nanorods and nanocubes in a Microfluidic System

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Materials and Interfaces

Ultrafast, Continuous and Shape-controlled Preparation of CeO2 Nanostructures: nanorods and nanocubes in a Microfluidic System Hongbao Yao, Yujun Wang, Yu Jing, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Industrial & Engineering Chemistry Research

Ultrafast, Continuous and Shape-controlled Preparation of CeO2 Nanostructures: nanorods and nanocubes in a Microfluidic System

Hongbao Yaoa, Yujun Wanga,*1, Yu Jing a,*2, Guangsheng Luoa

a

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua

University, Beijing 100084, China

1

Corresponding author* : Tel: 86-10-62798447, Fax: 86-10-62770304 Email address: [email protected]

2

Corresponding author* : Tel: 86-10-62795335, Fax: 86-10-62770304 Email address: [email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Ceria nanorods enclosed with {110}/{100} planes and ceria nanocubes with {100} planes have been successfully prepared using a home-made microfluidic system in a continuous, ultrafast and shape-controllable manner. Only 8 minutes of reaction time are needed rather than days to synthesize ceria nanostructures in the traditional batch hydrothermal method. During the synthesis, reaction temperatures and base concentration have been demonstrated as the key factors responsible for the shape evolution. Accordingly, a morphological phase diagram was determined. In addition, polyvinyl pyrrolidone (PVP) was introduced to realize the transformation from ceria nanorods to nanocubes under unfavorable hydrothermal conditions. Catalytic performance of different CeO2 architectures was also examined in decomposing hydrogen peroxide and a reactivity trend (nanoparticles < nanorods < nonocubes) was observed. This is assumed to be related with different surface oxygen vacancy amounts as a result of {110}/{100} preferential planes, as confirmed by X-ray photoelectron spectroscopy (XPS) and Raman analyses as well as density functional theory (DFT+U) calculations.

Key word Ceria, nanorod, nanocube, microreaction, DFT+U calculation

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1. Introduction As one of the most important rare earth metal oxides, ceria (CeO2) has been widely used in tremendous redox catalytic applications, such as automotive three-way converters, water-gas shift (WGS) reactions, wet catalytic oxidation and so on considering its desirable combination of chemical and physical properties1-7. In many of these catalytic applications, shape-dependency activity of ceria has been observed likely due to the selective exposition of surface planes, being the {100} surfaces catalytically more active than the {111} and {110} surfaces8-12. For example, Zhou et.al13 claimed that CeO2 nanorods with well-defined reactive planes {001} and {110} show 3-fold improvement in CO oxidation rates over CeO2 nanoparticles. Leandro and co-workers14 reported that the activity of the CeO2 nanocubes exposed with planes {100} is shown to be in the order of 400 times higher per gram of oxide than powder CeO2 for the CO oxidation reaction. Consequently, many approaches have been followed to synthesize CeO2 nanostructures with different morphologies including hydrothermal method15-19, precipitation20, sol-gel method21, electrochemical deposition22, nanoparticle self-assembling routes23,

24

etc. Among them, the

hydrothermal synthesis is considered as the most popular and effective method but special organic reagents, surfactants, or templates are generally needed. In addition, it is also a batch reaction process with long reaction time that typically lasts up to 12-24 h

10, 14, 25, 26

. Therefore,

development of facile, controllable, and effective methods for creating ceria architectures with defined exposed surfaces remains an important challenge. Nowadays, microfluidic synthesis has attracted extensive attention to prepare nanomaterial because of the advantages of supeorior mass, heat and momentum transfer properties20,

27-30

.

