Thermal Decomposition of Silver Acetate: Physico ... - ACS Publications

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Thermal Decomposition of Silver Acetate: Physico-Geometrical Kinetic Features and Formation of Silver Nanoparticles Masayoshi Nakano, Takayuki Fujiwara, and Nobuyoshi Koga* Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan S Supporting Information *

ABSTRACT: The thermal decomposition of silver acetate (CH 3 COOAg) was investigated to reveal the factors controlling the formation of Ag nanoparticles (NPs). The overall kinetic behavior was interpreted as partially overlapping two reaction steps using systematic kinetic and morphological analyses. Although the apparent activation energies were comparable (approximately 75 kJ mol−1), the initial reaction step was regulated by the first order law because of the consumption of reactive sites on the end surfaces of columnar crystals, whereas the subsequent reaction step advanced by shrinkage of the side surfaces of the crystals with an accelerating linear shrinkage rate, resulting in slimming of the crystals. A large surface area of the reactant crystals was exposed to the reaction atmosphere during the course of the reaction by the self-induced migration of the Ag product to the surfaces of the Ag-NP aggregates formed at certain parts of the reactant surfaces. As a result, the atmospheric water vapor affected the kinetic behavior by significantly lowering the reaction temperature. As a possible explanation for these phenomena, a physical mechanism involving evaporation of the reactant and simultaneous condensation of the product is proposed herein.

1. INTRODUCTION

The thermal decomposition of solids is largely regulated by the reaction geometry and the chemical and physical processes at the reaction interface, including destruction of the reactant crystal lattice, nucleation and crystal growth of the solid product, diffusional removal of gaseous product, and heat transfer.1−3 The actual reaction occurring at the reaction interface is thus largely influenced by the self-generated reaction conditions caused by the interactions of the component chemical and physical processes and the related mass and heat-transfer phenomena. In addition, various physicochemical and physico-geometrical events occur, which regulate the overall kinetics of the thermal decomposition. The typical macroscopic events are structural phase transitions of the reactant solid,4,5 gelation of the reactant solid,6 establishment of chemical equilibrium in the form of a core−shell structure comprising the internal reactant covered in a surface product layer and the consequent arrest of the overall reaction,4,5,7−9 condensation of gaseous product (water vapor),9−12 and crack formation in the surface product layer and sudden blowout of the gaseous product,4−15 among others. Therefore, it is quite difficult to determine the most appropriate reaction conditions of the thermal decomposition for producing solid products with desired properties. The practical reaction

Because of the range of potential applications of metal and metal-oxide nanoparticles (NPs), and the structural materials comprised thereof, much attention has been paid to the controlled syntheses of nanomaterials with desired morphological, chemical, and physical properties that are useful for applications in various fields. In the solution-mediated synthesis of nanomaterials, many parameters can be controlled in the synthetic processes, including the temperature, concentration, pH, pressure, additives, and templates. In addition, these parameters influence the reactions and the nanomaterial products homogeneously. In contrast, the direct synthesis of nanomaterials via the thermal decomposition of solid precursors is a classical method that still has merits for producing larger amounts of material via a single heating process. For synthesis via thermal decomposition, a suitable precursor compound should be selected and its morphology controlled carefully. For the heat treatment, heating rate, annealing temperature, annealing time, and reaction atmosphere are the most important factors controlling the formation of nanomaterials with desired properties. However, influences of these factors on the reaction processes involved in thermal decomposition appear to be more complex than they are in many solution-mediated processes because of the heterogeneous characteristics of the reactions that occur in solid-state and solid−gas systems. © 2016 American Chemical Society

Received: March 7, 2016 Revised: April 6, 2016 Published: April 7, 2016 8841

DOI: 10.1021/acs.jpcc.6b02377 J. Phys. Chem. C 2016, 120, 8841−8854

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The Journal of Physical Chemistry C

1600, JEOL, 30 mA, 30 s). Powder XRD patterns of the sample (5° ≤ 2θ ≤ 60°) were measured using a diffractometer (RINT2200 V, Rigaku Co.) with monochromatic Cu Kα radiation (40 kV, 20 mA) at a scan speed of 4° min−1. The FTIR spectra were recorded using a spectrometer (FT-IR 8400s, Shimadzu Co.) by the diffuse reflectance method after diluting the sample with KBr. The thermally induced mass-change process was tracked using TG−DTA (DTG-50M, Shimadzu Co.) by heating the sample (sample mass m0 = 2.0 mg) in a Pt pan (6 mm diameter and 2.5 mm height) at a heating rate β of 5 K min−1 under a N2 flow (80 cm3 min−1). 2.2. Characterization of Reaction Process. Changes in the XRD pattern of the sample during thermal decomposition were tracked using the aforementioned XRD instrument by equipping it with a programmable heating chamber (PTC-20A, Rigaku Co.). The sample was fitted by pressing onto a Pt plate and was heated under flowing N2 (100 cm3 min−1) according to a stepwise isothermal heating program with heating linearly at β = 5 K min−1 and 15 min constant temperature steps every 50 K from 323 to 773 K. The diffraction measurements were carried out during the isothermal heating steps at different temperatures. Separately, changes in the XRD pattern during isothermal heating at 443 K in flowing N2 (100 cm3 min−1) were also tracked by recording repeatedly every 15 min. The mass-loss process during the thermal decomposition of the sample was tracked under different measurement conditions. The TG−DTA curves under N2 or air flow (80 cm3 min−1) were recorded using a DTG-50M for approximately 2.0 mg of sample in a Pt pan (6 mm diameter and 2.5 mm height) at β = 5 K min−1. The TG−DTA measurements for the samples (2.02 ± 0.03 mg) in a Pt pan (5 mm diameter and 2.5 mm height) under controlled O2 concentrations in the ranges 0 ppm ≤ c(O2) ≤ 800 ppm and 1.0% ≤ c(O2) ≤ 21.7% in a flowing N2−air mixture (500 cm3 min−1) were also recorded using a TG−DTA instrument (TG-8121, Rigaku Co.) at β = 5 K min−1, in which the O2 and CO2 concentrations in the outlet gas were continuously monitored using O2 (LC-750, Toray Co.) and CO2 (LX-720, IIJIMA Electronics Co.) concentration meters. For tracking the mass spectrum of the evolved gas during the thermal decomposition, TG−DTA measurements (TG-8120, Rigaku Co.) were carried out at β = 5 K min−1 in flowing He or He−O2 mixed gas (21% O2) at a rate of 200 cm3 min−1 for an approximately 5.0 mg sample in a Pt pan (5 mm diameter and 2.5 mm height). The outlet gas was introduced into a quadrupole mass spectrometer (M-200QA, Anelva Co.) through a silica capillary tube (0.075 mm internal diameter) heated at 500 K, and the mass spectrum of the gas was repeatedly measured in the mass range from 10 to 70 amu (EMSN, 1.0 A; SEM, 1.0 kV). The influence of atmospheric water vapor on the mass-loss process was also investigated using TG−DTA (TG-8120, Rigaku Co.) by introducing a N2−H2O mixture (400 cm3 min−1) with different controlled water vapor pressures (0.2 kPa ≤ p(H2O) ≤ 16.3 kPa). Samples of 1.99 ± 0.03 mg in the Pt pan (5 mm diameter and 2.5 mm height) were heated to 333 K in flowing N2 (400 cm3 min−1), and the flow gas was switched to N2−H2O gas (400 cm3 min−1) with a controlled p(H2O) generated using a humidity controller (HUM-1, Rigaku Co.).49 After stabilizing the measuring system for 30 min at 333 K, the sample was heated in flowing gas with controlled p(H2O) at β = 5 K min−1. 2.3. Kinetic Measurements. The mass-loss data for the kinetic analysis of the thermal decomposition process were

