Effects of Heating Rate on the Nucleation, Growth, and Transformation

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Effects of Heating Rate on the Nucleation, Growth and Transformation of InOOH and InO via Solvothermal Reactions 2

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Shunxi Tang, Jian Zhang, Si Wu, Chunyuan Hu, Yingai Li, Lina Jiang, and Qiliang Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp505459z • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on September 1, 2014

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The Journal of Physical Chemistry

Effects of Heating Rate on the Nucleation, Growth and Transformation of InOOH and In2O3 via Solvothermal Reactions Shunxi Tang, Jian Zhang,* Si Wu, Chunyuan Hu, Yingai Li, Lina Jiang and Qiliang Cui

State Key Laboratory of Superhard Materials, Jilin University, 130012 Changchun, P.R. China

Keywords: InOOH nanowire, In2O3 nanocube, self-assembly, Ostwald-ripening

ABSTRACT: A solvothermal reaction is generally considered to be governed by the chemical and thermodynamic parameters. Yet, the effects of heating rate on the nucleation and growth of the target materials within solvothermal processes have been rarely reported. In this work, taking the solvothermally synthesized InOOH/In2O3 as the sample system, we intend to illustrate that the heating rate plays an important role in the nucleation, growth, and transformation in solvothermal reactions. It is shown that with the heating rate changing from 4 °C/min to 8 °C/min, the initial nucleation temperature for ultrathin InOOH nanowires drops greatly from 160 °C to 120 °C. At a heating rate of 4 °C/min, the transformation from InOOH nanowires to In2O3 nanocubes in the one-step solvothermal system begins at 170 °C and completes at 210 °C. While at a heating rate of 8 °C/min, the transformation begins at 130 °C and completes at 180 °C. It is also found that heating rate may trigger different growth mechanisms in the solvothermal system,

1 Environment ACS Paragon Plus


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and subsequently influence the microstructure of the products. Thus it is anticipated that controlling the heating rate may be a potential route to tailor the morphology, microstructure, and even the properties of materials via solvothermal processes.

INTRODCTION During the past three decades, solvothermal reactions have been developed into versatile and powerful techniques in a large domain of applications, such as materials synthesis, crystal growth, and thin films deposition, etc.. A solvothermal process is especially useful in preparing micro– or nanoparticles with controllable size, shape, and dimension. Typically, solvothermal reactions are considered to be mainly governed by different chemical parameters (nature of the reagents and of the solvents) and thermodynamic parameters (temperature and pressure).1–3 The effects of these parameters on the phase composition and morphological characteristics including sizes, shapes, and exposed surfaces, of the products have been intensively and extensively studied.4–10 However, to the best of our knowledge, the influence of the heating rate on the nucleation and growth of the target materials, is rarely reported. In the present work, we intend to illustrate that the heating rate plays an important role in the nucleation, growth, and phase transformation in solvothermal reactions. InOOH/In2O3 are chosen as the sample system under study. In the literature, indium compounds, especially oxyhydroxide (InOOH) and oxide (In2O3) were investigated intensively. By varying the precursors, solvents, surfactants, treating temperature, and reaction time, numerous morphologies of InOOH, such as


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nanoparticles,11 nanowires,12 nanorods,13 nanofibers,14 hollow spheres,15 and 3-D architectures,16 etc., have been synthesized via solvothermal methods. Usually, these InOOH nanostructures are employed as precursors for synthesis of In2O3 nanocrystals with well-defined geometric shapes.17,18 At the meantime, In2O3 nanoparticles have been prepared via one-pot aqueous routes.5 In particular, Wang et al. prepared ultrathin InOOH nanowires and uniform c-In2O3 nanocubes at different temperatures by one-step solvothermal processes.19 However, in these reports, much attention has been devoted to the chemical and thermodynamic aspects of the solvothermal reactions employed, while the heating rate have attracted little interest. Herein, take the nucleation, growth, and transformation from ultrathin InOOH nanowires to c-In2O3 nanocubes for example, we intend to explore whether heating rate has an impact on solvothermal processes. It is shown that a higher heating rate can reduce the critical temperature of nucleation and promote the transformation at relatively lower temperatures. Two kinds of growth mechanisms are found to compete to play the predominant role at different heating rates.

