Chinese Journal of Catalysis 38 (2017) 1307–1314
催化学报 2017年 第38卷 第8期 | www.cjcatal.org
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Article
Synthesis of graphene/tourmaline/TiO2 composites with enhanced activity for photocatalytic degradation of 2‐propanol Lili Yin a, Ming Zhao a, Huilin Hu a, Jinhua Ye a,b,c, Defa Wang a,b,* TJU‐NIMS International Collaboration Laboratory, Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China c International Center of Materials Nanoarchitectonics (WPI‐MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305‐0044, Japan a
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2016 Accepted 28 January 2017 Published 5 August 2017
Keywords: Photocatalysis Graphene Tourmaline TiO2 Composite 2‐Propanol Degradation
We report the construction of a graphene/tourmaline/TiO2 (G/T/TiO2) composite system with enhanced charge‐carrier separation, and therefore enhanced photocatalytic properties, based on tailoring the surface‐charged state of graphene and/or by introducing an external electric field arising from tourmaline. A simple two‐step hydrothermal method was used to synthesize G/T/TiO2 composites and poly(diallyldimethylammonium chloride)‐G/T/TiO2 composites. In the photocata‐ lytic degradation of 2‐propanol (IPA), the catalytic activity of the composite containing negatively charged graphene was higher than of the composite containing positively charged graphene. The highest acetone evolution rate (223 mol/h) was achieved using the ternary composite with the optimum composition, i.e., G0.5/T5/TiO2 (0.5 wt% graphene and 5 wt% tourmaline). The involve‐ ment of tourmaline and graphene in the composite is believed to facilitate the separation and transportation of electrons and holes photogenerated in TiO2. This synergetic effect could account for the enhanced photocatalytic activity of the G/T/TiO2 composite. A mechanistic study indicated that O2• radicals and holes were the main reactive oxygen species in photocatalytic degradation of IPA. © 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis using a semiconductor and solar light is widely regarded as an ideal green technique for dealing with the global energy crisis and environmental issues [15]. In re‐ cent decades, many researchers in the field of photocatalysis have focused their attention on improving the photocatalytic efficiency to enable practical applications [610]. The photo‐ catalytic efficiency is influenced by many factors, among which the separation and transportation of photoexcited charge car‐ riers (electrons/holes) play crucial roles. Previous studies have
shown that constructing composite semiconducting materials is an effective way of achieving good separation of photoexcited charge carriers and subsequent redox reactions on the catalyst surface, thus improving the photocatalytic efficiency [1115]. Based on the above considerations, we developed a novel graphene/tourmaline/TiO2 (G/T/TiO2) composite system. TiO2 has been widely studied as a photocatalyst for water splitting and photodegradation of organic pollutants [14,16,17] be‐ cause of its chemical inertness, cost effectiveness, environmen‐ tal friendliness, and stability against light and chemical corro‐ sion [18]. However, bare TiO2 usually has the drawbacks of
* Corresponding author. Tel/Fax: +81‐22‐27405065; E‐mail:
[email protected] This work was supported by the National Basic Research Program of China (973 Program, 2014CB239300), the National Natural Science Foundation of China (51572191), and the Natural Science Foundation of Tianjin (13JCYBJC16600). DOI: 10.1016/S1872‐2067(17)62795‐5| http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 8, August 2017
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limited light‐absorption ability (UV only) and rapid recombina‐ tion of photoinduced charge carriers, leading to a poor photo‐ catalytic performance when the entire solar spectrum is taken into account. Because of their intrinsic stoichiometry, graphene oxide (GO) exfoliated nanosheets are anionic two‐dimensional materials with large surface areas [19,20]. This unique two‐dimensional structure makes graphene an excellent cata‐ lyst support with good electronic conductivity/mobility. The use of semiconductor/graphene materials as photocatalysts with enhanced activities has been reported [21,22]. Graphene has also been used as an efficient cocatalyst with high activity in the photodegradation of organic pollutants [23] and photo‐ catalytic hydrogen evolution [24]. Tourmaline is a borosilicate mineral with the R3m space group [25]. The general chemical formula of tourmaline is XY3Z6Si6O18(BO3)3W4, where X is K, Na, Ca2, or