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Yingying Fan, Weiguang Ma, Dongxue Han,* Shiyu Gan, Xiandui Dong, and Li Niu* Serious water pollution concerns, including various organic contaminants and carcinogenic hexavalent chromium, are faced by modern society. Photocatalytic pollution mitigation with inexhaustible solar energy has already been widely put into effect and is expected to be devoted into practical applications.[1] In this regard, silver halide photocatalysts, as photosensitive semiconductors, are considered to be one of the most promising photocatalysts.[2] In that the silver halide can preliminarily decompose into Ag0,[3] it is able to demonstrate strong plasmon resonance and enhance the photocatalytic ability.[4] In order to fully utilize the photocatalyst and reduce the overall production cost, successive recycling application becomes necessary.[5,6] It is widely known that, owing to the nanoscale morphology of photocatalyst nanoparticles (NPs), miscellaneous and sophisticated operations (such as centrifugation, sonication, and drying) are frequently used during the photocatalyst recovery process. Simultaneously, a serious deficiency of this recovery method is the quantitative loss of the photocatalyst NPs during the repeated circulation process.[2] In further commercial processes, the recovery of photocatalyst is very significant and should not be neglected. Just following the economical point of view, during practical commercial applications, rational experimental conditions should be employed, while operations that do not satisfy the industry status should be removed as far as possible. Normally, during traditional photocatalytic experiments, powerful agitation is an inevitable experimental procedure used to disperse the photocatalytic NPs into aqueous pollutant solution through strong turbulence, which will ensure sufficient contact between the photocatalyst NPs and the contaminants. However, this punchy mechanical stirring treatment does not conform to the practical industrial demand. Therefore, a novel state escalation toward photocatalyst to avoid the employment of the force convection treatment is of great significance and will further facilitate the industrial degradation process. Y. Fan, W. Ma, Dr. D. X. Han, S. Gan, X. Dong, Prof. L. Niu State Key Laboratory of Electroanalytical Chemistry c/o Engineering Laboratory for Modern Analytical Techniques Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022, Jilin, China E-mail: [email protected]; [email protected] Y. Fan, W. Ma, Dr. D. X. Han, S. Gan, X. Dong, L. Niu University of Chinese Academy of Sciences Beijing 100039, China
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Convenient Recycling of 3D AgX/Graphene Aerogels (X = Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants
3D graphene aerogels (GAs) have aroused wide interest recently.[7–9] Their macroscopic block appearance makes them an easily and conveniently recycling operated photocatalytic candidate, which can be treated just using tweezers. Meanwhile, the hierarchically porous structure of 3D GAs can provide an ideal support for the deposition of photocatalyst NPs.[10] Moreover, compared with 2D graphene, the shapes, volumes, and densities of 3D GAs can be easily controlled through the modulation of the reaction vessel features, which indicates the possibility, as well as the feasibility, of large production.[11] Herein, we report a novel 3D AgX/GA (X = Br, Cl) structure, in which the AgX NPs are firmly and uniformly distributed throughout the surface of the 3D GA’s hierarchically porous structure. The recycling process gets very easy just with tweezers directly taking the 3D photocatalytic composite from one reaction system to another, which does not introduce any centrifugation, sonication, and drying processes. In addition, due to their light weight, the 3D AgX/GAs are capable of being suspended in the aqueous pollutant solution without any assistance of external force and the hierarchically porous morphology ensures that the photocatalyst NPs have effective contact with the contaminant molecules. Beyond those benefits of experimental operation, the introduction of a GA substrate also improves the photocatalytic performance of silver halide semiconductor significantly. In this work, compared with the pristine AgX, the preferable photocatalytic property of AgX/GAs is investigated and confirmed through two degradation processes, the oxidative degradation of methyl orange (MO) and the reduction of CrVI. No matter the convenient mode of operation or the promoted photocatalytic performance of 3D AgX/GAs, they both provide robust support for the future industrial application of photocatalysts. Figure 1 illustrates the fabrication scheme of the 3D photocatalytic composite AgBr/GAs and the corresponding images of resulting macroscopic materials with different shapes. The formation mechanism of such GAs is described and explained in the Supporting Information. Then the 3D GAs were immersed into a solution of cetyltrimethylammonium bromide (CTAB). Owing to the amphipathy of CTAB, the alkyl chain of the CTAB is likely to connect to the GAs leaving the ion segment (Br−) toward the solution. Then, the Ag+ can react evenly with the Br−, which results in a homogeneous in situ growth of AgBr NPs on the surface of the hierarchical pores of the GAs. The outstanding advantage of this synthetic method is that the amount of photocatalyst can be adjusted by the volume of the GAs, which in a row can be easily controlled by the reaction vessels (Figure 1b). What is more, such an in situ growth method ensures the firm binding of AgBr NPs between the GAs, which can be further certified by successive photodegradation tests.
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Figure 1. a) Fabrication of the AgBr/GAs 3D photocatalytic composite materials. b) The different sizes of the AgBr/GAs adjusted by the volumes of the reaction vessels.
