Z-scheme Photocatalytic CO2 Conversion on Three

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Z-scheme Photocatalytic CO Conversion on Three-Dimensional BiVO/Carbon-Coated CuO Nanowire Arrays under Visible Light 4

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Chansol Kim, Kyeong Min Cho, Ahmed Al-Saggaf, Issam Gereige, and Hee-Tae Jung ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00003 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Catalysis

Z-scheme Photocatalytic CO2 Conversion on ThreeDimensional BiVO4/Carbon-Coated Cu2O Nanowire Arrays under Visible Light Chansol Kim †‡〦, Kyeong Min Cho†‡〦, Ahmed Al-Saggaf§, Issam Gereige§ and Hee-Tae Jung*†‡

†

Department of Chemical & Biomolecular Engineering (BK-21 plus), Korea Advanced Institute

of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea ‡

KAIST Institute for Nanocentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea

§

Saudi Aramco, Research and Development Center, Dhahran 31311, Saudi Arabia

ABSTRACT

Cuprous oxide (Cu2O) is one of the most promising material for photoreduction of CO2 because of high conduction band and low band gap, which enable to produce high-potential electrons under visible-light irradiation. However, it is difficult to reduce the CO2 using Cu2O based photocatalyst due to fast charge recombination and low photostability. In this work, we enhanced the photocatalytic CO2 conversion activity of Cu2O by hybridization of Cu2O NWAs, carbon

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layer and BiVO4 nanoparticles. By constructing a Z-scheme charge flow on 3-D NWAs structure, the BiVO4/carbon-coated Cu2O (BVO/C/Cu2O) NWAs shows significantly enhanced charge separation and light harvesting property. As a result, CO formation rate of BVO/C/Cu2O was 9.4 and 4.7 times those of Cu2O mesh and Cu2O NWAs, respectively, under visible light irradiation. In addition, it retained 98% of its initial photocatalytic CO2 conversion performance after five reaction cycle (20 h) because of protective carbon layer and Z-schematic charge flow. We believe that this work provides a promising photocatalyst system that combines 3-D NWAs structure and a Z-scheme charge flow for efficient and stable CO2 conversion.

Carbon dioxide conversion, Photocatalyst, Z-scheme, Cu2O nanowire arrays, Visible light


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Climate change due to global warming arising from the greenhouse effect of CO2 poses a threat to our future. The depletion of fossil resources is another large issue that has compelled the development of sustainable energy solutions. From this viewpoint, the photocatalytic conversion of CO2 into useful chemicals or fuels is one of the most attractive and promising areas of research that can resolve both environmental issues and the energy crisis.1,2 Generally, semiconducting materials such as TiO2, CdS, ZnO, and g-C3N4 are commonly used to generate electrons and holes using sunlight.3-6 Unfortunately, these semiconducting photocatalysts suffer from very low CO2 photoconversion efficiencies because of electron–hole recombination, their narrow scope of light absorption, small surface area, and low conduction-band position. Thus, the visible-light-driven photocatalyst that can overcome the low photoconversion efficiency must be developed to achieve efficient and stable photocatalytic CO2 conversion.7,8 Among the various semiconducting materials used for CO2 photoconversion, cuprous oxide (Cu2O) is of particular interest because of its suitable band gap of ~2.0 eV, which imparts the ability to absorb visible light and produce electrons with sufficient potential energy for CO2 photoreduction due to its high conduction band. It is an abundant, inexpensive, and environmentfriendly material.9,10 Specifically, three-dimensional (3-D) Cu2O nanowire arrays (NWAs) based on mesh substrates are highly attractive building blocks for use in CO2 photoconversion. Individual one-dimensional (1-D) Cu2O nanowire possesses an efficient photoinduced electron– hole transport property that is induced by modulating the actual carrier diffusion length and lightabsorption depth.11-16 A large surface area with enhanced light absorption capabilities can be obtained with this material by forming 3-D Cu2O NWAs. However, photocatalysts based on 3-D Cu2O NWAs such as Au–Cu/graphene/Cu2O NWAs and carbon-coated Cu2O NWAs studied thus far exhibit insufficient CO2 conversion efficiencies and stabilities.15,16


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Herein, the photocatalytic conversion of CO2 under visible light irradiation was significantly enhanced by incorporating a Z-scheme charge flow using a 3-D Cu2O NWAs platform. To construct the Z-scheme using a Cu2O based photocatalyst, BiVO4 particles and conductive carbon were coated onto Cu2O NWAs to form water oxidation photocatalyst and an electron mediator, respectively. This Z-scheme light-harvesting system can generate electron–hole pair with a high redox potential and a low recombination rate.17-23 In addition, this photocatalyst with 3-D NWAs structure showed twice the photocatalytic activity relative to that of the mesh structure because of its large surface area, enhanced charge-transport properties, and lightscattering or reflecting effect.


