
trophotometer reading before and after the coatings were
irradiated with UV light. All experimental runs were repeated
three times.
3. Results and Discussion
3.1. Characterization of TiO2Nanoparticles. Figure 1(a)
depicts the TEM micrographs of TiO2nanoparticles, while
4Journal of Nanomaterials
100 nm 50 nm
(a)
20 25 30 35 40 45 50 55 60
Intensity
A
AAAA
2θ(deg)
(b)
Figure 1: (a) TEM micrograph and (b) XRD pattern of TiO2nanoparticles synthesized via MOCVD method.
Figure 1(b) illustrates the XRD pattern of the sample. The
micrograph shows that the nominal size of the nanoparticles
was less than 50 nm and that the nanoparticles appeared to
be relatively homogeneous and uniform in size albeit fairly
agglomerated.
The BET surface area of the sample was determined to
be 86.9 m2/g. The mean particle size calculated from the BET
data was found to be 17.7 nm. Meanwhile, the XRD pattern
for TiO2nanoparticles in Figure 1(b) shows peaks at 2θ
values of 25.3, 37.8, 48.0, 53.8, and 54.9, corresponding to the
diractions of the (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1)
planes of anatase, respectively [18,19]. There were no other
detectable peaks corresponding to the rutile crystal structure.
These findings confirmed that the TiO2nanoparticles were in
the pure anatase crystal structure.
3.2. Aqueous Suspensions
3.2.1. Eect of Ultrasonication. Figures 2(a) and 2(b),respec-
tively show the particle size distribution and zeta potential
of the TiO2suspensions with respect to ultrasonication
amplitude. The average cluster size in each suspension
is shown inset in Figure 2(a).FromFigure 2(a),itcan
be seen that the particle size distribution of the TiO2
nanoparticle suspension prepared without ultrasonication
shows a bimodal curve indicating that the nanoparticles
agglomerate in two size ranges of approximately 531–
3580 nm and 3580–5560 nm. When ultrasonication with an
amplitude of 20% was applied, the particle size distribution
curve still appeared as a bimodal shape. However, the first
curve (left curve) was shifted to the left indicating that the
average cluster size was decreased. The height of the second
curve (right curve) was decreased indicating that the volume
of large agglomerates was reduced. This implied that large
agglomerates, though not all, were broken up into smaller
agglomerates, suggesting that the break up process was a
result of a rupture mechanism.
As the amplitude was further increased to 30%, the par-
ticle size distribution curve became unimodal and narrower
with a decreased particle size distribution range of 190–
1281 nm. The curve shifted slightly to the left indicating
that the average cluster size was decreased. This implied that
smaller fragments were sheared ofrom large agglomerates,
and as a result, the volume of smaller cluster increased. Thus,
the main break up mechanism was most likely to be erosion.
As the amplitude was further increased to 40%, the curve
became bimodal again. The particle size range was addi-
tionally shifted to the left, indicating that the agglomerates
were further eroded to smaller size. Again, this implied that
the break up mechanism was erosion and that increasing
the ultrasonication amplitude improved the eciency of
the deagglomeration process. From Figure 2(a),itcanbe
Journal of Nanomaterials 5
0
2
4
6
8
10
12
14
16
18
20
10 100 1000 10000
Volume (%)
Particle size (nm)
20% 30% 40% Without ultrasonication
Without ultrasonication =
1284 nm
20% amplitude =620 nm
30% amplitude =526 nm
40% amplitude =425 nm
Average cluster size
(a)
0% 10% 20% 30% 40%
Zeta potential (mV)
Amplitude
60
50
40
30
20
10
0
(b)
Figure 2: (a) Particle size distribution and (b) zeta potential plot for dierent ultrasonication amplitude values.
deduced that the break up mode through ultrasonication
predominantly occurs by rupture followed by erosion.
The stability of the suspensions was studied in more
detail with measurements of zeta potential. Zeta potential
gives an indication of the potential stability of the suspension
and indicates the degree of repulsion between adjacent,
similarly charged particles in the suspension. It is also an aid
in predicting long-term stability. The higher the magnitude
of the zeta potential, the more stable the suspensions can
be. Figure 2(b) shows the eect of ultrasonication amplitude
on the zeta potential of each suspension. The zeta potential
increased from 35.0 to 50.3 mV as the ultrasonication
amplitude was increased indicating the improvement in the
suspensions potential stability, the amplitude was increased.
Thus, it can be deduced that high ultrasonication amplitude
did not only enhance the eciency of the deagglomeration
process but also the potential stability of the suspension.
3.2.2. Eect of Type and Amount of Dispersant. Figure 3
illustrates the eect of type and amount of dispersant on
the average cluster size of the TiO2suspensions. The inset
in Figure 3 shows, the average cluster size of the TiO2
nanoparticles as the amount of dispersant was varied from
1 to 5% in smaller scale. From this figure, it can be seen
that PAA 2000 produced TiO2suspensions with smallest
average cluster size followed by PAA 5000 and Darvan
C. This finding suggests that the lower molecular weight
dispersant produced suspensions with smaller average cluster
size compared to the larger one. This is most likely due to
the fact that polymers of high molecular weight have longer
carbon chain that can adsorb onto the surfaces of many
nanoparticles and bond them with each other [13], resulting
in a larger average cluster size.
Theamountofdispersantrequiredtoproducethe
smallest average cluster size for all types of dispersants
was found to be 3 wt.%. It can be seen from Figure 3
that below 3 wt.%, the average cluster sizes were relatively
large most likely due to lack of dispersant to prevent the
nanoparticles from being agglomerated. The average cluster
sizes reduced as the amount of dispersant was increased to
3 wt.%, where the optimum average cluster size was achieved.
