HfO2 Hybrid Electrospun Fibers and Calcined

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Properties of PVA/HfO2 Hybrid Electrospun Fibers and Calcined Inorganic HfO2 Fibers Daehwan Cho,†,z Woo Jin Bae,‡,z Yong Lak Joo,^ Christopher K. Ober,§ and Margaret W. Frey*,† †

Department of Fiber Science and Apparel Design, Cornell University, Ithaca, New York 14853, United States Department of Polymer Science and Engineering, University of Massachusetts Amherst, Conte Center for Polymer Research, 120 Governor Drive, Amherst, Massachusetts 01003, United States § Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States ^ School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States ‡

ABSTRACT: New organic-inorganic hybrid fibers (polyvinyl alcohol (PVA)/HfO2) were prepared by an electrospinning method using DI water as a solvent. The HfO2-acetate nanoparticles could be loaded up to 80% by weight in PVA polymer to fabricate uniform PVA/HfO2 electrospun hybrid fibers. After annealing at 140 C for 12 h, electrospun PVA/ HfO2 fibers were stable when soaked in water. Calcination of these hybrid fibers at temperatures ranging from 450 to 700 C resulted in the formation of pure inorganic HfO2 fibers with diameters ranging from 0.1 to 0.8 μm depending on the concentration of HfO2-acetate nanoparticles in the spinning dope. Fourier transform infrared spectroscopy and Raman spectroscopy techniques were used to investigate the crystal structures of the HfO2 fibers, which were confirmed by X-ray diffraction (XRD) results. XRD results showed the presence of both monoclinic and tetragonal crystal structures in the HfO2 fibers. The type and size of the crystals formed depended on the concentration of HfO2-acetate nanoparticles in the initial spinning dope and on the calcination temperature. In particular, the formation of stable tetragonal crystal structures in the as-spun hybrid fibers with lower concentration of HfO2 was achieved at lower temperature than the monoclinic-to-tetragonal transition temperature.

1. INTRODUCTION One dimensional (1D) nanostructures of metal oxides and related materials have potential applications in areas, including photonics,1,2 nanoelectronics,3 sensing,4 and data storage.5 In many of these applications, the sensitivity or efficiency of the device is proportional to the specific surface area of metal oxide available. Nanorods or nanowires not only provide a larger surface area per unit mass compared with that of films or the bulk material but also offer the opportunity to study the material properties of one-dimensional structures. Many unique and fascinating properties have been proposed and demonstrated for these classes of materials in semiconductor systems. Metal oxides have unique properties that can be used in microelectronic systems; metalinsulator transitions can be used to control devices in electric and electronic circuits,6 to secure superior stability of the metal organic frameworks by inorganic brick,7 to produce highly efficient light-emitting diodes,8-10 and to reduce thermal conductivity in nanowires.11 Metal oxide systems with tailored optical, electrical, and magnetic properties have been studied as well. Nanorods and nanowires with different compositions have been developed using various methods, including the vapor-phase transport process,12 chemical vapor deposition,13 arc discharge,14 laser r 2011 American Chemical Society

ablation,15 solution aided growth,16,17 and a template-based method.8,9 All methods listed above are technically difficult and too costly to fabricate the nanorods and nanowires. 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 common and stable compounds of hafnium and has the potential for applications in optical coatings and microelectronics.18,19 In particular, the high dielectric constant and good thermal stability, when in contact with silicon, makes HfO2 film a leading candidate to replace the current SiO2 as a gate insulator in field-effect transistors.20,21 The advantage of HfO2 as a dielectric material is its high dielectric constant (25) as compared with the dielectric constant of SiO2 (3.9).19 Hafnium oxide can also be used as a refractory material in gate insulation because of its high melting point (2758 C). HfO2 nanofibers would have advantages over the currently used Al2O3 and ZrO2 fibers based on a greater dielectric constant and excellent photoluminescent properties.22-24 Under ambient conditions, HfO2 has a stable monoclinic (m) structure25,26 that transforms at high Received: December 16, 2010 Revised: January 31, 2011 Published: March 11, 2011 5535

