Materials Processing
Effect of Acetate Ion on the Morphology of Zinc Oxide Obtained from Layered Zinc Hydroxide Chloride
2024 Volume 65 Issue 5 Pages 560-567
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2024 Volume 65 Issue 5 Pages 560-567
A two-step aging process was investigated in which the first step aging was carried out in zinc acetate aqueous solution and the second step in deionized water, using layered zinc hydroxide, ZHC, as a precursor with chloride ions in the interlayer. When the first step of aging was performed using a zinc acetate solution, hexagonal plate-shaped ZnO seed crystals with and without chipped corners were obtained, and at the same time, chloride ions incorporated in the interlayer of ZHC were exchanged with acetate ions to form ZHA. When the obtained mixture of ZnO seed crystals and ZHA was aged again in ion-exchanged water, coin-shaped or hexagonal plate-shaped ZnO particles with a particle diameter of about 1 um were obtained in a single phase. When the amount of zinc acetate solution used in the first step aging was small, the ion exchange of chloride ions to acetate ions between the layers of ZHC was insufficient and so ZHC remained, thus resulting in columnar ZnO with small particle size along with plate-like ZnO during the second step aging.
Zinc-oxide ZnO is a hexagonal wurtzite-type oxide with a 3.37 eV band gap at room temperature. Since it has a large exciton binding energy of 60 meV and exhibits conductivity due to the introduction oxygen deficiencies and impurities, it is utilized for semiconductor-laser devices in the ultraviolet range and gas sensors, etc.1–4) In addition, it is also used in cosmetics such as sunscreen because of its low toxicity to the human body and excellent ultraviolet absorbing characteristics.5–7) These properties are influenced by ZnO particle morphology (shape/size). Various morphologies and their properties have been reported, for example, in ZnO thin films using long-width rod-like ZnO, the electronic mobility is improved by a factor of 50 compared with spherical particles.8) Further, the hexagonal plate-like ZnO particles shows excellent UV absorbing properties, because hexagonal surfaces, which are the polar plane, are highly exposed.6,7) In addition, the smaller the particle size, the better ZnO is for transparency and photocatalytic properties.9,10) Therefore, the control of ZnO shapes and particle size is very useful in controlling their properties and can contribute to the performance improvement of products.
As shown in Fig. 1, ZnO has a structure in which Zn2+ and O2− of the hexagonal closest structure are stacked in the c-axis direction. It is composed of a Zn2+ plane (001) and an O2− plane (001), which are polar planes perpendicular to the c-axis, and nonpolar plane of the side.11) Since Zn2+ surface is constantly positively charged, the negatively charged growing unit Zn(OH)42− is easy to adsorption.12) Therefore, it has been suggested that growth parallel to the c-axis is fastest and that hexagonal columnar ZnO is easily formed, and indeed it has been reported that ZnO grows preferentially in the c-axis direction.13) The morphological control seems to be possible by promoting or inhibiting the adsorption of growth units to the 001 surface, and synthetic methods of ZnO with various forms have been reported by mixing additives.14)
Structure of ZnO.
For the synthesis of ZnO, hydrothermal synthesis using layered zinc hydroxide as a precursor has been extensively studied. Layered zinc hydroxide (Zn5(OH)10−yAy·nH2O, A: anion) is composed of anion (CH3COO−, NO3−, SO42− and Cl− etc.) and water molecules coordinated between zinc hydroxide layers.15) Synthesis of zinc oxide using layered zinc hydroxide as a precursor is expected to be widely used industrially because the particle morphology can be easily controlled compared to the conventional method.14,16,17) In this study, ZnO was synthesized by using layered zinc hydroxide chloride ZHC with Cl− coordinated between layers as precursors and aging under hydrothermal conditions. Deionized water and zinc acetate aqueous solution were used for the aging, and the effects of the acetate ion in the aging solution on the morphology of ZnO particles formed were studied.
