Ag2CO3 Three-Dimensional

Research Article pubs.acs.org/journal/ascecg

A Facile Method To Prepare Novel Ag2O/Ag2CO3 Three-Dimensional Hollow Hierarchical Structures and Their Water Purification Function Xiaole Zhao,† Yingchun Su,† Xiaodong Qi,‡ and Xiaojun Han*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, and ‡Key Laboratory of Microsystems and Micronanostructures Manufacturing, Harbin Institute of Technology, 92 West Da-Zhi Street, Harbin 150001, China S Supporting Information *

ABSTRACT: A facile, green, and low-cost method to prepare novel Ag2O/Ag2CO3 three-dimensional hollow flowerlike hierarchical microspheres has been demonstrated. The microspheres were obtained by simple injection of Na2CO3 aqueous solution into AgNO3 ethanol solution. The surface morphology of the microspheres was relatively smooth, trumpet flowerlike hole, and flowerlike shallow hole when the reaction temperature was 8, 25, and 70 °C, respectively. They were assembled by nanoparticles. The microsphere prepared at 25 °C was composed of 86.3% Ag2CO3 and 13.7% Ag2O. The HTEM image indicates that the Ag2CO3 microsphere is wrapped by a thin Ag2O layer, and there is a heterojunction between Ag2CO3 and Ag2O. The study of the formation mechanism demonstrates the first example of using water-soluble Na2CO3 as templates to prepare 3D hierarchical microstructures. The Ag2O/Ag2CO3 microspheres prepared at 25 °C have excellent adsorption and photocatalysis activity for Congo red (CR). The adsorption behavior of the Ag2O/Ag2CO3 microsphere follows the Langmuir isothermal adsorption model. The maximal adsorption capacity of the Ag2O/Ag2CO3 microsphere was about 100.84 mg g−1 for CR in water. Here, 87.5% of CR was removed within 20 min under visible light irradiation at an initial concentration of CR of 80 mg L−1 with a photodegradation rate constant, k, of 0.06814. The radicals (O2·−,·OH) and hole (h+) were proved to be the active species for photocatalyzing CR. The microspheres were reused five times without distinct loss of photocatalysis activity, which indicated that they have stable photocatalysis activity. KEYWORDS: One step, Na2CO3 template, Hierarchical microspheres, Ag2O/Ag2CO3, Water treatment

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INTRODUCTION In recent years, three-dimensional (3D) hierarchical nanostructures exhibited unique properties and potential applications.1−3 In the energy field, they were used as supercapacitors,4 fuel cells,5 and catalysis6−9 materials. They were also exploited for gas detection,10 CO2 reduction,11 and treatment of water pollutants.12 Various approaches were developed to synthesize 3D hierarchical nanostructures, such as polymer-assisted selfassembly,13 antisolvent crystallization,14 and the widely used classical hydrothermal and solvothermal synthesis method.4,15,16 Generally, those methods need toxic organic ligands, solvent, high pressure, or tedious procedures. Moreover, they were environmentally harmful and consumed a lot of energy. Those disadvantages greatly limit their practical production and application. Therefore, it is highly desirable but challenging to develop a facile and environmentaly friendly method instead of a traditional strategy to produce 3D hierarchical nanostructures. Ag2CO3 catalysts have drawn more and more attention because of their excellent catalytic activity in organic synthesis17,18 and visible light photodegradation of organic contaminants19,20 in environmental fields. In previous reports, © 2017 American Chemical Society

most of Ag2CO3 catalysts were single micron-sized crystalline grains20−22 or nanorods23 without a particular 3D hierarchical nanostructure, and they displayed deactivated photocatalic properties due to their photocorrosion.22,24 In order to significantly improve Ag2CO3 particle adsorption performance and solve a serious photocorrosion problem, different strategies have been proposed, such as adding inhibitors in the photocatalytic reaction system25,26 or coupling Ag2CO3 with other materials like graphene oxide,27 Ag,28,29 and UiO-66.30 However, these solutions are complicated and expensive. In this paper, we developed an environmentally friendly method to synthesize Ag2O/Ag2CO3 3D hollow hierarchical structures by injecting an Na2CO3 aqueous solution into a homogeneous AgNO3 ethanol solution at 25 °C. This method is simple, nontoxic, and low energy consumption. The 3D hollow hierarchical structures are assembled by ∼30 nm Ag2O/ Ag2CO3 nanoparticles. These particles possess great stable Received: April 6, 2017 Revised: May 19, 2017 Published: May 30, 2017 6148

