Significantly Enhanced Visible-Light-Induced Photocatalytic

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Significantly Enhanced Visible-Light-Induced Photocatalytic Performance of Hybrid Zn−Cr Layered Double Hydroxide/Graphene Nanocomposite and the Mechanism Study Meng Lan, Guoli Fan,* Lan Yang, and Feng Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P. O. Box 98, Beijing, 100029, P. R. China S Supporting Information *

ABSTRACT: In the present work, hybrid nanocomposites of Zn-Cr layered double hydroxide (ZnCr-LDH) and graphene were assembled successfully via a simple one-step coprecipitation method. The assembly process included the nucleation and growth of ZnCr-LDH crystals and the simultaneous reduction of GO in the absence of additional reducing agents. The experimental results revealed that ZnCr-LDH nanoplatelets with the diameter size of ∼6 nm were well dispersed on the graphene surface, and as-assembled hybrid ZnCr-LDH/graphene nanocomposites exhibited significantly improved visible-light-driven photocatalytic activity in the degradation of Rhodamine B, in comparison with pure ZnCr-LDH, which was attributable to the unique heteronanostructure of ZnCr-LDH/graphene, facilitating the efficient transportation and separation of photogenerated charges and thus continuously generating reactive oxygen species. The present work could open a new doorway for fabricating visiblelight-deriven graphene-based photocatalysts for pollutant degradation via an advanced oxidation process.

1. INTRODUCTION Over the past decade, increasing attention has been focused on environmental pollution, which is becoming a serious ecological problem. Effluents discharged from industries and households contain many categories of biotoxics and dyestuffs, which are hardly chemically decomposed and may be transmuted to toxic compounds hurtful to ecosystem and human being.1,2 At present, semiconductor photocatalysts are gaining great interest in eliminating pollutants from aqueous solutions.3−5 Especially, a large number of research interests have been drawn to the photocatalytic degradation of organic pollutants under visible light irradiation.6−9 The most popular photocatalyst, TiO2, however, only exhibits photocatalytic activity under ultraviolet (UV) irradiation due to its wide band energy.10 Doping heteroatoms into TiO2 and incorporating other metal and/or semiconductor with TiO2 to form photoactive heterojunction are commonly used methods to enhance its sensitivity toward visible light region.11−15 For instance, due to the formation of a Fe2O3−TiO2 heterojunction, α-Fe2O3@TiO2 core−shell photocatalyst is found to able to improve the optical response in the range of visible light.16 However, in most cases, the transition efficiency is limited because of inherent physicochemical characters of TiO2. Although other semiconductors or semiconductor-based composites (e.g., Cu2O,17 Bi2WO6,18 CdS, 19 CuS, 20 BiVO 4 , 21 ZnO/In 2 S 3 , 22 WO 3 /g-C 3 N 4 , 23 BiOCl/BiVO424) also have been applied as photocatalysts for the treatment of pollutants under visible light in recent years, they often suffer from the drawbacks such as low activity and poor stability. Therefore, the design of efficient and stable visible-light-active photocatalysts always is an important issue. Among a new generation of heterogeneous photocatalysts, layered double hydroxides (LDHs, [MII1−xMIIIx(OH)2]x+(An‑)x/n·mH2O) are very much promising photocatalysis materials. LDHs belong to a class of highly © XXXX American Chemical Society

