graphene composite

Journal of Molecular Catalysis A: Chemical 398 (2015) 399–406

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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Variant effect of graphene sheets and ribbons on photocatalytic activity of TiO2 sheets/graphene composite Zhizhong Han a,b , Liyuan Wei b , Haibo Pan b,c,∗ , Chunyan Li a , Jinghua Chen a a b c

School of Pharmacy, Fujian Medical University, Fuzhou, Fujian 350108, China College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China Fujian Key Lab of Medical Instrument and Pharmaceutical Technology, Fuzhou 350002, China

a r t i c l e

i n f o

Article history: Received 19 September 2014 Received in revised form 4 January 2015 Accepted 6 January 2015 Keywords: Graphene sheets Graphene ribbons Width of graphene ribbon TiO2 nanosheets Photocatalysis

a b s t r a c t To investigate the effect of graphene sheets and ribbons on photocatalysis, the composite of anatase TiO2 nanosheets (TiO2 -NS) and graphene materials was prepared. Graphene oxide sheet (GOS) was synthesized by a modified Hummers’ method, and graphene oxide ribbon (GOR) was formed by unzipping carbon nanotubes. UV–vis absorption spectrum and X-ray photoelectron spectroscopy (XPS) reveal that GOSs and GORs were photoreduced with TiO2 -NS under ultraviolet irradiation. The as-prepared TiO2 nanosheets/graphene sheets (TS-GNSs) or ribbons (TS-GNRs) display drastically quenching of photoluminescence (PL) intensity and lower electron impedance, and TS-GNSs show better results than TS-GNRs. It is due to the excellent mobility of charge carriers and large surface area with graphene, which facilitate the separation of electron–hole pairs. Thus, TS-GNSs and TS-GNRs exhibit prominent photocatalytic activity under ultraviolet irradiation. The degradation efficiency of RhB is nearly 80% for TS-GNRs and 100% for TS-GNSs within 25 min. The lower degradation efficiency of TS-GNRs is due to the lower charge mobility of GNRs, which demonstrates that the excellent charge mobility of graphene plays a crucial role in composites for photodegradation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic efficiency is mainly affected by the recombination of photogenerated electron–hole pairs. Thus, one of the most challenging issues in photocatalysis is to reduce the quick recombination of electron–hole pairs. A variety of strategies have been used to improve these problems such as coupling with different bandgap semiconductors [1], doping with nonmetal elements [2], and loading noble metals [3]. A novel way to improve the photocatalytic efficiency is the introduction of carbon materials, especially graphene [4,5]. Graphene is strictly expected to comprise a single layer, acts as a semimetal or zero band-gap semiconductor, and exhibits unusual properties, such as excellent mobility of charge carriers (200,000 cm2 V−1 s−1 ) [6], large surface area (theoretical value, ∼2600 m2 /g) [7], half-integer quantum Hall effect and ballistic electron transport [8]. Graphene can enhance photoinduced charge transfer as electron efficient acceptor and restrain the

∗ Corresponding author at: College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China. Tel.: +86 59122866127; fax: +86 59122866127. E-mail address: [email protected] (H. Pan). http://dx.doi.org/10.1016/j.molcata.2015.01.006 1381-1169/© 2015 Elsevier B.V. All rights reserved.

recombination of photogenerated electron–hole pairs for improved catalytic activity [9,10]. The extension of graphene in one direction yields graphene ribbons (GNRs), which can be considered as elongated strips [11]. GNRs inherit the excellent properties from graphene, but create a band gap due to the quantum constraint and gradually convert from semiconductors to semimetals with increasing width. For example, the energy gap of a 15 nm wide graphene ribbon is around 0.2 eV [12], while ∼50 meV for ∼100 nm wide [13]. Because of their tunable electronic properties, GNRs have quickly become a popular subject of research toward the design of graphene-based nanostructures for technological applications in future optoelectronic devices, such as field-effect transistors. Moreover, compared with zero band-gap graphene, GNRs show a higher chemical reactivity due to dangling bond of edge carbon atoms, active nonbanding  electrons, and special energy band structure [14]. Therefore, GNRs can be quite susceptible to adsorption of other organic molecules. There are three main approaches for the synthesis of structurally well-defined graphene ribbons, etching graphene or graphite, unzipping carbon nanotubes and bottom–up approaches [15–17]. The method of unzipping carbon nanotubes is very appealing, since it is simple and i ed as a photocatalyst because of its stable, nontoxic, and inexpensive [18,19]. The unique physical

