# Hydrothermal Preparation of Ag/Ag1.69Sb2.27O6.25 Sesame-Hollow-Ball-Type Nanocomposites: The Formation Mechanism of Metallic Ag in the Ag-H2O System at 400 K

Authors: Haomei Liu; Xihai Hao; Yuejun Liu; Aiguo Yan
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

Ag/Ag1.69Sb2.27O6.25 sesame-hollow-ball-type nanocomposites were prepared via a facile one-step hydrothermal method at 400 K. Power X-ray diffraction analysis shows that all diffraction peaks were well consistent with JCPDS card no: 89-6552 of Ag1.69Sb2.27O6.25. Scanning electron microscopy and high-resolution transmission electron microscopy images of the composites indicate that some smaller metallic Ag particles with size∼18.3 nm uniformly dense on the surface of Ag1.69Sb2.27O6.25 hollow nanospheres with a mean size of about 170 nm, producing Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites. The surface chemical state of Ag/Ag1.69Sb2.27O6.25 is investigated by XPS, and all peaks of Ag 3d, O 1s, and Sb 3d show their different chemical states. The BET surface area of the sample is 7.268 m2/g, and the pore sizes of nanocomposites are more than 5 nm. The light absorption property of as-prepared materials is studied by UV-vis/DRS, and the adsorption band is located at 445 nm, and the estimated energy band gap (Eg) is 2.55 eV. The calculated partial φ-pH diagrams in the Ag-H2O system at 400 K predict that the Ag+ ion can react with H2 to form metallic Ag.


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## Body

## 1. Introduction

The fabrication of silver antimonite nanoparticles has been intensively investigated in recent years due to their high electron mobility and photocatalytic activity, which are technological applications in visible-light-sensitive photocatalysts [1–5]. Due to the conduction band bottom mainly comprised of the hybridized hybridized Ag 5s and Sb 5s orbitals and the valence band top of Ag 4d and O 2p, silver antimonite was reported to possess higher visible-light sensitivity compared with Ag-containing d-block metal complex oxides such as AgNbO3 and AgVO3 [6, 7]. However, the single-phase silver antimonites have quite low reduction potential and suffer from the easy recombination of photogenerated electrons and holes. Combining two semiconductors to construct silver antimonite-based heterostructure photocatalysts such as AgSbO3/NaNbO3 [8] and AgSbO3/AgNbO3 [9] and designing a “Z-scheme” such as AgSbO3/Ag/g-C3N4 [10] have been considered to be efficient methods to prepare photocatalysts for producing enhanced photocatalytic activity.In contrast to the above process, visible-light-triggered plasmonic photocatalysts have been recognized as another promising alternative to the traditional photocatalysts [11]. In these metal-semiconductor composites, metal nanoparticles (NPs) can strongly absorb visible light due to the surface plasmon resonance (SPR) effect and facilitate the separation of eCB− and hVB− generated on the semiconductor [12]. Recently, it has been reported that many semiconductors modified by nanosized noble metals such as Au enhance the overall photocatalytic efficiency due to their unique surface plasmon resonance (SPR) features originating from the collective oscillations of the electrons on the surface of the NPs. For example, Haruta and coworkers [13, 14] demonstrated that gold nanoparticles in the 2–5 nm range show unusually high catalytic activities. As a cheap noble metal, some studies about Ag, selected as the mediator to shuttle the electrons between the two semiconductor components in the Z-scheme, have been reported [15–20], but relative reports about Ag-based metal-semiconductor, especially Ag/Ag antimonite composites, were quietly scarce. And few researchers investigated about the mechanism of reduction of silver ions to metallic silver in the aqueous system.In the present study, we have prepared sesame-hollow-ball-type Ag/Ag1.69Sb2.27O6.25 nanocomposites through a simple one-step hydrothermal method. The mechanism of reduction of Ag+ to Ag in the hydrometallurgy system on the basis of the partial potential-pH diagram for the silver-water system at a higher temperature of 400 K is investigated. This work will result in a general and economic synthetic strategy for obtaining various Ag-based semiconductors and a better understanding of the mechanism of their formation.


## 2. Experimental Sections

### 2.1. Materials

Potassium pyroantimonate (KSbO6H6) was purchased from Xilong Scientiﬁc Co., Ltd. (China). AgNO3 was obtained from Xilong Scientiﬁc Co., Ltd. (China). All chemical reagents are of analytic grade without further purification. Deionized water was prepared with ultrapure water of 18.2 MΩ (Milli-Q, Millipore).