dx.doi.org/10.1021/jp111964f | J. Phys. Chem. C 2011, 115, 5535–5544

The Journal of Physical Chemistry C temperatures to a tetragonal (t) phase and, on further heating, to a cubic (c) phase.27 The t- and c-HfO2 phases can be stabilized in doped crystals,28 nanostructured materials,29 and thin films30 at ambient conditions. These types of HfO2 crystal structures are important for applications in fields, such as microelectronics or photocatalysis, and for the fabrication of advanced multifunctional nanomaterials.29,30 Electrospinning is a technique by which fibers, with diameters ranging from micrometers to a few nanometers, can be produced from an electrically driven jet of polymeric fluid.31 Electrospinning has been widely studied during the past decade, and spinning conditions for many polymers have been reported.32 The mechanism of jet thinning33 and the effects of a variety of parameters34 on electrospinning have been investigated to provide a better understanding of the electrospinning process. Electrospinning has been in extensive use in the production of submicrometer fibers from polymeric and biomaterials for numerous applications. Formation of PVA nanofibers has been reported extensively.35-37 PVA electrospinning is convenient experimentally and environmentally friendly because uniform micro- and nanofibers can be formed using water as a solvent. The PVA fibers themselves, however, have limited usefulness because they are readily soluble in water. In recent years, electrospinning from polymer solutions carrying inorganic nanoparticles has been used to produce small-scale metal oxide fibers.38 Through the use of sol-gels, organic-inorganic hybrid precursor nanofibers can be formed by electrospinning, and calcination of the precursor fibers results in inorganic nanofibers.39,40 Many kinds of inorganic nanofibers, such as Al2O3, ZrO2, NiCO2O4, and TiO2, have been formed from organic-inorganic hybrid precursor nanofibers,41-43 but no research has been reported on HfO2 nanofibers prepared by electrospinning. In the current study, HfO2-acetate nanoparticles were highly loaded into the PVA polymer solutions to concentrations as high as 80 wt % compared to the PVA polymer. Organic/inorganic (PVA/HfO2) hybrid fiber webs were fabricated from the prepared PVA/HfO2 hybrid spinning dopes by the electrospinning method. Two levels of heat treatment were used to modify the as-spun fiber structure. Heat treatment at 140 C induced a cross-linking reaction between HfO2 and PVA, rendering the fibers stable in water. Heat treatment at temperatures above 450 C resulted in calcination to inorganic pure HfO2 fibers. The morphology of the calcined inorganic fibers was assessed via microscopy, infrared and Raman spectroscopies, and X-ray diffraction to ascertain the effects of HfO2 loading in the as-spun fibers and calcination temperature on the crystal structure and size in the inorganic fibers.

2. EXPERIMENTAL SECTION 2.1. Materials. PVA with a molecular weight of 78 000 was purchased from Polysciences, Inc. (Warrington, PA). The polymer is 99.7% hydrolyzed so that it has nearly the same number of corresponding hydroxyl groups as the degree of polymerization. HfO2-acetate nanoparticles were synthesized in the presence of acetic acid from hafnium isopropoxide that was purchased from Sigma-Aldrich. The size of HfO2-acetate nanoparticles is around 3-5 nm (refer to our previous paper in detail).22 The nanoparticles are stabilized by acetate anions and dispersible in water at high loadings. All procedures for synthesizing the HfO2-acetate nanoparticles were well described in our previous paper.22 The nonionic surfactant, Triton X-100 (p-tertiary-octylphenoxy polyethyl alcohol), was purchased from Sigma Aldrich Company. Distilled (DI) water was used as a solvent to dissolve the PVA polymers and to disperse the HfO2-acetate nanoparticles.

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Figure 1. Schematic representation of the loading of HfO2 nanoparticles into PVA polymer chains.