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and chemical properties of TiO2 crystals are tunable with electronic structure, size, shape, organization, and surface properties. Moreover, the photocatalytic and related application largely depend on the surface atomic configuration and the degree of exposure of reactive crystal facets [20]. The dominating exposed facet for typical anatase TiO2 crystals is the {1 0 1} facets, which are thermodynamically stable due to a low surface energy (0.44 J/m2 ) [21]. However, {0 0 1} facets with a higher energy (0.90 J/m2 ) are more interesting and important for higher reactivity and considered to be more reactive than the {1 0 1} facets in heterogeneous reactions [22,23]. It is attributed to the low atomic coordination numbers of exposed atoms and more oxygen deficiency on {0 0 1} facets [24]. Therefore, there is a great interest in the development of controllable synthesis of anatase TiO2 nanostructures with dominant {0 0 1} high-energy facets used as photocatalyst [25–28]. And it is reported that pure TiO2 with {0 0 1} facets extend the photoactivity down to visible light region due to a unique “lattice-work structure”, which mediate the creation of the electron and hole for the photochemical reaction [29]. In this work, anatase TiO2 nanosheets (TiO2 -NS) with a high percentage of {0 0 1} facets were prepared by solvothermal method using hydrofluoric acid (HF) as a morphology control agent, and then TiO2 sheets combined with graphene oxides (GOs) through self-assembly. The graphene oxides include graphene oxide sheets (GOSs) synthesized by a modified Hummers’ method and graphene oxide ribbons (GORs) formed from unzipping carbon nanotubes. TiO2 sheets are capable of interacting with (GOs) via carboxylic acid functional groups [30]. When adding GOs, TiO2 -NS can be easily grafted on the GOs surfaces via chemical bond because of abundant oxygen functional groups on the GOs surfaces. Thus, the mixing of GOs and TiO2 -NS results in the binding of oxide particles to graphene oxide flakes and minimizes any aggregation effects of GOs. Then, a facile strategy was used to reduce GOs via photoreduction with TiO2 -NS as reducing agent under ultraviolet irradiation. The TiO2 -NS with {0 0 1} facets decorated with graphene sheets or ribbons were investigated in detail, such as morphology, structure, and the influence of graphene on properties. The obtained composites of TiO2 nanosheets and graphene material exhibit a higher photocatalytic activity compared with pure TiO2 -NS. 2. Experimental 2.1. Preparation of the sample 2.1.1. Fabrication of {0 0 1} facet TiO2 nanosheets In this work, anatase TiO2 -NS with a high percentage of {0 0 1} facets were prepared through a simple hydrothermal method by using tetrabutyl titanate (Ti(OBu)4 , 98%, Aldrich) as Ti source and hydrofluoric acid (HF, 48%, Sigma–Aldrich) as capping agent. All of the chemicals were used as received without further purification. For a typical experiment, 10 mL of Ti(OBu)4 and 1.4 mL of HF were added dropwise into a dried Teflon-lined stainless steel autoclave with a capacity of 50 mL. Then the autoclave was sealed and heated at 200 ◦ C for 24 h in an electric oven. After completion of the reaction, the autoclave was taken out and cooled to room temperature. The product were collected by centrifugation, washed thoroughly with absolute ethanol and deionized water for several times to remove the residual contamination. Then the anatase TiO2 NS dominated with {001} facets were finally dispersed in distilled water and then kept for the following experiments. 2.1.2. Synthesis of graphene oxide sheets GO sheets (GOSs) were synthesized by a modified Hummers’ method. It started with graphite pretreatment. To accomplish this, 2 g of graphite powder were added to an 85 ◦ C solution of 10 mL of

