CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2570–2575
Articles Controlling Low Temperature Aqueous Synthesis of ZnO. 1. Thermodynamic Analysis Jacob J. Richardson and Frederick F. Lange* Materials Department, UniVersity of California Santa Barbara, Santa Barbara, California 93106 ReceiVed January 22, 2009; ReVised Manuscript ReceiVed March 26, 2009
ABSTRACT: A thermodynamic model based on published data is presented for calculating the solubility and speciation of ZnO in aqueous solutions as a function of pH, ammonia concentration, and temperature. These calculations are used to explain how solubility and speciation control nucleation and growth behavior from an aqueous solution. The nucleation and growth behavior, in turn, can lead to the different morphologies of epitaxial and particulate ZnO obtained experimentally using low temperature aqueous methods. The calculations indicate that the presence of the zinc amine complex, Zn(NH3)42+, causes ZnO to display a substantial decrease in solubility when the temperature is raised from room temperature to 90 °C. This retrograde solubility is the basis for a novel continuous circulation reactor for synthesizing ZnO, which is presented in part 2 of this series.
1. Introduction The aqueous synthesis of zinc oxide is well-known; high temperature hydrothermal growth of ZnO has become a standard method for producing large single crystals,1-4 and recent interest in ZnO as a wide bandgap semiconductor has led to many publications exploring the synthesis of ZnO films,5-9 powders,10-12 and nanostructures13-16 from aqueous solutions. Low temperature aqueous synthesis is an especially attractive method of producing ZnO due to advantages in cost and the environmental impact.17 In recent years, Lange and co-workers have demonstrated the ability to grow epitaxial films of ZnO, which are critical for many optoelectronic applications, from aqueous solutions at temperatures as low as 90 °C.18-21 Relative to vapor phase deposition techniques, low temperature aqueous synthesis of epitaxial films can be less damaging to sensitive substrates and has the added advantage of the ability to form template structures from the bottom up. Despite the increased interest and work with the low temperature aqueous synthesis of ZnO, a thorough presentation of the thermodynamic mechanisms that allow ZnO crystals to form and grow is lacking. As a result, methods of controlling the growth rate, total yield, and morphology of the ZnO synthesized from aqueous solutions have been largely empirical. Here, in part 1 of this two part series, a thermodynamic model is detailed for analyzing the solution chemistry often utilized to synthesize ZnO at low temperatures. The knowledge of the growth solution behavior provided by these calculations has enabled a better understanding of the factors that control growth * To whom correspondence should be addressed. Phone: (805) 893-8248. Fax: (805) 893-8486. E-mail:
[email protected].
rate, yield, and morphology. The calculations have also led to the development of a unique continuous circulation reactor, capable of producing ZnO epitaxial films and templated structures from low temperature aqueous solutions. The design details of the reactor and experimental results are discussed in part 2. Most low temperature aqueous methods for producing ZnO utilize ammonia in the growth solution, as ammonium hydroxide,11,18 an ammonium salt,7,18 or as the decomposition product of another species, e.g., hexamine.5,13 Due to the fact that the ∼9.25 pKa of ammonia lies within the range of pH typically used to synthesize ZnO in aqueous solutions and because ammonia can form stable zinc complexes, treating ammonia (or ammonium or hexamine) simply as a source of hydroxide ignores much of the complexity of the solution behavior. Therefore, the current study emphasizes the role of ammonia concentration, as well as pH and temperature, on zinc oxide solubility and speciation. Several other authors have previously treated ZnO growth from aqueous solutions giving consideration to the formation of zinc amine complexes.22,23 In the current work, however, ammonia is the only complexing ligand evaluated and the use of redox chemistry is completely avoided. This allows the analysis to focus on the variables that are common to many aqueous ZnO synthesis techniques, i.e., pH, ammonia concentration, and temperature. To keep the analysis as simple as possible, any other solution additives, such as so-called structure directing agents used to modify the morphology, are also omitted in the current analysis. However, even without these additives, substantially different morphologies of ZnO can be obtained simply by altering the pH, ammonia concentration, and temperature of synthesis.
