a novel nanocomposite based on tio2/cu2o/reduced graphene oxide with enhanced solar-light-driven...
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Accepted Manuscript
Title: A novel nanocomposite based on TiO2/Cu2O/reducedgraphene oxide with enhanced solar-light-drivenphotocatalytic activity
Author: Bruna M. Almeida Maurıcio A. Melo Jr JeffersonBettini Joao E. Benedetti Ana F. Nogueira
PII: S0169-4332(14)02348-4DOI: http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.105Reference: APSUSC 28970
To appear in: APSUSC
Received date: 15-8-2014Revised date: 18-10-2014Accepted date: 19-10-2014
Please cite this article as: B.M. Almeida, M.A.M. Jr, J. Bettini, J.E. Benedetti, A.F.Nogueira, A novel nanocomposite based on TiO2/Cu2O/reduced graphene oxide withenhanced solar-light-driven photocatalytic activity, Applied Surface Science (2014),http://dx.doi.org/10.1016/j.apsusc.2014.10.105
This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
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A novel nanocomposite based on TiO2/Cu2O/reduced graphene 1
oxide with enhanced solar-light-driven photocatalytic activity2
3
Bruna M. Almeida a, Maurício A. Melo Jr a, Jefferson Bettini b, João E. 4
Benedetti a and Ana F. Nogueira a,*5
6
a Institute of Chemistry, University of Campinas, UNICAMP, P.O. Box 6154, 13084-971 7
Campinas, São Paulo, Brazil8
b National Nanotechnology Laboratory, National Center for Energy and Materials9
Research, CNPEM, 13083-970, Campinas, São Paulo, Brazil10
*Corresponding author: Phone: +55 19 3521-3029; E-mail: [email protected]
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GRAPHICAL ABSTRACT13
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HIGHLIGHTS15
A novel nanocomposite 16
was obtained by coupling TiO2 and Cu2O nanoparticles with RGO.17
The method led to well-18
dispersed nanoparticles on the RGO nanosheets.19
Small Cu2O 20
nanoparticles with an average diameter of 5 nm were obtained.21
The final composite 22
enhanced the photoactivity of the separate intermediates.23
Photoelectrochemical 24
experiments confirmed the superiority of TiO2/Cu2O/RGO.25
26
ABSTRACT27
A novel nanocomposite composed of TiO2 and Cu2O nanoparticles combined with reduced 28
graphene oxide (RGO) was synthesized and characterized. X-ray diffraction (XRD), 29
scanning electron microscopy (SEM), high-resolution transmission electron microscopy 30
(HRTEM), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron 31
spectroscopy (XPS), thermogravimetry (TG) and elemental analysis were employed to 32
investigate the structure, morphology, optical properties and composition of the33
nanocomposite and the intermediate materials. The photocatalytic activity of 34
TiO2/Cu2O/RGO and the individual materials were studied through the photodegradation of 35
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methylene blue under solar radiation. A considerable increase in the photodegradation 36
activity using the nanocomposite was obtained after 5 h (~95% of MB degradation). 37
Photoelectrochemical studies were carried out and confirmed the superiority of the novel 38
nanocomposite in the photocurrent generation. The highest activity resulted from the 39
synergy of this carbonaceous structure with TiO2 and Cu2O, which could absorb a wider 40
portion of the solar spectrum, adsorb higher quantities of methylene blue on the surface and 41
improve the effective separation of the generated electron-hole pairs.42
43
Keywords: reduced graphene oxide, titanium dioxide, cuprous oxide, photocatalysis, 44
methylene blue45
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1. Introduction63
The textile industries are primarily responsible for the discharge of significant 64
amounts of colored effluents into water sources, followed by the leather, cosmetic, pulp, 65
food, and cellulose industries. Most of the dyes constitute a problematic group of pollutants 66
because they are non-degradable and harmful to the flora and fauna and because certain 67
dyes have been reported to be carcinogenic and mutagenic. Among the many dyes used in 68
these industries, methylene blue (MB) is one of the most common because it is widely used 69
for printing fabrics, such as calico, cotton and tannin, for dyeing leather, and in the 70
production of polymers, such as nylon. MB is also used for medicinal purposes as an 71
antiseptic agent when in its purified zinc-free form[1–3].72
The residues derived from the activities involving methylene blue are difficult to 73
treat by conventional methods because of the strong interaction of this compound with a 74
large variety of substrates. These MB residues are also toxic to aquatic living beings 75
because they cause a decrease in the amount of dissolved oxygen and modify the water 76
properties[1,4]. As a method to overcome these environmental issues, light-assisted 77
degradation of organic pollutants, such as dyes, has been extensively employed as a 78
promising strategy for remediation[5–7].79
The direct ultraviolet light or sunlight photodegradation of organic pollutants, such 80
as methylene blue, can be performed with the aid of a specific semiconductor, which acts as 81
a catalyst of the process. Thus, the catalytic photodegradation of methylene blue using this 82
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approach highly depends on the photoreactivity of the specific solid used as the 83
catalyst[8,9]. The semiconductor titanium dioxide (TiO2) is widely known as one of the 84
most efficient and environmentally friendly photocatalysts because of its good 85
photocatalytic performance, low cost, high stability and nontoxicity. Unfortunately, TiO286
also possesses disadvantages, such as low absorption of visible radiation because of its 87
wide bandgap (3.2 eV), high recombination rate of the photogenerated electron-hole pairs 88
and difficult recovery from treated water[10]. Many efforts have been made to enhance the 89
titanium dioxide photocatalytic activity, attempting to shift the light absorption towards 90
visible light and to increase the lifetime of the photogenerated electron-hole pairs[11,12].91
Sensitization of TiO2 by MB molecules in solar cells and for H2 generation from water 92
splitting, for example, have already been reported[13,14].93
Sensitization of the titanium oxide nanoparticles with low-bandgap semiconductors 94
has recently been reported as one of the best options for harvesting the ultraviolet and the95
visible portion of the solar spectrum[10]. Thus, a higher photocatalytic activity can be 96
achieved because a low-bandgap semiconductor can potentially generate multiple electron-97
hole pairs per incident photon. Many different reports have described visible-bandgap 98
semiconductors for sensitization, such as CdS, SnS2, PbSe and CdSe[15–20]. Although 99
these materials have presented a satisfactory photodegradation performance, it was not 100
sufficient for the development of commercial photocatalytic devices. Alternatively, Cu2O is 101
a highly promising low-bandgap semiconductor because of its reasonable bandgap value 102
(2.2 eV), which offers new opportunities for higher solar light harvesting, and a higher 103
stability, especially compared with the chalcogenide semiconductors[21].104
Previous reports have shown that carbonaceous materials, such as carbon nanotubes, 105
active carbon and carbon black, have certain beneficial effects on the photocatalytic activity 106
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of TiO2[22,23]. However, certain complications remain, hindering the additional upgrade of 107
the efficiency of the TiO2-carbon composites because of the lack of reproducibility 108
resulting from the preparation and treatment variations. More recently, graphene and its 109
derivatives have been widely studied to overcome the disadvantages of these previous 110
carbonaceous materials, with an emphasis on increasing the light absorption and charge 111
transport[24–26]. The unique mechanical, electrical, thermal and optical properties of 112
graphene, a two dimensional sheet of sp2-hybridized carbon, have attracted significant 113
attention for presenting better conductivity and higher surface area than carbon nanotubes 114
and active carbon[27–29]. Furthermore, composites composed of graphene (and its 115
derivatives) and titanium dioxide have also been exhaustively investigated[30–34].116
One example of an important graphene derivative is graphene oxide (GO), 117
composed of a layered structure with reactive functional groups containing oxygen located 118
on the basal planes and edges[25]. The oxygen-containing functional groups in this 119
structure make graphene oxide an excellent support to anchor the TiO2 nanoparticles, 120
promoting an enhancement of its photocatalytic activity[35].121
A diversified range of potential applications is also expected from another graphene 122
derivative, referred to as reduced graphene oxide (RGO), because of its remarkable 123
properties, including high electron mobility at room temperature, large theoretical specific 124
surface area, excellent thermal conductivity and good optical transparency. RGO can play a 125
crucial role in photocatalysis, enhancing the separation of the photogenerated electron-hole 126
pairs because of its electron-capturing ability[24,36,37].127
In this study, we report, for the first time, the synthesis and characterization of a 128
novel nanocomposite composed of titanium dioxide, cuprous oxide nanoparticles and 129
reduced graphene oxide sheets. An efficient methylene blue photodegradation process 130
