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Growth kinetics and morphological evolution of ZnO precipitated from solution Yin Liu, Kaiping Tai, and Shen J Dillon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303522z • Publication Date (Web): 10 Jul 2013 Downloaded from http://pubs.acs.org on July 15, 2013
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Chemistry of Materials
Growth kinetics and morphological evolution of ZnO precipitated from solution Yin Liu, Kaiping Tai and Shen J Dillon* Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Abstract This work characterizes the nucleation and growth kinetics of zinc oxide (ZnO) precipitated from aqueous hexamethylenetetramine (HMTA) zinc nitrate (Zn(NO3)2) solutions observed by in-situ and ex-situ transmission electron microscopy. Quantitative comparisons between in-situ beam-induced precipitation, insitu thermally activated precipitation, ex-situ thermally activated precipitation, and ex-situ electrochemistry provide insights into the rate limiting mechanism and the chemistry governing the reactions. All experiments indicate that isotropic ZnO precipitates directly from solution. These particles begin to aggregate and grow anisotropically shortly after nucleation. The conversion to anisotropic growth does not rely on coalescence despite the fact that the two are often observed to occur in concert. The results indicate that the reaction pathway for in-situ beam-induced growth more closely mimics ex-situ electrochemistry than ex-situ chemistry. In-situ and ex-situ thermally activated growth processes proceed in a similar manner, although particle transport and aggregation is limited by the in-situ geometry. Keywords: in-situ, transmission electron microscopy, liquid-phase, electron beam-induced crystal growth, nanoparticles, Zinc oxide Corresponding author: Shen J. Dillon,
[email protected], phone: 1 217 244 5622, fax: 1 217 333 2736
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Chemistry of Materials
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Introduction Semiconducting nanostructures hold great promise in applications ranging from energy harvesting and storage to environmental remediation and health care1, 2. ZnO is a leading candidate material for many such applications owing to its low cost, abundance, morphological and chemical flexibility, and bandgap energy. These materials find application in transparent electronics, lasers, piezoelectric devices, photocatalysts, and chemical sensors.3-5 Various techniques synthesize ZnO nanostructures, amongst them; solution growth methods are most attractive owing to their low cost, catalyst free, and scalable synthesis.6, 7 Understanding the associated nucleation and growth behavior is critical to optimizing the processing, properties, and performance of these materials and has been the focus of intensive investigation. The wealth of ZnO nanostructures accessible through solution growth motivates investigation of its nucleation and growth.8-10 Low temperature (60 oC).12 However, it can be electrodeposited chemically at room temperature from similar solutions.40, 41 We hypothesize that the electrochemical reaction is a reasonable indicator for the possibility of electron beam-induced reactions and that it should be possible to use electron beam-induced reactions to produce ZnO at room temperature. This would allow ZnO precipitation to be characterized in-situ under ambient conditions at high beam current density, in-situ at high temperature and low beam current density, and ex-situ under comparable thermal conditions. A quantitative analysis of the different conditions should provide insights into both the nature of the underlying chemical reaction and the validity of the technique. In this work, ZnO is precipitated under conditions where diffusional transport limits the beam-induced reaction rate. These results are compared to ex-situ results of ZnO precipitated from heated solutions, in-situ precipitation of ZnO from heated solution at low beam current densities, and ex-situ electrodeposition. The experiments target a range of precursor compositions in order to establish trends in the nucleation and growth behavior with varying chemistry. Experimental Procedure 400 mM Zn(NO3)2 (99.99% purity, Sigma Aldrich) and 100 mM HMTA (99.9% purity, Sigma Aldrich) aqueous solutions were prepared, separately. The solutions and water were mixed in appropriate proportions to produce solutions with different Zn(NO3)2 and HMTA concentrations. Polyvinylpyrrolidone (PVP) (99.9% purity, Fluka MW 40000) was used as surfactant in certain reactions. A custom designed environmental cell was used for in-situ TEM investigation of liquid samples. The aqueous solution was confined between two silicon grids with 50 nm thick amorphous silicon nitride (SiNx) membrane windows (Ted Pella) and sealed in the cell with viton o-rings. The hydrophilicity of the grids was enhanced by atomic layer deposition of 5 nm hydroxylated alumina. A schematic of the wet cell is shown in reference [37]. For comparison with the e-beam-induced precipitation, thermally activated ZnO precipitation from solution was studied in-situ using a custom-designed temperature controlled environmental stage. The environmental cell attaches to a copper rod, which links to an external heat source. Resistive heating modulated by a thermocouple, near the environmental cell, that was calibrated to the internal cell temperature. A low beam current density (500 A m-2), were performed under conditions where gas partially displaced the liquid. These imaging conditions have been utilized in previous in-situ studies by other groups 31, 42, and provide a reasonably consistent set of