UDK 54.057:542.9	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 49(2)281(2015)
MICROWAVE-ASSISTED HYDROTHERMAL SYNTHESIS OF Ag/ZnO SUB-MICROPARTICLES
HIDROTERMIČNA SINTEZA PODMIKROMETRSKIH DELCEV Ag/ZnO Z MIKROVALOVI
Lukaš Münster1'2, Pavel Bažant12, Michal Machovsky2, Ivo Kuritka12
1Polymer Centre, Faculty of Technology, Tomas Bata University in Zlin, Nam. T. G. Masaryka 275, 762 72 Zlin, Czech Republic 2Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Nad Ovcirnou 3685, 760 01 Zlin, Czech Republic
l_munster@ft.utb.cz
Prejem rokopisa - received: 2013-10-02; sprejem za objavo - accepted for publication: 2014-04-03
doi:10.17222/mit.2013.223
A fast and environmentally friendly, microwave-assisted, hydrothermal synthesis was utilized for the preparation of a Ag/ZnO hybrid system by using zinc acetate dihydrate and silver nitrate as the sources of Zn2+ and Ag+, and hexamethylentetramine as the reducing and precipitating agent. The influence of the concentration was investigated by X-ray diffraction analysis, scanning electron microscopy and energy-dispersive analysis. It was found that the concentration has a strong effect on the morphology and proportion between the Ag and ZnO components of the prepared particulate materials. With a decreasing concentration, the morphology of the ZnO changed from twinned or single frustums to rod-like microparticles, whereas the silver morphology changed from large polygon-shaped microparticles to very small, spherical nanoparticles.
Keywords: zinc oxide, silver, nanoparticle, hydrothermal, microwave synthesis
Uporabljena je bila hitra in okolju prijazna mikrovalovna hidrotermična sinteza hibridnega sistema Ag/ZnO z uporabo cink-acetat dihidrata in srebrovega nitrata kot vira Zn2+ in Ag+ ter heksametilentetramina kot reducenta in sredstva za izločanje. Z rentgensko difrakcijo, vrstično elektronsko mikroskopijo in energijsko disperzijsko analizo je bil preučevan vpliv koncentracije. Ugotovljeno je bilo, da ima koncentracija močan vpliv na morfologijo in razmerje med komponentama Ag in ZnO pripravljenega zrnatega materiala. Z zmanjšanjem koncentracije se je morfologija ZnO spremenila iz dvojčičnih ali stožčastih mikrodelcev v paličaste, medtem ko seje morfologija srebra iz velikih poligonalnih mikrodelcev spremenila v majhne sferične nanodelce.
Ključne besede: cinkov oksid, srebro, nanodelec, hidrotermičen, sinteza z mikrovalovi
1 INTRODUCTION	have been exploited for their antibacterial properties for
millennia before the realization that bacteria are the
Various metals (Au, Ag) and metal oxides (ZnO,	agents of the infection2. Silver nanoparticles are com-
TiO2, SnO2, CuO) have been the subjects of investigation	mercially available on the antibacterial-additives market,
for a long time because of their unique optical, electrical,	exhibiting a broad spectrum of antibacterial activity
and mechanical properties that can be modulated by	against gram-positive, gram-negative bacteria, fungi, cer-
varying the size and the morphology of the particles on	tain viruses, including antibacterial-resistant strains.1214
the nano- and sub-micro scales1,2. Recently, hybrid	Therefore, with the combination of Ag and ZnO in a
metal-semiconductor materials have attracted great	hybrid material on the nano- or sub-micro scales could
attention because their metal and semiconductor inter-	achieve a synergic effect of antibacterial and photocata-
face possesses a unique electronic band structure result- lytic activity, biosensing, electrochemical properties,
etc.
ing in a specific chemical activity3. Therefore, great efforts have been devoted to the synthesis of hybrid
Hence, we developed a simple, fast and efficient
materials by combining various metals and semiconductors such as Ag/TiO2, Ag/SiO2, Pt/SnO2 and Ag/ZnO.4-7	preparation of a Ag/ZnO hybrid via a microwave (MW) Among them, Ag/ZnO is of great interest because of	assisted hydrothermal synthesis using a modified
potential applications in many fields, such as catalysis	open-vessel laboratory microwave-oven system18. Intro-
and medicine.7,8 This particulate material has been	ducing microwaves instead of the conventional heating
successfully demonstrated as a suitable filler for an	allowed us to shorten the reaction time to 12 min, com-
inorganic antibacterial polymer system9. ZnO is an im-	pared to hours reported in references for conventional
portant semiconducting material because of its unique	heating17. The concentration of reactants is one of the
optoelectronic and piezoelectric properties.1011 More-	key factors affecting the properties of the prepared
over, it possesses strong antibacterial activity towards	hybrid material. Therefore, a study of the influence of
gram-negative bacteria and it is both non-toxic and	the reactants' concentrations on the morphology and
biocompatible10. Metallic silver, its salts and complexes	particles size was performed.
