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Wet chemistry synthesis of ZnO crystals with hexamethylenetetramine(HMTA): Understanding the role of HMTA in the formation of ZnO crystals
Materials Science in Semiconductor Processing 41 (2016) 462–469
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Wet chemistry synthesis of ZnO crystals with hexamethylenetetramine (HMTA): Understanding the role of HMTA in the formation of ZnO crystals Weiliang Feng a, Baocai Wang b, Pei Huang b,n, Xiaodong Wang b, Juan Yu b, Cunwen Wang a,nn a
School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, Hubei, PR China State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2015 Received in revised form 14 October 2015 Accepted 14 October 2015
ZnO crystals have been prepared via a simple aqueous solution route using zinc acetate-hexamethylenetetramine (HMTA) solution. To understand the role of HMTA in the formation processes of ZnO crystals, concentration measurements of Zn(II) remaining in the solution as well as SEM and XRD analyzes of the solid product have been made at regular intervals. The results demonstrated that the HMTA concentration has determinative effects on the growth rate of obtained ZnO products by controlling the composition of zinc complex species and mediating the transformation rate of building blocks. XRD results revealed the single phase nature with the wurtzite structure of the as prepared ZnO rods. X-ray photoelectron spectroscopy confirmed high purity of the ZnO rods. By varying the concentration of HMTA in the solution, the average size of rods was varied from 5 μm to 10 μm. Photoluminescence spectra indicated that the photoluminescent intensity of the emission peak at 380 nm increase to different degree with increasing HMTA concentration. These results revealed that the concentration of HMTA plays a vital role to control the properties of ZnO crystals. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide HMTA Transformation process Crystal growth
1. Introduction One-dimensional ZnO crystals possessing a direct electron pathway and high surface-to-volume ratio have received intensive attention due to their high electron mobility and novel size-dependent optical properties [1]. The various morphologies of ZnO 1D nanostructures include nanowires [2], nanotubes [3], nanorods [4] and nanoflower-like [5]. In recent years, ZnO is a wide band gap (3.37 eV) semiconductor used for applications such as lightemitting diodes [6], field-effect transistors [7], photodetectors [8,9], gas sensors [10], bistable memory devices [11] and solar cells [12]. Among various approaches, the wet chemistry synthesis method gained attention because of its simple experimental setup, cost effectiveness, and good potential for scaling up [13]. With respect to vacuum techniques, the wet chemistry synthesis method has the drawbacks of lack of good control over growth kinetics, usually resulting in contrasting results reported in the literature [14]. Beyond the intrinsic limit of the solution phase n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Huang), [email protected] (C. Wang). nn
http://dx.doi.org/10.1016/j.mssp.2015.10.017 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
techniques, there is also a partial understanding of the whole process underlying the growth of nanostructures. As far as the wet chemistry synthesis of ZnO crystals is concerned, the role of the most used reducing agent, hexamethylenetetramine (HMTA), is currently under debate. Heo et al. [15] described that one-dimensional single crystal ZnO nanorods were successfully synthesized on ZnO seed layer substrates by wet chemical method. The effect of HMTA concentration on the functional properties of ZnO nanostructures was studied. However, the precise effect mechanism of HMTA concentration on the synthesis of ZnO nanostructures has not been revealed. Govender et al. [16] proposed that HMTA slowly releases the OH ions by its thermal decomposition to formaldehyde and ammonia. Ashfold et al. [17] reported that the rate of decomposition of HMTA at 90 °C calculated for a zincfree solution is the same as that observed experimentally in a solution containing zinc nitrate and HMTA, implying that HMTA decomposition does not depend on the reactions taking place during ZnO deposition and that HMTA is an effective pH buffer. By using attenuated total reflection Fourier transform infrared spectroscopy performed on a solution containing zinc nitrate and HMTA, Sugunan et al. [18] proposed that HMTA acts as a nonpolar chelating agent which preferentially attaches to the lateral facets of ZnO nanorods, inducing anisotropic growth along the c-axis.
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Fig. 1. The schematic diagram illustrating the ZnO crystals formation processes.
Thus, up to now it is not clear what the role of HMTA in ZnO rods growth wet chemistry synthesis method is. In order to investigate the role of HMTA in the formation of ZnO crystals, ZnO crystals were prepared via a simple aqueous solution route. Having monitored the growth processes of resultant products, the formation mechanism of ZnO crystals has been preliminary presented. In addition, optical properties of the ZnO crystals prepared with different HMTA concentrations were also investigated. Our aim is to explore the role of HMTA in the formation of ZnO crystals and present the formation mechanism of ZnO crystals at different molar ratios of Zn(CH3COO)2:HMTA
2. Experimental 2.1. Synthesis of ZnO A typical procedure for the preparation of ZnO crystals was as follows: the reaction solution of 100 mL of 20 mM zinc acetate and 100 mL of 20 mM HMTA was first prepared, while keeping the molar ratio¼1:1. After that, the solution was kept at room temperature for 0.5 h under vigorous stirring. Then the wet chemistry synthesis process was carried out at 85 °C for 3 h. During the reaction the pH of the solution was continually measured with a portable pH meter. The white powders were collected from the wet chemistry synthesis process at regular intervals and thoroughly washed with deionized water to eliminate residual salts, dried in air at 60 °C for 12 h at last. The growth process of ZnO crystals was monitored by determining the concentration of Zn(II) and the phase composition of precipitate. Other two series of samples at different molar ratios of zinc acetate and HMTA are 1:2 (20 mM zinc acetate and 40 mM HMTA) and 2:1 (20 mM zinc acetate and 10 mM HMTA) were prepared in our experiment, while other conditions were kept constant. The schematic diagram illustrating the ZnO crystals formation processes is shown in Fig. 1.
