High-Quality, Reproducible ZnO Nanowire Arrays Obtained by a

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High quality, reproducible ZnO nanowire arrays obtained by a multi-parameter optimization of chemical bath deposition growth George Syrrokostas, Katerina Govatsi, and Spyros N. Yannopoulos Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01812 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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Crystal Growth & Design

High quality, reproducible ZnO nanowire arrays obtained by a multi-parameter optimization of chemical bath deposition growth

George Syrrokostas,1* Katerina Govatsi,1,2 and Spyros N. Yannopoulos1*

1 Foundation for Research and Technology, Hellas – Institute of Chemical Engineering Sciences (FORTH/ICE-HT), P.O. Box 1414, GR-26504, Rio–Patras, Greece 2 Department of Chemistry, University of Patras, GR-26504, Rio–Patras, Greece

Keywords: ZnO, nanowires, chemical bath deposition, seed layer, SEM

ACS Paragon Plus Environment

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Abstract

ZnO nanowire arrays represent an important multifunctional platform in nanotechnology with a diverse set of emerging applications. Many parameters and conditions are involved in the growth process affecting quality and reproducibility and hence the functionality of the nanowire arrays. Therefore, the fabrication of high-quality arrays is challenging and demands fastidious optimization. A multi-parameter optimization of the growth process is presented here using the chemical bath deposition method of ZnO nanowires on seeded substrates. Parameters such as the seed layer morphology, ammonia and the polyelectrolyte capping agent concentrations, the molecular weight of the polyelectrolyte capping agent, the ammonia evaporation, the substrate placement into the reactor vessel and the growth duration were adjusted to provide optimum nanowire arrays. Criteria used for selecting the optimum growth conditions include high aspect ratio, high nanowire length growth rate, good alignment, and minimum defect density in the crystal lattice. A critical comparison of the current achievements with the relevant current literature is presented. ZnO nanowire arrays fulfilling all these criteria hold promise for the fabrication of devices with wide functionalities including solar cells, water splitting, photocatalysis for water purification, nanophotonics and so on.

1.

Introduction As a wide bandgap semiconductor, zinc oxide (ZnO), has drawn extreme attention over

the last fifteen years not only due to its physical properties, but also due to an impressive variety of nanostructures this material can be transformed in, with the dominant ones being the onedimensional (1D) structures. Typical morphologies of nanostructures that have been grown include: nanobelts,1 nanorods and nanowires,2 nanotubes,3 nanorings,4 nanocombs and


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nanotetrapods,5 and several other as have been described in various reviews6,7. Control over the morphology of the nanoscale building block is essential since several, important for applications, physical properties of ZnO, such as optical, electrical, piezoelectric, pyroelectric, photocatalytic, etc., can be appropriately tuned. The nanowire (NW) building block has emerged as the most prominent structure for a number of applications. The long dimension of the NW provides a pathway for the fast and lossless transport of electrons, holes and photons, rendering NWs favorable candidates for micro/nano-electronics and nano-photonics. Rational growth methods have been developed allowing today the fabrication of arrays/assemblies of nanowires of various geometries and orientations, either vertical to the substrate or in-plane geometries, and achieve control over their self-assembly characteristics.8,9 A number of growth methods have been developed for the controlled, bottom-up, growth of ZnO nanostructures. These methods belong to two main categories: vapor phase and solution phase approaches. Sputtering, chemical vapor deposition, molecular beam epitaxy, pulsed laser deposition and thermal evaporation are among the vapor phase methods. The solution phase methods involve mainly hydrothermal decomposition, sol-gel approaches and electrochemical reactions. Vapor phase methods usually involve high temperatures, strict chamber conditions (high vacuum) and costly equipment, requiring also single crystal substrates. However, these methods provide ZnO nanostructures of high quality and purity, high growth rate, facilitate controlled doping, and offer flexibility in the morphology of the structures. On the contrary, solution phase methods are cost effective and eco-friendly, having also the prospects of scalability and integration of NW arrays onto flexible organic substrates, which could lead to the fabrication of foldable devices. However, there are certain disadvantages associated with the chemical bath deposition (CBD) techniques as they require long growth times, entail post-growth treatment and result in nanostructures with moderate aspect ratios and nanowire diameters larger


