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Preparation of hexagonally aligned inorganic nanoparticles from diblock copolymer micellar systems
Colloids and Surfaces A: Physicochem. Eng. Aspects 331 (2008) 213–219
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation of hexagonally aligned inorganic nanoparticles from diblock copolymer micellar systems Edit Pál a , Albert Oszkó b , Petra Mela c , Martin Möller c , Imre Dékány a,d,∗ a
Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary Department of Solid State and Radiochemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary c DWI at the RWTH Aachen, 52056 Aachen Pauwelsstr. 8, Germany d Supramolecular and Nanostructured Materials Research Group of The Hungarian Academy of Sciences, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary b
a r t i c l e
i n f o
Article history: Received 14 May 2008 Received in revised form 6 August 2008 Accepted 7 August 2008 Available online 26 August 2008 Keywords: Nanoparticles Zinc silicate Zinc oxide Surface patterning Diblock copolymers
a b s t r a c t The aim of this work was to prepare hexagonally aligned zinc oxide nanoparticles using poly(styrene)block-poly(2-vinyl-pyridine) block copolymer spherical micellar system. Various zinc salt were used with different zinc ion/pyridine unit ratio (loading factor, L) to obtain patterned surface. The loaded micelles were deposited on surface of silicon wafer by immersion method. Oxygen plasma treatment was used to generate nanoparticles in periodic patterns on the surface of Si-substrate. The morphology of the loaded micelles and the particles were studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The chemical composition and the crystalline structure of samples were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. On basis of the results of the XPS and XRD measurements we assumed that during the plasma treatment the zinc precursor salt reacted with the oxide layer on the surface of the silicon wafer and formed zinc silicate (Zn2 SiO4 ) instead of ZnO nanoparticles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its advantageous optical, structural and catalytic properties, ZnO is a widely used semiconductor. It has a direct wide band gap (3.37 eV). It is utilized as photocatalyst [1], UV-light emitter [2], photovoltaic equipment [3,4], oxygen sensor [5] and as a component of solar cells [6]. There are numerous examples for the preparation of ZnO nanoparticles in literature. A classical colloid chemical method is the sol–gel procedure, i.e. generation of ZnO nanoparticles by alkaline hydrolysis of a zinc precursor [7–9]. Preparations in liquid media often include various stabilizing agents, which help prevent aggregation of the nascent particles and ensure the synthesis of ZnO particles with controlled size. Examples include preparations in boronitride nanocapsules [10], in reversed micelles [11], in microemulsion [12], by suspension polymerisation [13] or using surfactants such as sodium dodecyl sulfate [14] or polymers such as polyvinyl pyrrolidone [15]. The application of polymers allows the preparation of a great variety of struc-
∗ Corresponding author at: Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary. Tel.: +36 62 544 210; fax: +36 62 544 042. E-mail address: [email protected] (I. Dékány). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.08.015
tures. Peng et al. used polyacrylamide (PAM) and polyacrylamide functionalized with carboxyl groups (PAM-COOH) as additives for the preparation of ZnO nanorings and nanodiscs [16]. Öner et al. studied the morphology of ZnO crystallites formed upon the hydrolysis of zinc nitrate in aqueous medium, in the presence of poly(ethylene-oxide) (PEO), poly(methyl-methacrylate) (PMMA) and poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMAA) diblock copolymers [17]. Taubert et al. obtained, also in aqueous medium, prismatic ZnO particles in the presence of PEO-b-PMAA and “stack of pancakes” structures when using poly(ethylene oxide)-block-poly(styrene sulfonic acid) (PEO-b-PSSH) diblock copolymer [18,19]. Bai et al. used poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) ((PEO)20 –(PPO)70 –(PEO)20 ) copolymer and obtained clewlike ZnO superstructures in the course of the hydrolysis of the zinc salt [20]. Copolymers are preferred additives in nanoparticle syntheses not only in aqueous but also in organic media. Since the micelles are capable of self-assembly, they can be used for the formation of periodic nanostructures. The method is based on spontaneous micelle formation by diblock copolymer chains. The micellar corona consists of the organophilic apolar chain and its core by the polar chain region. The metal salt added to the system (that is to form the nanoparticle later) is incorporated into this polar core. Monolayers of the loaded micelles, deposited by dipping on the surface of a solid support give rise to a periodic nanostructure,
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from which the polymer serving as template is removed by plasma treatment, and nanoparticles of ordered arrangement are formed. The poly(styrene)-b-poly(2-vinyl-pyridine) (PS-b-P2VP) diblock copolymer is widely utilized in organic medium syntheses. Möller et al. used this diblock copolymer as template for the synthesis of hexagonally arranged gold nanoparticles of controlled size [21–24]. Carrot et al. obtained gold nanoparticles with the help of poly(acrylic acid)/polystyrene graft copolymer (PAAg-PS) [25]. Antonietti et al. prepared non-spherical gold [26] and cobalt [27] nanoparticles using poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer. Rodriguez-Abreu et al. obtained silver nanoparticles by in situ synthesis in a reverse micellar system of poly(dimethylsiloxane)-graft-poly(ethylene oxide) (PDMS-g-PEO) block copolymer [28]. However, nanoparticles consisting of not only metals but also oxides, e.g. TiO2 [29], SiO2 [30] and Fe2 O3 [31] can be synthesized with the help of various diblock copolymers. ZnO nanoparticles were prepared from zinc chloride and PS-b-P4VP in toluene by Yoo et al. [32]. The loading factor was 0.5. Monolayer film of micelles in hexagonal order was spin-coated on silicon wafer. Oxygen plasma treatment was used to eliminate the polymer. During the oxidation treatment ZnO nanoparticles with diameter of 16 and 8 nm high were obtained. The photoluminescent property of the particles was investigated, but the chemical composition was not determined. Since the periodic-, and aperiodic-patterned metal and metal oxide nanoscale structures have advantageous physical, chemical and biological properties, they are capable for fabrication of nanoelectronics, biosensors and analytical devices [33]. The main requirements for these patterned structures are the controlled particle size, the monodispersity and the precise spatial order on the surface of the support material.
