Journal of Physics and Chemistry of Solids 72 (2011) 817–823

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

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

Characterizations of octahedral zinc oxide synthesizedby sonochemical method

Chat Pholnak n, Chitnarong Sirisathitkul, David J. Harding

Molecular Technology Research Unit, School of Science, Walailak University, Nakon Si Thammarat 80160, Thailand

a r t i c l e i n f o

Article history:

Received 24 August 2010

Received in revised form

7 March 2011

Accepted 11 April 2011Available online 21 April 2011

Keywords:

A. Oxides

C. X-ray diffraction

D. Optical properties

97/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jpcs.2011.04.005

esponding author. Tel.: þ6675 673230; fax:

ail address: chat432@hotmail.com (C. Pholna

a b s t r a c t

The ultrasonic reaction of zinc nitrate hexahydrate (Zn(NO3)2 �6H2O) and hexamethylenetetramine

(C6H12N4) was investigated by varying the concentration of the reactants, the irradiation time, and the

type of sonicator. The morphology, composition, and phase structure of the products were character-

ized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR)

and ultraviolet–visible (UV–vis) spectroscopy. Octahedral zinc oxide (ZnO) micropowders were formed

at low concentrations, 0.05 M, of Zn(NO3)2 �6H2O and C6H12N4 in both lab-made sonicator and

commercial ultrasonic bath. However, at concentrations between 0.1 and 1.0 M Zn(NO3)2–C6H12N4

mainly plate-like zinc hydroxide nitrate hydrate (Zn5(OH)8(NO3)2(H2O)2) resulted with only a small

fraction of ZnO, irrespective of the irradiation time employed, highlighting the sensitivity of the system

to the concentration of the starting materials. Heat treatment of Zn5(OH)8(NO3)2(H2O)2 at 350 1C in air

affords a ZnO phase of irregular morphology. Octahedral ZnO is found to exhibit slightly lower IR

absorption and similar UV absorption to that of commercial prismatic hexagonal ZnO, although an extra

peak due to small quantities of Zn5(OH)8(NO3)2(H2O)2 is observed.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Rapid development in ultrasound technology now permitssonochemical preparation of many materials such as gold parti-cles, nickel alumina, cadmium sulfide, zinc sulfide, and zinc oxide(ZnO) [1–5]. In this technique, ultrasound waves supply the highenergy needed for chemical reactions via the process of acousticcavitations involving the formation, growth, and implosive col-lapse of bubbles in the liquid. During the cavitational collapse,intense local heating of the bubbles occurs for a few microsecondsresulting in high-velocity interparticle collisions whose impactcan be used for synthesis [6]. The sonochemical method hasseveral advantages over other methods. Being a seedless, surfac-tant, and template free method, complicated procedures for theremoval of seeds, surfactants or templates can be avoided.Furthermore, it is suitable for large-scale production in terms ofrapidity, low cost, simplicity, and low energy demand [7].

Zinc oxide is one of the most important multifunctional semi-conductors which can be grown at low temperature. It has a wideenergy bandgap and large excitation binding energy with highchemical stability and radiation resistance. For these reasons, it hasbeen studied for applications in short-wavelength lasers and lightemitting diodes [8–12], sensors [13–17], solar cells [18–23], transis-tors [24,25], varistors [26–29], piezoelectric devices [30,31], and

ll rights reserved.

þ6675 672004.

k).

