Synthesis of organo tin halide perovskites via simple aqueous acidic solution-based method

In this paper, we report on the synthesis of organo tin mixed halide perovskites CH 3 NH 3 SnBr x Cl 3-x at room temperature in an aqueous acidic mixture between.. HCl and H 3 PO 2 witho[r]

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Original Article

Thuat Nguyen-Trana,b,*, Ngoc Mai Ana, Ky Duyen Nguyenc, Thi Duyen Nguyenc, Thanh Tu Truongc,**

aNano and Energy Center, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

bDepartment of Fundamental and Applied Sciences, University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang

Quoc Viet, Cau Giay, Hanoi, Viet Nam

cFaculty of Chemistry, VNU University of Science, 19 Le Thanh Tong, Hoan Kiem, Hanoi, Viet Nam

a r t i c l e i n f o

Article history: Received 30 April 2018 Received in revised form 20 August 2018 Accepted 24 August 2018 Available online 31 August 2018 Keywords:

Lead free

Sn-based halide perovskite Raman

Aqueous acid solution Low-cost precursor

a b s t r a c t

Organometal halide perovskites have been studied extensively during the last ten years for their interesting applications in solar cells and optoelectronics One drawback of these materials is the presence of lead inside the compound, thus limiting their practical applications Replacing lead with tin has been one of the implemented approaches for lead-free perovskites In this paper, we report on the synthesis of organo tin mixed halide perovskites CH3NH3SnBrxCl3-xat room temperature in an aqueous acidic mixture between

HCl and H3PO2without the need of protecting perovskites against moisture X-ray diffraction patterns show

that the tin mixed halide perovskites adopt the trigonal phase A detailed analysis of Raman scattering measurements has identiﬁed several low frequency Sn-Cl and Sn-Br modes of these perovskites These results show that the high-quality CH3NH3SnBrxCl3-xcrystals have been successfully synthesized by this

aqueous solution-based method, demonstrating a low-cost approach to replace lead in organo metal halide perovskites for photovoltaic and optoelectronic applications

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Organometal halide perovskites so far have attracted a lot of attention in the academic community, and their excellent prop-erties in solar cells have been proved due to high absorption, long balanced carrier diffusion length, tuneable energy gap and rela-tively simple fabrication processes[1e3] The photovoltaic prop-erties of solar cells depend strongly on the fabrication process, hole transport layers, electron transport layers, nanoporous layers, interfacial microstructures and crystal structures of pe-rovskites[4,5] There are still several key challenges that need to be carefully addressed before organo-metal halide perovskites become feasible for practical application in solar cells One of

these challenges is to synthesize lead-free perovskites with good stability because it is well known that lead is harmful to human's health For example, lead interferes with a variety of body pro-cesses and is toxic to many organs and tissues; including heart, bones, intestines, kidneys, reproductive and nervous systems[6] Candidates for the replacement of Pb in the perovskites include elements in the same group 14 of the periodic table, such as Sn or Ge[7e10] However, it is well known that the stability of the 2ỵ oxidation state decreases when going up the group 14, thus the major problem with the use of these metals is their chemical instability in the required oxidation state Sn-based perovskites have shown excellent mobility in transistors[11], but can also be intentionally or unintentionally doped to become metallic[12,13] It has been demonstrated that when the Sn2ỵion is oxidized to Sn4ỵ, the Sn4ỵacts as a p-type dopant within the material in a process referred to as “self/doping” [12] The ﬁrst report of completely Pb-free and Sn-based perovskite (CH3NH3SnI3) in

so-lar cells was done by Noel and co-workers in 2014 and showed efﬁciencies of over 6% under one sun illumination[14] A recent study by Ogomi et al reported a mixed metal, Sn-Pb, perovskite which allowed the tunability of the band gap of the perovskite

* Corresponding author Nano and Energy Center, VNU University of Science, Room 503, 5thﬂoor, T2 building, 334 Nguyen Trai street, Thanh Xuan, Hanoi, Viet Nam Fax:ỵ84 435 406 137

