Tailoring the active surface sites of ZnO nanorods ... - SIMAKIP

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Surfaces and Interfaces

journal homepage:www.elsevier.com/locate/surﬁn

Tailoring the active surface sites of ZnO nanorods on the glass substrate for photocatalytic activity enhancement

Liszulfah Roza

^a

, Vivi Fauzia

^b,⁎

, Mohd. Yusri Abd Rahman

^c

aJurusan Pendidikan Fisika, Fakultas Keguruan dan Ilmu Pendidikan, Universitas Muhammadiyah Prof. Dr. Hamka, Jakarta Timur, Indonesia

bDepartemen Fisika, Fakultas MIPA, Universitas Indonesia, Depok 16424, Indonesia

cInstitute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor 43600, Malaysia

A R T I C L E I N F O

Keywords:

ZnO Nanorods Facets

Ultrasonic spray pyrolysis Photocatalysts

A B S T R A C T

The urgent need for inexpensive, eﬃcient, ease in use and reusable photocatalyst has attracted the attention of many researchers. Here, the photocatalyst ZnO nanorods were synthesized with a simple, low cost and rapid ultrasonic spray pyrolysis method and then grown with a hydrothermal method for 2, 4, 6 and 8 h. The ZnO nanorods grown for 6 h shows the highest photocatalytic eﬃciency although do not have the largest surface area.

This is probably due to its surface is dominated by (002) facets as indicated by the highest texture coeﬃcient (TC) value. The (002) polar facets consisting of a positive Zn-terminated (002) facets and a negative O-terminated (00-2) facets may play an important role in more eﬃcient UV absorption and photogenerated charges separation. Moreover, the highest crystallite size as shown in XRD result provide pathways for electrons and holes for redox reactions on the surface of ZnO nanorods. The synthesized ZnO nanorods also contain high concentration of oxygen interstitial as a source of holes that react with the OH−ions that easily adsorbed on (002) facets.

1. Introduction

Over the past decade, many studies were focused on photocatalyst materials for degradation of chemical pollutants in the wastewater for a better and green environment[1,2]. In the typical photocatalytic process, the oxidation and reduction reactions occur at the material's surface in contact with the wastewater[3]. The photocatalytic activity is sensitive to its surface morphology because the photocatalytic reaction is a surface reaction[4,5]. Wide band gap semiconductors such as TiO2

and ZnO have been used as the photocatalysts for the degradation of various organic dye pollutants[4–9]. ZnO is a promising semiconductor with a wide-bandgap (3.37 eV) and large exciton binding energy (60 meV) due to it is environmental friendly, chemically stable and low cost[10]. High surface area of hexagonal ZnO nanorods demonstrates an enhanced photocatalytic eﬃciency in comparison to a commercial ZnO powder because of well-aligned ZnO nanorods have a favorable electron transfer property with high surface area[4,11].

Generally, the photocatalyst ZnO were synthesized in powder form [12–15]. This is complicated and costly becauseﬁltering, washing, and drying processes are needed to separate the ZnO nanocatalyst from the polluted solution[16]. The use of substrate to support photocatalysts makes theﬁltering process is unnecessary hence the procedure is much

easier and faster because it is only need to lift the photocatalyst from the polluted solution. Several studies have tried to use different types of substrate, such as silicon, open-cell aluminum foams,fluorine doped tin oxide, quartz, Ti foil and glass substrates [14,17–23]. The surface condition of the substrate strongly influences the morphology of ZnO nanorods. The diameter of the nanorods was observed to be less uni- form and randomly distributed over the FTO surface in comparison to the silicon substrate since the FTO substrate had a relatively rough surface[24]. Moreover, the ZnO nanocatalyst can be reused without any significant quality degradation of ZnO. Recently, our group reports the use of Mn-doped ZnO nanorods on glass substrate can be reused easily for four cycles in the degradation of methyl blue[10]while the others also report the recycling of photocatalyst ZnOfive up to ten consecutive cycles[25,26].

The synthesis of ZnO nanorods on the substrates generally consists of two steps i.e. the deposition of seed layer and the growth step. The seed deposition process can be carried out by the complex deposition system such as sputtering[27,28] and pulsed laser deposition (PLD) [29]or by the simple and low cost wet chemical methods such as drop casting, dip coating, spin coating and sol gel method[19,21,30–33].