1 mol/L zinc chloride (ZnCl2) aqueous solution was prepared by dissolving ZnCl2 (reagent-specific grade, Wako) in deionized water. Precursors ZHC were obtained by adding ZnO (particle size: 0.6 µm, Sakai Chemical Co., Ltd.) 5.00 g to 150 mL of 1 mol/L ZnCl2 aqueous solution and stirring for 24 hours using a magnetic stirrer. For washing, the resulting precipitate was dispersed in deionized water and then centrifuged at a 10,000 rpm rate for 20 minutes (H-200n Cokusan Co., Ltd.). This washing precipitate was performed three times.
2.2 AgingThe ZHC synthesized in 2.1 and deionized water 150 mL were mixed and enclosed in a Teflon-inner cylindrical stainless-steel container and heated in an oil bath at 120°C for 24 hours with stirring using a magnetic stirrer. After heating, the mixture was rapidly cooled to room temperature, filtrated under suction, and washed 3 times with deionized water. The residue was dried in an air bath at 50°C to obtain a sample powder.
In addition, for the first step of aging ZHC was aged at 120°C for 24 h in 150 mL of 1 mol/L ZnCl2 aqueous solution or 1 mol/L zinc acetate (ZnAc) aqueous solution and then rapidly cooled to room temperature and washed 3 times. For the second step of aging, the residue was aged again in 150 mL of deionized water. After rapidly cooling to room temperature and washing with deionized water 3 times, the sample powder was obtained by drying in an air bath at 50°C. Hereinafter, this aging process is defined as a two-step aging.
In addition, the same two-step aging as described above was performed by mixing ZHC synthesized in 2.1 and 1 mol/L ZnAc aqueous solution 75 mL, 150 mL, and 300 mL in the first step, respectively.
2.3 Sample characterizationThe identification of the constituents of each sample powder was carried out using an X-ray powder diffractometer (XRD) (RINT2000, Rigaku Co., Ltd.). Measuring conditions in as follows; the voltage 40 kV, current 40 mA, scan speed 4°/min, scan range 5° < θ < 65°. Thermal decomposition behaviors of various sample powders were measured using a differential thermal and thermogravimetric analyzer (TG-DTA) (Thermo plus TG8120, Rigaku Co., Ltd.) from room temperature to 600°C at a heating rate of 10°C/min. The microstructure of samples was observed at 15 kV using a scanning electron microscopy (SEM) (JSM-7600F, Japan Electronics Co.). The specific surface area of each sample powder was measured by a BET5 point measuring method using an automated specific surface area measuring device (TriStar II PLUS, Shimadzu Corporation).
Figure 2 shows X-ray diffraction patterns of the precursor ZHC. A particularly strong peak appears around 11.5°, which is the diffracted peak of ZHC in the (003) plane. Other less intense diffractive peaks are also peaks assigned to ZHC.18)
X-ray diffraction pattern of ZHC.
Figure 3 shows TG-DTA curves of the same sample. Endothermic peaks were found around 140°C, 170°C, 210°C and 470°C. The thermal decomposition of ZHC is considered to proceed in three steps as follows.19) In the first step, the desorption of water occurs between 110°C and 165°C as shown in the following eq. (1). In the second step, the OH group leaves between 145–190°C, as shown in eq. (2), and Zn5(OH)8Cl2 decomposes into ZnO and Zn(OH)Cl. This leads to the formation of an intermediate Zn(OH)Cl, and thus thermal decomposition of Zn(OH)Cl as in eq. (3) occurs between 190 and 220°C. In the third step, thermal decomposition of ZnCl2 shown in eq. (4) occurs between 220°C and 600°C.