DOI: 10.1021/acssuschemeng.7b01040 ACS Sustainable Chem. Eng. 2017, 5, 6148−6158

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ACS Sustainable Chemistry & Engineering

Figure 1. XRD pattern (a) and its partial enlarged detail (b) of products. XPS spectra of survey (c), O 1s (d), and Ag 3d (e) in the products. TGA, DTG profile (f) of products.

respectively. Figure 1c displays the survey spectrum of the asobtained Ag2O/Ag2CO3 products, which exhibit peaks of Ag 3p, Ag 3d, C 1s, and O 1s. The O 1s peaks at 529.7 and 531.2 eV are attributed to the O element in Ag2O and Ag2CO3, respectively.32 The Ag 3d3/2 and Ag 3d5/2 peaks at 367.9 and 374.0 eV, respectively, are highly consistent with the values reported for Ag+.32 Combined with HRTEM (Figure 2f) and XRD data (Figure 1a, b), Ag2O and Ag2CO3 were confirmed to form microstructures. The morphology and size of Ag2O/Ag2CO3 microstructures were investigated by SEM and TEM. SEM images (Figure 2a, b) and the TEM image (Figure 2c) show that they are hollow flowerlike microspheres. Their average diameter is 8.3 μm (Figure S2) from diameter distribution and fitted curves of Ag2O/Ag2CO3 microstructures. This is an amazing result because nobody can directly make Ag2O/Ag2CO3 into a microsphere yet, let alone into beautiful flowerlike structures. More surprisingly, when the single microsphere is magnified (Figure 2b), it is noted that the microsphere surface blooms many uniform and ordered arrangement petals. One magnified hole (as shown in Figure 2e) is like a trumpet flower, as compared with a real trumpet flower shown in the inset picture. The inner wall of the hole is very rough, and the depth of the hole is ∼1 μm. The external diameter of the trumpet flowerlike hole is about ∼1um. Previously reported methods for flowerlike structures are usually complicated,13,15,16,33−35 while our protocol is very simple with only a one-step process. The

abilities in organic dye adsorption and visible light photodegradation activity of organic dyes in wastewater, which indicates their great potential in water purification.

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RESULTS AND DISCUSSION Preparation and Characterization of Ag2O/Ag2CO3 3D Hollow Hierarchical Structures. The products produced at 25 °C were measured with an XRD instrument. Their XRD patterns, as shown in Figure 1a, indicates that the major composition of the product is Ag2CO3 (JCPDS no. 70-2184). Figure 1b is a partial enlargement of an XRD spectrum showing a small amount of Ag2O (JCPDS no. 41-1104) in the samples. To determine the respective percentage of Ag2CO3 and Ag2O, thermal gravimetric analysis (TGA) was carried out as shown in Figure 1f. TGA curve indicates two weight loss regions from 130 to 230 °C and from 250 to 425 °C, respectively. The two peaks of the DTG curve represent the maximum decomposition rate of the product at 185 and 375 °C, respectively. After 435 °C, the weight of the substrate is kept constant. Total weight loss is 19.7%. According to a previous report,31 the first weight loss region represents the decomposition of Ag2CO3 into Ag2O, and the second region represents further reduction of Ag2O into Ag. The XRD pattern of the products calcined at 400 °C for 1 h also confirms the conversion of Ag2CO3 into Ag (JCPDS no. 99-0094) (Figure S1). According to the weight loss percentages in the two temperature ranges, we can calculate the percentages of Ag2CO3 and Ag2O to be 86.3% and 13.7%, 6149


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Figure 2. Different magnification SEM (a, b, d, e) images and TEM (c, f) images of the as-obtained Ag2O/Ag2CO3 produced at 25 °C. Images g, h, and i are EDX componential maps of product for C, O, and Ag, respectively. Inset images of (b) and (e) are the simulation image and a real trumpet flower picture of the Ag2O/Ag2CO3 microsphere, respectively.

