H. Hou et al. /Solid State Sciences 7(2005)45-51 447nm, was contributed by the near band edge emission of Zn2++20H-- Zn(OH)2, the wide bandgap. The lack of green band in Fig. 5 is mainly indicative of a low surface area to volume ratio [ 22]. Further Zn(OH)2=Zno H20(pH=8 study is in progress to identify the origin of observed peaks At pH=8, the amount of ammonia cannot form the and the changes introduced by different ZnO morphologies. Zn(NH3)42+ but come into being the Zn(OH)2, which can Generally, the growth of the tubular whiskers and epitax be validated by the white turbidity of the initial system ial whiskers are suggested to follow a mechanism combined Therefore, the Zno crystal structure is crucial to synthe- by the effect of CTAB, the pH of the system, the structure size epitaxial whiskers at lower pH value as pH=8, under of ZnO wurtzite under the hydrothermal growth conditions. the effect of structure-directing agent CTAB. The illustration But, careful analysis indicated that the formation mechanism shown in Fig 9(a, b)clearly indicates the Zno crystal struc of ZnO tubular and epitaxial whiskers are totally different ture. As shown in Fig. 9(a), the face of tetrahedron Zn-O4 under the different pH value of the system parallel to positive polar c(0001), and the vertex angle is In the formation of tubular ZnO whiskers at pH= 10, in the face of negative polar (0001)[24]. Fig. 9(b)exhibits he initial tubular ZnO seed crystal come into being from the projection of tetrahedron Zn-O4 in [1120]. The distri- the hydration of Zn(NH3)4-[10, 19]. Under the pH= 10, bution of Zn in c is not symmetrical, which lean to(0001) droxyl OH- and the Zn(NH3)42+ form the initial Zno but apart from(0001) Zn and O are symmetrically distrib- eed crystal according to ute in [1100][25-28. In the same way, appropriate amount NH3+H,O: NH3- H2O= NH4++OH- (1) of CTAB also acts as the structure-directing agent in the for- Zn2++4NH3- Zn(NH3)42+ (2) the 1D epitaxial whiskers cannot shape even if with the ex Zn(NH3 )4++20H" istence of ammonia. Our experiments conform that other ZnO↓+4NH3↑+H2O(pH=10) surfactants, such as sodium dodecyl benzene sulphonate At the beginning of the reaction, because of the lower tem- perature and the production of small amounts of NH,, the ZnO seed crystal is liable to form. At the elevated tempe ature, the production of NH3, the existence of a gas-liquid equilibrium and the structure-directing agent of CTAB in the airtight autoclave make the ZnO seed crystal grow along certain orientation to form hollow structure. without the presence of ammonia, the ID tubular whiskers cannot form even if the presence of structure-directing agent CTAB. Our complement experiments show that when sodium hydrox de is used as a substitute for the ammonia in the pres- ence of CTAB. the tubular whiskers also cannot come forth and only irregular short rods can be obtained[23]. Ap- propriate amount of CTAB acts as the structure-directing agent in the formation of tubular Zno whiskers. While ab- sence of CTAB, the ID tubular whiskers cannot shape even if with the existence of ammonia. Our experiments con- form that other surfactants, such as sodium dodecyl benzene sulphonate(SDBS) and sodium dodecyl sulfate(SDS),can- not come into being the tubular whiskers, indicating that CTAB plays an undeniable and irreplaceable role. It has been found that the optimal molar ratio of zinc(l) acetate dihy drate to CTAB(2: 1)favors the growth of Zno whiskers The ID Zno tubular whiskers can only be formed with the cooperation of structure-directing agent of CTAB and the hydration of Zn(NH3)42+ under pH= 10 in hydrothermal growth conditions In the formation of epitaxial ZnO whiskers at pH=8, the initial epitaxial ZnO seed crystals come into being from the dehydration of Zn(OH)2. Under the pH=8, hydroxyl OH- Fig 9. The schematic diagram of ZnO crystal structure (a) The projec- and the Zn(OH)2 form the initial ZnO seed crystal according tion of ZnO crystal structure in c(0001)face. (b)The projection of Zn-O4 tetrahedron in(1120)face. (c) Intergrowth on the positive and negative po- lar faces. (d)Zn-O4 tetrahedrons connection on positive and negative polar NH3+H2O- NH3-H2O= NH4++OH (4) faces along c-axis.H. Hou et al. / Solid State Sciences 7 (2005) 45–51 49 447 nm, was contributed