Notably, Hoang et.al.31 proposed a breakthrough synthetic approach in an ionic liquids assisted microfluidic system, where nanoporous ZSM-5 and γ-AlOOH were synthesized in only tens of minutes rather than days usually needed. Ionic liquids here mainly play roles in maintaining the temperatures of reaction systems over 100 °C under ambient pressure but with drawbacks of high cost and difficulty of separation. Still, it provides us with a solution using microfluidic systems to synthesize unaccommodating inorganic nanomaterial that are usually prepared under harsh experimental conditions. On the other hand, curved tube reactor applied in nanomaterial synthesis with enhanced performance due to the generation of secondary flow has been a subject of intensive investigations32-34. Most recently, Wu et.al.35 successfully synthesized narrow sized Ag 3



nanoparticles in a helical microreactor with the absence of capping ligands. Nevertheless, there is little literature available concerning faceted nanocrystal preparation in similar coiled tube microreactors. Herein, an ultrafast, continuous and shape-controlled preparation method of ceria nanostructures with different exposed active planes was first time reported using a homemade microfluidic reaction system. This microfluidic reaction system mainly consists of a T-type micromixer and a helical microreactor with a cooling zone, which is designed to withstand reaction temperatures up to 250 °C. Morphology evolution of CeO2 nanomaterials synthesized in the micro-scale conditions was accordingly discussed in detail. Furthermore, polyvinyl pyrrolidone (PVP) as a surfactant was introduced into the micro-reaction system to realize the transformation from ceria nanorods to nanocubes under unfavorable hydrothermal conditions. In addition, the catalytic oxidation performance of prepared CeO2 architectures with different active planes was also examined in the decomposition of hydrogen peroxide. Accordingly, XPS, Raman and DFT+U studies explained the reasons for the different planar reactivity. This research aims to fill the gap in the combination of ceria nanorods/nanocubes fabrication and superior micro-technology. The success may also lead to a new synthetic pathway for other similar inorganic nanostructures in a continuous and effective manner.

2. Experimental 2.1. Materials and Chemicals Analytical-grade cerium nitrate (Ce(NO3)3·6H2O), sodium hydroxide (NaOH), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 28%) and anhydrous ethanol were all purchased from Beijing Chemical Plant and used as received without further purification process. Deionized water was used throughout the experiments. 2.2. Experimental Set-up and Catalyst Preparation Fig. 1 shows the scheme of experimental set-up. Cerium nitrate aqueous solution as continuous phase, and sodium hydroxide aqueous solution as dispersed phase were pressed by the advection pump and mixed in a T-type micro-mixer with bore size of 50 µm. Then the mixture flowed through a stainless helical tube micro-reactor with inner diameter of 1 mm. Reaction temperatures ranging from 100 °C to 230 °C were controlled by the oil bath here and the pressure was adjusted by a counterbalance valve at the end of whole system. After water bath cooling in 4


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another helical tube with same inner size, the resulting yellow product was collected and washed with water and ethanol several times, followed by a drying process at 60 °C for 12 h in vacuum. Particularly, CeO2 nanorods prepared at 150 °C with NaOH concentration of 2 mol/L and CeO2 nanocubes prepared at 230 °C with NaOH concentration of 2 mol/L were hereafter denoted as CeO2_nr and CeO2_nc, respectively. For comparison, CeO2 nanoparticles were also prepared following the similar procedure but at room temperature and ambient pressure, as well with additional calcination process at 500 °C for 4 h with a heating rate of 2 °C /min20, 36. This sample was denoted as CeO2_np.

Fig. 1 Schematic diagram of the reaction system for synthesis of CeO2 nanomaterials [1: NaOH tank, 2: Ce(NO3)3 tank, 3 and 4: advection pumps, 5: T-type micro-mixer, 6: oil-bath heating, 7: thermocouple, 8: water-bath cooling, 9: back pressure valve, 10: sample]. 2.3. Characterization Transmission electron microscopy (TEM, JEOL-2010, Japan) was applied to observe the morphology of prepared CeO2 samples. The Brunauer-Emmett-Teller (BET) surface areas of different samples were measured at 77 K on a Quantachrome Autosorb-1-C chemisorption– physisorption analyzer based on the adsorption branches in the relative pressure range of 0.05– 0.25. X-ray diffraction (XRD, Model D8 ADVANCE, Bruker) was used to determine the crystal structures. The Scherrer equation was used to determine the particle size of the crystals as follows: D