### 2.2. Preparation of Sesame-Ball-Type Ag/Ag1.69Sb2.27O6.25 Hollow Nanocomposites

Firstly, 1.0192 g (3.88 mmol) of KSbO6H6 was dispersed in 50 mL deionized water and was kept stirring at 80°C for 0.5 h. Secondly, 0.6796 g (4.0 mmol) of AgNO3 was dissolved in 5 mL deionized water and was slowly added into the above mixture and then kept stirring for 5 minutes. Thirdly, the mixture (pH value is 6.8 measured using a pH meter) was sealed in a teflon-lined stainless-steel autoclave (100 ml capacity) and heated at 127°C (400 K) for 24 h. After the reaction, the autoclave was cooled down to room temperature naturally. After washing (with deionized water and 99.5% ethanol) and drying (at 70°C for 3 h), the sample was obtained.


### 2.3. Characterization

The obtained samples were characterized on a Bruker D8 Advance X-ray powder diffractometer (XRD) with Cu-Kα radiation (λ = 1.5418 Å). The size and morphology of the as-synthesized products were determined by a Zeiss Gemini 500 scanning electron microscope (SEM). High-resolution transmission electron microscopy (HRTEM) images were collected by JEOL JEM2010. The chemical state of samples was investigated by X-ray photoelectron spectroscopy (XPS). UV-visible diffuse reflectance spectra were obtained on the Shimadzu UV-2600 spectrophotometer.


## 2.1. Materials

Potassium pyroantimonate (KSbO6H6) was purchased from Xilong Scientiﬁc Co., Ltd. (China). AgNO3 was obtained from Xilong Scientiﬁc Co., Ltd. (China). All chemical reagents are of analytic grade without further purification. Deionized water was prepared with ultrapure water of 18.2 MΩ (Milli-Q, Millipore).


## 2.2. Preparation of Sesame-Ball-Type Ag/Ag1.69Sb2.27O6.25 Hollow Nanocomposites

Firstly, 1.0192 g (3.88 mmol) of KSbO6H6 was dispersed in 50 mL deionized water and was kept stirring at 80°C for 0.5 h. Secondly, 0.6796 g (4.0 mmol) of AgNO3 was dissolved in 5 mL deionized water and was slowly added into the above mixture and then kept stirring for 5 minutes. Thirdly, the mixture (pH value is 6.8 measured using a pH meter) was sealed in a teflon-lined stainless-steel autoclave (100 ml capacity) and heated at 127°C (400 K) for 24 h. After the reaction, the autoclave was cooled down to room temperature naturally. After washing (with deionized water and 99.5% ethanol) and drying (at 70°C for 3 h), the sample was obtained.


## 2.3. Characterization

The obtained samples were characterized on a Bruker D8 Advance X-ray powder diffractometer (XRD) with Cu-Kα radiation (λ = 1.5418 Å). The size and morphology of the as-synthesized products were determined by a Zeiss Gemini 500 scanning electron microscope (SEM). High-resolution transmission electron microscopy (HRTEM) images were collected by JEOL JEM2010. The chemical state of samples was investigated by X-ray photoelectron spectroscopy (XPS). UV-visible diffuse reflectance spectra were obtained on the Shimadzu UV-2600 spectrophotometer.


