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Oxygen vacancies in shape controlled Cu2O/reduced graphene oxide/In2O3 hybrid for promoted photocatalytic water oxidation and degradation of environmental pollutants Jie Liu, Jun Ke, Degang Li, Hongqi Sun, Ping Liang, Xiaoguang Duan, Wenjie Tian, Moses O. Tade, Shaomin Liu, and Shaobin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01605 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017
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ACS Applied Materials & Interfaces
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Oxygen vacancies in shape controlled Cu2O/reduced graphene
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oxide/In2O3 hybrid for promoted photocatalytic water oxidation
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and degradation of environmental pollutants
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Jie Liu,a,b* Jun Ke,b Degang Li,c Hongqi Sun,d Ping Liang,b Xiaoguang Duan,b Wenjie
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Tian,b Moses O. Tadé,b Shaomin Liu,b Shaobin Wang b,* a
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Department of Environmental Science & Engineering, North China Electric Power University, Baoding 071003, China
7 b
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WA 6845, Australia
9 10
c
School of Chemical Engineering, Shandong University of Technology, Zibo, 255049, China
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Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth,
d
School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia
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*Corresponding authors: Dr. Jie Liu (
[email protected]) Prof. Shaobin Wang (
[email protected])
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Abstract
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A
novel
shape
controlled
Cu2O/reduced
graphene
oxide/In2O3
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(Cu2O/RGO/In2O3) hybrid with abundant oxygen vacancies was prepared by a facile,
20
surfactant-free method. The hybrid photocatalyst exhibits an increased photocatalytic
21
activity in water oxidation and degradation of environmental pollutants (methylene
22
blue and Cr6+ solutions) compared with pure In2O3 and Cu2O materials. The presence
23
of oxygen vacancies in Cu2O/RGO/In2O3 and the formation of heterojunction between
24
In2O3 and Cu2O induce extra diffusive electronic states above the valence band (VB)
25
edge and reduce the band gap of the hybrid consequently. Besides, the increased
26
activity of Cu2O/RGO/In2O3 hybrid is also attributed to the alignment of band edge, a
27
process that is assisted by different Fermi levels between In2O3 and Cu2O, as well as
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the charge transfer and distribution onto the graphene sheets, which causes the
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downshift of VB of In2O3 and the significant increase in its oxidation potential.
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Additionally, a built-in electric field is generated on the interface of n-type In2O3 and
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p-type Cu2O, suppressing the recombination of photo-induced electron-hole pairs and
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allowing the photo-generated electrons and holes to participate in the reduction and
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oxidation reactions for oxidizing water molecules and pollutants more efficiently.
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Keywords: Oxygen vacancies, p-n heterojunction, shape controlled synthesis,
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photocatalysis, indium oxide
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Introduction
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Photocatalysis has been considered as a promising and green technology for
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energy conversion and degradation of environmental pollutants, in which the
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fabrication of novel nanostructured materials with unique physical/chemical
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properties and high efficiency is the crucial step and has attracted much attention.1,2
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The effective harvesting of solar energy and inhibition of charge carrier
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recombination are the two key aspects in photocatalysis.3 Recently, oxygen vacancies
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in oxide semiconductors have been reported to both increase solar light harvesting
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through narrowing the band gap and improve the carrier separation efficiency by
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serving as the active sites.4,5 Indium oxide (In2O3), a typical n-type semiconductor
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with a band gap of ~2.8 eV, possesses a low electrical resistivity, favorable surface
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properties, and low toxicity, making it a good candidate for electronic and
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photocatalytic applications, such as environmental remediation, solar hydrogen
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production, and nanodevices.6-8 Liang et al. demonstrated that oxygen vacancies
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could be induced by some special morphologies of In2O3 materials, and that new
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energy levels could be formed in the band gap thereafter.9 The abundant oxygen
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vacancies were detected on the ultrathin In2O3 porous sheets, and were considered to
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play key roles in the high performance for water splitting on In2O3 under visible-light
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illumination.4
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Besides the oxygen vacancies, the sensitization by a narrow band-gap
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semiconductor also favors the broadening of light response of In2O3 to visible-light,
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and the realignment of energy level makes the hybrid better matching for some 3
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specific reactions. The p-type Cu2O, with a narrow band gap of ~2.0 eV, is often used
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as a photosensitizing component, and the possible formation of p-n heterojunctions
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between In2O3 and Cu2O benefits to improve the efficiency of the charge separation,
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by forming an inner electric field between the n-type and p-type semiconductors.8,10
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Besides acting as a sensitizer for In2O3 to enhance the photocatalytic degradation of
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environmental pollutions,11 Cu2O is also considered as a perfect candidate for water
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splitting.12-14 More importantly, the controlled morphology of Cu2O makes it to grow
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along with the specific crystal facets more easily, and the readily accumulation of
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photo-induced electrons on the (111) facets of Cu2O and its distinctive energy levels
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will accelerate the photocatalytic reactions.15-17 Furthermore, introduction of an
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electron-trapping material, such as 2D carbonaceous graphene, is also an effective
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way to enhance the transportation of photo-induced charge, which acts as a flexible
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conducting channel owing to its unique electronic property, and graphene is reported
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to provide the protective function to Cu2O.18-21
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The unique electronic structures of In2O3, Cu2O and graphene inspire us to
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design a novel Cu2O/reduced graphene oxide/In2O3 (Cu2O/In2O3/RGO) hybrid.
