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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

4

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

8

WA 6845, Australia

9 10

c

School of Chemical Engineering, Shandong University of Technology, Zibo, 255049, China

11 12 13

Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth,

d

School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia

14 15 16

*Corresponding authors: Dr. Jie Liu ([email protected]) Prof. Shaobin Wang ([email protected])

1

ACS Paragon Plus Environment


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Abstract

17 18

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A

novel

shape

controlled

Cu2O/reduced

graphene

oxide/In2O3

19

(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

28

the charge transfer and distribution onto the graphene sheets, which causes the

29

downshift of VB of In2O3 and the significant increase in its oxidation potential.

30

Additionally, a built-in electric field is generated on the interface of n-type In2O3 and

31

p-type Cu2O, suppressing the recombination of photo-induced electron-hole pairs and

32

allowing the photo-generated electrons and holes to participate in the reduction and

33

oxidation reactions for oxidizing water molecules and pollutants more efficiently.

34 35

Keywords: Oxygen vacancies, p-n heterojunction, shape controlled synthesis,

36

photocatalysis, indium oxide

2


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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

41

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

43

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

45

through narrowing the band gap and improve the carrier separation efficiency by

46

serving as the active sites.4,5 Indium oxide (In2O3), a typical n-type semiconductor

47

with a band gap of ~2.8 eV, possesses a low electrical resistivity, favorable surface

48

properties, and low toxicity, making it a good candidate for electronic and

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photocatalytic applications, such as environmental remediation, solar hydrogen

50

production, and nanodevices.6-8 Liang et al. demonstrated that oxygen vacancies

51

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

53

vacancies were detected on the ultrathin In2O3 porous sheets, and were considered to

54

play key roles in the high performance for water splitting on In2O3 under visible-light

55

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

66

along with the specific crystal facets more easily, and the readily accumulation of

67

photo-induced electrons on the (111) facets of Cu2O and its distinctive energy levels

68

will accelerate the photocatalytic reactions.15-17 Furthermore, introduction of an

69

electron-trapping material, such as 2D carbonaceous graphene, is also an effective

70

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.

75

Though some works about Cu2O/RGO, In2O3/RGO and Cu2O/In2O3 hybrids have

76

been reported, the photocatalytic activity of In2O3 decorated by single component of

77

Cu2O is still not satisfied, and their application in the photocatalytic oxygen evolution

78

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

80

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

86

the activity of the three-component photocatalyst, we proposed a possible mechanism

87

for the enhanced photocatalytic activity, based on a series of characterizations. The

88

factors influencing the photo-chemical properties of Cu2O/RGO/In2O3 were also

89

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

94

NaOH solution (3 mol/L) was added drop by drop. After the addition of 1 g glucose,

95

the obtained blue suspension was heated at 60 ºC for 30 min under stirring. The

96

obtained red slurry was filtered, washed by H2O and ethanol, and then dried at 60 ºC

97

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-

101

lined stainless steel autoclave (125 mL) and reacted at 125 ºC for 4 h. The obtained

102

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

107

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

110

steel autoclave (45 mL) and reacted at 180 ºC for 6 h. After the reaction, the obtained

111

slurry was washed by H2O and ethanol, and dried at 60 ºC for 12 h. The loading of

112

GO and Cu2O were both at 10 wt.%.

113

2.1.4 Synthesis of Cu2O/RGO, RGO/In2O3 and RGO

114

The synthesis of Cu2O/RGO, RGO/In2O3 and RGO was conducted similarly to

115

that of Cu2O/RGO/In2O3, while the weights of GO, octahedral Cu2O and In2O3 were

116

0.03 g, 0.03 g and 0.3 g, respectively, when used in the synthesis.

