pH-dependent growth of zinc oxide nanorods

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2549–2554

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pH-dependent growth of zinc oxide nanorods Sunandan Baruah, Joydeep Dutta  Centre of Excellence in Nanotechnology, School of Engineering and Technology, Asian Institute of Technology, P.O. Box-4, Klong Luang, Pathumthani 12120, Thailand

a r t i c l e in fo

abstract

Article history: Received 4 November 2008 Received in revised form 26 January 2009 Accepted 28 January 2009 Communicated by S. Uda Available online 4 February 2009

Here we study the effect of pH variation on the dimension and morphology of zinc oxide (ZnO) nanorods grown through hydrothermal process at temperatures less than 100 1C. ZnO nanorods were grown on pre-seeded glass substrates using zinc nitrate hexahydrate as the source of Zn ions and hexamethylenetetramine as the source of hydroxyl ions. The pH of the reaction bath was found to change gradually from 6.4 to 7.3 in 5 h during the growth process. The growth of the ZnO nanorods was observed to be faster, both laterally and longitudinally, when the growth solution was in basic conditions. However, flower petal like ZnO nanostructures were obtained when the growth process was initiated in basic condition (pH 8–12), indicating that initial acidic conditions were required to obtain nanorods with well-defined hexagonal facets. ZnO is known to erode in acidic condition and the final dimension of the nanorods is determined by a competition between crystal growth and etching. ZnO nanorods of different dimensions, both laterally (diameters ranging from 220 nm to 1 mm) and longitudinally (lengths ranging from 1 to 5.6 mm) were successfully synthesized using the same concentration of zinc nitrate and hexamine in the reaction bath and the same growth duration of 5 h simply through appropriate control of the pH of the reactant solution between 6 and 7.3. & 2009 Elsevier B.V. All rights reserved.

PACS: 61.46.HK 61.46.Km 81.07. b 81.16.Dn Keywords: A1. PH A2. Hydrothermal B1. Hexamine B1. Nanoparticle B1. Nanorod B2. Zinc oxide

1. Introduction Numerous reports on the growth and characterization of onedimensional nanowires of elemental and compound semiconductors such as silicon (Si) [1], germanium (Ge) [2], indium phosphide (InP) [3], gallium arsenide (GaAs) [4] and zinc oxide (ZnO) [5–7] are available in the literature. Nanostructures of ZnO such as nanowires and nanorods [8], nanocombs [9], nanorings [10], nanoloops and nanohelices [11], nanobows [12], nanobelts [13] and nanocages [14] have been reported. These structures have been synthesized under controlled growth conditions [15,16]. Zinc Oxide nanostructures can be synthesized either through gas-phase synthesis or through solution-phase synthesis. Gas-phase synthesis is carried out in a gaseous environment in closed chambers at high temperatures (500–1500 1C). Some commonly used gas-phase methods are vapor-phase transport, which includes vapor solid (VS) [17] and vapor liquid solid (VLS) [18] growth, physical vapor deposition (PVD) [19], chemical vapor deposition (CVD) [20], metalorganic chemical vapor deposition (MOCVD) [21] and thermal oxidation of pure Zn [22] followed by condensation. In the solution-phase synthesis, the growth process

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E-mail address: [email protected] (J. Dutta). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.135

is carried out in a liquid. Normally aqueous solutions are used and the process is then referred to as hydrothermal growth process. Some of the solution-phase synthesis processes reported are the zinc acetate hydrate (ZAH) derived nano-colloidal sol–gel route [23], ZAH in alcoholic solutions with sodium hydroxide (NaOH) [24], tetra methyl ammonium hydroxide (TMAH) [25] or lithium hydroxide (LiOH) [26], template-assisted growth [27] or spray pyrolysis [28,29] and electrophoresis [30]. One of the most energy-efficient strategies for synthesizing ZnO nanorods is the hydrothermal process that does not require high temperature and complex vacuum environment. The hydrothermal process induces an epitaxial, anisotropic crystal growth in a solution [26,31] (normally aqueous solution). The hydrothermal process is usually substrate independent [32] and the morphology of the nanorods can be easily controlled through slight changes in the reaction conditions [33]. There are reports on the successful growth of ZnO nanowires on flat substrates like Si [15], glass [33,34], TCO [35], polyethylene fibers [8], carbon cloth [36] and Al foil [37]. In one of the reported processes [31], an equimolar solution of zinc nitrate hexahydrate [Zn(NO3)2
6H2O] and hexamethylene tetramine [C6H12N4], popularly known as hexamine is utilized to epitaxially grow ZnO rods on substrates by fixing pre-synthesized ZnO nanoparticles on the substrate as seeds. The ZnO crystal is hexagonal wurtzite and exhibits partial polar characteristics [16]

ARTICLE IN PRESS S. Baruah, J. Dutta / Journal of Crystal Growth 311 (2009) 2549–2554

2. Experimental The ZnO nanorods were synthesized modifying a method initially suggested by Vayssieres et al. [31] and proposed by Sugunan et al. [33]. The method consisted of seeding substrates with ZnO nanocrystallites followed by a chemical bath growth of the nanorods as described in Section 2.3.