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Though some works about Cu2O/RGO, In2O3/RGO and Cu2O/In2O3 hybrids have
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been reported, the photocatalytic activity of In2O3 decorated by single component of
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Cu2O is still not satisfied, and their application in the photocatalytic oxygen evolution
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reaction (OER), which is a multi-electrons transfer reaction, is rarely reported.11,21-23
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In the present study, a novel corn-shaped In2O3 material was synthesized by a facile
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method, which was further decorated by reduced graphene oxide (RGO) and Cu2O 4
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with a high exposition of (111) facets to fabricate a three-component
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Cu2O/RGO/In2O3 hybrid with abundant oxygen vacancies. The OER performance of
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the Cu2O/RGO/In2O3 was tested, and the photocatalytic activity of the hybrid was
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also evaluated in the degradation of environmental pollutants (methylene blue and
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Cr6+). By investigating the functions of oxygen vacancies and the heterojunctions for
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the activity of the three-component photocatalyst, we proposed a possible mechanism
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for the enhanced photocatalytic activity, based on a series of characterizations. The
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factors influencing the photo-chemical properties of Cu2O/RGO/In2O3 were also
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discussed systemically.
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2. Experimental Section
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2.1 Materials Synthesis
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2.1.1 Synthesis of Cu2O octahedra
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Cu(NO3)2·2H2O at 1.208 g was dissolved in 50 mL of H2O, and then 10 mL of
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NaOH solution (3 mol/L) was added drop by drop. After the addition of 1 g glucose,
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the obtained blue suspension was heated at 60 ºC for 30 min under stirring. The
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obtained red slurry was filtered, washed by H2O and ethanol, and then dried at 60 ºC
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for 12 h.
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2.1.2 Synthesis of corn-like In2O3
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In(NO3)3·H2O (3.0835 g) and urea (5.7658 g) were dissolved in 80 mL of H2O,
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mixed and stirred for 15 min. Then the mixed solution was transferred into a Teflon-
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lined stainless steel autoclave (125 mL) and reacted at 125 ºC for 4 h. The obtained
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slurry was separated by filtration, washed by H2O and ethanol, dried at 60 ºC for 12 h, 5
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and calcined at 350 ºC for 4 h with a heating rate of 5 ºC·min-1.
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2.1.3 Synthesis of Cu2O/RGO/In2O3,
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Graphene oxide (GO) was synthesized by a modified Hummers’ method.24,25 For
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the synthesis of RGO/In2O3, 0.03 g of GO was dissolved in the mixture of 13.5 mL
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ethanol and 24 mL H2O, and ultrasonically treated for 60 min. After that, 0.03 g
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octahedral Cu2O and 0.3 g In2O3 were added, stirred, and ultrasonically treated for
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another 30 min. The obtained suspension was transferred into a Teflon-lined stainless
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steel autoclave (45 mL) and reacted at 180 ºC for 6 h. After the reaction, the obtained
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slurry was washed by H2O and ethanol, and dried at 60 ºC for 12 h. The loading of
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GO and Cu2O were both at 10 wt.%.
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2.1.4 Synthesis of Cu2O/RGO, RGO/In2O3 and RGO
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The synthesis of Cu2O/RGO, RGO/In2O3 and RGO was conducted similarly to
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that of Cu2O/RGO/In2O3, while the weights of GO, octahedral Cu2O and In2O3 were
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0.03 g, 0.03 g and 0.3 g, respectively, when used in the synthesis.
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2.2 Characterization methods
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The physicochemical properties of all the as-prepared catalysts were
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characterized by a series of techniques, including field-emission scanning electron
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microscope (FE-SEM), transmission electron microscope (TEM), high-resolution
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transmission electron microscope (HR-TEM), X-ray diffraction (XRD), Raman,
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electron spin resonance (ESR), Fourier transform infrared spectroscopy (FTIR), X-ray
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photoelectron spectroscopy (XPS), valence band XPS (VB-XPS), UV-vis diffuse
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reflection spectroscopy (DRS), and photoluminescence (PL) spectra. The details are 6
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presented in Supporting Information.
