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Thermal stability of HfO2 nanotube arrays
Applied Surface Science 257 (2011) 4075–4081
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Thermal stability of HfO2 nanotube arrays Xiaofeng Qiu a,1 , Jane Y. Howe b , Harry M. Meyer III b , Enis Tuncer c , M. Parans Paranthaman a,∗ a
Materials Chemistry Group, Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Fusion Energy Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b
a r t i c l e
i n f o
Article history: Received 5 November 2010 Received in revised form 28 November 2010 Accepted 29 November 2010 Available online 4 December 2010 Keywords: Hafnium oxide HfO2 Nanotube arrays Anodic oxidation Thermal stability Dielectric properties
a b s t r a c t Thermal stability of highly ordered hafnium oxide (HfO2 ) nanotube arrays prepared through an electrochemical anodization method in the presence of ammonium fluoride is investigated in a temperature range of room temperature to 900 ◦ C in flowing argon atmosphere. The formation of the HfO2 nanotube arrays was monitored by current density transient characteristics during anodization of hafnium metal foil. Morphologies of the as-grown and post-annealed HfO2 nanotube arrays were analyzed by powder Xray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although monoclinic HfO2 is thermally stable up to 2000 K in bulk, the morphology of HfO2 nanotube arrays degraded at 900 ◦ C. A detailed X-ray photoelectron spectroscopy (XPS) study revealed that the thermal treatment significantly impacted the composition and the chemical environment of the core elements (Hf and O), as well as F content coming from the electrolyte. Possible reasons for the degradation of the nanotube at high temperature were discussed based on XPS study and possible future improvements have also been suggested. Moreover, dielectric measurements were carried out on both the as-grown amorphous film and 500 ◦ C post-annealed crystalline film. This study will help us to understand the temperature impact on the morphology of nanotube arrays, which is important to its further applications at elevated temperatures. Published by Elsevier B.V.
1. Introduction In recent years, electrochemical anodization techniques have been successfully developed to prepare a variety of self-organized metal oxide nanotube arrays directly from metal substrates [1,2]. Although nanotube arrays have been successfully fabricated on a number of valve metals such as Ti, Zr, Hf, Nb, Ta, and W [1,3–10], the majority of the studies focused on the titanium oxide (TiO2 ) nanotube arrays, due to its great potential in energy related research. The biggest advantage of this technique is the ability to produce and manipulate highly ordered metal oxide nanotube structures through a simple one-step template-free approach. Therefore, this method attracts a great deal of attention in the development of nanotube based applications, such as dye-sensitized solar cells [11–13], catalysis [14,15], Li-battery [16], biomedical [17–19], and gas sensors [20,21]. Metal oxides with aligned nanotube structures are promising as catalysts or as a support for the catalysts. However, it
∗ Corresponding author at: Materials Chemistry Group, Chemical Science Division, Oak Ridge National Laboratory, Building 4500 South, MS-6100, 1 Bethel Valley Road, Oak Ridge, TN 37831-6100, USA. Tel.: +1 865 574 5045; fax: +1 865 574 4961. E-mail address: [email protected] (M.P. Paranthaman). 1 Present address: IBM TJ Watson Research Center, P.O. Box 218, Rte 134, Yorktown Heights, NY 10598, USA. 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.11.178
is quite challenging to design a catalyst or a catalyst support with proper chemical and thermal properties [22], which is particularly true in the case of anodic nanotube arrays. In order to operate at higher temperatures, the key requirement of such nanostructures is the structural stability against morphological transformations [23]. However, thermal stability of anodic nanotube arrays has been relatively less studied. There are only a few studies reported in the literature. As the most studied system, TiO2 nanotube arrays are normally post-treated at a temperature below 500 ◦ C in order to avoid the structural transformation due to the anatase to rutile phase transition [2,24–26]. In some cases, the TiO2 nanotube structures were reported to survive even up to 600 ◦ C in the presence of NH3 [27]. Guo et al. recently reported the collapse of ZrO2 nanotubes at 800 ◦ C [28]. We found that the HfO2 nanotube arrays can still preserve nice tubular structures at 700 ◦ C [9]. It was not until recently, Li and coworkers reported the morphological transformation of Ti–Al–V–O nanotube arrays from room temperature to 700 ◦ C [29]. However, a systematic study of thermal annealing effects on the structural and chemical stability of anodic nanotube arrays is still required. Herein, we investigated the morphological and surface chemical changes of anodic HfO2 nanotube arrays over a wide temperature range between room temperature to 900 ◦ C. It was found that the as-prepared F containing HfO2 nanotube structure was preserved until 700 ◦ C and then began to collapse at 900 ◦ C in an argon atmosphere. XPS studies of Hf 4f, F 1s and O 1s core level
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spectra indicate that thermal treatment has significant impacts on the coordination of core elements. The possible reasons for the nanotube collapse at 900 ◦ C are also discussed based on the XPS data. HfO2 is a well studied material with high-k dielectric constant, high refractive index, chemical stability, physical hardness, high bulk modulus and high melting temperature [30,31]. Therefore, it is a promising material for a variety of fields such as gate insulators in field-effect transistors, optical coating for UV to IR laser application and high temperature applications [31]. The synthesis of HfO2 nanotube arrays through electrochemical anodization was first reported in 2005 [8]. In 2009, highly ordered hexagonal HfO2 nanotube arrays were developed in the presence of F− by our group and Schmuki’s group [9,32]. In our previous study, we found that the as-prepared amorphous HfO2 can be converted into a crystalline HfO2 at temperatures higher than 500 ◦ C. We have also found that the initially ordered HfO2 nanotube arrays contain significant amount of fluorine and can be removed only at elevated temperatures. Bulk HfO2 is monoclinic (P21 c) at ambient conditions and transforms to a tetragonal phase (P42 nmc) at ∼2000 K, and a cubic phase (Fm3m) at higher temperatures and pressures [33], which makes HfO2 an important material for high temperature applications. Very recently, Shandalov et al. studied the size-dependent polymorphism in HfO2 nanotube and nanoscale thin films grown using atomic layer deposition [34]. However, the structural and chemical stability of HfO2 in the form of highly ordered nanotube arrays have not been studied and such a study can impact the further applications of this unique HfO2 nanostructures. In this paper, we report the detailed morphological and chemical properties of anodic oxidation of HfO2 nanotube arrays over a wide temperature range using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The dielectric properties of amorphous and crystalline HfO2 nanotube arrays are also presented. This study can be a valuable addition to the further applications of anodic nanotube arrays at elevated temperatures.
