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BY-NC-ND 3.0 license Open Access Published by De Gruyter August 26, 2015

Optical properties and photocatalytic activity of CdS-TiO2/graphite composite

  • Fitria Rahmawati EMAIL logo , Rini Wulandari , Eti Nofaris and Mudjijono

Abstract

TiO2 film was applied to graphite substrate through chemical bath deposition (CBD) method with TiCl4 as precursor. CdS was deposited on TiO2/graphite (TiO2/G) by CBD with precursors of CdSO4. A UV-Vis diffuse reflectance (UV-Vis DR) analysis was used to analyze the optical properties and band gap energy. Meanwhile, photocatalytic activity was determined from the quantum yield (QY) value of isopropanol degradation. The X-ray diffraction pattern shows that the TiO2 film contains rutile and anatase phases. UV-Vis DR spectrum shows the photoactivity in the visible light area and provides lower band gap value due to CdS deposition. TiO2/G provide energy gap at 3.0 eV, which indicates that rutile phase is dominant. Meanwhile, CdS-TiO2/G shows multiple energy gaps representing CdS at 2.4 eV, rutile at 3.0 eV, and anatase at 3.2 eV. The QY values of CdS-TiO2/G are 7.98×10-3 and 8.62×10-4 at 450 and 380 nm of radiation light, respectively. These values are higher than the QY of TiO2/G and TiO2-P25/graphite (TiO2-P25/G), which are 1.19×10-4 and 5.0×10-4, respectively. The photocatalytic reaction follows the first-order reaction. CdS deposition allows the rate constant to increase from 2×10-3 to 13×10-3 under UV radiation.

1 Introduction

Titanium dioxide is a photocatalyst for multipurpose works, such as for Escherichia coli disinfection [1, 2], degradation of dyes [3], and photocatalytic reduction of CO2 [4, 5]. The potential application of TiO2 is very important because of the fact that the oxidation potential of TiO2 (around 3.0 V versus SCE) is higher than other conventional oxidizing agents such as ozone (2.07 V versus SCE), and it is combined with the inertness and nontoxicity properties of TiO2 [6]. However, the quantum yield (QY) of TiO2 is still low as it was found by Ishibashi et al. [7] that the QY of hydroxyl radical (·OH) during photocatalysis is estimated to be 7×10-5 in aqueous solution and the QY of hole generation estimated by iodide ion oxidation is 5.7×10-2. Meanwhile, Wang et al. [8] found that the QY of formaldehyde formation with neat colloidal TiO2 (2.4 nm of particle size) is 0.032. It was founded under a high-energy radiation of 351 nm light and with laser pulse assistance. It means that to increase the QY of TiO2, some special treatments are required during application. Therefore, some efforts to enhance the photocatalytic activity of TiO2 are required to reach effective photocatalytic degradation without any specific treatment in the degradation cell.

The low QY may occur because of the recombination process of the excited electrons in the conduction band with a generated hole in the valence band, before those reducing agents (the excited electrons) and those oxidizing agents (the holes) move to the surface and react with the targeted molecules. Many attempts have been made to improve photocatalytic activity by suppressing the photogenerated charged carriers, such as noble metal and metal oxide deposition [9, 10], metal ion implantation into TiO2 [11], TiO2 preparation as a composite with other semiconductor such as CdS [4, 12], or TiO2 composite with CdS and CuWO3 [13]. Research on combining CdS-graphene/TiO2 has also been conducted to produce a cooperative reaction between the increase photoabsorption effect of graphene and the photocatalytic effect of CdS [14]. Graphene is a single layer of graphite that consists of sp2-bonded carbon atoms arranged in a hexagonal lattice. Graphene is a very strong compound, has a high conductivity of heat and electricity, and also has a high level of light absorption. However, graphene is still expensive for commercial uses because of the high cost of good quality graphene production. The production involves the use of toxic chemicals, and it is also impossible to grow graphene layer on a large scale without metallic substrate. Meanwhile, it is impossible to separate graphene layer from the metallic substrate without damaging the graphene layer. Therefore, this research used graphite rather than graphene because graphite is a naturally occurring carbon compound that also has a hexagonal lattice structure with high electrical conductivity and thermal conductivity and also has a very high resistance to chemical attack. In addition, the graphite used in this research is a waste graphite that was founded from zinc-carbon primary battery waste. It is in agreement with the environment-friendly production of materials. The other advantage is that the immobilized form of catalyst will reduce the cost of separation from the treated solution. The catalyst tablet of TiO2/G was produced by a chemical bath deposition method, as described in our previous papers. This paper discusses the research of CdS deposition on the TiO2/G with the objective to improve the photocatalytic activity in the visible light area. It was found by Li et al. [15] that the composite powder of CdS-TiO2 shows an excellent synergistic effect between CdS and TiO2 under visible light. This research investigated the immobilized form of CdS-TiO2. The photocatalytic activity was investigated through isopropanol degradation, and the QY was calculated as the QY of acetone production. Photocatalytic degradation was conducted under radiation of 380 and 450 nm of light to study their activity under light radiation, which is suitable with the gap energy of TiO2 (3.2 eV) and CdS (2.4 eV).