concentrated H2 SO4 , 2 g of K2 S2 O8 , and 2 g of P2 O5 . The mixture was reacted for 5 h, after which it was diluted with 500 mL of water, filtered, and washed using a 0.2 ␮m Nylon Millipore filter and dried in air overnight. For the following oxidation step, 1000 mL of H2 SO4 were chilled to 0 ◦ C using an ice bath. The oxidized graphite was added to the acid and stirred. Then, 10 g of KMnO4 were added slowly with temperature controlled below 20 ◦ C. This mixture was allowed to react at 35 ◦ C for 24 h, after which 500 mL of distilled water were added slowly so as to keep the temperature below 50 ◦ C. After further reaction for 2 h, 20 mL of 30% H2 O2 were added, resulting in a brilliant yellow color along with bubbling. The mixture was allowed to settle for at least a day before the supernatant was decanted. The remaining mixture was then filtered and washed with 10 wt% HCl solution followed by water to remove the acid. The GO product was suspended in distilled water to, which was subjected to dialysis to completely remove metal ions and acids. Exfoliation was achieved by dilution of the GO dispersion with deionized water, followed by 6 h sonication. The resulting homogeneous yellow–brown sol was stable for a period of months and was used for film preparation. 2.1.3. Synthesis of graphene oxide ribbons Graphene oxide ribbons (GORs) were prepared by a simple solution-based oxidative process [11]. We suspended multi-walled carbon nanotubes (MWCNTs, Shenzhen Nanometer Gang Co., Ltd., China) in 150 mL of concentrated sulphuric acid (H2 SO4 ) for a period of 10–12 h and then treated them with 750 mg of potassium permanganate (KMnO4 ). The H2 SO4 conditions aid in exfoliating the nanotubes and the subsequent graphene structures. The reaction mixture was stirred at room temperature for 1 h, and then heated to 55 ◦ C for 30 min and to 70 ◦ C for 15 min. When all of the KMnO4 had been consumed, the reaction mixture was quenched by pouring over ice containing a small amount of hydrogen peroxide (H2 O2 ). The solution was filtered over a polytetrafluoroethylene (PTFE) membrane, and the remaining solid was washed with acidic water followed by ethanol/ether. Then the solid was dispersed in distilled water, and GORs solution was obtained. 2.1.4. Fabrication of s TiO2 sheets/reduced graphene sheets or ribbons nanocomposites The GOSs or GORs aqueous solution and TiO2 -NS suspension were added into a quartz reactor after sonication for 1 h. Under N2 , the mixtures were irradiation by the light from 200 nm to 390 nm (Perfect light PLS-SXE300C, China) for 1 h. During the reaction, it was stirring. Then TiO2 -NS were integrated with graphene, and the oxide graphene materials were reduced by the photoelectron from TiO2 -NS. 2.2. Materials characterizations Crystalline structures of the samples were analyzed by powder X-ray diffraction (MiniFlexTMII, Japan). Surface morphologies of the samples across the entire substrate were characterized by fieldemission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan), while the transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, 200 kV, FEI, USA) was used to examine the morphology of the composites. UV–vis absorption spectra were measured on a Shimadzu UV-2500 UV–vis spectrophotometer. Raman scattering was performed on a Renishaw Invia Raman spectrometer using a 532-nm laser source. PL spectra were recorded at room temperature employing a Cary Eclipase (Varian, USA). The surface properties of samples were characterized by XPS in a PHI Quantum 2000 Scanning ESCA Microprobe (PHI. Corp., USA) system with a monochromatic Al K␣ 1,2 source (1486.60 eV). All binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. Electrochemical impedance

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Fig. 1. Synthetic procedures for the preparation of TiO2 sheets on graphene.

