Photocatalysis is attracting intensive attention in the field of pollutant degradation as a green chemistry technology [1-8]. Photocatalysis is deemed a particularly effective and sustainable technology in water purification [9-12]. Many photocatalysts have the powerful ability to decompose organic pollutants in water. The development of visible-light-driven photocatalysts is particularly desired, as these allow the utilization of natural solar light for both environmental remediation and energy generation [13-20].
In recent years, silver-based semiconductors, such as Ag2O [21], AgX (X = Cl, Br, I)[22-25], Ag2WO4[26], Ag3PO4 ,[27] Ag3VO4 [28], and Ag3AsO4 [29] have aroused great research interest because of their considerable visible-light response. Ag2CO3 is another typical silver-based semiconductor, which can decompose various organic dyes under visible light irradiation [30-32]. However, its application is greatly limitedby its high operational cost and serious susceptibility to photocorrosion, the latter of which causes very poor stability and recyclability [33]. Various strategies have been proposed to improve the stability of Ag2CO3 against photocorrosion. For example, the design of heterojunctions, e.g. TiO2/Ag2CO3 [20, 33], Ag2CO3/ZnO [34], Ag2O/Ag2CO3, and AgI/Ag2CO3 [35, 36], can effectively restrain the photocorrosion and enhance the stability of Ag2CO3. A well-defined heterojunction can suppress the recombination of photogenerated electrons (e-) and holes (h+) and thereby improve the photocatalytic performance [3, 37]. Photochemical corrosion can also be efficiently inhibited by adding AgNO3 into the photocatalytic reaction system [38]. Deposition of silver nanoparticles (NPs) to form a plasmonic photocatalyst is another effective means to improve the photocatalytic stability and activity [39, 40]. Because silver is a noble metal, the cost of fabrication of Ag2CO3 is relatively high. Bare Ag2CO3 cannot be reused, which further increases its operational cost. Moreover, the practical application of Ag2CO3 as a photocatalyst requires the addition of AgNO3 as well as the deposition of noble metal NPs, thus further limiting its viability as a result of increased cost. On the other hand, photocatalytic performance is closely related to the morphological structures of catalysts [41, 42]. Recent investigations have indicated that microsphere photocatalysts can display remarkably improved aqueous photocatalytic performance because of their efficient light-harvesting and carrier separation ability, high surface area, ease of settling and delivery, and high surface permeability [43-47].
In this work, CaMg(CO3)2@Ag2CO3 composite microspheres weresuccessfully prepared by a simple ion exchange route using CaMg(CO3)2 microspheres as the hard template. The relationships between the photocatalytic activity and preparation conditions for the CaMg(CO3)2@Ag2CO3 microspheres were discussed. The results showed that under the optimum conditions, i.e., an ion exchange time of 4 h and reaction temperature of 40 ℃, a high degree of ion exchange took place. The produced CaMg(CO3)2@Ag2CO3 microspheres showed a unique hierarchical morphology and high photocatalytic performance. More importantly, the fabrication cost of the Ag2CO3 photocatalyst was greatly reduced because of the low content of Ag2CO3 in CaMg(CO3)2@Ag2CO3.
All chemicals were of analytical grade and used without further purification. The CaMg(CO3)2 microsphere template was synthesized according to the literature [48]. Typically, 0.01 mol Mg(NO3)2·6H2O was dissolved in 20 mL distilled (deionized, DI) water, and then 0.01 mol Ca(NO3)2·4H2O was added to the above solution. After stirring for 10 min, the mixture became a clear solution. Then, 20 mL of 0.02 mol Na2CO3 solution was added dropwise into the above clear solution under continuous stirring and a white emulsion precipitate appeared. The suspension was vigorously stirred for 30 min at room temperature. Then, the suspension was heated at 60 ℃ for 24 h. The product was washed with DI water several times and dried at 100 ℃ for 6 h to obtain the CaMg(CO3)2 template.