of the AgBr nanoparticles in AgBr/GAs is given in Figure S1 (Supporting Information): the sizes of the AgBr nanoparticles are mainly distributed in the 70–150 nm range. As a comparison, AgBr without GAs is synthesized and corresponding SEM measurements are also performed (as shown in Figure S2, Supporting Information). It exhibits obviously that the pristine AgBr particles are much larger in size and tend to agglomerate into irregular shaped blocks, while the AgBr particles in the AgBr/GA composites are well scattered onto the GA surface. Therefore, the existence of the GAs dramatically inhibits the agglomeration of AgBr during the synthesis process, which results in the high photocatalytic activity of the AgBr. The energy-dispersive X-ray analysis (EDAX) spectra (Figure 2c) and the corresponding element mapping (Figure 2d–g) are listed to identify the element distribution of AgBr/GAs. Three elements (C, Ag, and Br) are entirely recognized in the sample and the other two unlabeled peaks therein are ascribed to O and Au. As shown in the element mapping, the three elements of C (Figure 2e), Ag (Figure 2f), and Br (Figure 2g) can be observed to exist homogeneously within the selected area on the AgBr/GAs (Figure 2d). X-ray diffraction (XRD) patterns (as shown in Figure 3a) reveal the crystal texture Figure 2. a,b) Typical SEM images of the AgBr/GAs under the different magnifications. c) The EDAX result of the AgBr/GAs. d–g) High-angle dark-field scanning transmission electron changes of AgBr/GAs before and after photocatalytic reaction. The distinct diffraction microscopy (d) and the element mapping images for C (e), Ag (f), and Br (g). The morphology of AgBr/Gas was examined by scanning electron microscopy (SEM) measurements. As shown in Figure 2a,b, it can be seen obviously that the AgBr nanoparticles are uniformly distributed throughout the graphene sheet surface of the GAs. Meanwhile, a histogram showing the size distribution
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peaks (2θ) at 25.3°, 31.2°, 44.6°, 55.5°, 64.7°, and 73.4° are attributed to the (111), (200), (220), (222), (400), and (311) crystal planes of AgBr (JCPDS file: 6-438). The peaks at 38.3°, 44.5°, and 64.5° are assigned to the (111), (200), and (220) crystal planes of metallic Ag0 (JCPDS file: 65-2871). Obviously, it can be concluded that metallic Ag0 has been produced after this photocatalytic reaction, demonstrating the photocatalytic performance improvement of AgBr/GAs during the cycling tests (as shown in Figure 4b). X-ray photoelectron spectroscopy (XPS) of AgBr/GAs before and after reaction with pristine AgBr was carried out and the results are shown in Figure 3b. Here, the two bands at ≈367.7 and 373.8 eV should be attributed to Ag 3d5/2 and 3d3/2, respectively.[12,13] It can be recognized that distinct band shifts to higher binding energy occur with incorporation of GAs and after the photocatalytic reaction. Such shifts reveal the decrease of charge density around the Ag atoms with the introduction of highly conductive GAs and the generation of metallic Ag0 after the photocatalytic reaction.[4] The result further confirms that AgBr in the composite of AgBr/GAs indeed acts as the electron donor and the GAs work as the electron acceptor. The peaks of Ag 3d5/2 and 3d3/2 after the photocatalytic reaction (Figure S3a, Supporting Information) could be deconvoluted into peaks at 368.5, 370.2 eV and 374.5, 376.0 eV, respectively.[14] Those peaks at 370.2 and 376.0 eV could be attributed to the metallic Ag0.[2] Meanwhile, as shown in Figure S3f (Supporting Information), the Br 3d peaks of AgBr are located at 67.5 (3d5/2) and 68.5 (3d3/2) eV. Similar to Ag 3d, a band shift of Br 3d can also be observed in AgBr/GAs before and after this
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photocatalytic reaction. The slight change in the system energy band might be attributed to the generation of Ag0. The Raman spectra of graphene oxide (GO) and AgBr/GAs are shown in Figure 3c, where the well-defined G-bands at ≈1612.8 cm−1 are attributed to the sp2 carbon type structure of the graphene nanosheets[15] and the D-bands at ≈1361.8 cm−1 confirm the existence of some defects within the graphene sheets.[16] It is also noticed that the typical G-band of GO at ≈1612.8 cm−1 shifts to a lower frequency at ≈1603.6 cm−1 for the AgBr/GA composite, suggesting that some structural perfections of GAs were acquired through hydrothermal reduction.[17] Since the intensity ratio of the D- and G-bands represents the order degree of graphene, ID-band/IG-band of GO and AgBr/GAs in Figure 3c are calculated to be 0.68 and 0.91, respectively, which signify a relatively low ordered carbon structure of the AgBr/GAs.[18] This could be attributed to the aggregation of graphene during the formation of GAs with the hydrothermal treatment process. Meanwhile, the reduction degree of the GAs is also investigated in Figure S4 (Supporting Information). Figure 3d displays the nitrogen adsorption–desorption isotherm of AgBr/GAs, which exhibits typical Type IV property and H2-type hysteresis loop. The presence of macropores is demonstrated by the prompt increase of the adsorption at relatively high pressure (P/P0 = 0.8–1.0).[19] The existence of mesopores is also confirmed by the inset figure in Figure 3d, which indicates that the pore size mainly lies in the range of 3–5 nm. The multipoint Brunauer–Emmett– Teller (BET) specific surface area of the AgBr/GAs is further measured to be 105.7 m2 g−1.
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Figure 4. a) Photocatalytic oxidative curves of MO by AgBr/GAs and AgBr under visible light (λ > 420 nm) and the absorptive curve of AgBr/GAs in the dark. b) Eight successive photodegradation dynamic curves of MO over AgBr/GAs. c) Photocatalytic reductive curves of CrVI by AgBr/GAs and AgBr under visible light (λ > 420 nm) and the adsorptive curve of AgBr/GAs in the dark. d) Five successive photoreductive dynamic curves of CrVI over AgBr/GAs. e) The quality of AgBr/GAs before photocatalytic reaction. f) The quality of AgBr/GAs after photocatalytic reaction. g) Tweezers are just used to take the AgBr/GAs from completed reaction system. h) Tweezers are just used to take the AgBr/GAs to a fresh system without centrifugation, ultrasound, and drying process. i) SEM image of AgBr/GAs after photocatalysis reaction. j) The AgBr/GAs composite suspends in the aqueous solution.
