Controlling Low Temperature Aqueous Synthesis of ZnO. 2. A Novel

Apr 23, 2009 - ABSTRACT: A novel low temperature continuous circulation reactor for synthesizing ZnO from aqueous solutions has been developed...
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Controlling Low Temperature Aqueous Synthesis of ZnO. 2. A Novel Continuous Circulation Reactor Jacob J. Richardson and Frederick F. Lange* Materials Department, UniVersity of California Santa Barbara, Santa Barbara, California 93106

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2576–2581

ReceiVed January 22, 2009; ReVised Manuscript ReceiVed March 26, 2009

ABSTRACT: A novel low temperature continuous circulation reactor for synthesizing ZnO from aqueous solutions has been developed. The reactor is based on the solubility behavior of ZnO predicted by the thermodynamic calculations presented in part 1 of this series. The chemistry used is simple and nontoxic, and because the reactor operates in a closed loop, no waste byproducts are produced. Experimental results confirm that the reactor is capable of synthesizing epitaxial ZnO at atmospheric pressure and temperatures well below the boiling point of water. It was possible to control the growth rate, yield, and morphology of the ZnO produced by adjusting variables, such as pH, ammonia concentration, temperature, heating rate, hold time, and recirculation rate. The experimental results obtained are shown to be in qualitative agreement with the calculations reported in part 1.

1. Introduction In the first part of this series, a thermodynamic model describing the behavior of aqueous ZnO growth solutions was detailed. The model used published thermodynamic data1,2 to calculate the composition of a solution in equilibrium with solid ZnO as a function temperature, pH, and ammonia concentration. Within a certain range of pH and ammonia concentration, the calculations predicted that the solubility of ZnO should drastically decrease with increased solution temperature. This temperature dependence provides the basis for the low temperature continuous circulation reactor presented in the present work. To our knowledge, all previous research on low temperature aqueous synthesis of ZnO has been based on batch processing. When ZnO is synthesized using a batch process, the initial conditions of the growth solution, such as the concentrations of solutes and the pH, are fixed by the solution preparation. However, once ZnO formation is initiated, these conditions will change. In a typical batch process, ZnO synthesis occurs either immediately after the solution is prepared or after being initiated by some event, e.g., a change in temperature or an increase in pH. Initially, the rate of ZnO synthesis is often limited only by the kinetics of the reactions that are taking place but synthesis must always slow and eventually stop as the solution equilibrates. The nature of batch processing presents several problems for the synthesis of high quality epitaxial ZnO. During the period of ZnO growth, the changes in the aqueous environment that occur as the solution equilibrates will result in variations in growth rate, morphology, and defect incorporation, yielding a nonuniform film. A second problem intrinsic to batch processing is that the amount of ZnO produced will always be limited by the initial conditions of the solution. Once the solution has equilibrated, further synthesis of ZnO requires that the specimen be removed from solution and the growth process repeated in a new solution. Third, the nonlinear growth rate inherent to batch growth makes precise control of either the thickness of an epitaxial film or the dimensions of three-dimensional structures difficult to control. Applications utilizing epitaxial ZnO films are likely to require uniform material of controllable thickness, composition, etc., rendering aqueous batch processing a problematic method of production. The continuous circulation * To whom correspondence should be addressed. Phone: (805) 893-8248. Fax: (805) 893-8486. E-mail: [email protected].