ordered two-dimensional layered anionic clays, where divalent and trivalent cations are orderly prearranged in the brucite-like layers at the atomic level and charge-compensating anions are present in the interlayer space.25−27 They can be artificially synthesized under laboratory conditions at low cost.28 It was reported that ZnM-LDHs (M = Ti, Cr, Fe) were effective visible-light active photocatalysts for pollutant degradation and water splitting,29−31 where the charge transfer and separation lead to the generation of electrons and holes in laminar structure of LDHs. Recently, carbonate-intercalated binary MIICr-LDHs (M = Ni, Co, Cu) and ternary NiZnCr-LDHs were found to be excellent photocatalysts in the degradation of organic pollutants and H2 production from water under visible light irradiation,32,33 respectively, due to the existence of metal linkages of MII−O-CrIII in the brucite-like layers. As for LDHbased photocatalysts, how to improve their performance or endue novel functionality still remains a big challenge. In an earlier study, our group found that ZnAl-LDH/carbon nanotubes nanocomposites exhibited enhanced photocatalytic activity in the degradation of methyl orange molecules,34 which was associated with unique hybrid nanostructures and tunable composition. As a zero band gap semiconductor carbon nanomaterial, graphene with a two-dimensional (2D) honeycomb-like structure and long-range π-conjugation possesses remarkable electrical, thermal, and mechanical properties.35−38 Especially, its sheet-like structure with excellent carrier mobility greatly facilitates shuttling electrons.37 Recently, graphene-based photocatalysts have attracted increasing attention,39−45 taking Received: April 22, 2014 Revised: July 4, 2014 Accepted: August 5, 2014

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dx.doi.org/10.1021/ie501650g | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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0.12 M. Subsequently, the above solution was titrated with a mixed alkali solution of NaOH (0.2 M) and Na2CO3 (0.1 M) under continuous stirring at room temperature until pH = 9.0. The obtained suspension was stirred at 95 °C for 24 h. The black solid was filtered and washed with deionized water and finally dried in a vacuum oven at 60 °C for 24 h. As reference catalyst, ZnCr-LDH was also synthesized without the addition of GO under identical experimental conditions. 2.3. Characterization. Powder X-ray diffraction (XRD) was analyzed using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu Kα source (λ = 0.15418 nm). Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) observations were performed using a JEOL JEM-2010 electron microscope with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was carried out using a Hitachi S4700 instrument. The accelerating voltage applied was 20 kV. Room-temperature Fourier transform infrared (FT-IR) spectra were collected on a Bruker Vector 22 spectrometer. Solid state UV−vis diffuse absorption spectra were recorded at room temperature and in the air on Shimadzu UV-2501PC spectrometer. Raman spectra were recorded at room temperature on a Jobin Yvon Horiba HR800 spectrometer using an Ar+ laser of 532 nm wavelength as excitation source. X-ray photoelectron spectra (XPS) were collected on a Thermo VG ESCALAB2201-XL X-ray photoelectron spectrometer with a monochromatic Al Kα X-ray excitation (1486.6 eV) as the source radiation at a base pressure of 2 × 10−9 Pa. Atomic force microscopy (AFM) observations were performed on DI nanoscope IV using a tapping mode. The decrease in total organic carbon (TOC) was evaluated by Apollo 9000 TOC analyzer. Photoluminescence data (PL) were acquired using a FLUOROMAX-4 spectrophotometer at room temperature. Electron spin resonance (ESR) signals of the active radicals spin-trapped by 5,5-dimethylpyrroline-N-oxide (DMPO) were recorded on a Bruker EPR 300E spectrometer, and the concentration of dye is 1 × 10−5 M, including 2 mg of catalyst; λ = 532 nm. Transient photocurrent response and electrochemical impedance spectroscopy (EIS) measurements were performed in 0.1 M Na2SO4 electrolyte solution using a CHI 660 C electrochemical workstation with a conversional three-electrode system at room temperature. A platinum foil electrode was used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Indium−tin oxide (ITO) glasses were ultrasonically cleaned in ethanol and acetone for 30 min, respectively, and dried at 60 °C for 2 hours. Then, 10 mg sample was ultrasonically dispersed in 1 mL of ethanol, and the resulting slurry was spread onto the pretreated ITO glasses served as the working electrode with an exposed area of 4 cm2. As for transient photocurrent response test, the light source was a 300 W Xe arc lamp equipped with a 420 nm UV cutoff filter, and the transient photocurrent of the photocatalysts responded to the light on and off was measured at 0.0 V to stimulate real photocatalysis reaction condition. EIS data were obtained at open circuit potential in the frequency range of 10 m Hz−100 kHz with an ac perturbation of 10 mV. 2.4. Photocatalytic Test. Photodegradation of RhB was carried out to investigate the catalytic activities of the prepared