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2.3 Photoelectrochemical (PEC) tests
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The PEC measurements were carried out using a Zahner Zennium
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electrochemical workstation operated in a standard three-electrode cell with a
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fluorine-doped tin oxide (FTO) electrode deposited with Cu2O/RGO/In2O3 samples as
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a photoanode, a saturated calomel electrode (SCE) as the reference electrode and a Pt
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wire as the counter electrode, respectively. For the fabrication of the working
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electrode, 40 mg catalyst was mixed with a certain amount of ethanol and Nafion
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solution homogeneously, and the obtained sample was deposited as a thin film on the
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FTO glass with a controlled area of 1 cm2 using spin-coating, and then dried at 60 °C
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for 30 min to form a film electrode. The PEC performance of the catalysts was
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measured under a 300 W Xe-lamp, and 0.5 M Na2SO4 solution (40 mL, pH = 6.8,
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25 °C) was used as the electrolyte. The electrode potential versus saturated calomel
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electrode (SCE) can be converting to reversible hydrogen electrode (RHE) potential
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by the Nernst equation as follows:
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VNHE = VSCE + V0SCE
(1)
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where VNHE is the converted potential vs NHE, V0SCE = 0.245 V at pH = 6.8, 25 °C,
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and VSCE is the experimental potential measured against the Hg/Hg2Cl2/saturated KCl
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reference electrode.
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2.4 Photocatalytic Measurement
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The photocatalytic activity of various catalysts was measured by OER and the
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degradation of methylene blue (MB) and Cr6+ in an aqueous solution at 25 ºC, 7
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illuminating by a high-pressure Xe-lamp (300 W, Philips). The photocatalytic activity
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of OER was evaluated by oxidizing water molecules with AgNO3 as an electron
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scavenger. In a typical procedure, 0.1 g of catalyst was added to 200 mL solution,
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including AgNO3 (0.03 M) and La2O3 (0.2 g). Prior to irradiation, the suspensions
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were mixed under vigorous stirring for 30 min in the dark and degassed to remove O2
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in solution. The O2 concentration in the reactor was in situ taken using a NEOFOX O2
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probe. For the photodegradation of MB,the concentrations of MB and catalyst were
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10 and 25 mg·L-1, respectively. Before the illumination, the MB solution with the
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catalyst was mixed in the dark to establish the adsorption-desorption equilibrium. The
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concertation of MB depending on reaction time was analyzed by a UV-vis
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spectrophotometer (JASCOV-670) at its characteristic absorption wavelength of 664
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nm. For the photoreduction of Cr6+, the concentrations of Cr6+ and catalyst were 50
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and 25 mg·L-1, respectively, with a pH value of 3, and the activity was measured at the
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characteristic absorption wavelength of 325 nm.
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3. Results and Discussion
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3.1 Morphology of as-prepared catalysts
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Figure 1a displays SEM images of the as-prepared In2O3. The typical
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morphology of In2O3 support is a hollow corn-like structure consisting of uniform
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nanorods, which is further confirmed by the magnified SEM (inset in Figure 1a) and
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the TEM image (Figure 1b), indicating that the length and width of each corn unit are
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at ~750 and ~170 nm, respectively. In Figure 1c, the interplanar spacing of 0.296 nm
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corresponds to the (222) plane of In2O3.26 From Figure 1d, the as-prepared Cu2O 8
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nanocrystals exhibit an octahedral structure with a side length of approximate 350 nm.
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In previous reports,27,28 it has been proved that every two Cu atoms have a dangling
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bond perpendicular to the (111) plane because of the termination of the crystal cell
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(Figure 1e), and the exposure of Cu atoms with the dangling bonds resulting in the
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high energy status of the (111) surface. The better photocatalytic activity of octahedral
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Cu2O has been widely reported.15,29
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Figure 1 (a) SEM image of corn-shaped In2O3, inset: magnified SEM image of corn-
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shaped In2O3, (b) TEM image of corn-shaped In2O3, (c) HRTEM of corn-shaped
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In2O3 from the yellow box in Figure 1b, (d) SEM image of Cu2O octahedra, and (e)
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three-dimensional model and atomic arrangement of the (111) planes of Cu2O
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octahedra (red: O atoms, orange: Cu atoms). 9
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Figure 2 (a) SEM of Cu2O/RGO/In2O3 hybrid, inset: magnified SEM images of
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Cu2O/RGO/In2O3 hybrid, (b) the elemental mapping image of In, Cu, O and C of
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Cu2O/RGO/In2O3 hybrid from (a), (c) TEM image of Cu2O/RGO/In2O3 hybrid, inset:
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the EDS spectra from the yellow box in Figure 2c, and (d) HRTEM image of
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Cu2O/RGO/In2O3 hybrid from the yellow box in Figure 2c.