a vacancy; Y is Mg2, Fe3, Al3, Cr3, V3, or Ti4; Z is Al3, Fe3, Mg2, Cr3, V3, or Fe2; and W is OH, O, or F. Tour‐ maline has a single‐symmetry polar axis and shows both pyro‐ electric and piezoelectric properties [26]. The electrostatic field on the surface of tourmaline arises from silicon‐oxygen octa‐ hedral distortion [26,27] and its direction is parallel to the c axis, i.e., opposing charges are present at different ends of tourmaline particles. The electric field strength increases with decreasing tourmaline particle size. When the particles are micron sized, the tourmaline surface has an electric field of strength 106107 V/m [25]. It is supposed that the electric field formed on the tourmaline surface plays an important role in the separation/transportation of photoexcited charge carriers, and therefore affects the photocatalytic activity of a composite semiconductor system containing tourmaline [28]. Here, we report the fabrication of G/T/TiO2 composites us‐ ing a two‐step hydrothermal method. The mass percentage of each component in the composite was optimized based on the composite’s performance in the photocatalytic degradation of 2‐propanol (IPA). We found that the usually negatively charged surface of GO obtained by chemical delamination [20] could be modified to become positively charged [29,30]. We therefore investigated and compared the effects of the surface charged state of GO on the photocatalytic performances of the compo‐ sites. We found that the electrostatic field on the tourmaline surface played an important role in the photocatalytic activity, and the activity of the composite with negatively charged GO was higher than that of the composite with positively charged GO. The related photophysical and photochemical mechanisms were also investigated. The results of our study provide an effective method for the development of composite photocata‐ lytic systems in which the charge carrier separation, and therefore the photocatalytic properties, can be modified by tailoring the surface charged state (e.g., GO) and/or by intro‐ ducing an external field (electric or magnetic) arising from a specific component (e.g., tourmaline) in the composite. 2. Experimental 2.1. Synthesis of GO GO was prepared from natural graphite powder using a
modified Hummers method. Typically, graphite powder (3.0 g) was dropped into a beaker of concentrated sulfuric acid (120 mL), which was cooled in an ice‐water bath. Then potassium permanganate (15 g) was gradually added to the mixture. After stirring in the ice‐water bath for 2 h, the mixture was trans‐ ferred to water at 308 K and stirring was continued for 1 h. The temperature was raised to 338 K, water (250 mL) was added, and the mixture was stirred gently for 2 h. The mixture was diluted with water to 1400 mL, H2O2 aqueous solution (30 wt%, 30 mL) was added, and the reaction was continued for 20 min. The suspension was centrifuged and washed with aqueous hydrochloric acid solution (10 wt%) until no sulfate ion was detected. The obtained GO was freeze dried [31]. 2.2. Synthesis of PDDA‐Functionalized GO GO was modified with poly(diallyldimethylammonium chlo‐ ride) (PDDA; 20 wt% in water, Mw = (2–3) × 105) as follows. GO (100 mg) and water (100 mL) were placed in a 200 mL beaker and the mixture was ultrasonicated for 2 h until the dispersion became clear, without any visible particles. The PDDA (30 mg) solution was mixed with water (100 mL). The GO suspension was slowly added dropwise, at a rate of 2 L/min, to the PDDA solution under stirring and the mixture was stirred overnight. Excess polymer was removed by repeated centrifugation (1 × 104 r/min, 10 min) with deionized (DI) water and the PDDA‐modified GO (denoted by P‐GO) was dried in a vacuum oven for 10 h [30]. 2.3. Synthesis of G/T/TiO2 Composites The ternary composites were synthesized using a two‐step hydrothermal method. Briefly, a mixture of GO (50 mg) or P‐GO (50 mg) and tourmaline (schorl, 500 mg) was dispersed in wa‐ ter (26 mL). The dispersion was transferred to a 50 mL Tef‐ lon‐lined stainless‐steel autoclave; the autoclave was sealed tightly, and heated at 453 K for 10 h. After cooling naturally, the black‐gray precipitates were collected by centrifugation, washed alternately with DI water and ethanol several times, and dried in a vacuum oven at 343 K for 4 h. The same method was used with different amounts of GO or P‐GO (0, 0.1%, 0.5%, and 1% by mass) and different amounts of tourmaline (0, 1%, 5%, and 10% by mass) in the starting material solution. Gra‐ phene/tourmaline powders were obtained after washing and drying at 353 K for 10 h. The prepared graphene/tourmaline powders were mixed with tetrabutyl titanate (TBT; 1.28 mL) and dispersed in water (26 mL). The aqueous solution was transferred to a 50 mL Teflon‐lined stainless‐steel autoclave and heated at 453 K for 10 h. Finally, the products were washed alternately with ID water and ethanol several times. The sam‐ ples were denoted by GX/TY/TiO2 and P‐GX/TY/TiO2, where X (01) and Y (010) are the mass percentages of graphene and tourmaline, respectively. For comparison, TiO2 powders were also synthesized using the hydrothermal method under the same conditions but without adding GO and tourmaline. 2.4. Material Characterization
