Enhanced Photodegradation of Methyl Orange Synergistically by

Article pubs.acs.org/JPCC

Enhanced Photodegradation of Methyl Orange Synergistically by Microcrystal Facet Cutting and Flexible Electrically-Conducting Channels Liangliang Sun,† Xinglong Wu,*,† Ming Meng,† Xiaobin Zhu,† and Paul K. Chu‡ †

Collaborative Innovation Center of Advanced Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: By performing precise facet cutting during hydrothermal synthesis, single-morphological and uniformsized octahedral and cubic Cu2O microcrystals respectively with {111} and {100} facets are synthesized and subsequently encapsulated with reduced graphene oxide (rGO). Electrochemical impedance spectroscopy shows that the rGO/Cu2O polyhedral composite has excellent conductivity, indicating that rGO can serve as a flexible electrically conducting channel. On account of the accumulation of a large amount of photoexcited electrons on the {111} facets of the octahedrons and efficient electron transfer to the rGO sheet, photodegradation of methyl orange by the rGO/Cu2O octahedral composite is enhanced by a factor of 4 compared to both bare Cu2O octahedrons and rGO-encapsulated cubes with hole accumulation on the {100} facets, and the stability of the rGO/Cu2O octahedrons is obviously improved due to no direct touch with water molecules in comparison with Cu2O microcrystals without rGO wrap reported previously. This work shows that the combination of crystal facet cutting and conducting channels is an effective strategy to design new composites with enhanced photocatalytic properties.

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INTRODUCTION Nanostructured materials with high catalytic efficiency for photodegration of pollutants, solar energy conversion, and gas sensing have attracted much research interest. Cuprous oxide (Cu2O), a p-type semiconductor with a direct band gap of 2.0− 2.2 eV, is a good photocatalyst because it can be produced on a large scale economically and is nontoxic.1 Moreover, Cu2O not only absorbs visible light directly but also can be used as a sensitized semiconductor in solar cells.2−4 Recently, Yang proposed that the photocatalytic properties of materials might be increased by crystal cutting.5 It was also discovered that Cu2O delivered good performance in photodegradation of methyl orange (MO) after crystal cutting.6 Sun et al. demonstrated that a polyhedral (26-facets) Cu 2 O−Cu heterogeneous architecture with selectively exposing copper nanoparticles on {111} facets shows better adsorption and photodegradation of MO dye than that of original 26-facet Cu2O architectures.7 Wu and co-workers observed enhanced photocatalytic activity from {100} truncated octahedral In2O3 compared to regular octahedrons.8 Meanwhile, graphene or reduced graphene oxide (rGO) has recently been used as a flexible conducting channel to encapsulate nanocrystals such as TiO2,9 ZnO,10 MnO2,11 CdS,12 and CdSe13 to improve the photocatalytic performance. Herein, the rGO-encapsulated © 2014 American Chemical Society

Cu2O octahedral composite is shown to have enhanced photodegradation capability of MO. Owing to the unique surface structure of the Cu2O octahedrons and excellent conductivity of rGO, the photodegradation capability increases by a factor of 4 compared to both bare Cu2O octahedrons and rGO-encapsulated cubes with hole accumulation on the {100} facets. This work clearly shows possible applications for the combined effects of rGO and crystal facet cutting in such as photocatalysis and photodegradation of inorganic materials.

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EXPERIMENTAL SECTION Graphene Oxide (GO) Preparation. GO was synthesized by the modified Hummers’ method14,15 from natural graphite powders. Graphite powders (3 g) were put into a mixture of concentrated H2SO4 (12 mL), K2S2O8 (2.5 g), and P2O5 (2.5 g) and kept at 80 °C in an oil bath for 5 h under stirring. After cooling to room temperature, 0.5 L of deionized (DI) water was introduced. The product was filtered with DI water using a 0.2 μm nylon film and dried under ambient conditions. The obtained preoxidized graphite was oxidized using the Received: October 27, 2014 Revised: November 10, 2014 Published: November 11, 2014 28063

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1.0 M Na2SO4 was the electrolyte. The impedance versus frequency spectra were acquired at an excitation signal with a 5 mV amplitude over the frequency range of 10−1 to 106 Hz. Characterization. Field-emission scanning electron microscopy (FE-SEM) was performed on the Hitachi S4800 SEM and high-resolution transmission electron microscopy (HR-TEM) was conducted on the JEOL-2100 electron microscope. UV− vis absorption spectra were acquired on the Shimadzu UV-3600 spectrophotometer and X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher Scientific K-Alpha photoelectron spectrometer. The powder X-ray diffraction (XRD) patterns were acquired on a Philips X’pert diffractometer and atomic force microscopy (AFM) images were taken on the JPK nanowizard. The thickness of the Cu2O film on the FTO was determined using the Zeta 20 (Zeta instrument, United States).