method described below can eliminate these issues by growth under steady-state conditions, similar to what is achievable in most vapor-phase deposition techniques commercially used to produce epitaxial films. Besides the drastically lower temperature and pressure utilized, the operation of the continuous circulation reactor utilized in this work is in many ways analogous to that of the high temperature hydrothermal reactors often used to synthesize bulk ZnO single crystals.3-6 Any continuous growth process not only requires the creation of a thermodynamic driving force for ZnO synthesis but also requires the maintenance of that thermodynamic driving force throughout the growth period. In the case of high temperature hydrothermal growth, the driving force is due to a temperature gradient between two zones within the growth vessel. In the hot portion of the vessel (∼420 °C), ZnO powder, often called the nutrient, is dissolved, saturating the solution. Dissolved Zn species are transported via convection to the cold portion of the vessel (∼380 °C). The drop in temperature decreases the solubility of ZnO, forcing the deposition of new material onto a seed crystal. The zinc depleted solution is then convectively recycled to the cooler zone to dissolve more of the ZnO nutrient. Theoretically, once steady state is achieved, the growth is continuous until either the nutrient is exhausted or the growing crystals are removed. A temperature gradient also provides the driving force for growth in the low temperature continuous circulation reactor. However, rather than the negative temperature gradient used for the high temperature hydrothermal method, the low temperature aqueous method presented here uses a positive temperature gradient to synthesize ZnO.

2. Reactor Design and Experimental Details To test the validity of the thermodynamic model detailed in part 1 and to ascertain the feasibility of low temperature steady-state aqueous synthesis of ZnO, a prototype continuous circulation reactor was constructed. The principle of operation and basic design of this reactor are shown in Figure 1. To minimize contaminates in the solution, the reactor was constructed entirely of chemically inert PFA Teflon parts (Savillex Corporation). The reactor consists of two vessels: a “cold” dissolution vessel, where the ZnO powder is dissolved, and a “hot” deposition vessel, where ZnO is synthesized. This two vessel design differs from the high temperature hydrothermal reactors, which consist of a single high pressure vessel. For the supercritical temperature and pressure conditions used in high temperature hydrothermal growth of ZnO, mechanical pumping of the solution is impractical, so circulation

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Synthesis of ZnO: Novel Continuous Circulation Reactor

Crystal Growth & Design, Vol. 9, No. 6, 2009 2577 Table 1. Growth Solution Formulations

Figure 1. (a) Schematic diagram of the principle of operation behind a continuous circulation reactor showing the directions of flow for the growth solution and solid ZnO. (b) Drawing of the reactor setup used in the experiments to synthesize ZnO. between the hot and cold zones must depend only on convection and diffusion. At the near ambient temperatures and pressures used in the current study, convection and diffusion are far less substantial. Fortunately, pumping, which produces better control over the circulation rate, is relatively straightforward. A peristaltic pump was used so that the solution was only exposed to the inside of the Teflon vessel and the tubing. The solution was pumped from the dissolution vessel through two consecutive porous (0.45 µm) PFA membranes to prevent ZnO powder (nutrient) from being transferred to the deposition vessel. Return flow to the dissolution vessel was accomplished by an overflow mechanism, ensuring that the volume in each vessel remained constant. The deposition vessel was heated by electrical heating tape wrapped around the vessel. The heating tape was connected to a controller in a feedback loop with a PTFE Teflon-coated thermocouple measuring the temperature of the deposition vessel solution. This allowed for the deposition solution to be heated at a constant rate and held at a constant temperature. To maintain a uniform concentration and temperature profile, the solution within the deposition vessel was continuously stirred with a PTFE Teflon-coated magnet. The variables affecting the growth of ZnO in the reactor can be divided into two categories: (a) the solution variables, namely, pH and ammonia concentration, and (b) the reactor variables, namely, the temperatures of the deposition and dissolution vessels, the hold period at constant temperature, the heating rate, and the circulation rate. The solution variables determine the equilibrium ZnO solubility and the concentrations of the different soluble species in the solution at a given temperature. Thus, along with the temperatures of the deposition and dissolution vessels, the solution variables determine the differential solubility. The other reactor variables modify the growth rate and the total yield of ZnO produced in the deposition vessel by changing the way the solubility differential is used. Only a portion of the possible

solution

pH

ammonia concentration (mol L-1)