2.5. Photocatalytic Activity Evaluation In the degradation of gaseous IPA, acetone is usually pro‐ duced as an intermediate before complete oxidation to CO2. IPA is therefore commonly used as a model organic pollutant for photocatalytic decomposition [32]. The photocatalytic decom‐ position of IPA was performed in a cylindrical static Pyrex glass vessel (total volume 500 mL). A certain amount of catalyst powder (the amount of TiO2 in all samples was 50 mg to enable activity comparisons) was evenly dispersed on a circular glass dish to give a uniform area and the dish was mounted in the vessel, which was sealed with a quartz cover and a rubber O‐ring. After carefully washing with artificial air, a drop of liq‐ uid IPA was injected into the vessel, which was kept in the dark for 3 h to achieve adsorption‐desorption equilibrium. The reac‐ tion was initiated by light irradiation using a xenon lamp oper‐ ated at 140 W. Gaseous samples (5 µL) were periodically ex‐ tracted from the reaction vessel and analyzed using a gas chromatography system (GC‐2014, Shimadzu) equipped with a flame ionization detector and a methanizer. 3. Results and discussion
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Crystal structures were determined by X‐ray diffraction (XRD; D8 Advanced, Bruker, Germany) using Cu‐K radiation ( = 0.154178 nm) at a scanning rate of 0.02/s. Ultraviolet‐visible (UV‐vis) diffuse reflectance spectra were recorded at room temperature (UV‐2700, Shimadzu, Japan) using BaSO4 as a ref‐ erence and converted to absorption spectra using the Kubel‐ ka‐Munk method. The Brunauer‐Emmett‐Teller surface areas were determined using a surface area analyzer (BET‐BJH‐AsiQcovoo 24, Quantachrome, USA). The morphol‐ ogies and microstructures of the samples were examined using scanning electron microscopy (SEM; S4800, Hitachi, Japan) and transmission electron microscopy (TEM; Technai G2 F20, FEI, the Netherlands). The surface charges were determined using a zeta potential analyzer (Nano ZS, Malvern, UK). During the measurements, the samples were suspended in DI water at low concentrations and the pH was kept constant at 6.5. Fouri‐ er‐transform infrared (FTIR) spectroscopy was performed using an FTIR spectrometer (Nicolet6700, Thermo Electron Corporation, USA).
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Intensity
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(1) Anatase (JCPDS No. 71-1166)
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2/( ) o
Fig. 1. XRD patterns of TiO2 (1), G0.5/T1/TiO2 (2), and G0.5/T5/TiO2 (3) powders.
Fig. 1 shows the XRD patterns of TiO2, and the G0.5/T5/TiO2 and G0.5/T1/TiO2 composites. The patterns all clearly show the characteristic peaks of the hexagonal anatase phase (JCPDS No.71‐1166), and the introduction of graphene and/or tourma‐ line had almost no effect on the XRD pattern and TiO2 crystal‐ linity. No peaks from either graphene or tourmaline were iden‐ tified because of the relatively small mass percentages of these two components in the composites. The microstructure of G0.5/T5/TiO2 was examined using SEM and TEM. As shown in Fig. 2(a), the SEM image of the G0.5/T5 binary composite before loading with TiO2 nanoparti‐ cles shows that graphene was evenly decorated with tourma‐ line particles. The TEM image of the G0.5/T5/TiO2 composite (Fig. 2(b)) shows that the TiO2 nanoparticles were uniformly loaded on the graphene/tourmaline support. The high‐resolution TEM image of the ternary composite in Fig. 2(c) shows a lattice spacing of 0.35 nm, corresponding to the (101) plane of anatase TiO2 (JCPDS No. 71‐1166) [24,33], and a lattice spacing of 0.398 nm, corresponding to the (220) plane of tour‐ maline (JCPDS No. 85‐1811). These results confirm the for‐ mation of a G0.5/T5/TiO2 ternary composite with good crystal‐ linity and an interfacial structure. As mentioned above, clear diffraction peaks from graphene
(c)
TiO2 (101) d=0.350 nm
Tourmaline (220) d=0.398 nm 5 nm Fig. 2. (a) SEM image of G0.5/T5, (b) TEM, and (c) high‐resolution TEM images of G0.5/T5/TiO2 composite.
