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reduced graphene oxide nanocomposites: A simple template-free synthesis and their high photocatalytic performance
Materials Research Bulletin 51 (2014) 244–250
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Mesoporous anatase TiO2/reduced graphene oxide nanocomposites: A simple template-free synthesis and their high photocatalytic performance Qi Zhou a,1, Yong-Hui Zhong a,b,1, Xing Chen b, Xing-Jiu Huang b, Yu-Cheng Wu a,* a b
School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 23000, PR China Laboratory of Nanomaterials and Environmental Detection, Hefei Institute of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 October 2013 Received in revised form 2 December 2013 Accepted 16 December 2013 Available online 25 December 2013
Mesoporous anatase phase TiO2 was assembled on reduced graphene oxide (rGO) using a template-free one-step hydrothermal process. The nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Brunauer–Emmett–Teller (BET) surface area. Morphology of TiO2 was related to the content of graphene oxide. TiO2/rGO nanocomposites exhibited excellent photocatalytic activity for the photo-degradation of methyl orange. The degradation rate was 4.5 times greater than that of pure TiO2 nanoparticles. This difference was attributed to the thin two-dimensional graphene sheet. The graphene sheet had a large surface area, high adsorption capacity, and acted as a good electron acceptor for the transfer of photo-generated electrons from the conduction band of TiO2. The enhanced surface adsorption characteristics and excellent charge transport separation were independent properties of the photocatalytic degradation process. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors Composites Nanostructures Solvothermal Catalytic properties
1. Introduction Clean and sustainable energy resources are needed as traditional energy sources are depleted and environmental impact is better appreciated. Fujishima and Honda first described the photocatalytic splitting of water using TiO2 electrodes in 1972, a method that uses sunlight and water to produce hydrogen gas. Significant progress has been made since then in the field of semiconductor photocatalysts [1,2]. TiO2 has a number of advantages including its lack of reactivity with the environment, strong oxidizing power, low cost, and long-term stability against photo and chemical corrosion [3]. UV illumination of TiO2 energizes electrons to move from the valence band to the conduction band on the surface of the metal. Water reacts with the charged surface metal to form OH and O2 free radicals. Redox reactions occur, ultimately yielding hydrogen and oxygen gas [4]. The large surface area contributes to the number of possible reaction sites on the photocatalyst surface [5,6]. Performance of the catalyst is enhanced by suppressing the fast recombination rate of photogenerated electron–hole pairs [7–9].
* Corresponding author. Tel.: +86 551 62901012; fax: +86 551 62904517. E-mail addresses: [email protected] (X.-J. Huang), [email protected] (Y.-C. Wu). 1 These authors contributed equally to this work. 0025-5408/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.034
An effective approach to producing high-surface-area TiO2 is the creation of mesoporous (pore diameter from 2 to 50 nm) materials. Mesoporous TiO2 facilitates the rapid diffusion of ions and molecules within the material so that the rate of photocatalytic reactions is substantially improved [10,11]. Conventional synthesis of mesoporous TiO2 uses hard (such as mesoporous silica or colloidal spheres) [12,13] or soft porous templates (such as Pluronic surfactant P123) [14]. The template method is generally tedious, time-consuming, and results in micron sized TiO2 pores [10]. A one-step template-free method was developed to overcome this limitation and produce mesoporous structures [15–17]. Graphene consists of a single layer of sp2 hybrid carbon atoms that are tightly packed into a two dimensional honeycomb network. It exhibits unique and remarkable properties, including a large specific surface area (2600 m2 g 1), high mobility of the charge carriers (2 105 cm2 V 1 s 1), and high chemical stability [18,19]. These properties make graphene attractive as a photocatalytic agent. The large number of oxygen containing functional groups act as nucleation sites [20–22]. Graphene has a large potential sink of electrons that are freely mobile and available to react [23,24]. These properties led to the evaluation of graphene– TiO2 composites as photocatalytic agents [25–30]. Zhang et al. prepared graphene encapsulated mesoporous hollow TiO2 nanospheres using a graphene protected calcination process. The composites exhibited significant photocatalytic activity for degrading RhB [25]. Zhang et al. prepared a TiO2 (P25)-graphene
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composite with enhanced photocatalytic activity for the degradation of methylene blue [26]. Du et al. produced hierarchically ordered macro-meso-porous TiO2 films using a confinement selfassembly method [27]. Yu et al. demonstrated that the TiO2– graphene nanocomposites exhibit exceptional photocatalytic reactivity and selectivity towards removing azo dyes in water relative to the pristine TiO2 [29]. The manufacturing procedure for mesoporous structures was complex and time-consuming and the TiO2 nanoparticles tended to agglomerate. The manufacture of mesoporous anatase compounds assembled on graphene using a template-free method has not been well studied and may increase the photocatalytic activity of composites by increasing the surface area of TiO2. We reported a simple template-free method to produce mesoporous anatase TiO2 on a rGO platform using a one-step hydrothermal process. This composite had excellent photocatalytic activity (degradation of organic pollutants) in comparison to pure TiO2. Through adsorption experiment and photo-electrochemical behaviours investigation, we found that the equilibrium adsorption capacity of TG composites were greater than that of pure TiO2, and under illumination the TG composites exhibited a much higher photocurrent density than pure TiO2. The TiO2/rGO composite exhibited increased adsorption of pollutants and facile charge transportation and separation. There was a synergistic relationship between enhanced surface adsorption and charge transportation-separation during photocatalytic degradation.
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was added with vigorous stirring for another 30 min. The homogeneous solution was transferred to a Teflon-sealed autoclave and maintained at 180 8C for 6 h. The product was rinsed and dried in a vacuum at 60 8C for 24 h. The GO content in the prepared composite was adjusted by changing the volume of the added GO solution. The prepared composites were named ‘‘TG-x’’ where ‘‘x’’ was the volume of the added GO solution (5 mL, 10 mL, 15 mL, and 30 mL). Pure TiO2 was prepared using the same procedure, but without the addition of GO. 2.4. Characterization
Natural graphite flake (325 mesh) was purchased from Alfa Aesar. Analytical grade sulfuric acid (H2SO4, 95.0–98.0%), hydrochloric acid (HCl, 36.5–38.0%), potassium permanganate (KMnO4, 99.5%), H2O2 (30 wt%), sodium nitrate (NaNO3, 99.0%), ammonium hexafluorotitanate ((NH4)2TiF6, 99.0%), and boric acid (H3BO3, 99.5%), were obtained from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. P25 was purchased from Degussa. Distilled water (18.2 MV cm) was purified using the NANO pure1DiamondTM UV water system.
