Photocatalytic performance of Cu2O-loaded TiO2/rGO nanoheterojunctions obtained by UV reduction
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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.
KeywordsTiO2 Graphene Oxide Cu2O Methyl Orange Graphite Oxide
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 TiO2 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]. TiO2 nanosheets exposing (001) facet, which had excellent photocatalytic performance, made itself a stable substrate for building TiO2-based heterojunctions architecture, while its wide band gap of 3–3.2 eV limited the absorption of sun light. Loading dot-like semiconductor with response of visible light on TiO2 nanosheets might be an optimum architecture.
Cuprous oxide (Cu2O) 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 TiO2, which facilitated separation of photo-induced electron–hole pairs [13, 14, 15]. However, TiO2 nanosheet/dot-like Cu2O crystal heterojunction still had poor electron conductivity . 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 TiO2 or Cu2O combining with rGO had enhanced charge shuttle and transfer performance [13, 19, 20]. So the dot-like Cu2O-loaded TiO2/rGO nanoheterojunction might become one of the most efficient TiO2-based photocatalysts.
General method for loading dot-like Cu2O crystal on the TiO2 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 Cu2O on TiO2 frame or arrays using cupric acetate as precursor and glucose as reducing reagent. Gao et al.  loaded Cu2O particle on rGO sheet using l-ascorbic acid as reductive reagent in mild condition. Compared to chemical liquid reduction, photochemical synthesis of Cu2O had advantages of free chemical reagents addition, room temperature, atmospheric pressure, free of pH adjustment via alkali or acid. In previous reports, Cu2O was synthesized via γ-ray radiation [25, 26]. However, γ-ray radiated by 60Co source is very environmental unfriendly, harmful and strictly restricted by laws.
In this work, γ-ray was alternated by a ultraviolent (UV) light (main peak 254 nm, 25 W), and dot-like Cu2O crystal with size of ca. 5 nm was successfully deposited on TiO2 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 aromatic-containing macromolecules existing in waste water were selected to evaluate its photocatalytic activity .
More importantly, various cupric salts with different anions such as SO4 2−, Cl−, CH3COO−, NO3 − were employed to synthesize Cu2O 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.
Natural graphite was purchased from Qingdao Baichun graphitic Co., Ltd. Fluorine tin oxide (FTO)-coated glass (resistivity <10 Ω 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 . In details, 2 g of natural graphite flakes was mixed with 1 g sodium nitrate in the ice bath. Then, 50 mL concentrated H2SO4 was slowly added into the mixture under stirring to keep temperature under 5 °C. 0.3 g potassium permanganate was slowly put into the mixture under stirring to maintain temperature below 20 °C. Then, 7 g potassium permanganate was slowly added into the mixture for 1 h to keep temperature below 20 °C. Successively, the mixed solution was stirred at 35 °C for 2 h, followed by slow addition of deionized (DI) water (90 mL). After that, the solution was heated to 98 °C and kept for 15 min. The suspension was further diluted with 55 mL DI warm water, and then, 7 mL H2O2 was added to terminate the reaction. The mixture was filtered and washed with 10% HCl (1 L) and DI water (1 L) until pH 7. The graphite oxide product was vacuum-dried at 40 °C for 12 h.
Synthesis of TiO2 nanosheets
The TiO2 nanosheets were synthesized by hydrothermal method . In a typical experimental procedure, 5 mL of tetrabutyl titanate [Ti(OBu)4, ≥98%] and 0.6 mL of hydrofluoric acid (HF) (≥40%) were mixed in a dried Teflon autoclave with a capacity of 20 mL, and kept at 180 °C for 24 h. The powder was separated by centrifugation, washed by water and ethanol several times, consecutively. The final product was vacuum-dried at 80 °C for 6 h. Caution! HF is extremely corrosive and a contact poison, and it should be handled with extreme care. Hydrofluoric acid solution should be stored in plastic container and used in a fume hood.
Synthesis of TiO2/Cu2O composite
20 mg TiO2 nanosheets were sonicated in 100 mL ethanol for 15 min and then poured into 100 ml CuSO4 aqueous solution (containing 3 mmol CuSO4·5H2O) with fiercely stirring. The following procedure was the same with the Cu2O synthesis (shown in supporting information), and this obtained TiO2/Cu2O composite was labeled as TC.