2. Experimental procedure Anodic oxidation of HfO2 nanotube arrays has been reported in detail in our previous publication [9]. Briefly, Hf foil (0.25 mm thick, 99.5% metal base, Alfa Aesar) 0.5 cm × 2.5 cm in size was first cleaned using ultrasonication in deionized water, isopropanol and acetone, respectively, and then dried in air. A solution containing 0.1 M NH4 F (99%, J.T. Bakers Inc.), 80 mL of ethylene glycol (99+%, Alfa Aesar) and 20 mL of deionized water was used as the electrolyte. A Keithley 2612 source meter (Keithley Instrument Inc.) was used as the power source and the results were recorded using a Lab Trace 2.0 (Keithley Instrument Inc.). A typical reaction was carried out at 20 ◦ C in a voltage range of 10–60 V for 20 min and the set voltages were instantly applied to the working electrode. The asprepared HfO2 nanotube arrays were cleaned in isopropanol using ultrasound bath and dried in air. Annealing of nanotube arrays was carried in a tube furnace under flowing Ar gas to prevent the further oxidation of Hf substrate during the post-treatment. The temperature ranges used in this study were from 300 ◦ C to 900 ◦ C and the annealing time was 1 h for all the samples. The powder X-ray diffraction results were collected using a SIEMENS D5005 X-ray Diffractometer (Cu K␣). SEM analysis was carried out using a Hitachi S4800 FEG-SEM (Hitachi High Technologies America Inc.) at an acceleration voltage of 20 kV. TEM analysis was carried out using a Hitachi HF-3300 TEM/STEM with an energy-dispersive X-ray spectrometer at 300 kV. XPS spectra were obtained using a Thermo Scientific K-Alpha XPS system (Al K␣ X-rays at 1486.6 eV) operated at a base pressure of 1 × 10−7 mbar
Fig. 1. Current density–time behavior of 0.25 mm thick Hf foil anodized (10–60 V) in an electrolyte containing 0.1 M NH4 F, 20 mL of deionized water and 80 mL of ethylene glycol. The inset is the enlarged profiles during the first 20 s of the reaction.
while running charge compensation system consisting of low E Arions and low E electrons. Impedance measurements were carried out using a QuadTech 1920 (QuadTech Inc.) between 20 Hz and 1 MHz at room temperature. 3. Results and discussion Electrochemical profile of the anodization provides very important information of the oxidation process. It was found in previous work on anodic oxidation of TiO2 nanotube arrays that the current density–time behavior is very sensitive to the reaction conditions such as voltage [35], electrolyte [7,36,37] and solvent [36]. In most cases, the current density undergoes a rapid decrease at the beginning of the reaction and then recovers back to certain values then slowly decreases as reaction continues [37]. These processes are believed to correspond to the fast formation of an insulating oxide layer followed by the dissolution of the oxide layer by electrolyte, and then reach equilibrium between these two factors and maintain the steady nanotube formation [2]. In the present study, we found a similar electrochemical profile when a bias voltage was applied to the Hf metal foil in NH4 F solution as reported before [32]. Fig. 1 shows the current density transient characteristics as a function of the applied voltage. Clearly, the initial current density increases dramatically with increase of the applied voltage. At higher voltages, current density undergoes a more rapid drop, which may be due to the fast accumulation of an oxide layer and hence creating a high electrical resistance on the film surface. Within 5 s, all the current densities start to recover. This phenomenon is believed to be related to the etching of the oxide layer under biased conditions. In about 7 s, the current density reaches a steady state and decays slowly. It can be seen from the inset of Fig. 1 that the drop and recovery of the current density takes a longer time when the applied voltage is higher. It is believed that the formation of nanotube arrays is the combination of oxide growth and dissolution [30]. These two processes may involve the field assisted ejection of Hf4+ into the electrolyte and the migration of O2− into the Hf substrate as suggested previously [38,39]. A faster migration rate of O2− due to high mobility compared to that of Hf4+ at higher voltages may explain the faster accumulation of the oxide layer and relatively slower dissolution of HfO2 . Therefore, it takes longer for the current density to reach the minimum and recover back to the steady growth condition. Interestingly, the current density profile obtained at 60 V still maintains relatively high current density during the first 200 s of reaction, and then starts to decay quickly compared to the current density transient profiles at
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Fig. 2. Typical powder XRD patterns of the as-prepared, 700 ◦ C and 900 ◦ C postannealed HfO2 nanotube arrays.
