Fabrication of TiO2 Nanosheet Aarrays/Graphene/Cu2O Composite Structure for Enhanced Photocatalytic Activities

TiO2 NSAs/graphene/Cu2O was fabricated on the carbon fiber to use as photocastalysts by coating Cu2O on the graphene (G) decorated TiO2 nanosheet arrays (NSAs). The research focus on constructing the composite structure and investigating the reason to enhance the photocatalytic ability. The morphological, structural, and photocatalytic properties of the as-synthesized products were characterized. The experimental results indicate that the better photocatalytic performance is ascribed to the following reasons. First, the TiO2 NSAs/graphene/Cu2O composite structure fabricated on the carbon cloth can form a 3D structure which can provide a higher specific surface area and enhance the light absorption. Second, the graphene as an electron sink can accept the photoelectrons from the photoexcited Cu2O which will reduce the recombination. Third, the TiO2 nanosheet can provide more favorable carrier transportation channel which can reduce the recombination of carriers. Finally, the Cu2O can extend the light absorption range.


Background
Recently, application of semiconductor photocatalyst titanium dioxide (TiO 2 ) in environmental purification has attracted great attention owing to its tremendous advantages, such as stability, nontoxicity, and low cost [1][2][3]. Based on the fact that photocatalytic reactions mainly take place on the surfaces of the photocatalysts, the morphology is the crucial factor to determine the efficiency [4]. Up to now, many efforts have been made to fabricate TiO 2 nanocrystals, nanowires, nanorods, and nanotubes [5][6][7][8]. However, the application of TiO 2 nanosheet arrays with larger specific surface area in photocatalytic degradation is rarely reported. Especially, TiO 2 nanosheet arrays grown on the carbon cloth can construct three-dimensional (3D) structures to improve the specific surface area.
Unfortunately, the wide bandgap of TiO 2 limits the effective absorption of visible light [9]. In order to overcome the shortcoming, one strategy is to modify TiO 2 with narrow bandgap semiconductors [10][11][12]. Among them, cuprous oxide (Cu 2 O) with narrow bandgap can be a promising candidate for expanding the absorption spectra range. [13][14][15][16][17][18] Moreover, the built-in electric field between P-type Cu 2 O and N-type TiO 2 can accelerate the separation of carriers.
However, the poor interfacial feature between different semiconductors directly influences the separation efficiency of carriers. Therefore, the interfacial optimization is an effective way to enhance photocatalytic degradation efficiency [19,20]. The previous researches indicate that graphene (G) shows excellent interfacial optimization function between different semiconductors due to its high conductivity and two-dimensional structure, which facilitates the interfacial contact and carrier transportation [21][22][23][24][25]. However, it is difficult to make graphene well disperse between different semiconductors. In this paper, a modified method is applied to fabricate the homogeneously dispersed G between TiO 2 nanosheet and Cu 2 O.
In this study, the composite structure of TiO 2 NSAs/ G/Cu 2 O has been prepared (Fig. 1). The significant enhancement of photocatalytic activities was observed, and the corresponding results were analyzed. The proposed mechanism of the photocatalytic degradation was also discussed. As we know, there are no related reports on TiO 2 NSAs/G/Cu 2 O photocatalysts up to now, so and thus it will be a meaningful reference for designing and fabricating this kind of photocatalyst used in photocatalytic degradation.

