Curcumin-sensitized anatase TiO2 nanoparticles for
photodegradation of methyl orange with solar
radiation
Ahed Zyoud* and Hikmat Hilal
Semiconductor & Solar Energy Research Laboratory (SSERL), Department of Chemistry, An-Najah N. University, PO Box 7,
Nablus, West Bank, Palestine. E-mail: ahedzyoud@najah.edu
Abstract. Curcumin, a non toxic yellow food additive, has been used
here to sensitizeTiO2 in photodegradation of methyl orange
contaminant in water with solar light. Using natural dyes is a
promising replacement for the hazardous heavy metal-based systems,
such as CdS and Ru-compounds, in sensitizing wide band gap
semiconductors. Naked (TiO2/Curcumin) and activated-carbon
supported (AC/TiO2/Curcumin) catalyst systems were prepared and
investigated here. The effects of different reaction parameters on
reaction rate, such as amount of catalyst, contaminant concentration
and pH, were all studied in terms of turn number (T.N.) and quantum
yield (Q.Y.) values. The results show that curcumin can sensitize
TiO2 particles to solar light in methyl orange photo-degradation
processes.
complicated purification and nontoxic food additive. The
structural formula of curcumin molecule is shown in Scheme
(I).
Curcumin is a yellow color pigment present in curcumin
plant roots. Only one layer of adsorbed dye molecules would
be anchored to the semiconductor particle surface. The light
conversion efficiency of dye mono-layered molecules should
be small. However, this may not be a difficulty, as the surface
areas in powder semiconducting materials are relatively high.
Thus, sensitization is more pronounced in nanoparticle
semiconductors than in bulk systems.
Keywords: Curcumin; Sensitization; Photo-degradation, TiO2
1.
INTRODUCTION
TiO2 is wide band gap (~3.2 eV) semiconductor that is
widely used as catalyst in photo-degradation of organic
contaminants in water. Its wide band gap limits TiO2
application to the costly UV irradiation. The challenge is to
sensitize wide band gap semiconductors, including TiO2, to
the freely abundant visible solar light. Dye-sensitized solar
cells (DSSC) have been widely investigated for many years,
and showed soundly high efficiency (~10%). The idea of
sensitizing semiconductors could then be applied to solardriven water purification with visible solar light [1]. Most of
synthetic dyes, such as CdS particles [2] or ruthenium dyes
[3], are hazardous and costly. This makes synthetic dyes
unfavorable in water purification. The need for alternative safe
photosensitizers in TiO2-based photodegradation of water
contaminants is thus clear. Therefore, finding safe, low cost
and readily available sensitizers still remains a scientific
challenge [4]. Natural dyes, obtained from plant sources, have
been investigated as sensitizers in solar cells. Examples are
chlorophyll derivatives, natural porphyrins [5] and
anthocyanins [5-9], and to a lesser extent in water purification.
Anthocyanin was described here as sensitizer for TiO2 in
photo-degradation of methyl orange with visible light [7].
Another alternative natural dye, which is worth to investigate,
is curcumin. Curcumin was chosen here because it is
available, easy to extract, applicable without further
Scheme I: Structure for curcumin molecule
In this work the curcumin natural dye was extracted from
crushed turmeric (Curcuma longa) root. The curcumin root is
available at local markets and is commonly used as food
additive in many Asian and Middle East countries.
The sensitization process in natural dyes occurs by absorption
of suitable photons, which causes excitation to the dye
molecules. In molecular terminology, an electron jumps from
Highest Occupied Molecular Orbital (HOMO) to Lowest
Unoccupied Molecular Orbital (LUMO). Then, the excited
electron immigrates to the TiO2 conduction band. The dye is
then left as a positively charged molecule (cation). The dye
cation conveys its positive charge to a redox species or an
organic contaminant molecule. The electrons in the TiO2
conduction band would then reduce molecules dissolved
inside water. For the dye sensitization to be effective, the
conduction band edge of the TiO2 must be lower (more
positive) than the LUMO level of dye molecule, so that an
electron can be injected during relaxation process. The
reduction potential of the organic contaminant must be higher
(more negative) than the HOMO of the dye molecule. In terms
of photo-excitation, the HOMO-LUMO gap resembles band
gap energetics in semiconductors [10]. Scheme (II)
summarizes these ideas.