A B C D E F

11 11 11 11 10 12

0.0 0.25 0.5 1.0 0.5 0.5

specimen(s) 1 2 3, 7-20 4 5 6

growth conditions created by these variables are explored in the current work, but by correlating the observed trends with the predictions made by the model presented in part 1, much of the remaining behavior can be inferred. Table 1 lists the solution variables used in the current study. All growth solutions were prepared by allowing an aqueous solution of the desired ammonia concentration to equilibrate with an excess of 99.9% pure ZnO powder (Aldrich) in a sealed container for a period of ∼2 days. During this period, the solutions were agitated and occasionally readjusted to the desired pH using either NaOH or HNO3. Once the pH of the solution appeared stable, the solution was considered to be equilibrated and ready for use. For all of the synthesis experiments, the dissolution vessel was maintained at room temperature, namely, between 22 and 24 °C. Synthesis was performed on epitaxial ZnO-seeded MgAl2O4 spinel substrates, the preparation of which is described below. The seeded substrates were inserted into the deposition vessel either at room temperature (specimens 1-16) or after the solution was preheated to the hold temperature (specimens 17-19). When the substrate was inserted at room temperature, heating and pumping of the solution were started immediately. For specimens 17-19, the solution was heated to the hold temperature without recirculation and held at that temperature for 2 h before the substrate was inserted. After the substrate was inserted and the reactor resealed, a period of 10 min was allowed for the temperature to stabilize before recirculation was started. All of the reactor conditions used in this study are listed in Table 2. All synthesis experiments were performed on 〈111〉 MgAl2O4 single crystal substrates coated with an epitaxial ZnO seed layer. As detailed elsewhere,7 the seed layer was formed by inserting the single crystal substrate into a preheated aqueous solution of zinc nitrate and ammonium nitrate, the pH of which is then immediately increased by adding aqueous ammonia. As discussed in part 1, the rapid increase in pH causes the solubility of ZnO to decrease, providing a thermodynamic driving force for the nucleation and growth of the epitaxial ZnO seed layer on the substrate. Using photolithography, the seeded substrates were patterned with a photoresist mask containing periodic 5 µm diameter “windows” spaced 20 µm apart, as shown in Figure 2a. The mask confines the synthesis of the ZnO to the periodic growth windows, leading to the formation of the vertical posts seen in Figure 2b. The average vertical and lateral growth rates were easily determined by making measurements of the dimensions of a number of these posts. Images of the posts were obtained using a FEI XL30 Sirion field emission gun digital scanning electron microscope. The ZnO powder remaining in the deposition vessel after the experiments was collected by centrifuging, and after decanting away the growth solution, rinsing with distilled water, and vacuum drying.

3. Results For each specimen, SEM micrographs were used to determine the dimensions of five different posts, each located in a different region on the masked substrate. The average post dimensions are listed in Table 3. Post heights were measured from the top of the photoresist mask to the highest point on the post. The diameters were measured at the widest point, which typically corresponded to the corner to corner distance parallel to the 〈112j0〉, or a-direction, in the hexagonal wurtzite structure of ZnO. The amount of a-direction growth was then calculated as half of the difference between the post diameter and the initial diameter (5 µm) of the circular growth window. The amount of growth in the 〈0001〉, or c+-direction, was considered to be the same as the post height.

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Table 2. Synthesis Conditions for Each Specimen specimen

solution name

hold temp (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

A B C D E F C C C C C C C C C C C C C

90 90 90 90 90 90 60 75 90 90 90 90 90 90 90 90 90 90 90

hold time (min)

ramp time (min)

insert time (min)

insert temp (°C)

pump rate (mL min-1)

growth time (min)

