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Research ArticleEquilibrium Isotherm Studies of Adsorption of PigmentsExtracted from Kuduk-kuduk (Melastoma malabathricum L.)Pulp onto TiO2 Nanoparticles

N. T. R. N. Kumara,1 Nurulhayah Hamdan,1 Mohammad Iskandar Petra,2

Kushan U. Tennakoon,3 and Piyasiri Ekanayake1

1 Applied Physics Program, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam2 Faculty of Integrated Technology, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam3 Biology Program, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam

Correspondence should be addressed to Piyasiri Ekanayake; [email protected]

Received 14 May 2013; Revised 25 September 2013; Accepted 24 October 2013; Published 12 January 2014

Academic Editor: Mohammad El-Shahawi

Copyright © 2014 N. T. R. N. Kumara et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The adsorption of natural pigments onto TiO2nanoparticles was investigated. The pigments were extracted from the dark purple

colored pulp of the berry-like capsule of Kuduk-kuduk (Melastoma malabathricum L.). The Langmuir, Freundlich, Redlich-Peterson, and Sips isotherm models were used to analyze the equilibrium experimental data. Data correlated well with the Sipsisotherm model, where the heterogeneity factor (𝑛 = 0.24) indicated heterogeneous adsorption characteristics, with a maximumadsorption capacity of 0.0130mg/g. The heterogeneous adsorption character was further supported by results obtained from zeta-potential measurements. When a dye-sensitized solar cell (DSSC) was sensitized with the extracted pigment, the photo-energyconversion efficiency was measured to be 0.83%, thus proving the suitability of Kuduk-kuduk fruit pulp as a sensitizer in DSSCs.

1. Introduction

Commercially, the quality of a product depends on theproduct value and acceptability. However, consumers arealso attracted to the physical appearances of the product,especially to its colors. In food, beverages, and textile indus-tries, the pigments are employed as coloring agents and fordyeing purposes. Natural colorants extracted from plantshave been used for centuries before the first synthetic colorantwas introduced. Low cost and ease of application havedriven the drift from natural colorants to synthetic. However,due to environmental and health concerns associated withthe applications of synthetic colorants the interest of usingnatural and organic sources as colorants has increased lately.

Anthocyanins are natural, water-soluble, and nontoxicpigments which are responsible for the red, orange, pink,blue, or purple color of plants. The resultant color variationdepends on the relative pH of the respective plant [1]. Theanthocyanin pigments are expressed as red or pink under

acidic media, purple in neutral media, and blue, green, oryellow in alkaline media. It turns colorless under extremealkaline conditions [2]. Thus, anthocyanins also can be usedas a pH indicator. Anthocyanins have been employed asadditives or colorants in the food industry with limitedusage due to its instability with varying pH levels [1]. Inthe alternative energy sector, natural dye extracts containinganthocyanins are integrated as sensitizers in dye-sensitizedsolar cells (DSSCs) because of their ability to absorb light[3, 4] and low cost associated with extraction procedures.

Many researches had been conducted on analyzing thepotential anthocyanins sensitizers extracted from naturalresources [5, 6], and currently an efficiency of natural sen-sitized DSSC of about 2% [4, 7] has been achieved. Theperformance and the efficiency of theDSSCs can be enhancedby increasing the adsorption capacity of the anthocyaninssensitizer onto the TiO

2nanoparticles surface and selecting

pigments with broader absorption capacity. Adsorption isdefined as the binding of molecules or particles (adsorbates)

Hindawi Publishing CorporationJournal of ChemistryVolume 2014, Article ID 468975, 6 pageshttp://dx.doi.org/10.1155/2014/468975

2 Journal of Chemistry

onto a surface (adsorbent). In this study adsorbate andadsorbent are anthocyanin pigments and TiO

2nanoparticles,

respectively.Kuduk-kuduk (KK) also known as Malabar Melastome

(Melastoma malabathricum L.), is a shrub belonging tothe family Melastomataceae. This is naturally grown inthe disturbed tropical and subtropical habitats under fullsunlight [8]. The flower is purple (sometimes pink), whilethe fruit a berry-like capsule filled with purple color pulpwith numerous seeds. The purple-pink color of the flowerpetals as well as the fruit pulp is due to anthocyanin pigments[9]. These features make it a valuable ornamental plant and apotential source for natural colorants.

