Pressure-Induced Synthesis and Evolution of Ceria Mesoporous

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Pressure-Induced Synthesis and Evolution of Ceria Mesoporous Nanostructures with Enhanced Catalytic Performance Yujing Geng,†,# Gang Lian,*,†,§,# Jun Wang,‡ Haibin Si,† Qilong Wang,‡ Deliang Cui,*,† and Ching-Ping Wong*,§ †

State Key Lab of Crystal Materials and ‡Key Lab for Special Functional Aggregated Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P. R. China § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Evaluating the effect of pressure for the formation of nanomaterials is significant in solvothermal methods. In this study, a pressure-dependent template-free solvothermal method is developed to controllably synthesize four kinds of uniform CeO2 mesoporous nanostructures in a single reaction system, i.e., mesoporous nanospheres, nanoporous mesocrystals, hollow nanospheres, and nanowires. They all comprise small nanoclusters (3−5 nm). Properly adjusting the reaction pressure allows for achieving the transition between them. Furthermore, the corresponding pressure-induced self-assembly (Ostwald ripening, reconstruction) mechanisms are proposed to illustrate the morphological evolution process. In addition, they also display large specific surface area and excellent catalytic activity for CO oxidation.

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which provide better control over the uniformity and morphology.17 However, residual templates may degrade their catalytic activity, which requires clean surfaces with maximum reactive sites. In addition, although a number of CeO2 mesoporous nanostructures have been obtained by various methods, it is still a great challenge to controllably synthesize multiple kinds of uniform CeO2 mesoporous nanostructures in a single reaction system and investigate the evolution between them. This is crucial for obtaining morphology-dependent properties. Herein, we report a high-pressure solvothermal method for the synthesis of four kinds of uniform CeO2 mesoporous nanomaterials in a mixed solvent composed of water, propanoic acid, and ethylene glycol without using any template. More interestingly, the morphology evolution from mesoporous nanospheres to mesocrystals, hollow nanospheres, and nanowires was achieved and the growth mechanism was revealed under high-pressure conditions. These as-obtained CeO2

eria (CeO2), as a typical rare earth oxide, has been extensively applied in fuel cells,1 sensors,2 three-way catalysts for toxic vehicular exhaust gases,3,4 and other catalysts for water−gas shift reaction,5,6 because of its high-performance oxygen storage capacity and easy-releasing characteristics via the conversion of Ce4+ and Ce3+ oxidation states.7 In previous studies, various dispersible CeO2 nanostructures have been synthesized, such as nanoparticles,8 nanowires,9 nanoplates,10 nanospheres,11 and nanocubes.12 Among them, CeO2 mesoporous nanostructures possess well-defined morphology, low density, high specific surface area (SSA), abundant porous nature, and preferable penetrability, which allow the quick ingestion of many chemical reagents, and facilitate the contact between the catalyst and the reactants. For this reason, considerable efforts have focused on their synthesis. As known, typical CeO2 mesoporous nanomaterials mainly include mesoporous nanospheres,13 hollow nanospheres with mesoporous shell,14 nanoporous mesocrystals,15 and polycrystalline nanowires.16 For the construction of these specific CeO2 nanostructures, the most common approaches are the hard and soft template based (i.e., polyvinylpyrrolidone, triblock copolymers, mesoporous silicas and others) strategies, © XXXX American Chemical Society

Received: November 11, 2015 Revised: April 12, 2016

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DOI: 10.1021/acs.cgd.5b01602 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. SEM, TEM, and HRTEM images of four kinds of CeO2 mesoporous nanostructures: (a) SEM, (b) TEM, and (c) HRTEM images of CeO2 mesoporous nanospheres. Insets are magnified SEM image of single nanosphere, the corresponding SAED pattern, and magnified lattice fringes, respectively. (d) TEM, (e) magnified TEM, and (f) HRTEM images of CeO2 nanoporous mesocrystals. Insets are the corresponding SAED and FFT patterns, respectively. The arrows in (f) show some atomic defects in the mesocrystal. (g) TEM, (h) magnified TEM, and (i) HRTEM images of CeO2 hollow nanospheres. Inset is the corresponding SAED pattern. (j) TEM, (k) magnified TEM, and (l) HRTEM images of CeO2 nanowires.

