J Sol-Gel Sci Technol DOI 10.1007/s10971-017-4407-y

ORIGINAL PAPER: NANO-STRUCTURED MATERIALS (PARTICLES, FIBERS, COLLOIDS, COMPOSITES, ETC.)

Synthesis of CeO2 nanocrystals with controlled size and shape and their influence on electrochemical performance Li Wang1 Yanhong Li1 Min Shi1 Wenle Ma1 Hongtao Cui1 ●

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Received: 21 February 2017 / Accepted: 5 May 2017 © Springer Science+Business Media New York 2017

Abstract In this work, nanosphere-like and nanowire shaped CeO2 nanocrystals were prepared at room temperature utilizing the epoxide precipitation reaction in aqueous and ethanolic solutions. It was found that the shape of CeO2 nanocrystals depends on the reaction kinetics of epoxide in solution. The isotropic growth of nanocrystals in aqueous solution resulted in the formation of CeO2 nanocrystals with sphere-like shape, while the anisotropic growth of nanocrystals in ethanolic solution led to the nanowire shape of CeO2 nanocrystals. The anisotropic growth of nanocrystals was attributed to the high surface energy of their {110} facet induced from the active reaction of epoxide in ethanolic solution. The reaction kinetics of epoxide affected not only the size and crystallinity of CeO2 nanocrystals, but also their shape. As a result, the produced CeO2 nanocrystals had different electrochemical performance. As an electrode material for application in supercapacitor, the nanosphere-like CeO2 nanocrystals presented better electrochemical performance and longer cycle life than the packed CeO2 nanowires. Graphical Abstract Nanosphere-like and nanowire shaped CeO2 nanocrystals were prepared utilizing the epoxide precipitation reaction in aqueous and ethanolic solutions. It was found that the shape of CeO2 nanocrystals depends on the reaction kinetics of epoxide in solution. The isotropic growth of nanocrystals in aqueous solution produced CeO2 nanocrystals with sphere-like shape, while the

* Hongtao Cui [email protected] 1

College of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China

anisotropic growth of nanocrystals in ethanolic solution produced nanowire shaped CeO2. As an electrode material for application in supercapacitor, the nanosphere-like CeO2 nanocrystals presented better electrochemical performance and longer cycle life than the packed CeO2 nanowires.

Keywords Ceria Nanosphere Nanowire Supercapacitor ●

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1 Introduction Nanostructured CeO2 is a technologically important functional material, which has leading applications in various areas such as catalysis, optics, sensors, fuel cells, and so on [1, 2]. It is well known that the performance of nanostructured materials depends on some characteristics including particle size and shape, micro-texture, crystallinity, surface properties, etc. [3–7]. Among these characteristics, particle shape is a particularly interesting and frequently investigated one for CeO2 [8–14]. The performance of CeO2 nanoparticles would be shape-dependent in some applications, especially in catalysis. The distribution

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of various crystallographic facets exposed on the surface of CeO2 nanocrystals is closely related with their shape, while the dominant crystal facets decides the catalytic properties of CeO2. As a result, the properties of CeO2 nanocrystals could be tuned directly by their shape in this case [9, 12, 14]. The shape determination of nanocrystals follows the rule of anisotropic growth [15–17]. The selective anisotropic growth of nanocrystals initiates from the packing of monomer or attachment of other nanocrystals on specific crystal facet having high surface energy, resulting in the formation of final shape. As compared with other facets, the higher surface energy of a certain facet results from its higher atomic density and number of dangling bonds. Therefore, the shape of nanocrystals could be tailored by controlling the surface energy of specific crystallographic facets. In general, the control of surface energy is thermodynamic or kinetic growth regimes-governed, or their balanced result [17]. The shape of nanocrystals controlled by thermodynamic growth regimes-governed mechanism is the development result of nuclei with thermal dynamically stable crystal phase. While, kinetic growth regimescontrolled shape tailoring on nanocrystals could realize diversified shapes. The shape control is achieved by the selective surface capping, oriented attachment, crystal phase of nucleated seeds etc. [18, 19]. The nanocrystals with well controlled shape and narrow size distribution are usually produced by nonhydrolytic methods at promoted temperature in the presence of organic surfactants [20–22]. It has been accepted that nanocrystals prepared by hydrolytic method frequently suffer from relatively poor crystallinity, size polydispersity, or irregular shape, especially for metal oxide [20]. Due to the inherent characteristics of metal oxide, the preparation has to be performed at elevated temperature or the produced intermediate has to be heat-treated at high temperature to form the required phase. These would induce issues such as high cost, irregular shape, and aggregation. Therefore, the shape control of metal oxide by hydrolytic methods is still an interesting and challenging topic in the area of nanochemisry. In this work, CeO2 nanocrystals with well-defined shapes of nanosphere-like and nanowire were prepared at room temperature by an epoxide precipitation route in aqueous or ethanolic solution. The preparation utilized the hydrolysis and condensation reactions occurring between [Ce(H2O)6]3+ ions and propylene oxide. The different reaction kinetic in aqueous or ethanolic solution resulted in the different shapes of CeO2 nanocrystals. The shape influence of CeO2 nanocrystals on their electrochemical performance was investigated in 3 M KOH aqueous solution. It was found that the CeO2 nanocrystals with nanosphere-like shape had higher electrochemical performance than CeO2 nanowires.

