A simple route to vertical array of quasi-1D ZnO nanofilms on FTO surfaces: 1D-crystal growth of nanoseeds under ammonia-assisted hydrolysis process
© Umar et al; licensee Springer. 2011
Received: 11 August 2011
Accepted: 25 October 2011
Published: 25 October 2011
A simple method for the synthesis of ZnO nanofilms composed of vertical array of quasi-1D ZnO nanostructures (quasi-NRs) on the surface was demonstrated via a 1D crystal growth of the attached nanoseeds under a rapid hydrolysis process of zinc salts in the presence of ammonia at room temperature. In a typical procedure, by simply controlling the concentration of zinc acetate and ammonia in the reaction, a high density of vertically oriented nanorod-like morphology could be successfully obtained in a relatively short growth period (approximately 4 to 5 min) and at a room-temperature process. The average diameter and the length of the nanostructures are approximately 30 and 110 nm, respectively. The as-prepared quasi-NRs products were pure ZnO phase in nature without the presence of any zinc complexes as confirmed by the XRD characterisation. Room-temperature optical absorption spectroscopy exhibits the presence of two separate excitonic characters inferring that the as-prepared ZnO quasi-NRs are high-crystallinity properties in nature. The mechanism of growth for the ZnO quasi-NRs will be proposed. Due to their simplicity, the method should become a potential alternative for a rapid and cost-effective preparation of high-quality ZnO quasi-NRs nanofilms for use in photovoltaic or photocatalytics applications.
PACS: 81.07.Bc; 81.16.-c; 81.07.Gf.
KeywordsZnO quasi-NRs nanofilms vertical array hydrolysis process seed-mediated method
ZnO nanocrystals, such as nanorods, nanowires and nanoparticles, have been receiving a growing research attention in the last few decades due to their unique electrical and optical properties [1–6]. ZnO is characterised by a wide direct band gap of 3.37 eV that indicates the potential use in blue light-emitting  devices application. Their high electron mobility (bulk ZnO 150 to 350 cm2 V-1 s-1), high exciton binding energy (60 meV) and long diffusion length  make them great material candidates for electronics , optoelectronics [10, 11] devices and solar cell and photocatalyst applications [12–14]. The synthesis of ZnO in the form of nanorods or nanowires is expected to further enhance their intrinsic property as the results of quantum effect.
Many approaches have been demonstrated for the preparation of ZnO nanorods and nanowires on solid substrate so far. They include, but are not limited to, vapour-liquid-solid (VLS) , metal organic vapour phase epitaxy [16, 17], plasma-enhanced chemical vapour deposition [18, 19] and a simple vapour-solid process . Amongst the available techniques, a vapour-liquid-solid (VLS) has been recognised as a versatile method to prepare high-quality ZnO oxide nanorods. The detail of the process and the promising properties of ZnO nanostructures prepared using these methods have also been well summarised in [1–6]. Although high-quality ZnO nanorods and nanowires can be successfully realised, such as controlled structures, growth orientation and properties, these techniques are recognised to comprise several major drawbacks, such as high-temperature process (typically approximately 1,000°C) to facilitate liquidifying and evaporating the zinc precursor and the growth. In addition, since usual procedure requires metal catalysts to promote and direct the ZnO nanorods growth, the ZnO product certainly is seriously contaminated by them. In many applications, this is definitely unexpected since they may superimpose the intrinsic properties of the ZnO itself. Thus, the unique properties of ZnO nanorods could not be harvested. After growth effort to remove them has also been demonstrated, but has come up with limited success. Due to the unique properties of ZnO nanorods and their potential function in currently existing applications, a low-temperature process and catalyst-free growth for nanorods on the surface should be continuously demonstrated.