2 Materials and methods

The chemicals used in this research were pro analysis grade. The graphites were isolated from zinc-carbon primary battery wastes. The graphite waste was then cleaned with AquaDes (Chemistry Laboratorium, Sebelas Maret University, Suakarta, Jawa Tengah, Indonesia) and followed by ethanol before heating at 105°C. The TiO2 film on graphite substrate, TiO2/G, was prepared by chemical bath deposition, as explained in our previous paper [2]. The precursor is TiCl4 (Sigma Aldrich, Singapore), which was dissolved in an acid solution of HCl, and then cetyl trimethyl ammonium bromide (CTAB) surfactant was added (Sigma Aldrich, Singapore). This surfactant molecule serves as linking agents between the graphite substrate and the titanium dioxide network formed while dipping the substrate in the precursor solution at 60°C for 4 days. Then it was washed with deionized water and calcination at 450°C for 4 h.

A 500-ml solution of CdSO4 (0.665 mmol) and thiourea (1.33 mmol) and (NH4) 2SO4 (Sigma Aldrich, Singapore) as a complexion agent was used as a precursor for CdS deposition. An ammonium salt was added to the precursor solution as a buffer. The TiO2/G tablet was dipped once and four times into the precursor solution, with 15 min for each dipping at 70°C. The as-prepared CdS-TiO2/G tablets were then washed with deionized water and dried at room temperature.

X-ray diffraction (Bruker D8, Germany) was used to identify the prepared materials in comparison with a standard taken from an established database. Morphological analysis was conducted by scanning electron microscopy (SEM, JEOL JSM-6510, Germany). Particle size was determined through SEM images by MeasureIT software (free edition; Olympus Soft Imaging Solutions GmbH, Germany). EDS was conducted by SEM Integrated EDS (Phenom Pro-X). A high-resolution UV-Vis spectrophotometer (UV 1700 Pharmaspec, Shimadzu, Japan) was used to analyze the diffuse reflection properties of the prepared materials at 200–500 nm of light. The diffuse reflection data were used to investigate the photoactivity of the prepared materials through the calculation of energy gap by plotting Kubelka-Munk function, F(R′∞), as defined in Equation (1) [16]:

(1)F(R)=(1-R2R)2=αS,

where R′∞ is the R∞(sample)/R∞(standard) and corresponds to the diffused reflection of a given wavelength of a dense layer of a nontransparent infinite material, α is the absorption coefficient (cm-1), and α can be calculated by using Equation (2), as follows:

(2)α=A(E-Eg)γ,

where A is a constant that depends on the properties of the material, E is the energy of the photon, Eg is the band gap, and (is a constant that depends on the type of electronic transition. γ=1/2 for a permitted direct transition, (=2 for a permitted indirect transition, γ=3/2 for a prohibited direct transition, and (=3 for a prohibited indirect transition [16]. Therefore, the equation is defined in Equation (3) for direct transition and Equation (4) for indirect transition, as follows:

(3)F(R)2=(AS)2(E-Eg),
(4)F(R)1/2=(AS)1/2(E-Eg).

Meanwhile, for samples with opacities >75%, the Kubelka-Munk function, F(R′∞) may be calculated as Equation (5):

(5)F(R)=(1-0.01R)22(0.01R),

where R is the reflection in percent.