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Figure 1. Morphology of BVO/C/Cu2O NWs. a) Schematic of the synthesis of BVO/C/Cu2O NWAs. b) Large-area SEM image and c) SEM image with inset showing the optical image of BVO/C/Cu2O NWAs. d) ECSA determined from the electrical double-layer capacitance obtained by CV with a changing scan rate. Figure 1a shows the overall procedure for fabricating the 3-D BiVO4/carbon layer coated onto the Cu2O NWAs (BVO/C/Cu2O NWA). First, Cu(OH)2 NWAs were grown on copper mesh through electrochemical anodization in a 3 M NaOH solution at a constant current density of 10 mA/cm2. The carbon-coated Cu2O NWAs were obtained by immersing Cu(OH)2 NWAs in a glucose solution for 12 h followed by thermal annealing at 550 °C under an Ar atmosphere for 4 h. In this step, the dehydration and oxygen removal of Cu(OH)2 and the coating of the carbon


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layer via the carbonization of glucose occurred simultaneously.24 Next, BiVO4 particles were incorporated into the carbon-coated Cu2O (C/Cu2O) NWAs through the sequential ionic-layer adsorption reaction (SILAR) method using sequential immersion in Bi(NO3)3 and NH4VO3 solutions.25,26 Finally, the hybrid material was post-annealed at 450 °C under an argon atmosphere to increase its crystallinity and to remove organic impurities. An image obtained by scanning electron microscopy (SEM) (Figure 1b) shows that highly dense BVO/C/Cu2O NWAs uniformly covered the copper mesh. In a magnified image, the hybrid materials are shown to be composed of a network structure of individual 1-D BVO/C/Cu2O NWs with irregular rough surfaces ~40 nm in diameter and a few micrometers in length. This irregular rough surface is a result of the dehydration and oxygen removal processes of the smooth Cu(OH)2 NWs. However, no fracture in or damage to the 1-D structure was observed because of the existence of the protective carbon layer (Figures 1c and S1).11 Enlarged surface area of 3-D Cu2O NWAs was also determined through the electrochemical surface area (ECSA) method, in which the electrical double-layer capacitance is measured using cyclic voltammetry (CV) with a changing scan rate (Figure S2).27 The corresponding capacitance (CdI) of the 3-D Cu2O NWA structure was then determined and compared with that of Cu2O on the mesh (Figure 1d). Because we obtained Cu2O on the mesh by anodization at relatively low current density, rough Cu2O nanocrystals formed on the copper mesh (Figure S3). The corresponding capacitance of 3-D Cu2O NWAs was found to be 88.75 mF/cm2, which is 3.57 times that of Cu2O on mesh (24.87 mF/cm2). This marked increase in capacitance indicates a large surface area achieved with the uniformly grown, dense 3-D NWA structure.


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Figure 2. Characterization of BVO/C/Cu2O NWAs. a) High-resolution TEM image of a BVO/C/Cu2O NW. b) SEM–EDX elemental mapping of Cu, O, C, Bi, and V (scale bar is 1 µm). c) XRD pattern and d) FT-IR spectrum of each sample. e) High-resolution C 1s XPS spectra for BVO/C/Cu2O. In order to identify and visualize the formation of the carbon layer and BiVO4 nanoparticles, high-resolution transmission electron microscopy (HRTEM) was performed (Figure 2a). Clearly, a carbon layer with a thickness of 3 nm is well coated onto the surface of the Cu2O NWs. BiVO4 particles with radii of 3 nm are also attached to the carbon layer, with 0.301 nm interlayer spacing of the lattice fringes, which correspond to the (121) plane of BiVO4.28 In the lowmagnification TEM image (Figure S4), the BiVO4 particles are well-dispersed on the surface of the carbon-coated Cu2O NW. To verify the formation of BiVO4 particles and the carbon layer on the Cu2O NWs, energy-dispersive X-ray (EDX) spectroscopy was conducted (Figure 2b). In the


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EDX elemental mapping of BVO/C/Cu2O, colored dots representing Cu, O, C, Bi, and V follow the shape of the NW; they suggest a uniform distribution of BiVO4 on the carbon layer of the Cu2O NWAs. The chemical composition and crystal structure of the fabricated material were examined by Xray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. The XRD patterns of the Cu2O and BVO/C/Cu2O NWAs are presented in Figure 2c. The spectrum of the Cu2O NWAs shows representative peaks of Cu2O with a dominant (111) orientation and a Cu peak resulting from the mesh substrate.29 The spectrum of the BVO/C/Cu2O NWAs shows additional peaks at 2θ = 28.95°, which are assigned to the (121) plane of monoclinic scheelite BiVO4.26 This is in agreement with the interlayer spacing of BiVO4 shown in the HRTEM image in Figure 2a. These XRD results suggest that crystalline BiVO4 was indeed loaded onto the C/Cu2O NWAs. Because of the low BiVO4 content, only the most intense peak can be seen (Figure S5). Furthermore, distinct peaks at the Bi 4f and V 2p bands corresponding to BiVO4, as well as the Cu 2p band of Cu2O, can be identified in the XPS spectra in Figure S6. The O 1s peaks shown in this figure belong to two different species, namely, Cu2O and the BiVO4 lattice oxygen, which are located at about 529.9 and 530.8 eV, respectively.30 Fourier transform infrared (FT-IR), XPS, and Raman spectral analyses were carried out to determine the chemical states of the carbon layer on Cu2O. In the FT-IR spectra shown in Figure 2d, the Cu2O NWAs produced only one intense absorption peak, observed at 630 cm−1. This peak is assigned to the Cu–O stretching vibration indicating the formation of crystalline Cu2O. The characteristic peaks of the carbon layer in BVO/C/Cu2O and C/Cu2O NWAs can be observed at four different locations: 748, 1057, 1120, and 1430 cm−1, which indicate graphitic carbon out-of-plane bending, alkoxy C–O stretching, carbonyl C–O stretching vibration, and