Synthesis of TiO2/rGO/Cu2O composites
10 mg graphite oxide was sonicated in 100 mL deionized water at 30–40 °C for 30 min to obtain clear suspension; then, 3 mmol CuSO4·5H2O was added and dissolved. The following synthesis procedure was as same as that of TC and labeled as TGC (6 h UV light irradiation).
Other samples with irradiation of 2, 4, 12 h (donated as TGC-2, TGC-4, TGC-12 h) are also prepared.
Several other cupric salts [such as CuCl2, Cu(CH3COO)2, Cu(NO3)2] were employed to substitute CuSO4 to obtain final products labeled as TGC-Cl2, TGC-A, TGC-N (6 h UV light irradiation).
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 Na2SO4 aqueous solution as electrolyte. The working electrode was prepared as follows: 10 mg product powder was mixed with 22 μL 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 1×1 cm2, 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.
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).
Powder X-ray diffraction (XRD) was performed on DX-2700 X-ray diffractometer (Dandong Fangyuan, China) with monochromatized Cu-Kα radiation (λ = 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 Kα (hv = 1486.6 eV) radiation and beam spot of 500 μm 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
Characterization of IR and Raman spectrum
In Fig. 3b, four strong vibration peaks at 143 cm−1 (Eg), 393 cm−1 (B1g), 512 cm−1 (A1g) and 615 cm−1 (E1g) were ascribed to the five Raman active modes (A1g + B1g + 3Eg) of anatase . 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 Γ 15 (1) (LO) infrared (ir)-allowed mode in perfect Cu2O crystals and a small peak for Eg mode of anatase as shown in Fig. 3c . The peak at 505 and 621 cm−1 should be assigned to the overlapping of Raman vibration mode of crystalline Cu2O and TiO2 [45, 47]. The peak at 394 cm−1 was the B1g mode of anatase TiO2 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 . G band was a prominent feature of the pristine graphite, corresponding to the first-order scattering of the E2g mode . 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].
Selective adsorption and photocatalytic performance
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 Cu2O particles is one of predominant factors for photochemical applications. Exposed to UV light (irradiated by varied lamp), Cu2O can be reduced to metal Cu [53, 54]. However, in this work under standard simulated solar light, Cu2O was very stable and no metal copper was found even after 12 h with the characterization of X-ray diffraction (Fig. 5f).
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 TiO2, 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 TiO2 that inferred Cu2O could accept the photocharge from TiO2. Furthermore, lowest intensity of TGC demonstrated that rGO could further accept the photo-induced electron and enhance separation efficiency of electron–hole pairs.
TiO2 electrode in Na2SO4 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 Cu2O (also demonstrated in Fig. S2b). In addition, Cu2O could also enhance the charge transportation in TC, compared to the current baseline of TiO2. 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 . CPEdl 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 Cu2O could decrease the R ct from 9.4×105 to 1.2×104 ohm cm2 for TC because of the smaller semicircle at the medium frequency, in comparison with TiO2 (Fig. 6d) . The resistance of TGC dramatically decreased to 4.4 ohm cm2 because of high conductivity of rGO as shown in inset of Fig. 6d. Its high charge shuttle and transfer enhanced the degradation ability.
Reduction mechanism of TGC via UV
Several other cupric salts [such as CuCl2, Cu(CH3COO)2, Cu(NO3)2] were tried to synthesize Cu2O products, labeled as TGC-Cl2, TGC-A, TGC-N (other synthesizing conditions were the same as that of sample TGC). There was CH3COOCu (PDF#28-0392) formed under UV reduction with (CH3COO)− involvement, shown in Fig. 8b. As NO3 − used, the final phases of TGC-N could not be identified at present. As Cl− used, 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 Cl− could chelate Cu+ preferentially instead of OH−.
Pure Cu2O, TiO2/Cu2O, TiO2/rGO/Cu2O nanoheterojunctions were fabricated by novel UV reduction method, and large amounts of dot-like Cu2O nanocrystals with size of ca. 5 nm were formed on the rGO or TiO2 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 Cu2O to metal copper. In comparison with CH3COO−/NO3 −/Cl−, SO4 2− is the optimum anion for synthesis of pure Cu2O phase under UV condition.
The financial support for this study by National Natural Science Foundation of China (No. 21476262), the Technology Development Plan of Qingdao (No. 14-2-4-108-jch) and Research Funds for the Central Universities (15CX05032A, 15CX05056A) is gratefully acknowledged.
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