TiO2-Intercalated Graphene Oxides with Highly Efficient Photocatalytic Degradation for Methylene Blue

The low photocatalytic decomposition activity of TiO2 toward industrial pollutants at room temperature is one of themain obstacles for its practical application. TiO2-intercalated graphene oxide (GO) composites were prepared by in situ hydrolysis of butyl titanate in a GO aqueous solution, followed by hydrothermal reaction to improve their photoelectron separation efficiency. The in situ generated TiO2 nanocrystals could grow and adhere to the GO walls, thereby greatly improving the contact area and binding strength among them and resulting in low photoelectron transfer resistance. The photocatalytic activities of the as-prepared catalyst were evaluated via photodegradation of methylene blue (MB). The TiO2-intercalated GOs displayed much higher catalytic activity than GO, TiO2, and TiO2-adsorbed GOs. The degradation efficiency of MB by TiO2-intercalated GOs increased with increasing bath ratio of TiO2-intercalated GOs to MB solution, but it decreased with increasing initial concentration of MB. Degradation of MB by UV light was much faster than by simulated sunlight. The degradation time by sunlight was only 5% of degradation time by UV light. Cyclic catalytic experiments indicated that TiO2-intercalated GO maintained 99.97% degradation activity after repeated degradation (five times), thereby indicating the good decomposition durability.

To improve the photocatalytic efficiency, several methods, such as modification of TiO 2 nanoparticles with conductive organic materials, metal nanoparticles, and carbon materials, were developed to prevent electron-hole pair recombination [4,[20][21][22][23][24]. The integration of TiO 2 and graphene (GR) is intensively investigated, and advancement in this technology is obvious in recent studies because of the super photoelectric properties of GR. For example, TiO 2 -GR nanocomposites have been prepared via a facile hydrothermal reaction of graphene oxide (GO) and TiO 2 in an ethanol-water solvent. Such TiO 2 -GR nanocomposites exhibited much higher photocatalytic activity and stability toward benzene gas, which is a volatile aromatic pollutant in air, than bare TiO 2 [25]. Wen et al. prepared TiO 2 -Ag-GR nanocomposites that showed significantly increased visible light absorption and photocatalytic activity compared with Ag-TiO 2 and TiO 2 -GR nanocomposites [26]. Fan et al. compared TiO 2 -reduced GO nanocomposites prepared by UV, hydrazine, and hydrothermal reductions. TiO 2 -reduced GO composites prepared by the hydrothermal method exhibited optimal photocatalytic performance [27]. Nanakkal and Alexander developed graphene/BiVO 4 /TiO 2 ternary nanocomposites through a facile, ultrasonic wave-assisted one-pot hydrothermal method. The as-prepared composite exhibited enhanced photocatalytic degradation of methylene blue (MB) under visible light irradiation [28].
However, the photocatalytic efficiency of TiO 2 -GR/TiO 2 -GR is still limited because of the high contact resistance and low binding strength between prefabricated TiO 2 nanocrystals due to their large morphology mismatch and lack of chemical interactions that lead to low contact area [29]. To solve this problem, we synthesized closely integrated TiO 2intercalated GO (TiO 2 -GO-TiO 2 sandwich-like) composite nanosheets. TiO 2 could grow in situ on GOs and form the sandwich-like TiO 2 -intercalated GO nanosheets by following hydrothermal reaction, when the hydrolysis and condensation of tetrabutyl titanate (TBOT) in GO solution is carefully controlled. The contact resistance and contact strength of TiO 2 nanocrystals can be greatly improved, because they grow by adhering to the GO surfaces [29]. The resultant TiO 2 -intercalated GOs possessed a sandwich-like structure with an ultrathin nature and highly crystallized TiO 2 nanocrystals. Most importantly, the TiO 2 -GO-TiO 2 nanosheets provide high photodecomposition efficiency and durability for the model dye, MB, compared with TiO 2 and TiO 2adsorbed GOs [30][31][32][33][34][35][36][37] 2.2. Synthesis of GOs, TiO 2 -Intercalated GOs, and TiO 2 -Adsorbed GOs. GOs were prepared from purified natural graphite by the modified Hummers method [38]. A total of 20 g of graphite and 460 mL of H 2 SO 4 were mixed in a beaker container at 0°C under vigorous stirring. Then, 60 g of KMnO 4 was added to the suspension sample, maintaining a temperature of below 5°C. After addition of KMnO 4 , the rising temperature should be maintained below 30°C for 30 min. The mixture was diluted with 1000 mL of DI water and heated to 95°C, maintaining this temperature for 15 min. Finally, 3000 mL of DI water and 100 mL of H 2 O 2 were added to the mixture to terminate the reaction. The resultant mixture was washed with 10% of HCl aqueous solution, ultrasonicated for 30 min, and stored overnight. The suspension mixture was finally filtered, washed by DI water several times, and dried in a dry oven at 60°C.