2.2. Characterization of the samples The pH was measured using a pH-meter (PHSJ-3F, Leici). The concentration of Zn(II) was determined with zinc portable photometer (HI96731, Hanna). The phase composition of the precipitate was recorded by X-ray powder diffraction (D8 Advance, Bruker) with a Cu Kɑ (λ ¼0.15406 nm) radiation source at 40 kV and 30 mA. The morphologies of the as-synthesized samples were observed using scanning electron microscopy (S-4800, Hitachi). X-ray photoelectron spectroscopy (XPS) were performed using a Vacuum Generator Mutilab 2000 spectrometer with an excitation source of Al Ka ¼ 1486.6 eV. Photoluminescence (PL) measurements were carried out on a Varian Cary Eclipse spectrophotometer by a 325 nm excitation from Xe lamp at room temperature.
Fig. 2. pH value in the solution as a function of heating time for different molar ratios of Zn2 þ : HMTA: (a)-2:1, (b)-1:1, and (c)-1:2.
3. Results and discussion 3.1. The role of HMTA during ZnO crystals grown ZnO crystals were fabricated by wet chemistry synthesis method from a aqueous solution containing HMTA precursor. The set of overall reactions most often referenced in the literature for ZnO crystals deposition is [19].
C6 H12 N4 + 6H2 O → 6CH2 O + 4NH3
(1)
NH3 + H2 O ↔ NH4 + + OH−
(2)
2OH− + Zn2 + ↔ ZnO + H2 O
(3)
HMTA decompose upon heating to form formaldehyde and ammonia (Eq. (1)). Ammonia reacts with water to produce OH (Eq. (2)), which drives the crystallization of ZnO (Eq. (3)). It is a general acceptance that HMTA provide continuous source of hydroxide to drive the precipitation reaction [20]. The role of the pH value in ZnO crystals growth by wet chemistry synthesis method has been largely discussed, concluding that by changing pH value different ZnO nanostructures can be formed. Fig. 2 shows the pH values of a 20 mM solution of Zn(CH3COO)2 with 10–40 mM concentration of HMTA. Without any amine, the zinc acetate solution is acidic (pH¼ 6.0) due to the hydrolysis of Zn2 þ (aq) complex. HMTA shows a strong pH regulation activity, quickly stabilizing the pH after heating 1.5 h at 85 °C (Fig. 2a–c). Söztürk et al. suggested that HMTA primary role in the wet chemistry synthesis process is as a pH buffer [21]. The present results lend support to such proposal. HMTA can slow release of OH group, consistent with the observed stable pH of the reactive solutions at longer time in the present experiments. 3.2. Effect of HMTA concentration According to the present study, the HMTA concentration determines not only the composition of the precursor but also the
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Fig. 3. The SEM image and XRD pattern of white solids synthesized at room temperature for 30 min (Zn2 þ :HMTA ¼1:2).
morphology and size of the final product. Before the heating step, the reaction solutions are clear all along when Zn2 þ :HMTA¼1:1 and 2:1 (Zn(II)¼ 20 mM), while some white solids appear when Zn2 þ :HMTA ¼1:2. As can be seen from SEM image in Fig. 3a, the morphologies of the solids at Zn2 þ :HMTA¼1:2 are irregular shapes. All of them can be indexed to Zn(OH)2 (JCPDS no. 89–0138) by the diffraction peaks positions in the XRD spectrum (Fig. 3b). In the synthesis systems, HMTA serves as a pH buffer to release OH ; OH subsequently reacts with Zn2 þ to form Zn(OH)2 or other soluble zinc hydroxide species. Therefore, it might be concluded that the precursors are Zn(OH)2-zinc hydroxide species mixture for Zn2 þ :HMTA¼ 1:2 under the present conditions. Coincidentally, Zn(OH)2 serving as an adjuster are important units for the wet chemistry synthesis of ZnO, the reason for which will be discussed later.
Fig. 4 show the representative SEM images and XRD patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. Fig. 4a–c shows the SEM images of ZnO rods with different molar ratios of Zn2 þ :HMTA, (a) 2:1, (b) 1:1 and (c) 1:2 synthesized by wet chemistry at constant reaction temperature of 85 °C for reaction time of 3 h. The average rods sizes were found to be 5, 7 and 10 μm for particles synthesized by varying HMTA concentrations of 10, 20 and 40 mM, respectively. It can be seen that most of the ZnO samples with different HMTA concentration show rod-like shape, with the only difference being the size of the ZnO rods. It indicates that the concentration of HMTA is the important factor that governed the size of the rods, due to the critical diffusion of the monomers and subsequent limited growth [22]. Fig. 4d shows the XRD patterns of the products synthesized with
Fig. 4. SEM images and XRD patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. (a)-2:1, (b)-1:1, and (c)-1:2.
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Table 1 The lattice parameters of ZnO samples with different HMTA concentration. Sample
a (nm)
c (nm)
Zn2 þ :HMTA ¼ 2:1 Zn2 þ :HMTA ¼ 1:1 Zn2 þ :HMTA ¼ 1:2
0.315 0.317 0.32
0.513 0.514 0.517
different concentration of HMTA. The patterns are in accordance with the typical wurtzite hexagonal structure in the reference data (JCPDS no. 36-1451). No other uncertain diffraction peaks are represented, suggesting that high-purity ZnO products have been synthesized.