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other lower voltages. After 500 s, the decay of current density slows down. This may be because there is a second stage oxide growth at this particular condition, which is still not fully understood and worth further investigation. The as-prepared HfO2 nanotube arrays are amorphous and may contain as much F as Hf according to our previous study [9]. Strictly speaking; the initially formed nanotube arrays are mostly hafnium oxide nanotube arrays containing significant amounts of fluorine. Fig. 2 shows that an as-prepared HfO2 film is amorphous and the HfO2 films post-annealed at 700 ◦ C and 900 ◦ C in Ar for 1 h exhibit a monoclinic phase (JCPDS #43-1017). The reason in this present study for annealing the anodized nanotube array in Ar is to prevent further oxidization of the Hf substrate. The oxidation of Hf foil introduces HfO2 structures and thin films, other than HfO2 nanotube arrays, thus making the permittivity measurement of HfO2 nanotube arrays inaccurate and complicated. It is very important to preserve the nanostructures against any thermal treatments. SEM images of the as-prepared and post heattreated HfO2 nanotube arrays are shown in Fig. 3. All of the samples shown in Fig. 3 were prepared at 20 V for 20 min. From the topview SEM images of the as-prepared HfO2 nanotube arrays, the diameter of the nanotube was determined to be around 50 nm with a pore size around 10 nm. The cross-section view of the
Fig. 3. SEM images of the top view and cross-section view of the as-prepared (RT) HfO2 nanotube arrays and the cross-section view of HfO2 nanotube arrays annealed at 300 ◦ C, 500 ◦ C, 700 ◦ C and 900 ◦ C in Ar for 1 h. Insets are the enlarged view of the HfO2 nanotube arrays.
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Fig. 4. TEM images of the as-prepared HfO2 nanotubes (A, C) and 700 ◦ C post-annealed nanotube arrays (B, D). Inset in B is the selected area electron diffraction (SAED) patterns of the post-annealed HfO2 nanotubes. Inset D is the Fourier transferred diffractogram from high resolution TEM image D.
as-prepared nanotubes reveals the length of the nanotubes to be ∼4 m. The thermal treatment of HfO2 at temperatures up to 700 ◦ C in Ar mostly maintains the morphology of the nanotube structures. The nanotube arrays treated at temperatures lower than 700 ◦ C still preserve very smooth surfaces. However, the nanotube surface becomes a little rougher when the temperature increases to 700 ◦ C as shown in Fig. 3. Although the material maintains a nanotube shape, most of the nanotube surfaces become rougher. At 900 ◦ C the nanotubes turn into small particles and completely lose their tubular structures. This study shows that the anodic HfO2 nanotube arrays survive only below 900 ◦ C. As mentioned in the introduction, bulk HfO2 is regarded as one of the promising thermally stable materials and could remain at monoclinic phase up to 2000K. However, in the case of nanotube arrays, the tubular structure survives only at temperatures lower than 900 ◦ C. The detailed structures of the nanotubes have been investigated using TEM. Fig. 4A and C shows the TEM images of the as-prepared HfO2 nanotubes. Fig. 4A is the middle and upper part of the HfO2 nanotubes. Compared to the bottom part of the nanotubes shown in Fig. 4C, the wall of the bottom part of the nanotube is thicker than the middle or upper parts. This is mainly due to the exposure of the top of the nanotube for longer times to the corrosive NH4 F solution as compared to the bottom of the tube. It can also be seen that the as-prepared nanotube is completely amorphous (Fig. 4C). Our previous study indicated that the amorphous tubes are formed by fluorine containing hafnium oxide where the F:Hf ratio can reach 1:1 [9]. Fig. 4B and D is the high resolution TEM images of the HfO2 nanotubes thermally treated at 700 ◦ C in Ar for 1 h. Fig. 4B and D shows the different parts of the nanotube after post-annealing. Fig. 4B is the lower part of the nanotube, which underwent less chemical etching and most of the wall materials after the reaction
was preserved. This is probably why the wall of the nanotube after thermal treatment is relatively thick. The top part of the nanotube (Fig. 4D) has been etched for longer time and the thickness of the nanotube wall is apparently thinner than the lower part. Fig. 4B inset SAED patterns proved that the 700 ◦ C annealing can effectively transform the amorphous HfO2 into well defined crystalline monoclinic phase. For most of the nanotube arrays developed through an electrochemical anodization approach, a F containing electrolyte is most commonly used. Therefore, F content plays an important role in the formation and maintaining the nanotube scaffold. In the case of HfO2 , an almost equal amount of F to Hf was found in the system according to the elemental analysis using TEM [9]. However, the simple elemental analysis and crystal structure analysis cannot track detailed information about the chemical properties of the involved elements as temperature increases. Herein, we utilized XPS as a tool to probe the chemical environment changes and quantitative elemental analysis during the thermal treatment [40–43]. Fig. 5A is the surface element ratios of F to Hf as a function of annealing temperature. It is believed that as the temperature increased, the initial hafnium oxide/fluoride mixture is most possibly transformed to hafnium oxide by releasing F. Fig. 5A F/Hf ratio shows that the loss of F happened mostly after 500 ◦ C under Ar and dropped rapidly from 500 ◦ C to 700 ◦ C. This ratio stayed at a low level when the temperature was increased from 700 ◦ C to 900 ◦ C. This result indicated that the chemical composition on the surface started to dramatically change when the temperature reached 500 ◦ C, which is consistent with the crystal structure formation temperature investigated by XRD [9]. Fig. 5B, C and D shows the core-level spectra of Hf 4f, F 1s and O 1s. As seen in Fig. 5, increasing temperature affects the binding structures of Hf, F and O significantly. Fig. 5B shows the Hf 4f of the samples with and
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Fig. 5. (A) F to Hf atomic ratio obtained from XPS as a function of annealing temperatures (the line guides the eye), (B) Hf 4f, (C) F 1s and (D) O 1s binding energy of the as-prepared samples and post heat-treated from 300 ◦ C to 900 ◦ C.