| \begin{equation} \text{Zn$_{5}$(OH)$_{8}$Cl$_{2}$}{\cdot }\text{H$_{2}$O}\to \text{Zn$_{5}$(OH)$_{8}$Cl$_{2}$} + \text{H$_{2}$O} \end{equation} | (1) |
| \begin{equation} \text{Zn$_{5}$(OH)$_{8}$Cl$_{2}$}\to \text{2 Zn(OH)Cl} + \text{3 ZnO} + \text{3 H$_{2}$O} \end{equation} | (2) |
| \begin{equation} \text{2 Zn(OH)Cl}\to \text{ZnO} + \text{ZnCl$_{2}$} + \text{3 H$_{2}$O} \end{equation} | (3) |
| \begin{equation} \text{ZnCl$_{2}$} + \text{H$_{2}$O}\to \text{ZnO} + \text{2HCl} \end{equation} | (4) |
TG-DTA curves of ZHC.
From the above, it is considered that the endothermic peak around 140°C corresponds to the first step shown in eq. (1), the endothermic peaks around 170°C and 210°C correspond to the second step shown in eqs. (2) and (3). Moreover, the endothermic peak around 470°C corresponds to the third step shown in eq. (4).
The total weight loss of the synthesized ZHC was 29%, which was close to the theoretical value calculated based on eqs. (1) to (4). The weight loss of synthesized ZHC was slightly higher than the theological value of 26.3%, which is thought to be due to the desorption of physically adsorbed water on the particle surface, resulting in a large weight loss in the first step. This result suggests that the total weight loss is close to the theoretical value. From the results of XRD and TG-DTA, it was revealed that the powder of ZHC single phase was obtained in this experiment.
Figure 4 shows SEM images of the same samples. Plate-like ZHC particles of about 1–3 µm in size were observed. The specific surface area value was 2.15 m2/g.
SEM image of ZHC.
Figure 5 shows X-ray diffraction patterns of the sample aged in 150 mL of deionized water at 120°C for 24 hours with stirring. All diffraction peaks are attributed to ZnO.20) The major peak intensities were (101) > (100) > (002).
X-ray diffraction pattern of ZnO.
Figure 6 shows TG-DTA curves of the same sample. No weight loss and no thermal decomposition behavior were found from room temperature to 600°C. From these results, it is considered that no precursor ZHC remained in the sample, and a single phase ZnO was obtained.
TG-DTA curves of ZnO.
Figure 7 shows SEM images of the same sample. Columnar particles with length less than 1 µm extended in a c-axis were observed. When ZHC is aged in deionized water, a formation reaction of ZnO probably proceeds according to the following scheme.21,22) First, ZHC is dissolved and Zn(OH)3− and Zn(OH)42− are generated according to eqs. (5) and (6). Hydroxide ions are desorbed from the produced Zn(OH)3− and Zn(OH)42− to form zinc hydroxide Zn(OH)2 following eqs. (7) and (8). As in eq. (9), Zn(OH)2 releases H2O and the ZnO nuclei are formed. Thereafter, ZnO particles grow while the growth unit, Zn(OH)42−, adsorbs on the ZnO of nuclei.
| \begin{equation} \text{Zn$_{5}$(OH)$_{8}$Cl$_{2}$}+\text{7OH$^{-}$}\to \text{5Zn(OH)$_{3}{}^{-}$} + \text{2Cl$^{-}$} \end{equation} | (5) |
| \begin{equation} \text{Zn$_{5}$(OH)$_{8}$Cl$_{2}$}+\text{12OH$^{-}$}\to \text{5Zn(OH)$_{4}{}^{2-}$} + \text{2 Cl$^{-}$} \end{equation} | (6) |
| \begin{equation} \text{Zn(OH)$_{3}{}^{-}$}\to \text{Zn(OH)$_{2}$} + \text{OH$^{-}$} \end{equation} | (7) |
| \begin{equation} \text{Zn(OH)$_{4}{}^{2-}$}\to \text{Zn(OH)$_{2}$} + \text{2OH$^{-}$} \end{equation} | (8) |
| \begin{equation} \text{Zn(OH)$_{2}$}\to \text{ZnO} + \text{H$_{2}$O} \end{equation} | (9) |
In the growth of ZnO particles, the negatively charged growth units, Zn(OH)42−, is easy to adhere onto the positively charged polarity plane (Zn2+ plane) of ZnO. Therefore, ZnO particles grow in the c-axis direction, resulting in columnar particles as shown in Fig. 1.