These chemical methods usually need a repeated deposition process that require a longer time. Here we propose the ultrasonic spray

https://doi.org/10.1016/j.surﬁn.2019.02.009

Received 15 October 2018; Received in revised form 2 February 2019; Accepted 14 February 2019

⁎Corresponding author.

E-mail address:[email protected](V. Fauzia).

Available online 15 February 2019

T

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pyrolysis (USP) method as a simple, low cost and rapid process because it is the one-step process that can be used for large area coverage substrates[34,35]. Moreover, ZnO nanorods could be strongly bound to the substrates since the seed layer deposited with a relatively high temperature (450 °C) in an ambient atmosphere[10]. This condition is required to decompose the chemical precursors and deposite on the substrate surface with best coating properties[36].

Meanwhile, many researchers have tried to enhance the photocatalytic activity of ZnO nanorods by adjusting the synthesis parameters such as types of precursor, types and molar ratio of surfactants, pH, growth time and heat post treatment to control the morphological properties and surface area of ZnO[37–40]. Recent study shows that high surface area of ZnO do not lead the higher photocatalytic performance, but other more important is the surface active sites [41–43].

The relation between the surface oxygen vacancies population and the exposed (002) facets have been regarded as the main criterion for high photocatalytic activity[44]. Shuwang et al. shows that the polar (002) plane and/or the nanorod tips aﬀect the photocatalytic activity for charge-transfer process[41]. The polar planes also aﬀect the population of surface defects population that plays an important role in adsorption and surface reactivity[42].

In order to optimize the photocatalitic activity, here we present that the active (002) facet of ZnO nanorods can be controlled by simply adjusting the growth time in hydrothermal method. The eﬀect of growth time on the morphological, structural, and optical properties and the photocatalytic activity of the ZnO nanorods were investigated in detail.

2. Experimental

The ZnO nanorods on glass substrates were prepared using ultrasonic spray pyrolysis (USP) and hydrothermal methods. The seeding process was carried out by deposition of a ZnO seed layer via the USP technique using 0.2 M zinc acetate dehydrate (Zn(CH3COO)2•2H2O).

Firstly, the seeding solution was placed into a container in a commercial ultrasonic nebulizer. Theﬁne sprays of the solution were generated by ultrasonic waves and sprayed on the clean substrates which were heated at 450 °C for 15 min. The samples were then annealed at 450 °C for 1 h on a hot plate and then coolled down to room temperature. After the seeding process, the samples were subsequently immersed in a growth solution containing an equimolar (40 mM) of zinc nitrate hex- ahydrate and hexamethylenetetramine for growth times ranging from 2 h to 8 h at 90 °C in an oven. The substrates were placed in the growth solutions at an angle of 45° to the horizontal plane. The samples were then taken out and washed several times using ultrapure water in order to remove any precipitation. They were then dried using nitrogen gas ﬂow.

A Zeiss Supra 55VP Field emission scanning electron microscope (FESEM) was employed to observe the morphological properties of ZnO. The structures of ZnO were examined using Bruker D8 Advance X- ray diffraction (XRD) equipment. An optical spectrophotometer ultraviolet-visible (UV–Vis) Lambda 900 Perkin Elmer, UV–Vis Diffuse Reflectance U-3900H spectrophotometer and FLS920 photoluminescence spectrometer Edinburgh instruments were employed to study the optical properties of the ZnO samples. X-ray photoelectron spectroscopy (XPS) measurements were carried out using the Ulvac-PHI Quantera II with an Al KαX-ray beam at 1486.6 eV. The binding energies were calibrated by taking the carbon C 1 s peak (285.0 eV) as the reference. The Raman spectra were recorded using the WITec Raman microscope equipped with a 532 nm laser source.

The ZnO nanorods were then used as a photocatalyst to degrade the methyl blue (MB) aqueous solution. All samples were immersed in 20 mL 10 mM MB solution while irradiated with a UV lamp (20 W). The photocatalytic activity of the ZnO nanorods was observed by recording the absorption intensity of the MB solution at 665 nm in the speciﬁc time intervals after the ZnO photocatalyst were simply removed from

the MB solution. The degradation of MB under UV irradiation without ZnO photocatalyst was also observed as a baseline.