The Ag2CO3 composite microspheres were prepared via an ion exchange route. In this procedure, the as-prepared CaMg(CO3)2 template powder was dispersed ultrasonically in DI water (30 mL) for 10 min. Then, 15 mL AgNO3 aqueoussolution with stoichiometric AgNO3 (CO32-in CaMg(CO3)2) was added dropwise to the above mixture at 40 ℃ and the reaction time was variously controlled as 2, 4, 6, 12, and 24 h. The resulting products were obtained by vacuum filtration, washed with DI water and ethanol, then collected after drying at 60 ℃ for 6 h. With a fixed 4 h reaction time, the CaMg(CO3)2@Ag2CO3 microspheres were also fabricated at different temperatures. The controlled reaction temperatures were 30, 40, 50, and 60 ℃. The product was obtained by vacuum filtration, washed with DI water and ethanol, and then collected after drying at 60 ℃ for 6 h.
The crystal properties of the fabricated products were characterized by powder X-ray diffraction (XRD) on a Bruker D8-Advance X-ray diffractometer at 40 kV and 40 mA using monochromatized Cu Kα (λ = 1.5418 Å) radiation. The morphology of the samples was observed by a Philips XL30 scanning electron microscope (SEM). The UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrophotometer (UV-2550, Shimadzu). The absorption spectra were referenced to BaSO4. Fourier transform infrared (FT-IR) spectra were collected on a Nicolet-470 Frontier. Samples were pressed into KBr disks using a disk preparation apparatus. The Brunauer-Emmett-Teller (BET) surface areas of the samples were analyzed from N2 adsorption-desorption isotherms determined at liquid nitrogen temperature (-196 ℃) on an automatic analyzer (ASAP 2020). Photocurrent measurements were performed on a CHI 660E electrochemical workstation (Chenhua Instruments, China) in a conventional three-electrode configuration with a Pt foil as the counter electrode and a Ag/AgCl (saturated KCl) as the reference electrode. A 300-W xenon lamp (PLS-SXE 300) served as the light source. The electrolyte was a Na2SO4 aqueous solution (0.1 mol/L). The working electrodes were prepared as follows: 15 mg photocatalyst and 0.5 mL Nafion dispersing reagent were added to 5 mL absolute ethanol, and sonicated for 30 min. The slurry was then spread on a 1.0 cm × 1.0 cm indium-tin oxide (ITO) glass substrate and dried in air. The photoresponses of the samples, with light on and light off, were measured at 0.3 V.
Under xenon lamp irradiation, the photocatalytic activities of the fabricated samples were examined by photodegradation of Acid Orange Ⅱ in aqueous solution. In a typical photodegradation process, 50 mg of photocatalysts was added into 80 mL of Acid Orange Ⅱ solution (20 mg/L). Prior to irradiation, the suspensions were magnetically stirred in the dark for 40 min to ensure the adsorption-desorption equilibrium of Acid Orange Ⅱ on the surface of the photocatalysts. Using a 500-W xenon lamp (solar 500N) as the light source, the above suspension was vigorously stirred during the photocatalytic reaction process at room temperature. The reaction mixture was sampled at given time intervals during light illumination. After centrifugation, the Acid Orange Ⅱ concentration was measured on a UV-vis spectrophotometer (Agilent, HP 8453). To investigate the photocatalytic activity of the samples against typical organic phenolic contaminants, activity tests were carried out with 10 ppm of phenol as the target contaminant.