** Corresponding author

E-mail addresses: thuatnt@vnu.edu.vn (T Nguyen-Tran), tutt@vnu.edu.vn

(T.T Truong)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.08.004

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absorber by varying the Sn:Pb ratio, thus indicating that Sn could be a good choice for metal cation, especially for having lower band gap solar cells[15] Another approach for replacing lead was the anion splitting method in order to obtain “mixed metal halide-chalcogenide” [16] The halogen anions (X ¼ Cl, Br, I) were partially substituted by chalcogenides (Ch ¼ S, Se, Te), i.e one atom per formula unit, to obtain IeIIIeVIeVII2etype semi-conductors with the formula CH3NH3BiChX2such as CH3NH3

Bi-SeI2 and CH3NH3BiSI2 [17] The cation splitting approach have

been also reported in double perovskites such as Cs2InAgCl6[18]

and A2BiXO6 (A¼ Ca, Sr, Ba; X ¼ Br, I) [19] These approaches

were more or less limited because of the chemical stability of these new quaternary perovskites Recently, chloride-based two-dimensional perovskite has drawn huge attention for yielding broadband white photoluminescence [20]; thus tin chloride based perovskites may exhibit interesting properties for opto-electronic applications

Here, we have attempted to synthesize Sn-based perovskites starting with CH3NH3SnxPb1-xI3 for comparison purposes, and

then arriving to the synthesis of CH3NH3SnBrxCl3-x The highlight

of this paper is the simple synthesis of CH3NH3SnBrxCl3-xin acidic

aqueous solution at room temperature by using the low-cost tin (II) chloride dihydrate precursor According to our understanding, there exist very few reports of organo tin halide perovskites uti-lizing an acidic aqueous solution as a reaction environment since Sn2ỵis easily oxidized to Sn4ỵunder a moisture condition The acidic solution was composed of a mixture between HCl and H3PO2, which had remarkable advantages, such as affordable cost

and availability, compared to other organic solvents, such as dimethylformamide (DMF), gamma-Butyrolactone (GBL) and dimethyl sulfoxide (DMSO) [21], in synthesizing Sn-based perovskites

2 Experimental

2.1 Synthesis of precursors

CH3NH3Br was synthesized by putting 45 ml of CH3NH2(25%)

into a 2-neck round bottomﬂask and 67 ml of 4.5 M HBr into a dropping funnel The reaction was taken place under a nitrogen environment at 0C When the temperature of methylamine in the round bottomﬂask was cooled down to 0C, the dropping funnel

was slowly opened to let HBr drop into the round bottomflask The reaction was kept at C for 150 After the reaction had completely taken place, the solution was transferred to an evapo-ratingflask and the solvent was removed by rotary evaporation at 60C After almost all solvent had been evaporated, yellow crystals left behind was taken out of the evaporating flask, filtered and washed by diethyl ether Finally, the product was dried overnight in a vacuum oven After the synthesis, the product was kept in a refrigerator at 0C

CH3NH3I was synthesized from 10 ml of CH3NH2(25%) and 10 ml

of 57% HI in a 2-neck round bottom ﬂask following the process similar to the one used for synthesizing CH3NH3Br described above

The obtained CH3NH3I white powder was stored in a refrigerator at

0C

CH3NH3Cl synthesis process was similar to the one used for

synthesizing CH3NH3Br and CH3NH3I ml of CH3NH2(25%) were

put into a 2-neck round bottomﬂask ml of concentrated HCl was mixed with ml of distilled water and transferred to a dropping funnel The reaction was taken place at room temperature The product CH3NH3Cl in this experiment was white powder but less

shining than CH3NH3I

2.2 Synthesis of perovskites

The synthesis of CH3NH3SnBrxCl3-xwas carried out as follows:

6 ml of distilled water were put into a 2-neck round bottomﬂask, followed by 4.3 ml of concentrated HCl and 1.3 ml of H3PO2(50%) to

form an aqueous solution of HCl and H3PO2with the molar ratio of

HCl:H3PO2¼ 3:1 This acidic mixture was heated to 100C under

nitrogen environment before 1.128 g of tin (II) chloride dihydrate (SnCl2.2H2O) was added and stirred until the solution was

completely transparent Then 0.340 g of CH3NH3Cl (for

synthesiz-ing CH3NH3SnCl3) or 0.560 g of CH3NH3Br (for synthesizing

CH3NH3SnBrCl2) was added and kept for 30 After the reaction

had taken place, the solvent was evaporated until about ml of solution left Cooling down the solution allowed CH3NH3SnBrxCl3-x

crystals to grow gradually in 24 h Finally, white rod-shaped crystals appeared and the product wasﬁltered and dried under vacuum at 60C