10.1021/cg900082u CCC: $40.75 2009 American Chemical Society Published on Web 04/23/2009
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Table 1. Thermodynamic Data Used in the Calculations species H2O(l) ZnO(c) OH-(aq) NH3(aq) NH4+(aq) Zn2+(aq) Zn2+(aq) ZnOH+(aq) Zn(OH)2(aq) Zn(OH)32-(aq) Zn(OH)42-(aq) ZnO22-(aq) ZnO2H-(aq) Zn(NH3)2+(aq) Zn(NH3)22+(aq) Zn(NH3)32+(aq) Zn(NH3)42+(aq)
∆Hf /J mol-1 S/J K-1 mol-1 Cp/J K-1 mol-1 source -285 830 -350 460 -229 987 -80 291 -132 633 -153 971 -153 385 -363 970 -613 410 -881 520 -1 124 380 -384 300 -457 144 -185 770 -225 099 -264 429 -533 500
69.95 43.639 -10.878 111.294 113.386 -112.131 -109.62 62.76 61.55 2.98 -27.51
301.248
75.375 41.05 -122.601 29.518 30.017 -25.94 41.84 33.47 159.83 89.54
-150.615
13 13 13 13 13 13 14 14 14 14 14 13 13 13 13 13 13
2. Thermodynamic Calculations The solution is modeled using thermodynamic equilibrium calculations that rely on standard thermodynamic data.24,25 Throughout, equilibrium is assumed between pure solid ZnO and the aqueous solution. Equilibrium with the vapor above the solution and any other solid phases is ignored. Aside from water, the components of the solution taken into account are hydronium (written as H+ for brevity) and hydroxide ions, ammonia and ammonium ions, and several soluble zinc species. The aqueous zinc species used in the model, namely, the mononuclear zinc hydroxide and amine complexes, were selected because they are the simplest and because they are assumed to be the most prevalent zinc species in dilute ammoniacal solutions. The species examined here are also the zinc species most often cited in the aqueous growth literature, and thus, those whose thermodynamic values were readily available. It is worth mentioning that there is some ambiguity in the literature as to whether ZnO2H- (also written as ZnO(OH)- or HZnO2-) and ZnO22- are truly distinct from the species represented by Zn(OH)3- and Zn(OH)42-. This confusion may arise from the fact that it is common practice to neglect bound waters when the formulas of aqueous metal complexes are written. In this case, subtracting a water molecule from the latter set of soluble species results in the formulas of the former. This, however, does not exclude the possibility of two sets of complexes, and the vastly different thermodynamic data that can be found in the literature for ZnO2H- relative to Zn(OH)32- and ZnO22- relative to Zn(OH)42- seems to support the existence of distinct species. Therefore, the thermodynamic values for all four complexes are included in Table 1, along with the values for all the other species used for the current study. In order to construct the model, a set of equilibrium equations is formed using the selected chemical species. The relevant equilibria, listed in Table 2, are those between solid ZnO and each soluble species containing zinc along with the self-dissociation reactions of water and ammonium. Assuming the values of enthalpy, entropy, and heat capacity to be independent of temperature, the data in Table 1 are used to calculate a chemical potential of each species as a function of temperature according to
(
µ ) ∆Hf + Cp(T - 298.15) - T ∆Sf + Cp log
T 298.15 (1)
)
The chemical potentials are then used to determine the temperaturedependent Gibbs free energy and an equilibrium constant for each equation in Table 2. Setting the equilibrium constants equal to the reaction quotients at equilibrium results in a set of expressions that can then be solved simultaneously for all the activities as a function of temperature, pH, and total ammonia concentration. Using dilute solution approximations, the calculated activity of each species is assumed to be equivalent to its respective equilibrium concentration in solution. The total solubility of ZnO is then calculated as the sum of the concentrations of all the zinc containing species.