However, as the amount of dispersant was further increased
80
130
180
230
280
330
380
430
0% 1% 2% 3% 4% 5%
Average cluster size (nm)
Amount of dispersant (wt.%)
PAA 2000
PAA 5000
Darvan C
90
100
110
120
130
1% 2% 3% 4% 5%
Figure 3: Eect of type of dispersant and amount of dispersant on
the average cluster size of TiO2suspensions.
to above 3 wt.%, excess of dispersant led to an increase in
average cluster sizes probably due to agglomeration caused
by polymeric bridging of free polyelectrolytes [14].
Figure 4 shows the eect of 3 wt.% of PAA 2000 addition
on the particle size distribution. The addition of the
dispersant caused the particle size distribution of TiO2
nanoparticles suspension to become unimodal, generating
monodisperse suspension of TiO2nanoparticles. The curve
was shifted to the left indicating that the average cluster size
was decreased. This implied that large agglomerates were
eroded to aggregates that could not be broken up further
and that the deagglomeration process was almost complete.
The particle size range was greatly reduced to 21–295 nm.
The average cluster size was significantly decreased from 425
to 118 nm. Hence, it can be deduced from Figures 3and 4
that the addition of dispersant with the aid of ultrasonication
contributed towards substantial enhancement of the deag-
glomeration process by improving the separation between
nanoparticles and hindering agglomeration.
Furthermore, Figure 5 demonstrates the eect of type
and amount of dispersant on the zeta potential of TiO2
6Journal of Nanomaterials
0
2
4
6
8
10
12
14
16
18
20
10 100 1000 10000
Volume (%)
Particle size (nm)
Average cluster size
Without dispersant (40% A) =424.5 nm
3 wt.% PAA 2000 (40% A) =117.6 nm
Without dispersant (40% A)
3 wt% PAA 2000 (40% A)
Figure 4: Eect of optimum dispersant addition on the particle size
distribution of TiO2suspensions.
Zeta potential (mV)
70
60
50
40
30
0% 1% 2% 3% 4% 5%
Amount of dispersant (wt.%)
PAA 2000
PAA 5000
Darvan C
Figure 5: Eect of type of dispersant and amount of dispersant on
the zeta potential of TiO2suspensions.
suspensions. It can be seen that PAA 2000 provides better
potential stability of the TiO2suspensions followed by PAA
5000 and Darvan C. This suggests that the most ideal disper-
sant to stabilize suspension with small particles (nanometer
sized particle) is low-molecular-weight dispersant that has
shorter carbon chain. Polymers of high molecular weight, on
the other hand, have longer carbon chains that can adsorb
onto the surfaces of many particles and bond them with each
other causing flocculation that results in destabilization of
the suspensions [13]. The same is true for excess amounts
of dispersants.
The amount of dispersant that gives the highest zeta
potential and hence highest potential stability was 3 wt.%
for all types of dispersant. Below 3 wt.%, the suspensions
have low zeta potential and therefore the nanoparticles
were expected to easily flocculate. This is probably due to
inadequate repulsive force to overcome the Van der Waals
attraction between nanoparticles [5]. However, the zeta
potential started to increase as the amount of dispersant
was increased until it reached 3 wt.%, where the optimum
potential stability was achieved. This is consistent with the
fact that PAA is a polyelectrolyte that provides electrosteric
stabilization when it is adsorbed onto the particles’ surface
[5]. Meanwhile, as the amount of dispersant was further
increased, excess of dispersant led to agglomeration and
destabilization as was discussed earlier. Therefore, the zeta
potential decreased, indicating reduction in the potential
stability of the suspensions.
The amount and type of dispersant that provides opti-
mum potential stability (Figure 5) is similar to the amount
and type of dispersant that produces the smallest average
cluster size (Figure 3). In fact, the potential stability is at its
maximum when the average cluster size is minimum. Thus,
it can be deduced that 3 wt.% of PAA 2000 is the optimum
amount and type of dispersant to disperse and stabilize TiO2
suspensions. Finding the optimum dispersant is important
not only to improve the eciency of the deagglomeration
process but also to improve the potential stability of the
suspension.
TEM was used to study the morphology of the TiO2
nanoparticles extracted from the suspensions as illustrated
in Figures 6(a)6(c).ItcanbeclearlyseenfromFigure 6(a)
that without ultrasonication, the nanoparticles are highly
agglomerated, with the agglomerates being relatively large,
more than 500 nm size structures. In contrast, Figure 6(b)
shows that ultrasonication caused a considerable reduction
on the agglomerate size and that the agglomeration of the
nanoparticles had been greatly reduced.
However, Figure 6(c) shows that the addition of the
dispersant had further reduced the size of the agglomerates
to approximately 100nm size structures. The TEM images
support the findings that the addition of dispersant improves
the ultrasonication break up process of the nanoparticles
preventing the agglomeration of the nanoparticle aggregates
and/or agglomerates as was discussed earlier.
Furthermore, a stability test was done to determine
the ability of the TiO2nanoparticle suspensions to remain
suspended over a period of time. The results are shown
in Figure 7. Since zeta potential only gives an indication
of the potential stability of the suspension, a stability test
was conducted to determine the long-term stability of the
suspension. The preparation of stable suspension is crucial
in determining the quality of the coatings.
Figure 7 shows that without ultrasonication, the sta-
bility of the suspension was fairly low. The nanoparticles
completely settled down at the bottom of the container
after around 3-week time. Nonetheless, with ultrasonication,
the stability of the suspension was fairly improved. The
nanoparticles in the suspension started to settle down at
around 1 week and were completely settled down upon
reaching week 5.