conditions are generally selected by considering the empirical relationships among the reactant, reaction conditions, and product, as well as the overall rate behavior, as revealed using thermoanalytical measurements. In turn, such complex reaction behaviors and special events can sometimes afford NPs and structural materials comprised thereof. One example is the formation of Au-NPs by the rapid and violent fragmentation of reactant particles during the thermal decomposition of gold(III) acetate as reported by Bakrania et al.16 The other is the generation of a Ag2CO3−Ag2O core−shell structure during the thermal decomposition of silver carbonate,4,5 which exhibits high visible light efficiency in the photocatalytic degradation of pollutants.17 To identify the major factors that control the formation kinetics of nanosized and structural materials and to determine the most appropriate reaction conditions of the heat treatment in more sophisticated manner, it is necessary to reveal the kinetic characteristics of the thermal decomposition from the viewpoints of physico-chemistry and physicogeometry. Because of their utilities for various purposes,18 syntheses of Ag-NPs and their structural and composite materials via thermal decomposition of solid precursors have been extensively studied using different Ag compounds as precursor materials.19−47 Among others, the thermal decomposition of silver acetate20,23,28−30,33,36,37,41,44−46 has been studied for preparing nanowires,33,45 nanorods,41 and catalysts,46 and these products have been used in electric devices.23,29,44 Logvinenko et al.30 and Siffiqui et al.34,37 determined the apparent kinetic parameters for the thermal decomposition under linearly increasing temperatures in flowing inert gas and in air. Further extension of the kinetic characterization of the thermal decomposition process from the viewpoints of physicochemistry and physico-geometry would provide direct information useful for controlling the reaction process and the properties of the Ag-NP product. Along this line, the present study focused on the characteristics of the physicochemical and physico-geometrical mechanisms of the thermal decomposition of silver acetate to form Ag-NPs and the overall kinetics of the process. The kinetic analyses were carried out through systematic measurements of kinetic rate data and logically coordinated kinetic calculations involving kinetic deconvolution analysis.48 The results were interpreted with the aid of microscopic observations of morphological changes of the sample during the reaction. The investigation was further extended to evaluate the impact of atmospheric water vapor on the kinetics of the overall reaction and on each reaction step. Through these investigations, the rarely observed special features of the thermal decomposition were revealed, and the possible causes were examined to gain information essential for the sophisticated design of the controlled synthesis of Ag-NPs and structural materials comprised thereof via thermal decomposition of silver acetate.

2. EXPERIMENTAL SECTION 2.1. Sample and Characterization. Reagent grade silver acetate (>99.99%, Sigma-Aldrich, USA) was used as received. The sample was characterized by morphological observation using scanning electron microscopy (SEM), powder X-ray diffractometry (XRD), Fourier transform infrared (FT-IR) spectroscopy, and thermogravimetry−differential thermal analysis (TG−DTA) measurements. The morphologies of the sample particles were observed using SEM (JSM-6510, JEOL) after coating the sample particles with Pt via sputtering (JFC8842


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The Journal of Physical Chemistry C recorded using suspension-type TG (TGA-50, Shimadzu Co.) for samples (1.99 ± 0.03 mg) in a platinum pan (6 mm diameter and 2.5 mm height) in a N2 flow (80 cm3 min−1). Three different temperature program patterns were applied to the measurement of the kinetic rate data: isothermal, linear nonisothermal, and constant transformation rate thermal analysis (CRTA)50 modes. The sample mass m0 = 2.0 mg and N2 flow rate of 80 cm3 min−1 for the thermal analysis were determined through preliminary measurements at different m0, gas flow rates, and heating programs to be suitable experimental settings for recording the mass-change curves that were the subject of kinetic study.51 Under these conditions, the sample particles were dispersed over the bottom of the sample pan without forming any significant layer of sample particles. The flow rate of N2 was within a range that enabled the experimentally resolved mass-change curve not to be practically influenced by changing the flow rate. Therefore, the undesired influences of mass and heat-transfer phenomena on the experimentally resolved shapes of mass-change curves and apparent kinetic behavior, which may possibly be caused by the diffusion of product gases through the sample bed52−55 and the thermal effects of the reaction,51,56 respectively, were expected to be negligible. The isothermal mass-loss traces were recorded at various constant temperatures (438 K ≤ T ≤ 453 K) after the sample was heated at β = 10 K min−1 to the programmed constant temperature. Linear nonisothermal measurements were performed at different β (0.5 K min−1 ≤ β ≤ 5 K min−1). For the CRTA measurements, a self-constructed CRTA controller57−61 was attached to the TGA-50 instrument. The sample was heated at β = 2 K min−1, and during the massloss process, the mass-loss rate was regulated to different constant rates C (2.5 μg min−1 ≤ C ≤ 15.0 μg min−1). The kinetic rate data for the thermal decomposition in flowing N2−H2O (400 cm3 min−1) with controlled p(H2O) were also recorded at different β using the aforementioned humidity-controlled TG−DTA instrument at p(H2O) of 0.2, 4.0, and 16.3 kPa. After the measurement system was stabilized at 333 K and the selected p(H2O) for 30 min, the sample was heated at different β (0.5 K min−1 ≤ β ≤ 5 K min−1) as for the measurements of kinetic rate data in flowing dry N2. 2.4. Morphological Study of Reaction Process. Samples partially decomposed in flowing N2 or N2−H2O were subjected to SEM observations. The original samples (1.99 ± 0.04 mg) were heated at β = 5 K min−1 to different temperatures in the flow of N2 (80 cm3 min−1) using a DTG-50, to obtain the samples decomposed to different fractional reactions α (α = 0.05, 0.49, and 1.00). The α values were determined from the ratio of the mass-loss value with respect to the total mass-loss value of the overall thermal decomposition reaction. Similarly, partially decomposed samples with α = 0.3 were prepared by heating the original samples at different constant temperatures (T = 438, 453, and 468 K). Furthermore, partially decomposed samples with different α (0.10 ≤ α ≤ 1.00) were prepared under different controlled p(H2O) conditions (p(H2O) = 4.0 and 16.3 kPa, 400 cm3 min−1) by heating the samples at β = 5 K min−1 using the aforementioned humidity-controlled TG− DTA. Samples partially decomposed in the instruments were immediately cooled to room temperature and were observed using SEM after coating with thin Pt layers.