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Figure 2a presents SEM images of Cu2O/RGO/In2O3 hybrid, illustrating the
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stable structure of Cu2O octahedra, which is inserted into the In2O3 particles. Some
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small particles scatter on the surface of Cu2O octahedra and corn-shaped In2O3 are
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speculated as InOOH deriving from In2O3. The elemental mapping image (Figure 2b)
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confirms the uniform distributions of In, Cu, O and C in the Cu2O/RGO/In2O3 sample,
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though the thin layered RGO is difficult to be observed in the SEM. However, TEM
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image (Figure 2c) shows that some Cu2O octahedra are contiguous with corn-shaped 10
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In2O3, and both of them are covered by few-layered RGO, which is further confirmed
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by the EDS spectra (inset image in Figure 2c). In Figure 2d, the lattice fringes with d
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spacing of 0.296 and 0.423 nm are assigned to the (222) and (211) planes of In2O3,
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respectively, and the interplane distance of 0.242 nm matches well with the (111)
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plane of Cu2O octahedra. The (222) and (211) planes of In2O3 are closely contacted to
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the (111) plane of Cu2O, suggesting the formation of heterojunction between In2O3
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and Cu2O. The interfacial connections could serve as migration paths to facilitate the
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charge separation, and induce synergistic effects for the enhanced photocatalytic
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performance.30-32
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3.2 Structure and Oxygen Vacancies in Cu2O/RGO/In2O3
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207 208
Figure 3 (a) XRD patterns and (b) Raman spectra of various catalysts, (c) ESR 11
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spectra of In2O3 and Cu2O/RGO/In2O3 samples measured at T = 100 K, and (d) FTIR
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spectra of GO, Cu2O/RGO and Cu2O/RGO/In2O3 samples.
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The XRD patterns of various catalysts are presented in Figure 3a. The as-
213
prepared Cu2O is ascribed to the cubic phase Cu2O, possessing a lattice constant of a
214
= 4.260 Å (PDF#65-3288). The diffraction peak at 36.5° is assigned to the (111) facet
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of Cu2O, and its much stronger intensity compared with that of other peaks indicates
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its high exposure. The XRD pattern of Cu2O/RGO is similar to that of Cu2O
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octahedra, suggesting the excellent stability of the (111) facet. For In2O3, the
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diffraction peaks at 21.4º, 30.5º, 35.4º, 45.6º, 50.9º and 60.5º correspond to the (211),
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(222), (400), (134), (440) and (622) planes of a cubic In2O3 (PDF#63-3170),
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respectively. When Cu2O and RGO are coupled with pristine In2O3, the characteristic
221
peaks of Cu2O at 36.5º, 42.4º and 61.5º are still observed clearly. Some new peaks
222
appear at 25.9º, 32.2º, 33.7º, 48.7º, 51.8º and 56.2º, and are attributed to the (110),
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(101), (011), (211), (121) and (002) planes of orthorhombic InOOH, respectively
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(PDF#17-0549). InOOH is speculated to be formed by the partial hydrolysis of In2O3
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crystals during the second hydrothermal process. Comparing the XRD patterns of
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In2O3 and Cu2O/RGO/In2O3, the intensities of the peaks assigned to In2O3 are quite
227
similar. Thus, it is speculated that the transformation of In2O3 to InOOH is minor,
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which most possibly occurs on the surface of In2O3. As a side product, the accurate
229
content of InOOH is hardly to be confirmed.
230
The structure of various catalysts is further confirmed by Raman spectra, as 12
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presented in Figure 3b. For the octahedral Cu2O, the characteristic vibrations of Cu2O
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are detected at 218 and 265 cm-1.24 In the case of the pure In2O3 sample, the features
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at 306, 364, 493 and 626 cm-1 are interpreted as the vibration mode of In2O3, and the
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peak of 364 cm-1, which is assigned to the stretching vibrations of In‒O‒In, reflects
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the oxygen vacancies in the structure of In2O3.33,34 After the introduction of RGO, two
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new peaks appear on Cu2O/RGO and Cu2O/RGO/In2O3 samples, centered at 1329 and
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1590 cm-1, and are assigned to the disorder band associated with structural defects
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generated in graphene (D band) and the well-ordered E2g phonon scattering of sp2
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carbon atoms of graphene (G band), respectively.22 Compared with pure In2O3, the
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characteristic wavenumbers attributed to In2O3 on Cu2O/RGO/In2O3 hybrid slightly
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shift to the lower frequency, setting at 293, 352, 483 and 608 cm-1, respectively,
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suggesting the probable formation of the heterojunction between In2O3 and Cu2O
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and/or RGO, as well as the formation of few InOOH. The intensity of the peak at 352
244
cm-1 is increased, indicating the presence of more oxygen vacancy species on
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Cu2O/RGO/In2O3. The absence of Cu2O features on Cu2O/RGO/In2O3 could be
246
attributed the low loading of Cu2O (10 wt.%).
