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Growth process, crystal size and alignment of ZnO nanorods synthesized under neutral and acid conditions
Journal of Alloys and Compounds 629 (2015) 84–91
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Growth process, crystal size and alignment of ZnO nanorods synthesized under neutral and acid conditions Yangsi Liu ⇑, Wei Gao Department of Chemicals and Materials Engineering, The University of Auckland, PB 92019, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 26 October 2014 Accepted 23 December 2014 Available online 3 January 2015 Keywords: ZnO nanorods pH Crystal size Alignment Nucleation Crystal growth
a b s t r a c t ZnO nanorods were synthesized by hydrothermal methods. The pH value is an important factors which affects the growth behavior and morphology of ZnO one dimensional (1D) nanostructures. ZnO nanorods immobilized on glass substrates were formed in aqueous solutions under neutral (pH = 7) and acid (pH = 5) conditions. In the neutral solution, well aligned ZnO nanorods with very small size, vertical orientation, high density and uniform distribution were produced, whereas ZnO nanorods with large dimensions were sparsely and obliquely arranged in the acid solution. Their growth process, chemical composition, crystal structure, and optical property were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL). It is revealed that the pH can change the chemical reactions and influence the extent of nucleation and crystal growth, thereby having a great effect on the crystal size and the alignment of ZnO nanorods. The crystal quality, structural defects, microstructural development and potential applications were also discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction ZnO has generated considerable interests in material researches owing to its attractive properties, such as direct and wide band gap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, excellent thermal stability, and specific electrical and optoelectronic properties [1,2]. This material also possesses a rich family of nanostructures, which exhibits splendid and abundant configurations as platforms for nanotechnology [3]. Among ZnO nanostructures, one dimensional (1D) structures, e.g. nanorods, nanowires and nanobelts, become one of the major focuses, and demonstrate potential in various fields, including light emitting diodes [3–5], Schottky diode [6], field-effect transistor [7], UV sensor [8], solar cells [9], and photocatalysts [10–12]. ZnO 1D nanostructures have been fabricated by a variety of methods, including spray pyrolysis [13], chemical vapor deposition [14], vapor phase transport [15,16], thermal annealing method [17] and thermal evaporation [18,19]. These physical routes usually have a low yield and require complex processes, high temperature and vacuum conditions and sophisticated equipment, which limit their technological applications. On the contrary, chemical based approaches, such as aqueous solution or hydrothermal methods, show advantages of low temperature, low pressure, easy ⇑ Corresponding author. Tel.: +64 9 3737599x89840. E-mail addresses: [email protected], [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jallcom.2014.12.139 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
scaling-up, simple operation and environmental friendliness, and they have been frequently used to produce ZnO nanorods [20–25]. The pH value is one of the most important preparation parameters in hydrothermal method, and has proved to have a remarkable effect on the growth and morphology of ZnO nanostructures. Huang et al. [26] reported that the free-standing ZnO crystals were apt to be rod-like at near neutral condition, whereas flower-like ZnO crystals were preferred to be formed at basic condition. Yang et al. [27] found that the shape of ZnO nanorod arrays changed from hexagon to tapered shape gradually as the pH value increasing from 6 to 10. In the previous work of our group, Yan et al. [21] focused on controlling the surface distribution density of ZnO nanorods by adjusting the initial pH value of the reaction solution and formed ZnO rods with low densities in acidic solutions (pH = 3–5), while dense array films were obtained when the pH was increased to 6–7. However, little attention has been put to the growth process of ZnO nanorods in solutions with different pH. The detailed investigation on how the pH affects their final morphology is lacking. In this paper, we synthesized ZnO nanorods on glass substrates via a facile hydrothermal method at low temperature in neutral and acid aqueous solutions. The morphology and growth process of the ZnO nanorods were studied; and how the pH influenced the crystal size and the alignment through nucleation and growth processes was discussed. Their optical properties and potential applications were also explored.
Y. Liu, W. Gao / Journal of Alloys and Compounds 629 (2015) 84–91 2. Experimental 2.1. Sample preparation All chemicals of analytical grade and de-ionized (DI) water were used throughout this study. Glass substrates were coated with ZnO seed layers by magnetron sputtering for 0.5 h [28], and the detailed processes have been described in our previous reports [10,29]. An aqueous solution containing 25 mM zinc nitrate (Zn(NO3)26H2O, 98%) and hexamethylenetetramine (HMT) (C6H12N4, 99%) with an equal molar ratio was prepared. The solution was in neutral condition as the pH value was about 7. The solution was transferred into a sealable glass jar, in which the glass substrates were held face-down. The whole set was put into an oven at 95 °C for a certain time and glass substrates were withdrawn, rinsed and dried. After that, the ZnO nanorods immobilized on the glass substrates were obtained under neutral condition. For the ZnO nanorods grown from acid condition, a few drops of diluted nitric acid (HNO3, 5 M) were added to the solution to adjust the initial pH value to 5, and the other parameters were kept the same. 2.2. Sample characterization Morphology and composition analyzes were carried out with a field-emission gun scanning electron microscope (FEG-SEM, Philips XL-30S) equipped with an energy dispersive spectroscope (EDS). The crystal structure of the ZnO nanorods was identified by X-ray diffraction (XRD, Bruker D2 phaser) using Cu Ka radiation. Room temperature photoluminescence (PL) testing was performed with a 325 nm He–Cd laser source.