Figure 1. SEM images of the as-received silver acetate sample.

particle was a columnar crystal with a length approximately between 10 and 50 μm (Figure 1a). The particle surfaces appear smooth (Figure 1b). The XRD pattern and FT-IR spectrum of the sample are shown in Figure S1 in the Supporting Information. The XRD pattern (Figure S1a) corresponds well with the reported pattern for a triclinic crystal structure (S.G. = P1, a = 5.5810, b = 9.960, c = 21.587, α = 89.10, β = 97.40, γ = 97.26, JCPDS 14-0733).62 The FT-IR spectrum (Figure S1b) exhibits absorption peaks corresponding to the vibrational modes of the acetic ion, as has been observed for acetate salts of transition metals.63−66 The absorption peaks at 1409 and 1568 cm−1 are attributable to the symmetric and antisymmetric stretching of −CO2−. The absorptions because of the rocking and in-plane bending (or deformation) of −CH3 appear at 1018 and 1342 cm−1. As shown in Figure S2, smooth mass-loss curves were recorded upon heating the sample in an inert atmosphere. The total mass change was determined to be −35.68 ± 1.06%, which is in good agreement with the calculated value (−35.37%) assuming the formation of metallic Ag by thermal decomposition. 3.2. Thermal Decomposition Process. Figure S3 shows the changes of the XRD pattern of a sample recorded at different temperatures during stepwise isothermal heating in flowing N2. The XRD pattern attributed to silver acetate starts to attenuate at a temperature between 423 and 448 K (Figure S3a), while the diffraction peaks of the solid product start to appear at the same temperature. The XRD pattern changes to that of a single phase of metallic Ag with the cubic crystal structure (Figure S3b, S.G. = Fm3m, a = 4.0862, JCPDS 40783) at a temperature between 473 and 498 K. In the reaction temperature range, no crystalline intermediate phase can be identified, indicating the formation of metallic Ag directly by the thermal decomposition of silver acetate. The changes of the XRD pattern during isothermal heating at 443 K are shown in Figure 2. The time-dependent XRD pattern also indicates attenuations of the XRD peaks of silver acetate and gradual growth of those of metallic Ag (Figure 2a). The changes in the crystallite sizes of silver acetate and metallic Ag calculated using the Scherrer equation67 (Figure 2b) clearly indicate the formation of Ag-NPs, which gradually grow from 15 to 24 nm as the reaction advances. These crystallite sizes are in close agreement with those determined previously by transmission electron microscopy (TEM) observations of the Ag product by the thermal decomposition of silver acetate,46 which were approximately 20 and 35 nm for samples calcined for 1 h at 523 and 873 K, respectively. Figure 3 compares the TG−DTG−DTA curves recorded in flowing He and He−O2 (21% O2) at a rate of 200 cm3 min−1 and the mass spectra of the evolved gases at 546 K. The massloss values are comparable for the reactions in flowing He and

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. Figure 1 shows SEM images of particles of the as-received silver acetate sample. Each 8843


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reaction. The mass spectra of the evolved gases indicate largely different gas compositions in the main part of the reactions (Figure 3b). In the mass spectra of the gases evolved from the reaction in flowing He, the evolution of acetic acid and acetone is revealed from the parent ion (m/z = 60) and the fragment ions (m/z = 58, 43, 28, and 15),49,68 respectively, in addition to the evolution of CO2 (m/z = 44), CO (m/z = 28), and H2O (m/z = 18). All traces of the evolution of acetic acid and acetone disappear in the mass spectra of the evolved gases from the reaction in flowing He−O2, indicating the combustion of these organic gases after evolution. Therefore, the exothermic DTA peak observed at the end of the reaction in flowing He− O2 can be attributed to the combustion reactions, and the overall reaction in flowing He−O2 can be expressed by assuming the complete combustion of evolved gases as 4CH3COOAg(s) + 7O2 (g) → 4Ag(s) + 8CO2 (g) + 6H 2O(g)

(1)

Although the composition of gases evolved by the reaction in the atmosphere of an inert gas, N2 or He, could not be determined quantitatively in the present study, the following stoichiometric equation has been proposed for the reaction in flowing N2:20

Figure 2. Changes in XRD pattern during isothermal heating of silver acetate at 443 K in flowing N2 (100 cm3 min−1): (a) changes in XRD pattern with time and (b) changes in the crystallite sizes of silver acetate and metallic Ag as the reaction advances.

2CH3COOAg(s) → 2Ag(s) + CH3COOH(g) + CO2 (g) + H 2(g) + C(s)

(2)

However, eq 2 does not necessarily agree with the present observation of comparable mass-loss values during thermal decomposition in air and in inert gas, or with the formation of acetone (Figure 3). Figure S4 shows the TG−DTG−DTA curves for samples under different atmospheric conditions. Comparing the thermal behaviors of the samples under different atmospheres (Figure S4a), the thermal effects during the reactions are opposite for N2 and air, as was expected from Figure 3. With increasing air flow rate, the exothermic effect is diminished, and the reaction temperature region shifts to higher temperatures (Figure S4b). These behaviors can be explained by the influence of the combustion reaction on the experimentally resolved TG−DTG curves as exothermic and consequent self-heating effects. Actually, the consumption of O2 and evolution of CO2 during the reaction were monitored by the changes in the O2 and CO2 concentrations during the reaction (Figure S4c). To avoid any undesirable influence of the combustion reaction of gases evolved by the thermal decomposition on the experimentally resolved TG−DTG curves, the measurements used for kinetic analysis were performed in flowing N2. The influences of p(H2O) on the TG−DTG curves are summarized in Figure 4. The reaction temperature region is unexpectedly and systematically shifted to lower temperatures with increasing p(H2O) (Figure 4a). When comparing the changes in the extrapolated onset temperature Teo and peak top temperature Tp in the DTG curve, the shift of Tp to lower temperatures is more significant than that of Teo, indicating that the main part of the reaction is more significantly affected by the atmospheric p(H2O) (Figure 4b). Several examples of an increase in the overall reaction rate of thermal decomposition by the effect of atmospheric p(H2O) have been reported, such as the thermal decomposition of synthetic malachite,69 sodium hydrogen carbonate,70 sodium percarbonate,9,10 synthetic hydrozincite,71 and silver carbonate.4 In these examples,