Hummers’ method and exfoliation was carried out by ultrasonic treatment of a graphite oxide suspension (0.2 mg/mL) for 1 h. Cu2O Polyhedral Microcrystal Synthesis. Cu2O polyhedral microcrystals were synthesized according to the method reported by Huang et al.16 Cubic and octahedral Cu2O microcrystals were obtained by adjusting the amounts of the reducing agent and DI water. In details, 95.5 and 90.5 mL of DI water were used, respectively, and 1 mL of 0.1 M CuCl2, 2 mL of 1 M NaOH, and 0.87 g sodium dodecyl sulfate (SDS) were added to the DI water under vigorous stirring. After dissolving the powder completely, 2 and 7.1 mL of 0.2 M NH2OH·HCl were introduced to the solutions, respectively. After aging under ambient conditions for 2 h, the precipitate was filtered, washed several times with DI water and absolute ethanol, and dried under vacuum at room temperature. rGO/Cu2O Polyhedral Composite Synthesis. Fifty milligrams of the cubic and octahedral Cu2O were put in 10 mL of 1 g/L poly(allylamine hydrochloride) (PAH), kept for 0.5 h, and filtered. The PAH solution is an effective surface amination agent that can modify the Cu2O polyhedral surface with positively charged amine end groups so that Cu2O polyhedrons can easily combine with negatively charged GO sheets. The precipitate was dispersed in water and a calculated amount (2 wt %) of 0.2 mg/mL of GO was added to the solution and stirred vigorously overnight. The mixture was filtered, transferred to a 25 mL Teflon-lined autoclave, and heated to 180 °C for 6 h. After cooling to room temperature, the black precipitate was filtered and dried in vacuum. Here we stress that if there is no special description, the “rGO/Cu2O polyhedral composites” represent the “rGO/Cu2O polyhedral composites with 2 wt % rGO”. Photocatalytic Reactions. The photocatalytic activity was assessed by analyzing the degradation of MO under light irradiation. Twenty-five milligrams of the photocatalyst was dispersed in 100 mL of the 15 mg/L MO solution. Before illumination, the mixture was stirred in the dark for 2 h to fully adsorb MO. For the photodegradation experiments, the reaction vessel was constantly stirred and irradiated by a 400 W xenon lamp placed 18 cm away to provide both UV and visible light. The irradiation light intensity reaching the reaction vessel was 360 mW/cm2. The reaction vessel was put in a sink to minimize the heat effect. UV−visible absorption spectra were taken at 20 min intervals for up to 120 min. The degree of MO photodegradation was determined by calculating the change in concentration (C/Co) according to the variation in the absorbance (A/Ao) at 462 nm, where Co and Ao are the initial concentration and absorbance of MO, respectively. To perform the cycling tests, we set the initial concentration of MO to be 15 mg/L. Every run lasts for 2 h and the whole 6 runs were taken. UV−visible absorption spectra were taken at 40 min intervals. After each run of photocatalytic reaction, the suspensions were filtered and the resulting precipitates were collected for next run. Electrochemical Impedance Spectroscopy (EIS). The Cu2O octahedron and rGO/octahedrons powders were deposited as a thin film on fluorine-doped tin oxide (FTO, 7 Ω/square) by spin coating in ethanol. The film with an approximate thickness of 3.5 μm and active area of 1 cm2 were calcinated at 200 °C for 2 h under Ar to acquire better electrical contact. EIS was performed using a three-electrode cell connected to a CHI 660D work-station. The Cu2O materials, Ag/AgCl, and Pt wire served as the working electrode, reference electrode, and counter electrode, respectively, and

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RESULTS AND DISCUSSION By appropriately adjusting and controlling the fabrication conditions, large areas of Cu2O cubes and octahedrons with single morphology and uniform size can be produced as shown in Figure 1a,b. The field-emission scanning electron microscopy

Figure 1. (a,b) Low-magnification FE-SEM images of the Cu2O cubes and octahedrons. (c,d) High-magnification FE-SEM images of the Cu2O cube and octahedron. (e,f) SAED patterns of the {100} facets of Cu2O cube and {111} facets of the Cu2O octahedron.

