Three-dimensional titanate–Graphene oxide composite gel with enhanced photocatalytic activity synthesized from nanofiber networks

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Three-dimensional titanate–Graphene oxide composite gel with enhanced photocatalytic activity synthesized from nanofiber networks Ruirui Liu a , Xiuyan Li a,b , Shichao Li a , Guowei Zhou a,∗ a Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, PR China b School of Chemical Engineering and Environment, Weifang University of Science and Technology, Weifang 262700, PR China

a r t i c l e

i n f o

Article history: Received 26 October 2016 Received in revised form 21 December 2016 Accepted 30 December 2016 Available online xxx Keywords: Three-dimension Titanate/graphene oxide composite gel Nanofibers Photocatalytic activity

a b s t r a c t We report the synthesis of a novel three-dimensional titanate–graphene oxide (TiGO) composite gel with controllable morphology through a facile one-pot solvothermal approach. This technique directly utilizes tetra-butyl titanate as a titanium source and acetic acid (HAc) as a stabilizer and cross-linking agent. Gel formation at the macro level and morphological evolution of flower-like networks into nanofibers at the micro level were controlled by regulating the amounts of graphene oxide (GO) and HAc added to the reaction system. TiGO composite is composed of nanofiber bundles and its photocatalytic activity was evaluated by photocatalytic decolorization of methyl orange aqueous solution under UV irradiation. The improved photocatalytic performance was attributed mainly to synergistic effects of titanate and GO. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Graphene oxide (GO) is widely used as a unique building block in different supramolecular interactions, such as hydrogen bonding, coordination, electrostatic interaction, and – stacking assembly, for 3D hydrogel structure formation [1–5]. Three-dimensional networks based on GO sheets exhibit higher mechanical strength, higher specific surface area, better electrical conductivities, and improved thermal, chemical, and electrochemical stability than 2D GO nanosheets. Previous reports showed that 2D GO nanosheets easily form into irreversible agglomerates or restack into graphitic structures, thereby significantly reducing the accessible surface area and deteriorating the actual performance [6,7]. As such, individual 2D GO nanosheets were integrated into 3D macroscopic structures, such as porous films, scaffolds, and networks, to obtain high-performance graphene-based materials for practical monolithic applications. Gelation, a main strategy for construction of 3D GO, was developed to facilitate the formulation of highperformance graphene materials with a wide range of practical applications [8–12].

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Zhou).

To date, GO-based functional materials are fabricated through various processes, such as solution mixing [13], in situ growth [14], impregnation [15,16], hydrothermal, and/or solvothermal methods [17,18]. Solvothermal method is an effective and reliable method used to produce 3D GO-based gel because of its stable reaction condition and easy adjustment of microstructure and properties by changing the concentration of the GO dispersion and solvothermal reaction time. A GO sheet can be recognized as a single-layered graphite bearing various hydrophilic oxygenated functional groups [19]. Thus, various small molecules, polymers, or ions can promote the gelation of GO through different supramolecular interactions even at very low GO concentrations [20–23]. For example, Bai et al. [24] demonstrated the preparation of GO-conducting polymer composite hydrogels via in situ polymerization of aromatic monomers in GO dispersions; the resulting hydrogels exhibited high conductivities and electrochemical activities and could be used to fabricate highly sensitive ammonia sensors. Tang et al. [25] showed that noble metals (e.g., Au, Ag, Pd, Ir, Rh, Pt, etc.) promoted the assembly of GO into macroscopic porous structures; the authors found that combination of noble-metal nanocrystals and GO layers result in excellent Heck reaction catalytic activity and selectivity. The applications of graphene-based materials in energy, environment, sensing, and biological fields often require 2D graphene sheet assembly into 3D architectures [26–29]. As such,

http://dx.doi.org/10.1016/j.cattod.2016.12.046 0920-5861/© 2016 Elsevier B.V. All rights reserved.