The morphology and microstructure of the samples were verified using field-emission scanning electron microscopy (FESEM, FEI-200) and transmission electron microscopy (TEM, JEM-2010). Powder X-ray diffraction (XRD) was carried out using Philips X’Pert Pro X-ray diffraction with Cu Ka radiation. The specific surface area and gas adsorption isotherms of the composite were determined using a Coulter Omnisorp 100CX Brunauer–Emmett–Teller (BET) with nitrogen adsorption and a degassing temperature of 80 8C. X-ray photoelectron spectroscopy (XPS) analyses of the samples were conducted using a VG ESCALAB MKII spectrometer with a Mg Ka X-ray source (1253.6 eV, 120 W). The energy scale was internally calibrated by referencing the binding energy of the C 1s peak at 284.6 eV for contaminated carbon. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CHI660C, ShangHai) at ambient temperature (about 22 8C). The electrochemical cell had a three electrode configuration with an Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode. A 5 mm diameter polished glassy carbon working electrode (GCE) was used as a current collector. An electroactive sample was mixed with ethanol to make a slurry. A small amount of Nafion solution (0.5 wt%) was added to thicken the slurry on the GCE. Photocurrent measurements were carried out using a CHI660C workstation and a xenon lamp. A threeelectrode configuration was employed, consisting of an ITO working electrode, Ag/AgCl electrode and a platinum wire electrode. The ITO electrodes were coated with TiO2 or TG composites, using a spin-coating method.
2.2. Synthesis of graphene oxide (GO) sheets
2.5. Evaluation of photocatalytic activity
GO was synthesized from natural graphite flake using a modified Hummers method. In a typical experiment, 1 g natural flake graphite powder and 1 g NaNO3 were added to 46 mL concentrated H2SO4 in an ice-bath. 6 g KMnO4 was gradually added while stirring. The mixture was then stirred at 35 8C for 2 h. 92 mL distilled water was then slowly added to the mixture while stirring. The resultant mixture was stirred at 90 8C for 30 min. The reaction was terminated by adding 200 mL distilled water and 6 ml H2O2 (30 wt%). This addition turned the mixture bright yellow. The mixture was filtered and washed with 150 mL 5% aqueous HCl to remove metal ions, and then washed with distilled water to completely remove metal ions and acids. The GO was added to distilled water and ultrasonically exfoliated in a bath sonicator for 1 h to achieve a brown viscous dispersion with a concentration of 3 mg/mL.
Photocatalytic activities of the composites were evaluated using methyl orange (MO) degradation under UV-light irradiation. Twenty micrograms samples were dispersed in 50 mL MO solutions (30 ppm). Solutions were magnetically stirred in the dark for 1 h to ensure the establishment of adsorption–desorption equilibrium of the MO dye. The suspensions were then exposed to UV irradiation from a 300 W high pressure Hg lamp with the main wave crest at 365 nm. Two microliters test samples were removed every 5 min for MO absorbance analysis after centrifugation. The percentage of degradation was reported as C/C0, where C was the absorption of dye solution at each time point and C0 was the absorption of the initial concentration.
2. Experimental 2.1. Materials
3. Results and discussion 3.1. Synthesis and structural characterizations of TG composites
2.3. Assembled mesoporous anatase on rGO Mesoporous anatase on a rGO platform was manufactured using the hydrothermal method. In a typical preparation, GO dispersion (3 mg/mL) was dissolved in deionized water to a total volume of 60 mL. 0.01 mol (NH4)2TiF6 was added to the dilute GO solution. The mixture was stirred for 30 min and 0.01 mol H3BO3
The synthesis of TG composites (Fig. 1) typically involved two steps: (1) synthesis of GO using the modified Hummers method, followed by uniformly grafting of Ti4+ on GO sheets using chemisorptions [31]. (2) H3BO3 was added to the dispersion heated in a Teflon-sealed autoclave in order to transform Ti4+ into anatase TiO2 and to reduce the GO.
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Fig. 1. Schematic representation of the formation of TG composites.
The crystallographic structures of the samples were confirmed using X-ray powder diffraction (XRD) analysis. Fig. 2 shows the XRD pattern of the samples. The synthesized GO had a distinct large peak at 2u = 10.88, corresponding to the (0 0 2) diffraction of GO. Samples with different GO content exhibited similar TiO2 crystal composition, primarily anatase phase. The peaks at 25.38, 36.98, 48.08, 53.98, 55.08, 62.78, 68.88 and 70.38, were assigned to the diffractions of the (1 0 1), (1 0 3), (2 0 0), (1 0 5), (2 1 1), (1 1 8), (1 1 6) and (2 2 0) crystal planes of the anatase phase (JCPDS NO. 21-1272) in TiO2, respectively. No diffraction peaks of rGO were observed in the TG composites. This might be attributed to the relatively low diffraction intensity of rGO [32]. The main peak of anatase at 25.38 could have been hidden by the large graphene peak at 24.58. The morphology of the GO and TG composites was examined using field-emission SEM. Fig. 3 presents SEM images of the different samples. Pure TiO2 was composed of spherical particles with a diameter of 2–3 mm. These particles aggregated to form larger particles (Fig. 3b). The morphology of TG composites significantly differed from that of pure TiO2. SEM imaging (Fig. 3f) showed that the TG-30 composite was composed of TiO2 particles uniformly dispersed on sheets of GO. The TiO2 particles had a diameter of 20–40 nm. Composite morphology varied with GO content. TG-5 (Fig. 3c) consisted of spherical TiO2, similar to pure TiO2, but with different sized particles. Fig. 3d, e, and f show that spherical TiO2 was seen
Fig. 2. XRD patterns of different samples.