SEM image of ZnO.
Figure 8 shows X-ray diffraction patterns ZHC aged in 150 mL of 1 mol/L ZnAc or ZnCl2 aqueous solution at 120°C for 24 hours with stirring before aged in 150 mL of deionized water at 120°C for 24 hours with stirring. When ZHC was aged in 1 mol/L ZnCl2 aqueous solution in the first step, a diffractive peak of ZHC was observed. After the second aging in deionized water, only ZnO diffraction peak was observed. The intensities of the major diffraction peaks were (101) > (100) > (002). When 1 mol/L ZnAc aqueous solution was used in the first step, a (001) plane diffraction peak of ZHA and a diffraction peak of ZnO were observed.23) As shown in Fig. 8, it was formed that ZHC used as a precursor disappeared to form ZHA and ZnO during aging. ZHA is probably generated by the interchange of chloride ions coordinated between the layers of ZHC and acetate ions in the solutions. After the second aging in deionized water, only ZnO diffraction peak was observed. The intensities of the major diffraction peaks were (101) > (100) > (002).
X-ray diffraction patterns of ZnO obtained by aging ZHC in 150 mL of 1 mol/L ZnCl2 and ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
Figure 9 shows TG-DTA curves of the same samples as shown in Fig. 8. For the sample aged in ZnCl2 aqueous solution in the first step, weight loss and thermal decomposition behavior derived from ZHC were observed. The total weight loss was 31.9%, which is greater than the theoretical weight loss of 26.3%. There was greater weight loss at 300–500°C compared to the TG curve of ZHC shown in Fig. 3. Since this weight loss is due to the thermal decomposition of ZnCl2 shown in eq. (4), it is considered that more Cl− ions are incorporated between the layers of ZHC compared to ZHC used as a precursor. The result indicates that little ZnO is formed during aging in the ZnCl2 aqueous solution in the first step. That is in good agreement with the XRD result shown in Fig. 8. After the second aging in deionized water, no weight loss and no thermal decomposition behaviors were observed, suggesting that the precursors ZHC did not remain and that ZnO is almost a single phase. For ZHC aged in ZnAc aqueous solution in the first step, weight loss and thermal decomposition behavior derived from ZHA were observed. The total weight loss was 15.9%, and the production ratio of ZnO calculated from the theoretical weight loss of 34.1% of ZHA is 53.4%. A weight loss of about 1% and an exothermic peak were observed around 530°C in the sample subjected to two-step aging. They are probably a thermal decomposition of Teflon container from the inner cylinder of a hydrothermal vessel. As shown above, it was found that after the two-step aging of ZHC using ZnCl2 or ZnAc aqueous solution, ZnOs are almost single phase. Regardless of the aqueous solution type in the first-step aging, ZnO was obtained by the second step of aging in deionized water again.
TG-DTA curves of ZnO obtained by aging ZHC in 150 mL of 1 mol/L ZnCl2 and ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
Figure 10 shows SEM images of the same samples. After aging ZHC in ZnCl2 aqueous solution in the first step, the plate-like ZHC particles 2–3 µm in size were seen. After the second aging in deionized water, Columnar ZnO particles with a length of 0.5–1.5 µm was observed. On the other hand, after aging ZHC in ZnAc aqueous solution in the first step, the coin-like ZnO particle of less than 1 µm in size and the irregular-shaped ZHA particles were seen. In addition to SEM images, it is desirable to distinguish ZHA from ZnO from TEM images and electron diffraction data. However, since ZHA and ZnO produced in this study are very different in morphology, ZHA and ZnO were distinguished only from SEM images. After second step aging in deionized water, only coin-like ZnO particles of about 1 µm in size were observed in the sample obtained by two-step aging.
