Synthesis of textured discontinuous-nanoisland Ca3Co4O9 thin films

Controllable engineering of the nanoporosity in layered Ca3Co4O9 remains a challenge. Here, we show the synthesis of discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles with controlled porosity and morphology. These discontinuously dispersed textured Ca3Co4O9 nanoparticles may be a candidate for hybrid thermoelectrics.

The mist-layered calcium cobaltate Ca 3 Co 4 O 9 has a complex crystal structure composed of CoO 2 conductive layers and oxygen decient rock-salt type Ca 2 CoO 3 insulating layers. 1 Ca 3 Co 4 O 9 can be used in various energy-harvesting systems because of its high thermal stability and oxidation resistance. This material is an attractive p-type thermoelectric material with a high Seebeck coefficient S, moderate electrical conductivity s and low thermal conductivity. It also can be used as an active material in Li-ion-battery anodes, 2,3 hydrogen evolution and oxygen reduction reactions 4,5 and supercapacitors with high cycling stability. 6,7 Nanostructures such as nanoparticles 8 and nanoporous lms 9,10 are common means to alter electrical, catalytic, and thermal properties of inorganic materials. In previous work, we have shown that nanoporous Ca 3 Co 4 O 9 lms on sapphire exhibit a thermal conductivity of 0.82 W m À1 K À1 , which is nearly twofold lower than that obtained from comparable nonporous Ca 3 Co 4 O 9 lms. 11 Furthermore, nanoporous Ca 3 Co 4 O 9 lms grown on mica can be obtained by reactions in hydrated CaO/CoO multilayers. 12 The volume shrinkage in Ca(OH) 2 /Co 3 O 4 multilayers and the out-of-plane orientation relationship between Ca(OH) 2 and Co 3 O 4 induce the formation of faceted and oriented nanopores in textured Ca 3 Co 4 O 9 lms.
Here, we show control of morphology and porosity in textured Ca 3 Co 4 O 9 lms, to form discontinuous lms with islands of highly textured Ca 3 Co 4 O 9 , effectively constituting distributed nanoparticles. The discontinuous lms with islands of highly textured Ca 3 Co 4 O 9 were synthesized by radiofrequency (rf) sputtering followed by post-deposition annealing without any templates. Such lms of discontinuously dispersed Ca 3 Co 4 O 9 nanoparticles may be a promising ller in polymer matrixes for hybrid and composite materials in, e.g., thermoelectrics. [13][14][15][16] The Ca 3 Co 4 O 9 nanoparticles were obtained by a similar method to that published in our earlier work. 12 First, the CaO/ Co 3 O 4 multilayer lms were deposited on muscovite mica (00l) and sapphire substrates (001) at 600 C by reactive radiofrequency magnetron sputtering. The multilayers consisted of eight alternative bilayers of CaO (top layer) and Co 3 O 4 . The overall Ca : Co elemental ratio in the multilayer lms was varied and set to 1 : 1.38 (close to stoichiometric Ca 3 Co 4 O 9 ), 1 : 0.82, 1 : 0.67, and 1 : 0.52 by varying CaO and Co 3 O 4 deposition times of their respective layer. Then, all the as-deposited multilayer lms were exposed to a humid environment (0.88 relative humidity at constant temperature) to form Ca(OH) 2 /Co 3 O 4 multilayer lms at room temperature for two days, as described earlier. 12,17 At the nal stage, the different Ca(OH) 2 /Co 3 O 4 multilayer lms were annealed at 700 C in air for 2 h. X-ray diffraction (XRD) measurements were performed using an X'Pert PRO MRD diffractometer from PANalytical using Cu K a1,2 radiation with a nickel lter in the Bragg-Brentano conguration (q-2q scans). The surface morphology of the lms was studied by scanning electron microscopy (SEM) using a LEO Gemini 1550 Zeiss with a 10 kV operating voltage. Transmission electron microscopy (TEM) was carried out on an FEI Tecnai G2 TF20 UT instrument operated at 200 kV. The Ca/Co elemental ratio was determined using energy-dispersive X-ray spectroscopy (EDS) by measuring at several positions on each sample growing on sapphire. The surface porosity fraction or coverage was determined from the SEM micrographs analysed using the soware ImageJ (Java version). 