For comparison purposes, the synthesis of CH3NH3SnxPb1-xI3

was carried out as follows: A mixture of solution of SnI2and PbI2

(the molar ratio of SnI2:PbI2¼ x:(1-x)) and CH3NH3I in

gamma-Butyrolactone (GBL) was heated to 130 C under nitrogen envi-ronment for 2.5 h After the reaction had taken place, the obtained solution exhibited high viscosity and the perovskite black powder precipitation was observed by adding dichloromethane (DCM) into the solution Then the powder was ﬁltered and ﬁnally dried at 100C under vacuum for 24 h

2.3 Characterization

Structural properties of perovskite crystals were characterized on a X-ray diffractometer, D8 ADVANCE Brucker system, by using Cu-Karadiation at the wavelength of 1.5406 Å, and on a Raman spectroscopy system, Horiba LabRAM, with the excitation wave-length of 632 nm The morphology was studied by scanning elec-tron spectroscopy (SEM), on a Nova Nanosem 450 SEM FEI system, and also on a conventional optical microscope

3 Results and discussion

Fig 1a shows X-Ray diffraction (XRD) patterns of the synthe-sized CH3NH3Sn0.5Pb0.5I3powder sample in comparison with those

previously reported in the literature, for CH3NH3SnI3 with cubic

structure[22]and for CH3NH3PbI3with tetragonal structure[23]

These results from the literature suggest that the crystalline structure of CH3NH3SnxPb1-xI3perovskite would change from cubic

to tetragonal when the ratio of Sn:Pb (or the parameter x) decreases from to Therefore, determining the phase structure of the synthesized product could be considered as an indirect method to conﬁrm the existence of tin in the perovskite compound The most obvious feature that helps us distinguish CH3NH3PbI3(tetragonal

structure I4/mm) from CH3NH3SnI3 (cubic structure Pm3m) is to

investigate XRD peaks with diffraction angle around 28.Fig 1b shows high-resolution XRD patterns of the same sample CH3NH3Sn0.5Pb0.5I3, in comparison with CH3NH3PbI3 and

CH3NH3SnI3 For CH3NH3PbI3, as previously reported [23], we

observed two peaks, corresponding to the reﬂection planes 004 and 220 For CH3NH3SnI3, our simulated XRD pattern shows only one

peak, corresponding to the planes 200[22] There are two remarks that we could draw from the XRD shown onFig Firstly, tin does contribute to the perovskite structure with a concentration value being lower than the intended Sn:Pb ratio of 1:1, as described in the synthesis section above, so the structure is tetragonal Secondly, the synthesized powder has the crystalline structure of CH3NH3PbI3

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requires more advanced techniques In this study, a small and very expensive quantity of SnI2 has been provided, thus limiting our

further characterization Still from the XRD pattern, our calculation of lattice parameters of tetragonal structure of CH3NH3Sn0.5Pb0.5I3

shows that a¼ 8.832 Å and c ¼ 12.598 Å

Fig 2shows the energy-dispersive X-ray spectrum (EDX) and SEM micrographs of the synthesized CH3NH3Sn0.5Pb0.5I3powder

The existence of tin in the obtained product is clearly conﬁrmed Another noticeable feature onFig 2is that the oxygen peak found in the EDX spectrum, indicating that a part of the synthesized powder has been oxidized It is highly likely that tin has been oxidized On the other hand, we also attempted to synthesize CH3NH3SnxPb1-xI3with different values of x such as 0.3, 0.75 and

However, in the case of x ¼ 0.75 and of x ¼ 1, the synthesized

Fig (a) XRD patterns of the synthesized CH3NH3Sn0.5Pb0.5I3compared with those of the reported CH3NH3SnI3[22]and CH3NH3PbI3[23](b) High resolution XRD patterns from 27

to 30 degrees of the synthesized CH3NH3Sn0.5Pb0.5I3, CH3NH3SnI3[22]and CH3NH3PbI3[23]