2.2. Preparation of Spinning Dopes. HfO2-acetate nanopar-

ticles were put in DI water and agitated to disperse them with a Genie Vortex mixer for 2 min. Because of the good solubility of HfO2-acetate nanoparticles in DI water, the nanoparticles could be input up to 90 wt % over DI water. To make PVA solutions, 10 wt % of PVA was dissolved in DI water in an oven of 95 C for 4 h. After the PVA solution was cooled to room temperature, the HfO2 dispersed solution was added into the PVA solution and then mixed together with a Genie Vortex mixer for 5 min. The Triton-X 100 surfactant was added into the mixed solution at 0.5% on the basis of the solution volume and agitated with the Arm-Shaker for 6 h to make a homogeneous spinning dope. Several spinning dopes were prepared for the electrospinning. Pure PVA spinning dope (10 wt %) was prepared to compare its spinnability with the hybrid spinning dopes. Several types of PVA/HfO2 hybrid spinning dopes were prepared from the different blending ratios of the nanoparticles to PVA. All material contents reported in the paper were based on weight percent. 2.3. Fabrication of PVA/HfO2 Hybrid Fibers. A 3 mL plastic syringe with an 18 gauge needle (inner diameter: 0.84 mm) was loaded with the prepared dope. A high-voltage power supply (Gamma) applied the positive charge to the needle. To collect the electrospun fibers in this study, a silicon (Si) wafer was used as a collector that was grounded. A micropump (Harvard Apparatus, Holliston, MA) was used to infuse the solution and to eject it toward to the collector. A voltage of 12 kV was maintained at the tip of the needle. The distance between the collector and the needle tip was set at 13 cm, and a constant flow rate of solution was set to 0.54 mL/h. The electrospinning temperature was maintained at room temperature. 2.4. Annealing and Calcination of the Hybrid Fibers. The electrospun fibers fabricated from pure PVA and hybrid PVA/ HfO2 spinning dopes were treated at 140 C for 12 h to investigate the thermal stability of the fibers and to induce the hydrolysis within the polymer chains in the fibers. After the heat treatment, the fibers were soaked in DI water for 5 min and dried under ambient condition. The PVA/HfO2 hybrid fiber webs for calcination were directly used after electrospinning without any heat treatment. Calcination of the fibers was performed by thermal treatment at 40 C/min to the target temperature using a standard muffle furnace under air. The webs on the Si wafer were placed in the furnace, in which the target temperature was already attained, and the calcination time was set at 20 min. 2.5. Characterization. The morphology of all electrospun fibers was evaluated with a Leica 440 scanning electron microscope (SEM) after the fiber webs were coated with Au-Pd. Image analysis software (ImageJ 1.41) was used to measure the electrospun fiber diameter. The EDX (energy dispersive X-ray) analysis was used in conjunction with SEM and caused X-rays to be emitted from a targeted point of the material, in which the energy of the beam was 10 keV. X-ray diffraction (XRD) experiments were performed using a Bruker General Area 5536

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The Journal of Physical Chemistry C

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Figure 2. (A) FTIR spectra of pure PVA fibers, PVA/HfO2 hybrid fibers with different ratios of HfO2 to PVA. (B) Magnified FTIR spectra (1200900 cm-1). Code: 0 wt %, pure PVA fibers; 80 wt %, PVA/HfO2 (20/80) hybrid fibers; 100 wt %, HfO2-acetate nanoparticles.

Detector Diffraction Systems (GADDS) at a wavelength of 1.5406 Å. The electrospun fiber webs on the Si wafer were loaded directly and scanned in the X-ray chamber. Infrared absorptions were measured for the samples with a Br€uker Optics Vertex80 spectrometer. The surface of the electrospun fiber webs on the Si wafer was characterized using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Ramanscattering spectroscopy was performed using a Renishwa InVia Confocal Raman microscope equipped with a cooled charge coupled device. The probe light was obtained from a linearly polarized 488 nm laser, which provided the focused light spots with a diameter of ∼1 μm. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 at a heating rate of 10 C min-1 under N2.