Moreover, curcumin molecule has carbonyl and hydroxyl
groups, which may bind to the surface of TiO2 particles,
making way for electron transfer from the excited curcumin
molecule to the conduction band of TiO2 [11]. It is thus
anticipated that supporting curcumin, and other suitable
natural dyes, like anthocyanin [7] or curcumin here, onto TiO2
particles would sensitize them to visible light in water photodegradation of water contaminants.
Scheme II: Formalism showing sensitization mechanism
To further increase their catalytic activity, nanoparticles of
sensitized and non sensitized catalysts were supported on
different substrates, the most common of which are activated
carbon, clay, glass pellets, sand particles and others [1, 7]. In
this work activated carbon was used as a solid support for the
prepared catalyst.
2. EXPERIMENTAL:
2.1. Materials and Chemicals:
Commercial anatase TiO2 nano-powder (Catal. no. 23,203-3,
density 3900 kg/m3, particle size range <40 nm, BET surface
area >20 m2/g, purity of 99.9%) was purchased from Aldrich.
Methyl orange (a model organic contaminant) and ethyl
acetate were purchased from Aldrich. Turmeric (Curcuma
longa) root powder was purchased from local markets.
Curcumin extraction:
Turmeric root powder (10.00 g) was soaked with 200.0 mL
ethyl acetate in a conical flask. The mixture was then heated
for 30 min at 75oC with continued magnetic stirring. After
cooling, the mixture was filtered. The electronic absorption
spectra for the resulting yellowish dye was measured on a
Shimadzu UV-1601 spectrophotometer. The absorption
spectrum matched earlier reports [12-13]. The solution was
then stored in refrigerator for further use.
Preparation of TiO2/Curcumin catalyst:
Anatase TiO2 powder (20.00 g) was refluxed for 30 min with
50.0 mL of ethyl acetate extract of curcumin (2.2X10 -3 M
curcumin). The mixture was then cooled in ice water for 20
min. Suction filtration (with sintered glass) was used to filter
the prepared catalyst system. The collected solid catalyst was
washed with cold water, dried under air in the dark and stored
for further use in dark. The adsorption of curcumin on the
TiO2 surface by the carbonyl adjacent groups was described
earlier [7, 14-15].
Preparation of AC/TiO2/Curcumin catalyst:
A mixture of 32.0 g TiO2 and 8.0 g of activated carbon AC
(845 m2/g) in 80.0 mL water were magnetically stirred for an
hour. The mixture was then filtered by suction filtration. The
solid AC/TiO2 (with ratio of 1:5 by mass) was dried for 3
hours at 120oC. Curcumin solution (100 mL, 2.2X10-3 M) was
magnetically stirred with 10.0 g of the composite AC/TiO2
solid for 60 min. The mixture was then suction-filtered
through sintered glass. The filtrate was left to dry under air in
dark and kept for further use.
2.2. Catalyst characterization:
Different techniques were used to characterize the prepared
catalyst systems. The electronic absorption spectrum for
extracted curcumin solution is shown in Figure (1a). The
spectrum shows a typical absorption band at λ max 530 nm,
characteristic for curcumin as measured for in situ species.
The solid absorption spectrum for TiO2/Curcumin, Figure
(1b), shows an absorption band at λ max 550 nm for curcumin
with a red shift (~20 nm) compared to curcumin solution band.
The shift is attributed to chemisorption onto TiO2 surface. An
absorption edge for TiO2 at λmax ~380 nm is also observed. TG
analysis was conducted for AC/TiO2 on a TA 2950HR V5-3
TGA apparatus, at ICMCB, University of Bordeaux. The
analysis shows 20% weight loss at temperature > 500 oC. The
loss is attributed to AC combustion. The result is compatible
with the nominal AC/TiO2 ratio, Figure (2).