spectroscopies (EIS) of the samples were performed with an electrochemical workstation (CHI 660D, CH Instrument Company, China). The electrochemical analyses were executed using a conventional three-electrode system. The working electrode was the as-prepared TiO2 -NS/graphene material composite based on a glass carbon electrode. A platinum wire and a saturated Ag/AgCl electrode were used as a counter and a reference electrode, respectively. The electrolyte solution was 0.1 M Na2 SO4 solution. 2.3. Photoelectrochemical and photocatalytic measurements For photocatalytic measurement, 2 mL the as-prepared TiO2 NS/graphene material suspension was dispersed into 33 mL RhB aqueous solution (5 ␮M), and subsequently irradiated using a xenon lamp (36 W) within 200–390 nm. Then at a definite time interval, some of the samples were withdrawn for analysis using a UV–vis spectrophotometer (Shimadzu UV-2500). The degradation efficiency was defined as  = (1 –C/C0 ) × 100%, where C and C0 were the equilibrium concentration of RhB after and before ultraviolet irradiation. And the concentration of RhB was linear proportion to absorption (A), thus C/C0 = A/A0 . 3. Results and discussion The overall fabrication procedures of the reduced graphene materials consisting of uniform TiO2 -NS are schematically illustrated in Fig. 1. It starts with the dispersion of graphene oxide in aqueous TiO2 -NS suspension, proceeding with the self-assembly of TiO2 -NS on GOSs. Then under ultraviolet irradiation, the electrons transfer from valence band to conduction band of TiO2 -NS, and emigrate to GOSs, reducing the graphene oxides to GNs without any additive reductant. To better study the interface structure between the two phases, the as-prepared samples were further examined with TEM and high-resolution transmission electron microscopy (HRTEM). TEM

images of TS-GNS reveal that the TiO2 -NS are well deposited on the surface of graphene (Fig. 2(a)). The pure graphene sheet is intensively flexible and wrinkled because of its strong free energy, and its width is up to micron (the inset of Fig. 2(a)). But GNSs in the composite of TS-GNS are planar and only wrinkled at the brim, which may be attributed to the decrease of free energy of graphene with TiO2 -NS, avoiding the aggregation for graphene. The image also presents that the obtained TiO2 -NS product consists of uniform, well-defined sheet-shaped structures possessing a rectangular outline. Statistically, anatase TiO2 -NS have a side length of 45 nm and a thickness of 5 nm on the average. Fig. 2(b) is the HRTEM image of blue circle in Fig. 2(a). It indicates that the lattice spacing parallels to the top and bottom facet is ca. 0.236 nm, corresponding to the (0 0 1) planes of anatase TiO2 . The SAED pattern viewed along [1 0 0]/[0 1 0] and [0 0 1] crystallographic directions (Fig. 2(c)) further confirms that the rectangular surfaces are {0 0 1} facets of the single-crystalline anatase TiO2 -NS. On the basis of TEM results, the percentage of exposed {0 0 1} facets on TiO2 -NS is ca. 81%. Further, it can be concluded that anatase TiO2 -NS with dominant {0 0 1} facets have been readily prepared and can be used for the following photocatalysis experiments. As concluded from Fig. 2(d) investigations, the carbon nanotubes are sliced either longitudinally or in a spiral manner affording straight-edged ribbons. Since the carbon nanotubes as source materials are multiwall, the ribbons are with a few layers (the inset of Fig. 2(d)). The width of the GORs is ca. 210 nm, which is quite narrower than that of GOSs. In the Fig. 2(e), it shows that TiO2 -NS are inserted between GNRs and lay one by one. The sandwich structure is beneficial to binding TiO2 -NS on the surface of GNRs and restraining the congregation of TiO2 -NS, resulting in a strong interaction between TiO2 -NS and GNRs. The crystallographic structure of as-prepared samples was determined by X-ray diffraction measurements. XRD patterns of as-prepared pure TiO2 -NS, TS-GNRs, GORs and GOSs are shown in Fig. 3(a), respectively. For GOSs and GORs, a diffraction peak at 10.8◦ (0 0 2) can be observed, the d-spacing of the lattice is calculated