Moreover, it can be seen that with ultrasonication and
the addition of dispersant, the stability of the suspensions
was greatly enhanced. The nanoparticles remain suspended
up to 4 weeks. After 5 weeks, the suspensions prepared with
the addition of PAA 5000 and Darvan C started to settle
down with Darvan C being relatively faster compared to
Journal of Nanomaterials 7
500 nm
(a)
500 nm
(b)
500 nm
(c)
Figure 6: TEM micrographs of TiO2nanoparticles extracted from the suspensions prepared (a) without ultrasonication, (b) with 40%
amplitude ultrasonication, and (c) with addition of 3 wt.% dispersant.
(a) (b) (c) (d)
(e) (f) (g) (h)
Figure 7: Photos of TiO2nanoparticle suspensions on (a) day 1, (b) week 1, (c) week 2, (d) week 3, (e) week 4, (f) week 5, (g) week 6, and
(h) week 8.
8Journal of Nanomaterials
PAA2000
TiO
TiO -PAA2000
400900140019002400290034003900
(a)
(b)
(c)
Wavenumber (cm1)
–C=O
–C=O
C–O
C–O
O–H
O–H
Ti–O–Ti
Ti–O–Ti
2
2
Figure 8: FTIR spectra of TiO2nanoparticles before and after
adsorption of PAA2000.
PAA 5000. This is due to the longer carbon chain (higher
molecular weight) of Darvan C compared to PAA 5000.
As discussed earlier, longer carbon chain dispersant can
adsorb onto the surfaces of many particles and bond them
with each other causing flocculation and destabilization of
the suspensions. The addition of the PAA 2000 dispersant
was found to produce TiO2suspension with the best
stability, thus supporting previous potential stability findings
(Figure 5). The suspension remained stable for more than 2
months.
3.2.3. FTIR Analysis. FTIR analysis was done on PAA 2000
as well as TiO2nanoparticles before and after adsorption
of 3 wt.% PAA 2000 as can be seen from Figure 8.The
assignments of various bands and peaks made in this study
are in reasonable agreement with those reported in the
literature for similar functional groups [12,13,20]. The
FTIR spectrum of PAA 2000 shows a strong absorption
band at 1650 cm1indicates –C=O stretching vibration for
carboxylic acids. The band at 1377 cm1is due to the C–
O stretching vibration. The broad peak at 3420 cm1is
attributed to the hydroxyl group (O–H) due to water. Note
that the FTIR spectrum of PAA 2000 was taken in liquid
form.
Meanwhile, the FTIR spectrum of TiO2nanoparticles
shows intense broadband in the vicinity of 400 to 800 cm1
attributed to the stretching vibration of Ti–O–Ti as expected
for TiO2samples. The spectrum does not show peak
attributed to O–H group because the nanoparticles were in
solid form and that they have been dried overnight to remove
the moisture.
In contrast to TiO2spectrum before adsorption, the
TiO2spectrum after adsorption of PAA 2000 shows new
absorption peaks at around 1690 and 1390 cm1indicating
–C=O and –C–O stretching vibrations, respectively. These
findings proved that PAA was adsorbed onto the surface
of the nanoparticles. Moreover, there is also absorption
peak attributed to stretching vibration of O–H at around
3350 cm1which may originate from the adsorbed PAA
2000.
3.3. Coatings
3.3.1. Uniformity of the Coatings. An EDX spectrum imaging
was performed on the coated glass to characterize the unifor-
mity of TiO2coatings prepared without and with dispersant.
Figure 9 shows the titanium (Ti) elemental distribution on
the surface of the coatings. It can be clearly seen that the
addition of the dispersant addition results in a higher surface
concentration of Ti and more uniform Ti distribution. These
findings indicate that the dispersant facilitated the TiO2
nanoparticles to distribute evenly in the suspensions and
hence produced better coatings.
3.3.2. Photocatalytic Activity. TiO2suspensions without and
with dispersant were coated on quartz glass using an air
brush, and the coatings were tested for degradation of
formaldehyde gas under UV light irradiation. The degrada-
tion percentage of formaldehyde gas at a certain period of
time was estimated and plotted in Figure 10.
It can be seen from Figure 10 that all the coatings
were photocatalytically active because the formaldehyde was
able to be degraded after a certain period of time. The
degradation percentage of formaldehyde increased as the
irradiation time of the coatings was increased. The degrada-
tion percentage almost reached a plateau after the coatings
were irradiated under UV light for 20 minutes demonstrating
that the formaldehyde degradation was almost completed
within that time. The degradation of formaldehyde from
both types of coatings was almost comparable with each
other indicating that the addition of dispersant does not
aect the photocatalytic activity of the coatings. Nonetheless,
it is believed that the TiO2coatings prepared with the
addition of dispersant have better adhesion to the glass
compared to that prepared without dispersant owing to the
fact that PAA is a polymer adhesive that can adhere or bond
the coating onto the glass.
4. Conclusion
PAA and Darvan C were employed to disperse and stabi-
lize TiO2nanoparticles produced using MOCVD method
in aqueous suspensions. The use of ultrasonication was
found to assist the agglomerates to break down to smaller
agglomerates and aggregates. Rupture followed by erosion
were found to be the main break up mechanisms when
ultrasonication was employed. The deagglomeration process
and the stability of the suspensions were improved by
increasing the ultrasonication amplitude.
Moreover, the addition of dispersant was found to
improve the deagglomeration process via ultrasonication
by enhancing the separation between nanoparticles and
hindering the agglomeration of the nanoparticles. Low-
molecular-weight dispersant, PAA 2000, produced the most
stable suspension with the smallest average cluster size. The
optimum amount of dispersant to disperse and stabilize the
TiO2aqueous suspensions was found to be 3 wt.%.
The TiO2coatings produced from suspension pre-
pared with optimum dispersant result in a higher sur-
face concentration of Ti and more uniform Ti elemental
Journal of Nanomaterials 9
(a) (b)
Figure 9: EDX spectra of TiO2coating (a) without dispersant and (b) with PAA 2000 dispersant.
