The kinetics of the hydrothermal growth of ZnO nanostructures

Thin Solid Films 515 (2007) 8679 – 8683 www.elsevier.com/locate/tsf

The kinetics of the hydrothermal growth of ZnO nanostructures Michael N.R. Ashfold, Rachel P. Doherty, N. George Ndifor-Angwafor, D. Jason Riley ⁎, Ye Sun School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK Available online 1 April 2007

Abstract The immersion of a material, seeded with ZnO nanoparticles, in an aqueous solution of Zn(NO3)2 and hexamethylenetetramine (HMT) at 90 °C yields an extended array of one-dimensional ZnO on the substrate surface. The structure of the ZnO evolves with reaction time. Initially nanorods are formed. At longer times the rods are tipped with nanotubes. Here we report a series of experiments in which both the composition of the reaction solution; concentrations of H+, Zn2+ and HMT; and the structure of ZnO deposited on the substrate are monitored as a function of reaction time. It was found that the change from ZnO rod to tube growth arises when the solution composition is such that it is no longer thermodynamically favorable to precipitate Zn(OH)2. © 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Nanotubes; Nanorods; Hydrothermal growth

1. Introduction Zinc oxide is a wide band gap semiconductor, 3.37 eV, with a large exciton binding energy, 60 meV. Applications of the material in electronic devices include blue light emitting diodes [1], piezoelectric transducers [2,3], gas sensors [4,5] and transparent conducting substrates for dye-sensitized solar cells [6,7]. Nanostructured zinc oxide films that exhibit increased surface to volume ratio and, owing to quantum confinement, modified electronic properties could enhance the performance of such devices and are expected to lead to new applications for the material. Hydrothermal [8–18], chemical vapour [7,19–21], pulsed laser [22–26] and electrochemical deposition [27–29] syntheses of macroscopic, aligned, arrays of 1-dimensional ZnO nanoparticles have recently been developed. A simple, low temperature, 90 °C, preparation of zinc oxide 1-dimensional nanomaterials involves the reaction between hexamethylenetetramine (HMT, also termed methenamine) and aqueous zinc ions [8,9,11,12,15,17,21]. When substrates, precoated with a laser ablated thin-layer of zinc oxide, are placed in ⁎ Corresponding author. Present address: Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK. Tel.: +44 207 594 6751; fax: +44 207 594 6757. E-mail address: [email protected] (D.J. Riley). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.122

this reaction solution arrays of aligned 1-dimensional, crystalline zinc oxide nanostructures are formed. Empirical changes to the reaction conditions have resulted in the growth of nanorods, nanowires and nanotubes on such substrates. Investigations in which ZnO has been grown over extended periods of time, several hours, have shown that the structures continually evolve, e.g., nanorods terminated with nanotubes have been observed. To gain increased understanding of the mechanism of the growth of these 1-dimensional zinc oxide structures we have studied the concentrations of the chemical species in the system as a function of reaction time. Here we report how the pH, concentration of HMT and zinc ions change during the experiment. The results are discussed in relation to the nanostructures formed. 2. Experimental The 1-dimensional nanostructures were grown on Si wafers coated with a thin film of ZnO. The substrate coatings were prepared using pulsed laser deposition, details of which are provided elsewhere [26]. The following experimental procedure was employed to prepare the ZnO nanoparticle arrays. Two sealed Schott bottles, the first containing 50 cm3 of a 0.1 mol dm− 3 Zn(NO3)2 solution plus the substrate and the second 50 cm3 of a 0.1 mol dm− 3 HMT solution, were placed in an oil bath at 90.0 ±

