Effect of Anionic Amphiphiles on the Morphology of Hexagonal Plate-like ZnO Particles

a ZnO was also using a surfactant as an ad-ditive. Hosono reported the fabrication and morphology control of mesoporous ZnO using cetyltrimethylammonium chloride （ CTAC ） , one of the most common surfactants used in preparing mesoporous materials ） . Jaramillo reported that mesoporous ZnO could be prepared using a nonionic surfactant 41 ） . In these studies, the nonionic surfactant functioned as a structure-directing agent. Recently, we successfully prepared hexagonal plate-like ZnO single-crystal particles free of mesopores via hydrothermal treatment in the presence of an anionic surfactant （ AS ） 42 ） . Therein, the surfactant determined the crystal growth direction. However, the mechanism by which the surfactant acts as a crystal growth-directing agent is unclear. In this study, the fabrication of ZnO particles using ASs with different alkyl chain lengths is investigated. Fur-thermore, the effects of the AS and the molar ratio of the AS to the ZnO precursor on the morphologies of the obtained ZnO particles are investigated. Finally, the role of ASs as a crystal-growth-directing agent is discussed based on the above results. Abstract: Zinc oxide (ZnO) particles were synthesized in the presence of anionic surfactants (ASs). The effect of ASs on the morphology of the ZnO particles was investigated by using ASs with various alkyl chain lengths and changing the molar ratio of AS/ZnSO 4 . Hexagonal plate-like ZnO particles were formed in the presence of ASs. Adsorption of the AS on the c face of the ZnO crystals inhibited (promoted) crystal growth along the c -axis ( ab -axis) direction. Increasing the molar ratio of AS/ZnSO 4 decreased the particle thickness, owing to the resulting increase in the coverage of the c face with AS. The particle diameter of the hexagonal plate-like ZnO particles (the diagonal length of the hexagonal plate) increased with increasing alkyl chain length of AS as a result of the van der Waals interactions between the alkyl chains. The data indicate that the particle diameter and thickness can be controlled by fine-tuning the van der Waals interactions between the alkyl chains and the coverage of the c face of the ZnO particles with AS.


Introduction
Zinc oxide ZnO is a well-known semiconductor used in photocatalysis to decompose organic compounds 1 6 and in electronic devices 7 11 , as it generates holes and electrons under UV irradiation. The crystallinity and particle size of ZnO significantly influence its properties. Thus, methods of controlling the crystal growth and morphology of ZnO have been investigated. One of the most favorable morphologies is the one-dimensional morphology, including the rod-like shape 12 14 , since in this case, ZnO grows preferentially in the c-axis direction because of the thermodynamic stability of the crystal. The crystallinity, particle size, morphology, and orientation can be controlled by varying the reaction conditions, such as the species of zinc oxide precursor, pH, reaction time, reaction temperature, and additives. Thus, variously shaped ZnO particles can be obtained, such as tubes 15 17 , tetrapods 18 21 , and flower-like particles 22 25 . Because surfactants form various self-assemblies, they are often used as additives for the synthesis of inorganic materials. Mesoporous materials 26 28 with highly ordered pore structures can be fabricated using self-assemblies of a surfactant as a template. Syntheses of mesoporous silica 26 30 , titania TiO 2 31 36 , and zirconia ZrO 2 37 39 have been re-

Preparation of zinc oxide particles
Each AS was added to 45 mL of ion-exchanged water and stirred at 343 K for 2 h. Five milliliters of an aqueous ammonia solution and 10 mL of a 1 M aqueous ZnSO 4 solution were mixed with 45 mL of the aqueous AS solution and stirred at 343 K for 24 h. The ZnSO 4 concentration was fixed. The molar ratio of AS/ZnSO 4 AS / ZnSO 4 was varied from 0 to 0.50. After stirring at 343 K for 24 h, hydrothermal treatment was carried out at 423 K for 24 h. A certain portion of the obtained particles was centrifuged and washed with ion-exchanged water. The morphology of the particles was observed by scanning electron microscopy SEM; Keyence, VE-7800 . The remainder was filtered, washed with ion-exchanged water, and dried at 333 K for 24 h. The samples were characterized by X-ray diffraction XRD; Rigaku, MiniFlex600 Cu-Kα radiation and Fouriertransform infrared spectroscopy FT-IR; JASCO, FT/IR-4200 .