(a)
(b)
Figure 1: Electronic absorption spectra for a) Curcumin in
water, b) urcumin supported onto TiO2
Figure 2: TGA thermograph for AC/TiO2/Curcumin
2.3 Photocatalytic Experiments
Photocatalytic experiments were conducted in a 100 mL
thermostated glass beaker. A jacket was used to control the
temperature around the reaction vessel, and an aluminum foil
was used to cover the outside walls of the beaker in order to
reflect back astray radiation. A 50.0 mL aqueous solution of
methyl orange contaminant, with known nominal
concentration and known catalyst amount, were placed in the
reactor. The mixture was magnetically stirred for a period of
time in dark, while drops of HCl and NaOH dilute solutions
were used to control the solution pH. A solar simulator
halogen spot lamp was used to irradiate the reaction mixture
[16]. The spot lamp was placed directly above the catalytic
solution, which was left open to atmospheric air, and the
illumination intensity was measured to be 0.0212 W/cm2.
The photocatalytic study was started immediately after
exposing the sample to irradiation. Aliquots (~3 ml each)
were syringed out of the reaction mixture, at different times,
and centrifuged at 4500 rounds/min for 6 min in the dark. The
supernatant was then spectrophotometrically analyzed at λ480
nm and the remaining contaminant concentration inside the
reaction mixture was measured using a calibration curve.
Control experiments were conducted. Control experiments
were conducted with catalyst in dark for 90 min, to check the
amount of contaminant lost by adsorption on catalyst system
surfaces. TiO2 and TiO2/curcumin systems showed no
significant contaminant adsorption with time, while significant
adsorption occurred at the AC/TiO2/curcumin system surface.
Exposing the contaminant solution to light for 90 min in the
absence of catalyst indicated no significant contaminant loss.
Control experiments with a cut-off filter (blocking 400 nm and
shorter wavelengths) placed between the reaction surface and
the light source, confirmed the ability of curcumin to sensitize
TiO2 in visible light with waves longer than 400 nm, as will be
discussed later on.
Effects of containment concentration, catalyst type & amount,
and pH on the photodegradation rate were all studied. For
efficiency comparison, values of reacted contaminant
molecules per incident photon (quantum yield, QY) and values
of reacted contaminant moles per nominal TiO2 mole
(turnover number, TN) were calculated after 60 min in each
experiment.
Complete mineralization of reacted molecules was confirmed
by the disappearance of the absorption band at λ480 nm (which
is attributed to MO azo group) during the photodegradation
experiments. The absorption band between 200-400 nm
(typical for aromatic ring derivatives) also decreased,
indicating that the aromatic ring groups of the MO were
totally degraded during the photodegradation process. No
traces of new organic species (such as carboxylic acids,
aldehydes, alcohols or ketones) were detected inside the
reaction mixture.
3. RESULTS AND DISCUSSION
Quantum yield and turnover number values were calculated
to evaluate the catalyst efficiency and were used for
comparison with earlier catalyst systems. Efficiencies of
catalyst systems were studied under solar simulator light.
Control experiments showed no significant contaminant
adsorption on TiO2/Curcumin catalyst in the dark, whereas
relatively high adsorption occurred onto AC/TiO2/curcumin
system. In the absence of catalyst, no observable methyl
orange degradation occurred, even under irradiation. Results
of
both
catalysts
systems
TiO2/Curcumin
and
AC/TiO2/Curcumin will are discussed below.
3.1 TiO2/Curcumin catalyst system:
The TiO2/curcumin catalyst (with cut off filter) showed higher
photocatalytic efficiency than the naked TiO2 in methyl
orange photodegradation, Figure (3). Values of T.N. and Q.Y.
confirmed this observation. Small values for T.N. and Q.Y.