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the unique properties of graphene-based materials must be maximized for their practical applications. Inorganic nanomaterial GO composite, as an important member of functionalized GO materials, exhibits a myriad of potential applications [15,18,30,31]. The integration of nanostructured materials into macroscopic devices, which can translate phenomena at the nanoscale to the macroscopic level, is a vital aspect for applications of nanomaterials. Studies have reported on GO-based gels, but the use of excellent photocatalytic properties of titanate has been rarely investigated; titanate is used to construct 3D TiGO composite gel for degradation of organic contaminants in liquid phase. Considering this observation, we fabricated a novel 3D TiGO composite gel with controllable morphology through a one-pot solvothermal approach. The photocatalytic performance of the resultant product was investigated. Mechanisms underlying the formation of TiGO composite gels were also studied. Macrolevel gel formation and microlevel flower-like nanofiber morphological evolution can be controlled by regulating GO and HAc concentrations in the reaction system. The photocatalytic performance of the TiGO composites is seriously affected by the GO/tetrabutyl titanate (TBT) ratio. In this monolithic structure, the large surface area and unique 3D structure of the TiGO nanocomposites can offer numerous active adsorption sites and photocatalytic reaction centers. The resulting 3D TiGO composite gel displays the synergistic effects of the assembled Ti and GO, displaying improved photocatalytic performance, including high photocatalytic activity and good photocatalytic stability and reusability. 2. Experimental section 2.1. Chemicals Graphite powder, phosphorus pentoxide (P2 O5 , 98%), potassium persulfate (K2 S2 O8 , 99.5%), acetic acid (HAc, 99.5%), and potassium permanganate (KMnO4 ) were obtained from Tianjin Kermel Chemical Co., Ltd. (China). TBT and aqueous hydrogen peroxide solution (H2 O2 , 30%) were obtained from Sinopharm Chemical Co., Ltd. (China). All of the reagents were of analytical grade and used without further purification. 2.2. Synthesis of GO GO was synthesized from expanded graphite powder using a modified Hummers method [32,33]. This two-step synthetic technique began with pre-oxidation. Briefly, 3.0 g of graphite was added to a mixture of 30 mL of 98% H2 SO4 with 3 g each of K2 S2 O8 and P2 O5 . The solution was sonicated for 5 min, placed in an oil bath, heated to 80 ◦ C, and then stirred for 6 h. The product was subsequently diluted with 500 mL of deionized water, filtered through a 0.2 ␮m nylon film, and dried under ambient conditions. The product obtained from this first step was used for the second step, i.e., oxidation. Exactly 150 mL of 98% H2 SO4 and 20 mL of 85.0% H3 PO4 were added slowly to the obtained product with stirring over an ice water bath. Then, 18 g of KMnO4 was gradually added to the mixture with stirring for 4 h at 20 ◦ C. The mixture was removed from the ice bath and subsequently stirred for another 8 h at 35 ◦ C. Addition of 1000 mL of distilled water and 24 mL of 30% H2 O2 solution terminated the reaction. Finally, the GO was obtained through filtration, ultrasonication, and vacuum drying. 2.3. Synthesis of TiGO composite gel In a typical synthesis, 36.75 mL (V1 ) of HAc was kept under static conditions for 0.5 h at 40 ◦ C. After heating, 1 mL of TBT was added dropwise to the HAc with continuous stirring. The resulting white suspension was stirred for 24 h at 350 rpm and 40 ◦ C. To produce the