The CaMg(CO3)2@Ag2CO3 composites were obtained via the following ion exchange reaction: 4 Ag++ CaMg(CO3)2 → 2 Ag2CO3 + Ca2++ Mg2+. The reaction degree of the photocatalysts prepared by this method was obtained by analyzing the mass fraction of the generated Ag2CO3 as a percentage of the total mass, which was estimated by measuring the concentration of Ag+ ions by inductively coupled plasma emission spectroscopy (ICP). The process was as follows. First, 20 mg sample was dissolved in 20 mL dilute HNO3 solution and the obtained solution was diluted 100 times. The concentration of Ag+ ions in the obtained solution was measured by ICP. The mass fraction of Ag2CO3 in the composite was calculated using the molar concentration of Ag+. Table 1 gives the calculated Ag2CO3 contents in the produced composites. The mass fraction of Ag2CO3 in the CaMg(CO3)2@Ag2CO3 samples ranged from 1.40% to 2.56%. The solubility of Ag2CO3 (8.45 × 10-12 mol/L) ismuch smaller than that of CaCO3 (3.32 × 10-9 mol/L) and MgCO3 (6.82 × 10-6 mol/L). Therefore, we can infer that Ag+ was able to replace Mg2+ and Ca2+ in the CaMg(CO3)2. Moreover, the ion exchange reaction first took place on the outer layer of the CaMg(CO3)2 crystals, after which the Ag2CO3 phase gradually replaced the core of CaMg(CO3)2. We can thus infer that the obtained CaMg(CO3)2@Ag2CO3 samples were core-shell-like composites. The CaMg(CO3)2 template partly remained as the shrinking core. Therefore, the fabrication cost of the CaMg(CO3)2@Ag2CO3 photocatalyst was greatly reduced because of the low content of Ag2CO3 in these composites.
XRD was used to investigate the phase composition of the catalysts synthesized via ion exchange at different ion exchange temperatures and times. Fig. 1 shows the XRD patterns of the obtained samples. For the pure template sample, a clear diffraction peak at 30.6° appeared, which was indexed to the (104) plane of the rhombohedral phase of CaMg(CO3)2(JCPDS No. 73-2444). For the CaMg(CO3)2@Ag2CO3 composites, all the samples exhibited sharp and intense diffraction peaks at 2θ of 18.6°, 20.5°, 32.5°, 33.7°, 37.0°, and 39.7°. These peaks were attributed to the monoclinic phase of Ag2CO3 (JCPDS No. 07-2184). In contrast, no characteristic peaks for CaMg(CO3)2 could be clearly detected in the CaMg(CO3)2@Ag2CO3 composites, possibly because of either the weakness of the diffraction peaks of CaMg(CO3)2, or interference from the coated Ag2CO3 phase. Fig. 1(c) shows the XRD patterns of CaMg(CO3)2@Ag2CO3 (4 h, 40 ℃) before and after reaction. It can be found that no obvious phase veriation was observed.
The Scherrer equation was used to estimate the average crystallite sizes of the Ag2CO3 phase in the composites, as follows: D = 0.89λ/(βcosθ), where λ is the wavelength of the X-rays, β is the width in radian of the XRD peak at half the peak height for the (130) plane, and θ is the measured diffraction angle. The results are shown in Tables 2 and 3. From Table 2, it can be seen that the average crystallite size of the Ag2CO3 phase decreases with the increase of reaction time. Table 3 indicates that the average crystallite size of the Ag2CO3 phase in the CaMg(CO3)2@Ag2CO3 samples prepared at 40 ℃ is larger than that of the other samples.
The influence of different preparation conditions on the surface area of the CaMg(CO3)2@Ag2CO3 was further investigated by N2 physical adsorption-desorption. These results are also presented in Tables 2 and 3. We can see that all the catalysts exhibit small specific surface areas (0.03-1.27 m2/g), and the preparation conditions have little effect on the specific surface areas of the samples.
The morphology of typical samples of each product was characterized by SEM. From Fig. 2(a), we can see that the CaMg(CO3)2 template is composed ofmicrospheres with diameters of ~10 μm and very smooth surfaces. From Fig. 2(b) to (e) it can be seen, to our surprise, that the morphologies are considerably altered after the ion exchange reaction, and the smooth surfaces have disappeared. Although the products still maintain their spherical morphology, the microspheres are transformed into stacked aggregations of small crystal particles, and the size of these spherical aggregations is not uniform. From the previous XRD analysis, we can infer that the small crystal particles are mainly composed of Ag2CO3 crystals. The variation in temperature (30 to 60 ℃) exerted only minor influence on the morphology. The CaMg(CO3)2@Ag2CO3 samples prepared at 50 or 60 ℃ for 4 h display irregular spherical morphologies. Fig. 2(f) shows that, in contrast to the other samples, the CaMg(CO3)2@Ag2CO3 prepared at 40 ℃ for 8 h shows no microsphere morphology. From the SEM analysis, we can see that the CaMg(CO3)2@Ag2CO3 fabricated at 40 ℃ for 4 h shows the most regular microsphere morphology. These CaMg(CO3)2@Ag2CO3 microspheres are hierarchical composite nanostructures constructed of very small and irregular nanocrystals. The SEM images of the CaMg(CO3)2@Ag2CO3 (40 ℃, 4 h) sample after the reaction are shown in Fig. 2(g) and (h). It's obvious that this sample still has the regular spherical morphology, which suggests the good stability of the CaMg(CO3)2@Ag2CO3.