(61x10-6 and 22x10-6 respectively) were observed when using
TiO2 catalyst under solar simulator radiation, due to small UV
fraction in the irradiation light reaching earth. Higher values
for T.N. and Q.Y. (73x10-6 and 26x10-6 respectively) were
observed when using TiO2/curcumin under direct simulator
light even with cut off filter (visible light only). This is due to
sensitizing effect of curcumin on TiO2, in similar manner to
other earlier natural dye systems [7, 16-18]. The T.N. and
Q.Y. values for TiO2/curcumin under direct sun light (without
cut off filter) were higher (122x10-6 and 44x10-6 respectively).
Photodegradation in this case is attributed to two concurrent
routes: the first one is by excitation of TiO2 with the small UV
fraction in solar light, and the second one is by curcumin
sensitization of TiO2 to visible region [17-18]. Effects of
different parameters on reaction rate have been investigated
for the TiO2/curcumin catalyzed photodegradation of methyl
orange.
Figure 3: Profiles showing decay of contaminant methyl
orange under solar light using a) Naked TiO2, b)
TiO2/Curcumin, and c) TiO2/Curcumin with cutoff filter only.
Effect of pH on photodegradation rate:
Effect of pH on reaction rate was studied. Three solutions of
methyl orange (50.0 mL, 5 ppm contaminant with 0.10 g
catalyst) were used with different pH values (4.5, 11, and 7.0).
The prepared reaction mixtures were exposed to direct spot
lamp radiation for 90 min. Figure (4) shows the effect of pH
on reaction rate. The catalytic efficiency was higher in acidic
solution (T.N. = 93x10-6 and Q.Y.= 256x10-6) than in basic or
neutral solutions. The T.N. and Q.Y. values in the basic
solution (62x10-6 and 171x10-6 respectively) were higher than
in the neutral one (49x10-6 and 134x10-6). The results are
consistent with other earlier systems [16, 19-22]. Under
neutral conditions, methyl orange is more stable to
photodegradation, than under acidic or basic conditions [2324]
Figure 4: Effect of pH on photo-degradation of methyl orange
(50 ml, 5 ppm solution) using TiO2/Curcumin (0.1 g) under
(a) Neutral, (b) pH = 4.5, and (c) pH = 11.
Effect of catalyst amount:
The effect of the amount of loaded catalyst on the
degradation rate was investigated. Three different amounts of
TiO2/Curcumin catalyst (0.05, 0.10, and 0.20 g) were added to
three separate containers having 50.0 ml of 5 ppm Methyl
Orange solution each. The results showed a slight increase in
photodegradation rate with increasing catalyst amount, as
shown in Figure (5). The Q.Y. values were (171x10 -6, 134x106
, 79x10-6) respectively for (0.05, 0.10, 0.20 g). The T.N.
values for the 0.05, 0.10 and 0.20 g experiments were 31x10-6,
49x10-6, 57x10-6 respectively. The Q.Y. values decreased with
increasing the loaded catalyst amount. Earlier studies indicate
that light screening occurs by increasing the loaded catalyst
amount [16, 19, 25]. The initial reaction rate order was found
to be (0.5) with respect to loaded catalyst.
Effect of contaminant concentration:
Effect of contaminant methyl orange concentration on rate of
photodegradation reaction
was
investigated.
Three
experiments with different methyl orange concentrations (5,
7.5, and 10 ppm) were conducted for this purpose, using 0.10
g catalyst in 50.0 mL solution. The measured spot light
simulator power was 0.0212 W/cm2. The calculated reaction
order with respect to nominal contaminant concentration was
0.54. The measured T.N. values for different methyl orange
concentrations 5, 7.5 and 10 ppm were 135x10 -6, 183x10-6 and
220x10-6 respectively, and the Q.Y. values were 49x10-6,
66x10-6 and 80x10-6 respectively. Within the working range of
methyl orange concentrations, the T.N. and Q.Y. values
increased with increasing the contaminant concentration. The
results are consistent with earlier reports [7, 16, 19, 25],
Figure (6).
