超声辅助法合成的石墨烯_TiO_2复合材料声催化降解罗丹明B_英文_

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1、Enhanced Sonocatalytic Degradation of Rhodamine B by Graphene-TiO2Composites Synthesized by an Ultrasonic-Assisted MethodZHU Lei, Trisha GHOSH, Chong-Yeon PARK, MENG Ze-Da, OH Won-Chun*Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, KoreaAbstract: A serie

2、s of graphene-TiO2 composites was fabricated from graphene oxide and titanium n-butoxide (TNB) by an ultra-sonic-assisted method. The structure and composition of the nanocomposites were characterized by Raman spectroscopy, BET surface area measurements, X-ray diffraction, transmission electron micr

3、oscopy, and ultraviolet-visible absorption spectroscopy. The average size of the TiO2 nanoparticles on the graphene nanosheets was controlled at around 1015 nm without using surfactant, which is attributed to the pyro- lysis and condensation of dissolved TNB into TiO2 by ultrasonic irradiation. The

4、catalytic activity of the composites under ultrasonic irradia- tion was determined using a rhodamine B (RhB) solution. The graphene-TiO2 composites possessed a high specific surface area, which in- creased the decolorization rate for RhB solution. This is because the graphene and TiO2 nanoparticles

5、in the composites interact strongly, which enhances the photoelectric conversion of TiO2 by reducing the recombination of photogenerated electron-hole pairs.Key words: grapheme; ultrasonication; sonocatalytic degradation; adsorption; rhodamine BCLC number: O643Document code: AReceived 18 February 20

6、21. Accepted 20 March 2021.*Corresponding author. Tel: +82-41-660-1337; Fax: +82-41-688-3352; E-mail: wc_ohhansec.krEnglish edition available online at Elsevier ScienceDirect (w.sciencedirect /science/journal/18722067).TiO2-based materials are the most commonly used semi-conductor oxide photocatalys

7、ts because of their low envi- ronmental impact. However, there are numerous obstacles preventing the photocatalytic activity of these materials from being maximized, including low adsorption ability, detrimental recombination of charge carriers, and poor light utilization 1,2. In the past few decade

8、s, doping with metal ions, coupling with a second semiconductor, and anchoring TiO2 particles onto materials with a large surface area, such as mesoporous materials, zeolites or carbon-based materials, have all been used to improve the photodegradation ability of semiconductor oxide photocatalysts 3

9、5. TiO2-carbon nanotube composites show the potential as photocatalysts for use in both water and air purification 68. Carbon nanotubes enhance the photocatalysis activity both by acting as an electron sink to prevent charge carrier recombination 6 and by acting a photosensitizer to generate a great

10、er density of electron/hole pairs 7. Carbon nanotubes also behave as impurities and form TiOC bonds, which ex- tends the absorption of light to longer wavelengths 8.Graphene is a single-atom-thick sheet containing sp2-bonded carbon atoms in a hexagonal lattice and is of interest for applications in

11、condensed-matter physics, elec- tronics, and materials science, such as carbon nanotubes and buckminsterfullerene 9. Such interest is because graphene shows outstanding mechanical, thermal, optical, and elec- trical properties. Graphene-based materials have been usedin diverse fields such as nanoele

12、ctronic devices, biomate-rials, intercalation materials, drug delivery, and catalysis1017.Graphene-TiO2 composites have been successfully fabri- cated in various ways. Williams et al. 18 prepared a gra- phene-TiO2 composite by illuminating a suspension of gra- phene oxide (GO) and TiO2 under N2 with

13、 ultraviolet (UV) light 18. Inhibition of the UV light resulted in reducing conditions. Liang et al. 17 fabricated a graphene-TiO2 nanocrystal hybrid material by directly growing TiO2 nanocrystals on GO sheets. In this two-step method, TiO2 was first coated on GO sheets by hydrolysis and then crys-