Currently, many researchers are focusing to achievehighly efficient DSSCs with natural sensitizers [4]. In orderto achieve high efficiency in natural sensitized DSSCs, inaddition to absorption capacities, the adsorption propertiesof the pigments onto TiO

2nanoparticles have to be investi-

gated and improved [10].The aim of this work is to investigatethe adsorption properties of anthocyanin pigments extractedfrom KKP (KK fruit pulp) onto the surface of TiO

2nanopar-

ticles using the Langmuir, Freundlich, Redlich-Peterson, andSips isothermmodels and to evaluate its potential as a naturalsensitizer in DSSCs.

2. Experimental

2.1. Preparation and Characterization of Adsorbate. The nat-ural anthocyanin-containing dye, used in this study, wasextracted from KKP. 20 g (fresh weight) of the clean pulp wasground with a small amount of 70% ethanol (diluted fromScharlau 99.9%with distilled water).The crushed sample wasmixed with 150mL of 70% ethanol solution using a vortexmixture and kept at room temperature (25∘C) overnightcovered with an aluminum foil to ensure no exposure tooutside light. The residues of the filtrate were removed byfiltering through a filter paper (Whatman, Grade 1). Theabsorption spectra of the KKP extract were investigated inthe range 400 to 700 nm using UV-vis spectrophotometer(1601 PC Shimadzu). According to absorption data in [11]the predominant anthocyanin present in KKP extract iscyanidin-3-glucoside. The total monomeric concentration ofanthocyanin pigments in the dye extract was determinedby using the pH-differential method [11] which is based onthe difference in the pigments absorbance measurement atwavelengths of 520 nm and 700 nm:

Anthocyanin pigment content (mg/L)

=

𝐴 ×MW × DF × 103

𝜀 × 𝑙

,

(1)

where 𝐴 = (𝐴520 nm –𝐴

700 nm) pH 1.0 − (𝐴520 nm –𝐴

700 nm)pH 4.5, MW (molecular weight) = 449.2 g/mol for cyanidin-3-glucoside (cyd-3-glu), DF = dilution factor, 𝜀 (molar extinc-tion coefficient) = 26900 L mol−1 cm−1, 103 is the factor forconverting g to mg, and 𝑙 is the path length in cm.

2.2. Adsorption Studies. Adsorption isotherms of this studywere carried out at different concentrations of KKP extract;

a dilution series of six samples was prepared by mixing 0.25to 0.5mL of KKP extracts with absolute ethanol up to avolume of 10mL. 20mg of commercially available Degussa P-25 (mean particle diameter of 30 nm, 70% anatase, 30% rutile,and specific surface area of 50 gm−2) TiO

2powder wasmixed

into each sample and kept overnight at room temperature(25∘C) without exposing to external light to ensure a goodadsorption. The liquid phase of each mixture was separatedafter centrifuging for 10min at 3000 rpm in a centrifuge(SiGma 3–18 k). The absorbance of KKP extract pigments inthe solution, before and after adsorption, was measured at520 nm. The adsorbed amount of KKP extract pigments atequilibrium, 𝑞

𝑒(mg/g), was calculated using themass balance

equation [12]:

𝑞𝑒=

V (𝐶𝑖− 𝐶𝑒)

𝑚

, (2)

where 𝐶𝑖and 𝐶

𝑒(mg/L) are the initial and equilibrium con-

centrations of KKP extract pigments solution, respectively,𝑉is the volume of the solution used (L), and 𝑚 is the mass ofthe dry adsorbent (TiO

2nanoparticles) (g). The Langmuir,

Freundlich, Redlich-Peterson, and Sips isotherms were usedto analyze the equilibrium data.

2.3. Zeta-Potential and I-VMeasurements. The zeta-potentialof the KKP extract at original pH was measured usingZetasizer (Nano MAN 0317) at a temperature of 25.0∘C inethanol solution. In order to prepare theTiO

2paste, amixture

of 0.20 g of TiO2(Degussa P-25), 12 drops (0.15 g) of glacial

acetic acid, one drop (0.02 g) of triton X-100, and 2mL ofethanol were ground in a ball-mill machine (Paul O. AbbeRoller Mill, Model 202421) for 20 h. Subsequently the TiO

2

paste was coated on precleaned fluorine-doped conductingtin oxide (FTO) glasses (Nippon sheet glass 10–12Ω sq−1)by Doctor Blade method. The coated film was sintered for45min at 450∘C. The thickness of sintered TiO

