Phenomenological Interpretation of the Multistep Thermal

Article pubs.acs.org/JPCC

Phenomenological Interpretation of the Multistep Thermal Decomposition of Silver Carbonate To Form Silver Metal Masahiro Yoshikawa,† Shuto Yamada,‡ and Nobuyoshi Koga*,† †

Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan ‡ Department of Applied Science, National Defense Academy of Japan, 1-10-20 Hashirimizu, Yokosuka 239-8686, Japan S Supporting Information *

ABSTRACT: Thermal decomposition of Ag2CO3 to form Ag occurs via a multistep reaction, and the reaction pathway varies drastically with the sample and reaction conditions. Understanding this complex reaction behavior has significance today with respect to the formation of Ag nanoparticles via the thermal decomposition of Ag compounds. In this study, the thermal decomposition of three different Ag2CO3 samples that exhibited different reactivities and reaction pathways was investigated via thermal analyses under linearly increasing temperatures and by studying the morphology of the reacting particles. The thermal decomposition was kinetically deconvoluted into four or five partially overlapping reaction steps, in which the contributions of each reaction step to the overall thermal decomposition of Ag2CO3 to Ag varied as a function of the properties of the sample particles and the heating rate. This complex reaction behavior resulted from the competitive interaction of two physicochemical processes, i.e., sintering of the product particles in the surface product layer and the diffusional removal of the generated gases, during the thermal decomposition of Ag2CO3 and the intermediate compound Ag2O in the geometrical reaction scheme for a contracting volume induced by surface reactions. The high sintering ability of the Ag2O and Ag formed in the surface product layer causes the complex reaction behaviors during the thermal decomposition and disturbs the formation of Ag nanoparticles.

1. INTRODUCTION Throughout recorded history, the thermal decomposition of Ag compounds has been an important process for producing Ag as a noble metal for use in jewelry goods and currencies. Many useful physical and electrical properties of Ag support the electronic society we know today. Since the discovery of the special physical and chemical properties of metal nanoparticles, gaining an understanding of the thermal decomposition of Ag compounds has grown in importance. Among the different synthetic methods of Ag nanoparticles, spray pyrolysis and ultrasonic spray pyrolysis techniques are relevant to the thermal decomposition of Ag compounds.1−3 The thermal decompositions of organometallic Ag compounds and Ag compounds in polymer matrices are also known to directly provide Ag nanoparticles or composite materials.4−17 Even the very simple Ag compound silver acetate has been extensively studied as a possible precursor for Ag nanoparticle production via thermal decomposition.18−22 For these compounds, the appropriate reaction temperatures and reaction pathways for the formation of Ag nanoparticles have been experimentally clarified, and the changes in size and shape of the product nanoparticles as a function of the reaction conditions have been discussed.4−22 Some studies have been focused on the determination of the kinetic parameters and possible physicochemical or physicogeometrical reaction models through formal kinetic analyses of thermal decomposition processes.3,5,19,21 The kinetics of the © 2014 American Chemical Society

thermal decomposition of solids are controlled by interactions between consecutive and concurrent processes associated with different physicochemical events, including surface nucleation, destruction of reactant crystals, crystal growth of the product solid, and diffusional removal of gaseous product.23,24 It must be noted that these physicochemical events take place with time and are strongly regulated by the geometrical factors originating from the heterogeneity of the reaction. Such features have been clearly described for the thermal decomposition of silver malonate by Galwey and Mohamed.5 Accordingly, knowledge of the physicochemical and physicogeometrical events taking place during the thermal decomposition of Ag compounds is critical for understanding the generation of Ag nanoparticles. A good example is the rapid and violent fragmentation of the reactant particles observed in the thermal decomposition of gold acetate during the generation of gold nanoparticles, as reported by Bakrania et al.25 This study focuses on the physicochemical and physicogeometrical events occurring during the thermal decomposition of an Ag compound to form Ag and the effects of those events on the overall kinetics in order to collect essential information on the formation process for Ag via thermal decomposition. The Received: February 9, 2014 Revised: March 22, 2014 Published: March 27, 2014 8059

dx.doi.org/10.1021/jp501407p | J. Phys. Chem. C 2014, 118, 8059−8070

The Journal of Physical Chemistry C

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Table 1. Information on the Ag2CO3 Samples Used in This Study

a

type

manufacturer

code

lot no.

purity description

SBETa/m2 g−1

avg particle sizeb/μm

I II III

Sigma-Aldrich, U.S. Chempure, Germany STREM CHEM, U.S.

178647 006972 93-4706

86096DJ Ch.290905 A8265048

99% 99.7% 99+%-Ag

0.37 ± 0.05 0.49 ± 0.07 0.42 ± 0.02

4.26 ± 0.27 3.48 ± 0.26 3.99 ± 0.39

Specific surface area determined by the single-point BET method. bDetermined by laser diffraction method.

