1、离子交换法制备廉价而高效的可见光驱动 CaMg (CO3) 2Ag2CO3 微球光催化剂（英文） 田坚 吴榛 刘珍 余长林 杨凯 朱丽华 黄微雅 周阳 江西理工大学冶金与化学工程学院 五邑大学化学与环境工程学院 福州大学能源与环境光催化国家重点实验室 摘 要： Ag2CO3是一种典型的银基半导体, 可在可见光照射下降解各种有机染料, 但制备成本高, 光腐蚀严重, 稳定性差, 难以循环利用等, 因而限制了它的实际应用.针对这些问题, 目前多数的改进措施是构建异质结, 有效的分离光生电子与空穴来提高 Ag2CO3的光催化性能.比如典型的异质结光催化剂有 TiO2/Ag2CO3, Ag2CO3/Zn
2、 O, Ag2O/Ag2CO3和 Ag X/Ag2CO3等.也有在表面化学沉积, 光化学还原Ag 等贵金属形成等离子体等方式提高其光催化性能, 但是很少通过特殊形貌控制以提高 Ag2CO3的光催化性能.最近的研究表明, 由于多尺度微球结构催化剂具有高效的光捕能力, 同时具有比表面积大、易沉降, 良好的物质传输能力和表面的渗透性, 因而在液相光催化反应中具有明显的优势.因此, 我们期望制备出一个多尺度微球结构 Ag2CO3光催化剂.CaMg (CO 3) 2是一种具有微球结构的半导体, 它与 Ag2CO3有相同的阴离子结构, 但是两者在水溶液中的溶解度相差较大, 利用这个特性理论上可以将两个不同
3、的半导体结合在一起, 得到一种新型的复合微球.本文以 CaMg (CO3) 2微球为硬模板, 通过简单的离子交换成功制备了粒径约为 10mm 的 CaMg (CO3) 2Ag2CO3微球.利用 X 射线衍射、N 2物理吸附、扫描电镜、傅里叶变换红外光谱和紫外-可见漫反射吸收光谱、光电流等手段对在不同反应时间与温度下制得的 CaMg (CO3) 2与 Ag2CO3的复合物进行了表征.结果表明, 在 40C 下 Ag+与 Ca2+、Mg 2+离子交换 4 h 后, 得到了一种多尺度 CaMg (CO3) 2Ag2CO3复合微球.此时, 微球中 Ag2CO3的含量约为 2.56%.结果表明, 这种具
4、有多尺度结构的复合微球能够增强可见光的吸收.电化学阻抗测试和光电流测试表明, CaMg (CO 3) 2核的存在可以降低光生载流子的迁移阻力, 进而促进光生电子与空穴的分离.在光降解酸性橙 II 的测试中, 核壳结构的 CaMg (CO3) 2Ag2CO3复合微球表现出了更高的催化活性, 而且具有更好的循环使用性能.同时, 相对于纯 Ag2CO3光催化剂来说, CaMg (CO 3) 2Ag2CO3复合微球制备的成本大幅度降低.ESR 测试证明了OH 为 CaMg (CO3) 2Ag2CO3复合微球光催化过程中的主要活性物质.关键词： 硬模板; 离子交换; CaMg (CO3) 2Ag2CO3
5、 复合微球; 光催化性能; 作者简介：余长林 电话/传真: (0797) 8312334;电子信箱:收稿日期：31 July 2017基金：supported by the National Natural Science Foundation of China (21567008, 21607064, 21707055, 21763011) Low-cost and efficient visible-light-driven CaMg (CO3) 2Ag2CO3 microspheres fabricated via an ion exchange routeJian Tian Zhen
6、Wu Zhen Liu Changlin Yu Kai Yang Lihua Zhu Weiya Huang Yang Zhou School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology; Abstract： CaMg (CO3) 2 microspheres were prepared and used as hard templates to fabricate a series of Ca Mg (CO3) 2Ag2CO3 composite microspher
7、es via a fast and low-cost ion exchange process. The effects of ion exchange time and temperature on the physicochemical properties and photocatalytic activities of the composite microspheres were studied through photocatalytic degradation of Acid Orange II under xenon lamp irradiation. The obtained
8、 samples were analyzed by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, UV-vis diffuse reflectance spectroscopy, N2 physical adsorption, and photocurrent tests. The Ca Mg (CO3) 2Ag2CO3 sample with the highest activity was obtained with an ion exchange time
9、 of 4 h and temperature of 40 C. The degradation rate of Acid Orange II by this sample reached 83.3% after 15 min of light irradiation, and the sample also performed well in phenol degradation. The Ca Mg (CO3) 2Ag2CO3 produced under these ion exchange conditions showed a well-ordered hierarchical mo
10、rphology with small particle sizes, which was beneficial to light absorption and the transfer of photoelectrons (e-) and holes (h+) to the catalyst surface. Moreover, the separation of photogenerated carriers over the composites was greatly improved relative to bare Ca Mg (CO3) 2. Despite the very l
11、ow content of Ag2CO3 (2.56%) , excellent photocatalytic performance was obtained over the Ca Mg (CO3) 2Ag2CO3 microspheres.Keyword： Hard template; Ion exchange; Ca Mg (CO3) 2Ag2CO3 microspheres; Photocatalytic performance; Received： 31 July 20171. IntroductionPhotocatalysis is attracting intensive a
12、ttention in the field of pollutant degradation as a green chemistry technology18.Photocatalysis is deemed a particularly effective and sustainable technology in water purification912.Many photocatalysts have the powerful ability to decompose organic pollutants in water.The development of visible-lig
13、ht-driven photocatalysts is particularly desired, as these allow the utilization of natural solar light for both environmental remediation and energy generation1320.In recent years, silver-based semiconductors, such as Ag2O21, Ag X (X=Cl, Br, I) 2225, Ag2WO426, Ag3PO427, Ag3VO428, and Ag3As O429have
14、 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 irradiation3032.However, its application is greatly limited by its high operational cost and seriou