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Fig. 2. (a) TEM image of TS-GNSs, (b and c) high-resolution TEM image of TS-GNSs, and (d and e) TEM image of GOR and TS-GNRs. The insets of (a), (c) and (d) show a pure graphene sheet, the corresponding SAED pattern, and the partial enlarged drawing, respectively. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

as 3.4 Å in agreement with the typical distance of – stacking of the graphenes, which indicates that pristine graphite and carbon nanotubes were oxidized into graphene oxides with a well ordered, lamellar structure. A broad minimal diffraction peak at 25.5◦ is also observed for GORs contributed by MWCNTs. XRD pattern of TSGNSs is similar to that of TS-GNRs, so only XRD pattern of TS-GNSs is shown in Fig. 3(a). For TS-GNSs, the intensity of the peak at 25.5◦ is strengthened after photoreduction with TiO2 -NS and the diffraction peak at 10.8◦ is absent, validating that GOSs and GORs are completely reduced. However, all the diffraction peaks in TS-GNSs are the same as the crystal structure of pure TiO2 -NS, and match well with the crystal structure of the anatase phase TiO2 (space

group: I41/amd, JCPDS No. 21-1272), indicating the formation of TiO2 -graphene sheets/ribbons composite. The formation of reduced graphene enables the reaction process to be monitored with UV–vis spectroscopy. The GNSs and GNRs dispersions are obtained after the centrifugation of TS-GNSs and TS-GNRs dispersions. As shown in Fig. 3(b), with the reduction progress, the absorption peak of GOSs dispersion at 231 nm red-shifts to 261 nm and the intensities of absorption spectra from 260 to 350 nm increases after photoreduction. Similar to the GOSs, the absorption peak of the GORs dispersion at 231 nm red-shifts to 261 nm and covers the whole spectral region (>231 nm) after photoreduction. Both bathochromic shift and hyperchromicity

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Wavenumber (cm ) Fig. 3. (a) XRD patterns of different samples, (b) UV–vis spectra of graphene materials before and after photoreduction, and (c) Raman spectra of different samples. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

indicate that the electronic conjugation within the graphene ribbons was restored upon photoreduction with TiO2 -NS [31]. This experiment also illustrates that the electronic conjugation level of graphene is chemically controllable as conjugated polymers, offering possibility to tailor the optical and electrical properties of graphene ribbons. To further investigate the crystalline phase of TiO2 and quality of graphene, Raman spectroscopy is a powerful nondestructive tool to distinguish ordered and disordered crystal structures of carbon materials. The measurements were performed at room temperature with a Renishaw spectrometer at 532 nm, with notch filters cutting at 100 cm−1 . In Fig. 3(c), the peak about 1604.5 cm−1 corresponds to the first-order scattering of the E2 g mode observed for in-phase stretching vibration sp2 carbon domains in aromatic rings, which red-shifts comparing to the G band of graphite (1579 cm−1 ) and mainly is caused by stress [32]. The G band does not obviously shift for as-prepared samples. The peaks at 1353.1 cm−1 for GOSs and GORs are attributed to D band, and the D band of GORs is a slight hypsochromic shift. However, the D bands for TS-GNSs and TS-GNRs are blue shifted to 1338.8 cm−1 . The pronounced D band is a breathing mode of k-point phonons of A1 g symmetry. Moreover, it is a disordered band associated with structural defects, amorphous carbon or edges that can break the symmetry and selection rule [33]. The intensity ratio (the D band to the G band) increases substantially, indicating the decrease in size of the in-plane sp2 domains, which is possibly due to the extensive oxidation and ultrasonic exfoliation. In addition, the spectra of TS-GNSs exhibit the typical optical modes of anatase, namely, Eg(1) peak (139.3 cm−1 ), B1g(1) peak (394.0 cm−1 ), Eg(2) peak (634.9 cm−1 ), and the A1 g + B1g(2) modes centered around 514.4 cm−1 , respectively. There is only a peak at 148.2 cm−1 for TS-GNR because of sandwich structure, where TiO2 -NS insert between graphene ribbons. The