Figure 3. Results of TG−DTG−DTA−MS measurements for thermal decomposition of silver acetate in flowing He and He−O2 (200 cm3 min−1) at β = 5 K min−1 (m0 = 4.97 mg and 5.04 mg): (a) comparison of TG−DTG−DTA curves and (b) comparison of mass spectra of evolved gas at 546 K.

He−O2, but the shapes of the TG and DTG curves under the different atmospheric conditions are different (Figure 3a). In flowing He, the DTA curve during the mass-loss process exhibits endothermic behavior, whereas in flowing He−O2 a significant exothermic effect is observed at the end of the 8844


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Figure 4. Influence of p(H2O) on TG−DTG curves for thermal decomposition of silver acetate (m0 = 1.99 ± 0.03 mg) at β = 5 K min−1: (a) comparison of TG−DTG curves and (b) change in Teo and Tp in the DTG curve.

atmospheric p(H2O) was found to affect the kinetics of the surface reaction during thermal decomposition as well as the morphology of the resulting surface product layers. 3.3. Formal Kinetic Analysis. Figure 5 summarizes the mass-change traces for the thermal decomposition recorded using the suspension-type TG in isothermal (Figure 5a), linear nonisothermal (Figure 5b), and CRTA (Figures 5c and d) modes. Superficially, the mass-loss traces under isothermal and linear nonisothermal conditions indicate single-step mass-loss behavior with well-formed single DTG peak. The mass-loss rate is satisfactorily controlled at the programmed constant rates in the measurements of CRTA. Irrespective of the applied temperature control mode, the mass-loss traces shift systematically with the respective measurement parameters T, β, and C. Because of the smooth single-step mass-loss behavior observed irrespective of the temperature control mode, the following simple kinetic equation for the single-step reaction was applied for the kinetic analysis:72,73 ⎛ E ⎞ dα = A exp⎜ − a ⎟f (α) ⎝ RT ⎠ dt

Figure 5. Mass-change traces for the thermal decomposition of silver acetate (m0 = 1.99 ± 0.03 mg) in flowing N2 (80 cm3 min−1) under (a) isothermal, (b) linear nonisothermal, and (c) CRTA conditions (C = 5.0 μg min−1) and (d) change in temperature profile with C in the CRTA measurements.

E ⎛ dα ⎞ ln⎜ ⎟ = ln[Af (α)] − a ⎝ dt ⎠ α RTα

(3)

where α, A, Ea, and R are the fractional reaction, Arrhenius preexponential factor, apparent activation energy, and gas constant, respectively. The kinetic model function f(α) accommodates the change in the reaction rate behavior as the reaction advances, and many different functions have been derived by considering the physico-geometrical features of the solid-state reaction. Based on eq 3, the differential isoconversional method, known as the Friedman method,74 was applied universally to all of the kinetic rate data obtained from the mass-loss traces under different temperature program modes:75−78

(4)

where (dα/dt)α and Tα are the rate of conversion and T at a fixed α, respectively. Figure 6 shows the results of kinetic analysis based on eq 3. The Friedman plots, ln(dα/dt)α versus Tα−1, are acceptably linear for each fixed α extracted from the kinetic rate data under different temperature program modes (Figure 6a). However, the slopes of the plots, which are related to Ea according to eq 4, are not necessarily constant during the course of the reaction. The trend of the variation in the slope of the Friedman plot is clearly observable from the change in Ea as the reaction advances (Figure 6b). Ea initially increases from 8845


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The Journal of Physical Chemistry C ⎛E ⎞ dα dα exp⎜ a ⎟ = Af (α), θ = = ⎝ RT ⎠ dθ dt

∫0

t

⎛ E ⎞ exp⎜ − a ⎟ dt ⎝ RT ⎠ (5)

where θ is Ozawa’s generalized time,79,80 which denotes the hypothetical reaction time at infinite temperature. Adopting this kinetic consideration, an experimental master plot of (dα/ dθ) versus α based on eq 5 was drawn using the average Ea of 77.9 ± 1.8 kJ mol−1 (0.05 ≤ α ≤ 0.90) and is shown in Figure 6c. The experimental master plot exhibits a convex shape; however, a nearly constant reaction rate is observed in the range 0.2 ≤ α ≤ 0.6. This behavior cannot be predicted by the physico-geometrical models for single-step solid-state reactions. 3.4. Reaction Morphology. Figure 7 shows the changes in the SEM images of the surface texture of the sample as the

Figure 6. Results of formal kinetic analyses for overall thermal decomposition reaction of silver acetate assuming single-step reaction: (a) Friedman plots at different α from 0.1 to 0.9 in steps of 0.1, (b) variation of Ea as reaction advances, and (c) experimental master plot of (dα/dθ) versus α drawn using average Ea (0.05 ≤ α ≤ 0.90).

approximately 70 to 80 kJ mol−1 for α < 0.25 and attains a nearly constant value, with an average of 79.0 ± 0.5 kJ mol−1 (0.3 ≤ α ≤ 0.8). Ea decreases gradually to approximately 60 kJ mol−1 in the final part of the reaction (α ≥ 0.8). Practically the same trend of the Ea variation was confirmed when the kinetic rate data recorded under different temperature control conditions were separately analyzed using the isoconversional method, indicating that the kinetic rate data recorded in this study under the different conditions are equally useful for obtaining the reliable kinetic results. In a strict sense, a constant Ea is a prerequisite for the kinetics of single step reaction, as expressed by eq 3. Two interpretations of the present results of the isoconversional kinetic analysis (Figure 6a and b) are possible, which can be used to design further kinetic analysis. One is a simplified interpretation that the major part of the reaction is characterized by a constant Ea; therefore, the kinetics are approximately described using eq 3. The other is that each part of the reaction with a different Ea, or its variation trend, can be attributed to different kinetic processes and mechanisms, which leads to the approach of a partially overlapping multistep reaction. When the former interpretation is assumed, the following kinetic equation is derived by extrapolating the kinetic rate behavior to infinite temperature:75−78

