All the samples S2-S15 in this work are indexed to a pure cubic phase of MnO according to the standard card of JCPDS 07-0230 (Figure S1). Different temperatures including 200 oC, 250 oC, 280 oC, 300 oC are used to synthesize nanoparticles (S1, S2, S3, and S4). No nanoparticles are obtained for S1 since the reaction temperature is too low for the decomposition of metal 2, 4-pentanedionates. Figure 1a, b, and c show the TEM images of S2, S3, and S4. It can be seen that the size distribution of S2 prepared at 250 oC is very broad. The sizes are in the range of 100–200 nm. The average size of S2 is 98 nm. The averages sizes increase to 387 nm and 525 nm for the reaction temperatures of 280 oC and 300 oC, respectively (Fig. 1b S3 and c S4). The nanoparticle sizes increase from 98 nm to 525 nm as the reaction temperature increases from 250 oC to 300 oC. The increased sizes are explained by the increasing temperatures.[20–22]
ODE is utilized as a noncoordinating ligand in this reaction to study the influence of precursor concentrations on the size and morphology of MnO nanoparticles. S5 and S6 are prepared with ODE of 20 and 40 mL, respectively, with the experimental condition of S4. Figure 1d and e show the TEM images of S5 and S6. The average sizes of S5 and S6 are 415 nm and 263 nm, respectively. It can be seen that the sizes are 525 nm, 415 nm, and 263 nm as the reaction precursor concentrations decrease (S4, S5, and S6). Therefore, the sizes decrease with the increase of the precursor concentration. The distance between seed crystals increases as the precursor concentration increases, leading to decreased seed crystal aggregation and smaller sizes of the nanoparticles.[23] It is also related to the ligands oleylamine and oleic acid.[23] It is reported that the ratio of OLA to OA can play a role in capping the nanoparticles, leading to different morphology and sizes.[24–26] The amount of OLA ranging from 0 to 30 mL is employed to investigate the influence of OLA on the size and morphology of the MnO nanoparticles. S7 does not exhibit any nanoparticles as the only OA is used in the reaction (without OLA). Figure 1f-j shows the TEM images of S8-S12. The OLA amount increases from 0 to 30 mL as the OA amount is kept to be 10 mL. S12 is prepared with 10 mL OLA, without OA. The average sizes of the S8-S11 are 481 nm, 426 nm, 73 nm, and 83 nm, respectively. It can be observed that the sizes of the nanocrystals decrease as the OLA amount increases. The average size of S12 is 46 nm. Reaction time is another experimental parameter to control nanoparticle growth. Usually, a long reaction time leads to large nanoparticles and increased crystallinity. Figure 1k-m show the TEM images of S13-S15. The average sizes of the S13, S4, S14, and S15 are 76 nm, 525 nm, 200 nm, and 250 nm, corresponding to the reaction time of 1 h, 2 h, 4 h, and 6 h, respectively. The sizes increase from 76 nm to 525 nm as the reaction time increases from 1h to 2h. The sizes drop to 200 nm and 250 as the reaction time further increases to 4 h and 6 h. However, the crystallinity improves significantly as the reaction time increases to 4 h and 6 h, which is reflected from the sharp and smooth edges of the cubes in the TEM images (Fig. 1l and m).
Figure 2 shows the magnetic hysteresis loop of S3 and S12. Typical paramagnetic properties of MnO can be confirmed from the curves, which is consistent with the results of Yu.[27] The magnetization of S12 at the 40000 Oe is much higher than that of the S3. This is explained by the smaller size of S12 (46 nm) compared to S3 (387 nm).
The same experimental conditions are also used to prepare Co and CoO nanoparticles. Only three samples with pure phases can be obtained including S16-S18. Other samples prepared with similar experimental parameters show the mixture phase of Co and CoO. Figure 3a shows the TEM image of S16 prepared with 10 mL OA and 20 mL OLA at 300 oC. The size of S16 is ~ 50 nm. Figure 3d shows the XRD diffraction patterns of S16, which can be indexed into hexagonal close-packed phases according to the standard card of JCPDS 45-1027. The inset of Fig. 3a is the M-H loops of S16. Typical magnetic properties are observed with a saturation magnetization of 88.2 emu/g, which is consistent with literature results of metallic Co.[28] Figs. 3b and e show the TEM image and XRD diffraction patterns of S17, respectively. It can be seen that the size of S17 increases to ~ 500 nm compared to S16, which is explained by the amount of OLA. More OLA results in nanoparticle growth toward large sizes.[25, 29–32] Fig. 3c and f show the TEM image and XRD diffraction patterns of S18, respectively. XRD result shows that cubic CoO nanocrystals (JCPDS card no. 48- 1719) are obtained. The average size of ~ 20 nm is obtained from the TEM image. The inset of Fig. 6c shows the M-H loop of S18. A coercivity of 250 Oe is obtained due to the uncompensated spins on the surface of the nanoparticles.[18]
Similar experimental procedures are employed to prepare Ni nanoparticles. Only S19, S20, and S21 are obtained, Other samples are mixture phases of Ni and NiO. Figure 4a, b, and c show the TEM images S19-S21. The average sizes of S19, S20, and S21 are 80 nm, 80 nm, and 200 nm, respectively. S19 and S20 show large nanoparticles composed of smaller nanoparticles of ~ 10 nm. All the samples S19, S20, and S21 are indexed to cubic phase Ni (JCPDS: 45-1027) nanoparticles shown in Fig. 4d, e, f.
Co and CoO nanoparticles can be used for magnetic concentration cells.[33, 34] Magnetic nanoparticles are attached with citric acid molecules and serve as carriers for the citric acid molecules.[33, 34] Fig. 5 shows the device performance of the concentration cells based on S16 and S18. It can be seen that the current and voltages are 0.000004 A and 0.04 V, for the devices based on Co nanoparticles S16. However, the current and voltages are 0.0000004 A and 0.005 V, for the devices based on CoO nanoparticles S18. This is explained by the magnetic properties of the Co and CoO nanoparticles.[33, 34]