3. Results and discussion 3.1. Morphology of ZnO nanorods Fig. 1 shows SEM photographs of the ZnO nanorods hydrothermally grown from neutral and acid conditions for 4 h. The top-section images show that the ZnO nanorods cover entire surface uniformly with high density under neutral condition (Fig. 1a). However, the ZnO nanorods under acid condition distribute sparsely with a lot of empty space. Moreover, the nanorod bodies have a hexagonal prism shape, and most of them randomly tilt from the glass substrate to different extents (Fig. 1b). The shape of ZnO nanorods under neutral condition is difficult to identify under the same magnification due to the much smaller crystal size. The cross section views indicate the dimensions and alignment of ZnO nanorods. The average length of the ZnO nanorods grown
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from neutral condition is 1 lm and the diameters are 60– 80 nm, whereas the dimensions of ZnO nanorods produced from acid condition are much larger as they are 800–1000 nm in diameter and several micron in length. The nanorods in Fig. 1c are almost vertical to the substrate, whereas the nanorods in Fig. 1d arranged obliquely in general. These results indicate that the pH value not only altered the distribution and density of ZnO nanorods, but also affected their crystal size and alignment. To investigate the role of the pH, time-dependent experiments were carried out and the morphology variation of ZnO crystals was monitored during the ZnO nanorod growth processes at each pH value. 3.2. Growth process of ZnO Nanorods Fig. 2 represents SEM images of ZnO nanorods grown under neutral condition as a function of growth time. Prior to the hydrothermal method, glass substrates were coated by a layer of ZnO seeds, on which many fine nanograins spaced compactly and uniformly, forming a rather even surface (Fig. 2a). After immersing the substrates in aqueous solution for 15 min, some short and irregular nanoribbons can be observed besides the original nanograins. This would be the primary shape of the ZnO nuclei generated on the seed layer by heterogeneous nucleation (Fig. 2b). After 0.5 h, many nanoparticles appeared and they would be transformed from the nanoribbon and became the embryos for ZnO nanorods (Fig. 2c). Some rod-like particles emerged after 1 h, which would be the initial nanorods started to grow (Fig. 2d). The ZnO nanorods grew quickly, and a large number of rods can be seen from 1.5 h. The ZnO nanorods dispersed with a high density and covered the whole area with the size of 60–80 nm in diameter (Fig. 2e). The distribution and diameter of the ZnO nanorods did not change much with growth time (Fig. 2f), but their length increased from 600 nm to 1 lm in the latter half period (Fig. 2g–i). The morphological evolution of ZnO nanorods grown under acid condition for different periods of reaction time is demonstrated in Fig. 3. No nanostructures can be found in the early stage except several pits appeared on the substrate (Fig. 3a). Some clusters with
Fig. 1. SEM images of the ZnO nanorods grown from neutral condition (a and c) and acid condition (b and d).
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Fig. 2. SEM images of the ZnO nanorods grown from neutral condition for (a) 0 h, (b) 15 min, (c) 0.5 h, (d) 1 h, (e) 1.5 h, (f) 3 h, and (g–i) 2–4 h.
various sizes were formed after 1 h. These ball-like nanostructures consist of numerous nanoslices with porous feature. It looks like some large clusters were ‘‘dividing’’ into smaller ones and nanorods started to sprout from some of the porous nanostructures (Fig. 3b–d). More and more nanorods generated from the ball-like clusters can be seen with time of hydrothermal growth; and the dimensions of the nanorods kept enlarging into an obvious hexagonal prism shape (Fig. 3e–h). In the last stage, not many porous clusters can be seen, instead many oblique nanorods formed. They were finally grown into much bigger size than their counterparts under neutral condition, but the alignment was much poor since most of them tilted from the normal direction of the substrate. EDS was applied on two selected areas (1 lm 1 lm) to analyze the elemental composition of the nanorod and the cluster as shown in Fig. 4. In the EDS spectra, Apart from Si and Ga from the glass substrates and Pt from the pre-SEM coating, Zn and O are the main elements detected, proving that the nanorod and the cluster are made of the same elements. The intensity of Si peak is even stronger than Zn peak in area #1, while the Zn peak is the highest one in area #2. This can probably be explained by the different morphology of the two parts. The porous structures of the cluster would be easier for the electron beam to penetrate than the solid rod, therefore, more signals can come be reflected from the glass substrate.
all diffraction peaks can be well indexed to the standard diffraction pattern of ZnO (JCPDS card No. 36-1451), indicating a typical hexagonal wurtzite ZnO crystal. Only the (0 0 2) peak is obvious for the ZnO nanorods grown from neutral condition, which is in accordance with our previous report [10]. This unique diffraction feature reveals that the ZnO nanorods have a preferential growth in [0 0 0 1] orientation. For the ZnO nanorods grown from acid condition, obvious ZnO diffraction peaks started to appear in the second half growth period of time. The highest (1 0 0) peak of ZnO is resulted from the oblique alignment of hexagonal rods, allowing abundant side surfaces with {1 0 1 0} planes to be exposed to the incident X-ray. It is also notable that the intensity of the strongest peaks for the ZnO nanorods enhances with time, confirming the crystallographic development of the individual nanorods in their own orientation and alignment during the hydrothermal growth process. No peaks of other Zn complexes, such as the reaction intermediate Zn(OH)2 and the precursor Zn(NO3)2, are observed, proving of the purity of ZnO phase.
3.3. Crystal structure of ZnO nanorods
C6 H12 N4 þ 6H2 O ! 6CH2 O þ 4NH3
ð1Þ
The variation of crystal structure of ZnO nanorods during their growth process was investigated by XRD. As depicted in Fig. 5,
NH3 þ H2 O $ NHþ4 þ OH
ð2Þ
3.4. Chemical process in hydrothermal method The mechanism for the formation of ZnO crystals in Zn(NO3)2 and HMT aqueous system has been well investigated and the chemical reactions can be summarized as follow [21,30–32]:
Y. Liu, W. Gao / Journal of Alloys and Compounds 629 (2015) 84–91
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Fig. 3. SEM images of the ZnO nanorods grown from acid condition for (a) 0.5 h, (b–d) 1 h, (e and f) 1.5 h, (g and h) 2 h, and (i) 3 h.