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than those obtained by the vapor phase growth techniques. Besides, the presence of defects in the crystal lattice is also a concern when applications in electrical devices are foreseen. In general, there are three main requirements for a successful growth of ZnO nanowires for a number of emerging applications in energy harvesting, optoelectronics and nanophotonics: (a) to control the preferred (vertical) orientation of the grown NWs, (b) to maximize their surface area and aspect ratio, by maintaining the mean diameter as small as possible while keeping at the same time a uniform diameter size distribution and (c) to minimize the defects concentration. The morphology of ZnO NWs grown by CBD depends on a large number of parameters, such as the seed layer characteristics (grain size and orientation, thickness, etc.), the concentrations of the reactants in the nutrition solution, the growth time, the bath temperature, the position of the substrate in the reactor, the presence of capping agents (usually polyelectrolyte molecules), the solution pH, the mechanical stirring, etc. Obviously, a simultaneous optimization of these parameters toward achieving NW arrays with the desired properties is a formidable task. To make this paper self-contained, we provide below a brief survey on the current literature dealing with the optimization of ZnO NW array growth by exploring the role of a typical polyelectrolyte molecule used as capping agent, i.e. polyethyleneimine (PEI) and ammonia, as they represent the two best optimized parameters in this work. Then we present our data concerning the optimization of these and other experimental parameters in conjunction with the three requirements mentioned above.

II.

Brief survey on the influence of capping agents in the growth of ZnO NW arrays The optimization of only one of the growth parameters is usually realized in the vast

majority of the published articles. Few works have so far appeared where a more systematic growth parameter optimization exploration has been undertaken. In one of the first attempts to


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systematize the controlled growth of the morphology and the alignment of ZnO NW arrays were undertaken by Guo et al.,10 who investigated several growth parameters such as the seed layer details, the growth temperature, the precursors concentration, and the growth time. Tak et al.11 employed a multistep method where the morphology of the zinc metal seed layer, pH, growth temperature, and concentration of zinc salt were optimized. It was found that at each growth step the NW diameter and length increase by 15 nm and 1 µm, respectively, resulting in an increase of the aspect ratio, r, from r=47 (as grown) to r=56 (after three steps). Qiu et al.

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used

preheating of the nutrient solution to grow ultralong ZnO nanowires, i.e. 40 µm at 150 h of growth time. They optimized the NW length by varying the growth time, the concentration of the capping agent and the nutrient solution, the preheating time, and the thickness of the seed layer. Overall, the lowest NW diameters achieved was of ~120 nm, while the highest aspect ratio was of about r=330 at 150 h of heating time. However, a very high density of defects was found in the as-grown ultralong NWs. Building upon the work of Xu et al.13 optimization of the concentration of ammonia was attempted by Zhitao et al.,14 in order to adjust properly the degree of the supersaturation of the solution and the capping agent. A linear increase of the NW length vs. the growth time was observed reaching 25 µm in 10 h with aspect ratios of about r=80. Finally, in another systematic investigation by Xu et al.15 adopted a systematic statistical design and analysis to optimize the aspect ratio of ZnO NWs by controlling the reaction temperature, the growth time, the precursor concentration and the presence of capping agent. It was found that for precursor concentration ~1 mmol/L, at a reaction temperature ~80 °C, and growth time of about 30 h, the aspect ratio of the ZnO NWs achieved their highest value of nearly 23. Capping agents, such as PEI, are used mainly in order to control the diameter of the NWs. In one of the first attempts to explore its influence on the growth of ZnO NWs Law et al.16


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reported the preparation of high density NW arrays with r≈125. The role of PEI is to adhere on the lateral facets of ZnO due to electrostatic affinity, depending on the pH of the growth solution. 17–19

For pH values of the solution higher than that of the isoelectric point, PEI becomes

protonated, and as positively charged adsorbs onto the negatively facets of the ZnO NWs. PEI has been introduced in the nutrient solution by several groups, while the effect of its concentration on the features of the NWs has been examined in a limited number of cases. Zhou et al.,17 achieved reduction of the NW diameter from 300 to 40 nm by varying the amount of PEI from 0 to 6 ml in 50 ml solution; at the same time, they observed also a decrease in length from 2.5 µm to 1.5 µm after 2 h of growth time. Optimization of PEI concentration by Wu et al.,18 resulted in a ten-fold decrease in the NW diameter, while a limit of PEI concentration beyond which no crystal growth occurs was found. The variation in both the diameter and the length was also explored when different concentrations of PEI were added.12,13 A change in the morphology from hexagonally faceted to cylindrical needle like, resulting also in an increase of the aspect ratio, has been reported at certain PEI concentrations.20,21 Finally, Sun et al.22 examined the effect of PEI and ammonia concentrations on the growth of branches for the development of ZnO nanoforests. Besides the role of PEI in dictating the morphology of ZnO NWs, it has been observed that it can reduce the defect density of the nanocrystals as observed in photoluminescence spectra,23 or its presence in the growth solution can alter the nature of defects by increasing the oxygen interstitial atoms.21 The use of PEI for the preparation of the seed layer has been also reported.24 More recently, an additional function of PEI, than that being simply an adsorbent on the lateral facets, was proposed by considering its role into various chemical reactions taking place in the course of the crystal growth.25