SEM images of ZnO obtained by aging ZHC in 150 mL of 1 mol/L ZnCl2 and ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
Table 1 shows BET specific surface area of the samples after two-step aging and ZnO particles obtained by aging with only deionized water obtained in 3.2.
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Specific surface measurement and SEM observation results revealed that two-step aging using ZnCl2 or ZnAc aqueous solutions produced ZnO with a larger particle size than the single-step aging with deionized water.
As mentioned above, in the two-step aging using ZnCl2 aqueous solution, columnar particles were obtained as in the case of the single-step aging with deionized water, while coin-shaped ZnO particles were obtained in the two-step aging using ZnAc aqueous solution. The following are possible reasons for the difference in the shape of the ZnO particles obtained depending on whether ZnCl2 and ZnAc aqueous solutions is used in the first step of the two-step aging process. The formation of ZnO from the layered zinc hydroxide precursor during aging is thought to occur by dissolution-redeposition. The Zn2+ ions dissolved from the precursor form complex ions coexisting such as chloride and acetate ions.
These zinc complexes with the chloride ion and the acetate ion are negatively charged so they adsorb on the Zn2+ plane of ZnO, which is a positively charged growth surface. The Zinc complexes adsorbed on the Zn2+ plane of ZnO inhibit the adsorption of the growth units Zn(OH)42− as shown in the schematic diagram of Fig. 11, and consequently, the growth in the c-axis direction is suppressed.
The schematic diagram of ZnO crystal growth in the presence of complexes.
However, when comparing the chloride and acetate complexes of zinc, as shown in Table 2,24,25) the acetate ion has a much higher complex stability constant than the chloride ion and is, therefore, more stable in the aging solution. Therefore, acetate complexes adsorb more easily on the Zn2+ plane of ZnO than chloride complexes, and as a result, fine coin-shaped ZnO seed crystals with suppressed c-axis growth are easily formed during the first step of aging. In addition, during the first step of aging in ZnAc solution, the chloride ions between the layers of ZHC are exchanged with acetate ions to form ZHA at the same time as the coin-like ZnO seed crystals are formed. Therefore, in the second step of aging in deionized water, the zinc acetate complex formed by the dissolution of ZHA adsorbs on the Zn2+ plane of the ZnO seed crystal, which inhibits growth in the c-axis direction and allows the crystal to grow in a coin-like shape.
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On the other hand, as shown in Fig. 8, almost no seed crystals of ZnO were formed in the first step of aging using a ZnCl2 aqueous solution. Furthermore, it is difficult to form stable complexes between acetate and chloride ions in the aging solution due to the extremely low stability constants of the complexes as shown in Table 2. Therefore, in the second step of aging with deionized water, ZnO particles elongated in the c-axis direction are expected to be formed as in the case of one-step aging with only deionized water.
3.4 Effect of amount of aging solutions in the two-step aging of ZHC using aqueous zinc-acetate and deionized waterFigure 12 shows X-ray diffraction patterns ZHC aged in 75 mL, 150 mL, and 300 mL of 1 mol/L ZnAc aqueous solution at 120°C for 24 hours with stirring before aged in 150 mL of deionized water at 120°C for 24 hours with stirring. In this section, the sample labeled 150 mL sample is the same one as the two-step aged with ZnAc aqueous solution shown in section 3.3. Aged in 75 mL of zinc acetate aqueous solution, which is labeled 75 mL sample, diffracted peaks of ZHA, ZHC, and ZnO were found.
X-ray diffraction patterns of ZnO obtained by aging ZHC in 75, 150, and 300 mL of 1 mol/L ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
On the other hand, for ZHC aged in 300 mL of ZnAc aqueous solution, which is labeled 300 mL sample, and 150 mL samples, the ZHA and ZnO diffraction peaks were observed, with no diffraction peaks for ZHC. The intensity of the diffraction peaks for ZHA decreased with increasing amount of ZnAc aqueous solution. After the second step of aging with 150 mL of deionized water, diffraction peaks for only ZnO were observed in all samples.