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30
Degradation percentage (%)
Time (min)
Without dispersant
With 3% PAA2000 dispersant
Figure 10: Degradation percentage of formaldehyde for TiO2
coatings produced from the suspensions prepared without and with
dispersant suspensions under UV light irradiation.
distribution. The degradation of formaldehyde for TiO2
coatings produced from the suspensions prepared without
and with dispersant was almost comparable with each other,
indicating that the addition of dispersant does not aect the
photocatalytic activity of the coatings.
Acknowledgment
This work was financially supported by Fundamental
Research Grant Scheme, Universiti Putra Malaysia (Grant no.
5523767).
References
[1] J. O. Carneiro, V. Teixeira, A. Portinha et al., “Iron-doped
photocatalytic TiO2sputtered coatings on plastics for self-
cleaning applications,Materials Science and Engineering B,
vol. 138, no. 2, pp. 144–150, 2007.
[2] H. Lachheb, E. Puzenat, A. Houas et al., “Photocatalytic
degradation of various types of dyes (Alizarin S, Crocein
Orange G, Methyl Red, Congo Red, Methylene Blue) in water
by UV-irradiated titania,Applied Catalysis B,vol.39,no.1,
pp. 75–90, 2002.
[3] H. Zhang, H. Liu, C. Mu, C. Qiu, and D. Wu, “Antibacterial
properties of nanometer Fe3+-TiO2thin films,” in Proceedings
of the 1st IEEE International Conference on Nano Micro
Engineered and Molecular Systems, pp. 955–958, January 2006.
[4] H. Yu, K. Zhang, and C. Rossi, “Experimental study of
the photocatalytic degradation of formaldehyde in indoor
air using a nano-particulate titanium dioxide photocatalyst,
Indoor and Built Environment, vol. 16, no. 6, pp. 529–537,
2007.
[5] R. Greenwood and K. Kendall, “Selection of suitable disper-
sants for aqueous suspensions of zirconia and titania powders
using acoustophoresis,Journal of the European Ceramic
Society, vol. 19, no. 4, pp. 479–488, 1999.
[6] J. Cesarano 3rd and I. A. Aksay, “Processing of highly
concentrated aqueous α-alumina suspensions stabilized with
polyelectrolytes,Journal of the American Ceramic Society, vol.
71, no. 12, pp. 1062–1067, 1988.
[7] K. Sato, J. G. Li, H. Kamiya, and T. Ishigaki, “Ultrasonic
dispersion of TiO2nanoparticles in aqueous suspension,
Journal of the American Ceramic Society,vol.91,no.8,pp.
2481–2487, 2008.
[8] K. Higashitani, K. Yoshida, N. Tanise, and H. Murata, “Dis-
persion of coagulated colloids by ultrasonication,Colloids and
Surfaces A, vol. 81, pp. 167–175, 1993.
[9] N. G. ¨
Ozcan-Tas¸kin, G. Padron, and A. Voelkel, “Eect of
particle type on the mechanisms of break up of nanoscale
particle clusters,Chemical Engineering Research and Design,
vol. 87, no. 4, pp. 468–473, 2009.
[10] H. Sun, C. Wang, S. Pang et al., “Photocatalytic TiO2
films prepared by chemical vapor deposition at atmosphere
pressure,Journal of Non-Crystalline Solids, vol. 354, no. 12-
13, pp. 1440–1443, 2008.
[11] W. Owpradit and B. Jongsomjit, “A comparative study on
synthesis of LLDPE/TiO2nanocomposites using dierent
TiO2by in situ polymerization with zirconocene/dMMAO
catalyst,Materials Chemistry and Physics, vol. 112, no. 3, pp.
954–961, 2008.
[12] Q. Ran, S. Wu, and J. Shen, “Eects of poly(acrylic acid)
on rheological and dispersion properties of aqueous TiO2
suspensions,Polymer—Plastics Technology and Engineering,
vol. 46, no. 11, pp. 1117–1120, 2007.
[13] S. Liufu, H. Xiao, and Y. Li, “Adsorption of poly(acrylic acid)
onto the surface of titanium dioxide and the colloidal stability
of aqueous suspension,Journal of Colloid and Interface
Science, vol. 281, no. 1, pp. 155–163, 2005.
10 Journal of Nanomaterials
[14] S. Fazio, J. Guzm´
an, M. T. Colomer, A. Salomoni, and R.
Moreno, “Colloidal stability of nanosized titania aqueous
suspensions,Journal of the European Ceramic Society, vol. 28,
no. 11, pp. 2171–2176, 2008.
[15] S. H. Othman, S. Abdul Rashid, T. I. Mohd Ghazi, and
N. Abdullah, “Eect of postdeposition heat treatment on
the crystallinity, size, and photocatalytic activity of TiO2
nanoparticles produced via chemical vapour deposition,
Journal of Nanomaterials, vol. 2010, Article ID 512785, 10
pages, 2010.
[16] C. S. Kuo, Y. H. Tseng, C. H. Huang, and Y. Y. Li, “Carbon-
containing nano-titania prepared by chemical vapor deposi-
tion and its visible-light-responsive photocatalytic activity,
Journal of Molecular Catalysis A, vol. 270, no. 1-2, pp. 93–100,
2007.
[17] T. Nash, “The colorimetric estimation of formaldehyde by
means of the Hantzsch reaction,The Biochemical Journal, vol.
55, no. 3, pp. 416–421, 1953.
[18] W. Li, S. Shah, C. P. Huang, O. Jung, and C. Ni, “Metallorganic
chemical vapor deposition and characterization of TiO2
nanoparticles,Materials Science and Engineering B, vol. 96,
no. 3, pp. 247–253, 2002.
[19] X. W. Zhang, M. H. Zhou, and L. C. Lei, “Co-deposition of
photocatalytic Fe doped TiO2coatings by MOCVD,Catalysis
Communications, vol. 7, no. 7, pp. 427–431, 2006.