14、tallized to form anatase nanocrystals by hydrothermal treatment. Wang et al. 19 prepared TiO2-graphene com- posites in aqueous solution through self-assembly using anionic sulfate as a surfactant to stabilize graphene. Chen et al. 20 prepared a visible light-responsive GO-TiO2 com- posite with a p-n

15、 heterojunction by adding sodium dodecyl sulfate to an aqueous of TiCl3 and GO, in which TiO2 could be excited by visible light 510 nm 20. Graphene-TiO2 composites with high photocatalytic activity have also been fabricated from GO and TiO2 P25 using a facile one-step hydrothermal method 21,22.Nanos

16、ized TiO2 powder has been used as a sonocatalyst to degrade organic pollutants under ultrasonic irradiation 23,24. It is difficult to completely mineralize opaque or translucent wastewater by photodegradation because of the chxZHU Lei et al.: Sonocatalytic Degradation of Rhodamine B by Graphene-TiO2

17、 Composites1277poor ability of light to penetrate such solutions. In the fewsuccessful examples of sonocatalysis reported to date, strict conditions or special catalysts have been required 25,26. Cavities and hot spots appear during ultrasonic treatment of aqueous solutions, leading to the dissociat

18、ion of water molecules. The hydroxyl (OH) radicals formed during such ultrasonic treatment possess high oxidative activity and can degrade toxic and industrial dyes. Ultrasound has been used to degrade polymers, and has received much attention as an advanced oxidation process to remove contaminants

19、from water. Ultrasound can allow complicated reactions to be performed using inexpensive equipment, and often requires fewer steps than conventional methods 2729.In this study, titanium n-butoxide (TNB) was used as a precursor of TiO2 and deposited on a graphene surface by chemical reduction of GO i

20、n ethanol-water using a sono- chemical method 30,31 that proceeds at high temperature. The effect of structural variation, surface state, and elemen- tal composition on the catalytic activity of the composite was investigated. The sonocatalytic activity of the gra- phene-TiO2 composite confirmed tha

21、t it was an excellent catalyst.increase the degree of oxidation of GO. The resultant brightyellow suspension was quenched by addition of distilled water (230 ml) followed by a solution of hydrogen peroxide (30%, 250 ml). The precipitate was separated by centrifuga- tion at 3000 r/min, and then washe

22、d initially with 5% HCl until sulfate ions were no longer detectable with barium chloride. The solid was then washed three times with ace- tone, and dried overnight in a vacuum oven. The prepared GO was transformed into GO sheets by sonication for 30 min at 308 K 33. This preparation procedure is ou

23、tlined in Fig. 1.KMnO4 30 g1ExperimentalUltrasonication for 30 min at308 KDrying in vaccum oven MaterialsFig. 1. Flow chart outlining the synthesis of GO.C16H36O4Ti (TNB) was used as a source of titanium toprepare TiO2 and graphene-TiO2 composites and was pur- chased from Kanto Chemical Company (Tok

24、yo, Japan). Rhodamine B (RhB, C28H31ClN2O3, 99.99%) was used as a model pollutant and was purchased from Samchun Pure Chemical Co., Ltd, Korea. TiO2 nanopowder ( 25 nm,99.7%) with an anatase structure used as control sample waspurchased from Sigma-Aldrich, USA. All chemicals were used without furthe

25、r purification. Experiments were carried out using distilled water. Synthesis of graphene-TiO2 composite catalystThe direct growth of TiO2 on GO sheets was achieved bya sonochemical method 27,30. TiO2 precursors were fab- ricated by preparing a solution with a molar ratio of etha- nol:H2O:TNB of 35:

26、15:4. GO g) was added and the resulting mixture was stirred for h at ambient tempera- ture. The suspension was sonicated at room temperature for3 h using a controllable serial-ultrasonic apparatus (Ultra- sonic Processor, VCX 750, Korea). The products were fil- tered, washed repeatedly with distille

27、d water and ethanol, and then vacuum dried at 373 K. The dried catalyst was ground in a ball mill and calcined at 773 K for 3 h to obtain a graphene-TiO2 composite. For comparison, TiO2 nanopar- ticles were prepared by a similar process without adding GO. Three kinds of graphene-TiO2 composites were