microwave absorbers [32,33]. Furthermore, nanoscale ZnO has beenimplemented in functional composites such as an additive to modifypiezoelectric materials [34] and polymers [35]. For instance, PZTthick films have been doped with ZnO nanowhiskers to enhancepiezoelectric and mechanical properties [36]. Thus the particularapplication envisaged typically requires ZnO of a specific shape andZnO particles in the form of wires, rods, tubes, sheets, disks, flowers,cups, tetrapods, multipods, octahedrons, spheres, and hollow spherescan be prepared by sonication [5,7,25,37–44]. Ultrasonic irradiationunder aerobic conditions commonly uses a frequency of 20–40 kHzwith a power between 50 and 1500 W. The morphologies of theproducts depend on the starting materials employed, mixture con-centrations and its pH, the nature of any additive surfactant orchelating reagent, and the irradiation conditions. For example, Zhanget al. [37] used this technique under aerobic conditions at 280–300 1Cto fabricate ZnO nanorods and trigonal-shaped ZnO ultrafine particlesfrom zinc acetate dihydrate (Zn(CH3COO)2 �2H2O) and stearic acidadded in paraffin oil. Mazloumi et al. [38] synthesized cauliflower-like ZnO nanostructures from a mixture of Zn(CH3COO)2 �2H2O andtriethanolamine (TEA). Post-treatment by ultrasonic irradiation for2 h gave rise to intense green emission because of an increase inthe density of surface defects. In contrast, Wahab et al. [39]synthesized single crystalline needle-shaped ZnO nanorods usingZn(CH3COO)2 �2H2O and sodium hydroxide (NaOH). Xiao et al. [40]fabricated rod-like and sheet-like ZnO nanopowders using a variety ofZn compounds and NaOH, and the effect of the pH on the morphol-ogy of these ZnO powders was observed. Yadav et al. [45] synthesizedZnO nanoparticles with an ethylenediamine (EDA) chelating agent in

C. Pholnak et al. / Journal of Physics and Chemistry of Solids 72 (2011) 817–823818

a mixed solution of zinc nitrate (Zn(NO3)2) and NaOH. Hu et al. [7]synthesized linked ZnO rods via the sonochemical method at 90 1Cfor 10–120 min, using zinc nitrate hexahydrate (Zn(NO3)2 �6H2O)and hexamethylenetetramine (C6H12N4) as starting materials. Withsimilar reagents, Jung et al. [41] fabricated highly crystalline ZnOnanostructures in the forms of nanorods, nanocups, nanodisks, nano-flowers, and nanospheres.

The chemical reactions between Zn(NO3)2 �6H2O and C6H12N4

in aqueous solution are summarized in Eqs. (1)–(5) below

Zn NO3ð Þ26H2OþH2O-Zn NO3ð Þ2þ7H2O ð1Þ

C6H12N4þ6H2O-4NH3þ6HCHO ð2Þ

Zn NO3ð Þ2þ7H2O-Zn2þþ2NO3

�þ7H2O ð3Þ

NH3þH2O-NH4þþOH� ð4Þ

NO3-þH2Oþ2e�-NO2

�þ2OH� ð5Þ

The precipitation from the solution is initiated from crystallinenuclei. By ultrasonic irradiation, the nuclei growth results in acrystalline ZnO structure [39] according to either Eq. (6) or (7)

Zn2þþ4OH�-Zn OHð Þ4

2-��!ÞÞÞ

ZnOþH2Oþ2OH� ð6Þ

Zn2þþ2OH�-Zn OHð Þ2��!

ÞÞÞZnOþH2O ð7Þ

In the reaction, a unique symmetry superstructure like octa-hedral ZnO may be formed. Such structures have a large surface tovolume ratio and are advantageous in gas sensors and photo-catalytic applications [15–17,46–49]. From a structural point ofview, octahedral ZnO is of interest because of its relation to eighttetrahedral crystals, i.e. being bounded by a negatively charged(0 0 0 1) surface [50]. Furthermore, the formation of a tetrapodZnO structure is based on octahedral nuclei according to the octa-twin model [50,51]. Thus far, there have been relatively fewreports on the synthesis of octahedral ZnO structures. Zhang et al.[42] fabricated octahedral ZnO particles sonochemically fromZn(CH3COO)2 �2H2O and ethylene glycol (EG) in a 2-step polyolprocess with heat treatment at 350–600 1C and hydrolysis.According to the work by Qu and Jia [46], heat treatment alsogives rise to the formation of octahedral ZnO mesoscale super-structures from the as-prepared zinc hydroxide. In this work, wesynthesized octahedral ZnO micropowders via sonochemicalroute using Zn(NO3)2 �6H2O and C6H12N4 as starting materials.The effects of the concentrations of the reagents, irradiation time,and the heat treatment process on the structural, morphological,and optical properties of ZnO are examined.