3. Results Parts a and b of Figure 1 show three-dimensional plots of ZnO solubility versus pH and ammonia concentration at 25 and 90 °C, respectively. Without ammonia, the calculated ZnO solubility is in good agreement with the solubility data of Baes and Mesmer26 and displays behavior typical of an amphoteric oxide, namely, high solubility at both high and low pH. When ammonia is added, ZnO also becomes highly soluble within an intermediate range of pH, in general agreement with the calculations of Hubert et al.23 The speciation plots shown in Figure 2a,b indicate that the increased solubility is due to the formation of the zinc tetra-amine complex, Zn(NH3)42+, as it is the dominant Zn(II) species in that pH range. Figure 2 also shows that at pH above the range where Zn(NH3)42+ is the dominate Zn species, the ammonia ligands are freed as dissolved NH3. As this happens, the dominate Zn(II) species in solution transitions to Zn(OH)2, Zn(OH)3-, and then Zn(OH)42-. At lower pH, NH4+ is the stable form of ammonia and the dominant zinc containing species become ZnOH+ and then Zn2+. The calculations indicate that temperature plays an especially crucial role on ZnO solubility in ammoniacal aqueous solutions. The surfaces in Figure 1c,d now represent ZnO solubility vs pH and temperature at constant ammonia concentrations of 0 and 1.0 M. As one might expect, without ammonia in solution, the solubility changes little or increases with temperature, depending on the pH. However, when ammonia is added, the behavior is more unexpected and interesting. Figure 2 shows how the pH range where Zn(NH3)42+ is stable shrinks and is shifted to a lower pH as the solution is heated. Comparing parts a and b of Figure 1, we see that ZnO solubility follows the same trend. The result of this behavior is a regime of pH and ammonia concentration where the solubility of ZnO decreases rather than increases with temperature, a counterintuitive condition referred to as retrograde solubility. For certain conditions, the calculations predict that the retrograde solubility of ZnO should be quite large.
Table 2. Equilibrium Expressions Used in the Calculations reaction
∆G (298 K)/J mol-1
log Keq (298 K)
H2O(l) T H+(aq) + OH-(aq) ZnO(c) + 2H+(aq) T Zn2+(aq) + H2O(l) ZnO(c) + H+(aq) T ZnOH+(aq) ZnO(c) + H2O(l) T Zn(OH)2(aq) ZnO(c) + 2H2O(l) T Zn(OH)3-(aq) + H+(aq) ZnO(c) + 3H2O(l) T Zn(OH)42-(aq) + 2H+(aq) ZnO(c) + H2O(l) T ZnO22-(aq) + 2H+(aq) ZnO(c) + H2O(l) T ZnO2H-(aq) + H+(aq) ZnO(c) + NH3(aq) + 2H+(aq) T Zn(NH3)2+(aq) + H2O(l) ZnO(c) + 2NH3(aq) + 2H+(aq) T Zn(NH3)22+(aq) + H2O(l) ZnO(c) + 3NH3(aq) + 2H+(aq) T Zn(NH3)32+(aq) + H2O(l) ZnO(c) + 4NH3(aq) + 2H+(aq) T Zn(NH3)42+(aq) + H2O(l) NH4+(aq) T NH3(aq) + H+(aq)
79 942 -63 754 -19 211 38 395 94 434 167 350 285 856 213 013 -15 511 58 633 132 776 -112 638 52 965
-14.0 11.2 3.4 -6.7 -16.5 -29.3 -50.1 -37.3 2.7 -10.3 -23.3 19.7 -9.3
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Figure 1. Solubility of ZnO in aqueous solution versus pH and ammonia concentration at (a) 25 °C and (b) 90 °C and versus pH and temperature at (c) 0 mol L-1 and (d) 1 mol L-1 ammonia concentration.
