{'fraction_non_alphanumeric': 0.03510622269097484, 'fraction_numerical': 0.025915323188187435, 'mean_word_length': 4.528946272386506, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
CuO/Cu2O nanowire arrays grafted by reduced graphene oxide: synthesis, characterization, and application in photocatalytic reduction of CO2

CuO/Cu2O nanowire arrays (NWAs) were prepared on a Cu foil substrate through a simple thermal process. The NWAs were then grafted with reduced graphene oxide (rGO) nanosheets via self-assembly. The combination of various characterization techniques including SEM, TEM, XRD, and XPS revealed the unique integration between CuO/Cu2O NWAs and umbrella-like rGO nanosheets. The CuO/Cu2O NWAs@rGO sample was found to be more active than bare CuO/Cu2O NWAs for the photocatalytic reduction of CO2 under simulated solar light. The improved photocatalytic activity was attributed to the slow recombination of charge carriers and the efficient transfer of photo-generated electrons through rGO nanosheets.


Introduction
With the rapid industrialization in the past century, the concentration of CO 2 in the atmosphere is increasing tremendously due to fossil fuel combustion, thus leading to global climate changes and a serious threat to the environment. 1,2 Photocatalytic conversion of CO 2 into useful chemicals or fuels by semiconductors has been actively studied. 3,4 Efficient photocatalysts for the reduction of CO 2 are needed for this purpose. 5-8 TiO 2 -based photocatalysts have attracted much attention because of their high stability, low cost, strong oxidation activity and outstanding electronic properties. [9][10][11] However, the fast photoinduced electron-hole pair recombination and wide band gap (3.2 eV) are limitations for its largescale application. 12,13 Thus, it is important to develop visiblelight-driven semiconductors for articial photosynthesis for CO 2 reduction.
CuO and Cu 2 O, with narrow band gaps of 1.2 eV and 2 eV, respectively, are attractive p-type semiconductors for visiblelight-driven photocatalysis. CuO and Cu 2 O have been widely investigated in gas sensors, 14 solar cells, 15 water splitting, 16 electronic materials, 17 and photocatalysts. 5,7,18 Thus, the CuO/ Cu 2 O system may be a good candidate for the photocatalytic reduction of CO 2 . Nevertheless, the low electric conductivity of copper oxides may lead to rapid recombination of photo-generated electrons and holes limits their photocatalytic performance. 7 The modi-cation of copper oxides with some highly conductive carbon materials could improve their performance. 19,20 Reduced graphene oxide (rGO) is a promising additive for hybrid materials, because of its unique properties such as superior mobility of charge carriers, extremely high specic surface area, and excellent conductivity. 21,22 Thus, rGO may be added to the CuO/ Cu 2 O system to improve the photocatalytic performance for CO 2 reduction.
Although studies on the photocatalytic performance of CuO-rGO or Cu 2 O-rGO have been reported frequently, there are less research on CuO/Cu 2 O-rGO system. We anticipate that an ideal approach to construct copper oxide based high-performance photocatalyst is to synthesize a CuO/Cu 2 O-rGO nanostructure in which the CuO/Cu 2 O is tightly covered with ultrathin graphene nanosheets.
In this study, we successfully prepared CuO/Cu 2 O NWAs@rGO composites through a simple thermal process and a self-assembly procedure. The CuO/Cu 2 O NWAs@rGO composite showed enhanced photocatalytic activity for converting CO 2 to CO under simulated solar light, owing to the presence of ultrathin rGO nanosheets that possesses high conductivity and results in slow recombination of charge carriers and efficient transfer of photo-generated electrons. Chemical and structural features of nanocomposites were characterized by XRD, XPS, Raman, SEM and TEM.

Materials
Hydrochloric acid (36.0-38.0%), ethanol (99.7%) and methanol (99.5%) were purchased from Shanghai Chemical Corp. All chemicals involved here were of analytical grade and used without further purication. Copper foil (99.9%, 1 mm thickness) were purchased from Shengjili Metals Sales Company. Partially reduced graphene oxide (rGO) was purchased from Hengqiu Technique Company.

Synthesis of CuO/Cu 2 O NWAs
CuO/Cu 2 O NWAs were fabricated by a simple thermal oxidation method. Copper foil (2 Â 2 cm 2 ) was carefully cleaned with a 6.0 M HCl solution in an ultrasound bath for 30 min to remove the CuO layer from the surface, then rinsed with deionized water and absolute ethanol for several times, and nally dried in air naturally. The bare Cu foil was calcinated at 600 C in air for 4 h with a speed of 10 C min À1 to obtain CuO/Cu 2 O NWAs.