180 180 180 180 180 180 180 180 0 60 360 180 180 180 180 180 60 180 360

120 120 120 120 120 120 65 83 120 120 120 60 240 120 120 120 120 120 120

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 240 240 240

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 90 90 90

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.0 0.25 1.0 0.5 0.5 0.5

300 300 300 300 300 300 245 263 120 180 480 240 420 300 300 300 60 180 360

3.1. Solution Variables. All of the experiments examining the effects of the solution variables, i.e., pH and ammonia concentration, on ZnO synthesis were performed under identical reactor conditions. These conditions entailed inserting the substrate into the deposition vessel solution at room temperature, ramping the deposition solution temperature to 90 °C over 120 min, and then holding at 90 °C for another 180 min before removing the specimen. During these experiments, the recirculation rate of the solution was held constant at 0.5 mL min-1. The effect of varying the ammonia concentration at constant pH (specimens 1-4) is shown in Figure 3a-d. Without ammonia (solution A), ZnO was not synthesized on the substrate. As shown in Figure 3a, only the seed layer was observed though the growth window in the photoresist. However, as shown in Figure 3b, post formation is clearly evident at an ammonia concentration of 0.25 mol L-1 (solution B). The posts became taller and wider and had flatter tops as the ammonia concentration was increased to 0.5 mol L-1 (solution C) and 1.0 mol L-1 (solution D), as shown respectively in parts c and d of Figure 3. Note that ZnO powder was also formed in the deposition vessel, and like the size of the ZnO posts, the amount of powder increased with the ammonia concentration.

Figure 2. (a) SEM image of an epitaxial ZnO seeded MgAl2O4 substrate patterned with a photoresist mask layer containing periodic 5 µm diameter windows separated by 20 µm. (b) ZnO posts grown on a specimen masked with the same photoresist layer using room temperature insertion into a pH 11, 0.5 mol L-1 ammonia solution, heating to 90 °C over 2 h, and then holding for 3 h, all while the solution was circulated through the reactor at 0.5 mL min-1.

This observation suggests that a higher ammonia concentration leads to a greater nucleation rate as well as a greater growth rate. The effect of changing the pH at constant ammonia concentration appears to be less dramatic but is still easily observed, comparing the images of specimens 5, 3, and 6 in parts e, c, and f of Figure 3, respectively. Of the three pH conditions used (pH 10, 11, and 12), the solution at pH 10 (solution E, specimen 5) resulted in both the largest posts and the most powder production. Relative to the other conditions, the posts grown at pH 12 (solution F, specimen 6) not only were smaller but also had much rougher top surfaces. 3.2. Reactor Variables. The results for the different reactor conditions were all acquired using a solution with a pH of 11 and containing 0.5 mol L-1 ammonia (solution C). It was found that the synthesis of ZnO at these solution conditions was heavily dependent on the deposition vessel temperature. Compared to results for specimen 3 (Figure 2c), parts a and b of

Figure 3. SEM images showing the effect of different solution conditions on the growth of epitaxial ZnO posts. (a) Specimen 1 (pH 11, 0 mol L-1 NH3), (b) specimen 2 (pH 11, 0.25 mol L-1 NH3), (c) specimen 3 (pH 11, 0.5 mol L-1 NH3), and (d) specimen 4 (pH 11, 1 mol L-1 NH3) show the effect of changing the ammonia concentration of the solution. (e) specimen 5 (pH 10, 0.5 mol L-1 NH3) and (f) specimen 6 (pH 12, 0.5 mol L-1 NH3) show the effect of changing the pH of the solution.

Synthesis of ZnO: Novel Continuous Circulation Reactor

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Table 3. Average Dimensions of Five ZnO Posts Measured on Each Specimen specimen

diameter (µm)

height (µm)

aspect ratio

volume (10-16 m3)

c-growth (µm)

a-growth (µm)

c/a-growth ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

6.60 7.38 8.01 7.88 6.50 5.36 7.26 6.65 7.37 7.47 6.70 6.85 6.96 7.18 7.10 5.64 6.09 6.23

7.16 10.90 13.66 12.07 9.41 0.89 7.30 3.60 8.82 13.44 8.07 10.77 8.41 10.90 11.00 0.84 2.15 4.78

1.08 1.48 1.71 1.53 1.45 0.17 1.01 0.54 1.20 1.80 1.20 1.57 1.21 1.52 1.55 0.15 0.35 0.77