Our objective was to prepare ZnO (or ZnO derivative) nanoparticles of ordered hexagonal arrangement with controlled size on the surface of a solid support by using poly(styrene)-block-poly(2vinyl-pyridine) diblock copolymer micellar system and various zinc precursors (zinc acetate dihydrate and zinc nitrate hexahydrate), furthermore to investigate the spatial order, the morphology and the chemical composition of the nanostructures. 2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O, Fluka, puriss), zinc acetate dihydrate (C4 H6 O4 Zn·2H2 O, Zn(OAc)2 ·2H2 O, Sigma, puriss), freshly distilled toluene (Merck, p.a.) and poly(styrene)-blockpoly(2-vinyl-pyridine) (PSx -P2VPy x = 1350 and y = 400) was used to prepare zinc salt-loaded micelle solutions. Silicon wafer (CrysTec) were cut into pieces (1 cm × 1 cm) and cleaned in ultrasonic bath using acetone (Reanal, a.r.), distilled water and 2-propanol (Reanal, a.r.). After the cleaning procedure the substrates were dried in nitrogen flow. 2.2. Preparation of the loaded micelles and particles on silicon substrate The poly(styrene)-block-poly(2-vinyl-pyridine) (PS-b-P2VP) diblock copolymer was synthesized by means of living, anionic polymerization as published before [21]. The PS-b-P2VP polymer were dissolved in 2 ml dried toluene and stirred for 24 h before loading. The concentration was 5 mg/mL. For preparation of the loaded micelles equivalents of zinc precur-
Fig. 1. (a) Schematic representation of the preparation of the loaded micelles and (b) schematic drawing of the monolayer preparation.
E. Pál et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 331 (2008) 213–219
sor salt per pyridine unit (L = 0.1–0.6) were added to the polymer solution and stirred for 48 h. Afterwards a previously cleaned silicon substrate was dipped into the solution to obtain a monolayer of loaded micelles. The dipping velocity was 10 mm/min. The silicon wafer was dried in air at room temperature. Oxygen plasma was used to remove the polymer from the surface. Oxygen plasma was generated by a plasma etcher (PVA TePla America Inc., P = 150 W, p = 0.4 mbar, t = 60 min).
X-ray diffraction (XRD) measurements of the nanoparticles were carried out on a Bruker D8 Advance diffractometer with Cu K␣ radiation ( = 0.1542 nm, 40 kV, 30 mA, 0.1 mm slit) using the Diffracplus Basic software. The analyses were carried out in the 30–40◦ (2) ranges. The Scherrer equation was used to determine the average particle diameter from the half width of the corresponding diffraction peak: d=
2.3. Measurement methods For the TEM measurements a Zeiss Lybra 120 electron microscope was used applying 100 kV accelerating voltage. To obtain a micellar monolayer, the loaded micelle solution was dropped onto a carbon-coated copper grid and the excess was soaked with a tissue. To evaluate images and the mean core diameter the Soft Imaging Viewer and Image Tool 3.00 software were used, respectively. The AFM measurements were taken on a Digital Instruments Atomic Force Microscope Nanoscope III in tapping mode, with a scanner capability of 12.5 m in x-, and y-direction and 5 m in zdirection. Silicon cantilever (Veeco Nanoprobe Tips RTESP model, 125 m length, 300 kHz) were used. The scanning rate was 1 Hz. Nanoscope 5.15r5 program was used to evaluate the images.