The optical properties of the as-prepared catalysts were measured by UV-vis DRS, as shown in Fig. 3. The pure CaMg(CO3)2 microspheres show no light absorption in the visible light range. After the ion exchange reaction, despite their low Ag2CO3content, the obtained CaMg(CO3)2@Ag2CO3 composites display wide and intense absorption in the visible light range, which is essential for significant photocatalytic activity [49]. In Fig. 3(a), we also find that at the fixed temperature of 40 ℃, the ion exchange reaction time slightly influences the light absorption ability. Among the CaMg(CO3)2@Ag2CO3 samples prepared at 40 ℃, the sample prepared for 4 h exhibits the strongest light absorption, which could be related to the nanostructures of the CaMg(CO3)2@Ag2CO3 microspheres or the overall Ag2CO3 content. Fig. 3(b) indicates that the temperature of ion exchange also slightly affects the light absorption ability. However, the variation in light absorption ability with the change of temperature does not follow a regular trend. But all of samples show better light absorption than the bare Ag2CO3 prepared by precipitation method. The band gap energy (Eg) for CaMg(CO3)2 and Ag2CO3 was determined by Eg = 1240/λg(eV), where λg is the absorption edge of the samples and λg was obtained from the intercept between the tangent of the absorption curve and the abscissa [8]. The calculated band gap energies for CaMg(CO3)2 and Ag2CO3 are 4.92 eV and 2.52 eV, respectively. We can see that Ag2CO3 is the active phase because the band gap of CaMg(CO3)2 is very large and is almost the insulator.
The FT-IR spectra of the CaMg(CO3)2@Ag2CO3 samples are presented in Fig. 4. All of the samples showed three main absorption bands. The broad peak at 3000-3500 cm-1was ascribed to the O-H stretching vibrations of physically adsorbed water [4]. The broad peak at 3200 cm-1 was considered to be the stretching vibration of surface O-H groups on the catalyst. Surface O-H groups can trap photogenerated holes (h+) to form hydroxyl radicals (·OH), which are powerful oxidizing agents. Therefore, the increased surface hydroxyl groups on the catalyst can be expected to accelerate the degradation of dye molecules. The peaks at 1640 cm-1 originate from the H-O-H bending vibration of the adsorbed water. The peaks at around 1350 and 1450 cm-1 were attributed to the characteristic absorption of the carbonate ion (CO32-). Careful comparison of the spectra suggested that the CaMg(CO3)2@Ag2CO3 prepared at 40 ℃ for 4 h shows the strongest intensity of O-H groups, which would be expected to promote the photocatalytic degradation reaction, as mentioned above. The FT-IR of the CaMg(CO3)2@Ag2CO3 sample (40 ℃, 4 h) before and after reaction are shown in Fig. 4(c). We can still find the obvious peaks in 1350 cm-1 and 1450 cm-1 which belong to CO32-. The peaks of ·OH and the bending vibration of H-O-H of the absorbed water molecules are relatively enhanced, which could be due to the humidity of the sample.