Zn2þ þ 4NH3 $ ZnðNH3 Þ2þ 4
ð3Þ
Zn2þ þ 4OH $ ZnðOHÞ2 4
ð4Þ
ZnðNH3 Þ2þ 4 þ 2OH ! ZnðOHÞ2 ðNH3 Þ4
ð5Þ
ZnðOHÞ2 4 ! ZnðOHÞ2 þ 2OH
ð6Þ
ZnðOHÞ2 ðNH3 Þ4 ! ZnO þ 4NH3 þ H2 O
ð7Þ
ZnðOHÞ2 ! ZnO þ H2 O
ð8Þ
The overall reaction can be simplified as follow:
Zn2þ þ 2OH $ ZnðOHÞ2 ! ZnO þ 2H2 O
Fig. 4. EDS spectra from the selected areas of the ZnO cluster and the ZnO nanorod.
ð9Þ
It has been generally accepted that Zn(NO3)2 provides Zn2+ ions and HMT acts as a pH-buffer agent, which would slowly hydrolyze in the water and produce a gradual and continuous supply of OH ions required for the building of ZnO [33–35]. With the movement of the reaction equilibrium to the right side, the intermediate complex, such as Zn(OH)2, will be firstly generated by Zn2+ and OH ions. Solid ZnO nuclei are then formed by the dehydration of Zn(OH)2, which controls the mass supply for the growth of ZnO nanorods.
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Fig. 5. XRD patterns of the ZnO nanorods grown from neutral condition (a) and acid condition (b) for various periods of time.
From the above SEM results, it is clearly seen that ZnO crystals were finally developed into rod shapes no matter in acid or neutral condition, which related to the anisotropic growth along the [0 0 0 1] direction of ZnO crystal. The wurtzite ZnO crystal has hexagonal structure, which contains both nonpolar prismatic {1 0 1 0} faces and polar base faces. The polar surfaces are thermodynamically less stable than nonpolar surfaces, giving the {0 0 0 1} planes the highest level of energy. This character suggests that the growth rate of ZnO crystal along c-axis could be the fastest, and the preferred grown feature of ZnO is the 1D rod-like structure along this direction in most cases [36–38]. Once the rod-like ZnO crystals were formed, the shape of ZnO crystals did not change with further prolonged growth time. However, the ZnO nanorods experienced different growth process under different pH conditions, which resulted in the distinct crystal size and alignment. We assume that the pH value would influence the equilibrium of the above chemical reactions by altering the total amount of reactants, e.g. Zn2+ ions and OH ions, thereby exerting a profound effect on the growth process and final morphology of the ZnO nanorods.
both homogeneous and heterogeneous nucleation would happen in large rate at a high level of supersaturation [20,23,41]. The degree of supersaturation in a solution is related to the solubility of solid phases: the lower solubility, the higher supersaturation, and vice versa. According to the phase stability diagram for aqueous ZnO species as shown in Fig. 6 [23], the solubility of ionic zinc species depends on the pH of the solution. When the pH value reduces from 7 to 5, the solubility of Zn2+ increases steeply. Thus, the degree of supersaturation can be tuned by varying the pH of the aqueous system. Because of the high degree of supersaturation, both the homogeneous and heterogeneous nucleation are favorable in neutral condition. The uniform ZnO seed layers facilitate the heterogeneous nucleation on the substrates, becoming the plenty nuclei of the ZnO nanorods (Fig. 2c). Spontaneous crystallization in the bulk solution can also occur through homogeneous nucleation. This would explain why there were a lot of white precipitates at the bottom of the reactor when the entire experiment finished. Their XRD pattern indicates the white precipitates are wurtzite ZnO powders (Supporting information, Fig. S1). In this case, the total amount of Zn2+ and OH ions will be partially consumed by the ZnO precipitates, and the ‘‘nutrients’’ for the growth of ZnO nanorods on the substrates are impaired, which will further restrict the size of the ZnO nanorods. The large number of heterogeneous nuclei on the substrates also prevents individual nuclei from growing very large. In acid condition, the chances for the homogeneous nucleation in the bulk solution for ZnO precipitates are rare due to the low supersaturation. In other words, there is not much ‘‘waste’’ of the Zn2+ and OH ions. The ZnO nuclei formed via heterogeneous nucleation on the substrates are also not as many as those in neutral condition (Fig. 3b). Therefore, more zinc and oxygen species will be transferred from solution to the growth fronts, offering favorite condition for the formation of ZnO nanorods with large dimensions. On the other hand, as HNO3 was added, the interaction of ZnO with H+ ions resulted in the partial dissolution of seed layers at the early stage, leaving the etching pits on the substrates (Fig. 3a). H+ ions would also lower the efficiency of the above reactions to move to the right side and slow down the initial nucleation of ZnO crystals, which explains the late formation of ZnO nuclei compared with those in neutral condition. The Zn2+ ions dissolved from the seed layers become the extra resource for rod growth, and the ZnO nuclei are in a relatively small number, benefiting the growing large of individual nanorods.
3.5. Effect of pH on crystal size Based on classical growth theory, a requisite for crystal development from a solution is the continued presence of a supersaturation to provide the thermodynamic driving force for the spontaneous growth of nuclei and grain [39]. Crystallization is a two-step process including nucleation and crystal growth. Initial nucleation can occur either in solution (homogeneously) or on the surface of substrate (heterogeneously), depending on the degree of supersaturation [40]. Generally, crystallization through heterogeneous nucleation occurs with a low rate in a low level of supersaturation because of the lower nucleation free energy, while
Fig. 6. Phase stability diagram for aqueous ZnO species as a function of the pH in a solution.