less frequently as GO content increased. TiO2 nanoparticles were instead uniformly dispersed on the surface of the rGO. GO acted as a dispersant and stabilizing agent, preventing TiO2 aggregation. A large number of TiO2 nuclei were absorbed on the surface of the GO nanosheets as a result of the large surface energy of grapheme [33]. The preferential nucleation on the surface of GO combined with the modulated growth of TiO2 nuclei led to the formation of a mesoporous anatase nanostructure assembled on the rGO. The morphology of the TG composites was evaluated using TEM analysis. GO consisted of a crumpled layered structure stacked in sheets (Fig. 4a). The TiO2 nanoparticles in TG-15 had a relatively uniform size of about 30 nm and were dispersed on the surface of the GO sheets (Fig. 4b). Some TiO2 nanoparticles were agglomerated together. There was an internal mesoporous structure (Fig. 1 C). The spacing of the clear lattice fringes was 0.35 nm, corresponding to the d-spacing of (1 0 1) crystalline plane anatase TiO2 (Fig. 4d). This suggests that TiO2 nanoparticles were highly crystallized on the GO sheets [10]. The orientation of the (1 0 1) lattice fringes was fairly uniform indicating the presence of a single-crystal-like microstructure within the single TiO2 nanoparticles. These findings support the synthesis of TiO2 nanocrystals with a mesoporous structure assembled on rGO using facile hydrothermal processes. The single crystal of anatase was synthesized using Ti4+ hydrolyzed during heat treatment. Corrosive F in the reaction system acted to etch the TiO2 crystal [16,17,34]. The interaction of crystallization and corrosion worked to form mesoporous anatase TiO2. Nitrogen adsorption–desorption analysis was performed to determine pore sizes, their distribution, and surface area of the synthesized TG composites. Fig. 5 shows pore size distribution plots and nitrogen adsorption–desorption isotherms for TG-15. TG-15 demonstrated a type-IV isotherm with one hysteresis loop at a relative pressure between 0.65 and 0.95 (inset in Fig. 5), indicating that mesoporous structures were present [35]. The Brunauer–Emmett–Teller (BET) method was used to determine surface area. The specific surface area of TG-15 was 124 m2 g 1. The pore size distribution of TG-15 was in the narrow range of 2– 20 nm. As shown in Fig. 5, there were two sharp peaks at 3.5 nm and 11.2 nm, representative of the two types of mesoporous structures in TG-15. This was consistent with the TEM observations. The mesoporous structures consisting of single anatase TiO2 nanoparticles had a relatively uniform size of about 3.5 nm. Some anatase nanoparticles agglomerated on the rGO sheets, resulting in particles about 11.2 nm in diameter. XPS was used to evaluate the chemical states of the carbon in the GO and TG composites. The C1s XPS spectra of GO and TG-15 are shown in Fig. 6a and b. Four Gaussian curves were seen to be centred at 284.6, 285.7, 286.7, and 288.5 eV, corresponding to graphitic carbon (C–C), alcohol or ether carbon (C–OH, C–O–C), carbonyl groups (C5 5O), and carboxyl carbon (O5 5C–O), respectively [36]. Synthesis of the composite was associated with a decrease in the intensity of the peaks at 286.7 eV (C5 5O group) and 288.5 eV (O5 5C–O group) in GO, indicating the content of oxygen-containing functional groups was decreased. The hydrothermal reaction reduced the GO sheets to rGO, increasing the number of defective carbon atoms. Raman spectroscopic measurement was used to further elucidate the structure of the composites. The Raman spectra of GO, pure TiO2 and TG-15 are shown in Fig. 6. The D and G bands of GO were seen at 1350 cm 1 and 1595 cm 1. TG-15 exhibited additional Raman peaks at 148 cm 1, 398 cm 1, 514 cm 1, and 630 cm 1. These peaks were ascribed to anatase TiO2 [37]. These findings support the assembly of anatase TiO2 on the rGO sheets. The increased D/G ratio intensity of TG-15 suggested there was a decrease in the average size of the sp2 domains after GO reduction
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Fig. 3. SEM images of (a) GO, (b) TiO2, (c) TG-5, (d) TG-10, (e) TG-15 and (f) TG-30.
Fig. 4. TEM images of (a) GO and (b, c, d) TG-15.
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Fig. 5. Pore size distribution curves of the TG-15 composite. The inset shows the nitrogen-adsorption–desorption isotherms.
[32]. This might be due to the removal of oxygen-containing groups during the hydrothermal treatment. These findings were consistent with the XPS spectra analysis. 3.2. Photocatalytic activity of TG composites Methyl orange was selected as a model organic pollutant to evaluate the photocatalytic performance of TG composites. The photocatalytic activities of TG composites and P25 were evaluated using UV light illumination after dark adsorption. The time dependence of MO dye degradation is shown in Fig. 7a. All of
Fig. 7. (a) Photocatalytic degradation of MO using UV light. (b) Photocatalytic degradation reaction rates of MO in different samples.
the photocatalysts demonstrated activity with UV light. TG composites had the highest photo-activity. TG-30 decomposed 95% of the MO after 30 min of irradiation. P25 had lower photocatalytic activity, with only 68% of MO decomposed after 30 min irradiation. The photocatalytic activity of TG composites increased as rGO content increased. In order to compare reaction rates, we re-plotted the previous results as the first-order reaction rate equation ln(C/C0) = kt, where k was the apparent first-order rate constant [37]. Fig. 7b shows the linear relationship of ln(C/C0) and reaction time t for different samples. P25 had the lowest k value, 0.039 min 1. The photodegradation rate of TG composites was much greater than that of P25. Incorporation of rGO resulted in an increase in the photocatalytic degradation rate constant. k was 0.058 min 1, 0.087 min 1, 0.093 min 1 and 0.122 min 1 for TG-5, TG-10, TG-15 and TG-30, respectively. rGO benefited MO photocatalytic degradation. 3.3. Reasons for the high photocatalytic activity of TG composites
Fig. 6. C1s XPS spectra of GO (a) and TG-15 (b). Raman spectra of different samples (c).
The adsorption of contaminant dye molecules, charge transportation and charge separation are crucial factors during photocatalysis [26]. The improvement in photocatalysis seen with TG composites was attributed to their enhanced adsorptivity. As shown in Fig. 8, the remaining concentration fraction of MO after dark adsorption by TG composites was lower than that for pure
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Fig. 8. Bar plot showing the remaining methyl orange in non-illuminated solutions after 1 h stirring (left). A line plot (right) shows the equilibrium adsorption capacity of different samples.