15、s susceptibility to photocorrosion, the latter of which causes very poor stability and recyclability33.Various strategies have been proposed to improve the stability of Ag2CO3 against photocorrosion.For example, the design of heterojunctions, e.g.Ti O2/Ag2CO320, 33, Ag2CO3/Zn O34, Ag2O/Ag2CO3, and A
16、g I/Ag2CO335, 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 performance3, 37.Photochemical corrosion can also be ef
17、ficiently inhibited by adding Ag NO3 into the photocatalytic reaction system38.Deposition of silver nanoparticles (NPs) to form a plasmonic photocatalyst is another effective means to improve the photocatalytic stability and activity39, 40.Because silver is a noble metal, the cost of fabrication of
18、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 Ag NO3 as well as the deposition of noble metal NPs, thus further limiting its viability as a result of incre
19、ased cost.On the other hand, photocatalytic performance is closely related to the morphological structures of catalysts41, 42.Recent investigations have indicated that microsphere photocatalysts can display remarkably improved aqueous photocatalytic performance because of their efficient light-harve
20、sting and carrier separation ability, high surface area, ease of settling and delivery, and high surface permeability4347.In this work, Ca Mg (CO3) 2Ag2CO3 composite microspheres were successfully prepared by a simple ion exchange route using Ca Mg (CO3) 2 microspheres as the hard template.The relat
21、ionships between the photocatalytic activity and preparation conditions for the Ca Mg (CO3) 2Ag2CO3 microspheres were discussed.The results showed that under the optimum conditions, i.e., an ion exchange time of 4 h and reaction temperature of 40C, a high degree of ion exchange took place.The produc
22、ed Ca Mg (CO3) 2Ag2CO3 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 Ca Mg (CO3) 2Ag2CO3.2. Experimental2.1. Preparation of Ca Mg
23、(CO3) 2 microsphere templateAll chemicals were of analytical grade and used without further purification.The Ca Mg (CO3) 2 microsphere template was synthesized according to the literature48.Typically, 0.01mol Mg (NO3) 26H2O was dissolved in 20 m L distilled (deionized, DI) water, and then 0.01 mol C
24、a (NO3) 24H2O was added to the above solution.After stirring for 10 min, the mixture became a clear solution.Then, 20 m L 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 sti
25、rred for 30 min at room temperature.Then, the suspension was heated at 60C for 24h.The product was washed with DI water several times and dried at 100C for 6 h to obtain the Ca Mg (CO3) 2 template.2.2. Preparation of Ca Mg (CO3) 2Ag2CO3 composite microspheres at different conditionsThe Ag2CO3 compos
26、ite microspheres were prepared via an ion exchange route.In this procedure, the as-prepared Ca Mg (CO3) 2 template powder was dispersed ultrasonically in DI water (30 m L) for 10 min.Then, 15 m L Ag NO3 aqueous solution with stoichiometric Ag NO3 (CO32in Ca Mg (CO3) 2) was added dropwise to the abov
27、e mixture at 40C 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 60C for 6 h.With a fixed 4 h reaction time, the Ca Mg (CO3) 2Ag2CO3microspheres were al
28、so fabricated at different temperatures.The controlled reaction temperatures were 30, 40, 50, and 60C.The product was obtained by vacuum filtration, washed with DI water and ethanol, and then collected after drying at 60C for 6 h.2.3. CharacterizationThe crystal properties of the fabricated products
29、 were characterized by powder X-ray diffraction (XRD) on a Bruker D8-Advance X-ray diffractometer at 40 k V and 40 m A using monochromatized Cu K (=1.5418) radiation.The morphology of the samples was observed by a Philips XL30 scanning electron microscope (SEM) .The UV-vis diffuse reflectance spectr
30、a (DRS) were recorded on a UV-vis spectrophotometer (UV-2550, Shimadzu) .The absorption spectra were referenced to Ba SO4.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-Tel
31、ler (BET) surface areas of the samples were analyzed from N2 adsorption-desorption isotherms determined at liquid nitrogen temperature (196C) on an automatic analyzer (ASAP 2020) .Photocurrent measurements were performed on a CHI 660E electrochemical workstation (Chenhua Instruments, China) in a con