into account the fact that photogenerated electrons can pass from the catalytic surface to graphene sheets, thus enhancing the photo conversion efficiency via the depression of the charge recombination. To date, a number of graphene-based nanocomposites containing metal nanoparticles, metal oxides, and polymers were reported.46−51 However, the synthesis of LDH/ graphene composites is rarely reported.52−56 Very recently, NiTi-LDH/reduced graphene oxide was reported to be able to show visible-light-responsive photocatalytic performance toward water oxidation.55 On the other side, a variety of methods have been established to prepare graphene sheets from monolayer to a few layers through exfoliating natural graphite flakes.39,57−60 Among them, the most widely adopted way for the bulk production of graphene is the chemical exfoliation of graphite to graphene oxide (GO) by strong oxidizing agents.60 However, in this case, additional reducing agents are necessary for the subsequent reduction of GO, which unavoidably brings some drawbacks such as complicated preparation procedure and high energy consumption. As for photocatalysts, increasing the lifetime of photogenerated charge carriers is always of crucial importance in enhancing the photocatalytic efficiency. In this work, with the aim of combining the outstanding physicochemical properties of graphene with LDHs to fabricate highly efficient visible-lightdriven photocatalysts, for the first time, hybrid nanocomposites of ZnCr-LDH and graphene were directly synthesized through a simple one-step coprecipitation route (Scheme 1), which Scheme 1. Schematic Illustration of the Assembly Process of ZnCr-LDH/Graphene Nanocomposite

included the nucleation and growth of ZnCr-LDH and the simultaneous reduction of GO in the absence of any additional reducing agents. As-assembled ZnCr-LDH/graphene nanocomposites, where ZnCr-LDH nanoplatelets with the diameter of ∼6 nm were well dispersed on the graphene surface, displayed significantly improved visible-light-induced photocatalytic activity in degrading organic Rhodamine B (RhB) dye, and the reason for the enhanced photodegradation activity was discussed systemically.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powder with a purity of 99.99% was purchased from Sigma-Aldrich. Other chemical reagents (analytical grade) were purchased from Beijing Chemical Reagent Co., Ltd., and directly used without further purification. 2.2. Assembly of ZnCr-LDH/Graphene Nanocomposites. GO was prepared from graphite powder by a modified hummers method as reported previously.60 ZnCr-LDH/ graphene nanocomposites with different amount of GO were fabricated by the coprecipitation method. Typically, a certain amount of GO, Zn(NO3)2·6H2O and Cr(NO3)3·9H2O with a Zn2+/Cr3+ molar ratio of 2.0 were ultrasonically dissolved in 50 mL of deionized water to give a total cationic concentration of B



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graphene surface and accompanying the formation of interfacial interaction between them. The surface/near-surface chemical states of carbon species of the synthesized samples were investigated by the XPS technique. Figure 2 shows the C1s XPS of pristine GO and

photocatalysts. A 300 W Xe arc lamp (PLS-SXE 300UV) with 420 nm cutoff filter was used as the light source, and the working current was set at 20 A. Typically, 100 mL of aqueous solution of RhB (10 ppm) containing 0.1 g of catalyst sample was dispersed in a quartz beaker by sonication, and then the suspension was vigorously stirred for 60 min in the dark for complete adsorption−desorption of RhB on the surface of the catalyst before the irradiation. Then the above suspension was exposed to visible light with stirring, and the samples of the reaction solution were taken out and analyzed by a UV−vis spectrophotometer at given time intervals. A blank reaction was performed under the identical experimental conditions without adding any photocatalyst.

3. RESULTS AND DISCUSSION 3.1. Characterization of ZnCr-LDH/Graphene Nanocomposites. Raman spectroscopy is an effective tool to determine the characteristics of graphitic microstructure. It can be seen from Figure 1 that GO exhibited two prominent

Figure 2. C1s XPS of samples: (a) GO and (b) LDH/G-1.