GO dispersions, 6 mg of GO was mixed with 10 mL (V2 ) of HAc solution by sonicating for 0.5 h. The GO dispersions were added to the white suspension obtained previously and stirred for 2 h at 40 ◦ C. Finally, the solutions were placed in a Teflon-lined stainless-steel autoclave for 24 h at 150 ◦ C. After cooling to room temperature, the products were retrieved. Five composite gel samples with varying GO weights were synthesized and designated as TiGOx (i.e., samples to which 0, 6, 12, 18, and 24 mg of GO were added were designated TiGO0, TiGO1, TiGO2, TiGO3, and TiGO4, respectively). Another set of composite gel samples with varying HAc V2 and only 6 mg of GO were also prepared (i.e., samples to which 5, 10, 20, 30, and 40 mL of HAc were added were designated as TiGOA1, TiGOA2, TiGOA3, TiGOA4, and TiGOA5, respectively). 2.4. Characterization To determine the phase purity and crystal structure of the samples, X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer (40 kV, 40 mA) using Cu K␣ ( = 0.15406 nm) radiation. The data were collected from 5◦ to 80◦ (2) with a resolution step size of 0.1◦ s−1 . Raman measurements were recorded on a JY Lab-Ram HR800 spectrometer with a 488 nm Ar+ ion laser. Thermogravimetric analysis (TGA) was carried out on a TGA1500 DSP-SP instrument at a heating rate of 20 ◦ C min−1 from room temperature to 800 ◦ C under an air atmosphere. To observe the morphologies of the prepared TiGOx and TiGOAx samples, high-resolution transmission electron microscopy (HRTEM) was performed using a JEM-2100 electron microscope with an acceleration voltage of 200 kV. The sample powder was dispersed in ethanol by sonication, dropped onto a copper grid, and air-dried. Field emission scanning electron microscopy (FESEM) micrographs of samples were obtained by a Nova NanoSEM 450 microscope operated at an acceleration voltage of 10.0 kV. Sample powder was dispersed in ethanol, dropped onto the surface of a silicon wafer, and then sputter-coated for two cycles with gold to avoid charging under the electron beam prior to examination. UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded on a SHIMADZU UV-2600 spectrophotometer. The specific surface areas of freezedried gels were measured on TriStar 3020 Surface Area and Porosity Analyzer and calculated by Brunauer-Emmett-Teller (BET) method. Samples were degassed for 6 h at 180 ◦ C before measurements. Fourier transform infrared (FT-IR) spectra were recorded on an IRPrestige-21 spectrometer with a resolution of 4 cm−1 and a scan number of 32 by using compressed KBr pellets containing 1 wt% of the sample. Sample composition was determined using X-ray photoelectron spectroscopy (XPS) on a ESCALAB 250 spectrometer with a monochromatic Mg Ka X-ray source. 2.5. Photocatalytic activity Methyl orange (MO) photodegdradation was performed in an aqueous solution under UV light irradiation at room temperature. The reaction was conducted in a cylindrical quartz reactor with a water circulation feature on its outer wall. A 375 W highpressure mercury lamp was used as the UV light source, and the reactant–light source distance was 20 cm. To test photocatalysis, 1.5 g of the undried gel (containing 1.48 g of HAc) was mixed with 100 mL of MO solution (30 mg L−1 ) with stirring. Prior to irradiation, the mixture was placed in the dark for 0.5 h to achieve the adsorption–desorption equilibrium. The absorbance of the target organic compounds was constant over time, indicating that the adsorption–desorption equilibrium was reached [34]. Subsequently, the solution was illuminated with UV lamp and bubbled with oxygen at a constant flow rate. To determine change in MO solution concentration during UV irradiation, 10 mL of the solution was obtained at 0.5 h intervals. The absorbance of the correspond-

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Fig. 1. Gel macrographs of TiGO1 (a–c) and TiGO3 (d–f) taken at various angles.

Table 1 Formability of the TiGOx and TiGOAx gels by regulating the weight of the GO and the volume of HAc. Acetic acid V2 /ml

20

20

20

20

20

Graphene oxide/mg TiGOx gel Graphene oxide/mg Acetic acid V2 /ml TiGOAx gel

0 × 6 5 ×

6 √

12 √

18 √

6 10 √

6 20 √

6 30 √

24 × 6 40 √

ing target organics was monitored by measuring with a UV–vis spectrophotometer. 3. Results and discussion 3.1. Morphological evolution at the macro level TiGOx and TiGOAx gels were synthesized through a simple solvothermal method. The as-prepared TiGOx and TiGOAx gels had a diameter of around 4 cm and contained about 1.41 wt.% titanate–GO and 98.59 wt.% HAc. TiGOx gels showed a well-defined 3D cylindrical structure when 1 mL of TBT, 20 mL HAc (V2 ), and 0–24 mg of GO were combined (Fig. 1). Fig. 1 shows the gel macrographs of TiGO1 and TiGO3 taken at various angles. The as-obtained gels exhibited well-defined cylindrical forms that were successfully self-assembled from the initial GO-TBT-HAc solution. Gelation of TiGOx into a cylindrical form occurred successfully at the initial stage of formation but was inhibited by addition of excess GO. Addition of over 24 mg GO inhibited gelation and formed a gray solution instead. Fig. 1 further shows that the gel coloration changed from light gray to dark gray as the weight of GO added increased. The relationship between TiGOx gelation and GO quantity is shown in Table 1. Well-defined 3D cylindrical TiGOAx gels were also formed from 1 mL of TBT, 6 mg of GO, and 5–40 mL of HAc (V2 ) (Table 1). TiGOx gelation was enhanced by increasing the HAc V2 up to 40 mL. The

Fig. 2. Effects of pH, NaCl, and organic solvents on TiGO1 stability. From left to right: water (pH = 7), HAc, NaCl aqueous solution (1 M), and acetone. The samples were allowed to stand for 72 h at room temperature.