Ag 2CO3 → Ag 2O + CO2

(1)

Ag 2O → 2Ag + (1/2)O2

(2)

and the overall reaction under linearly increasing temperatures. The thermal decomposition of three Ag2CO3 samples that exhibited different reactivity and reaction pathways under linearly increasing temperatures in a flow of an inert gas was studied by thermal analyses and morphological observations. The physicochemical and physicogeometrical events that occurred at different reaction stages and the changes in the reacting particles during the multistep reaction were investigated. In addition, through the kinetic deconvolution of the overall reaction into different physicogeometrical reaction steps, the kinetic characteristics of the respective reaction steps, and the changes due to different heating conditions were evaluated. Consequently, a phenomenological reaction model was proposed to interpret the overall kinetic behavior. The fundamental information about the process of Ag formation via the classical thermal decomposition of an inorganic Ag compound revealed in this study may be useful for interpreting the physicogeometrical kinetics of the formation of Ag nanoparticles via the thermal decomposition of organometallic compounds.

However, both the reactivity and reaction pathway drastically vary depending on the sample preparation method34,35 and the presence of impurities.36−38 The decarbonation reaction in eq 1 begins on the surfaces of the Ag2CO3 particles, resulting in the formation of a surface product layer.39 Immediately following this surface reaction, structural phase transitions of the internal Ag2CO3 to two high-temperature phases occur successively.39−46 The reactivity of the internal Ag2CO3 then increases due to fragmentation into smaller grains as a result of the structural phase transition, but the overall reaction rate is determined by the properties of the surface product layer and its changes as the reaction advances. Many physicochemical and physicogeometrical events take part in the decarbonation reaction, such as crystal growth of Ag2O and the sintering of those crystals, diffusion of the generated CO2 through the surface product layer, establishment of an internal chemical equilibrium for the reacting particles, the arrest of the reaction, and pore formation in the surface product layer. Thus, the decarbonation reaction proceeds via complex interactions that occur between those physicochemical and physicogeometrical events.39 In addition, the contribution of each event to the overall kinetics of the thermal decomposition varies systematically with the reaction conditions, such as the heating rate and reaction atmosphere. Consequently, different intermediate products, such as Ag2O and Ag2CO3−Ag2O with a core shell structure, are produced depending on the starting sample and the reaction conditions. Because the final product (Ag) is produced via the thermal decomposition of these intermediate products, which have differing compositions and morphological characteristics, the process for the formation of Ag and the morphological characteristics of the as-produced Ag may vary significantly with the sample and the reaction conditions. Herein, we describe the extension of our previous study for the thermal decomposition of Ag2CO3 to Ag2O (eq 1)39 to include the subsequent reaction for the formation of Ag (eq 2)

2. EXPERIMENTAL SECTION 2.1. Samples. Three commercially available Ag2CO3 samples that exhibited different thermal behaviors were selected based on the results of our previous study.39 Table 1 lists the information of the samples used in this study. The samples have been previously characterized using powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetry−differential thermal analysis (TG− DTA). The sample morphologies and sizes have also been characterized via scanning electron microscopy (SEM) and measurement of specific surface areas and particle size distributions. The crystallite size of the samples calculated from XRD peaks using the Scherrer equation was approximately 50 nm irrespective of the samples. The three samples are referred to as type I, II, and III according to their thermal decomposition reactivity as revealed previously (Table 1). The type I sample is the most reactive and decomposes to Ag via Ag2O via the clearly defined two-step reaction sequence shown in eqs 1 and 2. The type II sample is the least active, with the thermal decomposition to Ag2O arrested at the very beginning of the reaction and the residual Ag2CO3 decomposing at a higher temperature along with the thermal decomposition of Ag2O. The type III sample exhibits intermediate reactivity between that of the type I and II samples. 2.2. Thermal Decomposition Behavior. Each sample was weighed (5.0 mg) in a platinum pan (5 mm ϕ × 2.5 mm). The thermal decomposition behavior of the sample was then recorded via TG−DTA (TG8120, Rigaku Co.) at different heating rates β (0.5 ≤ β ≤ 5 K min−1) in flowing He (200 cm3 min−1). The gases evolved during the thermal decomposition were transferred to a mass spectrometer (MS; M-200Q, Anelva Co.) through a silica capillary (0.075 mm inner diameter) heated at 500 K. The mass spectrum was obtained every 5 s in the amu range from 10 to 50 (EMSN: 1.0 A; SEM: 1.0 kV). For

thermal decomposition of silver carbonate (Ag2CO3) was selected for the following reasons. First, it was expected that the chemical processes involved in the thermal decomposition of Ag2CO3 would be simpler than those for organometallic Ag compounds; thus, the relationships between the physicochemical and physicogeometrical events and the overall kinetics of Ag formation could be more clearly observed. Second, this reaction has been widely studied since the very early days of the 20th century,26−30 and the results obtained from those studies have contributed significantly to the development of the fundamental theory of solid-state reactions.23,24,31−33 However, fairly complex reaction behaviors during the thermal decomposition of Ag2CO3 have been revealed more recently using novel and advanced physicochemical techniques. Ideally, the thermal decomposition is understood to proceed via readily distinguished two-step chemical reaction:

