## 4 Conclusions  
This paper investigated the impact of reactant type and addition sequence on the microscopic morphology of aluminum oxide. Three aluminum sources, namely, octadecabydrated aluminum sulfate, hexahydrated aluminum chloride, and nonhydrated aluminum nitrate, were employed along with precipitants such as ammonium bicarbonate, sodium bicarbonate, and potassium bicarbonate. Precursors were synthesized using a hydrothermal method, and aluminum oxide powders were prepared through calcination. The key findings are summarized as follows:  
Figure 16: SEM images of alumina prepared by different agent addition sequences. (**a**) Aluminum chloride added dropwise to ammonium bicarbonate; (**b**) ammonium bicarbonate added dropwise to aluminum chloride.  
Figure 17: N2 adsorption–desorption isotherms and BJH pore size distribution plots of alumina prepared with different addition sequences. (a) Aluminum chloride added dropwise to ammonium bicarbonate; (b) ammonium bicarbonate added dropwise to aluminum chloride.  
Using sodium bicarbonate, ammonium bicarbonate, and potassium bicarbonate as precipitants and hexahydrated aluminum chloride as the aluminum source, spherical aluminum oxide particles with an average diameter of 80 nm were prepared. These particles exhibited mutual adhesion. Additionally, irregular short rod-shaped aluminum oxide with a length of approximately 200 nm and hexagonal plate-shaped aluminum oxide with long sides and short angles were obtained.
2. Using octadecahydrate aluminum sulfate, hexahydrate aluminum chloride, and non-abydrate aluminum nitrate as aluminum sources and ammonium bicarbonate as a precipitant, single-crystal particles of aluminum oxide were prepared. These particles had diameters ranging from 60 to 80 nm but tended to aggregate. Additionally, irregular short rod-shaped aluminum oxide with a length of approximately 200 nm and molten-adhered aluminum oxide were obtained.
3. The order of reactant addition significantly influences the crystal precursor products. Using hexahydrated aluminum chloride and ammonium bicarbonate as raw materials, when aluminum chloride was dripped into the ammonium bicarbonate solution, the resulting precursor was mainly sodium aluminum hydroxide carbonate, with some bechmite present. On the other hand, when ammonium bicarbonate was dripped into the aluminum chloride solution, the primary precursor formed was boehmite, which contained a small amount of sodium aluminum hydroxide carbonate. After calcination, the boehmite underwent dehydration, transitioning from a dense plate-like structure to a porous plate-like aluminum oxide, maintaining its overall morphology. Conversely, sodium aluminum hydroxide carbonate transformed from irregular rod-shaped structures with a length of 800 nm and a diameter of 60 nm into short rod-shaped aluminum oxide structures with a length of approximately 200 nm, indicating a reduction in size.  
Conceptualization, W.W. and Y.B.; methodology, W.W., Y.B. and P.Y.; formal analysis, W.W. and Y.B.; resources, Y.G. and H.X.; data curation, W.W., M.X. and Y.B.; writing--original draft preparation, W.W.; writing--review and editing, Y.B.; supervision, P.Y. and Y.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Project No. 51674137) and the Natural Science Foundation of Shandong Province (Project No. ZR2022 ME013) are sincerely appreciated. And the APC was funded by the Natural Science Foundation of Shandong Province (Project No. ZR2022 ME013).  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.