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 (TiO2) in environmental purification has attracted great attention owing to its tremendous advantages, such as stability, nontoxicity, and low cost [1-3]. 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 TiO2 nanocrystals, nanowires, nanorods, and nanotubes [5-8]. However, the application of TiO2 nanosheet arrays with larger specific surface area in photocatalytic degradation is rarely reported. Especially, TiO2 nanosheet arrays grown on the carbon cloth can construct three-dimensional (3D) structures to improve the specific surface area.

Unfortunately, the wide bandgap of TiO2 limits the effective absorption of visible light [9]. In order to overcome the shortcoming, one strategy is to modify TiO2 with narrow bandgap semiconductors [10-12]. Among them, cuprous oxide (Cu2O) with narrow bandgap can be a promising candidate for expanding the absorption spectra range. [13-18] Moreover, the built-in electric field between P-type Cu2O and N-type TiO2 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 [21-25]. 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 TiO2 nanosheet and Cu2O.

In this study, the composite structure of TiO2 NSAs/G/Cu2O 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 TiO2 NSAs/G/Cu2O 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.

Fig. 1

Schematic illustration for the preparation of TiO2 NSAs/G/Cu2O

Methods

Preparation of TiO2 NSAs/G/Cu2O

The fabrication of TiO2 NSAs is the following. TiO2 sol was prepared using a previously reported method [10]. In brief, TiO2 seed layer was deposited on carbon cloth (2 cm × 3 cm) by immersing in TiO2 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 TiO2 NSAs/G, 0.2 g of graphene replacing activated carbon was added into the Teflon-lined stainless steel autoclave ethanol solution.

Cu2O layer was deposited by the following procedures. 2.3 mmol of Cu(CH3COO)2 and 2.3 mmol of CH3CONH2 were dissolved into 100 mL of diethylene glycol (DEG) under ultrasonication to prepare the reaction solution. Then, TiO2 NSAs or TiO2 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, TiO2 or TiO2 NSAs/G substrate coated with Cu2O 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 Na2SO4 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/cm2) 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.

Results and Discussion

Figure 2a–c, the obtained TiO2 NSAs characterized by FE-SEM, shows that the uniform TiO2 NSAs with about 40–60 nm in width and 1.5 μm in height vertically grew on the surface of carbon cloth. These results illustrate that the morphology in favor of the enhancement of photocatalytic performance. As shown in Fig. 2d–f, Cu2O particles have been successfully deposited on the surface of the nanosheets. Unfortunately, G cannot be directly observed in Fig. 1d, e, though it could be confirmed by XPS measurement.

Fig. 2

SEM images of a–c TiO2 NSAs with different magnification, d–f TiO2 NSAs/G/Cu2O with different magnifications

XRD patterns were performed to investigate the crystal phase structure of photocatalyst. The XRD patterns of TiO2 NSAs, TiO2 NSAs/G, and TiO2 NSAs/G/Cu2O are shown in Fig. 3. For TiO2 NSAs (curve a), five distinctive peaks match well with TiO2. In curve b (for TiO2 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 (TiO2 NSAs/G/Cu2O), the diffraction peaks on Cu2O are observed.

Fig. 3

XRD patterns of a TiO2 NSAs, b TiO2 NSAs/G, and cTiO2 NSAs/G/Cu2O

X-ray photoelectron spectroscopy (XPS) was used to confirm the existence of G. The XPS survey spectrum of TiO2 NSAs/G composite shows the elements C, O, and Ti (Fig. 4a–c). The presence of these elements proves the method to fabricate TiO2 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 Ti4+ species (Fig. 4c) [26, 27]. In order to confirm the existence of graphene, furtherly, the Raman spectrum was characterized. The typical Raman spectra of TiO2 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.

Fig. 4

XPS spectra of a C 1s, b O 1s, c Ti 2p of TiO2 NSAs/G, and d Raman spectra of TiO2 NSAs/G

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 TiO2 NSAs/G/Cu2O, whereas 50 and 40% of RhB was decomposed by TiO2 NSAs and TiO2 NSAs/ Cu2O, respectively. All of these measurements show that TiO2 NSAs/G/Cu2O exhibits more prominent photocatalytic activity compared with other samples. A very important reason for the advantage of TiO2 NSAs/Cu2O on the photodegradation of RhB is that Cu2O plays a significant role in extending light absorption spectrum. In addition, TiO2 NSAs/G/Cu2O exhibits a better photocatalytic activity than TiO2 NSAs/Cu2O which results from the presence of graphene. The graphene as an electron sink to accept the photoelectrons from the photoexcited Cu2O will reduce the recombination of photoelectron-hole pairs, resulting in a higher photocatalytic activity. The stability of the TiO2 NSAs/G/Cu2O was carried out, and the results (Fig. 5b) show that TiO2 NSAs/G/Cu2O has good stability.

Fig. 5

a Photocatalytic degradation of RhB in the presence of various catalysts. b Recycle of TiO2 NSAs/G/Cu2O under Xe lamp irradiation

Generally, responding ability to light is one of the most important factors for evaluating photocatalytic performance. Therefore, UV–vis absorption spectra of samples were characterized as shown in Fig. 6. The absorption of TiO2 NSAs is located in the UV region. Compared with TiO2 NSAs, the absorption edge of TiO2 NSAs/Cu2O shows redshift. This larger absorption would result in the improvement of the photocatalytic property of TiO2 NSAs/Cu2O.

Fig. 6

UV–vis absorption spectra of TiO2 NSAs and TiO2 NSAs/Cu2O

To further understand the improvement of photocatalytic activity, the I–t response of TiO2 NSAs, TiO2 NSAs /Cu2O, and TiO2 NSAs/G/Cu2O were observed, as shown in Fig. 7. It can be found that TiO2 NSAs/G/Cu2O exhibits enhanced photocurrents compared with TiO2 NSAs and TiO2 NSAs/Cu2O. The higher photocurrent density of TiO2 NSAs/G/Cu2O indicates an enhanced light absorption and higher separation efficiency of photogenerated electrons and holes.

Fig. 7

Photoinduced I–t curves of different samples

The improved photocatalytic property of TiO2 NSAs/G/Cu2O may be attributed to the following factors. First, the introduction of Cu2O 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 Cu2O which will reduce the recombination. Besides, TiO2 nanosheet structure can provide more favorable carrier transportation channel. Third, the better photocatalytic property can take advantage of large specific surface area. TiO2 NSAs/G/Cu2O 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. Both TiO2 and Cu2O can be photoexcited under the light irradiation. Because the E VB of TiO2 is more positive than that of Cu2O, holes in the VB of TiO2 can migrate to the VB of Cu2O by the interface. Similarly, the E CB of Cu2O is higher than that of TiO2, so the electrons in the CB of Cu2O can transfer to the CB of TiO2. More importantly, graphene as electronic exchange medium can promote electron transfer ability between Cu2O and TiO2.

Fig. 8

The possible photocatalytic mechanisms of TiO2 NSAs/G/Cu2O

Conclusions

In summary, the novel 3D TiO2 NSAs/G/Cu2O 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.