18 The electrical conductivity s Cite this: Nanoscale Adv., 2022, 4, 3318 was calculated from the sheet resistance measured by using a four-point probe Jandel RM3000 station, and the lm thickness was determined from the cross-sectional SEM images. The Seebeck coefficient was determined from the slope of the temperature gradient-voltage characteristics measured using a homemade Seebeck measurement setup system described elsewhere. 12,17 Fig . 1 shows the X-ray diffraction patterns of the Ca 3 Co 4 O 9 lms on mica and sapphire as a function of the Ca/Co ratio: 1 : 1.38, 1 : 0.82, 1 : 0.67, and 1 : 0.52, respectively. The Ca/Co elemental ratios were measured from the different annealed lms grown on sapphire by EDS and were the same as those in the as-deposited multilayer lms grown on sapphire and mica and the annealed lms on mica, respectively. Diffraction peaks for 001, 002, 003, 004, 005, and 006 reections from Ca 3 Co 4 O 9 and 111 and 222 reections from Co 3 O 4 can be observed in the lm on mica with the initial composition (1 : 1.38, the closest to the stoichiometric Ca 3 Co 4 O 9 ) in Fig. 1a. With decreasing Co content (Ca : Co 1 : 0.82 / 1 : 0.52), pure-phase Ca 3 Co 4 O 9 can be identied in the lms on mica from the XRD patterns (Fig. 1a). The intensity of peaks of Ca 3 Co 4 O 9 growing on mica remains approximately the same in Fig. 1a. As known and observed from our earlier work, 19-21 the excess Ca migrates and is incorporated in an amorphous layer between the nanoporous Ca 3 Co 4 O 9 lms and the mica substrate and will be discussed below. However, the pure-phase Ca 3 Co 4 O 9 can be seen from the lm growing on sapphire with Ca : Co ¼ 1 : 1.38 (Fig. 1b). With increasing Ca content, additional CaO can be observed for the lms on sapphire (Fig. 1b). This result indicates that some CaO remained in the annealed lms on sapphire.
The SEM images of the morphology of Ca 3 Co 4 O 9 lms on mica are shown in Fig. 2. The lm on mica with the initial composition (1 : 1.38) shows morphology with few nanopores (Fig. 2a and e). With decreasing Co content, the morphology of  Ca 3 Co 4 O 9 in the annealed lms change from a nanoporous continuous lm morphology (Fig. 2b and f), via larger pores ( Fig. 2c and g), to a discontinuous lm of textured islands ( Fig. 2d and h). The surface porosity fraction increases from 1.2% and 22% to 37% for the rst three lms (Table 1). For the discontinuous lms, the corresponding value obtained from image analysis is an apparent "porosity" of 46% (Table 1), i.e. a surface coverage of 54%. This morphology is fundamentally different from the nanoporous lms, though, in that the lm is discontinuous and cannot be described as a porous lm. The size of the nanoislands is mainly distributed from 50 nm to 1000 nm, as shown in Fig. S1. † The electrical conductivity and the Seebeck coefficient of the Ca 3 Co 4 O 9 /Co 3 O 4 lm are 27 S cm À1 and 139 mV K À1 , respectively. The electrical conductivity of the nanoporous pure Ca 3 Co 4 O 9 lm decreases from 112 to 38 S cm À1 with increasing the porosity from 22% up to 37%. The Seebeck coefficient of the nanoporous Ca 3 Co 4 O 9 lms with different porosities is approximately 127 mV K À1 , essentially the same for both. The Seebeck coefficient and electrical conductivity of the discontinuous lm of textured Ca 3 Co 4 O 9 islands cannot be measured by using these setups since there is no continuous conduction path.
The cross-sectional SEM micrographs (Fig. 2i-l) reveal that the lms are composed of a crystalline layer on top of an amorphous layer. As is known from our earlier work, 20,21 this amorphous layer forms due to a reaction between the mica substrate and the initial lms during annealing. The amorphous layer contains O, Al, and Si elements from mica and Ca element from the initial lms. The lm growing on mica with the initial composition (1 : 1.38) shows a crystalline layer with a thickness of 121 nm and an amorphous layer with a thickness of 81 nm (Fig. 2i). This indicates that the formation of Ca 3 Co 4 O 9 and amorphous layers occurs at same time during annealing. With increasing Ca content in the initial lms, the pure crystalline Ca 3 Co 4 O 9 layer of the last three lms shows a similar apparent thickness of around 170 nm for all the lms, but the thickness of the amorphous layer increases from 130 nm to 240 nm (Fig. 2j-l and Table 1).