EXPERIMENTAL SECTION The study of the nucleation, growth, and transformation from ultrathin InOOH nanowires to cIn2O3 nanocubes was performed following the solvothermal method described elsewhere.19 In a typical reaction, 0.1 g of indium(III) chloride tetrahydrate (InCl3•4H2O, 99.99%) was dissolved in 5 mL of oleylamine (C18-content 80–90%, Acros). Then, 8 mL of ethanol was dropped in



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under ultrasonicating. When a clear solution was formed, it was transferred into a Teflon-lined stainless steel autoclave. To study the effects of heating rate on the nucleation, growth, and phase transformation in the solvothermal processes, the autoclave was heated at two different heating rates, 4 °C/min and 8 °C/min, in the two sets of parallel experiments, respectively. The solvothermal reactions were carried out in the temperature range of 110–210 °C. The autoclave was maintained at the desired temperature for 24 h and then allowed to cool naturally to room temperature. The white precipitates were separated by centrifugation at 3000 rpm for 20 min, and washed repeatedly with absolute ethanol to remove the residues and/or impurities. The collected products were dried at 50 °C for 4 h before further characterization. In order to determine the phase composition of the products, a powder X-ray diffractometer (Shimadu XRD-6000) with Cu Kα radiation (λ=1.5418 Å) was used to record the X-ray diffraction (XRD) patterns at a scanning speed of 2 °/min. The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) micrographs and the selected area electron diffraction (SAED) patterns were taken to observe the morphology, size and structure of the samples via a JEM-2200FS transmission electron microscope with an accelerating voltage of 200 kV, which is equipped with an energy dispersive spectrometer (EDS). A HITACHI S4800 field emission scanning electron microscope (FESEM) working at 25.0 kV was used to characterize the morphology. Photoluminescence (PL) spectra were


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examined using a fluorescence spectrophotometer (Edinburgh FLS920) with a Xe lamp excitation at room temperature.

RESULTS AND DISCUSSION From the classical thermodynamics point of view, it is generally accepted that the driving forces for the nucleation and growth depend largely on the reaction temperature, pressure, and the concentration of precursors. Yet, as one of the main findings of this work, it can be shown below that the heating rate will promote the nucleation and growth of ultrathin InOOH nanowires in a solvothermal process. With the heating rate changing from 4 °C/min to 8 °C/min, the initial nucleation temperature for ultrathin InOOH nanowires drops greatly from 160 °C to 120 °C. In addition, the heating rate will also affect profoundly the transformation from pure InOOH nanowires to In2O3 nanocubes in the same solvothermal system. At a heating rate of 4 °C/min, the transformation begins at 170 °C and completes at 210 °C. While at a heating rate of 8 °C/min, the transformation begins at 130 °C and completes at 180 °C. The typical XRD patterns of the as-prepared samples with increasing temperatures at the heating rate of 4 °C/min and 8 °C/min are shown in Figure 1(a) and (b), respectively. According to the previous report,19 the whole solvothermal process can be expressed by a hydrolysis reaction as follow: In3+ + 3H2O → In(OH)3 + 3H+ (1) In(OH)3 → InOOH + H2O

(2)

2InOOH → In2O3 + H2O

(3)



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Figure 1. The typical XRD patterns of the samples prepared at the heating rate of (a) 4 °C/min and (b) 8 °C/min, respectively, at various temperatures for 24 h.

As shown in Figure 1(a), at the heating rate of 4 °C/min, the hydrolysis reaction almost did not happen until the temperature reached 160 °C. At 160 °C, all the broadened diffraction peaks could be deconvoluted (as shown in Figure S1(a) in the Supporting Information) and indexed according to the orthorhombic InOOH lattice with the cell parameters a=5.23 Å, b=4.54 Å, and c=3.29 Å, which are in good agreement with the values from the standard card (JCPDS No. 712283).


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Figure 2. (a, b) The low and high-magnification TEM images of InOOH nanowires synthesized at 160 °C with the heating rate of 4 °C/min. (c) The HRTEM image of an isolated InOOH nanowire synthesized at 160 °C and corresponding FFT pattern. (d) The TEM images of InOOH nanowires synthesized at 120 °C with the heating rate of 8 °C/min.