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3.6. Effect of pH on alignment The alignment of ZnO nanorods is determined in a great extent by the crystallographic orientation and the spatial arrangement of ZnO nuclei formed during the initial growth stage. The flat seed layers on the glass substrates contains a lot of nanograins (Fig. 2a), which have a strong c-axis texture as there is only (0 0 2) peak in their XRD pattern (Fig. 5a). The seed layers can be well preserved in neutral aqueous solution. ZnO nuclei formed through the heterogeneous nucleation on the substrates would subsequently follow the same orientation as the seed layers, whose (0 0 0 1) planes are parallel to the substrates. The ZnO nanorods will then grow along the c-axis in [0 0 0 1] direction from the ZnO nuclei by virtue of their anisotropic growth character, making the ZnO nanorods perpendicular to the substrates. The large number of nuclei would also trigger the spatially confined oriented growth [27,42]. Podlogar et al. [42] proposed that when the crystals grow anisotropically, there is a strong spatial competition during crystal growth – the crystals oriented perpendicular to the substrate have much better chances to grow longer, whereas all other oriented crystals soon suffer the impingement by neighboring crystals; and their growth is stopped. Since the ZnO nanorods are not only in a large quantity but also adjacent, the free space among each other is very limited. When the nanorods deviate from the normal direction of the substrates, their growth will be easily obstructed by neighbors and difficult to be continued. However, if their growth direction obeys the normal direction, they remain unrestrained all the way up, thus the ZnO nanorods are apt to be perpendicular to the substrates. Fig. 7 displays the effect of space on the growth orientation of ZnO nanorods at different positions in an array. Compared with the nanorods in the inner part, those at the edge can still grow laterally owing to the weak spatial confinement. When the H+ ions were added to reduce the pH value to 5, the seed layers were partially destroyed, hence the nucleation positions are not as compact as those in neutral solution. The sparsely distribute nuclei create a lot of space among ZnO nanorods; and there will be more opportunity for them to grow freely as there are much less spatial competition. The porous ball-like clusters and their ‘‘dividing’’ performance could result from the competition between the erosion by H+ ions and the nucleation from reactants. These porous clusters are likely to contain lattice defects, such as dislocations and stacking faults, which would offer active sites for crystal growth and become the embryos of ZnO nanorods. The growth of ZnO crystals would take place preferentially on the clusters with defects to reduce the total surface energy through surface reconstruction [43]. The disordered atomic arrangement and the non-uniform and uneven surface of the clusters cannot provide a rigid and stable support, so the ZnO
Fig. 7. SEM image of a ZnO nanorod array grown from neutral condition.
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nanorods originated from them would grow obliquely in random directions. The shape transformation from the porous cluster to the solid rod is a diffusion and integration process [40]. During this process, the transfer of materials from the clusters into the crystal lattice of the nanorods may cause the local stress on their interfaces, the bottoms of ZnO nanorods, ascribe to the significant structure mismatch. The stress may provide the driving force for the nanorods to be tilted and haphazardly aligned. 3.7. Photoluminescence studies Photoluminescence (PL) studies give an insight into the optical and photochemical properties of ZnO and the information about the quality of crystals; and the structural defects can be reflected from PL property [1,44,45]. The room temperature PL emission spectra of the ZnO nanorods are exhibited in Fig. 8. Both the ZnO samples display an emission peak with the maximal intensity in the ultraviolet (UV) band. The narrow peak for the ZnO nanorods grown from neutral condition is higher than those grown from acid condition. The UV emission band is well known as the near band edge (NBE) emission of the wide band gap of ZnO, namely the excitonic recombination of the photo-generated holes in the valence band and the electrons in the conduction band through an exciton–exciton collision process [1]. In addition to the NBE peak, a broad emission band appears in the visible range for the ZnO nanorods under acid condition, which is too weak to be registered for the ZnO nanorods under neutral condition. The visible luminescence was believed to mainly derive from the deep-level defects, such as singly ionized oxygen vacancies, zinc interstitials or their complexes [46–48]. It is also accepted that the intensity NBE emission is closely associated with the crystal quality of ZnO samples [5,49]. The weaker NBE emission and the stronger deep level emission of the ZnO nanorods grown from acid condition suggest that they may possess more structural defects and their crystal quality is worse than those grown from neutral condition. 3.8. Microstructural development The large crystal size, oblique alignment and free inter-rod space of the ZnO nanorods grown from acid condition offer them more opportunities and high possibility to be modified and developed into various complex configurations. The thorn-like and brush pen-like ZnO nanostructures were fabricated on the basis
Fig. 8. PL spectra of the ZnO nanorods grown from neutral and acid conditions.
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authors would also like to thank the financial support of scholarship from China Scholarship Council (CSC). 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.jallcom.2014.12. 139. References
Fig. 9. SEM images of ZnO nanostructures: (a) thorn-like, and (b) brush pen-like.