without the heat-treatment. In general, the spectra show an overall narrowing of the spectral envelope and increased peak resolution. These observations are consistent with converting a mixed oxide/fluoride film into a more ordered single component (i.e., oxide) film. The Hf 4f core level consist of two peaks (a doublet) that are separated by ∼1.7 eV. The peak at lower BE (4f7/2) is centered at ∼17.5 eV and is associated primarily with Hf–O bonding. For the as-prepared sample (bottom spectrum) the overall peak shape is broad with poor resolution between the two peaks. This indicates a second (or more) bonding configuration for the Hf, most likely Hf–F bonding. As the film is heated to higher temperatures, there is a loss of F and hence a loss of Hf–F bonding. The Hf 4f spectra for the 700 ◦ C and 900 ◦ C films show a well resolved, narrow peak structure, consistent with HfO2 [40–43]. The binding energy of the single F 1s feature around 685.6 eV is consistent with an inorganic fluoride and is attributed to the Hf–F bond [43]. Fig. 5C shows that F 1s peak position remains almost unchanged, but the peak intensity decreases as anneal temperature increases. It is notable that the peak intensity around 685.6 eV remained at the same level at a temperature lower than 500 ◦ C and significantly reduced when the temperature went beyond 700 ◦ C. This is evident that the Hf–F bonds are essentially stable until 500 ◦ C under Ar in the nanotube structure. Hf–F bonds break up at a temperature higher than 700 ◦ C. In O 1s region, a predominate band around 531 eV contributions from lattice oxygen of Hf–O bonds [40–43] and a broad shoulder band near 532 eV peak is most likely –OH and/or water and it decreases with annealing [43]. As from Fig. 5D, the most likely replacement of Hf–F back-
bones by Hf–O bonds happened at 700 ◦ C. Since the system was thermally treated under an oxygen free environment, the sources of O for Hf–O bond formation could only come from the nanotube itself or the surface adsorbed organic residues. Although the monoclinic phase formation initiates at 500 ◦ C for anodic HfO2 nanotubes, further refining of the crystal structure occurs at 700 ◦ C. At this temperature, most of the Hf–F bonds were replaced by Hf–O bonds. The transition from Hf–F to Hf–O seems to maintain the well defined nanotube array structures at 700 ◦ C, but a higher temperature of 900 ◦ C seems to break up the ordering of nanotubes. It is an interesting phenomenon that the structural collapse did not happen at 700 ◦ C when the F was rapidly released from the system. A further increase of 200 ◦ C leads to only a small loss of F from the system compared to what happened at 700 ◦ C, but resulted in a significant change of the morphology of nanostructures. There are few possible reasons for the observed phenomenon: (1) insufficient oxygen available in the environment limits further replacement of Hf–F by Hf–O and at 900 ◦ C, some of the F vacancies may not be filled by O leading to the collapse of the nanostructures; or (2) decomposition of HfO2 occurs at 900 ◦ C. As reported by Zhan et al. [44], the decomposition of HfO2 and release of O2 could happen even at a temperature as low as 500 ◦ C under rapid thermal annealing conditions. XPS studies uncover the complicated scenario behind thermal annealing of anodic metal oxides. Most of the nanotube structures prepared through electrochemical anodization directly from the metal substrates exhibit initial amorphous structures and contains
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ble and strong metal oxide in bulk, HfO2 nanotube arrays lost well defined nanostructures at 900 ◦ C in Ar. Detailed XPS studies revealed that F impurity originated from the electrolyte can significantly affect the chemical composition and morphologies of the HfO2 nanotube arrays even at a temperature higher than 700 ◦ C when annealed in Ar. At a temperature as high as 900 ◦ C, factors like decomposition of metal oxides and reaction with substrate can also be responsible for the morphological degradation of the nanotube arrays. Further stabilization of the metal oxide nanostructures is desired for a broader range of applications at a wider operation temperature and atmosphere ranges. Dielectric measurements on the amorphous and crystalline HfO2 nanotube arrays were conducted to evaluate the temperature impact to the nanotube array properties. Acknowledgements
Fig. 6. Complex resistivity plots of HfO2 samples. As-prepared and 500 ◦ C postannealed samples were labeled with amorphous and crystalline, respectively. The inset shows the loss tangent of and the relative permittivity of the samples.
impurities originated from the electrochemical reactions. The postthermal annealing is the most common approach to remove any impurities present and crystallize the matrix. For most of the cases, the annealing temperatures are relatively lower and the composition of the matrix during the thermal treatment is paid very little attention. In order to further explore the potential applications for such unique structures, morphological, thermal and chemical stability, as well as surface condition requires more extensive investigation. This work suggested that even for the strong material such as HfO2 , the thermal and chemical stability of nanotube arrays is not necessarily guaranteed at higher temperatures and could be impacted by multiple factors. To further improve the stability of nanotube arrays, selective doping [45] and more targeted temperatures and operation atmospheres are approaches for future considerations. In order to evaluate the temperature impact on the intrinsic properties of the HfO2 nanotube arrays, dielectric measurements on both as-grown and 500 ◦ C post-annealed HfO2 tubes were performed using an impedance analyzer. The complex resistivity level ( = + i ) plots (Fig. 6) showed that the crystalline material (posttreatment at 500 ◦ C and labeled crystalline in Fig. 6) had lower resistivity than the untreated sample (amorphous). The dielectric losses shown in the set indicate that the crystalline material has higher losses at intermediate frequencies. However, the amorphous sample has a clear ohmic loss with dc resistivity value approximately 6.7 × 106 /m. The crystalline sample indicated that the conductive relaxation processes were not finalized because of the lack of bending of the complex resistivity representation. Relative dielectric permittivities of the HfO2 materials were close to each other at high frequencies. We estimated that the permittivities of the amorphous and crystalline tubes were 18.0 and 17.2, respectively. In the literature, the relative permittivity of HfO2 has been reported to be around 16 [46] and 17 [47]. 4. Conclusions Thermal and chemical stability studies of the HfO2 nanotube arrays through electrochemical anodization were investigated using SEM, TEM, XRD and XPS techniques. As a well known sta-
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Thermal stability of HfO2 nanotube arrays Xiaofeng Qiu a,1 , Jane Y. Howe b , Harry M. Meyer III b , Enis Tuncer c , M. Parans Paranthaman a,∗ a
