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AFM2 2017

Effect of synthesis parameters on the morphology of nanocrystal thermoelectric M（= Mg, Mn, Cu) doped Ca3Co4O9 fibers by

electrospinning

Yun Oua,b,c,*, Daifeng Zoud, Fang Wanga, Juanjuan Chenga

aHunan Provincial Key Laboratory of Health Maintenance for Mechanical Equipment, Hunan University of Science and Technology, Xiangtan City, Hunan Province, China 411201b Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, Guangdong, China cEngineering Research Center of Nano-Geo Materials of Ministry of Education,China University of Geosciences, Wuhan 430074,China dSchool of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan, China 411201

Abstract

The thermoelectric M (=Mg, Mn, Cu) doped Ca3Co4O9 (CCO) fibers were synthesized by electrospinning based on sol-gel method. The effect of various substrates, electrospinning time, and different doping on the morphology of M doped CCO fibers were studied systematically. The results showed that the morphology of calcined fibers with substrate was kept better and the grain size was smaller than that of without substrate, simultaneously. It is observed that the shorter electrospinning time, the better preserved nanofiber configuration, and hollow structure was appeared in the manganese doped CCO fibers.

© 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

Keywords: sol-gel, electrospinning, thermoelectric, layered oxide;

** Corresponding author. Tel.: 0731-58290265.E-mail address: ouyun101@163.com

2214-7853 © 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

2 Author name / Materials Today: Proceedings 00 (2018) 0000–0000

Introduction

Thermoelectric (TE) material is a kind of functional material which can convert exhaust heat energy into

electrical energy directly,[1-4] and the figure of merit can evaluate the properties of the TE materials. A

promising TE material requires a large S and σ , and a low κ simultaneously, which includes the contribution

from phonon thermal conductivity and electronic thermal conductivity. It has been shown that the phonon thermal

conductivity can be reduced significantly in nanocomposites due to the increased phonon interface and boundary

scattering.[5] It was reported that the ZT values reach 2.4 in layered nanoscale structures at 300 K, [6] and 3.2 for a

bulk semiconductors with nanoscale inclusions at about 600 K.[7] Since a large Seebeck coefficient was found in

layered oxide NaCo2O4 exhibiting a promising TE performance, p-type-layered cobalt oxides have attracted more

and more attention owing to having similar structure with NaCo2O4, high thermal stability, low cost and low

toxicity. CCO is a distorted CaO-CoO-CaO rock-salt-type layer and a CdI2-type CoO2 layer stacked along the c-axis

direction.[8-10] However, at present the ZT value of CCO is too low to be used practically. In order to improve the

thermoelectric performance of CCO, nanocrystal CCO ceramics were manufactured by different techniques.[11-14]

In recent years, one-dimensional nanostructured materials have attracted much attention owing to their

significant potential in nanodevices.[9,10] Among the techniques fabrication one-dimensional nanofiber,

electrospinning is one effective approach with high efficiency and convenient operation. Encouraged by these

developments and our recent works on spark plasma sintering nanocrystalline nanofibers, [11,15] The effect of various

substrates, electrospinning time, different doping on the morphology of nanocrystalline thermoelectric oxide M

doped CCO nanofibers were investigated.

Experimental

The M doped CCO sol-gel precursor was prepared by dissolving C4H6O4Ca·H2O, Co(CH3COO)2·4H2O,

Cu(NO3)2·3H2O, MgCl2·6H2O, and C4H6MnO4·4H2O with stoichiometric proportions into a hybrid solution of

methanol (CH3OH) and propionic acid (CH3CH2COOH) with a volume ratio of 3/7. Poly (vinyl pyrrolidone) (PVP,

Mw= 1300000 g·mol-1) was then added to the sol-gel precursor with 0.03 g·mL-1 concentration, and stirred

continuously to form a 0.2 mol·L-1 homogeneous M doped CCO electrospinning precursor. The solution was loaded

into a plastic syringe equipped with a stainless steel needle connected to a high-voltage supply (Spellman

SL40P300), and then electrospun with the feed rate of the solution controlled at 0.015 mL·min-1 and the electric

field set around 1.4 kV·cm-1. The as-spun nanofibers were collected by a glass flake with aluminum foil or

Pt/Ti/SiO2/Si (PT) substrate, dried at 120 °C for 4 h, and then calcinated at 750 °C for 2 h. For comparison, CCO

ceramic powders were also synthesized using the conventional sol-gel process, by drying the CCO precursor

solution without PVP in an oven at 120 °C for 2 days to obtain xerogel, and then sintered the xerogel at 800 °C for 2

h. M doped CCO powders with different doping have been synthesized by sol-gel method.

(d)

(e) (f)

(c)

(b)

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X-ray diffraction (XRD) (Rigaku D/max-rA) with CuKa radiation was used to identify their crystalline

structures, and scanning electron microscopy (SEM) (FEI Sirion) was used to examine the morphology of M doped

CCO nanofibers and powders with different substrates and collecting time.