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during the lower temperature process. With decreasing β, the mass loss ratio of the lower temperature process systematically increases, which is compensated by a decrease in the mass loss ratio of the higher temperature process. The type III sample exhibits thermal behavior between that of the type I and II samples, with the mass loss ratio of the lower and higher temperature processes changing with β in a manner similar to that observed for the type II sample. However, a wider temperature range for the higher temperature process than that observed for the type I and II samples is characteristic for the type III sample. These observations are in good agreement with the results obtained previously.39 To separate the mass loss steps during thermal decomposition in more detail, the DTG curves in Figure 1 were deconvoluted using a statistical function (Weibull function).47−51 Figure 2 presents typical examples of the

the SEM observations of partially reacted sample particles, the samples were heated to different temperatures at different β values in flowing N2 (80 cm3 min−1) using a TG instrument (TGA-50, Shimadzu Co.) under conditions otherwise identical to those used for TG/DTA−MS measurements. The samples were allowed to partially decompose to a characteristic reaction stage and were then observed via SEM (JSM-6510, Jeol) after being sputter-coated with Pt (30 mA, 60 s). At the same time, the energy dispersive X-ray (EDX) spectrum of the partially decomposed samples was recorded using an instrument (X-act, Oxford) attached to the SEM.

3. RESULTS AND DISCUSSION 3.1. Changes in the Thermal Decomposition Pathway. Figure 1 shows the TG−derivative TG (DTG) curves for the

Figure 1. TG−DTG curves for the thermal decomposition of Ag2CO3 (m0 = 5.0 mg) in flowing He (200 cm3 min−1) at different β values: (a) type I, (b) type II, and (c) type III samples. Figure 2. Deconvolution of the mass loss steps using a statistical function (Weibull): (a) type I, (b) type II, and (c) type III samples.

thermal decomposition of the three types of samples at different β values. Despite the sample and β, the thermal decomposition of Ag2CO3 to Ag proceeds via two distinguishable mass loss processes, one at temperatures below 550 K and one at temperatures above 550 K. The mass loss values for each mass loss step for the type I sample are nearly in agreement with the ideal reaction pathway for the formation of Ag via Ag2O, and the TG−DTG curves systematically shift to higher temperature with increasing β. The reaction of the type II sample, on the other hand, is initiated at a lower temperature than that of the type I and III samples but is arrested at the very beginning when the sample is heated at a higher β. The thermal decomposition of the residual portion of Ag2CO3 occurs via a higher temperature process along with the Ag2O produced

mathematical deconvolution. The overall mass loss process for the type I and III samples is empirically separated into five partially overlapping mass loss steps (Figures 2a and 2c). Conversely, the thermal decomposition of the type II sample occurs via four mass loss steps (Figure 2b). In all of the samples, the thermal decomposition of Ag2CO3 to Ag2O mainly involves the first two mass loss steps, as was previously revealed.39 The last two mass loss steps are considered to relate to the formation of Ag via the thermal decomposition of the intermediates produced during the preceding mass loss steps. The third mass loss step for the type I and III samples is 8061



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Figure 3. Mass chromatograms of the evolved gases (m/z 44 and m/z 32) during the thermal decomposition of Ag2CO3 at different β values: (a) type I, (b) type II, and (c) type III samples.

in the first mass loss step systematically shifts to higher temperature with increasing β. This shift is also observed for O2 evolution in the fourth mass loss step for the type I sample. Conversely, the β-dependent changes in the peak temperature for Ag formation with simultaneous evolution of O2 and CO2 in the type II and III samples are very complex. In the type II sample, the initiation temperature for the simultaneous evolution of O2 and CO2 systematically decreases as β decreases from 5 to 2 K min−1. In addition, the peak temperature for Ag formation in the type II sample is lower by 50 K than that of the type I sample at corresponding β. Furthermore, at β = 1 K min−1, the major reaction step for Ag formation in the type II sample suddenly shifts to higher temperature, and nearly corresponds to the temperature region of that in the type I sample. With decreasing β, the residual portion of Ag2CO3 available for direct decomposition to Ag decreases, which contributes to the shift of the temperature region for Ag formation to higher temperatures at lower β. As mentioned above, a wide temperature range for Ag formation process is characteristic of the type III sample, but the trend for the β-dependent change in the initiation temperature for Ag formation is similar to that observed for the type II sample. In the type III sample, the irregular shift of the reaction temperature region for Ag formation to higher temperature is observed at β = 2 K min−1, and the relationship between the reaction temperature region and the residual amount of Ag2CO3 can be seen more clearly. 3.2. Kinetic Deconvolution of the Apparent Reaction Steps. To simulate the kinetic behavior of the respective reaction steps and the changes influenced by the sample and β, the kinetic rate data for the thermal decomposition of Ag2CO3 obtained via the TG−DTG measurements at different β values (Figure 1) were kinetically deconvoluted using the following cumulative kinetic equation, assuming the independence of the respective reaction steps.48,52−56