3. Results and discussions

The XRD patterns of all the samples are shown inFig. 1. Based on the reference JCPDS No. 79-2205, the peaks confirm that ZnO has a polycrystalline hexagonal wurtzite structure. Each pattern demon- stratesfive prominent peaks at 2θof 31.7°, 34.4°, 36.25°, 47.5°, and 56.5° that correspond to (100), (002), (101), (102), and (110) lattice planes, respectively. No peaks related to other zinc complexes or other impurities were seen, confirming the phase purity of ZnO[45,46]. The ZnO diffraction patterns also show that the (002) plane is the highest peak for all the samples. This indicates that the (002) plane is the preferred growth orientation. In order to further identify a preferred growth orientation quantitatively, a parameter texture coefficient (TC) was defined by using the following simple calculation[47]:

=

⎡⎣∑ ⎤

⎦ TChkl

I hkl I hkl n

n I hkl I hkl

( )

( ) 1

1 ( ) ( ) m O

m

O (1)

wheren is the number of peaks and Imand Io are the intensity of measured and standard peaks, respectively. Generally, a TC value less than 1 means that the growth orientation is random, while the highest TC value means that the lattice plane is dominant and can be con- sidered to be the preferred orientation. As shown inTable 1, the TC value of all the peaks of the ZnO grown for 2 h is less than 1; it reﬂects the random growth orientation. While for other samples, the (002) lattice plane has the highest TC value thus it can be concluded that the c-axis direction is the preferred growth orientation. Moreover, ZnO nanorods that were grown for 6 h have the highest TC value of the (002) plane among all samples.

The lattice parameters were then evaluated using the (002) lattice plane data and presented inTable 2. The measured lattice parameters of ZnO for all the samples are similar, namely a = 3.25 Å and c = 5.20 Å.

These ﬁndings are in good agreement with the standard data for wurtzite ZnO (a = 3.249 Å, c = 5.205 Å). By using Scherer's formula [48], the average crystallite size of ZnO grown for 2, 4, 6 and 8 h are Fig. 1.X-ray diﬀraction patterns of ZnO nanorods with four diferrent growth times.

Table 1

Texture coeﬃcient of ZnO nanorods with four diﬀerent growth times.

Growth time (hours) Texture Coeﬃcient (TC)

(100) (002) (101) (102) (103) (112)

2 0.392 0.666 0.301 0.497 0.298 0.525

4 0.877 1.491 0.674 1.113 0.667 1.177

6 0.169 2.801 0.385 1.290 0.877 0.477

8 0.303 1.573 0.477 1.638 0.932 1.072

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98.67 Å, 136.88 Å, 241.12 Å, and 246. 94 Å, respectively. Generally, this result demonstrates that the increase in the growth times increases the crystallinity of ZnO.

The FESEM images of ZnO nanorods with four diﬀerent growth times are shown inFig. 2. It is clearly observed that generally the ZnO nanorods have hexagonal shape and grow perpendicularly to the substrates. ZnO nanorods grown for 2 h and 4 h have a slightly random growth direction, while when the growth time is increased to 6 h and 8 h, ZnO nanorods are grown more perpendicularly to the substrate.

The density of the ZnO nanorods are also increased from around 15, 22, 24 up to 25 nanorods/μm²substrate area for the ZnO grown with 2, 4, 6 and 8 h, respectively. This indicates that the nucleation of ZnO on the glass substrate still continue during the growth time. Moreover, the diameter of ZnO nanorods is also increased as the increase of growth time. The average diameter of the ZnO nanorods was approximately 90–165 nm and 120–167 nm for the growth time of 2–4 h and 6–8 h, respectively.

Cross-sectional images of the ZnO nanorods are shown inFig. 3. It is clearly seen that the height of the nanorods increases signiﬁcantly as the growth time increases. The average height of the ZnO nanorods grown for 2, 4, 6 and 8 h is 353, 1067, 1573 and 1800 nm, respectively.

By combining with the surface image inFig. 2, it can be obtained that the calculated surface area of ZnO nanorods grown for 2, 4, 6 and 8 h is around 14, 20, 25 and 27 μm² perμm²substrate, respectively. The FESEM images clearly show that during the growth process, the Zn and O ions in the precursor solution dominantly continue to grow on the

existing ZnO nanorods in c-axis direction; while the growth on a-b plane and new nucleation are also occur. This is in accordance with the XRD pattern inFig. 1which demonstrates that the TC value of (002) plane increase in the increase of growth time. From the FESEM images and the XRD patterns, it can be concluded that the increase in the growth time from 2 h to 8 h greatly enhance the surface area of ZnO nanorods but the highest TC value of the (002) plane is ZnO nanorods grown for 6 h.

