JES-00089; No of Pages 9 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 4 ) XX X–XXX
Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
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Yaxin Zhang1,2 , Zeyu Zhou2 , Tan Chen2 , Hongtao Wang2,⁎, Wenjing Lu2
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1. College of Environmental Science and Engineering, Hunan University, Changsha 410082, China. E-mail: [email protected] 2. School of Environment, Tsinghua University, Beijing 100084, China
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Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol
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AR TIC LE I NFO
ABSTR ACT
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Article history:
A series of graphene–TiO2 photocatalysts was synthesized by doping TiO2 with graphene oxide 15
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Received 15 November 2013
via hydrothermal treatment. The photocatalytic capability of the catalysts under ultraviolet 16
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Revised 26 March 2014
irradiation was evaluated in terms of sodium pentachlorophenol (PCP-Na) decomposition and 17
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Accepted 28 March 2014
mineralization. The structural and physicochemical properties of these nanocomposites were 18
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characterized by X-ray diffraction, N2 adsorption–desorption, transmission electron micros- 19
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copy, scanning electron microscopy, Ultraviolet–visible diffuse reflectance spectroscopy, X-ray 20 Keywords:
photoelectron spectroscopy, electron paramagnetic resonance spectra, and Fourier-transform 21
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Photocatalysis
infrared spectroscopy. The graphene–TiO2 nanocomposites exhibited higher photocatalytic 22
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Graphene
efficiency than commercial P25 for the degradation of PCP-Na, and 63.4% to 82.9% of the total 23
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TiO2
organic carbon was fully mineralized. The improved photocatalytic activity may be attributed 24
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PCP-Na
to the accelerated interfacial electron-transfer process and the significantly prolonged lifetime 25
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Degradation
of electron‒hole pairs imparted by graphene sheets in the nanocomposites. However, 26
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excessive graphene and the inhomogeneous aggregation of TiO2 nanoparticles may decrease 27 28
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photodegradation efficiency.
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© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 29
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Introduction
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Heterogeneous nanocrystalline titania (TiO2) is commonly used because it is readily available, inexpensive, nontoxic, and relatively chemically stable. However, two problems limit the use of pure TiO2 in photocatalysis: the fast recombination of electron–hole pairs and the mismatch between band gap energy and solar radiation spectrum (Zhang et al., 2010b). The wide band gap causes selective ultraviolet (UV, λ < 380 nm) absorption by the catalyst (Khalid et al., 2013), and the short lifetime of photogenerated electron–hole pairs (2 to 3 μS) (Homa et al., 2009) causes the loss of approximately 90% of the generated carriers (Venkatachalam et al., 2007). To overcome
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Published by Elsevier B.V. 30
these disadvantages, studies have focused on doping or co-doping of metal and non-metal elements into TiO2 to enhance quantum efficiency and extend the light-response range (Liu et al., 2011a). The recently discovered carbonaceous material graphene has elicited extensive attention in the fields of condensedmatter physics, electronics, and material science (Wang et al., 2013). Graphene shows various superior properties, such as high charge carrier mobility (>200,000 cm2/(V·sec), specific surface area (2630 m2/g), optical transmittance (~97.7%) (Zhang et al., 2012a), and Fermi velocity (106 m/sec) (Song et al., 2011). Thus, graphene–TiO2 (GT) nanocomposites have become a promising material for photocatalytic applications. GT has high stability,
⁎ Corresponding author. E-mail: [email protected] (Hongtao Wang).
http://dx.doi.org/10.1016/j.jes.2014.08.011 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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1. Material and methods
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1.1. Chemicals and reagents
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PCP was purchased from Dima (USA). Commercial P25 TiO2 (80% anatase, 20% rutile; Brunauer–Emmett–Teller (BET) area, 54.77 m2/g) was supplied by Degussa Corporation (Germany). Graphene oxide (>99%) was purchased from Nanjing Xianfeng Nanomaterial Technology Co., Ltd. (China). All chemicals used in this study were of analytical grade and used without further purification. Double-distilled water was used in catalyst preparation and subsequent catalytic tests.
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1.2. Catalysts preparation
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Graphene oxide (100 mg) was sonicated in 250 mL of double-distilled water at room temperature for at least 1 hr to ensure thorough dispersion of graphene oxide in the solution. Then, a certain amount of P25 TiO2 was added to the above solution with magnetic stirring (120 r/min). The weight addition ratios of graphene oxide to P25 were 1:10, 1:20, 1:50, and 1:100. After stirring for 2 hr, the suspension was transferred into a 100 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 180°C, for 6 hr. The hydrothermal process ensured sufficient reduction of graphene oxide to graphene (Yang et al., 2013). The obtained precipitates were centrifuged at 10,000 r/min for 10 min and then washed with double-distilled water until the
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Crystal structures of samples were analyzed using X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer with Cu Ka radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. BET specific surface area, Barrett–Joyner–Halenda (BJH) pore volume, average pore diameter, and fractal dimension of photocatalyst calculated from the N2 adsorption–desorption isotherms were obtained with a Quantachrome gas adsorption instrument (Autosorb AS-1N2). The samples were vacuum-degassed for 4 hr at 300°C before measurements. Ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS) were obtained using a Hitachi U-3010 spectrophotometer equipped with an integrating sphere accessory for diffuse reflectance. Transmission electron micrographs were obtained with a Hitachi H-800 transmission electron microscope operating at an accelerating voltage of 200 kV. Scanning electron micrographs were acquired by a Hitachi S-5500 scanning electron microscope at an accelerating voltage of 20 kV. Surface composition of the nanocomposite was analyzed by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra System with monochromatic Al Ka X-rays (1486.6 eV). Electron paramagnetic resonance spectra (EPR) were recorded at 70 K on a JEOL JES-FA200 spectrometer operating in the X-band with 30 mg of the sample introduced into an EPR quartz probe cell under N2 atmosphere. The FT-IR spectra of the catalysts were obtained using a spectrometer (Spectrum GX, PerkinElmer, USA) within the 4000 to 400 cm−1 region at a 4 cm−1 resolution. Approximately 2 mg of the milled sample was ground with 200 mg of KBr (FT-IR grade) and compressed into a pellet under vacuum at a pressure of 75 kN/cm for 3 min. A total of 32 scans were performed, averaged for each spectrum, and then corrected against air as background in each acquisition. The spectra were subjected to a Savitsky–Golay second derivative filter (using a third-order polynomial and seven-point smoothing) to discriminate the emerging peaks (Savitzky and Golay, 1964).
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1.4. Photocatalytic test
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The photocatalytic degradation experiments were carried out in a vigorously stirred batch reactor containing 500 mL of PCP-Na (50 mg/L) solution and 100 mg of catalyst (GT or P25). The reactor was equipped with a high-pressure mercury lamp (300 W), and the temperature was maintained at 25°C. The photocatalytic degradation experiments were performed in a simple targeted PCP-Na aqueous system. Before irradiation, the suspension was stirred in darkness for 30 min to obtain an adsorption–desorption equilibrium, which proved to be sufficient (Zhang et al., 2012b). At designated time intervals, 1 mL aliquots of the suspension were collected and transferred into centrifuge tubes (CNW, 8 mL) with Teflon-lined screw caps. To quantitatively recover the analytes adsorbed on the catalyst, an equal volume of methanol was added to the suspension sample, which was then thoroughly shaken. The processed suspension was filtered through a 0.45 μm pore size membrane
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1.3. Characterization
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pH of the supernatant was neutral. The sediment samples were dried at 333 K for 24 hr in an electric oven and then ground in an agate mortar. The as-synthesized photocatalysts of different ratios were denoted as GT 1:10, 1:20, 1:50, and 1:100.