test conditions for the beam-induced reaction. It should be noted that such thin liquid films typically exhibit diffusivities that differ from the bulk values and may contain molecular structural arrangements that, on average, differ from the bulk liquid. The hydrothermal precipitation reactions were performed under conditions where no gas is evolved due to electron beam-induced reactions. This is most consistent with an ex-situ hydrothermal reaction. The effects of the electron beam on gas evolution and chemical reactions are described in more detail in the supplemental information along with a more thorough description of the imaging conditions for the different experiments (Figure S1-S3). The thickness of the liquid within the cell, in absence of bubbles, was quantified using electron energy-loss spectroscopy (EELS) (Figure S4). EELS was performed using a JEOL 2010F STEM/TEM equipped with Gatan Image Filter. Sample thickness was determined from Beer’s law: I = I0 exp(-t/λ) where I is the number of unscattered electrons , I0 is the number of total incident electrons, t is the liquid thickness, λ is the inelastic mean free path. λ was estimated based on references 43, 44. This work characterizes deionized water. The thickness of the liquid within the cell was 310-480 nm, which is consistent with results of prior studies.43 Electrochemical deposition of ZnO was carried out in aqueous solutions of varying Zn(NO3)2 and HMTA concentrations. Soda-lime glass coated with gold, via electron beam evaporation, was used as an anode, with a 15 µm gold wire cathode, and a Ag/AgCl reference electrode. Cyclic voltammetry measurements were carried out using a potentiostat (SP200, Biologic Co.) without stirring at temperatures between 25oC and 60oC with a sweep rate of 10 mVs-1. Constant voltage electrodeposition at -1 V vs Ag/AgCl was also performed at temperatures between 25oC and 60oC. Elevated temperature was necessary to produce ZnO chemically.45, 46 Ex-situ thermally activated precipitation was performed in a stirred solution at temperatures between 65oC and 85oC. Samples of the solution were collected with pipettes at different times, and deposited directly onto SiNx windows for TEM observation. Results Video S1 (Supporting Information) shows the nucleation and growth of particles from solution at four beam current densities varying by 2 orders of magnitude. Figure 1 plots the per unit area of particles as a function of time for each condition and shows example images from each condition. The number of particles per unit area growing from solution is sensitive to beam current density at low beam current densities, but is relatively insensitive at high beam current densities (> 500 A m-2). The finding suggests that the reaction transitions from reaction rate limited to diffusion limited with increasing beam current density, as has been reported for other beam-induced reactions.24, 34 At a beam current density of 1 A m-2 no beam-induced precipitation is observed. This regime is useful for investigating thermally activated chemical precipitation. Figure 2 and Video S2 show the evolution of nanoparticles in 10 mM Zn(NO3)2 aqueous solutions containing 0-30 mM HMTA during in-situ observation at 3000 A m-2. In the absence of HMTA, limited nanoparticle nucleation is observed. Nanoscale circular white features are observed in most of the data (figure 2). We hypothesize that these are nanoscale bubbles that exist in the thin liquid film that wets the SiNx windows as they can be observed more clearly in liquid films between bubbles and there are no other expected
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Chemistry of Materials
reaction products that should produce a decrease in sample mass thickness. A more detailed description is provided in Figures S2-S3 and the associated supplemental text. Increasing the HMTA content increases both the number of nanoparticles nucleated and the overall volume of precipitated ZnO. Figure 3 plots the area fraction, in the field of view, containing ZnO nanoparticles as a function of time. In-situ lattice imaging (Video S3 and Figure S5) reveals ~0.25 nm fringes associated with the wurzite phase of ZnO. This d-spacing is not associated with the stable phases of Zn(OH)2 and indicates that the electron beam-induced reaction promotes the direct precipitation of ZnO. Zn(OH)2 was not observed under the test conditions described here, but its formation can not be discounted under different but related experimental conditions. Based on the volume of ZnO evolved during the initial stages of beam-induced precipitation and assuming that the process consumes either electrons or holes resulting from the primary beam, in some manner, approximately 10-5 of the of the beam current produces reaction resulting in ZnO. This is significantly lower than the direct decomposition of non-aqueous liquids, ~10-2, analyzed under similar conditions. The peak and subsequent decrease in the total area of particles precipitated as a function of time shown in Figure 3a results from an image projection effect. Figure 3b shows this explicitly by characterizing the number of particles per unit area and size of nanoparticles as a function of time in the 10 mM Zn(NO3)2 and 30 mM HMTA solution. The average diameter of the particles increases continuously. The number of particles per unit area initially increases due to beam-induced nucleation and then decreases as coarsening processes begin to dominate the response. Two different coarsening processes, namely particle coalescence and Ostwald ripening are observed in-situ (Figure S6). These two processes (ripening versus coalescence) occur in a relative proportion of 4 to 1, in the early stages of growth (
