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Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method
Powder Technology 203 (2010) 243–247
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method N. Samaele a, P. Amornpitoksuk a,⁎, S. Suwanboon b a b
Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 22 April 2010 Accepted 8 May 2010 Available online 21 May 2010 Keywords: Zinc oxide Sodium dodecyl sulfate Precipitation
a b s t r a c t ZnO particles were synthesized directly from an aqueous solution of zinc acetate dihydrate in the presence of sodium dodecyl sulfate (SDS) and sodium hydroxide at 70 °C. The morphological changes were investigated in the range of pH 8–12. The hexagonal prism-like shape was formed at pH 8 and 10 by inhibition of growth along the c direction whereas the small rod-like shape was observed at pH 12. The estimated band gap and the room temperature photoluminescence intensity in a visible region are dependent upon the geometrical shape and size of the ZnO particles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is one of the more important materials with extensive applications in many fields such as medicines, pigments, ceramics and rubber additives [1–3]. ZnO has a wide direct-band gap of 3.37 eV and a high exciton binding energy of about 60 meV [4]. With this unique property, ZnO has also attracted attention for electrical and optical applications such as light-emitting diodes, photocatalysts, photodetectors, solar cells, piezoelectronic devices and sensors [5]. Because the shape of the ZnO particles depends on the reaction conditions during their formation, many methods have been used to synthesize ZnO particles, for example, sol–gel [6,7], hydrothermal [8] and co-precipitation [9]. Nowadays, various wet chemical methods have been consolidated and are now fashionable for synthesizing ZnO nanoparticles because they provide a simple and economic route as well as requiring low temperature. Not only this method is effective but also the addition of appropriate surfactants can help to control the ZnO morphology. To date, many surfactants have been investigated for their role in controlling the morphology of ZnO particles, for example, triethanolamine (TEA) [10], cetyltrimethylammonium bromide (CTAB) [11] and sodium dodecyl sulfate (SDS) [12]. Among these surfactants, SDS has the potential to change the shape of ZnO particles to a nanowire [13], nanorod [14], nanoflake [15], nanowhisker [11] and hexagonal prismatic shapes [12]. In order to reduce the reaction steps and ⁎ Corresponding author. Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand. Tel.: + 66 74 28 84 38; fax: + 66 42 12 918. E-mail address: [email protected] (P. Amornpitoksuk). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.05.014
costs, a singlet step synthesis has attracted attention. When SDS is the stabilizer, the as-prepared ZnO particles form a single phase using a hydrothermal method [14,15] or from a highly alkaline solution (precipitation method) [16,17]. These ZnO formation mechanisms are easily explained by the transformation of Zn(OH)2− 4 to ZnO. Using the precipitation method (b100 °C), the solution with SDS at low pH does not produce ZnO particles in a single step [17]. In this condition, the impurity compound is Zn(OH)2 which is an intermediate specie occurring at low pH. This process has not been fully understood. In this study we report the potential of high concentrations of SDS or higher concentrations of critical micelles (CMC) to control the shape of ZnO particles at various pH values and demonstrate that SDS molecules stabilize Zn(OH)2. Furthermore, we have also studied the correlation between their structural properties and optical characteristics at room temperature. 2. Experimental All chemicals used in this experiment are analytical grade and used without further purification. Nanocrystalline ZnO particles were synthesized through hydrolysis of zinc acetate dihydrate (Zn(C2H3O2)2·2H2O, Fluka) in the presence of sodium dodecyl sulfate (CH3(CH2)11OSO3Na, Fluka) acting as an anionic surfactant in aqueous solution. In a typical procedure, 6.58 g Zn(C2H3O2)2·2H2O was first dissolved in 150 mL of distilled water with continuous stirring until a homogeneous solution was obtained. Then 8.64 g SDS was added so that the mole ratio of Zn2+:SDS = 1:1 was obtained. Finally, 0.2 M NaOH was added dropwise to the SDS-modified Zn(C2H3O2)2·2H2O precursor solution until the pH reached 8, 10 or 12. The white precipitates obtained at each pH were continuously
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stirred at 70 °C for 1 h before being cooled to room temperature. These precipitates were filtered, rinsed with distilled water and ethanol several times then collected and dried at 100 °C for 1 h. The structural identification of ZnO particles was carried out using an X-ray diffractometer (XRD, X'Pert MPD, Philips) with Cu Kα radiation at a wavelength (λ) of 0.15406 nm and the crystallite size was calculated by the Scherrer equation [18]: D=
0:9 λ β cos θ
