Production of zinc oxide with variable morphology by two novel wet-chemical methods
Ben Fulcher, Barry Tang, Dalibor Frutnik
Electron microscope Unit, Faculty of Science, University of Sydney Abstract Zinc oxide powders were synthesised by two wet-chemical methods featuring low temperature (£100°C) and atmospheric pressure. We investigated a hydrothermal method using reagents Zn(NO 3 ) 2 .6H 2O and hexamethylenetetramine (HMT) as well as an alkali precipitation method using reagents ZnO, and NaOH. The morphologies observed by scanning electron microscopy (SEM) include “cabbage-like”, “belt-like”, “flower-like” and “stringy-ball-like”. These morphologies were found to vary with substrate material and initial concentration of reagents.

1. Introduction Zinc oxide is a unique compound that demonstrates a large variety of nanostructures[1]. It is an ntype semiconducting material with a large band gap of 3.37eV and exhibits piezoelectric, pyroelectric, and photocatalytic properties. As such, ZnO has been investigated for potential in such diverse fields as optics, optoelectronics, catalysis and piezoelectricity. More specifically, ZnO has also found applications in solar cells[2-4], luminescence[5, 6], and as a rubber additive[7]. The large band gap of ZnO implies a strong absorbance in the UV for wavelengths lower than 360nm, such that it has long been an effective ingredient in sunscreens[8]. Among the sophisticated methods hitherto used to create ZnO structures are spray pyrolysis[9], metalorganic chemical vapour deposition (MOCVD)[10-12], magnetron spluttering[13, 14], ethanol-in-oil microemulsion[15], electrodeposition[16], oxidation of zinc[17], the sol-gel process[18], atomic layer deposition[3] and pulsed laser deposition[19]. Generally, these complicated methods require specialist machinery and expensive conditions such as elevated temperature and pressure. Recently, solutionbased methods[4, 20-23] have been reported, including alkali precipitation[9] and hydrothermal synthesis[24, 25] . Although concerns have been raised as to the presence of water and the subsequent formation of hydroxylated surfaces hindering catalytic performance[26], wet-chemical methods have the advantages of convenience, low cost, low temperature, use of atmospheric pressure, and therefore the prospect of large-scale production. Each method used to produce nanocrystalline ZnO involves a specific set of parameters that can be manipulated so as to modify the morphology and hence properties of the resultant ZnO powder. Nanorings, nanohelixes, nanobows, nanobelts, nanowires and nanocages are some of the nanostructures that have been produced via a solid-vapour phase thermal sublimation technique[1], whilst electrodeposition techniques can produce nanocolumnar ZnO[16]. Unique structures have also been formed by simpler thermal solution-based methods; ellipsoidal and needle aggregates, rod-like particles[9], pencil-like, twinned pyramidal, shortened prismatic structures[24], flower-, snowflake-, prism-, prickly sphere-, and rodlike samples[21], as well as nanorod[20, 27]. Zinc oxide is a well-known photocatalyst[28, 29]. The CO shift reaction ( CO + H 2O Æ CO2 + H 2 ) uses ZnO and copper as a catalyst[30], where the decomposition of methanoic acid is catalysed by ZnO alone[31]. Furthermore, it has been shown that ZnO morphology has direct influence on its photocatalytic properties[28, 29]. Enhanced catalytic properties of ZnO in the photodecomposition of volatile organic compounds has been attributed to higher surface area and features of surface nanostructure[27]. It is also suggested that chemical, electrical and optical properties are reliant upon crystalline morphology and surface area. Control of surface area and morphology is a vital aspect in other applications such as photo-emitters, sensors, varistors and transducers[27]. Singhal et al. showed that the critical voltage of the varistor is directly related to the size of ZnO particles[15]. Further, the use of ZnO as a humidity sensor requires a sparse, porous microstructure[32]. It is therefore vital to control the nanostructure of ZnO to allow its use in novel applications that may demand a specific morphology to be produced and subsequently utilised on a large-scale, with methods based on the principles presented in this paper potentially contributing to this end.

