Photocatalytic performance of Cu2O-loaded TiO2/rGO nanoheterojunctions obtained by UV reduction

A novel dot-like Cu2O-loaded TiO2/reduced graphene oxide (rGO) nanoheterojunction was synthesized via UV light reduction for the first time. Cu2O with size of ca. 5 nm was deposited on rGO sheet and TiO2 nanosheets. The products were characterized by infrared spectroscopy, Raman spectrum, UV–Vis diffuse reflectance spectra, XPS techniques, photoluminescence spectra. The results demonstrated that Cu2O and rGO enhanced the absorption for solar light, separation efficiency of electron–hole pairs, charge shuttle and transfer, and eventually improved photoelectrochemical and photocatalytic performance for contaminants degradation. The reaction time and anion precursor could affect the final copper-containing phase. As extending UV irradiation time, Cu2+ was be first reduced to Cu2O and then transformed to metal Cu. In comparison with CH3COO− (copper acetate), NO3 − (copper nitrate) and Cl− (copper chloride), SO4 2− (copper sulfate) was the optimum for synthesizing pure Cu2O phase. Electronic supplementary material The online version of this article (doi:10.1007/s10853-017-0911-2) contains supplementary material, which is available to authorized users.


Introduction
Exploration of an optimum semiconductor nanoheterojunction architecture for enhanced photoelectrochemical properties had been developed with great efforts for years [1][2][3][4][5]. Varied architectures, such as bulk crystal/bulk crystal, core/shell, bulk crystal/dotted crystal et al., had been intensively studied [3,6,7]. Architecture of bulk crystal/dotted crystal was similar with a component of dye-sensitized or semiconductor quantum dot-sensitized TiO 2 in solar cell, owning high photoelectrochemical performance [8,9]. For this architecture, dotted crystal with special structure and size had a tunable contact area on the surface of matrix [4,10,11]. TiO 2 nanosheets exposing (001) facet, which had excellent photocatalytic performance, made itself a stable sub-the absorption of sun light. Loading dot-like semiconductor with response of visible light on TiO 2 nanosheets might be an optimum architecture.
Cuprous oxide (Cu 2 O) was a relative stable p-type semiconductor with direct band gap of 2.0-2.2 eV which could absorb visible light below 600 nm [12,13]. In addition, its conduction and valence band positions matched well with those of n-type TiO 2 , which facilitated separation of photo-induced electron-hole pairs [13][14][15]. However, TiO 2 nanosheet/dot-like Cu 2 O crystal heterojunction still had poor electron conductivity [16]. Reduced graphene oxide (rGO) owning graphitic sp 2 and sp 3 -hybrid structures had comparable conductivity of metal and large surface area as a substrate for building heterojunctions [17,18]. It is reported that particles of TiO 2 or Cu 2 O combining with rGO had enhanced charge shuttle and transfer performance [13,19,20]. So the dot-like Cu 2 O-loaded TiO 2 /rGO nanoheterojunction might become one of the most efficient TiO 2 -based photocatalysts.
General method for loading dot-like Cu 2 O crystal on the TiO 2 or rGO is reduction of various cupric salts with strong chemical reagents in alkaline condition at high temperature [14,21,22]. For example, Wang or Geng et al. [14,23] synthesized nanocrystalline Cu 2 O on TiO 2 frame or arrays using cupric acetate as precursor and glucose as reducing reagent. Gao et al. [24] loaded Cu 2 O particle on rGO sheet using L-ascorbic acid as reductive reagent in mild condition. Compared to chemical liquid reduction, photochemical synthesis of Cu 2 O had advantages of free chemical reagents addition, room temperature, atmospheric pressure, free of pH adjustment via alkali or acid. In previous reports, Cu 2 O was synthesized via c-ray radiation [25,26]. However, c-ray radiated by 60 Co source is very environmental unfriendly, harmful and strictly restricted by laws.
In this work, c-ray was alternated by a ultraviolent (UV) light (main peak 254 nm, 25 W), and dot-like Cu 2 O crystal with size of ca. 5 nm was successfully deposited on TiO 2 nanosheet/rGO. To our knowledge, this has never been reported before. The results revealed that the newly designed nanoheterojunction had strong absorption of solar light, high separation efficiency of electron-hole pairs and high performance of charge shuttle and transfer. Surfactants such as sodium dodecyl benzene sulfonate (SDBS) existing in cleaning agents, dyes such as methyl orange (MO), rhodamine B (RhB) as the aromaticcontaining macromolecules existing in waste water were selected to evaluate its photocatalytic activity [27].
More importantly, various cupric salts with different anions such as SO 4 2-, Cl -, CH 3 COO -, NO 3 were employed to synthesize Cu 2 O in previous works [14,[28][29][30]. In this report, taken different stabilities, chemical activities and chelating ability with positive ion into consideration, these cupric salts as precursors were studied to explore the synthetic mechanism under photochemical condition.