0.89

1 cos

where D is the grain size; λ is the X-ray wavelength, which was maintained at 0.15406 nm in our experiments; and θ is the Bragg angle. Surface chemical composition of the samples was detected by X-ray photoelectron spectroscopy (XPSPHI Quantera SXM，ULVAC-PHI, Japan) and binding energy was calibrated with C1s at 284.8 eV. The Raman spectra were recorded in the region of 200-1000 cm-1 on a spectrometer (LabRAM HR Evolution, France) equipped with a helium-neon 5



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laser operating at 50 mW for the 532 nm output as the excitation source. 2.4. Measurement of Catalytic Performance The catalytic performance of prepared CeO2 samples was evaluated by decomposing H2O2. Typically, 0.05 g of prepared CeO2 was added to the 100-mL H2O2 aqueous solution with an initial concentration of 71.5 mmol/L at 50 °C under vigorous stirring. At the given time intervals (4 min), 5-mL reaction products were taken from the mixture and immediately centrifuged. The residual concentration of H2O2 in supernatant was analyzed by a KMnO4 titration method

37

. Blank

experiment was also conducted in the absence of CeO2 sample under same experimental conditions. The decomposition efficiency in this work was defined as the percentage reduction in the H2O2 concentration relative to the initial value.

3. Results and Discussion 3.1 Characteristics of Representative Samples Fig. 2(a) shows the resulting TEM images of CeO2_nr samples synthesized within only a reaction time of 8 min. These nanorods have typical diameters of 4-6 nm and lengths in the range of 15-35 nm. In addition, three kinds of lattice fringes of (111), (200), and (220) could be identified in the HRTEM images, as depicted in Fig. 2(b), with respective interplanar spacing of 0.312, 0.271, and 0.191 nm, respectively. It indicates that the synthesized nanorods show a 1D growth structure with a preferred growth direction along [110], and are enclosed by (220) and (200) planes, which are identical to the case of CeO2 nanorods in previous reported literatures16, 38. Selected area electron diffraction (SAED) pattern shown in Fig. S1(a) further confirms that as-prepared ceria nanorods has fluorite structure with high crystallinity. Similarly, several noteworthy peaks at 2θ of 28.7, 33.2, 47.6, 56.5, 59.1, 69.4, 76.7 and 79.1 degrees were also observed in the XRD pattern, as shown in Fig. 3, which are typical signals of cubic CeO2 (fluorite structure, JCPDS 34-0349).

6


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Fig. 2 (a) TEM images of CeO2_nr (b) HRTEM image of a CeO2 nanorod [reaction temperature: 150 °C, continuous phase: Ce(NO3)3, concentration: 0.05 mol/L, flow rate: 5 mL/min; dispersed phase: NaOH, concentration: 2 mol/L, flow rate: 5 mL/min; residence time: 8 min]. CeO2_np CeO2_nr CeO2_nc

Intensity

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20

40

60

80

2-Theta

Fig. 3 XRD patterns of CeO2_np, CeO2_nr and CeO2_nc, respectively. To reveal the superiority of this novel procedure coupled with the microreactor over traditional hydrothermal methods, two parallel groups of CeO2 nanorods were prepared under identical hydrothermal conditions but with different reaction time. The resulting TEM images were shown in the Supporting Information. As shown in Fig. S2, it can be clearly observed that only CeO2 nanoclusters can be acquired at a reaction time of 30 min in the traditional hydrothermal method; and it takes at least two hours to obtain well defined CeO2 nanorods. However, in the microfluidic system, most CeO2 nuclei have already grown into the rod-like at a residence time of 3.5 min, as depicted in Fig. S3. Undoubtedly, excellent heat transfer as well as mass transfer properties helps control the growth of nanostructures30, 31. On the other hand, Fig. 4(a) shows the resulting TEM images of CeO2_nc samples synthesized with a reaction time of 8 min. The statistical average diameter from the TEM results is 14.3 ± 1.2 nm, which is consistent with the grain size data (14.7 nm, as shown in Table 1) 7