So far, well known and widely used techniques of catalyst-free and low-temperature growth process for 1D ZnO nanostructures on the surface are represented by anodic aluminium oxide (AAO) template electrochemical  and hydrothermal [22–24] methods. For the case of the AAO template method, high-quality vertical array tubular ZnO nanostructures on the surface have been normally realised at a room-temperature processing. However, despite the fact that after growth templates removal indicates a diminutive problem and effect on the grown-up nanostructures, this method shows a strict limitation on the reducing of the nanorods or nanotubes diameter as an inadequacy in controlling the dimension of the AAO template itself. A hydrothermal method seems to be the potential approach for a better synthetic control for a catalyst-free 1D ZnO growth on the substrate surface. This technique realises the growth of vertically oriented ZnO nanorods on the surface from the nanoseeds under a low-temperature hydrothermal process (approximately 60°C to 150°C) in an autoclave. Typical growth time is approximately 4 to 12 h. Highly ordered ZnO nanorods on the surface have been produced by coupling with a lithography seeding process . Improved results could be likely further obtained via coupling with a sonochemical  or microwave-assisted  hydrothermal process. In contrast to such interesting properties, however, hydrothermal techniques actually impose a tight control over the preparation process, such as temperatures and atmosphere (normally using autoclave), to obtain preferred ZnO products. Also, in the growth process, this technique is relatively time-consuming (typical time for projecting 50-nm nanorods is approximately >4 h) so that the preparation of ZnO nanorods with high aspect ratio is a challenging process. In addition, since the nature of this technique produces ZnO product not only on the target surface but also throughout the container, it requires an appropriate position of the target surface for obtaining a desired ZnO nanorods structure, inferring that it is a complex procedure. Therefore, considering the broad spectrum of ZnO nanorods applications, the preparation of ZnO nanorods with a simple and rapid process is highly demanded.
Here, we demonstrate an alternative method for preparing high-density, vertically oriented quasi-1D ZnO nanofilms on the surfaces via a 1D crystal growth of nanoseeds under a simple ambient-temperature hydrolysis process of zinc salt in the presence of ammonia with a relatively short growth period. In a typical process, the growth time to project the nanoseed into quasi-NRs morphology was approximately 3 min and this can produce quasi-NRs with a final length of up to approximately 150 nm. The morphology of the quasi-NRs was noticed to depend on the concentration of the ammonia and the zinc precursor in the reaction. X-ray diffraction (XRD) characterisation on the as-prepared sample surprisingly discovered that the samples had a phase purity of ZnO without the presence of any zinc complexes. A room-temperature optical absorption spectroscopy analysis surprisingly revealed that the nanostructures were high-degree crystallinity in nature, which was indicated by the presence of two distinct excitonic characters, namely A- and B-excitons, on the spectrum. Although better shape control is not yet achieved in the present report, due to the simplicity of the process, the present method should become a potential approach for the preparation of vertically oriented quasi-NRs ZnO nanofilms on the surface for use in currently existing applications.
Quasi-1D ZnO nanostructures on FTO (Solartron, Oak Ridge, TN, USA) surface were prepared via 1D crystal growth of nanoseeds on the surface in the presence of ammonia, adopting our previous approach in preparing CuO nanowires on the surface . This method consists of two steps, namely seeding and growth processes. The following are typical procedures for the preparation of ZnO quasi-NRs on the FTO surface.
ZnO nanoseeds on the FTO surface were prepared using an alcohothermal seeding method. In the typical process, a thin layer of ethanoloic solution of zinc acetate dihydrate (Zn (CH3COO)2 2H2O, Across) on a clean FTO surface was firstly prepared using a two-step spin-coating process at 400 and 2,000 rpm for 6 and 30 s, respectively. The concentration of Zn (CH3COO)2 2H2O used was 0.01 M. The sample was then dried up at 100°C on a hot-plate for 15 min. This procedure was repeated three times. After that, the sample was annealed in air at 350°C for 1 h. This process may produce high-density ZnO nanoseeds with sizes ranging from 5 to 10 nm on the surface.