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powder was degraded so quickly by oxidation that on XRD patterns we obtained only the signature of amorphous tin oxide In the case of x ¼ 0.3, the synthesized powder's XRD pattern had similar feature in comparison with the case x¼ 0.5 shown onFig This suggests that CH3NH3SnxPb1-xI3perovskite with high tin

concen-tration (x 0.75) is very sensitive and easily decomposed when exposed to air, thus tin iodide perovskites need extremely special conditions for applications

In contrast with the instability of tin iodide perovskite, tin mixed Br-Cl (CH3NH3SnBrxCl3-x) crystals were very stable after being

synthesized from a tin (II) chloride dehydrate precursor and methylammonium halide (CH3NH3X, X ¼ Cl, Br) in an acidic

aqueous solution of HCl/H3PO2.Fig 3, and respectivelyFig 4, show

SEM micrographs, EDX spectra and elemental analysis of perovskite

powder of preparation formula CH3NH3SnCl3, and respectively of

CH3NH3SnBrCl2, after gradually crystallized from the solution for

24 h A photograph taken on an optical microscope of CH3NH3SnCl3

powder is illustrated on Fig 2S of the supporting information, showing obtained crystals with transparent appearance and an elongated shape For the sample corresponding to the preparation formula CH3NH3SnBrCl2, an elemental analysis revealed 28.4 w% of

Cl and 29.2 w% of Br, corresponding to a molar halide ratio Br:Cl of about 0.94:2.06, or a deduced formula CH3NH3SnBr0.94Cl2.06 This

composition will be further discussed with XRD powder reﬁnement in the next part

Fig 5shows experimental XRD patterns of CH3NH3SnCl3, the tin

(II) chloride dehydrate precursor SnCl2.2H2O and CH3NH3Cl, in

comparison with the simulated XRD pattern of CH3NH3SnCl3

Fig (a,b,c) SEM micrographs and (d) EDX spectrum of the synthesized CH3NH3SnCl3powder

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triclinic phase[22] There is a pronounced mismatch between the experimental XRD pattern of CH3NH3SnCl3 with the precursors'

one, implying that the obtained crystals are clearly not SnCl2.2H2O

nor CH3NH3Cl but theﬁnal product of the equimolar reaction

be-tween these precursors It is well known that organometallic halide perovskite materials undergo phase transitions when decreasing temperature In the case of CH3NH3SnCl3 there are three phase

transitions when temperature is decreased from above 463 K to below 307 K Theﬁrst phase transition from cubic to rhombohedral is around 463 K The second is around 331 K, where the rhombo-hedral phase changes to the monoclinic phase And around 307 K, CH3NH3SnCl3 is transferred to the triclinic phase [22] or to the

trigonal phase[24] Since we let CH3NH3SnCl3crystallize at room

temperature, we expected that the structure of the product would be triclinic or trigonal A comparison between CH3NH3SnCl3

triclinic, trigonal simulation and experimental CH3NH3SnCl3XRD

patterns[22]is illustrated onFig The reﬁnement of the triclinic phase yielded following parameters a¼ 5.7316 Å, b ¼ 8.2538 Å, c¼ 7.9227 Å,a¼ 90.3608,b¼ 93.0415,g¼ 90.2468whereas the

reﬁnement of the trigonal phase gave a ¼ b ¼ c ¼ 5.7173 Å and

a¼b¼g¼ 92.1060 Details of the trigonal parameters are shown

onTable 1, and the corresponding Rietveld analysis can be found on Figure S3from the provided supplementary information The XRD pattern of the synthesized CH3NH3SnCl3 crystals was perfectly

matched with the pattern based on the reference[24], which im-plies the trigonal structure of the synthesized CH3NH3SnCl3

pow-der For further XRD powder Rietveld analysis, we chose the trigonal phase as it possesses higher crystalline order than the triclinic one

For CH3NH3SnBrxCl3-x, after 24 h of crystallizing, obtained

crystals also had an elongated shape but its colour is slightly different in comparison to CH3NH3SnCl3 While CH3NH3SnCl3

crystals were rather transparent, CH3NH3SnBrxCl3-xexhibited pale

yellow appearance, as shown inFigure S1 Since we have mixed SnCl2.2H2O and CH3NH3Br with an equimolar ratio, the synthesized

perovskites formula is expected to be CH3NH3SnBrCl2 As shown on

Fig 6, the experimental XRD pattern of CH3NH3SnBrCl2is similar to

that of CH3NH3SnCl3 A perfect match between the experimental

and simulation of CH3NH3SnBrCl2trigonal structure indicating that

the material, with the preparation formula CH3NH3SnBrCl2, is in the

trigonal phrase The performed reﬁnement of the preparation for-mula CH3NH3SnBrCl2, by adjusting also the halide site occupation

factor (SOF) of Br and Cl, gave a¼ 5.7833 Å anda¼ 91.5462(as shown on Table 1, and the corresponding Rietveld analysis is illustrated onFigure S4) We can see that the unit size of experi-mental formula CH3NH3SnBrCl2 is higher than that of