[20] D. Santhiya, S. Subramanian, K. A. Natarajan, and S. G. Mal-
ghan, “Surface chemical studies on the competitive adsorption
of poly(acrylic acid) and poly(vinyl alcohol) onto alumina,
Journal of Colloid and Interface Science, vol. 216, no. 1, pp. 143–
153, 1999.
Submit your manuscripts at
http://www.hindawi.com
Scientifica
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Corrosion
International Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Polymer Science
International Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Ceramics
Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Composites
Journal of
Nanoparticles
Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Nanoscience
Journal of
Textiles
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Journal of
Nanotechnology
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Journal of
Crystallography
Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
The Scientic
World Journal
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Coatings
Journal of
Advances in
Materials Science and Engineering
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Smart Materials
Research
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Metallurgy
Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
BioMed
Research International
Materials
Journal of
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com Volume 2014
Journal of
Nanomaterials
... The average ZP was −13.6 mV at pH 6.5 ( Figure 3A). The negative values elucidate the repulsion among the nanoparticles and thus the accomplishment of superior constancy of the Au/cellulose nanocomposite, which prevents agglomeration in aqueous solutions [46]. Major positive or negative data of nanocrystals determined by zeta potential accurately designate the physical constancy of nanosuspensions because of the electrostatic repulsion of unique particles [47]. ...
... FTIR was utilized to study the chemical groups of TiNT and their transformation when adding graphene. The FTIR spectra of TiNT and Gr/TiNT hybrid system are presented in Figure 4. Several common peaks were identified, most importantly: at 840 cm À1 corresponding to the Ti-O-Ti vibration of the TiO 2 phase, [26] at 1030 cm À1 corresponding to C-O-C stretching vibration, [27] at 1630 cm À1 which is associated to bending vibration Ti-OH [28] and at 1720 cm À1 originating from the C ¼ O elongation vibration mode. In addition, FTIR spectra of TiNT shows a strong and broad peak at 3400 cm À1 , indicating the presence of surface O-H stretching vibrations of the C-OH groups and water. ...
Article
The development of multi-functional nanomaterials is nowadays more than a necessity to address both energy and environmental needs of our society. In this perspective, the hybridization of graphene (Gr) with TiO2 nanotubes (TiNT) is very promising to meet these issues. In the present work, bi-functional graphene-modified TiO2 nanotubes electrodes (Gr/TiNT) were successfully prepared. A simple and effective approach for graphene production, by electrochemical exfoliation of pure graphite foil in inorganic salt (Na2SO4) is used. Graphene sheets were formed on the surface of TiO2 nanotube arrays through electrodeposition process by chronoamperometry. Mesoporous graphene of high surface area is promising material for photocatalytic applications. Gr/TiNT electrode displays significantly improved photodegradation efficiency to MB, achieving a degradation percentage of 60% in 2 hours, which is about 50% higher than that of TiNT. Moreover, Gr/TiNT with high conductivity demonstrated good capability to deliver extremely high energy and power in a very short period of time, the relaxation time constant was only about 48 ms, which is far lower than that of most conventional activated carbon-based electrochemical capacitors. Capacitance retention of 98% was also demonstrated after 1000 cycles.
... Sene et al. [35] immobilized TiO 2 onto clinoptilolite support, and the presence of ultrasound guaranteed a uniform morphology and a more homogeneous dispersion. Othman et al. [36] have highlighted the effect of increased ultrasonic amplitude in improving the stability of suspensions and uniformity in the glass coating process. It can be inferred that the frequency with which the ultrasonic waves are emitted promotes vibrations that allow for better dispersion of TiO 2 in ethanol. ...
Article
Full-text available
Water scarcity is one of the major concerns of this century. The photocatalysis through TiO2 can be suitable for improving liquid wastewater treatment. However, TiO2 is used as a powder (nanoparticles), which is a drawback for full-scale applications. To overcome this, in the present work, powder TiO2 was impregnated on ceramic material. Several parameters, such as support cleanliness, support load, TiO2 suspension concentration, powder dispersion in a solvent, contact method, and drying temperature, were evaluated on the impregnation method. The influence of TiO2 concentration in suspensions was tested from 1 to 10% w/w. The results showed that the preparation conditions impact the TiO2 impregnation yield. The 10%TiO2/Leca was the most effective in photocatalysis but had a relevant loss of TiO2 from the support by erosion. For 3.6%TiO2/Leca and 5%TiO2/Leca, at TiO2 concentrations of 86.6 and 102.5 mg/L promoted 71 to 85% of sulfamethoxazole removal in 6 h under UVA radiation, respectively. Scanning electron microscopy (SEM) revealed the TiO2 adhesion onto the surface of the ceramic material, and the thickness of the TiO2 layer over the support can attain 7.64 to 10.9 μm. The work showed that the TiO2 impregnation method over Leca could be suitable for obtaining cost-effective photocatalysts.
... 15 In addition, the photocatalyst particles inevitably aggregate into large clusters because of the strong electrostatic attraction between the photocatalyst particles. 16,17 Though many efforts have been made to explore the impact of particle aggregation and aggregate structure on photocatalytic activity, 18 with feasible strategy developed for well-dispersed and stabilized photocatalyst in aqueous suspension, 19,20 how the hydrodynamic conditions, usually applied for the scaled up photocatalysis system, interact with the photocatalyst still needs to be clarified. It is anticipated that the underlying mechanisms of the aggregation of photocatalyst particles can be elucidated from the perspective of solid−liquid interaction in the suspension to guide the reliable design of photocatalytic systems with the optimization of the operating conditions for efficient photocatalytic reactions. ...