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calculated from XRD results. The HRTEM image in Fig. 4(b) presents clear (200) and (220) lattice fringes with the interplanar spacing of 0.271 and 0.191 nm, respectively, implying that the CeO2 nanocubes are only enclosed by (200) planes10, 39, 40. The corresponding SAED pattern shown in Fig. S1(b) and XRD patterns in Fig. 3 both confirms that as-prepared ceria nanocubes have cubic structure with high crystallinity.

Fig. 4 (a) TEM images of CeO2_nc and relative particle size distribution (b) HRTEM image of a CeO2 nanocube [reaction temperature: 230 °C, continuous phase: Ce(NO3)3, concentration: 0.05 mol/L, flow rate: 5 mL/min; dispersed phase: NaOH, concentration: 2 mol/L, flow rate: 5 mL/min; residence time: 8 min]. In addition, the TEM image of CeO2_np sample is shown in the Supporting Information (Fig. S4) with an average crystal size of ~10.1 nm seen in Table 1. Those particles present typical cubic structure confirmed by the XRD patterns shown in Fig. 4. The specific surface areas of three representative samples are also summarized in Table 1. CeO2_nr sample exhibits the highest specific surface area with a value of 98.7 m2/g, while CeO2_nc sample has the smallest value of 29.8 m2/g. Moreover, CeO2_np sample has a relative smallest crystal size but with a specific surface area value only reaching 61.4 m2/g, which may be due to the serious particle aggregation. Table 1 Summary of characterization results of different samples by XRD, BET, XPS and Raman analyses. Catalyst

Specific surface

Crystal size

Ce3+ concentration

area (m2/g)a

(nm)b

(%)c 8


Oβ/Oαc

I593/I463 (×10-2)d

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CeO2_np

61.4

10.1

26.8

0.34

1.9

CeO2_nr

98.7

13.8

28.3

0.47

3.5

CeO2_nc

29.8

14.7

31.3

0.85

5.1

a

Determined by BET analysis; b Determined by XRD using Scherrer’s equation; c Determined by XPS analysis;

d

Determined by Raman analysis.

3.2 Morphological Phase Diagram In order to study the morphology evolution systematically, various CeO2 nano-samples were prepared at a range of NaOH concentrations (ranging from 0.5 mol/L to 8 mol/L) with different reaction temperatures (from 100 °C to 230 °C) and characterized mainly by the TEM analysis. The resulting morphological phase diagram was summarized in Fig. 5. It should be noted that at low temperatures (< 100 °C), only CeO2 nanoparticles can be observed, which are not included in the plot. Also, the phase boundaries did not show sharp transitions to pure phases. For example, the CeO2 nanoparticles and nanorods co-exist under the conditions of NaOH concentrations ranging from 1 mol/L to 2 mol/L, while pure nanorods start to come out over 4 mol/L NaOH at 100 °C. In addition, pure CeO2 nanorods can also be obtained with increasing temperatures at a wide range of 150-180 °C and increasing NaOH concentrations at a range of 1-4 mol/L. Furthermore, CeO2 nanocubes will be formed at even higher reaction temperatures and base concentrations. Similar morphological phase evolution mechanism controlled basically by reaction temperatures and base concentrations was also reported in traditional hydrothermal methods12. Actually, the formation of CeO2 nanostructures in a hydrothermal condition has been ruled by the well-known dissolution/recrystallization mechanism41, 42. At low temperatures and base concentrations, the dissolution/recrystallization rate is low, and there seem to exist inadequately high chemical potential for driving the anisotropic growth of Ce(OH)3 nuclei, resulting in the formation of CeO2 nanoparticles.