3. RESULTS AND DISCUSSION 3.1. Characterization of As-Spun Fibers. Spinning dopes made with high concentrations of nanoparticles relative to PVA polymer (PVA/HfO2, 20/80 wt/wt %) were spinnable because hydrogen bonding occurs between acetate groups on HfO2acetate nanoparticles (referred to as HfO2 in the text below) and the PVA polymers in spinning dopes so that the particles are miscible with PVA polymer solutions. Fibers were successfully spun from dopes containing PVA/HfO2 ratios over the full range from 100/0 to 80/20. In Figure 1, a schematic illustrates the interaction between PVA and HfO2 in the spinning dope. Strong hydrogen bonding between the acetate group of HfO2 and the hydroxyl group of PVA chains enables the high loading of HfO2 into PVA polymer solutions and the formation of uniform, highly loaded fibers. This hydrogen bonding can be measured via FTIR spectroscopy (Figure 2). Shifts in both the O-H and the C-H stretching bands of the PVA and the C-O stretching band were attributed to HfO2acetate. The C-O stretching band observed at 1546 cm-1 from HfO2-acetate is shifted significantly to 1577 cm-1. This blue shift can be attributed to hydrogen bonding between acetate groups on HfO2-acetate and hydroxyl groups in PVA. The CO stretching peak is expected to become larger as the concentration of HfO2 increased. With the increase of HfO2 contents in the hybrid fiber webs, however, the location of the PVA O-H peak was not significantly shifted but decreased in intensity and then

Figure 3. Raman spectra of as-spun PVA/HfO2 hybrid fibers with various HfO2 weight fractions (2.5, 5, 10, 30, 60, and 80 wt %). Code: 2.5 wt %, PVA/HfO2 (97.5/2.5 wt/wt %) hybrid fibers.

disappeared at the HfO2 80 wt % loading. As depicted in Figure 1, incorporation of HfO2 into the PVA fibers resulted in strong association of the PVA hydroxyls with HfO2. Addition of HfO2 also disrupted PVA crystallization during electrospinning. PVA typically forms small, dense, and closely packed monoclinic crystallites.44 The degree of crystallinity of PVA fibers strongly affects the FTIR C-O stretching peak at 1141 cm-1. Addition of inorganic materials to electrospinning dopes has been broadly shown to enhance the crystallization behavior of PVA and other polymers.45-47 Interestingly, the crystal growth of PVA was suppressed by incorporation of HfO2. Incorporation of 30 wt % HfO2 in the PVA polymer completely suppressed crystal formation and resulted in an amorphous PVA phase.46 As the PVA polymer chains are aligned and folded to make the crystalline structure, the PVA hydroxyl groups typically form intramolecular and intermolecular hydrogen bonds between PVA chains.48 As mentioned earlier, the position and peak at 1141 cm-1 depend on the degree of crystallization. The decrease in the crystallinity is also verified by Raman spectra (Figure 3). In the Raman spectrum, a vibrational peak at 1147 cm-1 is an indicator for the degree of crystallinity of the PVA polymer. As the HfO2 weight 5537



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Figure 4. EDX spectrum results at the 1 and 2 points on the PVA/HfO2 (70/30) hybrid fibers.

ratio increases from 2.5 to 10 wt %, the gradual decrease in 1147 cm-1 is clearly observed in the Raman spectra, and no discernible peak at 1147 cm-1 is observed at PVA/HfO2 (70/30). The existence of the HfO2 at the surface of the PVA/HfO2 (70/ 30) hybrid fibers was proven by EDX analysis, as shown in Figure 4. EDX spectra show that identical Hf peak intensities were measured at multiple points along the nanofiber. Through the elemental maps of C at the K peak and Hf at the M peak, the existence of HfO2 in nanofibers and their uniform distribution might be verified. 3.2. Water Stabilization of As-Spun Fibers. Although PVA is readily spun into uniform nanofibers, the usefulness of these nanofibers is limited by their water solubility. For example, the PVA hydroxyl functional groups can be used for virus capture or enzyme immobilization.49 Cross-linking the fibers via a condensation reaction between the PVA and HfO2 reduced the water swelling and solubility of the PVA/HfO2 hybrid fibers. The electrospun nanofiber webs of pure PVA and hybrid PVA/HfO2 were heat-treated at 140 C for 12 h to induce a thermal cross-linking without any other additives. Figure 5A shows PVA and PVA/HfO2 (70/30 wt/wt %) nanofiber webs after heat treatment and immersion in water. After the heat treatment, the color of the pure PVA fiber webs was changed from white to light brown, indicating that PVA was degraded by the heat treatment. However, the hybrid fiber webs did not discolor even at low nanoparticle loadings. This shows that the addition of HfO2 improved the thermal stability of the nanofibers. When these fibers were soaked in water for 5 min, neither fiber dissolved (Figure 5B). However, the SEM images compared before and after soaking in water show obvious swelling and collapse of the pure PVA fiber webs (Figure 5D). The morphology of the hybrid fiber webs was maintained, as shown in Figure 5F. HfO2 incorporation in nanofibers, which induced the thermal cross-linking within PVA polymer chains, resulted in the HfO2-incorporated PVA nanofibers that improved the thermal stability and were stable in water. From the FTIR spectra, the decrease in the O-H stretching vibration (3300 cm-1) was observed in the PVA/HfO2 hybrid fibers after heat treatment, even though pure PVA fiber did not show any significant change in the O-H peak before and after heat treatment. This result confirms that the thermal cross-linking occurred in the PVA/HfO2 hybrid fibers during heat treatment.