We carried out photocurrent tests to explore the influence of different preparation temperatures and reaction times on the separation efficiency of the photogenerated electron and holes in CaMg(CO3)2@Ag2CO3 [50]. When the electrodes were illuminated under a xenon lamp light, clear photocurrent responses were observed [51]. It can be seen from Fig. 5(a) that the intensity of the photocurrents of CaMg(CO3)2@Ag2CO3 samples prepared for different ion exchange times follows the order 4 > 6 > 2 > 24 > 12 h. All the CaMg(CO3)2@Ag2CO3 samples produce larger photocurrents than that of bare Ag2CO3 prepared by precipitation. At the fixed temperature of 40 ℃, the CaMg(CO3)2@Ag2CO3 sample obtained for an ion exchange time of 4 h displays the strongest photocurrent. Fig. 5(b) shows the influence of fabrication temperature on the photocurrent response. Clearly, CaMg(CO3)2@Ag2CO3 synthesized at 40 ℃ shows a much higher photocurrent than the samples fabricated at other temperatures. Furthermore, the distinct photocurrent responses of all the samples of CaMg(CO3)2@Ag2CO3 are again stronger than that of bare Ag2CO3. According to the literature [50, 52], a high photocurrent response suggests a high separation efficiency of the photogenerated electrons and holes. The photocurrent test results thus demonstrated that an ion exchange reaction temperature of 40 ℃ and reaction time of 4 h were the optimal conditions for synthesizing CaMg(CO3)2@Ag2CO3, producing asample with high photocatalytic performance because of the enhanced separation efficiency of charge carriers.
Electrochemical impedance spectroscopy (EIS) is a well-established approach to analyze the charge transfer resistance and separation efficiency of the photogenerated electrons and holes of photocatalysts. As shown in Fig. 6, in the EIS tests, the sample of Ag2CO3 prepared by precipitation produced a larger arc radius than that of CaMg(CO3)2@Ag2CO3 prepared by ion exchange at 40 ℃ for 4 h. The smaller the radius, the smaller the migration resistance of the carriers [53], and the higher the separation efficiency of photogenerated electrons and holes. Hence, the results indicate that the existence of CaMg(CO3)2 reduced the migration resistance of carriers in Ag2CO3 to improve its photocatalytic performance.
Under xenon lamp irradiation, the photocatalytic activities of the as-prepared samples were evaluated by photodegradation of a dye, Acid Orange Ⅱ. Fig. 7 shows the photocatalytic performance test over CaMg(CO3)2@Ag2CO3 samples. Firstly, from Fig. 7(a) and (b), we can see that the adsorption capacity of all the as-prepared catalysts to acid orange was weak in the degradation process, indicating that the reaction temperature and time had little effect on the adsorption performance of CaMg(CO3)2@Ag2CO3, which could due to their small specific surface area. Fig. 7(a) indicates that the CaMg(CO3)2 template exhibits no photocatalytic activity, while all the CaMg(CO3)2@Ag2CO3 samples display clearly perceptible activity in dye degradation. Evidently, the ion exchange time and temperature greatly influenced the activity. At the fixed temperature of 40 ℃, the ion exchange time of 4 h produced the CaMg(CO3)2@Ag2CO3 sample with the highest activity. Further increases in reaction time exerted an adverse effect on the activity, and a decrease in photocatalytic performance was observed. After light irradiation for 15 min, the degradation rates of Acid Orange Ⅱ were 77%, 84%, 69%, 72%, and 61% over CaMg(CO3)2@Ag2CO3 prepared for ion exchange times of 2, 4, 6, 12, and 24 h, respectively. Compared with pure Ag2CO3 prepared by the precipitation method, all the CaMg(CO3)2@Ag2CO3 samples show better photocatalytic performance.
Fig. 7(b) shows the influence of preparation temperature on the photocatalytic activity. After light irradiation for 15 min, the degradation rates of Acid Orange Ⅱ were 66%, 82%, 78%, and 41% over CaMg(CO3)2@Ag2CO3 prepared at 30 ℃, 40 ℃, 50 ℃, and 60 ℃, respectively. Once again, therefore, the sample prepared at 40 ℃ for 4 h displays the highest catalytic activity.
Activity tests were also carried out for degradation of another typical organic contaminant, phenol. As shown in Fig. 7(c), bare Ag2CO3 exhibited low degradation activity. In contrast, excellent photocatalytic performance was obtained over the CaMg(CO3)2@Ag2CO3 composite microspheres, which further confirms the superior properties of these core-shell-structured microspheres.