Results and Discussion
3.1 Preparation of zinc oxide particles using various anionic surfactants First, ZnO particles were synthesized using various ASs and the morphology of the resulting particles was examined. The molar ratio of AS to ZnSO 4 was 0.50. For comparison, ZnO particles were synthesized without the addition of ASs. Figure 1 shows the XRD patterns of the particles synthesized in the presence and absence of the ASs. All XRD patterns have five diffraction peaks at approximately 31.8 , 34.4 , 36.2 , 47.5 , and 56.6 due to the 100 , 002 , 101 , 102 , and 110 reflections, respectively, indicating the formation of zinc oxide 13,15 . The intensity of the 002 peak for the particles synthesized with ASs was much higher than that of the particles obtained in the absence of ASs. The I 002 /I 101 intensity ratio was calculated to normalize the 002 peak intensity with respect to the 101 peak intensity. The I 002 /I 101 ratio for the particles fabricated in the absence of AS was 0.21, compared to an I 002 /I 101 ratio of 24 for all samples synthesized in the presence of ASs. These results suggest that the particles synthesized with ASs are zinc oxide particles with a high c face orientation.
SEM images of the particles synthesized in the a absence and b d presence of ASs are shown in Fig. 2. The particles fabricated without AS had rod-like shapes Fig. 2 a . In contrast, the use of AS additives yielded particles with hexagonal plate-like shapes Figs. 2 (b) -2 (d) . Wurtzite is a thermodynamically stable crystal structure of ZnO. The data indicate that the hexagonal structure was formed  by preferential growth of ZnO along the c face. These SEM images clearly show that the ZnO particles fabricated with the ASs had a high c face orientation. These results are in good agreement with the XRD patterns of the particles fabricated using ASs. The dispersion properties of the ZnO particles were also investigated using a solution comprising two phases water and 1,3,5-trimethylbenzene . Figure 3 shows photographs of the solutions after addition of the ZnO particles synthesized in the absence and presence of SHS. The particles fabricated with and without SHS were dispersed in 1,3,5-trimethylbenzene and water, respectively. Thus, the hydrophilic and hydrophobic groups of SHS were adsorbed on and modified the surface of the ZnO particles, respectively. These results indicate that hexagonal plate-like ZnO particles with a high c face orientation were formed owing to adsorption of the ASs.
The mechanism of formation of the hexagonal plate-like ZnO particles was evaluated. A Zn atomic layer was present along the c face of the ZnO crystal structure. ASs can be easily adsorbed on the c faces of the ZnO particles owing to the electrostatic interaction between the Zn atoms and the hydrophilic groups of the ASs during growth of the ZnO crystals. Adsorption of the AS inhibits promotes crystal growth along the c-axis ab-axis direction. As a result, hexagonal plate-like ZnO particles are formed when ASs are present in the solution. These results indicate that ASs can serve as crystal-growth-directing agents.