3.3. Calcined Inorganic HfO2 Fibers. Beyond stabilization of the PVA nanofibers, calcinations of highly loaded PVA/HfO2 fibers were used as a route to produce inorganic HfO2 fibers. SEM images of the as-electrospun fibers and the fibers calcined at different temperatures are shown in Figure 6. All of the fibers showed good fiber morphology despite the highest loading of HfO2 of 80 wt %, and the calcined fibers also maintained their fibrous structures as well. To our knowledge, there has been no report on the fabrication of inorganic fibers from the spinning dopes with high loading of inorganic particles up to approximately 50 wt % over polymer. The actual loading ratio of inorganic nanoparticles in the polymer is inferred from the fact that HfO2-acetate nanoparticles have around 40% organic acetate groups, according to the TGA results in Figure 7. As shown in Figure 1, the hydrogen bonding between HfO2 and the carrier PVA polymer can achieve the good miscibility that allows the homogeneous spinning dopes to experience the whipping and thinning motion during the electrospinning and to form a good fiber morphology. The diameter of the electrospun fibers increased as the ratio of HfO2 nanoparticles increased, as shown in Figures 6 and 8, possibly due to the increase in solution viscosity. The diameter increase between pure PVA and PVA/HfO2 70/30 fibers was not statistically significant. The fiber diameter of 40/60 PVA/HfO2 was twice the diameter of the pure PVA, and 20/80 PVA/HfO2 fibers were 6 times as large. The spinning dopes with 80 wt % HfO2 nanoparticles were visibly more viscous. The large proportion of nanoparticles in the dope hindered the thinning of the polymer jet, resulting in thicker spun fiber diameters. The as-spun fibers were calcined at temperatures ranging from 450 to 700 C. As is expected, weight loss from the polymer decomposition and degradation sequentially occurred at all calcinations temperatures. Figure 8 shows the diameters of the hybrid fibers and the inorganic fibers before and after calcination, respectively. All of them show that the diameter of the fibers became thinner after the calcination of the as-spun fibers. It is interesting to note that the diameter decreases dramatically after calcination at 450 C, which might be due to the collapse and relocation of the inorganic nanoparticles as the PVA polymers decompose away under 300 C. The diameter decreases can be attributed to both removal of PVA and repacking of the HfO2 particles into one or more of the three known crystalline structures and amorphous 5538



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Figure 5. Photographs and SEM images of PVA and PVA/HfO2 (70/30 wt/wt %) fibers: (A) after heat treatment and (B) after soaking in water for 5 min; PVA nanofibers (C) before and (D) after soaking; PVA/HfO2 hybrid fibers (E) before and (F) after soaking.

Figure 6. SEM images of electrospun fibers and their calcined fibers according to the different HfO2 ratios in the hybrid fibers (scale bar represents 3 μm). 5539



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Figure 7. TGA results of HfO2-acetate nanoparticles and pure PVA nanofibers.