In this paper, two aqueous solvent-based methods involving the precipitation of ZnO are investigated to determine factors affecting the morphology of the resultant ZnO powder. 2. Experimental 2.1 Hydrothermal synthesis using a metallic substrate A variable mass of hexamethylenetetramine (HMT) (Aldrich Chemical Company Inc., 99%) was weighed and placed into a Teflon lined autoclave followed by 1.18g Zn(NO 3 ) 2 .6H 2O (Asia Pacific Specialty Chemicals, Ltd., 98%). 40mL of deionised water was then added to each autoclave without stirring, and the mixture aged at 95°C for 9 hours. Copper strips made into rings were rinsed ultrasonically in acetone for 20 minutes and then added to each autoclave. For experiments in which the role of silicon was investigated, a small piece of silicon metal was also attached to the top of the copper ring. After a further 12 hours at 95°C, the solutions were allowed to cool to room temperature in air, the substrates removed, flushed gently in deionised water and then dried at 95°C for several hours. A sample was then prepared for scanning electron microscope (SEM) examination. 2.2 Alkali precipitation A solution of ZnO powder (Merck, 99%) in 5M NaOH (Asia Pacific Specialty Chemicals Ltd., 97%) was prepared. A variable volume of this solution of was taken and made up to 40mL with deionised water. This dilution was then placed in a Teflon-lined autoclave, which itself was positioned into a steel autoclave and then aged at 100°C for 8 hours. After being allowed to cool to room temperature in air, the precipitate deposited at the base of the autoclave was concentrated by centrifuging several times in water and then once in ethanol. The result was washed into a ceramic crucible using ethanol and then dried in the oven for several hours at 95°C. The resulting thin layer of white powder was prepared for SEM examination. 2.3 Examination The silicon substrate was cut to an appropriate size using a diamond-tipped knife, while the copper substrate was similarly cut using a scalpel. The powders were scraped gently and then sprinkled onto adhesive tape in such a way as to obtain a very thin layer of powder. All samples were goldcoated prior to examination on both the Phillips/FEI XL30 SEM and the Phillips/FEI 505 SEM. 3. Results and Discussion 3.1 Hydrothermal synthesis The hydrothermal procedure followed here initially focused on varying the HMT concentration in the reaction flask. No ZnO was precipitated when the HMT concentration was less than 0.8g/40mL (ie. 0.110M). The presence of a deposition was inconsistent at a concentration 0.8g/40mL, occurring in only one of three identical trials. At HMT concentrations over this value, depositions were always observed, the sizes of which are depicted in Table 1. Table 1. Summary of Hydrothermal Experimental Results
Mass of HMT Substrate material Deposits observed Average particle size (microns) 0.7g Copper no N/A 0.8g Copper variable 120 0.9g Copper yes 60 1.0g Copper yes 55 1.0g Silicon yes 80 1.2g Copper yes 100 1.2g Silicon yes 60 1.4g Copper yes 100 1.4g Silicon yes 80

All morphologies produced by the hydrothermal method on either copper or silicon substrate were found to deposit primary cabbage-like structures connected into secondary belt structures (Figure 1, 2). As the HMT concentration increased, the belt-like linkages became less prominent, and the particles became more particle-like and independent. This can be seen by comparing the strong belt-like linkages of Figure 3A to the independent cabbage-like particular nature shown in Figure 3C.

Figure 1. SEM micrographs of samples prepared via hydrothermal synthesis on a copper substrate using (A) 0.8g, (B) 0.9g HMT These observations are consistent with the conclusion that increasing concentrations of HMT within the reagent vessel reduce the attraction between adjacent ZnO particles. The formation of ZnO nanorings, spirals and helixes can be explained by the minimisation of total energy due to polar electrostatics, surface area and elastic deformation[1]. ZnO is known to form polar structures[1], and the attraction between particles can thus be attributed to electrostatics. For two ZnO particles to attract each other and join together in a belt-like linkage is energetically favourable due to a minimisation of surface area and electrostatic potential. It is clear that an increased concentration of HMT inhibits this attractive force by ‘shielding’ adjacent ZnO particles from each other. The choice of substrate is important, as it has in some cases been shown to have a significant impact upon crystal growth and consequential structure. ZnO has been shown to produce different microstructures due even to different substrate orientation grown by metalorganic chemical vapour deposition (MOCVD)[10]. Substrate materials including sapphire[10, 19], crystalline quartz and amorphous glass[20], polycrystalline F-SnO2 glass, and Si/Si2 wafers[22, 27] have all been investigated. The substrate material, whether quartz or glass, was claimed to have no effect on crystal structure by Wang et al.[20]. The effect of substrate in our hydrothermal method was investigated using copper and silicon metal (lower six rows of Table 1). All other experimental variables were controlled by placing the two substrates into the same Teflon-lined autoclave. As mentioned above, Wang et al. found no correlation between substrate and ZnO morphology[20]. We, however, have found different structures on copper substrate compared to those deposited on silicon substrate when prepared under the same conditions (Figure 2). The particle sizes differed, as did the specific morphologies, although both substrate materials gave the same general morphology, whether cabbage-like or belt-like. The most pronounced example of this can be observed by comparison of Figure 2A,D. These were both synthesised using 1.0g HMT under the same conditions with (A) deposited onto copper and (D) onto silicon. It can be seen that (A) is belt-like, while (D) is cabbage-like. While other morphologies obtained are similar, the substrate material is certainly a contributing factor towards the growth and deposition of the zinc oxide crystal.