2. Experimental

2.1. Synthesis of ZnO micropowder

Zinc nitrate hexahydrate (Zn(NO3)2 �6H2O, Z99.0% purity),hexamethylenetetramine (C6H12N4, Z99.0% purity), and deio-nized water of analytical grade were used as starting materialswithout further purification. Two series of ZnO micropowderwere prepared sonochemically. In the first series (samples A1–A4),the molar concentrations of Zn(NO3)2 �6H2O and C6H12N4 were0.05–1.0 M. The concentration of the reagents was kept the samein the series at a ratio of 1:1 and an irradiation time of 60 min. Inorder to investigate the growth mechanism and morphologies of theprecursors with different reaction times, another series (samplesB1–B4) was fabricated with varying reaction times between 30 and240 min. In this series, the concentrations of Zn(NO3)2 �6H2O andC6H12N4 were fixed at 1.0 M.

In a typical procedure for samples A1–A4, an aqueous solutionof zinc nitrate was prepared by adding Zn(NO3)2 �6H2O (0.05 M,0.744 g) into 50 ml deionized water at room temperature in a100 ml glass beaker without sealing under magnetic stirring.Subsequently, C6H12N4 (0.05 M, 0.350 g) was added to the solu-tion under magnetic stirring at 50 1C. Once the solution turned toa stable viscous white colloid, the stirring was continued foranother 10 min. The colloid was then sonicated under ambientconditions for 60 min using an ultrasonic apparatus. This appa-ratus consisted of a lead zirconate titanate (PZT) ultrasonictransducer (Chanel Industries, Inc.), a function generator, a poweramplifier, and a digital multimeter. The disk-shaped transducer,12 mm thick with a diameter of 63 mm, was driven at itsresonance frequency of 35 kHz with a maximum power outputof up to 40 W at room temperature in air. The vibrating energywas transferred from the transducer to the colloid by the ultra-sonic gel. During sonication, the temperature of the colloid rose toaround 70 1C. After the reaction completed, a white precipitatewas collected by filtration and thoroughly rinsed with distilledwater and ethanol. Finally, the precipitate was dried in an oven at60 1C in an air atmosphere. In the case of samples B1–B4, thedifference in the procedure was that the precipitate was centri-fuged and then subjected to heat treatment at 350 1C for 12 h inan air atmosphere with a heating rate of 30 1C/min to comparethe phase and morphology before and after annealing.

2.2. Characterization of ZnO micropowder

X-ray diffraction (XRD, Philips X’ Pert MPD) of the synthesizedproducts was carried out with the CuKa radiation (l¼1.54056 A).Their size and morphology were inspected by scanning electronmicroscopy (SEM, FEI Quanta 400) operating at 25 kV. The Fouriertransform infrared (FTIR) spectra of samples A1 and B4 wereacquired using a Perkin Elmer Spectrum One spectrophotometerfrom pellets of potassium bromide (KBr) of the synthesizedpowder. A milligram of each sample was mixed and crushed withabout 0.2 g of dried KBr. The homogeneous powder was then filledin a hollow-cylindrical mold 13 mm in diameter, and pressed at6 tons under ambient conditions to obtain a pellet of around500 mm thickness. The FTIR measurements were performed onthe pellets placed in a window of IR radiation with measure-ments recorded between 350 and 4000 cm�1 with a resolution of4 cm�1. The optical absorption spectra of the samples A1–A4 wereinspected using an ultraviolet–visible (UV–vis) spectrophotometer(Shimadzu, UV-1700) with a double beam system of halogen anddeuterium light sources. To prepare samples for the spectroscopicanalysis, 5 mg of powder was dispersed in 10 ml NaOH (0.1 M)solution at room temperature and then transferred to a quartz cellof the spectrophotometer.

To allow comparison of the results with another ultrasonicapparatus, the same experimental procedure was conductedusing 0.05 M Zn(NO3)2–C6H12N4 in a commercial ultrasonic bath(Kerry KC3-38 kHz, 75 W). Morphology, UV–vis, and FTIR spectraof our products were also compared with those of commercialZnO powder.