Table 1: Sets of samples and amounts of precursors and reducing/precipitating agent used in MW synthesis and yields of the powder product Tabela 1: Sklopi vzorcev in koli~ina predhodnikov in reducimega/izlo~evalnega sredstva, uporabljenih v MW-sintezi, in izkoristki proizvodov v obliki prahu
Set
Sample
Zn(Ac)2-2H2O mol
AgNOj mol
HMTA mol
Yield
Theoretical Yield
g
I-ZnO
0.0500
0.0500
0.740
4.070
I-Ag
0.0050
0.0500
0.501
0.539
I-AgZnO
0.0500
0.0050
0.0500
1.160
4.609
II
II-ZnO
0.0050
0.0050
0.190
0.407
II-Ag
0.0005
0.0050
0.048
0.054
II-AgZnO
0.0050
0.0005
0.0050
0.182
0.461
III
III-ZnO
0.0010
0.0010
0.025
0.041
III-Ag
0.0001
0.0010
0.003
0.005
III-AgZnO
0.0010
0.0001
0.0010
0.026
0.046
2 EXPERIMENTAL WORK
2.1	Sample preparation
Zinc acetate dihydrate Zn(Ac)2 ■ 2H2O (PENTA) and silver nitrate AgNO3 (PENTA) were used as the sources of Zn2+ and Ag+, respectively. Hexamethylentetramine (HMTA), (CH2)6N4 (LACHNER) was used as the source of OH-, acting as a precipitating agent and CH2O as a reducing agent. All the chemicals were of analytical grade and used as received without any further purification. Demineralised water with a conductivity 0.1 ^S cm-1 was used throughout the experiment.
The synthesis was carried out using an open-vessel microwave-oven laboratory system MWG1K-10 operated at 800 W and 2.45 GHz (RADAN) with an external reflux cooler. In a typical experimental procedure a given amount of precursor (Zn2+, Ag+ or both) and reducing/precipitating agent were dissolved in 100 mL and 50 mL of demineralised water, respectively. The precursor solutions were first preheated in the microwave oven for 2 min, then the solution of HMTA was added via a dropping funnel, and then microwave heating continued for another 10 min. Hence, the total time of the microwave synthesis was 12 min for each prepared sample. The reaction mixture was always left to cool to approximately 50 °C and then the material was collected and washed by microfiltration. Samples were dried at 40 °C in a laboratory oven until a constant weight was achieved. In all the experiments, the molar ratio of Zn2+/Ag+ was set to 10/1. In order to investigate the influence of concentration, the initial solution mixture concentration, which is close to the saturation limit for zinc acetate (set I), was lowered ten (set II) and fifty times (set III), giving rise to three sets of samples. The concentration of HMTA was kept at a constant ratio to the precursors as well. For the reaction-mixture compositions (Table 1).
2.2	Characterization methods
Crystal-phase structure identification was conducted using the multifunctional X-ray diffractometer PANaly-tical X'Pert Pro MPD (PANalytical) and Cu-Ka X-ray source (X = 0.15418 nm), at 40 kV and 30 mA and the 20
diffraction angle range 10-90 The morphology was observed in a scanning electron microscope (SEM) Vega II LMU (TECSAN) at an acceleration voltage of 10 kV. Semi-quantitative elemental analysis was performed with the energy-dispersive X-ray (EDX) analyzer (Oxford Instruments, INCA, UK) mounted on the microscope.
3 RESULTS AND DISCUSSION
All the samples were examined by powder X-ray diffraction (XRD) analyses. As an example of the crystal-phase structure investigation, X-ray patterns of the samples prepared at the lowest concentration, i.e., pure ZnO, Ag and hybrid Ag-ZnO (set III) are shown in Figure 1. Other sets of the experiment with higher concentrations of precursors and reducing/precipitating agents (set I and II) showed similar XRD patterns. Specific diffraction lines (labeled by #) were observed at approximately 20 = (31.7, 34.4, 36.2, 47.5, 56.5, 62.8, 66.2, 67.8, 72.5, 76.8, 81.3 and 89.7) According to the
Figure 1: X-ray diffractogram of samples from set III, the 20 range is cropped at lower values in the graph as no peaks were detected between 10 ° and 30 °
Slika 1: Rentgenski difraktogram vzorcev iz sklopa III, podro~je 20 je odrezano pri nižjih vrednostih, ker v obmo~iu 10-30 ° ni bilo vrhov
I
Figure 2: SEM micrographs of prepared samples of ZnO, Ag and AgZnO particles arranged in dependence of the concentration of precursors and HMTA in the reaction mixture
Slika 2: SEM-posnetki pripravljenih vzorcev ZnO, Ag in AgZnO delcev, razvr{~enih v odvisnosti od koncentracije predhodnikov in HMTA v reakcijski zmesi
JCDD PDF-2 entry 01-079-0207, these peaks can be unambiguously assigned to the ZnO wurtzite hexagonal crystal phase. Other observed diffraction peaks (labeled by *) at approximately IQ = (38.2, 44.4, 64.6, 77.6 and 81.8) ° were assigned to a metallic silver cubic crystal phase according to JCDD PDF-2 entry 01-087-0720. Moreover, the sharpness and the narrowness of the diffraction peaks imply well-developed crystalline structures for both the Ag and ZnO, with no level traces of crystalline impurities.