by the near band edge emission of the wide bandgap. The lack of green band in Fig. 5 is mainly indicative of a low surface area to volume ratio [22]. Further study is in progress to identify the origin of observed peaks and the changes introduced by different ZnO morphologies. Generally, the growth of the tubular whiskers and epitax￾ial whiskers are suggested to follow a mechanism combined by the effect of CTAB, the pH of the system, the structure of ZnO wurtzite under the hydrothermal growth conditions. But, careful analysis indicated that the formation mechanism of ZnO tubular and epitaxial whiskers are totally different under the different pH value of the system. In the formation of tubular ZnO whiskers at pH = 10, the initial tubular ZnO seed crystal come into being from the hydration of Zn(NH3)4 2+ [10,19]. Under the pH = 10, hydroxyl OH− and the Zn(NH3)4 2+ form the initial ZnO seed crystal according to: NH3 + H2O
NH3·H2O
NH4 + + OH−, (1) Zn2+ + 4NH3 → Zn(NH3)4 2+, (2) Zn(NH3)4 2+ + 2OH− → ZnO↓ + 4NH3↑ + H2O (pH = 10). (3) At the beginning of the reaction, because of the lower tem￾perature and the production of small amounts of NH3, the ZnO seed crystal is liable to form. At the elevated temper￾ature, the production of NH3, the existence of a gas–liquid equilibrium and the structure-directing agent of CTAB in the airtight autoclave make the ZnO seed crystal grow along certain orientation to form hollow structure. Without the presence of ammonia, the 1D tubular whiskers cannot form even if the presence of structure-directing agent CTAB. Our complement experiments show that when sodium hydrox￾ide is used as a substitute for the ammonia in the pres￾ence of CTAB, the tubular whiskers also cannot come forth and only irregular short rods can be obtained [23]. Ap￾propriate amount of CTAB acts as the structure-directing agent in the formation of tubular ZnO whiskers. While ab￾sence of CTAB, the 1D tubular whiskers cannot shape even if with the existence of ammonia. Our experiments con￾form that other surfactants, such as sodium dodecyl benzene sulphonate (SDBS) and sodium dodecyl sulfate (SDS), can￾not come into being the tubular whiskers, indicating that CTAB plays an undeniable and irreplaceable role. It has been found that the optimal molar ratio of zinc(II) acetate dihy￾drate to CTAB (2:1) favors the growth of ZnO whiskers. The 1D ZnO tubular whiskers can only be formed with the cooperation of structure-directing agent of CTAB and the hydration of Zn(NH3)4 2+ under pH = 10 in hydrothermal growth conditions. In the formation of epitaxial ZnO whiskers at pH = 8, the initial epitaxial ZnO seed crystals come into being from the dehydration of Zn(OH)2. Under the pH = 8, hydroxyl OH− and the Zn(OH)2 form the initial ZnO seed crystal according to: NH3 + H2O
NH3·H2O
NH4 + + OH−, (4) Zn2+ + 2OH−
Zn(OH)2, (5) Zn(OH)2
ZnO + H2O (pH = 8). (6) At pH = 8, the amount of ammonia cannot form the Zn(NH3)4 2+ but come into being the Zn(OH)2, which can be validated by the white turbidity of the initial system. Therefore, the ZnO crystal structure is crucial to synthe￾size epitaxial whiskers at lower pH value as pH = 8, under the effect of structure-directing agent CTAB. The illustration shown in Fig. 9(a,b) clearly indicates the ZnO crystal struc￾ture. As shown in Fig. 9(a), the face of tetrahedron Zn–O4 parallel to positive polar c (0001), and the vertex angle is in the face of negative polar (0001) [24]. Fig. 9(b) exhibits the projection of tetrahedron Zn–O4 in [1120]. The distri￾bution of Zn in c is not symmetrical, which lean to (0001) but apart from (0001). Zn and O are symmetrically distrib￾ute in [1100] [25–28]. In the same way, appropriate amount of CTAB also acts as the structure-directing agent in the for￾mation of epitaxial ZnO whiskers. While absence of CTAB, the 1D epitaxial whiskers cannot shape even if with the ex￾istence of ammonia. Our experiments conform that other surfactants, such as sodium dodecyl benzene sulphonate Fig. 9. The schematic diagram of ZnO crystal structure. (a) The projec￾tion of ZnO crystal structure in c (0001) face. (b) The projection of Zn–O4 tetrahedron in (1120) face. (c) Intergrowth on the positive and negative po￾lar faces. (d) Zn–O4 tetrahedrons connection on positive and negative polar faces along c-axis