3. Results and discussion

3.1. Structural characterization

The XRD spectra of the products with various concentrationsof Zn(NO3)2–C6H12N4 aqueous solutions are shown in Fig. 1 andthe phase is summarized in Table 1. In Fig. 1(a), the sample A1prepared from 0.05 M Zn(NO3)2–C6H12N4 aqueous solutions pos-sesses a hexagonal wurtzite ZnO structure with a space group of

C. Pholnak et al. / Journal of Physics and Chemistry of Solids 72 (2011) 817–823 819

P63mc (ICSD no. 10-080-0074). The yield of the octahedral ZnOmicropowder is very low at approximately 5.4%. However, samplesA2–A4 prepared with higher concentrations of Zn(NO3)2–C6H12N4

solutions (0.1–1.0 M) have different diffraction profiles character-ized as zinc hydroxide nitrate hydrate (Zn5(OH)8(NO3)2(H2O)2)(JCPDF no. 01-072-0627) as exemplified in Fig. 1(b). According tothe reaction in Eq. (6), the ultrasonic energy density might not beenough to disrupt the Zn(OH)4

2� in the concentrated solutionbecause the solution has higher viscosity and it does not transferthe ultrasonic energy thoroughly. Since the formation of othercompounds may competitively occur in the solution [50], theformation of Zn5(OH)8(NO3)2(H2O)2 by Eq. (8) tends to be dominantin the case of highly concentrated zinc nitrate solutions

5Zn2þþ8OH�þ2NO3

�þ2H2O-Zn5 OHð Þ8 NO3ð Þ2 H2Oð Þ2 ð8Þ

Fig. 1(c) shows the XRD spectrum of sample B4, prepared from1.0 M Zn(NO3)2–C6H12N4 solutions with an irradiation time of240 min. The as-synthesized sample is identified as monoclinicZn5(OH)8(NO3)2(H2O)2 (JCPDF no. 00-024-1460). However, thephase transforms to the hexagonal wurtzite ZnO structure (ICSDno. 01-089-7102) after heat treatment at 350 1C as shown inFig. 1(d). As a result of thermal decomposition, Zn5(OH)8(NO3)2

(H2O)2 releases water molecules, nitrous oxide and oxygen gasto the atmosphere. The ZnO phase is formed by the reaction

Fig. 1. XRD patterns of (a) sample A1 (ZnO), (b) sample A2 (Zn5 OHð Þ8ðNO3Þ2

ðH2OÞ2), (c) sample B4 before annealing (Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2), and (d) sample

B4 after annealing at 350 1C (ZnO).

Fig. 2. SEM micrographs of sample A1 showing the octahedral morphology

(a) a cluster of octahedral ZnO and (b) magnified image of a single ZnO

octahedron.

Table 1Effect of the molar concentrations of the reagents on the phase of the samples prepared with 60 min irradiation, and the irradiation times as well as heat treatment on the

phase of the samples prepared from 1.0 M solutions.

Sample Irradiationtime (min)

Molar concentration (M) Crystalline phase Morphology

ZnðNO3Þ2�6H2O C6H12N4 As-synthesized Annealing As-synthesized Annealing

A1 60 0.05 0.05 ZnO � Octahedron �

A2 60 0.1 0.1 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 � Plate-like �

A3 60 0.5 0.5 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 � Plate-like �

A4 60 1.0 1.0 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 � Plate-like �

B1 30 1.0 1.0 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 ZnO Plate-like Irregular

B2 60 1.0 1.0 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 ZnO Plate-like Irregular

B3 120 1.0 1.0 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 ZnO Plate-like Irregular

B4 240 1.0 1.0 Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 ZnO Plate-like Irregular

C. Pholnak et al. / Journal of Physics and Chemistry of Solids 72 (2011) 817–823820

in Eq. (9).