positioned in the intermediate temperature region between the lower and higher temperature processes and hence is attributed to the decomposition of residual Ag2CO3 remaining after the first two mass loss steps. The lack of a corresponding mass loss step for the type II sample indicates the higher stability of the arrested state for the thermal decomposition of Ag2CO3 to Ag2O. Notably, the contributions of the respective mass loss steps to the overall mass loss process change depending on β (Table S1 in the Supporting Information). This result suggests that the respective mass loss steps are not stoichiometric processes, but physicogeometrically controlled processes. Figure 3 shows the mass chromatograms (m/z 44 and 32) of the gases that evolved during the thermal decomposition process. The first three mass loss steps observed for the type I and III samples and the first two mass loss steps observed for the type II sample are characterized by the evolution of CO2 as a result of the thermal decomposition of Ag2CO3 to Ag2O. In the type I sample, only the evolution of O2 (sharp peak of the mass chromatogram of m/z 32) is observed for the last two mass loss steps, indicating the decomposition of Ag2O to Ag. The gas evolution behavior of the last two mass loss steps for the type II sample is relatively different with the simultaneous evolution of CO2 and O2 for all of the measurements at different β values. Because the coexistence of α-Ag2CO3 and Ag2O phases prior to initiation of the Ag formation process was previously confirmed via high-temperature XRD,39 the third and fourth mass loss steps are interpreted to involve the simultaneous thermal decomposition of the α-Ag2CO3 and intermediate crystalline Ag2O. The characteristics of the thermal behavior of the type III sample, which is intermediate between those of the type I and II samples, are also confirmed based on the evaluation of its gaseous evolution behavior. At lower β, the evolution of CO2 and O2 is clearly separated for the different mass loss steps, as in the case of the type I sample. With increasing β, the simultaneous evolution of CO2 and O2 is observed during the high-temperature process, as in the case of the type II sample. The characteristics of the thermal decomposition of each sample are also investigated by evaluating the change in the temperature region of each reaction step as a function of β. For all of the samples, the initiation temperature for CO2 evolution

dα = dt

⎛ Ea, i ⎞ ⎟ f (α ) ⎝ RT ⎠ i i

N

∑ ciAiexp⎜− i=1 N

with

i=1

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N

∑ ci = 1 and ∑ ciαi = α i=1

(3)



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Figure 4. Results of the kinetic deconvolution for the thermal decomposition of Ag2CO3 at different β values: (a) type I, (b) type II, and (c) type III samples.

where N is the number of reaction steps in the overall thermal decomposition reaction of Ag2CO3 to Ag. The values for ci, Ai, and Ea,i are the relative contribution, the Arrhenius preexponential factor, and the apparent activation energy of the ith reaction step, respectively. For the kinetic model function of the ith reaction step, f i(αi), an empirical kinetic model known as the Šesták−Berggren model (SB(m,n,p))57 was employed, because there is sufficient flexibility for the function to accommodate various types of reactions.58−61 SB(m , n , p): fi (αi) = αim(1 − αi)n [−ln(1 − αi)]p

(4)

The detailed procedures of the kinetic computation based on eqs 3 and 4 are described in section S2 in the Supporting Information. Figure 4 shows the results of the kinetic simulations for the thermal decomposition of the three samples at different β values. The optimized values for ci for each reaction step change depending on β, with a special trend for each sample type, as shown in Figure 5. In the type I sample (Figures 4a and 5a), the values for ci for each reaction step do not largely change with β in comparison with the ci values for the type II and III samples, because the decomposition proceeds via a nearly ideal two-step mass loss process, as described in eqs 1 and 2. However, the third reaction step in the intermediate temperature region between these established reactions is detectable when the sample is heated at a higher β, indicating the decomposition of residual Ag2CO3 to Ag2O. The fourth reaction step in the type I sample appears most significantly at β = 2 K min−1 and precedes the fifth reaction step, which is indicated by a sharp peak and is characterized as the major reaction step of Ag formation. A significant decrease in the ci value for the second reaction step with an increase in β is observed for the type II and III samples (Figures 5b and 5c), which is compensated by a corresponding increase in the fourth reaction step for the type II sample (Figures 4b and 5b) and in the third, fourth, and fifth reaction steps for the type III sample (Figures 4c and 5c). These β-dependent changes in the ci values for the type II and III samples describe well the β-dependent changes in the residual amount of Ag2CO3 that was not decomposed in the

Figure 5. Change in the contribution of each reaction step, ci, to the overall decomposition of Ag2CO3 to Ag as a function of β: (a) type I, (b) type II, and (c) type III samples.