CH3NH3SnCl3 This is due to the presence of Br atoms, replacing Cl

atoms in the lattice of the synthesized crystals, which causes the lattice parameter to increase when increasing the ion radii of the halogen atoms The refinement figured out also that the SOF of Cl was 0.6493, and the SOF of Br was 0.3507 This corresponds to a molar halide ratio Br:Cl of 1.05:1.95, or a refined formula CH3NH3SnBr1.05Cl1.95 We performed supplementary refinement of

XRD powder pattern of the preparation formula CH3NH3SnBrCl2by

using: (i) an isoelectronic dummy (V) for the average of BrCl2

yielding a¼ 5.7827 Å anda¼ 91.571(as shown onTable S1, and

the corresponding Rietveld analysis is illustrated onFigure S5), (ii) an isoelectronic dummy (Cu) for the average of Br2Cl yielding

a ¼ 5.7832 Å and a ¼ 91.5610 (as shown on table S1, and on

Figure S6) We can see a very slight difference of the obtained unit size (a) and the angle (a) of the trigonal structure of both three different choices of reﬁnement parameters for the halide site If we compare the values of a and ofawith the one in the literature, for example in the reference [24], we see that a combination of a¼ 5.783 ± 0.001 Å anda¼ 91.56± 0.01should be corresponded

Table

Details of Rietveld analysis of XRD powder patterns of the preparation formula CH3NH3SnCl3and CH3NH3SnBrCl2

Preparation formula Atom/Unit x y z a () a(degree) Site occupation factor CH3NH3SnCl3 CH3NH3ỵa 0.017 (4) ¼ x ¼ x 5.717 (3) 92.106 (0)

Sn 0.517 (4) ¼ x ¼ x

Cl 0.460 (6) ẳ x 0.017 (4)

CH3NH3SnBrCl2 CH3NH3ỵ 0.034 (7) ¼ x ¼ x 5.783 (3) 91.546 (2)

Sn 0.575 (6) ¼ x ¼ x

Cl 0.502 (4) ¼ x 0.069 (5) 0.649 (3)

Br 0.350 (7)

aIsoelectronic K (19) was used as a dummy.

Fig Experimental XRD patterns of SnCl2.2H2O, CH3NH3Cl, and CH3NH3SnCl3in

comparison with the simulated XRD patterns of CH3NH3SnCl3(triclinic and trigonal)

Fig Experimental XRD patterns of CH3NH3SnCl3and CH3NH3SnBrCl2in comparison

with the precursor CH3NH3Br and the simulated XRD pattern of CH3NH3SnBrCl2

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to a chemical formula CH3NH3SnBrCl2 (or x ¼ for

CH3NH3SnBrxCl3-x) The Rietveld analysis that we used in this

report may not be sensitive enough when changing the halide Br:Cl composition On the contrary, we had proved that the halide composition had been quite sensitive when reﬁning XRD of single crystals of CH3NH3PbI3-xBrx[25] In this study, we observed that the

values of a and a could reveal the halide composition when comparing with similar results So we decided to use a value x¼ 1, which is in agreement with the EDX elemental analysis giving CH3NH3SnBr0.94Cl2.06 and with the SOF reﬁnement giving