The photocatalytic activities of AgBr/GAs are estimated by the oxidative degradation of MO and the reduction of CrVI. In the light of the Lambert–Beer law, the actual concentration changes of MO (C/C0, C0 = 10 mg L−1) are directly proportional to the normalized absorption value (A/A0, λ = 463 nm) during the photodegradation at given time intervals.[20,21] Throughout the entire photocatalytic degradation procedure, weak stirring is utilized to simulate gentle water flow. Typical real-time absorption spectra of the MO dye during the photocatalytic process upon different photocatalyst samples are shown in Figure S5 (Supporting Information). It can be easily observed that the degradation rate of MO presents a regular tendency in the AgBr/GAs system (Figure S5a, Supporting Information), while an irregular degradation rate is noticed in the AgBr system (Figure S5b, Supporting Information). This should be attributed to the mild agitation conditions, which cannot be enough to sufficiently disperse the AgBr NPs in aqueous solution. Conversely, due to the light weight and porosity of GAs in AgBr/GAs composites, the loaded AgBr NPs are able to entirely suspend in solution and adequately contact with contaminant molecules (Figure 4j). As shown in Figure 4a, after visible light irradiation (>420 nm) for 8 min, the MO can be completely degraded by the AgBr/GAs, while only 65% is removed by pristine AgBr during the same interval. Since the photocatalytic oxidation process derived from silver halide is considered as a pseudo-first-order reaction,[4] the corresponding degradation rate constant (κ) has been calculated through the average level of the entire degradation process. Compared
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with bare AgBr without the GA component (κ = 0.12 min−1), the photocatalytic performance of AgBr/GAs (κ = 0.72 min−1) increases six times. Figure 4b demonstrates the photodegradation effect of the cycling tests of AgBr/GAs on MO. The photocatalytic activity of AgBr/GAs has a distinct growth along with the increase of the cycling number in the first four circulation process. This should be attributed to the localized surface plasmon resonance (SPR) of Ag0, which is inevitably produced during the photocatalytic process. The photocatalytic abilities of AgBr/GAs basically remain unchanged in the last four circulation process. Thus, it is inferred that the metallic Ag reaches a maximum after four cycles, as the photocatalytic activity reaches a maximum. In addition, X-ray photoelectron spectroscopy of Ag element have also been measured after two, four, six, and eight cycles (Figure S3, Supporting Information). It is observed that the content of Ag0 can reach a maximum of about 4.4%, which is consistent with previous reports (3–6%).[6,22] The mechanism of the stopping formation of Ag0 after four cycles has been considered as follows: the SPR-excited electrons on the surface of metallic Ag develop polarization fields, which produce many regions of negative and positive charges closing to the surface of AgBr.[23–25] The polarization fields can force the excited electrons further away from the surface of AgBr and close to the graphene sheet of the GAs, which prevents the surface electrons from combining with Ag+ ions to continuously generate Ag0. For enhancement of the photocatalytic ability of the AgBr/ GAs compared with pristine AgBr, three possible reasons are involved upon our theoretical analysis: i) the adsorption
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AgBr/GAs and pristine AgBr. As shown in Figure 4c, the incorporation of GAs improves the reduction ability of AgBr distinctly in comparison with the pristine sample and the reductive amount of CrVI by AgBr/GAs is calculated to be 1.5 times larger than that by bare AgBr at the same time interval. The cycling experiments regarding the photoreductive property of AgBr/GAs are similarly investigated and the results are shown in Figure 4d. Unlike the cycling tests on photodegradation of MO, no evident change in photocatalytic performance can be observed along with the increase of cycling number during the photoreduction process of CrVI. That should be attributed to the relatively long photoreductive time (80 min) in one photocatalytic reaction period, which has already resulted in the production of enough Ag0 in the first cycle. Moreover, the state of the Ag0 produced during the photocatalytic experiment is examined further by TEM and element mapping, which are shown in Figure S7 (Supporting Information). Here, the photocatalytic degradation of MO by AgBr/GAs is chosen as a model to explain the photocatalytic oxidation mechanism. Typical Mott–Schottky plots, UV–vis absorption spectra and transformed Kubelka–Munk function plots have been collected to accurately calculate the band construction of AgBr (Figure S8, Supporting Information). The conduction band (CB) and valence band (VB) of AgBr are estimated to locate at −0.67 and 1.93 eV, respectively (Figure 5). Here, when the AgBr/GAs composite is irradiated under visible light (>420 nm), the photoexcited electrons transfer into the CB of AgBr and the VB preserves the remaining holes.[26–28] Simultaneously, the existence of GAs not only acts as the substrate of AgBr NPs but also plays a significant role as an electro-reservoir, which leads to a highly efficient separation of the electrons and holes. The electrons located on the CB of AgBr or stored in the GAs, can produce •OH through a two-electron oxygen reduction route (O2 + 2H+ + 2e−→ H2O2 → •OH, O2/H2O2: 0.695 eV vs normal hydrogen electrode (NHE)) and O2•− by one-electron reduction (O2 + e− → O2•−, O2/O2•− = −0.33 eV).[6] Thus, both of the radicals (•OH, O2•−) play important roles in the photocatalytic degradation of MO. The remaining photogenerated holes can be transferred to oxide Br− ions into Br0 or directly participate in degradation of MO.[4] During the next degradative circulation of AgBr/GAs, the metal Ag0 gradually emerges and its influence on photodegradation seems remarkable (as illustrated in Figure 5b).