From the TEM images shown in Figure 2(a) and (b), it can be seen that the length of the InOOH nanowires is about 100 nm and the diameter of the InOOH nanowires can be estimated to be around 2.5 nm. A “V”-shaped structure which can be explained by the oriented attachment mechanism is observed.20–23 The HRTEM image (Figure 2(c)) shows that the InOOH nanowires grow along the [001] direction, which can also be verified by the XRD pattern with a relatively stronger (002) peak. The one-dimensional preferential growth of InOOH along the [001] direction may be attributed to the anisotropic crystallographic feature.24 However, when the heating rate changes to 8 °C/min, the InOOH nanowires can also be obtained at a temperature as low as 120 °C (Figure 1(b)). As shown in the typical TEM image



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(Figure 2(d)), the diameter of the ultrathin InOOH nanowires is almost the same with that of the nanowires synthesized at the heating rate of 4 °C/min, but the length gradually increases as the temperature rises. This phenomenon gives a strong hint that the nucleation temperature of InOOH nanowires tend to decrease with increasing heating rate. At the heating rate of 4 °C/min, when the reaction temperature is increased to 170 °C, some weak peaks of c-In2O3 emerge, as shown in Figure 1(a). The peaks from InOOH and c-In2O3 coexist in the pattern until the temperature reaches 200 °C, at which all the peaks of InOOH disappear and the sample transforms to some c-In2O3 nanoparticles with irregular morphology completely (Figure S2(a) in the Supporting Information). In addition, no additional peaks from contaminants or other phases can be detected, indicating that the prepared product is entirely composed of c-In2O3. The sharp peaks also indicate that the prepared samples have perfect crystallinity. As shown in the typical SEM image (Figure 3(a)) and TEM images (Figure 3(b) and (c)), when the temperature rises to 210 °C, the uniform c-In2O3 nanocubes with a side length of about 90 nm are prepared.


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Figure 3. (a) The SEM image of uniform c-In2O3 nanocubes synthesized at 210 °C with the heating rate of 4 °C/min. (b, c) The low- and high-magnification TEM images of c-In2O3 nanocubes synthesized at 210 °C with the heating rate of 4 °C/min. (d) The HRTEM and corresponding SAED pattern of one c-In2O3 nanocube shown in (c).

At the heating rate of 8 °C/min and the temperature of 130 °C, a few nanoparticles come into being in the product (Figure S2(b) in the Supporting Information). From the deconvoluted narrow-range XRD pattern of the sample synthesized at 130 °C (Figure S1(b) in the Supporting Information), it can be deduced that those nanoparticles are c-In2O3. With the temperature increasing, the intensity of the peaks of c-In2O3 grows stronger and that of InOOH prefers the opposite. At 180 °C, all the peaks of InOOH disappear, indicating the transformation from InOOH to c-In2O3 is completed. In this case the prepared uniform c-In2O3 nanocubes have a side length of about 120 nm (Figure 4(a)–(c)). And the sample synthesized at 190 °C are almost the same as that synthesized at 180 °C (Figure S2(c) and (d) in the Supporting Information). These



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results also indicate the nucleation temperature of uniform c-In2O3 nanocubes and transformation temperature from InOOH nanowires to uniform c-In2O3 nanocubes tend to decrease with increasing heating rate.

Figure 4. (a) The SEM image of uniform c-In2O3 nanocubes synthesized at 180 °C with the heating rate of 8 °C/min. (b, c) The low- and high-magnification TEM images of c-In2O3 nanocubes synthesized at 180 °C with the heating rate of 8 °C/min. (d) The HRTEM and corresponding SAED pattern of one c-In2O3 nanocube shown in (c).

In addition to XRD results, the chemical composition of the prepared samples was further characterized by energy dispersive X-ray analysis (EDS). As shown in Figure S3 in the Supporting Information, it can be seen that the pure InOOH and In2O3 samples contain mainly O and In elements. Cu element coming from the Cu supporting mesh is also detected. In a wide spectrum range, no signals for other elements can be observed.


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Figure 5. (a) The narrow-range XRD pattern of the sample synthesized at 170 °C with the heating rate of 4 °C/min (solid black trace), and the deconvoluted (222) and (400) peaks of c-In2O3 (dotted pink trace), and (110), (101), (011) and (200) peaks of InOOH (dotted violet trace). The red trace is the superposition of the deconvoluted peaks. (b) The average content of c-In2O3 prepared at different heating rates.