(FE-SEM) images show that the cube has six square {100} surfaces with a side length of about 700 nm (Figure 1c), whereas the octahedron has eight triangular {111} surfaces with a side length of about 900 nm (Figure 1d). The selected-area electron diffraction (SAED) patterns impart the information about the exposed facets as shown in Figure 1e,f. The tetragonal and symmetrical diffraction spots acquired along the [100] direction of the cube confirm that the exposed facets on the Cu2O cube are {100} and meanwhile, the hexagonal diffraction spots along the [111] direction of the octahedron indicate that the exposed facets on the Cu2O octahedron are {111}.16 The corresponding atomic structures of Cu2O (100) and (111) surfaces are shown in Figure S1a,b in Supporting Information, respectively, consistent with the SAED results. To produce the rGO/Cu2O polyhedral composites, welldispersed GO is prepared. According to the atomic force microscopy (AFM) image (Figure S2 in Supporting Information), the thickness of the exfoliated GO sheet is approximately 1.1 nm. To obtain the rGO-encapsulated Cu2O polyhedrons, the synthesized Cu2O polyhedrons are dipped in the poly(allylamine hydrochloride) (PAH) solution for 0.5 h to allow the positively charged amine functional group to modify the surface.17 GO contains negatively charged −COOH, 28064

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−CHO functional groups and thus can spontaneously assemble onto the surface of the Cu2O polyhedrons via electrostatic interactions between the two materials.18,19 Afterward, the GO is reduced to rGO by a hydrothermal reaction.20 As shown in Figure S3 in Supporting Information, the C 1s XPS spectrum of pure GO shows the presence of three types of carbon bonds: C−C (284.88 eV), C−O (286.98 eV), and CO (288.18 eV). Although the C 1s XPS spectra of the rGO-Cu2O octahedrons show the same species, the intensity of the oxide species is smaller than that of pure GO and the C−O bond even vanishes, suggesting effective deoxygenation during reduction. The XRD patterns of both the Cu2O polyhedrons and rGO/ Cu2O polyhedral composites are depicted in Figure S4 in Supporting Information. The strong and sharp peaks indicate that the obtained Cu2O crystals are highly crystalline. The intensity ratio between the (111) and (200) diffraction peaks from Cu2O cubes is lower than that from Cu2O octahedrons (1.642 versus 2.885), indicating that the cubes have more {100} facets. This is consistent with the TEM result. Both sides on Cu2O (111) peak have two CuO (111̅) and (111) diffraction peaks (JCPDS NO. 48-1548) (Supporting Information Figure S4a,b). Their intensities are relatively weak. In addition, the XRD results also show that the encapsulation of rGO does not introduce any structural change of Cu2O polyhedrons. Owing to the small amount of rGO, no obvious diffraction peaks belonging to graphene can be observed from the rGO/Cu2O polyhedral composites and similar results are observed from P25-rGO nanocomposites.21,22 The morphology and structure of the rGO/Cu2O polyhedral composites are shown in Figure 2. The low- and high-

bright orange color of the initial solution gradually disappears with exposure time (Figure 3a). Figure 3b shows the MO

Figure 3. (a) Evolution of MO degradation in the presence of rGO/ octahedrons under light irradiation for different time from 0 to 120 min. (b) Corresponding profiles of MO absorbance for different light exposure durations.

absorbance spectra acquired after different times. The characteristic absorption band of MO at 462 nm weakens rapidly with exposure time. However, for the rGO-encapsulated Cu2O cubes the MO absorbance variation is not obvious, as shown in Figure S5 in Supporting Information. This indicates that crystal facet cutting alters photodegradation. To compare the degradation of MO using different catalysts, the change in the main peak of MO at 462 nm at a given time interval is monitored. As known well, the photodegradation capability of rGO-based nanocomposites changes with the amount of the introduced rGO.9,21,23 To achieve the best photodegradation performance, we fabricated the rGO/ octahedrons with the different amount of rGO and carried out the photodegradation tests (Figure S6 in Supporting Information). We found that the photodegradation of the rGO/octahedrons with 2 wt % rGO is the most efficient, thus we use such composites to test the photodegradation capability. Figure 4a displays the plot of photodegradation of MO as a function of irradiation time using the blank and four kinds of catalysts. In our calculation, the remaining concentration fraction of MO after photodegradation was normalized to that after the adsorption equilibrium in dark for 2 h. It can be clearly seen that MO degrades slightly in the presence of bare Cu2O cubes by about 5% and the degradation ability of the bare Cu2O octahedrons is about 17.6%. After rGO incorporation, both the rGO/cubes and rGO/octahedrons show enhanced photodegradation of MO and approximately 21 and 81% of MO decompose within 2 h, respectively. Here, we can see that for the rGO/octahedrons under light irradiation, the remaining concentration fraction of MO is only 19%, indicating that the photodegradation process is accompanied by the adsorption of MO. Hence, rGO can significantly improve the photodegradation ability of the Cu2O polyhedrons. In fact, the rGO/octahedrons show a factor of 4 times improvement compared to rGO/cubes. Here, we would like to mention that