CH3NH3SnBr1.05Cl1.95, for the subsequence of this report

Fig 7shows Raman spectra of CH3NH3SnBrCl2and CH3NH3SnCl3

powder withﬁtted curves (dash lines) obtained from multi Gaussian peaks adjustment The inset shows the superposition of the normalized intensity at the lowest frequency (about 70 cm1) of these two samples The nearly perfect match of the normalized intensity of this peak reveals that this is an artefact of the Raman measurement at the low frequency range We suggest that this peak may be due to the edge of the notchﬁlter and the broadening of the excited laser As a consequence, this peak is excluded from the multi Gaussian peaks adjustment.Table 2summarizes detailed results of the adjustment of Raman spectra of CH3NH3SnBrCl2 and CH3NH3SnCl3 and

corre-sponding Raman mode suggestion In contrast to Fourier transform infrared absorption spectra of these two samples, on Figure S7, showing little difference of peak positions, which characterize vibrational modes of the organic cation CH3NH3ỵ, Raman spectra

illustrated better vibrational modes of inorganic bonds It is quite surprised that no Raman mode relating to Sn-O has been found on these two samples, thus indicating those tin mixed halide perovskites are really stable against oxidation by moisture For the CH3NH3SnCl3

sample, the adjustment deduced a Sn-Cl rocking mode at around 114.8± 0.7 cm1[26], a Sn-Cl symmetrical stretching mode at around 140.2 ± 0.2 cm1 [26], a Sn-Cl bending mode at around 179.2± 0.2 cm1[27], and a Sn-Cl asymmetrical stretching mode at

around 261± 0.4 cm1[28] For CH3NH3SnBrCl2, from the adjustment

results, we suggest that the Sn-Br symmetrical stretching mode is at around 100± cm1[29], and that the Sn-Br bending mode is at

around 127± cm1[27] It is quite interesting that the signature of Cl

found in the CH3NH3SnBrCl2was relatively weak A Sn-Cl bending

mode at around 184.6± 0.6 cm1[27], and respectively a Sn-Cl

rocking mode at 115± cm1[26], were revealed with a relative

height of about 0.8, and respectively 3.2, over 10 Conclusion

The paper shows various processes for synthesizing different types of perovskite containing tin Apart from using organic solvent-based methods for obtaining CH3NH3Sn0.5Pb0.5I3, we

illustrated that lead-free tin halide perovskites CH3NH3SnBrxCl3-x

(x¼ and 1) have been successfully synthesized via an acidic aqueous solution at room temperature from the low-cost tin (II) chloride dehydrate precursor The structural properties of the

Table

Details of Raman modes obtained after multi Gaussian peaks adjustment of Raman spectra CH3NH3SnCl3(Relative

adjustment errora0.04)

Frequency (cm1) 114.8± 0.7 140.2± 0.2 179.2± 0.2 261.0± 0.4

FWHM (cm1) 69.5 12.9 44.8 16.9

Relative height 4.0 1.1 10.0 0.8

Mode description Sn-Cl rocking[26] Sn-Cl symmetrical stretching[26]

Sn-Cl bending[27] Sn-Cl asymmetrical stretching[28]

CH3NH3SnBrCl2(Relative

adjustment error 0.005)

Frequency (cm1) 100± 115± 127± 184.6± 0.6

FWHM (cm1) 67 18 20 23

Relative height 10.0 3.2 5.2 0.8

Mode description Sn-Br symmetrical stretching[29]

Sn-Cl rocking[26] Sn-Br bending[27] Sn-Cl bending[27]

aThe relative adjustment error is equal to the adjustment value ofc2divided by total peaks area.

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obtained perovskites have been investigated and characterized by XRD and Raman spectroscopy Raman spectroscopy results show the existence of tin in the perovskite structure with clearﬁtted results of Sn-Cl and Sn-Br rocking, bending, and stretching modes XRD patterns show that CH3NH3Sn0.5Pb0.5I3adopts the tetragonal

structure, while CH3NH3SnCl3 and CH3NH3SnBrCl2crystals adopt

the trigonal phase These results pave the way for our future study for applications of organo tin halide perovskites in optoelectronics as well as in solar cells

Acknowledgments

This research is funded by the Vietnam National University, Hanoi (VNU) under project number QG.17.26 The authors acknowledge fruitful SEM measurements carried out by Mr Sai Cong Doanh and Raman measurements carried out by Dr Nguyen Viet Tuyen from the Faculty of Physics, VNU University of Science The authors would like to thank the Vietnam National University Hanoi for research equipment from the project named “Strength-ening research and training capacity inﬁelds of Nanoscience and Technology, and Application in Medical, Pharmaceutical, Food, Biology, Environmental protection and Climate Change adaptation in the direction of sustainable development”

Appendix A Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.jsamd.2018.08.004

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