Effects of AS alkyl chain length and AS/ZnSO 4 molar ratio on ZnO particles
The effect of the AS/ZnSO 4 molar ratio on the morphology of the ZnO particles was investigated. AS / ZnSO 4 was varied from 0 to 0.50. Figure 4 shows the XRD patterns of the ZnO particles synthesized using SDS. The XRD patterns of the particles fabricated using STS and SHS are shown in Figs. S2 and S3, respectively. The diffraction peaks assigned to ZnO were observed at all molar ratios, regardless of the type of AS. The intensity of the 002 peak increased with increasing AS / ZnSO 4 . The I 002 /I 101 intensity ratio was then calculated. Figure 5 shows the plot of I 002 /I 101 versus AS / ZnSO 4 . I 002 /I 101 gradually increased for SDS / ZnSO 4 0-0.15. At SDS / ZnSO 4 0.20, I 002 /I 101 was constant. The same trends were observed when ZnO particles were prepared using STS and SHS. These results suggest that hexagonal plate-like ZnO particles are formed when AS / ZnSO 4 is 0.20 or higher.
SEM was used to observe the effects of changing the AS species and AS / ZnSO 4 on the particle shape. Figure 6 shows the SEM images of the ZnO particles synthesized using SDS. When SDS / ZnSO 4 was changed from 0 to 0.15, ZnO particles with hexagonal rod-like shapes were formed. The average particle diameter diameter means the diagonal length of the hexagonal plate and thickness were 0.8 and 2.9 µm, respectively. When SDS / ZnSO 4 was 0.20 or higher, hexagonal plate-like particles were formed,  where the particle diameter was 1.8 µm, with a particle thickness of 0.8 µm. SEM images of the particles prepared using STS and SHS are shown in Figs. S4 and S5, respectively. Regardless of the AS species, the particle shape showed the same trends as in the case of SDS. These results indicate that the morphology of the ZnO particles can be controlled by varying the ASs/ZnSO 4 molar ratio. The average particle diameter and thickness were esti-mated from the SEM images. Figure 7 shows the relationship between the average particle diameter, thickness, and AS / ZnSO 4 . With increasing AS / ZnSO 4 , the average particle diameter thickness gradually increased decreased and became saturated at AS / ZnSO 4 ≥ 0.20. These results demonstrate successful control of the particle diameter and thickness by changing the amount of AS within the range of AS / ZnSO 4 0-0.20. In this study, we focused on the average particle diameter obtained at AS / ZnSO 4 ≥ 0.20. The diameters of the particles synthesized with SDS, STS, and SHS were 1.8, 3.4, and 5.2 µm, respectively. The particles prepared with STS and SHS are approximately two and three times larger than those fabricated with SDS. This confirms successful control of the particle diameter by varying the AS species.
Finally, the effects of the ASs on the particle diameter and thickness were evaluated. First, the effect of the ASs on the particle thickness was considered. AS is adsorbed on the c face of ZnO and results in the formation of hexagonal plate-like particles. When AS / ZnSO 4 was below 0.20, only a small amount of AS was adsorbed onto the c face. This enhances crystal growth along the c-axis direction, and thus the formation of thick ZnO particles. The thickness decreased with increasing AS / ZnSO 4 . This is because adsorbed ASs inhibit crystal growth along the c-axis direction given that the AS coverage increases as the amount of AS adsorbed on the c face of ZnO increases. The thickness was constant for AS / ZnSO 4 ≥ 0.20, presumably because the coverage of the c face was saturated. There was no significant variation in the average particle thickness with the AS type. The ASs with different alkyl  The effect of the ASs on the particle diameter was also considered. When AS / ZnSO 4 was below 0.20, the particle diameter increased as the amount of AS increased. The particle diameter was constant for AS / ZnSO 4 ≥ 0.20. As with the particle thickness, this is assumed to be because increasing the coverage of the c face inhibits promotes crystal growth along the c-axis ab-axis direction. Meanwhile, increasing the alkyl chain length resulted in the formation of larger particles. The van der Waals interaction between the alkyl chains of the ASs increases with increasing number of carbon atoms in the alkyl chain 43,44 . Thus, the interaction between ASs having longer alkyl chains is stronger. The stronger hydrophobic interaction due to ASs with longer alkyl chains adsorbed on the c faces promotes crystal growth along the ab-axis parallel to the c face . As a result, the diameters of the particles prepared using an AS with a longer alkyl chain are larger. ASs can act as crystal-growth-directing agents owing to interactions such as van der Waals and electrostatic attraction.

Conclusions
In this study, hexagonal plate-like ZnO particles were synthesized through a hydrothermal process in the presence of ASs with different alkyl chain lengths. The effect of the ASs on the morphology of the ZnO particles was investigated. AS adsorption on the c face of ZnO inhibited crystal growth along the c-axis direction, resulting in the formation of hexagonal plate-like particles. The diameter and thickness of the ZnO particles were determined by the coverage of the c face, that is, the amount of adsorbed AS. It was also demonstrated that the alkyl chain length of the AS affected the particle diameter. When ZnO particles were synthesized with an AS having a long alkyl chain length, large ZnO particles were formed. The van der Waals interactions between the alkyl chains of the AS determined the particle diameter. These findings suggest that the particle diameter and thickness can be controlled by fine-tuning the van der Waals interactions and the coverage of the c face. To the best of our knowledge, this is the first report on the control of the shape, diameter, and thickness of metal oxide particles using surfactants. The present technique is expected to play an important role in the fabrication of inorganic particles that require precise control of the shape, diameter, and thickness.

Supporting Information
This material is available free of charge via the Internet at https://dx.doi.org/jos.70.10.5650/jos.ess21062