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ACS Catalysis

carbonyl O–H stretching, respectively. The peak at 1698 cm−1 is assigned to an in-plane vibration of sp2-hybridized C–C bonds.15,31 Therefore, the carbon layer obtained corresponds to graphitic carbon with some functional groups. To further investigate the chemical and bonding states of the carbon layer, high-resolution XPS spectra were obtained. The C 1s spectrum of BVO/C/Cu2O NWAs shows a strong dominant C–C peak at 285.0 eV, indicating the formation of a carbon layer on the Cu2O NW surface. The lowintensity peaks at 286.2 and 288.3 eV respectively correspond to C–OH and O–C=O, which are attributed to the surface oxygen functional groups in the carbon layer (Figure 2e).32 Moreover, the Raman spectrum shows distinct D and G bands located respectively at 1365 and 1595 cm−1, which correspond to amorphous and graphitic carbon layer (Figure S7).33,34 Therefore, the results obtained from the FT-IR, XPS, and Raman spectra confirm the formation and chemical state of the conductive carbon layer.


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Figure 3. Light-harvesting property and band-edge position characterization of BVO/C/Cu2O NWAs. a) UV–vis diffuse-reflectance spectra of Cu2O-based photocatalysts; inset shows Tauc plot of Cu2O and BiVO4. b) PL emission spectra for BVO/C/Cu2O NWAs with an excitation wavelength of 375 nm. c) Transient photocurrent under visible light (>420 nm) irradiation for BVO/C/Cu2O NWAs and other samples. Ultraviolet photoelectron spectra of d) Cu2O and e) BiVO4 and f) schematic energy-level diagram. Figure 3 shows the light absorption and harvesting properties of BVO/C/Cu2O NWAs under visible light irradiation. The optical absorption properties of four different Cu2O-based samples (Cu2O mesh, Cu2O NWAs, C/Cu2O NWAs, and BVO/C/Cu2O NWAs) were measured using UV–vis diffuse-reflectance spectra subjected to Kubelka–Munk transformation (Figure 3a). Because of the band gap (Eg) of Cu2O (2.07 eV, obtained from the Tauc plot shown in the inset), the samples demonstrated visible-light absorption at wavelengths shorter than 620 nm. The visible-light absorption of the Cu2O NWAs was markedly enhanced as compared with Cu2O


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ACS Catalysis

mesh. This enhancement is due to a structural effect, which can be explained by light reflection and scattering.35 The additional exposed Cu2O NWs on the lateral surface of the copper mesh can also contribute to light absorption.36 C/Cu2O NWAs showed a slight decrease in the amount of visible light absorbed due to light shielding induced by the carbon layer.11,16 Attachment of BiVO4 onto the C/Cu2O NWAs enhanced light absorption at around 520 nm because of the presence of BiVO4 (which possesses Eg of 2.41 eV). The PL emission spectra shown in Figure 3b display the different PL intensities among samples based on Cu2O NWAs. The PL intensity of C/Cu2O NWAs was half of that of Cu2O; this may be explained by the reduced effects of recombination in the former sample due to the rapid transfer of electrons to the conductive carbon layer. Furthermore, the PL intensity decreased significantly in comparison with both Cu2O and C/Cu2O NWAs upon attachment of BiVO4 onto C/Cu2O NWAs. This decrease may be explained by the formation of a heterojunction between the conductive carbon layer and semiconductors, which resulted in an efficient charge transfer pathway that can strongly inhibit the fast recombination of photoinduced electron–hole pairs.16,37,38 The light-harvesting property of the materials was determined by plotting the photocurrent density under visible light (>420 nm) irradiation as a function of irradiation time (Figure 3c). BVO/C/Cu2O NWAs showed the highest photocurrent density, about 4.16 times that of Cu2O mesh. The addition of a carbon layer coating and the incorporation of BiVO4 resulted in photocurrent densities 1.61 and 2.87 times, respectively, that of Cu2O NWAs. This significant improvement suggests the enhanced light harvesting due to the efficient electron–hole pair separation via heterojunction charge flow in BVO/C/Cu2O NWAs. On the other hands, conventional heterojunction of BVO/Cu2O NWAs without carbon layer induced only 1.36 times photocurrent increment than Cu2O NWAs. Regarding the structural effect, the 3-D Cu2O NWAs