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The presence of oxygen vacancies in Cu2O/RGO/In2O3 is further proved by the
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low-temperature ESR spectra. As shown in Figure 3c, both the In2O3 and
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Cu2O/RGO/In2O3 exhibit a remarkable ESR signal at g = 2.0003, corresponding to a
250
typical signal of oxygen vacancies.32,35 The signal intensity (H) of oxygen vacancies
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on Cu2O/RGO/In2O3 is 126010, much higher than that of 78224 of In2O3,
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demonstrating the generation of more oxygen vacancies caused by the coupling of 13
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In2O3, Cu2O and RGO. Figure S1 (Supporting Information) compares the PL spectra
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of In2O3 and Cu2O/RGO/In2O3. The peaks at 466 and 492 nm are corresponded to the
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oxygen vacancies as the shallow donor and those in deeper level, respectively.17,36,37
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The decreased intensity of the peak at 466 nm over Cu2O/RGO/In2O3 means the less
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recombination of the electrons with the oxygen vacancies at shallow, which is mainly
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caused by the rapid transfer of electrons by RGO sheets and the interface between
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In2O3 and Cu2O. For the signal at 492 nm, the PL intensity over Cu2O/RGO/In2O3 is
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enhanced, which is speculated to be caused by the much more oxygen vacancies in
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deep level in Cu2O/RGO/In2O3, probably generated by the formation of
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heterojunctions between In2O3 and Cu2O and/or RGO. The oxygen vacancies on
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Cu2O/In2O3/RGO hybrid is speculated to be caused by the surface defects formed
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during the hydrothermal process, mainly derived from the reduction of GO and the
265
partial hydrolysis of In2O3. According to previous reports,38,39 the formation of
266
oxygen vacancies may reduce the band edge by introducing new energy levels, and
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the oxygen vacancies can serve as traps for photo-induced charge to inhibit the hole-
268
electron recombination. The improved photoabsorption capacity caused by the
269
presence of oxygen vacancies has also been reported elsewhere.40 Herein, the
270
enhanced formation of oxygen vacancies in Cu2O/RGO/In2O3 could produce benefits
271
to promote photocatalytic performance.
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Additionally, the reduction of GO during the synthesis of Cu2O/RGO and
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Cu2O/RGO/In2O3 samples is confirmed by the FTIR spectra. As presented in Figure
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3d, the characteristic bands at 1716, 1585, 1403 and 1046 cm-1 are assigned to the 14
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vibrations of C=O (carboxyl) stretching, C=C stretching from nonoxidized sp2 C
276
hydridization, C‒OH and C‒O (alkoxy), respectively, and the broad band around 3400
277
cm-1 corresponds to the O-H stretching vibrational mode of the GO sheets.41,42 In
278
contrast, the intensity of the C=O, C‒OH, C‒O and O‒H groups over Cu2O/RGO and
279
Cu2O/RGO/In2O3 samples decreased significantly, indicating the removal of oxygen
280
containing groups and partial reduction of GO after the hydrothermal reaction. And
281
the low loading of GO (10 wt.%) in Cu2O/RGO/In2O3 leads to the further weaker
282
signals of these species compared with those over Cu2O/RGO, where the GO loading
283
is 50 wt.%.
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285 286
Figure 4 XPS spectra of O 1s (a), In 3d (b), C 1s (c) and Cu 2p (d) of as-prepared
287
catalysts. 15
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XPS measurement has been carried out to investigate the elemental composition
289
and chemical state of the as-prepared samples. The XPS survey spectrum of
290
Cu2O/RGO/In2O3 sample shows the presence of Cu, C, In and O elements (Figure S2
291
in Supporting Information). The XPS spectra of O 1s of In2O3 and Cu2O/RGO/In2O3
292
are compared in Figure 4a, which have been fitted into three sub-bands depending on
293
their different properties, and denoted as Oα, Oα’ and Oβ, respectively. The band
294
centered at 531.2 eV (Oα) is assigned to the O-atoms in the vicinity of an oxygen
295
vacancy or hydroxyl-like group, and the shoulder band around 531.6 eV (Oα’) is
296
attributed to the chemisorbed water. The peak at 529.5 eV (Oβ) is deemed as the
297
oxygen bond of In‒O‒In.4,43 Different from In2O3, the oxygen vacancy and
298
chemisorbed water exist as the main oxygen species over Cu2O/RGO/In2O3 hybrids,
299
illustrating that the introduction of RGO and the heterojunction of In2O3 and Cu2O
300
and/or RGO accelerate the formation of oxygen vacancies, which is in agreement with
301
the results from Raman, low-temperature ESR and PL spectra.