TiO2. This indicates that TG composites possessed better adsorption strength than pure TiO2. In order to further visually compare the adsorption capacity of different composites, the equilibrium adsorption capacity was calculated as Qe = (C0 C)V/m, where Qe was the amount adsorbed (mg/g), C0 and C the initial and equilibrium MO concentrations (mg/L), V the solution volume (L), and m the samples dose (mg) [38]. Differences were seen in the equilibrium adsorption capacity of different samples (Fig. 8). The Qe of TiO2, TG-5, TG-10, TG-15 and TG-30 was 1.6 mg/g, 4.34 mg/g, 11.65 mg/g, 14.57 mg/g and 19.06 mg/g, respectively. The equilibrium adsorption capacity of TG composites was greater than that of pure TiO2. Increasing the proportion of rGO was associated with an increase in adsorption capacity of the TG composites. The enhanced adsorptivity was largely related to the selective adsorption of MO on the TG composites. MO was adsorbed on the rGO surface of the TG composites via p–p conjugation between MO molecules and aromatic regions of the rGO [26]. The rGO surface contained a number of oxygen-containing functional groups, such as O5 5C–O and C–O, which acted as active adsorption sites and enhanced adsorption of MO [37]. Adsorption ability of the composite increased with rGO content. Mesoporous anatase assembled on rGO had a unique structure that allowed strongly oxidative holes (h+) in valence of TiO2 to be generated and transferred to the nearby MO when under UV light. As a result, the redox reactions that degrade MO could occur. Degradation of MO shifted the adsorption equilibrium, allowing more MO to transfer from solution to the interface of the TG composites. There was a synergy between surface adsorption and photocatalysis during degradation of MO on TG composites, resulting in a significant improvement in the photocatalysis of MO. Efficient charge separation and transfer are crucial for enhancing the photocatalytic activity of photocatalysts during degradation reactions. Electrochemical impedance spectroscopy (EIS) was performed to further investigate the photocatalytic activity (Fig. 9a). A typical EIS analysis is presented as a Nyquist plot. As electrolyte and electrode reactions are similar over the length of each electrode, a semicircle will relate the resistance of electrode material [39]. TG composites demonstrated a much lower resistance than pure TiO2. The size of the semicircle in the plot decreased with increasing rGO content, indicating a similar reduction in resistance. The interfacial charges transferred rapidly with efficient separation of the photogenerated charge [40]. These results are further independent corroboration of the photoelectrochemical experiments. The photoelectrochemical behaviours of pure TiO2 and the TG composites are presented in Fig. 9b. In order to analyze the photocurrents induced by UV
Fig. 9. (a) Electrochemical impedance spectroscopy (EIS) Nynquist plots of different samples. (b) Photocurrents of TiO2 and TG-15 composites using intermittent irradiation with a 150 W high-pressure Xenon lamp and a bias potential of 0.2 V.
irradiation, dark currents were subtracted. As shown in Fig. 9b, the TG composites exhibited a much higher photocurrent density than pure TiO2. The photocurrent in pure TiO2 rapidly increased to a constant value as soon as light was turned on. The photocurrent rapidly decreased to zero when the light was turned off. The photocurrent in TG composites gradually increased to a constant value when the light was turned on, and the photocurrent gradually decreased to zero when the light was off. There is a heterojunction at the interface of mesoporous TiO2 and rGO, allowing photogenerated electrons to migrate from the higher Fermi level of anatase to the lower Fermi level of grapheme [37]. As a result, photocurrent was generated by stored electrons on rGO rather than electrons transferred from the conduction band of TiO2. This resulted in a gradual increase in photocurrent. Similarly, when the light was turned off, the electrons stored in rGO were gradually released, leading to the gradual decline in photocurrent [35]. The observed enhancement in the photocurrent represents improved charge transportation from TiO2 to rGO. In this way, the photogenerated electron–hole recombination rate was suppressed. As a result, TG composites had a higher rate of decomposition of MO than pure TiO2. The high photocatalytic activity of TG composites was attributed to several factors (Fig. 10), such as mesoporous structure, interface structure and composition. First of all, the adsorption ability of TG composites for dye can be largely enhanced owing to the unique structure. Because of graphene, dye molecules can easily adsorb to the surface of the TG composite owing to p–p conjugation between dye and aromatic regions of the graphene. As mentioned, TG composites have mesoporous
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electron–hole pairs. Second, graphene oxide nanosheets were reduced during the hydrothermal reaction. The rGO nanosheets acted as a thin surface that supported substrate. It contained a large surface area with a high adsorption capacity for MO. Most importantly, the photo-generated electrons from the conduction band of TiO2 transferred rapidly to the rGO nanosheets. This increased the efficiency of photocatalysis remarkably. This unique nanostructure holds promise for environmental remediation and energy conversion. References [1] [2] [3] [4] [5] Fig. 10. Schematic illustration of photocatalysis with TG composites.