relationship between gelation and relative HAc V2 is also shown in Table 1. Results obtained thus far reveal that the GO dispersion concentration and HAc volume are the two main factors influencing gel formation. Direct relationships were observed between 2D GO assembly into 3D gel structures and these two factors, although only to a certain extent. Beyond optimal conditions, 3D gels could not be obtained. Compared with other self-assembling hydrogels, TiGOx and TiGOAx composite gels are stable in various harsh conditions. As shown in Fig. 2, TiGO1 maintained its form and integrity after 72 h of immersion in NaOH (pH = 12), water (pH = 7), HAc (analytically pure), 1 M NaCl aqueous solution, and several organic solvents, such as n-heptane, hexamethylene, and acetone. The impressive environmental stability of TiGO1 could be attributed to strong multiple non-covalent interactions between the titanate precursor chains and GO sheets. These non-covalent interactions include ␲–␲

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Fig. 3. Schematic showing the procedure for the preparation of TiGOx and TiGOAx gel.

stacking, hydrophobic interactions between titanate bases and GO graphitic domains, and electrostatic/hydrogen bonding interactions between titanate bases and the oxygen-containing groups of GO.

3.2. Mechanism discussion To reveal the mechanism of titanate precursor-promoted GO gelation, the titanate-GO weight ratio was analyzed by adjusting the amount of GO added to the reaction system. The gelation process strongly depended on the ratio of titanate-GO weight with the existence of both the lowest and highest critical gelation concentrations for GO. Gelation occurred through van der Waals interactions or hydrogen bonding by subtle adjustment of the hydrophilic–hydrophobic balance. A tentative mechanism describing the formation of TiGO composite gel is proposed in Fig. 3. In the early stage of the reaction, TBT can react with CH3 COOH to produce numerous nanocrystallites of Ti complex intermediates [Ti6 O6 (CH3 COO)6 (OH)6 ] and the nanocrystallites form linear macromolecule [Ti6 O9 (CH3 COO)6 ]m nanofibers through condensation and aggregation[35–38]. The titanate precursor comprises the linear macromolecules [Ti6 O9 (CH3 COO)6 ]m . GO can provide steric and electrostatic stabilization because of the hydrophilic parts coating the nanocrystallite surfaces through weak steric force [15,30,39]. Thus, GO promotes the condensation and aggregation of nanocrystallites to form a stable nanofiber structure under GO protection. The addition of the appropriate concentration of GO to the fiber-like titanate precursor allowed one nanofiber to bind to two or more GO sheets through hydrogen bonding between the oxygenated groups of the GO sheets and the hydroxyl moieties of the titanate precursor [40,41]. The active component of the fiber-like titanate precursor in the hybrid gel may act as a cross-linker that promotes the formation of cross-linking sites among GO sheets. A sufficiently high number of cross-linking sites induces TiGO network formation and generates TiGO composite gels. However, some of the titanate nanofiber precursors are adsorbed into the GO surfaces with high coverage because of a small quantity of GO solution introduced to the titanate precursors, thereby reducing the availability of cross-linking sites among the GO sheets. Gel-sol transition is believed to have resulted from enhanced electrostatic repulsion between adjacent GO sheets instead of conformational changes in these sheets. These results further confirm the supposition that GO gelation is controlled by the balance between electrostatic repulsion and bonding forces among GO sheets.

Fig. 4. Cross-sectional FESEM images of TiGOA2.

3.3. Morphological evolution at the micro level FESEM images show that micro-level morphological evolution of the gels from flower-like to nanofiber is controlled by regulation of the added quantities of GO and HAc. Figs. 4 and 5 demonstrate that the TiGO nanofibers are uniform in diameter (30–60 nm) and dozens of microns in length. These nanofibers formed bundles, and the resulting nanofiber networks could encapsulate HAc to form gels. The red circles in Figs. 5 and 6 indicate the presence of the GO sheets in TiGOx and TiGOAx gel. These FESEM images (Figs. 5, 6, and S1) reveal that the small GO sheets become more obvious with increasing proportion of GO in the composites. The small GO sheets are dispersed randomly in the composite, thereby connecting or wrapping the titanate nanofibers, instead of supporting them. A similar composite structure was found in the carbon nanofiber–GO composite [42]. The randomly distributed GO sheets without significant alignment in an aqueous dispersion could be self-assembled to form a 3D network upon introducing a proper cross-linker [24]. This process led to a sharp increase in the viscosity of the GO dispersion and formation of a GO-based hydrogel and determined the formation mechanism of the TiGO gel. Fig. 5b–e demonstrates easy cluster-formation of fibers with the addition of 6–24 mg of GO to the reaction system. High concentrations of GO limit the space between two adjacent GO sheets and consequently hinder the formation of cross-linking sites required for 3D GO sheets formation. Increased GO concentrations did not alter the diameter of the nanofibers. Further micro-level assessment revealed that the titanate flower-like morphology is achieved without addition of GO (no gel formed) under similar conditions (Fig. 5a). This result is in agreement with findings by Ye et al. [35], who observed that GO is essential in altering the structure and morphology of samples. EDX analysis (Fig. 5f) confirmed that the TiGOx gel was composed of Ti, O, and C, with a Ti C atomic ratio of nearly