Figure 7. Typical SEM images of partially decomposed silver acetate obtained by heating the sample to different temperatures at β = 5 K min−1 in flowing N2 (80 cm3 min−1): (a, b) α = 0.05 (453 K), (c, d) α = 0.49 (468 K), and (e, f) α = 1.00 (573 K).

reaction advances. At a very early stage of the reaction (α = 0.05), nanosized spherical particles of the solid product are observed on the end surfaces of the columnar crystals (Figure 7a and b). It can be concluded from the crystallite sizes calculated from the XRD peaks and the TEM images reported previously30,46 that the spherical particles are aggregates of AgNPs. As the overall reaction advances, the Ag-NP aggregates start to appear on the side surfaces of the reactant crystals (Figure 7c, α = 0.49). The time-lag of the reaction initiations in the end and side surfaces is interpreted by the different reactivities characterized by the different nucleation rates in each surface, as have been reported for some thermal decomposition reactions.81 The initially formed spherical AgNP aggregates grow gradually to submicron-sized spherical 8846


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was selected as the probable route of the present kinetic analysis. The possible contribution of the third reaction step in the final part of the reaction can subsequently be investigated during the detailed kinetic analysis of the separated second reaction step. For such a two-step reaction that results from the different reaction sites and geometries, the overall reaction rate can be expressed by a cumulative kinetic equation with an assumption of negligible interdependence between the component reaction steps:48,82

particles, and these are positioned on certain parts of the reactant surface (Figure 7d). It is noted that a large area of the reactant surface is not covered with the solid product layers. Therefore, the thermal decomposition proceeds on the newly developed surfaces of the reactant crystals that are exposed to the reaction atmosphere, even in the established reaction stage, resulting in slimming of the columnar reactant crystals. The final product forms a chain of Ag-NP aggregates along with the original columnar crystals of the reactant (Figure 7e). The chain is formed by the sintering of aggregates of submicronsized spherical particles (Figure 7f). A significant effect of the temperature program mode and control parameters on morphological changes during thermal decomposition cannot be observed in the SEM images of the samples partially decomposed (α = 0.3) under isothermal conditions at different temperatures (Figure S5). It is also worth noting that a practically comparable sequence of the morphological changes was observed for the thermal decomposition in flowing air. In many thermal decomposition reactions of solids, the surfaces of the reactant crystals are readily covered with the surface product layer.1−3 Therefore, the subsequent reaction is influenced more or less by the diffusional removal of the gaseous product through the surface product layer, and the reaction advances under the self-generated atmospheric conditions at the internal reaction interface. In contrast, a characteristic of the present reaction is that the surfaces of the reactant solid were exposed to the reaction atmosphere throughout the majority of the reaction, rendering the reaction free from the diffusion of gaseous products. Conversely, the reaction might be directly influenced by the applied reaction atmosphere conditions. The significant shift of the reaction temperature region to lower temperatures with increasing p(H2O) (Figure 4) can be explained by this mechanistic feature of the reaction. 3.5. Kinetic Deconvolution Analysis. From the morphological changes of the sample as the reaction advances, it is apparent that the initial and subsequent reactions occurred on different surfaces, namely the end surfaces and side surfaces of the columnar crystals, respectively. The slight but detectable variation in Ea observed in the initial stages of the reaction (Figure 6b) can be explained by considering the contribution of the reaction on the end surfaces. Therefore, the overall reaction can be understood as two partially overlapping reaction steps comprising reactions on the end and side surfaces of the columnar crystals. The unexpectedly depressed shape of the experimental master plot (Figure 6c) is also interpretable as resulting from the multistep reaction caused by the different reaction sites and geometries. From varying Ea observed as the reaction advances (Figure 6b), the third reaction step may be considered for the final reaction part characterized by the distinguishable decrease in the Ea value. However, the difference in the reaction sites and geometrical mechanism of the final reaction part from those of the established reaction part was not clearly distinguished from the morphological study, although the reactant crystals were covered by Ag-NP aggregates in the final reaction part. In addition, it was concerned over the possibility that separation of the final reaction part from the established reaction part results in the superficial fitting through the kinetic deconvolution analysis because of the rapidly decelerating rate behavior in the final reaction part. In this situation, a kinetic deconvolution of the overall process into the initial and the latter parts as the reactions in the end and side surfaces of the columnar crystals

dα = dt

⎛ E ⎞ ∑ ciAi exp⎜− a ,i ⎟fi (αi) with ⎝ RT ⎠ i=1 2

2

∑ ci = 1 and i=1

2

∑ ciαi = α i=1

(6)

where c is the contribution of each reaction step to the overall reaction. The subscript i denotes the component reaction step, where the first reaction step on the end surfaces is i = 1 and the established reaction step on the side surfaces is i = 2. An empirical kinetic model, known as the Šesták−Berggren model with three kinetic exponents (SB(m, n, p); f(α) = αm(1 − α)n[−ln(1 − α)]p),83−85 was employed for f(α) for each reaction step because it affords great flexibility in accommodating any possible physico-geometrical kinetic behavior into the kinetic equation. Therefore, a total of 12 kinetic parameters in eq 6 should be determined simultaneously for the kinetic characterization of the multistep reaction. To optimize the kinetic parameters for the reaction under linear nonisothermal conditions, the initial kinetic parameters in eq 6 were determined with reference to the results of former kinetic analysis (Figure 6). The initial ci values were determined by considering the α range of initial variation of Ea to be (c1, c2) = (0.20, 0.80). The average Ea values in the α ranges of 0.05 ≤ α ≤ 0.10 and 0.30 ≤ α ≤ 0.80 were tentatively used as the initial Ea,i values, that is, (Ea,1, Ea,2) = (73.4 kJ mol−1, 79.0 kJ mol−1). Furthermore, SB(0, 1, 0) was applied as the initial f(α) for each reaction step by assuming first-order kinetic behavior. The order of the initial Ai values was then determined by graphically comparing the experimental kinetic rate data with that calculated from eq 6. After all of the initial kinetic parameters were set in eq 6, nonlinear least-squares analysis was performed to minimize the sum of squares of the differences between the overall experimental reaction rate, (dα/dt)exp, and that calculated from eq 6, (dα/dt)cal, at different t during the overall reaction, by using 2 ⎡⎛ ⎞ ⎛ dα ⎞ ⎤ α d −⎜ ⎟ ⎥ F = ∑ ⎢⎜ ⎟ ⎢⎝ dt ⎠exp, j ⎝ dt ⎠cal, j ⎥⎦ j=1 ⎣ M

