## 3 Results and Discussion
### Characteristic of Palm Oil Ash Morphology and Particle Size Distribution  
The particle size distribution of the POA nanoparticles under predicted optimal condition from the Taguchi method was illustrated in the histogram graph in Figure 7. A study by [PERSON] and [PERSON] [36] proposed that to obtain reliable and valid frequency versus particle size curve, hundreds of particles should be measured for an optimum sample size. The histogram graph for particle size distribution shows the intensity or frequency versus the size range typically. It was observed that the majority of the POA particle size distributed between 70-110 nm of particle sizes. From the histogram profile, the average diameter of the POA nanoparticles was estimated to be 80 nm. The smallest particles were obtained with the lowest intensity and were estimated to be eight (8) nm. Meanwhile, the largest particles exhibited at 0.2% intensity were estimated to be 200 nm. Although the histogram reveals the POA nanoparticle's distribution, the information is still inadequate to characterize the behaviour of the POA nanoparticles. Moreover, the distribution of the POA nanoparticles probably influences by the agglomeration resulting in variation of POA nanoparticles sizes. This behaviour occurred because of the robust and attractive interaction between nanoparticles derived from strong Van Der Waals force. A recent study by [PERSON] and [PERSON] [37] revealed that a high concentration of nanoparticles with extraordinary surface area commonly aggregated more rapidly, hence formed larger nanoparticles. Therefore, the preparation of POA nanoparticles mainly depends on the optimal parameters that could increase the number of nanoparticles and significantly decrease the agglomeration of nanoparticles. Thus, transmission electron microscope will be used to view the image of POA nanoparticles individually.  
Transmission electron microscope (TEM) micrograph visualized the morphology and distribution of POA nanoparticles. Moreover, the TEM image supplies quantitative information about the shape, size, and distribution of particles on a very local scale. Figure 7 showed the TEM image of POA nanoparticles, which is in accordance with the particle size distribution, which is evaluated by laser diffraction particle size analyzer. From the TEM image, the POA nanoparticles appear in black colour.  
Figure 7: TEM micrograph (200 nm) and particle size distribution graph of palm oil ash nanoparticles.  
The black colour was probably due to that less transparent towards electron beam and electronic density of POA nanoparticles element as reported by [PERSON], [PERSON] [38], [PERSON], and [PERSON] [39]. The TEM micrograph reveals that the POA nanoparticles possess an irregular size and uniformly distributed within 70 to 200 nm range without having agglomeration. This similar finding also reported by [PERSON] and [PERSON] [11] that ground POA particles had an irregular shape and the size was measured less than 50 nm. In addition, within that nanometric range, there was slightly an agglomeration that occurred between POA nanoparticles. It was assumed that the agglomeration occurred between POA nanoparticles due to the increase in surface area and energy of POA nanoparticles. In this study, the ball milling process involved complex mechanical forces such as particles that are repeatedly fractured and flattened. In the case of the spherical shape of POA nanoparticles that can be observed in the TEM image, it is probably due to the balanced formation of milling force between balls, milling jar, and POA particles. Hence, the POA particles resist being fractured and flattened, causing particles to obtain a size more than 100 nm. Therefore, the confirmation test indicated that the results obtained from the selection of optimal parameters could produce a significant size of palm oil ash (POA) nanoparticles. The results confirming the success of Taguchi statistical analysis in obtaining optimal parameters to produce POA nanoparticles were presented in Table 6.