LDH/G-1 hybrid. As for GO, the C1s spectrum was decomposed into five contributions corresponding to carbon atoms in different functional groups: the sp2 graphitized carbon (284.7 eV), the −C-OH group (286.6 eV), the C−O−C group (287.8 eV), the >CO group (289.4 eV), and −COOH group (291.2 eV) .64,65 Compared with those for GO, the peak intensities of carbon atoms related to oxygen-containing groups for LDH/G-1 significantly decreased because of the deoxygenation of GO. The ratio of carbon atoms bonded with oxygen atoms to the C−C skeletal carbon atoms were 2.07 for GO and 0.26 for LDH/G-1, respectively, calculated from their relevant XPS peak areas. The aforementioned results demonstrated that most of GO was reduced to graphene in the course of assembly, and a small quantity of oxygenic functional groups still remained on the surface of graphene, well in agreement with the FT-IR (see Supporting Information and Figure S2) and Raman results. Under the present synthesis conditions, NaOH severed as a precipitating agent for the formation of ZnCr-LDH and simultaneously provided alkaline ambient for the deoxygenation of surface oxygenic functional groups in the GO.66 Figure 3 exhibits the XRD patterns of GO and ZnCr-LDH/ graphene composites with different graphene content. As for GO, the characteristic (002) diffraction at 2θ of about 10.5° was observed, indicating that the interlayer distance of GO is 0.9 nm.61 All ZnCr-LDH/graphene composites displayed the characteristic (003), (006), (012), and (110) diffractions of two-dimensional hydrotalcite-like materials.67 The basal spacing value (d003) of LDH phase was about 0.75 nm, demonstrating the intercalation of CO32− anions into the interlayer of LDH. For all composites, the characteristic (002) diffraction of GO or graphene was hardly observed. This was because the formed graphene in composites was significantly exfoliated by ZnCrLDH crystals.68 Furthermore, with increasing the graphene content, the broadening of remaining XRD lines revealed the

Figure 1. Raman spectra of samples: (a) GO and (b) LDH/G-1. The inset is an enlarged Raman spectrum of LDH/G-1.

Raman peaks for G band at about 1576 cm−1 and the D band at about 1330 cm−1, corresponding to the in-plane vibration of sp2-bonded carbon and defects, respectively. As for LDH/G-1 sample, the Raman peaks of D- and G-bands shifted to higher wave numbers of 1588 and 1340 cm−1, respectively, suggestive of the conversion of graphite to graphene sheets.61 Besides, as shown in the inset of Figure 1, the second-order band (2D band) was also observed at around 2694 cm−1, and the peak position of 2D band corresponded to three or four layers of graphene.62 Further, the thickness of the graphene sheets measured from the AFM image (Supporting Information, Figure S1) was about 1.96 nm, well consistent with the thickness of 2−3 graphene layers. The Raman and AFM results demonstrated the successful reduction of GO and the formation of LDH/graphene composite. In addition, the intensity ratio value of the D and G band (R = ID/IG) for LDH/G-1 was about 1.05, higher than that of GO (0.97). Usually, the value of R indicates the disorder degree and average size of the sp2 domains in graphite material.63 The higher this value, the higher the distortion degree of the graphite phase. Thus, the increase of R indicated more structural defects of carbon materials, which was associated with the anchoring of ZnCr-LDH nanoplatelets onto the C



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sponding to the (012) plane of LDH phase. Under the present synthetic conditions, metal cations were first adsorbed on the surface of GO via electrostatic attraction, and then ZnCr-LDH nuclei were uniformly formed on the surface of GO; meanwhile, the GO was reduced to graphene in alkali medium. With increasing the aging time, the newly formed ZnCr-LDH nuclei gradually grew into larger crystals without significant migration and accumulation due to the confined effect of the residual oxygenic functional groups on the surface of GO. Finally, ZnCr-LDH nanoplatelets with uniform particle size were evenly dispersed on the surface of the graphene supporting matrix. The optical absorption spectra of ZnCr-LDH and LDH/G-1 were tested by UV−vis diffuse reflectance spectroscopy. As shown in Figure 5, in the case of ZnCr-LDH, two intense