Figure 9. XRD patterns of (A) PVA/HfO2 (70/30), (B) PVA/HfO2 (40/60), and (C) PVA/HfO2 (20/80) hybrid fibers after calcination at different temperatures (450, 550, and 700 C) for 15 min.

Figure 8. Diameters of the as-spun fibers and their calcined fibers.

regions. Further analysis via XRD and Raman spectroscopy was used to elucidate the fiber morphology. It is important to understand the structural transformation of HfO2 nanoparticles in PVA/HfO2 hybrid fibers under variations in calcination temperature. The tetragonal phases have much greater dielectric constants than monoclinic phases.50 Usually, HfO2 can exist under atmospheric pressure in the form of three packing structures (the monoclinic, the tetragonal, and the cubic modifications). Under heating, the monoclinic phase of HfO2 transforms, first, into the tetragonal domains and then to the cubic structure. X-ray diffraction patterns (XRD) of PVA/HfO2 hybrid fibers and the HfO2 inorganic nanofibers formed by the calcination at temperatures ranging from 450 to 700 C for 15 min are shown in Figure 9. The calcination temperatures and HfO2 concentrations are variables in this analysis. All the XRD data show amorphous traces below 500 C and relatively sharp peaks at 21 (Figure 9). To prevent a misunderstanding, we have to note here that the peak at 21 is not attributed to the crystallinity of PVA (∼20) but is attributed to the background of the SiO2 thin layer formed on the Si wafer.51 At room temperature, the amorphous trace confirms that neither

PVA nor HfO2 crystallize measurably in the as-spun fiber. Calcination at 450 C does not result in measurable crystallization of the HfO2 nanoparticles as the PVA is removed. Samples calcined at 550 C have some crystalline regions with monoclinic crystalline peaks evident in XRD spectra for samples calcined from 20/80 and 40/60 PVA/HfO2 fibers and tetragonal crystalline peaks for samples calcined from 70/30 PVA/HfO2 fibers along with a significant amorphous background. As can be seen from Figure 9, the calcination temperature and the ratio of HfO2 in the hybrid fibers are factors to influence the structural transformation. The calcination at below 550 C formed the broad peaks assigned as amorphous domains, whereas the calcination at above 550 C started to make the distinct peaks. In the case of the concentration of HfO2 in the PVA/HfO2 hybrid fiber system, the XRD peaks between 30 and 60 wt % HfO2 showed the different patterns. Especially with 30 wt % HfO2, the peaks developed less sharp than with 60 wt %. Otherwise, the structural transformations of 60 and 80 wt % HfO2 according to the temperature displayed almost the same results. In Figure 9A (PVA/HfO2 70/30 wt/wt %), relatively broad, but distinct, diffraction peaks are observed above 550 C, and these diffraction peaks are ascribed to the tetragonal phase of HfO2. The reflection at 2θ = 30.02 (111) and the slight splitting of the 110 and 112 peaks at 35 and 50 provide evidence for the existence of the tetragonal phase in the HfO2 nanofiber at 700 C. Calcination at 700 C results in strong crystalline peaks and a decrease of the amorphous background. The observation of characteristic peaks for the HfO2 tetragonal phase is significant and unexpected. The tetragonal phase is usually observed only at temperatures above 800 C.26 In Figure 9A (PVA/HfO2 70/30 wt/wt %), the relatively broad, but distinct, diffraction peaks are observed in samples calcined at 5540



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Figure 10. FESEM images of (A) 70/30 wt/wt % PVA/HfO2 hybrid fibers and (B) 20/80 wt/wt % PVA/HfO2 hybrid fibers after calcination at 450 C for 15 min.

Figure 11. FTIR spectra of before and after calcination of HfO2 nanoparticles and PVA/HfO2 hybrid fibers (HfO2 30 wt %) at 450 C.