Experiment Materials
Natural graphite was purchased from Qingdao Baichun graphitic Co., Ltd. Fluorine tin oxide (FTO)coated glass (resistivity \10 X sq -1 ) was purchased from Zhuhai Kaivo Electronic Components Co., Ltd. The other chemical reagents were purchased from Sinopharm chemical reagent Co., Ltd. And all the chemicals were used without further purification.

Synthesis of graphite oxide
Graphite oxide was synthesized by the typical modified Hummers' method [31]. In details, 2 g of natural graphite flakes was mixed with 1 g sodium nitrate in the ice bath. Several other cupric salts [such as CuCl 2 , Cu(CH 3-COO) 2 , Cu(NO 3 ) 2 ] were employed to substitute CuSO 4 to obtain final products labeled as TGC-Cl2, TGC-A, TGC-N (6 h UV light irradiation).

Photoelectrochemical performance
The photoelectrochemical measurement was performed by a CHI 760E electrochemical workstation (Shanghai CH instrument Co., Ltd, China), with Pt plate as counter electrode, Ag/AgCl (filled with 3.5 M KCl aqueous solution) as reference electrode and 0.2 M Na 2 SO 4 aqueous solution as electrolyte. The working electrode was prepared as follows: 10 mg product powder was mixed with 22 lL PVDF poly(vinylidene fluoride) solution, and that PVDF was dissolved in N-methyl-2-pyrrolidone (wt% 5%) with weight ratio of 90:10 to make slurry. The film was made by doctor blade method on FTO for area of 191 cm 2 , then vacuum-dried at 100°C for 12 h. The uncovered area of FTO which would be immersed in electrolyte was protected by insulting glue.

Photocatalytic performance
The photocatalytic performance was measured by photodegradation of MO, RhB and SDBS. In a typical process, 20 mg of photocatalysts and 100 mL MO/ SDBS/RhB solution (20 mg L -1 ) were sonicated for 10 min to obtain homogeneous suspension. Before light irradiation, the suspension was stirred for 0.5 h in dark to achieve adsorption and desorption equilibrium. Then, 5 mL of the solution was extracted every 0.5 h for UV-Vis absorption measurement. The photoreaction was carried out in the protection of cycling cool water. The light source is 350 W Xenon lamp to simulate solar light (range of spectrum is from 200 to 2500 nm).

Characterization
Powder X-ray diffraction (XRD) was performed on DX-2700 X-ray diffractometer (Dandong Fangyuan, China) with monochromatized Cu-Ka radiation (k = 1.5418 Å ) at 40 kV and 30 mA. Transmission electron microscopy (TEM) images were taken with JEOL JEM-2100 transmission electron microscope at 200 kV. The concentration of MO was analyzed by measuring the light absorption at 484 nm UV-Vis 756PC Spectrophotometer (Shanghai Spectrum Instruments Co., Ltd. China). Fourier transform infrared (FTIR) spectra were obtained using BRUKER Tensor II spectrometer in the frequency range of 4000-400 cm -1 with a resolution of 4 cm -1 . Measurement of Raman spectra was performed on a Raman DXR Microscope (Thermo Fisher, USA) with excitation laser beam wavelength of 532 nm. PL spectrum was measured at room temperature on a 7-PLSpec fluorescence spectrophotometer (Saifan, China). The wavelength of the excitation light is 325 nm. Optical absorption spectra were recorded on a UV-Vis spectrometer (UV-2600, Shimadzu, Japan) over a spectral range of 200-1400 nm. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) with Al Ka (hv = 1486.6 eV) radiation and beam spot of 500 lm was operated at 150 W. The Brunauer-Emmett-Teller (BET) surface areas were characterized by a surface area analyzer (Micromeritics, ASAP2020 M, USA) with nitrogen adsorption at 77 K.