The quasi-quantitative analysis of the In2O3 contents during the transformation process was performed using the most pronounced XRD peaks in the region 2θ≈23–37°.25,26 In this range, the XRD patterns mainly consist of a mixture of c-In2O3 (222) and (400), and InOOH (110), (101), (011) and (200) reflections. The deconvolution of the XRD peaks of the sample synthesized at 170 °C with the heating rate of 4 °C/min is shown in Figure 5(a), as an example. Performed by means of RIR quantitative analysis using the Formula S1 and S2 in the Supporting Information, the results of quantitative analysis of the composition of the samples are shown in Figure 5(b). From Figure 5(b), it is clear that the result is consistent with the XRD analysis. At a given heating rate, the weight ratio of In2O3 will increase with increasing temperature. Equally, at a given temperature, the weight ratio of In2O3 will be higher with a higher heating rate.



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Another important finding of this study is that heating rate may trigger different growth mechanisms in the solvothermal system, and subsequently influence the microstructure of the product. As two kinds of common mechanisms, Ostwald-ripening and self-assembly were widely adopted to explain the growth process of In2O3 nanomaterials. For example, Franzman et al. found that well-defined indium oxide nanocrystals were prepared in the solution phase by a slow Ostwald ripening process via a new and reproducible peroxide-mediated route at relatively low temperatures (120–180 °C).27 Ye et al. found that the truncated octahedral indium oxide were self-assembled into either zigzag lines or pentagram patterns, and the regular octahedral and truncated cubic indium oxide were self-assembled into hexagonally packed nanocrystal arrays with different reaction media.6 In the present case, as shown in Figure 6, the c-In2O3 nanocubes grow via a typical Ostwald-ripening mechanism at the heating rate of 4 °C/min. While at the heating rate of 8 °C/min, self-assembly process plays the dominant role.

Figure 6. The Ostwald-ripening mechanism and self-assembly process play the dominant role respectively at different heating rates.

As shown in the high magnification TEM (Figure 3(c) and 4(c)) and HRTEM (Figure 3(d) and 4(d)) images, the c-In2O3 nanocubes are not identical. The nanocubes obtained at the heating rate


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of 8 °C/min have a relatively rougher surface. They are likely to be assembled by a large number of highly oriented nanowires. In order to investigate the effects of heating rate on the growth mechanism of c-In2O3 nanocubes, a series of experiments with various reaction times were systematically carried out. Figure S4(a) and (b) in the Supporting Information show the typical XRD patterns of the samples synthesized at 210°C with the heating rate of 4 °C/min and 180 °C with the heating rate of 8 °C/min for different reaction times. In both cases, it can be seen that the samples transform from InOOH to In2O3 gradually with time.

Figure 7. (a), (b–d), (e–g), (h), (i), (j–k), (l) depict the time-dependent morphologies of the samples synthesized at 210 °C with the heating rate of 4 °C/min for 1 h, 2 h, 4 h, 8 h, 12 h, 18 h and 24 h, respectively.



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At the heating rate of 4 °C/min, the transformation process from InOOH to c-In2O3 is demonstrated by a series of typical TEM images shown in Figure 7. As shown in Figure 7(a), in the primary stage, the nucleation and growth of InOOH nanowires occurred rapidly within 1 h, when the temperature was brought up to 160 °C. The sample consist almost entirely of ultrathin InOOH nanowires (Figure 7(a)), and this is consistent with the XRD pattern of the sample synthesized at 210 °C for 1 h (Figure S4(a) in the Supporting Information). With the elongation of reaction time, a lot of “nanodots” appeared (Figure 7(b)). From the higher magnification TEM images (Figure 7(c) and (d)), it could be seen clearly that those “nanodots” are actually some rings with an irregularly shape and an average size of ~20 nm. Those rings were formed by one or several InOOH nanowires connected end to end. Those rings are indicated to be c-In2O3 by the (222) peak appearing in the XRD pattern for 2 h (Figure S4(a) in the Supporting Information). When the reaction time was prolonged to 4 h (Figure 7(e)–(g)), the interaction of the primary cIn2O3 rings resulted in their aggregation into many spherical particles with a diameter of ~50 nm. It is generally believed that, for a spherical single-phase crystal with a small size, its surface must be a polyhedron containing high-index crystallography planes which possibly result in a higher surface energy.28 Once small spherical particles are formed in the solution with high degrees of supersaturation, they are active and aggregate to form larger nanocrystals to minimize the surface energies. As the reaction proceeds, more and more c-In2O3 spherical particles were formed with the consumption of the InOOH nanowires (Figure 7(h)). When the reaction time was prolonged to 12 h (Figure 7(i)), all the InOOH nanowires disappeared, instead a lot of coarse-surfaced