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high photocatalytic activity, and enhanced photovoltaic properties (Long, 2013) because of its enhanced quantum efficiency and decreased band gap energy (Geng et al., 2013). Several studies have been focused on the photocatalytic activity of GT. However, most of these studies utilized dyes or simple substances as target pollutants, such as methylene blue (Zhang et al., 2010a), benzene (Zhang et al., 2010c), Rhodamine B (Li et al., 2013), propanol (Kamegawa et al., 2010), and Acid Orange (An and Yu, 2011), which cannot fully represent the toxic substances existing in the environment. In aqueous systems, the harmful effect of persistent organic pollutants (POPs) on human health is becoming a global concern. Pentachlorophenol (PCP), one of the common POPs, is widely used as a wood preservative, anticorrosive, fungicide, and pesticide, leaving serious contamination in the environment (Xie et al., 2013). PCP is highly toxic, mutagenic, and carcinogenic, and was listed as one of the priority pollutants in the US, European Union, and China (Liu et al., 2011a). PCP is resistant to biodegradation and photolytic degradation in aqueous systems because of its stable chemical structure; thus, investigating the photocatalytic degradation of PCP using GT as a catalyst is of great interest. In this study, GT nanocomposites of different carbon material ratios were prepared via a facile hydrothermal treatment. The function of graphene in the enhancement of light absorption, quantum efficiency, and photoactivity of the photocatalysts was systematically investigated. The photocatalytic activity of the as-prepared nanoparticles was examined by the photodegradation of sodium pentachlorophenol (PCP-Na) under UV irradiation and the mineralization of the target pollutant. This study provides a new approach for the decomposition of persistent hazardous compounds.
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Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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filter. The concentrations of PCP-Na and surfactants were determined using high-performance liquid chromatography 181 Q16 (Agilent 1260) fitted with a UV detector and an Agilent TC-C18 182 column (4.5 mm × 250 mm; 5 μm). The mobile phase of high183 performance liquid chromatography was methanol–water 184 (80:20) at a flow rate of 1.00 mL/min. Chromatography was 185 performed at 35°C. UV wavelengths were set at 249 nm for 186 PCP-Na. During photodegradation, 10 mL of the sample was 187 collected at certain time intervals and filtered through a 0.45 μm 188 pore size membrane filter for total organic carbon testing 189 Q17 (Shimadzu TOC-VCPH). All data were expressed as the means 190 of two replicates with ±5% deviation.
2.2. BET surface areas and pore distributions
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The specific surface areas of P25 and GT with different addition ratios were determined by the BET method using adsorption data, whereas the pore volume and pore size distribution were derived using the BJH model based on desorption isotherms. Fig. 2 (inset) shows the adsorption–desorption isotherms and the corresponding pore size distribution curves of the catalysts. All catalysts exhibited similar type IV isotherms (Sing et al., 1985) with an H3 hysteresis loop at a relative pressure close to unity, which are typical characteristics of mesoporous materials (Liu et al., 2011b). The pore size distributions (inset in Fig. 2) of the GT samples were broad (from 2 to over 100 nm), indicating the presence of mesopores and macropores (Xiang et al., 2011b). As shown in Table 1, the specific surface area and pore volume of the as-prepared catalysts were primarily dominated by graphene compared with those of P25. However, the pore diameter was uniform and lower than 5 nm. The BET surface area of the GT samples significantly increased from 53.74 to 72.73 m2/g with increasing graphene content. This result can be attributed to the fraction of bare graphene in the composites. However, the BET surface area of the GT samples was still relatively low compared with that of pure graphene (2600 m2/g). This result can be ascribed to the minimal doping level of graphene. Thus, surface area and porosity are mainly dominated by the TiO2 component. Another parameter that can provide additional information on the catalyst structure in terms of “roughness exponent” is the surface fractal dimension (D) (Zhu et al., 2009). Fractal dimension describes the topography of the real surface based on fractal geometry. This factor is usually between 2 (smooth surface) and 3 (rough surface). The complex pore and surface structure can be adequately described by D values (Zhang et al., 2012c). D was derived using the Frenkel– Halsey–Hill and Neimark–Kiselev methods (Table 1). The D values of the GT samples increased with increasing graphene content. This result suggests that the doping with graphene,
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The XRD patterns of the as-prepared GT composites and P25 TiO2 are shown in Fig. 1. GT nanocomposites with different weight addition ratios of graphene and P25 clearly had similar XRD patterns. Peaks located at 25.3°, 37.8°, 48.0°, 55.1°, and 62.7° can be indexed to the (101), (004), (200), (211), and (204) crystal planes of anatase TiO2 (JCPDS No. 21-1272), respectively. No typical diffraction peaks of graphene were observed in the nanocomposites. This result can be attributed to the main characteristic peak of graphene at 25.0° overlapping with the (101) peak of anatase TiO2 (Zhang et al., 2011) and to the relatively low weight fraction of graphene in the nanocomposites. However, the GT composites slightly increased the amount of rutile phase TiO2 (JCPDS No. 21-1276). The XRD results indicate that doping graphene with TiO2 promotes the phase transformation from anatase to rutile. In other words, graphene has a catalytic function in the phase transformation from anatase to rutile during the hydrothermal treatment because of its special structural properties (Li et al., 2013).
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GT 1:20 GT 1:50
P25 GT 1:10
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dV/dW (cm3/(g.min))
Adsorbed volume (cm3/g, STP)
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(224)
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(215)
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(204)
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(116)
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(211) (220)
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(200)
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Intensity (a. u.)
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2.1. XRD patterns of the photocatalysts
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2. Results and discussion
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2θ (°) Fig. 1 – XRD patterns of the graphene–TiO2 (GT) photocatalyst and P25.
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Relative pressure (P/P0) Fig. 2 – Nitrogen adsorption–desorption isotherm and pore size distribution curve (inset) of graphene–TiO2 (GT) and P25.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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Table 1 – BET specific surface area, pore volume, average pore diameter, and fractal dimension of graphene–TiO2 nanocomposites and P25. Sample
BET surface area (m2/g)
Total pore volume (mL/g)
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P25 GT 1:10 GT 1:20 GT 1:50 GT 1:100
t1:11 t1:13 t1:12 t1:14 t1:15
GT: graphene–TiO2. D values calculated at P/P0 > 0.35. b NK: D was derived using Neimark–Kiselev method. c FHH: D was derived using Frenkel–Halsey–Hill method.