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Fig. 5. FESEM images of (a) TiGO0, (b) TiGO1, (c) TiGO2, (d) TiGO3, (e) TiGO4, and (f) the corresponding EDX spectrum of TiGO3. Red circles indicate GO sheets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1:1 (e.g., TiGO3). Ti originating from the titanate and C atoms were contributed by graphene oxide; no other impurities were detected in the samples. Furthermore, we have added the XPS spectra of asprepared TiGO3 sample and the results give the further evidence of the sample composition (see Fig. S4 in the Supporting information). The aforementioned results clearly show that the HAc V2 added to the samples could also influence TiGO gelation (Table 1). A series of samples with varying HAc V2 was characterized by FESEM (Fig. 6a–e). Increasing HAc V2 increased nanofiber lengths but not widths. Further increases in HAc V2 resulted in extremely long TiGO nanofibers. Figs. 5 and 6 indicate that the TiGOAx samples show only slight increases in nanofiber length. Another important observation was that titanate flower-like and fiber morphologies were still present in the sample regardless of whether gelation had been completed if HAc V2 was 5 mL (Fig. 6a, Table 1). This phenomenon may have occurred because HAc promoted morphological flower-like–fiber transition of TiGO and helped achieve uniform gel formation. Consequently, HAc could also gradually enhance ␲–␲ interactions among GO sheets to ultimately improve gelation [22,43]. GO nanosheets were stacked and precipitated from their dispersion by adding HAc, which inhibited the formation of cross-linking sites among these sheets, as shown in previous results [10,44]. Fig. 6f shows the EDX spectrum of TiGOA2. The

fibers were mainly composed up of Ti, C, and O. Compared with the corresponding EDX spectrum of TiGO3 (Fig. 5f), elemental Ti content increased whereas elemental C content in TiGOA2 (Fig. 6f) decreased significantly. This result is consistent with the fact that the amount of GO added to TiGOA2 was less than that added to TiGO3 during sample synthesis. Fig. 6 To describe the sample microstructures further, the TiGOx and TiGOAx samples were characterized by HRTEM (Figs. 7 a–d, 8 a–d, and S2 ). Fig. 7a–d shows that 6–24 mg of GO facilitated relatively easier fiber clustering. The insets in Figs. 7 and 8 confirm that a single fiber obtained from a nanofiber cluster was about 30–60 nm in diameter. The red circles in Figs. 7 and 8 also indicate the GO sheets in TiGOx and TiGOAx gel. The TEM Figures of TiGO4 under different magnification values are shown in Fig. S2 to clearly observe the morphology of the GO in the samples. Fig. S2 shows that small GO sheets are dispersed randomly in the composite and can only connect or wrap the titanate nanofibers. This finding is consistent with the FESEM test results. Morphological transition of TiGO from flower-like to fibers occurred when HAc V2 ranged from 5 mL to 40 mL (Fig. 8a–d). The TBT–GO–HAc system did not undergo gelation when HAc V2 was less than 5 mL, likely because inhibition of GO nanosheet crosslinking occurs at this volume. The sizes of the fibers were clearly

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Fig. 6. FESEM images of (a) TiGOA1, (b) TiGOA2, (c) TiGOA3, (d) TiGOA4, (e) TiGOA5, and (f) the corresponding EDX spectrum of TiGOA2. Red circles indicate GO sheets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

unchanged despite increases in HAc V2 , which is consistent with the FESEM results.