(7)

where M is the total number of data points. Figure 8 shows a typical result of the kinetic deconvolution analysis for the reaction at β = 5 K min−1. The overall rate behavior is satisfactorily described by the sum of those two reaction steps, irrespective of β. The optimized kinetic parameters for each reaction step are acceptably independent of β. Table 1 lists the values averaged over various β. The kinetic results indicate that the contribution of the first reaction step, attributed to the reaction on the end surface, is 15% of the overall reaction. Ea value for the first reaction step is slightly smaller than it is for the second reaction step, but the difference is practically negligible. This observation is understandable 8847


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Figure 8. Typical result of kinetic deconvolution for thermal decomposition of silver acetate under linear nonisothermal conditions at β = 5 K min−1.

because the chemical reactions that occur in the first and second steps are identical. On the other hand, the larger A for the first reaction step compared with the second reaction step characterizes well the difference in reactivity between the reaction sites on the end and side surfaces. Because of the different geometrical features of the first and second reaction steps, the kinetic exponents in SB(m, n, p) determined for the respective reaction steps are different, as is clearly observable in the f i(αi) versus αi plots shown in Figure 9. The experimental master plot of f i(αi) versus αi for the first reaction step indicates an initial acceleration and a subsequent linear deceleration (Figure 9a). The linear deceleration region is satisfactorily fitted using the JMA(m) function (f(α) = m(1 − α)[−ln(1 − α)]1−1/m)86−89 with m = 0.94, which is very close to the first-order kinetic model. This rate behavior is in accordance with the characteristic of the first reaction step revealed from morphological observations that the reaction occurs at specific reactive sites positioned on the end surfaces of the columnar reactant crystals. Therefore, the first-order rate behavior of the established part of the first reaction step is understood to involve the consumption of the specific reactive sites. The experimental master plot for the second reaction step (Figure 9b) shows an unexpected shape with a maximum midway through the reaction, which does not correspond to the two-dimensional shrinkage of the reactant crystals as the reaction advances. By assuming a possible autocatalytic rate behavior, attempts were made to fit the experimental master plots using SB(m, n, 0), which is understood to be the empirical expression of the autocatalytic rate behavior known as the Prout−Tompkins model, expressed by SB(1, 1, 0),90 resulting in a good fitting using SB(0.44, 0.51, 0) (Figure 9b). As a possible kinetic model that describes the autocatalytic reaction in the scheme of the contracting reaction area, the following

Figure 9. Experimental master plots drawn using optimized kinetic exponents in SB(m, n, p) and fitting using different physicogeometrical kinetic models for (a) first and (b) second reaction steps.

kinetic model function in integral form has been proposed by Galwey and Hood:91 g (α ) =

∫0

α

dα = [1 − (1 − α)1/2 ]1/2 f (α )

(8)

The differential form of this kinetic model function with different dimensions of interface shrinkage can be expressed by10 f (α) = 2n(1 − α)1 − 1/ n [1 − (1 − α)1/ n ]1/2

(9)

where n is the interface shrinkage dimension. The fit of the experimental master plot for the second reaction step is more appropriate when using the model with n = 3 (Galwey− Hood(3); Figure 9b), rather than the reaction geometry observed microscopically, n = 2 (Galwey−Hood(2)). The disagreement of the experimental curve fitted using Galwey− Hood(2) is significant in the latter half of the second reaction step. In the results of the former kinetic analysis for the overall reaction using the Friedman plot (Figure 6), a distinguishable decrease in the apparent value of Ea is observable in the final part of the overall reaction, α > 0.8. It is also evident from the microscopic observation of the reacting particles (Figure 7) that the reactant particles are covered by significantly sintered Ag in the final stages of the reaction. Because the product layer constitutes the limiting factor for the diffusional removal of the

Table 1. Average Optimized Kinetic Parameters for Thermal Decomposition of Silver Acetate under Linear Nonisothermal, Isothermal, and CRTA Conditions SB(m, n, p) i

condition nonisothermal isothermalb CRTAc

a

a

1 2 1 2 1 2

−1

ci 0.15 0.85 0.16 0.84 0.15 0.85

± ± ± ± ± ±

Ea,i/kJ mol 0.01 0.01 0.01 0.01 0.03 0.03

74.7 76.1 76.6 76.6 78.6 76.9

± ± ± ± ± ±

0.5 0.4 0.9 0.8 1.2 0.6

5

−1

Ai/10 s 3.00 1.38 2.99 1.27 2.96 1.23

± ± ± ± ± ±

0.01 0.02 0.01 0.07 0.09 0.15

mi 0.16 0.09 0.18 0.09 0.11 0.09

± ± ± ± ± ±

ni 0.01 0.01 0.02 0.01 0.15 0.01

1.21 0.65 0.90 0.55 0.83 0.49

± ± ± ± ± ±

R2

pi 0.06 0.09 0.10 0.07 0.26 0.04

0.08 0.35 0.09 0.30 0.07 0.29

± ± ± ± ± ±

0.01 0.03 0.01 0.04 0.04 0.03

0.998 ± 0.001 0.998 ± 0.002 0.950 ± 0.038

Average values for the reactions at different β. bAverage values for the reactions at different T. cAverage values for the reactions at different C. 8848


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Figure 11 shows typical SEM images of the sample decomposed to different α, which were obtained by heating

gaseous product, as in the thermal decomposition of many other solids, the kinetic rate behavior during the second half of the second reaction step is possibly influenced by the geometrical construction of the residual reactant particle covered by the sintered Ag. The acceleration of the linear advancement rate of the two-dimensional shrinkage of the reactant particle, and the subsequent deceleration of the linear rate by the effect of the surface Ag layer, could explain the overall reaction behavior of the second reaction step. Deconvolution of the overall rate behavior under isothermal and CRTA conditions using the same procedure of kinetic deconvolution based on eq 6 was also attempted. As the initial kinetic parameters in eq 6, the average kinetic parameters optimized for the reactions under linear nonisothermal conditions listed in Table 1 were used for the subsequent optimization. Figure 10 shows graphically the typical results of

Figure 11. SEM images of partially decomposed silver acetate after heating in flowing N2−H2O (p(H2O) = 4.0 kPa): (a) end surface at α = 0.11 and (b) side surface at α = 0.46.

