Study of the surface modification with oleic acid of nanosized HfO2 synthesized by the polymerized complex derived sol–gel method

doi:10.1016/j.apsusc.2012.02.122

Applied Surface Science

Volume 258, Issue 16, 1 June 2012, Pages 6034-6039

https://doi.org/10.1016/j.apsusc.2012.02.122 Get rights and content

Abstract

The synthesis of nanosized hafnium oxide by the polymerized complex derived sol–gel method is reported. The structural and morphological characterization of the HfO₂ was carried out by X-ray diffraction and scanning electron microscopy. The surface of hafnium oxide nanoparticles was modified by capping with oleic acid. The nanoparticle surface area was measured by the gas adsorption technique in order to determine the minimal amount of oleic acid needed to obtain a uniform coverage of the hafnium oxide. The existence of organic layer can be confirmed by Fourier transform spectroscopy, solid state nuclear magnetic resonance spectroscopy, thermal gravimetric analysis and transmission electron microscopy. The FTIR and solid state NMR results reveal that oleic acid is chemisorbed as a carboxylate onto the HfO₂ nanoparticle surface and confirm the formation of a monomolecular layer of oleic acid surrounding the HfO₂. The cover density of oleic acid on the HfO₂ increases with the amount of oleic acid used to modify the nanoparticles and the surface properties of HfO₂ nanoparticles modified with oleic acid change from hydrophilic to hydrophobic.

Graphical abstract

Highlights

► Synthesis and characterization of nanosized HfO₂ by the polymerized complex derived sol–gel method. ► The HfO₂ nanoparticles were coated with oleic acid. ► The minimum concentration of oleic acid necessary to cover HfO₂ particles was determined. ► The oleic acid was chemisorbed as a carboxylate onto the HfO₂ nanoparticle surface. ► The surface properties of HfO₂ modified with oleic acid changed from hydrophilic to hydrophobic.

Introduction

As hafnium oxide (HfO₂) is a quite inert, inorganic compound and has good chemical resistance to strong acids and bases, it is one of the most stable compounds of hafnium and has the potential for applications in microelectronics [1], as an alternative to silicon dioxide [2]; and optical coatings [1], due to its high refractive index and transparent spectral range from the infrared to the ultraviolet [3]. Hafnium oxide is used as dielectric material due to its high dielectric constant (∼16–29) and band gap (∼5.8 eV), making it ideal for use in dielectrics with high capacitance and low dielectric breakdown and leakage current [4], [5].

Nanosized metal oxides have received a considerable attention because of the physical and chemical properties [6], [7], and the interest in the MO₂ oxides, where M = Hf, Ti and Zr, is stimulated by their wide range of industrial applications [8]. However, inorganic nanoparticles tend to agglomerate and aggregate [9], [10], and often present hydrophilic properties [11] which makes them to have low dispersal ability into non-polar solvents, therefore the applications of many nanosized particles are largely limited. Fortunately, this problem could be solved by modifying the nanoparticle surface to avoid agglomeration when the particles are filled into a hydrophobic matrix [11], [12].

There are two basic requirements to use a nanoparticle surface modification agent to stabilize the particles in non-polar solvents. (1) Anchor groups are needed for adsorption or covalent binding of the modifier agent over the particle surface. And (2) the other part must have an aliphatic ending which provides affinity with the non-polar medium. Functional groups like single bond NH₂, SH, COOH, SO₂OH, and PO(OH)₂ are capable to interact with inorganic nanoparticles by coordinative, electrostatic or hydrogen bonding and act as reactive anchor [11]. However, amphiphilic molecules such as oleic acid (OA) have been used to chemically modify metal oxide nanoparticles, such as SiO₂, ZnO, Fe₃O₄, and Al₂O₃ which have sufficient hydroxyl groups ( single bond OH) on its surface, allowing to conduct an esterification reaction between the carboxyl group (COOH) present in the acid molecule and the hydroxyl groups of the nanoparticle surface [12], [13], [14], [15].