53.74 72.73 64.39 58.39 55.63
Fractal dimension (D) a
Pore diameter (nm)
0.264 0.361 0.372 0.338 0.332
3.092 3.109 3.117 3.113 3.115
NK b
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FHH c
R2
2.459 2.568 2.527 2.493 2.488
0.998 0.979 0.980 0.990 0.989
2.555 2.517 2.512 2.504 2.491
0.993 0.995 0.995 0.997 0.977
t1:5
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2.3. Scanning electron microscopy and transmission electron microscopy The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of GT 1:10 and 1:100 are shown in Fig. 3. As shown in the figure, the TiO2 nanoparticles did not easily complex with graphene and were entrapped
inside the graphene sheets. However, the TEM images of GT reveal inhomogeneous dispersion of TiO2 particles in the graphene matrix. The particles accumulated along the wrinkles and edges of the graphene sheets. This accumulation limited the interfacial contact between TiO2 and graphene, which resulted in insufficient utilization of the excellent electron conductivity of graphene (Zhang et al., 2011). Hence, further studies should focus on achieving homogenous dispersion of solid TiO2 particles on a graphene sheet to effectively utilize the “structure-directing” ability of graphene
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with its wrinkles and stacking structure, forms composites with a “rougher” surface.
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Fig. 3 – TEM and SEM images of graphene–TiO2 (GT). (a) TEM of GT 1:10, (b) TEM of GT 1:100; (c) SEM of GT 1:10; (d) SEM of GT 1:100. Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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oxide, which possesses abundant oxygen-containing functional groups on the basal plane and edge that can provide reactive sites.
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2.4. UV–Vis DRS
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Fig. 4 shows the UV–Vis DRS of the as-prepared GT composites. For comparison, P25 TiO2 was also tested under the same condition. P25 showed a mild response within 200 to 400 nm, which can be attributed to the electron transition from the valence band to the conduction band. The addition of graphene increased the light absorption intensity in both UV and visible light regions. Furthermore, a qualitative red shift to higher wavelength was observed in the absorption edge of GT nanocomposites (50 nm for GT 1:100). This shift can be attributed to the hybridization of C2p and O2p atomic orbits to form a new valence band; hence, the band gap energy was reduced in the GT system (Yang et al., 2013). Within the 400 to 800 nm range, light absorption intensity increased with increasing graphene content. This phenomenon can be attributed to the surface plasmon resonance effect of the graphene unit (Li et al., 2013). These findings suggest that the introduction of graphene can promote the visible light response of the nanocomposites.
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2.5. XPS
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The XPS surface probe technique was adopted to study the chemical states of Ti, O, and C species and the interaction of graphene and TiO2 on the surface of GT composites. Fig. 5 shows the XPS of P25 and GT 1:100 composites in the Ti 2p, O 1s, and C 1s binding energy regions. For GT, the measured binding energies of Ti 2p3/2 and Ti 2p1/2 were 458.3 and 464.0 eV, respectively, consistent with P25 TiO2. The deconvoluted peaks centered at the binding energies of 529.5, 530.4, and 531.7 eV were assigned to Ti\O, O\H, and C\O functional groups, respectively. Fig. 5b shows the shift in the O 1s binding energies, illustrating that electron transformation at the interface between graphene and TiO2 perturbs the O electronic environment (Li et al., 2013).
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Intensity (a. u.)
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Fig. 5 – High-resolution XPS spectra of the synthesized graphene–TiO2 (GT 1:100) and P25. (a) Ti 2p region; (b) O 1s region; (c) C 1s region.
Wavelength (nm) Fig. 4 – UV–Vis diffuse reflectance spectra of the synthesized graphene–TiO2 (GT) and P25.
Fig. 5c shows the high-resolution C 1s XPS spectra of the 301 GT 1:100 sample in comparison with that of P25. The binding 302 energy of 284.2 eV can be attributed to the carbon, C\C, and 303
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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2.6. EPR analysis
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An in situ EPR study was performed for GT composites and P25 at liquid N2 temperature (77 K) under UV irradiation. Photo-induced EPR signals of trapped electrons and holes on GT nanocomposites and P25 are shown in Fig. 6. Two sets of features were observed in the EPR spectrum: signals from electrons (A) and holes (B). EPR signals due to GT were characterized by g values. The sharp signal at g┴ = 1.995 and g// = 1.970 corresponded to surface electron trapping sites and are represented as signal A (Gopal et al., 2008; Kumar et al., 2006). EPR signals with g factors g1 = 2.030, g2 = 2.016, and g3 = 2.008 corresponded to surface hole trapping sites and are represented as signal B (Ribbens et al., 2011; Shkrob et al., 2011). Fig. 6 demonstrates apparent variation in EPR intensities between trapped electrons and holes of GT and P25. For P25, a very weak signal was observed presumably due to hole traps. This result indicates that the lifetime of electron and hole radicals is too short to be observed under such testing conditions. By contrast, the EPR intensities for the GT nanocomposites were higher and more intense, especially for GT 1:20, followed by GT 1:100. These results suggest that graphene doping can effectively prolong the lifetime of photoelectronhole pairs.
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2.8. Photocatalytic degradation of PCP-Na
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The photocatalytic activity of GT with different carbon ratios and P25 catalysts was evaluated through PCP-Na (initial concentration of 50 mg/L) degradation and TOC variation in aqueous solution under UV irradiation for 2 hr (Fig. 8a, b, respectively). After 120 min of UV irradiation, 31.1% of PCP-Na was degraded using P25 as the catalyst. Remarkable improvements in photodegradation were observed in all GT nanocomposites, wherein more than 97% of PCP-Na was decomposed. However, GT with different graphene ratios showed similar degradation efficiencies. The degradation curves were fitted to the pseudo-firstorder kinetic law described by the following equation:
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c − ln ¼ kt c0
ð1Þ
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The FT-IR spectra of GT and P25 TiO2 are described in Fig. 7. The broad peak at 3400 cm−1 in all samples indicates the presence of surface O\H stretching vibrations (Nguyen-Phan et al., 2011). Peaks at around 1393 and 1250 cm−1 can be assigned to tertiary C\OH and epoxy C\O groups, respectively (Nguyen-Phan et al., 2011; Shen et al., 2011). The absorption peak at around 1600 cm−1 is contributed by the skeletal vibration of graphene (Zhang and Pan, 2011). For the GT composites, the C\O stretching vibration peak at 1052 cm−1 and the C_O stretching vibration of \COOH group peak at 1720 cm−1 of graphene oxide completely disappeared. Thus, the oxygen-containing functional groups were completely removed through the thermal reaction. Meanwhile, strong absorption bands observed in the 400 to 1000 cm−1 region suggest the presence of Ti\O\Ti and Ti\O\C bonds, indicating the chemical interaction between the surface hydroxyl groups of TiO2 and the functional groups of graphene oxide (Nguyen-Phan et al., 2011).
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A
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Intensity (a.u.)