28、 prepared by changing the amount of GO, as listed in Table 1. A proposed mechanism for ultrasonic-assisted synthesis of the compos- ites is presented in Fig. 2. Synthesis of GOGO was prepared from graphite (KS-6) according to theHummers-Offeman method 32. In brief, graphite powder(10 g) was disperse

29、d in concentrated sulfuric acid (230 ml,98 wt%) cooled in a dry ice bath. Potassium permanganate (KMnO4, 30 g) was gradually added with continuous vig- orous stirring. The temperature of the mixture was pre- vented from exceeding 293 K by cooling. The dry ice bath was removed and replaced with a wat

30、er bath. The mixture was then heated to 308 K in 30 min under continuous stir- ring. Slow addition of deionized water (460 ml) caused a rapid increase in solution temperature up to a maximum of371 K. The reaction was maintained at 371 K for 40 min to Characterization of sonocatalystsRaman spectra we

31、re used to detect possible structural de-fects in the graphene flakes. The measurements were carriedGOGraphite oxide and distilled water solution30% H2O2 250 mlGraphite oxideDistilled water 230 mlPristine Graphite powder 10 gSulfuric acid 230 mlPreparation of Graphene oxide (GO)催化学 报1278Chin. J. Cat

32、al., 2021, 33: 12761283Table 1 Preparation condition and sample namesonicate the RhB solution. Control sample or graphene-TiO2composite g/100 ml of solution after addition of RhBsolution) was added to water (90 ml) and then sonicated for30 min to disperse the particles. RhB solution (2 10-4mol/L, 10

33、 ml) was then added to the suspension. The initialPreparation methodSample nameHummers-Offeman methodGONanoscale TiOEthanol:H O:TNB + ultrasound22Graphene oxide (0.2 g) + ethanol:H2O:TNB+ ultrasound + heat treatmentGraphene oxide (0.3 g) + ethanol:H2O:TNB+ ultrasound + heat treatmentGraphene oxide (

34、0.4 g) + ethanol:H2O:TNB+ ultrasound + heat treatmentGR-TiO -12-5concentration of RhB (c0) was 10 mol/L. The reac-tor was placed on a magnetic stirrer, and then stirred for 120 min in the dark to establish an adsorption-desorption equi- librium. The concentration of RhB was recorded as ct, and then

35、ultrasonic irradiation was started to induce degradation. The temperature of the reactor was controlled around room temperature (293 K) using a water bath. Degradation reac- tions were performed in a glass reactor (diameter = 5 cm, height = 7 cm) placed on a magnetic stirrer. The diameter of the ult

36、rasonic tip was cm, and the surface area of the ultrasonic probe was cm2. The mixture was exposed to ultrasound for 150 min and samples (3 ml) were with- drawn from the reactor every 30 min. Dispersed powders were removed using a centrifuge and then the filtrates wereanalyzed using a UV-Vis spectrop

37、hotometer (Optizen PopMecasys Co., Ltd., Korea).GR-TiO -22GR-TiO2-3out using a Horiba Jobin Yvon LabRAM spectrometer witha 100 objective lens and 532 nm laser excitation. The crystallographic structures of the composite photocatalysts were observed by X-ray diffraction (XRD, Shimadzu XD-D1, Japan) a

38、t room temperature with Cu K radiation. Diffuse reflectance ultraviolet-visible light (UV-Vis) spectra (DRS) were obtained using a scan UV-Vis spectrophotome- ter (Neosys-2000) equipped with an integrating sphere as- sembly. The morphologies of the sonocatalysts were ana- lyzed by a scanning electro

39、n microscope (SEM, JSM-5200JOEL, Japan) operating at keV that was equipped with an energy-dispersive X-ray analysis (EDX) system. A transmission electron microscopy (TEM, JEOL, JEM-2021, Japan) with an accelerating voltage of 200 kV was used to examine the size and distribution of the photocatalysts