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The lattice constants for hexagonal crystal structure are expressed by the following relation [23,24]:
1 4 (h2 + hk + k 2) l2 = + 2 2 2 3a d c
(4)
1 4 sin2 θ = 2 d λ2
(5)
where d is the lattice spacing parameter, hlk are the miller indices, ‘a’ and ‘c’ are the lattice parameters, λ is the wavelength of the X-ray source, θ is the Bragg angle of diffraction peak. The lattice parameters of ZnO samples with different HMTA concentration are shown in Table 1.
Fig. 5. XPS patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. (a–a′)-2:1, (b–b′)-1:1, and (c–c′)-1:2.
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Fig. 6. SEM images, XRD patterns and PL spectra of the time-dependent evolution in the formation of ZnO crystals synthesized with different molar ratios of Zn2 þ :HMTA:(a– f)-1:2, (a′–f′)-2:1.
The XPS spectra of Fig. 5 can provide further significant information about the quality and composition of the crystals prepared at different molar ratios of zinc acetate:HMTA. Binding were corrected for specimen charging, through referencing the C1s to 285.0 eV. XPS measurement of ZnO rods revealed the binding state of Zn and O in the crystals and also the stoichiometric nature of the ZnO. In three cases, from Fig. 5(a–c), two strong peaks centered on 1021.1 and 1044.2 eV, which are in agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively. These results indicate that the chemical valence of Zn at the surface of ZnO rods is þ2 oxidation state[25]. However, the deconvolution of the XPS spectra for the O1s core level line is shown in Fig. 5(a′–c′) for different molar ratios of zinc acetate:HMTA, respectively. The deconvolution of those bands shows the presence of three different peaks. The first peak on the low binding energy side of the O1s
spectra can be attributed to the O2 ions in the ZnO lattice [26]. The second peak is assigned to OH group absorbed onto the surface of the ZnO rods [27]. The higher binding energy peak is usually attributed to H2O [28]. To further understand the role of HMTA in the formation processes of these ZnO crystals, concentration measurements of Zn(II) remaining in the solution as well as SEM and XRD analyzes of the solid product have been made at regular intervals throughout each reaction. Fig. 6(a–d) describe the morphology of the products with the reaction at the molar ratio of Zn2 þ :HMTA ¼ 1:2 for 30, 60, 90 and 120 min, respectively. Fig. 6a shows that a part of rod-like ZnO and the irregular shape of Zn(OH)2 agglomerates coexist after 30 min. With the reaction time increasing, the morphology is less irregular shape and more rod-like (Fig. 6b). When reaction time is
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prolonged to 90 min, the products are composed solely of ZnO rods (Fig. 6c), while the Zn(OH)2 precursors disappear. Further increasing of the reaction time, no obvious distinction has been observed in the morphology of the ZnO rods (Fig. 6d). The corresponding XRD patterns described in Fig. 6e shows that the discernible peaks of ZnO appear from 30 min; moreover, ZnO alone up to 90 min. Fig. 6f demonstrates that the PL peak intensity in the UV gradually increased with reaction time. It indicated the samples have high crystal quality, which is well matched with the obtained XRD results (Fig. 6e). The formation process of ZnO rods synthesized at Zn2 þ :HMTA ¼2:1 is slight different from that at Zn2 þ :HMTA ¼1:2. It is observed that precipitation took place ca. 30 min after heating. A low-magnification view of the products shown in Fig. 6a′ demonstrates that the products are composed of uniform rod-like structures with an average diameter of 500 nm. Further increasing the reaction time, no obvious distinction had been observed in the morphology of the ZnO rods (Fig. 6b′–d′). Fig. 6e′ shows the XRD patterns of the as-prepared products. All the peaks can be readily indexed to hexagonal wurtzite ZnO. Peak sharpening, indicating an increase in the sample crystallinity is observed as the heating time increased. The PL spectra of ZnO samples synthesized at Zn2 þ :HMTA ¼ 2:1 as a function of reaction time show similar PL features(Fig. 6f′). Very similar results are obtained for the molar ratios of Zn2 þ :HMTA ¼1:1 (Fig. S1). In addition, a plot of the concentration of the Zn(II) in reactive solution as a function of growth time is shown in Fig. 7. During the first 0.5 h reaction at this room temperature, the concentration of Zn(II) in solution (Zn2 þ :HMTA ¼1:2) decreases from 20 mM to 17 mM, subsequently decreasing much more fast during the heating process. However, on examining the curve (Zn2 þ :HMTA ¼2:1), it is evident that the concentration of Zn(II) in solution changes mildly in the heating process and levells off at 13 mM after 90 min. The similar slowly change of trend of concentration of Zn(II) also occurs at Zn2 þ :HMTA ¼ 1:1. It is very interesting to notice that the change of trend of concentration of Zn(II) (Zn2 þ :HMTA ¼1:2) different from those of Zn2 þ :HMTA ¼1:1 and 2:1. It may be concluded that this relate to different precursors. Zhao et al. reported the white precipitates (Zn(OH)2) which could act as nuclei for ZnO growth are formed immediately when the solutions of Zn(NO3)2 and HMTA are mixed [29].Thus, at higher HMTA concentration, the white precipitates mentioned above lead to the higher nucleation rate and growth rate. The concentration of Zn(II) is consumed too fast for growth of ZnO when molar ratios of Zn2 þ :HMTA ¼ 1:2.