Materials Chemistry Group, Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Fusion Energy Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b
a r t i c l e
i n f o
Article history: Received 5 November 2010 Received in revised form 28 November 2010 Accepted 29 November 2010 Available online 4 December 2010 Keywords: Hafnium oxide HfO2 Nanotube arrays Anodic oxidation Thermal stability Dielectric properties
a b s t r a c t Thermal stability of highly ordered hafnium oxide (HfO2 ) nanotube arrays prepared through an electrochemical anodization method in the presence of ammonium fluoride is investigated in a temperature range of room temperature to 900 ◦ C in flowing argon atmosphere. The formation of the HfO2 nanotube arrays was monitored by current density transient characteristics during anodization of hafnium metal foil. Morphologies of the as-grown and post-annealed HfO2 nanotube arrays were analyzed by powder Xray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although monoclinic HfO2 is thermally stable up to 2000 K in bulk, the morphology of HfO2 nanotube arrays degraded at 900 ◦ C. A detailed X-ray photoelectron spectroscopy (XPS) study revealed that the thermal treatment significantly impacted the composition and the chemical environment of the core elements (Hf and O), as well as F content coming from the electrolyte. Possible reasons for the degradation of the nanotube at high temperature were discussed based on XPS study and possible future improvements have also been suggested. Moreover, dielectric measurements were carried out on both the as-grown amorphous film and 500 ◦ C post-annealed crystalline film. This study will help us to understand the temperature impact on the morphology of nanotube arrays, which is important to its further applications at elevated temperatures. Published by Elsevier B.V.
1. Introduction In recent years, electrochemical anodization techniques have been successfully developed to prepare a variety of self-organized metal oxide nanotube arrays directly from metal substrates [1,2]. Although nanotube arrays have been successfully fabricated on a number of valve metals such as Ti, Zr, Hf, Nb, Ta, and W [1,3–10], the majority of the studies focused on the titanium oxide (TiO2 ) nanotube arrays, due to its great potential in energy related research. The biggest advantage of this technique is the ability to produce and manipulate highly ordered metal oxide nanotube structures through a simple one-step template-free approach. Therefore, this method attracts a great deal of attention in the development of nanotube based applications, such as dye-sensitized solar cells [11–13], catalysis [14,15], Li-battery [16], biomedical [17–19], and gas sensors [20,21]. Metal oxides with aligned nanotube structures are promising as catalysts or as a support for the catalysts. However, it
∗ Corresponding author at: Materials Chemistry Group, Chemical Science Division, Oak Ridge National Laboratory, Building 4500 South, MS-6100, 1 Bethel Valley Road, Oak Ridge, TN 37831-6100, USA. Tel.: +1 865 574 5045; fax: +1 865 574 4961. E-mail address: [email protected] (M.P. Paranthaman). 1 Present address: IBM TJ Watson Research Center, P.O. Box 218, Rte 134, Yorktown Heights, NY 10598, USA. 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.11.178
is quite challenging to design a catalyst or a catalyst support with proper chemical and thermal properties [22], which is particularly true in the case of anodic nanotube arrays. In order to operate at higher temperatures, the key requirement of such nanostructures is the structural stability against morphological transformations [23]. However, thermal stability of anodic nanotube arrays has been relatively less studied. There are only a few studies reported in the literature. As the most studied system, TiO2 nanotube arrays are normally post-treated at a temperature below 500 ◦ C in order to avoid the structural transformation due to the anatase to rutile phase transition [2,24–26]. In some cases, the TiO2 nanotube structures were reported to survive even up to 600 ◦ C in the presence of NH3 [27]. Guo et al. recently reported the collapse of ZrO2 nanotubes at 800 ◦ C [28]. We found that the HfO2 nanotube arrays can still preserve nice tubular structures at 700 ◦ C [9]. It was not until recently, Li and coworkers reported the morphological transformation of Ti–Al–V–O nanotube arrays from room temperature to 700 ◦ C [29]. However, a systematic study of thermal annealing effects on the structural and chemical stability of anodic nanotube arrays is still required. Herein, we investigated the morphological and surface chemical changes of anodic HfO2 nanotube arrays over a wide temperature range between room temperature to 900 ◦ C. It was found that the as-prepared F containing HfO2 nanotube structure was preserved until 700 ◦ C and then began to collapse at 900 ◦ C in an argon atmosphere. XPS studies of Hf 4f, F 1s and O 1s core level
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spectra indicate that thermal treatment has significant impacts on the coordination of core elements. The possible reasons for the nanotube collapse at 900 ◦ C are also discussed based on the XPS data. HfO2 is a well studied material with high-k dielectric constant, high refractive index, chemical stability, physical hardness, high bulk modulus and high melting temperature [30,31]. Therefore, it is a promising material for a variety of fields such as gate insulators in field-effect transistors, optical coating for UV to IR laser application and high temperature applications [31]. The synthesis of HfO2 nanotube arrays through electrochemical anodization was first reported in 2005 [8]. In 2009, highly ordered hexagonal HfO2 nanotube arrays were developed in the presence of F− by our group and Schmuki’s group [9,32]. In our previous study, we found that the as-prepared amorphous HfO2 can be converted into a crystalline HfO2 at temperatures higher than 500 ◦ C. We have also found that the initially ordered HfO2 nanotube arrays contain significant amount of fluorine and can be removed only at elevated temperatures. Bulk HfO2 is monoclinic (P21 c) at ambient conditions and transforms to a tetragonal phase (P42 nmc) at ∼2000 K, and a cubic phase (Fm3m) at higher temperatures and pressures [33], which makes HfO2 an important material for high temperature applications. Very recently, Shandalov et al. studied the size-dependent polymorphism in HfO2 nanotube and nanoscale thin films grown using atomic layer deposition [34]. However, the structural and chemical stability of HfO2 in the form of highly ordered nanotube arrays have not been studied and such a study can impact the further applications of this unique HfO2 nanostructures. In this paper, we report the detailed morphological and chemical properties of anodic oxidation of HfO2 nanotube arrays over a wide temperature range using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The dielectric properties of amorphous and crystalline HfO2 nanotube arrays are also presented. This study can be a valuable addition to the further applications of anodic nanotube arrays at elevated temperatures.