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2.7. FT-IR spectra
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C\H bonds in the composite. The peak at 285.1 eV can be ascribed to carbon and defect-containing sp2 hybridized carbon atoms (Xiang et al., 2011a). The relatively weak peak at 288.3 eV was contributed by carboxyl carbon (O_C\O), indicating the esterification reaction of \OH on TiO2 with \COOH on the graphene oxide surface and the formation of O_C\O\Ti bonds (Xiang et al., 2011b). The peak located at 287.0 eV, which corresponded to the oxygen-containing carbonaceous bands, disappeared in the C 1s XPS spectra of GT. Therefore, graphene oxide was effectively reduced to graphene during the hydrothermal treatment. Moreover, the sp2 carbon network was retained in graphene, as indicated by the C\C peak at 284.2 eV (Murugan et al., 2009).
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3393 1720
g2 = 2.016g3 =2.008 g1 = 2.030
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B
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Magnetic field (G) Fig. 6 – EPR spectra of the synthesized graphene–TiO2 (GT) and P25 at 70 K.
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Wavenumber (cm-1) Fig. 7 – FT-IR spectra of the synthesized graphene–TiO2 (GT) and P25.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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where, c is the PCP-Na concentration at time t, and k is the apparent first-order rate constant. The values of k for the initial 373 reaction under UV irradiation with different catalysts are shown 374 in Table 2. All degradation curves were fitted well by Eq. (1), with 2 −3 −1 375 Q18 all R values ≥ 0.946. For P25, k was 1.81 × 10 min , and the 376 degradation rate for GT 1:100 was 18.34 times higher than that 377 of P25. The order of photodegradation efficiency of PCP-Na 378 under UV light is as follows: GT 1:100 > GT 1:50 > GT 1:20 > GT 379 1:10 > P25. This result suggests that the GT 1:100 nanocompos380 ite is more efficient than P25 for PCP-Na degradation. In 381 addition, graphene oxide doping can significantly enhance 382 photodegradation efficiency. However, reaction rates decreased 383 with increasing graphene ratio (Table 2). Thus, excessive 384 graphene can be detrimental for the photocatalytic degradation 385 reaction. Although graphene doping can retard the recombina386 tion of electrons and holes, it can also obstruct photon 387 absorption, thereby decreasing degradation efficiency (Wang 388 and Zhang, 2011). 389 Fig. 8b shows the TOC variation in GT catalysts with 390 different graphene ratios. After 120 min of irradiation, 63.4% 391 to 82.9% of TOC was mineralized into CO2 and H2O, which was 392 lower than the decomposition rate of PCP-Na (97.7% to 99.0%). 393 GT 1:100 exhibited the highest mineralization efficiency. The 394 comparatively lower mineralization rate than degradation 395 rate suggests that dechlorination is stepwise and that the 396 dechlorinated intermediates are subsequently oxidized to
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Fig. 8 – Photodegradation of PCP-Na (A) with graphene–TiO2 and P25 catalyst and first-order fitting curves (inset), and (B) TOC variation.
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Table 2 – Apparent first-order rate constant k values for the photocatalytic degradation of PCP-Na with graphene–TiO2 and P25 as catalysts.
t2:5
Catalyst
k (10−3 min−1)
R2
t2:6 t2:7 t2:8 t2:9 t2:10
P25 GT 1:10 GT 1:20 GT 1:50 GT 1:100
1.81 26.94 30.77 32.10 33.19
0.946 0.977 0.984 0.989 0.990
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A schematic electronic structure and degradation process for PCP-Na by GT was proposed based on the abovementioned characterization and photodegradation results (Fig. 9). Under UV irradiation, the photogenerated electrons can be effectively conducted to the graphene sheet, leaving holes with positive charges on the surface of TiO2 particles. Graphene is an electron acceptor that accelerates the interfacial electron-transfer process. Graphene can strongly hinder the recombination of e−–h+ because of the Schottky barrier effect. As proved by EPR results, where signals with much higher intensity were shown for GT composites, the lifetime of electron–hole pairs can be prolonged, providing more radicals for the degradation of PCP-Na into H2O, CO2 and other intermediates. Thus, the photocatalytic activity of GT could be significantly enhanced. Though the BET surface area of the GT samples increased with increasing graphene content, GT 1:100 showed the highest degradation efficiency other than GT 1:10. This phenomenon can be ascribed to the fact that the degradation was not mainly based on the surface absorption of PCP-Na onto catalysts. Meanwhile, both the PCP-Na decomposition and TOC mineralization corresponded well with the UV–Vis DRS results, where the addition of graphene significantly increased the light absorbance, and GT 1:100 gives the highest intensity under UV light irradiation. However, the GT precursor P25 accumulated along the wrinkles and edges of the graphene oxide sheets during hydrothermal treatment because of the abundant carboxylic acid groups on graphene oxide, as shown in the SEM and TEM results. The inhomogeneous aggregation of TiO2 nanoparticles on graphene creates new e−–h+ recombination centers, thereby decreasing the photocatalytic efficiency.
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open the aromatic ring (Liu et al., 2011a). However, after 2 hr of reaction, the decreasing tendency of TOC was apparent in all GT cases. The continuous decrease in TOC can be attributed to the decomposition of small molecule acids, such as formic acid and acetic acid (Zhang et al., 2012b).
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UV Electron e-
Hole h+
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Graphene UV PCP-Na
PCP-Na
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H2O, CO2 and other mineralization
H2O, CO2 and other mineralization
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (No. 41371472).
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Fig. 9 – Schematic of the PCP-Na photocatalytic degradation under UV irradiation with graphene–TiO2.
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Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
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Yaxin Zhang1,2 , Zeyu Zhou2 , Tan Chen2 , Hongtao Wang2,⁎, Wenjing Lu2
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1. College of Environmental Science and Engineering, Hunan University, Changsha 410082, China. E-mail: [email protected] 2. School of Environment, Tsinghua University, Beijing 100084, China
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Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol
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Article history:
A series of graphene–TiO2 photocatalysts was synthesized by doping TiO2 with graphene oxide 15
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Received 15 November 2013
via hydrothermal treatment. The photocatalytic capability of the catalysts under ultraviolet 16
12
Revised 26 March 2014
irradiation was evaluated in terms of sodium pentachlorophenol (PCP-Na) decomposition and 17
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Accepted 28 March 2014
mineralization. The structural and physicochemical properties of these nanocomposites were 18
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characterized by X-ray diffraction, N2 adsorption–desorption, transmission electron micros- 19
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copy, scanning electron microscopy, Ultraviolet–visible diffuse reflectance spectroscopy, X-ray 20 Keywords:
photoelectron spectroscopy, electron paramagnetic resonance spectra, and Fourier-transform 21
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Photocatalysis
infrared spectroscopy. The graphene–TiO2 nanocomposites exhibited higher photocatalytic 22
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Graphene
efficiency than commercial P25 for the degradation of PCP-Na, and 63.4% to 82.9% of the total 23
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TiO2
organic carbon was fully mineralized. The improved photocatalytic activity may be attributed 24
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PCP-Na
to the accelerated interfacial electron-transfer process and the significantly prolonged lifetime 25
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Degradation
of electron‒hole pairs imparted by graphene sheets in the nanocomposites. However, 26
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excessive graphene and the inhomogeneous aggregation of TiO2 nanoparticles may decrease 27 28
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Introduction
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Heterogeneous nanocrystalline titania (TiO2) is commonly used because it is readily available, inexpensive, nontoxic, and relatively chemically stable. However, two problems limit the use of pure TiO2 in photocatalysis: the fast recombination of electron–hole pairs and the mismatch between band gap energy and solar radiation spectrum (Zhang et al., 2010b). The wide band gap causes selective ultraviolet (UV, λ < 380 nm) absorption by the catalyst (Khalid et al., 2013), and the short lifetime of photogenerated electron–hole pairs (2 to 3 μS) (Homa et al., 2009) causes the loss of approximately 90% of the generated carriers (Venkatachalam et al., 2007). To overcome
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these disadvantages, studies have focused on doping or co-doping of metal and non-metal elements into TiO2 to enhance quantum efficiency and extend the light-response range (Liu et al., 2011a). The recently discovered carbonaceous material graphene has elicited extensive attention in the fields of condensedmatter physics, electronics, and material science (Wang et al., 2013). Graphene shows various superior properties, such as high charge carrier mobility (>200,000 cm2/(V·sec), specific surface area (2630 m2/g), optical transmittance (~97.7%) (Zhang et al., 2012a), and Fermi velocity (106 m/sec) (Song et al., 2011). Thus, graphene–TiO2 (GT) nanocomposites have become a promising material for photocatalytic applications. GT has high stability,
⁎ Corresponding author. E-mail: [email protected] (Hongtao Wang).