rough inner wall is assembled by many nanoparticles. The nanoparticle size is ∼30 nm from the magnified image of the inner wall (Figure 2d). The HTEM image (Figure 2f) implies that the Ag2CO3 microsphere is wrapped by a thin Ag2O layer, and there is a heterojunction between Ag2CO3 and Ag2O. Figure 2g, h, and i are EDX componential maps of the single sample, which represent C, O, and Ag element distribution, respectively. The EDX spectroscopy spectrum of this product is shown in Figure S3. Morphology Control of Ag2O/Ag2CO3 3D Hollow Hierarchical Structures. In order to investigate the influence of the reaction temperature on the product morphology, a series of experiments were carried out at different reaction temperatures while keeping other conditions unchanged. Reaction temperatures of 8 and 70 °C were chosen to study the morphology of the products. When the sample solutions were cooled to 8 °C, they were mixed rapidly and kept at 8 °C for another 30 min. From SEM images, it is noted that hollow microspheres (Figure 3a, b) were obtained, but the trumpet flowerlike structure disappeared. The surfaces of the products are also hierarchitectures and assembled with nanoparticles (Figure 3d). When the sample solutions were heated to 70 °C, the products are also hollow flowerlike microspheres similar to products made at 25 °C, but the holes are less pronounced. The magnified image (Figure 3g) indicates that the microspheres are assembled with nanoparticles. The nanoparticle size is nonuniform and bigger than those in the products prepared

Figure 3. SEM and simulation images of products at reaction temperatures of 8 °C (a, b, c, d) and 70 °C (e, f, g).

at 8 °C (Figure 3d) and 25 °C (Figure 2d). The hole is shallower than that in the product prepared at 25 °C (Figure 6150


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Figure 4. XRD patterns and SEM images of the products obtained by adding Na2CO3 aqueous solution into anhydrous ethanol (a, c, d, e), and into AgNO3 ethanol solution (b, f, g, h), respectively. Schematic illustration (i, j, k) of the formation process of Ag2O/Ag2CO3 microspheres produced at 25 °C.

M Na2CO3 aqueous solution was rapidly injected into 15 mL of anhydrous ethanol at 25 °C. Instantaneously, a vast white precipitation was generated in solution due to the less solubility of Na2CO3 in the ethanol solution. The XRD pattern (Figure 4a) indicates that the products are pure sodium carbonate (Na2CO3) (JCPDS no. 72-0628). SEM images (Figure 4c, d, e) show a similar flowerlike structure to that of the Ag2O/Ag2CO3 microspheres at 25 °C (Figure 2b). The difference is that the inner wall of the single petal was very smooth (Figure 4e). This similarity implies a templating function of the Na2CO3 microstructure for Ag2O/Ag2CO3 microspheres. When the ethanol contains AgNO3, the products after washing with ethanol shown in Figure 4f, g and h were obtained. After washing with ethanol, the residues of AgNO3 on the

2e). We also measured their constituents and element distribution. The EDX componential maps and XRD patterns are shown in Figures S4 and S5. We found that both products prepared at 70 and 8 °C contained evenly distributed C, O, and Ag elements. Both XRD patterns of these two products indicate that the major composition is also Ag2CO3 (JCPDS no. 702184) with a small amount of Ag2O (JCPDS no. 41-1104). Possible Formation Mechanism of Ag2O/Ag2CO3 3D Hollow Hierarchical Structures. Our method for microstructure preparation is very simple with the injection of an Na2CO3 aqueous solution into an AgNO3 ethanol solution followed by washing the precipitations with ethanol and water subsequently. In order to understand its mechanism, the following experiments were carried out. Here, 100 μL of a 1.35 6151


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Figure 5. UV−vis absorption spectra of CR solutions after being treated by different amounts of Ag2O/Ag2CO3 microspheres (a) and different times at the concentration of microspheres of 1.2 g L−1 (b). (c) Residue concentration and remove rate of CR as a function of adsorption time. (d) Amount of adsorbed CR against adsorption time. (e) Pseudo-first-order and pseudo-second-order kinetics fitting. (f) Adsorption isotherm curves for the adsorption of CR on the Ag2O/Ag2CO3 microspheres. The initial concentration of CR is 80 mg L−1 (a, b, c, d, e), and the dosage of Ag2O/ Ag2CO3 microspheres is 1.2 g L−1 (b, c, d, e, f).