Fig. 8. Graph of fraction f of Zn(II) existing as Zn2 þ (aq), Zn(OH) (aq), Zn(OH)2(aq), Zn(OH)3 , and Zn(OH)42 (aq) over a range of pH at 25 � [32].
3.3. The growth mechanization at different HMTA concentration In dilute solutions, zinc(II) can exist as several monomeric hydroxyl species [30]. These species include ZnOH þ (aq), Zn(OH)2 (aq), Zn(OH)3 (aq), and Zn(OH)42 (aq). At a given zinc(II) concentration, the stability of these complexes is dependent on pH and temperature of the solution [31]. In our system, the solution (Zn2 þ :HMTA ¼ 1:1 and 2:1) has a pH that stabilizes at 6.34–6.46, it is reasonable to expect that large numbers of Zn2 þ (aq) ions along with few Zn(OH) þ (aq) solute species in the solution from the phase diagram (Fig. 8) [32]. This postulation is consistent with many reports [33–35] showing that the aqueous solution is stable. The following main reactions are involved in the crystal growth of ZnO in the solution [36].
Zn2 + + OH− ↔ Zn (OH)+
(6)
Zn (OH)+ + OH− ↔ Zn (OH)2 ↓
(7)
Zn (OH)2 ↓ ↔ Zn (OH)2 (aq)
(8)
Zn (OH)2 ↓ + OH− ↔ Zn (OH)3−
(9)
Zn (OH)2 ↓ + 2OH− ↔ Zn (OH)42 −
(10)
With the continual increases in the reaction temperature, HMTA is believed to decompose quickly and the concentration of OH also rapidly increases. Here, Zn2 þ or Zn(OH) þ are known to react readily with OH to form more soluble predominantly Zn(OH)42 complexes under given reaction conditions, which act as the growth unit of ZnO crystals. When the concentration of zinc hydroxide complex the formation of reached supersaturation, under the thermal conditions, the reaction between the Zn(OH)42 species leads to large zinc hydroxide species Zn2O(OH)64 , which form the ZnO crystal nuclei, as described by the following equation [37]:
Zn (OH)42 − + Zn (OH)42 − ↔ Zn2 O (OH)64 − + H2 O
Fig. 7. Zn(II) concentration in the solution as a function of time for different molar ratios of Zn2 þ :HMTA at 85 °C.
467
(11)
The crystal structure of ZnO was constructed by this dehydration between OH on the surface of the growing crystals and the OH ligands of the hydroxyl complexes. However, when molar ratios of Zn2þ : HMTA¼ 1:2, the solution became turbid due to the formation of white Zn(OH)2 precipitation. Very recently, the growth of well-defined ZnO crystals from Zn(OH)2 with solution-phase method has been reported, in which most authors suggested that such transformations may take place by
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take place by the dissolution–reprecipitation mechanism. XRD results revealed the single phase nature with the wurtzite structure of the as prepared ZnO rods. X-ray photoelectron spectroscopy confirmed high purity of the ZnO rods. By varying the concentration of HMTA in the solution, the average size of rods was varied from 5 μm to 10 μm. The ZnO crystals prepared at different HMTA concentrations show similar PL features in the present study. These results revealed that the concentration of HMTA plays a vital role to control the properties of ZnO crystals.
Acknowledgments This work was supported by the Youths Science Foundation of Wuhan Institute of Technology (No. K201454). Fig. 9. Room-temperature photoluminescence of ZnO at different molar ratios of Zn2 þ : HMTA : (a)-2:1, (b)-1:1, and (c)-1:2.
dissolution-reprecipitation [38–40]. During the wet chemistry synthesis process at high temperature, the Zn(OH)2 colloids gradually dissolves into zinc soluble species according to Eqs. (7–9). The subsequent growth then takes place in a regular well-controlled fashion. Owing to the same initial concentration of Zn(II) in every reaction system (20 mM), it is speculated that the superabundant alkali and relatively high heating temperature enable the regular Zn(OH)2, serving as an adjuster [41,42], to control the amounts of Zn(OH)42 in the reaction solution. It should be stated that essential understanding of this demands more in-depth experimental and theoretical work, and further study is under way. 3.4. Optical properties
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2015.10.017.
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The typical room-temperature photoluminescence (PL) spectra of the ZnO crystals were obtained at different concentrations of HMTA (Fig. 9). The PL spectra show similar PL features, in which two obvious luminescence bands were observed, including a strong UV, a weak blue emission bands centered at about 380 nm and 430 nm, respectively. The strong UV emission band results from near-band-gap emission, namely the recombination of free excitons through an exciton–exciton collision process [43]. The weak blue emission peak can be due to the existence of intrinsic defects in the ZnO [44]. Furthermore, the strong UV emission band and weak visible emission band indicate that the as-synthesized ZnO samples have high crystal quality and low concentration of defect. It also can be seen clearly that the intensity of peak in the spectra is a 4b 4c. Combining with the previous XRD results, it might be concluded that this result is related to the crystal quality of ZnO samples [45]. Currently, there are reports that ZnO samples have more perfect crystallization, the UV emission would has much sharper peak [46–48]. When the molar ratios of Zn2 þ : HMTA were changed from 2:1 to 1:2, the PL peak intensity in the UV gradually increased (Fig. 9(b,c)). It indicated the samples have high crystal quality, which is well matched with the obtained XRD results (Fig. 4d).