2.05 3.61 5.41 4.74 2.59 0.17 2.40 1.01 2.99 4.84 2.28 3.16 2.61 3.63 3.53 0.17 0.51 1.18

7.16 10.90 13.66 12.07 9.41 0.89 7.30 3.60 8.82 13.44 8.07 10.77 8.41 10.90 11.00 0.84 2.15 4.78

0.80 1.19 1.51 1.44 0.75 0.18 1.13 0.83 1.19 1.24 0.85 0.93 0.98 1.09 1.05 0.32 0.54 0.62

8.94 9.18 9.08 8.39 12.55 4.99 6.46 4.35 7.43 10.86 9.49 11.62 8.60 10.02 10.45 2.62 3.95 7.77

Figure 4 clearly show reduced growth on specimens 7 and 8, where the temperature was lowered to 60 and 75 °C, respectively. Using a deposition vessel temperature of 90 °C, parts c and d of Figure 4 (specimens 12 and 13) show the change in post size achieved by altering the heating rate. Parts e and f of Figures 4 (specimens 14 and 16) show the effect of, respectively, turning off and doubling the recirculation to 1 mL min-1. Along with the changes in post dimensions, an increase in powder production was also observed for increased temperature, heating rate, and recirculation rate. The effect of changing the hold time for specimens inserted at room temperature is shown in parts a (0 h), b (1 h), and c (6 h) of Figure 5. Unsurprisingly, increasing the hold time from zero to six hours caused the posts to become larger. A sizable fraction of the ZnO growth occurred during heating, before the solution was held at 90 °C, but clearly the posts continued to grow while being held at 90 °C. Unlike all other specimens, those shown in parts d (specimen 17), e (specimen 18), and f (specimen 19) of Figure 5 were not inserted until the solution

Figure 4. SEM images showing the effect of different reactor conditions on the growth of epitaxial ZnO posts. (a) Specimen 7 (60 °C) and (b) specimen 8 (75 °C) show the effect of changing the hold temperature. (c) Specimen 12 (1 h to 90 °C) and (d) specimen 13 (4 h to 90 °C) show the effect of the ramp rate. (e) Specimen 14 (0 mL/min) and (f) specimen 16 (1.0 mL/min) show the effect of recirculation rate.

had already reached 90 °C. Once the specimens were inserted, ZnO was respectively deposited for 1, 3, and 6 h using a recirculation rate of 0.5 mL min-1. The resulting posts are substantially smaller and have smoother, more faceted surfaces relative to specimens inserted before heating the solution.

4. Discussion The fact that ZnO has been synthesized using the reactor clearly indicates that the calculations presented in part 1 were qualitatively correct in predicting that the solubility of ZnO in aqueous ammonia solutions would decrease with increasing temperature. As discussed below, the results also demonstrate that the yield, growth rate, and morphology of the ZnO synthesized can be controlled using different solution and reactor conditions. These changes were also qualitatively predicted by the thermodynamic model reported in part 1. 4.1. Solution Variables. The lack of ZnO formation on specimen 1 indicates that the presence of ammonia in the growth solution is necessary for the synthesis of ZnO using this reactor. This is in agreement with the model presented in part 1, which shows no retrograde solubility and, thus, no driving force for ZnO synthesis without ammonia in the growth solution. At increased ammonia concentrations, the calculations predict larger decreases in ZnO solubility upon heating. Experimentally, this translates to greater ZnO synthesis, and hence, larger posts. However, the increase in post size observed when going from an ammonia concentration of 0 to 1 mol L-1 is moderate compared to the change in ZnO synthesis predicted by the model. This discrepancy is likely related to the simultaneous increase in the amount of powder synthesized in these experiments. Increased powder formation results from a higher nucleation rate of ZnO particles, which is consistent with the increased thermodynamic driving force for ZnO synthesis resulting from a larger decrease in solubility. The precipitation of powder competes with the growth of the expitaxial posts, and thus, as powder formation increases, a smaller fraction of the total ZnO synthesized contributes to increasing the post dimensions. The calculations also qualitatively predicted the experimental trend in growth vs pH, namely, a decrease in ZnO synthesis as the pH was increased from 10 to 12. However, for a pH of 12 and an ammonia concentration of 0.5 mol L-1 (solution F, specimen 6), the model actually predicts a small increase in ZnO solubility with temperature and, thus, no ZnO synthesis. This discrepancy may be caused by the assumptions and

