Controlling the Particle Size of ZrO2 Nanoparticles in Hydrothermally

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Controlling the Particle Size of ZrO Nanoparticles in Hydrothermally Stable ZrO/MWCNT Composites Chang-Chang Liu, Sungchul Lee, Dong Su, Byeongdu Lee, Sungsik Lee, Randall E Winans, Chunrong Yin, Stefan Vajda, Lisa D. Pfefferle, and Gary L. Haller Langmuir, Just Accepted Manuscript • DOI: 10.1021/la303545y • Publication Date (Web): 13 Nov 2012 Downloaded from http://pubs.acs.org on November 26, 2012

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MWCNT

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Controlling the Particle Size of ZrO2 Nanoparticles in Hydrothermally Stable ZrO2/MWCNT Composites Changchang Liu1, Sungchul Lee2, Dong Su3, Byeongdu Lee4, Sungsik Lee4, Randall E. Winans4, Chunrong Yin5, Stefan Vajda1,5, Lisa Pfefferle1, Gary L. Haller1* 1

Department of Chemical and Environmental Engineering, Yale University, New Haven, CT

06520-8682, USA 2

Energy Laboratory, Corporate R&D Center, Samsung SDI Co., Ltd, 428-5, Gongse-dong,

Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Republic of Korea 3

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973,

USA 4

Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne,

IL 60439, USA 5

Materials Science Division & Center for Nanoscale Materials, Argonne National Laboratory,

9700 South Cass Avenue, Argonne, IL 60439, USA *To whom correspondence should be addressed. E-mail: [email protected] Abstract The composite of multi-walled carbon nanotubes (MWCNT) decorated with ZrO2 nanoparticles, synthesized by a grafting method followed by high-temperature annealing, was studied. The oxygen functionalized MWCNT surface uniformly disperses and stabilizes the oxide nanoparticles to an extent that is controlled by the metal oxide loading and thermal annealing temperature. This ZrO2/MWCNT also withstands decomposition in a hydrothermal environment providing potential applications in the catalysis of biomass conversion (e.g., aqueous phase reforming). The ZrO2/MWCNT have been characterized by (Scanning) transmission electron microscopy ((S)TEM), X-ray diffraction (XRD), in-situ small-angle X-ray scattering (SAXS), in-situ wide-angle X-ray scattering (WAXS), and near edge X-ray fine structure (NEXAFS) for the purpose of a comprehensive analysis of the ZrO2 particle size and particle size stability. Keywords: nano-composite, zirconium oxide, multi-walled carbon nanotubes, particle size, insitu WAXS.

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Introduction Since their discovery by Ijima, 1 carbon nanotubes (CNT) have been attractive candidates for applications in many fields, including heterogeneous catalysis,2 where CNT are used as catalyst supports due to their high surface area, tunable hydrophilicity through functionalization, and stability in both acidic and basic media.3, 4, 5 Recent research interest has been focused on composite systems which combine metal oxides and CNT. For example, nanocomposites of conducting CNT and transition metal oxides are considered promising electrode materials for supercapacitors,6, 7 composites of rare earth oxides and CNTs are useful as highly functionalized laser materials, phosphors, and catalysis materials,8 and Pt/metal oxide composites supported on CNT have exhibited improved activity towards methanol and ethanol oxidation in direct alcohol fuel cell (DAFC) applications.9, 10 In the literature, frequently coatings have been used to make metal oxide/CNTs composite systems, producing either uniform multilayers or thin layers of highly dispersed nanoparticles on CNT. L. Fu et al. reported coating MWCNT with rare earth oxide using a high-pressure method that involves supercritical CO2, resulting in multilayered Eu2O3-coated MWCNT with ten concentric cylinders coated on a MWCNT with an interlayer spacing of 0.31 nm, as confirmed by HRTEM. 8 W. Han et al., on the other hand, reported a room-temperature chemical route to coat single-walled carbon nanotubes (SWCNT) with a thin layer of SnO2, where this coating is demonstrated, by XRD and TEM, to be composed of interconnected SnO2 nanoparticles of 1-6 nm.11 The sol-gel method has also been introduced for the preparation of CNT composite materials.12 Starting with nitric acid treatment on the pristine MWCNT to produce oxygen functionalities, J. Y. Lee et al. synthesized a NiO/MWCNT composite through chemical precipitation followed by thermal annealing, yielding NiO crystallites in the composite of 17 nm with poor uniformity.6 Procedures similar to Lee et al.6 have been repeated thereafter to synthesize ZrO2/MWCNT composite materials for applications in heterogeneous catalysis. Inspired by the fact that Pt/ZrO2 catalysts have excellent catalytic activity in low-temperature water-gas shift reactions, PtZrO2/MWCNT composites have been used as the electrode materials 13, and have demonstrated better performance in DAFC because of hypothesized to increased CO tolerance of the catalysts.14, 15 Moreover, sulfated ZrO2 (S-ZrO2) is a strong solid acid catalyst;16 S-ZrO2 supported on MWCNT has also been proposed to be a water-tolerant solid acid catalyst that has 2 ACS Paragon Plus Environment