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showed photocurrent density 1.45 times higher than that of Cu2O mesh. This enhancement of the photocurrent can be ascribed to the efficient visible-light absorption by the 3-D NWAs and the efficient charge-carrier transfer in the 1-D NW structure.35 Further, electrochemical impedance spectroscopy (EIS) under dark condition was conducted to investigate the charge transfer property (Figure S8). Charge transfer resistance could be determined by semicircle diameter of Nyquist plot. While the Cu2O NWAs shows very large diameter due to its low conductivity, carbon coating (C/Cu2O NWAs) and formation of heterojunction with BiVO4 (BVO/Cu2O NWAs) shows smaller diameter by effective charge transfer pathway. Moreover, semicircle of BVO/C/Cu2O NWAs was significantly smaller than others, implying much reduced resistance by fast transport of charge carriers among BiVO4, carbon and Cu2O. To determine the charge flow and the energy level of the heterojunction, the band edge configuration of BVO/C/Cu2O NWAs was investigated by ultraviolet photoelectron spectroscopy (UPS). The upper onset and secondary onset in the UPS spectra for Cu2O was found to 16.5 and 0.61 eV, respectively (Figure 3d). From the UPS spectra for BiVO4, the upper onset and secondary onset was determined to be 16.3 and 2.3 eV, respectively (Figure 3e). Thus, the valance band level (Ev) was calculated to be -5.31 and -7.2 eV (vs. vacuum level) by subtracting the width of the He I UPS spectra from the excitation energy of 21.2 eV. In addition, Fermi levels (Ef) of Cu2O and BiVO4 were determined by subtracting upper onset from the excitation energy, which were -4.7 and -4.9 eV vs. vacuum level, respectively. Finally, the conduction band level (Ec) was estimated using the equation Ec = Ev - Eg. These energy values (vs. vacuum) in electron volts are converted into normal electrode potential (vs. NHE) in volts, using the formula (Evac(eV) = E°NHE(V) - 4.44).39,40 The estimated band positions for Cu2O and BiVO4 strongly agree with the requirement for a Z-scheme system in which the photoinduced


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electrons in the BiVO4 conduction band can easily transfer to the Cu2O valence band (Figure 3f).21,41 Therefore, the photocatalytic reduction of CO2 and water oxidation may be efficiently carried out because of the high redox potential of the electron–hole pairs resulting from the high conduction band and low valence band of Cu2O and BiVO4, respectively.


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Figure 4. Photocatalytic CO2 reduction of the various catalysts. a) CO formation rate and b) CH4 formation rate for each composite. c) The structural effect on the photocatalytic CO2 reduction activity; the inset shows SEM images of the Cu2O mesh and Cu2O NWAs. d) Results of the test for photocatalyst recycling stability for BVO/C/Cu2O and C/Cu2O NWAs. The CO2 photoconversion property of BVO/C/Cu2O NWAs is presented in Figure 4. The photocatalytic CO2 conversion test was conducted under visible light (>420 nm) irradiation in a CO2 atmosphere with water vapor at 40 °C. The formation of CO and CH4 was detected by gas chromatography following 4 h of irradiation. Control experiments were conducted without light, CO2 and photocatalyst (Figure S9). The reaction with

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CO2 was also carried out that

shows predominant signal at m/z = 29 which could be assigned to

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CO in the gas


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chromatography–mass spectrometry (GC-MS) (Figure S10). These provide confirmation that produced CO was originated from CO2 photoconversion reaction. The Cu2O-based photocatalysts used in this study showed photocatalytic CO2 conversion with CO as the main product. The CO formation rate of the BVO/C/Cu2O NWAs, in particular, reached ~3.01 µmol/g/h, much higher than that of other Cu2O-based photocatalysts. This value is about 4.7 and 9.4 times higher than that of Cu2O NWAs and Cu2O mesh, respectively. Among the 3-D Cu2O NWA-based photocatalysts, Cu2O coated with conductive carbon was found to enhance the CO formation rate by only 1.4 times. Interestingly, the Z-scheme charge flow obtained by the incorporation of BiVO4 led to a formation rate 3.3 times higher than that of C/Cu2O NWAs. However, BiVO4 alone produced a trace amount of CO and CH4 because its conduction band level (0.36 V vs. NHE) is more positive than the standard redox potential Eθ (CO2/CO) (−0.53 V vs. NHE). Moreover, conventional heterojunction of BVO/Cu2O NWAs showed adverse effect in photocatalytic CO2 reduction which is ~28% decreased CO formation rate than Cu2O NWAs. (Figure 4a).42 This decrease is mainly due to electron flow to conduction band of BiVO4 where the electrons possess less negative potential. Therefore, this exceptional enhancement is mainly due to the induction of the Z-scheme that reduced recombination by enhancing the charge separation and transfer, which was previously verified by PL intensity and photocurrent density analyses. Moreover, CH4 formation was achieved only with BVO/C/Cu2O NWAs, the formation of which requires more electrons than CO formation (two electrons are required for CO and eight electrons are required for CH4; Figure 4b). This result may also be ascribed to Z-scheme charge flow, which allows stacking of the photogenerated electrons on the conduction band of Cu2O.37,43 Furthermore, the BVO/C/Cu2O photocatalyst was optimized by changing the carbon layer thickness and the amount of BiVO4 particles (Figure S11). The carbon layer and BiVO4