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k ˇ cos
(1)
where d is the average particle diameter, k is related to the crystallite shape, is the radiation wavelength, ˇ is the line broadening (ˇ = ˇs − ˇo , where ˇs and ˇo are the half-widths of the XRD peak of the sample and the silicon standard), and is the Bragg angle [34]. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Specs, PHoibos 150 MCD9 X-ray photoelectron spectrometer. The excitation source was the non-monochromatic K␣ radiation of magnesium anode (h = 1253.6 eV). The gun was operated at 225 W power (15 kV, 15 mA). The pressure during the spectrum acquisition in the analyzing chamber was less than 5 × 10−9 mbar. The C 1s binging energy of adventitious carbon was used as energy reference: it was taken 285.1 eV. For data acquisition and evaluation SpecsLab2 software was used.
Fig. 2. (a) AFM image (1 m × 1 m) and the cross-section analysis of zinc nitrate hexahydrate-loaded micelles (L = 0.4). (b) AFM image (1 m × 1 m) and the cross-section analysis of zinc acetate dihydrate loaded micelles (L = 0.4). Table 1 The geometric data of the micellar cores and the particles L
Height of the loaded cores (nm) determined from AFM Zn(NO3 )2 precursor
0.1 0.2 0.3 0.4 0.6
14.9 15.3 15.8 16.2 16.9
± ± ± ± ±
0.8 1.2 1.4 0.9 1.3
Height of the particles (nm) determined form AFM Zn(OAc)2 precursor 9.3 10.5 11.2 12.1 12.6
± ± ± ± ±
1.0 1.7 1.9 1.8 1.2
Zn(NO3 )2 precursor 0.9 1.4 3.5 2.3 2.5
± ± ± ± ±
0.1 0.1 0.5 0.3 0.3
Average diameter of the particles (nm) determined form XRD Zn(OAc)2 precursor 1.6 1.7 0.9 1.0 1.8
± ± ± ± ±
0.2 0.3 0.2 0.1 0.2
Zn(NO3 )2 precursor
Zn(OAc)2 ·precursor
18.8 20.9 24.8 20.3 17.7
20.6 22.8 22.3 21.9 23.3
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Fig. 3. TEM images of zinc nitrate hexahydrate loaded (a) and zinc acetate dihydrate loaded (b) micelles (L = 0.2).
Samples deposited on Si substrates were used for the AFM, XRD and XPS measurements. 3. Results and discussion As the first step of the procedure the polymer was dissolved in toluene. It is well known the toluene preferably dissolves the PS blocks, so above the critical micelle concentration (cmc) the PS-bP2VP polymer chains spontaneous form micelles with a polar P2VP cores and a nonpolar PS corona [21]. The micelles formation was followed by the loading of the polar core of the micelles (Fig. 1a) with different type of inorganic zinc salts. Dipping method was used to obtain a monolayer of the loaded micelles on the previously cleaned silicon wafers. The dipping and the lifting out were at controlled velocity. With this method a selfassembled monolayer of hexagonally packed micelles forms on the surface of the substrate. Oxygen plasma treatment was used to eliminate the polymer and to oxidize the zinc salt to obtain hexagonally oriented, uniform oxide particles (Fig. 1b). AFM measurements showed that the uniform, loaded spherical PS-b-P2VP micelles are oriented hexagonally on the surface of the silicon substrate (Fig. 2). In case of nitrate loading, the average height of the micellar cores changes between 14.9 ± 0.8 nm and 16.9 ± 1.3 nm when the loading factor is 0.1 and 0.6, respectively (Table 1). When zinc acetate dihydrate is used as precursor the height of the micelles decreases, the average height alters between 9.3 ± 1.0 nm and 12.6 ± 1.2 nm (Table 1). This difference can be seen from the cross-section analysis of the zinc nitrate hexahydrate
loaded (Fig. 2a) and the zinc acetate dihydrate loaded (Fig. 2b) micelles in case L = 0.4. The reason for this may be that the higher crystal water content of zinc nitrate makes the polar core more extensively swelled. The average distance of cores (lcore ) can also be determined from the AFM images: this value is 98.6 ± 6.9 nm and does not change significantly with type and amount of zinc precursor salt. The loaded micelles were investigated by TEM. The TEM images also display hexagonally arranged, uniform, spherical micellar cores (Fig. 3a and b). The average diameter of the nitrate loaded cores is 28.4 ± 2.7 nm and for the acetate loaded one is 27.1 ± 2.6 nm when loading factor is 0.2. These values do not change with the loading significantly. Particles were prepared from the loaded micelles by oxygen plasma treatment. During oxidation the polymer is eliminated and oxide particles are formed from the precursor salt on surface of the silicon substrate. The morphology of the particles was investigated also by AFM. The height of the particles determined from the cross-section analysis are summarised in Table 1. The height of the hexagonally packed particles prepared from the nitrate salt first increases with the increasing amount of added precursor salt till L = 0.3 then it decreases. It means that the L = 0.3 is an ideal loading value for the zinc nitrate salt. With this loading factor particles with average height 3.5 ± 0.5 nm are formed (Fig. 4). Smaller particles are obtained when zinc acetate is used as precursor salt. The average height of the particles does not vary parallel with the loading factor between 0.9 ± 0.2 nm and 1.8 ± 0.2 nm (Table 1).