Fig. 4 shows the typical room temperature optical absorption spectrum of ZnO nanorods based on various growth times. Strong absorption in the UV region with an absorption edge at a wavelength about 390 nm is the ZnO characteristics as a wide band gap semiconductor [49]. Generally, the absorption intensity in the UV and visible regions increases as the growth time increases, but there is no difference in UV absorption intensity between ZnO nanorods grown for 6 h and 8 h although the surface of both is slightly different. Although ZnO nanorods grown for 6 h have a smaller surface area but have more atoms in the (002) plane, so their electrons can absorb the UV light more effectively and excited into the conduction band.

Fig. 5(a) shows the UV–Vis diffuse-reflectance spectra recorded at room temperature. Each spectra show a sharp edge at a wavelength about 390 nm, which correlates with the absorption edge in the absorbance spectra. The reflectance spectra were then used to calculate the band gap energy (Eg) using the Kubelka-Munk equation[50,51]:

= −

αhν A hν( Eg)1/2

(2)

= = −

α F R R

( ) (1 R ) 2

2

(3) whereα, h, v, A,Egand R are the absorption coeﬃcient, the Plank constant, the light frequency, the constant, the bandgap energy and%

reﬂectance, respectively. The value of F(R) is proportional to an absorption coeﬃcientαin the Tauc equation, soαcan be replaced byF (R).

Bandgap energy was determined using the Tauc plot, which is the plot between (αhν)²and the photon energy (hν) as shown inFig. 5(b).

By extrapolating the linear part of the plot, the band gap is deﬁned Table 2

ZnO lattice parameters in (002) lattice plane.

Growth time (hour)

2θ Volume (Å³) a (Å) c(Å) Crytallite Size (Å)

FWHM

2 34.488 47.57 3.25 5.20 98.67 0.28817

4 34.525 47.68 3.25 5.20 136.88 0.46298

6 34.494 47.66 3.25 5.20 241.12 0.32746

8 34.552 47.64 3.25 5.20 246.94 0.30995

Fig. 2.FESEM images of ZnO nanorods with four diﬀerent growth times.

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whenα=F(R) = 0 or at the intersection of the linear slope with the photon energy axis[52]. By using this method, it is found that the es- timated bandgap energy of ZnO nanorods with growth times of 2 h, 4 h, 6 h, and 8 h are 3.25, 3.24, 3.25 and 3.24 eV, respectively. It can be seen that the energy gap of ZnO nanorods does not signiﬁcantly increase as the growth time increases.

The room temperature photoluminescence spectra of the ZnO nanorods are shown inFig. 6. For all the samples, the photoluminescence spectra of ZnO show three prominent emission bands at 379–420 nm, 450–500 nm, and 600–650 nm. The emission in the range of 430–450 nm are believed to come from the glass substrate. The UV emission in the range of 379–420 nm is related to a near band-edge transition (NBE), which is due to the recombination of photoelectrons in a conduction band with the holes in a valence band [53,54]. Two other broad emission peaks in the visible region are commonly called as deep level emission (DLE) band. These emissions are attributed to the

presence of a new energy state from several crystal defects, such as oxygen vacancy (VO), zinc-vacancy (VZn), oxygen-interstitial (Oi), and zinc-interstitial (Zi)[55]. Although there is a presumption that PL is not enough for characterizing the surface defects precisely, but it is generally believed that the surface oxygen vacancies whose presence is shown as green emission (500–550 nm) and the yellow emission (620 nm) could be assigned to the oxygen interstitial[44]. All samples show a relatively high yellow emission which is attributed as a result of recombination of delocalized electrons with holes in oxygen interstitials [56]. The existence of oxygen interstitials indicates a considerable availability of holes that are beneﬁt for photocatalitic activity of ZnO.

Generally, the UV and visible emissions increase as the growth time increases up to 6 h. This corresponds to the absorbance spectra inFig. 4.

The high absorbance in the UV region shows the more excited electrons leads the more recombination of electrons to the ground states[57,58].