Figure 3. XRD patterns of samples: (a) GO, (b) ZnCr-LDH, (c) LDH/G-0.5, (d) LDH/G-1, and (e) LDH/G-2.

lowered crystallinity or reduced ZnCr-LDH particle sizes in ZnCr-LDH/graphene composites, which was attributed to the segregation and inhibition effects of exfoliated graphene on the growth of ZnCr-LDH crystallites. The microstructure of GO and representative LDH/G-1 composite were also revealed by SEM and TEM (Figure 4). It

Figure 5. UV−vis diffuse reflectance spectra of samples: (a) ZnCrLDH and (b) LDH/G-1.

absorption peaks appeared; one at about 400 nm was assigned to O 2p → Cr-3dt2g (ligand-to-metal charge-transfer) transition of CrO6 octahedral units in the lattice of ZnCr-LDH, and the other at about 550 nm was assigned to Cr-3dt2g → Cr-3deg (d− d transition) of Cr3+.69,70 Due to the fact that the high level Cr3deg orbitals were not filled, under visible light irradiation, electrons could be easily excited from the partially filled Cr3dt2g orbitals to the Cr-3deg orbitals. The band gap for ZnCrLDH, calculated from the plots of (αhν)2 vs hν (Supporting Information, Figure S4), was about 2.12 eV, similar to the previously reported value,32,33 indicative of the capability of harvesting visible-light photons. However, as for LDH/G-1 composite, it was difficult to accurately obtain the band gap due to the introduction of the graphene component, resulting in the broad absorption peaks with equivocal absorption edge in the UV−vis diffuse reflectance spectrum. Compared with ZnCrLDH, LDH/G-1 composite was more sensitive toward both the UV and visible light because of the presence of ZnCr-LDH/ graphene coupling system. 3.2. Photocatalytic Performance of ZnCr-LDH/Graphene Nanocomposites. It is well-known that electron−hole pairs are generated during the photocatalytic process, which is closely bounded with the redox reactions of species adsorbed on the surface of photocatalyst. Considering that as-assembled hybrid ZnCr-LDH/graphene nanocomposites had much stronger visible light absorption ability, more accessible

Figure 4. TEM image (a) of GO, SEM (b), TEM (c) images of LDH/ G-1, and the size distribution (d) of ZnCr-LDH nanoparticles over LDH/G-1. The inset of (c) is a typical HRTEM image of an individual ZnCr-LDH nanparticle on the graphene surface.

was clearly noted that uniform nanoplatelets with mean diameter of about 6 nm were even dispersed on the surface of flexible and transparent graphene sheets, suggestive of the high structural matching degree and good affinity between them. The thickness of the LDHs platelets estimated from the TEM image was about 2−3 nm (Supporting Information, Figure S3). Close inspection of a representative HRTEM image depicted the interplanar distance of about 0.261 nm for an individual ZnCr-LDH nanoplatelet in the LDG/G-1, correD



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LDH/G-1. Especially, the degradation percentage of RhB after 140 min over LDH/G-1 was as high as 93%, which was ascribed to more efficient photogenerated charge separation. Furthermore, the degradation percentage over LDH/G-1 could reach nearly 100% when the irradiation time was prolonged to 200 min. As for LDH/G-2 with the highest graphene content, a lower content of ZnCr-LDH in composite could lead to fewer reaction centers, consequently resulting in a relatively lower photocatalytic activity. The commercially available standard Degussa P25 was used as a reference photocatalyst. The degradation percentage over P25 was about 35%, much lower than that over ZnCr-LDH/graphene photocatalysts under the same condition. Further, it was noted that the photodegradation process of RhB over different photocatalysts obeyed pseudo-first-order kinetics model (Supporting Information, Figure S6). As listed in Table 1, LDH/G-1 showed a