Figure 2. Speciation in an aqueous solution of dissolved Zn(II) versus pH at (a) 25 °C and (b) 90 °C and of dissolved ammonia at (c) 25 °C and (d) 90 °C, all with 0.5 mol L-1 total ammonia.
4. Discussion Contrary to the usual assumption of aqueous solubility, that it should increase with temperature, the analysis shows that ZnO solubility in ammoniacal solutions can actually be much greater at lower temperatures than at higher temperatures. This knowledge is important because most procedures for producing ZnO powders or films from low temperature aqueous solutions involve heating the growth solution. When one analyzes the growth of crystalline material from a solution, whether in an aqueous solvent, a nonaqueous solvent, or a molten salt, the most important parameter is the solubility of the synthesized material. According to classical growth theory, a requisite for growth from a solution is the development and continued presence of a supersaturation to provide the thermodynamic driving force for the spontaneous growth of nuclei. The
retrograde solubility predicted by the current model indicates that the heating of an ammonical solution containing dissolved Zn(II) can actually create the thermodynamic driving force for ZnO synthesis, as opposed to simply increasing the reaction kinetics. Because it is generally believed that crystal growth at near-equilibrium conditions produces better quality crystals,3 this understanding has implications for the ability to grow higher quality material in low temperature aqueous solutions. The retrograde solubility of the solution also allows for continuous and steady-state growth from a low temperature reactor, analogous in principle to the high temperature hydrothermal autoclaves used to grow bulk ZnO single crystals. The details of such a continuous low temperature reactor and the experimental results of its use to grow epitaxial ZnO are presented in part 2 of this series.
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Figure 3. Solubility of ZnO versus (a) pH for the first step growth conditions (90 °C and 0.3 mol L-1 ammonia) and (b) temperature for the second step growth conditions (pH 10.9 and 0.75 mol L-1 ammonia).
Figure 4. (a) Epitaxial ZnO seed layer on 〈111〉 MgAl2O4 (crosssectional view inset) and (b) ZnO powder particles, both produced with the first growth step (rapid pH increase at 90 °C and 0.3 mol L-1 ammonia).
Even though the analysis may indicate that a solution of a given pH and ammonia concentration will display retrograde solubility, care must be exercised when one predicts if a real solution will become supersaturated when heated. This is partially due to the fact that the pH of the solution is also dependent on temperature, generally decreasing with temperature due to the increased self-dissociation of water. However, given the number of other unknowns in the calculation, it was necessary to treat pH as invariant with temperature in order to obtain an analytically tractable solution. The expectation of constant pH, along with the other assumptions and approximations mentioned earlier, means that the model should not be expected to provide quantitative predictions of experimental results. However, as discussed below, the model does provide a qualitative description of the effects of pH, ammonia concentration, and temperature which correlates with experimental results. In the following paragraphs, the model presented above is used to analyze the two step growth process, briefly described below, which was developed by Lange and co-workers for producing epitaxial ZnO films and periodic structures from low temperature aqueous solutions.18,21,27 The first step of this process serves to produce a thin epitaxial seed layer on either a single crystal 〈111〉 MgAl2O4 or GaN substrate by utilizing conditions that produce a high nucleation density but only a small amount of film growth. In practice, this is accomplished by preheating an aqueous solution of zinc nitrate and ammonium nitrate to 90 °C in a sealed vessel. Once heated, the substrate is submerged in the vessel. This is immediately followed by the amount of dilute ammonia previously determined to give the solution a room temperature pH of 7.5. Figure 3a shows the decrease in ZnO solubility with increased pH under the conditions typically used to produce the seed layer, namely, 0.3 M ammonia and 90 °C. Because the amount of aqueous ammonia added to increase the pH is small compared to the amount of ammonia already present from the ammonium nitrate, we can ignore any change in solubility due to the change in