The schematic of the preparation of TiO 2 -intercalated GOs is presented in Figure 1. A 0.1 g of GOs was dissolved in 30 mL of DI water, sealed by a hydrothermal reactor (50 mL), and hydrothermally reacted at 135°C for 6 h. Then, the GO aqueous solution was ultrasonicated for 30 min, filtered, dried at 60°C, and then dissolved in tetrahydrofuran  with ultrasound treatment for three to four hours. Subsequently, 10-50 mL of butyl titanate was added into the GO solution and ultrasonicated for 30 min. The mixture was added into a hydrothermal reactor and crystallized at 180°C for 12 h. Finally, the resultant TiO 2 -intercalated GO powder was filtered, washed, and finally dried at 60°C. To synthesize TiO 2 -adsorbed GOs (control), 40 mL of butyl titanate was added to 100 mL of absolute ethanol solution, after which 5 g of GOs was added. TiO 2 -adsorbed GO solution was prepared by stirring for 5 h and then crystallized in a hydrothermal reactor at 180°C for 12 h. The obtained solution was centrifuged and dried at 60°C.

Characterizations.
To identify the crystalline phase of synthesized samples, all samples were subjected to X-ray diffractometer (XRD; Shimata XD-D1, Japan) experiments at a scan speed of 2°/m −1 . The morphology of TiO 2 nanoparticles and GO sheets was observed by an S-3400N scanning electron microscope (SEM; Hitachi, Japan) and JEOL 2100F transmission electron microscope (TEM; JEOL, Japan) that operate at 200 kV accelerating voltage. The diffuse reflectance spectra were recorded on a TU-1901 ultravioletvisible (UV-vis) spectrophotometer (PG Scientific, USA). Fourier transform infrared (FTIR) spectra were obtained on a Thermo Nicolet AVATAR 370 FTIR spectrometer (Bruker, Germany). The X-ray photoelectron spectra (XPS) were studied by the ESCALAB 250 XI X-ray photoelectron spectroscopy (Thermo Scientific, USA).

Photocatalytic Degradation of MB.
The samples were added into 50 mL of the MB solutions and stored in the dark for 24 hours to reach the adsorption equilibrium. The obtained mixtures were placed onto a XPA-4 photochemical reactor (Xujiang Electromechanical Plant, China) with a 300 W UV light type mercury lamp and/or a 350 W solarsimulated xenon lamp. All light sources were located at 15 cm above the reaction solution. In the catalytic decomposition, 1 mL mixture was sampled and centrifuged at regular intervals. The supernatants were obtained and measured using a UV-vis spectrophotometer at a maximum absorption wavelength (λ max) of 664 nm to detect the MB concentration. To study the photocatalytic degradation of TiO 2 -intercalated GOs toward MB in aqueous solution, we examined the initial MB concentration (ranging from 100-300 mg/L), the bath ratio between TiO 2 -intercalated GOs and MB solution (1 : 50, 1 : 100, and 1 : 200), and the light source (without light, UV light, and simulated sunlight). The catalytic performances of the controls, TiO 2 , GOs, and TiO 2 -adsorbed GOs (prepared by adsorption of TiO 2 to GOs), were studied by following the experimental method.
The concentration C (mg/L) and the degradation rate D (%) were calculated according to the standard curve of MB. The calculation formula is as follows (1): where C 0 is the initial concentration of MB, and C t is the concentration of MB at time t.