of the ZnO nanorods grown from acid condition using a combination of magnetron sputtering and hydrothermal method as we reported before [29]. As illustrated in Fig. 9, the hierarchical constructions and multi-level morphologies would endue the ZnO samples with vast light harvesting and large surface area, which will benefit their applications as dye sensitized solar cells [50,51], sensors [52] and photocatalysts [29,53]. 4. Conclusions ZnO nanorods with hexagonal wurtzite phase were synthesized on glass substrates by hydrothermal method under neutral and acid conditions. The ZnO nanorods grown from neutral condition were almost vertical to the substrates and densely packed with small crystal size, whereas the acid condition produced sparsely and obliquely arranged ZnO nanorods with large dimensions. The pH can adjust the occurrence of the homogeneous and heterogeneous nucleation by varying the degree of supersaturation in the aqueous system, which affects the reactant source for the growth of ZnO nanorods and further influences their crystal size. The quantity, distribution, arrangement and orientation of ZnO nuclei can be also tailored by the pH, which plays a crucial role in the crystal development, and lead to the different alignment of ZnO nanorods. Therefore, the pH value of the solution has a significant effect on the crystal size and the alignment of ZnO nanorods. The defects and unique morphological feature of the ZnO nanorods grown from acid condition are profitable for their technological applications as novel devices. Acknowledgements The authors gratefully appreciate the assistance from staff members in the Department of Chemical and Materials Engineering and the Research Centre for Surface and Materials Science at the University of Auckland, especially Ms. Catherine Hobbis. The
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Growth process, crystal size and alignment of ZnO nanorods synthesized under neutral and acid conditions Yangsi Liu ⇑, Wei Gao Department of Chemicals and Materials Engineering, The University of Auckland, PB 92019, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 26 October 2014 Accepted 23 December 2014 Available online 3 January 2015 Keywords: ZnO nanorods pH Crystal size Alignment Nucleation Crystal growth
a b s t r a c t ZnO nanorods were synthesized by hydrothermal methods. The pH value is an important factors which affects the growth behavior and morphology of ZnO one dimensional (1D) nanostructures. ZnO nanorods immobilized on glass substrates were formed in aqueous solutions under neutral (pH = 7) and acid (pH = 5) conditions. In the neutral solution, well aligned ZnO nanorods with very small size, vertical orientation, high density and uniform distribution were produced, whereas ZnO nanorods with large dimensions were sparsely and obliquely arranged in the acid solution. Their growth process, chemical composition, crystal structure, and optical property were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL). It is revealed that the pH can change the chemical reactions and influence the extent of nucleation and crystal growth, thereby having a great effect on the crystal size and the alignment of ZnO nanorods. The crystal quality, structural defects, microstructural development and potential applications were also discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction ZnO has generated considerable interests in material researches owing to its attractive properties, such as direct and wide band gap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, excellent thermal stability, and specific electrical and optoelectronic properties [1,2]. This material also possesses a rich family of nanostructures, which exhibits splendid and abundant configurations as platforms for nanotechnology [3]. Among ZnO nanostructures, one dimensional (1D) structures, e.g. nanorods, nanowires and nanobelts, become one of the major focuses, and demonstrate potential in various fields, including light emitting diodes [3–5], Schottky diode [6], field-effect transistor [7], UV sensor [8], solar cells [9], and photocatalysts [10–12]. ZnO 1D nanostructures have been fabricated by a variety of methods, including spray pyrolysis [13], chemical vapor deposition [14], vapor phase transport [15,16], thermal annealing method [17] and thermal evaporation [18,19]. These physical routes usually have a low yield and require complex processes, high temperature and vacuum conditions and sophisticated equipment, which limit their technological applications. On the contrary, chemical based approaches, such as aqueous solution or hydrothermal methods, show advantages of low temperature, low pressure, easy ⇑ Corresponding author. Tel.: +64 9 3737599x89840. E-mail addresses: [email protected], [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jallcom.2014.12.139 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
scaling-up, simple operation and environmental friendliness, and they have been frequently used to produce ZnO nanorods [20–25]. The pH value is one of the most important preparation parameters in hydrothermal method, and has proved to have a remarkable effect on the growth and morphology of ZnO nanostructures. Huang et al. [26] reported that the free-standing ZnO crystals were apt to be rod-like at near neutral condition, whereas flower-like ZnO crystals were preferred to be formed at basic condition. Yang et al. [27] found that the shape of ZnO nanorod arrays changed from hexagon to tapered shape gradually as the pH value increasing from 6 to 10. In the previous work of our group, Yan et al. [21] focused on controlling the surface distribution density of ZnO nanorods by adjusting the initial pH value of the reaction solution and formed ZnO rods with low densities in acidic solutions (pH = 3–5), while dense array films were obtained when the pH was increased to 6–7. However, little attention has been put to the growth process of ZnO nanorods in solutions with different pH. The detailed investigation on how the pH affects their final morphology is lacking. In this paper, we synthesized ZnO nanorods on glass substrates via a facile hydrothermal method at low temperature in neutral and acid aqueous solutions. The morphology and growth process of the ZnO nanorods were studied; and how the pH influenced the crystal size and the alignment through nucleation and growth processes was discussed. Their optical properties and potential applications were also explored.
Y. Liu, W. Gao / Journal of Alloys and Compounds 629 (2015) 84–91 2. Experimental 2.1. Sample preparation All chemicals of analytical grade and de-ionized (DI) water were used throughout this study. Glass substrates were coated with ZnO seed layers by magnetron sputtering for 0.5 h [28], and the detailed processes have been described in our previous reports [10,29]. An aqueous solution containing 25 mM zinc nitrate (Zn(NO3)26H2O, 98%) and hexamethylenetetramine (HMT) (C6H12N4, 99%) with an equal molar ratio was prepared. The solution was in neutral condition as the pH value was about 7. The solution was transferred into a sealable glass jar, in which the glass substrates were held face-down. The whole set was put into an oven at 95 °C for a certain time and glass substrates were withdrawn, rinsed and dried. After that, the ZnO nanorods immobilized on the glass substrates were obtained under neutral condition. For the ZnO nanorods grown from acid condition, a few drops of diluted nitric acid (HNO3, 5 M) were added to the solution to adjust the initial pH value to 5, and the other parameters were kept the same. 2.2. Sample characterization Morphology and composition analyzes were carried out with a field-emission gun scanning electron microscope (FEG-SEM, Philips XL-30S) equipped with an energy dispersive spectroscope (EDS). The crystal structure of the ZnO nanorods was identified by X-ray diffraction (XRD, Bruker D2 phaser) using Cu Ka radiation. Room temperature photoluminescence (PL) testing was performed with a 325 nm He–Cd laser source.