Results and discussion
Characterization of phase and morphology The XRD peaks of crystalline Cu 2 O were observed in Fig. S2a and 1 for TC and TGC, which indicated Cu 2 O (PDF#05-0667) could be synthesized under UV radiation directly without assistance of any chemical reagent at room temperature. Peaks of crystallized anatase TiO 2 (PDF#21-1272) were observed in the TC and TGC samples. Graphene oxide (GO) fabricated by Hummers' method in aqueous solution was reduced to rGO under the UV irradiation [31,33,34], which was also demonstrated by IR spectrum shown in Fig. 3a. However, no GO or rGO peak was found since ordered stacking of rGO sheets had been disrupted by loading TiO 2 nanosheets and Cu 2 O [33].
TiO 2 nanosheets were prepared by the classical hydrothermal method [32], which had rectangular shape with the length of ca. 50 nm and thickness of 5 nm, as shown in Fig. S1. As Cu 2 O composited with TiO 2 forming sample TC, large amounts of Cu 2 O nanocrystals were deposited on TiO 2 nanosheets and even self-aggregated because of large quantities, as shown in Fig. 2a. After further compositing with rGO, it is observed that large amounts of Cu 2 O nanocrystal with size of ca. 5 nm adhered on TiO 2 nanosheets and rGO sheet in Fig. 2b, c, which was ascribed to residual oxygen-containing groups of rGO facilitating dispersion of Cu ? . This new morphology was achieved only via UV irradiation without addition of any chemical reducing reagent, so this work provides a novel way for synthesizing nano-Cu 2 O and its composites.

Characterization of IR and Raman spectrum
Chemical bond and phases of composites were characterized by FRIT and Raman spectrum (Fig. 3). In Fig. 3a, around 3420 cm -1 corresponded to the O-H stretching vibration of alcoholic or phenolic groups as well as intercalated or adsorbed water molecular for all samples [35][36][37][38]. The peak of 1631 cm -1 was attributed to bending mode of surface -OH or water for TiO 2 , TC [39]. The broadband around 560 cm -1 between 880 and 400 cm -1 showed the vibration of Ti-O-Ti bonds of sample TiO 2 , TC, TGC [40]. The sharp peak of 623 cm -1 was attributed to the stretching of copper(I)-O bond in TC and TGC, which indicated the formation of Cu 2 O [41]. In comparison with stretching modes of carbonyl (C=O) bond (1724 cm -1 ), conjugation absorption for bending mode of water and C=C sp 2 hybrid(1622 cm -1 ), tertiary alcohol (C-OH) bending (1375) and alkoxy (C-O) vibrations (1060) in IR spectrum of GO, most oxygen-containing groups of rGO in TGC were removed by the UV reduction [42,43], its absorption peak of C=C bond was shifted to 1578 cm -1 . The 1240 cm -1 peak should be attributed to stretching modes of the epoxy (C-O-C) group that can hardly removed by UV irradiation [44].
In Fig. 3b, four strong vibration peaks at 143 cm -1 (E g ), 393 cm -1 (B 1g ), 512 cm -1 (A 1g ) and 615 cm -1 (E 1g ) were ascribed to the five Raman active modes (A 1g ? B 1g ? 3E g ) of anatase [45]. The peaks around 120-180 cm -1 for TC and TGC were decomposed to the sharp peak at 152.6 cm -1 , which should be mainly attributed to the C 15 (1) (LO) infrared (ir)-allowed mode in perfect Cu 2 O crystals and a small peak for E g mode of anatase as shown in Fig. 3c [46]. The peak at 505 and 621 cm -1 should be assigned to the overlapping of Raman vibration mode of crystalline Cu 2 O and TiO 2 [45,47]. The peak at 394 cm -1 was the B 1g mode of anatase TiO 2 for TC and TGC. Phase determination of Raman spectra agreed with the results of XRD (Fig. 1). D band provided information about defect of graphitic structure and the presence of sp 3 -hybridized domain [17]. G band was a prominent feature of the pristine graphite, corresponding to the first-order scattering of the E 2g mode [48]. In Fig. 3d, the position of D and G band of GO was about 1357 and 1586 cm -1 , respectively. After reduction, the D and G band for rGO of TGC shifted to 1350, 1592 cm -1 , respectively, and I D /I G increased from 0.91 to 1.13 after UV reduction of GO, which indicated a decrease in average size and increase in numbers of the sp 2 -hybridized domains of rGO in TGC, comparing with that of GO [48,49]. Figure 4a demonstrates the full spectrum of sample TGC, while Fig. 4b-d focuses on the specific binding energy of element Ti, Cu and C, respectively. The Ti 2p peaks located at the binding energies of 459.0 and 464.8 eV were attributed to Ti 2p 3/2 and Ti 2p 1/2 , which corresponded to Ti 4? [50]. In Fig. 4c, the peak for Cu 2p 3/2 was decomposed into two peaks, the main peak of which at 932.6 eV was the characteristic of Cu ? in Cu 2 O [28], and the peak at 934.1 eV indicated the existence of Cu 2? in CuO [51]. XPS could only detect the shallow surface elements composition, so the observation of Cu 2? indicated the oxidation of a small portion of Cu 2 O during sample drying and handing under normal ambient condition. This phenomenon had been reported by many researchers on the synthesis of Cu 2 O nanoparticles. From C 1s XPS spectrum of GO (Fig. 4d), the peaks at 282.8, 286.7, 288.5 eV were assigned to the sp 2 -hybrid bond (C-C, C=C, C-H), C-O and O-C=O bond, respectively [52]. After UV reduction, a large amount of C-O and O-C=C bonds were removed as indicated by the decrease in intensities of these two peaks, which was consistent with results of IR spectra.