Results and Discussion
The integration of TiO 2 and GOs may influence the adsorption band of TiO 2 because of the special electrochemical properties of GOs. The UV-vis absorption spectra in Figure 2 showed that the light absorption edge of TiO 2 -intercalated GOs extended to about 426 nm while that of TiO 2 was only about 394 nm. The redshifts into the visible regions demonstrated the significant influence of GOs on the optical characteristics in which the adding of GOs narrowed the bandgap of TiO 2 owing to the formation of Ti-O-C chemical bonding in their interface [39,40]. The FTIR spectra of graphite, GOs, and TiO 2 -intercalated GOs are shown in Figure 3. The FTIR spectra of the natural flake graphite show a very smooth curve, indicating its poor infrared activity. A broad absorption band, which is a characteristic of the hydroxyl groups in GOs, can be observed at approximately 3430 cm −1 . In particular, 3500-3800 cm −1 exhibited a wide range of absorption peaks, which can be attributed to the absorbed water by GOs [41]. In addition, due to the hydroxyl groups in GOs, the FTIR spectra exhibited an absorption band in the vicinity of 1634 cm −1 , corresponding to the absorption peak for the bending vibration of O-H [42]. Furthermore, the band at the vicinity of 1720 cm −1 is attributed to the absorption of carboxylic acid and/or the carbonyl stretching vibration of C=O, which is attributed by -COOH and C=O in the surface and/or edge and of the GOs. The band observed at 1380 cm −1 is the stretching vibration of the epoxy groups (C-O-C) and carboxyl C-O in GOs [43]. The band in the vicinity of 1045 cm −1 is generated by the C-OH stretching vibration of GOs. All these absorption bands derived from the oxygencontaining groups in GO nanosheets. This indicates that graphite was fully oxidized and it generated abundant hydrophilic oxygen-containing groups. Hydrogen bonds were readily formed with water molecules because the GO surfaces contain these polar functional groups, thereby explaining the hydrophilic nature of GOs after the oxidation. For TiO 2intercalated GOs, the absorption band near 3430 and 1634 cm −1 , representing the OH stretching and blending vibrations of GOs, respectively, weakened, whereas the C-O 3 Journal of Nanomaterials adsorption at approximately 1045 cm −1 broadened. The OH groups in GOs were likely to form Ti-O-C during the hydrothermal reaction [29], suggesting the possible chemical interactions between TiO 2 and GOs.
The photocatalytic capability of TiO 2 -intercalated GOs was derived from TiO 2 nanocrystals, which were mainly influenced by their crystal form, degree, and morphology. Figure 4 shows the XRD patterns of the natural flake graphite, GOs, and TiO 2 -intercalated GOs. The characteristic diffraction peak was located at 2θ = 30°for the natural flake graphite. The XRD pattern of GO showed diffraction peaks at 2θ = 10°, with lattice spacing of 0.887 nm. However, for  [44]. In addition, the XRD signals of TiO 2 nanocrystals were extremely strong that the GO signals were completely concealed, thereby suggesting the high content of wellcrystallized TiO 2 nanoparticles.
To examine the chemical composition and binding states of the samples, XPS spectra were collected, as displayed in Figure 5. All binding energies in the XPS experiments are trued by referring to the C1 (284.6 eV) peak. Figure 5(a) presents the wide-scan XPS spectra of GOs and TiO 2 -intercalated GOs. TiO 2 -intercalated GOs showed three strong peaks at 530.1, 454.8, and 284.1 eV, corresponding to O1s, Ti2p, and C1s [5,45], respectively. However, only O1 and C1 signals were detected for GOs, indicating that Ti was successfully combined to GO surfaces. Figure 5(b) shows the C1 XPS spectra of TiO 2 -intercalated GOs. The C1 XPS spectrum of TiO 2 -intercalated GOs could be deconvoluted into three peaks, which were ascribed to sp 2 bonded carbon (C-C, 284.6 eV), epoxy/hydroxyls (C-O, 286.5 eV), and carboxyl (O=C-O, 288 eV) in line with GOs before the reaction [46,47]. Therefore, GOs maintained their chemical construction after the hydrothermal reaction [48].