... The size of the zeta potential confers particle stability. The high potential zeta nanoparticles have enhanced stability, which implies nanoparticle aggrégation and agglomeration may be prevented by dispersion and solution [44]. In the present study, they revealed an average particle size of 53 nm, the ZE of -24 mV, and PDI (Polydispersity Index) of 0.275 ( Fig. 4a and b). ...
Article
Full-text available
In the present work, we report the biosynthesis of Nickel oxide (NiO) nanoparticles using Senna auriculata aqueous flower extract and their evaluated to the photocatalyst and antimicrobial activities. The synthesized nanoparticles were analysed using UV–Visible (UV–Vis), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Photoluminescence (PL), High resolution transmission electron microscopy (HR-TEM), Dynamic light scattering (DLS), Zeta potential (ZE) and Surface area analysis. The XRD shows the obtained peaks are indexed to the crystalline planes (111), (200), (220), (311) and (222) confirmed that the NiO nanoparticles possessed a good crystalline nature. UV–Vis confirmed that NiO nanoparticles had a direct bandgap at 3.25 eV. DLS analysis and ZE values validated the stability of NiO nanoparticles (53 nm and − 24 mV). HR-TEM analysis revealed that the synthesized NiO nanoparticles had a spherical structure with uniform distribution along the surface. The FT-IR analyses show that the surface of synthesized nanoparticles acts as a reducing and stabilising agent for proteins, carboxyl and hydroxyl groups. The antibacterial activities of biosynthesized NiO nanoparticles are greater against Gram-negative (Escherichia coli) than the Gram-positive microorganism. The catalyst activity of synthesized nanoparticles regarding methylene blue (MB) dye degradation under direct visible light irradiation was studied. The degradation effectiveness against MB dye was found to be 97% for 90 min. Overall, these studies show that Senna auriculata is an effective sell being planted and has the highest probability of being used in the design and development of nanoparticles for environmental pollution and human health.
... In contrast, the presence of carboxymethyl cellulose (CMC) or polyvinylpyrrolidone (PVP) significantly enhances the stability of uncoated nTiO 2 , as determined by the zeta potential values measured at pH 7, with substantial shape changes that result in spherical particles and relatively small nTiO 2 sizes [57]. Similar substantial shape transformations induced by stabilizers have been found in other studies [58,59]. Inorganic UVFs generally have a small particle size, strong hydrophobicity, and are insoluble in water; thus, Brownian motion, eddy motion, and runoff shear force result in some inorganic UVF particles remaining in suspension [60]. ...
Article
Full-text available
An increasing number of inorganic ultraviolet filters (UVFs), such as nanosized zinc oxide (nZnO) and titanium dioxide (nTiO2), are formulated in sunscreens because of their broad UV spectrum sunlight protection and because they limit skin damage. However, sunscreen-derived inorganic UVFs are considered to be emerging contaminants; in particular, nZnO and nTiO2 UVFs have been shown to undergo absorption and bioaccumulation, release metal ions, and generate reactive oxygen species, which cause negative effects on aquatic organisms. We comprehensively reviewed the current study status of the environmental sources, occurrences, behaviors, and impacts of sunscreen-derived inorganic UVFs in aquatic environments. We find that the associated primary nanoparticle characteristics and coating materials significantly affect the environmental behavior and fate of inorganic UVFs. The consequential ecotoxicological risks and underlying mechanisms are discussed at the individual and trophic transfer levels. Due to their persistence and bioaccumulation, more attention and efforts should be redirected to investigating the sources, fate, and trophic transfer of inorganic UVFs in ecosystems.
... Over the past few years, TiO 2 catalyzed photodegradation of PAHs has been reported in enormous studies due to its high photocatalytic efficiency, cost-effectiveness, relatively low toxicity, and good chemical stability in the aqueous medium (Schneider et al., 2014). However, it should be noted that spherical TiO 2 NPs tend to agglomerate, thereby showing poor photocatalytic efficiency in the water phase (Othman et al., 2012). Further, pristine TiO 2 NPs suffer from the low surface area, light absorption in the UV region, and rapid recombination of photoexcited e − − h + pairs. ...
Article
The mitigation of polycyclic aromatic hydrocarbons (PAHs) in the aqueous environment by physical and biological processes is a considerable challenge. Among various remediation approaches, photocatalysis is considered an efficient and eco-friendly option to convert PAHs into less harmful and/or biodegradable end products. This review provides a comprehensive overview of various nanophotocatalysts used to degrade PAHs in water based on their photocatalytic performances (e.g., in terms of quantum efficiency, space-time yield (SY), and figure-of-merit). Among these photocatalysts, semiconductor metal oxides (e.g., TiO2 and ZnO nanophotocatalyst) and graphene-doped noble metal nanoparticles are identified as the most effective options due to their high photoactivity, low-cost, and tailorable structural/functional characteristics. The present and futuristic challenges in the development of photodegradation of PAHs in aqueous medium are discussed to deliver a blueprint to construct high performance photocatalysts for their removal.
Article
This paper reports the successful synthesis of mesoporous carbon/titanium dioxide (MC/TiO2) composite electrodes via the hydrothermal method for supercapacitor (SC) applications. The morphology and structural properties of MC/TiO2 composites were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectra (FTIR). The electrochemical properties were recorded by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) with an electrolyte (6 M KOH) in symmetric/asymmetric configuration.The specific capacitance (Cs) evaluated by CV is about 280 F/g for composite electrode (95 % capacitance retention after 1000 cycles) and pristine has 150 F/g @ 10 mV/s. Enhancement in capacitance is owing to faster charge dynamics within electrode material. The fabricated asymmetric device demonstrates high energy density (30.31 Wh/kg), than the symmetric configuration (27 Wh/kg). Finally, both symmetric/asymmetric supercapacitors have illuminated a red LED, and strengthens the candidature of composite electrode for energy storage technology.