under a controlled water vapor pressure (p(H2O) = 4.0 kPa). In the initial stages of the reaction, the solid product is selectively produced on the end surfaces of the columnar crystals (Figure 11a). In the subsequent reaction step, the reaction occurs on the side surfaces of the columnar crystals, forming aggregates of Ag-NPs (Figure 11b), and a large area of the reactant surface remains exposed to the reaction atmosphere. The reaction scheme is practically identical to those observed for the reaction under flowing dry N2. However, in both the initial and subsequent reaction steps, the spheres of the Ag-NP aggregates on the surfaces of the reactant crystals are apparently larger than those observed for the reaction under flowing dry N2 at the same α (Figures 7b and 7d). Therefore, it is expected that atmospheric water vapor promotes the migration and aggregation of Ag-NPs that were originally produced at each reaction site on the reactant surfaces, resulting in a larger area of the reactant surface being exposed to the reaction atmosphere and a larger diffusion path for the product gases through the aggregated product when the reactant surfaces are covered with Ag-NP aggregates. Figure 12 shows the TG−DTG curves for the thermal decomposition of silver acetate at different β under three selected p(H2O) (0.2, 4.0, and 16.3 kPa). The trend in the reaction temperature shift caused by p(H2O) that was observed in Figure 4 remains unchanged even at different β. The Ea values at different α determined by the Friedman plots for each reaction under different p(H2O)s are shown in Figure 13. Overall, the Ea values increase with increasing p(H2O). Irrespective of applied p(H2O), in the initial stages of the reaction (approximately α ≤ 0.2), Ea decreases to the nearly constant value observed during the major reaction stage. In contrast, Ea slightly increases from the constant value in the final stage of the reaction (approximately α ≥ 0.8). These behaviors are opposite to those observed for the reaction under flowing dry N2 (Figure 6b). Because of the different reaction sites in the initial and established reactions and the change in Ea in the initial reaction stage, a kinetic deconvolution analysis assuming the two overlapping reaction steps, as applied to the reaction under flowing dry N2, was performed. The initial values of the kinetic parameters were determined as described above for the reaction under flowing dry N2. The initial values of the kinetic parameters used for the kinetic deconvolution analysis for each reaction under different p(H2O) are listed in the Supporting Information (Table S1). Figure 14 shows the fit to the experimental kinetic data assuming two overlapping

Figure 10. Typical results of kinetic deconvolution analysis for thermal decomposition of silver acetate under isothermal and CRTA conditions: (a) isothermal condition at T = 450 K and (b) CRTA condition at C = 5.0 μg min−1.

the kinetic deconvolution for the reaction under isothermal (Figure 10a) and CRTA (Figure 10b) conditions. The optimized kinetic parameters averaged over different T and different C for the reactions under isothermal and CRTA conditions, respectively, are also listed in Table 1. The optimized kinetic parameters for the reactions under isothermal and CRTA conditions do not deviate so significantly from those under linear nonisothermal conditions, providing acceptable fits to the experimental rate data (Figure 10). The findings indicate that the contributions of the reactions on the end and side surfaces to the overall reaction, and their reactivities, remain unchanged over the temperature conditions covered by the present thermoanalytical measurements under linear nonisothermal, isothermal, and CRTA conditions. 3.6. Impact of Atmospheric Water Vapor on the Kinetics. The overlapping two-step reaction behavior and the reaction geometry characterized by the reaction sites exposed to the reaction atmosphere during the course of the overall reaction stimulated our interest in the effects of atmospheric water vapor on the kinetics of the respective reaction steps. 8849


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Figure 14. Comparison of results of kinetic deconvolution analysis for thermal decomposition of silver acetate under different p(H2O) (β = 5 K min−1): (a) flowing dry N2 (80 cm3 min−1; Figure 8), (b) flowing N2−H2O (400 cm3 min−1, p(H2O) = 0.2 kPa), (c) flowing N2−H2O (400 cm3 min−1, p(H2O) = 4.0 kPa), and (d) flowing N2−H2O (400 cm3 min−1, p(H2O) = 16.3 kPa).

by parallel changes in A, in which a linear correlation between Ea and ln A, known as the apparent kinetic compensation effect,92−97 is observed (Figure S6). Figure 15 graphically compares the kinetic model functions optimized for the reaction under different p(H2O). In the first reaction step, enhancement of the initial acceleration part with higher p(H2O) is expected based on the shapes of f(α)−α curves, whereas in the second reaction step the shapes of f(α)−α curves are practically identical among the reactions under different p(H2O). The change in the rate of each reaction step with p(H2O) can be compared by constructing Arrhenius plots using the Arrhenius parameters of each reaction step determined at different p(H2O) (Figure S7). In the practical reaction temperature region, the increase in the rate constant at a constant temperature with p(H2O) is clearly represented by the Arrhenius plots in both the first and second reaction steps. The major difference in the dependence of the Arrhenius plots on p(H2O) between the first and second reaction steps is in their slopes. For the first reaction step, the slope of the Arrhenius plot increases gradually with increasing p(H2O), as expected from the change in Ea value (Table 2). In contrast, the Arrhenius plots exhibit a parallel shift with nearly constant slope for the second reaction step. Therefore, the decrease of the reaction temperature in the first reaction step with increasing p(H2O) is characterized by the mutually correlated increase in Ea and A, whereas that in the second reaction step is mainly explained by the increase in A. From the morphological and kinetic studies of the reactions under different p(H2O), it is concluded that the overall promotion of the thermal decomposition by p(H2O) results from different degrees of impact of p(H2O) on the first and second reaction steps. In the first reaction step, which occurs on the end surfaces, the reaction sites are activated by the effect of atmospheric water vapor, as expected from the detectable increase in the contribution c1 with p(H2O), but the increase in the number of reaction sites that contribute to the first reaction step is limited. Similarly, water vapor activates the reaction sites

Figure 12. Mass-change traces for thermal decomposition of silver acetate (m0 = 2.01 ± 0.03 mg) at different β in flowing N2−H2O (400 cm3 min−1) with different p(H2O): (a) 0.2 kPa, (b) 4.0 kPa, and (c) 16.3 kPa.