Figure 2. SEM micrographs of samples prepared hyrothermally on either copper (A,B,C) or silicon (D,E,F) substrate using (A,D) 1.0g, (B,E) 1.2g, (C,F) 1.4g HMT

The hydrothermal method presented here utilises zinc nitrate hexahydrate ( Zn(NO 3 ) 2 .6H 2O ) and hexamethylenetetramine (HMT – C 6H12N 4 ) (Figure 3). These reagents have been used previously in a number of aqueous routes[4, 9, 20, 22, 23, 27, 33], but they have never been employed via the method presented here. Li et al.[9] reported rodlike particles, Vayssieres et al.[22, 33] synthesised nanowires and nanorods, while Tian et al.[27] were able to produce nanorods, nanoplates and nanocolumns. Clearly the end result is related in a sensitive way to the conditions in which the ZnO crystal is grown. It has been shown for many different methods that the morphology and even composition of the resultant powder is extremely sensitive to even minor changes to the conditions in which it is produced[34]. This result is confirmed in both methods presented here, for which every tested variation in initial conditions yielded a different final structure.

Figure 3. Structure of HMT (C6H12N4) It has been shown by energy dispersive spectroscopy (EDS) in similar experimental procedures that the product of the thermal treatment of Zn(NO 3 ) 2 .6H 2O and HMT is ZnO without the presence of carbon or nitrogen contamination from the HMT[22]. Others have made use of X-ray powder diffraction (XRD) to come to the same conclusion[20] with regard to alkali precipitation. A similar method could have been carried out here to make certain that the product was in fact ZnO. Making use of the above references, however, the product has been assumed to be ZnO without impurity throughout the paper. Wither regard to length of thermal treatment we explored a secondary thermal treatment time of 2 hours, but no deposition could be recorded. Once a precipitate could be obtained by a particular method, a variation in thermal treatment time was not altered. Although this variable could be expected to influence the particle structures, it has previously been shown that prolonging the reaction time does not significantly vary particle size, nor enhance crystal growth[25]. When exploring a new methodology for the formation of novel zinc oxide microstructures, every variable must be controlled. In this case, all other variables have been controlled other than the initial concentration of HMT. A definite effort has been made to control all variables that are suspected to influence morphology. For example, HMT powder was always weighed out and added to the autoclave before Zn(NO 3 ) 2 .6H 2O and the reaction flask was never stirred. Being such a delicate growth process, changing these variables could possible influence the structure of the final ZnO crystal. We also placed the autoclaves in the oven at room temperature and allowed it to heat up in that way every time. What differentiates this procedure so markedly from other solution deposition methods is the addition of powders together followed by their dilution with water. In other wet-chemical methods[9, 20-22, 24, 25, 27, 33, 34] , the reagents are first dissolved in solution at some concentration and then added together at some volume ratio while stirring vigorously. This detail is proposed to be one of the factors influencing the growth of ZnO crystals in our hydrothermal method. The benefit of investigating ZnO over other metal oxides is the convenience of its single oxidation state of Zn2+[34]. HMT is a water soluble, non-ionic tetradentate cyclic tertiary amine[22, 35]. The behaviour of HMT around zinc though, is relatively unexplored compared to the large amount of research into the structure of HMT-silver complexes[36-39]. With regard to the chemistry behind the presence of HMT causing a precipitation of ZnO from Zn2+, I propose that perhaps hydrogen ions can attach to the four lone electron pairs on the HMT molecule forming ZnO by the reactions (1), (2) and (3) below. Reaction (4) gives the overall ionic process, while (5) shows the entire process including the nitrate spectator ions: 2Zn 2+ + 4H 2O Æ 2Zn(OH) 2 + 4H + (1) Zn(OH) 2 Æ ZnO + H 2O (2) C6 H12 N 4 + 4H + Æ C6 H16 N 4 4 + (3)