Figure 2. (a,b) Low-magnification FE-SEM images of the rGO/cubes and rGO/octahedrons. (c,d) High-magnification FE-SEM images of the rGO/cubes and rGO/octahedrons. (e,f) High-resolution TEM images of the rGO/cubes and rGO/octahedrons that show the rGO layers on the Cu2O {100} and {111} facets.

magnification FE-SEM images disclose that the rGO wraps around the octahedral and cubic microcrystals effectively (Figure 2a−d). The two-dimensional rough texture indicates the presence of flexible and ultrathin rGO sheets on the composites after the hydrothermal reduction. The HR-TEM images in Figure 2e,f also show that the rGO sheets adhere tightly to the Cu2O polyhedral surfaces. Owing to the distribution of oxygen-related groups on rGO, the Cu2O polyhedrons with surface amine functional groups can be dispersed well on the rGO forming wrinkles and edges. The rGO sheets can serve as a electrically conducting channel to link adjacent Cu2O polyhedrons.22 The photodegradation activities of the rGO/Cu2O polyhedral composites are assessed by using MO as an example. When the rGO/octahedrons are used as the photocatalyst, the 28065

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Cu2O {100} facets under light irradiation.30 Electrons are consumed by adsorbing O2 molecules to generate O−2 , and O−2 further reacts with electrons and H2O to yield H2O2 and •OH. H2O2 and •OH are powerful oxidizing agents that can degrade MO but the photogenerated holes cannot oxidize MO due to the low redox potential.31 In this respect, the Cu2O octahedrons provide abundant photogenerated electrons. The conduction band of Cu2O lies at −4.22 eV and the vacuum level work function of graphene is 4.42 eV.32,33 In the integrated Cu2O/rGO system, the photoexcited electrons accumulating on the {111} facets can effectively transfer from the conduction band of Cu2O to rGO via a percolation mechanism.34 Although the hydrothermally produced rGO has a complicated structure consisting of sp2 and sp3 hybridized carbon atoms and many other oxygen-containing groups, the excellent conductivity makes electron transfer on the rGO sheets faster thus resulting in rapid electron transport through the rGO.23 Electrochemical impedance spectroscopy (EIS) is an effective technique to evaluate electron transfer at a solid/electrolyte interface. To confirm the rapid transport ability of electrons in the rGO/Cu2O polyhedrons, EIS is performed. Owing to the unique sp2 carbon network, rGO has excellent conductivity. Therefore, the introduction of rGO to the Cu2O octahedrons, which have the capability of accumulating electrons on the {111} facets, gives rise to enhanced electrical conductivity and interfacial charge transfer. Figure 5 shows the Nyquist plots of

Figure 4. (a) Photodegradation of MO under light irradiation with Cu2O cubes, octahedrons, rGO/cubes, and rGO/octahedrons, respectively. (b) Remaining concentration fraction of MO in the Cu2O cubes, octahedrons, rGO/cubes, and rGO/octahedrons after stirring for 2 h in dark.

a little amount of CuO also exists in the rGO/cubes (Supporting Information Figure S4b). Thus, compared to the situation from the bare cubes, the degradation ability of 21% from the rGO/cubes has nothing to do with the CuO component. During photocatalysis, increased adsorption of pollutants, extended light absorption, and trapping and shuttling of photogenerated electrons are the three main factors that can improve the photocatalytic performance of graphene-based composites.21,22,24−27 Figure 4b shows the remaining concentration fractions of MO after the adsorption equilibrium in dark for 2 h. Owing to π−π stacking between MO and aromatic regions of the rGO with the giant π-conjunction system and two-dimensional planar structure,28 more MO molecules adsorb onto the surface of the rGO/Cu2O polyhedral composites but the adsorption abilities of the rGO/cubes and rGO/octahedrons are almost the same (54.17% for rGO/cubes and 56.56% for rGO/octahedrons). The UV−visible absorption spectra of the polyhedrons and rGO/Cu2O polyhedral composites are shown in Figure S7 in Supporting Information. The absorption intensities in both ultraviolet and visible regions are obviously enhanced due to the introduction of rGO but keep almost the same for different polyhedrons. This is in good agreement with the result in Figure 4b. Moreover, the same content of rGO attached to the Cu2O polyhedrons does not result in large differences in conductivity between the rGO/ cubes and rGO/octahedrons based on our measurements.29 Hence, it is believed that the improved photocatalytic performance rendered by the rGO/octahedron composite stems from the synergistic effects of facet cutting and incorporation of rGO. Cu2O octahedrons are bound by eight {111} surfaces whereas the Cu2O cube is bound by six {100} surfaces. On account of the distinctive energy levels, the Cu2O {111} facets accumulate abundant electrons but holes accumulate on the