## 3. Results and Discussion

### 3.1. Characterizations of the Ag/Ag1.69Sb2.27O6.25 Sesame-Ball-Type Hollow Nanocomposites

The phase and crystal structures of the as-prepared sample were analyzed by XRD, and the result is shown in Figure1. All diffraction peaks are well consistent with JCPDS card no: 89-6552 of Ag1.69Sb2.27O6.25, and the intense diffraction peak at 2θ = 30.0° is corresponding to the (222) plane. Compared to the peaks of Ag/Ag1.69Sb2.27O6.25, the peak of metallic Ag is not observed, which may be due to that the amount of silver particles is too low to detect by XRD [15].Figure 1
XRD patterns of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites.Figure2 shows SEM images of Ag/Ag1.69Sb2.27O6.25 composites. As shown in Figure 2(a), it could be found that the Ag/Ag1.69Sb2.27O6.25 composites exhibit a sphere-type structure with a diameter of about 170 nm, and there exist many broken spheres, which show that they are hollow inside. Figure 2(b) is a higher-magnification SEM image obtained from a selected area of Figure 2(a). Herein, a large number of smaller particles uniformly dispersing on the surface of the hollow nanospheres can be clearly observed. The whole shape of the nanocomposites looks like a Chinese delicacy hollow-sesame-ball. The subtle structure and composition of the sesame-ball-type hollow particles were further investigated by HRTEM. Figure 2(c) shows the HRTEM image obtained from a partial area of an individual hollow sphere. There are two smaller spheres, identified as “a” and “b,” respectively, with size average 18.3 nm embedded in the particle. The crystal lattice of the “a” sphere is measured as 0.23 nm, which is corresponding to (111) lattice planes of metal Ag. Besides for Ag, there is an interplanar distance of 0.3 nm which can be observed on the individual hollow sphere, which is identical with the (222) face of silver antimonite. The elemental distribution of the silver/silver antimonite is verified by the EDS elemental mapping, as illustrated in Figure 2(d), and suggests that there exist Sb, O, Ag elements of these broken hollow spheres but with only Ag and without Sb and O of all the smaller nanospheres, indicating that Ag nanoparticles embed into the Ag1.69Sb2.27O6.25 hollow sphere to form a hollow-sesame-ball.Figure 2
(a) SEM. (b) High-magnification SEM. (c) HRTEM. (d) EDS elemental mapping images of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites.
(a)(b)(c)(d)The surface chemical state of Ag/Ag1.69Sb2.27O6.25 was investigated by XPS. The detailed XPS spectrum of Ag 3d in Figure 3(a) shows four peaks assigned to Ag+ and Ag0, respectively. The peaks of Ag 3d at round 373.9 eV and 368.0 eV are assigned to Ag+ corresponding to Ag 3d3/2 and Ag 3d5/2, respectively, whereas the peaks at round 373.3 eV and 367.9 eV are assigned to Ag0, suggesting the existence of metallic Ag [10, 20]. The O 1s XPS spectrum in Figure 3(b) shows two peaks at 530.4 eV and 530.8 eV. The former peak of 530.4 eV is derived from crystal lattice oxygen of Ag/Ag1.69Sb2.27O6.25, and the latter one comes from hydroxyl [10]. Figure 3(c) shows the Sb 3d XPS spectrum, and it consists of three fitted peaks. The peaks at round 530.4 eV and 530.5 eV are assigned to oxygen in the form of Sb3+ and Sb5+, and the peak of 539.8 eV corresponds to antimony-bonded silver [10]. All peaks of Ag 3d, O 1s, and Sb 3d show their different chemical states, revealing the hydrothermal synthesis mechanism of Ag/Ag1.69Sb2.27O6.25.Figure 3
XPS patterns of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites. (a) Ag 3d. (b) O 1s. (c) Sb 3d.
(a)(b)(c)Figure4 shows the N2 adsorption-desorption curves for Ag/Ag1.69Sb2.27O6.25 nanocomposites, and the characteristics of the adsorption curve are a classic second kind of physical adsorption curve [21] in which the adsorption capacity increases slowly in the first half of the isotherm, showing an upward convex shape, and then violently increases in the second half of the one. The BET surface area of the as-fabricated sample is 7.268 m2/g, and the pore sizes of nanoparticles, investigated by the BJH model (see the inset), are more than 5 nm.Figure 4
Nitrogen adsorption-desorption isotherms for Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites (inset, pore size distributions).The light absorption property of as-prepared materials was studied by UV-vis/DRS, and the result is shown in Figure5 in which the adsorption band is located at 445 nm, and the estimated energy band gap (Eg) is 2.55 eV according to the following formula:(1)αhυ=Ahυ−Eg1/2,where Eg, h, α, v, and A are the band gap, plank constant, absorption coefficient, light frequency, and constant, respectively.Figure 5
UV-vis DRS light adsorption spectra and band gap calculation diagram of Ag/Ag1.69Sb2.27O6.25.


### 3.2. The Formation Mechanism of the Ag/Ag1.69Sb2.27O6.25 Sesame-Ball-Type Hollow Nanocomposites

The mechanism leading to the formation of the Ag/Ag1.69Sb2.27O6.25 sesame-ball-type hollow nanocomposites is not yet clear. However, our own experimental evidence has led us to believe the formation process of Ag/Ag1.69Sb2.27O6.25, schematically illustrated in Figure 6. Firstly, potassium pyroantimonate and silver nitrate are separately dissociated into SbO6H6− and K+ (equation (2)) and Ag+ and NO3−(equation (3)) in warm aqueous solution. Lots of tiny Ag1.69Sb2.27O6.25 crystal nuclei are formed by the reaction between SbO6H6− and Ag+ (equation (4)) and preferentially adsorbed on the gas-liquid interface on the bubbles, produced in the boiling aqueous in the reclusive reactor at 400K, because of the enhanced surfce tension of gas-liquid interface layer by the presence of inorganic ions such as K+ and NO3− [21].(2)KSbO6H6=SbO6H6−+K+(3)AgNO3=NO3−+Ag+(4)SbO6H6−+Ag+⟶Ag1.69Sb2.27O6.25↓Figure 6
Schematic illustration for the formation process of Ag/Ag1.69Sb2.27O6.25 sesame-hollow-ball-type nanocomposites.Then, the generated Ag1.69Sb2.27O6.25 nucleus grows upon the ball-type interface on the bubbles and forms hollow-sphere-structure Ag1.69Sb2.27O6.25 nanoparticles. Metallic Ag nanoparticles, with size average 18.3 nm, which were reduced from Ag+ in the hydrothermal reaction system, gather on the surface of Ag1.69Sb2.27O6.25 hollow-sphere to form sesame-hollow-ball-type nanocomposites.