structure and high specific area, which can also strengthen the surface absorbed dye. In addition, the interface structure in the TG composite directly affects its photoelectric properties. Because of the heterojunction in the interface of TiO2 and graphene, photogenerated electrons tend to migrate from the higher Fermi level of anatase to lower Fermi level of graphene. In this way, the photogenerated electron–hole recombination rate could be suppressed effectively, and photogenerated holes and electrons could better react with H2O and O2 to form reactive oxygen species. Additional redox reactions with MO and reactive oxygen species occurred. The OH radicals are highly reactive and often deemed to be the major species responsible for the photocatalytic oxidation of dye molecules. Yu et al. [41] reported the presence of OH radicals on the surface of irradiated commercial Degussa P25, could be detected by the photoluminescence technique using coumarin as a probe molecule. As the photo-decomposition of dye, the adsorption equilibrium broke, so more dye would transfer from solution to the surface of TG composite and subsequently be decomposed by a series of redox reactions. The synergistic relationship between enhanced surface adsorption characteristics and excellent charge transportation-separation led to TG composites with high photocatalytic activity. 4. Conclusions Mesoporous anatase phase TiO2 nanoparticles were assembled on rGO using a single step hydrothermal method. The mesoscale pore sizes of the TG-15 nanocomposite were distributed in a narrow range, 2–20 nm. The TiO2-rGO nanocomposites exhibited excellent photocatalytic activity. The MO degradation rate of the nanocomposites was 4.5 times faster than that of P25. Through calculating the equilibrium adsorption capacity and photoelectrochemical behaviours, the excellent properties of the nanocomposites were attributed to several factors. First, mesoporous TiO2 was synthesized using a template-free method. The unique structure possessed a large surface area for generation of
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Mesoporous anatase TiO2/reduced graphene oxide nanocomposites: A simple template-free synthesis and their high photocatalytic performance Qi Zhou a,1, Yong-Hui Zhong a,b,1, Xing Chen b, Xing-Jiu Huang b, Yu-Cheng Wu a,* a b
School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 23000, PR China Laboratory of Nanomaterials and Environmental Detection, Hefei Institute of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 October 2013 Received in revised form 2 December 2013 Accepted 16 December 2013 Available online 25 December 2013
Mesoporous anatase phase TiO2 was assembled on reduced graphene oxide (rGO) using a template-free one-step hydrothermal process. The nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Brunauer–Emmett–Teller (BET) surface area. Morphology of TiO2 was related to the content of graphene oxide. TiO2/rGO nanocomposites exhibited excellent photocatalytic activity for the photo-degradation of methyl orange. The degradation rate was 4.5 times greater than that of pure TiO2 nanoparticles. This difference was attributed to the thin two-dimensional graphene sheet. The graphene sheet had a large surface area, high adsorption capacity, and acted as a good electron acceptor for the transfer of photo-generated electrons from the conduction band of TiO2. The enhanced surface adsorption characteristics and excellent charge transport separation were independent properties of the photocatalytic degradation process. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors Composites Nanostructures Solvothermal Catalytic properties
1. Introduction Clean and sustainable energy resources are needed as traditional energy sources are depleted and environmental impact is better appreciated. Fujishima and Honda first described the photocatalytic splitting of water using TiO2 electrodes in 1972, a method that uses sunlight and water to produce hydrogen gas. Significant progress has been made since then in the field of semiconductor photocatalysts [1,2]. TiO2 has a number of advantages including its lack of reactivity with the environment, strong oxidizing power, low cost, and long-term stability against photo and chemical corrosion [3]. UV illumination of TiO2 energizes electrons to move from the valence band to the conduction band on the surface of the metal. Water reacts with the charged surface metal to form OH and O2 free radicals. Redox reactions occur, ultimately yielding hydrogen and oxygen gas [4]. The large surface area contributes to the number of possible reaction sites on the photocatalyst surface [5,6]. Performance of the catalyst is enhanced by suppressing the fast recombination rate of photogenerated electron–hole pairs [7–9].
* Corresponding author. Tel.: +86 551 62901012; fax: +86 551 62904517. E-mail addresses: [email protected] (X.-J. Huang), [email protected] (Y.-C. Wu). 1 These authors contributed equally to this work. 0025-5408/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.034
An effective approach to producing high-surface-area TiO2 is the creation of mesoporous (pore diameter from 2 to 50 nm) materials. Mesoporous TiO2 facilitates the rapid diffusion of ions and molecules within the material so that the rate of photocatalytic reactions is substantially improved [10,11]. Conventional synthesis of mesoporous TiO2 uses hard (such as mesoporous silica or colloidal spheres) [12,13] or soft porous templates (such as Pluronic surfactant P123) [14]. The template method is generally tedious, time-consuming, and results in micron sized TiO2 pores [10]. A one-step template-free method was developed to overcome this limitation and produce mesoporous structures [15–17]. Graphene consists of a single layer of sp2 hybrid carbon atoms that are tightly packed into a two dimensional honeycomb network. It exhibits unique and remarkable properties, including a large specific surface area (2600 m2 g 1), high mobility of the charge carriers (2 105 cm2 V 1 s 1), and high chemical stability [18,19]. These properties make graphene attractive as a photocatalytic agent. The large number of oxygen containing functional groups act as nucleation sites [20–22]. Graphene has a large potential sink of electrons that are freely mobile and available to react [23,24]. These properties led to the evaluation of graphene– TiO2 composites as photocatalytic agents [25–30]. Zhang et al. prepared graphene encapsulated mesoporous hollow TiO2 nanospheres using a graphene protected calcination process. The composites exhibited significant photocatalytic activity for degrading RhB [25]. Zhang et al. prepared a TiO2 (P25)-graphene
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composite with enhanced photocatalytic activity for the degradation of methylene blue [26]. Du et al. produced hierarchically ordered macro-meso-porous TiO2 films using a confinement selfassembly method [27]. Yu et al. demonstrated that the TiO2– graphene nanocomposites exhibit exceptional photocatalytic reactivity and selectivity towards removing azo dyes in water relative to the pristine TiO2 [29]. The manufacturing procedure for mesoporous structures was complex and time-consuming and the TiO2 nanoparticles tended to agglomerate. The manufacture of mesoporous anatase compounds assembled on graphene using a template-free method has not been well studied and may increase the photocatalytic activity of composites by increasing the surface area of TiO2. We reported a simple template-free method to produce mesoporous anatase TiO2 on a rGO platform using a one-step hydrothermal process. This composite had excellent photocatalytic activity (degradation of organic pollutants) in comparison to pure TiO2. Through adsorption experiment and photo-electrochemical behaviours investigation, we found that the equilibrium adsorption capacity of TG composites were greater than that of pure TiO2, and under illumination the TG composites exhibited a much higher photocurrent density than pure TiO2. The TiO2/rGO composite exhibited increased adsorption of pollutants and facile charge transportation and separation. There was a synergistic relationship between enhanced surface adsorption and charge transportation-separation during photocatalytic degradation.
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was added with vigorous stirring for another 30 min. The homogeneous solution was transferred to a Teflon-sealed autoclave and maintained at 180 8C for 6 h. The product was rinsed and dried in a vacuum at 60 8C for 24 h. The GO content in the prepared composite was adjusted by changing the volume of the added GO solution. The prepared composites were named ‘‘TG-x’’ where ‘‘x’’ was the volume of the added GO solution (5 mL, 10 mL, 15 mL, and 30 mL). Pure TiO2 was prepared using the same procedure, but without the addition of GO. 2.4. Characterization
Natural graphite flake (325 mesh) was purchased from Alfa Aesar. Analytical grade sulfuric acid (H2SO4, 95.0–98.0%), hydrochloric acid (HCl, 36.5–38.0%), potassium permanganate (KMnO4, 99.5%), H2O2 (30 wt%), sodium nitrate (NaNO3, 99.0%), ammonium hexafluorotitanate ((NH4)2TiF6, 99.0%), and boric acid (H3BO3, 99.5%), were obtained from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. P25 was purchased from Degussa. Distilled water (18.2 MV cm) was purified using the NANO pure1DiamondTM UV water system.