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Although the effect of adding different concentrations of PEI on the morphology and the properties of ZnO NWs has been amply documented in the literature, the comparison of the influence of the molecular weight keeping all other parameters the same, has not yet been explored. A diverse set of PEI molecular weights has been studied, with values ranging from low i.e. 800,13,21 4000,18 4500,17 to as high as 70000,24 or 750000,26 whereas in several other works the molecular weight is not specified.12,14,16,20,22,23,25 The brief survey presented above demonstrates that despite the vast number of attempts to optimize the growth parameter of the CBD, further exploration is still indispensable for improving the physical properties of the ZnO nanowire arrays including the concurrent control of their aspect ratio, the NW height growth rate and the density of defects. In this article, we reexamine in a systematic way the optimization of few growth parameters and conditions, paying particular attention to the concentration and the molecular weight of PEI as well as the concentration of ammonia. Other parameters explored include the morphology of the seed layer, the role of ammonia evaporation, the growth time and the placement of the substrate into the reactor vessel.

2.

Experimental Section ZnO NW arrays were grown on glass substrates. The substrates were cleaned initially

with a soft detergent. Ultrasound treatment followed, first in a solution containing a small amount of acetone (5% v/v) and then in a solution containing ethanol (5% v/v). They were then rinsed with 3-D water and dried with air. Calcination at 150 oC for 15 min completed the cleaning process.


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The precursor solution for the seed layer growth was prepared by dissolving 0.005 M zinc acetate dehydrate, Zn(CH3COO)2 2H2O (ZnAc, purity > 98%, Aldrich) in absolute ethanol. The resultant solution was vigorously stirred at 60 oC for 60 min to yield a clear and homogeneous solution. Certain volumes of the solution (5 drops) were spin-coated onto the glass substrates at 1000 rpm for 30 sec, at ambient conditions and the procedure was repeated up to 6 times. The films were dried at 150 oC for 20 min and a second deposition was followed, with the same conditions, in order to achieve the desired thickness and morphology of the seed layer. Finally, films with the precursor compound were annealed at 300 oC for 20 min to achieve its decomposition to ZnO seed nanocrystals. A schematic of the procedure described above is shown in Figure 1.

Figure 1. Schematic of the seed layer formation via the spin-coating and thermal decomposition of a precursor zinc salt. The seeded glass substrates were placed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Merk, 99.5%), hexamethylenetetramine (C6H12N4, HMTA, Riedel-de Haen, 99.5%), polyethylenimine branched (PEI, Aldrich) and ammonium hydroxide


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(NH4OH).13 Two different molecular weights of PEI were used, i.e. m.w.≈800 and 25.000. The reactants concentrations were 50 mM Zn(NO3)2·6H2O and 25 mM HMTA. The PEI content was changed from 0 to 0.1 g, keeping constant the solution volume at 15 ml. In the presence of these reactants, the pH of the solution depends on the amount of ammonium hydroxide added. The seeded substrates were placed in the growth solution face down and inserted in an oven equilibrated at 88 oC for 7 h, except if otherwise stated. Finally, after growth the substrates were rinsed with 3-D water. A lidded glass bottle with a small hole on the cup was used for the growth. The morphologies of the deposited materials were characterized by FE-SEM (Zeiss SUPRA 35VP-FEG) operating at 5 – 20 keV. The crystal structures of the composites were investigated using X-ray diffraction (XRD Bruker D8 diffractometer, operating at 40 kV and 40 mA, employing Cu Kα radiation (λ = 1.54056 Å). The optical properties (photoluminescence, PL) were studied at ambient temperature with the aid of a PerkinElmer (LS45) luminescence spectrometer using the 325 nm as the excitation wavelength. Film thicknesses were measured with a stylus profilometer, while the seed layer morphology was obtained with the aid of an optical profilometer (Veeco NT1000).

3.