where D is the average crystallite size, λ is the wavelength of the Cu Kα line, θ is the Bragg angle and β is the full-width at half maximum (FWHM) in radians. The morphology of the ZnO particles was determined using a scanning electron microscope (SEM, JSM-5800, JEOL). The FT-IR spectra of the as-powder samples were measured by the FT-IR spectrophotometer (Spectrum BX, Perkin Elmer) in a transmission mode at 400–4500 cm−1. The UV–Vis diffuse reflectance spectra in the range of 200–800 nm were carried out with a UV–Vis spectrophotometer (UV-2401, Shimadzu) using BaSO4 as a reflectance standard. Room temperature photoluminescence (PL) measurements were performed by a luminescence spectrophotometer (LS/55, Perkin Elmer). 3. Results and discussion 3.1. Structure and morphology of prepared ZnO powder In the absence of any SDS molecule, the XRD patterns of all asprepared ZnO powders obtained at different pH values are shown in Fig. 1. All diffraction peaks of all samples were quite similar to the hexagonal or wurtzite ZnO structure. The obtained structure is in good agreement with the JCPDS file of ZnO (JCPDS 36-1451). No impurity phase other than ZnO was observed in the XRD patterns. Fig. 2 shows the XRD patterns of the as-powders prepared from the SDS-modified Zn(C2H3O2)2·2H2O solutions in the pH range of 8–12. As seen from these patterns, only one sample obtained from the solution of pH 12 showed a single phase of ZnO structure (Fig. 2(c)). At pH 8 and 10, the as-prepared products, however, show the diffraction patterns of both ZnO and Zn(OH)2 which are clearly seen in Fig. 2(a) and (b), respectively. From these samples, it seems that the diffraction peak intensities of the ZnO phase increased with the pH of the solution. Fig. 3 describes the morphologies of the as-prepared products obtained for pH 8 and 10. For an aging time of 1 h, the solid products at pH 8 are amorphous (Fig. 3(a)) and can be assigned to Zn(OH)2. However, the rate of the transformation of Zn(OH)2 to
Fig. 2. XRD patterns of as-powders prepared from the SDS-modified Zn2+ solutions at (a) pH = 8, (b) pH = 10 and (c) pH = 12.
ZnO increased with the pH then some hexagonal bi-prism particles were formed after 1 h of aging time at pH 10 (Fig. 3(b)). Owing to the presence of Zn(OH)2 as an intermediate phase in the XRD pattern, the thermal behavior of sample was recorded by TGA with a heating rate of 10 °C/min under a nitrogen atmosphere. Fig. 4 shows the nature of the pyrolysis process of the SDS-modified asparticles prepared from the precursor solution with a mole ratio of Zn2+:SDS equal to 1:1 at a pH of 8. The first weight loss of 3.45% near 100 °C is ascribed to the dehydration of crystal water. The second weight loss of 29.5% between 153 and 185 °C is ascribed to the decomposition of Zn(OH)2 to ZnO [19]. The third weight loss of 13.14% between 577 and 900 °C indicates that the entire SDS had been pyrolized so the SDS molecules bonded with the as-precipitates are eliminated by the heat and the ZnO phase formed simultaneously and completely. The last weight loss at a temperature of over 1100 °C might be referred to the evaporation of the nanocrystalline ZnO particles during sintering process [20]. This can be seen in ZnO chemical or commercial product (data is not shown). From this figure, the calcination temperature was selected at 900 °C. Under this condition, the XRD patterns of the calcined samples (pH = 8 and 10) show only a single ZnO phase (data is not shown). The crystallite size of the calcined ZnO particles are 62.00 (pH 8), 55.64 (pH 10) and 48.10 (pH 12, aspowder) nm. The SEM images of the calcinated ZnO particles prepared from the SDS-modified- Zn(C2H3O2)2.2H2O solution are presented in Fig. 5. At pH 8 and 10, the ZnO particles tend to form a hexagonal prism-like structure as shown in Fig. 5(a) and (b), respectively. At pH 8, the ZnO particles have a hexagonal diameter of ∼1 μm and a thickness of ∼0.25 μm. The diameter of this hexagonal facet decreases to ∼ 3 μm and a thickness of ∼0.2 μm when the pH of solution was increased to 10. A small rod-like (agglomerated by sphere-like shape) structure however made an appearance at pH 12 as shown in Fig. 5(c). These various morphologies found at different pH are explained in the next section. 3.2. Possible growth mechanism
Fig. 1. XRD patterns of all as-prepared ZnO without SDS addition.
The formation of ZnO particles from the hydrolysis of Zn2+ ions in aqueous media is known to be a complex process. Many polyvalent cationic species can be formed between Zn2+ ions with OH− ions and are strongly dependent upon the pH of the solution. However, the precipitation of ZnO particles has been usually described through a growth unit that might be either Zn(OH)2 or Zn(OH)2− ions [21–24] 4 depending on the pH, temperature and synthetic methods. The growth of ZnO from Zn(OH)2 has been usually suggested to occur through the dissolution–reprecipitation mechanism, however this is ambiguous. Although there are many explanations for the growth of ZnO from Zn(OH)2− ions that are very simple to understand, this 4
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Fig. 3. SEM images of solid products prepared from (a) pH = 8, (b) pH = 10.