adsorption sites, and more photoreactive centers resulting from better dispersion behavior of active ZnCr-LDH nanoplatelets as compared with ZnCr-LDH, the photocatalytic performance of ZnCr-LDH/graphene nanocomposites was investigated. The catalytic activity of the as-synthesized catalysts was evaluated by the degradation of RhB aqueous solution under visible light irradiation. The evolution of the adsorption spectra of RhB aqueous solution as a function of irradiation time over LDH/G-1 sample is shown in Supporting Information, Figure S5. It can seen that the intensity of the absorption spectra of RhB decreased gradually with increasing the irradiation time, demonstrating that the RhB dye molecules were destroyed. The remarkable temporal changes in the concentration of RhB also definitely showed that absorption of RhB was quite weak after irradiation for 140 min. Under visible light irradiation, the photodegradation of RhB over different photocatalysts was also investigated. As shown in Figure 6A, the self-degradation of RhB could be neglected,

Table 1. Reaction Rate Constant of the RhB Photodegradation over Different Photocatalysts photocatalyst

k (10−3 min−1)

refs

ZnO/In2S3 Fe2O3@TiO2 GdFeO3 TiO2 microspheres ZnCr-LDH LDH/G-0.5 LDH/G-1 LDH/G-2

17.4 2.7 5.7 28.0 1.4 7.3 18.4 11.1

22 16 80 81 this work this work this work this work

maximum of the reaction rate constant (k) value, which was significantly higher than those over ZnCr-LDH and some previously reported semiconductor photocatalysts.16,22,80,81 In addition, a TOC experiment was also performed to evaluate the degree of mineralization of RhB over different catalysts. After 140 min irradiation, the removal efficiency of TOC over LDH/ G-1, LDH/G-2, LDH/G-0.5, and pure ZnCr-LDH was about 42, 38, 31, and 12%, respectively, suggesting that the RhB molecules might be partly degraded into carbon dioxide and water over ZnCr-LDH/graphene photocatalysts. More importantly, LDH/G-1 maintained excellent photocatalytic activity with nearly 88% photodegradation percentage of RhB after recycling for five times (Figure 6B). Additionally, Raman, XPS, and XRD analyses confirmed that no obvious structural change of LDH/G-1 catalyst was observed after five cycles of photocatalytic test. After five runs of recycling experiments, no obvious agglomeration and accumulation of the LDH nanoplatelets on the graphene surface could be observed from TEM image of LDH/G-1 (Supporting Information, Figure S7), which was attributable to the excellent structural flexibility, large surface area, and residual oxygencontaining groups of graphene working as interaction centers to anchor the guest ZnCr-LDH nanoplatelets. It was confirmed that the as-synthesized photocatalyst possessed excellent photocatalytic stability under visible light. To reveal the predominant reactive oxygen species in the degradation of RhB over ZnCr-LDH/graphene photocatalysts, benzoquinone and tert-butyl alcohol (t-BuOH) were chosen as scavengers for superoxide a radical (•O2−) and a hydroxyl radical (•OH), respectively.72,73 As shown in Figure 7, the introduction of benzoquinone into the photodegradation system led to a slight reduction in the degradation of RhB after 140 min irradiation; on the contrary, the photo-

Figure 6. Photodegradation of RhB (A) as a function of irradiation time over different photocatalysts: without catalyst (a) blank, (b) graphene, (c) ZnCr-LDH, (d) standard Degussa P25, (e) LDH/G-0.5, (f) LDH/G-2, and (g) LDH/G-1. (B) Reproducibility of LDH/G-1 in the photodegradation of RhB.