302
Figure 4b displays the In 3d XPS spectra of In2O3 and Cu2O/RGO/In2O3. The
303
characteristic doublets of In3+ centered at 452.1 and 444.5 eV are assigned to In 3d3/2
304
and In 3d5/2.44 The location of the characteristic doublets of In3+ over
305
Cu2O/RGO/In2O3 hybrid shifts to the higher binding energy by ~0.5 eV, meaning the
306
change of chemical state or coordinated environment.23,45 Herein, the strong chemical
307
bonding between In2O3 and Cu2O and/or RGO is considered to be formed, though this
308
may also be caused by the minor formation of InOOH. The heterojunction between
309
In2O3 and Cu2O and/or RGO results in the possible formation of an electron transfer 16
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channel, benefiting the separation of photo-induced charges during the photocatalytic
311
process.
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Figure 4c shows the high resolution peak of the C 1s of GO and
313
Cu2O/RGO/In2O3 samples. The peaks centered at 288.8, 286.9 and 284.8 eV are
314
assigned to O-C=O (carboxyl groups), C-O-H (epoxy and alkoxy) and C=C-C (sp2-C
315
and sp3-C atoms), respectively.42,46 After the hydrothermal treatment, the intensity of
316
the carbon binding to oxygen in Cu2O/RGO/In2O3 sample decreases, suggesting that
317
most oxygen containing functional groups in layered GO have been reduced to form
318
RGO, well in agreement with the FTIR results. By comparing the XPS spectra of Cu
319
2p of Cu2O octahedra and Cu2O/RGO/In2O3 hybrid (Figure 4d), Cu+ is still the
320
predominant valance of copper species in the hybrid, which is characterized by a Cu
321
2p3/2 binding energy of 932.7 eV and Cu 2p1/2 binding energy of 952.2 eV, though
322
some characteristic peaks of Cu2+ with a weak intensity appear at 934.8, 954.2 and
323
938.5–946.5 eV.47,48
324
3.3 Energy band structure analysis
325
Figure 5a shows the DRS of various samples to indicate the light response and
326
absorption of photocatalysts to the solar light, and the corresponding band-gap
327
energies can be estimated from the Tauc’s plots by (αhν) = A(hν-Eg)1/2, as presented in
328
Figure 5b.19 When energy of photons are equal to even larger than the band gap of
329
photocatalysts, the electrons at ground state are excited and moved to high energy
330
levels. Different peaks in UV-vis DRS reflect the different transition paths of the
331
excited electrons. When the absorption rapidly increases with the decreasing 17
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wavelength, it means that the corresponding energy of photon is equal to the band gap
333
of photocatalysts, where the electrons move from the top of valence band to the
334
bottom of conduction band. For the pristine In2O3 and Cu2O samples, the absorption
335
rapidly raises at the beginning of 400 and 623 nm, respectively, and the corresponding
336
band gaps are 3.09 and 1.99 eV, respectively. Meanwhile, these broad and obscure
337
peaks positioned at smaller wavelength are ascribed to the transition of electrons from
338
lower valence band level to higher conduction band level. For the Cu2O/RGO sample,
339
its great light response ability to the entire UV-visible light is due to the high
340
absorption coefficient of graphene. The absorption of the Cu2O/RGO/In2O3 hybrid is
341
apparently enhanced for the visible light range compared with the pristine In2O3
342
sample, and the approximate band-gap energy is 2.76 eV, significantly reduced from
343
3.09 eV of In2O3. In addition, there is an obvious leaping at 350 nm in Figure 5a,
344
which is caused by switching of different light sources in the equipment.
345
To further describe the band structure of various catalysts, the Mott-Schottky (M-
346
S) plots and valence band XPS (VB-XPS) curves were measured. The M-S plots for
347
electrodes made of Cu2O, In2O3, and Cu2O/In2O3/RGO (Figure 5c) indicate that the
348
flat band potentials of Cu2O and In2O3 are ~0.83 and 0.31 V vs NHE in the dark,
349
respectively, obtained from the x intercept of the linear region. The flat band
350
potentials reflect the differences between the Fermi levels and water-reduction
351
potential of various samples49. From the VB-XPS (Figure 5d), the distances from VB
352
edge levels for the samples of Cu2O, In2O3 and Cu2O/RGO/In2O3 are determined to be
353
0.37, 2.26 and 2.38 eV, respectively, corresponding to the Fermi level (Ef). It is 18
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interesting that the VB-XPS spectra of Cu2O/RGO/In2O3 hybrid exhibit remarkable
355
VB tail states, suggesting the generation of additional diffusive electronic states above
356
the VB edge, which are caused by the presence of oxygen vacancies and the
357
heterojunctions among In2O3 and Cu2O and/or RGO. Then the VB width is broadened
358
due to the rising of valance band maximum (VBM), and the band gap of
359
Cu2O/RGO/In2O3 is narrowed consequently.40,50,51 Combining the results from DRS,
360
M-S plots and VB-XPS, the approximate band positions of various catalysts are
361
presented in Figure 6, in which the CB edge levels were calculated by the equation of
362
ECB = EVB – Eg.52
363 364
Figure 5 (a) DRS of Cu2O, Cu2O/RGO, In2O3 and Cu2O/RGO/In2O3 photocatalysts 19
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and (b) their corresponding plots of (αhν)2 vs. photon energy, (c) Mott-Schottky plots
366
for Cu2O, In2O3 and Cu2O/RGO/In2O3 electrodes, (d) valance band XPS spectra of
367
Cu2O, In2O3 and Cu2O/RGO/In2O3 samples.