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In summary, ZnO crystals were prepared via a simple aqueous solution route. Experimental results showed that the HMTA concentration has determinative effects on the growth rate of obtained ZnO products by controlling the composition of zinc complex species and mediating the transformation rate of building blocks. The phase transformation from Zn(OH)2 to ZnO crystal may
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Wet chemistry synthesis of ZnO crystals with hexamethylenetetramine (HMTA): Understanding the role of HMTA in the formation of ZnO crystals Weiliang Feng a, Baocai Wang b, Pei Huang b,n, Xiaodong Wang b, Juan Yu b, Cunwen Wang a,nn a
School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, Hubei, PR China State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2015 Received in revised form 14 October 2015 Accepted 14 October 2015
ZnO crystals have been prepared via a simple aqueous solution route using zinc acetate-hexamethylenetetramine (HMTA) solution. To understand the role of HMTA in the formation processes of ZnO crystals, concentration measurements of Zn(II) remaining in the solution as well as SEM and XRD analyzes of the solid product have been made at regular intervals. The results demonstrated that the HMTA concentration has determinative effects on the growth rate of obtained ZnO products by controlling the composition of zinc complex species and mediating the transformation rate of building blocks. XRD results revealed the single phase nature with the wurtzite structure of the as prepared ZnO rods. X-ray photoelectron spectroscopy confirmed high purity of the ZnO rods. By varying the concentration of HMTA in the solution, the average size of rods was varied from 5 μm to 10 μm. Photoluminescence spectra indicated that the photoluminescent intensity of the emission peak at 380 nm increase to different degree with increasing HMTA concentration. These results revealed that the concentration of HMTA plays a vital role to control the properties of ZnO crystals. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide HMTA Transformation process Crystal growth
1. Introduction One-dimensional ZnO crystals possessing a direct electron pathway and high surface-to-volume ratio have received intensive attention due to their high electron mobility and novel size-dependent optical properties [1]. The various morphologies of ZnO 1D nanostructures include nanowires [2], nanotubes [3], nanorods [4] and nanoflower-like [5]. In recent years, ZnO is a wide band gap (3.37 eV) semiconductor used for applications such as lightemitting diodes [6], field-effect transistors [7], photodetectors [8,9], gas sensors [10], bistable memory devices [11] and solar cells [12]. Among various approaches, the wet chemistry synthesis method gained attention because of its simple experimental setup, cost effectiveness, and good potential for scaling up [13]. With respect to vacuum techniques, the wet chemistry synthesis method has the drawbacks of lack of good control over growth kinetics, usually resulting in contrasting results reported in the literature [14]. Beyond the intrinsic limit of the solution phase n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Huang), [email protected] (C. Wang). nn
http://dx.doi.org/10.1016/j.mssp.2015.10.017 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
techniques, there is also a partial understanding of the whole process underlying the growth of nanostructures. As far as the wet chemistry synthesis of ZnO crystals is concerned, the role of the most used reducing agent, hexamethylenetetramine (HMTA), is currently under debate. Heo et al. [15] described that one-dimensional single crystal ZnO nanorods were successfully synthesized on ZnO seed layer substrates by wet chemical method. The effect of HMTA concentration on the functional properties of ZnO nanostructures was studied. However, the precise effect mechanism of HMTA concentration on the synthesis of ZnO nanostructures has not been revealed. Govender et al. [16] proposed that HMTA slowly releases the OH ions by its thermal decomposition to formaldehyde and ammonia. Ashfold et al. [17] reported that the rate of decomposition of HMTA at 90 °C calculated for a zincfree solution is the same as that observed experimentally in a solution containing zinc nitrate and HMTA, implying that HMTA decomposition does not depend on the reactions taking place during ZnO deposition and that HMTA is an effective pH buffer. By using attenuated total reflection Fourier transform infrared spectroscopy performed on a solution containing zinc nitrate and HMTA, Sugunan et al. [18] proposed that HMTA acts as a nonpolar chelating agent which preferentially attaches to the lateral facets of ZnO nanorods, inducing anisotropic growth along the c-axis.
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Fig. 1. The schematic diagram illustrating the ZnO crystals formation processes.
Thus, up to now it is not clear what the role of HMTA in ZnO rods growth wet chemistry synthesis method is. In order to investigate the role of HMTA in the formation of ZnO crystals, ZnO crystals were prepared via a simple aqueous solution route. Having monitored the growth processes of resultant products, the formation mechanism of ZnO crystals has been preliminary presented. In addition, optical properties of the ZnO crystals prepared with different HMTA concentrations were also investigated. Our aim is to explore the role of HMTA in the formation of ZnO crystals and present the formation mechanism of ZnO crystals at different molar ratios of Zn(CH3COO)2:HMTA
2. Experimental 2.1. Synthesis of ZnO A typical procedure for the preparation of ZnO crystals was as follows: the reaction solution of 100 mL of 20 mM zinc acetate and 100 mL of 20 mM HMTA was first prepared, while keeping the molar ratio¼1:1. After that, the solution was kept at room temperature for 0.5 h under vigorous stirring. Then the wet chemistry synthesis process was carried out at 85 °C for 3 h. During the reaction the pH of the solution was continually measured with a portable pH meter. The white powders were collected from the wet chemistry synthesis process at regular intervals and thoroughly washed with deionized water to eliminate residual salts, dried in air at 60 °C for 12 h at last. The growth process of ZnO crystals was monitored by determining the concentration of Zn(II) and the phase composition of precipitate. Other two series of samples at different molar ratios of zinc acetate and HMTA are 1:2 (20 mM zinc acetate and 40 mM HMTA) and 2:1 (20 mM zinc acetate and 10 mM HMTA) were prepared in our experiment, while other conditions were kept constant. The schematic diagram illustrating the ZnO crystals formation processes is shown in Fig. 1.