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capacity of the 3D GAs plays an important role. As revealed in Figure 3d, there is no doubt that the large BET specific surface owing to the porosity of GAs can greatly improve the adsorption property of MO. From the AgBr/GAs in the dark adsorptive curve in Figure 4a, the AgBr/GAs displays an admirable adsorptive capacity, which has even adsorbed 16% of MO in 10 min without any light irradiation. Nevertheless, this adsorption capacity is much lower than the photodegradation quantity at the same time, indicating that the photocatalytic activity is the main driving force in removing the pollutants and the preferable adsorption property of GAs servers as an effective assistance. ii) Since AgBr NPs in situ grow tightly onto the GAs, the photogenerated electrons could promptly inject into the graphene sheets through a percolation process under light irradiation.[2,20] Thereby, the charge recombination of electron–hole pairs is suppressed and the photocatalytic activity is therefore prompted. iii) The quality and morphology of the AgBr/GAs composite material can be maintained almost completely after photocatalytic reactions (Figure 4e,f,i). From Figure 4e,f, only 0.8% quality loss can be found during the whole degradation cycling process. Furthermore, the morphology after the cycling tests also keeps intact without any distinct change (Figure 4i). As is well known, the quality and morphology of a photocatalyst will definitely affect its activity. All these extraordinary properties of such an AgBr/GAs photocatalytic material demonstrate excellent performance during the degradation process, even for successive recycling. In addition, the fascinating appearance of a macroblock structure is also endowed to this 3D composite. Such AgBr/GAs bulk structure makes separation from a liquid reaction medium much easier just with tweezers (Figure 4g,h). When a photocatalytic cycle finished, the bulk photocatalyst is just taken out by using tweezers, rinsed and then employed into another photocatalytic cycle. Between the two cycles, it is unnecessary to perform tedious centrifugation, ultrasonication, and drying processes. Therefore, this bulk photocatalyst material exhibits great suitability for the practical applications of large scale sewage treatment due to its excellent stability, reproducibility, and operation convenience. The reduction activities of such AgBr/GAs on pollutant treatments have also been investigated through the reduction of CrVI to CrIII. Figure S6 (Supporting Information) shows the realtime absorption spectra of CrVI for photocatalytic reduction by
Figure 5. Schematic illustration of photocatalytic enhancement mechanism of AgBr/GAs in photocatalytic oxidation of MO before (a) and after (b) the emergence of Ag0. c) The mechanism of AgBr/GAs in photocatalytic reduction of CrVI.
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In principle, a Schottky barrier formed with combination of the AgBr semiconductor and metallic Ag due to the different work functions of the AgBr and Ag (ΦAgBr = 5.3 eV, ΦAg = 4.25 eV).[6] The electrons are inclined to transfer from the low work function (high Fermi level) to the high work function (low Fermi level) until their Fermi levels are equilibrated.[29] Because of the strong SPR of Ag, the electron would be enriched on the surface of the Ag0 under visible light irradiation, which lifts the Fermi level of Ag.[30] Subsequently, the SPR-excited electrons would transfer from Ag0 to AgBr or directly migrate to the GAs. Meanwhile, a similar degradation process with only AgBr/GAs can also occur in Ag@AgBr/GAs system. For the degradation effects of radicals and hole in a photodegradation system, detailed investigations are listed in the Supporting Information (Figure S9 and S10, Supporting Information). The photocatalytic reduction mechanism of AgBr/GAs on carcinogenic CrVI has also been investigated, as illustrated in Figure 5c. Owing to the lengthy reaction time (80 min), the generation of Ag0 should be initiated at the first photocatalytic reduction circulation. Thus, throughout the course of the photocatalytic cycles, the SPR effect of Ag NPs is always helpful for the photoreduction of CrVI. Similar to the migration of electrons in photocatalytic degradation of MO, free electrons on the surface of Ag NPs would prefer to transfer into the n-type AgBr semiconductor and the GA substrate. Different from the photocatalytic oxidation of MO, no apparent demand of an intermediate state is listed in the photoreduction of CrVI.[31] The photoexcited electrons enriched on the surface of AgBr and GAs would be directly trapped by CrVI in solution to form CrIII. Meanwhile, the remaining holes in the valence band of AgBr and Ag NPs would be depleted by the hole scavenger ethylene diamine tetraacetic acid (EDTA) to be converted to EDTA+.[32] In addition, the AgCl/GAs have also been synthesized. The SEM image of AgCl/GAs sample in Figure S11 (Supporting Information) shows that the AgCl NPs are also homogeneously distributed into the GAs, which exhibits outstanding capability of GAs as excellent substrate material. The photocatalytic activities of AgCl/GAs on the degradation of MO and reduction of CrVI are measured and presented in Figure S12 (Supporting Information). Highly similar to the AgBr/GAs system, compared with bare AgCl, preferable photocatalytic abilities are achieved upon AgCl/GAs. In summary, novel 3D structured AgX/GA (X = Br, Cl) composites with a macroscopic block appearance are synthesized successfully. Owing to the unique structure of AgX/GAs, this bulk composite material could just be recycled by directly clipping out using tweezers and washed with deionized water several times. After the cycling photocatalysis, only very little quality loss is observed and the morphology also remains invariant. Excellent cycling performance has also been well maintained even after multiple cycles on photocatalytic reactions. The corresponding photocatalytic degradation mechanism has been interpreted both theoretically and experimentally. Overall, in this photocatalytic system, the introduction of GAs has significantly promoted the photocatalytic performance. As a capable substrate for a photocatalyst, the GAs possess a general applicability and are anticipated to promote the evolution of photocatalysis toward commercial applications.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are most grateful to the Natural Science Fund Achievements (NSFC), China (Grant Nos. 21225524, 21205112, 21475122, 211175130, 21127006, 21127007, and 21127010), the Department of Science and Techniques of Jilin Province (Grant Nos. 20120308, 201215091, and SYHZ0006), and Chinese Academy of Sciences (YZ201354 and YZ201355) for their financial support. Received: January 25, 2015 Revised: April 3, 2015 Published online: May 20, 2015