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potential applications in aqueous phase reactions in biomass processing, 17 and it has been incorporated in Pt/S-ZrO2/MWCNT to enhance electron and proton conductivity so as to improve the catalytic activity of Pt electrocatalysts.18 These electrochemical studies used a solgel method to synthesize the ZrO2/MWCNT composites that results in either an amorphous ZrO2 layer on MWCNT (as in the case of the studies by H. Song et al. 13 and by J. C. Juan et al. 17) or ZrO2 particles of ~ 8 nm with uncertain uniformity and dispersity (as in the case of the studies by D. J. Guo et al. 18). These studies indicate that controlling the size and dispersion of hydrothermally stable ZrO2 particles would benefit the catalysis activity for the PtZrO2/MWCNT system. In the report by F. Lupo et al., 19 ZrO2/MWCNT composites were prepared by a hydrothermal crystallization method of zirconium hydroxide onto pyrolysed MWCNT (no oxygen functionalities compared to the nitric acid-oxidized MWCNT). XRD results show that the ZrO2 particles formed in the composites (coated on the MWCNT) are monoclinic (< 40 nm), and the near edge fine structure at the carbon K-edge and Zr M2,3 edge indicate no Zr–C bond formation at the interface between ZrO2 and MWCNT. They suspect the formation of such bonds is not to be expected at the low temperatures (200°C) of the hydrothermal process. The importance of the study of particle size in catalyst systems that involve nanoparticles as the catalytic active phase and/or catalyst support cannot be overemphasized since the particle size affects the interfacial interaction, where electron transfer across the boundary occurs, that leads to the stability of particle dispersion, and therein, ultimately, the catalytic activity.20, 21, 22, 23 In our previous work, we developed a grafting procedure (in ethanol at 180°C) followed by thermal annealing to synthesize a ZrO2/MWCNT composite with highly dispersed and uniform cryatalline ZrO2 nanoparticles.24 We have also taken advantage of near-edge X-ray absorption spectroscopy (NEXAFS) to probe the strong interfacial interactions between the ZrO2 nanoparticles and MWCNT, and have examined the ZrO2 particle size by XRD and TEM. 25 In this study we focus on a detailed analysis of the tuning of ZrO2 particle size in the ZrO2/MWCNT composites using SEM, TEM, STEM, XRD, NEXAFS, SAXS and WAXS, and probe the thermal stability, as well as hydrothermal stability, of such materials.