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G
D Eg(1)
(2) 1634 1471 1399 1399
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3500
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Wavenumber (cm ) Fig. 5. FTIR spectra of GO (1) and PDDA‐functionalized GO (2).
and tourmaline were not detected because of their relatively small percentages in the composite. However, Raman scatter‐ ing is a more sensitive tool and to detect each component in the G/T/TiO2 composite. Fig. 3 shows Raman spectra of the syn‐ thesized GO, TiO2, and the G0.5/T5/TiO2 composite. The spec‐ tra contain bands at 150, 397, 515, and 638 cm–1, which can be assigned to the Eg(1), B1g(1), A1g + B1g(2), and Eg(2) modes, respec‐ tively, of anatase TiO2 [34]. Two bands, at about 1342 (D band) and 1599 cm–1 (G band), can also be observed, proving the presence of graphene in the G0.5/T5/TiO2 composite [35]. The ratio of the D and G band intensities (ID/IG) for G0.5/T5/TiO2 was larger than that for GO, indicating reduction of GO in the composite. Fig. 4 shows the UV‐vis diffuse reflectance spectra of TiO2, and the G0.5/T1/TiO2 and G0.5/T5/TiO2 composites. The main absorption edge for all three samples appears at around 370 nm, corresponding to the typical energy band gap of TiO2 (Eg: 3.35 eV). This implies that the introduction of gra‐ phene/tourmaline into the composite did not affect the absorp‐ tion range of TiO2. However, the enlarged spectra (inset) clearly show enhanced absorption in the visible‐light region for the
absorption ranges of graphene (500 nm) [26,36] and tourma‐ line (600800 nm) [28]. The FTIR spectra of GO and PDDA‐functionalized GO in Fig. 5 show vibrations from carboxylic groups in both positively and negatively charged GO at around 16271631 cm–1. This can be attributed to the absorption of water and skeletal vibrations of unoxidized graphite [37]. The absorption peak at around 1400 cm–1 corresponds to the carboxyl bending vibration [29], and the new peak, at around 1472 cm–1, in the PDDA‐modified GO spectrum arises from NH bond bending [30]. The surface charge properties of a material can be deter‐ mined based on the zeta potential. We used the electrokinetic method to measure the zeta potentials of GO, PDDA‐GO, G0.1/T1, PDDA‐G0.1/T1, and TiO2 samples in water. The zeta potentials of PDDA‐GO and GO were 60 and −35 mV, respec‐ tively, implying that GO was successfully functionalized with PDDA. The measured zeta potential of GO (35 mV) is close to that previously reported in the literature [38]. The results for G0.1/T1 (15.3 mV) and PDDA‐G0.1/T1 (26 mV) show electro‐ static attraction between tourmaline and oppositely charged graphene. The zeta potential of anatase TiO2 in water was 28 mV, which is consistent with the previously reported value [39]. The graphene and tourmaline contents in the composite were optimized by performing photocatalytic IPA decomposi‐ tion to acetone over various samples (i.e., G0.1/T1/TiO2, P‐G0.1/T1/TiO2, G0.5/T1/TiO2, G0.5/T5/TiO2, G0/T5/TiO2, and G0.5/T0/TiO2), in designed orthogonal experiments; the results are shown in Fig. 6. After irradiation for 1 h, the amount of acetone evolved over G0.1/T1/TiO2 (187 mol) was higher than that over P‐G0.1/T1/TiO2 (139 mol) (Fig. 6(a)). An elec‐ trostatic field is present on the surfaces of tourmaline particles as a result of spontaneous polarization [27] therefore the posi‐ tively charged end of tourmaline can attract negatively charged graphene, and the negatively charged end electrostatically at‐ tracts TiO2 nanoparticles, which usually have a positive surface charge [40,41]. Conversely, when the graphene surface is posi‐ tively charged, the negative end of tourmaline is attracted by graphene, leaving the positive end of tourmaline exposed. As a
Absorbance
Absorbance
Fig. 3. Raman spectra of GO (1), TiO2 (2), and G0.5/T5/TiO2 (3).