2electrode

was ∼9 𝜇m (Dektak Profilometer; Veeco, Dektak 3). Thiselectrode was dipped in the KKP extract overnight at roomtemperature (25∘C) and air-dried at the same temperature[13]. The active solar cell area was 0.25 cm2. The cell wasassembled using a Test Cell Assembly Machine (Dyesol)with the Surlyn (50𝜇m, Dyesol) as the spacer. A mixtureof tetrabutylammonium iodide (TBAI; 0.5M)/I

2(0.05M),

acetonitrile, and ethylene carbonate (6 : 4, V/V) was used asredox electrolyte [14].The assembled solar cell was kept under1 sun (Dyesol Solar Simulator LP-156B) for about 4 h forlight soaking. Finally, 𝐼-𝑉 characteristic of the DSSC wasmeasured.

3. Results and Discussion

3.1. The Optical Properties. The UV-vis absorption spectraof KKP extract confirmed the presence of anthocyaninpigments. It produced a prominent peak at the wavelength of520 nm when a drop of HCl was added into the original KKPextract [11].

Journal of Chemistry 3

Table 1: The parameters required for kinetic adsorption isothermstudy.

Vol. of dye extract (mL) 𝐶𝑖(mg/L) 𝐶

𝑒(mg/L) 𝑞

𝑒(mg/g)

0.25 0.02145 0.00765 0.006860.30 0.02574 0.00914 0.008260.35 0.03003 0.00963 0.009950.40 0.03433 0.01112 0.011780.45 0.03860 0.01459 0.011660.50 0.04290 0.01756 0.01280

3.2. Kinetic Adsorption Studies. The parameters required forkinetic adsorption isotherm study are summarized in Table 1.The occurrence of the adsorption process was signified fromthe differences shown among the values of 𝐶

𝑖and 𝐶

𝑒, where

a decrease in the concentration of anthocyanin pigments wasobserved.The amount of anthocyanin pigments adsorbed perunit mass of TiO

2is shown in Table 1.

Adsorption isothermsmodels describe the distribution ofthe adsorbate species between liquid and adsorbent throughlinearly plotted graphs based on a set of assumptions y relatedto the heterogeneity or homogeneity of adsorbents, the typeof coverage, and the possibility of interaction between theadsorbates [15].

3.2.1. Langmuir Isotherm. The Langmuir adsorptionisotherm model explains the variation of adsorption ofmolecules (adsorbates) with pressure. The isotherm is basedon the assumption that maximum adsorption occurs when asaturated monolayer of adsorbate molecules is present on theadsorbent surface, the energy of adsorption is constant, andthere is no migration or interaction between the adsorbatemolecules in the surface plane [16, 17]. The linear expressionof the Langmuir isotherm model is defined by

1

𝑞𝑒

=

1

𝑞𝑚𝐾𝐿

⋅

1

𝐶𝑒

+

1

𝑞𝑚

. (3)

By plotting the linear graph of 1/𝑞𝑒against 1/𝐶

𝑒, the values

of𝐾𝐿and 𝑞𝑚can be determined from the intercept and slope

of the plot (Figure 1), where the parameter 𝑞𝑚(mg/g) is the

maximumadsorption capacity of dye per unitmass of sorbentto form a complete monolayer on the surface bound at high𝐶𝑒and 𝐾

𝐿(L/mg) is the Langmuir energy constant which is

related to the heat of adsorption. Table 2 presents the values ofLangmuir constants (𝑞

𝑚= 0.0468mg/g, 𝐾

𝐿= 28.489 L/mg).

The essential characteristics of the Langmuir isotherm canbe expressed in terms of a dimensionless constant separationfactor 𝑅

𝐿that is given by

𝑅𝐿=

1

(1 + 𝐾𝐿𝐶𝑖)

, (4)

where 𝐶𝑖(mg/L) is the highest initial concentration of

adsorbate and 𝑅𝐿values between 0 and 1 imply favorable

adsorption. The value of 𝑅𝐿in the present investigation was

calculated to be 0.45 which indicates favorable adsorption ofKKP extract pigments onto TiO

2nanoparticles surface.

1.4

1.2

1.0

0.8

0.6 0.8 1.0 1.2 1.4

1/q

e×102

(g/m

g)

1/Ce × 102 (L/mg)

Experimental dataLangmuir isotherm

Figure 1: Langmuir adsorption isotherm and experimental data ofKKP extract.