The morphology of the prepared particles is shown in Figure 2. A back-scattered electron (BSE) detector was utilized to enhance the contrast between the Ag and ZnO particles. It is clear that the concentration strongly influenced the particles size and the morphology. For pure ZnO synthesized at the highest concentration (set I), the particles form typical twinned frustums joined by their apical bases, in many cases hollow if single frustums. The dimensions of these particles are diameter up to 1.5 ^m and in length up to 2 ^m. The particles with a hollow core have a wall thickness starting from tens of nanometers. Lowering the concentration to one tenth of the ZnO precursor (set II) resulted in an increased aspect ratio from approximately 1 : 1 observed for pure ZnO at the highest concentration (set I) to 6 : 1. The approximate diameter and length were 500 nm and 3 ^m. At the lowest concentration (set III), rod-shaped ZnO particles have the same aspect ratio; however, there is observable
a slight increase of the particles' length and diameter. The morphology of the Ag particles prepared by using the highest concentration of the precursor (set I), displayed polygon-shaped particles with a diameter up to 1.5 ^m. The lowering of the concentration of the Ag precursor (set II) results in spherically shaped Ag particles with an approximate diameter of 250 nm. The lowest concentration of Ag (set III) displayed the smallest particles with a size in the range of tens of
Figure 3: EDX spectra of samples from set III Slika 3: EDX-spektri vzorcev iz sklopa III
nanometers. For the hybrid AgZnO system, the trend is very similar as that observed for the pure Ag and ZnO components in the prepared material.
Figure 3 shows typical spectra obtained from the EDX analyses. For the sake of brevity, only spectra for the lowest concentration (set III) are presented. The presence of elemental zinc (peaks at (1.0, 8.6 and 9.5) keV) and oxygen (peak at 0.5 keV) from the ZnO particles was confirmed in the samples ZnO, Ag/ZnO. Elemental silver (peaks at 2.6 keV and 3.0 keV) was confirmed even within the lowest concentration used in the samples Ag and Ag/ZnO. Elemental carbon with a peak at 0.3 keV can be assigned to the carbon tape used for fixation of the sample to the holder inside the microscope chamber or to other organic impurities. The semi-quantitative results from the EDX in the weight percentage of Ag, Zn and O content are listed in Table 2. It is clear that the weight percentage of Zn increases with lowering of the concentration, whereas the content of Ag decreases with lowering the concentration, which is in accordance with observations based on the SEM micrographs. The large observed content of O points towards the presence of water adsorbed at the surface or in the pores of the particles.
Table 2: EDX results of prepared samples, content of Zn, Ag and O in mass fractions, w/%
Tabela 2: EDX-rezultati pripravljenih vzorcev, vsebnost Zn, Ag in O
v masnih deležih, w/%
	EDX (w/%)		
Set	ZnO	Ag	AgZnO
I	Zn: 59.1; O: 40.9	Ag: 100	Ag: 39.5; Zn: 31.7; O: 28.8
II	Zn: 60.9; O: 39.1	Ag: 100	Ag: 28.2; Zn: 36.4; O: 35.5
III	Zn: 65.2; O: 34.8	Ag: 100	Ag: 3.3; Zn: 62.2; O: 34.4
4 CONCLUSION
A simple one-pot open-vessel microwave-assisted hydrothermal synthesis of a ZnO, Ag and Ag/ZnO hybrid combination of both types of particles was introduced. The influence of the concentration of precursors and the precipitating/reducing agent on the properties and on the morphology was investigated. Powder XRD analysis showed wurtzite crystalline phases of zinc oxide in all the samples prepared using Zn(Ac)2 ■ 2H2O as a precursor and cubic crystalline phases of silver prepared using the precursor AgNO3, even at the lowest concentrations. The alternation of the concentration has a marked effect on the particle morphology: (i) the decrease of the concentration of the zinc precursor results in prolongation and an increased size of the ZnO particles, whereas (ii) the size of the silver particles decreases directly with the concentration decrease, from micrometer down to tens of nanometers, and the highest percentage yield was almost 93 % of pure silver particles.
Acknowledgment
The authors wish to thank the internal grant of TBU in Zlin No. IGA/FT/2013/014 funded from the resources of specific university research for financial support.
This article was written with the support of the Operational Program "Research and Development for Innovations" co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the Centre of Polymer Systems project (reg. number: CZ.1.05/2.1.00/03.0111).
This article was written with the support of the Operational Program "Education for Competitiveness" co-funded by the European Social Fund (ESF) and the national budget of the Czech Republic, within the "Advanced Theoretical and Experimental Studies of Polymer Systems" project (reg. number: CZ.1.07/2.3.00/20.0104).
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