http://dx.doi.org/10.1016/j.jes.2014.08.011 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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1. Material and methods
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1.1. Chemicals and reagents
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PCP was purchased from Dima (USA). Commercial P25 TiO2 (80% anatase, 20% rutile; Brunauer–Emmett–Teller (BET) area, 54.77 m2/g) was supplied by Degussa Corporation (Germany). Graphene oxide (>99%) was purchased from Nanjing Xianfeng Nanomaterial Technology Co., Ltd. (China). All chemicals used in this study were of analytical grade and used without further purification. Double-distilled water was used in catalyst preparation and subsequent catalytic tests.
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Graphene oxide (100 mg) was sonicated in 250 mL of double-distilled water at room temperature for at least 1 hr to ensure thorough dispersion of graphene oxide in the solution. Then, a certain amount of P25 TiO2 was added to the above solution with magnetic stirring (120 r/min). The weight addition ratios of graphene oxide to P25 were 1:10, 1:20, 1:50, and 1:100. After stirring for 2 hr, the suspension was transferred into a 100 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 180°C, for 6 hr. The hydrothermal process ensured sufficient reduction of graphene oxide to graphene (Yang et al., 2013). The obtained precipitates were centrifuged at 10,000 r/min for 10 min and then washed with double-distilled water until the
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Crystal structures of samples were analyzed using X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer with Cu Ka radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. BET specific surface area, Barrett–Joyner–Halenda (BJH) pore volume, average pore diameter, and fractal dimension of photocatalyst calculated from the N2 adsorption–desorption isotherms were obtained with a Quantachrome gas adsorption instrument (Autosorb AS-1N2). The samples were vacuum-degassed for 4 hr at 300°C before measurements. Ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS) were obtained using a Hitachi U-3010 spectrophotometer equipped with an integrating sphere accessory for diffuse reflectance. Transmission electron micrographs were obtained with a Hitachi H-800 transmission electron microscope operating at an accelerating voltage of 200 kV. Scanning electron micrographs were acquired by a Hitachi S-5500 scanning electron microscope at an accelerating voltage of 20 kV. Surface composition of the nanocomposite was analyzed by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra System with monochromatic Al Ka X-rays (1486.6 eV). Electron paramagnetic resonance spectra (EPR) were recorded at 70 K on a JEOL JES-FA200 spectrometer operating in the X-band with 30 mg of the sample introduced into an EPR quartz probe cell under N2 atmosphere. The FT-IR spectra of the catalysts were obtained using a spectrometer (Spectrum GX, PerkinElmer, USA) within the 4000 to 400 cm−1 region at a 4 cm−1 resolution. Approximately 2 mg of the milled sample was ground with 200 mg of KBr (FT-IR grade) and compressed into a pellet under vacuum at a pressure of 75 kN/cm for 3 min. A total of 32 scans were performed, averaged for each spectrum, and then corrected against air as background in each acquisition. The spectra were subjected to a Savitsky–Golay second derivative filter (using a third-order polynomial and seven-point smoothing) to discriminate the emerging peaks (Savitzky and Golay, 1964).
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The photocatalytic degradation experiments were carried out in a vigorously stirred batch reactor containing 500 mL of PCP-Na (50 mg/L) solution and 100 mg of catalyst (GT or P25). The reactor was equipped with a high-pressure mercury lamp (300 W), and the temperature was maintained at 25°C. The photocatalytic degradation experiments were performed in a simple targeted PCP-Na aqueous system. Before irradiation, the suspension was stirred in darkness for 30 min to obtain an adsorption–desorption equilibrium, which proved to be sufficient (Zhang et al., 2012b). At designated time intervals, 1 mL aliquots of the suspension were collected and transferred into centrifuge tubes (CNW, 8 mL) with Teflon-lined screw caps. To quantitatively recover the analytes adsorbed on the catalyst, an equal volume of methanol was added to the suspension sample, which was then thoroughly shaken. The processed suspension was filtered through a 0.45 μm pore size membrane
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pH of the supernatant was neutral. The sediment samples were dried at 333 K for 24 hr in an electric oven and then ground in an agate mortar. The as-synthesized photocatalysts of different ratios were denoted as GT 1:10, 1:20, 1:50, and 1:100.
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high photocatalytic activity, and enhanced photovoltaic properties (Long, 2013) because of its enhanced quantum efficiency and decreased band gap energy (Geng et al., 2013). Several studies have been focused on the photocatalytic activity of GT. However, most of these studies utilized dyes or simple substances as target pollutants, such as methylene blue (Zhang et al., 2010a), benzene (Zhang et al., 2010c), Rhodamine B (Li et al., 2013), propanol (Kamegawa et al., 2010), and Acid Orange (An and Yu, 2011), which cannot fully represent the toxic substances existing in the environment. In aqueous systems, the harmful effect of persistent organic pollutants (POPs) on human health is becoming a global concern. Pentachlorophenol (PCP), one of the common POPs, is widely used as a wood preservative, anticorrosive, fungicide, and pesticide, leaving serious contamination in the environment (Xie et al., 2013). PCP is highly toxic, mutagenic, and carcinogenic, and was listed as one of the priority pollutants in the US, European Union, and China (Liu et al., 2011a). PCP is resistant to biodegradation and photolytic degradation in aqueous systems because of its stable chemical structure; thus, investigating the photocatalytic degradation of PCP using GT as a catalyst is of great interest. In this study, GT nanocomposites of different carbon material ratios were prepared via a facile hydrothermal treatment. The function of graphene in the enhancement of light absorption, quantum efficiency, and photoactivity of the photocatalysts was systematically investigated. The photocatalytic activity of the as-prepared nanoparticles was examined by the photodegradation of sodium pentachlorophenol (PCP-Na) under UV irradiation and the mineralization of the target pollutant. This study provides a new approach for the decomposition of persistent hazardous compounds.