368
369 370
Figure 6 Proposed band positions of Cu2O, In2O3, and Cu2O/RGO/In2O3 catalysts
371
according to the band gaps, flat band potentials, and VB positions obtained from
372
Figure 5.
373 374
3.4 Photocatalytic Activity Tests
375
The photocatalytic activity of various samples was evaluated by the
376
photodegradation of MB and the photo-reduction of Cr6+, and the results are presented
377
in Figure 7. By comparison, Cu2O/RGO/In2O3 exhibits the best photocatalytic activity
378
both in the degradation of MB and Cr6+, achieving the degradation efficiency of
379
95.1% and 86.2% after an illumination of 2 h and 6 h, respectively. The reaction rate
380
is represented by the apparent rate constants (k) from the degradation curves of –
381
ln(C/C0’) vs irradiation time, illustrating that the photocatalytic rates of various
382
samples decrease in the following order: Cu2O/RGO/In2O3 > Cu2O/RGO > Cu2O > 20
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In2O3 in the degradation of both MB and Cr6+. Besides, Figure S3 (Supporting
384
Information) displays the photocatalytic activity of Cu2O/RGO/In2O3, Cu2O/In2O3,
385
RGO/In2O3, physically mixed Cu2O and In2O3 (denoted as Cu2O/In2O3 (M)) and a
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physical mixture of all the three components (denoted as Cu2O/RGO/In2O3 (M)). The
387
performance comparison among the referred catalysts confirms the formation of
388
heterojunctions in Cu2O/RGO/In2O3 as well as the heterojunction-assisted charge
389
separation, and the interface between Cu2O and In2O3 as the crucial contributing
390
factor in the enhanced activity of Cu2O/RGO/In2O3 hybrid.
391
392 393
Figure 7 (a) Photocatalytic degradation of MB as a function of illumination time over
394
Cu2O, In2O3, Cu2O/RGO, Cu2O/RGO/In2O3 samples, (b) the corresponding plots of –
395
ln(C/C0’) versus t for MB degradation, (c) photocatalytic reduction of Cr6+ over 21
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various catalysts, and (d) the corresponding plots of –ln(C/C0’) versus t for Cr6+
397
reduction, where C0’ is the concentration of MB or Cr6+ after adlayer equilibration.
398
399 400
Figure 8 Photocatalytic activity dependence on reaction time (a) and average water
401
oxidation rate (b) of various samples for OER under simulated solar light irradiation.
402 403
The photocatalytic activity of Cu2O/RGO/In2O3 catalyst is further verified by the
404
OER, as its band structure matches well to the water oxidation reaction (see Figure 6),
405
and its abundant oxygen vacancies and unique photophysical and electrochemical
406
properties make it as a potential candidate for the water oxidation reaction. From
407
Figure 8 the Cu2O/RGO/In2O3 catalyst exhibits an apparent photocatalytic activity
408
with 26.5 µmol·L-1 after illumination for 150 min under simulated solar light, with an
409
average O2 evolution rate of 212.0 µmol·L-1·g-1. By comparison, the performances of
410
Cu2O, In2O3 and Cu2O/RGO samples are not satisfied, obtaining an O2 evolution of
411
9.8, 5.3 and 13.5 µmol·L-1 at 150 min, with the average O2 evolution rates of 74.7,
412
42.6 and 107.7 µmol·L-1·g-1, respectively. The average O2 evolution rate of
413
Cu2O/RGO/In2O3 catalyst is 4.98 and 1.97 times than that of In2O3 and Cu2O/RGO, 22
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respectively. Then the recycling test was carried out for both the OER and degradation
415
of MB and Cr6+ to evaluate the stability of Cu2O/RGO/In2O3 (Figures S4 and S5 in
416
the Supporting Information). For the water oxidation reaction, the yield of oxygen is
417
decreased to 52% compared with the fresh catalyst, when the catalyst is recycled three
418
times. The reason could be the hindering of photon absorption caused by the reductive
419
deposition of silver ions on the surface of catalysts. While for the photodegradation of
420
MB and Cr6+, due to the uniform distribution of Cu2O, RGO and In2O3, which is
421
confirmed by the elemental mapping image (Figure 2b), the catalyst exhibits much
422
better stability.