Results and Discussion

3.1 Morphology of the seed layer The basis to achieve control over the morphology of ZnO NW arrays is to prepare the proper seed layer which will bridge mismatch gaps between the substrate and the grown NWs. Indeed, the use of a ZnO-based seed layer has been shown to exhibit beneficial effects for the controlled growth of ZnO NWs as it facilitates the homoepitaxial growth on a diverse set of substrates, such as Si, polymers (PMMA, polyurethanes), paper, fibers, and so on.27 The seed


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layer provides an effective lowering of the nucleation energy barrier decreasing also the lattice mismatch, thus making heterogeneous nucleation on the seed crystals more favorable than the homogeneous nucleation in the solution at lower levels of supersaturation.28 The surface roughness further facilitates nucleation by offering more nucleation sites. The morphology of the seed layer affects significantly the growth of the NWs. For example the diameter and the density of the NWs are affected by the thickness and the grain size of the seed layer

29–33

, whereas a critical thickness is necessary in order to initiate the crystal growth. In

general a thicker seed layer results in less dense NW arrays and the diameter of the NWs arrays increases with the seed layer thickness, due to the following increase of the seed layer grain size. The above are not valid when the thickness is below 3.5 nm.29 The fine orientation of the NWs arrays is influenced also by the thickness, the crystallinity and the surface roughness of the seed layer.34–37 Moreover the presence of defects at the seed layer can affect the growth rate and the PL properties.38 While the grain size is frequently considered to dictate the width of the nanorods, this is not generally observed experimentally. The use of an optical profilometer is a fast and convenient method to observe the morphology of the seed layer. The plane view and a 3D visualization of a typical seed layer, as observed with the aid of an optical profilometer, are displayed in Figure 2. The ZnO seed crystals form islands with almost uniform density, as has also been reported in previous studies,34 with sizes reaching up to few tens of µm. The thickness of the seed layer varies from few nm to nearly 100 nm, making them suitable for crystal growth.29 Apparently, the density and the size of the islands dictate the diameter and the density of the NWs arrays.


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ZnO nanocrystals

(a) Glass Substrate

(b)

Figure 2. Plane view (a) and 3D view (b) of a seed layer prepared on a glass substrate.

3.2 Effect of the concentration of ammonium hydroxide In order to examine the combined role of ammonia and PEI on the growth of ZnO NWs, the concentration of ammonium hydroxide was varied from 0.2 M to 1 M, while the amount of PEI (m.w: 800) added in the solution was kept constant at 0.06 g /15 ml solution. Representative


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FE-SEM images showing the morphology of ZnO NWs arrays grown varying the concentration of NH4OH in the growth solution are illustrated in Figure 3.

Figure 3. FE-SEM images of ZnO NW arrays grown with different concentrations of ammonium hydroxide: a)0.2M, b)0.45M, c)0.6M, d)0.7M, e)0.8M, f)1M (growth temperature: 88 oC, growth time: 7h).

We observe that the presence of NH4OH affects significantly the morphology of the NWs arrays. We observe that increasing the concentration of NH4OH causes increase in the diameter of the NWs while their density is reduced; however, without appreciably affecting the NW alignment. Besides, for concentrations ≥0.7 M the NWs acquire an obelisk-shaped top with a layered structure, as also observed elsewhere.39


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As Figure 3 illustrates the mean diameter of the NWs increases from ~60 nm to a value greater than 200 nm, over the concentration range of NH4OH 0.2 to 1M. The length of the NWs exhibits an increasing trend and saturates at ~10 µm, at the NH4OH concentration of 0.8 M. The concentration dependence of the NW mean diameter and length are presented in Figure 4.

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50 0

0 0.2

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Concentration of NH4OH (M) Figure 4. NH4OH concentration dependence of the mean length and diameter of ZnO NW arrays. Inset: aspect ratio of the NWs vs. the NH4OH concentration.

Each point in the diagrams shown in Figure 4 represents the average value 2 to 4 experiments at each concentration of ammonium hydroxide, in order to verify the reproducibility of our results. Combining these parameters we observe that the aspect ratio of the NWs is maximized at the concentration of ~0.7 M, see Fig. 4 (inset graph), resulting in an overall


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increment of almost one order of magnitude in comparison to the aspect ratio obtained for the lowest concentration of ammonium hydroxide. An important outcome of this study relates to the near absence of defects in the crystal lattice of the single-crystalline NWs prepared by the current method. Defects in ZnO are known to affect optical emission. The optical properties of the ZnO NW arrays were explored by means of PL spectroscopy. Figure 5 presents the ambient temperature PL spectra of ZnO NW arrays prepared using different NH4OH concentrations. In general, the emission spectra of ZnO nanostructures are composed of two main bands; a rather narrow UV band located at about 370380 nm and another broader band, which depending on the morphology of the nanostructure, covers (partly) the visible part of the spectrum over the range 450-650 nm.40 The UV band 50

40

PL Intensity [arb. units]

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0.45 M 0.6 M 0.7 M 0.8 M 1M

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Figure 5. Representative normalized PL spectra of the ZnO NW arrays prepared under various NH4OH concentrations as indicated in the label. corresponds to the near-band edge (NBE) emission of the wide band gap ZnO (3.37eV or 369 nm), while the visible band, when present, corresponds to deep level emission originating from