10 Chemicals replenished

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with lattice parameters a=0.3296 and c=0.52065 nm. The anisotropy in the crystal structure of ZnO assists the growth of the nanorods. The most common polar surface is the basal plane (0 0 1). One end of the basal polar plane terminates in partially positive Zn lattice points and the other end terminates in partially negative oxygen lattice points. The anisotropic growth of the nanorods takes place along the c-axis in the [0 0 0 2] direction. It was reported by Sugunan et al. [33] that hexamine, a nonionic tertiary amine derivative and a non-polar chelating agent, would preferentially attach to the non-polar facets of the ZnO crystal as it builds up, thereby exposing only the (0 0 1) plane for epitaxial growth [33]. Thus a preferential growth along the [0 0 0 2] direction is made possible. Seeding of the substrate with ZnO nanoparticles was found to lower the thermodynamic barrier by providing nucleation sites, further improving the aspect ratio of the synthesized nanorods. Seeding of the substrate is thus an important parameter for the uniform growth of ZnO nanorods through hydrothermal process. Seeding can be done by dip coating [8] and spin coating [32] using a colloidal solution of ZnO nanoparticles or sputtering a thin layer of ZnO on the substrate [38]. A lot of factors come into play during the growth of the ZnO nanorods like concentration of the chemical bath, temperature, duration of growth, pH, etc., which directly affect the final morphology of the rods grown. There are reports available in the literature about the synthesis of ZnO nanorods and other morphologies through a variation in pH of the reaction bath [39–41]. However, this study is aimed at optimizing the pH conditions to obtain ZnO nanorods of different dimensions starting with the same concentration of the reactant mixture. It was possible to grow ZnO nanorods of different dimensions (both lateral and longitudinal), with the same concentration of Zn(NO3)2 and hexamine in the chemical bath and the same growth duration, simply by varying the pH of the growth solution between 6 and 7.3.

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2.1. Materials used All chemicals used in this study were analytical grade and was used without further purification. Zinc acetate dihydrate [(CH3COO)2Zn
2H2O] procured from Merck was used as the zinc ion source, sodium hydroxide [NaOH] from Merck as the reducing agent and ethanol [C2H5OH] from J.T.Baker as the solvent for the synthesis of ZnO nanoparticles. Zinc nitrate hexahydrate [Zn(NO3)2
6H2O, Aldrich, 99% purity] and hexamethylene tetramine [C6H12N4, Carlo Erba, 99.5%] were used as the reactants in the chemical bath for the ZnO nanorod growth. 2.2. Synthesis of ZnO nanoparticles The synthesis of ZnO nanoparticles that were used as seeds was carried out following a procedure reported by Bahnemann et al. [42] 1 mM zinc acetate solution was prepared in 2-propanol under rigorous stirring at 50 1C. The solution was then further diluted and cooled, after which aliquots of 20 mM sodium hydroxide in 2-propanol was added under continuous stirring. The mixture was then kept in a water bath at 60 1C for 2 h [43]. 2.3. Hydrothermal growth of ZnO nanorods

Fig. 1. HRTEM image showing the ZnO nanoparticles synthesized in isopropanol. Measurements of lattice spacings done on images of different particles indicated the presence of the (1 0 0), (0 0 2), (1 0 1) and (1 0 2) planes of the wurtzite structure.

ZnO nanorods were grown on glass slides, which were first thiolated by dipping in a 1% solution of dodecane thiol in ethanol

ARTICLE IN PRESS S. Baruah, J. Dutta / Journal of Crystal Growth 311 (2009) 2549–2554

[8] as the surface functionalization of silica using thiol allows the irreversible binding of metal oxide particles from a colloidal solution [44]. The substrates were seeded by dipping the thiolated

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substrates into a concentrated colloidal solution of ZnO nanoparticles in isopropanol for 15 min. Three dippings were made and the substrates were heated at 150 1C for 15 min after each dipping

Fig. 4. SEM images of ZnO nanowires grown hydrothermally for 15 h with different concentration of the reactants (A) 1 mM (B) 5 mM and (C) 10 mM. Scale bars=1 mm.

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Fig. 5. Dimensions of ZnO nanorods grown over a period of 15 h using 1, 5 and 10 mM solution of zinc nitrate and hexamine (A) diameters (B) lengths.