Interestingly, the absorption intensity of ZnO nanorods grown in 6 and 8 h is almost similar, but the emission of ZnO grown for 8 h decreases sharply. This indicates that the ZnO grown for 8 h have a lot of photo excited electrons recombinate non-radiatively; the electrons lose its energy maybe in the form of lattice vibrations or phonons. This may be related to the structural properties obtained from XRD data where the ZnO nanorods grown for 4 and 8 h have a TC value greater than 1 for the lattice planes (102) and (112). The photoexited electrons in both lattice planes may contribute to high non-radiative recombinations, whereas the photoexcited electrons in the lattice plane (002) dominated the ZnO nanorods grown for 6 h tend to undergo radiative recombinations. It is also in accordance with the previous result that show a positive correlation between the PL intensity and the proportion of exposed (002) facet[44].

Fig. 7shows the room temperature Raman spectra of the ZnO nanorods with a growth time of 6 h. The spectra showsﬁve modes of ZnO at 96.9 cm⁻¹, 334.7 cm⁻¹, 376.8 cm⁻¹, 436 cm⁻¹, and 579.8 cm⁻¹, which correspond to E2(low), E2(high)–E2(low), A1(TO), E2(high), and A1(low), respectively[39,59]. As seen inFig. 8, the main dominant sharp peak occurs at 436 cm⁻¹, demonstrating the characteristic peak Fig. 3.FESEM cross-sectional images of ZnO nanorods with four diﬀerent growth times. Scale bar is 500 nm.

Fig. 4.UV-VIS optical absorption spectra of ZnO nanorods grown with four diﬀerent growth times.

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Fig. 5.(a) Reﬂectance spectra and (b) Tauc plot of ZnO nanorods with four diﬀerent growth times.

Fig. 6.Photoluminescence spectra of ZnO nanorods with four diferrent growth

times. Fig. 7.Room temperature Raman spectra of ZnO nanorods grown for 6 h.

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of the hexagonal wurtzite phase ZnO. The peak at 96.9 cm⁻¹refers to the presence of Zn vibration[60], the peak at 334.7 cm⁻¹comes from the vibrations of Zn element associated with oxygen elements[61], the peak at 376.8 cm⁻¹is assigned to the second-order Raman spectrum, originating from the zone-boundary phonons of hexagonal ZnO, and the peak at 579.8 cm⁻¹is assigned to the E1 (LO) mode, which is attributed to the formation of oxygen deﬁciency or other defect states in ZnO[41].

The XPS spectra of ZnO nanorods grown for 6 h is shown inFig. 8. It reveals the highest peak of Zn 2p3/2with a binding energy of 1021.1 eV and Zn 2p1/2with a binding energy of 1044.2 eV that conﬁrmed that the Zn ions exist mostly in the form of Zn⁺²[62]. The spectrum of O 1 s demonstrate an asymmetric peak which indicates the presence of oxygenated-bonded Zn with binding energy of 529.9 eV, oxygenated- bonded CeO/C]O with a binding energy of 531.1 eV and oxygenated bonding with hydroxyl ions OH⁻with a binding energy of 532.7 eV.

The existence of the CeO/C]O bonds is generally associated with the presence of oxygen vacancies while the binding with hydroxyl ions closely related to the photocatalytic activity of ZnO[63]. The presence

of hydroxyl ions facilitates the trapping of photoexcited electrons and holes, thus improves the photocatalytic activity of ZnO[39].

The photocatalytic activity of ZnO nanorods for the photodegradation of MB is shown inFig. 9. The photodegradation eﬃciency is calculated using the following equation[42]:

− × Degradation Efficiency (%): A A

A 100(%)

o (t)

o (4)

whereAoandAtare the MB solution absorbance at the initial time and after a duration time (t), respectively.

The photocatalytic degradation of MB solution can be explained as follows. The photogenerated electrons and holes in ZnO by ultraviolet radiation react with the oxygen and water and result in the free radicals (•O2−) and (•OH). These highly reactive species then break the chemical bond of MB dye structure with the following reaction[21,64–66]:

+ → +

+ →

+ → +

+ →

− +

− −

+ −

+ +

−

h ZnO hv e (CB) (VB)

e O •O

h OH •OH

h H O •h •OH

• OH MB degradation product

• O MB degradation product

2 2

2

Peroxide ion (O2−) is formed when the dissiolved oxygen interact with photogenerated electrons. Then this peroxide takes one proton to yield a superoxide (HO2−) followed by the formation of hydrogen peroxide (H2O2). In other hand, a hydroxyl radical was also produced by the attack of a photogenerated electron to hydrogen peroxide. These reactive radicals and intermediate species react with dyeband degrade them into notoxic organic compounds.