ammonia concentration. A calculated solubility of ∼0.3 mol L-1 at the initial pH of 4.5 is shown to decrease to ∼0.0002 mol L-1 by pH 7.5. The second growth step results in substantial epitaxial growth on the seed layer created by the first step with little new heterogeneous nucleation or ZnO powder being produced. Before the second growth step is performed, the film is covered by a layer of photoresist patterned with periodic circular growth windows that expose the seed layer. This patterning step is performed so that ZnO will grow through the windows to produce posts that will help determine the anisotropic nature of the growth. Once patterned, the film is inserted into a sealed vessel containing a room temperature solution of zinc nitrate and ammonia with a pH of ∼10.9. Synthesis of ZnO occurs only after the growth vessel has been placed in a 90 °C oven to heat. The calculated change in solubility with temperature of the pH 10.9, 0.75 M ammonia solution typically used for the second growth step is shown in Figure 3b. Here, the solubility is decreased from ∼2.5 to 0.05 mmol L-1 as the solution heats from 25 to 90 °C. Parts a and b of Figure 3 appear similar in form, but the decrease in solubility is over 2 orders of magnitude larger for the first synthesis step (Figure 3a). During the first step, the decrease in ZnO solubility is also very rapid, occurring as the sudden addition of ammonia immediately increases the pH of the solution. This large and rapid decrease in solubility leads to a supersaturation not only large enough to initiate the nucleation of epitaxial ZnO on the single crystal substrate but also large enough to precipitate ZnO particles. The extremely large number of nuclei, formed both on the substrate and in the solution, prevents each individual nuclei from growing very large before ZnO synthesis ceases as the solution approaches equilibrium. Thus, the first growth step is observed to result in only a thin ZnO film composed of many nanosized epitaxial nuclei that have not completely coalesced (Figure 4a). Rather than increasing the thickness of the film, most of the ZnO
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Figure 5. (a) Periodic epitaxial ZnO posts on a photoresist templated substrate and (b) large powder particles produced with the second growth step (slow temperature increase at pH 10.9 and 0.75 mol L-1 ammonia).
synthesized in this step goes toward the precipitation of the ZnO particles that grow from nonepitaxial nuclei (Figure 4b). In comparison, the decrease in solubility during the second step, which is caused by the solution being gradually heated from room temperature to 90 °C, occurs at a much slower rate. While slowly heating, the solution can remain near equilibrium by depositing ZnO onto the pre-existing seeds that were produced on the substrate in the first step. The growth of these existing seeds keeps the supersaturation from ever becoming large enough to initiate the nucleation of new precipitates. Thus, without the competition from numerous precipitates, the second step leads to a larger amount of growth on the substrate despite less ZnO being synthesized overall. Because the seeded substrate was patterned with a periodic mask, the second step results in the formation of the tall ZnO posts shown in Figure 6a. Inevitably, a small number of particles are present during the second step, and these particles also experience a large amount of growth, as shown in Figure 6b. The speciation of the solution during the first and second steps can also provide information about the growth processes. During growth, the different species present in the solution are in contact with the surfaces of the crystalline ZnO. These species may adsorb on the surfaces of the crystals and, in the case of Zn containing complexes, react to synthesize new layers of ZnO on the crystals. Like all oxides in contact with an aqueous solution, the sign and density of charged surface sites on a ZnO crystal are a function of pH. This dependence is generally characterized by the isoelectric point (IEP) of a material; below the IEP, more positive sites are present, and above the IEP, the surface contains more negative sites. However, IEP is generally determined from the average surface charge of a particle, and thus, it assumes that the particle is uniform and isotropic. For ZnO, the IEP has been determined from powder specimens to occur at approximately pH 9.5,18,28 but this only indicates the pH at which the average surface charge of a typical ZnO particle transitions from positive to negative. Because ZnO crystallizes in the polar wurtzite structure, its surfaces are anisotropic. Thus,
Richardson and Lange
Figure 6. Speciation of Zn(II) versus (a) pH for the first step growth conditions (90 °C and 0.3 mol L-1 ammonia) and (b) temperature for the second step growth conditions (pH 10.9 and 0.75 mol L-1 ammonia).