Fig. 4. AFM image (1 m × 1 m) of particles prepared from zinc nitrate hexahydrate (L = 0.3).
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Fig. 5. XPS spectra of the Zn 2p peak of the zinc nitrate hexahydrate (a) and the zinc acetate dihydrate (b) loaded samples (L = 0.3) before and after the oxygen plasma treatment.
The chemical composition of the particles was monitored by XPS measurements. In the Zn 2p spectrum of the zinc nitrate hexahydrate loaded micelles the binding energy of the Zn 2p3/2 core level electrons appears at 1022.6 eV, which shifts to 1022.8 eV after the oxygen plasma treatment (Fig. 5a). Similar results were obtained in case of the zinc acetate dihydrate loading: the original
binding energy of the Zn 2p3/2 core level electrons in zinc acetate at 1023.1 eV shifts to 1022.8 eV after the surface treatment (Fig. 5b). Due to the oxidation treatment an increase in the intensity of Zn 2p3/2 peaks also can be observed. Only one peak located at 532.5 eV appears in the O 1s XPS spectra of the samples (Fig. 6). This peak is identified as the binding energy
Fig. 6. XPS spectra of the O 1s peak of the zinc nitrate hexahydrate (a) and the zinc acetate dihydrate (b) loaded samples (L = 0.3) before and after the oxygen plasma treatment.
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4. Conclusion Hexagonally ordered Zn2 SiO4 particles were prepared using PSb-P2VP diblock copolymer micellar system as templates and zinc nitrate hexahydrate or zinc acetate dihydrate as precursor salts in organic medium. During the synthesis the loading factor was varied. Monolayer of loaded micelles was prepared on Si-wafer by dipping method. Oxygen plasma treatment was used to remove the polymer and to oxidize the precursor salt. The geometric data of the particles were determined from AFM and XRD measurements. It was found that the particles have lens like morphology. The highest particles (3.5 ± 0.5 nm) were obtained when zinc nitrate was used and the loading factor was 0.3. In case of zinc acetate loading smaller particles were formed. The lack of peaks characteristic of ZnO in the XPS spectra and Zn–O–Zn binding energy in the XPS results indicates that no ZnO particles formed. Therefore it could be assumed that during the plasma treatment the zinc ions reacted with the surface of the Si substrate forming Zn2 SiO4 instead of ZnO nanoparticles. Acknowledgements The authors are very thankful for the financial support of the MÖB-DAAD project No. 14. 2005-2006, and for the Hungarian National Scientific Fund (OTKA) No. K73307. References
Fig. 7. The XRD patterns of the Si support, the loaded micelles and the particles prepared from zinc nitrate hexahydrate (L = 0.3, 0.6).
of the O 1s core level electrons in pure SiO2 [35]. The O 1s peak belongs to the Zn–O–Zn bonding located at 530.4 eV could not be identified. In order to explain the lack of Zn–O–Zn binding XRD measurements were used to determine the crystalline structure of the particles. The measurements were carried out between 30◦ and 40◦ 2. Surprisingly, in this 2 region instead of the three expected characteristic peaks correspond to d(100) , d(002) and d(101) crystal planes of the ZnO with hexagonally structure only one diffraction peak is obtained at 33.0◦ (2) (Fig. 7). This peak is identified as the d(112) diffraction peak of the Zn2 SiO4 with bodycentered lattice structure [24-1467 JCPDS card]. This XRD result can explain that why the binding energy of the Zn 2p3/2 core level electrons in our plasma treated samples located at higher energies than in pure ZnO (1021.8 eV) [35]. On basis of the results of XPS and XRD measurements it was established that the zinc ions due to the plasma treatment react with the silicon oxide layer on the surface of the Si support, and form Zn2 SiO4 instead of ZnO particles. The average diameter of the particles can be determined from the half width of the d(112) diffraction peak using the Scherrer equation (1). The average diameter of the particles varies between 17 and 25 nm (Table 1.). Since the average diameter of the particles is larger than their height, we suppose that the adhesion forces between the support and the zinc precursor and the interfacial surface tension could deform the original half sphere shape during the oxygen plasma treatment. Thus, combining the AFM and the XRD results it is concluded that the deposited particles are wide and flat, therefore the particles have lens like morphology.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation of hexagonally aligned inorganic nanoparticles from diblock copolymer micellar systems Edit Pál a , Albert Oszkó b , Petra Mela c , Martin Möller c , Imre Dékány a,d,∗ a
Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary Department of Solid State and Radiochemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary c DWI at the RWTH Aachen, 52056 Aachen Pauwelsstr. 8, Germany d Supramolecular and Nanostructured Materials Research Group of The Hungarian Academy of Sciences, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary b
a r t i c l e
i n f o
Article history: Received 14 May 2008 Received in revised form 6 August 2008 Accepted 7 August 2008 Available online 26 August 2008 Keywords: Nanoparticles Zinc silicate Zinc oxide Surface patterning Diblock copolymers