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under solar light was realized using a successful combination of these three materials in an 131
attempt to harvest a wider portion of solar radiation, including ultraviolet light and visible 132
light, and to minimize the recombination of the photogenerated electron-role pairs during 133
the photocatalytic process. Photoelectrochemical studies were also carried out and the 134
results corroborate with the photocatalytic investigation.135
136
2. Experimental137
2.1. Chemicals138
The titanium dioxide precursor with a particle diameter of approximately 15 nm 139
(P90) was a high-grade reagent purchased from Evonik, Brazil. Copper(II) acetate 140
monohydrate P. A. (Vetec), ascorbic acid (Aldrich) and sodium hydroxide P. A. (Synth) 141
were used in the cuprous oxide nanoparticles synthesis. For the reduction of graphene 142
oxide, ammonium hydroxide (Synth) and hydrazine hydrate (Aldrich) were used. All of the 143
other reagents, such as ethylene glycol P. A. (Reagen), ethanol P. A. (Synth) and methylene 144
blue P. A. (Synth), were also high-grade reagents and were used as received without prior 145
purification. The methylene blue solutions were prepared in deionized water (ultra-pure 146
Milli-Q Millipore, 18.2 M cm).147
148
2.2. Reduction of graphene oxide 149
The graphene oxide (GO) used in this study was synthesized from graphite (with a 150
particle diameter ranging from 3 to 11 μm) using Hummer’s method[38]. The graphene 151
oxide reduction was performed as follows[39]: graphene oxide (0.2385 g) was added to 100 152
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mL of deionized water in a round-bottom flask and sonicated for 30 min. Subsequently, the 153
pH of the system was set to 10 by adding 10 drops of ammonium hydroxide (NH4OH), and 154
then 1.3 mL of hydrazine hydrate (80%) was added. The round-bottom flask was connected 155
to a reflux system and heated at 80°C for 24 h.156
157
2.3. Preparation of the Cu2O nanoparticles158
The cuprous oxide (Cu2O) nanoparticles was synthesized according to the procedure 159
described by Li et. al.[40]. Copper(II) acetate (0.4 g) was dissolved in 30 mL of deionized 160
water. To this solution, 40 mL of 0.2 mol L-1 NaOH was added under stirring followed by 161
20 mL of 0.1 mol L-1 ascorbic acid, and the system was heated at 50°C for 30 min. Finally, 162
the material was centrifuged, washed with deionized water and ethanol and dried in an oven 163
for 2 h under vacuum.164
165
2.4. Synthesis of the TiO2/RGO nanocomposite166
The synthesis of the composite formed by the combination of the titanium dioxide 167
(TiO2) nanoparticles and the reduced graphene oxide (RGO) was performed according to 168
the procedure described by Williams et al.[35]. Initially, a suspension of titanium dioxide 169
was prepared in ethanol (80 mg of TiO2 per liter of alcohol). To every 1 L of this solution, 170
14.25 mg of graphene oxide (GO) was added. Next, the resultant solution was sonicated for 171
40 min to avoid agglomerations and deposited on a Petri dish. This system was placed in an 172
ultraviolet (UV) chamber in which it was exposed to the UV radiation until the composite 173
became completely dark. The color change of the material was monitored by diffuse 174
reflectance spectroscopy. The dark solid formed was named TiO2/RGO.175
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176
2.5. Synthesis of the Cu2O/RGO nanocomposite177
The synthesis of the Cu2O/RGO nanocomposite, composed of the combination of 178
the reduced graphene oxide and the cuprous nanoparticles, was performed in two steps, 179
adapted from the procedure described by Xu et al.[41]. In the first step, 100 mg of graphene 180
oxide and 60 mg of copper(II) acetate were dispersed in 200 mL of ethanol and sonicated. 181
The mixture was maintained under continuous stirring at 25°C for 4 h followed by 182
centrifugation. The solid was washed four times with ethanol to remove the impurities and 183
dried in an oven at 60°C until the ethanol was completely evaporated. 184
In the second part of the synthesis, 50 mg of the solid obtained in the first step and 185
50 mL of ethylene glycol were added in a round-bottom flask connected to a reflux system, 186
maintained under constant stirring and heated to 140°C. Then, 2.5 mL of deionized water 187
was added, and the temperature of the system was increased to 160°C. After 2 h, the 188
suspension was cooled to room temperature, centrifuged and washed with ethanol to ensure 189
that all of the ethylene glycol and the other soluble impurities were removed. The obtained 190
solid, named Cu2O/RGO, was dried in an oven at 60°C under vacuum.191
192
2.6. Preparation of the TiO2/Cu2O/RGO nanocomposite193
The synthesis of the final TiO2/Cu2O/RGO nanocomposite was performed in two 194
steps adapted from the procedures used to prepare TiO2/RGO and Cu2O/RGO. The 195
preparation started with a partial photoreduction of graphene oxide in the presence of 196
titanium dioxide. Initially, a suspension composed of TiO2 and GO was prepared in 197
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ethanol, sonicated for 30 min, and deposited on a Petri dish. The entire system was placed 198
in an ultraviolet light chamber and was exposed to radiation for 4 h under stirring until part 199
of the graphene oxide was photoreduced by the ultraviolet radiation, forming the 200
TiO2/RGO nanocomposite with the remaining non-reduced graphene oxide. The time of 4 h 201
was previously determined by diffuse reflectance spectroscopy because after this period,202
the graphene oxide was partially reduced. The obtained composite was dried in an oven at 203
60°C under vacuum until the ethanol was completely evaporated.204
The second step of the preparation of the final nanocomposite was performed205
according to the procedure used to synthesize Cu2O/RGO, replacing the pure graphene 206
oxide with the partially photoreduced TiO2/RGO.207
208
2.7. Methylene blue photodecomposition209
The photodegradation tests of methylene blue were performed using the TiO2, 210
Cu2O, TiO2/RGO, Cu2O/RGO and TiO2/Cu2O/RGO solids according to the following 211
procedure: approximately 80 mg of the synthesized catalyst was dispersed in 5 mL of 212
ethanol and sonicated for 2 min. Then, the suspension was deposited on a Petri dish 213
(diameter of 22 cm). The entire system was placed in an oven at 60°C under vacuum for the 214
solvent evaporation. A film of the catalyst was formed at the bottom of the Petri dish. Then, 215
100 mL of an aqueous solution of 25 mg L-1 methylene blue was added to the Petri dish,216
and it was covered with PVC film to avoid the evaporation of water. Prior to the beginning 217
of the tests, the entire system was maintained in the dark at room temperature for 2 h to 218
reach the adsorption-desorption equilibrium of methylene blue on the photocatalysts, and 219
afterwards, the system was exposed to the radiation of a solar simulator (Solsim) for 5 h. 220
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Aliquots (3 mL) were obtained from the photodegradation system at the beginning of the 221
experiment (0 h) and after 1, 2, 3, 4 and 5 h. Each aliquot was diluted with deionized water 222
to 8 mL and analyzed with a UV-Vis spectrophotometer.223
224
2.8. Photoelectrochemical studies225
The photoelectrochemical experiments were carried out in an Eco Chimie-Autolab 226
PGSTAT 12 potentiostat. Chronoamperometric measurements were performed using a 227
three electrode configuration cell (Ag/AgCl as reference electrode, the synthesized 228
semiconductors and nanocomposites films as working electrodes and a platinum wire as 229
counter electrode) at room temperature. 1 mol L-1 solution of Na2SO3 in water was used as 230
electrolyte. The photoelectrochemical cells were placed in an optical bench consisting of an 231
Oriel Xe(Hg) 250 W lamp coupled with an AM 1.5 filter (Oriel), collimating lenses and a 232
water filter (Oriel). The light intensity was calibrated with an optical power meter; model 233
1830-C (Newport) to 100 mW cm-2.234
2.9. Instrumentation235
The powder X-ray diffraction patterns were used to unveil the phase information 236
and the crystalline quality of the prepared nanocomposites and were collected on a 237
Shimadzu model XRD-7000 diffractometer using Cu Kα radiation (0.154 nm) at 40 kV and 238
30 mA with a rate of 2.0° min-1 for 2θ measurements over the range of 5 to 80°.239
The spectra of the diffuse reflectance spectroscopy (DRS) were acquired using a 240
Varian spectrophotometer model Cary 5G UV-Vis, NIR with a diffuse reflectance 241
accessory.242
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Nitrogen adsorption-desorption analyses were obtained in a Quantachrome Nova 243
4200 instrument at 77 K. The samples were previously outgassed at 80 ºC for 12 h. The 244
Brunauer-Emmett-Teller (BET) method was employed to calculate the specific surface 245
areas.246
The surface morphology was studied using the secondary electron images acquired 247
on a JEOL JSM 6340 F scanning electron microscope, operating at a voltage of 5 kV and a 248
current of 12 µA. The samples were fixed onto double-faced carbon tape adhered to an 249
aluminum support and carbon-coated in a Bal-Tec MD20 instrument.250
The high-resolution transmission electron microscopy (HRTEM) images were 251
obtained using a HRTEM-JEM 3010 URP microscope at an accelerating voltage of 300 kV 252
at the Electron Microscopy Laboratory (LME) located at the Brazilian National 253
Nanotechnology Laboratory (CNPEM). The sample for the TEM analysis was prepared by 254
