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2013 1st International Conference & Exhibition on the Applications of Information Technology to Renewable Energy Processes and Systems

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Curcumin-Sensitized Anatase TiO2 Nanoparticles for

Photodegradation of Methyl Orange with Solar

Radiation

Ahed Zyoud

Department of Chemistry

Semiconductor & Solar Energy Research Laboratory

An-Najah N. University

Nablus, West Bank, Palestine

Hikmat Hilal

Department of Chemistry

Semiconductor & Solar Energy Research Laboratory

An-Najah N. University

Nablus, West Bank, Palestine

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.

Keywords—Curcumin; Sensitization; Photo-degradation, TiO2

I. 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 solar-driven 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. Thus, 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 complicated purification and nontoxic food additive. The structural formula of curcumin molecule is shown in Fig. 1.

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.

Fig. 1: 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]. Fig. 2 summarizes these ideas.

978-1-4799-0712-0/13/$31.00 ©2013 IEEE


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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 photo-degradation of water contaminants.

Fig. 2. 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.

II. EXPERIMENTAL

A. 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.

B. 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 75

oC 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.

C. Preparation of catalysts 1) 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].

2) Preparation of AC/TiO2/Curcumin catalyst: A mixture of 32.0 g TiO2 and 8.0 g of activated carbon AC (845 m

2/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 120

oC. 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.

D. Catalyst characterization

Different techniques were used to characterize the prepared catalyst systems. The electronic absorption spectrum for extracted curcumin solution is shown in Fig. 3a. The spectrum shows a typical absorption band at λmax 530 nm, characteristic for curcumin as measured for in situ species. The solid absorption spectrum for TiO2/Curcumin, Fig. 3b, shows an absorption band at λmax 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, Fig. 4.

(a) (b)

Fig. 3. Electronic absorption spectra for a) Curcumin in water, b) Curcumin

supported onto TiO2.

Fig. 4. TGA thermograph for AC/TiO2/Curcumin.


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E. 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/cm

2.

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.

III. 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.

A. TiO2/Curcumin catalyst system

The TiO2/curcumin catalyst (with cut off filter) showed higher photocatalytic efficiency than the naked TiO2 in methyl orange photodegradation, Fig. 5. 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.

1) 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. Fig. 6 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 (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 [23-24].

Fig. 5. 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.


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Fig. 6. 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.

2) 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 Fig. 7. The Q.Y. values were (171x10

-6, 134x10

-6, 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.

3) 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/cm

2. 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], Fig. 8.

Fig. 7. Effect of TiO2/curcumin amount on photo-degradation 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.

Fig. 8. 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.

B. 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.

1) 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, Fig. 9. 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.

2) 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.=168x10

-6)

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, Fig. 10. The results are consistent with earlier

studies [16, 19-22].

AC/TiO2/Curcumin catalyst showed higher catalytic activity in photodegradation of methyl orange than TiO2/Curcumin


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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. Also the activated carbon has the other advantage of making catalyst recovery easier by simple filtration.

Fig. 9. Effect of contaminant initial concentration on its photo-degradation reaction rate using 0.12 g AC/TiO2/Curcumin (Containing 0.1 g TiO2) using different methyl orange concentrations (a) 20 (b) 25, and (c) 30 ppm (n= 0.3).

Fig. 10. Effect of medium acidity on photo-degradation rate of methyl orange (50 ml, 25 ppm) using 0.12 g AC/TiO2/curcumin. The pH values were: (a) 7, (b) 4.5, and (c) 11.

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.

ACKNOWLEDGMENT

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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