−1.8

−1.9

−2.0

−2.1

−2.2−2.1 −2.0 −1.9 −1.8 −1.7

Log Ce (mg/L)

Logqe

(mg/

g)

Experimental dataFreundlich isotherm

Figure 2: Freundlich adsorption isotherm and experimental data ofKKP extract.

3.2.2. Freundlich Isotherm. TheFreundlich isotherm explainsthat the extent of adsorption varies directly with pressure.This empirical relationship describes the multilayer adsorp-tion of heterogeneous systems and assumes that different siteshave several adsorption energies involved [18]. The linearmodel of the isotherm can be expressed logarithmically as in

log 𝑞𝑒=

1

𝑛

log𝐶𝑒+ log𝐾

𝐹. (5)

The values of parameters 𝐾𝐹and 𝑛 (listed in Table 2)

can be determined from the intercept and slope of theplot log𝐶

𝑒against log 𝑞

𝑒(Figure 2), where 𝐾

𝐹(1/g) is the

Freundlich adsorption capacity constant and 1/𝑛 is related


Table 2: Parameters and correlation coefficient (𝑅2) of equilibrium isotherm models for the adsorption of KKP extract pigments onto TiO2nanoparticles.

Isotherm Equation Parameters 𝑅2

Langmuir1

𝑞𝑒

=

1

𝑞𝑚𝐾𝐿

⋅

1

𝐶𝑒

+

1

𝑞𝑚

𝐾𝐿= 28.489 L/mg 0.915𝑞𝑚= 0.047mg/g

Freundlich log 𝑞𝑒=

1

𝑛

log𝐶𝑒+ log𝐾

𝐹

𝐾𝐹= 0.225 (1/g) 0.8931

𝑛

= 0.69

Redlich-Peterson ln(𝐾𝑅

𝐶𝑒

𝑞𝑒

− 1) = 𝑏𝑅ln𝐶𝑒+ ln 𝑎

𝑅

𝐾𝑅= 10mg/g

0.657𝑎𝑅= 45.814mg/L𝑏𝑅= 0.335

Sips 1

𝑞𝑒

=

1

𝑄max𝐾𝑠(

1

𝐶𝑒

)

1/𝑛

+

1

𝑄max

𝐾𝑆= 6.97 × 10

8 L/mg0.971𝑄max = 0.0130mg/g

𝑛 = 0.24

2.8

2.1

1.4

0.7

0.0

−4.8 −4.5 −4.2 −3.9

ln Ce (mg/L)

ln(K

R(C

e/q

e)−1

(mg/

L)

KR = 3.5

KR = 5.5

KR = 10

R-P isotherm for KR = 3.5

R-P isotherm for KR = 5.5

R-P isotherm for KR = 10

Figure 3: Redlich-Peterson adsorption isotherm and experimentaldata of KKP extract.

to the adsorption intensity constant that varies with theheterogeneity of the adsorbate.

3.2.3. Redlich-Peterson Isotherm. TheRedlich-Peterson (R-P)isotherm is a three-parameter empirical adsorption modelthat incorporates elements from both the Langmuir andFreundlich isotherms [15] and amends the inaccuracies. Theadsorption mechanism is unique and does not follow idealmonolayer adsorption characteristics. The linear expressionof the R-P isotherm model is defined as [15]

ln(𝐾𝑅

𝐶𝑒

𝑞𝑒

− 1) = 𝑏𝑅ln𝐶𝑒+ ln 𝑎𝑅, (6)

where 𝐾𝑅is the Redlich-Peterson adsorption capacity con-

stant determined via trials and errors to obtain themaximumlinear regression value of the isotherm graph (Figure 3).The parameter 𝑎

𝑅is the R-P isotherm constant and 𝑏

𝑅

is the exponent between 0 and 1. Numerous studies [19–24] have shown that the R-P isotherm was more accuratethan the Langmuir and Freundlich isotherms as it containsthree unknown parameters. The reason for this is that theLangmuir and Freundlich isotherms can be derived from theR-P isotherm. When 𝑏

𝑅= 1, the R-P equation becomes the

Langmuir isotherm equation and when 𝑏𝑅= 0, it is closer

to the Freundlich equation. However, the accuracy of theseinterpretations strongly depends on the fitting method [25–28].