Zn5 OHð Þ8 NO3ð Þ2 H2Oð Þ2-5ZnOþ6H2OþN2Oþ2O2 ð9Þ

3.2. Morphological characterization

The effect of the reagent concentrations on the morphology ofthe samples A1–A4 seen by SEM is summarized in Table 1. InFig. 2(a), ZnO synthesized from 0.05 M Zn(NO3)2–C6H12N4 has anoctahedral structure whose baseline is over 5 mm. Fig. 2(b) showsthe magnified images of a single octahedral particle that issymmetrically formed from identical tetrahedral structures [50].The octa-twin model illustrated in the literature [46,50,51] showsthat tetrahedral Zn(OH)4

2� or Zn(OH)2 cores are formed bychemical bonds between Zn2þ and OH� and grow as a result ofprecursor interaction, diffusion, and layered deposition. Due to

Fig. 3. Schematic diagram illustrating the formation of octahedral ZnO crystals fo

Zn(OH)42� core (adapted from [46]).

Fig. 4. Typical SEM micrographs of powder obtained from 0.1–1.0 M Zn NO3ð Þ2- C6H1

annealing at 350 1C (ZnO).

polar Zn–O bonds, positive zinc and negative oxygen planesreconstruct at an atomic level to maintain electrical neutrality[52]. During ultrasonic irradiation, these cores become ZnO nucleiwith unchanged geometry, and then the aggregation of the growthunits is in an ordered manner due to specific interaction of theparticles’ surface [46]. The growth of octahedral ZnO crystals isthought to involve aggregation of two tetrahedral crystals to formtrigonal bipyramids two of which fuse to form octahedral ZnO(Fig. 3). The octa-twin crystal then becomes a bigger octahedralmicrocrystal via layered deposition. It is also noted that thesubmicron particulates still exist in the SEMs even after repeatedrinsing. Rezende et al. [53] demonstrated that such particulatesmay be removed by additional heat treatments at 800 1C.

While low concentrations of the reagents lead to microscaleoctahedral ZnO, Zn5(OH)8(NO3)2(H2O)2 obtained from 0.1–1.0 MZn(NO3)2–C6H12N4 (samples A2–A4 and B1–B4) has a differentmorphology as well as XRD pattern. According to Fig. 4(a), their

rmed by Coulombic interactions and ultrasonic irradiation from a tetrahedral

2N4Zn(NO3)2–C6H12N4 (a) before annealing (Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2) and (b) after

Fig. 6. FTIR spectra of (a) sample A1 (octahedral ZnO) and (b) commercial sample

(hexagonal ZnO).

Fig. 5. SEM micrographs of (a) a single ZnO octahedron synthesized in an ultrasonic bath and (b) commercial ZnO sample with a prismatic hexagonal morphology.

Fig. 7. FTIR spectra of sample B4 (a) before annealing (plate-like Zn5 OHð Þ8

ðNO3Þ2ðH2OÞ2) and (b) after annealing (irregular ZnO).

C. Pholnak et al. / Journal of Physics and Chemistry of Solids 72 (2011) 817–823 821

shapes are microplates with a width of ca. 0.3–2 mm and athickness of ca. 0.1 mm. After annealing, the morphology ofplate-like Zn5(OH)8(NO3)2(H2O)2 is changed to an irregular ZnOstructure with some grains detached from the porous plates, asshown in Fig. 4(b). This change in morphology is related to thethermal decomposition of the Zn5(OH)8(NO3)2(H2O)2 structurewith an increase in porosity due to the release of water, nitrousoxide, and oxygen gases at high temperature. The phases andmorphology of samples B1–B4 before and after annealing aresummarized in Table 1. It follows that the crystalline phase andmorphology are independent of the reaction time and concentra-tion at concentrations above 0.1 M, but a change in the crystallinephase is observed after thermal annealing of the as-preparedsamples at 350 1C.

For comparison, the sample prepared by the reaction of 0.05 MZn(NO3)2–C6H12N4 solution in the ultrasonic bath has an

imperfect octahedral-shape morphology as shown in Fig. 5(a).This emphasizes the relation between the formation of octahedralZnO and molar concentration of the reagents. Typical SEMmicrographs of the commercial ZnO powder shown in Fig. 5(b)illustrate the shape of a hexagonal prism elongated along c-axis.The diameter of prismatic ZnO is about 0.05–0.4 mm and itslength is about 0.2–0.5 mm.