550 C and the sharper peaks evident in XRD spectra of samples calcined at 700 C are ascribed to the tetragonal phase of HfO2. In particular, the reflection at 2θ = 30.02 (111) and the slight splitting of the 110 and 112 peaks at 35 and 50 provide strong evidence for the existence of tetragonal phase crystals in the HfO2 nanofiber at 700 C. On the other hand, the XRD data in HfO2 nanofibers calcined from 20/80 and 40/60 PVA/HfO2 have XRD spectra indicating a polycrystalline, (111) textured, monoclinic phase (Figure 9B,C). Consistently, calcinations of these samples at 550 C resulted in some crystallization within largely amorphous fibers. Calcination at 700 C resulted in significant crystallization with little or no amorphous background evident in the XRD spectra. No discernible characteristic peak of the tetragonal phase at 30.02 was observed for these HfO2 nanofibers. The XRD spectra of the monoclinic phase formed after the calcination at 700 C agrees well with the literature.52,53 The tetragonal phase appearing in the HfO2 nanofiber (calcined with 70/30 PVA/HfO2 hybrid fibers (30 wt %)) compared to higher concentrations of HfO2 is an extraordinary case. The large amount of PVA affects the crystallization behavior of the HfO2 nanoparticles during the calcinations; moreover, the encapsulated HfO2 by a relatively large amount of PVA favors the formation of a

tetragonal crystal structure over the monoclinic structure during the calcination. As can be seen in FESEM images in Figure 10, the HfO2 nanofibers calcined with 70/30 wt/wt % PVA/HfO2 hybrid fibers appeared to have the packed structures on their surface; otherwise, the HfO2 nanofibers calcined with 20/80 wt/wt % PVA/HfO2 hybrid fibers had lots of pores, showing the poorly formed crystals. The materials produced in this study are pure hafnium fibers that have a porous morphology. The porosity depends on the precursor formulation and the calcining conditions. FESEM confirmed that HfO2 nanofibers could be fabricated to form either a more or a less packed structure than the starting materials (HfO2 nanoparticles). The crystallite size was calculated using the Scherrer equation, t = (K 3 λ)/(β 3 cos θ), and the average crystallite size in the HfO2 nanofiber calculated from the monoclinic (111) diffraction peak was approximately 8 nm in the HfO2 (60 wt %) nanofibers and HfO2 (80 wt %) nanofibers at 700 C. The tetragonal structure in the HfO2 (30 wt %) nanofiber is relatively poorly formed such that the crystallite size is about 3 nm calculated from the tetragonal (111) diffraction peaks. The FTIR spectra of the PVA/HfO2 (70/30) hybrid fibers and pure HfO2 before and after calcination are shown in Figure 11. The peaks of interest are assigned in Table 1. The intensity of the 5541



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Table 1. FTIR Peak Assignments of PVA and HfO2-Acetate chemical group

wavenumber (cm-1)

assignments

PVA

3550-3100

O-H from the intermolecular and intramolecular hydrogen bonds

PVA

3000-2840

C-H from alkyl groups

HfO2-acetate

1710

CdO from acetate

HfO2-acetate, PVA

1570, 1546, 1450, 1420

C-O from acetate and PVA

PVA

1236

CH deformation

PVA

1141

C-O (crystallinity)

PVA

1097

C-O-C stretching

HfO2-acetate

651

Hf-O-Hf

Figure 12. FTIR spectra of (A) PVA/HfO2 hybrid fibers (70/30), (B) PVA/HfO2 hybrid fibers (40/60), and (C) PVA/HfO2 hybrid fibers (20/80) before and after the calcination at different temperatures (450 and 700 C) for 15 min.

Figure 13. Raman spectra of (A) PVA/HfO2 hybrid fibers (70/30), (B) PVA/HfO2 hybrid fibers (40/60), and (C) PVA/HfO2 hybrid fibers (20/80) before and after the calcination at different temperatures (450 and 700 C) for 15 min.