423
3.5 Proposed Mechanism for the Enhanced Photocatalytic Activity
424
425 426
Figure 9 (a) The formation and transfer of photo-generated electrons and holes on the
427
pure In2O3 and Cu2O samples, and (b) the proposed mechanism for the enhanced
428
separation of photo-induced electron-hole pairs on Cu2O/RGO/In2O3 hybrid.
429 430
We propose a model to illustrate the increased photocatalytic activity caused by
431
the introduction of Cu2O and RGO to In2O3, and a schematic representation is shown
432
in Figure 9. The poor photocatalytic activity of In2O3 is mainly attributed to its weak 23
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433
light absorbance caused by the wide band gap, and the rapid recombination of
434
electron-hole pairs easily occurs on the pure semiconductors of In2O3 and Cu2O. The
435
as-prepared In2O3 and Cu2O are typical n-type and p-type semiconductors,
436
respectively, and the Fermi level of In2O3 is higher than that of Cu2O. Once the In2O3
437
particles are decorated with p-type Cu2O, the surface contact between In2O3 and Cu2O
438
leads to the formation of p-n heterojunction and the realignment of the valence and
439
conduction bands, due to the thermal equilibrium of different Fermi levels and the
440
formation of a built-in electric field. This allows the Fermi levels of In2O3 and Cu2O
441
to move down and up, respectively, until an equilibrium state forms.53-55 From the
442
view of band structure, the realigning of band edge of In2O3 leads to a downshift of
443
the In2O3 valence band, and the expected significant increase in its oxidation power
444
might promote water oxidation. When the Cu2O/RGO/In2O3 heterojunction system is
445
irradiated by visible light, the narrow band gap of Cu2O favors the excitation to
446
produce electrons and holes. The photo-induced electrons on the Cu2O conduction
447
band transfer to that of n-type In2O3, and the built-in electric field between p-n
448
heterojunction suppresses the recombination of the electron-hole pairs. The addition
449
of RGO allows delocalization of the transferred charge over a much larger volume,
450
and the photothermal effect (PTE) of RGO endows the electrons more energy and
451
faster moving,19 further improving the separation efficiency of the photo-generated
452
carries and accelerating the photocatalytic reaction. Besides, the oxygen vacancies in
453
Cu2O/RGO/In2O3 hybrids should also play key roles in the enhanced photocatalytic
454
activity, by reducing the band edge of the hybrids via inducing new defect levels (as 24
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discussed in Sections 3.2 and 3.3), and also serving as traps for photo-induced charge
456
to inhibit the hole-electron recombination.
457
Conclusions
458
In summary, a Cu2O/RGO/In2O3 hybrid with high photocatalytic water oxidation
459
activity was successfully prepared by a simple, surfactant-free hydrothermal method,
460
that induces the formation of abundant oxygen vacancies. The hybrid exhibits
461
improved photoelectrochemical and photocatalytic performances. We attributed this
462
improvement to (i) a narrowed band gap and the improved light absorbance, a result
463
of the introduction of additional diffusive electronic states above the valence band
464
edge, caused by the oxygen vacancies and the heterojunction among In2O3, Cu2O and
465
RGO; (ii) the lowered valance band of In2O3 and the increase in the oxidation power,
466
caused by the decoration of Cu2O and RGO and especially favoring the OER; (iii) the
467
heterojunction between n-type In2O3 and p-type Cu2O and the resulting built-in
468
electric field, which suppresses the recombination of photo-induced electron-hole
469
pairs efficiently; (iv) the synergism of RGO, the excellent conductivity and
470
photothermal effect of RGO lead to the much faster transfer of photo-electrons and
471
endow the photo-electrons more energy to accelerate the photocatalytic reaction.
472 473
Associated Content
474
Supporting Information
475
Additional details concerning the characterization methods, results of the PL
476
spectra and XPS survey spectrum. And the photocatalytic activity comparison among 25
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Cu2O/In2O3,
RGO/In2O3,
Cu2O/In2O3
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477
Cu2O/RGO/In2O3,
(M)
and
478
Cu2O/RGO/In2O3(M), as well as the recycle test for Cu2O/RGO/In2O3 are also
479
provide in the Supporting Information. This material is available free of charge via the
480
Internet at http://pubs.acs.org.
481
Acknowledgements
482
This work was supported financially by the National Nature Science Foundation
483
of China (21507029, 21501138), Nature Science Foundation of Hebei Province
484
(B2016502063), Open Foundation of Key Laboratory of Industrial Ecology and
485
Environmental Engineering (KLIEEE-15-02), China Ministry of Education and the
486
Fundamental Research Funds for the Central Universities (2016MS109), and
487
Australian Research Council (DP150103026). The authors also acknowledge the use
488
of equipment, scientific and technical assistance of the WA X-Ray Surface Analysis
489
Facility, funded by the Australian Research Council LIEF grant LE120100026.
490
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