Photocatalytic activity was determined through isopropanol degradation. The 2.5 m of isopropanol solution was irradiated with light at 380 and 450 nm for 30, 60, and 90 min. Degradation was conducted in a cuvette with a catalyst tablet (diameter, 0.8 cm) dipped into it by using a wire. The cuvette then was installed into sample holder of UV-Vis spectrophotometer (Single-Beam UV-Vis Mini 1240 Spectrophotometer, Kasugawa, Shimazu, Japan). The Spectronic was tuned in the photometric menu. The degradation product was analyzed by Fourier transform infrared (FT-IR Prestige 21 8201 PC, Shimadzu, Japan) to investigate the changes of functional groups. The QY of acetone production, Φ, is calculated using Equation (6):

(6)Φ=Nmol(mol/s)Nphoton(molEinstein/s),

where Nmol is the number of isopropanol molecules transformed into a product or the number of products formed during degradation. This was determined through its UV-Vis absorbance spectra, which is plotted based on the Lambert-Beer equation. Meanwhile, the Nphoton is the number of photons absorbed by a photocatalyst material. It was determined from UV-Vis diffuse reflectance data, which was recorded with 50 W of lamp at 380 and 450 nm, as those wavelengths were used as a photocatalytic radiation source and those represent UV light with 3.2 eV energy as the band gap of TiO2 and represent the visible light, respectively. The photocatalytic activity investigation was also done for TiO2 P25 Degussa film on graphite substrate, a well-known commercial titanium dioxide, as a comparison.

On the basis of the result of Xu and Raftery [17], who found that the product of isopropanol photocatalytic degradation is acetone, the maximum absorbance, λmax, of pure acetone was therefore determined in this research, that is, 265 nm. However, the change of solution polarity during degradation may shift the λmax of acetone, as Kumar [18] stated that the λmax of acetone is shifted to 279 nm in hexane solvent. This pure acetone UV-Vis spectrum was used as a standard compared with the degradation product.

Kinetics analysis was conducted by applying first- and second-order reactions. Higher linearity constant represents the suitable order for the reaction. The kinetic data used were started at 30 min because the initial rate usually has a high slope that might not represent the whole reaction rate.

3 Results and discussion

The XRD pattern of the TiO2/G tablet in comparison with P25 Degussa, rutile, and anatase standard diffraction is depicted in Figure 1. The pattern shows that the prepared TiO2/G contains rutile and anatase phases. It is compared with rutile diffraction standard of ICSD 23697 and to anatase diffraction standard ICSD 9852. There are peaks that are indicated as CTAB at 2θ of 21.3202° and 2θ of 24.0444° based on comparison with JCPDS 48-2454 and a peak at 2θ of 77.800°, which is identified as graphite, based on comparison with ICSD 28417.

Figure 1: XRD pattern of TiO2/G, which was synthesized by the chemical bath deposition method and the TiO2 P25 Degussa powder in comparison with standard rutile ICSD 23697 and anatase ICSD 9852. R refers to rutile, A refers to anatase, • is indicated as CTAB, and ○ is indicated as graphite peak.
Figure 1:

XRD pattern of TiO2/G, which was synthesized by the chemical bath deposition method and the TiO2 P25 Degussa powder in comparison with standard rutile ICSD 23697 and anatase ICSD 9852. R refers to rutile, A refers to anatase, • is indicated as CTAB, and ○ is indicated as graphite peak.

The XRD pattern of CdS-TiO2/G is depicted in Figure 2 in comparison with a TiO2/G diffraction pattern of CdS ICSD 81925. CdS peak is identified at 2θ of 26.6°. Other peaks are in agreement with peaks of TiO2/G. It indicates that there is no secondary product as the result of CdS formation. The CdS formation from precursors follows the reactions depicted in Equations (8)–(13) [19]:

Figure 2: The XRD pattern of CdS-TiO2/G, which was synthesized by the chemical bath deposition method in comparison with CdS standard diffraction ICSD 81925. Sign O refers to the indicated CdS peak.
Figure 2:

The XRD pattern of CdS-TiO2/G, which was synthesized by the chemical bath deposition method in comparison with CdS standard diffraction ICSD 81925. Sign O refers to the indicated CdS peak.

(8)CdSO4 (Cd2++SO42-),
(9)NH4OH (NH4++OH-),
(10)Cd2++NH4+Cd(NH3)42++S2-4NH3+Cd2+.

Thiourea SC(NH2)2 was provided as an S2- precursor after hydrolysis, as described in Equations (11)–(13):

(11)SC(NH2)2+OH-SH-+CH2N2+H2O,
(12)SH-+OH-S2-+H2O.