microspheres were removed. They have a similar structure as the Na2CO3 microstructure; however, the XRD result (Figure 4b) reveals that there are two phases of the product including AgNaCO3 (JCPDS no. 30-1141) and Na2CO3 (JCPDS no. 720628), which indicates that AgNaCO3 tends to crystallize in the ethanol environment. AgNaCO3/Na2CO3 microstructures, as precursors, became Ag2O/Ag2CO3 microspheres after washing with water (Figures 1 and 2). On the basis of the above-mentioned experimental observations and analysis, the possible formation mechanism of Ag2O/Ag2CO3 microspheres is illustrated in Figure 4i, j, and k. When an Na2CO3 aqueous solution was rapidly injected into an AgNO3 anhydrous ethanol solution, a number of water droplets were generated in an ethanol solution.36 Due to the miscible property between water and ethanol, CO32− and Na+ ions along with water diffuse out, while Ag+ ions along with ethanol diffuse into the water droplets (Figure 4i), thereby forming an intersection region of CO32−/Ag+/Na+/ethanol/ water at the interface (Figure 4j).37 Because water droplets (0.1 mL) were surrounded with a large amount of ethanol (15 mL), this lead to the ethanol concentration rapidly increasing in the intersection region. So, crystallization rapidly occurs in the intersection region, and AgNaCO3/ Na2CO3 precursors were obtained through the templating function of the Na2CO3

microstructure, as shown in Figure 4k. The morphology similarity between AgNaCO3/Na2CO3 and Na2CO3 microstructures obtained at 8 °C (Figure S6a, c), 25 °C (Figure 4h, e), and 70 °C (Figure S6b, d) confirmed the templating function of the Na2CO3 microstructure. When treating precursors with water, Ag+ reacts with CO32− to form Ag2CO3 and OH− ions react with Ag+ to form AgOH in the basic solution due to the dissolution of Na2CO3 in water, which turn into Ag2O subsequently. Adsorption Activity of Ag2O/Ag2CO3 Microstructures. Because Ag2O/Ag2CO3 microspheres produced at 25 °C possess a special structure and can be prepared at room temperature and atmosphere pressure, they were used to treat organic pollutants in water to investigate their adsorption and photocatalysis activity. Congo red was used as a model pollutant. Figure 5a shows the adsorption spectra of CR solutions after being treated by different amounts of Ag2O/Ag2CO3 microspheres with the initial CR concentration fixed at 80 mg L−1. With an increase in the concentration of microspheres, the amount of CR left in the solution became less and less. Here, 83.2% of the CR was removed with a concentration of microspheres of 1.2 g L−1. Maybe the good adsorption capacity of the microspheres is due to their unique porous structure. 6152


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ACS Sustainable Chemistry & Engineering Table 1. Kinetic Parameters for Adsorption of CR on Ag2O/Ag2CO3 Microspheres pseudo-first-order

pseudo-second-order

c0 (mg L−1)

qe,exp (mg g−1)

k1 (min−1)

qe,cal (mg g−1)

R2

k2 (g mg−1 min−1)

qe,cal (mg g−1)

R2

80

56.78

0.0228

19.89

0.9599

0.0028

57.87

0.9989

Figure 6. Photodegradation curves (a). Photodegradation kinetics of CR under visible light irradiation (b). Photocatalysis stability test of Ag2O/ Ag2CO3 microspheres against CR under visible light irradiation (c). Photodegradation curves of phenol and TC under visible light irradiation (d).

coefficients (R2 > 0.9989) and similarity of qe,cal (57.87 mg g−1) and qe,exp. This indicates that CR adsorption kinetics follows the pseudo-second-order model. In order to understand adsorption behavior, the adsorption isotherms of CR were obtained at the concentration of microspheres of 1.2 g L−1 as shown in Figure 5f. The Langmuir (eq 3) and Freundlich (eq 4) isotherm models were commonly used to describe adsorption behavior.