a b s t r a c t The aim of this work was to prepare hexagonally aligned zinc oxide nanoparticles using poly(styrene)block-poly(2-vinyl-pyridine) block copolymer spherical micellar system. Various zinc salt were used with different zinc ion/pyridine unit ratio (loading factor, L) to obtain patterned surface. The loaded micelles were deposited on surface of silicon wafer by immersion method. Oxygen plasma treatment was used to generate nanoparticles in periodic patterns on the surface of Si-substrate. The morphology of the loaded micelles and the particles were studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The chemical composition and the crystalline structure of samples were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. On basis of the results of the XPS and XRD measurements we assumed that during the plasma treatment the zinc precursor salt reacted with the oxide layer on the surface of the silicon wafer and formed zinc silicate (Zn2 SiO4 ) instead of ZnO nanoparticles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its advantageous optical, structural and catalytic properties, ZnO is a widely used semiconductor. It has a direct wide band gap (3.37 eV). It is utilized as photocatalyst [1], UV-light emitter [2], photovoltaic equipment [3,4], oxygen sensor [5] and as a component of solar cells [6]. There are numerous examples for the preparation of ZnO nanoparticles in literature. A classical colloid chemical method is the sol–gel procedure, i.e. generation of ZnO nanoparticles by alkaline hydrolysis of a zinc precursor [7–9]. Preparations in liquid media often include various stabilizing agents, which help prevent aggregation of the nascent particles and ensure the synthesis of ZnO particles with controlled size. Examples include preparations in boronitride nanocapsules [10], in reversed micelles [11], in microemulsion [12], by suspension polymerisation [13] or using surfactants such as sodium dodecyl sulfate [14] or polymers such as polyvinyl pyrrolidone [15]. The application of polymers allows the preparation of a great variety of struc-
∗ Corresponding author at: Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Aradi vt. 1, Hungary. Tel.: +36 62 544 210; fax: +36 62 544 042. E-mail address: [email protected] (I. Dékány). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.08.015
tures. Peng et al. used polyacrylamide (PAM) and polyacrylamide functionalized with carboxyl groups (PAM-COOH) as additives for the preparation of ZnO nanorings and nanodiscs [16]. Öner et al. studied the morphology of ZnO crystallites formed upon the hydrolysis of zinc nitrate in aqueous medium, in the presence of poly(ethylene-oxide) (PEO), poly(methyl-methacrylate) (PMMA) and poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMAA) diblock copolymers [17]. Taubert et al. obtained, also in aqueous medium, prismatic ZnO particles in the presence of PEO-b-PMAA and “stack of pancakes” structures when using poly(ethylene oxide)-block-poly(styrene sulfonic acid) (PEO-b-PSSH) diblock copolymer [18,19]. Bai et al. used poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) ((PEO)20 –(PPO)70 –(PEO)20 ) copolymer and obtained clewlike ZnO superstructures in the course of the hydrolysis of the zinc salt [20]. Copolymers are preferred additives in nanoparticle syntheses not only in aqueous but also in organic media. Since the micelles are capable of self-assembly, they can be used for the formation of periodic nanostructures. The method is based on spontaneous micelle formation by diblock copolymer chains. The micellar corona consists of the organophilic apolar chain and its core by the polar chain region. The metal salt added to the system (that is to form the nanoparticle later) is incorporated into this polar core. Monolayers of the loaded micelles, deposited by dipping on the surface of a solid support give rise to a periodic nanostructure,
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from which the polymer serving as template is removed by plasma treatment, and nanoparticles of ordered arrangement are formed. The poly(styrene)-b-poly(2-vinyl-pyridine) (PS-b-P2VP) diblock copolymer is widely utilized in organic medium syntheses. Möller et al. used this diblock copolymer as template for the synthesis of hexagonally arranged gold nanoparticles of controlled size [21–24]. Carrot et al. obtained gold nanoparticles with the help of poly(acrylic acid)/polystyrene graft copolymer (PAAg-PS) [25]. Antonietti et al. prepared non-spherical gold [26] and cobalt [27] nanoparticles using poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer. Rodriguez-Abreu et al. obtained silver nanoparticles by in situ synthesis in a reverse micellar system of poly(dimethylsiloxane)-graft-poly(ethylene oxide) (PDMS-g-PEO) block copolymer [28]. However, nanoparticles consisting of not only metals but also oxides, e.g. TiO2 [29], SiO2 [30] and Fe2 O3 [31] can be synthesized with the help of various diblock copolymers. ZnO nanoparticles were prepared from zinc chloride and PS-b-P4VP in toluene by Yoo et al. [32]. The loading factor was 0.5. Monolayer film of micelles in hexagonal order was spin-coated on silicon wafer. Oxygen plasma treatment was used to eliminate the polymer. During the oxidation treatment ZnO nanoparticles with diameter of 16 and 8 nm high were obtained. The photoluminescent property of the particles was investigated, but the chemical composition was not determined. Since the periodic-, and aperiodic-patterned metal and metal oxide nanoscale structures have advantageous physical, chemical and biological properties, they are capable for fabrication of nanoelectronics, biosensors and analytical devices [33]. The main requirements for these patterned structures are the controlled particle size, the monodispersity and the precise spatial order on the surface of the support material.