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introduced into the Cu2O nanowires possess conflicting roles in the efficient Z-schematic charge transfer and blockage of light absorption (Figure S12).11,37 To clarify the effect of the 3-D NWAs in photocatalytic activity, each mesh and NWAs structure consisting of the same photocatalytic component (Figure 4c) were compared. Notably, a ~1.8 to 2 times enhancement in activity was obtained by constructing a 3-D NWA structure; this implies the efficient lightabsorption and large surface area characteristics this structure enables. Moreover, BVO/C/Cu2O NWAs were stable while undergoing CO2 photoconversion. To establish the recycling stability of the photocatalysts, the aforementioned photocatalytic CO2 conversion experiment was conducted for five cycles (~20 h; Figure 4d). The activity of Cu2O decreased to 60% relative to that in the first cycle; this degradation is a result of serious photocorrosion due to self-oxidation involving holes. This can be verified by the XPS spectrum in Figure S13, which shows the formation of Cu2+ from Cu+ oxidation following the CO2 photoconversion reaction.42,43 On the other hand, BVO/C/Cu2O NWAs retained 98% of the activity in the first cycle of testing. The negligible change in the position and intensity of the XPS peak after the reaction supports the high stability of this material during the photocatalytic reaction. Stable formation of CO was also verified by detection of CO amount over time (Figure S14). This photostability originates from the carbon layer that serves as a protective layer, which prevents Cu2O oxidation to CuO. In addition, a hole that is successively consumed could help prevent Cu2O self-oxidation, as a consequence of the Z-schematic electron transfer from BiVO4 to Cu2O.16,45,46


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Figure 5. Probe-molecule PL experiment for the Z-scheme mechanism. a) PL spectral change of the aqueous coumarin solution with BVO/C/Cu2O NWAs under irradiation. b) Comparison of PL intensities for Cu2O, BiVO4, and BVO/C/Cu2O NWAs at 490 nm under irradiation. c) Schematic of the Z-scheme and a conventional heterojunction. The marked enhancement of photocatalytic CO2 conversion is due to the Z-scheme charge flow in BVO/C/Cu2O NWAs. To confirm the Z-scheme charge-transfer mechanism, the energy level of the photoinduced hole was investigated by PL experiment using coumarin as a probe molecule in water.47,48 Coumarin can be transform to 7-hydroxylcoumarin when the photocatalyst produces hydroxyl radicals from the photooxidation of the hydroxyl ion. This reaction is easily detected by the PL signal because of its on–off switching behavior. While coumarin does not


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produce a significant PL peak under excitation at 325 nm, 7-hydroxylcoumarin has intense PL at 490 nm. The PL spectral change of the coumarin solution with BVO/C/Cu2O NWAs shows a substantially intensified PL peak during visible-light irradiation (Figure 5a). No peak was observed in the case of Cu2O because of its higher valence band potential (0.87 V vs. NHE) in comparison with the OH−/OH oxidation potential (2.3 V vs. NHE), which is insufficient for producing a hydroxyl radical. BiVO4 could generate a PL peak because of its lower valence band potential (2.76 V vs. NHE). Interestingly, BVO/C/Cu2O NWAs caused the intensity of the peak to increase faster than BiVO4 did, in spite of the lower amount of BiVO4 (Figures 5b and S15). In other words, the holes in the BVO/C/Cu2O photocatalyst tended to accumulate faster at the valence band of BiVO4 in comparison with BiVO4 photocatalyst. However, the photoinduced hole at the BiVO4 valence band at the conventional heterojunction is transferred to the more negative valence band of Cu2O. Therefore, the PL spectra obtained from reaction of coumarin indicate that BVO/C/Cu2O NWAs exhibit the Z-scheme charge flow. Consequently, the electrons and holes with high reducing and oxidizing potentials are obtained with the Z-scheme BVO/C/Cu2O NWAs (Figure 5c).


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In summary, we fabricated 3-D BVO/C/Cu2O NWAs via the facile method of electrochemical anodization and carbonization followed by the SILAR method in order to produce a highly efficient CO2 conversion photocatalyst. To our knowledge, no other report on CO2 photoconversion using a Z-scheme approach with BiVO4, a carbon layer and 3-D Cu2O NWAs currently exists. The construction of a Z-scheme on a 3-D NWA structure played a significant role in enhancing the photocatalytic CO2 conversion activity. The key parameters that led to the improved photocatalytic activity are the following: (1) an increase in the surface area and enhancement of light harvesting by the 3-D Cu2O NWAs structure, (2) efficient charge separation and transfer by the Z-scheme and conductive carbon layer, and (3) strong reduction and oxidation potentials attained with the Z-scheme. Through the above strategies, the photocatalytic CO2 conversion performance of BVO/C/Cu2O NWAs featured a CO formation rate of 3.01 µmol/g/h, which is 9.4 and 4.7 times those on Cu2O mesh and Cu2O NWAs, respectively. After five cycles, this photocatalyst also exhibited outstanding stability while retaining 98% of the original formation rate (20 h) because of both the incorporation of a carbon protective layer and the use of the Z-scheme. This newly developed photocatalyst is an appealing model for the conversion of CO2 using low-cost materials and solar energy.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Description of fabrication method, material characterization and photoconversion test. Additional data for SEM and TEM images, XRD, EDX, Raman, XPS, UV-vis, EIS and ECSA measurement and photocatalytic CO2 conversion results (PDF) AUTHOR INFORMATION Corresponding Author *E-mail for H.-T.J: [email protected] Author Contributions C.K.〦 and K.M.C.〦 contributed equally to this work.