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Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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filter. The concentrations of PCP-Na and surfactants were determined using high-performance liquid chromatography 181 Q16 (Agilent 1260) fitted with a UV detector and an Agilent TC-C18 182 column (4.5 mm × 250 mm; 5 μm). The mobile phase of high183 performance liquid chromatography was methanol–water 184 (80:20) at a flow rate of 1.00 mL/min. Chromatography was 185 performed at 35°C. UV wavelengths were set at 249 nm for 186 PCP-Na. During photodegradation, 10 mL of the sample was 187 collected at certain time intervals and filtered through a 0.45 μm 188 pore size membrane filter for total organic carbon testing 189 Q17 (Shimadzu TOC-VCPH). All data were expressed as the means 190 of two replicates with ±5% deviation.
2.2. BET surface areas and pore distributions
212
The specific surface areas of P25 and GT with different addition ratios were determined by the BET method using adsorption data, whereas the pore volume and pore size distribution were derived using the BJH model based on desorption isotherms. Fig. 2 (inset) shows the adsorption–desorption isotherms and the corresponding pore size distribution curves of the catalysts. All catalysts exhibited similar type IV isotherms (Sing et al., 1985) with an H3 hysteresis loop at a relative pressure close to unity, which are typical characteristics of mesoporous materials (Liu et al., 2011b). The pore size distributions (inset in Fig. 2) of the GT samples were broad (from 2 to over 100 nm), indicating the presence of mesopores and macropores (Xiang et al., 2011b). As shown in Table 1, the specific surface area and pore volume of the as-prepared catalysts were primarily dominated by graphene compared with those of P25. However, the pore diameter was uniform and lower than 5 nm. The BET surface area of the GT samples significantly increased from 53.74 to 72.73 m2/g with increasing graphene content. This result can be attributed to the fraction of bare graphene in the composites. However, the BET surface area of the GT samples was still relatively low compared with that of pure graphene (2600 m2/g). This result can be ascribed to the minimal doping level of graphene. Thus, surface area and porosity are mainly dominated by the TiO2 component. Another parameter that can provide additional information on the catalyst structure in terms of “roughness exponent” is the surface fractal dimension (D) (Zhu et al., 2009). Fractal dimension describes the topography of the real surface based on fractal geometry. This factor is usually between 2 (smooth surface) and 3 (rough surface). The complex pore and surface structure can be adequately described by D values (Zhang et al., 2012c). D was derived using the Frenkel– Halsey–Hill and Neimark–Kiselev methods (Table 1). The D values of the GT samples increased with increasing graphene content. This result suggests that the doping with graphene,
213
O
The XRD patterns of the as-prepared GT composites and P25 TiO2 are shown in Fig. 1. GT nanocomposites with different weight addition ratios of graphene and P25 clearly had similar XRD patterns. Peaks located at 25.3°, 37.8°, 48.0°, 55.1°, and 62.7° can be indexed to the (101), (004), (200), (211), and (204) crystal planes of anatase TiO2 (JCPDS No. 21-1272), respectively. No typical diffraction peaks of graphene were observed in the nanocomposites. This result can be attributed to the main characteristic peak of graphene at 25.0° overlapping with the (101) peak of anatase TiO2 (Zhang et al., 2011) and to the relatively low weight fraction of graphene in the nanocomposites. However, the GT composites slightly increased the amount of rutile phase TiO2 (JCPDS No. 21-1276). The XRD results indicate that doping graphene with TiO2 promotes the phase transformation from anatase to rutile. In other words, graphene has a catalytic function in the phase transformation from anatase to rutile during the hydrothermal treatment because of its special structural properties (Li et al., 2013).
C
E
R
250
0.010
Anatase Rutile
GT (1:100) GT (1:50) GT (1:20)
GT 1:100
GT 1:20 GT 1:50
P25 GT 1:10
0.008
200
150
dV/dW (cm3/(g.min))
Adsorbed volume (cm3/g, STP)
211
(224)
210
R
209
(215)
208
(204)
207
(116)
206
(211) (220)
205
N C O
204
(200)
203
(210)
202
(101) (004)
201
(111)
200
U
199
(101)
198
(110)
197
Intensity (a. u.)
196
R O
194
P
2.1. XRD patterns of the photocatalysts
D
193
E
2. Results and discussion
T
192 191
195
F
180
0.006
0.004
0.002
100 0.000
10
100 Pore diameter (nm)
50
GT (1:10) P25 20
30
40
50
60
70
80
90
100
2θ (°) Fig. 1 – XRD patterns of the graphene–TiO2 (GT) photocatalyst and P25.
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 2 – Nitrogen adsorption–desorption isotherm and pore size distribution curve (inset) of graphene–TiO2 (GT) and P25.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247
4
t1:3
t1:4
Table 1 – BET specific surface area, pore volume, average pore diameter, and fractal dimension of graphene–TiO2 nanocomposites and P25. Sample
BET surface area (m2/g)
Total pore volume (mL/g)
t1:6 t1:7 t1:8 t1:9 t1:10
P25 GT 1:10 GT 1:20 GT 1:50 GT 1:100
t1:11 t1:13 t1:12 t1:14 t1:15
GT: graphene–TiO2. D values calculated at P/P0 > 0.35. b NK: D was derived using Neimark–Kiselev method. c FHH: D was derived using Frenkel–Halsey–Hill method.
53.74 72.73 64.39 58.39 55.63
Fractal dimension (D) a
Pore diameter (nm)
0.264 0.361 0.372 0.338 0.332
3.092 3.109 3.117 3.113 3.115
NK b
R2
FHH c
R2
2.459 2.568 2.527 2.493 2.488
0.998 0.979 0.980 0.990 0.989
2.555 2.517 2.512 2.504 2.491
0.993 0.995 0.995 0.997 0.977
t1:5
a
250 251 252 253 254
2.3. Scanning electron microscopy and transmission electron microscopy The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of GT 1:10 and 1:100 are shown in Fig. 3. As shown in the figure, the TiO2 nanoparticles did not easily complex with graphene and were entrapped
inside the graphene sheets. However, the TEM images of GT reveal inhomogeneous dispersion of TiO2 particles in the graphene matrix. The particles accumulated along the wrinkles and edges of the graphene sheets. This accumulation limited the interfacial contact between TiO2 and graphene, which resulted in insufficient utilization of the excellent electron conductivity of graphene (Zhang et al., 2011). Hence, further studies should focus on achieving homogenous dispersion of solid TiO2 particles on a graphene sheet to effectively utilize the “structure-directing” ability of graphene
a
T
E
255
with its wrinkles and stacking structure, forms composites with a “rougher” surface.
P
249
D
248
R O
O
F
t1:1 t1:2
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 4 ) XXX –XXX
b
TEM-GT 1:100
U
c
N
C
O
R
R
E
C
TEM-GT 1:10
Q1
SEM GT 1:10
d
SEM GT 1:100
Fig. 3 – TEM and SEM images of graphene–TiO2 (GT). (a) TEM of GT 1:10, (b) TEM of GT 1:100; (c) SEM of GT 1:10; (d) SEM of GT 1:100. Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
256 257 258 259 260 261 262 263 264 265
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J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 4 ) XX X–XXX
oxide, which possesses abundant oxygen-containing functional groups on the basal plane and edge that can provide reactive sites.