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Experimental The preparation of ZrO2/MWCNT of various loadings follows the procedure previously published by our group. 24, 25 Thermal annealing was performed in continuous He flow (Ultra High Purity He purchased from Airgas) at different temperatures (450°C, 550°C, 650°C and 750°C) in a quartz flow reactor. The final weight loading of ZrO2 was measured using thermogravimetric analysis (TGA), which was conducted on a Setaram Setsys 1750 instrument in flowing air (Ultra Zero grade purchased from Airgas). During measurement, the temperature was held at 200°C for 30 min to remove the adsorbed water, ramped to 1000°C at a rate of 10°C/min, and then held at 1000°C for 30 min. A second ramp on the oxidized sample was repeated to establish the baseline. For different loadings of ZrO2/MWCNT, we were interested in comparing the difference (e.g., particle size, thermal stability) between low loading (with nominal loading of ZrO2 of about 15 wt%) and high loading (with nominal loading of ZrO2 of about 40 wt%) ZrO2/MWCNT composites. The final loadings analyzed by TGA have 1 ~ 2 wt% systematic error for repeated synthesis and measurement, for simplicity, 14 wt% ZrO2/MWCNT will be called “low loading” and 42 wt% ZrO2/MWCNT will be called “high loading”. An intermediate loading 30 wt% was also included for comparison. X-ray diffraction (XRD) patterns were collected using the Bruker D8 Focus powder XRD with Cu-Kα radiation at λ = 0.154 nm. Transmission electron microscopy (TEM) was performed using a JEOL2100F microscope operated at 200kV. Secondary electron (SE) and high angle annular dark-field (HAADF) images were taken using a dedicated scanning transmission electron microscope (STEM) Hitachi HD-2700C with an aberration correction system. In this work, we used a probe resolution of 1.3 Å. The collection angle for high angle annular dark-field (HAADF)-STEM imaging is between 114 to 608(±2) mrad. Under this imaging condition, the bright contrast represents an atomic column and the contrast value can be proportionally related to the average atomic number. All microscopy work was carried out at the Center for Functional Materials, Brookhaven National Laboratory. Oxygen K-edge NEXAFS data were collected at Beamline U4B at the National Synchrotron Light Source, Brookhaven National Laboratory. Each powder sample was pressed into a pellet and attached to a piece of copper tape. Total electron yield (TEY) signal was measured in ultra-high vacuum. The data were aligned to the FeNiCo reference channel, and normalized to the oxygen edge-step, that is, intensity per oxygen 4 ACS Paragon Plus Environment

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atom in the sample. SAXS and WAXS data were collected at Beamline 12-ID-B at the Advanced Photon Source, Argonne National Laboratory. SAXS and WAXS data were recorded simultaneously in-situ, performed in a quartz capillary plug-flow apparatus that was adapted from the reaction cell designed by Clausen et al., 26 but instead of using heating gas and a Kapton “hood”, the sample was heated by Pt heating wires. The sample was thermally pre-annealed in flowing He (Ultra High Purity grade from Airgas) and in-situ annealed in flowing He again to test their thermal stability of the ZrO2/MWCNT composites. The temperature was ramped from room temperature to 550°C at a rate of 20°C/min, and SAXS and WAXS spectra were recorded every minute. SAXS data before and after the in-situ heating were modeled to obtain information on ZrO2 particle size distribution; time-resolved WAXS spectra were plotted to observe the change in ZrO2 particle size over time. Hydrothermal stability experiments were carried out by mixing ZrO2/MWCNT materials with liquid water at different temperatures – mixing with stirring at ambient temperature autoclaving at 95°C and 220°C, followed by filtering and drying. Final loading of ZrO2 after the hydrothermal treatment was measured by TGA and particle size of ZrO2 was measured by XRD line-broadening. Results and Discussion In Figure 1, a SE micrograph taken at low magnification gives an overview of the structure of 14 wt% ZrO2/MWCNT composite, revealing the intertwined network of MWCNT uniformly decorated with ZrO2 particles. The STEM-HAADF image of Figure 2 (a) shows a single MWCNT with highly dispersed ZrO2 particles attached to the surface and Figure 2 (b) shows ZrO2 particles in well-crystallized phases with the particle size being less than 5 nm. In our previous work, 25 we compared the XRD patterns of the annealed ZrO2/MWCNT (at 450°C in He for 2hr) with those of functionalized MWCNT, monoclinic bulk ZrO2 and tetragonal bulk S-ZrO2. The results suggest that, on the one hand, the ZrO2 particles in the composite materials have small particle size compared to the bulk materials, as indicated by linebroadening; on the other hand, the ZrO2 particles have crystalline features that resemble tetragonal (111)t phase, as further confirmed by the HRTEM results. Using these two differing but complementary particle size analysis methods, i.e., XRD and TEM, we are able to acquire information about the ZrO2 nanoparticles – where XRD analysis gives information on the 5 ACS Paragon Plus Environment

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Figure 1. SE image of 14 wt% ZrO2/MWCNT.

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Figure 2. (a) and (b) HAADF-STEM images of 14 wt% ZrO2/MWCNT at high magnifications. (c) Particle size number distribution based on (b) and four HAADF-STEM images in Figure S1 (Supplementary Materials) at different regions on the same sample, in comparison to the XRD fitting result (marked with green line). (d) Particle size number distribution in frequency (%).