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single-crystalline c-In2O3 pre-cubes emerged. Thereafter, particles smaller than the average size disappeared, indicating a typical Ostwald-ripening mechanism (Figure 7(j)–(k)). At the same time, the surfaces of the ripened c-In2O3 nanocrystals converges to specific crystal facets to reduce the surface energy. As a result, truncated nanocubes are formed (Figure 7(l)). The cube facets of {200} and the truncated facets of {420} were verified by HRTEM (Figure 3(d)). According to Wulff construction, the shape of a crystal is determined by the relative specific energy of crystalline planes.28 As is known, the growth rate of low index crystallographic planes is proportional to their surface energies. Thus, after growth, the high energy planes would preferentially disappear and the obtained crystal would be terminated with low energy planes. For In2O3 with bcc structure, the surface energy relationship among three low-index crystallographic planes should correspond to γ{111} < γ{100} < γ{110}.29 Thus the growth rates of the corresponding growth directions should have the following relationships: r < r < r. Under dynamic equilibrium conditions, the grown In2O3 nanoparticles would be terminated with equivalent low energy (111) planes and possess octahedral configuration. However, in our case, In2O3 nanoparticles grow into cubes. It can be seen that the fast growth direction was along , but not . These results revealed that the growth of In2O3 nanocubes occurred under non-equilibrium conditions.



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Figure 8. (a), (b), (c, d), (e), (f, g), (h) depict the time-dependent morphologies of the samples synthesized at 180 °C with the heating rate of 8°C/min for 1 h, 2 h, 4 h, 8h, 12h and 24 h, respectively.

When the heating rate was raised to 8 °C/min (Figure 8), the transformation process also began with the pure InOOH nanowires (Figure 8(a)). Then, some rings with an irregularly shape were formed (Figure 8(b)), and then aggregate into many spherical particles (Figure 8(c) and (d)).This section is almost the same with the process at the heating rate of 4 °C/min. Different process happened subsequently, in which the self-assembly growth mechanism played dominant roles. The similar transformation process from nanowires to nanocubes via the self-assembly mechanism have been reported in Zn-MS-Zn coordination polymers system.30 At the relative lower temperature, the nanowires are more likely to attach to the surface of the nanoparticles. When the treating time reached 8 h (Figure 8e), the spherical nanoparticles transformed to less perfect nanocubes with the minimized surface energies. As shown in Figure 8(f), 8(g) and S5 in the Supporting Information, the self-assembly process can be seen clearly. At last, the uniform c-


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In2O3 nanocubes with rough surface were obtained (Figure 8(h)). The cube facets of {200} with no obvious truncated facets were verified by HRTEM (Figure 4(d)). During the whole growth process, there was no evidence for the coexistence of particles with different sizes, which excluded the Ostwald-ripening mechanism at this heating rate. As a side evidence, the PL emission behaviors of the InOOH nanowires and c-In2O3 nanocubes (shown in Figure S6 in the Supporting Information) may also reflect the difference in growth mechanism at different heating rates. As compared to In2O3 nanocubes obtained at the heating rate of 4 ºC/min, the intensity of the emission band of those obtained at the heating rate of 8 ºC/min is significantly strengthened, with the maximum of the emission band blue-shifting from 496 nm to 469 nm. In the case of InOOH, the nanowires obtained at the heating rate of 8 ºC/min also exhibit stronger PL emission. However, red shift of the PL emission with increasing heating rate could be observed. These phenomena indicate that defects of different types and densities may result from the different growth mechanisms triggered by the heating rate.

CONCLUSIONS In summary, the effects of heating rate on the nucleation, growth, and transformation from InOOH nanowires to c-In2O3 nanocubes via a one-step solvothermal reaction route were studied. The XRD and TEM results suggested that high heating rate could lower the initial nucleation temperature and promote the nucleation and growth of ultrathin InOOH nanowires in a



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solvothermal process. In addition, with a high heating rate, the transformation from InOOH nanowires to In2O3 nanocubes in the same solvothermal system would also occur at a lower temperature. It was found that during the growth of In2O3 nanocubes, the competition between different mechanisms could be induced by the heating rates. The c-In2O3 nanocubes grew via a typical Ostwald-ripening mechanism at lower heating rate. While at high heating rate, selfassembly process played the dominant role. Hence, the microstructures of the InOOH nanowires and the c-In2O3 nanocubes produced at different heating rates got imperceptible differences, which could be reflected by their PL emissions. This work strongly indicate that the heating rate may provide potential pathways to manipulate the solvothermal processes, as well as the chemical and thermodynamic parameters.