2.2. Characterization of the samples The pH was measured using a pH-meter (PHSJ-3F, Leici). The concentration of Zn(II) was determined with zinc portable photometer (HI96731, Hanna). The phase composition of the precipitate was recorded by X-ray powder diffraction (D8 Advance, Bruker) with a Cu Kɑ (λ ¼0.15406 nm) radiation source at 40 kV and 30 mA. The morphologies of the as-synthesized samples were observed using scanning electron microscopy (S-4800, Hitachi). X-ray photoelectron spectroscopy (XPS) were performed using a Vacuum Generator Mutilab 2000 spectrometer with an excitation source of Al Ka ¼ 1486.6 eV. Photoluminescence (PL) measurements were carried out on a Varian Cary Eclipse spectrophotometer by a 325 nm excitation from Xe lamp at room temperature.
Fig. 2. pH value in the solution as a function of heating time for different molar ratios of Zn2 þ : HMTA: (a)-2:1, (b)-1:1, and (c)-1:2.
3. Results and discussion 3.1. The role of HMTA during ZnO crystals grown ZnO crystals were fabricated by wet chemistry synthesis method from a aqueous solution containing HMTA precursor. The set of overall reactions most often referenced in the literature for ZnO crystals deposition is [19].
C6 H12 N4 + 6H2 O → 6CH2 O + 4NH3
(1)
NH3 + H2 O ↔ NH4 + + OH−
(2)
2OH− + Zn2 + ↔ ZnO + H2 O
(3)
HMTA decompose upon heating to form formaldehyde and ammonia (Eq. (1)). Ammonia reacts with water to produce OH (Eq. (2)), which drives the crystallization of ZnO (Eq. (3)). It is a general acceptance that HMTA provide continuous source of hydroxide to drive the precipitation reaction [20]. The role of the pH value in ZnO crystals growth by wet chemistry synthesis method has been largely discussed, concluding that by changing pH value different ZnO nanostructures can be formed. Fig. 2 shows the pH values of a 20 mM solution of Zn(CH3COO)2 with 10–40 mM concentration of HMTA. Without any amine, the zinc acetate solution is acidic (pH¼ 6.0) due to the hydrolysis of Zn2 þ (aq) complex. HMTA shows a strong pH regulation activity, quickly stabilizing the pH after heating 1.5 h at 85 °C (Fig. 2a–c). Söztürk et al. suggested that HMTA primary role in the wet chemistry synthesis process is as a pH buffer [21]. The present results lend support to such proposal. HMTA can slow release of OH group, consistent with the observed stable pH of the reactive solutions at longer time in the present experiments. 3.2. Effect of HMTA concentration According to the present study, the HMTA concentration determines not only the composition of the precursor but also the
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Fig. 3. The SEM image and XRD pattern of white solids synthesized at room temperature for 30 min (Zn2 þ :HMTA ¼1:2).
morphology and size of the final product. Before the heating step, the reaction solutions are clear all along when Zn2 þ :HMTA¼1:1 and 2:1 (Zn(II)¼ 20 mM), while some white solids appear when Zn2 þ :HMTA ¼1:2. As can be seen from SEM image in Fig. 3a, the morphologies of the solids at Zn2 þ :HMTA¼1:2 are irregular shapes. All of them can be indexed to Zn(OH)2 (JCPDS no. 89–0138) by the diffraction peaks positions in the XRD spectrum (Fig. 3b). In the synthesis systems, HMTA serves as a pH buffer to release OH ; OH subsequently reacts with Zn2 þ to form Zn(OH)2 or other soluble zinc hydroxide species. Therefore, it might be concluded that the precursors are Zn(OH)2-zinc hydroxide species mixture for Zn2 þ :HMTA¼ 1:2 under the present conditions. Coincidentally, Zn(OH)2 serving as an adjuster are important units for the wet chemistry synthesis of ZnO, the reason for which will be discussed later.
Fig. 4 show the representative SEM images and XRD patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. Fig. 4a–c shows the SEM images of ZnO rods with different molar ratios of Zn2 þ :HMTA, (a) 2:1, (b) 1:1 and (c) 1:2 synthesized by wet chemistry at constant reaction temperature of 85 °C for reaction time of 3 h. The average rods sizes were found to be 5, 7 and 10 μm for particles synthesized by varying HMTA concentrations of 10, 20 and 40 mM, respectively. It can be seen that most of the ZnO samples with different HMTA concentration show rod-like shape, with the only difference being the size of the ZnO rods. It indicates that the concentration of HMTA is the important factor that governed the size of the rods, due to the critical diffusion of the monomers and subsequent limited growth [22]. Fig. 4d shows the XRD patterns of the products synthesized with
Fig. 4. SEM images and XRD patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. (a)-2:1, (b)-1:1, and (c)-1:2.
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Table 1 The lattice parameters of ZnO samples with different HMTA concentration. Sample
a (nm)
c (nm)
Zn2 þ :HMTA ¼ 2:1 Zn2 þ :HMTA ¼ 1:1 Zn2 þ :HMTA ¼ 1:2
0.315 0.317 0.32
0.513 0.514 0.517
different concentration of HMTA. The patterns are in accordance with the typical wurtzite hexagonal structure in the reference data (JCPDS no. 36-1451). No other uncertain diffraction peaks are represented, suggesting that high-purity ZnO products have been synthesized.