nanomaterials also present high SSA and promising advanced catalytic performance for CO oxidation. The high-pressure solvothermal strategy might offer an excellent approach to design other nanomaterials. Four kinds of uniform CeO2 mesoporous nanomaterials, i.e., mesoporous nanospheres, mesocrystals, hollow nanospheres, and nanowires, were obtained by varying the pressure as a function of time. The scanning electron microscope (SEM), transmission electron microscope (TEM), and high resolution transmission electron microscope (HRTEM) images show their typical morphologies (Figure 1). First, large-scale CeO2 nanospheres were obtained after 3 h under 45 MPa and 180 °C (Figure 1a). They possess narrow size distribution and an average size of ∼70 nm. The rough surface implies that they are composed of small nanoparticles. The TEM image (Figure 1b) and corresponding selected area electron diffraction (SAED) pattern (inset) show their polycrystalline and mesoporous nature (Figure 1b), which is further verified by the HRTEM test. As shown in Figure 1c, the nanospheres are mainly composed of tiny nanoclusters with diameter of 3−4 nm. Moreover, there are abundant nanopores existing between these stacked nanoclusters in the nanospheres. The distinct lattice fringes indicate a highly crystalline nature. The interplanar spacing is ∼0.31 nm, corresponding to the (111) plane of cubic CeO2 (inset in Figure 1c).18

When the reaction pressure was increased to 90 MPa, monodisperse nanoporous mesocrystals were synthesized instead of mesoporous nanospheres (Figure 1d−f). The magnified TEM image of these aggregates confirms that they consist of nanosized units with well-distributed nanopores (Figure 1e). The SAED pattern (inset in Figure 1e) exhibits a single-crystal structure of the whole particle, which suggests the subunits are assembled via highly oriented attachment.19 The diffraction spots are slightly elongated, indicating that there is small lattice mismatch between the boundaries of these subunits when they are assembled along the same orientation, typical for mesocrystals. The HRTEM image (Figure 1f) and fast Fourier transform (FFT) pattern (inset) further confirm their single-crystal nature. However, some atomic defects are still visible (labeled by arrows), in good agreement with the SAED pattern. Compared to mesoporous solid nanospheres (Figure 1b) obtained under 45 MPa for 3 h, when the reaction was continued for other 4 h under 60 MPa, named 45 MPa (3 h)− 60 MPa (4 h) for short, hollow nanospheres were prepared with mesoporous shells of ∼20 nm in thickness (Figure 1g−i). The magnified TEM and HRTEM images of hollow nanospheres confirm that the shells also comprise small nanoclusters and present a porous nature. Furthermore, the SEM images show that the surfaces of the hollow nanospheres are rather B