the pH where the transition from positive to negative occurs for a specific ZnO crystal plane may not be represented by the IEP. It has been observed that three different crystal facets are typically expressed in ZnO crystals grown using the low temperature aqueous method, namely, (0001), (0001j), and {11j00}. Surfaces that appear to be higher index facets are typically revealed upon closer inspection to be composed of many steps of these lower index planes. The (0001) and (0001j) surfaces, which are respectively referred to as either the c+ or Zn terminated and the c- or O terminated basal planes, are polar in their unreconstructed state and, thus, are not stable without a charge compensation mechanism.29 In aqueous solution, compensation by OH- on the positive (0001) surface and H+ on the negative (0001j) surface are likely to be the most favorable mechanisms. This would give the two surfaces very different effective IEP as OH- could more easily be removed from the (0001) surface at low pH and H+ removed from the (0001j) surface at high pH. For this reason, it is expected that the (0001) surface will maintain a more positive character than either the (0001j) or the nonpolar (101j0) (m-plane) at a pH above the IEP. The electrostatic interactions of these same three ZnO crystal surfaces with charged species were measured by Golovko et al.30 Using a functionalized latex probe attached to an atomic force microscope, they found that the (0001) surface indeed had an attractive interaction with negatively charged acid groups under conditions where the interactions with the (0001j) and (101j0) surfaces were repulsive. In other words, the (0001) surface appeared to be positive, and the (0001j) and (101j0) surfaces appeared to be negative. Assuming that ionic species will be attracted to surfaces of the opposite charge, the surface chemistry of a ZnO crystal in solution is also expected to be anisotropic with respect to the adsorption and condensation of the different soluble species. Panels a and b of Figure 6 show the fractional distribution of dissolved zinc as functions of pH and temperature, respectively, for the first and second step synthesis conditions described above. Throughout the lower pH range seen in the first step,
Synthesis of ZnO: Thermodynamic Analysis
positively charged complexes, Zn2+, ZnOH+, and Zn(NH3)42+, dominate. However, for the second step, the dissolved zinc transitions from positive Zn(NH3)42+ to negative Zn(OH)3- and Zn(OH)42- as the solution is heated. It is believed that the preferential adsorption of these negatively charged species onto the positively charged (0001) surface of ZnO leads to the anisotropic growth of the high aspect ratio epitaxial posts and needlelike particles shown in Figure 5. In comparison, the low pH of the first step growth produces more isotropic growth. Because this step is performed below the isoelectric point of ZnO, the (101j0) as well as the (0001) will be positively charged. This makes for a similar interaction between the soluble Zn species and the orthogonal (101j0) and (0001) ZnO surfaces.
5. Conclusions Thermodynamic calculations have been presented for predicting the solubility of ZnO and the speciation of dissolved zinc in aqueous solution as a function of pH, ammonia concentration, and temperature. The model predicts retrograde ZnO solubility with temperature under certain ranges of pH and ammonia concentration. By heating a solution in this regime, ZnO synthesis can take place under thermodynamic rather than kinetic control. This conclusion has enabled the authors to build a steady-state growth reactor, the details of which will be discussed in part 2 of this series. In this part, the model was also used to analyze two different experimental growth procedures. The ZnO synthesized by these procedures was consistent with an explanation based on the supersaturation rates and preferential adsorption of the soluble Zn species predicted by the calculations. Acknowledgment. The authors gratefully acknowledge the support of the Solid State Lighting and Energy Center, College of Engineering, University of California Santa Barbara. This work made use of the MRL Central Facilities supported by the MRSEC Program of the National Science Foundation (NSF) under award No. DMR05-20415. A portion of this work was done in the UCSB nanofabrication facility, part of the NSF funded NNIN network.
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