117

2.2 Characterization methods

118

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

120

microscope (FE-SEM), transmission electron microscope (TEM), high-resolution

121

transmission electron microscope (HR-TEM), X-ray diffraction (XRD), Raman,

122

electron spin resonance (ESR), Fourier transform infrared spectroscopy (FTIR), X-ray

123

photoelectron spectroscopy (XPS), valence band XPS (VB-XPS), UV-vis diffuse

124

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

127

The PEC measurements were carried out using a Zahner Zennium

128

electrochemical workstation operated in a standard three-electrode cell with a

129

fluorine-doped tin oxide (FTO) electrode deposited with Cu2O/RGO/In2O3 samples as

130

a photoanode, a saturated calomel electrode (SCE) as the reference electrode and a Pt

131

wire as the counter electrode, respectively. For the fabrication of the working

132

electrode, 40 mg catalyst was mixed with a certain amount of ethanol and Nafion

133

solution homogeneously, and the obtained sample was deposited as a thin film on the

134

FTO glass with a controlled area of 1 cm2 using spin-coating, and then dried at 60 °C

135

for 30 min to form a film electrode. The PEC performance of the catalysts was

136

measured under a 300 W Xe-lamp, and 0.5 M Na2SO4 solution (40 mL, pH = 6.8,

137

25 °C) was used as the electrolyte. The electrode potential versus saturated calomel

138

electrode (SCE) can be converting to reversible hydrogen electrode (RHE) potential

139

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,

142

and VSCE is the experimental potential measured against the Hg/Hg2Cl2/saturated KCl

143

reference electrode.

144

2.4 Photocatalytic Measurement

145

The photocatalytic activity of various catalysts was measured by OER and the

146

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

148

of OER was evaluated by oxidizing water molecules with AgNO3 as an electron

149

scavenger. In a typical procedure, 0.1 g of catalyst was added to 200 mL solution,

150

including AgNO3 (0.03 M) and La2O3 (0.2 g). Prior to irradiation, the suspensions

151

were mixed under vigorous stirring for 30 min in the dark and degassed to remove O2

152

in solution. The O2 concentration in the reactor was in situ taken using a NEOFOX O2

153

probe. For the photodegradation of MB，the concentrations of MB and catalyst were

154

10 and 25 mg·L-1, respectively. Before the illumination, the MB solution with the

155

catalyst was mixed in the dark to establish the adsorption-desorption equilibrium. The

156

concertation of MB depending on reaction time was analyzed by a UV-vis

157

spectrophotometer (JASCOV-670) at its characteristic absorption wavelength of 664

158

nm. For the photoreduction of Cr6+, the concentrations of Cr6+ and catalyst were 50

159

and 25 mg·L-1, respectively, with a pH value of 3, and the activity was measured at the

160

characteristic absorption wavelength of 325 nm.

161

3. Results and Discussion

162

3.1 Morphology of as-prepared catalysts

163

Figure 1a displays SEM images of the as-prepared In2O3. The typical

164

morphology of In2O3 support is a hollow corn-like structure consisting of uniform

165

nanorods, which is further confirmed by the magnified SEM (inset in Figure 1a) and

166

the TEM image (Figure 1b), indicating that the length and width of each corn unit are

167

at ~750 and ~170 nm, respectively. In Figure 1c, the interplanar spacing of 0.296 nm

168

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.

170

In previous reports,27,28 it has been proved that every two Cu atoms have a dangling

171

bond perpendicular to the (111) plane because of the termination of the crystal cell

172

(Figure 1e), and the exposure of Cu atoms with the dangling bonds resulting in the

173

high energy status of the (111) surface. The better photocatalytic activity of octahedral

174

Cu2O has been widely reported.15,29

175

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Figure 1 (a) SEM image of corn-shaped In2O3, inset: magnified SEM image of corn-

178

shaped In2O3, (b) TEM image of corn-shaped In2O3, (c) HRTEM of corn-shaped

179

In2O3 from the yellow box in Figure 1b, (d) SEM image of Cu2O octahedra, and (e)

180

three-dimensional model and atomic arrangement of the (111) planes of Cu2O

181

octahedra (red: O atoms, orange: Cu atoms). 9



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182 183

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

192

speculated as InOOH deriving from In2O3. The elemental mapping image (Figure 2b)

193

confirms the uniform distributions of In, Cu, O and C in the Cu2O/RGO/In2O3 sample,

194

though the thin layered RGO is difficult to be observed in the SEM. However, TEM

195

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

197

by the EDS spectra (inset image in Figure 2c). In Figure 2d, the lattice fringes with d

198

spacing of 0.296 and 0.423 nm are assigned to the (222) and (211) planes of In2O3,

199

respectively, and the interplane distance of 0.242 nm matches well with the (111)

200

plane of Cu2O octahedra. The (222) and (211) planes of In2O3 are closely contacted to

201

the (111) plane of Cu2O, suggesting the formation of heterojunction between In2O3

202

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

204

performance.30-32

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3.2 Structure and Oxygen Vacancies in Cu2O/RGO/In2O3

206

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

210

spectra of GO, Cu2O/RGO and Cu2O/RGO/In2O3 samples.