while the degradation percentage of RhB was low in the absence of catalyst. The photodegradation percentage of RhB over pure ZnCr-LDH only reached 18% after irradiation for 140 min, indicative of a low photocatalytic activity of pure ZnCr-LDH. The degradation percentage over pristine graphene was only about 9%, which was mainly attributed to the existence of electron transfer between graphene and excited dye (dye*).71 Notably, all ZnCr-LDH/graphene nanocomposites showed enhanced photocatalytic activity, and the photocatalytic activity increased gradually from LDH/G-0.5 to LDH/G-2 and E



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Figure 7. Photodegradation of RhB over LDH/G-1: (a) alone, (b) in the presence of 1 mM benzoquinone, and (c) in the presence of 10 mM t-BuOH.

degradation of RhB was significantly declined due to the introduction of t-BuOH. The above results indicated that the •OH radical was the main active oxidizing species rather than the •O2− radical in the photogradation process. The formation of •OH radical was also determined by PL spectra in the presence of terephthalic acid (TA) as a probe reagent instead of RhB in the photocatalytic test.78 It is well known that •OH radical can react readily with TA to quantitatively generate a strong fluorescent product, 2hydroxyterephthalic acid (TAOH), and the intensity of PL for TAOH is proportional to the amount of produced •OH radicals. As shown in Figure 8A, on the basis of the intensity of PL for TAOH (λex = 315 nm), the amount of the produced •OH radicals over LDH/G-1 was much larger than that over ZnCr-LDH after 30 min visible light irradiation, suggestive of the higher separation efficiency of photogenerated charge. Furthermore, the generation of active •OH radicals from ZnCrLDH and LDH/G-1 was also confirmed by ESR spin-trap technique using dimethylpyridine N-oxide (DMPO) as a spintrapping reagent. As shown in Figure 8B, four responsive peaks of DMPO-•OH with an intensity ratio of 1:2:2:1 appeared after visible light irradiation, in good agreement with the previously reported results.74,75 No such signals were observed in the dark, which meant visible light should be indispensable to the generation of •OH on the catalyst surface. In addition, the peak intensity of DMPO-•OH for LDH/G-1 was much higher than that for pure ZnCr-LDH. The above results agreed well with the photocatalytic activity of ZnCr-LDH and LDH/G-1. EIS measurements were also utilized to investigate electrical conductivity of the prepared samples. The EIS of ZnCr-LDH and LDH/G-1 substrates are shown in Figure 9A. Compared with that of ZnCr-LDH, the impedance spectrum of LDH/G-1 exhibited a smaller radius, indicating that the charge transfer of LDH/G-1 was greatly improved due to the good charge transport property of graphene. Further, to demonstrate the electronic interaction between graphene and LDH, the transient photocurrent response versus time was recorded with three on−off cycles (Supporting Information, Figure S8). The results revealed that the photocurrent over LDH/G-1 (about 2.56 μA cm−2) is about two times higher than that over ZnCr-LDH due to the enhanced separation and transfer of photogenerated charge carriers originating from the intimate

Figure 8. Room-temperature photoluminescence (PL) spectra (A) of TAOH from reaction solution without addition of RhB in the presence of 5 × 10−4 M of TAOH and 2 × 10−3 M NaOH after 30 min irradiation: (a) blank, (b) ZnCr-LDH, and (c) LDH/G-1. ESR signals (B) of DMPO-•OH adducts generated in the suspensions of pure ZnCr-LDH and LDH/G-1 samples at ambient temperature with visible light on and off.

contact at the interface between ZnCr-LDH and the graphene sheets. In addition, the PL emission behavior was further investigated to clarify the transfer, separation, and recombination of photogenerated charge carriers. As shown in Figure 9B, ZnCr-LDH showed an emission peak at approximately 470 nm (λex =366 nm), corresponding to the recombination of photoinduced electron−hole pairs, whereas the observed PL intensity of LDH/G-1 was much weaker than that of ZnCrLDH. As for LDH/G-1 sample, the recombination of photoinduced electron−hole pairs was inhibited greatly due to superior electrical conductivity of graphene and highly hybridized nanostructure. The above results demonstrated a lower electron−hole recombination occurring in the LDH/G-1, thus accounting for the significantly improved degradation efficiency under visible light irradiation. As a new type of photocatalyst, ZnCr-LDH differs from traditional semiconductor photocatalysts. In any case, it is the same as that the fast intrinsic recombination of photogenerated electron−hole pairs leads to the low catalytic activity. According to the above experimental results, the photoexcitation of electrons in CrO6 octahedron under visible light irradiation played a crucial role in the photodegradation of RhB F