Figure 5. Nyquist plots (Zre versus Zim) of the Cu2O octahedron and rGO/octahedron electrodes with and without irradiation.

the bare Cu2O octahedrons and rGO/octahedrons in darkness and under illumination. The intercept of the semicircle with the real axis represents bulk resistance of the electrode, whereas the semicircle in the Nyquist plot represents charge transfer and its diameter can be regarded as the charge transfer resistance. It can be seen that the presence of rGO hardly increases bulk resistance of the electrode (the bulk resistance from the rGO/ octahedrons is even smaller) but successfully decreases the semicircle in comparison with the bare Cu2O octahedrons with and without light suggesting increased conductivity of rGO/ octahedrons and reduced interfacial electron transfer resistance between the catalyst and electrolyte. Therefore, electrons can transfer from the Cu2O {111} facets to rGO, then to the electrolyte effectively. Because rGO is a flexible conducting channel that encapsulates the Cu2O octahedrons effectively, photodegradation of MO is improved largely. 28066

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electrons moving on the rGO sheets can be consumed by oxygen molecules and react to produce a large amount of H2O2 and •OH as a result of the low interfacial electron-transfer resistance thereby promoting the MO photodegradation efficiency on rGO.

To see the stability of the photodegradation performance of rGO/octahedrons, we performed the cycling tests of MO photodegradation and found that adsorption equilibrium is reached before light irradiation by constantly stirring in dark for 2 h in each run. Figure 6 shows the corresponding cycling data.

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CONCLUSIONS rGO-encapsulated Cu2O octahedrons are produced by crystal cutting. By taking advantage of strong electron accumulation on the {111} facet and flexible conducting channels composed of rGO, the rGO/octahedral composite boasts high electron output, good electrical conductance, and enhanced photodegradation of MO. The novel metallic oxide polyhedrons/ rGO composites have many potential applications in degradation of pollutants, reduction of CO2, water splitting, and other environmental applications.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic structures of Cu2O (100) and (111) surfaces, AFM image of graphene oxide (GO), high-resolution C 1s XPS spectra of GO and rGO/octahedrons, XRD patterns of Cu2O cubes, octahedrons, rGO/cubes and rGO/octahedrons, timedependent profiles of MO absorbance spectra in the presence of rGO/cubes, photodegradation of MO over Cu2O octahedrons and rGO/octahedrons with different wt % of rGO, UV− visible absorption spectra of Cu2O cubes, octahedrons, rGO/ cubes, and rGO/octahedrons. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Photodegradation of MO over rGO/octahedrons during repeated degradation experiments of 12 h under light irradiation.

We can observe that after the first run for 2 h, the 80.02% MO is degraded. After the sixth run, the degraded concentration fraction of MO is at 72.13%. This means that the performance of photodegradation over rGO/octahedrons declines 9.62% after a measurement period of 12 h. The decline rate is obviously smaller than that of Cu2O microcrystals without rGO wrap reported previously,30 indicating that the surface coating of rGO on Cu2O polyhedrons largely improves the stability of Cu2O particle during photodegradation. This is understandable because in the process of photodegradation, a large number of electrons are consumed, and the remaining holes will not oxidize Cu2O easily due to no direct touch with water molecules.35 The enhanced photocatalytic performance of the rGO/ octahedrons is illustrated in Figure 7. After crystal cutting, the photoexcited electrons accumulate on the Cu2O octahedral {111} facets under light irradiation to provide an abundant source of electrons. The electrons can transfer to rGO by the percolation mechanism and hop through the rGO sheets that serve as a flexible conducting channel. The large amount of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-25-83595535. Tel: 8683686303. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Programs of China under Grants Nos. 2011CB922102, 2013CB932901, and 2014CB339800. Partial support was also from National Natural Science Foundation (Nos. 11404162 and 11374141) and Guangdong-Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ.

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Figure 7. Mechanism of MO degradation on the rGO/octahedrons: Electrons accumulating on {111} facets effectively transfer from Cu2O to rGO via the percolation mechanism. The rGO serving as a flexible conducting channel promotes the generation of reactive species thus accelerating degradation of MO. 28067

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dx.doi.org/10.1021/jp510772u | J. Phys. Chem. C 2014, 118, 28063−28068