novel mechanism can occur only under strongly basic conditions. J. Xie et al. [23] proposed that ZnO can grow by the Zn(OH)2–Zn(OH)2− 4 species when the mole ratio of Zn2+:OH− is within a range of 1:4–1:7. 2+ However, Zn(OH)2− :OH− in the range 4 ions are the precursors of Zn of 1:8–1:9. At pH 12, a white colloidal precipitant of Zn(OH)2 was observed after mixing the Zn2+ and OH− solutions. This species is rather stable and can be further dehydrated and transformed to ZnO as has been previously described in many publications [21–24]. The rate of the last step depends on the temperature and pH of the solution. In this work, the ZnO formation process was completed within 1 h. However the experimental results for the reaction time of less than 1 h were not carried out. At this point, the Zn(OH)2 phase was not observed in the XRD patterns from these samples (pH 12) with or without addition of SDS. Based on the speciation diagram of ZnO [25], Zn(OH)+ ion is a dominant species at pH 8. It can interact with OH− easily to form Zn(OH)2 and this species is the most prevalent at pH = 10 in the speciation diagram. However, using the simple calculation at 25 °C, the maximum Zn(OH)2 precipitation was achieved only when the pH was between 8 and 11 [21]. From the as-prepared ZnO at pH 8 and 10 without SDS molecules, no secondary phase other than ZnO was observed in the XRD patterns but the existence of both ZnO and Zn(OH)2 phases can be detected in the as-powder from the SDSmodified Zn(C2H3O2)2.2H2O solution. We propose that the SDS molecules are probably adsorbed onto the Zn(OH)2 surface and this stabilizes the Zn(OH)2 to depress its conversion to ZnO. However, this species can be destabilized by its dissolution at a higher pH as a competition reaction. Then, the ZnO phase is the more prevalent as
Fig. 4. TGA curve of as-powder prepared from the SDS-modofied Zn2+ solution at pH = 8.
seen from the XRD pattern for the sample at pH = 10. In the absence of SDS molecules, there is no stabilizer therefore Zn(OH)2 can be transformed to ZnO. This is a reason why this phase cannot be seen in the XRD patterns. P. Li et al. [12] prepared the ZnO microparticles with an SDS additive from aqueous solution at pH ∼ 9. Using 101 °C for 4 h, all the diffraction peaks are well assigned to the hexagonal ZnO and no characteristic peaks of Zn(OH)2 are observed. This result is maybe a consequence of the increasing dissolution rate at a higher reaction temperature. Zn(OH)2 is indeed a good adsorbent and possibly adsorbs the SDS molecules. This adsorption can be detected by FT-IR spectrophotometry. The comparison of the IR transmittance spectra of SDS and all aspowders with the SDS-modified Zn(C2H3O2)2·2H2O solution at different pH values is displayed in Fig. 6. The band at around 1200 cm−1 is assigned to an S = O stretching vibration of –SO4 from the SDS molecule [4,26]. The peaks at around 2920, 2852, 1468 and
Fig. 5. SEM of calcinated ZnO particles prepared at a mole ratio of Zn2+:SDS = 1:1 from the SDS-modified Zn2+ solutions at (a) pH = 8, (b) pH = 10, (c) pH = 12 (as-powder) and (d) pH = 12 (as-powder, FE–SEM).
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Fig. 6. FT-IR spectra of SDS (a) and as-powders prepared from the SDS-modified Zn2+ solutions at (b) pH = 8, (c) pH = 10 and (d) pH = 12.
1083 cm−1 are due to C–H stretching and bending. A broadening peak at around 3440 cm−1 is proposed to be due to H–OH stretching. The peak at 470 cm−1 is a characteristic peak of Zn–O stretching [4], and is not observed in the spectrum of pure SDS. It is noteworthy that no characteristic peak of SDS is observed from the IR spectrum of the ZnO particles prepared from the solution at pH 12. The results are in good agreement with the results from the XRD previously mentioned. From these results, the characteristic vibrational frequencies of SDS can be found in the samples prepared from pH 8 and 10 only. This confirms the adsorption of SDS by Zn(OH)2. On the other hand, if the rate of Zn (OH)2 dissolution to form Zn(OH)2− is very fast, the SDS molecules 4 will not be adsorbed because of the electrostatic repulsion between the negative charge of the hydrophilic groups of SDS and Zn(OH)2− 4 species. Then, no SDS was detected in the as-prepared sample in the case of pH = 12. On its face, ZnO is a polar crystal with a positive polar (0001) plane rich in Zn2+ cations and a negative polar (0001 ̅) plane rich in O2− anions. In the solution method, the velocity of crystal growth in the different directions is reported to be: V(0001) N V(1 ̅011 ̅) N V(1 ̅010) N V(1 ̅011) N V(0001 ̅) [12]. The hydrophilic group of SDS at pH 8 and pH 10 may adsorb with Zn2+ in the Zn(OH)2 due to electrostatic interaction. After Zn(OH)2 transforms to ZnO, this position is in the (0001) plane of the ZnO. It means that growth (0001) along c-axis must be retarded. Furthermore, the growth along the opposite face cannot be favorable for the lowest growth rate on the (0001 ̅) face. It could be grown in the ab or prism plane and then the hexagonal prism-like shape can be formed. If the rate of Zn(OH)2 dissolution is faster, the smaller diameter of the hexagonal prism shape may be formed (pH = 10). The agglomerated rod-like structure was formed distinctly at pH 12 as shown in Fig. 5(d). It was observed that small rod-like structures were mostly made up of particles by self-alignment of the spherical particles [27]. Moreover, we also found that the individual rod-like structure within a local region seemed to be randomly oriented. In the case of pH 12, SDS cannot be adsorbed onto the growth unit (Zn (OH)2− 4 ) because of the electrostatic repulsion, and then growth along the c-axis of the anisotropic crystal is favored [28]. This is in agreement with other publications in which no hexagonal prism shape (high diameter and short length) could be formed at a high pH [17]. The incorporation of SDS in as-prepared ZnO powder (pH 12) is also not detected in the XRD pattern and IR spectrum.