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0.3 °C. After the solutions had reached thermal equilibrium they were mixed, t = 0. The combined volume of solution in the Schott bottle, a vessel with a volume of 135 cm3, was 100 cm3. The solution was kept sealed and maintained at a temperature of 90 °C throughout the reaction, time t N 10 h. During the reaction the pH was continually monitored, using a pH electrode sealed in the reaction vessel. In addition 1.0 cm3 aliquots of the reaction solution were removed, through a rubber septum embedded in the lid of the Schott bottle, at regular intervals for analysis. The aliquots removed were immediately placed in ice cold vials to quench the reaction. The concentration of zinc ions and HMT in the aliquots was determined using electroanalysis, atomic absorption spectroscopy and NMR. The zinc ion concentration was monitored using both electrochemical analysis and atomic absorption spectroscopy. Electroanalysis was performed using anodic stripping voltammetry at a glassy carbon electrode. 0.05 cm3 of the solution from the reaction vessel was placed in an electrochemical cell and 0.1 cm3 and 10 cm3 aqueous solutions containing 0.05 mol dm− 3 HgCl2 and 0.1 mol dm− 3 NaCl respectively were added. Potential control was achieved using an EG&G 273 potentiostat and a three-electrode system: a glassy carbon working electrode, a platinum gauze counter electrode and a saturated calomel reference electrode. All potentials are quoted with respect to the calomel reference electrode. The solution was purged with argon for 5 min. Then a potential of − 1.2 V was applied to the polished glassy carbon electrode (GCE) for exactly 80 s. After deposition of the zinc containing mercury film, an anodic scan from − 1.2 to 0 V at a scan rate of 20 mV s− 1 was performed. The area of the zinc stripping peak was compared to that of standard zinc nitrate

solutions and the concentration of zinc ions in the aliquot removed from the reaction vessel calculated. Atomic absorption spectra were recorded on a Unicam 919 supplied by Unicam Cambridge, UK. The sensitivity to zinc with this system was 1 ppm. Three determinations were performed for each diluted aliquot. The instrument was calibrated by diluting a 1000 ppm zinc standard solution serially to 0.5 ppm, 1 ppm, 2 ppm, 3 ppm and 4 ppm. The zinc concentrations determined using AAS and electroanalysis were in agreement. NMR was used to monitor the concentration of HMT through the reaction. The hydrothermal preparation of ZnO was performed using D2O as the solvent and aliquots removed at regular intervals, as detailed above. 0.7 cm3 of each aliquot was mixed with 0.1 cm3 of a stock solution of TMS salt dissolved in D2O. Proton NMR spectra (non-solvent suppressed) were recorded for each sample using a JEOL GX270 instrument. The relative concentration of HMT in each sample was calculated using Jeol Spec NMR software to ratio the integrated area of the HMT peak to that of the TMS peak. Actual concentrations of HMT were estimated by comparison to the NMR spectrum obtained from a sample prepared by adding 0.7 cm3 of a 0.05 mol dm− 3 HMT in D2O solution to 0.1 cm3 of the stock TMS in D2O solution. The as-deposited products were characterized and analysed by scanning electron microscopy (SEM, JEOL 6300LV) and transmission electron microscopy (TEM, JEOL 1200EX). 3. Results Syntheses of ZnO NRs on ZnO seeds have been reported previously [12,17]. Fig. 1 shows how in the present experiments

Fig. 1. Electron microscope images of ZnO nanoarrays prepared as described in the text. Samples were removed from the reaction solution at different times. (a) A plan view SEM image of a substrate removed from the reaction solution after 2 h. (b) A 30° tilt SEM image of a substrate removed from the reaction solution after 3 h. (c) A 30° tilt SEM image of a substrate removed from the reaction solution after 10 h. (d) A TEM image of the nanotubes formed on the substrate after 10 h immersion.

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the structure of the ZnO films, grown on Si substrates precoated with ZnO seeds deposited using PLD, evolved with time. For substrates removed from the growth solution at t b 2 h, Fig. 1(a), well-aligned ZnO hexagonal nanorods are obtained. For longer immersion times the morphologies of many of these nanorods begin to evolve from hexagonal rods to “syringe”-like structures. Fig. 1(b) shows an SEM image of a ZnO sample grown for t = 3 h, ‘volcano-like’ structures with a central hole are evident on the tops of some but not all of the rods. At extended times, t N 6 h, these craters develop into ultra thin nanotubes. An SEM image of a substrate removed from the growth solution after a period of 10 h is shown in Fig. 1(c). The growth of tubular structures was confirmed by TEM. Fig. 1(d) shows a TEM image of a selection of nanostructures removed from a substrate that had been immersed in the growth solution for a period of 10 h. The as-grown nanotubes are typically 20–40 nm in external diameter and possess wall thicknesses of 5–15 nm. In previous studies we have