dispersing the nanocomposite powder in isopropanol via sonication for 15 min followed by 255
drop-casting onto a carbon-coated copper TEM grid (400 mesh).256
The Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive 257
Spectroscopy (EDS) mappings were acquired using a JEOL JEM 2100F microscope 258
equipped with an EDS detector and a GIF Tridiem 863 filter.259
A ThermoVGScientific Sigma Probe instrument was used in the XPS 260
measurements. The monochromatic Al Kα X-ray source (1486.6 eV) was focused on the 261
samples and the high resolution spectrum was collected with pass energy of 20 eV, energy 262
step size of 0.1 eV with flood gun on to eliminate surface charge.263
The absorbance of the methylene blue solutions during the photodegradation tests 264
was analyzed in a UV-Vis spectrophotometer (model Hewlett Packard 8453). Cuvettes with265
an optical path of 1 cm were used.266
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3. Results and Discussion267
As described in Section 2.6, the procedure used in this study to synthesize the final268
TiO2/Cu2O/RGO nanocomposite is a combination of well-known methods used to prepare 269
composites that combine reduced graphene oxide sheets with TiO2 and Cu2O nanoparticles 270
separately.271
The mechanism involved in the combination of RGO and the TiO2 nanoparticles to 272
form the intermediate TiO2/RGO nanocomposite can be explained by the reactions 273
described in Eq. (1) and (2), as follows[35]:274
TiO2 + UV light (hʋ) → TiO2 (h+ + e-) + C2H5OH → TiO2 (e
-) + ●C2H4OH + H+ (1)275
276
According to these equations, irradiation of the semiconductor TiO2 with ultraviolet 277
light promotes the excitation of electrons to the conduction band (CB), creating the 278
electron-hole pairs. The electrons photogenerated in this process are responsible for the 279
reduction of the graphene oxide. In addition, the ethanol present in the reaction medium 280
acts as a hole (h+) scavenger, producing ethoxy radicals[35]. 281
The process involved in the synthesis of the Cu2O/RGO nanocomposite is called the282
polyol process, which is often used to prepare metallic nanoparticles on the surface of 283
carbonaceous materials. In this procedure, polyethylene glycol acts as a solvent, stabilizer 284
and reducing agent, and it limits the size of the nanoparticles and prevents285
agglomeration[42]. In addition to reducing copper(II) to copper(I), ethylene glycol reduces 286
graphene oxide to reduced graphene oxide[41].287
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In summary, the surface of the exfoliated GO sheets is covered by hydroxyl, 288
carboxyl and epoxy groups, resulting from the oxidation process. Because of these289
functional oxygen-containing groups, when the GO was suspended with Cu(Ac)2, the Cu2+290
ions adsorbed onto its surface, bonding to the available O atoms via electrostatic forces. 291
The Cu2O nanoparticles were then formed from the reduction of these species dispersed on292
the GO sheets, and GO was reduced to RGO by ethylene glycol at 160°C[40,42].293
The intermediate composites, the precursor nanoparticles TiO2 and Cu2O and the 294
final TiO2/Cu2O/RGO composite were characterized by X-ray diffractometry, scanning and 295
transmission electron microscopy and diffuse reflectance spectroscopy before being used in296
the methylene blue photodegradation tests. Furthermore, the final nanocomposite was 297
characterized through elemental analysis and thermogravimetry, which indicated a 298
composition of 69% TiO2, 12% RGO and 19% Cu2O, which is in accordance with the 299
molar quantities of the reagents used in the synthesis.300
Graphite, pre-oxidized graphite and graphite oxide were the precursor materials for 301
the synthesis of reduced graphene oxide (RGO). In the last step, graphene oxide was 302
chemically reduced with hydrazine hydrate to yield the reduced graphene oxide. Thus, 303
acquiring information about the crystalline planes of these precursor materials used in the 304
synthesis of RGO through the X-ray diffraction technique is of fundamental importance to 305
ensure the quality of the structure of the final composites.306
The diffraction pattern acquired for graphite (Fig. S1, Supplementary data) presented307
an intense peak at 26.6°, which is related to the reflection plane (002) with a basal distance 308
of 3.35 Å. This peak is evidence of the crystallinity of this structure[43]. The pre-oxidized 309
graphite pattern (Fig. S1, Supplementary data) also presented one intense peak at 26.6°, 310
attributed to the (002) plane, which was broader than the corresponding peak in the XRD 311
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pattern of graphite. The broadening of this peak provides evidence that the graphite pre-312
oxidation favors the next step of the oxidation. After the oxidation of graphite through 313
Hummer’s method to form graphite oxide, only one peak is observed at 11.5°, showing that 314
the basal spacing increased from 3.35 to 7.97 Å. The increase in the distance between the 315
graphite planes results from the introduction of hydrophilic functional groups and the 316
intercalation of water molecules between the graphite layers after the oxidation process[44]. 317
The diffraction pattern of graphene oxide reduced by hydrazine hydrate (Fig. S1, 318
Supplementary data) only shows a broad peak at 25.0°, which implies that either the 319
formation of the reduced graphene oxide or structures with few graphene stacked layers320
were obtained. Furthermore, the basal distance of the RGO (3.37 Å) is slightly higher than 321
that found in pure graphite (3.35 Å), suggesting the presence of a few residual functional 322
groups between the layers after the chemical reduction stage[45].323
Fig. 1 shows the X-ray diffraction patterns of the precursor TiO2 and Cu2O324
nanoparticles and the TiO2/RGO, Cu2O/RGO and TiO2/Cu2O/RGO nanocomposites. 325
Certain peaks observed in the TiO2 diffraction pattern (Fig. 1a) are related to the anatase 326
phase at 25.4, 36.1, 38.0 and 48.3°, and the other peaks located at 27.4 and 41.4° are 327
characteristic of the rutile phase[46]. Higher quantities of TiO2 anatase compared with TiO2328
rutile present in the precursor material favor the photocatalytic activity, according to 329
previous reports[47,48].330
The diffraction pattern of the cuprous oxide (Cu2O) nanoparticles is shown in Fig. 1b. 331
This diffractogram presents peaks at 29.4, 36.7, 42.4, 61.5, 73.5 and 77.4 °, which are332
related to cuprous oxide (Cu2O) in the cuprite form[42].333
Fig. 1c shows the diffraction pattern of the TiO2/RGO composite after 4 h of 334
ultraviolet light exposure. The characteristic signals of the TiO2 anatase phase at 25.6, 38.1, 335
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48.5, 55.6, 62.9, 70.8 and 75.4° and the TiO2 rutile phase at 27.9, 54.3 and 69.0° are 336
observed in this diffractogram combined with signals related to graphene oxide at 10.7° and 337
reduced graphene oxide located between 20.0 and 30.0°. The presence of all of these peaks 338
in this diffractogram proves the formation of the expected intermediate nanocomposite.339
As expected, graphene oxide (GO) in the TiO2/RGO sample was not completely 340
reduced after 4 h of exposure to ultraviolet light. The peak at 10.4° is characteristic of 341
graphene oxide. 342
The intermediate Cu2O/RGO composite was also characterized by XRD analysis,343
and its diffraction pattern is shown in Fig. 1d. This pattern displays peaks associated with344
the crystallographic planes of cuprous oxide at 29.7, 36.5, 42.1, 61.4 and 73.6° and one 345
broad peak between 20 and 30 o attributed to the reduced graphene oxide. The combination 346
of these peaks implies that the synthesis of this intermediate nanocomposite was successful.347
Finally, the diffractogram of the TiO2/Cu2O/RGO composite, shown in Fig. 1e,348
presents the same peaks related to the TiO2 anatase and rutile precursor materials as those 349
described for TiO2/RGO along with certain peaks related to the Cu2O nanoparticles and the 350
reduced graphene oxide. The polyol process performed on TiO2/RGO with the graphene 351
oxide partially reduced was successful for the preparation of the TiO2/Cu2O/RGO352
nanocomposite. The graphene oxide present in the sample was completely reduced because 353
the signal observed at 10.0° in the TiO2/RGO pattern was suppressed. The presence of all 354
of these peaks in this diffractogram confirms the effective formation of the expected 355
nanocomposite.356
X-ray photoelectron microscopy (XPS) was used for qualitative analysis of the 357
composition and chemical states of the final nanocomposite containing reduced graphene 358
oxide, TiO2 and Cu2O nanoparticles, and the spectra are displayed in Fig. S4 359
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(Supplementary data). The spectrum presented a signal at the C 1s region (Fig. S4a, 360
Supplementary data), which can be deconvoluted into four peaks. The strongest peak 361
centered at 284.6 eV corresponds to the overlapping of the signals related to the C-C, C=C 362
and C-H groups, whereas the other peaks are observed at 285.9, 287.5 and 288.5 eV and are 363
attributed to the C-OH, C=O (and/or C-O-C) and HO-C=O groups, respectively[49,50], 364
which are groups characteristic of reduced graphene oxide. Two XPS peaks centered at 365
458.8 and 464.5 eV can be detected at the Ti 2p region. These peaks are attributed to Ti 366
2p3/2 and Ti 2p1/2 of the Ti4+ chemical state in TiO2 framework, respectively. Moreover, the 367
binding energy difference between both peaks corresponds to 5.7 eV, which indicates a Ti4+368