ACKNOWLEDGMENT This work was funded by Saudi Aramco-KAIST CO2 Management Center. In addition, this research was supported by a grant from the National Research Foundation of Korea funded by the Ministry of Science, ICT, and Future Planning (grant no. 2015R1A2A1A05001844). REFERENCES (1) Dimitriou, I.; Garcia-Gutierrez, P.; Elder, R. H.; Cuellar-Franca, R. M.; Azapagic, A.; Allen, R. W. K. Carbon Dioxide Utilization for Production of Transport Fuels: Process and Economic Analysis. Energy Environ. Sci. 2015, 8, 1775-1789.


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Page 21 of 28 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

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(2) White, J. L.; Baruch, M. F.; Pander, J. E.; Hu, Y.; Fortmeyer, I. C.; Park, J. E.; Zhang, T.; Liao, K.; Gu, J.; Yan, Y.; Shaw, T. W.; Abelev, E.; Bocarsly, A. B. Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes. Chem. Rev. 2015, 115, 12888-12935. (3) Wang, W. N.; An, W.-J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276-11281. (4) Cho, K. M.; Kim, K. H.; Park, K.; Kim, C.; Kim, S.; Al-Saggaf, A.; Gereige, I.; Jung, H.-T. Amine-Functionalized Graphene/CdS Composite for Photocatalytic Reduction of CO2. ACS Catal. 2017, 7, 7064-7069. (5) He, Y. M.; Wang, Y.; Zhang, L. H.; Teng, B. T.; Fan, M. H. High-Efficiency Conversion of CO2 to Fuel over ZnO/g-C3N4 Photocatalyst. Appl. Catal., B 2015, 168, 1-8. (6) Yu, J.; Wang, K.; Xiao, W.; Cheng, B. Photocatalytic Reduction of CO2 into Hydrocarbon Solar Fuels over g-C3N4-Pt Nanocomposite Photocatalysts. Phys. Chem. Chem. Phys. 2014, 16, 11492-11501. (7) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem. Int. Ed. 2013, 52, 7372-7408. (8) Tu, W.; Zhou, Y.; Zou, Z. Photocatalytic Conversion of CO2 into Renewable Hydrocarbon Fuels: State-of-the-Art Accomplishment, Challenges, and Prospects. Adv. Mater. 2014, 26, 46074626. (9) An, X.; Li, K.; Tang, J. Cu2O/Reduced Graphene Oxide Composites for the Photocatalytic Conversion of CO2. ChemSusChem 2014, 7, 1086-1093.


21

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Page 22 of 28

(10) Wang, M.; Sun, L.; Lin, Z.; Cai, J.; Xie, K.; Lin, C. p-n Heterojunction Photoelectrodes Composed of Cu2O-Loaded TiO2 Nanotube Arrays with Enhanced Photoelectrochemical and Photoelectrocatalytic Activities. Energy Environ. Sci. 2013, 6, 1211-1220. (11) Zhang, Z.; Dua, R.; Zhang, L.; Zhu, H.; Zhang, H.; Wang, P. Carbon-Layer-Protected Cuprous Oxide Nanowire Arrays for Efficient Water Reduction. ACS Nano 2013, 7, 1709-1717. (12) Mahmoud, M. A.; Qian, W.; El-Sayed, M. A. Following Charge Separation on the Nanoscale in Cu2O-Au Nanoframe Hollow Nanoparticles. Nano Lett. 2011, 11, 3285-3289. (13) In, S.-I.; Vaughn, D. D.; Schaak, R. E. Hybrid CuO-TiO2-xNx Hollow Nanocubes for Photocatalytic Conversion of CO2 into Methane under Solar Irradiation. Angew. Chem. Int. Ed. 2012, 51, 3915-3918. (14) Dubale, A. A.; Su, W.-N.; Tamirat, A. G.; Pan, C.-J.; Aragaw, B. A.; Chen, H.-M.; Chen, C.H.; Hwang, B.-J. The Synergetic Effect of Graphene on Cu2O Nanowire Arrays as a Highly Efficient Hydrogen Evolution Photocathode in Water Splitting. J. Mater. Chem. A 2014, 2, 18383-18397. (15) Hou, J.; Cheng, H.; Takeda, O.; Zhu, H. Three-Dimensional Bimetal-GrapheneSemiconductor Coaxial Nanowire Arrays to Harness Charge Flow for the Photochemical Reduction of Carbon Dioxide. Angew. Chem. Int. Ed. 2015, 54, 8480-8484. (16) Yu, L.; Li, G.; Zhang, X.; Ba, X.; Shi, G.; Li, Y.; Wong, P. K.; Yu, J. C.; Yu, Y. Enhanced Activity and Stability of Carbon-Decorated Cuprous Oxide Mesoporous Nanorods for CO2 Reduction in Artificial Photosynthesis. ACS Catal. 2016, 6, 6444-6454. (17) Li, H.; Tu, W.; Zhou, Y.; Zou, Z. Z-Scheme Photocatalytic Systems for Promoting Photocatalytic Performance: Recent Progress and Future Challenges. Adv. Sci. 2016, 3, 1500389.