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demonstrated using HRTEM and XRD that both the nanorods and nanotubes grow along the [0001] direction and exhibit Zn-polar surfaces with both Zn and O termination. The role of the surface termination in determining nanostructure has been discussed elsewhere. In this paper we are primarily concerned with establishing a relationship between the concentration of reactants in the bulk solution and the evolving nanostructures. Fig. 2 shows the pH, zinc ion and HMT concentration of the bulk solution as a function of time. The pH at t = 0 in Fig. 2(a), 5.66, is that of the HMT solution. On addition of the Zn(NO3)2 solution (that exhibited a pH of 4.16 immediately prior to mixing) the pH falls to 5.51. Over the following 2 h the pH increases until a pH of 5.64 is achieved. A pH of ~ 5.64 is maintained for the remainder of the experiment. The zinc ion concentration, see Fig. 2(b), shows a rapid decrease from its initial value of 0.05 mol dm− 3. The inset in Fig. 2(b) shows that at longer times (t N 2 h) the zinc ion concentration decreases linearly with time. The zeroth order rate constant for the reaction at a constant pH of 5.64 is 7.4 × 10− 4 mol dm− 3 s− 1. The HMT concentration, see Fig. 2(c), decays rapidly in the first 2 h of the reaction but shows a much slower decay thereafter. The concentration of HMT does not tend to zero during the 9 h. The inset in Fig. 2(c) indicates that, for t N 2 h, when the pH is constant, the rate of decomposition of HMT is first order with a rate constant of 0.059 h− 1. 4. Discussion The key reactions in the formation of zinc oxide nanoparticle arrays are the thermal decomposition of HMT to formaldehyde and ammonia, the latter acting as a base in aqueous solution: − C6 H12 N4 þ 10H2 O⇌6CH2 O þ 4NHþ 4 þ 4OH

and the precipitation of ZnII ions: ZnII þ 2OH− ⇌ZnO þ H2 O: We first consider the role of HMT in the reaction. It is often stated that HMT simply acts as a source of hydroxide to drive the precipitation reaction. However it has also been argued that HMT's role is that of a buffer, the rate of hydrolysis decreasing with increasing pH and vice versa [11]. Tada [30] and Strom and Won Jun [31] have demonstrated that the observed first order rate constant, kobs, for hydrolysis of HMT, in the absence of zinc species, is given by the expression: kobs ¼ ðkw þ kh ½Hþ Þ f þ

Fig. 2. Concentration of reactants versus time: (a) pH, (b) [Zn2+], the inset is an expansion of the data in the time interval 2 to 8 h, (c) HMT, the inset shows ln[HMT] over the time period in which the pH is constant.

where kw and kh are the water solvolysis and hydrogen-ioncatalysed reaction rate constants respectively and f+ is the fraction of HMT in its protonated form. At the solution pH of this experiment kw ≫ kh[H+]. The Arrhenius analysis presented in Ref. [31] yields a kw value of 2.36 h− 1 for a reaction temperature of 90 °C. Van't Hoff analysis, using standard bond enthalpies for protonation of an amine, gives a pKa of 4.01 for HMT at this temperature. The predicted rate of decomposition of HMT at 90 °C in a zinc free solution of pH 5.64 is 0.055 h− 1.

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Fig. 3. Speciation diagram showing the percent of Zn(II) present in the labelled form at each pH. Only species that are present at a ratio of greater than 10% in the pH range 2–13.5 are displayed.

The close agreement between this value and that observed experimentally for t N 2 h indicates that the rate of decomposition of HMT is independent of the reaction that yields zinc oxide, i.e., HMT serves as a kinetic buffer of solution pH. The continual decomposition of HMT means that the reaction solution will contain ammonium ions and formaldehyde in addition to the starting material and zinc complexes. A speciation diagram [32,33] for a solution at 25 °C containing 5.9 × 10− 3 mol dm− 3 of Zn2+, 0.1 mol dm− 3 NO3− and 0.1 mol dm− 3 NH4+, approximately corresponding to the composition of the growth solution at t = 3 h, is shown in Fig. 3. Consideration of reaction entropies suggests that at higher temperatures complexation will be less favoured, thus the diagram indicates that in the pH and temperature range of these experiments the majority of zinc present in solution phase is of the form Zn2+(aq). Zinc oxide can be precipitated from aqueous solutions either directly: Zn2þ þ 2OH− ⇌ZnO þ H2 O