2400
269
2.4. UV–Vis DRS
1200
270
286
Fig. 4 shows the UV–Vis DRS of the as-prepared GT composites. For comparison, P25 TiO2 was also tested under the same condition. P25 showed a mild response within 200 to 400 nm, which can be attributed to the electron transition from the valence band to the conduction band. The addition of graphene increased the light absorption intensity in both UV and visible light regions. Furthermore, a qualitative red shift to higher wavelength was observed in the absorption edge of GT nanocomposites (50 nm for GT 1:100). This shift can be attributed to the hybridization of C2p and O2p atomic orbits to form a new valence band; hence, the band gap energy was reduced in the GT system (Yang et al., 2013). Within the 400 to 800 nm range, light absorption intensity increased with increasing graphene content. This phenomenon can be attributed to the surface plasmon resonance effect of the graphene unit (Li et al., 2013). These findings suggest that the introduction of graphene can promote the visible light response of the nanocomposites.
287
2.5. XPS
288
The XPS surface probe technique was adopted to study the chemical states of Ti, O, and C species and the interaction of graphene and TiO2 on the surface of GT composites. Fig. 5 shows the XPS of P25 and GT 1:100 composites in the Ti 2p, O 1s, and C 1s binding energy regions. For GT, the measured binding energies of Ti 2p3/2 and Ti 2p1/2 were 458.3 and 464.0 eV, respectively, consistent with P25 TiO2. The deconvoluted peaks centered at the binding energies of 529.5, 530.4, and 531.7 eV were assigned to Ti\O, O\H, and C\O functional groups, respectively. Fig. 5b shows the shift in the O 1s binding energies, illustrating that electron transformation at the interface between graphene and TiO2 perturbs the O electronic environment (Li et al., 2013).
282 283 284 285
289 290 291 292 293 294 295 296 297 298 299 300
Intensity (a. u.)
1.5
1.2
0.9
F 466
462
460
458
456
454
452
Binding energy (eV)
2200 1650
2750
O1s
C-O 531.7
O-H 530.4
1100
b
Ti-O 529.5
GT 1:100
2750
P25
2200 1650 1100 536
534
532
530
528
526
524
Binding energy (eV) 3500
Celement, C-H, C-C GT 1:100
2800
c
284.2
C1s
2100 1400
GT 1:10
464
O
0 468
R O
281
P25
2250
P
280
3000
D
279
Intensity (a. u.)
278
600
750
E
277
Intensity (a. u.)
276
Ti2p
1500
T
275
C
274
a
Ti 2p3/2 458.3 eV Ti 2p1/2 464.0 eV
E
273
R
272
GT 1:100
1800
R
271
N C O
268
700
C=O 288.3
C-OH 285.1
30000 P25
2250
GT 1:20
1500
U
267
Absorbence (a. u.)
266
GT 1:50 GT 1:100
0.6
750 0 292
290
288
286
284
282
280
278
276
Binding energy (eV) 0.3 P25
0.0 200
300
400
500
600
700
800
Fig. 5 – High-resolution XPS spectra of the synthesized graphene–TiO2 (GT 1:100) and P25. (a) Ti 2p region; (b) O 1s region; (c) C 1s region.
Wavelength (nm) Fig. 4 – UV–Vis diffuse reflectance spectra of the synthesized graphene–TiO2 (GT) and P25.
Fig. 5c shows the high-resolution C 1s XPS spectra of the 301 GT 1:100 sample in comparison with that of P25. The binding 302 energy of 284.2 eV can be attributed to the carbon, C\C, and 303
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
6
2.6. EPR analysis
318
An in situ EPR study was performed for GT composites and P25 at liquid N2 temperature (77 K) under UV irradiation. Photo-induced EPR signals of trapped electrons and holes on GT nanocomposites and P25 are shown in Fig. 6. Two sets of features were observed in the EPR spectrum: signals from electrons (A) and holes (B). EPR signals due to GT were characterized by g values. The sharp signal at g┴ = 1.995 and g// = 1.970 corresponded to surface electron trapping sites and are represented as signal A (Gopal et al., 2008; Kumar et al., 2006). EPR signals with g factors g1 = 2.030, g2 = 2.016, and g3 = 2.008 corresponded to surface hole trapping sites and are represented as signal B (Ribbens et al., 2011; Shkrob et al., 2011). Fig. 6 demonstrates apparent variation in EPR intensities between trapped electrons and holes of GT and P25. For P25, a very weak signal was observed presumably due to hole traps. This result indicates that the lifetime of electron and hole radicals is too short to be observed under such testing conditions. By contrast, the EPR intensities for the GT nanocomposites were higher and more intense, especially for GT 1:20, followed by GT 1:100. These results suggest that graphene doping can effectively prolong the lifetime of photoelectronhole pairs.
323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338
2.8. Photocatalytic degradation of PCP-Na
357
The photocatalytic activity of GT with different carbon ratios and P25 catalysts was evaluated through PCP-Na (initial concentration of 50 mg/L) degradation and TOC variation in aqueous solution under UV irradiation for 2 hr (Fig. 8a, b, respectively). After 120 min of UV irradiation, 31.1% of PCP-Na was degraded using P25 as the catalyst. Remarkable improvements in photodegradation were observed in all GT nanocomposites, wherein more than 97% of PCP-Na was decomposed. However, GT with different graphene ratios showed similar degradation efficiencies. The degradation curves were fitted to the pseudo-firstorder kinetic law described by the following equation:
358
R O
322
P
321
D
320
T
319
C
315
E
314
R
313
c − ln ¼ kt c0
ð1Þ
R
312
340
O
311
14000
P25 GT 1:100 GT 1:50 GT 1:20 GT 1:10
12000
C
310
10000
N
309
The FT-IR spectra of GT and P25 TiO2 are described in Fig. 7. The broad peak at 3400 cm−1 in all samples indicates the presence of surface O\H stretching vibrations (Nguyen-Phan et al., 2011). Peaks at around 1393 and 1250 cm−1 can be assigned to tertiary C\OH and epoxy C\O groups, respectively (Nguyen-Phan et al., 2011; Shen et al., 2011). The absorption peak at around 1600 cm−1 is contributed by the skeletal vibration of graphene (Zhang and Pan, 2011). For the GT composites, the C\O stretching vibration peak at 1052 cm−1 and the C_O stretching vibration of \COOH group peak at 1720 cm−1 of graphene oxide completely disappeared. Thus, the oxygen-containing functional groups were completely removed through the thermal reaction. Meanwhile, strong absorption bands observed in the 400 to 1000 cm−1 region suggest the presence of Ti\O\Ti and Ti\O\C bonds, indicating the chemical interaction between the surface hydroxyl groups of TiO2 and the functional groups of graphene oxide (Nguyen-Phan et al., 2011).