Article
Full-text available
This work describes a potential selective adsorption of epigallocatechin gallate by Ga-doped TiO2 nanoparticles (NPs) and showed excellent adsorption (∼99%) and desorption capacity even after the 7th run. The work reports about a surfactant mediated template based synthesis of both pristine and Ga-doped TiO2 mesoporous NPs to study the adsorption behavior EGCG. EGCG is the most abundant and bio-active compound capable of showing free radical scavenging activity, anticancer, antibacterial, and several other physiological functions. Therefore, extraction of EGCG is highly desirable for incorporation in food items that can effectively increase their nutritional and medicinal values. The pore diameter of the synthesized materials was found to be in the range 8.76–10.38 nm and the specific surface area was found to be around 25.80–58.72 m²/g. The particle size decreased from 25 nm to 15 nm in presence of Ga. A mixture of anatase and brookite phase was observed for all the synthesized TiO2 NPs. The band gap value initially (3.40 eV) decreased in presence of minute amounts of Ga (3.44 eV) but then increased (3.59 eV) with the increase of Ga percentage. The point of zero charge (PZC) value of the materials lies in the range of 6.6–7.2. The adsorption study was carried out at different pH (1, 3, 5, 7 and 9) with variation of shaking time (1h, 3h, 5h and 7h). The material containing maximum Ga in the Ga-doped set (Set 3 TiO2 NP) showed highest adsorption percentage (99.45%) in pH 1 medium after 5 h shaking. The adsorption isotherm and the kinetics follow the Langmuir model and pseudo-second order respectively. Desorption was studied under high energy gamma rays, shaking, sonication and UV irradiation. The highest desorption (∼96%) was obtained with sonication for 30 min.
Chapter
Colloidal science is often classified as an intriguing complex fluid system due to its unique composition of two or more elements, comprising of at least two heterogeneous phases. It has a significant role in coating formulations of many applications ranging from macroscopic to nanoscopic levels. Colloidal stability is the long-reign origin of effective and superior coatings, especially coatings of specific functionalities. Unlike decorative coatings, functional coating materials require excellent performance in harsh environments. Therefore, the suitability of the base coating materials, stabilizers and other ingredients, and their polymerization routes are vital to preserving the quality of the dispersion/emulsion/suspension produced, and eventually the superiority of the functional coatings during service and applications. In this article, the background of polymer colloids at a molecular level will be discussed to relate the concept with the polymerization methods available to manufacture coatings with specialized functionalities. Some examples of trendy functional coatings will be also discussed that include conductive and self-healing coatings, and nanoscopic polymer coating techniques such as self-assembly and polymer brushes. Interestingly, regardless of the coating method or specializations, microscopic and nanoscopic functional coatings are very valuable in many industries including medical, construction, electronic, automotive, architecture, food, cosmetics and pharmaceutical.
Article
Full-text available
Aggregation and dispersion behavior of nanometer and submicrometer scale TiO2 particles in aqueous suspension were investigated using three kinds of mechanical dispersion methods: ultrasonic irradiation, milling with 5-mm-diameter balls, and milling with 50 μm beads. Polyacrylic acids with molecular weights ranging from 1200 to 30 000 g/mol were used as a dispersant, and the molecular weight for each dispersion condition was optimized. Viscosities and aggregate sizes of the submicrometer powder suspensions were not appreciably changed in the ultrasonic irradiation and 5-mm-ball milling trials. In contrast, in the trials in which nanoparticle suspension was used, ultrasonic irradiation produced better results than 5-mm-ball milling. Use of ultrasonication enabled dispersion of aggregates to primary particle sizes, which was determined based on the specific surface area of the starting TiO2 powders, even for relatively high solid content suspensions of up to 15 vol%. Fifty-micrometer-bead milling was also able to disperse aggregates to the same sizes as the ultrasonic irradiation method, but 50-μm-bead milling can be used only in relatively low solid content suspensions. It was concluded that the ultrasonic dispersion method was a useful way to prepare concentrated and highly dispersed nanoparticle suspensions.
Article
In this work the colloidal behaviour of three different TiO2 nanopowders in water is studied. A commercial powder of anatase and another of rutile were used for this study. For comparison purposes, a cryogel of anatase synthesised by a particulate sol–gel route and freeze-dried was also studied. All three powders were characterised by scanning electron microscopy, specific surface area measurements and X-ray diffraction. Diluted aqueous suspensions were prepared and characterised in terms of particle size distribution and zeta potential, using dynamic light scattering and laser Doppler velocimetry principles, respectively. All suspensions were prepared using an ultrasounds probe for mixing times ranging from 0 to 5min. Colloidal stability was studied as a function of pH, type and concentration of dispersants (polyacrylic-based deflocculant and citric acid) and mixing time. Stable suspensions of commercial nanosized powders were obtained with polyelectrolyte contents of 1.0–1.5wt.%. No stable suspensions of the cryogel were obtained with polyelectrolyte, requiring in this case the use of citric acid as deflocculant. It was observed that neither the size distribution nor the zeta potential values were affected by the sonication time.
Article
Acoustophoresis was used to study the effect of adding various commercially available dispersants onto aqueous suspensions of two zirconia and two titania powders. These powders were characterised for elemental composition by X-ray fluorescence (XRF) spectroscopy and for surface area by BET single point nitrogen adsorption. From the maximum value of the zeta potential, it was possible to select the most promising dispersants. From the shape of the curve the minimum amount of dispersant required to stabilise the powder particles was noted. The iso electric point (i.e.p) of the powders was also identified. Several dispersants can be recommended for the first titania powder, whilst none can be recommended for the second titania as the final zeta potentials on addition of the dispersants were low. The two powders had different surface chemistries which was reflected in a large difference in their i.e.p; the first at pH 7·5 and the second at pH 6·1. This was due to different coatings on the powder surfaces; alumina and an organic material respectively. Removal of this organic coating by calcinatian then enabled the dispersants to fully adsorb. Similarly dispersants for the first zirconia powder could be identified and the i.e.p identified at pH 5·4. However, no dispersants can be recommended for the second zirconia powder as yttria dissolves out of the powder under the naturally occurring weakly acidic conditions. The i.e.p was estimated to be pH i.e.p 7–7·5.