2 Experimental procedure 2.1 Synthesis of CeO2 nanoparticles Propylene oxide (C3H6O, ≥99.5%) and cerium (III) chloride heptahydrate (CeCl3.7H2O, ≥99%), ethanol (C2H6O, ≥99.5%) were purchased from Aladdin and used asreceived. In a typical procedure, 3 mL propylene oxide as precipitant was added to a 20 mL 0.3 M aqueous or ethanolic solution of CeCl3.7H2O. The reaction was performed at room temperature under vigorous stirring for 24 h. After reaction, final product was obtained by the sequent steps of precipitate collection through centrifugation, washing with water and ethanol, and drying at 80 °C. The products respectively prepared in aqueous and ethanolic solution were denoted as CeO2-W and CeO2-E. 2.2 Characterization The crystalline phase of samples was measured with a Shimadzu LabX XRD-6100 diffractometer using Cu Kα radiation (λ = 1.5406). The morphology of samples was observed using a JEOL JEM-1400 transmission electron microscope (TEM). The TEM samples was prepared by the dispersing of a little amount of powder sample in ethanol under ultrasonification and the following drying of one drop suspension on a 400-mesh carbon-coated copper grid. 2.3 Preparation of working electrodes and electrochemical measurement The electrochemical measurements were carried out using an IVIUMSTAT electrochemical workstation in a threeelectrode cell equipped with a working electrode, a platinum-plate counter electrode and a Hg/HgO reference electrode. The cycle stability of electrode materials was tested on a LANHE CT2001A battery testing system connected with the above mentioned three-electrode cell and electrodes. The working electrode was prepared by inserting a paste into a nickel foam substrate. A paste was formed by adding a few drops of ethanol to a mixture of 80 wt% powder sample and 20 wt% acetylene black. After brief evaporation of ethanol, the paste was pressed at 10 MPa to the nickel foam with a nickel wire for electrical connection. The obtained electrode was dried in air for 3 h at 80 °C. Each electrode contained ~3.0 mg sample and had a geometric surface area of 1 cm2. After drying, the electrodes were activated by 100 cycles of cyclic voltammetry (CV) from 0 to 0.6 V vs. Hg/HgO at 100 mV s−1 in 3 M KOH aqueous solution and then used as a working electrode in the electrochemical measurements. The electrochemical tests on the electrodes including CV, galvanostatic charge–discharge and electrochemical impedance

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spectroscopy (EIS) were performed in 3 M KOH aqueous solution. The calculations about the specific capacitance were based on the description in literature [23].