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crystallite size (rather than the particle size), a volume average across the entire sample, TEM provides more localized size information on individual particles shown in a micrograph. For a 14 wt% ZrO2/MWCNT composite annealed at 450°C in He for 2hr, both XRD and TEM results allow estimation of the ZrO2 particle size to be less than 3 nm. By counting the particles in HAADF-STEM images at high magnification (Figure 2 (b) and Figure 1S), we obtained a number distribution of different particle size in Figure 2 (c) and (d). The number average distribution indicates that most of the particles are around 2 nm (49%) and 3 nm (31%), and the volume average would be weighed in favor of 2 nm by XRD, as shown in Figure 3 below. Of course, it may be that 2 nm (XRD crystallite size) and 3 nm (TEM particle size) is a real difference, i.e., that the crystalline domain size is smaller than the TEM particle size. The implication would be that even at 450°C, the particles are not fully crystallized. We cannot fully resolve this issue but our interpretation of the XRD line broadening and TEM particle size distribution suggest nearly fully crystallized particles after 450°C sintering. Two major factors are suggested to affect the ZrO2 particle size in the resulting ZrO2/MWCNT composites – (1) ZrO2 loading, and (2) the thermal annealing temperature. To probe these two effects, we prepared three ZrO2/MWCNT catalysts with different loadings – (a) 14 wt% ZrO2/MWCNT, (b) 30 wt% ZrO2/MWCNT, (c) 42 wt% ZrO2/MWCNT – and applied thermal annealing at four different temperatures – 450°C, 550°C, 650°C, 750°C – for each of the above three composites. The XRD patterns of the annealed samples are illustrated in Figure 3 (a) – (c). To perform particle size analysis, the diffraction intensity was first normalized to the (002) MWCNT, 2θ = 26°, a 3-peak Gaussian fitting to deconvolute the three peaks at 2θ = 26°, 2θ = 30° and 2θ = 35°, the latter two peak being the (111)t and the overlapping (200)t and (002)t of tetragonal ZrO2, and used the line-broadening at the ZrO2 tetragonal (111)t peak (2θ = 30°) in the Scherrer equation to calculate the particle size of ZrO2. The correlation between particle size and the annealing temperature is shown in Figure 3 (d). While for the lower loading sample (14 wt%), the ZrO2 particle size remains constant at around 2 nm, with a slight increase in size when annealed at 750°C; the higher loading sample (42 wt%) has larger ZrO2 particle size as synthesized at 450°C (3 nm), and the particles grow in size when the annealing temperature increases. After annealed at 750°C, the particle size reaches ~ 9 nm. In other words, sintering occurs more severely with the higher loading ZrO2/MWCNT at higher annealing temperature.


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These particle size results calculated by line-broadening fitting are confirmed and complemented by high-resolution transmission electron microscopy (HRTEM) images, as shown in Figure 4. At synthesis annealing temperature (450°C), both the low loading (14 wt%) and the high loading (42 wt%) ZrO2/MWCNT have crystalline ZrO2 particles of the size of 2 – 3 nm, but the higher loading ZrO2/MWCNT has more densely packed ZrO2 particles. Using a simple geometric model, and measured surface areas, it is estimated that about 8 and 30% of the MWCNT surface is covered by ZrO2 for the 14 and 42 wt% ZrO2/MWCNT (details are given in Supplementary Information). However, when they are both annealed at high temperature (i.e., 750°C), the 42 wt% ZrO2/MWCNT has large ZrO2

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Figure 3. Particle size analysis of ZrO2 in the ZrO2/MWCNT composites with various loadings based on the XRD patterns of (a) 14 wt% ZrO2/MWCNT, (b) 30 wt% ZrO2/MWCNT, and (c) 42 wt% ZrO2/MWCNT, with each sample annealed at four different temperatures (450°C, 550°C, 650°C, 750°C); (d) particle size correlation with annealing temperature.