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Wavelength (nm) Fig. 4. UV‐vis diffuse reflectance spectra of TiO2 (1), G0.5/T1/TiO2 (2), and G0.5/T5/TiO2 (3) (inset: enlargement).
Lili Yin et al. / Chinese Journal of Catalysis 38 (2017) 1307–1314
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Fig. 6. Comparative activities for IPA photodegradation over (a) G0.1/T1/TiO2 (1) and P‐G0.1/T1/TiO2 (2), (b) TiO2, G0.1/T1/TiO2, G0.5/T1/TiO2, and G1/T1/TiO2, (c) TiO2, G0.5/T1/TiO2, G0.5/T5/TiO2, and G0.5/T10/TiO2, and (d) TiO2, T5/TiO2, G0.5/T5/TiO2, and G0.5/TiO2 under UV‐vis irradiation ( 300 nm).
result, the positive TiO2 particles are repelled. Apparently, a much closer neighborhood of the G/T/TiO2 composite can be achieved when the graphene is negatively charged directly by the hydrothermal process. It is believed that this composite configuration favors the separation/transfer of photogenerated electrons and holes, and this enhances the photocatalytic activ‐ ity in IPA degradation. Fig. 6(b) shows IPA degradation over ternary composites with different amounts of graphene. The amount of tourmaline was 1 wt%. Pure TiO2 gave a low acetone evolution rate (148 mol/h) because of easy electron‐hole pair recombination. The inclusion of graphene, i.e., in the G/T1/TiO2 hybrid, improved the IPA degradation activity. The optimum composition was G0.5/T1/TiO2, which gave the maximum acetone evolution (209 mol) after irradiation for 1 h. Further increases in the amount of graphene led to decreased activity because the in‐ troduced graphene blocked the light incident on TiO2. We also adjusted the weight content of tourmaline in the composite from 0 to 10 wt%, with a graphene content of 0.5 wt%. Fig. 6(c) shows that the composite with 5.0 wt% tourmaline gave the highest acetone evolution rate (223 mol/h). The acetone evo‐ lution rate decreased with increasing tourmaline content be‐
cause too much gray tourmaline reduced light absorption by TiO2. Generally, the photocatalytic activity of a semiconductor is affected by several parameters, e.g., the surface area, crystallin‐ ity, absorbance, and the presence of an electric field. The full width at half maximum (FWHM) of the XRD pattern can be used as a semi‐quantitative index of the crystallinity. Our measurements showed that all the composite samples had sim‐ ilar surface areas (160 m2/g) and FWHM values (1.26), re‐ gardless of their compositions. These results together with the UV‐vis spectra (see Fig. 4) imply that the surface area, crystal‐ linity, and absorbance were not the key factors accounting for the different activities in IPA photodegradation. Fig. 6(d) shows that the activity of the ternary composite G0.5/T5/TiO2 was higher than that of the binary systems G0.5/TiO2 and T5/TiO2, or TiO2 alone. It should be noted that no obvious decrease in the activity was observed during long time periods and re‐ peated reactions, confirming that the composites were stable in photocatalytic reactions under the current experimental condi‐ tions. The enhanced photocatalytic activity of the G/T/TiO2 com‐ posite can be attributed to the synergetic effect of tourmaline
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PL intensity
(2)
(3)
(4)
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Wavelength (nm) Fig. 7. Photoluminescence spectra of TiO2 (1), T5/TiO2 (2), G0.5/TiO2 (3), and G0.5/T5/TiO2 (4).
and graphene, which facilitates separation and transportation of the electrons and holes photogenerated in TiO2. This is sup‐ ported by the photoluminescence (PL) intensity decrease and the slight blue shift of the photoluminescence peak for the composite compared with those for TiO2 (see Fig. 7), indicating photoexcited electron transfer from TiO2 to graphene and/or tourmaline. A similar phenomenon has also been observed for two nanocomposite systems consisting of PbS quantum dots and carbon nanotubes [12] or TiO2 nanobelts [13]. To investigate the mechanism of IPA photodegradation over G0.5/T5/TiO2, we used t‐butyl alcohol (TBA), benzoquinone (BQ), and ammonium oxalate (AO) as scavengers of the species for •OH, O2•, and holes, respectively. Fig. 8 shows that the in‐ troduction of TBA does not affect the degradation rate, but the presence of BQ and AO significantly decreases the IPA photo‐ degradation rate. These observations imply that O2• and holes are the main reactive oxygen species in IPA photodegradation, and the holes are less important than O2• in the photocatalytic process. Based on these results, we propose that IPA degrada‐
Acetone evolution (mol)
200 G0.5/T5/TiO2 TBA-G0.5/T5/TiO2 AO-G0.5/T5/TiO2 BQ-G0.5/T5/TiO2
150
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Scheme 1. Mechanism of IPA photodegradation over G/T/TiO2 compo‐ site under UV‐vis irradiation.