The Cd(NH3)42+ and sulfide ions reacted to form CdS on the substrate surface:

(13)Cd(NH3)42++S2-CdS+4NH3.

Ammonia would be eliminated during washing treatment and leaved CdS deposit on the substrate surface. The absorption spectrum of TiO2/G as recorded by UV-Vis diffuse reflectance is described in Figure 3, which shows that it has a gap energy of 3.0 eV, which indicates that rutile phase plays as a dominant photoactive mechanism. Meanwhile, TiO2 Degussa P25/G (Figure 3) shows a gap energy of 3.2 eV, which indicates that anatase plays dominant photoactive in this material because anatase is the major component in this material, as described in its diffraction pattern depicted in Figure 1. The UV-Vis DR spectrum of TiO2/G (Figure 3) also shows a peak at around 4.0–5.0 eV. This may contribute to the ππ* transition of graphite. Graphite has an aromatic hydrocarbon compound containing benzene rings in its structure with the large delocalized orbitals around the benzene rings. Some ππ* transitions are possible from the singlet ground state to the singlet excited states. The optical absorption spectrum of graphite at visible range from 0 up to 5 eV originates from transition among π bands [20]. A calculation by Johnson and Dresselhaus [21] on the π bands in graphite, which were determined by McClure parameters along with the addition of in plane neighbor’s interactions, resulted in the allowed optical transitions near 5 eV.

Figure 3: The absorption spectra of (A) TiO2/G and (B) TiO2 Degussa P25/G. α is the absorption coefficient.
Figure 3:

The absorption spectra of (A) TiO2/G and (B) TiO2 Degussa P25/G. α is the absorption coefficient.

The absorbance coefficient shows sharp peaks that indicate the high-density transitions of free excitons, as described in Figure 4. E1 represents the gap energy of CdS (2.4 eV), E2 represents the gap energy of rutile phase, E3 represents the gap energy of anatase phase, and E4 represents the energy of ππ* transition in graphite. The absorption of photons by an interband transition in a semiconductor excites an electron to the conduction band and leaves a hole in the valence band. The opposite charged particles that were created at the same point can attract each other through mutual Coulomb interaction that increases the probability of the formation of an electron-hole pair and therefore increases the optical transition rate. If the right conditions are satisfied, a bound electron-hole pair can be formed. This neutral bond pair is called an exciton. At low density, the separation between excitons is large, and the exciton-exciton interactions may be negligible. Meanwhile, as the power of the photon is increased, the density of excitons increases, the exciton wave functions begin to overlap, and the exciton-exciton interactions become significant. The peak at 4.6 eV, E5, represents an asymmetry π band transition as it was predicted through calculation by Johnson and Dresselhaus [21] at 4.8 eV. Interband transitions occurred in CdS-TiO2/G because each component has their own optical properties and interaction between them that may complicate the electronic transitions inside the photocatalyst material. Besides that, the existence of Si and Al as impurities, as detected by EDX (Figure 5), may allow the new absorption mechanisms that contribute to the complexity of absorption spectra. Figure 5 shows the morphology of TiO2/G and CdS-TiO2/G. The SEM images show that TiO2 film consists of round form particles with a diameter of 161±0.051 nm. The form was changed after CdS deposition and became a rod with a length of 204±0.040 nm. Si element is an impurity embedded in titanium chloride solution, the precursor of titanium dioxide networks. CdS deposited on TiO2 layer is around 2.2% based on its EDX data.

Figure 4: The absorption spectra of CdS-TiO2/G that was synthesized by chemical bath deposition method.
Figure 4:

The absorption spectra of CdS-TiO2/G that was synthesized by chemical bath deposition method.

Figure 5: SEM images of (A) TiO2/G and (B) CdS-TiO2/G and the result of EDX analysis on CdS-TiO2/G.
Figure 5:

SEM images of (A) TiO2/G and (B) CdS-TiO2/G and the result of EDX analysis on CdS-TiO2/G.