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particles of ~ 10 nm. This suggests that the ZrO2 particles are more thermally stable against sintering when the loading remains low. When we look at the 14 wt% ZrO2/MWCNT and 42 wt% ZrO2/MWCNT samples before they were annealed, as shown in Figure 5, Zr precursor particles are visible but they are amorphous in the 14 wt% sample (Figure 5 (b) and (c)); while most of the precursor particles in the 42 wt% are amorphous (Figure 5 (e)), some crystalline particles are also present (Figure 5 (f)), as indicated by the red circle. Presumably during the synthesis of the higher loading ZrO2/MWCNT composites, crystalline particles can be formed in the liquid phase, but these particles lack interaction of the MWCNT surface and are by nature physical deposition. This is further

(a) 14 wt% 450°C annealed

(b) 42 wt% 450°C annealed

(c) 14 wt% 750°C annealed

(d) 42 wt% 750°C annealed


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Figure 4. HRTEM of (a) 14 wt% ZrO2/MWCNT annealed at 450°C; (b) 42 wt% ZrO2/MWCNT annealed at 450°C; (c) 14 wt% ZrO2/MWCNT annealed at 750°C; (d) 42 wt% ZrO2/MWCNT annealed at 750°C.

confirmed by the XRD results of the 42 wt% sample (before annealing) in Figure 5 (g), where broadened diffraction peak is present. As noted previously, the grafting precursor zirconium acetylacetonate, Zr(acac)4, may undergo ligand exchange with the ethanol solvent, EtOH, at reaction conditions (180°C, 20 atm, 6 hr) to form Zr(EtO)4. 25 We hypothesize that a byproduct of this reaction is decomposition of Zr(acac)4 to ZrO2, that is probably nucleated on the MWCNT.

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(g) Figure 5. (a) TEM of 14 wt% ZrO2/MWCNT before annealing, (b) and (c) are the HRTEM of (a). (d) TEM of 42 wt% ZrO2/MWCNT before annealing, (e) and (f) are the HRTEM of (d). In (f), the red circle indicates a crystalline Zr precursor particle of about 3 nm. (g) XRD of 14 and 42 wt% ZrO2/MWCNT (before and after annealing).

(a)

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Figure 6. Oxygen K-edge NEXAFS of (a) 14 wt% ZrO2/MWCNT and (b) 42 wt% ZrO2/MWCNT, each sample was compared through before-annealing, annealed at 450°C and 750°C.

Figure 6 shows the oxygen K-edge NEXAFS of the comparison among the same samples for the TEM images in Figure 4 and 5, i.e., the grafted composite before and after annealing (450°C and 750°C) samples of 14 wt% and 42 wt% ZrO2/MWCNT. As discussed in our previous paper, 11 ACS Paragon Plus Environment

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25

the ~532 eV peak (absorption edge) and ~536 eV peak correspond to the transition from O 1s

to the Zr 4d – O 2pπ and the Zr 4d – O 2pσ molecular orbitals of the Zr–O bond, respectively; the ~540 eV peak primarily reflects the surface oxygen on MWCNT (transition from 1s O to C– O σ* orbital). As the annealing temperature increases, the intensity of the transitions contributed by the Zr–O bond is enhanced while at the same time the intensity at ~540 eV decreases; this is to be expected as the ZrO2 particles further agglomerate, they lose their interaction with the MWCNT surface. It should be noted that in both sets of spectra, an isosbestic point is formed for the phase change that increases the intensity for the transitions to empty Zr–O hybrid molecular orbitals involving d atomic orbitals (of ZrO2) and decreases that for the transitions to the empty C–O σ∗ orbitals (of MWCNT). Moreover, the before-annealing 42 wt% sample adopts the lineshape of the 14 wt% sample annealed at 450°C, which provides more spectroscopic evidence to support the discussion with regard to Figure 5 – crystalline particles start to form in the 42 wt% sample even before thermal annealing. Note also that the initial intensity of the oxygen functionalized MWCNT peak (540 eV) is lower for 42 wt% loading than for 14 wt% loading reflecting greater consumption of oxygen functional groups to form grafted Zr precursors to ZrO2 crystalline nanoparticles. To further test the thermal stability of the ZrO2/MWCNT composites with variation of loadings, we performed small-angle scattering (SAXS) on the 14 wt%, 30 wt% and 42 wt% ZrO2/MWCNT. All three samples were annealed at 450°C in He for 2hr as synthesized, and during the SAXS experiments, they were first measured at room temperature (25°C), heated to 550°C in-situ, and measured again. SAXS on functionalized MWCNT was taken as the blank (background scattering) and it was confirmed that the MWCNT was stable over the entire temperature range (25°C ~ 550°C). The scattering signal from MWCNT was then subtracted from each scattering curve of the three ZrO2/MWCNT samples and the background-subtracted data were fit with the Maximum Entropy Method 27 to obtain the volume size distribution of the ZrO2 particles. The results are shown in Figure 7. The size distribution for 14 wt% ZrO2/MWCNT is mono-modal and is stable up to 550°C, while for both 30 wt% ZrO2/MWCNT and 42 wt% ZrO2/MWCNT, bi-modal size distribution is observed. In the case of the latter two samples, the distribution at smaller particle size (radius = 1.5 nm) is reasonably stable, and loading- and temperature-independent, but the larger particles are not and shift toward larger radius with increasing temperature, indicating sintering. 12 ACS Paragon Plus Environment