Fig. 6. FESEM images showing the nanostructures obtained when growth was carried out at basic pH using 10 mM reaction bath and growth duration of 5 h (A) pH of 7.3 (B) pH of 8 (C) pH of 10 and (D) pH of 12.

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to ensure that the seeds were securely attached. The wires were grown in a sealed chemical bath containing an equimolar solution of zinc nitrate hexahydrate and hexamethylene tetramine at 90 1C. Different concentrations of the precursor solution were used to study the growth of the ZnO nanowires. As the growth rate was observed to decrease after about 5 h [8], the precursor solution was changed every 5 h and growth was continued for up to 15 h. pH of the reaction bath was controlled by titrating the reactant solution with HCl and NaOH. The samples were then heated at 250 1C for 30 min to vaporize any organic deposits. The characterization of the nanowires was performed using scanning electron microscopy (SEM, IE350FSG FESEM) images through imageprocessing software. High-resolution transmission electron microscopy (HRTEM) was carried out using a JEOL JEM 2010 operating at 100 kV. pH measurements were carried out with a pH meter from Denver Instrument.

3. Results and discussions Organometallic synthesis method was selected for the growth of the ZnO seed nanoparticles as their nucleation and growth are much faster in alcohol (isopropanol in the present case) than in aqueous solution, water being a highly polar solvent. The ZnO seed nanoparticles were almost spherical and about 5–6 nm in diameter, as can be observed from the HRTEM image shown in Fig. 1. The nanoparticles synthesized by this method exhibit the wurtzite structure of ZnO [8] and lattice spacing measurements

done using Scion image-processing software show almost comparable dominance of the (1 0 0), (0 0 2), (1 0 1) and (1 0 2) crystallographic planes [33]. The ZnO nanorods were initially synthesized using a reaction bath containing 20 mM aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine for 20 h, recording the length after every 5 h. It was observed that the growth was very fast in the initial 5 h and the growth rate showed a gradual decrease after that as was reported earlier [8]. This may be accounted to the decrease in concentration of Zn2+ ions in the chemical bath. To maintain a high growth rate, the chemicals in the reaction bath was replenished after every 5 h. Fig. 2 shows the comparison between the lengths of the nanorods with and without replenishment of the reaction bath. It was observed that the growth of the ZnO nanorods was not consistent throughout and the growth rate at any particular concentration is not a linear function of time. Hexamine is known to degrade upon prolonged thermal treatment thereby releasing hydroxyl ions, which gradually changes the pH of the reaction bath [45,46]. To understand the role of pH in the growth of ZnO nanorods, pH variations during the growth process was carefully followed for 3 different concentrations (1, 5 and 10 mM) of the chemical bath and a plot of pH versus time recorded over a period of 5 h is shown in Fig. 3. It can be concluded from Fig. 3 that the pH of the reaction bath changes from acidic to basic in about an hour of growth time of the ZnO nanorods. After 5 h the pH stands at about 7.3 for all concentrations. To correlate the lateral and longitudinal growth of the ZnO nanorods with the pH, measurements were done on SEM

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Fig. 7. Length and width of the ZnO nanorods after a growth of 5 h as a function of the time for which the initial growth was carried out in different pH conditions and the final pH of 7.3: (A) initial pH 6, (B) initial pH 6.5, (C) initial pH 6.8 and (D) initial pH 7.

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images of the nanorods grown over a total duration of 15 h. Some of the SEM images are shown in Fig. 4. The chemical reaction bath was replenished after every 5 h with similar concentration of reactants. Fig. 5 shows the length and diameter of the nanorods plotted as a function of time over a period of 15 h. The growth rate, both lateral and longitudinal, was observed to be higher in basic conditions than in acidic conditions, which is understandable as ZnO nanorods are known to get eroded in acidic conditions and the final growth depends upon the competition between growth and etching [47]. The initial phase of nanorod growth is slow (Fig. 5) as the nucleation sites are limited and these smaller rods would merge during growth to form bigger rods. The enhanced rate of lateral and longitudinal growth during the period between 8 and 10 h can be attributed to the higher availability of surface atoms on the ZnO nanorods for the crystal growth. Once the nanorods reach a thermodynamically stable size, the growth rate shows a decline (after about 10 h of growth) that arises due to the larger number of surface adatoms to which the available Zn2+or O2 ions can bond as compared to the thinner rods. In continuation with the observation that the growth rate was faster in basic pH conditions, a few controlled experiments were conducted to monitor the growth and morphology of the grown nanorods. Hydrothermal crystallization of ZnO carried out at various basic pH values (7.3, 8, 10, 12) showed the transition of morphology from rod like to flower like with a shift towards more basic pH. Fig. 6 shows the SEM images of the nanostructures obtained at basic pH. Comparing the rods obtained with growth