40、. The BET surface areas of the photocatalysts were determined by measuring nitrogen adsorption isotherms at 77 K using a BET analyzer (Monosorb, USA).2Results and discussion Physicochemical propertiesRaman spectra can be used to quickly and accurately de-termine the number of layers and the crystal

41、structure of graphene after chemical treatment 34. Thus, Raman spec- troscopy was used to compare the crystal structures of GR-TiO2-2 and GO, as shown in Fig. 3. The Raman spec-trum of GO contains D and G bands at 1345 and 1592 cm1,respectively. The G band is common to all materials con- taining sp2

42、-bonded carbon atoms 35 and provides infor- Ultrasonic degradation of dye solutionsA controllable serial-ultrasonic apparatus (UltrasonicProcessor, VCX 750, Korea) operating at an ultrasonic fre- quency of 20 kHz and output power of 750 W was used toH2SO4 (NH4) 2SO4 TNBIntercalationEthanol H2OGraphi

43、teGraphite oxideSonochemicalGraphite layerFunctional groupTiO2 particleTiO2 precursorFig. 2. Proposed formation mechanism of TiO2 nanoparticles on graphene sheets by a sonochemical method. chxZHU Lei et al.: Sonocatalytic Degradation of Rhodamine B by Graphene-TiO2 Composites1279O(4)12001600 2000240

44、02800012345678910Raman shift (cm-1)Raman spectra of GR-TiO2-2 (1) and GO (2).Energy (keV)Fig. 4. EDX analysis of TiO2 (1) and composite photocatalystsGR-TiO2-1 (2), GR-TiO2-2 (3), and GR-TiO2-3 (4).Fig. 3.mation on their in-plane vibration 36. The presence of theD band suggests the GO contains sp3 d

45、efects 37. The sec- ond-order D band (2D band) at about 2720 cm1 is very sensitive to the stacking order of the graphene sheets along the c-axis as well as the number of layers, and becomes more structured (often a doublet) as the number of graphene layers increases. The stacking structure and agglo

46、merated morphology of the GO nanosheets are therefore consistent with those reported previously 34,38. In the Raman spec-trum of GR-TiO2-2, the D band is broadened and shifted to1354 cm1 compared with that for GO. The G band also shifted to around 1602 cm1. The relative intensity of the D/G bands wa

47、s increased after hydrothermal reaction, which is in agreement with the results of Lambert et al. 39 and Stankovich et al. 40. This further confirmed that GO was reduced to graphene and indicated that hydrothermal reaction considerably increased the size of the in-plane sp2 domains and thickness of

48、the graphitic structure.EDX and elemental analysis (wt%) of TiO2 and the gra- phene-TiO2 composites indicate that materials with high purity were successfully synthesized. The signal for C should mainly originate from the graphene sheets, while those for O and Ti are from the TiO2 nanoparticles. Fig

49、ure 4 shows that strong K and K peaks from Ti appear at and keV, respectively, while a moderate K peak from O appears at keV 41. Elemental analysis (wt%) of the samples revealing the ratio of C:O:Ti are shown in Table 2. Compared with the EDX results, the intensity of the TiO2 peak has decreased. In

50、 addition, the intensity of the signals TiO2 decreases from GR-TiO2-1 to GR-TiO2-3. Therefore, introduction of graphene can prevent the growth of anatase crystals.XRD patterns of graphene and GR-TiO2 composites are shown in Fig. 5. For graphene, peaks were observed at 2 of26 and 43 that could be ind

51、exed to the characteristic peaks (002) and (100) plane reflections, respectively, of graphene (JCPDS 01-0646) 42. Moreover, no typical dif- fraction peaks of graphite were found in these patterns, in-Table 2 Element mass of different samplesElement mass (%)Sample C O Ti TiO2GR-TiO2-1GR-TiO2-2GR-TiO2