3.3. IR and UV–vis absorptions

Room temperature FTIR spectra of octahedral and commercialZnO are compared in Fig. 6. Both samples exhibit the intensivecharacteristic band of wurtzite ZnO in the range of 400–600 cm�1

[39,54]. The IR absorption by octahedral ZnO is less than that ofcommercial ZnO with a lower absorption peak at 446 cm�1. Inaddition, the broad band centered at around 3435 cm�1 corre-sponds to the O–H stretching mode of the hydroxyl group and the

Fig. 8. UV–vis spectra of (a) sample A1 (octahedral ZnO), (b) sample A4 (plate-like

Zn5 OHð Þ8ðNO3Þ2ðH2OÞ2 and (c) commercial sample (hexagonal ZnO).

C. Pholnak et al. / Journal of Physics and Chemistry of Solids 72 (2011) 817–823822

band at around 1637 cm�1 is related to the bending mode ofwater [55,56]. The sharp peak at around 1385 cm�1 is associatedwith residual NO3

� ions [58], present from the reaction of theprecursors.

Fig. 7 shows the IR spectra of sample B4 before and afterannealing. For the as-prepared sample, the absorption peaks inthe range of 350–600 cm�1 correspond to the ZnO bond [57]. Thestrong absorption bands at 3480 and 1638 cm�1 are indicative ofwater whereas the intense peak at 1384 cm�1 belongs to NO3

�

[58]. The peaks within 639–1053 cm�1 may be due to thevibration of O–H and C–N bonds, and other free ions [54,58].After annealing, the ZnO band with a peak at 448 cm�1 increasessignificantly and the IR absorption is about twice that of the as-synthesized sample. The broad bands of water and the sharp peakof the nitrate ions are reduced significantly and associated withthe loss of water and the release of N2O and O2 gases.

The UV–vis absorption spectra in the range of 200–800 nm ofoctahedral ZnO (sample A1), plate-like Zn5(OH)8(NO3)2(H2O)2

(sample A4) and commercial ZnO are compared in Fig. 8. Thecommercial ZnO with a prismatic hexagonal shape (Fig. 8(c))exhibits only one broad band centered at 382 nm (3.24 eV)corresponding to the exciton absorption over the bandgap ofbulk ZnO [59,60]. The absorption spectrum of octahedral ZnO(Fig. 8(a)) shows a broad exciton absorption band centered at379 nm (3.27 eV) close to that of commercial ZnO. Sample A1 alsopossesses UV absorption ability at 220 nm. This sharp absorbancepeak with a higher energy than that of bulk ZnO, also found inFig. 8(b), is a characteristic of plate-like Zn5(OH)8(NO3)2(H2O)2.Conversely, each plate-like Zn5(OH)8(NO3)2(H2O)2 sample showsa trace of an exciton absorption band around 365–375 nmindicating the existence of a small fraction of ZnO in thesesamples.

4. Conclusions

Octahedral ZnO micropowders can be synthesized via sono-chemical methods with a low power sonicator under ambientconditions using 0.05 M Zn(NO3)2–C6H12N4 solutions. The synthe-sized ZnO octahedra exhibit similar IR and UV absorption proper-ties to that of prismatic hexagonal commercial ZnO. Theconcentration of the reagents plays a key role in the phase and

morphological control of the products. A low reagent concentra-tion, 0.05 M, can also be used in the synthesis of octahedral ZnOutilizing a commercial ultrasonic bath. In contrast, an intermedi-ate Zn5(OH)8(NO3)2(H2O)2 phase with a plate-like morphology isobtained at high reagent concentrations (0.1–1.0 M) because theultrasonic energy is attenuated in the highly viscous solution.Nevertheless, Zn5(OH)8(NO3)2(H2O)2 can be converted to ZnO byheating at 350 1C under aerobic conditions although an octahe-dral structure is not obtained. Overall, the ability to control themorphology of ZnO, the relative simplicity of the syntheticprocedure, and the fact that samples do not have to be annealedmake this a promising alternative to traditional techniques forfabricating ZnO.

Acknowledgments

This work was funded by Walailak University (Grant no.WU53301). The authors would like to thank the Scientific Equip-ment Center, Prince of Songkhla University and Faculty of Science,Thaksin University for their XRD and UV–vis facilities.

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