Fig. 7. Absorption spectra (a) and evolution of (αhv)2 vs. hv (b) of ZnO at pH = 8–12.
to the conduction band. The estimated direct-band gap according to the K–M model was calculated from a plot of (αhv)2 vs. photon energy (hv) as shown in Fig. 7(b). Extrapolation of the linear part until it intersects the hv-axis gives Eg. The estimated band gap energies are 3.205, 3.207 and 3.257 eV for the ZnO powder prepared at pH= 8, 10 and 12, respectively. The blue shift in the absorption edge is observed for the samples formed with the increase of pH due to their changing morphologies and surface microstructures. The estimated band gap energies of ZnO at pH= 8 and 10 are in the same order of another ZnO hexagonal prism shape that was prepared using PSS as a stabilizer (less than 3.21 eV) [5]. Comparison of the room temperature PL spectra of ZnO particles prepared at different pH is presented in Fig. 8. The entire PL spectra
3.3. Optical properties The UV–Vis diffuse reflectance spectra of ZnO samples prepared with the mole ratio of Zn2+:SDS= 1:1 at pH 8–12 are shown in Fig. 7(a). The absorption below 400 nm is assigned to the intrinsic band gap absorption of ZnO, due to electron transitions from the valence band
Fig. 8. Room temperature PL spectra of ZnO particles at pH = 8–12.
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show two emission peaks. The UV emission peak centered at about 390 nm is well understood as being the near-band-edge emission, whereas the broad visible emission originates from a variety of deep level defects, e.g. oxygen vacancies and zinc interstitials [13]. The diameter of the hexagonal face decreases with the increasing pH and changes to a rod-like shape (agglomerated by sphere-like shape) indicating that there is an increase of the surface area of its particle. With a higher surface area, the photogenerated holes in the valence band are possibly trapped into the surface defects and are increasingly returned to oxygen vacancies. Therefore the intensity of emission in the visible region increased with the increase of the surface area [13]. The intensity of emission in the visible region also increases as a function of pH. 4. Conclusion From the experimental results we proposed that the rate of Zn(OH)2 dissolution to form ZnO particles was inhibited by the adsorption of SDS molecules onto the Zn(OH)2 surface. Both Zn(OH)2 and ZnO phases were detected in the XRD from the as-powder prepared from the SDSmodified Zn(C2H3O2)2·2H2O solution at pH 8. However, this stabilized species can undergo more dissolution when the pH of the solution is increased. The observed Zn(OH)2 phase must decrease as a function of the increase in pH and only a single phase of ZnO was found at pH 12. At this point, the SDS molecules cannot adsorb onto the Zn(OH)2− as a 4 growth unit, because of the electrostatic repulsion. As the pH of the solution is increased, the particle sizes of ZnO are observed to decrease. A blue shift in the absorption edge was found. The increase of the energy band gap results in a slight blue shift of the PL emission. From this work, the size and growth direction of the ZnO particles can be controlled by the addition of SDS that is adsorbed onto the intermediate colloidal species surface. At a low pH, the hexagonal prism shape is formed. This makes it useful as a photocatalyst. The increase of the hexagonal facet (0001) plane of ZnO increases the photocatalytic RhB degradation [29]. The high band gap of ZnO can be found at high pH. This makes it applicable for many photophysical frame work reactions such as a modified TiO2 electrode in the dye sensitized solar cell (DSSC) and as a UV absorber etc. Acknowledgements Financial support from the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education is gratefully acknowledged. We would like to thank Dr. Brian Hodgson for English corrections. References [1] A.D. Hardy, H.H. Sutherland, R. Vaishnav, A study of the composition of some eye cosmetics (kohls) used in the United Arab Emirates, J. Ethnophrma. 80 (2002) 137–145.