2Zn 2+ + 2H 2O + C6 H12 N 4 Æ 2ZnO + C6 H16 N 4 4 + (4) 2Zn(NO3 ) 2 + 2H 2O + C6 H12 N 4 Æ 2ZnO + C6 H16 N 4 (NO3 ) 4 (5)
3.2 Alkali Precipitation Variables explored in this method are summarized in Table 2, and representative morphologies in Figure 4 and Figure 5. The particles of the powder produced from 1mL of supersaturated ZnO powder

in 5M NaOH solution with 39mL water (powder 1) were all primarily stringy-ball-like of about 4mm in diameter (Figure 4A). As the concentration of this reagent solution became more concentrated, the morphology became increasingly diverse. At 3mL of solution to 37mL of water (powder 2), there were two main morphologies simultaneously present – both spiky- and fluffy-flower-like (Figure 4B,C). The 5mL result (powder 3) featured an average of the two forms (Figure 5D), while at 10mL (powder 4), the structures were unstable and only fragments of flower-like particles were observed (Figure 5E). Table 2. Summary of Alkali Precipitation Results
Name Mass of ZnO dissolved in NaOH Volume of solution used Average particle size (microns) Powder 1 Powder 2 Powder 3 Powder 4 Powder 5 Powder 6 Powder 7 Powder 8 -Supersaturated Solution2g 4g 2g 4g 1mL 3mL 5mL 10mL 3mL 10mL 3mL 10mL 4 10 7 12 5 8 6 N/A

Figure 4. SEM micrographs of samples prepared via alkali precipitation using (A) 1mL, (B,C) 3mL, (D) 5mL, (E) 10mL of supersaturated ZnO in 5M NaOH solution The second group of powders (powders 5-9) that were created using an unsaturated solution gave a novel result (Figure 5). There was a partial suspension of ZnO powder in the reaction mixture due to insufficient stirring. Small, suspended particles are observed to attach themselves onto the flower-like and rod-like ZnO nanostructures that have been precipitated from solution (Figure 5A,B,C). This is important because it demonstrates the polar attraction between solid ZnO particles and shows that the undissolved ZnO does not play a significant role in the crystal growth and precipitation process. From the observations, I infer that the dissolved solution forms these morphologies and then, in a secondary process, the suspended particles are attracted to them. This process is similar to the formation of belt linkages in the substrate method in that the nanoparticles are created, feel an attraction to each other and then aggregate. With regard to powders 7 and 8 (Figure 5D,E), no structural features were observed, only large aggregates of suspended ZnO solid. It is suspected that the increased amount of suspended ZnO solid (4g in 20mL rather than 2g in 20mL) has been attracted to what structure may have been formed by this process and in doing so completely covered it, thereby preventing its examination.

Figure 5. SEM micrographs of samlpes prepared via alkali precipitation using (A) 3mL, (B,C) 10mL of 2g ZnO in 20mL 5M NaOH solution; and (D) 3mL, (E) 10mL of 4g ZnO in 20mL 5M NaOH solution In considering the chemistry of the alkali precipitation method, we have ZnO dissolved in an alkaline solution, and thus have a solution consisting of Zn 2+ ,O2- ,OH - , Na + for which we would expect ZnO + 2NaOH + H 2O Æ Na2 [ Zn(OH) 4 ]. Due to heat convection and the diffusion of ions through solution, the complex Zn(OH) 4 2- can form clusters of Zn x Oy (OH) z(z +2y -2x )- by dehydration:
[40] OH - + OH - Æ H 2O + O2- . Such clusters will grow and eventually reach a critical size for the formation of ZnO powder, at which point ZnO is precipitated. The following equations are those put forward by Li et al.[41], which are used in a similar way elsewhere[20, 24]. The variables x, y, z, represent the number of Zn2+, O2- and OH- ions respectively in the crystal: Zn(OH) 4 2- + Zn(OH) 4 2- Æ Zn 2O(OH) 6 4 - + H 2O (6) Zn x Oy (OH) z(z + 2y - 2x )- + Zn(OH) 4 2- Æ Zn x +1Oy +1 (OH)(z++22y - 2x + 2)- + H 2O (7). z