3 Results and discussion In this work, two nanosized CeO2 samples were prepared at room temperature by an epoxide precipitation route under exactly same procedure. The only difference was the solvents used for dissolving CeCl3.7H2O (water and ethanol were, respectively, used as solvent). In the following characterizations, it will be proven that the ring-opening reaction of propylene oxide in water or ethanol would produce CeO2 nanocrystals with different shapes and size. In the XRD patterns (Fig. 1a), sample CeO2-W, prepared in aqueous solution, shows well-defined and widened X-ray diffraction lines, which is well indexed to the cubic fluorite structure of CeO2 (JCPDS 34-0394). The sample CeO2-E, prepared in ethanolic solution, also shows the same cubic fluorite structure of CeO2 (Fig. 1), however the intensity of its diffraction lines is much lower than the former sample. The TEM images of sample CeO2-W shown in Fig. 2a and b presents well-defined sphere like-shaped CeO2 nanocrystals with average size of ~5 nm. While in the TEM image of sample CeO2-E (Fig. 2c), it shows much larger CeO2 particles with irregular shape. It is clearly observed in the TEM image at high magnification (Fig. 2d) that these large particles are composed of parallel packed nanowires with average diameter of ~5 nm, forming bundles. These TEM results demonstrate that the reactions between epoxide and cerium salt in aqueous or ethanolic solution would produce CeO2 with same phase, however with different shapes. In brief, aquo complexes of metal ions, [M(H2O)6]n+, react with propylene oxide to form M–OH bonds [24–27]. The hydrolyzed ions undergo condensation, releasing water and forming M–O–M bonds. Propylene oxide acts as a proton scavenger, reacting with the protons coming from the hydrolysis of aquo complexes. It was found that the reaction activity of epoxide depends on the acidity of aquo complexes of metal ions in solution. Because aquo complexes presents higher acidity in ethanolic solution than in aqueous solution, it was observed that precipitation occurred earlier in the former solution. Herein, it is proposed that the shape of CeO2 nanocrystals depends on the different reaction kinetics in aqueous and ethanolic solution. It is known that in the case of no interference such as surface capping, the shape of nanocrystals should be based on the natural development of thermal dynamically stable crystal phase, following the thermodynamic growth regimes-governed mechanism. The {200} and {111} enclosed truncated-octahedral shape is the dominant and thermal-dynamically stable shape for CeO2 nanocrystals.

Fig. 1 XRD patterns of CeO2-W and CeO2-E samples

This sphere-like shape is developed from the isotropic growth of CeO2 unit cell with isotropic structural property [13]. Obviously, the nanowire shaped CeO2-E sample is the result of anisotropic growth, following the kinetic growth regimes-governed mechanism. According to the established theory [13, 28, 29], one-dimensional shape of CeO2 should be formed by the oriented attachment of adjacent nanocrystals sharing a common crystallographic orientation and the following joining of nanocrystals at {110} facet having higher surface energy. It is reasonable to believe that the {110} facet of CeO2 nanocrystals produced in ethanolic solution has higher atomic density and number of dangling bonds than those produced in aqueous solution due to the more active reactions occurring in ethanolic solution. This results in the higher surface energy of CeO2 nanocrystals produced in ethanolic solution. Therefore, the attachment and joining of nanocrystals at {110} facet could reduce the overall surface energy by eliminating {110} facet. As a result, the nanowire shape of CeO2 is developed in ethanolic solution. The CV behavior of CeO2-W and CeO2-E was measured in 3 M KOH aqueous solution within a potential window of 0–0.6 V at scan rate ranging from 5 to 100 mV s−1. As shown in the normalized CV curves (Fig. 3a and b), both samples present a pair of intensive and symmetrical redox peaks, corresponding to the conversion between different cerium oxidation states (Eqs. 1–2) [30]. This is the typical electrochemical behavior of Faraday-type electrode materials. It is observed that both samples have almost identical shape of CV curves and location of redox peaks at each scan rate. However, the peak intensity of CeO2-W is much higher than that of CeO2-E, indicating the more reactive faradaic reactions occurring in CeO2-W. It is also observed that the anodic and cathodic peaks shift in the opposite direction with the increase of scan rate due to the relatively

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Fig. 2 TEM images of CeO2-W a and b and CeO2-E c and d samples at different magnification

slower diffusion of electrolyte ions inside the material at higher scan rates. As shown in the plots of anodic peak current vs. scan rate (Fig. 3c), the curve for CeO2-W exhibits higher slop rate than CeO2-E, indicating the shorter distance of ions diffusion in former sample. Ce3þ OOHe þ OH ! CeO2 þ H2 O