Generally, the photodegradation efficiency is increased by in- creasing the growth time and the ZnO nanorods grown for 6 h has the highest efficiency. The degradation efficiency of MB with ZnO nanorods grown for 2, 4, 6 and 8 h is 77%, 78%, 83%, and 81%, respectively. It has been known that the activity and selectivity of heterogeneous solid catalysts are determined by the surface structure especially depends strongly on their exposed lattice plane [67,68]. The highest Fig. 8.XPS spectra of the ZnO sample grown for 6 h for (a) C1s spectrum, (b) O 1 s and (c) Zn 2p.

Fig. 9.Photodegradation rate of MB by ZnO nanorods grown with four diferrent growth times.

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photocatalytic activity of ZnO nanorods grown for 6 h may correlates with the (002) facets dominated surface with various mechanisms.

First, the exposed (002) facets absorp UV radiation efficiently as shown inFig. 4. It may be due to the honeycomb networks structure scatters light beyond the optical path length hence increases the trapping and harvesting of light[69]. Second, the (002) polar surface consists of a positive Zn-terminated (002) facets and a negative O-terminated (00-2) facets and result in an electric dipole. An internal electricfield induces the photogenerated electrons and holes move to the respective polar facets. This charges separation could reduce the probability of recombination hence these polar lattices planes are highly reactive for reduction and oxidation reactions in the degradation process of dyes [44,70]. Moreover, the highest cystallite size as shown in XRD result gives more electron pathways which accelerates the active redox reactions in the surface area of ZnO[21]. Third, the exposed (002) facets are attributed to the increase of oxygen interstitial on these facets as shown in PL measurements. This condition is advantageous because (002) facets is also known to be facile for adsorption of OH−ions, hence the reaction between holes from oxygen interstitial with OH- ions could increase the photocatalytic efficiency[44]. In addition, a positive Zn-terminated (002) facets can easily adsorb the negatively charged MB molecules. Other researchers also said that the (002) surface facets could also provide the photogenerated electrons and holes with higher redox ability for catalytic reaction[43,71]. Fourth, the high density of the Zn⁺²ionic sites on the (002) facets that adsorb the oxygen will form a thin O2−layer. A thin layer O2−could act as a trapping center of the hole and prevent the electron-hole pair recombination rate in ZnO sample[21].

For the reusability and stability of the ZnO nanorods, our study focused on the ZnO nanorods grown for 6 h because it showed the highest photocatalytic activity among the others. Our previous work showed the stability of the ZnO nanorods in the degradation of methylene blue under UV light irradiation in four successive cycles. It was shown that after four times cycles, the degradation efficiency only decreases by 6%, but after thefifth cycle, efficiency decreases sharply to 35%. It may be due to the saturated dye adsorption on ZnO nanorods as indicated by the appearance of more swelling ZnO nanorods[72].

4. Conclusions

In the present study, ZnO nanorods were synthesized using seed- mediated growth and hydrothermal methods on glass substrates with four different growth times. The ZnO nanorods grown for 6 h have the most aligned and the most perpendicular nanorods to the substrates. It is consistent with the XRD result showing the highest TC value of the (002) plane with the largest crystallite size. Although the ZnO nanorods grown for 6 h has fewer surface area but it has more (002) facets on the surface that results in the highest photocatalytic efficiency (83% within 45 min). This may be due to the exposed polar (002) facets absorb UV radiation more efficiently, promote the reduction of recombination rate, more favorable for oxygen interstitial and facile to the adsorption of OH⁻ions and anionic methyl blue molecules. All of these mechanisms work together to enhance the photocatalytic activity of ZnO nanorods.

Acknowledgements

This research study wasﬁnancially supported by Hibah Publikasi International Terindeks untuk Tugas Akhir Mahasiswa (PITTA) 2017 no. 694/UN2.R31/HKP.05.00/2017 from Universitas Indonesia.

Conﬂict of interest

The authors declare that they have no conﬂict of interest.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, atdoi:10.1016/j.surﬁn.2019.02.009.

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