Article
The present study revealed the effect of different TiO2 nanoparticles employed on catalytic and characteristic properties of LLDPE/TiO2 nanocomposites synthesized by the in situ polymerization with zirconocene/dMMAO catalyst. It was found that the presence of rutile phase in titania apparently resulted in decreased activities due to low intrinsic activity of active sites being present. Based on 13C NMR results, all LLDPE/TiO2 samples exhibited the random copolymer having different degree of 1-hexene insertion. The highly dispersion of titania can enhance the degree of 1-hexene insertion resulting in decreased crystallinity.
Article
Formaldehyde in the indoor environment may be degraded using nano-particulate titanium dioxide (TiO2) photocatalysis to improve air quality. In the work described, a polytetrafluoroethylene filter is employed as the substrate for a nano-particulate TiO2 coating. This is mounted in an experimental setup developed for the tests, similar to an actual air purification system, which are conducted at room temperature. The effects on the formaldehyde photocatalytic degradation rate of some key factors are investigated, including initial concentration, stream flow rate, reaction temperature, light source intensity, and relative humidity. Within the experimental ranges studied, the degradation rate increases with the enhancement of initial concentration and light intensity. The stream flow rate and reaction temperature have dual effects on the degradation rate. It is shown that the degradation rate is relatively high under low relative humidity.
Article
Nanoparticles tend to form large clusters (aggregates and agglomerates) which need to be broken up when dispersing in a liquid. The dispersion of nanoparticle clusters has been studied to investigate the kinetics and mechanisms of break up with two types of particles: fumed silica and aluminium oxide. Results obtained under different processing conditions using an in-line rotor-stator are reported. It could be concluded that break up occurs predominantly through erosion in the case of silica and shattering with aluminium oxide.
Article
Ultraviolet and visible-light-responsive titania was synthesized and employed in the NOx photomineralization. A thermal decomposition reaction of titanium isopropoxide was carried out with a metal-organic chemical vapor deposition (MOCVD), enabling continuous production of TiO2 nanoparticles. Carbon-containing titanium dioxide with the anatase phase prepared at 500°C under nitrogen atmosphere exhibited high photocatalytic activity for NO oxidation under visible-light illumination. Experimental results indicate that up to 48% removal of NOx can be achieved in a continuous flow type of reaction system under visible-light illumination (green LED). The chamber temperature in this MOCVD process plays an important role in lattice structure formation, and also affected TiO2 carbon content. The carbonaceous species on the TiO2 surface, shown by X-ray diffractometry (XRD), and Raman, UV–vis, and X-ray photoelectron spectroscopies (XPS), is important to the visible-light absorption and visible-light-catalytic mineralization of NOx.
Article
The characteristics of ultrasonic deflocculation are investigated quantitatively by measuring, with a Coulter counter, the change in the size distribution of flocs of polystyrene latex spheres exposed to an ultrasonic field. It is found that the degree of deflocculation is determined by the total sonic energy per unit volume radiated to the floc solution, irrespective of the volume and shape of the container, the ultrasonic radiation period and the intensity of the sound, provided that the sizes of the constituent particles of the flocs are the same. The mechanism of sonic deflocculation differs from the mechanism of hydrodynamic deflocculation.
Article
Titanium dioxide is among the few semiconductors that have good chemical/photochemical stabilities and high oxidation power. However, its relatively high band gap makes it only effective when exposed under UV light. It has been found that the addition of transition metals to TiO 2 can improve the photocatalytic activity by UV irradiation and extend its use in the visible region of the electromagnetic spectrum. In this work, the reactive magnetron sputtering method was used to prepare pure and Fe-doped titanium dioxide thin films. The films were deposited onto microscope glass slides and polycarbonate plates at different total pressures and iron-doping concentrations. The morphology of the films was analysed by atomic force microscopy (AFM) and their structure by X-ray diffraction (XRD). The effect of Fe-doping and total sputtering pressure on the photocatalytic activity, was evaluated by measuring the degradation rates of the rhodamine-B (RhB) dye under UV irradiation. The experimental results show that the deposited TiO 2 films on the glass substrate were of the anatase phase with a 0 0 4 preferred orientation. On contrary, for TiO 2 films deposited on the polycarbonate substrate only an amorphous structure was obtained. The crystallite sizes of the films were 4.1 and 7.1 nm for TiO 2 films deposited at total sputtering pressures of 0.4 and 0.5 Pa, respectively. The iron doped films resulted in the light absorbance shifting to the visible range of the electromagnetic spectrum. In general, iron doping led to a decrease in the photocatalytic activity of TiO 2 films deposited on both substrates. However, the highest photodegradation rates were obtained for films deposited on the polymer substrate under the lower total pressure of 0.4 Pa.
Article
The poly(acrylic acid) was used as dispersant to prepare aqueous TiO2/poly(acrylic acid) suspensions. The poly(acrylic acid) was adsorbed on the surface of the TiO2 particles. The zeta potential of the TiO2 particles in TiO2/poly(acrylic acid) suspensions was higher than that of the TiO2 particles in TiO2 suspensions, and the zeta potential of the TiO2 particles increased with increasing poly(acrylic acid) content. At the same shear rate, the viscosity of TiO2/poly(acrylic acid) suspensions was lower than that of TiO2 suspensions, and the liquidity was improved. The dispersion of TiO2 particles in TiO2/poly(acrylic acid) suspensions was improved compared with that of TiO2 particles in TiO2 suspensions.