ð1Þ

CeO2 þ e þH2 O ! Ce3þ OOH þ OH

ð2Þ

In the galvanostatic charge-discharge curves (Fig. 3d) measured at current density of ~2.0 A g−1 in 3 M KOH

solution, both CeO2-W and CeO2-E samples present the asymmetry shape including potential plateau and sloping potential regions, which is typical for faradaic reactions. In the discharge curves, the potential plateau region is attributed to the battery-type behavior of material and the sloping potential region corresponds to the pseudocapacitive contributions from surface or near-surface charge storage of material. It is observed that CeO2-W has longer chargedischarge time than CeO2-E at same current density, which proves the higher electrochemical performance of former sample. In Fig. 3e, the specific capacitance for both samples

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Fig. 3 CV curves of CeO2-W a and CeO2-E b; the plots of anodic peak current vs. scan rate c for CeO2-W and CeO2-E derived from their CV curves; Galvanostatic charge-discharge curves d of CeO2-W

and CeO2-E at current density of ~2.0 A g−1 in 3 M KOH solution; Specific capacitance e of CeO2-W and CeO2-E calculated from their charge-discharge curves measured at different current density

is calculated according to the galvanostatic chargedischarge curves measured at different current density. It is known from this figure, the CeO2-W sample presents much higher specific capacitance at each current density

than the other sample. This sample shows the maximum specific capacitance of 372.6 F g−1 at low current density of 6.3 A g−1, while CeO2-E sample has lower capacitance of 306.5 F g−1 at 3.6 A g−1. The CeO2-W sample has excellent

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Fig. 4 Nyquist plots a of CeO2-W and CeO2-E measured in 3 M KOH electrolyte solution; Cycle stability b of CeO2-W and CeO2-E measured at a charge-discharge current density of ~2.0 A g−1

high rate performance (188.7 F g−1 at 125.0 A g−1), much better than the other sample. The high electrochemical performance of CeO2-W could be attributed to the large interface originating from its small particle size, while the lower performance of CeO2-E is attributed to the limited interface resulting from the packing of nanowires. EIS is used to characterize the resistance of electrodes. The Nyquist plots analyzed from the EIS obtained at various potential ranges in the frequency range of 0.01 Hz–1 M Hz were fitted by software ZVIEW using the equivalent circuit given in Fig. 4a. As shown in this figure, both CeO2-W and CeO2-E samples show the similar nearly linear pattern, which demonstrates their similar electrochemical impedance behavior. The intersecting point of the curves with the real-axis gives the value of series resistance (Rb), accounting for the contact resistance between nanocrystals and current collector. The figure indicate that the Rb value for CeO2-W and CeO2-E samples is 0.24 and 0.36, respectively. The lower contact resistance of CeO2-W could be attributed to the compact contact of its nanocrystals with the current collect due to its small particle size as compared with CeO2-E. The cycle stability of both samples was tested for 4000 cycles by repeating the charge–discharge process under applied current density of ~2.0 A g−1 within the potential window of 0–0.6 V. In the obtained results (Fig. 4b), the specific capacitance of both samples is decreased gradually with the charge-discharge cycles. However, the cycle stability of CeO2-W is higher than CeO2-E. It remains 91% of its initial specific capacitance after 4000 cycles, while the other sample remains 74%.

4 Conclusions In summary, CeO2 nanocrystals with shapes of nanosphere and nanowire were prepared at room temperature by an

epoxide precipitation route utilizing the hydrolysis reaction between aquo complexes of cerium ions and propylene oxide. The different reaction kinetics in aqueous and ethanolic solutions resulted in the different shapes of CeO2 nanocrystals. The more active reaction of epoxide in ethanolic solution than in aqueous solution produced CeO2 nanocrystals with higher surface energy at {110} facet. As a result, oriented attachment and the following joining of nanocrystals at {110} facets occurred, leading to the formation of nanowire shape for CeO2 nanocrystals. In aqueous solution, sphere-like CeO2 nanocrystals was grown following the isotropic growth mechanism due to the isotropic structural property of unit cell. Because of larger interface, sphere-like CeO2 nanocrystals had much better electrochemical performance than the nanowire-shaped CeO2 nanocrystals. Compliance with ethical standards Conflict of interest interests.

The authors declare that they have no competing

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