Our objective was to prepare ZnO (or ZnO derivative) nanoparticles of ordered hexagonal arrangement with controlled size on the surface of a solid support by using poly(styrene)-block-poly(2vinyl-pyridine) diblock copolymer micellar system and various zinc precursors (zinc acetate dihydrate and zinc nitrate hexahydrate), furthermore to investigate the spatial order, the morphology and the chemical composition of the nanostructures. 2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O, Fluka, puriss), zinc acetate dihydrate (C4 H6 O4 Zn·2H2 O, Zn(OAc)2 ·2H2 O, Sigma, puriss), freshly distilled toluene (Merck, p.a.) and poly(styrene)-blockpoly(2-vinyl-pyridine) (PSx -P2VPy x = 1350 and y = 400) was used to prepare zinc salt-loaded micelle solutions. Silicon wafer (CrysTec) were cut into pieces (1 cm × 1 cm) and cleaned in ultrasonic bath using acetone (Reanal, a.r.), distilled water and 2-propanol (Reanal, a.r.). After the cleaning procedure the substrates were dried in nitrogen flow. 2.2. Preparation of the loaded micelles and particles on silicon substrate The poly(styrene)-block-poly(2-vinyl-pyridine) (PS-b-P2VP) diblock copolymer was synthesized by means of living, anionic polymerization as published before [21]. The PS-b-P2VP polymer were dissolved in 2 ml dried toluene and stirred for 24 h before loading. The concentration was 5 mg/mL. For preparation of the loaded micelles equivalents of zinc precur-
Fig. 1. (a) Schematic representation of the preparation of the loaded micelles and (b) schematic drawing of the monolayer preparation.
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sor salt per pyridine unit (L = 0.1–0.6) were added to the polymer solution and stirred for 48 h. Afterwards a previously cleaned silicon substrate was dipped into the solution to obtain a monolayer of loaded micelles. The dipping velocity was 10 mm/min. The silicon wafer was dried in air at room temperature. Oxygen plasma was used to remove the polymer from the surface. Oxygen plasma was generated by a plasma etcher (PVA TePla America Inc., P = 150 W, p = 0.4 mbar, t = 60 min).
X-ray diffraction (XRD) measurements of the nanoparticles were carried out on a Bruker D8 Advance diffractometer with Cu K␣ radiation ( = 0.1542 nm, 40 kV, 30 mA, 0.1 mm slit) using the Diffracplus Basic software. The analyses were carried out in the 30–40◦ (2) ranges. The Scherrer equation was used to determine the average particle diameter from the half width of the corresponding diffraction peak: d=
2.3. Measurement methods For the TEM measurements a Zeiss Lybra 120 electron microscope was used applying 100 kV accelerating voltage. To obtain a micellar monolayer, the loaded micelle solution was dropped onto a carbon-coated copper grid and the excess was soaked with a tissue. To evaluate images and the mean core diameter the Soft Imaging Viewer and Image Tool 3.00 software were used, respectively. The AFM measurements were taken on a Digital Instruments Atomic Force Microscope Nanoscope III in tapping mode, with a scanner capability of 12.5 m in x-, and y-direction and 5 m in zdirection. Silicon cantilever (Veeco Nanoprobe Tips RTESP model, 125 m length, 300 kHz) were used. The scanning rate was 1 Hz. Nanoscope 5.15r5 program was used to evaluate the images.