The morphology and microstructure of the samples were verified using field-emission scanning electron microscopy (FESEM, FEI-200) and transmission electron microscopy (TEM, JEM-2010). Powder X-ray diffraction (XRD) was carried out using Philips X’Pert Pro X-ray diffraction with Cu Ka radiation. The specific surface area and gas adsorption isotherms of the composite were determined using a Coulter Omnisorp 100CX Brunauer–Emmett–Teller (BET) with nitrogen adsorption and a degassing temperature of 80 8C. X-ray photoelectron spectroscopy (XPS) analyses of the samples were conducted using a VG ESCALAB MKII spectrometer with a Mg Ka X-ray source (1253.6 eV, 120 W). The energy scale was internally calibrated by referencing the binding energy of the C 1s peak at 284.6 eV for contaminated carbon. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CHI660C, ShangHai) at ambient temperature (about 22 8C). The electrochemical cell had a three electrode configuration with an Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode. A 5 mm diameter polished glassy carbon working electrode (GCE) was used as a current collector. An electroactive sample was mixed with ethanol to make a slurry. A small amount of Nafion solution (0.5 wt%) was added to thicken the slurry on the GCE. Photocurrent measurements were carried out using a CHI660C workstation and a xenon lamp. A threeelectrode configuration was employed, consisting of an ITO working electrode, Ag/AgCl electrode and a platinum wire electrode. The ITO electrodes were coated with TiO2 or TG composites, using a spin-coating method.
2.2. Synthesis of graphene oxide (GO) sheets
2.5. Evaluation of photocatalytic activity
GO was synthesized from natural graphite flake using a modified Hummers method. In a typical experiment, 1 g natural flake graphite powder and 1 g NaNO3 were added to 46 mL concentrated H2SO4 in an ice-bath. 6 g KMnO4 was gradually added while stirring. The mixture was then stirred at 35 8C for 2 h. 92 mL distilled water was then slowly added to the mixture while stirring. The resultant mixture was stirred at 90 8C for 30 min. The reaction was terminated by adding 200 mL distilled water and 6 ml H2O2 (30 wt%). This addition turned the mixture bright yellow. The mixture was filtered and washed with 150 mL 5% aqueous HCl to remove metal ions, and then washed with distilled water to completely remove metal ions and acids. The GO was added to distilled water and ultrasonically exfoliated in a bath sonicator for 1 h to achieve a brown viscous dispersion with a concentration of 3 mg/mL.
Photocatalytic activities of the composites were evaluated using methyl orange (MO) degradation under UV-light irradiation. Twenty micrograms samples were dispersed in 50 mL MO solutions (30 ppm). Solutions were magnetically stirred in the dark for 1 h to ensure the establishment of adsorption–desorption equilibrium of the MO dye. The suspensions were then exposed to UV irradiation from a 300 W high pressure Hg lamp with the main wave crest at 365 nm. Two microliters test samples were removed every 5 min for MO absorbance analysis after centrifugation. The percentage of degradation was reported as C/C0, where C was the absorption of dye solution at each time point and C0 was the absorption of the initial concentration.
2. Experimental 2.1. Materials
3. Results and discussion 3.1. Synthesis and structural characterizations of TG composites
2.3. Assembled mesoporous anatase on rGO Mesoporous anatase on a rGO platform was manufactured using the hydrothermal method. In a typical preparation, GO dispersion (3 mg/mL) was dissolved in deionized water to a total volume of 60 mL. 0.01 mol (NH4)2TiF6 was added to the dilute GO solution. The mixture was stirred for 30 min and 0.01 mol H3BO3
The synthesis of TG composites (Fig. 1) typically involved two steps: (1) synthesis of GO using the modified Hummers method, followed by uniformly grafting of Ti4+ on GO sheets using chemisorptions [31]. (2) H3BO3 was added to the dispersion heated in a Teflon-sealed autoclave in order to transform Ti4+ into anatase TiO2 and to reduce the GO.
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Fig. 1. Schematic representation of the formation of TG composites.
The crystallographic structures of the samples were confirmed using X-ray powder diffraction (XRD) analysis. Fig. 2 shows the XRD pattern of the samples. The synthesized GO had a distinct large peak at 2u = 10.88, corresponding to the (0 0 2) diffraction of GO. Samples with different GO content exhibited similar TiO2 crystal composition, primarily anatase phase. The peaks at 25.38, 36.98, 48.08, 53.98, 55.08, 62.78, 68.88 and 70.38, were assigned to the diffractions of the (1 0 1), (1 0 3), (2 0 0), (1 0 5), (2 1 1), (1 1 8), (1 1 6) and (2 2 0) crystal planes of the anatase phase (JCPDS NO. 21-1272) in TiO2, respectively. No diffraction peaks of rGO were observed in the TG composites. This might be attributed to the relatively low diffraction intensity of rGO [32]. The main peak of anatase at 25.38 could have been hidden by the large graphene peak at 24.58. The morphology of the GO and TG composites was examined using field-emission SEM. Fig. 3 presents SEM images of the different samples. Pure TiO2 was composed of spherical particles with a diameter of 2–3 mm. These particles aggregated to form larger particles (Fig. 3b). The morphology of TG composites significantly differed from that of pure TiO2. SEM imaging (Fig. 3f) showed that the TG-30 composite was composed of TiO2 particles uniformly dispersed on sheets of GO. The TiO2 particles had a diameter of 20–40 nm. Composite morphology varied with GO content. TG-5 (Fig. 3c) consisted of spherical TiO2, similar to pure TiO2, but with different sized particles. Fig. 3d, e, and f show that spherical TiO2 was seen
Fig. 2. XRD patterns of different samples.