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Yingying Fan, Weiguang Ma, Dongxue Han,* Shiyu Gan, Xiandui Dong, and Li Niu* Serious water pollution concerns, including various organic contaminants and carcinogenic hexavalent chromium, are faced by modern society. Photocatalytic pollution mitigation with inexhaustible solar energy has already been widely put into effect and is expected to be devoted into practical applications.[1] In this regard, silver halide photocatalysts, as photosensitive semiconductors, are considered to be one of the most promising photocatalysts.[2] In that the silver halide can preliminarily decompose into Ag0,[3] it is able to demonstrate strong plasmon resonance and enhance the photocatalytic ability.[4] In order to fully utilize the photocatalyst and reduce the overall production cost, successive recycling application becomes necessary.[5,6] It is widely known that, owing to the nanoscale morphology of photocatalyst nanoparticles (NPs), miscellaneous and sophisticated operations (such as centrifugation, sonication, and drying) are frequently used during the photocatalyst recovery process. Simultaneously, a serious deficiency of this recovery method is the quantitative loss of the photocatalyst NPs during the repeated circulation process.[2] In further commercial processes, the recovery of photocatalyst is very significant and should not be neglected. Just following the economical point of view, during practical commercial applications, rational experimental conditions should be employed, while operations that do not satisfy the industry status should be removed as far as possible. Normally, during traditional photocatalytic experiments, powerful agitation is an inevitable experimental procedure used to disperse the photocatalytic NPs into aqueous pollutant solution through strong turbulence, which will ensure sufficient contact between the photocatalyst NPs and the contaminants. However, this punchy mechanical stirring treatment does not conform to the practical industrial demand. Therefore, a novel state escalation toward photocatalyst to avoid the employment of the force convection treatment is of great significance and will further facilitate the industrial degradation process. Y. Fan, W. Ma, Dr. D. X. Han, S. Gan, X. Dong, Prof. L. Niu State Key Laboratory of Electroanalytical Chemistry c/o Engineering Laboratory for Modern Analytical Techniques Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022, Jilin, China E-mail: [email protected]; [email protected] Y. Fan, W. Ma, Dr. D. X. Han, S. Gan, X. Dong, L. Niu University of Chinese Academy of Sciences Beijing 100039, China
DOI: 10.1002/adma.201500391
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Convenient Recycling of 3D AgX/Graphene Aerogels (X = Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants
3D graphene aerogels (GAs) have aroused wide interest recently.[7–9] Their macroscopic block appearance makes them an easily and conveniently recycling operated photocatalytic candidate, which can be treated just using tweezers. Meanwhile, the hierarchically porous structure of 3D GAs can provide an ideal support for the deposition of photocatalyst NPs.[10] Moreover, compared with 2D graphene, the shapes, volumes, and densities of 3D GAs can be easily controlled through the modulation of the reaction vessel features, which indicates the possibility, as well as the feasibility, of large production.[11] Herein, we report a novel 3D AgX/GA (X = Br, Cl) structure, in which the AgX NPs are firmly and uniformly distributed throughout the surface of the 3D GA’s hierarchically porous structure. The recycling process gets very easy just with tweezers directly taking the 3D photocatalytic composite from one reaction system to another, which does not introduce any centrifugation, sonication, and drying processes. In addition, due to their light weight, the 3D AgX/GAs are capable of being suspended in the aqueous pollutant solution without any assistance of external force and the hierarchically porous morphology ensures that the photocatalyst NPs have effective contact with the contaminant molecules. Beyond those benefits of experimental operation, the introduction of a GA substrate also improves the photocatalytic performance of silver halide semiconductor significantly. In this work, compared with the pristine AgX, the preferable photocatalytic property of AgX/GAs is investigated and confirmed through two degradation processes, the oxidative degradation of methyl orange (MO) and the reduction of CrVI. No matter the convenient mode of operation or the promoted photocatalytic performance of 3D AgX/GAs, they both provide robust support for the future industrial application of photocatalysts. Figure 1 illustrates the fabrication scheme of the 3D photocatalytic composite AgBr/GAs and the corresponding images of resulting macroscopic materials with different shapes. The formation mechanism of such GAs is described and explained in the Supporting Information. Then the 3D GAs were immersed into a solution of cetyltrimethylammonium bromide (CTAB). Owing to the amphipathy of CTAB, the alkyl chain of the CTAB is likely to connect to the GAs leaving the ion segment (Br−) toward the solution. Then, the Ag+ can react evenly with the Br−, which results in a homogeneous in situ growth of AgBr NPs on the surface of the hierarchical pores of the GAs. The outstanding advantage of this synthetic method is that the amount of photocatalyst can be adjusted by the volume of the GAs, which in a row can be easily controlled by the reaction vessels (Figure 1b). What is more, such an in situ growth method ensures the firm binding of AgBr NPs between the GAs, which can be further certified by successive photodegradation tests.
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Figure 1. a) Fabrication of the AgBr/GAs 3D photocatalytic composite materials. b) The different sizes of the AgBr/GAs adjusted by the volumes of the reaction vessels.