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Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method N. Samaele a, P. Amornpitoksuk a,⁎, S. Suwanboon b a b
Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 22 April 2010 Accepted 8 May 2010 Available online 21 May 2010 Keywords: Zinc oxide Sodium dodecyl sulfate Precipitation
a b s t r a c t ZnO particles were synthesized directly from an aqueous solution of zinc acetate dihydrate in the presence of sodium dodecyl sulfate (SDS) and sodium hydroxide at 70 °C. The morphological changes were investigated in the range of pH 8–12. The hexagonal prism-like shape was formed at pH 8 and 10 by inhibition of growth along the c direction whereas the small rod-like shape was observed at pH 12. The estimated band gap and the room temperature photoluminescence intensity in a visible region are dependent upon the geometrical shape and size of the ZnO particles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is one of the more important materials with extensive applications in many fields such as medicines, pigments, ceramics and rubber additives [1–3]. ZnO has a wide direct-band gap of 3.37 eV and a high exciton binding energy of about 60 meV [4]. With this unique property, ZnO has also attracted attention for electrical and optical applications such as light-emitting diodes, photocatalysts, photodetectors, solar cells, piezoelectronic devices and sensors [5]. Because the shape of the ZnO particles depends on the reaction conditions during their formation, many methods have been used to synthesize ZnO particles, for example, sol–gel [6,7], hydrothermal [8] and co-precipitation [9]. Nowadays, various wet chemical methods have been consolidated and are now fashionable for synthesizing ZnO nanoparticles because they provide a simple and economic route as well as requiring low temperature. Not only this method is effective but also the addition of appropriate surfactants can help to control the ZnO morphology. To date, many surfactants have been investigated for their role in controlling the morphology of ZnO particles, for example, triethanolamine (TEA) [10], cetyltrimethylammonium bromide (CTAB) [11] and sodium dodecyl sulfate (SDS) [12]. Among these surfactants, SDS has the potential to change the shape of ZnO particles to a nanowire [13], nanorod [14], nanoflake [15], nanowhisker [11] and hexagonal prismatic shapes [12]. In order to reduce the reaction steps and ⁎ Corresponding author. Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand. Tel.: + 66 74 28 84 38; fax: + 66 42 12 918. E-mail address: [email protected] (P. Amornpitoksuk). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.05.014
costs, a singlet step synthesis has attracted attention. When SDS is the stabilizer, the as-prepared ZnO particles form a single phase using a hydrothermal method [14,15] or from a highly alkaline solution (precipitation method) [16,17]. These ZnO formation mechanisms are easily explained by the transformation of Zn(OH)2− 4 to ZnO. Using the precipitation method (b100 °C), the solution with SDS at low pH does not produce ZnO particles in a single step [17]. In this condition, the impurity compound is Zn(OH)2 which is an intermediate specie occurring at low pH. This process has not been fully understood. In this study we report the potential of high concentrations of SDS or higher concentrations of critical micelles (CMC) to control the shape of ZnO particles at various pH values and demonstrate that SDS molecules stabilize Zn(OH)2. Furthermore, we have also studied the correlation between their structural properties and optical characteristics at room temperature. 2. Experimental All chemicals used in this experiment are analytical grade and used without further purification. Nanocrystalline ZnO particles were synthesized through hydrolysis of zinc acetate dihydrate (Zn(C2H3O2)2·2H2O, Fluka) in the presence of sodium dodecyl sulfate (CH3(CH2)11OSO3Na, Fluka) acting as an anionic surfactant in aqueous solution. In a typical procedure, 6.58 g Zn(C2H3O2)2·2H2O was first dissolved in 150 mL of distilled water with continuous stirring until a homogeneous solution was obtained. Then 8.64 g SDS was added so that the mole ratio of Zn2+:SDS = 1:1 was obtained. Finally, 0.2 M NaOH was added dropwise to the SDS-modified Zn(C2H3O2)2·2H2O precursor solution until the pH reached 8, 10 or 12. The white precipitates obtained at each pH were continuously
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stirred at 70 °C for 1 h before being cooled to room temperature. These precipitates were filtered, rinsed with distilled water and ethanol several times then collected and dried at 100 °C for 1 h. The structural identification of ZnO particles was carried out using an X-ray diffractometer (XRD, X'Pert MPD, Philips) with Cu Kα radiation at a wavelength (λ) of 0.15406 nm and the crystallite size was calculated by the Scherrer equation [18]: D=
0:9 λ β cos θ