3.4. Crystal structures of the samples The crystal structure and orientation of the as-prepared samples were studied by XRD. Fig. 9A shows the XRD patterns of GO, TiGO0, TiGO1, TiGO2, TiGO3, and TiGO4. Fig. 9A(a) shows a peak at approximately 10.8◦ , which may be attributed to the (002) reflection of GO with an interlayer distance (d) of 0.82 nm, as determined using the Bragg equation. Oxygen functional groups attached to both sides of the graphite flakes create atomic defects (sp3 bonding) in graphite structure and tend to exfoliate this component into individual or a few layers in aqueous medium [45]. In addition to the several weak peaks observed at high 2, as shown in Fig. 9A(b), a single sharp peak at 7.6◦ (d = 1.15 nm), which corresponds to titanate, was also observed [46]. After solvothermal treatment, this peak weakened in the TiGOx samples and a new peak centered at 2␪ = 11.5◦ formed (Fig. 9A). This new peak corresponds to the d-spacing of GO at 0.26 nm, which means the lattice structure of GO was distorted by its interaction with titanate [47]. Fig. 9A(c)–(f) shows both TiGOx GO and titanate peaks, although slight differences compared to the peaks of pure GO and titanate may be seen. These devia-

tions may be due to the nature of the interaction between GO and titanate [48]. The XRD patterns of TiGOAx in Fig. 9B(a)–(d) indicate that TiGO samples were successfully obtained and that the intensity and sharpness of their characteristic peaks increased as HAc V2 increased. Thus, HAc V2 not only affected theTiGO morphology but also the resultant gel structure. Raman spectroscopy is widely used to characterize the electronic structures and defects of various carbon composite materials. Fig. 10A shows the Raman spectra of TiGO0, TiGO1, TiGO2, TiGO3, and TiGO4 composites taken from 100 cm−1 to 3500 cm−1 . Changes in Raman band intensity and characteristic peak sharpness were observed in the TiGOx samples as GO contents varied (Fig. 10A). The Raman spectrum of pure titanate we obtained shows strong agreement with that reported in a previous study [49]. After solvothermal treatment, the intensity and frequency of characteristic titanate peaks decreased as GO content in the composites increased (other than TiGO3), which can be attributed to titanate–GO interactions. Fig. 10A(b)–(e) demonstrates the appearance of D (1337 cm−1 ) and G (1608 cm−1 ) bands of the TiGO composites as a result of the presence of GO in the samples [50]. The D band suggests a common feature for sp3 defects in carbon, and the G band provides information regarding the in-plane vibrations of sp2 -bonded carbon atoms [47,51]. As the weight ratio of

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Fig. 7. HRTEM images of (a) TiGO1, (b) TiGO2, (c) TiGO3, and (d) TiGO4. Red circles indicate GO sheets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

GO increased, the intensities of characteristic GO peaks increased whereas those of titanate decreased. This implies alterations in the crystallinity of the TiGO composites [45]. The ratio of D G band intensities is generally used to determine the order of defects in GO or graphene [47]. Fig. 10B shows the calculated ID /IG of the TiGOAx composites to which 1.03, 1.05, 1.06, and 1.18 mL of HAc had been added. Our previous study reported that the ID /IG of GO is 0.844 [45]. However, this value increased to 1.18 in TiGOA5. The increasing magnitude of ID /IG in the composites indicates decrease in the average size of the sp2 domains formed during the solvothermal reaction [52]. Thus, changes in Raman band intensity and decreases in Raman peak numbers clearly indicate the presence of GO in the gel composites. TG and DSC analyses were performed after drying of the TiGOx and TiGOAx samples. The results of simultaneous TG and DSC analyses of the TiGOx and TiGOAx composites are shown in Fig. 11. TiGOx exhibited about 12% weight loss from 30 ◦ C to 150 ◦ C, which can be attributed to loss of water or organic solvent molecules. This result corresponds to the inconspicuous endothermic peak at 75 ◦ C in the DSC curve shown in Fig. 11B and D. The apparent exothermic peak at 275 ◦ C corresponds to the decomposition of some of the oxygen-containing functional groups of GO, similar to the results of thermal treatment of GO in air or inert conditions [53,54]. The DSC curves of the TiGOx and TiGOAx samples from 350 ◦ C to 500 ◦ C are shown in Fig. 11B and D. Here, two apparent endothermic peaks may be observed at 350 ◦ C and 420 ◦ C, which represent complete oxidation of the GO carbon skeleton and titanate–anatase trans-