The adsorption kinetics of CR on microspheres was also investigated at the concentration of 1.2 g L−1, as shown in Figure 5b, c, d, and e. Generally speaking, the amount of CR in the solution became less against adsorption time (Figure 5b) and leveled off at 80 min (Figure 5c). After adsorption for 180 min, the removal percentage of CR was 83.2%. Figure 5d represents the amount of adsorbed CR at different times. At equilibrium state, the amount of adsorbed CR was 56.78 mg g−1 (Figure 5d). To understand the characteristics of the adsorption process, the kinetics of CR adsorption was investigated by using two well-known pseudo-first-order (eq 1) and pseudo-second-order (eq 2) kinetic models. log(qe − qt) = log qe − t 1 t = + 2 qt qe k 2qe

k1t 2.303

Ce C 1 = + e qe bqm qm log qe = log K f +

(1)

(3)

1 log Ce n

(4)

where qm (mg g−1) is the maximum adsorption capacity corresponding to complete monolayer coverage, b is the equilibrium constant (L mg−1), Kf is roughly an indicator of the adsorption capacity, and n is the adsorption intensity. The adsorption data of CR is found to fit better to the Langmuir model with R2 = 0.9806 than to the Freundlich model with R2 = 0.8471, as shown in Table S1, suggesting the monolayer adsorption behavior of CR on the microspheres. The maximal adsorption capacity of Ag2O/Ag2CO3 microspheres was about 100.84 mg g−1 for CR. This is the first time to report that Ag2O/Ag2CO3 microspheres have high efficiency for the removal of CR in water through the adsorption process. This is maybe attributed to the special architectures of Ag2O/ Ag2CO3 leading to an increase in specific surface area. As shown in Figure S7, nitrogen adsorption−desorption measurements give the specific surface area of Ag 2 O/Ag 2 CO 3

(2)

where qe and qt (mg g−1) are the amounts of CR adsorbed at equilibrium at a fixed initial CR concentration and at any time t (min), respectively. Here, k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The kinetic parameters and the correlation coefficients (R2) can be determined by linear regression (Figure 5e), as summarized in Table 1. From Table 1, the large differences between qe values obtained from experiments (56.78 mg g−1) and using the pseudo-first-order model (19.89 mg g−1) suggests that the adsorption kinetics of CR on microspheres do not obey the pseudo-first-order model. In contrast, the experimental data fit well to the pseudo-second-order model with high correlation 6153


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ACS Sustainable Chemistry & Engineering Table 2. Summary of Reported Photocataysts for Degradation of Congo Red in Recent Years photocatalyst Ag2O/Ag2CO3 ZnS-Co Sm(OH)3 chitosan/nano-CdS

[catalyst] 0.8 0.8 1.0 1.5

g/L g/L g/L g/L

pollutant Congo Congo Congo Congo

red red red red

[pollutant]

removal %

light source

time

refs

80 mg/L 5 mg/L 30 mg/L 20 mg/L

87.5 94 100 85.9%

visible light UV solar light visible light

20 min 120 min 340 min 180 min

this work 41 42 43

Figure 7. (a) Effects of various scavengers on the degradation of CR over Ag2O/Ag2CO3 microspheres. Mott−Schottky plots at frequencies of 2 and 3 kHz for Ag2CO3 (b) and Ag2O (c) in 0.5 M Na2SO4 at pH 7. The x-intercept of the linear region (red dotted line) shows the measured flat-band potential. (d) Possible photocatalysis mechanism of Ag2O/Ag2CO3 toward organic pollutants under visible light irradiation.

microspheres to be 10.946 m2 g −1, which is 7 times than the value of 1.5 m2 g −1of Ag2CO3 powders.38 Visible Light Photocatalysis Activity of Ag2O/Ag2CO3 Microstructures. To demonstrate the photocatalytic activity of obtained Ag2O/Ag2CO3 microspheres as a novel integrated photocatalytic adsorbent, photodegradation experiments of CR was carried out under visible light illumination. The suspensions of CR and microspheres were magnetically stirred in the dark for 120 min to establish adsorption/desorption equilibrium of CR on the catalysts. Ag2O/Ag2CO3 microspheres showed significant photocatalytic efficiency for CR. As shown in Figure 6a, the blue line is the result of catalysts (0.8 g L−1) in a CR solution (80 mg L−1). After magnetically stirring in the dark for 60 min, the adsorption/desorption equilibrium of CR on the catalysts was established since the adsorption rate of CR maintained almost a straight line after stirring in the dark for another 60 min. Starting from this moment, visible light was turned on. The red line rapidly decreased from 0.4 to 0.05 within 20 min, followed by complete degradation in 3 h. The black line in Figure 6a was the result of a control experiment without catalysts. All of the above-mentioned results imply the excellent photocatalytic property of Ag2O/Ag2CO3 microspheres. Pseudo-first-order reaction kinetics (eq 5) was applied to simulate the experimental data.39,40