### 3.3. The Formation Mechanism of Ag in the Silver-Water System at 400 K

Recently, the fabrication and photocatalytic activity of Ag-based composites including Ag-containing multimetal oxides such as AgMO2 (M = Al, Cr) [22], AgGaO2 [23], and AgSbO3 [11] and the hierarchical Z-scheme photocatalysts such as g-C3N4@Ag/BiVO4 [24], Ag@AgBr/g-C3N4 [15], and Ag2CrO4/Ag/g-C3N4 [20], in which metallic Ag is used as an electron mediator, have been intensively investigated. In these studies, the action and mechanism of metallic silver have been fully studied. However, few researchers investigate the formation mechanism of reduction of silver ions, dissociated fromation silver nitrate, to silver in the hydrometallurgy system at 400 K. In the search, we construct the partial potential-pH diagram based on the thermodynamic calculation of the aqueous solution at 400 K and standard pressure (1pΘ) and reveal that when the pH value is 6.8, the oxidant Ag+ ion can react with the reductant H2 to form Ag and H+ ion.


#### 3.3.1. Thermodynamic Calculations

The following half-cell reaction represents a general electrochemical reaction in the hydrothermal reaction system [25]:(5)aA+mH++ze−=bB+cH2Owhere a, m, b, and c are the stoichiometric coefficients for species A, H+, B, and H2O in the electrochemical reaction, respectively, and z is the electron transfer number of the electrode reaction. In the dilute solution, when aH2O = 1 for the activity of water, the equilibrium potential can be simplified as follows according to the Nernst equation:(6)φT=φΘT−2.30RTmzFpH−2.30RTmzFlgaBbaAa,where φ(T) or φΘ(T) is the electrochemical potential or standard electrochemical potential for the reaction at a certain temperature T; F is Faraday’s constant; R is the molar gas constant; and aA or aB is the overall activity of species A or B.The relationship betweenφΘ(T) and standard free energy of the reaction in equation (5) is shown explicitly:(7)ΔrGmΘT=−zFφΘT,where ΔrGmΘT is the standard molar free energy of the reaction at temperature T.At arbitrary temperature and standard pressure, the thermodynamic parameters includingΔrHmΘT, ΔrSmΘT, and ΔrGmΘT of general electrochemical reaction (5) are calculated through the relationship as follows:(8)ΔrHmΘT=ΔrHmΘ298K+∫298TΔrCPTdT=∑BvBΔfHmΘB,298K+∫298T∑BvBCp,mΘB,TdT,where △fHmΘB,298K is the standard enthalpy of formation of substance B; ΔrHmΘ298K or ΔrHmΘT is the molar reaction enthalpy of the reaction at 298 K or temperature T/K under the same standard pressure; Cp,mΘB,T or ΔrCpΘT is the standard heat capacity at standard pressure of substance B or the reaction at temperature T; and νB is the stoichiometry of substance B, and νB is positive for the reactant and negative for the product.(9)ΔrSmΘT=ΔrSmΘ298K+∫298KT∑BvBCP,mBdTT=∑BvBSmB,298K+∫298KT∑BvBCP,mBdTT,where SmΘB,298K is the third law entropy of substance B at 298 K; ΔrSmΘ298K or ΔrSmΘT is the molar reaction entropy of the reaction at 298 K or temperature T/K under the same standard pressure.(10)ΔrGmΘT=ΔrHmΘT−TΔrSmΘT.The data of the thermodynamic calculation in this paper are obtained from [26], except as otherwise indicated, and the thermodynamic data of the main substances in the Ag-H2O system are listed in Table 1.Table 1
Thermodynamic data of the main substances in the Ag-H20 system.
SpeciesΔfHmΘ298K/kJ⋅mol−1SmΘ298K/J⋅mol−1⋅K−1Cp,mΘ298K/J⋅K−1⋅mol−1H2(g)0130.68428.836O2(g)0250.13829.378H2O (l)−285.83069.9175.291H+(aq)000Ag(s)042.5525.351Ag+(aq)−105.57977.10721.8In Table1, ΔfHmΘ298K is the standard enthalpy of formation at 298 K; SmΘ298K is the third law entropy at 298 K; and Cp,mΘ298K is the standard heat capacity at standard pressure at 298 K.