9



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Fig. 5 Morphological phase diagram of CeO2 nanosamples prepared at different conditions. [ , , and When

stand for nanoparticles, nanorods, and nanocubes, respectively]. the reaction

temperatures or base concentration

are high

enough,

the

dissolution/recrystallization rate is considerably promoted and CeO2 nanorods are accordingly formed. At even higher temperatures or base concentrations, the Ce(OH)3 nuclei become unstable and are oxidized to CeO2 with a nanocube morphology16, 43. Interestingly, nanocubes appear to not show up with a low NaOH concentration of 0.5 mol/L even at an ultrahigh reaction temperatures of 230 °C under our experimental conditions, implying the significance of the presence of strong basic conditions. Similarly, Wu et.al. 38 also claims that strong basic condition is necessary during the morphological conversion from CeO2 nanorods to nanocubes, which suggests that the dissolution/recrystallization equilibrium was hard to be broken under mild alkaline conditions. Furthermore, the effect of reaction temperatures or base concentrations on the aspect ratio of CeO2 nanorods and the size of CeO2 nanocubes was accordingly examined in detail. As shown in Fig. 6(a), the CeO2 rod length changes from 6.8 nm to 23.9 nm, an increase of ~2.5 times and rod width changes from 2.2 nm to 4.1 nm, increasing by ~92.6% when the reaction temperature increases from 70 °C to 180 °C. Accordingly, the aspect ratio of CeO2 nanorod varies from 3.1 to 5.8, which is a ~87.1% increase. Identical phenomenon that aspect ratio of CeO2 nanorod increases with an increasing temperature has also been reported in previous literatures44. The growth of nanorods appears to follow an oriented crystallite attachment mechanism, where faster incorporation of the crystallites at high temperatures will result in increased rod lengths45.

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16

25

(a) Length Width Aspect Ratio

15 8 10 4

5 0 80

160

o

0 200

Reaction Temperature ( C)

120

(b) Mean Size (nm)

120

Aspect Ratio

12

9

Length Width Aspect Ratio

80

6

40

3

Aspect Ratio

Mean Size (nm)

20

0

0 0

3

6

9

NaOH Concentration (mol/L) 20

(c) Mean Size (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

10

5 0

3

6

9

NaOH Concentration (mol/L)

Fig. 6 Effect of different experimental conditions on the size of as-prepared samples (a) CeO2 nanorods [Ce(NO3)3 concentration: 0.05 mol/L, flow rate: 5 mL/min; NaOH concentration: 1 mol/L, flow rate: 5 mL/min], (b) CeO2 nanorods [reaction temperature: 180 °C; Ce(NO3)3 concentration: 0.05 mol/L, flow rate: 5 mL/min; NaOH flow rate: 5 mL/min], (c) CeO2 nanocubes [reaction temperature: 230 °C; Ce(NO3)3 concentration: 0.05 mol/L, flow rate: 5 mL/min; NaOH flow rate: 5 mL/min] . In addition, the use of NaOH seems to decrease the tendency of hydrogen bonding between crystallites, which will favor one-dimensional growth of CeO2 nanorods46. As shown in Fig. 6(b), CeO2 rod length increase with an increasing NaOH concentration, but with a steady aspect ratio of ~4.7 under experimental conditions considering the similar increase speed in rod width. On the other hand, nanoceria samples obtained from a highly alkaline system are generally heavily 11