−ln(C /C0) = kt

microspheres were carried out as shown in Figure 6c. The recovered Ag2O/Ag2CO3 sample for the degradation of CR exhibits high photocatalysis stability and good recyclability as the removal percentage is just a slight decline even after five cycling tests from 95% for the first run to 87% for the fifth run. The performances of other photocatalysts for CR removal are listed in Table 2. From Table 2, it is noted that our catalyst displayed higher removal rate and shortor removal time than other catalysts with similar/higher concentrations of catalyst. Therefore, our microstructures possess an excellent visible light photocatalysis property of organic pollutants. Phenol and tetracycline (TC) are also chosen as the degradation targets. The initial concentration of phenol and TC is 10 mg/L. The suspension is irradiated under visible light for 75 min after stirring for 60 min in a dark environment. The results are shown in Figure 6d. The photodegradation rates of phenol and TC are 100% after 75 min irradiation. These results clearly demonstrated that Ag2O/Ag2CO3 microspheres can be used as an efficient integrated photocatalytic adsorbent for the purification of contaminated water. The removal rates of CR are 54%, 60%, and 58% for the microstructures obtained from 8, 25, and 70 °C respectively, which indicates that the adsorption activity of the product with a trumpet flowerlike hole produced at 25 °C is better than morphologies of products produced at 8 and 70 °C. We found there was almost no difference on photocatalytic activity among these three products. Photodegradation Mechanism of Ag2O/Ag2CO3 Hollow Flowerlike Hierarchical Microspheres. To explore the mechanism of the photocatalysis activity of Ag2O/Ag2CO3

(5)

where k is the pseudo-first-order rate constant. From Figure 6b, it is noted that the decomposition of CR followed the pseudofirst-order model very well, and the rate constant k was 0.06814. The photocatalysis stability and reusability of Ag2O/Ag2CO3 6154


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The solubility of pure Ag2CO3 (Ag+: 2.5 × 10−4 M), Ag2O (Ag+: 1.1 × 10−4 M), and Ag2O/Ag2CO3 (Ag+: 1.35 × 10−4 M) in aqueous solution is different. The coating of a surface Ag2O layer (Figure 2f) over Ag2CO3 could greatly prevent the dissolution of Ag2CO3. Thus, Ag2O/Ag2CO3 photocatalyst exhibits good stability during the visible photocatalysis process.

hollow flowerlike hierarchical microspheres, trapping experiments of reactive species and h+ were carried out. Two different quenchers, i.e., sodium oxalate 44 (Na 2 C 2 O 4 4 mM, h + scavenger) and sodium bicarbonate45 (NaHCO3 4 mM, ·OH scavenger) were added into the solution before photodegradation of CR. The photodegradation process was carried out under an N2 environment to confirm O2·− generation.46 The experimental results (Figure 7a) showed that the addition of Na2C2O4 or NaHCO3 under an N2 environment caused obvious reduction of the photocatalytsis activity of Ag2O/ Ag2CO3 microspheres for the degradation rate of CR from 95% to 82.5% (Na2C2O4), 80.4% (NaHCO3), and 76.2% (N2) in 20 min, which proved that h+, ·OH, and O2·− were generated during the process of photocatalysis.44−46 In order to explain the excellent visible light photocatalysis activity of Ag2O/Ag2CO3 microspheres, a UV−vis diffuse reflectance absorption spectrum measurement was carried out, as shown in Figure S8a. The result revealed that the microspheres can absorb visible light. Furthermore, their band gap energy (Eg) was estimated to be 1.16 eV, inset of Figure S8a. This band gap value was significantly less than 3.2 eV of TiO2 materials,47 which explains the reason for the visible light photocatalysis activity of Ag2O/Ag2CO3 microspheres. The flat band potentials of Ag2CO3 and Ag2O semiconductors were estimated by electrochemical impedance spectroscopy (EIS) based on the Mott−Schottky (MS) equation.48−50 As shown in Figure 7c and d, the linear part of the data was extrapolated to the point where C−2 equals 0. The negative value of the slope in the lower potential region implys they are p-type semiconductors.51 The flat-band potentials for Ag2CO3 and Ag2O semiconductors were estimated to be 2.65 and 1.37 V versus a normal hydrogen electrode (NHE) from the x-intercept of the linear region of the curves at different frequency, respectively. It is well known that the VB potentials of a p-type semiconductor is about 0.10 V more positive than the flat-band potentials; therefore, the valence band energy (EVB) positions of Ag2CO3 and Ag2O are about 2.75 and 1.47 eV, respectively. While the conduction band energy (ECB) positions were calculated to be around 0.45 and 0.17 eV for Ag2CO3 and Ag2O, respectively, the band gaps of Ag2CO3 and Ag2O are 2.3 eV52 and 1.3 eV.53 Figure S8b shows the detail of the electronic band structures of Ag2CO3 and Ag2O. On the basis of above the experimental results and theoretical analysis, a possible catalytic mechanism for CR by Ag2O/Ag2CO3 was proposed, as illustrated in Figure 7d. Due to the existence of heterojunction (Figure 2f) and the unique bandgap structure (Figure S8b), under visible light irradiation, Ag2CO3 and Ag2O adsorb energy of photons and more efficiently generate e−/h+ pairs. The electrons could be trapped by molecular oxygen in solution to form O2·− and meanwhile to form h+. Holes can react with H2O to generate ·OH. All O2·− and ·OH radicals and h+ can degrade organic pollutant. Ag2O has a more negative potential of the conduction band (CB: 0.17 eV) and valence band (VB: 1.47 eV) than those of Ag2CO3 (CB: 0.45 eV, VB: 2.75 eV). Therefore, the photogenerated electrons in Ag2O can be easily transferred to Ag2CO3, and the photoinduced holes of Ag2CO3 can migrate to Ag2O resulting in carriers between Ag2CO3 and Ag2O. This result can reduce the recombination rate of e−/h+ pairs, which is also responsible for the high photocatalysis activity of Ag2O/ Ag2CO3 microspheres.32,54,55 Furthermore, the high adsorption performance facilitates CR degradation.21