#### 3.3.2. The Partial Potential-pH Diagrams

The partially relevant reactions are listed in Table2. The diagrams for other values of activity and pressure can be easily obtained by pH-potential functions listed in Table 2. The partial potential-pH diagrams of the Ag-H2O system are plotted in Figure 7. It can be seen that when the pH value is 6.8 (lower than 7.0), the line of Ag+/Ag equilibrium is higher than H+/H2 equilibrium which means that, in these two electric pairs, the stronger oxidant Ag+ ion can react with the stronger reductant H2 to form the weaker reductant Ag and the weaker oxidant H+:(11)2Ag++H2=Ag↓+2H+Table 2
Electrode reactions andφ-pH equations.
Electrode reactionsφ-pH equationsφΘ400K (V)O2+4H++4e−=2H2Oφ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgaH202/pO2/pΘ=1.418V−0.0794pHwhenaH20=1andpO2=pΘ.1.4182H++2e−=H2φ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgpH2/pΘ=0.273V−0.0794pHwhenpH2=pΘ.0.273Ag++e−=Agφ400K=φΘ400K−2.303R×400/FlgCΘ/CAg=0.861VwhenCΘ=1.0mol⋅l−1andCAg=0.07mol⋅l−1.0.951Figure 7
Partial potential-pH diagrams of the Ag-H2O system at 400 K.


## 3.1. Characterizations of the Ag/Ag1.69Sb2.27O6.25 Sesame-Ball-Type Hollow Nanocomposites

The phase and crystal structures of the as-prepared sample were analyzed by XRD, and the result is shown in Figure1. All diffraction peaks are well consistent with JCPDS card no: 89-6552 of Ag1.69Sb2.27O6.25, and the intense diffraction peak at 2θ = 30.0° is corresponding to the (222) plane. Compared to the peaks of Ag/Ag1.69Sb2.27O6.25, the peak of metallic Ag is not observed, which may be due to that the amount of silver particles is too low to detect by XRD [15].Figure 1
XRD patterns of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites.Figure2 shows SEM images of Ag/Ag1.69Sb2.27O6.25 composites. As shown in Figure 2(a), it could be found that the Ag/Ag1.69Sb2.27O6.25 composites exhibit a sphere-type structure with a diameter of about 170 nm, and there exist many broken spheres, which show that they are hollow inside. Figure 2(b) is a higher-magnification SEM image obtained from a selected area of Figure 2(a). Herein, a large number of smaller particles uniformly dispersing on the surface of the hollow nanospheres can be clearly observed. The whole shape of the nanocomposites looks like a Chinese delicacy hollow-sesame-ball. The subtle structure and composition of the sesame-ball-type hollow particles were further investigated by HRTEM. Figure 2(c) shows the HRTEM image obtained from a partial area of an individual hollow sphere. There are two smaller spheres, identified as “a” and “b,” respectively, with size average 18.3 nm embedded in the particle. The crystal lattice of the “a” sphere is measured as 0.23 nm, which is corresponding to (111) lattice planes of metal Ag. Besides for Ag, there is an interplanar distance of 0.3 nm which can be observed on the individual hollow sphere, which is identical with the (222) face of silver antimonite. The elemental distribution of the silver/silver antimonite is verified by the EDS elemental mapping, as illustrated in Figure 2(d), and suggests that there exist Sb, O, Ag elements of these broken hollow spheres but with only Ag and without Sb and O of all the smaller nanospheres, indicating that Ag nanoparticles embed into the Ag1.69Sb2.27O6.25 hollow sphere to form a hollow-sesame-ball.Figure 2
(a) SEM. (b) High-magnification SEM. (c) HRTEM. (d) EDS elemental mapping images of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites.
(a)(b)(c)(d)The surface chemical state of Ag/Ag1.69Sb2.27O6.25 was investigated by XPS. The detailed XPS spectrum of Ag 3d in Figure 3(a) shows four peaks assigned to Ag+ and Ag0, respectively. The peaks of Ag 3d at round 373.9 eV and 368.0 eV are assigned to Ag+ corresponding to Ag 3d3/2 and Ag 3d5/2, respectively, whereas the peaks at round 373.3 eV and 367.9 eV are assigned to Ag0, suggesting the existence of metallic Ag [10, 20]. The O 1s XPS spectrum in Figure 3(b) shows two peaks at 530.4 eV and 530.8 eV. The former peak of 530.4 eV is derived from crystal lattice oxygen of Ag/Ag1.69Sb2.27O6.25, and the latter one comes from hydroxyl [10]. Figure 3(c) shows the Sb 3d XPS spectrum, and it consists of three fitted peaks. The peaks at round 530.4 eV and 530.5 eV are assigned to oxygen in the form of Sb3+ and Sb5+, and the peak of 539.8 eV corresponds to antimony-bonded silver [10]. All peaks of Ag 3d, O 1s, and Sb 3d show their different chemical states, revealing the hydrothermal synthesis mechanism of Ag/Ag1.69Sb2.27O6.25.Figure 3
XPS patterns of Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites. (a) Ag 3d. (b) O 1s. (c) Sb 3d.
(a)(b)(c)Figure4 shows the N2 adsorption-desorption curves for Ag/Ag1.69Sb2.27O6.25 nanocomposites, and the characteristics of the adsorption curve are a classic second kind of physical adsorption curve [21] in which the adsorption capacity increases slowly in the first half of the isotherm, showing an upward convex shape, and then violently increases in the second half of the one. The BET surface area of the as-fabricated sample is 7.268 m2/g, and the pore sizes of nanoparticles, investigated by the BJH model (see the inset), are more than 5 nm.Figure 4
Nitrogen adsorption-desorption isotherms for Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites (inset, pore size distributions).The light absorption property of as-prepared materials was studied by UV-vis/DRS, and the result is shown in Figure5 in which the adsorption band is located at 445 nm, and the estimated energy band gap (Eg) is 2.55 eV according to the following formula:(1)αhυ=Ahυ−Eg1/2,where Eg, h, α, v, and A are the band gap, plank constant, absorption coefficient, light frequency, and constant, respectively.Figure 5
UV-vis DRS light adsorption spectra and band gap calculation diagram of Ag/Ag1.69Sb2.27O6.25.