tion over the G0.5/T5/TiO2 composite mainly involves the fol‐ lowing three processes: (1) electrons and holes are excited from the semiconductor upon irradiation; (2) electrons com‐ bine with O2 molecules to produce O2• radicals, which oxidize IPA to acetone; and (3) holes directly oxidize IPA to acetone [42,43]. Scheme 1 shows the proposed reaction mechanism for IPA photodegradation over the G0.5/T5/TiO2 composite. On irradi‐ ation, electrons are generated in TiO2 and transferred to the surface. In a G/TiO2 system, the electrons generated in TiO2 are rapidly injected into graphene, which has a slightly lower po‐ tential. The superior electronic conductivity of graphene effec‐ tively suppresses the recombination of electrons and holes generated in TiO2 because the electrons and holes migrate in opposite directions, driven by the electric field on tourmaline [26], i.e., the electrons diffuse to the positively charged end of tourmaline and the holes migrate toward the negatively charged end of tourmaline. During this process, electrons are transferred to graphene and combine with a molecule of O2 to produce the O2• radical, and then the O2• radical oxidizes IPA to acetone [41]. In addition, some TiO2 nanoparticles are pre‐ sent on the tourmaline surface, and the photogenerated elec‐ trons can reduce O2 to form O2• radicals, which oxidize IPA on the surfaces of TiO2 and tourmaline. The composite can effec‐ tively suppress the recombination of electron‐hole pairs gener‐ ated in TiO2 in three ways, and can therefore prolong the life‐ times of the charge carriers and improve the photocatalytic activity. 4. Conclusions
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Irradiation time (min) Fig. 8. Photodegradation of IPA to acetone over G0.5/T5/TiO2 compo‐ site in presence of various scavengers under UV‐vis irradiation ( 300 nm).
G/T/TiO2 composites and PDDA‐G/T/TiO2 composites were prepared using a simple two‐step hydrothermal method. In the photocatalytic degradation of IPA, the performance of the ter‐ nary composite (G/T/TiO2) was better than that of the binary systems (G/TiO2, T/TiO2) or TiO2 alone. The optimum ternary composite composition was G0.5/T5/TiO2 (0.5 wt% graphene and 5 wt% tourmaline), over which the highest acetone evolu‐
Lili Yin et al. / Chinese Journal of Catalysis 38 (2017) 1307–1314
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Graphical Abstract Chin. J. Catal., 2017, 38: 1307–1314 doi: 10.1016/S1872‐2067(17)62795‐5 Synthesis of graphene/tourmaline/TiO2 composites with enhanced activity for photocatalytic degradation of 2‐propanol Lili Yin, Ming Zhao, Huilin Hu, Jinhua Ye, Defa Wang * Tianjin University, China; Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), China; National Institute for Materials Science, Japan A graphene/tourmaline/TiO2 composite system with enhanced photocatalytic activity in the degradation of 2‐propanol was syn‐ thesized. The superior electronic conductivity of graphene and the electrostatic field on the tourmaline nanoparticle surfaces facili‐ tate the separation and transportation of electrons and holes pho‐ togenerated in TiO2.
tion rate (223 mol/h) was achieved. The enhanced photo‐ catalytic activity of the G/T/TiO2 composites can be ascribed to the synergetic effect of tourmaline and graphene, which facili‐ tates separation and transportation of electrons and holes photogenerated in TiO2. A mechanistic study indicated that O2• and holes were the main reactive oxygen species in photocata‐ lytic degradation of IPA. The results of this work show that for a composite system, the charge carrier separation, and hence the photocatalytic properties, can be effectively modified by tailor‐ ing the surface charged state and/or by introducing an external field (electric or magnetic) via material design.