But as mentioned in the introduction, this growth process is extremely sensitive to the reaction conditions. So while other wet-chemical methods[24] feature the same complex in solution, the conditions in which it grows and precipitates is different, and so it produces different morphologies of ZnO. The crystal growth process for zinc oxide is not well documented. While there have been some success in the complicated mathematical modelling of crystal growth[42], these have been simplified to 2-dimensional growth. But while this simplification for rod-like morphologies is acceptable, to apply the same model for the products reported here would not yield success due to their complex 3dimensional form. No conclusive trends with regard to particle size could deduced from either set of experimental results (Tables 1, 2). Thus the ultimate growth size does not simply depend on the concentration of a particular reagent or a substrate material. There is certainly a connection between particle size and the variables explored, but it evidently varies with such a magnitude of variables that it cannot be explicitly realized from the set of data obtained here. 3.3 Similarities to other reported structures The obelisk like ZnO nanorods reported by Wang et al.[20] have structural similarities to the powders prepared here by alkali precipitation. On close examination, it can be seen that the hexagonal obelisk-like rods are present in powders 2, 3, 4, 5 and 6 (Figure 4, Figure 5), but oriented in a flowerlike morphology. Wang et al.[20] were able to achieve a flower-like arrangement of these obelisk-like rods using a solution of Zn(NO 3 ) 2 .6H 2O and NH 4 Cl . It was stated that a decrease in the amount of ZnO nucleus on the substrate corresponded to the formation of this flower-like cluster, which was achieved by lowering the concentration of zinc nitrate below 0.02M, the reaction temperature to 8090°C or the pH to the range 9.5-10.2 or 10.8-11.2[20]. Their reaction occurred over 30 minutes on a quartz substrate. Our reaction occurred at a pH ranging from 13 to 14.1, at 100°C for 8 hours without substrate. It is curious that on one hand we observe similar morphologies formed by very different experimental methods, yet on the other hand, morphologies change dramatically with only small changes in initial conditions.

Xu et al.[24] also report morphological similarities with those reported here by alkali precipitation. The precipitation of ZnO from clusters of Zn x Oy (OH) z(z + 2y - 2x )- is considered the critical formation reaction for Xu et al., as it is here. Although the structure of powder 1 was mostly stringy-ball-like as mentioned above, it also showed unusual defects in form, exhibiting nanorods, block-like (Figure 6) and prism-like structures. A nanorods obtained in powder 6 is similar to the rod-like morphology reported by Xu et al. (Figure 7). Again the method was very different to ours, featuring zinc acetate with water, KOH, or ammonia maintained at 200∞C for 2 hours without substrate.

A

B

Figure 6. SEM micrograph by Xu, Wang et al. (A) with similar block-like morphology to that of our sample (B)

A

B

Figure 7. SEM micrograph by Xu, Wang et al. (A) showing similar rod-like morphology to our micrograph (B) Zhang et al. [21] used a simple solution-based method to create prism-like ZnO using ethanol as a solvent at 100°C for 13 hours. We were also able to create this structure as another unusual defect in our powder 1 (Figure 8).

A

B

Figure 8. SEM micrograph by Zhang, Sun et al. (A) that shows a similar prism-like morphology to our sample (B) through a very different experimental procedure What can be said is that all morphologies that contained comparable features to our powders were produced using a Zn(OH) 4 2- precursor. Since correlations were formed between the unusual defects in our powders 1 and 6, I infer that random isolated thermochemical areas contained different conditions during the reaction process thereby producing these defects. I claim also that the precursor is the important species in determining the range of crystal morphologies that can be produced. The particles deposited onto metallic substrate by our novel hydrothermal method have not to our knowledge been reported previously. We are also the first to report ball-like nanostructures produced alkali precipitation (powder 1). 5. Conclusions Zinc oxide particles with variable morphology were produced by two convenient wet-chemical methods. The growth, deposition subsequent morphology of ZnO was found to depend on the initial concentration of reagents and the substrate material. Cabbage-like structures produced by our

hydrothermal method featured belt-like linkages with greatest prominence when the added mass of HMT was less than 1.2 grams. It is proposed that the increased presence of HMT decreases the adjacent electrostatic attraction between ZnO particles. Evidence for electrostatic attraction between ZnO particles was observed also by the attraction of suspended ZnO particles to precipitated structures. The morphologies of ZnO produced by alkali precipitation varied from ball-like through to flower-like with variation in reagent concentration. Due to isolated areas in the reaction flask of atypical conditions, unusual defects in uniform structure were reported in powders 1 and 6. A viable chemical explanation for the formation of ZnO by both methods has been put forward. Ultimately it is hoped that a particular ZnO morphology will be suited to a specific application for which ZnO could be safely and conveniently produced by a novel wet-chemical similar to one reported here on a large scale for a specific application. 6. Acknowledgements We wish to thank firstly the Sydney University Science Department and the Talented Students Program for allowing us to undertake in this investigation. In particularly Lisa Stadtmueller for organising and overseeing the project, Mr. Zheng for spending many hours teaching and advising us experimentally, Dr. Zhu for overseeing the project and teaching us valuable scientific techniques, and Tony Romeo for giving up his time to teach us how the SEM works and how to use it effectively. We acknowledge the methodology of both wet-chemical methods to Mr. Zheng.

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