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In our previous work, 25 we have used carbon K-edge NEXAFS to study the interfacial interactions between the ZrO2 and MWCNT, which indicated bonding at their interface, leading to thermal stability of the ZrO2 nanoparticles. Based on the SAXS results, it is likely that the 3 nm radius particles are more extensively bonded to the MWCNT surface, and therefore they are stable in all three samples at both low and high temperatures. However, the large particles in the 30 wt% and 42 wt% samples have less interaction with the MWCNT surface and behave differently under heat treatment at 550°C. These large particles may have formed in the liquid phase, as discussed with respect to Figure 5 (f). We note that there is a slight difference in the particle size relative to XRD particle size analysis. There also appears to be a modest discrepancy within the analysis in Figure 7, that may result from the synthesis varables for the 30 wt % data (solid, blue dotted line). Were these data shifted about 10% to larger size, they would align with the data for 14 and 42 wt% of the as-synthesized particles. However, it would not change the general observations that 1) in all loadings there is a population of small-size particles that are thermally stable and at higher loadings (≥ 30 wt%) there is a population of larger particles that sinter at a modest increase in temperature above the synthesis temperature (550°C instead of 450°C).


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• : r. t. ◦ : 550°C

Figure 7. Size distribution of ZrO2 particles of 14 wt%, 30 wt% and 42 wt% ZrO2/MWCNT modeled on SAXS results. •, closed symbol: calcined at 450°C (SAXS measured at room temperature), ◦ , open symbol: calcined at 450°C and after in-situ annealed to 550°C (SAXS measured at 550°C).

In-situ wide-angle scattering (WAXS) was carried out on the 14 wt% and 42 wt% ZrO2/MWCNT composites to provide further evidence on sintering (Figure 8). Both samples were annealed during synthesis at 450°C in He for 2hr. Each sample was placed in the capillary setup with flowing He. After purging with He, the sample-capillary was ramped to 550°C at a rate of 20°C/min (blue lines), held at 550°C (red lines) and cooled to room temperature (green lines). A WAXS spectrum was recorded every 1 minute. At q = 1.9 Å-1, the scattering peak for MWCNT, the intensity is constant during the entire temperature range, except for Figure 8 (a), where a kink (sudden increase in overall intensity) at 178°C appeared. We propose that this is due to disturbance in the capillary caused by He flow. At q = 2.2 Å-1, the scattering peak represents the ZrO2 particle (equivalent with ZrO2 (111)t peak at 2θ = 30° in XRD), and significant growth in the in-situ WAXS spectra for the 42 wt% ZrO2/MWCNT composite is shown, again reinforcing the idea of severe sintering for the composite with high ZrO2 loading.


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(a)

(b) Figure 8. In situ WAXS of (a) 14 wt% ZrO2/MWCNT annealed at 450°C and (b) 42 wt% ZrO2/MWCNT annealed at 450°C thermally treated at 550°C. (Blue lines: heating ramp from r. t. to 550°C; red lines: annealing at 550°C; and green lines: during cooling ramp.)

The 14 wt% and 42 wt% ZrO2/MWCNT that were annealed at 750°C (He, 2hr), were examined at the low magnification (at the scale of µm) TEM images (of the same two samples as Figure 4 (c) and (d)) in Figure 9. The high loading composite material has very large agglomerates of ZrO2, but they are absent in the low loading composite. These results are consistent with our hypothesis that the sample with higher ZrO2 loading, due to the lack of 15 ACS Paragon Plus Environment

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interfacial interaction of the MWCNT surface, sinters severely under high-temperature annealing. While this was also illustrated by the XRD, SAXS and WAXS data that provide overall averaged information on particle size, the micrographs probe more locally in a straightforward sense on the samples, herein they complement each other in our particle size analysis with good consistency.