8000 6000 4000 2000
0 -2000
P25 GT 1:10 GT 1:20 GT 1:50 GT 1:100
A
g=1.995
g =1.970
1600
U
308
Intensity (a.u.)
307
339
F
317
306
2.7. FT-IR spectra
O
316
C\H bonds in the composite. The peak at 285.1 eV can be ascribed to carbon and defect-containing sp2 hybridized carbon atoms (Xiang et al., 2011a). The relatively weak peak at 288.3 eV was contributed by carboxyl carbon (O_C\O), indicating the esterification reaction of \OH on TiO2 with \COOH on the graphene oxide surface and the formation of O_C\O\Ti bonds (Xiang et al., 2011b). The peak located at 287.0 eV, which corresponded to the oxygen-containing carbonaceous bands, disappeared in the C 1s XPS spectra of GT. Therefore, graphene oxide was effectively reduced to graphene during the hydrothermal treatment. Moreover, the sp2 carbon network was retained in graphene, as indicated by the C\C peak at 284.2 eV (Murugan et al., 2009).
305
E
304
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1393 1250
3393 1720
g2 = 2.016g3 =2.008 g1 = 2.030
-4000
B
-6000 -8000 316
318
320
322
324
326
328
330
332
334
Magnetic field (G) Fig. 6 – EPR spectra of the synthesized graphene–TiO2 (GT) and P25 at 70 K.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 7 – FT-IR spectra of the synthesized graphene–TiO2 (GT) and P25.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356
359 360 361 362 363 364 365 366 367 368 369
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1.0
1.0
a
b
0.8
0.8
TOC (%)
4 3
0.6
0.4
2 1 0
0
20
40 60 80 100 120 Time (min)
0.0 0
20
40
GT 1:100 GT 1:50 GT 1:20 GT 1:10
0.2
60
80
100
0.0
120
0
20
40
Time (min)
F
0.2
O
Ct /C0
0.4
5
-ln(Ct /C0)
P25 GT 1:100 GT 1:50 GT 1:20 GT 1:10
0.6
60
80
100
120
R O
Time (min)
where, c is the PCP-Na concentration at time t, and k is the apparent first-order rate constant. The values of k for the initial 373 reaction under UV irradiation with different catalysts are shown 374 in Table 2. All degradation curves were fitted well by Eq. (1), with 2 −3 −1 375 Q18 all R values ≥ 0.946. For P25, k was 1.81 × 10 min , and the 376 degradation rate for GT 1:100 was 18.34 times higher than that 377 of P25. The order of photodegradation efficiency of PCP-Na 378 under UV light is as follows: GT 1:100 > GT 1:50 > GT 1:20 > GT 379 1:10 > P25. This result suggests that the GT 1:100 nanocompos380 ite is more efficient than P25 for PCP-Na degradation. In 381 addition, graphene oxide doping can significantly enhance 382 photodegradation efficiency. However, reaction rates decreased 383 with increasing graphene ratio (Table 2). Thus, excessive 384 graphene can be detrimental for the photocatalytic degradation 385 reaction. Although graphene doping can retard the recombina386 tion of electrons and holes, it can also obstruct photon 387 absorption, thereby decreasing degradation efficiency (Wang 388 and Zhang, 2011). 389 Fig. 8b shows the TOC variation in GT catalysts with 390 different graphene ratios. After 120 min of irradiation, 63.4% 391 to 82.9% of TOC was mineralized into CO2 and H2O, which was 392 lower than the decomposition rate of PCP-Na (97.7% to 99.0%). 393 GT 1:100 exhibited the highest mineralization efficiency. The 394 comparatively lower mineralization rate than degradation 395 rate suggests that dechlorination is stepwise and that the 396 dechlorinated intermediates are subsequently oxidized to
P
Fig. 8 – Photodegradation of PCP-Na (A) with graphene–TiO2 and P25 catalyst and first-order fitting curves (inset), and (B) TOC variation.
371 370
t2:4
Table 2 – Apparent first-order rate constant k values for the photocatalytic degradation of PCP-Na with graphene–TiO2 and P25 as catalysts.
t2:5
Catalyst
k (10−3 min−1)
R2
t2:6 t2:7 t2:8 t2:9 t2:10
P25 GT 1:10 GT 1:20 GT 1:50 GT 1:100
1.81 26.94 30.77 32.10 33.19
0.946 0.977 0.984 0.989 0.990
397
2.9. Photocatalytic mechanism
402
A schematic electronic structure and degradation process for PCP-Na by GT was proposed based on the abovementioned characterization and photodegradation results (Fig. 9). Under UV irradiation, the photogenerated electrons can be effectively conducted to the graphene sheet, leaving holes with positive charges on the surface of TiO2 particles. Graphene is an electron acceptor that accelerates the interfacial electron-transfer process. Graphene can strongly hinder the recombination of e−–h+ because of the Schottky barrier effect. As proved by EPR results, where signals with much higher intensity were shown for GT composites, the lifetime of electron–hole pairs can be prolonged, providing more radicals for the degradation of PCP-Na into H2O, CO2 and other intermediates. Thus, the photocatalytic activity of GT could be significantly enhanced. Though the BET surface area of the GT samples increased with increasing graphene content, GT 1:100 showed the highest degradation efficiency other than GT 1:10. This phenomenon can be ascribed to the fact that the degradation was not mainly based on the surface absorption of PCP-Na onto catalysts. Meanwhile, both the PCP-Na decomposition and TOC mineralization corresponded well with the UV–Vis DRS results, where the addition of graphene significantly increased the light absorbance, and GT 1:100 gives the highest intensity under UV light irradiation. However, the GT precursor P25 accumulated along the wrinkles and edges of the graphene oxide sheets during hydrothermal treatment because of the abundant carboxylic acid groups on graphene oxide, as shown in the SEM and TEM results. The inhomogeneous aggregation of TiO2 nanoparticles on graphene creates new e−–h+ recombination centers, thereby decreasing the photocatalytic efficiency.
403
E
T
C
E
R
R
N C O
U
t2:1 t2:2 t2:3 Q2
open the aromatic ring (Liu et al., 2011a). However, after 2 hr of reaction, the decreasing tendency of TOC was apparent in all GT cases. The continuous decrease in TOC can be attributed to the decomposition of small molecule acids, such as formic acid and acetic acid (Zhang et al., 2012b).
D
372
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
398 399 400 401
404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432
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UV Electron e-
Hole h+
TiO2
Graphene UV PCP-Na
PCP-Na
Graphere
Graphere
F
H2O, CO2 and other mineralization
H2O, CO2 and other mineralization
O
PCP-Na
449 448
Acknowledgments
450 451
This work was supported by the National Natural Science Foundation of China (No. 41371472).
45 2
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3. Conclusions
T
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R O
Fig. 9 – Schematic of the PCP-Na photocatalytic degradation under UV irradiation with graphene–TiO2.
Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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P
R O
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Please cite this article as: Zhang, Y., et al., Graphene TiO2 nanocomposites with high photocatalytic activity for the degradation of sodium pentachlorophenol, J. Environ. Sci. (2014), http://dx.doi.org/10.1016/j.jes.2014.08.011
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