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The lattice constants for hexagonal crystal structure are expressed by the following relation [23,24]:
1 4 (h2 + hk + k 2) l2 = + 2 2 2 3a d c
(4)
1 4 sin2 θ = 2 d λ2
(5)
where d is the lattice spacing parameter, hlk are the miller indices, ‘a’ and ‘c’ are the lattice parameters, λ is the wavelength of the X-ray source, θ is the Bragg angle of diffraction peak. The lattice parameters of ZnO samples with different HMTA concentration are shown in Table 1.
Fig. 5. XPS patterns of ZnO samples with different molar ratios of Zn2 þ :HMTA at 85 °C for 3 h. (a–a′)-2:1, (b–b′)-1:1, and (c–c′)-1:2.
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Fig. 6. SEM images, XRD patterns and PL spectra of the time-dependent evolution in the formation of ZnO crystals synthesized with different molar ratios of Zn2 þ :HMTA:(a– f)-1:2, (a′–f′)-2:1.
The XPS spectra of Fig. 5 can provide further significant information about the quality and composition of the crystals prepared at different molar ratios of zinc acetate:HMTA. Binding were corrected for specimen charging, through referencing the C1s to 285.0 eV. XPS measurement of ZnO rods revealed the binding state of Zn and O in the crystals and also the stoichiometric nature of the ZnO. In three cases, from Fig. 5(a–c), two strong peaks centered on 1021.1 and 1044.2 eV, which are in agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively. These results indicate that the chemical valence of Zn at the surface of ZnO rods is þ2 oxidation state[25]. However, the deconvolution of the XPS spectra for the O1s core level line is shown in Fig. 5(a′–c′) for different molar ratios of zinc acetate:HMTA, respectively. The deconvolution of those bands shows the presence of three different peaks. The first peak on the low binding energy side of the O1s
spectra can be attributed to the O2 ions in the ZnO lattice [26]. The second peak is assigned to OH group absorbed onto the surface of the ZnO rods [27]. The higher binding energy peak is usually attributed to H2O [28]. To further understand the role of HMTA in the formation processes of these ZnO crystals, concentration measurements of Zn(II) remaining in the solution as well as SEM and XRD analyzes of the solid product have been made at regular intervals throughout each reaction. Fig. 6(a–d) describe the morphology of the products with the reaction at the molar ratio of Zn2 þ :HMTA ¼ 1:2 for 30, 60, 90 and 120 min, respectively. Fig. 6a shows that a part of rod-like ZnO and the irregular shape of Zn(OH)2 agglomerates coexist after 30 min. With the reaction time increasing, the morphology is less irregular shape and more rod-like (Fig. 6b). When reaction time is
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prolonged to 90 min, the products are composed solely of ZnO rods (Fig. 6c), while the Zn(OH)2 precursors disappear. Further increasing of the reaction time, no obvious distinction has been observed in the morphology of the ZnO rods (Fig. 6d). The corresponding XRD patterns described in Fig. 6e shows that the discernible peaks of ZnO appear from 30 min; moreover, ZnO alone up to 90 min. Fig. 6f demonstrates that the PL peak intensity in the UV gradually increased with reaction time. It indicated the samples have high crystal quality, which is well matched with the obtained XRD results (Fig. 6e). The formation process of ZnO rods synthesized at Zn2 þ :HMTA ¼2:1 is slight different from that at Zn2 þ :HMTA ¼1:2. It is observed that precipitation took place ca. 30 min after heating. A low-magnification view of the products shown in Fig. 6a′ demonstrates that the products are composed of uniform rod-like structures with an average diameter of 500 nm. Further increasing the reaction time, no obvious distinction had been observed in the morphology of the ZnO rods (Fig. 6b′–d′). Fig. 6e′ shows the XRD patterns of the as-prepared products. All the peaks can be readily indexed to hexagonal wurtzite ZnO. Peak sharpening, indicating an increase in the sample crystallinity is observed as the heating time increased. The PL spectra of ZnO samples synthesized at Zn2 þ :HMTA ¼ 2:1 as a function of reaction time show similar PL features(Fig. 6f′). Very similar results are obtained for the molar ratios of Zn2 þ :HMTA ¼1:1 (Fig. S1). In addition, a plot of the concentration of the Zn(II) in reactive solution as a function of growth time is shown in Fig. 7. During the first 0.5 h reaction at this room temperature, the concentration of Zn(II) in solution (Zn2 þ :HMTA ¼1:2) decreases from 20 mM to 17 mM, subsequently decreasing much more fast during the heating process. However, on examining the curve (Zn2 þ :HMTA ¼2:1), it is evident that the concentration of Zn(II) in solution changes mildly in the heating process and levells off at 13 mM after 90 min. The similar slowly change of trend of concentration of Zn(II) also occurs at Zn2 þ :HMTA ¼ 1:1. It is very interesting to notice that the change of trend of concentration of Zn(II) (Zn2 þ :HMTA ¼1:2) different from those of Zn2 þ :HMTA ¼1:1 and 2:1. It may be concluded that this relate to different precursors. Zhao et al. reported the white precipitates (Zn(OH)2) which could act as nuclei for ZnO growth are formed immediately when the solutions of Zn(NO3)2 and HMTA are mixed [29].Thus, at higher HMTA concentration, the white precipitates mentioned above lead to the higher nucleation rate and growth rate. The concentration of Zn(II) is consumed too fast for growth of ZnO when molar ratios of Zn2 þ :HMTA ¼ 1:2.
Fig. 8. Graph of fraction f of Zn(II) existing as Zn2 þ (aq), Zn(OH) (aq), Zn(OH)2(aq), Zn(OH)3 , and Zn(OH)42 (aq) over a range of pH at 25 � [32].