results match with the previous TEM and XRD analysis, confirming TiO2 -NS successfully decorated with graphene ribbons. To investigate the chemical state of TiO2 -NS/graphene and the interactions between TiO2 -NS and graphene in composites, then XPS measurements were carried out (Fig. 4). Fig. 4(a) exhibits that only the peaks of Ti, C and O in the survey spectrum of TS-GNSs can be clearly detected. Meanwhile, the oxidation state of the Ti in TS-GNSs is shown in Fig. 4(b). Two bands located at 459.3 and 465.0 eV can be observed, and assigned to the Ti2p3/2 and Ti2p1/2 signals, respectively. Compared with TiO2 nanoparticles, the higher binding energy reveals that more Ti are in the Ti3+ chemical state for TS-GNSs [34]. The main C 1s XPS spectra for GOSs and TS-GNSs are presented in Fig. 4(c), where the C 1s peaks of TS-GNSs are obviously weaker in comparison with those of GOSs. Fig. 4(d) shows a high resolution asymmetric C 1s XPS spectrum. After the subtraction of a Shirley background followed by fitting with a mixture function of Lorentzian and Gaussian, the C 1s peak of TS-GNRs can be mainly deconvoluted into three sub-peaks at 284.6, 286.4 and 288.9 eV, respectively. The sub-peak centered at 284.6 eV is attributed to the C C, C C, and C H bonds (sp2 ), the sub-peak at 286.4 eV is assigned to C O and C O C, and the signal at 288.9 eV is corresponding to O C O [35]. The C 1s peak of GNSs is similar to GOSs, but the middle peak shifts to 286.2 eV. The peak intensities of the oxygen functional groups substantially decrease in the C 1s XPS spectrum of GNS (Fig. 4(d)), confirming that GOSs has been converted to reduced graphene. In Fig. 4(e) and (f), three deconvoluted peaks are also observed for the graphene ribbons, and the peak intensities of the oxygen functional groups decay after photoreduction. The middle deconvoluted peak of GORs is located at 286.8 eV, which is higher than those of TS-GNRs and GOSs. The C 1s peak of GORs is similar to TS-GNRs, but the middle deconvoluted peak of GORs located at 286.8 eV is higher than that of TS-GNRs. Because GORs are unzipped

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Fig. 4. (a) The survey XPS spectra of TS-GNSs, (b) Ti2p XPS spectrum, (c) XPS spectra and (d) fitted XPS spectra of C 1s for TS-GOSs and TS-GNSs, (e) XPS spectra and (f) fitted XPS spectra of C 1s for GORs and TS-GNRs.

from MWCNTs, there is a strong interaction as the O C O band between graphene ribbons. For TS-GNRs, the O C O band is broken with TiO2 -NS and substituted by Ti O C band. Therefore, the middle deconvoluted peak of TS-GNRs shifts to 286.2 eV after photoreduction. The peak intensities of the oxygen functional groups substantially decrease in the C 1s XPS spectrum of GNRs (Fig. 4(e) and (f)), confirming that GORs have been converted to GNRs after photoreduction by TiO2 -NS. Electrochemical impedance spectroscopy (EIS) was used to measure the current response to the application of an ac voltage as a function of the frequency and characterizing electrochemical interfacial reactions. Fig. 5(a) shows the Nyquist plots of TiO2 NS, TS-GNSs and TS-GNRs under dark conditions. The diameter of

the arc radius on the Nyquist plots of the composite of TiO2 and graphene is smaller than that of pure TiO2 -NS. It is due to the excellent mobility of charge carriers and large surface area with graphene, which facilitate the separation of electron–hole pairs as well as the interfacial transfer of carriers. It could be found that the arc radius for TS-GNRs is faintly bigger than TS-GNSs, demonstrating that the electron mobility of GNRs is lower than GNSs [36]. As shown in Fig. 5(b), a strong fluorescence emission peak is centered at 362 nm for TiO2 -NS, which is related to the near band-edge emission of TiO2 . The emission originates in the recombination of free excitons through an exciton–exciton collision process. Compared with TiO2 -NS, TS-GNSs and TS-GNRs exhibit notably weaker intensity with a featureless emission around 362 nm and nearly