preceding lower temperature process. In the type II sample (Figures 4b and 5b), the third reaction step, characterized as the simultaneous evolution of CO2 and O2, begins at lower temperatures with increasing β. In addition, at β ≥ 2 K min−1, the fourth step of the type II sample is perfectly overlapping with the third reaction step. However, this feature suddenly changes at β = 1 K min−1, with an irregular shift of the fourth reaction step to higher temperatures, which results in clearly 8063



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Table 2. Average Kinetic Parameters for Different β Values (0.5 ≤ β ≤ 5 K min−1) Optimized for Each Reaction Step in the Thermal Decomposition of Ag2CO3 SB(m, n, p) sample

step i

I

1 2 3 4 5 1 2 3 4 1 2 3 4 5

II

III

Ea/kJ mol−1 109.0 140.4 123.7 133.4 150.3 101.0 64.9 140.8 150.8 103.3 89.6 100.6 122.2 172.6

± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.8 1.3 4.0 5.4 3.9 2.6 2.1 12.3 5.7 2.4 3.5 21.5 12.6 2.6

A/s−1 (7.22 (8.53 (9.33 (1.47 (1.71 (3.01 (1.06 (6.00 (5.78 (4.47 (8.24 (5.54 (1.31 (1.66

± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.88) 0.15) 0.31) 0.35) 0.55) 2.51) 1.10) 5.39) 2.50) 2.25) 7.11) 5.14) 2.06) 0.36)

m × × × × × × × × × × × × × ×

109 1012 107 108 1010 109 105 109 1010 109 107 108 109 1011

separated reaction steps. From the β-dependent changes in the third and fourth reaction steps, it is concluded that a larger amount of residual Ag2CO3, coexisting with Ag2O, accelerates the entire Ag formation process. In the type III sample (Figures 4c and 5c), a wide temperature range for the formation of Ag that consists of the fourth and fifth reaction steps is characteristic, as described above. The relatively sharp peak for the third reaction process is positioned at the early stage of the fifth reaction step, and the entire wide temperature range for Ag formation is covered by the fifth reaction step. With decreasing β and decreasing residual Ag2CO3 content, the fourth reaction step is gradually attenuated. The irregular shift of the last reaction step at lower heating rates that was observed for the type II sample is also observed for the type III sample at β ≤ 2 K min−1. Although Ag formation in the type III sample occurs with the evolution of only O2 in the fifth reaction step at 0.5 K min−1, the fifth reaction step occurs over a wider temperature range compared to the fifth reaction step for the type I sample at corresponding β. Despite such β-dependent changes in the value of ci, the optimized kinetic parameters for each reaction step in each sample were not largely influenced by β. The average optimized kinetic parameters for different β values are listed in Table 2. For all of the samples, the optimized Arrhenius parameters for the first and second reaction steps are in good agreement with those previously determined under isothermal conditions.39 The optimized Ea values for Ag formation processes are in the range from 130 to 160 kJ mol−1 for all of the samples, without any clear indication of differences in the O2 evolution for the type I sample and the simultaneous O2 and CO2 evolutions in the type II and III samples. However, a relatively large distribution of the optimized Ea values at different β values is observed for the third reaction steps in the type II and type III samples. Given the β-dependent changes in the ci values for these reaction steps, changes in the kinetic behavior as a function of β should be expected for these reaction steps. Using the average values of the kinetic exponents in SB(m,n,p), the rate behaviors of the respective reaction steps for each sample were then simulated as the plot of SB(mi,ni,pi) versus α (Figure 6). Despite the sample type, the first and second reaction steps indicate the maximum rate occurred within the first-half of each reaction step, and the rate subsequently decelerates with a convex shape. The third reaction step for the type I and III

0.14 0.05 −0.29 0.07 0.02 0.10 0.29 −0.01 0.23 0.23 0.01 0.08 0.01 0.04

± ± ± ± ± ± ± ± ± ± ± ± ± ±

n 0.03 0.09 0.18 0.14 0.30 0.04 0.02 0.25 0.14 0.12 0.01 0.44 0.57 0.15

0.95 2.08 1.14 0.87 0.95 1.50 1.31 0.95 1.16 1.50 1.61 1.57 1.30 1.05

± ± ± ± ± ± ± ± ± ± ± ± ± ±

p 0.14 0.62 0.25 0.28 0.21 0.24 0.17 0.14 0.21 0.50 0.60 0.52 0.28 0.05

0.21 0.25 −0.35 0.10 0.93 0.09 0.39 −0.01 0.70 0.16 0.64 −0.03 0.03 0.19

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.06 0.07 0.21 0.16 0.25 0.05 0.09 0.24 0.12 0.06 0.20 0.24 0.70 0.10

Figure 6. Plots of the optimized SB(mi,ni,pi) versus α for each reaction step: (a) type I, (b) type II, and (c) type III samples.