3.2.4. Sips Isotherm. The Sips isotherm is derived fromthe limiting behavior of the Langmuir and Freundlichisotherms. The model is valid for localized adsorptionwithout adsorbate-adsorbate interactions [15]. When 𝐶

𝑒

approaches a low value, the Sips isotherm effectively reducesto Freundlich, while at high 𝐶

𝑒, it predicts the Langmuir

monolayer sorption characteristic. The Sips linear equationmodel is expressed as [15]

1

𝑞𝑒

=

1

𝑄max𝐾𝑆(

1

𝐶𝑒

)

1/𝑛

+

1

𝑄max, (7)

where 𝐾𝑆(1/mg) and 𝑄max (mg/g) are the Sips equilibrium

constant and maximum adsorption capacity values obtainedfrom the slope and the intercept of the plot (Figure 4). TheSips isotherm equation is characterized by the dimensionlessheterogeneity factor, 𝑛, which can also be employed todescribe the system’s heterogeneity when 𝑛 is between 0 and1. When 𝑛 = 1, the Sips equation reduces to the Langmuirequation and it implies a homogeneous adsorption process[21].

The parameters and correlation coefficient (𝑅2) for eachof the equilibrium isothermmodels of the adsorption processare shown in Table 2. The Redlich-Peterson model clearlyshows poor correlation (𝑅2 = 0.657) to the experimentaldata. Similarly, the Freundlich isotherm model that assumesan exponential increase in the adsorption amount does notfit well with data (𝑅2 = 0.893). Both the Langmuir (𝑅2 =0.915) and Sips isotherm (𝑅2 = 0.971) models show goodfit to experimental data. However, heterogeneous adsorption

Journal of Chemistry 5

1.4

1.2

1.0

0.8

1/q

e×102

(g/m

g)

Experimental dataSips isotherm

(1/Ce)1/n

× 108 (L/mg)

0 2 4 6

Figure 4: The Sips adsorption isotherm and experimental data ofKKP extract.

0.8

0.4

0.0

0.0 0.2 0.4

Voltage (V)

Curr

ent d

ensit

y (m

A/c

m2)

Figure 5: 𝐼-𝑉 characteristics of DSSC sensitized with KKP extract.

process is evidently indicated from the heterogeneity factorvalue (𝑛 = 0.24). Thus, the isotherm that best describes theadsorption behavior of KKP extract pigments onto the TiO

2

nanoparticles is the Sips isotherm.

3.3. Zeta-Potential and Photovoltaic Performance in DSSC.For KKP extract, zeta-potential of −9.48mV was obtained.This indicates the tendency of anthocyanin pigmentmolecules coagulating at its original pH (∼5), whichcorrelates well with the pigment’s adsorption characteristicsas explained by the Sips isotherm.

The DSSC performance was examined using the current-voltage characteristics, as shown in Figure 5. The valuesobtained for open circuit voltage (𝑉OC), short circuit currentdensity (𝐽SC), and fill factor (𝑓𝑓)were 0.448V, 3.177mA/cm2,and 0.52, respectively. Although monolayer adsorption con-dition is favorable for the photoconversion of DSSC anode

[29], the KKP extract that showed multilayer adsorptioncharacteristics has demonstrated its capability of sensitizingDSSCs while giving a photoenergy conversion efficiency of0.83%.

4. Conclusions

The adsorption behavior of KKP extract containing antho-cyanin onto TiO

2nanoparticles surface was investigated.

Experimental data were modeled using the Langmuir, Fre-undlich, Redlich-Peterson, and Sips isotherms. Equilibriumadsorption of KKP extract pigments onto TiO

2was well

described by the Sips model with a heterogeneous adsorptioncapacity of 0.0130mg/g. Zeta-potential data revealed the ten-dency of pigment molecule coagulation in the solvent. Thisphenomenon correlated well with its adsorption behavior.From 𝐼-𝑉 analysis, a photo-energy conversion efficiency of0.83% was obtained indicating that the Kuduk-kuduk fruit-pulp extraction has the potential to be integrated into the dye-sensitized solar cell system as a viable sensitizer.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

The Universiti Brunei Darussalam Research GrantUBD/PNC2/2/RG/1(176) and Brunei Research Council(BRC) Science and Technology Grant (S & T 17) areacknowledged for financial support.

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