chemical state typical of TiO2 present in composites with reduced graphene oxide[31,51]. 369
The Cu 2p region of TiO2/Cu2O/RGO (Fig. S4c, Supplementary data) showed two peaks 370
located at 932.4 and 952.3 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 peaks of Cu (I), 371
which proves the presence of Cu2O in the nanocomposite. In addition, Cu (II) signals could 372
also be detected mainly because of the oxidation after the exposure of the TiO2-Cu2O-RGO 373
sample to air and also because the remaining adsorbed Cu2+ ions on the surface that have 374
not undergone reduction[52,53].375
The scanning electron microscopy images of graphite, graphite oxide and reduced 376
graphite oxide are presented in Fig. S2 (Supplementary data). 377
As inferred by the images, the graphite used for the preparation of the reduced 378
graphene oxide is an ordered structure composed of aggregates of different sizes containing 379
periodically stacked layers. This arrangement is a characteristic structure of graphite, with 380
particles having an average diameter ranging from 4 to 75 m. After oxidation, the 381
morphology of the graphite underwent a modification and became a disordered layered 382
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structure, characteristic of graphite oxide[54,55]. Generally, the reduction of graphite oxide 383
using hydrazine hydrate leads to significant changes in the resulting morphology of this 384
carbonaceous material. In this specific case, the morphological aspect of the reduced 385
graphene oxide resembles “a crumpled sheet of paper” with curling edges[56], which is 386
beneficial because this form leads to the attainment of a higher surface area[57].387
The scanning electron microscopy images of TiO2 (Fig. S3a, Supplementary data)388
show that the TiO2 sample has interconnected pores in a sponge-like structure, which favors 389
the increase of light scattering. It is also possible to estimate the average nanoparticle 390
diameter from this image of approximately 15 nm, which is in accordance with the 391
information given by the supplier. This small particle diameter is beneficial for 392
photocatalytic processes because particles with smaller radii have higher surface areas[58]. 393
The increase in the surface area can be up to 100 times higher compared with a compact 394
film of TiO2[42]. Because the photocatalytic process generally occurs at the nanoparticle 395
surface, the increase in the surface area favors a higher activity of the nanocomposite 396
formed[59].397
A SEM micrograph of the pure Cu2O nanoparticles at 80,000 obtained in the 398
absence of GO sheets (Fig. S3b, Supplementary data) confirms that the cuprous oxide399
nanoparticles possess a spherical morphology with an average diameter of 100 nm. A 400
portion of these particles tend to aggregate into larger particles to reduce their surface 401
energies[41].402
The scanning electron microscopy images of the TiO2/RGO and Cu2O/RGO403
nanocomposites are presented in Fig. 2. The images show that after the reduction processes, 404
the two-dimensional sheet structure containing wrinkles on the micrometer scale 405
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characteristic of pure RGO was retained in both of the nanocomposites, even in the 406
presence of the TiO2 and Cu2O nanoparticles. In both of the cases, the TiO2 and Cu2O 407
nanoparticles are well-dispersed on the RGO sheets and are primarily accumulated along 408
the wrinkles and edges rather than on the basal planes because of the distribution of the 409
carboxylic acid groups on the graphite oxide sheets, which are likely situated at these 410
positions. The oxygen-containing functional groups of GO interact with the surface 411
hydroxyl groups of the P90 nanoparticles and with the Cu2+ ions before the reduction412
processes, forming the chemically bonded species. In addition, the attached nanoparticles 413
might prevent the aggregation and restacking of this as-synthesized graphene[41]. 414
In contrast to Fig. 2a and b in which the TiO2 nanoparticles in TiO2/RGO are 415
spherical, resembling the TiO2 nanoparticles without RGO, the images of the Cu2O/RGO416
nanocomposite, displayed in Fig. 2c and d show that the graphene sheets are decorated with417
cubic Cu2O nanoparticles randomly distributed on the surface of the RGO with edge 418
lengths of approximately 200 nm. Only the spherical Cu2O nanoparticles were obtained 419
when the same method was used to synthesize Cu2O without RGO (Fig. S3b, 420
Supplementary data). It is proposed that in systems containing graphene derivatives, the 421
carbon sheets act as a template, forcing the Cu2O crystals to grow along definite directions, 422
forming cube-like crystals. However, an in-depth study of the role of the reduced graphene 423
oxide sheets in the process of the growth of the cubic Cu2O nanoparticles remains to be 424
performed[41]. Surface area was analyzed according to BET method and the values 425
indicate that, as expected, the addition of RGO increases the surface area: TiO2 (131); Cu2O 426
(43); TiO2/RGO (194) and Cu2O/RGO (141 m2 g-1).427
The morphology of the final TiO2/Cu2O/RGO composite was also analyzed through428
scanning electron microscopy, and the images are displayed in Fig. 3. It is possible to 429
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visualize a dispersion of the particles with three different morphologies and sizes on the 430
RGO sheets, identified as titanium dioxide nanoparticles (P90) and two different forms of431
cuprous oxide synthesized in situ onto the RGO sheets. The spherical particles with an 432
average diameter of 15 nm are the deposited TiO2 nanoparticles, which were directly 433
incorporated onto the RGO nanosheets, maintaining the original morphology and size of 434
the P90 particles. The in situ prepared Cu2O nanoparticles presented the following two 435
different morphologies: one similar to that observed for the Cu2O/RGO composite, i.e., 436
cubic nanoparticles with an average diameter of 200 nm, and a second spherical 437
morphology, presenting nanoparticles significantly smaller than the cubic particles, with an 438
average diameter of 5 nm, primarily situated around the TiO2 nanoparticles. These 439
morphologies are better visualized by the TEM images, and the compositions of the 440
particles were determined by the EELS and EDS techniques.441
The microstructure of the final TiO2/Cu2O/RGO composite was further investigated 442
through high-resolution transmission electron microscopy (HRTEM), and the images are 443
displayed in Fig. 4. This figure shows that the Cu2O and TiO2 nanoparticles were 444
distributed over the entire RGO sheets, proving that the expected nanocomposite was 445
successfully prepared.446
The HRTEM images show that the RGO sheets in the nanocomposite are rippled 447
and resemble crumpled silk veil waves, which is appropriate for immobilizing the 448
nanoparticles[60]. Furthermore, a highly dense deposit of the TiO2 and Cu2O nanoparticles 449
on the RGO sheets can be observed in Fig 4a and b, which was consistent with the SEM 450
images and the XRD patterns. This morphology is expected to be beneficial for the 451
photoinduced electron transfer between the TiO2 and Cu2O nanoparticles to the reduced 452
graphene oxide sheets.453
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In addition, the HRTEM images clearly show the presence of smaller particles with 454
diameters of approximately 5 nm, evidenced by Fig. 4c, composed of in situ synthesized 455
Cu2O nanoparticles and larger particles of approximately 15 nm, displayed in Fig. 4d, 456
which are the TiO2 nanoparticles derived from the P90 material. The composition of these 457
particles was analyzed by EELS and EDS.458
Transmission electron microscopy was also useful for the identification of the 459
compositions of these nanoparticles with different morphologies dispersed on the RGO 460
nanosheets in the final TiO2/Cu2O/RGO nanocomposite using the energy dispersive 461
spectroscopy (EDS) and electron energy loss spectroscopy (EELS) modes[61].462
According to the copper and titanium EDS mappings of the sample displayed in Fig.463
5, the small particles with diameters of approximately 5 nm are primarily composed of464
copper, inferring that these particles are cuprous oxide, whereas the larger particles, with 465
diameters close to 15 nm, are composed of titanium, inferring that these particles are 466
titanium dioxide. The largest cubic particles are too large to be shown simultaneously with 467
the other nanoparticles in the same images; however, they were also verified by EDS, and 468
their composition as Cu2O was confirmed.469
Further information is given in the titanium and copper EELS mappings exhibited in 470
Fig. 6. This technique can indicate the oxidation state of the nanoparticles and can confirm 471
the composition of the particles dispersed on the RGO nanosheets[62].472
As observed in the images in Fig. 6, the information given by the EDS mappings 473
that the small particles are composed of Cu2O, whereas the larger particles are composed of 474
TiO2 is reinforced by EELS, according to the generated images. The EELS results also 475
confirm that no metallic titanium or copper nanoparticles are present in the mapped regions.476
The cuprous oxide nanoparticles acquired a small spherical morphology in the 477
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TiO2/Cu2O/RGO final nanocomposite in addition to the larger cubic morphology observed 478