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have narrow peaks at 3−5 nm calculated using the desorption branch of the isotherms. The nanostructures’ growth habits are always influenced by their growth conditions. To shed light on their formation and evolution mechanism, a series of pressure-dependent experiments were conducted (Figure 3). First, the Ce3+ ions were hydrolyzed and were oxidized by NO3− to form precursors in solvothermal condition.11 Because high pressures reduce the nucleation barrier as well as the size of critical nucleus,21−23 copious nucleation at rather small size (average size: ∼3 nm) occurred under 45 MPa for 1 h (Figure 3a). Upon prolonging the reaction time to 2−3 h, the initially formed nanoclusters underwent collision and gradually aggregated into lager secondary nanospheres (2 h: ∼50 nm, Figure 3b and Figure S3; 3 h: ∼70 nm, Figure 3c). As a result, nearly monodisperse spherical aggregates were obtained. The diffraction ring pattern indicated that the nanoclusters aggregated randomly (inset in Figure 1b). The reaction between the ethylene glycol and propanoic acid adsorbed on the nanoclusters’ surface to form the ester enlarged the modifying effect on the surface state, which finally resulted in the formation of open mesoporous structures.11 When the imposed pressure was raised from 45 to 90 MPa, loosely packed mesoporous nanospheres still appeared. However, under the higher-pressure condition, the free-energy change of the materials system may allow one to tune the configuration of the materials. The response in the direction of a free energy minimum resulted in the alignment into a singlecrystal structure of the nanoclusters in the aggregates via oriented attachment, corresponding to CeO2 nanoporous mesocrystals (Figure 3d). This illustrates that external pressure has a great influence on the nanoclusters’ self-assembly. Moreover, the single crystal nonporous structure may be beneficial for catalysis because of improved transport properties. Upon increasing the pressure to 120 MPa, coalescence and coarsening behavior between adjacent nanoclusters occurred and more dense CeO2 spherical crystal were prepared (Figure 3e and Figure S4). These structures could be used as abrasive materials for chemical−mechanical planarization of advanced integrated circuits.24 As mentioned above, only mesoporous solid nanospheres could be prepared under constant pressure conditions, even when the reaction time was prolonged. However, if the reaction was conducted via two-step pressure treatment, i.e., 45 MPa (3 h)−60 MPa (2 h), CeO2 hollow nanospheres with mesoporous shells were instantly prepared (Figure 3f). Furthermore, prolonging the reaction time of the second step could effectively induce the formation of more defined hollow structures (Figure 3g). Actually, in the transition from solid to hollow nanospheres, both the interfacial energy of the nanoclusters and the penetration of solvent should be carefully considered. With a sudden increase of pressure in the second step, the interfacial energy greatly decreased and the penetrating force strengthened.9 The solvent more easily diffused into the mesoporous nanospheres and accelerated the dissociation of internal nanoclusters. The soluble Ce species were transported and precipitated on the outer surface. This roughened the outer surface and formed many granular protrusions (Figure S5). The dissociation−recrystallization process implies that the pressure-induced Ostwald ripening is the governing mechanism for the formation of hollow nanospheres.

rough and apparently built of small nanoparticles (Figure S1). The broken nanospheres further illustrate their well-defined hollow structure. Furthermore, when the reaction was conducted via three-step pressure treatment, namely, 45 MPa (3 h)−60 MPa (4 h)−90 MPa (17 h), the hollow nanospheres would completely transform into nanowires. As shown in Figure 1j−k, uniform CeO2 nanowires were obtained with 10−20 nm width and 1−2 um length. The HRTEM image of an individual nanowire indicates that they also comprise well-defined crystalline nanoclusters (3−5 nm) without any specific growth direction (Figure 1l). However, compared to the above-mentioned spherical aggregates, the polycrystalline nanowires are more densely stacked to minimize the interfacial energy of materials system in the high-pressure condition. In addition, the small crystallite size of these nanoclusters and textural porosities in these mesoporous nanostructures could be illustrated by X-ray diffraction (XRD) pattern and N2 sorption measurements. The XRD patterns (Figure S2) clearly show that all the samples can be indexed to face-centered cubic CeO2 (JCPDS 34−3094). Analysis of the broadening of the (111) peaks using the Scherer equation indicates a crystallite size of 3−5 nm, which is in agreement with the electron microscopy analysis. The N2 adsorption−desorption isotherms and mesopore size distribution plots are given in Figure 2. The

Figure 2. (a) N2 sorption isotherms and (b) BJH pore size distribution curves of CeO2 mesoporous nanostructures.

four samples all exhibit typical type IV isotherms with H3 hysteresis loops, implying the presence of mesopores.20 The calculated Brunauer−Emmett−Teller (BET) specific surface area (SSA) and Barrett−Joyner−Halenda (BJH) pore volume are shown in Table 1. They display large SSA (158.8−197.6 m2g−1) and pore volume (0.22−0.40 m3g−1), which should be attributed to aggregation of small nanoclusters. The plots also C



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Table 1. Series of Experiments Illustrating the Condition−Morphology Relationship pressure−time 1 2 3 4 5 6 7 8 9