3. Results and discussion 3.1. Morphology of ZnO nanorods Fig. 1 shows SEM photographs of the ZnO nanorods hydrothermally grown from neutral and acid conditions for 4 h. The top-section images show that the ZnO nanorods cover entire surface uniformly with high density under neutral condition (Fig. 1a). However, the ZnO nanorods under acid condition distribute sparsely with a lot of empty space. Moreover, the nanorod bodies have a hexagonal prism shape, and most of them randomly tilt from the glass substrate to different extents (Fig. 1b). The shape of ZnO nanorods under neutral condition is difficult to identify under the same magnification due to the much smaller crystal size. The cross section views indicate the dimensions and alignment of ZnO nanorods. The average length of the ZnO nanorods grown
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from neutral condition is 1 lm and the diameters are 60– 80 nm, whereas the dimensions of ZnO nanorods produced from acid condition are much larger as they are 800–1000 nm in diameter and several micron in length. The nanorods in Fig. 1c are almost vertical to the substrate, whereas the nanorods in Fig. 1d arranged obliquely in general. These results indicate that the pH value not only altered the distribution and density of ZnO nanorods, but also affected their crystal size and alignment. To investigate the role of the pH, time-dependent experiments were carried out and the morphology variation of ZnO crystals was monitored during the ZnO nanorod growth processes at each pH value. 3.2. Growth process of ZnO Nanorods Fig. 2 represents SEM images of ZnO nanorods grown under neutral condition as a function of growth time. Prior to the hydrothermal method, glass substrates were coated by a layer of ZnO seeds, on which many fine nanograins spaced compactly and uniformly, forming a rather even surface (Fig. 2a). After immersing the substrates in aqueous solution for 15 min, some short and irregular nanoribbons can be observed besides the original nanograins. This would be the primary shape of the ZnO nuclei generated on the seed layer by heterogeneous nucleation (Fig. 2b). After 0.5 h, many nanoparticles appeared and they would be transformed from the nanoribbon and became the embryos for ZnO nanorods (Fig. 2c). Some rod-like particles emerged after 1 h, which would be the initial nanorods started to grow (Fig. 2d). The ZnO nanorods grew quickly, and a large number of rods can be seen from 1.5 h. The ZnO nanorods dispersed with a high density and covered the whole area with the size of 60–80 nm in diameter (Fig. 2e). The distribution and diameter of the ZnO nanorods did not change much with growth time (Fig. 2f), but their length increased from 600 nm to 1 lm in the latter half period (Fig. 2g–i). The morphological evolution of ZnO nanorods grown under acid condition for different periods of reaction time is demonstrated in Fig. 3. No nanostructures can be found in the early stage except several pits appeared on the substrate (Fig. 3a). Some clusters with
Fig. 1. SEM images of the ZnO nanorods grown from neutral condition (a and c) and acid condition (b and d).
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Fig. 2. SEM images of the ZnO nanorods grown from neutral condition for (a) 0 h, (b) 15 min, (c) 0.5 h, (d) 1 h, (e) 1.5 h, (f) 3 h, and (g–i) 2–4 h.
various sizes were formed after 1 h. These ball-like nanostructures consist of numerous nanoslices with porous feature. It looks like some large clusters were ‘‘dividing’’ into smaller ones and nanorods started to sprout from some of the porous nanostructures (Fig. 3b–d). More and more nanorods generated from the ball-like clusters can be seen with time of hydrothermal growth; and the dimensions of the nanorods kept enlarging into an obvious hexagonal prism shape (Fig. 3e–h). In the last stage, not many porous clusters can be seen, instead many oblique nanorods formed. They were finally grown into much bigger size than their counterparts under neutral condition, but the alignment was much poor since most of them tilted from the normal direction of the substrate. EDS was applied on two selected areas (1 lm 1 lm) to analyze the elemental composition of the nanorod and the cluster as shown in Fig. 4. In the EDS spectra, Apart from Si and Ga from the glass substrates and Pt from the pre-SEM coating, Zn and O are the main elements detected, proving that the nanorod and the cluster are made of the same elements. The intensity of Si peak is even stronger than Zn peak in area #1, while the Zn peak is the highest one in area #2. This can probably be explained by the different morphology of the two parts. The porous structures of the cluster would be easier for the electron beam to penetrate than the solid rod, therefore, more signals can come be reflected from the glass substrate.
all diffraction peaks can be well indexed to the standard diffraction pattern of ZnO (JCPDS card No. 36-1451), indicating a typical hexagonal wurtzite ZnO crystal. Only the (0 0 2) peak is obvious for the ZnO nanorods grown from neutral condition, which is in accordance with our previous report [10]. This unique diffraction feature reveals that the ZnO nanorods have a preferential growth in [0 0 0 1] orientation. For the ZnO nanorods grown from acid condition, obvious ZnO diffraction peaks started to appear in the second half growth period of time. The highest (1 0 0) peak of ZnO is resulted from the oblique alignment of hexagonal rods, allowing abundant side surfaces with {1 0 1 0} planes to be exposed to the incident X-ray. It is also notable that the intensity of the strongest peaks for the ZnO nanorods enhances with time, confirming the crystallographic development of the individual nanorods in their own orientation and alignment during the hydrothermal growth process. No peaks of other Zn complexes, such as the reaction intermediate Zn(OH)2 and the precursor Zn(NO3)2, are observed, proving of the purity of ZnO phase.
3.3. Crystal structure of ZnO nanorods
C6 H12 N4 þ 6H2 O ! 6CH2 O þ 4NH3
ð1Þ
The variation of crystal structure of ZnO nanorods during their growth process was investigated by XRD. As depicted in Fig. 5,
NH3 þ H2 O $ NHþ4 þ OH
ð2Þ
3.4. Chemical process in hydrothermal method The mechanism for the formation of ZnO crystals in Zn(NO3)2 and HMT aqueous system has been well investigated and the chemical reactions can be summarized as follow [21,30–32]:
Y. Liu, W. Gao / Journal of Alloys and Compounds 629 (2015) 84–91
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Fig. 3. SEM images of the ZnO nanorods grown from acid condition for (a) 0.5 h, (b–d) 1 h, (e and f) 1.5 h, (g and h) 2 h, and (i) 3 h.