namically unstable. At this stage any Zn(OH)2 formed on the substrate will dissolve and further growth of nanostructures on the substrate will occur by the direct deposition of ZnO. Comparison of the measured pH and Zn2+ concentration with the calculated equilibrium concentration of Zn2+ at each pH indicates that the Zn(OH)2 is thermodynamically unstable with respect to hydrolysis at t ≥ 2.5 h. The fact that the ‘volcano-like’ structures first emerge between t = 2 h and t = 3 h suggests the change in precipitation mechanism, from deposition via a hydroxide intermediate to formation of zinc oxide directly, encourages the emergence of nanotube structures. Whilst the results reported here provide evidence that the change of structure of arrays of zinc oxide nanorods grown on PLD seeded substrates is related to the change in the mechanism of deposition they do not explain the fact that nanotubes tend to be located at the centers of the initially formed rods or the observation that nanotubes do not grow on all rods. This stems from the fact that we have only considered the bulk concentrations of HMT, Zn2+ and H+; to understand the non-

or via the hydroxide: Zn2þ þ 2OH− ⇌ZnðOHÞ2 ⇌ZnO þ H2 O: Using published thermodynamic data [34,35] and the Clarke– Glew equation the solubility of zinc oxide (K, ZnO + 2H+ ⇌ Zn2+ + 2H2O) and solubility product for zinc hydroxide (K, Zn(OH)2 ⇌Zn2+ + 2OH− ) at 90 °C were approximated as 2.74× 108 and 2.73 × 10− 16 respectively. The approximate equilibrium concentration of Zn2+(aq) for both the Zn(OH)2(s)/Zn2+(aq) and ZnO(s)/Zn2+(aq) equilibria at 90 °C are displayed in Fig. 4 for a range of pH values. The measured pH and Zn2+ concentrations, recorded at 30-minute intervals during zinc oxide formation, are also plotted in Fig. 4. In oxide formation, the phase that is less stable when in contact with the media will generally precipitate first [11]. In the initial stages of nanostructure formation the pH and [Zn2+] concentration are such that formation of ZnO on the substrate will occur via Zn(OH)2. As the pH of the solution increases and the zinc ion concentration decreases the eventual composition of the solution is such that Zn(OH)2 is thermody-

Fig. 4. Stability of Zn(OH)2 and ZnO as a function of pH. Solid lines represent the calculated Zn(OH)2/Zn2+ and ZnO/Zn2+ equilibrium concentrations. The symbols are the measured pH and [Zn2+] at 30-minute intervals, starting at t = 1 h.

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uniform emergence of nanotubes it is necessary to consider the local concentrations of the reactants, as discussed, for example, in Refs. [17,36]. 5. Conclusions The combined study of the temporal evolution of solution composition and ZnO nanostructure provides evidence that the change in the architecture of the ZnO array arises as a result of a change in the precipitation mechanism of the oxide. Nanorods are formed when Zn(OH)2 is a stable intermediate and nanotubes when the solution composition is such that Zn(OH)2 is thermodynamically unstable. This correlation indicates how reactant concentrations can be used to control zinc oxide structure at the nanoscale and, for other metal oxides, points to the concentration regimes where evolving structures may occur. Acknowledgements The authors are grateful to University of Bristol and the Overseas Research Scholarship (ORS) scheme for postgraduate scholarships (Y. Sun and N.G. Ndifor-Angwafor). References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [2] Z.C. Tu, X. Hu, Phys. Rev., B 74 (2006) 035434. [3] J.H. Song, J. Zhou, Z.L. Wang, Nano Lett. 6 (2006) 1656. [4] X.D. Wang, C.J. Summers, Z.L. Wang, Nano Lett. 4 (2004) 423. [5] C.H. Wang, X.F. Chu, M.W. Wu, Sens. Actuators, B, Chem. 113 (2006) 320. [6] P. Ravirajan, A.M. Peiro, M.K. Nazeeruddin, M. Graetzel, D.D.C. Bradley, J.R. Durrant, J. Nelson, J. Phys. Chem., B 110 (2006) 7635. [7] E. Galoppini, J. Rochford, H.H. Chen, G. Saraf, Y.C. Lu, A. Hagfeldt, G. Boschloo, J. Phys. Chem. B 110 (2006) 16159. [8] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Chem. Mater. 13 (2001) 4395. [9] D.S. Boyle, K. Govender, P. O'Brien, Chem. Commun. (2002) 80.

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