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initiation in mild acidic condition (Fig. 3) and the ones grown at basic condition (Fig. 6(a) and (b)), it can be concluded that in the former case the growth was much better with sharp hexagonal facets clearly visible. Further, for the nanorods shown in Fig. 3, initial growth was in mild acidic condition, which gradually changed to basic condition after about 1 h (for 5 and 10 mM samples) and 1.5 h for the 1 mM sample (Fig. 3). In order to observe the growth variation with changes in pH, a set of experiments were conducted using 10 mM growth solution and growth duration of 5 h for all the samples. Starting in a slightly acidic media (pH 6, 6.5 and 6.8) and also at a neutral pH of 7 for different durations of time (10, 30, 40 and 60 min), further growth was continued after transferring the substrates to a reactant mixture at a pH of 7.3 (pH at which fastest growth was observed). Observations were done on FESEM images and measurements were carried out using Scion image-processing software. Figs. 7 show the length and width of the nanorods as functions of the time when the sample was kept in slightly acidic condition (pH 6, 6.5, 6.8 and 7). From Fig. 7, we can observe that the maximum growth, both in the lateral and longitudinal directions, was obtained when the growth was started with a pH of 6.8 for 10 min followed by growth at pH 7.3 (Fig. 7(A)). The smallest rods, both in diameter and length, was obtained with a starting pH of 6.5 for 10 min and the rest of the growth at a pH of 7.3 (Fig. 7(B)). Fig. 7(D) shows the length and diameter of the nanorods grown with an initial neutral pH and continued at a pH of 7.3. Negligible variations in the

Fig. 8. ZnO nanorods grown under different initial pH with same concentration (10 mM) and growth duration (5 h): (A) pH 6 for 40 min and 7.3 for 4 h 20 min, (B) pH 6.8 for 10 min and 7.3 for 4 h 50 min, (C) pH 6 for 1 h and 7.3 for 4 h, and (D) pH 6.5 for 40 min and 7.3 for 4 h 20 min. Scale bars: 1 mm.

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dimensions were observed, which can be accounted to the lack of etching of the ZnO crystal facets at neutral pH. The growth rate of the ZnO crystal at pH 7 and 7.3 are observed to be comparable. The rods obtained after keeping the reaction bath at pH 7 for different durations and continuing in pH 7.3 were therefore of similar dimensions (Fig. 7(D)). The ZnO nanorods obtained a tapered morphology when the initial growth was carried out at a pH of 6.8 for 30–40 min and continued for 5 h at pH 7.3 (Fig. 7(C) and 8(D)). This tapering of the rods may result from the growth of multiple layers of the ZnO crystal with decreasing width along the c-axis. The growth rate of ZnO is known to be the fastest along the caxis [48]. The disappearance rate of the (0 0 0 1) face of ZnO nanorods is also reported to be the fastest compared to the other faces when eroded in acidic medium [47]. This corroborates with the observation that the ZnO nanorods grown initially with a pH of 6 for 60 min followed by another 4 h growth at pH 7.3 gives the shortest length (3.5 mm) and the largest width (1.8 mm). The length was probably checked by the erosion of the (0 0 0 1) face leading to a higher lateral growth. An interesting point to note is that, in just 5 h, it was possible to grow ZnO nanorods with length (5.6 mm in 5 h using 10 mM concentration) up to twice that obtained using the conventional method (2.8 mm in 5 h using 10 mM concentration) simply through controlling the pH of the reactant bath. However, the maximum width that could be obtained in 5 h is 1.8 mm as compared to 242 nm using the conventional method (an increase of over 7 times) that needs closer look for future studies.

4. Conclusion ZnO nanorods with diameters varying between 220 nm and 1.8 mm and lengths between 1 and 5.6 mm were successfully synthesized through a simple hydrothermal method, using the same concentration and the same growth duration, simply by controlling the pH of the reaction bath during the growth process. It was clearly demonstrated that the pH plays a major part in the morphology and dimensions of the nanorods grown through hydrothermal process. The growth of the nanorods was observed to be faster in basic medium than in acidic medium. However, slightly acidic conditions during the initial growth from the nucleation sites resulted in appreciable variations in the lateral and longitudinal dimensions, opening up possibilities of tailoring the width and length of the nanorods to suit different applications. This may be an attractive option for manufacturing ZnO nanorod-based devices at a commercial level as the dimension of the nanorods can be controlled simply through the variation in a single parameter i.e. the pH of the reaction bath, keeping all other parameters like concentration of the bath, duration of growth and temperature constant.

Acknowledgements The authors would like to acknowledge partial financial support from the Centre of Excellence in Nanotechnology at the Asian Institute of Technology and the National Nanotechnology Center (NANOTEC) belonging to the National Science & Technology Development Agency (NSTDA), Thailand.

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