The distribution state of TiO 2 on/in the GO-GO surfaces and interlayers is significant for the catalytic activity of TiO 2 nanocrystals. The severe agglomeration of TiO 2 not only leads to low specific surface area but also to low contact efficiency of GO surfaces, thereby greatly influencing the electron-hole separation efficiency. The surface morphology and microscopic structure of the GOs and as-prepared TiO 2 -intercalated GOs are initially observed by SEM ( Figure 6). GOs were loosely stacked to form microsized layers ( Figure 5(a)). Such loose structure was mainly caused by strong electrostatic repulsion among GOs due to the negatively charged oxygen-containing functional groups such as -OH and -COOH in GOs. After the reaction, as shown in Figure 5(b), the as-prepared TiO 2 -intercalated GOs had abundant wrinkles and slight curling, indicating good flexibility. Most importantly, TiO 2 nanocrystals were almost uniformly monodispersed on the GO surfaces because these TiO 2 nanocrystals grow on OH, COOH-containing defect sites in the GO nanosheets, restricting their selfagglomeration. Such well-dispersed TiO 2 nanocrystals improved the photocatalytic efficiency of TiO 2 -intercalated GO nanosheets by facilitating the generation and separation of electron-hole pairs.
The TiO 2 nanocrystals were not only attached to the GO surfaces but also to their interlamination. As shown in Figure 7(b), dense TiO 2 nanocrystals were found on and/or in the GOs, thereby suggesting that TiO 2 has good affinity to GOs [49]. To analyze the detailed hierarchical structure, the GO edge was observed in detail by HRTEM. As shown in Figure 6(a), two GO nanosheets were clearly observed. The nanosheets consisted of 8-10 one-atom thick GOs (arrows 1 and 2 in Figure 7(a)). Such multilayered GO nanosheets preserved certain conductivity and rigidity after oxidation. Notably, TiO 2 nanocrystals were found not only on the outer surface of GOs but also in the interlamination. As shown in box 1, these TiO 2      5 Journal of Nanomaterials and GO nanosheets. As shown in the circle, TiO 2 nanocrystals were clamped by two GO nanosheets, evidencing the sandwich structure. Similarly, as shown in box 2, the TiO 2 nanocrystals covered by bottomed GOs proved that they are on the lower GO surfaces.

Photocatalytic Degradation of MB.
The photocatalytic activities of TiO 2 -intercalated GOs, TiO 2 -adsorbed GOs, GOs, and TiO 2 for MB were evaluated. As shown in Figure 8(a), GOs and TiO 2 showed relatively weak photocatalytic activity toward MB. For TiO 2 -adsorbed GOs, onlỹ 60% of MB was decomposed after 1 h UV radiation. On the contrary, the TiO 2 -intercalated GOs exhibited much higher photocatalytic degradation efficiency than other catalysts (TiO 2 -intercalated GOs > TiO 2 -adsorbed GOs > GOs > -TiO 2 ). They could degrade nearly 99.9% of MB within 18 min under the UV light which was much faster than reported literature [25,33,[50][51][52][53]. Because of in situ growth of TiO 2 nanocrystals by adhering to GO surfaces and/or GO-GO interfaces, the contact resistance between TiO 2 and GOs was significantly reduced. As a result, the photoelectron generated by TiO 2 could be efficiently led away by GOs, thereby greatly improving the separation of electron-hole pairs on TiO 2 surfaces. In addition, GO nanosheets could help TiO 2 to adsorb MB organic molecules due to their huge surface area and abundant anionic OH and COOH groups.
To study the effect of MB initial concentration on the photocatalytic degradation efficiency, the photocatalytic degradation experiments were conducted with MB concentrations that range from 100 mg/L to 300 mg/L, while maintaining other parameters constant. As shown in Figure 8  Journal of Nanomaterials calculated photodegradation efficiency of TiO 2 -intercalated GOs showed significant decrease with increase in MB concentration because of the incident of UV light depth which became shallower with increasing MB concentration, thereby greatly reducing the photodegradation efficiency in the low solution.
To study the effect of the bath ratio between the TiO 2intercalated GO catalyst and MB solution on the photocatalytic degradation performance, 0.5-2 g TiO 2 -intercalated GOs was added into 100 g of 100 mg/L MB solution, while maintaining the other parameters constant. As presented in Figure 8(c), although the photodegradation ratio of MB changed with increasing bath ratio, the corresponding photodegradation rate of TiO 2 -intercalated GOs toward MB increased. The nearly complete photodegradation time for MB stretched from 18 min to 23 min and then to 32 min. Thus, the increase in the mass of TiO 2 -intercalated GOs could improve the photodegradation efficiency toward MB.