52、-35.110.829.444.811.019.938.355.283.969.332.4dicating thatGO has beenreduced tographene duringhydrothermal reaction, which is consistent with previousreports 43-45. The patterns of both the nanoscale TiO2and GR-TiO2 composites exhibited diffraction peaks around2 of 37.9, 47.8, 54.3, 55, and 62.7, wh

53、ich could be indexed to the characteristic (004), (200), (105), (211), and (204) peaks of anatase TiO2 (JCPDS 21-1272), respectively 46. Peaks belonging to rutile TiO2 were not observed (JCPDS 21-1276) 47. These results suggest that the ana- tase form of TiO2 is dominant in all of the composites pre

54、- pared by sonochemical reaction.(211)(204)(100)210203040502q/( o )607080Fig. 5. XRD patterns of graphene, TiO2, and GR-TiO2 photocata-lysts.IntensityIntensityIntensity(101GR-TiO2-3GR-TiO -2)(002)(200) (004) (105)GR-TiO2-1TiO2GRTiC Ti (1) (2)(3) CTiD G2D (1) (2)(e(e)(a)(b)(c)(f)(g)(h)(d)(e)(e(e)催 化学

55、报1280Chin. J. Catal., 2021, 33: 127612831.5The surface microstructures and morphologies of theas-prepared composites were characterized by SEM and TEM (Fig. 6). GO possessed a flaky texture, reflecting its layered microstructure, as shown in Fig. 6(a). Large inter- spaces between layers and thin lay

56、er edges of GO were ob- served. The sonochemical method used to synthesize nano- scale TiO2, the GR-TiO2 composites, produced a favorable morphology of TiO2 with a slight tendency to agglomerate, as depicted in Fig. 6(b). Introduction of graphene caused the TiO2 particles to disperse on the graphene

57、 so that TiO2 ap- peared uniform (Fig. 6(c). A TEM image of GO (Fig. 6(d) showed that it consisted of thin stacked flakes with a well-defined structure containing a few layers at its edge. The nanoscale TiO2 control sample was composed of well-dispersed nanoparticles with an average size of around10

58、 to 15 nm, as shown in Fig. 6(e). The graphene-TiO2 composites exhibited a homogeneous dispersion of TiO2 nanoparticles attached to the almost transparent graphene sheets, which may support the growth of TiO2 crystals (Fig.6(f-h). In this approach, the reduction of GO and deposi-tion of TiO2 nanopar

59、ticles on graphene occurred simultane- ously. Once the reaction was complete, decoration with TiO2 helps to prevent both the aggregation of graphene sheets and TiO2. The formation mechanism of TiO2 nanoparticles and the exact role of graphene sheets in this process require further study to be fully

60、understood.The light-absorbance properties of the samples were characteristed by UV-Vis spectroscopy, as shown in Fig. 7. The absorption bands of GR-TiO2-2 were quite different from those of nanoscale TiO2. Because GR-TiO2-2 is black and absorbs strongly, the Kubelka-Munk theory could not be applied

61、 to this sample. Therefore, the DRS of TiO2 and GR-TiO2-2 cannot be compared quantitatively. The en- hanced light-harvesting intensity of GR-TiO2-2 compared with that of TiO2 could be possibly explained by the forma-1.00.50.200 300 400 500 600 700 800 900 1000 11000Wavelength (nm)Fig. 7. UV-Vis spec

62、tra of TiO2 and GR-TiO2-2.tion of chemical bonds between TiO2 and graphene, i.e.,TiOC, which facilitate charge transfer upon light excita- tion 48. Degradation of RhB Adsorption abilityTo evaluate the adsorption ability of TiO2 and theGR-TiO2 composite catalysts, the degradation of RhB was performed

63、 in the dark, as presented in Fig. 8. GR-TiO2 ad- sorbed more RhB than the TiO2 powder. This can be attrib- uted to the large surface area of the GR-TiO2 catalysts, as shown in Table 3, which improves adsorption ability. The enhanced adsorption ability can also be related to the amount of graphene in the GR-TiO2 catalysts 20. Sonocatalytic activityAn interesting alternative to the photo