of the AgBr nanoparticles in AgBr/GAs is given in Figure S1 (Supporting Information): the sizes of the AgBr nanoparticles are mainly distributed in the 70–150 nm range. As a comparison, AgBr without GAs is synthesized and corresponding SEM measurements are also performed (as shown in Figure S2, Supporting Information). It exhibits obviously that the pristine AgBr particles are much larger in size and tend to agglomerate into irregular shaped blocks, while the AgBr particles in the AgBr/GA composites are well scattered onto the GA surface. Therefore, the existence of the GAs dramatically inhibits the agglomeration of AgBr during the synthesis process, which results in the high photocatalytic activity of the AgBr. The energy-dispersive X-ray analysis (EDAX) spectra (Figure 2c) and the corresponding element mapping (Figure 2d–g) are listed to identify the element distribution of AgBr/GAs. Three elements (C, Ag, and Br) are entirely recognized in the sample and the other two unlabeled peaks therein are ascribed to O and Au. As shown in the element mapping, the three elements of C (Figure 2e), Ag (Figure 2f), and Br (Figure 2g) can be observed to exist homogeneously within the selected area on the AgBr/GAs (Figure 2d). X-ray diffraction (XRD) patterns (as shown in Figure 3a) reveal the crystal texture Figure 2. a,b) Typical SEM images of the AgBr/GAs under the different magnifications. c) The EDAX result of the AgBr/GAs. d–g) High-angle dark-field scanning transmission electron changes of AgBr/GAs before and after photocatalytic reaction. The distinct diffraction microscopy (d) and the element mapping images for C (e), Ag (f), and Br (g). The morphology of AgBr/Gas was examined by scanning electron microscopy (SEM) measurements. As shown in Figure 2a,b, it can be seen obviously that the AgBr nanoparticles are uniformly distributed throughout the graphene sheet surface of the GAs. Meanwhile, a histogram showing the size distribution
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peaks (2θ) at 25.3°, 31.2°, 44.6°, 55.5°, 64.7°, and 73.4° are attributed to the (111), (200), (220), (222), (400), and (311) crystal planes of AgBr (JCPDS file: 6-438). The peaks at 38.3°, 44.5°, and 64.5° are assigned to the (111), (200), and (220) crystal planes of metallic Ag0 (JCPDS file: 65-2871). Obviously, it can be concluded that metallic Ag0 has been produced after this photocatalytic reaction, demonstrating the photocatalytic performance improvement of AgBr/GAs during the cycling tests (as shown in Figure 4b). X-ray photoelectron spectroscopy (XPS) of AgBr/GAs before and after reaction with pristine AgBr was carried out and the results are shown in Figure 3b. Here, the two bands at ≈367.7 and 373.8 eV should be attributed to Ag 3d5/2 and 3d3/2, respectively.[12,13] It can be recognized that distinct band shifts to higher binding energy occur with incorporation of GAs and after the photocatalytic reaction. Such shifts reveal the decrease of charge density around the Ag atoms with the introduction of highly conductive GAs and the generation of metallic Ag0 after the photocatalytic reaction.[4] The result further confirms that AgBr in the composite of AgBr/GAs indeed acts as the electron donor and the GAs work as the electron acceptor. The peaks of Ag 3d5/2 and 3d3/2 after the photocatalytic reaction (Figure S3a, Supporting Information) could be deconvoluted into peaks at 368.5, 370.2 eV and 374.5, 376.0 eV, respectively.[14] Those peaks at 370.2 and 376.0 eV could be attributed to the metallic Ag0.[2] Meanwhile, as shown in Figure S3f (Supporting Information), the Br 3d peaks of AgBr are located at 67.5 (3d5/2) and 68.5 (3d3/2) eV. Similar to Ag 3d, a band shift of Br 3d can also be observed in AgBr/GAs before and after this
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photocatalytic reaction. The slight change in the system energy band might be attributed to the generation of Ag0. The Raman spectra of graphene oxide (GO) and AgBr/GAs are shown in Figure 3c, where the well-defined G-bands at ≈1612.8 cm−1 are attributed to the sp2 carbon type structure of the graphene nanosheets[15] and the D-bands at ≈1361.8 cm−1 confirm the existence of some defects within the graphene sheets.[16] It is also noticed that the typical G-band of GO at ≈1612.8 cm−1 shifts to a lower frequency at ≈1603.6 cm−1 for the AgBr/GA composite, suggesting that some structural perfections of GAs were acquired through hydrothermal reduction.[17] Since the intensity ratio of the D- and G-bands represents the order degree of graphene, ID-band/IG-band of GO and AgBr/GAs in Figure 3c are calculated to be 0.68 and 0.91, respectively, which signify a relatively low ordered carbon structure of the AgBr/GAs.[18] This could be attributed to the aggregation of graphene during the formation of GAs with the hydrothermal treatment process. Meanwhile, the reduction degree of the GAs is also investigated in Figure S4 (Supporting Information). Figure 3d displays the nitrogen adsorption–desorption isotherm of AgBr/GAs, which exhibits typical Type IV property and H2-type hysteresis loop. The presence of macropores is demonstrated by the prompt increase of the adsorption at relatively high pressure (P/P0 = 0.8–1.0).[19] The existence of mesopores is also confirmed by the inset figure in Figure 3d, which indicates that the pore size mainly lies in the range of 3–5 nm. The multipoint Brunauer–Emmett– Teller (BET) specific surface area of the AgBr/GAs is further measured to be 105.7 m2 g−1.
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Figure 4. a) Photocatalytic oxidative curves of MO by AgBr/GAs and AgBr under visible light (λ > 420 nm) and the absorptive curve of AgBr/GAs in the dark. b) Eight successive photodegradation dynamic curves of MO over AgBr/GAs. c) Photocatalytic reductive curves of CrVI by AgBr/GAs and AgBr under visible light (λ > 420 nm) and the adsorptive curve of AgBr/GAs in the dark. d) Five successive photoreductive dynamic curves of CrVI over AgBr/GAs. e) The quality of AgBr/GAs before photocatalytic reaction. f) The quality of AgBr/GAs after photocatalytic reaction. g) Tweezers are just used to take the AgBr/GAs from completed reaction system. h) Tweezers are just used to take the AgBr/GAs to a fresh system without centrifugation, ultrasound, and drying process. i) SEM image of AgBr/GAs after photocatalysis reaction. j) The AgBr/GAs composite suspends in the aqueous solution.