## 3.2. The Formation Mechanism of the Ag/Ag1.69Sb2.27O6.25 Sesame-Ball-Type Hollow Nanocomposites

The mechanism leading to the formation of the Ag/Ag1.69Sb2.27O6.25 sesame-ball-type hollow nanocomposites is not yet clear. However, our own experimental evidence has led us to believe the formation process of Ag/Ag1.69Sb2.27O6.25, schematically illustrated in Figure 6. Firstly, potassium pyroantimonate and silver nitrate are separately dissociated into SbO6H6− and K+ (equation (2)) and Ag+ and NO3−(equation (3)) in warm aqueous solution. Lots of tiny Ag1.69Sb2.27O6.25 crystal nuclei are formed by the reaction between SbO6H6− and Ag+ (equation (4)) and preferentially adsorbed on the gas-liquid interface on the bubbles, produced in the boiling aqueous in the reclusive reactor at 400K, because of the enhanced surfce tension of gas-liquid interface layer by the presence of inorganic ions such as K+ and NO3− [21].(2)KSbO6H6=SbO6H6−+K+(3)AgNO3=NO3−+Ag+(4)SbO6H6−+Ag+⟶Ag1.69Sb2.27O6.25↓Figure 6
Schematic illustration for the formation process of Ag/Ag1.69Sb2.27O6.25 sesame-hollow-ball-type nanocomposites.Then, the generated Ag1.69Sb2.27O6.25 nucleus grows upon the ball-type interface on the bubbles and forms hollow-sphere-structure Ag1.69Sb2.27O6.25 nanoparticles. Metallic Ag nanoparticles, with size average 18.3 nm, which were reduced from Ag+ in the hydrothermal reaction system, gather on the surface of Ag1.69Sb2.27O6.25 hollow-sphere to form sesame-hollow-ball-type nanocomposites.


## 3.3. The Formation Mechanism of Ag in the Silver-Water System at 400 K

Recently, the fabrication and photocatalytic activity of Ag-based composites including Ag-containing multimetal oxides such as AgMO2 (M = Al, Cr) [22], AgGaO2 [23], and AgSbO3 [11] and the hierarchical Z-scheme photocatalysts such as g-C3N4@Ag/BiVO4 [24], Ag@AgBr/g-C3N4 [15], and Ag2CrO4/Ag/g-C3N4 [20], in which metallic Ag is used as an electron mediator, have been intensively investigated. In these studies, the action and mechanism of metallic silver have been fully studied. However, few researchers investigate the formation mechanism of reduction of silver ions, dissociated fromation silver nitrate, to silver in the hydrometallurgy system at 400 K. In the search, we construct the partial potential-pH diagram based on the thermodynamic calculation of the aqueous solution at 400 K and standard pressure (1pΘ) and reveal that when the pH value is 6.8, the oxidant Ag+ ion can react with the reductant H2 to form Ag and H+ ion.