The SEM images of the morphology of Ca 3 Co 4 O 9 lms on sapphire are shown in Fig. 3a-d. A dense lm can be observed in the annealed lm with Ca : Co ¼ 1 : 1.38 (Fig. 3a). The nanoporous morphology with a nanopore size of $200 nm can be observed in the annealed lm with low Ca : Co ¼ 1 : 0.82 (Fig. 3b). Upon further decreasing the Co content, the surface morphology seems to be composed of a mixture of two families Table 1 The amorphous layer apparent thickness, Ca 3 Co 4 O 9 layer apparent thickness, and the apparent porosity fraction calculated from Fig. 2 as a function of the Ca : Co ratio in the films  of grains (Fig. 3c). The similar round grains with a "size" ($100 nm) can be observed on the top of the at grains and nanopores at lms 1 : 0.82 and 1 : 0.6 in Fig. 3b and c. For the lowest Co containing lm, different family of grains can be observed forming a dense lm (without apparent nanopores) (Fig. 3d). The cross-sectional TEM images of the annealed lm with the ratio of Ca : Co ¼ 1 : 0.82 deposited on sapphire are shown in Fig. 4a and b. The nanopore structure in the Ca 3 Co 4 O 9 layer with an apparent thickness of 185 nm can be observed in Fig. 4a, with the EDS maps of Co and Ca elements showing a uniform distribution in the Ca 3 Co 4 O 9 layer but a higher Ca concentration in the nanopores. At the interface lm substrate, a thin Ca x CoO 2 layer can be seen near the sapphire substrate in Fig. 4b. The formation of Ca x CoO 2 has been observed in earlier work. 22 The lattice images for the Ca 3 Co 4 O 9 layer and the SAED patterns (Fig. 4b) conrm that the (001) basal planes are oriented parallel to the lm surface, corroborating the XRD results.
The dense Ca 3 Co 4 O 9 lm can be synthesized with the right Ca : Co elemental ratio (close to stoichiometric Ca 3 Co 4 O 9 ) when the lm grows on sapphire. The nanoporous lm but a nonphase pure lm mixing CaO and Ca 3 Co 4 O 9 can form on sapphire with increasing Ca content. Comparing the results for the lms grown on sapphire with those on mica allows determining the mechanism of the increase in the porosity fraction and formation of a discontinuous lm of islands, effectively constituting distributed nanoparticles.
This discontinuous structure is correlated with the reaction between Ca in the Ca(OH) 2 /Co 3 O 4 multilayer lms with the mica layer. In our previous work, pore formation could be attributed to the basal plane removal driven by local densication of textured Ca 3 Co 4 O 9 nuclei during growth. 12 In the present case, this mechanism yields formation of a discontinuous lm of islands, i.e., distributed nanoparticles, for the high initial Ca content in the starting multilayers. A schematic illustration is shown in Fig. 5. When Ca : Co ¼ 1 : 1.38 (close to 3 : 4), the lm with few nanopores is composed of a crystalline Ca 3 Co 4 O 9 /Co 3 O 4 layer on top of a thin amorphous layer, which proves that Ca diffuses and reacts with the mica substrate to form an amorphous layer during formation of Ca 3 Co 4 O 9 . During annealing and with increased Ca content, the excess Ca from Ca(OH) 2 will be attracted to the interface substrate/lm where the reaction occurs to form a thicker amorphous layer underneath phase pure crystalline Ca 3 Co 4 O 9 layers with nanopores. With further increase of Ca content, the nanopore size and porosity signicantly increase, while the apparent thickness of the crystalline Ca 3 Co 4 O 9 layer remains constant. This result indicates that the volume shrinkage of Ca 3 Co 4 O 9 Fig. 4 The cross-sectional TEM images of the annealed film with the ratio of Ca/Co ¼ 1 : 0.82 deposited on sapphire: (a) low-magnification TEM image and the inset of EDS spectral maps of Ca and Co for nanopores, and (b) the corresponding high-resolution TEM images and inset of the SAED patterns capturing the layered atomic structure of Ca 3 Co 4 O 9 . preferentially occurs in the in-plane direction and not in the out-of-plane direction. As expected, the more excess Ca results in a thicker amorphous layer with even lower Co content. Instead of forming nanoporous Ca 3 Co 4 O 9 , the Ca 3 Co 4 O 9 instead forms a discontinuous lm of islands, constituting distributed nanoparticles with a larger apparent "porosity".
The growth of discontinuous lms with islands of highly textured Ca 3 Co 4 O 9 effectively constituting distributed nanoparticles has been demonstrated by sequential sputteringannealing without any templates. The volume shrinkage in the initial Ca(OH) 2 /Co 3 O 4 multilayers with different Ca/Co overall ratios can be used to tailor morphology and surface coverage porosity in textured Ca 3 Co 4 O 9 lms. Such lms of discontinuously dispersed Ca 3 Co 4 O 9 nanoparticles may be a promising ller in polymer matrixes for hybrid and composite materials in, e.g., hybrid thermoelectrics.

Conflicts of interest
There are no conicts to declare.