The photocatalytic activities of AgBr/GAs are estimated by the oxidative degradation of MO and the reduction of CrVI. In the light of the Lambert–Beer law, the actual concentration changes of MO (C/C0, C0 = 10 mg L−1) are directly proportional to the normalized absorption value (A/A0, λ = 463 nm) during the photodegradation at given time intervals.[20,21] Throughout the entire photocatalytic degradation procedure, weak stirring is utilized to simulate gentle water flow. Typical real-time absorption spectra of the MO dye during the photocatalytic process upon different photocatalyst samples are shown in Figure S5 (Supporting Information). It can be easily observed that the degradation rate of MO presents a regular tendency in the AgBr/GAs system (Figure S5a, Supporting Information), while an irregular degradation rate is noticed in the AgBr system (Figure S5b, Supporting Information). This should be attributed to the mild agitation conditions, which cannot be enough to sufficiently disperse the AgBr NPs in aqueous solution. Conversely, due to the light weight and porosity of GAs in AgBr/GAs composites, the loaded AgBr NPs are able to entirely suspend in solution and adequately contact with contaminant molecules (Figure 4j). As shown in Figure 4a, after visible light irradiation (>420 nm) for 8 min, the MO can be completely degraded by the AgBr/GAs, while only 65% is removed by pristine AgBr during the same interval. Since the photocatalytic oxidation process derived from silver halide is considered as a pseudo-first-order reaction,[4] the corresponding degradation rate constant (κ) has been calculated through the average level of the entire degradation process. Compared
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with bare AgBr without the GA component (κ = 0.12 min−1), the photocatalytic performance of AgBr/GAs (κ = 0.72 min−1) increases six times. Figure 4b demonstrates the photodegradation effect of the cycling tests of AgBr/GAs on MO. The photocatalytic activity of AgBr/GAs has a distinct growth along with the increase of the cycling number in the first four circulation process. This should be attributed to the localized surface plasmon resonance (SPR) of Ag0, which is inevitably produced during the photocatalytic process. The photocatalytic abilities of AgBr/GAs basically remain unchanged in the last four circulation process. Thus, it is inferred that the metallic Ag reaches a maximum after four cycles, as the photocatalytic activity reaches a maximum. In addition, X-ray photoelectron spectroscopy of Ag element have also been measured after two, four, six, and eight cycles (Figure S3, Supporting Information). It is observed that the content of Ag0 can reach a maximum of about 4.4%, which is consistent with previous reports (3–6%).[6,22] The mechanism of the stopping formation of Ag0 after four cycles has been considered as follows: the SPR-excited electrons on the surface of metallic Ag develop polarization fields, which produce many regions of negative and positive charges closing to the surface of AgBr.[23–25] The polarization fields can force the excited electrons further away from the surface of AgBr and close to the graphene sheet of the GAs, which prevents the surface electrons from combining with Ag+ ions to continuously generate Ag0. For enhancement of the photocatalytic ability of the AgBr/ GAs compared with pristine AgBr, three possible reasons are involved upon our theoretical analysis: i) the adsorption
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AgBr/GAs and pristine AgBr. As shown in Figure 4c, the incorporation of GAs improves the reduction ability of AgBr distinctly in comparison with the pristine sample and the reductive amount of CrVI by AgBr/GAs is calculated to be 1.5 times larger than that by bare AgBr at the same time interval. The cycling experiments regarding the photoreductive property of AgBr/GAs are similarly investigated and the results are shown in Figure 4d. Unlike the cycling tests on photodegradation of MO, no evident change in photocatalytic performance can be observed along with the increase of cycling number during the photoreduction process of CrVI. That should be attributed to the relatively long photoreductive time (80 min) in one photocatalytic reaction period, which has already resulted in the production of enough Ag0 in the first cycle. Moreover, the state of the Ag0 produced during the photocatalytic experiment is examined further by TEM and element mapping, which are shown in Figure S7 (Supporting Information). Here, the photocatalytic degradation of MO by AgBr/GAs is chosen as a model to explain the photocatalytic oxidation mechanism. Typical Mott–Schottky plots, UV–vis absorption spectra and transformed Kubelka–Munk function plots have been collected to accurately calculate the band construction of AgBr (Figure S8, Supporting Information). The conduction band (CB) and valence band (VB) of AgBr are estimated to locate at −0.67 and 1.93 eV, respectively (Figure 5). Here, when the AgBr/GAs composite is irradiated under visible light (>420 nm), the photoexcited electrons transfer into the CB of AgBr and the VB preserves the remaining holes.[26–28] Simultaneously, the existence of GAs not only acts as the substrate of AgBr NPs but also plays a significant role as an electro-reservoir, which leads to a highly efficient separation of the electrons and holes. The electrons located on the CB of AgBr or stored in the GAs, can produce •OH through a two-electron oxygen reduction route (O2 + 2H+ + 2e−→ H2O2 → •OH, O2/H2O2: 0.695 eV vs normal hydrogen electrode (NHE)) and O2•− by one-electron reduction (O2 + e− → O2•−, O2/O2•− = −0.33 eV).[6] Thus, both of the radicals (•OH, O2•−) play important roles in the photocatalytic degradation of MO. The remaining photogenerated holes can be transferred to oxide Br− ions into Br0 or directly participate in degradation of MO.[4] During the next degradative circulation of AgBr/GAs, the metal Ag0 gradually emerges and its influence on photodegradation seems remarkable (as illustrated in Figure 5b).