Zn2þ þ 4NH3 $ ZnðNH3 Þ2þ 4
ð3Þ
Zn2þ þ 4OH $ ZnðOHÞ2 4
ð4Þ
ZnðNH3 Þ2þ 4 þ 2OH ! ZnðOHÞ2 ðNH3 Þ4
ð5Þ
ZnðOHÞ2 4 ! ZnðOHÞ2 þ 2OH
ð6Þ
ZnðOHÞ2 ðNH3 Þ4 ! ZnO þ 4NH3 þ H2 O
ð7Þ
ZnðOHÞ2 ! ZnO þ H2 O
ð8Þ
The overall reaction can be simplified as follow:
Zn2þ þ 2OH $ ZnðOHÞ2 ! ZnO þ 2H2 O
Fig. 4. EDS spectra from the selected areas of the ZnO cluster and the ZnO nanorod.
ð9Þ
It has been generally accepted that Zn(NO3)2 provides Zn2+ ions and HMT acts as a pH-buffer agent, which would slowly hydrolyze in the water and produce a gradual and continuous supply of OH ions required for the building of ZnO [33–35]. With the movement of the reaction equilibrium to the right side, the intermediate complex, such as Zn(OH)2, will be firstly generated by Zn2+ and OH ions. Solid ZnO nuclei are then formed by the dehydration of Zn(OH)2, which controls the mass supply for the growth of ZnO nanorods.
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Fig. 5. XRD patterns of the ZnO nanorods grown from neutral condition (a) and acid condition (b) for various periods of time.
From the above SEM results, it is clearly seen that ZnO crystals were finally developed into rod shapes no matter in acid or neutral condition, which related to the anisotropic growth along the [0 0 0 1] direction of ZnO crystal. The wurtzite ZnO crystal has hexagonal structure, which contains both nonpolar prismatic {1 0 1 0} faces and polar base faces. The polar surfaces are thermodynamically less stable than nonpolar surfaces, giving the {0 0 0 1} planes the highest level of energy. This character suggests that the growth rate of ZnO crystal along c-axis could be the fastest, and the preferred grown feature of ZnO is the 1D rod-like structure along this direction in most cases [36–38]. Once the rod-like ZnO crystals were formed, the shape of ZnO crystals did not change with further prolonged growth time. However, the ZnO nanorods experienced different growth process under different pH conditions, which resulted in the distinct crystal size and alignment. We assume that the pH value would influence the equilibrium of the above chemical reactions by altering the total amount of reactants, e.g. Zn2+ ions and OH ions, thereby exerting a profound effect on the growth process and final morphology of the ZnO nanorods.
both homogeneous and heterogeneous nucleation would happen in large rate at a high level of supersaturation [20,23,41]. The degree of supersaturation in a solution is related to the solubility of solid phases: the lower solubility, the higher supersaturation, and vice versa. According to the phase stability diagram for aqueous ZnO species as shown in Fig. 6 [23], the solubility of ionic zinc species depends on the pH of the solution. When the pH value reduces from 7 to 5, the solubility of Zn2+ increases steeply. Thus, the degree of supersaturation can be tuned by varying the pH of the aqueous system. Because of the high degree of supersaturation, both the homogeneous and heterogeneous nucleation are favorable in neutral condition. The uniform ZnO seed layers facilitate the heterogeneous nucleation on the substrates, becoming the plenty nuclei of the ZnO nanorods (Fig. 2c). Spontaneous crystallization in the bulk solution can also occur through homogeneous nucleation. This would explain why there were a lot of white precipitates at the bottom of the reactor when the entire experiment finished. Their XRD pattern indicates the white precipitates are wurtzite ZnO powders (Supporting information, Fig. S1). In this case, the total amount of Zn2+ and OH ions will be partially consumed by the ZnO precipitates, and the ‘‘nutrients’’ for the growth of ZnO nanorods on the substrates are impaired, which will further restrict the size of the ZnO nanorods. The large number of heterogeneous nuclei on the substrates also prevents individual nuclei from growing very large. In acid condition, the chances for the homogeneous nucleation in the bulk solution for ZnO precipitates are rare due to the low supersaturation. In other words, there is not much ‘‘waste’’ of the Zn2+ and OH ions. The ZnO nuclei formed via heterogeneous nucleation on the substrates are also not as many as those in neutral condition (Fig. 3b). Therefore, more zinc and oxygen species will be transferred from solution to the growth fronts, offering favorite condition for the formation of ZnO nanorods with large dimensions. On the other hand, as HNO3 was added, the interaction of ZnO with H+ ions resulted in the partial dissolution of seed layers at the early stage, leaving the etching pits on the substrates (Fig. 3a). H+ ions would also lower the efficiency of the above reactions to move to the right side and slow down the initial nucleation of ZnO crystals, which explains the late formation of ZnO nuclei compared with those in neutral condition. The Zn2+ ions dissolved from the seed layers become the extra resource for rod growth, and the ZnO nuclei are in a relatively small number, benefiting the growing large of individual nanorods.
3.5. Effect of pH on crystal size Based on classical growth theory, a requisite for crystal development from a solution is the continued presence of a supersaturation to provide the thermodynamic driving force for the spontaneous growth of nuclei and grain [39]. Crystallization is a two-step process including nucleation and crystal growth. Initial nucleation can occur either in solution (homogeneously) or on the surface of substrate (heterogeneously), depending on the degree of supersaturation [40]. Generally, crystallization through heterogeneous nucleation occurs with a low rate in a low level of supersaturation because of the lower nucleation free energy, while
Fig. 6. Phase stability diagram for aqueous ZnO species as a function of the pH in a solution.