The ZnO quasi-NRs were grown from the attached nanoseeds by simply immersing the nanoseeds-attached FTO into a 35-ml glass vial containing 10 mL of 10 mM aqueous solution of zinc acetate dihydrate (Zn (CH3COO)2 2H2O, Aldrich Chemical Co., Milwaukee, WI, USA). The sample was kept in a vertical position in the vial during the reaction by hanging it using adhesive tape. The solution was then mildly stirred during the reaction using a 10-mm magnetic stirrer bar. After that, a 30 μL of 30% ammonia solution (NH3, Aldrich) was added drop wisely into the reaction using a micropipette. This composition is referred as standard reaction later. The time interval for the additions of NH3 drops was approximately 1 min. The clear solution of zinc acetate immediately changed to a translucent bluish colour for the first 1 to 3 min of the process (inferring a rapid hydrolysis of zinc complexes in the growth solution) and then disappeared, a reflection of complete olation process of zinc complexes on the nanoseeds surface. This phenomenon was again obtained every time the ammonia was added into the solution. A tiny whitish suspension was sometimes observed if the reaction time was extended or a high concentration of ammonia was used. The reaction was allowed to continue for up to 5 min for a growth process. The effect of ammonia concentration on the structural growth of ZnO nanostructures was examined by using several variations of ammonia additions into the reaction, namely from 30 to 300 μL. If we used, for example, 30 μL of ammonia, the final ammonia concentration in the reaction is 36 mM. The experiment was carried out at room temperature.
The sample was then removed and vigorously washed several times using pure water to remove any precipitate on the surface and dried using a flow of nitrogen gas. The sample was also subjected to an annealing process at 350°C in air for 1 h to obtain the effect of annealing treatment on the structures and the morphology.
The morphology of the as-prepared samples was obtained using a field emission scanning electron microscope (FESEM) machine model ZEISS SUPRA 55VP that was operated at an acceleration voltage of 3 kV. The structure and phase purity of the as prepared and the annealed samples were characterised using a Bruker D8 Advance XRD diffractometer with CuKα radiation operated at 40 kV and 40 mA. The optical property of ZnO quasi-NRs on FTO surface was characterised using a Perkin Elmer double-beam UV/VIS/NIR spectrophotometer model Lambda 900.
Results and discussion
Meanwhile, on the dimension of the quasi-NRs, in spite of such intense aggregates amongst the nanostructures, on the basis of available free-standing individual quasi-NRs (see dotted circles in high-resolution image in Figure 1D); the diameter can be estimated to be approximately 30 nm. It is true that the present quasi-NRs are relatively inferior in terms of morphology and orientation control compared to those currently obtained using other synthetic methods. However, the present technique at least provides an alternative way for a rapid formation of quasi-1D ZnO nanostructures films directly on the surface. Improved and controlled morphology might be achieved later if suitable conditions are obtained, for example via a surfactant modification.
It is important to note here that the nanoseeds are necessary for the preparation of quasi-NRs morphology. If they were absent on the surface, no quasi-NRs products were obtained. Irregular and big nanostructures sometimes were found on the surface instead. However, these could be the precipitates that formed in the solution which then attached onto the surface.
On the quasi-NRs crystals growth direction, as is evident from the XRD results, the preferred growth orientation of the quasi-NRs might be towards  direction judging from the appearance of relatively higher peaks belonging to this crystallographic plane on the spectrum. The peak ratio between this plane and (101) is as high as approximately 1.5 to 2.0, which is much higher compared to the standard ZnO XRD data (JCPDS 01-079-2205), namely approximately 0.5. This result agrees well with those obtained from most ZnO nanorods prepared using, e.g. hydrothermal or other techniques [22, 23] in which the  is the main crystal growth orientation of the ZnO nanorods. It is true that HRTEM analysis is required for determining the growth orientation of the quasi-NRs. Since the apparatus is unavailable at the moment, a detailed analysis on the crystal growth orientation is being pursued and will be reported in a separate publication.
On the basis of the experimental results, we confirmed that the present approach has successfully promoted the formation of ZnO quasi-NRs from the nanoseed particles. However, at the moment, the mechanism of growth is still not yet well understood. Though, we thought that the growth characteristic of the present system seems identical to the formation of CuO nanowires as reported in . As has been well known, when an aqueous metal salts solution, such as Zn(CH3OO)2 here, was introduced to the NH3, unstable zinc-ammonium complexes might be formed at the first instance. They then rapidly transformed into zinc hydroxides, more stable zinc complexes in solution. In the presence of ZnO nanoseeds on the surface, as confirmed by the XRD shown in Figure 2, these complexes might transform into tetragonal ZnO4 phases that initiates the formation of O-Zn-O bridges with the nanoseeds via an olation process [31, 33]. Thus, the nanorod structures were projected. In the present work, unsuccessful coordinated zinc hydroxide complexes might apparently be formed, but remained in bulk solution in the form of white-bluish suspension. If attached onto the surface, it can be easily washed out by rinsing with excessive water.