22

Page 23 of 28 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

ACS Catalysis

(18) Li, H.; Gao, Y.; Zhou, Y.; Fan, F.; Han, Q.; Xu, Q.; Wang, X.; Xiao, M.; Li, C.; Zou, Z. Construction and Nanoscale Detection of Interfacial Charge Transfer of Elegant Z-Scheme WO3/Au/In2S3 Nanowire Arrays. Nano Lett. 2016, 16, 5547-5552. (19) Wang, Q.; Hisatomi, T.; Suzuk, Y.; Pan, Z.; Seo, J.; Katayama, M.; Minegishi, T.; Nishiyama, H.; Takata, T.; Seki, K.; Kudo, A.; Yamada, T.; Domen, K. Particulate Photocatalyst Sheets Based on Carbon Conductor Layer for Efficient Z-Scheme Pure-Water Splitting at Ambient Pressure. J. Am. Chem. Soc. 2017, 139, 1675-1683. (20) Li, P.; Zhou, Y.; Li, H.; Xu, Q.; Meng, X.; Wang, X.; Xiao, M.; Zou, Z. All-Solid-State ZScheme System Arrays of Fe2V4O13/RGO/CdS for Visible Light-Driving Photocatalytic CO2 Reduction into Renewable Hydrocarbon Fuel. Chem. Commun. 2015, 51, 800-803. (21) Zhou, P.; Yu, J.; Jaroniec, M. All-Solid-State Z-Scheme Photocatalytic Systems. Adv. Mater. 2014, 26, 4920-4935. (22) Jiang, W.; Zong, X.; An, L.; Hua, S.; Miao, X.; Luan, S.; Wen, Y.; Tao, F. F.; Sun, Z. Consciously Constructing Heterojunction or Direct Z-Scheme Photocatalysts by Regulating Electron Flow Direction. ACS Catal. 2018, 8, 2209-2217. (23) Sun, S.; Hisatomi, T.; Wang, Q.; Chen, S.; Ma, G.; Liu, J.; Nandy, S.; Minegishi, T.; Katayama, M.; Domen, K. Efficient Redox-Mediator-Free Z-Scheme Water Splitting Employing Oxysulfide Photcatalysts under Visible Light. ACS Catal. 2018, 8, 1690-1696. (24) Hou, J.; Yang, C.; Cheng, H.; Jiao, S.; Takeda, O.; Zhu, H. High-Performance p-Cu2O/nTaON Heterojunction Nanorod Photoanodes Passivated with an Ultrathin Carbon Sheath for Photoelectrochemical Water Splitting. Energy Environ. Sci. 2014, 7, 3758-3768. (25) Yan, L.; Zhao, W.; Liu, Z. 1D ZnO/BiVO4 Heterojunction Photoanodes for Efficient Photoelectrochemical Water Splitting. Dalton Trans. 2016, 45, 11346-11352.


23

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Page 24 of 28

(26) Odling, G.; Robertson, N. BiVO4-TiO2 Composite Photocatalysts for Dye Degradation Formed Using the SILAR Method. ChemPhysChem 2016, 17, 2872-2880. (27) Schreier, M.; Heroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J. S.; Mayer, M. T.; Luo, J. S.; Gratzel, M. Solar Conversion of CO2 to CO Using Earth-Abundant Electrocatalysts Prepared by Atomic Layer Modification of CuO. Nat. Energy 2017, 2, 17087. (28) Patil, S. S.; Mali, M. G.; Hassan, M. A.; Patil, D. R.; Kolekar, S. S.; Ryu, S.-W. One-Pot in Situ Hydrothermal Growth of BiVO4/Ag/rGO Hybrid Architectures for Solar Water Splitting and Environmental Remediation. Sci. Rep. 2017, 7, 8404. (29) Luo, J.; Steier, L.; Son, M.-K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Lett. 2016, 16, 1848-1857. (30) Deng, Y.; Tang, L.; Zeng, G.; Feng, C.; Dong, H.; Wang, J.; Feng, H.; Liu, Y.; Zhou, Y.; Pang, Y. Plasmonic Resonance Excited Dual Z-scheme BiVO4/Ag/Cu2O Nanocomposite: Synthesis and Mechanism for Enhanced Photocatalytic Performance in Recalcitrant Antibiotic Degradation. Environ. Sci.: Nano 2017, 4, 1494-1511. (31) Kim, K. H.; Jun, Y.-S.; Gerbec, J. A.; See, K. A.; Stucky, G. D.; Jung, H.-T. Sulfur Infiltrated Mesoporous Graphene-Silica Composite as a Polysulfide Retaining Cathode Material for Lithium-Sulfur Batteries. Carbon 2014, 69, 543-551. (32) Pan, Y.-X.; You, Y.; Xin, S.; Li, Y.; Fu, G.; Cui, Z.; Men, Y.-L.; Cao, F.-F.; Yu, S.-H. Goodenough, J. B.; Photocatalytic CO2 Reduction by Carbon-Coated Indium-Oxide Nanobelts. J. Am. Chem. Soc. 2017, 139, 4123-4129. (33) Gao, L.; Liu, R.; Hu, H.; Li, G.; Yu, Y. Carbon-Decorated Li4Ti5O12/Rutile TiO2 Mesoporous Microspheres with Nanostructures as High-Performance Anode Materials in Lithium-Ion Batteries. Nanotechnology 2014, 25, 175402.