2. Experimental procedure Anodic oxidation of HfO2 nanotube arrays has been reported in detail in our previous publication [9]. Briefly, Hf foil (0.25 mm thick, 99.5% metal base, Alfa Aesar) 0.5 cm × 2.5 cm in size was first cleaned using ultrasonication in deionized water, isopropanol and acetone, respectively, and then dried in air. A solution containing 0.1 M NH4 F (99%, J.T. Bakers Inc.), 80 mL of ethylene glycol (99+%, Alfa Aesar) and 20 mL of deionized water was used as the electrolyte. A Keithley 2612 source meter (Keithley Instrument Inc.) was used as the power source and the results were recorded using a Lab Trace 2.0 (Keithley Instrument Inc.). A typical reaction was carried out at 20 ◦ C in a voltage range of 10–60 V for 20 min and the set voltages were instantly applied to the working electrode. The asprepared HfO2 nanotube arrays were cleaned in isopropanol using ultrasound bath and dried in air. Annealing of nanotube arrays was carried in a tube furnace under flowing Ar gas to prevent the further oxidation of Hf substrate during the post-treatment. The temperature ranges used in this study were from 300 ◦ C to 900 ◦ C and the annealing time was 1 h for all the samples. The powder X-ray diffraction results were collected using a SIEMENS D5005 X-ray Diffractometer (Cu K␣). SEM analysis was carried out using a Hitachi S4800 FEG-SEM (Hitachi High Technologies America Inc.) at an acceleration voltage of 20 kV. TEM analysis was carried out using a Hitachi HF-3300 TEM/STEM with an energy-dispersive X-ray spectrometer at 300 kV. XPS spectra were obtained using a Thermo Scientific K-Alpha XPS system (Al K␣ X-rays at 1486.6 eV) operated at a base pressure of 1 × 10−7 mbar
Fig. 1. Current density–time behavior of 0.25 mm thick Hf foil anodized (10–60 V) in an electrolyte containing 0.1 M NH4 F, 20 mL of deionized water and 80 mL of ethylene glycol. The inset is the enlarged profiles during the first 20 s of the reaction.
while running charge compensation system consisting of low E Arions and low E electrons. Impedance measurements were carried out using a QuadTech 1920 (QuadTech Inc.) between 20 Hz and 1 MHz at room temperature. 3. Results and discussion Electrochemical profile of the anodization provides very important information of the oxidation process. It was found in previous work on anodic oxidation of TiO2 nanotube arrays that the current density–time behavior is very sensitive to the reaction conditions such as voltage [35], electrolyte [7,36,37] and solvent [36]. In most cases, the current density undergoes a rapid decrease at the beginning of the reaction and then recovers back to certain values then slowly decreases as reaction continues [37]. These processes are believed to correspond to the fast formation of an insulating oxide layer followed by the dissolution of the oxide layer by electrolyte, and then reach equilibrium between these two factors and maintain the steady nanotube formation [2]. In the present study, we found a similar electrochemical profile when a bias voltage was applied to the Hf metal foil in NH4 F solution as reported before [32]. Fig. 1 shows the current density transient characteristics as a function of the applied voltage. Clearly, the initial current density increases dramatically with increase of the applied voltage. At higher voltages, current density undergoes a more rapid drop, which may be due to the fast accumulation of an oxide layer and hence creating a high electrical resistance on the film surface. Within 5 s, all the current densities start to recover. This phenomenon is believed to be related to the etching of the oxide layer under biased conditions. In about 7 s, the current density reaches a steady state and decays slowly. It can be seen from the inset of Fig. 1 that the drop and recovery of the current density takes a longer time when the applied voltage is higher. It is believed that the formation of nanotube arrays is the combination of oxide growth and dissolution [30]. These two processes may involve the field assisted ejection of Hf4+ into the electrolyte and the migration of O2− into the Hf substrate as suggested previously [38,39]. A faster migration rate of O2− due to high mobility compared to that of Hf4+ at higher voltages may explain the faster accumulation of the oxide layer and relatively slower dissolution of HfO2 . Therefore, it takes longer for the current density to reach the minimum and recover back to the steady growth condition. Interestingly, the current density profile obtained at 60 V still maintains relatively high current density during the first 200 s of reaction, and then starts to decay quickly compared to the current density transient profiles at
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Fig. 2. Typical powder XRD patterns of the as-prepared, 700 ◦ C and 900 ◦ C postannealed HfO2 nanotube arrays.