211 212

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

215

of Cu2O, and its much stronger intensity compared with that of other peaks indicates

216

its high exposure. The XRD pattern of Cu2O/RGO is similar to that of Cu2O

217

octahedra, suggesting the excellent stability of the (111) facet. For In2O3, the

218

diffraction peaks at 21.4º, 30.5º, 35.4º, 45.6º, 50.9º and 60.5º correspond to the (211),

219

(222), (400), (134), (440) and (622) planes of a cubic In2O3 (PDF#63-3170),

220

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),

223

(101), (011), (211), (121) and (002) planes of orthorhombic InOOH, respectively

224

(PDF#17-0549). InOOH is speculated to be formed by the partial hydrolysis of In2O3

225

crystals during the second hydrothermal process. Comparing the XRD patterns of

226

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,

228

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

232

are detected at 218 and 265 cm-1.24 In the case of the pure In2O3 sample, the features

233

at 306, 364, 493 and 626 cm-1 are interpreted as the vibration mode of In2O3, and the

234

peak of 364 cm-1, which is assigned to the stretching vibrations of In‒O‒In, reflects

235

the oxygen vacancies in the structure of In2O3.33,34 After the introduction of RGO, two

236

new peaks appear on Cu2O/RGO and Cu2O/RGO/In2O3 samples, centered at 1329 and

237

1590 cm-1, and are assigned to the disorder band associated with structural defects

238

generated in graphene (D band) and the well-ordered E2g phonon scattering of sp2

239

carbon atoms of graphene (G band), respectively.22 Compared with pure In2O3, the

240

characteristic wavenumbers attributed to In2O3 on Cu2O/RGO/In2O3 hybrid slightly

241

shift to the lower frequency, setting at 293, 352, 483 and 608 cm-1, respectively,

242

suggesting the probable formation of the heterojunction between In2O3 and Cu2O

243

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

245

Cu2O/RGO/In2O3. The absence of Cu2O features on Cu2O/RGO/In2O3 could be

246

attributed the low loading of Cu2O (10 wt.%).

247

The presence of oxygen vacancies in Cu2O/RGO/In2O3 is further proved by the

248

low-temperature ESR spectra. As shown in Figure 3c, both the In2O3 and

249

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

251

on Cu2O/RGO/In2O3 is 126010, much higher than that of 78224 of In2O3,

252

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

254

of In2O3 and Cu2O/RGO/In2O3. The peaks at 466 and 492 nm are corresponded to the

255

oxygen vacancies as the shallow donor and those in deeper level, respectively.17,36,37

256

The decreased intensity of the peak at 466 nm over Cu2O/RGO/In2O3 means the less

257

recombination of the electrons with the oxygen vacancies at shallow, which is mainly

258

caused by the rapid transfer of electrons by RGO sheets and the interface between

259

In2O3 and Cu2O. For the signal at 492 nm, the PL intensity over Cu2O/RGO/In2O3 is

260

enhanced, which is speculated to be caused by the much more oxygen vacancies in

261

deep level in Cu2O/RGO/In2O3, probably generated by the formation of

262

heterojunctions between In2O3 and Cu2O and/or RGO. The oxygen vacancies on

263

Cu2O/In2O3/RGO hybrid is speculated to be caused by the surface defects formed

264

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

267

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.

272

Additionally, the reduction of GO during the synthesis of Cu2O/RGO and

273

Cu2O/RGO/In2O3 samples is confirmed by the FTIR spectra. As presented in Figure

274

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.%.

284

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

288

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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310

channel, benefiting the separation of photo-induced charges during the photocatalytic

311

process.

312

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

332

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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354

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

365

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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383

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

386

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

396

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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414

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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


Page 24 of 35

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455

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu2O/In2O3,

RGO/In2O3,

Cu2O/In2O3

Page 26 of 35

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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Oxygen Vacancies in Shape Controlled Cu2O ... - ACS Publications

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