where D is the average crystallite size, λ is the wavelength of the Cu Kα line, θ is the Bragg angle and β is the full-width at half maximum (FWHM) in radians. The morphology of the ZnO particles was determined using a scanning electron microscope (SEM, JSM-5800, JEOL). The FT-IR spectra of the as-powder samples were measured by the FT-IR spectrophotometer (Spectrum BX, Perkin Elmer) in a transmission mode at 400–4500 cm−1. The UV–Vis diffuse reflectance spectra in the range of 200–800 nm were carried out with a UV–Vis spectrophotometer (UV-2401, Shimadzu) using BaSO4 as a reflectance standard. Room temperature photoluminescence (PL) measurements were performed by a luminescence spectrophotometer (LS/55, Perkin Elmer). 3. Results and discussion 3.1. Structure and morphology of prepared ZnO powder In the absence of any SDS molecule, the XRD patterns of all asprepared ZnO powders obtained at different pH values are shown in Fig. 1. All diffraction peaks of all samples were quite similar to the hexagonal or wurtzite ZnO structure. The obtained structure is in good agreement with the JCPDS file of ZnO (JCPDS 36-1451). No impurity phase other than ZnO was observed in the XRD patterns. Fig. 2 shows the XRD patterns of the as-powders prepared from the SDS-modified Zn(C2H3O2)2·2H2O solutions in the pH range of 8–12. As seen from these patterns, only one sample obtained from the solution of pH 12 showed a single phase of ZnO structure (Fig. 2(c)). At pH 8 and 10, the as-prepared products, however, show the diffraction patterns of both ZnO and Zn(OH)2 which are clearly seen in Fig. 2(a) and (b), respectively. From these samples, it seems that the diffraction peak intensities of the ZnO phase increased with the pH of the solution. Fig. 3 describes the morphologies of the as-prepared products obtained for pH 8 and 10. For an aging time of 1 h, the solid products at pH 8 are amorphous (Fig. 3(a)) and can be assigned to Zn(OH)2. However, the rate of the transformation of Zn(OH)2 to
Fig. 2. XRD patterns of as-powders prepared from the SDS-modified Zn2+ solutions at (a) pH = 8, (b) pH = 10 and (c) pH = 12.
ZnO increased with the pH then some hexagonal bi-prism particles were formed after 1 h of aging time at pH 10 (Fig. 3(b)). Owing to the presence of Zn(OH)2 as an intermediate phase in the XRD pattern, the thermal behavior of sample was recorded by TGA with a heating rate of 10 °C/min under a nitrogen atmosphere. Fig. 4 shows the nature of the pyrolysis process of the SDS-modified asparticles prepared from the precursor solution with a mole ratio of Zn2+:SDS equal to 1:1 at a pH of 8. The first weight loss of 3.45% near 100 °C is ascribed to the dehydration of crystal water. The second weight loss of 29.5% between 153 and 185 °C is ascribed to the decomposition of Zn(OH)2 to ZnO [19]. The third weight loss of 13.14% between 577 and 900 °C indicates that the entire SDS had been pyrolized so the SDS molecules bonded with the as-precipitates are eliminated by the heat and the ZnO phase formed simultaneously and completely. The last weight loss at a temperature of over 1100 °C might be referred to the evaporation of the nanocrystalline ZnO particles during sintering process [20]. This can be seen in ZnO chemical or commercial product (data is not shown). From this figure, the calcination temperature was selected at 900 °C. Under this condition, the XRD patterns of the calcined samples (pH = 8 and 10) show only a single ZnO phase (data is not shown). The crystallite size of the calcined ZnO particles are 62.00 (pH 8), 55.64 (pH 10) and 48.10 (pH 12, aspowder) nm. The SEM images of the calcinated ZnO particles prepared from the SDS-modified- Zn(C2H3O2)2.2H2O solution are presented in Fig. 5. At pH 8 and 10, the ZnO particles tend to form a hexagonal prism-like structure as shown in Fig. 5(a) and (b), respectively. At pH 8, the ZnO particles have a hexagonal diameter of ∼1 μm and a thickness of ∼0.25 μm. The diameter of this hexagonal facet decreases to ∼ 3 μm and a thickness of ∼0.2 μm when the pH of solution was increased to 10. A small rod-like (agglomerated by sphere-like shape) structure however made an appearance at pH 12 as shown in Fig. 5(c). These various morphologies found at different pH are explained in the next section. 3.2. Possible growth mechanism
Fig. 1. XRD patterns of all as-prepared ZnO without SDS addition.
The formation of ZnO particles from the hydrolysis of Zn2+ ions in aqueous media is known to be a complex process. Many polyvalent cationic species can be formed between Zn2+ ions with OH− ions and are strongly dependent upon the pH of the solution. However, the precipitation of ZnO particles has been usually described through a growth unit that might be either Zn(OH)2 or Zn(OH)2− ions [21–24] 4 depending on the pH, temperature and synthetic methods. The growth of ZnO from Zn(OH)2 has been usually suggested to occur through the dissolution–reprecipitation mechanism, however this is ambiguous. Although there are many explanations for the growth of ZnO from Zn(OH)2− ions that are very simple to understand, this 4
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Fig. 3. SEM images of solid products prepared from (a) pH = 8, (b) pH = 10.