### 3.3.1. Thermodynamic Calculations

The following half-cell reaction represents a general electrochemical reaction in the hydrothermal reaction system [25]:(5)aA+mH++ze−=bB+cH2Owhere a, m, b, and c are the stoichiometric coefficients for species A, H+, B, and H2O in the electrochemical reaction, respectively, and z is the electron transfer number of the electrode reaction. In the dilute solution, when aH2O = 1 for the activity of water, the equilibrium potential can be simplified as follows according to the Nernst equation:(6)φT=φΘT−2.30RTmzFpH−2.30RTmzFlgaBbaAa,where φ(T) or φΘ(T) is the electrochemical potential or standard electrochemical potential for the reaction at a certain temperature T; F is Faraday’s constant; R is the molar gas constant; and aA or aB is the overall activity of species A or B.The relationship betweenφΘ(T) and standard free energy of the reaction in equation (5) is shown explicitly:(7)ΔrGmΘT=−zFφΘT,where ΔrGmΘT is the standard molar free energy of the reaction at temperature T.At arbitrary temperature and standard pressure, the thermodynamic parameters includingΔrHmΘT, ΔrSmΘT, and ΔrGmΘT of general electrochemical reaction (5) are calculated through the relationship as follows:(8)ΔrHmΘT=ΔrHmΘ298K+∫298TΔrCPTdT=∑BvBΔfHmΘB,298K+∫298T∑BvBCp,mΘB,TdT,where △fHmΘB,298K is the standard enthalpy of formation of substance B; ΔrHmΘ298K or ΔrHmΘT is the molar reaction enthalpy of the reaction at 298 K or temperature T/K under the same standard pressure; Cp,mΘB,T or ΔrCpΘT is the standard heat capacity at standard pressure of substance B or the reaction at temperature T; and νB is the stoichiometry of substance B, and νB is positive for the reactant and negative for the product.(9)ΔrSmΘT=ΔrSmΘ298K+∫298KT∑BvBCP,mBdTT=∑BvBSmB,298K+∫298KT∑BvBCP,mBdTT,where SmΘB,298K is the third law entropy of substance B at 298 K; ΔrSmΘ298K or ΔrSmΘT is the molar reaction entropy of the reaction at 298 K or temperature T/K under the same standard pressure.(10)ΔrGmΘT=ΔrHmΘT−TΔrSmΘT.The data of the thermodynamic calculation in this paper are obtained from [26], except as otherwise indicated, and the thermodynamic data of the main substances in the Ag-H2O system are listed in Table 1.Table 1
Thermodynamic data of the main substances in the Ag-H20 system.
SpeciesΔfHmΘ298K/kJ⋅mol−1SmΘ298K/J⋅mol−1⋅K−1Cp,mΘ298K/J⋅K−1⋅mol−1H2(g)0130.68428.836O2(g)0250.13829.378H2O (l)−285.83069.9175.291H+(aq)000Ag(s)042.5525.351Ag+(aq)−105.57977.10721.8In Table1, ΔfHmΘ298K is the standard enthalpy of formation at 298 K; SmΘ298K is the third law entropy at 298 K; and Cp,mΘ298K is the standard heat capacity at standard pressure at 298 K.


### 3.3.2. The Partial Potential-pH Diagrams

The partially relevant reactions are listed in Table2. The diagrams for other values of activity and pressure can be easily obtained by pH-potential functions listed in Table 2. The partial potential-pH diagrams of the Ag-H2O system are plotted in Figure 7. It can be seen that when the pH value is 6.8 (lower than 7.0), the line of Ag+/Ag equilibrium is higher than H+/H2 equilibrium which means that, in these two electric pairs, the stronger oxidant Ag+ ion can react with the stronger reductant H2 to form the weaker reductant Ag and the weaker oxidant H+:(11)2Ag++H2=Ag↓+2H+Table 2
Electrode reactions andφ-pH equations.
Electrode reactionsφ-pH equationsφΘ400K (V)O2+4H++4e−=2H2Oφ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgaH202/pO2/pΘ=1.418V−0.0794pHwhenaH20=1andpO2=pΘ.1.4182H++2e−=H2φ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgpH2/pΘ=0.273V−0.0794pHwhenpH2=pΘ.0.273Ag++e−=Agφ400K=φΘ400K−2.303R×400/FlgCΘ/CAg=0.861VwhenCΘ=1.0mol⋅l−1andCAg=0.07mol⋅l−1.0.951Figure 7
Partial potential-pH diagrams of the Ag-H2O system at 400 K.