formation, respectively. Fig. 11A and C reveal that the TiGOx and TiGOAx samples were basically completely weightless. The total weight loss of the TiGOx composites increased from 46% to 65% as the GO concentration increased, and the total weight loss of the TiGOAx composites was fairly similar despite variations in HAc volume added. Fig. 12 shows the UV–vis diffuse reflectance spectra of TiGOx and TiGOAx. The amount of GO and HAc added to the reaction system significantly affected the light absorption properties of the TiGO nanocomposites. Fig. 12A shows that the absorbance intensity of the TiGO samples is higher than that of pure titanate over the entire range surveyed. An apparent red-shift of about 25 nm was observed in the absorption edge of TiGO3 compared with that of pristine titanate, which can be attributed to narrowing of the band gap through chemical bonding between titanate and GO, i.e., formation of Ti O C bonds, which is similar to that observed in carbon-doped TiO2 composites [13,45,55]. This also could be further confirmed and demonstrated by FTIR and XPS spectra (see Figs. S3 and S4 in the Supporting information). The added volume of HAc also affected the sample absorption intensity and caused some red-shifting in the sample (Fig. 12B). Significant adsorption intensity enhancement and absorption edge red-shifting are essential features for applicability of TiGOx and TiGOAx materials in photocatalytic reactions, such as degradation of organic pollutants or water splitting into hydrogen. The data of BET specific surface area of TiGOx samples are listed in Table 2. All TiGOx composite samples exhibit large specific sur-

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Fig. 8. HRTEM images of (a) TiGOA1, (b) TiGOA3, (c) TiGOA4, and (d) TiGOA5. Red circles indicate GO sheets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. (A) XRD patterns of (a) GO, (b) TiGO0, (c) TiGO1, (d) TiGO2, (e) TiGO3, and (f) TiGO4; (B) XRD patterns of (a) TiGOA1, (b) TiGOA3, (c) TiGOA4, and (d) TiGOA5.

Fig. 10. (A) Raman spectra of (a) TiGO0, (b) TiGO1, (c) TiGO2, (d) TiGO3, and (e) TiGO4; (B) Raman spectra of (a) TiGOA1, (b) TiGOA3, (c) TiGOA4, (d) TiGOA5.

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Fig. 11. (A)TG and (B) DSC curves of TiGOx; (C)TG and (D) DSC curves of TiGOAx.

Fig. 12. UV–vis DRS of (A)TiGOx and (B) TiGOAx.

Table 2 BET specific surface area of the prepared samples. sample

BET surface area (m2 /g)

GO Titanate TiGO1 TiGO2 TiGO3 TiGO4

60.7 127.8 189.6 211.9 225.8 207.6

face areas than pure titanate and GO because of the presence of GO, which has an extremely high surface area [19]. Interestingly, the BET specific surface area initially increased and then decreased with increasing amount of GO. In addition, the BET surface area of TiGO3 exhibits the largest specific surface area among the samples (225.8 m2 /g). The appropriate amount of the titanate load can effectively prevent the agglomeration of the GO sheets during preparation of TiGOx composites. The high specific surface areas can provide numerous surface-active sites and promote the

effective separation of photoinduced electron hole pairs, leading to enhanced photocatalytic performance [19]. 3.5. Photocatalytic activity and stability The photocatalytic activity of the prepared TiGOx samples was evaluated by photocatalytic decolorization of MO aqueous solution under UV under pH = 3, irradiation and room temperature. We placed a block of 1.5 g of undried gel (with 1.48 g of HAc) in MO (100 mL, 30 mg/L) aqueous solution for photocatalytic degradation. To separate the contribution of adsorption and degradation in the experiment, we characterized the adsorption ability of the TiGOx samples for MO in the dark; the adsorption–desorption equilibration was almost established within 0.5 h (Table S1). At the beginning of UV irradiation, the adsorption saturation was almost achieved; thus, the adsorption minimally contributed to photodegradation. As shown in Table S1, the TiGO3 hybrids exhibit the highest adsorption ability for MO because of differences in specific surface areas. The adsorption capacity may also influence the diffusion of the MO molecules from the solution to the catalyst surface, thereby affecting the photocatalytic activity [56].

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Fig. 13. Changes in the absorbance of MO aqueous solutions in the presence of (A) TiGO1, (B) TiGO2, (C) TiGO3, and (D) TiGO4 under pH = 3, UV irradiation and room temperature. The insets in each photograph show the color change of the MO aqueous solutions with different photocatalysts at a given time interval.

Fig. 14. Changes in the absorbance of MO aqueous solutions in the presence of TiGO3 under various pH, (A) pH = 3, (B) pH = 5, and (C) pH = 7; (D) Changes in the C/C0 value of TiGO3 as a function of time under various pH levels for degradation of MO aqueous solutions.