Preparation of TiO 2 NSAs/G/Cu 2 O
The fabrication of TiO 2 NSAs is the following. TiO 2 sol was prepared using a previously reported method [10]. In brief, TiO 2 seed layer was deposited on carbon cloth (2 cm × 3 cm) by immersing in TiO 2 sol for 10 min. Then, the seed layer was calcined at 400°C for 1 h. The Teflon-lined stainless steel autoclave (100 mL in volume) filled with 40 mL of aqueous solution of 10 M NaOH and 0.2 g of activated carbon was placed in an oven at 180°C for 24 h. After the autoclave cooled down to room temperature, the prepare samples were rinsed with DI water to remove the residual activated carbon, followed by soaking with 0.1 M hydrochloric acid for 1 h, then washed to neutral with DI water.
For the composite structure of TiO 2 NSAs/G, 0.2 g of graphene replacing activated carbon was added into the Teflon-lined stainless steel autoclave ethanol solution.
Cu 2 O layer was deposited by the following procedures. 2.3 mmol of Cu(CH 3 COO) 2 and 2.3 mmol of CH 3 CONH 2 were dissolved into 100 mL of diethylene glycol (DEG) under ultrasonication to prepare the reaction solution. Then, TiO 2 NSAs or TiO 2 NSAs/G substrate was immersed into the solution. Subsequently, it was heated to 120°under magnetic stirring and kept at this temperature for 6 h. After cooling down to room temperature in air, TiO 2 or TiO 2 NSAs/G substrate coated with Cu 2 O was washed with absolute ethanol and DI water for five times in sequence and dried in air.

Characterization
The morphologies of the samples were investigated by a field emission scanning electron microscopy (FE-SEM, Quanta FEG250). The crystal structure of samples was examined by X-ray diffraction (XRD, D8 Advance) with Cu K α at λ = 0.15406 nm radiation. XPS spectra were recorded on a Thermo Fisher ESCALAB 250Xi system with Al Kα radiation, operated at 150 W. The absorption spectrum of the samples was measured using a UV-vis spectrophotometer (TU-1901). The Raman spectrum of the sample was characterized by Raman spectroscopy (LabRAM HR800).

Photoelectrochemical Measurement
Photocurrent density was measured using an electrochemical workstation (CS2350) in a three-electrode electrochemical cell with 1 M Na 2 SO 4 as the electrolyte, in which the as-prepared samples were acted as the working electrode, Pt and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The I-t curve was recorded under Xe lamp (153 mW/cm 2 ) irradiation.

Measurement of Photocatalytic Activity
The photocatalytic activity was evaluated toward the photodegradation of RhB. A 500 W Xe lamp was used as the light source. The samples with same size 2 cm × 3 cm were placed into 20 ml RhB solution (10 mg/L). After irradiation for a designated time (30 min), 3 mL of RhB solution was taken out to identify the concentration of RhB using UV-vis spectrophotometer (TU-1901). All of these measurements were carried out at room temperature.  surface of the nanosheets. Unfortunately, G cannot be directly observed in Fig. 1d, e, though it could be confirmed by XPS measurement.