Synthesis of CuO/Cu 2 O NWAs@rGO
CuO/Cu 2 O NWAs@rGO was fabricated via a two-step selfassembly procedure. In that procedure, 30 mg reduced graphene oxide (rGO) was added into 300 mL of methanol, and aer being ultrasonically treated for 3 h, rGO was exfoliated into thin sheets and disintegrated into a homogeneous suspension. CuO/Cu 2 O NWAs grown on the Cu foil were then put on a platform which was hung in the suspension and the suspension was stirred at room temperature for 24 h. Finally, the obtained sample was collected and dried in a vacuum oven at 60 C.

Chemical and structural characterizations of materials
X-ray diffraction (XRD) experiments were conducted on a Bruker D8 Advance X-ray diffractometer (Bruker Optics, Ettlingen, Germany) with Cu-Ka radiation, operated at 40 kV and 40 mA (scanning step: 0.02 s À1 ) in 2q ¼ 10-80 . Scanning electron microscopy (TEM) and transmission electron microscopy (TEM) experiments were conducted on a Tescan Vega3 scanning electron microscope (TESCAN Ltd., Czech) and a JEM-2010 transmission electron microscope (JEOL Ltd., Japan) at an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 XPS meter (Thermo Scien-tic) with monochromized Al Ka irradiation at 250 W. Raman spectra were obtained with a Dilor LabRam-1B microscopic Raman Spectrum. Electrochemical impedance spectroscopy (EIS) experiments were conducted on a VSP three-electrode electrochemical analysis system (Bio-logic Science Instruments, France) in 6 M KOH aqueous solutions and obtained at the frequency from 0.01 Hz to 100 kHz with amplitude of 5 mV. The products supported on copper foil (2 Â 2 cm 2 ) were directly used as the working electrode without any binder or conductive additive. A Pt plate (2 Â 2 cm 2 ) and an Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively.

Photocatalytic experiments
The photocatalytic experiments for CO 2 and H 2 O reduction were carried out using CuO/Cu 2 O NWAs and CuO/Cu 2 O NWAs@rGO as the photocatalysts in a gas-closed circulation system with the volume of 2700 mL. For each test, 4 Â 4 cm 2 catalyst was placed onto the stainless-steel lm which was xed in the center of reactor. A Xe arc lamp (500 W) with sunlike radiation spectrum was put at the top of quartz window with the distance of 8 cm. The circulated cooling water was used to keep the photoreactor at ambient temperature during the reaction. Before the illumination, the reactor was vacuumed and purged with the CO 2 and H 2 O mixture at about 20 mL min À1 for 2 h to establish an adsorption-desorption equilibrium. During the irradiation, the gas phase products were taken at a given time interval (1 h) for subsequent quantitative analysis of product by using an on-line gas chromatograph. The much stronger peaks related to graphitic sp 2 carbon emerging from rGO skeleton suggests considerable content of graphitic carbon. The peak component at 286.6 eV was originated from the residual oxygen functionalities of rGO. The high energy peak component with low intensity at 288.4 eV was due to the environmental contamination to the sample. The O 1s spectrum in Fig. 1c consists of three oxygen contributions at 530.0, 531.4, and 533.4 eV, corresponding to oxygen bonded to Cu, carbonyl oxygen and oxygen atoms in hydroxyl groups, and oxygen atoms in carboxyl groups. 26 Fig. 1d presents the Cu 2p XPS spectrum. Two Cu peaks at 932.2 and 952.0 eV, corresponding to the Cu 2p 3/2 and Cu 2p 1/2 peaks of Cu + , respectively, conrm the existence of Cu 2 O. 27 The peaks at 933.5 and 953.5 eV with two extra shake-up satellites are assigned to the Cu 2p 3/2 and Cu 2p 1/2 peaks of Cu 2+ , respectively. 28,29 The samples were further characterized by Raman spectroscopy (Fig. 1e). Compared to the smooth spectrum of CuO/ Cu 2 O NWAs, two strikingly similar peaks of rGO and CuO/Cu 2 O NWAs@rGO (centered at $1580 cm À1 which represents the G-line and $1350 cm À1 representing the D-line) indicate the presence of rGO in CuO/Cu 2 O NWAs@rGO.