Results and discussion

The morphology of CCO fibers retained on the two different substrates and lifted off the substrate is shown in

Fig. 1. It is observed in Fig. 1(a) that the diameter of the green fibers on the PT substrate is in the range of 400–2000

nm with straight and smooth surface, each nanofiber is rather uniform across its length, and almost fibers are column

shape. The morphology of green fibers on aluminum foil substrate is shown in Fig. 1(b), while some are belts and

majority are nanofibers, the width of the belts is between 2 and 5 μm with nanosize thickness, the diameter of

nanofibers is clearly less than that of nanobelts and their size distribution is more inhomogenous than that of PT

substrate due to the inhomogeneous electric field distribution of aluminum foil. However, when the fibers were

lifted off the substrate and then dried at 120 °C for 4 h, the continuous fibers change to short stick, and their length

is in the range of 1-20 μm with nanometer diameter as seen in Fig. 1(c), which indicates that substrate can provide

binding force to keep fiber with stable morphology, and stress is large in the evaporation of the solvent, the huge

tensile stress shuts off the long nanofibers.

Fig. 1. SEM images of CCO green fibers collected on different substrate and ceramic fibers with different collecting time(a) PT (b) Al foil (c) lift off PT (d) 1 min (e) 30 min. (f) 2 h.

The SEM images of calcined CCO ceramic fiber collected on PT substrate with different collecting time are

shown in Figs. 1(d)-(f). It is shown that after being calcined the diameter of nanofibers on PT substrate is reduced to around 100–300 nm, there are partial continuous fibers when collecting only 1

(g) (f)

(c)(a) (b)

(e)

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min as shown in Fig. 1(d), fractional nanofibers are broken and the grain size is typically nanocrystal. When

collecting time extend to 30 min, the CCO ceramic preserved fiber structure, but the diameter of fibers increased to

about 200–400 nm as some fibers are bonding together, which enhanced the average grain size of ceramic as shown in Fig. 1 (e). However, in Fig. 1 (f) few fibers structure was kept in 2 h collecting time

sample, because the mutual binding of nanofibers increased with increasing collecting time, which results the grown

of grain size and the disappear of fiber morphology.

Fig. 2 compared the crystal size of the fibers on or without the PT substrate with sol-gel powders, and the

particle size distribution was analyzed by image J. It is confirmed that the grain size derived by fibers is much

smaller than that of sol-gel, and the crystal size of fibers on PT is the smallest among the three samples. The average

particle size of the CCO powders by sol-gel is the largest, which is in the range of 200-1100 nm with wider

distribution. The average particle size of the powders grinded by calcined fibers without PT substrate is around 80

nm with normal distribution. The average size of the particle with PT substrate is reduced to approximately 60 nm

with centralized distribution. Fig.2 also reveals the lamellar morphology of CCO powders expected from the layered

lattice structure.

Fig. 2. SEM images of M doped CCO sol-gel powders and fibers(a) sol-gel (b) fibers without PT (c) fibers on PT

(e) (f) (g) corresponding crystal size distribution of (a) (b) (c).

Fig. 3 gives the SEM images of the fibers doped with different elements and corresponding XRD results. It is

obvious that the morphology of fibers various with different doping. The morphology of Ca2.8Mg0.2Co4O9 fiber (Fig.

3(a)) is similar with that of CCO fiber as the Mg ions substituting the Ca site not Co site.[16] The Ca3Co3.8Mn0.2O fiber

(Fig. 3(b)) presents a obvious hollow structure, that is because the Mn substitutes for Co in CoO2 layers and the

melting point of MnO2 is much lower than that of CoO2, phase separation promotes the formation of hollow

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structure. The different morphology between Fig. 3(b) and Fig. 3(c) is being Mn ions mainly occupy Co sites in

CoO2 layers whereas Cu ions substitutes for Co sites in Ca2CoO3 layers.[17] It is indicated that different doping

elements results various morphology owing to different doping sites, which results different structure.

The crystalline structure of M doped CCO ceramic fibers with different doping were examined by XRD, as

shown in Fig. 3(d). All the diffraction peaks of different samples are identical to the standard JCPDS card 21-0139

of CCO. Excellent crystallinity was observed in the four kinds nanofibers calcinated at 750 °C, and no visible

impurity phase was detected because of the low doping ratio.

Fig. 3. SEM images and XRD of the fibers with different doping (a) Ca2.8Mg0.2Co4O9

(b) Ca3Co3.8Mn0.2O9 (c) Ca3Co3.8Cu0.2O9 (d) XRD of the M doped CCO

Conclusions

In conclusion, nanocrystalline M doped CCO fibers have been successfully fabricated by electrospinning, and

the effect of major process parameters on the morphology of the fibers were investigated systematically. It is found

that the particle sizes with limited collecting time and PT substrate are much smaller than that of without substrate

and longer collecting time, and the particle size of fibers derived powders is also much smaller than the powders

prepared by sol-gel method. The average particle size of the fibers derived powders with the substrate is about 80

nm. Our experiments further demonstrate that the particle size synthesized by electrospinning is much smaller than

the particles by sol-gel method.

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Acknowledgments

We acknowledge support from Natural Science Foundation of China (Approval No. 11702092 and 11762016)

and Opening Project of Engineering Research Center of Nano-Geo Materials of Ministry of Education of China

University of Geosciences (NGM2018KF005).

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Highlights

1. The M doped thermoelectric Ca3Co4O9 (CCO) fibers with nanocrystal size were synthesized by

electrospinning, hollow structure were formed in Mn doped fiber.

2. The substrate can provide binding force to keep the small crystal in electrospinning, results the formation of

nanocrystal fibers.