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capacity of the 3D GAs plays an important role. As revealed in Figure 3d, there is no doubt that the large BET specific surface owing to the porosity of GAs can greatly improve the adsorption property of MO. From the AgBr/GAs in the dark adsorptive curve in Figure 4a, the AgBr/GAs displays an admirable adsorptive capacity, which has even adsorbed 16% of MO in 10 min without any light irradiation. Nevertheless, this adsorption capacity is much lower than the photodegradation quantity at the same time, indicating that the photocatalytic activity is the main driving force in removing the pollutants and the preferable adsorption property of GAs servers as an effective assistance. ii) Since AgBr NPs in situ grow tightly onto the GAs, the photogenerated electrons could promptly inject into the graphene sheets through a percolation process under light irradiation.[2,20] Thereby, the charge recombination of electron–hole pairs is suppressed and the photocatalytic activity is therefore prompted. iii) The quality and morphology of the AgBr/GAs composite material can be maintained almost completely after photocatalytic reactions (Figure 4e,f,i). From Figure 4e,f, only 0.8% quality loss can be found during the whole degradation cycling process. Furthermore, the morphology after the cycling tests also keeps intact without any distinct change (Figure 4i). As is well known, the quality and morphology of a photocatalyst will definitely affect its activity. All these extraordinary properties of such an AgBr/GAs photocatalytic material demonstrate excellent performance during the degradation process, even for successive recycling. In addition, the fascinating appearance of a macroblock structure is also endowed to this 3D composite. Such AgBr/GAs bulk structure makes separation from a liquid reaction medium much easier just with tweezers (Figure 4g,h). When a photocatalytic cycle finished, the bulk photocatalyst is just taken out by using tweezers, rinsed and then employed into another photocatalytic cycle. Between the two cycles, it is unnecessary to perform tedious centrifugation, ultrasonication, and drying processes. Therefore, this bulk photocatalyst material exhibits great suitability for the practical applications of large scale sewage treatment due to its excellent stability, reproducibility, and operation convenience. The reduction activities of such AgBr/GAs on pollutant treatments have also been investigated through the reduction of CrVI to CrIII. Figure S6 (Supporting Information) shows the realtime absorption spectra of CrVI for photocatalytic reduction by
Figure 5. Schematic illustration of photocatalytic enhancement mechanism of AgBr/GAs in photocatalytic oxidation of MO before (a) and after (b) the emergence of Ag0. c) The mechanism of AgBr/GAs in photocatalytic reduction of CrVI.
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In principle, a Schottky barrier formed with combination of the AgBr semiconductor and metallic Ag due to the different work functions of the AgBr and Ag (ΦAgBr = 5.3 eV, ΦAg = 4.25 eV).[6] The electrons are inclined to transfer from the low work function (high Fermi level) to the high work function (low Fermi level) until their Fermi levels are equilibrated.[29] Because of the strong SPR of Ag, the electron would be enriched on the surface of the Ag0 under visible light irradiation, which lifts the Fermi level of Ag.[30] Subsequently, the SPR-excited electrons would transfer from Ag0 to AgBr or directly migrate to the GAs. Meanwhile, a similar degradation process with only AgBr/GAs can also occur in Ag@AgBr/GAs system. For the degradation effects of radicals and hole in a photodegradation system, detailed investigations are listed in the Supporting Information (Figure S9 and S10, Supporting Information). The photocatalytic reduction mechanism of AgBr/GAs on carcinogenic CrVI has also been investigated, as illustrated in Figure 5c. Owing to the lengthy reaction time (80 min), the generation of Ag0 should be initiated at the first photocatalytic reduction circulation. Thus, throughout the course of the photocatalytic cycles, the SPR effect of Ag NPs is always helpful for the photoreduction of CrVI. Similar to the migration of electrons in photocatalytic degradation of MO, free electrons on the surface of Ag NPs would prefer to transfer into the n-type AgBr semiconductor and the GA substrate. Different from the photocatalytic oxidation of MO, no apparent demand of an intermediate state is listed in the photoreduction of CrVI.[31] The photoexcited electrons enriched on the surface of AgBr and GAs would be directly trapped by CrVI in solution to form CrIII. Meanwhile, the remaining holes in the valence band of AgBr and Ag NPs would be depleted by the hole scavenger ethylene diamine tetraacetic acid (EDTA) to be converted to EDTA+.[32] In addition, the AgCl/GAs have also been synthesized. The SEM image of AgCl/GAs sample in Figure S11 (Supporting Information) shows that the AgCl NPs are also homogeneously distributed into the GAs, which exhibits outstanding capability of GAs as excellent substrate material. The photocatalytic activities of AgCl/GAs on the degradation of MO and reduction of CrVI are measured and presented in Figure S12 (Supporting Information). Highly similar to the AgBr/GAs system, compared with bare AgCl, preferable photocatalytic abilities are achieved upon AgCl/GAs. In summary, novel 3D structured AgX/GA (X = Br, Cl) composites with a macroscopic block appearance are synthesized successfully. Owing to the unique structure of AgX/GAs, this bulk composite material could just be recycled by directly clipping out using tweezers and washed with deionized water several times. After the cycling photocatalysis, only very little quality loss is observed and the morphology also remains invariant. Excellent cycling performance has also been well maintained even after multiple cycles on photocatalytic reactions. The corresponding photocatalytic degradation mechanism has been interpreted both theoretically and experimentally. Overall, in this photocatalytic system, the introduction of GAs has significantly promoted the photocatalytic performance. As a capable substrate for a photocatalyst, the GAs possess a general applicability and are anticipated to promote the evolution of photocatalysis toward commercial applications.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are most grateful to the Natural Science Fund Achievements (NSFC), China (Grant Nos. 21225524, 21205112, 21475122, 211175130, 21127006, 21127007, and 21127010), the Department of Science and Techniques of Jilin Province (Grant Nos. 20120308, 201215091, and SYHZ0006), and Chinese Academy of Sciences (YZ201354 and YZ201355) for their financial support. Received: January 25, 2015 Revised: April 3, 2015 Published online: May 20, 2015
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