characteristic peaks of HfO2 in the as-spun fibers increases as the HfO2 ratio increases (Figure 2A). The spectrum of HfO2 shows a weak band at 1710 cm-1, which corresponds to the CdO double bond formed by the acetate of nanoparticles. It is inferred that the HfO2 nanoparticles were bound to acetate groups and surrounded by them during HfO2-acetate nanoparticle synthesis.22 In FTIR spectra of all the PVA/HfO2 hybrid fibers, the weakly bonded acetate completely disappears, as shown in Figure 11. After calcination of the hybrid fibers, the acetate band is again evident and has red shifted back to 1548 cm-1 as there are no hydrogen-bonding sites left after PVA has been removed at this high temperature. The FTIR spectra of HfO2 nanofibers before and after calcination at various temperatures are shown in Figure 12. After the calcinations, all of the PVA characteristic peaks disappear even at the 450 C

temperature. FTIR spectra of calcined fibers provide further evidence for the crystalline structure formation. XRD spectra indicated that HfO2 nanofibers calcined from 40/60 and 20/80 wt/wt % PVA/ HfO2 hybrid fibers at 700 C formed monoclinic crystals. The FTIR absorption bands at 755, 601, and 512 cm-1 were observed (Figure 12B,C). These bands have been previously reported as characteristic of the monoclinic structure.44,45 No discernible absorbance bands were observed in FTIR spectra of the nanofibers calcined from 70/30 wt/wt % PVA/HfO2 hybrid fibers at 700 C. Raman spectroscopy has also been useful in confirming and identifying HfO2 crystalline phases. In the case of the monoclinic structure, 18 Raman-active modes (9Ag þ 9Bg) and 15 IR-active modes (8Au þ 7Bu) are detected. The tetragonal structure of 5542


The Journal of Physical Chemistry C HfO2, however, has only three IR-active modes (A2u þ 2Eu) and three Raman-active modes (A1g, B1g, and Eg), and all these modes lie in the far IR region and are also very weak.46 Figure 13 shows the Raman spectra of the as-spun PVA/HfO2 hybrid fibers and inorganic fibers calcined at 450 and 700 C. The strong peak around 498 cm-1 (partly obscured by the strong Si Raman mode at 520 cm-1) can be assigned to the strongest Au phonon mode of the monoclinic phase for HfO2 in Figure 12B,C. These results confirm the presence of a monoclinic phase in the PVA/HfO2 nanofibers (40/60 and 20/80) calcined at 700 C, in agreement with FTIR and XRD analyses in the present study. The small peaks around 550, 582, 641, and 675 cm-1 in the PVA/HfO2 fibers (40/60 and 20/80) calcined at 700 C are all characteristic peaks of the monoclinic phase of HfO2.

4. CONCLUSIONS The organic/inorganic (PVA/HfO2) hybrid fibers were fabricated to form the fiber morphology from the prepared hybrid spinning dopes in which HfO2-acetate nanoparticles were loaded into PVA solutions up to 80 wt % over PVA. Strong hydrogen bonding between PVA and HfO2-acetate nanoparticles resulted in stable spinning dopes and spinnable systems even at extremely high HfO2-acetate proportions. The as-spun fibers were uniform, smooth, and bead-free. Annealing these fibers resulted in waterstable PVA/HfO2 fibers. Pure inorganic HfO2 fibers were formed by the calcination of the as-spun hybrid fibers at 450, 550, and 700 C. Fibers calcined at 450 C were largely amorphous with no measurable crystalline structure formation. Calcination at 550 C resulted in a low degree of crystallization in the monoclinic phase for samples calcined from 20/80 and 40/60 wt/wt % PVA/HfO2 fibers and in the tetragonal crystal for samples calcined from 70/30 wt/wt % PVA HfO2 fibers. Calcination at 700 C led to a significantly higher crystallinity in the same crystal forms developed at 550 C. The type and size of the crystals are related to the concentration of HfO2acetate nanoparticles in the initial spinning dopes and to the calcination temperature. Confirmation of tetragonal crystal formation at 550 and 700 C is facilitated by incorporation of the HfO2 nanoparticles into the PVA fibers; formation of this structure has been previously reported only at temperatures greater than 800 C. The formation of stable tetragonal crystal structures in the as-spun hybrid fibers with lower concentrations of HfO2 was achieved at lower temperature treatments than the monoclinic-to-tetragonal transition temperature (800 C). ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 607-255-1937. Fax: 607255-1093. Author Contributions z

The authors contributed equally to this work.

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HfO2 Hybrid Electrospun Fibers and Calcined

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