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k ˇ cos
(1)
where d is the average particle diameter, k is related to the crystallite shape, is the radiation wavelength, ˇ is the line broadening (ˇ = ˇs − ˇo , where ˇs and ˇo are the half-widths of the XRD peak of the sample and the silicon standard), and is the Bragg angle [34]. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Specs, PHoibos 150 MCD9 X-ray photoelectron spectrometer. The excitation source was the non-monochromatic K␣ radiation of magnesium anode (h = 1253.6 eV). The gun was operated at 225 W power (15 kV, 15 mA). The pressure during the spectrum acquisition in the analyzing chamber was less than 5 × 10−9 mbar. The C 1s binging energy of adventitious carbon was used as energy reference: it was taken 285.1 eV. For data acquisition and evaluation SpecsLab2 software was used.
Fig. 2. (a) AFM image (1 m × 1 m) and the cross-section analysis of zinc nitrate hexahydrate-loaded micelles (L = 0.4). (b) AFM image (1 m × 1 m) and the cross-section analysis of zinc acetate dihydrate loaded micelles (L = 0.4). Table 1 The geometric data of the micellar cores and the particles L
Height of the loaded cores (nm) determined from AFM Zn(NO3 )2 precursor
0.1 0.2 0.3 0.4 0.6
14.9 15.3 15.8 16.2 16.9
± ± ± ± ±
0.8 1.2 1.4 0.9 1.3
Height of the particles (nm) determined form AFM Zn(OAc)2 precursor 9.3 10.5 11.2 12.1 12.6
± ± ± ± ±
1.0 1.7 1.9 1.8 1.2
Zn(NO3 )2 precursor 0.9 1.4 3.5 2.3 2.5
± ± ± ± ±
0.1 0.1 0.5 0.3 0.3
Average diameter of the particles (nm) determined form XRD Zn(OAc)2 precursor 1.6 1.7 0.9 1.0 1.8
± ± ± ± ±
0.2 0.3 0.2 0.1 0.2
Zn(NO3 )2 precursor
Zn(OAc)2 ·precursor
18.8 20.9 24.8 20.3 17.7
20.6 22.8 22.3 21.9 23.3
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Fig. 3. TEM images of zinc nitrate hexahydrate loaded (a) and zinc acetate dihydrate loaded (b) micelles (L = 0.2).
Samples deposited on Si substrates were used for the AFM, XRD and XPS measurements. 3. Results and discussion As the first step of the procedure the polymer was dissolved in toluene. It is well known the toluene preferably dissolves the PS blocks, so above the critical micelle concentration (cmc) the PS-bP2VP polymer chains spontaneous form micelles with a polar P2VP cores and a nonpolar PS corona [21]. The micelles formation was followed by the loading of the polar core of the micelles (Fig. 1a) with different type of inorganic zinc salts. Dipping method was used to obtain a monolayer of the loaded micelles on the previously cleaned silicon wafers. The dipping and the lifting out were at controlled velocity. With this method a selfassembled monolayer of hexagonally packed micelles forms on the surface of the substrate. Oxygen plasma treatment was used to eliminate the polymer and to oxidize the zinc salt to obtain hexagonally oriented, uniform oxide particles (Fig. 1b). AFM measurements showed that the uniform, loaded spherical PS-b-P2VP micelles are oriented hexagonally on the surface of the silicon substrate (Fig. 2). In case of nitrate loading, the average height of the micellar cores changes between 14.9 ± 0.8 nm and 16.9 ± 1.3 nm when the loading factor is 0.1 and 0.6, respectively (Table 1). When zinc acetate dihydrate is used as precursor the height of the micelles decreases, the average height alters between 9.3 ± 1.0 nm and 12.6 ± 1.2 nm (Table 1). This difference can be seen from the cross-section analysis of the zinc nitrate hexahydrate
loaded (Fig. 2a) and the zinc acetate dihydrate loaded (Fig. 2b) micelles in case L = 0.4. The reason for this may be that the higher crystal water content of zinc nitrate makes the polar core more extensively swelled. The average distance of cores (lcore ) can also be determined from the AFM images: this value is 98.6 ± 6.9 nm and does not change significantly with type and amount of zinc precursor salt. The loaded micelles were investigated by TEM. The TEM images also display hexagonally arranged, uniform, spherical micellar cores (Fig. 3a and b). The average diameter of the nitrate loaded cores is 28.4 ± 2.7 nm and for the acetate loaded one is 27.1 ± 2.6 nm when loading factor is 0.2. These values do not change with the loading significantly. Particles were prepared from the loaded micelles by oxygen plasma treatment. During oxidation the polymer is eliminated and oxide particles are formed from the precursor salt on surface of the silicon substrate. The morphology of the particles was investigated also by AFM. The height of the particles determined from the cross-section analysis are summarised in Table 1. The height of the hexagonally packed particles prepared from the nitrate salt first increases with the increasing amount of added precursor salt till L = 0.3 then it decreases. It means that the L = 0.3 is an ideal loading value for the zinc nitrate salt. With this loading factor particles with average height 3.5 ± 0.5 nm are formed (Fig. 4). Smaller particles are obtained when zinc acetate is used as precursor salt. The average height of the particles does not vary parallel with the loading factor between 0.9 ± 0.2 nm and 1.8 ± 0.2 nm (Table 1).