CaMg(CO3)2@Ag2CO3 and Ag2CO3 were also subjected to stability tests. Fig. 8 compares the activity of the two catalysts during repeated cycling, suggesting that despite the low content of Ag2CO3 in the composite microspheres, the stability of CaMg(CO3)2@Ag2CO3was significantly higher than pure Ag2CO3. The active phase of CaMg(CO3)2@Ag2CO3 is Ag2CO3, so it still suffers photocorrosion; however, the photocorrosion rate is much lower than that of bare Ag2CO3. Therefore, the existence of the CaMg(CO3)2 core not only brings about a large increase in activity but also in stability.
To explore the radicals involved in the photodegradation process over CaMg(CO3)2@Ag2CO3. We used electron spin resonance (ESR) to test the active species. 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as a capture agent and the results are shown in Fig. 9. In this figure, before irradiation, it can be found that there is no ESR signal for the sample. After illumination, the sample has a weak ESR signal of DMPO-·O2-, indicating that ·O2- is an active species in the process of photocatalytic reaction. Moreover, we can see the significant ESR characteristic signals of DMPO-·OH in the field of about 3488 to 3516 (G). Therefore, it can be concluded that ·OH plays an important role in the photodegragation process.
To account for the superior photocatalytic performance of the CaMg(CO3)2@Ag2CO3 microspheres prepared under various conditions, we must first understand the formation mechanism of the core-shell composite microspheres. Fig. 10 illustrates the preparation procedure and the photodegradation reaction. After ion exchange reaction of Ag+ for Ca2+ and Mg2+, the compact CaMg(CO3)2 microspheres transform into more loosely structured, hierarchical CaMg(CO3)2@Ag2CO3 microspheres. The outer layers of the microspheres contain nano-sized Ag2CO3 crystals. Under irradiation by visible light, Ag2CO3 produces photogenerated electrons and holes. The holes migrate to the surface to generate hydroxyl radicals (·OH), which oxidize the dye molecules to CO2 and H2O. Meanwhile, the adsorbed O2 can capture individual photogenerated e- to generate photocatalytically active O2·- radicals [6]. XRD and ICP analysis indicate that the obtained samples underwent a relatively large degree of ion exchange. The UV-vis absorption spectra indicate that the CaMg(CO3)2@Ag2CO3 microspheres absorbed much more strongly in the visible region than did the bare Ag2CO3 prepared by the precipitation method. Moreover, the results of the photocurrent test for CaMg(CO3)2@Ag2CO3 implied that the photogeneration of electrons and holes was enhanced over the composites. The intimate interfacial contact between CaMg(CO3)2 and Ag2CO3 improved the separation efficiency of charge carriers. In addition, the small particle size and hierarchical structure of the Ag2CO3 phase deposited on the outer layer of the microspheres benefitted the harvesting of light and adsorption of dye molecules.
Core-shell-like CaMg(CO3)2@Ag2CO3 composite microspheres with low content of Ag2CO3 were prepared via a facile ion exchange route. The influences of ion exchange time and temperature on the morphology and structure of the composite microspheres and photocatalytic performance of the photocatalysts were investigated. The ICP test for decomposition of Acid Orange Ⅱ demonstrated that the content of Ag2CO3 is 2.56% in CaMg(CO3)2@Ag2CO3 microspheres produced by ion exchange (of Ag+ for Ca2+ and Mg2+) for 4 h at 40 ℃. These composite microspheres possess a hierarchical structure and enhanced visible light absorption. Moreover, the existence of the CaMg(CO3)2 core promotes the separation of photogenerated electrons and holes. Compared with bare Ag2CO3, the cost for the fabrication and operation of the CaMg(CO3)2@Ag2CO3 composite microspheres was greatly decreased, and superior photocatalytic performance was obtained over the microspheres. We believe these low-cost CaMg(CO3)2@Ag2CO3 composite microspheres have potential applications as photocatalysts in environmental remediation.
2017,
Vol. 38 Issue (11): 1899-1908 
PDF
2017,
Vol. 38 Issue (11): 1899-1908 PDF