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other lower voltages. After 500 s, the decay of current density slows down. This may be because there is a second stage oxide growth at this particular condition, which is still not fully understood and worth further investigation. The as-prepared HfO2 nanotube arrays are amorphous and may contain as much F as Hf according to our previous study [9]. Strictly speaking; the initially formed nanotube arrays are mostly hafnium oxide nanotube arrays containing significant amounts of fluorine. Fig. 2 shows that an as-prepared HfO2 film is amorphous and the HfO2 films post-annealed at 700 ◦ C and 900 ◦ C in Ar for 1 h exhibit a monoclinic phase (JCPDS #43-1017). The reason in this present study for annealing the anodized nanotube array in Ar is to prevent further oxidization of the Hf substrate. The oxidation of Hf foil introduces HfO2 structures and thin films, other than HfO2 nanotube arrays, thus making the permittivity measurement of HfO2 nanotube arrays inaccurate and complicated. It is very important to preserve the nanostructures against any thermal treatments. SEM images of the as-prepared and post heattreated HfO2 nanotube arrays are shown in Fig. 3. All of the samples shown in Fig. 3 were prepared at 20 V for 20 min. From the topview SEM images of the as-prepared HfO2 nanotube arrays, the diameter of the nanotube was determined to be around 50 nm with a pore size around 10 nm. The cross-section view of the
Fig. 3. SEM images of the top view and cross-section view of the as-prepared (RT) HfO2 nanotube arrays and the cross-section view of HfO2 nanotube arrays annealed at 300 ◦ C, 500 ◦ C, 700 ◦ C and 900 ◦ C in Ar for 1 h. Insets are the enlarged view of the HfO2 nanotube arrays.
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Fig. 4. TEM images of the as-prepared HfO2 nanotubes (A, C) and 700 ◦ C post-annealed nanotube arrays (B, D). Inset in B is the selected area electron diffraction (SAED) patterns of the post-annealed HfO2 nanotubes. Inset D is the Fourier transferred diffractogram from high resolution TEM image D.
as-prepared nanotubes reveals the length of the nanotubes to be ∼4 m. The thermal treatment of HfO2 at temperatures up to 700 ◦ C in Ar mostly maintains the morphology of the nanotube structures. The nanotube arrays treated at temperatures lower than 700 ◦ C still preserve very smooth surfaces. However, the nanotube surface becomes a little rougher when the temperature increases to 700 ◦ C as shown in Fig. 3. Although the material maintains a nanotube shape, most of the nanotube surfaces become rougher. At 900 ◦ C the nanotubes turn into small particles and completely lose their tubular structures. This study shows that the anodic HfO2 nanotube arrays survive only below 900 ◦ C. As mentioned in the introduction, bulk HfO2 is regarded as one of the promising thermally stable materials and could remain at monoclinic phase up to 2000K. However, in the case of nanotube arrays, the tubular structure survives only at temperatures lower than 900 ◦ C. The detailed structures of the nanotubes have been investigated using TEM. Fig. 4A and C shows the TEM images of the as-prepared HfO2 nanotubes. Fig. 4A is the middle and upper part of the HfO2 nanotubes. Compared to the bottom part of the nanotubes shown in Fig. 4C, the wall of the bottom part of the nanotube is thicker than the middle or upper parts. This is mainly due to the exposure of the top of the nanotube for longer times to the corrosive NH4 F solution as compared to the bottom of the tube. It can also be seen that the as-prepared nanotube is completely amorphous (Fig. 4C). Our previous study indicated that the amorphous tubes are formed by fluorine containing hafnium oxide where the F:Hf ratio can reach 1:1 [9]. Fig. 4B and D is the high resolution TEM images of the HfO2 nanotubes thermally treated at 700 ◦ C in Ar for 1 h. Fig. 4B and D shows the different parts of the nanotube after post-annealing. Fig. 4B is the lower part of the nanotube, which underwent less chemical etching and most of the wall materials after the reaction
was preserved. This is probably why the wall of the nanotube after thermal treatment is relatively thick. The top part of the nanotube (Fig. 4D) has been etched for longer time and the thickness of the nanotube wall is apparently thinner than the lower part. Fig. 4B inset SAED patterns proved that the 700 ◦ C annealing can effectively transform the amorphous HfO2 into well defined crystalline monoclinic phase. For most of the nanotube arrays developed through an electrochemical anodization approach, a F containing electrolyte is most commonly used. Therefore, F content plays an important role in the formation and maintaining the nanotube scaffold. In the case of HfO2 , an almost equal amount of F to Hf was found in the system according to the elemental analysis using TEM [9]. However, the simple elemental analysis and crystal structure analysis cannot track detailed information about the chemical properties of the involved elements as temperature increases. Herein, we utilized XPS as a tool to probe the chemical environment changes and quantitative elemental analysis during the thermal treatment [40–43]. Fig. 5A is the surface element ratios of F to Hf as a function of annealing temperature. It is believed that as the temperature increased, the initial hafnium oxide/fluoride mixture is most possibly transformed to hafnium oxide by releasing F. Fig. 5A F/Hf ratio shows that the loss of F happened mostly after 500 ◦ C under Ar and dropped rapidly from 500 ◦ C to 700 ◦ C. This ratio stayed at a low level when the temperature was increased from 700 ◦ C to 900 ◦ C. This result indicated that the chemical composition on the surface started to dramatically change when the temperature reached 500 ◦ C, which is consistent with the crystal structure formation temperature investigated by XRD [9]. Fig. 5B, C and D shows the core-level spectra of Hf 4f, F 1s and O 1s. As seen in Fig. 5, increasing temperature affects the binding structures of Hf, F and O significantly. Fig. 5B shows the Hf 4f of the samples with and
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Fig. 5. (A) F to Hf atomic ratio obtained from XPS as a function of annealing temperatures (the line guides the eye), (B) Hf 4f, (C) F 1s and (D) O 1s binding energy of the as-prepared samples and post heat-treated from 300 ◦ C to 900 ◦ C.