in the Cu2O/RGO intermediate because of the spatial hindrance caused by the high 479
concentration of the TiO2 nanoparticles. This assumption results from the in situ growth of 480
the cuprous oxide nanoparticles on the RGO sheets after the incorporation of the TiO2481
nanoparticles. As expected, the surface area of the nanocomposite TiO2/RGO/Cu2O is 482
higher than the surface area of the single or the dual component composite and equal to 214 483
m2 g-1.484
A semiconductor that presents a high-intensity optical absorbance is crucial for the 485
photocatalytic process with a high yield[63]. In addition to ultraviolet light, absorbance in 486
the visible region is important to cover a larger portion of the solar spectrum. The 487
absorbance spectra of the TiO2, Cu2O, TiO2/RGO, Cu2O/RGO and TiO2/Cu2O/RGO488
nanocomposites were recorded using a UV-Vis spectrophotometer over the range of 190 –489
800 nm and are depicted in Fig. 7. An intense absorbance in the ultraviolet region for the 490
TiO2 nanoparticles (200 to 320 nm) is credited to the intrinsic bandgap energy absorption, 491
which was calculate and equal to 3.2 eV, whereas the pure Cu2O nanoparticles have a 492
strong absorption in the UV and the visible regions, represented by a broad absorption band 493
from 250 to 500 nm in the spectrum (the band gap was calculated and it is equal to 2.0 eV).494
Because of the combination of TiO2 with Cu2O and RGO, the absorption edge shifts 495
toward the visible and near-infrared (NIR) regions, not only because of the incorporation of 496
the Cu2O nanoparticles but also because of the introduction of the black body properties 497
typical of graphite-like material, such as reduced graphene oxide. The increased absorbance 498
in the visible and NIR regions is expected to improve the light harvesting capacity and 499
should enhance the photocatalytic performance of the synthesized nanocomposites500
compared with the pure TiO2 and Cu2O nanoparticles[18].501
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3.1. Photocatalytic performance502
The photocatalytic activities toward methylene blue under solar radiation of the 503
synthesized TiO2/Cu2O/RGO nanocomposite and all of the intermediate materials prepared 504
in this study were evaluated under the same experimental conditions. Fig. 8 presents the 505
ultraviolet-visible spectra of the methylene blue solutions that underwent photodegradation 506
by solar radiation before the beginning of the experiment and after 1, 2, 3, 4 and 5 h for 507
each material.508
The graphs show that degradation rates of methylene blue were slow for the Cu2O, 509
TiO2 and Cu2O/RGO materials compared with TiO2/RGO, and particularly, compared with 510
TiO2/Cu2O/RGO. Especially in the first hour of the experiment, the photodegradation 511
capacity increased considerably when the TiO2/RGO and TiO2/Cu2O/RGO nanocomposites 512
were applied. The concentration of the methylene blue in solution decreased 48 and 70% in 513
the first hour of the experiment and 79 and 95% after 5 h of irradiation for TiO2/RGO and 514
TiO2/Cu2O/RGO, respectively. These values were 33, 21 and 18% in the first hour of the 515
tests and 77, 69 and 72% after 5 h for TiO2, Cu2O and Cu2O/RGO, respectively. These 516
results highlight the superiority of the photoactivity of the new TiO2/Cu2O/RGO517
nanocomposite compared with the intermediates because only 5% of the methylene blue 518
remained in solution after 5 h of the experiment.519
The graph in Fig. 9 shows the relative variation in the absorbance of the methylene 520
blue solution over time under the influence of the synthesized photocatalysts under solar 521
light illumination. The relative absorbance is given by the relationship 100*A/A0, where A 522
is the absorbance of the methylene blue solution at irradiation time t, and A0 is the 523
absorbance of the solution before irradiation (t = 0). 524
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The results show that the photodegradation efficiency of the final TiO2/Cu2O/RGO525
nanocomposite improved compared with that of the precursors and the intermediate 526
nanocomposites. The concentration of methylene blue was significantly lower in all of the 527
aliquots obtained from the solution during the photodegradation tests on TiO2/Cu2O/RGO. 528
More specifically, Fig. 9 shows that the TiO2 nanoparticles without RGO presented 529
higher photodegradation efficiency than the pure Cu2O nanoparticles. It appears that there 530
is an additional importance of the absorption of the UV portion of the solar spectrum in the 531
MB photodegradation provided by TiO2. This fact can be related to the results in the study 532
by Xu et al.[64] using a Cu2O-TiO2 system in which the Cu2O nanoparticles were deposited 533
on the TiO2 structure, and the increase in the Cu2O concentration on the TiO2 above a 534
certain limit led to a decrease in the photocatalytic efficiency of the material. Xu et al. have535
claimed that this result could be associated with a screen effect of the absorption of the 536
photocatalyst in the UV portion. Moreover, it is also important to consider the higher 537
dielectric constant of TiO2 compared with Cu2O, resulting in less charge 538
recombination.[65,66].539
The combination of these nanoparticles with reduced graphene oxide in the 540
TiO2/RGO and Cu2O/RGO nanocomposites led to an increase in the photocatalytic 541
efficiency in both of the cases. There are different known aspects that explain the enhanced 542
catalytic activity of the nanocomposites formed by the combination of the TiO2 or Cu2O 543
nanoparticles with the reduced graphene oxide sheets. 544
It was expected that because of the increase in surface area after RGO introduction, 545
this could lead to a more capability of adsorbing higher quantities of MB and as 546
consequence to an improvement in the efficiency of the photodegradation process. 547
However, Figure 8 shows that the initial absorbance amplitude for all samples is very close 548
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even after 2 hours of equilibrium in the dark. It suggests that the increase in the surface 549
area, as the only one factor, cannot explain the photocatalytic efficiency. We cannot 550
discard, however, that during illumination, the implement in the surface area because of the 551
RGO sheets can cause some changes in the adsorption-desorption equilibrium of the MB 552
dyes and this could in turn, be important for the photocatalytic process.553
The conduction band (CB) and the valence band (VB) of the Cu2O (2.0 eV) lie 554
above those of the TiO2 (3.2 eV). After photoexcitation, the electrons excited to the CB of 555
Cu2O would be transferred to TiO2, whereas the holes generated in the VB in TiO2 can be 556
transferred to Cu2O[67,68]. The spatial separation of the photogenerated h-e pairs in 557
different semiconductors reduces charge recombination and it is beneficial for 558
photocatalysis and photoelectrochemical cells. The presence of RGO sheets supporting 559
both catalysts enhances the photocatalytic mechanisms in several ways. RGO is an electron 560
accepting material, which presents good conductivity because of the two-dimensional 561
planar structure. These properties leads to a faster electron transfer from both TiO2 and 562
Cu2O nanomaterials to the RGO sheets (via titania conduction band). The rapid transport of 563
the charge carriers in the graphene sheets promotes an even better charge separation. This 564
process eventually decreases the rate of recombination of the photogenerated electron-hole 565
pairs. The degradation of methylene blue is expected to occur by reactive species as566
hydroxyl radicals (OH), superoxide radical anions (O2-) and direct reaction with holes. 567
The OH radicals are generated by the holes in VB of the Cu2O after reaction with 568
adsorbed H2O or OH- and the electrons in the CB are picked up by oxygen to generate569
superoxide O2- radicals. These oxidative reactions would result in the degradation of the 570
MB. Besides, a second path for MB degradation is introduced by the presence of RGO 571
sheets that contain a great amount of adsorbed H2O and O2 molecules, thus more reactive 572
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species are also expected to be generated and this can explain the enhancement of the 573
photocatalytic activity after RGO introduction[30,69]. Furthermore, RGO provides a good 574
base, making the dispersion of the TiO2 and Cu2O nanoparticles more uniform within a 575
certain range of sizes[30,70].576
It is also noticeable that the photodegradation capacities of the photocatalysts Cu2O 577
and Cu2O/RGO considerably increase after 3 h of the test, leading to the conclusion that the 578
rate of the photocatalytic reaction is slower in Cu2O than in the TiO2 nanoparticles, most 579
likely because of the difference in the surface area.580
Comparing all of the photodegradation results, there was a remarkable increase in 581
the photoactivity for the final TiO2/Cu2O/RGO nanocomposite compared with the 582
intermediate composites and the precursor nanoparticles. These results confirm the 583
efficiency of combining in the same composite the capacities of the TiO2 and Cu2O 584
nanoparticles to absorb ultraviolet light and visible light from solar radiation, respectively, 585
and the high ability of the reduced graphene oxide in capturing the photogenerated 586
electrons, and into suppress the electron-hole pair recombination. Furthermore, this higher 587
photodegradation capacity of TiO2/Cu2O/RGO may also be associated with the obtainment 588