UV-Vis spectrophotometer analysis during isopropanol degradation with TiO2/G and CdS-TiO2/G at 380 nm of light irradiation produces the spectrums depicted in Figure 6. On the basis of the UV-Vis spectra, it can be concluded that the degradation of isopropanol by TiO2/G and CdS-TiO2/G produced new peaks at 221–222 and at 266–267 nm. Those peaks are indicated as acetone peak. Kumar [18] mentioned that the ketone functional group shows an electronic transition of ππ* at 180–195 nm and nπ* at 270–295 nm. The transition of nπ* is frequently shifted to shorter wavelength in more polar solvent; meanwhile, the transition of ππ* is shifted to longer wavelength. In this research, isopropanol has been diluted in water, a polar solvent, until a concentration of 250×10-2m was reached. Therefore, peaks of the product, acetone, shifted to shorter wavelength at 266–267 nm for the nπ* transition and shifted to longer wavelength at 221–222 nm for the ππ* transition. Meanwhile, degradation with light irradiation <450 nm using TiO2/G does not show new peaks in the UV-Vis spectrum, as described in Figure 7A. This is because TiO2 is not photoactive at a photon energy <450 nm. However, the UV-Vis spectrum of isopropanol degradation using CdS-TiO2/G shows new peaks (Figure 7) at 221 and 267 nm, as its present degradation is <380 nm. It indicates that CdS with a gap energy of 2.4 eV plays significant role in activating this composite material under visible light irradiation.

Figure 6: UV-Vis spectrum of degraded solution with (A) TiO2/G and (B) CdS-TiO2/G as a photocatalyst of monochromatic light at 380 nm produced by UV-Vis spectrophotometer at the photometric menu. The initial solution is 2.5 m of isopropanol solution and irradiation time is 30, 60, and 90 min.
Figure 6:

UV-Vis spectrum of degraded solution with (A) TiO2/G and (B) CdS-TiO2/G as a photocatalyst of monochromatic light at 380 nm produced by UV-Vis spectrophotometer at the photometric menu. The initial solution is 2.5 m of isopropanol solution and irradiation time is 30, 60, and 90 min.

Figure 7: UV-Vis spectrum of degraded solution with (A) TiO2/G and (B) CdS-TiO2/G as a photocatalyst of monochromatic light at 450 nm produced by UV-Vis spectrophotometer at the photometric menu. The initial solution is 2.5 m of isopropanol solution, and irradiation time is 30, 60, and 90 min.
Figure 7:

UV-Vis spectrum of degraded solution with (A) TiO2/G and (B) CdS-TiO2/G as a photocatalyst of monochromatic light at 450 nm produced by UV-Vis spectrophotometer at the photometric menu. The initial solution is 2.5 m of isopropanol solution, and irradiation time is 30, 60, and 90 min.

Figure 8 is the FTIR spectrum of isopropanol at initial, after degradation <380 nm using TiO2/G and CdS-TiO2/G. It is described in Figure 8 that the CH aliphatic peak at 2922 cm-1 disappears after degradation. The OH stretching at 3450 cm-1 becomes stronger and sharper after degradation by CdS-TiO2/G. The pure H2O peak at 1697 cm-1 and the CH bending at 1398 cm-1 are also disappearing, and a shift in carbonyl vibration at 1043 cm-1 to 1138 cm-1 after degradation by TiO2/G and at 1144 cm-1 after degradation by CdS-TiO2/G was found. The carbonyl vibration in the initial isopropanol solution might come from the formaldehyde functional group.

Figure 8: FTIR spectrum of isopropanol at (A) initial, (B) after being degraded at 380 nm by TiO2/G, and (C) after being degraded at 380 nm by CdS-TiO2/G.
Figure 8:

FTIR spectrum of isopropanol at (A) initial, (B) after being degraded at 380 nm by TiO2/G, and (C) after being degraded at 380 nm by CdS-TiO2/G.