It needs to be noted here that to produce quasi-NRs morphology, the stirring process is necessary in this procedure. If there were no stirring, no quasi-NRs growths were obtained, but a thin films structure composed of quasi-spherical particles instead. It is typical in the present procedure that the zinc complexes were rapidly hydrolysed in the solution upon the addition of ammonia (see growth process in section 2.2.). The hydrolysed complexes easily aggregate on each other forming a bluish colour in solution and at a certain condition they precipitate down to the bottom of the vials. In order to maintain the formation of ZnO quasi-NRs on the surface, the zinc complexes precursors' availability near the nanoseed surface should be sufficient and be controlled. For that reason, the zinc complexes have to be quickly transported to the vicinity of the nanoseed surface by means of stirring shortly after being hydrolysed. Thus, quasi-1D morphology can be formed.
The concentrations of ammonia and zinc salt used in the reaction were found to noticeably affect the structural growth (diameter and length) of the ZnO quasi-NRs on the surface. For the case of the ammonia, firstly, it is noted that the concentration which promotes the formation of quasi-NRs morphology is in the range of 36 to 360 mM. If the ammonia concentration is outside this range, for example lower than this value, no quasi-NRs were obtained, but instead irregular shape particles film formed on the surface. This could probably be associated with the limited precursor availability as a result of a weak hydrolysis process under such low ammonia concentration. Meanwhile, when the ammonia is higher (>360 mM), no or limited quasi-NRs growth was obtained. At this condition, highly compact quasi-spherical nanostructures films were obtained. This could be the result of solution instability under such high ammonia concentration in which the zinc complexes extremely formed and agglomerated in solution that in turn hindered the olation process on the nanoseed surface.
In addition, besides modifying the diameter and the length, the variation of ammonia also significantly alters the overall nanorod density on the surface; namely it improves with the increasing of ammonia concentration. Unfortunately, contrary to such enhancement in the density, the augmentation of ammonia induced extreme coalescence amongst the quasi-NRs at their top-end as the result of surface energy minimisation, generating bigger or irregular-shaped nanostructures on the surface that hides the underneath structure of individual quasi-NRs (see Figure 3).
An alternative method for the formation of vertically oriented ZnO quasi-NRs growth on the surface via 1D crystal growth of nanoseeds under a rapid hydrolysis of zinc complexes in the presence of ammonia has been demonstrated. In a typical process, high-density vertically oriented ZnO quasi-NRs with diameter and length in the range of approximately 30 and 110 nm, respectively, was the characteristic of the products. Quasi-NRs were found not to freely stand but leant on each other and combined at the top of the nanarods probably as the results of coalescing process of several quasi-NRs. The growth process was very quick; namely in the range of 4 to 5 min. The quasi-NRs morphology was influenced by the concentration of ammonia used in the reaction. In typical results, the quasi-NRs shape becomes more rounded and fatter with the increasing of ammonia concentration. Meanwhile, the diameter of the quasi-NRs decreased with the increasing of ammonia concentration. The as-prepared quasi-NRs products were pure ZnO phase without the presence of any zinc complexes and feature a relatively high-crystallinity property as confirmed by XRD and optical absorption spectroscopy results, respectively.
As for the mechanism, the quasi-NRs were projected from the nanoseeds probably due to an olation process of zinc complex[31, 33], such as zinc hydroxide, on the surface of ZnO nanoseeds, a process that is similar to what has been obtained in CuO nanorods .
At present, ZnO quasi-NRs with free-standing and a controlled morphology has not yet been achieved; however, the present method may become a potential alternative for the preparation of ZnO nanorods on the surface. Since the quasi-NRs morphology exhibited a relative dependence on the ammonia and zinc salt concentrations, ZnO quasi-NRs with controlled morphology will be realised if suitable conditions were obtained; for example by utilising the surfactants. The study on this effect is in progress.
We acknowledge the support from the Universiti Kebangsaan Malaysia and Ministry of Higher Education of Malaysia under research grant UKM-GUP-NBT-08-25-086 and UKM-RRR1-07-FRGS0037-2009 and the Universiti Tenaga Nasional and Ministry of Science and Technology and Innovation Malaysia under Science Fund 03-02-03-SF0196 project.
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