of Cu2O nanoparticles with smaller than average diameters (5 nm) onto the RGO 589
nanosheets compared with the pure Cu2O (100 nm) and the Cu2O/RGO (200 nm), as 590
evidenced by the TEM images associated with the EELS and EDS mappings because the 591
larger diameters of the Cu2O nanoparticles could have been the primary disadvantage of 592
these intermediate materials for this application. 593
The photocatalytic results is also supported by the photoelectrochemical 594
measurements carried out in the TiO2/Cu2O/RGO nanocomposite and also in the TiO2, 595
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Cu2O, TiO2/RGO, Cu2O/RGO films (Fig. 10) that were employed as working electrodes in 596
a typical photoelectrochemical cell.597
As observed in Fig. 10, all samples exhibit anodic photocurrent indicating that the 598
electron flux is towards the electrode and then driven to the platinum wire by the external 599
circuit. Cu2O films usually present p-type behavior because of the presence of Cu 600
vacancies[71]. In our work, both Cu2O and Cu2O/RGO materials present n-type behavior. 601
Yu and co-workers have also reported the synthesis of n-type Cu2O nanoparticles in acid 602
medium using a solvothermal procedure[72]. Other groups have also reported n-type Cu2O 603
films deposited on various conducting substrates using neutral or slightly acid aqueous 604
electrolytes[73,74]. The Cu2O nanoparticles synthesis occurred using ascorbic acid and this 605
might explain the anodic photocurrent displayed by this electrode. In the other hand, the 606
Cu2O/RGO nanocomposite was prepared by the polyol method where the ethylene glycol 607
(EG), acts as both solvent and reduction agent. The pK value of the EG is 15.4, close to 608
H2O (16) and the temperature of the experimental procedure varied from 140 to 160oC, thus 609
it is possible that oxygen vacancies were created in the solid, resulting in the observed n-610
type behavior, although the origin of this n-type behavior is still under debate[75,76].611
The films prepared with TiO2 nanoparticles and TiO2/RGO only showed a small 612
photoactivity, i.e, a low photocurrent owing to its limited band gap in the ultraviolet region 613
[77,78]. Besides, the electrolyte medium employed was chosen to be more suitable to Cu2O 614
photoelectrochemistry instead. For the Cu2O, Cu2O/RGO and TiO2/Cu2O/RGO films, 615
appreciable values of photocurrent appeared immediately under illumination (ON) and 616
dropped down under dark conditions (OFF). This effect is due to a strong absorption of 617
Cu2O nanoparticles in the visible region, followed by a fast electron transfer to the TiO2618
nanoparticles, and then to the RGO sheets[40,67,79,80]. 619
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An enhancement in the anodic photocurrent values was verified for the ternary 620
nanocomposite TiO2/Cu2O/RGO corroborating with the photocatalytic data. It is difficult to 621
quantitatively evaluate the contribution of each component separately. We strong believe 622
that electrons are transported more efficiently through the RGO sheets, inhibiting the 623
recombination between the electrons in the TiO2 CB and holes in the Cu2O VB. The 624
beneficial effect of the RGO sheets was observed after their introduction to all 625
nanomaterials, independent of their chemical and electronic structure, size and porosity; 626
and in both photodegradation and photoelectrochemical tests. Recently, Zhu and co-627
workers provided a basic understanding of how RGO sheets can effectively promote 628
electron transport in titania films using IMPS, IMVS and EIE techniques[81]. Coupling two 629
semiconductors (TiO2 and Cu2O) with complementary absorption profiles and suitable 630
energy levels that allowed an efficient electron transfer between them was also decisive for 631
the success of the ternary nanocomposite.632
633
4. Conclusions634
In summary, a novel photoactive composite composed of reduced graphene oxide, 635
titanium dioxide nanoparticles and cuprous oxide nanoparticles was successfully prepared,636
and its properties were evaluated. The XRD diffraction patterns and the EDS and EELS 637
techniques confirmed that the final nanocomposite is a combination of the TiO2, RGO and 638
Cu2O nanoparticles, and the SEM and TEM images showed that the TiO2 and Cu2O 639
nanoparticles are uniformly distributed across the surface of the RGO sheets. The 640
absorption edges of TiO2 were extended into the visible light and near infrared regions by 641
the Cu2O and RGO incorporation, as proven by the diffuse reflectance spectroscopy. The 642
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photodegradation activity of the TiO2/Cu2O/RGO nanocomposite was higher than that643
observed for the other intermediate composite samples because it decomposed 95% of the 644
methylene blue present in aqueous solution under the solar radiation influence. Therefore, 645
these differences in the degradation activity indicated that the photodegradation ability646
depended on the proper combination of Cu2O and RGO to the TiO2 semiconductor and the 647
particle size of these components. A further explanation is being explored with the648
investigation of the effect of the variation of the RGO, TiO2 and Cu2O concentrations on 649
the photoactivity of the resultant nanocomposites. The photoelectrochemistry studies were 650
in accordance to the photocatalytic tests showing the beneficial effect of the RGO sheets in 651
accepting electrons and also into inhibiting charge recombination between the electrons in 652
the TiO2 CB and holes in the Cu2O VB. 653
654
Acknowledgments655
The authors would like to acknowledge FAPESP and CNPq for their financial 656
support and fellowships, the National Center for Energy and Materials Research (CNPEM) 657
for the use of the high-resolution transmission electron microscopy (HRTEM) facilities at 658
LNNano, and Andréia de Morais for the HRTEM images.659
660
Supplementary data661
The supplementary data associated with this article, such as the XRD patterns, XPS 662
spectra and the SEM images of the reduced graphene oxide and its precursors and the SEM 663
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images of the pure TiO2 and the Cu2O nanoparticles can be found in the online version at664
http://dx.doi.org/j.apsusc.665
666
References667
[1] E. Forgacs, T. Cserháti, G. Oros, Removal of synthetic dyes from wastewaters: a 668
review, Environ. Int. 30 (2004) 953–971.669
[2] S.-T. Yang, S. Chen, Y. Chang, A. Cao, Y. Liu, H. Wang, Removal of methylene 670
blue from aqueous solution by graphene oxide, J. Colloid Interface Sci. 359 (2011) 671
24–29.672
[3] T.-J. Whang, M.-T. Hsieh, H.-H. Chen, Visible-light photocatalytic degradation of 673
methylene blue with laser-induced Ag/ZnO nanoparticles, Appl. Surf. Sci. 258 674
(2012) 2796-2801.675
[4] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann, 676
Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B-677
Environ. 31 (2001) 145–157.678
[5] G. Yang, T. Wang, B. Yang, Z. Yan, S. Ding, T. Xiao, Enhanced visible-light 679
activity of F-N co-doped TiO2 nanocrystals via nonmetal impurity, Ti3+ ions and 680
oxygen vacancies, Appl. Surf. Sci. 287 (2013) 135–142.681
[6] G. Lui, J.-Y. Liao, A. Duan, Z. Zhang, M. Fowler, A. Yu, Graphene-wrapped 682
hierarchical TiO2 nanoflower composites with enhanced photocatalytic performance, 683
J. Mater. Chem. A 1 (2013) 12255–12262.684
Page 31 of 51
Accep
ted
Man
uscr
ipt
31
[7] M. Sookhakian, Y.M. Amin, W.J. Basirun, Hierarchically ordered macro-685
mesoporous ZnS microsphere with reduced graphene oxide supporter for a highly 686
efficient photodegradation of methylene blue, Appl. Surf. Sci. 283 (2013) 668–677.687
[8] Y.J. Acosta-Silva, R. Nava, V. Hernández-Morales, S.A. Macías-Sánchez, M.L. 688
Gómez-Herrera, B. Pawelec, Methylene blue photodegradation over titania-689
decorated SBA-15, Appl. Catal. B-Environ. 110 (2011) 108–117.690
[9] A. McLaren, T. Valdes-Solis, G. Li, S.C. Tsang, Shape and size effects of ZnO 691
nanocrystals on photocatalytic activity, J. Am. Chem. Soc. 131 (2009) 12540–12541.692
[10] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. 693
Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O`Shea, M.H. Entezari, D.D. Dionysiou, A 694
review on the visible light active titanium dioxide photocatalysts for environmental 695
applications, Applied Catal. B-Environ. 125 (2012) 331–349.696
[11] A.A. Ismail, D.W. Bahnemann, Mesostructured Pt/TiO2 nanocomposites as highly 697
active photocatalysts for the photooxidation of dichloroacetic acid, J. Phys. Chem. C 698
115 (2011) 5784–5791.699
[12] Y. Wang, C. Feng, Z. Jin, J. Zhang, J. Yang, S. Zhang, A novel N-doped TiO2 with 700
high visible light photocatalytic activity, J. Mol. Catal. A-Chem. 260 (2006) 1–3.701
[13] K.M. Gangotri, R.C. Meena, Use of reductant and photosensitizer in photogalvanic 702
cells for solar energy conversion and storage: oxalic acid–methylene blue system, J. 703
Photochem. Photobiol. A-Chem. 141 (2001) 175–177. 704
[14] D. Chatterjee, Effect of excited state redox properties of dye sensitizers on hydrogen 705
production through photo-splitting of water over TiO2 photocatalyst, Catal. 706
Commun. 11 (2010) 336–339.707
Page 32 of 51
Accep
ted
Man
uscr
ipt
32
[15] S.Y. Ryu, W. Balcerski, T.K. Lee, M.R. Hoffmann, Photocatalytic production of 708
hydrogen from water with visible light using hybrid catalysts of CdS attached to 709
microporous and mesoporous silicas, J. Phys. Chem. C 111 (2007) 18195–18203.710
[16] M.D. Hernández-Alonso, F. Fresno, S. Suárez, J.M. Coronado, Development of 711