3.3. The growth mechanization at different HMTA concentration In dilute solutions, zinc(II) can exist as several monomeric hydroxyl species [30]. These species include ZnOH þ (aq), Zn(OH)2 (aq), Zn(OH)3 (aq), and Zn(OH)42 (aq). At a given zinc(II) concentration, the stability of these complexes is dependent on pH and temperature of the solution [31]. In our system, the solution (Zn2 þ :HMTA ¼ 1:1 and 2:1) has a pH that stabilizes at 6.34–6.46, it is reasonable to expect that large numbers of Zn2 þ (aq) ions along with few Zn(OH) þ (aq) solute species in the solution from the phase diagram (Fig. 8) [32]. This postulation is consistent with many reports [33–35] showing that the aqueous solution is stable. The following main reactions are involved in the crystal growth of ZnO in the solution [36].
Zn2 + + OH− ↔ Zn (OH)+
(6)
Zn (OH)+ + OH− ↔ Zn (OH)2 ↓
(7)
Zn (OH)2 ↓ ↔ Zn (OH)2 (aq)
(8)
Zn (OH)2 ↓ + OH− ↔ Zn (OH)3−
(9)
Zn (OH)2 ↓ + 2OH− ↔ Zn (OH)42 −
(10)
With the continual increases in the reaction temperature, HMTA is believed to decompose quickly and the concentration of OH also rapidly increases. Here, Zn2 þ or Zn(OH) þ are known to react readily with OH to form more soluble predominantly Zn(OH)42 complexes under given reaction conditions, which act as the growth unit of ZnO crystals. When the concentration of zinc hydroxide complex the formation of reached supersaturation, under the thermal conditions, the reaction between the Zn(OH)42 species leads to large zinc hydroxide species Zn2O(OH)64 , which form the ZnO crystal nuclei, as described by the following equation [37]:
Zn (OH)42 − + Zn (OH)42 − ↔ Zn2 O (OH)64 − + H2 O
Fig. 7. Zn(II) concentration in the solution as a function of time for different molar ratios of Zn2 þ :HMTA at 85 °C.
467
(11)
The crystal structure of ZnO was constructed by this dehydration between OH on the surface of the growing crystals and the OH ligands of the hydroxyl complexes. However, when molar ratios of Zn2þ : HMTA¼ 1:2, the solution became turbid due to the formation of white Zn(OH)2 precipitation. Very recently, the growth of well-defined ZnO crystals from Zn(OH)2 with solution-phase method has been reported, in which most authors suggested that such transformations may take place by
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take place by the dissolution–reprecipitation mechanism. XRD results revealed the single phase nature with the wurtzite structure of the as prepared ZnO rods. X-ray photoelectron spectroscopy confirmed high purity of the ZnO rods. By varying the concentration of HMTA in the solution, the average size of rods was varied from 5 μm to 10 μm. The ZnO crystals prepared at different HMTA concentrations show similar PL features in the present study. These results revealed that the concentration of HMTA plays a vital role to control the properties of ZnO crystals.
Acknowledgments This work was supported by the Youths Science Foundation of Wuhan Institute of Technology (No. K201454). Fig. 9. Room-temperature photoluminescence of ZnO at different molar ratios of Zn2 þ : HMTA : (a)-2:1, (b)-1:1, and (c)-1:2.
dissolution-reprecipitation [38–40]. During the wet chemistry synthesis process at high temperature, the Zn(OH)2 colloids gradually dissolves into zinc soluble species according to Eqs. (7–9). The subsequent growth then takes place in a regular well-controlled fashion. Owing to the same initial concentration of Zn(II) in every reaction system (20 mM), it is speculated that the superabundant alkali and relatively high heating temperature enable the regular Zn(OH)2, serving as an adjuster [41,42], to control the amounts of Zn(OH)42 in the reaction solution. It should be stated that essential understanding of this demands more in-depth experimental and theoretical work, and further study is under way. 3.4. Optical properties
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2015.10.017.
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The typical room-temperature photoluminescence (PL) spectra of the ZnO crystals were obtained at different concentrations of HMTA (Fig. 9). The PL spectra show similar PL features, in which two obvious luminescence bands were observed, including a strong UV, a weak blue emission bands centered at about 380 nm and 430 nm, respectively. The strong UV emission band results from near-band-gap emission, namely the recombination of free excitons through an exciton–exciton collision process [43]. The weak blue emission peak can be due to the existence of intrinsic defects in the ZnO [44]. Furthermore, the strong UV emission band and weak visible emission band indicate that the as-synthesized ZnO samples have high crystal quality and low concentration of defect. It also can be seen clearly that the intensity of peak in the spectra is a 4b 4c. Combining with the previous XRD results, it might be concluded that this result is related to the crystal quality of ZnO samples [45]. Currently, there are reports that ZnO samples have more perfect crystallization, the UV emission would has much sharper peak [46–48]. When the molar ratios of Zn2 þ : HMTA were changed from 2:1 to 1:2, the PL peak intensity in the UV gradually increased (Fig. 9(b,c)). It indicated the samples have high crystal quality, which is well matched with the obtained XRD results (Fig. 4d).
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In summary, ZnO crystals were prepared via a simple aqueous solution route. Experimental results showed that the HMTA concentration has determinative effects on the growth rate of obtained ZnO products by controlling the composition of zinc complex species and mediating the transformation rate of building blocks. The phase transformation from Zn(OH)2 to ZnO crystal may
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