aggregated, resulting in larger particle sizes20, 40. As depicted in Fig. 6(c), the mean size of CeO2 nanocubes changes from 9.8 nm to 16.7 nm, an increase of ~70.4% when the NaOH concentration rises from 1 mol/L to 8 mol/L. 3.3 Effect of PVP Introduction Considering the {100} terminated surfaces are inherently more reactive than {111} and {110} surfaces47, the conversion from CeO2 nanorods to CeO2 nanocubes was also examined in our microfluidic system. CeO2 nanorod surrounded by CeO2 nanocube samples were prepared as the blank group following the previous discussed protocol with a NaOH concentration of 5 mol/L and a reaction temperature of 230 °C. The relevant TEM image was listed in Fig. 7(a) and the CeO2 nanocube percentage could be easily determined as ~ 65.3% based on a statistical number-weighted method by surveying more than 200 particles. The other three experimental groups also followed the identical procedure except that PVP surfactant with different concentrations pre-mixed with the Ce(NO3)3 aqueous solution were used as the continuous phase. The resulting TEM images were shown in Fig. 7(b-d). Obviously, with an increasing PVP concentration, CeO2 nanorods begin to disappear with decreasing quantities. The CeO2 nanocube percentage increases from ~ 82.5% to ~ 91.7% when the PVP/Ce molar ratio changes form 0.5:1 to 1:1. Specifically, pure CeO2 nanocubes can be obtained while PVP/Ce molar ratio reaching 2:1. In the previous reported literatures, PVP surfactant was used to directly synthesize CeO2 nanowires48 and CeO2 nanoflowers49. Here it was found that CeO2 nanorods can also turn into nanocubes morphologically with the aid of PVP under disadvantageous hydrothermal conditions. This is most probably because PVP serve as capping agents selectively absorbed on the {100} planes of CeO2 nuclei, leaving {110} planes open and resulting in the formation of CeO2 nanocubes enclosed by {100} planes. It is well-known that the use of capping reagent or template can break the shape restriction and favor the crystal growing anisotropically50. The adsorbed layer will change the surface free energies of CeO2 crystals, thus kinetically control the growth rates of different facets. Similarly, NO3- 38 and oleic acid40 was also reported previously and considered to play the same roles in promoting the conversion from CeO2 nanorods to nanocubes.

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Fig. 7 TEM images of CeO2 nanostructures prepared with different PVP/Ce molar ratios (a) 0:1 (b) 0.5:1 (c) 1:1 (d) 2:1 [reaction temperature: 230 °C, Ce(NO3)3 concentration: 0.05 mol/L, flow rate: 5 mL/min; NaOH concentration: 5 mol/L, flow rate: 5 mL/min; residence time: 8 min]. 3.4 Catalytic Performance for H2O2 Decomposition The catalytic performance of three types of representative samples was examined by decomposing H2O2. As shown in Fig. 8, the pure H2O2 aqueous solution with an initial concentration of 71.45 mmol/L (calibrated by the mentioned KMnO4 titration method) actually kept stable and hardly decompose even at a reaction temperature of 50 °C. However, it started degrade with the addition of all three catalysts. Apparently, the catalytic reactivity of different catalysts follow the sequence: CeO2_nc > CeO2_nr > CeO2_np. Specifically, it was observed that H2O2 concentration decrement can be well-fittted in apparent zero-order kinetics. The calculated apparent reaction rate constant (ka) for CeO2 nanoparticles is 0.9 mmol L-1 min-1. On the other hand, the ka value for CeO2 nanorods and CeO2 nanocubes reach 1.6 mmol L-1 min-1 and 2.6 mmol L-1 min-1, which improves by 0.78 and 1.8 times, respectively, than that in the case of CeO2 nanoparticles. In addition, the ka value for CeO2 nanorods in this work is about 80.2% higher than that prepared by traditional batch reactions39. This seems to be because it is easier to generate nano-structures with smaller particle size in microfluidic reaction system, resulting in larger specific surface areas and more excellent catalytic performance20, 51. It is assumed that the Ce(III) state of cerium dioxide, accompanied with the oxygen vacancy is the reactive site for catalyzing H2O239, 52. Additionally, previous studies strongly suggest that the 13



exposure of more reactive (100) surfaces, followed by (110) surfaces, should facilitate the formation of oxygen vacancies on CeO211, 47. Therefore, CeO2 nanocubes with well-defined {100} facets show the most favorable catalytic performance in the decomposition in H2O2. 80

C H 2O2 (m m ol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

blank CeO2_np

40

CeO2_nr CeO2_nc 20 0

6

12

18

Reaction time (min) Fig. 8 H2O2 decomposition process and corresponding linear fitting over reaction time in the presence of different catalysts [H2O2 initial concentration: 71.45 mmol/L, initial volume: 200 mL; reaction temperature: 50 °C; catalyst dosage: 0.05 g; stirring rate: 500 rpm].