samples, which was observed between the lower and higher temperature processes, is apparently characterized as a diffusion-controlled process given that the rate decelerates with a concave shape. This observation is in accordance with the physicogeometrical considerations of the third reaction step, which is impeded by the Ag2O surface product layer. The fourth reaction step for the type I and III samples and the third 8064



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reaction step for the type II sample, which are the Ag formation processes, are mainly characterized by linear deceleration of the rate. The subsequent reaction steps, the fifth reaction step for the type I and III samples and fourth reaction step for the type II sample, exhibit different rate behaviors. The maximum rate is observed in the second-half, in the middle, and in the first-half of the processes for the type I, II, and III samples, respectively. 3.3. Morphological Characteristics of Each Reaction Stage. To correlate the complex reaction pathways with the physicogeometrical characteristics of the reactions at each reaction stage, SEM observations of the surface textures of the samples at different stages of the decomposition reaction were performed by selecting the reaction conditions that indicated the most complex reaction pathway for each type of sample. Figure 7 shows SEM images of the type I sample (Figure 7A, EDX: Supporting Information Figure S4A) at different reaction stages indicated on the accompanying TG−DTG curves. When Ag2CO3 is decomposing to Ag2O in the type I sample, separate particles of Ag2O are clearly observed on surfaces of the reacting particles (Figure 7B). The CO2 that evolved during the first reaction step (Figure 4a) possibly diffuses through the interstices of the surface product layer, resulting in the nearly quantitative formation of Ag2O. At the end of the first reaction step, the Ag2O particles are gradually sintered and form a smooth surface (Figure 7C). This product layer with a smooth surface then impedes the diffusional removal of CO2 generated by the internal reaction. This inhabitation of CO2 release may be the possible cause of the long reaction tail that appears as the second reaction step. A small amount of residual Ag2CO3 then decomposes in the third reaction step, and the evolved CO2 escapes through pores formed in the surface product layer (Figure 7D, EDX: Supporting Information Figure S4D). After the third reaction step, the particle surface is further sintered, once again creating a smooth surface (Figure 7E). Just prior to the initiation of the thermal decomposition of the intermediate Ag2O in the fourth reaction step, a slight swelling of the surface is observed (Figure 7F). This swelling suggests an increase in the internal pressure of the particles due to the evolution of O2 produced in the internal reaction and a higher mobility of the surface product layer at this reaction stage. It should be noted that micron-sized particles appear on the surfaces of the reacting particles just prior to the thermal decomposition of the as-produced Ag2O. After the initiation of the thermal decomposition of the surface Ag2O, the smooth surface layer is divided into blocks (Figure 7G). Significantly, boundaries between the blocks are accompanied by holes in some areas that possibly serve as diffusion channels for the removal of the O2 evolved as a result of the internal reaction or as a trace of the diffusion pathway. It should also be noted that the submicron particles are deposited at the boundaries of the blocks. For the thermal decomposition of Ag2O, a mechanism involving dissociative evaporation of Ag2O with simultaneous condensation of Ag has been proposed by L’vov.62 This evaporation−condensation mechanism is supported by the observed deposition of Ag particles at the boundaries of the blocks. In this scenario, the formation of reacting particles with a hollow structure is expected at this reaction stage. The hollow structure of the reacting particles has been frequently observed during the thermal decomposition of solids comprised of particles with core shell structures and that have an internal reactant with a higher mobility.54 Upon further heating, the produced Ag is sintered, forming a smooth surface and causing

Figure 7. TG−DTG curves for the thermal decomposition of the type I sample and SEM images of the type I samples heated to different temperatures indicated on the DTG curve using labels A−H.

shrinkage of the particle volume (Figure 7H, EDX: Supporting Information Figure S4H). The surface textures of the type II sample at different reaction stages are shown in Figure 8. The type II sample is composed of smaller particles (Figure 8A, EDX: Supporting Information Figure S5A) than the type I sample (Figure 7A). Formation of Ag2O on the surfaces of the particles at the beginning of the first reaction step is also evident in the type II sample (Figure 8B), but the particle size of the product Ag2O is apparently smaller than that observed for the type I sample (Figure 7B). Because the reaction is arrested at this stage, it is assumed that the surface product layer impedes the diffusion of 8065



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α‐Ag 2CO3 ⇄ Ag 2O + CO2

(6)