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CONCLUSION We developed a novel, green, and low-cost method in a green solvent system, i.e., water/ethanol at 25 °C for the synthesis of Ag2O/Ag2CO3 3D hollow hierarchical structures which have a good morphology with an orderly arranged trumpet flowerlike holes on the surface. These microspheres are assembled by amounts of ∼30 nm Ag2O/Ag2CO3 nanoparticles. The surface morphology of an Ag2 O/Ag2CO3 microsphere can be controlled by modulation of reaction temperature to form an ordered shallow hole (70 °C) and relatively smooth surface (8 °C). The formation mechanism of the Ag2O/Ag2CO3 microsphere was also investigated to confirm the templating effect of Na2CO3, while the templates are easily removed by washing with water. The prepared Ag2O/Ag2CO3 microstructures display excellent and stable adsorption and visible light photocatalysis activity. Ag2O/Ag2CO3 microstructures can remove CR in wastewater through adsorption. The adsorption behavior of Ag2O/Ag2CO3 follows the Langmuir isothermal adsorption model. They also can take advantage of visible light to degrade CR in water. The superoxide radical (O2·−), hole (h+) and hydroxy radical (·OH) are the active species for photocatalysis oxidation. The thin layer of Ag2O on the microsphere is beneficial for the excellent and stable photocatalysis degradation performance for CR. Those results suggest their potential in practical water treatment. The proposed green method may be extended to prepare other hollow hierarchical metal oxide microstructures.