Fig. 4. AFM image (1 m × 1 m) of particles prepared from zinc nitrate hexahydrate (L = 0.3).
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Fig. 5. XPS spectra of the Zn 2p peak of the zinc nitrate hexahydrate (a) and the zinc acetate dihydrate (b) loaded samples (L = 0.3) before and after the oxygen plasma treatment.
The chemical composition of the particles was monitored by XPS measurements. In the Zn 2p spectrum of the zinc nitrate hexahydrate loaded micelles the binding energy of the Zn 2p3/2 core level electrons appears at 1022.6 eV, which shifts to 1022.8 eV after the oxygen plasma treatment (Fig. 5a). Similar results were obtained in case of the zinc acetate dihydrate loading: the original
binding energy of the Zn 2p3/2 core level electrons in zinc acetate at 1023.1 eV shifts to 1022.8 eV after the surface treatment (Fig. 5b). Due to the oxidation treatment an increase in the intensity of Zn 2p3/2 peaks also can be observed. Only one peak located at 532.5 eV appears in the O 1s XPS spectra of the samples (Fig. 6). This peak is identified as the binding energy
Fig. 6. XPS spectra of the O 1s peak of the zinc nitrate hexahydrate (a) and the zinc acetate dihydrate (b) loaded samples (L = 0.3) before and after the oxygen plasma treatment.
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4. Conclusion Hexagonally ordered Zn2 SiO4 particles were prepared using PSb-P2VP diblock copolymer micellar system as templates and zinc nitrate hexahydrate or zinc acetate dihydrate as precursor salts in organic medium. During the synthesis the loading factor was varied. Monolayer of loaded micelles was prepared on Si-wafer by dipping method. Oxygen plasma treatment was used to remove the polymer and to oxidize the precursor salt. The geometric data of the particles were determined from AFM and XRD measurements. It was found that the particles have lens like morphology. The highest particles (3.5 ± 0.5 nm) were obtained when zinc nitrate was used and the loading factor was 0.3. In case of zinc acetate loading smaller particles were formed. The lack of peaks characteristic of ZnO in the XPS spectra and Zn–O–Zn binding energy in the XPS results indicates that no ZnO particles formed. Therefore it could be assumed that during the plasma treatment the zinc ions reacted with the surface of the Si substrate forming Zn2 SiO4 instead of ZnO nanoparticles. Acknowledgements The authors are very thankful for the financial support of the MÖB-DAAD project No. 14. 2005-2006, and for the Hungarian National Scientific Fund (OTKA) No. K73307. References
Fig. 7. The XRD patterns of the Si support, the loaded micelles and the particles prepared from zinc nitrate hexahydrate (L = 0.3, 0.6).
of the O 1s core level electrons in pure SiO2 [35]. The O 1s peak belongs to the Zn–O–Zn bonding located at 530.4 eV could not be identified. In order to explain the lack of Zn–O–Zn binding XRD measurements were used to determine the crystalline structure of the particles. The measurements were carried out between 30◦ and 40◦ 2. Surprisingly, in this 2 region instead of the three expected characteristic peaks correspond to d(100) , d(002) and d(101) crystal planes of the ZnO with hexagonally structure only one diffraction peak is obtained at 33.0◦ (2) (Fig. 7). This peak is identified as the d(112) diffraction peak of the Zn2 SiO4 with bodycentered lattice structure [24-1467 JCPDS card]. This XRD result can explain that why the binding energy of the Zn 2p3/2 core level electrons in our plasma treated samples located at higher energies than in pure ZnO (1021.8 eV) [35]. On basis of the results of XPS and XRD measurements it was established that the zinc ions due to the plasma treatment react with the silicon oxide layer on the surface of the Si support, and form Zn2 SiO4 instead of ZnO particles. The average diameter of the particles can be determined from the half width of the d(112) diffraction peak using the Scherrer equation (1). The average diameter of the particles varies between 17 and 25 nm (Table 1.). Since the average diameter of the particles is larger than their height, we suppose that the adhesion forces between the support and the zinc precursor and the interfacial surface tension could deform the original half sphere shape during the oxygen plasma treatment. Thus, combining the AFM and the XRD results it is concluded that the deposited particles are wide and flat, therefore the particles have lens like morphology.
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