Upon further heating, the surfaces of the particles gradually change to a smooth texture due to sintering of the Ag2O surface product layer (Figure 8C, EDX: Supporting Information Figure S5C). The formation of aggregates with necks is also characteristic of this reaction stage (Figure 8D). Under these conditions, the residual Ag2CO3 inside the particles is stabilized, and an Ag2CO3−Ag2O core shell structure is established that is preserved until higher temperatures are reached. The thickness of the Ag2O surface product layer at this stage decreases with increasing β because of the larger amount of residual Ag2CO3 in the core. Accordingly, the thermal stability of the Ag2CO3− Ag2O core shell structure decreases with increasing β, resulting in the initiation of the third reaction step at lower temperature. Formation of submicrometer-sized Ag particles and holes on the surfaces of the particle is observed during Ag formation in the third and fourth reaction steps (Figure 8E), which are characterized by the simultaneous evolution of CO2 and O2 (Figure 3b, β = 2 K min−1). Those holes enable the rapid escape of the product gases. A smooth surface is then recovered upon further heating of the Ag product (Figure 8F). When the sample was heated at a lower β (1 K min−1), Ag formation is divided into two distinguishable reaction steps. During the arrested stage after partial decomposition of Ag2CO3 to Ag2O, the surface texture of the sample (Figure 8G) is comparable to that observed at higher β (2 K min−1; Figure 8D). After the first reaction step for Ag formation process (the third reaction step), the sample particles are aggregated with smooth surfaces (Figure 8H, EDX: Supporting Information Figure S5H). The impedance of the diffusional removal of O2 generated as the internal thermal decomposition of Ag2CO3 and Ag2O proceeds by the sintered Ag surface product layer, and aggregated reacting particles may be the cause of the separation of the third and fourth reaction steps. Changes in the surface texture during the thermal decomposition of the type III sample include changes observed for both the type I and II samples, as shown in Figure 9, because the sample particles are a mixture of larger and smaller particles (Figure 9A, EDX: Supporting Information Figure S6A). The texture of the larger particles changes as in a manner similar to those of the type I sample, while the smaller particles behave like those in the type II sample. Larger Ag2O particles are observed on the surfaces of the larger particles at the early stage of the first reaction step (Figure 9B), and the surface product layer on these particles gradually becomes smooth during the second reaction step (Figure 9C), resulting in the arrest of the thermal decomposition of the internal Ag2CO3. During the third reaction step, pores are detected on the surfaces of the larger particles (Figure 9D), which are necessary for the diffusional removal of the evolved CO2. Despite the sizes of the sample particles, the Ag2O particles in the surface product layer are largely sintered, and the reacting particles are heavily aggregated after the third reaction step (Figure 9E, EDX: Supporting Information Figure S6E). At the Ag formation stage with simultaneous evolution of CO2 and O2 at approximately 600 K, the particles are significantly aggregated due to the sintering of the Ag surface product layer (Figure 9F). This phenomenon is responsible for the wider reaction temperature range for Ag formation for the type III sample. During the fifth reaction step, many holes are observed on the sintered particles (Figure 9G), indicating that traces of gas evolved. The holes then disappear upon further

Figure 8. TG−DTG curves for the thermal decomposition of the type II sample and SEM images of the type II samples heated to different temperatures indicated on the DTG curves using labels A−H.

the CO2 generated in the internal reaction in the second reaction step. In addition, because successive structural phase transitions of Ag2CO3 to the high-temperature β- and α-phases occurs at this stage,40−46 it is assumed that the internal αAg2CO3 has reactive sites at the grain boundaries. Further reaction at these grain surfaces then led to a rapid increase in the partial pressure of CO2 inside the reacting particles resulting in an equilibrium condition. 8066



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layer, structural phase transitions occur, resulting in the formation of α-Ag2CO3 via β-Ag2CO3 in the interior of the reacting particles.39−46 Consequently, fine Ag2CO3 grains are formed in the interior of the reacting particles, generating reactive sites at the grain boundaries. Under these conditions, the thermal decomposition of Ag2CO3 subsequently proceeds at the grain boundaries of the interior Ag2CO3 grains. From a physicogeometrical perspective, the change in the reaction geometry because of the structural phase transition temporary accelerates the thermal decomposition, which is accompanied by an increase in the internal CO2 pressure. The differences in the reaction mechanisms for the type I and II samples may be due to the possibility of (or lack of) diffusional removal of the CO2 gas produced by the internal reaction (stage B). In the type I sample, diffusion channels are retained as pores and interstices between well-grown Ag2O particles in the surface product layer. The larger temporary increase in the CO2 partial pressure in the interior of the reacting particles is also expected for the type I sample because of the larger size of the particles and the greater amount of internal Ag2CO3 grains. This increased CO2 pressure may also be a driving force for diffusional removal when the diffusion channels exist. In contrast, the smooth texture of the surface product layer in the type II sample at this reaction stage, which is constructed of densely arranged smaller Ag2O particles, suggests greater inhabitation of the gaseous diffusion. Under these conditions, a chemical equilibrium between Ag2CO3 and Ag2O (eq 6) is established in the interior of the reacting particles. When the type II sample is heated at a lower heating rate, the Ag2O particles in the surface product layer gradually grow, and interstices between the Ag2O particles form. The chemical equilibrium of eq 6 then shifts to the right with the evolution of CO2 through these interstices in the surface product layer. Consequently, reaction behavior similar to that of the type I sample is observed for the type II sample at this reaction stage, although a small amount of Ag2CO3 remains in the interior until higher temperatures are reached. In contrast, at a higher β, the sample quickly passes the temperature region for the crystal growth of Ag2O and moves to the temperature region for the sintering of the Ag2O particles. Accordingly, an Ag 2 CO 3 −Ag 2 O core shell structure and the chemical equilibrium in eq 6 are preserved until higher temperatures. Despite the sample type, the sintering of the Ag2O surface product layer occurs prior to the thermal decomposition of Ag2O (stage C). The evolution of O2 during the thermal decomposition of the intermediate Ag2O (stage D) begins at lower temperatures in the type II sample. An increase in the internal pressure of the reacting particles with a core−shell structure due to the thermal decomposition of the residual Ag2CO3 in the core may be possible cause. In the type II sample, evolution of CO2 and O2 as a result of the reactions in eqs 1 and 2, respectively, occurs simultaneously. It is therefore deduced that the reactivity of the Ag2O produced during the thermal decomposition of Ag2CO3 at this reaction stage is sufficient to immediately transform the Ag2O to Ag. It should be noted that the surface Ag2O produced in the previous reaction stage also decomposes to Ag in the same temperature region as that of the successive reactions of the internal Ag2CO3, which also result in the formation of Ag. It is also thought that the reactivity of the thin Ag2O surface product layer in the type II sample is also high due to interaction with the CO2 generated in the chemical equilibrium described in eq 6. In contrast, the surface Ag2O layer is thicker in the type I