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Fig. 9 Ce 3d XPS spectra of (a) CeO2_np, (b) CeO2_nr and (c) CeO2_nc, respectively. To prove above assumption, XPS was further used to evaluate the surficial Ce(III) concentration as well as oxygen vacancy amounts of different catalysts. Fig. 9 shows the Ce 3d XPS spectra and its deconvolution results. Apparently, all three samples contained predominantly surficial Ce(IV) but with different Ce(III) quantities depending on their morphologies. As listed in Table 1, CeO2_nc contains the highest Ce(III) amounts of 31.3%, coming to CeO2_nr of 28.3% and being the lowest for CeO2_np of 26.8% (details seen in Supporting Information). O1s XPS spectra are described in Fig. S5. The XPS Oβ/Oα ratio is assumed to be a common way to roughly assess the amount of oxygen vacancies where Oα stands for the low binding energy peak at ~530 eV ascribed to lattice oxygen and Oβ stands for the high binding energy peak at ~532 eV assigned to oxygen vacancy as well with hydroxyl groups12. Identical conclusion can also be drawn from O1s XPS analyses. Namely, the surface oxygen vacancy concentration follow the sequence: 15



CeO2_nc > CeO2_nr > CeO2_np. This is also consistent with what observed in the H2O2 decomposition experiment. Complementary data could be obtained from the Raman analysis of three different CeO2 catalysts. As shown in Fig. 10, all samples exhibit a single dominant band at a frequency of ~ 463 cm-1, which is attributed to the active vibrational mode (F2g) of the O atoms around each Ce(IV) ion18. The weak peak at ~ 251 cm-1 is associated with disorder in the system while low intense peak at ~ 593 cm-1 is considered be related to the oxygen vacancies or point defects due to the presence of Ce(III). Therefore, through comparing the ratio of Raman peak intensity at 593 cm-1 to that at 463 cm-1 (denoted as I593/I463), the relative oxygen vacancy concentration in CeO2 can be estimated19,

53

. The corresponding I593/I463 values listed in Table 1 also indicate the defect

concentration over the three catalysts follow the order: CeO2_nc > CeO2_nr > CeO2_np and it is consistent with what observed in XPS analysis as well as H2O2 decomposition experiment.

Fig. 10 Raman spectra of CeO2_np, CeO2_nr and CeO2_nc, respectively. 3.5 Calculation of Vacancy Formation Energies Density functional theory calculations with the inclusion of on-site electronic correlations (DFT + U) were carried out to further examine the electronic structure of three low-index lattice planes on the surface of CeO2 nanocrystals: (111), (110) and (100). The purpose of adding U (Hubbard term) is to diminish the self-interaction error and properly localize the Ce 4f states54, 55. Computational details were presented in the Supporting Information. Fig. 11 shows the calculated density of states of bulk CeO2 and three defective low-index CeO2 surfaces. The low lying Ce 5s band found at -33 eV is not included in the plot. For bulk CeO2, a narrow unoccupied band just situated above zero level mainly derives from Ce 4f states. 16


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The conduction band is made up of Ce 5d states. The valence band is composed of predominantly O 2p states with small contribution from Ce5d and 4f. In addition, the band found between -20 eV and -10 eV below the valence band arises from interactions between Ce 5p and O 2s states. The calculated energy gaps of 1.5 eV between O 2p and Ce 4f states, and 5 eV between O 2p and Ce 5d states show the usual underestimation of band gaps using DFT calculations56,

57

; the

experimental band gaps are 3 eV and 6 eV, respectively58. Comparing the DOS of three defective surfaces with the bulk, the main changes occur in the valence states, those being predominantly oxygen 2p. Specifically, the shift of Ce 5d states to the valence results in the reduced energy gaps between valence band and Ce 4f, which will lead to higher catalytic reactivity59, 60.