Fig. 13 shows that all of the TiGOx samples promoted nearly 100% degradation after 2.5 h of UV irradiation. These results are comparable with those of other carbon nanomaterials [50,57]. The insets in Fig. 13 demonstrate color changes in the samples from red to white as the UV irradiation time increased. The color of the MO solution also changed from orange to red after addition of TiGOx catalyst. Fig. 13A–D shows obvious red-shifts (about 25 nm) in the position of the absorption peak, which implies the occurrence of chemical reactions between the MO aqueous solution and the TiGOx gel. Furthermore, it is clear that the amounts of GO has a significant effect on the photodegradation of TiGOx hybrids for MO.

TiGO3 showed the highest photocatalytic activity under UV light illumination, which could be attributed to strong coupling between titanate and GO in this sample. On the one hand, GO can efficiently separate photo-induced electrons and holes from titanate to the GO acceptor through interfacial interactions to reduce electron–hole recombination [58,59]. On the other hand, the large surface area and unique 3D structure of the TiGO nanocomposites can offer more active adsorption sites and photocatalytic reaction centers, thereby resulting in enhanced photocatalytic activity [16]. These superior properties contribute to enhancement of the photocatalytic activity of the composite gels [60]. Under low GO content, synergistic effects

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dozens of microns. By regulating the addition of GO and HAc to the reaction system, the formation rate and microstructure of the gel may be easily controlled. TiGO composite gels can behave as highperformance photocatalysts because of their unique 3D structure and abundance of chemical bonds between titanate and GO. This study provides new insights into the assembly of functionalized graphene with other building blocks, especially inorganic nanomaterials, which is essential for rational design and preparation of hierarchical graphene-based materials.

Acknowledgments

Fig. 15. Photocatalytic performance for MO of TiGO3 during the four-cycle photodegradation.

increase with increase in GO content. However, excess GO content rapidly deteriorates the photocatalytic performance of the TiGO nanocomposites. This phenomenon can be attributed to decreased synergistic effects resulting from increased hole–electron recombination and decreased active adsorption sites and photocatalytic reaction centers [61,62]. Usually, pH plays an important role in photocatalytic degradation of various organic compounds because the generation of hydroxyl radicals is affected by pH. To study the effect of pH on the photocatalytic degradation of MO, we studied different pH values, varying from 3 to 7. The addition of other acidic or alkaline solutions to the reaction system may induce additional reactions; thus, the pH of the MO solution (30 mg L−1 , 100 mL) was adjusted by adding gel (1.5 g) with different HAc contents. Fig. 14 shows the effect of pH on the degradation of MO solution under UV light irradiation with TiGO3 as catalyst. As shown in Fig. 14A–C, the absorption peaks are located 504, 500, and 476 nm at pH = 3, 5, and 7, respectively. This finding implies that pH may affect the chemical reactions between the MO aqueous solution and TiGOx gel. The amount of MO solution degraded reached 100% after UV light irradiation for 2 h at pH 3 (Fig. 14A). For different pH levels, the photocatalytic activity varies in the order of pH 3.0 > 5.0 > 7.0. High pH levels do not induce the degradation of MO, which may be relevant to the point of zero charge (isoelectric point, IEP) of TiGOx [63,64]. The TiGOx surface is negatively charged when the pH is higher than the IEP but positively charged when the pH is lower than the IEP. MO is a typically negatively charged organic dye; hence, the acidic solution favors the adsorption of the MO molecules onto the TiGOx surface, increasing the photocatalytic activity. An ideal photocatalyst should maintain photochemical durability under repeated irradiation conditions [65]. To evaluate the stability and reusability of the TiGO3 hybrid photocatalysts, we measured the recycled photoactivities by repeated photodegradation reaction of MO over TiGO3 for four times under UV light irradiation. The results are shown in Fig. 15. C0 and C are the MO concentrations before and after illumination, respectively. As observed from Fig. 15, no distinct activity decay was observed after four recycling runs. These results indicate that the composite photocatalyst is relatively stable and cannot be easily photocorroded during photocatalysis. 4. Conclusion In summary, we have synthesized 3D TiGO composite gels with controllable morphology through a simple one-pot solvothermal process. TiGO composite gels can be fabricated via a cross-linking mechanism through nanofiber networks. A single nanofiber synthesized in this study had a diameter of 30–60 nm and length of

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51372124, 51572134) and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.12. 046.

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