novel mechanism can occur only under strongly basic conditions. J. Xie et al. [23] proposed that ZnO can grow by the Zn(OH)2–Zn(OH)2− 4 species when the mole ratio of Zn2+:OH− is within a range of 1:4–1:7. 2+ However, Zn(OH)2− :OH− in the range 4 ions are the precursors of Zn of 1:8–1:9. At pH 12, a white colloidal precipitant of Zn(OH)2 was observed after mixing the Zn2+ and OH− solutions. This species is rather stable and can be further dehydrated and transformed to ZnO as has been previously described in many publications [21–24]. The rate of the last step depends on the temperature and pH of the solution. In this work, the ZnO formation process was completed within 1 h. However the experimental results for the reaction time of less than 1 h were not carried out. At this point, the Zn(OH)2 phase was not observed in the XRD patterns from these samples (pH 12) with or without addition of SDS. Based on the speciation diagram of ZnO [25], Zn(OH)+ ion is a dominant species at pH 8. It can interact with OH− easily to form Zn(OH)2 and this species is the most prevalent at pH = 10 in the speciation diagram. However, using the simple calculation at 25 °C, the maximum Zn(OH)2 precipitation was achieved only when the pH was between 8 and 11 [21]. From the as-prepared ZnO at pH 8 and 10 without SDS molecules, no secondary phase other than ZnO was observed in the XRD patterns but the existence of both ZnO and Zn(OH)2 phases can be detected in the as-powder from the SDSmodified Zn(C2H3O2)2.2H2O solution. We propose that the SDS molecules are probably adsorbed onto the Zn(OH)2 surface and this stabilizes the Zn(OH)2 to depress its conversion to ZnO. However, this species can be destabilized by its dissolution at a higher pH as a competition reaction. Then, the ZnO phase is the more prevalent as
Fig. 4. TGA curve of as-powder prepared from the SDS-modofied Zn2+ solution at pH = 8.
seen from the XRD pattern for the sample at pH = 10. In the absence of SDS molecules, there is no stabilizer therefore Zn(OH)2 can be transformed to ZnO. This is a reason why this phase cannot be seen in the XRD patterns. P. Li et al. [12] prepared the ZnO microparticles with an SDS additive from aqueous solution at pH ∼ 9. Using 101 °C for 4 h, all the diffraction peaks are well assigned to the hexagonal ZnO and no characteristic peaks of Zn(OH)2 are observed. This result is maybe a consequence of the increasing dissolution rate at a higher reaction temperature. Zn(OH)2 is indeed a good adsorbent and possibly adsorbs the SDS molecules. This adsorption can be detected by FT-IR spectrophotometry. The comparison of the IR transmittance spectra of SDS and all aspowders with the SDS-modified Zn(C2H3O2)2·2H2O solution at different pH values is displayed in Fig. 6. The band at around 1200 cm−1 is assigned to an S = O stretching vibration of –SO4 from the SDS molecule [4,26]. The peaks at around 2920, 2852, 1468 and
Fig. 5. SEM of calcinated ZnO particles prepared at a mole ratio of Zn2+:SDS = 1:1 from the SDS-modified Zn2+ solutions at (a) pH = 8, (b) pH = 10, (c) pH = 12 (as-powder) and (d) pH = 12 (as-powder, FE–SEM).
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Fig. 6. FT-IR spectra of SDS (a) and as-powders prepared from the SDS-modified Zn2+ solutions at (b) pH = 8, (c) pH = 10 and (d) pH = 12.
1083 cm−1 are due to C–H stretching and bending. A broadening peak at around 3440 cm−1 is proposed to be due to H–OH stretching. The peak at 470 cm−1 is a characteristic peak of Zn–O stretching [4], and is not observed in the spectrum of pure SDS. It is noteworthy that no characteristic peak of SDS is observed from the IR spectrum of the ZnO particles prepared from the solution at pH 12. The results are in good agreement with the results from the XRD previously mentioned. From these results, the characteristic vibrational frequencies of SDS can be found in the samples prepared from pH 8 and 10 only. This confirms the adsorption of SDS by Zn(OH)2. On the other hand, if the rate of Zn (OH)2 dissolution to form Zn(OH)2− is very fast, the SDS molecules 4 will not be adsorbed because of the electrostatic repulsion between the negative charge of the hydrophilic groups of SDS and Zn(OH)2− 4 species. Then, no SDS was detected in the as-prepared sample in the case of pH = 12. On its face, ZnO is a polar crystal with a positive polar (0001) plane rich in Zn2+ cations and a negative polar (0001 ̅) plane rich in O2− anions. In the solution method, the velocity of crystal growth in the different directions is reported to be: V(0001) N V(1 ̅011 ̅) N V(1 ̅010) N V(1 ̅011) N V(0001 ̅) [12]. The hydrophilic group of SDS at pH 8 and pH 10 may adsorb with Zn2+ in the Zn(OH)2 due to electrostatic interaction. After Zn(OH)2 transforms to ZnO, this position is in the (0001) plane of the ZnO. It means that growth (0001) along c-axis must be retarded. Furthermore, the growth along the opposite face cannot be favorable for the lowest growth rate on the (0001 ̅) face. It could be grown in the ab or prism plane and then the hexagonal prism-like shape can be formed. If the rate of Zn(OH)2 dissolution is faster, the smaller diameter of the hexagonal prism shape may be formed (pH = 10). The agglomerated rod-like structure was formed distinctly at pH 12 as shown in Fig. 5(d). It was observed that small rod-like structures were mostly made up of particles by self-alignment of the spherical particles [27]. Moreover, we also found that the individual rod-like structure within a local region seemed to be randomly oriented. In the case of pH 12, SDS cannot be adsorbed onto the growth unit (Zn (OH)2− 4 ) because of the electrostatic repulsion, and then growth along the c-axis of the anisotropic crystal is favored [28]. This is in agreement with other publications in which no hexagonal prism shape (high diameter and short length) could be formed at a high pH [17]. The incorporation of SDS in as-prepared ZnO powder (pH 12) is also not detected in the XRD pattern and IR spectrum.