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3.6. Effect of pH on alignment The alignment of ZnO nanorods is determined in a great extent by the crystallographic orientation and the spatial arrangement of ZnO nuclei formed during the initial growth stage. The flat seed layers on the glass substrates contains a lot of nanograins (Fig. 2a), which have a strong c-axis texture as there is only (0 0 2) peak in their XRD pattern (Fig. 5a). The seed layers can be well preserved in neutral aqueous solution. ZnO nuclei formed through the heterogeneous nucleation on the substrates would subsequently follow the same orientation as the seed layers, whose (0 0 0 1) planes are parallel to the substrates. The ZnO nanorods will then grow along the c-axis in [0 0 0 1] direction from the ZnO nuclei by virtue of their anisotropic growth character, making the ZnO nanorods perpendicular to the substrates. The large number of nuclei would also trigger the spatially confined oriented growth [27,42]. Podlogar et al. [42] proposed that when the crystals grow anisotropically, there is a strong spatial competition during crystal growth – the crystals oriented perpendicular to the substrate have much better chances to grow longer, whereas all other oriented crystals soon suffer the impingement by neighboring crystals; and their growth is stopped. Since the ZnO nanorods are not only in a large quantity but also adjacent, the free space among each other is very limited. When the nanorods deviate from the normal direction of the substrates, their growth will be easily obstructed by neighbors and difficult to be continued. However, if their growth direction obeys the normal direction, they remain unrestrained all the way up, thus the ZnO nanorods are apt to be perpendicular to the substrates. Fig. 7 displays the effect of space on the growth orientation of ZnO nanorods at different positions in an array. Compared with the nanorods in the inner part, those at the edge can still grow laterally owing to the weak spatial confinement. When the H+ ions were added to reduce the pH value to 5, the seed layers were partially destroyed, hence the nucleation positions are not as compact as those in neutral solution. The sparsely distribute nuclei create a lot of space among ZnO nanorods; and there will be more opportunity for them to grow freely as there are much less spatial competition. The porous ball-like clusters and their ‘‘dividing’’ performance could result from the competition between the erosion by H+ ions and the nucleation from reactants. These porous clusters are likely to contain lattice defects, such as dislocations and stacking faults, which would offer active sites for crystal growth and become the embryos of ZnO nanorods. The growth of ZnO crystals would take place preferentially on the clusters with defects to reduce the total surface energy through surface reconstruction [43]. The disordered atomic arrangement and the non-uniform and uneven surface of the clusters cannot provide a rigid and stable support, so the ZnO
Fig. 7. SEM image of a ZnO nanorod array grown from neutral condition.
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nanorods originated from them would grow obliquely in random directions. The shape transformation from the porous cluster to the solid rod is a diffusion and integration process [40]. During this process, the transfer of materials from the clusters into the crystal lattice of the nanorods may cause the local stress on their interfaces, the bottoms of ZnO nanorods, ascribe to the significant structure mismatch. The stress may provide the driving force for the nanorods to be tilted and haphazardly aligned. 3.7. Photoluminescence studies Photoluminescence (PL) studies give an insight into the optical and photochemical properties of ZnO and the information about the quality of crystals; and the structural defects can be reflected from PL property [1,44,45]. The room temperature PL emission spectra of the ZnO nanorods are exhibited in Fig. 8. Both the ZnO samples display an emission peak with the maximal intensity in the ultraviolet (UV) band. The narrow peak for the ZnO nanorods grown from neutral condition is higher than those grown from acid condition. The UV emission band is well known as the near band edge (NBE) emission of the wide band gap of ZnO, namely the excitonic recombination of the photo-generated holes in the valence band and the electrons in the conduction band through an exciton–exciton collision process [1]. In addition to the NBE peak, a broad emission band appears in the visible range for the ZnO nanorods under acid condition, which is too weak to be registered for the ZnO nanorods under neutral condition. The visible luminescence was believed to mainly derive from the deep-level defects, such as singly ionized oxygen vacancies, zinc interstitials or their complexes [46–48]. It is also accepted that the intensity NBE emission is closely associated with the crystal quality of ZnO samples [5,49]. The weaker NBE emission and the stronger deep level emission of the ZnO nanorods grown from acid condition suggest that they may possess more structural defects and their crystal quality is worse than those grown from neutral condition. 3.8. Microstructural development The large crystal size, oblique alignment and free inter-rod space of the ZnO nanorods grown from acid condition offer them more opportunities and high possibility to be modified and developed into various complex configurations. The thorn-like and brush pen-like ZnO nanostructures were fabricated on the basis
Fig. 8. PL spectra of the ZnO nanorods grown from neutral and acid conditions.
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authors would also like to thank the financial support of scholarship from China Scholarship Council (CSC). 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.jallcom.2014.12. 139. References
Fig. 9. SEM images of ZnO nanostructures: (a) thorn-like, and (b) brush pen-like.
of the ZnO nanorods grown from acid condition using a combination of magnetron sputtering and hydrothermal method as we reported before [29]. As illustrated in Fig. 9, the hierarchical constructions and multi-level morphologies would endue the ZnO samples with vast light harvesting and large surface area, which will benefit their applications as dye sensitized solar cells [50,51], sensors [52] and photocatalysts [29,53]. 4. Conclusions ZnO nanorods with hexagonal wurtzite phase were synthesized on glass substrates by hydrothermal method under neutral and acid conditions. The ZnO nanorods grown from neutral condition were almost vertical to the substrates and densely packed with small crystal size, whereas the acid condition produced sparsely and obliquely arranged ZnO nanorods with large dimensions. The pH can adjust the occurrence of the homogeneous and heterogeneous nucleation by varying the degree of supersaturation in the aqueous system, which affects the reactant source for the growth of ZnO nanorods and further influences their crystal size. The quantity, distribution, arrangement and orientation of ZnO nuclei can be also tailored by the pH, which plays a crucial role in the crystal development, and lead to the different alignment of ZnO nanorods. Therefore, the pH value of the solution has a significant effect on the crystal size and the alignment of ZnO nanorods. The defects and unique morphological feature of the ZnO nanorods grown from acid condition are profitable for their technological applications as novel devices. Acknowledgements The authors gratefully appreciate the assistance from staff members in the Department of Chemical and Materials Engineering and the Research Centre for Surface and Materials Science at the University of Auckland, especially Ms. Catherine Hobbis. The
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