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approximations used in the calculations, in particular, the assumption that the pH of the growth solution is independent of temperature. In reality, the pH of an aqueous solution will decrease as the solution is heated due to the increased selfdissociation of water molecules. Although the temperature dependence of the dissociation of water is well-known, the calculations presented in part 1 required that the pH remain constant in order to simultaneously solve for the concentrations of all other soluble species. In this case, it appears the error introduced was large enough to change the sign of the solubility difference when going from room temperature to 90 °C. Note that the posts of specimen 6, grown at pH 12 (solution F), show the highest average c/a-direction growth ratio ()12.6) of all of the specimens. Under otherwise identical growth conditions, synthesis at pH 10 and 11 produced c/a-growth ratios of only 8.4 and 9.2, respectively. This result agrees with the hypothesis, presented in part 1, that the anisotropic growth rate observed in a high pH aqueous ZnO synthesis is due to the preferential adsorption and condensation of negatively charged soluble zinc species on the relatively positively charged (0001), or the so-called Zn-polar basal plane surface. According to the calculations in part 1, the average charge of soluble zinc species becomes more negative at higher pH values. The isoelectric point (iep) of ZnO is reported to be pH 9.5.7,8 Above this pH, the presence of negatively charged ionic species increases the probability of adsorption on the more positive (0001) surface relative to the (101j0), or m-plane surfaces, which are expected to contain more negative surface sites. 4.2. Reactor Variables. Using the thermodynamic calculations detailed in part 1, the rate of growth and total yield of ZnO can be estimated for different solution and reactor conditions. Assuming that ZnO always remains dissolved near its equilibrium concentration, the synthesis of ZnO in the deposition vessel of the reactor can be approximated with the differential equation

∂Ce ∂T ∂σ )V + rp(Ce(Ti) - Ce(T)) ∂t ∂T ∂t g

Figure 5. SEM images showing the effect of different hold times on the growth of epitaxial ZnO posts. (a) Specimen 9 (0 h), (b) specimen 10 (1 h), and (c) specimen 11 (6 h) were inserted into a room temperature deposition solution and then heated to 90 °C over 2 h before holding. (d) Specimen 17 (1 h), (e) specimen 18 (3 h), and (f) specimen 19 (6 h) were inserted into a preheated 90 °C deposition vessel solution and held. All experiments used a pH 11, 0.5 mol L-1 NH3 growth solution and a constant recirculation rate of 0.5 mL min-1.

(1)

where σ is the supersaturation in moles of ZnO, t is time, Ce(T) is the equilibrium concentration of zinc in solution in mol L-1 for the given solution conditions as a function of temperature, T and Ti are the respective temperatures of the deposition and dissolution vessels, Vg is the volume of solution in the deposition vessel, and rp is the rate that solution is circulated between the dissolution vessel and deposition vessel. The first term in eq 1 represents the rate of supersaturation due to the temperature change of the solution in the deposition vessel. The second term represents the supersaturation rate due to the circulation of solution from the lower temperature dissolution vessel to the hotter deposition vessel. For the solution to maintain an equilibrium concentration of dissolved ZnO, the supersaturation rate must be equivalent to the rate at which ZnO is synthesized in the deposition vessel. Therefore, the supersaturation rate can be integrated over the growth period to predict the total yield of ZnO synthesized by the reactor. Using the integration method described above, parts a and c, respectively, of Figure 6 show the calculated amount of ZnO synthesized from an aqueous solution containing 0.5 M ammonia with a pH of 11 (solution C) as a function of time after inserting the substrate into the deposition vessel at either 25 or 90 °C. Corresponding to experimental reactor conditions used, this calculation assumes that 90 mL of solution in the deposition vessel was heated to 90 °C over a period of 2 h while subjected to a constant recirculation rate of 0.5 mL min-1. Because the