Figure 9. TG−DTG curves for the thermal decomposition of the type III sample and SEM images of the type III samples heated to different temperatures indicated on the DTG curve using labels A−H.

heating due to the sintering of the Ag (Figure 9H, EDX: Supporting Informaion Figure S6H). 3.4. Phenomenological Interpretation of the Thermal Decomposition. Integrating the above findings on the reaction pathways and the kinetic behaviors, a physicogeometrical mechanism for the thermal decomposition of Ag2CO3 was proposed and is illustrated in Figure 10. The reaction pathway for the thermal decomposition can be interpreted by dividing the process into five reaction stages, A−E. Despite the sample type, the thermal decomposition of Ag2CO3 to Ag2O is initiated on the surface of the reactant particles (stage A). After the reacting particles are covered with an Ag2O surface product 8067



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Figure 10. Schematic illustration of the physicogeometrical mechanism of the thermal decomposition of Ag2CO3.

sample, as has been suggested by the isothermal kinetics for the two-step reaction in eq 1.39 In addition to the higher stability of this surface Ag2O layer, an increase in the internal pressure as a result of the reaction in eq 2 also occurs at higher temperatures. The effect of the residual Ag2CO3 in the core of the reacting particles, which is deduced from the different reaction behaviors of the type I and II samples in reaction stage D, was also confirmed by the changes in the thermal decomposition behavior as a function of β that were observed for the type II sample. Finally, the as-produced Ag is significantly sintered at higher temperatures (stage E).

nanoparticle formation via the thermal decomposition of Ag compounds.

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ASSOCIATED CONTENT

S Supporting Information *

Contributions of each reaction step evaluated by mathematical deconvolution (Table S1); procedures of the kinetic deconvolution (Figures S1−S3, Table S2); EDX spectra of the samples at different reaction stages (Figures S4−S6). This material is available free of charge via the Internet at http:// pubs.acs.org.

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4. CONCLUSION The complex reaction behavior of the thermal decomposition of Ag2CO3 to form Ag and the changes in the thermal decomposition behavior that depend on the properties of the sample particles and the heating rate β result from competitive interactions of two physicochemical processes, i.e., sintering of the product particles in the surface product layer and the diffusional removal of product gases, in the reactions in eqs 1 and 2, within the framework of the reaction geometry for a surface reaction-induced contracting geometry reaction. The structural phase transition of the internal Ag2CO3 after the formation of the Ag2O surface product layer and the formation of an Ag2CO3−Ag2O core shell structure make the reaction behavior more complex and result in the establishment of an equilibrium state (eq 6) in the interior of the reacting particles. The high sintering ability of the initially produced Ag2O and Ag particles on the surfaces of the reacting particles in the respective reactions in eqs 1 and 2 is one of the major reasons for the regulation of the complex reaction behavior by the physicogeometry. In the formation of Ag nanoparticles via the thermal decomposition of Ag compounds, the growth and sintering of product Ag particles during the reaction are undesirable physicochemical phenomena. For the formation of Ag nanoparticles via the thermal decomposition, critical physicochemical and physicogeometrical phenomena are necessary to fragment the reacting particles, destroy the physicogeometrical reaction scheme, and inhibit the growth and sintering of the as-produced Ag particles. Investigation of such specific phenomena from physicochemical and physicogeometrical viewpoints is necessary for understanding Ag

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was partially supported by a grant-in-aid for scientific research (A) (25242015) and (C) (25350202, 25350203) from Japan Society for the Promotion of Science.

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REFERENCES

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