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intra gap defects states. It is noteworthy that in the vast majority of PL studies of CBD-grown NW arrays, there is a strong visible PL band signifying the presence of significant defect density,10–12 which in most times is hardly diminishing after thermal annealing,12,41 or else a stepping heating procedure during growth must be used in order to decrease their density.25 On the contrary, the current PL spectra demonstrate the absence of defects whose presence would be detrimental to device performance especially when carrier mobility is involved, e.g. in solar cells. The ambient temperature NBE emission peak is located at ~378 nm bearing a full-width ah half maximum of ~12 nm; its spectral shape is insensitive to the NH4OH concentration. To discuss these findings in terms of the growth mechanism we recall that the following reactions intervene in a typical growth experiment of ZnO NW arrays by the CBD method,

+ 6 ↔ 4 + 6

(1)

+ ↔ ∙ ↔ +

(2)

+ 2 →

(3)

→ +

(4)

where HMTΑ ((CH2)6N4) is the source of hydroxide ions, with the rate of its hydrolysis depending on the solution pH and temperature.7,42,43 The addition of ammonia results in the formation of zinc amine complexes, according to the following reactions (5-8), and thus the degree of the supersaturation of the solution decreases. As a result, the formation of ZnO nanocrystals in the bulk solution, or else the homogeneous nucleation is less favorable. In particular, the precipitation of ZnO proceeds in a basic solution according to the reactions (5-8),39,42 + ↔ , = 1,2,3,4 (5) + 2 ↔ + +

(6)


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Or + 4 ↔

(7)

↔ + + 2

(8)

According to speciation diagrams,44,45 the most probable complexes are , n=4, when pH > 10. The complexes serve as a buffer for Zn2+ releasing these ions during growth, when NH3 volatilizes from the solution. Initially, in a closed system the rate of volatilization is fast, until the vapors cover the reactor vessel free space above the solution. Similarly the release of Zn2+ is fast, resulting in an initially high growth rate for the NWs.13,46 When equilibrium is established, then both the release of Zn2+ and the growth stop,47 unless consumption of the reactants has already taken place. According to Deng et al.47 the appropriate volume of the free space above the solution has to be three times larger than the volume of the solution, to allow for adequate volatilization of NH3. Wang et al.48 observed the growth of a layer of ZnO nuclei floating on the surface of the solution after uncovering the solution for 10 min, which was attributed to the rapid volatilization of ammonia. In addition, Chen et al.46 reported a significant increase of the ZnO NWs length when the volume of the free space above the solution decreased from 30% to 5% of the total solution volume. This observation contradicts the findings of Deng et al.47 presumably due to the lower concentration of the reactants (zinc nitrate and HMT) used in the former work and the absence of PEI. Moreover, etching of the NW surface was observed when the growth duration was more than 8 h, due to the high concentration of ammonia, while the grown NWs acquired a polycrystalline structure. If the reaction vessel is open to the air since the beginning of the growth, then the release of Zn2+ is faster than their consumption in the growth process of ZnO NWs.47 Thus, the homogeneous nucleation rate becomes faster than that in the closed system, due to the high


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degree of the supersaturation of the solution. In addition, evaporation of solvent will affect the concentration of the reactants. In our case, the equilibrium described by reaction (6) will not be established due to the small hole drilled on the cap enabling controlled evaporation of ammonia. Hence, the supply of Zn2+ is constant during growth and the volume of the free space above the solution has no effect on the growth process. As the concentration of NH4OH increases at values beyond those reported in the literature,13 due to constant volatilization, the homogeneous nucleation is suppressed more efficiently, thus accounting for the variation in the aspect ratio of ZnO NWs in the current work. No significant growth was observed with a closed vessel, when the concentration of ammonium hydroxide was 0.7 M, verifying the role of NH3 volatilization assisted by the small hole drilled on the cap of our reaction vessel.

3.3 Effect of the concentration and the molecular weight of PEI Having optimized the concentration of NH4OH in obtaining the highest aspect ratio ZnO NW arrays, its value was kept constant at 0.7 M, while the concentration of PEI was varied in order to further optimize the ZnO NWs properties. We provide here for the first time a comparison between the growth details of ZnO NWs for two different molecular weights of PEI, i.e. 800 and 25,000, whilst all other growth parameters were maintained the same. Figure 6(a) shows the XRD patterns of two typical ZnO NW arrays prepared using the low and high-


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25

m.w. 800 m.w. 25,000

20 15 10 5 0 350

400

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500

550

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Wavelength [nm]