32、ventional three-electrode configuration with a Pt foil as the counter electrode and a Ag/Ag Cl (saturated KCl) as the reference electrode.A 300-W xenon lamp (PLS-SXE300) served as the light source.The electrolyte was a Na2SO4aqueous solution (0.1 mol/L) .The working electrodes were prepared as follo
33、ws:15 mg photocatalyst and 0.5 m L Nafion dispersing reagent were added to 5 m L absolute ethanol, and sonicated for 30 min.The slurry was then spread on a 1.0 cm1.0 cm indium-tin oxide (ITO) glass substrate and dried in air.The photoresponses of the samples, with light on and light off, were measur
34、ed at 0.3 V.2.4. Photocatalytic testUnder 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 m L of Acid Orange II sol
35、ution (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 vigorousl
36、y 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 photoca
37、talytic activity of the samples against typical organic phenolic contaminants, activity tests were carried out with 10 ppm of phenol as the target contaminant.3. Results and discussion3.1. Ag2CO3 content in Ca Mg (CO3) 2Ag2CO3 compositesThe Ca Mg (CO3) 2Ag2CO3 composites were obtained via the follow
38、ing ion exchange reaction:4 Ag+Ca Mg (CO3) 22Ag 2CO3+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 indu
39、ctively coupled plasma emission spectroscopy (ICP) .The process was as follows.First, 20 mg sample was dissolved in 20 m L 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
40、 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 Ca Mg (CO3) 2Ag2CO3 samples ranged from 1.40%to 2.56%.The solubility of Ag2CO3 (8.451012 mol/L) is much smaller than that of Ca CO
41、3 (3.32109 mol/L) and Mg CO3 (6.82106mol/L) .Therefore, we can infer that Ag+was able to replace Mg2+and Ca2+in the Ca Mg (CO3) 2.Moreover, the ion exchange reaction first took place on the outer layer of the Ca Mg (CO3) 2crystals, after which the Ag2CO3 phase gradually replaced the core of Ca Mg (C
42、O3) 2.We can thus infer that the obtained Ca Mg (CO3) 2Ag2CO3 samples were core-shell-like composites.The Ca Mg (CO3) 2 template partly remained as the shrinking core.Therefore, the fabrication cost of the Ca Mg (CO3) 2Ag2CO3photocatalyst was greatly reduced because of the low content of Ag2CO3 in t
43、hese composites.Table 1Ag2CO3 content in Ca Mg (CO3) 2Ag2CO3 determined by ICAP. 下载原表 3.2. XRD and surface area analysisXRD 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
44、 obtained samples.For the pure template sample, a clear diffraction peak at 30.6appeared, which was indexed to the (104) plane of the rhombohedral phase of Ca Mg (CO3) 2 (JCPDS No.73-2444) .For the Ca Mg (CO3) 2Ag2CO3 composites, all the samples exhibited sharp and intense diffraction peaks at 2of18
45、.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 Ca Mg (CO3) 2could be clearly detected in the Ca Mg (CO3) 2Ag2CO3 composites, possibly because of either the weakness of the diffraction pea
46、ks of Ca Mg (CO3) 2, or interference from the coated Ag2CO3phase.Fig.1 (c) showstheXRDpatternsof Ca Mg (CO3) 2Ag2CO3 (4 h, 40C) 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 Ag2CO
47、3 phase in the composites, as follows:D=0.89/ (cos) , whereis the wavelength of the X-rays, is the width in radian of the XRD peak at half the peak height for the (130) plane, andis the measured diffraction angle.The results are shown in Tables 2 and 3.From Table 2, it can be seen that the average c
48、rystallite 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 Ca Mg (CO3) 2Ag2CO3 samples prepared at 40C is larger than that of the other samples.Fig.1.XRD patterns of the samples. (a) Ca Mg (CO3) 2 te
49、mplate and Ca Mg (CO3) 2Ag2CO3 composites obtained at 40C for different ion exchange times; (b) Ca Mg (CO3) 2 template and Ca Mg (CO3) 2Ag2CO3 composites obtained after ion exchange time of 4 h at different temperatures; (c) The Ca Mg (CO3) 2Ag2CO3 sample (40C, 4 h) before and after reaction. 下载原图The influence of different preparation conditions on the surface area of the Ca Mg (CO3) 2Ag2CO3 was further investigated by N2 physical adsorption