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EXPERIMENTAL SECTION

Materials. Sodium carbonate, silver nitrate, ethanol, disodium oxalate, sodium bicarbonate, BaSO4, and Congo red (CR) were obtained from Sigma-Aldrich. Ultrapure water (18.2 MΩ cm) was used for solution preparation in all experiments. All chemicals were used without further purification. Preparation of Ag2O/Ag2CO3 Hollow Flowerlike Hierarchical Microspheres. A typical protocol to prepare Ag2O/Ag2CO3 hollow flowerlike hierarchical microspheres is described below. Here, 0.7155 g of Na2CO3 was dissolved in 5 mL of ultrapure water to form an aqueous solution (1.35 M, at 25 °C), and 0.5096 g of AgNO3 was added into anhydrous ethanol to form 75 mL of an ethanol solution (0.04 M, at 25 °C) under vigorous stirring for 3 h. Subsequently, 100 μL of an aqueous Na2CO3 solution (1.35 M) was mixed with 15 mL of a homogeneous AgNO3 ethanol solution (0.04 M) through a rapid injection method by a microsyringe (100 μL, ϕ = 0.7) with a blunt needle at 25 °C. After 30 min, the precipitates were collected by centrifuging and washed with ethanol and ultrapure water in turn, then dried at 60 °C for 24 h in a vacuum drying oven. The dry products were used as typical products. By changing the reaction temperature to 8 or 70 °C from 25 °C, products with other morphologies were obtained. Characterizations. Scanning electron microscopy (Quanta 200 FEG; Netherlands) was applied to examine the structure and morphology of the products. X-ray powder diffraction (XRD) was measured in the reflection mode (Cu Kα radiation) on a diffractometer (D/Max-RB, Japan). A thermal gravimetric analysis (TGA) was carried out using a synchronized thermal analyzer (SDTA85IE) at a heating rate of 10 °C min−1 from 25 to 600 °C. A diffuse reflectance spectrum was recorded on a Hitachi-4100 UV− vis spectrophotometer (Japan). BaSO4 was used as a reference. UV− 6155


ACS Sustainable Chemistry & Engineering vis spectra were obtained using a Cary 60 spectrophotometer (Agilent, US). A nitrogen adsorption−desorption experiment was carried out on a Quantachrome automated surface area and pore size analyzer. The type of product was confirmed through the previous method. Adsorption Activity Measurements. Different dosages (0.8, 1, 1.2 g L−1) of the typical product were added to the stock aqueous solutions containing 80 mg L−1 of organic azo dye CR. After stirring for 3 h in a dark environment, the typical products were removed by centrifugation, and the supernatant solutions were analyzed with UV− vis spectrophotometer to ascertain the concentrations of the remaining CR in the solution. To study adsorption kinetics, the initial concentration of the dye solution was 80 mg L−1 with a dosage of 1.2 g L−1 of the products. A 4 mL suspension was taken out to determine the concentration of CR in a certain time interval. In order to confirm the maximum adsorption capacity and adsorption model, initial concentrations of 20, 40, 70, 100, 150, 200, and 300 mg L−1 aqueous dye solution with a dosage of 1.2 g L−1 products were also investigated. Photocatalysis Activity Measurements. Photocatalytic activity of the typical products was tested through the decomposition of CR with visible light illumination after 120 min adsorption in the dark. For the purpose of photodegradation of CR, 40 mg of a typical product was dispersed in 50 mL of an 80 mg L−1 CR aqueous solution in a specific photocatalytic reaction apparatus (XPA-7, China) equipped with an ultraviolet cutoff filter (λ ≥ 420 nm). The metal halide lamp is 500 W. Prior to irradiation, the suspension was magnetically stirred in the dark for 120 min to ensure adsorption/desorption equilibrium. Afterward the above suspensions were exposed to the visible light irradiation under magnetic stirring. At suitable time intervals, 4 mL of suspension was sampled and centrifuged to remove the photocatalyst. The concentrations of the remaining CR were analyzed with a UV−vis spectrophotometer. Moreover, recycle experiments were also carried out for five consecutive cycles to estimate the stability of the photocatalyst. After each cycle, the catalyst was washed by ethanol several times and then dried at 60 °C for the next test. In order to demonstrate the active species produced in the photocatalysis process, the trapping experiment was carried out. The disodium oxalate (Na2C2O4 4 mM, h+ scavenger) and sodium bicarbonate (NaHCO3 4 mM, ·OH scavenger) was added before illuminating CR. The photodegradation process was carried out under an N2 environment to demonstrate O2·− generation.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Key Research and Development Programme (2016YFC0401104), HIT Environment and Ecology Innovation Special Funds (HSCJ201617), and State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2017DX05).

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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/acssuschemeng.7b01040. XRD pattern of calcined product; dameter distribution and fitted curves; EDX spectroscopy spectrum; N2 adsorption−desorption isotherm and pore size distribution of Ag2O/Ag2CO3 microspheres prepared at 25 °C; SEM image, EDX componential maps, and XRD patterns of product prepared in 8 and 70 °C; isotherm parameters for adsorption of CR on the Ag2O/Ag2CO3; electronic band structures of Ag2CO3 and Ag2O. (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaojun Han: 0000-0001-8571-6187 Notes

The authors declare no competing financial interest. 6156


Research Article

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Ag2CO3 Three-Dimensional

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