Fig. 11 Density of states (DOS) for (a) bulk CeO2, (b) defective (111) surfaces, (c) defective (110) surfaces and (d) defective (100) surfaces. The zero energy is set to the top of the valence band. To gain deep sight into the difference of three low index surfaces, the calculated surface energies of the three pure surfaces were listed in the first column of Table 2. It was observed that the surface energies increase in the order (111) > (110) > (100); namely, the stability decreases in the order (111) > (110) > (100). The (111) surface has the smallest surface energy with a value of 0.68 J/m2, consistent with the fact that (111) is the most compact of the three surfaces, with all the surficial O ions being three coordinated and Ce ions seven coordinated. The (110) surface is next in stability with a surface energy of 1.02 J/m2 for the relaxed surface, while the highest surface 17



energy of 1.43 J/m2 for (100) surface. Similar phenomena have also been reported in literature58. Table 2 Calculated surface energies and vacancy formation energies using DFT+U

a

surface

Es (J/m2)a

Evac (eV)b

Corrected Evac (eV)c

(111)

+0.68

+2.63

+3.33

(110)

+1.02

+2.21

+2.91

(100)

+1.43

+2.04

+2.74

Surface energies of the pure surfaces; b DFT+U formation energies; c DFT+U formation energies corrected for the

DFT error in the O2 bonding energy.

Furthermore, calculated DFT+U formation energies per oxygen vacancy for the three defective surfaces were listed in the fourth column of Table 2. An increase of 0.70 eV in the vacancy formation energy compared with Evac was due to the error correction of well-known O2 bonding energy calculation by DFT. The results show that the ordering of vacancy formation energies of the three low index surface follow the trend indicated by the stabilities of the pure surfaces. In other words, the catalytic reactivity of different CeO2 surface planes follow the sequence: {100} > {110} > {111}, which is consistent with what observed both in XPS analysis and H2O2 decomposition experiments.

4. Conclusion In this work, the controllable preparation of ceria nanomaterials with well-defined {110}/{100} planes using a home-made microfluidic system has been examined in detail. A morphological phase diagram was accordingly determined basing on the key factors responsible for the shape evolution, namely, reaction temperatures and base concentration. The effect of PVP introduction was also studied specifically and it seems to serve as a capping agent onto {100} planes, thus promoting the conversion of CeO2 nanorods to CeO2 nanocubes. In addition, representative samples of nanoparticles, nanorods and nanocubes have also been characterized by TEM, SAED, XRD, BET analysis in detail. Most importantly, 0.78-fold and 1.8-fold improvements in catalytic activity of H2O2 decomposition were observed for CeO2 nanorods and nanocubes, respectively, than that of nanoparticles. Consequently, the catalytic reactivity of different exposed surfaces follow the sequence: (100) > (110) > (111). The reason seems to be related with different surficial oxygen 18


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vacancy concentration, as demonstrated by XPS, Raman and DFT+U studies.

ASSOCIATED CONTENT Supporting Information Available SEAD pattern, TEM images and O1s XPS spectra of prepared ceria samples, determination process of surficial Ce(III) concentration and DFT+U computational details.

Conflicts of interest There are no conflicts to declare.

Acknowledgements We gratefully acknowledge the support of the National Basic Research Program of China (2013CB733600) and the National Natural Science Foundation of China (21276140, 20976069 and 21036002).

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Graphic Abstract

23


nanorods and nanocubes in a Microfluidic System

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