3.2 AC/TiO2/Curcumin catalyst system:
The photodegradation efficiency of AC-supported
TiO2/curcumin was investigated for methyl orange
contaminant. Effects of other reaction parameters, such as pH
and contaminant concentration, on reaction rates were also
studied using AC/TiO2/curcumin catalyst. Values of T.N. and
Q.Y. were calculated as well.
Figure 5: Effect of TiO2/curcumin amount on photodegradation of methyl orange (50 ml, of 5.0 ppm solution).
Nominal catalyst amounts (a) 0.05 g (b) 0.1 g, and (c) 0.2 g.
Calculated n= 0.5.
Figure 6: Effect of methyl orange initial concentration on its
photo-degradation reaction rate. (a) 5 ppm (b) 7.5 ppm (c) 10
ppm, n = 0.54. Reactions conducted using 0.1 g
TiO2/Curcumin.
Effect of contaminant concentration:
Three different contaminant solutions (50 mL of 20, 25 and 30
ppm, each with 0.12 g AC/TiO2/curcumin catalyst) were
stirred in the dark for 30 min before exposure to spot light.
This was to check the amount of adsorbed contaminant onto
the AC. The remaining contaminant equilibrium concentration
were measured and found to be ~5, 8, and 12 ppm
respectively. The adsorbed contaminant concentration was 15
ppm. The three solutions were then exposed to the light. The
measured T.N. values respectively were 232x10-6, 354x10-6,
and 391x10-6; and Q.Y. values respectively were 64x10-6,
128x10-6, and 142x10-6, Figure (7). The T.N. and Q.Y. values
increased with increasing the contaminant concentration. The
calculated reaction order with respect to contaminant nominal
concentration was 0.3.
Effect of pH on AC/TiO2/curcumin photocatalytic efficiency:
The effect of pH on photodegradation rate of methyl orange
(50.0 mL of 25 ppm) by Ac/TiO2/Curcumin (0.12 g) under
visible irradiation was investigated. Three 50.0 mL solutions
of methyl orange with different pH values (7, 4.5, and 11)
were used to study the effect of pH on reaction rate. The
prepared solutions were stirred in the dark for 30 min before
exposure to light, and the remaining contaminant
concentrations after adsorption (onto AC) were measured. The
remaining contaminant concentrations were 8, 10, and 18 ppm
respectively. Minimum adsorption occurred in the basic
solution. The solutions were then exposed to direct spot lamp
radiation for 90 min. The photodegradation efficiency was
higher in acidic solution (T.N. = 465x10 -6 and Q.Y.= 168x106
) than in basic and neutral solutions, and the
photodegradation in the basic solution (T.N. =366x10 -6 and
Q.Y.= 133x10-6) was close to neutral medium (T.N. =
354x10-6 and Q.Y.= 128x10-6, Figure (8). The results are
consistent with earlier studies [16, 19-22].
catalyst. This is evident inform values of T.N. and Q.Y. The
reason is due to ability of AC surface to adsorb contaminant
molecules and bringing them into close proximity to catalyst
sites therein. Moreover the activated carbon has the other
advantage of making catalyst recovery easier by simple
filtration.
CONCLUSION
Curcumin, a well-known low-cost non-hazardous dye,
effectively sensitized the TiO2 particles in solar-driven methyl
orange degradation in water. Curcumin is thus a promising
replacement for hazardous sensitizing dyes such as CdS and
Ru-compounds. The photodegradation efficiency was further
enhanced by supporting the TiO2/Curcumin catalyst on
activated carbon (AC). The AC speeds up the catalytic process
by adsorption, and moreover, makes the recovery of the
catalyst from the reaction mixture easier.
ACKNOWLEDGEMENT
The core activities have been conducted at SSERL, ANU.
Assistance from the technical staff of Chemistry Department,
An-Najah N. University, is acknowledged. XRD and TGA
services from ICMCB, Bordeaux University, France, are also
acknowledged. The authors wish to thank Al-Maqdisi Project
for partial financial support to this work.
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Figure 7: Effect of contaminant initial concentration on its
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