Morphological and structural analyses
The surface morphologies of CuO/Cu 2 O NWAs and CuO/ Cu 2 O NWAs@rGO were characterized by SEM (Fig. 2). The Cu foil shown in Fig. 2a has a smooth surface with unidirectional nicks. Fig. 2b and c report SEM images of nanowires grown with high density, as obtained by the oxidation of a Cu foil for 4 h at 600 C. Aer heating the Cu foil in air for 4 h, the large-scale, dense, and directional CuO/Cu 2 O NWAs were directly grown in situ on the copper foil. The growth of the nanowires was preceded by a deformation of the substrate due to a straining force, with the formation of a hill and valley structure, in accordance with the literature. 30 SEM analysis in cross-view reveals the absolutely straight nanowires, which allows us to determine that the average length of the nanowires is 25-35 mm (Fig. 2d). SEM images of CuO/Cu 2 O NWAs@rGO at different magnications are shown in Fig. 2e-h. Plenty of ultrathin rGO nanosheets ock together and accumulate on the top of CuO/Cu 2 O NWAs like umbrellas (Fig. 2e). Furthermore, long and uniform CuO/Cu 2 O NWAs are coated with well-spread rGO nanosheets (Fig. 2f-h). Numerous tips of CuO/Cu 2 O NWAs stretch directionally and were closely wrapped by the rGO nanosheets, suggesting that the two materials are well combined through self-assembly procedure.
The structure of the sample obtained by annealing the copper foil at 600 C in air for 4 h and CuO/Cu 2 O NWAs@rGO were further studied by TEM. Typical TEM images of a single CuO/Cu 2 O NWAs as synthesized are shown in Fig. 3a-c. The products have wire-like shapes and the nanowires are solid and straight with smooth surfaces and uniform diameters of $115.8 nm (Fig. 3a and b). The interlayer fringe of the selected area is 0.23 nm, which agrees well with the (111) plane of CuO (Fig. 3c), 17,28 whereas there is no interlayer fringe associated with Cu 2 O. Thus, these nanowires are composed of CuO phase and the underlying lm is Cu 2 O. 31 As seen from Fig. 3d, wrinkled and little aggregated rGO nanosheets were observed. The thickness of the ultrathin rGO nanosheets is about 40 nm.

Photocatalytic activity
Electrochemical impedance spectroscopy (EIS) was further used to investigate the photogenerated charge separation process. Fig. 4a shows the EIS Nyquist plots of CuO/Cu 2 O NWAs and CuO/Cu 2 O NWAs@rGO. A smaller arc radius of the EIS Nyquist plot suggests an effective separation of photogenerated electron-hole pairs and fast interfacial charge transfer. 32,33 Evidently, the diameters of the arc radius of CuO/Cu 2 O NWAs@rGO is smaller than that of CuO/Cu 2 O NWAs, further conrming that the ultrathin layer of rGO with good conductivity facilitates the electron migration to the reaction sites on the surface of the composite. 34 CuO/Cu 2 O NWAs and CuO/Cu 2 O NWAs@rGO were tested for the photocatalytic reduction CO 2 with H 2 O under simulated solar irradiation. Fig. 4a shows the evolution of the main product (CO) as function of irradiation time. No CO is observed in the absence of photocatalyst, whereas CuO/Cu 2 O NWAs and CuO nanomaterials absorb visible light of solar spectrum very efficiently and generate electron-hole pairs. However, the fast recombination of electron-hole pairs (which is crucial for the electron-dominated reduction reaction) owing to the narrow band gap makes them unavailable for photocatalytic reaction. 5 Therefore, CuO/Cu 2 O NWAs showed lower photocatalytic conversion. CuO/Cu 2 O NWAs@rGO revealed improved photocatalytic activity, most likely ascribed to the improvement of the conductivity of CuO/Cu 2 O NWAs@rGO by introducing rGO and the nearly equal position of conduction band of CuO ($4.1 eV) and LUMO of graphene ($4.4 eV). 35,36 rGO platelets with high conductivity provide a direct pathway for efficient separation and transfer of photoexcited electrons, which means that the photo-generated electron in the conduction band of CuO/Cu 2 O NWAs can easily transfer to the rGO nanosheets, thus inhibiting the recombination of electron-hole pairs and facilitating the electron transport to the catalytic sites for the reduction of CO 2 to CO. Photo-generated holes can oxidize the water molecules to give protons (H + ). 11 Furthermore, the existence of mixture of Cu + /Cu 2+ oxidation states on the surface of catalysts according to XPS analysis. Cu + may also can interact with holes and suppress the combination of photogenerated electrons and holes.

Conclusions
In conclusion, the large-scale, dense, and directional CuO/Cu 2 O nanowires on Cu foil were graed by ultrathin rGO nanosheets.

Conflicts of interest
There are no conicts to declare.