## 3.3.1. Thermodynamic Calculations

The following half-cell reaction represents a general electrochemical reaction in the hydrothermal reaction system [25]:(5)aA+mH++ze−=bB+cH2Owhere a, m, b, and c are the stoichiometric coefficients for species A, H+, B, and H2O in the electrochemical reaction, respectively, and z is the electron transfer number of the electrode reaction. In the dilute solution, when aH2O = 1 for the activity of water, the equilibrium potential can be simplified as follows according to the Nernst equation:(6)φT=φΘT−2.30RTmzFpH−2.30RTmzFlgaBbaAa,where φ(T) or φΘ(T) is the electrochemical potential or standard electrochemical potential for the reaction at a certain temperature T; F is Faraday’s constant; R is the molar gas constant; and aA or aB is the overall activity of species A or B.The relationship betweenφΘ(T) and standard free energy of the reaction in equation (5) is shown explicitly:(7)ΔrGmΘT=−zFφΘT,where ΔrGmΘT is the standard molar free energy of the reaction at temperature T.At arbitrary temperature and standard pressure, the thermodynamic parameters includingΔrHmΘT, ΔrSmΘT, and ΔrGmΘT of general electrochemical reaction (5) are calculated through the relationship as follows:(8)ΔrHmΘT=ΔrHmΘ298K+∫298TΔrCPTdT=∑BvBΔfHmΘB,298K+∫298T∑BvBCp,mΘB,TdT,where △fHmΘB,298K is the standard enthalpy of formation of substance B; ΔrHmΘ298K or ΔrHmΘT is the molar reaction enthalpy of the reaction at 298 K or temperature T/K under the same standard pressure; Cp,mΘB,T or ΔrCpΘT is the standard heat capacity at standard pressure of substance B or the reaction at temperature T; and νB is the stoichiometry of substance B, and νB is positive for the reactant and negative for the product.(9)ΔrSmΘT=ΔrSmΘ298K+∫298KT∑BvBCP,mBdTT=∑BvBSmB,298K+∫298KT∑BvBCP,mBdTT,where SmΘB,298K is the third law entropy of substance B at 298 K; ΔrSmΘ298K or ΔrSmΘT is the molar reaction entropy of the reaction at 298 K or temperature T/K under the same standard pressure.(10)ΔrGmΘT=ΔrHmΘT−TΔrSmΘT.The data of the thermodynamic calculation in this paper are obtained from [26], except as otherwise indicated, and the thermodynamic data of the main substances in the Ag-H2O system are listed in Table 1.Table 1
Thermodynamic data of the main substances in the Ag-H20 system.
SpeciesΔfHmΘ298K/kJ⋅mol−1SmΘ298K/J⋅mol−1⋅K−1Cp,mΘ298K/J⋅K−1⋅mol−1H2(g)0130.68428.836O2(g)0250.13829.378H2O (l)−285.83069.9175.291H+(aq)000Ag(s)042.5525.351Ag+(aq)−105.57977.10721.8In Table1, ΔfHmΘ298K is the standard enthalpy of formation at 298 K; SmΘ298K is the third law entropy at 298 K; and Cp,mΘ298K is the standard heat capacity at standard pressure at 298 K.


## 3.3.2. The Partial Potential-pH Diagrams

The partially relevant reactions are listed in Table2. The diagrams for other values of activity and pressure can be easily obtained by pH-potential functions listed in Table 2. The partial potential-pH diagrams of the Ag-H2O system are plotted in Figure 7. It can be seen that when the pH value is 6.8 (lower than 7.0), the line of Ag+/Ag equilibrium is higher than H+/H2 equilibrium which means that, in these two electric pairs, the stronger oxidant Ag+ ion can react with the stronger reductant H2 to form the weaker reductant Ag and the weaker oxidant H+:(11)2Ag++H2=Ag↓+2H+Table 2
Electrode reactions andφ-pH equations.
Electrode reactionsφ-pH equationsφΘ400K (V)O2+4H++4e−=2H2Oφ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgaH202/pO2/pΘ=1.418V−0.0794pHwhenaH20=1andpO2=pΘ.1.4182H++2e−=H2φ400K=φΘ400K−2.303R×400/FpH−2.303R×400/FlgpH2/pΘ=0.273V−0.0794pHwhenpH2=pΘ.0.273Ag++e−=Agφ400K=φΘ400K−2.303R×400/FlgCΘ/CAg=0.861VwhenCΘ=1.0mol⋅l−1andCAg=0.07mol⋅l−1.0.951Figure 7
Partial potential-pH diagrams of the Ag-H2O system at 400 K.


## 4. Conclusions

In summary, a facile one-step hydrothermal method was developed to synthesize Ag/Ag1.69Sb2.27O6.25 sesame-hollow-ball-type nanocomposites in large quantities at 400 K. Structural characterization technique indicates that large-quantity smaller, about 18.3 nm, metallic Ag particles uniformly dense on the surface of ∼170 nm Ag1.69Sb2.27O6.25 hollow nanospheres to form the novel Ag/Ag1.69Sb2.27O6.25 hollow-sesame-ball nanocomposites. The calculated partial φ-pH diagrams predicted that the Ag+ ion can react with H2 to form Ag in the Ag-H2O system at 400 K, which will provide new deep insights into the silver-containing photocatalyst with enhanced photocatalytic activity.



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*Source: 1582861-2020-11-30.xml*