less frequently as GO content increased. TiO2 nanoparticles were instead uniformly dispersed on the surface of the rGO. GO acted as a dispersant and stabilizing agent, preventing TiO2 aggregation. A large number of TiO2 nuclei were absorbed on the surface of the GO nanosheets as a result of the large surface energy of grapheme [33]. The preferential nucleation on the surface of GO combined with the modulated growth of TiO2 nuclei led to the formation of a mesoporous anatase nanostructure assembled on the rGO. The morphology of the TG composites was evaluated using TEM analysis. GO consisted of a crumpled layered structure stacked in sheets (Fig. 4a). The TiO2 nanoparticles in TG-15 had a relatively uniform size of about 30 nm and were dispersed on the surface of the GO sheets (Fig. 4b). Some TiO2 nanoparticles were agglomerated together. There was an internal mesoporous structure (Fig. 1 C). The spacing of the clear lattice fringes was 0.35 nm, corresponding to the d-spacing of (1 0 1) crystalline plane anatase TiO2 (Fig. 4d). This suggests that TiO2 nanoparticles were highly crystallized on the GO sheets [10]. The orientation of the (1 0 1) lattice fringes was fairly uniform indicating the presence of a single-crystal-like microstructure within the single TiO2 nanoparticles. These findings support the synthesis of TiO2 nanocrystals with a mesoporous structure assembled on rGO using facile hydrothermal processes. The single crystal of anatase was synthesized using Ti4+ hydrolyzed during heat treatment. Corrosive F in the reaction system acted to etch the TiO2 crystal [16,17,34]. The interaction of crystallization and corrosion worked to form mesoporous anatase TiO2. Nitrogen adsorption–desorption analysis was performed to determine pore sizes, their distribution, and surface area of the synthesized TG composites. Fig. 5 shows pore size distribution plots and nitrogen adsorption–desorption isotherms for TG-15. TG-15 demonstrated a type-IV isotherm with one hysteresis loop at a relative pressure between 0.65 and 0.95 (inset in Fig. 5), indicating that mesoporous structures were present [35]. The Brunauer–Emmett–Teller (BET) method was used to determine surface area. The specific surface area of TG-15 was 124 m2 g 1. The pore size distribution of TG-15 was in the narrow range of 2– 20 nm. As shown in Fig. 5, there were two sharp peaks at 3.5 nm and 11.2 nm, representative of the two types of mesoporous structures in TG-15. This was consistent with the TEM observations. The mesoporous structures consisting of single anatase TiO2 nanoparticles had a relatively uniform size of about 3.5 nm. Some anatase nanoparticles agglomerated on the rGO sheets, resulting in particles about 11.2 nm in diameter. XPS was used to evaluate the chemical states of the carbon in the GO and TG composites. The C1s XPS spectra of GO and TG-15 are shown in Fig. 6a and b. Four Gaussian curves were seen to be centred at 284.6, 285.7, 286.7, and 288.5 eV, corresponding to graphitic carbon (C–C), alcohol or ether carbon (C–OH, C–O–C), carbonyl groups (C5 5O), and carboxyl carbon (O5 5C–O), respectively [36]. Synthesis of the composite was associated with a decrease in the intensity of the peaks at 286.7 eV (C5 5O group) and 288.5 eV (O5 5C–O group) in GO, indicating the content of oxygen-containing functional groups was decreased. The hydrothermal reaction reduced the GO sheets to rGO, increasing the number of defective carbon atoms. Raman spectroscopic measurement was used to further elucidate the structure of the composites. The Raman spectra of GO, pure TiO2 and TG-15 are shown in Fig. 6. The D and G bands of GO were seen at 1350 cm 1 and 1595 cm 1. TG-15 exhibited additional Raman peaks at 148 cm 1, 398 cm 1, 514 cm 1, and 630 cm 1. These peaks were ascribed to anatase TiO2 [37]. These findings support the assembly of anatase TiO2 on the rGO sheets. The increased D/G ratio intensity of TG-15 suggested there was a decrease in the average size of the sp2 domains after GO reduction
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Fig. 3. SEM images of (a) GO, (b) TiO2, (c) TG-5, (d) TG-10, (e) TG-15 and (f) TG-30.
Fig. 4. TEM images of (a) GO and (b, c, d) TG-15.
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Fig. 5. Pore size distribution curves of the TG-15 composite. The inset shows the nitrogen-adsorption–desorption isotherms.
[32]. This might be due to the removal of oxygen-containing groups during the hydrothermal treatment. These findings were consistent with the XPS spectra analysis. 3.2. Photocatalytic activity of TG composites Methyl orange was selected as a model organic pollutant to evaluate the photocatalytic performance of TG composites. The photocatalytic activities of TG composites and P25 were evaluated using UV light illumination after dark adsorption. The time dependence of MO dye degradation is shown in Fig. 7a. All of
Fig. 7. (a) Photocatalytic degradation of MO using UV light. (b) Photocatalytic degradation reaction rates of MO in different samples.
the photocatalysts demonstrated activity with UV light. TG composites had the highest photo-activity. TG-30 decomposed 95% of the MO after 30 min of irradiation. P25 had lower photocatalytic activity, with only 68% of MO decomposed after 30 min irradiation. The photocatalytic activity of TG composites increased as rGO content increased. In order to compare reaction rates, we re-plotted the previous results as the first-order reaction rate equation ln(C/C0) = kt, where k was the apparent first-order rate constant [37]. Fig. 7b shows the linear relationship of ln(C/C0) and reaction time t for different samples. P25 had the lowest k value, 0.039 min 1. The photodegradation rate of TG composites was much greater than that of P25. Incorporation of rGO resulted in an increase in the photocatalytic degradation rate constant. k was 0.058 min 1, 0.087 min 1, 0.093 min 1 and 0.122 min 1 for TG-5, TG-10, TG-15 and TG-30, respectively. rGO benefited MO photocatalytic degradation. 3.3. Reasons for the high photocatalytic activity of TG composites
Fig. 6. C1s XPS spectra of GO (a) and TG-15 (b). Raman spectra of different samples (c).
The adsorption of contaminant dye molecules, charge transportation and charge separation are crucial factors during photocatalysis [26]. The improvement in photocatalysis seen with TG composites was attributed to their enhanced adsorptivity. As shown in Fig. 8, the remaining concentration fraction of MO after dark adsorption by TG composites was lower than that for pure
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Fig. 8. Bar plot showing the remaining methyl orange in non-illuminated solutions after 1 h stirring (left). A line plot (right) shows the equilibrium adsorption capacity of different samples.