Figure 13. Ea at different α for thermal decomposition of silver acetate in flowing N2−H2O (400 cm3 min−1) with different p(H2O).

reaction steps (eq 6) with the optimized kinetic parameters listed in Table 2. Although a shift of the reaction temperature to lower temperatures with increasing p(H2O) is observable for both the deconvoluted first and second reaction steps, the shift of the second reaction step is more significant. Therefore, the overlapping area of the derivative kinetic data increases with increasing p(H2O). This increase is accompanied by the slight but detectable increase in the contribution of the first reaction step c1. Ea also increases with p(H2O) in both the first and second reaction steps, but the variation is more significant for the first reaction step. These variations in Ea are accompanied 8850


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Table 2. Average Optimized Kinetic Parameters for Thermal Decomposition of Silver Acetate under Flowing N2−H2O Gas SB(m, n, p) p(H2O)/kPa

i

0.2

1 2 1 2 1 2

4.0 16.3

Ea,i/kJ mol−1

ci 0.15 0.85 0.21 0.79 0.25 0.75

± ± ± ± ± ±

0.02 0.02 0.01 0.01 0.04 0.04

86.7 ± 1.2 76.1 ± 0.4 93.6 ± 0.5 81.3 ± 0.4 108.4 ± 1.2 84.8 ± 0.8

Ai/s−1 (1.98 (4.56 (2.01 (4.80 (1.98 (1.73

± ± ± ± ± ±

0.11) 0.31) 0.01) 0.25) 0.06) 0.26)

mi × × × × × ×

107 105 108 106 1010 107

0.12 ± 0.07 0.16 ± 0.05 0.02 ± 0.04 0.04 ± 0.18 0.20 ± 0.11 −0.25 ± 0.29

ni 1.10 0.77 0.80 0.87 1.37 0.94

± ± ± ± ± ±

R2

pi 0.27 0.07 0.15 0.05 0.44 0.09

0.13 0.29 0.09 0.50 0.05 0.78

± ± ± ± ± ±

0.08 0.06 0.01 0.17 0.09 0.27

0.999 ± 0.001 0.999 ± 0.001 0.999 ± 0.001

original reaction site to the surfaces of the Ag-NP aggregates is also possible via the evaporation−deposition process. In this scenario, by enhancing the reactivity at the reactant surfaces and promoting the growth of the aggregates of Ag-NPs, atmospheric water vapor can affect the dissociative evaporation of silver acetate from the surface of the reactant crystals, the condensation of the Ag product on the surfaces of the reactant crystals during the thermal decomposition process, and the evaporation of previously deposited Ag and its subsequent deposition on the surfaces of growing Ag-NP aggregates.

4. CONCLUSIONS The thermal decomposition of silver acetate (columnar crystals) to form Ag-NPs occurs with smooth mass-loss behavior under isothermal, linear nonisothermal, and CRTA conditions. However, the overall reaction is kinetically characterized by two partially overlapping reaction steps because of the difference in reactivity between the end and side surfaces of the reactant crystals. The reaction is initiated on the end surfaces with an apparent Ea of approximately 75 kJ mol−1 in flowing N2, and is regulated by a first-order rate law due to the consumption of reactive sites. The subsequent reaction occurs on the side surfaces with Ea comparable to that in the initial reaction step and contributes as the established reaction. The product Ag migrates on the reactant surfaces from the original reaction sites to the certain regions to form aggregates of Ag-NPs. This phenomenon results in the continuous development of new reactant surfaces and in the slimming of the columnar reactant crystals as the reaction advances. The established reaction is initially accelerated, despite the shrinkage of the reactant surface area. Therefore, the overall rate behavior of the established reaction is interpreted as autocatalytic behavior for the linear advancement rate of reactant crystal shrinkage of the reactant crystals and deceleration due to the shrinkage of the reactant surface area. The aggregates of Ag-NPs deposited on the reactant surfaces also contribute to the deceleration of the overall reaction by covering the reaction sites. As a result, chains of aggregated AgNPs develop in the final product along the original columnar reactant crystals by sintering. In addition, the overall reaction is significantly accelerated by the effect of water vapor, resulting in a systematic shift of the reaction temperature to lower temperatures with increasing p(H2O). The shift of the reaction temperature is described by the compensative increases in Ea and A values for the first reaction step, and mainly in A for the second reaction step. The water vapor activates both reactions on the end and side surfaces, but the influence of the second reaction step on the side surfaces more significantly contributes to the phenomenon, because the reaction sites are always exposed to the reaction atmosphere. At the same time, the growth of the Ag-NP aggregates is also enhanced by water vapor in the reaction atmosphere. A physical mechanism of

Figure 15. Experimental master plots under linear nonisothermal conditions in flowing N2−H2O (400 cm3 min−1) for (a) first and (b) second reaction steps.

on the side surfaces of the columnar crystals, but the effect continues to the end of the reaction because the large area of the reactant surface newly developed by the advancement of the reaction is exposed to the atmospheric water vapor throughout the second reaction step. At the same time, the migration of Ag-NPs from their original reaction site and their subsequent aggregation at certain regions on the reactant surface are also promoted by the effect of water vapor. Although the actual physical and chemical mechanisms of the activation of reaction sites by the water vapor and migration− aggregation of Ag-NPs cannot be revealed from the experimental evidence in the present study, a physical mechanism involving dissociative evaporation of the reactant with simultaneous condensation of Ag, as has been proposed by L’vov for the thermal decomposition of Ag2O, Ag2CO3, and other inorganic compounds,98−100 explains well the phenomenology of the kinetic behavior and the morphological changes observed for the thermal decomposition of silver acetate. The dissociative evaporation of the reactant from the surface and the condensation of Ag explain the special characteristics of the reaction with the newly developed reactant surface during the course of the reaction. It was also reported by Lu et al. in their study of laser-induced thermal decomposition of silver acetate23 that the Ag already deposited on the reactant surface can also once again evaporate and be deposited elsewhere. Because the substrate of the deposited Ag is the reactant crystal surface, the Ag deposited on the reactant surface can evaporate together with the silver acetate. Thus, the migration of Ag from the 8851


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dissociative evaporation of the reactant and simultaneous condensation of the product is tentatively proposed to explain the physico-geometrical characteristics of the reaction and the variation in kinetic behavior with p(H2O). The growth of AgNP aggregates is also explained by the vaporization of the Ag product in the vicinity of the original reaction site on the reactant surfaces, followed by subsequent deposition on the surface of growing Ag-NP aggregates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02377. Sample characterization (powder XRD pattern, FT-IR spectrum, and TG−DTG curve); thermal decomposition process (TG−DTG−DTA curves under different atmospheric conditions and SEM images of the partially decomposed sample); impact of atmospheric water vapor (initial kinetic parameters for the kinetic deconvolution analysis, mutual dependence of the Arrhenius parameters, and simulated Arrhenius plots) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-82-424-7092. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The present work was supported by JSPS KAKENHI Grant Nos. 25242015, 25350202, and 25350203. REFERENCES

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