[15] R. Marschall, Adv. Funct. Mater., 2014, 24, 24212440. [16] J. W. Tang, H. D. Quan, J. H. Ye, Chem. Mater., 2007, 19, 116122. [17] M. Grandcolas, J. H. Ye, Sci. Technol. Adv. Mater., 2010, 11,
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石墨烯/电气石/二氧化钛复合材料的制备及其光催化降解异丙醇性能 尹利利a, 赵
明a, 胡慧林a, 叶金花a,b,c, 王德法a,b,*
天津大学-日本国立物质材料研究机构国际合作实验室, 先进陶瓷与加工技术教育部重点实验室, 天津市材料复合与功能化重点 实验室, 天津大学材料科学与工程学院, 天津300072, 中国 b 天津化学化工协同创新中心, 天津300072, 中国 c 日本国立物质材料研究机构, 筑波茨城305-0044, 日本
a
摘要: 光催化是一种理想的应对全球能源短缺和环境污染问题的绿色化学技术, 可以实现有机物降解、水分解和二氧化碳 光还原等. 光催化反应效率受诸多因素影响, 其中光生载流子(电子和空穴)的分离和传输具有至关重要的作用. 以往研究 表明, 构筑多元复合光催化材料体系有利于光生电子和空穴有效分离和传递, 促进催化剂表面的还原和氧化反应, 从而提 高其光催化效率. 基于以上考虑, 我们提出了一种新型的石墨烯/电气石/TiO2三元复合光催化材料体系, 其中TiO2因其价格 低廉、无毒和抗光腐蚀等优点而被广泛用作光催化材料; 石墨烯(G)拥有独特的二维结构、高的电子迁移率、大的比表面 积, 是一种优异的催化剂载体; 电气石(T)的一个重要性质是表面存在自发极化的静电场, 该静电场将会影响光激发载流子 的分离、传递和光催化反应过程. 利用水热法合成了不同成分的石墨烯/电气石/TiO2三元复合材料体系. 为了对比研究石墨烯表面电荷性质的影响, 其 中一组的石墨烯(氧化石墨)为直接采用改良的Hummers法所制备, 其表面带负电; 另一组的石墨烯经聚二烯丙基二甲基氯 化铵(PDDA)修饰, 使其表面带正电. X射线衍射结果显示, 三元复合材料中TiO2为锐钛矿相, 其结晶性没有因为与石墨烯 和电气石的复合而受到影响. 扫描和透射电子显微分析表明, TiO2的平均颗粒大小为15 nm左右, 并且与石墨烯和电气石均 匀复合. 傅里叶变换红外光谱和zeta电位表征分析证实, PDDA可以有效地对石墨烯进行功能化改性, 使其表面带正电. 紫 外-可见分光光谱显示, 石墨烯/电气石/TiO2三元复合材料与TiO2的吸收带边一致, 复合材料中石墨烯和电气石并没有改变 TiO2的光吸收特征. 光催化降解异丙醇实验表明, 石墨烯/电气石/TiO2三元复合材料优于单纯的TiO2、石墨烯/TiO2以及电气石/TiO2二元复 合材料, 当石墨烯和电气石的质量百分比分别为0.5%和5%时, 三元复合材料降解异丙醇产生丙酮的速率达到最高(223 μmol/h). 特别值得指出的是, 由表面带负电的石墨烯组成的复合材料比由带正电荷的PDDA-石墨烯组成的复合材料具有 更高的光催化性能, 原因如下: 在水溶液中显示正zeta电位值的TiO2与带负电的石墨烯/电气石复合物静电吸引而均匀紧密 复合, 有利于TiO2中光生电子和空穴的快速分离和传递, 从而使得石墨烯/电气石/TiO2三元复合材料具有较高的光催化性 能; 而带正电的PDDA-石墨烯/电气石复合物和TiO2颗粒相互排斥而不宜复合, 导致PDDA-石墨烯基复合材料的光催化活 性降低. 机理研究揭示, 在三元复合材料光催化降解异丙醇的反应中起主要作用的是光生电子和空穴. 基于以上研究结 果, 我们提出了三元复合材料光催化降解异丙醇的反应机理. 关键词: 光催化; 石墨烯; 电气石; 二氧化钛; 复合材料; 异丙醇; 降解 收稿日期: 2016-12-29. 接受日期: 2017-01-28. 出版日期: 2017-08-05. *通讯联系人. 电话/传真: (022)27405065; 电子信箱:
[email protected] 基金来 源 : 国家 重 点基 础 研究 发 展 计 划 (973计 划 , 2014CB239300); 国家自然科学基金(51572191); 天津市自然科学 基 金 (13JCYBJC16600). 本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).