Figure 6. (a) XRD patterns and (b) PL spectra of ZnO NW arrays grown using the low (800) and the high (25,000) molecular weight PEI as a diameter limited agent. molecular weight PEI. Evidently, only one diffraction peak, assigned to (002) crystal plane, is evident for both cases indicating perfect alignment of the NW long axis normal to the substrate and very good single-crystalline nature of the ZnO NWs. The high quality of the ZnO nanocrystals becomes evident from the PL spectra shown in Fig. 6(b). As in the case discussed above for ZnO NWs prepared under different NH4OH concentrations, the PL spectra exhibit only the UV band associated with the NBE emission, whilst the deep-level emission band is practically negligible. ACS Paragon Plus Environment

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Figure 7. FE-SEM images of ZnO NWs grown for different concentrations of low-molecular weight (800) PEI: a) 0.03g, b) 0.045g, c) 0.06g, d) 0.08g, e) 0.1g. (Growth temperature: 88 oC, Growth time: 7 h, c[NH4OH] = 0.7 M). Representative FE-SEM images revealing the morphology of ZnO NWs arrays grown at various PEI concentrations are shown in Figures 7 and 8 for low- and high-molecular weight PEI, respectively.


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Figure 8. FE-SEM images of ZnO NWs grown for different concentrations of high-molecular weight (25,000) PEI: a) 0.03g, b) 0.045g, c) 0.06g, d) 0.08g, e) 0.1g. (Growth temperature: 88 o

C, Growth time: 7 h, c[NH4OH] = 0.7 M).

In general, we observe that the mean diameter, orientation and density of the ZnO NWs do not significantly vary with the low m.w. PEI concentration, apart from the NWs grown using the concentration of 0.1g/15 ml solution. The changes in these properties are more obvious and systematic in the ZnO NWs grown using the high-molecular weight PEI where a tendency for the mean diameter reduction is evident with increasing PEI concentration. The quantitative results are presented in Figure 9.


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Aspect ratio

12 10


500

80

m.w.:800

60


40

400

20 0 0.02

0.04

0.06

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Concentration of PEI [g / 15 ml solution]

8

300

6 200 4 2


(a)

100

0 0.02

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0.10


Aspect ratio

8

400

80

m.w.:25.000

60


40 20

300 0 0.02

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0.10


6 200 4

100 2

0 0.02


(b)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0.04

0.06

0.08

0.10


Figure 9. Dependence of the length and the diameter of ZnO NW on the concentration of (a) low and (b) high molecular weight PEI. Inset graphs: aspect ratio of NWs vs. the concentration of PEI.

On the other hand, the variation in the length is more notable. For the low-molecular weight PEI the NW length passes through a maximum i.e. ~9.5 µm at the concentration of ~0.08


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g / 15 ml thereafter decreasing at higher concentrations. Similar behavior, though following a sharper profile, is exhibited in the NW length dependence for the high-molecular weight PEI. However, the maximum value reached in this case is ~7 µm. Notably, no crystal growth was observed for concentrations higher than 0.1g/15 ml, in the case of the high-molecular weight PEI. Again, each point in Figure 9 represents the average of 2 to 4 growth experiments, for both molecular weights, to verify the reproducibility of the results. These findings demonstrate a trade-off between the successful reduction of the diameter against the increase of the length of ZnO NWs. While low m.w. PEI favors the growth of long NWs up to ~10 µm, their mean diameter cannot be diminished below ~120 nm. On the contrary, high m.w. PEI is capable of keeping the NW diameter appreciably below 100 nm, but at the same time it does not favor the growth of long NWs. The calculation of the aspect ratio in both cases provides a useful quantitative measure of the optimum conditions for growing ZnO NW arrays. Figure 9 (inset graphs) displays the aspect ratio for the low- and high m.w. PEI nanowire arrays for various concentrations. It is obvious that the low m.w. polyelectrolyte works better for low concentrations, while the use of high m.w. PEI results in higher aspect ratios at higher concentrations. The best aspect ratios for the low and high m.w. PEI are ~70 and ~80, respectively. The current findings indicate that the high m.w. PEI exhibits the tendency to confine the growth of ZnO NWs at both the long and the lateral direction more efficiently than the low m.w. polyelectrolyte. To account for these results we have to consider that the high m.w. macromolecule with a mean chain length larger than the low m.w. one, by a factor of 30 (at least), provides efficient wrapping of the growing NW thus limiting growth at both directions. In addition, the longer chain molecule in the case of the high m.w. PEI affects stronger the supersaturation degree of the solution, due to coordination of PEI with Zn2+ ions.17 As a result,


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for a given concentration of NH4OH and the same PEI concentration, homogeneous nucleation becomes more favorable using the high m.w. PEI in comparison to the low m.w. polymer. This is observed as a larger amount of ZnO forming in the solution for the high m.w. polyelectrolyte and due to the faster depletion of the reactants, the size of the NWs is limited. In summary, it is demonstrated that the combined role of ammonia and PEI is crucial for the morphological properties of ZnO NWs.13