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ZnCr-LDH nanoplatelets, could pass the photogenerated electrons from ZnCr-LDH, which facilitated the separation and antirecombination of photogenerated electron−hole pairs. Consequently, the adsorbed oxygen could scavenge excited electrons passed by graphene to yield •O2− radicals,77 which were further converted to reactive •OH radicals in the present aqueous reaction environment taking part in the degradation of RhB.78 In addition, π−π stacking interactions between the aromatic domains of graphene and the aromatic RhB molecules could significantly improve the RhB concentration near the photocatalyst surface,79 thus further favoring the photocatalytic degradation behavior.

4. CONCLUSIONS We developed a simple and effective one-step coprecipitation approach to synthesize hybrid nanocomposites of ZnCr-LDH/ graphene. The results revealed that ZnCr-LDH nanoplatelets with the mean particle size of about 6 nm were well dispersed on the exfoliated graphene in nanocomposites. Compared with pristine ZnCr-LDH, as-assembled ZnCr-LDH/graphene nanocomposites showed superior catalytic activity in the photodegradation of RhB under visible light irradiation. Cr3+ ion in the octahedral site of ZnCr-LDH played a significant role in the photoexcitation of electrons by splitting the Cr-3dt2g to Cr-3deg orbital under visible light. The heteronanostructure of ZnCrLDH and graphene coupling system facilitated the efficient transportation and separation of the photogenerated charge carriers, which was an antichange recommendation mechanism. It is expected that as-synthesized graphene-based photocatalysts with rather high photocatalytic activity can be potentially applied in the field of dyes removal under visible light.

Figure 9. EIS (A) and room temperature PL emission spectra (B) of ZnCr-LDH (a) and LDH/G-1 (b).

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ASSOCIATED CONTENT

S Supporting Information *

in the system of ZnCr-LDH/graphene nanocomposites. On one hand, after migration of the excited electrons, active •OH radicals could be formed via the reaction between the photogenerated holes remained at Cr-3dt2g orbital and hydroxyl groups (OH−) in the solution, which directly led to the oxidation of organic pollutants. The charge transfer and photocatalytic mechanism over ZnCr-LDH/graphene nanocomposite are schematically illustrated in Scheme 2. On the other hand, the higher conductivity usually results in more effective charge separation and antirecombination of photogenerated electron−hole pairs during the photocatlytic process.76 Therefore, graphene nanosheets with good conductivity, which were effectively and intimately decorated by

AFM image and cross-section analyses of graphene, FT-IR spectra of prepared samples, TEM image of LDH/G-1, absorption band edges obtained from UV−vis diffuse reflectance spectra, absorption profiles of RhB solution during the photocatalysis on over the LDH/G-1 sample under visible light irradiation, pseudo-first-order kinetic for the RhB photodegradation over different samples, typical TEM image of LDH/G-1 after recycling for five times, and transient photocurrent responses for ZnCr-LDH and LDH/G-1 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Scheme 2. Schematic Illustration of Photocatalytic Reaction Mechanism over ZnCr-LDH/Graphene Nanocomposite

AUTHOR INFORMATION

Corresponding Authors

*G. F.: Tel., 8610-64451226; Fax, 8610-64425385. E-mail, fangl@mail. buct. edu. cn. *F. L.: Tel., 8610-64451226; Fax, 8610-64425385. E-mail, lifeng@mail. buct. edu. cn Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported financially by 973 Program (2011CBA00506) and the National Natural Science Foundation of China. G



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