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a line in visible emission. The results demonstrate that graphene can block both direct and trap-related charge carrier recombination pathways, since graphene can act as electron efficient acceptors and accelerate the photogenerated electrons transfer. Moreover, due to the better electron mobility, the PL intensity of TS-GNSs is much weaker than that of TS-GNRs. Additionally, the PL intensity of TSGNSs and TS-GNRs is drastically quenched, displaying the existence of a direct interaction between TiO2 -NS and graphene that enabled the nonirradiative relaxation of excitons formed in TiO2 -NS. The photocatalytic activitives of as-prepared TiO2 -NS, TS-GNSs and TS-GNRs on the RhB degradation were investigated in the presence of UV light radiation. Fig. 6 shows the degradation rates of RhB without catalysts and using different photocatalysts. After irradiation for 25 min, the degradation efficiency of RhB is found to be only 56.0% for pristine TiO2 -NS. This result confirms the advantage of dominant {0 0 1} facets in photocatalysis. TS-GNRs and TS-GNSs show higher photocatalystic efficiency, 80.0% for TS-GNRs and nearly 100% for TS-GNSs. Obviously, graphene sheets and ribbons can improve the photocatalystic activity.

Fig. 7. Schematic reaction mechanism for photocatalytic degradation of RhB.

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The carbon platform plays important roles during the photodegradation of RhB because graphene is an important candidate for a charge acceptor attributed to its 2D planar-conjugation structure and excellent charge mobility [9,37]. Moreover, the bandgap of TiO2 (Eg = 3.2 eV) is smaller than the work function of graphene (ϕ = 4.42 eV). The conducting band of TiO2 is more negative than the work function of graphene, such that the photogenerated electrons energetically transfer from TiO2 to graphene. Thus, GNSs and GNRs could act as acceptors of the photogenerated electrons by TiO2 , as shown in Fig. 7. Additionally, graphene may ensure fast charge transportation due to its high conductivity. And the adsorptivity of dyes for TS-GNRs and TS-GNSs increases compared to that of bare TiO2 -NS, because RhB molecules could be adsorbed to the surface of as-prepared composites with offset face-to-face orientation via – conjugation between RhB and aromatic regions of graphene. Overall, the effective charge separation can be achieved and dramatically enhance the photocatalytic activity. Moreover, the band gaps of semiconducting GNRs are inversely proportional to the width, and the electronic properties are highly dependent on the width [15]. The width of GNRs are much narrower than that of GNSs, thus the priors have higher band gap, decreasing the electron mobility which agrees on the result of EIS. Therefore, TS-GNSs show higher photocatalytic efficiency. 4. Conclusions In this work, rectangular anatase TiO2 nanosheets with a high percentage (ca. 81%) of {0 0 1} facets were prepared by solvothermal method using hydrofluoric acid (HF) as a morphology control agent. Graphene oxide sheets were synthesized by a modified Hummers’ method and graphene oxide ribbons were formed by unzipping carbon nanotubes, and then the two graphene oxides were photoreduced by TiO2 -NS under ultraviolet irradiation. Compared to pure TiO2 -NS, TS-GNSs and TS-GNRs show a drastically quenching of PL intensity and lower electron impedance, indicating that graphene can enhance the separation of electron–hole pairs due to their excellent charge mobility. In addition, the anatase {0 0 1} facets show more oxygen deficiency and are reactive for photocatalysis. Thus, TS-GNSs and TS-GNRs exhibit higher photocatalytic activity. Within 25 min, the degradation efficiency of RhB is 80% for TS-GNRs and nearly 100% for TS-GNSs, but only 56.0% for pristine TiO2 -NS. The smaller width of GNRs induces higher band gap and slower charge mobility, therefore the degradation efficiency for TS-GNRs exhibits a bit decrease. Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21375017, 21105012 and 21201035), Fujian Natural Science Foundation (2012J01204), National Science Foundation for Distinguished Young Scholars of Fujian Province (2013J06003), the Key Project of Fujian Science and Technology (2013Y0045), Program for New Century Excellent Talents of Colleges and Universities in Fujian

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