We varied the light conditions (simulated sunlight and UV light) to test their influence on the photocatalytic degradation of MB. As shown in Figure 8(d), 99.99% of MB was degraded by TiO 2 -intercalated GOs for only 18 min under UV light, whereas only 99.89% degradation efficiency was obtained under 360 min sunlight irradiation. The degradation time under UV light is only 5% of that under simulated sunlight. The low degradation efficiency is caused by the UV light, which only accounts for a small fraction (3%-5%) of the sunlight, whereas the TiO 2 -intercalated GOs only showed photocatalytic degradation activity in the UV band. However, the TiO 2 -intercalated GOs could degrade 99.89% of MB under 360 min (<6 h) simulated sunlight irradiation, still indicating their good applicability. Figure 9 showed MB with and without UV radiation in the presence of TiO 2 -intercalated GOs. The color of MB with UV radiation soon became shallow and finally colorless, whereas that without UV radiation showed no obvious change after 20 min magnetic stirring in the dark. This indicates that physical adsorption by GOs was negligible during short UV irradiation probably because a number of OH and COOH groups reacted with TiO 2 to form Ti-O-C groups. The high treatment efficiency for MB should mainly attribute to the high photodegradation of TiO 2 -intercalated GOs.
To further demonstrate the decomposition durability, the circulating runs of the TiO 2 -intercalated GO catalyst in the photocatalytic degradation of MB were carried out. After the complete photodegradation of MB every time, TiO 2 -intercalated GO catalyst was collected, washed, dried at 60°C, and used for the next photodegradation cycle. As shown in Figure 10, the TiO 2 -intercalated GO catalyst did not exhibit evident significant loss of photocatalytic activity after four runs of MB degradation. The photodegradation rate of MB reached up to 99.98%, 99.98%, 99.99%, and 99.97% after two to four times of circulation. The relative degradation time was 24, 32, 41, and 70 min. Nevertheless, after five times of circulation photodegradation, the TiO 2 -intercalated GOs obtained only 55.47% photodegradation rate even after 330 min UV light irradiation. The results indicated that the optimal cycle time of TiO 2 -intercalated GOs was four times. To examine the possible damage for the chemical structure of TiO 2 -intercalated GOs during prolonged exposure to UV light, FTIR spectra of TiO 2 -intercalated GOs before and after six-cycle UV irradiation were measured ( Figure 11). The adsorption intensity of OH groups for TiO 2 -intercalated GOs slightly weakened after six time cycles while the chemical construction remained unchanged, indicating the high chemical stability. The decrease in hydroxyl, small molecular contamination, and interlayer blocking may be responsible for the weakened photocatalytic performance after five time cycles.

Conclusion
In summary, we demonstrate a synthesis strategy of sandwich-like TiO 2 -intercalated GO nanosheets to improve the contact area between TiO 2 and GOs. Our FIIR, XPS, and XRD measurements showed that the as-prepared TiO 2 -intercalated GOs maintained the inherent chemical construction of GOs and generated high content and high   Journal of Nanomaterials crystallized TiO 2 nanoparticles after the hydrothermal reaction. The SEM and TEM results indicated that TiO 2 nanocrystals were uniformly dispersed on and in GO nanosheets, forming an independent sandwich-like structure. Notably, GOs consisted of 8-10 layered one-atom thick GOs. Such structure was beneficial to maintain certain electric conductivity of GOs. In addition, the photocatalytic activities of the as-prepared catalyst were further studied using methylene blue as a degradation sample. The results indicated that the catalytic activity of the TiO 2 -intercalated GOs was much stronger than pure TiO 2 and TiO 2 -adsorbed GOs. Moreover, decreasing the initial concentration of MB and increasing the bath ratio of TiO 2 -intercalated GOs could increase the degradation efficiency of MB. The contrast tests further indicated that the degradation of MB by UV light was much faster than that by simulated sunlight. The final cyclic photodegradation test showed that the TiO 2 -intercalated GOs maintained 99.97% degradation activity after four times of repeated degradation, indicating good decomposition durability.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no competing interests.   Journal of Nanomaterials