Otherwise, a variety of methods for the production of metal oxide nanoparticles have been developed [16], [17], [18]. Among those techniques sol–gel processing is known to have several advantages, including low temperature processing, good composition control and the ability to develop processes for large area applications [19]. Synthesis of materials through sol–gel process ordinarily requires metal alkoxides as precursors. Unfortunately, these ones show intense chemical reaction with water resulting in the occurrence of precipitates. An alternative approach to form nanoparticles is the polymerized complex (PC) derived sol–gel method, also known as Pechini method, which mainly employs inorganic salts as precursors, citric acid as chelating agent and ethylene glycol as cross-linking agent [20], [21].

In this study, HfO₂ nanoparticles were prepared by PC derived sol–gel method. The surface of HfO₂ was coated with oleic acid. The presence of oleic acid was confirmed by FTIR, solid state ¹³C NMR, TGA and TEM analysis. The results reveal the chemisorption of oleic acid onto the HfO₂ nanoparticles and the presence of a monolayer surrounding the HfO₂ nanoparticle surface. After the surface modification with oleic acid, the HfO₂ surface properties changed from hydrophilic to hydrophobic.

Section snippets

Experimental

Hafnium oxide nanoparticles were prepared following the procedure reported in our previous publication [19]. To modify the surface of HfO₂ nanoparticles with oleic acid, we used a procedure based on that described elsewhere [12], [13]. First, oleic acid (Sigma–Aldrich) was dissolved in hexane (Fluka), and then appropriate amount of HfO₂ was added. The mixture was reacted for 4 h at 70 °C. Nanosized HfO₂ modified with oleic acid was recovered by centrifugal separation at 15,500 rpm for 30 min and

Results and discussion

The pure HfO₂ particles were analyzed by XRD in order to investigate the crystalline phase. Fig. 1 gives the XRD pattern of HfO₂ nanoparticles synthesized by PC sol–gel method. All observed reflections present a significant widening in the bases with an acceptable intensity of the peaks, indicating a good crystallinity of the material. The reflections observed can be associated with the monoclinic phase of HfO₂ as compared with diffraction pattern presented in Fig. 1 (vertical lines) [22]. The

Conclusions

The nanosized HfO₂ particles were synthesized by the polymerized complex derived sol–gel method. The diameter and the surface area of the obtained particles were ∼12 nm and 17 m²/g, respectively. The HfO₂ nanoparticles were coated with oleic acid. The oleic acid surface modified HfO₂ nanoparticles were obtained by the reaction between single bond OH group, present on the HfO₂ surface, and COOH of the oleic acid. The oleic acid was chemisorbed as a carboxylate onto the HfO₂ nanoparticle surface and forms a

Acknowledgements

The authors thank the Mexican Council of Science and Technology (CONACyT) for its financial support to carry out this investigation project (Grant No. 133991). Rodolfo Ramos-González acknowledges CONACyT for the scholarship (No. 170601) to carry out his doctorate studies and the economic support for his stay as graduate research scholar at The University of Texas at Dallas under the program “becas mixtas”.

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Study of the surface modification with oleic acid of nanosized HfO2 synthesized by the polymerized complex derived sol–gel method

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Surf. Coat. Technol.

Appl. Surf. Sci.

Chem. Eng. J.

J. Magn. Magn. Mater.

Appl. Surf. Sci.

Biomaterials

Properties of PVA/HfO2 hybrid electrospun fibers and calcined inorganic HfO2 fibers

J. Phys. Chem. C

ALD of hafnium oxide thin films from tetrakis(ethylmethylamino)hafnium and ozone

J. Electrochem. Soc.

Hf-based High-k Dielectrics: Process Development, Performance Characterization, and Reliability

Study on the formation of self-assembled monolayers on sol–gel processed hafnium oxide as dielectric layers

Langmuir

Properties

Basic properties and measuring methods of nanoparticles

Properties of small TiO2, ZrO2 and HfO2 nanoparticles

J. Mater. Chem.

Synthesis and stabilization of colloidal gold nanoparticle suspensions for SERS

Nanoparticles: building blocks for nanotechnology

Functional inorganic nanofillers for transparent polymers

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Dispersion of nano-sized γ-alumina powder in non-polar solvents

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Chem. Rev.

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Chem. Rev.

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Study of the surface modification with oleic acid of nanosized HfO₂ synthesized by the polymerized complex derived sol–gel method

Properties of PVA/HfO₂ hybrid electrospun fibers and calcined inorganic HfO₂ fibers

Properties of small TiO₂, ZrO₂ and HfO₂ nanoparticles