without the heat-treatment. In general, the spectra show an overall narrowing of the spectral envelope and increased peak resolution. These observations are consistent with converting a mixed oxide/fluoride film into a more ordered single component (i.e., oxide) film. The Hf 4f core level consist of two peaks (a doublet) that are separated by ∼1.7 eV. The peak at lower BE (4f7/2) is centered at ∼17.5 eV and is associated primarily with Hf–O bonding. For the as-prepared sample (bottom spectrum) the overall peak shape is broad with poor resolution between the two peaks. This indicates a second (or more) bonding configuration for the Hf, most likely Hf–F bonding. As the film is heated to higher temperatures, there is a loss of F and hence a loss of Hf–F bonding. The Hf 4f spectra for the 700 ◦ C and 900 ◦ C films show a well resolved, narrow peak structure, consistent with HfO2 [40–43]. The binding energy of the single F 1s feature around 685.6 eV is consistent with an inorganic fluoride and is attributed to the Hf–F bond [43]. Fig. 5C shows that F 1s peak position remains almost unchanged, but the peak intensity decreases as anneal temperature increases. It is notable that the peak intensity around 685.6 eV remained at the same level at a temperature lower than 500 ◦ C and significantly reduced when the temperature went beyond 700 ◦ C. This is evident that the Hf–F bonds are essentially stable until 500 ◦ C under Ar in the nanotube structure. Hf–F bonds break up at a temperature higher than 700 ◦ C. In O 1s region, a predominate band around 531 eV contributions from lattice oxygen of Hf–O bonds [40–43] and a broad shoulder band near 532 eV peak is most likely –OH and/or water and it decreases with annealing [43]. As from Fig. 5D, the most likely replacement of Hf–F back-
bones by Hf–O bonds happened at 700 ◦ C. Since the system was thermally treated under an oxygen free environment, the sources of O for Hf–O bond formation could only come from the nanotube itself or the surface adsorbed organic residues. Although the monoclinic phase formation initiates at 500 ◦ C for anodic HfO2 nanotubes, further refining of the crystal structure occurs at 700 ◦ C. At this temperature, most of the Hf–F bonds were replaced by Hf–O bonds. The transition from Hf–F to Hf–O seems to maintain the well defined nanotube array structures at 700 ◦ C, but a higher temperature of 900 ◦ C seems to break up the ordering of nanotubes. It is an interesting phenomenon that the structural collapse did not happen at 700 ◦ C when the F was rapidly released from the system. A further increase of 200 ◦ C leads to only a small loss of F from the system compared to what happened at 700 ◦ C, but resulted in a significant change of the morphology of nanostructures. There are few possible reasons for the observed phenomenon: (1) insufficient oxygen available in the environment limits further replacement of Hf–F by Hf–O and at 900 ◦ C, some of the F vacancies may not be filled by O leading to the collapse of the nanostructures; or (2) decomposition of HfO2 occurs at 900 ◦ C. As reported by Zhan et al. [44], the decomposition of HfO2 and release of O2 could happen even at a temperature as low as 500 ◦ C under rapid thermal annealing conditions. XPS studies uncover the complicated scenario behind thermal annealing of anodic metal oxides. Most of the nanotube structures prepared through electrochemical anodization directly from the metal substrates exhibit initial amorphous structures and contains
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ble and strong metal oxide in bulk, HfO2 nanotube arrays lost well defined nanostructures at 900 ◦ C in Ar. Detailed XPS studies revealed that F impurity originated from the electrolyte can significantly affect the chemical composition and morphologies of the HfO2 nanotube arrays even at a temperature higher than 700 ◦ C when annealed in Ar. At a temperature as high as 900 ◦ C, factors like decomposition of metal oxides and reaction with substrate can also be responsible for the morphological degradation of the nanotube arrays. Further stabilization of the metal oxide nanostructures is desired for a broader range of applications at a wider operation temperature and atmosphere ranges. Dielectric measurements on the amorphous and crystalline HfO2 nanotube arrays were conducted to evaluate the temperature impact to the nanotube array properties. Acknowledgements
Fig. 6. Complex resistivity plots of HfO2 samples. As-prepared and 500 ◦ C postannealed samples were labeled with amorphous and crystalline, respectively. The inset shows the loss tangent of and the relative permittivity of the samples.
impurities originated from the electrochemical reactions. The postthermal annealing is the most common approach to remove any impurities present and crystallize the matrix. For most of the cases, the annealing temperatures are relatively lower and the composition of the matrix during the thermal treatment is paid very little attention. In order to further explore the potential applications for such unique structures, morphological, thermal and chemical stability, as well as surface condition requires more extensive investigation. This work suggested that even for the strong material such as HfO2 , the thermal and chemical stability of nanotube arrays is not necessarily guaranteed at higher temperatures and could be impacted by multiple factors. To further improve the stability of nanotube arrays, selective doping [45] and more targeted temperatures and operation atmospheres are approaches for future considerations. In order to evaluate the temperature impact on the intrinsic properties of the HfO2 nanotube arrays, dielectric measurements on both as-grown and 500 ◦ C post-annealed HfO2 tubes were performed using an impedance analyzer. The complex resistivity level ( = + i ) plots (Fig. 6) showed that the crystalline material (posttreatment at 500 ◦ C and labeled crystalline in Fig. 6) had lower resistivity than the untreated sample (amorphous). The dielectric losses shown in the set indicate that the crystalline material has higher losses at intermediate frequencies. However, the amorphous sample has a clear ohmic loss with dc resistivity value approximately 6.7 × 106 /m. The crystalline sample indicated that the conductive relaxation processes were not finalized because of the lack of bending of the complex resistivity representation. Relative dielectric permittivities of the HfO2 materials were close to each other at high frequencies. We estimated that the permittivities of the amorphous and crystalline tubes were 18.0 and 17.2, respectively. In the literature, the relative permittivity of HfO2 has been reported to be around 16 [46] and 17 [47]. 4. Conclusions Thermal and chemical stability studies of the HfO2 nanotube arrays through electrochemical anodization were investigated using SEM, TEM, XRD and XPS techniques. As a well known sta-
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