Selective adsorption and photocatalytic performance
Photocatalytic performances of samples were characterized by degradation of MO, SDBS and RhB (Fig. 5a-c). All experiments were carried out in nearly neutral solution, shown in  contaminations, which indicated that only some special contaminations could be decomposed by nanoheterojunction effectively. The adsorption of photocatalysts for contaminations played an important role in the process of photocatalytic reactions. Contaminants owning opposite charge with the photocatalysts could easily adsorb on the surface of photocatalysts preferentially. However, given that the charge of MO/SDBS/RhB (molecule structures are shown in Fig. 5e) in aqueous solution and adsorption abilities of the synthesized photocatalysts, the reverse charge principle was not perfect for explaining adsorption difference. Taken some special groups of contaminants into consideration, nitrogen-containing groups may also affect the eventual adsorption outcome.
Chemical stability of Cu 2 O particles is one of predominant factors for photochemical applications. Exposed to UV light (irradiated by varied lamp), Cu 2 O can be reduced to metal Cu [53,54]. However, in this work under standard simulated solar light, Cu 2 O was very stable and no metal copper was found even after 12 h with the characterization of X-ray diffraction (Fig. 5f).

Photocatalytic mechanism
As shown in Fig. 6a, TiO 2 could only absorb UV light below 387 nm. Cu 2 O enhanced absorption of visible light below 640 nm in TC. Moreover, with incorporation of rGO, TGC had highest light absorbance, so it induced more photo-induced charge carriers and higher degradation rate. Optical band gap of composite was calculated using Tauc plot shown in Fig. S3d [55]. Comparing with pure TiO 2 (3.2 eV), optical band gap of TC and TGC decreased to 2.72 and 2.64 eV, respectively.
PL spectrum was employed to characterize separation efficiency of photo-generated electron-hole pairs. As shown in Fig. 6b, several peaks such as 400, 434, 470, 544 nm were observed in PL spectrum of TiO 2 , which attributed to electron transition from the conduction band to valence band, band-edge free excitons, oxygen vacancies or surface defect [56,57]. TC had lower intensity of PL than pure TiO 2 that inferred Cu 2 O could accept the photocharge from TiO 2 . Furthermore, lowest intensity of TGC demonstrated that rGO could further accept the photo-induced electron and enhance separation efficiency of electron-hole pairs. TiO 2 electrode in Na 2 SO 4 aqueous solution had a positive photocurrent, indicating its n-type semiconductor nature, shown in inset of Fig. 6c. Oppositely, sample TC and TGC exhibited a negative current response and larger photocurrent, which was a sign of p-type semiconductor of Cu 2 O (also demonstrated in Fig. S2b). In addition, Cu 2 O could also enhance the charge transportation in TC, compared to the current baseline of TiO 2 . The rGO further enhanced the conductivity of TGC. As shown in Fig. 6d, the typical electrochemical impedance spectra were presented as Nyquist plots. For fitting the EIS, equivalent circuit [model: R(Q(RW))(Q(RW))] was demonstrated that the simulating results fitted the experimental very well. Q is constant phase element (CPE). R b represented the bulk resistance, CPEs should be considered in the nonhomogeneous condition of the composites, associating with the capacitor, and R s are the resistance of the solid-state interface layer which is formed at the highly charged state due to the passivation reaction