15 MPa (1 h) 45 MPa (1 h) 45 MPa (2 h) 45 MPa (3 h) 90 MPa (3 h) 120 MPa (3 h) 45 MPa (3 h)−60 MPa (2 h) 45 MPa (3 h)−60 MPa (4 h) 45 MPa (3 h)−60 MPa (4 h)−90 MPa (5 h)

10

45 MPa (3 h)−60 MPa (4 h)−90 MPa (10 h) 45 MPa (3 h)−60 MPa (4 h)−90 MPa (17 h)

11

T (°C) 180

morphology nanoclusters nanoclusters mesoporous nanospheres mesoporous nanospheres mesocrystals spherical crystals hollow nanospheres hollow nanospheres hollow nanospheres, short nanowires hollow nanospheres, nanowires long nanowires

size (nm) 4−5 3−5 ∼50 ∼70 ∼80 ∼70 ∼70 ∼80 ∼50; ∼200 (length) ∼30; 500−800 (length) 1000−2000 (length)

SSA (m2 g−1)

pore volume (cm3 g−1)

158.8 180.6 -

0.34 0.37 -

197.6 -

0.40 -

-

-

176.7

0.22

Figure 3. P−t map showing the morphological evolution of CeO2 nanostructures. (a) Nanoclusters: 45 MPa (1 h). (b) Mesoporous nanospheres: 45 MPa (2 h). (c) Mesoporous nanospheres: 45 MPa (3 h). (d) Mesocrystals: 90 MPa (3 h). (e) Spherical crystals: 120 MPa (3 h). (f) Hollow nanospheres: 45 MPa (3 h)−60 MPa (2 h). (g) Hollow nanospheres: 45 MPa (3 h)−60 MPa (4 h). (h) Hollow nanospheres and short nanowires: 45 MPa (3 h)−60 MPa (4 h)−90 MPa (5 h). (i) Hollow nanospheres and nanowires: 45 MPa (3 h)−60 MPa (4 h)−90 MPa (10 h). (j) Long nanowires: 45 MPa (3 h)−60 MPa (4 h)−90 MPa (17 h).

Furthermore, when a three-step pressure treatment was imposed in the reaction, i.e., 45 MPa (3 h)−60 MPa (4 h)−90 MPa (17 h), the hollow nanospheres completely transformed into uniform one-dimensional nanowires (Figure 3j). The growth process should also be attributed to dissociation and reconstruction of nanoclusters to form nanowires under higherpressure condition.9 Upon increasing the reaction time from 5 to 17 h in the third step, the nanowires grew longer and longer, but in contrast, the hollow nanospheres became smaller and gradually diminished (Figure 3h−i and Figure S6). Finally, pure densely stacked CeO2 polycrystalline nanowires were prepared. The fusion of nanoclusters in the nanowires not only smoothed the surface, but also was energetically favorable compared to loosely stacked nanostructures. For intuitively illustrating the whole evolution process, Scheme 1 summarizes the mechanism from nanoclusters to mesoporous nanospheres, mesocrystals, hollow nanospheres, and nanowires. Generally, noble metal (Au, Ag, Pt, and Pd) loaded CeO2 nanomaterials exhibit high catalytic activity for CO oxidation.25,26 However, in recent years, investigation on the catalytic activity of pure or transition metal loaded CeO2 nanomaterials has attracted much attention instead of the costly noble metal loaded nanomaterials.27−30 Herein, taking the CO oxidation as

Scheme 1. Schematic Diagram Showing the Morphological Evolution from Nanoclusters to Mesoporous Nanospheres, Mesocrystals, Hollow Nanospheres, and Nanowires

the indicator reaction, the catalytic performance of the four of CeO2 mesoporous nanostructures was examined (Figure 4). Figure 4a shows the CO conversion as a function of the temperature of the various catalysts. These mesoporous nanostructures all exhibit much higher activity than that of D