alternative photocatalysts to TiO2: challenges and opportunities, Energy Environ. 712
Sci. 2 (2009) 1231–1257. 713
[17] F. Yang, G. Han, D. Fu, Y. Chang, H. Wang, Improved photodegradation activity of 714
TiO2 via decoration with SnS2 nanoparticles, Mater. Chem. Phys. 140 (2013) 398–715
404. 716
[18] Y. Medina-Gonzalez, W.Z. Xu, B. Chen, N. Farhanghi, P.A. Charpentier, CdS and 717
CdTeS quantum dot decorated TiO2 nanowires. Synthesis and photoefficiency, 718
Nanotechnology 22 (2011) 1-8. 719
[19] Y.-C. Nah, I. Paramasivam, P. Schmuki, Doped TiO2 and TiO2 nanotubes: synthesis 720
and applications, ChemPhysChem. 11 (2010) 2698–2713. 721
[20] Q. Zhang, J. Su, X. Zhang, J. Li, A. Zhang, Y. Gao, Chemical vapor deposition of a 722
PbSe/CdS/nitrogen-doped TiO2 nanorod array photoelectrode and its band-edge level 723
structure, New J. Chem. 36 (2012) 2302-2307. 724
[21] A.R. Zainun, S. Tomoya, U.M. Noor, M. Rusop, I. Masaya, New approach for 725
generating Cu2O/TiO2 composite films for solar cell applications, Mater. Lett. 66 726
(2012) 254–256. 727
[22] S.M. Miranda, G.E. Romanos, V. Likodimos, R.R.N. Marques, E.P. Favvas, F.K. 728
Katsaros, K.L. Stefanopoulos, V.J.P. Vilar, J.L. Faria, P. Falaras, A.M.T. Silva, Pore 729
structure, interface properties and photocatalytic efficiency of hydration/dehydration 730
derived TiO2/CNT composites, Appl. Catal. B-Environ. 147 (2014) 65–81. 731
Page 33 of 51
Accep
ted
Man
uscr
ipt
33
[23] R. Leary, A. Westwood, Carbonaceous nanomaterials for the enhancement of TiO2732
photocatalysis, Carbon 49 (2011) 741–772. 733
[24] U.N. Maiti, W.J. Lee, J.M. Lee, Y. Oh, J.Y. Kim, J.E. Kim, J. Shim, T.H. Han, S.O. 734
Kim, 25th anniversary article: chemically modified/doped carbon nanotubes & 735
graphene for optimized nanostructures & nanodevices, Adv. Mater. 26 (2014) 40–67.736
[25] C.K. Chua, M. Pumera, Chemical reduction of graphene oxide: a synthetic chemistry 737
viewpoint, Chem. Soc. Rev. 43 (2014) 291–312. 738
[26] D.A.C. Brownson, D.K. Kampouris, C.E. Banks, Graphene electrochemistry: 739
fundamental concepts through to prominent applications, Chem. Soc. Rev. 41 (2012) 740
6944-6976.741
[27] N. Mahmood, C. Zhang, H. Yin, Y. Hou, Graphene-based nanocomposites for 742
energy storage and conversion in lithium batteries, supercapacitors and fuel cells, J. 743
Mater. Chem. A 2 (2014) 15-32.744
[28] N. Zhang, M.-Q. Yang, Z.-R. Tang, Y.-J. Xu, Toward improving the graphene-745
semiconductor composite photoactivity via the addition of metal ions as generic 746
interfacial mediator, ACS Nano 8 (2014) 623–633.747
[29] N. Zhang, Y. Zhang, Y.-J. Xu, Recent progress on graphene-based photocatalysts: 748
current status and future perspectives, Nanoscale 4 (2012) 5792-5813. 749
[30] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, P25-graphene composite as a high 750
performance photocatalyst, ACS Nano 4 (2010) 380–386.751
[31] Y. Cong, M. Long, Z. Cui, X. Li, Z. Dong, G. Yuan, J. Zhang, Anchoring a uniform 752
TiO2 layer on graphene oxide sheets as an efficient visible light photocatalyst, Appl. 753
Surf. Sci. 282 (2013) 400–407. 754
Page 34 of 51
Accep
ted
Man
uscr
ipt
34
[32] P. Gao, D.D. Sun, Hierarchical sulfonated graphene oxide–TiO2 composites for 755
highly efficient hydrogen production with a wide pH range, Appl. Catal. B-Environ. 756
147 (2014) 888–896.757
[33] N. Zhang, Y. Zhang, X. Pan, M.-Q. Yang, Y.-J. Xu, Constructing ternary 758
CdS−graphene−TiO2 hybrids on the flatland of graphene oxide with enhanced 759
visible-light photoactivity for selective transformation, J. Phys. Chem. C 116 (2012)760
18023-18031.761
[34] M.-Q. Yang, Y.-J. Xu, Selective photoredox using graphene-based composite 762
photocatalysts, Phys. Chem. Chem. Phys. 15 (2013) 19102–19118.763
[35] G. Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted 764
photocatalytic reduction of graphene oxide, ACS Nano 2 (2008) 1487–1491.765
[36] H. Sun, S. Liu, S. Liu, S. Wang, A comparative study of reduced graphene oxide 766
modified TiO2, ZnO and Ta2O5 in visible light photocatalytic/photochemical 767
oxidation of methylene blue, Appl. Catal. B-Environ. 146 (2014) 162–168. 768
[37] X. Zeng, J. Bao, M. Han, W. Tu, Z. Dai, Quantum dots sensitized titanium dioxide 769
decorated reduced graphene oxide for visible light excited photoelectrochemical 770
biosensing at a low potential, Biosens. Bioelectron. 54 (2014) 331–338. 771
[38] W.S. Hummers Jr., R.E. Offeman, Preparation of Graphitic Oxide, J. Am. Chem. 772
Soc. 80 (1958) 1339-1339.773
[39] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nat. 774
Nanotechnol. 4 (2009) 217–224. 775
[40] B. Li, T. Liu, L. Hu, Y. Wang, A facile one-pot synthesis of Cu2O/RGO 776
nanocomposite for removal of organic pollutant, J. Phys. Chem. Solids. 74 (2013) 777
635–640. 778
Page 35 of 51
Accep
ted
Man
uscr
ipt
35
[41] C. Xu, X. Wang, L. Yang, Y. Wu, Fabrication of a graphene–cuprous oxide 779
composite, J. Solid State Chem. 182 (2009) 2486–2490. 780
[42] Z.C. Orel, A. Anzlovar, G. Drazic, M. Zigon, Cuprous oxide nanowires prepared by 781
an additive-free polyol process, Cryst. Growth Des. 7 (2007) 453-458.782
[43] A. Kaniyoor, S. Ramaprabhu, Thermally exfoliated graphene based counter 783
electrode for low cost dye sensitized solar cells, J. Appl. Phys. 109 (2011) 124308. 784
[44] H. Wang, Y.H. Hu, Effect of oxygen content on structures of graphite oxides, Ind. 785
Eng. Chem. Res. 50 (2011) 6132–6137. 786
[45] J. Shen, B. Yan, M. Shi, H. Ma, N. Li, M. Ye, One step hydrothermal synthesis of 787
TiO2-reduced graphene oxide sheets, J. Mater. Chem. 21 (2011) 3415-3421.788
[46] Y. Zhang, Z.-R. Tang, X. Fu, Y.-J. Xu, Engineering the unique 2D mat of graphene 789
to achieve graphene-TiO2 nanocomposite for photocatalytic selective transformation: 790
what advantage does graphene vave over its forebear carbon nanotube?, ACS Nano 5791
(2011) 7426–7435.792
[47] X. Shen, B. Tian, J. Zhang, Tailored preparation of titania with controllable phases 793
of anatase and brookite by an alkalescent hydrothermal route, Catal. Today 201 794
(2013) 151–158. 795
[48] H. Yu, B. Tian, J. Zhang, Layered TiO2 composed of anatase nanosheets with 796
exposed {001} facets: facile synthesis and enhanced photocatalytic activity, Chem. 797
Eur. J. 17 (2011) 5499–5502. 798
[49] S. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S. Ruoff, Hydrazine-799
reduction of graphite- and graphene oxide, Carbon 49 (2011) 3019–3023. 800
[50] Y. Zhang, Z.-R. Tang, X. Fu, Y.-J Xu, TiO2-graphene nanocomposites for gas-phase 801
photocatalytic degradation of volatile aromatic pollutant: is TiO2-graphene truly 802
Page 36 of 51
Accep
ted
Man
uscr
ipt
36
different from other TiO2-carbon composite materials?, ACS Nano 4 (2010) 7303–803
7314.804
[51] G.Y. He, J. Huang, W.F. Liu, X. Wang, H.Q. Chen, X.Q. Sun, ZnO-Bi2O3/graphene 805
oxide photocatalyst with high photocatalytic performance under visible light, Mater. 806
Technol. 27 (2012) 278–283. 807
[52] A.A. Ismail, R.A. Geioushy, H. Bouzid, S.A. Al-Sayari, A. Al-Hajry, D.W. 808
Bahnemann, TiO2 decoration of graphene layers for highly efficient photocatalyst: 809
impact of calcination at different gas atmosphere on photocatalytic efficiency, Appl. 810
Catal. B-Environ. 129 (2013) 62–70. 811
[53] C.J. Barbé, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. 812
Grätzel, Nanocrystalline titanium oxide electrodes for photovoltaic applications, J. 813
Am. Ceram. Soc. 80 (1997) 3157–3171.814
[54] A.E. Eliyas, L. Ljutzkanov, I.D. Stambolova, V.N. Blaskov, S. V. Vassilev, E.N. 815
Razkazova-Velkova, D.R. Mehandjiev, Visible light photocatalytic activity of TiO2816
deposited on activated carbon, Cent. Eur. J. Chem. 11 (2013) 464–470. 817
[55] D. Liang, C. Cui, H. Hu, Y. Wang, S. Xu, B. Ying, P. Li, B. Lu, H. Shen, One-step 818
hydrothermal synthesis of anatase TiO2/reduced graphene oxide nanocomposites 819
with enhanced photocatalytic activity, J. Alloys Compd. 582 (2014) 236–240. 820
[56] M.J.-F. Guinel, N. Brodusch, Y. Verde-Gómez, B. Escobar-Morales, R. Gauvin, 821
Multi-walled carbon nanotubes decorated by platinum catalyst nanoparticles--822
examination and microanalysis using scanning and transmission electron 823
microscopies, J. Microsc. 252 (2013) 49–57. 824
Page 37 of 51
Accep
ted
Man
uscr
ipt
37
[57] H. Kim, G. Moon, D. Monllor-Satoca, Y. Park, W. Choi, Solar photoconversion 825
using graphene/TiO2 composites: nanographene shell on TiO2 core versus TiO2826
nanoparticles on graphene sheet, J. Phys. Chem. C 116 (2012) 1535–1543.827
[58] Y. Zhang, N. Zhang, Z.-R. Tang, Y.-J. Xu, Graphene transforms wide band gap ZnS to 828
a visible light photocatalyst. The new role of graphene as a macromolecular 829
photosensitizer, ACS Nano 6 (2012) 9777–9789.830
[59] S. Liu, M.-Q. Yang, Y.-J. Xu, Surface charge promotes the synthesis of large, flat 831
structured graphene-(CdS nanowire)-TiO2 nanocomposites as versatile visible light 832
photocatalysts, J. Mater. Chem. A. 2 (2014) 430-440.833
[60] L.M. Pastrana-Martínez, S. Morales-Torres, V. Likodimos, J.L. Figueiredo, J.L. 834
Faria, P. Falaras, A.M.T. Silva, Advanced nanostructured photocatalysts based on 835
reduced graphene oxide-TiO2 composites for degradation of diphenhydramine 836
pharmaceutical and methyl orange dye, Appl. Catal. B-Environ. 123-124 (2012) 837
241–256.838
[61] X. Dong, K. Wang, C. Zhao, X. Qian, S. Chen, Z. Li, H. Liu, S. Dou, Direct 839
synthesis of RGO/Cu2O composite films on Cu foil for supercapacitors, J. Alloys 840
Compd. 586 (2014) 745–753.841
[62] M. Zong, Y. Huang, H. Wu, Y. Zhao, P. Liu, L. Wang, Facile preparation of 842
RGO/Cu2O/Cu composite and its excellent microwave absorption properties, Mater. 843