between the electrolyte and the surface of the electrode, corresponding to the first semicircle at high frequency [58]. CPE dl and R ct are the double-layer capacitance and the charge-transfer resistance, corresponding to the second semicircle at medium frequency. Nyquist plots of EIS showed that nanocrystalline Cu 2 O could decrease the R ct from 9.4910 5 to 1.2910 4 ohm cm 2 for TC because of the smaller semicircle at the medium frequency, in comparison with TiO 2 (Fig. 6d) [40]. The resistance of TGC dramatically decreased to 4.4 ohm cm 2 because of high conductivity of rGO as shown in inset of  Photocatalytic performance of TGC was the best in the photocatalysts for MO and SDBS degradation. The schematic illustration is shown in Fig. 7: (1) The band gap of Cu 2 O determined by UV-Vis diffuse reflection spectroscopy (Fig. S3b) is 1.75 eV, which could enhance visible light absorption below 708 nm, generating more electron-hole pairs; (2) the matching energy band structure facilitated the separation of electron-hole pairs of heterojunction [15,29,62]; (3) rGO facilitated the dispersion of nanocrystals, transfer and shuttle of photo-generated electron in metal oxide particles [63]. Figure 8a shows the effect of irradiation time on phase of products. No Cu 2 O could be detected by the XRD after 2-h reaction. If the reaction prolonged for 4 h, a weak peak (111) of Cu 2 O could be observed, then more Cu 2 O were synthesized after 6 h. However, when the UV reduction time extended to 12 h, part of Cu 2 O particle converted to metal Cu.

Reduction mechanism of TGC via UV
Several other cupric salts [such as CuCl 2 , Cu(CH 3 COO) 2 , Cu(NO 3 ) 2 ] were tried to synthesize Cu 2 O products, labeled as TGC-Cl2, TGC-A, TGC-N (other synthesizing conditions were the same as that of sample TGC). There was CH 3 COOCu (PDF#28-0392) formed under UV reduction with (CH 3 COO)involvement, shown in Fig. 8b. As NO 3 used, the final phases of TGC-N could not be identified at present. As Clused, all XRD peaks for TGC-Cl2 were assigned to anatase and no copper-containing phase was detected. Furthermore, if NaCl was added to the TGC synthesis process, no diffraction peaks of copper-containing phase were found either (shown in Fig. 8b donated as TGC-Cl). It demonstrated that Clcould chelate Cu ? preferentially instead of OH -. Above all, the mechanism of synthesizing Cu 2 O could be proposed as follows: If CH3COOwas employed as the precursor: Cu 2þ þ e À aq ! Cu þ Cu þ þ OH À þ CH 3 COO À ! CH 3 COOCu þ OH À If Clwas added into the reaction system of TGC, the reaction was changed to: Cu 2þ þ e À aq ! Cu þ Cu þ þ OH À þ Cl À ! CuCl aq þ OH À So compared to CH 3 COO -/NO 3 -/Cl -, SO 4 2is the optimum anion for UV reduction synthesis of Cu 2 O.

Conclusion
Pure Cu 2 O, TiO 2 /Cu 2 O, TiO 2 /rGO/Cu 2 O nanoheterojunctions were fabricated by novel UV reduction method, and large amounts of dot-like Cu 2 O nanocrystals with size of ca. 5 nm were formed on the rGO or TiO 2 nanosheets. Sample TGC achieved the strongest absorption for solar light, highest separation efficiency of photo-induced electron-hole pairs. It had p-type photocurrent response under solar light and excellent photocatalytic performance. The adsorption abilities for catalysts varied with different dyes or surfactant, determined by nitrogen-containing groups and surface charge. Extending irradiation time could convert Cu 2 O to metal copper. In comparison with CH 3 COO -/NO 3 -/ Cl -, SO 4 2is the optimum anion for synthesis of pure Cu 2 O phase under UV condition.