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be identified, which corresponds to the defect-induced (D) mode of CeO2.26 A comparison between the four catalysts shows that the D band of the hollow nanospheres presents the strongest relative intensity (ID/IF2g). This suggests that there are more oxygen vacancies in the hollow nanospheres. It is generally proposed that the nature of oxygen vacancies plays an important role in the catalytic activity of CeO2.26,29 It is widely accepted that the migration of oxygen in CeO2 takes place via a vacancy hopping mechanism.29 The small size surface oxygen vacancies on CeO2 (111) were immobile at room temperature, but linear clusters of these vacancies formed at higher temperature.30 The linear clusters proved to be favorable for migration of oxygen. When the diffusion of anions is sufficiently fast, a continuous supply of oxygen from the bulk to the surface will guarantee an enhanced CO oxidation.29 In summary, four kinds of typical CeO2 mesoporous nanostructures (i.e., mesoporous nanospheres, nanoporous mesocrystals, hollow nanospheres, and nanowires) were controllably synthesized by a pressure-dependent solvothermal method without any template. They are all composed of tiny nanoclusters with diameter of 3−4 nm and present high specific surface area. More interestingly, the morphology evolution from mesoporous nanospheres to nanoporous mesocrystals, hollow nanospheres, and nanowires can be achieved via adjusting the reaction pressure. These different microstructures help us to understand the morphology-dependent properties. The corresponding pressure-induced self-assembly (Ostwald ripening, reconstruction) mechanisms are proposed to illustrate their evolution. They also present enhanced catalytic properties for CO oxidation. Moreover, it is anticipated that the pressuredependent solvothemal strategy should be applicable in producing other mesoporous nanostructures for catalysis, advanced energy transformation, and other related applications.

Figure 4. (a) CO conversion as a function of reaction temperature and (b) Raman spectra of various CeO2 catalysts.

commercial products. In particular, hollow nanospheres show enhanced catalytic activity than the other obtained structures and reported CeO2 nanospheres (Table S1). Here, we compare the catalytic activity difference between mesoporous and hollow nanospheres. CeO2 hollow nanospheres become very active above 170 °C and achieve 90% of CO conversion at 215 °C. However, the activation and 90% CO conversion temperatures of mesoporous nanospheres are ∼228 and 260 °C, respectively. There is an obvious difference between the two catalysts. The conversion−temperature curve of hollow nanospheres also reveals that the catalyst is temperature sensitive for the CO oxidation reaction. The CO conversion rate strikingly increases in a very narrow temperature interval, from 10% at 196 °C to 90% at 215 °C. The hollow nanospheres’ excellent performance should be attributed to the following factors: first, the large SSA, high pore volume, and well-defined hollow structure promote the adsorption of CO and O2 on the surfaces of these samples (Table 1); second, defects play a critical role in the surface reactions of CeO2 nanostructures. There may be high concentrations of oxygen vacancy defects on the surface of small nanoclusters.27,28 The defects benefit the relatively easy shuttles between the III and IV oxidation states, which can obviously enhance the catalytic activity.25−30 For the asobtained hollow nanospheres, the lower conversion temperature suggests lower barriers for CO oxidation. So, in addition to oxygen vacancies, there should be other defects to influence the catalytic activity, but this is still unclear. Figure 4b shows the Raman spectra of the catalysts. They all exhibit a strong peak at ∼456 cm−1, which corresponds to the F2g mode of cubic CeO2.28 Compared to the bulk CeO2 value (∼466 cm−1), there is obvious red-shift and broadening because of the defects. Meanwhile, a weak peak at 597 cm−1 could also

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01602. Experimental section, SEM images, TEM images, HRTEM images and XRD patterns of CeO2 mesoporous nanostructures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

# The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Yujing Geng and Gang Lian contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from the NSFC (grant numbers 51102151, 51372143, 50990061, 21073107), Natural Science Foundation of Shandong Province (ZR2011EMQ002, 2013GGX10208), and Independent Innovation Foundation of Shandong University (2012GN051). E



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Pressure-Induced Synthesis and Evolution of Ceria Mesoporous

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