The QY values at 60 min degradation by TiO2/G, CdS-TiO2/G, and P25 film/G are listed in Table 1. The QY values were calculated based on the absorbance at 265–279 nm, which indicates the acetone peak, as recorded by UV-Vis from acetone solution. Table 1 shows that 380 nm of photons absorbed by TiO2/G is higher than that was absorbed by CdS-TiO2/G; however, the QY value is lower than CdS-TiO2/G. It indicates that CdS does not increase the absorption of photon, but CdS mainly inhibits the electron-hole recombination. The CdS-TiO2/G also has higher QY than P25/G although the capability of photon absorption is lower than P25/G. Table 1 also shows that TiO2/G is not photoactive in light irradiation <450 nm. Meanwhile, CdS-TiO2/G absorbed 450 nm of photons and was photocatalytically active. It indicates that CdS with 2.4 eV of gap energy played significant contribution on the photoactive mechanism. Although the photons absorbed by CdS-TiO2/G were small, i.e. 8.28×10-5 mol/s, the QY value, i.e. 7.98×10-3, is higher than QY at 380 nm irradiation, i.e. 8.62×10-4. It indicates the different mechanisms between irradiation <380 nm and irradiation <450 nm on CdS-TiO2/G. Because the conduction band of CdS (approximately 0.5 V) is more negative than that of TiO2 [22], electrons from the valence band of CdS excite the conduction band when 450-nm photons of light radiates, and then the excited electrons from CdS can quickly transfer to TiO2. Meanwhile, when the composite material is irradiated with 380 nm of light, the electrons from the valence band of CdS and also of TiO2 excite their conduction bands, and the electrons of the CdS conduction band cannot be transferred to the TiO2 conduction band because the conduction band has been filled. Therefore, it allows recombination to the valence band.

Table 1

The number of photons absorbed, Nphoton (mol/s), and the QY of the prepared photocatalyst materials at 380 and 450 nm of light at 60 min of degradation.

Wavelength of light (nm)MaterialsNPhoton (mol/s)QY
380TiO2/G6.58×10-31.19×10-4
P25/G2.43×10-35.00×10-4
CdS-TiO2/G4.86×10-48.62×10-4
450TiO2/G1.11×10-50
P25/G1.18×10-40
CdS-TiO2/G5.54×10-57.98×10-3

Table 1 shows that in P25/G, TiO2/G absorbed photons with 450-nm radiation; however, the QY values are zero because no peak appears in its UV-Vis spectra at 265–280 nm. It indicates that the absorbed photon cannot be used to excite electrons in their conduction band because of the low energy of its photons. However, those 450-nm photons could excite electrons in the conduction bands of CdS-TiO2/G, resulting in high QY values.

Table 2

First-order kinetics calculation of isopropanol degradation with TiO2/G and CdS-TiO2/G. The kinetic analysis was based on acetone production, which is defined by peak at 265 nm.

MaterialsWavelength of photon source (nm)Rate constant, kLinearity, RStandard deviation
TiO2/G3800.0020.9450.025
CdS-TiO2/G3800.0130.9130.253
TiO2/G4500.0010.9190.024
CdS-TiO2/G4500.0040.9980.013

By analyzing the reaction kinetics of isopropanol degradation with light irradiation <380 and 450 nm, it was found that the rate constant of isopropanol degradation with CdS-TiO2/G is higher than degradation with TiO2/G. The results of the first-order kinetics are listed in Table 2, and the plots are drawn in Figure 9. This kinetics analysis shows that CdS deposition increases the photocatalytic activity, whether under UV or visible radiation.

Figure 9: First-order plot of isopropanol degradation with (○) TiO2/G and (•) CdS-TiO2/G as a photocatalyst under irradiation of (A) 380 nm of light and (B) 450 nm of light.
Figure 9:

First-order plot of isopropanol degradation with (○) TiO2/G and (•) CdS-TiO2/G as a photocatalyst under irradiation of (A) 380 nm of light and (B) 450 nm of light.

4 Conclusion

The CdS deposition changes the optical properties of TiO2/G shown by the changes of gap energy and its photoexcitation mechanism. It also increases the rate constant of isopropanol degradation. The presence of CdS does not increase the photon absorption of TiO2/G; however, it increases the photocatalytic activity by decreasing the recombination possibility of the excited electrons back to their valence band.


Corresponding author: Fitria Rahmawati, Research Group of Solid State Chemistry and Catalysis, Department of Chemistry, Sebelas Maret University, Jl. Ir. Sutami 36 A Kentingan, Surakarta 57126, Indonesia, e-mail:

Acknowledgments

This research is a continuation of the Hibah Bersaing Project batch XIII, which was funded by the Directorate General of Higher Education, Republic of Indonesia, and part of the International Collaborative Research Project 2014, which was funded by PNBP Sebelas Maret University (grant no. 501/UN27.11/PN/2014) and continued with the PNBP mandatory research on 2015. The authors express gratitude for the support. The authors also thank Irvinna M. Murni for good collaboration during experimental section.

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Received: 2015-4-10
Accepted: 2015-6-16
Published Online: 2015-8-26
Published in Print: 2017-3-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

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