(a) 14 wt% 750°C annealed

(b) 42 wt% 750°C annealed

Figure 9. Low magnification TEM images of (a) 14 wt% ZrO2/MWCNT annealed at 750°C and (b) 42 wt% ZrO2/MWCNT annealed at 750°C (they are the same samples as in Figure 4 (c) and (d)).

The hydrothermal stability of ZrO2/MWCNT was tested in liquid water for 6 hours at ambient temperature, at 95°C and at 200°C (Table 1). The particle size (measured by line-width of the ZrO2 (111)t XRD peak) did not change from its initial 2.6 nm. Likewise, the ZrO2 loading (measured by TGA) remained constant after the sample was filtered and dried. Further spectroscopic analyses, as well as catalytic testing of aqueous phase reactions, are required but physical analysis implies hydrothermal stability at temperatures where reactions, such as aqueous phase reforming of biomass products,5 are performed appear to be feasible.


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Table 1. Particle size and ZrO2 loading of before and after different hydrothermal treatments. Hydrothermal Treatment

Particle Size

ZrO2 Loading

Untreated [email protected]. Autoclaved@95°C Autoclaved@200°C

2.6±0.1 nm 2.4±0.1 nm 2.6±0.1 nm 2.6±0.1 nm

17±2 wt% 16±2 wt% 19±2 wt% 19±2 wt%

Conclusions In summary, we have demonstrated that the ZrO2/MWCNT composite materials synthesized by a high-temperature (180°C) grafting method in ethanol followed by annealing at moderate temperature (450°C) produce very well-dispersed ZrO2 crystalline nanoparticles on the functionalized MWCNT surface. Multiple characterization methods have been applied to analyze the size and morphology of the nanoparticles, including HAADF-STEM, TEM, XRD, NEXAFS, SAXS and WAXS, and the results show that ZrO2 nanoparticles have a size of less than 3 nm and resist annealing to 750°C, i.e., remain crystalline in structure for low loading (14 wt%). The metal oxide loading and annealing temperature affect the particle size, i.e., the higher loading samples (42 wt%) annealed at high temperatures (750°C) increase the mobility of the particles and cause more severe sintering. Using SAXS, WAXS and XRD, we suggest that the thermal instability of the higher loading composite is due to the lack of interfacial interaction between the larger particles and the MWCNT surface. Using TGA and XRD on samples that underwent simple hydrothermal treatment, the composite materials proved to be stable in aqueous media. In other words, by controlling the metal oxide loading and annealing temperature, the synthesis can be tuned to produce a composite that has uniform dispersion, small-sized ZrO2 nanoparticles decorating the MWCNT, which have potential applications in the area of catalysis in fuel cell and biomass processing. Acknowledgment. The authors – Changchang Liu, Sungchul Lee, Lisa Pfefferle and Gary L. Haller of Yale University – are grateful to the DOE, Office of Basic Energy Sciences, grant DE-FG0201ER15183, and AFOSR MURI grant FA9550-08-0309 for financial support. Electron microscopy work was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic 17 ACS Paragon Plus Environment

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Energy Sciences, under Contract No. DE-AC02-98CH10886. Chunrong Yin and Stefan Vajda gratefully acknowledge the support by the US Department of Energy, BES Materials Sciences and Engineering under Contract DE-AC02-06CH11357 with UChicago Argonne, LLC, operator of Argonne National Laboratory. The use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886, and we thank the beamline scientists Dario Arena, George Sterbinsky and Jing Liu for their onsite help.

Supporting Information Available Supporting information on the HAADF-STEM images used for particle size counting, calculations on particle coverage on the low (14 wt%) and high (42 wt%) loading composites are described. A broad 2Θ XRD sample scan, relevant to Figure 3, is presented (and can also be found in Figure 3a of Ref. 25. This information is available free of charge via the Internet at http://pubs.acs.org/.


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Controlling the Particle Size of ZrO2 Nanoparticles in Hydrothermally

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