3.4 Effect of growth time A more effective indicator of an optimized growth process that is often overlooked is the growth rate of the ZnO NWs. Notably, the m.w. of the polyelectrolyte molecules affects not only the final NW length, but the growth rate as well. As we observe from Figure 10 the growth rate was 1.7 µm/h and 0.95 µm/h for the two molecular weights, 800 and 25,000 respectively for the first 5 h. For longer growth times was significantly reduced for both cases as has also been observed in other studies.13,46 No etching was observed though, as was reported elsewhere.46 This indicates that a significant part of the reactants have been consumed during the first 5 h. In addition, the solution pH was decreased from the range 10-11 to 7-8, after 15 h of growth, due to volatilization of NH3. We observe that further volatilization of ammonia, after 7 h of growth, has a minor effect in crystal growth, due to the depletion of the reactants. The growth rate exhibited in the present study is comparable to that reported in the literature, where the growth rate usually ranges from 0.3 to 2.5 µm/h.13,14,19,21,46,47,49


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600

(a)

500

m.w.: 800

8

400


6

300 4 200



10

2 100 0 0

2

4

6

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Growth time (h) 200

(b) 7

175

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5

m.w.: 25,000 Average thickness Average diameter

4

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3 100 2


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75 1 0

50 0

2

4

6

8

10

12

14

16

Growth time (h)

Figure 10. Growth time dependence of the length and the diameter of the ZnO NWs arrays grown with m.w. of PEI 800 (a) and 25,000 (b). [NH4OH] = 0.7M, amount of PEI: 0.08 g.

3.5 Effect of position of the sample in the growth reactor The location of the seeded substrate into the reactor vessel has also been considered as a factor affecting the growth details.50 It was reported that the distance between the substrate and the growth reactor affects the morphology of the NW arrays and more specifically the diameter of the NWs, since nucleation doesn’t initiate at the same time inside the whole volume of the


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solution. In our case the presence of ammonia into the nutrient solution and its volatilization through the partially open reactor lid implies that the placement of the substrate into the reactor will further affect the growth conditions. In order to verify the role of ammonia volatilization we have explored this effect. Based on the findings described in previous sections we expect that growth of the NW arrays will be less efficient near the surface of the solution in comparison to the bottom as the concentration gradient of ammonia entails a lower concentration near the surface. Figure 11 shows representative FE-SEM images revealing the morphology of ZnO NWs arrays keeping the same all the growth conditions, expect the height of the substrate into the reactor vessel. As Figure 11 reveals the average NW diameter increases from 130 nm to 210 nm for PEI with m.w. 800, while the effect is far more drastic for PEI with m.w. 25,000 increasing from 84 nm to 250 nm when the substrate is placed higher in the reactor vessel. On the contrary, the NW length is not appreciably affected by the placement of the substrate as it slightly decreases from 9.5 µm to 8.4 µm and slightly increases from 6.6 µm to 7.1, for PEI with m.w. 800 and 25,000, respectively. In summary, the lower ammonia concentration near the surface of the solution – originating from the continuous volatilization of ammonia – results in a significant increment in the diameter of the ZnO NWs. This arises from the larger availability of Zn2+ ions, released from 51 2+ the complexes. As reported elsewhere there is a critical concentration of Zn ions

for the growth of the different crystal planes. The critical concentration is lower for the growth of the (002) crystal plane, while at higher concentration a simultaneous growth of the other planes occurs, resulting in the lateral growth of the NWs arrays.


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m.w. : 800

solution

m.w.: 800

Page 26 of 36

m.w. : 25,000

m.w.: 25,000

solution

Figure 11. Representative FE-SEM images of ZnO NW arrays grown at various heights in the reactor vessel, while keeping all other growth conditions constant.


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The crystal orientation and quality does not depend on the position of the substrate into the reactor vessel as becomes evident from the XRD data shown in Fig. 12 (a). The sharpness of the 002 peak and the absence of any other diffraction peak manifest excellent single-crystal growth, toward a direction normal to the substrate.

Intensity [arb. units]

(a) m.w. 25,000; up m.w. 25,000; down

m.w. 800; up m.w. 800; down 26

28

30

32

34

36

38

40

42

44

46

48

50

2 Theta [degrees]

(b) PL Intensity [arb. units]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m.w. 25,000; up m.w. 25,000; down

m.w. 800; up m.w. 800; down 400

450 500 Wavelength [nm]

550

600

Figure 12. Comparison of (a) XRD patterns and (b) PL spectra of ZnO NW arrays grown using the low (800) and the high (25,000) molecular weight PEI for substrate position at the top and the bottom of the reaction vessel.