ASSOCIATED CONTENT Supporting Information

Narrow-range XRD patterns of the samples synthesized at 160 °C with the heating rate of 4 °C/min and at 130 °C with the heating rate of 8 °C/min; TEM images of samples synthesized at 200 °C with the heating rate of 4 °C/min, 130 °C with the heating rate of 8 °C/min and 190 °C with the heating rate of 8 °C/min; XRD pattern of c-In2O3 nanocubes synthesized at 190 °C with the heating rate of 8 °C/min; EDS spectra of the InOOH and In2O3 obtained at the heating rate of 4 ºC/min and 8 ºC/min; XRD patterns of samples hydrothermally treated with the heating of 4


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°C/min and 8 °C/min for different times; TEM images of samples synthesized at 180 °C with the heating rate of 8 °C/min for 12 h; The photoluminescence spectra of c-In2O3 nanocubes and InOOH nanowires synthesized at the different heating rates. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported in part by the Natural Science Foundation of China (grant nos. 50772043, 51172087, and 11074089) and the National Basic Research Program of China (grant no. 2011CB808200).

AUTHOR INFORMATION Corresponding Author

* Corresponding Author: E-mail: [email protected]; Fax: +86 431 85168346; Tel: +86 43185168881.

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The typical XRD patterns of the samples prepared at the heating rate of (a) 4 °C/min and (b) 8 °C/min, respectively, at various temperatures for 24 h. 104x78mm (600 x 600 DPI)

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(a, b) The low and high-magnification TEM images of InOOH nanowires synthesized at 160 °C with the heating rate of 4 °C/min. (c) The HRTEM image of an isolated InOOH nanowire synthesized at 160 °C and corresponding FFT pattern. (d) The TEM images of InOOH nanowires synthesized at 120 °C with the heating rate of 8 °C/min. 70x70mm (300 x 300 DPI)


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(a) The SEM image of uniform c-In2O3 nanocubes synthesized at 210 °C with the heating rate of 4 °C/min. (b, c) The low- and high-magnification TEM images of c-In2O3 nanocubes synthesized at 210 °C with the heating rate of 4 °C/min. (d) The HRTEM and corresponding SAED pattern of one c-In2O3 nanocube shown in (c). 70x70mm (300 x 300 DPI)



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(a) The SEM image of uniform c-In2O3 nanocubes synthesized at 180 °C with the heating rate of 8 °C/min. (b, c) The low- and high-magnification TEM images of c-In2O3 nanocubes synthesized at 180 °C with the heating rate of 8 °C/min. (d) The HRTEM and corresponding SAED pattern of one c-In2O3 nanocube shown in (c). 70x70mm (300 x 300 DPI)


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(a) The narrow-range XRD pattern of the sample synthesized at 170 °C with the heating rate of 4 °C/min (solid black trace), and the deconvoluted (222) and (400) peaks of c-In2O3 (dotted pink trace), and (110), (101), (011) and (200) peaks of InOOH (dotted violet trace). The red trace is the superposition of the deconvoluted peaks. (b) The average content of c-In2O3 prepared at different heating rates. 49x17mm (600 x 600 DPI)



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The Ostwald-ripening mechanism and self-assembly process play the dominant role respectively at different heating rates. 42x11mm (300 x 300 DPI)


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(a), (b–d), (e–g), (h), (i), (j–k), (l) depict the time-dependent morphologies of the samples synthesized at 210 °C with the heating rate of 4 °C/min for 1 h, 2 h, 4 h, 8 h, 12 h, 18 h and 24 h, respectively. 106x79mm (300 x 300 DPI)



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(a), (b), (c, d), (e), (f, g), (h) depict the time-dependent morphologies of the samples synthesized at 180 °C with the heating rate of 8°C/min for 1 h, 2 h, 4 h, 8h, 12h and 24 h, respectively. 70x35mm (300 x 300 DPI)


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Table of Contents Graphic 36x25mm (300 x 300 DPI)


Effects of Heating Rate on the Nucleation, Growth, and Transformation

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