Lett. 109 (2013) 112–115.844
[63] M.T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M.M. Müller, H.-J. Kleebe, K. 845
Rachut, Jürgen Ziegler, A. Klein, W. Jaegermann, Preparation of RuO2/TiO2846
mesoporous heterostructures and rationalization of their enhanced photocatalytic 847
Page 38 of 51
Accep
ted
Man
uscr
ipt
38
properties by band alignment investigations, J. Phys. Chem. C 117 (2013) 22098-848
22110.849
[64] Y. Xu, D. Liang, M. Liu, D. Liu, Preparation and characterization of Cu2O–TiO2: 850
efficient photocatalytic degradation of methylene blue, Mater. Res. Bull. 43 (2008) 851
3474–3482. 852
[65] V. Bessergenev, High-temperature anomalies of dielectric constant in TiO2 thin 853
films, Mater. Res. Bull. 44 (2009) 1722–1728. 854
[66] W.Y. Ching, Y.-N. Xu, K.W. Wong, Ground-state and optical properties of Cu2O 855
and CuO crystals, Phys. Rev. B. 40 (1989) 7684–7695.856
[67] Y. Bessekhouad, D. Robert, J.-V. Weber, Photocatalytic activity of Cu2O/TiO2, 857
Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions, Catal. Today 101 (2005) 315–321.858
[68] Y. Hou, X. Li, X. Zou, X. Quan, G. Chen, Photoeletrocatalytic activity of a Cu2O-859
loaded self-organized highly oriented TiO2 nanotube array electrode for 4-860
chlorophenol degradation, Environ. Sci. Technol. 43 (2009) 858–863.861
[69] F. Meng, J. Li, S.K. Cushing, J. Bright, M. Zhi, J.D. Rowley, Z. Hong, A. 862
Manivannan, A.D. Bristow, N. Wu, Photocatalytic water oxidation by 863
hematite/reduced graphene oxide composites, ACS Catal. 3 (2013) 746–751. 864
[70] A. Wang, X. Li, Y. Zhao, W. Wu, J. Chen, H. Meng, Preparation and 865
characterizations of Cu2O/reduced graphene oxide nanocomposites with high photo-866
catalytic performances, Powder Technol. 261 (2014) 42–48. 867
[71] A.E. Rakhashani, Preparation, characteristcs and photovoltaic properties of cuprous 868
oxide – a review, Solid State Electron. 29 (1986) 7–17.869
Page 39 of 51
Accep
ted
Man
uscr
ipt
39
[72] L. Xiong, S. Huang, X. Yang, M. Qiu, Z. Chen, Y. Yu, p-Type and n-type Cu2O 870
semiconductor thin films: controllable preparation by simple solvothermal method 871
and photoelectrochemical properties, Electrochim. Acta. 56 (2011) 2735–2739.872
[73] W. Siripala, J.R.P. Jayakody, Sol. Energy Mater. 14 (1986) 23-27.873
[74] W. Siripala, L.D.R.D. Perera, K.T.L. De Silva, J.K.D.S. Jayanetti, I.M. Dharmadasa, 874
Sol. Energy Mater. Sol. Cells 44 (1996) 251–260.875
[75] R. Garuthara, W. Siripala, Photoluminescence characterization of polycrystalline n-876
type Cu2O films, J. Lumin. 121 (2006) 173–178. 877
[76] D.O. Scanlon, G.W. Watson, Undoped n-type Cu2O: fact or fiction?, J. Phys. Chem. 878
Lett. 1 (2010) 2582–2585.879
[77] X. Liu, L. Pan, T. Lv, Z. Sun, CdS sensitized TiO2 film for photocatalytic reduction 880
of Cr(VI) by microwave-assisted chemical bath deposition method, J. Alloys 881
Compd. 583 (2014) 390–395.882
[78] W. Siripala, A. Ivanovskaya, T.F. Jaramillo, S.-H. Baeck, E.W. Mcfarland, A 883
Cu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis, Sol. Energy 884
Mater. Sol. Cells 77 (2003) 229–237.885
[79] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.-J. Xu, Assembly of CdS nanoparticles 886
on the two-dimensional graphene scaffold as visible-light-driven photocatalyst for 887
selective organic transformation under ambient conditions, J. Phys. Chem. C 115 888
(2011) 23501–23511.889
[80] Y.-B. Tang, C.-S. Lee, J. Xu, Z.-T. Liu, Z.-H. Chen, Z. He, Y.-L. Cao, G. Yuan, H. 890
Song, L. Chen, L. Luo, H.-M. Cheng, W.-J. Zhang, I. Bello, S.-T. Lee, Incorporation 891
of graphenes in nanostructured TiO2 films via molecular grafting for dye-sensitized 892
solar cell application, ACS Nano 4 (2010) 3482-3488.893
Page 40 of 51
Accep
ted
Man
uscr
ipt
40
[81] Y. Zhu, X. Meng, H. Cui, S. Jia, J. Dong, J. Zheng, J. Zhao, Z. Wang, L. Li, Li 894
Zhang, Z. Zhu, Graphene frameworks promoted electron transport in quantum dot-895
sensitized solar cells., ACS Appl. Mater. Interfaces. 6 (2014) 13833–13840.896
897
Figure Captions898
Fig. 1. XRD patterns of TiO2 (a), Cu2O (b), TiO2/RGO (c), Cu2O/RGO (d) and 899
TiO2/Cu2O/RGO (e); the peaks marked with ■ are related to graphene oxide, ● to anatase 900
phase, ♦ to rutile phase of titanium dioxide and ▲ to cuprous oxide.901
Fig. 2. Scanning electron microscopy images of TiO2/RGO at (a) 20,000 and (b) 100,000902
and the Cu2O/RGO nanocomposite at (c) 20,000 and (d) 100,000.903
Fig. 3. Scanning electron microscopy images of the TiO2/Cu2O/RGO nanocomposite at (a)904
20,000 and (b) 100,000.905
Fig. 4. Transmission electron microscopy images of the TiO2/Cu2O/RGO nanocomposite.906
Fig. 5. High angle annular dark field image (a) and energy dispersive spectroscopy 907
mappings of copper (b) titanium (c) and titanium and copper overlapped (d) on the 908
TiO2/Cu2O/RGO surface.909
Fig. 6. High angle annular dark field image (a) zoomed in on the analyzed area (b) and 910
electron energy loss spectroscopy mappings of titanium (c), copper (d) and an overlapping 911
of titanium and copper mappings (e) on the TiO2/Cu2O/RGO surface.912
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Fig. 7. Optical absorbance spectra of TiO2 (a), TiO2/RGO (b), TiO2/Cu2O/RGO (c), Cu2O 913
(d) and Cu2O/RGO (e).914
Fig. 8. UV-Vis absorbance spectra of the methylene blue solution during the photocatalytic 915
degradation under solar radiation using the TiO2 (a), Cu2O (b), TiO2/RGO (c), Cu2O/RGO 916
(d) and TiO2/Cu2O/RGO (e) photocatalysts after 0, 1, 2, 3, 4 and 5 h.917
Fig. 9. Relative optical absorbance of the methylene blue solutions during the 918
photodecomposition tests on TiO2, Cu2O, TiO2/RGO, Cu2O/RGO and TiO2/Cu2O/RGO.919
Fig. 10. Photocurrent generation from FTO/TiO2/Cu2O/RGO nanocomposite film and also 920
in the FTO/TiO2, FTO/Cu2O, FTO/TiO2/RGO and FTO/Cu2O/RGO films. The experiments 921
were done under visible light irradiation using an aqueous electrolyte containing 1 mol L-1922
Na2SO3 at short circuit conditions.923
924
925
926
927
928
929
930
931
932
933
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Figures934
935
936
Fig. 1. XRD patterns of TiO2 (a), Cu2O (b), TiO2/RGO (c), Cu2O/RGO (d) and 937
TiO2/Cu2O/RGO (e); the peaks marked with ■ are related to graphene oxide, ● to anatase 938
phase, ♦ to rutile phase of titanium dioxide and ▲ to cuprous oxide.939
940
941
942
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943
944
Fig. 2. Scanning electron microscopy images of TiO2/RGO at (a) 20,000 and (b) 100,000945
and the Cu2O/RGO nanocomposite at (c) 20,000 and (d) 100,000.946
RGOsheets
Cu2O
TiO2
RGOsheets
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947
Fig. 3. Scanning electron microscopy images of the TiO2/Cu2O/RGO nanocomposite at (a)948
20,000 and (b) 100,000.949
950
951
952
953
954
TiO2
Cu2O
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955
956
Fig. 4. Transmission electron microscopy images of the TiO2/Cu2O/RGO nanocomposite.957
Cu2O
TiO2
RGOsheets
TiO2
RGOsheets
Cu2O
TiO2
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958
Fig. 5. High angle annular dark field image (a) and energy dispersive spectroscopy 959
mappings of copper (b) titanium (c) and titanium and copper overlapped (d) on the 960
TiO2/Cu2O/RGO surface.961
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962
Fig. 6. High angle annular dark field image (a) zoomed in on the analyzed area (b) and 963
electron energy loss spectroscopy mappings of titanium (c), copper (d) and an overlapping 964
of titanium and copper mappings (e) on the TiO2/Cu2O/RGO surface.965
966
967
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200 300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
0,0
0,2
0,4
0,6
0,8
1,0
0,0
0,2
0,4
0,6
0,8
1,0
Abs
orba
nce
/ a. u
.
Wavelength / nm
(c)
(b)
(a)
0,0
0,2
0,4
0,6
0,8
1,0
200 300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
200 300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
Abs
orba
nce
/ a. u
.
Wavelength / nm
(c)
(e)
(d)
968
Fig. 7. Optical absorbance spectra of TiO2 (a), TiO2/RGO (b), TiO2/Cu2O/RGO (c), Cu2O 969
(d) and Cu2O/RGO (e).970
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400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Irradiation
time
(a)
Abs
orba
nce
Wavelength / nm
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
(b)
Irradiation
time
Abs
orba
nce
Wavelength / nm971
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Irradiation
time
(c)
Abs
orba
nce
Wavelength / nm
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Irradiation
time
(d)A
bsor
banc
e
Wavelength / nm972
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orba
nce
Wavelength / nm
Irradiation
time
(e)
973
Fig. 8. UV-Vis absorbance spectra of the methylene blue solution during the photocatalytic 974
degradation under solar radiation using the TiO2 (a), Cu2O (b), TiO2/RGO (c), Cu2O/RGO 975
(d) and TiO2/Cu2O/RGO (e) photocatalysts after 0, 1, 2, 3, 4 and 5 h.976
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0 1 2 3 4 50
10
20
30
40
50
60
70
80
90
100R
elat
ive
abso
rban
ce
Time / h
TiO2
Cu2O
TiO2/RGO
Cu2O/RGO
TiO2/Cu
2O/RGO
977
Fig. 9. Relative optical absorbance of the methylene blue solutions during the 978
photodecomposition tests on TiO2, Cu2O, TiO2/RGO, Cu2O/RGO and TiO2/Cu2O/RGO.979
980
981
982
983
984
985
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986
Fig. 10. Photocurrent generation from FTO/TiO2/Cu2O/RGO nanocomposite film and also 987
in the FTO/TiO2, FTO/Cu2O, FTO/TiO2/RGO and FTO/Cu2O/RGO films. The experiments 988
were done under visible light irradiation using an aqueous electrolyte containing 1 mol L-1989
Na2SO3 at short circuit conditions.990
991