TiO2. This indicates that TG composites possessed better adsorption strength than pure TiO2. In order to further visually compare the adsorption capacity of different composites, the equilibrium adsorption capacity was calculated as Qe = (C0 C)V/m, where Qe was the amount adsorbed (mg/g), C0 and C the initial and equilibrium MO concentrations (mg/L), V the solution volume (L), and m the samples dose (mg) [38]. Differences were seen in the equilibrium adsorption capacity of different samples (Fig. 8). The Qe of TiO2, TG-5, TG-10, TG-15 and TG-30 was 1.6 mg/g, 4.34 mg/g, 11.65 mg/g, 14.57 mg/g and 19.06 mg/g, respectively. The equilibrium adsorption capacity of TG composites was greater than that of pure TiO2. Increasing the proportion of rGO was associated with an increase in adsorption capacity of the TG composites. The enhanced adsorptivity was largely related to the selective adsorption of MO on the TG composites. MO was adsorbed on the rGO surface of the TG composites via p–p conjugation between MO molecules and aromatic regions of the rGO [26]. The rGO surface contained a number of oxygen-containing functional groups, such as O5 5C–O and C–O, which acted as active adsorption sites and enhanced adsorption of MO [37]. Adsorption ability of the composite increased with rGO content. Mesoporous anatase assembled on rGO had a unique structure that allowed strongly oxidative holes (h+) in valence of TiO2 to be generated and transferred to the nearby MO when under UV light. As a result, the redox reactions that degrade MO could occur. Degradation of MO shifted the adsorption equilibrium, allowing more MO to transfer from solution to the interface of the TG composites. There was a synergy between surface adsorption and photocatalysis during degradation of MO on TG composites, resulting in a significant improvement in the photocatalysis of MO. Efficient charge separation and transfer are crucial for enhancing the photocatalytic activity of photocatalysts during degradation reactions. Electrochemical impedance spectroscopy (EIS) was performed to further investigate the photocatalytic activity (Fig. 9a). A typical EIS analysis is presented as a Nyquist plot. As electrolyte and electrode reactions are similar over the length of each electrode, a semicircle will relate the resistance of electrode material [39]. TG composites demonstrated a much lower resistance than pure TiO2. The size of the semicircle in the plot decreased with increasing rGO content, indicating a similar reduction in resistance. The interfacial charges transferred rapidly with efficient separation of the photogenerated charge [40]. These results are further independent corroboration of the photoelectrochemical experiments. The photoelectrochemical behaviours of pure TiO2 and the TG composites are presented in Fig. 9b. In order to analyze the photocurrents induced by UV
Fig. 9. (a) Electrochemical impedance spectroscopy (EIS) Nynquist plots of different samples. (b) Photocurrents of TiO2 and TG-15 composites using intermittent irradiation with a 150 W high-pressure Xenon lamp and a bias potential of 0.2 V.
irradiation, dark currents were subtracted. As shown in Fig. 9b, the TG composites exhibited a much higher photocurrent density than pure TiO2. The photocurrent in pure TiO2 rapidly increased to a constant value as soon as light was turned on. The photocurrent rapidly decreased to zero when the light was turned off. The photocurrent in TG composites gradually increased to a constant value when the light was turned on, and the photocurrent gradually decreased to zero when the light was off. There is a heterojunction at the interface of mesoporous TiO2 and rGO, allowing photogenerated electrons to migrate from the higher Fermi level of anatase to the lower Fermi level of grapheme [37]. As a result, photocurrent was generated by stored electrons on rGO rather than electrons transferred from the conduction band of TiO2. This resulted in a gradual increase in photocurrent. Similarly, when the light was turned off, the electrons stored in rGO were gradually released, leading to the gradual decline in photocurrent [35]. The observed enhancement in the photocurrent represents improved charge transportation from TiO2 to rGO. In this way, the photogenerated electron–hole recombination rate was suppressed. As a result, TG composites had a higher rate of decomposition of MO than pure TiO2. The high photocatalytic activity of TG composites was attributed to several factors (Fig. 10), such as mesoporous structure, interface structure and composition. First of all, the adsorption ability of TG composites for dye can be largely enhanced owing to the unique structure. Because of graphene, dye molecules can easily adsorb to the surface of the TG composite owing to p–p conjugation between dye and aromatic regions of the graphene. As mentioned, TG composites have mesoporous
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electron–hole pairs. Second, graphene oxide nanosheets were reduced during the hydrothermal reaction. The rGO nanosheets acted as a thin surface that supported substrate. It contained a large surface area with a high adsorption capacity for MO. Most importantly, the photo-generated electrons from the conduction band of TiO2 transferred rapidly to the rGO nanosheets. This increased the efficiency of photocatalysis remarkably. This unique nanostructure holds promise for environmental remediation and energy conversion. References [1] [2] [3] [4] [5] Fig. 10. Schematic illustration of photocatalysis with TG composites.
structure and high specific area, which can also strengthen the surface absorbed dye. In addition, the interface structure in the TG composite directly affects its photoelectric properties. Because of the heterojunction in the interface of TiO2 and graphene, photogenerated electrons tend to migrate from the higher Fermi level of anatase to lower Fermi level of graphene. In this way, the photogenerated electron–hole recombination rate could be suppressed effectively, and photogenerated holes and electrons could better react with H2O and O2 to form reactive oxygen species. Additional redox reactions with MO and reactive oxygen species occurred. The OH radicals are highly reactive and often deemed to be the major species responsible for the photocatalytic oxidation of dye molecules. Yu et al. [41] reported the presence of OH radicals on the surface of irradiated commercial Degussa P25, could be detected by the photoluminescence technique using coumarin as a probe molecule. As the photo-decomposition of dye, the adsorption equilibrium broke, so more dye would transfer from solution to the surface of TG composite and subsequently be decomposed by a series of redox reactions. The synergistic relationship between enhanced surface adsorption characteristics and excellent charge transportation-separation led to TG composites with high photocatalytic activity. 4. Conclusions Mesoporous anatase phase TiO2 nanoparticles were assembled on rGO using a single step hydrothermal method. The mesoscale pore sizes of the TG-15 nanocomposite were distributed in a narrow range, 2–20 nm. The TiO2-rGO nanocomposites exhibited excellent photocatalytic activity. The MO degradation rate of the nanocomposites was 4.5 times faster than that of P25. Through calculating the equilibrium adsorption capacity and photoelectrochemical behaviours, the excellent properties of the nanocomposites were attributed to several factors. First, mesoporous TiO2 was synthesized using a template-free method. The unique structure possessed a large surface area for generation of
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