Fig. 7. Absorption spectra (a) and evolution of (αhv)2 vs. hv (b) of ZnO at pH = 8–12.
to the conduction band. The estimated direct-band gap according to the K–M model was calculated from a plot of (αhv)2 vs. photon energy (hv) as shown in Fig. 7(b). Extrapolation of the linear part until it intersects the hv-axis gives Eg. The estimated band gap energies are 3.205, 3.207 and 3.257 eV for the ZnO powder prepared at pH= 8, 10 and 12, respectively. The blue shift in the absorption edge is observed for the samples formed with the increase of pH due to their changing morphologies and surface microstructures. The estimated band gap energies of ZnO at pH= 8 and 10 are in the same order of another ZnO hexagonal prism shape that was prepared using PSS as a stabilizer (less than 3.21 eV) [5]. Comparison of the room temperature PL spectra of ZnO particles prepared at different pH is presented in Fig. 8. The entire PL spectra
3.3. Optical properties The UV–Vis diffuse reflectance spectra of ZnO samples prepared with the mole ratio of Zn2+:SDS= 1:1 at pH 8–12 are shown in Fig. 7(a). The absorption below 400 nm is assigned to the intrinsic band gap absorption of ZnO, due to electron transitions from the valence band
Fig. 8. Room temperature PL spectra of ZnO particles at pH = 8–12.
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show two emission peaks. The UV emission peak centered at about 390 nm is well understood as being the near-band-edge emission, whereas the broad visible emission originates from a variety of deep level defects, e.g. oxygen vacancies and zinc interstitials [13]. The diameter of the hexagonal face decreases with the increasing pH and changes to a rod-like shape (agglomerated by sphere-like shape) indicating that there is an increase of the surface area of its particle. With a higher surface area, the photogenerated holes in the valence band are possibly trapped into the surface defects and are increasingly returned to oxygen vacancies. Therefore the intensity of emission in the visible region increased with the increase of the surface area [13]. The intensity of emission in the visible region also increases as a function of pH. 4. Conclusion From the experimental results we proposed that the rate of Zn(OH)2 dissolution to form ZnO particles was inhibited by the adsorption of SDS molecules onto the Zn(OH)2 surface. Both Zn(OH)2 and ZnO phases were detected in the XRD from the as-powder prepared from the SDSmodified Zn(C2H3O2)2·2H2O solution at pH 8. However, this stabilized species can undergo more dissolution when the pH of the solution is increased. The observed Zn(OH)2 phase must decrease as a function of the increase in pH and only a single phase of ZnO was found at pH 12. At this point, the SDS molecules cannot adsorb onto the Zn(OH)2− as a 4 growth unit, because of the electrostatic repulsion. As the pH of the solution is increased, the particle sizes of ZnO are observed to decrease. A blue shift in the absorption edge was found. The increase of the energy band gap results in a slight blue shift of the PL emission. From this work, the size and growth direction of the ZnO particles can be controlled by the addition of SDS that is adsorbed onto the intermediate colloidal species surface. At a low pH, the hexagonal prism shape is formed. This makes it useful as a photocatalyst. The increase of the hexagonal facet (0001) plane of ZnO increases the photocatalytic RhB degradation [29]. The high band gap of ZnO can be found at high pH. This makes it applicable for many photophysical frame work reactions such as a modified TiO2 electrode in the dye sensitized solar cell (DSSC) and as a UV absorber etc. Acknowledgements Financial support from the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education is gratefully acknowledged. We would like to thank Dr. Brian Hodgson for English corrections. References [1] A.D. Hardy, H.H. Sutherland, R. Vaishnav, A study of the composition of some eye cosmetics (kohls) used in the United Arab Emirates, J. Ethnophrma. 80 (2002) 137–145.
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