Figure 6. (a) Calculated ZnO yield and (b) experimental ZnO post height obtained for substrate insertion into a 25 °C deposition vessel solution which was then heated to 90 °C over 2 h and then held for up to 6 h. (c) The calculated ZnO yield and (d) experimental ZnO post height obtained for substrate inserted into a preheated 90 °C solution and then held for up to 6 h. All calculated and experimental results were obtained using a pH 11, 0.5 mol L-1 ammonia solution with a constant dissolution to deposition vessel recirculation rate of 0.5 mL min-1. The dashed lines in (b) and (d) have the same slope of 0.8 µm h-1.

90 mL of solution in the deposition vessel is relatively large compared to the 0.5 mL min-1 rate of recirculation, the first term of eq 1 dominates during the initial heating stage of the experiment. As a result, Figure 6a shows a high but variable rate of synthesis during heating. The first term in eq 1 becomes zero when the hold temperature is reached, giving a lower but constant rate of synthesis. Figure 6b shows the average height of the ZnO posts experimentally produced using the same conditions (specimens 9, 10, 3, and 11). Corresponding to the synthesis calculation in

Synthesis of ZnO: Novel Continuous Circulation Reactor

Figure 6a, the observed growth rate also became constant after an initial period of more rapid growth. However, it appears that the observed growth rate lags behind what is predicted by eq 1. The experimental data increases slower than predicted during the heating period, and the growth rate does not level out until sometime after the solution reaches constant temperature. This may be due to the kinetics of ZnO formation limiting the ability of the solution to remain near equilibrium as the solubility changes rapidly during the initial heating period. It may also be possible that during the initial heating more of the ZnO is being synthesized as powder, reducing growth on the posts. An hour or two after heating, the solution appears to reach a steadystate condition, with the posts growing in the c-direction at a nearly constant rate of ∼0.8 µm h-1. When the substrate was inserted after the solution had time to equilibrate (specimens 17, 18, and 19), the same steady-state growth rate of ∼0.8 µm h-1 appeared to be achieved immediately, as shown in Figure 6d. This corresponds to the linear ZnO synthesis seen in the calculation made for the same conditions in Figure 6c.

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The continuous circulation reactor and the thermodynamic calculations, presented here and in part 1, will allow for greater understanding and control over the low temperature aqueous synthesis of ZnO. Unlike other low temperature aqueous methods, the continuous reactor presented here allows ZnO to be synthesized under steady-state conditions. This is expected to improve the uniformity of the ZnO produced. Lastly, because other transition metal oxides besides ZnO are likely to show similar solubility behavior in aqueous ammonia solutions, it is expected that the same general technique presented here can also be used to synthesize other metal oxides and hydroxides of technological interest. 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.

5. Conclusions A new low temperature method has been developed for synthesizing epitaxial ZnO from aqueous solutions using a continuous circulation reactor. With this method, the total yield, growth rate, and morphology of the ZnO synthesized were controlled by altering the pH, ammonia concentration, temperature, heating rate, recirculation rate, and substrate insertion method. The experimental results were in qualitative agreement with the predictions of the thermodynamic calculations presented in part 1. A steady-state 〈0001〉, or c-direction, growth rate of ∼0.8 µm h-1 was achieved by recirculating an aqueous solution with a pH of 11, containing 0.5 mol L-1 ammonia and saturated with ZnO, between a room temperature dissolution vessel and a 90 °C deposition vessel at 0.5 mL min-1. These growth conditions have not been fully optimized, and the calculations suggest that substantially faster growth rates are possible. If high quality material can be synthesized at faster rates, it would then become feasible to grow bulk single crystals of ZnO from low temperature solutions within time spans similar to those used in high temperature hydrothermal runs.

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