Figure 13 shows TG-DTA curves of the same samples. For the 75 mL sample, weight loss and thermal decomposition behavior derived from ZHA were seen, and the total weight loss rate was 19.9%. From X-ray diffraction patterns, the precursors ZHC partially remained, it is considered that the generation rate of ZnO is less than 50%. For the 300 mL sample, weight loss and thermal decomposition behavior derived from ZHA were seen. Since the total weight-loss rate was 5.25%, the production rate of ZnO was 84.6%. The weight loss was smaller for the samples after the first step of aging as the amount of ZnAc aqueous solution increased. On the other hand, after the second step of aging, the weight loss of all samples was approximately 2%, regardless of the amount of the first step aging solution, indicating that the layered zinc hydroxide used as the precursor was almost entirely converted to ZnO. These TG-DTA results are in good agreement with the X-ray diffraction analysis results shown in Fig. 12.
TG-DTA curves of ZnO obtained by aging ZHC in 75, 150, and 300 mL of 1 mol/L ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
Figure 14 shows SEM images of the same samples. For the 75 mL sample in the first step of aging, irregular-shaped fine ZHA was observed around the plate-like ZnO particles less than 1 µm in size. After the second step of aging, a hexagonal plate-like ZnO with a size of about 1 µm and irregular plate-like ZnO particles smaller than that were observed. In the 150 mL sample, the first step of aging produced hexagonal plate-like ZnO particles with chipped corners and irregularly shaped ZHA. In comparison, in the 300 mL sample, relatively well-shaped hexagonal plate-like ZnO particles were observed. These plate-like hexagonal ZnO particles were observed in 150 mL and 300 mL samples after the second step of aging, the 75 mL and 150 mL samples showed thick hexagonal plate-like or coin-like particles with a particle size of less than 1 µm, while the 300 mL sample showed thinner ZnO particles with a particle size greater than 1 µm. In addition, in the 75 mL samples, a small amount of columnar ZnO particles were also mixed.
SEM images of ZnO obtained by aging ZHC in 75, 150, and 300 mL of 1 mol/L ZnAc aqueous solutions at 120°C for 24 h with stirring, before aging in 150 mL of deionized water at 120°C for 24 h with stirring.
In the two-step aging of ZHC with a zinc acetate solution, the formation of ZnO seed crystals and the exchange reaction between acetate ions and chloride ions present in the interlayer of ZHC proceed simultaneously during the first step aging. At this time, the zinc acetate complexes adsorb on the Zn2+ plane of the ZnO seed crystals, inhibiting growth in the c-axis direction and causing the seed crystals to grow into plate-like particles. However, since acetate ions remain in the aging solution, the dissolution of ZHA formed by ion exchange is suppressed, and as a result, ZnO and ZHA are assumed to coexist. When the amount of aging solution is small, the ion exchange between acetate and chloride ions is insufficient. ZHC remains in addition to ZHA, resulting in the coexistence of fine columnar particles in the sample after the second step of aging in deionized water.
On the other hand, if the amount of the first-stage aging solution is large, dissolution of ZHC is more likely to occur, and more seed crystals of ZnO are likely to be generated. In addition, because the amount of zinc complexes of acetate ions produced is large, the suppression of crystal growth in the c-axis direction becomes more effective, and more well-shaped hexagonal plate-like ZnO particles with larger particle sizes are likely to be produced in the first-step aging.
A two-step aging process was investigated in which the first step of aging was carried out in zinc chloride or zinc acetate aqueous solutions and the second step of aging in deionized water, using layered zinc hydroxide, ZHC, as a precursor with chloride ions in the interlayer. The results obtained are summarized as follows;