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The PL spectra illustrated in Fig. 12(b) further validate the above conclusion given that no evident deep-level emission is present in any of the ZnO NW arrays grown at various positions in the reaction vessel. In summary, while the morphological properties, and in particular the NW diameter, seem to depend of the substrate location – being optimum for the bottom position – no effect on orientation and crystal quality (defect density) is observed.

4.

Conclusions The controlled growth of high quality ZnO NWs has been explored by employing a

multi-parameter optimization of miscellaneous growth parameters involved in the chemical bath deposition method. Optimization the CBD growth of ZnO NWs is a challenging procedure as certain parameters and experimental conditions exert a profound effect on the morphology of the nanostructures and their physical properties. The influence of the seed layer morphology, the concentration of ammonia, the concentration and the molecular weight of the polyelectrolyte capping agent, the ammonia evaporation, the substrate placement into the reactor vessel and the growth duration on the morphology of ZnO NWs are among the parameters which have been examined in this article. The main findings of this study are summarized as follows. (i)

The concentration of ammonia exerts a significant effect on the aspect ratio of NWs which seems to be maximized at 0.7 M of NH4OH. The allowance of ammonia evaporation is of paramount importance for the growth of ZnO NWs. Hindering ammonia evaporation limits the growth procedure. This observation is related to the finding that the placement of the substrate near the bottom of the solution engenders higher throughput in the growth of NWs.

(ii)

The presence of polyelectrolyte molecules, i.e. PEI, play dominant role on the NW morphology. Apart from the concentration dependence, the m.w. was explored for the


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first time here in a comparative way. For the low m.w. PEI the ZnO NW aspect ratio is maximized (~72) at the concentration of ~0.07-0.08 g / 15 ml. A sharper dependence is found for the high m.w. PEI where the aspect ratio is maximized (~78) at the concentration of 0.08 g / 15 ml. Although the low m.w. PEI results in slightly longer NWs both the aspect ratio and the density of NWs are higher for the high molecular weight polyelectrolyte molecules. (iii)

The growth rate was found to appreciably depend on the m.w. of the polyelectrolyte. A decreased growth rate was observed in the case of the high m.w. PEI, resulting from the efficient wrapping of the NWs by the longer chains of the high m.w. PEI. Nevertheless, even in this case, the growth rate was comparable to the values reported in the literature.

(iv)

The PEI with m.w. 800 exhibits a substantially higher NW growth rate, i.e. 1.7 µm/h in comparison to the growth rate, 0.95 µm/h, dictated by the high m.w. PEI. Those rates, measured during the first 5 h of growth, are comparable with the rates reported so far in previous works and exhibit substantial decrease at longer growth times.

(v)

Finally, despite that all parameters optimized in the current work did indeed had a significant effect on the NW morphology, none of these parameters influenced the orientation and the optical properties of the prepared ZnO NW arrays. In all cases, vertical to the substrate growth took place and negligible defect density was observed.


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AUTHOR INFORMATION Corresponding Author *George Syrrokostas, E-mail: [email protected] ; Spyros N. Yannopoulos, E-mail: [email protected]

Funding Sources Financial support from the European-funded FP7 project ‘‘SMARTPRO’’ (grant agreement No.: 607295) is highly acknowledged. ACKNOWLEDGMENT Prof. G. Leftheriotis (University of Patras, Physics Department) and Prof. N. Bouropoulos (University of Patras, Department of Materials Science) are thanked for providing access to the mechanical profilometer and the PL instrument, respectively.

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For Table of Contents Use Only High quality, reproducible ZnO nanowire arrays obtained by a multi-parameter optimization of chemical bath deposition growth

George Syrrokostas,1* Katerina Govatsi,1,2 and Spyros N. Yannopoulos1* 1 Foundation for Research and Technology, Hellas – Institute of Chemical Engineering Sciences (FORTH/ICE-HT), P.O. Box 1414, GR-26504, Rio–Patras, Greece 2 Department of Chemistry, University of Patras, GR-26504, Rio–Patras, Greece

Synopsis: A multi-parameter optimization of the chemical bath deposition method for growing ZnO nanowire arrays is presented. Emphasis is placed on the influence of ammonia concentration/evaporation and the concentration and molecular weight of the polyelectrolyte capping agent, among other parameters. A protocol for the reproducible preparation of wellaligned, high-aspect ratio nanowire arrays, devoid of crystal-lattice defects is provided.


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High-Quality, Reproducible ZnO Nanowire Arrays Obtained by a

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