24

Page 25 of 28 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

ACS Catalysis

(34) Kim, K. H.; Yang, M.; Cho, K. M.; Jun, Y.-S.; Lee, S. B.; Jung, H.-T. High Quality Reduced Graphene Oxide through Repairing with Multi-Layered Graphene Ball Nanostructures. Sci. Rep. 2013, 3, 3251. (35) Li, Y.; Takata, T.; Cha, D.; Takanabe, K.; Minegishi, T.; Kubota, J.; Domen, K. Vertically Aligned Ta3N5 Nanorod Arrays for Solar-Driven Photoelectrochemical Water Splitting. Adv. Mater. 2013, 25, 125-131. (36) Liu, Z.; Zhang, Q.; Zhao, T.; Zhai, J.; Jiang, L. 3-D Vertical Arrays of TiO2 Nanotubes on Ti Meshes: Efficient Photoanodes for Water Photoelectrolysis. J. Mater. Chem. 2011, 21, 1035410358. (37) Wang, J.-C.; Zhang, L.; Fang, W.-X.; Ren, J.; Li, Y. Y.; Yao, H. C.; Wang, J. S.; Li, Z. J. Enhanced Photoreduction CO2 Activity over Direct Z-Scheme α-Fe2O3/Cu2O Heterostructures under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2015, 7, 8631-8639. (38) Cho, K. M.; Kim, K. H.; Choi, H. O.; Jung, H.-T. A Highly Photoactive, Visible-LightDriven Graphene/2D Mesoporous TiO2 Photocatalyst. Green Chem. 2015, 17, 3972-3978. (39) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970-974. (40) Xiao, G.; Wang, X.; Li, D.; Fu, X. InVO4-Sensitized TiO2 Photocatalysts for Efficient Air Purification with Visible Light. J. Photochem. Photobiol. A 2008, 193, 213-221. (41) Qiu, B.; Zhu, Q.; Du, M.; Fan, L.; Xing, M.; Zhang, J. Efficient Solar Light Harvesting CdS/Co9S8 Hollow Cubes for Z-Scheme Photocatalytic Water Splitting. Angew. Chem. Int. Ed. 2017, 56, 2684-2688.


25

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Page 26 of 28

(42) Chang, X.; Wang, T.; Gong, J. CO2 photo-reduction: Insights into CO2 Activation and Reaction on Surfaces of Photocatalysts. Energy Environ. Sci. 2016, 9, 2177-2196. (43) Li, M.; Zhang, L.; Fan, X.; Zhou, Y.; Wu, M.; Shi, J. Highly Selective CO2 Photoreduction to CO over g-C3N4/Bi2WO6 Composites under Visible Light. J. Mater. Chem. A 2015, 3, 51895196. (44) Bessekhouad, Y.; Robert, D.; Weber, J.-V. Photocatalytic Activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 Heterojunctions. Catal. Today 2005, 101, 315-321. (45) Aguirre, M. E.; Zhou, R.; Eugene, A. J.; Guzman, M. I.; Grela, M. A. Cu2O/TiO2 Heterostructures for CO2 Reduction through a Direct Z-scheme: Protecting Cu2O from Photocorrosion. Appl. Catal. B 2017, 217, 485-493. (46) He, J.; Shao, D.W.; Zheng, L.C.; Zheng, L.J.; Feng, D.Q.; Xu, J.P.; Zhang, X.H.; Wang, W.C.; Wang, W.-H.; Lu, F.; Dong, H.; Cheng, Y.H.; Liu, H.; Zheng, R.K. Construction of Zscheme Cu2O/Cu/AgBr/Ag Photocatalyst with Enhanced Photocatalytic Activity and Stability under Visible Light. Appl. Catal. B. 2017, 203, 917-926. (47) Jin, J.; Yu, J.; Guo, D.; Cui, C.; Ho, W. A Hierarchical Z-Scheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity. Small 2015, 11, 5262-5271. (48) He, Y.; Zhang, L.; Wang, X.; Wu, Y.; Lin, H.; Zhao, L.; Weng, W.; Wan, H.; Fan, M. Enhanced Photodegradation Activity of Methylorange over Z-Scheme Type MoO3-g-C3N4 Composite under Visible Light Irradiation. RSC Adv. 2014, 4, 13610-13619.


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Tertiary BiVO4/carbon coated Cu2O nanowire arrays show enhanced photocatalytic reduction of carbon dioxide under visible light due to its unique 3D structure consisting of nanowires and zschematic charge flow.


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Z-scheme Photocatalytic CO2 Conversion on Three

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