Results and Discussion
XRD patterns were performed to investigate the crystal phase structure of photocatalyst. The XRD patterns of TiO 2 NSAs, TiO 2 NSAs/G, and TiO 2 NSAs/G/Cu 2 O are shown in Fig. 3. For TiO 2 NSAs (curve a), five distinctive peaks match well with TiO 2 . In curve b (for TiO 2 NSAs/G), there are the same diffraction peaks with curve a. The phase on G is not detected because the content is low. From the curve c (TiO 2 NSAs/G/Cu 2 O), the diffraction peaks on Cu 2 O are observed.
X-ray photoelectron spectroscopy (XPS) was used to confirm the existence of G. The XPS survey spectrum of TiO 2 NSAs/G composite shows the elements C, O, and Ti ( Fig. 4a-c). The presence of these elements proves the method to fabricate TiO 2 NSAs/G composite is feasible. XPS spectrum of C1s located at 284.77 eV can be corresponded to carbon-containing species on the surface, which is the dominant existence form (Fig. 4a). Moreover, the 288.37 eV indicates the tiny existence of C-O bond. The peak located at 529.78 eV is related to the oxygen bonded with metal as Ti-O, and the 534.98 eV is the adsorbed oxygen or hydroxyl species (Fig. 4b). It can be seen that the spectra of catalysis showed two peaks at 458.38 and 464.03 eV. These peaks can be assigned to 3d5/2 and 3d3/2 spin orbital components of Ti 4+ species (Fig. 4c) [26,27]. In order to confirm the existence of graphene, furtherly, the Raman spectrum was characterized. The typical Raman spectra of TiO 2 NSAs/G are shown in Fig. 4d. There are two typical Raman peaks corresponding to the typical D band and G band of graphene, respectively.
The photocatalytic properties of the as-obtained samples were investigated by decomposition of RhB (Fig. 5a). Before irradiation started, the system was kept in dark for 1 h to reach the adsorption-desorption equilibrium. There is almost no change in the concentration of the solution when RhB solution is irradiated without any catalysts. After 180 min, the degradation ratio of RhB was almost 80% in the presence of TiO 2 NSAs/G/Cu 2 O, whereas 50 and 40% of RhB was decomposed by TiO 2 NSAs and TiO 2 NSAs/ Cu 2 O, respectively. All of these measurements show that TiO 2 NSAs/G/Cu 2 O exhibits more prominent photocatalytic activity compared with other samples. A very important reason for the advantage of TiO 2 NSAs/Cu 2 O on the photodegradation of RhB is that Cu 2 O plays a significant role in extending light absorption spectrum. In addition, TiO 2 NSAs/G/Cu 2 O exhibits a better photocatalytic activity than TiO 2 NSAs/Cu 2 O which results from the presence of graphene. The graphene as an electron sink to accept the photoelectrons from the photoexcited Cu 2 O will reduce the recombination of photoelectron-hole pairs, resulting in a higher photocatalytic activity. The stability of the TiO 2 NSAs/G/Cu 2 O was carried out, and the results (Fig. 5b) show that TiO 2 NSAs/G/Cu 2 O has good stability. To further understand the improvement of photocatalytic activity, the I-t response of TiO 2 NSAs, TiO 2 NSAs /Cu 2 O, and TiO 2 NSAs/G/Cu 2 O were observed, as shown in Fig. 7. It can be found that TiO 2 NSAs/G/ Cu 2 O exhibits enhanced photocurrents compared with TiO 2 NSAs and TiO 2 NSAs/Cu 2 O. The higher photocurrent density of TiO 2 NSAs/G/Cu 2 O indicates an enhanced light absorption and higher separation efficiency of photogenerated electrons and holes.
The improved photocatalytic property of TiO 2 NSAs/G/ Cu 2 O may be attributed to the following factors. First, the introduction of Cu 2 O can extend the light absorption range, and thus, the photocatalytic activities are enhanced. Second, the limitation of carrier recombination is a key factor to enhance the photocatalytic property. The graphene as an electron sink can accept the photoelectrons from the photoexcited Cu 2 O which will reduce the recombination. Besides, TiO 2 nanosheet structure can provide more favorable carrier transportation channel. Third, the better photocatalytic property can take advantage of large specific surface area. TiO 2 NSAs/G/Cu 2 O structure fabricated on the carbon cloth can form a 3D structure which can provide a higher specific surface area. The high surface area of the 3D structure allows not only more surfaces to be reached by the incident light but also more sites on the surface for the adsorption and photodegradation of RhB, which results in enhanced photocatalytic performance. Finally, the 3D structure can enhance the photon utilization efficiency. The structure allows a great number of the photons to penetrate deep inside the photocatalyst, and most photons are trapped within the 3D structure until being completely absorbed.
The corresponding mechanism of electron transfer has been illustrated in Fig. 8

Conclusions
In summary, the novel 3D TiO 2 NSAs/G/Cu 2 O structure is prepared via a simple and efficient method. Importantly, the composite structure exhibits excellent photocatalytic degradation properties. The enhanced performance can be ascribed to its extended light absorption range, large specific surface area, enhanced photon utilization efficiency, improved charge transfer efficiency and suppressed photoelectron-hole recombination. Furthermore, the photocatalysts grown on carbon cloths make the collection and recycle of photocatalysts much easier.